WO2005119814A2

WO2005119814A2 - Method for producing a battery case, a battery case, and metal strip for producing said battery case

Info

Publication number: WO2005119814A2
Application number: PCT/EP2005/006073
Authority: WO
Inventors: Arnoud Cornelis Adriaan De Vooys; Lene Hviid; Manja Boon; Catharina Hillegonda Maria De Moel
Original assignee: Hille & Müller GMBH
Priority date: 2004-06-04
Filing date: 2005-06-03
Publication date: 2005-12-15
Also published as: WO2005119814A3; EP1756888A2

Abstract

The invention relates to a method for producing a battery case comprising the steps of providing a strip of a first metal having a first surface and a second surface; providing said strip on the first surface with a layer of a second metal; providing said strip on the second surface with an electrically insulating polymer layer; forming a battery case from said strip in one or more forming steps wherein said battery case has an inner surface and an outer surface and wherein the first surface of the strip is turned to the inside of the battery case and the second surface of the strip is turned to the outside of the battery case. The invention further relates to a battery case produced with said method and a battery produced with said battery case. The invention further relates to a method for producing a strip.

Description

METHOD FOR PRODUCING A BATTERY CASE, A BATTERY CASE, AND METAL STRIP FOR PRODUCING SAID BATTERY CASE

The invention relates to a method for producing a battery case. The invention further relates to a battery case produced with said method and a battery produced with said battery case. The invention further relates to a method for producing a strip. In recent years, the demand for batteries has increased considerably, for instance for use in portable devices. These batteries comprise a metal battery case, an electrolyte, a cathode and an anode. Most battery cases have an electrically insulating layer on the outside to minimise the risk of shortcuts and associated catastrophic failures. In known batteries this electrically insulating layer is typically provided by means of a sleeve which is shrunk around the battery case, by means of a stick-on label or by means of a paint layer. The electrically insulating layer is often also used as identification means to identify the producer or the brand of said batteries. A commonly used method is imprinting the brand name or producer on the sleeve which is shrunk around the battery case, on the stick-on label or by painting it on the case. The application of a shrink sleeve requires the application of the sleeve over the battery case or battery followed by a treatment to shrink the sleeve. In addition, the shrink sleeves are quite expensive as well, adding to the cost of a commodity product. Since the external diameter is determined by the original equipment manufacturers and is fixed, this leaves less space for the electro-active materials and therefore results in a decrease of battery capacity. When using a stick-on label instead of a shrink sleeve, the internal diameter of the battery decreases as well. Also, in order to avoid that the label is applied incorrectly or skewed, a very precise process control is required. These arguments become even more important when the sleeve or label overlap to cover the battery continuously, as in the overlap zone the thickness of the sleeve or label is doubled. An additional disadvantage of using a shrink sleeve or a stick-on label is the risk of counterfeiting. It is possible to apply such a sleeve or label to batteries of inferior quality thereby passing for higher grade or higher quality batteries. The problem of the increased diameter of the battery when using shrink sleeves as identification means can be solved by painting the identification directly on the metal of the battery case. This however has the disadvantage that when the paint is damaged for instance as a result of wear or scratching, the electrical insulation of the battery is compromised. The application of the paint also necessitates an additional process step in the manufacture of batteries. It is an object of this invention to provide a method for producing battery cases without the need of an additional process step to apply the electrically insulating layer. It is another object of this invention to provide a method for producing battery cases to minimise the possibility of counterfeiting the identification of the brand or producer of the battery after it has been produced. It is also an object of this invention to provide a method for producing battery cases with a reduced increase in thickness as a result of the electrically insulating layer. According to the invention, one or more of these objects are reached with a method for producing