Molten Salt Electrolyte Research Articles

A freeze-thaw molten salt battery for seasonal storage

Grid-level storage of seasonal excess can be an important asset to renewable electricity. By applying the freeze-thaw thermal cycling strategy, here, we report Al-Ni molten salt batteries with effective capacity recovery over 90% after a period of 1–8 weeks as a proof-of-concept. We explore three activation methods of the nickel cathode in a molten-salt battery: (1) heat treating the cathode granules under H2/N2, (2) incorporating a partially charged NiCl2/Ni cathode, and (3) doping the molten salt electrolyte with sulfur. In particular, sulfur doping, a cost-efficient method suitable for large-scale applications, is not only effective in activating the Ni cathode initially but also invaluable for energy retention during thermal cycling. Overall, these Al-Ni molten salt batteries under thermal cycling show high retention in cell capacity over weeks, setting a direction for scalable seasonal storage.

Layer coupling between solutal and thermal convection in liquid metal batteries

For longer than one decade, liquid metal batteries (LMBs) are developed with the primary aim to provide economic stationary energy storage. Featuring two liquid metal electrodes separated by a molten salt electrolyte, LMBs operate at elevated temperature as simple concentration cells. Therefore, efficient mass transfer is a basic prerequisite for their economic operation. A thorough understanding of the relevant mechanisms cannot be achieved by studying single layers in isolation. With this motivation, the effects of solutal- and thermally-driven flow are studied, as well as the flow coupling between the three liquid layers of the cell. It is shown that solutal convection appears first and thermal convection much later. While the presence of solutal flow depends on the mode of operation (charge or discharge), the occurrence of thermal convection is dictated by the geometry (thickness of layers). The coupling of the flow phenomena between the layers is intriguing: while thermal convection is confined to its area of origin, i.e. the electrolyte, solutal convection is able to drive flow in the positive electrode and in the electrolyte.

Low-Temperature and High-Energy-Density Li-Based Liquid Metal Batteries Based on LiCl–KCl Molten Salt Electrolyte

Low-Temperature and High-Energy-Density Li-Based Liquid Metal Batteries Based on LiCl–KCl Molten Salt Electrolyte

Integration of ceramic felt as separator / electrolyte in lithium salt thermal batteries and the prospect of rechargeability

Ceramic felts as an immobilizing structure for molten salt electrolytes in thermal batteries are an effective alternative to widely used MgO. One of the significant benefits of ceramic felts is their high porosity and low weight. In this work Al2O3 and YSZ ceramic felts with bulk porosity of 97% and 96%, respectively, were investigated. The LiCl-KCl eutectic electrolyte was incorporated in ceramic felts at different loading levels, and the performance of the cells (capacity, specific energy, and charge/discharge behavior) was evaluated. LiSi/FeS2 thermal cells with YSZ and Al2O3 ceramic felt electrolyte/separators reported specific energy of 58.47 ± 0.05 Wh kg−1 and 43.96 ± 0.05 Wh kg−1, respectively, which are two times the specific energy for a typical MgO-pellet design thermal cell (22 Wh kg−1).

Molten salt electrolytes for electrochemical capacitors with energy densities exceeding 50 W h kg−1

Inorganic molten salts are reported as a new electrolyte category for electrochemical capacitors with ultrahigh energy densities of over 50 W h kg−1.

Transport Performance of Molten Salt Electrolyte in a Fractal Porous FeS2 Electrode: Mesoscale Modeling and Experimental Characterization

Transport Performance of Molten Salt Electrolyte in a Fractal Porous FeS₂ Electrode: Mesoscale Modeling and Experimental Characterization

Increasing interfacial infiltration between cathode materials and solid molten salt for high power thermal batteries

