The Storage Life Of LiFePO₄ Battery At Full State Of Charge

Lithium ion battery was considered to be the ideal power source for electric vehicles and military equipments. LiFePO4, as a cathode material with the advantages of excellent cycle performance and high security, had wide application prospect in batteries of civilian and military equipment. While in the process of storage or use, the life of the battery was the key to its range of application. Especially in some disposable military equipment, the storage life was a decisive factor in its application. Aiming at this problem, this paper mainly studied the storage life of Li FePO4 battery and its aging mechanism at full SOC, also reseached the corresponding ways to extend storage life。First, the aging mechanism was investigated and mathematical model was built based on the aging mechanism. Then, through the use of A-XXX cell with LiFePO4 as cathode material for full SOC storage experiments, the aging mathematical model was validated, and storage life was predicted. Also, storage experiments and life predict of the Li-rich LiFePO4 battery was carrying on. And at the same time, the storage life of the battery was prolonged by adding electrolyte additives.According to the analysis of the aging process of the battery, the capacity fading was link with the Li loss in the anode. On the assumption that:(1) the battery had the same aging mechanism during the experiment process, the main reaction of the anode Li loss was LixC(or metal Li) generating into Li+.(2) The reaction is a slow one, and its rate can be regarded as only concerned with Li+.(3) Li loss and capacity fading met the type of constant current discharge reaction, the battery capacity loss and the storage time and temperature met the equation: Q=exp(A1-B1/T) × t ρ.During the storage process of A-XXX secondary battery, the crystal structure of cathode and anode material remained unchanged. Impedance analysis indicated that the impedance changed mainly due to changes in the electrode SEI film. Through the mathematical model, the average life A-XXX batteries was about 3.5 years.The structure of Li-rich LiFePO4 battery was different from A-XXX LiFePO4 battery. After charge, a layer of metal Li was formed on the anode surface of Li-rich LiFePO4 battery. And during its storage, the main reason for the capacity fade of battery was the continuous losses of metal Li. As a result of less activation, the formation of SEI of Li-rich LiFePO4 battery was incomplete, making the impedance changes divided into fast stabilization and slow growth during storage. Fitting the data into the model, the average life of Li-rich LiFePO4 battery was about 2.4 years.VC, FEC and VTAS electrolyte analogues can produce active groups in the electrolyte, which lead to reaction with Li or self polymerization, affecting the composition and stability of SEI film. The VC can effectively improve the storage life, and the best proportion of VC is 0.5%, the average storage life of the modified Li-rich battery was extended to 7-8 years.