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. Na0.66Ni0.1Co0.1Mn0.8O2||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).