Breaking the Specific Capacity Limit: 409% Boost in Manganese-Based Redox Flow Batteries at High Current Densities with a MnO2 Semi-Solid Slurry Electrolyte.

The common-ion effect of SO42- limits the solubility of MnSO4 to ≤ 1 M in conventional H2SO4 electrolytes, resulting in low specific capacity of manganese (Mn)-based redox flow batteries (RFBs). Although MnO2 semi-solid RFBs are expected to improve the specific capacity, the sluggish electrochemical reaction kinetics leads to low operating current densities (≤ 1 mA cm-2). Herein, we employ a reverse-design strategy by introducing additional high-concentration MnO2 into the conventional Mn-based electrolyte. Leveraging the MnO2/Mn2+ electrochemical reaction successfully circumvents the common-ion effect, raising the concentration of soluble Mn species in the electrolyte to 3.76 M and achieving a specific capacity of 156.2 Ah L Catholyte - 1 -a 409% increment over reported MnSO4-based electrolytes. Subsequently, the reverse disproportionation of electrolytic MnO2 generated during the second charging cycle shifts the redox mechanism from the MnO2(s)/Mn2+ couple to the solution-phase Mn3+(aq)/Mn2+ pair. This transformation not only elevates the discharge voltage but also enables stable operation at 30 mA cm-2, representing a current density 30-fold higher than reported Mn-based semi-solid RFBs. This work demonstrates a rational design strategy for semi-solid slurry electrolytes to enhance the specific capacity of RFBs, thereby advancing their applicability in grid-scale renewable energy storage.