Lithium-ion battery self-discharge reaction can not be prevented, and its existence not only leads to the reduction of the capacity of the battery itself, but also seriously affects the battery grouping and cycle life. The self-discharge rate of lithium-ion batteries is generally 2% to 5% per month, which can fully meet the requirements of the use of a single battery.
However, once the single lithium-ion battery is assembled into a module, because the characteristics of each single lithium-ion battery is not completely consistent, so each time after charging and discharging, the terminal voltage of each single lithium-ion battery can not be completely consistent, which will appear in the lithium-ion battery module overcharge or over discharge of the single battery, single lithium-ion battery performance will deteriorate. With the addition of the number of charge and discharge, the deterioration will be further exacerbated, and the cycle life will be significantly reduced compared to the unpaired single cell. Therefore, an in-depth study of the self-discharge rate of lithium-ion batteries is an urgent need for battery production.
First, the impact of self-discharge factors
Battery self-discharge phenomenon refers to the battery is in the open circuit shelved, the phenomenon of spontaneous loss of capacity, also known as charge retention capacity. Self-discharge can generally be divided into two types: reversible self-discharge and irreversible self-discharge. The reversible self-discharge is the one where the lost capacity can be reversibly compensated, and its principle is similar to the normal discharge reaction of the battery. Self-discharge that cannot be compensated for lost capacity is irreversible self-discharge, which is caused by irreversible reactions inside the battery, including reactions between positive electrode and electrolyte, negative electrode and electrolyte, reactions caused by impurities in the electrolyte, and irreversible reactions caused by micro-short circuits caused by impurities carried when it is made. The factors affecting self-discharge are described below.
1. Cathode material
Yah-MeiTeng et al. studied the physical and electrochemical properties of two LiFepO4 cathode materials. It was found that the batteries with high content of iron impurities in the raw materials and during charging and discharging have high self-discharge rate and poor stability because of the gradual reduction and precipitation of iron in the negative electrode, which punctures the diaphragm and leads to internal short-circuiting of the battery, resulting in higher self-discharge.
2. Cathode material
The influence of negative electrode material on self-discharge is important because of the irreversible reaction between negative electrode material and electrolyte. As early as 2003, Aurbach et al. proposed that the electrolyte is reduced and the gas is released, exposing part of the graphite surface to the electrolyte. During charging and discharging, the graphite layered structure is easily damaged when lithium ions are embedded and dislodged, resulting in a large self-discharge rate.
3. Electrolyte
The influence of electrolyte is important as follows: corrosion of negative electrode surface by electrolyte or impurities; dissolution of electrode material in electrolyte; electrode is covered by insoluble solid or gas decomposed by electrolyte, forming passivation layer, etc. At present, a large number of researchers have devoted themselves to developing new additives to suppress the effect of electrolyte on self-discharge. JunLiu et al. added additives such as VEC to the electrolyte of MCN111 battery and found that the high-temperature cycling performance of the battery improved and the self-discharge rate generally decreased. The reason for this is that these additives can improve the SEI film and thus protect the negative electrode of the battery.
4. Storage state
The general factors affecting the storage state are storage temperature and battery SOC. generally speaking, 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 ion batteries under static conditions. The results showed that the capacity retention rate gradually decreased with the shelving time as the temperature increased and the self-discharge rate of the battery increased.
Liu Yunjian et al. used the commercial lithium manganate power battery and found that with the addition of the battery charge state, the relative potential of the positive electrode is getting higher and higher, and its oxidation is getting stronger; the relative potential of the negative electrode is getting lower and lower, and its reduction is getting stronger, both of which can accelerate the precipitation of Mn, leading to an increase in self-discharge rate.
5 Other factors
There are many factors affecting the self-discharge rate of the battery, in addition to the above-mentioned, there are also the following important aspects: in the production process, the burr when slitting the pole piece, the impurities introduced in the battery due to the production environment, such as dust, metal powder on the pole piece, etc., which may cause the internal micro-short circuit of the battery; the external environment is humid, the insulation of the external circuit is not complete, the battery shell isolation is poor, etc. The battery storage with external electronic circuit, which leads to self-discharge; long time storage process, the active material of the electrode material and the bonding failure of the collector fluid, resulting in the active material shedding and stripping, etc. leading to capacity reduction and self-discharge increase. Each of the above factors or a combination of factors can cause self-discharge behavior of lithium-ion batteries, which causes difficulties in finding the cause of self-discharge and estimating the storage performance of the battery.
