LAUSR

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 charge/discharge cycle. In this work, we investigated the electrochemical performance of various MnO2 polymorphs in Mn2+ /MnO2 redox reactions and determined ɛ-MnO2 with low conductivity to be the primary electrochemically deposited phase in normal acidic aqueous electrolyte. We found that increasing the temperature can change the deposited phase from ɛ-MnO2 with low conductivity to γ-MnO2 with two orders of magnitude increase in conductivity. We demonstrated that the highly conductive γ-MnO2 could be effectively exploited for ultrahigh areal loading electrode, and a normalized areal loading of 33 mAh cm-2 was achieved. At a mild temperature of 50 °C, cells were 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. This article is protected by copyright. All rights reserved.