Kyoo-Seung Han

Direct fabrication of thin-film LiNiO2 electrodes in LiOH solution by electrochemical-hydrothermal method

Abstract The formation of the ternary oxide thin films on nickel substrates in LiOH solution was studied under hydrothermal or electrochemical–hydrothermal conditions. While the purely hydrothermal treatment of nickel substrates at 150°C leads to the formation of Ni(OH) 2 films, by the use of the electrochemical–hydrothermal approach under supplementary galvanostatic charge with the same hydrothermal conditions, LiNiO 2 films are directly fabricated. The structural purity and chemical composition of the obtained films were confirmed by X-ray diffraction pattern analysis and X-ray photoelectron spectroscopy, respectively. The fabricated films show, according to X-ray diffraction, a good crystallinity without any post-synthesis annealing. In comparison with the fabrication of Ni(OH) 2 films by the hydrothermal method, it has been found that the electrochemical anodic process is necessary to obtain the desired LiNiO 2 films.

Effect of LiOH concentration change on simultaneous preparation of LiCoO2 film and powder by hydrothermal method

Abstract LiCoO2 film and powder applicable to rechargeable lithium microbatteries have been directly prepared at the same time by hydrothermal treatment of a cobalt metal plate in a concentrated LiOH solution. The effect of LiOH concentration (0.1–6 M) on the structural change in film and powder has been examined using the XRD, micro-Raman and SEM analyses. It is found that all the powders with a layered hexagonal structure are precipitated by homogeneous nucleation regardless of LiOH concentration, whereas the dependence of film structure on the concentration is revealed by a selection of phase such as cobalt hydroxide species, hexagonal or cubic spinel phases formed by heterogeneous nucleation.

Recent developments in soft, solution processing: one step fabrication of functional double oxide films by hydrothermal-electrochemical methods

Based upon thermodynamic considerations, we propose ‘soft, solution processing’, as a means to fabrication of shaped, sized, and controlled advanced materials from aqueous solutions without excess heat, energy consumption, expensive equipment, and precursor(s) as one of the most economical and environmentally friendly preparation techniques for advanced materials. We have succeeded in the fabrication of double oxide thin films such as BaTiO 3 , SrTiO 3 , their solid solutions, and LiNiO 2 at fixed temperatures between 60 and 150 °C by hydrothermal–electrochemical methods. Nucleation and growth of the films takes place at the interface between the alkaline (earth) component in the solutions and anodically treated metal plates. The structures and properties of these films are described. A novel method to fabricate BaTiO3, SrTiO3, their solid solutions, and LiNiO2 at fixed temperatures between 60 and 150 °C by hydrothermal-electrochemical methods. Nucleation and growth of the films takes place at the interface between the alkaline (earth) component in the solutions and anodically treated metal plates. The structures and properties of these films are described. A novel method to fabricate BaTiO 3 /SrTiO 3 layered thin films by changing the solution compositions in flow-cell equipment is presented.

Fabrication temperature and applied current density effects on the direct fabrication of lithium nickel oxide thin-film electrodes in LiOH solution by the electrochemical-hydrothermal method

Abstract Lithium nickel oxide thin-film electrodes for lithium rechargeable microbatteries are directly fabricated from nickel substrates using the electrochemical-hydrothermal method in 4 M LiOH solution in the temperature range between 100 and 200°C and a current density between 0.1 and 5.0 mA/cm 2 for 20 h. The film properties show the obtained lithium nickel oxide films to be possible cathode films. However, the properties depend on the film formation conditions. Investigation of the effects of the film fabrication conditions gives fundamental information on film formation pathways and optimum conditions for lithium nickel oxide film formation. The successful separation of two different intermediate stages and the subsequent obtaining of lithium nickel oxide films confirms the film formation mechanism.

In situ visible Raman spectroscopic study of phase change in LiCoO2 film by laser irradiation

Abstract A phase change in LiCoO2/Co films in a spot of 1 μm diameter has been investigated under ambient conditions using 514.5 nm radiation by laser Raman spectroscopy. Laser irradiation resulted in a remarkable phase change of the film from hexagonal to cubic spinel, forming a lithium deficient phase Li1−xCoO2, which was enhanced with laser power. Based on comparative Raman studies, incorporation of cobalt atoms from the Co substrate by the local heating effect of laser irradiation has been suggested for the phase change. The in situ tool is expected to be useful for applications in control of micro-structure of cathode films for micro-batteries and micro-structural analysis.

