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 Al 2 O 3 filler. The “polymer‐in‐salt” strategy disrupts PEO crystallinity via concentrated Li salt to form continuous ion channels, while Al 2 O 3 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//LiFePO 4 coin cell retains 70.7% capacity after 300 cycles at 0.1C. Moreover, Li//LiFePO 4 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.