Local Structure and Dynamics in Solvent‐Free Molten Salt Ca2+‐Electrolytes

Here molten salt electrolytes (MSEs) and specifically their physico‐chemical properties as a function of composition are reported on, with a special emphasis on the compositional entropy and targeting calcium battery application. By using MSEs, several problematic issues associated with organic electrolytes, such as the blocking of Ca 2+ transfer at the electrode/electrolyte interfaces and electrolyte flammability, are avoided. Ca(FSI) 2 salt in combination with the analogous Li‐, Na‐, and KFSI salts are used in equimolar compositions to first create several ternary MSEs, melting at (ca.) 60–75 °C, a melting temperature which is further reduced to (ca.) 55 °C for the unique quaternary MSE. This is ascribed to an increased entropy of mixing, which also contributes to an improved stability (re−)crystallization, as shown by Raman spectroscopy. Furthermore, molecular dynamics simulations of the quaternary MSE alongside density functional theory calculations targeting the ion‐ion interactions are used to elucidate the local structure in more detail, demonstrating that both the ionic radii and valence influence the coordination and solvation of the cations. These MSEs are stepping‐stones towards completely solvent‐free, semi‐solid, and ideally room‐temperature Ca‐conducting electrolytes.

Rechargeable aluminum–sulfur (Al–S) batteries are considered promising candidates for next-generation energy storage owing to the natural abundance, intrinsic safety, low cost, and high gravimetric capacities of aluminum and sulfur (2980 mAh g⁻¹ and 1675 mAh g⁻¹, respectively). However, their practical deployment has been hindered by the scarcity of electrolytes that can simultaneously support reversible aluminum electrodeposition and sulfur electroreduction, while also minimizing polarization losses. Alkali chloroaluminate molten salts (AlCl₃–NaCl–KCl) were recently used as electrolytes for Al–S batteries instead of conventional AlCl₃–EMImCl (1-ethyl-3-methylimidazolium chloride) ionic liquids, demonstrating significantly reduced polarization at elevated temperature (e.g., 110 °C), though its origins are poorly understood.In this work, we investigate how electrolyte speciation, ion mobility, and soluble intermediates in these two electrolyte systems influence polarization losses in rechargeable Al–S batteries. Variable-temperature liquid-state 27Al, 23Na, and 1H single-pulse and relaxation nuclear magnetic resonance (NMR) measurements were conducted under quantitative conditions to probe electrolyte speciation and ion dynamics. 23Na and 1H pulsed-field-gradient (PFG) NMR experiments were also performed to quantify cation diffusion coefficients and their activation energies for diffusion. Additionally, integrated 27Al and 23Na NMR signal intensities of the molten salt electrolyte enabled accurate measurements of its freezing and melting temperature range, which coupled with changes in ion mobility, yielded insights into the optimum operating temperature. Differential scanning calorimetry (DSC) was also performed to measure these thermodynamic phase transitions and analyzed with respect to the NMR data. Solid-state 27Al and 23Na single-pulse NMR measurements of both pristine and cycled molten salt electrolytes provided mechanistic insights into the electrochemical conversion reaction, including the formation of electrolyte soluble polysulfide-like intermediates. In addition, we formulated quaternary electrolyte mixtures comprising AlCl₃, NaCl, KCl, and a fourth component (e.g., [EMIm]Cl, urea, or LiCl) with the objective of lowering the operating temperature and understanding the effects on cell polarization. Al-S batteries were prepared using the different electrolytes to compare their overpotentials, as well as specific capacities, cycle life, and cell impedance. Overall, the roles of electrolyte speciation, ion mobility and soluble intermediates in governing the overpotential in Al-S batteries were elucidated, providing new insights for the rational design of low cost, energy dense electrolytes for Al-S batteries.

