Description
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`Button cells and method for producing same
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`[0001]
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`The present invention relates to button cells comprising two metallic housing half-
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`parts, which are separated from one another by an electrically insulating seal and which form a
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`housing with a flat bottom area and a flat top area parallel to it, as well as within the housing, an
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`electrode-separator assembly comprising at least one positive and at least one negative electrode,
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`which are in the form of flat layers and are connected to one another by at least one flat
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`separator, and to a method for producing such button cells.
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`[0002]
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`Button cells normally have a housing consisting of two housing half-parts, a cell cup
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`and a cell top. By way of example, these may be produced from nickel-plated deep-drawn metal
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`sheet as stamped and drawn parts. The cell cup normally has positive polarity, and the housing
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`top negative polarity. The housing may contain widely differing electrochemical systems, for
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`example zinc/MnO2, primary and secondary lithium systems, or secondary systems such as
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`nickel/cadmium or nickel/metal hydride.
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`[0003]
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`By way of example, rechargeable button cells based on nickel/metal hydride or
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`lithium-ion systems are in widespread use.
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`In the case of lithium-ion button cells, the
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`electrochemically active materials are normally not arranged within the button cell housing in the
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`form of individual electrodes, in the form of tablets, separated from one another by a separator.
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`Instead of this, prefabricated electrode-separator assemblies are preferably inserted flat into the
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`housing. In this case, a porous plastic film is preferably used as a separator, onto which the
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`electrodes are laminated or adhesively bonded flat. The entire assembly comprising the
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`separator and the electrodes in this case generally has a maximum thickness of a few hundred
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`µm. In order to allow button cell housings of normal dimensions to be filled, a plurality of such
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`assemblies are therefore frequently placed flat one on top of the other. This allows stacks of any
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`desired height, in principle, to be produced, in each case matched to the available dimensions of
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`the button cell housing into which the stack is intended to be installed. This ensures optimum
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`utilization of the available area within the housing.
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`[0004]
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`By virtue of the design, however, various problems also occur in the case of button
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`cells which contain such stacks of electrode-separator assemblies. On the one hand, it is
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`necessary, of course, for the electrodes of the same polarity each to be connected to one another
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`within the stack, and then each to make contact with the corresponding pole of the button cell
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`housing. The required electrical contacts result in material costs, and the space occupied by
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`them is, furthermore, no longer available for active material. In addition, the production of the
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`electrode stacks is complicated and expensive since faults can easily occur when the assemblies
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`make contact with one another, increasing the scrap rate. On the other hand, it has been found
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`that button cells having a stack of electrodes and separators very quickly start to leak.
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`[0005]
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`Traditionally, button cells have been closed in a liquid-tight manner by beading the
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`edge of the cell cup over the edge of the cell top in conjunction with a plastic ring, which is
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`arranged between the cell cup and the cell top and at the same time acts as a sealing element and
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`for electrical insulation of the cell cup and of the cell top. Button cells such as these are
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`described, for example, in DE 31 13 309.
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`[0006]
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`However, alternatively, it is also possible to manufacture button cells in which the
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`cell cup and the cell top are held together in the axial direction exclusively by a force-fitting
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`connection, and which do not have a beaded-over cup edge. Button cells such as these and
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`methods for their production are described in the still unpublished German patent application
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`with the file reference 10 2009 017 514.8. Irrespective of the various advantages which button
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`cells such as these without beading may have, they can, however, not be loaded as heavily in the
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`axial direction as comparable button cells with a beaded-over cup edge, in particular with respect
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`to axial mechanical loads which are caused in the interior of the button cell. For example, the
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`electrodes of rechargeable lithium-ion systems are continually subject to volume changes during
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`charging and discharging processes. The axial forces which occur in this case can, of course,
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`lead to leaks more readily in the case of button cells without beading than in the case of button
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`cells with beading.
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`[0007]
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`The present invention was based on the object of providing a button cell in which the
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`problems mentioned above do not occur, or occur only to a greatly reduced extent. In particular,
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`the button cell is intended to be more resistant to mechanical loads which occur in the axial
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`direction than conventional button cells, in particular even when they are manufactured as button
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`cells without a beaded-over cup edge.
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`[0008]
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`This object is achieved by the button cell having the features of claim 1. Preferred
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`embodiments of the button cell according to the invention are defined in the dependent claims 2
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`to 10. The method according to claim 11 also contributes to the solution of the problem
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`according to the invention. Preferred embodiments of the method according to the invention are
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`defined in dependent claims 12 to 14. The wording of all the claims is hereby included by
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`reference to the content of this description.
