Regulating Li+ Solvation, Transport, and Interfacial Robustness via Voltage Resistant Cationic Copolymer Design for Safe Lithium Metal Batteries

The application of polymer electrolytes in high-performance lithium metal batteries (LMBs) is usually restricted by their sluggish ion conduction, and inferior electrochemical stability compatibility with electrodes. Here, a cationic copolymer-based electrolyte PMC is developed. The copolymer of acryloyloxyethyl trimethylammonium bis(trifluoromethanesulfonyl)imide (AET⁺TFSI⁻), hexafluorobutyl acrylate (HFBA), and N, N′-methylenebisacrylamide (MBA) is synthesized by photopolymerization with carbonate electrolytes. The copolymer facilitates the lithium salt dissociation, adjusts Li⁺ interaction with the polymer chain, regulates Li⁺ solvation environment, and thus promotes fast ion transport (ionic conductivity of 7.19 × 10⁻⁴ S cm⁻¹, Li⁺ transference number of 0.84) and uniform Li deposition. PMC is electrochemically stable up 4.43 V versus Li⁺/Li and forms stable solid electrolyte interphase (SEI) with the anode, supporting long-term stability (1500 h) of lithium plating/stripping test at 0.2 mA cm⁻², 0.2 mAh cm⁻². The Li/PMC/LiFePO₄ cell shows excellent stability at 1C and a high specific capacity of 134.2 mAh g⁻¹ even at 5C. PMC forms voltage-resisting, LiF-rich cathode electrolyte interface (CEI) with LiCoO₂. The Li/PMC/LiCoO₂ cell shows excellent stability over 100 cycles with a capacity retention of 96%. Nonflammability of PMC and high safety of PMC-based pouch type cells are confirmed. This work provides a facile method toward high-performance and safe LMBs.