Long‐Life Lithium Metal Batteries Enabled by In Situ Solidified Polyphosphoester‐Based Electrolyte

Abstract The practical application of lithium metal batteries (LMBs) is hindered by the imbalanced periodic oscillatory distribution of cations/anions in liquid electrolytes (LEs) and thus the formed mechanically vulnerable solid electrolyte interphase (SEI), which collectively exacerbate lithium (Li) dendrite formation and degrade electrochemical stability. To overcome these issues, a polyphosphoester electrolyte (PPUM‐PE) is designed through a dual‐ion regulation strategy. The ‒NH‒ moieties in PPUM polymer effectively anchor anions, while its P═O/C═O functional groups reconstruct Li + solvation architecture, collectively enabling an exceptional Li + transference number (0.82) and improved reductive stability of the solvation sheath. A bilayer SEI layer formed on Li anodes—composed of an outer lithium‐containing alkyl phosphate polymer and an inner LiF‐enriched inorganic phase—exhibits high Young's modulus, effectively suppressing Li dendrite propagation and continuous electrolyte decomposition. Impressively, the as‐assembled LMBs employing LiFePO 4 cathodes retain 91.28% capacity retention after 1000 cycles at 1C. The electrolyte also demonstrates good compatibility with high‐voltage cathodes (LiCoO 2 , LiNi 0.8 Co 0.1 Mn 0.1 O 2 ) and substantially improves battery thermal safety. This dual‐ion synergistic regulation provides a scalable pathway toward high‐energy‐density LMBs.