GB1568598A

GB1568598A - Process for the production of sintered battery electrodes

Info

Publication number: GB1568598A
Application number: GB49133/77A
Authority: GB
Original assignee: Daimler Benz AG
Current assignee: Daimler Benz AG
Priority date: 1976-11-27
Filing date: 1977-11-25
Publication date: 1980-06-04
Also published as: SE7712445L; DE2653983A1; FR2372514A1; JPS5367836A

Description

(54) PROCESS FOR THE PRODUCTION OF SINTERED BATTERY ELECTRODES (71) We, DAIMLER-BENZ AKTIEN GESELLSCHAFT, a German body corporate organised under the laws of Germany, of Mercedesstrasse 136, 7000 Stuttgart 60, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to a process for the production of sintered battery electrodes; more particularly, it relates to such a process in which a foam plastics material which contains metal powder is decomposed by pyrolysis, for example at from 300 to 4500C. and the metal powder which is left behind, which has the framework structure of the foam, is sintered to form a metal framework having a high pore content.

For the production of nickel hydroxide electrodes for electrolytic cells, it has been disclosed in German Offenlegungsschrift No. 2,427,422 to cover the surface of an open-celled foam with nickel powder and to subject the resulting coated foam to a heat treatment at ca. 1000"C in a protective gas atmosphere so that the basic structure formed by the foam is decomposed and a high bulk nickel structure is left behind. Sintered nickel electrodes having a high volume of pores may be produced in this way. However, the coating of the surface of open-celled foam may lead to problems due to the accumulation of dust which is difficult to remove. This is partly due to the fact that the foam is coated with the nickel powder in thin layers in order to obtain a uniform covering of metal powder on the surface of the foam.

It is an object of the present invention to provide a process for the production of sintered electrodes for electrolytic cells, in which process the advantages associated with the use of a foam may be realised under conditions allowing of easy technical production. In particular, problems due to dust are to be obviated.

The present invention is characterised in that the metal powder is mixed with the reactive mixture for producing the foam or a component or components thereof before this mixture is foamed so that the mixture is foamed in the presence of the metal powder and hardened.

The present invention enables large quantities of metal powder to be mixed with the reaction mixture from which the foam is to be produced and, moreover, by a method which is simple to carry out. The foam sheeting or individual plates of foam cut up to the size of the electrode therefore no longer need to be coated separately with metal powder. The proportion of metal powder in the foam may be varied within wide limits, whereby the mechanical strength, volume of pores and other properties of the sintered body may be varied accordingly. Generally, the metal powder is introduced into the foam-forming reactive mixture or one or more components thereof in a ratio, by weight, of from 0.5:1 to 1.5:1, preferably ca. 0.8:1, based on the total weight of the components of the mixture. The foam used is preferably a duroplast which may be pyrolysed without considerable softening first taking place.

Moreover, pyrolysis of the foam should not give rise to the evolution of gases liable to harm the metal. Suitable foams are, inter alia, the so-called "semi-rigid" polyurethane foams. The incorporation of metal powder imparts to the foams a mechanical strength which improves the processibility thereof so that, for example, plates of the required size may easily be cut off a relatively large foam block containing metal powder. Also, if desired, a foamed band may, after hardening, be sliced into two pieces each being half the thickness of the original. Alternatively, the metal-containing foam may be foamed inside the mould which is to be used for the subsequent sintering process.

If a polyurethane foam is used, the metal powder is preferably mixed with the polyol component. In that case, it is immaterial whether the metal powder is completely free from moisture or not. If desired, however, the metal powder may be mixed with the isocyanate component.

In sintered electrodes, the layers of sintered metal are generally held by a supporting structure. In order that the supporting structure and the sintered metal may be firmly bonded together, the structure may be covered on both sides with a plate of foam containing the metal so that a firm bond between the supporting structure and the sintered layer is produced in the course of the sintering process. Alternaitvely, the foam may be foamed round the supporting structure.

The process according to the present invention is particularly suitable for the production of nickel electrodes and iron electrodes. The particle size and the sintering temperature may be adapted to the most suitable conditions in each case. The pyrolysis which precedes sintering may be carried out while the electrodes are being heated to the sintering temperature or it may be carried out in a separate temperature stage adjusted to the type of foam used in each case. Pyrolysis is carried out in an atmosphere which allows of complete disintegration of the foam without destroying the metal structure.

