Electrochemical Insertion/Deinsertion of Mg2+ Ions into MnO2 Nanowires

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.