LiMnPO<SUB>4</SUB>: Review on Synthesis and Electrochemical Properties

The olivine structured mixed lithium-transition metal phosphates LiMPO <SUB>4</SUB> (M = Fe, Mn, Co) have attracted tremendous attention of many research teams worldwide as a promising cathode material for lithium batteries and for studying their magnetic and electrical properties. High energy density is required for mind- to large-scale batteries because the mounting spaces are quite small for vehicles and other energy storage applications. This constraint necessitates synthesis to yield high volumetric energy density and reliable battery performance. Development of eco-friendly, low cost and high energy density, LiMnPO <SUB>4</SUB> cathode material became attractive due to its high operating voltage, which can be located inside of the electrochemical stability window of conventional electrolyte solutions and it offers more safety features due to the presence of a strong P-O covalent bond. This review is an overview of current research activities on LiMnPO <SUB>4</SUB> cathodes and its carbon coating synthesized by various synthetic techniques.

• Synthesis
• Properties
• LiMnPO<SUB>4</SUB>

1. Introduction

Since the pioneering work of Goodenough et al. [1] , the Phospho-olivine LiMPO₄ compound (with M equal Fe, Mn, Co or Ni) have been recognized as a potential positive electrode material for used in lithium rechargeable batteries. This family of materials has numerous advantages over the layered rock salt oxides (e.g. LiCoO₂ and LiNiO₂) that are currently used in commercial batteries. Besides its high discharge capacity (≈170 mA∙h∙g⁻¹), LiMPO₄ cathode material shows high stability during lithium extraction/insertion and does not deteriorate when used at moderately high temperatures. Recently, a lot of effort has been devoted to optimize LiFePO₄ material. In particular, particle minimization with intimate carbon contact resulted in almost the theoretical capacity at moderate current densities with LiFePO₄ cathode material [2] . Surprisingly, only a few papers indicate the applicability of LiMnPO₄ [3] - [5] and, the results remain controversial.

The origin of the different electrochemical activities of isostructural LiFePO₄ and LiMnPO₄ is not quite clear at the moment. Yamada et al. [3] suggested that the low electrochemical activity of LiMnPO₄ is due to slow kinetics and internal friction during lithium extraction/insertion. Akin et al. has recently performed a comparative structural study [6] of these two materials and found minute differences in the oxygen vibrations in the Li layer. Yet another important factor is the magnetism in these compounds [7] - [9] . The magnetic properties are determined by the magnetic moment and may thus reflect the potential differences within the LiMPO₄ family. Magnetism is therefore indirectly important for electrochemical properties as well.

Figure 1 presents the schematics of the LiMnPO₄ olivine structure. LiMnPO₄ has an olivine structure, where Mn and Li occupy octahedral 4c and 4a sites, and P atom is in 4c site, respectively. The O atoms are in a hexagonal close-packed arrangement. The MnO₆ octahedral is separated by PO₄ plain that leads to the significant reduction of the electrical conductivity of the material. It results in a poor rate capability and a slow utilization of Li ions in the olivine host structure. Therefore, similarly to LiFePO₄, the successful preparation of high performance LiMnPO₄ could be achieved through the preparation of the conductive composites of fine particles of this material. However, the current durability of LiMn_xPO₄ is orders-of-magnitude inferior to that of Li_xFePO₄ [10] , which make the preparation of the high conductivity LiMnPO₄/C composites, more difficult than in the case of the lithium-iron phosphate.

In the olivine structures the magnetic ions occupy only the so-called M₂ site, i.e. the M₂₊ ion sits in the center of MO₆ unit. Each MO₆ distorted octahedron is connected to four other MO₆ via vertices forming a layered network perpendicular to the crystal (100) direction. Successive M₂₊ layers are separated by PO₄ tetrahedral and LiO₆ octahedral.

Only few routes synthesis for LiMnPO₄ has been proposed. Powder properties are strongly affected by the preparation technique. In this review, special attention is focused on obtaining LiMnPO₄.

2. Synthesis Method and Properties of LiMnPO 4

2.1. Solid-State Reaction

Some research groups have tried to use solid-state reactions to synthesize LiMnPO₄. The solid state reaction is a conventional synthesis method, which usually needs a two-step heating treatment, including the first firing in a temperature range of 300˚C - 400˚C and subsequent one between 600˚C and 800˚C. The solid state reaction method has been a developed technology and being used much frequently since it’s simple to synthesis and easy to make mass production. The need for high temperature unfortunately upgrades cost; what is more, the product size always cannot be small. So other measures are added to circumvent those problems to process olivine

Figure 1

Structure of the LiMnPO 4 olivine with Pnma space group [ 10 ]

phosphates, such as slighter temperatures or adding carbon. Table 1 shows the precursors used in the solid-state reaction of the resultant LiMnPO₄ and composites of this phase.

