What Causes Self-discharge of Lithium ion Batteries

The self-discharge reaction of the lithium ion battery is inevitable. Its existence not only leads to the reduction of the battery's own capacity, but also seriously affects the battery's assembly and cycle life. The self-discharge rate of lithium ion batteries is generally 2% to 5% per month.

Classification of self-discharge

Self-discharge can be generally divided into two types: reversible self-discharge and irreversible self-discharge.

The loss capacity can be compensated reversibly for reversible self-discharge, and the principle is similar to the normal battery discharge reaction. Irreversible self-discharge is the self-discharge cannot be compensated for the loss of capacity. The main reason is that an irreversible reaction has occurred inside the battery, including the reaction of the positive electrode with the electrolyte, the reaction of the negative electrode with the electrolyte, the reaction caused by impurities in the electrolyte, and the irreversible reactions caused by micro short circuits caused by impurities carried when manufacturing.

Factors affecting self-discharge

1. Physical micro-short circuit

The physical micro-short circuit is the direct cause of the low voltage of the lithium battery. Its direct performance is that the battery voltage is lower than the normal cut-off voltage after the battery is stored at normal temperature and high temperature for a period of time. Compared with self-discharge caused by chemical reaction, self-discharge caused by physical micro-short circuit will not cause irreversible loss of lithium battery capacity.

2. Chemical reaction

A. Moisture

Moisture causes the electrolyte to decompose and release a large number of electrons, then embedded in the positive electrode oxidation structure, causing the positive electrode potential to drop and cause low pressure. In addition, moisture is a catalyst in the entire battery, which generates a strong Lewis acid, thus constantly corroding the SEI film, consuming lithium source and causing low voltage of the battery.

B. Electrolyte solvent

The addition of certain electrolyte solvents will cause the voltage of the lithium battery to drop too quickly. Possible mechanism: these solvents are not resistant to oxidation, a slow chemical reaction occurs during storage, and the capacity is consumed to cause the voltage to drop.

C. SEI film is not well formed

During the storage, the warehouse has a certain temperature, which makes the SEI film to fall off and react again, causing flattening and low pressure of the lithium battery.

D. Poor packaging

Excessive sealing at the location of the tab may cause corrosion of the tab and consume the low pressure of the lithium source. At other locations, the electrolyte may corrode the aluminum foil through the CPP layer, causing perforation of the aluminum-plastic film and entering the water to cause low-pressure inflation.

3. Temperature

The higher the ambient temperature, the higher the activity of the electrochemical material of the lithium battery, and the secondary reactions of the positive electrode material, negative electrode material, and electrolyte of the lithium power battery will be more intense, causing more capacity loss in the same period. The chemical self-discharge of lithium batteries is more significant at high temperatures. It is more effective to use high-temperature storage to judge the self-discharge of lithium batteries.

4. SOC

By comparing the influence of SOC on self-discharge rate, the general trend is that the higher the SOC of lithium battery, the higher the self-discharge rate. That is, the higher the SOC of the lithium battery, the higher the positive electrode potential and the lower the negative electrode potential. In this way, the stronger the oxidizability of the positive electrode and the stronger the reducibility of the negative electrode, the more intense the side reaction.

Why you test the self-discharge?

1. Predict the problem cell.

In the same batch of batteries, the materials and manufacturing controls are basically the same. When the self-discharge of individual batteries is obviously too large, the reason is likely to be a serious micro short circuit due to impurities and burrs piercing the separator. Because the effect of micro-short circuit on the battery is slow and irreversible, in the short term, the performance of this type of battery will not be much different from that of a normal battery, but as the internal irreversible reaction gradually deepens after long-term storage, the performance of the battery will be far lower.

2. Assemble the batteries.

Since the self-discharge level is obviously different under different initial voltages, it is necessary to ensure that the primary voltage of the battery is within a small range.

3. Help formulate the battery factory voltage and factory capacity

Some customers have such requirements: no matter how much the battery's factory voltage and factory capacity are, it only requires the battery to be shipped to the customer with a capacity of 60%. At this time, it is necessary to evaluate the degree of self-discharge that the battery will generate during transportation to determine the factory voltage or capacity of the battery.

Misunderstanding of self-discharge

Self-discharge after charging: Some people said that the voltage of the lithium battery is dropped very fast after charging. They thought this is self-discharge too fast. The reason for this is the polarization of the lithium battery during the charging process, which causes the charging voltage to be higher than the actual battery voltage. The process of voltage drop after charging is the process of battery voltage drops from charging voltage to its own voltage.

And the result of the charging voltage-the actual voltage of the battery, called the overpotential, not what is called "virtual power", and there is no name of virtual power in electrochemical terms. Therefore, the voltage drop after charging is mainly the disappearance of overpotential, and the proportion of self-discharge in it is very very small and can be ignored.

In addition, the data shows that after charging, the voltage needs to be basically stable for at least 4h, and whether the charging ends with constant current or constant voltage, the difference in rest time is not very large.