Hydrogen storage alloy electrode and method for preparing the same

Disclosed is a hydrogen storage alloy electrode prepared by integrating a mixture of a hydrogen storage alloy powder, an electrical conducting powder and a polymer binder with a current collector, characterized in that the polymer binder is composed of a polyacrylic acid salt and polytetrafluoroethylene as essential components, and that the hydrogen storage alloy powder is coated with polyacrylic acid salt, wherein the three-dimensional reticulate molecular chain of the polyacrylic acid salt itself is partially severed. Disclosed also is a method of preparing a hydrogen storage alloy electrode which comprises the steps of preparing a paste of a mixture containing hydrogen storage alloy powder, electrical conducting powder and a polymer binder, coating a current collector with the paste, drying it and pressure molding the current collector coated with the paste, the method being characterized in that at least a polyacrylic acid salt and polytetrafluoroethylene are used as the polymer binder, and after the paste is stirred at a high speed, the current collector is coated with the paste.

BACKGROUND OF THE INVENTION 
This invention relates to a hydrogen storage alloy electrode used as a 
negative electrode of an alkaline secondary battery and a method for 
preparing the hydrogen storage alloy electrode, and more particularly it 
relates to a paste type hydrogen storage alloy electrode in which a great 
deal of hydrogen storage alloy is present per unit volume and which 
permits the preparation of a battery having great capacity, is 
inexpensive, and can be mass-produced, and a method for preparing the 
same. 
Much attention is now widely paid to a hydrogen battery which mainly 
consists of various hydrogen storage alloys, and uses a hydrogen storage 
alloy electrode, as a negative electrode, since the above hydrogen battery 
has high energy density. In particular, the hydrogen storage alloys of 
LaNi.sub.5 series, CaNi.sub.5 series and the like are expected as base 
materials for the above electrode, since they have low hydrogen 
equilibrium pressure. 
In this kind of hydrogen storage alloy electrode, charge and discharge 
reactions take place as follows: At the time of the charge, hydrogen 
generated due to the electrolysis of water on the surface of the electrode 
is stored in the hydrogen storage alloy, and inversely at the time of the 
discharge, the stored hydrogen is released from that alloy and is then 
reacted with a hydroxyl ion, whereby the charge/discharge reaction is 
achieved. 
Thus, in order to increase the energy density of the battery, there have 
been suggested two methods. One method intends to increase the amount of 
stored hydrogen per unit weight of the hydrogen storage alloy in the 
electrode, and another method intends to increase the amount of the 
hydrogen storage alloy which can be contained in the unit volume of the 
electrode. 
As a method for preparing this hydrogen storage alloy electrode, for 
example, there is known a method which comprises first mixing hydrogen 
storage alloy powder with electrical conducting powder such as carbon 
black and then sintering the resulting mixture to a porous material 
(Japanese Unexamined Patent Publication No. 46827/1983) or a method 
comprises rolling a mixture obtained by kneading hydrogen storage alloy 
powder, polytetrafluoroethylene (PTFE) and viscosity increasing agent, 
followed by contact bonding to a current collector (see Japanese 
Unexamined Patent Publication No. 66366/1986). 
In addition, the following other methods are also known: 
(i) A method comprising the steps of kneading hydrogen storage alloy powder 
with Teflon grains, forming the mixture into a sheet, and pressing the 
sheet against a net-like current collector; (ii) a method comprising the 
step of packing a three-dimensional electrode core with a hydrogen storage 
alloy in the state of powder or a paste of the powder; (iii) a method 
comprising the steps of mixing a binder with hydrogen storage alloy powder 
and then pressing the mixture into pellets; (iv) a method comprising the 
step of rolling a hydrogen storage alloy; and (v) a method comprising the 
steps of kneading hydrogen storage alloy powder with a polymer binder such 
as polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA) or 
carboxymethyl cellulose (CMC) and an electrical conducting material to 
form a paste, coating a net-like current collector such as a punched metal 
with the paste, drying it, and then subjecting the entire current 
collector to a pressure molding treatment by use of a roller press. The 
pressure molding treatment in the last method (v) contemplates increasing 
the amount of the hydrogen storage alloy included in the unit volume of 
the prepared electrode. 
