Polyaniline and water pre-intercalated V 2 O 5 cathodes for high-performance planar zinc-ion micro-batteries

,


Introduction
The rise of the Internet of Things (IoT) has firmly established miniaturization, flexibility, and integration as dominant trends in the evolution of microelectronics.Notably, significant progress has been made in the development of microelectronic devices designed for wearables and implants, including micro-robots and micro-sensors, poised to become integral parts of our daily lives.These diminutive devices excel in intricate tasks like data processing and wireless signal transmission within a space smaller than a few cubic millimetres, holding immense potential in fields such as health monitoring, medical diagnosis, and disease treatment [1].For these devices to function seamlessly, a crucial component is the energy supply unit.The ongoing trend of reducing the size of wearable and implantable microelectronics while enhancing their capabilities necessitates corresponding micro-power sources, such as micro-batteries capable of delivering substantial energy outputs.Conventional micro-battery structures, resembling layered sandwiches with positive and negative electrodes separated by separators, pose various challenges.Thin electrodes face limitations in terms of energy density, while 3D thick electrodes encounter slow ion diffusion.Additionally, the use of 3D thick electrodes and interdigitated patterns demands precise alignment and poses integration challenges when seamlessly incorporating them with on-chip microelectronics [2].An alternative design concept, known as planar-type device configurations, provides a solution.In this approach, electrodes are organized in a planar pattern on the same substrate, resulting in a flat device structure.This planar arrangement offers several advantages, including better control over critical battery attributes like internal resistance and ionic diffusion distance, all without the need for a separator [2,3].Most importantly, it provides a practical solution for reducing battery size and seamlessly integrating them with on-chip microelectronic devices.
Among potential energy systems, zinc-ion batteries (ZIBs) have recently garnered increased attention due to their cost-effectiveness, safety, and availability of materials compared to lithium-ion batteries [4][5][6].Additionally, in ZIBs, the use of metal zinc as anode, which boasts high capacities (820 mAh/g, 5855 mAh/cm 3 ) and a low redox potential (− 0.76 V vs SHE), allows for utilization in aqueous electrolytes [7].This characteristic enables the use of high-capacity oxide-based materials that are more stable in the air, simplifying the assembly process and reducing manufacturing difficulty.In recent years, various cathode materials have been explored for ZIBs, including Mn-based materials, Vbased materials, PBA-based materials, and others [7].However, certain cathode materials often experience degradation and exhibit small layer distances, resulting in poor long-term lifespan, unstable efficiency, and low capacity [8][9][10].Among these, layered V-based materials are popular cathode material candidates, offering the flexibility to modify and enhance energy storage through strategies such as altering the interlayer gap and modifying the valences of V [11][12][13][14].For instance, Wan et al. reported a Zn/NVO (NaV 3 O 8 ⋅1.5H 2 O) reversible battery by introducing hydrated sodium ions into V 3 O 8 layers to increase the interlayer gap [15].Furthermore, there are additional examples of altering valence states of V to boost rate stability and capability [16,17].However, it is noteworthy that these strategies are typically applied in conventional ZIBs with a coin cell configuration, and there is limited attention paid to their application in planar micro-batteries.Furthermore, augmenting the mass loading of high-capacity materials onto planar electrodes is crucial for enhancing the overall energy storage performance within the constraints of the limited device footprint.Therefore, it is imperative to adopt comprehensive strategies that address these diverse aspects in order to mitigate associated issues and prolong the operational lifespan of high-performance, environmentally safer planar micro-batteries.
In our study, we investigate approaches to enhance the capacity of V 2 O 5 materials.This involves synthesizing V 2 O 5 nanowires from commercial V 2 O 5 powder, followed by pre-intercalating polyaniline and water into the V 2 O 5 nanowires.These materials are then applied as cathode materials for planar zinc-ion micro-batteries (ZIMBs).As anticipated, the ZIMBs exhibit a remarkable areal capacity of 409 μAh/ cm 2 at 50 mA/g, coupled with cycling stability and capacity retention of 78 % even after 500 cycles.Simultaneously, the efficiency of this ZIMBs device yields a high areal energy density of 306.7 μWh/cm 2 , a power density of 3.44 mW/cm 2 and slow self-discharge performance.This study effectively improves the storage capabilities of ZIMBs, steering them toward achieving high-performance, highly secure planar microbatteries designed to power microelectronics.

