Why Do Lithium-ion Batteries Self-discharge, How To Measure Self-discharge?

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, which can fully meet the use requirements of single cells.

However, once the single lithium battery is assembled into a module, the characteristics of each single lithium battery are not completely consistent, so after each charge and discharge, the terminal voltage of each single lithium battery cannot reach complete consistency, which will cause If overcharged or overdischarged cells appear in the module, the performance of the cell lithium battery will deteriorate. As the number of charging and discharging increases, the degree of deterioration will be further exacerbated, and the cycle life will be greatly reduced compared to unassembled single cells. Therefore, in-depth research on the self-discharge rate of lithium-ion batteries is an urgent need for battery production.

1

Factors affecting self-discharge

The self-discharge phenomenon of the battery refers to the phenomenon of spontaneous loss of the capacity of the battery when it is left in an open circuit, and is also called the charge retention capacity. 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, the principle is similar to the normal battery discharge reaction. The self-discharge that cannot be compensated for the loss of capacity is irreversible self-discharge. The main reason is that an irreversible reaction has occurred inside the battery, including the reaction of the positive electrode and the electrolyte, the reaction of the negative electrode and the electrolyte, the reaction caused by the impurities in the electrolyte, and the time of manufacture Irreversible reactions caused by micro short circuits caused by impurities carried. 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 are precipitated at the negative electrode to 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 impurity content in the raw materials and during charging and discharging have a high self-discharge rate and poor stability. The reason is that iron gradually reduces and precipitates at the negative electrode, pierces the separator, and causes a short circuit in the battery, resulting in higher self-discharge .

2 Anode material

The influence of the negative electrode material 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 be reduced to release gas, exposing the graphite surface to the electrolyte. In the process of charging and discharging, when lithium ions are inserted and extracted, the graphite layer structure is easily damaged, resulting in a large self-discharge rate.

3 Electrolyte

The influence of the electrolyte mainly includes: corrosion of the surface of the negative electrode by the electrolyte or impurities; dissolution of the electrode material in the electrolyte; the electrode is covered with an insoluble solid or gas decomposed by the electrolyte to form a passivation layer. At present, a large number of researchers are devoted to the development of new additives to suppress the influence of electrolyte on self-discharge. Jun Liu et al. added additives such as VEC to the NCM111 battery electrolyte, and found that the battery's high-temperature cycle performance improved and the self-discharge rate generally declined. The reason is that these additives can improve the SEI film, thereby protecting the battery negative electrode.

4 Storage status

The general influencing factors of storage status are storage temperature and battery SOC. In general, the higher the temperature and the higher the SOC, 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 a commercial lithium manganate power battery and found that as the battery's state of charge increased, the relative potential of the positive electrode became higher and higher, and its oxidizability became stronger and stronger; the relative potential of the negative electrode became lower and lower, Its reducibility is getting stronger and stronger, 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 environment problems, such as Dust, metal powder on the pole pieces, etc., may cause internal micro short circuit of the battery; the external environment is humid, the external circuit is not completely insulated, and the battery enclosure has poor isolation. There is an external electronic circuit during storage, which leads to self-discharge; During long-term storage, the bonding between the active material of the electrode material and the current collector fails, resulting in the shedding and peeling of the active material, 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 the lithium battery, which makes it difficult to find the cause of self-discharge and estimate the storage performance of the battery.

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Measuring method of self-discharge rate

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

1 Traditional measurement method of self-discharge rate

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

1.1 Direct measurement method

First, charge the battery under test to a certain state of charge, and keep it open for a period of time, then discharge the battery to determine the capacity loss of the battery. Self-discharge rate:

In the formula: C is the rated capacity of the battery; C1 is the discharge capacity. After leaving the circuit open, the remaining capacity of the battery can be obtained by discharging the battery. At this time, the battery is recharged and discharged several times to determine the full capacity of the battery. This method can determine the irreversible capacity loss and reversible capacity loss of the battery.

1.2 Open-circuit voltage attenuation rate measurement method

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

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

1.3 Capacity maintenance method

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

2 Self-discharge rate rapid measurement method

Since the traditional measurement method takes a long time and the measurement accuracy is insufficient, the self-discharge rate is only used as a method for screening the battery in the battery detection process in most cases. The emergence of a large number of novel and convenient new measurement methods saves a lot of time and effort for the measurement of battery self-discharge.

2.1 Digital control technology

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

2.2 Equivalent circuit method

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

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Significance of measuring self-discharge rate

As an important performance index of lithium ion batteries, self-discharge rate has an important influence on the selection and grouping of batteries. Therefore, it is of far-reaching significance to measure the self-discharge rate of lithium batteries.

1 Predict the problem cell

In the same batch of batteries, the materials and 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 due to impurities and burrs piercing the separator. Because the effect of micro-short circuit on the battery is slow and irreversible. Therefore, in the short term, the performance of such batteries will not be much different from that of normal batteries, but as the internal irreversible reaction gradually deepens after long-term storage, the battery performance will be far lower than its factory performance and other normal battery performance. Therefore, in order to ensure the quality of the factory batteries, batteries with large self-discharge must be eliminated.

2 Group the batteries

Lithium batteries need better consistency, including capacity, voltage, internal resistance, and white discharge rate. The impact of the battery's self-discharge rate on the battery pack is mainly as follows: once the module is assembled, due to the different self-discharge rates of the individual lithium batteries, the voltage will drop to varying degrees during the shelving or cycling process, and charge in series Under the current, the current will be equal again, so after each charge, there may be overcharged or undercharged single cells in the lithium battery module. As the number of charge and discharge increases, the battery performance will gradually deteriorate, and the cycle life Compared with unassembled single cells, it has dropped significantly. Therefore, battery assembly requires accurate measurement and screening of the self-discharge rate of lithium-ion batteries.

3 Correction of battery SOC estimation

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