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

Abstract Manganese‐based aqueous batteries utilizing Mn 2+ /MnO 2 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 MnO 2 , 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 MnO 2 polymorphs in Mn 2+ /MnO 2 redox reactions is investigated, and ɛ‐MnO 2 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‐MnO 2 with low conductivity to γ ‐MnO 2 with two order of magnitude increase in conductivity. It is demonstrated that the highly conductive γ ‐MnO 2 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.