Low Temperature Molten Salt Research Articles

Gradient design for Si-based microspheres as ultra-stable Li-storage anode

High-capacity Si-based microspheres are being spotlighted as a promising substitute for commercial spherical graphite anodes in the development of high-energy lithium-ion batteries. Nevertheless, the formidable challenge of their severe mechanochemical degradation during the (de)lithiation process remains unaddressed currently. Herein, we present a Si-based microsphere prepared by the oxygen pumping mechanism under a cost-efficiently low-temperature (250 °C) molten salt reduction environment. By optimally controlling oxygen gradient distribution, the resulted Si-based microspheres exhibit the unique coherent architecture ranging from ordered crystalline Si core to disordered SiO2(v) shell. Their structural coherence but regional difference in function achieves a perfect combination of structural compatibility and optimized chemo-mechanical effect, endowing the obtained Si-based microspheres with a nearly intact morphology after 1500 cycles and a 97% capacity retention after 1000 cycles at 2 A g-1. Our design broadens research directions for Si anode material design, which will accelerate the practical application of micro-sized Si anode materials.

Rapid Preparation of Platinum Catalyst in Low-Temperature Molten Salt Using Microwave Method for Formic Acid Catalytic Oxidation Reaction.

In this paper, platinum nanoparticles with a size of less than 50 nm were rapidly and successfully synthesized in low-temperature molten salt using a microwave method. The morphology and structure of the product were characterized by SEM, TEM, EDX, XRD, etc. The TEM and SEM results showed that the prepared product was a nanostructure with concave and uniform size. The EDX result indicated that the product was pure Pt, and the XRD pattern showed that the diffraction peaks of the product were consistent with the standard spectrum of platinum. The obtained Pt/C nanoparticles exhibited remarkable electrochemical performance in a formic acid catalytic oxidation reaction (FAOR), with a peak mass current density of 502.00 mA·mg-1Pt and primarily following the direct catalytic oxidation pathway. In addition, in the chronoamperometry test, after 24 h, the mass-specific activity value of the Pt concave NPs/C catalyst (10.91 mA·mg-1Pt) was approximately 4.5 times that of Pt/C (JM) (2.35 mA·mg-1Pt). The Pt/C NPs exhibited much higher formic acid catalytic activity and stability than commercial Pt/C. The microwave method can be extended to the preparation of platinum-based alloys as well as other catalysts.

Ultrafast Synthesis of MXenes in Minutes via Low-Temperature Molten Salt Etching.

Developing green and efficient preparation strategies is a persistent pursuit in the field of 2D transition metal nitrides and/or carbides (MXenes). Traditional etching methods, such as HF-based or high-temperature Lewis-acid-molten-salt etching route, require harsher etching conditions and exhibit lower preparation efficiency with limited scalability, severely constraining their commercial production and practical application. Here, an ultrafast low-temperature molten salt (LTMS) etching method is presented for the large-scale synthesis of diverse MXenes within minutes by employing NH4HF2 as the etchant. The increased thermal motion and improved diffusion of molten NH4HF2 molecules significantly expedite the etching process of MAX phases, thus achieving the preparation of Ti3C2Tx MXene in just 5minutes. The universality of the LTMS method renders it a valuable approach for the rapid synthesis of various MXenes, including V4C3Tx, Nb4C3Tx, Mo2TiC2Tx, and Mo2CTx. The LTMS method is easy to scale up and can yield more than 100g Ti3C2Tx in a single reaction. The obtained LTMS-MXene exhibits excellent electrochemical performance for supercapacitors, evidently proving the effectiveness of the LTMS method. This work provides an ultrafast, universal, and scalable LTMS etching method for the large-scale commercial production of MXenes.

Boosting Li-Ion Conductivity of Fluoride Solid Electrolyte by Low-Temperature Molten Salt Ablation and Particle Boundary Doping.

