In this study, the effect of oxygen vacancy in the CoMn2O4 on pseudocapacitive characteristics was examined, and two tetragonal CoMn2O4 spinel compounds with different oxygen vacancy concentrations and morphologies were synthesized by controlling the mixing sequence of the Co and Mn precursors. The mixing sequence was changed; thus, morphologies were changed from spherical nanoparticles to nanoflakes and oxygen vacancies were increased. Electrochemical studies have revealed that tetragonal CoMn2O4 spinels with a higher number of oxygen vacancies exhibit a higher specific capacitance of 1709 F g−1 than those with a lower number of oxygen vacancies, which have a higher specific capacitance of 990 F g−1. Oxygen vacancies create an active site for oxygen ion intercalation. Therefore, oxidation–reduction reactions occur because of the diffusion of oxygen ions at octahedral/tetrahedral crystal edges. The solid-state asymmetric pseudocapacitor exhibits a maximum energy density of 32 Wh-kg−1 and an excellent cyclic stability of nearly 100%.
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