Electrode for primary or secondary battery and method for producing such an electrode

Electrode for a primary of secondary battery the electrochemically active part of which comprises a porous layer of a hydride of an intermetallic compound which is sintered to a metallic support, the pores being filled with a hydrophylic water-insoluble macromolecular material.

The invention relates to an electrode for a primary or secondary battery, 
the electrochemical active part of which comprises an intermetallic 
compound which can absorb and release reversibly hydrogen while forming 
hydrides. 
Such an electrode may, for example, constitute the negative electrode in a 
primary or secondary battery. During the supply of electric energy the 
reaction H .fwdarw. H.sup.+ + e takes place. The inermetallic compounds 
used for this purpose are, generally, brittle. With a repeated charging 
and discharging such as this takes place with a secondary battery or 
accumulator the material expands and shrinks respectively which may cause 
the material to pulverize. For this reason it is substantially impossible 
to fabricate an electrode having a suitable life from the material alone, 
for example by means of sintering. It is known to increase the mechanical 
strength of the electrodes by applying the active electrode material on a 
metallic support. In itself this method appears insufficient for 
electrodes in which the electrochemically active part consists of an 
intermetallic compound and which, during charging and discharging absorb 
and release reversible hydrogen while forming hydrides to maintain the 
cohesion in the electrochemically active part of the electrode. To fully 
utilize the good electrochemical loadability of the intermetallic 
compounds to be used it is furthermore desirable to give the electrodes a 
large surface area and to choose the distance between the electrodes to be 
small in view of the internal resistance. This may be achieved by making 
the positive and negative electrode in the form of a strip and, for 
example, to wind them into a roll while separating them from one another 
by means of a separator. With this construction the electrodes must, of 
course, be flexible. It is also possible to combine a plurality of 
strip-shaped electrodes to a package, separated by means of separator 
layers. 
It is an object of the invention to provide a flexible electrode which has 
a life which is sufficient for the desired purpose, in which the 
electrochemically active part of the electrode consists of an 
intermetallic compound which can absorb and release reversible hydrogen. 
According to the invention this object is fulfilled by means of an 
electrode consisting of a porous metal support provided with a porous, 
sintered layer of a powder of the intermetallic compound in which the 
pores of the sintered layer contain a hydrophylic, water-insoluble 
macromolecular substance. 
An electrode according to the invention may, for example, be obtained in 
the following manner: 
An intermetallic compound is pulverized by charging it with hydrogen and by 
liberating it from hydrogen once or several times. Suitable intermetallic 
compounds are, for example, lanthanum-nickel (LaNi.sub.5) and 
lanthanum-nickel compounds in which the lanthanum is partly replaced by 
another rare earth metal and in which nickel is, for example partly, 
replaced by cobalt, copper and iron. It is recommended that compounds in 
which the so-called plateau-pressure does not exceed 1 atmosphere at the 
operating temperature be used. A suitable compound is, for example, 
LaNi.sub.4 Cu, the plateau pressure of which is between 0.7 and 0.8 
atmosphere at 20.degree. C. In this connection plateau pressure must be 
understood to mean that hydrogen pressure at a given temperature over the 
intermetallic compound which is independent of the hydrogen concentration 
in the compound. The powder of the intermetallic compound may be applied 
to the metal support in various manners, for example by means of a binder. 
To this end the powder, which may consist fully or partly of the hydride 
is, for example, suspended in an organic solvent, in which also the binder 
can be dissolved. Suitable organic solvents are for example: toluene, 
xylene, propanol. 
Thereafter an organic binder is added to the suspension obtained, in a next 
step it must be possible to remove this binder by means of firing without 
leaving any residue. For this purpose polysterene and nitrocellulose may, 
for example be used as binder in a quantity of, for example, 20 g per 100 
ml of solvent. Thereafter a strip of metal gauze is uniformly coated on 
both sides or on one side, what ever the requirement, with the suspension 
and dried. The metal gauze may, for example, consist of nickel or 
stainless steel. A perforated metal plate may also be used as porous metal 
support. Now the binder is first removed by firing in a furnace and 
thereafter the powder is sintered. To this end the temperature is kept 
constant for some time at a value at which the binder evaporates or 
decomposes. For the binders mentioned this temperature is generally 
250.degree.-300.degree. C. Sintering takes place at approximately 
900.degree. C for, for example, at least five minutes. Sintering is 
preferably done in vacuum or in a reducing atmosphere. The powder of the 
intermetallic compound may also be applied to the metal gauze by means of 
electrophoresis. The powder is then suspended in a polar organic solvent 
such as methanol. The metal support is placed in the suspension and 
connected, for example as cathode. After a layer of the desired thickness 
has been obtained it is sintered as described above. The porous layer 
obtained in this way is thereafter impregnated with a solution of a 
macromolecular substance which is either hydropylic and which, in a next 
processing, has been made water-insoluble, for example by means of a heat 
treatment or radiation, or which is insoluble in water and is made 
hydrophylic in a following process, for example by means of 
saponification, in which the macromolecular substance must, of course, 
remain insoluble in water. As hydrophylic macromolecular substances, 
especially macromolecular substances may be used with alcoholic hydroxyl 
groups which can be made water-insoluble in, for example, an aqueous 
electrolyte, by means of a physical treatment such as a heat treatment 
either, or not, in the presence of an auxiliary substance which promotes, 
or effects hardening, or crosslinking. Polyvinylalcohol has, for example, 
proved particularly suitable for use as an electrode according to the 
invention in an aqueous electrolyte. In that case ammonium chloride or 
sodium hydrosulphate may, for example, be used as auxiliary material. The 
heat treatment consists in this case of heating to 120.degree.-150.degree. 
C for 10 to 20 minutes in a furnace in air. Water-insoluble macromolecular 
substances which may be used in the production are, for example, 
sponifiable cellulose derivatives such as cellulose acetate butyrate which 
are soluble in organic solvents. After impregnating the macromolecular 
substance is saponified by means of, for example, an alcoholic lye. 
By means of the sintering operation described above it is achieved that the 
hydrogen transfer between the particles of the intermetallic compounds can 
take place in a sufficient degree. The hydrophylic macromolecular 
substance serves as a binder which guarantees a permanent cohesion of the 
porous layer during charging and discharging. 
Owing to the hydrophylic character of the binder, an aqueous electrolyte 
can penetrate into the sintered layer which enables ion transport. The 
aqueous electrolyte may, for example, consist of a solution of potassium 
hydroxide. In that case the counter electrode then consists, for example, 
of NiOOH or manganese dioxide. The hydrophylic binder generally does not, 
or hardly, increase the internal resistance.

