Aqueous nickel hydroxide paste of high flowability

An aqueous nickel-hydroxide paste of high flowability for the vibration filling of foam-structure and fiber-structure electrode plaques has a content of about 30 to 50% by volume of nickel hydroxide, a maximum particle size of about 0.04 mm, a plastic viscosity of about 0.1 to 1 Pa.s and a yield value of between about 10 and 120 Pa, a pH of between about 9 and 12 and a content of about 0.5 to 5, in particular about 1 to 5, % by weight, based on the nickel hydroxide content, of a dispersant from the group comprising the water-soluble salts of polhphosphoric acids or di- and polyphosphonic acids and their derivatives. Very particularly suitable as a dispersant is 1-hydroxyethane-1, 1-diphosphonic acid in the form of its cobalt/alkali salt complex. This paste makes it possible to completely fill foam-structure and fiber-structure electrode plaques in one operation.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to an aqueous nickel hydroxide paste of high 
flowability for the vibration filling of foam-structure and 
fiber-structure electrode plaques. 
Industrial nickel oxide electrodes (the correct terms are nickel hydroxide 
electrodes or nickel oxide hydroxide electrodes) can be classified, 
according to their current collecting structure, as tubular electrodes, 
pocket plate electrodes, sintered nickel electrodes and fiber plaque 
electrodes. In button cells, a round electrode of pressed active material 
without a current collecting structure is used. Plastic bound electrodes 
have not achieved any relatively great importance. The so called 
"controlled microgeometry" electrode, in which layers of nickel hydroxide 
material are held between a multiplicity of perforated nickel foils, have 
also failed to achieve any dissemination. 
In the case of tubular electrodes, the prefabricated nickel hydroxide is 
ground in powder form, and in the case of the pocket plate electrode, 
prefabricated compacts, so called briquets, are used. In the case of the 
narrow-pore sintered nickel plaque, the active nickel hydroxide material 
is formed in situ in the pores by precipitation from nickel salt solutions 
by chemical means (using alkalis) or by electrochemical means (cathodic 
polarization). The chemical precipitation process achieves the necessary 
concentration of nickel hYdroxide in the pores only by repeating the 
soaking and precipitation several times with intermediate washing and 
drying. The electrochemical process achieves the filling in one step, the 
residence time of the electrode plate in the salt bath being about 1 hour. 
However, the salt bath alters its composition and has to be discarded from 
time to time. Both precipitation processes are expensive but are 
indispensable for powder sintered plaque. 
Foam-structure plaque and fiber-structure plaque have been used for about 
15 years for supporting the active material. The are composed of metal 
only or they contain additionally the structure providing plastic or 
carbon basic body. The impregnation with active material is normally 
performed using a precipitation process, but descriptions have also been 
given of mechanical filling processes using prefabricated material which 
have become possible as a result of the substantially larger pore size 
compared with powder sintered plaque. 
The first mention of vibration filling of foam and fiber plaque with dry 
pulverulent iron sulphide for lithium/sulphur cells is found in U.S. Pat. 
No. 3,933,520. Pulverulent dry nickel hydroxide is not suitable for a 
fluidized bed process of this type. At values of between 0.4 and 1 
g/cm.sup.3, the bulk density is too low, the flow properties are 
unfavorable and the health hazard is high. Attempts have therefore already 
been made to avoid these disadvantages by using nickel hydroxide 
suspensions. 
The suitability of nickel oxide electrodes is assessed, inter alia, by how 
much material has been accommodated in the electrode volume. In general, a 
range of 1.2 to 2.2 g of nickel hydroxide/cm.sup.3 of empty volume is 
regarded as suitable in the literature (for example, G. Crespy, R. 
Schmitt, M. A. Gutjahr and H. Saufferer in Power Sources 7, page 219 and 
page 225, Academic Press 1979). However, very high degrees of filling 
around 2 g/cm.sup.3 are not suitable because of the considerable expansion 
of the electrode. The density of nickel hydroxide (3.94 g/cm.sup.3) means 
that a paste (suspension, slurry) which is intended to achieve the 
specified range of 1.2 to 2.2 g of nickel hydroxide/cm.sup.3 in a single 
filling operation must have a proportion by volume of nickel hydroxide of 
at least 30.5%. 
In German Published, Unexamined Patent Application (DE-OS) 2,427,421, the 
proposal was made to allow freshly precipitated nickel hydroxide suspended 
in a mother liquor to act on a horizontally mounted fiber plaque. The 
application of vacuum to the lower side of the plaque and the excitation 
by an ultrasonically driven vibrator electrode in the suspension promote 
the penetration of the nickel hydroxide into the carrier plaque. The 
filling is, however, inadequate since an additional soaking with nickel 
nitrate melt and a chemical preciPitation of further nickel hydroxide by 
alkali is carried out. This is not surprising since the nickel hydroxide 
is precipitated in very bulky form with considerable quantities of water 
of crystallization and anion residues and the necessary density and 
proportion by volume is far from being achieved in the suspension. 
