The impact of recycling in low temperature environment on the life of lithium iron phosphate battery

Lithium iron phosphate batteries are favored in the new energy industry. The battery cycle life can reach about 3000 times, and the discharge is stable. It is widely used in power batteries and energy storage fields. However, the speed of its promotion and the breadth and depth of its application are not satisfactory. In addition to the price and batch consistency caused by the battery material itself, the factors hindering its rapid promotion, its temperature performance is also an important factor. 1. Lithium iron phosphate battery cell (module) room temperature cycle summary The cycle life of the room temperature test battery can be seen, the long life advantage of the lithium iron phosphate battery, currently 3314 cycles, the capacity retention rate is still 90%, and To reach 80% of the end of life may be about 4000 times. 1. The lithium iron phosphate battery cell cycle test has been completed: 3314cyc, with a capacity retention rate of 90%. The inconsistency of the battery has been formed after the PACK is completed. The more sophisticated the process, the smaller the internal resistance of the group, and the smaller the difference between the cells. However, when the assembled battery pack is used, it will not only be affected by the difference of the battery cell itself, but also affected by environmental factors, such as the heat temperature inside and outside the battery pack. In this way, the BMS is required for the battery pack during use. Perform regular equalization to reduce the difference between the cells and prolong the service life. 2. The module cycle test of the assembled lithium iron phosphate battery pack has been completed: 2834cyc, and the capacity retention rate is 67.26%. 2. Lithium iron phosphate battery The cycle summary of the monomer in a high temperature environment accelerates the aging of the battery and shortens the service life under high temperature conditions. The high and low temperature performance of lithium iron phosphate batteries is much worse than that of ternary lithium batteries and polymer lithium batteries. Low-temperature performance is also the biggest bottleneck encountered by lithium iron phosphate batteries. 1. Under high temperature environment, the charging and discharging curve of lithium iron phosphate battery monomer 2. Cycle test of lithium iron phosphate battery under high temperature environment The high temperature cycle completed 1100 cyc, and the capacity retention rate was 73.8%. 3. Low temperature affects the charge and discharge performance of lithium batteries. The discharge capacity of the battery at a temperature of 0～-20℃ is equivalent to 88.05%, 65.52% and 38.88% of the discharge capacity at 25℃; the average discharge voltage is 3.134 and 2.963 V respectively. And 2.788 V, the average discharge voltage at 20°C is 0.431 V lower than that at 25°C. From the above analysis, it can be seen that as the temperature decreases, the average discharge voltage and discharge capacity of the lithium battery decrease, especially when the temperature is -20°C, the discharge capacity and average discharge voltage of the battery decrease faster. Figure 1 The discharge curve of lithium iron phosphate battery at different temperatures is analyzed from an electrochemical point of view. The solution resistance and SEI film resistance change little over the entire temperature range, and have little effect on the battery's low-temperature performance; the charge transfer resistance is significant with the decrease of temperature Increase, and the change with temperature in the entire temperature range is significantly greater than the solution resistance and the SEI film resistance. This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, and the SEI film resistance and the electrochemical reaction resistance increase, resulting in an increase in ohmic polarization, concentration polarization and electrochemical polarization at low temperatures. Large, the average voltage and discharge capacity of the battery are shown on the discharge curve as the temperature decreases. Fig. 2 After charging and discharging the battery 5 times at low temperature, it can be seen from Fig. 2 that if the battery is cycled at -20°C for 5 times and then cycled at 25°C, the capacity and discharge platform of the battery are reduced. This is because as the temperature decreases, the ionic conductivity of the electrolyte decreases, and the ohmic polarization, concentration polarization and electrochemical polarization in the low-temperature charging process increase, which leads to the deposition of metal lithium, which decomposes the electrolyte, and finally leads to The thickening of the SEI film on the electrode surface and the increase of the resistance of the SEI film appear on the discharge curve as the discharge plateau and the discharge capacity decrease. 4. The influence of low temperature environment on the cycle performance of lithium iron phosphate battery Figure 3 Li-ion battery 0.5C rate cycle curve at room temperature Figure 4 Li-ion battery 0.5C rate cycle curve at -10°C It can be seen from the figure that the battery is in The capacity decays rapidly in an environment of -10°C. After 100 cycles, the capacity is only 59mAh/g, and the capacity decays 47.8%. The battery that has been discharged at low temperature will be charged and discharged at room temperature to investigate the capacity recovery performance during the period. Its capacity was restored to 70.8mAh/g, with a capacity loss of 68%. It can be seen that the low-temperature cycle of the battery has a greater impact on the recovery of the battery capacity. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous: Features and precautions of lithium battery PACK