The effect of Fe₊₂ substitutions was successfully employed by Martha et al. [11] to form a solid solution between olivine phases of Fe and Mn. A solid state method was employed for the preparation of such a LiMn_0.8Fe_0.2PO₄ phase along with 10 wt% carbon by ball milling before the heat treatment at 550˚C for 3 h in Ar atm. The olivine phase LiMn_0.8Fe_0.2PO₄comprised 25 - 60 nm size particulates with ≈5 nm thick carbon coating on the surface. Yang et al. [12] reported the synthesis of Co doped LiMnPO₄ (LiMn_0.95Co_0.5PO₄) by an oleic acid assisted approach. The source materials were initially ball milled before the heat treatment in N₂ atmosphere at various calcination temperatures.

The sample prepared at 600˚C performed better when compared to other temperature conditions and it showed discharge capacities of 103 and 144 mA∙h∙g⁻¹ for LiMnPO₄ and LiMn_0.95Co_0.5PO₄ respectively at a 0.05 C rate between 2.5 and 4.5 V is using the CC-CV protocol. LiMnPO₄ with off-stoichiometry (LiMn_0.9P_0.95O_4-5) was reported by Kan and Ceder [13] with a particle size of <50 nm. The preparation phase contains some impurity phases like Li₃PO₄ and Li₄P₂O₇; the concentration of those phases was totally less than 5% of native compound, hence the theoretical capacity of LiMnPO₄ was assumed to be 162 mA∙h∙g⁻¹. To prepare Fe and Mg co-doped LiMnPO₄ with 14 wt% of sucrose, a solid-state route has been adopted with subsequent ball milling for 6 h by Hu et al. [14] [15] . Four different synthesis temperatures, namely, 650˚C, 750˚C, 800˚C and 850˚C were studied. When the temperature exceeds 800˚C it leads to the formation of Fe₂P impurity during Fe substitution. Hence, 800˚C for 10 h was well suited to obtain the best performing single phase material. The final comprised ≈ 7.5 wt% carbon for all the three temperatures except 850˚C (≈5.7 wt%). The electrochemical properties of C-LiMn_0.9Fe_0.1-xMg_xPO₄ where x = 0.01, 0.02 and 0.05 were evaluated using CC-CV mode at 4.5 V vs Li.

Capacity profiles increased with increasing synthesis temperature and it could be attributed to the increased electronic conductivity of the carbon film. Hong et al. [16] reported Fe substituted olivine phase, LiMn_1-xFe_xPO₄ (x = 0, 0.05, 0.1, 0.15. and 0.2) prepared with an appropriate amount of citric acid as the carbon source and subsequently employed planetary ball milling for about 3.5 days before the heat treatment. All the prepared showed similar diffraction patterns, except for a slight shift towards higher angles with increasing concentration of Fe ions and the unit cell shrank continuously as iron was introduced into the system. The cell experienced a discharge capacity of ≈133 and ≈138 mA∙h∙g⁻¹ at 0.1 C between 2.8 and V by CC mode for 15 and 20% of Fe concentration, and those cells presented solid-solution behavior when the Fe concentration exceeds 10% when compared to the native compound (55 mA∙g⁻¹ at 0.067 C). Muraliganth and Manthiram [17] reported the solid-solution between LiMnPO₄ and either LiCoPO₄ or LiFePO₄ by a high energy ball milling procedure with 20% carbon in the final product. The discharge capacity increases significantly from 91 mA∙h∙g⁻¹ in LiMnPO₄ to 142 mA∙h∙g⁻¹ in LiMn_0.75Fe_0.25PO₄, and 155 mA∙h∙g⁻¹ for LiMn_0.5Fe_0.5PO₄ at 0.05 C, whereas LiCoPO₄ and LiMnPO₄ solid solution experienced severe capacity fade due to the poor compatibility of the electrolyte.

