Mechanochemical Synthesis and Characterization of K2+xZr1–xYxCl6: Potassium-Ion-Conducting Chloride

K-ion batteries are attracting attention as rechargeable batteries following Li-ion and Na-ion batteries. However, their safety is of concern owing to the use of organic electrolytes. The use of solid electrolytes instead of organic electrolytes can resolve this problem. Chloride solid electrolytes have attracted attention in recent years as Li- and Na-ion conductors, but reports on chloride K-ion conductors have been scarce. In this study, new K-ion-conducting chlorides, K2+xZr1–xYxCl6 (x = 0–1), were prepared by a mechanochemical method. The X-ray powder diffraction (XRPD) patterns of the prepared solid electrolytes exhibited broad Bragg peaks, and the high-resolution transmission electron microscopy observations revealed the presence of nanocrystals dispersed in amorphous matrixes. New crystalline phases, closely related to monoclinic K2TeCl6, were precipitated in the K2+xZr1–xYxCl6 matrixes at x = 0–0.4. The Rietveld refinements of the XRPD patterns for the new phases revealed that the excess of K, introduced by Y substitution, occupied the vacant B′ octahedral site of the cation-deficient double perovskite A2BB′X6 structure. The conductivity at 25 °C of the K2+xZr1–xYxCl6 chlorides exceeded 10–7 S cm–1 at x = 0–0.4, and a maximum conductivity of 9.2 × 10–7 S cm–1 was achieved at x = 0.3. Heat treatment above 400 °C of the K2+xZr1–xYxCl6 electrolyte with x = 0.3 resulted in crystallization of the amorphous components, decomposition of the phase into a mixture of K2ZrCl6 and K3YCl6, and a decrease of the ionic conductivity. In addition to the K concentration and crystalline phase in the electrolytes, the amorphous components also play a key role in the high ionic conductivity of the K2+xZr1–xYxCl6 chlorides.