CA1064104A - Storage battery having positive nickel oxide and negative iron electrodes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A storage battery having positive nickel oxide electrodes of compressed powder and negative iron electrodes of sintered iron powder.
Preferably, a negative electrode is positioned between every two positive electrodes, and a metal fabric armature encloses the positive pressed powder electrodes.

Description

~064~4 The invention relates to an electrlcal storage battery with positive nickel oxide electrodes and negative iron electrodes.
Under the influence of the limited resources of natural energy and in furtherance of environmental protection, persistent efforts are being directed toward the application of known electrochemical current sources to different fields of use, and particularly to electrical traction.
A promising electrochemical system for vehicle storage batteries consists of the chargeable system dating back to Edison having a positive nickel oxide electrode and a negative iron electrode in aqueous potassium hydroxide solution. Although this is the oldest alkaline storage system, it has subsequently undergone insignificant improvements in the technical reali-~;~ zation of the practical cell, as well as in the preparation and electrochemi-cal yield of the active mass.
Among the drawbacks of the nickel oxide/iron system, there is its thermodynamic instability which results from the fact that the potentials wh~ch are required for charging the nickel CII) oxide electrode, or rather the FeCOH)2 electrode lie outside the limits of the thermodynamic stability range of the water. This manifests itself in a parasitic gas evolution during charging, particularly from the iron electrode, and this in turn requires a

- 2~ disproportionately high charging input. That charging of the electrodes is possible under those conditions is attributable to kinetic impedances.
In contra~t to a theoretical load capacity of 260 Wh/kg, based upon an open cell potential of 1.33 volts, practical nickel oxide/iron cells have reached during five hour discharge only values between 20 and 25 ~h/kg.
There are known the classical electrode constructions of the nickel oxide/iron storage battery, namely the positiye tubular electrodes and the negative plate electrodes, in which the iron mass which consists of metallic iron, iron oxide, or mixtures thereof is pressed into a metallic holder. In order to increase the hydrogen excess potential, it is customary to provide the iron mass also ~ith a predetermined quantity of mercury oxide.

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1064~04 Measures intended to raise the load capacity of the nickel oxide/iron storage battery have been principally addressed to the negative iron electrode, whose theoretical capacity is utilized only at the 28 per cent level and therefore would appear to provide wide latitude for improvement.
Thus, German patent publication (DT-AS) 1,696,570 teaches the combination in a storage battery of a positive sinter electrode with a nega-tive iron sinter electrode which is doped with small quantities of sulfur compounds whose presence is intended to prevent passivation of the iron.
According to German patent publication (DT-OS) 2,261,997 an iron electrode is produced by cathodic deposition of iron from an iron (II) nitrate solution upon an electrically conductive carrier while being simultaneously brought into contact with a sulfur salt.
United States patent 3,507,696, teaches the production, upon the particles of an iron oxide powder, of a thin cover layer of molten elements of the sulfur group, after which a metal fiber plate serving as carrier is impregnated with this material which has previously been made into a slurry with water.
However, these and other measures have not sufficed to raise the load capacity of the respective commercial cells above the values indicated.
A significant obstacle to doing so is represented by the high weight contri-bution of those cell components which are not directly involved in the delivery of current and which correspondingly detract from the load capacity.
- In particular, 20 to 35 per cent of the total cell weight are attributable to the inactive structure of conventional electrodes (support, armature, frame).
Accordingly, it is a primary object of the invention to provide a nickel oxide/iron cell which, in addition to a good voltage level particularly during discharge, conforms to the high capacity requirements of electric traction, particularly through reduction of dead weight.
According to the present invention, there is provided an electric storage battery having positive nickel oxide electrodes and negative iron electrodes, wherein the positive electrodes are of compressed powder, and the negative iron electrodes are of sintered iron powder.

