The thermodynamic transition levels as calculated with the HSE06 functional for Li 3 OCl are plotted in Figure showing both intrinsic (upper panels -(a) and (b)) and extrinsic hydrogen defects (lower panels (c) and (d)) at anion-poor/Li-rich conditions (point A, in Figure ) and anion-rich/Li-poor conditions (point B in Figure ) respectively. Additionally, the equilibrium concentrations of the dominant defects (≥ 1 × 10 8 cm -3 ) are given in Figure as a function of oxygen chemical potential (µ O ) as one travels from point A to point B. Even though hydrogen incorporation is typically unavoidable in Li 3 OCl we will still refer to it as "doping" for the case of simplicity in this work. Intrinsic Defects: The intrinsic defects of Li 3 OCl are given in Figure (a) and (b) for Li-rich and Li-poor conditions respectively. Equilibrium concentrations when undoped and under H-doping are given in Figure (a) and (b) respectively. Under both regimes, lithium vacancies (V Li ) are the dominating defect possessing a formation energy of ∼ 1.02 eV at the Fermi level (E F ∼ 3.71 eV and ∼ 1.37 eV for Li-rich and Li-poor conditions respectively when undoped). If we compare the Fermi level calculated at µ O = -1.08 and -1.34 eV corresponding to the "experimentally accessible" oxygen chemical potentials of Li-rich and Li-poor conditions set out in the work by Squires et al. we obtain formation energies of 2.05eV and 1.89 eV respectively in good agreement with their values of ∼ 2.20eV and ∼ 1.90eV. A thermodynamic determination of the "experimentally accessible" region of oxygen chemical potentials is given in SI Section 5. Upon H-doping, E F increases in energy to ∼ 4.12 eV and ∼ 1.72 eV for Li-rich and Li-poor conditions respectively. Pushing E F higher in energy, such as through doping results in an ease in the formation of V Li , increasing the concentration and therefore the ionic conductivity. Under the most Li-rich/anion-poor conditions, point A, the dominant intrinsic defects are seen to be V Li , V Cl , and V O , indicative of fully Schottky disorder. This is partially in agreement with previous literature findings where three types of Schottky disorder are predicted, either "LiCl" partial Schottky: