Lithium iron manganese phosphate cathode material chemical structure, preparation process, performance advantages, application direction
Aug,16,24
Lithium iron phosphate market share has increased significantly, but energy density faces a bottleneck in the short term.
The current power lithium battery cathode materials present lithium iron phosphate and ternary two major routes to fight the situation.
Thanks to the progress of battery technology to make up for the energy density,
as well as high prices of lithium carbonate on the economic demands of the enhancement, since 2021,
lithium iron phosphate battery by virtue of the absolute advantage in terms of safety and cost,
the installed share of all the way up, to achieve the ternary cathode of the overtaking, in 2022, the installed share reached 62.4%.
However, at present, the actual energy density of lithium iron phosphate battery is close to the upper limit of theoretical energy density,
and in the future of the car end of the range requirements gradually improved,
restricting the lithium iron phosphate anode penetration rate of further enhancement.
Through manganese doping, lithium manganese iron phosphate achieves higher operating voltage and energy density.
Lithium manganese iron phosphate is a new type of cathode material obtained by doping
a certain proportion of manganese elements on the basis of lithium iron phosphate.
Lithium manganese iron phosphate and lithium iron phosphate have the same theoretical gram capacity,
but the theoretical voltage platform of lithium iron phosphate is about 3.4-3.5V,
and the voltage platform is increased to 3.8-4.1V after manganese doping.
According to the energy density = gram capacity × working voltage,
the theoretical energy density of lithium manganese iron phosphate can be increased by 10-20% compared with lithium iron phosphate,
which is close to the level of 5-series ternary.
LMFP Chemical Structure
Compared to ternary materials, LMFP has a more stable structure, higher cycle times and lower cost.
Lithium manganese iron phosphate (LMFP), like lithium iron phosphate (LFePO4), has an olivine-type structure,
which is more stable than ternary materials with a layered structure,
and therefore retains LFePO4's excellent safety performance and higher cycle life than ternary materials.
In addition, due to avoiding the use of nickel and cobalt precious metals in ternary materials,
the cost of LiFeMnPo is much lower than that of ternary.
Advantages of LMFP
The energy density of LMFP is 20% higher than LFP, and the safety is higher than that of Saintyear.
LMFP (chemical formula LiFexMn1-xPO4) is the solid solution of LiFePO4
(chemical formula LiFePO4, abbreviated as LFP) and LiMnPO4 (chemical formula LiMnPO4, abbreviated as LMP),
and the crystal structure of LMFP and LFP is an ordered perovskite structure,
and the migration of lithium ions through the channels in the structure has a high degree of stability.
Both LMFP and LFP have an ordered olivine crystal structure, and lithium ions migrate through the channels in the structure,
which is highly safe and chemically stable.
The theoretical specific capacity of LMFP and LFP is 170mAh/g,
and the theoretical energy density of LMFP is 20% higher than that of LFP due to its higher voltage plateau,
which is able to break through the bottleneck of energy density faced by LFP at present to a certain extent.
Compared with ternary materials, LMFP has similar energy density as ternary five-system materials,
while being safer, cheaper and more environmentally friendly.
As one of the internal components of LMFP, LMP has the advantages of high energy density, high safety and stability,
but the defects of electrochemical performance obviously lead to the obstruction of application.
LMP has a theoretical voltage of 4.1V, which is 0.7V higher than that of LFP's 3.4V, and the theoretical energy density of LMP is 697Wh/kg,
which is about 578Wh/kg higher than that of LFP, based on the measurement of similar specific capacity of discharge and compact density.
he theoretical energy density of LMP is 697Wh/kg, which is about 20% higher than that of LFP's 578Wh/kg.
However, the electrical conductivity and cycling performance of LMP are extremely poor,
which results in its actual specific capacity and multiplication performance being far inferior to that of LFP, as shown in the following:
(1) The electronic conductivity and ion diffusion coefficient of LMP are very low, which makes it difficult to utilise the capacity of the material;
(2) LMP will have side reactions with electrolyte, generating Li4P2O7 and other products,
and some manganese ions will be dissolved in the electrolyte by disproportionation reaction, which reduces the cycling performance;
(3) The manganese phosphate after delithiation will be affected by the Jahn-Teller effect, resulting in distortion of crystal structure and loss of capacity.
