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

Abstract 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 (LiPF 6 ), and lithium tetrafluoroborate (LiBF 4 ) are employed as the catalysts for the copolymerization. These lithium salts can modulate the compositions of the solid‐electrolyte interphase (SEI) layer. PEA‐LiPF 6 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‐LiPF 6 demonstrates noteworthy capacity retention rates of 96.6% (0.5 C, 200th/first cycle in LiFePO 4 ||Li), 95.6% (0.5 C, 100th/first cycle in LiMn 0.6 Fe 0.4 PO 4 ||Li), 95.1% (1 C, 100th/first cycle in LiCoO 2 ||Li), and 87.0% (1 C, 100th/first cycle in LiNi 0.6 Co 0.2 Mn 0.2 O 2 ||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.