The ionic interphases of the lithium anode in solid state batteries

Solid state battery (SSB) performance is largely governed by processes occurring at electrolyte–electrode interfaces. At the Li metal anode, where the overwhelming majority of solid electrolyte (SE) are unstable against Li metal, the interface can readily react to form emergent Li-solid electrolyte interphases (SEI) with ionic, electronic, chemical, mechanical, and electrochemical properties substantially distinct from the parent phase. Facing similar challenges with liquid electrolytes, the Li battery community underwent a half century-long effort, still in progress, to illuminate fundamental properties of the Li SEI—including chemistry, morphology, transport, and sources of Li loss upon cycling—from which guiding principles have emerged to drive improvement in electrolyte and interface design. The Li metal SEI with solid electrolytes presents both similarities and differences to that in liquid electrolytes, with differences defining unique research needs. Here, we examine current understanding of the Li-SE interface as well as learnings from the liquid electrolyte community that we propose might be adopted to help rationalize and improve SE integration with Li anodes. Through this lens, we inspect current state-of-understanding of Li SEI composition, structure, and properties, along with Coulombic efficiency values reported so far for Li cycling with SE. We also highlight potential Li modification strategies for SSB, which are informed by and exploit understanding of the ionic SEI phases; in some instances, engineering strategies utilize a liquid electrolyte SEI directly, making liquid-derived SEI knowledge of immediate relevance.