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.