Influence of Mg on the Li ion mobility in Li 4-2 x Mg x P 2 S 6 .

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 Li 4 P 2 S 6 by a combination of nuclear magnetic resonance experiments on 31 P and 7 Li 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 4-2 x Mg x P 2 S 6 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 P 2 S 6 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.