Effect of electric fields on tungsten distribution in Na2WO4-WO3 molten salt.

Tungsten coatings have unique properties such as high melting points and hardness and are widely used in the nuclear fusion and aviation fields. In experiments, compared to pure Na2WO4 molten salt, electrolysis with Na2WO4-WO3 molten salt results in a lower deposition voltage. Herein, an investigation combining experimental and computational approaches was conducted, involving molecular dynamics simulations with deep learning, high-temperature in situ Raman spectroscopy and activation strain model analysis. The results indicated that the molten salt system's behaviour, influenced by migration and polarization effects, led to increased formation of Na2W2O7 in the Na2WO4-WO3 molten salt, which has a lower decomposition voltage and subsequently accelerated the cathodic deposition of tungsten. We analyzed the mechanism of the effect of the electric field on the Na2W2O7 structure based on the bond strength and electron density. This research provides crucial theoretical support for the effect of electric field on tungsten in molten salt and demonstrates the feasibility of using machine learning-based DPMD methods in simulating tungsten-containing molten salt systems.

Operando probing and adjusting of the complicated electrode process of multivalent metals at extreme temperature.

Nearly half of the elements in the periodic table are extracted, refined, or plated using electrodeposition in high-temperature melts. However, operando observations and tuning of the electrodeposition process during realistic electrolysis operations are extremely difficult due to severe reaction conditions and complicated electrolytic cell, which makes the improvement of the process very blind and inefficient. Here, we developed a multipurpose operando high-temperature electrochemical instrument that combines operando Raman microspectroscopy analysis, optical microscopy imaging, and a tunable magnetic field. Subsequently, the electrodeposition of Ti-which is a typical polyvalent metal and generally shows a very complex electrode process-was used to verify the stability of the instrument. The complex multistep cathodic process of Ti in the molten salt at 823 K was systematically analyzed by a multidimensional operando analysis strategy involving multiple experimental studies, theoretical calculations, etc. The regulatory effect and its corresponding scale-span mechanism of the magnetic field on the electrodeposition process of Ti were also elucidated, which would be inaccessible with existing experimental techniques and is significant for the real-time and rational optimization of the process. Overall, this work established a powerful and universal methodology for in-depth analysis of high-temperature electrochemistry.

Ultra-High Temperature Molten Oxide Electrochemistry.

Recently, the ultra-high temperature electrochemistry (UTE, about >1000 °C) has emerged, which represents an exploration to extend the temperature limit of human technology in electrochemical engineering. UTE has far-reaching impact on revolutionary low-carbon metal extraction and the in situ production of oxygen for deep-space exploration. It is hence of urgency to systematically summarize the development of UTE. In this Review, the basic concepts of UTE and the physicochemical properties of molten oxides are analyzed. The principles in the design of inert anodes for the oxygen evolution reaction in molten oxides are discussed, which forms a solid basis for the in situ production of oxygen from simulated lunar regolith by UTE. Furthermore, liquid metal cathodes for revolutionary titanium extraction and ironmaking/steelmaking are highlighted. With emphasis on the key challenges and perspectives, the Review can provide valuable inspiration for the rapid advancement of UTE.

A 4D x-ray computer microtomography for high-temperature electrochemistry.

High-temperature electrochemistry is widely used in many fields. However, real-time observations and an in-depth understanding of the inside evolution of this system from an experimental perspective remain limited because of harsh reaction conditions and multiphysics fields. Here, we tackled this challenge with a high-temperature electrolysis facility developed in-house. This facility permits in situ x-ray computer microtomography (µ-CT) for nondestructive and quantitative three-dimensional (3D) imaging. In an electrorefining system, the µ-CT probed the dynamic evolution of 3D morphology and components of electrodes (4D). Subsequently, this 4D process was visually presented via reconstructed images. The results monitor the efficiency of the process, explore the dynamic mechanisms, and even offer real-time optimization. This 4D analysis platform is notable for in-depth combinations of traditional electrochemistry with digital twin technologies owing to its multiscale visualization and high efficiency of data extraction.

Stable Quasi-Solid-State Aluminum Batteries.

Nonaqueous rechargeable aluminum batteries (RABs) of low cost and high safety are promising for next-generation energy storage. With the presence of ionic liquid (IL) electrolytes, their high moisture sensitivity and poor stability would lead to critical issues in liquid RABs, including undesirable gas production, irreversible activity loss, and an unstable electrode interface, undermining the operation stability. To address such issues, herein, a stable quasi-solid-state electrolyte is developed via encapsulating a small amount of an IL into a metal-organic framework, which not only protects the IL from moisture, but creates sufficient ionic transport network between the active materials and the electrolyte. Owing to the generated stable states at both positive-electrode-electrolyte and negative-electrode-electrolyte interfaces, the as-assembled quasi-solid-state Al-graphite batteries deliver specific capacity of ≈75 mA h g-1 (with positive electrode material loading ≈9 mg cm-2 , much higher than that in the conventional liquid systems). The batteries present a long-term cycling stability beyond 2000 cycles, with great stability even upon exposure to air within 2 h and under flame combustion tests. Such technology opens a new platform of designing highly safe rechargeable Al batteries for stable energy storage.

Quantificational 4D Visualization of Industrial Electrodeposition.

Electrodeposition is a fundamental technology in modern society and has been widely used in metal plating and extraction, etc. However, extreme reaction conditions, including wide operation temperature ranges and corrosive media (molten salt/oxide systems as a particular example), inhibit direct in situ observation of the electrodeposition process. To visualize the electrode kinetics in such "black box," X-ray tomography is employed to monitor the electrochemical processes and three-dimensional (3D) evolution of morphology. Benefiting from the excellent penetration of X-ray, a non-destructive and non-contact in situ four-dimensional (4D) visualization of Ti deposition is realized. Real-time 3D reconstructed images reveal that the counterintuitive nucleation and growth process of a mesoscale Ti dendrite at both solid and liquid cathodes. According to 3D morphology evolution, unusual mechanism based on synergetic effect of the diffusion of metallic Ti and local field enhancement is achieved utilizing a simulation method based on a finite element method. This approach allows for timely and accurately regulating the electrodeposition process upon in situ monitored parameters. More importantly, the 4D technique upon operando X-ray tomography and numerical simulation can be easily applied to other electrodeposition systems, which will help deeply understand the internal kinetics and the precise optimization of the electrodeposition conditions.

Nonmetal Current Collectors: The Key Component for High-Energy-Density Aluminum Batteries.

As one of the emerging safe energy-storage devices with high energy-to-cost ratio, nonaqueous aluminum batteries with enhanced energy density are intensively pursued by researchers. Although significant progress has been made on positive electrode materials, the effective energy density of aluminum batteries is still limited by the presence of high-density refractory metal current collectors, which are known to be electrochemically inert in highly acidic ionic-liquid electrolytes. To address such critical issues, here, a novel low-density (<2 g cm-3 ) nonmetal current collector is presented, which uses poly(ethylene terephthalate) (PET) substrates coated with indium tin oxide (ITO), with the purpose of significantly reducing the ratio of nonactive components in the electrodes. In addition to the excellent chemical and electrochemical stability (with voltage as high as ≈2.75 V vs Al3+ /Al), this nonmetal current collector, also encompassing a carboxymethyl cellulose (CMC) binder, allows as-assembled pouch cells to deliver a reversible specific capacity of ≈120 mAh g-1 at a current density of 50 mA g-1 . In comparison with the high-density refractory metal Mo or Ta current collectors, these nonmetal current collectors offer a novel strategy for constructing high-energy-density aluminum batteries by substituting the key components, with the aim of boosting the energy density of nonaqueous aluminum batteries.

Stable Interface between a NaCl-AlCl3 Melt and a Liquid Ga Negative Electrode for a Long-Life Stationary Al-Ion Energy Storage Battery.

Intermediate temperature NaCl-AlCl3-based Al-ion batteries are considered as a promising stationary energy storage system due to their low cost, high safety, etc. However, such a cheap electrolyte has a critical feature, i.e., strong corrosion, which results in the short cycle life of the conventional Al-metal anode and also limits the development of the NaCl-AlCl3-based Al-ion batteries. A noncorrosive electrolyte may be a good choice for addressing the above challenge, while it is difficult to obtain the electrolyte that has advantages of both noncorrosion and low cost. Therefore, here, we report a Ga-metal anode in the affordable NaCl-AlCl3 electrolyte for constructing a long-life stationary Al-ion energy storage system. This featured liquid metal anode shows good alloying and dealloying processes between metallic Ga and Al, as well as renders superior stability of the interface between the electrolyte and the anode (e.g., smoothly running for over 580 h at 2 mA cm-2). No-corrosion and no-pulverization problems appear in this novel liquid/liquid interface. Those advantages demonstrate that the liquid Ga-metal anode has a great promise for the improvement of the NaCl-AlCl3-based Al-ion batteries for large-scale stationary energy storage applications.

Rechargeable Nickel Telluride/Aluminum Batteries with High Capacity and Enhanced Cycling Performance.

Rechargeable aluminum-ion batteries (AIBs) possess significant advantages of high energy density, safety performance, and abundant natural resources, making them one of the desirable next-generation substitutes for lithium battery systems. However, the poor reversibility, short lifespan, and low capacity of positive materials have limited its practical applications. In comparison with semiconductors, the metallic nickel telluride (NiTe) alloy with enhanced electrical conductivity and fast electron transmission is a more favorable electrode material that could significantly decrease the kinetic barrier during battery operation for energy storage. In this paper, the NiTe nanorods prepared through a simple hydrothermal routine enable an initial reversible capacity of approximately 570 mA h g-1 (under the current density of 200 mA g-1) to be delivered on the basis of the ionic liquid electrolyte, along with the average voltage platform of about 1.30 V. Moreover, the cycling performance could be easily enhanced using a modified separator to prevent the diffusion of soluble intermediate species to the negative electrode side. At a high rate of 500 mA g-1, the NiTe nanorods could retain a specific capacity of about 307 mA h g-1 at the 100th cycle. The results have important implications for the research of transition metal tellurides as positive electrode materials for AIBs.

Production of Ti-Fe alloys via molten oxide electrolysis at a liquid iron cathode.

This work studies the direct electrochemical preparation of Ti-Fe alloys through molten oxide electrolysis (MOE) at a liquid iron cathode. Cyclic voltammetry and potentiostatic electrolysis have been employed to study the cathodic process of titanium ions. The results show that cathodic behavior happens during the negative sweep at a potential range from -0.80 to -1.25 V (vs. QRE-Mo), corresponding to the electro-reduction of titanium ions. Importantly, Ti-Fe and titanium-rich Ti-Fe alloys have been successfully produced by galvanostatic electrolysis at different current densities of 0.15 and 0.30 A cm-2, respectively. The results show that it is feasible to directly prepare Ti-Fe alloys by the MOE method at a liquid iron cathode.

A Novel Ultrafast Rechargeable Multi-Ions Battery.

An ultrafast rechargeable multi-ions battery is presented, in which multi-ions can electrochemically intercalate into graphite layers, exhibiting a high reversible discharge capacity of ≈100 mAh g-1 and a Coulombic efficiency of ≈99% over hundreds of cycles at a high current density. The results may open up a new paradigm for multi-ions-based electrochemical battery technologies and applications.

A rechargeable Al-ion battery: Al/molten AlCl3-urea/graphite.

A new Al-ion battery based on an affordable and nontoxic liquid electrolyte made from molten AlCl3/urea was assembled. As the cathode material, natural graphite shows two well-defined discharge voltage plateaus at about 1.9 and 1.5 V with a high specific capacity of 93 mA h g-1 and excellent coulombic efficiency (>99%). The attractive capacity (about 78 mA h g-1) is retained even at a high current density of 1000 mA g-1. Moreover, no faster fading in capacity is observed after 500 cycles. This electrolyte could provide a new system for Al ion batteries, which can be used for large scale energy storage, owing to its cost advantages, high-rate capability and durability.

Direct Conversion of Greenhouse Gas CO2 into Graphene via Molten Salts Electrolysis.

Producing graphene through the electrochemical reduction of CO2 remains a great challenge, which requires precise control of the reaction kinetics, such as diffusivities of multiple ions, solubility of various gases, and the nucleation/growth of carbon on a surface. Here, graphene was successfully created from the greenhouse gas CO2 using molten salts. The results showed that CO2 could be effectively fixed by oxygen ions in CaCl2-NaCl-CaO melts to form carbonate ions, and subsequently electrochemically split into graphene on a stainless steel cathode; O2 gas was produced at the RuO2-TiO2 inert anode. The formation of graphene in this manner can be ascribed to the catalysis of active Fe, Ni, and Cu atoms at the surface of the cathode and the microexplosion effect through evolution of CO in between graphite layers. This finding may lead to a new generation of proceedures for the synthesis of high value-added products from CO2, which may also contribute to the establishment of a low-carbon and sustainable world.

A sodium ion intercalation material: a comparative study of amorphous and crystalline FePO4.

Due to their low cost, high abundance and eco-friendly features, Na-ion batteries are becoming alternative choices for rechargeable batteries, especially in large scale applications. Generally, the well-crystallized materials have many advantages over amorphous materials, such as long cycle life, high rate performance and other electrochemical properties. However, the amorphous FePO4 we report here exhibits outstanding cycling stability and rate performance which are derived from its amorphous nature and wafer-like porous morphology. A comparative study of amorphous and crystalline FePO4 has been carried out as cathode materials for Na-ion batteries. The present study not only reports a synthetic method which is facile, inexpensive, and scalable for mass production, but it also motivates further exploration of other amorphous materials for Na-ion batteries.