Salt Systems Research Articles

An efficient molten‐salt electro‐deoxidation strategy enabling fast‐kinetics and long‐life aluminum–selenium batteries

AbstractAluminum–selenium (Al–Se) batteries have been considered as one of the most promising energy storage systems owing to their high capacity, energy density, and cost effectiveness, but Se falls challenges in addressing the shuttle effect of soluble intermediate product and sluggish reaction kinetics in the solid–solid conversion process during cycling. Herein, we propose an unprecedented design concept for fabricating uniform Se/C hollow microspheres with controllable morphologies through low‐temperature electro‐deoxidation in neutral NaCl–AlCl3 molten salt system. Such Se/C hollow microspheres are demonstrated to hold a favorable hollow structure for hosting Se, which can not only suppress the dissolution of soluble intermediate products into the electrolyte, thereby maintaining the structural integrity and maximizing Se utilization of the active material, but also promote the electrical/ionic conductivity, thus facilitating the rapid reaction kinetics during cycling. Accordingly, the as‐prepared Se/C hollow microspheres exhibit a high reversible capacity of 720.1 mAh g−1 at 500 mA g−1. Even at the high current density of 1000 mA g−1, Se/C delivers a high discharge capacity of 564.0 mAh g−1, long‐term stability over 1100 cycles and high Coulombic efficiency of 98.6%. This present work provides valuable insights into short‐process recovery of advanced Se‐containing materials and value‐added utilization for energy storage.

Applications of Thermochemical Modeling in Molten Salt Reactors.

The extensively evaluated and consistent thermodynamic database, the Molten Salt Thermal Properties Database-Thermochemical (MSTDB-TC), was used along with additional thermodynamic values from other sources as examples of ways to examine molten salt reactor (MSR) fuel behavior. Relative stability with respect to halide potential and temperature for likely fuel and fission product components were mapped in Ellingham diagrams for the chloride and fluoride systems. The Ellingham diagrams provide a rich, visual means for identifying halide-forming components in proposed fuel/solvent salt systems. Thermochemical models and values from MSTDB-TC and ancillary sources were used in global equilibrium calculations to provide compositions for a close analysis of the behavior of a possible Molten Chloride Salt Fast Reactor and a Molten Salt Reactor Experiment-type system at high burnup (100 GWd/t). The results illustrated the oxidative nature of burnup in MSRs and provided information about redox behavior and possible control.

Strong Oxidizing Molten Salts for Strengthening Structural Restoration Enabling Direct Regeneration of Spent Layered Cathode.

As a mainstream technology for recycling spent lithium-ion batteries, direct regeneration is rapidly developed due to its high efficiency and green characteristics. However, efficient reuse of spent LiNixCoyMn1- x - yO2 cathode is still a significant challenge, as the rock salt/spinel phase on the surface hinders the Li replenishment and phase transformation to the layered structure. In this work, the fundamental understanding of the repair mechanism is confirmed that the oxidizing atmosphere is the crucial factor that can greatly improve the rate and degree of phase restoration. Particularly, a ternary-component molten salt system (LiOH-Li2CO3-LiNO3) is proposed for direct regeneration of LiNi0.5Co0.2Mn0.3O2 (NCM523), which can in situ generate the strong oxidizing intermediate of superoxide radicals. Additionally, it shows a liquid-like reaction environment at a lower temperature to acceclerate the transport rate of superoxide-ions. Therefore, the synergistic effect of LiOH-Li2CO3-LiNO3 system can strengthen the full restoration of rock salt/spinel phases and achieve the complete Li-supplement. As anticipated, the regenerated NCM523 delivers a high cycling stability with a retention of 91.7% after 100 cycles, which is even competitive with the commercial NCM523. This strategy provides a facile approach for the complete recovery of layer structure cathode, demonstrating a unique perspective for the direct regeneration of spent lithium-ion batteries.