LMP and LFP have the same crystal structure and can dissolve each other to form LMFP solid solution with arbitrary ratio,
which has the advantages of both high voltage and performance.
The low voltage of LFP leads to the limited space to improve the energy density,
and the technology of mutual doping modification of transition metal phosphates
has been widely researched with reference to the design idea of ternary materials.
LMP and LFP have the same crystal structure and can dissolve each other to form LMFP solid solution with arbitrary ratio.
LMP and LFP have the same crystal structure and can form LMFP solid solution by mutual solubility in any ratio.
Several studies have proved that iron doping can improve the electrochemical activity of manganese in LMP,
thus improving the discharge specific capacity, multiplicity performance and cycling performance of the material,
while the high voltage of LMP can improve the energy density of the material.
In the actual charging and discharging process, unlike the single voltage plateau of LFP,
there are two voltage plateaus in LMFP, corresponding to the 4.1V voltage formed by manganese redox
and the 3.4V voltage formed by iron redox,
and the first voltage plateau in the discharging process is the 4.1V voltage plateau, reflecting the embedding process of lithium ions in LMP.
After the lithium embedding in LMP is completed, the voltage plateau will drop to 3.4V, reflecting the lithium ion embedding process of LFP.
Significant effect of Mn-Fe ratio on material performance
The introduction of manganese enhances the voltage plateau of the material, but also brings other problems:
1) the presence of manganese makes it difficult for lithium ions to de-embed and move,
and the electronic conductivity and lithium ion mobility are lower, which affects the capacity and multiplicity performance of the material;
2) manganese ions have the John-Teller effect,
which leads to irreversible changes in the crystal structure during the process of charging and discharging,
and at the same time, manganese ions precipitate and deposit on the surface of the anode,
which will cause significant impacts on the material's performance.
At the same time, the precipitation of manganese ions on the surface of the negative electrode will damage the SEI film,
so the specific capacity of the material is low and the decay is rapid, and the cycling performance is affected to a certain extent;
3) the voltage platforms of manganese and iron are inconsistent,
which leads to a double-voltage plateau in the discharging, which may bring about the instability of the output power.
Therefore, it is very important to choose a suitable manganese doping ratio for the electrochemical performance of LiMnFePO4 materials:
if the ratio of manganese to iron is too low, it will not be able to obtain the effect of improving the voltage plateau and energy density;
if the ratio of manganese to iron is too high, the multiplicity performance of the material and the cycling performance will be affected adversely.
At present, in the research and application practice, the ratio of manganese to iron ranges from 2:8 to 8:2.
Determining the optimal manganese to iron ratio and achieving better consistency and stability of the material
on this basis has also become one of the difficulties for
the lithium manganese iron phosphate (LMFP) anode to move towards the mass production and commercialisation.
LMFP Production Process
LMFP production process and modification technology has been accumulated through R&D in recent years,
which can greatly overcome the defects of LMFP electrochemical performance.
LMFP is a member of the family of phosphate-based anode materials developed by Goodenough's group in 1997;
in 2012, the Dow Chemical Company in the United States claimed to have developed
a new lithium manganese oxide material (LMFP) with an energy density in the range of 150+ Wh/kg,
which is higher than that of LFP. /kg range, an increase of 10% to 15% over LFP materials;
BYD made a large R&D investment and attempted to mass produce it from 2013,
but due to its low yield rate and the policy pointing to energy density, the development of LMFP was put on hold;
in 2014, Hongse Technology (now ICP Materials) realised the mass production of LMFP,
and in the same period, DeFang Nano, Tianjin Strand and others also had small batch production. Starting from 2017,
Ningde Times, ATL, Guoxuan Gaoke, De Fang Nano, LiTai Li-energy and other companies have carried
out a large number of technology and patent reserves,
and the defects of low conductivity, low cycling performance
and low multiplication performance that constrain the application of LMFPs have been greatly improved
with the support of process innovations and modification technologies.
The preparation process of LMFP is similar to that of LFP, which mainly consists of solid-phase method and liquid-phase method.
