Research progress on lithium manganese phosphate as cathode material for olivine type lithium-ion batteries

As a new type of energy storage device, lithium-ion batteries have been widely used in fields such as electric vehicles, mobile devices, computers, etc. 

due to their high energy density, good safety performance, good cycle stability, excellent rate performance, and long service life. 

The positive electrode material of lithium-ion batteries is a key factor affecting battery performance, and the commercialized positive electrode material is lithium iron phosphate

The relatively low working voltage cannot meet the usage requirements of high-capacity lithium-ion batteries.

 Olivine type lithium manganese phosphate, as a current research hotspot for positive electrode materials, 

has the advantages of low cost, low toxicity, good cycling performance, stable structure, and high energy intensity. 

However, similar to lithium iron phosphate materials,

 lithium manganese phosphate materials are severely limited in their electronic conductivity and lithium ion diffusion ability due to the influence of their own structure. 

Therefore, improving the electrochemical performance of lithium manganese phosphate, 

enhancing its synthesis method, and strengthening modification research are the research focuses in this field. 

The author reviewed the research progress on the preparation and modification methods of lithium manganese phosphate cathode materials, 

and summarized the advantages and disadvantages of different preparation methods.

1. Preparation method of lithium manganese phosphate cathode material

1.1 Sol gel method

The sol gel method is to dissolve the raw materials in water or ethanol solution, and make each component form a uniform sol by violent stirring. 

With the help of chelating agents such as citric acid and oxalic acid, the system gel is heated to obtain the finished product by high-temperature sintering. 

The samples synthesized by sol-gel method have uniform size and high purity, but the preparation cost is high and the process is complex.

Esmezjan et al. synthesized carbon coated lithium manganese phosphate (LiMnPO4/C) cathode material with orthogonal olivine structure by sol-gel method with lithium acetate dihydrate, 

manganese acetate tetrahydrate and ammonium dihydrogen phosphate as main raw materials.

 Scanning electron microscopy shows that the particles on the surface of the material are spherical, with a particle size distribution between 100-200nm and uniform size distribution. 

The electrochemical performance test results show that the positive electrode material has a voltage plateau of about 4.1V and a charging capacity of up to 135mAh/g. 

Fang et al. used the sol-gel method to prepare LiMn1-xFexPO4/C composites. 

The particles on the surface of the materials are mostly spherical, with uniform particle size distribution, and the main particle size is about 300nm. 

Compared with lithium manganese phosphate material, this composite material exhibits superior electrochemical performance, 

with discharge specific capacities of 152.2mAh/g and 130.5mAh/g at discharge rates of 0.1C and 1C, respectively.

The sol gel method can also be combined with other processes to prepare lithium manganese phosphate cathode materials. 

Liu et al. synthesized the nanocomposite LiMnxFe1-xPO4/C by the freeze drying assisted sol-gel method. 

The introduction of the freeze drying process can effectively solve the agglomeration problem generated in the traditional drying process. 

Scanning electron microscopy analysis showed that the surface of the composite material was uniformly dispersed with nano rod-shaped particles, and no agglomeration was observed.

1.2 Solid phase synthesis method

The solid-phase synthesis method has a relatively simple process and is suitable for large-scale production and industrial applications. 

The common preparation process is to first uniformly grind the raw materials and then perform high-temperature treatment under inert gas protection to form lithium manganese phosphate products. 

However, the surface of the material prepared by this method is prone to the formation of larger particles or agglomeration phenomena.

Hu et al. used lithium dihydrogen phosphate, manganese acetate tetrahydrate, oxalic acid, ferrous oxalate,

 and magnesium acetate tetrahydrate as the main raw materials to obtain LiMn0.9Fe0.09Mg0.01PO4/C composite material through solid-state synthesis. 

The composite material exhibited high discharge capacity and excellent cycling performance at room temperature and high temperature. 

After 120 charge discharge cycles at 50 ℃ and 1C, the discharge capacity still remained at 135mAh/g, and the capacity decay was less than 1%. 

Experimental results have shown that Fe Mg co doping effectively inhibits the dissolution of Mn3+,

 making the olivine structure of the material more stable and enhancing its high rate performance and cycling stability at high temperatures.

The calcination temperature has a significant impact on the structure and properties of lithium manganese phosphate materials. 

When the calcination temperature is too high, excessive calcination can easily lead to blurred grain boundaries, abnormal grain formation, and a decrease in the quality factor of the material. 

Wang et al. prepared lithium manganese phosphate ceramics using solid-state synthesis method with lithium carbonate, manganese carbonate, 

and ammonium dihydrogen phosphate as raw materials, and calcined them at 700-800 ℃. 

Scanning electron microscopy analysis shows that as the temperature increases, the surface pores of the material gradually shrink, 

and the relative density and quality factor of the material gradually increase. 

But when the temperature rises above 750 ℃, the quality factor of the material begins to decrease. 

