Koichi Tanaka

Aromatic Compounds as Redox Shuttle Additives for 4 V Class Secondary Lithium Batteries

Redox shuttle, a chemical overcharge protection that consumes the excess current during overcharge, has been studied. Aromatic compounds with two methoxy groups and a halogen directly substituted on the benzene were found to be stable up to about 4 V against lithium, and oxidize above this potential by means of cyclic voltammetry. Of these compounds, compounds with methoxy groups at adjacent positions and opposite positions showed reversible redox reactions. These compounds worked as redox shuttle overcharge protection when added to the electrolyte of a Li/LiCoO{sub 2} coin-type cell. The voltage leveled off right above the full charge voltage even when overcharge current was supplied. The current supplied over the full-charge voltage was not stored in the cell or consumed in a side reaction, but completely converted into heat, and the operation of a redox shuttle in a 4 V class battery was confirmed.

Electron Spin Resonance Study of the Electrochemical Reduction of Electrolyte Solutions for Lithium Secondary Batteries

The reductive decomposition mechanism of electrolyte solutions for lithium secondary batteries has been investigated by electron spin resonance (ESR) with the aid of molecular orbital (MO) calculations. Discussion is focused on the initial reactions with the ultimate goal of suppressing decomposition. Solvent-related radical species were observed by ESR measurements in all of the electrolyte solutions after reductive electrolysis. The MO calculations suggest that the electron transfer from the electrode to the solvent molecule coordinated with the lithium cation upon cathodic polarization of the electrode is thermodynamically favorable. It is concluded that this electron transfer is the initial process in the decomposition of electrolyte solutions by electrochemical reduction. The radical species observed by ESR is attributed to the radical anion of the solvent coordinated with the lithium cation. Furthermore, ESR analysis indicated that the solvent radical anion and the neighboring solvent molecules coordinated with the same lithium cation are in electron-transfer equilibrium. The subsequent decomposition is initiated from this equilibrium.

Initial Reaction in the Reduction Decomposition of Electrolyte Solutions for Lithium Batteries

The rotating ring-disk electrode system and ab initio molecular orbital calculations were used to investigate the initial reaction in the reductive decomposition of electrolyte solutions for lithium batteries. It was confirmed that electron transfer is induced by the cathodic polarization of the electrode, forming soluble reduced species which can be reoxidized. This electron transfer is an exothermic reaction with a reaction energy of -3 eV, from the electrode to the solvent molecules coordinated with lithium ions. This is the initial reaction in the reductive decomposition of electrolyte solutions for lithium batteries, which must be controlled to improve the performance of lithium-ion and lithium secondary batteries.

Asymmetric direct aldol reactions catalyzed by chiral amine macrocycle–metal(II) complexes under solvent-free conditions

Solvent-free asymmetric aldol reactions between cyclohexanone and 4-nitrobenzaldehyde using chiral amine macrocycle–metal(II) complexes as catalysts in a ball mill afforded the anti-aldol product as the major isomer with up to 93% ee.

HPLC enantioseparation on a homochiral MOF–silica composite as a novel chiral stationary phase

The last frontier in the development of chiral stationary phases for chromatographic enantioseparation involves homochiral metal–organic frameworks (MOFs). Using enantiopure (R)-2,2′-dihydroxy-1,1′-binaphthalene-6,6′-dicarboxylic acid as a starting material, we prepared three homochiral MOFs that were further used as chiral stationary phases for high-performance liquid chromatography to separate the enantiomers of various kinds of racemic sulfoxides, sec-alcohols, β-lactams, benzoins, flavanones and epoxides. The experimental results showed excellent performances for enantioseparation, and highlighted that enantioseparation on homochiral MOF columns is practical.

Spin Trapping Study of Gradual Decomposition of Electrolyte Solutions for Lithium Secondary Batteries

Spin trapping was used to study the gradual decomposition of electrolyte solutions for lithium secondary batteries. After the reductive electrolysis of ester-based electrolyte solutions, continuous production of active alkyl radicals was confirmed by spin trapping, which suggests the existence of a chain reaction initiated by cathodic polarization of the electrode. Liquid chromatography confirmed the existence of the produced polymer in electrolyte solutions after 4 weeks of electrolysis. A gradual degradation mechanism of electrolyte solutions is proposed, in which the polymerization reaction initiating from the electron transfer equilibrium accompanies the continuous production of active alkyl radicals. This is one mechanism for the gradual decomposition of electrolyte solutions and could be a cause of the polymeric components in the surface film on the electrode.

Efficient HPLC enantiomer separation using a pillared homochiral metal–organic framework as a novel chiral stationary phase

HPLC enantioseparation of racemates using novel pillared homochiral metal–organic framework–silica composite as chiral stationary phase has been successfully demonstrated.

Intermolecular proton transfer in host–guest crystals: the case of pyrazole included in 1,1-di(2,4-dimethylphenyl)but-2-yn-1-ol, an X-ray and solid-state 13C/15N NMR study

The first example of a cyclic intermolecular solid-state proton transfer involving nitrogen and oxygen atoms is described, which takes place in a cyclic 2 :2 complex formed by inclusion of pyrazole in the host, 1,1-di(2,4-dimethylpheny)but-2-yn-1-ol; the system has been studied by a combination of X-ray crystallography and dynamic high resolution solid-state 13C/15N NMR spectroscopy.

Structure study of host–guest molecular association in solution and in the solid state

A correlation between the mechanism of 1H NMR shift of a chiral guest by association with an optically active host compound in solution and in the solid state is revealed by an X-ray analysis of the host-guest inclusion crystal.

Perkin communications. X-Ray structural study of a 1 : 1 complex of 4-methoxy-1-methylpyridone and (R,R)-(–)-1,6-bis(o-chlorophenyl)-1,6-diphenylhexa-2,4-diyne-1,6-diol the irradiation of which gives optically pure β-lactam

A 1 : 1 complex of 4-methoxy-1-methylpyridone and (R,R)-(–)-1,6-bis(o-chlorophenyl)-1,6-diphenylhexa-2,4-diyne-1,6-diol, on irradiation, gives a pure β-lactam. The mechanism for this photoconversion has been studied by means of an X-ray crystallographic analysis of the complex.