Inorganic Molten Salts Research Articles

Ternary Low-Temperature Molten Salt Electrolyte for Sodium Ion Batteries

To meet the increasing demand of electricity (which is forecasted to double by 2050)1 and to mitigate the climate crisis associated with using fossil fuels, renewable and clean energy sources are becoming the ultimate choices to generate electricity. Electrochemical energy storage devices such as batteries play an important role in the efficient use of renewable energy, providing an efficient and sustainable way to store energy.1,2 Beyond-lithium rechargeable batteries based on earth-abundant elements, namely sodium ion batteries (SIBs), are believed to be the alternatives for large-scale storage. Nevertheless, the commercial applications of SIBs are restricted by absence of reliable and low-cost electrolyte favouring low-temperature, high-efficiency, and reversible metal deposition/stripping. Interestingly, the newly reported molten salts containing several metal halides act as a low-cost electrolyte, which exhibits high electrical conductivity and quick electrode kinetics, and produce even better performances than ionic liquid electrolytes.3,4 Nonetheless, the conventional molten salt electrolytes normally operate at a relatively high temperature (over 260 ), which is significantly inconvenient for outdoor operation. Actually, reducing the operation temperature is not only beneficial to improving cycle life by restraining temperature-related degradation (i.e. undesired side reactions), but also profitable to using lower cost cell materials and reduce the cost of batteries fabrication. This paper reports our development of a low-temperature inorganic molten salt electrolytes with good electrochemical performances.5 The metal chloride eutectic mixture is used as electrolyte, whose melting points are dependent on its precursors and composition, so the relationship of mole ratio of metal chlorides and electrolyte’s melting points are investigated in our work. The influences of additives in chloride-based eutectic mixture are also studied, to decrease the melting point of the electrolyte. Herein, the electrochemical properties of this electrolyte are investigated in battery cells using Na metal as anode and metal fluoride cathode respectively. The battery charge /discharge behaviour and electrochemical impedance spectra (EIS) is tested using GAMRY potentiostat. Materials characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),X-ray spectroscopy (XPS) and Raman spectroscopy are used to characterise the electrode materials and to understand the effect of electrolyte on the electrode materials.

Nanocrystals in Molten Salts and Ionic Liquids: Experimental Observation of Ionic Correlations Extending beyond the Debye Length.

The nature of the interface between the solute and the solvent in a colloidal solution has attracted attention for a long time. For example, the surface of colloidal nanocrystals (NCs) is specially designed to impart colloidal stability in a variety of polar and nonpolar solvents. This work focuses on a special type of colloids where the solvent is a molten inorganic salt or organic ionic liquid. The stability of such colloids is hard to rationalize because solvents with high density of mobile charges efficiently screen the electrostatic double-layer repulsion, and purely ionic molten salts represent an extreme case where the Debye length is only ∼1 Å. We present a detailed investigation of NC dispersions in molten salts and ionic liquids using small-angle X-ray scattering (SAXS), atomic pair distribution function (PDF) analysis and molecular dynamics (MD) simulations. Our SAXS analysis confirms that a wide variety of NCs (Pt, CdSe/CdS, InP, InAs, ZrO2) can be uniformly dispersed in molten salts like AlCl3/NaCl/KCl (AlCl3/AlCl4-) and NaSCN/KSCN and in ionic liquids like 1-butyl-3-methylimidazolium halides (BMIM+X-, where X = Cl, Br, I). By using a combination of PDF analysis and molecular modeling, we demonstrate that the NC surface induces a solvent restructuring with electrostatic correlations extending an order of magnitude beyond the Debye screening length. These strong oscillatory ion-ion correlations, which are not accounted by the traditional mechanisms of steric and electrostatic stabilization of colloids, offer additional insight into solvent-solute interactions and enable apparently "impossible" colloidal stabilization in highly ionized media.

