Contact element coated by electron beam evaporation of a steel strip with nickel for use in an alkaline battery

TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to a contact element of an alkaline battery having a water-based alkaline electrolyte, said element being made of a material from a metal strip product.

Any type of contact element used in an alkaline battery having a water-based electrolyte is comprised. A current collector adapted to conduct current from an anode of one cell to the cathode of an adjacent cell of a said battery having battery cells connected in series and a so-called contact in such a battery may be mentioned as examples.

The battery in which said contact element is arranged may be any type of alkaline battery having a water-based alkaline electrolyte, especially such batteries being rechargeable and in particular batteries of battery driven tools, electrical wheel chairs, motor bikes, cars and the like. Examples of such batteries are rechargeable Ni-metal hydride batteries having a cathode of Ni(OH) ₂ and an anode of a metal hydride.

The feature that the contact element is made of a material from a metal strip product indicates that it is a question of comparatively thin contact elements.

Such known contact elements are made of Ni, since no other element or compound suitable to be used as a contact element is able to only deliver a current therethrough in a said water- based electrolyte of an alkaline battery without producing hydro- gen gas.

Nickel has become a very costly element, so that it has been an aim to make such contact elements as thin as possible. However, there is a lower thickness limit, mostly in the range of about 40 μm - 50 μm, below which a strip of Ni will be too flabby and easily folded when handled, such as when assembling such batteries. Furthermore, such contact elements used as current collectors are in some batteries subjected to pressures resulting in comparatively high mechanical loads, so that they have to have a certain stiffness requiring a corresponding thickness thereof.

A contact element according to the preamble of appended claim 1 is known through US 5 069 989 A1 and EP 0 175 149 A2.

SUMMARY OF THE INVENTION

The object of the invention is to provide a contact element of an alkaline battery according to the preamble of appended claim 1 , which may be manufactured to a lower cost compared to such contact elements already known while maintaining electrical properties and mechanical stability.

This object is according to the invention obtained by providing such a contact element, in which said strip substrate has a tensile strength of at least 1300 MPa, the strip substrate has been bright cold rolled and the surface roughness Ra of said at least one coating layer of Ni is max 0.2 μm, preferably max 0.1 μm, most preferred max 0.06 μm.

It is pointed out that a side of said contact element which may come into contact with the water-based alkaline electrolyte of said alkaline battery has of course to be coated by Ni, but when said contact element is for instance an end plate in a stack of battery cells it would be sufficient to have only one of the two large sides thereof coated by a coating layer of Ni.

Thanks to the use of a said steel substrate a high stiffness of the contact element may be obtained without any need to make the layer of Ni thick for contributing to this stiffness, but it is sufficient to select such a thickness of the Ni coating layer that it will reliably completely cover the steel substrate so as to prevent this to enter into contact with a said electrolyte. This means that the thickness of the Ni coating layer may be chosen to be several times lower than in known contact elements of this type being made of pure Ni and by that considerable costs may be saved.

Thanks to the high tensile strength of the steel of the strip sub- strate of a contact element according to the invention the thickness of this contact element may be kept small and a stiffness making it easy to handle such contact elements when assembling a battery and making it able to withstand high pressures possibly applied thereon in a said battery may nevertheless be obtained.

The bright cold rolling of said strip substrate used for the contact element results in a possibility to apply a coating layer of Ni thereon with a small surface roughness reducing the thickness of such a layer necessary for reliably cover the entire surface of the steel substrate. The small surface roughness of said at least one coating layer of Ni means that this coating layer may be given a thickness in the order of 1 μm while reliably totally covering the steel substrate.

According to an embodiment of the invention said substrate is made of steel with a tensile strength of min 1500 MPa, preferably min 1700 MPa and most preferred min 1900 MPa. Such a high tensile strength of the substrate steel makes it possible to make the contact element thin and still obtain a high mechanical stability thereof.

According to another embodiment of the invention said steel substrate has a thickness of 20-200 μm, preferably 30-100 μm, which results in a contact element having a desired high mechanical stability without any contribution thereto from other layers than said steel substrate thanks to the high tensile strength of the latter.

According to another embodiment of the invention said at least one side of the substrate is coated by a coating layer of Ni having a thickness of 0.5 - 10 μm, preferably max 5 μm, more preferred max 2 μm and most preferred 0.5-1 .5 μm. It has turned out that such a small thickness of said Ni coating layer is suffi- cient to with a high reliability completely cover the steel substrate, which results in a considerable reduction of Ni required for such a contact element with respect to contact elements already known and by that a possibility to substantially reduce the costs of such a contact element.

