Lithium Storage Properties of Rocksalt-Type Li-Excess Titanium Sulfides

Abstract

In recent years, the demands for lithium-ion batteries has increased rapidly all over the world. To further increase the energy density, Li-excess electrode materials which deliver higher reversible capacities have been actively studied as advanced positive electrode materials. A Li-excess sulfide, Li2TiS3, delivers a large reversible capacity on the basis of reversible anion redox.1) Li2TiS3 has the advantage of high electronic conductivity even though it is composed of Ti4+ ions without valence electrons. This is a characteristic feature of sulfides that is different from an oxide counterpart, Li2TiO3. In this study, we study factors affecting on electrode performance of Li2TiS3 and discuss reaction mechanisms as the potential high-capacity positive electrode material for lithium-ion batteries.Li2TiS3 was prepared by mechanical milling. Li2S and TiS2 were used as precursors, and the mixture of the precursors were mixed by using a zirconia pot and zirconia balls. Crystal structures of the samples were examined by using an X-ray diffractometer. Composite electrodes consisted of 80 wt% active materials, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride, pasted on aluminum foil as a current collector. Metallic lithium was used a negative electrode. The electrolyte solutions used were 1.0 M LiPF6 dissolved in ethylene carbonate:dimethyl carbonate (3:7 by volume) and 5.3 M LiFSA dissolved in trimethylphosphine as a concentrated electrolyte solution.2) Two-electrode cells were used and cycled at a rate of 20 mA g-1 at room temperature.Figure 1 shows the X-ray diffraction patterns of the precursors and the sample after mechanical milling. From Fig. 1, diffraction lines of the starting materials with high crystallinity disappear after mechanical milling and broad diffraction lines after milling are assigned into a cation-disordered rocksalt-type structure with low crystallinity. Although Li2TiO3 is electrochemically inactive, Li2TiS3 delivers a large reversible capacity of over 300 mA h g-1 in Li cells. However, the reversible capacity of Li2TiS3 is reduced to approximately 150 mA h g-1 after the 30 cycles test with the aprotic electrolyte solution with 1 M LiPF6 as shown in Fig. 2. On the other hand, Li2TiS3 delivers a reversible capacity of 230 mA h g-1 even after 50 cycles with the concentrated electrolyte solution (5.3 M LiFSA) because dissolution of the sulfide could be suppressed for the concentrated electrolyte solution with less free solvents.3) From these results, we discuss the factors affecting electrode performance of Li2TiS3 for advanced positive electrode materials with sulfide ions.References(1) Atsushi Sakuda et al., Scientific Reports. 8, 15086 (2018).(2) Jianhui Wang et al., Nature Energy. 3, 22–29 (2018).(3) Takayuki Doi et al., ChemistrySelect. 2, 8824-8827 (2017). Figure 1