Reference Correlations for the Viscosity of 13 Inorganic Molten Salts

In 1988, reference correlations for the viscosity of a selection of molten inorganic salts were proposed by Janz and have been used extensively. During the last 31 years, many additional measurements have been published. In a very recent paper, new reference correlations for the thermal conductivity of 13 inorganic molten salts were proposed. In this paper, reference correlations for the viscosity of those same salts are proposed. All available experimental data for the viscosity of 13 inorganic molten salts have been critically examined with the intention of establishing improved or new reference viscosity correlations. All experimental data have been categorized into primary and secondary data according to the quality of measurement specified by a series of criteria. Standard reference correlations are proposed for the following molten salts (with estimated uncertainties at the 95% confidence level given in parentheses): LiNO3 (6.7%), NaNO3 (3.0%), KNO3 (3.0%), NaBr (1.6%), KBr (2.0%), RbBr (2.2%), LiCl (3.7%), NaCl (2.4%), KCl (1.6%), RbCl (3.6%), CsCl (1.1%), NaI (1.5%), and RbI (1.5%).

Effect of Inorganic Salt Blending on the CO2 Separation Performance and Morphology of Pebax1657/Ionic Liquid Gel Membranes

Gel membranes comprising inorganic salt, molten salt or ionic liquid (IL), and Pebax1657 were prepared by blending the three components in ethanol/water mixture, and their carbon dioxide separation...

Corrosion monitoring and mitigation techniques on advanced thermal energy storage materials for CSP plants

High temperature corrosion is one of the most important issues for materials selection, structure design and service life prediction of engineering parts that are exposed to high temperature environments. The prevention of high temperature corrosive attacks on materials plays a critical role in aspects such as reliability, quality, safety and profitability of any industrial sector associated with high temperature process and in the study case, concentrated solar power (CSP) market, using inorganic molten salts as storage material. The aim of this research is the development of corrosion tests through conventional gravimetric techniques focussed on thermal energy storage (TES) materials as well as electrochemical impedance spectroscopy (EIS) analysis for storage systems monitoring. To mitigate corrosion, different alumina forming austenitic (AFA) alloys were exposed to the most promising molten salts to be proposed for the new generation of CSP plants, nitrate, carbonate and chloride molten salts. Ni base alloys showed a better behaviour in the corrosive environments and electrochemical impedance tests revealed the formation of a protective layer in the first 100 h. The gravimetric results and the scanning electron microscopy (SEM) analyses that were performed, confirmed the excellent behaviour of the steels tested in nitrate salts reducing the corrosion in these promising novel molten salts compared with the solar salt currently used in commercial CSP. On the other hand, carbonate and chloride molten salts showed a higher corrosion rate in the materials proposed, especially chloride molten salts.

Experimental evaluation of chemical systems for CO<sub>2</sub> capture by CaO in eutectic CaF<sub>2</sub>-CaCl<sub>2</sub>

CO2 capture by CaO in molten salts is a variant of calcium looping in which the active substances (CaO/CaCO3) are dissolved or in a slurry with inorganic molten salts. One of the main advantages is the nonexistence of degradation in the reactivity between the active material and CO2. Previous research has revealed good absorption and desorption characteristics with CaO contents up to 20 wt% in eutectic CaF2-CaCl2. The hypothesis is that the formed CaCO3 continuously dissolves in the melt, leaving highly reactive CaO readily available for the incoming CO2. In the present study, the CaO content is increased to 40 wt%, and the absorption characteristics is investigated with focus on the sorption capacity and CO2 removal rate. The chemical system is also evaluated experimentally with regards to viscosity and solubility of the formed CaCO3 during CO2 absorption, with the aim of determining chemical upscaling limitations. The results show that the practical CaO content limit is 30 wt%, in which a sorption capacity of 20 g CO2/100 g sorbent is observed, without any deterioration of the reaction kinetics. For 40 wt% CaO, the sorption capacity is higher, but on the expense of the CO2 removal rate and CaO conversion. This is attributed to a significant increase in viscosity and the solubility limit of CaCO3 being exceeded.

A sub-100 °C aluminum ion battery based on a ternary inorganic molten salt.

By using a low-cost, ternary AlCl3/LiCl/KCl inorganic molten salt as the electrolyte, an aluminum ion battery is able to work stably below 100 °C. When intergrated with a graphite paper cathode, the battery exhibits close performance and similar electrochemical transformations to inorganic electrolyte-based batteries operating at ≥120 °C.

