Lithium ion batteries are widely used for their high specific energy, high working voltage, low self-discharge rate, and long lifespan. Whether used as power sources for electronic products or as energy storage power sources for satellites, they all face battery storage problems in practical use. Due to the self discharge during storage, lithium-ion batteries will cause battery performance degradation.

The storage performance of batteries is related to factors such as their state of charge (SOC), storage time, and storage temperature. According to literature reports, storing the battery at an open circuit voltage of 3.8-3.9V has excellent overall performance, but the storage temperature studied is room temperature and the storage time is only 3 months. This article investigates the performance of lithium-ion batteries stored at two different states of charge and storage temperatures, providing a theoretical reference for the storage conditions of lithium-ion batteries.

1 Test

Discharge the test battery to 2.75V, then charge it to different states of charge (50% and 80%), and store it at different temperatures. The battery is maintained every 6 months during storage. Five sample batteries were used for parallel experiments under each experimental condition, and the results were averaged. Use battery testing chamber and other equipment to test the recoverable capacity, self discharge rate, and cycling performance of batteries.

2 Test results

Store batteries with a state of charge of 50% and 80% at different temperatures for 18 months.

2.1 Recoverable capacity

Through testing, it can be seen that both 50% SOC and 80% SOC of the battery decrease in recoverable capacity with the extension of storage time; And at the same temperature, the difference between the recoverable capacity of the second storage period battery and the recoverable capacity of the first storage period battery should be less than the difference between the recoverable capacity of the first storage period battery and the capacity of the battery before storage; The difference between the recoverable capacity of the third storage period battery and the recoverable capacity of the second storage period battery should be less than the difference between the recoverable capacity of the second storage period battery and the recoverable capacity of the first storage period battery.

This indicates that the irreversible capacity loss of the battery is relatively large in the initial stage and gradually decreases in the later stage. The battery is stored at 10, 0, and -10 ℃, with 50% SOC batteries having a recoverable capacity of over 98%, and 80% SOC batteries having a recoverable capacity of over 97%; Under the same state of charge, when stored at 0 and -10 ℃, the recoverable capacity of the battery is basically the same, which is slightly higher than that stored at 10 ℃. Based on the above analysis, it can be concluded that when the battery is stored at 0 and -10 ℃, the 50% SOC battery has a higher recoverable capacity, both of which are above 98%.

2.2 Battery self discharge rate

The self discharging capacity of the battery decreases significantly in the initial stage and decreases in the later stage with the extension of storage time. From Figures 3 and 4, it can be seen that the self discharge rate of the battery is high in the initial stage, but decreases and tends to stabilize in the later stage. Regardless of the state of charge of the battery, the self discharge rate of the battery is below 8%, and as the storage time increases, the self discharge rate of the battery almost drops to below 2%, indicating that the self discharge rate of the battery significantly decreases after 12 months of storage.

Under the same state of charge, the self discharge rate of the battery stored at 0 and -10 ℃ is lower than that stored at 10 ℃. This is because the side reaction rate of the battery will intensify with the increase of temperature, and the self discharge rate will also increase with the increase of temperature. Low environmental temperature will suppress the side reaction of the battery, thus reducing the self discharge rate. Therefore, the lower temperature during the storage process of the battery is more conducive to its storage.

Through data comparison, it was found that at the same temperature, the self discharge rate of 50% SOC batteries is lower than that of 80% SOC batteries, indicating that 50% SOC is more likely to maintain battery performance. In summary, it can be seen that 50% SOC batteries are more conducive to maintaining battery performance under storage conditions of 0 and -10 ℃.

2.3 Battery cycling performance after storage

Cycle performance testing was conducted on batteries stored for 18 months. The battery cycle charging and discharging system was as follows: 0.5C constant current charging to 4.1V, constant voltage charging to current dropping to 0.01 C, and after standing still, 0.5C discharging to 2.75V.

The capacity degradation trend of the unstored battery is basically the same as that of the stored battery. After 400 cycles, the capacity retention rates of the new battery, 50% SOC battery, and 80% SOC battery are 90.1%, 89.0%, and 86.3%, respectively. It can be seen that the capacity of the stored battery has decreased compared to the unstored battery, and the capacity retention rate of the 50% SOC battery is higher than that of the 80% SOC battery.

3 Conclusion

By testing the recoverable capacity and self discharge rate of lithium-ion batteries under different states of charge and storage temperatures, it was found that storage temperature and state of charge are important factors affecting battery performance. The higher the storage environment temperature, the higher the state of charge, the smaller the recoverable capacity of the battery, and the higher the self discharge rate of the battery; The capacity decay of batteries during storage mainly occurs in the first 6 months, and thereafter, the capacity decay with storage time is minimal. The battery is stored at 50% SOC and stored at 0 and -10 ℃, maintaining good performance. Therefore, when lithium-ion batteries require long-term storage, they should be stored in a semi charged state and at low temperature.