Sluggish anion transport provides good kinetic stability to the anhydrous anti-perovskite solid electrolyte Li₃OCl

Some lithium oxyhalides have been proposed as low-cost solid electrolytes for having room-temperature Li⁺ conductivity close to commercial liquid electrolytes, but with the advantages of enabling higher energy densities through the use of the Li metal anode and not being flammable. However, the stability of anhydrous anti-perovskite lithium oxyhalides, such as Li₃OCl, is not well understood yet: whereas theoretical calculations show they should decompose into lithium halides and Li₂O (except at high temperatures), there is no experimental evidence of such decomposition. Thus, here we use a combination of analytical calculations and force-field-based atomistic modelling to investigate the role of kinetics in the stability of anhydrous Li₃OCl. The results show that due to sluggish Cl^- and O^2- transport this material has good kinetic stability below ∼400 K under high concentration gradients, below ∼450 K under typical cell voltages, and at all temperatures against local composition fluctuations. Furthermore, the good kinetic stability explains the apparent discrepancy between theoretical thermodynamics calculations and experimental observations and contributes to enlighten the nature and extent of this material's stability. The methods presented here can also be extended to other battery materials that are predicted to decompose, to access the safe temperature range they can undergo without degrading.