Halide solid electrolytes (SEs) are attracting great attention, owing to their high ionic conductivity and excellent high-voltage compatibility. However, severe moisture sensitivity, poor thermal stability, and instability at the lithium metal anode interface with chloride and bromide SEs retard their applications in solid-state lithium metal batteries. Fluoride SEs are expected to solve these problems, but they are now plagued by inadequate room-temperature (RT) ionic conductivity. Herein, a low-temperature molten salt (LiCl+1.33AlCl3) ablation method is proposed to enhance the ionic conductivity of monoclinic Li3GaF6 by particle boundary doping. The RT ionic conductivity of Li3GaF6 is correspondingly increased by 2 orders of magnitude, and the conductivity reaches 10-4 S cm-1 at 60 °C. The improved ionic conductivity benefits from the enhancement of interfacial ion transport, with the formation of more conductive chlorine-doped Li3GaF6-xClx and in situ binder LiAlCl4 to cement surrounding nanoparticles. The as-synthesized Li3GaF6 demonstrates outstanding humidity tolerance without conductivity degradation after exposure to a relative humidity of up to 35%. It also exhibits the widest electrochemical stability window experimentally (close to 6 V) compared with other state-of-the-art SEs. The solid-state Li/Li3GaF6/LiFePO4 cell with a stable Li+-conductive polymer interface is successfully driven for at least 200 cycles at 0.5C. Our study provides a solution to various chemical and electrochemical stability issues encountered by the halide SE family.

Ce-doped cobalt-based hydroxide assisted with low-temperature molten salt for industrial oxygen evolution reaction

Developing low cost and high-performance oxygen evolution electrocatalysts is significant to improve the efficiency of water electrolysis for large-scale hydrogen production. Cobalt hydroxide is a promising electrocatalyst for oxygen evolution reaction (OER), but its poor conductivity and activity seriously restrict the practical application. A simple one-step low temperature molten salt method was applied to successfully synthesize the Ce-doped cobalt hydroxide nitrate (Ce-CoNH/CF), which exhibits outstanding OER performance with a low overpotential of 448 mV at the current density of 1000 mA/cm2 in 1 mol/L KOH. The remarkable performance of Ce-CoNH/CF electrode in OER may be the comprehensive result of fast reaction kinetics, large electrochemical active specific surface area (ECSA) and small charge transfer resistance (Rct) as revealed by the Tafel, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) analysis. Under the simulated industrial test conditions (6 mol/L KOH, 70 °C), the Ce-CoNH/CF electrode still displays excellent OER performance.

Repair and Reuse of Spent Lithium Battery Electrode Materials

With the popularity of new energy vehicles and various electronic devices, the use of lithium-ion batteries (LIBs) has shown explosive growth, which has resulted in a large number of spent LIBs. Spent LIBs contain a large amount of metal resources, and improper disposal will not only cause waste of resources, but also have potential environmental risks. The existing commercial recycling methods are mainly pyrometallurgy and hydrometallurgy recycling methods, both of which require re-extraction from the electrode material after destroying them to the atomic level for the preparation of new electrode materials, which is a long process with high cost, and involves the application of extreme conditions such as high temperature and strong acid, with inferior economic and environmental benefits. It is a major challenge in the field of battery recycling to develop innovative and clean recycling methods, to simplify the recycling process and to develop ways to reuse the recovered products. In view of the challenge of existing recycling methods, the reporters proposed the idea of direct recycling of electrode materials at the molecular scale, and designed innovative recycling methods such as direct repair of degraded lithium cobalt oxides with deep eutectic solvent (DES), repair of Ni-Mn-Co ternary (NCM) cathode with high failure degree by low temperature molten salt, and thermal regeneration of degraded lithium iron phosphate (LFP) with multifunctional solvent; and closed-loop design of the direct recycling process. In addition, we propose a high-value utilization path for the conversion of degraded electrode materials to high-performance electrode materials and nano-catalysts. It greatly simplifies the recycling process, avoids the use of extreme conditions, and provides a new technical system and theoretical guidance for battery recycling research and industry.

CO2 transformed into highly active catalysts for the oxygen reduction reaction via low-temperature molten salt electrolysis

The implementation of a technology capable of capturing and converting CO2 into valuable products is one of the key requirements for limiting the effects of our carbon-intensive industries. At the same time, future CO2 emissions need to be reduced to combat climate change, meaning that new devices capable of storing and converting energy without CO2 emissions have to be adopted widely. In this work, we demonstrate catalysts made directly from CO2 for fuel cells and zinc-air batteries. The molten salt electrolysis process is used to electrodeposit solid carbon from CO2 in two mixtures, a known eutectic mixture of Li2CO3, Na2CO3, K2CO3 and a new mixture containing 0.1 mol of LiOH in addition. The effects of the electrolyte towards the final carbon product and its electrocatalytic activity are analysed using the rotating disk electrode method, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The porosity of the materials is described by N2 adsorption and the best performing catalyst is compared to the activity of a commercial 20 wt% PtRu/C material in a zinc-air battery.

