Enhancement of Low Temperature Superionic Conductivity by Suppression of Li Site Ordering in Li 7 Si 2-x Ge x S 7 I.

Ge 4+ substitution into the recently discovered superionic conductor Li 7 Si 2 S 7 I is demonstrated by synthesis of Li 7 Si 2-x Ge x S 7 I, where x≤1.2. The anion packing and tetrahedral silicon location of Li 7 Si 2 S 7 I are retained upon substitution. Single crystal X-ray diffraction shows that substitution of larger Ge 4+ for Si 4+ expands the unit cell volume and further increases Li + site disorder, such that Li 7 Si 0.88 Ge 1.12 S 7 I has one Li + site more (sixteen in total) than Li 7 Si 2 S 7 I. The ionic conductivity of Li 7 Si 0.8 Ge 1.2 S 7 I (x=1.2) at 303 K is 1.02(3)×10 -2  S cm -1 with low activation energies for Li + transport demonstrated over a wide temperature range by AC impedance and 7 Li NMR spectroscopy. All sixteen Li + sites remain occupied to temperatures as low as 30 K in Li 7 Si 0.88 Ge 1.12 S 7 I as a result of the structural expansion. This differs from Li 7 Si 2 S 7 I, where the partial Li + site ordering observed below room temperature reduces the ionic conductivity. The suppression of Li + site depopulation by Ge 4+ substitution retains the high mobility to temperatures as low as 200 K, yielding low temperature performance comparable with state-of-the-art Li + ion conducting materials.