Low interfacial contact resistance material, use thereof and method of producing said material

Abstract

Method of producing a low interfacial contact resistance material for use in batteries or connectors and a low interfacial contact resistance material for use in batteries or connectors produced thereby.

Claims

1. A method of producing a low interfacial contact resistance material for use in batteries or connectors comprising the following steps: providing a steel substrate in the form of a cold-rolled strip; providing a nickel or nickel-based layer on one or both sides of the steel substrate to form a plated substrate; electrodepositing a molybdenum oxide layer from an aqueous solution onto the nickel or nickel-based layer of the plated substrate wherein the plated substrate acts as a cathode, wherein the aqueous solution comprises a molybdenum salt and an alkali metal phosphate and wherein the pH of the aqueous solution is adjusted to between 4.0 and 6.5; wherein the plated substrate provided with the molybdenum oxide layer is subjected to an annealing step in a reducing atmosphere to, at least partly, reduce the molybdenum oxide in the molybdenum oxide layer to molybdenum metal in a reduction annealing step and to form, simultaneously or subsequently, in the annealing step a diffusion layer which contains nickel and molybdenum, wherein the nickel originates from the nickel or nickel-based layer and the molybdenum originates from the molybdenum oxide layer.

2. The method according to claim 1, wherein the molybdenum salt is ammonium molybdate.

3. The method according to claim 1, wherein the phosphate is sodium dihydrogen phosphate.

4. The method according to claim 1, wherein the nickel or nickel-based layer provided on the substrate is between 0.5 μm and 5 μm in thickness, and/or wherein the diffusion layer has a thickness of between 10 nm and 200 nm.

5. The method according to claim 1, wherein the method comprises at least one feature selected from the group consisting of: the temperature of the aqueous solution for the electrodeposition of the molybdenum oxide layer onto the nickel plated steel substrate is between 40° C. and 75° C., the plating time for the electrodeposition of the molybdenum oxide layer onto the nickel plated steel substrate is between 5 and 30 seconds, the current density for the electrodeposition of the molybdenum oxide layer onto the nickel plated steel substrate is between 2 and 25 A/dm.sup.2, the maximum annealing temperature during the annealing step is between 500 and 1050° C., and the annealing time is between 6 and 10 hours for a batch annealing process and between 10 and 120 seconds for a continuous annealing process.

6. The method according to claim 1, wherein the aqueous solution for the electrodeposition of the molybdenum oxide layer onto the nickel plated steel substrate comprises: between 10 and 50 g/l of (NH.sub.4).sub.6Mo.sub.7O.sub.24, and/or between 20 and 80 g/l of NaH.sub.2PO.sub.4.

7. The method according to claim 1, wherein the low interfacial contact resistance material is used in a battery that comprises a steel can and/or a cap and the battery is produced by a process comprising: a deep drawing and/or wall-ironing step to produce the steel can from a blank produced from the low interfacial contact resistance material wherein the side of the blank provided with the nickel-molybdenum alloy layer becomes the inside of the can, and/or a forming step to produce the cap to cover the open end of the steel can from the low interfacial contact resistance material wherein the side of the cap provided with the nickel-molybdenum alloy layer becomes the inside of the cap.

8. The method according to claim 1, wherein the low interfacial contact resistance material is provided in a form suitable for use as a connector material, and wherein the connector material connects individual batteries into a battery pack, and wherein the side of the connector material provided with the nickel-molybdenum alloy layer is connected to the anode or cathode of the individual batteries.

9. A low interfacial contact resistance material for use in batteries or connectors, produced according to the method of claim 1, comprising a steel strip substrate provided on one side with a nickel layer, and provided on the other side with a nickel-molybdenum alloy layer which is a diffusion layer which contains nickel and molybdenum wherein the nickel originates from the nickel or nickel-based layer and the molybdenum originates from the at least partly fully reduced molybdenum oxide layer, wherein the interfacial contact resistance of the nickel-molybdenum alloy layer is at most 20 mΩ.Math.cm.sup.−2 when measured at a pressure P of 1.37 MPa (200 psi) according to the method described in the description.

10. A low interfacial contact resistance material for use in batteries or connectors, produced according to the method of claim 1, comprising a steel strip substrate provided on both sides with a nickel-molybdenum alloy layer which are diffusion layers which contain nickel and molybdenum wherein the nickel originates from the nickel or nickel-based layers and the molybdenum originates from the at least partly fully reduced molybdenum oxide layers, wherein the interfacial contact resistance of the nickel-molybdenum alloy layers is at most 20 mΩ.Math.cm.sup.−2 when measured at a pressure P of 1.37 MPa (200 psi) according to the method described in the description.

