Published papers

Copyright © 2019 American Chemical Society. To elucidate the properties of binary system of room temperature ionic liquids (RTILs), we measured and compared the temperature dependence of densities, viscosities, and refractive indices of seven binary RTILs as well as neat RTILs. The observed densities, viscosities, and refractive indices of the binary systems showed intermediate properties between those of two starting RTILs. The values observed for the binary RTILs were only deviated from the values estimated based on the data of neat RTILs (±0.5%), and this deviation depended on both the combination of RTILs and the structures of the cation/anion. These results indicate that the properties of RTILs can be easily tuned by mixing RTILs depending on their applications.

© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. The formation of the solid electrolyte interphase (SEI) on a Cu electrode was investigated in 50.0-50.0 mol% lithium bis(trifluoromethylsulfonyl)amide (LiTFSA)-triglyme (G3, CH3(OCH2CH2)3OCH3) solvate ionic liquid, [Li(G3)]TFSA, using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). The Li+ conductive SEI was formed on a Cu electrode by keeping the potential at 0 V vs Li|Li(I). The resistance and capacitance attributable to the SEI changed over a period of 24 h in EIS spectra, suggesting the sluggish SEI formation on the Cu electrode in [Li(G3)]TFSA. XPS and FT-IR spectra indicated the existence of G3, TFSA-, [Li(G3)]+, and/or their decomposition products in the SEI. In contrast, LiF was not found in the SEI, suggesting LiF was not formed by the cathodic decomposition of [Li(G3)]TFSA on the Cu electrode at 0 V. The whisker-like growth of Li on a Cu electrode was suppressed after the SEI formation at 0 V probably due to the formation of homogeneous and low-conductive SEI layer. The coulombic efficiency for deposition and dissolution of Li was improved with the SEI formation at 0 V prior to the measurements, suggesting the SEI on a Cu electrode affected the reversibility of the Li anode.

The electrochemical properties of a sulfur positive electrode in equimolar glyme-Li salt mixtures were investigated. Cyclic voltammograms indicated that the insertion of Li into the sulfur/carbon composite electrode took place over at least three steps during the reduction process. In contrast, the broad anodic current suggested that the electrode kinetics for the extraction of Li were relatively slow. Stable charge-discharge operation of the Li-S cell consisting of a Li negative electrode with [Li(triglyme)][bis(trifluoromethanesulfonyl)amide] as the electrolyte was achieved at 800 cycles. This indicates that dissolution of Li2Sx into the electrolyte was effectively suppressed in this system.

Control of interfacial properties between the electrode and electrolyte is important for obtaining long life-cycle lithium-ion secondary batteries. To control interfacial degradation, we investigated the effect of changing the composition ratio of lithium salt (LiN(SO2CF3)2) and low-molecular-weight ether (CH3-O-(C2H4O)3-CH3) in the system. A highly electrochemically stable lithium salt glyme solvated ionic liquid electrolyte was realized, with long-lived, robust lithium solvation complexes by increasing the lithium salt composition ratio. Lithium secondary batteries using the electrolyte show notable suppression of the degradation of interfacial resistance at the interface of the electrolyte and the positive electrode (high oxidation state), which leads to a longer life-cycle of the batteries.

Abstract Electrochemical quartz crystal microbalance (EQCM) measurement is conducted with a Li4Ti5O12 (lithium titanium oxide, LTO)-coated quartz crystal electrode in a carbonate electrolyte (ethylene carbonate + dimethyl carbonate; 50: 50 vol%) containing 1 M LiPF6. In-situ monitoring of the mass change during the charge and discharge of the LTO electrode can be achieved quantitatively because of the "zero-strain" property of LTO with Li+ insertion and the probably low reactivity between LTO and the electrolyte. The local changes of viscosity and density of the electrolyte contacting the LTO electrode are detected via the resonance resistance of the quartz crystal electrode, suggesting the local concentrations of Li+ and counter anion changed significantly during insertion and extraction of Li+ in the organic electrolyte.

We have measured physicochemical properties of five alkyltrimethylammonium cation-based room-temperature ionic liquids and compared them with those obtained from computational methods. We have found that static properties (density and refractive index) and transport properties (ionic conductivity, self-diffusion coefficient, and viscosity) of these ionic liquids show close relations with the length of the alkyl chain. In particular, static properties obtained by experimental methods exhibit a trend complementary to that by computational methods (refractive index ∞ [polarizability/molar volume]). Moreover, the self-diffusion coefficient obtained by molecular dynamics (MD) simulation was consistent with the data obtained by the pulsed-gradient spin-echo nuclear magnetic resonance technique, which suggests that computational methods can be supplemental tools to predict physicochemical properties of room-temperature ionic liquids.

