Quantum and Classical Molecular Dynamics of Ionic Liquid Electrolytes for Na/Li-based Batteries: Molecular Origins of the Conductivity Behavior

Simulations of electric field gradient fluctuations and dynamics around sodium ions in ionic liquids

The T₁ 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-methylimidazolium tetrafluoroborate, [Im₂₁][BF₄], to provide a framework for future interpretation of NMR experiments. Our calculations demonstrate that the Sternheimer approximation holds for Na⁺ in [Im₂₁][BF₄], 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 T₁ times of monatomic ions in this ionic liquid. The EFG dynamics are strongly bi-modal, with 75%-90% of the de-correlation 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 [BF₄]^- anions and directly coordinated by ∼6 fluorine atoms. Exchange of [BF₄]^- anions is rare on the 25-50 ns timescale and suggests that motion of solvent-shell [BF₄]^- is the primary mechanism for the EFG fluctuations. Different couplings of [BF₄]^- translational and rotational diffusion to viscosity are shown to be the source of the non-hydrodynamic scaling of ⟨τ_EFG⟩.

Influence of Li-Salt on the Mesophases of Pluronic Block Copolymers in Ionic Liquid

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.

Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes

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.

Boosting Rechargeable Batteries R&D by Multiscale Modeling: Myth or Reality?

This review addresses concepts, approaches, tools, and outcomes of multiscale modeling used to design and optimize the current and next generation rechargeable battery cells. Different kinds of multiscale models are discussed and demystified with a particular emphasis on methodological aspects. The outcome is compared both to results of other modeling strategies as well as to the vast pool of experimental data available. Finally, the main challenges remaining and future developments are discussed.

Temperature Dependence of Static Structure Factor Peak Intensities for a Pyrrolidinium-Based Ionic Liquid

Static structure factors ( S( q)) for many ionic liquids show low-wavenumber peaks whose intensities increase with increasing temperature. The greater peak intensities might seem to imply increasing intermediate-range order with increasing temperature. Molecular dynamics (MD) simulations for a representative ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (C₄C₁pyrrTFSI), were used to calculate S( q) and partial S( q) (cation-cation, anion-anion, and cation-anion) at 298, 363, and 500 K. S( q) and partial S( q) were further decomposed into positive and negative components (which each indicate structural ordering) by separately summing positive and negative Fourier transform summands. Increasing temperature causes the negative components of each partial S( q) to decrease in magnitude more than the positive components, causing the total S( q) to increase in magnitude. Thus, structural ordering with periodicities corresponding to observed peaks in S( q) does not increase but instead decoheres with increasing temperature, even though S( q) peak heights increase. Fourier transform summands also show where in real space the positive and negative component contributions to S( q) change when the temperature increases. This new, detailed analysis based on Fourier transform summands comprising S( q) argues for great caution when interpreting S( q) intensities and highlights the value of simulations as a complement to X-ray (or neutron) scattering experiments.

Interfacial structure and electrochemical stability of electrolytes: methylene methanedisulfonate as an additive

The mechanism responsible for widening the electrochemical stability window of methylene methanedisulfonate (MMDS)-containing electrolytes compared to conventional carbonate electrolytes is suggested based on molecular dynamics (MD) simulations and density functional theory (DFT) calculations. We find that MMDS has a stronger reduction ability and higher affinity for the electrode surface than solvents, and these behaviors provide an important condition for priority decomposition of the additive. The addition of MMDS could reduce the probability of finding solvent-ion complexes at the electrolyte-electrode interface, which is especially beneficial for the stability of the solvent electrochemical window. This knowledge of the local electrolyte composition and structure at the surface plays a significant role in advancing our understanding of the relationships between interface structure and battery cycling performance, and expanding the operating windows of electrochemical devices.

Role of Ionic Liquid [EMIM]+[SCN]- in the Adsorption and Diffusion of Gases in Metal-Organic Frameworks

We study the adsorption performance of metal-organic frameworks (MOFs) impregnated of ionic liquids (ILs). To this aim we calculated adsorption and diffusion of light gases (CO₂, CH₄, N₂) and their mixtures in hybrid composites using molecular simulations. The hybrid composites consist of 1-ethyl-3-methylimidazolium thiocyanate impregnated in IRMOF-1, HMOF-1, MIL-47, and MOF-1. We found that the increase of the amount of IL enhances the adsorption selectivity in favor of carbon dioxide for the mixtures CO₂/CH₄ and CO₂/N₂ and in favor of methane in the mixture CH₄/N₂. We also provide detailed analysis of the microscopic organization of ILs and adsorbates via radial distribution functions and average occupation profiles and study the impact of the ILs in the diffusion of the adsorbates inside the pores of the MOFs. Based on our findings, we discuss the advantages of using IL/MOF composites for gas adsorption to increase the adsorption of gases and to control the pore sizes of the structures to foster selective adsorption.

Electrolytes, SEI Formation, and Binders: A Review of Nonelectrode Factors for Sodium-Ion Battery Anodes

Through intense effort in recent years, knowledge of Na-ion batteries has been advanced significantly, pertaining to electrodes. Often, such progress has been accompanied by using a convenient choice of electrolyte or binder. Nevertheless, it has been witnessed that "external" factors to electrodes, such as electrolytes, solid electrolyte interphase, and binders, affect the functions of electrodes profoundly. And generally, certain types of electrodes favor some electrolytes or binders. With a rapidly increasing number of publications in the area, trends in terms of electrolytes and binders are possibly exploitable. Unfortunately, the field has yet to see a review article that devotes itself to these nonelectrode aspects of Na-ion batteries. Here, the gap is filled by conducting a comprehensive review of these nonelectrode external factors, especially by looking into their correlation with electrochemical properties, such as cycle life, and first cycle coulombic efficiency. Not only are the representative reports reviewed, but also quantitative analyses on the database that are constructed are provided. With such analyses, some new data-driven perspectives are postulated, which are of great value to the community.

Electrochemical Reduction of Oxygen in Aprotic Ionic Liquids Containing Metal Cations: A Case Study on the Na-O2 system

Metal-air batteries are intensively studied because of their high theoretical energy-storage capability. However, the fundamental science of electrodes, electrolytes, and reaction products still needs to be better understood. In this work, the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) was chosen to study the influence of a wide range of metal cations (Mⁿ⁺ ) on the electrochemical behavior of oxygen. The relevance of the theory of Lewis hard and soft acids and bases to predict satisfactorily the reduction potential of oxygen in electrolytes containing metal cations is demonstrated. Systems with soft and intermediate Mⁿ⁺ acidity are shown to facilitate oxygen reduction and metal oxide formation, whereas oxygen reduction is hampered by hard acid cations such as sodium and lithium. Furthermore, DFT calculations on the energy of formation of the resulting metal oxides rationalize the effect of Mⁿ⁺ on oxygen reduction. A case study on the Na-O₂ system is described in detail. Among other things, the Na⁺ concentration of the electrolyte is shown to control the electrochemical pathway (solution precipitation vs. surface deposition) by which the discharge product grows. All in all, fundamental insights for the design of advanced electrolytes for metal-air batteries, and Na-air batteries in particular, are provided.

Reversibility of imido-based ionic liquids: a theoretical and experimental study

Theoretical and experimental methods were used to study the reversibility of a series of imido-based ionic liquids.