Salt‐Optimized Interface for Mitigating Cycle‐Induced Interface Fluctuation of In Situ Polymerized Solid State Lithium Metal Batteries

ABSTRACT In situ poly(1,3‐dioxolane) (PDOL) electrolytes hold great promise for improving initial interface contact (before cycling) in solid‐state batteries. However, the Li anode/in situ PDOL electrolyte interface suffers from the cycle‐induced interface fluctuation, causing renewed interfacial contact loss and poor dynamic interface stability. Herein, we systematically investigate the influence of lithium salt initiators on both physicochemical properties of the solid electrolyte interface (SEI) and dynamic interfacial contact. And we find that PDOL electrolyte initiated by lithium bis(fluorosulfonyl)imide, termed FPDOL, contributes to the formation of SEI with high viscoelasticity and Li‐ion conductivity. Specifically, the SEI outer layer with a high content of organic ─CF x species allows it to maintain conformal contact with the dynamically evolving Li surface, preventing contact loss at the interface during cycling. Meanwhile, the SEI inner layer containing Li 3 N and Li 2 S lowers the energy barriers of Li‐ion diffusion, accelerating interfacial dynamic kinetics. Therefore, the Li||FPDOL||Li cells achieve stable cycling performance of up to 1600 h at 0.1 mA cm −2 with 0.1 mAh cm −2 . Furthermore, the LiFePO 4 ||FPDOL||Li cells maintain a prolonged cycling life of over 250 cycles with a capacity retention of 97.8% at 1C. This work provides insights into advancing the dynamic interfacial stability of the in situ PDOL system.