Flame-Retardant PEO-Based Composite Solid Polymer Electrolyte with Modified Ammonium Polyphosphate for High-Safety and Long-Cycle Lithium-Ion Batteries

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) still face critical challenges in all-solid-state lithium-ion batteries, including insufficient ionic conductivity and inherent flammability. To overcome these limitations, a flame-retardant SPE (PA-SPE) was developed by incorporating ammonium polyphosphate (MAPP) encapsulated in a rigid-flexible polyurethane shell into the PEO. The rigid-flexible polyurethane shell surrounding MAPP serves as a protective barrier that mitigates undesirable reactions between the flame retardant and lithium metal, while simultaneously acting as a thermally responsive layer that enables the controlled release of flame-retardant species at elevated temperatures, allowing the PA-SPE to achieve rapid self-extinguishing within 1s. Compared with pristine PEO, the mechanical strength of PA-SPE is enhanced by a factor of 2.6. More importantly, the incorporation of MAPP markedly improves the ionic conductivity of the PA-SPE, reaching 1.012 × 10^–4 S cm⁻¹ at 30 °C. The electrochemical stability window of PA-SPE is extended to 4.8 V, while the Li⁺ transference number is significantly enhanced to 0.41. Furthermore, Li‖LiFePO₄ assembled with PA-SPE maintain 97.78% of the initial capacity after 200 cycles. The proposed strategy provides a universal solution for the development of all-solid-state lithium-ion batteries with high safety and long-cycle life.