Pulsed Laser Deposited Films for Microbatteries

This review article presents a survey of the literature on pulsed laser deposited thin film materials used in devices for energy storage and conversion, i.e., lithium microbatteries, supercapacitors, and electrochromic displays. Three classes of materials are considered: Positive electrode materials (cathodes), solid electrolytes, and negative electrode materials (anodes). The growth conditions and electrochemical properties are presented for each material and state-of-the-art of lithium microbatteries are also reported.


Introduction
It has been widely demonstrated that pulsed-laser deposition (PLD) based on the process of the transportation of a material (laser ablation) is a successful technique for the growth of stoichiometric multicomponent oxide films [1,2]. Indeed, PLD has shown unique advantages for the formation of dense films for energy storage and conversion, namely a high reproducibility, easy control of the growth rate, and a high film purity with a variety of substrates, such as amorphous glass, oriented silicon [3], stainless steel [4], (001)Al 2 O 3 [5], indium tin oxide (ITO)-and ZnO-coated glass, and ITO-coated Upilex polymer [6]. Generally, the stoichiometry of the target phase is preserved in PLD films of oxides but a deviation is observed for lithiated material that implies an Li-enriched target. Consequently, the loss of volatile Li during deposition is compensated for by using about a 15 wt.% excess of Li 2 O [7,8]. Accurate stoichiometry can be obtained by controlling several parameters of the process. The typical set-up for the fabrication of PLD films consists of a stainless-steel vacuum chamber evacuated down to a residual pressure less than 1 × 10 −4 Pa before material deposition. Energy (laser fluence) in the range of 1.0 to 3.0 J·cm −2 is generated by a pulsed-laser beam, which falls onto the target surface with an incidence angle of approximately 45 • (see Table 1 for laser characteristics). Indeed, four PLD parameters are of prime importance for the growth of films: The laser fluence; type of substrate; orientation and lattice parameters, which must match with those of the film for an efficient epitaxy process; substrate temperature (T s ); and the oxygen partial pressure (P O 2 ). In addition, as the capacity of the microbattery depends on the electrode thickness, the duration of the deposition (t p ) must also be considered. The activity of a thin-film electrode, i.e., specific discharge energy, is proportional to the thickness, thus an increase of the film thickness leads to a power limitation because of the slow transport kinetic of Li + ions. Consequently, PLD is a popular technique due to the growth of a compact and dense film, which is replaced by a thick and porous film. Another advantage of the physical vacuum-like deposition techniques is the possibility of depositing a thin layer on top of the microbattery, which protects the device against a reactivity toward moisture. Moreover, due to the well-defined surface area of PLD films, a direct comparison of the electrochemical activity of materials can be done for different morphologies, from amorphous to single-crystalline materials [9].  [14] lithium microbatteries using the PLD technique and showing the production of a multilayer structure with dense and smooth films [30]. There are many variations on the general scheme of microbatteries outlined in the literature. Two principal options are shown in Figure 1 [25]. Scheme (a) shows a four-layer design on a conducting substrate (i.e., oriented silicon wafer) that can act as a current collector. Scheme (b) shows a six-layer stack incorporating two metallic current collectors. There are two fast-ion conductor (FIC) layers in this design: A thick layer, which is the solid electrolyte itself, and a thin buffer film that acts as an electrolyte layer between the FIC and the Li metal film to prevent interface passivation. In 1992, a thin-film solid-state microbattery with an overall thickness of approximately 10 µm, including the TiS2 cathode, oxide-sulfide solid electrolyte, LiI buffer, and Li metal anode, was developed at the Technology Laboratory of Eveready Battery Company (EBC) [31]. Laïk et al. evaluated the performance of three 4-V commercial all-solid-state lithium microbatteries (200-µm thick) with a nominal capacity of 700 µAh based on a LiCoO2 cathode material [32]. Note that a typical 1-mWh battery weighs 2.5 mg and has a volume less than 1 µL, providing a specific energy and power of 400 Wh·kg −1 and 1 kWh·L −1 , respectively [26]. Regarding the manufacture of thin-film batteries, several start-up companies have marketed micropower sources. Enfucell developed a thin, printable, and flexible SoftBattery ® used in various wearable electronics products [33]. Cymbet Corporation fabricates the EnerChip TM battery, which is a battery bare die and can be embedded with other integrated circuits [34]. Excellatron announced a pilot production line (10,000 cells/month) of thin-film solid-state batteries (approximately 0.3 µm thick) made of cathode films of LiCoO2 or LiMn2O4, LiPON as the electrolyte, and Li metal or Sn3N4 as the anode based on the technology developed at Oak Ridge National Labs. Using a 2-µm thick positive electrode, these microbatteries have been cycled in excess of 2000 cycles [35]. Some industrial developments of thin film microbatteries are listed in Table 2. MnO2-based cell voltage rating >3 V [33] GS Caltex n/a 300 µm thick/3.9 V/ 8000 cycles [40] A sequential PLD technique was applied for the fabrication of a rechargeable thin-film lithium battery (2-µm thick, area of 0.23 cm 2 ) with partially crystallized LCO as the cathode, an Li6.1V0.61Si0.39O5.36 (LVSO) glassy electrolyte, and SnO film anode [41]. The ablation beam produced Regarding the manufacture of thin-film batteries, several start-up companies have marketed micropower sources. Enfucell developed a thin, printable, and flexible SoftBattery ® used in various wearable electronics products [33]. Cymbet Corporation fabricates the EnerChip TM battery, which is a battery bare die and can be embedded with other integrated circuits [34]. Excellatron announced a pilot production line (10,000 cells/month) of thin-film solid-state batteries (approximately 0.3 µm thick) made of cathode films of LiCoO 2 or LiMn 2 O 4 , LiPON as the electrolyte, and Li metal or Sn 3 N 4 as the anode based on the technology developed at Oak Ridge National Labs. Using a 2-µm thick positive electrode, these microbatteries have been cycled in excess of 2000 cycles [35]. Some industrial developments of thin film microbatteries are listed in Table 2.   12 32 µAh·cm −2 at 3.5 µA·cm −2 [43]

Positive Electrode PLD Films
The main parameter for a microbattery is the delivered specific capacity. Rather than being expressed as the conventional unit of mAh·g −1 , due to the uncertainty in the film density, technologists prefer the stored charge, Q (expressed in µAh or in coulomb), per film surface area and the film thickness, i.e., µAh·cm −2 ·µm −1 of mC·cm −2 ·µm −1 .The relation between the gravimetric capacity, Q m , of the material and the volumetric capacity of a film, Q f , is given by: where Q f is expressed in mC·cm −2 ·µm −1 , Q m in mAh·g −1 , and d is the density of the material in g·cm −3 . Table 4 summarizes the energetic quantities for the studied cathode compounds. Table 4. Characteristics of the oxide materials used as a positive electrode in Li batteries. ∆x m is the quantity of electrons transferred (or Li uptake).

LiCoO 2 (LCO)
Having a lamellar structure, LiCoO 2 (LCO) is the prototypal positive electrode material commonly used in Li-ion batteries that yields a practical specific capacity of 135 mAh·g −1 in the voltage range from~3.8 V (fully lithiated state) to~4.2 V (charge state at Li 0.5 CoO 2 ) [46]. Since the early work in 1996 by Berkeley's group [47], numerous studies have been devoted to the growth of LiCoO 2 thin films prepared by the PLD technique due in large to their high electrochemical performances. Further investigations of dense and well-defined PLD films described the phase evolution during Li extraction and the kinetics of Li + ions in the host lattice, which eventually found applications in the fabrication of the cathode element in microbattery stacks [48]. The two crystal forms, HT-and LT-LiCoO 2 phases, with the rock-salt (rhombohedral, R-3m space group) and spinel (cubic, Fd3m space group) structure, respectively, have been synthesized by pulsed-laser deposition. It was pointed out that the crystallographic texture for LCO films differs from one deposit technique to another, i.e., PLD versus sputtering, which influences the electrochemical properties due to the diffusion plane orientation [49]. Julien et al. stated that well-crystallized PLD-grown LCO thin films with a single layered structure can be obtained at substrate temperatures (T s ) as low as 300 • C [3].
The first growth of single phase LCO films by the PLD method was realized by Antaya et al. [4]. Films deposited on unheated stainless-steel substrates were amorphous but crystallized readily with heat treatment in air above 500 • C. Later, Striebel et al. [47] demonstrated the promise of PLD-grown films as cathodes for rechargeable lithium cells. Crystalline (003)-textured LCO films with thicknesses ranging from 0.2 to 1.5 µm were prepared without postdeposition treatment, which displayed a specific capacity of films of 62 µAh·cm 2 ·µm −1 and an Li diffusion coefficient of 1 × 10 −10 cm 2 ·s −1 . Highly dense LCO films were first elaborated by the PLD process using a KrF laser under oxygen flow rates of 30 sccm and the pressure was maintained at 260 Pa on (200)-textured F-doped SnO 2 on fa used silica substrate maintained at T s = 700 • C [49]. As-prepared LCO thin films were (00l) textured and had a density of 85% of the single crystal. The charge-discharge profile of the films was typical of the LCO bulk and presented an~18% capacity loss for a single cycle to 4.15 V. In the potential range of 4.14 to 4.19 V, the measured chemical diffusion coefficients ranged from 1.7 × 10 −12 to 2.6 × 10 −9 cm 2 ·s −1 for as-deposited films and films annealed at 700 • C, respectively. Structural analysis of nanostructured LCO films prepared with PLD has been conducted by several research groups. Julien et al. [3,8,50] analyzed changes of the stoichiometry (i.e., the absence of the Co 3 O 4 amorphous phase) as a function of the growth conditions using Raman spectroscopy. The inclusion of Co 3 O 4 impurity is detected by analysis of the Raman intensity of the A 1g modes. Impurity-free films exhibit a specific capacity as high as 195 mC·cm −2 ·µm −1 for polycrystalline films grown from an Li-rich target (i.e., excess of 15% Li 2 O). The work by Okada et al. revealed that a decrease of the amount of inclusions can be obtained by a lower laser fluence and lower T s [51]. Figure 2 presents the relationship between the impurity inclusions and growth conditions of PLD-grown LCO films established from spectroscopic Raman data. In this figure, the Co 3 O 4 phase is grown under the conditions of high P O 2 , i.e., above the gray dashed line.  Ohnishi et al. [52,53] stated that suppression of the Co 3 O 4 spinel phase can be ensured by the growth under a relatively low oxygen partial pressure. Zhang et al. [54] discussed the effect of the deposition conditions on the structure and morphology. The advantages of the preferential orientation of LCO films prepared by PLD has been discussed by numerous groups with the conclusion that dense uniaxial textured (003)-oriented films are obtained by a well-chosen substrate [6,53,[55][56][57][58][59]. However, Xia et al. [59] stated that the fast transport of Li + ions is obtained for LCO films with a random orientation, in contrast with the results obtained with films having (003)-preferred orientation. Contrastingly, Nishio et al. claimed an excellent electrochemical performance for epitaxially grown LCO (77-nm thick) with a (104)-orientation on a (100) Nb:SrTiO 3 substrate that exhibited a discharge capacity of 26 mAh·g −1 even at high rates up to 100C [60]. Huo et al. stated that the film orientation is strongly dependent on the thickness and size of grains and demonstrated that films structured with parallel (003) planes are grown for thicknesses up to 300 nm [11]. Liu et al. showed that under certain PLD conditions, such as a high repetition rate of 35 Hz and low oxygen partial pressure of P O 2 = 1 Pa, LCO films tend to grow LCO films with a random orientation [61].
Epitaxial LCO thin films deposited on (001)-Al 2 O 3 substrates remained in a single phase in a narrow range, 250 ≤ T s ≤ 300 • C, whereas secondary phases appeared at T s > 300 • C, i.e., Co 2 O 3 , Co 3 O 4 , and LiCo 2 O 4 [5]. Xia et al. established that thin LCO films can be easily grown with a (003) orientation because of the lowest surface energy for the (003) plane, while the minimized strain energy in thick LCO films allows preferential (101) and (104) textures. It seems that this last type of orientation favors the electrochemical performance of the LCO cathode [12]. The reduction of the laser fluence results in a decrease of the surface roughness of LCO films. With post annealing at 400 • C and optimized deposition conditions, LCO films exhibit an initial discharge capacity of 36 µAh·cm −2 ·µm −1 and a cycleability of 94% [57]. Composition control was monitored to prepare stoichiometric LCO films using an Li-enriched target with a high-rate growth via an increase of the laser fluence to 0.29 J·cm −2 and an adjustment of the P O 2 to scatter the excess lithium. Ohnishi et al. showed that by using a Li 1.1 CoO 2+δ target, the deposition of stoichiometric LCO with the highest crystallinity can be realized at the rate of 0.06 Å per pulse at the P O 2 of 0.1 Pa and T s = 800 • C ( Figure 3) [52,62].
Recently, Nishio et al. claimed that a high deposition rate of 1.2 Å·s −1 tends to form oxygen-deficient LCO films due to the destabilization of Co 3+ cations and showed that post-annealing in air cancels the impurity phase [63]. Funayama et al. studied the effects of mechanical stress applied to LCO films (200 nm thick) deposited on Li-glass ceramic by the PLD method at 600 • C under a 20 Pa oxygen partial pressure for 1 h. Due to the lattice volume change, the generated electromotive force was 6.1 × 10 −12 V·Pa −1 [64]. The electrode behavior shows an increase of the discharge capacity from 10 to 40 mAh·g −1 at a 2C rate with an increase of the T s from 600 to 750 • C, whereas a T s = 800 • C worsens the performance. Studies of the physico-chemistry of PLD-grown LCO thin films report the structural, surface morphology, optical, and electrical properties [65][66][67]. worsens the performance. Studies of the physico-chemistry of PLD-grown LCO thin films report the structural, surface morphology, optical, and electrical properties [65][66][67]. The electrochemical properties, i.e., thermodynamics and kinetics, of lithium intercalation in PLD LCO thin films have been widely investigated by the structure-electrochemistry relationship [59,[68][69][70]; structural evolution upon charge-discharge cycles [71]; effect of doping by Ti, Al, and Mg [49,[72][73][74]; characterization of the electrode/electrolyte interface [75]; and lithium-ion kinetics vs. basal plane orientation [76][77][78][79][80]. Highly (003)-oriented impurity-free LCO thin films grown by PLD on a stainless-steel substrate display an initial discharge capacity of 52.5 µAh·cm −2 ·µm −1 and a capacity loss of 0.18% per cycle at a moderate current density of 12.7 µA·cm −2 . These films show a very small lattice expansion upon charge, i.e., 0.09 Å for a charge of 4.2 V [81]. Figure 4 shows the typical electrochemical features of PLD LCO thin films grown on an Si wafer maintained at different temperatures. Both the specific discharge capacity and the mid-voltage increased with increasing Ts. For a film deposited at Ts = 300 °C under PO₂ = 15 Pa, the discharge capacity reached a value ~140 mC·cm −2 ·µm −1 .  The electrochemical properties, i.e., thermodynamics and kinetics, of lithium intercalation in PLD LCO thin films have been widely investigated by the structure-electrochemistry relationship [59,[68][69][70]; structural evolution upon charge-discharge cycles [71]; effect of doping by Ti, Al, and Mg [49,[72][73][74]; characterization of the electrode/electrolyte interface [75]; and lithium-ion kinetics vs. basal plane orientation [76][77][78][79][80]. Highly (003)-oriented impurity-free LCO thin films grown by PLD on a stainless-steel substrate display an initial discharge capacity of 52.5 µAh·cm −2 ·µm −1 and a capacity loss of 0.18% per cycle at a moderate current density of 12.7 µA·cm −2 . These films show a very small lattice expansion upon charge, i.e., 0.09 Å for a charge of 4.2 V [81]. Figure 4 shows the typical electrochemical features of PLD LCO thin films grown on an Si wafer maintained at different temperatures. Both the specific discharge capacity and the mid-voltage increased with increasing T s . For a film deposited at T s = 300 • C under P O 2 = 15 Pa, the discharge capacity reached a value~140 mC·cm −2 ·µm −1 . worsens the performance. Studies of the physico-chemistry of PLD-grown LCO thin films report the structural, surface morphology, optical, and electrical properties [65][66][67]. The electrochemical properties, i.e., thermodynamics and kinetics, of lithium intercalation in PLD LCO thin films have been widely investigated by the structure-electrochemistry relationship [59,[68][69][70]; structural evolution upon charge-discharge cycles [71]; effect of doping by Ti, Al, and Mg [49,[72][73][74]; characterization of the electrode/electrolyte interface [75]; and lithium-ion kinetics vs. basal plane orientation [76][77][78][79][80]. Highly (003)-oriented impurity-free LCO thin films grown by PLD on a stainless-steel substrate display an initial discharge capacity of 52.5 µAh·cm −2 ·µm −1 and a capacity loss of 0.18% per cycle at a moderate current density of 12.7 µA·cm −2 . These films show a very small lattice expansion upon charge, i.e., 0.09 Å for a charge of 4.2 V [81]. Figure 4 shows the typical electrochemical features of PLD LCO thin films grown on an Si wafer maintained at different temperatures. Both the specific discharge capacity and the mid-voltage increased with increasing Ts. For a film deposited at Ts = 300 °C under PO₂ = 15 Pa, the discharge capacity reached a value ~140 mC·cm −2 ·µm −1 .   The electrochemical behavior of doped LCO thin films show that the voltage plateau at 3.65 V disappears in the charge curve of LiTi 0.05 Co 0.95 O 2 due to the doping effect, which cancels the semiconductor-metal-like transition of the LCO framework [81]. The Al-doped LCO film (LiCo 0.5 Al 0.5 O 2 ) exhibits a steady increase in the voltage vs. Li extraction with the absence of a voltage plateau as observed in stoichiometric LCO films; however, such films suffered from a limited capability and an upper bound of the diffusion coefficient of Li (D* = 9 × 10 −13 cm 2 ·s −1 ) was observed [49]. Recent studies report on the improved electrochemical behavior of surface-modified LCO films using lithium tantalate (LTaO) and lithium niobite (LNbO). Coating with LNbO preserves the LCO surface and decreases the interfacial resistance, which indicates fast lithium transport [82]. LCO films modified by amorphous tungsten oxide (LWO) fabricated by PLD show a high capacity retention of Q r = 80% at a high rate of 20C, against Q r = 0% for bare LCO films cycled at the same C-rate. A slight increase of the superficial diffusion coefficient of Li + ions from 2.2 × 10 −13 and 3.0 × 10 −13 cm 2 ·s −1 was also observed, owing to the surface modification [83][84][85][86]. Note that LWO as well as LNBO are lithium ion conductors, which act as an efficient buffer between the electrolyte and LCO cathode. The structural degradation of cycled LCO films was investigated by Raman spectroscopy over 400 cycles, showing microstructural modification due to nanocrystallization and phase separation [87].
All-solid-state lithium microbatteries (SSLMB) using LiCoO 2 films have been developed using various inorganic solid electrolyte (SE) films, i.e., LiPON, Li 2 S-P 2 S 5 , and amorphous Li 3 PO 4 . The thin-film battery with an electrochemical chain Li/amorphous Li 3 PO 4 /LCO/Pt shows a columnar-like LCO cathode (see the cross-sectional SEM image in Figure 5a) [88]. The excellent electrochemical performance is displayed in Figure 5b. This microcell exhibited an increase in capacity of up to 240 µAh·cm −2 when increasing the LCO thickness to 6.7 µm, which is 54% of the theoretical specific capacity of LCO (69 µAh·cm −2 ·µm −1 ). Shiraki et al. fabricated an SSLMB with an epitaxial LCO thin-film cathode (200 nm thick) by using PLD with a polycrystalline Li 1.1 CoO 2 target ablated at a laser fluence of 1 J·cm −2 , LiPON solid electrolyte (2 µm thick), and Li film as the anode (0.5 µm thick) [89]. The authors reported cyclic voltammograms with six redox peaks, which drastically changed upon cycling but did not display the galvanostatic charge-discharge profile of the SSLMB. The electrochemical behavior of doped LCO thin films show that the voltage plateau at 3.65 V disappears in the charge curve of LiTi0.05Co0.95O2 due to the doping effect, which cancels the semiconductor-metal-like transition of the LCO framework [81]. The Al-doped LCO film (LiCo0.5Al0.5O2) exhibits a steady increase in the voltage vs. Li extraction with the absence of a voltage plateau as observed in stoichiometric LCO films; however, such films suffered from a limited capability and an upper bound of the diffusion coefficient of Li (D* = 9 × 10 −13 cm 2 ·s −1 ) was observed [49]. Recent studies report on the improved electrochemical behavior of surface-modified LCO films using lithium tantalate (LTaO) and lithium niobite (LNbO). Coating with LNbO preserves the LCO surface and decreases the interfacial resistance, which indicates fast lithium transport [82]. LCO films modified by amorphous tungsten oxide (LWO) fabricated by PLD show a high capacity retention of Qr = 80% at a high rate of 20C, against Qr = 0% for bare LCO films cycled at the same C-rate. A slight increase of the superficial diffusion coefficient of Li + ions from 2.2 × 10 −13 and 3.0 × 10 −13 cm 2 ·s −1 was also observed, owing to the surface modification [83][84][85][86]. Note that LWO as well as LNBO are lithium ion conductors, which act as an efficient buffer between the electrolyte and LCO cathode. The structural degradation of cycled LCO films was investigated by Raman spectroscopy over 400 cycles, showing microstructural modification due to nanocrystallization and phase separation [87].
All-solid-state lithium microbatteries (SSLMB) using LiCoO2 films have been developed using various inorganic solid electrolyte (SE) films, i.e., LiPON, Li2S-P2S5, and amorphous Li3PO4. The thin-film battery with an electrochemical chain Li/amorphous Li3PO4/LCO/Pt shows a columnarlike LCO cathode (see the cross-sectional SEM image in Figure 5a) [88]. The excellent electrochemical performance is displayed in Figure 5b. This microcell exhibited an increase in capacity of up to 240 µAh·cm −2 when increasing the LCO thickness to 6.7 µm, which is 54% of the theoretical specific capacity of LCO (69 µAh·cm −2 ·µm −1 ). Shiraki et al. fabricated an SSLMB with an epitaxial LCO thin-film cathode (200 nm thick) by using PLD with a polycrystalline Li1.1CoO2 target ablated at a laser fluence of 1 J·cm −2 , LiPON solid electrolyte (2 µm thick), and Li film as the anode (0.5 µm thick) [89]. The authors reported cyclic voltammograms with six redox peaks, which drastically changed upon cycling but did not display the galvanostatic charge-discharge profile of the SSLMB.

