Electrochemical separation of barium from multi-component molten salt electrolyte (BaCl₂-LiCl-CaCl₂-NaCl) at 500–700 °C is demonstrated using a liquid bismuth electrode which possesses strong chemical interactions with barium. While the standard emf analysis suggests Na to be the first species to deposit in this electrolyte followed by Ca, Li, and finally Ba, barium was found to be the first species to be reduced into the bismuth electrode followed by Ca. The exceptional deposition behavior of barium was ascribed to the activity of the constituent alkali/alkaline-earth metals in the bismuth metal. The activity of barium in bismuth was extremely low (as low as 10⁻¹⁵), shifting the redox potential of barium to the most positive potentials and enabling the separation of barium into liquid bismuth. By exploiting the differential interactions of constituent ions with the liquid bismuth, it was possible to separate conventionally non-separable barium species from the electrolyte solution. In addition, high coulombic efficiencies of the liquid bismuth electrode (>99%) suggest that electrode processes are chemically reversible for co-deposition of barium and calcium. The analyses of electrode potentials at various current densities and electrochemical impedance spectra indicate charge transfer as the most significant overpotential mechanism during electrolysis.

Calcium is an attractive material for the negative electrode in a rechargeable battery due to its low electronegativity (high cell voltage), double valence, earth abundance and low cost; however, the use of calcium has historically eluded researchers due to its high melting temperature, high reactivity and unfavorably high solubility in molten salts. Here we demonstrate a long-cycle-life calcium-metal-based rechargeable battery for grid-scale energy storage. By deploying a multi-cation binary electrolyte in concert with an alloyed negative electrode, calcium solubility in the electrolyte is suppressed and operating temperature is reduced. These chemical mitigation strategies also engage another element in energy storage reactions resulting in a multi-element battery. These initial results demonstrate how the synergistic effects of deploying multiple chemical mitigation strategies coupled with the relaxation of the requirement of a single itinerant ion can unlock calcium-based chemistries and produce a battery with enhanced performance.

The thermodynamic CALPHAD description of the Al–Pt system is remodeled. The four sub-lattice (4SL) model for the ordered/disordered fcc and bcc phases is adopted resulting in improved agreement with experiment and first-principles information compared to previous descriptions. First-principles calculations are performed and the results are used in addition to available experimental data as input for the modeling. Modeling results agree well with most experimental phase equilibria and thermochemical data compiled. Special attention is also paid to the metastable phase diagrams to ensure that the parameters obtained in the modeling do not present unphysical results in the metastable regime. The obtained fcc and bcc descriptions are converted to two sublattice (2SL) models to enable combination with available multi-component Ni-base superalloy descriptions. The converted Al–Pt system is extended into the Al–Ni–Pt ternary system to study its extrapolation characteristics using available thermodynamic and thermochemical data. It is found that the 2SL model is not adequate in capturing the thermodynamic behavior of the fcc-based phases found in the ternary Al–Ni–Pt system. Possible approaches for future work is discussed.

The performance of a calcium-antimony (Ca-Sb) alloy serving as the positive electrode in a Ca∥Sb liquid metal battery was investigated in an electrochemical cell, Ca(in Bi) | LiCl-NaCl-CaCl₂ | Ca(in Sb). The equilibrium potential of the Ca-Sb electrode was found to lie on the interval, 1.2–0.95 V versus Ca, in good agreement with electromotive force (emf) measurements in the literature. During both alloying and dealloying of Ca at the Sb electrode, the charge transfer and mass transport at the interface are facile enough that the electrode potential varies linearly from 0.95 to 0.75 V vs Ca(s) as current density varies from 50 to 500 mA cm⁻². The discharge capacity of the Ca∥Sb cells increases as the operating temperature increases due to the higher solubility and diffusivity of Ca in Sb. The cell was successfully cycled with high coulombic efficiency (∼100%) and small fade rate (<0.01% cycle⁻¹). These data combined with the favorable costs of these metals and salts make the Ca∥Sb liquid metal battery attractive for grid-scale energy storage.

