Break it down to speed it up: Na₂O-NaTaCl₆

Fast sodium-ion conductors hold strong potential for enabling all-solid-state sodium batteries, offering inherent advantages such as enhanced safety and cost-effectiveness. However, challenges remain in achieving fast ion transport for realizing high-power density. This work reports the synthesis of a novel sodium solid electrolyte, Na₂O-NaTaCl₆ (Na₂O-NTC), via an energy-efficient approach. We achieved a high ionic conductivity of 4.41 mS cm^-1 and activation energy of 0.32 eV with only 4 hours of mechanochemical milling. The conductivity of Na₂O-NTC surpassed that of crystalline NaTaCl₆ (NTC) synthesized under similar conditions by more than one order of magnitude. In addition, Na₂O-NTC exhibited a relatively low electronic conductivity of 6.72 × 10^-10 S cm^-1. Using XRD, Raman, and high-resolution NMR characterizations, the presence of Na₂O as a glass modifier was found to effectively amorphize the crystalline structure of NaTaCl₆, resulting in a glassy oxyhalide material with fast Na⁺ dynamics. This work demonstrates that leveraging inexpensive glass modifiers can effectively break down low-conductivity crystalline materials and tune the local structures to obtain highly conductive glassy solid electrolytes. Cost-effective and energy-efficient synthesis of glassy superionic conductors can aid the development and widespread adoption of high-performance rechargeable solid-state Na batteries.