LiNiO2 (LNO)
PLD-grown thin films of lithium nickel oxide (LNO), i.e., LixNi1−xO and stoichiometric LiNiO2, are applied as electrochromic and/or battery electrodes. In an early work, Rubin et al. established the complex relationship of the surface morphology and chemical composition of LixNi1−xO thin films vs. the deposition oxygen partial pressure, substrate temperature, and substrate-target distance as well [90]. LNO films produced at Ts < 600 °C immediately absorb CO2 and H2O when exposed to air, whereas they show long-term stability for Ts = 600 °C. LNO film with a composition of Li0.5Ni0.5O (cubic rock-salt Fd-3m structure instead of the rhombohedral R-3m structure for

LiNiO 2 (LNO)
PLD-grown thin films of lithium nickel oxide (LNO), i.e., Li x Ni 1−x O and stoichiometric LiNiO 2 , are applied as electrochromic and/or battery electrodes. In an early work, Rubin et al. established the complex relationship of the surface morphology and chemical composition of Li x Ni 1−x O thin films vs. the deposition oxygen partial pressure, substrate temperature, and substrate-target distance as well [90]. LNO films produced at T s < 600 • C immediately absorb CO 2 and H 2 O when exposed to air, whereas they show long-term stability for T s = 600 • C. LNO film with a composition of Li 0.5 Ni 0.5 O (cubic rock-salt Fd-3m structure instead of the rhombohedral R-3m structure for LiNiO 2 ) was obtained under a deposition atmosphere of P O 2 = 60 mTorr. This film (150-nm thick) showed excellent electrochemical reversibility as an electrochromic item in the range of 1.0 to 3.4 V vs. Li + /Li. An electrochromic device using WO 3 as the opposite electrode and PEO/LiTFSI as the solid polymer electrolyte (250-µm thick) showed an optical transmission range of ≈70% at 550 nm. Bouessay et al. optimized the PLD conditions to prepare NiO films, i.e., P O 2 = 0.1 mbar and T s = 25 • C, and analyzed the electrochromic reversibility associated with the Ni 3+ /Ni 2+ redox couple [91]. Using a laser fluence of 1 to 2 J·cm −2 , which corresponded to an ablation rate of 0.9 Å·s −1 , NiO films with a cubic rock-salt structure (Fm-3m space group) were formed. Porous PLD NiO films were prepared using nickel foil as the target in a low oxygen atmosphere (P O 2 = 50 Pa) [92,93] and were applied as the electrode for supercapacitors, showing a high specific capacitance of 835 F·g −1 at a 1 A·g −1 current density.
Similarly to LCO, the PLD growth of stoichiometric LiNiO 2 with an α-NaFeO 2 layered structure requests an Li-enriched target, i.e., LiNiO 2 + 15%Li 2 O. López-Iturbe and coworkers attempted to avoid Li loss by using an Ar atmosphere of P Ar = 10 mTorr and laser fluence of 15 J·cm −2 [94], while Rao et al. introduced pure oxygen (P O 2 = 0.1 Torr) in the PLD chamber and ablated the target at a laser fluence of 10 J·cm −2 [95]. The LNO films prepared at T s = 700 • C exhibited an initial discharge capacity of 175 mC·cm −2 ·µm −1 . Yuki et al. used, as the oxygen evolution reaction (OER), electrocatalysts, which were prepared LNO films from a target composed of Li 2 O and NiO 2 sintered at 1000 • C for 8.5 h in air ablated by a Nd:YAG laser operating at 532 nm [96]. Recently, PLD Li x Ni 2−x O 2 thin films with 0.15 ≤ x ≤ 0.45 deposited on a glass substrate under a pressure of 0.1 Pa and annealed at 350 • C were grown by PLD with an LiNiO 2 structure. The films appeared to be entirely made of particles even in the cross-section (grain size of 95 nm for x = 0.45). The average surface roughness estimated from the AFM measurements decreased with an increasing x, reaching a value of 0.615 nm for x = 0.45 [97].

LiNi 1−y Co y O 2 (NCO)
The LiNi 1−y Co y O 2 (0 < y <1) system with a layered α-NaFeO 2 structure belongs to a LiCoO 2 -LiNiO 2 solid solution with a higher reversible capacity than LCO and better cycleability than LNO. Among these substituted oxides, Ni-rich LiNi 0.8 Co 0.2 O 2 (NCO) has been identified as one the most attractive cathodes [98]. In this context, several works investigated the growth of NCO thin films using pulsed-laser deposition. Dense PLD NCO films grown at T s > 400 • C exhibited a gravimetric density of 4.8 g·cm −3 [99]. Ramana [100]. At T s ≤ 300 • C, the PLD film showed the highest intensity of the (00l) reflection, which indicates that the c-axis was normal to the film surface. The XRD (003) diffraction peak at 2θ = 18.5 • corresponds to an interplanar spacing of 0.145 nm. Phase diagram mapping ( Figure 6) was proposed to highlight the effect of the growth temperature on the microstructure of PLD LiNi 0.8 Co 0.2 O 2 films. Galvanostatic titration carried out at a rate of C/30 in the potential range of 2.5 to 4.2 V showed a discharge capacity of 82 µAh·cm −2 µm −1 , which compares with the theoretical value of 136 µAh·cm −2 ·µm −1 (490 mC·cm −2 ·µm −1 ) [101].
Hirayama et al. fabricated NCO films using the standard PLD conditions (Φ = 100-220 mJ, P O 2 = 3.3 Pa, target composition Li/Ni(Co) = 1.3, and T s = 600-650 • C) at a deposition rate of 0.3 nm·min −1 on an oriented SrTiO 3 (STO) substrate. Microstructural analysis shows a misfit of ca. 5% and roughness of 1 to 3 nm for the film grown with an in-plane orientation at T s = 600 • C. AFM imaging revealed the surface modification for films cycled in the voltage range of 2 to 5 V [102]. PLD NCO films (0.62 µm thick) were electrochemically characterized by galvanostatic titration (GITT), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Films grown at T s = 600 • C under P O 2 = 13 Pa with a laser fluence of Φ = 450 mJ per pulse exhibited an average specific capacity of~60 µAh·cm −2 ·µm −1 and the Li + diffusion coefficient varied from 3 × 10 −13 to 2 × 10 −10 cm 2 ·s −1 . After 100 cycles, the electrode showed a capacity retention of 85% [103]. The kinetics of the Li-ion intercalation in PLD NCO films grown on an Nb-doped STO substrate at T s = 600 • C under P O 2 = 3.3 Pa were investigated by EIS [104]. Nyquist plots showed changes of the electrode impedance as a function of the Li extraction/insertion with a larger value at a potential of 4.2 V. Baskaran et al. prepared NCO films on Pt and Si substrates heated at T s = 500 • C under a low oxygen partial pressure of 0.21 Pa [105]. The 40-min deposited films (120-nm thick) displayed a specific discharge capacity of 69.6 µAh cm −2 ·µm −1 (145 mAh·g −1 ) after 10 cycles. Based on these results, a Li-ion microbattery was fabricated with a LNCO/Li 3.4 V 0.6 Si 0.4 O 4 (LVSO)/SnO configuration with a thickness of~1.5 µm. Such a microcell delivered a capacity of 16.1 µAh·cm −2 ·µm −1 after 20 cycles.

LiNi1−yMnyO2 (NMO)
The substitution of Mn for Ni has been demonstrated to be beneficial for LiNiO2 cathode materials. LiNi0.5Mn0.5O2 (NMO) thin films were prepared on stainless steel and gold substrates from an Li-enriched target with an Li/(Ni + Mn) ratio of 1.5 in the NiO + MnO2 mixture. Under standard conditions (Φ = 2 J·cm −2 , Ts = 550 °C, and PO₂ = 266 Pa), impurity-free and (00l)-textured PLD films (300-500 nm thick) were obtained after an annealing process at 650 °C [106]. Galvanostatic charge-discharge tests showed that NMO films deposited on stainless steel displayed an electrochemical response, with a large voltage plateau between 2.5 and 3 V attributed to the presence of spinel phases (i.e., LiMn2O4 and LiNi0.5Mn1.5O4), while NMO films prepared on Au substrate showed the typical fingerprint of the LiMO2 layered compound with a single plateau at ca. 3.7 V. The analysis of the kinetics from CV measurements in the 2.5 to 4.5 voltage range led to a diffusion coefficient of the Li + -ions of D* = 3.13 × 10 −13 cm 2 ·s −1 for the Li insertion and 7.44 × 10 −14 cm 2 ·s −1 for the Li extraction. GITT results showed that D* is highly dependent on the electrode potential in the range of 10 −12 to 10 −16 cm 2 ·s −1 [107]. Sakamoto et al. addressed the growth of epitaxial PLD NMO films with an orientation of the basal layered plane (BLP) that depends on the SrTiO3 (STO) substrate plane: The BLP is parallel to the STO(110) substrate, while the BLP is perpendicular to the STO(111) substrate. The relationship between the lattice parameters and applied voltage highlights the charge-discharge processes for both orientations [108]. In a second article, Sakamoto et al. examined the structural properties of the surface and bulk of LiNi0.5Mn0.5O2 epitaxial thin films during an electrochemical reaction using in situ X-ray scattering [109]. In normal conditions, the two-dimensional diffusion of Li during (de)intercalation proceeds for the (110) plane. However, 3Ddiffusion activity can be observed for a high degree of cation mixing (Ni 2+ in Li(3a) sites).

Li(Ni, Co, Al)O2 (NCA)
The growth of LiNi0.8Co0.15Al0.05O2 (NCA) has been envisaged because Al-doping provides excellent structural and thermal stability for the electrode with the suppression of phase transitions. NCA thin films were prepared on Ni substrates at Ts = 500 °C by PLD with an energy and laserpulse repetition rate of 300 mJ and 10 Hz, respectively, under PO₂ = 18 Pa. The PLD target (i.e., pellet

LiNi 1−y Mn y O 2 (NMO)
The substitution of Mn for Ni has been demonstrated to be beneficial for LiNiO 2 cathode materials. LiNi 0.5 Mn 0.5 O 2 (NMO) thin films were prepared on stainless steel and gold substrates from an Li-enriched target with an Li/(Ni + Mn) ratio of 1.5 in the NiO + MnO 2 mixture. Under standard conditions (Φ = 2 J·cm −2 , T s = 550 • C, and P O 2 = 266 Pa), impurity-free and (00l)-textured PLD films (300-500 nm thick) were obtained after an annealing process at 650 • C [106]. Galvanostatic charge-discharge tests showed that NMO films deposited on stainless steel displayed an electrochemical response, with a large voltage plateau between 2.5 and 3 V attributed to the presence of spinel phases (i.e., LiMn 2 O 4 and LiNi 0.5 Mn 1.5 O 4 ), while NMO films prepared on Au substrate showed the typical fingerprint of the LiMO 2 layered compound with a single plateau at ca. 3.7 V. The analysis of the kinetics from CV measurements in the 2.5 to 4.5 voltage range led to a diffusion coefficient of the Li + -ions of D* = 3.13 × 10 −13 cm 2 ·s −1 for the Li insertion and 7.44 × 10 −14 cm 2 ·s −1 for the Li extraction. GITT results showed that D* is highly dependent on the electrode potential in the range of 10 −12 to 10 −16 cm 2 ·s −1 [107]. Sakamoto et al. addressed the growth of epitaxial PLD NMO films with an orientation of the basal layered plane (BLP) that depends on the SrTiO 3 (STO) substrate plane: The BLP is parallel to the STO(110) substrate, while the BLP is perpendicular to the STO(111) substrate. The relationship between the lattice parameters and applied voltage highlights the charge-discharge processes for both orientations [108]. In a second article, Sakamoto et al. examined the structural properties of the surface and bulk of LiNi 0.5 Mn 0.5 O 2 epitaxial thin films during an electrochemical reaction using in situ X-ray scattering [109]. In normal conditions, the two-dimensional diffusion of Li during (de)intercalation proceeds for the (110) plane. However, 3D-diffusion activity can be observed for a high degree of cation mixing (Ni 2+ in Li(3a) sites).

Li(Ni, Co, Al)O 2 (NCA)
The growth of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) has been envisaged because Al-doping provides excellent structural and thermal stability for the electrode with the suppression of phase transitions. NCA thin films were prepared on Ni substrates at T s = 500 • C by PLD with an energy and laser-pulse repetition rate of 300 mJ and 10 Hz, respectively, under P O 2 = 18 Pa. The PLD target (i.e., pellet pressed at 1.5 to 5.0 × 10 3 kg·cm −2 ), optimized by using bulk NCA Li-enriched with 15 mol% Li 2 O as the precursor, was sintered at 800 • C for 24 h [110]. The Li//NCA microcells delivered an initial specific capacity of 92 µAh·cm −2 ·µm −1 . The kinetics of Li + ions in PLD films measured by the GITT method in the voltage range of 2.50 to 4.25 V vs. Li + /Li revealed a diffusion coefficient of 4 × 10 −11 cm 2 ·s −1 with a maximum of 1 × 10 −10 cm 2 ·s −1 for the composition of Li 0.7 Ni 0.8 Co 0.15 Al 0.05 O 2 . Figure 7 displays the specific capacity of Li//LiNi 0.8 Co 0.15 Al 0.05 O 2 cells as a function of the substrate temperature. PLD films were grown onto various substrates at 25 ≤ T s ≤ 500 • C under a controlled O 2 atmosphere (P O 2 = 50 mTorr). NCA thin films prepared onto Ni foil at T s = 500 • C exhibited the best specific discharge capacity of 100 µA·cm −2 ·µm −1 .  Figure 7 displays the specific capacity of Li//LiNi0.8Co0.15Al0.05O2 cells as a function of the substrate temperature. PLD films were grown onto various substrates at 25 ≤ Ts ≤ 500 °C under a controlled O2 atmosphere (PO₂ = 50 mTorr). NCA thin films prepared onto Ni foil at Ts = 500 °C exhibited the best specific discharge capacity of 100 µA·cm −2 ·µm −1 . Figure 7. The specific capacity of LiNi0.8Co0.15Al0.05O2 thin films deposited onto Ni foil, Si wafer, and ITO-coated glass as a function of the substrate temperature.

