Talk about Lithium Manganese Iron Phosphate (LMFP)

In May 2023, Guoxuan High Tech announced at the 12th Science and Technology Conference 

the independently developed L600 Qichen battery cell and battery pack with a new LMFP (Lithium Manganese Iron Phosphate) system. 

The battery cell energy density is 240Wh/kg, and the system energy density reaches 190Wh/kg.

 It is planned to be mass-produced in 2024 without any unique partners. 

CATL also released a ternary positive electrode material system composed of lithium manganese iron phosphate. 

As a member of the positive electrode material phosphate series, what are its advantages, disadvantages, and solutions? Let's explore them below.

1： Introduction to Lithium Manganese Iron Phosphate (LMFP)

We know that the mainstream positive electrode materials for lithium batteries can be divided into transition metal oxides and polyanion salts. 

The former is represented by lithium cobalt oxide, lithium manganese oxide, and ternary materials, 

while the latter is represented by lithium iron phosphate material, which has a very stable olivine structure. 

Therefore, it has always been known for its long cycle, high safety, and low cost.

 Due to inherent advantages in cost and safety, the installed capacity of phosphate based cathode materials,

represented by lithium iron phosphate, has been steadily increasing this year. 

At the same time, due to structural innovation and battery cell innovation, 

the range of passenger cars equipped with lithium iron phosphate has entered the 600 kilometer range threshold.

 Has the performance of lithium iron phosphate reached its limit, and has the energy density reached its ceiling? 

The material upgrade and iteration speed of lithium iron phosphate has been very fast in the past decade. 

The currently mass-produced lithium iron phosphate material has a positive electrode compaction density of 2.6g/cm3, 

and a capacity of 145mA-h/g at 1C grams on the entire battery. 

The next generation of products can achieve a compaction density of 2.7 to 2.8g/cm3. 

It can be seen that both in terms of gram capacity and compaction, lithium iron phosphate has significant differences from ternary and lithium cobalt oxide, 

and it also has the disadvantage of low discharge voltage. 

Therefore, lithium manganese iron phosphate has emerged. 

Lithium iron phosphate and lithium manganese iron phosphate have similar structures, 

which can be simply understood as some of the iron sites inside lithium iron phosphate are replaced by manganese. 

They both belong to the olivine type oxide, with stable structure and good safety performance. 

However, their lithium ion migration is limited to one-dimensional transport channels, so compared to three-dimensional oxides, their ion conductivity is slightly lower. 

As the content of manganese element in lithium manganese iron phosphate increases, 

the voltage plateau will gradually increase (manganese metal can change valence 

and its oxidation-reduction potential is higher than iron) to compensate for the problem of low voltage plateau in lithium iron phosphate. 

Due to the similar radii of iron and manganese ions, they can be mixed at the atomic level to obtain lithium manganese iron phosphate that combines the advantages of both.

Here are its advantages: 

(1) The voltage platform of LMFP is 4.1V higher than that of LFP, which is 3.4V. The difference in gram capacity between the two is almost the same.

 According to the energy density=gram capacity * voltage, the theoretical energy density of LMFP is 21% higher than that of LFP,

 which is also the main starting point of its advantages. 

However, there is still a significant gap compared to ternary materials, 

so LMFP is not positioned to replace ternary materials (PS: in practical applications, due to its lower compaction density, the energy density will be greatly reduced).

 (2) The low-temperature performance is better than that of lithium iron phosphate, which can actually refer to the low-temperature performance of lithium manganese oxide. 

The apparent reason is that the low-temperature plateau capacity of manganese in lithium iron phosphate can be almost fully utilized, while lithium iron phosphate can only be utilized at half. 

The capacity utilization of lithium manganese iron phosphate Mn platform at -20 ℃ accounts for 95% of that at room temperature, 

while the capacity utilization of Fe platform at -20 ℃ accounts for about 50% of that at room temperature. 

So why can the Mn low-temperature platform capacity of LMFP be almost fully utilized, while LFP can only be utilized at half? 

The reason is difficult to explain, because from the intrinsic properties of the material, the electronic and ionic conductivity of LMFP is lower than that of LFP, 

and the polarization of LMFP at low temperatures will be greater, so the capacity it can exert should be less.

 The root cause may be related to the arrangement of 3D orbital electrons and the energy barrier of electrochemical reactions at low temperatures. 

(3) Good stability (compared to layered oxides): 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. 

This makes lithium manganese iron phosphate highly thermally stable, safe, and has a long service life.

 Meanwhile, lithium manganese iron phosphate avoids the use of precious metals, resulting in lower costs compared to ternary materials.

2： Introduction to the disadvantages of lithium manganese iron phosphate (LMFP)

Disadvantages: (1) Low conductivity: The energy of electrons in the transition of manganese iron lithium is much higher than that of iron lithium. 

Iron lithium can be compared to semiconductors, while manganese iron lithium is basically an insulator. 

In addition, the primary particles of manganese iron lithium are very small, making it much more difficult to process than iron lithium.

(2) Dual voltage platform: Due to the presence of iron and manganese elements, there are two different operating voltages, approximately 3.9V and 3.3V, and the dual voltage platform is unstable.

