Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids

A many-body polarizable force field has been developed and validated for ionic liquids (ILs) containing 1-methyl-3-alkylimidazolium, 1-alkyl-2-methyl-3-alkylimidazolium, N-methyl-N-alkylpyrrolidinium, N-alkylpyridinium, N-alkyl-N-alkylpiperidinium, N-alkyl-N-alkylmorpholinium, tetraalkylammonium, tetraalkylphosphonium, N-methyl-N-oligoetherpyrrolidinium cations and BF4−, CF3BF3−, CH3BF3−, CF3SO3−, PF6−, dicyanamide, tricyanomethanide, tetracyanoborate, bis(trifluoromethane sulfonyl)imide (Ntf2− or TFSI−), bis(fluorosulfonyl)imide (FSI−) and nitrate anions. Classical molecular dynamics (MD) simulations have been performed on 30 ionic liquids at 298, 333, and 393 K. The IL density, heat of vaporization, ion self-diffusion coefficient, conductivity, and viscosity were found in a good agreement with available experimental data. Ability of the developed force field to predict ionic crystal cell parameters has been tested on four ionic crystals containing Ntf2− anions. The influence of polarization on the structure and ion transport has been investigated for [emim][BF4] IL. A connection between the structural changes in IL resulting from turning off polarization and slowing down of ion dynamics has been found. Developed force field has also provided accurate description/prediction of thermodynamic and transport properties of alkanes, fluoroalkanes, oligoethers (1,2-dimethoxyethane), ethylene carbonate, propylene carbonate, dimethyl carbonate, hydrazine, methyhydrazine, dimethylhydrazine, acetonitrile, dimethyl amine, and dimethyl ketone

Relation between Heat of Vaporization, Ion Transport, Molar Volume, and Cation−Anion Binding Energy for Ionic Liquids

A number of correlations between heat of vaporization (Hvap), cation−anion binding energy (E±), molar volume (Vm), self-diffusion coefficient (D), and ionic conductivity for 29 ionic liquids have been investigated using molecular dynamics (MD) simulations that employed accurate and validated many-body polarizable force fields. A significant correlation between D and Hvap has been found, while the best correlation was found for −log(DVm) vs Hvap + 0.28E±. A combination of enthalpy of vaporization and a fraction of the cation−anion binding energy was suggested as a measure of the effective cohesive energy for ionic liquids. A deviation of some ILs from the reported master curve is explained based upon ion packing and proposed diffusion pathways. No general correlations were found between the ion diffusion coefficient and molecular volume or the diffusion coefficient and cation/anion binding energy

Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids

A many-body polarizable force field has been developed and validated for ionic liquids (ILs) containing 1-methyl-3-alkylimidazolium, 1-alkyl-2-methyl-3-alkylimidazolium, N-methyl-N-alkylpyrrolidinium, N-alkylpyridinium, N-alkyl-N-alkylpiperidinium, N-alkyl-N-alkylmorpholinium, tetraalkylammonium, tetraalkylphosphonium, N-methyl-N-oligoetherpyrrolidinium cations and BF4−, CF3BF3−, CH3BF3−, CF3SO3−, PF6−, dicyanamide, tricyanomethanide, tetracyanoborate, bis(trifluoromethane sulfonyl)imide (Ntf2− or TFSI−), bis(fluorosulfonyl)imide (FSI−) and nitrate anions. Classical molecular dynamics (MD) simulations have been performed on 30 ionic liquids at 298, 333, and 393 K. The IL density, heat of vaporization, ion self-diffusion coefficient, conductivity, and viscosity were found in a good agreement with available experimental data. Ability of the developed force field to predict ionic crystal cell parameters has been tested on four ionic crystals containing Ntf2− anions. The influence of polarization on the structure and ion transport has been investigated for [emim][BF4] IL. A connection between the structural changes in IL resulting from turning off polarization and slowing down of ion dynamics has been found. Developed force field has also provided accurate description/prediction of thermodynamic and transport properties of alkanes, fluoroalkanes, oligoethers (1,2-dimethoxyethane), ethylene carbonate, propylene carbonate, dimethyl carbonate, hydrazine, methyhydrazine, dimethylhydrazine, acetonitrile, dimethyl amine, and dimethyl ketone

Thermodynamic, Dynamic, and Structural Properties of Ionic Liquids Comprised of 1-Butyl-3-methylimidazolium Cation and Nitrate, Azide, or Dicyanamide Anions