A low-temperature molten salt synthesis strategy of Eu2Ti2O7 ceramic enabling ultra-durable glass-ceramic waste forms

The conventional solid-phase sintering method for the preparation of Ln2Ti2O7 ceramics using oxides as reactants generally requires higher temperature, larger pressure, and greater energy consumption. In this paper, we synthesize a Eu2Ti2O7 ceramic powder in relatively low-temperature molten salt (KCl–NaCl) using Eu2O3 powder and TiO2 powder as reactants at 1000 °C. The results of XRD, SEM, and element mapping indicate that the product have high phase purity, good crystal morphology and uniform chemical composition. Moreover, Eu2Ti2O7-based glass-ceramics are obtained by sintering with different contents of borosilicate glass. The SEM images of glass-ceramic show that almost no obvious defects or voids are found on the surface of the samples. The values of water absorption for all glass-ceramic waste forms are less than 0.5 %. With the increase of ceramic embedding rate, the hardness of the glass-ceramics first increases and then decreases, while the density gradually increases. When the embedding rate increased to 30 wt%, the hardness of glass-ceramic reaches a maximum of 19.53 GPa. The glass-ceramic waste form embedded with 30 wt% Eu2Ti2O7 exhibits very stable chemical durability with a low leaching rate of 1.42 × 10−8 g m−2 d−1 at 90 °C even after 28 days.

Ca/P in situ introduction for enhancing coating biocompatibility via plasma electrolytic oxidation in low-temperature molten salt

Plasma electrolytic oxidation (PEO) is one of the most promising methods for synthesizing ceramic coatings on metallic substrates. Recently, PEO in molten salt electrolytes was found to be advantageous due to overcoming system heating and the formation of contaminants. However, the PEO in molten salt was conducted at a high temperature (Tm∼220 °C), limiting the introduction of different components such as Ca and P. This study realizes the employment of a low-temperature electrolyte based on a ternary eutectic system of Ca(NO3)2–NaNO3–KNO3 (Tm∼130 °C) jointly with the in situ introduction of Ca/P in the form of ammonium dihydrogen phosphate (ADP). The surface morphology, phase and chemical composition, wettability, and anti-corrosion performance were examined. The results indicate improved surface performance with the addition of 1 wt% ADP to the electrolyte. This surface preferably comprises the anatase phase of TiO2 and exhibits enhanced biocompatibility and corrosion resistance in a biological environment.

Selective electro-oxidative separation of Sm, Yb, and Eu in low-temperature molten salt AlCl3-NaCl

The variable valence rare earth elements are difficult to separate due to their similar properties and intermediate valence states in molten salts. In order to realize their efficient and selectivity separation, the properties and separation methods of Sm, Yb, and Eu have been investigated in a low-temperature AlCl3-NaCl molten salt. It has been found that the oxidation reactions of different Ln(II) to Ln(III) have different potentials. The Ln(II)/Ln(III) reaction is also irreversible when the AlCl3 content is less than 50 mol%, Ln(III) exists in the form of LnCl3 precipitation. Therefore, selectively modulating the valence of Sm, Yb, and Eu by electrochemical methods can achieve their separation. The LnCl3 produced from the Ln(II) electro-oxidation precipitates and separates from the other unoxidized soluble Ln(II) in AlCl3-NaCl melt. The efficient separation of Ln(II) in AlCl3-NaCl can be achieved by pulse potentiostatic electrolysis. Under these conditions, the separation factors of Sm-Eu, Sm-Yb, and Yb-Eu can reach 103. The maximum removal of Ln(II) exceeded 99 % with a maximum current efficiency of more than 90 %. Most importantly, the separation of variable valence rare earth elements has been realized in a low-temperature AlCl3-NaCl melt.

Preparation of aluminum-tin alloy by electrodeposition in low-temperature molten salt system

Preparation of aluminum-tin alloy by electrodeposition in low-temperature molten salt system

Phase formation, microstructural and dielectric properties of bismuth ferrite nanoceramics synthesized by low-temperature molten salt flux route

BiFeO3 nano-ceramics were synthesized using the low-temperature molten salt synthesis route to study the influence of calcination temperature and salt ratios. The Rietveld refinement revealed the distorted perovskite structure of BFO. These granular-shaped BFO samples’ grains morphology was studied using the FESEM. The FTIR revealed the presence of Metal-Oxygen bonds in the BFO ceramics. The XPS studies exhibited the respective constituent elements’ oxidation states and validated the presence of oxygen ion vacancies. The frequency-dependent dielectric properties were measured at various temperatures. The electrical conductivity measurement showed a linear variation as a function of frequency which validated Jonscher’s power law.

An artificial aluminum-tin alloy layer on aluminum metal anodes for ultra-stable rechargeable aluminum-ion batteries.

