Dual‐Anion Sodium Halide‐based Solid Electrolytes With High Ionic Conductivity and High‐Voltage Stability

Sodium solid-state batteries (SSBs) offer a promising alternative to lithium-based systems due to their low cost, comparable energy density, and enhanced safety. However, the development of high-performance solid-state electrolytes (SEs) remains challenging due to low ionic conductivity and interfacial instability. Here, we report a halide-based SE, NaNb_xCl5_x-1O (NNCO, x is close to 1), synthesized via an anion mixing strategy by partially substituting Cl^- with O^2-. Compared to crystalline NaNbCl₆, the glassy NNCO shows a significantly enhanced Na⁺ ionic conductivity (>1.0 mS cm^-1 at 30 °C) and a low activation energy of 0.23 eV. Heat-treated NNCO (NNCO-HT) further improves structural order while retaining high ionic conductivity. The NNCO SE demonstrates excellent oxidative stability up to 4.3 V (vs. Na⁺/Na) and strong chemical compatibility with sulfide-based components. A full SSB incorporating doped-Na_2/3Ni_1/3Mn_2/3O₂ cathode, NNCO + Na₃PS₄ electrolyte, and Na₂Sn anode delivers an initial capacity of 95 mAh g^-1, with 80% capacity retention after 500 cycles and 73% after 1000 cycles at 0.3C. This work highlights anion mixing as an effective strategy to enhance halide-based SEs, providing a practical path toward high-performance Na SSBs.