Improved ionic conductivity and structural transition from (nLi2S-LiI)-(P2S5) solid solutions to LixP2SyI crystalline electrolytes

Abstract

In this study, LixP2SyI – type crystalline lithium superionic conductors have been synthesized from nLi2S-LiI (n = 1, 2, 3, and 4) solid solutions and P2S5 using the high energy ball milling process. First, different concentrations of Li2S were mixed and ball-milled with LiI at different molar ratios to obtain various Li2S-LiI solid solutions (nLi2S-LiI). Then, P2S5 was added to the prepared nLi2S-LiI solid solutions to get crystalline solid electrolytes. The crystalline nature and the composition of the prepared nLi2S-LiI solid solutions, and P2S5 added nLi2S-LiI (LixP2SyI) crystalline solids were examined using the powder X-ray diffraction technique. The evolution of the crystalline structural units was captured by laser Raman spectroscopy analysis. The surface morphology changes with varying Li2S concentrations as well as the addition of P2S5 were studied using field emission scanning electron microscopy. Electrochemical impedance spectroscopy analysis revealed that the prepared Li5S2I (3.1 ×10−5 S cm−1) and Li7P2S8I (1.07 ×10−3 S cm−1) solid electrolytes exhibited higher ionic conductivities than the other nLi2S-LiI solid solutions and LixP2SyI crystalline solids, respectively. The result of cyclic voltammetry analysis and DC polarization analysis proved that the prepared nLi2S-LiI solid solutions and LixP2SyI crystalline solids are stable against lithium metal. All-solid-state lithium batteries fabricated using nLi2S-LiI solid solutions and LixP2SyI crystalline solid electrolytes showed comparable electrochemical performances. In the present study, we demonstrated nLi2S-LiI solid solutions and LixP2SyI crystalline solids which could potentially be used as solid electrolytes for the fabrication of ASSBs with good performance.