Innovation in the design of electrolyte materials is crucial for realizing next-generation electrochemical energy storage devices such as Li-S batteries. The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li-ion batteries. However, Li-S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reaction at the S cathode. Herein, we present simple solvate ionic liquids, glyme-Li salt molten complexes, as excellent electrolyte candidates because they greatly suppress the dissolution of lithium polysulfides. The molten complexes do not readily dissolve other ionic solutes, which leads to the stable operation of the Li-S battery over more than 400 cycles with discharge capacities higher than 700 mAh g-sulfür-1 and with coulombic efficiencies higher than 98% throughout the cycles. Such high performance has not been realized to the best of our knowledge. fürthermore, the addition of a nonflammable fluorinated solvent, which does not break the solvate structure of the glyme-Li salt molten complexes, greatly enhances the power density of the Li-S battery. The strategic design of electrolyte properties provides opportunities for the development of new electrochemical devices with many different electrode materials.

The electrodeposition and dissolution of lithium were investigated in an equimolar mixture of glyme and lithium bis(trifluoromethylsulfonyl)amide. An impedance technique electrochemical quartz crystal microbalance (EQCM-A) was used to monitor the product of the viscosity (ηL) and density (ρL) of the electrolyte, ηLρL, near the electrode in addition to the mass change. The value of ηLρL decreased during Li deposition and sharply increased during Li dissolution because of the changes in the concentration and the dissolved state of Li ion around the electrode. The sharp increase in viscosity during Li dissolution may affect the diffusion of Li(I) near the electrode/electrolyte interface, which is a key issue for the Li secondary battery.

Lithium-aluminum alloying reaction was investigated in lithium bis(trifluoromethylsulfonyl)amide triglyme solvate ionic liquid as a model system for the negative electrode of lithium secondary batteries. The formation of β-phase LiAl was confirmed by X-ray diffraction. The potential of α+β coexisting region was approximately 0.35 V vs. Li/Li(I). The slow relaxation of open circuit voltage and scanning electron microscope images suggested the inhomogeneous alloying reaction along with the active sites. The mass changes with doping and undoping of Li were measured qualitatively by electrochemical quartz crystal microbalance technique.

The electrochemical and battery performances of a CH3-O-(C 2H4O)3-CH3/LiN(SO2CF 3)2 1:1 equimolar complex electrolyte were evaluated using [Li metal|Li metal], [LiFePO4|Li metal], and [LiNi 1/3Mn1/3Co1/3O2|Li metal] cells. A relatively favorable surface was formed at the Li metal/electrolyte interface revealed by observation of the storage time dependence of the electrochemical impedance spectroscopy. A 3V-class [LiFePO4|Li metal] cell exhibited favorable capacity retention with little degradation after 600 cycles (2.5-4.0 V). A 4V-class [LiNi1/3Mn1/3Co1/3O 2|Li metal] cell also exhibited good capacity retention of approximately 60% of the initial discharge capacity after 400 cycles (2.7-4.2 V), while the capacity retention decreased markedly upon increasing the upper charge cutoff voltage above 4.4 V.

The physicochemical (e.g., density, viscosity, ionic conductivity, and self-diffusion coefficient) and electrochemical (e.g., electrochemical window and electric double-layer capacitor performance) properties of two new types of room-temperature ionic liquids, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMIm-FAP) and 1-ethyl-3- methylimidazolium tetracyanoborate (EMIm-TCB), were investigated and compared. EMIm-FAP showed a wider electrochemical window with a carbon working electrode compared with EMIm-TCB, in spite of lower viscosity, larger self-diffusion coefficients and higher ionic conductivity. Also, the electric double-layer capacitor using EMIm-FAP shows better cycle reversibility and higher charge/discharge voltage operation (over 3 V) than that using EMIm-TCB.

Relationships between the lithium salt concentration in a low-volatility solvent (room-temperature ionic liquid) and lithium secondary battery performance characteristics were investigated by using [LMOpositive electrode |DMPIm-TFSA+LiTFSA binary electrolyte

The dissolution behavior of ammonia (NH 3) in a molten LiCl-KCl-CsCl system was studied in order to determine the feasibility of low-energy NH 3 production via electrochemical synthesis in a high temperature molten salt system. In the presence of nitride ion (N 3-), a portion of NH 3 chemically dissolved in the melt as imide (NH 2-) and amide (NH 2 -) anions formed by the reaction between N 3- and NH 3. Although the dissolved NH 2 - anion thermally decomposed to form NH 3 and NH 2- anion, this phenomena may lower the yield of electrochemical NH 3 production. The dissolved NH 3 can be recovered by supplying hydrogen or water vapor to the melt. The anodic reaction between N 3- and H 2 to form NH 3 at a hydrogen gas-diffusion electrode was also examined and the NH 3 yield was estimated. Although the competing N 2 evolution reaction occurred at potentials more positive than 1.2 V vs. Li(I)Li, the conversion of Li 3N approached 70%, following the recovery processes.

Refractive index, an optical property, and liquid density were measured for 17 types of room-temperature ionic liquids at various temperatures. Ab initio calculations were performed to yield theoretical polarizability of the respective ionic liquid composing ions. A highly linear correlation was found between the experimental refractive index and the predicted polarizability normalized in terms of the molar volume. This suggests that the electronic polarization of the ionic liquid composing ions predominantly contributes to the refractive index of the room-temperature ionic liquids.

