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-center dot 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.