Molten Alkali Metal Research Articles

Covalent Surface Alteration of MXenes and Its Effect on Superconductivity

Surface functional groups such as F, O, and OH attached to MXenes have a profound impact on the electronic properties. Generally, MXenes are synthesized by the aqueous acidic etching of the MAX phase, and these functional groups are inherently incorporated during the synthesis. Recently, covalent surface modification of MXene has been demonstrated at low temperature using molten inorganic salts.

Low-Temperature Multielement Fusible Alloy-Based Molten Sodium Batteries for Grid-Scale Energy Storage.

The sustainable future of modern society relies on the development of advanced energy systems. Alkali metals, such as Li, Na, and K, are promising to construct high-energy-density batteries to complement the fast-growing implementation of renewable sources. The stripping/deposition of alkali metals is compromised by serious dendrite growth, which can be intrinsically eliminated by using molten alkali metal anodes. Up to now, most of the conventional molten alkali metal-based batteries need to be operated at high temperatures. To decrease the operating temperature, we extended the battery chemistry to multielement alloys, which provide more flexibility for wide selection and rational screening of cost-effective and fusible metallic electrodes. On the basis of an integrated experimental and theoretical study, the depressed melting point and enhanced interfacial compatibility are elucidated. The proof-of-concept molten sodium battery enabled by the Bi–Pb–Sn fusible alloy not only circumvents the use of costly Ga and In elements but also delivers attractive performance at 100 °C, holding great promise for grid-scale energy storage.

Genetic Algorithm Driven Force Field Parameterization for Molten Alkali-Metal Carbonate and Hydroxide Salts

Molten alkali-metal carbonates and hydroxides play important roles in the molten carbonate fuel cell and in Earth's geochemistry. Molecular simulations allow us to study these systems at extreme conditions without the need for difficult experimentation. Using a genetic algorithm to fit ab intio molecular dynamics-computed densities and radial distribution functions, as well as experimental enthalpies of formation, we derive new classical force fields able to accurately predict liquid chemical potentials. These fitting properties were chosen to ensure accurate liquid phase structure and energetics. Although the predicted dynamics is slow when compared to experiments, in general the trends in dynamic properties across different systems still hold true. In addition, these newly parametrized force fields can be extended to the molten carbonate-hydroxide mixtures by using standard combining rules.

Ceramic/Metal-Supported, Tubular, Molten Carbonate Membranes for High-Temperature CO2 Separations

Dual-phase membranes, consisting of molten alkali metal carbonates infiltrated into an oxygen ion or mixed-conducting substrate, are a promising technology for high-temperature CO2 separations and other energy conversion applications, such as membrane reactors. In this paper, we report a new fabrication technique for triple-phase membranes capable of high-temperature CO2 transport using an oxygen ion-conducting ceramic combined with an electronic-conducting metal layer. An oxygen ion-conducting, 4YSZ porous substrate tube was coated with a thin layer of metal (Pd/Ag or Ni) using electroless plating to prepare a mixed-conducting (oxygen ion and electronic) support for the triple-phase membranes. The porous section was then filled with a molten carbonate (MC) salt consisting of a eutectic mixture of Li/K or Na/K carbonates. This enabled both the mixed oxygen and carbonate ion conductor (MOCC) and mixed electron and carbonate ion conductor (MECC) CO2 transport mechanisms. The highest CO2 permeance (flux/driving force) of 1.07 × 10–7 mol m–2 s–1 Pa–1 (a flux of 0.8 cm3(STP) cm–2 min–1) was measured on a 4YSZ support coated with a Pd/Ag layer and infiltrated with the eutectic Li/K carbonate salt; it was measured at 725 °C with a 50/50 CO2/O2 feed gas mixture at a transmembrane pressure of 110 kPa. The apparent activation energy for CO2 transport was calculated to be 57 kJ mol–1 between 450 and 600 °C, suggesting that the rate-limiting step was the diffusion of the carbonate ion in the molten carbonate; however, the activation energy was observed to increase to 74 kJ mol–1 (650–750 °C), which was attributed to the influence of oxygen anion transport at higher temperatures.