The Research Progress on Preparation of Al-Ti Alloy

Al-Ti alloys are widely used in various fields due to their excellent properties. There are mainly four production methods for Al-Ti alloys, namely direct dissolving method, sintering method, reduction method and electrolytic method. The first three preparation methods all have high-temperature melting steps, which increase energy consumption and production costs, generate hydrogen evolution or segregation, and reduce the quality of the Al-Ti alloy coating. There are three types of electrolysis systems, namely organic solvent systems, ionic liquid systems, and inorganic molten salt systems. Apart from the inorganic molten salt system, the organic solvent system and the ionic liquid system have the advantages of lower energy consumption, lower production cost, and uniform Al-Ti alloy coating prepared by electrolysis. Among them, the ionic liquid system has good chemical stability, good thermal stability, low energy consumption, and can be recycled.

Reference Correlations for the Thermal Conductivity of 13 Inorganic Molten Salts.

The available experimental data for the thermal conductivity of 13 inorganic molten salts have been critically examined with the intention of establishing thermal conductivity reference correlations. All experimental data have been categorized into primary and secondary data according to the quality of measurement specified by a series of criteria. Standard reference correlations are proposed for the following molten salts (with estimated uncertainties at the 95 % confidence level given in parentheses): LiNO3 (7 %), NaNO3 (7 %), KNO3 (15 %), NaBr (15 %), KBr (15 %), RbBr (15 %), LiCl (17 %), NaCl (20 %), KCl (17 %), RbCl (17%), CsCl (10 %), NaI (17 %), and RbI (20 %).

Colloidal Chemistry in Molten Salts: Synthesis of Luminescent In1- xGa xP and In1- xGa xAs Quantum Dots.

Control of composition, stoichiometry, and defects in colloidal quantum dots (QDs) of III-V semiconductors has proven to be difficult due to their covalent character. Whereas the synthesis of colloidal indium pnictides such as InP, InAs, and InSb has made significant progress, gallium-containing colloidal III-V QDs still remain largely elusive. Gallium pnictides represent an important class of semiconductors due to their excellent optoelectronic properties in the bulk; however, the difficulty with the synthesis of gallium-containing colloidal III-V QDs has largely prohibited their exploration as solution-processed semiconductors. Here we introduce molten inorganic salts as high-temperature solvents for the synthesis and manipulation of III-V QDs. We demonstrate cation exchange reactions on presynthesized InP and InAs QDs to form In1- xGa xP and In1- xGa xAs QDs at temperatures above 380 °C. This approach produces novel ternary alloy QDs with controllable compositions that show size- and composition-dependent absorption and emission features. Emission quantum yields of up to ∼50% can be obtained for In1- xGa xP/ZnS core-shell QDs. A comparison of the optical properties of InP/ZnS core-shells with In1- xGa xP/ZnS core-shells reveals that Ga incorporation leads to significant improvement in the optical properties of III-V/II-VI core-shell emitters which is of great importance for quantum dot-based lighting and display applications. This work also demonstrates the potential of molten inorganic salts as versatile solvents for the synthesis and processing of colloidal nanomaterials at temperatures inaccessible for traditional solvents.

Molten salt synthesis of Mn2O3 nanoparticle as a battery type positive electrode material for hybrid capacitor in KNO3-NaNO2-NaNO3 melts

An innovative kind of low-temperature inorganic molten salt hybrid capacitor was first put forward. Mn2O3 nanoparticle was selected as battery type positive electrode of hybrid capacitor. The high crystallinity Mn2O3 nanoparticles with large specific surface areas were successfully prepared by molten salt synthesis method using ternary alkali nitrates. Electrochemical performances of Mn2O3 electrode were evaluated by cyclic voltammetry, square wave voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge in KNO3 (53 wt%)-NaNO2 (40 wt%)-NaNO3 (7 wt%) melts at 423 K. The results illustrated that Mn2O3 electrode transferred one electron during the reduction reaction and the Mn2O3 nanoparticle exhibited a high specific capacity of 669.6 C g−1 (186.0 mAh g−1, 2.51 F cm−2) at a current density of 5 mA cm−2. Furthermore, Mn2O3 electrode exhibited excellent cycle stability, and the capacity retention was up to 90.2% after 5000 cycles at a current density of 20 mA cm−2. The hybrid capacitors with molten salt electrolyte could be considered as a potential application in some specific situation which requires the high-temperature demand.

