The storage of lithium-ion batteries is very common in practice, and they may be stored for a long time during the production and sales cycle of the battery. In practical use, they may also be stored for a long time. During the storage process of lithium-ion batteries, especially in high-temperature environments, the battery system is in a thermodynamic unstable state under full charge, and will continuously undergo a transition to an equilibrium state. When the changes accumulate to a certain extent, it will not only cause changes in the voltage and internal resistance of lithium-ion batteries, but also affect the rate performance and safety characteristics. Therefore, it is particularly important to study the storage performance of lithium-ion batteries in a certain environment. This article studies the performance degradation of lithium-ion batteries under full charge state and different temperature storage, including changes in voltage, resistance, capacity retention rate, impedance, and magnification after storage.
1 Test
1) Experimental object
18650 cylindrical battery
2) Charging and discharging test
Before battery storage, charge and discharge performance test. After standing at 45 ℃ for 24 hours, use a tester to test the performance of the battery at room temperature. First, charge the battery at a constant current of 0.1C to 4.1V, then charge it at a constant voltage of 4.1V until the current drops to 0.01C, let it stand for 10 minutes, and then discharge it at a constant current of 0.1C to 2.7V. After two cycles, charge the battery at a constant current of 0.1C to 4.1V, so that it has a fully charged state.
3) Storage test
The battery storage experiment will store lithium-ion batteries in a fully charged state in constant temperature chambers at 25 ℃, 45 ℃, 55 ℃, and 65 ℃, respectively. The storage experiment will be carried out after a certain interval of time, and the electrochemical performance analysis and testing related to voltage, internal resistance, etc. will be conducted at room temperature.
2 Test results
1) Battery capacity change
The capacity retention rates after 241 days of storage at 25 ℃, 45 ℃, and 55 ℃ were 91.47%, 80.19%, and 73.21%, respectively, while only 70.34% remained after 150 days of storage at 65 ℃. It can be observed that the open-circuit voltage of lithium-ion batteries stored at 65 ℃ decreases significantly faster than batteries stored at lower temperatures. This is because the battery undergoes a transition from a thermodynamic unstable state to an equilibrium state during storage, resulting in changes in the structure of the positive electrode material and self discharge reactions such as the loss of active lithium within graphite carbon. As the storage temperature increases, the reactions occurring inside the lithium-ion battery become more intense.
At the same time, the decomposition reaction rate of components in the electrolyte increases at high temperatures, leading to the rapid deposition of impurities and side reaction products on the positive and negative electrode plates. This also leads to a faster decrease in battery voltage at high temperatures. The results indicate that the temperature of lithium-ion batteries during storage will directly affect the rate of chemical reactions inside the battery, thereby affecting the storage performance of the battery. The higher the temperature, the more severe the performance degradation.
2) Change in battery internal resistance
The internal resistance of lithium-ion batteries refers to the resistance that lithium-ion batteries encounter when their current passes through various components inside the battery during operation. It is the sum of the resistance between the positive and negative ends, including the resistance of the positive and negative active substances, electrolyte, diaphragm, and external components of the current collector. When discharging a lithium-ion battery, if the internal resistance is small, the voltage drop generated during discharge will also be smaller, resulting in less capacity loss and more energy released. Therefore, the change in internal resistance of lithium-ion batteries is also an important factor to pay attention to during the storage process.
From the data, it can be seen that storage temperature has a significant impact on the internal resistance changes of lithium-ion batteries. The internal resistance of lithium-ion batteries will continue to increase during storage, and the higher the temperature, the more significant the increase. After storing at 25 ℃ for 241 days, the internal resistance of lithium-ion batteries only increased by 4.2m Ω (17.95%). At 45 ℃, the internal resistance of the batteries increased by 8.6 m Ω (37.07%) after 241 days. When the storage temperature reaches 55 ℃ or 65 ℃, the internal resistance of lithium-ion batteries will undergo a sharp change, increasing by 13.5 m (56.25%) and 16.9 mΩ (70.42%) respectively after 150 days of storage, with an increase of approximately 3.7 and 4.6 times that of 25 ℃ storage for the same time.
3) Battery rate performance
The discharge capacities of lithium-ion batteries at 0.2C, 1C, and 2C magnification before storage are 1742mAh, 1612mAh, and 1357mAh, respectively. After 30 days of storage at 65 ℃, the discharge capacities are 1594mAh, 1354mAh, and 1065mAh, respectively. It can be seen that after storage at 65 ℃, the discharge capacity of the battery at each rate has significantly decreased.
The discharge capacity at 2C rate before storage is 77.90% of 0.2C rate, while the discharge capacity at 2C rate after storage is 66.81% of 0.2C rate. In addition, it can be seen that after 30 days of storage at 65 ℃, the discharge plateau of each rate of the battery has decreased. This is due to the increase in polarization caused by internal reactions in the battery during storage, which reduces the diffusion rate of lithium ions in the positive and negative electrode materials and electrolyte after storage, resulting in poor rate performance of the battery after storage.
3 Conclusion
1) The temperature of lithium-ion batteries during storage will directly affect the rate of chemical reactions inside the battery, which in turn affects the battery’s storage performance. The higher the temperature, the more severe the performance degradation. After storage at 25 ℃, 45 ℃, and 55 ℃ for 241 days, the capacity retention rates were 91.47%, 80.19%, and 73.21%, respectively, while after storage at 65 ℃ for 150 days, only 70.34% remained. The internal resistance increases during storage. When the temperature reaches 55 ℃ or 65 ℃, it increases by 13.5mΩ(56.25%) and 16.9mΩ(70.42%) after 150 days of storage, respectively, with an increase of approximately 3.7 and 4.6 times that of storage at 25 ℃ for the same time.
2) The internal reactions of the battery during storage will cause polarization, and these reactions reduce the diffusion rate of lithium ions in the positive and negative electrode materials and electrolyte after storage, resulting in poor rate performance of the battery after storage. The discharge capacity at 2C magnification before storage is 77.90% of 0.2C, and after 30 days of storage at 65 ℃, the discharge capacity at 2C magnification is 66.81% of 0.2C.