Liquid-Phase Synthesis of Argyrodite-Type Li6PS5Br Solid Electrolyte with High Lithium-Ion Conductivity

Introduction Solid electrolytes are attracting attention as essential components for advanced lithium-ion batteries because of their great electrochemical stability, high lithium ion transport number and wide operating temperature. Therefore, all-solid-state batteries have been proposed as a strong candidate for various electrochemical storage devices. To improve the electrochemical performances of the all-solid-state batteries, solid electrolytes with high ionic conductivities have been investigated. Li7P3S11 [1] and Li6PS5X (X = Cl, Br) [2] have been reported to show an extremely high lithium ion conductivity of more than 10-3 S cm-1, which is comparable to that of liquid electrolytes. Unfortunately, it takes long time to prepare these solid electrolytes using a mechanical milling technique. Therefore, liquid-phase synthesis of the sulfide-based solid electrolytes is expected as more simple and effective processes. Recently, it was reported that β–Li3PS4 [3] with an ionic conductivity of 1.6×10-4 S cm-1 and Li7P2S8I [4] with an ionic conductivity of 6.4×10-4 S cm-1 were synthesized using tetrahydrofuran and acetonitrile, respectively. In addition, Matsuda and coworkers reported that Li3PS4 was synthesized by liquid-phase shaking process using ethyl acetate for 6 hours [5]. We have reported that argyrodite-type Li6PS5X (X=Cl, Br) solid electrolyte prepared by mechanical milling was dissolved in ethanol solution, and the ionic conductivity of the reprecipitated electrolyte was 10-5-10-4 S cm-1 at room temperature [6]. However, this approach needs the multi-step processes of mechanical milling and dissolution-reprecipitation. In this study, the Li6PS5Br solid electrolyte was directly synthesized from Li2S, P2S5 and LiBr by liquid-phase reaction using tetrahydrofuran and ethanol. Experimental Li2S and P2S5 with a stoichiometry of 3 to 1 were mixed in tetrahydrofuran (THF) at room temperature, and then a Li3PS4 precursor in THF suspension was obtained. An ethanol solution of Li2S and LiBr was also prepared. The mixture solution was obtained by the THF suspension and the ethanol solution. The mixture solution was dried at 150oC under vacuum to obtain solid powders or heat-treated at 550oC in a dry argon atmosphere to enhance their crystallinity. All-solid-state batteries with LiNi1/3Mn1/3Co1/3O2 (NMC) active material and Li6PS5Br solid electrolytes obtained by mechanical milling or liquid-phase synthesis were fabricated and characterized. Results and discussion A homogenous pale-green solution was obtained. Fine powders were precipitated after removing the solvents by drying at 150oC and/or heat treatment at 550oC. X-ray diffraction patterns showed that the obtained sample was mainly Li6PS5Br crystal. Especially, crystallinity of Li6PS5Br was enhanced by heat treatment at 550oC. The Li6PS5Br electrolyte synthesized using the liquid-phase technique showed a high lithium-ion conductivity of 1.1×10- 3 S cm-1 at 25oC, comparable to that prepared using a mechanical milling technique. The primary particle size of Li6PS5Br obtained using the liquid-phase technique was about 1 μm. All-solid-state batteries with the Li6PS5Br electrolyte showed the capacity of 140 mAh g-1. Acknowledgement The research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovation Next Generation Batteries (SPRING) Project, and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan References [1] Y. Seino et al., Energy Environ. Sci., 7 (2014) 627. [2] S. Boulineau, et al., Solid State Ionics, 3 (2012), 1. [3] Z. Liu et al., J. Am. Chem. Soc., 135 (2013) 975. [4] E. Rangasamy et al., J. Am. Chem. Soc., 137 (2015) 1387. [5] N. H. H. Phuc et al., Solid State Ionics (2015) in press. [6] S. Yubuchi et al., J. Power Sources, 293 (2015) 941.