The 69th special feature, titled “Frontiers of Molten Salts and Ionic Liquids,” will focus on the recent progress in the field of molten salts and ionic liquids. Molten salts include conventional high-temperature molten salts and room-temperature molten salts (more commonly referred to as ionic liquids), as well as molten metals, molten glasses, and molten hydrates in the broadest sense. In recent years, chemical and electrochemical reactions using molten salts and ionic liquids have become increasingly important for the realization of a carbon-neutral society. Major recent applications include storage batteries, green smelting, recycling, CO₂ utilization, hydrogen production, and nuclear fuel reprocessing. In this special feature, readers will see how electrochemical processes in molten salts and ionic liquids hold promise for realizing a carbon-neutral society in the future.

Primary aluminum is produced by the Hall-Heroult process which is based on electrolysis in molten fluoride electrolyte, Na₃AlF₆-AlF₃, at ∼960 °C in which the raw material alumina is dissolved and decomposed into pure aluminum and CO₂ gas due to the use of carbon anodes. Direct CO₂ emissions are due to the anode process including perfluoro carbon (PFC) formation during anode effect. An inert anode to produce oxygen may eliminate direct CO₂ emissions including PFC gases and give possibilities to improve the cell design. CO₂ emissions from generation of electricity are the most important issue globally. Also the use of pure metals to produce alloys may significantly increase the carbon footprint due to the primary production of alloying elements. A new approach to produce alloys directly during electrolysis is proposed, and results from lab experiments show that this method may give significant reduction of carbon footprint for the production of aluminum alloys. Other sources of CO₂ emissions are production and manufacture of alumina and carbon anodes as well as loss in current efficiency for aluminum. A new process based on aluminum chloride electrolysis and recycling of CO₂ may eliminate CO₂ emissions from the production process.

In situ white X-ray diffraction was used to investigate the electrochemical alloying and de-alloying processes of Dy–Ni alloys in molten LiCl–KCl–DyCl₃ at 723 K. X-ray diffraction peaks of Ni and DyNi₂ and fluorescence peaks of Dy were obtained. DyNi₂ was the only identified Dy–Ni alloy, although previous ex situ studies have reported the formation of several Dy–Ni alloys. During the de-alloying process, the apparent lattice constant of DyNi₂ decreased to a small value. Also, the area of Dy fluorescence peaks increased, which can be attributed to the formation of a Dy compound layer on the electrode surface. Electron backscatter diffraction was performed to clarify the relationship between the crystal orientations of Ni and DyNi₂ after electrochemical alloying or sequential alloying and de-alloying. The results confirmed that Dy–Ni alloying was faster at the Ni grain boundaries. In addition, DyNi₂ or porous Ni grains in reacted regions had similar crystal orientations as adjacent Ni grains in unreacted regions. These results help clarify the rapid electrochemical formation mechanism of Dy–Ni alloys in molten LiCl–KCl, which can help facilitate the separation and recovery of rare-earth elements, such as Dy, from spent magnets.

This study investigated the electrodeposition behavior of Al–Au alloys in AlCl₃–NaCl–KCl molten salt containing AuCl at 443 K, and porous gold was formed by potential pulse electrolysis in the molten salt. The deposition/dissolution potential of Au in the molten salt was approximately 2.0 V vs. Al/Al(III). For the Al–Au alloy electrodeposits obtained by constant potential electrolysis, the Au concentration was less than 1 at% at potentials lower than 0.0 V. Then the Au concentration increased sharply to 84 at% at 0.0 V and increased gradually to 92 at% at 0.3 V and 98 at% at 0.9 V. Porous gold was formed by potential pulse electrolysis with repeated Al–Au alloys electrodeposition and Al dissolution. The specific surface area of the electrodeposits obtained by potential pulse electrolysis increased with decreasing frequency. The specific surface area of porous gold obtained in this experiment was up to 9.2 m² g⁻¹.

