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

Summary

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 Na_2.25-xY_0.25Zr_0.75Cl_6-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.