Computational and Experimental Study of Li-Doped Ionic Liquids at Electrified Interfaces

Electric field induced associations in the double layer of salt-in-ionic-liquid electrolytes

Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications. Upon dissolving alkali metal salts, such as Li or Na based salts, with the same anion as the IL, an intrinsically asymmetric electrolyte can be created for use in batteries, known as a salt-in-ionic liquid (SiIL). These SiILs have been well studied in the bulk, where negative transference numbers of the alkali metal cation have been observed from the formation of small, negatively charged clusters. The properties of these SiILs at electrified interfaces, however, have received little to no attention. Here, we develop a theory for the electrical double layer (EDL) of SiILs where we consistently account for the thermoreversible association of ions into Cayley tree aggregates. The theory predicts that the IL cations first populate the EDL at negative voltages, as they are not strongly bound to the anions. However, at large negative voltages, which are strong enough to break the alkali metal cation-anion associations, these IL cations are exchanged for the alkali metal cation because of their higher charge density. At positive voltages, we find that the SiIL actually becomes more aggregated while screening the electrode charge from the formation of large, negatively charged aggregates. Therefore, in contrast to conventional intuition of associations in the EDL, SiILs appear to become more associated in certain electric fields. We present these theoretical predictions to be verified by molecular dynamics simulations and experimental measurements.

Impact of Li, Na and Zn metal cation concentration in EMIM-TFSI ionic liquids on ion clustering, structure and dynamics

We use molecular dynamics calculations to investigate the behavior of metal cations (Li, Na and Zn) within ionic liquids (ILs), specifically EMIM-TFSI, and their impact on key properties, particularly focusing on ion-ion correlations and their influence on diffusion and conductivity. The study explores the competition between metal cations and EMIM ions for binding to TFSI and analyzes ion pair dynamics, revealing that metal cation-TFSI pairs exhibit significantly longer lifetimes compared to TFSI-EMIM pairs. This competitive interaction and the increased stability of metal cation-TFSI pairs at higher concentrations leads to reduced ion exchange, resulting in decreased diffusion and conductivity. The observations underscore the importance of ion size and charge in determining their behavior regarding IL dynamics. Overall, this work provides valuable insights for designing ILs with customized properties, particularly in the context of optimizing conductivity and addressing energy storage challenges.

Understanding the Capacity Decay of Si/NMC622 Li-Ion Batteries Cycled in Superconcentrated Ionic Liquid Electrolytes: A New Perspective

Silicon-containing Li-ion batteries have been the focus of many energy storage research efforts because of the promise of high energy density. Depending on the system, silicon generally demonstrates stable performance in half-cells, which is often attributed to the unlimited lithium supply from the lithium (Li) metal counter electrode. Here, the electrochemical performance of silicon with a high voltage NMC622 cathode was investigated in superconcentrated phosphonium-based ionic liquid (IL) electrolytes. As a matter of fact, there is very limited work and understanding of the full cell cycling of silicon in such a new class of electrolytes. The electrochemical behavior of silicon in the various IL electrolytes shows a gradual and steeper capacity decay, compared to what we previously reported in half-cells. This behavior is linked to a different evolution of the silicon morphology upon cycling, and the characterization of cycled electrodes points toward mechanical reasons, complete disconnection of part of the electrode, or internal mechanical stress, due to silicon and Li metal volume variation upon cycling, to explain the progressive capacity fading in full cell configuration. An extremely stable solid electrolyte interphase (SEI) in the full Li-ion cells can be seen from a combination of qualitative and quantitative information from transmission electron microscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and magic angle spinning nuclear magnetic resonance. Our findings provide a new perspective to full cell interpretation regarding capacity fading, which is oftentimes linked almost exclusively to the loss of Li inventory but also more broadly, and provide new insights into the impact of the evolution of silicon morphology on the electrochemical behavior.

