Why do lithium-ion batteries self-discharge and how to measure self-discharge?

The self-discharge reaction of lithium-ion batteries is inevitable, and its existence not only leads to the reduction of the battery's own capacity, but also seriously affects the battery's 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 for the use of single batteries.

lithium-ion batteries self-discharge

However, once a single lithium battery is assembled into a module, because the characteristics of each single lithium battery are not completely the same, after each charge and discharge, the terminal voltage of each single lithium battery cannot be completely consistent, which will cause problems in the lithium battery. The performance of the single lithium battery will be deteriorated if there is an overcharged or overdischarged single battery in the module. As the number of charging and discharging increases, the degree of deterioration will be further aggravated, and the cycle life will be greatly reduced compared to unmatched single cells. Therefore, in-depth research on the self-discharge rate of lithium-ion batteries is an urgent need for battery production.

1. Influencing factors of self-discharge

The self-discharge phenomenon of a battery refers to the phenomenon that its capacity is spontaneously lost when the battery is left in an open circuit, and is also called the charge retention capability. Self-discharge can generally be divided into two types: reversible self-discharge and irreversible self-discharge. The loss of capacity can be reversibly compensated for reversible self-discharge, the principle of which is similar to the normal discharge reaction of a battery. The self-discharge whose loss of capacity cannot be compensated is irreversible self-discharge. 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 impurities in the electrolyte, and the Irreversible reactions caused by micro-short circuits caused by carried impurities, etc. The influencing factors of self-discharge are as follows.

1) Cathode material

The influence of the positive electrode material is mainly that the transition metal and impurities of the positive electrode material precipitate in the negative electrode and cause an internal short circuit, thereby increasing the self-discharge of the lithium battery. Yah-Mei Teng et al. studied the physical and electrochemical properties of two LiFePO4 cathode materials. The study found that batteries with high iron impurities in the raw materials and during the charging and discharging process have high self-discharge rate and poor stability. The reason is that iron is gradually reduced and precipitated in the negative electrode, piercing the diaphragm, causing a short circuit in the battery, resulting in high self-discharge .

2) Anode material

The influence of the negative electrode material on the 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 part of the graphite surface to the electrolyte. During the charging and discharging process, when lithium ions are inserted and extracted, the graphite layered structure is easily destroyed, which leads to a higher self-discharge rate.

3) Electrolyte

The influence of the electrolyte is mainly manifested as: the corrosion of the electrolyte or impurities on the surface of the negative electrode; the dissolution of electrode materials in the electrolyte; the electrode is covered by insoluble solids or gases decomposed by the electrolyte to form a passivation layer. At present, a large number of scientific researchers are committed to developing new additives to suppress the influence of electrolyte on self-discharge. Jun Liu and others added VEC and other additives to the electrolyte of the NCM111 battery, and found that the high temperature cycle performance of the battery improved, and 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.

4) Storage status

The general influencing factors of storage status 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 shelf time, and the battery self-discharge rate increases.

Liu Yunjian and others used commercial lithium manganese oxide power batteries and found that as the state of charge of the battery increases, the relative potential of the positive electrode becomes higher and higher, and its oxidizing property becomes stronger; the relative potential of the negative electrode becomes lower and lower. Its reducibility is getting stronger and stronger, and both can accelerate the precipitation of Mn, leading to an increase in the self-discharge rate.

5) Other factors

There are many factors that affect the self-discharge rate of the battery. In addition to the ones introduced above, there are mainly the following aspects: during the production process, the burrs generated when the pole pieces are cut, and the impurities introduced into the battery due to production environmental problems, such as Dust, metal powder on the pole piece, etc., all of which may cause the internal micro short circuit of the battery; the external electronic circuit caused by the humidity of the external environment, the incomplete insulation of the external circuit, and the poor isolation of the battery casing, etc., will cause self-discharge; During long-term storage, the bonding between the active material of the electrode material and the current collector fails, causing the active material to fall off and peel off, which leads to a decrease in capacity and an increase in self-discharge. Each of the above factors or a combination of multiple 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.

2. Measuring method of self-discharge rate

Through the above analysis, it can be known that the self-discharge rate of lithium batteries is generally low. The self-discharge rate itself is affected by factors such as temperature, cycle times, and SOC. Therefore, it is very difficult and time-consuming to accurately measure the self-discharge of the battery.

