Published papers

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.

The physicochemical properties of room temperature ionic liquids (RTILs) consisting of bis(trifluoromethanesulfonyl)amide (TFSA−) combined with 1-hexyl-1-methylpyrrolidinium (Pyr1,6+), 1-(butoxymethyl)-1-methylpyrrolidinium (Pyr1,1O4+), 1-(4-methoxybutyl)-1-methyl pyrrolidinium (Pyr1,4O1+), and 1-((2-methoxyethoxy)methyl)-1-methylpyrrolidinium (Pyr1,1O2O1+) were investigated using both experimental and computational approaches. Pyr1,1O2O1TFSA, which contains two ether oxygen atoms, showed the lowest viscosity, and the relationship between its physicochemical properties and the position and number of the ether oxygen atoms was discussed by a careful comparison with Pyr1,1O4TFSA and Pyr1,4O1TFSA. Ab initio calculations revealed the conformational flexibility of the side chain containing the ether oxygen atoms. In addition, molecular dynamics (MD) calculations suggested that the ion distributions have a significant impact on the transport properties. Furthermore, the coordination environments of the Li ions in the RTILs were evaluated using Raman spectroscopy, which was supported by MD calculations using 1000 ion pairs. The presented results will be valuable for the design of functionalized RTILs for various applications.

The electrode reactions of Ag(I)/Ag and ferrocenium/ferrocene (Fc+/Fc) were investigated in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA). The potential of Ag(I)/Ag depended on the logarithm of the concentration of Ag(I), as predicted by the Nernst equation, indicating the Ag(I)/Ag can be used as a reference electrode reaction in BMPFSA. The reversible electrode reaction of Fc+/Fc was observed in BMPFSA by cyclic voltammetry. The donor number of BMPFSA was estimated to be 13 from the difference in the formal potentials of Ag(I)/Ag and Fc+/Fc, indicating the coordination ability of FSA- was slightly stronger than that of bis(trifluoromethylsulfonyl)amide (TFSA-). The diffusion coefficients (D) of Fc and Fc+ were (5.7 ± 0.7) and (3.3 ± 0.2) × 10-7 cm2 s-1, respectively. The ratio of D of Fc+ against that of Fc was smaller than those in TFSA--type ionic liquids, reflecting the higher charge density of FSA-. The standard rate constant (k 0) of Fc+/Fc was estimated to be (5.4 ± 1.1) × 10-3 cm s-1. The apparent activation energy for k 0 was close to the activation energy for D, suggesting the electrode reaction of Fc+/Fc can be regarded as the outer sphere electron transfer reaction with a very small reorganization energy.

The formation of solid-electrolyte interphase (SEI) on a glassy carbon (GC) electrode in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Li+, Na+, K+, and Ni2+ was investigated using the redox reaction of ferrocene (Fc). The anodic peak potential (E pa) for the oxidation of Fc changed after holding the electrode at -1.4 to -1.5 V vs Ag|Ag(I) in BMPFSA in the presence of the alkali metal ions. The decomposition of FSA- was confirmed by X-ray photoelectron spectroscopy (XPS) on a GC electrode held at -1.5 V vs Ag|Ag(I) for 6 h in BMPFSA containing the alkali metal ions. The change in E pa may also suggest the formation of homogeneous SEI in the FSA--type ionic liquid. Moreover, E pa changed after holding the electrode at the potential more negative than -0.9 V vs Ag|Ag(I) in 50 mM Ni(TFSA)2/BMPTFSA while the decomposition of TFSA- was not confirmed by XPS on the electrode held at -1.1 V vs Ag|Ag(I) for 6 h, suggesting the bonds except C-F (e.g., S-C) were dissociated to form the SEI. Since the deposition potential of Ni in BMPTFSA was more negative than the SEI formation potential, the deposition of Ni may be inhibited by the SEI.

The formation and aging of the solid electrolyte interphase (SEI) formed on a Li and Cu electrode in two bis(fluorosulfonyl)amide (FSA-)-type ionic liquids (ILs), 1-methyl-1-butylpyrrolidinium bis(fluorosulfonyl)amide and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide, containing a high concentration of LiFSA were investigated using electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy. The intensity of peaks corresponding to the decomposition of the electrolyte in XPS spectra changed with time. The thicknesses of the SEI formed on a Cu electrode in both ILs were estimated to be 20-100 nm by TEM observation. The resistance of SEI increased with the lapse of time although the change in the thickness of the SEI was small, suggesting the aging of the SEI in the ILs was considered to depend on the composition of the SEI rather than the thickness of the SEI. The Li deposits shrank during dissolution. The Li anode became sparse and porous after deposition and dissolution cycles, probably due to the inhomogeneous reaction through the aged SEI. The SEI formation on the surface of the porous deposits was considered to lead to an increase in the polarization and the isolation of the deposits, i.e., the formation of “dead lithium”.

The formation of solid-electrolyte interphase (SEI) on a Pt electrode was investigated in ethylene carbonate (EC) and diethyl carbonate (DEC) containing LiPF6 and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) using the redox probe method. The formation and properties of the SEI were examined in the electrolytes containing ferrocene as a redox probe. Inhomogeneous and rigid SEI was found to form in 1 M (= mol dm−3) LiPF6/EC+DEC, probably due to the conversion of decomposition products to LiF by the reaction with HF derived from LiPF6. On the other hand, the formation of homogeneous and soluble (or dispersible) SEI was suggested in 1 M LiTFSA/EC+DEC.

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.

The physicochemical and electrochemical properties of an ionic liquid consisting of the dichlorocuprate anion, [CuCl2]-, were investigated. The equimolar mixture of CuCl and 1-butyl-1-methylpyrrolidinium chloride (BMPCl) was found to yield an ionic liquid at 298 K composed predominantly of BMP+ and [CuCl2]-. Fine deposits of metallic Cu were obtained on a glassy carbon electrode by potentiostatic and galvanostatic reduction of [CuCl2]- in CuCl-BMPCl (50.0-50.0 mol%). The overpotential for Cu nucleation was larger on a glassy carbon electrode than on a Pt electrode. The electrochemical deposition and dissolution of Cu were analyzed using an electrochemical quartz crystal microbalance. The local viscosity and density of the electrolyte near the electrode/electrolyte interface increased during both deposition and dissolution of Cu, probably reflecting the shift in the electrolyte composition to the local basic and acidic conditions, respectively.

The formation potentials of the solid-electrolyte interphase (SEI) on a platinum and glassy carbon electrode in two amide-type ionic liquids, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), containing some metal salts were investigated by cyclic voltammetry in the presence of ferrocene (Fc). It is considered that the SEI with the thickness of less than a few nm can be observed by monitoring the shift in the peak potential of Fc+/Fc. The SEIs began to form at ∼2.4 V vs. Li|Li(I) in the presence of lithium salts because of a lowering of the lowest unoccupied molecular orbital level of the anion by the interaction with lithium ion. In the case of BMPTFSA containing Ni(TFSA)2, the SEI began to form at -0.7 V vs. Ag|Ag(I), suggesting that the deposition of Ni may be hindered by the SEI formation.

Electrodeposition of nickel was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) containing Ni(TFSA)2 under a external magnetic field. The onset potential of cathodic reduction of Ni(II) with a magnetic field was more positive than that without the magnet field. Ni nanowires were deposited by potentiostatic cathodic reduction with the magnetic field, indicating the magnetic field influence not only the electrode kinetics of the magnetic species but also the morphology of the ferromagnetic metal.