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`[0009]
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`A button cell according to the invention always comprises two metallic housing half-
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`parts, which are separated from one another by an electrically insulating seal and form a housing
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`with a flat bottom area and a flat top area parallel to it. As already mentioned initially, the two
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`housing half-parts are generally a so-called housing cup and a housing top. In particular, parts
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`composed of nickel-plated steel or metal sheet are preferred as housing half-parts. Furthermore,
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`trimetals, for example with the sequence of nickel, steel ( or stainless steel) and copper (with the
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`nickel layer preferably forming the outer layer and the copper layer preferably forming the inside
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`of the button cell housing) are particularly suitable for use as the metallic material.
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`[0010]
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`By way of example, an injection-molded or film seal can be used as a seal. The latter
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`are described, for example, in DE 196 47 593.
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`[0011] Within the housing, a button cell according to the invention comprises an electric-
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`separator assembly with at least one positive and at least one negative electrode. These are each
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`in the form of flat electrode layers. The electrodes are connected to one another via a flat
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`separator. The electrodes are preferably laminated or adhesively bonded onto this separator.
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`The electrodes and the separator generally each have thicknesses only in the µm range.
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`In
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`general, a porous plastic film is used as the separator.
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`[0012]
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`In contrast to the button cells mentioned initially, the button cell according to the
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`invention is distinguished in particular by the electrode layers having a very particular
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`orientation, specifically being aligned essentially at right angles to the flat bottom and top areas.
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`While button cells known from the prior art with stacked electrode-separator assemblies always
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`contain these assemblies inserted flat, such that the electrode layers are aligned essentially
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`parallel to the flat bottom and top areas, the situation in a button cell according to the invention is
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`the opposite of this.
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`[0013]
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`The right-angled alignment of the electrode layers has an unexpectedly considerable
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`advantage, specifically because it has been found that this alignment results in a considerable
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`improvement in the sealing characteristics of a button cell according to the invention, particularly
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`for button cells based on lithium-ion systems. The electrodes of rechargeable lithium-ion
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`systems are continually subject to volume changes during charging and discharging processes.
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`Volume changes such as these also occur, of course, in the electrodes of a button cell according
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`to the invention. However, the mechanical forces which are created during this process no
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`longer act primarily axially, as in the case of a stack of electrode-separator assemblies which are
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`inserted flat. Because of the right-angled alignment of the electrodes, they in fact act radially.
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`Radial forces can be absorbed very much better than axial forces by the housing of a button cell.
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`The improved sealing characteristics are presumably a result of this.
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`[0014]
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`Particularly preferably, the electrodes and the flat separator of a button cell according
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`to the invention are each in the form of strips or ribbons. By way of example, the production of a
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`button cell according to the invention can be based on a separator material in the form of an
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`endless ribbon, onto which the electrodes are applied, in particular laminated, once again in
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`particular in the form of strips or at least rectangles.
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`[0015]
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`In the housing of a button cell according to the invention, this assembly is particularly
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`preferably in the form of a winding, in particular in the form of a spiral winding. Windings such
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`as these can be produced very easily using known methods (see for example DE 36 38 793), by
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`applying the electrodes flat, in particular in the form of strips, to a separator which is in the form
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`of an endless ribbon, in particular by laminating them on. In this case, the assembly comprising
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`electrodes and separators is generally wound onto a so-called winding mandrel. Once the
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`winding has been removed from the winding mandrel an axial cavity remains in the center of the
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`winding. This allows the winding to expand into this cavity, if necessary. However, in some
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`circumstances, this can lead to problems in making electrical contact between the electrodes and
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`the metallic housing half-parts, and this will be described in more detail in the following text.
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`[0016]
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`The electrode winding is preferably arranged within a button cell according to the
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`invention (in order that the electrode layers of the winding are aligned at right angles to the flat
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`bottom area and top area of the housing), such that the end faces of the winding face in the
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`direction of the flat bottom area and of the flat top area.
