Carbon disulfide: A redox mediator for organodisulfides in redox flow batteries

Organodisulfides (RSSR) are a class of promising active materials for redox flow batteries (RFBs). However, their sluggish kinetics and poor cyclic stability remain a formidable challenge. Here, we propose carbon disulfide (CS₂) as a unique redox mediator involving reversible C-S bond formation/breakage to facilitate the reduction reaction of organodisulfides in RFBs. In the discharge of RSSR, CS₂ interacts with the negatively charged RSSR_-• to promote cleavage of the S-S bond by reducing about one-third of the energy barrier, forming RSCS₂Li. In the recharge, CS₂ is unbonded from RSCS₂Li while RSSR is regenerated. Meanwhile, the redox mediator can also be inserted into the molecular structure of RSSR to form RSCS₂SR/RSCS₂CS₂SR, and these new active materials with lower energy barriers can further accelerate the reaction kinetics of RSSR. With CS₂, phenyl disulfide exhibits an exceptional rate capability and cyclability of 500 cycles. An average energy efficiency of >90% is achieved. This strategy provides a unique redox-mediating pathway involving C-S bond formation/breakage with the active species, which is different from those used in lithium-oxygen or other batteries.