Study On The Influence Factors And Mechanisms Of Li-ion Battery During High Temperature Storage

A lithium-ion battery, as the green auto power source, has attracted widely attentions in global scale. Generally, we except the life of the lithium-ion battery used on electronic vehicles could reach up to ten years. On one hand, the battery might be used in low-temperature, high-temperature and high-humidity extreme environment. The temperature can reach 70-80℃ when the car stored in the sunshine of summer. Lithium-ion battery using at this high-temperature face sever challenges. On the other hand, the power source battery was constructed by packing the cells in series and in parallel. The temperatures of cells were different and the self-discharge rates were also different.This will decrease the uniformity of the battery cell in power source, and finally affect the performance. Lithium-ion battery have advantage on environmental protection and energy density. Therefore, the thorough research on the influence factors and mechanisms of lithium-ion battery during high temperature storge is urgently needed.Based on the studies about storage time, storage voltage, storage temperature effects during lithium-ion battery high temperature storage.We found that storage time is the key factor to LiCoO2 and LiNiO2 batteries. When increasing the storage temperature, the performance degradation became most significant. For LiMn2O4 and LiFePO4 batteries, the key factor is the storage voltage. When the storage voltage is too low or too high, the performance degradation is serious.Through the study of the lithium-ion battery temperature storage performance and mechanisms of internal factors, we found that:1) Based on thickness swelling and capacity loss during high temperature storage, the LiFeP04 battery is the best one, the following are LiCo02 and LiMn2O4 batteries, the worst is NCA battery.2) Fully charged NCA battery during 85℃high temperature storage, cell thickness swelling is caused mainly by the cathode gas, the main component are CO2 and CO.3) Fully charged LiCoO2 battery during 85℃high temperature, the capacity loss is about 38%, the reversible capacity loss is 27%, the irreversible part is 11%. We test the EIS and polarization by using a three-electrode, RSEI and Rct of positive and negative are significantly increased, especially the positive. The battery (ohms+electrochemical) polarization and concentration polarization are significantly increased, and the concentration polarization amplification is much larger than (ohms+electrochemical) polarization.4) Capacity loss is partly caused by positive polarization increased and battery polarization increased. For LiCoO2 battery, cathode impedance increased sevenfold, the separator polarization is increased and its porosity is decreased to 45%. Dissembled the battery after high temperature storage, we found the separator near the cathode became yellow. Using the SEM we can see that the hole is blocked, which can cause the capacity loss.5) Irreversible capacity loss during storage is caused by the negative SEI film re-formation. The lithium salt LiPF6 is strong Lewis acid, if there is trace moisture in battery, it will react with the lithium salt and destroy the SEI film. The SEI reformation will consume the active Li+ and result in the irreversible capacity loss.Based on the studies about performance and mechanisms of the lithium-ion battery storage at high temperatures, we propose the improvement plan about high storage performance:i) Selecting the suitable solvent electrolyte additive to reduce reaction between the electrolyte solvent and positive;ii) Selecting the appropriate additives to enhance the stability of the anode surface SEI film, reducing the irreversible capacity loss.