Effect of Alkali Hydroxide on the Properties of PTFE Graphite Bonded Aluminium Titanium Composite Sheet

This study focused on the effect of some alkali hydroxide on the properties of a composite sheet prepared by power spray coating of clean aluminium-titanium sheets with PTFE/graphite emulsion. The blank sheets were washed with hot water and dried at 80 °C for 5-6 minutes before power spraying of colorless polytetrafluoroethylene PTFE/graphite emulsion in perfluoro kerosene followed by hot-rolling under a pressure of 150-200 KPa at the temperature 50 °C higher than the melting temperature of the PTFE polymer (Du Pont). The obtained sandwich was sintered at 550 °C for a few minutes. Alkali hydroxide was added to the polymer emulsion before spraying. Results showed that temperature and time enhanced the extent of adhesion of the two metals to form a homogeneous composite metal sheet. Alkali hydroxide inclusion deteriorates the stability of the prepared sheet. The alkali effect was because PTFE is inactive material, Incompatible with molten alkali metals and alkali hydroxide. The disability of the alkali hydroxide to be incompatible with the polymer material experienced changes in its intrinsic properties. Alkali halides have insignificant effect. Alkali halides just displayed a filling in material.

Rapidly Thinning Silicon Carbide By Anodization

SiC is a semiconductor material used to fabricate electronic devices that can operate under severe conditions, such as high temperatures, and which have some advantages over silicon-based devices (for example, high frequency and high power). The hardness of SiC is quite high (Mohs=9.5) and strong chemically inert, making it difficult be thinned. Up to now, since the majority carriers of n-type SiC are electrons, its application is more extensive, but it is also more difficult to thin. An effective etching method is by the reactive ion etching (RIE) method, which may produce satisfactory results in a smooth surface. In addition, according to reports, an efficiently etching processing can be performed by a high temperature (>700 C) molten alkali metal hydroxide (for example, NaOH). However, the equipment cost, operation convenience and thinning speed of these etching approaches are not as well as electrochemical etching, i.e., anodization. However, n-type SiC is very difficult to be anodized because of the lack of electric holes which are the key species for etching. In our report, we used a wafer bonding processing to form a temporary p-n junction by jointing to a p-type silicon, so that minority carriers, electric holes, in the mated p-type silicon can easily transfer to the SiC surface via a bias, resulting in a HF-based electrochemical etching. By the processing, the n-type SiC can be quickly thinned. After thinning, the p-type silicon subatrate can be easily removed without the chemical contamination on the thinned SiC substrate.Figure: The anodized surface of n-type SiC. Figure 1

Bench-Scale Demonstration of Molten Alkali Metal Borates for High-Temperature CO2 Capture

The molten alkali metal borates represent an opportunity for carbon dioxide capture and the on-going challenge of adapting to a carbon-constrained world with the immediacy of climate change. We demonstrate the performance of this new class of materials at the bench scale, highlighting the ability to operate isothermally at high temperatures through the use of steam as a sweep gas. Breakthrough curves for a wide set of conditions are presented and linked to the sorbents’ underlying thermodynamic and kinetic properties. We demonstrate the ability to capture over 99.9% of incoming CO2 even at low concentrations or to capture ∼90% CO2 under high flow rates and utilize a greater portion of the sorbent capacity. The advantages of the molten alkali metal borates scale well, and their unique position as high-temperature liquid phase sorbents may enable new process designs for efficient low-cost CO2 capture facilities.

Acid Gas Capture at High Temperatures Using Molten Alkali Metal Borates.

Materials designed for CO2 capture provide both an opportunity and a challenge in that industrial emissions typically contain an assortment of acid gasses, which may include SOx and NOx alongside CO2. Growing pressure to reduce emissions of all acid gasses, CO2 included, presents an opportunity for simultaneous capture and a challenge in handling the resultant products. Molten alkali metal borates embody a new class of high-temperature liquid-phase materials for carbon dioxide capture and we propose here that they can also be used to address the more general challenge of acid gas capture. We examine the melt capture performance at industrially relevant concentrations and mixtures, identifying the various reaction mechanisms and products, and propose designs for separating these products efficiently at high temperatures, so that they outperform the state-of-the-art CO2 capture technologies in handling this opportunity challenge. We also discuss the conditions to avoid and the challenges that lie ahead for these materials in the context of emission reduction and environmental protection.

Lithiophilic Three-Dimensional Porous Ti3C2Tx-rGO Membrane as a Stable Scaffold for Safe Alkali Metal (Li or Na) Anodes.

