Anion Redox in an Amorphous Titanium Polysulfide.

Abstract

Amorphous transition-metal polysulfides are promising positive electrode materials for next-generation rechargeable lithium-ion batteries because of their high theoretical capacities. In this study, sulfur anion redox during lithiation of amorphous TiS4 (a-TiS4) was investigated by using experimental and theoretical methods. It was found that a-TiS4 has a variety of sulfur valence states such as S2-, S-, and Sδ-. The S2- species became the main component in the Li4TiS4 composition, indicating that sulfur is a redox-active element up to this composition. The simulated a-TiS4 structure changed gradually by lithium accommodation to form a-Li4TiS4: S-S bonds in the disulfide units and polysulfide chains were broken. Bader charge analysis suggested that the average S valency decreased drastically. Moreover, deep lithiation of a-TiS4 provided a conversion reaction to metallic Ti and Li2S, with a high practical capacity of ∼1000 mAh g-1 when a lower cutoff voltage was applied.