Physico-Chemical and Electrochemical Properties of Lithium Bis(Oxalate)Borate Solutions in Sulfolane

The physico-chemical, electrochemical, and thermal properties of lithium bis(oxalate)borate solutions in sulfolane are studied. The sulfolane solutions of lithium bis(oxalate)borate were found to have lower specific ion conductivity and higher viscosity as compared with solutions of lithium perchlorate and lithium hexafluorophosphate. With an increase in the concentration of lithium bis(oxalate)borate solutions in sulfolane, the activation energies for ion conductivity and viscous flow increased. The association constant of lithium bis(oxalate)borate in sulfolane is lower than that of LiClO4 and close to LiPF6. From the dependence of the association constant on the donor number of lithium salts’ anions, the donor number value of the [B(C2O4)]– anion was estimated as ~3. The effective transference number of lithium ion in 1 M lithium bis(oxalate)borate solution in sulfolane is 0.46 ± 0.03, which is higher than in 1 M LiClO4 solution (0.39 ± 0.02) and close to 1 M LiBF4 solution (0.42 ± 0.03) in sulfolane. The sulfolane solutions of lithium bis(oxalate)borate are shown to be prone to supercooling and thermodynamically nonequilibrium states formation. The anodic stability boundary of 1 M lithium bis(oxalate)borate solution in sulfolane is as high as 5.65 V vs. Li/Li+ reference electrode. According to the anodic stability, sulfolane solutions of lithium salts are ranked in the following order: LiPF6 ~ LiBF4 > Li bis(oxalate)borate ~ LiClO4 > LiSO3CF3. The cycling life of lithium metal electrode in lithium bis(oxalate)borate sulfolane solution is 1.75 times longer than that in LiClO4 solution; it is determined by the dispersion rate of lithium metal rather than by the electrolyte decomposition rate.