Hydrothermally Processed Oxide Nanostructures and Their Lithium–ion Storage Properties
© The Author(s) 2010
Received: 22 June 2010
Accepted: 26 July 2010
Published: 13 August 2010
Y- and Si-based oxide nanopowders were synthesized by a hydrothermal reaction of Y or Si powders with NaOH or LiOH aqueous solution. Nanoparticles with different morphology such as elongated nanospheres, flower-like nanoparticles and nanowires were produced by a control of processing parameters, in particular, the starting composition of solution. The preliminary result of electrochemical examination showed that the hydrothermally processed nanowires exhibit high initial capacities of Li-ion storage: 653 mAh/g for Y2O3 nanowires as anode materials and 186 mAh/g for Li2Si2O5 nanowires as cathode materials in a Li secondary cell. Compared to the powder with elongated sphere or flower-like shapes, the nanowires showed a higher Li-ion capacity and a better cycle property.
Nano-sized metal oxides have interesting properties, which cannot be expected in conventional microcrystalline materials [1–5]. Due to high specific surface area and unique structures, they have attracted much attention among scientists for potential applications in electronic devices, chemical and physical sensors, photocatalysts, materials for energy conversion and energy storage, etc. In particular, one-dimensional (1-D) nanomaterials such as nanotubes and nanowires are expected to have novel properties due to higher specific surface area than 2-D or 3-D materials. In the field of energy storage, for instance, there is a strong demand to replace conventional carbonaceous and Li–M–O-based electrode materials to high-performance nanostructured electrodes in Li-ion rechargeable batteries. Recently, a very high Li-ion storage capacity combined with good cyclability was reported in nanowires such as Na–Ti–O . A variety of methods has been employed to synthesize 1-D nanomaterial [7–10]. The chemical methods, such as a hydrazine reduction route in aqueous ethanol solutions assisted by external magnetic fields, are very effective to synthesize nanowires . As well, hydrothermal process is considered as one of the most effective methods for the scaled-up to produce high-quality nanopowders. By a proper control of processing parameters such as the composition of starting solution, the morphology of nanopowders can be effectively controlled in this method [12–14].
In the present work, we synthesized Y- and Si-based oxide nanopowders with different morphology by a hydrothermal method using metallic Y and Si powders as starting materials. We also examined Li-ion storage property of the synthesized nanopowders to be potentially used as anode or cathode materials for Li-ion cells.
Pure Y (>99.5%, 10 μm), Si (>99.8%, 20 μm), NaOH and LiOH were used as starting materials. Y or Si powders were put into an aqueous solution of NaOH(1–5 M) or LiOH(1–5 M). The powder containing solution was then put into a Teflon-sealed mini-autoclave (80ϕ × 120 mm). The sealed mini-autoclave was put in a heated furnace for hydrothermal reaction. The reaction took place in the autoclave at 220–250°C for several hours. After the hydrothermal reaction, the solid products remaining in the solution were isolated by centrifugal separation, followed by washing with de-ionized water and ethanol for three times. The products were then dried at 100°C for 3 h. A portion of the synthesized products was further heat-treated at 500°C in air for an hour. Phase identification and structural examination were performed by a XRD and a field-emission SEM.
The Li-ion storage property was evaluated by an electrochemical test using the synthesized nanopowders as anode or cathode electrodes in Li-ion cell. The electrodes were made by dispersing 80 wt% nanopowders, 15 wt% carbon black and 7 wt% polyvinylidene fluoride (PVDF) binder in n-methyl pyrrolidone (NMP) solvent to form the slurry. The slurry was then spread onto a Cu foil, followed by drying in an oven under a vacuum pressure of 30 torr at 120°C for 12 h. The dried electrodes were then pressed at a pressure of 12 kg/mm2. The nanopowder electrodes were finally assembled to CR2032 coin cells in an argon-filled glove-box using lithium metal foil as the counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume, provided by MERCK, Germany). The cells were galvanostatically charged and discharged over a voltage range of 0–3.0 and 2.5–4.5 V for anode and cathode materials, respectively.
Results and Discussions
The synthesis condition for the nanopowders in this work
Y 1 g + 3 M LiOH
200°C, 24 h
Y(OH)5 (elongated shere)
Y 1 g + 1 M LiOH
200°C, 24 h
Si 1 g + 3 M NaOH
180°C, 24 h
Si 1 g + 1 M LiOH
180°C, 24 h
Y- and Si-oxide based nanoparticles with variable morphology were easily produced by a hydrothermal method using metallic Y or Si powders. The difference in resulting morphology might be related to the supercritical condition of metal hydroxide solution at high pressure, influencing the crystal growth, but the knowledge of detailed mechanism is required to clarify the formation of nanowires. The preliminary result of the Li-ion storage property of the hydrothermally nanopowders showed clearly that the nanowires exhibit much higher capacity of Li-insertion than 2-D (flower-shaped) or 3-D(spherical) nanostructures. Further study is needed to optimize further electrochemical characteristics to be practically applicable to Li-ion cell.
This research was supported by a grant from the Center for Advanced Materials Processing (CAMP) of the 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy (MKE), Korea.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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