The formation and breakdown of the solid electrolyte interphase (SEI) on a Pt electrode were investigated in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) containing a high concentration of LiFSA using a ferrocene/ferrocenium couple as a redox probe. The formation of the SEI was confirmed at the potential more negative than −1.1 V vs. Ag|Ag(I). However, the SEI was found to disappear at the open circuit potential. The SEI was suggested to form by the dissolution and/or dispersion of the decomposition products in the electrolyte.

Room temperature ionic liquids (RTILs) with ether oxygen atoms in the side chain have been investigated for electrochemical applications because they show favorable transport properties compared with alkyl analogs. However, the influence of the position and number of ether oxygen atoms on physical properties has not yet been fully clarified. In the present study, the physicochemical properties of RTILs consisting of bis(trifluoromethanesulfonyl)amide (TFSA⁻) with 1-methy-1-propoxyethylpyrrolidinium (Pyr_1,2O3⁺), 1-methyl-1-ethoxypropylpyrrolidinium (Pyr_1,3O2⁺), and 1-methyl-1-(methoxymethoxy)ethylpyrrolidinium (Pyr_1,2O1O1⁺) was investigated using experimental and computational approach. Pyr_1,2O3TFSA exhibited the lowest viscosity and highest ionic conductivity. Pyr_1,2O1O1TFSA with two oxygen atoms in the side chain showed relatively high viscosity, indicating the physicochemical properties were affected sensitively by the position and number of ether oxygen atoms. Furthermore, by comparing with the structural isomers reported so far, we were able to systematically discuss the introduction effect of ether oxygen on the physicochemical properties of RTILs.

Graphite is widely used as the negative electrode for alkali-metal ion secondary batteries because of its ability to accommodate various ions between its graphene layers resulting in the formation of graphite intercalation compounds (GICs). In this study, we investigated the intercalation of Cs⁺ ions into graphite using an ionic liquid (IL)-based electrolyte, Cs[FTA]–[C₄C₁pyrr][FTA] (FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide, C₄C₁pyrr = N-butyl-N-methylpyrrolidinium). In this electrolyte, the graphite negative electrode imparted an initial reversible capacity of 180 mAh g⁻¹ at 298 K. The formation of Cs-GICs was analyzed using an X-ray diffraction technique to reveal the formation of stage-1 CsC₈. The formation potentials of various alkali metal GICs were also compared with respect to the potential of ferrocenium/ferrocene redox couple, revealing that these GICs particularly for stage-1 compounds form in a similar potential range with small differences of 0.2–0.3 V.

The electrode reactions of copper species were investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl) amide (BMPFSA). The anodic current assignable to the dissolution of Cu to form monovalent Cu(I) complex was observed at −0.42 V vs. Ag|Ag(I). A pair of anodic and cathodic current peaks attributable to Cu(II)/Cu(I) was observed at 0.5 V, suggesting that a 4 V class redox flow battery combined with a Li metal anode is expected using the redox reaction of Cu(II)/Cu(I) in BMPFSA as the cathode reaction. The electrodeposition of Cu and the quasi-reversible redox reaction of Cu(II)/Cu(I) were affected by the electrode materials.

We fabricated lithium-air secondary batteries (LABs) employing amide-based ionic liquids (ILs) as electrolytes and evaluated their electrochemical characteristics. Lithium bis(trifluoromethanesulfonyl)amide (Li-TFSA) was employed as the lithium salt, N-methyl-N-propylpyrrolidinium-TFSA (Py₁₃ system) with a cyclic aliphatic cation in the ILs, and N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium-TFSA (DEME system) with an acyclic aliphatic cation. The constant-current discharge-charge tests with the capacity controlled at 200 mAh (g-carbon)⁻¹ showed that the overvoltage of the LABs using the Py₁₃ system was lower than those of LABs using the DEME system and the organic solvent-based system electrolyte. The cycling performance of the DEME system rapidly decreased at the 74th cycle, while the Py₁₃ system showed 200 mAh (g-carbon)⁻¹ up to the 100th cycle, indicating a high stability. Electrochemical impedance measurements showed that the LABs using the Py₁₃ system had the lowest interfacial resistance after the 1st charge. These results indicated that the use of the Py₁₃ system with a relatively high electrical conductivity and low viscosity as the electrolyte would stabilize the cycling performance of the LABs.

