Electrochemical Insertion/Deinsertion of Mg2+ Ions into MnO2 Nanowires
We investigated the electrochemical insertion/deinsertion behaviors of Mg2+ ions with nanostructured MnO2 by utilizing various spectroscopic and microscopic techniques. We determined the stoichiometry of Mg2+ ions and MnO2 at various charged states as well as with various electrode structures and electrolyte conditions by directly analyzing the inserted amount of Mg2+ ions with inductively coupled plasma – optical emission spectroscopy (ICP-OES). In addition, we studied the effect of the water content of the organic magnesium electrolyte, motivated by previous studies where water molecules had a positive effect on Mg2+ insertion behavior into layered-structure V2O5. We found that the sluggish Mg2+ insertion/deinsertion behaviors into MnO2 can be suppressed by the presence of a small amount of water in organic electrolytes, presumably due to the preferential solvation of Mg2+ ions by water molecules, which could “shield” the divalent charge of Mg2+ ions and reduce the electrostatic force between MnO2 and Mg2+. The cyclic voltammogram measurements showed that there was no considerable degree of Mg2+ insertion/deinsertion reactions in pure organic electrolytes whereas distinct peaks, responsible for Mg2+ insertion/deinsertion, were observed in water-containing electrolytes. In addition, we found that the nanostructured MnO2 electrode delivered improved Mg2+ insertion/deinsertion capabilities compared to the planar MnO2 electrode due to the shorter ionic diffusion length and high surface area of nanostructure, allowing the Mg2+ ions to overcome the electrostatic force.