What is lithium manganese iron phosphate!

1.1 What is lithium manganese iron phosphate?Similar in structure to lithium iron phosphate, manganese iron combines strengths and weaknesses. 

Lithium iron phosphate crystals are olivine shaped, with basic structural units consisting of LiO6 octahedra, FeO6 octahedra, and PO4 tetrahedra. 

Among them, FeO6 octahedra and PO4 tetrahedra are cross connected to form a polyanion framework structure, and Li+is transported along a single b-axis. 

Lithium manganese iron phosphate adds Mn element to the structure of lithium iron phosphate, but it is not a simple physical mixture of LiFePO4 and LiMnPO4. 

The difference in ionic radius between Fe2+and Mn2+is minimal. 

By relying on the synergistic effect between LiFePO4 and LiMnPO4, a stable and uniform solid solution is formed, 

which combines the stable electrochemical performance of LiFePO4 with the high potential of LiMnPO4. 

The Mn element can increase the discharge voltage to 4.1V, significantly improving the energy density of the positive electrode material.

The manganese iron ratio can be freely adjusted, and key indicators determine performance. 

In LiMnxFe1-xPO4 material, x represents the doping ratio of manganese and can take any value between 0 and 1. 

Due to the higher voltage platform of Mn and better conductivity of Fe, the performance of lithium manganese iron phosphate varies with different manganese iron ratios. 

When the manganese iron ratio is high, Mn significantly increases the battery voltage and energy density. 

However, excessive manganese content can damage the solid solution structure due to the Jahn Teller effect, 

leading to the dissolution of active materials and rapid degradation of cycling performance; 

When the ratio of manganese to iron is too low, the effect of voltage increase is limited and the advantage of energy density over lithium iron phosphate is not significant.

High manganese iron ratio is a trend. To fully leverage the performance advantages of lithium manganese iron phosphate, 

the Mn content in the material is often not less than 50%, and research on the manganese iron ratio mainly focuses on 5:5, 6:4, 7:3, 8:2, and 9:1. 

Currently, most of the developed products are 64 or 73. 

When Sheng Technology's manganese iron phosphate lithium product has a manganese content of 65%, its patented manganese content covers a range of 40% -90%; 

Rongbai Technology has released a product with a phosphorus manganese iron ratio of 7:3 and has achieved stable mass production of 100 tons; 

The research scope of Litai Lithium's patented manganese content is between 60% and 80%. 

From various products and patents, it can be seen that high manganese iron ratio is the direction for future efforts.

1.2 Lithium manganese iron phosphate has distinct characteristics and complements the advantages of other materials

Compared to lithium iron phosphate, LMFP has better energy density and low-temperature performance. 

Lithium manganese iron phosphate has two main advantages compared to lithium iron phosphate - improved energy density and excellent low-temperature performance. 

Firstly, the theoretical voltage platform of lithium iron phosphate is about 3.4-3.5V, 

and the introduction of manganese element enables the voltage platform of lithium iron phosphate to reach 4.1V. 

The theoretical energy density is 10-20% higher than that of lithium iron phosphate, which helps to improve the range of new energy vehicles. 

Secondly, under the condition of -20 ℃, the Mn platform capacity of lithium manganese iron phosphate is 95% of that at room temperature, 

while the Fe platform capacity is only about 50%, indicating good low-temperature performance.

Compared to ternary materials, LMFP has higher stability and outstanding cost advantages. 

Ternary materials belong to a layered structure, while lithium manganese iron phosphate has an olivine structure, 

which provides better stability during charge and discharge processes, and there will be no structural collapse problem when Li+is removed;

 Moreover, the P atom in lithium manganese iron phosphate forms PO4 tetrahedra through strong P-O covalent bonds, 

making it difficult for the O atom to detach from the structure, thus enhancing the stability and safety of lithium manganese iron phosphate. 

Meanwhile, the main elements of lithium manganese iron phosphate are manganese and iron, 

which avoids the use of precious metals nickel and cobalt in ternary materials and significantly reduces costs.

The voltage range of the material has widened, and it can be used purely or mixed. 

