Amorphous and nanocrystalline halide solid electrolytes with enhanced sodium-ion conductivity

Solid-state batteries are an emerging energy storage technology with the potential to offer improved safety, higher energy density, and longer cycle life. The solid electrolyte is a key component that determines the electrochemical performance of the solid-state battery, especially at room temperature. Herein, we report a series of nanocrystalline and amorphous chloride solid electrolytes with composition Na2.25..xY0.25Zr0.75Cl6..x(1.375 <= x <= 2.000) possessing enhanced ionic conductivity. This study employs X-ray diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance spectroscopy, and electrochemical impedance spectroscopy to study the relationship between composition, structure, and conductivity. These results indicate that NaCl-deficient compositions can form both nanocrystalline and amorphous phases. Moreover, preferred occupancy of prismatic Na+ local environments and fast exchange between such environments were also observed. These factors together contribute to a low activation energy for Na+ hopping, increased ionic conductivity, and improved electrochemical performance at higher cycling rates while operating at room temperature.