Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-11-28
2002-09-03
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C029S002000, C029S623500
Reexamination Certificate
active
06444366
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a non-sintered type electrode for use in storage batteries (secondary batteries), such as alkaline storage battery, lithium ion storage battery and polymer lithium storage battery.
BACKGROUND ART
Various types of electrodes are known for the storage battery (secondary battery). Non-sintered electrodes are defined to be the electrodes that can be obtained without sintering and are fabricated by coating or pressing an active material for battery or active material retaining medium onto an electrode support.
The process of fabricating a nickel positive electrode of alkaline storage battery as one example of sintered electrodes is described below. A microporous sintered plaque obtained by sintering nickel powder is impregnated with an aqueous solution of nickel nitrate or the like thereby to add nickel salt, and, after drying, the sintered plaque is immersed in caustic alkali aqueous solution to convert the nickel salt to nickel hydroxide. This method has the disadvantage that the process is complicated and the filling density of nickel hydroxide as an active material is reduced in comparison with the non-sintered electrode described later. In spite of this disadvantage, this electrode has an excellent high-rate discharge characteristic and a long cycle life, and finds wide application in a variety of field.
A method called pocket system was previously used for the manufacture of non-sintered electrode, while paste applying or pressing methods have become the mainstream in the recent years. In the paste applying method, the active material for battery itself or active material retaining medium is prepared into a paste with water or an organic solution, this paste is applied on an electrode support and dried. In the pressing method, on the other hand, the active material for battery or active material retaining medium in the form of powder directly fills the electrode support by being pressed thereonto with a press machine or the like.
A variety of materials are used for these non-sintered electrodes, including metal foil, perforated metal plate, metal net, expanded metal, foamed porous metal material and the like. These materials have been applied with their properties and forms varied-accordingly so as to suit each battery system.
For example, foamed nickel porous material having a three dimensionally reticulated structure has been commonly used as the electrode support for the positive electrode of non-sintered type used in alkaline storage batteries such as nickel-hydrogen storage battery or nickel-cadmium storage battery. For the negative electrode, on the other hand, punched metal has been mainly employed.
The method using foamed nickel porous material is a simple method of electrode production. Further, the availability of a foamed nickel porous material of high porosity makes it possible to fill it with nickel hydroxide to a high density and therefore a high-capacity battery can be produced. The foamed nickel porous material, however, needs to be produced by electroplating and therefore has the disadvantage of high material cost.
In view of this, a non-sintered electrode is under development using a low-cost punched metal or expanded metal in place of the foamed nickel porous material as an electrode support. These electrode supports have no three-dimensional structure unlike the sintered plaque or the foamed-nickel porous material. As a result, an electrode made of these electrode supports has a low ability to hold an active material and the active material is liable to fall off during electrode fabrication or repeated charging and discharging. Further, due to the low electronic conductivity in the electrode thickness direction and a poor electrode characteristic, which is a serious hindrance to the application in the nickel positive electrode of alkaline storage battery, these electrode supports find no practical applications except for special types of electrodes.
Japanese Laid-Open Patent Application No.7-130370 and No.9-7603 disclose technologies for improvement of these electrode supports. The electrode support according to JP Laid-Open No.7-130370 is constructed of flat metal sheet or flat metal foil and thus is weak in the adhesion between the active material layer and electrode support. Separation of active material from electrode support occurs particularly in the application as the electrode of a storage battery due to changes in volume of the active material caused by repeated charging and discharging. Current collecting ability decreases accordingly, as a result of which the battery characteristics are deteriorated.
As a countermeasure for this drawback, formation of minute irregularities using metal powder is proposed in JP Laid-Open No.9-7603. The adhesion between the active material layer and the electrode support is thereby improved. However, the production cost of electrode supports will be raised because of sintering in an inactive gas atmosphere or electroplating methods required for forming the minute irregularities layer.
In both of the above electrode supports, furthermore, in the case of corrugating the electrode supports so that they have a three-dimensional structure, they are more subjected to deformation and elongation during the compressing process for filling the active material at a high density. As a result, cracks or rupture occur in the electrode support, which leads to troubles such as decrease in current collecting ability of the electrode and micro-short circuit when assembled as a battery. Moreover, the above mentioned deformation and elongation of the electrode support also set a limit to the high-density filling and a battery of large discharge capacity cannot be obtained.
Meanwhile, efforts have been made to improve the electrode characteristics such as retaining property of active material and electronic conductivity for the negative electrode of alkaline storage battery (cadmium electrode or hydrogen-absorption alloy electrode), using inexpensive punched metal or expanded metal while exploiting their advantages. Further improvement is desired to achieve a more efficient high-rate discharge characteristic and a longer cycle life, which are still unsatisfactory in these negative electrodes.
These demands are also applicable to other types of batteries such as for example lithium ion storage battery or polymer lithium storage battery. There has generally been a desire for an electrode using a low cost electrode support while exhibiting excellent performance.
The above-mentioned method of electrode production using a punched metal or expanded metal as an-electrode support has the advantage that a powder of active material made into a paste with a solution of a high polymer binder and a conductive powder is coated and dried on the electrode support and thus the electrode can be easily produced. The adhesion between the metal substrate acting as the electrode support and the active material layer is generally weak so that the active material is liable to peel off from the metal substrate in an application using the electrode for batteries. In the case where the electrode support acts as a current collector, the electrical resistance of the electrode increases thereby causing a reduced discharge voltage and discharge capacity. In order to solve this problem, adding a great amount of binder to the active material layer suppresses the separation. The resultant reduced reactivity of the active material, however, has an adverse effect on the discharge characteristic.
In a method for strengthening the adhesion between the electrode support and the active material layer, a thermoplastic resin layer functioning as a binder is formed on the surface of the electrode support. Then, the active material is coated on the thermoplastic resin layer and the electrode is heated, to improve the adhesion between the electrode support and the active material layer. This method, however, has a disadvantage that a resin insulating layer is formed between the metal electrode support and the active material layer
Kawano Hiroshi
Matsumoto Isao
Moriwaki Yoshio
Chaney Carol
Greenblum & Bernstein P.L.C.
Tsang-Foster Susy
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