Renewable energy systems will need large-scale energy storage to ensure reliability and provide power when and where it is needed. Even though lithium-ion batteries are increasingly used for large-scale storage, their costs and competition in terms of materials needed for electric vehicles are driving the need for alternative batteries for stationary energy storage systems. Molten sodium-ion batteries that operate at intermediate temperatures, approximately 150 °C or less, offer an abundant and cost-effective solution to our energy storage issues. This review paper highlights the materials, enhancements, and performance of molten Na–ion batteries that operate at temperatures at or below 150 °C for use in energy storage systems as well as an outlook on future improvements for this energy storage system.

Low-temperature liquid metal batteries (LMB) are an up-and-coming technology for large-scale energy storage due to increased efficiency with liquid electrodes and decreased corrosion at lower temperatures. To reduce the operating temperature of LMBs, identification of electrode and electrolyte materials that maintain sufficient performance becomes important towards economic viability of these technologies. Sodium (Na) and mercury (Hg) are both advantageous materials for LMBs due to their low melting points of ~ 98°C and -39°C respectively, and above average electrochemical properties. This presentation includes the design and performance of a novel Na – Hg battery with a solid – state NaSICON electrolyte that operates at 110°C. Due to the difficulty of molten metal electrode wetting on the NaSICON electrolyte, specific interfacial coatings have been developed to decrease ionic resistance at the electrode – electrolyte interface. Determination of components, interfacial coatings, and cell design will be discussed to demonstrate the benefits of this battery type. The electrochemical performance of this battery system will be presented. Characterization of the system through ex-situ methods will be discussed to correlate the observed behaviors of each electrode and Na diffusion mechanism to performance. Finally, methods towards the scale-up of this battery design will be presented towards the goal of a high-performance utility-scale battery storage system.

For molten salt reactors and pyrochemical reprocessing systems, it is vital to monitor the concentration and speciation of U and fission products in the fuel salt as well as the basicity of the salt. We show that NIR spectroscopy allows for better monitoring of actinide and lanthanide fission product concentrations and speciation with greater sensitivity than UV and visible range absorption spectroscopy due to fewer interfering absorption lines, more tractable molar absorption coefficients of transitions present in this range, and less sensitivity to scattering from solid phases suspended in the salt. Additionally, to monitor the basicity of the molten salt mixture, we have explored the use of Pb2+ and Bi2+ as probe ions; shifts in the absorption maxima of their 6s→6p electronic transition are correlated to changes in the Lewis basicity of the molten salt. We show that this optical basicity method is viable in alkali chloride molten salts and may be of use for online monitoring of molten salt chemistry in nuclear molten salt applications. This work was funded by Department of Energy under award DE-EE0009822

Fiber optic-based Raman and UV–Vis spectroscopy systems were commissioned for use in an inert atmosphere for in situ analysis of high temperature molten salts. The speciation of samarium chloride was studied in the LiCl–KCl eutectic system at 500 °C. Raman and electronic absorption spectra indicated trivalent samarium forms an octahedral SmCl 6 3− complex with two detectable Raman features. Metallothermic and electrolytic reduction of Sm(III) to Sm(II) in the alkali halide solution was carried out to investigate the coordination of Sm(II) in the same eutectic salt. Formation of the Sm(II) was confirmed using UV–Vis absorption spectroscopy. The resulting spectra are reported and discussed in terms of complex formation, inelastic scattering, and electronic absorption of the reduced Sm(II) ions. The Raman spectrum of metallothermically produced Sm(II) exhibited similar shift to the Raman spectrum of metallothermically reduced Nd(II).

Argon ion sputtering for depth profiling in XPS results in the preferential sputtering of oxygen in certain oxide systems. Preferential oxygen removal results in the chemical reduction of the associated cation which is observed in spectra collected following sputtering. Characterizing the effects of different sputtering parameters and developing methods to account for and to minimize changes to chemical state during sputtering would allow for greater confidence in the qualitative and quantitative analysis of the chemical states observed after sputtering. In this study argon ion sputtering was performed with monatomic and clustered argon ions on thermally oxidized NiO. Parameters for monatomic and cluster sputtering were varied to observe their effects on the induced reduction. Results show that extensive reduction is experienced for monatomic sputtering irrespective of the energy and current chosen. For cluster argon ion sputtering, reduction was mitigated when selecting either large cluster sizes or low cluster energies. Sputtering for long periods of time with low energy clusters was observed to result in the removal of surface contaminants without concomitant reduction of the nickel oxide.