Metallic anodes have high theoretical specific capacities and low electrochemical potentials. However, short-circuit problems caused by dendritic deposition and low Coulombic efficiency limit the cyclic life and safety of metallic anode-based batteries. Herein, dendrite-free and flexible three-dimensional (3D) alkali anodes (Li/Na-Ti3C2Tx-rGO) are constructed by infusing molten lithium (Li) or sodium (Na) metal into 3D porous MXene Ti3C2Tx-reduced graphene oxide (Ti3C2Tx-rGO) membranes. First-principles calculations indicate that large fractions of functional groups on the Ti3C2Tx surface lead to the good affinity between the Ti3C2Tx-rGO membrane and molten alkali metal (Li/Na), and the formation of Ti-Li/Na, O-Li/Na, and F-Li/Na mixed covalent/ionic bonds is extremely critical for uniform electrochemical deposition. Furthermore, the porous structure in Li/Na-Ti3C2Tx-rGO composites results in an effective encapsulation, preventing dendritic growth and exhibiting stable stripping/plating behaviors up to 12 mA·cm-2 and a deeper capacity of 10 mA·h·cm-2. Stable cycling performances over 300 h (750 cycles) at 5.0 mA·cm-2 for Li-Ti3C2Tx-rGO and 500 h (750 cycles) at 3.0 mA·cm-2 for Na-Ti3C2Tx-GO are achieved. In a full cell with LiFePO4 cathodes, Li-Ti3C2Tx-rGO electrodes show low polarization and retain 96.6% capacity after 1000 cycles. These findings are based on 2D MXene materials, and the resulting 3D host provides a practical approach for achieving stable and safe alkali metal anodes.

Performance of Nano-3YSZ toughened β’’-Alumina solid electrolyte prepared by EDTA-Zr(IV)/Y(III) complex as surface modifier

Due to its excellent ionic conductivity and electronic insulation, β/β″-alumina is widely used as a solid electrolyte in molten salt electrochemical cell and alkali metal thermoelectric convertor (AMTEC). In this article, nano-3mol% Yttrium partial stabilized zirconia (3YSZ) toughened β″-alumina solid electrolytes (BASEs) were fabricated by surface modification of β/β’’-alumina precursor particles with ethylenediaminetetraacetic acid (EDTA) - Zirconium (IV)/Yttrium (III) complex. The performance of BASEs with 0-10 wt% amount of nano-3YSZ addition (Zr-BASE) was systematically investigated on their phase composition, density, microstructure, bending strength, and conductivity. It was found that nano-3YSZ grains were intergranular and uniformly distributed in BASE matrix even at as low as 3 wt% amount of addition, which resulted in higher density, more homogeneous microstructure, higher Weibull modulus, higher characteristic strength and increased grain boundary resistance of Zr-BASE than those of BASE without 3YSZ addition. Zr-BASE with 5 wt% amount of nano-3YSZ addition demonstrated good conductivity (0.071 S•cm−1 at 300 °C, 0.33 S cm−1 at 600 °C) and higher characteristic strength (283 MPa). The obtained results indicated that surface modification by EDTA-Zr(IV)/Y(III) complex advantaged in the preparation of better performance BASEs with a small amount of nano-3YSZ addition.

(Invited) Nanocrevasse-Rich Carbon Fibers for Scalable Production and Stable Performance of Lithium and Sodium Metal Anodes

