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![Radial distribution function between Na + /Li + and [Tf2N]-(O atom), from Ab Initio Molecular Dynamics and classical Molecular Dynamics with static force fields for a concentration of 1.0 M. Dashed lines stand for the spherical integration of the g(r) (coordination number).](profile/Jose-Vicent-Luna/publication/303027544/figure/fig5/AS:667889199742988@1536248602990/Radial-distribution-function-between-Na-Li-and-Tf2N-O-atom-from-Ab-Initio.jpg)
Radial distribution function between Na + /Li + and [Tf2N]-(O atom), from Ab Initio Molecular Dynamics and classical Molecular Dynamics with static force fields for a concentration of 1.0 M. Dashed lines stand for the spherical integration of the g(r) (coordination number).
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![Figure 1. Atomistic representation of IL [C4PYR] + [Tf2N]-used in this...](profile/Jose-Vicent-Luna/publication/303027544/figure/fig4/AS:667889199771653@1536248602917/Atomistic-representation-of-IL-C4PYR-Tf2N-used-in-this-work_Q320.jpg)
![Figure 2. Radial distribution function between Na + /Li + and [Tf2N]-(O...](profile/Jose-Vicent-Luna/publication/303027544/figure/fig5/AS:667889199742988@1536248602990/Radial-distribution-function-between-Na-Li-and-Tf2N-O-atom-from-Ab-Initio_Q320.jpg)
![Figure 8. Radial distribution function between [Tf2N] --[Tf2N] -(left)...](profile/Jose-Vicent-Luna/publication/303027544/figure/fig3/AS:362709892780034@1463488185701/Radial-distribution-function-between-Tf2N--Tf2N-left-and_Q320.jpg)
Compositional effects in the transport properties of electrolytes for batteries based on Room-Temperature Ionic Liquids (ILs) are well-known. However, understanding is required about the molecular origin of these effects. In this work we investigate the use of ILs in batteries, by means of both classical (MD) and quantum molecular dynamics (AIMD)....
Contexts in source publication
Context 1
... Figure 2, the radial distribution functions between Li + or Na + ions and [Tf2N] -anions, as obtained by both AIMD and classical MD, is presented. Good agreement is found between both kinds of calculations, showing that Lennard-Jones plus Coulombic potentials are accurate enough to describe correctly the short- range microscopic structure of these systems. ...
Context 2
... value is twice the coordination number, n, of the complexes [Na(Tf2N)n] -(n-1) and [Li(Tf2N)n] -(n-1) respectively. Results in Figure 2 indicate that this number is between 5 and 6, in agreement with the binding energies. As expected, we obtain the same coordination number from MD and AIMD. ...
Context 3
... in this kind of systems is slow and the time interval for which the diffusion regime is valid should be carefully selected. For the most unfavorable case of diffusion of sodium ions at the lowest temperature studied (298 K) and highest salt concentration (1.0 M), ion motion comprises between 2 and 3 ionic radiuses (see figure S2 in the Supporting Information) in the time span by the simulation. However, it is illustrative to compare this time with the time required for the system to reach thermodynamic equilibrium (relaxation time), which is when the diffusion regime is reached. ...
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... fact, it has been reported [44] that over 5 ns is sufficient to reach the diffusive regime at room temperature ILs. We have confirmed the realization of the diffusion regime (see figure S2 in the Supporting Information) and confirm that the mean squared displacements are purely diffusive in a time interval of 5-30 ns. To gain further assurance, we have carried out 3 additional simulations, two simulations for 50 ns and one with a time interval of 100 ns for the most unfavorable case (lowest temperature and higher salt concentration). ...
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Citations
... Much effort has been made to understand EFG fluctuations around monatomic ions in water, [23][24][25][26][27][28][29][30][31][32][33][34][35][36] and this body of work demonstrates that EFG fluctuations are the product of a subtle interplay between collective solvent rotations and translations, where no single solvent motion drives the dynamics. The dynamics of monatomic ions in ionic liquids are of particular interest, due to recent efforts toward the development of sodium-ion batteries using ionic liquid electrolytes; [37][38][39][40][41][42] however, NMR T 1 studies of monatomic ions in ILs are rare and have focused primarily on Li + . [43][44][45][46] The goal of the present report is to develop a protocol for simulating EFG dynamics around monatomic ions in ionic liquids, for future analysis of T 1 measurements and to investigate which solvent motions are correlated with the EFG dynamics. ...
