Ion solvation in carbonate-based lithium battery electrolyte solutions

Abstract

Partial molar volumes at infinite dilution V MX ∞ of three electrolytes (LiClO 4, LiCF 3SO 3 and LiPF 6) have been determined by vibrating-tube densimetry in pure propylene carbonate (PC), ethylene carbonate (EC) and dimethylcarbonate (DMC) and, for LiClO 4, in some mixtures of EC with DMC and PC. V mx ∞ are strongly dependent of the nature of the pure or mixed solvents. Absolute ionic molar volumes of the Li ion and associated anions have been deduced from the partial molar volume of tetra-alkylammonium salts in the same solvents. For this purpose the concept of van der Waals volume and packing density proposed by King has been employed. In all solvents under investigation, the volume of the Li ion has similar values: V Li + ∞ ≈ 5 cm 3/mol. This result is different from what is obtained using the tetraphenylarsonium tetraphenylborate assumption. The differences among polyatomic anions volumes are greater than expected and strongly dependent on the solvents' nature. These effects are discussed in terms of ion-solvent interactions: ion solvation, packing effect in the solvation shell and electrostriction. Implication of these findings for Li batteries is briefly discussed.