Ion Correlations Research Articles

Nonlinear conductivity of aqueous electrolytes: Beyond the first Wien effect.

The conductivity of strong electrolytes increases under high electric fields, a nonlinear response known as the first Wien effect. Here, using molecular dynamics simulations, we show that this increase is almost suppressed in moderately concentrated aqueous electrolytes due to the alignment of the water molecules by the electric field. As a consequence of this alignment, the permittivity of water decreases and becomes anisotropic, an effect that can be measured in simulations and reproduced by a model of water molecules as dipoles. We incorporate the resulting anisotropic interactions between the ions into a stochastic density field theory and calculate ionic correlations as well as corrections to the Nernst-Einstein conductivity, which are in qualitative agreement with the numerical simulations.

Intercalating Architecture for the Design of Charge Density Wave in Metallic MA2Z4 Materials.

We present a novel approach to induce charge density waves (CDWs) in metallic MA2Z4 materials, resembling the behavior observed in transition metal dichalcogenides (TMDCs). This method leverages the intercalating architecture to maintain the same crystal field and Fermi surface topologies. Our investigation reveals that CDW instability in these materials arises from electron-phonon coupling (EPC) between the d band and longitudinal acoustic (LA) phonons, mirroring TMDC's behavior. By combining α-MA2Z4 with 1H-MX2 materials in a predictive CDW phase diagram using critical EPC constants, we demonstrate the feasibility of extending CDW across material families with comparable crystal fields and reveal the crucial role in CDW instability of the competition between ionic charge transfer and electron correlation. We further uncover a strain-induced Mott transition in β2-NbGe2N4 monolayer featuring star-of-David patterns. This work highlights the potential of intercalating architecture to engineer CDW materials, expanding our understanding of CDW instability and correlation physics.

Charge regulation of nanoparticles in the presence of multivalent electrolytes.

We explore the charge regulation (CR) of spherical nanoparticles immersed in an asymmetric electrolyte of a specified pH. Using a recently developed reactive canonical Monte Carlo (MC) simulation method, titration isotherms are obtained for suspensions containing monovalent, divalent, and trivalent coions. A theory based on the modified Poisson-Boltzmann approximation, which incorporates the electrostatic ion solvation free energy and discrete surface charge effects, is used to compare with the simulation results. A remarkably good agreement is found without any fitting parameters, both for the ion distributions and titration curves, suggesting that ionic correlations between coions and hydronium ions at the nanoparticle surface play only a minor role in determining the association equilibrium between hydroniums and the functional sites on the nanoparticle surface. On the other hand, if suspension contains multivalent counterions, we observe a large deviation between theory and simulations, showing that the electrostatic correlations between counterions and hydronium ions at the nanoparticle surface are very significant and must be properly taken into account to correctly describe CR for such solutions.

A Gaussian field approach to the solvation of spherical ions in electrolyte solutions.

In this work, the electrostatic response of an electrolyte solution to a spherical ion is studied with a Gaussian field theory. In order to capture the ionic correlation effect in concentrated solutions, the bulk dielectric response function is described by a two-Yukawa response function. The modified response function of the solution is solved analytically in the spherical geometry, from which the induced charge density and the electrostatic energy are also derived analytically. Comparisons with results for small ions in electrolyte solutions from the hyper-netted chain theory demonstrate the validity of the Gaussian field theory.

Lithium Distribution and Site Disorder in Halide-Substituted Lithium Argyrodites: A Structural and Transport Study

Lithium argyrodite superionic conductor has recently gained significant attention as a potential solid electrolyte for all-solid-state batteries because of its high ionic conductivity and ease of processing. One promising aspect of these materials is the ability to introduce halide (Li6-xPS5-xY1+x, Y = Cl and Br ) into the crystal structure, which can greatly impact the lithium distribution over the wide range of accessible sites and structural site-disorder between the S2₋ and Y₋ anion on Wyckoff 4d site, strongly influences the ionic conductivity. However, the relationship between halide substitution, structural site-disorder, and lithium distribution is not fully understood. In this study, we investigate the effect of halide substitution on lithium argyrodite and engineer site-disorder by changing the synthesis protocol. We reveal the lithium substructure and ionic transport correlations using neutron diffraction, solid-state NMR, and electrochemical impedance spectroscopy, We find that higher ionic conductivity is correlated with a negative charge on the 4d site, as replacing the S2− with Br− leads to a lowered average charge on the 4d site and weaker interactions within the Li+ “cage”, promoting a migration pathway for Li+ ions across the Li+ cage. We also identify a new T4 Li+ site, which enables an alternative jump route (T5–T4–T5) with a lower migration energy barrier. The resulting expansion of Li+ cages and increased connections between cages leads to a maximum ionic conductivity of 8.55 mS cm-1 with higher site-disorder, an improvement of 11-fold compared to lower site-disorder. Overall, this work provides a deeper understanding of the structure-transport correlations in lithium argyrodite, specifically how site-disorder and halide substitution impact the lithium substructure and transport properties.