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`[0017]
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`According to the present invention, preferred embodiments of the button cell
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`according to the invention have a fixed winding core in the center of the winding, which at least
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`partially fills the axial cavity in the center of the winding. A winding core such as this fixes the
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`electrode winding in the radial direction and prevents possible implosion of the winding into the
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`axial cavity. When the winding expands in this way, this also results in the reduction in the
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`pressure which the end faces of the winding exert in the axial direction, and therefore in the
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`direction of output conductors which may be arranged there (this is described in more detail
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`further below).
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`If this is prevented, then there are generally also no problems with making
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`electrical contact between the electrodes and the metallic housing half-parts.
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`[0018]
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`In addition, a winding core such as this also makes the button cell according to the
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`invention more robust against external mechanical influences. In general, it is no longer possible
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`for the electrode winding in the button cell to be damaged by external mechanical pressure in the
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`axial direction.
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`[0019]
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`According to the preferred embodiment of the electrode winding as a spiral electrode
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`winding, the axial cavity which has been mentioned in the center of the winding is preferably
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`essentially cylindrical (in particular circular-cylindrical). On the casing side, it is bounded by the
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`winding, and at the end it is bounded by corresponding surfaces of the bottom area and of the top
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`area of the button cell housing.
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`[0020]
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`Correspondingly, the winding core which is contained in a button cell according to
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`the invention is preferably also in the form of a cylinder, in particular a hollow cylinder. The
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`height of a cylinder such as this preferably corresponds to the respective distance between the
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`flat bottom area and the flat top area, which is parallel to it.
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`[0021]
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`In particularly preferred embodiments, the winding core may have radially self-
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`expanding characteristics. For example, it is possible for the winding core to be inserted in a
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`radially compressed configuration into the axial cavity in the winding of a button cell according
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`to the invention. When the radially compressed winding core expands, it exerts a radial pressure
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`on the electrode winding surrounding it, thus ensuring a contact pressure in the axial direction as
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`well.
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`[0022]
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`By way of example, an axially slotted hollow cylinder may be used as a radially self-
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`expanding winding core. However, alternatively, it is also conceivable to use other radially self(cid:173)
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`expanding materials, for example based on plastic.
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`[0023]
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`Particularly preferably, the winding core is composed of a metal such as stainless
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`steel or plastic.
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`[0024]
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`Particularly preferably, the assembly compnsmg electrodes and a separator m a
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`button cell according to the invention has one of the following layer sequences:
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`•
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`or
`
`•
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`negative electrode/separator/positive electrode/separator
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`positive electrode/separator/negative electrode/separator.
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`[0025]
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`Assemblies such as these can be produced and wound very easily without short
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`circuits occurring between electrodes of opposite polarity.
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`[0026]
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`The separators which can be used in a button cell according to the invention are
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`preferably films composed of at least one plastic, in particular of at least one polyolefin. By way
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`of example, the at least one polyolefin may be polyethylene. However, it is also possible to use
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`multilayer separators, for example separators composed of a sequence of different polyolefin
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`layers, for example with the sequence polyethylene/polypropylene/polyethylene.
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`[0027]
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`It is not essential to use a plurality of separate separators in order to produce
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`assemblies with the abovementioned sequence. In fact, a separator can also be looped around the
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`end of one of the electrodes, thus resulting in both sides of this electrode being covered by the
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`separator.
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`[0028]
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`The separators which can preferably be used in a button cell according to the
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`invention preferably have a thickness of between 3 µm and 100 µm, in particular of between 10
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`µmand50 µm.
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`[0029]
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`The electrodes of a button cell according to the invention preferably have a thickness
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`of between 10 µm and 1000 µm, in particular of between 30 µm and 500 µm.
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`[0030]
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`In preferred embodiments of a button cell according to the invention, the negative
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`electrode and the positive electrode in the electrode-separator assembly are arranged offset with
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`respect to one another within the assembly. In this case, an offset arrangement is intended to
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`mean that the electrodes are arranged such that this results in a respectively different separation
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`between the electrodes and the flat bottom and top areas in the button cell according to the
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`invention. In the simplest case, for example, a positive and a negative electrode can be slightly
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`offset as strips of the same width applied to the opposite sides of a separator ribbon, as a result of
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`which the distance between the positive electrode and the upper separator edge is greater than the
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`comparable distance measured from the negative electrode. This then applies in the opposite
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`sense, of course, to the distance from the lower separator edge.
