Na₂ZrCl₆ enabling highly stable 3 V all-solid-state Na-ion batteries

Halide solid electrolytes (SEs) are emerging as an alternative to sulfide and/or oxide SEs for applications in all-solid-state batteries owing to the advantage fulfilling high (electro)chemical stability and mechanical sinterability at the same time. Thus far, the developments in halide SEs have focused on Li⁺ superionic conductors. Herein, the development of a new Na⁺-conducting halide SE, mechanochemically prepared Na₂ZrCl₆ (1.8 × 10⁻⁵ S cm⁻¹ at 30°C) with excellent oxidative electrochemical stability, is described. A trigonal crystal structure with the P3¯m1 symmetry is successfully identified by the Rietveld refinement of X-ray diffraction. Additionally, the bond valence sum energy level calculations disclose one-dimensional preferable Na⁺-diffusion channels in Na₂ZrCl₆. It is to be noted that despite the rather low Na⁺ conductivity of Na₂ZrCl₆, NaCrO₂ electrodes that uses Na₂ZrCl₆ in NaCrO₂/Na-Sn all-solid-state Na-ion batteries demonstrate an exceptionally high initial Coulombic efficiency of 93.1% and a high reversible capacity of 111 mA h g⁻¹ at 0.1C and 30 °C (98.4% and 123 mA h g⁻¹ at 60 °C), highlighting the excellent electrochemical stability of Na₂ZrCl₆.