Predicting the Properties of Ionic Liquids from a Polarizable Force Field in Molecular Dynamics Simulations
Oleg Borodin at the Army Research Laboratory has developed and validated a polarizable force field for a wide class of ionic liquids (ILs), which are being explored as additives to lithium-battery electrolytes for improved stability. The developed force field for a wide range of anions shown in Figure 1 will serve as a starting point for molecular dynamics simulations (MD) of liquid electrolytes doped with variety of conventional and novel lithium salts. This force field, when used in MD simulations, is a tool for predicting thermodynamic and transport properties of conceptual ILs and IL-solvent mixtures and may shed light on structure-property relationships. Using predictions of promising properties to direct materials design can accelerate the development of ILs suitable for battery applications.
To validate the quantum chemistry-based force field, MD simulations were performed on 30 cation and anion pairs for which physical properties have been experimentally determined (Figure 1). Several of the anions such as TFSI–, FSI–, N(CN)2–, C(CN)3–, and BF3CF3– were chosen because they yield ILs with low melting and glass transition temperatures, relatively low viscosity, and fast ion transport, which are properties desired for batteries, supercapacitors, and other applications. The different classes of ions also allow for a thorough investigation of force-field transferability, in particular, that of the repulsion-dispersion parameters. Transferability of these force-field parameters between various compounds and chemical environments is important for accurate predictions of properties. The MD simulations provided an accurate description of density, heat of vaporization, self-diffusion coefficients, ionic conductivity, and viscosity for most of the ILs, thereby verifying the force-field transferability.
This force field is an integral part of the Atomistic Polarizable Potential for Liquids, Electrolytes, & Polymers (APPLE&P) force-field database that is being developed by Borodin. More details on the force field development and the MD simulations can be found in the published paper (http://pubs.acs.org/doi/abs/10.1021/jp905220k), which is in the list of “Top 20 Most Cited articles for the Journal of Physical Chemistry B that were published within the Last 3 Years.”
More information on experimental efforts in synthesis and characterization of ILs can be found in the following article on research in the Henderson Lab at NCSU: http://batt.lbl.gov/2010/11/22/stabilizing-batteries-with-salts/
Reference: Borodin, O., Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids, J. Phys. Chem. B. 2009, 113, 11463-11478.