Lithium iron phosphate undergoes a two-phase reaction during charging and discharging, resulting in a single voltage plateau of 3.5V. 

The addition of manganese element causes manganese iron phosphate to have two voltage plateaus, namely 4.1V for manganese and 3.5V for iron; 

Ternary materials belong to single-phase reactions, and their voltage curves exhibit a ramp shape of 2.8-4.35V. 

From this, it can be seen that the voltage range of lithium manganese iron phosphate and the ternary material platform highly overlap, 

so the mixed use of the two can be used as one of the application solutions for lithium manganese iron phosphate. 

This solution can not only address the dual voltage platform issue and ternary safety issue of lithium manganese iron phosphate, 

but also improve other electrochemical properties such as capacity retention and charge discharge efficiency of composite materials.

2. Insufficient modification by various technologies, and the synthesis method is similar to lithium iron phosphate

2.1 Insufficient manganese iron lithium phosphate: low conductivity, poor cycle life

The structure restricts ion movement and low conductivity affects rate performance. Lithium manganese iron phosphate has a hexagonal close packed structure, 

and the discontinuous FeO6 (MnO6) octahedral network in the crystal, as well as the PO4 tetrahedra between them, 

affect electron transfer and Li+insertion and extraction; 

In addition, the diffusion path of Li+is easily blocked by Fe Li anti site defects, resulting in the diffusion coefficient of Li+being much lower than the theoretical value. 

The intrinsic low Li+diffusion coefficient and electronic conductivity of LiFePO4 batteries result 

in lower capacity retention during high rate (≥ 5C) charging and discharging processes, thus exhibiting poor rate performance.

The dual voltage platform increases the difficulty of BMS development. 

The voltage of lithium manganese iron phosphate has two characteristics: dual plateau and horizontal; 

When estimating the remaining charge of a battery, the Battery Management System (BMS) often uses OCV-SOC (the one-to-one correspondence between 

the open circuit voltage and the remaining charge of the battery) for calibration; 

The voltage platform is horizontal, which increases the difficulty and accuracy of estimation; 

Dual platforms often cause fluctuations in remaining range data, leading to increased difficulty in developing BMS; 

By blending with ternary materials, the gradient of the voltage platform can be effectively avoided.

The Jahn Teller effect affects cycling performance. When the manganese iron ratio is too high, manganese based materials are prone to Jahn Teller effect. 

The Jahn Teller effect refers to the asymmetric occupation of electrons in degenerate orbitals, resulting in a distortion of the geometric configuration of molecules. 

In the nonlinear MnO6 octahedron, the asymmetric distribution of Mn3+electrons leads to distortion of the MnO6 octahedron, 

and the acid generated by electrolyte decomposition corrodes the manganese ions in the positive electrode material, 

accelerating the Mn3+disproportionation reaction process. The Mn2+and Mn4+generated by the Mn3+disproportionation reaction dissolve in the electrolyte, 

resulting in loss of positive electrode active material and damage to the SEI film of the negative electrode. 

SEI film consumes active lithium ions during repair, leading to a decrease in battery capacity and affecting cycle life and stability.

2.2 Various improvement methods to assist in optimizing material properties

Lithium manganese iron phosphate and lithium iron phosphate have similar physical and chemical properties, 

as well as issues such as low ionic and electronic conductivity. 

Therefore, similar improvement methods have been adopted, including nano modification, carbon coating, and metal ion doping. 

Each technology can effectively improve the electrochemical performance of the material and meet market commercialization needs.

Carbon coating constructs a conductive network, significantly improving conductivity. 

Surface coating is the most commonly used modification method, among which carbon coating is the most widely studied and applied.

 By uniformly coating carbon on the surface of the material through high-temperature carbonization, 

on the one hand, the uniform carbon coating layer provides a medium for electron transfer, which can improve the electronic conductivity between particles; 

On the other hand, encapsulation can prevent particle growth, suppress particle aggregation, thereby shortening the Li+transport distance and improving ion conductivity. 

The content of carbon coating directly affects the conductivity of the material. 