The process described in the U.S. Pat. No. 4,217,939 starts from 
commercially obtainable nickel hydroxide powder with 10% nickel powder 
added as a conduction aid. An aqueous paste is formed which has a dry 
material concentration of 17% by weight for which a proportion by volume 
of 34.4% by volume of nickel hydroxide can be calculated. A reticulate 
metal foam plaque is passed horizontally on a perforated plate over the 
paste container in which the paste is agitated by stirring and is forced 
upwards into and around the plaque while paste is spread into the plaque 
from above by doctor blading. Before impregnation, it is necessary to fill 
the pores of the plaque with water since the paste cannot otherwise be 
introduced into all the pores. If necessary, additional dry nickel 
hydroxide is applied to the upper side of the plaque in order to improve 
the filling result. From this information it is apparent that the transfer 
of the paste is not achieved with a homogenous working material. The 
changes in concentration (displacement of the pure water situated in the 
pores by paste, or additionally applying dry Powder) make it difficult to 
carry out the process continuously. 
In one publication (W. A. Ferrando and W. W. Lee, Proc. of the 31st Power 
Sources Symposium 1984, page 177), the starting point is also 
prefabricated nickel hydroxide which is made up with ethylene glycol to 
form a paste. The specified ratio by mass of 1:3 results, after a 
conversion to percentages by volume, in a proportion by volume of nickel 
hydroxide of 8.6%. The paste (heavy cream) is rubbed into a nickel fiber 
plaque. After drying, the procedure is repeated in order to increase the 
loading. The necessity for this results from the nickel hydroxide content 
of the paste. In addition, the use of an organic fluid instead of water is 
uneconomical, it is necessary to recover the fluid during drying and, in 
addition, disposal problems may arise for the solvent vapors produced. 
In the Japanese published specification No. (Kokai) 81-82,577, a paste 
composed of nickel hydroxide, cobalt hydroxide, methylcellulose, 
nickel-plated polyethylene fibers, nickel powder and water is applied to a 
nickel-plated perforated iron plate and calandered to the desired 
thickness after drying. The properties of the paste, in particular, the 
fiber content, make it unsuitable for filling the pores of foam-structure 
or fiber-structure electrode plaque. 
According, the object of the invention is to provide an aqueous nickel 
hydroxide paste of high flowability for the vibration filling of 
foam-structure and fiber structure electrode plaque which makes it 
possible to fill said plaque completely in one operation. 
These and other objects are achieved by a paste having a content of nickel 
hydroxide of about 30% to 50% by volume, a maximum particle size of about 
0.04 mm, a plastic viscosity of about 0.1 to 1 Pa.s and a yield value of 
between about 10 and 120 Pa, and also a pH of between about 9 and 12, and 
a content of about 0.5% to 5% by weight, based on the nickel hydroxide 
content, of a dispersant selected from the group consisting of 
water-soluble salts of polyphosphoric acid, wherein the polyphosphoric 
acid has about 3 to 20 phosphorus atoms per molecule. 
The high content of nickel hydroxide of about 30% to 50% by volume is 
necessary in order to be able to produce, in one filling step, electrodes 
which have the concentrations of active material and consequently capacity 
values per unit volume which have been achievable only with a plurality of 
steps by the methods hitherto introduced. The particularly preferred range 
is between about 35% and 45% by volume. Such a high concentration can only 
be achieved with the aid of specific, very effective dispersants. 
Suitable dispersants are the water-soluble salts, in particular the alkali 
salts, of polyphosphoric acids or of di- and polyphosphonic acids and 
their derivatives. Particularly suitable are polyphosphates containing 
about 2 to 20 phosphorus atoms per molecule, and in particular, the 
polyphosphates containing about 16 to 20 phosphorus atoms in the molecule 
are preferred. Dispersants from the group comprising di- and 
polyphosphonic acids and their derivatives should not have more than about 
two carbon atoms per phosphorus atom per molecule since the conversion of 
the carbon atoms in the course of the electrode reactions would otherwise 
produce an intolerably high impurity content. Of this group, 
1-hydroxyethane-1, 1-diphosphonic acid (HEDP) or 
aminotrismethylenephosphonic acid in the form of their alkali salts are 
particularly suitable. 
