Notes on Lithium Manganese Iron

The application of M3P battery in the pure electric sedan Luxeed S7 jointly developed by Chery and Huawei. 

M3P battery is a new type of battery material with better energy density, low-temperature performance than lithium iron phosphate, and cost than ternary batteries. 

Ningde Times believes that M3P is not lithium manganese iron phosphate, but a "ternary" battery of phosphate system, 

and its synthesis process is based on the lithium manganese iron phosphate process. Lithium manganese iron phosphate plays an important role in M3P batteries.

The background and purpose of searching for ideal positive electrode materials require high energy density, high power density, long lifespan, high safety, and low cost. 

Lithium manganese iron phosphate is an ideal positive electrode material with a small molecular weight and an ideal 4-volt voltage plateau. 

Although its diffusion coefficient and conductivity are relatively poor, it can be improved through nanomaterialization and carbon coating. 

It has a stable olivine phase structure, and the constituent elements iron, manganese, and phosphorus are all elements with high crustal abundance. 

The theoretical energy density of the material end is 431Wh/kg, and the energy density of the battery cell is above 210. 

As an upgraded product of lithium iron, the main characteristics of manganese iron lithium are high energy density and excellent low-temperature performance. 

Compared with lithium iron, its gram capacity is around 170, but in low-temperature environments, especially at minus 20 degrees Celsius, 

its capacity retention rate is much higher than that of lithium iron, approaching ternary. 

Its voltage plateau is 3.7V, which is 0.4V higher than that of lithium iron. 

In addition to excellent low-temperature performance, its rate is also better than that of iron lithium. 

Under similar designs, for small soft pack batteries, the rate of lithium iron at 2C current is difficult to exceed 90%, while manganese iron lithium can exceed 95%. 

From the low-temperature discharge curve, it can be seen that the low-temperature retention rate of manganese iron lithium is relatively high, 

but the voltage plateau drop in the manganese region is still relatively large.

The advantages of LMFP: Its low temperature and rate performance are much better than LFP, 

and its voltage plateau is smooth at close to ternary low temperature or high rate. 

The voltage drop is worth paying attention to.

Basic parameters and characteristics of manganese iron lithium batteries. The ratio of manganese to iron mainly affects energy density. 

The higher the manganese content, the higher the energy density. 

However, excessive manganese content can affect electrochemical activity and lifespan decay. 

The commonly used ratio of manganese to iron is above 1:1, such as manganese 5 iron 5, manganese 6 iron 4, manganese 7 iron 3, etc. 

The particle size of manganese iron lithium batteries is smaller than that of iron lithium batteries, generally between 100-200 nanometers. 

Small particles can bring higher capacity and better magnification, but they can affect processing performance and increase manganese leaching. 

In terms of gram capacity, compared to iron lithium batteries with a capacity of around 160mAh/g, manganese iron lithium batteries generally have a capacity of around 150, 

and their full charge gram capacity is around 140. 

However, there are three major pain points for manganese iron lithium batteries: firstly, the low diffusion ability of lithium ions; 

Secondly, the compaction density is relatively low due to the small particle size and high carbon content; 

Finally, there is a significant Jiang Taylor effect, which is present in all positive electrodes containing trivalent manganese, 

from spinel lithium manganese oxide to lithium rich manganese to manganese iron lithium.

The disadvantages of manganese iron lithium: low electronic conductivity, low lithium ion diffusion rate, LMFP conductivity of only 10-11S/cm,

 lithium ion diffusion rate of 10-9cm2/S; 1-2 orders of magnitude lower than LFP. 

The low compaction density affects the energy density utilization. 

The small particle size of LMFP material results in a lower compaction density, with a maximum compaction of 2.3g/cc.

 John Teller effect of Mn3+- dissolution of trivalent Mn.

      The modification of lithium manganese iron (LiMnFePO4) usually involves methods such as carbon coating, nanomaterialization, and doping. 

Carbon coating helps to enhance its conductivity, control particle growth, and inhibit the dissolution of Mn.

Due to the low calcination temperature (about 700 ℃) and fast particle growth rate of LiMnFePO4, there are special requirements for the selection of carbon sources, 

which require the use of carbon sources with lower cracking temperatures and wider cracking temperature ranges. 

Nanotechnology can increase the active area, but it can affect processing and cycling performance, 

so a balance needs to be found between increasing capacity and maintaining good processing performance. 

The doping strategy is similar to that of iron lithium, and commonly used doping elements such as titanium, vanadium, magnesium, niobium, etc. are also applicable to LiMnFePO4. 

However, the compatibility of Mn needs to be considered, and new doping element systems need to be explored to enhance the interaction with Mn and protect the Mn platform.

The synthesis method of LiMnFePO4 is similar to that of lithium iron, including phosphate method, oxalate method, sol gel method and hydrothermal method,

 but it needs to introduce manganese source, which can be independent manganese source or ferromanganese precursor. 

The performance comparison of different synthesis methods is not yet clear and further research is needed.

The three major problems LiMnFePO4 is currently facing include: processing difficulties. 

Due to small particles and high carbon content, the slurry viscosity is high, easy to gel, and the core moisture is difficult to bake; 

The lifespan attenuation caused by Mn leaching is manifested as loss of active lithium and deposition of ferromanganese, 

as well as gas production and voltage plateau attenuation at high temperatures; 

And the sudden increase in DCR and charge discharge asymmetry caused by the 0.4V pressure difference of two valence changing elements (Mn and Fe). 

In terms of lifespan decay, different LiMnFePO4 materials exhibit different behaviors. 

Some stabilize in the manganese region while the iron region decays during cycling, while others do the opposite. 