Results and discussion
The synthetic process of polyaniline (PANI) and water intercalated V 2 O 5 is depicted in schematic Fig. 1a.The presence of a layered structure in α-V 2 O 5 is apparent, however, the diffusion of Zn 2+ ions is hindered by a substantial energy barrier between the layers and restricted interlayer distance, resulting in sluggish diffusion.In this study, water molecules are initially introduced into the gap between the layers during synthesis, resulting in the formation of V 2 O 5 ⋅nH 2 O.Following this step, aniline is added and subjected to heating, leading to polymerization and intercalation in V 2 O 5 ⋅nH 2 O, thus forming PANI-V 2 O 5 ⋅nH 2 O.The detailed synthesis procedure can be found in the experimental section.Consequently, it is anticipated that the co-intercalation of H 2 O and PANI into V 2 O 5 could broaden the interlayer distance, subsequently reducing the energy barrier between layers and facilitating effective Zn 2+ intercalation, thereby enhancing charge storage.The preparation process of ZIMBs is depicted in Fig. 1b.Initially, a piece of adhesive polyimide (PI) tape serves as the substrate, placed at the bottom.The pre-patterned graphene paper, coated with cathode material and zinc anode, is positioned on the PI tape with a 500 μm gap, using a mask for assistance.Subsequently, a glass fibre paper is affixed over the electrodes, and a consistent amount of aqueous electrolyte is dispensed onto the paper.To prevent leakage, a piece of parafilm is applied to create a space for the separator and aqueous electrolyte.Fig. 1c presents a digital photograph of a ZIMB, with further details available in the experimental section.As illustrated in the schematic (Fig. 1d), the diffusion of Zn 2+ ions in ZIMBs differs from conventional sandwich-type batteries.In ZIMBs, in-plane diffusion of Zn 2+ ions occurs, facilitating effective intercalation/deintercalation into the cathode and stripping/plating of Zn 2+ into the Zn anode during the discharging and charging processes.noteworthy that the intercalation of PANI into V 2 O 5 , increases the surface roughness and, consequently, the specific surface area (as discussed later), without fundamentally changing the overall morphologies.As depicted in Fig. 1e (i), CV 2 O 5 exhibits a smooth surface on the particles, while the addition of PANI increases the surface roughness of V 2 O 5 particles without altering their morphologies.When water is intercalated in V 2 O 5 , the nanostructure changes from particles to nanofiber structures due to a recrystallization process [18].TEM images (Fig. 1f) further confirm that the addition of PANI does not disrupt the original structure of the sample but enhances the roughness of nanomaterials, which is because the intercalated PANI is situated around the V 2 O 5 layers.Fig. S1 illustrates the elemental mappings of the sample.
