In‐Built Reactive Polymer as Versatile Electrolyte to Shield the Bi‐Electrode Surfaces for Practical Li‐Metal Batteries

High-voltage lithium metal batteries (LMBs) represent promising candidates for next-generation energy storage systems, yet are fundamentally constrained by interfacial instability at both high-voltage cathodes (≥4.3 V vs. Li/Li⁺) and lithium metal anodes. Herein, we in situ prepared a reactive polymer electrolyte (PTGI) with a narrow orbital energy gap and good electrode affinity, which is preferentially reacted prior to solvent to create thermo-electrochemically robust cathode electrolyte interphase/solid electrolyte interphase, significantly suppressing the interfacial parasitic reactions and the structure degradation of single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ (SC-NCM) in a wide temperature range. Furthermore, the isocyanurate groups endow PTGI with enhanced capability to scavenge HF, while reducing the coordination power of the solvent molecules to promote a loose solvation shell for Li⁺ migration. Consequently, the Li|PTGI|SC-NCM cells deliver ultra-long and stable cycle life over 1000 cycles at 1 C. LMBs with LiFePO₄ can also stably cycle for 1700 cycles with 76.1% capacity retention. The assembled 1.5 Ah pouch cells, featuring a high-loading SC-NCM cathode and lean PTGI electrolyte, also exhibit good cycling stability. The in-built reactive strategy in this work provides new insights into developing advanced LMBs with high cycle stability and safety.