The use of lithium-ion batteries is limited in low-temperature environments, as their discharge capacity will severely decline and they cannot be charged at low temperatures. During low-temperature charging, the insertion of lithium ions on the graphite electrode of the battery and the lithium plating reaction coexist and compete with each other. Under low temperature conditions, the diffusion of lithium ions in graphite is inhibited, and the conductivity of the electrolyte decreases, leading to a decrease in insertion rate. On the surface of graphite, the lithium plating reaction is more likely to occur.
Research has shown that a battery with a capacity of 3500mAh, if operated in an environment of -10 ℃, after less than 100 charging and discharging cycles, will experience a sharp decline in battery capacity to 500mAh, and is basically scrapped. That is to say, in a working environment of -10 ℃, if an electric vehicle is charged and discharged once a day, the battery will have to be scrapped and replaced with a new one after three months.
The reasons that affect the low-temperature performance of lithium iron phosphate batteries:
1. Positive electrode structure
The three-dimensional structure of the positive electrode material restricts the diffusion rate of lithium iron phosphate batteries, especially at low temperatures. Different positive electrode materials have different three-dimensional structures. Currently, important positive electrode materials used in lithium-ion batteries for electric vehicles are lithium iron phosphate, nickel cobalt manganese ternary materials, and lithium manganese oxide. The discharge capacity of lithium iron phosphate batteries can only reach 67.38% of room temperature capacity at -20 ℃, while nickel cobalt manganese ternary batteries can reach 70.1%.
2. High melting point solvent
Due to the presence of high melting point solvents in the mixed solvent of electrolyte, the viscosity of lithium-ion battery electrolyte increases at low temperatures. When the temperature is too low, electrolyte solidification occurs, leading to a decrease in the transmission rate of lithium ions in the electrolyte.
3. Lithium ion diffusion rate
The diffusion rate of lithium ions in graphite negative electrodes decreases under low temperature conditions. The increase in charge transfer impedance of lithium-ion batteries in low-temperature environments leads to a decrease in the diffusion rate of lithium ions in the graphite negative electrode, which is an important reason affecting the low-temperature performance of lithium iron phosphate batteries.
4. SEI membrane
In low-temperature environments, the SEI film on the negative electrode of lithium iron phosphate batteries thickens, and the impedance of the SEI film increases, leading to a decrease in the conduction rate of lithium ions in the SEI film. Ultimately, the polarization formed during charging and discharging in low-temperature environments reduces the efficiency of charging and discharging.
5. Production environment
As a high-tech product with numerous chemical raw materials and complex processes, lithium iron phosphate batteries have high requirements for temperature, humidity, dust, and other factors in their production environment. If not properly controlled, battery quality will fluctuate.
Summary: Currently, multiple factors affect the low-temperature performance of lithium iron phosphate batteries, such as the structure of the positive electrode, the migration rate of lithium ions in various parts of the battery, the thickness and chemical composition of the SEI film, and the selection of lithium salts and solvents in the electrolyte. The low-temperature performance limits the application of lithium-ion batteries in the field of electric vehicles, special fields, and extreme environments. Developing lithium-ion batteries with excellent low-temperature performance is an urgent demand in the market.