Theoretical study on the effective dehydrochlorination of 1,2-dichloroethane catalyzed by tetraalkylphosphonium chlorides: electrostatically controlled reactivity

Herein, we report a DFT study on how and why tetraalkylphosphonium chlorides efficiently catalyze the dehydrochlorination of 1,2-dichloroethane.

Multicationic Sr4Mn3O10 mesostructures: molten salt synthesis, analytical electron microscopy study and reactivity

Molten salt synthesis of anisotropic Sr4Mn3O10 particles.

Synthesis and statistical modelling of dry sliding wear of Al 8011/6 vol.% AlB2 in situ composite

The present study has used response surface methodology (RSM) and central composite design (CCD) for modelling, using wear parameters to predict the wear performance of an Al 8011/6.0 vol.% AlB2 composite. The effect of applied load and sliding velocity was studied at five levels for a fixed sliding distance. To understand wear behaviour, sliding wear tests were planned according to CCD and performed on a pin-on-disc apparatus at ambient temperature. An analysis of variance (ANOVA) was conducted to show the relative significance of the parameters. A second-order regression model was developed to predict the wear loss and to establish the relationships between wear parameters. Response surface and contour plots were drawn to analyse the wear results. Worn surfaces were examined under scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS) was used to interpret the operative wear mechanisms. Validation tests results show good agreement between experimental and predicted data.As an initial step of this study, AlB2 particles were reinforced in Al 8011 alloy by an in situ technique to synthesise an Al 8011/6.0 vol.% AlB2 composite. During synthesis an in situ reaction takes place between molten alloy and inorganic salt KBF4 at 850 °C, which leads to the formation of AlB2 particles. The composite was analysed by x-ray diffractometer (XRD) to detect the phases present, while optical and scanning electron microscopy (OM & SEM) were carried out to ascertain morphology and particle distribution. Hardness was evaluated by a Vickers hardness testing machine.

Organics- and Surfactant-Free Molten Salt Medium Controlled Synthesis of Pt-M (M = Cu and Pd) Bi- and Trimetallic Nanocubes and Nanosheets

We present a novel synthetic strategy for the shape-controlled synthesis of Pt-based alloy nanoparticles (NPs) in inorganic molten salt without using any organic surfactants or capping agents. Graphene oxide (GO) was chosen as the stabilizer in the inorganic molten salt synthetic strategy, due to the existence of various oxygen-containing functional groups on its surface, which adsorb, anchor, and stabilize the metal ions or NPs. After GO was added in molten salt, the PtPd nanosheets were formed on its surface in H2 atmosphere. In addition, when KI was chosen as the shape-inducing agent to selectively adsorb on and fully protect the (100) facets of alloys, PtPd nanocubes with core–shell structure, PtCu nanohemicubes, and PtPdCu nanocubes were prepared successfully on the surface of GO in molten salt. Introduction of GO as the stabilizer in molten salt proves a new approach in synthesis of Pt-based nanocrystals with controlled morphologies.

Stable colloids made with molten inorganic salts

A colloidal solution is a uniform dispersion of two different types of components—particles or droplets of one phase, the solute, in a second (typically liquid) phase, the solvent. Milk, a colloid ...

Stable colloids in molten inorganic salts.

A colloidal solution is a homogeneous dispersion of particles or droplets of one phase (solute) in a second, typically liquid, phase (solvent). Colloids are ubiquitous in biological, chemical and technological processes, homogenizing highly dissimilar constituents. To stabilize a colloidal system against coalescence and aggregation, the surface of each solute particle is engineered to impose repulsive forces strong enough to overpower van der Waals attraction and keep the particles separated from each other. Electrostatic stabilization of charged solutes works well in solvents with high dielectric constants, such as water (dielectric constant of 80). In contrast, colloidal stabilization in solvents with low polarity, such as hexane (dielectric constant of about 2), can be achieved by decorating the surface of each particle of the solute with molecules (surfactants) containing flexible, brush-like chains. Here we report a class of colloidal systems in which solute particles (including metals, semiconductors and magnetic materials) form stable colloids in various molten inorganic salts. The stability of such colloids cannot be explained by traditional electrostatic and steric mechanisms. Screening of many solute-solvent combinations shows that colloidal stability can be traced to the strength of chemical bonding at the solute-solvent interface. Theoretical analysis and molecular dynamics modelling suggest that a layer of surface-bound solvent ions produces long-ranged charge-density oscillations in the molten salt around solute particles, preventing their aggregation. Colloids composed of inorganic particles in inorganic melts offer opportunities for introducing colloidal techniques to solid-state science and engineering applications.