Interfacial infiltration between cathode materials and ionic conduction additive is a key factor for reducing contact resistance in thermal battery. And structure of cathode materials has a huge impact on interfacial infiltration. Hence, NiS2 with nano structure, globular micro-nano structure and three dimensional micro-nano hierarchical structure are synthesized and used as cathode materials of high power thermal battery. A sintering treatment similar to the hot-pressing of powder is used to improve the infiltration between cathode materials and solid molten salt. The pulse test results show that nano NiS2 electrode has minimum resistance, which means nanocrystallization is beneficial to improve interfacial infiltration. Further studies indicate that microvoid of micro-nano hierarchical structures is not filled by solid molten salt electrolyte, which obstructs transmission of electronic and Li+, and sintering treatment could improve interfacial infiltration. The nano NiS2 cathode has excellent specific capacity with 484.2 mAh g − 1 under 0.2 A cm−2 after sintering treatment, which is higher than 431.7 mAh g − 1 of globular micro-nano NiS2 and 271.2 mAh g − 1 of three dimensional micro-nano NiS2 cathodes. The discharge results further show that hierarchical structures do not apply to the high power thermal battery based on non-flowing character of molten salt under the natural condition.

Electrochemical production of hydrogen in molten salt

Clean and economical production of hydrogen is a key step for the decarbonization of energy systems. Current water electrolysis technologies developed for the green hydrogen production require high voltage values of around 2.0 V, making them uneconomical for large-scale applications. The voltage required for the production of hydrogen can be substantially reduced to around 1.0 V by performing the electrolysis at high temperatures. This article concerns the low-cost and green electrolytic production of hydrogen operating at such a low voltage performed by the dissolution of steam in high temperature molten salt electrolytes, and the subsequent electrochemical reduction of the dissolved hydrogen ions. The electro-generation of hydrogen is confirmed by performing cyclic voltammetry using molten lithium chloride as the electrolyte at 660 °C under a flow of moisturized argon. Furthermore, hydrogen is generated by the electrolysis of molten salt containing hydrogen ions using a nickel foam cathode at the constant cell voltage of 1.5 V with a high Faradaic efficiency of 83.2%. The results obtained provide crucial insights for further development of molten salt technologies for clean and efficient hydrogen production.

A Molten-Salt Electrolysis Approach for the Recycling of Spent Lithium-Ion Batteries

Recycling spent lithium-ion batteries can close the strategic metal cycle while averting ecological and environmental footprints. High-temperature molten salts could act as both a promising electrolyte and a solvent to recover valuable Li and transition metals assisted by an electrochemical method that use electrons to break down the crystal structure and thereby sperate the Li and transition metal. In this paper, the molten salt paired electrolysis method employs electrons as both reducing and oxidizing agents and the molten salt as the solvent to separate the desired products without producing any hazardous secondary waste. Since the valence of Fe in LiFePO4 is 2+, the valence can be moved at both directions if proper electrolytes are chosen and appropriate potentials are applied. Moreover, molten salt electrolytes have a wide electrochemical potential range (e.g., electrochemical window) and considerable capability for dissolving lithium salt, which can be used as an electrolyzer to entail specific electrochemical reactions and solvent extraction processes. A molten salt electrolyzer with a solid LiFePO4 cathode and three types of anodes. All the three electrolyzers work well to electrochemically split LiFePO4 to Fe, Li+, and PO4 3 −. Besides molten carbonates, other types of molten salts such as halides, hydroxide, sulfates can be used. The anodic products are CO2, O2 and Fe3O4 if graphite, Ni10Cu11Fe alloy and LiFePO4 anodes are employed, respectively. To minimize the energy consumption and CO2 emissions, the paired electrolysis cells with dual LiFePO4 electrodes is preferable. In all types of eletrolyzers, the recovery rates of both Li and Fe are over 95.2%. Beside LiFePO4, the molten-salt electrolysis approach can be used to treat a variety of cathode materials of LIBs.

Lithiation Capabilities of Bulk Silicon at Elevated Temperature

Pure silicon, despite its high intrinsic capacity for lithium storage, is not typically utilized in Li-based battery systems due to severe swelling and subsequent fracturing of the material associated with the lithiation/delithiation process. This phenomenon, known as pulverization, significantly limits the capacity of the material over multiple cycles. Typical routes to overcome pulverization require nanostructured high aspect ratio wires or compositing with carbon. We present a differing approach reliant on elevated temperature operation to demonstrate lithiation in bulk silicon at elevated temperature. The move to elevated temperature shows the effect of increased thermal energy on Si structural stability during alloying and de-alloying processes. A fully functioning Swagelok half-cell of lithium versus silicon has been fabricated and tested at 250°C using a molten salt (LiTFSI) electrolyte, demonstrating the lithiation and delithiation of bulk Si at an elevated temperature. Results show increased reversible lithium capacity in bulk silicon, which may be of interest in applications where inexpensive, commodity electrode materials are favored.

