Hoang Phuong Khanh Ngo’s research while affiliated with Université Savoie Mont Blanc and other places

The increasing need of portable electrical resources requires to develop post-Li batteries, in which redox reactions are then based on the different alkali or earth alkaline ions. Keeping in mind the specific advantages of electrolytes based on ionic liquid (electrochemical properties, safety, stability …), this paper focuses on the specific properties of those obtained by mixing alkali or earth alkaline salts (Li⁺, Na⁺, K⁺, Cs⁺ and Mg²⁺) in a prototypical ionic liquid, the 1-butyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl) imide (BMImTFSI). Transport properties of these electrolytes are deciphered from viscosity, ionic conductivity and individual self-diffusion coefficients. At room temperature, change of electrolyte composition (nature of ion, concentration) induces some large variations of these transport properties. However, if these measurements are scaled towards glass transition temperatures, master curves are obtained with only slight differences between monovalent alkali and divalent earth alkaline ions. Further information is obtained from the Walden approach and evidences that these electrolytes are 'good ionic conductors'. These results are confirmed from self-diffusion coefficients which also allows to retrieve contributions of each species to ionic transport and then dissociation ratio inside these mixtures. Slight differences between alkali and earth alkali ions may be related to solvation mechanisms, as proposed from infrared spectroscopy measurements.

A new class of electrolyte based on TFSI- and triphenolate borohydride anions was designed and produced which fulfills all requirements of easy synthesis, high ionic conductivity, wide potential window and non-corrosion of Al current collector. The electrolyte composed of magnesium triphenolate borohydride and Mg(TFSI)2 in glyme displays simultaneously high conductivity with 5.5 mS cm-1 at 25°C and a reversible Mg plating/stripping with high current density and coulombic efficiency at room temperature. By addition of a slight amount of MgCl2 to this electrolyte, a coulombic efficiency of 90% in SS/Mg cell, a stable cycling performance, and wide anodic potential of 3.4 V vs. Mg2+/Mg on Al current collector can be reached. Reversible and efficient Mg insertion/de-insertion with high capacity of 94 mAh g-1 and 96% of coulombic efficiency were obtained with Mo6S8 Chevrel cathode phase.