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`[0031]
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`In particularly preferred embodiments, preferably as a result of this offset
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`arrangement, the positive electrode, in particular an edge of the positive electrode, rests directly
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`on the cup part, in particular in the flat bottom area of the cup part, while the negative electrode,
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`in particular an edge of the negative electrode, rests directly on the top part, in particular in the
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`flat top area of the top part. In this embodiment, a direct electrical and mechanical contact is
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`made between the electrodes and the cup and top parts. The offset arrangement of the electrodes
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`with respect to one another therefore makes it possible for the electrodes to make contact with
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`the respective housing parts, without any need to use additional electrical contacts and
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`connecting means.
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`[0032]
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`However, in alternative preferred embodiments, it is also preferable for at least one of
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`the electrodes, preferably both the at least one negative electrode and the at least one positive
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`electrode, in a button cell according to the invention, to be connected to the flat bottom and top
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`areas via one or more output conductors. By way of example, the output conductors may be
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`output-conductor lugs composed of copper or some other suitable metal. On the electrode side,
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`the output conductors may, for example, be connected to a current collector. The output
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`conductors can be connected to the housing and/or to the current collectors by, for example,
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`welding or via an clamped joint.
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`[0033]
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`In the simplest case, the current collectors of the positive and negative electrodes can
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`also themselves act as output conductors. Collectors such as these are generally metallic films or
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`meshes which are embedded in the respective electrode material. Uncovered subareas, in
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`particular end pieces, of collectors such as these can be bent around and can be brought into
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`contact with the button cell housing.
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`[0034]
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`The use of output conductors may be particularly advantageous if the negative
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`electrode and the positive electrode within the assembly are arranged with respect to one another
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`such that this results in the electrodes each being at the same distance from the flat bottom and
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`top areas. Or, in other words, if the electrodes are not arranged offset with respect to one another
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`within the electrode-separator assembly, as has been described above.
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`[0035]
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`However, if the distance between electrodes of opposite polarity and the flat bottom
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`and top areas is the same, this results in the risk of a positive and a negative electrode touching
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`the metallic cup or top part at the same time, thus resulting in a short circuit.
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`In preferred
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`embodiments, the button cell according to the invention may therefore comprise at least one
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`insulating means, which prevents a direct mechanical and electrical contact between the end
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`faces of the winding and the flat bottom and top areas.
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`[0036]
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`In one development, it is preferable for the electrodes in a button cell according to the
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`invention such as this to be connected via the already mentioned separate output conductors to
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`the flat bottom and top areas. These ensure the electrical contact between the electrodes and the
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`housing.
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`[0037]
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`In this case, it is preferable for at least a subsection of the output conductor or
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`conductors in the bottom area and in the top area of the housing to rest flat on the inside of the
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`housing half-parts.
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`Ideally, the output conductors naturally make electrical contact with the
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`insides of the housing when they are at least slightly pressed against the housing (if they are not
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`welded to it in any case). This can be achieved surprisingly efficiently by a suitable arrangement
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`of the winding core that has been mentioned, in a button cell according to the invention.
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`[0038]
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`By way of example, the insulating means may be a flat layer composed of plastic, for
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`example a plastic film, which is arranged between the end faces of the winding and the flat
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`bottom and top areas of the housing of a button cell according to the invention.
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`[0039]
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`Corresponding to the above statements, the button cell according to the invention is,
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`in particular, a rechargeable button cell. A button cell according to the invention particularly
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`preferably has at least one lithium-intercalating electrode.
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`[0040]
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`The ratio of the height to the diameter of button cells is, by definition, less than 1.
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`For a button cell according to the invention, this ratio is particularly preferably between 0.1 and
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`0. 9, in particular between 0 .15 and 0. 7. In this case, the height means the distance between the
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`flat bottom area and the flat top area parallel to it. The diameter means the maximum distance
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`between two points on the casing area of the button cell.
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`[0041]
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`The button cell according to the invention is particularly preferably a button cell
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`which is not beaded over, as is described in the patent application with the file reference 10 2009
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`017 514.8, which has already been mentioned in the introduction. Correspondingly, there is
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`preferably an exclusively force-fitting connection between the housing half-parts. Therefore, the
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`button cell according to the invention does not have a beaded-over cup edge, as is always the
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`case with button cells known from the prior art. The button cell is closed without being beaded
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`over.
