Hitoshi Onodera

Flux growth of patterned LiCoO2 crystal arrays directly on a Pt substrate in molten LiNO3

We demonstrate a new way to prepare hollow-structured LiCoO2 crystals directly on a Pt substrate for the first time through a combination of semi-additive electrodeposition of a Co core and subsequent flux growth in molten LiNO3. The reaction process was characterized by time-dependent X-ray diffraction and scanning electron microscopy. The vertically oriented crystals having a platelet shape grew densely on the Co dot surface. The crystal growth was driven by supersaturation in the same manner as the flux growth. Significantly slower oxidation of the Co core and rapid lithiation of Co3O4 lead to pore formation, which suggests that slow oxygen diffusion in the Co core is rate limiting. Galvanostatic tests revealed that the LiCoO2 crystal array exhibited typical capacity–voltage profiles with no heavy capacity loss during the first three cycles without any additives.

Low-temperature growth of idiomorphic cubic-phase Li7La3Zr2O12 crystals using LiOH flux

Micrometer-sized Li7La3Zr2O12 crystals with well-developed facets were grown from a LiOH flux at 700 °C. Supersaturation-controlled crystallization driven by the cooling of a homogeneous hot solution with a Li/Zr ratio of 70 achieved one-step formation of cubic-phase Li7La3Zr2O12 that potentially exhibits high lithium-ion conductivity. Excess LiOH flux decreased the reaction temperature dramatically to 700 °C, lower than that of solid-state-reaction processes. The flux growth of the Li7La3Zr2O12 crystals in response to varying reaction conditions was studied systematically and indicated that the initial Li/Zr ratio and the holding temperature significantly affected the crystal phase, shape, and size. We further demonstrated Li7La3Zr2O12 crystal growth at 500 °C, beginning with La2Zr2O7 powders that dissolve readily in hot LiOH. This will be examined in greater depth in the near future.

Growth of hollow-structured LiMn2O4 crystals starting from Mn metal in molten KCl through the microscale Kirkendall effect

Hollow-structured LiMn2O4 crystals are grown in molten KCl (flux) for the first time via an oxidation reaction of metallic Mn followed by a lithiation reaction. Characterization of the formation process, which is achieved using thermogravimetric differential thermal analysis, X-ray diffraction, and scanning electron microscopy observation, reveals that the hollow structures are formed via a mechanism analogous to the microscale Kirkendall effect, which results from the difference between the solid-state diffusion rates of the core materials and the rate of O2 diffusion through the shell at elevated temperatures occurring during oxidation. Interestingly, the development of well-defined crystal facets is observed on the surfaces of the LiMn2O4 crystals, implying that the crystal growth is driven by supersaturation in the same manner as the flux growth. We also examine two possible approaches to reduce the volume of the inner space in the crystal: crystal growth under O2 flow for enhancement of the O2 diffusion and insertion of a Co and Ni layer between Mn and the substrate to induce the formation of the LiNi1/3Co1/3Mn1/3O2 phase.

Thin and Dense Solid-solid Heterojunction Formation Promoted by Crystal Growth in Flux on a Substrate

In this work, we demonstrate the direct growth of cubic Li5La3Nb2O12 crystal layer on the LiCoO2 substrate through the conversion of ultra-thin Nb substrate in molten LiOH flux. The initial thickness of the Nb layer determines that of the crystal layer. SEM and TEM observations reveal that the surface is densely covered with well-defined polyhedral crystals. Each crystal is connected to neighboring ones through the formation of tilted grain boundaries with Σ3 (2–1–1) = (1–21) symmetry which show small degradation in lithium ion conductivity comparing to that of bulk. Furthermore, the sub-phase formation at the interface is naturally mitigated during the growth since the formation of Nb2O5 thin film limits the whole reaction kinetics. Using the newly developed stacking approach for stacking solid electrolyte layer on the electrode layer, the grown crystal layer could be an ideal ceramic separator with a dense thin-interface for all-solid-state batteries.