Exploring the Underlying Correlation between the Structure and Ionic Conductivity in Halide Spinel Solid-State Electrolytes with Neutron Diffraction

The development of cutting-edge solid-state electrolytes (SSEs) entails a deep understanding of the underlying correlation between the structure and ionic conductivity. Generally, the structure of SSEs encompasses several interconnected crystal parameters, and their collective influence on Li⁺ transport can be challenging to discern. Here, we systematically investigate the structure-function relationship of halide spinel Li_xMgCl_2+x (2 ≥ x ≥ 1) SSEs. A nonmonotonic trend in the ionic conductivity of Li_xMgCl_2+x SSEs has been observed, with the maximum value of 8.69 × 10^-6 S cm^-1 achieved at x = 1.4. The Rietveld refinement analysis, based on neutron diffraction data, has revealed that the crystal parameters including cell parameters, Li⁺ vacancies, Debye-Waller factor, and Li-Cl bond length assume diverse roles in influencing ionic conductivity of Li_xMgCl_2+x at different stages within the range of x values. Besides, mechanistic analysis demonstrates Li⁺ transport along three-dimensional pathways, which primarily governs the contribution to ionic conductivity of Li_xMgCl_2+x SSEs. This study has shed light on the collective influence of crystal parameters on Li⁺ transport behaviors, providing valuable insights into the intricate relationship between the structure and ionic conductivity of SSEs.