Preface: This paper describes the effects of cathode material, cathode material, electrolyte and storage environment on the self-discharge rate of lithium-ion batteries. At the same time, it introduces the measurement methods of traditional lithium battery self-discharge rate and new self-discharge rate rapid measurement methods. From Guoxuan High-tech engineers, welcome to exchange and share!
The self-discharge reaction of Li-ion battery is inevitable, and its existence not only leads to the reduction of the battery capacity, but also seriously affects the battery configuration and cycle life. The self-discharge rate of lithium-ion batteries is generally 2% to 5% per month, which can fully meet the requirements of the use of single cells.
However, once the single lithium battery is assembled into a module, because the characteristics of each single lithium battery is not completely consistent, so each time after charging and discharging, the terminal voltage of each single lithium battery can not be completely consistent, which will appear in the lithium battery module overcharge or over discharge of the single battery, the single lithium battery performance will deteriorate. With the increase in the number of charge and discharge, the deterioration will be further aggravated, and the cycle life will be significantly reduced compared with the unpaired single cell. Therefore, an in-depth study of the self-discharge rate of lithium-ion batteries is an urgent need for battery production.
First, the impact of self-discharge factors
Self-discharge phenomenon of the battery is the phenomenon of spontaneous loss of capacity when the battery is in open-circuit shelving, also known as charge retention capacity. Self-discharge can generally be divided into two types: reversible self-discharge and irreversible self-discharge. The lost capacity can be reversibly compensated for reversible self-discharge, the principle of which is similar to the normal battery discharge reaction. The self-discharge that cannot be compensated for is irreversible self-discharge, which is mainly due to 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 impurities in the electrolyte, and the irreversible reaction caused by the micro-short circuit caused by impurities carried by the battery when it is made. The factors affecting self-discharge are described below.
1 Anode material
The influence of cathode material is mainly due to the precipitation of transition metals and impurities in the cathode, which leads to internal short circuit and thus increases the self-discharge of Li-ion battery, Yah-Mei Teng et al. studied the physical and electrochemical properties of two LiFePO4 cathode materials. It was found that the batteries with high content of iron impurities in the raw material and during charging and discharging have high self-discharge rate and poor stability due to the gradual reduction and precipitation of iron in the negative electrode, which punctures the diaphragm and leads to internal short circuit, resulting in higher self-discharge.
2 cathode material
The influence of cathode material on self-discharge is mainly due to the irreversible reaction between cathode material and electrolyte. As early as 2003, Aurbach et al. proposed that the electrolyte was reduced and the gas was released, exposing part of the graphite surface to the electrolyte. During charging and discharging, the graphite layered structure is easily damaged when lithium ions are embedded and dislodged, resulting in a large self-discharge rate.
3 Electrolyte
The influence of electrolyte is mainly manifested as follows: corrosion of negative electrode surface by electrolyte or impurities; dissolution of electrode material in electrolyte; electrode covered by insoluble solids or gases decomposed by electrolyte, forming passivation layer, etc. At present, a large number of researchers are devoted to developing new additives to suppress the effect of electrolyte on self-discharge. Jun Liu et al. added additives such as VEC to the electrolyte of MCN111 battery and found that the high-temperature cycling performance of the battery improved and the self-discharge rate generally decreased. The reason for this is that these additives can improve the SEI film and thus protect the negative electrode of the battery.
4 Storage state
In general, the higher the temperature, the higher the SOC, and the higher the self-discharge of the battery, Takashi et al. conducted capacity decay experiments on lithium iron phosphate batteries under static conditions. The results showed that with the increase of temperature, the capacity retention rate gradually decreased with the shelving time and the self-discharge rate of the battery increased.
Liu Yunjian et al. used commercial lithium manganate power batteries and found that as the charge state of the battery increases, the relative potential of the positive electrode becomes higher and its oxidation becomes stronger; the relative potential of the negative electrode becomes lower and its reduction becomes stronger, both of which can accelerate the precipitation of Mn and lead to an increase in self-discharge rate.
5 Other factors
There are many factors affecting the self-discharge rate of the battery, in addition to the above-mentioned, there are also the following aspects: in the production process, the burr generated when slitting the pole piece, impurities introduced in the battery due to the production environment, such as dust, metal powder on the pole piece, etc., which may cause the internal micro-short circuit of the battery; the external environment is humid, external circuit insulation is not complete, poor isolation of the battery shell, etc. The battery storage with external electronic circuit, which leads to self-discharge; long time storage process, the active material of the electrode material and the bonding failure of the collector fluid, resulting in active material shedding and stripping, etc. lead to capacity reduction and self-discharge increase. Each of the above factors or a combination of factors can cause the self-discharge behavior of lithium batteries, which makes it difficult to find the cause of self-discharge and estimate the storage performance of the battery.
