Influencing factors and measurement methods of lithium battery self-discharge - 1 views

The self-discharge reaction of lithium-ion batteries is inevitable. Their existence not only leads to a reduction in the capacity of the battery itself, but also seriously affects the battery's 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.

Influencing factors of 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-shorts caused by carried impurities. 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 charge and discharge have 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 the 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 scientific 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 electrolyte of the MCN111 battery, and found that the high-temperature cycle performance of the battery 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 standing time, and the battery self-discharge rate increases.

Liu Yunjian and others used commercial lithium manganate power batteries and found that as the battery's state of charge increases, the relative potential of the positive electrode becomes higher and higher, and its oxidizability becomes stronger; Its reducibility is also getting 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 piece is cut, the impurities introduced into the battery due to the production environment problems, such as Dust, metal powder on the pole pieces, etc., may cause internal micro short circuits of the battery; the external environment is humid, the external circuit is not completely insulated, and the battery housing is poorly isolated. 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.

The measurement 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, so 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:

● 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, perform multiple charge and discharge cycles on the cell 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.

● 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, only need to record the voltage of the battery in any time interval, and then according to the correspondence between the voltage and the battery SOC, the state of charge of the battery can be obtained at this time. 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.

● 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 whether the battery is qualified during the battery detection process. 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.

● Digital control technology

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

● 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.