Constant Potential Molecular Dynamics Research Articles

Cation-Specific interfacial behavior in organic electrolytes for enhanced energy storage

Energy storage systems such as electrical double-layer capacitors have achieved significant attention due to their high power density and cyclability, attained through charge accumulation at the electrode interfaces. Organic electrolytes are widely investigated for energy storage applications owing to their broad potential windows and high specific energy. In this study, we employed constant potential molecular dynamics simulations at various applied potential differences from 0.0 V to 4.0 V to investigate the interfacial structure between organic electrolytes and graphene electrodes. The electrolytes consisted of propylene carbonate (PC) as the solvent, various cations (Li+, Na+, and K+), and bis(trifluoromethanesulfonyl)imide (TFSI−) as the anion. We observed significant structural reorientation of the PC solvent near the electrode surface under high applied potentials, which altered the K+ layer position near the cathode surface and influenced the capacitive behavior of the system. Additionally, K+ ions exhibited a high desolvation rate, lower ion pair interactions, and faster diffusion capabilities compared to Li+ and Na+ ions. These insights are crucial for advancing the understanding and development of organic electrolyte-based metal ion energy storage systems.

Accounting for the Quantum Capacitance of Graphite in Constant Potential Molecular Dynamics Simulations.

Molecular dynamics (MD) simulations at a constant electric potential are an essential tool to study electrochemical processes, providing microscopic information on the structural, thermodynamic, and dynamical properties. Despite the numerous advances in the simulation of electrodes, they fail to accurately represent the electronic structure of materials such as graphite. In this work, a simple parameterization method that allows to tune the metallicity of the electrode based on a quantum chemistry calculation of the density of states (DOS) is introduced. As a first illustration, the interface between graphite electrodes and two different liquid electrolytes, an aqueous solution of NaCl and a pure ionic liquid, at different applied potentials are studied. It is shown that the simulations reproduce qualitatively the experimentally-measured capacitance; in particular, they yield a minimum of capacitance at the point of zero charge (PZC), which is due to the quantum capacitance (QC) contribution. An analysis of the structure of the adsorbed liquids allows to understand why the ionic liquid displays a lower capacitance despite its large ionic concentration. In addition to its relevance for the important class of carbonaceous electrodes, this method can be applied to any electrode materials (e.g. 2D materials, conducting polymers, etc), thus enabling molecular simulation studies of complex electrochemical devices in thefuture.

Modeling of Nanomaterials for Supercapacitors: Beyond Carbon Electrodes.

Capacitive storage devices allow for fast charge and discharge cycles, making them the perfect complements to batteries for high power applications. Many materials display interesting capacitive properties when they are put in contact with ionic solutions despite their very different structures and (surface) reactivity. Among them, nanocarbons are the most important for practical applications, but many nanomaterials have recently emerged, such as conductive metal-organic frameworks, 2D materials, and a wide variety of metal oxides. These heterogeneous and complex electrode materials are difficult to model with conventional approaches. However, the development of computational methods, the incorporation of machine learning techniques, and the increasing power in high performance computing now allow us to tackle these types of systems. In this Review, we summarize the current efforts in this direction. We show that depending on the nature of the materials and of the charging mechanisms, different methods, or combinations of them, can provide desirable atomic-scale insight on the interactions at play. We mainly focus on two important aspects: (i) the study of ion adsorption in complex nanoporous materials, which require the extension of constant potential molecular dynamics to multicomponent systems, and (ii) the characterization of Faradaic processes in pseudocapacitors, that involves the use of electronic structure-based methods. We also discuss how recently developed simulation methods will allow bridges to be made between double-layer capacitors and pseudocapacitors for future high power electricity storage devices.

Induced charges in a Thomas–Fermi metal: insights from molecular simulations

We study the charge induced in a Thomas–Fermi metal by an ion in vacuum, using an atomistic description employed in constant-potential molecular dynamics simulations, and compare the results with the predictions from continuum electrostatics. Specifically, we investigate the effects of the Thomas–Fermi screening length l TF and the position d of the ion with respect to the surface on the induced charge distribution in a graphite electrode. The continuum predictions capture most of the features observed with the atomistic description (except the oscillations due to the atomic sites of the graphite lattice), provided that d and l TF are larger than the inter-atomic distances within the electrode. At large radial distance from the ion, the finite l TF case can be well approximated by the solution for a perfect metal using an effective distance d + l TF . This requires a careful definition of the effective interface between the metal and vacuum for the continuum description. Our atomistic results support in particular an early analytical prediction (Vorotyntsev and Kornyshev, Sov. J. Exp. Theor. Phys. 51, 509 (Mar., 1980)) for a single charge at the interface between a Thomas–Fermi metal and a polarisable medium, which remains to be tested in atomistic simulations with an explicit solvent.