The metallic lithium (Li) and sodium (Na) anodes have long been considered as ideal anodes for high energy density batteries due to its high theoretical capacity and lowest reduction potential. However, the challenges induced by dendritic growth on metallic Li and Na anodes hinder the practical applications. Finding a stable host structure with polar functional groups is an essential strategy to prevent the Li and Na dendrite growth with improved electrochemical performance. Recently, many researchers have concentrated on development of host structure for storing the Li/Na metal. Nevertheless, complexity and inconvenience of the synthetic process still require further improvement of Li/Na metal anodes. In this talk, we will introduce a facile synthetic method to fabricate alkali metal/carbon composites which contain nanocrevasses in carbon fibers to facilitate the penetration of molten alkali metal via capillary forces. When the molten alkali metal is contacted to the carbon fibers with rGO-like crevasses, the metallic liquid infiltrates into the fine gaps within a few seconds. As well, the nanocrevasses provide a pretty large surface area which is enough to lower the specific current, ultimately leading to suppression of dendritic formation. The resulting alkali metal/carbon composites exhibit stable long-term cycling over hundreds of cycles. In addition, Unlike previous studies that focused on improving the performance of metal batteries, we carried out the test after applying the composite to actual equipment with scalable production process using recycled metal waste. Thus, the addition of nanocrevasses to carbon fiber as a scaffold for alkali metals can generate environmentally friendly and cost-effective composites for practical electrode applications References Go, W.; Kim, M.-H.; Park, J.; Lim, C.H.; Joo, S.H.; Kim, Y.; Lee, H.-W. Nano Lett. 2019, 19, 1504-1511.Lin, D.; Liu, Y.; Liang, Z.; Lee, H.-W.; Sun, J.; Wang, H.; Yan, K.; Xie, J.; Cui, Y. Nat. Nanotechnol. 2016, 11, 626-632.Zhang, R.; Chen, X.; Chen, X.; Cheng, X.; Zhang, X.; Yan, C.; Zhang, Q. Chem. int. Ed. 2017, 56, 7764-7768.Yun, J.H.; Kim, J.H.; Kim, D.K.; Lee, H.-W. Nano Lett. 2018, 18, 475–481. Figure 1

Multigram Syntheses of Magnesium(I) Compounds Using Alkali Metal Halide Supported Alkali Metals as Dispersible Reducing Agents

The alkali metal halide supported alkali metal materials, ca. 5% w/w Na/NaCl and 5% w/w K/KI, are prepared without specialist equipment by rapidly stirring molten alkali metal with finely ground al...

Nanocrevasse-Rich Carbon Fibers for Stable Lithium and Sodium Metal Anodes.

Metallic lithium (Li) and sodium (Na) anodes have received great attention as ideal anodes to meet the needs for high energy density batteries due to their highest theoretical capacities. Although many approaches have successfully improved the performances of Li or Na metal anodes, many of these methods are difficult to scale up and thus cannot be applied in the production of batteries in practice. In this work, we introduce nanocrevasses in a carbon fiber scaffold which can facilitate the penetration of molten alkali metal into a carbon scaffold by enhancing its wettability for Li/Na metal. The resulting alkali metal/carbon composites exhibit stable long-term cycling over hundreds of cycles. The facile synthetic method is enabled for scalable production using recycled metal waste. Thus, the addition of nanocrevasses to carbon fiber as a scaffold for alkali metals can generate environmentally friendly and cost-effective composites for practical electrode applications.

Temperature and composition dependences of shear viscosities for molten alkali metal chloride binary systems by molecular dynamics simulation

Structures and properties of molten alkali chlorides is the foundation of electrolyte process. In the paper, shear viscosities of molten LiCl-NaCl, LiCl-KCl, LiCl-RbCl, LiCl-CsCl, NaCl-KCl, NaCl-RbCl and NaCl-CsCl in temperature range of near melting point to 1200 K within full composition range have been calculated intensively using equilibrium molecular dynamics (EMD) method. The calculated values are in good agreement with available experimental results. Simple databases for melt shear viscosity have been built based on the verified algorithm. Shear viscosities of these melts exhibit negative dependences on temperature and more complicated variation trends on compositions. Under the effects of local structure and interatomic force simultaneously, shear viscosities of molten LiCl-NaCl, LiCl-KCl, LiCl-RbCl, LiCl-CsCl, NaCl-RbCl and NaCl-CsCl decrease firstly and then fluctuate slightly as the contents of LiCl or NaCl are increased. However, addition of NaCl into molten KCl deteriorates the melt fluidity. Expressions of melt shear viscosity vs. temperature & composition have also been obtained according to the calculated data finally.

