Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1997-05-28
2001-04-10
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S234000, C429S235000
Reexamination Certificate
active
06214491
ABSTRACT:
This application claims the priority of German patent application No. 196 21 316.9, the disclosure of which is expressly incorporated by reference herein.
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to fabricated porous metallized fiber structure framework plates for use as an electrode or recombination element (gas diffusion bodies) in electric accumulators, and to a process for manufacturing such plates.
Accumulators having a type of electrode made of a framework of metallized synthetic fibers, as well as different partial aspects or manufacturing steps therefor, are known from various patent publications:
German Patent Document DE-PS 40 04 106, for example, describes a nonwoven metallized synthetic-fiber electrode framework for battery electrodes, with an increased load capacity.
Processes for activation and chemical metallization of nonwoven and needle felt webs are disclosed in German Patent Documents DE-PS 36 31 055 and DE-PS 36 37 130.
German Patent Documents DE-PS 38 17 827, DE-PS 38 17 826 and DE-PS 40 10 811 disclose aqueous nickel hydroxide or cadmium oxide pastes for the vibration filling of foam and fiber structure electrode frameworks.
German Patent Document DE-PS 38 22 197 describes processes for continuous filling of fiber structure electrode frameworks, and for cleaning away excess paste from the electrode plate, preferably by means of brushes, after mechanical filling.
German Patent Document DE-PS 36 32 352 discloses a fiber structure electrode plate with a welded-on current tapping tab.
German Patent Document DE-PS 40 18 486 discloses a process for manufacturing fiber structure electrodes, in which the framework is calibrated before the filling, and is calibrated again afterward, by means of a whole-surface pressing-together.
German Patent Documents DE-PS 40 40 017 and DE-PS 41 03 546 respectively describe a process for filling fiber structure electrode frameworks for accumulators with an active-mass paste while simultaneously calibrating the framework; in the former, the framework is rolled during the filling operation, and in the latter, the plate is pressed during the filling operation.
Despite the intensive treatment of the topic which is of interest here and which is documented by the above-mentioned list (which is to serve only as an example, and is by no means complete), difficulties and inadequacies in fiber structure electrode plates which become apparent only during the operation of the cells, continue to occur. As a result of the cycle-dependent volume mainly of the positive electrodes and the separators, and the resulting displacement of the components with respect to one another due to charging and discharging, occasionally premature capacity losses of the accumulator will occur.
The manufacture of such fiber structure electrodes is based on a nonwoven made of synthetic fibers. According to experience, nonwovens and needle felts with starting porosities of from 45% to 96% with a fiber diameter of between 5 &mgr;m and 40 &mgr;m are particularly suitable for the manufacture of fiber structure electrodes and recombination elements with a metallic fiber structure. After chemical activation, wet-chemical metallizing and galvanic reinforcing of the metal coating, the pieces finished to this extent are cut to a useful size and the plates are provided with a current tapping tab on a portion of a dimensionally stable edge which is reinforced during the galvanizing and was left standing in areas. Before filling with the active mass, the fiber structure framework plates are calibrated in order to be able to manufacture electrode frameworks with a defined filling while the scattering is low.
During fabrication of the fiber structure framework plates, the fiber structure will fan out on the cut edges because of the separating, sawing or cutting. That is, the spatial fiber composite will be partly lost in the area of such cutting, etc., and individual fibers or fiber bundles may come to the surface already because of their inherent tension and may project from it more or less steeply. When the active mass is entered into the fiber structure framework by vibration filling, the fibers exposed in the cutting area are further relaxed because of the vibrations. By means of the vibrations and the active mass penetrating into the fiber structure, the fanning-out may be increased. A superficially adhering excess of active mass paste must then be mechanically removed, which also exacerbates the fanning-out of individual fibers from the fiber structure in the area of the cut edges. In this case, the ends of fibers which are not completely linked into the composite are occasionally torn out and may, under certain circumstances, even project from the electrode surface at a right angle.
Fiber structure framework plates of the type addressed here are used in the accumulator construction not only as positive or negative electrode plates (after the filling-in of the active mass), but also in the form of metallized fiber frameworks (not filled with an active mass) as a gas diffusion framework and recombination element on which the oxygen is reduced relatively rapidly. The gas diffusion frameworks should rest against the electrodes or against individual negative electrodes in a full-surface and electrically conductive manner. The recombination elements therefore do not require a current tapping tab. Also when fiber structure framework plates of this type are manufactured, individual metallized fibers may protrude from the flat side of the framework plate in a more or less steep fashion and can also not be pressed back permanently into the composite by means of a calibrating operation.
Such metallized fibers and fiber composites which are pulled up or spread out from the fiber composite would result in a certain failure rate of the cells, because of internal short circuits. For this reason, such interfering fibers are normally pushed back by means of another calibration after the filling and drying of the fiber structure electrodes and before the assembly to an accumulator. However, this technique represents another working step during which, because of a possible formation of dust as the result of the crushed dry active material, additional special workers' protection measures are necessary.
Despite the calibration of the (filled or unfilled) framework plates, afterwards individual fibers may still project from the flat side of the framework plate. During the combined mechanical stressing of the plate composite which occurs during the assembly of the different framework plates and separators to an accumulator, as a result of pressing directed transversely to the plates and a thrust directed in parallel to the layer construction, if unfavorable circumstances coincide, individual fibers may penetrate into the separators and may even penetrate through the thin separators to the opposite electrode plate. Moreover, mainly during the mounting of the current tapping tabs, stresses occur in the area of the starting portion of the current tapping tabs on the electrode plates, which promote such a piercing of individual fibers through the separators. In the dry condition of the freshly assembled cell, a possible subsequent short circuit can usually not yet be diagnosed or localized. However, after a few cycles during the formation, or more probably later in the accumulator operation, because of a constant volume change in the positives as the result of the charge and discharge operation or because of the expanding of the plate stack by the addition of electrolyte, a projecting fiber, at first hidden, may cause a failure of the cell because of a local short circuit. Even if a fiber itself may not have completely penetrated the separators after a certain number of cycles, needle-like, electrically conductive dendrites grow up from such local anomalies, and may penetrate the separators, leading to a short circuit.
It is an object of the invention to provide an improved fiber structure for framework plates which exhibits enhanced operational reliability, and eliminat
Henke Dieter
Imhof Otwin
Kistrup Holger
Kitzhofer Wilhelm
Schaffrath Uwe
Deutsche Automobilgesellschaft mbH
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Maples John S.
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