Electrolyte Solvation and Ionic Association I. Acetonitrile-Lithium Salt Mixtures: Intermediate and Highly Associated Salts

Solution structure is the key determinant for electrolyte properties, but little is known about the ion solvate structures present in liquids. A detailed exploration of this topic is begun here utilizing acetonitrile (AN) due to the simplicity of this solvent's interactions with Li+ cations. Phase diagrams have been prepared for (AN)(n)-LiClO4 and -LiBF4 mixtures (salts with intermediate ionic association). The solvate species present in the solid and liquid phases have been analyzed utilizing single crystal solvate structures and Raman spectroscopy to determine how the anion identity influences the solvate species equilibrium (i.e., ionic association and solvation number). The phase behavior and solvation interactions of these mixtures are compared with those for (AN)(n)-LiCF3SO3, -LiNO3 and -LiCF3CO2 mixtures (salts which are highly associated). Quantum chemical calculations for the (AN)(n)-LiBF4 and -LiClO4 solvates have been performed to aid in the analysis. Results from MD simulations for (AN)(n)-LiBF4 and -LiClO4 mixtures have been compared to the experimental work to explore both the insight gained and limitations of the experimental work and simulations for electrolyte characterization. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.jes112264] All rights reserved.