3D Wettable Framework for Dendrite‐Free Alkali Metal Anodes

AbstractThe infinite volume change and dendritic behavior in alkali metal anodes lead to low Coulombic efficiency and short‐circuit issues that significantly hamper renewed efforts at commercialization. Here, a dendrite‐free alkali metal anode, made by thermally preloading molten Li or Na into a 3D framework with high alkali wettability, is reported. In the mechanically robust 3D framework, carbon fiber (CF) serves as an electrical highway that provides fast charge transfer for the redox reaction. Through a facile solution‐based process, a SnO2 coating is introduced to modify the poor wetting behavior of the carbon framework and drastically improve both the electrochemical performance and reliability. The kinetic barrier to adhesion of molten alkali metals on the CF framework is eliminated by the mixed reaction with SnO2. The growth of dendrites is effectively repressed under the decreased local current density of the 3D framework. In full‐cell configurations with LiFePO4 cathodes, the Li–CF electrode shows reduced polarization and 90% capacity retention after 500 cycles in traditional carbonate electrolyte. Comparable improvements are also observed in 3D electrodes for Na metal batteries. These findings on a stable 3D carbon framework with improved wetting behavior provide significant practical implications for achieving safe and commercially viable alkali metal anodes.

Wetting of a Charged Surface of Glassy Carbon by Molten Alkali-Metal Chlorides

Values of the contact angle of wetting of a surface of glassy carbon by molten chlorides of lithium, sodium, potassium, and cesium are measured by the meniscus weight method to determine the common factors of wettability of solid surfaces by ionic melts upon a change in the salt phase composition and a jump in electric potential. It is found that with a potential shift in the positive direction the shape of the curve of the contact angle’s dependence on the potential varies upon substitution of one salt by another: the angle of wetting shrinks monotonously in lithium chloride but remains constant in molten cesium chloride. This phenomenon is explained by the hypothesis that the nature of the halide anion adsorption on the positively charged surface of an electrode is chemical and not electrostatic. It is shown that the adsorption process is accompanied by charge transfer through the interface, with covalent bonding between the adsorbent and adsorbate.

Selective Dissolution of Brass in the Molten Eutectic Mixture of Lithium, Sodium, and Potassium Carbonates

The dissolution of L63 brass in molten alkali metal carbonates is shown to occur at an operating temperature of 773 K in the potentiostatic and galvanostatic regimes. The size and number of pores are found to depend on the electrochemical parameters, namely, the applied potential and the current density.

The Properties of Carbons Derived through the Electrolytic Reduction of Molten Carbonates under Varied Conditions: Part I. A Study Based on Step Potential Electrochemical Spectroscopy

Carbons have been produced through the electrolytic reduction of molten alkali metal carbonate salts under a range of conditions. The resultant carbons consist of both graphitized and amorphous phases. The carbon surface has also been shown to be heavily functionalized with oxygen containing groups. Their charge storage capabilities as electrochemical capacitor materials have been investigated using cyclic voltammetry and step potential electrochemical spectroscopy (SPECS). Trends in carbon performance with electrodeposition conditions have been identified, and carbons with a specific capacitance as high as 375 F.g−1 have been obtained at scan rates of 10 mV.s−1. With respect to temperature of carbon deposition, carbon electrodeposited at 500°C has been identified as the best electrical double layer capacitor type material, and as a superior material for pseudocapacitive applications. The performance of a control activated carbon has been compared with the electrodeposited carbons, and it has been shown that, when electrodeposited at low current densities, the resultant carbon performance is superior to the activated carbon.

Estance of gold in molten LiCl and the effect of the nature of cation on the estance zero potentials of Au in molten alkali metal chlorides

The dependences estance vs. potential in molten LiCl are obtained. They have three zeros of estance near the melting temperature of the salt. When the temperature increases by 400 K, only one cathode zero is retained for equilibrium estance. A comparison of the potentials of this zero of estance in a row of alkali metal chlorides at the same temperature indicates their nonmonotonic dependence on the cation radius, in contrast to the potentials of the ECC maxima of liquid Pb, Bi, and In and the PZC of gold in this row.

Effect of the electrolyte composition on the corrosion of 12Kh17 steel in molten alkali metal carbonates

The corrosion of 12Kh17 steel in the melts of alkali metal carbonates containing additions of various chemical types is studied using anodic potentiostatic polarization. The morphology of the 12Kh17 steel surface is analyzed by electron-probe microanalysis. The introduction of corrosion passivators and activators affects the mechanism of local corrosion damages. The corrosion of the steel proceeds according to an electrochemical mechanism. Carbonate anions serve as oxidizers of the steel along with oxygen dissolved in the melt. Treatment of a corrosive medium can be used to protect metallic constructions against corrosion in high-temperature salt electrolytes. Combined introduction of corrosion activators and passivators in a salt phase makes it possible to form protective layers with a high corrosion resistance.