Anion Redox in an Amorphous Titanium Polysulfide

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 S^2-, S^-, and S^δ-. The S^2- 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.