The mass change of the electrode and the local physicochemical properties of the electrolyte at the dynamic electrochemical interface during electrode reactions in non-aqueous electrolytes were analyzed in situ using an impedance-type electrochemical quartz crystal microbalance (EQCM). The changes in the local properties of the electrolyte (viscosity and density), reflecting the concentration distribution of the dissolved species formed near the electrode, were observed during the electrochemical deposition and dissolution of metals in an ionic liquid and the insertion/extraction of Li+ in an organic electrolyte used in commercial lithium-ion secondary batteries. It is necessary to subtract the contribution of the changes in the local viscosity and density of the electrolyte from the observed change in the resonant frequency to evaluate the mass change of the quartz crystal electrode, especially when the local properties of the electrolyte change significantly during the electrode reaction. The dissolved species of metals in solvate ionic liquids and chlorocuprate ionic liquids depend on the composition. The observed changes in the local properties of the electrolyte during the electrode reactions in these electrolytes were considered to reflect the transient changes in the liquid structure at the dynamic electrochemical interface in addition to the concentration distribution of the metal complexes.

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.

Electrochemical behavior of zirconium tetrachloride (ZrCl4) was investigated in an amide-type hydrophobic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA). ZrCl4 was found to be soluble in BMPTFSA with or without addition of 1-butyl-1-methylpyrrolidinium chloride (BMPCl) as a Lewis base. Cyclic voltammograms recorded at a glassy carbon (GC) electrode showed two noticeable reduction peaks at about –1.9 V and –2.3 V for the ZrCl4 in ionic liquid without addition of BMPCl at room temperature, while no reduction peak was noticed for the ZrCl4 in ionic liquid in the presence of BMPCl at room temperature. The absence of any cathodic current peak corresponding to the reduction of Zr(IV) in ionic liquid with BMPCl at room temperature is presumably ascribed to the formation of a stable anion, [ZrCl6 ]2– by addition of excess chloride ion to the electrolyte. Electrodeposition of Zr from ZrCl4 on a GC electrode substrate in BMPTFSA ionic liquid with or without addition of BMPCl at elevated temperature or room temperature was observed. Scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy results confirmed the presence of Zr in the deposits obtained by electrolysis of ZrCl4 in BMPTFSA.

The electrode reactions of haloargentates were investigated in an aprotic and hydrophobic amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) in the presence of the halide ions. Silver halides, AgX (X- = Cl-, Br-, and I-) were found to be soluble in BMPTFSA containing 0.5 M BMPX to form halogenocomplex, [AgX3]2-. The cathodic reduction of [AgX3]2- to metallic Ag was observed within the electrochemical potential window of the ionic liquid, while the reduction potential was lower than that in the ionic liquid in the absence of X-. The equilibrium potentials of [AgX3]2-/Ag were in the order of [AgCl3]2- > [AgBr3]2- > [AgI3]2-, probably reflecting the thermodynamic stability of the complexes. The diffusion coefficients of [AgCl3]2-, [AgBr3]2-, and [AgI3]2- were 2.5, 2.0, and 1.6 × 10-7 cm2 s-1, respectively. The morphology of deposits strongly depended on the reduction potential. The nucleation and growth mechanism of Ag deposition was considered to be progressive rather than instantaneous. Formation of Ag nanoparticles dispersed in the ionic liquids was confirmed after potentiostatic cathodic reduction at -2.5 V vs Ag|Ag(I) using a transmission electron microscope.

The formation potentials of solid-electrolyte interphase (SEI) were investigated in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing LiFSA and LiTFSA, respectively, by monitoring the redox reaction of ferrocene with high sensitivity. The SEI was found to affect the electrode kinetics of the outersphere electron transfer reaction. The SEIs with the thickness of less than a few nm formed in LiFSA/BMPFSA and LiTFSA/BMPTFSA at 2.4 and 2.5 V vs Li|Li(I), respectively. The cathodic decomposition of FSA- and TFSA- was considered to be promoted by the interaction with Li+ based on the ab-initio calculation.

