The charge-discharge cycle of a lithium-ion battery pack is the process of lithium ions moving back and forth between the positive and negative electrode materials through the electrolyte. During this cycle, in addition to redox reactions at the positive and negative electrodes, numerous side reactions also occur. If these side reactions can be minimized, ensuring smooth movement of lithium ions between the positive and negative electrode materials through the electrolyte, the battery's cycle life can be extended.
The properties of the current collectors at the positive and negative electrodes also affect the battery's capacity and cycle life. The commonly used current collector materials for the positive and negative electrodes of lithium-ion battery packs are aluminum and copper, respectively, both of which are easily corroded metals. Corrosion of the current collector leads to the formation of a passivation film, poor adhesion, localized corrosion (pitting), and overall corrosion, all of which increase the battery's internal resistance, resulting in capacity loss and reduced discharge efficiency. Pretreatment methods such as acid-alkali etching and conductive coating can enhance its adhesion and corrosion resistance.
The utilization process of a lithium-ion battery pack is essentially a charge-discharge cycle. The magnitude of the charge-discharge current, the selection of the charge-discharge cutoff voltage, and the charging-discharge method used all significantly impact the cycle life of the lithium-ion battery. Blindly increasing the battery's operating current, raising the charge cutoff voltage, or lowering the discharge cutoff voltage will all degrade the performance of the lithium-ion battery pack.
The charging and discharging cutoff voltages differ between lithium-ion batteries with different electrochemical systems. During the charging process, any voltage exceeding the charging cutoff voltage is considered overcharge. When a lithium-ion battery is overcharged, excess lithium ions released from the positive electrode deposit or embed into the negative electrode. The deposited active lithium readily reacts with the solvent, releasing heat and raising the battery temperature. When the discharge voltage of a lithium-ion battery falls below the discharge cutoff voltage, over-discharge occurs.
