Review on Synthesis, Characterizations, and Electrochemical Properties of Cathode Materials for Lithium Ion Batteries

The development of cutting-edge cathode materials is a challenging research topic aiming to improve the energy and power densities of lithium ion batteries (LIB) to cover the increasing demands for energy storage devices. Therefore, highly needed further improvements in the performance characteristics of Li-ion batteries are largely dependent on our ability to develop novel materials with greatly improved Li ion storage capacities. Three different types of cathode materials including intercalation, alloying and conversion materials are reviewed in this paper in order to orientate our researches towards highly performant LIBs batteries. This includes characteristics of different cathode materials and approaches for improving their performances. *Corresponding author: Bensalah N, Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, Doha, Qatar, Tel: 97444036540; E-mail: nasr.bensalah@qu.edu.qa Received April 06, 2016; Accepted May 26, 2016; Published June 05, 2016 Citation: Bensalah N, Dawood H (2016) Review on Synthesis, Characterizations, and Electrochemical Properties of Cathode Materials for Lithium Ion Batteries. J Material Sci Eng 5: 258. doi:10.4172/2169-0022.1000258 Copyright: © 2016 Bensalah N, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Since their commercialization by SONY in 1991 [1], lithium ion batteries (LIBs) have made significant progress in terms of safety, electrochemical properties such as capacity, power, and cycling stability and it has the highest energy density comparing to other secondary batteries such as nickel-cadmium and nickel-metal hydride. A great part of such progress can be attributed to the introduction of new materials with higher capacity, higher potential, and enhanced thermal stability. Along this journey, numerous studies have been conducted in order to find cathode materials for LIBs with higher capacity to allow for practical applications in plug-in hybrid electric vehicles, large-scale power generation systems, and critical space and aeronautical applications.
In this paper, a comprehensive literature review of cathode materials used or proposed for LIBs is attempted. Since the focus is in this paper on capacity, three areas will be highlighted through the review: synthesis methods, characterization techniques, and electrochemical properties related to capacity, namely initial discharge capacity and cyclability. Out of several possible categorization schemes of cathode materials for LIBs, a structure-based scheme (layered, spinel, olivine, etc.) seems to be dominant as used by most reviewers such as Whittingham [2] and Xu et al. [3]. Also other kinds of cathodes have been studied (air, sulfur, organics and conversion like transition metal FeF 3 ).
Carbon-coated LiCoO 2 thin film with PVDF-HFP gel electrolyte Park et al. [11] Sol-gel process and Screen printing XRD; SEM 110 -  2 Lu et al. [12] In situ sol-gel process followed by calcination at 1123    the safe limits, costly to manufacture and have aging issues, especially in hot places.
Lithium-ion rechargeable batteries have great achievement because of their characteristics as high energy density, long-term stability and its effectiveness as a solution for huge applications. This journal is completed to motivate reviewers who are interested in LIB, types of cathodes specifically and looking for environment-friendly, inexpensive and charge/discharge long-term cycles materials. Also, open the challenges front of researchers to discover new materials with better properties, characteristics, and features for LIB.

Electrochemistry
The basic working mechanism based on which LIBs functions is associated with the transfer of lithium ions from the positive electrode (cathode) to the negative one (anode) and vice versa. During discharging process, lithium ions travel through an electrolyte, often an organic solution of lithium salt such as LiPF 6 , from the cathode side to the anode side. The exact opposite occurs during charging as an external current is applied. Although all LIBs work according to this basic mechanism, there are two different processes by which lithium ions associate with cathodes or anodes: intercalation and reversible chemical reaction or alloying. Since the focus here is on the cathode side, a straightforward explanation of both processes follows.

Cathode materials
Despite the fact that layered LiCoO 2 has been the dominant cathode materials in commercialized LIBs, many important alternatives have attracted many researchers for potential use. Such substitutes, some of which have been already introduced to the market, include layered LiNiO 2 and LiMnO 2 along with their derivatives such as LiNi x Co y O 2 , LiCo x Ni y Mn 1-x-y O 2 , spinel-structured LiMn 2 O 4 along with its derivatives such as LiNi x Mn 2-x O 4 and LiCr x Mn 2-x O 4 , and olivine-structured LiFePO 4 . However, for any of these substitutes to be widely adapted, some challenges have to be overcome ( Figure 3).