2.2. Spray Pyrolysis.

Spray pyrolysis is an important method for the preparation of ultrafine powders [18] [19] and it is based on the generation of droplets in a continuous way from a solution containing precursor colloidal particles. Droplets can

Table 1

Precursors used in the solid state reaction method of the resulting powder

Li Precursor	Mn Precursor	PO4 Precursor	Metal Dopant	Carbon Source	Product	Discharge Capacity (mA∙h/g)	Ref.
LiH2PO4	MnCO3		Fe(C2O4)∙2H2O	Carbon black	LiMn0.8Fe0.2PO4/C		[11]
LiOH∙H2O	Mn2O4	(NH4)2PO4	Co3O4	Oleic acid.	LiMn0.95Co0.5PO4/C		[12]
Li2CO3	MnC2O4∙2H2O	(NH4)2PO4			LiMnPO4	103 - 144	[13]
LiH2PO4	MnC4H6O4∙4H2O		Fe(C2O4)∙2H2O, Mg(C4H6)∙4H2O	sucrose	LiMn0.9Fe0.1-xMgxPO4/C	162	[14] [15]
Li2CO3	MnCO3	(NH4)2PO4	Fe(C2O4)∙2H2O	Citric Acid	LiMn1-xFexPO4/C	133 - 138	[16]
Li2CO3	MnC2O4∙2H2O	(NH4)2PO4	Fe(C2O4)∙2H2O		LiMn0.5Fe0.5PO4	91 - 142 - 155	[17]

be generated by using different techniques, such as ultrasonic transduction [20] . In spray pyrolysis, the generation of droplets is a key step because the droplets act as the nucleation centers and eventually evolve to well-crystallized, dense, and pure particles. Table 2 shows the precursors used in the Spray Pyrolysis method of the resultant LiMnPO₄ and composites of this phase. The pyrolysis product is collected in a series of water bubblers at the reactor outlet where the salt by product dissolves leaving LiMnPO₄. In the recent past, Taniguchi and co-workers [21] - [26] reported the synthesis of LiMnPO₄ by spray pyrolysis followed by wet ball milling with 10% acetylene black and ethanol to make the carbon composite. Finally, the composite was fired at 500˚C for 4 h in a N₂ + 3% H₂ environment and found that discharge capacity increases with the charge cut-off voltage. The influence of surface area on the electrochemical performance of LiMnPO₄ was also reported by the same authors by adjusting the firing temperature and increasing acetylene black concentration (20%). The solid solution between LiMnPO₄ and LiCoPO₄ was also reported by the same group using the spray pyrolysis technique [26] . The prepared LiCoxMnx⁻¹PO4 (x = 0, 0.2, 0.5, 0.8 and 1) powders were milled with 10% wt% acetylene black in ethanol by high energy ball-milling t form composites with carbon. The cells were tested at a 0.5 C rate and delivered an initial discharge capacity of 165 mA∙h∙g⁻¹ at x = 0, 136 mA∙h∙g⁻¹ at x = 0.2, 132 mA∙h∙g⁻¹ at x= 0.5, 125 mA∙g∙h⁻¹ at x = 0.8 and 132 mA∙g∙h⁻¹ at x = 1.0, respectively.

Oh et al. [27] - [29] continued the work of ultrasonic spray pyrolysis initiated by Taniguchi and co-workers with spherical size LiMnPO₄ particulates. Carbon coating over the particulates was employed by mixing with sucrose, which was followed by ball milling at a speed of 100 rpm for 20 h and subsequently heat treated with different calcination temperatures (550˚C, 650˚C and 700˚C in Ar-H₂ atm) ball milling followed by a sintering process led to the destruction of the spherical shape morphology and the final carbon content in the composite was found to be ≈3 wt%. It is obvious that increasing the calcination temperature results in an increase in crystallite size (52, 56 and 60 nm for 550˚C, 650˚C and 700˚C) and decrease in specific surface area.

2.3. Co-Precipitation

This method needs a shorter reaction time and lower temperature compared with the solid state reaction. The particle size can reach the nanometer level and this reduced size can help to enhance the charge-discharge performance especially at big current condition. The inadequacies inhibiting the mass production are the complex process and large power consumption. Table 3 shows the precursors used in the co-precipitation method of the

