(1) During the use of lithium ion battery, the number of charge and discharge increases, and the capacity gradually decreases. The so-called recession refers to an intuition that power is gradually being exhausted. Like our mobile phones, the first charge may take a whole day, but it only takes half a day to fully charge. This is the reason why the capacity of lithium ion battery decreases in use. This kind of product can be replaced quickly and solved better. Before the battery runs out, we may have replaced a new mobile phone, but electric vehicles, which are durable and have a long service life, cannot be solved well. Generally speaking, the service life of vehicles may reach about 10 years. During this period, it may be necessary to charge 1000 to 2000 times (assuming charging every other day). In order to meet the use demand of electric vehicles, certain requirements must be put forward for the service life of lithium-ion batteries for electric vehicles

　　(2) There are many factors that affect the service life of lithium ion batteries. The use temperature, charge discharge current and charge discharge blocking voltage will affect the deterioration rate of lithium ion battery. The mechanism of lithium ion battery capacity decline can be divided into three categories: the increase of internal resistance and polarization, the loss of positive and negative active materials and the loss of lithium. External factors also have different influences on these three factors. For example, LiFePO4 batteries usually have good cycle performance, but Draken of the University of Texas Arenton pointed out that if 266650 LiFePO4 batteries are used for 15C pulse discharge and 15C continuous discharge, the impact on LiFePO4 batteries will be completely different. The lithium iron phosphate battery with 15C pulse discharge can not discharge 15C after the capacity is reduced by 40 times, but the discharge at 1C is reduced by 6% to 20 times. The capacity of 15C continuous discharge battery decreases slowly, and 15C can be discharged after 60 times, but the decay rate of 1C ratio is 14% to 20% faster than that of 15C pulse discharge. The mechanism study shows that the content of LIF in the positive SEI film of 15C pulsed discharge battery is high, which has great interference on lithium ion diffusion, and the lithium ion diffusion impedance and charge exchange impedance increase rapidly. Therefore, during the charging and discharging process, the polarization voltage is too large, which leads to the rapid reduction of the large current discharge capacity of LiFePO4

　　(3) The discharge system of lithium ion battery is heavily dependent on users, and a good discharge system may not be suitable for all users. However, since the charging agent is mainly controlled by the designer, the study of the impact of the charging agent on the decline of battery life can better guide the design of lithium ion batteries. Yang Ge of Beijing Jiaotong University studied the impact of various charging systems on the decline of lithium ion battery life, studied its mechanism, and proposed a model for the decline of lithium ion battery life. Yangguau research shows that if the charging current and blocking voltage exceed a certain value, the deceleration of lithium ion battery may be greatly accelerated. In order to reduce the decay rate of lithium ion battery, the charge discharge current and blocking voltage suitable for various systems must be selected

　　(4) In this test, Xiangyang uses commercial 18650 battery, LiCoO2 cathode material and graphite cathode material. Test the influence of other charging currents on the attenuation speed of the battery, as shown in the figure below. As shown in Figure a below, the charging current has a great influence on the decay rate of lithium ion batteries. According to the charging ratio of 0.5c, the attenuation rate of the battery is 0.020% cycles for the first 150 cycles, 0.0156% cycles for 150-800 cycles, and 0.0214% cycles after 800 cycles. According to the charge ratio of 0.8C, the attenuation rate of the battery is 0.0243% cycle for the first 150 times, 0.175% cycle for 150-800 times, and 0.0209% cycle for 800 times. For 1C proportional filling, the first 150 times of decline rate is 0.032% cycle, 150 to 600 times of decline rate is 0.0188% cycle, and after 600 times of decline rate is 0.0271% cycle. 1.2C charging makes the attenuation speed of the first 100 times be 0.0472% cycle, the attenuation source speed is 100-400 times 0.0226% cycle, and the attenuation speed after 400 times is 0.0356% cycle. A battery charged at a ratio of 1.5C is very different from a battery charged at a different ratio. The average decay rate is much faster than charging with 0.078% loop, and much faster than other proportion of batteries. According to the above data, with the increase of charging ratio, the decay rate of lithium ion battery is also increasing rapidly. From the slope of the curve, the decay speed of the battery can be divided into three stages: the first stage has a fast decay speed (the first stage), the middle stage (the second stage) has a slow decay speed, and the later stage (the third stage) has an accelerated decay speed. According to the research on the attenuation mechanism of the third stage battery, the growth of the battery SEI film will consume some lithium, so the attenuation rate of the first stage battery may be faster. In the second stage, the SEI membrane structure is stable, the internal is relatively stable, and the descending speed is slow. In the third stage, the battery ages, the active substances begin to drain, the active interface of the electrode decreases, and the battery is very sensitive to the current response. Figure C shows the experiment of the influence of different cut-off voltages on the falling speed of the battery. The test results show that increasing the cut-off voltage to 4.3 v will lead to a sharp decline in the cycle performance of the battery, and reducing the cut-off voltage will effectively improve the cycle performance of the battery

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