However, each of the above methods for preparing the electrode has some 
drawbacks. That is, in the method (i), it is difficult to continuously 
prepare the electrodes. In the method (ii), the three-dimensional core 
which is a current collector is very expensive, and thus the industrial 
value of the electrode containing such a core is low. In the method (iii), 
it is difficult to make a large-scale electrode, enough to make a wound 
from electrode used in a cylindrical secondary battery. In the method (iv) 
of rolling of alloy, its use application is limited by a selected kind of 
alloy, and this method cannot be applied to hard and brittle alloys such 
as La series hydrogen storage alloys. Moreover, in preparing the hydrgen 
storage alloy electrode in accordance with the method (v) (a paste 
system), the following problem is posed: 
That is, when a current collector net is coated with such a paste as 
mentioned above and is then dried, if the selected binder is CMC, the 
coated paste is solidified after the drying and is then easily peeled down 
from the current collector net in the subsequent pressure molding process. 
In such a state, the amount of the hydrogen storage alloy contained in the 
unit volume of the electrode is not so much as expected. 
For the purpose of solving this problem, there have been suggested methods 
of using, as the current collectors, various kinds of foamed metals having 
a three-dimensional structure and sintered metallic fibers, but these 
current collectors all are very expensive, and so their industrial value 
is low. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a hydrogen storage alloy 
electrode prepared by means of pasting, having a large amount of the 
hydrogen storage alloy to be contained in the unit volume of the 
electrode, which can facilitate mass production at a low cost and the 
method of preparing the same. 
The present inventors have intensively conducted researches with the 
intention of achieving the above-mentioned object, and as a result, they 
have found the fact that when a polyacrylic acid salt and PTFE are used as 
binders, a paste including these binders is not peeled down from a current 
collector coated therewith after drying, even if pressure molding is 
performed. 
Furthermore, when the paste which has been previously stirred at a high 
speed is used, the surface of the hydrogen storage alloy grains is covered 
with the polyacrylic acid salt, whereby the oxidation of the alloy grains 
is prevented. That is, in the case that the alloy surfaces is not covered 
with the polyacrylic acid salt, three-phase interfaces of the alloy, water 
and air come out during the drying of the paste, and on these three-phase 
interfaces, a local battery reaction represented by the following formula 
occurs, so that the hydrogen storage alloy is rapidly oxidized, with the 
result that the hydrogen storage ability of the alloy is finally lost: 
##STR1## 
However, the present inventors have also found the following fact: Among 
the possible contacts between the alloy, water and air which occurs during 
the drying of the paste, the contact between the alloy and air can be 
prevented in case that the alloy surface is covered with the polyacrylic 
acid salt film. So that the alloy is scarcely oxidized. 
On the basis of the above facts, the present inventors have developed the 
hydrogen storage alloy electrode and the method of preparing the same of 
this invention. 
That is, according to this invention, there are provided a hydrogen storage 
alloy electrode prepared by integrally joining a mixture of hydrogen 
storage alloy powder, electrical conducting powder and a polymer binder to 
a current collector, the aforesaid hydrogen storage alloy electrode being 
characterized in that the polymer binder is composed of a polyacrylic acid 
salt and polytetrafluoroethylene as essential components; and a method for 
preparing a hydrogen storage alloy electrode which comprises the steps of 
preparing a paste of a mixture containing hydrogen storage alloy powder, 
electrical conducting powder and a polymer binder, coating a current 
collector with the paste, drying it, and pressure molding the current 
collector with the paste, the aforesaid method being characterized in that 
at least a polyacrylic acid salt and polytetrafluoroethylene are used as 
the polymer binder, and after the paste is stirred at a high speed, the 
current collector is coated with the paste.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the preparation of the electrode of this invention, hydrogen storage 
alloy powder, electrical conducting powder, a polymer binder and water are 
first mixed to prepare a paste, and the latter is then stirred at a high 
speed. 
Raw materials for the hydrogen storage alloy are not particularly limited, 
but they must have the function to store hydrogen electrochemically 
generated in an electrolyte and to easily release the stored hydrogen 
therefrom at the time of discharge. Examples of the alloy materials 
include LaNi.sub.5 ; MnNi.sub.5 (Mm: misch metal); LmNi.sub.5 (Lm: misch 
metal rich in lanthanum); multi-element series alloys in which Ni atoms of 
the above compounds are partially replaced with other metals such as Al, 
Mn, Fe, Co, Ti, Cu, Zn, Zr and Cr; Mg.sub.2 Ni series alloys; TiNi series 
alloys; and TiFe series alloys. 
These alloys, when used, may be usually ground into powder having an 
average powder diameter of about 10 to about 30 .mu.m. 
Examples of the above-mentioned electrical conducting powder materials 
include carbon black and graphite powder. 