Table 2

Precursors used in the spray pyrolysis method of the resultant powders

Li Precursor	Mn Precursor	PO4 Precursor	Metal Dopant	Carbon Source	Product	Discharge Capacity (mA∙h/g)	Ref.
LiNO3	Mn(NO3)2∙6H2O	H3PO4		Acetylene Black	LiMnPO4/C	165	[21]
LiNO3	Mn(NO3)2∙6H2O	H3PO4		Acetylene	LiMnPO4/C	136	[22]
LiNO3	Mn(NO3)2∙6H2O	H3PO4		Acetylene Black	LiMnPO4/C	132	[23]
LiNO3	Mn(NO3)2∙6H2O	H3PO4			LiMnPO4	125	[24]
LiNO3	Mn(NO3)2∙6H2O	H3PO4			LiMnPO4	132	[25]
LiNO3	Mn(NO3)2∙6H2O	H3PO4		Carbon Black	LiMnPO4/C	132	[26]
LiH2PO4	Mn(NO3)2∙4H2O		Fe(NO3)3∙9H2O		LiMn1-xFexPO4		[27]
LiH2PO4	Mn(NO3)2∙4H2O				LiMnPO4		[28]
LiH2PO4	Mn(NO3)2∙4H2O			Acetylene Black	LiMnPO4/C		[29]

Table 3

Precursors used in the co-precipitation method of the resultant powders

Li Precursor	Mn Precursor	PO4 Precursor	Metal Dopant	Carbon Source	Product	Discharge Capacity (mA∙h/g)	Ref.
LiCO3	MnC2O4∙2H2O	NH4H2PO4		Acetylene Black	LixMnPO4/C		[30] [31]
LiH2PO4	Mn(CH3CO2)2∙4H2O		Fe(CH3CO2)2∙4H2O	Acetylene Black	LiMn0.5Fe0.5PO4/C	75 - 110	[32]
LiH2PO4	Mn(CH3CO2)2∙4H2O		Fe(CH3CO2)2∙4H2O		LiMn0.85Fe0.15PO4	125 - 140	[33] [34]

resultant LiMnPO₄ and composites of this phase. Non-stoichiometries proportions of lithium deficient (Li_0.5MnPO₄ and Li_0.8MnO₄) and rich phases (Li_1.1MnPO₄ and Li_1.2MnPO₄) were synthesized and reported by Xiao et al. [30] using co-precipitation followed by a solid-state approach with ball milling of carbon.

In all the non-stoichiometric compounds of Li_xMnPO₄ phases, traces of impurities like Mn₂P₂O₇ or Li₃PO₄ unavoidably co-existed with the native phase. The observed impurity phases were consistent with the phases observed in another olivine compound such as LiFePO₄ [30] [31] . The half-cells comprising LiMnPO₄, Li_1.1MnPO₄ and Li_1.2MnPO₄ phases displayed almost the same initial discharge capacity of ≈124 mA∙h∙g⁻¹ at 0.05 C rate between 2 and 4.5 V by CC-CV mode, whereas Li_0.5MnPO₄ and Li_0.8MnO₄ exhibited ≈75 and 110 mA∙h∙g⁻¹, respectively. Oh et al. [32] reported the reduction of polarization of the electrode and the least amount of Mn dissolution in C-LiMn_0.5Fe_0.5PO₄ by a precipitation technique. Initially, olivine LiMn_0.5Fe_0.5PO₄ was carbon coated with carbon (3.3 wt%) and it delivered a discharge capacity of ≈125 and ≈140 mA∙h∙g⁻¹ at 0.05 C rate at room temperature and 55˚C, respectively, between 2.7 and 4.5 V vs Li by CC-CV mode. The same group reported the performance of micron-sized nanoporous C-LiMn_0.85Fe_0.15PO₄ with high volumetric capacity [33] . The capacity delivered by the micron-LiMn_0.85Fe_0.15PO₄ electrode was nearly 1.4 times higher than that of the nano-LiMn_0.85Fe_0.15PO₄ electrode, i.e., 369.3 mA∙h∙cm⁻³ versus 261.1 mA∙h∙cm⁻³ under above test conditions. Double structured LiMn_0.85Fe_0.15PO₄ coordinated with LiFePO₄ was also reported by Oh et al. [34] . The core contained LiMn_0.85Fe_0.15PO₄ to enable high energy density and the outer layer was composed of LiFePO₄ to facilitate high rate capability. Carbon was also coated over the outer layer to increase the electronic conductivity.

