Lithium ion battery is a type of secondary battery (rechargeable battery) that mainly relies on the insertion and extraction of Li+between two electrodes. With the continuous development of downstream industries such as new energy vehicles, the production scale of lithium-ion batteries is constantly expanding. So, what are the general usage conditions that affect lithium-ion batteries?
1. Temperature
The influence of temperature on the internal resistance is obvious. The lower the temperature, the slower the ion transport inside the battery, and the greater the internal resistance of the battery. Battery impedance can be divided into bulk impedance, SEI film impedance, and charge transfer impedance. Bulk impedance and SEI film impedance are mainly affected by the ion conductivity of the electrolyte, and their variation trend at low temperatures is consistent with the variation trend of electrolyte conductivity.
Compared to the increase in bulk impedance and SEI film resistance at low temperatures, the charge reaction impedance increases more significantly with decreasing temperature. Below -20 ℃, the proportion of charge reaction impedance to the total internal resistance of the battery reaches almost 100%.
2. SOC (State of Charge)
When the battery is at different SOC, its internal resistance also varies, especially the DC internal resistance directly affects the power performance of the battery, which in turn reflects the battery performance in the actual state: the DC internal resistance of lithium batteries increases with the increase of the battery discharge depth DOD, and the internal resistance remains basically unchanged in the discharge range of 10%~80%. Generally, the internal resistance increases significantly at deeper discharge depths.
3. Storage
As the storage time of lithium-ion batteries increases, the batteries continue to age and their internal resistance increases. The degree of internal resistance variation varies among different types of lithium batteries. After a long period of storage from September to October, the internal resistance increase rate of LFP batteries is higher than that of NCA and NCM batteries. The increase rate of internal resistance is related to storage time, storage temperature, and storage SOC
4. Cycle
Whether it is storage or cycling, the effect of temperature on battery internal resistance is consistent. The higher the cycling temperature, the greater the rate of increase in internal resistance. And different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery accelerates with the increase of charge and discharge depth, and the increase in internal resistance is proportional to the strengthening of charge and discharge depth.
In addition to the influence of charging and discharging depth in the cycle, the charging cut-off voltage also has an impact: too low or too high a charging voltage upper limit will increase the interface impedance of the electrode, too low an upper limit voltage cannot form a passivation film well, and too high a voltage upper limit will cause the electrolyte to oxidize and decompose on the surface of LiFePO4 electrode, forming products with low conductivity.