Cost-Effective Polysulfide and Polyiodide Redox Flow Battery for Large-Scale Energy Storage

Renewable resources have the potential to compete with coal, but provide energy only intermittently. Grid-scale electrical energy storage cost-effectively smooths out these intermittencies, which is critical to enable deeper market penetration of renewable energy sources. Redox flow batteries (RFBs) are one such technology that will help renewable energy sources gain wider use. The active species in typical RFBs are expensive and relatively rare. In contrast, this work examines sulfur and iodine as the energy storage compounds, both of which are inexpensive, earth-abundant elements. The work presented herein demonstrates the feasibility of the polysulfide-polyiodide flow battery (SIFB). The system has an open circuit voltage of 1.0 volts with a peak power density of 65 mW/cm². SIFBs use sodium cations as the working ion to balance charges between the electrolytes; this is advantageous because the ion-selective membranes used here facilitate the transfer of positive charges while repelling negative ones, ensuring minimal crossover occurs as determined by cyclic voltammograms of the post-cycling anolyte and catholyte. Indeed, the reactor demonstrates nearly 100% coulombic efficiency for 200 charge-discharge cycles, or 530 hours, implying minimal impact of the solutions on reactor performance. The solutions were refreshed every 50 cycles. Finally, it is shown through a techno-economic analysis that SIFBs can reach the U.S. Department of Energy’s price target for electrical energy storage of $100/kWh. Not only can they reach this target, but they are currently less expensive than even the future cost of vanadium RFBs; they outperform existing batteries and can compete with other energy storage methods.