Lithium - polymer energy storage batteries, a type of lithium - ion battery, are known for their high energy density and flexibility in design. However, like all batteries, they are subject to capacity fade over time, which is a gradual reduction in their ability to store and release electrical energy.

One of the main causes of capacity fade in lithium - polymer batteries is the formation of a solid - electrolyte interphase (SEI) layer on the surface of the electrodes. During the charging and discharging processes, chemical reactions occur at the electrode - electrolyte interface. Over time, these reactions can lead to the deposition of a layer of insoluble compounds, which forms the SEI layer. As the SEI layer thickens, it increases the resistance between the electrodes and the electrolyte, impeding the movement of lithium ions. This results in a reduced ability of the battery to store and release energy, causing the capacity to fade.

Another factor contributing to capacity fade is the degradation of the active materials in the electrodes. The cathode and anode materials in lithium - polymer batteries are prone to structural changes over repeated charge - discharge cycles. For example, in the cathode material, the crystal structure can gradually break down, reducing the number of available sites for lithium - ion insertion and extraction. This leads to a decrease in the battery's capacity. In addition, the anode material, typically graphite, can experience exfoliation or cracking over time, which also affects the battery's performance and capacity.

The operating temperature of lithium - polymer batteries has a significant impact on capacity fade. High temperatures accelerate the rate of chemical reactions within the battery, including the formation of the SEI layer and the degradation of the active materials. Batteries operating in hot environments, such as in some outdoor electronic devices or in poorly ventilated battery packs, will experience more rapid capacity fade. On the other hand, low temperatures can also cause problems. At low temperatures, the electrolyte's viscosity increases, which reduces the mobility of lithium ions and can lead to uneven lithium - ion plating on the anode. This uneven plating can cause irreversible damage to the anode and contribute to capacity fade.

The charge - discharge rate also plays a role in capacity fade. Fast charging and discharging at high currents can cause stress on the battery's internal components. High - current charging can lead to over - potential at the electrodes, which can accelerate the degradation of the active materials and the formation of the SEI layer. Similarly, high - current discharging can cause a rapid depletion of lithium ions at the electrodes, leading to uneven distribution of lithium within the battery and contributing to capacity fade. In applications where lithium - polymer batteries are used in devices with high - power demands, such as in some high - performance drones, proper management of the charge - discharge rate is essential to minimize capacity fade and extend the battery's lifespan.