Tuning Mn^sup 2+^ additive in the aqueous electrolyte for enhanced cycling stability of birnessite electrodes

Long-term cycling stability is a main issue for the application of manganese-based electrodes in pseudocapacitors. Herein, we demonstrate a facile approach to enhance the cycling stability of birnessite electrodes by pre-adding slight Mn2+ into the aqueous electrolyte. The cycling performance of the electrode and the morphology of birnessite change with Mn2+ concentration, which are ascribed to the electrochemical oxidation of Mn2+ or the reduction of birnessite during the charge/discharge cycling process. By controlling a low concentration of Mn2+, the birnessite electrode exhibits enhanced capacitance and cycling stability, while a hierarchical nanosheets assembled nanowall array architecture is achieved after the prolonged cycling. The resulting electrode delivers a superior rate capability (60% retention at 20 A g−1) and a high cycling stability (93% retention after 10000 cycles). This work opens up a new strategy for the development of manganese-based electrodes with high rate performance and long-term cycling stability.