Anionic redox reaction in layered NaCr_2/3Ti_1/3S₂ through electron holes formation and dimerization of S–S

The use of anion redox reactions is gaining interest for increasing rechargeable capacities in alkaline ion batteries. Although anion redox coupling of S²⁻ and (S₂)²⁻ through dimerization of S–S in sulfides have been studied and reported, an anion redox process through electron hole formation has not been investigated to the best of our knowledge. Here, we report an O3-NaCr_2/3Ti_1/3S₂ cathode that delivers a high reversible capacity of ~186 mAh g⁻¹ (0.95 Na) based on the cation and anion redox process. Various charge compensation mechanisms of the sulfur anionic redox process in layered NaCr_2/3Ti_1/3S₂, which occur through the formation of disulfide-like species, the precipitation of elemental sulfur, S–S dimerization, and especially through the formation of electron holes, are investigated. Direct structural evidence for formation of electron holes and (S₂)ⁿ⁻ species with shortened S–S distances is obtained. These results provide valuable information for the development of materials based on the anionic redox reaction.