Current salt containing liquid lithium electrolytes show both cationic and anionic conductivity with lithium transference numbers below 50% leading to formation of the ion concentration gradients inside a lithium battery cell. Moreover, the possibility of indirect (vehicular) transport of lithium ions from one electrode to the other via ion pair diffusion is usually neglected when typical DC polarization measurements are performed.[1] If the concept of Conservative Ensembles [2] is employed, effective conductivity can be deconvoluted into transport contributions from free ions and ion pairs even when higher order aggregates are present.[3] We demonstrate experimentally that a combination of DC polarization, impedance spectroscopy and PFG-NMR can enable such analysis for an electrolyte composed of poly(ethylene glycol) dimethyl ether (PEGDME-150) and lithium trifluoromethanesulfonate (LiTf) salt at ambient temperature.[4] In this case, the influence of ion pairs is substantial at all concentrations with values exceeding pure lithium conductivity (in 1M LiTf/PEGDME-150, only 5% of the current is transported by solvated Li⁺ while 21% is transported via Li-Tf ion pair under applied voltage). By including IR spectroscopy in the arsenal of techniques, mobilities of all species as well as effective mass action constants are calculated for the first time.

 

1. J. Maier, Advanced Functional Materials, 21, 1448, 2011.
2. J. Maier, Journal of the American Ceramic Society, 76, 1212, 1993.
3. J. Maier, Electrochimica Acta, 129, 21, 2014.
4. J. Popovic, C. Pfaffenhuber, J. Melchior, J. Maier, Electrochemistry Communications, 60, 195–198, 2015.