Experimental investigation and thermodynamic assessment of the BaCl2–CeCl3 system

The thermodynamic and thermo-physical properties of the molten salt system ▪ have been investigated using an experimental and modelling approach. This molten salt system includes a single intermediate compound ▪, whose structure has been investigated using X-ray and neutron diffraction. Furthermore, this system exhibits solubility of ▪ in ▪ at high temperatures up to a concentration of around 25% ▪ at 1060 K. Additionally, our measurements show solubility of ▪ in ▪ up to about 5% ▪ at 973 K. The investigation of these solid solutions has been performed using quenching experiments and subsequent post-characterisation by X-ray diffraction (XRD). Phase diagram equilibria have also been investigated using differential scanning calorimetry (DSC). Using the aforementioned information on phase transitions, intermediate compound formation, and mutual solid solubility, a thermodynamic assessment of the system has been performed using the CALPHAD method. The model for the Gibbs energy of the liquid solution is the quasi-chemical formalism in the quadruplet approximation, while the model for the Gibbs energy of the solid solutions is a two-sublattice polynomial model.

Construction of three-dimensional porous graphene with N,S co-doping in KOH-K2CO3 eutectic active molten salt system for supercapacitors

We prepared a three-dimensional porous graphene (3DPG) with N or/and S doping (N-3DPG, S-3DPG, NS-3DPG) in the KOH-K2CO3 eutectic active molten salt system. Graphene oxide, urea, and 3-mercaptopropionic acid are adopted as the 3DPG, nitrogen, and sulfur precursors, respectively. As a result, a continuous 3D network with dense cross-linked architecture is gained because the KOH-K2CO3 medium works as template to accelerate the conversion of GO to 3D architecture. A specific capacitance of 521 F/g is obtained for the NS-3DPG, which is much larger than that of 3DPG, N-3DPG, S-3DPG. A symmetric supercapacitor NS-3DPG||NS-3DPG delivers an energy density of 21.7 Wh/kg at a power density of 300 W/kg, and displays the capacitance retention ratio of 98.0 % after 10,000 cycles. An asymmetric supercapacitor 3DPG||NS-3DPG is assembled, which shows an energy density as high as 33.6 Wh/kg at a power density of 400 W/kg and little capacitance fading for 10,000 cycles. Outstanding supercapacitor performances of the 3DPG||NS-3DPG are attributed to its excellent 3D hierarchical porous structure and the synergistic effect of N,S co-doping.

Molecular dynamics simulation of microstructure and thermophysical properties of LiCl–CaCl2eutectic molten salt

AbstractChloride molten salt systems are widely used as electrolytes for molten salt electrolysis because of their relatively low eutectic temperatures and good thermal stability, but there is a serious lack of data on the thermophysical properties of chloride molten salts at elevated temperatures, whereas the nature of the electrolyte is very important for the electrolysis process. In this paper, the variation of the microstructure and thermophysical properties of the binary mixed LiCl–CaCl2molten salt system with temperature and composition is calculated using molecular dynamics (MD) simulations based on the BMH potential. The microscopic conformations observed in LiCl–CaCl2molten salts are mainly irregular, distorted tetrahedra and octahedra, which dynamically coexist, as analyzed by the radial distribution function, coordination number and angular distribution function. In addition, the effects of temperature and composition on the density, ionic self-diffusion coefficient, shear viscosity, and ionic conductivity of the molten salts were investigated, and the relationships of the thermophysical properties of LiCl–CaCl2molten salts with temperature and composition were obtained, which provide fundamental thermophysical data for the molten salt electrolytes.

First-principles molecular dynamics study on the behaviors of Cs in a mixed system of liquid metal and LiCl-KCl molten salt.