Solid-phase method is divided into high-temperature solid-phase reaction method and carbothermal reduction method,
which has the advantages of simple equipment and process, low cost, and suitable for industrial production,
and the disadvantages of solid-phase inhomogeneity,
difficult to control the crystalline form of the product and the size of the particles, and poor consistency.
Liquid-phase methods are divided into hydrothermal synthesis, gel sol method and co-precipitation method,
the advantage is that it can make the mixing of raw materials at the molecular level more homogeneous,
and the size and morphology of the product can be controlled,
the disadvantage is that the process is complicated, and it needs high temperature and high pressure resistant reaction equipment, the cost is high,
and the difficulty of large-scale production is large.
Methods to improve the electrochemical activity of LMFP include carbon coating, material nanosizing, metal ion doping,
and compounding with other materials, etc.
The electronic conductivity and ion mobility of LMFP are very low, which directly limits its development and application,
while the advancement of the modification technology can effectively improve its electrochemical activity,
and increase its specific capacity and cycle life.
At present, the main modification principles of LMFP include reducing primary particle size,
improving material cleanliness and elemental composition uniformity,
and coating doped materials with better conductivity to reduce resistance.
LMFP Application Direction
LMFP is mainly used in power batteries, which can broaden the application scope of phosphate anode materials.
Due to the advantages in energy density and low-temperature performance compared with lithium iron phosphate,
lithium manganese iron phosphate can meet the needs of 600km+ high range models and high latitude areas,
and broaden the application scenarios of phosphate anode materials in power batteries.
However, due to the short board of low cycle life, LiMnFePO4 is currently more difficult to be applied in the field of energy storage.
Blending with ternary materials can achieve complementary advantages and improve the performance of diversified applications.
Lithium manganese iron phosphate has a large specific surface area and a small particle size,
so it can be filled in the gap of ternary material or attached to its surface, forming a composite material system.
At the same time, due to their similar voltage platform, mixing can solve the problem of lithium manganese iron phosphate dual-voltage platform.
The use of composite material battery will have both high energy density,
high power characteristics of ternary and lithium manganese iron phosphate high safety, low cost advantages.
The cost of lithium manganese iron phosphate is slightly higher than lithium iron phosphate, and much lower than three yuan.
The cost difference between LiMnFePO4 and LiFePO4 mainly comes from the addition of manganese source,
a certain proportion of iron source can be replaced by manganese source to measure the cost of LiMnFePO4.
Taking the ratio of manganese to iron 8:2 and German nano liquid phase process as an example,
according to the calculation of raw material price on 30th June 2023,
the direct material cost of lithium manganese iron phosphate is about 86,500 yuan per tonne,
which is 3.6% higher than that of lithium iron phosphate, and the direct material cost of medium
and low nickel ternary cathode is higher than 20.0 million yuan per tonne.
The increase in performance is greater than the increase in cost, and lithium manganese iron phosphate is a more cost-effective anode route.
Compared with ternary, the energy density of lithium manganese iron phosphate is similar to that of low and medium nickel ternary,
while the cost is significantly lower. Compared with lithium iron phosphate, the energy density of lithium manganese iron phosphate is more than 10%,
while the cost of raw materials per ton is only less than 4% higher,
the performance improvement is much larger than the cost increase,
so the cost of a single watt-hour of the battery end should be lower than that of lithium iron phosphate after the maturity of mass production.
Battery cost reduction demand exists for a long time, lithium manganese iron phosphate industrialisation power is strong.
2023 Since the beginning of the year, vehicle manufacturers price cuts and promotions are frequent,
led by BYD new energy car enterprises have played ‘oil and electricity at the same price’ strategy,
the new model pricing to the fuel car on par. In the long run,
‘oil and electricity at the same price’ is an electric car to replace the fuel car must go through,
the industrial chain to reduce the demand for the continued existence of the battery
as the core of the cost of the vehicle will inevitably bear the greatest pressure to reduce costs.
Therefore, for the comprehensive cost-effective lithium manganese iron phosphate,
the battery factory has a strong willingness to switch to lithium manganese iron phosphate industrialisation of the core driving force.
