1. Field of the Invention
The present invention relates to a method of manufacturing an electrode used for lithium secondary batteries.
2. Description of Related Art
Silicon is a material that can be alloyed with lithium. Because of its large theoretical capacity, silicon has received attention as an electrode material that achieves high energy density for lithium secondary batteries. Published PCT Application WO01/029912, which is assigned to the assignee of the present invention, proposes a lithium secondary battery electrode in which a thin film composed of silicon is formed on a current collector, as an electrode for a lithium secondary battery that exhibits high charge-discharge capacity and good charge-discharge cycle performance.
However, in the process of forming the thin film used for the electrode, it is difficult to form a flat and smooth surface, and protrusions form on the surface. When such an electrode is used to fabricate a battery, the protrusions formed on the electrode surfaces penetrate the separator and come into contact with the counter electrode, causing a short-circuit failure of the battery.
Examples of the method of forming the thin film include CVD, sputtering, evaporation, and thermal spraying. In the following, vacuum evaporation is discussed, in which the above-described problem tends to occur noticeably.
Electron beam evaporation, which is one type of the vacuum evaporation, uses an evaporation system as illustrated in FIG. 1. The evaporation system has a supporting roller 2, a crucible 4, an electron beam gun 5, and rollers 6 and 7, all of which are disposed in a thin-film deposition system 8. The supporting roller 2 supports a current collector 1 while conveying it around the outer circumferential surface. The crucible 4 supplies and melts an evaporation source material 3, which is an evaporation source. The electron beam gun 5 is for heating the evaporation source. The rollers 6 and 7 are for transferring the current collector in a direction A or a direction B. In this kind of evaporation system, as illustrated in FIG. 1, an electron beam C emitted from the electron beam gun 5 is applied to the evaporation source material 3 so that the evaporation source material 3 is heated. The heated evaporation source material 3 melts and evaporates, and the evaporated vapor condenses on the current collector 1. Thereby a thin film composed of the evaporation source material 3 is deposited on the current collector 1. By evaporating the evaporation source material 3 while transferring the current collector 1 at a constant velocity, a thin film with a uniform film thickness is deposited over the entire surface of the current collector 1.
When the electron beam is applied to the evaporation source material 3 and the evaporation source material 3 is heated to evaporate, so-called bumping of the evaporation source material 3 can take place because of the influence of gases and impurities that contaminate the interior of the evaporation source material 3, and the phenomenon of scattering particles, known as splash, may occur. These particles adhere onto the electrode surface, and result in formation of protrusions on the electrode surface.
Techniques for preventing the bumping of the evaporation source material have been proposed. Japanese Published Unexamined Patent Application Nos. 5-171412 and 7-34224 describe a technique to control a fabrication method of the evaporation source material and a technique to control the shape and composition of the evaporation source material, both of which are intended for use in forming a silicon monoxide thin film. Japanese Published Unexamined Patent Application No. 2004-204314 describes a technique to control the procedure of heating the evaporation source material for use in forming a metal thin film. Nevertheless, it has been difficult to completely prevent the bumping of evaporation source material. In particular, in the case of depositing a silicon thin film, techniques to prepare the evaporation source material and to control the procedure of heating the evaporation source material for preventing the bumping have not yet been established and it has been difficult to prevent the bumping phenomenon over a long time period.