With the use of lithium batteries, battery performance may deteriorate over time, mainly showing decreased capacity, increased internal resistance, and decreased power. The change in battery internal resistance is affected by various usage conditions such as temperature and depth of discharge.
Resistance is the resistance that lithium batteries encounter when current flows through the battery during operation. Typically, the internal resistance of a lithium battery is divided into ohmic resistance and polarization resistance. Ohmic resistance consists of electrode materials, electrolytes, membrane resistance, and contact resistance of various parts. Polarization resistance refers to the resistance caused by polarization during electrochemical reactions, including electrochemical polarization resistance and concentration polarization resistance. The ohmic resistance of a battery is determined by the total conductivity of the battery, and the polarization resistance of a battery is determined by the solid-phase diffusion coefficient of lithium ions in the electrode active material.
The effect of temperature on the size of internal resistance of lithium batteries is evident. The lower the temperature, the slower the ion transport inside the lithium battery, and the larger the internal resistance of the battery. Battery impedance can be divided into bulk impedance, SEI film impedance, and charge transfer impedance. The bulk impedance and SEI film impedance are mainly affected by the ion conductivity of the electrolyte, and their change trend at low temperatures is consistent with the trend of the electrolyte conductivity. Compared with the increase in bulk impedance and SEI film impedance at low temperatures, the increase in charge reaction impedance with decreasing temperature is more significant. Below -20℃, the charge reaction impedance accounts for almost 100% of the total internal resistance of the battery.
When lithium batteries are at different SOC, the size of their internal resistance also varies, especially the DC internal resistance directly affects the battery's power performance, reflecting the battery's performance in actual state: the DC internal resistance of lithium batteries increases with the increase of DOD of the battery. The internal resistance size is basically unchanged in the discharge range of 10% to 80%, and generally increases significantly at deeper discharge depths.
As the storage time of lithium-ion batteries increases, the battery continues to age, and its internal resistance continues to increase. Different types of lithium batteries have different degrees of internal resistance. After long-term storage from September to October, the rate of increase in internal resistance of LFP batteries is higher than that of NCA and NCM batteries. The rate of increase in internal resistance is related to storage time, storage temperature, and storage SOC.
Whether it is storage or cycling, the effect of temperature on the internal resistance of lithium batteries is consistent. The higher the cycling temperature, the greater the rate of increase in internal resistance. Different cycling intervals also have different effects on the battery's internal resistance. Battery internal resistance increases faster as the charge and discharge depth increases, and the increase in internal resistance is proportional to the increase in charge and discharge depth. In addition to the effect of charge and discharge depth during cycling, the upper limit of the charging cutoff voltage also has an effect: a too low or too high upper limit voltage will increase the interface impedance of the electrode, and a too low upper limit voltage cannot form a good passivation film, while a too high voltage limit will cause the electrolyte to oxidize and decompose on the surface of the LiFePO4 electrode, forming a product with low conductivity.