Power density vs ionic radii correlations in REBa2Cu3O6+ẟ (RE = La, Nd, Sm, Gd and Y) IT – SOFC potential cathodes

Power density vs ionic radii correlations in REBa2Cu3O6+ẟ (RE = La, Nd, Sm, Gd and Y) IT – SOFC potential cathodes

Identifying the hydrochemical features, driving factors, and associated human health risks of high-fluoride groundwater in a typical Yellow River floodplain, North China.

Fluoride enrichment (> 1.5mg/L) in groundwater has become a global threat, particularly given the hazards to human health. This study collected 58 unconfined groundwater samples from Fengpei Plain in June 2022 for hydrochemical and stable isotope analyses combined with multiple methods to explore sources, influencing factors, and potential health hazards of groundwater F-. The results showed that groundwater F- concentration ranged from 0.08 to 8.14mg/L, with an average of 1.91mg/L; over 41.4% of them exceeded the acceptable level of 1.5mg/L prescribed by the World Health Organization (WHO). The dominant hydrochemical facies changed from Ca·Mg-HCO3 and Ca·Mg-SO4·Cl type in low-F- groundwater to Na-HCO3 and Na-SO4·Cl water types in high-F- groundwater. The Self-Organizing Map (SOM) and ionic correlation analysis indicated that F- is positively correlated to pH, EC, Na+, K+, SO42-, and TDS, but negatively to Ca2+ and δ18O. Groundwater F- accumulation was primarily driven by F--bearing minerals dissolution such as fluorite. Simultaneously, the carbonates precipitation, positive cation exchange processes, and salt effect were conducive to groundwater F- enrichment. However, competitive adsorption between OH-/HCO3- and F-, evaporation, and anthropogenic activities only had a weak effect on the F- enrichment in groundwater. The hazard quotient (HQ) assessment results show that 67.2% of groundwater samples pose a non-carcinogenic risk (HQ > 1) for infants, followed by 53.4% for children, 32.8% for females, and 25.9% for males. The Monte Carlo simulation results agreed with those of the deterministic model that minors are more susceptible than adults. These findings are vital to providing insights into the geochemical behavior, driving factors, and drinking water safety of high-F- groundwater worldwide.

Mechanistic Insights into Ion Transport in Polymerized Ionic Liquids

Polymerized ionic liquids (polyILs) continue to be of great interest as electrolytes in energy conversion and storage devices. However, challenges in enhancing the ionic conductivity to ~10-3 S/cm still hinder the application of these materials. Therefore, it is urgent to improve the conductivity in these electrolytes, which strongly relies on advances in molecular-level understanding of ion transport mechanisms governing charge transport. To this end, ionic correlations and the mechanisms of ion transport in the polyILs and their corresponding ILs have been systematically studied by altering the molecular chemistry of the cation and the anion using classical molecular dynamics simulations.We first hypothesized that immobilization of one of the ions in the polymer may lead to a significant increase in negative correlations in the dynamics of the mobile ions. To verify this hypothesis, we used a polymethylmethacrylate (PMMA) based backbone with attached imidazolium cations and mobile anions, including TFSI−, PF6 −, BF4 −, and Br−. The increase in temperature enhances both intrachain and interchain ion hopping while reducing the rattling of mobile ions. The analysis of ionic correlations in both ILs and polyILs systems at various temperatures revealed that the inverse Heaven ratio was not temperature dependent in either of the systems. We further investigated the effect of side-chain flexibility of polyILs on ionic correlations by changing the side-chain length. The higher flexibility enhances local mobility of the imidazolium cations, leading to an increasing number of associating cations from more polymer chains to the anions. Another key factor in tuning ionic correlations in concentrated ionic systems is electrostatic interactions, whose role was investigated by changing the dielectric constant of the simulated systems. We believe these molecular-level insights will advance the rational design of novel polyILs for electrochemical applications.