The shear relaxation spectra of the solutions of lithium salts in ionic liquids composed of N-methyl-N-propylpyrrolidinium cation paired with bis(trifluoromethanesulfonyl)amide (TFSA-) or bis(fluorosulfonyl) amide (FSA-) anions are determined from 5 to 205 MHz at various concentrations of lithium salts. The addition of lithium salt retards the shear relaxation, together with the increase in the shear viscosity. The normalized spectra reduce to a single curve when plotted against the product of the frequency and zero-frequency shear viscosity, which indicates that the increase in the shear viscosity by lithium salts is ascribed to the increase in the relaxation time. The difference in the shear viscosity of TFSA- and FSA-based ionic liquids is also elucidated in terms of the shear relaxation time. The relationship with previous studies on ionic mobility and liquid structure is also discussed.

Physicochemical properties such as density, viscosity, ionic conductivity, and self-diffusion coefficient of two types of room-temperatureionic liquids composed of cations with N-P bonds were investigated. One is an ionic liquid composed of cations with N-P-O bond, namely, tri(N,N-dimethylamino)-n- butoxyphosphonium bis(trifluoromethanesulfonyl)amide (TABP-TFSA) and has a relatively low viscosity (38 mPa s at 30C) due to the high internal mobility of the cation, despite its large ion size. TABP-TFSA showed a characteristic electrochemical oxidation behavior, such as a wide electrochemical potential window of stability without gas evolution. Formation of passivation film on a Pt electrode was suggested from the results of potential sweep experiments. The lithium secondary battery ([LiCoO2Li] cell) using TABP-TFSA as the electrolyte achieved stable charge-discharge operations in over 500 cycles. For another ionic liquid with N-P bond, namely, TAEP-TFSA (TAEP+ tri(N-methyl-N-(n-butylamino))ethylphosphonium), less battery cycle stability and low rate performances were observed, despite the higher ionic dissociation than TABP-TFSA, probably because of the high viscosity and low electrochemical oxidative stability. It is shown that the introduction of oxygen in the cation like N-P-O produces better performances compared with the cation having N-P structure.

The compatibility of SrTi 1-xCo xO 3 perovskites (0 ≤ × ≤ 0.2) was evaluated for use as interconnect materials in solid oxide fuel cells (SOFCs). Although SrTi 1-xCo xO 3 perovskites have a single perovskite phase in the range of 0 ≤ × ≤ 0.2, it was observed for SrTi 0.8Co 0.2O 3 that Co element agglomerated at the grain boundary during sintering. The dense SrTi 0.8Co 0.2O 3 sample was destroyed and included Sr 2TiO 4 as a secondary phase after reducing treatment at 1000°C. As a result of Co doping, the linear thermal expansion coefficient (TEC) increased remarkably with increasing Co content, but the TEC of SrTi 0.9Co 0.1O 3 was comparable with those of SOFC cathodes and anodes. Co doping of SrTiO 3 effectively increased electrical conductivity in air, whereas the conductivity of Co-doped SrTiO 3 in a reducing atmosphere was much lower than that in air. This suggests that the Co ions 3+/4+ in the perovskites were earlier reduced into Co 2+ ions, compared to Ti 4+ ions.

Ag(I)/Ag electrode reaction was investigated in some amide-type room-temperature ionic liquids composed of different cations. The morphology of silver deposits and the electrochemical behavior were not sensitive to the difference in the cations of ionic liquids. On the other hand, it was suggested that the adsorption of bis(trifluoromethylsulfonyl)amide (TFSA-) is more important for electrodeposition of silver in both ionic liquids and aqueous solutions. The diffusion coefficients of silver cation in the ionic liquids indicated the silver cation is surrounded by TFSA- to form a bulky species. The rate of crystal growth of silver particles in the ionic liquids by electrochemical Ostwald ripening was much slower than that in a nitrate aqueous solution, suggesting the charge transfer in the ionic liquids is slower than that in the aqueous solution.

The physicochemical and electrochemical properties of three 1-alkyl-2,3,5-trimethylpyrazolium cation-based room-temperature ionic liquids with various alkyl chain lengths were investigated. The temperature dependences of density, viscosity, and ionic conductivity were obtained by precise measurements. Electrolyte properties of these room-temperature ionic liquids were also examined from the viewpoint of their uses in lithium secondary batteries ([LiCoO2 positive electrode|electrolyte|lithium metal negative electrode]). It was found that the alkyl chain length affects the charge-discharge performances of cells.

Electrodeposition of silver was investigated using an impedance technique (separately excited, passive technique) electrochemical quartz crystal microbalance (EQCM) in a room-temperature ionic liquid. The mass changes during silver deposition and dissolution were observed with the current efficiencies of nearly 100% during potential sweep, constant potential step, and constant current step experiments. The product of the viscosity and density of the electrolyte near the electrode, νLρL, can be estimated by the resonance resistance, which can be monitored simultaneously with the resonance frequency. The change in the νLρ L value during silver deposition was consistent with the change in the calculated concentration of Ag(I) near the electrode. During the outer-sphere electron-transfer reaction between ferrocenium and ferrocene, no significant changes in the mass and the Lρ L value were observed.