Ceramic Felt Separator and Modified Iron (II) Disulfide Cathodes for Lithium Salt Thermal Batteries

Ceramic felts as an immobilizing structure for molten salt electrolytes in thermal batteries are shown to be an effective alternative to widely used MgO. One of the major benefits of ceramic felts is their high porosity and low weight. In this work Al2O3 and YSZ ceramic felts with bulk porosity of 97% and 96%, respectively, were investigated. LiCl-KCl eutectic electrolyte was incorporated in ceramic felts at different loading levels and cells’ performance (capacity, specific energy, and charge/discharge behavior) was evaluated. LiSi/FeS2 thermal cells with YSZ and Al2O3 ceramic felt electrolyte/separators reported specific energy of 58.47 ± 0.05 Wh kg-1 and 43.96 ± 0.05 Wh kg-1, respectively, which are 2 times the specific energy for a typical MgO-pellet design thermal cell (22 Wh kg-1).Pellet design pyrite (FeS2) cathodes for thermal batteries usually has low electronic conductivity introducing a large amount of internal ohmic resistance, especially when used in a battery stack with multiple thermal cells. Several alternative materials to pyrite and pellet design alternatives such as tape casting thin film have been introduced, however, due to the simple manufacturing process and their low-cost pyrite pellets are still known as the main type of cathode systems for thermal batteries. By adding chemically compatible and conductive particles into the pyrite powder, the properties of the cathode were modified while the fabrication process remained simple. Iron particles (≤74 µm) as additives for the cathode pellet of thermal batteries were successfully investigated. By adding 11 wt.% Fe particles to the cathode the ohmic polarization was reduced by 17.5 % while the available capacity was increased by 78% over the cell with cathode pellet with no electrically conductive particle additives.

Nitrate Molten Salt Electrolytes with Iron Oxide Catalysts for Open and Sealed Li-O2 Batteries.

Li-O2 batteries with nitrate molten salt electrolytes are attracting considerable attention owing to their various electrochemical pathways to form a discharge product upon the open and sealed systems. Here, we investigate nitrate molten salt electrolyte-based open and sealed Li-O2 batteries with pristine and iron oxide catalysts. Through the systematic analysis of various Li-O2 battery characteristics, we observe the irreversible electrochemical reactions of the open Li-O2 battery with an iron oxide catalyst that erodes the battery performance due to the detrimental parasitic reaction of H2 gas evolution from the Li anode. In contrast, the sealed Li-O2 system with cathodes containing the iron oxide catalyst exhibits the formation and decomposition of Li2O discharge products without significant side reactions, which guarantees long cycle endurance, high-rate performance, and a gravimetric energy density. Thus, promising electrochemical results from the sealed Li-O2 system with the iron oxide catalyst provide a viable strategy for the high-performance molten salt-based Li-O2 battery.

The Future of Nuclear Energy: Electrochemical Reprocessing of Fuel Takes Center Stage

Nuclear power plants use energy-dense fuel and provide dependable baseload energy without generating greenhouse gas emissions. Despite these advantages, the long-term management of used nuclear fuel (UNF) remains a key challenge due to its lifetime (hundreds of thousands of years) and radiotoxicity. The components of UNF that contribute the most to this challenge are the actinide elements. A potential solution to this issue is to separate these radioisotopes from the bulk of the UNF and recycle them as fuel in advanced nuclear reactors. These separations can be achieved using electrochemical reprocessing, which employs electrochemical conversion and electrodeposition in a high-temperature molten salt electrolyte medium to separate the actinides from UNF. In this short perspective, we review the current status, fundamental challenges, and future prospects of electrochemical reprocessing as they relate to UNF recycling.