When the carbon content is too low, the conductivity cannot be effectively improved, while when the content is too high, the tap density of the material will decrease. 

An excessively thick carbon layer will hinder the transport of Li+.

Therefore, carbon coating can improve the conductivity of materials, but it is necessary to choose an appropriate carbon content to balance conductivity and tap density.

Nanotechnology shortens the particle size of ion transport and improves material activity. 

The Li+of lithium iron phosphate with olivine structure can only undergo one-dimensional diffusion along the plane direction, 

and charge transfer mainly occurs on this plane, resulting in extremely low lithium ion diffusion coefficient.

Therefore, adjusting the particle size and ensuring a shorter diffusion plane orientation have a significant impact 

on improving the performance of lithium manganese iron phosphate materials. 

When the particle size of the material is at the nanometer level, the migration path of lithium ions is effectively shortened, accelerating the migration rate. 

At the same time, the sufficient contact between the material and the electrolyte increases the specific surface area, 

exhibiting better discharge specific capacity and obtaining excellent electrochemical performance.

Ion doping changes the structure and broadens the Li+migration channel. 

Body doping is the fundamental way to improve the intrinsic conductivity of materials, 

while ion doping involves adding trace amounts of other elements (such as Mg, Co, Zn, etc.) to 

the lattice structure to enhance its properties at the material structure level. 

Taking Mg element as an example, in the case of Mg2+doping, Mg will preferentially form a new LiFe1-xMgxPO4 solid solution with Fe sites. 

Due to the smaller ionic radius of Mg2+and Fe2+compared to Mn2+, 

the bond length in the MO6 (M=Mn, Fe, Mg) octahedron of the doped olivine structure becomes shorter, 

and the Li-O bond length in the LiO6 octahedron becomes longer. 

The extension of Li-O bonds in LiO6 octahedra widens the diffusion channel of Li+, 

making it easier for Li+to migrate and increasing the carrier density of the material, 

which is beneficial for multi-component olivine structured cathode materials to have better electrochemical performance.

2.3 Synthesis process similar to lithium iron phosphate, coexistence of solid-liquid method

The preparation method of lithium manganese iron phosphate is similar to that of lithium iron phosphate; 

In terms of raw material composition, lithium manganese iron phosphate only has one more manganese source 

than lithium iron phosphate; In terms of material properties, 

both have issues such as low ionic and electronic conductivity; 

From the perspective of improvement methods, 

it is necessary to adopt nanotechnology and carbon coating methods to enhance its electrochemical performance. 

Therefore, the preparation process is highly similar, 

and the preparation method of lithium manganese iron phosphate can draw on the synthesis process of lithium iron phosphate, 

which requires processes such as ball milling, granulation, crushing, sintering, etc; 

According to the communication announcement of Hunan Yuneng, 

the existing lithium iron phosphate production line can produce lithium manganese phosphate after renovation, with good compatibility.

There are many synthesis methods, including solid-phase and liquid-phase methods. 

The preparation methods of lithium manganese iron phosphate can be divided into solid-phase method and liquid-phase method; 

Solid phase method is a process that involves heat treatment of uniformly mixed reactants 

at high temperatures to allow them to interact and form the desired materials. 

It is divided into high-temperature solid-phase method and carbon thermal reduction method. 

The liquid phase method is to prepare a solution of soluble metal salts according to the metered composition of the prepared materials, 

select a suitable precipitant or use evaporation, 

sublimation, hydrolysis and other methods to make metal ions precipitate or crystallize evenly, 

and finally dehydrate or heat the precipitation or crystallization to obtain the required material powder, 

including hydrothermal method, sol gel method and coprecipitation method. 

The advantages of solid-phase method are simple process, low cost, and suitable for large-scale production. 

The disadvantages are poor product consistency and difficulty in controlling particle size distribution and morphology; 

The liquid-phase method has the advantages of uniform mixing of raw materials, fast reaction rate, and good product consistency,

 but the process is complex, requires high equipment, and is difficult to control. Solid phase and liquid-phase technologies are becoming increasingly mature,

 with accelerated commercialization processes, dual line parallel production, and conditions for large-scale production.