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 SO 4 2− limits the solubility of MnSO 4 to ≤ 1 M in conventional H 2 SO 4 electrolytes, resulting in low specific capacity of manganese (Mn)-based redox flow batteries (RFBs). Although MnO 2 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 MnO 2 into the conventional Mn-based electrolyte. Leveraging the MnO 2 /Mn 2+ 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 L Catholyte - 1 —a 409% increment over reported MnSO 4 -based electrolytes. Subsequently, the reverse disproportionation of electrolytic MnO 2 generated during the second charging cycle shifts the redox mechanism from the MnO 2 (s)/Mn 2+ couple to the solution-phase Mn 3+ (aq)/Mn 2+ 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.