Toward the Practical Integration of a Face-Centered Cubic (FCC)-Type Superionic Conductor in Solid-State Batteries

Li-ion conducting oxides with high electrochemical stability and bulk conductivity represent a critical component for solid-state Li metal batteries. Among them, the recently reported face-centered cubic (FCC) oxide with face-sharing Li configuration offers broad chemical space, good compatibility with oxide-type cathodes fast Li+ transport and exceptional oxidative stability. However, the investigation of this emerging family of superionic conductors has largely been limited at the material level, and their incorporation into practical solid-state battery (SSB) systems remains unexplored. In this work, we focus on the cell-level application of a model FCC-type superionic conductor, Li17In9SnO24 (LISO). We identify a critical challenge of incorporating this emerging class of material into practical SSB due to the severe interfacial incompatibility with Li metals, primarily caused by the reduction of In3+. We further demonstrate that such instability can be mitigated via a polymer−ceramic composite strategy. By encapsulating LISO particles with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), interfacial alloying with Li is effectively suppressed, enabling stable Li plating/stripping with low interfacial resistance. The resulting dual-functional composite electrolyte can then be stable cycling in both solid-state and quasi-solid-state batteries with various cathodes. This work marks the first demonstration of an FCC-type oxide superionic conductor in practical cell configuration, bridging the gap between material discovery and device application. Our composite strategy provides a viable path toward deploying high-conductivity FCC-structured electrolytes in next-generation solid-state battery technologies.