Access to M3+/M2+ Redox Couples in Layered LiMS2 Sulfides (M=Ti, V, Cr) as Anodes for Li-Ion Battery

The insertion of Li into layered LiMS2 (M=Ti, V, Cr) sulfides has been reinvestigated with ethylene-carbonate-based carbonate electrolytes. The Ti3+/Ti2+ and V3+/V2+ redox couples in a sulfide were found to give a reversible insertion reaction at about 0.5 and 1.0 V vs Li+/Li-0, respectively, while Cr atoms are displaced from LiCrS2 at 0.9 V vs Li+/Li-0. Comparison to a graphite anode indicates that a solid-electrolyte-interface (SEI) passivation layer is formed below 0.9 V during the first discharge of the LiTiS2 before reaching the Ti3+/Ti2+ redox couple, while an SEI layer was formed after reaching the V3+/V2+ redox couple at 1.0 V during the first discharge of the Li1+xVS2 electrode. The formation of a suitable SEI layer and therefore the reversibility and capacity of sulfide anodes are shown to depend on the composition of the electrolyte.