Layered Lithium Transition Metal Oxides
The layered lithium transition metal oxide with the formula LiMO 2 , where M=Co, Mn, Ni or a combination of two or more, have been arguably the most successful category of cathode materials for LIBs. Their superior electrochemical behavior can be ascribed to their layered structure which allows for a large number of diffusion paths for lithium ions (Figure 4).

LiCoO 2
As stated earlier, LiCoO 2 is the earliest and the most commonlyused cathode material for commercial LIBs. Suggested first by Mizushima et al. [6] in 1980, this material has several desirable features including high discharge potential, low molecular weight, high energy capacity, good charge/discharge performance, relative ease of synthesis and treatment, and stable and high discharge voltage [7]. However, extensive research has been conducted during the last two decades to find cathode materials with larger capacity and higher potential than LiCoO 2 . This was further motivated by the high cost, chemical hazards, and the environmental impact associated with cobalt. The preparation of LiCoO 2 was done by means of solid state reaction [8][9][10], sol-gel technique [11,12], ultrasonic spray pyrolysis process [13], combustion synthesis [14,15], co-precipitation method [16], molten salt synthesis   [17], freeze-drying method [18], complex formation method [19], hydrothermal synthesis [20], mechanochemical, and microwave synthesis [21,22], and other methods. Depending on the synthesis method, LiCoO 2 could have either hexagonal layered for hightemperature LiCoO 2 or cubic spinel-like structure for low-temperature Despite its high theoretical capacity of 274 mAhg -1 , reported practical discharge capacities of LiCoO 2 are relatively low, in the range of 135-150 mAhg -1 [14,18,24], only 50-55% of its theoretical capacity. In order to enhance the ionic conductivity and cycling performance of the cathode, some approaches such as carbon coating [11,25] [35,37]. Having achieved a mixed record of success, some coatings remarkably increased the initial discharge capacity of LiCoO 2 as high as 190 mAhg -1 while others significantly enhanced the cycling behavior. For instance, Lu et al. [11] reported an initial discharge capacity of 195 mAhg -1 for Al 2 O 3 -co ated LiCoO 2 (1.0 wt% Al 2 O 3 ) prepared by in situ sol-gel method with a good capacity retention upon cycling (85% after 30 cycles) [12]. In another study by Li et al. [

LiMnO 2
Although LiMnO 2 has been proposed as a cathode material in LIBs almost as early as LiCoO 2 , its use has not spread mainly due to performance limitations such as low capacity, difficulty of mass production, and power charge/discharge performance, especially at high temperatures. However, years of extensive research has led to significant improvement of its performance. Compared to LiCoO 2 , LiMnO 2 has major advantages such as high safety and low cost which make it a promising substitute in the future.

LiNiO 2
One of the early cathode materials to be explored was lithium nickel oxide (LiNiO 2 ) which has a comparable layered structure and charge-discharge characteristics to those of LiCoO 2 . Although nickelbased cathodes are currently feasible for commercial use, their major drawback is poor solubility in organic electrolyte solutions, particularly at high temperature. Also, synthesis and treatment of LiNiO 2 often require harsh temperature conditions which further limit its current use in commercial LIBs despite its superior capacity [40]. Nickel has higher energy density than cobalt does; 50% of lithium ions can be transferred between anode and cathode for cobalt at the maximum voltage of a typical battery (4.7 V), while 70% of lithium ions can be mobilized for nickel at only 4.2 V.

Derivative compounds
The electrochemical behaviors of various layered derivative compounds haven been extensively studied by numerous research groups. For more than two decades, researchers have been working on developing derivatives of nickel, cobalt, and/or manganese oxides in order to enhance the stability and improve the electrochemical behavior of layered cathode materials. Some of these derivatives are as LiNi x Co 1-x O 2 , LiCo x M 1-x O 2 , LiNi x M x O 2 , and LiNi x M y Co 1-x-y O 2 (where M=Al or Mn). Substantial improvement on the cycling performance has been reported by optimizing the composition of these derivative materials. Also, since manganese is less expensive and safer to use than cobalt or nickel, these derivatives could provide low-cost alternatives.
Surface modification by either coating with metal oxides or doping with metal cations proved to be an effective method for improving the chemical stability of layered derivative compounds.

