Lithium-ion secondary batteries (LIBs) are the mainstream energy storage devices in the field of new energy today. Lithium iron phosphate (LiFePO4) has become one of the most widely used cathode materials for lithium-ion batteries due to its high energy density, low cost, stable charging and discharging platform, environmental friendliness, and high safety.

　　As a manufacturer with nearly 20 years of experience in lithium battery customization services, SES Power has been paying attention to the development of the lithium battery industry, especially the development of lithium battery cathode materials. Because the cathode material is the core of almost all raw materials of lithium batteries. When SES Power first made a lithium battery pack in 2008, it used the 18650 model of lithium iron phosphate battery with a capacity of only 1200mah. By 2021, the capacity of this model can reach 2000mah, which is almost doubled. times. Our energy storage products, such as 12V100Ah, 12V200Ah, 24V100Ah, 24V200Ah, 36V100Ah, 48V50Ah, 48V100Ah, and the home energy storage system HESS (3KW inverter output) using square aluminum lithium batteries, were unimaginable ten years ago. A single cell reaches 100Ah or even 280Ah. Similarly, SES Power's custom lithium-ion batteries can now be used in -60 degrees and -40 degrees Celsius. These products were almost unimaginable before.

　　With the development of the new energy industry, customers have put forward higher requirements for the electrochemical performance of batteries such as cycle stability, high-rate performance, and energy density. How to improve its output power, energy density and service life at low temperature is the main challenge for lithium iron phosphate cathode materials. According to the research of SES Power, element doping improves the electrochemical performance of the material, the protection mechanism of different capping agents for lithium iron phosphate, and the high-capacity lithium-replenishing material. These three methods can effectively improve the electronic conductivity and ionic conductivity of lithium iron phosphate cathode materials. Diffusion rate to achieve higher energy density, longer cycle life and higher rate capability of the material.

　　(Crystal structure of LiFePO4 viewed from the Li+ one-dimensional diffusion channel)

　　A: Challenges faced by LiFePO4

　　LiFePO4 material has stable charging and discharging platform, high specific capacity, good safety, less self-discharge, low cost and no pollution to the environment. However, LiFePO4 is limited by its own crystal structure, poor conductivity and slow ion migration rate, which greatly limit the improvement of its electrochemical performance. In a sub-zero temperature environment, the energy density of LiFePO4 batteries decreases, shortening the battery life and increasing the operating cost of the battery system.

　　B: LiFePO4 improvement method

　　Due to the low ion diffusion rate and poor electrical conductivity of LiFePO4, it has a great impact on the rate capability and low temperature performance of LiFePO4. Therefore, how to improve the ion diffusion rate and electrical conductivity of materials, so as to further effectively improve the rate performance and low temperature performance, has always been the focus of researchers.

　　b.1 Element doping modification

　　Element doping is considered to be an important method to improve the internal electronic conductivity and ionic diffusivity of LiFePO4 materials. This method can improve the charge-discharge performance of LiFePO4 material at high current density.

　　The electrochemical data suggest that a moderate amount of Na doping can improve the electrochemical performance. Li0.99Na0.01FePO4 exhibits excellent rate performance and cycling stability, with an initial discharge specific capacity of 80.9 mA h/g at a current density of 10 C and a capacity retention rate of 86.7% after 500 cycles.

　　A small amount of Mn doping can improve the electrochemical performance of the material, but too much doping will cause Li/Fe inversion defects and even destroy the material structure. Although LiFePO4 doped with Mn element has better electrical conductivity, its structural stability is relatively poor. With the increase of Mn doping amount, in addition to the decrease in rate performance, the dissolution of Mn2+ will change the structure of the cathode material and reduce the discharge capacity.

　　Two-element doping is a good method, and Li site doping and Fe site doping are usually the most studied.

　　Multi-ion co-doping mainly incorporates two or more metal elements into the LiFePO4 structure, and integrates the advantages of each doped metal ion to improve the electrochemical performance.

　　All in all, element doping can improve the internal conductivity of LiFePO4 material particles and accelerate the diffusion of lithium ions, and is still the mainstream choice for optimizing material properties.

　　b.2 Material cladding

　　To prepare LiFePO4 with excellent electrochemical performance, only doping is not enough. The conductivity of LiFePO4 is extremely poor. By coating the surface of the material with a suitable conductive/ion-conducting material, the electronic and ionic conduction between the particles of LiFePO4 can be improved.

　　The types of coating agents mainly include carbon materials, metal or metal oxide materials, and ion conductive materials. Among them, coating LiFePO4 material with conductive material is an important measure to improve its rate and low temperature performance, and carbon material is the simplest and cheapest kind of excellent material.

　　Graphene has the advantages of high electrical conductivity and porous structure, and it is also a good direction. The improved LiFePO4 with graphene has excellent cycling and rate performance. The discharge specific capacity was 160 mA h/g at 0.2 C rate, 107 mA h/g at 60 C high rate, and the capacity retention rate over 2000 cycles was 95%.

　　Metal or metal oxide and carbon composite metal material coating is also feasible, the battery conductivity is significantly improved, and the tap density is also improved. However, the metal coating has an oxidation problem, and the metals introduced are generally precious metals, which are not suitable for mass production.

　　Ion conducting materials can also be used to improve LiFePO4 cathode materials.

　　b.3 Adding Lithium Supplementary Materials

　　During the first charging process of LiFePO4 batteries, due to the formation of a solid electrolyte interface (SEI) on the surface of the negative electrode, about 5% to 20% of the lithium in the positive electrode material is consumed, resulting in excessive irreversible capacity loss. We can add lithium supplementary material to the lithium iron phosphate positive electrode material. During the charging process of the battery, the lithium supplementary material decomposes and releases excess lithium to compensate for the irreversible lithium loss caused by the formation of the SEI film on the negative electrode. Lithium supplementation materials usually have the characteristics of strong lithium supplementation ability, easy synthesis, strong stability and low cost. Common lithium iron phosphate cathode supplementary materials include Li2O, LiF, Li3N and Li2S.

　　C: Summary and Outlook

　　After nearly 30 years of development, LiFePO4, as a commercialized cathode material, still has many aspects worthy of further research. In the future, the research on LiFePO4 cathode materials can focus on the following aspects.

　　(1) Industrialized production improvement. The cathode material of LiFePO4 has been industrially produced, but there are disadvantages such as high energy consumption and poor rate performance of the produced material, and there is still a lot of room for improvement.

　　(2) The combination of various improvement strategies can further improve the rate performance of LiFePO4, and realize the ultra-fast charging characteristics and excellent low-temperature long-cycle performance of the material.

　　(3) Further in-depth theoretical research, to study the thermodynamic and kinetic transformation process of LiFePO4 material in the process of charging and discharging.

　　(4) Develop solid-state electrodes. Combining the LiFePO4 cathode material with a solid electrolyte produces a new type of lithium battery that is safer and can be used in flexible wearable electronic devices.

　　SES Power believes that LiFePO4, a cathode material for lithium ion batteries, has broad application and development prospects in the commercial field. It is believed that through the joint efforts of researchers, the further performance improvement of LiFePO4 cathode material can maximize people's production and life needs.