Li(Ni, Mn, Co)O2 (NMC)
Lithiated nickel-manganese-cobalt oxides, LiNi1−y−zMnyCozO2 (NMC), is a complex LiNiO2-LiMnO2-LiCoO2 solid solution, which displays the same structure as rock salt, α-NaFeO2, with a valence state of cations as Ni 2+ , Mn 4+ , and Co 3+ . LiNi1/3Co1/3Mn1/3O2 (NMC333) thin film electrodes were prepared by pulsed laser deposition on a Pt/Ti/SiO2/Si substrate (where the 150-nm thick Pt and the 100-nm thick Ti films act as the current collector and buffer layer, respectively) at room temperature under a PO₂ of 6.6 Pa. The deposition rate was about 4.4 nm·min −1 . Impurity-free NMC333 films were obtained using an Li-enriched target (15% excess Li2O). The charge-discharge profiles strongly depended on the film morphology that was tuned by increasing the annealing temperature from 400 to 500 °C. The best electrochemical features were obtained for annealing at 450 °C, showing a discharge plateau of about 3.7 and 3.6 V [111]. Epitaxial and highly textured LiNi0.5Mn0.3Co0.2O2 (NMC532) thin film cathodes were deposited by a one-step PLD process [112]. Using a laser fluence of Φ = 6 J·cm −2 , Ts = 750 °C, and an Li-enriched target (20% Li2CO3 excess), the films were deposited on silicon (111), stainless steel (SS), and c-cut sapphire (0001) substrates. It was stated that highly dense and epitaxial films with a strong (003) reflection peak intensity are achieved at high Ts. Films grown on an Au-SS substrate delivered 125 mAh·g −1 at 0.5C and demonstrated a 72% capacity retention after 100 cycles. Furthermore, 3D-electrode architectures were fabricated, and are applicable to power hearing aids. Structured NMC333 film electrodes (50-80 µm thick) were prepared by laser printing using a pulsed laser (λ = 355 nm) at a fluence of 50 to 100 mJ·cm −2 . Such films exhibit a stable discharge capacity at a low C-rate (0.1C), but for a current density of 1C, the electrode reached 40% of the initial capacity [113]. Recently, Abe et al. elucidated the deterioration mechanism of pristine ZrO2-coated NMC333 thin films prepared by PLD on a (110)STO substrate maintained at two temperatures of 25 and 700 °C; the oxygen pressure ranged from 3.3 to 10 Pa [114]. Characterization was performed by cyclic voltammetry, in situ X-ray diffraction, and in situ neutron reflectometry. As a result, ZrO2 coating suppressed the low activity

Li(Ni, Mn, Co)O 2 (NMC)
Lithiated nickel-manganese-cobalt oxides, LiNi 1−y−z Mn y Co z O 2 (NMC), is a complex LiNiO 2 -LiMnO 2 -LiCoO 2 solid solution, which displays the same structure as rock salt, α-NaFeO 2 , with a valence state of cations as Ni 2+ , Mn 4+ , and Co 3+ . LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NMC333) thin film electrodes were prepared by pulsed laser deposition on a Pt/Ti/SiO 2 /Si substrate (where the 150-nm thick Pt and the 100-nm thick Ti films act as the current collector and buffer layer, respectively) at room temperature under a P O 2 of 6.6 Pa. The deposition rate was about 4.4 nm·min −1 . Impurity-free NMC333 films were obtained using an Li-enriched target (15% excess Li 2 O). The charge-discharge profiles strongly depended on the film morphology that was tuned by increasing the annealing temperature from 400 to 500 • C. The best electrochemical features were obtained for annealing at 450 • C, showing a discharge plateau of about 3.7 and 3.6 V [111]. Epitaxial and highly textured LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) thin film cathodes were deposited by a one-step PLD process [112]. Using a laser fluence of Φ = 6 J·cm −2 , T s = 750 • C, and an Li-enriched target (20% Li 2 CO 3 excess), the films were deposited on silicon (111), stainless steel (SS), and c-cut sapphire (0001) substrates. It was stated that highly dense and epitaxial films with a strong (003) reflection peak intensity are achieved at high T s . Films grown on an Au-SS substrate delivered 125 mAh·g −1 at 0.5C and demonstrated a 72% capacity retention after 100 cycles. Furthermore, 3D-electrode architectures were fabricated, and are applicable to power hearing aids. Structured NMC333 film electrodes (50-80 µm thick) were prepared by laser printing using a pulsed laser (λ = 355 nm) at a fluence of 50 to 100 mJ·cm −2 . Such films exhibit a stable discharge capacity at a low C-rate (0.1C), but for a current density of 1C, the electrode reached 40% of the initial capacity [113]. Recently, Abe et al. elucidated the deterioration mechanism of pristine ZrO 2 -coated NMC333 thin films prepared by PLD on a (110)STO substrate maintained at two temperatures of 25 and 700 • C; the oxygen pressure ranged from 3.3 to 10 Pa [114]. Characterization was performed by cyclic voltammetry, in situ X-ray diffraction, and in situ neutron reflectometry. As a result, ZrO 2 coating suppressed the low activity of the spinel phase formed at the surface and hence improved the cycleability of the thin film electrode.

Li-Rich Layered Oxides
The growth of Li-rich layered oxide Li 1.2 Mn 0.55 Ni 0.15 Co 0.1 O 2 on SRO/STO(100) and SRO/STO (111) substrate (where SRO and STO are SrRuO 3 and SrTiO 3 , respectively) heated at T s = 600 • C was reported by Bendersky et al. [115]. The transmission electron microscopy (TEM) images recorded using a high-angle annular dark field (HAADF) mode showed a predominant Li 2 (Mn,Ni,Co)O 3 monoclinic phase. Johnston-Peck et al. confirmed the growth of monoclinic Li-rich thin films (C2/m space group) using a target with the composition of Li 1.2 Mn 0.55 Ni 0.15 Co 0.1 O 2 and displayed the CV response with a non-aqueous electrolyte at a scan rate of 0.1 mV·s −1 [116]. Yan et al. intended to develop a PLD thin film electrode using an Li-rich layer structured oxide with a composition of Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 (or written as 0.55Li 2 MnO 3 ·0.45LiNi 1/3 Co 1/3 Mn 1/3 O 2 ) [117]. Films deposited at T s = 650 • C under P O 2 = 46 Pa and annealed at 800 • C showed the best electrochemical performance with an initial specific discharge capacity of 70 µAh·cm −2 ·µm −1 . However, the differential capacity curves, dQ/dV, indicated that the layered structure gradually changed to the spinel phase during the charge-discharge cycling.

LiMn 2 O 4 (LMO)
Since the early report in 1996 [47], numerous studies have been devoted to LiMn 2 O 4 (LMO) thin films grown by laser ablation. A spinel structure (Fd3m space group) was successfully prepared using different PLD conditions and applied as a positive electrode in thin-film lithium microbatteries (see, for example, [121]). Most prior works focused on the fundamental properties of PLD-grown LMO cathode films, aiming to deposit the highly structured and porous morphology required for a good operating electrode [7,[122][123][124][125][126][127][128][129][130][131][132][133][134][135][136]. Morcrette et al. analyzed PLD film composition as a function of T s and P O 2 using the Rutherford backscattering method and nuclear reaction analysis [122]. A stoichiometric LMO film was obtained at T s = 500 • C and P O 2 = 20 Pa, while the Li/Mn ratio decreased with T s and increased with P substrates. Impurity-free well-crystallized samples with a crystallite size of 300 nm were obtained at T s as low as 300 • C and P O 2 = 10 Pa using an Li-enriched target (15% Li 2 O excess) to avoid Li deficiency in the film [123]. The electrochemical features of the Li cell showed a specific capacity as high as 120 mC·cm −2 ·µm −1 in the voltage range of 3.0 to 4.2 V vs. Li + /Li, which was attributed to the high degree of film crystallinity [7,123]. Simmen et al. studied the relationship between Li 1+x Mn 2 O 4−δ thin films and Li excess in the target and concluded that a film deposited from a composite target of Li 1.03 Mn 2 O 4-δ + 7.5 mol% Li 2 O was the best, exhibiting a discharge capacity of 42 µAh·cm −2 ·µm −1 [127]. Various substrates were successfully used for the epitaxial growth of LiMn 2 O 4 (LMO) spinel thin films, such as Pt, Si, Au, MgO, Al 2 O 3 , and SrTiO 3 . Gao et al. [128] reported a detailed mechanism of the epitaxial LMO film/substrate (current collector) interface formation. A coherent hetero-interface was formed with the substrate but a tetragonal Jahn-Teller distortion was observed, induced by oxygens' non-stoichiometry and the lattice misfit strain. PLD epitaxial LMO thin films were deposited on oriented Nb:SrTIO 3 substrates maintained at 950 • C from a sintered target with 100 wt.% excess Li 2 O with different surface morphologies and orientations, such as (100)-oriented pyramidal, (110)-oriented rooftop, or (111)-oriented flat structure. The pyramidal-type LMO was cycled at a 3.3C rate, demonstrating a specific capacity of 90 mAh·g −1 after 1000 cycles [129]. Using oriented substrates, i.e., (111) reported a detailed mechanism of the epitaxial LMO film/substrate (current collector) interface formation. A coherent hetero-interface was formed with the substrate but a tetragonal Jahn-Teller distortion was observed, induced by oxygens' non-stoichiometry and the lattice misfit strain. PLD epitaxial LMO thin films were deposited on oriented Nb:SrTIO3 substrates maintained at 950 °C from a sintered target with 100 wt. % excess Li2O with different surface morphologies and orientations, such as (100)-oriented pyramidal, (110)-oriented rooftop, or (111)-oriented flat structure. The pyramidal-type LMO was cycled at a 3.3C rate, demonstrating a specific capacity of 90 mAh·g −1 after 1000 cycles [129]. Using oriented substrates, i.e., (111)  By applying an elevated-temperature PLD technique, Tang et al. [134] studied the influence of the substrate temperature (Ts) and the oxygen partial pressure (PO₂) on LMO film crystallinity. LMO thin films deposited on Si (001)/0.2 µm-SiO2 substrates at 575 °C under 13 Pa oxygen had a flat and smooth surface and exhibited mainly a (111) out-of-plane preferred texture ( Figure 9a). Such films of the 300 nm thickness showed a very dense cross-section (density ~4.3 g·cm −3 ) (see Figure 9b) [135]. The effect of stoichiometric deviations on the electrochemical performance of an LMO thinfilm cathode was investigated by Morcrette et al. [136], while the kinetics of Li + ions in the LMO thin-film framework were documented by Yamada and coworkers [137]. A high-activation barrier of 50 kJ·mol −1 for Li-ion transfer was identified at the electrode/electrolyte interface for films deposited at Ts = 700 °C and PO₂ = 27 Pa. Albrecht et al. [138] reported the minimum crystallization temperature of spinel LMO thin films in a narrow annealing temperature range of around 700 °C. Electrochemical tests carried out with the galvanostatic cycling with the potential limits (GCPL) method proved that Li-ions are (de-)intercalated in different tetrahedral sites for which the processes occur at potentials that are slightly shifted by U ≈ 100 mV, which is similar to the previous results by Julien et al. [7]. By applying an elevated-temperature PLD technique, Tang et al. [134] studied the influence of the substrate temperature (T s ) and the oxygen partial pressure (P O 2 ) on LMO film crystallinity. LMO thin films deposited on Si (001)/0.2 µm-SiO 2 substrates at 575 • C under 13 Pa oxygen had a flat and smooth surface and exhibited mainly a (111) out-of-plane preferred texture ( Figure 9a). Such films of the 300 nm thickness showed a very dense cross-section (density~4.3 g·cm −3 ) (see Figure 9b) [135]. The effect of stoichiometric deviations on the electrochemical performance of an LMO thin-film cathode was investigated by Morcrette et al. [136], while the kinetics of Li + ions in the LMO thin-film framework were documented by Yamada and coworkers [137]. A high-activation barrier of 50 kJ·mol −1 for Li-ion transfer was identified at the electrode/electrolyte interface for films deposited at T s = 700 • C and P O 2 = 27 Pa. Albrecht et al. [138] reported the minimum crystallization temperature of spinel LMO thin films in a narrow annealing temperature range of around 700 • C. Electrochemical tests carried out with the galvanostatic cycling with the potential limits (GCPL) method proved that Li-ions are (de-)intercalated in different tetrahedral sites for which the processes occur at potentials that are slightly shifted by U ≈ 100 mV, which is similar to the previous results by Julien et al. [7]. Studies of the structure and electrochemical reactivity of heteroepitaxial LiMn2O4/La0.5Sr0.5CoO3 (LMO/LSCO) bilayer thin films deposited on crystalline SrTiO3 substrates show that LSCO reduced the lattice misfit strain with the substrate and favored a lower LMO surface roughness. However, a decrease of the electrical conductivity occurred during the electrochemical test (after first cycle) due to the lattice oxygen loss at the outermost layer (40 nm) [139]. Tang et al. reported a comparative investigation of the structures, morphologies, and properties of Li insertion for LMO films with different crystallizations. At Ts = 400 °C, LMO films consisted of nanocrystallites < 100 nm in size with rough surfaces that exhibited a discharge capacity of 61 µAh·cm 2 ·µm −1 with a capacity loss of 0.032% per cycle up to 500 cycles, while for Ts = 600 °C and PO₂ = 10 Pa, highly crystallized films showed an initial discharge capacity of 54.3 µAh·cm 2 ·µm −1 [134]. The intrinsic properties of PLDgrown LMO have been investigated by several techniques. Electrical measurements of LMO films showed that the conductivity is sensitive to Ts, as the activation energy that followed the Mott's rule increased with Ts up to Ea = 0.64 eV at Ts = 600 °C [133]. Singh et al. characterized the crystallinity and texture of LMO films deposited at Ts = 650 °C. Here, (111)-oriented films were grown on a doped Si substrate, while films deposited on a stainless-steel substrate exhibited a (001) orientation [140]. The thermo-power (or Seebeck coefficient) of PLD LMO films was reported to be 70 µV·K −1 [141].
PLD LMO films were subjected to an overcharge (5 V vs. Li + /Li), which did not modify the structure and preserved the well-resolved voltage peaks at 4.1 and 4.2 V, while an overdischarge (2 V vs. Li + /Li) led to a loss of capacity due to the structural disorder associated with the tetragonal transition, i.e., Jahn-Teller distortion [142]. Singh patented the fabrication of PLD Li1−xMyMn2−2zO4 films, where M is a doping element (M = Al, Ni, Co, Cr, Mg, etc.) and x, y, and z vary from 0.0 to 0.5 [143]. These defective spinel structures enhanced the oxygen content as compared to LiMn2O4 crystal. In particular, the oxygen-rich Li1−δMn2−δO4 films were superior cathode films, leading to excellent rechargeable battery performances. It is claimed that a high discharge rate of 25C produces only a 25% capacity loss and a specific capacity >150 mAh·g −1 remains after 300 cycles. Rao et al. reported the preparation of well-crystallized LMO films at a high substrate temperature of Ts = 700 °C and PO₂ = 13 Pa that delivered a capacity of 133 mC·cm −2 ·µm −1 at a very slow C/100 rate [144]. Several workers reported the evolution of the thin-film electrode/electrolyte interface, as the planar form of the film is the ideal geometry for such investigations [145][146][147][148]. Room temperature impedance measurements were carried out to identify the formation of the solid electrolyte interface (SEI) layer on a PLD LMO film cathode and the degradation mechanism during cycling in an aprotic electrolyte containing LiPF6 salt. A reversible disproportionation reaction was suggested with the formation of the Li2Mn2O4 and λ-MnO2 phases at the surface [146]. Using epitaxial-film model electrodes, Hirayama studied the surface reaction and the formation of the SEI layer and the interfacial structural reconstruction during an initial battery process using in situ surface X-ray diffraction and reflectometry [147]. TEM images confirmed the surface reconstruction that occurred during the first charge, i.e., when a potential was applied. After 10 cycles, the SEI layer was observed on both the (111) and (110) surfaces and Mn dissolution appeared at the (110) surface [148]. Inaba et al. [149] investigated the surface morphology evolution of PLD LMO thin films Studies of the structure and electrochemical reactivity of heteroepitaxial LiMn 2 O 4 /La 0.5 Sr 0.5 CoO 3 (LMO/LSCO) bilayer thin films deposited on crystalline SrTiO 3 substrates show that LSCO reduced the lattice misfit strain with the substrate and favored a lower LMO surface roughness. However, a decrease of the electrical conductivity occurred during the electrochemical test (after first cycle) due to the lattice oxygen loss at the outermost layer (40 nm) [139]. Tang et al. reported a comparative investigation of the structures, morphologies, and properties of Li insertion for LMO films with different crystallizations. At T s = 400 • C, LMO films consisted of nanocrystallites < 100 nm in size with rough surfaces that exhibited a discharge capacity of 61 µAh·cm 2 ·µm −1 with a capacity loss of 0.032% per cycle up to 500 cycles, while for T s = 600 • C and P O 2 = 10 Pa, highly crystallized films showed an initial discharge capacity of 54.3 µAh·cm 2 ·µm −1 [134]. The intrinsic properties of PLD-grown LMO have been investigated by several techniques. Electrical measurements of LMO films showed that the conductivity is sensitive to T s , as the activation energy that followed the Mott's rule increased with T s up to E a = 0.64 eV at T s = 600 • C [133]. Singh et al. characterized the crystallinity and texture of LMO films deposited at T s = 650 • C. Here, (111)-oriented films were grown on a doped Si substrate, while films deposited on a stainless-steel substrate exhibited a (001) orientation [140]. The thermo-power (or Seebeck coefficient) of PLD LMO films was reported to be 70 µV·K −1 [141].
PLD LMO films were subjected to an overcharge (5 V vs. Li + /Li), which did not modify the structure and preserved the well-resolved voltage peaks at 4.1 and 4.2 V, while an overdischarge (2 V vs. Li + /Li) led to a loss of capacity due to the structural disorder associated with the tetragonal transition, i.e., Jahn-Teller distortion [142]. Singh patented the fabrication of PLD Li 1−x M y Mn 2−2z O 4 films, where M is a doping element (M = Al, Ni, Co, Cr, Mg, etc.) and x, y, and z vary from 0.0 to 0.5 [143]. These defective spinel structures enhanced the oxygen content as compared to LiMn 2 O 4 crystal. In particular, the oxygen-rich Li 1−δ Mn 2−δ O 4 films were superior cathode films, leading to excellent rechargeable battery performances. It is claimed that a high discharge rate of 25C produces only a 25% capacity loss and a specific capacity >150 mAh·g −1 remains after 300 cycles. Rao et al. reported the preparation of well-crystallized LMO films at a high substrate temperature of T s = 700 • C and P O 2 = 13 Pa that delivered a capacity of 133 mC·cm −2 ·µm −1 at a very slow C/100 rate [144]. Several workers reported the evolution of the thin-film electrode/electrolyte interface, as the planar form of the film is the ideal geometry for such investigations [145][146][147][148]. Room temperature impedance measurements were carried out to identify the formation of the solid electrolyte interface (SEI) layer on a PLD LMO film cathode and the degradation mechanism during cycling in an aprotic electrolyte containing LiPF 6 salt. A reversible disproportionation reaction was suggested with the formation of the Li 2 Mn 2 O 4 and λ-MnO 2 phases at the surface [146]. Using epitaxial-film model electrodes, Hirayama studied the surface reaction and the formation of the SEI layer and the interfacial structural reconstruction during an initial battery process using in situ surface X-ray diffraction and reflectometry [147]. TEM images confirmed the surface reconstruction that occurred during the first charge, i.e., when a potential was applied. After 10 cycles, the SEI layer was observed on both the (111) and (110) surfaces and Mn dissolution appeared at the (110) surface [148]. Inaba et al. [149] investigated the surface morphology evolution of PLD LMO thin films grown on a PT substrate at T s = 600 • C by electrochemical scanning tunneling microscopy (STM) with voltage cycling in the range of 3.5 to 4.25 V. The original LMO grains of 400 nm in size coexisted with small particles 120 to 250 nm in size, which appeared after 20 cycles and decreased to~70 nm after 75 cycles through a kind of dissolution/precipitation process. LMO thin-film electrodes with a grain size of <100 nm deposited on a stainless steel substrate at T s = 400 • C under a 26 Pa oxygen partial pressure displayed an excellent capacity of 62.4 µAh·cm −2 ·µm −1 when cycled at a 20 µA·cm −2 current density in the voltage range of 3.0 to 4.5 V. A very low capacity fading was recorded for up to 500 cycles at 55 • C. Li + -ion diffusion coefficients evaluated from EIS measurements were around 2.7 × 10 −12 cm 2 ·s −1 for an electrode charged at 4.0 V and 2.4 × 10 −11 cm 2 ·s −1 for 4.2 V [150]. Xie et al. [151] investigated the Li + -ion transport in LMO thin films (~100 nm thick) grown on Au substrates at 600 • C at a deposition rate of 0.14 nm·min −1 . The chemical diffusion coefficients determined by the EIS, GITTm, and PITT methods were in the range of 10 −14 to 10 −11 cm 2 ·s −1 in the voltage range of 3.9 to 4.2 V. Table 5 lists some typical results on the kinetics of Li + ions in pulsed-laser deposited LMO thin films. Table 5. Diffusion coefficients of Li + ions in PLD LMO thin film frameworks.