Therefore, lithium manganese phosphate materials prepared at appropriate calcination temperatures have lower dielectric constants and higher quality factors and relative densities.

1.3 Hydrothermal synthesis method

The hydrothermal synthesis method belongs to a type of liquid-phase preparation method. 

The main process is to put the raw materials into a hydrothermal synthesis reactor and directly generate lithium manganese phosphate material under high temperature and high pressure conditions. 

The surface particle distribution of the material prepared by this method is uniform and the morphology is controllable, so it is widely used in the preparation of nanoscale cathode materials.

Priyadharsini et al. continuously stirred lithium dihydrogen phosphate, manganese acetate tetrahydrate, 

and citric acid in a solvent polyethylene glycol 400, and then poured them into a high-pressure reaction vessel for hydrothermal synthesis reaction to obtain lithium manganese phosphate powder. 

The purity and crystallinity of lithium manganese phosphate particles were improved by heat treatment at 400 ℃. 

The surface particle size of the prepared product presents a rod-shaped structure, with a particle size ranging from 100 to 200nm. 

Chang et al. first dissolved lithium phosphate, manganese sulfate monohydrate, and magnesium chloride hexahydrate in a mixed solution of PEG-400 and water, 

and synthesized LiMn1-xMgxPO4 powder by hydrothermal method. 

After uniformly mixing the powder material with ascorbic acid, LiMn1-xMgxPO4/C material was obtained by grinding and high-temperature calcination. 

The introduction of an appropriate amount of Mg element effectively reduces the impedance of the material, promotes its reversibility, increases the diffusion rate of Li+, 

and makes it easier for the material to achieve high capacity and high rate performance.

By appropriate heat treatment methods, the particle size and morphology of materials can be effectively controlled, 

which has a significant impact on improving their conductivity, rate performance, and cycling performance. 

However, the hydrothermal synthesis method has a complex process, strict synthesis conditions, and often uses organic solvents as reaction solvents, 

which can cause environmental pollution. Therefore, this method currently faces certain difficulties in industrial development.

2. Modification method of lithium manganese phosphate cathode material

2.1 Nanomaterialization of Materials

The size of material grains has a significant impact on the activity of electrodes. 

Reducing the grain size can effectively shorten the diffusion distance of Li+, reduce the adverse effects of defect areas on Li+diffusion, 

and thus effectively improve the performance of lithium manganese phosphate materials. 

At present, liquid phase methods, such as sol gel method and hydrothermal synthesis method, are the main methods to prepare nano lithium manganese phosphate particles.

Cao et al. prepared nano LiMnPO4/C composite materials with porous layered structure by a simple hot solvent method. 

Hexamethyltetramine was used as a mineralizer to adjust the size and structure of the nano material particles. 

The discharge capacities of the prepared composite materials at 1C and 2C were 126.3 mAh/g and 110.2 mAh/g, respectively. 

At the same time, the battery capacity remained at 93% after 500 cycles at 1C. Han et al. synthesized lithium manganese phosphate nanosheets using acetic acid assisted hot solvent method. 

The study showed that the reduction of particle size significantly improved the electrochemical performance of the material. 

The discharge capacities of the material at 1C and 20C were 148.8mAh/g and 96.4mAh/g, respectively. After 100 cycles at 5C, the battery capacity retention rate was 76.1%.

The size of material particles can also be adjusted by changing the molar ratio of lithium, phosphorus, 

and manganese sources in the raw materials. Zhu et al. adjusted the molar ratio of LiAc (lithium acetate): H3PO4: MnCl3 to 6:1:1, 

and synthesized nano lithium manganese phosphate particles with a particle size of about 100nm by hot solvent method. 

After adding a carbon coating, the discharge capacity of the material was 156.9mAh/g, 122.8mAh/g, and 89.7mAh/g under 0.05C, 1C, and 10C conditions, respectively. 

After 200 cycles at 1C, the capacity retention rate reached 85%.

2.2 Carbon coating

Using carbon for surface modification of materials can not only enhance the conductivity between particles, 

but also effectively suppress the increase in particle size of lithium manganese phosphate, shorten the diffusion path of Li+, 

and thus improve the charge discharge performance and rate performance of the material.

There are currently multiple methods for preparing LiMnPO4/C composite materials, 

and different preparation methods have different effects on the electrochemical performance of lithium manganese phosphate. 

Li et al. prepared four different LiMnPO4/C composites by spray drying, ball milling, spray drying+ball milling and spray drying+electrospinning, respectively. 

Among them, LiMnPO4/C composites prepared by spray drying+ball milling have the best electrochemical performance. 

Under the conditions of discharge rate of 0.1C, temperature of 25 ℃ and 50 ℃, 

the discharge capacities of the composites are 114.90mAh/g and 132.14mAh/g respectively, and the capacity retention rate is 89.8% after 50 discharge cycles.