a battery case comprising the steps of providing a strip of a first metal having a first surface and a second surface; - providing said strip on the first surface with a layer of a second metal; - providing said strip on the second surface with an electrically insulating polymer layer; - forming a battery case from said strip in one or more forming steps wherein said battery case has an inner surface and an outer surface and wherein the first surface of the strip is turned to the inside of the battery case and the second surface of the strip is turned to the outside of the battery case. According to the invention, a battery case or container is obtained having a layer of a second metal on at least the inside of the case, and a polymer layer on at least the outside of the case. This layer of a second metal should be corrosion resistant to the contents of the battery when the battery case is used in a battery. The forming steps may also comprise a blanking step in which a blank is formed from said strip of a first metal (hereinafter referred to in this description as metal strip or strip) and wherein the battery case is formed from said blank. The polymer layer is electrically insulating and is applied on the metal strip before forming a battery case from said metal strip. Consequently, the insulating layer does not need to be applied to the individual batteries or battery cases one by one. Since a polymer layer is provided on the metal strip, the thickness of the metal strip increases only with the thickness of the polymer layer. The polymer layer may be provided to the metal strip by means of techniques such as roll-coating, laminating or co-extrusion. This ensures a consistent quality of the metal strip comprising the polymer layer and thereby a consistent quality of the battery case. Another advantage of the invention is that the second metal can be chosen independently of the first metal. In case the battery case consists essentially of a first metal, the corrosion resistant properties of the second metal can be chosen independently from the properties of the first metal, which may be optimised for its forming properties. In the case the battery case consists essentially of a formable metal (which may also be an alloy), the forming properties and mechanical properties such as strength of the first metal are combined with the corrosion resistance of the layer of second metal which is present upon the first metal forming the battery case. During conventional forming of a battery case from a metal strip, for instance by a draw-and-redraw (DRD) method, a draw-and-iron (Dl) method or a draw-and-thin- redraw (DTR) forming method, said strip having a layer of a second metal on its second surface, lubrication during forming is required to facilitate the forming process and to reduce the wear of the forming tools. Often an oil-emulsion is used for lubrication. After use, this emulsion has to be treated as chemical waste. Also, the oil needs to be removed from the battery case, which necessitates a cumbersome and expensive cleaning operation. During forming of a battery case according to the invention, the presence of the polymer layer enables to use a small amount of wax as lubricant which does not have to be treated as chemical waste. Moreover, the wax can be removed during a simple heating operation, which could be included for other reasons as well. Cooling, if required, can be achieved by water, which is easily removed from the case and disposed of after use. There is no limit to the shape of the battery case. Known types such as AAA, AA, A, B, C, D size cylindrical battery cases for cylindrical batteries or prismatic battery cases for prismatic batteries can be easily produced. The battery case, when used in a battery, is able to function at least as an electrical current collector, or otherwise enables transport of electrical current to one of the electrical contact positions of the battery such as the anode or the cathode. In an embodiment of the invention the polymer layer is a thermoplastic polymer layer, preferably comprising PP and/or PET. If the polymer is also present on the inner surface of the battery case, it is preferable that the polymer is resistant to the electrolyte in the battery. It may be preferable that the polymer layer comprises a plurality of layers, for example an adhesion layer for adhesion to the battery case and a top layer. In an embodiment of the invention the first metal is steel or a steel alloy suitable for forming operations, e.g. a low carbon steel, an extra-low carbon steel, an ultra-low carbon steel, an interstitial free steel, a microalloyed steel or a high strength steel. In table 1 , suitable ranges for these steel alloys are given (all percentages in weight percent).