Density functional theory of alkali metals at the IL/graphene electrochemical interface

The mechanism of charge transfer between metal ions and graphene in the presence of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate) is investigated by means of density functional theory calculations. For that purpose, two different comparisons are established: (i) the behavior of Li⁺ and K⁺ when adsorbed onto the basal plane of graphene and (ii) the differences between Li⁺ approaching the carbon surface from the basal plane and being intercalated through the edge plane of trilayer graphene. In the first case, it is found that the metal ions must overcome high energy barriers due to their interaction with the ionic liquid before reaching an equilibrium position close to the interface. In addition, no significant charge transfer between any of the metals and graphene takes place until very close energetically unfavorable distances. The second configuration shows that Li⁺ has no equilibrium position in the proximity of the interface but instead has an equilibrium position when it is inside the electrode for which it has to cross an energy barrier. In this case, the formation of a LiC₁₂ complex is observed since the charge transfer at the equilibrium distance is achieved to a considerable extent. Thus, the interfacial charge transfer resistance on the electrode in energy devices based on ionic liquids clearly depends not only on the binding of the ionic liquid to the metal cations and their ability to form a dense solvation shell around them but also on the surface topography and its effect on the ion packing on the surface.

A molecular dynamics study of a fully zwitterionic copolymer/ionic liquid-based electrolyte: Li+ transport mechanisms and ionic interactions

The development of polymer electrolytes (PEs) is crucial for advancing safe, high-energy density batteries, such as lithium-metal and other beyond lithium-ion chemistries. However, reaching the optimum balance between mechanical stiffness and ionic conductivity is not a straightforward task. Zwitterionic (ZI) gel electrolytes comprising lithium salt and ionic liquid (IL) solutions within a fully ZI polymer network can, in this context, provide useful properties. Although such materials have shown compatibility with lithium metal in batteries, several fundamental structure-dynamic relationships regarding ionic transport and the Li⁺ coordination environment remain unclear. To better resolve such issues, molecular dynamics simulations were carried out for two IL-based electrolyte systems, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) with 1 M LiTFSI salt and a ZI gel electrolyte containing the IL and a ZI copolymer: poly(2-methacryloyloxyethyl phosphorylcholine-co-sulfobetaine vinylimidazole), poly(MPC-co-SBVI). The addition of ZI polymer decreases the [TFSI]^- -[Li]⁺ interactions and increases the IL ion diffusivities, and consequently, the overall ZI gel ionic conductivity. The structural analyses showed a large preference for lithium-ion interactions with the polymer phosphonate groups, while the [TFSI]^- anions interact directly with the sulfonate group and the [BMP]⁺ cations only display secondary interactions with the polymer. In contrast to previous experimental data on the same system, the simulated transference numbers showed smaller [Li]⁺ contributions to the overall ionic conductivities, mainly due to negatively charged lithium aggregates and the strong lithium-ion interactions in the systems.

Tuning the Formation and Structure of the Silicon Electrode/Ionic Liquid Electrolyte Interphase in Superconcentrated Ionic Liquids

The latest advances in the stabilization of Li/Na metal battery and Li-ion battery cycling have highlighted the importance of electrode/electrolyte interface [solid electrolyte interphase (SEI)] and its direct link to cycling behavior. To understand the structure and properties of the SEI, we used combined experimental and computational studies to unveil how the ionic liquid (IL) cation nature and salt concentration impact the silicon/IL electrolyte interfacial structure and the formed SEI. The nature of the IL cation is found to be important to control the electrolyte reductive decomposition that influences the SEI composition and properties and the reversibility of the Li-Si alloying process. Also, increasing the Li salt concentration changes the interface structure for a favorable and less resistive SEI. The most promising interface for the Si-based battery was found to be in P₁₂₂₂FSI with 3.2 m LiFSI, which leads to an optimal SEI after 100 cycles in which LiF and trapped LiFSI are the only distinguishable lithiated and fluorinated products detected. This study shows a clear link between the nanostructure of the IL electrolyte near the electrode surface, the resulting SEI, and the Si negative electrode cycling performance. More importantly, this work will aid the rational design of Si-based Li-ion batteries using IL electrolytes in an area that has so far been neglected, reinforcing the benefits of superconcentrated electrolyte systems.