The sodium and potassium salts of 1-hydroxyethane-1, 1-diphosphonic acid 
(HEDP) exhibit a stronger liquefying action in nickel hydroxide pastes 
than the polyphosphates, i.e. for the same viscosity, a paste containing 
an HEDP salt as dispersant contains more nickel hydroxide, or for the same 
nickel hydroxide content, a paste containing HEDP salt as dispersant has a 
lower viscosity. Hereinafter, the tetrabasic acid anion is abbreviated to 
HEDP and the hydrogen atoms written separately. While the free acid 
H.sub.4 (HEDP) produces very viscous paste, an optimum in the effect is 
between NaH.sub.3 (HEDP) and Na.sub.2 H.sub.2 (HEDP) The pH of the paste 
is about 10 or 11.2 and is consequently within the claimed range or 
between about 9 and 12. The pH of the paste containing Na.sub.3 H (HEDP) 
is already about 12.1. At this value, the absorption of CO.sub.2 from the 
air reaches a level which jeopardizes the use of the filled electrode in 
storage cells. 
A surprising further increase in the nickel hydroxide concentration or 
reduction in the viscosity (reduction in the yield value or the plastic 
viscosity) is achieved by using an alkali-metal/cobalt complex of 
1-hydroxyethane-1, 1-diphosphonic acid. Within the general formula 
Co.sub.x K.sub.y H.sub.z (HEDP), with 2x +y+z=4, the following ranges are 
permissible: x=about 0.5 to 1.25, y=about 0.5 to 1.5 and z=about 0.2 to 2. 
Outside these ranges, the pH is already too high and the yield value 
increases. The fluid can be produced by dissolving cobalt hydroxide in a 
suitable quantity in aqueous 1-hydroxyethane-1, 1-diphosphonic acid and 
adding a suitable quantity of alkali hydroxide. 
The dispersant is used in quantities of about 0.5% to 5% by weight, based 
on nickel hydroxide, in particular in quantities of about 1% to 5% by 
weight. A higher addition only increases the number of foreign ions later 
present in the electrolyte without, however, achieving an improved effect. 
Below about 0.5% by weight, the effect of the dispersant is in some cases 
already too low and the viscosity of the paste is therefore too high. 
In order to be able to penetrate the pores of the foam-structure or 
fiber-structure plaque, the paste must have an adequate flowability. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
From a rheological point of view, the nickel hydroxide paste according to 
preferred embodiments of the present invention belongs to the plastic 
bodies. Usually an ideal plastic behavior is found (so called Bingham 
bodies). The viscosity is not a material constant and it can be 
represented only in a flow diagram (shear stress in Pa against velocity 
gradient in s.sup.-1). Below a certain shear stress, the paste is solid, 
it having a yield value. In the case of ideal plastic behavior, there is 
linearity between shear stress and velocity gradient after exceeding the 
yield value. The plastic viscosity is the quotient of the shear stress 
less the yield value and the velocity gradient. The yield value and the 
plastic viscosity describe the system completely. 
For an explanation of the rheological terms and measuring methods, 
reference may be made to the Contraves Company publication entitled 
"Messung rheologischer Eigenschaften" ("Measurement of rheological 
properties:) (Bulletin T 990 d-7309), Section 6.3 (Plastic flow behavior) 
and also the the publication by A. Fincke and W. Heinz entitled "Zur 
Bestimmung der Fliessgrenze grobdisperser Systeme" ("On the determination 
of the yield value of coarsely dispersed systems"), Rheologica Acta, 1 
(1961), 530. The measurements were carried out with the rotation 
viscometer Rotovisco RV 12 manufactured by Haake using the measuring 
devices NV and MV I. Shear rates (velocity gradients) of at least about 
100 s.sup.-1 should be achieved. For the purpose of evaluation, use was 
made of the relaxation curve. The measurements were carried out at 20 C. 
The paste is suitable for filling the pores if the yield value is between 
about 10 and 120 Pa and the plastic viscosity is about 0.1 to 1 Pa.s, the 
preferred range being between about 0.15 and 0.25 Pa.s. The paste should 
preferably have a slight thixotropy (time dependence of the viscosity). 
After the filling operation, the paste should still remain liquid for a 
time in order to facilitate the removal of excess paste from the surface 
of the filled body. However, it should not be so liquid that it is able to 
escape from the pores again and form troublesome drainage drops. 
The maximum particle size of the paste must be markedly below the mean pore 
size of the substrate to be filled in order to avoid blockages. The 
maximum particle size is about 0.04 mm and the preferred range is between 
about 0.015 and 0.03 mm. To determine the particle size, use is made of a 
so called grindometer in which a paste coating with decreasing thickness 
is assessed. The maintenance of the pH between about 9 and 12, in 
particular between about 10 and 12, is also important for the viscosity of 
the paste and the behavior during filling. If the pH is too high, such a 
strong carbonation already occurs in the paste that the finished 
electrodes are subsequently unusable. 