Temporal response of the conductivity of electrolytes

We study the temporal response of the electric current in an electrolyte under a sudden switch on or switch off of an external electric field of arbitrary magnitude. We use Stochastic Density Functional Theory including hydrodynamic interactions to express the current as a function of the ionic correlations. Assuming small density fluctuations, we linearize the field theory to compute the correlations in the transient regime. We first show that the correlations do not follow the same trajectory when the field is switched on or switched off. Accordingly, the behavior of the current differs in the two cases: it decays exponentially when the field is switched off, but it relaxes algebraically to its stationary value when the field is switched on. This difference is a non-linear effect since an exponential relaxation is recovered in both cases in the weak field limit.

Using stable isotopes (δ2H and δ18O) and hydrochemistry to understand the genesis and hydrochemical processes of groundwater in Chongming Island, Yangtze Estuary.

Groundwater is an indispensable freshwater resource and its quality is significant in supporting sustainable social and economic development, particularly in estuarine islands where aquifers are complicated. In this study, a total of 19 groundwater and 4 surface water samples were collected in September 2022 to identify the origin and hydrogeochemical evolution processes of groundwater using stable isotopes and hydrochemistry in Chongming Island, which is the largest estuarine alluvial island in the world. The stable isotopic composition indicated that shallow groundwater and surface water are all derived from precipitation recharge under a humid climate, and the evaporative effect incurs the enrichment of isotopic compositions. The shallow groundwater and surface water were primarily of Ca-HCO3 type. Gibbs diagram, ionic correlation analysis, ionic ratios analysis, and mineral saturation indices suggested that water-rock interactions like carbonate and silicate weathering play a vital role in groundwater chemistry, but cation exchange reactions are weak. Revelle index (RI) result indicated that 10.5% of shallow groundwater samples were found to suffer seawater intrusion. The NO3- concentrations were between l2.0 and 180.8mg/L with 31.6% of groundwater samples exceeding the World health organization (WHO) standards (50mg/L). Agricultural activities and industrial activities were found to be mainly responsible for groundwater pollution in shallow groundwater. The findings of this study provide a scientific basis for better managing groundwater resources on coastal estuarine islands.

Molecular-Weight Dependence of Center-of-Mass Chain Diffusion in Polymerized Ionic Liquid Melts

Polymerized ionic liquids (PILs) with flexible polymer chains and weakly interacting ionic liquid (IL) groups have received great attention for their desirable properties in electrochemical applications such as ionic conductivity. Less is known about their dynamic properties such as center-of-mass chain diffusion and how it depends on molecular weight in the presence of IL groups. In this work, a series of acrylic PILs with imidazolium cations and bis(trifluoromethanesulfonyl)imide (TFSI) anions (TFSI-f-PILN) were synthesized via reversible addition–fragmentation chain-transfer polymerization with degrees of polymerization N ranging from 40 to 236. A fluorescent acrylic monomer with the 7-nitrobenzofurazan group was copolymerized at trace levels as a probe of chain motion, and the diffusion coefficient (D) of TFSI-f-PILN was determined by fluorescence recovery after photo bleaching at Tg + 45 K. Within the uncertainty of 3–20%, a scaling relationship of D ∼ N–2 was observed which is the same as the scaling of linear neutral polymers. Wide-angle X-ray scattering exhibited no peak at ∼5 nm–1, indicating no long-range imidazolium-TFSI ionic correlations. Our results indicate that the molecular weight dependence of center-of-mass diffusion is not affected by electrostatic interactions of IL groups. No transition from a Rouse regime (D ∼ N–1) to reptation regime (D ∼ N–2) was observed within the studied N range.