Molecular dynamics simulations of ionic liquids (IL) comprised of 1-butyl-3-methylimidazolium [bmim] cation and nitrate [NO3], azide [N3], or dicyanamide [N(CN)2] anions were conducted using the polarizable APPLE&P force field. Comparison of thermodynamic properties such as densities, enthalpies of vaporization, and ion binding energies as well as structural correlations obtained from simulations at atmospheric pressure and temperature range 298−393 K showed that IL with the N(CN)2 anion shows significantly different characteristics as compared to ILs with the N3 and NO3 anions. [bmim][N(CN)2] IL was found to have the lowest enthalpy of vaporization and the weakest ion−ion structural correlation as compared to ILs with the other two ions. This trend was further manifested in dynamical properties characterized by self-diffusion coefficients and molecular rotational relaxation times, where IL with the N(CN)2 anion showed the fastest dynamics as compared to other ILs. We also examine the dynamic correlations between the ions’ translational and rotational motions as well as discuss the anisotropy of the latter

LiTFSI Structure and Transport in Ethylene Carbonate from Molecular Dynamics Simulations

Molecular dynamics (MD) simulations using a many-body polarizable force field were performed on ethylene carbonate (EC) doped with lithium bistrifluoromethanesulfonamide (LiTFSI) salt as a function of temperature and salt concentration. At 313 K Li+ was coordinated by 2.7−3.2 EC carbonyl oxygen atoms and 0.67−1.05 TFSI- oxygen atoms at EC:Li = 10 and 20 salt concentrations. In completely dissociated electrolytes, however, Li+ was solvated by approximately 3.8 carbonyl oxygen atoms from EC on average. The probability of ions to participate ion aggregates decreased exponentially with an increase in the size of the aggregate. Ion and solvent self-diffusion coefficients and conductivity predicted by MD simulations were in good agreement with experiments. Approximately half of the charge was transported by charged ion aggregates with the other half carried by free (uncomplexed by counterion) ions. Investigation of the Li+ transport mechanism revealed that contribution from the Li+ diffusion together with its coordination shell to the total Li+ transport is similar to the contribution arising from Li+ exchanging solvent molecules in its first coordination shell with solvents from the outer shells

Development of Many−Body Polarizable Force Fields for Li-Battery Components: 1. Ether, Alkane, and Carbonate-Based Solvents

Classical many-body polarizable force fields were developed for n-alkanes, perflouroalkanes, polyethers, ketones, and linear and cyclic carbonates on the basis of quantum chemistry dimer energies of model compounds and empirical thermodynamic liquid-state properties. The dependence of the electron correlation contribution to the dimer binding energy on basis-set size and level of theory was investigated as a function of molecular separation for a number of alkane, ether, and ketone dimers. Molecular dynamics (MD) simulations of the force fields accurately predicted structural, dynamic, and transport properties of liquids and unentangled polymer melts. On average, gas-phase dimer binding energies predicted with the force field were between those from MP2/aug-cc-pvDz and MP2/aug-cc-pvTz quantum chemistry calculations

Molecular Dynamics Simulations of Comb-Branched Poly(epoxide ether)-Based Polymer Electrolytes

Molecular dynamics (MD) simulations using many-body polarizable force field were performed on comb-branched poly(epoxide ether) (PEPE) polymer electrolytes doped with lithium bistrifluoromethanesulfonamide (LiTFSI) salt as a function of temperatures from 333 to 423 K at ether oxygen (EO) to lithium ratio of 20. MD simulations predicted electrolyte conductivity in good agreement with experiments. The fraction of solvent-separated ions and lithium cation environment for PEPE/LiTFSI were similar to those found for the linear poly(ethylene oxide) (PEO)/LiTFSI electrolyte. The Li+ cations had the highest probability to be coordinated by EOs near the PEPE polymer backbone and the lowest probability being coordinated by EO's at the end of side chains. Segmental dynamics of the backbone was slower by 2 orders of magnitude compared to the dynamics of side-chain ends. The Li+ self-diffusion coefficient was approximately an order of magnitude lower than the TFSI- anion self-diffusion coefficient. Visualization of the lithium motion revealed that the most mobile Li+ cations moved by hopping from a side chain to another without being complexed by the backbone. The influence of the backbone−Li+ interactions and the backbone stiffness on ion transport was investigated in MD simulations performed on the PEPE/LiTFSI-like electrolytes with the same PEPE architecture but a very stiff backbone that does not complex lithium cations. The ion transport in these model electrolytes was compared to that of the original PEPE/LiTFSI electrolyte

Interfacial Structure and Dynamics of the Lithium Alkyl Dicarbonate SEI Components in Contact with the Lithium Battery Electrolyte