The structure and dynamic properties of Cs in the mixed system of LiCl-KCl molten salt and a liquid metal (Bi and Pb) electrode are investigated through first-principles molecular dynamics simulation. It is found that the dynamic properties of different ions in molten salt could be significantly affected when the liquid metal electrode is coupled and this influence varies with the type of liquid metal applied. The microstructures of the mixed systems of molten salt and liquid metal electrode: MS-Cs-Bi, MS-CsCl-Bi, MS-Cs-Pb, and MS-CsCl-Pb, are also investigated by the bond angle distribution function, Voronoi tessellation analysis, five-fold symmetry parameter, and bond-orientational order parameter. The comparison study of the microstructures of the mixed systems when different liquid metal electrodes are applied provides information on the liquid metal electrode selection when conducting electrolysis. The present study represents the first demonstration of the study of a mixed system of salt and liquid electrode for practical applications and would be greatly beneficial to the development of pyroprocessing of spent nuclear fuel.

Study on the characteristics of molten salt heat storage in the fast peak regulation of coal-fired power unit

The load variation rate of the coal-fired power unit in China is generally around 2%, and the new technology is needed to further improve the load variation rate and to increase the peak regulation benefits. In this paper, the molten salt is utilized to constructed the ?Carnot battery? based on the coal-fired power unit, in order to increase the load variation rate of the coal-fire power units, and the regulatory characteristics of the molten salt system during load variation processes and the coupling characteristics with the boiler and turbine are investigated. The results indicate that, the load variation rate in the 20-100% load can be improved to 6% Pe per minute for a 300 MW coal-fired power unit when molten salt system with power of 301 MW and thermal storage duration of 83.6 MWh is coupled, in which the maximum molten salt temperature is, and the steam produced by molten salt is added into the low pressure cylinder of turbine. The average power generation efficiency of the ?Carnot battery? constructed by the boiler and molten salt during the variable load regulation process is 38.72%, and the average power generation efficiency of the molten salt system is 24.57%. This study indicates that the small-scale molten salt systems can be used to improve the load variation rate of the coal-fired power units.

Effect of oxide impurities on the dissolution behavior of Th4+, Be2+ and U4+ in fluoride salts.

Oxides are one of the most important impurities in the fuel salt of molten salt reactors (MSRs), and excessive oxide impurities pose a risk to the safe operation of MSRs. This study focused on investigating the precipitation behavior between Th4+, U4+, and Be2+ with O2- in the 2LiF-BeF2 (FLiBe) eutectic salt system. The results showed that the solubility of UO2 was 5.52 × 10-3 mol kg-1, and the solubility product (Ksp) of UO2 was 6.14 × 10-7 mol3 kg-3 in FLiBe salt at 650 °C. It was also found that the O2- ion would firstly react with U4+ to form UO2, and then the excessive O2- would react with Be2+ to generate BeO in the FLiBe system. Despite conducting the solubility experiment of ThO2 and titration experiment of FLiBe-ThF4, the system failed to achieve the solubility and the Ksp of ThO2. The main reason for this was that O2- preferentially reacted with Be2+ over Th4+ to form precipitates, in other words, Be2+ exerted a protective effect against Th4+. Above all, this work experimentally demonstrated that in the FLiBe system, O2- preferentially combines with U4+ to form a precipitate, followed by Be2+, while Th4+ is relatively inert.

Solubility of Nd2O3 in LiF and LiF–NdF3 molten salts

An experimental device was developed with a diaphragm and NdOF was used instead of Nd2O3 to solve the problem of analysis errors. A more accurate method for measuring the dissolution of Nd2O3 in the LiF–NdF3 molten salt system was reported.

Synthesis of graphite-based lead composites and modification of their physicochemical and electrochemical properties.

The present work describes preparation of a graphite lead composite, its modification and the examination of basic physicochemical and electrochemical properties. Graphite lead composites are the products of reaction of lead chloride with flaky graphite performed in a molten salt system. The process was carried out at 450 °C for 96 hours. In the second stage, the obtained composites were subjected to thermal or chemical treatment in order to modify their physicochemical properties. The structure of the as prepared material has been examined by X-ray diffraction analysis. Transmission electron microscopy analysis (TEM) along with energy dispersive spectroscopy (EDS) have been used to determine the size as well as the distribution of Pb particles. To study the electrochemical properties of graphite-based lead composites, cyclic voltammetry and galvanostatic methods have been used. It has been proved that the thermally modified compound at 600 °C contains on its surface spherical particles of lead chloride and/or oxide with diameters varying from hundreds of nanometers to several micrometers. The acquired electrochemical results revealed that graphite/Pb composites exhibit good electrochemical activity towards the reversible reaction of Pb → Pb2+ oxidation. Charge associated with the reversible transformation of Pb to Pb2+ amounts to 15.72 C g-1 and 14.62 C g-1 for the original compound and the compound heated at 600 °C, respectively. It has been also proved that the highest level of structure modification of the composite is reached by its chemical treatment with hydrogen peroxide. However, the mentioned modification leads to the removal of the entire lead from the structure of the graphite matrix.

