Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-01-14
2002-01-01
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S217000, C429S215000, C429S213000
Reexamination Certificate
active
06335120
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-sintered nickel electrode such as that which is used in secondary cells containing an alkaline electrolyte, such as, for example, nickel-cadmium, nickel-iron, nickel-hydrogen and nickel-hydridable-metal rechargeable batteries, and to the cell containing it.
2. Description of the Prior Art
Several types of electrode exist, in particular sintered electrodes and non-sintered nickel electrodes, also referred to as impasted or plasticized electrodes. The electrodes most widely used nowadays are of the non-sintered type. Compared with other electrodes, a non-sintered electrode contains a larger amount of active material, its specific capacity is thus increased and its manufacturing cost is lower.
A non-sintered electrode is composed of a support which serves as a current collector, which is coated with a paste containing the active material and a binder, to which is usually added a conductive material. It is usually made by depositing the paste in a porous three-dimensional conductive support such as a felt or a foam made of metal or carbon.
European Patent Application EP-0 726 607 mentions an electrode comprising a porous support coated with a paste. The support is not a critical component, and can be two-dimensional or three-dimensional. The paste contains the active material, a conductive agent, a fluorinated resin and a thickener. The fluorinated resin used as binder can be a mixture of fluorinated resin and of thermoplastic resin. This document does not mention the electrochemical functioning of the electrode.
For cost reasons, the trend is nowadays toward use of a two-dimensional conductive supports.
Japanese Patent Application JP-3 165 469 proposes a nickel electrode comprising a two-dimensional porous conductive support coated with a paste containing nickel hydroxide, a conductive material and a thermoplastic binder, such as a butylene/ethylene/styrene copolymer. In order to ensure binding of the active material to the support, a separator is hot-pressed onto each face of the electrode.
European Patent Application EP-0 750 358 describes a non-sintered nickel electrode whose support is a corrugated metal sheet on which teeth have been formed in order to catch in a layer with microrugosity. A paste comprising carboxymethylcellulose (CMC) and a styrene/butadiene copolymer (SBR) is deposited on this layer.
The known binders used to make an electrode containing a three-dimensional support prove to be unsuitable for a two-dimensional support. In the two previous examples, it was necessary to use a means other than the binder to give the electrode its mechanical strength.
The object of the present invention is to propose a non-sintered nickel electrode containing a two-dimensional, or flat, support, whose mechanical strength and chemical resistance to electrochemical oxidation are enhanced.
SUMMARY OF THE INVENTION
The subject of the present invention is a non-sintered nickel electrode containing a two-dimensional conductive support and a paste comprising an electrochemically active material containing nickel hydroxide and a binder which is a mixture of an elastomer and a crystalline polymer, wherein the proportion of the elastomer is in the range 25% to 60% by weight of the binder and the proportion of the crystalline polymer is in the range 40% to 75% by weight of the binder.
The binder is an essential component of the electrode since its role is both mechanical and electrochemical.
The binder has the function of ensuring cohesion of the grains of active material with each other and with the electrode support, before assembly of the rechargeable battery and during its functioning. On the one hand, the binder must be of sufficient chemical stability. Firstly, it must be chemically inert with respect to the components of the cell; next, it must be capable of withstanding electrochemical oxidation under the cycling conditions to which the electrode is subjected. However, certain binders suffer degradation of their adhesive properties during cycling. On the other hand, the binder must be capable of being deformed in order to match the variations in size of the electrode during cycling throughout its life.
The function of the binder is also to maintain the electrical contact between the grains of active material and to promote the ionic exchanges with the electrolyte. On the one hand, the electrochemically active area of an electrode depends on the area wetted by the electrolyte. To promote the wetability of the electrode by the aqueous electrolyte, the binder should have a hydrophilic nature. If the electrode is not sufficiently wetted, the active area is decreased, which leads to an increase in the local current density and a lower charged capacity. On the other hand, the area accessible to the electrolyte depends on the manner in which the grains of active material are coated and bound by the polymer. The polymer film should have discontinuities to allow the electron exchanges.
An elastomer is a polymer which has elastic properties. It is defined as a polymer which has a viscoelastic state at ambient temperature Ta, which means that its glass transition temperature Tg is below ambient temperature Ta. The use of an elastomer as a binder makes it possible to obtain a nickel electrode with suitable mechanical properties. However, when used alone, it forms a film which coats the grains of active material and greatly reduces the electrical conductivity of the electrode.
The elastomer is preferably chosen from a copolymer of styrene, of ethylene, of butylene and of styrene (SEBS), a terpolymer of styrene, of butadiene and of vinylpyridine (SBVR) and a copolymer of styrene and of butadiene (SBR). The copolymer of styrene and of butadiene preferably contains from 25 to 35% by weight of styrene.
In contrast with other elastomers, a crosslinkable elastomer will instead form lumps of polymer distributed on and around the grains of active material. Crosslinking makes it possible to limit the creep of the polymer. Advantageously, the elastomer is a crosslinkable carboxylated copolymer of styrene and of butadiene (carboxylated SBR), i.e. an SBR bearing —COOH groups which allow its crosslinking. A crystalline polymer is characterized by the fact that it has a melting point.
This polymer is solid at ambient temperature. A crystalline polymer does not form a film: when used alone, it does not have sufficient cohesion to keep the active material on the support.
Preferably, the crystalline polymer is chosen from a fluorinated polymer and a polyolefin, such as polyethylene (PE).
If the crystalline polymer is a fluorinated polymer, it is preferably chosen from a fluorinated copolymer of ethylene and of propylene (FEP), polytetrafluoroethylene (PTFE) and polyhexafluoropropylene (PHFP).
From a mechanical point of view, the greater the proportion of elastomer, the better the cohesion of the electrode. The addition of a crystalline polymer has the function of breaking the continuity of the elastomer film, and thus of preserving the electrochemical performance of the electrode. The binder according to the invention should contain at least 25% by weight of elastomer. Below this proportion, the mechanical strength of the electrode will no longer be sufficient, in particular in the case of a coiled electrode.
To ensure the cohesion and electrochemical functioning of the electrode throughout its period of use, the proportion of the crosslinkable elastomer should be in the range 25% to 60% by weight of the binder, and the proportion of the crystalline polymer should be in the range 40% to 70% by weight of the binder.
According to a preferred embodiment, the binder is composed of 40% to 60% by weight of the elastomer and of 40% to 60% by weight of the crystalline polymer.
If a high content of binder is introduced into the paste, the electrical conductivity of the electrode is decreased, which produces a lowering of the energy density of the power source. It is thus essential to minimize the inevitable loss of capacity wh
Alcorta José
Bernard Patrick
Cocciantelli Jean-Michel
Coco Isabelle
Dennig Corinne
Alcatel
Chaney Carol
Dove Tracy
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