The decomposition of ethylene carbonate (EC) during the initial growth of solid-electrolyte interphase (SEI) films at the solvent-graphitic anode interface is critical to lithium ion battery operations. Ab initio molecular dynamics simulations of explicit liquid EC/graphite interfaces are conducted to study these electrochemical reactions. We show that carbon edge terminations are crucial at this stage, and that achievable experimental conditions can lead to surprisingly fast EC breakdown mechanisms, yielding decomposition products seen in experiments but not previously predicted.Improving the fundamental scientific understanding of lithium ion batteries 1-3 is critical for electric vehicles and efficient use of solar and wind energy. A key limitation in current batteries is their reliance on passivating solid electrolyte interphase (SEI) films on graphitic anode surfaces. 1-5 Upon first charging of a pristine battery, the large negative potential applied to induce Li + intercalation into graphite decomposes ethylene carbonate (EC, Fig. 1) molecules in the solvent, yielding a self-limiting, 30-50 nm thick, passivating SEI layer containing Li 2 CO 3 , lithium ethylene dicarbonate ((CH 2 CO 3 Li) 2 ), 2,4-6 and salt decomposition products. C 2 H 4 and CO gases have also been detected 7,8 and shown to come from EC. 9 Similar reactions occur during power cycling when the SEI film cracks and graphite is again exposed to EC. 2 If instead the solvent is pure propylene carbonate (PC), a stable SEI film does not materialize 1,2 and the battery fails. Our work shows that novel mechanisms for the initial stages of SEI-growth at electrode-electrolyte interfaces can be simulated within time scales accessible to ab initio molecular dynamics (AIMD), 10 which have successfully modelled liquid-solid interfaces. 11 AIMD is likely also applicable to shed light on cosolvent/additives which must decompose more readily than EC to alter and improve SEI structure, Li + transport, and passivating properties. 1,2 EC-decomposition mechanisms under electron-rich conditions have been proposed (e.g., Refs. 4,5) and investigated using gas cluster Density Functional Theory calculations with and without dielectric continuum approximation of the liquid environment. 12-15 Thus "EC − ", coordinated to Li + or otherwise, has been predicted to undergo ethylene carbon (C E )-oxygen (O 1 ) bond cleavage to form a more stable radical anion (Figs. 1a-b). The potential energy barrier involved is at least 0.33 eV. 12,14 Carbonyl carbon (C C )-O 1 bond-"breaking" (or elongation) in the gas phase EC − -Li + complex yields a lower barrier, but metastable products. 14 Unlike these previous work, AIMD simulations can include explicit liquid state environments and EC/graphite interfaces. Unlike classical force field-based simulations, 16,17 AIMD accounts for covalent bondbreaking. We apply the VASP code, 18,19 the PerdewBurke-Ernzerhof (PBE) functional, 20 Γ-point Brillouin zone sampling, 400 eV planewave energy cutoff, tritium masses for all protons to allow B...

We applied static and dynamic hybrid functional density functional theory (DFT) calculations to study the interactions of one and two excess electrons with ethylene carbonate (EC) liquid and clusters. Optimal structures of (EC) n and EC ð Þ À n clusters devoid of Li þ ions, n ¼ 1-6, were obtained. The excess electron was found to be localized on a single EC in all cases, and the EC dimeric radical anion exhibits a reduced barrier associated with the breaking of the ethylene carbon-oxygen covalent bond compared to EC À . In ab initio molecular dynamics (AIMD) simulations of EC À solvated in liquid EC, large fluctuations in the carbonyl carbon-oxygen bond lengths were observed. AIMD simulations of a two-electron attack on EC in EC liquid and on Li metal surfaces yielded products similar to those predicted using nonhybrid DFT functionals, except that CO release did not occur for all attempted initial configurations in the liquid state.