11. A low interfacial contact resistance material according to claim 9, wherein the interfacial contact resistance of the nickel-molybdenum alloy layer or layers is at most 15 mΩ.Math.cm.sup.−2.

12. A low interfacial contact resistance material according to claim 9, wherein the interfacial contact resistance of the nickel-molybdenum alloy layer or layers is at most 10 mΩ.Math.cm.sup.−2.

13. A battery or connector material produced using the low interfacial contact resistance material according to claim 9.

14. A battery according to claim 13, comprising a can and/or a cap, wherein the can and/or the cap is produced from the low interfacial contact resistance material.

15. The connector material comprising a connector strip or connector produced from the low interfacial contact resistance material according to claim 9.

16. The method according to claim 1, wherein the plated substrate provided with the molybdenum oxide layer is subjected to the annealing step in the reducing atmosphere to fully reduce the molybdenum oxide in the molybdenum oxide layer to molybdenum metal in the reduction annealing step.

17. The low interfacial contact resistance material of claim 9, wherein the at least partly reduced molybdenum oxide layer is a fully reduced molybdenum oxide layer.

18. The low interfacial contact resistance material of claim 10, wherein the at least partly reduced molybdenum oxide layer is a fully reduced molybdenum oxide layer.

19. A battery according to claim 13, comprising a steel can and/or a cap, wherein the steel can and/or the cap is produced from the low interfacial contact resistance material; wherein the battery is produced by a process comprising: a deep drawing and/or wall-ironing step to produce the steel can from a blank produced from the low interfacial contact resistance material wherein the side of the blank provided with the nickel-molybdenum alloy layer becomes the inside of the can, and/or a forming step to produce the cap to cover an open end of the steel can from the low interfacial contact resistance material wherein a side of the cap provided with the nickel-molybdenum alloy layer becomes the inside of the cap.

Description

EXAMPLES

(1) For the various nickel layers a conventional Watts plating bath is used. The Watts electrolyte combines nickel sulphate, nickel chloride and boric acid. The pH is maintained between 3.5 and 4.2 (aim 3.7) and the temperature of the bath is between 60 and 65° C. Nickel sulphate is the source of most of the nickel ions and is generally maintained in the range of 150-300 g/L. It is the least expensive nickel salt, and the sulphate anion has little effect on deposit properties. Nickel chloride improves the conductivity of the plating bath. The typical operating range is 30-150 g/L. Boric acid buffers the hydrogen ion concentration (pH) in the cathode film. If it were not for this buffering action, the cathode film pH in the higher-current-density regions would very quickly exceed 6.0, and nickel hydroxide would be precipitated and codeposited along with hydrogen, resulting in a green nodulation or burned deposit.

(2) An aqueous solution was prepared consisting of 30 g/l of (NH.sub.4).sub.6Mo.sub.7O.sub.24 (0.024 mol/l) and 50 g/l of NaH.sub.2PO.sub.4 (0.42 mol/l) with a pH of 5.5 and maintained at 60° C. A molybdenum-oxide layer was deposited on different nickel plated low-carbon steel strips using a current density of 20 A/dm.sup.2 and a plating time of 15 and 10 seconds. This material was then annealed in a reducing hydrogen atmosphere for 7.3 hours in a batch annealing furnace. The resulting Ni—Mo-diffusion layer has a thickness of about 150 nm at the surface of the coated substrate.

(3) Experiments using the conditions above result in the following linear dependence of the amount of Mo deposited on plating time (measured after annealing using Atomic Absorption Spectroscopy after dissolution of the layer of the substrate in HCl (1:1)).

(4) TABLE-US-00001 Plating time (s) A/dm.sup.2 mg (Mo)/m.sup.2  5 20 94 10 20 128 15 20 174 20 20 220

(5) FIG. 1 shows a non-limitative example of the implementation of the process according to the invention. The hot-rolled starting product is pickled to remove the oxides from the strip and clean the surface. After pickling the strip is cold-rolled. The steel grade used is DC04 (EN10139). In the plating step the various layers are electrodeposited. In the annealing step the diffusion annealing takes place. The cold-rolling can obviously also take place elsewhere when the cold-rolled coil is bought from a supplier of cold rolled coil.