Impedance of nanocapacitors from molecular simulations to understand the dynamics of confined electrolytes

Nanoelectrochemical devices have become a promising candidate technology across various applications, including sensing and energy storage, and provide new platforms for studying fundamental properties of electrode/electrolyte interfaces. In this work, we employ constant-potential molecular dynamics simulations to investigate the impedance of gold-aqueous electrolyte nanocapacitors, exploiting a recently introduced fluctuation-dissipation relation. In particular, we relate the frequency-dependent impedance of these nanocapacitors to the complex conductivity of the bulk electrolyte in different regimes, and use this connection to design simple but accurate equivalent circuit models. We show that the electrode/electrolyte interfacial contribution is essentially capacitive and that the electrolyte response is bulk-like even when the interelectrode distance is only a few nanometers, provided that the latter is sufficiently large compared to the Debye screening length. We extensively compare our simulation results with spectroscopy experiments and predictions from analytical theories. In contrast to experiments, direct access in simulations to the ionic and solvent contributions to the polarization allows us to highlight their significant and persistent anticorrelation and to investigate the microscopic origin of the timescales observed in the impedance spectrum. This work opens avenues for the molecular interpretation of impedance measurements, and offers valuable contributions for future developments of accurate coarse-grained representations of confined electrolytes.

S.O.S: Shape, orientation, and size tune solvation in electrocatalysis.

Current models to understand the reactivity of metal/aqueous interfaces in electrochemistry, e.g., volcano plots, are based on the adsorption free energies of reactants and products, which are often small hydrophobic molecules (such as in CO2 and N2 reduction). Calculations played a major role in the quantification and comprehension of these free energies in terms of the interactions that the reactive species form with the surface. However, solvation free energies also come into play in two ways: (i) by modulating the adsorption free energy together with solute-surface interactions, as the solute has to penetrate the water adlayer in contact with the surface and get partially desolvated (which costs free energy); (ii) by regulating transport across the interface, i.e., the free energy profile from the bulk to the interface, which is strongly non-monotonic due to the unique nature of metal/aqueous interfaces. Here, we use constant potential molecular dynamics to study the solvation contributions, and we uncover huge effects of the shape and orientation (on top of the already known size effect) of small hydrophobic and amphiphilic solutes on their adsorption free energy. We propose a minimal theoretical model, the S.O.S. model, that accounts for size, orientation, and shape effects. These novel aspects are rationalized by recasting the concepts at the base of the Lum-Chandler-Weeks theory of hydrophobic solvation (for small solutes in the so-called volume-dominated regime) into a layer-by-layer form, where the properties of each interfacial region close to the metal are explicitly taken into account.

Probing the electric double layer structure at nitrogen-doped graphite electrodes by constant-potential molecular dynamics simulations

Electric double layer (EDL) is a critical topic in electrochemistry and largely determines the working performance of lithium batteries. However, atomic insights into the EDL structures on heteroatom-modified graphite anodes and EDL evolution with electrode potential are very lacking. Herein, a constant-potential molecular dynamics (CPMD) method is proposed to probe the EDL structure under working conditions, taking N-doped graphite electrodes and carbonate electrolytes as an example. An interface model was developed, incorporating the electrode potential and atom electronegativities. As a result, an insightful atomic scenario for the EDL structure under varied electrode potentials has been established, which unveils the important role of doping sites in regulating both the EDL structures and the following electrochemical reactions at the atomic level. Specifically, the negatively charged N atoms repel the anions and adsorb Li+ at high and low potentials, respectively. Such preferential adsorption suggests that N-doped graphite can promote Li+ desolvation and regulate the location of Li+ deposition. This CPMD method not only unveils the mysterious function of N-doping from the viewpoint of EDL at the atomic level but also applies to probe the interfacial structure on other complicated electrodes.

