Poly(ester-alt-acetal) Electrolyte via In Situ Copolymerization for High-Voltage Lithium Metal Batteries: Lithium Salt Catalysts Deciding Stable Solid-Electrolyte Interphase

The in situ-formed polymer electrolytes provide a vital solution for improving both safety and performance in the high-voltage lithium metal batteries. This study reports new poly(ester-alt-acetal) (PEA) electrolytes, synthesized through in situ alternating copolymerization of glutaric anhydride and 1,3-dioxane. In the presence of 25 wt.% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), three lithium salts, lithium difluoro(oxalate)borate (LiDFOB), lithium hexafluorophosphate (LiPF₆), and lithium tetrafluoroborate (LiBF₄) are employed as the catalysts for the copolymerization. These lithium salts can modulate the compositions of the solid-electrolyte interphase (SEI) layer. PEA-LiPF₆ exhibits outstanding SEI chemistry, with observing the highest LiF content, thereby achieving a remarkable critical current density of up to 2.5 mA cm⁻², a Li⁺ transference number of 0.81, and an expansive electrochemical stability window of 6.0 V. Furthermore, PEA-LiPF₆ demonstrates noteworthy capacity retention rates of 96.6% (0.5 C, 200th/first cycle in LiFePO₄||Li), 95.6% (0.5 C, 100th/first cycle in LiMn_0.6Fe_0.4PO₄||Li), 95.1% (1 C, 100th/first cycle in LiCoO₂||Li), and 87.0% (1 C, 100th/first cycle in LiNi_0.6Co_0.2Mn_0.2O₂||Li full-cells). This work demonstrates a facile in situ route to fabricate polymer electrolytes for high-voltage lithium-metal batteries with balanced and comprehensive performance.