Large-scale molecular dynamics simulations and the reactive force field ReaxFF were used to study shock-induced initiation in crystalline pentaerythritol tetranitrate (PETN). In the calculations, a PETN single crystal was impacted against a wall, driving a shockwave back through the crystal in the [100] direction. Two impact speeds (4 and 3 km/s) were used to compare strong and moderate shock behavior. The primary difference between the two shock strengths is the time required to exhibit the same qualitative behaviors with the lower impact speed lagging behind the faster impact speed. For both systems, the shock velocity exhibits an initial deceleration due to onset of endothermic reactions followed by acceleration due to the onset of exothermic reactions. At long times, the shock velocity reaches a steady value. After the initial deceleration period, peaks are observed in the profiles of the density and axial stress with the strongly shocked system having sharp peaks while the weakly shocked system developed broad peaks due to the slower shock velocity acceleration. The dominant initiation reactions in both systems lead to the formation of NO(2) with lesser quantities of NO(3) and formaldehyde also produced.

The effects of sequential cross-linking and scission of polymer networks formed in two states of strain are investigated using molecular dynamics simulations. Two-stage networks are studied in which a network formed in the unstrained state (stage 1) undergoes additional cross-linking in a uniaxially strained state (stage 2). The equilibrium stress is measured before and after removing some or all of the original (stage 1) cross-links. The results are interpreted in terms of a generalized independent network hypothesis. In networks where the first-stage cross-links are subsequently removed, a fraction (quantified by the stress transfer function Φ) of the second-stage cross-links contribute to the effective first-stage cross-link density. The stress transfer functions extracted from the MD simulations of the reacting networks are found to be in very good agreement with the predictions of Flory and Fricker. It was found that the fractional stress reduction upon removal of the first-stage cross-links could be accurately calculated from the slip tube model of Rubinstein and Panyukov modified to use the theoretical transfer functions of Fricker.

The permanent set of cross-linking networks is studied by molecular dynamics. The uniaxial stress for a bead-spring polymer network is investigated as a function of strain and cross-link density history, where cross-links are introduced in unstrained and strained networks. The permanent set is found from the strain of the network after it returns to the state-of-ease where the stress is zero. The permanent set simulations are compared with theory using the independent network hypothesis, together with the various theoretical rubber elasticity theories: affine, phantom, constrained junction, slip-tube, and double-tube models. The slip-tube and doubletube models, which incorporate entanglement effects, are found to be in very good agreement with the simulations.

Comprehensive investigation of lithium ion complexation with 15N-labeled polyphosphazenes 15 N-poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (15 N-MEEP) and 15 N-poly-[((2-allylphenoxy)0.12(4-methoxyphenoxy)1.02(2-(2-methoxyethoxy)ethoxy)0.86)phosphazene] (15 N-HPP)was performed by NMR, IR, and Raman spectroscopies. Previous studies characterized the ionic transport through the polymer matrix in terms of “jumps” between neighboring polymer strands utilizing the electron lone pairs of the etherial oxygen nuclei with the nitrogen nuclei on the polyphosphazene backbone not involved. However, noteworthy changes were observed in the NMR, IR, and Raman spectra with the addition of lithium trifluoromethanesulfonate (LiOTf) to the polyphosphazenes. The data indicate that the preferred association for the lithium ion with the polymer is with the nitrogen nuclei, resulting in the formation of a “pocket” with the pendant groups folding around the backbone. NMR temperature-dependent spin−lattice relaxation (T 1) studies (13C, 31P, and 15N) indicate significant lithium ion interaction with the backbone nitrogen nuclei. These studies are in agreement with molecular dynamics simulations investigating lithium ion movement within the polyphosphazene matrix.

The hypothetical polyethylene melt at room temperature is of interest for the purposes of developing the atomic level modeling of gas solubilities in rubbery polymers. In the present study, the gas solubilities in completely amorphous polyethylene are predicted by extrapolating alkane behavior to the long chain limit in a Flory−Huggins context. The results are seen to agree well with a recent simulation study.

We performed molecular dynamics simulations of chain systems to investigate general relationships between the system mobility and computed scalar quantities. Three quantities were found that had a simple one-to-one relationship with mobility: packing fraction, potential energy density, and the value of the static structure factor at the first peak. The chain center-of-mass mobility as a function of these three quantities could be described equally well by either a Vogel-Fulcher type or a power law equation.