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`[0042]
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`Button cells such as these which are not beaded over generally make use of
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`conventional cell cups and cell tops, which each have a bottom area and a top area, a casing area,
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`an edge area which is arranged between the bottom and top areas and the casing area, and a cut
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`edge. Together, the cell cup and cell top form a housing, which forms a receptacle for the
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`conventional internal components of a button cell, such as electrodes, separator etc. As in the
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`normal way, the bottom area of the cell cup and the top area of the cell top are aligned essentially
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`parallel to one another in this housing. The casing areas of the cell cup and cell top in the
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`finished button cell are aligned essentially at right angles to the bottom and top areas, and
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`preferably have an essentially cylindrical geometry. The internal and external radii of the cell
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`cup and cell top are preferably essentially constant in the casing areas. The edge areas, which
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`have been mentioned, of the cell cup and cell top form the transition between the casing areas
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`and the top and bottom areas. They are preferably therefore bounded on the one hand by
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`essentially flat bottom and top areas, and on the other hand by the essentially cylindrical casing
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`areas, which are arranged at right angles to them. By way of example, the edge areas may be in
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`the form of a sharp edge, or else may be rounded.
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`[0043]
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`The procedure for producing a button cell which is not beaded over is generally to
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`first of all apply a seal to the casing area of a cell top. In a further step, the cell top is then
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`inserted, with the seal fitted, into a cell cup thus resulting in an area in which the casing areas of
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`the cell cup and cell top overlap. The size of the overlap area and the ratio of the overlapping
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`area to the non-overlapping areas are in this case governed by the respective height of the casing
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`areas of the cell cup and cell top, and by the depth of the insertion. With regard to the casing
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`area of the cell top, it is preferable for between 20% and 99%, in particular between 30% and
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`99%, particularly preferably between 50% and 99%, to overlap the casing area of the cell cup
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`(the percentages each relate to the height of the casing or of the casing area). Before being
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`inserted into the housing cup and/or the housing top, the other conventional components of a
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`button cell ( electrodes, separator, electrolyte etc.) are inserted. After the cell top has been
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`inserted completely into the cell cup a pressure is exerted on the casing area of the cell cup, in
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`particular in the area of the cut edge, in order to seal the housing. In this case, a joined-together
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`housing part should as far as possible not be subjected to any loads, or only to very small loads,
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`in the axial direction. Therefore, the pressure is applied in particular radially. Apart from the
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`sealing of the housing which has already been mentioned the external diameter of the cell
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`housing can therefore also be calibrated.
`
`[0044]
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`It is particularly important for the heights of the casing areas of the cell cup and cell
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`top to be matched to one another such that the cut edge of the cell cup is pressed against the
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`casing area of the cell top by the pressure on the casing area of the cell cup. The heights of the
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`casing areas are therefore preferably chosen such that it is impossible to bend the cut edge of the
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`cell cup around inward over the edge area of the cell top which has been completely inserted into
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`the cell cup. Correspondingly, the edge of the cell cup is not beaded over the edge area of the
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`cell top. In consequence, the cell cup of a button cell manufactured using the method according
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`to the invention has a casing area with an essentially constant radius in the direction of the cut
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`edge.
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`[0045]
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`In the case of button cells produced using a method such as this, there is preferably an
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`exclusively force-fitting connection between the housing components comprising the cell cup,
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`the cell top and the seal. This ensures that the components are therefore held together in a
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`preferred manner, essentially only by static-friction force.
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`[0046]
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`Button cells without any beading over are particularly preferably produced using a
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`cell cup which is conical at least in one subarea of its casing, such that at least its internal
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`diameter increases in the direction of the cut edge. This makes it considerably easier to insert the
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`cell top into the cell cup. The dimensions of the cell cup and cell top are preferably matched to
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`one another such that relatively large opposing forces preferably do not occur until the top has
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`been inserted virtually completely into the cup. The cone angle in this case is preferably
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`between 10 minutes and 3°, in particular between 30 minutes and 1 ° 30 minutes.
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`[0047]
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`The cell top, which is inserted into the cell cup with the applied seal, is cylindrical, at
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`least in a part of the casing area, in preferred embodiments. This may relate in particular to that
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`part of the casing area which overlaps the conical subarea of the cell cup casing that has been
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`mentioned, after the cell top has been inserted into the cell cup. The casing of the cell top, and
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`therefore also the casing area, is particularly preferably entirely cylindrical. The cell top
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`therefore preferably has a constant external radius in the casing area. This may relate in
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`particular to that part which overlaps the conical part of the casing area of the cell cup after the
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`cell top has been inserted.