Additionally, in understanding the crystal structure of the samples, XRD patterns reveal identical patterns, with observed additional peaks in PANI intercalated samples.In Fig. 2a, the observed peaks for all the samples are attributed to diffraction peaks around 15.3  , corresponding to (2 0 0), (0 0 1), (1 1 0), (4 0 0), (0 1 1), (3 1 0), and (1 1 1) planes for layered V 2 O 5 [19][20][21][22].In CPVO and SPVO, two additional peaks are detected at 14.6 • and 25.5 • , representing (0 1 1) and (2 0 0) crystal planes due to the presence of PANI, which corresponds to the peaks of PANI itself [23].Therefore, XRD patterns confirm that the introduction of H 2 O and PANI molecules maintains a similar crystal structure.In Fig. 2b, the FT-IR spectra of CV 2 O 5 , CPVO and SPVO are presented within the region of 2000-500 cm − 1 .The bands in the range of 900 cm − 1 to 500 cm − 1 represent the symmetric and asymmetric vibrations of V-O, and the peak at approximately 830 cm − 1 indicates the characteristic absorption of V-O-V bonds [24].The peak at 1613 cm − 1 in CPVO represents the O-H bond [21], indicating the absence of water molecules inserted into V 2 O 5 layers and suggesting adsorbed water.In SPVO, two water-related peaks appear at positions 1632 cm − 1 and 1611 cm − 1 , indicating the coexistence of both adsorbed and structural water in the microstructure and confirming the successful intercalation of water molecules [21].Upon introducing PANI to V 2 O 5 , a series of characteristic peaks appear between 1100 cm − 1 and 1600 cm − 1 [20,21].The peaks at 1760 cm − 1 and 1474 cm − 1 are attributed to the benzene ring of PANI.The peak at 1563 cm − 1 corresponds to the quinoid stretching of C --C bonds.The peaks at 1309 cm − 1 and 1246 cm − 1 represent C-N and C --N stretching vibrations, respectively.The peak at 1168 cm − 1 represents C-H in-plane bending.Furthermore, the shift of the V --O stretching peak, from 1016 cm − 1 in CV 2 O 5 to 1004 cm − 1 in CPVO and 988 cm − 1 in SPVO, suggests that the V 5+ cations have been partially reduced to V 4+ cations (see further) [25][26][27].Fig. 2c displays the Raman spectra of CV 2 O 5 , CPVO and SPVO.The characteristic vibrations of V 2 O 5 manifest as peaks below 1100 cm − 1 .The VO 5 -VO 5 bending vibration is represented by peaks at 139 cm − 1 and 197 cm − 1 .V --O bending produces peaks at 288 cm − 1 and 404 cm − 1 .The stretching of V-O-V bonds is evident at 528 cm − 1 , while the stretching of V-O bonds and V --O bonds is observed at 700 cm − 1 and 998 cm − 1 , respectively [20,21].Following synthesis, new peaks in the 1100 cm − 1 to 1700 cm − 1 range indicate PANI stretching modes [19,28,29].The peaks at 1350 cm − 1 and 1570 cm-1 are assigned to D (defect and disorder) and G (graphitic) bands of carbon material, respectively [30].In-plane C-H bending vibration is denoted by peaks at 1176 cm − 1 .Peaks at 1350 cm − 1 and 1397 cm − 1 signify the stretching modes of --NH + and -NH + , respectively.Various peaks in the series confirm the presence of the benzene ring, including aromatic ring vibration at 1500 cm − 1 , C --C quinocide unit stretching at 1565 cm − 1 , and C --C stretching in the benzenoid unit at 1570 cm − 1 .The peak positions and the corresponding bonds are listed in Tables S1 and S2.The presence of water and PANI molecules in the CPVO and SPVO composite was further validated through TGA.In Fig. 2d, noticeable distinctions are evident before and after the introduction of PANI.CV 2 O 5 exhibits only a 0.2 wt% weight loss between 100 • C and 200 • C, indicating the evaporation of a small amount of adsorbed water.In contrast, CPVO and SPVO experience approximately 0.6 wt% and 3.8 wt% weight losses in the same temperature range, attributed to water insertion.Both CPVO and SPVO exhibit weight losses of about 2.7 wt% and 6.1 wt% between 350 • C and 450 • C, signifying the degradation of PANI.During the TGA process, the V 2 O 5 phase in PVO is initially reduced to the V 2 O 3 phase during annealing [21].Fig. S2 depicts the N2 adsorption-desorption isotherms of the materials.All the curves indicate that the four samples exhibit type III isotherms, as per the IUPAC classification.This suggests that the c value associated with the adsorption energy of the first monolayer is less than 1 [31,32].This implies that CV 2 O 5 , CPVO and SPVO all possess multilayer structures without the formation of a monolayer structure.Notably, the volume adsorbed and desorbed in the synthesized materials, as indicated on the Y-axis, are considerably higher than those of the commercial counterparts − 1.2, 4.9 and 8 ml/g for CV 2 O 5 , CPVO and SPVO, respectively.These BET results clearly highlight that the surface area of SPVO (53.6 m 2 /g) is significantly greater than that of CV 2 O 5 (9.6 m 2 /g) and CPVO (24.2 m 2 /g), attributable to the nanofiber nanostructure.
