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 TiS 4 (a-TiS 4 ) was investigated by using experimental and theoretical methods. It was found that a-TiS 4 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 Li 4 TiS 4 composition, indicating that sulfur is a redox-active element up to this composition. The simulated a-TiS 4 structure changed gradually by lithium accommodation to form a-Li 4 TiS 4 : 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-TiS 4 provided a conversion reaction to metallic Ti and Li 2 S, with a high practical capacity of ∼1000 mAh g -1 when a lower cutoff voltage was applied.