The extraction and purification of metals such as aluminum has relied on the electrowinning process for decades. The Hall-Héroult process, developed in 1885, utilizes a molten salt electrolyte to electrochemically produce aluminum metal (Al) and carbon dioxide (CO2) from aluminum oxide (Al2O3) and carbon.1–3 Similar molten salt techniques have been developed for a variety of other metals including the rare earth metal Neodymium (Nd). Neodymium is of particular interest recently with significant increases in demand being driven by the increased production of new technologies such as electrified vehicles and magnetic data storage that require neodymium in the form of neodymium–iron–boron (Nd–Fe–B) permanent magnets. 4,5 The state of the art procedure for neodymium processing, similar to the aluminum process, utilizes a neodymium and lithium fluoride molten salt electrolyte with a sacrificial carbon anode to convert neodymium oxide (Nd2O3) and carbon to neodymium metal and carbon dioxide.4,6 As an unfortunate byproduct of this process performed in a fluoride containing molten salt, perfluorocarbons (PFCs) can also be produced simultaneously alongside carbon dioxide from the sacrificial anode. The formation of PFCs combined with the emission of a significant amount of greenhouse gas (carbon dioxide) make the current process for neodymium electrowinning undesirable from an environmental standpoint.7 An alternative molten salt process has been proposed in which the fluoride salts have been replaced with chloride salts consisting of lithium chloride (LiCl) and potassium chloride (KCl). Rather than directly converting neodymium oxide to neodymium metal, the oxide is first converted to chloride salt form by reaction with hydrochloric acid. The neodymium salt is then dissolved into the LiCl-KCl molten salt and neodymium is electroplated via the below set of reactions.8,9 Cathode: 2NdCl3 + 6e- → 2Nd(solid) + 6Cl- Anode: 6Cl- → 3Cl2 + 6e- Overall: 2NdCl3 → 2Nd(solid) + 3Cl2 This process has several distinct advantages. Utilizing the chlorine reaction eliminates the need for a sacrificial anode material as well as the production of carbon dioxide. The chloride based molten salt also eliminates the formation of PFCs. The chlorine produced could then be recycled to make more hydrochloric acid for use in converting neodymium oxide to chloride. Our work evaluates anode and cathode behavior during this neodymium chloride molten salt process in order to determine its viability. Overpotential and stability of various anode materials are investigated in order to minimize energy consumption and ensure long life of process materials. The effect of various plating conditions such as current density and substrate material are investigated to determine impact on deposit quality, coulombic efficiency, and metal purity. Additional purification techniques such as vapor distillation procedures are developed to ensure a product that is viable for industrial use. This proof of concept work aims to develop a safe, sustainable and environmentally friendly path towards large scale production of rare earth elements. Reactor design and cathode efficiency results are based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Advanced Manufacturing Office, Award Number DE-EE0009434. Anode design work was supported through a subcontract from the Ames Laboratory with funding from the Department of Energy - Energy Efficiency and Renewable Energy under contract No. DE-AC02-07CH11358; Agreement No. 26110-AMES-CMI. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. T. R. Beck, Electrochem. Soc. Interface, 23, 36–37 (2014).G. G. Botte, Electrochem. Soc. Interface, 23, 49–55 (2014).W. E. Haupin, J. Chem. Educ., 60, 279–282 (1983).M. F. Chambers and J. E. Murphy, Electrolytic production of neodymium metal from a molten chloride electrolyte.B. Sprecher, R. Kleijn, and G. J. Kramer, Environ. Sci. Technol., 48, 9506–9513 (2014).V. S. Cvetković et al., Met. 2020, Vol. 10, Page 576, 10, 576 (2020).H. Vogel, B. Friedrich, H. Vogel, and B. Friedrich, Int. J. Nonferrous Metall., 6, 27–46 (2017).R. Akolkar, J. Electrochem. Soc., 169, 043501 (2022).D. Shen and R. Akolkar, J. Electrochem. Soc., 164, H5292–H5298 (2017).

As the operating period of a nuclear power plant increases, the demand for dismantling of nuclear facilities has been rising, and the dismantling market is expected to continue to expand steadily. The Inconel 600, which was used as a material for the steam generator, has problem with crack and corrosion, and it is currently replaced with a steam generator of Inconel 690. It has been reported that if this steam generator is corroded or cracked, the cooling water contaminated with radioactive elements such as cobalt, strontium, may penetrate into the cracks. Therefore, development of decontamination technology for radioactive metal waste is essential to minimize waste during dismantling process.This study investigates the decontamination possibility of Inconel 600 in LiCl-KCl-CoCl2-NiCl2 molten salt electrolyte. Inconel 600 – cobalt alloy was used for anode as a simulated radioactive metal waste. After electrorefining process, the cobalt remains in the molten salt. Therefore, CoCl2 was added to the electrolyte to imitate the case when the radioactive metal waste was dissolved and concentrated in the electrolyte. Electrochemical analysis was performed by cyclic voltammetry (CV) to confirm the redox behavior of Ni ions in molten salt electrolytes. Decontamination experiments were performed by chrono potentiometry (CP) to analyze the oxidation behavior of Inconel 600 – cobalt alloy (anode) and the reduction behavior of Ni on Ni plate electrodes (cathode). Decontamination factor(DF) of the deposits produced by electrorefining was analyzed by ICP.