Iron electrodes are usually produced using iron powder having an average particle size of less than 30 micron, preferably from 3 to 10 micron. Iron electrodes are substantially ready for use once the sintering process, for example at from 800 to 820"C for from 1 to 2 hours, has been completed, although they may subsequently be mechanically compressed and accurately cut to the final dimensions. Nickel electrodes are usually produced using nickel powder having an average particle size of less than 10 micron, preferably from 2 to 7 micron. In the case of nickel elec trodes, on the other hand, sintering, for example at between 900 and 1000"C for from 1 to 2 hours. is followed by a process of activation by impregnation with nickel hydroxide. This impregnation may be carried out in known manner. Galvanic impregnation is preferred. This may be carried out by connecting a sintered nickel electrode as cathode in an electrolytic bath containing nickel nitrate and possibly other additives, and using a sacrificial nickel electrode as anode. It is particularly advantageous to impregnate several sintered nickel electrodes simultaneously by placing two or more sintered nickel electrodes in a bath and connecting them to a source of direct voltage with alternating reversal of the polarity. With this method, the sacrificial electrode is unnecessary; moreover, the composition of the bath may easily be kept constant. So long as the sintered electrodes are connected as anodes, oxygen is liberated at these anodes and some nickel is removed. As soon as the polarity is reversed, this nickel may be redepositied as active mass, whereby impregnation is assisted. Generally, current reversal is effected at least every three minutes, preferably at least every 60 seconds, more preferably every from 4 to 45 seconds.

It is therefore preferable to begin this process of activation using a slightly thicker sintered nickel electrode than is finally desired. The production of a sintered nickel electrode is found to be an advantage here, because the integration of the metal powder with the plastics foam provides numerous possibilities of variation by which the structural framework of the sintered layer which is to be impregnated may be influenced.

In one preferred embodiment of the present process, after electrolytic impregnation, the impregnated electrodes are additionally activated by immersion of about 1 hour in an approximately two-molar ammonium sulphate solution at room temperature.

The present invention is illustrated by the following Examples.

Example I Mond nickel powder was mixed with the polyol component used for the production of the semi-rigid, heat-stable polyurethane foam. The components for the foam were then mixed together in known manner and poured into a rectangular mould. The ratio, by weight, of nickel to foam was 80:100. Foaming was completed after 5 minutes and was accompanied by an increase in volume to from 3 to 4 times the original volume. After a further 20 minutes at room temperature, the foam had hardened sufficiently to enable it to be processed. It was cut into plates approximately 8 mm in thickness.

The polyurethane foam had been stiffened by the nickel powder embedded in it. Two of the foam plates were then placed together with inter-position of a conductive framework of expanded nickel and lightly weighted down with a ceramics plate. This sandwich structure was then heated in an oven to a temperature of from 300 to 400"C and maintained at this temperature for 5 minutes to bring about pyrolysis of the polyurethane foam. The nickel electrode, which still had a loose and friable structure, was then sintered by being maintained at a temperature of 950"C for 2 hours in a protective gas atmosphere of ammonia decomposition gas.

During this time, the electrode shrunk so that its thickness was reduced by about 30% and its length and width were also somewhat reduced. The sintered structure of metal powder was firmly bonded to the conductive framework used as support. In the sintered layers, the electrode had a pore volume of from 90 to 95%. It was mechanically slightly compressed in order to strengthen the structure and was then cut to the required final dimensions.

In order to be able to use the nickel electrode as positive electrode in an electrolytic cell, it was then activated by electrolytic impregnation with nickel hydroxide. This activation may be carried out using a sacrificial nickel electrode. The method of activation described in Example 2 is preferred.

Example 2 Two nickel electrodes 8mm in thickness having a supporting structure of expanded nickel and a porous coating of sintered nickel, produced as described in Example 1, were immersed in an electrolytic bath containing 500 g per litre of Ni(NO3)2 . 6 H2O and 30 g per litre of Co(NO,)2 . 6 H20 in a weak nitric acid solution. The pH is 5.0.

