There are many factors that affect the capacity of lithium-ion batteries, such as the use temperature, charge-discharge current, charge-discharge cut-off voltage and other factors will affect the decay rate of lithium-ion batteries. The mechanism of capacity decay of lithium-ion batteries can be divided into three categories: internal resistance and polarization addition, positive and negative active material loss, and Li loss.
Different external factors have different effects on all three. For example, LiFepO4 lithium-ion batteries have very good cycling performance, but different use conditions have an important impact on the cycle life of lithium-ion batteries. The test proves that the 26650 lithium-ion battery is discharged at 15C pulse discharge and 15C continuous discharge, and the two discharge regimes have completely different effects on the 26650 lithium-ion battery. The capacity of the 26650 lithium-ion battery with 15C pulse discharge decays very quickly, and after 40 times of charging and discharging, it can no longer be discharged at 15C, but it can still be discharged at 1C. The capacity decay of 15C continuous discharge battery is slower, and 15C discharge can still be carried out after 60 times, but the decay rate of 1C rate is faster than that of 15C pulse discharge.
The charging strategy of lithium-ion batteriesThe research on the effect of life decay of lithium-ion batteries can better guide the design of lithium-ion batteries. In the following, the influence of different charging control strategies on the life decay of lithium-ion batteries is studied, and the life decay model of lithium-ion batteries is proposed. The research test shows that when the charging current and cut-off voltage exceed a certain value, the attenuation of the lithium-ion battery will be greatly accelerated, in order to reduce the attenuation rate of the lithium-ion battery, it is necessary to select the appropriate charge-discharge current and cut-off voltage for different systems.
Effect of rate discharge on battery performance
From the data, it can be seen that with the increase of the charging rate, the decay rate of lithium-ion batteries is also increasing rapidly, and from the slope of the curve, there are three different stages of the decay rate of the battery, the stage with a faster decay rate in the early stage (stage 1), the stable stage with a slower decay rate in the middle (phase 2), and the decay rate acceleration stage in the later stage (phase 3). According to the study of the decay mechanism of the three-stage battery, stage 1 may be due to the fact that the growth of the battery SEI film consumes a part of Li+, so the decay rate is relatively fast. In stage 2, with the stabilization of the SEI membrane structure, the internal stability is relatively stable, so the decay rate is slower, and in stage 3, as the battery ages, the loss of active material begins to occur, and the active interface of the electrode decreases, resulting in the battery being very sensitive to current. Figure C is an experiment on the effect of different cut-off voltages on the decay speed of the battery, and it can be seen from the experimental results that when the charging cut-off voltage is increased to 4.3V, the cycling performance of the battery will deteriorate sharply, and reducing the charging cut-off voltage can effectively improve the cycling performance of the battery.
Effect of lithium-ion battery yield on internal resistance
The analysis of the dynamic internal resistance of the battery is shown in the figure, from the test results of Figure A, when the charging current is less than 1C, the dynamic internal resistance of the battery is almost the same as the change trend of the battery cycle, but when the charging current exceeds 1C, the new speed of the dynamic internal resistance of the battery will increase rapidly with the increase of the charging rate. According to the test results in Figure B, when the charging cut-off voltage is 4.3V, the dynamic internal resistance of the battery increases very quickly, indicating that the high cut-off voltage will worsen the kinetic conditions of the battery, and the dynamic internal resistance of the battery is added slowly when the cut-off voltage is 4.1V and 4.2V.
From the above analysis, we can note that there is a value both the charging current and the charging cut-off voltage, when the charging current or voltage exceeds this value, it will lead to the acceleration of battery attenuation, about the above battery this value is 1C and 4.2V, when the charging current and cut-off voltage exceed this value, it will accelerate the decay of the battery, when it is less than this value, increasing the charging current and cut-off voltage will not significantly increase the decay speed of the new battery. The mechanism study on the influence of charging current and cut-off voltage on the decay speed of the battery shows that when the charging current is lower than 1C, the loss of positive and negative active materials is important, and when the cut-off voltage is lower than 4.2V, the loss of positive and negative active materials and Li will be significantly accelerated.