The arrangement of ions at the interface between highly oriented pyrolytic graphite (HOPG) and Aluminum chloride–1-ehtyl-3-methylimidazolium chloride (AlCl₃–EmImCl) ionic liquid was evaluated using Atomic Force Microscopy (AFM) force curve measurements. In the force curve measurements, three steps were observed towards the HOPG electrode. The width of each step was 0.3, 0.4, and 0.5 nm in order from the final arrival point. Their step widths were regarded as the thicknesses of EmIm⁺, Al₂Cl₇⁻, or their mixed layers. The force curve measurements at each potential demonstrated that the width of the first layer close to the HOPG tended to decrease as the potential shifted towards the less noble side.

In this article, electrode behavior of commercially available exfoliated graphite material, GRAFOIL^®, was examined in Lewis acidic AlCl₃–1-ethyl-3-methylimidazolium chloride ([C₂mim]Cl) ionic liquid electrolytes (AlCl₃ molar fraction: >50 mol%). Cyclic voltammograms and chronopotentiograms recorded at the GRAFOIL^® electrode strongly suggested that the electrode behavior is very similar to that for the graphite cathode active materials employed for the Al metal anode rechargeable batteries and that the intercalation reaction of the [AlCl₄]⁻ into the GRAFOIL^® proceeds electrochemically. From XRD data of the [AlCl₄]⁻-intercalated GRAFOIL^® obtained, a lot of information was obtained about its crystal structure. Relatively high discharge capacity over 80 mAh g⁻¹ of GRAFOIL^® cathode would be due to the formation of graphite intercalation compound with stage 3 structure. The laminate-type Al metal anode–GRAFOIL^® cathode rechargeable batteries created based on the obtained knowledge showed a favorable battery performance, if the appropriate charging condition that suppress Cl₂ generation was applied.

During the electrorefining of spent nuclear metallic fuels for recycling actinides, fission products of rare-earths, alkaline-earths and alkalis accumulate in LiCl–KCl melt in the form of their chlorides. For the purpose of periodically removing the accumulated fission product chlorides from the melt and stabilizing them in a waste form, a novel used-salt treatment process was proposed wherein electrochemical rare-earth silicide formation on a Si cathode was used as one of the main reactions. In this study, electrochemical measurements using a Si plate electrode were performed in LiCl–KCl melt containing lanthanide chlorides at 723 K to obtain basic knowledge of designing the Si cathode configuration and evaluating the composition of the recovered fission products in the proposed used-salt treatment process. A linear relationship between the thickness of the dense silicide layer and the square root of the electrolysis time in potentiostatic electrolysis at −1.5 V vs. Ag/AgCl was seen, which suggested the dense silicide layer growth governed by a diffusion in the layer. We also found a dependence of the distribution of lanthanides between the silicides and the melt on the applied cathodic current density during galvanostatic electrolysis for their recovery as silicides from the melt containing multi-lanthanide chlorides.

Post-combustion CO₂ capture is a promising method for removing CO₂ from processes where emissions cannot be mitigated by renewable energy input and where the chemical reactions required for production emit CO₂, e.g. calcination of calcium carbonate (CaCO₃) for cement production. One promising capture method is carbon capture in molten salts (CCMS). CCMS is a thermal swing gas-liquid process that utilizes CaO carbonation to absorb CO₂. The molten salt used in this work is 15 wt% CaO in eutectic CaCl₂-CaF₂ (86.2 : 13.8 wt%). The CaCl₂-CaF₂-CaO system has been found to have high cyclic absorption capacity (0.6 g CO₂/g CaO), though reaction kinetics has yet to be studied. By utilizing a novel experimental setup, data is collected, and a kinetic model is developed, which can be used in a techno-economic evaluation. The model proposes a simplified description of the CaCl₂-CaF₂-CaO system, with the assumption that the reaction is a first order elementary reaction where CaO and CO₂ react to form CaCO₃ without any solubility of CO₂ in the molten salt. CO₂ concentration, temperature, wt% CaO and surface area of molten salt are parameters in the proposed kinetic model. The result is a kinetic model that accurately fits the experimental data with an R² value above 0.98. It has been found that increasing the CO₂ concentration and decreasing the temperature yield a higher CaO to CaCO₃ equilibrium conversion.

