Determination of individual contributions to the ionic conduction in liquid electrolytes: Case study of LiTf/PEGDME-150

Deconvolution of the contributions of free cations, free anions, neutral ion pairs, and ion pair dimers to the overall ionic conductivity is demonstrated for poly(ethylene glycol) dimethyl ether containing lithium trifluoromethanesulfonate with concentrations between 0.01 M and 1.0 M. Experimentally, this is achieved by combining impedance spectroscopy, DC polarization, and pulsed-field gradient (PFG) NMR techniques. The theoretical basis is the concept of conservative ensembles. Ion pair influence is found to be substantial for all salt concentrations with its individual transport coefficient values above conductivity of free lithium ions. Lithium transference number measured by DC polarization exceeds the value of true lithium transference number emphasizing the importance of indirect (vehicular) transport mechanism under current, in particular at higher salt concentrations. By including IR spectroscopy in the arsenal of techniques, mobilities of all species as well as effective mass action constants are calculated. Heterogeneous doping with SiO2 varies the parameters as expected from space charge theory.