Effect of alkaline and alkaline–earth cations on the supercapacitor performance of MnO2 with various crystallographic structures
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The electrochemical performances of the α-, γ-, and δ-MnO2 with different crystallographic structures were systematically investigated in 0.5 mol/L Li2SO4, 0.5 mol/L Na2SO4, 1 mol/L Ca(NO3)2, and 1 mol/L Mg(NO3)2 electrolytes. The results showed that the electrochemical performances of the manganese dioxides depended strongly on the crystallographic structures of MnO2 as well as the cation in the electrolytes. Because the δ-MnO2 consists with layers of structure and the interlayer separation is 7 Å, which is suitable for insertion/extraction of some alkaline and alkaline–earth cations, the δ-MnO2 electrode showed the higher specific capacitance than that of α-MnO2 and γ-MnO2. We also found that the α-, γ-, and δ-MnO2 electrodes in the Mg(NO3)2 electrolyte showed a higher specific capacitance, while all the α-, γ-, and δ-MnO2 electrodes in the Li2SO4 electrolyte exhibited a better cycle life. The reason for the different behavior of Li+ and Mg2+ during the charge/discharge process can be ascribed to the charge effect of the cations in the electrolytes. The ex situ X-ray diffraction (XRD) and long-time cyclic voltammogram measurements were used to systematically study the energy storage mechanism of MnO2-based electrodes. A progressive crystallinity loss of the materials is also observed upon potential cycling at the oxidized states. A reasonable charge/discharge mechanism is proposed in this work.