Transport properties of alkali/alkaline earth cations in ionic-liquid based electrolytes
Résumé
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.
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