Growth Conditions
The electrochemical behavior of Li-rich spinel Li 1.1 Mn 1.9 O 4 thin films grown by PLD on an Au substrate was reported by several workers [156,157]. The best performance was reported at a discharge current density of the 36C-rate for LMO films deposited for 30 min in P O 2 = 30 Pa and T s = 600 • C with an Nd:YAG laser (266 nm) adjusted to an energy fluence of 1 J·cm −2 [156]. Nanocrystalline LMO films with grains less than 100 nm were deposited on a stainless-steel substrate at T s = 400 • C and P O 2 = 26 Pa using a PLD pulse power of 100 mJ at the frequency of 10 Hz. The film cycled over 100 cycles delivered a specific capacity of 118 mAh·g −1 at a current density of 100 A·cm −2 [157]. Using reflectometry measurements, Hirayama et al. [158] studied the structural modifications at the electrode/electrolyte interface of a lithium cell, in which the LMO electrodes were prepared as epitaxial films by the PLD method with different orientations. The respective orientation of the LMO film corresponded to that of the substrate plane, i.e., the (111), (110), and (100) planes of the SrTiO 3 substrate. No density change was observed for the (110) and (100) planes, whereas a defect layer was detected in the (111) plane. ZrO 2 -modified LiMn 2 O 4 thin films prepared via PLD consisting of amorphous ZrO 2 formed on the grain boundary and the outer layer of the LMO matrix [159]. The high capacity retention of 82% after 130 cycles of films of xZrO 2 -(1−x)LiMn 2 O 4 (x = 0.025) monitored at the 4C rate was attributed to the decrease of the charge transfer resistance (R ct ).
Epitaxial LiMn 2 O 4 /La 0.5 Sr 0.5 CoO 3 (LMO/LSCO) bilayer thin films with sub-nano flat interfaces were deposited on (111)-oriented STO substrates at T s = 650 • C in P O 2 = 4 Pa. After the first charge-discharge cycle, the decrease of the electrical conductivity of the LSCO buffer layer due to lattice oxygen loss induced capacity fading [139]. The PLD growth of a multilayer LMO thin film electrode demonstrated the compensation of lithium loss during deposition [160].  [163]. The LiSn x/2 Mn 2−x O 4 films were prepared on a Pt/Ti/SiO 2 /Si(100) substrate in the conditions of T s = 450 • C, P O 2 = 26.7 Pa, Φ = 4.6 J·cm −2 , 10 Hz pulse frequency, and 4 cm target-substrate distance. XPS and EXAFS measurements showed that Sn 2+ cations replace Mn 3+ ions, which resulted in an increase of the valence of Mn in the spinel lattice. A high specific capacity of ∼120 mAh·g −1 and cycleability with a capacity retention >81% at the 4C rate after 90 cycles was attributed to the Mn-deficient structure. A multi-layer PLD process was utilized to deposit LMO films (90 nm thick) on Si-based substrates coated with Pt as the current collector [164]. A reversible capacity of 2.6 µAh·cm −2 (corresponding to a specific capacity of ≈28 µAh·cm −2 ·µm −1 or 66 mAh·g −1 assuming a dense film with 4.3 g·cm −3 ) was reached at an extremely high current density of 1889 µA·cm −2 (equivalent to the 348C rate) with a capacity retention of 86% over 3500 cycles. A significant non-diffusion-controlled contribution (pseudocapacitive-like) was evidenced by cyclic voltammetry; however, the two typical voltage plateaus in the GCD of LMO (around 4 V) indicates that the faradaic redox reaction is the main process. For an easy comparison, Table 6 lists the electrochemical properties of PLD-prepared LMO thin film electrodes. Table 6. Electrochemical properties of PLD-prepared LMO thin film electrodes. J is the current density, δ is the film thickness, and ∆C c is the capacity fading per cycle.

MnO 2
Due to its environmental compatibility and low cost, manganese oxides are promising candidate materials for supercapacitor applications using neutral aqueous solution as the electrolyte (i.e., 0.5 mol·L LiFePO 4 (LFP) thin-film electrodes have been successfully fabricated by pulsed-laser deposition [156,[177][178][179]. It was shown that, due to the film thickness and carbon content, the electrochemical performances are very sensitive, i.e., electronic conductivity and Li-ion diffusion. Iriyama et al. reported the PLD growth of olivine structured LFP thin films and their electrochemical properties characterized by cyclic voltammetry and charge-discharge tests [180,181]. The typical olivine features were evidenced by CV measurements in the range of 2.0 and 5.0 V vs. Li + /Li, i.e., a single couple of anodic and cathodic peaks at~3.4 V. Song et al. synthesized PLD LFP films with a low carbon content (<1 wt.%) on stainless steel substrates utilizing an Ar atmosphere [182]. The 75-nm thick films showed reversible cycling of more than 80 mAh·g −1 after 60 cycles. Furthermore, 156-nm thick films grown using a target-substrate distance reduced to 5 cm had a layered surface texture and delivered more than 120 mAh·g −1 with a good capacity retention. LFP thin films with a needle-like morphology were prepared by an off-axis PLD technique [183]. The effect of the substrate on the structure and morphology was examined by Palomares et al. for PLD film deposited under argon gas kept at a pressure of 8 Pa [184]. Stainless steel was demonstrated to be the best substrate for the single-phase olivine (Pnma space group) with a temperature set at 500 • C.
LiFePO 4 deposited by pulsed-laser deposition proved to be effective as a thin film electrode. Tang et al. stated that a well-crystallized pure olivine phase was grown using optimized deposition parameters (T s = 500 • C, P Ar = 20-30 Pa, pulse power of 120 mJ, pulse frequency of 10 Hz, λ = 248 nm) [185]. An electrochemical capacity of 38 µAh cm −2 ·µm −1 at the C/20 rate (36 µAh·cm −2 ·µm −1 at a rate of C/4) was measured at 25 • C. High substrate temperatures (500 ≤ T s ≤ 700 • C) favored the presence of Fe 3+ impurities, i.e., Li 3 Fe 2 (PO 4 ) 3 and Fe 4 (P 2 O 7 ) 3 . In a second article, the same group analyzed the kinetics of Li + ions in PLD LFP films using CV, GITT, and EIS measurements [179]. CV data provided average D* values of 10 −14 cm 2 ·s −1 , while D* deduced from both GITT and EIS techniques was in the range of 10 −14 to 10 −18 cm 2 ·s −1 . A maximum D* value was observed at x = 0.5 for Li x FePO 4 . Lu et al. prepared different composite thin films, i.e., LiFePO 4 -Ag and LiFePO 4 -C, with the aim of enhancing the electronic conductivity [177,178]. It was found that films grown with 2 mol% carbon and annealed at 600 • C for 6 h had an improved coulombic efficiency. Well-crystallized olivine-type structure LFP films were obtained by PLD coupled with high temperature annealing of 650 • C. The first discharge capacity was 27 mAh·g −1 with a retention of only 49% after 100 cycles. The low reversible capacity and poor cycling performance was attributed to the existence of an Fe 2 O 3 impurity produced by the high temperature treatment and poor intrinsic conductivity [186]. Sauvage et al. published several reports on the electrochemical properties of PLD LFP thin films grown in different configurations [187][188][189][190]. First, it was shown that well-crystallized and homogeneous 300-nm thick LFP films deposited on Pt-capped Si substrates have intrinsic Li insertion properties evaluated both in aqueous and non-aqueous electrolytes, i.e., voltage plateau at 3.42 V vs. Li + /Li [187]. Second, the influence of the film thickness was studied in the range of 12 to 600 nm [188]. Third, the effect of the texture on the electrochemical performance was analyzed for PLD films deposited on a polycrystalline α-Al 2 O 3 substrate coated with a 20-nm thick Pt layer from an LiFePO 4 pellet as the target. The standard PLD conditions were used (i.e., (Φ = 2 J·cm −2 , P Ar = 8 Pa, T s = 600 • C) [189]. Finally, the electrochemical stability of LFP films was analyzed as a function of the exposition to the most common lithium salt and for different current collectors (i.e., Si, Pt, Ti, Al, and (304)-stainless steel) [190]. A 270-nm thick film tested by CV at a 2 mV·s −1 scanning rate in 1 mol·L −1 LiClO 4 in EC/DMC solution delivered a specific capacity of 1.52 µAh cm −2 after 150 cycles. Recently, Raveendran et al. reported the properties of FeSe and LiFeO 2 /FeSe bi-layers prepared by PLD as cathode materials [191]. Mangano-olivine LiMnPO 4 (LMP) thin films were fabricated on Pt-coated SiO 2 glass substrates using PLD parameters, e.g., Φ = 1.58 J·cm −2 , T s = 400-700 • C, and P Ar = 2-100 Pa [45]. LMP films (50-nm thick, 0.09 cm 2 area) were applied in Li/Li 3 PO 4 /LiMnPO 4 microbatteries for 500 cycles. From the CV measurement, a capacity of 28 mAh·g −1 at 20 mV min −1 was reported.

