Diffusion Mechanism of Li Argyrodite Solid Electrolytes for Li-Ion Batteries and Prediction of Optimized Halogen Doping: The Effect of Li Vacancies, Halogens, and Halogen Disorder

Using density functional theory molecular dynamics simulations, the origin of the Li-ion conductivity in argyrodite solid electrolytes is investigated. The simulations show that besides Li-ion vacancies in Li6PS5Cl and Li6PS5Br, the influence of halogen atoms on their local surroundings also plays an important role in Li-ion diffusion. The difference in Li-ion conductivity between Li6PS5Cl and Li6PS5I, which is several orders of magnitude, is caused by the distribution of the halogen ions over the available crystallographic sites. This suggests that altering the halogen distribution in Li argyrodites during synthesis could increase the Li-ion conductivity of these materials. For Li6PS5Cl, the simulations predict an optimal Cl distribution of 1:3 over sites 4a and 4c, resulting in a Li-ion conductivity that is 2 times larger than that of the currently prepared materials. On the basis of these results, simulations were performed on Li5PS4X2 (X = Cl, Br, or I), which show Li-ion conductivities similar to those of Li6PS5Cl and Li6PS5Br, suggesting that the Li5PS4X2 compounds are interesting new compositions for solid state electrolytes.