Spinel Lithium Transition Metal Oxides (LiMn 2 O 4 ) LiMn 2 O 4
The spinel lithium manganese oxide LiMn 2 O 4 has been one of the most prospective cathode materials as a non-toxic, environmentallyfriendly, the high natural abundance of Mn and low-cost candidate [42]. This material has a theoretical capacity of 148 mAh/g for an equivalent weight (M) is 180 [73]. The latter approach demonstrated a significant improvement on the cycling behavior of LiMn 2 O 4 approaching 100% after 100 cycles [73].

Derivative Compounds
Much research work has been conducted on the electrochemical properties of derivatives of LiMn 2 O 4 in order to enhance the specific capacity and power and optimize the operational range of temperature for the original spinel compound. Two derivatives which have attracted many researchers since the early 90's, LiNi x Mn 2-x O 4 [74], and LiCr x Mn 2- 75,76], have demonstrated a remarkable improvement on the cyclability of spinel magnesium oxides.

Olivine Lithium Transition Metal Phosphates and Silicates (LiMPO 4 and LiMSiO 4 ) LiMPO 4
The electrochemical behavior of olivine-structured transition poly anion compounds of the structural formulas of LiMPO 4 and LiMSiO 4 (M=Co, Fe, Mn, Ni or V) have been attracted a great deal of interest as potential cathode materials. A comparative study of lithium intercalation potential in olivine-structured transition metal compounds has been reported by Zhou et al. [77].
Olivine-structured compounds have several advantages over other cathode materials including its structure of material hardly changes while Li ion intercalation and deintercalation; (2). It holds a long voltage platform.
Among phosphate compounds, LiFePO 4 has received the greatest amount of attention due to a number of desirable features such as low cost, non-toxicity, and good thermal and chemical stability [78]. For example, the main drawback of lithium iron phosphates, LiFePO 4 , is their low electrochemical performance at room temperature due to low lithium ion diffusion and poor electronic conductivity [78]. In order to overcome this drawback, several material processing techniques, including solid solution doping in metals and nanocoatings of phosphate particles with carbon, have been proposed. LiFePO 4 has been prepared through different synthetic methods as reviewed by Zhang et al. [79] such as solid-state reaction, sol-gel synthesis, hydrothermal synthesis, carbothermal reduction, microwave synthesis, and spray pyrolysis. A comparative study of the synthetic routes of LiFePO 4 and their effect of the electrochemical properties was conducted by Franger et al. [80].
Another widely-investigated phosphate compound that has recently attained strong interest is LiCoPO 4 . LiCoPO 4 has been prepared by means of solid-state reaction, sol-gel synthesis, coprecipitation, hydrothermal synthesis, optical floating zone method, radio frequency magnetron sputtering, spray pyrolysis, and microwave synthesis. Zhao et al. [81] reported an initial capacity of 156.7 mAhg −1 and a capacity loss of 3.5% after 50 cycles for LiFePO 4 prepared by solid state reaction at 650°C.
Yang et al. studied mesoporous FePO 4 as a potential cathode material reporting an initial discharge capacity of 160 mAhg -1 with a capacity loss of 10% after 20 cycles. A significant amount of research has suggested that the migration of lithium ions, and consequently charge/discharge performance LiFePO 4 , can be greatly enhanced by carbon coating [81,[82][83][84][85][86].

Vanadium oxides
The vanadium-based oxides have attracted strong interest from researchers due to their good electronic conductivity, excellent chemical stability in polymeric electrolytes, and high energy density [104]. Since vanadium can exist in a number of oxidation states from 2 + in VO to 5 + in V 2 O 5 , vanadium oxides could offer a wide range of capacities as cathode materials [105]. Vanadium oxides which have been studied as potential cathode materials in LIBs are VO 2 [ [149,150], and Li 6 V 10 O 28 [151]. Chernova et al. [152] reviewed in detail the structural and electrochemical features of different vanadium oxides as well as the process of lithium insertion in them [153][154][155][156]. Among these oxides, V 2 O 5 , LiV 3 O 8 and Li 3 V 2 (PO 4 ) 3 have shown the most promising electrochemical behavior as indicated by their high discharge capacities and good capacity retention. Li 3 V 2 (PO 4 ) 3 has been previously reviewed along with other phosphate compounds.

