Vacancies and antisites have well-defined positions in a periodic crystal, corresponding to atomic positions in the ideal stoichiometric material. In contrast, interstitial sites do not have a priori well-defined positions, but instead these must be specified through some additional procedure. In this study, we have considered a total of five interstitial defects, comprising one lithium interstitial, which occupies a "dumbbell" position described by Emly et al. ; and four anion interstitials, with O or Cl occupying one of two possible sites identified using Voronoi decomposition as implemented in Pymatgen (fig. ).