The authors focus on the development of a molten salt processing technology for extracting white phosphorus (P₄) from sewage sludge ash, which is recognized as a potential secondary P source due to its composition of various metal phosphates. This work offers insights into the interaction between solid AlPO₄ and molten CaCl₂. The results show that solid AlPO₄ exhibits favorable solubility in molten CaCl₂, although at a slower rate compared to solid Ca₃(PO₄)₂ due to the presence of stable Al–O bonds. The study also demonstrates the feasibility of extracting high-purity P₄ from the molten mixture through electrolysis. The purity of the obtained electrolyzed P₄ from AlPO₄ is comparable to the commercially available high-purity-grade product. These findings underscore the potential of sewage sludge ash containing AlPO₄ as a viable source for extracting high-purity P₄. This research contributes to the advancement of sustainable P utilization technologies from secondary sources.

The permeation tendency of La, Ce, Pr, Nd, Gd, and Tb was examined using a rare earth (RE)–Ni alloy diaphragm in LiCl–KCl eutectic melts at 450 °C. This experiment was conducted as a part of an ongoing study on a new recycling process for Nd–Fe–B permanent magnets, because wasted magnets may contain La, Ce, Pr, Gd, and Tb as RE impurities. The permeation experiments were performed under two conditions that were expected to enable the selective permeation of Nd and Dy, respectively, which were determined on the basis of our previous studies confirming the selective permeation of Nd and Dy. As a result, Gd and Tb showed a similar permeation tendency to Dy, whereas Ce and Pr behaved like Nd. Under both experimental conditions, La hardly permeated. These tendencies are discussed on the basis of the applied potential and the equilibrium potential for RENi₂/RENi₃ (RE = Ce, Pr, Nd, Gd, Tb, and Dy) and La₇Ni₁₆/LaNi₃. The cross sections of the alloy diaphragm after the permeation experiments were analyzed via scanning electron microscopy/energy-dispersive X-ray spectroscopy. Relatively high concentrations of Gd, Tb, and Dy were detected in the alloy diaphragm, whereas the concentrations of La, Ce, Pr, and Nd were low in both experiments.

Na-ion batteries (NIBs) with ionic liquid (IL) electrolytes are promising candidates for large-scale energy storage devices owing to the abundance of Na resources and the safety of ILs. In our previous study, we have demonstrated the improved rate capability of NIBs consisting of a hard carbon negative electrode, an NaCrO₂ positive electrode, and an FSA-based IL electrolyte (FSA = bis(fluorosulfonyl)amide) by increasing the Na⁺ ion concentration in the IL. However, this phenomenon is not consistent with the trend observed for the ionic conductivities of bulk ILs. In this study, to clarify the unexplained behavior particularly for electrolytes with high Na⁺ concentrations, we performed in-situ Raman spectroscopic analysis in the vicinity of the electrode/electrolyte interface. The results of discharge rate capability tests indicated that a rate-determining step existed in the reaction at the positive electrode, where Na⁺ insertion occurred during discharge. In-situ Raman spectroscopy for Na/NaCrO₂ half-cells using an IL electrolyte of low Na⁺ concentration (∼1 mol dm⁻³) revealed that the Na⁺ ion concentration at the interface (inside the NaCrO₂ composite electrode) locally decreased as the discharging proceeded. In contrast, a high Na⁺ concentration electrolyte (∼2.2 mol dm⁻³) considerably suppressed the decrease in the Na⁺ ion concentration at the interface. Therefore, the improved performance of the electrolyte with a high Na⁺ concentration can be explained by the local Na⁺ ion concentration near the electrode/electrolyte interface, rather than by the bulk properties of the IL electrolytes.