XPS analysis was also processed to further confirm the transformation of V 5+ to V 4+ .Fig. 2e and f shows the XPS spectra of V2p and O1s for CV 2 O 5 , CPVO and SPVO.In Fig. 2e, the binding energy of CV 2 O 5 is at E B (V 3/2p ) = 517.4eV, which is a single peak in this area, standing for the single V 5+ cations existence.By comparison, there are two peaks in the V 3/2p area in the spectra of CPVO and SPVO, which are positioned at E B (V 3/2pA ) = 517.7 eV and E B (V 3/2pB ) = 516.3eV, referring to two oxidation states, V 5+ and V 4+ , respectively [33,34].The ratio between the V 4+ /V 5+ peak area of SPVO is 0.412, while that of CPVO is 0.199, confirming the higher concentration of V 4+ in SPVO.An additional oxygen component can be observed in Fig. 2f.The peaks at 530.5 eV represent the lattice oxygen, while the additive peaks at 531.8 eV in CPVO and SPVO belong to adsorbed oxygen [35,36].
The electrochemical performance of these materials was initially evaluated in coin cells.Fig. 3a presents the comparative cyclic voltammetry (CV) curves of CV 2 O 5 , CPVO and SPVO at scan rates of 0.5 mV/s, revealing two main peaks between 1.05 and 1.2 V and 0.75-0.9V, along with two minor peaks positioned at 0.7-0.8V and 0.35-0.5 V.These correspond to the multiple redox reactions in V 2 O 5 [37].The SPVO samples exhibit higher response to specific currents compared to the pristine CV 2 O 5 and CPVO counterparts, evident in the specific capacity difference shown in Fig. 3b and c, depicting the Galvanostatic charge-discharge (GCD) curve at specific currents of 200 mA/g and rate tests, respectively.Consistent with the CV curves, SPVO demonstrates a significantly higher specific capacity of 384 mAh/g compared to CPVO and CV 2 O 5 , which register only 312 mAh/g and 235 mAh/g at a specific current of 200 mA/g.Moreover, SPVO displays excellent rate ability, as depicted in Fig. 3c, with specific capacities of 430, 415, 383, 326, and 286 mAh/g at specific currents of 50, 100, 200, 500, and 1000 mA/g, respectively.Additionally, SPVO maintains a recovered capacity of 395 mAh/g when the current density is stepwise reduced back to 100 mA/g.In contrast, the rate ability of CPVO is only 366, 335, 311, 275, and 243 mAh/g at the corresponding specific currents.Similarly, the specific capacity of SPVO and CPVO surpasses that of CV 2 O 5 , and the rate test results are higher as well, confirming the enhanced specific capacity.The enhanced specific capacities observed in SPVO may be attributed to several factors, including a reduced energy barrier due to preintercalation of H 2 O and PANI, the presence of mixed valance states of V 5+ and V 4+ (confirmed from XPS results), as well as higher specific surface areas (as indicated by BET results) [38][39][40].
Furthermore, the long cycling results at 1000 mA/g have been segmented into the activation process (Fig. 3d) and the extended cycling process after complete activation (Fig. 3e).Notably, the activation process of SPVO is notably shorter than that of CPVO and CV 2 O 5around 40 cycles for SPVO but requiring around 100 cycles for CPVO and 245 cycles for CV 2 O 5 .The duration of the activation process is closely tied to the interlayer spacing and energy barrier of the interlayers in cathode materials.As the redox reaction progresses, the distance between the layers expands until reaching a balance, coinciding with the maximum specific capacity due to the intercalation mechanism.In this study, the introduction of H 2 O and PANI molecules reduces the energy barrier as well as increases the gap between the layers.This implies that the necessary activation process has been expedited to varying degrees based on the size of the molecules inserted into the layers.This explains why SPVO has the shortest activation process, while CPVO and CV 2 O 5 have longer ones.In Fig. 3e, the stability of the materials is evident.All materials experience capacity drops in the first 200 cycles, entering a plateau period thereafter.The activation propels CV 2 O 5 and CPVO to a higher specific capacity compared to SPVO, but the retention of the former two does not reach the same level as the latter one.CPVO and SPVO can maintain 71.9 % and 64.4 %, respectively, after 500 cycles, while CV 2 O 5 experience a short circuit within 500 cycles.CPVO also experiences a short circuit at around 800 cycles, while SPVO can still retain 55.9 % after 1000 cycles.