The ability to store energy on the electric grid would greatly improve its efficiency and reliability while enabling the integration of intermittent renewable energy technologies (such as wind and solar) into baseload supply. Batteries have long been considered strong candidate solutions owing to their small spatial footprint, mechanical simplicity and flexibility in siting. However, the barrier to widespread adoption of batteries is their high cost. Here we describe a lithium–antimony–lead liquid metal battery that potentially meets the performance specifications for stationary energy storage applications. This Li||Sb–Pb battery comprises a liquid lithium negative electrode, a molten salt electrolyte, and a liquid antimony–lead alloy positive electrode, which self-segregate by density into three distinct layers owing to the immiscibility of the contiguous salt and metal phases. The all-liquid construction confers the advantages of higher current density, longer cycle life and simpler manufacturing of large-scale storage systems (because no membranes or separators are involved) relative to those of conventional batteries. At charge–discharge current densities of 275 milliamperes per square centimetre, the cells cycled at 450 degrees Celsius with 98 per cent Coulombic efficiency and 73 per cent round-trip energy efficiency. To provide evidence of their high power capability, the cells were discharged and charged at current densities as high as 1,000 milliamperes per square centimetre. Measured capacity loss after operation for 1,800 hours (more than 450 charge–discharge cycles at 100 per cent depth of discharge) projects retention of over 85 per cent of initial capacity after ten years of daily cycling. Our results demonstrate that alloying a high-melting-point, high-voltage metal (antimony) with a low-melting-point, low-cost metal (lead) advantageously decreases the operating temperature while maintaining a high cell voltage. Apart from the fact that this finding puts us on a desirable cost trajectory, this approach may well be more broadly applicable to other battery chemistries.

Calcium is an attractive electrode material for use in grid-scale electrochemical energy storage due to its low electronegativity, earth abundance, and low cost. The feasibility of combining a liquid Ca–Bi positive electrode with a molten salt electrolyte for use in liquid metal batteries at 500–700 °C was investigated. Exhibiting excellent reversibility up to current densities of 200 mA cm⁻², the calcium–bismuth liquid alloy system is a promising positive electrode candidate for liquid metal batteries. The measurement of low self-discharge current suggests that the solubility of calcium metal in molten salt electrolytes can be sufficiently suppressed to yield high coulombic efficiencies >98%. The mechanisms giving rise to Ca–Bi electrode overpotentials were investigated in terms of associated charge transfer and mass transport resistances. The formation of low density Ca₁₁Bi₁₀ intermetallics at the electrode-electrolyte interface limited the calcium deposition rate capability of the electrodes; however, the co-deposition of barium into bismuth from barium-containing molten salts suppressed Ca–Bi intermetallic formation thereby improving the discharge capacity.

The thermodynamic properties of calcium–magnesium alloys were determined by electromotive force (emf) measurements using a Ca(in Bi)|CaF₂|Ca(in Mg) cell over the temperature range 713–1048 K. The activity and partial molar Gibbs free energy of calcium in magnesium were calculated for nine Ca–Mg alloys, calcium mole fractions varying from x_Ca = 0.01 to 0.80. Thermodynamic properties of magnesium in calcium and the molar Gibbs free energy of mixing were estimated using the Gibbs–Duhem relationship. In the all-liquid region at 1010 K, the activity of calcium in magnesium was found to range between 8.8 × 10⁻⁴ and 0.94 versus pure calcium. The molecular interaction volume model (MIVM) was used to model the activity coefficient of Ca and Mg in Ca–Mg liquid alloys. Based on this work, Ca–Mg alloys show promise as the negative electrode of a liquid metal battery in which calcium is the itinerant species: alloying with Mg results in both a decrease in operating temperature and suppression of Ca metal solubility in the molten salt electrolyte.

The thermodynamic properties of Ca–Sb alloys were determined by emf measurements in a cell configured as Ca(s)|CaF₂|Ca–Sb over the temperature range 550–830 °C. Activity coefficients of Ca and Sb, enthalpy, Gibbs free energy, and entropy of mixing of Ca–Sb alloys were calculated for x_Ca < 0.55. To explain the connection between short-range order of liquid Ca–Sb alloys and the strong deviation from ideality in the thermodynamic properties, two thermodynamic models were invoked and reconciled: the regular associated solution model, assuming the presence of a CaSb₂ associate, and the molecular interaction volume model (MIVM). For the first time, the MIVM was used successfully to model the activity coefficients of a system with high-melting intermetallics, reducing the number of fitting parameters necessary from 5 (regular associated model) to 2 (MIVM). From the interaction parameters optimized by fitting at 800 °C, the activity coefficient of Ca was predicted at 650 °C, with an average error of less than 0.6% in the emf value.

Batteries are an attractive option for grid-scale energy storage applications because of their small footprint and flexible siting. A high-temperature (700 °C) magnesium–antimony (Mg||Sb) liquid metal battery comprising a negative electrode of Mg, a molten salt electrolyte (MgCl₂–KCl–NaCl), and a positive electrode of Sb is proposed and characterized. Because of the immiscibility of the contiguous salt and metal phases, they stratify by density into three distinct layers. Cells were cycled at rates ranging from 50 to 200 mA/cm² and demonstrated up to 69% DC–DC energy efficiency. The self-segregating nature of the battery components and the use of low-cost materials results in a promising technology for stationary energy storage applications.