A long-life rechargeable Al ion battery based on molten salts

In this study, we established a rechargeable aluminum ion super battery with high-rate capability using a low temperature inorganic molten salt which is much cheaper, safer and environmentally friendly.

Corresponding-states behavior of an ionic model fluid with variable dispersion interactions.

Guggenheim's corresponding-states approach for simple fluids leads to a remarkably universal representation of their thermophysical properties. For more complex fluids, such as polar or ionic ones, deviations from this type of behavior are to be expected, thereby supplying us with valuable information about the thermodynamic consequences of the interaction details in fluids. Here, the gradual transition of a simple fluid to an ionic one is studied by varying the relative strength of the dispersion interactions compared to the electrostatic interactions among the charged particles. In addition to the effects on the reduced surface tension that were reported earlier [F. Leroy and V. C. Weiss, J. Chem. Phys. 134, 094703 (2011)], we address the shape of the coexistence curve and focus on properties that are related to and derived from the vapor pressure. These quantities include the enthalpy and entropy of vaporization, the boiling point, and the critical compressibility factor Zc. For all of these properties, the crossover from simple to characteristically ionic fluid is seen once the dispersive attraction drops below 20%-40% of the electrostatic attraction (as measured for two particles at contact). Below this threshold, ionic fluids display characteristically low values of Zc as well as large Guggenheim and Guldberg ratios for the reduced enthalpy of vaporization and the reduced boiling point, respectively. The coexistence curves are wider and more skewed than those for simple fluids. The results for the ionic model fluid with variable dispersion interactions improve our understanding of the behavior of real ionic fluids, such as inorganic molten salts and room temperature ionic liquids, by gauging the importance of different types of interactions for thermodynamic properties.

Intermediate Temperature Molten Salt Lithium Batteries, New Chemistries and Beyond

A current focus in battery research, particularly for electric vehicles and stationary energy storage applications, is identifying new chemistries that could enable a higher energy density and lower cost battery than currently available Li-ion batteries. Li-ion electrode materials, which dictate attainable battery energy densities, used in these batteries have changed little in the past decades, and therefore physical limits in Li-ion battery pack energy density are being approached given the enormous development efforts in the field. In this work, we take a close look at molten salt lithium batteries and in particular alkali metal nitrate/nitrite molten salt electrolytes which exhibit low melting point temperature, high ionic conductivity, high thermal stability and good interfacial properties against lithium metal anode. More specifically, we investigated the O2 electrode electrochemical behavior as well as the reduction of nitrates on metal and metal oxide nanoparticles to form lithium oxide and nitrites. The results reported herein provide a potentially transformative approach to enable a rechargeable Li-O2 battery chemistry through the use of inorganic molten salts as electrolytes. Building on our previous foundational research, in which we identified that all organic electrolytes are incompatible with the reversible Li-O2 electrochemistry, the electrolyte compositions reported here do not contain unstable organic solvents. As a result, the reversibility of the Li-O2 electrochemistry reported here is unprecedented. Another approach developed at Liox Power, Inc. is the use of nitrates as electroactive material, or in other words, as both the electrolyte and the cathode material. In this case the battery discharge reaction proceeds as the electrochemical reduction of nitrate anions NO3 - present in the electrolyte phase. High Li+ and NO3 -concentration and ionic conductivity allow extremely fast kinetics at 150 °C resulting in very low discharge overpotentials. Furthermore, we demonstrate that such electrochemical reaction is reversible and lithium oxide particles (octahedron equilibrium Wulff shape) can be recharged with energy efficiencies greater than 97%. Figure 1. Top, voltage profile (first 2 cycles) of a cell employing a Li metal anode, a LiNO3-KNO3 eutectic and a Ni cathode, cycling at 150 °C at 0.32 mA/cm2, under Ar gas. Bottom, voltage profiles of (black) organic electrolyte Li-O2 cell and (red) LiNO3-KNO3 molten salt Li-O2 cell; both cells used a Li metal anode, a Super P carbon black cathode (5% PTFE binder) and cycled at 0.32 mA/cm2 under pure O2 gas. Figure 1