Li+ Transference Numbers in Liquid Electrolytes Obtained by Very-Low-Frequency Impedance Spectroscopy at Variable Electrode Distances

Abstract

The Li+ transference numbers of three different liquid electrolytes for Li-ion batteries were measured in a symmetrical Li | electrolyte | Li cell by means of very-low-frequency impedance spectroscopy (VLF-IS). The electrolytes were: (i) The standard battery electrolyte LP30; (ii) an equimolar mixture of tetraglyme (G4) and lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI); (iii) Li-TFSI dissolved in the ionic liquid 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI). We found that the Li+ transference numbers of the two electrolytes LP30 and G4/Li-TFSI are much smaller than the Li+ transport numbers obtained from pulsed-field gradient NMR measurements. On the other hand, in the case of BMP-TFSI/Li-TFSI, the values for and are more similar. In order to rationalize the large differences between and found for LP30 and G4/Li-TFSI, we combined the Onsager reciprocal relations with linear response theory, and we derived expressions for , which take into account all correlations between ionic movements in the electrolyte. Thereby, we show that can be considerably smaller than , if strong directional correlations exist between the movements of cations and anions. Finally, we discuss differences in Li+ transference numbers obtained by VLF-IS and by potentiostatic polarization measurements.