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

Application of Screening Functions as Cutoff-Based Alternatives to Ewald Summation in Molecular Dynamics Simulations Using Polarizable Force Fields

The range-dependent screening of the charge–charge, charge-induced dipole, and induced dipole–induced dipole interactions was examined for a variety of liquids modeled using polarizable force fields. A cutoff-based method for calculation of the electrostatic interactions in molecular dynamics (MD) is presented as an alternative to Ewald-type summation for simulations of the disordered materials modeled using many-body polarizable force fields with permanent charges and induced point dipoles. The proposed approach was tested on bulk water, room-temperature ionic liquids, and solutions of ions in polar solvents. The smooth, short-range, and atom-type independent screening functions for interactions between the charges and induced dipoles were obtained using the force matching approach. An excellent agreement for both the magnitude and directionality of forces, structural and dynamic properties, was found in MD simulations utilizing the developed screening functions, compared to those with Ewald summation. While similar in shape and range, the charge–charge screening functions were somewhat dependent on the material chemistry. On the other hand, the charge-induced dipole and induced dipole-induced dipole screening functions were found to be nearly universal for the tested materials

Influence of electrostatic interactions on the properties of cyanobiphenyl liquid crystals predicted from atomistic molecular dynamics simulations

<p>The influence of force field details in all-atom molecular dynamics (MD) simulations on the predicted thermodynamic, structural, and dynamic properties of bulk 4-cyano-4ʹ-pentylbiphenyl (5CB) systems have been investigated in the 292–368 K temperature range. The effect of the molecular dipole moment and the details of dihedral potential for biphenyl unit were investigated using both polarisable (POL) and non-polarisable (NP) versions of the quantum chemistry-based force field. The predicted densities for the nematic and isotropic phases of bulk 5CB were found to be in excellent agreement with available experimental data. The nematic-isotropic transition temperature (<i>T</i><i>_NI</i>) showed strong sensitivity to the force field details, MD simulations with partial atomic charge distributions and molecular dipole moment corresponding to high-level quantum chemistry calculations predicted an overestimation of the <i>T</i><i>_NI</i> by about 30 K. Rescaling the charges to allow the molecular dipole to be closer to experimentally reported values of 5CB dipole in condensed phases, significantly improved the prediction of <i>T</i><i>_NI</i> as well as other thermodynamic and dynamic properties of 5CB. We also discuss how the structural, thermodynamic, and dynamic properties of bulk 5CB are affected by the flexibility of the central biphenyl dihedral and the inclusion of induced polarisation effects.</p

Role of Plasticity in Mechanical Failure of Solid Electrolyte Interphases on Nanostructured Silicon Electrode: Insight from Continuum Level Modeling

Understanding the failure mechanisms of solid electrolyte interphases (SEI) is important for silicon electrodes because their volume expands substantially during lithiation. This work discusses material point method simulations of SEI failure during lithiation of silicon nanopillars. We demonstrate that considering SEI films as brittle, elastic materials does not allow fracture that would be consistent with experimental observations. However, constitutive models that include plastic deformation and result in ductile fracture are in very good agreement with trends observed in experiments. The insight gained from these results allows suggestion of possible strategies for design of SEI with improved failure resistance under lithiation-induced electrode expansion, where modification of the SEI leading to increased Young’s modulus and/or strain hardening without compromising the underlying ductility of the material presents a desirable outcome for chemical and/or processing modifications designed to modify SEI response

Li⁺ Transport and Mechanical Properties of Model Solid Electrolyte Interphases (SEI): Insight from Atomistic Molecular Dynamics Simulations

