Kaoru Dokko

Oxidative-Stability Enhancement and Charge Transport Mechanism in Glyme–Lithium Salt Equimolar Complexes

The oxidative stability of glyme molecules is enhanced by the complex formation with alkali metal cations. Clear liquid can be obtained by simply mixing glyme (triglyme or tetraglyme) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1. The equimolar complex [Li(triglyme or tetraglyme)(1)][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a room-temperature ionic liquid consisting of a [Li(glyme)(1)](+) complex cation and a [TFSA](-) anion, exhibiting high self-dissociativity (ionicity) at room temperature. The electrochemical oxidation of [Li(glyme)(1)][TFSA] takes place at the electrode potential of ~5 V vs Li/Li(+), while the oxidation of solutions containing excess glyme molecules ([Li(glyme)(x)][TFSA], x > 1) occurs at around 4 V vs Li/Li(+). This enhancement of oxidative stability is due to the donation of lone pairs of ether oxygen atoms to the Li(+) cation, resulting in the highest occupied molecular orbital (HOMO) energy level lowering of a glyme molecule, which is confirmed by ab initio molecular orbital calculations. The solvation state of a Li(+) cation and ion conduction mechanism in the [Li(glyme)(x)][TFSA] solutions is elucidated by means of nuclear magnetic resonance (NMR) and electrochemical methods. The experimental results strongly suggest that Li(+) cation conduction in the equimolar complex takes place by the migration of [Li(glyme)(1)](+) cations, whereas the ligand exchange mechanism is overlapped when interfacial electrochemical reactions of [Li(glyme)(1)](+) cations occur. The ligand exchange conduction mode is typically seen in a lithium battery with a configuration of [Li anode|[Li(glyme)(1)][TFSA]|LiCoO(2) cathode] when the discharge reaction of a LiCoO(2) cathode, that is, desolvation of [Li(glyme)(1)](+) and insertion of the resultant Li(+) into the cathode, occurs at the electrode-electrolyte interface. The battery can be operated for more than 200 charge-discharge cycles in the cell voltage range of 3.0-4.2 V, regardless of the use of ether-based electrolyte, because the ligand exchange rate is much faster than the electrode reaction rate.

Particle morphology, crystal orientation, and electrochemical reactivity of LiFePO4 synthesized by the hydrothermal method at 443 K

LiFePO4 (space group: Pnma) was synthesized by the hydrothermal method at 443 K. The pH of the precursor solution was systematically changed between 2.5 and 9.5. The particle morphology, crystal orientation, and electrochemical reactivity of the prepared LiFePO4 particles changed depending on the concentration of the Li source and pH of the precursor. The particles obtained from acidic solutions (pH ≈ 3.5) were needle-like particles. On the other hand, plate-like crystals were obtained from weak acidic solutions of 4 < pH < 6.5. At higher pH than 7.2, the particles became randomly shaped. The plate-like crystal had a large facet in the ac-plane, while the needle-like particles had a large facet in the bc-plane. The electrochemical properties of the prepared LiFePO4 were characterized in a mixed solvent of ethylene carbonate and diethyl carbonate with volume ratio of 1 : 1 containing 1.0 mol dm−3 LiClO4 at room temperature. The plate-like crystals exhibited the highest electrochemical reactivity among the prepared samples, and the discharge capacity was 163 mA h g−1 measured at a current density of 17 mA g−1.

Kinetic Characterization of Single Particles of LiCoO2 by AC Impedance and Potential Step Methods

This is the first report of impedance technique run on single particle electrodes with the aim of clarifying its electronic and ionic transport properties. Measurements were successfully conducted on a particle of 15 μm diam resulting in impedance magnitude on the order of MΩ. The impedance spectra exhibited (i) one semicircle in the high frequency region, (ii) Warburg impedance in low frequencies, and finally, (iii) a limiting capacitance in the very low frequencies. The spectra were analyzed using a modified Randles-Ershler circuit, so that the reaction kinetics could be precisely evaluated. The charge transfer resistance decreased as the potential increased, whereas the double layer capacitance was almost invariant with the potential. Thus, the apparent chemical diffusion coefficient of lithium ions was determined to be to as function of electrode potential. These results are in agreement with those obtained by potential step chronoamperometry technique.

