Catholyte Modulation and Prussian Blue/Berlin Green Redox Mediator Enabling Efficient High-Potential Mn²⁺/MnO₂ Reaction for Aqueous Hybrid Batteries

The use of multielectron Mn2+/MnO2 cathode reaction achieves high-energy aqueous acid-base asymmetric hybrid batteries. However, the ion crossover through the membrane and the accumulation of unreacted MnO2 lead to serve potential and capacity decay of the cathode during cycling. Herein, the catholyte modulation using the KCl additive and the design of a Prussian blue/Berlin green (PB/BG) redox mediator on the cathode current collector are developed to promote high-potential Mn2+/MnO2 reaction. The addition of saturated KCl greatly suppresses the ion crossover to maintain the catholyte acidity, thus enabling high-potential Mn2+/MnO2 reaction and stable high-voltage discharge plateau of approximate to 2.2 V for the hybrid battery at a prolonged cycling. The reversible MnO2 deposition is effectively improved by the PB/BG redox mediator, which contributes to significant enhancements in the reversible capacity, rate capability, and cycling stability of the cathode. Importantly, the as-assembled hybrid battery exhibits the highest energy density of approximate to 260 Wh kg(-1) (based on the mass of the aqueous MnCl2 catholyte and a hydrogen storage anode material) among the previously reported Mn-based acid-base asymmetric batteries.