(6) FIG. 2 shows a GDOES-measurement of the surface after depositing the molybdenum oxide on the nickel layer. The X-axis gives the thickness in nm and the Y-axis gives the concentration in wt. %. Note that the values for carbon and sulphur are in fact 10 times as low as presented. Clearly visible is the layer of molybdenum oxide on top of the nickel layer. The nickel layer is 2 μm (i.e. 2000 nm), whereas the molybdenum oxide layer is about 60 nm.

(7) FIG. 3 shows a GDOES-measurement of the surface after annealing the layers of FIG. 2. Note that the values for carbon and sulphur are in fact 10 times as low as presented. The clearly discernible layer of molybdenum oxide on top of the nickel layer has vanished, and a diffusion layer comprising nickel and molybdenum is shown. There is still a degree of oxygen present in the surface layers, but this is believed to be associated with re-oxidation of the surface, and with the presence of the phosphates, and not with the molybdenum oxide which has reduced to metallic molybdenum.

(8) FIG. 4 shows a schematic drawing of the layer structure before (A) and after (B) the diffusion annealing. In this example the steel substrate 1 is provided on both sides with a nickel layer 2a and 2b. The thickness of both nickel layers may be different. Usually the nickel layer which becomes the outside of the can (2b) is thicker than the nickel layer which becomes the inside of the can (2a) after deep-drawing the low interfacial contact resistance material into a can for use in a battery. A molybdenum oxide layer 3 is subsequently provided on nickel layer 2a.

(9) After the diffusion annealing (B) the layer 2b is practically unaltered, possibly with some minor interdiffusion of iron and nickel at the interface between the steel substrate and the nickel layer 2b, but the molybdenum oxide layer has been reduced to metallic molybdenum and also interdiffusion of nickel and molybdenum has taken place leading to the formation of a NiMo alloy layer on top of the steel substrate. Clearly, the mechanism would be equal on both sides if a molybdenum oxide layer 3 is provided on both nickel layer 2a and 2b. In that case the resulting material would essentially consist of a steel substrate provided with a NiMo alloy layer on both sides of the substrate. Initially all molybdenum is located on top of the nickel layer (2a), and during the annealing the molybdenum diffuses into the nickel layer. The concentration of molybdenum therefore decreases when moving through the diffusion layer to the steel substrate, as clearly visible when comparing FIG. 2 with FIG. 3. If the nickel layer is very thick compared to the molybdenum layer and/or the annealing time not very long, and/or the annealing temperature not very high, then the molybdenum concentration near the steel substrate will be very low indeed. Consequently, the schematic indication of the situation after annealing (B) in FIG. 4 which indicates the presence of a NiMo alloy layer (3′) on top of the steel substrate means that the molybdenum has diffused into the nickel layer. Depending on the relative thicknesses of the original nickel layer and the molybdenum layer, the annealing time and the annealing temperature, the concentration of molybdenum in the original nickel layer near the steel substrate may be negligible, so that in effect the situation after annealing is that the steel substrate is provided with a nickel layer, which is practically unaltered, possibly with some minor interdiffusion of iron and nickel at the interface between the steel substrate and the nickel layer, and that on top of this practically unaltered nickel layer a nickel-molybdenum diffusion layer is provided, wherein the concentration of molybdenum decreases and the nickel concentration increases when moving from the surface of the NiMo alloy layer towards the steel substrate as depicted in FIG. 3, and as schematically depicted in FIG. 11. FIG. 11 shows the initial situation before the diffusion of the Ni (dashed line) and Mo (solid line) concentration, and the situation after a degree of diffusion has taken place. A fully Mo-layer (left) and a fully Ni-layer (middle) is deposited on a steel substrate (Fe, right). After annealing at a certain temperature for a certain period of time a diffusion profile will have been established in which Ni has diffused into the Mo-layer and Mo has diffused into the Ni-layer (any difference in diffusion speed of one element in the other v.v. has been ignored). As the initial Mo-layer is thinner than the initial Ni-layer, the Mo has not penetrated into the full thickness of the Ni-layer, leaving the Ni-layer nearest to the Fe-substrate substantially Mo-free, but this is still considered part of layer 3′ in FIG. 4B. The layers 2 and 3 as well as 3′ as depicted in FIG. 4A and B are also shown in FIG. 11. When the annealing is at a sufficiently high temperature for a sufficiently long time there may be Mo that reaches the Ni—Fe interface.

(10) FIG. 5 shows a typical AA-battery showing the elements of a battery including the steel can and (anode) cap which jointly form the structural basis of the battery.

