Both supervalent and subvalent doping strategies have been proposed to enhance the ionic conductivity of Li 3 OCl . Supervalent dopinge.g., where a cation such as Mg 2+ or Al 3+ replaces Li +has been proposed as an effective route to increase the concentration of lithium vacancies , while subvalent doping-e.g., where an anion such as S 2- replaces Cl --has been proposed as an effective route to increase the concentration of lithium interstitials ; in this case it has been suggested that because of the much higher mobility of interstitials relative to that of the intrinsically dominant lithium vacancies, a relatively small increase in lithium interstitial concentration might produce a disproportionately large increase in ionic conductivity.