The thermodynamic properties of Ca–Bi alloys were determined by electromotive force (emf) measurements to assess the suitability of Ca–Bi electrodes for electrochemical energy storage applications. Emf was measured at ambient pressure as a function of temperature between 723 K and 1173 K using a Ca(s)|CaF₂(s)|Ca(in Bi) cell for twenty different Ca–Bi alloys spanning the entire range of composition from x_Ca = 0 to 1. Reported are the temperature-independent partial molar entropy and enthalpy of calcium for each Ca–Bi alloy. Also given are the measured activities of calcium, the excess partial molar Gibbs energy of bismuth estimated from the Gibbs–Duhem equation, and the integral change in Gibbs energy for each Ca–Bi alloy at 873 K, 973 K, and 1073 K. Calcium activities at 973 K were found to be nearly constant at a value of a_Ca = 1 × 10⁻⁸ over the composition range x_Ca = 0.32–0.56, yielding an emf of ∼0.77 V. Above x_Ca = 0.62 and coincident with Ca₅Bi₃ formation, the calcium activity approached unity. The Ca–Bi system was also characterized by differential scanning calorimetry over the entire range of composition. Based upon these data along with the emf measurements, a revised Ca–Bi binary phase diagram is proposed.

Molten oxide electrolysis (MOE) is a carbon-free, electrochemical technique to decompose a metal oxide directly into liquid metal and oxygen gas. From an environmental perspective what makes MOE attractive is its ability to extract metal without generating greenhouse gases. Hence, an inert anode capable of sustained oxygen evolution is a critical enabling component for the technology. To this end, iridium has been evaluated in ironmaking cells operated with two different electrolytes. The basicity of the electrolyte has been found to have a dramatic effect on the stability of the iridium anode. The rate of iridium loss in an acidic melt with high silica content has been measured to be much less than that in a basic melt with high calcia content.

Stress corrosion cracking (SCC) of Alloy 625 (UNS N06625) has been investigated in pH 2 aqueous solution at high temperatures (300°C to 426°C) and high pressure (24.1 MPa) to understand the corrosion behavior in supercritical water oxidation (SCWO) systems, which can destroy aqueous organic wastes with high efficiency with no harmful byproducts. Alloy 625 was exposed to 11 operational (chemical, thermal, pressure) cycles. SCC at subcritical temperature comes from the chemical stability of the elements, which produces a dealloyed oxide layer where Ni is selectively dissolved and Cr forms stable oxides. Its growth is accelerated along the grain boundaries, where SCC develops during the operational cycles. As a result of the defective dealloyed oxide layer structure, the diffusivity of Ni is fast, intermediate between the surface and grain boundary diffusivities. SCC at supercritical temperature comes from the direct chemical attack of associated hydrochloric acid (HCl) molecules.

The unique properties of supercritical water (SCW) have enabled its application in the destruction of aqueous organic wastes. However, drastic changes in properties of SCW as it approaching the critical point also affect the degradation behavior of constructional alloys. The corrosion behavior of Ni-base alloys was studied at pH 2, water temperature in the range 300–425 °C, and at a constant pressure of 24.1 MPa. Aggressive corrosion is observed at high subcritical temperature and minimal corrosion is observed at supercritical temperature. At subcritical temperatures, selective dissolution of Ni and the oxidation of Cr have been observed, and increasing Cr content improved corrosion resistance.

The properties of water change significantly across the critical point of water and these changes affect the degradation behavior of constructional alloys. The corrosion behavior of nickel-base alloys is investigated across the critical temperature of water at a constant pressure in aqueous solutions. At subcritical temperatures, the corrosion behavior of alloys of 625 and C-276 is influenced by the pH values of the aqueous solutions. In neutral pH, a thin surface oxide film develops on the substrate. In pH 2, Ni and Fe are selectively dissolved, and Cr and Mo form stable oxides. In pH 1, Cr, Ni, and Fe are dissolved from the substrate and Mo forms a stable oxide. At supercritical temperatures, no dealloying is observed independent of pH and a thin surface oxide film develops. Changes in water properties and thermodynamics explain the dealloying behavior of elements and the highest corrosion rate at high subcritical temperature.

Molten oxide electrolysis (MOE) is a carbon-neutral, electrochemical technique to decompose metal oxide directly into liquid metal and oxygen gas upon use of an inert anode. What sets MOE apart from other technologies is its potential environmental advantage of no greenhouse gas emissions. Therefore, the primary challenge for carbon-free molten oxide electrolysis is the development of an inert anode. In the quest for an inert anode that can sustain the aggressive conditions of the process, iridium has been evaluated in two different slags for ironmaking. The basicity of the electrolyte proves to have a dramatic effect on the stability of the iridium anode, where iridium corrosion in an acidic slag with high silica content is less pronounced than the corrosion rate in a basic slag with high calcia content.