1~6410g In another aspect, the invention provides the method of making an iron electrode for a storage battery comprising pasting onto an iron expanded metal support an aqueous dispersion of iron powder, water soluble filler material, and organic thickener, sintering the above at temperatures from about 600 to 800C, and thereafter dissolving out the filler material.
Preferably, the positive electrodes of pressed powder electrodes, and the negative iron electrodes of sintered iron powder are positioned so that a negative electrode is between every two positive electrodes.
Such an electrode combination makes it possible to match the high surface capacity of the iron electrode with a counter-electrode of correspond- -ing capacity.
This requirement cannot be met by the positive pocket or tubular electrodes heretofore used, because the open spaces provided by the perfora-tions of these electrode armatures amount to only about 15 per cent of the geometrical electrode surface so that a substantial fraction of the active mass is more or less covered. It is therefore reached by the charged carries of the electricity movement only through detours, i.e. with a voltage drop.
It is therefore particularly advantageous to provide, in accordance with the invention, a metal fabric armature which encloses the positive mass which has been pressed into it under high pressure. In this armature, which is preferably a nickel fabric, the open surface portion, at 36 per cent, is about 2.4 times greater and the individual openings at 1.44X10 2mm2 are only one-third as large as with conventional pockets. In this way, the filter effect, that is the retention of the active mass, is substantially improved.
The fraction of the total weight of the electrode constituted by the armature amounts to only about 20 per cent instead of the 50% of a conventional pocket electrode; the surface capacity of such an electrode amounts to about 6.5 Ah/dm2.
In accordance with the invention, the negative counter-electrode consists of sintered iron powder which contains a supporting structure, for example expanded iron metal. It is preferably produced by applying to iron expanded metal iron powder which has been made into a paste by means of an

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1064~04 alcohol-water-methyl cellulose mixture and subsequent sintering.
When ready for use the paste contains, in addition to traces of what may be a defoamer, 70-80 per cent by weight iron powder, 15-20 per cent by weight liquid (alcohol and water, for example in the relatioDship 1:15) : -3a-L~

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~064~04 0.5-1 per cent by weight methyl cellulose as thickener, 3-10 per cent by weight filler material (water soluble inorganic salts, particularly sodium chloride, sodium carbonate).
The pure iron powder used has a BET surface of 0.1 to 0.3 m2/g.
By BET surface is meant the surface area per unit weight, as calculated by a procedure attributed to the physicists Brunauer, Emmet, and Teller (hence the acronym BET) based on the measured volume of nitrogen absorbed by powders or porous bodies. The predominant portion, e.g. more than 80 per cent, of the powder has a grain size of less than 30 microns.
After pasting,the electrode is sintered for about one-half hour at 600-800C, preferably at 700C, in an atmosphere of protective gas.
The defoamer may be a boundary-layer active or surfactant material, which displaces foam forming substances from the bou~dary layer, without itself creating a foam. Alternatively, it may be a material which raises the surface tension of the water. These may be fats or oils, or long-chain alcohols, such as 2-ethyl hexanol or acetyl alcohol.
The thickener may also be carboxy methyl cellulose, polyvinyl alcohol, a poly wax or an alginate.
In this electrode, too, theidead weight portion (expanded metal and lead-out conductor) amounts to only 20 per cent of the total electrode weight, the surface capacity to abo~t 15-16 Ah/dm2. Consequently, there still remains a small capacity excess relative to the adjacent positive electrodes. These therefore limit the cell capacity.
Worthy of note with regard to the iron electrode according to the invention is its charging characteristic, which differs from that of a conventional pocket electrode by a more positive voltage level and a definite voltage step upon complete charging, resembling that of cadmium! This means that such an iron electrode acts more like cadmium with respect to hydrogen evolution, that is the gasing rate remains relatively low until the voltage step is reached. For full charging of the electrode a charge factor of 1.4 suffices.

