Investigations into the origin of pseudocapacitive behavior of Mn3O4 electrodes using in operando Raman spectroscopy

A detailed understanding of the phase changes of manganese oxides in a pseudocapacitor during cycling is vital to reveal the mechanism by which they store energy and to achieve a rational design of improved electrode materials. Here we report the results of our probing the phase changes of a Mn3O4 electrode material in a pseudocapacitor during a charging/discharging process using in operando Raman spectroscopy. Along with enhancing performance, the spinel Mn3O4 was transformed to a layered birnessite-type MnO2 upon potential cycling and two types of processes were shown to contribute to the energy storage: intercalation/deintercalation of Na+ and oxidation/reduction of Mn2+. After charging and discharging for similar to 10 000 cycles, the specific capacitance of manganese oxide increased almost 3-fold; and the maximum specific capacitance approached 230 F g (1) at a cycling rate of 0.5 A g(-1).