Aliovalent doping of solid electrolytes with the intention of increase the concentration of charge carrying mobile defects is a common strategy for enhancing their ionic conductivities. For the antiperovskite lithium-ion solid electrolyte Li3OCl, both supervalent (donor) and subvalent (acceptor) doping schemes have previously been proposed. The effectiveness of these doping schemes depends on two conditions: first, that aliovalent doping promotes the formation of mobile lithium vacancies or interstitials rather than competing immobile defects; and second, that any increase in lithium defect concentration gives a corresponding increase in ionic conductivity. To evaluate the effectiveness of aliovalent doping in Li3OCl, we have performed a hybrid density-functional theory study of the defect chemistry of Li3OCl and the response to supervalent and subvalent doping. In nominally stoichiometric Li3OCl the dominant native defects are predicted to be VLi, OCl, and VCl. Supervalent doping increases VLi and OCl concentrations, with the preferentially formed defect species dependent on synthesis conditions. Subvalent doping increases the concentration of VCl more than the concentration of Lii under all accessible synthesis conditions. While supervalent doping is predicted to be effective at increasing ionic conductivity, particularly under Li-poor synthesis conditions, subvalent doping is predicted to decrease room-temperature ionic conductivities at low-to-moderate doping levels. This effect is due to a reduction in the number of lithium vacancies formed during synthesis, and increased [VLi + Lii] Frenkel-pair recombination upon cooling to room temperature. The strongly asymmetric doping response of Li3OCl with respect to supervalent versus subvalent doping is explained as a consequence of the low [VLi + VCl] Schottky pair formation energy, suggesting analogous behaviour should be expected in other Schottky-disordered solid electrolytes.
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