V 2 O 5
As layered compounds with a high theoretical capacity of 442 mAhg −1 , V 2 O 5 compounds are among the most promising high-capacity cathode materials under development. Several synthetic methods have been employed to prepare V 2 O 5 including sol-gel method, solvothermal route, precipitation process, and electrodeposition. A novel synthetic approach by Pomerantseva et al. enabled for nanostructured V 2 O 5 thin films by biotemplated synthesis using Tobacco mosaic virus particles [114].
Studies on V 2 O 5 have shown high discharge capacity mostly in the range of 250-300 mAhg −1 for V 2 O 5 /polypyrrole composites which represent 57-67% of its theoretical capacity with good capacity retention (15-20% after 50 cycles) [104]. When the structure and composition of lithiated V 2 O 5 nanocomposites are optimized, their electrochemical behavior can be significantly enhanced. For example, in a study by Semenenko et al. [153] thin Li x V 2 O 5 (x~0.8) nanorods with thickness of 5-10 μm were synthesized using hydrothermal treatment of V 2 O 5 gel and lithium ions [157]. An initial discharge capacity of 490 mAhg -1 was reported with capacity loss of about 18% after 50 cycles [158].
Despite these advantages of V 2 O 5 compounds, two drawbacks still persist: low power density due to their intrinsic low ionic conductivity, and poor cyclability as a result of microstructural failure upon cyclic lithium ion intercalation-deintercalation [104]. Since thermal stability in polymeric electrolytes is one of the main advantages for vanadium oxide, a variety of conductive polymeric materials have been suggested to be used as hybrid hosts of V 2 O 5 such as polypyrrole [104,112,114], poly(ethylene glycol) [116], polythiophene [109], polyphosphazene [159], and polyaniline [115]. Such polymeric hybrid materials have been found to increase the electronic conductivity of the original oxides and improve the cycling behavior by enhancing the microstructural stability. Among these polymeric materials, polypyrrole has been the most extensively studied due to its high electric conductivity once doped with oxidizing agents and good electrochemical activity with a theoretical capacity of 72 mAhg -1 [160].

LiV 3 O 8
Another lithium vanadium oxide that has been widely investigated over the last two decades for its good electrochemical properties is LiV 3 O 8 . Such good electrochemical properties include high discharge capacity, high specific energy density, and long cycle life [120].
A wide range of synthetic routes have been used to prepare LiV 3 O 8 in different forms including spherical particles [120], polymeric composites [123,125], and nanostructured materials of various morphologies such as nanorods [128], thin films [127,161] nanocrystals [162], and porous nanoparticles [126]. The synthetic routes studied in literature include solid-state reaction [124], spray pyrolysis method [163], sol-gel process [126], hydrothermal synthesis [105,122], radio frequency magnetron sputtering [156,127], hydrothermal treatment [128], rheological phase reaction method [164] and microwave synthesis [165]. In order to enhance the electrochemical properties of the material synthesized by increasing surface porosity and lowering crystallinity, several techniques have been suggested such as ultrasonic treatment [166] and partial crystalline modification by introducing small amounts of H 2 O, CO 2 and NH 3 [167]. Such techniques, although have improved the electrochemical behavior of LiV 3 O 8 to some degree, need further optimization in order to produce satisfactory power and cycling efficiency and to be applicable in large-scale production systems [159].
It was found by several researchers that the synthetic routes affect greatly the capacity of LiV 3 O 8 cathodes [159]. Even though the initial discharge capacity could even reach more than 100% of its theoretical capacity (280 mAhg -1 ) [157] when certain nanostructuring techniques or polymeric alloying are applied. Idris et al. [129] reported an initial discharge capacity of 227 mAhg -1 with low capacity loss (~ 15% after 100 cycles) for LiV 3 O 8 /carbon nanocomposites prepared by hydrothermal synthesis followed by a carbon-coating process. Other researchers such as Feng et al. [123] and Tian et al. [125] obtained higher initial discharge capacity (~ 300 mAhg -1 ) with good cycling efficiency (8-14% capacity loss after 30-40 cycles) for polypyrrole-LiV 3 O 8 composites.

