Which degradation mechanism is primarily responsible for capacity fade in lithium-ion batteries?

The formation and growth of the Solid Electrolyte Interphase (SEI) layer is a primary degradation mechanism responsible for capacity fade in lithium-ion batteries. The SEI layer is a thin film that forms on the surface of the anode (typically graphite) due to the decomposition of the electrolyte during the initial charging cycles. This layer is crucial for the battery's operation because it prevents further decomposition of the electrolyte while still allowing lithium ions to pass through. However, over time, the SEI layer continues to grow, consuming lithium ions from the electrolyte and increasing the internal resistance of the battery. The consumption of lithium ions reduces the amount of cyclable lithium, which directly contributes to capacity fade, reducing the battery's ability to store charge. The SEI layer's growth is accelerated by high temperatures, high charging voltages, and the presence of impurities in the electrolyte. In addition to SEI formation, other degradation mechanisms, such as lithium plating (the deposition of metallic lithium on the anode surface), electrolyte decomposition, and structural changes in the electrode materials, also contribute to capacity fade, but SEI layer growth is often considered the dominant factor, especially during the early stages of battery life.