Synergic effect of CaI₂ and LiI on ionic conductivity of solution-based synthesized Li₇P₃S₁₁ solid electrolyte

Li₇P₃S₁₁ doped with CaX₂ (X = Cl, Br, I) and LiI solid electrolytes were successfully prepared by liquid-phase synthesis using acetonitrile as the reaction medium. Their structure was investigated using XRD, Raman spectroscopy and SEM-EDS. The data obtained from complex impedance spectroscopy was analyzed to study the ionic conductivity and relaxation dynamics in the prepared samples. The XRD results suggested that a part of CaX₂ and LiI incorporated into the structure of Li₇P₃S₁₁, while the remaining part existed at the grain boundary of the Li₇P₃S₁₁ particle. The Raman peak positions of PS₄^3- and P₂S₇^4- ions in samples 90Li₇P₃S₁₁-5CaI₂ and 90Li₇P₃S₁₁-5CaI₂-5LiI had shifted as compared to the Li₇P₃S₁₁ sample, showing that CaI₂ addition affected the vibration of PS₄^3- and P₂S₇^4- ions. EDS results indicated that CaI₂ and LiI were well dispersed in the prepared powder sample. The ionic conductivity at 25 °C of sample 90Li₇P₃S₁₁-5CaI₂-5LiI reached a very high value of 3.1 mS cm^-1 due to the improvement of Li-ion movement at the grain boundary and structural improvement upon CaI₂ and LiI doping. This study encouraged the application of Li₇P₃S₁₁ in all-solid-state Li-ion batteries.