Coatings of Cathode Materials
A comprehensive review of surface coatings of cathodes in LIBs was conducted by Li et al. [163]. Such coatings were found to enhance the structural stability of the cathode material by limiting the contact with electrolyte solution, suppress phase transition, and stabilize the cations in their crystal sites. Nanocoatings of LiMn 2 O 4 include SiO 2 [168][169][170][171][172].

Nanostructured Composites
Nanostructured carbon-oxide composites Wang and Dai [165] developed an approach towards functionalized porous carbon-oxide composite materials by using ionic liquid (ILs) as solvents in nonhydrolytic sol-gel processing and this to limited and oxide-catalyzed carbonization of ILs trapped within an oxide framework. BET, TEM, XRD and XPS characterization measurements were applied. Zhang et al. prepared LiFePO 4 /C composite fibers by a combination of electrospinning and sol-gel techniques using Polyacrylonitrile (PAN) as an electrospinning media. XRD and SEM measurements were carried out to characterize the structure of the fibers formed [173].

Lithium/Air
Recently, a growing attention has been directed to lithium/air batteries which utilize mesoporous carbon as cathode materials. These devices represent a special category of LIBs as they enjoy very high energy density compared to other conventional types of lithium-ion batteries.

Organosulfur Materials
Nowadays, organosulfur compounds usually with the organothiol (-SH)/disulfide (S-S) redox couple, it's characterized by their ability to store big amounts of charges per unit mass to use as cathode materials for rechargeable lithium batteries and featuring of being safety, cheap, synthetic it easily and chemically stable. The theoretical energy of organosulfur compounds goes beyond that of as intercalation compounds, conducting polymers and conventional battery materials [166,167]. Organosulfur compounds containing disulfide bonds showed a high discharge capacity equal to 500 mAh/g [168].

Organic cathodes
Most of the recent lithium batteries made of inorganic compounds as a cathode are produced from nonrenewable resources and because of that they are highly cost [174][175][176][177]. Scientists searched for another candidate to improve the power and energy density, safety of Liion cells and greener Li-ion batteries. Organic electrodes have been proposed as one of the best electrodes for Li-ion batteries due to its inherently flexible, non-toxic, cheap, abundant nature and also their limitation of cycle life, thermal stability, low energy density values and rate capability led to a huge improvement of it [170]. Nakahara et al. proposed a high-energy organic cathode material; poly(2,2,6,6tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) for use in lithium rechargeable batteries, it is obtained good power capability, cycling efficiency (retaining more than half of original capacity after 1000 cycles), fast charging and discharging (less than 1.5 min) and can transfer specific capacity over 100 mAh.g −1 [171].

Sulfur compounds Li 2 S
Except the air cathode, the sulfur element has the cheapest cathode material for lithium batteries and top theoretical capacity density of 1672 mA/g between all known cathode materials [172]. Li 2 S cathode material shows a great potential of high-performance rechargeable lithium batteries comparing with other resources of elemental sulfur in nature, like micro-batteries for power sources for electric vehicles and small-size electronic devices emphasizing high charge density [169]. However, because sulfur element has the dissolution of its reaction product polysulfides into the electrolytes and highly insulating nature, it cannot be used directly at low temperature as an electrode material for lithium batteries, which caused various problems, such as rapid fall of the capacity and short utilization of active material [178]. Yang et al. [174] found that Li 2 S can be an active material and reached a large potential barrier (∼ 1 V) at the beginning of charging by applying a higher voltage cutoff. It's obtained a greater initial discharge capacity (~ 800 mAh/g) which becomes stabilized after (10 cycles with around 500-550 mAh/g, ~ 0.25% per cycle capacity decay rate).