2.4. Hydrothermal and Solvothermal Routes

Hydrothermal and solvothermal synthesis is a chemical process that occurs in an aqueous solution of mixed precursors above the boiling temperature of water and alcohol respectively. On these routes, it is possible to avoid the calculation step and obtain pure powders directly from the heated solution. However, if the carbon coating is desired, it is necessary to carry out the calcination step at high temperatures. During the synthesis, heated water or alcohol accelerates the diffusion of particles and the crystal growth is relatively fast. Both routes are typically carried out in a closed system called autoclave and there are less environmental concerns than many other powder production technologies.

Therefore, hydrothermal and solvothermal synthesis is a simple, clean and relatively low-cost methods that can be used to produce powders with high uniformity and purity [35] - [37] . Table 4 shows the precursors used in the hydrothermal and solvothermal routes of the resultant LiMnPO₄ and composites of this phase. Fang et al. [38] [39] reported the successful preparation of LiMnPO₄ plates by a simple hydrothermal route in a basic medium at

Table 4

Precursors used in the hydrothermal and solvothermal routes of the resultant powders

Li Precursor	M n Precursor	PO4 Precursor	Metal Dopant	Carbon Source	Product	Discharge Capacity (mA∙h/g)	Ref.
Li2SO4	MnSO4	NH4H2PO4			LiMnPO4	68	[38] [39]
LiOH	MnSO4∙H2O	H3PO4	Mg(NO3)26H2O NiSO4∙6H2O CuSO4∙5H2O		LiMn1-x(Mg Ni Cu)xPO4	107	[40]
LiPO4	MnPO4			Carbon black	LiMnPO4/C	49 - 107	[41] [42]
LiH2PO4	MnCO3		FeC2O4∙2H2O	Acid lactone	LiMn11-xFexPO4/C	50 - 65	[43]
LiOH	Mn(CH3COO)2∙4H2O	H3PO4		Multiwalled Carbon nanotubes (MWCNT)	LiMnPO4/MWCNT		[44] - [46]
LiOH	MnSO4∙H2O	H3PO			LiMnPO4		[47]
LiOH	Mn(CH3COO)2∙4H2O	H3PO4	CuSO4∙5H2O		LiMn0.98Cu0.02PO4 LiMn0.95Cu0.98PO4	76 - 101 - 121	[48]
LiOH∙H2O	MnSO4∙H2O	H3PO4			LiMnPO4	92 - 120 - 126 - 147	[49]
Li3PO4	MnSO4∙5H2O				LiMnPO4	135	[50]

200˚C. The results clearly indicate that the prepared materials were in the nanometric range and actively participated in the electrochemical reaction in which plate-like LiMnPO₄ ball milled with 20% of carbon delivered a discharge capacity of 68 mA∙h∙g⁻¹ between 3 and 4.5 V at 1.5 mA g⁻¹. Divalent cation doping (10%) such as Mg, Ni, Co, Zn, and Cu, was reported by Chen et al. [40] via a hydrothermal approach. The substitution of Mn sited leads to a decrease in lattice parameter values. The cell volume also decreased 0.8% for Mg₂₊, 0.6% for Ni₂₊, 0.3% for Cu₂₊, and 0.4% for Zn₂₊ substitution consistent with their smaller ionic radii. A citric acid assisted facile one step solvothermal (equiv. volume mixture of water and ethanol) procedure was adopted by Wang et al. [41] [42] to synthesize the microspherical LiMnPO₄ at 300˚.