A direct voltage was then applied to the two electrodes and its polarity was reversed symmetrically every 8 seconds. The current density was 15 Aids2 and the temperature 90"C. During electrolysis, the bath was agitated and its composition was kept constant. The time of impregnation was 5 hours. Deposition of the active nickel hydroxide mass proceeded rapidly and some metallic nickel from the sintered layer was also converted into nickel hydroxide.

Other additives which, like cobalt nitrate, influence the composition of the active mass may also be added to the bath, e.g. magnesium nitrate (e.g. from 2 to 10%, by weight, preferably from 3 to 5%, by weight, based on the weight of the bath) or lithium nitrate. Other additives may be used to influence the process of electrolysis. Thus, halides may be added as depassivating ions, for example chloride ions may be added in quantities of from 1 to 5 g per litre, e.g. in the form of potassium chloride. Sulphate ions and persulphate ions also have a depassivating effect. Hydrogen peroxide, e.g. as perhydrol, may also be added to the bath.

This causes darkening of the colour of the electrolytic bath, presumably due to the oxidation of nickel to the trivalent state.

Such an addition of hydrogen peroxide has an advantageous effect on the activity of the impregnation.

Example 3 Iron carbonyl powder having an average particle size of ca. 10 micron was dried and mixed in approximately equal parts with the polyol component and the isocyanate component of a reactive mixture used for producing a heat-resistant polyurethane foam. Drying of the iron powder was carried out to prevent premature reaction of the isocyanate component due to moisture. The two components for producing the foam were mixed together and the mixture was extruded through two flat sheeting dies placed parallel to each other so that it was applied as continuous sheets to the two sides of perforated steel plates passing between the sheeting dies, and the extruded mixture was foamed at a slightly elevated temperature and hardened. The plates coated with the foam were continuously carried on a conveyor belt where the foam was kept to the required dimensions, inter alia by means of spacers. When the foam had hardened, the individual electrode plates coated with foam were cut to the required length. Pyrolysis of the foam was carried out in the same way as in Example 1 in a slightly oxidizing atmosphere adjusted so that it did not attack the iron to a significant extent. Sintering, however, was carried out at a lower temperature, within the relatively narrow range of from 800 to 820"C in a reducing atmosphere. After sintering, gas discharge grooves were stamped into the sintered electrode and the edges were then trimmed to adjust the electrode to the correct measurements. No sparate process of activating impregnation is required for the sintered iron electrodes.

WHAT WE CLAIM IS: - 1. A process for the production of a sintered battery electrode which comprises decomposing metal powder-containing foam plastics material by pyrolysis, leaving the metal powder having the framework structure of the foam, and sintering the metal powder to form a metal structure having a high pore content, the metal powder being mixed with the reactive mixture for producing the foam or one or more components thereof before foaming, so that the reactive mixture is foamed in the presence of the metal powder and then hardened.

2. A process as claimed in claim 1 in which the metal powder is mixed into a reactive mixture used for producing a heat-stable duroplasti foam or one or more components thereof.