In an embodiment of the invention the layer of the second metal essentially consists of nickel. Nickel has excellent corrosion resistance properties and is able to resist the contents of a battery when the battery case is used in a battery. In an embodiment of the invention said layer of a second metal is applied to the strip of a first metal by means of a plating step, wherein preferably the second metal is chosen from the group of metals comprising one or more of nickel, cobalt, iron, tin, indium, palladium, bismuth and/or alloys thereof. This metal layer may also comprise other metals such as Au, Ir, Rh, Zn, B, Fe, Si, Ge, Ln, Ti, P, Ga and their alloys and possibly combined with embedded electrically conducting substances such as graphite, soot, titaniumsulfide, titaniumnitride etc, or combinations thereof, as disclosed in EP1142041-B1. In an embodiment of the invention, the inner and the outer surface are provided with a layer of the second and a third metal respectively. These metal layers may protect the battery case against the electrolyte and/or it may serve as an adhesion layer to improve the adhesion between the metal of the battery case and the polymer layer. In an embodiment of the invention, the second metal and the third metal is the same metal. In an embodiment of the invention, the layer of the second and the third metal essentially consists of nickel because the inventors surprisingly found that a nickel-based coating provides excellent adhesion for the polymer layer. In this embodiment no additional process step of modification of the metal strip with for instance an electrolytic chromium coating is required, thus reducing costs. In an embodiment of the invention the layer of a second metal on the first surface of the strip comprises a brittle metal layer, such as a bright nickel layer and a cobalt or cobalt alloy layer deposited thereupon as disclosed in German patent application 103 16 612.2, which is included herein by reference. The advantage of having this brittle metal layer on the inner surface of the battery case is that the brittle metal layer gets cracked during the forming of the battery case resulting in an increased roughness and surface increase of the inner surface. This higher roughness decreases the contact resistance between the active cathode material and the inner side of the battery case, thereby improving the performance of the battery produced from said battery case. It should therefore be understood that on top of the layer of the second metal additional layers may be provided. These additional layers may be polymer layers such as a PET or PP, or metal layers chosen from the group as disclosed hereinabove. In an embodiment of the invention said layer of a second metal is applied to the strip of a first metal by non-electrochemical methods, for instance by Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD) or as disclosed in WO04/002634-A1. In an embodiment of the invention the battery case is heated at least once to heat-treat the polymer layer, optionally to above the melting temperature of the polymer layer, for instance between two forming steps and/or after the last forming step of the battery case. This embodiment enables the polymer layer to recover from previous forming steps. This heating of the battery case is preferably provided by induction heating, thereby indirectly heating the polymer layer. In an embodiment of the invention, the outer surface of the battery case is provided with an imprint. This imprint may be provided with known printing methods. The battery case thus produced does not need an additional shrink sleeve or label to identify the brand or producer and is therefore protected against counterfeiting or rebranding. Also, since the polymer layer with the imprint is thinner than a shrink sleeve, the thickness of the battery case is reduced, thereby allowing a larger internal diameter of the battery produced from said battery case and thus potentially increasing the battery's capacity. The imprint may be provided after forming of the battery case, but it may also be provided on the second surface of the strip prior to forming of the battery case. In the latter case, the shape of the imprint has to be given such a shape so as to take the deformation during forming of the battery case into account so as to form the desired imprint of the proper final dimensions after forming of the battery case. The imprint may also be provided to the second surface of the strip prior to the application of the polymer layer, whereby the imprint is covered by the polymer layer. The imprint may have a decorative function, but it may also have an identification function. Identification marks such as a bar code could be imprinted on the batter case for example for identifying the batch or producer of the battery. The electrically conducting contact at the location of the electrical contact positions when using the battery case in a battery may be provided by locally removing the polymer layer of the outer surface of the battery case or battery. This removal of the polymer layer may be done by known methods such as mechanical grinding or chemical dissolving. In an embodiment of the invention a method is provided for producing a battery case comprising at least one electrical contact position and wherein the polymer layer is removed at the location of the contact position by using a laser for enabling electrical contact. The inventors found that it is beneficial to use a laser to locally remove the polymer layer of the outer surface of the battery case or battery. No solid or liquid wastes are produced, which has environmental and economical advantages. In addition, the surface of the battery case is not oxidised, as the removal takes place in an effective reducing atmosphere, leading to a lower contact resistance and better battery performance. Also an advantage is that the removal of the polymer layer creates a carbon rich environment, which allows the formation of surface carbides on the electrical contact positions, which in turn are electrically conductive, less prone to oxidation and relatively brittle so a good contact is more easily made. Also, since the metal strip has a high heat capacity, a high heat conductivity and a high reflectivity, the effect of the laser on the surface of the metal strip is minimal and the process is more stable, which is also beneficial for geometrical accuracy. According to a second aspect of the invention there is provided a battery case produced as described hereinabove, having an inner surface and an outer surface, wherein at least the inner surface of the case is provided with the layer of the second metal and wherein at least the outer surface is provided with the polymer layer which is electrically insulating. The polymer layer could form the outermost surface of the battery case, or could be coated. This additional coating or coatings can be applied by any known method, such as varnishing, lacquering, printing or a combination thereof, and said additional coating or coatings could cover the entire outer surface of the battery case or part of the outer surface of the battery. In an embodiment of the invention, the inner and the outer surface are provided with a metal plating layer. This metal layer may protect the metal of the battery case against the electrolyte and/or it may serve as an adhesion layer to improve the adhesion between the metal of the battery case and the polymer layer. In an embodiment of the invention, the metal in the metal plating layer on the inner and the outer surface is the same metal. In a further embodiment of the invention, the layer of a second metal essentially consists of nickel because the inventors surprisingly found that a nickel-based coating provides excellent adhesion for the polymer layer. In an embodiment of the invention the polymer layer on the battery case comprises PET and/or PP. In a preferred embodiment of the invention the layer of a second metal essentially consists of nickel and/or the polymer layer comprises PET and/or PP. In a preferred embodiment of the invention at least the polymer layer on the outer surface of the battery case is provided with an imprint. This imprint may be provided with known methods. The battery case thus produced does not need an additional shrink sleeve to identify the brand or producer and is therefore protected against counterfeiting or rebranding. Also, since the polymer layer with the imprint is thinner than a shrink sleeve, the thickness of the battery is reduced. The imprint may be provided after forming of the battery case, but it may also be provided on the second surface of the strip prior to forming of the battery case. The imprint may have a decorative function, but it may also have an identification function. Identification marks such as a bar code could be imprinted on the battery case for example for identifying the batch or producer of the battery. The electrically contact position or positions of the battery may be provided by removing the polymer layer of the outer surface of the battery case at the location of the contact position or positions for enabling electrical contact. In an embodiment of the invention the metal of the metal strip is steel or a steel alloy suitable for forming operations, e.g. a low carbon steel, an extra-low carbon steel, an ultra-low carbon steel, an interstitial free steel, a microalloyed steel or a high strength steel (see table 1 for typical ranges).

