Toward a Unified Mechanistic Understanding of Polymer Electrolytes for Advanced Solid-State Batteries.

Polymer electrolytes (PEs) are widely regarded as a promising platform for solid-state batteries (SSBs), offering the potential to simultaneously achieve high energy density with improved safety. However, in current literature, PEs spanning liquid-percolated gels, liquid-assisted quasi-solids, and truly polymer-governed solids are often indiscriminately grouped as solid polymer electrolytes (SPEs), obscuring their distinct ion transport mechanisms, interfacial behaviors, and practical performance constraints, and leading to misleading performance comparisons and unrealistic expectations regarding solid-state operation. Herein, we establish a mechanistic framework that categorizes PEs into gel polymer electrolytes (GPEs), quasi-solid polymer electrolytes (QSPEs), and all-solid polymer electrolytes (ASPEs) based on their dominant ion-solvation environment and transport pathways. By systematically analyzing the ion-transport mechanisms, interfacial behaviors, and performance-limiting features associated with each PE class, we clarify their defining characteristics and mechanism-imposed limitations. Accordingly, we outline category-specific research priorities and highlight the necessity of mechanism-driven materials design, transparent definitions and reporting, and application-relevant benchmarking. This unified Perspective lays a foundation for consistent interpretation, meaningful comparison across PE systems, and more rational materials design toward the advancement of PE-enabled SSBs.