Organic-inorganic composite solid-state electrolytes: Filler-specific design strategies and practical prospects

Organic-inorganic composite solid-state electrolytes (CSSEs) represent a promising class of electrolyte materials for solid-state lithium batteries, as they effectively integrate the processability of polymers with the ionic transport, mechanical, and interfacial properties of inorganic solid-state electrolytes. However, their design usually does not rely on straight-forward filler-addition strategy, because each category of inorganic phases exhibits distinct conductivities, surface chemistries, structural connectivity, and processing compatibility. In this review, we discuss CSSEs based on specific functions and limitations of filler materials. For polymer-oxide systems, of the analysis focuses on local interfacial regulation, continuous ceramic pathways, and multilayer architectures tailored for electrode-specific interfaces. Polymer-sulfide systems are discussed with an emphasis on the preservation of intrinsic ionic conductivity during thin-film formation, which critically depends on binder chemistry, solvent compatibility, film density, and sulfide-sulfide contact. Polymer-halide systems are examined as an emerging direction for high-voltage cathode environments and interfacial stabilization, where growing opportunities emerge to extend their use toward thin, stable, and low-resistance electrolyte films. Finally, this review links filler-specific design with scalable, low-resistance thin films and practical cell integration, while identifying quantitative filler-polymer interfacial relationships, mechanical reliability, low-pressure operation, and dry-room processability as key future directions.