Electrochemical and structural characterization of Mg ion intercalation into Co3O4 using ionic liquid electrolytes

The electrochemical behavior of magnesium perchlorate in ionic liquids is characterized using Co3O4 as the working electrode and Mg foil as the anode material. The cations of the ionic liquids were 1,2-dimethyl-3-R-imidazolium with R = butyl (MMBI) or octyl (MMOI), and the anion was bis(trifluoromethanesulfonyl)imide (TFSI). The TFSI ionic liquids exhibited large electrochemical windows, with MMOITFSI possessing an electrochemical window of 5 V. Ionic conductivity studies indicated a higher ionic conductivity of 3.67 mS/cm for 1.0 M Mg(ClO4)2 in MMBITFSI when compared to 2.16 mS/cm for 1.0 M Mg(ClO4)2. MMOITFSI was selected as the electrolyte for the electrochemical characterizations of Co3O4, due to its larger electrochemical window and its inability to intercalate, and therefore degrade, graphitic material added to the Co3O4 electrode. Electrochemical results indicated that the maximum attainable value for x in MgxCo3O4 was 0.33, with an initial capacity of only 78 mAh/g. X-ray diffraction studies of the Mg intercalated Co3O4 indicated a 20% expansion of the unit cell and a decrease in the symmetry of the metal oxide from cubic to orthorhombic symmetry. Charge/discharge experiments at a C/3 rate were performed vs. Mg metal. Results indicated that after 10 cycles, the capacity decreased linearly with cycling, most likely due to the inability to deintercalate the more strongly interacting Mg++.