Solid–Gas Interphase Formation in Anode-Free Solid-State Batteries

The solid electrolyte interphase (SEI) is known to play an essential role in battery performance. However, for highly reactive components such as Li metal anodes, additional side reactions may also contribute to interfacial stability. In particular, for solid-state batteries (SSBs), there exists a unique interface where exposed Li metal surfaces come into contact with the surrounding gas-phase molecules during operation, which is distinct from the solid-liquid interface that exists in a liquid electrolyte system. In this work, we study the dynamic formation of a solid-gas interphase (SGI) layer on the exposed surfaces of the Li metal anode during plating, stripping, and open-circuit rest periods. A customized environmental chamber was fabricated to allow for control of the background gas composition and pressure while cycling SSBs under stack pressure. The formation of an SGI was found to depend on the plated Li capacity, ambient gas composition, and aging time, which has a direct impact on Coulombic efficiency. To reveal the dynamic formation of the SGI layer with trace background reactant gases, operando X-ray photoelectron spectroscopy (XPS) was performed under ultrahigh vacuum conditions. Solid-state pouch cells were assembled in a commercially relevant dry room environment, revealing the influence of SGI formation on calendar life in practical cell configurations. These findings highlight the importance of SGI formation, which is distinct from the classical SEI layer that arises from electrolyte decomposition, in the analysis of SSBs.