Electrochemical separation is one of the most promising methods for recycling used nuclear fuel, in which uranium is isolated from other fission products using a molten salt electrolyte1. The used fuel is immersed in molten salt electrolyte in an electrochemical cell referred to as an electrorefiner. In an ideal electrorefiner cell the used fuel is dissolved anodically, releasing uranium ions into a ternary chloride salt (LiCl-KCl-10wt%UCl3). The uranium ions are then reduced at an inert cathode, forming uranium metal for reuse in new uranium fuel pellets2. However, in actual electrorefiner systems, uranium is not the only element from the used fuel that is oxidized into the electrolyte. Alkali/alkaline-earth elements, such as cesium, barium, and strontium, are also oxidized because they have lower standard reduction potentials than uranium in the chloride system3. As alkali/alkaline-earth elements accumulate in the electrolyte, they change the physical and chemical properties of the electrolyte due to their short half-lives and high heat densities. The electrolyte is periodically removed from the electrorefiner to maintain the recycling process, thereby generating excessive volumes of nuclear waste. Lichtenstein et al. have demonstrated that alkali/alkaline-earth elements exhibit low thermodynamic activity (6.6 × 10-16 at x Ba = 0.05, 773 K) in liquid metals such as bismuth, implying strong atomic interactions4. These interactions cause a shift in the elements’ reduction potentials and can be leveraged to selectively remove the accumulated elements from the electrolyte by depositing them into liquid metal electrodes. To determine the viability of removing alkali/alkaline-earth elements using liquid metals such as lead, the thermodynamic properties of Sr-Pb alloys were determined by electromotive force (emf) measurements using a solid state electrolyte (binary CaF2-Sr­F­2, 97-3 mol%). Emf values were measured at 723 K-1073 K using a Sr(s)|CaF2-SrF­2|Sr(in Pb) electrochemical cell and were used to determine the thermodynamic properties of Sr-Pb, including activity, partial molar entropy, and partial molar enthalpy for x Sr = 0.05-0.75. At 973 K, activity values of Sr in Pb were as low as 5 × 10-9 at xSr = 0.05, implying strong atomic interactions between Sr and Pb. According to Okamoto et al., the liquid-state solubility of Sr in Pb is approximately 37 mol% at 973 K5. With the high solubility of Sr in Pb in the liquid state, combined with strong Sr-Pb atomic interactions, Pb has demonstrated promise for use as a liquid metal electrode material to remove Sr2+ ions from molten salt electrolytes. [1] Electrometallurgical Techniques for DOE Spent Fuel Treatment: Final Report, National Academy Press Washington, D.C, 2000.[2] Willit, J. L., Miller, W.E., Battles, J.E. Electrorefining of uranium and plutonium – A literature review, J. Nucl. Mater. 195 (3), 229-249 (1992). [3] Roine, A., Outokumpu HSC Chemistry® 5.1; Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database, (2002). [4] Lichtenstein, T., Smith, N.D., Gesualdi, J., Kumar, K., Kim, H. Thermodynamic Properties of Barium-Bismuth Alloys Determined by emf Measurements, Electrochim. Acta. 228, 628-635 (2017). [5] Okamoto, H. Sr-Pb (Strontium-Lead), 2nd ed., ASM International, Materials Park, (1990).

A potential working electrode based on graphite and montmorillonite for electrochemical applications in both aqueous and molten salt electrolytes