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`[0048] When a cell top with a cylindrical casing area is being inserted into a cell cup which
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`is conical at least in one subarea of its casing, as has been described above, a gap which is open
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`at the top is generally created between the cell cup and the cell top. This gap is generally closed
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`again by the pressure on the casing area of the cell cup. Thus, the pressure on the casing area of
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`the cell cup may be chosen such that the conical part of the casing area of the cell cup is pushed
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`inward until the inside of the cell cup and the outside of the cell top are essentially at the same
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`distance from one another in the overlapping area. The resultant button cell has casing areas
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`which are aligned parallel to one another, in particular in the overlapping area.
`
`[0049]
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`One important aspect in this case is the choice of the seal which connects the cell cup
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`to the cell top. The seal is preferably a plastic seal which connects the cell cup to the cell top.
`
`The seal is preferably a plastic seal composed of a thermoplastic.
`
`[0050]
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`The plastic seal is particularly preferably a film seal, for example as is described in
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`the already cited DE 196 47 593, in particular a film seal composed of a thermoplastic.
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`[0051]
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`Film seals can be produced with a very uniform thickness. When a suitable pressure
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`is applied to the casing area of the cell cup, this results in an interference fit, as a consequence of
`
`which the button cell that has been produced has highly excellent sealing characteristics. Not
`
`least, the use of film seals makes it possible to dispense with the edge of the cell cup being
`
`beaded over without this on the other hand resulting in a need to accept disadvantages in other
`
`important characteristics.
`
`[0052]
`
`It is very particularly preferable to use plastic seals, in particular plastic films, based
`
`on polyamide or based on polyether ether ketones in the present case.
`
`[0053]
`
`It is preferable for the seal for a cell which is not beaded over to have an initial
`
`thickness in the range between 50 µm and 250 µm, particularly preferably between 70 µm and
`
`150 µm, in particular about 100 µm. The term "initial thickness" is in this case intended to mean
`
`the thickness of the seal before it is applied to the casing of the cell top. In contrast to this, the
`
`term "final thickness" is intended to mean the thickness of the seal in the finished cell. It is clear
`
`that, at least in the overlapping area, this generally corresponds to the distance between the inside
`
`of the cell cup and the outside of the cell top.
`
`[0054]
`
`In order to allow a sufficiently large amount of friction to be produced between the
`
`cell cup and the cell top, both the external and internal radii of the cup and top should be
`
`matched to one another and to the thickness of the film seal. This is the only way to create a
`
`sufficiently high contact pressure to hold the two individual parts together. It is preferable for
`
`the parts used in this case for the difference between the external radius of the cell top, which is
`
`to be inserted into the cell cup, on the cut edge of the cell top and the smallest internal radius of
`
`the cell cup in that part of the casing area which overlaps the casing area of the cell top to be less
`
`than the initial thickness of the seal that is used. The difference is particularly preferably
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`15
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`VARTA Ex. 2032 Page 15 of 99
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`between 10% and 90% of the initial thickness, in particular between 30% and 70%, and very
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`particularly preferably about 50%.
`
`[0055]
`
`After the cell top has been inserted into the cell cup, a part of the casing area of the
`
`cell cup can be drawn radially inward. In particular, this relates to that part of the casing area
`
`which does not overlap the casing area of the cell top.
`
`[0056]
`
`It has been found that this process of drawing in radially makes it possible to achieve
`
`considerably better sealing characteristics. Drawing in the cup casing results in a radial pressure
`
`being exerted on the edge section which rests on the inner wall of the housing cup and on the
`
`seal which is arranged between the housing top and the housing cup, with the seal in
`
`consequence being compressed in this area.
`
`[0057]
`
`The drawing-in process can be carried out at the same time as the already mentioned
`
`exertion of pressure on the casing area of the cell cup, although the drawing-in process is
`
`preferably carried out in a subsequent, separate step.
`
`[0058]
`
`The method according to the invention for producing a button cell can be used in
`
`particular to produce button cells as have been described above, that is to say button cells having
`
`a housing with a flat bottom area and a flat top area parallel to it. It is suitable for producing not
`
`only button cells which are not beaded over, but also for those which are beaded over.
`
`[0059] With respect to the preferred embodiments of the individual components which are
`
`used in the method according to the invention (housing parts and dimensions, electrodes,
`
`separator etc.), reference can therefore be made to the above statements and explanations in their
`
`entirety.
`