Ionic Microenvironment Regulator Enables Enhanced Anion Trapping in Cationic Poly(Ionic Liquid) Solid Electrolyte for Low‐Temperature Lithium Metal Batteries

Despite the considerable promise of solid polymer electrolytes (SPEs) for enabling high‐energy‐density and intrinsically safe lithium metal batteries, their practical deployment in cold environments remains hindered by sluggish Li + transport at sub‐zero temperatures. Herein, methyl trifluoromethanesulfonate (TMS) is introduced as an ionic microenvironment regulator into a cationic poly(ionic liquid) solid electrolyte, enabling enhanced Li + transport under extreme low‐temperature conditions. Notably, TMS not only enhances lithium salt dissociation but also induces an anion‐trapping effect by modulating the local charge distribution of the polycationic backbone, thereby facilitating uniform lithium deposition. As a result, the optimized cationic poly(ionic liquid) solid electrolyte (PIL‐TMS 1 ) enables stable cryogenic operation of high‐voltage Li||PIL‐TMS 1 ||LiNi 0.8 Co 0.1 Mn 0.1 O 2 and Li||PIL‐TMS 1 ||LiFePO 4 batteries, delivering capacity retentions of 89.8% after 500 cycles and 80.1% after 800 cycles at −15°C, respectively. This work presents an effective strategy for designing low‐temperature solid‐state lithium metal batteries through additive‐regulated anion trapping and accelerated Li + transport.