Over several decades, extensive research has been devoted to developing the optical basicity concept to quantify Lewis basicity of oxidic media. Spectroscopy (ultraviolet (UV), infrared (IR), and fluorescence) was originally used to experimentally determine optical basicity, but linear refractive index and x-ray photoelectron spectroscopy (XPS) have been more recent techniques for determining basicity of glasses. While glasses have been one media of study, slags, oxyanion melts, and molten halide salts are also of interest. The concept has been only briefly explored in molten salts, but optical basicity could be a valuable tool for assessing its corrosivity. This review summarizes different techniques used to determine optical basicity, their pitfalls, and the preliminary work outlined for molten salts.

Stainless steel 316 L exposed to LiCl-Li2O molten salt containing Li metal for 500 and 1000 h experiences deep intergranular attack and bulk void formation. This degradation is due to a synergistic corrosion mechanism, different than other forms of molten salt corrosion, where Cr and Mn oxides and carbides form along grain boundaries and are dissolved by Li metal. Synchrotron transmission X-ray microscopy 3D imaging is used to show corrosion morphology and Cr enrichment and depletion in a novel format that has not previously been used for bulk corrosion samples.

Liquid metal batteries (LMB) have the potential to be ideal grid-scale storage batteries; they maintain their performance and exhibit high efficiencies because they do not structurally degrade like lithium-ion batteries. While current commercial LMBs operate at temperatures above 500°C, low temperature LMBs are more attractive because they experience less corrosion of structural components and require less external heating for operation. In this work, we will detail a sodium – mercury battery with a NaSICON solid state electrolyte that operates at 110 °C. Described will be the manufacturing of the solid-state electrolyte, assembly of the full cell, and performance.

Liquid metal batteries (LMB) have the potential to be ideal grid-scale storage batteries, though many designs require a high operating temperature which may decrease economic viability and increase corrosion of structural components. To circumvent these issues, development of room temperature LMBs can offer a more affordable and long-term energy storage solution. In this work, we will detail a Na-K || Hg battery with a 1M NaClO4 in DME:FEC organic electrolyte that operates at room temperature. Described will be the manufacturing of the Na-K electrode, assembly of the full cell, and performance.

Currently, there is a lack of reliable measurement techniques for understanding the basicity of molten chloride salts. Optical basicity, an ultraviolet-visible (UV-vis) spectroscopic method for measuring Lewis basicity, is explored for its applicability to molten chloride salts. Shifts in probe ion (Pb2+ and Bi3+) electronic transitions are observed that show that cations in chloride salts follow the same basicity series as in oxide melts, and an increase in basicity is observed with increasing temperature. Pb2+ and Bi3+ are validated as effective probe ions in alkali, alkaline earth, and aluminum-sodium chloride molten salts. However, the utility of Bi3+ is limited by the volatility of BiCl3 at high temperatures, posing a challenge for use in high-temperature molten salt applications. Since optical basicity measures the extent of electron donation, it may be a useful metric for electrochemical corrosion.

The capture of long-lived radioactive iodine (129I) from oxidizing off-gasses produced from reprocessing used nuclear fuel is paramount to human health and environmental safety. Bismuth has been investigated as a viable iodine getter but the phase stability of bismuth-based sorbents in an oxidizing environment have not yet been researched. In the current work, bismuth nanoparticle-based sorbents, as free particles (Bi-NPs) and embedded within silica xerogel monoliths made with a porogen (TEO-5), were exposed to I2(g) before and after aging in 1 v/v% NO2 at 150 degrees C. For unaged sorbents, BiI3 was the dominant phase after iodine capture with 8-30 mass% BiOI present due to native Bi2O3 on the surface of the unaged nanoparticles. After 3 h of aging, 82 mass% of the Bi-NPs was converted to Bi2O3 with only a small amount of iodine captured as BiOI (18 mass%). After aging TEO-5 for 3 h, iodine was captured as both BiI3 (26 %) and BiOI (74 %) and no Bi2O3 was detected.". Additionally, bismuth lining the micrometer-scale pores in the TEO-5 led to enhanced iodine capture. In a subsequent exposure of the sorbents to NO2 (secondary aging), all BiI3 converted to BiOI. Thus, direct capture of iodine as BiOI is desired (over BiI3) to minimize loss of iodine after capture.

