In Li-ion batteries, what is the role of the SEI layer?

• A. A passivation layer on the anode that prevents further electrolyte reduction while allowing Li+ transport, improving longevity.
• B. A protective coating on the cathode to prevent lithium plating.
• C. A conductor layer to increase electron mobility.
• D. A barrier to prevent solvent evaporation.

Answer: (D)

The SEI layer is a passivation film that forms on the anode surface during initial charging. It arises from the reduction of electrolyte components and stabilizes the interface by halting further electrolyte decomposition, while still allowing lithium ions to pass through. This selective ion conductivity (ionic, not electronic) lets Li+ intercalate into the anode while stopping electrons from continuing to react with the electrolyte, which would wear away the electrolyte and degrade performance. As a result, the SEI protects the electrode, reducing capacity fade and improving long-term cycle life. It’s not a protective coating on the cathode, not a conductor layer, and not primarily a barrier to solvent evaporation.