Durable and Adjustable Interfacial Engineering of Polymeric Electrolytes for Both Stable Ni‐Rich Cathodes and High‐Energy Metal Anodes

Abstract Achieving stable cycling of high‐voltage solid‐state lithium metal batteries is crucial for next‐generation rechargeable batteries with high energy density and high safety. However, the complicated interface problems in both cathode/anode electrodes preclude their practical applications hitherto. Herein, to simultaneously solve such interfacial limitations and obtain sufficient Li⁺ conductivity in the electrolyte, an ultrathin and adjustable interface is developed at the cathode side through a convenient surface in situ polymerization (SIP), achieving a durable high‐voltage tolerance and Li‐dendrite inhibition. The integrated interfacial engineering fabricates a homogeneous solid electrolyte with optimized interfacial interactions that contributes to tame the interfacial compatibility between LiNi_xCo_yMn_zO₂ and polymeric electrolyte accompanied by anticorrosion of aluminum current collector. Further, the SIP enables a uniform adjustment of solid electrolyte composition by dissolving additives such as Na⁺ and K⁺ salts, which presents prominent cyclability in symmetric Li cells (>300 cycles at 5 mA cm⁻²). The assembled LiNi_0.8Co_0.1Mn_0.1O₂ (4.3 V)||Li batteries show excellent cycle life with high Coulombic efficiencies (>99%). This SIP strategy is also investigated and verified in sodium metal batteries. It opens a new frontier for solid electrolytes toward high‐voltage and high‐energy metal battery technologies.