The anodic dissolution reaction of Al–Zn alloys containing a Zn solid solution in Lewis acidic AlCl₃–1-ethyl-3-methyl-imidazolium chloride (EmImCl) ionic liquids at various potentials is investigated herein. An enrichment layer was formed on the anode surface after applying constant potentials of 0.3 and 0.7 V in the 60 mol%AlCl₃–40 %EmImCl (60 % AlCl₃) electrolyte and 0.2 V in the 67 mol%AlCl₃–33 %EmImCl (67 %AlCl₃) electrolyte. Consequently, metallic Zn was detected in the enriched layer using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Notably, in the 60 %AlCl₃ electrolyte, high-purity Al was obtained at a much nobler potential than the dissolution potential of pure Zn, suggesting that the anodic dissolution of Zn from the Al–Zn alloy into the electrolyte was difficult. When electrorefining Al–Zn alloy anodes containing a Zn solid solution using 60 %AlCl₃, which has lower acidity than the 67 %AlCl₃ electrolyte, it is possible to increase the anodic potential from 0.2 to 0.7 V. At this higher potential, high-purity Al is obtained at the cathode.

In the pyro-reprocessing of spent nuclear fuel, salt bath is normally used in several times, but at final moment, spent salt containing small amount of chloride nuclear fuel material is generated. In terms of managing nuclear fuel materials, it is desirable that the nuclear fuel materials should be recovered from the spent salt. We are proposing methods for recovering nuclear fuel materials using precipitation by oxide addition and distillation for reducing pressure techniques. In this study, we have focused on the behavior of manganese(II), which is one of the radioactivated products. As a result of experiments and thermodynamic simulation, it was found that manganese(II) is likely to be entrained in nuclear fuel materials. Therefore, it is necessary to add a step to separate manganese(II) from nuclear fuel materials.

The electrochemical behavior of neptunium in NaCl–2CsCl melt at 823–923 K was investigated by cyclic voltammetry, differential pulse voltammetry, and open-circuit chronopotentiometry after polarization. The results show that Np⁴⁺ ions are reduced to Np metal by a two-step mechanism via Np³⁺ ions in the NaCl–2CsCl melt. The diffusion coefficients of Np³⁺ and Np⁴⁺ ions were determined from cyclic voltammograms. The apparent standard potentials of Np³⁺/Np⁰ and Np⁴⁺/Np³⁺ redox couples have been determined to be E*(Np³⁺/Np⁰) = −3.353 + 7.67 × 10⁻⁴ T and E*(Np⁴⁺/Np³⁺) = −1.175 + 4.99 × 10⁻⁴ T vs. Cl₂/Cl⁻ (V), respectively. The activity coefficients of Np³⁺ and Np⁴⁺ ions were also determined using the reported data on the Gibbs free energy of formation for NpCl₃ and NpCl₄ in a supercooled liquid state.

To enhance the efficiency of water electrolysis, this study focuses on the NaOH–KOH hydrate melt (NaOH : KOH : H₂O = 9 : 61 : 30 mol%) at temperatures ranging from 100 to 200 °C. We examined the behaviors of hydrogen and oxygen evolution reactions (HER and OER) on a Ni electrode at atmospheric pressure. In the Tafel plots, the upper limit of the Tafel region for both HER and OER expanded with increasing temperature, especially at 200 °C. Additionally, changes in the rate-determining step for HER and OER were observed compared to a 30 wt% KOH aqueous solution at 80 °C. The total overpotential for HER and OER was compared to the value of 1133 mV obtained in the 30 wt% KOH aqueous solution (80 °C, 500 mA cm⁻²). The total overpotentials in the NaOH–KOH hydrate melt at 200 °C and 500, 1000, and 2000 mA cm⁻² were 545, 619, and 714 mV, respectively. The reduction in overpotentials was 52 %, 45 %, and 37 %, respectively. Water electrolysis utilizing the NaOH–KOH hydrate melt shows promising potential for significantly enhancing energy efficiency even at higher current densities relative to traditional alkaline water electrolysis.

