Extending the Electrochemical Window of Na⁺ Halide Nanocomposite Solid Electrolytes for 5 V-Class All-Solid-State Na-Ion Batteries

This study introduces a Na⁺ fluorinated halide nanocomposite solid electrolyte (HNSE), ZrO₂-2Na₂ZrCl₅F, synthesized through a mechanochemical reaction using Na₂O. This HNSE exhibits a substantial improvement in Na⁺ conductivity (2.1 × 10^-5 S cm^-1 at 30 °C) compared to Na₂ZrCl₅F (2.0 × 10^-7 S cm^-1). The significant reduction in ionic conductivity of Na₂ZrCl₅F relative to Na₂ZrCl₆ (2.0 × 10^-5 S cm^-1) is elucidated through synchrotron pair distribution function (PDF) analysis. Structural insights, including the fine structure of the ZrO₂ nanograins embedded in an amorphous Na₂ZrCl₅F matrix and the potential O-substituted interphase, are revealed through X-ray absorption spectroscopy, PDF, and cryogenic transmission electron microscopy. Fluorinated HNSEs offer exceptional electrochemical oxidative stability up to 5 V (vs Na/Na⁺), enabling high-voltage cathode applications. Na_0.66Ni_0.1Co_0.1Mn_0.8O₂||Na₃Sn all-solid-state cells using ZrO₂-2Na₂ZrCl₅F as the catholyte demonstrate enhanced performance at 30 °C compared to cells using Na₂ZrCl₆ (47.4% capacity retention after 100 cycles vs 35.3% using Na₂ZrCl₆).