Integrated Multiphase Polymer Electrolyte Enhances Bilateral Interface Compatibility in High Nickel Ternary Solid‐State Lithium Metal Batteries

Abstract As the core component in solid‐state lithium metal batteries (SSLMBs) using high nickel ternary (Ni‐rich NCM) cathode, solid electrolytes must exhibit good interfacial compatibility with both electrodes to withstand their strong oxidation and reduction, respectively. However, meeting these requirements remains challenging for single‐electrolyte systems. Herein, a multiphase polymer electrolyte is designed by integrating polyethylene oxide (PEO)‐based composite polymer electrolyte with nitrile‐based gel polymer electrolyte (GPEs). This strategy enhances the bilateral interface compatibility of the overall electrolyte through the complementarity of two polymer systems. Binding energy calculations reveal that PEO component effectively anchors nitrile‐based molecules, thereby mitigating their corrosion on Li metal. The polar nitrile‐based GPEs enhance the antioxidation of the polymer matrix with an expanded electrochemical window. Additionally, introduced surface‐modified nanofillers bridge with polymer chains to construct efficient Li⁺ transport channels for improving ionic conductivity (1.03 × 10⁻³ S cm⁻¹ at 25 °C). Utilizing this electrolyte, the LiNi_0.9Co_0.05Mn_0.05O₂/Li cell delivers an initial capacity of 225.1 mAh g⁻¹ (0.3 C) and 87.9% retention after 150 cycles, the corresponding pouch cell output with 40 mAh (0.3 C). Furthermore, the LiNi_0.9Co_0.05Mn_0.05O₂/graphite pouch cell demonstrates remarkable safety while cycling (119 mAh at 0.3 C). This strategy is promising for developing safe and high‐performance Ni‐rich NCM SSLMBs in the future.