Mechanochemically prepared sodium-ion conducting fluorides in the system NaF–TaF<sub>5</sub>

We report a new sodium-ion conducting fluorides, NaTaF6, Na2TaF7, and Na3TaF8 as the possible candidates for a solid electrolyte in composite positive electrode for all-solid-state sodium batteries with high operating voltage. The mechanochemically prepared Na2TaF7 sample shows the ionic conductivity of 1.1 × 10−6 S cm−1 at 100 °C and the activation energy of 0.67 eV. The Na2TaF7 sample has the electrochemically stability between 1.6 and 4.2 V vs. Na2Sn. This report pioneeringly develops the sodium-ion conducting fluorides with high electrochemical stability for all-solid-state batteries.