(Invited) Combining Constant Potential Molecular Dynamics and Experiments to Study Electrolyte Effects at Solid/Liquid Interfaces

Liquid electrolytes are at the heart of many devices in energy storage and conversion applications. Bringing an atomistic picture of the composition-properties relation is essential to understand how their reactivity can be tuned, both in the bulk and at interfaces, by variation of their composition.In this talk, I will present some recent works where we elucidated the role of the electrolyte, by using state-of-the-art constant potential classical Molecular Dynamics [1] in combination with experiments. I will first introduce some developments that we did for treating metallic surfaces within the constant potential approach, which allows computing interfacial properties, e.g. capacitance, in good agreement with experiments.[2] Then, I will show a study about the insertion process of Li+ into LiFePO4 and K+ into Prussian blue for Li-ion and K-ion batteries, in aqueous vs non-aqueous electrolytes. I will describe the molecular aspects of the electrode-electrolyte interface that dictate ion transport and insertion processes, which strongly depend on the nature of the electrolyte.[3]I will finally illustrate some exciting perspectives for the design of more complex electrolytes, where water and an organic solvent are mixed, to control the reactivity of water at a solid/liquid interface. This finds direct applications in practical devices such as Li-ion batteries, multivalent batteries, or even electrolyzers. Our strategy was to confine the reactive species, i.e. water, in a non-reactive organic matrix and different salts. Modifying the nature of the salt as well as the salt/water ratio can be used to fine tune the water reactivity at the interface by controlling its coordination environment at both short- and long-range.[4,5] References [1] A. Marin-Laflèche et al., J. Open Source Softw., 5, 2373, 2020.[2] A. Serva, L. Scalfi, B. Rotenberg, M. Salanne, J. Chem. Phys., 155, 044703, 2021.[3] P. Lemaire, A. Serva et al., ACS Appl. Mater. Interfaces, 14, 20835, 2022.[4] N. Dubouis, A. Serva et al., Nat. Catal., 3, 656, 2020.[5] A. Serva, N. Dubuois, A. Grimaud, M. Salanne, Acc. Chem. Res., 54, 1034, 2021.

Structural Evolution Governs Reversible Heat Generation in Electrical Double Layers.

Electrical double layer (EDL) formation determines the reversible heat generation of supercapacitors. While classical theories suggest an exothermic nature, experiments revealed that it can be endothermic, depending on the polarization and electrolyte. Here, we perform constant-potential molecular dynamics simulations and develop a lattice gas model to explore the reversible heat of EDL formation in aqueous and ionic liquid (IL) electrolytes. Our Letter reveals that EDL formation in aqueous electrolytes exhibits endothermicity under negative polarization; it shows new complexity of endothermicity followed by exothermicity in ILs, regardless of electrode polarity. These thermal behaviors are determined by the structural evolution during EDL formation, dominated by adsorbed solvent molecules rather than ions in aqueous electrolytes but governed by "demixing" and "vacancy occupation" phenomena in ILs. This Letter provides new insights into the reversible heat of supercapacitors and presents a theoretical approach to investigating thermal behaviors involving the dynamics of EDLs.

Constant chemical potential-quantum mechanical-molecular dynamics simulations of the graphene-electrolyte double layer.

We present the coupling of two frameworks-the pseudo-open boundary simulation method known as constant potential molecular dynamics simulations (CμMD), combined with quantum mechanics/molecular dynamics (QMMD) calculations-to describe the properties of graphene electrodes in contact with electrolytes. The resulting CμQMMD model was then applied to three ionic solutions (LiCl, NaCl, and KCl in water) at bulk solution concentrations ranging from 0.5 M to 6 M in contact with a charged graphene electrode. The new approach we are describing here provides a simulation protocol to control the concentration of electrolyte solutions while including the effects of a fully polarizable electrode surface. Thanks to this coupling, we are able to accurately model both the electrode and solution side of the double layer and provide a thorough analysis of the properties of electrolytes at charged interfaces, such as the screening ability of the electrolyte and the electrostatic potential profile. We also report the calculation of the integral electrochemical double layer capacitance in the whole range of concentrations analyzed for each ionic species, while the quantum mechanical simulations provide access to the differential and integral quantum capacitance. We highlight how subtle features, such as the adsorption of potassium graphene or the tendency of the ions to form clusters contribute to the ability of graphene to store charge, and suggest implications for desalination.

Constant-potential molecular dynamics simulation and its application in rechargeable batteries

Molecular dynamics reveals structures, properties, and reactions at battery interfaces on the atomic scale. Imposing a constant electrode potential in the simulation brings the model one step closer to physical reality and battery working conditions.

Constant-potential molecular dynamics simulations of molten salt double layers for FLiBe and FLiNaK.

