What is the impact of charging rate on the degradation of lithium-ion batteries

There are many influencing factors that affect the life of lithium-ion batteries. Factors such as operating temperature, charge and discharge current, and charge-discharge cut-off voltage will all affect the rate of degradation of lithium-ion batteries. The mechanism that causes the capacity decline of lithium-ion batteries can also be divided into three categories: internal resistance and polarization increase, positive and negative active material loss, and Li loss. Different external factors have different effects on these three. For example, batteries made of LiFePO4 materials generally have very good cycle performance, but studies by Derek N. Wong of the University of Texas at Arlington found that the use conditions have an important impact on their cycle life. The LiFePO4 battery performs 15C pulse discharge and 15C continuous discharge. The two discharge systems have completely different effects on the LiFePO4 battery. The capacity of the lithium iron phosphate battery with 15C pulse discharge decays very quickly. After 40 times, the 15C discharge cannot be carried out, but the 1C discharge can still be carried out. The decay rate of the 1C discharge is 6%/20 times. The 15C continuous discharge battery capacity decays slowly, and 15C discharge can still be performed after 60 times, but the decay rate of 1C rate is faster than 15C pulse discharge, reaching 14%/20 times. Mechanism studies have shown that the 15C pulse discharge battery contains more LiF in the SEI film of the negative electrode, and LiF hinders the diffusion of lithium ions more, which makes the Li+ diffusion resistance and charge exchange resistance of the battery increase rapidly, thus making the battery more effective. The polarization voltage is too large during charging and discharging, which leads to the rapid decline of LiFePO4 high current discharge capacity. The discharge system of lithium-ion batteries is largely dependent on users, and a good discharge system is not necessarily applicable to some users. But the charging system is important to the designer to control, so the research on the impact of the charging system on the degradation of battery life can better guide our design of lithium-ion batteries. Yang Gao of Beijing Jiaotong University et al. aimed at the impact of different charging systems on the life decline of lithium-ion batteries, and studied the mechanism of their use, and proposed a life decline model of lithium-ion batteries. YangGao’s research shows that when the charging current and cut-off voltage exceed a certain value, the decay of the lithium-ion battery will be greatly accelerated. In order to reduce the decay rate of the lithium-ion battery, it is necessary to select the appropriate charging for different systems. Discharge current and cut-off voltage. In the test, YangGao used a commercial 18650 battery with LiCoO2 as the cathode material and graphite as the anode material. The effect of different charging currents on the battery decay rate was tested. The results are shown in the figure below. From the figure a below, we can see that the charging current has a great influence on the decay rate of the lithium-ion battery. At a charging rate of 0.5C, the decay rate of the battery in the first 150 cycles is 0.020%/cycle, at 150 Times-800 times is stable at 0.0156%/cycle, after 800 times, it is 0.0214%/cycle. At 0.8C charging rate, the battery decay rate is 0.0243%/cycle for the first 150 times, 0.175%/cycle for 150-800 times, and 0.0209%/cycle after 800 times. Regarding 1C rate charging, the decay rate for the first 150 times is 0.032%/cycle, the decay rate for 150-600 times is 0.0188%/cycle, and the decay rate after 600 times is 0.0271%/cycle. Charging at 1.2C, the first 100 times decay rate is 0.0472%/cycle, 100-400 times decay rate is 0.0226%/cycle, after 400 times, the decay rate is 0.0356%/cycle. 1.5C rate charging is very different from batteries charged at other rates. The average attenuation rate is 0.078%/cycle, which is much faster than batteries charged at other rates. It can be seen from the above data that as the charging rate increases, the decay rate of lithium-ion batteries is also rapidly increasing, and from the slope of the curve, there are three different stages in the decay rate of the battery. The stage with a faster decay rate (stage 1), the stable stage with a slower decay rate in the middle (stage 2), and the later stage of decay rate acceleration (stage 3). According to the research on the decay mechanism of the three-stage battery, stage 1 may be because the growth of the battery SEI film consumes a part of Li+, so the decay rate is faster. In stage 2, as the structure of the SEI membrane becomes stable, the interior is relatively stable, so the decay rate is slower. In stage 3, as the battery ages, the loss of active materials begins to occur, and the active interface of the electrode decreases, making the battery very sensitive to current. Figure C is an experiment on the impact of different cut-off voltages on the battery decay speed. From the experimental results, it can be seen that when the charge cut-off voltage is increased to 4.3V, the cycle performance of the battery will deteriorate sharply. Lowering the charge cut-off voltage can be effective Improve the cycle performance of the battery. The analysis of the dynamic internal resistance of the battery is shown in the figure below. From the test results of Figure a, when the charging current is less than 1C, the dynamic internal resistance of the battery changes with battery cycles almost the same, but when the charging current exceeds 1C , The increase speed of the dynamic internal resistance of the battery will increase rapidly with the increase of the charging rate. From the test results in Figure b, when the charge cut-off voltage is 4.3V, the dynamic internal resistance of the battery increases very quickly, indicating that the high cut-off voltage will deteriorate the dynamic conditions of the battery. When the cut-off voltage is 4.1V and 4.2V, the dynamic internal resistance of the battery is increased. The increase in resistance is slower. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. 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