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.
(© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)