A biphase coupled cathode enables all-organic rocking-chair lithium ion batteries based on crystalline AB-stacked covalent triazine-based frameworks

Crystalline AB-stacked CTFs were synthesized in the AlCl3–NaCl–KCl ternary eutectic salt system, and a CTF-based biphase coupled cathode (BPCC) has been proposed and proved to show significantly enhanced electrochemical performance.

An artificial aluminum-tin alloy layer on aluminum metal anodes for ultra-stable rechargeable aluminum-ion batteries.

Rechargeable aluminum ion batteries (RAIBs) exhibit great potential for next-generation energy storage systems owing to the abundant resources, high theoretical volumetric capacity and light weight of the Al metal anode. However, the development of RAIBs based on Al metal anodes faces challenges such as dendrite formation, self-corrosion, and volume expansion at the anode/electrolyte interface, which needs the rational design of an aluminum anode for high-performance RAIBs. This work proposes a novel and low-cost strategy by utilizing an alloy electrodeposition method in a low-temperature molten salt system to fabricate an aluminum-tin (AlSn) alloy coating layer on copper foil as the anode for RAIBs, which successfully addresses the issues of dendrite formation and corrosion at the anode/electrolyte interface. The artificial AlSn alloy layer could enhance the active sites for metal Al homogeneous deposition and effectively retard the dendrite formation, which was verified by an in situ optical microscopy study. The symmetric AlSn@Cu cell demonstrates a low average overpotential of ∼38 mV at a current density of 0.5 mA cm-2 and a long-term lifespan of over 1100 h. Moreover, the AlSn@Cu//Mo6S8 full cells deliver a high capacity of 114.9 mA h g-1 at a current density of 100 mA g-1 and maintain ultra-stable cycling stability even over 1400 cycles with a ∼100% coulombic efficiency (CE) during the long-term charge/discharge processes. This facile alloy electrodeposition approach for designing high-performance Al-based anodes provides insights into the understanding of artificial interface chemistry on Al-based anodes and potentially accelerates the design of high-performance RAIBs.

Experimental investigation and thermodynamic modelling assessment of the AECl2-NdCl3 (AE = Sr, Ba) systems.

The thermodynamic and thermo-physical properties of the binary salt systems AECl2-NdCl3 (AE = Sr, Ba) have been investigated using an experimental and modelling approach. The binary salt systems both include a single intermediate salt, i.e. Sr9Nd5Cl33 and Ba3Nd2Cl12, respectively. The structure of these intermediates has been investigated with X-ray diffraction (XRD). Furthermore, these systems exhibit mutual solubility of NdCl3 in BaCl2 and SrCl2. The investigation of these solid solutions has been performed using quenching experiments and subsequent post-characterisation by XRD. Phase diagram equilibria have also been investigated using differential scanning calorimetry (DSC). Using the aforementioned information on phase transitions, intermediate compound formation, and solid solubility, thermodynamic assessment of the systems has been performed using the CALPHAD method. The model for the Gibbs energy of the liquid solution is the quasi-chemical formalism in the quadruplet approximation, while the model for the Gibbs energy of the solid solutions is the two-sublattice polynomial model.

