Mechanochemical Synthesis of Na3BS3 Glass Electrolyte

Abstract

All-solid-state batteries using non-flammable inorganic solid electrolytes are expected to solve safety issues in commercially available batteries by removing flammable organic liquid electrolytes. Moreover, sodium-ion batteries are attracting for use in energy storage systems from the viewpoint of production costs based on the abundance of sodium sources. Sodium-ion conducting solid electrolytes are important materials for all-solid-state sodium batteries. We reported that Na3PS4 glass-ceramics, which are obtained via the mechanochemical treatment and consecutive heat treatment, exhibited higher ionic conductivity than the crystal Na3PS4 [1].Inorganic solid electrolytes are mainly classified into oxides and sulfides. Recently, we reported that Na2O-B2O3 glasses solid electrolytes and the ionic conductivity of Na3BO3 glass showed over 10-8 S cm-1 [2]. Among sulfides, some Na2S-B2S3 glasses were prepared by the melt quenching method and their glass-forming region and local structure were reported [3,4]. However, Na3BS3 glass at the ortho-composition has not been studied yet.In this report, we prepared Na3BS3 glass and glass-ceramic electrolytes via mechanochemical treatment and consecutive heat treatment. Their ionic conductivities and formability were examined. Their structures were analyzed by X-ray diffraction, Raman spectroscopy and MAS-NMR measurement.The mixture of Na2S, B, and S was placed into a carbon crucible and subsequently sealed into a quartz tube under vacuum. The tube was heat-treated at 700oC and held for 12 h, and then cooled down to room temperature to obtain Na3BS3 crystal. The Na3BS3 glass was obtained by the mechanochemical treatment of the crystal using a planetary ball milling.A halo pattern was observed for the Na3BS3 crystal after the mechanochemical treatment. From the 11B MAS-NMR spectrum, the peaks attributable to the BS3 3- unit were observed. The ionic conductivity of the Na3BS3 glass was evaluated by the AC impedance measurement. The ionic conductivity at room temperature and activation energy of the cold-pressed pellet were 1.1×10-5 S cm-1 and 39 kJ mol-1, respectively. This conductivity was about three orders of magnitude higher than that of the Na3BO3 glass [2]. After heated at 300oC, the crystalline Na3BS3 (C 2/c, SG No. 15) phase was observed, and its ionic conductivity was about 10-10 S cm-1. On the other hand, the XRD pattern of the glass-ceramic heated at 200oC showed a new metastable phase. The ionic conductivity of the glass-ceramic was about 10-8 S cm-1, which is lower than that of Na3BS3 glass, but higher than that of the Na3BS3 crystal. Reference s: [1] A. Hayashi, K. Noi, A. Sakuda, M. Tatsumisago, Nat. Commun., 3 (2012) 856.[2] K. Suzuki, Y. Nakamura, N. Tanibata, A. Hayashi, M. Tatsumisago, J. Asian Ceram. Soc., 163 (2016) 6–10.[3] S. Susman, L. Boehm, K. J. Volin, C. J. Delbecq, Solid State Ionics, 5 (1981) 667-669.[4] H. K. Patel, S. W. Martin, Solid State Ionics, 53 (1992) 53-56. Acknowledgements: This work was supported by Element Strategy Initiative of MEXT, Grant Number JPMXP0112101003 and JSPS KAKENHI Grant Number 18H01713 and 19H05816.