Search for a Grotthuss mechanism through the observation of proton transfer

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between ‘chains’ of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5–0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

Resurrection of Hückel’s idea: Decoupling ion–ion and ion–water terms in activity coefficients via the state-dependent dielectric constant

Although Hückel proposed the basic idea of using a concentration dependent dielectric constant, ϵ(c), to compute the activity coefficients of ions in electrolytes in 1925 (Hückel, 1925), a large amount of modeling studies appeared in the literature only after 2010 when we published our II+IW theory (Vincze et al., 2010) that splits the excess chemical potential into two terms corresponding to interactions between ions (II) and interactions between ions and water (IW). In this approach, the two terms are decoupled, which means that they can be computed independently with ϵ(c) being the only link between them. Here, we review our theory and other works based on Hückel’s suggestion by discussing several issues that are partly cornerstones of the theory, partly make it possible to put the theory into a larger context. These issues include the role of ϵ(c) and the ionic radii used in the II and IW term, the statistical mechanical methods to estimate the II term, the existence and interpretation of individual activities, phenomena associated with strong ionic correlations in multivalent electrolytes, and explicit-water models used in molecular dynamics simulations.

Nonequilibrium molecular dynamics for accelerated computation of ion–ion correlated conductivity beyond Nernst–Einstein limitation

Condensed matters with high ionic conductivities are crucial in various solid devices such as solid-state batteries. The conduction is characterized by the cooperative ionic motion associated with the high carrier density. However, the high cost of computing correlated ionic conductivities has forced almost all ab initio molecular dynamics (MD) to rely on the Nernst–Einstein dilute-solution approximation, which ignores the cross-correlation effect. Here we develop a chemical color-diffusion nonequilibrium MD (CCD-NEMD) method, which enables to calculate the correlated conductivities with fewer sampling steps than the conventional MD. This CCD-NEMD is demonstrated to well evaluate the conductivities in the representative solid electrolyte bulk Li10GeP2S12 and Li7La3Zr2O12. We also applied CCD-NEMD to the grain boundary of Li7La3Zr2O12 and demonstrated its applicability for calculating interfacial local conductivities, which is essential for investigating grain boundaries and composite electrolytes. CCD-NEMD can provide further accurate understanding of ionics with ionic correlations and promote developing solid devices.

Electrostatics of soft (bio)interfaces: Corrections of mean-field Poisson-Boltzmann theory for ion size, dielectric decrement and ion-ion correlations

HypothesisElectrostatics of soft (ion-permeable) (bio)particles (e.g. microorganisms, core/shell colloids) in aqueous electrolytes is commonly formulated by the mean-field Poisson-Boltzmann theory and integration of the charge contributions from electrolyte ions and soft material. However, the effects connected to the size of the electrolyte ions and that of the structural charges carried by the particle, to dielectric decrement and ion-ion correlations on soft interface electrostatics have been so far considered at the margin, despite the limits of the Gouy theory for condensed and/or multivalent electrolytes. ExperimentsAccordingly, we modify herein the Poisson-Boltzmann theory for core/shell (bio)interfaces to include the aforementioned molecular effects considered separately or concomitantly. The formalism is applicable for poorly to highly charged particles in the thin electric double layer regime and to unsymmetrical multivalent electrolytes. FindingsComputational examples of practical interests are discussed with emphasis on how each considered molecular effect or combination thereof affects the interfacial potential distribution depending on size and valence of cations and anions, size of particle charges, length scale of ionic correlations and shell-to-Debye layer thickness ratio. The origins of here-evidenced pseudo-harmonic potential profile and ion size-dependent screening of core/shell particle charges are detailed. In addition, the existence and magnitude of the Donnan potential when reached in the shell layer are shown to depend on the excluded volumes of the electrolyte ions.

A Maxwell–Ampère Nernst–Planck Framework for Modeling Charge Dynamics

Understanding the properties of charge dynamics is crucial to many practical applications, such as electrochemical energy devices and transmembrane ion channels. This work proposes a Maxwell–Ampère Nernst–Planck (MANP) framework for the description of charge dynamics. The MANP model with a curl-free condition on the electric displacement is shown to be energy dissipative with respect to a convex free-energy functional, and demonstrated to be equivalent to the Poisson–Nernst–Planck model. By the energy dissipation law, the steady state of the MANP model reproduces the charge conserving Poisson–Boltzmann (PB) theory, providing an alternative energy stable approach to study the PB theory. In order to achieve the curl-free condition, a companion local curl-free relaxation algorithm, which is shown to naturally preserve the discrete Gauss’s law and converge robustly with linear computational complexity, is developed for the MANP model. One of the main advantages of our development is that it can efficiently deal with space-dependent permittivity instead of solving the variable-coefficient Poisson’s equation. Many-body effects such as ionic steric effects and Coulomb correlations can be incorporated within the MANP framework to derive modified MANP models for problems in which the mean-field approximation fails. Numerical results on the charge dynamics with such beyond mean-field effects in inhomogeneous dielectric environments are presented to demonstrate the performance of the MANP models in the description of charge dynamics, illustrating that the proposed MANP model provides a general framework for modeling charge dynamics.

