New hybrid redox flow battery with high energy density using V–Mn/V–Mn multiple redox couples

A new hybrid redox flow V–Mn/V–Mn battery is introduced for enhancing the energy density of a V/V system. The energy density of the V–Mn/V–Mn system is high because the system has multiple redox reactions involving both V and Mn ions, and the operating cell potential increases owing to the high standard potential of the Mn(II)/Mn(III) reaction. Consequently, the discharge energy density is increased by 66.8% compared with that of the V/V system. Additionally, because the V–Mn/V–Mn system contains the same components in the negative and positive electrolytes, a reduction in energy density due to ion crossover with cycling can be achieved through a rebalancing process. The V–Mn/V–Mn system is not simply a mixture of V and Mn electrolytes but has a unique advantage. The presence of V ions reduces the particle size of the MnO₂ produced by the disproportionation reaction during the charging process. The small MnO₂ particles can flow into the cell together with the electrolyte and can be reduced to Mn ions. As a result, the reversibility and durability of the system are greatly improved. Therefore, the V–Mn/V–Mn system can complement the low energy density of the V/V system.