V 2 O 5
Another candidate material for the cathodes of microbatteries is V 2 O 5 , in which about 1 mol of Li + ions can be inserted and extracted without the phase transformation of V 2 O 5 , leading to a theoretical specific capacity of 147 mAh·g −1 . Due to its stable layered structure and its ability to accommodate large amounts of Li ions, V 2 O 5 has been widely studied for the development of electrochromic displays, color memory devices, and lithium-battery cathodes [192]. Extensive works have evidenced the advantages of PLD for the preparation of V 2 O 5 films with a good reproducible stoichiometry similar to the target material [193][194][195][196][197][198][199][200][201][202][203][204][205][206][207]. The first work related to the growth of V 2 O 5 thin film by PLD as an electrode for a thin-film lithium battery was reported by the National Renewable Energy Labs (USA) [193] followed by Julien's group [194,195]. A major advantage of laser ablation deposits is that it is possible to prepare thin layers of crystallized V 2 O 5 under oxygen at a relatively low temperature of 200 • C [193]. The growth mechanism of PLD V 2 O 5 thin films has been proposed by Ramana and coworkers [196]. It was reported that the grain size, surface roughness, and global morphology are highly sensitive to the nature and temperature of the substrate for films deposited in an oxygen partial pressure of P O 2 = 13 Pa. The functional influence of the growth temperature on the grain size for films deposited onto various substrates was also evidenced. Two main features should be pointed out: (i) The exponential variation of the grain size over the substrate temperature range of 25 to 500 • C; (ii) the variation is dependent on the substrate material, which is larger for the Si(00) wafer. McGraw et al. reported that pulsed-laser deposited V 2 O 5 films can be grown on a number of low-cost substrates, including SnO 2 -coated glass, on which highly textured (001) films are obtained at T s = 500 • C under P O 2 in the range of 0.2 to 0.5 Pa [52,197,198]. PLD thin films of V 2 O 5 were prepared for applications in lithium batteries using a ceramic V 2 O 5 target and a KrF laser of a wavelength of 248 nm. Depending on the temperature of the substrates and the oxygen pressure during deposition, amorphous or crystallized layers are obtained. PLD-grown amorphous films exhibited a low capacity loss of~2% over 100 discharge-charge cycles in the voltage range 4.1 to 1.8 V compared to 20% for crystalline film [45,199]. Thin layers of V 2 O 5 were also prepared using a V 2 O 3 target [200]. By making deposits at 200 • C with the same V 2 O 3 target, amorphous layers were obtained in the absence of oxygen and layers crystallized in the presence of oxygen. Madhuri et al. [201] reported the successful crystallization of laser-ablated V 2 O 5 thin films at T s = 200 • C. These films were grown in the orthorhombic structure and exhibited a predominant (001) orientation. The growth of crystalline thin dense films without post-deposition annealing was claimed and the good electrochemical performance of PLD films was demonstrated. Iida et al. [202] addressed the electrochromic properties of V 2 O 5 films deposited onto ITO glass as a function of the PLD parameters. The film recrystallization occurred in the range of 400 ≤ T s ≤ 500 • C and the best morphology was obtained for P O 2 = 13.3 Pa. McGraw et al. deposited thin films of V 2 O 5 for applications in lithium batteries using a ceramic V 2 O 5 target and a KrF laser, with a wavelength of 248 nm. Depending on the temperature of the substrates and the oxygen pressure during deposition, amorphous or crystallized layers were obtained [193,199]. Thin layers of V 2 O 5 were also prepared using a V 2 O 3 target. By making deposits at 200 • C with the same V 2 O 3 target, amorphous layers were obtained in the absence of oxygen and layers crystallized in the presence of oxygen. Stoichiometric amorphous V 2 O 5 films can be grown onto substrates maintained at low temperatures (T s < 100 • C) using a sintered V 2 O 5 target. Ramana et al. revealed that stoichiometric V 2 O 5 films can be grown with a layered structure onto amorphous glass substrates at temperatures as low as 200 • C and an oxygen partial pressure of 100 mTorr [203]. The onset of crystallization occurred at 200 • C with an activation energy of 0.43 to n0.55 eV [204]. Correlations between the growth conditions, microstructure, and optical properties were investigated for V 2 O 5 thin films deposited over a wide substrate temperature range of 30 to 500 • C by Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and UV−vis−NIR spectral measurements. As shown in Figure 11, the film grain size follows a power law of the substrate temperature and the optical energy bandgap decreases from 2.47 to 2.12 eV with the increase of T s from 30 to 500 • C [205]. Bowman and Gregg investigated the effect of the applied strain on the resistance of V 2 O 5 thin films grown from both metallic vanadium and a ceramic V 2 O 5 target using a laser fluence of~3.0 and~1.5 J cm −2 , respectively [208]. Deng et al. compared the growth of V 2 O 5 films using femtosecond (f-PDL) and nanosecond (n-PDL) pulsed laser deposition using SEM, XRD, and Raman spectroscopy. Prior to annealing, f-PLD films showed a rougher texture and nano-crystalline character, while n-PLD films were much smoother and predominantly amorphous [209]. size follows a power law of the substrate temperature and the optical energy bandgap decreases from 2.47 to 2.12 eV with the increase of Ts from 30 to 500 °C [205]. Bowman and Gregg investigated the effect of the applied strain on the resistance of V2O5 thin films grown from both metallic vanadium and a ceramic V2O5 target using a laser fluence of ~3.0 and ~1.5 J cm −2 , respectively [208]. Deng et al. compared the growth of V2O5 films using femtosecond (f-PDL) and nanosecond (n-PDL) pulsed laser deposition using SEM, XRD, and Raman spectroscopy. Prior to annealing, f-PLD films showed a rougher texture and nano-crystalline character, while n-PLD films were much smoother and predominantly amorphous [209]. The PLD growth conditions were refined by an analysis of the surface properties for the production of high-quality V2O5 films. The investigations were carried out by AFM, SEM, FTIR, and XRD. AFM measurements showed a surface roughness of ~12 nm with a Gaussian-like height distribution of surface grains for films deposited at Ts = 200 °C under PO₂ = 10 Pa [210]. The local structure of 0.3-µm thick films grown on Si(100) substrates was characterized by Raman spectroscopy [211]. The influence of the deposition temperature on the microstructure was investigated by an examination of the rigid layer-like mode at 145 cm −1 , which showed a frequency shift with increasing Ts. The ability of the V2O5 thin film lattice to accommodate Li + ions was also investigated by Raman spectroscopy. The appearance of the δ-and γ-phases of LixV2O5 gave additional insight into the structural changes of lithiated films. From the photoluminescence spectra, Iida et al. evidenced a blue shift of the vanadyl V = O peak upon Li + insertion for different electric charge in the range of 0 ≤ Q ≤ 20 mC of Li + [212]. From X-ray diffraction and Raman spectroscopy data, Shibuya et al. derived a PO₂ -Ts phase diagram for V-O films grown on Si(100) substrates ( Figure 12). The composition of V-O films was as follows: (i) A VO2 monoclinic phase was formed at Ts ≥ 450 °C and PO2 in the range of 5 to 20 mTorr; (ii) a V2O5 orthorhombic phase was obtained under oxidative conditions, i.e., at high PO₂; (iii) a V6O13 phase was grown under PO₂ between oxidative and reductive conditions; and (iv) metastable V4O9 and VO2(B) phases were formed for lower Ts (≤400 °C) and lower PO₂ (≤30 mTorr) [213].
V2O5 thin films have been widely used as electrochromic electrodes but few reports are devoted to PLD-grown films. Fang et al. obtained thin films deposited on In2O3:SnO2 (ITO)-coated glass and (111)Si wafer from a V2O5 target using an XeCl laser with a wavelength of 308 nm for applications in electrochromic devices [214,215]. Electrochromic tests over 60,000 cycles showed that a significant change in the optical density (bleached and colored states) was evaluated to be 0.13 at λ = 600 nm for as-prepared films at Ts = 200 °C. Crystallized c-axis oriented V2O5 films were obtained under oxygen and at a substrate temperature of 200 °C. The durability without long-term degradation of the electrochromic V2O5 films was tested over 8000 cycles in the voltage range of 1.2 to 1.4 V [216]. Ti-doped V2O5 thin films prepared by the pulsed laser ablation technique at Ts = 200 °C and Φ = 2 J·cm −2 were studied as the electrode for an electrochromic display that exhibits a neutral brownish blue color. The long-term durability was verified over 8000 cycles of a voltage The PLD growth conditions were refined by an analysis of the surface properties for the production of high-quality V 2 O 5 films. The investigations were carried out by AFM, SEM, FTIR, and XRD. AFM measurements showed a surface roughness of~12 nm with a Gaussian-like height distribution of surface grains for films deposited at T s = 200 • C under P O 2 = 10 Pa [210]. The local structure of 0.3-µm thick films grown on Si(100) substrates was characterized by Raman spectroscopy [211]. The influence of the deposition temperature on the microstructure was investigated by an examination of the rigid layer-like mode at 145 cm −1 , which showed a frequency shift with increasing T s . The ability of the V 2 O 5 thin film lattice to accommodate Li + ions was also investigated by Raman spectroscopy. The appearance of the δand γ-phases of Li x V 2 O 5 gave additional insight into the structural changes of lithiated films. From the photoluminescence spectra, Iida et al. evidenced a blue shift of the vanadyl V = O peak upon Li + insertion for different electric charge in the range of 0 ≤ Q ≤ 20 mC of Li + [212]. From X-ray diffraction and Raman spectroscopy data, Shibuya et al. derived a P O 2 -T s phase diagram for V-O films grown on Si(100) substrates ( Figure 12). The composition of V-O films was as follows: (i) A VO 2 monoclinic phase was formed at T s ≥ 450 • C and P O2 in the range of 5 to 20 mTorr; (ii) a V 2 O 5 orthorhombic phase was obtained under oxidative conditions, i.e., at high P O 2 ; (iii) a V 6 O 13 phase was grown under P O 2 between oxidative and reductive conditions; and (iv) metastable V 4 O 9 and VO 2 (B) phases were formed for lower T s (≤400 • C) and lower P O 2 (≤30 mTorr) [213]. V 2 O 5 thin films have been widely used as electrochromic electrodes but few reports are devoted to PLD-grown films. Fang et al. obtained thin films deposited on In 2 O 3 :SnO 2 (ITO)-coated glass and (111)Si wafer from a V 2 O 5 target using an XeCl laser with a wavelength of 308 nm for applications in electrochromic devices [214,215]. Electrochromic tests over 60,000 cycles showed that a significant change in the optical density (bleached and colored states) was evaluated to be 0.13 at λ = 600 nm for as-prepared films at T s = 200 • C. Crystallized c-axis oriented V 2 O 5 films were obtained under oxygen and at a substrate temperature of 200 • C. The durability without long-term degradation of the electrochromic V 2 O 5 films was tested over 8000 cycles in the voltage range of 1.2 to 1.4 V [216]. Ti-doped V 2 O 5 thin films prepared by the pulsed laser ablation technique at T s = 200 • C and Φ = 2 J·cm −2 were studied as the electrode for an electrochromic display that exhibits a neutral brownish blue color.
The long-term durability was verified over 8000 cycles of a voltage cycled in the range from −1.0 to +1.0 V vs. SCE showing a charge of 35 mC·cm −2 . The good cycleability was attributed to the layered structure of PLD crystalline films with a parallel orientation to the substrate, suitable for Li + -ions' transport [215]. PLD thin films of the system, WO 3 -V 2 O 5 , were prepared with a laser fluence of 1 to 2 J·cm −2 on SnO 2 /F-coated glass substrates at T s = 25 • C under P O 2 = 0.1 mbar. Such films with low V contents cycled in the protonic medium. The true color neutrality is the main advantage of V-based WO 3 thin films; however, the cell capacity and coloration efficiency decrease with an increase of the V content [217]. The orthorhombic V 2 O 5 phase is also applied as electrodes for sensors. Huotari reported that pure PLD films were obtained at Φ = 2.6 J cm −2 , T s = 400 • C, and P O 2 = 1.0 Pa with a post-annealing treatment at 400 • C for 1 h in normal ambient conditions [218]. The efficient response to NH 3 at part-per-billion levels. indicates these films use as possible sensing materials for ammonia gas [219]. The good cycleability was attributed to the layered structure of PLD crystalline films with a parallel orientation to the substrate, suitable for Li + -ions' transport [215]. PLD thin films of the system, WO3-V2O5, were prepared with a laser fluence of 1 to 2 J·cm −2 on SnO2/F-coated glass substrates at Ts = 25 °C under PO₂ = 0.1 mbar. Such films with low V contents cycled in the protonic medium. The true color neutrality is the main advantage of V-based WO3 thin films; however, the cell capacity and coloration efficiency decrease with an increase of the V content [217]. The orthorhombic V2O5 phase is also applied as electrodes for sensors. Huotari reported that pure PLD films were obtained at Φ = 2.6 J cm −2 , Ts = 400 °C, and PO₂ = 1.0 Pa with a post-annealing treatment at 400 °C for 1 h in normal ambient conditions [218]. The efficient response to NH3 at part-per-billion levels. indicates these films use as possible sensing materials for ammonia gas [219]. The electrochemical properties of V2O5 thin-film cathode material have been widely studied in cells with aprotic electrolytes (typically LiClO4 dissolved in propylene carbonate). The electrochemical charge-discharge profiles of PLD V2O5 films were also found to be dependent on Ts, exhibiting a marked difference for V2O5 films grown at Ts < 200 °C when compared to those grown at Ts ≥ 200 °C. The effect of the substrate temperature and hence the microstructure on the kinetics of the lithium intercalation process in V2O5 films is remarkable. The applicability of the grown PLD V2O5 films in lithium microbatteries indicates that PLD V2O5 films in the temperature range of 200 to 400 °C offer better electrochemical performance than films grown at other temperatures due to their excellent structural quality and stability [25,220]. As an experimental fact, pulsed laser deposited V2O5 thin films exhibit a higher initial voltage than the crystalline material, i.e., ~4.1 vs. ~3.5 V (Li + /Li). For instance, V2O5 thin-film cathodes, deposited from a V6O13 target at a fluence of ~12 J cm −2 on SnO2-coated glass at Ts = 200 °C, were efficient for Li + -ion incorporation. In (h00)textured films, the specific capacity reached values between 50% and 80% of the theoretical value. On the other hand, amorphous films display a stable capacity corresponding to 1.2 F mol −1 in the voltage range of 4.1 to 1.5 V. Prior textured V2O5 films discharged beyond the threshold to 2.0 V vs. Li + /Li showed an immediate and continuous capacity fading and a quasi-total amorphization after 10 cycles [193,197]. The chemical diffusion coefficient of Li + ions, D*, measured by PITT was found to be in the range of 1.7 × 10 −12 to 5.8 × 10 −15 cm²·s −1 in crystalline V2O5 films, which compares well to the value found in LixV2O5 phases, whereas D* displayed a smooth and continuous decrease as the Li content increased in amorphous films [198].
In an attempt to apply PLD V2O5 films in SSMB, a thin-film microbattery was constructed using a glassy Li1.4B2.5S 0.1O4.9 electrolyte film with an ionic conductivity of 5 × 10 −6 S·cm −1 at 25 °C The electrochemical properties of V 2 O 5 thin-film cathode material have been widely studied in cells with aprotic electrolytes (typically LiClO 4 dissolved in propylene carbonate). The electrochemical charge-discharge profiles of PLD V 2 O 5 films were also found to be dependent on T s , exhibiting a marked difference for V 2 O 5 films grown at T s < 200 • C when compared to those grown at T s ≥ 200 • C. The effect of the substrate temperature and hence the microstructure on the kinetics of the lithium intercalation process in V 2 O 5 films is remarkable. The applicability of the grown PLD V 2 O 5 films in lithium microbatteries indicates that PLD V 2 O 5 films in the temperature range of 200 to 400 • C offer better electrochemical performance than films grown at other temperatures due to their excellent structural quality and stability [25,220]. As an experimental fact, pulsed laser deposited V 2 O 5 thin films exhibit a higher initial voltage than the crystalline material, i.e.,~4.1 vs.~3.5 V (Li + /Li). For instance, V 2 O 5 thin-film cathodes, deposited from a V 6 O 13 target at a fluence of~12 J cm −2 on SnO 2 -coated glass at T s = 200 • C, were efficient for Li + -ion incorporation. In (h00)-textured films, the specific capacity reached values between 50% and 80% of the theoretical value. On the other hand, amorphous films display a stable capacity corresponding to 1.2 F mol −1 in the voltage range of 4.1 to 1.5 V. Prior textured V 2 O 5 films discharged beyond the threshold to 2.0 V vs. Li + /Li showed an immediate and continuous capacity fading and a quasi-total amorphization after 10 cycles [193,197]. The chemical diffusion coefficient of Li + ions, D*, measured by PITT was found to be in the range of 1.7 × 10 −12 to 5.8 × 10 −15 cm 2 ·s −1 in crystalline V 2 O 5 films, which compares well to the value found in Li x V 2 O 5 phases, whereas D* displayed a smooth and continuous decrease as the Li content increased in amorphous films [198].
In an attempt to apply PLD V 2 O 5 films in SSMB, a thin-film microbattery was constructed using a glassy Li 1.4 B 2.5 S 0.1 O 4.9 electrolyte film with an ionic conductivity of 5 × 10 −6 S·cm −1 at 25 • C and an Li anode film. This Li/Li 1.4 B 2.5 S 0.1 O 4.9 /V 2 O 5 cell delivered a capacity of~400 mC·cm −2 ·µm −1 at a current density of 15 µA·cm −2 [221]. Ag 0.3 V 2 O 5 and LiPON thin films with a smooth surface were grown by PLD in an N 2 and O 2 atmosphere, respectively. The Li/LiPON/Ag 0.3 V 2 O 5 SSMB displayed good cycleability at a current density of 7 µA·cm −2 in the voltage window of 1.0 to 3.5 V. The specific capacity was maintained at 40 µAh·cm −2 ·µm −1 after 100 cycles [222]. Recently, amorphous vanadium oxide a-VO x PLD films (650 nm thick) were grown on stainless steel substrates from a V 2 O 5 PLD-target under P O 2 in the range of 0 to 30 Pa. Films prepared under P O 2 = 13 Pa had a smooth surface and bore an O/V atomic ratio of 2.13 with a higher atomic percentage of V 5+ than that of V 4+ . Electrochemical tests carried out in Li cells with 1 mol·L −1 LiPF 6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) as the electrolyte showed a reversible specific capacity as high as 300 mAh·g −1 at the C/10 current rate and a capacity retention of 90% after 100 cycles [223]. Such studies were initiated by Zhang et al. in 1997 to obtain VO x films PLD grown at 200 • C and exhibiting a specific capacity of 340 mAh·g −1 at a current density of 0.1 mA·cm −2 and a capacity loss <2% at the end of 100 cycles [201]. A summary of the electrochemical properties of PLD-grown vanadium oxide thin film electrodes is given in Table 7. Table 7. Electrochemical properties of PLD-prepared vanadium oxide thin film electrodes. J is the current density, δ is the film thickness, and ∆C c is the capacity fading per cycle. 3.14. V 6 O 13 With the ability of vanadium cations (two V 4+ every V 5+ ) to be reduced, the mixed-valence vanadium oxide, V 6 O 13 , the structure of which is formed by alternated single and double layers of VO 6 units, can insert reversibly about 6 mol of Li, giving a specific capacity of 311 mAh·g −1 . It makes this compound a good candidate for the cathode material of rechargeable batteries [224]. V 6 O 13 films were fabricated by the PLD technique using a pulsed KrF excimer laser (λ = 248 nm, 20 ns pulse duration, 10 Hz frequency, and 4 J·cm −2 laser fluence). A (100)-oriented Si substrate was maintained at a temperature of 500 • C. During the PLD process, the formation of crystalline V 6 O 13 films (dark-bluish color) and the vanadium oxidation state (+2.166) was monitored by controlling the processing temperature and O 2 partial pressure. The (002)-oriented V 6 O 13 thin films (50 nm thick) were obtained after a post annealing at 400 • C under an oxygen partial pressure of 100 Pa [221]. The discharge profiles for Li//V 6 O 13 thin-film cells were recorded in the voltage range of 3.3 to 2.5 V at a current density of 5 µA cm −2 (Figure 13). A film as-grown at T s = 250 • C exhibited a steady discharge curve with an insertion uptake of 6Li per V 6 O 13 formula unit, whereas the cell voltage decay was faster for a film deposited at T s = 25 • C. However, the films deposited at T s = 250 • C and annealed at 300 • C in an Ar atmosphere displayed a stepped discharge profile with the appearance of a voltage plateau at ca

FeF2
Iron fluoride is a conversion-type cathode material with a high theoretical specific capacity of 571 mAh·g −1 . Several groups reported electronic additive-free FeF2 films grown by the PLD technique at low temperatures [225][226][227][228]. The electrochemical properties of FeFx films were reported to be dependent on the substrate temperature. Using an FeF3 target, crystallized-like FeF2 film (P42/mnm space group) was obtained at Ts = 600 °C, while a mixed FeF3-FeF2 phase was grown at Ts = 25 °C and single FeF3 phase was prepared at Ts = −50 °C [226]. FeFx deposited on stainless steel substrates under vacuum (5 × 10 −5 Pa) exhibited a capacity of ~600 mAh·g −1 at a current density of 0.56 µA·cm −2 . Santos-Ortiz et al. reported the PLD growth of polycrystalline FeF2 thin films on oxide-etched Si(100) and glass substrates using standard conditions (Ts = 400 °C, Φ = 8 J·cm −2 , growth rate of ~6 nm·min −1 ) [227]. A 50-nm thick PLD FeF2 film on stainless steel substrates held at 400 °C showed an initial specific discharge capacity of 167 mAh·g −1 when cycled 200 times in the potential range of 1 to 4 V vs. Li + /Li at the 1C current rate [228].

MoO3
MoO3 is an attractive cathode material for microbattery technology from several standpoints: (i) The orthorhombic a-phase is a layered structure favorable for Li insertion between slabs; (ii) Mo has the highest +6 oxidation state, making the high structural stability; (iii) the lattice can be reversibly inserted up to 1.5Li per mole of oxide, yielding a specific capacity of 280 mAh·g −1 ; and (iv) the capacity of the dense film can reach a value of ≈130 µAh·cm −2 ·µm −1 , almost twice the value for LiCoO2 [229]. In addition to the use as cathode batteries, MoO3 is a material applied in electrochromics, gas sensors, and electro-optics. For certain applications, high-quality films grown by PLD are required.
Currently, PLD MoO3 thin films are grown using a KrF excimer laser (λ = 248 nm) with a fluence of 2 J cm −2 (energy of 300 mJ per pulse) and deposited on various substrates heated in the range of 25 ≤ Ts ≤ 500 °C under an atmosphere of O2 flow maintained at a pressure of 0.1 ≤ PO₂ ≤ 20 Pa. In the prior report, Julien et al. showed that the structure analyzed by optical spectroscopy strongly depends on Ts: For Ts < 150 °C, an amorphous phase is formed, the β-MoO3 phase grows at Ts ≈ 200  C, and the layered α-MoO3 phase appears at Ts = 300 °C [230][231][232][233]. Al-Kuhaili et al. reported the growth of polycrystalline MoO3 films on unheated substrates using both XeF and KrF excimer lasers. By tuning the annealing temperature in the range of 300 to 500 °C, both the grain size and surface roughness increased. Films formed using the XeF laser (λ = 351 nm) and annealed at 400 °C have the best stoichiometry of MoO2.95 [233]. Analyzing the growth mechanism, Ramana and Julien concluded that the thermochemical reaction during ablation strongly influences the structural characteristics of PLD MoO3 films. Above Ts = 400 °C, the formation of compositional defects induces structural disorder, i.e., α-β-MoO3−x phase mixture [234,235].

FeF 2
Iron fluoride is a conversion-type cathode material with a high theoretical specific capacity of 571 mAh·g −1 . Several groups reported electronic additive-free FeF 2 films grown by the PLD technique at low temperatures [225][226][227][228]. The electrochemical properties of FeF x films were reported to be dependent on the substrate temperature. Using an FeF 3 target, crystallized-like FeF 2 film (P4 2 /mnm space group) was obtained at T s = 600 • C, while a mixed FeF 3 -FeF 2 phase was grown at T s = 25 • C and single FeF 3 phase was prepared at T s = −50 • C [226]. FeF x deposited on stainless steel substrates under vacuum (5 × 10 −5 Pa) exhibited a capacity of~600 mAh·g −1 at a current density of 0.56 µA·cm −2 . Santos-Ortiz et al. reported the PLD growth of polycrystalline FeF 2 thin films on oxide-etched Si(100) and glass substrates using standard conditions (T s = 400 • C, Φ = 8 J·cm −2 , growth rate of 6 nm·min −1 ) [227]. A 50-nm thick PLD FeF 2 film on stainless steel substrates held at 400 • C showed an initial specific discharge capacity of 167 mAh·g −1 when cycled 200 times in the potential range of 1 to 4 V vs. Li + /Li at the 1C current rate [228].

