Factors affecting self discharge of lithium batteries

Factors affecting self discharge of lithium batteries
The self discharge reaction of lithium-ion batteries is inevitable, which not only reduces the capacity of the battery itself, but also seriously affects the assembly and cycle life of the battery. The self discharge rate of lithium-ion batteries is generally 2% to 5% per month, which can fully meet the usage requirements of individual batteries. However, once individual lithium batteries are assembled into modules, their characteristics are not completely consistent. Therefore, after each charge and discharge, the terminal voltage of each individual lithium battery cannot reach complete consistency, resulting in overcharging or overdischarging of individual batteries in the lithium battery module, and the performance of individual lithium batteries will deteriorate. As the number of charge and discharge cycles increases, the degree of deterioration will further intensify, and the cycle life will significantly decrease compared to the unassembled individual battery. Therefore, in-depth research on the self discharge rate of lithium-ion batteries is an urgent need for battery production.
1、 The self discharge phenomenon of a battery refers to the spontaneous loss of its capacity when the battery is in an open circuit state, also known as the charge holding capacity. Self discharge can generally be divided into two types: reversible self discharge and irreversible self discharge. The reversible self discharge can compensate for the loss of capacity, and its principle is similar to the normal discharge reaction of a battery. The self discharge that cannot be compensated for the loss of capacity is irreversible self discharge, mainly due to irreversible reactions occurring inside the battery, including reactions between the positive electrode and the electrolyte, reactions between the negative electrode and the electrolyte, reactions caused by impurities in the electrolyte, and irreversible reactions caused by micro short circuits caused by impurities carried during production. The influencing factors of self discharge are described below. The main impact of positive electrode materials is the precipitation of transition metals and impurities in the negative electrode, which leads to internal short circuits and increases the self discharge of lithium batteries. Yah Mei Teng et al. investigated the physical and electrochemical properties of two LiFePO4 cathode materials. Research has found that batteries with high levels of iron impurities in raw materials and during charging and discharging processes have high self discharge rates and poor stability. The reason is that iron gradually reduces and precipitates in the negative electrode, pierces the separator, and causes a short circuit inside the battery, resulting in high self discharge. The impact of negative electrode materials on self discharge is mainly due to the irreversible reaction between the negative electrode material and the electrolyte. As early as 2003, Aurbach et al. proposed that the electrolyte is reduced to release gas, exposing the surface of graphite to the electrolyte. During the charging and discharging process, the graphite layered structure is easily damaged when lithium ions are inserted and removed, resulting in a higher self discharge rate. The influence of electrolyte mainly manifests as: corrosion of the negative electrode surface by electrolyte or impurities; The dissolution of electrode materials in electrolyte; The electrode is covered by insoluble solids or gases decomposed by the electrolyte, forming a passivation layer, etc. Currently, a large number of researchers are dedicated to developing new additives to suppress the impact of electrolytes on self discharge. Jun Liu et al. found that adding additives such as VEC to the electrolyte of MCN111 battery improved the high-temperature cycling performance of the battery, but the self discharge rate generally decreased. The reason is that these additives can improve the SEI film, thereby protecting the negative electrode of the battery. The general influencing factors of storage state are storage temperature and battery SOC. Generally speaking, the higher the temperature, the higher the SOC, and the greater the self discharge of the battery. Takashi et al. conducted capacity decay experiments on lithium iron phosphate batteries under static conditions. The results show that as the temperature increases, the capacity retention rate gradually decreases with the idle time, and the self discharge rate of the battery increases. Liu Yunjian et al. used commercialized lithium manganese oxide power batteries and found that as the battery’s state of charge increased, the relative potential of the positive electrode became higher and its oxidation became stronger; The relative potential of the negative electrode is decreasing, and its reducibility is also increasing. Both can accelerate the precipitation of Mn, leading to an increase in self discharge rate. There are many other factors that affect the self discharge rate of batteries. In addition to the ones introduced above, there are mainly the following aspects: burrs generated during the cutting of electrode pieces during the production process, impurities introduced into the battery due to production environment issues, such as dust, metal powder on electrode pieces, etc., all of which may cause internal micro short circuits in the battery; The external environment is humid, the insulation of the external circuit is incomplete, and the isolation of the battery casing is poor, resulting in external electronic circuits during battery storage, which leads to self discharge; During long-term storage, the adhesion between the active substance of the electrode material and the current collector fails, leading to the detachment and peeling of the active substance, resulting in a decrease in capacity and an increase in self discharge. Each of the above factors or a combination of multiple factors can cause self discharge behavior in lithium batteries, which makes it difficult to identify the causes of self discharge and estimate the storage performance of the battery.