According to another embodiment of the invention said strip substrate is made of a steel with a Cr content of min 10%, preferably min 12%, more preferred min 13% and most preferred min 15%, which means that the substrate material will have a good general corrosion resistance, which is a preferred feature of a substrate used for such a contact element.

According to another embodiment of the invention both sides of the strip substrate are coated by a said coating layer of Ni. As already mentioned, this is a requirement if both said sides may come into contact with said water-based electrolyte of the bat-

tery to which the contact element belongs, such as when the contact element is a current collector adapted to conduct current from an electrode of one cell to the electrode of an adjacent cell of a said battery, which constitutes another embodiment of the invention.

According to another embodiment of the invention the contact element is adapted to be applied with one side thereof in direct contact to a said electrode of a said one cell and the opposite side thereof in direct contact to said electrode of a said adjacent cell for conducing charge carriers transversely therethrough from one cell to the other, in which it is preferred that said contact element is thin for reducing the electrical impedance of the contact element, at the same time as it may be desired to have a comparatively high stiffness of a contact element in the form of such a bipolar current collector should it be subjected to high mechanical loads, such as for participating in sealing adjacent battery cells with respect to each other.

The invention also relates to a use of a metal strip product comprising a substrate of steel with a tensile strength of at least 1300 MPa and on at least one side thereof coated by at least one coating layer of Ni for producing a contact element according to the invention. Such a contact element of an alkaline bat- tery having a water-based electrolyte, especially a current collector, may then be produced.

The invention also relates to a use of a contact element according to the invention as current conducing element in an al- kaline battery having a water-based alkaline electrolyte, and such a use has the advantages mentioned above when discussing a contact element according to the present invention, and a use of the contact element as a current collector for conducting current from an anode of one cell to a cathode of an adjacent cell of a said battery is particularly preferred and constitutes another embodiment of the invention.

The invention also relates to an alkaline battery having a water- based alkaline electrolyte, which is characterized in that is comprises at least one contact element according to the invention. One important feature of such a battery is that the use of a contact element according to the present invention opens up for a possibility to reduce the costs of such a battery.

According to embodiments of the invention said battery is a Ni- metal hydride battery, and it is a rechargeable battery having a capacity of 3-200 Ah, preferably at least 7 Ah.

Further advantages as well as advantageous features of the invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows a schematic cross-section of a contact element according to an embodiment of the invention,

Fig 2 shows schematically a production line for manufacturing of a metal strip product in the form of a coated metal strip material to be used for manufacturing a contact element according to the invention,

Fig 3 shows very schematically the construction of an alkaline battery provided with contact elements according to the invention in the form of current collectors, and

Fig 4 is a graph of current versus potential in a comparing electrochemical corrosion testing of a Ni-foil and a Ni-coated steel substrate foil according to the invention.

It is pointed out that some features have been exaggerated in the Figures to illustrate the invention more clearly. Therefore, the Figures 1 -3 should not be considered drawn to scale.

DETAILED DESCRIPTION OF THE INVENTION

Fig 1 shows a steel strip substrate 2 which is coated with a metallic coating layer 1 , 3 of Ni on both sides of the steel strip. The thickness of the coating of the two sides may be the same or different. This steel strip is to be used for producing a contact element of an alkaline battery having a water-based alkaline electrolyte, and in the case that this contact element is a so called end plate in a stack of battery cells or otherwise having only one side thereof exposed to said electrolyte, it is possible to only coat one side thereof by a said coating layer of Ni.

Nickel is used for contact elements of alkaline batteries having a water-based electrolyte, since presently no other element or compound suitable to be used as a contact element is known, which is able to only deliver a current therethrough in a said water-based alkaline electrolyte without generating hydrogen gas through electrolyte reduction, which would be detrimental to the function of the battery due to drying-out of the electrolyte.

By applying a thin layer of nickel onto a steel substrate, as for the contact element according to the present invention, it is possible to combine the unique properties of nickel according to the above with the excellent and versatile mechanical bulk proper- ties of steel, and especially to use only a fraction of the amount of nickel for such a contact element otherwise necessary for obtaining a required mechanical stability of the contact element if it were made of pure nickel. This results in a possibility to reduce the material costs of such a contact element considerably.

Physical Vapor Deposition, PVD, is a very suitable technology that is able to produce coatings of nickel in both a qualitative and productive manner. The coatings may be produced through both sputtering and evaporation routes, however, evaporation would be the most suitable method for high-rate production.

