Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-04-16
2004-08-03
VerSteeg, Steven (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298060, C204S298240, C204S298260, C204S192120
Reexamination Certificate
active
06770175
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for and a method of forming an electrode for a lithium secondary cell, and more specifically, it relates to an apparatus for and a method of forming an electrode for a lithium secondary cell for forming an active material layer on the surface of a collector.
2. Description of the Prior Art
In a lithium secondary cell actively subjected to research and development in recent years, the cell characteristics such as a charge/discharge voltage, an operating cycle lifetime and shelf stability remarkably depend on an electrode employed therein. Therefore, an active material employed for the electrode is improved for upgrading and improving the cell characteristics.
For example, a lithium secondary cell employing aluminum, Si or tin electrochemically alloyed with lithium in charging as a negative electrode active material is proposed in general, as reported in Solid State Ionics, 113-115, p 57 (1998) or the like. Among aluminum, Si and tin, Si having large theoretical capacity is particularly promising as a negative electrode active material for a cell exhibiting high capacity. Therefore, various lithium secondary cells having negative electrode active materials of Si are proposed.
In order to form the aforementioned negative electrode active material consisting of Si, Si is generally deposited on a collector from a sputtering source containing a target consisting of Si, thereby forming a negative electrode active material layer of Si on the collector.
In the aforementioned negative electrode active material layer consisting of only Si formed on the collector, however, the volume of Si alloyed with lithium thereby storing lithium is remarkably expanded and shrinked following charge/discharge reaction. In charging/discharging, therefore, Si is so powdered (pulverized) that the negative electrode active material layer consisting of Si is disadvantageously separated from the collector to deteriorate the operating cycle characteristics.
In general, therefore, a technique of preventing pulverization of Si by adding a foreign element to Si is developed. Pulverization of a thin film of Si can conceivably be suppressed by introducing the foreign element into the Si thin film and changing the mechanical/physical properties of the thin film. Therefore, it is important that the thin film homogeneously contains the foreign element. In the aforementioned conventional sputtering, however, it is difficult to form such a negative electrode active material layer containing Si and the foreign element added thereto on the collector. In other words, it is difficult to form the negative electrode active material containing Si and the foreign element added thereto in the conventional sputtering employing only a single target (single sputtering source) consisting of Si.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for forming an electrode for a lithium secondary cell capable of readily forming an active material layer constituted by at least two elements and controlling the composition of the active material layer.
Another object of the present invention is to form the active material layer constituted by at least two elements to have homogeneous concentration distribution in the aforementioned apparatus for forming an electrode for a lithium secondary cell.
Still another object of the present invention is to provide a method of forming an electrode for a lithium secondary cell capable of readily forming an active material layer constituted by at least two elements and controlling the composition of the active material layer.
A further object of the present invention is to form the active material layer constituted by at least two elements to have homogeneous concentration distribution in the aforementioned method of forming an electrode for a lithium secondary cell.
In order to attain the aforementioned objects, an apparatus for forming an electrode for a lithium secondary cell according to a first aspect of the present invention, employed for forming an active material layer on the surface of a collector, comprises a first sputtering source for sputtering a first material forming the active material layer onto the surface of the collector and a second sputtering source for sputtering a second material forming the active material layer onto the surface of the collector. A plasma region of the first sputtering source and a plasma region of the second sputtering source are arranged to overlap with each other.
The apparatus for forming an electrode for a lithium secondary cell according to the first aspect is provided with the first sputtering source and the second sputtering source for sputtering the first material and the second material forming the active material layer respectively onto the surface of the collector while arranging the plasma region of the first sputtering source and the plasma region of the second sputter region to overlap with each other as hereinabove described, whereby an active material layer constituted by at least two elements can be readily formed with excellent reproducibility. When power applied to the first sputtering source and that applied to the second sputtering source are controlled independently of each other in this case, the composition of the active material layer constituted by at least two elements can be readily controlled.
In this case, a resulting thin film preferably forms a solid solution of the first material (e.g., Si) and the second material (e.g., Co). In the above example, Co is preferably contained in Si not as an intermetallic compound of Si and Co but in the form of a solid solution. The term “intermetallic compound” stands for a compound formed by combining metals at a specific ratio to have a specific crystal structure. The solid solution is preferably in a non-equilibrium state. Only Ge is known as an element forming a solid solution with Si in an equilibrium state, and the solid solution of Si and a foreign element according to the aforementioned aspect exists only in a non-equilibrium state. In this point of view, formed Si is preferably in an amorphous or microcrystalline state, and sputtering which is a thermally non-equilibrium process, CVD, vacuum deposition or the like is preferably employed for forming Si.
The aforementioned apparatus for forming an electrode for a lithium secondary cell according to the first aspect preferably further comprises a collector holding part capable of holding the collector and relatively moving the collector with respect to the first sputtering source and the second sputtering source. The collector holding part capable of relatively moving the collector with respect to the first sputtering source and the second sputtering source is so provided that the composition ratio (concentration distribution) of the active material layer can be homogenized by moving the collector holding part when forming the active material layer constituted by at least two elements on the surface of the collector. Thus, the active material layer constituted by at least two elements can be readily formed with excellent reproducibility and homogeneous composition.
In this case, the collector holding part preferably includes means cooling the collector. According to this structure, the collector can be cooled when the active material layer constituted by at least two elements is formed thereon. Thus, the collector component can be prevented from excessively diffusing into the active material layer when the collector is at a high temperature. Further, strain (internal stress) resulting from difference in thermal expansion can also be prevented. Consequently, it is possible to prevent deterioration of charge/discharge characteristics resulting from excessive diffusion of the collector component in the active material layer or internal stress of the temperature leading to separation of the active material layer.
In the aforementioned structure comprising the collector holding part, the collector holding pa
Domoto Yoichi
Tarui Hisaki
Yagi Hiromasa
McDermott & Will & Emery
Sanyo Electric Co,. Ltd.
VerSteeg Steven
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