After establishing the superior capacities of CPVO and SPVO compared to CV 2 O 5 and recognizing their reliability as cathode materials in ZIBs, they are subsequently employed in planar ZIMBs.In Fig. 4a and b, the CV comparative curves at 0.5 mV/s and 1.0 mV/s for CPVO and SPVO planar ZIMBs are presented.Consistent with the electrochemical tests in coin cells, SPVO-based ZIMBs exhibit superior capacities compared to CPVO-based ZIMBs, characterized by higher specific currents and a larger area under CVs.Furthermore, in the GCD measurements (Fig. 4c and d), a unit, areal specific capacity (mAh/g.cm 2 ), is introduced for meaningful capacity comparisons of planar ZIMBs.At a current density of 50 mA/g (Fig. 4c), SPVO planar ZIMBs demonstrate an areal specific capacity of 472 mAh/g.cm 2 (equal to 409 μAh/cm 2 ), significantly surpassing CPVO, which records 433 mAh/g.cm 2 .Likewise, at 200 mA/g, the measured areal specific capacities are 437 mAh/g.cm 2  and 370 mAh/g.cm 2 , respectively, for CPVO and SPVO-based ZIMBs (Fig. 4d).Notably, these areal specific capacities values of SPVO-based ZIMBs are either considerably larger or comparable to those of highperformance reported ZIMBs, for instance, 178 μAh/cm 2 in Jiang's Zn//α-MnS planar zinc ion battery [41], 250 μAh/cm 2 in Li's in-plane Zn-polyaniline micro-battery [42], 159 μAh/cm 2 in Zhou's planar Zn@VG//MnO2@VG (vertical graphene) micro-battery [43], respectively.Additionally, rate tests of the ZIMBs exhibit superior charge storage performance of SPVO compared to CPVO, as depicted in Fig. 4e.
Furthermore, the SPVO planar ZIMB undergo long cycling test as illustrated in Fig. 5a, where the SPVO planar ZIMB maintains 78.7 % of the specific area capacity after 500 cycles.Remarkably, the retention of the SPVO planar ZIMB exceeds 60 % even after 1000 cycles (Fig. S3 shows coulombic efficiency plot).To understand the self-discharge behavior of our SPVO planar ZIMB, we conducted a study by charging the ZIMB to its maximum voltage of 1.6 V and subsequently measuring self-discharge over time, as depicted in Fig. 5b.Intriguingly, we observed only a 20.7 % voltage reduction, indicating 79.3 % retention, even after 200 h of self-discharging.This level of voltage retention surpasses that of previously reported planar energy storage microdevices.For example, a 50 % voltage reduction occurred within 3 h in a Fe-MnO 2 symmetric micro-supercapacitor [44].AC-based symmetric micro-supercapacitors have a 50 % voltage drop after 2.5 h [45].The voltage of a laser-writing GO film-based symmetric microsupercapacitor used 13 h to drop over 50 % [46], and Laser-writing rGO-based symmetric micro-supercapacitors experienced a 50 % voltage drop within less than 1 h [47], respectively.Additionally, our ZIMBs exhibited outstanding charge storage performance with areal energies (at areal powers) of 306.72 μWh/cm 2 (at 0.08 mW/cm 2 ) and 144.74 μWh/cm 2 (at 3.44 mW/cm 2 ).These values surpass the performance of most previously reported high-performance planar microbatteries, as shown in Fig. 5c.For instance, a Zn//MnO 2 planar battery shows a peak areal energy of 201.5 μWh/cm 2 and a peak areal power of 0.067 mW/cm 2 [43].A Zn//VO 2 micro-battery performs the peak data at 188.8 μWh/cm 2 and 0.61 mW/cm 2 [48].The Zn//VS 2 batteries can show quite a high areal power of 2.85 mW/cm 2 , but the areal energy can only reach 30.8 μWh/cm 2 [49].A few more devices are also shown in the Ragone plot [41,42,[50][51][52], proposing the high efficiency of our Zn//SPVO planar ZIMBs.As depicted in Fig. 5d, a single SPVO ZIMB is applied to the circuit, and the voltage of the planar ZIMB is sufficient to power a moisture sensor.Another stability test is done for the SPVO ZIMBs.As shown in Fig. S4, the SPVO ZIMBs are bent to 90 • and − 90 • 0, 10, and 20 times before CV tests [53][54][55].Compared to the battery without bending, the area of the CV curves is slightly decreased, but the influence of bending the batteries is limited, confirming the flexibility and stability of our ZIMBs.Moreover, Fig. S5 demonstrates the morphological alterations of the SPVO cathode following multiple cycling cycles.Because the SPVO is combined with carbon black and PVDF as the cathode material, distinct nanofiber structures are not observable.Nonetheless, the micro 3D structure within the electrodes remains distinguishable.Even after 150 cycles and 500 cycles of cycling tests, the microstructure largely retains its original form.However, due to repetitive cycling, some gaps between particles become noticeable.
