Mechanism-Guided Multiscale Functional Design of Solid Polymer Electrolytes for Balanced Lithium Salt Dissociation and Ion Conduction

Solid polymer electrolytes (SPEs) are central to next-generation high-energy-density solid-state lithium metal batteries (LMBs). However, their practical deployment is severely hindered by intrinsic trade-off between efficient lithium salt dissociation and rapid Li⁺ conduction, which originates from strong ion–ion/ion–polymer interactions, low dielectric environment, and sluggish segmental dynamics. This review provides comprehensive, mechanism-guided overview to address this long-standing dilemma. We first elucidate fundamental constraints governing lithium salt dissociation (coulombic association, dielectric limitation, donor number matching) and key bottlenecks for Li⁺ migration (segmental coupling/decoupling, interfacial desolvation barrier, ion clustering). We then summarize four categories of multiscale optimization strategies: (1) polymer matrix engineering via functional group modulation and topological regulation; (2) lithium salt innovation and concentration optimization; (3) multifunctional filler design for promoted salt dissociation and fast-ion highways; (4) multi-component synergistic integration toward balanced dissociation and conduction. Emphasis is placed on discussing structure-mechanism-performance relationships and design principles for overcoming difficulties in lithium salt dissociation and ion migration. Finally, we outline remaining challenges in wide-temperature adaptation, high-voltage compatibility, and interfacial stability, and propose directions including machine learning-accelerated design, dynamic adaptive electrolytes, bio-inspired ion channel fillers, and operando characterization. This review provides theoretical and practical guidance for developing SPEs toward industrial LMBs applications.