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

Abstract 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, NaNbxCl5x−1O (NNCO, x is close to 1), synthesized via an anion mixing strategy by partially substituting Cl− with O2−. 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‐Na2/3Ni1/3Mn2/3O2 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.