The positive electrode of a lithium iron phosphate battery is composed of LiFePO4 with a olivine structure, the negative electrode is composed of graphite, and the middle is a polyolefin PP/PE/PP separator, which is used to isolate the positive and negative electrodes, prevent electrons, and allow lithium ions to pass through.
During charging, lithium ions are deintercaled from the positive electrode through the electrolyte and enter the negative electrode. At the same time, electrons move from the positive electrode to the negative electrode in the external circuit to ensure charge balance between the positive and negative electrodes. During discharge, lithium ions are deintercaled from the negative electrode and embedded into the positive electrode through the electrolyte. This microstructure provides a good voltage platform and longer service life for lithium iron phosphate batteries: during the charging and discharging process of the battery, the positive electrode transitions between the rhombic LiFePO4 and hexagonal FePO4 phases. Due to the coexistence of FePO4 and LiFePO4 in solid and molten form below 200 ℃, there is no obvious two-phase turning point during the charging and discharging process. Therefore, the charging and discharging voltage platform of lithium iron phosphate batteries is long and stable; In addition, after the charging process is completed, the volume of the positive electrode FePO4 is only reduced by 6.81% compared to LiFePO4, while the volume of the carbon negative electrode slightly expands during the charging process, which plays a role in regulating volume changes and supporting internal structure. Therefore, lithium iron phosphate batteries exhibit good cycling stability during charging and discharging, and have a long cycling life.
Advantages of lithium iron phosphate batteries
Lithium iron phosphate batteries have won the trust of many manufacturers due to their low price and strong safety. The positive electrode material accounts for over 40% of the total cost of lithium-ion batteries, and under current technological conditions, the energy density of the overall battery mainly depends on the positive electrode material. Therefore, the positive electrode material is the core material for the development and research of lithium-ion batteries. Currently, mature positive electrode materials include lithium cobalt oxide, nickel cobalt manganese oxide, lithium iron phosphate, and lithium manganese oxide.
Lithium cobalt oxide
There are layered structures and spinel structures, commonly used layered structures with a theoretical capacity of around 270mAh/g. Layered lithium cobalt oxide is mainly used in digital products such as mobile phones, aircraft models, car models, electronic cigarettes, and smart wearables. The energy density and compaction density of lithium cobalt oxide have basically reached their limits, and there is still a lot of room for improvement in its specific capacity compared to the theoretical capacity. However, due to the limitations of the current overall chemical system, especially the easy decomposition of the electrolyte in high voltage systems, further methods to increase specific capacity by increasing the charging cutoff voltage are limited. Once electrolyte technology is breakthrough in the future, there is still room for improvement in its energy density.
Lithium nickel cobalt manganese oxide
It generally has the advantages of green environmental protection, low cost (equivalent to only 2/3 of lithium cobalt oxide), good safety (safe working temperature can reach 170 ℃), and long cycle life (extended by 45%). The current research on monocrystalline nickel cobalt manganese lithium mainly aims to further improve the energy density of products by continuously increasing the nickel content and charging cutoff voltage. However, this puts higher requirements on the technical capabilities of electrolyte and related supporting materials, as well as lithium-ion battery manufacturers.
Lithium manganese oxide
There are spinel structures and layered structures, and spinel structures are commonly used. The theoretical capacity is 148 mAh/g, and the actual capacity is between 100-120 mAh/g. It has the characteristics of good capacity utilization, stable structure, superior low-temperature performance, and low cost. However, its crystal structure is prone to distortion, causing capacity decay and short cycle life. Mainly used in markets with high safety and cost requirements, but low energy density and cycle requirements. Such as small communication devices, power banks, electric tools and bicycles, and special scenarios (such as coal mines).
Lithium iron phosphate
Generally, it has a stable olivine skeleton structure, with a discharge capacity of over 95% of the theoretical discharge capacity, excellent safety performance, good resistance to overcharging, long cycle life, and low price. But its energy density limitation is difficult to solve, while electric vehicle users are constantly increasing their range requirements.
Lithium iron phosphate cathode material exhibits good thermal stability, safety and reliability, and low carbon environmental friendliness when applied, making it the preferred cathode material for large battery modules. However, the stacking density of lithium iron phosphate cathode materials is low, the volume energy density is not high, and the application range is limited. In response to the application limitations of lithium iron phosphate cathode materials, relevant personnel can improve the conductivity of this material by doping high valent metal cations and coating conductive materials on the surface. After a period of development, lithium iron phosphate cathode materials have gradually matured and been widely used in various fields, such as electric vehicles, electric bicycles, mobile power equipment, energy storage power supply, etc.