The present invention relates to a capacitor and a method of manufacture thereof.

In capacitors like electrolytic tantalum or niobium capacitors, the cathode in form of an electrolyte is typically electrically contacted from the outside via an electrically conductive housing acting as cathode current collector, while the anode, e.g. in form of a sintered and anodized metal body, is contacted form the outside via an electrical feedthrough electrically insulated from the housing. For electrically contacting the cathode, such capacitors commonly utilize an electrically conductive coating applied on the inside of the capacitor’s housing, which can be wetted by the electrolyte. Electrolytic capacitors further comprise at least one separator element, e.g. in form of a pouch enveloping the anode, which provides an electric insulation between anode and housing or coating, respectively, in order to prevent an undesired shout circuit.

However, in such common electrolytic capacitor, the separator element usually is in direct substantial physical contact with the electrically conductive coating, exposing the coating potentially to mechanical stress applied by the separator element, e.g. in case of vibrations, shocks or mechanical pressure. In addition, such coatings exhibit rather inferior mechanical stability compared to, for example, the separator element or the housing. Consequently, the coating may be mechanical impaired or damaged by the separator element, e.g. by abrasion, which may result in short circuits and loss of the capacitor. In order to overcome the above problem, one solution known in the art is to adjust the thickness of the separators. However, this solution is rather unsure, and in addition, by using thicker separator elements, the electrical series resistance is increased, which in turn decreases the efficiency of the capacitor.

Accordingly, it is an objective of the present invention to provide means and method for improving the reliability of capacitors.

This objective is attained by a capacitor having the features of claim 1 and a method for manufacturing such capacitor with features of claim 10. Appropriate embodiments thereof are stated in the respective dependent claims and in the descriptions below.

According claim 1, a capacitor is provided. The capacitor comprises:

- an electrically conductive housing having a housing inner surface and a housing interior substantially defined by the housing inner surface,

- at least one first electrode having a first polarity and being arranged in the housing interior,

- at least one separator element being arranged between the housing inner surface and said at least one first electrode, and

- an electrically conductive coating being arranged between the housing inner surface and the at least one separator element.

According to the invention, it is particularly envisioned that the housing inner surface comprises at least one recess or depression, and the electrically conductive coating is arranged in the at least one recess or depression. Advantageously, the at least one separator element is not in substantial or is only in neglectable physical contact with the electrically conductive coating, wherein particularly the at least one separator element does not exercise physical stress on the electrically conducting coating, which can impair the integrity of the coating.

Advantageously, the reliability of the capacitor can be improved in general, and in addition, tribologically lower performing, but more costs and manufacturing efficient coatings may be used while keep the reliability of the capacitor. Particularly, such recess or depression in the housing inner surface may be provided by techniques such as machining, stamping, embossing, deep drawing or the like. Also possible are techniques such as etching or ablating, e.g. with a laser, in order to provide the depression. In this context, the skilled person will appreciate that such recess or depression may also be provide by arranging protruding elements such as beams, struts or like on the housing inner surface.

Particularly, the at least one recess or depression is limited by a protrusion, web, or bar, particularly a circumferential protrusion, web or bar, extending from the housing inner surface into the interior of the capacitor, wherein the at least one separator element is supported essentially by the protrusion, web or bar, particularly mechanically or physically supported. In other words, the at least one separator element is pressed essentially against the protrusion, web or bar in the assembled state.

In one embodiment, the capacitor of the invention is configured as an electrolytic capacitor, wherein a second electrode having a second polarity is configured as an electrolyte, and the electrolyte is in electrically conductive communication with the electrically conductive coating.

In one embodiment of the capacitor of the invention, the at least one recess or depression has a depth in range of 10 pm to 500 pm.

In one embodiment of the capacitor of the invention, the electrically conductive coating has a thickness in the range of 2 pm to 400 pm.

In one embodiment of the capacitor of the invention, the electrically conductive coating comprises electrically conductive carbon, particularly in form of graphite, graphene, activated carbon, charcoal, carbon black, a carbon nanotube or a fullerene, or a conductive polymer. In one embodiment, the electrically conductive coating comprises a binder. In one embodiment, the binder is selected from polyvinylidene fluoride (PVDF) polytetrafluoroethylene (PTFE), carbomethyl cellulose (CMC) or a rubber, particularly acryl rubber, nitrile butadiene rubber (NBR), styrene butadiene rubber (SBR) or butyl rubber

In one embodiment of the capacitor of the invention, the electrolyte is an aqueous electrolyte. In one embodiment, the electrolyte comprises ethylene glycol and optionally an acid, particularly boric acid or acetic acid, particularly in case of the anode is formed by tantalum. In one embodiment, the electrolyte comprises ethylene glycol, acetic acid and ammonium acetate. In one embodiment, the electrolyte comprises dimethylformamide, dimethylacetamide and/or gamma-butyrolactone. In one embodiment, the electrolyte comprises tetracyanoquinodimethane, polypyrrole, or poly(3,4-ethylenedioxythiophene).

