First-principles molecular dynamics simulations on the local structure and thermo-kinetic properties of molten magnesium chloride

Diffusion Behaviors of CaCl2-NaCl Molten Salt under an Electric Field: A Deep Potential Molecular Dynamics Simulation

CaCl2-NaCl molten salt is one of the widely used electrolytes, and the effect of the electric field on it needs to be considered in the environment of manufacturing metals and alloys as well as in battery applications. To be closer to the state of the CaCl2-NaCl molten salt system in practical applications, this study uses the training-based deep potential, combined with the ionic structure, to analyze the electric field effects on the drift velocity, ionic mobility, ion diffusion, and viscosity of the CaCl2-NaCl system by molecular dynamics simulations. It is shown that the electric field has a stronger effect on the ion diffusion behavior parallel to the direction of the electric field in the CaCl2-NaCl molten salt system. Due to the looser coordinate structure of Na+, the interaction force between Na-Cl is small compared with that between Ca-Cl. Na+ is easily driven and more sensitive to the electric field, whose drift velocity, ionic mobility, and diffusion coefficient are larger than those of the other two ions, with an order of Na+ > Cl- > Ca2+. All ionic drift velocities increase linearly as electric field intensity increases and the ionic mobilities tend to be constant. The diffusion coefficient parallel to the direction of the electric field exponentially increases and the viscosity tends to exponentially decrease with the increase in the electric field intensity. This work is important for accurately predicting the properties and performance of molten salts and their improvement for applications such as solar thermal energy conversion.

Development of NaCl-MgCl2-CaCl2 Ternary Salt for High-Temperature Thermal Energy Storage Using Machine Learning

NaCl-MgCl2-CaCl2 eutectic ternary chloride salts are potential heat transfer and storage materials for high-temperature thermal energy storage. In this study, first-principles molecular dynamics simulation results were used as a data set to develop an interatomic potential for ternary chloride salts using a neural network machine learning method. Deep potential molecular dynamics (DPMD) simulations were performed to predict the microstructure and thermophysical properties of the NaCl-MgCl2-CaCl2 ternary salt. This work reveals that DPMD simulations can accurately calculate the microstructure and thermophysical properties of ternary chloride salts. The association strength of chloride ions and cations follows the order of Mg2+ > Ca2+ > Na+, and the coordination number decreases gradually with increasing temperature, indicating a progressively looser and more disordered molten structure. Furthermore, thermophysical properties, such as density, specific heat capacity, thermal conductivity, and viscosity, are in good agreement with the experimental measurements. Machine learning molecular dynamics will provide a feasible multivariate molten salt exploration method for the design of next-generation solar power plants and thermal energy storage systems.

Structure and Dynamics of NaCl/KCl/CaCl2-EuCln (n = 2, 3) Molten Salts

Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu²⁺ and Eu³⁺ ions in CaCl₂, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K⁺ to Na⁺ to Ca²⁺, the coordination number (CN) of Cl^- in the first solvation shell increases from 5.6 (Eu²⁺) and 5.9 (Eu³⁺) in KCl to 6.9 (Eu²⁺) and 7.0 (Eu³⁺) in CaCl₂. This coordination change is validated by the EXAFS measurements, in which the CN of Cl^- around Eu increases from 5 in KCl to 7 in CaCl₂. Our simulation shows that the fewer Cl^- ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu²⁺/Eu³⁺ are related to the rigidity of their first coordination shell of Cl^-: the more rigid the first coordination shell is, the slower the solute cations diffuse.

Computational simulation-driven discovery of novel zeolite-like carbon materials as seawater desalination membranes

