Review of Inorganic Separator Engineering for Next-Generation Lithium-Sulfur Batteries: Compromise or Cornerstone?

Lithium-sulfur (Li-S) batteries are promising next-generation energy-storage systems, but their practical application remains limited by polysulfide shuttling, sluggish redox kinetics, and interfacial instability. Separator engineering offers an effective route to regulate sulfur chemistry. This review summarizes metal compound-based separators as active interfacial architectures that promote polysulfide adsorption and catalytic conversion while regulating ion and electron transport. To evaluate diverse separator systems, we propose a Practical Relevance Index (PRI)-guided framework that integrates intrinsic electrochemical improvements with system-level constraints. A localized figure of merit is further introduced to interpret performance trends without direct cross-study ranking. Recent advances in metal oxides, sulfides, nitrides, carbides, and MXene-based separators are discussed, with emphasis on heterostructures, defect engineering, electronic-structure regulation, and atomic-scale design. Particular attention is given to constraint-aware strategies that connect interfacial chemistry with practical energy density. Finally, major challenges and future directions are outlined, including reaction-pathway regulation, operando characterization, system integration, and scalable manufacturing. This review provides a unified framework for designing practical, high-energy-density Li-S batteries.