Phase Stability and Transport Mechanisms in Antiperovskite Li₃OCl and Li₃OBr Superionic Conductors

We investigate phase stability and ionic transport mechanisms in two recently discovered superionic conductors, Li 3 OX (X = Cl, Br), from first principles. These compounds, which have an antiperovskite crystal structure, have potential applications as solid electrolytes in Li-ion batteries. We identify a low-barrier three-atom hop mechanism involving Li interstitial dumbbells. This hop mechanism is facile within the (001) crystallographic planes of the perovskite crystal structure and is evidence for the occurrence of concerted motion, similar to ionic transport in other solid electrolytes. Our first-principles analysis of phase stability predicts that antiperovskite Li 3 OCl (Li 3 OBr) is metastable relative to Li 2 O and LiCl (LiBr) at room temperature. We also find that although the band gap of Li 3 OCl exceeds 5 eV, the metastable antiperovskite becomes susceptible to decomposition into Li 2 O 2, LiCl and LiClO 4 above an applied voltage of 2.5 V, suggesting that these compounds are most suited for low-voltage Li batteries provided the formation of Li 2 O can be suppressed.