Published papers

Silicon micropillars with tunable sizes are successfully fabricated on copper foils by using nanosecond-pulsed laser irradiation and then used as anodes for lithium-ion batteries. The size of the silicon micropillars is manipulated by using different slurry layer thicknesses ranging from a few microns to tens of microns. The effects of the pillar size on electrochemical properties are thoroughly investigated. The smaller the pillars, the better the electrochemical performance. A capacity of 1647 mAh g at 0.1 C current rate is achieved in the anode with the smallest pillars, with 1215, 892, and 582 mAh g at 0.2, 0.5, and 1.0 C, respectively. Although a significant difference in discharge capacity is observed in the early period of cycling among micropillars of different sizes, this discrepancy becomes smaller as a function of the cycle number. Morphological studies reveal that the expansion of micropillars occurred during long-term cycling, which finally led to the formation of island-like structures. Also, the formation of a solid electrolyte interphase film obstructs Li diffusion into Si for lithiation, resulting in capacity decay. This study demonstrates the importance of minimizing the pillar size and optimizing the pillar density during anode fabrication. -1 -1 +

The electrochemical behavior of a chlorocomplex of nickel (Ni) was investigated in a Lewis basic and hydrophobic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Cl . NiCl dissolved in BMPTFSA with addition of 0.5 M BMPCl. The dissolved species of Ni(II) in BMPTFSA was identified as [NiCl ] by UV-visible spectrometry and magnetic susceptibility balance. The cathodic and anodic currents assignable to deposition and dissolution of Ni, respectively, were observed in cyclic voltammetry. The granular deposits of Ni were obtained on a glassy carbon electrode at 25 and 100 °C. Ni nanoparticles were dispersed in the electrolyte by galvanostatic cathodic reduction. The initial stage of Ni deposition was found to be regarded as instantaneous. The diffusion coefficient of [NiCl ] was calculated to be 6.6 × 10 cm s - 2- 2- -8 2 -1 2 4 4

Electrochemical preparation of Co-Sm nanoparticles was conducted in an aprotic room temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Co(TFSA) and Sm(TFSA) . The cyclic voltammetry on a glassy carbon (GC) electrode indicated the electrochemically generated Sm(II) reacted with Co(II) at 25 C. Potentiostatic cathodic reduction on a GC electrode in BMPTFSA containing 30 mM Co(TFSA) and 5 mM Sm(TFSA) at 25 C gave the deposits, which were found to be composed of Co and Sm by energy dispersive X-ray spectroscopy (EDX). The deposits were found to be the aggregates of SmCo nanoparticles by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The formation of SmCo nanoparticles dispersed in the ionic liquid was also confirmed by TEM. SmCo nanoparticles were considered to form by the disproportionation reaction of Sm(II) in the presence of elementary Co, which was formed by the reduction of Co(II) by Sm(II). 2 3 2 3 7 7 7

Electrodeposition of cadmium (Cd) was investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) using CdCl as the source of Cd species in the presence of excess chloride ion. Raman spectroscopy and potentiometric measurement suggested formation of a cadmium tetrachlocomplex, [CdCl ] , in BMPCl/BMPTFSA. Cyclic voltammetry showed the possibility of electrochemical reduction from [CdCl ] to Cd(0) with interesting unusual electrochemical behavior, probably derived from the potential-dependent electric double layer structure typical to the ionic liquid. Electrodeposition of Cd was performed by potentiostatic electrolysis and the deposits were characterized by X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy. 2 4 4 2- 2-

Electrochemical behavior of Sn was investigated on a GC electrode in an aprotic ionic liquid, 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing BMPCl and SnCl at 298K. SnCl was found to dissolve as [SnCl ] by potentiometry. The formal potential of [SnCl ] /Sn was -1.67V vs. Ag|Ag(I) (-1.24V vs. ferrocene|ferrocenium), which was more negative than that in BMPTFSA without chloride ions. Electrodeposition of Sn was possible by reduction of [SnCl ] in the ionic liquid. The dissociation of [SnCl ] was suggested from the diffusion coefficient of [SnCl ] . Tubular deposits with an inner diameter of 1-1.5μm were obtained in addition to granular deposits by potentiostatic cathodic reduction at -2.0V. 2 2 4 4 4 4 4 2- 2- 2- 2- 2-

