Ultrahigh-Loading Manganese-Based Electrodes for Aqueous Batteries via Polymorph Tuning

Manganese-based aqueous batteries utilizing Mn²⁺ /MnO₂ redox reactions are promising choices for grid-scale energy storage due to their high theoretical specific capacity, high power capability, low-cost, and intrinsic safety with water-based electrolytes. However, the application of such systems is hindered by the insulating nature of deposited MnO₂ , resulting in low normalized areal loading (0.005-0.05 mAh cm^-2 ) during the charge/discharge cycle. In this work, the electrochemical performance of various MnO₂ polymorphs in Mn²⁺ /MnO₂ redox reactions is investigated, and ɛ-MnO₂ with low conductivity is determined to be the primary electrochemically deposited phase in normal acidic aqueous electrolyte. It is found that increasing the temperature can change the deposited phase from ɛ-MnO₂ with low conductivity to γ-MnO₂ with two order of magnitude increase in conductivity. It is demonstrated that the highly conductive γ-MnO₂ can be effectively exploited for ultrahigh areal loading electrode, and a normalized areal loading of 33 mAh cm^-2 is achieved. At a mild temperature of 50 °C, cells are cycled with an ultrahigh areal loading of 20 mAh cm^-2 (1-2 orders of magnitude higher than previous studies) for over 200 cycles with only 13% capacity loss.