Influence of Mg on the Li ion mobility in Li_4-2MgP₂S₆

Aliovalent substitution of Li in salts by Mg can generate Li vacancies and thus in principle improve the ionic conductivity. In fact the influence of substitution on ionic conductivity is far more complex. Here the impact of Mg substitution on Li ion mobility is studied in the example of Li₄P₂S₆ by a combination of nuclear magnetic resonance experiments on ³¹P and ⁷Li at variable temperatures, impedance spectroscopy and X-ray powder diffraction to elucidate the relationship with structural changes and the effect on mobility on different length scales. It is found that substituting Li ions with Mg ions in Li_4-2MgP₂S₆ with 0 ≤ x ≤ 0.2 increases the local Li ion mobility up to a certain concentration at which a phase transition induces a different structural realignment of the P₂S₆^4- units. The determined activation energies can be assigned to vacancy hopping processes by comparison with nudged elastic band calculations at the density functional level of theory, which shows not only the possibilities but also limitations of substituting Li with Mg.