Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2000-10-20
2001-12-25
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06332908
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for modifying a surface of a hydrogen storage alloy, and more particularly to a method for modifying a hydrogen storage alloy for an Ni/MH (nickel-metal hydride) secondary battery using flake type metal powder to enhance discharge capacity of an electrode and to lengthen electrode life duration.
2. Description of the Related Art
A hydrogen storage alloy refers to a metal or an alloy capable of absorbing or discharging hydrogen reversibly at a certain temperature and a certain pressure, and it must have large storage capacity of hydrogen being reversibly available and exhibit high hydrogenation velocity in an electrolyte in order to be applied in practice to an electrochemical battery.
Such a hydrogen storage alloy is used in, for example, a negative electrode of an electrochemically rechargeable battery whose positive electrode is typically made of nickel hydroxide material. A rechargeable secondary battery of this type is commonly referred to as “a nickel-metal hydride (Ni/MH) secondary battery” due to the nickel hydroxide positive electrode and the hydride nature of the negative electrode metal.
The hydrogen storage alloys for the Ni/MH secondary battery developed so far can be largely divided into three types: (1) AB.sub.5 type including La—Ni and Mn—Ni based alloys, wherein A is an element having a strong affinity for hydrogen, for example, alkali earth metal, such as, La, Ti, Zr, Ce, Pr, Nd, etc., and B is a transition element or a transition metal, such as, Ni. Mn, Co, Fe, Al, etc.; (2) AB.sub.2 type including Zr—Ni and Ti—Ni based alloys; and (3) AB type including V—Ti based alloys. The first AB.sub.5 type has a drawback in its low energy storage density while the second AB.sub.2 type is poor in nearly all of its performances. Also, in spite of its largest hydrogen storage capacity, the third AB type still has a problem in its incapability of charging/discharging in an alkali aqueous solution. Thus, in order to cope well with coming development of a high capacity and high performance Ni/MH secondary battery, it is necessary to precede researches for realizing high performances of the AB.sub.2 or AB type hydrogen storage alloys in which higher capacity is guaranteed than in the AB.sub.5 type hydrogen storage alloys.
In general, on a surface of the hydrogen storage alloy electrode is formed an oxide film which causes reduction of activated materials and acts as a barrier to the absorbance/discharge of hydrogen. This results in deterioration of every performance of alloy due to increases of contact resistance and charge transfer resistance. In the end, the oxide film becomes a major factor that reduces the discharge capacity and degrades the life duration of electrode.
To solve the above-mentioned problems, there have been developed various techniques for improving of surface characteristics of the hydrogen storage alloy, such as, an alloy design technique, an alloy surface-coating/etching technique, a surface characteristic-improving technique by means of additives, and a surface-modifying technique using a ball milling. The alloy design technique, however, has a difficulty in designing the alloy to maintain its thermodynamic characteristic, i.e., its large hydrogen storage capacity and at the same time to have excellent surface characteristics. Also, the alloy surface-coating/etching technique not only requires additional processes due to use of solutions, but is scarcely applied in practice because it is inevitably performed under a noxious atmosphere. On contrary, the surface characteristic-improving technique by means of additives and the surface-modifying technique using a ball milling are advantageous in that they can enhance all of the performances only by modifying the alloy surface without having an influence on the thermodynamic characteristic of the alloy. In connection with this, it is reported by M. A. Fetcenko et al., J. Electrochemical Society, 15 (1991) that nickel is necessary material for these surface-modifying or surface characteristic-improving techniques. Since the report, there have been proposed a technique using nickel as an additive in production of an electrode or a ball milling technique using nickel powder. Nevertheless, these surface-modifying techniques using normal type of nickel did not succeed in enhancing efficiency by lack of uniform contact of the nickel on the alloy surface.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made to overcome the above-mentioned problems, and it is an object of the present invention to provide a novel method for modifying a surface of a hydrogen storage alloy for an Ni/MH secondary battery using flake type metal powder having larger surface area over ordinary sphere type metal powder to improve surface characteristics of the alloy, thereby enhancing all performances of an electrode as well as increasing discharge capacity and life duration thereof.
To achieve this object, there is provided a method for modifying a surface of a hydrogen storage alloy for an Ni/MH secondary battery using flake type metal in accordance with an aspect of the present invention, the method comprising the steps of:
ball-milling metal powder to produce flake type metal powder; and
ball-milling the flake type metal powder together with hydrogen storage alloy powder to obtain mixture powder.
The present invention is directed to the improvement of surface characteristics of the above-mentioned AB.sub.2 or AB type hydrogen storage alloys.
Nickel, copper, palladium, chrome and other commonly used metal can be preferably used as raw material of the flake type metal in producing the flake type metal powder according to the present invention without any limitation of its shape. More preferably, the flake type metal powder is nickel powder.
This flake type metal powder functions as a catalyst that is required for charge/discharge of the hydrogen storage alloy in an electrolyte.
Because of larger contact area of the flake type metal powder than that of conventional metal powder (metal powder of any other type than the flake type, e.g., sphere type metal powder), use of the flake type metal powder according to the present invention offers significant advantages over use of the conventional type metal powder.
Also, it is preferred that the hydrogen storage alloy is a Zr based alloy or a Ti based alloy, particularly, a V—Ti based alloy, and is added as a current collector in producing the hydrogen storage alloy electrode.
Now, a description will be given for a method of producing the flake type metal powder according to the present invention.
In accordance with the present invention, the flake type metal powder is produced by means of a ball milling technique. A ball mill to be used in the technique is preferably a SPEX mill or an attrition mill. If raw material of the metal powder is put into the SPEX mill or the attrition mill and then the ball milling is performed, the powder collides with the balls to generate heat and so to proceed mechanical alloying owing to the heat. Since the flake type metal powder produced by the ball milling technique is characterizes in that a particle size is increased in proportional to a time of ball milling, any size of the metal powder is properly selected in accordance with an intention of use of the relevant alloy.
According to the present invention, any technique can be used as a technique for adding the flake type metal powder with a view to modifying the surface of the hydrogen storage alloy, including a case of mixing the flake type metal powder with the hydrogen storage alloy powder and then ball-milling the mixture powder in the SPEX mill, a case of adding the flake type metal powder as a current collector during production of a pellet-type or paste-type electrode and so forth.
REFERENCES:
patent: 5916381 (1999-06-01), Sapru et al.
patent: 5951945 (1999-09-01), Komada et al.
patent: 5962165 (1999-10-01), Tsuruta et al.
patent: 6030724 (2000-02-01), Sawa et al.
patent: 6040087 (2000-
Kim Seoung Hoe
Lee Ho
Lee Jai Young
Lee Sang Min
Park Jeong Gun
Korea Advanced Institute of Science and Technology
Mai Ngoclan
Rosenberg , Klein & Lee
LandOfFree
Method for modifying surface of hydrogen storage alloy for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for modifying surface of hydrogen storage alloy for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for modifying surface of hydrogen storage alloy for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2586095