Choosing a suitable carbon source is one of the key factors in synthesizing high-performance lithium manganese phosphate cathode materials. 

Generally, glucose, sucrose, cyclodextrin, and other organic carbon sources are used. 

In addition, some inorganic substances such as acetylene black and quercetin black are also used as carbon sources. 

Graphene is considered an ideal carbon coating for cathode material modification due to its excellent conductivity, flexibility, and chemical stability.

 Li et al. modified Li0.95Na0.05MnPO4 nanoparticles with graphene oxide as a carbon source. 

The addition of graphene oxide significantly improved the electrochemical performance of the material, with a discharge capacity of 150.4mmAh/g at a discharge rate of 0.05C, 

while the unmodified material had a discharge capacity of only 122.3mAh/g.

The carbon content in the material plays an important role in improving the rate performance of lithium manganese phosphate. 

Li Li'e et al. carbonized lithium manganese phosphate with different contents of β - cyclodextrin to obtain LiMnPO4/C nanocomposites, 

and studied the effect of carbon content on the electrochemical properties of the composites. 

Research has shown that when the carbon mass fraction is 3.80%, the initial discharge capacity of the composite material can reach 140mAh/g at 0.1C, 

and the electrochemical performance of the composite material is optimal under this condition. 

When the carbon content is too low, the surface of lithium manganese phosphate cannot be completely coated, 

while when the carbon content is too high, it will have a negative impact on the capacity of the material.

2.3 Ion doping

Ion doping mainly improves the performance of materials by changing their structure.

 Ionic doping mainly includes Li doping, Mn doping, and P doping, with Mn doping being the most common.

Li doping is the use of ions with larger ionic radii to replace some Li+, thereby expanding the diffusion channel radius of Li+, reducing charge transfer resistance,

 and improving the rate performance of materials. El et al. prepared Li0.97Na0.03MnPO4/C positive electrode material using solution combustion method. 

Compared with LiMnPO4/C, the maximum discharge capacity of Li0.97Na0.03MnPO4/C has increased from 126.9mAh/g to 136.7mAh/g. 

Liu Bijiao et al. synthesized Li1-xNaxMnPO4 nanomaterials in one step using high-temperature solid-phase method. 

The electrochemical impedance spectroscopy and charge discharge test results show that when the Na mole fraction is 20%, the charge transfer impedance of the material is the smallest, 

and the battery charge discharge performance is more stable.

Compared with Li doping, doping metal ions such as Fe2+, Co2+, Zn2+on Mn sites can suppress the Jahn Teller effect of Mn3+, inhibit the dissolution of Mn3+, 

and more effectively improve the electrochemical performance of the material. 

Li et al. prepared LiMn0.8Fe0.2PO4/C composite material using citric acid as a carbon source. 

The discharge capacities of the composite material reached 164mAh/g, 122mAh/g, and 106mAh/g at discharge rates of 0.1C, 1C, and 5C, respectively. 

At the same time, the composite material exhibited excellent capacity retention ability. After 100 discharge cycles at 1C, the battery capacity remained at 99.6%. 

The appropriate metal ion doping ratio is also a key factor in improving the ion conductivity and high rate performance of positive electrode materials. 

Zhang et al. prepared LiMn0.94Nd0.06PO4/C neodymium doped nanomaterials with a porous spherical structure. 

The composite material exhibits high rate discharge capability, with discharge capacities of 155.2mAh/g and 128.0mAh/g at 0.05C and 10C, respectively.

At present, there is relatively little research on P-doping, 

mainly by replacing a small amount of P element with non-metallic elements, causing slight changes in the anionic groups,

 thereby increasing the width of Li+diffusion channels and reducing the diffusion energy barrier of Li+.

3. Conclusion and Prospect

Due to the influence of olivine type structure, the electronic conductivity and lithium ion diffusion performance of lithium manganese phosphate are severely restricted. 

At present, lithium manganese phosphate is still in the research stage and has not yet achieved large-scale production at home and abroad. 

The reason is that a simple and suitable method for large-scale production, as well as an effective solution to the problem of material high performance, has not yet been found. 

The main methods to enhance the electrochemical performance of lithium manganese phosphate are to control the grain size, 

shorten the diffusion path of Li+, expand the diffusion channel radius of Li+, and reduce the charge transfer resistance. 

The nanomaterialization of materials, carbon coating, 

and ion doping methods have all played a significant role in improving the electrochemical performance of lithium manganese phosphate, 

and are also important for its industrial development. 

Considering cost, safety, service life, and rate performance, lithium manganese phosphate is currently the ideal choice for lithium-ion battery cathode materials. 

In the future, it is still necessary to solve the difficulties in large-scale synthesis of lithium manganese phosphate, improve its conductivity and lithium ion diffusion ability.

 The breakthrough of lithium manganese phosphate materials will have an important impact on the development of lithium-ion batteries.