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 S^2- 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.