Enhancement of Low Temperature Superionic Conductivity by Suppression of Li Site Ordering in Li7Si2–xGexS7I

Ge4+ substitution into the recently discovered superionic conductor Li7Si2S7I is demonstrated by synthesis of Li7Si2–xGexS7I, where x≤1.2. The anion packing and tetrahedral silicon location of Li7Si2S7I are retained upon substitution. Single crystal X‐ray diffraction shows that substitution of larger Ge4+ for Si4+ expands the unit cell volume and further increases Li+ site disorder, such that Li7Si0.88Ge1.12S7I has one Li+ site more (sixteen in total) than Li7Si2S7I. The ionic conductivity of Li7Si0.8Ge1.2S7I (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 7Li NMR spectroscopy. All sixteen Li+ sites remain occupied to temperatures as low as 30 K in Li7Si0.88Ge1.12S7I as a result of the structural expansion. This differs from Li7Si2S7I, where the partial Li+ site ordering observed below room temperature reduces the ionic conductivity. The suppression of Li+ site depopulation by Ge4+ 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. Isovalent substitution of Ge4+ for Si4+ in Li7Si2–xGexS7I is observed for x≤1.2. Control of unit cell volume via this substitution enhances Li+ site disorder beyond Li7Si2S7I. This extensive disorder is stabilised to 30 K with very low activation energies observed by 7Li NMR, yielding low temperature Li+ transport in highly substituted (x=1 & 1.2) materials that is comparable to best‐in‐class solid‐state electrolytes.