The electrode reaction of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was investigated in a solvate ionic liquid composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4), [Li(G4)]TFSA. A quasi-reversible redox reaction of the oxoammonium (TEMPO ) and TEMPO was observed with a formal potential of 0.19 V vs ferrocene|ferrocenium. The diffusion coefficient of TEMPO (2.1 × 10 cm s ) was smaller than that of neutral TEMPO (1.3 × 10 cm s ), reflecting the coulombic interaction between monovalent TEMPO and TFSA in the solvate ionic liquid. The standard rate constant for TEMPO /TEMPO redox reaction was estimated to be 1.9 × 10 cm s at 25 C by electrochemical impedance spectroscopy. The solubility of TEMPO in [Li(G4)]TFSA was found to be 4.4 M at 25 C. + + -8 2 -1 -7 2 -1 + - + -4 -1

The solid electrolyte interphase (SEI) between Li and ionic liquid electrolytes was investigated in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (MPPFSA) and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide (MOEMPFSA) with different LiFSA concentrations. The physicochemical properties and the coordination states of Li+ with FSA- with increasing the LiFSA concentration were similar between xLiFSA-MPPFSA and MOEMPFSA. On the other hand, the SEI resistance obtained by electrochemical impedance spectroscopy using a Li|Li symmetrical cell increased with the lapse of time in xLiFSA-MPPFSA more significantly than in xLiFSA-MOEMPFSA, suggesting that the SEI formed in xLiFSA-MOEMPFSA was more conductive than that in xLiFSA-MPPFSA. The SEI formed on Li was considered to be composed mainly of the ions in the electrolytes and their decomposed products by X-ray photoelectron spectroscopy. The cyclability of deposition and dissolution of Li was improved with increasing the LiFSA concentration in both xLiFSA-MPPFSA and MOEMPFSA. An increase in the conductivity of the SEI with increasing the LiFSA concentration may lead to the promotion of homogeneous growth of Li rather than dendritic growth of Li.

Electrochemical recovery of metallic Co was attempted from CoO in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing 0.5 M BMPCl. CoO was dissolved in 0.5 M BMPCl/BMPTFSA with a carbochlorination reagent, C2Cl6, to form [CoCl4]2- at 150 °C. After the removal of electroactive C2Cl6 and byproducts, the electrochemical reduction of [CoCl4]2- was found to be possible on a glassy carbon electrode in the ionic liquid. Co nanowires were deposited by electrochemical reduction of [CoCl4]2- in 0.5 M BMPCl/BMPTFSA under an external magnetic field, which promoted the cathodic reduction and affected the morphology of the deposits. The deposited Co nanowires were found to be composed of crystalline Co nanoparticles by transmission electron microscopy.

Electrodeposition of Co was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), under an external magnetic field. Neodymium magnets were used as a source of the magnetic field, which was parallel to the ionic current during the electrodeposition of Co on a glassy carbon electrode. Potentiostatic cathodic reduction applying -1.6 and -2.0 V vs Ag|Ag(I) under the magnetic field in BMPTFSA containing Co(TFSA)2 at 25 C gave nanowire-shaped deposits on the electrode surface. The deposits were found to be composed of Co, which was confirmed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. However, no diffraction peak was observed in the deposits by X-ray diffraction. The nanowires were further examined by transmission electron microscopy. The lattice fringe corresponding to (111) plane of Co was found in the deposits, indicating the nanowires were composed of metallic Co nanoparticles. Therefore, crystalline Co nanoparticles were considered to be formed in the presence of the magnetic field.

Electrochemical reaction of samarium (Sm) species was explored in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), at 25 and 100°C. The cathodic and anodic current peaks corresponding to the reduction of Sm(III) and the oxidation of Sm(II), respectively, were observed in BMPTFSA containing Sm(TFSA)3 by cyclic voltammetry at 25°C. However, the anodic current peak corresponding to the oxidation of Sm(II) diminished significantly at lower scan rates when the temperature was elevated to 100°C, suggesting the disproportionation of Sm(II) at higher temperatures. Deposits were obtained on a glassy carbon electrode by potentiostatic cathodic reduction at -1.6 and -2.5 V in BMPTFSA containing Sm(TFSA)3 at 100°C. The deposits were found to contain Sm by energy dispersive X-ray spectroscopy (EDX), although no diffraction peak was observed by X-ray diffraction. The formation and dispersion of nanoparticles in the ionic liquid were also confirmed by transmission electron microscopy with and without exposure of the samples to air. The nanoparticles were considered to be metallic Sm, which was identified by electron diffraction and EDX. Sm nanoparticles were anticipated to be formed by the disproportionation reaction of Sm(II) to Sm(III) and Sm in the ionic liquid.