Single-step fabrication of Li1−xNi1+xO2 and LiCoO2 films by Soft Solution-Processing at 20–200°C

Abstract Li 1− x Ni 1+ x O 2 and LiCoO 2 films as cathodes for lithium rechargeable microbatteries were fabricated in a concentrated LiOH solution at 20–200°C using different Soft Solution-Processing methods. While LiCoO 2 films can be obtained by a purely hydrothermal reaction, a supplementary electrochemical charge with the same hydrothermal conditions is necessary to produce Li 1− x Ni 1+ x O 2 films. A film formation mechanism study elucidated the necessity of such different activation methods for the preparation of Li 1− x Ni 1+ x O 2 and LiCoO 2 films.

Soft solution processing for fabrication of lithiated thin-film electrodes in a single synthetic step

A process called ‘soft solution processing’ was used to prepare lithiated thin-film electrodes as a cathode for lithium rechargeable microbatteries in a single synthetic step. The well crystallized Li1–xNi1+xO2 films were fabricated by the electrochemical–hydrothermal treatment of nickel plates in 4 m LiOH solution at fixed temperatures around 100 °C with no post-synthesis annealing. The prepared films exhibit prospective electrochemical activity; however, it is dependent on the synthetic conditions. The film formation mechanism study elucidates the effect of the synthetic conditions and also shows that soft solution processing is capable of preparing advanced inorganic materials with desired properties through the active control of the reaction conditions.

Pure tetravalent nickel in γ-type nickel oxyhydroxide as secondary battery electrode

Pure tetravalent nickel in γ-type cobalt substituted nickel oxyhydroxide, Ni 0.70 Co 0.30 O 2 K 0.30 (H 2 O) 0.42 , could be obtained by the “chimie douce” reaction. The presence of tetravalent nickel is confirmed by comparing the Ni K-edge XANES spectrum of the sample with those of reference compounds having various nickel valency and similar layer structure. The Co K-edge XANES spectrum indicates that the trivalent cobalt remains unchanged regardless of the nickel valency. The structural modification during chimie douce reaction observed from XRD patterns and the result of iodometric titration are consistent with the Ni and Co K-edge XANES data.

'Soft solution processing' in situ fabrication of morphology- controlled advanced ceramic materials in low temperature solutions without firing

A process called ‘soft solution processing’ has been introduced to fabricate advanced solid state materials in an economical, environmental friendly, and energy and material efficient way. A concise discussion of how to improve the synthetic conditions and how to extend the applied system is given. The successful examples show that soft solution processing is capable of preparing advanced materials with planned properties through the easy control of reaction conditions in a suitable aqueous solution in a single synthetic step without huge energy consumption for sintering or melting and without any sophisticated equipment.

The Effect of Lithium Intercalation on the Crystal Structure and Magnetic Property of Layered FeWO4Cl

LiFeWO4Cl has been prepared by reductive intercalation of lithium into FeWO4Cl. FeWO4Cl crystallizes into a tetragonal system (space group P4/nmm) with cell dimensions of a=6.677(5) A and c=5.270(5) A, while lithium intercalation gives rise to the formation of a monoclinic phase ( P21/m) with cell dimensions of a=7.050(0) A, b=6.926(2) A, c=5.043(0) A and β=92.54(2)°. Infrared (IR) spectroscopic analyses of FeWO4Cl and LiFeWO4Cl show that the symmetry of the tetrahedral WO42- group in FeWO4Cl becomes reduced as the lithium intercalate into its two-dimensional lattice, indicating an alternately ordered lithium occupancy of half of the octahedral interlayer sites. The effective magnetic moments of FeWO4Cl and LiFeWO4Cl were estimated to be 5.95 and 5.10 B.M., respectively, which can be attributed to the selective reduction of a high-spin ferric ion to a high-spin ferrous one upon lithium intercalation. It was also found that the two-dimensional magnetic property of FeWO4Cl was changed to a three-dimensional one in LiFeWO4Cl owing to the c-axis contraction.