3. A process as claimed in claim 2 in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. pyrolysis of the polyurethane foam. The nickel electrode, which still had a loose and friable structure, was then sintered by being maintained at a temperature of 950"C for 2 hours in a protective gas atmosphere of ammonia decomposition gas. During this time, the electrode shrunk so that its thickness was reduced by about 30% and its length and width were also somewhat reduced. The sintered structure of metal powder was firmly bonded to the conductive framework used as support. In the sintered layers, the electrode had a pore volume of from 90 to 95%. It was mechanically slightly compressed in order to strengthen the structure and was then cut to the required final dimensions. In order to be able to use the nickel electrode as positive electrode in an electrolytic cell, it was then activated by electrolytic impregnation with nickel hydroxide. This activation may be carried out using a sacrificial nickel electrode. The method of activation described in Example 2 is preferred. Example 2 Two nickel electrodes 8mm in thickness having a supporting structure of expanded nickel and a porous coating of sintered nickel, produced as described in Example 1, were immersed in an electrolytic bath containing 500 g per litre of Ni(NO3)2 . 6 H2O and 30 g per litre of Co(NO,)2 . 6 H20 in a weak nitric acid solution. The pH is 5.0. A direct voltage was then applied to the two electrodes and its polarity was reversed symmetrically every 8 seconds. The current density was 15 Aids2 and the temperature 90"C. During electrolysis, the bath was agitated and its composition was kept constant. The time of impregnation was 5 hours. Deposition of the active nickel hydroxide mass proceeded rapidly and some metallic nickel from the sintered layer was also converted into nickel hydroxide. Other additives which, like cobalt nitrate, influence the composition of the active mass may also be added to the bath, e.g. magnesium nitrate (e.g. from 2 to 10%, by weight, preferably from 3 to 5%, by weight, based on the weight of the bath) or lithium nitrate. Other additives may be used to influence the process of electrolysis. Thus, halides may be added as depassivating ions, for example chloride ions may be added in quantities of from 1 to 5 g per litre, e.g. in the form of potassium chloride. Sulphate ions and persulphate ions also have a depassivating effect. Hydrogen peroxide, e.g. as perhydrol, may also be added to the bath. This causes darkening of the colour of the electrolytic bath, presumably due to the oxidation of nickel to the trivalent state. Such an addition of hydrogen peroxide has an advantageous effect on the activity of the impregnation. Example 3 Iron carbonyl powder having an average particle size of ca. 10 micron was dried and mixed in approximately equal parts with the polyol component and the isocyanate component of a reactive mixture used for producing a heat-resistant polyurethane foam. Drying of the iron powder was carried out to prevent premature reaction of the isocyanate component due to moisture. The two components for producing the foam were mixed together and the mixture was extruded through two flat sheeting dies placed parallel to each other so that it was applied as continuous sheets to the two sides of perforated steel plates passing between the sheeting dies, and the extruded mixture was foamed at a slightly elevated temperature and hardened. The plates coated with the foam were continuously carried on a conveyor belt where the foam was kept to the required dimensions, inter alia by means of spacers. When the foam had hardened, the individual electrode plates coated with foam were cut to the required length. Pyrolysis of the foam was carried out in the same way as in Example 1 in a slightly oxidizing atmosphere adjusted so that it did not attack the iron to a significant extent. Sintering, however, was carried out at a lower temperature, within the relatively narrow range of from 800 to 820"C in a reducing atmosphere. After sintering, gas discharge grooves were stamped into the sintered electrode and the edges were then trimmed to adjust the electrode to the correct measurements. No sparate process of activating impregnation is required for the sintered iron electrodes. WHAT WE CLAIM IS: -

1. A process for the production of a sintered battery electrode which comprises decomposing metal powder-containing foam plastics material by pyrolysis, leaving the metal powder having the framework structure of the foam, and sintering the metal powder to form a metal structure having a high pore content, the metal powder being mixed with the reactive mixture for producing the foam or one or more components thereof before foaming, so that the reactive mixture is foamed in the presence of the metal powder and then hardened.

3. A process as claimed in claim 2 in

which the foam-forming reactive mixture is one used for producing a duroplast foam which may be pyrolysed substantially without destruction of the foam structure of the metal.

4. A process as claimed in claim 1 in which the metal powder is mixed into a reactive mixture used for producing a polyurethane foam or one or more components thereof.

5. A process as claimed in claim 4 in which the reactive mixture is one used for producing a semi-rigid polyurethane foam.

6. A process as claimed in claim 4 or claim 5 in which the metal powder is mixed into the polyol component and/or the isocyanate component.

7. A process as claimed in any of claims 1 to 6 in which the metal powder is introduced into the foam-forming reactive mixture or one or more components thereof in a ratio, by weight, of from 0.5:1 to 1.5:1, based on the total weight of the components ofthe mixture.

8. A process as claimed in claim 7 in which the ratio is ca 0.8:1.

9. A process as claimed in any of claims 1 to 8 in which the metal powder-containing foam is foamed into relatively large blocks from which plates of the required size are cut.

10. A process as claimed in any of claims 1 to 8 in which the metal powdercontaining - foam is foamed continuously in the forkn of a band of the required width and thickness.

11. A process as claimed in claim 10 in which, after hardening, the foamed band is sliced into two pieces each being half the thickness of the original.

12. A process as claimed in any one of claims 1 to 11 in which a conductive supporting plate is covered on both sides with plates of metal powder-containing foam and is then subjected to pyrolysis and sintering.

