The intrinsic electrochemical characteristics of needle-like LiCoO₂ crystals with a hexagonal cylindrical shape were studied by the single particle measurement technique. The needle-like LiCoO₂ crystals were synthesized by the flux growth method. Single-particle Raman spectroscopy and galvanostatic charge-discharge tests at different electrical contact points in one needle-like LiCoO₂ crystal revealed that the crystal has homogeneous intercalation/deintercalation characteristics throughout the long axis. This suggests that the electron conductivity is high on a 100 µm scale along that axis and that lithium ions are efficiently transferred from the electrolyte to the layer structure of the crystal. The charge rate characteristics of the needle-like LiCoO₂ crystals were also evaluated and compared to those of powdered LiCoO₂. The polarization curve analysis indicates that the needle-like LiCoO₂ particles had almost same exchange current density, i₀, as powdered LiCoO₂, which has a much smaller volume. These excellent electrochemical characteristics are considered to be due to the orientation of the {104} crystal face on the crystal sides, which is the preferential face for lithium ion transfer, and the larger effective surface area of the particles.

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Thermal stability of LiCoO₂, LiMn₂O₄ and LiFePO₄ with Li_6.25Al_0.25La₃Zr₂O₁₂ (Al-LLZ) solid electrolyte was investigated at 300–800°C. In the case of LiCoO₂/Al-LLZ mixture, the XRD patterns and charge-discharge behavior did not change after the heat treatment at 800°C. In contrast, the formation of impurities and degradation of cathode performance were observed for LiMn₂O₄/Al-LLZ mixture and LiFePO₄/Al-LLZ mixture. The decomposition of Al-LLZ and the formation of some impurities were observed from 600°C for LiMn₂O₄/Al-LLZ mixture, leading to decrease in the discharge capacity. In the case of LiFePO₄/Al-LLZ mixture, the decomposition of both materials was started at 400°C. These results indicate that the limit of heat treatment temperature for the formation of electrode/solid electrolyte system strongly depends on the kind of electrode material.

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Direct electron transfer (DET)-type bioelectrocatalytic waves of bilirubin oxidase (BOD)-catalyzed O₂ reduction and [NiFe] hydrogenase (H₂ase)-catalyzed H₂ oxidation are very small and un-detectable using glassy carbon (GC) electrodes, respectively; however, clear catalytic waves are observed when the enzymes are adsorbed on Ketjen black-modified GC (KB-GC) electrodes, in which KB provides mesopores for DET-type bioelectocatalysis. To explain the phenomena, we focus on the curvature effect of mesoporous structures on long range electron transfer kinetics and simulate steady-state voltammograms catalyzed by model redox enzymes adsorbed with a random orientation on planar and mesoporous electrodes based on a three-dimensional model. In the simulation, we assume a spherical enzyme with a radius of r, an active site located at a certain distance from the center of the enzyme, and a spherical pore with a radius of R_p in mesoporous electrodes in which the enzyme is trapped and adsorbed. The simulation reveals that mesoporous electrodes provide platforms suitable for DET-type bioelectrocatalysis of enzymes when R_p becomes close to r. Such curvature effects of mesoporous electrodes become especially notable for larger sized enzymes. Furthermore, the simulation reproduces the experimental data of BOD- and H₂ase-catalyzed DET-type waves by considering the crystal structures of the enzymes. This work will open a route to improve the kinetic performance of the DET-type bioelectrocatalysis that has become very important in its practical application to a variety of bioelectrochemical devices.

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Recently, hydride ion (H⁻) has come to be recognized as new charge carrier for the transport of hydrogen in solids by realization of pure H⁻ conduction in BaH₂ and La_2−x−ySr_x+yLiH_1−x+yO_3−y oxyhydride system (LSLHO). In this study, the H⁻ conductive oxyhydride, LaSrLiH₂O₂ (x = 0, y = 1 in LSLHO), was synthesized by a conventional solid-state reaction at ambient pressure. The crystal structure of LaSrLiH₂O₂, as well as the H⁻ conductivity and bonding state of hydrogen, was examined by Rietveld analysis using X-ray and neutron diffraction data, attenuated total reflection Fourier transform spectroscopy (ATR-FTIR), and electrochemical impedance spectroscopy (EIS). The sample synthesized at ambient pressure had a K₂NiF₄-type layered perovskite structure composed of alternately stacked tetragonal (LiH₂)⁻ and (LaSrO₂)⁺ layers, and exhibited a conductivity of H⁻ of 3.2 × 10⁻⁶ S cm⁻¹ at 300°C, which were the same structure and property as that reported previously for a sample synthesized by high-pressure synthesis.

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