Anodic dissolution and electropolishing of Sn were investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide. The rate of anodic dissolution was considered to be determined by the diffusion of anodically dissolved Sn(II) species. A large increase in the local viscosity during dissolution was observed in-situ by the impedance-type electrochemical quartz crystal microbalance, reflecting an increase in the local concentration of Sn(II) near the electrode. A shiny and smooth surface was obtained after anodic dissolution at 0.1 V vs Ag|Ag(I) with agitation. A decrease in the surface roughness estimated by confocal laser scanning microscopy suggested electropolishing of Sn was possible in the ionic liquid within the electrochemical potential window probably due to the formation of the viscous layer near the electrode.

The redox reaction of 2, 2, 6, 6-tetramethylpiperidine-l-oxyl (TEMPO) has been investigated in a solvate ionic liquid composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4). The one-electron transfer and quasi-reversible redox reaction of TEMPO+/TEMPO was observed in the stable electrochemical potential window of the solvate ionic liquid. The formal potential of TEMPO'S/TEMPO was 0.19 V vs. Ferrocene/ferrocenium. The diffusion coefficients of TEMPO and TEMPO at 298 K were 1.8 × 10 and 7.6 × 10 cm s , respectively, reflecting the coulombic interaction between the charged species with the anion of the solvate ionic liquid. The standard rate constant for the redox reaction of TEMPO /TEMPO at 298 K was 1.9 × 10 cm s . Charge-discharge of a lithium secondary cell using a lithium-ion conductive glass-ceramics (LICGC) as a diaphragm and [Li(G4)]TFSA containing TEMPO as a catholyte was found to be possible with the average discharge voltage of 3.4 V. + -7 -8 2 -1 + -4 -1

Room temperature ionic liquids (RTILs) containing ether oxygen atoms have been investigated for a gamut of science and technology applications owing to their superior physicochemical properties. However, the effect of the position of the ether oxygen atom in the side chain on their physicochemical properties is not clearly understood. This study investigates, using both experimental and computational approaches, the effect of ether oxygen atoms on the physicochemical properties of RTILs consisting of bis(trifluoromethylsulfonyl)amide (TFSA-) with 1-methyl-1-propylpyrrolidinium (MPP+), 1-butyl-1-methylpyrrolidinium (BMP+), 1-methoxymethyl-1-methylpyrrolidinium (MOMMP+), 1-ethoxymethyl-1-methylpyrrolidinium (EOMMP+), and 1-methoxyethyl-1-methylpyrrolidinium (MOEMP+). The viscosity of the RTILs containing the ether oxygen atom was lower than that of the alkyl analogues. Moreover, the viscosity of EOMMPTFSA was lower than that of MOEMPTFSA, albeit EOMMPTFSA and MOEMPTFSA have the same molecular weight with ether oxygen atoms at different positions. Ab initio calculations reveal that the number of methylene groups between nitrogen and oxygen atoms in the cation structure profoundly impacts the local stable structure of the cation. Furthermore, we discussed the relationship between the transport properties and the spatial distribution of ions obtained by MD simulations. This result will be valuable in the design of functionalized RTILs, via the judicious tuning of the conformational flexibility of ether oxygen atoms in related ionic liquids.

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.

Deposition and dissolution of copper were investigated by electrochemical quartz crystal microbalance (EQCM) technique using a Pt-coated quartz crystal resonator in contact with a separator in order to simulate the configuration of practical batteries. The admittance analysis indicated the quartz crystal resonated even in contact with a glass separator impregnated with the electrolyte under pressure. The mass change estimated from the change in the resonance frequency during deposition and dissolution of Cu on a Pt-coated resonator in contact with the separator was linearly related to the passed electric charge and closed to that without the separator, suggesting the mass monitoring according to Saurbrey equation is possible in the presence of the separator on the electrode. The highly sensitive mass monitoring with an overtone resonance was also found to be possible in the presence of the separator using the third harmonic vibration mode.