The paste regarding this invention is characterized by containing, as the 
essential components, the polyacrylic acid salt and PTFE which are the 
polymer binders, and by being stirred at a high speed. In this case, 
functions of the respective components are not clearly elucidated, but in 
terms of phenominalism, the polyacrylic acid salt is effective to bind the 
hydrogen storage alloy powder together during the drying of the coated 
paste, and PTFE becomes fibrous by itself at the time of pressure molding 
and effectively retains the hydrogen storage alloy powder. 
Here, important things are as follows: PTFE becomes fibrous by a roller 
press, and thus the hydrogen storage alloy must be retained on this 
fibrous PTFE by another binder. The polyacrylic acid salt is considered to 
adhesively contribute to the above retention of the fibrous PTFE. The 
reason why the surfaces of the hydrogen storage alloy powder is covered 
with the polyacrylic acid salt by stirring the paste at a high speed would 
be that molecular chains of the polyacrylic acid salt are severed due to 
the high-speed stirring operation to increase the flowability of the 
paste. That is, it can be presumed that the above-mentioned two components 
exert their specific effects in the respective unit steps of the 
preparation of the electrode, so that the hydrogen storage alloy powder is 
prevented from peeling down and the amount of the hydrogen storage alloy 
in the paste is increased. 
Examples of the polyacrylic acid salts include sodium polyacrylate and 
ammonium polyacrylate, and an example of PTFE is dispersion type PTFE. 
In the paste regarding this invention, the amount of the polyacrylic acid 
salt is preferably in the range of 0.1 to 0.8 parts by weight with respect 
to 100 parts by weight of the hydrogen storage alloy powder. When the 
amount of the polyacrylic acid salt is less than 0.1 parts by weight, the 
above-mentioned effect cannot be exerted sufficiently, and it is 
impossible to uniformly coat the current collector with the paste. 
Moreover, the effect of the polyacrylic acid salt reaches its upper limit 
at an amount of 0.8 parts by weight, and thus the addition of the 
polyacrylic acid salt in excess of this amount is useless and doesn't 
improve the spreadability. In addition, the excessive amount of the 
polyacrylic acid salt disturbs electrical conduction among the hydrogen 
storage alloy powder. 
The amount of PTFE is preferably in the range of 0.5 to 4 parts by weight 
in terms of solid. When the amount of PTFE is less than 0.5 parts by 
weight, the binding effect of PTFE is low in the pressure molding step, 
and when it is more than 4 parts by weight, conductive properties of the 
electode deteriorate all over, and PTFE aggromerates as it becomes fibrous 
during the high-speed stirring, so that the paste cannot be applied onto 
the current collector uniformly. 
The amount of the electrical conducting material is usually in the range of 
0.1 to 5 parts by weight. 
The paste may be prepared by mixing predetermined amounts of the above 
respective components and then stirring the mixture at a high speed. At 
this time, CMC and PVA which have been heretofore used as binders may be 
added thereto in suitable amounts. Moreover, water may be added thereto so 
as to adjust the viscosity of the paste. 
In stirring the paste at a high speed, for example, while an electromotive 
stirring machine may be used. Revolutions and the like of the stirring 
machine may be varied depending on the conditions employed so long as it 
is operated at a sufficient speed to achieve severing of the 
three-dimensional reticulate structure of the polyacrylic acid salt. It 
should be noted that the polyacrylic acid salt assumes a sol state by 
carrying out this high-speed stirring. 
With the thus prepared paste, the current collector is coated in a 
predetermined thickness, and the paste is dried at a temperature of about 
80.degree. C., followed by pressure molding by use of a roller press, 
thereby obtaining the hydrogen storage alloy electrode of this invention. 
The density of the hydrogen storage alloy in the electrode obtained may be 
4.0 g/cm.sup.3 or higher, preferably between 4.5 and 6.0 g/cm.sup.3. The 
reason for the above is that a density of at least 4.0 g/cm.sup.3 is 
required in order to secure electric connection of the hydrogen storage 
alloy powder itself. 
If the density is less than 4.0 g/cm.sup.3, electric connection of the 
hydrogen storage powder itself may not be obtained; whereas if the density 
is more than 6.0 g/cm.sup.3, retention of the electrolytic solution will 
be difficult. 
Examples 1 to 5 
Prepared was 100 g of hydrogen storage alloy powder having a composition of 
LaNi.sub.4.7 Al.sub.0.3 and an average powder diameter of 20 .mu.m, and to 
this alloy, sodium polyacrylate and PVA or CMC were added in ratios of 
Examples 1 to 5 in Table 1. In addition, carbon was added thereto as an 
electrical conducting material, and water and a PTFE dispersion were 
further added thereto, followed by high-speed stirring, thereby obtaining 
pastes. 