MoO 3
MoO 3 is an attractive cathode material for microbattery technology from several standpoints: (i) The orthorhombic a-phase is a layered structure favorable for Li insertion between slabs; (ii) Mo has the highest +6 oxidation state, making the high structural stability; (iii) the lattice can be reversibly inserted up to 1.5Li per mole of oxide, yielding a specific capacity of 280 mAh·g −1 ; and (iv) the capacity of the dense film can reach a value of ≈130 µAh·cm −2 ·µm −1 , almost twice the value for LiCoO 2 [229]. In addition to the use as cathode batteries, MoO 3 is a material applied in electrochromics, gas sensors, and electro-optics. For certain applications, high-quality films grown by PLD are required.
Currently, PLD MoO 3 thin films are grown using a KrF excimer laser (λ = 248 nm) with a fluence of 2 J cm −2 (energy of 300 mJ per pulse) and deposited on various substrates heated in the range of 25 ≤ T s ≤ 500 • C under an atmosphere of O 2 flow maintained at a pressure of 0.1 ≤ P O 2 ≤ 20 Pa. In the prior report, Julien et al. showed that the structure analyzed by optical spectroscopy strongly depends on T s : For T s < 150 • C, an amorphous phase is formed, the β-MoO 3 phase grows at T s ≈ 200 • C, and the layered α-MoO 3 phase appears at T s = 300 • C [230][231][232][233]. Al-Kuhaili et al. reported the growth of polycrystalline MoO 3 films on unheated substrates using both XeF and KrF excimer lasers. By tuning the annealing temperature in the range of 300 to 500 • C, both the grain size and surface roughness increased. Films formed using the XeF laser (λ = 351 nm) and annealed at 400 • C have the best stoichiometry of MoO 2.95 [233]. Analyzing the growth mechanism, Ramana and Julien concluded that the thermochemical reaction during ablation strongly influences the structural characteristics of PLD MoO 3 films. Above T s = 400 • C, the formation of compositional defects induces structural disorder, i.e., α-β-MoO 3−x phase mixture [234,235].
The applicability of PLD films to an Li microbattery was demonstrated by the best electrochemical features: A discharge capacity of 90 µAh cm −2 µm −1 was obtained for T s = 400 • C, while only 53 µAh·cm −2 ·µm −1 was delivered for T s = 200 • C [236]. Puppala et al. investigated the microstructure and morphology of PLD MoO 3−x thin films' growth for catalytic applications using a femtosecond laser (f-PLD) and a nanosecond excimer-laser (n-PLD). Substantially textured films with a partially crystalline phase prior to annealing were obtained by the f-PDL laser, while the n-PLD-grown MoO 3−x films were predominantly amorphous with a smooth surface [237]. Sunu et al. claimed that as-deposited PLD films (T s = 400 • C, Φ = 4-5 J·cm −2 , repetition rate of 15 to 20 Hz, and P O 2 = 500 Pa) are suboxide-like, i.e., mixture of η-Mo 4 O 11 and χ-Mo 4 O 11 , which transformed to MoO 3 after annealing at 500 • C in air for 5 h [238]. Several works reported the PLD growth of films (MoO 3 ) 1−x (V 2 O 5 ) x with 0.0 ≤ x ≤ 0.3 prepared at room temperature under an oxygen pressure of 13.3 Pa. The effect of the V 2 O 5 content on the coloring switching properties for thermochromic, gasochromic, photochromic, and electrochromic applications was investigated [239,240]. Contrary to pure MoO 3 , the electrochromism of MoO 3 -V 2 O 5 films showed that the Mo oxidation state (+6) did not change considerably upon Li + insertion, while V 5+ was reduced considerably to V 4+ [239]. A similar improvement of the gas-sensing properties, i.e., the shortest response time and highest transmittance change, was observed for V 2 O 5 -doped MoO 3 films under an H 2 atmosphere [240].

WO 3
Tungsten oxide (WO 3 ) belongs to the class of "chromogenic" materials, i.e., materials exhibiting coloration effects through electro-, photo-, gas-, laser-, and thermochromism processes, which requires the high homogeneity provided by the PLD technique. Preliminary studies of the growth of WO 3 thin films by PLD were first attempted by Haro-Poniaowski et al. [233] in 1998. Later, Rougier et al. reported the PLD conditions for the growth of efficient WO 3 films as electrochromics (EC) components [241]. The microstructure of films deposited on SnO 2 :F coated glass substrate is strongly sensitive to both the oxygen pressure and substrate temperature: (i) Crystallized films are formed for T s = 400 • C and P O 2 = 10 Pa; (ii) amorphous films are obtained for P O 2 = 1 Pa at any T s ; (iii) for T s = 25 • C and P O2 = 1 Pa, WO 3 films are blue colored and conductive; and (iv) colorless insulator films are grown for T s = 25 • C and P O 2 = 10 Pa, which display the best electrochromic properties. Qiu and Lu showed that oxygen deficient WO 3−δ films with a deviated monoclinic structure were produced using PLD parameters as 2.5 J·cm −2 , P O 2 = 26 Pa, and a target-Si (100)  They showed that films, prepared at T s < 300 • C, are amorphous and polycrystalline phases were obtained at T s > 400 • C, while the crystallinity of the film on glass substrates was not dependent on P O 2 [247]. Films deposited at 400 • C were porous with a nanocrystalline triclinic structure and showed the best cycleability [216,248,249].
The suitability of PLF WO 3 films for EC applications was investigated as a function of the partial oxygen pressure during deposition. Studies of the texture and morphology of PLD 30-nm thick WO 3 films deposited on Si(100) and SrTiO 3 (100) substrates under an O 2 background of 2.5 Pa showed that: (i) The laser fluence (in the range of 5 to 15 J·cm −2 ) strongly influences the texture, (ii) the films grown on STO are biaxially textured with a smooth surface, and (iii) films deposited on Si are granular [250]. The fabrication of WO 3 thin films with color neutrality for applications as EC materials was realized by the deposition of films containing 20% of vanadium onto SnO 2 :F coated glasses at T s = 20 • C under P O 2 = 10 Pa. The blue color in the reduced state (−0.4 V) of the W-O-V films lost intensity and turned grey-blue (transmittance of 50%) as the V concentration increased [251]. Highly transparent WO 3 films exhibiting strong coloration and fast and full bleaching were prepared under PLD conditions (Φ = 1 J·cm −2 , T s = 250 • C, P O 2 = 16 Pa, and d = 40 mm) [252]. WO 3 films were also prepared using similar PLD parameters for applications in gas sensors [253][254][255].

Solid Electrolyte PLD Films
For the development of solid-state thin film batteries, thin films of solid electrolytes with excellent performances, i.e., high ionic conductivity (σ i ), good stability against the lithium anode, large electrochemical window (∆V), and poor electronic conductivity (σ e ), are currently required. To fulfill these requirements, the thin films of oxide-, phosphate-, or sulphide-based solid electrolytes were grown by the PLD technique [256][257][258]. The facile manufacture of such thin films is due to the easy control of the PLD chamber's atmosphere. Table 8 lists some typical solid electrolyte thin films prepared by PLD [41,[259][260][261][262][263][264]. Table 8. Electrical properties of PLD-grown solid-electrolyte thin films.

LiPON
In the early 1990s, Bates et al. prepared Li 3 PO 4 thin films using a sputter-deposition technique in the presence of N 2 gas that resulted in a nitrogen-doped lithium phosphate (called LiPON) of a typical chemical composition, Li 3.3 PO 3.9 N 0.17 to Li 2.9 PO 2.9 N 0.7 . The structure consists of doubly and triply coordinated nitrogen atoms, which form cross-links between the phosphate chains [20]. LiPON displays a high chemical stability and an ionic conductivity of 2 × 10 −6 S·cm −1 at 25 • C [265]. The growth of LiPON thin films by pulsed-laser deposition is also realized in nitrogen partial pressure with a moderate laser power influence [259,266].
Zhao et al. reported the growth LiPON thin films on three different substrates (i.e., Si wafer, Au-coated Si, and Al-coated glass plate) by reactive PLD in an N 2 gas atmosphere in the range of 50 to 200 mTorr using a Li 3 PO 4 target. The target was ablated by the beam of a Nd:YAG laser at the fluence of 5 to 20 mJ·cm −2 . The influence of the ambient N 2 pressure and the laser fluence on the ionic conductivity was systemically examined and the best result of 1.6 × 10 −6 S·cm −1 with an activation energy of 0.58 eV at 25 • C was obtained for a film prepared under 200 mTorr at Φ = 15 J·cm −2 . The mechanism of the nitridation of Li 3 PO 4 was carried out by XPS measurements, showing that σ i increases with the N/P ratio [259]. West et al. showed that a 17-nm thick layer of LiPON deposited at the solid electrolyte-electrode interface decreased the charge-transfer resistance from 4470 to 760 cm −2 in a Li/LiPON/LNM cell. The PLD amorphous films with σ i = 1.5 × 10 −8 S·cm −1 at 25 • C were deposited from a crystalline Li 2 PO 2 N target under the flow of N 2 gas at P N 2 = 1 Pa [267].

Li x La 2/3+y TiO 3−d (LLTO)
Solid electrolytes, such as lithium lanthanum titanium oxides, Li x La 2/3+y TiO 3−δ (LLTO), based on a perovskite-like structure can accept vacancies at the Li (or La) and oxygen sites and show properties depending on the composition, with an electronic conductivity when Ti 3+ cations (instead of Ti 4+ ) are present and an ionic conductivity for Li-rich material. The typical growth of LLTO thin films fabricated by the laser ablation technique is obtained at deposition temperatures in the range of 600 to 800 • C under a controlled oxygen pressure from 0.1 to 100 Pa [268]. LLTO films, such as Li 3x La (2/3)−x TiO 3 , exhibit a high ionic conductivity of up to 10 −5 S·cm −1 when deposited with pulsed laser deposition [269][270][271]. Li 0.5 La 0.5 TiO 3 (LLTO) PLD thin films, prepared at 400 to 600 • C, are amorphous and show an ionic conductivity of~2 × 10 −5 S·cm −1 at room temperature. Contrary to crystalline films, the amorphous LLTO exhibits good stability in contact with lithium metal anodes. Half-cells based on LiCoO 2 films covered with LLTO films deposited by pulsed laser deposition could be cycled for hundreds of cycles [269]. Furusawa et al. prepared amorphous LLTO films at T s = 25 • C with a uniform thickness (0.46-0.63 µm) using a laser energy of 180 mJ per pulse at 10 Hz [270]. The authors stated a controlled pressure of~10 −6 Torr but did not mention the presence of O 2 gas. The highest σ i of 1.2 × 10 −3 S·cm −1 (E a = 0.35 eV) obtained for Li 0.5 La 0.5 TiO 3 films deposited on an Ag substrate was due to the absence of grain boundaries. Maqueda optimized the PLD growth parameters to prepare La 0.57 Li 0.29 TiO 3 dense films at T s = 700 • C under P O 2 = 15 Pa with smooth surfaces [271]. The obtained nano-crystalline films exhibited domains, which are cubic and tetragonal modifications of the perovskite phase. Transport measurements showed an ionic conductivity of 8.2 × 10 −4 S·cm −1 at 25 • C with E a = 0.34 eV. Epitaxial Li 0.33 La 0.56 TiO 3 solid electrolyte thin films were grown on NdGaO 3 (110) by PLD at T s higher than 900 • C under P O 2 = 5 Pa [272]. These films showed a conductivity σ i of 3.5 × 10 −5 S·cm −1 at 25 • C with E a = 0.35 eV (Figure 14).  The influence of different substrates and excess lithium in the target on the microstructure and ionic conductivity of PLD LLTO thin films was examined by Aguesse et al. [273] Despite a large lattice mismatch of up to +8.8% with the substrate, the epitaxial growth of LLTO is possible on different (001) oriented LaAlO3, SrTiO3, and MgO substrates using a sintered Li0.37La0.54TiO3 target and PLD parameters, such as Ts = 750-880 °C, PO₂ = 4-20 Pa, and laser fluence of 1.07 J cm −2 . An ionic conductivity as high as 19.2 × 10 −3 mS·cm −1 at 25 °C was obtained for 170-nm thick LLTO films grown on an STO substrate from an ablated 10 mol% lithium excess target. PLD LLTO films with a σi of 3 × 10 −4 S·cm −1 and σe of 5 × 10 −11 S·cm −1 were obtained by controlling the background PO₂ and Ts. Amorphous LLTO films were utilized in SSMB cycled up to 4.8 V vs. Li + /Li with high voltage LiNi0.5Mn1.5O4 spinel cathode thin films [274].
Another class of LLTO electrolytes consists of Ti-based solid electrolytes with a garnet-like structure, first reported by Weppner et al. [275]. Li6BaLa2Ta2O12 thin films were deposited on an MgO The influence of different substrates and excess lithium in the target on the microstructure and ionic conductivity of PLD LLTO thin films was examined by Aguesse et al. [273] Despite a large lattice mismatch of up to +8.8% with the substrate, the epitaxial growth of LLTO is possible on different (001) oriented LaAlO 3 , SrTiO 3 , and MgO substrates using a sintered Li 0.37 La 0.54 TiO 3 target and PLD parameters, such as T s = 750-880 • C, P O 2 = 4-20 Pa, and laser fluence of 1.07 J cm −2 . An ionic conductivity as high as 19.2 × 10 −3 mS·cm −1 at 25 • C was obtained for 170-nm thick LLTO films grown on an STO substrate from an ablated 10 mol% lithium excess target. PLD LLTO films with a σ i of 3 × 10 −4 S·cm −1 and σ e of 5 × 10 −11 S·cm −1 were obtained by controlling the background P O 2 and T s . Amorphous LLTO films were utilized in SSMB cycled up to 4.8 V vs. Li + /Li with high voltage LiNi 0.5 Mn 1.5 O 4 spinel cathode thin films [274].
Another class of LLTO electrolytes consists of Ti-based solid electrolytes with a garnet-like structure, first reported by Weppner et al. [275]. Li 6 BaLa 2 Ta 2 O 12 thin films were deposited on an MgO(100) substrate by the ablation of a target with a 5 mol% Li 2 O excess. In standard PLD conditions films via PLD and studied the impact of PLD parameters (fluence of 1 to 4 J·cm −2 , T s in the range of 50 to 700 • C, and a post-annealing process) on the structural and transport properties. The ionic conductivity was measured by impedance spectroscopy. It was concluded that σ i is not dependent on T s but is strongly affected by the laser fluence [276]. Li 7 La 3 Zr 2 O 12 (LLZO) garnet-like thin films were deposited on Si 3 N 4 /Si substrates at temperatures in the range of 50 ≤ T s ≤ 750 • C under a fixed background of P O 2 = 1.3 Pa with a KrF excimer laser set at 0.6 J·cm −2 [277]. The best material, which exhibited an ionic conductivity of 6.3 × 10 −3 S cm −1 at 400 • C (E a = 0.6 eV), was obtained at T s = 300 • C. The review of garnet-like solid electrolyte thin films grown via PLD is summarized in Table 9. Table 9. Literature review of garnet-like solid electrolyte thin films grown via PLD.

P-and Si-Based Electrolytes
Several phosphorus-or silicon-based oxides and sulfides are solid electrolytes for lithium batteries, such as Li 3 [14,282]. Amorphous PLD thin films of LiSICON display higher conductivities than that of Li 4 SiO 4 and Li 3 PO 4 films. The solid electrolyte 0.5Li 4 SiO 4 -0.5Li 3 PO 4 dense films deposited on an Si wafer at Φ = 2-6 J·cm −2 under an argon gas of P Ar = 0.01-5 Pa had an ionic conductivity of 1.6 × 10 −6 S·cm −1 at 25 • C and an activation energy of 52 kJ·mol −1 [283]. Nakagawa et al. determined that PLD Li 2 SiO 3 films stable to CO 2 have an ionic conductivity of 2.5 × 10 −8 S·cm −1 at 25 • C lower than that of Li 2 SiO 3 films (i.e., 4.1 × 10 −7 S·cm −1 ), which are unstable to CO 2 [284]. PLD thin films of lithium meta-silicate (LSO) deposited at a growth rate of 0.17 Å per pulse on various substrates (i.e., SiO 2 , quartz, sapphire, Al 2 O 3 ceramic, and MgO) from an Li 2 SiO 3 sintered tablet were grown in the amorphous state. The ionic conductivity slightly depends on the substrate species with the best results (σ i = 4.5 × 10 −4 S·cm −1 at 300 • C, E a = 0.88 eV) found for an 80-nm thick film deposited on SiO 2 glass [262,285]. The PLD conditions for the growth of thio-LiSICON Li 3.25 Ge 0.25 P 0.75 S 4 solid electrolyte thin films were carefully chosen (especially the Li content of 3.2 in the target, which maintains the number of Li vacancies) to obtain a high σ i value of 1.7 × 10 −4 S·cm −1 at 25 • C [265]. PLD 80Li 2 S-20P 2 O 5 thin film prepared under P Ar = 5 Pa exhibited an ionic conductivity and activation energy of 7.9 × 10 −5 S·cm −1 and 43 kJ mol −1 at 25 • C, respectively. Heat treatment increased the σ I to 2.8 × 10 −4 S·cm −1 [286]. To avoid the formation of a Li-deficient phase, such as Li 4 P 2 S 6 , an Li 2 S-enriched Li 3 PS 4 target was used to grow PLD solid-electrolyte thin films. Using an Li 3.42 PS 4.21 target, PLD Li 3 PS 4 films exhibited a higher ionic conductivity of 5.3 × 10 −4 S·cm −1 at 20 • C [287].