In summary, a pre-intercalation strategy was employed to synthesize high-capacity V 2 O 5 , enabling the realization of high-performance planar ZIMBs.The PANI and H 2 O-intercalated V 2 O 5 nanowires obtained through this method exhibit remarkable capacities, lower activation energy, and a larger surface area in comparison to commercial V 2 O 5 particles and PANI and H 2 O-intercalated commercial V 2 O 5 particlebased electrode materials.Undoubtedly, the charge storage performance of the PANI and H 2 O pre-intercalated V 2 O 5 nanowires as cathode and Zn as anode in planar ZIMBs demonstrates an impressive areal capacity of 409 μAh/cm 2 , along with areal energy density 306.7 μWh/cm 2 , power density 3.44 mW/cm 2 , as well as a slow self-discharge profile.While the SPVO ZIMBs exhibit distinctive electrochemical performance compared to currently applied ZIMBs, there is still a considerable journey ahead to integrate these ZIMBs into real-life applications.Firstly, the packaging process requires standardization and a scale-up approach, including the adoption of suitable electrolytes such as semisolid or solid-state for packaging improvements.Additionally, there are opportunities to enhance specific capacity and stability to achieve effective energy storage performance within the limited device footprint.

Fig. 1 .
Fig. 1.(a) Schematic depiction of the synthesis process for high-capacity cathode materials based on layered V 2 O 5 .(b) Schematic representation of the processes involved in the fabrication of planar ZIMBs.(c) A digital image showcasing a ZIMB.(d) Schematic illustration delineating the diffusion of Zn 2+ ions during charging and discharging processes.(e) SEM images of various samples, including (i) commercial V 2 O 5 (referred to as CV 2 O 5 ), (ii) PANI intercalated commercial V 2 O 5 (CPVO), and (iii) PANI intercalated synthesized V 2 O 5 (SPVO).(f) TEM images of the samples -(i) CV 2 O 5 , (ii) CPVO, and (iii) SPVO.

Fig. 3 .
Fig. 3. (a) CV curves for CV 2 O 5 , CPVO and SPVO coin cells at scan rates of 0.5 mV/s.(b) Comparable GCD curves for CV 2 O 5 , CPVO and SPVO coin cells at a specific current of 200 mA/g are presented.(c) Showcases comparable rate tests for CV 2 O 5 , CPVO and SPVO coin cells at specific currents of 50, 100, 200, 500, and 1000 mA/ g.The activation process of CV 2 O 5 , CPVO and SPVO coin cells is depicted in (d), while (e) illustrates the long-term charge and discharge tests conducted at 1000 mA/g.

Fig. 5 .
Fig. 5. (a) Long-term cycling outcome for SPVO planar ZIMB tested at 1000 mA/g.(b) Self-discharge tests specifically conducted on the SPVO ZIMB.(c) An illustrated Ragone plot comparing the areal energy density and power densities of our SPVO planar ZIMB with reported planar micro-batteries utilizing various cathode and anode materials [41-43,48-52].(d) A digital image demonstrating the capability of our ZIMB in powering a moisture sensor (Indoor and Outdoor Thermometer with Hygrometer clock).