稲本 純一

<jats:title>Abstract</jats:title><jats:p>In this review, fundamental aspects of the electrochemical intercalation of anions into graphite have been first summarized, and then described the electrochemical preparation of covalent‐type GICs and application of graphite as the cathode of dual‐ion battery. Electrochemical overoxidation of anion GICs provides graphite oxide and covalent‐fluorine GICs, which are key functional materials for various applications including energy storage devices. The reaction conditions to obtain fully oxidized graphite has been mentioned. Concerning the application of graphite for the cathode of dual‐ion battery, it stably delivers about 110 mA h g<jats:sup>−1</jats:sup> of reversible capacity in usual organic electrolyte solutions. The combination of anion and solvent as well as the concentration of the anions in the electrolyte solutions greatly affect the performance of graphite cathode such as oxidation potential, rate capability, cycling properties, etc. The interfacial phenomenon is also important, and fundamental studies of charge transfer resistance, anion diffusion coefficient, and surface film formation behavior have also been summarized. The use of smaller anions, such as AlCl<jats:sub>4</jats:sub><jats:sup>−</jats:sup>, Br<jats:sup>−</jats:sup> can increase the capacity of graphite cathode. Several efforts on the structural modification of graphite and development of electrolyte solutions in which graphite cathode delivers higher capacity were also described.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The effects of soft X-ray irradiation and atomic hydrogen annealing (AHA) on the reduction of graphene oxide (GO) to obtain graphene were investigated. To clarify the interaction between soft X-rays and GO, soft X-rays of 300 eV and 550 eV were used for C 1s and O 1s inner-shell electron excitation, respectively at the NewSUBARU synchrotron radiation facility. Low-temperature reduction of the GO film was achieved by using soft X-ray at temperatures below 150 °C at 300 eV, and 60 °C at 550 eV. O-related peaks in X-ray photoelectron spectroscopy, such as the C–O–C peak, were smaller at 550 eV than those at 300 eV. This result indicates that excitation of the core–shell electrons of O enhances the reduction of GO. Soft X-rays preferentially break C–C and C–O bonds at 300 and 550 eV, respectively.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Graphene-like graphite (GLG) is a promising anode material for sodium-ion batteries, and is believed to have unique kinetic properties compared to hard carbon due to its different intercalation mechanism. In this study, electrochemical impedance spectroscopy was used to investigate the kinetic properties of sodium-ion intercalation in GLG. Our results indicate that the activation energies for interfacial sodium-ion transfer in GLGs were nearly identical to those reported for graphite, regardless of the heat treatment temperature applied to the GLGs. Furthermore, these activation energies were lower than those observed for hard carbon, suggesting better sodium-ion intercalation kinetics. In addition, the diffusion coefficient of sodium ions in the GLG was similar to that of graphite, with the highest value observed for GLG800, the GLG heat-treated at the highest temperature of 800°C. This may indicate that the diffusion coefficient increases with the presence of nanopores in the graphene layer of GLG. It has also been reported that GLG800 is superior in terms of reversible capacity and working potential compared to GLGs synthesized at other temperatures. Consequently, the results clearly demonstrate that GLG800 has the best electrochemical properties in terms of both thermodynamics and kinetics among the GLGs investigated in this study.</jats:p>

Abstract The reduction of graphene oxide (GO) through atomic hydrogen annealing (AHA) and soft X-ray irradiation is investigated using microwell substrates with μm-sized holes with and without Ni underlayers. The GO film is reduced through AHA at 170 °C and soft X-ray irradiation at 150 °C. In contrast, some GO films are not only reduced but also amorphized through soft X-ray irradiation. The effect of the Ni underlayer on GO reduction differs between AHA and soft X-ray irradiation. In AHA, the difference in GO reduction between SiO₂ and Ni underlayer originates from the atomic hydrogen density on the sample surface. On the other hand, in soft X-ray irradiation, the difference in GO reduction between SiO₂ and the Ni underlayer originates from the excited electrons generated by soft X-ray irradiation. Reduction without damage is more likely to occur in the suspended GO than in the supported GO.