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For further details reference is made to the discussion which follows in the light of the accompanying drawings wherein:
Figure 1 shows the operating characteristics of a cell embodying the invention, and Figure 2 shows a physical embodiment of such a cell.
Referring to Figure 1, curve 1 shown therein shows the charging voltage while curve 2 shows the discharge voltage as a function of time of a cell in accordance with the invention. The load during discharge equals 0.2 CA, where C is the numerical value of the capacity of the cell. For example, if the cell has a capacity of 50 Ah, then C is equal to 1501, and the dis-charge current 0,2 CA will be equal to 0,2 x 50 A ~ 10 A.
The negative electrode is separated from the positive electrode by separators of synthetic material, preferably in the form of a grate, the spacing between vertical rods being about 8 millimeters and the rod diameter about 2 millimeters.
The electrode spacing determined by the rod thickness is advanta-geous relative to the electrolyte quantity and the heat capacity of the cell.
~ue to the relatively close rod spacing, deformation of the electrode under the influence of expansion pressure can be better counteracted so that the 2C rod separator prevents contact between the positive and negative electrode and thereby the formation of short circuit connections.
To reduce the fraction of the cell housing relative to the total weight, particularly for multi-cell storage batteries, in a battery box, the enclosure for each individual cell, or rather for each individual electrode set, is a synthetic plastic tube which exhibits sufficient chemical, thermal and mechanical stability and which can be welded shut together with a compact bottom plate and a compact lid which insure pole positioning protected from disldcation. Polyethylene may, for example, be suitable as this material.
With this synthetic plastic envelope as the cell container the total weight of a cell embodying the invention divides among its various components as ollows:

~ 5 -51.5 perccent for the active mass 13.5 per cent for mass armature and vanes 23.0 per cent for the electrolyte 12.0 per cent for the cell container, separators, pole.
This clearly shows that an arrangement according to the invention, particularly has a very high proportion by weight of active materials. Corr-espondingly, the energy density of the nickel oxide/iron cell can be raised by the technique embodying the invention from the conventional figure of about 24 Wh/kg by a factor of 2, namely to about 50 Wh/kg for five hour discharge current.
Figure 2 shows a cell embodying the invention and containing in this case four positive and three negative electrodes, the three front electrodes being shown partly broken away. These show the principle of the arrangement.
Between each two positive electrodes 1 having metal fabric armatures 4, and separated from these by gratelike separators 3, there is a negative electrode 2.
The current take-off vanes 5 of the positive electrodes 1, which are attached by spot welds to the metal fabric armature 4, are riveted to the 2~ positive pole shoe 6, whereas the current take-off vanes 7 of the negative electrodes are similarly attached to the negative pole shoe 8.
Knobs 9, which engage the frame of the rod separator and extend over the edges of the electrodes, prevent relative sliding of the plates.
To the compact lid 10, which may consist for example of poly-ethelene, there are welded, spaced by foil material serving as cell housing 11, the two pole lead-throughs 12 and the filler plug 13, all these elements being of the same synthetic plastic.
After insertion of the negative and positive pole bolts 14 and 15 through lead-throughs 12, the poleshoes are so attached to the underside of the lid between ribs 16 that they do not rotate during tightening of hexagonal nuts 17.

10~4104 A rigid bottom plate 18, for example of polyethelene, gives-,,*he electrode package additional mechanical protection at its lower end.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electric storage battery having positive nickel oxide electro-des and negative iron electrodes, wherein the positive electrodes are of compressed powder, and the negative iron electrodes are of sintered iron powder.

2. The battery of claim 1 wherein a negative electrode is positioned between every two positive electrodes.

3. The battery of claim 1 further comprising a metal fabric envelope for the positive pressed powder electrodes.

4. The battery of claim 1 wherein the negative electrodes have a sintered support structure.

5. The battery of claim 4 wherein the support structure is of iron expanded metal.

6. The battery of claim 1 comprising between the electrodes a separator in the form of a synthetic plastic grate.

7. The battery of claim 1 further comprising a tube of welded synthetic plastic enclosing the electrodes which form a cell block.

8. The method of making an iron electrode for a storage battery comprising pasting onto an iron expanded metal support an aqueous dispersion of iron powder, water soluble filler material, and organic thickener, sintering the above at temperatures from about 600 to 800°C, and thereafter dissolving out the filler material.

9. The process of claim 8 wherein the thickening material is methyl cellulose.

10. The process of claim 8 wherein the iron powder has a BET surface of about 0.15 to 0.25 m2/g.

11. The process of claim 10 wherein the major portion of the iron particles has a grain size below 30 microns.