High‐Performance Poly(ethylene Oxide)‐Based Composite Solid Electrolyte with Enhanced Room‐Temperature Ionic Conductivity for Lithium‐Metal Batteries

Poly(ethylene oxide) (PEO)-based solid-state polymer electrolytes have emerged as one of the most promising candidates for high-energy-density lithium-metal batteries in energy storage applications. However, their notably low ionic conductivity has significantly hindered PEO-based polymer electrolytes' practical application and development, particularly at room temperature (RT). To address the critical challenge of low RT ionic conductivity in PEO-based solid-state electrolytes, this study develops a composite solid electrolyte (CSE) synergistically reinforced by high-concentration lithium salts and Al₂O₃ filler. The "polymer-in-salt" strategy disrupts PEO crystallinity via concentrated Li salt to form continuous ion channels, while Al₂O₃ further suppresses crystallization and bridges ionic cluster transport. This dual optimization achieves an ultrahigh ionic conductivity of 9.62 × 10^-4 S cm^-1 and a 4.8 V electrochemical stability window at RT. The Li symmetric cell exhibits stable cycling for 1000 h at 1 mA cm^-2/1 mAh cm^-2, and the all-solid-state Li//LiFePO₄ coin cell retains 70.7% capacity after 300 cycles at 0.1C. Moreover, Li//LiFePO₄ pouch cell maintains 96.4% capacity over 100 cycles at RT. This work establishes a new paradigm for material design and performance enhancement in practical solid-state lithium batteries.