A comprehensive knowledge of the coordination, bonding, and speciation of elements in molten salt mixtures is necessary to understand and predict the chemical and physical properties of the salt. Absorption spectroscopy can yield information about the chemistry of species of interest in alkali halide molten salt mixtures by revealing information about the electronic structure and transitions of those species. In this study, ultraviolet (UV), visible (vis), and near-infrared (NIR) absorption spectroscopy was used to examine changes to the electronic structure of trivalent Nd, Sm, and Dy in LiCl–KCl eutectic molten salt with changes in temperature and the anion composition of the melt. With increasing temperature, changes to spectral features suggest a distortion of the coordination complexes. Changes to lineshape with the substitution of alternative halide anions were examined and analyzed, revealing differences in the coordination for I− versus F− with the lanthanides. Gaussian peak fitting was used to show that the changes in lineshape with the progressive addition of F− anions can be explained by the superposition of a set of absorption bands from complexes with all Cl− anion ligands and a set of blueshifted absorption bands from complexes containing both F− and Cl− anion ligands. This work yields a new method to analyze and interpret change to electronic absorption spectra for f-block elements dissolved in alkali halide molten salts as well as new observations of the interactions of larger and smaller halide anions with lanthanides in Cl−-based molten salts.

Electroanalytical determination of chlorides in molten salts are often found to not have the accuracy needed for nuclear material monitoring. This was also observed in our studies on samarium trichloride. It was determined that a poorly soluble and relatively stable oxychloride that formed during our studies accounted for the loss of accuracy in the quantification of samarium trichloride. In this study, the spectroscopic and electrochemical properties of synthesized samarium oxychloride were investigated as they relate to the pyrochemical reprocessing of used nuclear fuel in the molten LiCl–KCl of eutectic composition. A qualitative, in situ investigation of oxychloride formation was conducted using electroanalytical voltammetry and Raman spectroscopy. These results were confirmed using synthesized oxychloride. The nature of oxychloride formation, solubility in the molten eutectic, implications in electrochemical processing, and the management of nuclear material in high-temperature systems are discussed.

Determining the concentration of the dissolved lanthanide species in LiCl–KCl eutectic salt is important to the development of pyrochemical reprocessing of used nuclear fuel. In this process, lanthanide fission products are found dissolved in the electrorefiner electrolyte in their trivalent oxidation state. The presence of dissolved trivalent lanthanides increases the liquidus temperature of the electrolyte mixture and can lead to the formation of insoluble oxide or oxychloride phases and must therefore be continuously monitored and controlled during the operation. Absorbance spectroscopy is a promising method for continuous measurement of the concentration of lanthanides and other elements dissolved in the electrolyte. The absorption of light by elements is linearly proportional to the concentration of the element for relatively dilute solutions according to the Beer-Lambert law. Although measurement of the absorption of ultraviolet and visible range light by lanthanides in LiCl–KCl eutectic molten salt have been explored previously, near infrared (NIR) absorption spectroscopy has received far less attention. It may, however, provide a better analytical signal when insoluble phases are present due to less Raleigh scattering compared to shorter wavelength radiation. Additionally, it may allow for concentration determination for certain elements using NIR absorption features where UV and visible range features are overlapping with features from other species. In this study, we report the UV–Vis–NIR spectra of the trivalent lanthanide chlorides of neodymium, samarium, and dysprosium in LiCl–KCl eutectic. Molar absorption coefficients are reported for analytically useful absorption maxima, with a focus on the molar absorption coefficients for NIR absorption maxima which have not been reported previously. Additionally, we observe a NIR-range absorption band of Nd3+ which was previously predicted but never experimentally observed. We compare the calculated crystal field levels to the newly observed absorbance band and find them to be in good agreement with previous predictions.

Advanced nuclear technologies using high-temperature molten salts are poised to increase the efficiency, flexibility, and acceptability of nuclear energy. To realize these advances, additional understanding of the chemistry and thermochemical and thermophysical properties of the molten salt mixtures is required. Additionally, methods to rapidly and non-destructively monitor and determine the concentration and chemistry of actinides and fission products must be developed. Electronic and vibrational spectroscopy are promising methods to both interpret the structure and speciation of dissolved species in molten salts and to determine concentrations of species of interest. We have determined the molar absorbance coefficients of selected absorbance features for several lanthanides of interest in the ultraviolet, visible, and near infrared ranges and show that some NIR absorbance features, which were previously neglected as a means of concentration determination, are analytically useful. In fact, these NIR absorbance features may have advantages over UV and visible range features for concentration determination in industrial systems. Additionally, we will present our analysis of the effects of the coordination environment on lanthanide electronic structure and transitions, yielding insight into the chemistry and coordination of these elements in molten alkali halide mixtures, which is crucial for understanding the corrosion issues in these salts amongst other requirements. Acknowledgement: This research is being performed using funding received from the DOE Office of Nuclear Energy's Nuclear Energy University Programs under award DE-NE0008889 and the US Nuclear Regulatory Commission (USNRC) under contract 31310018M0032. Dr. Kenny Osborne and Ms. Nancy Hebron-Isreal serve as the program managers for the DOE and NRC awards, respectively. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1447692. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation, the U.S. Department of Energy, or the United States Government.

