Structural determination of amorphous AlCl3–<i>x</i>Na2O solid electrolytes

Abstract

Transforming crystalline solid electrolytes into an amorphous state offers several advantages. However, many details regarding the amorphous structures remain unknown. In this work, amorphous sodium solid electrolytes, AlCl3–xNa2O (x = 0.50, 0.60, 0.75, 0.80, and 0.85), were synthesized by incorporating oxide anions into the NaAlCl4 lattice via the ball-milling process. The introduction of oxide anions significantly enhanced the Na-ion conductivity. At room temperature, it reached approximately 10−4 S cm−1, representing an increase in two orders of magnitude compared to crystalline NaAlCl4. Furthermore, structural analysis using pair distribution function, solid-state nuclear magnetic resonance, x-ray absorption fine spectral, and molecular dynamics simulations revealed the formation of Al–O–Al bonds within the amorphous framework. This bridge oxygen was further identified as a crucial factor for enhanced Na-ion conductivity in amorphous AlCl3–xNa2O. This work presents a robust research approach for the low-temperature synthesis and comprehensive structural analysis of amorphous solid electrolytes.