Competitive coordination of lithium ion

Mixtures of solvents are commonly used in nonaqueous lithium battery electrolytes. Structures comprised of the solvents and ions determine many of the electrolyte properties. Solvation structures which could exist in the electrolyte include separated-solvated ions, solvent-separated ion pairs, and solvated contact ion pairs. An understanding of these solvent-ion interactions aid in the selection of solvent mixtures. Three molecules are used to investigate the competition between solvents to bind to Li+. Ethylene carbonate has a high dielectric constant, is a common component in lithium battery electrolytes, and has a carbonyl functional group to bind to the Li+. Water has a lower dielectric constant, is a common contaminant in electrolytes, and has an ether-like oxygen. Acetaldehyde has the lowest dielectric constant and also has a carbonyl functional group. This paper attempts to identify the primary solvation structure using the experimental observation that water excludes ethylene carbonate from close contact with the lithium ion and the calculated energetics of the solvation structures. The preference of binding to Li+ is determined by calculating the energies of the possible solvation structures and comparing the energetics. For separated-solvated structures of Li+, ethylene carbonate and acetaldehyde are found to displace water. Ethylene carbonate is the preferred solvent for the contact ion pair. Water is the preferred solvent for solvent-separated complexes. Experimentally water is found to bind to Li+ in ethylene carbonate/water mixtures. To explain the complete exclusion of ethylene carbonate from Li+ solvation it is necessary to postulate the existence of aggregates of solvated, solvent-separated ion pairs, which are similar to structures common to highly hydrogen-bonded water.