Molecular dynamics simulations were performed on the dilithium ethylene dicarbonate (Li₂EDC) and dilithium butylene dicarbonate (Li₂BDC) components of the lithium battery solid electrolyte interphase (SEI) in contact with mixed solvent electrolyte: ethylene carbonate (EC):dimethyl carbonate (DMC) (EC:DMC = 3:7) doped with LiPF₆. The many-body polarizable APPLE&P force field was used in the simulations. Examination of the SEI–electrolyte interface revealed an enrichment of EC and PF₆^– molecules and a depletion of DMC at the interfacial layer next to the SEI surface compared to bulk electrolyte concentrations. The EC and DMC molecules at the interfacial layer next to the SEI demonstrated a preferential orientation of carbonyl oxygens directed toward the SEI surface. The process of the Li⁺ ion desolvation from electrolyte and intercalation into the SEI was examined. During the initial desolvation step, the Li⁺ cation showed a preference to shed DMC molecules compared to losing the EC or PF₆^– moieties. The PF₆^– anion was involved in the Li⁺ cation desolvation process at high temperatures. The activation energies for the Li⁺ solvation–desolvation reaction were estimated to be 0.42–0.46 eV for the Li₂EDC–electrolyte and Li₂BDC–electrolyte interfaces

Quantum Chemistry and Molecular Dynamics Simulation Study of Dimethyl Carbonate: Ethylene Carbonate Electrolytes Doped with LiPF₆

Quantum chemistry studies of ethylene carbonate (EC) and dimethyl carbonate (DMC) complexes with Li+ and LiPF6 have been conducted. We found that Li+ complexation significantly stabilizes the highly polar cis−trans DMC conformation relative to the nearly nonpolar gas-phase low energy cis−cis conformer. As a consequence, the binding of Li+ to EC in the gas phase is not as favorable relative to binding to DMC as previously reported. Furthermore, quantum chemistry studies reveal that, when complexation of LiPF6 ion pairs is considered, the DMC/LiPF6 complex is about 1 kcal/mol more stable than the EC/LiPF6 complex. The EC3DMC(cis−cis)/Li+ complex was found to be the most energetically stable among ECnDMCm/Li+ (n + m = 4) investigated complexes followed by EC4/Li+. Results of the quantum chemistry studies of these complexes were utilized in the development of a many-body polarizable force field for EC:DMC/LiPF6 electrolytes. Molecular dynamics (MD) simulations of EC/LiPF6, DMC/LiPF6, and mixed solvent EC:DMC/LiPF6 electrolytes utilizing this force field were performed at 1 M salt concentration for temperatures from 298 to 363 K. Good agreement was found between MD simulation predictions and experiments for thermodynamic and transport properties of both pure solvents and the electrolytes. We find increased ion pairing with increasing DMC content; however, both EC and DMC were found to participate in Li+ solvation in mixed EC:DMC electrolytes despite a huge difference in their dielectric constants. In contrast to previous NMR studies, where dominance of EC in cation solvation was reported, we find a slight preference for DMC in the cation solvation shell for EC:DMC (1 wt:1 wt) electrolytes and show that reanalyzed Raman spectroscopy experiments are in good agreement with results of MD simulations. Finally, analysis of solvent residence times reveals that cation transport is dominated by motion with solvating DMC and approximately equal contributions from vehicular motions with the first solvation shell and solvent exchange with respect to solvating EC

Structure and Dynamics of <i>N</i>-Methyl-<i>N</i>-propylpyrrolidinium Bis(trifluoromethanesulfonyl)imide Ionic Liquid from Molecular Dynamics Simulations

Molecular dynamics (MD) simulations were performed on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (mppy+TFSI-) from 303 to 393 K to improve understanding of the structure and ion transport of this ionic liquid. The density, ion self-diffusion coefficients, conductivity, and viscosity of mppy+TFSI- predicted from MD simulations are in good agreement with experimental measurements. The time-dependent shear modulus of the ionic liquids was calculated and compared with that for nonionic liquids. On average each mppy+ cation was found to be coordinated by four TFSI- anions. The angular distributions of NTFSI−−Nmppy+−NTFSI− and Nmppy+−NTFSI-−Nmppy+ exhibit a maximum at 80−90° and a second maximum at 180°. Correlation of ion motion was found to lower ionic conductivity by approximately one-third from the expected value based upon ion self-diffusion coefficients. Rotational motion of the cation and anion are anisotropic with the degree of anisotropy increasing with decreasing temperature. Electrostatic interactions are responsible for slowing down the dynamics of the ionic liquid by more than an order of magnitude and a dramatic decrease of the time-dependent shear modulus