The Ultrahigh-Rate Performance of Alkali Ion-Pre-Intercalated Manganese Oxides in Aqueous Li2SO4, Na2SO4, K2SO4 and MgSO4 Electrolytes

This work focuses on the capacitive characteristics of Na0.3K0.03MnO1.57 and K0.29MnO1.58 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, Na0.3K0.03MnO1.57 mainly consists of the amorphous region mixed with several small crystallites in a loose microstructure while K0.29MnO1.58 is a mixture closely arranged with large-area crystalline and amorphous domains. The capacitive behavior of Na0.3K0.03MnO1.57 and K0.29MnO1.58 is evaluated in sulfate electrolytes containing Li+, Na+, K+, and Mg2+ 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 Na0.3K0.03MnO1.57 in the Na2SO4 electrolyte is ideally surface-control without the significant influence from the solid-state diffusion in a very wide range of CV scan rates.