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.