The obtained microspherical particles were mixed with glucose followed by heat treatment for carbon coating. The Li/LiMnPO₄ cell displayed a discharge a capacity of 107 mA∙h∙g⁻¹ at 0.01 C with an appreciable plateau around ≈4.2 vs Li. As expected, increasing the current rate leads to poor cell performance; for example, at 1 C test cell presented a discharge capacity of 49 mA∙h∙g⁻¹. By introducing cetyltrimethylammonium bromide (CTAB) as a chelating agent in the above solvent mixture at relatively low temperature conditions (240˚C) LiMnPO₄ nanorods can be obtained. The storage performance of nanoplatelets of LiMnPO₄ and LiFe_0.5Mn_0.5PO₄ was studied by Saravanan et al. [43] using a simple solvothermal method at 250˚C for 8 h with gluconic acid p-lactone. The final LiMnPO₄ and Fe doped phases comprised of 10 wt% carbon. The test cells exhibited a discharge capacity of ≈50 and 65 mA∙h∙g⁻¹ between 2.3 and 4.5 V by CC mode. Further, the same group of authors continued the work on solid-solutions of LiMn_11-xFe_xPO₄ nanoplates by tuning various parameters, such as source materials, precursor concentration, effect of solvent, synthesis temperature and conducting coatings using the same approach. Manthiram and co-workers [44] - [46] reported a facile route to produce nanostructured LiMnPO₄ by a microwave assisted hydrothermal or solvothermal approach. In the hydrothermal approach, glucose is used as the source material for carbon, whereas MWCNTs were painted on the surface to enable a conducting network for LiMnPO₄ nanorods by the solvothermal route. LiMnPO₄ prepared by the above mentioned rousted displayed a monotonous charge-discharge curve and exhibited a discharge capacity of ≈15 to 45 mA∙h⁻¹. Ji et al. [47] also followed a similar approach to yield LiMnPO₄ nanostructures using citric acid and sodium dodecyl benzene sulfonate (SDBS) as an additive to tune the morphology of the final product. Also reported the kinetic behavior during Mg₂₊ substitution on Mn site compounds were prepared by a hydrothermal reaction and consequently ball milled carbon (20 wt%) before making electrodes. Increasing the concentration of Mg₂₊ (LiMn_1-xMg_xPO₄, x = 0 ≤ 5) provides substantial improvement in the thermal stability of the phase. Ni and Gao [48] reported the synthesis of Cu₂₊ substituted LiMnPO₄ by an ascorbic acid mediated synthesis. The obtained particles (≈100 nm) were carbon coated by carbonization of sucrose at 600˚C and carbon content of 2.4 wt% was obtained in the final product. In the range of 2.2 - 4.5 V (CC-CV mode), test cells delivered a reversible capacity of 101, 121 and 76 mA∙h∙g⁻¹ for LiMnPO₄, LiMn_0.98Cu_0.02PO₄ and LiMn_0.95Cu_0.98PO₄ phases, respectively. Wang et al. [49] reported the solvothermal synthesis of LiMnPO₄ nanoplates and rods using sodium dodecyl benzene sulfonate (SDBS). The synthesized powders were carbon coated through chemical vapor deposition (CVD) using methylbenzene as the carbon source and argon as the carrier gas. The authors compared the carbon coating technique with the conventional high energy ball milling (BM) procedure.

The discharge capacities of plate-CVD, Rod-CVD, Plate-BM and Rod-BM were 147, 126, 120 and 92 mA∙h∙g⁻¹, respectively, between 2.5 and 4.9 V at 0.05C at room temperature by CC mode. Recently, Dokko et al. [50] suggested the synthesis of LiMnPO₄ nanoparticles by reacting Li₃PO₄ (solid) with molten aqua-complexes of MnSO₄ under hydrothermal conditions at 190˚C. Later, the obtained LiMnPO₄ particles were carbon coated using sucrose with subsequent heat treatment at 700˚C. The molar ratio of Mn_x and H₂O in the reactor was varied by adding water to the reaction mixture. When x = 7.8, 50 nm size LiMnPO₄ particles were obtained which delivered a discharge capacity of ≈135 mA∙h∙g⁻¹ at 0.1 C between 2 and 4.5 V by CC-CV mode.

2.5. Sol-Gel Synthesis

Sol-gel synthesis is a low temperature, wet chemical approach, which is often used for the preparation of metal oxides or other specific compositions. Standard sol-gel synthesis involves the formation of a sol, i.e. a stable colloidal suspension of solid particles in a solvent, and the gelation of the sol to form a gel consisting of interconnected rigid skeleton with pores made o colloidal particles. The properties of the gel are determined by the particle size and cross-linking ratio. The gel can then be dried to form xerogel, which shows reduced volume [51] - [54] .

To obtain the final powder products, all liquids need to be removed from the surfaces of pores by a heat treatment carried out at elevated temperatures, which also reduces the number and connectivity of pores, known as densification [53] [54] . Reaction parameters such as temperature, time, pH, precursor, solvent, concentration and viscosity, etc., are of importance for the formation and ultimate morphology (particle size and shape, pore size, and porosity) of the obtained powders. In sol-gel synthesis, the surfaces of the powder products are controlled from the beginning of the reactions. In addition, sol-gel synthesis is low cost and does not require high processing temperature, and powders produced by this method have the advantages or precise stoichiometry control, high purity, uniform structure and very small size. Table 5 shows the precursors used in the sol-gel method of the resultant LiMnPO₄ and composites of this phase. Ethylene glycol assisted synthesis of LiMnPO₄ (M = Fe, Mn and Co) nanostructures was reported by Yang and Xu [55] to enable in situ carbon coating. In situ carbon coating over LiMnPO₄ particles by using organic precursors was successful and it was found that 3.3 wt% of carbon was covered on the surface after the phase formation at 700˚C in a N₂ atmosphere. The Li/LiMnPO₄ cell was cycled between 2.5 and 4.5 V in CC mode and delivered a reversible capacity of 85 and 42 mA∙g⁻¹ at 0.01 and 0.05 C rates, respectively. Kwon et al. [56] successfully synthesized nanosized (140 - 130 nm) LiMnPO₄ particles by a glycolic acid assisted sol-gel approach.