A contact element thus produced is suitable for use as current collector in alkaline batteries, especially as bipolar current collector applied with one side thereof in direct contact to an anode of a battery cell and the opposite side thereof in direct contact to a cathode of an adjacent battery cell in a rechargeable battery for conducting current transversely therethrough from one cell to the other. Such a battery is preferably a Ni-metal hydride battery. The contact element may also be a contact in an alkaline battery and/or a contact end plate of a stack of said battery cells of such a battery.

The substrate steel material to be coated has preferably a good general corrosion resistance, which means that the material has a chromium content of at least 10% by weight, preferably mini- mum 12%, more preferred minimum 13% and most preferred minimum 15% chromium. Furthermore, the material must be alloyed in a way that allows for a high tensile strength, which shall be at least 1300 MPa, more preferred minimum 1500 MPa, preferably minimum 1700 MPa and most preferred minimum 1900 MPa. The mechanical strength may be achieved by cold deformation, such as for steels of the ASTM 200 and 300 series, or by thermal hardening as for hardenable martensitic chromium steels, e.g. certain ASTM 400 series steels. Other suitable substrate materials are precipitation hardenable (PH) steels of the type 13-8PH, 15-5PH, 17-4PH and 17-7PH. Yet another group of suitable substrate materials are stainless maragin steels that are characterized by their low carbon- and nitrogen-containing martensitic matrix that is hardened by the precipitation of substitutional atoms, such as copper, aluminium, titanium, nickel etc.

Hardenable carbon steel such as ASTM 1095, ASTM 1074, ASTM 1055 or equivalent, are also conceivable as material for the steel substrate.

Examples of suitable substrate materials and their compositions are listed in Table 1 below.

Table 1 .

The coating layer consists of pure Ni and has a thickness of 0.5- 10 μm, in which it is necessary that it has a thickness being enough for reliably completely covering the entire surface of the steel substrate that may be in contact with said water-based electrolyte. The thickness necessary for this is dependent on the surface roughness R ₃ of the coating layer. It is an aim to make said coating layer as thin as possible while obtaining said reliable covering and by that saving costs for nickel. The lesser roughness the thinner the Ni-coating may be made, and it has preferably a thickness of max 5 μm, more preferred max 2 μm and most preferred a thickness of 0.5-1 .5 μm. It has been found to be advantageous to for this sake have a surface roughness R ₃ of said coating layer of Ni being max 0.2 μm, preferably max 0.1 μm, most preferred max 0.06 μm, and such a low surface

roughness of the coating layer is preferably obtained by bright cold rolling the strip substrate used. Bright cold rolling is here defined as a rolling process resulting in a surface roughness R ₃ being lower than 0.10 μm. A person with skill in the art knows how to carry out such rolling for obtaining such fine surface roughnesses.

The coating process for obtaining the metal strip product to be used for manufacturing a contact element according to the in- vention will now be described with reference made to Fig 2. The coating process is carried out in a roll-to-roll strip production line, which is an advantageous solution for high-rate production of coated stainless steel strip. This stainless steel strip has preferably a thickness in the range of 20-200 μm, preferably 30- 100 μm, and it depends on the mechanical stability requirements of a contact element to be manufactured from said steel strip. As mentioned, it has preferably previously been bright cold rolled, and it is now provided by a roll 6 delivered as a metal strip product 20 to another roll 7 after having been coated. The substrate should preferably first be cleaned from oil residues resulting from the previous production steps of the substrate, i.e. the rolling. This may for example be made in a degreasing bath 8. Thereafter, the substrate is introduced into the coating production line. An etching chamber 9 is placed as a first step in the production line, and the strip is here exposed to ion-assisted etching in order to remove the oxide scale on the steel strip and thereby to achieve good adhesion of the surface layer. The nickel layer is deposited by means of PVD in a chamber 10 in the second step of the roll-to-roll process. The PVD process may preferably be electron beam evaporation. The contact elements may then be produced out of said strip through punching etc.

By exchanging a pure nickel metal product for a nickel-coated steel product in a contact element according to the invention first of all of course cost savings are offered owing to the high

price of nickel. Furthermore, a coated product makes it possible to combine the excellent inert properties of a nickel surface in an alkaline battery having a water-based electrolyte and the excellent mechanical properties of steel. Using a steel with a higher tensile strength than nickel makes it possible to obtain a contact element with an increased mechanical stability for the same thickness. Alternatively, for the same mechanical stability it is possible to reduce the thickness of the contact element. This means that comparatively thin foils of said metal strip prod- uct according to the invention may be provided while still enabling both a comfortable handling thereof by suction cups and the like without any risk of folding when assembling a said battery and a use of small amounts of the expensive nickel metal. Thus, a gain of weight and volume of the batteries may by that be obtained.

