Papers by Author: Kaoru Dokko

Abstract: Macroporous LiCoO2 was prepared by a colloidal crystal templating method. Colloidal crystal consisting of monodisperse polystyrene particles was used as the template for the synthesis of macroporous LiCoO2. A Li-Co-O sol was injected into the template, and it was calcined at high temperatures. A porous membrane of LiCoO2 with three dimensionally ordered macroporous (3DOM) structure was obtained. The prepared material had a rock-salt type crystallographic structure with R3m space group. The interconnected macropores with relative uniform size (0.8~0.9 ;m) were observed on entire part of the membrane. The electrochemical properties of the 3DOM LiCoO2 were characterized with galvanostatic charge-discharge measurements in an organic electrolyte containing a lithium salt. The 3DOM LiCoO2 exhibited charge and discharge capacities of 136 and 107 mA h g-1, respectively, at around 3.9 V vs. Li/Li+, indicating that 3DOM LiCoO2 electrode had solid state redox reaction accompanying with Li+ ion insertion and extraction to CoO2 frameworks.

Abstract: Three dimensionally ordered macroporous (3DOM) Li4Ti5O12 was successfully prepared by a colloidal crystal templating process. Colloidal crystal consisting of monodisperse polystyrene particles (1 9m diameter) was used as a template for the synthesis of macroporous Li4Ti5O12. A precursor sol consisting of titanium isopropoxide and lithium acetate was injected into the template, and it was calcined at high temperatures. A macroporous membrane of Li4Ti5O12 with inverse-opal structure was obtained. The prepared material had a spinel-related crystallographic structure. The interconnected pores with uniform size (0.8 9m) were clearly observed on entire part of the membrane. The electrochemical properties of the 3DOM Li4Ti5O12
were characterized with cyclic voltammetry and glavanostatic charge-discharge in an organic electrolyte containing a lithium salt. 3DOM Li4Ti5O12 exhibited a discharge capacity of 160 mA h g-1 at the electrode potential of 1.55 V vs. Li/Li+ due to the solid state redox of Ti3+/4+ accompanying with Li+ ion insertion and extraction. The discharge capacity was close to the theoretical capacity (167 mA h g-1). This means that the Li+ insertion and extraction took place at all part of the 3DOM Li4Ti5O12 membrane.

Abstract: A new proton-conducting membrane was prepared consisting of uniformly macroporous silica matrix and a proton-conducting gel polymer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). Three-dimensionally ordered macroporous silica membrane was fabricated by use of a colloidal template method with mono-dispersed polystyrene beads. Surface sulfonation of the pores in the silica matrix was performed by using 1,3-propanesultone. The sulfonated silica matrix exhibited about 400 times higher proton conductivity than unmodified one. The proton conductivity of the composite membrane was also successfully enhanced by using the sulfonated silica matrix.

Abstract: Thin films of lithium ion conductive ceramic Li4+xAlxSi1-xO4 were fabricated on Au substrate using sol-gel process. The sol of Li-Al-Si-O was spread on Au substrate using a spin coater, and it was gelated at room temperature. The gel was calcinated at 400 °C and heat-treated at high temperatures between 500 °C and 800 °C in air. The addition of poly(vinylpyrrolidone) (PVP) was effective in stabilizing the sol. Furthermore, the morphology of the obtained thin film was changed by the PVP additive. Li4+xAlxSi1-xO4 thin film prepared at 800 °C exhibited a Li+ ion conductivity of 10-8 S cm-1 at room temperature.