Influence mechanism of interfacial organic matter and salt system on carbon dioxide hydrate nucleation in porous media

CO2 carbon sequestration in marine gas hydrate occurrence areas is a potential way to mitigate the continued growth of CO2 in the atmosphere. The organic matters of marine sediment can affect the phase equilibrium conditions of CO2 hydrate in salt system, and the organic salt system in pore medium has effect on the nucleation growth of CO2 hydrate. The organic matters on the surface of the sediment is mainly composed of lipids, proteins, carbohydrates, and unsaturated hydrocarbon organic compounds, and these polar functional groups weaken the inhibitory effect of salt ions on hydrates through coordination bonds and Ca ion complexation. The nucleation kinetics of CO2 hydrate under organic salt system in pore medium in marine environment is an important parameter determining the carbon sequestration of hydrate. In addition, experimental results show that clay surface organic matters (CSOM) can reduce the surface tension of pore water, and thus pore water migration occurs during hydrate growth and expansion. The basic background in our findings will bring a much better recognition of phase equilibrium conditions among the offshore marine sediment CO2 sequestration process and further assess the location of potential marine CO2 storage sites.

Coupled Computational Fluid Dynamics and Computational Thermodynamics Simulations for Chemically Reacting Flows with Phase Transformations: A Fluoride Volatility Process Example

This study presents the development of a novel computational capability for chemically reacting flows with phase transformation that can be applied to complex multi-component systems. This is done by coupling Computational Fluid Dynamic (CFD) using OpenFOAM and Computational Thermodynamics (CT) using Thermochimica and the Joint Research Centre Molten Salt Database (JRCMSD). To illustrate this capability, the Fluorite Volatility Process (FVP) is modelled because it is the first step to clean up irradiated fuel salt in the Molten Salt Fast Reactor design and needs to be better understood. Local chemical equilibrium is assumed, which means that chemical kinetics is solely governed by mass transport. FVP recovers nuclear fuel UF4(liq) by injection of fluorine gas (F2), forming UF6(gas). The vaporization rate of UF4 is investigated on four different molten salt systems, using different dynamic viscosities and temperatures. The vaporization rate was shown to be clearly dependent on both thermochemical and thermophysical properties. Higher viscosities promoted larger bubbles, which delayed the chemical reaction. Results were compared to already available experimental data, showing good agreement. The applicability of coupling CFD with Thermochimica showed to be promising and expands interpretations on multiphysics systems and will be later applied to more complex scenarios.

Pulse Electrodeposition in the Presence of Corrosion Reactions during the Off-Time

Pulse electrodeposition of metals and alloys offers one route to enhance or tune material properties. However, an often-neglected period during pulse plating is the off-time, which may also influence deposit properties. Traditionally it is considered a relatively unimportant, allowing, for example, relaxation of concentration gradients, desorption of additives, recrystallization and discharge of the double later capacitance. Over the past few decades, however, studies have shown that it is possible to have ‘internal current’ processes occurring in the off-time which can strongly influence the deposit properties and composition. For example, in the pulse electrodeposition of binary alloys from aqueous solutions, displacement (corrosion) reactions have been observed in the off-time that can significantly alter the alloy composition. This has also been observed in many aqueous and molten salt systems, and appears a very general phenomenon. More recently we have observed corrosion arising from a comproportionation reactions during the pulse plating of copper from a deep eutectic solvent (DES) and noted some of their effect on deposit properties.The main purpose of this paper is to examine some of the similarities and differences in off-time corrosion caused by displacement and comproportionation processes during pulse electrodeposition. As is shown in the figure, corrosion may occur due to the presence of oxygen, or due the presence of a noble material leading to displacement reactions, as well as the presence of a stable species of differing valency in solution leading to comproportionation reactions. These differing mechanisms can produce different effects on the deposit. We will discuss how these different mechanisms can be understood and the influence on the deposit and their characteristics. Figure 1

Synthesis of Ternary and Quaternary Group III-Arsenide Colloidal Quantum Dots via High-Temperature Cation Exchange in Molten Salts: The Importance of Molten Salt Speciation.