The T 1 relaxation time measured in nuclear magnetic resonance experiments contains information about electric field gradient (EFG) fluctuations around a nucleus, but computer simulations are typically required to interpret the underlying dynamics. This study uses classical molecular dynamics (MD) simulations and quantum chemical calculations to investigate EFG fluctuations around a Na ⁺ ion dissolved in the ionic liquid 1-ethyl 3-methylimidizolium tetrafluoroborate, [Im 21 ][BF 4 ], to provide a framework for future interpretation of NMR experiments. Our calculations demonstrate that the Sternheimer approximation holds for Na ⁺ in [Im 21 ][BF 4 ] and the anti-shielding coefficient is comparable to its value in water. EFG correlation functions, C EFG ( t), calculated using quantum mechanical methods or from force field charges are roughly equivalent after 200 fs, supporting the use of classical MD for estimating T 1 times of monatomic ions in this ionic liquid. The EFG dynamics are strongly bi-modal with 75-90 % of the decorrelation attributable to inertial solvent motion and the remainder to a highly distributed diffusional processes. Integral relaxation times, <τ EFG >, were found to deviate from hydrodynamic predictions and were non-linearly coupled to solvent viscosity. Further investigation showed that Na ⁺ is solvated by four tetrahedrally arranged [BF 4 ] ⁻ anions and directly coordinated by ~6 fluorine atoms. Exchange of [BF 4 ] ⁻ anions is rare on the 25-50 ns timescale and suggests that motion of solvent-shell [BF 4 ] ⁻ are the primary mechanism for the EFG fluctuations. Different couplings of [BF 4 ] ⁻ translational and rotational diffusion to viscosity are shown to be the source of the non-hydrodynamic scaling of <τ EFG >.
... However, it appears that some effects of cation are evidenced on solubility, viscosity and ionic conductivity of the electrolytes. They may be related to some local structural modifications as it could be evidenced from IR/Raman spectroscopy 34 , and also confirmed from molecular simulations 26,35,36,37 : i) modification of alkali coordination shell (number of TFSIanion around the alkali); ii) formation of clusters in which two (or more) alkali ions are connected together through anion(s) and iii) equilibrium constant and lifetime of complex species. These modifications are related to the differential Soft/Hard characteristics of the components 38 which may be described as well in terms of size, surface charge density, polarizability or electronegativity of the different alkali/alkaline earth cations (Table 1). ...
... 2a) and this should correspond to stronger reduction of the lattice energy for the more asymmetric systems (smaller ions) in reducing the packing of the ions. 35,59 This is also confirmed by the measurements of electrolyte densities presented in table S3, which are larger when alkali/alkali earth is added to pure BMImTFSI, but rather independently to the nature of the salt itself. Tg also gradually increases with the increase of the salt concentration (Fig. 2b). ...
The increasing need of portable electrical resources requires to develop post-Li batteries, in which redox reactions are then based on the different alkali or earth alkaline ions. Keeping in mind the specific advantages of electrolytes based on ionic liquid (electrochemical properties, safety, stability …), this paper focuses on the specific properties of those obtained by mixing alkali or earth alkaline salts (Li⁺, Na⁺, K⁺, Cs⁺ and Mg²⁺) in a prototypical ionic liquid, the 1-butyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl) imide (BMImTFSI). Transport properties of these electrolytes are deciphered from viscosity, ionic conductivity and individual self-diffusion coefficients. At room temperature, change of electrolyte composition (nature of ion, concentration) induces some large variations of these transport properties. However, if these measurements are scaled towards glass transition temperatures, master curves are obtained with only slight differences between monovalent alkali and divalent earth alkaline ions. Further information is obtained from the Walden approach and evidences that these electrolytes are 'good ionic conductors'. These results are confirmed from self-diffusion coefficients which also allows to retrieve contributions of each species to ionic transport and then dissociation ratio inside these mixtures. Slight differences between alkali and earth alkali ions may be related to solvation mechanisms, as proposed from infrared spectroscopy measurements.
... A popular approach has been to combine DFT calculations of activation energies of different events, which are then used to implement kMC simulations [104]. DFT calculations of the migration mechanisms and activation barriers of Li-ions have also been combined with classical MD studies of Li-ion diffusion to gain a more complete analysis of the dynamic properties in LiB materials [784,785]. Quantum mechanical techniques, such as DFT, are also increasing being used to parameterise larger scale techniques, for example classical MD [143-145, 164, 776]. ...