Protic Ionic Liquids and Salts as Versatile Carbon Precursors

Instead of traditional polymer precursors and complex procedures, easily prepared and widely obtainable nitrogen-containing protic ionic liquids and salts were explored as novel, small-molecule precursors to prepare carbon materials (CMs) via direct carbonization without other treatments. Depending on the precursor structure, the resultant CMs can be readily obtained with a relative yield of up to 95.3%, a high specific surface area of up to 1380 m(2)/g, or a high N content of up to 11.1 wt%, as well as a high degree of graphitization and high conductivity (even higher than that of graphite). One of the carbons, which possesses a high surface area and a high content of pyridinic N, exhibits excellent electrocatalytic activity toward the oxygen reduction reaction in an alkaline medium, as revealed by an onset potential, half-wave potential, and kinetic current density comparable to those of commercial 20 wt% Pt/C. These low-cost and versatile precursors are expected to be important building blocks for CMs.

Reversibility of electrochemical reactions of sulfur supported on inverse opal carbon in glyme-Li salt molten complex electrolytes

Electrochemical reactions of sulfur supported on three-dimensionally ordered macroporous carbon in glyme-Li salt molten complex electrolytes exhibit good reversibility and large capacity based on the mass of sulfur, which suggests that glyme-Li salt molten complexes are suitable electrolytes for Li-S batteries.

Preparation of three dimensionally ordered macroporous carbon with mesoporous walls for electric double-layer capacitors

Three dimensionally ordered macroporous (3DOM) carbons with mesoporous walls were prepared by a colloidal crystal templating method. A three dimensionally ordered composite consisting of monodisperse polystyrene (PS) latex (100–450 nm) and colloidal silica (5–50 nm) was prepared by an evaporation process of suspensions containing PS latex and colloidal silica in water. In the course of the heat treatment of this composite membrane at 573 K under an inert atmosphere, the PS was melted and penetrated into the spaces between the colloidal silica. The penetrated PS was carbonized during further heat treatment to provide a very thin carbon layer on the colloidal silica, and the macropore corresponding to the PS particle size was formed simultaneously. After this procedure, the 3DOM carbon with mesoporous walls was obtained by removing the silica particles. From the results of scanning electron microscope observations and nitrogen adsorption-desorption measurements, it was confirmed that the prepared carbon had a bimodal porous structure, and the sizes of macropores and mesopores of prepared carbon were in good agreement with the sizes of the PS and silica particles used as templates, respectively. The bimodal porous carbon, which had a specific surface area of 1500 m2 g−1 and 5 nm mesopores, showed highest capacitance of 120 F g−1 in propylene carbonate solution containing 1 mol dm−3 (C2H5)4NBF4. The mesopore size rather than macropore size gave significant effects on the rate capability of carbon electrode during charge and discharge. The bimodal porous carbon having 5 nm mesopores showed an excellent rate capability and its capacitance at a high current density of 4 A g−1 was 109 F g−1.

Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices

Ionic liquids (ILs) are liquids consisting entirely of ions and can be further defined as molten salts having melting points lower than 100 °C. One of the most important research areas for IL utilization is undoubtedly their energy application, especially for energy storage and conversion materials and devices, because there is a continuously increasing demand for clean and sustainable energy. In this article, various application of ILs are reviewed by focusing on their use as electrolyte materials for Li/Na ion batteries, Li-sulfur batteries, Li-oxygen batteries, and nonhumidified fuel cells and as carbon precursors for electrode catalysts of fuel cells and electrode materials for batteries and supercapacitors. Due to their characteristic properties such as nonvolatility, high thermal stability, and high ionic conductivity, ILs appear to meet the rigorous demands/criteria of these various applications. However, for further development, specific applications for which these characteristic properties become unique (i.e., not easily achieved by other materials) must be explored. Thus, through strong demands for research and consideration of ILs unique properties, we will be able to identify indispensable applications for ILs.