Anion-Assisted Delivery of Multivalent Cations to Inert Electrodes

To understand and control key electrochemical processes-metal plating, corrosion, intercalation, etc.-requires molecular-scale details of the active species at electrochemical interfaces and their mechanisms for desolvation from the electrolyte. Using free energy sampling techniques we reveal the interfacial speciation of divalent cations in ether-based electrolytes and mechanisms for their delivery to an inert graphene electrode interface. Surprisingly, we find that anion solvophobicity drives a high population of anion-containing species to the interface that facilitate the delivery of divalent cations, even to negatively charged electrodes. Our simulations indicate that cation desolvation is greatly facilitated by cation-anion coupling. We propose anion solvophobicity as a molecular-level descriptor for rational design of electrolytes with increased efficiency for electrochemical processes limited by multivalent cation desolvation.

Adding salt to expand voltage window of humid ionic liquids

Humid hydrophobic ionic liquids-widely used as electrolytes-have narrowed electrochemical windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed molecular dynamics simulations to explore effects of adding Li salt in humid ionic liquids on the water adsorbed on the electrode surface. Results reveal that most of the water molecules are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li⁺, having significantly lowered activity. The Li⁺-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the reduction of electrochemical windows of humid hydrophobic ionic liquids. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liquids exhibit enlarged electrochemical windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liquids from electrochemical performance degradation.

Interfacial Structures in Ionic Liquid-Based Ternary Electrolytes for Lithium-Metal Batteries: A Molecular Dynamics Study

Lithium-metal batteries are promising candidates to fulfill the future performance requirements for energy storage applications. However, the tendency to form metallic dendrites and the undesirable side reactions between the electrolyte and the Li electrode lead to poor performance and safety issues in these batteries. Therefore, understanding the interfacial properties and the Li-metal surface/electrolyte interactions is crucial to resolve the remaining obstacles and make these devices feasible. Here, we report a computational study on the interface effects in ternary polymer electrolytes composed by poly(ethylene oxide) (PEO), lithium salts, and different ionic liquids (ILs) confined between two Li-metal slabs. Atomistic simulations are used to characterize the local environment of the Li⁺ ions and the transport properties in the bulk and at the interface regions. Aggregation of ions at the metal surface is seen in all investigated systems; the structure and composition are directly correlated to the IL components. The strong interactions between the electrolyte species and the Li-metal atoms result in the structuring of the electrolyte at the interface region, in which comparatively small and flat ions result in a well-defined region with extensive Li⁺ populations and high self-diffusion coefficients. In contrast, large ions such as [P222mom]⁺ increase the PEO density in the bulk due to large steric effects at the interface. Therefore, the choice of specific ILs in ternary polymer electrolytes can tune the structure-dynamic properties at the Li-metal surface/electrolyte interface, controlling the SEI formation at the electrode surface, and thereby improve battery performance.

Electric double layer formation and storing energy processes on graphene-based supercapacitors from electrical and thermodynamic perspectives

Atomistic molecular dynamics simulations were used to investigate the processes of electrical double layer formation and electrolyte confinement in graphene-based supercapacitors. For both processes, free energy calculations were used to analyze the thermodynamics involved in the electrolyte confinement and its re-arrangement in a double layer on the electrode surface. The value of the free energy of the formation of the double electric layer was related to the energy required to charge the supercapacitor, i.e., the energy density stored, and compared with values obtained using Poisson's electrostatic formalism, which is the conventionally employed approach. Both analyzes were consistent with each other, presenting compatible values for the stored energy.

