Study On High And Low Temperature Charge Discharge Performance Of Lithium Ion Battery
With high energy density, design flexibility, long cycle life, no memory effect, low self discharge rate and no pollution to the environment, lithium-ion batteries have been more and more widely used in mobile electronic equipment, electric vehicles, national defense industry and other high-tech fields. However, due to the harsh service environment in electric vehicles, Aerospace and military fields, Lithium ion battery is required to be used in a wide temperature range, which has attracted much attention because of its high and low temperature performance. This paper takes a domestic commercialized scientific research improved model of lithium-ion battery as the research object, and tests its performance under high and low temperature conditions, which provides a certain data reference for manufacturers to improve battery technology.
1.1 instruments and materials
Experimental instruments: high precision battery performance test system, ly-2480 high and low temperature explosion-proof test chamber, ly-2800 high and low temperature explosion-proof test chamber
Experimental battery: domestic battery cell, nominal voltage 37 V, nominal capacity 11 ah.
1.2 experimental methods and conditions
(1) Preparation before test:
a. The battery used in the experiment was stably cycled for 5 times at room temperature (23 ± 2) ℃. Charge and discharge conditions: charge the battery to 4.2 V with 3500 Ma constant current and 350 Ma with constant voltage; Set aside for 1H between charge and discharge; Discharge to 27 V with 3 500 mA constant current.
b. The battery was kept at - 20 ℃ and 65 ℃ for 6 hours, and then the charge and discharge experiments were carried out at different temperatures (- 20 ℃ and 65 ℃).
(2) High and low temperature charge and discharge
Place the battery in high-temperature explosion-proof box (65 ℃) or temperature and humidity regulation box (- 20 ℃), charge it to 42 V with 3500 Ma constant current and 350 Ma with constant voltage; Set aside for 1 h between charge and discharge; Discharge to 2 7 V with 3500 Ma constant current.
2. Experimental results and discussion
1.1 low temperature charge discharge performance of lithium ion battery.
The charging curve of the battery at room temperature (23 ± 2) ℃ and low temperature (- 20 ℃) is shown in Figure 1.
Table 1 shows the values of constant current and constant voltage charging capacity and total charging capacity of the battery at room temperature and low temperature. It can be seen from Figure 1 and table 1 that compared with the normal temperature charging process, the terminal voltage of the battery in the constant current charging process increases at low temperature (possibly because the polarization of the battery is relatively large under low temperature conditions, and the corresponding voltage change is relatively large), the constant current charging time decreases, and the constant voltage charging time is significantly prolonged, resulting in an increase in the average charging voltage of the battery, Reduced charging efficiency. At room temperature, the ratio of constant current charging capacity to total charging capacity is 52%; When the temperature drops to - 20 ℃, the ratio of constant current charging capacity to total charging capacity is only 6.2%. Cause analysis: at low temperature, the chemical activity of active substances in the battery decreases; Part of the solvent in the electrolyte solidifies, resulting in the reduction of the number of lit migration and the decrease of electrical conductivity; In the process of battery charging, a large amount of metal lithium deposition occurs, the concentration polarization increases, and the voltage increases rapidly.
The discharge curves of the battery at room temperature (23 ± 2) ℃ and low temperature - 20 ℃ are shown in Figure 2.
It can be seen from the figure that at - 20 ℃, the terminal voltage of lithium-ion battery during discharge decreases, the average discharge voltage decreases, and the discharge voltage platform decreases faster than that at room temperature. This may be due to the decrease of ionic conductivity of electrolyte at low temperature, resulting in the increase of ohmic polarization, concentration polarization and electrochemical polarization at low temperature, which is shown as the decrease of discharge voltage on the battery discharge curve. For its capacity, due to the low charging efficiency at low temperature, its discharge capacity also decreases accordingly. From the microscopic point of view of the battery, low temperature (< 0 ℃) charging will lead to the reduction of lithium ions into metal lithium dendrites. This lithium metal dendrite has sharp angles and is easy to pierce the internal diaphragm of the battery, causing short circuit in the battery, which has certain potential safety hazards.
The charging curves of the battery at room temperature (23 ± 2) ℃ and high temperature 65 ℃ are shown in Figure 3.
Table 2 shows the values of constant current, constant voltage charging capacity and total charging capacity of the battery at room temperature and low temperature.
It can be seen from Fig. 3 and table 2 that compared with the normal temperature charging process, the charging voltage of the battery at high temperature rises rapidly to the charging limit voltage; The constant current charging time is obviously reduced, while the constant voltage charging time is almost 0, and the whole charging process ends quickly. After the battery is stored at 65 ℃ for a period of time, the charged capacity under this condition is only 20.8% of the normal temperature. This may be due to the change of SEI film on the surface of positive and negative materials of the battery when charging at high temperature, certain side reactions of electrolyte may occur at high temperature, the reduction of the amount of active lithium and certain irreversible changes in the internal structure of the battery, resulting in the increase of the internal resistance of the battery. At the same time, some additives in the battery are conducive to the improvement of battery performance at room temperature, but they may not have the same effect at high temperature.
The discharge curves of the battery at room temperature (23 ± 2) ℃ and high temperature 65 ℃ are shown in Figure 4.
It can be seen from the figure that at 65 ℃, the discharge starting voltage of lithium-ion battery decreases significantly and the discharge time decreases; At the same time, the charging capacity is reduced due to the decrease of charging acceptance capacity, so the discharge capacity is low. Then the battery was placed at room temperature for a period of time. After it was stable, the charge and discharge experiment was carried out. It was found that the change caused by high temperature showed serious irreversibility. This may be due to the decomposition of battery electrolyte under high temperature conditions or irreversible changes in the structure of battery materials, resulting in the decrease of charge and discharge voltage platform and capacity under high temperature conditions. After the battery is charged and discharged at high temperature, the battery is placed at room temperature and observed without deformation, explosion and other phenomena. These indexes meet the requirements of battery standards.
In this paper, the charge and discharge performance of the battery at low temperature - 20 ℃ and high temperature 65 ℃ is tested. The research shows that the shape of the battery does not change after being put aside at low temperature - 20 ℃, and the charging capacity at low temperature is 83% of that at normal temperature; After the battery is put aside at 65 ℃, the capacity charged at high temperature is only 20.8% of the normal temperature, and the capacity cannot be restored after the battery is restored to normal temperature. It can be seen that the low-temperature charging and discharging performance of the battery is better than its high-temperature performance, but both are worse than the normal temperature charging and discharging performance. For the specific reasons of battery charge and discharge under high and low temperature conditions, we will do further research in subsequent tests.
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