Recent progress in understanding the solid electrolyte interphase of lithium and sodium metal anodes

The solid electrolyte interphase (SEI) is an indispensable component that enables the operation of non-aqueous lithium- and sodium-metal batteries (SMBs). Although alkali metals are thermodynamically unstable in organic electrolytes, reversible electrochemical operation becomes possible through the formation of an interphase that is electronically insulating yet ionically conductive, thereby suppressing continuous electrolyte reduction. This article reviews recent progress in understanding the structure, formation mechanisms, transport properties, and long-term evolution of the SEI on lithium- and sodium-metal anodes. Particular emphasis is placed on the classical two-layer model, which comprises a dense inorganic inner layer responsible for electronic passivation and a porous outer layer that accommodates mechanical stresses. Ion transport is shown to be dominated by grain boundaries and defects, explaining the high apparent Tafel slopes and SEI-controlled kinetics observed experimentally. Key differences between lithium and sodium SEIs are discussed, highlighting the higher solubility, poorer mechanical stability, and faster growth of sodium-derived interphases. Finally, electrolyte engineering strategies, including salt selection, sacrificial additives, high-concentration formulations, and metal-oxide nanoparticle additives, are evaluated as promising approaches for improving interfacial stability and durability. A mechanistic understanding of SEI formation and evolution remains central to achieving durable, high-energy alkali-metal batteries.