The dependence of the impedances of lithium phosphorous oxynitride (LiPON) thin film and solid electrolyte interphase (SEI) formed by decomposition of some electrolytes on the electrode potential was investigated by electrochemical impedance spectroscopy. A LiPON thin film was prepared on a Ni electrode by radio frequency magnetron sputtering of Li3PO4 under nitrogen atmosphere. The resistance of the LiPON thin film decreased with lowering the electrode potential in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) containing 1M LiTFSA. The similar potential dependence of the impedance of the SEI formed in 1M LiTFSA/BMPTFSA was observed, suggesting that the Li+ carrier density in the LiPON thin film and SEI increased with lowering the electrode potential probably due to the doping of Li+ from the electrolyte into the thin Li+ conductors in order to compensate the negative charge on the electrode. On the other hand, the potential dependence of the SEI formed in LiTFSA-tetraglyme (G4) solvate ionic liquid was insignificant because of the high concentration of Li+ in the SEI and electrolyte. The resistance of the SEI formed in 1M LiClO4/EC (ethylene carbonate) + DMC (dimethyl carbonate) (1 : 1 vol%) did not depend on the electrode potential, suggesting the thin and highly Li+ conductive SEI is formed in the organic electrolyte.

The redox reaction of tris(acetylacetonato)iron(III) (Fe(acac)3) on a glassy carbon electrode was investigated in 1- butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA). A one-electron transfer redox reaction of [Fe(acac) ] showed the relatively negative potential of -1.4V vs. Ag|Ag(I). The diffusion coefficient of Fe(acac) estimated by chronoamperometry was (1.4 + 0.1) × 10 cm s . The redox properties examined in this study suggested that the redox reaction of [Fe(acac) ] can be utilized in the negative half-cell reaction of redox flow batteries. 3 3 3 0/- -7 2 -1 0/-

The in-situ Fourier transform infrared spectroscopy of molten eutectic LiCl-KCl-CsCl was conducted in order to identify the dissolved ions in the molten salt electrolytes. The transflectance spectrum of the melts could be easily obtained at temperatures higher than 350°C using the commercially available diffuse reflectance optical system and an air-tight chamber with a built-in heater. The sharp absorption peak attributable to the stretching vibration of OH was observed in the melt containing LiOH, indicating the availability of identification of dissolved species in the melts. The intensities of transflection possibly assignable to N-H bonds in imide (NH ) and amide (NH ) anions in the melt containing Li N changed during supplying H gas because of the progress of the protonation reactions of these anions to form NH . The in-situ analysis of the dissolved ions in the melts under the reaction conditions by infrared spectroscopy with the diffuse reflectance optical system can be a simple and powerful tool for understanding the reaction mechanism. - 2- - 2 3 2 3

Aluminum species forming in Lewis acidic chloroaluminate and bis(trifluoromethylsulfonyl)amide (TFSA ) mixed ionic liquids having 1-butyl-3-methylimidazolium (BMI ) was investigated by potentiometry and Raman spectroscopy. A decrease in the concentration of [Al Cl ] and an increase in that of [AlCl ] with addition of BMITFSA into acidic BMICl-AlCl were observed by Raman spectroscopy. The redox potential of Al(III)/Al in BMICl-AlCl (1:2 in molar ratio) was found to be -0.68V vs. an Ag|Ag(I) reference electrode, which was corresponding to -0.25V vs. the ferrocene/ferrocenium couple at 25°C. The potentiometry of an Al electrode in the mixed ionic liquids with different compositions using the Ag|Ag(I) reference electrode suggested formation of a mixed ligand aluminum complex, Al(TFSA)Cl , indicating the instability of [Al Cl ] against TFSA . - + - - - - 2 7 4 3 3 2 2 7

Electrochemical formation of selenium (Se) nanoparticles in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing selenium tetrachloride (SeCl ) was investigated in the presence of excess chloride anion on a glassy carbon electrode. Electrochemical reduction of [SeCl ] resulted in deposition of Se on the electrode surface. The deposited Se was further reduced electrochemically at the more negative potential region to form [Se ] , which was suggested to exist by ultraviolet-visible spectroscopy. Se nanoparticles dispersed in the ionic liquid were formed by the proportionation reaction between [SeCl ] and [Se ] in the ionic liquid. The Se nanoparticles were characterized by energy dispersive X-ray spectroscopy and transmission electron microscopy. 4 6 2 6 2 2- 2- 2- 2-