Free-Energy and Structural Analysis of Ion Solvation and Contact Ion-Pair Formation of Li + with BF 4 − and PF 6 − in Water and Carbonate Solvents Munetaka Takeuchi, ‡ Nobuyuki Matubayasi,* ,§,⊥ Yasuo Kameda, ∥ Babak Minofar, ‡ Shin-ichi Ishiguro, ‡ and Yasuhiro Umebayashi* ,‡,† ‡ Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812-8581, Japan § Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ⊥ Japan Science and Technology Agency (JST), CREST, Kawaguchi, Saitama 332-0012, Japan ∥ Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Yamagata 990-8560, Japan ABSTRACT: Free energy of contact ion-pair (CIP) formation of lithium ion with BF 4 − and PF 6 − in water, propylene carbonate (PC), dimethyl carbonate (DMC) are quantitatively analyzed using MD simulations combined with the energy representation method. The relative stabilities of the mono-, bi-, and tridentate coordination structures are assessed with and without solvent, and water, PC, and DMC are found to favor the CIP−solvent contact. The monodentate structure is typically most stable in these solvents, whereas the configuration is multidentate in vacuum. The free energy of CIP formation is not simply governed by the solvent dielectric constant, and microscopic analyses of solute−solvent interaction at a molecular level are then performed from energetic and structural viewpoints. Vacant sites of Li + cation in CIP are solvated with three carbonyl oxygen atoms of PC and DMC solvent molecules, and the solvation is stronger for the monodentate CIP than for the multidentate. Energetically favorable solute−solvent configurations are shown to be spatially more restricted for the multidentate CIP, leading to the observation that the solvent favors the monodentate coordination structure. 1. INTRODUCTION Having the smallest ionic radius among monovalent metal cations, lithium ion is distinct in its structure and dynamics in solutions. 1−5 While the coordination number of the lithium ion is well accepted to be four in water, it is still a target of active investigations. For example, neutron/X-ray diffraction experi- ments have revealed that the coordination number varies from four to six, depending on the salt concentration and counteranion species. 6 The lithium ion dynamics cannot further be described by the classical Stokes−Einstein law, 7 and a new molecular approach is being sought. On the standpoint of electrochemistry and related industries, lithium has the most negative redox potential among all elements (Li/Li + = −3.06 V (vs NHE)), hence the lithium secondary battery is one of the most expected electric storages due to its high energy density. 8,9 In designing batteries, nonaqueous solvents of carbonates are often used, for instance, propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC) 10,11 and also their mixtures owing to their availability as a liquids in a wide temperature range, chemical and electrical stability, large lithium ion solubility, and low toxicity. 12 Contact ion-pair (CIP) formation ability of electrolyte in a lithium battery plays an important role in the lithium ion conduction. To increase the conductivity of the lithium ion, it is required to keep the single-ion concentration and prevent the CIP formation. Favorable anions are thus of weak coordination nature such as BF 4 − or PF 6 − . A molecular study of lithium solvation and CIP formation is thus not only interesting for their physical and chemical properties but also useful for designing a lithium salt for a high performance battery. The CIP formation has been studied by a number of experimental techniques such as static 13 and dynamic 14,15 ionic conductivity, Raman/IR 16−24 and NMR. 25 Sano et al. have investigated the LiPF 6 26 and LiBF 4 27 in PC solutions in terms of ionic conductivity, solution viscosity, and self-diffusion coefficient over the wide rang of salt concentration, and revealed that the higher conductivity of LiPF 6 in dilute solutions is related to the weaker CIP formation ability of the PF 6 − . The CIP stability is closely related to the CIP structure. Structural study on the lithium ion in highly concentrated LiPF 6 /PC solution has been performed by means of neutron di ffraction experiments with 6/7 Li isotopic substitution technique. 28 Also some theoretical studies about CIP have been reported. 29−32 Borodin and Smith carried out MD simulations of LiBF 6 in EC:DMC mixed solvents with many- body polarizable force fields. 33 In fact, there are a few candidates for the CIP structure (Chart 1), and the structure Received: February 4, 2012 Revised: May 1, 2012 Published: May 22, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 6476 dx.doi.org/10.1021/jp3011487 | J. Phys. Chem. B 2012, 116, 6476−6487