This is closely related to the synthesis process and characteristics of the material. 

Through the small current recovery experiment in the later stage of the cycle, it was found that the manganese region platform can be basically restored, 

while the iron region platform attenuates significantly. 

At the same time, the pressure difference increases in the later stage of the cycle, and the maximum value shifts towards low SOC, indicating that the loss of active lithium is the main cause of attenuation. 

After disassembling the battery in the later stage of the cycle, 

it was observed that there was severe manganese and iron deposition in the negative electrode, and the discharge voltage and pressure difference rebounded, 

reflecting the increase in battery polarization and internal resistance.

The manganese iron lithium in the later stage of the cycle was disassembled and the electrode pieces were taken out for separate half cell research. 

It can be seen that the manganese iron lithium positive electrode can be completely restored and basically remains unchanged. 

On the contrary, the negative electrode cannot be charged or discharged, which proves that its capacity decay is in the negative electrode, not in the positive electrode. 

As the cycle progresses, its pressure difference will gradually increase, from around 0.2V to above 0.4V, and the maximum pressure difference will also shift towards low SOC.

Based on the above phenomena, we have some basic conclusions. 

We believe that the direct contribution of manganese dissolution to capacity loss is minimal, and most manganese platforms can be restored during EOL. 

On the contrary, the loss of active lithium is the main factor. 

Manganese dissolves and transfers to the negative electrode, catalyzing the continuous growth, consumption, 

and thickening of the SEI film, resulting in a significant loss of active capacity.

We have conducted research on the DCR problem of lithium manganese iron, 

and found that its DCR in the manganese region is higher than that in the iron region, with obvious DCR protrusions in the transition zone. 

We have conducted a lot of research on manganese iron lithium and found that all proportions of manganese iron lithium have obvious DCR protrusions in the transition zone. 

At the same time, we conducted GITT analysis and found a significant polarization increase in the iron manganese transition zone in the GITT curve, 

which is consistent with the DCR trend

The difference in charge and discharge characteristics between lithium iron 

and lithium manganese iron mainly stems from their crystal structure and the diffusion mechanism of lithium ions in them. 

The diffusion of lithium ions in the lean lithium phase of iron lithium is easy, while the diffusion of lithium ions in the rich lithium phase is difficult, 

which makes it easier for lithium ions to enter the material during charging, but relatively difficult during discharging. 

On the contrary, manganese iron lithium has difficulty in diffusing lithium ions in the lean lithium phase and is easier to diffuse in the rich lithium phase. 

Therefore, lithium ions are more likely to leave the material during discharge, while it is more difficult during charging.

As the cycle progresses, the proportion and duration of the constant voltage (CV) segment of the manganese iron lithium battery increase, 

which usually leads to a decrease in battery life. 

Professor He's research divides the charging and discharging process of manganese iron lithium into four reaction zones, each corresponding to different SOC and reaction characteristics, 

which helps to understand the behavior of the material under different charging and discharging states.

During the charging process, lithium manganese iron tends to undergo two-phase reactions, while during discharge, it tends to undergo single-phase reactions. 

This difference explains its characteristics of difficult charging and easy discharging. 

The uneven distribution of elements in manganese iron may form micro regions rich in manganese or iron, which can affect the stability of the voltage platform. 

Different processes, ratios, and doping can all affect the distribution of ferromanganese.

In order to find the optimal manganese iron ratio, a series of computational studies were conducted to calculate the leaching energy and diffusion energy barrier of manganese.

 Research has shown that in different manganese iron ratios, the dissolution energy of the 020 crystal plane is the highest and relatively more stable. 

In the ratio of manganese 5 to iron 3, the diffusion energy barrier is the lowest, 

and the diffusion coefficient and conductivity are both high, making it an ideal manganese iron ratio. 

These computational studies help optimize the performance of lithium manganese iron and guide future material design and preparation.

For LMFP with different manganese iron ratios, the diffusion coefficient in the iron region is basically the same, 

while the diffusion coefficient in the manganese region varies with different Mn ratios. 

At the same time, there is a clear maximum diffusion coefficient in the iron manganese transition region.

We also did some band calculations, and in fact, for pure lithium iron, its Fermi level is located in the valence band, and its contribution mainly comes from iron. 

As the manganese content increases, its Fermi level gradually leaves the valence band and enters the bandgap. 

Its contribution comes not only from iron, but also from manganese, and even from oxygen. 

With the increase of manganese, its oxygen evolution energy will be lower. 

We can see that the oxygen evolution energy of pure lithium manganese phosphate is only about one-third of that of pure lithium iron. 

This is also why we have found that the safety performance of lithium manganese iron is weaker than that of lithium iron in some safety experiments.

The results of the manganese iron ratio experiment show that when the iron manganese ratio is 5:5, its cycling is better than 6:4. 

At the same time, the deposition of iron manganese lithium at 5:5 on the negative electrode manganese iron is better than 6:4. 

This conflicts with previous calculations and should be related to its particle morphology and crystallization. 

At the same time, we deliberately selected materials with different proportions of 020 crystal planes for comparison, and indeed,

 the higher the proportion of 020, the better the cycling performance.

By reducing the material ratio table and optimizing the carbon source, the proportion of CV segment capacity can be reduced, 

making it more stable with cycling and improving high-temperature cycling performance. 

By improving the quality of carbon coating, reducing free carbon, and lowering powder resistivity, 

its cycling performance can be effectively improved. In addition, the oxide coating method can also stabilize the interface and improve cycling performance.

By reducing the material ratio table and optimizing the carbon source, there has been a significant improvement in the proportion of CV segment capacity, 

and high-temperature cycling has been significantly enhanced.