Sulfides Electrolytes for All Solid State Metal Ion Batteries

Abstract

Replacing the organic, liquid, flammable electrolytes in conventional lithium ion batteries with inorganic, solid, nonflammable electrolytes can potentially eliminate the fire hazard of the batteries. Solid state batteries may also use simpler and lower cost manufacturing methods than what has been used for conventional lithium ion batteries. However, the development of solid state batteries previously has been limited by the low ionic conductivity of the solid electrolytes. Recently discovered superfast ionic conductors such as Li10GeP2S12 (LGPS) and Li10SnP2S12have demonstrated solid state electrolyte can have comparable or even higher Li ion conductivity than commercial liquid electrolyte. All solid state batteries with using sulfide electrolytes has also been reported by Kanno et al. with very high rates at room temperature and elevated temperatures. Currently many sulfides showed high Li and Na conductivities than their oxides counter parts. However, in general, sulfides are much less explored than oxides. Novel sulfides with good ionic conductivities may exist in the previously unexplored regions in the phase diagrams. We aim to design and discover novel sulfide Li and Na ionic conductors by systematically exploring a number of selected Na(Li)-M1-(M2)-S ternary and quaternary phase diagrams combining conventional solid state synthesis and variable temperature in situ XRD for synthesis. In this work, we report the synthesis, in situ XRD study and electrochemical testing of several sodium tin sulfides as solid electrolyte for sodium ion batteries. Na4SnS4 was synthesized by solid state reactions in pure phase. In situ XRD study on the synthesis process were used to reveal the formation mechanism of this compound. The Na ion conductivity of this compound at various temperatures was measured by using electrochemical impedance spectroscopy (EIS). The relationship between the conductivity and synthesis condition and starting materials are discussed. Several other sulfides were also successfully synthesized in pure phase. Their conductivities were also measured and compared with Na4SnS4. These novel Na ion conductors provide new promise for developing high performance all solid state sodium ion batteries. Figure caption: Figure 1. in situ X-ray diffraction patterns of the solid state synthesis process of Na4SnS4sample. Reference:[1] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nature Energy, (2016) 16030. Figure 1