Freshwater is a scarce and vulnerable resource that has never encountered such an extensive focus on a nearly worldwide scale as it does today. Recently, it has been found that desalination powered by two-dimensional (2D) carbon materials as separation membranes has significantly reduced the operational costs and complexity but presents heavy requirements for the structural stability and separation properties of the membrane materials. Here, we combined carbon materials with promising adsorption properties and zeolites characterized by a regular pore structure to obtain a zeolite-like structured carbon membrane Zeo-C and investigated the suitability of the Zeo-C membrane for seawater desalination based on the computational-simulation-driven approach. The results of molecular dynamics (MD) simulations and density functional theory (DFT) calculations revealed that the periodic pore distribution conferred favorable structural stability and mechanical strength to the Zeo-C desalination membrane. The rejection rate of Na⁺ and Cl^- is ensured at 100% under a pressure of 40-70 MPa, and that of Na⁺ could reach 97.85% even though the pressure increases to 80 MPa, exhibiting superior desalination properties. The porous nature of the zeolite-like structure and the low free energy potential barrier are conducive for reliable adsorption and homogeneous diffusion of salt ions, which facilitates the acquisition of desirable water molecule permeability and salt ion selectivity. In particular, the interlinked delocalized π-network imparts inherent metallicity to Zeo-C for self-cleaning in response to electrical stimulation, thereby extending the lifetime of the desalination membrane. These studies have greatly encouraged theoretical innovations and serve as a guiding reference for desalination materials.

First-principles molecular dynamics simulations of UCln-MgCl2 (n = 3, 4) molten salts

Molten chlorides are a preferred choice for fast-spectrum molten salt reactors. Molten MgCl₂ forms eutectic mixtures with NaCl and is considered as a promising dilutant to dissolve fuel salts such as UCl₃ and UCl₄. However, the structure and chemical properties of UCl_n (n = 3, 4) in molten MgCl₂ are not well understood. Here we use first-principles molecular dynamics to investigate the molten salt system UCl_n-MgCl₂ (n = 3, 4) at various concentrations of U³⁺ and U⁴⁺. It is found that the coordination environment of Cl^- around U³⁺, especially in the first coordination shell, varies only slightly with the uranium concentration and that both the 7-fold coordinate (UCl₇^4-) and 6-fold coordinate (UCl₆^3-) structures dominate at ∼40%, leading to an average coordination number of 6.6-6.7. A network or polymeric structure of U³⁺ cations sharing Cl^- ions is extensively formed when the mole fraction of UCl₃ is greater than 0.2. In contrast, the average coordination number of Cl^- around U⁴⁺ is about 6.4 for a mole fraction of UCl₄, x(UCl₄), of 0.1 but decreases to 6.0 for x(UCl₄) = 0.2 and then stays at about 6.0-6.2 with the uranium concentration. The 6-fold coordinate structure (UCl₆^2-) is the most populous in UCl₄-MgCl₂, at about 60%. U-Cl network formation becomes dominant (>50%) only when x(UCl₄) > 0.5. Unlike Na⁺, Mg²⁺ forms a network structure with Cl^- ions and when x(UCl₃) or x(UCl₄) < 0.5, over 90% of Mg²⁺ ions are part of a network structure, implying the complex influences from Mg²⁺ on the coordination of Cl around U. The present work reveals the impact of MgCl₂ as a solvent for UCl_n (n = 3, 4) on the U-Cl coordination and structure, and motivates further studies of their transport properties and the tertiary systems containing MgCl₂-UCl_n.

A Holistic Approach for Elucidating Local Structure, Dynamics, and Speciation in Molten Salts with High Structural Disorder

To examine ion solvation, exchange, and speciation for minority components in molten salts (MS) typically found as corrosion products, we propose a multimodal approach combining extended X-ray absorption fine structure (EXAFS) spectroscopy, optical spectroscopy, ab initio molecular dynamics (AIMD) simulations, and rate theory of ion exchange. Going beyond conventional EXAFS analysis, our method can accurately quantify populations of different coordination states of ions with highly disordered coordination environments via linear combination fitting of the EXAFS spectra of these coordination states computed from AIMD to the experimental EXAFS spectrum. In a case study of dilute Ni(II) dissolved in the ZnCl₂+KCl melts, our method reveals heterogeneous distributions of coordination states of Ni(II) that are sensitive to variations in temperature and melt composition. These results are fully explained by the difference in the chloride exchange dynamics at varied temperatures and melt compositions. This insight will enable a better understanding and control of ion solubility and transport in MS.

