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This work focuses on the capacitive characteristics of Na 0.3 K 0.03 MnO 1.57 and K 0.29 MnO 1.58 in aqueous neutral electrolytes containing various cations in order to clarify the complicated interactions between electrolytes and cation-pre-intercalated Mn oxides. The material properties of both oxides are examined by the inductively coupled plasma-mass spectrometer (ICP-MS), X-ray diffractometer (XRD), and transmission electron microscope (TEM). From the textural results, Na 0.3 K 0.03 MnO 1.57 mainly consists of the amorphous region mixed with several small crystallites in a loose microstructure while K 0.29 MnO 1.58 is a mixture closely arranged with largearea crystalline and amorphous domains. The capacitive behavior of Na 0.3 K 0.03 MnO 1.57 and K 0.29 MnO 1.58 is evaluated in sulfate electrolytes containing Li + , Na + , K + , and Mg 2+ by cyclic voltammetry (CV) and chronopotentiometry (CP). The capacitive responses of both oxides are significantly affected by the cation in the electrolyte because of the influences of conductivity, pH, charge-to-mass ratio, ionic radius, primary and secondary hydrated radii, and hydration ratio. We demonstrate that the charge storage behavior of Na 0.3 K 0.03 MnO 1.57 in the Na 2 SO 4 electrolyte is ideally surface-control without the significant influence from the solid-state diffusion in a very wide range of CV scan rates.

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