Discussion and analysis of the failure of lithium iron phosphate (LiFePO4) battery

　　Understanding the cause or mechanism of failure of lithium iron phosphate batteries is particularly important for improving battery performance and its large-scale production and use.

　　1. Failure in the production process

　　In the production process of LiFePO4 batteries, personnel, equipment, raw materials, methods, and environment are the main factors that affect product quality. We mainly discuss the last three influencing factors.

　　The impurity in the electrode active material causes the battery to fail: a small amount of Fe2O3, Fe and other impurities will be reduced on the surface of the negative electrode, which may pierce the diaphragm and cause an internal short circuit. The higher the impurity content of the graphite anode, the greater the irreversible capacity loss.

　　The failure of the battery caused by the formation method: research has found that the formation temperature is too high will cause more irreversible lithium ion loss. Nowadays, the formation mode of low current constant current-high current constant current and constant voltage is used more frequently, which can effectively reduce the influence of formation temperature on battery performance.

　　The failure of the battery caused by moisture in the production environment: In actual production, the battery will inevitably come into contact with the air. Since the anode and cathode materials are mostly micron or nanometer particles, it is easy to absorb moisture in the air.

　　2. Failure in shelving

　　During the service life of the battery, it is in a state of shelving most of the time. Generally, after a long time of shelving, the performance of the battery will decrease, generally showing an increase in internal resistance, a decrease in voltage, and a decrease in discharge capacity. There are many factors that cause the degradation of battery performance, of which temperature, state of charge and time are the most obvious influencing factors.

　　In the shelving state, poor storage conditions (high temperature and high state of charge) will increase the degree of self-discharge of the LiFePO4 power battery, making the battery aging more obvious.

　　3. Failure in recycling

　　Batteries generally emit heat during use, so the influence of temperature is very important. In addition, usage, ambient temperature, etc. will have different effects on the battery.

　　The capacity loss of LiFePO4 power battery during cycling is generally considered to be caused by the loss of active lithium ions. As the discharge rate increases, the capacity loss of the positive electrode increases more than that of the negative electrode. The loss of battery capacity during low-rate cycling is mainly due to the consumption of active lithium ions in the negative electrode, while the power loss of the battery during high-rate cycling is due to the increase in the impedance of the positive electrode.

　　The discharge capacity of LiFePO4 battery will drop rapidly when the temperature drops. Under low temperature conditions, the battery cannot be charged or discharged with high current, otherwise it will cause irreversible capacity loss.

　　Fourth, failure during charging and discharging

　　The battery is often inevitably overcharged during use. Relatively speaking, there is less overdischarge. The heat released during overcharge or overdischarge is likely to accumulate inside the battery, which will further increase the battery temperature. , It affects the service life of the battery and increases the possibility of fire or explosion of the battery. Even under normal charging and discharging conditions, as the number of cycles increases, the capacity inconsistency of single cells in the battery system will increase.

　　Although the thermal stability of LiFePO4 is the best compared to other cathode materials under different charging conditions, overcharging can also cause unsafe hidden dangers in the use of LiFePO4 power batteries. One of the main reasons for battery failure under overcharged conditions is the internal short circuit caused by lithium crystal branches piercing the diaphragm.

　　Lithium-ion battery (LIB) has become the main energy storage solution in modern social life. Among them, lithium iron phosphate batteries are a perfect substitute for lead-acid batteries, and are the first choice for grid-connected peak shaving, off-grid energy storage, photovoltaic energy storage, UPS, data center and other industries.