Developing “Polymer‐in‐Salt” High Voltage Electrolyte Based on Composite Lithium Salts for Solid‐State Li Metal Batteries

Abstract The stringent demands for lithium salts make the design of “polymer‐in‐salt” type solid electrolyte restricted since it was proposed in 1993. Herein, a novel polymer‐in‐salt solid electrolyte is developed via a supramolecular strategy based on poly(methyl vinyl ether‐alt‐maleic anhydride) (PME) and novel single‐ion lithiated polyvinyl formal (LiPVFM)/lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) composite salts (Dual‐Li). Hydroxyl of LiPVFM in Dual‐Li forms a strong hydrogen bond with the carboxylic acid group generated by the partial ring‐opening reaction of maleic anhydride in PME. Meanwhile, PME with abundant carbonyl enables the improved LiTFSI coordination in the polymer/salt composites. As a result, the greatly enhanced mutual solubility of PME and Dual‐Li is of importance to build a “polymer‐in‐salt” solid electrolyte (PISE), which exhibits high ionic conductivity of 3.57 × 10 –4 S cm –1 , wide electrochemical window beyond 5 V, and superior lithium‐ion transference number of 0.62 at 25 °C as well as excellent interfacial compatibility with electrodes. The as‐assembled LiCoO 2 ||Li solid batteries present prominent high‐voltage cyclability with 89.2% capacity retention in 225 cycles. Furthermore, LiNi 0.7 Mn 0.2 Co 0.1 O 2 ||Li pouch cells exhibit remarkable safety even under harsh conditions. The study offers a promising strategy to address the high voltage compatibility and interfacial issues using PISE in solid‐state batteries.