Thermodynamic, structural, and conductance studies of lithium coronand electrolytes relevant to lithium battery technology

Gibbs energies, enthalpies, and entropies of coronand macrocycles (15-crown-5 and 1-aza-12-crown-4) and lithium salts containing highly polarizable anions (hexafluoroarsenate, tetrafluoroborate, and trifluoromethane-sulfonate) in acetonitrile and in propylene carbonate at 298.15 K are reported. These titration calorimetry studies are accompanied by ¹³C and ¹H NMR measurements in acetonitrile (CD₃CN). On the basis of the stability of the complexes, six new coronand electrolytes were isolated. The thermochemical behavior of these electrolytes as assessed from the standard enthalpies of solution is compared with corresponding data for common lithium salts. Interpretation of these data reveals that the new coronand electrolytes are much less solvated by these solvents than the latter. The implications of these results on the conductivity enhancement observed by the addition of 15-crown-5 and 1-aza-12-crown-4 to nonaqueous lithium solutions are demonstrated leading to the conclusion that the use of these electrolytes in lithium batteries shows promise. Enthalpies of coordination referred to reactants and products in their pure physical state for these systems are first reported. A notable feature of the data is the higher enthalpic stability observed for the coordination of l-aza-12-crown-4 and lithium trifluoromethanesulfonate with respect to that of the same ligand and other lithium salts. The observed enhancement of stability is within the range expected for hydrogen bond formation likely to be attributed to a specific interaction between the hydrogen atom of the aza crown ligand and the anion. Further investigations in this area are suggested with particular emphasis on fundamental research where more efforts should be geared to overcome some of the problems encountered in lithium battery technology.