Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-04-19
2004-02-03
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C427S058000, C427S255230, C427S397700, C029S592100
Reexamination Certificate
active
06685804
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for fabricating a novel electrode for a rechargeable lithium battery.
BACKGROUND ART
The battery performance of rechargeable lithium batteries recently under intensive research and development, such as charge-discharge voltage, cycle life characteristics or storage characteristics, depends largely upon the types of the electrodes used. This has led to the attempts to better battery performance by improving electrode active materials.
The use of metallic lithium for the negative active material, although possible to construct a battery with high energy density per weight and volume, presents a problem that the lithium deposited on charge grows into dendrite which might cause internal short-circuiting.
Rechargeable lithium batteries are reported (Solid State Ionics, 113-115, p57 (1998)) which use an electrode consisting of aluminum, silicon, tin or the like that is electrochemically alloyed with lithium on charge. Among these, a silicon electrode provides a particularly high theoretical capacity and is promising as a high-capacity negative electrode. For this reason, various rechargeable batteries using silicon for the negative electrode are proposed (Japanese Patent Laid-Open No. Hei 10-255768). However, such alloying negative electrodes fail to provide sufficient cycle characteristics since alloys, as electrode active materials, are themselves pulverized on charge and discharge to reduce current-collecting capabilities.
As a rechargeable lithium battery which uses silicon for the electrode active material and exhibits good charge-discharge cycle characteristic, the present applicant has proposed a rechargeable lithium battery which incorporates a microcrystalline or amorphous silicon thin film deposited on a current collector by a CVD, sputtering or other thin-film forming processes (Japanese Patent Laying-Open No. Hei 11-301646 and others).
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a method for fabricating an electrode, for a rechargeable lithium battery, which uses a thin film of active material, such as a silicon thin film, and can provide a high charge-discharge capacity and good charge-discharge cycle characteristics.
A method for fabricating an electrode for a rechargeable lithium battery, in accordance with the present invention, includes depositing a thin film composed of active material capable of alloy formation with lithium on a current collector made of a metal incapable of alloy formation with lithium, using a process for depositing a thin film by supplying a material thereof from a gas phase, and is characterized in that the thin film of active material is deposited at such a temperature that enables formation, of a mixed layer via diffusion of a constituent of the current collector into the thin film in the vicinity of an interface therebetween.
Examples of processes that can deposit a thin film of active material by supplying the material from a gas phase include sputtering, CVD, vacuum evaporation and spraying processes.
In the present invention, any material can be used for the active material if it can form an alloy with lithium. Examples of such materials include silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, gallium, indium and the like.
In view of the easiness of thin-film deposition by the aforementioned deposition method, the active material composed mainly of silicon or germanium is preferred. In view of the ability to provide a high charge-discharge capacity, the active material composed mainly of silicon is particularly preferred. Also preferably, the thin film of active material has the amorphous or microcrystalline form. Accordingly, an amorphous or microcrystalline silicon thin film is preferred as the thin film of active material. The thin film is identified as an amorphous silicon thin film when Raman spectroscopy detects the substantial absence of a peak around 520 cm-
1
corresponding to a crystal region, and as a microcrystalline thin film when Raman spectroscopy detects the substantial presence of a peak around 520 cm
−1
corresponding to a crystalline region and a peak around 480 cm
−1
corresponding to an amorphous region. Other examples of preferred thin films include an amorphous germanium thin film, a microcrystalline germanium thin film, an amorphous silicon-germanium alloy thin film, and a microcrystalline silicon-germanium alloy thin film.
The current collector for use in the present invention is composed of a material incapable of alloy formation with lithium, such as copper.
In the present invention, the thin film of active material is deposited at such a temperature that enables formation of a mixed layer via diffusion of a constituent of the current collector into the thin film in the vicinity of an interface therebetween. That is, the diffusion of the current collector constituent into the thin film of active material is promoted as the temperature (thin-film forming temperature) at which the thin film of active material is deposited is increased. Accordingly, in the present invention, the thin film of active material is deposited at a temperature that enables sufficient diffusion of the current collector constituent into the thin film and sufficient formation, in the thin film, of the mixed layer consisting of the current collector constituent and the active material.
The formation of the mixed layer via diffusion of the current collector constituent into the thin film of active material improves adhesion of the thin film to the current collector. Also, the current collector constituent is a metal element which does not form an alloy with lithium. The diffusion of such a current collector constituent into the thin film of active material results in the relative reduction of expansion and shrinkage of the thin film of active material when it stores and releases lithium. A stress produced in the thin film of active material when it expands and shrinks is thus lowered in its location adjacent to the current collector. This prevents the thin film of active material, if its volume expands and shrinks, from separating from the current collector, and thus achieves further improvement of adhesion between the current collector and the thin film of active material.
In the mixed layer, the concentration of the current collector constituent in the thin film is found to be higher in the vicinity of an interface between the thin film and the current collector, and is lower at a location closer to the surface of the thin film of active material. This continuously decreasing concentration gradient of the current collector constituent in the mixed layer is considered to indicate the formation of a solid solution between the current collector constituent and the active material.
The higher thin film-forming temperature causes the excessive diffusion of the current collector constituent into the thin film and results in the increased tendency of the current collector constituent to form an intermetallic compound with the active material. The formation of such an intermetallic compound reduces the number of sites serving as the active material since the active material atoms are incorporated in the compound, so that a charge-discharge capacity of the thin film of active material is reduced. The formation of the intermetallic compound also reduces adhesion of the current collector to the thin film of active material. It is thus preferred that the thin film of active material is deposited on the current collector at such a temperature that does not produce, in the mixed layer, an intermetallic compound between the active material and the current collector constituent. Such a temperature is preferably below 300° C.
In the present invention, a heat treatment may be performed after the thin film of active material is deposited on the current collector. The heat treatment allows further diffusion of the current collector constituent into the thin film. Hence, in the case where the mixed layer is formed to an insufficient thickn
Domoto Yoichi
Fujimoto Masahisa
Fujitani Shin
Ikeda Hiroaki
Shima Masaki
Kubovcik & Kubovcik
Sanyo Electric Co,. Ltd.
VerSteeg Steven H.
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