It has been reported that lithium-rich cathode materials of LIB emit singlet oxygen during charging, which chemically oxidizes electrolyte solutions, and the decomposition products form surface film on the material. However, the detailed conditions and mechanism of the surface film formation and its effect on the electrochemical reaction at the electrode/electrolyte interface have not been clarified in detail. In this study, using 0.5LiCoO₂ • 0.5Li₂MnO₃ thin-film electrodes as the model electrodes of the lithium-rich cathode materials, the surface film formation behavior was investigated. After a constant current-constant voltage (CCCV) measurement to 4.8 V, passivation of the electrodes did not occur. On the other hand, the electrode after cyclic voltammetry (CV) up to 4.8 V showed complete passivation. The results of spectroscopic analyses revealed that decomposition products of the solvent formed thick surface film on the electrode after CV. From the results, it was concluded that the passivation surface film was formed by the simultaneous decomposition of the solvent via electrochemical oxidation at high potentials and chemical oxidation by singlet oxygen. Furthermore, the electrode with the surface film showed better cyclability than that without the surface film, indicating that it contributes to the suppression of side reactions at the electrode/electrolyte interface.

<jats:p>LiNi<jats:sub>0.5</jats:sub>Mn<jats:sub>1.5</jats:sub>O<jats:sub>4</jats:sub> shows promise as a positive electrode material for lithium-ion batteries. However, because the material has a higher operating potential than conventional cathodes, interfacial side reactions are accelerated during cycling, which degrades the outmost surface of the material. Although it has been reported that some additives offer effective protection against degradation, their protection mechanism has not been clarified in detail. Here, the effect of additives on the surface states of LiNi<jats:sub>0.5</jats:sub>Mn<jats:sub>1.5</jats:sub>O<jats:sub>4</jats:sub> was investigated using thin film model electrodes. It was found that surface film was not formed on LiNi<jats:sub>0.5</jats:sub>Mn<jats:sub>1.5</jats:sub>O<jats:sub>4</jats:sub> in additive-free electrolyte solution even after cycling at 55 °C, and severe dissolution of transition metal ions continuously occurred, leading to rapid capacity fading. Addition of ethylene glycol bis(propionitrile) ether (EGBE) effectively suppressed the capacity fading at 55 °C. Analysis with redox reaction of ferrocene on the electrodes, X-ray photoelectron spectroscopy, and scanning electron microscopy indicated that surface film hardly formed in EGBE-containing solution, but the dissolution was effectively suppressed. Because a nitrile group tends to adsorb on positive active material at high potential, it was concluded that the adsorption layer of EGBE impeded side reactions at the interface, resulting in improved cycleability of LiNi<jats:sub>0.5</jats:sub>Mn<jats:sub>1.5</jats:sub>O<jats:sub>4</jats:sub>.</jats:p>

Dual carbon batteries have recently attracted significant attention because of their ecofriendliness and reliability. In this study, graphene-like graphite (GLG) was prepared by thermal reduction of graphite oxide to be used as a cathode material, and the electrochemical PF₆⁻ anion-intercalation reaction into GLG was investigated. Decreasing the heat-treatment temperature of GLGs from 900 °C to 600 °C resulted in increasing the reversible capacities and interlayer distances of GLG samples. Among them, GLG synthesized at 700 °C (GLG700) showed the largest discharge capacity of 137 mAh g⁻¹, which was much larger than that of graphite (52 mAh g⁻¹). Variations in the X-ray diffraction patterns and Raman spectra of GLG700 indicated that the stage number reached 1 at 4.8 V (vs Li⁺/Li) while that of graphite was 2 at the same potential. This indicates that GLG could store PF₆⁻ anion in every interlayer, which is probably one of the main causes of the larger capacity. The charge–discharge cycling test of GLG700 showed that the capacity gradually increased during cycling, and the coulombic efficiency was approximately 97% at every cycle after the 5th cycle. These results clearly demonstrate that GLG can be used as a cathode material with a large capacity for dual carbon batteries.

Graphene-like graphite prepared by heating graphite oxide under vacuum at 800 degrees C was fluorinated by elemental fluorine in the presence of HF at room temperature. The interlayer spacing of the resulting material was 0.639 nm and it showed CxF type characteristics. The fluorine content of it (x = 1.7) was higher than that obtained from natural graphite (x = 2.3). The discharge capacity of it as a cathode of lithium primary battery reached 940 mAhg(-1) at a low current density, which was 50% larger than the theoretical capacity based on the 100% discharge of fluorine. (C) The Author(s) 2020. Published by ECSJ.