Considering the effect of aliovalent doping on ionic conductivity, supervalent doping produces a monotonic increase in the concentration of lithium vacancies, and therefore also produces a monotonic increase in ionic conductivity. The quantitative effect of supervalent doping on ionic conductivity depends on synthesis conditions. Under Li-poor conditions, where lithium vacancies are the highest concentration native defect species, the ionic conductivity behaves almost entirely as predicted by assuming direct charge-compensating formation of lithium vacancies. Under Li-rich conditions, however, where the dominant negatively charged native defect species is the oxygen-chlorine antisite O Cl , this non-charge-carrying defect species is preferentially formed. The positive effect of supervalent doping on ionic conductivity is therefore suppressed, and higher effective dopant concentrations are needed for a given increase in ionic conductivity than predicted by a simple lithium-defect chargecompensation model.