Computational modelling is a vital tool in the research of batteries and their component materials. Atomistic models are key to building truly physics-based models of batteries and form the foundation of the multiscale modelling chain, leading to more robust and predictive models. These models can be applied to fundamental research questions with high predictive accuracy. For example, they can be used to predict new behaviour not currently accessible by experiment, for reasons of cost, safety, or throughput. Atomistic models are useful for quantifying and evaluating trends in experimental data, explaining structure-property relationships, and informing materials design strategies and libraries. In this review, we showcase the most prominent atomistic modelling methods and their application to electrode materials, liquid and solid electrolyte materials, and their interfaces, highlighting the diverse range of battery properties that can be investigated. Furthermore, we link atomistic modelling to experimental data and higher scale models such as continuum and control models. We also provide a critical discussion on the outlook of these materials and the main challenges for future battery research.
... 49−51 DFT has increasingly been utilized in parametrization of larger-scale techniques, such as classical MD. 50,52−56 One popular approach has been to use DFT calculations of migration mechanisms and activation barriers of Li-ions, in conjunction with classical MD studies of Li-ion diffusion, to gain a more complete analysis of the dynamic properties. 57,58 In a similar vein, DFT calculations of activation energies for different events are used to construct the basis for kinetic Monte Carlo (kMC) simulations. 59 kMC is a natural technique to include different time-scale dynamic events. ...
... The surface activity of SAILs can be further increased by increasing the hydrophobicity of the SAIL. Furthermore, the interaction between ILs and water is important for applications in catalysis and electrochemical processes [16,17]. Alkylimidazolium based ILs, [C n MIM] + X − , are among the most popular type of SAILs. ...
Ionic liquids have drawn vast attention due to their extraordinary physicochemical properties and their relevance in wide range of applications. As a result of their inherent amphiphilic nature, many ionic liquids are emerging as novel surfactants, being referred to as surface active ionic liquids (SAILs). Under suitable conditions in aqueous solution, these SAILs self-assemble to form nanostructures such as micelles and vesicles. Here, we report on the structure and dynamics of one such SAIL micellar system, 1-Decyl-3-methylimidazolium Bromide (DMIMBr), as studied using fully atomistic molecular dynamics (MD) simulation and quasielastic neutron scattering (QENS). Results are compared with a conventional surfactant, dodecyltrimethylammonium bromide (DTAB) micelles. MD simulations showed that while the structure of the DMIMBr micelle is very similar to that of DTAB, the conformational flexibility of the alkyl chains is starkly different. The alkyl chains in the SAIL are more ordered, having lesser gauche defects. This is consistent with experimental results obtained using QENS which also showed slower alkyl chain segmental dynamics. MD results also show restricted dynamics of the hydration water in DMIMBr micelles, of a sub-diffusive nature, suggesting stronger affinity of water molecules to the imidazolium moiety. Thus the decreased flexibility in the alkyl chains could arise from the slower hydration dynamics. In contrast, the lateral motion of the surfactant is found to be relatively faster in DMIMBr micelles than in DTAB micelles. The understanding gained from this study using QENS and MD simulations provides a microscopic insight about the dynamical aspects of SAIL micelles, which might be useful for related applications including micellar catalysis, nanoparticle synthesis.
... It is precisely because of the increase in ion correlation that larger clusters are not conducive to transport, thus causing slow diffusion. [ 38 ] Increasing the operating temperature of battery can be expected to improve the diffusion coefficient. In addition, the high synthesis cost of IL electrolytes cannot be ignored. ...
Potassium-ion batteries (KIBs) are competitive alternatives to lithium-ion batteries (LIBs) due to the abundant K resources and high energy density. As an indispensable part of the battery, the electrolyte affects the battery capacity, rate capability, cycle life, and safety. Nevertheless, the researches on electrolytes and corresponding solid electrolyte interfaces (SEI) in KIB are still in its infancy and require further attention. In this review, recent progresses of various K⁺ containing electrolytes for KIBs are summarized comprehensively. Additionally, the effects of salts, solvents, additives, and concentrations on the properties of various electrolyte systems are discussed in detail. Thereafter, interface chemistry between electrode and electrolyte, as well as rational modification strategies for high-performance KIB are reviewed. Finally, the major challenges and the future perspectives are estimated for advanced KIB. This review will provide good directions for the development of high-performance KIB.
... It is precisely because of the increase in ion correlation that larger clusters are not conducive to transport, thus causing slow diffusion. [ 38 ] Increasing the operating temperature of battery can be expected to improve the diffusion coefficient. In addition, the high synthesis cost of IL electrolytes cannot be ignored. ...
... The diffusion coefficient D 1 decreases with increasing Li concentration, which is a common property of Li-salt/IL mixtures, explained by the relatively strong interaction of the Li ions with the anions. 54 Additionally, a larger fraction of the polymer might be dissolved in the IL/Li-salt enriched phase due to the addition of the Li ions, which would also reduce the diffusivities of the ions. ...