Pyroprocessing is a potential route to close the nuclear fuel cycle. Used nuclear fuel (UNF) is electrolytically reduced from UO2 to U0 at a stainless-steel cathode while oxygen evolution occurs at a platinum anode in a molten LiCl-Li2O environment. Platinum is consumed during this process as a result of the formation and spallation of lithium platinate. To increase the economic viability of pyroprocessing, alternative low-cost, electrochemically efficient materials are needed to replace platinum. In this study, metal-oxide coated 316L stainless streel rods were explored as potential replacements. The characteristics of these coatings in molten LiCl-Li2O was evaluated through electrochemical techniques. The surface chemistry of the coatings was explored through X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy and scanning electron microscopy before and after exposure to molten salts to understand the degradation of the coatings. Results detailing the performance of the coatings will be presented.

Understanding the speciation of dissolved lanthanides and actinides in molten salts and the chemical behavior of those species is vital to the development of homogenously fueled molten salt reactors and pyrochemical reprocessing of used nuclear fuel. The coupling of spectroscopy and electrochemical analysis can provide complimentary information about the chemistry and speciation of elements dissolved in molten salt mixtures, which is crucial for managing, amongst other concerns, corrosion issues in the process. We have developed a custom furnace with optical pathways for spectroscopic measurements and provisions for the insertion of electrodes into samples inside the furnace. Absorbance spectra were recorded during the electrolytic reduction of trivalent lanthanides in LiCl-KCl molten salt. The absorbance spectra yield insight into the progression of the electrolytic reduction of the species of interest while the electrolytic reduction provides a means of studying the electronic structure and transitions of soluble reduced species of the lanthanides and potentially monitoring changes to concentrations of the oxidized and reduced species. Acknowledgements: This research is being performed using funding received from the DOE Office of Nuclear Energy's Nuclear Energy University Programs under award DE-NE0008889 and the US Nuclear Regulatory Commission (USNRC) under contract NRC-HQ-13-G-38-0027. Dr. Kenny Osborne and Ms. Nancy Hebron-Isreal serve as the program managers for the DOE and NRC awards, respectively. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1447692. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation, the U.S. Department of Energy, or the United States Government.

The coordination chemistry of various fission and decay products, such as actinides and lanthanides, are crucial to the commercial deployment of molten salt reactors as they can affect the thermophysical properties. Here, we examined the structure, coordination environment, and physical properties such as the density and the vibrational density of states for three lanthanide species, namely Ce, Eu, and Sm in the LiCl-KCl eutectic system using a combination of quantum mechanics simulations and spectroscopic experiments. Quantum mechanics molecular dynamics (QM-MD) modelling was employed to determine the physical properties of each system resulting in accurate local coordination of each species. Then, the vibrational density of states (DOS) was determined using a two-phase thermodynamic modelling which was then compared to the experimentally obtained Raman spectra of the species in molten LiCl-KCl having the eutectic composition. We find that Ce3+, Eu3+ and Sm3+ all adopt octahedral local coordination environments in the eutectic salt composition in good agreement with experimental results. Ce3+ is found to fluctuate between an octahedral six-coordinated and a seven-coordinated structure due to the increased local proximity of Cl in the eutectic salt, resulting in a lower fluidicity/diffusivity than the other trivalent lanthanides studied. The thermophysical properties of the eutectic composition with trivalent lanthanides were not significantly different from the pure eutectic salt composition, but several changes were noted.

The molten salt reactor (MSR) is one of the leading advanced nuclear reactor candidates to replace current nuclear reactor technologies in the U.S. Besides having more economical and reliable designs, MSRs are amenable to a closed fuel cycle, in which electrochemical reprocessing can be performed to recycle the used nuclear fuel. This review intends to provide information about potential waste forms for metal and salt waste streams from these salt-based nuclear processes. Metal waste streams arise from reactor components and structural materials. Salt waste streams are generated during reactor operations as fission products build up in salt-fuelled systems. Waste forms that have the highest waste loading and/or have shown the most commercial promise are discussed with an emphasis on the current state of efforts to understand the synthesis and chemical durability of metal and ceramic waste forms.