The battery case as described hereinabove is particularly suitable for batteries of the alkaline type or of the rechargeable type such as the Ni-Cd or metal-hydride type. In the case of a Zn-carbon battery, the first metal and the second metal are the same metals, namely zinc. The outer surface of the Zn-battery case may then be provided with the electrically insulating polymer coating. The Zn-battery case may be enveloped by a thin sleeve of tinplate to protect the Zn-battery case against denting. In an embodiment of the invention there is provided a battery produced using a battery case as described hereinabove comprising at least one electrical contact position such as an anode or a cathode, wherein the polymer layer has been removed at the location of the contact position for enabling electrical contact. According to a third aspect of the invention there is provided a method for producing a strip comprising the steps of - producing a strip of a first metal, said strip having a first surface and a second surface; providing said strip with a layer of a second metal on the first surface, preferably wherein the second metal essentially consists of nickel; performing recrystallisation annealing of said strip; - coating said strip on the second surface with an electrically insulating polymer layer, preferably comprising PET and/or PP, preferably by co-extrusion or laminating rolling; so as be able to produce battery cases as described hereinabove. The strip is provided with a layer of the second metal on the first surface wherein the layer preferably essentially consists of nickel. The strip may also be provided with a layer of a third metal on the outer surface of the battery case, which is subsequently coated with the polymer layer. In an embodiment of the invention the third metal and the second metal are the same, effectively coating the inner and the outer surface of the battery case with the same metal, preferably a nickel-based coating essentially consisting of nickel. Preferably, the polymer layer is resistant against the electrolyte in when the battery case produced is used in a battery. Preferably, the first metal is a steel or a steel alloy suitable for forming operations, e.g. a steel selected from the steel types as described hereinabove. The strip of a first metal may be produced according to known methods, such as steel- making followed by hot-rolling. The hot-rolled strip is then cold-rolled to a final cold- rolling thickness. In an embodiment of the invention the layer of a second metal on the first metal is provided by means of a plating step. This way a very thin and homogeneous layer of a second metal may be applied to the first metal. Preferably the recrystallisation annealing step takes place after the provision of the layer of a second metal on the first metal and prior to coating said strip on the second surface with a polymer layer. This way a diffusion layer builds up in the nickel layer providing excellent adhesion to the first metal. A specific embodiment of the present invention will now be explained by the following non-limitative examples. The first metal that was used as a substrate was a low-carbon diffusion annealed nickel coated steel commonly used for battery cases. It is provided by Corus Special Strip, and available under the brand name Hilumin^®. The substrate was coated with PP or PET by either laminar rolling or co-extrusion. AA-size cases were made from this material by deep drawing in one cupping step and 5 redraw steps. Prior to the drawing operations both surfaces were coated with an appropriate wax for lubrication. Between the third and fourth redraw step the cases were heat treated to above the melting point of the polymer, and quenched in water. After deformation, the outer surface of the cases were imprinted with ink and cured in either an UV-curing step or a temperature curing step and proper adhesion of both the polymer and the print was confirmed. It is also possible to imprint the outer surface of the battery case with a curable printing system comprising an ink, a base coat, an over-print varnish etc, or suitable combinations thereof. It is even possible to use a suitable printing system which does not require an additional curing step, providing the adhesion between the printing system and the battery case is adequate. The cases produced in this example showed excellent adhesion of the polymer layer to the metal strip prior to and after forming of the battery case. Also, the polymer layer proved to provide an excellent basis for imprinting. It is of course to be understood that the present invention is not limited to the described embodiments and examples described above, but encompasses any and all embodiments within the scope of the description and the following claims.

