We all know that batteries will undergo self discharge reactions during use and storage, which is inevitable. High rate lithium batteries are no exception. Self discharge reactions not only lead to a decrease in battery capacity, but also seriously affect battery grouping and cycle life. The self discharge rate of lithium batteries is generally 2% to 5% per month, which can fully meet the usage requirements of individual batteries; However, after assembling individual lithium batteries into modules, the characteristics of each individual lithium battery are not 100% identical. Therefore, after each charge and discharge, the terminal voltage of each individual lithium battery cannot reach complete consistency, resulting in overcharged or over discharged individual batteries in the lithium battery module, and the performance of individual lithium batteries will deteriorate.
Over time, as the number of charge and discharge cycles of high rate lithium batteries increases, the deterioration of self discharge rate will further intensify, and the cycle life will be significantly reduced compared to unmatched individual batteries. Therefore, studying the self discharge rate of high rate lithium batteries is an urgent need for battery production.
1、 Factors affecting the self discharge of high rate lithium batteries
The self discharge phenomenon of high rate lithium batteries refers to the spontaneous loss of capacity when the battery is in an open circuit state, also known as charge holding capacity. Self discharge can generally be divided into two types: reversible self discharge and irreversible self discharge. The reversible self discharge can compensate for the capacity loss of high rate lithium batteries, which is similar to the normal discharge reaction of batteries. The self discharge that cannot be compensated for due to capacity loss is irreversible, mainly due to irreversible reactions occurring inside the battery, including reactions between the positive electrode and electrolyte, reactions between the negative electrode and electrolyte, reactions caused by impurities in the electrolyte, and irreversible reactions caused by micro short circuits caused by impurities carried during production due to process reasons. There are several factors that affect the self discharge of high rate lithium batteries:
1. Positive electrode material
The main impact of high rate lithium battery positive electrode materials is the precipitation of transition metals and impurities in the negative electrode, which leads to internal short circuits and increases the self discharge of lithium batteries. Researchers studied the physical and electrochemical properties of two LiFePO4 cathode materials. Research has found that batteries with high levels of iron impurities in raw materials and during charging and discharging processes have high self discharge rates and poor stability. The reason is that iron gradually reduces and precipitates at the negative electrode, piercing the separator and causing short circuits inside the battery, resulting in higher self discharge.
2. Negative electrode material
The influence of high rate lithium battery 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, it was proposed that the reduction of electrolyte would release gas, exposing the surface of graphite to the electrolyte. During the charging and discharging process, when lithium ions are embedded and removed, the graphite layered structure is easily damaged, leading to a higher self discharge rate.
3. Electrolyte
The impact of high rate lithium battery electrolyte is mainly manifested as: corrosion of the negative electrode surface by electrolyte or impurities; Dissolution of electrode materials in electrolyte; The electrode is covered by insoluble solids or gases decomposed by the electrolyte, forming a passivation layer, etc. At present, a large number of researchers are committed to developing new additives to suppress the impact of electrolyte on self discharge. By adding additives such as VEC to the electrolyte of NCM111 batteries, it has been found that the high-temperature cycling performance of the battery is improved, and the self discharge rate is generally reduced. 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 on the storage state of high magnification lithium batteries 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. Researchers conducted capacity decay experiments on lithium iron phosphate batteries under static conditions, and the results showed that as the temperature increased, the capacity retention rate gradually decreased with the storage time, and the battery self discharge rate increased.
Using commercialized lithium manganese oxide power batteries, it was found that as the state of charge of the battery increases, the relative potential of the positive electrode becomes higher and its oxidizing ability becomes stronger; The relative potential of the negative electrode is decreasing, and its reducibility is also increasing. Both can accelerate Mn precipitation, leading to an increase in self discharge rate.
5. Other factors
There are many factors that affect the self discharge rate of high rate lithium batteries. In addition to the several mentioned above, there are also the following aspects: burrs generated during the cutting of the electrode plates during the production process, impurities introduced into the battery due to production environment issues, such as dust, metal powder on the electrode plates, etc., which may cause internal micro short circuits in the battery; The presence of external electronic circuits during battery storage caused by humid external environments, incomplete insulation of external circuits, and poor isolation of battery casings can lead to self discharge; During long-term storage, the bonding failure between the active substance of the electrode material and the current collector leads to the detachment and peeling of the active substance, resulting in a decrease in capacity and an increase in self discharge. Each or a combination of the above factors can cause the self discharge behavior of high rate lithium batteries.
2、 Measurement of self discharge rate of high rate lithium batteries
Based on the above analysis, it can be concluded that the self discharge rate of lithium batteries is generally low. The self discharge rate itself is influenced by various factors, so precise measurement of battery self discharge is a very difficult and time-consuming task.
1. Traditional measurement methods for self discharge rate of high rate lithium batteries
At present, there are three traditional methods for self discharge detection:
Direct measurement method
Firstly, charge the tested battery cell to a certain state of charge and maintain an open circuit for a period of time. Then, discharge the battery cell to determine its capacity loss. The self discharge rate is:
Measurement method for self discharge rate of high rate lithium batteries
In the formula: C is the rated capacity of the battery; C1 is the discharge capacity. After the open circuit is set aside, discharging the battery cell can obtain the remaining capacity of the battery cell. At this point, perform multiple charge and discharge cycles on the battery cells to determine the full capacity of the garlic at this time. This method can determine the irreversible capacity loss and reversible capacity loss of the battery.
Measurement method for open circuit voltage attenuation rate
The open circuit voltage is directly related to the state of charge (SOC) of the battery, and it is only necessary to measure the rate of change of the battery's OCV over a period of time, that is:
Measurement method for open circuit voltage attenuation rate of high rate lithium batteries.This method is easy to operate and only requires recording the voltage of the battery at any time interval. Based on the correspondence between the voltage and the battery SOC, the state of charge of the battery at that time can be obtained. By calculating the attenuation slope of voltage and the corresponding attenuation capacity per unit time, the self discharge rate of the battery can ultimately be obtained.
Capacity retention method
Measure the expected open circuit voltage or SOC required by the battery to determine its self discharge rate. The charging current when maintaining the open circuit voltage of the battery is measured, and the self discharge rate of the battery can be considered as the measured charging current.
2. Rapid measurement method for self discharge rate
Due to the long time required and insufficient measurement accuracy of traditional measurement methods, the self discharge rate is mostly used as a method to screen whether the battery is qualified in the battery detection 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 traditional self discharge measurement methods using microcontrollers and others. This method has the advantages of short measurement time, high accuracy, and simple equipment.
Equivalent circuit method
The equivalent circuit method is a novel self discharge measurement method that simulates a battery as an equivalent circuit, which can quickly and effectively measure the self discharge rate of lithium-ion batteries.
As an important performance indicator of high rate lithium batteries, the self discharge rate has a significant impact on the selection and grouping of batteries. For the same batch of battery cells, the materials and manufacturing controls used are basically the same. When individual batteries have significantly higher white discharge, the reason is likely due to serious micro short circuits caused by impurities and burrs piercing the separator inside. Because the impact of micro short circuits on batteries is slow and irreversible. Therefore, measuring the self discharge rate of high rate lithium batteries has profound significance.
The materials and manufacturing controls used in the same batch of battery cells are basically the same. When there are individual batteries with significantly higher white discharge, the reason is likely due to serious micro short circuits caused by impurities and burrs piercing the separator inside. Because the impact of micro short circuits on batteries is slow and irreversible. So, in the short term, the performance of such batteries will not differ much from normal batteries, but after 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 batteries, batteries with high self discharge must be removed.