Second, the self-discharge rate measurement method
Through the above analysis, it is clear that the self-discharge rate of lithium-ion batteries is generally low. The self-discharge rate itself is affected by the temperature, the number of cycles and SOC, so it is very difficult and time-consuming to achieve accurate measurement of self-discharge of the battery.
Traditional self-discharge rate measurement methods
At present, there are three traditional self-discharge detection methods as follows.
● Direct measurement method
Firstly, charge the battery cell under test to a certain state of charge and maintain an open-circuit shelf for a period of time, then discharge the battery cell to determine the capacity loss of the battery cell. The self-discharge rate is
Where: C is the rated capacity of the battery; C1 is the discharged capacity. After open-circuit shelving, the remaining capacity of the cell can be obtained by discharging the cell. At this point, the cell is operated again for several charge/discharge cycles to determine the full capacity of the electro-garlic at this time. This method can determine the irreversible capacity loss and reversible capacity loss of the battery.
● Open-circuit voltage decay rate measurement method
There is a direct relationship between open-circuit voltage and battery charge state SOC, just measure the rate of change of OCV of the battery over a period of time, i.e.
This method is simple to operate, just record the voltage of the battery in any time period, and then according to the correspondence between the voltage and the battery SOC, the state of charge of the battery at that moment can be derived. The self-discharge rate of the battery can be finally obtained by calculating the slope of voltage decay and the decay capacity corresponding to the unit time.
Capacity retention method
The self-discharge rate of the battery is obtained by measuring the amount of power required to maintain the open-circuit voltage or SOC of the battery. That is, measuring the charging current when keeping the battery open-circuit voltage, the battery self-discharge rate can be considered as the charging current obtained from the measurement.
2 Self-discharge rate rapid measurement method
Due to the long time required by traditional measurement methods and the lack of measurement accuracy, the self-discharge rate in the battery testing process in most cases is only used as a screening method to determine whether the battery is qualified. The emergence of a large number of novel and convenient new measurement methods has saved 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 microcontroller and so on. This method has the advantages of short measurement time, high accuracy, and simple equipment.
Equivalent circuit method
The equivalent circuit method is a new self-discharge measurement method, which simulates the battery as an equivalent circuit and can measure the self-discharge rate of lithium-ion batteries quickly and effectively.
The significance of measuring self-discharge rate
As an important performance index of the lithium-ion battery, the self-discharge rate has an important impact on the screening and grouping of the battery, so measuring the self-discharge rate of lithium-ion batteries has far-reaching significance.
1. Predicting problematic cells
The same batch of cells, the materials used and made to control basically the same, when there is an individual battery white discharge is significantly large, the reason is likely to be internal due to impurities, burrs piercing the diaphragm, and a serious micro-short circuit. Because the impact of the micro-short circuit on the battery is slow and irreversible. Therefore, the performance of such batteries will not be too much different from normal batteries in a short period of time, but after long-term shelving with the gradual deepening of the internal irreversible reaction, the performance of the battery will be far below its factory performance and other normal battery performance. Therefore, in order to ensure the quality of the factory battery, a self-discharge large battery must be removed.
2. Grouping and matching of batteries
Lithium-ion batteries to better consistency, including capacity, voltage, internal resistance, and white discharge rate. Battery self-discharge rate on the battery pack is important to show: once assembled into a module, because of the different self-discharge rate of every single lithium-ion battery, in the process of shelving or cycling, the voltage will be different degrees of decline, and in series charging, the current will be equal, so each charge may be in the lithium-ion battery module overcharge or unfilled single cell, with the number of charges and discharge With the addition of more charges and discharges, the battery performance will gradually deteriorate, and the cycle life will be significantly reduced compared to the unpaired single cell. Therefore, battery pairing requires accurate measurement and screening of the self-discharge rate of lithium-ion batteries.
3. Battery SOC estimation correction
The state of charge, also known as the remaining power, represents the ratio of the remaining capacity of the battery after a period of use or long-term shelving and its fully charged state capacity, commonly expressed as a percentage. The self-discharge rate is an important reference value for the SOC estimation of Li-ion batteries. After the self-discharge current correction of the initial value of SOC can improve the accuracy of SOC estimation, on the one hand, the customer can estimate the product useable time or driving distance according to the remaining power; on the other hand, improving the accuracy of SOC prediction of BMS can effectively prevent the battery overcharge and over-discharge, thus prolonging the battery life.
