Evolution of the Lithium-Ion Conduction Mechanism in Solid-State Polymer Electrolytes

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Solid-state polymer electrolytes (SPEs) are central to the development of all-solid-state lithium metal batteries (ASSLMBs) with high safety, lightweight, and high energy density. Despite their mechanical and processing advantages, SPEs still suffer from limited room-temperature lithium-ion (Li⁺) conductivity, constraining fast-charging and high-power applications. This review traces the historical evolution of the lithium-ion conduction mechanisms in a series of polymer electrolytes over the past five decades, from the earliest amorphous-phase segmental motion model, through ceramic-filler-induced interfacial pathways, to the crystalline ion channels with molecularly engineered conduction routes, and finally to the synergistic mechanism frameworks. For each mechanistic stage, we correlate the transport physics with the material design strategies by focusing on the structure–property relationship. This review offers a coherent roadmap for achieving high room-temperature Li⁺ conductivity while maintaining mechanical integrity and electrochemical stability, serving as a reference for designing next-generation high-performance SPEs.