PLD Electrolyte as Buffer Layers
Solid-state electrolyte (SSE) thin films have been used as a conductive buffer layer for the reduction of high resistance at the electrode/SSE interface of high-power all-solid-state lithium batteries. Coating the Li 3 PO 4 thin films on electrode materials by the PLD method was found to be efficient for this purpose. Konishi et al. [288] reported the effect of surface Li 3 PO 4 coating on LiNi 0.5 Mn 1.5 O 4 epitaxial thin film electrodes. Amorphous Li 3 PO 4 film (1-4 nm thick) was deposited at 25 • C with a laser energy of 150 mJ under P O 2 = 3.3 Pa. It was also pointed out that such a coating reduces the Mn dissolution in the non-aqueous electrolyte. Yubuchi et al. [289] fabricated the same coated electrode with Φ = 2 J cm −2 but under a lower oxygen gas pressure of 0.01 Pa. With a 100-nm thick Li 3 PO 4 deposit, the total resistance of the Li cell decreased from 15 to 350 Ω. A PLD protective coating of 80Li 2 S-20P 2 S 5 solid electrolytes on LiCoO 2 particles was performed at room temperature under Ar gas at P Ar = 5 Pa with a fluence of ca. 2 J·cm −2 (200 mJ per pulse). After SSE deposition for 120 min, the deposited film was~150-nm thick, corresponding to 3 wt.% LiCoO 2 . Annealing the SSE deposit at 200 • C increased the capacity of the all-solid-state cell [42]. Another example of the buffer function of the Li 2 S−P 2 S 5 solid electrolyte is given by the PLD coating of NiS-carbon fiber composite electrodes. The high ionic conductivity of 80Li 2 S−20P 2 S 5 film deposited on an Si wafer was 7.9 × 10 −5 S·cm −1 at 25 • C [290]. A capacity of 300 mAh·g −1 was delivered after 50 cycles at a current density of 3.8 mA·cm −2 (1C-rate). This SSE coating favors the lithium ion and electron conduction paths in the NiS framework. Ito et al. [291] successfully deposited Li 2 S-GeS 2 thin films as the buffer electrolyte (σ i = 1.8 × 10 −4 S·cm −1 ) on LiCoO 2 particles by the PLD technique. The amorphous 78Li 2 S-22GeS 2 solid electrolyte thin films prepared using standard PLD conditions exhibited an ionic conductivity of 1.8 × 10 −4 S·cm −1 at 25 • C. These SSE films were applied to form an electrode-electrolyte buffer interface with LiCoO 2 [291]. The coating of a LiNbO 3 SSE buffer coated onto the LiMn 2 O 4 cathode resulted in an enhancement of the high rate capability and cycling stability of the electrode [292]. A similar process ensured a high thermal stability for the LiNi 0.8 Co 0.15 Al 0.05 O 2 electrode operating over 500 charge-discharge cycles at 150 • C [293].  (111) or fused silica plate exhibited an ionic conductivity of 10 −7 S cm −1 ·at 25 • C, which is one order higher than the value for PLD Li 2 TiO 3 film [295]. All-solid-state thin film batteries were fabricated using both LCO and LMO PLD film cathodes and amorphous LVSO solid electrolytes as shown in Figure 15 [121].

LiNbO3
Because of its high room-temperature ionic conductivity and low electronic conductivity (10 −5 and 10 −11 S·cm −1 , respectively), LiNbO3 (R3c crystal structure) is considered as a good SSE for electrode coating [296]. LiNbO3 was applied as a buffer layer between an LCO cathode and thio-LISICON electrolyte (Li3.25Ge0.25P0.75S4). The resultant electrochemical cell showed low interfacial resistance and a high-rate capability [297]. A high quality was obtained for PLD LiNbO3 thin films deposited at 730 °C on sapphire substrates by using a relatively high oxygen partial pressure of PO₂ = 133 Pa and a laser fluence of 3 to 5 J cm −2 [298]. Contrastingly, Perea et al. prepared PLD LiNbO3 films using a lower laser fluence (0.8 to 1.6 J·cm −2 ) in a residual pressure of ≈4 × 10 −4 Pa [299].

TiO2
Due to the theoretical capacity of ~335 mAh·g −1 of titanium dioxide (comparable to ~372 mAh·g −1 for graphite and the small volume expansion (~4% for anatase)) significant interest has been devoted to the applied anode material in Li-ion batteries. The tetragonal anatase polymorph of TiO2 is a good anode candidate due to its insertion potential of around 1.5 V vs. Li + /Li [300]. Several works of the literature report the growth of TiO2 thin films with either a rutile or anatase structure fabricated by the PLD technique [301,302]. The growth conditions were studied on TiO2 films deposited by PLD using an Nd:YAG laser (532 nm wavelength beam) and a rutile-type TiO2 target. The effects of the substrate temperature (Ts) and oxygen partial pressure (PO₂) were investigated by Raman spectroscopy [13]. The parameters of Ts = 300 °C and PO₂ = 50 mTorr were optimized to obtain crystalline TiO2 films with a preferential (110) orientation. Kim et al. discussed the effects of the target morphology and target density on the size and distribution density of crystalline in PLD rutile-type TiO2 films deposited on (100)-oriented Si wafers maintained at 700 °C in a chamber with an oxygen partial pressure of 1.33 Pa [303]. A nearly particulate-free film was obtained from a dense target and the laser shots were adjusted for clear ripple patterns from the target surface. The optical bandgap energies of TiO2 PLD films grown on an α-Al2O3 (0001) substrate with an anatase and rutile structure were evaluated to be 3.22 and 3.03 eV, respectively [304]. Inoue et al. reported that films deposited at Ts = 150 °C have an anatase structure, while Ts = 300 °C provides rutile-type TiO2 films [305]. Choi et al. [306] prepared anatase TiO2 thin films with nanograins of 11 to 28 nm using a TiC target with Ts = 500 °C under 4 Pa O2 gas.

LiNbO 3
Because of its high room-temperature ionic conductivity and low electronic conductivity (10 −5 and 10 −11 S·cm −1 , respectively), LiNbO 3 (R3c crystal structure) is considered as a good SSE for electrode coating [296]. LiNbO 3 was applied as a buffer layer between an LCO cathode and thio-LISICON electrolyte (Li 3.25 Ge 0.25 P 0.75 S 4 ). The resultant electrochemical cell showed low interfacial resistance and a high-rate capability [297]. A high quality was obtained for PLD LiNbO 3 thin films deposited at 730 • C on sapphire substrates by using a relatively high oxygen partial pressure of P O 2 = 133 Pa and a laser fluence of 3 to 5 J cm −2 [298]. Contrastingly, Perea et al. prepared PLD LiNbO 3 films using a lower laser fluence (0.8 to 1.6 J·cm −2 ) in a residual pressure of ≈4 × 10 −4 Pa [299].

TiO 2
Due to the theoretical capacity of~335 mAh·g −1 of titanium dioxide (comparable to~372 mAh·g −1 for graphite and the small volume expansion (~4% for anatase)) significant interest has been devoted to the applied anode material in Li-ion batteries. The tetragonal anatase polymorph of TiO 2 is a good anode candidate due to its insertion potential of around 1.5 V vs. Li + /Li [300]. Several works of the literature report the growth of TiO 2 thin films with either a rutile or anatase structure fabricated by the PLD technique [301,302]. The growth conditions were studied on TiO 2 films deposited by PLD using an Nd:YAG laser (532 nm wavelength beam) and a rutile-type TiO 2 target. The effects of the substrate temperature (T s ) and oxygen partial pressure (P O 2 ) were investigated by Raman spectroscopy [13]. The parameters of T s = 300 • C and P O 2 = 50 mTorr were optimized to obtain crystalline TiO 2 films with a preferential (110) orientation. Kim et al. discussed the effects of the target morphology and target density on the size and distribution density of crystalline in PLD rutile-type TiO 2 films deposited on (100)-oriented Si wafers maintained at 700 • C in a chamber with an oxygen partial pressure of 1.33 Pa [303]. A nearly particulate-free film was obtained from a dense target and the laser shots were adjusted for clear ripple patterns from the target surface. The optical bandgap energies of TiO 2 PLD films grown on an α-Al 2 O 3 (0001) substrate with an anatase and rutile structure were evaluated to be 3.22 and 3.03 eV, respectively [304]. Inoue et al. reported that films deposited at T s = 150 • C have an anatase structure, while T s = 300 • C provides rutile-type TiO 2 films [305]. Choi et al. [306] prepared anatase TiO 2 thin films with nanograins of 11 to 28 nm using a TiC target with T s = 500 • C under 4 Pa O 2 gas.

Li 4 Ti 5 O 12 (LTO)
Li 4 Ti 5 O 12 (LTO) cubic structure (Li[Li 1/3 Ti 5/3 ]O 4 in spinel notation), considered as a "zero-strain" anode material, exhibits the advantage of very minor volumetric changes (<0.2%) upon cycling. This electrode displays a large voltage plateau at~1.5 V vs. Li + /Li and a theoretical specific capacity of 175 mAh·g −1 [307]. The first PLD growth of LTO thin films deposited onto Pt/Ti/SiO 2 /Si substrates using a KrF excimer laser beam (248 nm, 250 mJ) were reported by Deng et al. [308]. Films annealed at 800 • C (410 nm thick) exhibited a cubic structure with a lattice constant 8.375 Å larger than that of the LTO crystal (8.359 Å). The SEM cross-section image (Figure 16a) revealed the porous morphology induced by the high temperature treatment. The discharge specific capacity was the largest for films annealed at 700 • C due to the optimized adhesion strength between the film and substrate (Figure 16b). The anode films discharged at a current density of 10 µA·cm −2 (0.58C rate) showed excellent cycleability; the discharge capacity remained as 149 mAh·g −1 after 50 cycles. Li 4 Ti 5 O 12 films (545 nm thick) deposited on conducting fluorine-doped tin oxide (LTO/FTO) with a crystallite size of 50 to 80 nm were investigated as electrochromic active material with the highest contrast at a wavelength of 705 nm (transmittance change of~48%) [309]. Epitaxial LTO thin-film grown on SrTiO 3 single crystal from an Li-rich target, Li 5.2 Ti 5 O 12 , have a structural orientation identical to the substrate and are impurity-free when deposited at T s = 700 • C. The electrochemical features of LTO film anodes (20 nm thick) exhibited discharge capacities of~200 and~250 mAh·g −1 for the (100)-and (111)-orientation, respectively [310]. Kim et al. prepared nano-sized epitaxial LTO(110) deposited on Nb:SrTiO 3 (110) substrate. These films (~28 nm thick) were tested by cyclic voltammetry at a scan rate of 1 mV·s −1 and exhibited redox peaks at 1.53 and 1.60 V, corresponding to the insertion and extraction of Li + ions. As-deposited films at a substrate temperature of 700 • C in a 6.6 Pa oxygen partial pressure exhibited a high initial capacity (~200 mAh·g −1 ) but poor stability [311]. Kumatani

Li4Ti5O12 (LTO)
Li4Ti5O12 (LTO) cubic structure (Li[Li1/3Ti5/3]O4 in spinel notation), considered as a "zero-strain" anode material, exhibits the advantage of very minor volumetric changes (<0.2%) upon cycling. This electrode displays a large voltage plateau at ~1.5 V vs. Li + /Li and a theoretical specific capacity of 175 mAh·g −1 [307]. The first PLD growth of LTO thin films deposited onto Pt/Ti/SiO2/Si substrates using a KrF excimer laser beam (248 nm, 250 mJ) were reported by Deng et al. [308]. Films annealed at 800 °C (410 nm thick) exhibited a cubic structure with a lattice constant 8.375 Å larger than that of the LTO crystal (8.359 Å). The SEM cross-section image (Figure 16a) revealed the porous morphology induced by the high temperature treatment. The discharge specific capacity was the largest for films annealed at 700 °C due to the optimized adhesion strength between the film and substrate (Figure 16b). The anode films discharged at a current density of 10 µA·cm −2 (0.58C rate) showed excellent cycleability; the discharge capacity remained as 149 mAh·g −1 after 50 cycles. Li4Ti5O12 films (545 nm thick) deposited on conducting fluorine-doped tin oxide (LTO/FTO) with a crystallite size of 50 to 80 nm were investigated as electrochromic active material with the highest contrast at a wavelength of 705 nm (transmittance change of ~48%) [309]. Epitaxial LTO thin-film grown on SrTiO3 single crystal from an Li-rich target, Li5.2Ti5O12, have a structural orientation identical to the substrate and are impurity-free when deposited at Ts = 700 °C. The electrochemical features of LTO film anodes (20 nm thick) exhibited discharge capacities of ~200 and ~250 mAh·g −1 for the (100)-and (111)-orientation, respectively [310]. Kim et al. prepared nano-sized epitaxial LTO(110) deposited on Nb:SrTiO3(110) substrate. These films (~28 nm thick) were tested by cyclic voltammetry at a scan rate of 1 mV·s −1 and exhibited redox peaks at 1.53 and 1.60 V, corresponding to the insertion and extraction of Li + ions. As-deposited films at a substrate temperature of 700 °C in a 6.6 Pa oxygen partial pressure exhibited a high initial capacity (~200 mAh·g −1 ) but poor stability [311]. Kumatani et al. investigated the PLD growth process of epitaxial LTO films deposited on an MgAl2O4 (111) substrate. With Ts = 800 °C and PO₂ = 1 × 10 −3 Torr, LTO films had excellent crystallinity and a low resistivity of 3.3 × 10 −4 Ω cm. at 25 °C. At lower PO₂, the PLD LiTi2O4 film was formed, while at higher PO₂, Ti was segregated as TiO2 rutile and Li0.74Ti3O6 [312]. Studies of the electrochemical performance and kinetic behavior of PLD LTO films deposited on Pt/Ti/SiO2/Si substrates were reported by Deng et al. [313]. Using an Li-rich target (i.e., excess 5 wt. % Li2O), the films annealed at 700 °C for 2 h in air were well-crystallized items with densely packed grains. The galvanic charge-discharge plateau was observed around 1.56 V and an initial specific capacity of 159 mAh g − 1 was delivered with a retention of 93.7% after 20 cycles. The diffusion coefficient of Li + ions in such an LTO framework was in the range of 10 − 15 to 10 − 12 cm 2 ·s − 1 . The energy barrier of the diffusion of lithium ions was estimated to be Ea = 0.11 eV in LTO (111)oriented PLD thin films (190 nm thick) grown on a spinel MgAl2O4 (111) substrate [314]. Studies of the electrochemical performance and kinetic behavior of PLD LTO films deposited on Pt/Ti/SiO 2 /Si substrates were reported by Deng et al. [313]. Using an Li-rich target (i.e., excess 5 wt.% Li 2 O), the films annealed at 700 • C for 2 h in air were well-crystallized items with densely packed grains. The galvanic charge-discharge plateau was observed around 1.56 V and an initial specific capacity of 159 mAh g −1 was delivered with a retention of 93.7% after 20 cycles. The diffusion coefficient of Li + ions in such an LTO framework was in the range of 10 −15 to 10 −12 cm 2 ·s −1 . The energy barrier of the diffusion of lithium ions was estimated to be E a = 0.11 eV in LTO (111)-oriented PLD thin films (190 nm  PLD films were obtained on various substrates at T s = 650 • C under a 0.3 Pa pure oxygen atmosphere using a commercially available LTO powder. As-prepared films (650 nm thick) revealed columnar growth that allowed a coulombic efficiency >97% after the second cycle and a discharge capacity of 33 µAh·cm −2 at a 3.5 µA cm −2 current density [43]. Pfenninger et al. demonstrated that LTO thin films deposited by PLD on an MgO substrate kept at 500 • C using a dense Li 7.1 Ti 5 O 12 target sintered at 1000 • C for 12 h are compatible with the Li 6.25 Al 0.25 La 3 Zr 2 O 12 electrolyte pellet. Such films display a stable structure and cycleability almost close to 175 mAh·g −1 . The typical voltage plateau at 1.57 V (oxidation) and 1.53 V (reduction) was observed at a rate of 2.5 mA·g −1 [316]. Among the Li 1+x Ti 1−x O 4 ternary system, LiTi 2 O 4 thin films were grown by the PLD route in the temperature range of 400 to 800 • C using a target with a higher Li/Ti ratio of 0.8 [317]. Chopdekar [318]. The authors state the PLD conditions with T s held at 450 to 600 • C in a vacuum of better than 5 × 10 −6 Torr without any mention of the oxygen partial pressure, while Kumatani determined that stoichiometric LiTi 2 O 4 thin films were obtained at a P O 2 of 5 × 10 −6 Torr with T s = 800 • C [312]. Recently, PLD LTO films grown on Nd-doped oriented STO substrates at T s = 700 • C under P O 2 = 20 Pa showed high discharge capacities of 280 to 310 mAh·g −1 . The best rate performance of 30C was obtained for the (100)-oriented Li 4 Ti 5 O 12 films [319].

LiNiVO 4
Amorphous LiNiVO 4 thin-film anodes for microbatteries were grown by pulsed laser deposition using a sintered Li 1.2 NiVO 4 target. The film grown at T s = 25 • C and P O 2 = 8 mTorr showed the best electrochemical performance with a retainable capacity as high as 410 µAh·cm −2 ·µm −1 after 50 cycles [320].

TiNb 2 O 7
An alternative to LTO, titanium-niobium oxide, TiNb 2 O 7 (TNO), is considered a promising anode material for long life Li-ion batteries, due to its high Li + ion transport, average voltage of 1.66 V, and theoretical capacity of ∼387 mAh·g −1 [321]. Fabrication of PLD TiNb 2 O 7 thin films as anode electrodes for Li-ion micro-batteries was demonstrated by the ablation of a Nb 2 O 5 + TiO 2 mixture as a target at a laser fluence of 4.6 J·cm −2 . Pure monoclinic TNO films were deposited on Pt/TiO 2 /SiO 2 /Si(100) substrates at 750 • C under an O 2 gas of P O 2 = 6-13 Pa. The 380-nm thick films grown at P O 2 = 13 Pa delivered an initial specific capacity of 142 µAh cm −2 µm −1 at a current density of 50 µA·cm −2 with a 58% capacity retention after 25 cycles [322]. Recently, the same co-workers reported a high specific discharge capacity of 226 µAh·cm −2 ·µm −1 (~460 mAh·g −1 ) at a current density of 17 µA·cm −2 for amorphous TNO films grown by PLD. Li + diffusion coefficient of ≈10 −13 cm 2 ·s −1 and an electronic conductivity of ≈10 −9 S·cm −1 were also reported [323].