The reference numerals in FIG. 1 have the following meaning. References 1 
and 2 indicate the crosswise wires of a metal gauze. A layer of granules 3 
of an intermetallic compound is sintered to both sides of this gauze 1-2. 
An hydrophylic macromolecular material is between the granules 3. 
EMBODIMENT I 
The relevant electrode may, for example, be produced as follows: 25 parts 
by volume of LaNi.sub.4 Cu powder which has been obtained by charging the 
relevant material with hydrogen and by thereafter withdrawing hydrogen 
from the hydride form, is suspended in 75 parts by volume of toluene. 20 g 
of polystyrene is added per 100 ml of the suspension. After the 
polystyrene has been fully dissolved while stirring, a strip of nickel 
gauze (the wire diameter 0.15 mm, 64 wires per cm.sup.2) is homogeneously 
coated with the suspension on both sides, whereafter the suspension is 
dried in air at 80.degree. C. Thereafter, the binder is expelled at 
250.degree. C whereafter the layer is sintered at 900.degree. C for 5 
minutes in vacuum. Thereafter, the porous layer is impregnated with a 
solution of polyvinyl alcohol in water (10 g per 100 ml) which contains 
NH.sub.4 Cl (0.1 g per 10 g of polyvinylalcohol). After drying in air at 
80.degree. C the polyvinyl alcohol is made insoluble by heating at 
120.degree. C for 15 minutes. The ammonium chloride which acts as hardener 
is thereafter washed away with water. The product obtained is now suitable 
for use as electrode. 
EMBODIMENT II 
10 g of LaNi.sub.4 Cu powder is suspended in 100 ml of methanol while 
stirring. The metal gauze to be coated is placed in the suspension 
together with a counter electrode. A dc voltage of 30 volts/cm is applied 
while the metal gauze to be coated acts as negative electrode. In these 
circumstances approximately 25 mg of LaNi.sub.4 Cu per cm.sup.2 is 
deposited on the metal gauze in 30 seconds. In this method a sufficient 
quantity of the intermetallic compound is also deposited on the edges and 
corners of the metal support, which sometimes entails difficulties when 
the method described in Embodiment I is used. 
After drying, the LaNi.sub.4 Cu-coated gauze is subjected to a sintering 
treatment (900.degree. C for 5 minutes in vacuum). After cooling the gauze 
is immersed in a solution of cellulose acetate butyrate in methylene 
chloride (15 g of cellulose acetate butyrate in 85 g of methylene 
chloride). After drying the cellulose acetate butyrate is saponified in an 
alcoholic lye (6 g of KOH in 94 g of ethanol) for 1 minute. The product 
obtained may now be used as electrode. 
The secondary battery shown diagrammatically in FIG. 2 comprises in a 
casing 21 which, for example, consists of nickel or nickel plated steel a 
package, composed at LaNi.sub.4 Cu-coated metal gauze strips 22 (negative 
electrode), of porous nickel layers 23 (positive electrode) which comprise 
Ni(OH).sub.2 and separators of polypropylene fibers (felty) 24. The strips 
22 and the layers 23 are each interconnected (not shown) and connected to 
the poles 25 and 26 respectively. The electrolyte 25 in the battery 
consists of an aqueous KOH solution which contains 40% by weight of KOH. 
The electrodes according to the invention show no evidence of a decline in 
capacity, also not after repeated charging and discharging.