In one embodiment of the capacitor of the invention, the at least one first electrode comprises or essentially consists of a valve metal. In one embodiment, the valve metal is selected from tantalum or niobium. In one embodiment, the capacitor of the invention comprises one first electrode, wherein the first electrode substantially is a body of sintered and anodized tantalum. In one embodiment, the body of sintered and anodized tantalum is substantially enclosed by one separator element, being particularly designed as a pouch.

In one embodiment of capacitor of the invention, the housing comprises or is formed by a beaker portion and a lid portion, wherein the at least one recess or depression is comprised within an inner surface of the beaker portion or the lid portion. In one embodiment, the lid portion comprises an electrical feedthrough configured to electrically contact the at least one first electrode, and optionally an opening for filling in the electrolyte.

In one embodiment of the capacitor of the invention, the housing is made of titanium or a titanium alloy.

In one embodiment of the capacitor of the invention, the housing comprises a plurality of recesses or depressions, each of which comprising the electrically conductive coating.

In one embodiment of the capacitor of the invention, the at least one separator element comprises or essentially consists of cellulose (paper), polyethylene, polypropylene, polytetrafluorethylene, polyvinylidene fluoride, ethylene chlorotrifluoroethylene, or polysulfone. According to claim 10, a method for manufacturing a capacitor is provided, particularly for manufacturing the capacitor of the invention as described above. The method comprises the steps of:

- providing an electrically conductive housing having a housing inner surface and a housing interior substantially defined by the housing inner surface, wherein at least one depression or recess is provided in the housing inner surface,

- applying an electrically conductive coating in the at least one recess or depression,

- arranging at least one first electrode in the housing interior, and

- arranging at least one separator element between the at least one first electrode and the housing inner surface.

Particularly, at least one recess or depression is limited by a protrusion, web, or bar, particularly a circumferential protrusion, web or bar, protruding from the housing inner surface into the interior of the capacitor, wherein arranging at least one separator element between the at least one first electrode and the housing inner surface includes that the at least one separator element is supported essentially by the protrusion, web or bar, particularly mechanically or physically supported or, in other words, the at least one separator element is pressed essentially against the protrusion, web or bar in the assembled state.

In one embodiment of the method of the invention, the at least one recess or depression is provided in said housing inner surface by deep drawing, etching, ablating, machining, stamping or embossing. In one embodiment, a plurality of recesses or depressions is provided in the housing inner surface, and the electrically conductive coating is applied in each of the plurality of recesses or depressions.

In one embodiment of the method of the invention, the electrically conductive housing is provided in form of a beaker portion and a lid portion. The beaker portion may be provided by suitable techniques such as machining or deep drawing.

In one embodiment of the method of the invention, the beaker portion and the lid portion are joined, particularly welded, particularly after assembly of the other above-mentioned components of the capacitor of the invention, e.g. the first electrode and/or the at least one separator element, in the housing.

In one embodiment, the lid portion is provided with an electrical feedthrough configured to electrically contact the at least one first electrode electrically insulated from the housing, and optionally an opening for filling-in an electrolyte.

Further advantages, features and embodiments of the present invention will be explained hereinafter with reference to the drawings, in which:

Fig. 1 shows a schematic drawing of a housing and an electrically coating of a prior art capacitor;

Fig. 2 shows a schematic drawing of a housing and an electrically coating according to the invention.

Fig. 3 shows an embodiment of the capacitor of the invention.

Figure 1 illustrates a prior art electrolytic capacitor, more precisely a part of an electrically conductive housing 10 that acts as a current collector for the cathode of the capacitor. For contacting the cathode (electrolyte), an electrically conductive coating 40 comprising, e.g. activated carbon and suitable binder, is applied on an inner surface 11 of the housing 10. In this configuration, a separator 30 enveloping the anode (not shown in Figure 1) may exercise substantial physical stress on the electrically conductive coating 40 and, thus, may damage the latter, e.g. by abrasion.

In contrast to the above, the capacitor of the invention comprises an electrically conductive housing 10, e.g. made of titanium, and having one or more recesses or depression 12, in which the electrically conductive coating 40 is applied, as shown in Figure 2. Thereby, the one or more recesses or depressions 12 are limited or confined by bars or webs 13. As can be seen in more detail in Figure 3, the separator 30 is substantially (mechanically or physically) supported by the bars or webs 13 but not or only in a neglectable amount by the electrically conductive coating 40. By that, the separator 30 may exercise a substantial physical stress on the bars or webs 13, but not on the electrically conductive coating 40. Accordingly, damage of the electrically conductive coating 40 by the separator 30 may be reduced or inhibited

Such recess or depression 12 may be provided in a beaker portion and/or a lid portion of the housing 10 by for example deep drawing, stamping or embossing, or alternative or additionally, by laser ablating or etching. After forming of the recess(es) or depression(s) 12, the electrically conductive coating 40 is applied in the recess(es) or depression(s) 12.