(11) FIG. 6 shows a typical and schematic use of connector material to produce battery packs.

Method for Determining the Interfacial Contact Resistance (ICR)

(12) FIG. 7 shows the set-up for the measurements of the interfacial contact resistance. This set-up is used also in measurements of the total electrical DC resistance, thus including interfacial contact resistance, of fuel cell stacks (Properties of Molded Graphite Bi-Polar Plates for PEM Fuel Cell Stacks, F. Barbir, J. Braun and J. Neutzler, Journal of New Materials for Electrochemical Systems 2, 197-200 (1999)).

(13) The interfacial contact resistance (ICR) test is based on Ohm's law, R=V/I, where R is the resistance in Ohms, V is the potential difference in Volts and I is the current in Amperes. A current of 10 Amperes is led through the sample, and the potential is measured, and this potential can then be used to calculate the resistance over the surface area of the sample. As backing plate a gas diffusion layer (GDL) is used on both sides of the sample (A). For the tests presented below Toray Paper TGP-H-120 was used as GDL. This is a carbon fibre composite paper suitable for use as a catalyst backing layer. It has a total thickness of 370 um (microns). By first placing the sample between two GDLs and then placing the GDL and sample between the two gold-plated copper pressure plates, the potential can be measured at certain pressure, the amount of pressure being applied to the sample is dependant of the size of the sample, for each new pressure value a 30 seconds interval is used before the current is determined. The dimensions of the gold-plated copper pressure plates is irrelevant because a pressure is imposed on the sample, but for the tests in this invention rectangular plates of 4×4 cm.sup.2 or 2×2.5 cm.sup.2 plates. The reference value of the pressure P is 200 psi (=13.8 bar, or 1.37 MPa). Prior to the testing of any sample A, several measurements were conducted with only two GDLs and no sample present, the average value of these measurement is then subtracted from the measurement done with the sample so that what remains is the ICR value of only the sample.

ICR RESULTS

(14) A 0.25 mm low-carbon cold-rolled steel strip (DC04 (EN 10139), 76% CR, annealing at 610° C.) was coated with a 1.8 μm Nickel on both sides and 77.5 mg/m.sup.2 Molybdenum on one side (sample 330). A 0.61 mm low-carbon cold-rolled steel strip (DC04 (EN 10139), double reduced CR1=78%, recrystallisation annealing, CR2=55%, annealing at 610° C.) was coated with a 3.2 μm Nickel on both sides, and 50 mg/m.sup.2 Molybdenum (sample 257). The Mo-content is determined after annealing, but as the Mo does not disappear, the amount is the same before and after annealing. All samples were temper rolled.

(15) Results at P=200 psi (1.37 MPa)

(16) TABLE-US-00002 330 257 (i) (ii) (iii) (iv) (v) (vi) #1 1.09 8.34 50.10 65.00 61.67 43.56 29.69 13.99 #2 1.15 7.99 — — — — — — #3 0.97 6.46 — — — — — — Avg 1.07 7.60 50.10 65.00 61.67 43.56 29.69 13.99 stdevp 0.07 0.82 — — — — — — (i) 1.50/0.60 μm Ni (inside/outside) (ii) 0.30 Ni + 0.10 Co/1.20 Ni (i/o) (iii) 1.20 Ni + 0.10 Co/0.5 Ni (i/o) (iv) 1.30 Ni + 0.20 Co/1.50 Ni (i/o) (v) 1.30 Ni + 0.20 Co/1.50 Ni (i/o) (vi) 1.80 Ni + 0.20 Co/0.20 Ni (i/o)

(17) FIG. 8 shows the ICR for NiMo layers with a different substrate thickness. It is apparent that the influence of the substrate causes the measured ICR to increase with increasing substrate thickness. However, for the substrate thicknesses that are applied for batteries and connector materials the value at 200 psi is well below the lowest value presented in the table for Ni and Ni+Co coatings.

(18) In FIG. 9 the curve for Ni (i) is compared with a NiMo alloy layer. It is immediately apparent that the ICR values for the dashed curve (330) is much lower.

(19) In FIG. 10 the curves for Ni+Co are compared with a NiMo alloy layer. It is immediately apparent that the values for the curve labelled “330” are much lower than even the lowest curve for Ni+Co. Hence even the Ni+Co layers are outperformed by the material according to the invention.

(20) Tests performed on deep drawn material battery can material revealed that the ICR showed similar and consistent improvements over the Ni+Co layers and Ni layers.