The Solid Electrolyte Interphase (SEI) is a passive interface that forms at the interface between the anode, which is carbon, and the electrolyte in some rechargeable batteries
, and is formed by the decomposition of the electrolyte. The Solid Electrolyte Interphase forms in lithium ion (LiIon) and lithium titanate batteries at the interface between the anode and electrolyte. The SEI layer, which forms a passivation layer on the anode, increases the internal resistance of the battery. The passivation layer prevents further decomposition of the electrolyte. This adverse effect can be reduced by appropriate choice of electrolytic materials and by chemical additives. There are also additives that prevent the formation of SEI layers and thus improve the intercalation of lithium ions in the anode, which is important for fast charging of the batteries.