The self-discharge reaction of lithium-ion batteries cannot be prevented. 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.
However, once a single lithium-ion battery is assembled into a module, because the characteristics of each single lithium-ion battery are not completely the same, after each charge and discharge, the terminal voltage of each single lithium-ion battery cannot be completely consistent, which will If overcharged or over-discharged single cells appear in the lithium-ion battery module, the performance of the single lithium-ion battery will deteriorate. As the number of charging and discharging increases, the degree of deterioration will be further intensified, and the cycle life will be greatly reduced compared with unmatched single cells. Therefore, in-depth research on the self-discharge rate of lithium-ion batteries is an urgent need for battery processing.
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 it 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, and its principle is similar to the normal discharge reaction of the battery. The self-discharge for which the loss of capacity cannot be compensated is irreversible self-discharge. The main reason is the irreversible reaction 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 impurity of the electrolyte, and the time of manufacture. 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 impact of the positive electrode material is 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-ion battery. Yah-MeiTeng et al. studied the physical and electrochemical properties of two LiFePO4 cathode materials. The study found that batteries with high content of 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 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 was reduced to release gas, exposing part of the graphite surface to the electrolyte. In the process of charging and discharging, when lithium ions are inserted and extracted, the graphite layered structure is easily destroyed, which leads to a higher self-discharge rate.
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 the electrode material in the electrolyte; the electrode is overturned by the insoluble solid or gas analyzed 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. JunLiu et al. added VEC and other additives to the MCN111 battery electrolyte, 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 tests on lithium iron phosphate batteries under static conditions. The results stated 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 lithium-ion battery and found that with the increase in the state of charge of the battery, the relative potential of the positive electrode is getting higher and higher, and its oxidizing property is getting stronger and stronger; the relative potential of the negative electrode is increasing. The lower the value, the stronger the reducibility, both of which 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 above analysis, there are also the following aspects: during the processing, the burrs that sprout when the pole pieces are cut, and the impurities introduced into the battery due to processing environmental issues, 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 environment is humid, the external circuit insulation is incomplete, and the battery casing is poorly insulated, etc., when the battery is stored, there is an external electronic circuit, which leads to 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-ion batteries, which makes it difficult to find the reason for self-discharge and estimate the storage performance of the battery.
Self-discharge rate measurement method
Through the above decomposition, it can be seen that the self-discharge rate of lithium-ion batteries is generally low. The self-discharge rate itself is affected by factors such as temperature, cycle times 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 test methods are as follows:
●Straight measurement method
First, charge the tested cell to a certain state of charge, and keep it open for a period of time, and then discharge the cell to determine the capacity loss of the cell. Self-discharge rate:
C is the rated capacity of the battery; C1 is the discharge capacity. After the open circuit is left aside, the remaining capacity of the battery can be obtained by discharging the battery cell. 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.
●Open circuit voltage attenuation rate measurement method
The open circuit voltage has a direct relationship with the SOC of the battery state of charge. As long as the rate of change of the battery's OCV over a period of time is measured, that is:
This method is simple to operate. It only needs to record the voltage of the battery in any time period, and then according to the corresponding relationship between the voltage and the battery SOC, the state of charge of the battery at that time can be obtained. 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.
●Capacity retention method
Measure the open circuit voltage that the battery expects to maintain 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.
2. Fast measurement method of self-discharge rate
Because the traditional measurement method takes a long time and the measurement accuracy is insufficient, the self-discharge rate is only used as a way to screen whether the battery is qualified in most cases in the battery test 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.
●Digital control technology
Digital control technology is a new type of self-discharge measurement method derived from the traditional self-discharge measurement method by using a single-chip microcomputer. 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, which simulates the battery as an equivalent circuit, which can quickly and effectively measure the self-discharge rate of lithium-ion batteries.
Self-discharge rate, as an important performance index of lithium-ion batteries, has a major impact on the selection and configuration of batteries. Therefore, it is of far-reaching significance to measure the self-discharge rate of lithium-ion batteries.
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 clearly too large, the reason is likely to be that the internal impurities and burrs pierce the diaphragm and cause serious micro-short circuits. Because the impact of micro-short circuit on the battery is slow and irreversible. Therefore, the performance of this type of battery in a short period of time will not differ too much from the normal battery, but after a long-term storage, as the internal irreversible reaction gradually deepens, 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 has to be eliminated.
2. Assemble the battery
Lithium-ion 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 critical: once assembled into a module, because the self-discharge rate of each single lithium-ion battery is different, the voltage will drop to different degrees during shelving or cycling, and in series Under charging, the current will be equal, so after each charge, there may be overcharged or unsaturated single cells in the lithium-ion battery module. As the number of charging and discharging increases, the battery performance will gradually deteriorate. The cycle life is greatly reduced compared to unmatched single cells. Therefore, the battery pack requires accurate measurement and screening of the self-discharge rate of lithium-ion batteries.
3. Revision of battery SOC estimation
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. It 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 value of SOC 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 overcharging. Over-discharge, thereby extending battery life.
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