A sodium superionic chloride electrolyte driven by paddle wheel mechanism for solid state batteries

Halides are promising solid electrolytes due to their high ionic conductivity and high oxidation potential. Here we report a superionic chloride material, NaTaCl₆, which exhibits a high ionic conductivity of 3.3 mS cm^-1 at 27 °C, being two-orders of magnitude higher than that of NaNbCl₆ (0.01 mS cm^-1). The considerably higher conductivity exhibited by NaTaCl₆ vs. NaNbCl₆ arises from the more facile rotational/reorientational dynamics of the [TaCl₆] polyanions in comparison to the [NbCl₆] anions. [TaCl₆] polyanion rotation is readily activated while [NbCl₆] polyanion reorientation is hindered at room temperature but can be turned on as the temperature increases or under prolonged mechanical milling. The higher degree of structural disorder exhibited by NaTaCl₆ compared to NaNbCl₆-likely attributed to its greater mechanical and phonon softness-is found to contribute to the more pronounced [TaCl₆] anion rotation. Anion rotation is coupled with, and facilitates, macroscopic Na⁺-ion diffusion. As a result, enhanced rotational dynamics are directly correlated with the higher Na⁺-ion conductivity observed in NaTaCl₆. The high ionic conductivity, combined with its electrochemical stability against positive electrode materials, enables good rate capability and long-term cycling performance in solid-state cells. These findings provide insights into ion transport mechanism in the newly emerging halide solid electrolytes.