A fundamental understanding of solid electrolyte interphase (SEI) properties is critical for enabling further improvement of lithium batteries and stabilizing the anode–electrolyte interface. Mechanical and transport properties of two model SEI components were investigated using molecular dynamics (MD) simulations and a hybrid MD-Monte Carlo (MC) scheme. A many-body polarizable force field (APPLE&P) was employed in all simulations. Elastic moduli and conductivity of model SEIs comprised of dilithium ethylene dicarbonate (Li₂EDC) were compared with those comprised of dilithium butylene dicarbonate (Li₂BDC) over a wide temperature range. Both ordered and disordered materials were examined with the ordered materials showing higher conductivity in the conducting plane compared to conductivity of the disordered analogues. Li₂BDC was found to exhibit softening and onset of anion mobility at lower temperatures compared to Li₂EDC. At 120 °C and below, both SEI model compounds showed single ion conductor behavior. Ordered Li₂EDC and Li₂BDC phases had highly anisotropic mechanical properties, with the shear modulus of Li₂BDC being systematically smaller than that for Li₂EDC

Why Do Sulfone-Based Electrolytes Show Stability at High Voltages? Insight from Density Functional Theory

Sulfone-based electrolytes have attracted a great attention due to their high oxidation stability comparing to conventional carbonates. However, the ab initio calculated oxidation potentials (<i>E</i>_ox) of isolated sulfones are lower than those for carbonates. To understand this contradiction, the oxidations of three carbonates and eleven sulfones in a presence of anions and other solvent molecules have been investigated by the density functional theory calculations with a polarized continuum model. Importantly, we found that the <i>E</i>_ox of some of the sulfones show surprisingly high stability toward the presence of anions and another solvent, which is the key factor of high oxidation stability of these electrolytes compared to carbonates. Finally, the way to design new high oxidation stability sulfones by modifying their functional groups is discussed

Effect of Organic Solvents on Li⁺ Ion Solvation and Transport in Ionic Liquid Electrolytes: A Molecular Dynamics Simulation Study

Molecular dynamics simulations of <i>N</i>-methyl-<i>N</i>-propylpyrrolidinium (pyr₁₃) bis­(trifluoromethanesulfonyl)­imide (Ntf₂) ionic liquid [pyr₁₃]­[Ntf₂] doped with [Li]­[Ntf₂] salt and mixed with acetonitrile (AN) and ethylene carbonate (EC) organic solvents were conducted using polarizable force field. Structural and transport properties of ionic liquid electrolytes (ILEs) with 20 and 40 mol % of organic solvents have been investigated and compared to properties of neat ILEs. Addition of AN and EC solvents to ILEs resulted in the partial displacement of the Ntf₂ anions from the Li⁺ first coordination shell by EC and AN and shifting the Li–Ntf₂ coordination from bidentate to monodentate. The presence of organic solvents in ILE has increased the ion mobility, with the largest effect observed for the Li⁺ cation. The Li⁺ conductivity has doubled with addition of 40 mol % of AN. The Li⁺–N^Ntf2 residence times were dramatically reduced with addition of solvents, indicating an increasing contribution from structural diffusion of the Li⁺ cations

Nanopatterning of Electrode Surfaces as a Potential Route to Improve the Energy Density of Electric Double-Layer Capacitors: Insight from Molecular Simulations

Electrostatic double-layer capacitors (EDLCs) with room-temperature ionic liquids (RTILs) as electrolytes are among the most promising energy storage technologies. Utilizing atomistic molecular dynamics simulations, we demonstrate that the capacitance and energy density stored within the electric double layers (EDLs) formed at the electrode–RTIL electrolyte interface can be significantly improved by tuning the nanopatterning of the electrode surface. Significantly increased values and complex dependence of differential capacitance on applied potential were observed for surface patterns having dimensions similar to the ions' dimensions. Electrode surfaces patterned with rough edges promote ion separation in the EDL at lower potentials and therefore result in increased capacitance. The observed trends, which are not accounted for by the current basic EDL theories, provide a potentially new route for optimizing electrode structure for specific electrolytes

Correction to “Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [C_<i>n</i>mim][TFSI] Ionic Liquids at Graphite Electrodes”

Correction to “Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [C_<i>n</i>mim][TFSI] Ionic Liquids at Graphite Electrodes