Conversion cathodes
To improve cathode materials, electrochemical conversion reactions have been used as another way to accomplishing the utilization of all the oxidation phases of a transition metal [179][180][181][182]. Fluorides metal are one of greatest transition metals that commonly studied in the research because of its stability and its ion considered as a strong and suitable ionic character of the (M-F bond) to transfer charges between two electrodes and produce high operating voltages and reversible capacity [176]. Iron fluoride (FeF 3 ) is one of the derivative of transition metal fluorides that safety and inexpensive, it is characterized by high theoretical capacity (712 mAh/g) [177,175]. In 1990, Arai et al. were the first researcher who reported the electrochemical activity of trifluorides with its high theoretical voltage and a specific capacity equal to 80 mAh/g [178].
Among various studies of fluorinated, fluorinated solvents are the most studied since it's used for increasing the safety and stability in LIB [183][184][185][186]. Different fluorine-doped intercalation cathodes are produced from fluorine which has been used as a dopant like layered transition metal oxyfluorides (Li 1+x Ni 1−x O 2−y F y ), spinel lithium manganese oxyfluorides (Li 1+x Mn 2−x O 4−y F y ) and orthorhombic lithium manganese oxyfluorides (Li 1.07 Mn 0.93 O 1.92 F 0.08 ) [187]. Layered transition metal oxyfluoride: As have been mentioned before about layered lithium transition metal oxides owing to their electrochemial performance for secondary Li-ion batteries as high potential electrode materials. However, fluoride cathodes have the highest average voltage between conversion reactions [179].  features on charge/discharge cycling compared to LiNiO 2 and showed enhanced in the capacity retention during cycling [188][189][190].
Spinel lithium manganese oxyfluorides: Spinel lithium manganese oxide LiMn 2 O 4 has been studied widely as cathode materials due to its features of being non-toxic, inexpensive and environmentally-friendly material. Spinel LiMn 2 O 4 can deliver 120 mAh/g capacity. Choi and Manthiram [182] reported the synthesized of spinel Li 4 Mn 5 O 12-n F n oxyfluoride cathodes at 500 and 600°C by substituted F-ions from employing LiF for oxygen (O 2− ) ions in the spinel Li 4 Mn 5 O 12 . XRD, electrochemical cycling, and chemical analysis measurements were carried out to characterize Li 4 Mn 5 O 12-n F n and it has been found the useful of oxyfluoride cathodes to increase and enhance the capacity [181]. However, many different types of research discussed different ways to improve the Spinel lithium manganese oxyfluorides [182][183][184].
Metal fluorides: Conversion reactions start to attract attention recently, like fluorides, nitrides, sulfides and phosphides. Fluoride showed a great reversible cathode electrode that used to react with Li at 2.5 V and produced 800 mWh/g of energy density [191][192][193]. Poizot et al. [179] studied the mechanism of Li reactivity depend on involves the formation and decomposition of Li 2 O, escorting the oxidation and reduction of metal nanoparticles. However, in the past and due to metal fluorides characteristic such as insulating nature and apparent irreversibility in structural conversion [175], it has been ignored as rechargeable cathode materials for lithium batteries. Malini et al. [186] reported two ways to exploit the electrochemical efficiency of metal fluorides: first one, by mixing metal fluorides with conducting carbon materials to improve electrical conductivity and the second one, mechanical ball milling of metal fluoride to reduce the particle size [194][195][196][197]. VF 3 and TiF 3 are also transition metal fluoride, they demonstrated their effectiveness with Li and generated as high as 500-600 mAh/g [175]. Amatucci et al. [187] studied the basic reactions of alkali nitrides with metal fluorides by prepared pre-lithiation agent, Li 3 N, to (FeF 3 , FeF 2 , and BiF 3 ) metal fluorides. XRD, DMC, and TEM measurements were carried out to characterize the structure of the nanocomposite product and showed 243mAh/g initial charge capacity.
Carbon fluorides: Carbon fluorides consider as a great theoretically materials for high energy batteries because of its high theoretical potential, low equivalent weight and also most of them produce a very low self-discharge and extraordinary stability which pays high attention for carbon fluorides [177].
Two main groups are classified carbon fluorides: high temperature (HT) up to 300°C and room or low temperature (LT). HT fabricated graphite fluorides compounds consisting of two stages, (CF) n and (C 2 F) n respectively while LT fabricated fluorine-graphite intercalation compounds of CF x [198,199]. Various studies have been studied the lithium batteries contained graphite fluoride as cathode material [200][201][202][203][204][205]. Therefore, the specific energy densities of covalent graphite fluorides of the (C-F bond) reached 900 W h kg −1 [191]. Many researchers have been discussed graphite fluorides in their both categories: HT [193,194], and at LT [195][196][197]. Electrochemical performance of LT fluorinated graphite's studied by Delabarre et al [198] where the compounds prepared at room temperature under fluorine gas, XRD, FT-IR, NMR and EPR measurements were carried out to characterize the electrochemical properties.
Theoretical capacity (156 mAh/g) with potential equal to 4 V was reported by Saidi and Barker [150]. Gover et al. [210] prepared LiVPO 4 as a cathode electrode in LIB using carbothermal reduction method, DSC, and electrochemical measurements were performed. The discharge capacity is about 140 mAhg -1 for the positive electrode with average discharge voltage around 4V [200]. Other research done by Zhang et al. [208] depended on prepared LiVPO 4 F/C nanosheets with homogeneous carbon coating by applying a hydrothermal approach and calcinations respectively. X-ray diffraction, SEM, TEM measurements and electrochemical tests have been performed and showed initial discharge capacity (143 mAh/g) with potential between (3.0-4.5 V) [216]. NaVPO 4 F: Sodium vanadium fluorophosphate characterized as a safer, economical and higher work potential comparing to other materials [209,210]. This includes Na 3 V 2 (PO 4 ) 2 F 3 with theoretical capacity equal to 128 mAh/g [170,211,212], NaVPO 4 F [209] and Na 1.5 VOPO 4 F 0.5 [213]. Generally, NaVPO 4 F materials synthesis required VPO 4 as the reaction intermediate phase, also it's successfully synthesized by three strategies: first, solid-state which require high temperature, long time-consuming process and complex operation procedure [209,212], second ion exchange or third, hydrothermal approach that considered as a complicated system to collect the results of compounds after the procedures finished [209]. Na 3 V 2 (PO 4 ) 2 F 3 classified as the most important among fluorophosphate materials due to the high theoretical capacity 192 mAh/g and its flexibility to be used as a cathode in both Li-ion batteries and Na-ion batteries [214]. While Na 1.5 VOPO 4 F 0.5 delivered through 3.6 and 4 V vs. Na + /Na theoretical capacity equal to 156 mAh/g [217][218][219][220][221].