The composition of platinum group alloys in insoluble residue liquid waste produced during spent nuclear fuel reprocessing depends on the type of fuel and the degree of burn-up. Consequently, for disposal of this waste by its vitrification in borosilicate glass, robustness of the vitrification process is necessary. It is very important in this process whether the platinum group alloy contained in the undissolved residue remains as a metal or blends into the glass. In this paper, the local structure and chemical state of each type of atom in borosilicate glass have been analyzed by SEM-EDS and EXAFS to evaluate the behavior of platinum group alloys in the glass. The SEM-EDS results showed that molybdenum migrate into the glass phase easily, while ruthenium, rhodium, and palladium atoms tend to stay in the alloy phase. The EXAFS showed that molybdenum (VI), ruthenium (IV), and palladium (0) were present in the same chemical form regardless of compositional change, while rhodium (III or IV) varied depending on the composition of the alloy.

Tungsten is a basic metal commodity that is classified as critical raw material (CRM) by the European Commission (EC) with the highest economical importance compared to other CRM. Thanks to its unique properties, secure W supply is critical to all industrial applications involving cutting or component wear, such as mining, machining, construction, tools and dies. Other important uses are in high-strength steels and high-temperature alloys, chemicals, mill products and lighting filaments. The production of W metal or carbide to be used in end products, requires a reduction process of the oxide which has been previously extracted from the primary (ores) resources by complex and energy intensive hydrometallurgical processes.

In the frame of the EC-funded TARANTULA project (GA 821159), innovative methods to obtain W metal directly from scheelite raw material have been investigated. The process comprises two steps, i.e., selective chlorination of the W ore in a molten chloride media using gaseous reactants, and subsequent electrolysis of the dissolved W electroactive species from the same reaction media. The chlorination of natural scheelite in a molten chloride has been demonstrated in the equimolar NaCl-KCl mixture at a working temperature of 727 °C, using both Cl₂ (g) and HCl (g). The dissolved W species in the molten chloride were found to be tri-tungstate: W₃O₁₀²⁻ and/or the chloro-complex, as e.g., W₃O₁₀Cl₂⁴⁻. Subsequent electrolysis trials demonstrated the recovery of WC₂ deposits on a carbonaceous cathode, while the anode reaction was evidenced to include the discharge of the oxide ions from the dissolved tri-tungstate species.

A polarizable ion model was applied to the solid and molten alkaline-earth halide MX₂, the parameters of which were determined by using first-principles calculations based on density functional theory, where M = Ca, Sr, Ba, and X = F, Cl, Br. The obtained parameters were used to evaluate the ionic conductivity, shear viscosity, and thermal conductivity in the molten and solid states by molecular dynamics simulations using the Green-Kubo relations. The calculated results were in good agreement with the experimental ionic conductivities and shear viscosities. The behaviors of all the calculated properties were well accounted for by ionic mass, number density, and packing fraction. Especially, the calculated thermal conductivities were well expressed by the empirical formula obtained for molten alkali halides. In addition, it was revealed that the reversal of cationic dependence in ionic conductivity of fluorides between solids and melts is due to the mass effects of carrier ions.

Eectrochemical glucose sensors is crucial for both environmental and human health, requiring rational nanoarchitectures with high electrochemical performance for glucose oxidation. Ni(OH)₂/Ni(DMG)₂ composite nanotubes were synthesized by etching nickel-dimethylglyoxime (DMG) nanorods with OH⁻, using Ni(DMG)₂ as a partially sacrificial template. The optimal Ni(OH)₂/Ni(DMG)₂ nonenzymatic glucose sensor was evaluated on both conventional and portable electrochemical workstations, showing high sensitivity and a low detection limit. The optimal Ni(OH)₂/Ni(DMG)₂ glucose sensors integrated into smartphones demonstrated a low detection limit of 3.3 µM (M = mol L⁻¹), a wide linear range (10 µM–8 mM), and a sensitivity of 262.80 µA mM⁻¹ cm⁻² for glucose detection. The sensors also exhibited favorable stability and reproducibility, along with preferable resistance to interference in the presence of uric acid, gluconate, proline, NaCl, valine, and lysine. Moreover, the portable sensor also demonstrated satisfactory glucose recovery (97.25–104 %) in serum samples, indicating its potential for future applications in real samples analysis.

