The low ionic conductivity in solid electrolytes is a limiting factor for commercial implementation of all solid state batteries. Through understanding Li⁺ ion diffusion by using first-principles molecular dynamics, significant insights in developing new battery materials can be achieved. For example, alloying of anions is a promising engineering strategy for tuning ionic conductivity in halide-based inorganic solid electrolytes. We explain the alloying effects in Li₃InBr_6−xCl_x, in terms of strain, chemistry, and microstructure, using first-principles molecular dynamics simulations and novel electronic structure analysis. We find that strain and bond chemistry can be tuned through alloying and affect the activation energy and maximum diffusivity coefficients. The similar conductivities of the x=3 and x=6 compositions can be understood by assuming the alloy separates into Br-rich and Cl-rich regions. Phase-separation increases diffusivity at the interface and in the expanded Cl-region, suggesting microstructure effects are critical.