We report the results of constant-potential molecular dynamics simulations of the double layer interface between molten 2LiF-BeF2 (FLiBe) and 23LiF-6NaF-21KF (FLiNaK) fluoride mixtures and idealized solid electrodes. Employing methods similar to those used in studies of chloride double layers, we compute the structure and differential capacitance of molten fluoride electric double layers as a function of applied voltage. The role of molten salt structure is probed through comparisons between FLiBe and FLiNaK, which serve as models for strong and weak associate-forming salts, respectively. In FLiBe, screening involves changes in Be-F-Be angles and alignment of the oligomers parallel to the electrode, while in FLiNaK, the electric field is screened mainly by rearrangement of individual ions, predominantly the polarizable potassium cation.

Regulating interfacial structure enables high-voltage dilute ether electrolytes

Poor oxidation stability of ether solvents at the cathode restricts the use of dilute ether electrolytes with conventional concentrations around 1 M in high-voltage, lithium-metal batteries. Here, we report an anion-adsorption approach to altering the ether solvent environment within the electrical double layer (EDL) at the cathode by adding a small amount of nitrate so that the oxidation tolerance of nitrate-containing dilute ether electrolytes is enhanced up to 4.4 V (versus Li/Li + ), leading to complete compatibility with high-voltage cathodes and exhibiting superior cycling stability. Constant potential molecular dynamics simulations reveal that ether molecules are mostly excluded from the cathode because of nitrate occupation in the inner layer of the EDL, thus suppressing ether oxidative decomposition. This work highlights that regulating the interfacial structure by adding surface adsorbates, rather than passivating the cathode-electrolyte interphase or changing ion solvation, can help to enhance the oxidation stability of ether solvents. • A high-voltage dilute ether electrolyte (at 1 M salt concentration) is developed • Molecular mechanism that surface adsorbates suppress solvent decomposition is revealed • A series of adsorption-type additives for high-voltage dilute ethers are proposed Wang et al. propose a simple and convenient approach to enhance the oxidation stability of dilute ether electrolytes (at 1 M salt concentration), by adding a small amount of nitrate (50 mM). This approach addresses a long-standing voltage limitation for dilute ether electrolytes.

Fully periodic, computationally efficient constant potential molecular dynamics simulations of ionic liquid supercapacitors.

Molecular dynamics (MD) simulations of complex electrochemical systems, such as ionic liquid supercapacitors, are increasingly including the constant potential method (CPM) to model conductive electrodes at a specified potential difference, but the inclusion of CPM can be computationally expensive. We demonstrate the computational savings available in CPM MD simulations of ionic liquid supercapacitors when the usual non-periodic slab geometry is replaced with fully periodic boundary conditions. We show how a doubled cell approach, previously used in non-CPM MD simulations of charged interfaces, can be used to enable fully periodic CPM MD simulations. Using either a doubled cell approach or a finite field approach previously reported by others, fully periodic CPM MD simulations produce comparable results to the traditional slab geometry simulations with a nearly double speedup in computational time. Indeed, these savings can offset the additional cost of the CPM algorithm, resulting in periodic CPM MD simulations that are computationally competitive with the non-periodic, fixed charge equivalent simulations for the ionic liquid supercapacitors studied here.

Origin of Enhanced Performance in Nanoporous Electrical Double Layer Capacitors: Insights on Micropore Structure and Electrolyte Composition from Molecular Simulations.

We explore the effect of solvation and micropore structure on the energy storage performance of electrical double layer capacitors using constant potential molecular dynamics simulations of realistically modeled nanoporous carbon electrodes and ionic liquid/organic solvent mixtures. We show that the time-dependent charging profiles of electrodes with larger pores reach the plateau regime faster, while the charging time has a nonmonotonic dependence on ion concentration, mirroring the composition dependence of bulk electrolyte conductivity. When the average pore size of the electrode is similar to or slightly larger than the size of a solvated ion, the solvation enhances ion electrosorption into nanopores by disrupting anion-cation coordination and decreasing the barrier to counterion penetration while blocking the co-ions. In these systems, areal capacitance exhibits a significant nonmonotonic dependence on ion concentration, in which capacitance increases with the introduction of solvent in the concentrated regime followed by a decrease with further dilution. This gives rise to a maximum in capacitance at intermediate dilution levels. When pores are significantly larger than solvated ions, capacitance maximum weakens and eventually disappears. These findings provide novel insights on the combined effect of electrolyte composition and electrode pore size on the charging kinetics and equilibrium behavior of realistically modeled electrical double layer capacitors. Generalization of the approach developed here can facilitate the rational optimization of material properties for electrical double layer capacitor applications.

