Preparation and characterization of sodium-ion conductive Na3BS3 glass and glass–ceramic electrolytes

Abstract

In order to find suitable solid electrolytes for all-solid-state sodium batteries, sulfide electrolytes composed of tetrahedral structural units such as PS₄, SnS₄ and SbS₄ have been widely studied. In this paper, the ionic conductivities of Na₃BS₃ortho-thioborate electrolytes with triangular BS₃ units are firstly reported. Na₃BS₃ glass was prepared via a mechanochemical process from crystalline Na₃BS₃ (monoclinic phase). The crystalline Na₃BS₃ was pre-synthesized from a mixture of Na₂S, B, and S due to the instability of the B₂S₃ compound. A new metastable phase of trigonal Na₃BS₃ was precipitated as the primary phase by crystallization of the Na₃BS₃ glass. The prepared glass–ceramic electrolyte showed a higher ionic conductivity than the monoclinic Na₃BS₃ phase. The Na₃BS₃ glass showed the highest conductivity of 1.1 × 10⁻⁵ S cm⁻¹, which was higher than that of conventional Na₃PS₄ glass. Futhermore, the Na₃BS₃ glass showed a superior formability and electrochemical stability to Na₁₅Sn₄ negative electrode. An all-solid-state cell with the Na₃BS₃ glass as an electrolyte successfully operated as a secondary battery at 60 °C. It is concluded that the Na₃BS₃ glass with triangular structural units has appropriate properties as a solid electrolyte for application to all-solid-state sodium batteries. The results of this study extend research on multi-component sulfide electrolytes with triangular BS₃ structural units and contribute to the development of solid electrolytes for all-solid-state batteries.