Silicon
Pulsed-laser deposited silicon thin films have been widely studied for applications in opto-electronics. With a large theoretical capacity (4200 mAh·g −1 ), silicon is also considered as a promising anode material for the replacement of graphite anode (LiC 6 , 372 mAh·g −1 ) for Li-ion batteries [324]. Despite the huge volume expansion of >300% during lithiation up to Li 22 Si 5 , it is possible to obtain anodes with Si thin films grown by physical vapor deposition (PVD), reaching a cycling life of up to 3000 cycles due to the limited volume change in the 2D film [325]. For example, a film deposited on Ni foil maintained a capacity of 3000 mAh·g −1 at a 12C rate over 1000 cycles [326]. PLD-grown Mg 2 Si thin film (30-380 nm thick) exhibited electrochemical activity with a stable cycling behavior in the voltage range of 0.1 to 1.0 V vs. Li + /Li; however, the initial irreversible capacity loss increased with the film thickness. The superior capacity of the 30-nm thick film was attributed to the formation of Li-Si alloys at the Si-rich surface [327]. Park et al. prepared PLD amorphous Si (a-Si) thin films on a stainless-steel substrate at temperature of 500 • C under an Ar gas pressure P Ar = 6.5 mPa. Furthermore, 1.5-µm thick a-Si films were obtained at the growth rate of 25 nm·min −1 . Electrochemical tests carried out in the voltage range of 0.005 to 1.5 V showed a first discharge capacity of 9 to 0.7 µAh·cm −2 with a 54.4% coulombic efficiency. Although, after 70 cycles, the 1-µm thick Si film exhibited a good cyclic performance [328]. Xia et al. reported the growth of a-Si using the standard conditions (T s = 25 • C, P = 1.3 mPa, fluence of 150 to 160 mJ per pulse, deposition time of 30 min). Electrochemical tests showed that 120-nm thick a-Si films exhibited an initial charge capacity of~64 µAh·cm −2 at a current density of 100 µA·cm −2 , a discharge capacity of~50 µAh cm −2 was maintained after 40 cycles, and the diffusion coefficient of Li ions determined from the cyclic voltammograms was~10 −13 cm 2 ·s −1 [329]. Some Si-based composite thin films prepared by PLD combine the advantages of both components. The most popular are the carbon-based composites [330][331][332][333]. Chou et al. obtained a flexible anode material by the deposition of Si film onto single-wall carbon nanotubes (SWCNTs) using standard PLD conditions (λ = 248 nm, T s ≈ 30 • C, Φ = 3 J·cm −2 , P Ar = 13 Pa, target-substrate distance of 50 mm). After 50 cycles, this Si/SWCNT nanocomposite delivered a specific capacity of 163 mAh·g −1 at a 25 mA·g −1 current density, which is 60% higher than for CNT [330]. The ultrathin film of Si grown by PLD was deposited on multilayer graphene (MLG) by CVD on an Ni foam substrate. The specific capacity of this binder-free Si/MLG anode appeared to be stable at ca. 1400 mAh·g −1 [331]. Silicon nitride SiN 0.92 thin films were prepared by PLD and investigated as a negative electrode in lithium batteries. A 200-nm thick film grown on buffed stainless-steel substrates kept at 25 • C from an Si 3 N 4 pellet as the target delivered a high specific capacity of 1300 mAh·g −1 after 100 cycles [334].

Graphene
Most of the commercial lithium batteries have a carbon anode. Graphene is the most conductive form of carbon, and as such, it is considered as a promising electrode, especially when it is doped with nitrogen [229]. A recent review was devoted to PLD-graphene synthesized from a solid carbon source [335]. Since nitrogen modifies the intrinsic properties of graphene, it is important to control its concentration. PLD, which allows for a one-step synthesis of N-doped carbon films, is particularly suited to this purpose. The first N-doped amorphous carbon film (a-C:N) synthesized by PLD dates from 2013 [336]. It contained 2 at. % of nitrogen. More recently, using the same approach, the upper nitrogen concentration in PLD a-C:N film was raised to 3.2 at.% [337]. These films, however, were not used as electrodes. On the other hand, a N-doped graphene (NG) electrode prepared by PLD coupled with in-situ thermal annealing (PLD-TA) was achieved by Fortgang et al. [338]. More recently, Bourquard et al. used the PLD-TA process to form an N-doped graphene film by high temperature condensation of the laser-induced carbon plasma plume onto the Si electrode previously covered by an Ni catalytic film [339], using a protocol published by the same group [340] Carbon was ablated at 780 • C from the graphite target using a femtosecond laser (λ = 800 nm, pulse width of 60 ns, repetition rate of 1 kHz, and Φ = 5 J·cm −2 ) at a distance of 36 mm from the graphite target, with P N = 10 Pa in the vacuum chamber. The electrochemical properties were measured with the thus-obtained 40 nm-thick film with a nitrogen concentration of 1.75% as the working electrode and an active area of 0.07 cm 2 , saturated calomel electrode as the reference electrode, and platinum as the counter electrode, in a 0.5 mol·L −1 1,1 ferrocene-dimethanol solution of 0.1 mol·L −1 NaClO 4 . Aiming to detect H 2 O 2 in 0.1 mol L −1 phosphate buffer saline (PBS) solution (pH 7.4), linear sweep voltammetry was used in the anodic range from 0 to 1000 mV with a scan rate of 100 mV·s −1 . The electrode showed excellent reversibility, 60 mV, close to the theoretical value, and a detection limit of 1 mM of hydrogen peroxide, which constitutes a major improvement of the electroanalytical oxidation of H 2 O 2 in comparison with undoped graphite electrodes. Such results are recent, and to our knowledge, no such electrode has been tested yet as an anode for lithium batteries.

Other PLD Anodes
Significant efforts have been devoted to the design and development of new PLD-grown anode materials for SSMBs, yielding a high energy density (from 500 to 1500 mAh·g −1 ) and electrochemical stability [341][342][343][344][345][346][347][348][349][350][351][352][353][354]. Table 10 summarizes the PLD-growth conditions and electrochemical properties of some new anodes proposed for lithium microbatteries. All transition-metal oxide M x O y materials are subjected to electrochemical lithiation via a conversion reaction, which implies a two-step process, i.e., first, fine metallic (M) nanoparticles embedded in an insulating matrix, such as Li 2 O, are in situ formed during the first (irreversible) discharge, and secondly, an alloying reversible reaction (Li-M) occurs on subsequent cycles [341].
Recently, Wu et al. proposed a novel anode consisting of a Li 3 P-VP nanocomposite fabricated by PLD [353] using a 5-Hz Nd:YAG laser (λ = 355 nm, Φ = 2 J·cm −2 ) concentrated on the target surface with an incident angle of 45 • , with P Ar =10 Pa in the vacuum chamber. The stainless-steel substrate was placed 3 cm from the surface and kept at 400 • C. The weight of the film thus obtained (without the substrate) was 0.10 mg·cm −2 . The excessive lithium in this composite was used to stabilize the VP 2 structure after the first charge. Electrochemical tests were made with this film as the working electrode, while lithium metal sheets were used as counter and reference electrodes with 1 mol·L −1 LiPF 6 in EC:DMC (1:1 in volume) electrolyte. When cycled in the range of 0.01 and 4 V vs. Li + /Li at a current density of 5 µA·cm −2 , a capacity of 1040 mAh·g −1 was delivered at the second discharge, 987 mAh·g −1 at the 50th cycle.

Discussion
There is general agreement on the beneficial results of the pulsed-laser deposition of thin films used as active materials in lithium microbatteries. This is a technique prone to fabricate thin film rapidly, from a small amount of target material, while maintaining the ideal stoichiometry by the control of different growth parameters. We observed ( Table 2) that there is a strong trend to develop microcell technologies using LiCoO 2 film (typical thickness of 4-µm) as the cathode, LiPON thin film (typical thickness of 1 µm) as the solid electrolyte, and Li thin film anode, which may have advantages in terms of the following key requirements: High energy density, high voltage, sustainability, and easy fabrication. For such microbatteries, let us recall the energy units used by authors. For a comparison of the volumetric specific energy, one generally refers to that of the cathode material (i.e., the limiting electrode); thus, considering a dense LCO film (d = 4.3 g·cm −3 ), a gravimetric specific energy of 100 mAh·g −1 is equivalent to 43 µAh·cm −2 ·µm −1 or 154.8 mC·cm −2 ·µm −1 .
Let us compare and discuss the growth conditions that allow the best electrochemical performance of each component, i.e., cathode, anode, and electrolyte. Regarding the growth of lithiated oxides used as cathode materials, an excess of lithium (at least~15 wt.%) is mandatory for any material to compensate the loss of volatile lithium species during the ablation process. Given the different materials' available candidates for the cathode, LiCoO 2 appears to be the most electrochemically efficient. From Figure 17, comparing the Ragone plots of LiCoO 2 and LiMn 2 O 4 thin films, each curve displays a series of discharge profiles for a lithium microbattery with the cathode thickness (in µm). As the capacity delivered by a cathode is proportional to the mass of the material, thicker films provide high energies, but often at the expense of the power performance [26]. The specific energy for a planar Li//LiCoO 2 cell with a thick cathode can reach 500 µWh·cm −2 ·µm −1 . However, the rate capability depends strongly on the plane orientation of the film, which can be controlled by the nature of the substrate. Thus, the preferred orientation is the (003)-plane parallel [29]. However, a surface-electrolyte interface (SEI) is formed on epitaxial LiCoO 2 films with different orientations of (104), (110), and (003) that result in anisotropic reactions of intercalation activity [102]. It has also been demonstrated that the minimized strain energy in thick LCO films allows preferential (101) and (104) textures [32]. Nevertheless, the best well-crystallized LCO thin films were fabricated in the following PLD conditions: T s = 500 • C, P O 2 = 13 Pa, Φ = 2 J·cm −2 , substrate-target distance of 30 to 40 mm, and laser beam-target incident angle of 45 • . Interesting results reported in Ref. [53] showed impurity-free LCO films, highly (003)-oriented with a very small lattice expansion during charge (at 4.2 V), when grown on stainless steel substrates at relatively low temperatures (T s ≈ 300 • C). In this case, the films had a texture between the amorphous and well-crystalline state with very small grains, which is suitable for short pathways for electrons and ions during the (de)intercalation reaction. the different materials' available candidates for the cathode, LiCoO2 appears to be the most electrochemically efficient. From Figure 17, comparing the Ragone plots of LiCoO2 and LiMn2O4 thin films, each curve displays a series of discharge profiles for a lithium microbattery with the cathode thickness (in µm). As the capacity delivered by a cathode is proportional to the mass of the material, thicker films provide high energies, but often at the expense of the power performance [26]. The specific energy for a planar Li//LiCoO2 cell with a thick cathode can reach 500 µWh·cm −2 ·µm −1 . However, the rate capability depends strongly on the plane orientation of the film, which can be controlled by the nature of the substrate. Thus, the preferred orientation is the (003)plane parallel [29]. However, a surface-electrolyte interface (SEI) is formed on epitaxial LiCoO2 films with different orientations of (104), (110), and (003) that result in anisotropic reactions of intercalation activity [102]. It has also been demonstrated that the minimized strain energy in thick LCO films allows preferential (101) and (104) textures [32]. Nevertheless, the best well-crystallized LCO thin films were fabricated in the following PLD conditions: Ts = 500 °C, PO₂ = 13 Pa, Φ = 2 J·cm −2 , substrate-target distance of 30 to 40 mm, and laser beam-target incident angle of 45°.
Interesting results reported in Ref. [53] showed impurity-free LCO films, highly (003)-oriented with a very small lattice expansion during charge (at 4.2 V), when grown on stainless steel substrates at relatively low temperatures (Ts ≈ 300 °C). In this case, the films had a texture between the amorphous and well-crystalline state with very small grains, which is suitable for short pathways for electrons and ions during the (de)intercalation reaction. To avoid the poor performance of LIBs derived from hindered lithium-ion diffusion at the interface between the LCO positive electrode and electrolyte, modifications of the cathode surface have been realized by the deposition of a thin layer of a fast-ionic Li + conductor, such as amorphous Li2WO4 or Li3PO4. This layer reduces the interfacial Li + -ion transfer resistance that results in a rapid charge-discharge rate. The a-Li2WO3/LCO/Pt/Cr/SiO2 electrode cycled at a high rate of 20C with a high capacity retention [84]. Another electrode exhibiting a fast charge-discharge rate as high as 348C has been fabricated by the multilayer PLD technique, but in this case, the LMO thin film exhibited a significant pseudocapacitive behavior (non-diffusion-controlled) instead of a faradaic mechanism. An additional promising improvement is the fabrication of an LCO thin film sandwiched between a PLD-prepared SrRuO3 film as the electronic conductor and the film of Li3PO4 (3.2 nm thick) as the ionic conductor with the result being limited surface structural change in the high voltage range (4.4 V) [71].
The influence of the PLD conditions on the texture of LiMn2O4 thin films has shown that Ts = 500 °C and PO₂ = 20 Pa are the optimum values that maintain the Li/Mn ratio close to 1, when an Lienriched target is used, and obtains the best mass transfer [122]. It was also noticed that any To avoid the poor performance of LIBs derived from hindered lithium-ion diffusion at the interface between the LCO positive electrode and electrolyte, modifications of the cathode surface have been realized by the deposition of a thin layer of a fast-ionic Li + conductor, such as amorphous Li 2 WO 4 or Li 3 PO 4 . This layer reduces the interfacial Li + -ion transfer resistance that results in a rapid charge-discharge rate. The a-Li 2 WO 3 /LCO/Pt/Cr/SiO 2 electrode cycled at a high rate of 20C with a high capacity retention [84]. Another electrode exhibiting a fast charge-discharge rate as high as 348C has been fabricated by the multilayer PLD technique, but in this case, the LMO thin film exhibited a significant pseudocapacitive behavior (non-diffusion-controlled) instead of a faradaic mechanism. An additional promising improvement is the fabrication of an LCO thin film sandwiched between a PLD-prepared SrRuO 3 film as the electronic conductor and the film of Li 3 PO 4 (3.2 nm thick) as the ionic conductor with the result being limited surface structural change in the high voltage range (4.4 V) [71].
The influence of the PLD conditions on the texture of LiMn 2 O 4 thin films has shown that T s = 500 • C and P O 2 = 20 Pa are the optimum values that maintain the Li/Mn ratio close to 1, when an Li-enriched target is used, and obtains the best mass transfer [122]. It was also noticed that any substrate does not strongly influence the stoichiometry, but affects the out-of-plane preferred texture. The applicability of the PLD-grown V 2 O 5 films in lithium microbatteries has been evidenced that, in the range of 200 < T s < 400 • C under P O 2 = 6 Pa, the films offer better electrochemical performance than those grown at other temperatures in terms of their structural quality and stability. Only two works were devoted to the fabrication of solid-state thin-film batteries with vanadium oxide as the cathode materials: The Li/Li 1.4 B 2.5 S 0.1 O 4.9 /V 2 O 5 cell delivered a capacity of~400 mC·cm −2 ·µm −1 [221], while the Li/LiPON/Ag 0.3 V 2 O 5 maintained a specific capacity of 40 µAh·cm −2 ·µm −1 after 100 cycles [222], but the low current density was due to the poor electronic conductivity of the positive electrode. Recently, a solid-state thin-film battery, Li/Li 3 PO 4 /LiMnPO 4 , was successfully fabricated by PLD [48]. Such a cell delivered a modest specific capacity of 10 µAh·cm −2 ·µm −1 , which was limited by the slow chemical diffusion coefficient of the Li + ion in the olivine framework (3 × 10 −17 cm 2 ·s −1 ).
Lithium phosphates, i.e., Li 3 PO 4 and LiPON, are the most widely used solid electrolytes in microbatteries; they are easily fabricated by PLD using an ArF excimer laser and show a good ionic conductivity. The LiPON electrolyte is known to exhibit a better chemical stability than Li 3 PO 4 [282]. However, the electrochemical stability of PLD-prepared Li 3 PO 4 thin films is greater than 4.7 V [265].
Few works have attempted to replace the lithium metal thin-film anode by other lithiated materials (i.e., intercalation compound or alloy) for the fabrication of microbatteries. The most stable insertion compound should be Li 4 Ti 5 O 12 spinel with minor volumetric changes but the high voltage plateau of 1.5 V is a great penalty for high energy density. The In/80Li 2 S-20P 2 S 5 /LiCoO 2 microbattery developed by Sakuda et al. seems to be promising as the Li-In alloy allows a high specific discharge capacity at moderate current density of 0.13 mA·g −1 [42].

Concluding Remarks
The results of the intensive research on the growth of thin films by pulsed laser deposition in recent years have been reviewed. Due to careful investigations of the mechanism of the sample preparation, optimized materials with adequate properties for energy storage and conversion have been obtained. The PLD technique is considered to be suitable for improving the density and adhesion properties of films. A huge effort has been mainly concentrated on the deposition of lithiated oxides, which require specific conditions due to the volatile character of lithium vapor species during the PLD process. Due to the outstanding performance of the conventional cathode materials, LiCoO 2 and LiMn 2 O 4 , PLD films exhibiting a specific capacity close to the theoretical one are the most popular. The progress concerns mainly the epitaxial films grown with an orientation favorable to a high rate of transport of Li ions at the electrode/electrolyte interface. For instance, the pyramidal-type LiMn 2 O 4 films cycled at the 3.3C rate demonstrate a specific capacity of 90 mAh·g −1 after 1000 cycles.
The PLD technique has proved to also be efficient for the preparation of thin films of anode materials. The best example is the production of LTO, which is a "zero-strain" compound. Other anode thin film materials, such as silicon and conversion-type oxides, are attractive due to their high specific capacity and easy PLD fabrication.
So far, solid-electrolyte thin-films have been fabricated essentially by thermal vacuum evaporation and rf-sputtering. The manufacture of solid-electrolyte thin films by PLD has brought improvements in their intrinsic properties. For example, the electronic conductivity of PLD films is small in comparison with rf-sputtered films. LiPON and Li-V-S-O are the most popular solid-electrolyte films.
In recent years, due to a strong demand for smaller power sources, the interest in rechargeable micro-batteries has gradually increased. The progress on lithium microbatteries is remarkable, mainly due to the PLD growth of high-quality, pinhole-free, solid-state electrolyte thin films, such as Li 6.1 V 0.61 Si 0.39 O 5. 36 . The rechargeable thin-film lithium-ion battery designed by the Japanese group at Tohoku University was fabricated using the sequential PLD technique. This microcell delivered a specific capacity of 9.5 Ah cm −2 discharged at a current density of 44 µA·cm −2 using an Li-Sn alloy film as the anode and showed good reversibility over 100 cycles.