One very preferred application of a contact element according to the invention is shown in Fig 3, which very schematically illustrates a rechargeable alkaline battery 1 1 , which may be a Ni- metal hydride battery. The battery 1 1 comprises a number of battery cells 12, 13, 14, such as for instance 20 such cells, connected in series between a negative pole 15 and a positive pole 16 of the battery. Each cell may provide a voltage of 1 .2 V, so that the battery voltage will then be 24 V. Each cell consists of two electrodes in the form of an anode 17 and a cathode 18 as well as a water-based electrolyte 19 for conduction of charge carriers therebetween. Contact elements according to the present invention in the form of bipolar current collectors 21 are arranged between each battery cell and applied with one side thereof in direct contact to a said anode 17 and the opposite side thereof in direct contact to a said cathode of an adjacent battery cell for conducting charge carriers transversely therethrough from one cell to the other as shown through arrows 22. This means that both sides of such a current collector may be exposed to said electrolyte, so that both sides of a said steel substrate of such a contact element are coated by a coating

layer of Ni. End plates 23, 24 of the stack only have one side thereof exposed to said electrolyte, so that these have only to be coated by a coating layer of Ni on one side thereof. It is in such a battery important to seal adjacent battery cells with re- spect to each other with a suitable sealing frame, and this is here obtained by arranging sealing frames 25 against which the current collectors 21 , 23, 24 according to the invention are pressed. Thus, a not neglectible mechanical pressure is applied on said contact elements, and it is in this context possible to benefit from a high tensile strength of a steel substrate used for such contact elements. A battery of this type has suitably a capacity of 3-200 Ah, preferably at least 7 Ah. It is pointed out that a contact element according to the invention may be used as current collectors or contacts in other types of alkaline batteries than the one shown in Fig 3, such as in batteries in which the current between adjacent battery cells is not led transversely through a current collector but in the extension thereof in a loop between adjacent battery cells.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A stainless steel strip of ASTM 301 with a tensile strength of 1900 MPa and a thickness of 50 μm was coated on both sides thereof with a coating layer of Ni with a thickness of 1 μm and a surface roughness Ra of 0.06 μm while using the method described above.

An electrochemical corrosion testing experiment was carried out for this sample and a reference sample in the form of a pure Ni foil with a thickness of 42 μm. The corrosion test was for each sample carried out by arranging the sample in an alkali electrolyte (30% KOH) in which a Ag/AgCI-reference electrode and a Pt counter-electrode were arranged. Slow-scan cyclic voltametry (CV) was carried out for 10 cycles at a scan rate of 10 mV/s in a scan interval of -1 .2 to 0.3 V vs the reference electrode. During

the potential sweep a current will flow between the working electrode and the counter-electrode when either oxidation (during positive scan) or reduction (during negative scan) occur.

Fig 4 shows a said current I versus the potential P between the reference electrode and the sample during the tenth cycle of said scanning for the two samples, in which the black curve shows the current for the pure Ni foil and the lighter one for the sample according to the embodiment of the invention. The scan- ning is started at a sample potential of approximately -1 .2 V and then carried out to a potential of approximately 0.3 V and then back again to the start potential. The peaks a) emanate from an oxidation of Ni to Ni(OH) ₂ , creating a current peak. The peaks b) emanate from oxidation of Ni(OH) ₂ to NiOOH. The peaks c) show a reversal, i.e. a reduction, of the reaction resulting in the peaks b). However, the reaction of Ni to Ni(OH) ₂ is not reversible, so that instead H ₂ is formed when the potential is reaching the region indicated by d). Thus, both samples are stable in said electrolyte at a potential within the range of approximately -1 .1 V to approximately 0.3 V.

This testing shows that the Ni-coated stainless steel sample of the embodiment according to the invention is stable in an alkali electrolyte over a large potential interval being substantially the same as for a foil of pure Ni. Thus, a contact element according to this embodiment of the invention will have a comparable operation as current collector in an alkaline battery as a Ni foil while requiring approximately 20 times less Ni metal and simultaneously having a considerably improved mechanical stability with respect to such a pure Ni foil.

The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.