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), ZrO 2 -2Na 2 ZrCl 5 F, synthesized through a mechanochemical reaction using Na 2 O. This HNSE exhibits a substantial improvement in Na + conductivity (2.1 × 10 –5 S cm –1 at 30 °C) compared to Na 2 ZrCl 5 F (2.0 × 10 –7 S cm –1 ). The significant reduction in ionic conductivity of Na 2 ZrCl 5 F relative to Na 2 ZrCl 6 (2.0 × 10 –5 S cm –1 ) is elucidated through synchrotron pair distribution function (PDF) analysis. Structural insights, including the fine structure of the ZrO 2 nanograins embedded in an amorphous Na 2 ZrCl 5 F 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 0.66 Ni 0.1 Co 0.1 Mn 0.8 O 2 ||Na 3 Sn all-solid-state cells using ZrO 2 -2Na 2 ZrCl 5 F as the catholyte demonstrate enhanced performance at 30 °C compared to cells using Na 2 ZrCl 6 (47.4% capacity retention after 100 cycles vs 35.3% using Na 2 ZrCl 6 ).