Theoretical Studies of Lithium Perchlorate in Ethylene Carbonate, Propylene Carbonate, and Their Mixtures

Abstract

Ab initio calculations have been used to study pair interactions between lithium ion and perchlorate ion, lithium perchlorate, and polar aprotic solvents: ethylene carbonate (EC), propylene carbonate (PC), and their mixtures. The resulting optimized structures and electrostatic potential‐derived charges serve as input parameters in molecular dynamics simulations used to determine collective structural effects. Little molecular association is detected in pure EC solutions. In EC/PC mixtures it is found, in agreement with previous experimental studies, that EC tends to substitute for PC in the first shell of the cation. Thus, the effective radius of the complex ion solvent is smaller for the EC/PC mixture than in the case of pure PC, leading to higher ionic conductivities. The potential energy surface for ion pair association of lithium and perchlorate ions is investigated with a self‐consistent polarizable continuum model. In solution, the minimum corresponding to the ion pair association is shifted toward larger values of the ion‐ion separation. The configuration of the lithium perchlorate ion pair in solution is found from molecular dynamics simulations to be intermediate between a monodentate and bidentate structure corresponding to one or two oxygens interacting with the lithium ion. Solvent‐separated species are present in dilute (∼0.15 M) solutions, whereas ion pairs and solvent‐separated complexes are found in concentrated (∼0.8 M) solutions. © 1999 The Electrochemical Society. All rights reserved.