Solid Composite Polymer Electrolyte Lithium‐Ion Battery With an Improved Electrochemical Stability Window and Cycle Characteristics

Polymer electrolytes for lithium‐ion batteries (LIBs) face challenges such as low ionic conductivity, insufficient mechanical flexibility, and limited thermal stability, which restrict their use in high‐energy density applications. To address these issues, a solid composite polymer electrolyte (SCPE) is developed by blending poly(ethylene oxide) (PEO) with poly(ethylene glycol) dimethyl ether (PEGDME) as a synthetic electrolyte and adding an F‐doped lithium aluminum titanium phosphate inorganic filler. As a result, the prepared CPE exhibits a high ionic conductivity of 1.24 × 10 −4 S/cm and excellent thermal stability, along with an extended electrochemical stability window (ESW) up to 6.8 V. When applied to LiFePO 4 half‐cells, the CPE enables charge and discharge capacities of 152.3 and 152.0 mA h/g, respectively, while maintaining a low voltage hysteresis of approximately 0.16 V throughout the cycles. Although the initial cycle shows a relatively high irreversible capacity of 3.7 mA h/g, the cell demonstrates outstanding cycling stability, retaining 98.9% of its initial capacity after 100 cycles. The developed composite electrolyte shows potential for use in safe and high‐performance all‐solid‐state LIBs, providing a viable pathway toward safer and high‐performance solid‐state LIBs suitable for advanced energy storage applications.