A). Traditional measurement method of self-discharge rate

At present, the traditional self-discharge detection methods are as follows:

a). Direct measurement method

First charge the tested cell to a certain state of charge, and maintain it for a period of open-circuit shelving, and then discharge the cell to determine the capacity loss of the cell. Self-discharge rate:

lithium-ion batteries self-discharge

In the formula: C is the rated capacity of the battery; C1 is the discharge capacity. After the open circuit is left, the remaining capacity of the battery can be obtained by discharging the battery. At this time, perform multiple charge and discharge cycles on the battery again to determine the full capacity of the electric garlic at this time. This method can determine the irreversible capacity loss and reversible capacity loss of the battery。

b). Open circuit voltage attenuation rate measurement method

The open circuit voltage is directly related to the SOC of the battery state of charge. It is only necessary to measure the rate of change of the battery's OCV over a period of time, namely:

This method is simple to operate, and only needs to record the voltage of the battery in any time interval, and then the state of charge of the battery at that time can be obtained according to the corresponding relationship between the voltage and the battery SOC. Through the calculation of the attenuation slope of the voltage and the corresponding attenuation capacity per unit time, the self-discharge rate of the battery can be finally obtained.

c). Capacity retention method

Measure the battery's expected open circuit voltage or the amount of power required by the SOC to obtain the battery's self-discharge rate. That is to measure the charging current while maintaining the open circuit voltage of the battery, and the battery self-discharge rate can be regarded as the measured charging current.

B). Fast measurement method of self-discharge rate

Because the traditional measurement method requires a long time and the measurement accuracy is insufficient, the self-discharge rate is only used as a method to screen whether the battery is qualified in most cases in the battery testing process. The emergence of a large number of novel and convenient measurement methods has saved a lot of time and energy for the measurement of battery self-discharge.

a). Digital control technology

Digital control technology is a new type of self-discharge measurement method derived from the traditional self-discharge measurement method using single-chip microcomputers. This method has the advantages of short measurement time, high accuracy, and simple equipment.

b). Equivalent circuit method

The equivalent circuit method is a brand-new self-discharge measurement method, which simulates the battery as an equivalent circuit, which can quickly and effectively measure the self-discharge rate of lithium-ion batteries。

3. the significance of measuring the self-discharge rate

As an important performance index of lithium-ion batteries, self-discharge rate has an important influence on the selection and configuration of batteries. Therefore, measuring the self-discharge rate of lithium batteries has far-reaching significance.

1). Predict problem cells

The same batch of batteries, the materials used and the manufacturing controls are basically the same. When the white discharge of individual batteries is obviously too large, the reason is likely to be a serious micro-short circuit caused by the internal impurities and burrs piercing the diaphragm. Because the impact of micro-short circuit on the battery is slow and irreversible. Therefore, in the short term, the performance of this type of battery will not be too different from that of a normal battery. However, as the internal irreversible reaction gradually deepens after being left for a long time, the performance of the battery will be far lower than its factory performance and other normal battery performance. Therefore, in order to ensure the quality of the factory battery, the battery with large self-discharge must be eliminated.

2). Assemble the battery

Lithium batteries need better consistency, including capacity, voltage, internal resistance, and white discharge rate. The self-discharge rate of the battery affects the battery pack mainly as follows: once assembled into a module, because the self-discharge rate of each single lithium battery is different, the voltage will drop to varying degrees during the shelving or cycling process, and the battery will be charged in series. The current will be equal again, so after each charge, overcharged or undercharged single cells may appear in the lithium battery module. As the number of charging and discharging increases, the battery performance will gradually deteriorate, and the cycle life will be similar. Compared with the unmatched single battery, it is significantly lower. Therefore, the battery pack requires accurate measurement and screening of the self-discharge rate of lithium-ion batteries.

3). Battery SOC estimation and correction

The state of charge is also called the remaining power, which represents the ratio of the remaining capacity after the battery has been used for a period of time or left unused for a long time to its fully charged state, and is often expressed as a percentage. The self-discharge rate has important reference value for the SOC estimation of lithium-ion batteries. The correction of the initial SOC value by the self-discharge current can improve the accuracy of SOC estimation. On the one hand, customers can estimate the usable time or driving distance of the product based on the remaining power; on the other hand, improving the SOC prediction accuracy of BMS can effectively prevent battery overcharge. Over-discharge, thereby extending battery life.

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