Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2001-05-30
2004-10-05
Dove, Tracy (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S239000, C429S243000, C429S245000, C429S218200
Reexamination Certificate
active
06800399
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a paste type thin electrode for a battery, in which the cost is reduced and the high rate discharge characteristics and the cycle life are improved, and to a secondary battery using this electrode.
BACKGROUND OF THE INVENTION
At present electrodes for batteries, used commercially for secondary batteries, are broadly categorized as sintered type electrodes and non-sintered type electrodes. In the sintered type electrodes, active material is filled into a highly porous three dimensional substrate where metal powder is sintered to have a large porosity on both sides of a two dimensional metal substrate. In the non-sintered type electrodes, the active material powder with a binder is coated on a two dimensional metal substrate or grid, or filled into a three dimensional substrate, such as foamed nickel, metal bag or tube, without employing a sintered substrate.
In general, the former exhibits excellent characteristics in electronic conductivity (high-rate charge and discharge characteristics) due to a large amount of metal used in the sintered plaque and has a long cycle-life with excellent mechanical strength and stability in the shedding of active material, while it has the defect that the electrode is heavy and has a small volumetric energy density due to a small amount of active material impregnated therein because of a large volume of the electrode substrate.
On the contrary, a representative and simple non-sintered type electrode is inexpensive and light weight, and has a large volumetric energy density because of using an inexpensive substrate of a small volumetric amount in the electrode, which is easy to manufacture, through the coating or direct filling process of active material powder, while it entails the problem that the entire electrode is inferior in current collection ability as a whole, in the mechanical strength and in the holding of the active material. These are significant problems in secondary batteries where charging and discharging is repeated and, therefore, a variety of ideas are incorporated into respective battery systems.
As a result, non-sintered types have a variety of substrates to improve the above problems, as represented by a paste type or an application type, wherein active material powder is mixed with conductive material or a binder which is then mixed together with a solution and the obtained paste or slurry is coated on a two dimensional substrate of a variety of shapes, or in some cases the active material powders are filled in a pocket type or a tube type substrate which has innumerable small pores for electrochemical reactions.
As examples of non-sintered type electrodes, which are of the former type, a cadmium negative electrode, a metal hydride negative electrode for alkaline storage batteries, the positive and negative electrodes for lithium ion batteries and the positive and negative electrodes for lead acid batteries are cited. Non-sintered type batteries which are of the latter type are, for example, employed in part of the nickel positive electrode for large scale alkaline storage batteries or for certain types of lead acid batteries. As a substrate of the electrodes described herein, punching metal, a metal screen, foamed metal, a metal grid or the like are individually utilized according to the battery systems or the purpose.
However, recently, new types of electrodes in which a paste of active materials is filled into a foamed nickel porous substrate or into a nickel fiber substrate, which have a three dimensional structure, in the high density (hereinafter abbreviated as 3DM type), have started being employed as proposed in U.S. Pat. No. 4,251,603, which belongs to another non-sintered type in classification. However, though these types of electrodes have a high capacity and a high reliability and are easily made to have higher capacity and to be lighter weight compared with the sintered type, due to a small amount of metal employed in the substrate, they have the technical problems that the mechanical strength is low and the electronic conduction of the entire electrode is inferior due to a large pore diameter within the substrate and, in addition, have the technical problem that the cost of the substrate is high.
Since the present invention of paste type electrode relates to an improvement of the three dimensional substrate used in the above described 3DM system, in particular for alkaline storage battery system currently, for the convenience of the detail technological description of prior art, a nickel positive electrode for a small sealed cylindrical Ni/MH batteries is focused on thereafter.
As for the nickel positive electrode for alkaline storage batteries, the sintered type electrode, which was developed in Germany during the Second World War, has a high performance and is durable, which replaced the previous non-sintered type electrode, that is to say, the pocket type electrode, and, therefore, a sintered type electrode started to be used for rectangular Ni/Cd batteries requiring high performance and high reliability. As for the negative electrode, a similar conversion to the sintered type has occurred. As for the electrodes of sealed cylindrical Ni/Cd batteries developed afterwards, sintered type positive and negative electrodes have become the most popular because they are easily processed into thin electrodes. The small sealed cylindrical batteries represented by this nickel-cadmium battery (Ni/Cd battery) have achieved a dramatic growth as a power supply for portable compact electronic equipments, such as camcorders or CD players, which have achieved a remarkable growth in Japan starting in the 1980's. However, in the 1990's, a new type of nickel-metal hydride storage battery (Ni/MH battery) and a lithium ion battery successively have been put into practical use so as to begin expansion into the market of nickel-cadmium batteries.
And, as for a new market, applications for power supplies as power tools, applications for mobile power supplies, that is to say, for electrical vehicles (EVs), hybrid electrical vehicles (HEVs), electric power assisted bicycles or the like have newly started growing in recent years, and for those power supplies mainly Ni/MH batteries have started being used. A nickel positive electrode is employed for the positive electrodes of the above described Ni/Cd batteries and Ni/MH batteries for which the growth recently has been remarkable and the sintered types and 3DM types are used respectively, according to the applications under the present circumstances.
As for the structure of this nickel positive electrode for a mass-production level, the non-sintered type was limited only to the pocket type, due to the electrode mechanical stability. The pocket type electrode has a structure wherein active material powder is filled into a electrolyte proof metal bag with innumerable small pores to prevent the shedding of active metal powders as described above. The sintered type adopts a structure wherein a solution of active material salt is impregnated into the space of a three dimensional sintered plaque, followed by the process of conversion to the active material with alkaline solution. Naturally, the active material in this case is not in a powder condition.
Another non-sintered 3DM type, which is different from the pocket type, is reported as a nickel positive electrode employing foamed nickel in the ECS Fall Meeting (Detroit) Abstract No. 10 in 1981. This electrode has a structure using a foamed nickel porous body as a substrate, into which active material powder is filled.
Though a light weight nickel positive electrode with a high capacity is realized by using this foamed nickel as a substrate, it has the problems that the high power drain of the entire active material is not sufficient due to the large diameter of the internal spherical space, which is approximately 450 &mgr;m in the case of even the smallest diameter, and it is expensive. Therefore, batteries using a sintered type nickel positive electrode which exhibit excellen
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