Charge Transport in Water-NaCl Electrolytes with Molecular Dynamics Simulations.

A systematic description of microscopic mechanisms is necessary to understand mass transport in solid and liquid electrolytes. From Molecular Dynamics (MD) simulations, transport properties can be computed and provide a detailed view of the molecular and ionic motions. In this work, ionic conductivity and transport numbers in electrolyte systems are computed from equilibrium and nonequilibrium MD simulations. Results from the two methods are compared with experimental results, and we discuss the significance of the frame of reference when determining and comparing transport numbers. Two ways of computing ionic conductivity from equilibrium simulations are presented: the Nernst-Einstein approximation or the Onsager coefficients. The Onsager coefficients take ionic correlations into account and are found to be more suitable for concentrated electrolytes. Main features and differences between equilibrium and nonequilibrium simulations are discussed, and some potential anomalies and critical pitfalls of using nonequilibrium molecular dynamics to determine transport properties are highlighted.

Impact of Ionic Correlations on Selective Salt Transport in Ligand-Functionalized Polymer Membranes

We present a theoretical formalism quantifying the interplay between cation–ligand interaction strength and dynamical ionic correlations on salt diffusivities in the transport of single salt and binary (mixed) salts through ligand-functionalized polymer membranes. The Onsager framework is utilized to capture the effect of correlated ionic motions on salt diffusivities. Such a framework is implemented in the context of a coarse-grained model of systems involving ligand-selective (LS) and/or ligand-agnostic (LA) salts in ligand-functionalized polymer membranes. Our results indicate that ionic correlations speed up the diffusivities of both LS and LA salts relative to the ideal, uncorrelated case. At small values of cation–ligand interaction strength, such a speedup arises from a general trend of positively correlated motion among the mobile ions. On the other hand, ionic correlation-mediated speedup in salt diffusivities at large values of cation–ligand interaction strength can be attributed to the correlated hopping of the cations bound to the ligands in both single and mixed salt systems. At low cation–ligand interaction strengths, ionic correlations reduce the ratio of salt diffusivities of LS to LA salts (relative to the uncorrelated case) in single salt systems. Disparately, ionic correlations enhance the ratio of salt diffusivities of LS to LA salts (relative to the uncorrelated case) at large values of the cation–ligand interaction strengths in both single and mixed salt systems. We observe that single salt systems exhibit higher LS to LA salt diffusivity ratios than their mixed salt analogs up to a certain value of the cation–ligand interaction strength, beyond which the salt diffusivity ratios for the two systems converge.

Coupled ion transport in concentrated PEO–LiTFSI polymer electrolytes

Ionic correlations are important in concentrated PEO–LiTFSI polymer electrolytes. Onsager coefficients are necessary to characterize the ionic motion. Li and TFSI are anticorrelated, increasing the ionic conductivity and Li-ion transport number.

Surface polarization enhances ionic transport and correlations in electrolyte solutions nanoconfined by conductors.

Layered materials that perform mixed electron and ion transport are promising for energy harvesting, water desalination, and bioinspired functionalities. These functionalities depend on the interaction between ionic and electronic charges on the surface of materials. Here we investigate ion transport by an external electric field in an electrolyte solution confined in slit-like channels formed by two surfaces separated by distances that fit only a few water layers. We study different electrolyte solutions containing monovalent, divalent, and trivalent cations, and we consider walls made of non-polarizable surfaces and conductors. We show that considering the surface polarization of the confining surfaces can result in a significant increase in ionic conduction. The ionic conductivity is increased because the conductors' screening of electrostatic interactions enhances ionic correlations, leading to faster collective transport within the slit. While important, the change in water's dielectric constant in confinement is not enough to explain the enhancement of ion transport in polarizable slit-like channels.