We study the complex mixture of a polyethylene
oxide-b-polypropylene oxide-b-polyethylene oxide triblock copolymer (Pluronic F127) with ionic liquid (IL) and Li-salt, which is potentially interesting as an electrolyte system with decoupled
mechanical and ion-transport properties. Small-angle X-ray
scattering (SAXS) and differential scanning calorimetry (DSC)
are employed to scrutinize the phase structures and elucidate the
ternary phase diagram. These data are combined with the ion
diffusivities obtained by pulsed field gradient (PFG) nuclear
magnetic resonance (NMR). Analyzing the partial ternary phase
diagram of F127/LiTFSI/Pyr14TFSI, hexagonal, lamellar, and
micellar mesophases are identified, including two-phase coexistence regions. While the PPO block is immiscible with the liquid, and forms the backbone of the mesostructured aggregates, the PEO blocks are not well miscible with the IL. Poorly solvated, the latter may still crystallize. At a higher IL content, PEO is further solvated, but a major solvation effect occurs due to addition of Li-salt. Li ions promote solubilization of the PEO chains in the IL, since they coordinate to the PEO chains. This was identified as the mechanism of a transition of the mesostructures, with increasing Li-salt content changing from a hexagonal to a lamellar and further to a micellar phase. In summary, both, the amount of IL and its compatibility with the PEO block, the latter being controlled by the Li-salt amount, influence the compositions of the formed mesophases and the ion diffusion in their liquid regions.
... The increased CN of Na-FSI at the higher NaFSI salt concentrations is also reported in the bulk-phase simulation of this IL, which was suggested to be a consequence of the formation of extended Na x (FSI) y ion aggregates 36 . The increase in Na-FSI CN not only results in a decrease in binding strength between Na + and [FSI]anions due to enlarged anion repulsive energy 30,31,[36][37][38][39] , but also promotes the local dynamics of [FSI]in the 50-mol% system, and consequently enhances the Na ion hopping events, demonstrated as a more frequent change in the number of Na + counted in the innermost layer near the electrode ( Supplementary Fig. 3). ...
Non-uniform metal deposition and dendrite formation in high-density energy storage devices reduces the efficiency, safety and life of batteries with metal anodes. Superconcentrated ionic-liquid electrolytes (for example 1:1 ionic liquid:alkali ion) coupled with anode preconditioning at more negative potentials can completely mitigate these issues, and therefore revolutionize high-density energy storage devices. However, the mechanisms by which very high salt concentration and preconditioning potential enable uniform metal deposition and prevent dendrite formation at the metal anode during cycling are poorly understood, and therefore not optimized. Here, we use atomic force microscopy and molecular dynamics simulations to unravel the influence of these factors on the interface chemistry in a sodium electrolyte, demonstrating how a molten-salt-like structure at the electrode surface results in dendrite-free metal cycling at higher rates. Such a structure will support the formation of a more favourable solid electrolyte interphase, accepted as being a critical factor in stable battery cycling. This new understanding will enable engineering of efficient anode electrodes by tuning the interfacial nanostructure via salt concentration and high-voltage preconditioning. Non-uniform metal deposition and dendrite formation reduce the efficiency, safety and life of batteries with metal anodes. The influence of these factors in a sodium electrolyte now shows how a molten-salt-like structure at the electrode surface results in dendrite-free metal cycling at higher rates.
... 7,19,23−26 Ion−ion interactions were studied by quantum chemical calculations. 12,19,24 Calculations for ions or ion aggregates in the gas phase or in effective solvent models can provide information on infrared or Raman spectra and possible effects of interactions, but they do not refer directly to the bulk electrolyte. ...
Classical and ab initio molecular dynamics simulations have been performed for series of electrolytes for Na-ion devices based on NaFSI solutions in EMIM-FSI ionic liquid. Viscosities and conductivities estimated in a polarizable force field agree with experimental data. Fourier transform of the autocorrelation function of the dipole moment obtained in ab initio simulations has been used to calculate the IR spectra of electrolytes. Computed shifts of the vibrational band originating from S-F stretch of the FSI anion are in very good agreement with experimental data measured for electrolytes with increasing NaFSI mole fraction. Fourier transform of geometrical parameters of FSI anions has been applied to probe locally the frequency changes. Results show the correlation between local environment of the FSI ion and vibrational frequencies, corroborating the deconvolution of experimental spectrum into contributions from “free” and “bound” ions.
