Anionic Redox Chemistry in Polysulfide Electrode Materials for Rechargeable Batteries

Classical Li-ion battery technology is based on the insertion of lithium ions into cathode materials involving metal (cationic) redox reactions. However, this vision is now being reconsidered, as many new-generation electrode materials with enhanced reversible capacities operate through combined cationic and anionic (non-metal) reversible redox processes or even exclusively through anionic redox transformations. Anionic participation in the redox reactions is observed in materials with more pronounced covalency, which is less typical for oxides, but quite common for phosphides or chalcogenides. In this Concept, we would like to draw the reader's attention to this new idea, especially, as it applies to transition-metal polychalcogenides, such as FeS2, VS4, TiS3, NbS3, TiS4, MoS3, etc., in which the key role is played by the (S-S)(2-)/2S(2-) redox reaction. The exploration and better understanding of the anion-driven chemistry is important for designing advanced materials for battery and other energy-related applications.