Search

Capacity decrease at low temperatures is one of the issues to be solved for secondary batteries especially for automobile applications and it is thus important to clarify the reaction kinetics in operating batteries and identify the rate determining step that governs the performance at low temperatures. Phase transitions in electrode active materials are important factors that affect the reaction kinetics particularly for thin electrodes used in high power applications. In this study, the phase transition dynamics of thin LiNi0.5Mn1.5O4 electrodes at various temperatures is examined using electrochemical methods combined with temperature-controlled in situ X-ray diffraction analysis to directly capture the reacting species and elucidate the reaction mechanism. The analysis shows that there occur consecutive phase transitions of LiNi0.5Mn1.5O4 (Li1 phase) ↔ Li0.5Ni0.5Mn1.5O4 (Li0.5 phase) and the Li0.5 phase ↔ Ni0.5Mn1.5O4 (Li0 phase) at room temperature and above. At lower temperatures the transition of Li1 → Li0.5 proceeds during the charging process but further delithiation to form the Li0 phase is restricted, leading to the capacity decrease. On the other hand, on discharging at low temperatures the amount of the Li0 phase to be lithiated is limited and this causes the capacity decrease. There is no Li0.5 phase formation on discharging at low temperatures, revealing remarkable kinetic asymmetry of the reaction processes for charging and discharging. It is suggested that the Li0.5 phase formed on discharging is instantly lithiated to form the Li1 phase, due to the small potential gap between the two transitions. These results indicate that the phase transition kinetics of Li0.5 ↔ Li0 is slower than that of Li1 ↔ Li0.5 and the former transition is the rate determining step at low temperatures.

Tantalum oxide (TaO3) nanosheets coated on the surface of a LiCoO2 cathode decrease its interfacial resistance in a solid-state battery by two orders of magnitude. Since the interfacial resistance is rate-determining in the solid-state system, the interfacial structure of the nanosheet is anticipated to pave the way for realising high-performance solid-state lithium batteries. The reduction in the interfacial resistance also strongly suggests that the TaO3 nanosheet is a self-standing solid electrolyte layer with an ultimate thinness of 1 nm. It has a wide band gap and a mesh structure with openings that are almost the same in size as the lithium ion, which prevents electronic conduction and allows the penetration of lithium ions, respectively.

We examined the photochemical properties of well-ordered multilayer films of titania nanosheets prepared on quartz-glass substrate using the layer-by-layer deposition method. The photocatalytic decomposition of gaseous 2-propanol and bleaching of Methylene Blue dye under UV light illumination were measured to evaluate the photocatalytic oxidation ability. Photoinduced hydrophilicity was also studied by measuring the contact angle of water droplets on the film. The results indicated that titania nanosheets had good photoinduced hydrophilicity. The monolayer film of titania nanosheets showed almost identical activity compared with well investigated sol-gel derived anatase TiO(2) film, while its photocatalytic oxidation activity was low by more than an order of magnitude. This fact suggests that photoinduced hydrophilicity could not be explained simply in terms of the photocatalytic removal of hydrophobic organic species adsorbed on the surface. The photocatalytic oxidation activity and the photoinduced hydrophilic conversion rate decreased with increasing number of nanosheet layers, suggesting that photogenerated carriers produced in the internal part of the multilayer films can hardly diffuse to the surface layer. Photochemical properties of ultrathin anatase films obtained simply by heating the titania nanosheet films were evaluated as well, and also revealed high photoinduced hydrophilicity.

Sodium cobalt oxide bilayer-hydrate, NaxCoO2·yH2O (x ≈ 0.35), was characterized by chemical and structure analyses. The formal valence of Co was found to be +3.4 from redox titration, which was much lower than +3.65 that had been estimated from the Na content, x. The deviation from the charge neutralization was considered to be compensated by the presence of oxonium ions in the CoO2 galleries, which was evidenced by Raman spectroscopy. Therefore, the composition of the superconducting phase was determined to be Na0.343(H3O)0.237CoO2·1.19H2O. Structural studies based on Raman spectroscopy and synchrotron X-ray powder diffraction suggested that the oxonium ions occupy the same crystallographic sites as the Na+ ions.