Polymer‐Metal Oxide Cluster Co‐Crystallization for the Synergy of Superionic Conduction and Mechanical Robustness

ABSTRACT Solid polymer electrolytes (SPEs) promise high safety for next‐generation energy storage, but practical applications are bottlenecked by the coupling of ion transport with sluggish polymer segmental dynamics. Herein, polymers are co‐crystallized with sub‐nm anionic metal oxide clusters (MOCs) using Li + counterions, achieving synergistic superionic conduction and mechanical robustness. Mixing MOC (Li 4 SiW 12 O 40 ) and poly(ethylene glycol) (PEG) affords face‐centered cubic crystalline structures at high MOC loadings (≥ 60 wt.%). The crystalline framework imposes spatial nanoconfinement that favors a pronounced shift of PEG chains toward densely packed zigzag‐like conformations. Relaxation‐dynamics studies reveal substantial decoupling between Li + transport and PEG segmental relaxation, thereby enabling a superionic transport regime. The optimized composite achieves an ionic conductivity of 1.1 mS cm −1 at 110°C with an apparent activation energy of 0.32 eV. Since the long‐range mobility of SiW 12 4− is constrained, the electrolyte exhibits pronounced single‐ion‐conducting character with a high lithium‐ion transference number (0.79). Furthermore, the composition with inorganic Li 4 SiW 12 O 40 enhances the mechanical modulus (25.04 MPa) and thermal stability with intrinsic flame retardancy. Solid‐state symmetric supercapacitors fabricated from the electrolyte exhibit predominantly electric‐double‐layer capacitive behavior with a high specific capacitance (76 F g −1 ) and excellent rate capability (87.16%).