A novel flexible composite polymer electrolyte with robust interfaces and synergistic ion channels for solid-state sodium metal batteries

The growth of sodium dendrites poses a significant challenge for solid-state sodium metal batteries (SSMBs), in which the solid-state electrolyte plays a crucial role. Composite polymer electrolytes (CPEs) receive extensive attention due to their combination of the advantages of inorganic and polymer electrolytes. Currently, CPEs still confront obstacles such as limited interfacial stability, insufficient ionic conductivity, and inadequate mechanical strength. Herein, a novel CPE is designed to integrate the electrospun Na₃Zr₂Si₂PO₁₂ (NZSP)/polyacrylonitrile (PAN) three-dimensional (3D) flexible framework and polyethylene oxide (PEO) to form a double-layer asymmetric structure with excellent electrode/electrolyte interface compatibility. Density functional theory calculations indicate that sodium ions exhibit lower migration energy barriers in NZSP/PAN compared to PAN. This CPE provides abundant and continuous inorganic-polymer synergistic fast ion channels, demonstrating high ionic conductivity (1.09 mS cm⁻¹) and Na⁺ migration number (0.55). The 3D NZSP/PAN nano-crosslinked structure offers a robust mechanical network with high Young's modulus. At 60 °C, the assembled Na symmetric cell can achieve a long cycle life of 3400 h at 0.2 mA cm⁻² and 0.2 mAh cm⁻². The assembled Na₃V₂(PO₄)₃||Na full cell retains high capacity retention of 91.2% after 1400 cycles at 1C. At room temperature, the full cell also demonstrates satisfactory cycling stability. During cycling, the resulting favorable interface significantly suppresses the growth of sodium dendrites and side reactions between the electrolyte and sodium anode. The solid electrolyte interphase is stable and thin with a balanced composition of inorganic and organic components, which facilitates rapid ionic transport at the interface.