Effect of the Cation Sublattice Configuration on Li Ion Dynamics for the Spinel Li–Mg–Al–Cl System

Spinel-based halide solid electrolytes (HSEs) are gaining recent attention as they incorporate more affordable elements (Mn, Mg, and Fe) compared to conventional layered Li3MCl6 (M = Y, Sc, Er, In, Yb). However, the lithium occupancy is either fully saturated or exhibits limited disorder at elevated temperatures, resulting in lower ionic conductivity. To address these issues, we introduced Al3+ doping into Li2MgCl4, which creates both disorder and vacancies that improve conductivity by more than 2 orders of magnitude (∼0.1 mS cm–1 at 30 °C), although this is still not on par with the best-layered HSEs. This discrepancy is primarily due to the migration of Mg2+ ions from 16d to new site 16c, which led to blocking effects for long-range Li+ diffusion, confirmed through neutron diffraction, solid-state NMR, hopping frequency, and DFT analysis. Nonetheless, this work provides valuable insights for the design of high-conductivity and cost-effective spinel halides for future solid-state battery applications.