Over the last century, anthropogenic activity has led to a significant increase in the concentration of carbon dioxide in the Earth's atmosphere. Today, the annual increase in CO2 is 3200-3600 million tons. Carbon dioxide together with other greenhouse gases (water vapor H2O, methane CH4, nitrous oxide N2O, sulfur hexafluoride SF6, etc.) causes global warming, which leads to serious environmental degradation - increasing the acidity of sea water, melting glaciers, strengthening the water cycle, which in some regions leads to heavy rainfall and floods, and in others - to extreme droughts. Therefore, the effective carbon dioxide utilization is becoming an urgent scientific and environmental task.One of the ways of carbon dioxide utilization is the electrochemical method. It consists in the electrochemical reduction of CO2 at the cathode. Depending on the conditions of electrolysis (composition of the electrolytic bath, temperature, current density, voltage in the bath, electrode materials), the chemical composition of cathode products changes dramatically.Report is a review of the present state of research concerning the electrochemical conversion of carbon dioxide from conducting liquid and solid media of different chemical composition. They include solid, aqueous, non-aqueous (organic and ionic liquids), and molten salt electrolytes. It contains a comparative analysis of the effectiveness of using these electrolytes, as well as products obtained by the electrochemical splitting of CO2 and the prospects for their use. Special emphasis has been made on the electrochemical decomposition of carbon dioxide from molten salts, several variants of decomposition have been considered, the advantages and disadvantages of each variant have been analyzed. An analysis of literature data on this subject as well as research results of the author on the direct electrochemical conversion of CO2 into valuable solid-phase chemicals with added value is presented.A feature of the electrochemical reduction of CO2 dissolved in molten salts, in contrast to aqueous, organic, solid electrolytes and ionic liquids is the possibility of forming a solid carbon phase at the cathode. Knowledge of the laws of electrode processes involving CO2 is necessary to create a scientific basis and control for the processes of electrochemical synthesis of nanosized carbon-containing inorganic compounds - nanotubes, nanofibers, graphene, amorphous carbon, carbides of refractory metals and numerous composites based on them.The voltammetric study of carbon electrowinning mechanism from carbon dioxide under excessive pressure and lithium carbonate in different gas media (air, argon, carbon dioxide) dissolved in chloride melts at temperatures of 550–800 °С was conducted. The characterization of the chemical and phase compositions, morphological and structural features of nanosized carbon phases obtained by electrolysis in the studied melts has been carried out; a correlation between the structure of the products and the synthesis conditions is established.Thermodynamic analysis of possible ways of electroreduction of CO2 and carbonates of alkali and alkaline earth metals in the temperature range 700 - 1000 K showed that the cathode product can be: (1) - solid carbon phase; (2) - carbon monoxide; (3) - carbide phase. At temperatures below 1000 K, the first path will be thermodynamically advantageous.Electrochemical behavior of CO2 in molten salt mixtures NaCl–KCl (0.5: 0.5 mol%); NaCl–KCl–CsCl (0.3: 0.245: 0.455 mol%); NaCl–KCl–NaF (0.423: 0.423: 0.154 mol.%) under gas excess pressure (1-17 atm.) was studied by cyclic voltammetry at different potential scan rates (0.02-10 Vs-1) and reverse potentials at Pt, Au and GC electrodes. It was established that CO2 electroreduction takes place in two stages, the electrode process is controlled by the charge transfer rate and the rate of CO2 diffusion in the melts. The solubility of CO2 in the melts of Na,K|Cl and Na,K|Cl,F was determined and the implementation of Henry's law in the studied pressure range was shown. An ECE (electrochemical–chemical–electrochemical) mechanism of cathodic reaction has been proposed for the cathode discharge of CO2. Oxidation of oxide- and carbonate- anions takes place at the anode, with the release of oxygen.Electrochemical reduction of Li2CO3 in molten equimolar mixture of Na,K|Cl in various gaseous media (air, argon, CO2) was studied by CV. Reduction of Li2CO3 to carbon in air occurs through the stage of preliminary chemical reaction of acid-base type with the formation of two electrochemically active particles of CO2 and {LixCO3}2-x at potentials -0.8 and -1.7 V, respectively, relative to Pb|PbCl2 reference electrode . Both processes are irreversible, and the limiting stage of reduction {LixCO3}2-x is the diffusion of the depolarizer to the electrode surface. In the atmosphere of argon and CO2, only {LixCO3}2-x participates in the electroreduction process.Promising areas of application of electrolytic carbon are presented.

The reductive and oxidative intercalation of ions into graphite from room‐temperature and low temperature molten salts is demonstrated. For this investigation, the molten salts use 1‐ethyl‐3‐methylimidazolium (EMI+) or 1,2‐dimethyl‐3‐propylimidazolium (DMPI+) as the cation and , , , , or as the anion. In a two‐electrode battery configuration, the molten salt electrolyte provides both the cation and anion which are intercalated into the graphite anode and cathode, respectively. A cell employing a electrolyte and two graphite rod electrodes achieved an open‐circuit voltage of 3.5 V and a cycling efficiency of 85%.

