Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-01-27
2001-08-21
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C029S623100, C420S900000
Reexamination Certificate
active
06277519
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for making hydrogen storage alloy powder having good hydrogen storage properties. More particularly, the invention relates to a method for making hydrogen storage alloy powder which exhibits good initial activity and ensures high capacity and prolonged life when applied to as a negative electrode of nickel-hydrogen secondary cells or batteries. The invention also relates to an electrode comprising the hydrogen storage alloy powder obtained by the method mentioned above.
2. Description of the Prior Art
After the discovery of hydrogen storage alloys capable of storing and releasing hydrogen, they have been applied not only to hydrogen storage means, but also to cells or batteries. In particular, alkaline secondary batteries have been already put into practice, and hydrogen storage alloys have been improved one by one to impart thereto higher capacity and more prolonged life. However, while intensive studies are continued, on one hand, on further improvements of the performance of lithium ion secondary batteries, which have been commercially sold in recent years, there is, on the other hand, a strong demand of development of a nickel hydrogen secondary battery which has higher capacity and longer life than existing counterparts.
As is well known in the art, hydrogen storage alloys have been predominantly prepared by a casting method and quenching method. It is also known that the properties of hydrogen storage alloys differ from each other depending on the manner of preparation.
In general, a negative electrode used in nickel hydrogen secondary batteries, makes use of hydrogen storage alloy powder obtained by breaking, into pieces, the hydrogen storage alloy prepared by the casting method. This electrode exhibits not only poor initial activity, but also insufficient capacity and life along with a poor shelf life.
Japanese Laid-open Patent Application Nos. 61-176063 and 5-225975 propose an electrode made of alloy powder of the type discussed above, which is further treated with an alkali and/or acid. Such electrode is improved in initial activity and capacity although not satisfactory, but its shelf life and cycle life are considerably degraded. Further, during the course of the surface treatment and in subsequent steps, the surface is inconveniently liable to deactivate. This could be avoided to a significant extent by further treatment of the electrode with a solution containing a specific type of compound, i.e. a highly conjugated unsaturated compound. However, satisfactory results have never been obtained with respect to the electrode life.
On the other hand, Japanese Laid-open Patent Application No. 63-291363 proposes an electrode using hydrogen storage powder, which is obtained by breaking, into pieces, the alloy prepared by the quenching method. This alloy suffers a reduced degree of segregation of elements with the electrode life being improved. However, the alloy surface appreciably undergoes oxidation, and the resultant electrode has initial activity and capacity lower than the case of the casting method, thus not withstanding use in practical application.
Under these circumstances, treatment with an alkali and/or acid has been studied on the hydrogen storage alloy powder obtained by the quenching method. In this case, the life is improved, but the initial activity, capacity and shelf life are not satisfactory from the standpoint of the practical use.
SUMMARY OF THE INVENTION
We have made intensive studies on the development of an electrode for high-performance nickel hydrogen storage alloy battery. As a result, it has been found that when the hydrogen storage alloy powder obtained by a quenching method is treated with a solution containing a highly conjugated unsaturated compound and is used to make an electrode of a nickel hydrogen storage alloy battery, the battery exhibits high capacity and long life.
It is accordingly an object of the invention to provide a method for making hydrogen storage alloy powder which is useful in making an electrode of a nickel hydrogen storage alloy secondary battery whereby a high capacity and long life of the battery are ensured.
It is another object of the invention to provide an electrode comprising the alloy powder of the type mentioned above.
The above objects can be achieved, according to the invention, by a method for making hydrogen storage alloy powder which comprises rapidly cooling or quenching a melt of a hydrogen storage alloy, breaking the alloy into fine pieces provided that the alloy is in non-powder form after the quenching, and subjecting the fine pieces to treatment with a solution containing a conjugated unsaturated compound which has five or more conjugated &pgr; bonds in the molecule and a molecular weight of 100 or above.
DETAILED DESCRIPTION OF THE INVENTION
The composition of the hydrogen storage alloy used in the present invention are not critical, and is appropriately selected from those hydrogen storage alloys ordinarily used as negative electrode. Among such known hydrogen storage alloys, it is preferred to use MmNi
5
hydrogen storage alloys from the standpoint of life cycle in case of the alloys being used in secondary batteries. In the MmNi
5
alloys, Mm is Misch metal made of a mixture of rare earth elements such as La, Ce, Pr, Nd and the like. In the MmNi
5
hydrogen storage alloy, it is preferred to replace part of Ni with Mn so as to obtain a better cycle life. More preferably, part of Ni should be further replaced by Al and also by Co in addition to Mn and Al.
Most preferably, the hydrogen storage alloy of the invention should be one represented by the general formula, (La)
x
R
1−x
Ni
a
M
b
, in which R represents at least one element selected from Ce, Pr and Nd, M represents at least one element selected from Al, Co, Cu, Fe, Mn, Ti and Zr, x is a value of 0.2 to 1, (a+b) is a value of 4.0 to 6.0 provided that 0<b≦2.0.
In the practice of the invention, the hydrogen storage alloy powder is obtained by a procedure which comprises mixing individual elements to provide a mixture having a desired composition, melting the mixture in an atmosphere of an inert gas such as Ar, He or the like, or in vacuum at a temperature ranging from 1300 to 1600° C., quenching the resultant melt, preferably in the form of a ribbon, followed by breaking the resultant alloy into fine pieces, if necessary. Any limitation is not placed on the manner of the quenching, and it is preferred from the standpoint of safety and efficiency to use any of a revolving roll method, a gas atomizing method, or a rotary disc method. The revolving roll method may make use of either a single roll or a twin roll.
The term “quenching” used herein means cooling at a rate of 100° C./second or higher, and the cooling rate is preferably in the range of 10
3
to 10
5
° C./second. If the cooling rate is lower than 10
2
° C./second, the cooling capacity becomes so low that the resultant alloy may undergo segregation, resulting in the great strain involved in the alloy pieces.
According to the revolving method, an alloy melt is run over a cooling roll or rolls. While rotating the roll, the alloy melt is quenched at such a cooling rate as defined above to obtain a ribbon alloy. The ribbon is subsequently subjected to a wet or dry milling machine such as a jet mill, ball mill or the like to obtain an alloy powder. The powder usually has an average size of 5 to 100 &mgr;m.
When using the gas atomizing method, a fine stream of an alloy melt is quenched while flowing a high-speed gas against the stream. The quenched alloy is obtained as powder in spherical form, similar round form and the like. The high-speed gas may be made of an inert gas such as argon, helium or the like, with the gas being pressurized at 10 kgf/cm
2
or above.
According to the rotary disc method, the alloy melt is vertically dropped onto a disc being rotated substantially at the center thereof, whereupon the melt is radially spread along the disc surface by the agency of the rotation o
Ishii Masatoshi
Kitamura Hajime
Miyamoto Hiroyuki
Noguki Genji
Chaney Carol
McDermott & Will & Emery
Shin-Etsu Chemical Co. , Ltd.
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