Toward an Inexpensive Aqueous Polysulfide–Polyiodide Redox Flow Battery

Redox flow batteries (RFBs) hold promise for large-scale energy storage to facilitate the penetration of intermittent renewable resources and enhance the efficiency of nonrenewable energy processes in the evolving electric power system. While all vanadium redox flow batteries (VRFBs) represent the current state-of-the-art, their system price is near 4-fold higher than the price targets outlined by the U.S. Department of Energy, inspiring research into cost reduction through increased energy density or reduced materials cost contributions. Motivated by the abundance and low cost of sulfur and iodine, herein, we explore the feasibility of an aqueous flow battery system using a polysulfide negative electrolyte and polyiodide positive electrolyte. The polysulfide–polyiodide flow battery (SIFB) has an open circuit voltage of 1 V and uses Na+ as the working ion to balance the charge in each electrolyte. Both positive and negative electrolytes display coulombic efficiency close to 100%. Without significant system optimization, the SIFB gives a peak power of 65 mW/cm2 and consistent cycling performance for more than 200 cycles (ca. 530 h) with a stabilized energy efficiency of ca. 50%. Techno-economic analysis shows that the proposed SIFB has the potential to achieve lower prices than the VRFB and to meet the established price targets, but further research is needed with a particular focus on reducing the power cost contributions and developing ion-exchange membranes with improved conductivity and species selectivity.