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

Ramifications of Water-in-Salt Interfacial Structure at Charged Electrodes for Electrolyte Electrochemical Stability

Development of safe aqueous batteries and supercapacitors critically relies on expanding the electrolyte electrochemical stability window. A novel mechanism responsible for widening the electrochemical stability window of water-in-salt electrolytes (WiSEs) compared to conventional salt-in-water electrolytes is suggested based on molecular dynamics (MD) simulations of the electrolyte–electrode interface. Water exclusion from the interfacial layer at the positive electrode provided additional kinetic protection that delayed the onset of the oxygen evolution reactions. The interfacial structure of a WiSE at negative electrodes near the potential of zero charge clarified why the recently discovered passivation layers formed in WiSEs are robust. The onset of water accumulation at potentials below 1.5 V vs Li/Li⁺ leads to formation of water-rich nanodomains at the negative electrode, limiting the robustness of the WiSE. Unexpectedly, the bis­(trifluoromethanesulfonyl)­imide anion adsorbed and trifluoromethanesulfonate desorbed with positive electrode polarization, demonstrating selective anion partitioning in the double layer

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

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

Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes

The oxidative stability and initial oxidation-induced decomposition reactions of common electrolyte solvents for batteries and electrical double layer capacitors were investigated using quantum chemistry (QC) calculations. The investigated electrolytes consisted of linear (DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate, and alkyl phosphate solvents paired with BF₄ ^–, PF₆ ^–, bis­(fluorosulfonyl)­imide (FSI^–), difluoro-(oxalato)­borate (DFOB^–), dicyanotriazolate (DCTA^–), and B­(CN)₄ ^– anions. Most QC calculations were performed using the M05-2X, LC-ωPBE density functional and compared with the G4MP2 results where feasible. The calculated oxidation potentials were compared with previous and new experimental data. The intrinsic oxidation potential of most solvent molecules was found to be higher than experimental values for electrolytes even after the solvation contribution was included in the QC calculations via a polarized continuum model. The presence of BF₄ ^–, PF₆ ^–, B­(CN)₄ ^–, and FSI^– anions near the solvents was found to significantly decrease the oxidative stability of many solvents due to the spontaneous or low barrier (for FSI^–) H- and F-abstraction reaction that followed the initial electron removal step. Such spontaneous H-abstraction reactions were not observed for the solvent complexes with DCTA^– or DFOB^– anions or for VC/anion, TMP/PF₆ ^– complexes. Spontaneous H-transfer reactions were also found for dimers of the oxidized carbonates (EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was found for dimers of sulfones (TMS and EMS). These reactions resulted in a significant decrease of the dimer oxidation potential compared to the oxidation potential of the isolated solvent molecules. The presence of anions or an explicitly included solvent molecule next to the oxidized solvent molecules also reduced the barriers for the oxidation-induced decomposition reaction and often changed the decomposition products. When a Li⁺ cation polarized the solvent in the EC_<i>n</i>/LiBF₄ and EC_<i>n</i>/LiPF₆ complexes, the complex oxidation potential was 0.3–0.6 eV higher than the oxidation potential of EC_<i>n</i>/BF₄ ^– and EC_<i>n</i>/PF₆ ^–

Correlating Li⁺ Solvation Sheath Structure with Interphasial Chemistry on Graphite

In electrolytes with unique electrochemical signature, the structure of Li⁺ solvation sheath was quantitatively analyzed in correlation with its electrochemical behavior on graphitic anodes. For the first time, a direct link between Li⁺ solvation sheath structure and formation chemistry of the solid electrolyte interphase (SEI) is established. Quantum chemistry calculations and molecular dynamics simulations were performed to explain the observed reversed preference of propylene carbonate (PC) over ethylene carbonate (EC) by Li⁺

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

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

Physicochemical Properties of Binary Ionic Liquid–Aprotic Solvent Electrolyte Mixtures

The properties of mixtures of ionic liquids (ILs) with a variety of different aprotic solvents have been examined in detail. The ILs selectedbis­(trifluoromethanesulfonyl)­imide (TFSI^–) salts with <i>N</i>-methyl-<i>N</i>-pentylpyrrolidinium (PY₁₅⁺), -piperidinium (PI₁₅⁺), or -morpholinium (MO₁₅⁺) cationsenabled the investigation of how cation structure influences the mixture properties. This study includes the characterization of the thermal phase behavior of the mixtures and volatility of the solvents, density and excess molar volume, and transport properties (viscosity and conductivity). The mixtures with ethylene carbonate form a simple eutectic, whereas those with ethyl butyrate appear to form a new IL–solvent crystalline phase. Significant differences in the viscosity of the mixtures are found for different solvents, especially for the IL-rich concentrations. In contrast, only minor differences are noted for the conductivity with different solvents for the IL-rich concentrations. For the solvent-rich concentrations, however, substantial differences are noted in the conductivity, especially for the mixtures with acetonitrile

Density Functional Theory Study of the Role of Anions on the Oxidative Decomposition Reaction of Propylene Carbonate

The oxidative decomposition mechanism of the lithium battery electrolyte solvent propylene carbonate (PC) with and without PF₆^– and ClO₄^– anions has been investigated using the density functional theory at the B3LYP/6-311++G(d) level. Calculations were performed in the gas phase (dielectric constant ε = 1) and employing the polarized continuum model with a dielectric constant ε = 20.5 to implicitly account for solvent effects. It has been found that the presence of PF₆^– and ClO₄^– anions significantly reduces PC oxidation stability, stabilizes the PC–anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. The primary oxidative decomposition products of PC–PF₆^– and PC–ClO₄^– were CO₂ and acetone radical. Formation of HF and PF₅ was observed upon the initial step of PC–PF₆^– oxidation while HClO₄ formed during initial oxidation of PC–ClO₄^–. The products from the less likely reaction paths included propanal, a polymer with fluorine and fluoro-alkanols for PC–PF₆^– decomposition, while acetic acid, carboxylic acid anhydrides, and Cl^– were found among the decomposition products of PC–ClO₄^–. The decomposition pathways with the lowest barrier for the oxidized PC–PF₆^– and PC–ClO₄^– complexes did not result in the incorporation of the fluorine from PF₆^– or ClO₄^– into the most probable reaction products despite anions and HF being involved in the decomposition mechanism; however, the pathway with the second lowest barrier for the PC–PF₆^– oxidative ring-opening resulted in a formation of fluoro-organic compounds, suggesting that these toxic compounds could form at elevated temperatures under oxidizing conditions