The prepared LiMnPO₄ was ball milled with 20 wt% carbon size reduction as well as to enable carbon coating. The cycling profiles indicate reduction of particle size from 830 to 140 nm which resulted in improved electrochemical performance (≈134 mA∙h∙g⁻¹ for 140 nm particles) between 2.3 and 4.5 V at 0.1 C. At 1 C, 140 nm particles exhibited a reversible capacity of ≈81 mA∙h∙g⁻¹, whereas 270 nm particles showed a reversible capacity of ≈5 mA∙h∙g⁻¹. A liquid phase synthesis was also reported by Doi et al. [57] to reduce the particle size in the presence of long chain oleic acids comprising 18 atoms of carbon. For this synthesis, the total reaction was completed at less than 300˚C resulting in the formation of LiMnPO₄. The obtained compound delivered a discharge capacity of 6 mA∙h∙g⁻¹ between 3 and 4.5 V at 0.01C by CC mode. After heat treatment at 500˚C for 1 h in Ar, the long chain oleic acid was burned and converted to carbon. Isovalent and aliovalent doping (Mg₂₊, Fe₂₊, Co₂₊, V₃₊ and Gd₃₊) on Mn₂₊ sited reported by Yang et al. [58] and such phases were prepared by solution phase reaction using citric acid as the carbon source. The precursors were ball-milled for 10 h in a planetary ball miller and fired at 700˚C for 20 h in a N₂ atmosphere. The substitution in Mn sites (LiMn_0.95M_0.95PO₄, M = Mg, V, Fe, Co and Gd) clearly revealed the increase in unit cell volume for all cases, except for Fe₂₊. The cell delivered a discharge capacity of 45, 61, 32, 102 and 59 mA∙h∙g⁻¹ at room temperature (25˚C), and 113, 120, 95, 149, and 117 mA∙h∙g⁻¹ at an elevated temperature (50˚C) for Mg, V, Fe, Co, and Gd substitutions, respectively, between 2.7 and 4.4 V in CC-CV mode. Preparation of LiMnPO₄ composites was suggested by Herrera et al. [59] .

The authors intended to increase the surface area, thereby achieving full performance of the material by utilizing a conventional coating technique. It is worth mentioning that addition of glucose. This indicates that the presence of carbon coating during the formation of crystallites increases the number of nucleation sited and

Table 5

Precursors used in the sol-gel method of the resultant powders

Li Precursor	Mn Precursor	PO4 Precursor	Metal Dopant	Carbon Source	Product	Discharge Capacity (mA∙h/g)	Ref.
Li(COOCH3)∙2H2O	Mn(CH3COO)2∙4H2O	H3PO4			LiMnPO4	42 - 85	[55]
LiOH	Mn(CH3COO)2∙4H2O	H3PO4			LiMnPO4	5 - 81 - 134	[56]
Li(COOCH3)∙2H2O	MnSO4∙H2O	NH4H2PO4			LiMnPO4		[57]
Li(COOCH3)∙2H2O	Mn(CH3COO)2∙4H2O	NH4H2PO4	Mg(NO3)2 V2O5 Co(CH3COO)2 FeCO4 Gd2O3		LiMn0.95M0.95PO4, M = Mg, V, Fe, Co and Gd* respectively	45, 61, 32, 10, 59 (at 25˚)* and 113, 120, 95, 149, 117 (at 50˚)*	[58]
LiOH	Mn(CH3COO)2∙4H2O	H3PO4		Glucose	LiMnPO4/C		[59]
LiNO3	Mn(NO3)2	H3PO4		SWNT	LiMnPO4/SWCN	70	[58]
Li(COOCH3)∙2H2O	Mn(CH3COO)2∙4H2O	NH4H2PO4		SWNT	LiMnPO4/SWCN	140	[58]
Li(COOCH3)∙2H2O	MnSO4∙H2O	NH4H2PO4	Co(CH3COO)2		LiMn0.91Co0.09PO4	148	[58]

