Solid fast-ion conductors represent a distinct class of ion-conducting materials characterized by the diffusion of a specific type of atom through the interstitial spaces within a solid lattice while other atoms remain stationary in their crystal sites. This unique behavior allows liquid-like conduction of the particular atom within a solid framework. Notably, solid fast ion conductors exhibit comparable ionic conduction to liquids, particularly organic solvents, such as 25 mS cm -1 for Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , and 70 mS cm -1 for Na 2 (CB 9 H 10 )(CB 11 H 12 ) . Additionally, these materials have extended operating temperature range compared to their liquid counterparts , higher energy density, longer life cycles, and enhanced safety due to the absence of organic liquid or polymer electrolytes . The technological applications of solid fast-ion conduction are of significant interest, particularly in the development of all-solid-state batteries (ASSBs), fuel cells, and gas sensors .