Theoretical studies of lithium perchlorate in ethylene carbonate, propylene carbonate, and their mixtures

Ab initio calculations have been used to study pair interactions between li thium ion and perchlorate ion, lithium perchlorate, and polar aprotic solve nts: ethylene carbonate (EC), propylene carbonate (PC), and their mixtures. The resulting optimized structures and electrostatic potential-derived cha rges serve as input parameters in molecular dynamics simulations used to de termine collective structural effects. Little molecular association is dete cted in pure EC solutions. In EC/PC mixtures ii is found, in agreement with previous experimental studies, that EC tends to substitute for PC in the f irst shell of the cation. Thus, the effective radius of the complex ion sol vent 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 a ssociation of lithium and perchlorate ions is investigated with a self-cons istent 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 soluti on is found from molecular dynamics simulations to be intermediate between a monodentate and bidentate structure corresponding to one or two oxygens i nteracting with the lithium ion. Solvent-separated species are present in d ilute (similar to 0.15 M) solutions, whereas ion pairs and solvent-separate d complexes are found in concentrated (similar to 0.8 M) solutions. (C) 199 9 The Electrochemical Society. S0013-4651(99)03-062-1. All rights reserved.