Breaking the Specific Capacity Limit: 409% Boost in Manganese-Based Redox Flow Batteries at High Current Densities...

The common-ion effect of SO₄^2- 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 ${\text{L}}_{\mathrm{Catholyte}}^{-1}$ -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.