LiFePO 4 F:
Another option about olivine-type, tavorite-structured lithium-metal-fluorophosphate as cathode material achieved a good alternative for LIBs [216]. In lithium iron fluorophosphate, one Li + can be cycled charge/discharge with the theoretical capacity of 153 mAh/g [222][223][224][225]. LiFePO 4 F was conducted by Ramesh et al. [218] as a new adequate material of fluorophosphate because of Li + feature that migrates easily without any barriers. LiFePO 4 F prepared by solid-state routes and produced a reversible capacity approximately 145 mAh.g -1 , with stable electrochemical cycling (40 cycles at room temperature and 55°C) [218]. In another study by Wang et [236][237][238][239][240]. Then reported by the Sauvage [213], who prepared in the tavorite type structure LiFePO 4 OH. Investing the large electronegativity of sulfur and fluorine of LiMSO 4 F in produce high-voltage cathode materials such as LiFeSO 4 F transfer around 140 mAh/g with a high redox potential of 3.6 V [241][242][243][244]. Different Li metal fluorosulfate compounds (M=Mg, Mn, Fe, Co, Ni, Zn, Cu) were studied and shown a great electrochemistry and crystal chemistry counting on the type of metal ions and on the synthesis methods [227].

Conclusion
Rechargeable batteries are considered of crucial role nowadays and become mandatory for most important electronics devices that most of the people use in communication, transportation, and monitoring. LIBs need an improvement in their characteristics for future applications where high energy and power density with long-term stability are required [307][308][309][310]. This paper summarized the characteristics of different types of cathode materials for LIBs and compare between their electrochemical performance such as specific capacity, thermal stability, synthesis method, and characterization techniques. The best cathode materials for LIBs should have high capacity, inexpensive, environment-friendly and charge/discharge long-term cycles for large and practical applications. However, as much as material exist, as well as challenges present, challenges of cathode materials include nanostructuring, switching from insertion to alloying and conversion materials, and improving cyclability and life types. Therefore, better understanding the mechanisms involved in charge-discharge of different cathode materials will certainly help scientists to overcome volume changes and hysteresis phenomena encountered with alloying and conversion materials. LIBs are expected to reach more commercial production in the future with better improvements in energy density and capacity.