Synthesis, Structure, and Electrochemical Properties of Anion-Doped Li10GeP2S12-Type Solid Electrolyte in the Li–Si–P–S System
Authors: Hori Satoshi , Li Yuxiang , Hayashi Daiki , Sun Xueying , Suzuki Kota , Hirayama Masaaki , Kanno Ryoji 
Published: February 2020

Lithium ion conductors attract much attention since they can potentially be used as a solid electrolyte in all-solid-state batteries. Especially Li9.54Si1.74P1.44S11.7Cl0.3 (LSiPSCl) with a Li10GeP2S12-type structure shows the highest Li-ion bulk-conductivity of 25 mS/cm at room temperature, while synthesis of LSiPSCl requires a strict composition control, making it difficult to obtain as a pure phase [1,2]. Indeed, impurity phases were observed for the synthesized product in the initial and following reports [1,3]. Assuming that phase purity of LSiPSCl correlates to high bulk-conductivity, we targeted to obtain single phase of LSiPSCl via doping oxygen into the crystal structure, and thereby investigated the effects of oxygen towards synthesis conditions, structure and electrochemical properties of final products.
Li9.54Si1.74P1.44S11.7–zCl0.3Oz (LSiPSClOz; 0 ≤ z ≤ 2) were synthesized by mechanical-milling followed by heating at 400–475°C. X-ray diffraction patterns of samples indicated that LSiPSClO0.6 provided LGPS-type single phase while the non-oxygen-doped counterpart included Argyrodite phase as a secondary phase [4]. The total conductivity combining contributions from the bulk and grain-boundary was measured by AC impedance method for compressed powder samples with ca. 80% of theoretical density. LSiPSClO0.6 that was synthesized at 475 °C showed conductivity of 8.2 mS cm−1, which was slightly higher than that of 7.8 mS cm−1 observed for non-oxygen-doped counterpart, which contained an Argyrodite impurity phase. The difference in conductivity and phase purity became more significant when synthesized at 400 °C; LSiPSClOz showed 1.5 times higher conductivity as well as much better phase-purity. The correlation observed between conductivity and purity of LGPS-type phase indicated that the larger proportion of LGPS-type phase in the sample contributed to higher conductivity, suggesting control of oxygen-dope can be useful for obtaining LGPS-type material with high conductivity.
This presentation will focus on the effect of oxygen-doping towards electrochemical stability, which was investigated by cyclic voltammetry and charge-discharge cycle tests of solid-state-cells using LSiPSCl or LSiPSClO0.6.
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