leads to increases in the size of the particle. Nithya et al. [60] reported synthesis of LiMn_0.91Co_0.09PO₄ by a glycine assisted sol-gel approach with the particle size ranging from 50 to 300 nm. The resultant phase was ball milled with the desired amount of carbon (acetylene black 30 wt%) to yield a high performance cathode. LiCo_0.09Mn_0.91PO₄/C delivered a maximum discharge capacity of 162 mA∙h∙g⁻¹ between 3 and 4.9 V vs Li (CC mode) at 0.1 C when compared to 70, 140 and 148 mA∙h∙g⁻¹ for LiMnPO₄, LiMnPO₄ and LiCo_0.09Mn_0.91PO₄ respectively.

As a final remark, it can be observed that the highest Discharge Capacity is reported for LiMnPO₄/C (165 mAh/g, Table 2) synthesized by spry pyrolysis, the same composite synthesized by hydrothermal or solvothermal (147 mAh/g, Table 4) and by solid state reaction (144 mAh/g, Table 1). High values are also reported for LiMn_0.5Fe_0.5PO₄ (155 mAh/g, Table 1) by solid state reaction, LiMn_0.85Fe_0.15PO₄ (140 mAh/g, Table 3) by chemical co-precipitation, and LiMn_0.95M_0.95PO₄, (M = Co) (149 mAh/g, Table 5) by sol-gel method.

These results suggest that those synthesis methods that allow a better control of particle size and morphology can offer a better potential to improve the Discharge Capacity. Another path can be combined Mn with Fe and Co. It also can be observed that the highest Discharge Capacity can be reached for the LiMnPO₄/C, and then the addition of Carbon source is also a topic that deserves attentions.

Other phosphates compounds can be used as cathodes in lithium ion batteries. LiCoPO₄ (100 mA∙h∙g⁻¹) [61] [62] has higher open circuit voltages, but lower dielectric capacity than those of LiMnPO₄. In addition, Co₂P₄O₇ has been observed to be formed in delithiated LiCoPO₄ electrode, which degrades the lifetime and can be a safety concern as oxygen is involved during the decomposition reaction. Mixtures of phosphates, including LiCoPO₄ or LiFePO₄ (170 mA∙h∙g⁻¹) [63] , with LiMnPO₄, have been used for cathode materials. In such mixtures, the operating voltage increases with increasing manganese content [64] [65] , while capacity increases with increasing iron content [1] . Although LiNiPO₄ also forms the olivine structure [1] it is no typically used as a cathode material. However, nickel has been added to other phosphate cathode materials. Not considering Olivine Phosphate, few examples can be mentioned that can be considered potentially useful: LiCoO₂ (140 mA∙h∙g⁻¹), Li_xMn₂O₄ (125 mA∙h∙g⁻¹) and nanostructured Vanadium Oxide (VO_x) with Lithium intercalation capacities [66] . It must be mentioned that VO_x can go through structural breakdown during redox cycles. In this scenery, LiMnPO₄ can be considered as a promising material for battery cathode.

3. Conclusions

LiMnPO₄ have been reviewed focusing mainly on the synthesis method and how to improve the electrochemical properties performance for batteries cathode. For LiMnPO₄ small particle size well-shaped crystals are important for enhancing the properties. In particles with a small diameter, the Li ions may diffuse over shorter distances between the surfaces and center during Li intercalation and de-intercalation contributing to the charge/ discharge reaction. The variety of synthesis methods offers more possibilities to control desirable structures and high electrochemical performance. However, it is often challenging to obtain all the desired properties.

Typically, solid state methods are of importance in terms of obtaining ordered crystal structure, but they require higher treatment temperature and longer process time, which may lead to larger particle size and lower electrochemical capacity. On the other hand, sol-gel method gives high purity, small particle size, uniform size distribution, and hence relatively higher electrochemical capacity, but additional solvent cost and environment issues are major disadvantages of these methods. Many solid-state and solution-based methods are good candidates for the mass production of LiMnPO₄ powders at industrial scale; however the cost, productivity, reproducibility and complexity of these methods must be taken into consideration.

Acknowledgements

The authors are in debt to the CONACYT-CNPq 174391 Nanostructure Multiferroic grant for supporting this work.

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