Electrochemical Insertion/Deinsertion of Mg2+ Ions into MnO2 Nanowires

Wednesday, 8 October 2014: 16:40
Sunrise, 2nd Floor, Galactic Ballroom 1 (Moon Palace Resort)


The increasing demand for rechargeable energy storage devices has generated great interest in battery systems that could replace the high-cost lithium based batteries. Among various candidates, magnesium-ion (Mg-ion) batteries are considered one of the most promising alternatives due to the abundance and better stability of magnesium compared to those of lithium. However, despite the similarity in ionic size between Li+ and Mg2+ ions, Mg2+ ions exhibit very slow solid-state insertion/deinsertion kinetics with the cathode materials currently used for Li-ion batteries. The sluggish Mg2+ insertion/deinsertion kinetics is presumed to be the result of the divalent characteristic of Mg2+ ions, which leads to a strong electrostatic force between the Mg2+ ions and the hosting material. For instance, poor electrochemical insertion/deinsertion performances of Mg2+ into manganese dioxide (MnO2) has been reported in previous studies which suggest that the strong ionic characteristic of MnO2 hampers the Mg2+ insertion into its lattice.  

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.