Kinetic Study of Li-Ion Extraction and Insertion at LiMn2 O 4 Single Particle Electrodes Using Potential Step and Impedance Methods

The kinetics of Li-ion extraction and insertion at LiMn 2 O 4 single particles (8-21 μm diam) were investigated by cyclic voltammetry, potential step chronoamperometry (PSCA), and electrochemical impedance spectroscopy (EIS) methods using a microelectrode technique. The EIS measurements in a frequency range from 110 kHz to 11 mHz were conducted successfully on a LiMn 2 O 4 single particle resulting in the magnitude of MΩ orders. The impedance spectra exhibited (i) a single semicircle in the high frequency region, (ii) a Warburg impedance in the low frequency region, and (iii) a limiting capacitance in the very low frequency region. The EIS spectra were fitted to a modified Randles-Ershler circuit, so that the reaction kinetics could be evaluated precisely. The dependences of the charge transfer resistance (R cl ) and the apparent diffusion coefficient of Li within the particle on the electrode potential were evaluated. Obtained values for D app were in the range of 10 -10 to 10 -6 cm2/s from EIS measurements, in fair agreement with those from PSCA results. Finally, the apparent chemical diffusion coefficients of Li-ion in the single crystal, thin-film, and single particle of LiMn 2 O 4 are compared.

Electrochemical investigation of LiNi0.5Mn1.5O4 thin film intercalation electrodes

This work provides kinetic and transport parameters of Li-ion during its extraction/insertion into thin film LiNi0.5Mn1.5O4 free of binder and conductive additive. Thin films of LiNi0.5Mn1.5O4 (0.2 μm thick) were prepared on electronically conductive gold substrate utilizing the electrostatic spray deposition technique. High purity LiNi0.5Mn1.5O4 thin film electrodes were observed with cyclic voltammetry, to exhibit very sharp peaks, high reversibility, and absence of the 4 V signal related to the Mn3+/Mn4+ redox couple. The electrode subjected to 100 CV cycles of charge/discharge delivered a capacity of 155 mAh g−1 on the first cycle and sustained a good cycling behavior while retaining 91% of the initial capacity after 50 cycles. Kinetics and mass-transport of Li-ion extraction at LiNi0.5Mn1.5O4 thin film electrode were investigated by means of electrochemical impedance spectroscopy. The apparent chemical diffusion coefficient (Dapp) value determined from EIS measurements changed depending on the electrode potential in the range of 10−10–10−12 cm2 s−1. The Dapp profile shows two minimums at the potential values close to the peak potentials of the corresponding cyclic voltammogram.

In situ Raman spectroscopic studies of LiNixMn2 − xO4 thin film cathode materials for lithium ion secondary batteries

Chemical states and structural changes accompanying the electrochemical Li extraction and insertion of LiNixMn2 − xO4 (0 < x < 0.5) thin films in LiBF4–EC–DMC solutions, studied by in situ Raman spectroscopy, are reported for the first time. Ex situ Raman measurements for the virgin electrodes revealed that the oxidation state of Ni in the pristine thin films was Ni2+. In situ Raman spectra of the thin films collected in the organic electrolyte during Li ion extraction and insertion in the potential range 3.4–5.0 V vs. Li/Li+ showed a new Raman band at 540 cm−1 appearing around 4.7 V, which is attributed to the Ni4+–O bond. In addition, from the in situ Raman spectral changes, it is suggested that Li ion extraction and insertion proceed as follows: the redox of Ni2+/3+ takes place in the potential range 4.4–4.7 V, and Ni3+/4+ in the 4.7–5.0 V range, while the redox at 3.8–4.4 V corresponds to Mn3+/4+. Furthermore, it was confirmed that these changes in the Raman spectra were reversible upon changing the electrode potential, and the Li ion extraction and insertion proceed in a reversible manner.