Anionic redox reaction in layered NaCr 2/3 Ti 1/3 S 2 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 (S ₂ ) ^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 _2/3 Ti _1/3 S ₂ 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 _2/3 Ti _1/3 S ₂ , 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 (S ₂ ) ^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.