Probing Electric Double-Layer Composition via in Situ Vibrational Spectroscopy and Molecular Simulations

At an electrode, ions and solvent accumulate to screen charge, leading to a nanometer-scale electric double layer (EDL). The EDL guides electrode passivation in batteries, while in (super)capacitors, it determines charge storage capacity. Despite its importance, quantification of the nanometer-scale and potential-dependent EDL remains a challenging problem. Here, we directly probe changes in the EDL composition with potential using in situ vibrational spectroscopy and molecular dynamics simulations for a Li-ion battery electrolyte (LiClO₄ in dimethyl carbonate). The accumulation rate of Li⁺ ions at the negative surface and ClO₄^- ions at the positive surface from vibrational spectroscopy compares well to that predicted by simulations using a polarizable APPLE&P force field. The ion solvation shell structure and ion-pairing within the EDL differs significantly from the bulk, especially at the negative electrode, suggesting that the common rationalization of interfacial electrochemical processes in terms of bulk ion solvation should be applied with caution.

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.

O2 reduction on a Au film electrode in an ionic liquid in the absence and presence of Mg2+ ions: Product formation and adlayer dynamics

Aiming at a detailed understanding of the interaction between an ionic liquid, O₂, and electrodes in Mg-air batteries, we performed a combined differential electrochemical mass spectrometry and in situ infrared spectroscopy model study on the interaction between the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) and a gold film electrode in the presence and absence of O₂ and Mg²⁺ ions in the potential range relevant for the oxygen reduction reaction (ORR) and evolution reaction. Detailed information on the dynamic exchange of adsorbed ions, on the stability/decomposition of the ionic liquid, and on the activity/selectivity/reversibility of the ORR is derived from measurements performed under potentiodynamic and potentiostatic conditions. In neat BMP-TFSI, we find the dynamics of the potential induced exchange of adsorbed ions to depend significantly on the exchange direction. In the presence of O₂, the anions formed in the ORR distinctly affect the adsorption characteristics of the IL ions and the exchange dynamics. Furthermore, the ORR changes from reduction to superoxide anions at moderate potentials to reduction to peroxide anion at more negative potentials. In the additional presence of Mg²⁺ ions, dominant magnesium peroxide and oxide formation result in an irreversible ORR, in contrast to the requirements of an efficient re-chargeable Mg-air battery. In addition, these ions result in the increasing formation of a blocking adlayer, reducing the coverage of adsorbed IL species.

Solvate Ionic Liquids at Electrified Interfaces

Solvate ionic liquids (SILs) are a promising electrolyte for Li-ion batteries; thus, their behavior at electrified interfaces is crucial for the operation of these batteries. We report molecular dynamics simulation results for a prototypical SIL of lithium triglyme bis(trifluoromethanesulfonyl)imide ([Li(G3)][TFSI]) sandwiched between electrified surfaces. At negatively charged as well as neutral electrodes, the electrolyte largely maintains the characteristics of SILs in terms of the interfacial Li⁺ ions' coordination by a similar number of oxygen atoms on G3 ligands as the bulk Li⁺ ions. The persistence of the complex ions is attributed to the 1:1 Li-G3 ratio in bulk SILs and the fact that G3 molecules readily adapt to the interfacial environment by aligning themselves with the surface to ensure good solvation of the interfacial Li⁺ ions. Nevertheless, the interfacial Li⁺ ions also display changes of solvation from that in bulk SIL by deviating from the molecular plane formed by the oxygen atoms on G3 ligands as electrodes become more negatively charged. Using density functional theory along with natural bond orbital calculations, we examine the effects of such structural distortion on the properties of the complex cation. Both the frontier orbital energies of the complex cation and the donor-acceptor interactions between Li⁺ ions and G3 ligands are found to be dependent on the deviation of Li⁺ ions from the molecular plane of the G3 ligands, which suggests that the electrochemical reduction of Li⁺ ions should be facilitated by the structural distortion. These results bear important implications for the nanostructures and properties of SILs near electrified interfaces during actual operations of Li-ion batteries and serve to provide guidance toward the rational design of new SIL electrolytes.

On the coordination of Zn2+ ion in Tf2N− based ionic liquids: structural and dynamic properties depending on the nature of the organic cation

The Zn²⁺ coordination structure changes when the Zn(Tf₂N)₂ salt is dissolved in ionic liquids resulting in more favorable interactions among solvent cations and anions.