A High Rate and Stable Hybrid Li/Na-Ion Battery Based on a Hydrated Molten Inorganic Salt Electrolyte.

Taking into the consideration safety, environmental impact, and economic issue, the construction of aqueous batteries based on aqueous electrolyte has become an indispensable technical option for large-scale electrical energy storage. The narrow electrochemical window is the main problem of conventional aqueous electrolyte. Here, an economical room-temperature inorganic hydrated molten salt (RTMS) electrolyte with a large electrochemical stability window of 3.1V is proposed. Compared with organic fluorinated molten salts, RTMS is composed of lithium nitrate hydrate and sodium nitrate with much lower cost. Based on the RTMS electrolyte, a hybrid Li/Na-ion full battery is fabricated from cobalt hexacyanoferrate cathode (NaCoHCF) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) anode. The full cell with the RTMS electrolyte exhibits a fantastic performance with high capacity of 139 mAh g-1 at 1 C, 90 mAh g-1 at 20 C, and capacity retention of 94.7% over 500 cycles at 3 C. The excellent performances are contributed to the unique properties of RTMS with a large electrochemical window, solvated H2 O free and high mobility of Li+ , which exhibits excellent Li-ions insertion and extraction capacity of NaCoHCF. This RTMS cell provides a new economic choice for large-scale energy storage.

Self-discharge mitigation in a liquid metal displacement battery

Recently, a disruptive idea was reported about the discovery of a new type of battery named Liquid Displacement Battery (LDB) comprising liquid metal electrodes and molten salt electrolyte. This cell featured a novel concept of a porous electronically conductive faradaic membrane instead of the traditional ion-selective ceramic membrane. LDBs are attractive for stationary storage applications but need mitigation against self-discharge. In the instant battery chemistry, Li|LiCl-PbCl2|Pb, reducing the diffusion coefficient of lead ions can be a way forward and a solution can be the addition of PbO to the electrolyte. The latter acts as a supplementary barrier and complements the function of the faradaic membrane. The remedial actions improved the cell’s coulombic efficiency from 92% to 97% without affecting the voltage efficiency. In addition, the limiting current density of a 500 mAh cell increased from 575 to 831 mA cm−2 and the limiting power from 2.53 to 3.66 W. Finally, the effect of PbO on the impedance and polarization of the cell was also studied.

Molten Salts CO2 Transformation: Lower Energy Input and High-Yield Carbon Nanotubes Production Induced by Zinc Oxide

Molten salt electrochemistry is cited as one of the most efficient ways to convert stable but unwanted CO2 into value-added chemical fuels and functional materials, in particular carbon nanotubes (CNTs). In this work, we report the high yield and scalable CO2-deriving CNTs by adding ZnO to molten carbonate electrolyte. The physicochemical properties such as crystal structure, morphology and element composition of the carbon products were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD) and Raman spectrometer (Raman). It is demonstrated that an appropriate amount of ZnO additives can greatly increase the content of CNTs in carbon products. However, the micromorphology and microstructure of the carbon products are found to be significantly changed when the ZnO content is insufficient or excessive. Compared with molten salt electrolyte without ZnO, ZnO-included electrolyte endows CNTs with a higher degree of graphitization and crystallinity. This work demonstrates the feasibility of improving CNTs content in carbon products and lowering required energy for CNTs production by adding ZnO as a promoter.

A high-voltage, low-temperature molten sodium battery enabled by metal halide catholyte chemistry

Despite its promise as a safe, reliable system for grid-scale electrical energy storage, traditional molten sodium (Na) battery deployment remains limited by cost-inflating high-temperature operation. Here, we describe a high-performance sodium iodide-gallium chloride (NaI-GaCl3) molten salt catholyte that enables a dramatic reduction in molten Na battery operating temperature from near 300°C to 110°C. We demonstrate stable, high-performance electrochemical cycling in a high-voltage (3.65 V) Na-NaI battery for >8 months at 110°C. Supporting this demonstration, characterization of the catholyte physical and electrochemical properties identifies critical composition, voltage, and state of charge boundaries associated with this enabling inorganic molten salt electrolyte. Symmetric and full cell testing show that the catholyte salt can support practical current densities in a low-temperature system. Collectively, these studies describe the critical catholyte properties that may lead to the realization of a new class of low-temperature molten Na batteries.

