Synthesis and Electrical Properties of Mixed Anion Fluorosulfide Ba18F18in8S21

Abstract

Fluoride-ion batteries that utilize the multi-electron reaction of metal cation have the potential to far improve the energy density compared with the current lithium-ion batteries.1) In addition, the all-solid-state fluoride-ion battery that uses solid electrolyte is attracting great expectations as a battery system that has both safety and high energy density and that can open up new applications. The fluoride-ion solid conductors such as PbSnF4 and Ba1-xLaxF2+x have been widely studied for decades. Although PbSnF4 has a high ionic conductivity of 1.6 mS cm-1, the potential window is narrow, and a high voltage secondary battery cannot be realized by using PbSnF4. Although Ba1-xLaxF2+x has a wide potential window, the conductivity is 0.2 mS cm-1 at 50°C, which is not suitable for practical use at an ambient temperature. To our knowledge, a solid electrolyte that has both high ionic conduction and a wide potential window has not been found at present and the further development of materials is required.In this study, we have extended the search area to mixed fluoride compounds containing sulfide ion due to its high polarizability. For the crystal structure of the fluorosulfide compound, Ba18F18In8S21 exhibits unique domain structure having Ba-F structure.2) We report the synthesis and electrical property of Ba18F18In8S21.According the literature, 2) we prepared Ba18F18In8S21 by solid state synthesis. In an Ar atmosphere, BaS:BaF2:In:S = 3:3:4:6 were weighed in molar ratio and mixed in a mortar. The pelletized sample was sealed in vacuum and calcined at 800 °C. From the result of XRD, it was confirmed that the prepared samples contained Ba18F18In8S21 crystal phase with BaF2 and BaS impurities. Therefore, it is necessary to improve the synthesis condition. Since sulfur exhibits a boiling point of 444 °C and indium metal has a melting point of 156 °C, two atoms might be deficient during the calcination. Therefore, In2S3 was used without using sulfur powder and indium metal, and synthesis was performed at a lower temperature of 700 °C. XRD pattern of the latter prepared powder can be indexed with a space group of the reported Ba18F18In8S21, and the lattice constants were a = b = 22.152 Å and c = 12.642 Å. Although this value is equivalent to the literature,2) there was small amount of BaF2 impurity.In order to evaluate the electrical property of the obtained sample, AC impedance measurement was performed by using two-electrode cell. The conductivity at 140 ° C was the order of 10-8 S cm-1. M. Reddy, M. Fichtner, J. Mater. Chem., 21 , 17059-17062 (2011). Z.-Z. Luo, C.-S. Lin, W.-L. Zhang, H. Zhang, Z.-Z. He and W.-D. Cheng, CrystEngComm, 16 , 2788-2794 (2014).