Molecular insights into capacitive deionization mechanisms inside hydrophobic and hydrophilic carbon nanotube channel electrodes

Constant-potential molecular dynamics simulations were performed to evaluate the influence of wettability and pore diameter of carbon-nanotube electrodes on deionization mechanisms.

Capacitance response and concentration fluctuations close to ionic liquid-solvent demixing

We investigate the concentration fluctuations in a simple model of electrolyte (two positively and negatively charged Lennard-Jones spheres in a solution of an uncharged Lennard-Jones liquid) confined between electrodes formed by parallel graphene layers. Using constant potential molecular dynamics simulations and extensive constant charge simulations the effect of the proximity to the demixing transition on the electric double layers is analyzed and compared to the results of our continuum mean field theory. In agreement with our previous theoretical findings, we observe a considerable enhancement of the capacitance when temperature is lowered approaching the demixing transition for dilute ionic solutions. This enhancement is less visible when the ionic concentration is increased. Moreover, we observe the new shape of the capacitance, with a maximum at potential of zero charge, and symmetric maxima for positive and negative voltages.

Structure of cholinium glycinate biocompatible ionic liquid at graphite electrode interface.

We use constant potential molecular dynamics simulations to investigate the interfacial structure of the cholinium glycinate biocompatible ionic liquid (bio-IL) sandwiched between graphite electrodes with varying potential differences. Through number density profiles, we observe that the cation and anion densities oscillate up to ∼1.5 nm from the nearest electrode. The range of these oscillations does not change significantly with increasing electrode potential. However, the amplitudes of the cation (anion) density oscillations show a notable increase with increasing potential at the negative (positive) electrode. At higher potential differences, the bulkier N(CH3)3CH2 group of cholinium cations ([Ch]+) overcomes the steric barrier and comes closer to the negative electrode as compared to oxygen atom (O[Ch]+ ). We observe an increase in the interaction between O[Ch]+ and the positive electrode with a decrease in the distance between them on increasing the potential difference. We also observe hydrogen bonding between the hydroxyl group of [Ch]+ cations and oxygens of glycinate anions through the simulated tangential radial distribution function. Orientational order parameter analysis shows that the cation (anion) prefers to align parallel to the negative (positive) electrode at higher applied potential differences. Charge density profiles show a positive charge density peak near the positive electrode at all the potential differences because of the presence of partially positive charged hydrogen atoms of cations and anions. The differential capacitance (Cd) of the bio-IL shows two constant regimes, one for each electrode. The magnitude of these Cd values clearly suggests potential application of such bio-ILs as promising battery electrolytes.

Symmetrizing cathode-anode response to speed up charging of nanoporous supercapacitors

Asymmetric behaviors of capacitance and charging dynamics in the cathode and anode are general for nanoporous supercapacitors. Understanding this behavior is essential for the optimal design of supercapacitors. Herein, we perform constant-potential molecular dynamics simulations to reveal asymmetric features of porous supercapacitors and their effects on capacitance and charging dynamics. Our simulations show that, counterintuitively, charging dynamics can be fast in pores providing slow ion diffusion and vice versa. Unlike electrodes with single-size pores, multi-pore electrodes show overcharging and accelerated co-ion desorption, which can be attributed to the subtle interplay between the dynamics and charging mechanisms. We find that capacitance and charging dynamics correlate with how the ions respond to an applied cell voltage in the cathode and anode. We demonstrate that symmetrizing this response can help boost power density, which may find practical applications in supercapacitor optimization.

Molecular understanding of charge storage and charging dynamics in supercapacitors with MOF electrodes and ionic liquid electrolytes.

We performed constant-potential molecular dynamics simulations to analyse the double-layer structure and capacitive performance of supercapacitors composed of conductive metal-organic framework (MOF) electrodes and ionic liquids. The molecular modelling clarifies how ions transport and reside inside polarized porous MOFs, and then predicts the corresponding potential-dependent capacitance in characteristic shapes. The transmission line model was adopted to characterize the charging dynamics, which further allowed evaluation of the capacitive performance of this class of supercapacitors at the macroscale from the simulation-obtained data at the nanoscale. These 'computational microscopy' results were supported by macroscopic electrochemical measurements. Such a combined nanoscale-to-macroscale investigation demonstrates the potential of MOF supercapacitors for achieving unprecedentedly high volumetric energy and power densities. It gives molecular insights into preferred structures of MOFs for accomplishing consistent performance with optimal energy-power balance, providing a blueprint for future characterization and design of these new supercapacitor systems.