Rechargeable aluminum ion batteries (RAIBs) exhibit great potential for next-generation energy storage systems owing to the abundant resources, high theoretical volumetric capacity and light weight of the Al metal anode. However, the development of RAIBs based on Al metal anodes faces challenges such as dendrite formation, self-corrosion, and volume expansion at the anode/electrolyte interface, which needs the rational design of an aluminum anode for high-performance RAIBs. This work proposes a novel and low-cost strategy by utilizing an alloy electrodeposition method in a low-temperature molten salt system to fabricate an aluminum-tin (AlSn) alloy coating layer on copper foil as the anode for RAIBs, which successfully addresses the issues of dendrite formation and corrosion at the anode/electrolyte interface. The artificial AlSn alloy layer could enhance the active sites for metal Al homogeneous deposition and effectively retard the dendrite formation, which was verified by an in situ optical microscopy study. The symmetric AlSn@Cu cell demonstrates a low average overpotential of ∼38 mV at a current density of 0.5 mA cm-2 and a long-term lifespan of over 1100 h. Moreover, the AlSn@Cu//Mo6S8 full cells deliver a high capacity of 114.9 mA h g-1 at a current density of 100 mA g-1 and maintain ultra-stable cycling stability even over 1400 cycles with a ∼100% coulombic efficiency (CE) during the long-term charge/discharge processes. This facile alloy electrodeposition approach for designing high-performance Al-based anodes provides insights into the understanding of artificial interface chemistry on Al-based anodes and potentially accelerates the design of high-performance RAIBs.

Batteries Worth Their Salt: Targeting Next Generation Stationary Storage with Molten Salt Electrolytes

There is an imminent need for batteries capable of meeting the evolving demands of a growing stationary storage landscape, one rich in renewable energy sources and challenged with unprecedented global electrification. Key to our future electrical grid, desirable large-scale, long-duration batteries must not only meet demands for high power and energy density, but they must be safe, cost-effective, and should be sustainably sourced from earth-abundant materials. Here, we will highlight our research and development of low temperature molten salt batteries that employ active materials such as sodium, aluminum, iron, chloride, and iodide. We will demonstrate the promise of our approach through a low-temperature (100°C) molten sodium battery that comprises a molten sodium anode, a solid-state NaSICON ceramic separator, and a low-cost molten metal-halide salt catholyte. These batteries, with remarkable voltages greater than 3.1V, are capable of months of cycling as well as high current charge and discharge behavior, greater than 50mA/cm2. Here, we will not only highlight these performance metrics, but we will describe how we used specialized electrochemical and chemical characterization technique to understand the impacts of molten salt electrolyte composition, electrochemistry, and interfacial interactions on battery performance. By understanding the connections between specific electrochemical phenomena and battery chemistry, we have learned to improve battery performance through tuning of properties, such as molten salt Lewis acidity and current collector composition. This scientifically-motivated battery design approach continues to enable agile innovation and advances to realize the potential of molten salt batteries as a new, impactful stationary storage technology.Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Molten salt mediated in-situ template formation of reticulated foams of complex oxides and carbides

A novel low temperature molten salt mediated in-situ template formation (MSMITF) strategy was developed to synthesise high quality reticulated foams of complex oxides and carbides from their single oxide and carbon counterparts, respectively. As working tools, CaZrO3 and TiC foams were prepared in CaCl2 and KCl at as low as 1300 and 1050 °C, respectively, which retained largely major macroporous features of the pristine ZrO2 and C template foams, demonstrating well the feasibility of the MSMITF strategy. It is believed that the proposed strategy would be universally applicable to preparing a range of complex oxide and carbide foams, potentially benefiting various important areas.

Passivation phenomenon and variable properties of ytterbium in different Lewis acid AlCl3-NaCl melts

To further explore the application of the low-temperature molten salt in processing heavy rare earths, the properties of ytterbium in AlCl3-NaCl melts have been investigated. In this melt, YbCl2 powder products are prepared by electrolysis based on the dissolution reaction of Yb metal. The properties of Yb(II) in AlCl3-NaCl melts have also been studied in detail. In-situ UV–VIS absorption spectroscopy demonstrates that the coordination of Yb(II) changes in AlCl3-NaCl melts with different Lewis acidity, leading to variations in its diffusion ability. In addition, the conversion of Yb(II)/Yb(III) shows typical soluble-soluble reversible electrochemical behavior in strongly acidic AlCl3-NaCl. However, the electrochemical behavior of Yb(II) on inert electrodes becomes complicated when the amount of AlCl3 in AlCl3-NaCl melts less than 52 mol%. This change is attributed to the formation of a YbCl3 passivation film on the inert electrode during the electro-oxidation of Yb(II). Soaking the passivated electrode in the melt can alleviate the passivation. Understanding the electrochemical and spectroscopic properties of ytterbium in AlCl3-NaCl melts paves the way for its subsequent applications.