Colloidal semiconductor nanocrystals are an important class of materials which have many desirable optoelectronic properties. In their bulk phases, gallium- and aluminum-containing III-V materials such as GaAs, GaP, and Al1-xGaxAs represent some of the most technologically important semiconductors. However, their colloidal synthesis by traditional methods is difficult due to the high temperatures needed to crystallize these highly covalent materials and the extreme reactivity of Ga- and Al- precursors toward organic solvents at such high temperatures. A recently developed paradigm shift in the synthesis of these materials is to use molten inorganic salts as solvents to prepare Ga- containing III-V colloidal nanocrystals by cation exchange of the corresponding indium pnictide (InPn) colloidal nanocrystals. There have been several successful applications of molten salt solvents to prepare III-phosphide colloidal nanocrystals. However, little is known about the nature of these reaction environments at the relevant reaction conditions and synthesis of III-arsenide colloidal nanocrystals remains challenging. Herein we report a detailed study on cation exchange of InPn nanocrystals using nominally Lewis basic molten salt solvents with added gallium halides. Surprisingly, these salt systems phase separate into two immiscible phases, and the nanocrystals preferentially segregate to one of the phases. Using a suite of in situ spectroscopy tools, we identify the phase the nanocrystals segregate to as Lewis neutral alkali tetrahalogallate molten salts. We apply in situ high-temperature Raman spectroscopy to identify the chemical species present in several molten salt compositions at experimentally relevant reaction conditions to elucidate a molecular basis for the reactivity observed. We then employ Lewis neutral KGaI4 molten salts to prepare high-quality In1-xGaxAs and In1-xGaxP nanocrystals and demonstrate that deviation from Lewis neutral conditions accelerate nanocrystal decomposition in the case of III-arsenide materials. Further, we expand to KAlI4-based molten salts to prepare In1-x-yGaxAlyAs nanocrystals which represent an example of solution-synthesized quaternary III-V nanocrystals. These insights provide a molecular basis for the rational development of molten salt solvents, thus allowing the preparation of a diverse array of multicomponent III-V colloidal nanocrystals.

Effect of the template particles size on structure and piezoelectric properties of 〈0 0 1〉‐textured BNT‐based ceramics

AbstractThis research investigated the effect of Bi4Ti3O12 (BiT) template particles size on the structure and piezoelectric properties of 〈001〉‐textured (Bi0.5Na0.5)TiO3 (BNT)‐based lead‐free piezoelectric ceramics. Results of X‐ray diffractometer analyses and scanning electron microscopy observations showed that high purity BiT template particles could be obtained with the KCl as the molten salt system. Highly 〈001〉‐textured (~87%) BNT‐based ceramics prepared with large size BiT templates (7.75 µm) exhibited a high inverse piezoelectric constant d33* (1100 pm/N), which was ∼3.38‐fold larger than that with 1.91 µm (325 pm/N). In addition, we have found that too small the template size can make it difficult to align the orientation of the template during the casting process, whereas too large the size can create problems such as too much hysteresis (∆S/Smax = 67%). This study has demonstrated that the use of high purity large size BiT template particles could provide a feasible route to fabricate BNT‐based textured ceramics with desired electro‐strain properties.

Rare earth removal from pyroprocessing fuel product for preparing MSR fuel

A series of experiments were performed to produce a fuel source for a molten salt reactor (MSR) through pyroprocessing technology. A simulated LiCl–KCl–UCl3-NdCl3 salt system was prepared, and the U element was fully recovered using a liquid cadmium cathode (LCC) by applying a constant current. As a result, the salt was purified with an UCl3 concentration lower than 100 ppm. Subsequently, the U/RE ingot was prepared by melting U and RE metals in Y2O3 crucible at 1473 K as a surrogate for RE-rich ingot product from pyroprocessing. The produced ingot was sliced and used as a working electrode in LiCl–KCl–LaCl3 salt. Only RE elements were then anodically dissolved by applying potential at −1.7 V versus Ag/AgCl reference electrode. The RE-removed ingot product was used to produce UCl3 via the reaction with NH4Cl in a sealed reactor.