Synthesis and characterization of lithium germanogallium sulfide, Li₂GeGa₂S₆

Highlights

• Crystalline Li₂S-GeS₂-Ga₂S₃ was synthesized by melting in a carbon coated silica tube.

• The experimental XRD pattern of the LiGeGa₂S₆ appears to closely match the calculated data.

• The host structure is built of GaS4 tetrahedra linkd by coners to GeS₄ tetrahedra to create a 3D framework forming tunnels along the c axis.

Abstract

A new thio-germanium sulfide Li₂GeGa₂S₆ (LGGS) has been synthesized for the first time. In order to determine the structure of the LGGS, the material was synthesized by being melted in a carbon-coated silica tube and being slowly cooled to make a crystalline structure. X-ray diffraction (XRD) was performed. The experimental XRD powder pattern of the LiGeGa₂S₆ appears to closely match the calculated data, Joint Committee on Powder Diffraction Standards (JCPDS). The host structure is built of GaS₄ tetrahedra linked by corners to GeS₄ tetrahedra to create a 3D framework forming tunnels along the c-axis, in which the Li⁺ ions are located. To further characterize the structure of the phase, the Raman and infrared (IR) spectra of LGGS were performed. The strongest intensity peak in the Raman spectra is observed at ~ 337 cm^–1 assigned to symmetric stretch vibrations of S atoms (Ge–S–Ge symmetrical bridge-stretching mode) in GeS₄ and GaS₄ units. This symmetric sulfur bridge stretching mode is also observed at ~ 338 cm^− 1 in the IR spectra. The ionic conductivities of the LGGS were conducted with various temperatures. The ionic conductivity of 3.8 × 10^− 8 S/cm was observed at room temperature. Electrochemical stability was evaluated from the cyclic voltammogram of a Li/Li₂GeGa₂S₆/Au cell with a lithium reference electrode. The material shows high decomposition potential up to 6 V.

Introduction

Sulfide materials can exhibit higher ionic conductivities compared to oxide materials [1]. Since the larger polarizability of sulfur produces weaker coulombic interactions with mobile ions than oxygen, lithium sulfide compounds, in particular, can display high ionic conduction. Recently, sulfide glasses have been investigated based on SiS₂ [2], GeS₂ [3], [4], P₂S₅ [5], and B₂S₃ [6]. Among these sulfide materials, this study uses GeS₂ as a base material because it is less hygroscopic (more oxidatively stable) and enables a more electrochemically stable matrix for lithium ion conduction to be prepared. Since GeS₂-based materials are more stable in air than other sulfide materials, GeS₂-based sulfide materials such as Li₂S–GeS₂ [7], Li₂S–GeS₂–P₂S₅ [1], Li₂S–GeS₂–Ga₂S₃ [8], GeS₂–Ga₂S₃–AgI–Ag [9], and LiI–Li₂S–GeS₂–Ga₂S₃ [10] have been widely studied. In addition, GeS₂-based oxy-sulfide material, Li₂S–GeS₂–GeO₂, has been also reported [11]. Classes of these materials are called LISICON (Lithium SuperIonic CONductor) in the case of oxides or thio-LISICON in the case of sulfides. Lithium-containing sulfide glasses have been extensively investigated for ionic conductivity and exhibit fast ionic conductivity. For the same reason, ionic conductivity is an important property for future applications as solid electrolytes for all solid-state lithium batteries and may solve the safety problems of the rechargeable lithium-ion batteries using non-aqueous liquid electrolytes.

From the conductivity standpoint, the highest value of 10^–2 S/cm at room temperature was obtained for the sulfide polycrystals in the 5Li₂S + GeS₂ + P₂S₅ system (Li₁₀GeP₂S₁₂) [1]. Amorphous or glassy materials often have superior ionic conductivities over corresponding crystalline materials because they can form over a wide range of compositions, have isotropic properties, do not have grain boundaries, and can form thin films easily. Because of their open and disordered structure, amorphous materials typically have higher ionic conductivities than the corresponding crystalline materials [12], [13]. In addition, unlike most organic polymeric conductors, single ion conduction can be realized because inorganic glassy materials belong to decoupled systems in which the mode of ion conduction relaxation is decoupled from the mode of structural relaxation. Amorphous or glassy materials are thus among the more promising candidates of solid electrolytes because of their properties of single ion conduction and high ionic conductivities.

However, crystalline materials might have higher conductivity than the corresponding glasses if their crystal structure is designed for high ionic conduction [1]. Furthermore, another advantage of crystalline materials is their stability at high temperature. A new material search is still necessary to find high lithium ionic conducting crystalline compounds. Only a few materials have been investigated previously in the crystalline sulfides: Li₃PS₄ [14], Li₂SiS₃ [15], and Li₄SiS₄ [15] were reported to have conductivities of 10^–6–10^–9 S/cm at room temperature. These compounds are interesting from a structural point of view, because their structures are characterized by the various anionic groups formed by the linkage of a basic structural unit, XY₄ tetrahedra. In case of silicates, two SiO₄ tetrahedra always share corners, never edges or faces. On the contrary, in the above mentioned chalcogenides, two XY₄ tetrahedra can share their edge.

In this study, the crystalline Li₂S–GeS₂–Ga₂S₃ system was synthesized and structural properties and their ionic conductivities were characterized by XRD, Raman and IR spectroscopy, impedance spectroscopy, and cyclic voltammetry to further the material search for stable crystalline ionic conductors at high temperature.