Why do rechargeable batteries lose capacity over time?

Rechargeable batteries lose capacity over time due to chemical reactions that degrade the battery's components.

Rechargeable batteries, such as lithium-ion batteries, work through a process of charge and discharge cycles. During a charge cycle, lithium ions move from the positive electrode to the negative electrode through an electrolyte. During discharge, the ions move back to the positive electrode, providing energy. This process is not perfectly efficient and with each cycle, a small amount of lithium ions fail to make the return journey, reducing the battery's capacity.

The degradation of battery capacity, also known as battery wear, is primarily caused by two chemical processes: solid electrolyte interface (SEI) layer formation and lithium plating. The SEI layer is a film that forms on the surface of the negative electrode during the first few charge cycles. While it helps to stabilise the battery, it also consumes lithium ions, reducing the number of ions available for energy storage. Over time, the SEI layer thickens, consuming more lithium ions and further reducing the battery's capacity.

Lithium plating is another process that contributes to battery wear. It occurs when lithium ions are deposited onto the negative electrode during charging, forming a metallic lithium layer. This process is particularly prevalent when the battery is charged quickly or at low temperatures. Like the SEI layer, lithium plating consumes lithium ions and reduces the battery's capacity.

In addition to these chemical processes, physical factors can also contribute to battery wear. For example, high temperatures can accelerate chemical reactions, leading to faster degradation. Similarly, overcharging or discharging a battery can cause physical damage to the battery's components, reducing its capacity.

In conclusion, the loss of capacity in rechargeable batteries is a complex process involving both chemical reactions and physical factors. While some degree of wear is inevitable, understanding these processes can help to develop strategies to extend the life of rechargeable batteries.