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

Abstract

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.

Anharmonicity and phase stability of antiperovskiteLi3OCl

“…It is, however, predicted to have a positive formation energy relative to a two-phase mixture of LiCl and Li 2 O 2,3 . Furthermore, to ensure a high concentration of diffusion mediating defects, the compound must be synthesized with a composition that deviates from perfect stoichiometry as the energy to form Frenkel pairs in the stoichiometric compound is too high to generate a sufficient number of Li interstitials and vacancies at room temperature 3 . A deeper understanding of the factors responsible for the observed stability of Li 3 OCl is therefore desirable.…”

“…Li 3 OCl has a perovskite crystal structure, but with the role of anions and cations reversed ( Figure 1a): The positively charged Li ions form the corner sharing octahedra while the negatively charged O ions occupy the center of the Li-octahedra; the negatively charged Cl occupy the large cages at the center of the unit cell coordinated by 12 Li ions. Several first-principles studies of Li transport in the anti-perovskite crystal structure have predicted low migration barriers for Li-vacancy exchanges, with values on the order of 350 meV 2,3 . An even lower migration barrier of approximately 160 meV was predicted for an interstitial dumbbell mechanism 3 .…”

“…Several first-principles studies of Li transport in the anti-perovskite crystal structure have predicted low migration barriers for Li-vacancy exchanges, with values on the order of 350 meV 2,3 . An even lower migration barrier of approximately 160 meV was predicted for an interstitial dumbbell mechanism 3 . The effect of alloying in Li 3 OCl 1−x Br x on the migration barriers of vacancy mediated Li diffusion was also recently investigated from first-principles 4 .…”

“…Firstprinciples studies predict that Li 3 OCl is metastable at zero Kelvin relative to decomposition into LiCl and Li 2 O 2,3 (Figures 1b and 1c). The ability to synthesize Li 3 OCl experimentally 1 , however, suggests that this compound is likely entropically stabilized at elevated temperatures, if not at room temperature, then at least at the higher synthesis temperatures of approximately 330 − 360…”

Computational studies of solid-state alkali conduction in rechargeable alkali-ion batteries

“…Zhao et al 131 reported the room temperature ionic conductivities for two samples, 0.85 mS cm − 1 for Li 3 OCl with Following the experimental work, Zhang et al 132 proved that Li vacancies and anion sublattice disorder are the driving forces for Li + diffusion in antiperovskites using AIMD simulations. Emly et al 133 later identified a low-barrier three-atom hopping mechanism involving Li interstitial dumbbells, for which the barriers calculated using NEB methods are ∼ 170 meV (Figure 13), 50% lower compared with those of vacancy-driven migration. However, defect formation energy calculations suggest that a high Li interstitial concentration can only be obtained by off-stoichiometry synthesis.…”