The beneficial effect of a cobalt doping on the nickel oxide electrode is 
known. The paste is doped with additives of cobalt powder or with cobalt 
compounds in the form of the oxides, hydroxides, borates, phosphates and 
the like. Because of the high density and the good effectiveness, cobalt 
powder is particularly preferred. The total quantity of cobalt is about 2 
to 12 atom percent, based on nickel. The use of the alkali-metal/cobalt 
complex of HEDP not only achieves, as already described, a surprising 
further rise in the nickel hydroxide concentration, but some of the cobalt 
required for the doping can also very advantageously be introduced into 
the paste. The cobalt introduced via the cobalt complex has a very 
particularly uniform distribution and is therefore very particularly 
effective in the electrode. 
The paste is best produced in a ball mill using grinding balls of ceramic 
material. If the paste is to be doped with metallic cobalt, the mill must 
be hermetically sealed owing to the sensitivity of cobalt to oxidation in 
alkaline medium. The optimum grinding time for producing the paste 
depends, inter alia, on the paste mix, on the degree of filling of the 
mill, on the grinding balls and on the rotary speed, but can be determined 
in an easy manner per se. In general the grinding time is up to about 24 
hours. Usually, the mixture passes through a phase at the beginning with a 
thin-bodied consistency. Sometimes, dilatant behavior, which disappears 
with the grinding time, is also observed. The paste is ready if the 
specified viscosity data and the grindometer value are in the claimed 
range. 
EXAMPLE 1 
About 405 g (about 36.7% by volume) of pulverulent nickel hydroxide 
(manufactured by Riedel-de-Haen), about 12.5 g (190 % by volume) of cobalt 
powder and about 182 g (about 176 ml, about 62.8% by volume) of an about 
5% aqueous sodium polyphosphate solution containing about 17 phosphorus 
atoms in the polyphosphate ion (Calgon 322 manufactured by 
Benckiser-Knapsack) were ground for about 16 hours in about a 1L porcelain 
ball mill with the aid of about 540 g of grinding balls composed of 
aluminium oxide ceramic and having a diameter of about 16 mm at a rotary 
speed of about 70/min. The following rheological data were determined from 
the flow curve of the paste by extrapolation and regression calculation. 
(Measuring apparatus: Viscometer RV 12 manufactured by Haake, measuring 
device MV I, maximum velocity gradient equals about 300 s.sup.-1, T= about 
20 C.: yield value =about 105 Pa, plastic viscosity= about 0.30 Pa.s. The 
maximum particle diameter determined with the grindometer was about 23 
.mu.m. 
EXAMPLE 2 value= 
Instead of sodium polyphosphate solution, an about 5% by weight aqueous 
solution of the disodium salt of 1-hydroxyethane-1, 1-diphosphonic acid, 
Na.sub.2 H.sub.2 (HEDP), was used as fluid. The mixture in the ball mill 
had the same composition as in Example 1. The grinding conditions were 
also unaltered. The viscosity data obtained for the finished paste were 
markedly lower: yield value =about 63 Pa, plastic viscosity= about 0.12 
Pa.s. The pH of the paste was about 11.2 and the maximum particle size was 
about 18 .mu.m. 
EXAMPLE 3 
On about 0.2 molar solution of the composition CoK.sub.1.5 H.sub.0.5 (HEDP) 
was used as dispersive solution. To produce a liter of this solution, 
about 68.7 g of about 60% aqueous HEDP acid (Turpinal SL, manufactured by 
Henkel) were weighed out into a beaker and approximately about 800 ml of 
deionised water was added. About 18.6 g of cobalt hydroxide were dissolved 
in it while stirring. About 35 g (about 23.7 ml) of about 47% KOH were 
then added while stirring, which increased the pH of the solution to about 
6. The solution was transferred to a volumetric flask and made up to about 
1 liter with water. 
To produce the paste, about 500 g (about 40% by volume) of nickel 
hydroxide, about 15.5 g (about 0.55% by volume) of cobalt powder and about 
195.3 g (about 188.5 ml) (about 59.45% by volume) of fluid were rolled 
together with about 650 g of grinding balls in a 1 liter porcelain ball 
mill for about 20 hours at about 70 rev/min. Despite the increased 
concentration of nickel hydroxide, a paste of lower viscosity resulted 
which had the yield value of about 18 Pa and the plastic viscosity of 
about 0.21 Pa.s. The grindometer value was about 20 .mu.m. The pH was 
about 11. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only, and is not to be taken by way of 
limitation. The spirit and scope of the present invention are to be 
limited only by the terms of the appended claims.