Claims

1. Method for producing a battery case comprising the steps of providing a strip of a first metal having a first surface and a second surface; providing said strip on the first surface with a layer of a second metal; providing said strip on the second surface with an electrically insulating polymer layer; forming a battery case from said strip in one or more forming steps wherein said battery case has an inner surface and an outer surface and wherein the first surface of the strip is turned to the inside of the battery case and the second surface of the strip is turned to the outside of the battery case.

2. Method according to claim 1 wherein the polymer layer is a thermoplastic polymer layer, preferably comprising PP and/or PET;

3. Method according to claim 1 or 2 wherein said layer of a second metal is applied to the strip of a first metal by means of a plating step, wherein preferably the second metal is chosen from the group of metals comprising one or more of nickel, cobalt, iron, tin, indium, palladium, bismuth and/or alloys thereof.

4. Method according to any one of the claims 1 to 3 wherein the battery case is heated at least once to heat-treat the polymer layer, optionally to above the melting temperature of the polymer layer, for instance between two forming steps and/or after the last forming step of the battery case.

5. Method according to any one of the claims 1 to 4 wherein the outer surface of the battery case is provided with an imprint.

6. Method according to any one of claims 1 to 5 wherein the battery case comprises at least one electrical contact position and wherein the polymer layer is removed at the location of the contact position by using a laser for enabling electrical contact.

7. Battery case produced according to any one of the claims 1 to 6, having an inner surface and an outer surface, wherein at least the inner surface of the case is provided with the layer of the second metal and wherein at least the outer surface is provided with the polymer layer which is electrically insulating.

8. Battery case according to claim 7 wherein the layer of a second metal essentially consists of nickel and/or wherein the polymer layer comprises PET and/or PP.

9. Battery case according to claim 7 or 8 wherein at least the polymer layer on the outer surface of the battery case is provided with an imprint.

10. Battery case according to any one of claims 7 to 9, said battery case comprising at least one electrical contact position, wherein the polymer layer has been removed at the location of the contact position by using a laser for enabling electrical contact.

11. Battery comprising a battery case according to any one of claims 7 to 10 said battery comprising at least one electrical contact position such as an anode or a cathode, wherein the polymer layer has been removed at the location of the contact position by using a laser for enabling electrical contact.

12. Method for producing a strip comprising the steps of producing a strip of a first metal, said strip having a first surface and a second surface; - providing said strip with a layer of a second metal on the first surface, preferably wherein the second metal essentially consists of nickel; performing recrystallisation annealing of said strip; coating said strip on the second surface with an electrically insulating polymer layer, preferably comprising PET and/or PP, preferably by co- extrusion or laminating rolling. so as to be able to produce battery cases according to any one of the claims 1 to 10.

13. Method according to claim 12 wherein the layer of a second metal on the first metal is provided by means of a plating step.

4. Method according to claim 12 or 13 wherein the recrystallisation annealing step takes place after the provision of the layer of a second metal on the first metal and prior to coating said strip on the second surface with a polymer layer.