Difference in chemical bonding between lithium and sodium salts

The desire to enhance performance as well as longevity of sodium- and lithium-ion rechargeable batteries has continued to soar in the recent years. The electrolyte used in these batteries is the main hazard as its volatile organic nature could finally lead to a thermal runaway. Therefore, ionic liquids have been proposed as better replacements for electrolytes in metal-ion batteries.

There has been more preliminary data, which is promising, but the whole issue of switching completely to ionic liquids is still unresolved. The inclusion of sodium and lithium salts have been faced with serious problems leading to the decrease in performance of ionic liquids. There has been reported a considerable decrease in conductivity and an associated increase in viscosity, both of which negatively affect the battery performance.

The concept of replacing the costly lithium ions with a cheaper version of sodium cations has been faced with serious unforeseen problems owing to poor solubility of the sodium salts in ionic liquids. It puzzles more that the solubility of lithium salts in ionic liquids is excellent irrespective of the much high charge density on the lithium cations that would necessitate a large solvation shell as compared to the sodium cations. Worse off is that the phenomenon of the striking differences in the solubility of sodium and lithium salts in ionic liquids has not been addressed.

Su Chen and Ekaterina Izgorodina at Monash University in Australia in collaboration with Jun Ishii, Shunsuke Horiuchi, and Masahiro Yoshizawa-Fujita at Sophia University in Japan demonstrated a high-level quantum chemical study of the chemical bonding of sodium and lithium salts in conjunction with ionic liquid atoms. Their study indicated that Lithium salts dissolve better in ionic liquids than sodium salts. Their research work is published in journal, Physical Chemistry Chemical Physics.

The authors conducted the solubility analyses of lithium and sodium salts in 6 ionic liquids. They checked the solubility at room temperature by mixing a selected amount of either sodium or lithium salt into the ionic liquid. When the mixture gave a clear solution, they added a given amount of each salt into the solution. They repeated the procedure until they obtained a turbid solution.

Owing to their proximity to the anion, the 1s² electrons on the lithium cation were observed to get polarized strongly by the presence of the anion in the sense that they began to participate in bonding, therefore making it more covalent than previously thought. However, for the sodium salts, the 2s² orbitals were much removed from the anion, therefore making its effect much weaker. This polarization led to 90kJ/mol of difference in the interaction level between lithium and sodium salts.

Theoretical results have actually confirmed that the increasing covalency of lithium salts is the principle cause of their superior solubility. However, this comes with a downside of decreased conductivity as lithium salts are unlikely to dissociate easily in ionic liquids. On the other hand, sodium salts maintain a higher degree of ionicity, which decreases their chances of being dissolved by ionic liquids. A/Prof. Ekaterina Izgorodina suggests that “Increasing the charge delocalisation on the anion through inclusion of electron-withdrawing groups in the structure is expected to improve the solubility of sodium salts, potentially with insignificant penalty in their conductivity. We are currently testing a couple of such anions in the laboratory.”

Lithium salts containing BF₄^– or PF₆^– indicated better solubility as opposed to sodium analogues by not less than one order of magnitude. The results of their study will be helpful for the future direction of the development of safe electrolytes for sodium- and lithium-ion secondary batteries.