Aqueous redox flow batteries (ARFBs) are an important electrochemical storage technology for grid-scale applications. Compared to conventional ARFBs, such as those based on vanadium, manganese-based systems are appealing because of their rich redox chemistry – and thus potentially high energy and/or power density operation – and affordability. However, most manganese-based ARFBs adopt a negative electrode made from metal, such as zinc, and suffer from poor cycling life with limited areal capacities (typically <10 mAh/cm2) due to detrimental dendrite formation and positive electrode passivation. Now, Yaqin Huang, Yi-Chun Lu and colleagues, at the Chinese University of Hong Kong and Beijing University of Chemical Technology, report a sulfur–manganese ARFB with a Mn2+/MnO2 posolyte and a S22−/S2− negolyte. They show that this battery achieves good reversibility at much higher areal capacities (>50 mAh/cm2) than zinc-manganese ARFBs.
The battery working principle is that upon charge, MnO2 deposits onto the positive electrode, while S22− is reduced to S2− on the negative electrode; reverse processes take place upon discharge. To facilitate the reversibility of MnO2 deposition and dissolution, the researchers introduce both acetate and iodide ions into the posolyte. They show that the acetate ions assist a direct one-step MnO2 deposition without the generation of intermediates such as Mn3+. They also show that through the reaction, MnO2 + 3I− + 4H+ → Mn2+ + I3− + 2H2O, during discharge, iodide serves as a mediator to help the dissolution of MnO2. The acetate and iodide additives ensure reversible cycling at high areal capacities. The researchers also report that due to the Earth’s abundance of sulfur and manganese species, the cost of the electrolytes in their battery is low.
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