Synthesis and catalytic mechanism for NiS/NiS2/NC nanoparticles towards oxygen reduction reaction and oxygen evolution reaction

The advancement of efficient and environmentally viable bifunctional electrocatalysts, capable of facilitating both the oxygen reduction reaction and oxygen evolution reaction, assumes a pivotal role in driving the commercial progression of metal-air batteries. This research employs the low-temperature molten salt template method to fabricate the NiS/NiS2/NC catalyst in an atmosphere devoid of noble gasses protection. BET characterization indicates that the composite of NiS/NiS2 and NC can enhance the specific surface area and effectively enhancing the reactive sites. The NiS/NiS2/NC catalyst exhibits remarkable bifunctional catalytic prowess in 1 M KOH electrolyte, manifesting a half-wave potential of 0.871 V and an overpotential of 195 mV for the OER at a current density of 10 mA cm−2. Moreover, the NiS/NiS2/NC catalyst demonstrates outstanding electrochemical stability. Furthermore, density functional theory calculations have substantiated a cooperative influence arising from the interaction between NiS2 and NC, which engenders a significant advancement in the bifunctional catalytic activity of the synthesized composite. These findings provide a novel avenue and theoretical insight for the design and synthesis of advanced bifunctional electrocatalysts.

High-quality SiC-HfC coating with interpenetrating structure based on a two-step low temperature molten salt method

High-quality SiC-HfC coating with interpenetrating structure based on a two-step low temperature molten salt method

Recovery of LiNi0.5Mn0.3Co0.2O2 cathode material from spent lithium-ion batteries with oxygen evolution reduction in ammonium sulfate low-temperature molten salt

The advancement and extensive application of lithium-ion batteries (LIBs) as an energy storage device has led to an increase in the number of spent LIBs, and recycling spent LIBs is an urgent need to address their resource and environmental sustainability issues. Herein, an efficient and comprehensive recycling method for spent LiNi0.5Mn0.3Co0.2O2 cathode material (NMC 532), one of the most important materials for electric vehicles and grid storage applications, is developed based on ammonium sulfate low-temperature molten salt (LTMS) reduction with oxygen evolution, and the feasibility of the strategy is systematically investigated. The results demonstrate that (NH4)2SO4 LTMS can effectively realize the sulfate conversion of spent NMC 532, in which the sulfate conversion rates of Li, Ni, Mn, and Co all reach more than 90%. Through mechanism analysis, it is found that the induced superoxide ion (O2·-) and H+ are the keys to the reduction of transition metal oxides in LTMS. Meanwhile, O2·-, as a single electron reducing agent in the intermediate state, can be used to explain the reduction behavior of transition metal oxides in other ammonium salts, mineral acids, and H-containing salts in molten. Based on (NH4)2SO4 LTMS reduction, water leaching, thermodynamic analysis of coprecipitation, and a series of precipitation and separation experiments, this study provides an effective strategy for recycling spent cathode materials, which can realize the combination of spent material recycling and electrode material regeneration.

Relaying alkaline hydrogen evolution over locally amorphous Ni/Co-based phosphides constructed by diffusion-limited phase-transition

Ni/Co-based phosphides are a family of promising electrocatalysts for hydrogen evolution reaction (HER), but the sluggish water dissociation kinetics impedes their applications in alkaline condition. Herein, locally amorphous NiCoP (LA-NiCoP) with enhanced orbital coupling to facilitate water dissociation is constructed by a low-temperature molten salt enabled diffusion-limited phase-transition strategy. Time-dependent experiment systematically clarifies the crystallinity regulation mechanism of NiCoP associated with its nonclassical crystallization process of amorphous-to-crystalline transition. As the alkaline HER electrocatalyst, LA-NiCoP exhibits a close to Pt/C activity (ƞ10 =45 mV). Both experimental and theoretical results show that the Co atoms in the amorphous phase display higher proportion of unoccupied 3d orbitals than the crystalline counterpart, which interacts with the O 2p of H2O for activating the cleavage of HO–H bond. This work not only provides a facile approach to prepare locally amorphous phosphides, but also offers a new protocol to accelerate the water dissociation kinetics.