Steric‐Hindrance Polyoxometalate Li 4 SiW 12 O 40 Enabling Fast Li + Conduction and Ultrastable Li Metal Interface Chemistry for Solid‐State Lithium Metal Batteries

ABSTRACT Solid‐state lithium metal batteries (SLMBs) hold promise for next‐generation high‐energy‐density energy storage batteries, but encounter critical challenges of intrinsically sluggish Li + transfer kinetics and poor electrolyte/electrode interfacial stability. Herein, a Keggin‐type polyoxometalate (POM), lithium silicotungstate (Li 4 SiW 12 O 40 , SiWLi), acting as a functional additive is introduced into the cationic poly(ionic liquid) (PIL) electrolyte to modulate the Li + coordination environment. The cationic groups of PIL and SiW 12 O 40 4− anions of SiWLi collectively promote the sufficient dissociation of LiTFSI through electrostatic interactions. Profiting from the anchoring effect of PIL toward TFSI − and the strong steric‐hindrance effect of large‐size SiW 12 O 40 4− , anions movement is effectively restrained to achieve a high Li + transference number of 0.82. Moreover, the introduction of SiW 12 O 40 4− helps to construct more contact ion pairs coordination, inducing an anions‐derived LiF‐rich SEI layer to facilitate the stable Li metal anode without dendrite formation. The designed composite solid‐state electrolyte supports the stable operation of Li||Li cells for over 3000 h at 0.3 mAh cm −2 . When paired with LiFePO 4 (LFP) cathodes, the full cells deliver specific capacities of 145.03 mAh g −1 (99.5% retention after 600 cycles). This work provides an effective strategy for using steric‐hindrance POMs to regulate Li + solvation structure, boosting the design of high‐performance SLMBs.