A Poly(ethylene oxide)-Na_3.2Zr₂Si_2.2P_0.8O₁₂ Composite Electrolyte Achieves a High-Performance All-Solid-State Sodium Battery

The electrochemical–mechanical properties of different solid-state electrolytes present diverse features; the ceramic solid-state electrolytes show excellent ionic conductivity and superior electrochemical stability, while the solid polymer electrolytes show advantages of excellent flexibility and simple preparing process. Unscalable preparation of ceramics and instability of polymers significantly restrict their commercial application in all-solid-state batteries. Here, a PEO-based solid polymer composite electrolyte (SPCE) incorporated with Na_3.2Zr₂Si_2.2P_0.8O₁₂ (NZSP) ceramic solid electrolyte particles is demonstrated to integrate the advantages of the two. The prepared PEO-NZSP SPCE presents an ionic conductivity of 2.71 × 10^–4 S cm^–1 (60 °C) accompanied by a moderate Na⁺ transference number of 0.34. The interfacial impedance of the Na||Na symmetric cell decreases from 136.13 to 82.72 Ω cm², indicating the improved interfacial contact of the designed Na/PEO-NZSP interface. The practicability of the SPCEs to diverse cathodes is validated, and the Na|PEO-NZSP|Na₄Fe₃(PO₄)₂P₂O₇ full cell presents a specific capacity of 97.4 mAh g^–1 with a high capacity-retention up to 98.77% after 500 cycles at 0.5C, and discharges a specific capacity of 122.3 mAh g^–1 even after 150 cycles at 0.1C, demonstrating significant advantages compared to the pristine PEO solid polymer electrolyte. A bipolar stacked full cell and a pouch cell are assembled, demonstrating the potential application on all-solid-state batteries with high energy/power density.