The self-discharge phenomenon of a battery refers to the phenomenon of spontaneous loss of its capacity when the battery is in an open circuit, which is also known as the charge retention capability. In general, self-discharge can be divided into reversible self-discharge and irreversible self-discharge. The reversible self-discharge can be reversibly compensated by the loss capacity, and the principle is similar to the normal discharge reaction of the battery. Irreversible self-discharge refers to the loss of capacity that cannot be compensated. The main reason is the irreversible reaction inside the battery, including the reaction between the positive electrode and the electrolyte, the reaction between the negative electrode and the electrolyte, the reaction caused by the impurity in the electrolyte, and the irreversible reaction caused by the micro-short circuits due to the carried impurities. The influencing factors of self-discharge are described as follows.
The influence of the cathode material of li ion batteries is mainly due to the internal short circuit caused by the precipitation of transition metal and impurities in the cathode material, which will increase the self-discharge of li ion batteries. According to the research, batteries with high content of iron impurities in the raw materials and during charging and discharging have a high self-discharge rate but poor stability. The reason is that iron is gradually reduced and precipitated in the anode electrode and pierced the diaphragm, which results in a short circuit in the battery and leads to a higher self-discharge.
The influence of the anode material on the self-discharge is mainly due to the irreversible reaction between the anode material and the electrolyte. As early as 2003, it was proposed that the graphite surface could be exposed to the electrolyte by reducing the electrolyte and releasing the gas. During the charging and discharging process, the lamellar structure of graphite is easy to be destroyed when lithium ions are inserted and extracted, which results in a higher self-discharge rate of li ion batteries.
The influence of the electrolyte is mainly reflected as the corrosion of the electrolyte or impurities on the surface of the anode electrode, the dissolution of the electrode material in the electrolyte, the electrode is covered by the insoluble solid or gas decomposed by the electrolyte, forming a passivation layer, etc. Currently, a large number of researchers are working on developing new additives to suppress the influence of electrolytes on self-discharge. The high-temperature cycle performance of the battery can be improved while the self-discharge rate generally is decreased by adding additives such as VEC to the NCM111 battery’s electrolyte. The reason is that these additives can improve the SEI film to protect the anode electrode of the battery.
The general influencing factors of the storage state of li ion batteries are storage temperature and battery SOC. In general, the higher the temperature, the higher the SOC and the greater the self-discharge of the battery. The capacity attenuation experiment of li ion phosphate battery in static condition shows that the capacity retention rate will decrease and the self-discharge rate will increase with the increase of temperature.
There are many factors that affect the self-discharge rate of li ion batteries. In addition to the mentioned factors, there are mainly the following aspects. The burrs generated when the pole pieces are cut during the production process and impurities such as dust and metal power on the electrode piece introduced into the battery due to production environmental problems all can cause the internal micro short circuit of the battery. Problems such as humid environment, incomplete insulation of external wiring, poor isolation of battery shell will cause external electronic circuits when the battery is stored, which will lead to self-discharge. The bonding between the active material of the electrode material and the current collector will fail during long-term storage, which will cause the shedding and stripping of active materials and lead to an increase in self-discharge.
Each of the above factors or a combination of multiple factors can cause the self-discharge of li ion batteries, which makes it difficult to find the cause of the self-discharge and estimate the storage performance of batteries.