The inconsistency between this work and previous calculations is likely down to both the choice of functional, as well as from the calculation of the chemical potential stability region for Li 3 OCl of which the defect formation energies are highly sensitive to. Hybrid functionals typically give a much better description of electron localisation for defects, as well as favourable description of the band gap of a material relative to experiment. Both of these are crucial when determining charged defect calculations in order to correctly describe the thermodynamic defect quantities of solid electrolytes. In this work, the chemical potential region was determined "temperature-dependently" through calculating the free energies of competing phases using QHA (Section 3 3.1), which has shown success previously in Na 2 Ti 3 O 7 . Confidence in this result arises from the fact that Li 3 OCl becomes "stable" around ∼ 750K, in excellent agreement with the experimental synthesis temperature (without H) of ∼ 773K. The unfortunate reality of Li 3 OCl is that it is highly unlikely to be purely Li 3 OCl, but a hydrated form Li 3 -x OH x Cl. Even when attempting to create dehydrated forms of Li OHCl agree with this conclusion. This "pseudo-V Li " can also be seen as a Frenkel defect, where the mobile Li becomes effectively an interstitial upon rotation of the OH species. Additionally, OH Cl , whilst neutral, acts to distort the structure slightly due to its smaller radii compared to Cl which may also aid Li mobility. Squires et al. calculated the conductivity of Li 3 OCl within their full defect model through: