Molten salts play multiple important roles in the electrolysis of solid metal compounds, particularly oxides and sulfides, for the extraction of metals or alloys. Some of these roles are positive in assisting the extraction of metals, such as dissolving the oxide or sulfide anions, and transporting them to the anode for discharging, and offering the high temperature to lower the kinetic barrier to break the metal-oxygen or metal-sulfur bond. However, molten salts also have unfavorable effects, including electronic conductivity and significant capability of dissolving oxygen and carbon dioxide gases. In addition, although molten salts are relatively simple in terms of composition, physical properties, and decomposition reactions at inert electrodes, in comparison with aqueous electrolytes, the high temperatures of molten salts may promote unwanted electrode-electrolyte interactions. This article reviews briefly and selectively the research and development of the Fray-Farthing-Chen (FFC) Cambridge Process in the past two decades, focusing on observations, understanding, and solutions of various interactions between molten salts and cathodes at different reduction states, including perovskitization, non-wetting of molten salts on pure metals, carbon contamination of products, formation of oxychlorides and calcium intermetallic compounds, and oxygen transfer from the air to the cathode product mediated by oxide anions in the molten salt.

Compared with solid metals, liquid metals are considered more promising cathodes for molten slat/oxide electrolysis due to their fascinating advantages, which include strong depolarization effect, strong alloying effect, excellent selective separation, and low operating temperature. In this review, we briefly introduce the properties of the liquid metal cathodes and their selection rules, and then summarize development in liquid metal cathodes for molten salt electrolysis, specifically the extraction of Ti and separation of actinides and rare-earth metals in halide melts. We also review recent attractive progress in the preparation of liquid Ti alloys via molten oxide electrolysis by using liquid metal cathodes. Problems related to high-quality alloy production and large-scale applications are cited, and several research directions to further improve the quality of alloys are also discussed to realize the industrial applications of liquid metal cathodes.

Molten salt is an excellent medium for chemical reaction, energy transfer, and storage. Molten salt innovative technologies should be developed to recover metals from secondary resources and reserve metals from primary natural sources. Among these technologies, molten salt electrolysis is an economic and environment-friendly method to extract metals from waste materials. From the perspective of molten salt characteristics, the application of molten salts in chemistry, electrochemistry, energy, and thermal storage should be comprehensively elaborated. This review discusses further directions for the research and development of molten salt electrolysis and their use for metal recovery from various metal wastes, such as magnet scrap, nuclear waste, and cemented carbide scrap. Attention is placed on the development of various electrolysis methods for different metal containing wastes, overcoming some problems in electrolytes, electrodes, and electrolytic cells. Special focus is given to future development directions for current associated processing obstacles.

The low O²⁻ diffusion rate in the electro-deoxidation of titanium containing compounds by either the OS process or the FFC process leads to a low reaction speed and a low current efficiency. In this study, Ca₃Ti₂O₇ was used as a precursor to improve the reduction speed of titanium. Because of the greater number of “diffusion channels” created in cathode as Ca²⁺ liberates from Ca₃Ti₂O₇ precursor in the electro-deoxidation process, the O²⁻ diffusion rate was improved significantly by using Ca₃Ti₂O₇ instead of CaTiO₃ as precursor. Parallel constant voltage electrolysis (3.2 V) confirms that Ca₃Ti₂O₇ and CaTiO₃ are reduced simultaneously because of their similar crystal structures. However, the reduction area of Ca₃Ti₂O₇ spreads much faster than that of CaTiO₃, indicating a difference in the O²⁻ diffusion rate. Constant voltage cyclic voltammetry (CV) and theoretical analysis of the crystal structure were also conducted to compare the differences between Ca₃Ti₂O₇ and CaTiO₃. The results indicate that using a precursor with a greater number of soluble cations, titanium reduction speed can be greatly improved in the electro-deoxidation process. Finally, a new electrolysis method for converting and recycling excess CaO from the Ca₃Ti₂O₇ precursor was proposed.

Solid oxide membrane-assisted electrolytic reduction of solid Cr₂O₃ to Cr in molten CaCl₂ was performed using a sintered porous Cr₂O₃ cathode paired with an yttria-stabilized zirconia (YSZ) tube anode containing carbon-saturated liquid copper alloy. Analyses of the reduction mechanism, ion migration behavior, and effects of cathode pellet porosity and particle size on the electrolysis products and reduction rate revealed that the cathode microstructure and electrolytic conditions were key factors influencing the electrolysis process. Optimal results were obtained when the cathode was characterized by high porosity and a small particle size because this combination of features contributed to ion migration. Good electrochemical activation was observed when cathode pellets prepared by 4 MPa molding followed by 2 h of sintering at 1150°C were applied. The electrode reduction process (Cr³⁺ → Cr²⁺ → Cr) was promoted by high electrode voltages, and Cr metal was efficiently formed. The proposed method appears to be well suited for electrolytic Cr production because it does not require expensive pre-electrolysis techniques or generate harmful by-products.

A new concept for producing highly pure Ti metal powder from ilmenite (FeTiO₃) is proposed in this article. Titanium nitride (TiN) or titanium oxycarbonitride (TiO_xC_yN_z) could be synthesized in the first step via the simultaneous carbothermal reduction and nitridation (CTRN) of FeTiO₃ to remove oxygen roughly. To separate oxygen completely, high-quality TiS₂ samples were then synthesized from TiN and TiC using S₂ gas, and the clean sulfides were finally reduced to α-Ti powders with spherical morphology using electrolysis in molten CaCl₂. X-ray diffraction (XRD), scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS), and elemental LECO analysis were used to study the phases and microstructures of the sulfides and the electrochemically reduced powders. The Ti powder showed no carbon contamination and consisted of high-purity foil-like Ti sheets with very low oxygen, carbon, and nitrogen contents of less than 0.15wt% O, 0.02wt% C, and 0.003wt% N, respectively. The quality of the Ti powder was much higher than that of the powder obtained using the conventional OS process (proposed by K. Ono and R.O. Suzuki) starting directly from the oxides.

The cathodic reduction mechanism of Hf(IV) ions in a fused NaCl–KCl–NaF–K₂HfF₆ salt system was studied in various NaF concentrations at 1073 K to obtain a purified dendritic Hf metal. The results of cyclic voltammetry and square wave voltammetry indicated that the reduction process comprised two steps of Hf(IV) → Hf(II) and Hf(II) → Hf at low NaF concentrations (0 < molar ratio of [F⁻/Hf ⁴⁺] ≤ 17.39) and one step of Hf(IV) → Hf at high NaF concentrations (17.39 < molar ratio of [F⁻/Hf ⁴⁺] < 23.27). The structure and morphology of the deposits obtained in potentiostatic electrolysis in the one-step reduction process were analyzed and verified by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectrometry. In the one-step reduction process, the disproportionation reaction between the Hf metal and Hf complex ions was inhibited, and a large dendrite Hf metal was achieved in molten salt electrorefining.

Electrorefining effectively separates metals from their corresponding alloys. To obtain Nd from Cu₆Nd alloy, cyclic voltammetry and square wave voltammetry were used to investigate the reduction behavior of Nd³⁺ and the anode dissolution behavior of Cu₆Nd in the NaCl–KCl–0.5mol%NdCl₃ melt at 1023 K. According to the analysis of the electrochemical behavior, the cell voltage was determined to be between 0.3 and 1.2 V for separating Nd from Cu₆Nd. After electrolysis at 0.6 V for 4 h, the Nd was found at the surface of the Mo cathode without any Cu. For the Fe cathode, a deposition with an atom ratio of Nd : Fe = 1:1 was formed on the surface. However, the low current density of separation remains a great experimental challenge that must be solved.

Sm extraction from a LiCl–KCl melt was carried out by forming alloys on various electrodes, including Al, Ni, Cu, and liquid Zn, and the electrochemical behaviors of the resultant metal products were investigated using different electrochemical techniques. While Sm metal deposition via the conventional two-step reaction process was not noted on the inert electrode, underpotential deposition was observed on the reactive electrodes because of the latter’s depolarization effect. The depolarization effects of the reactive electrodes on Sm showed the order Zn > Al > Ni > Cu. Sm–M (M = Al, Ni, Cu, Zn) alloys were deposited by galvanostatic and potentiostatic electrolysis. The products were fully characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM)–energy dispersive spectrometry  (EDS), and the stability of the obtained M-rich compounds was determined. Finally, the relationship between the electrode potential and type of Sm–M intermetallic compounds formed was assessed on the basis of the observed electrochemical properties and electrodeposits.

Carbon nanofiber (CNF) is considered a promising material due to its excellent physical and chemical properties. This paper proposes a novel way to transform CO₂ into heteroatom-doped CNFs, with the introduction of Fe, Co, and Ni as catalysts. When the electrolyte containing NiO, Co₂O₃, and Fe₂O₃ was employed, sulfur-doped CNFs in various diameters were obtained. With the introduction of Fe catalyst, the obtained sulfur-doped CNFs showed the smallest and tightest diameter distributions. The obtained sulfur-doped CNFs had high gravimetric capacitance (achieved by SDG-Fe) that could reach 348.5 F/g at 0.5 A/g, excellent cycling stability, and good rate performance. For comparison purposes, both Fe and nickel cathodes were tested, where the active metal atom at their surface could act as catalyst. In these two situations, sulfur-doped graphite sheet and sulfur-doped graphite quasi-sphere were the main products.

Tailings from the vanadium extraction process are discarded each year as waste, which contain approximately 30wt% of Fe. In our previous work, we extracted Fe and Mn from vanadium slag, and Fe and Mn existed in the form of FeCl₂ and MnCl₂ after chlorination by NH₄Cl to achieve effective and green usage of waste containing Fe and Mn. In this work, square wave voltammetry (SWV) and cyclic voltammetry (CV) were applied to investigate the electrochemical behaviors of Fe²⁺ and Mn²⁺ in NaCl–KCl melt at 800°C. The reduction processes of Fe²⁺ and Mn²⁺ were found to involve one step. The diffusion coefficients of FeCl₂ and MnCl₂ in molten salt of eutectic mixtures NaCl–KCl molten salt were measured. The electrodeposition of Fe and Mn were performed using two electrodes at a constant cell voltage. The Mn/Fe mass ratio of the electrodeposited product in NaCl–KCl–2.13wt%FeCl₂–1.07wt%MnCl₂ was 0.0625 at 2.3 V. After the electrolysis of NaCl–KCl–2.13wt%FeCl₂–1.07wt%MnCl₂ melted at 2.3 V, the electrolysis was again started under 3.0 V and the Mn/Fe mass ratio of the electrodeposited product was 36.4. This process provides a novel method to effectively separate Fe and Mn from simulated chlorinated vanadium slag.

Graphite materials are widely used as electrode materials for electrochemical energy storage. N-doping is an effective method for enhancing the electrochemical properties of graphite. A novel one-step N-doping method for complete and compact carbon paper was proposed for molten salt electrolysis in the LiCl−KCl−Li₃N system. The results show that the degree of graphitization of carbon paper can be improved by the electrolysis of molten salts, especially at 2.0 V. Nitrogen gas was produced at the anode and nitrogen atoms can substitute carbon atoms of carbon paper at different sites to create nitrogen doping during the electrolysis process. The doping content of N in carbon paper is up to 13.0wt%. There were three groups of nitrogen atoms, i.e. quaternary N (N-Q), pyrrolic N (N-5), and pyridinic N (N-6) in N-doping carbon paper. N-doping carbon paper as an Al-ion battery cathode shows strong charge‒recharge properties.

The Fe–Ni36 alloy was prepared via the one-step electrolysis of a mixed oxides precursor in a molten Na₂CO₃–K₂CO₃ eutectic melt at 750°C, where porous Fe₂O₃–NiO pellets served as the cathode and the Ni10Cu11Fe alloy was an inert anode. During the electrolysis, NiO was preferentially electro-reduced to Ni, then Fe₂O₃ was reduced and simultaneously alloyed with nickel to form the Fe–Ni36 alloy. Different cell voltages were applied to optimize the electrolytic conditions, and a relatively low energy consumption of 2.48 kW·h·kg⁻¹ for production of FeNi36 alloy was achieved under 1.9 V with a high current efficiency of 94.6%. The particle size of the alloy was found to be much smaller than that of the individual metal. This process provides a low-carbon technology for preparing the Fe–Ni36 alloy via molten carbonates electrolysis.

Pyrolysis of the Ta₂O₅/melamine mixture in molten chlorides is herein demonstrated as a facile and controllable method to nitridize and functionalize Ta₂O₅. The influence of the stoichiometry and composition of Ta₂O₅/melamine in molten salts on the nitridation process is rationalized to ensure the controllable preparation of Ta₃N₅ and Ta₃N₅/TaON. The characterization results, including scanning electron microscopy, transmission electron microscopy, elemental mapping, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy, all confirm the existence of the Ta₃N₅/TaON heterojunction, in which the TaON nanoparticles are closely anchored to the Ta₃N₅ nanorods. Benefiting from its composition and structure, the Ta₃N₅/TaON composites show enhanced photocatalytic activity for the degradation of methylene blue. The present study highlights that the molten salt method using a solid nitrogen source can be a new technique for rationalizing the design of nitrides and oxynitrides.

Aluminum storage systems with graphite cathode have been greatly promoting the development of state-of-the-art rechargeable aluminum batteries over the last five years; this is due to the ultra-stable cycling, high capacity, and good safety of the systems. This study discussed the change of electrochemical behaviors caused by the structural difference between flake graphite and expandable graphite, the effects of temperature on the electrochemical performance of graphite in low-cost AlCl₃–NaCl inorganic molten salt, and the reaction mechanisms of aluminum complex ions in both graphite materials by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, cyclic voltammetry, and galvanostatic charge−discharge measurements. It was found that flake graphite stacked with noticeably small and thin graphene nanosheets exhibited high capacity and fairly good rate capability. The battery could achieve a high capacity of ~219 mA·h·g⁻¹ over 1200 cycles at a high current density of 5 A·g⁻¹, with Coulombic efficiency of 94.1%. Moreover, the reaction mechanisms are clarified: For the flake graphite with small and thin graphene nanosheets and high mesopore structures, the reaction mechanism consisted of not only the intercalation of

anions between graphene layers but also the adsorption of

anions within mesopores; however, for the well-stacked and highly parallel layered large-size expandable graphite, the reaction mechanism mainly involved the intercalation of

anions.

The Ca–Pb electrode couple is considered to be one of the least expensive (~36 $/(kW·h)) among various optional materials for liquid–metal batteries (LMBs). The electrochemical properties of Ca–Pb alloy in a Ca|LiCl–NaCl–CaCl₂|Pb cell were investigated in this paper. The electrode potential maintained a linear relationship in the current density range of 50–200 mA·cm⁻², which indicates that the alloying and dealloying processes of Ca with Pb attained rapid charge transfer and mass transport in the interface between the liquid electrode and electrolyte. The Ca–Pb electrode exhibited remarkable properties with a high discharge voltage of 0.6 V, a small self-discharge current density (<2 mA·cm⁻² at 600°C), and a high coulombic efficiency (>98.84%). The postmortem analysis showed that intermetallics CaPb₃ and CaPb were uniformly distributed in the electrode with different molar fractions of Ca, which indicates that the nucleation of solid intermetallics did not hinder the diffusion of Ca in the electrode. This investigation on Ca–Pb electrode sheds light on the further research and the design of electrodes for Ca-based LMBs.