13. A process as claimed in claim 12 in which an electrically conductive supporting plate for the electrode is embedded in the structure during formation of the foam.

14. A process as claimed in claim 12 or claim 13, in which the supporting plate used is made of the same metal or the same metal alloy as the metal powder contained in the foam and has perforated framework structure.

15. A process as claimed in any one of claims 1 to 14 in which a nickel electrode is produced using nickel powder having an average particle size of less than 10 micron.

16. A process as claimed in claim 15 in which the nickel powder has an average particle size of from 2 to 7 micron.

17. A process as claimed in claim 15 or claim 16 in which, after pyrolysis, the nickel structure is sintered at a temperature of between 900 and 1000"C for from 1 to 2 hours.

18. A process as claimed in claim 17 in which the sintering temperature is about 950"C.

19. A process as claimed in any of claims 1 to 14 in which an iron electrode is produced using iron powder having an average particle size of less than 30 micron is used.

20. A process as claimed in claim 19 in which the iron powder has a particle size of from 3 to 10 micron.

21. A process as claimed in claim 19 or claim 20 in which, after pyrolysis, the iron structure is sintered at a temperature of from 800 to 820 C from 1 to 2 hours.

22. A process as claimed in any of claims 1 to 21 in which sintering is carried out in a reducing protective gas atmosphere directly after pyrolysis.

23. A process as claimed in claim 22 in which pyrolysis is carried out at a temperature of from 300 to 450"C.

24. A process as claimed in any of claims 1 to 18, 22 or 23 in which a positive nickel electrodes for an electrolytic cell is produced by subjecting the porous nickel electrode to an activating impregnation wtih nickel hydroxide in an electrolytic bath containing nickel nitrate.

25. A process as claimed in claim 24 in which the electrolytic bath also contains cobalt nitrate in a weak nitric acid solution.

26. A process as claimed in claim 24 or claim 25 in which at least two porous nickel electrodes are placed in the bath and simultaneously impregnated by applying a terminal of a source of direct voltage to each electrode and reversing the polarity of the direct voltage at a pulsating frequency.

27. A process as claimed in any of claims 24 to 26 in which only electrodes of the type which, in the form of porous nickel electrodes, are themselves activated with nickel hydroxide are used in the bath.

28. A process as claimed in any of claims 24 to 26 in which reversal of the current occurs at symmetric time intervals so that, in each electrode, the duration of the cathodic current load and the duration of the anodic current load during impregnation are equal.

29. A process as claimed in any of claims 24 to 28 in which reversal of the current is effected at least every three minutes.

30. A process as claimed in claim 29 in which reversal is effected at least every 60 seconds.

31. A process as claimed in claim 30 in which reversal is effected every from 4 to 45 seconds.

32. A process as claimed in any of claims 24 to 31, in which the starting porous nickel electrodes have a greater thickness than the desired nickel structure of the finished electrode.

33. A process as claimed in any of claims 24 to 32 in which the composition of the bath and the conditions of electrolysis are adjusted so that, at whichever electrode is connected as anode at any given moment, the liberation of oxygen takes place preferentially to the removal of metal.

34. A process as claimed in any of claims 24 to 33 in which hydrogen peroxide is added to the bath.

35. A process as claimed in claim 34 in which the hydrogen peroxide is added as perhydrol.

36. A process as claimed in any of claims 24 to 35 in which from 2 to 10% of magnesium nitrate, based on the weight of the bath, are added to the bath.

37. A process as claimed in claim 36 in which from 3 to 5% of magnesium nitrate are added.

38. A process as claimed in any of claims 24 to 37, in which depassivating anions of one or more mineral acids are added to the bath.

39. A process as claimed in claim 38 in which chlorides, sulphates and persulphates are added in quantities of from 1 to 5 g/l.

40. A process as claimed in any of claims 24 to 39 in which, after electrolytic impregnation, the impregnated electrodes are additionally activated by immersion for about 1 hour in an approximately twomolar ammonium sulphate solution at room temperature.

41. A process as claimed in claim 1 substantially as herein described.

42. A process as claimed in claim 1 substantially as herein described with reference to any one of the Examples.

43. A sintered battery electrode when produced by a process as claimed in any of claims 1 to 42.