(3) Similar to lithium manganese oxide, manganese leaching: 

The ginger Taylor effect of manganese ions causes manganese leaching, deposition on the negative electrode surface, and damage to the SEI film. 

So its cycling performance is naturally inferior to lithium iron phosphate.

(4) Processing is difficult because the particles are relatively small, the surface area is large, the carbon content is high, the viscosity of the slurry is high, and the moisture in the battery cells is difficult to dry.

(5) The 45 ℃ cycling and 60 ℃ high-temperature storage performance of LMFP are relatively poor.

3： Improvement plan for lithium manganese iron phosphate material and battery system

(1) Material aspect: 1. Optimization of manganese iron ratio: Currently, 6:4 or 7:3 are the most common, and high manganese is the direction of future development, 

but the Ginger Taylor effect of high manganese needs to be addressed. 2: 

The choice of process synthesis route: Both manganese iron and iron iron belong to the phosphate system, and the preparation process is similar. 

The solid-phase method is simple and suitable for industrial production, while the liquid-phase method is more complex but has better product performance.

 Manganese iron requires atomic level mixing, so liquid-phase method is naturally more suitable. 3: Modification: It refers to the commonly discussed doping coating nanomaterialization.

 Here, we will focus on the alleviation of the Jahn Teller effect by doping. Unlike coating, doping changes the conductivity and ion diffusion properties of the material from within the lattice. 

Doping ions can cause defects in the lattice and suppress the Jahn Teller effect, thereby improving material performance.

 Implementation: Doping with metal cations, commonly including Mg2+, Zn2+, Cu2+, Co3+, Ni2+, Cr3+, etc. 

The research on Mg2+is the most extensive, and isovalent doping has a positive impact on improving the performance of LFMP.

 Due to the consistent valence state, it will not cause defects in the lattice structure, resulting in the collapse of the structure during the cycling process.

(2) : Blending with other materials: 1. For example, Ningde Times' M3P battery adopts a ternary blending scheme. 

The reason for blending is mainly because the particle size of manganese iron lithium is small, close to the nanometer level, while ternary is at the micrometer level. 

By filling with particles of different sizes, the problem of low compaction density of manganese iron lithium can be solved. 

On the other hand, the higher specific energy of ternary materials compared to lithium iron phosphate can be utilized. 

In addition, this type of battery also has the advantages of good LMFP safety and good low-temperature performance. 2: Regarding the implementation of the compounding scheme: 

(1): The compounding of the two is carried out using conventional homogenization, which has strong processing operability. 

(2) The design basis for surface coating of ternary materials with lithium iron manganese phosphate is to suppress oxygen evolution of ternary materials 

and prevent electrolyte side reactions by surface coating with lithium iron manganese phosphate. 

(3) Layered coating: The design basis for the lower layer ternary and upper layer lithium iron phosphate is to isolate the electrolyte from direct contact with the ternary material, 

improve the stability of the ternary material, 

and maximize its energy density; Lithium iron manganese phosphate with olivine structure has high thermal stability and small volume change, and can withstand higher cold pressure as an outer layer.

(3) Optimization of battery cell electrolyte: Mainly aimed at the reasons for high temperature storage difference and high temperature cycle attenuation. 

On the one hand, the intrinsic specific surface area of the material is large, which produces more gas through side reactions with the electrolyte at high temperatures. 

On the other hand, the generation of HF and the dissolution of positive electrode manganese at high temperatures cause Mn2+to migrate to the negative electrode surface and be reduced for deposition, 

thereby damaging the SEI film, leading to continuous growth and thickening of the SEI film, 

consumption of active lithium, increased electrode impedance, and negative electrode gas production, ultimately resulting in continuous capacity degradation of the battery. 

When the root cause of the problem is clear, the solution is very clear: just like the solution of high nickel plus silicon oxide: 

on the one hand, the construction of high stability positive and negative electrode interface facial mask:

 reduce the oxidation and decomposition of electrolyte on the positive electrode surface, inhibit Mn dissolution, negative electrode film formation, 

and improve the interface stability through positive electrode surface film formation or complexation; 

The use of water and acid inhibiting additives reduces the HF content and slows down the corrosion of the positive electrode by HF, 

which leads to the dissolution of Mn ions. Another aspect is the optimization of solvent systems and the compounding of main salts to stabilize lithium hexafluorophosphate; 

The types of additives are shown in the following figure:

Stable negative electrode film: Combining organic and inorganic films to reduce impedance and increase stability. 

Enhanced positive electrode film: Combining inorganic polymer film to suppress impedance and prevent gas production.

The above is a general introduction to lithium manganese iron phosphate. In terms of lithium iron phosphate, previous research has mainly focused on changes in packaging methods, such as blade batteries. 

With the continuous development of technology, R&D personnel have also begun to focus on chemical systems and develop lithium manganese iron phosphate batteries. 

Overall, it is an important iterative direction for positive electrode phosphates, with clear advantages and disadvantages. 

Solving the disadvantages requires mutual cooperation between material factories and battery cell factories to better solve them. 

Whether the future prospects can be developed and expanded remains to be seen.