This chapter describes recent work on batteries and fuel cells using molten salt electrolytes. This entails a comparison with other batteries and fuel cells utilizing aqueous and organic electrolytes; for further details of these low-temperature systems a number of reference works are available(1,2) General background in molten salt properties may be found in several books,(3–7) some recent reviews,(8,9) and of course in other parts of the current series.

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

Experiments to determine the kinetics of the thermal decomposition of MgOHCl were performed. It was found that the decomposition of MgOHCl commenced at 649 K, and it directly converted into MgO and HCl without undergoing any intermediate step. Decomposition vs time data showed that the thermal decomposition of MgOHCl was a first-order process with respect to the amount of MgOHCl remaining, and the mass transfer of the product HCl gas away from the interface was likely the rate-limiting step. It was also found that the time required to completely decompose MgOHCl into MgO, more than 20 minutes, at the operating temperatures of electrolytic magnesium production processes, 600 °C±50 °C, was significantly longer than the time required, less than 1 minute, to digest the solid magnesium chloride containing feed material into the molten salt electrolyte in these processes. Such delay in the decomposition would mean that any MgOHCl produced during heating and digestion of the feed would not be decomposed by the heat of the electrolyte and thus the persistent MgOHCl would dissolve into the molten salt electrolyte with potentially severe negative consequences on electrolysis cell operation.

5454914 Method of removal of heavy metal from molten salt in IFR fuel pyroprocessing : Gay Eddie Park Forest, IL, United States Assigned to The United States of America as rep. by the United States Department of Energy

Coatings consisting of Ni aluminide/Ni bilayer on Nb–W and Nb–W–Mo alloys were prepared by the electrodeposition of Al using a molten salt electrolyte after the Ni electrodeposition. The coating specimen with a thin Ni layer showed a vigorous oxidation during the initial period. On the contrary, the coating specimen with a thick Ni layer showed a high oxidation resistance. For the coating specimen with a thick Ni layer, a surface layer consisting of NiAl was formed after the oxidation test. For this coating specimen, further, intermediate layers consisting of Ni–Nb alloys were formed by the diffusion of the Ni layer and the alloy substrate during the oxidation test. These intermediate layers contributed to the maintenance of a high concentration of Al in the surface layer and the suppression of spallation of the surface layers, leading to the improvement in the oxidation resistance of the coating.

Iron disulphide molten salt electrochemical cells are among the most promising technological options for batteries.The electrochemical behaviour of pyrite allies excellent cathodic characteristics to optimal performance and low operational costs.The cathodic iron disulphide mechanism involves many processes, encompassing from polysulphides formation to the reduction of iron to the metallic state.The use of X-ray diffraction together with scanning electronic microscopy analysis on cells of the Li/KCl-LiCl/FeS 2 system made possible to identify intrinsic and extrinsic parameters to the electrochemical process involved in the establishment of the polysulphides stoichiometry.It is necessary to note that the augmentation of the cell's internal resistance and loss of electrical capacity are directly related with the formation of these same substances.Researches in the electrochemistry of these phenomena aim to elucidate the cathodic interphase processes and the effect of every reaction in the global mechanism.

Molten salt electrolytes have been considered to be the most promising electrolytes for next-generation lithium secondary batteries, owing to their high safety and wide electrochemical windows. Perfluorosulfonylamide-based lithium salts exhibit the lowest melting temperatures among all of the lithium salts. Nevertheless, their melting point is above 100 °C, and lowering their melting point is still a considerable challenge for their application as secondary battery electrolytes. In this study, we prepared binary lithium salts by mixing lithium bis(fluorosulfonyl)amide (LiFSA, Li[F–SO2–N–SO2–F]) and lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide (LiFTA, Li[F–SO2–N–SO2–CF3]) using a ball mill and revealed a eutectic composition ratio of 35:65, a eutectic point of 76 °C, and a viscosity of 20800 mPa s–1 at 100 °C. This binary lithium molten salt electrolyte exhibited a low melting point, a wide electrochemical potential window of 5.1 V, no Li+ concentration gradient, and successful charge and discharge of a conventional graphite negative electrode with high Coulombic efficiency at a 0.05C rate. Our findings further advance the development of high-voltage and high-power lithium secondary batteries.