Here, first-principles calculations help us understand solid-state electrolytes within the framework of point defects, considering Schottky, cation, and anion Frenkel defects, Li as interstitial defects, Li deficiencies as vacancies, and isovalent/anisovalent substitutional defects by anions. The defect formation energies, which capture electrons and holes, the diffusivity of Li, the mechanical and thermal stability, and the electrochemical windows are studied for halide solid electrolytes with monoclinic and trigonal phases, respectively. The diffusion pathways along both the $c$ axis and $\mathit{ab}$ plane are observed and the activation energies ($\sim 0.44$ eV) and room-temperature Li-ion conductivities for all structures have been determined. It is observed that anisovalent substitution (${\mathrm{O}}_{\text{Cl}}$) is an effective way to promote Li-ion conductivity for ${\mathrm{Li}}_3{\mathrm{InCl}}_6$ increasing the ionic conductivity from 1.34 mS/cm to 6.23 mS/cm at room temperature, whereas isovalent substitution (${\mathrm{F}}_{\text{Cl}}$) and interstitial (${\mathrm{Li}}_i$) defects help achieve similar conductivity values as those shown by ${\mathrm{Li}}_3{\mathrm{YCl}}_6$ and ${\mathrm{Li}}_3{\mathrm{ErCl}}_6$. Furthermore, the defect structures exhibit a lower reduction potential than sulfide solid electrolytes and, due to the incorporation of defects, the band gaps increase. These results highlight the significance of defects in halide solid electrolytes.

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• Point defects
• Density functional theory
• First-principles calculations
• Molecular dynamics