Synthesis, structure, and ionic conductivity of solid solution, Li10+delta M1+delta P2-delta S12 (M = Si, Sn)

Solid solutions of the silicon and tin analogous phases of the superionic conductor Li10MP2S12 (M = Si, Sn) were synthesized by a conventional solid-state reaction in an evacuated silica tube at 823 K. The ranges of the solid solutions were determined to be 0.20 < delta < 0.43 and -0.25 < delta < -0.01 in Li10+delta M1+delta P2-delta S12 (0.525 <= k <= 0.60 and 0.67 <= k <= 0.75 in Li4-kM1-kPkS4) for the Si and Sn systems, respectively. The ionic conductivity of these systems varied as a function of the changing M ions: the Si and Sn systems showed lower conductivity than the Ge system, Li10+delta Ge1+delta P2-delta S12. The conductivity change for different elements might be due to the lattice size and lithium content affecting the ionic conduction. The relationship between ionic conduction, structure, and lithium concentration is discussed based on the structural and electrochemical information for the silicon, germanium, and tin systems.