Solid Electrolyte Interphase (SEI)
Solid electrolyte interphase (SEI) is a passivating layer of solid reaction products that forms where a negative electrode meets the electrolyte, especially during the first charging cycles of a lithium-ion battery.
Why it matters
A useful SEI lets lithium ions pass while largely blocking electron transfer and continued electrolyte reduction. This helps stabilize the reactive electrode surface. It is not a separately installed separator or one designed material: its composition and structure emerge from the electrode, electrolyte, additives, temperature, and formation process.
The layer must remain thin enough for ion transport but intact enough to protect the surface. If it cracks or is otherwise disrupted as electrode particles expand and contract, newly exposed material can react with the electrolyte and form more SEI. Continued growth consumes cyclable lithium and electrolyte and can raise resistance, contributing to Battery Degradation.
For electrode interfaces, formation reactions, and material trade-offs, see Cell Chemistry & Components.