Determination of kinetic parameters for TiO2 and Nb2O5 molten salt FFC reduction by modelling experimental sweep voltammograms

Abstract The production of Ti and Nb metals from its oxides is an important candidate to become fully scale-up to the FFC (Fray–Farthing–Chen) process. In this process, electrochemical reactions reduce the metals oxides cathodes through oxygen ionization and further elimination on anode. The oxygen diffusion from solid oxide phases toward molten salt electrolyte is the main kinetic obstacle and is dependent on process temperature, oxides stoichiometry and diffusion coefficients. In this paper, experimental sweep voltammograms and mathematical modelling were used to obtain kinetic parameters of Ti and Nb production by FFC process, by simple fitting of experimental/simulated voltammetry curves. Diffusion coefficients, exchange current densities and cathodic transfer coefficients were obtained for the oxides present in reduction pathway.

Molten salt electrodeposition of aluminum on mild steel: effect of process parameters on surface morphology and corrosion properties

The electrodeposition of aluminum on mild steel in a molten salt electrolyte consisting of a mixture of AlCl3/NaCl/KCl (weight percent ratio of 80:10:10) was studied. Parametric studies were carried out to evaluate the effect of different parameters such as current density, electrolysis time, and intermediate coating layer on the coating morphology and coating-to-substrate adhesion. The quality and morphology of the coating were investigated using scanning electron microscope (SEM) and x-ray map analyses. The effect of heat treatment of the coated samples on the interface stability and formation of intermetallic compounds at the Al-Fe interface was also investigated. Cross-sectional examination by SEM as well as energy dispersive spectroscopy (EDS) line scan showed that upon annealing at temperatures in the range of 350 °C–550 °C, brittle Fe-Al intermetallic layers were formed at the interface. This shows that high-temperature service conditions can adversely affect the coating properties. The apparent activation energy of the formation of such intermetallic layers was calculated based on thickness measurements on these layers. The optimum conditions for electroplating were determined as current density of 0.022 A.cm−2 and electroplating time of 60 min. Potentiodynamic polarization tests were used to evaluate the corrosion resistance of the samples in 3.5 wt% NaCl solution. Considering the corrosion rate of coated samples which is much lower than the bare substrate, it was concluded that the electrodeposited coatings could efficiently protect the steel substrate against corrosion in corrosive media.

Heat Treatment-Induced Microstructure and Property Evolution of Mg/Al Intermetallic Compound Coatings Prepared by Al Electrodeposition on Mg Alloy from Molten Salt Electrolytes.

A method of forming an Mg/Al intermetallic compound coating enriched with Mg17Al12 and Mg2Al3 was developed by heat treatment of electrodeposition Al coatings on Mg alloy at 350 °C. The composition of the Mg/Al intermetallic compounds could be tuned by changing the thickness of the Zn immersion layer. The morphology and composition of the Mg/Al intermetallic compound coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscattered diffraction (EBSD). Nanomechanical properties were investigated via nano-hardness (nHV) and the elastic modulus (EIT), and the corrosion behavior was studied through hydrogen evolution and potentiodynamic (PD) polarization. The compact and uniform Al coating was electrodeposited on the Zn-immersed AZ91D substrate. After heat treatment, Mg2Al3 and Mg17Al12 phases formed, and as the thickness of the Zn layer increased from 0.2 to 1.8 μm, the ratio of Mg2Al3 and Mg17Al12 varied from 1:1 to 4:1. The nano-hardness increased to 2.4 ± 0.5 GPa and further improved to 3.5 ± 0.1 GPa. The Mg/Al intermetallic compound coating exhibited excellent corrosion resistance and had a prominent effect on the protection of the Mg alloy matrix. The control over the ratio of intermetallic compounds by varying the thickness of the Zn immersion layer can be an effective approach to achieve the optimal comprehensive performance. As the Zn immersion time was 4 min, the obtained intermetallic compounds had relatively excellent comprehensive properties.