Coordination and Thermophysical Properties of Transition Metal Chlorocomplexes in LiCl-KCl Eutectic

Eutectic LiCl-KCl molten salt is often used in molten salt reactors as the primary coolant due to its high thermal capacity and high solubility of fission products. Thermophysical properties, such as density, heat capacity, and viscosity, are important parameters for engineering applications of molten salts but may be significantly influenced by metal solutes from corrosion of metallic structural materials. The behavior of the LiCl-KCl eutectic composition is well researched, yet the effects on these properties due to chlorocomplex formation from metals dissolved in the salt are less well known. These properties are often difficult to accurately measure from experimental methods due to the issues arising from the dissolved species, such as volatility. Here, we applied a combination of quantum mechanics molecular dynamics (QM-MD) and deep machine learning force field (DP-FF) molecular dynamics simulations to investigate the structural and thermophysical properties of LiCl-KCl eutectic as well as the influence of dissolved transition metal chlorocomplexes NiCl₂ and CrCl₃ at low concentrations. We find that the dissolution of Ni and Cr in the LiCl-KCl system forms the local tetrahedral (NiCl₄)^2- and octahedral (CrCl₆)^3- chlorocomplexes, respectively, which do not have a significant impact on the overall liquid salt structures. In addition, the thermodynamic properties including diffusion constant and specific heat capacity are not significantly affected by these chlorocomplexes. However, the viscosity significantly increases in the temperature range of 673-773 K. This study thus provides essential information for evaluating the effects of dissolved metals on the thermophysical and transport properties of molten salts.

Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Ab Initio Molecular Dynamics

In this work, we resolve a long-standing issue concerning the local structure of molten MgCl₂ by employing a multimodal approach, including X-ray scattering and Raman spectroscopy, along with the theoretical modeling of the experimental spectra based on ab initio molecular dynamics (AIMD) simulations utilizing several density functional theory (DFT) methods. We demonstrate the reliability of AIMD simulations in achieving excellent agreement between the experimental and simulated spectra for MgCl₂ and 50 mol % MgCl₂ + 50 mol % KCl, and ZnCl₂, thus allowing structural insights not directly available from experiment alone. A thorough computational analysis using five DFT methods provides a convergent view that octahedrally coordinated magnesium in pure MgCl₂ upon melting preferentially coordinates with five chloride anions to form distorted square pyramidal polyhedra that are connected via corners and to a lesser degree via edges. This is contrasted with the results for ZnCl₂, which does not change its tetrahedral coordination on melting. Although the five-coordinate MgCl₅^3- complex was not considered in the early literature, together with an increasing tendency to form a tetrahedrally coordinated complex with decreasing the MgCl₂ content in the mixture with alkali metal chloride systems, current work reconciles the results of most previous seemingly contradictory experimental studies.

Machine-Learning-Driven Simulations on Microstructure and Thermophysical Properties of MgCl2-KCl Eutectic

Theoretical studies on the MgCl₂-KCl eutectic heavily rely on ab initio calculations based on density functional theory (DFT). However, neither large-scale nor long-time calculations are feasible in the framework of the ab initio method, which makes it challenging to accurately predict some properties. To address this issue, a scheme based on ab initio calculation, deep neural networks, and machine learning is introduced. By training on high-quality data sets generated by ab initio calculations, a deep potential (DP) is constructed to describe the interaction between atoms. This work shows that the DP enables higher efficiency and similar accuracy relative to DFT. By performing molecular dynamics simulations with DP, the microstructure and thermophysical properties of the MgCl₂-KCl eutectic (32:68 mol %) are investigated. The structural evolution with temperature is analyzed through partial radial distribution functions, coordination numbers, angular distribution functions, and structural factors. Meanwhile, the estimated thermophysical properties are discussed, including density, thermal expansion coefficient, shear viscosity, self-diffusion coefficient, and specific heat capacity. It reveals that the Mg²⁺ ions in this system have a distorted tetrahedral geometry rather than an octahedral one (with vacancies). The microstructure of the MgCl₂-KCl eutectic shows the feature of medium-range order, and this feature will be enhanced at a higher temperature. All predicted thermophysical properties are in good agreement with the experimental results. The hydrodynamic radius determined from the shear viscosity and self-diffusion coefficient shows that the Mg²⁺ ions have a strong local structure and diffuse as if with an intact coordination shell. Overall, this work provides a thorough understanding of the microstructure and enriches the data of the thermophysical properties of the MgCl₂-KCl eutectic.