Redox flow batteries are promising candidates for large-scale energy storage because of their safety, long service life, and independently scalable power and energy capacity. Among them, manganese-based systems are particularly attractive due to the abundance, low cost, and environmental compatibility of manganese. However, conventional manganese electrolytes suffer from limited solubility, restricting the amount of energy they can store. Although solid MnO₂ can increase the active-material content, its slow reaction kinetics typically limit the operating current density of semi-solid flow batteries.
To address these challenges, researchers at Shanghai University have published a study in Nano-Micro Letters entitled "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 team developed a semi-solid slurry strategy that significantly increases the capacity of manganese-based flow batteries while enabling operation at much higher current densities. Their approach combines additional solid MnO₂ with a change in the electrochemical reaction pathway, overcoming two major limitations of conventional manganese-based systems.
The key innovation is a different way of increasing the amount of active manganese. In conventional sulfuric acid electrolytes, the common-ion effect limits the solubility of MnSO₄, restricting the concentration of active manganese species. Instead of relying solely on dissolved manganese salts, the researchers introduced additional MnO₂ particles into the electrolyte. During discharge, solid MnO₂ is electrochemically converted into soluble Mn²⁺, allowing the battery to access more active material without being constrained by the initial solubility limit.
This strategy produced a substantial improvement in capacity. With a 4 M MnO₂ slurry, the concentration of soluble manganese species reached 3.76 M, while the specific capacity increased to 156.2 Ah L⁻¹, a 409% improvement over the reported MnSO₄-based electrolyte benchmark. Importantly, the slurry maintained relatively low viscosity without requiring additional conductive carbon additives, supporting its operation in a flowing electrolyte system.
The battery also demonstrated excellent performance at higher current densities. At 30 mA cm⁻², the 4 M MnO₂ slurry retained a specific capacity above 151 Ah L⁻¹ and an energy efficiency exceeding 71%. This operating current density is 30 times that previously reported for MnO₂-based semi-solid redox flow batteries, demonstrating that high active-material loading does not necessarily have to come at the expense of reaction rate.
Beyond the performance gains, the researchers uncovered an important change in the battery's electrochemical reaction mechanism. Initially, commercial MnO₂ participates in a relatively slow solid-phase MnO₂/Mn²⁺ reaction. However, MnO₂ newly formed during subsequent charging exhibits a more disordered structure, a larger specific surface area, and higher electrochemical activity. This newly generated MnO₂ favors a reverse disproportionation reaction with Mn²⁺, producing soluble Mn³⁺.
As a result, the dominant redox process shifts from the slower solid-phase MnO₂/Mn²⁺ reaction to the faster solution-phase Mn³⁺/Mn²⁺ reaction. This transition improves reaction kinetics and discharge voltage, helping explain how the battery can maintain high capacity at substantially higher current densities. Structural characterization and electrochemical measurements provided evidence for this mechanism, including the disappearance of the low-voltage discharge plateau after the initial cycle.
The researchers also examined long-term cycling performance. A battery using a 1 M MnO₂ slurry operated for 100 cycles at 30 mA cm⁻² with approximately 99% Coulombic efficiency and 76% energy efficiency. However, capacity retention remains a challenge, particularly at higher MnO₂ concentrations. The study identified manganese crossover and particle accumulation as important causes of capacity loss, pointing to opportunities for further electrolyte and membrane optimization.
With an estimated electrolyte energy cost of approximately US$33 kWh⁻¹, the proposed approach also shows potential economic advantages. By overcoming the conventional capacity limitation and enabling faster electrochemical reactions, this work introduces a new route toward high-capacity, cost-effective semi-solid redox flow batteries. Further improvements in cycling stability could support their future application in large-scale renewable energy storage.