The sulfide solid electrolyte Li₄SnS₄ has gained attention owing to its high moisture durability. In this study, we quantitatively investigated the changes in the electrochemical properties and chemical/physical states of Li₄SnS₄ resulted from moisture exposure using the XRD, Raman spectroscopy, and high-frequency electrochemical impedance spectroscopy (HF-EIS). Li₄SnS₄ was subjected to Ar gas flow at a dew point ranging from −20 °C to 0 °C for 1 h, and sulfide hydrolysis generated only a minute amount of H₂S. The XRD patterns and Raman spectra revealed the formation of Li₄SnS₄·4H₂O with increasing dew point. The HF-EIS analysis, which was conducted to clarify the spatial distribution of the hydrate within the particle, revealed a significant decrease in the ionic conductivity of Li₄SnS₄; this result can be attributed to the increased grain-boundary (SE/SE particle contact) resistance due to the formation of Li₄SnS₄·4H₂O at the particle surface, despite the generation of a minute amount of H₂S. By combining these multifaceted analytical methods, we demonstrated that the thermodynamically stable surface hydrate Li₄SnS₄·4H₂O reduced the lithium-ion conductivity without H₂S generation owing to the hydrolysis of sulfide. Thus, we chemically, spatially, and quantitatively verified the mechanism underlying the observed decrease in the ionic conductivity.

Although Li-air batteries (LAB) have a high theoretical energy density (3500 Wh kg⁻¹), further developments are required to overcome their practical limitations. Regarding the Li-metal negative electrode (NE), we have previously reported on the reversibility of the Li dissolution/deposition reaction by using Li|Li symmetric cells with a tetraglyme (G4)-based electrolytic solution. Particularly, in the 1.0 M (= mol L⁻¹) LiNO₃/G4 electrolyte under an O₂ atmosphere, a Li₂O protective layer is efficiently formed on the Li-metal electrode at a current density of 0.40 mA cm⁻², and Li dendrite formation is suppressed. In the present study, we expanded the test conditions (current densities up to 2.0 mA cm⁻² and temperatures of 10 to 50 °C) to clarify the dissolution/deposition behavior of the Li-metal NE. The effects of two electrolyte solutions, namely LiTFSI/G4 and LiNO₃/G4, on the Li-metal NE were evaluated based on cyclical testing using Li|Li symmetric cells under an O₂ atmosphere. The NEs were also examined by scanning electron microscopy and X-ray photoelectron spectroscopy. The results indicated that not only LiNO₃ salt but also the supply of Li and nitrate ions at the Li electrode surface are critical factors in LAB performance.

Photoelectrodes have attracted significant attention in green hydrogen production via water electrolysis. Among others, titanium oxide (TiO₂) is a representative photoelectrode for the conversion of solar energy in the ultravisible region into electrical and chemical energies. However, the poor solar adsorption of this material and the low anode reaction rate have limited the efficiency of hydrogen production via water electrolysis. In addition, there is an urgent need for more ecofriendly hydrogen production systems. The composite films of TiO₂ and electronically conductive materials with various structures have been investigated to improve the photoelectrochemical activity of TiO₂ by increasing the effective surface area of TiO₂ and electronic conductivity of the films, which suppress the electron–hole pair combination. Herein, TiO₂ and multiwalled carbon nanotube (MWCNT) composite films with various structures are prepared using the sol–gel method and electrophoretic deposition simultaneously, i.e., sol–gel electrophoretic deposition, under different solution conditions. Scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) results reveals that additives and the mixing ratio of TiO₂ sol and MWCNT dispersion solution in the electrophoresis bath affect film structure. Thin-film electrodes with different structures show different photoelectrochemical activities. Most as-fabricated TiO₂/MWCNT composite thin-film electrodes outperform pristine TiO₂ thin-film electrodes. TiO₂ deposition on the MWCNT network surface with an antenna-like shape yields the best photoelectrochemical activity, achieving a low film resistance and ∼80 % anatase/rutile ratio.