As the number of charge and discharge cycles increases, the battery capacity will continue to decay. When the capacity decays to 75% to 80% of the rated capacity, the lithium-ion battery is considered to be in a failure state. Discharge rate, battery temperature rise, and ambient temperature have a greater impact on the discharge capacity of lithium-ion batteries.

This paper adopts the charging and discharging criteria of constant voltage and constant current charging and constant current discharging for the battery. The discharge rate, battery discharge temperature rise, and ambient temperature are successively used as variables and cyclic experiments are carried out quantitatively, and the discharge rate and battery discharge temperature are analyzed under different cathode materials. The influence of temperature, ambient temperature and cycle times on the discharge capacity of lithium-ion batteries.

1. The basic experimental program of the battery

The positive and negative materials are different, and the cycle life varies greatly, which affects the capacity characteristics of the battery. Lithium iron phosphate (LFP) and nickel-cobalt-manganese ternary materials (NMC) are widely used as cathode materials for lithium-ion secondary batteries with their unique advantages. It can be seen from Table 1 that the rated capacity, nominal voltage, and discharge rate of the NMC battery are higher than those of the LFP battery.

Charge and discharge LFP and NMC lithium-ion batteries according to certain constant current and constant voltage charging and constant current discharge rules, and record the charge and discharge cut-off voltage, discharge rate, battery temperature rise, experimental temperature, and battery capacity changes during the charge and discharge process Condition.

2. The influence of discharge rate on discharge capacity Fix the temperature and charge and discharge rules, and discharge the LFP battery and NMC battery at a constant current according to different discharge rates.

Adjust the temperature respectively: 35, 25, 10, 5, -5, -15°C. It can be seen from Figure 1 that at the same temperature, by increasing the discharge rate, the overall discharge capacity of the LFP battery shows a declining trend. Under the same discharge rate, changes in low temperature have a greater impact on the discharge capacity of LFP batteries.

When the temperature drops below 0 ℃, the discharge capacity decays severely and the capacity is irreversible. It is worth noting that LFP batteries aggravate the attenuation of discharge capacity under the dual influence of low temperature and large discharge rate. Compared with LFP batteries, NMC batteries are more sensitive to temperature, and their discharge capacity changes significantly with ambient temperature and discharge rate.

It can be seen from Figure 2 that at the same temperature, the overall discharge capacity of the NMC battery shows a trend of first decay and then rise. Under the same discharge rate, the lower the temperature, the lower the discharge capacity.

With the increase of the discharge rate, the discharge capacity of lithium-ion batteries continues to decline. The reason is that due to the serious polarization, the discharge voltage is reduced to the discharge cut-off voltage in advance, that is, the discharge time is shortened, the discharge is insufficient, and the negative electrode Li+ does not fall off. Embedded completely. When the battery discharge rate is between 1.5 and 3.0, the discharge capacity begins to show signs of recovery to varying degrees. As the reaction continues, the temperature of the battery itself will increase significantly with the increase of the discharge rate, the thermal movement capacity of Li+ is strengthened, and the diffusion speed is accelerated, so that the de-embedding speed of Li+ is accelerated and the discharge capacity rises. It can be concluded that the dual influence of the large discharge rate and the temperature rise of the battery itself causes the non-monotonic phenomenon of the battery.

3. The influence of battery temperature rise on discharge capacity. NMC batteries are respectively subjected to 2.0, 2.5, 3.0, 3.5, 4.0, 4.5C discharge experiments at 30℃, and the relationship curve between the discharge capacity and the temperature rise of the lithium-ion battery is shown in Figure 3. Shown.

It can be seen from Figure 3 that under the same discharge capacity, the higher the discharge rate, the more significant the temperature rise changes. Analyzing the three periods of the constant current discharge process under the same discharge rate shows that the temperature rise is mainly in the initial and late stages of the discharge.

Fourth, the influence of ambient temperature on discharge capacity The best operating temperature of lithium-ion batteries is 25-40 ℃. From the comparison of Table 2 and Table 3, it can be seen that when the temperature is lower than 5°C, the two types of batteries discharge rapidly and the discharge capacity is significantly reduced.

After the low temperature experiment, the high temperature was restored. At the same temperature, the discharge capacity of the LFP battery decreased by 137.1mAh, and the NMC battery decreased by 47.8mAh, but the temperature rise and discharge time did not change. It can be seen that LFP has good thermal stability and only exhibits poor tolerance at low temperatures, and the battery capacity has an irreversible attenuation; while NMC batteries are sensitive to temperature changes.

Fifth, the influence of the number of cycles on the discharge capacity Figure 4 is a schematic diagram of the capacity decay curve of a lithium-ion battery, and the discharge capacity at 0.8Q is recorded as the battery failure point. As the number of charge and discharge cycles increases, the discharge capacity begins to show a decline.

A 1600mAh LFP battery was charged and discharged at 0.5C and discharged at 0.5C for a charge-discharge cycle experiment. A total of 600 cycles were performed, and 80% of the battery capacity was used as the battery failure criterion. Use 100 as the interval times to analyze the relative error percentage of discharge capacity and capacity attenuation, as shown in Figure 5.

A 2000mAh NMC battery was charged at 1.0C and discharged at 1.0C for a charge-discharge cycle experiment, and 80% of the battery capacity was taken as the battery capacity at the end of its life. Take the first 700 times and analyze the discharge capacity and the relative error percentage of the capacity attenuation with 100 as the interval, as shown in Figure 6.

The capacity of LFP battery and NMC battery when the number of cycles is 0 is the rated capacity, but usually the actual capacity is less than the rated capacity, so after the first 100 cycles, the discharge capacity decays seriously. The LFP battery has a long cycle life, the theoretical life is 1,000 times; the theoretical life of the NMC battery is 300 times. After the same number of cycles, the NMC battery capacity decays faster; when the number of cycles is 600, the NMC battery capacity decays close to the failure threshold.

6. Conclusion

Through charging and discharging experiments on lithium-ion batteries, the five parameters of cathode material, discharge rate, battery temperature rise, ambient temperature and cycle number are used as variables, and the relationship between capacity-related characteristics and different influencing factors is analyzed, and the following are obtained in conclusion:

(1) Within the rated temperature range of the battery, an appropriate high temperature promotes the deintercalation and embedment of Li+. Especially for the discharge capacity, the greater the discharge rate, the greater the heat generation rate, and the more obvious the electrochemical reaction inside the lithium-ion battery.

(2) The LFP battery shows good adaptability to high temperature and discharge rate during charge and discharge; it has poor tolerance to low temperature, the discharge capacity decays severely, and cannot be recovered after heating.

(3) Under the same number of charge and discharge cycles, the LFP battery has a long cycle life, and the NMC battery capacity decays to 80% of the rated capacity more quickly. (4) Compared with the LFP battery, the discharge capacity of the NMC battery is more sensitive to temperature, and at a large discharge rate, the discharge capacity is not monotonic and the temperature rise changes significantly.