Each punched metal current collector was coated with the thus obtained 
paste, and after drying by warm air at 80.degree. C., and was subjected to 
a roller press operation. The press operation was carried out until the 
current collector was stretched 20% in the length and the amount of the 
hydrogen storage alloy reached about 4.5 g/cm.sup.3. 
Strengths of the electrodes thus prepared are set forth together with 
compositions of binders in Table 1. 
Comparative Examples 1 to 10 
For comparison, the same procedure as in Examples 1 to 5 was repeated in 
order to prepare hydrogen storage alloy electrodes having compositions set 
forth in Table 1. 
Strengths of the electrodes thus prepared are set forth together with 
compositions of binders in Table 1. 
TABLE 1 
______________________________________ 
Composition of Binder 
Sodium PTFE Strength 
Poly- Disper- of 
acrylate 
sion CMC PVA Electrode 
______________________________________ 
Example 1 
0.5 g 1.65 ml -- -- Good 
Example 2 
0.1 g 1.65 ml -- -- Good 
Example 3 
0.5 g 1.65 ml 0.5 g -- Good 
Example 4 
0.5 g 1.65 ml -- 0.5 g Good 
Example 5 
0.5 g 1.65 ml 0.25 g 
0.25 g 
Good 
Comparative 
0.05 g 1.65 ml -- -- Coating was 
Example 1 impossible 
Comparative 
0.05 g 1.65 ml 0.5 g -- Paste was 
Example 2 peeled down 
Comparative 
0.5 g -- -- -- Paste was 
Example 3 peeled down 
Comparative 
-- 1.65 ml -- -- Coating was 
Example 4 impossible 
Comparative 
-- -- 0.5 g -- Paste was 
Example 5 peeled down 
Comparative 
-- -- 1.0 g -- Paste was 
Example 6 peeled down 
Comparative 
-- -- 1.0 g 1.0 g Paste was 
Example 7 peeled down 
Comparative 
-- -- -- 1.2 g Paste was 
Example 8 peeled down 
Comparative 
-- 1.65 ml 0.2 g -- Paste was 
Example 9 peeled down 
Comparative 
-- 1.65 ml -- 0.2 g Paste was 
Example 10 peeled down 
______________________________________ 
Each figure above denotes an amount with respect to 100 g of the alloy. 
The PTFE dispersion contained 0.9 g of PTFE solid in 1 ml thereof. 
As shown in Table 1, the electrodes having good strengths are not obtained 
when contents of sodium polyacrylate and PTFE are not contained proper 
amount. Moreover, as is apparent from the results in Table 1, even when 
other binders are added, similar effects can be obtained, if sodium 
polyacrylate and PTFE are present. 
Next, the electrode (electrode A) of Example 1 and an electrode (electrode 
B) made without the high-speed stirring from a material in which the 
composition of the binder was the same as in Example 1 were each used to 
build up a test cell shown in FIG. 1, and tests were carried out to 
inspect charge/discharge efficiencies of hydrogen storage alloy 
electrodes. 
In FIG. 1, reference numeral 1 is a hydrogen storage alloy electrode, and 
this electrode 1, as well as the Ni electrodes 3, 3 disposed on both sides 
of said electrode 1 through separators 2, 2 comprising a polypropylene 
unwoven fabric, is supported between acrylic resin holding plates 4, 4. 
Numeral 5 is a reference electrode which consists of a mercury oxide 
electrode and is housed in an acylic resin battery vessel 7l which is 
filled with an electrolyte 6 comprising a 8N KOH aqueous solution, and the 
above-mentioned hydrogen storage alloy electrode 1 is also received in the 
vessel 7. In this case, the Ni electrode 3 is added in an excessive amount 
to the hydrogen storage alloy electrode. 
In the test conditions, charge was carried out at a current of 170 mA for 1 
hour with respect to 1 g of the hdyrogen storage alloy contained in the 
electrode, and discharge was carried out at the similar current until the 
potential of the hydrogen storage alloy electrode reached a level of -0.7 
V to a mercury oxide electrode. Under these test conditions, the cycle of 
the charge/discharge was repeated. The results are shown in FIG. 2. 
As is apparent from this drawing, the efficiency of the electrode B made 
without the high-speed stirring is as low as 60% or so, whereas that of 
the electrode A made performing the high-speed stirring is very excellent 
and is about 95% or more even after 200 cycles. 
As is apparent from the above description, the electrode of this invention 
can have high strength, even when the amount of the hydrogen storage alloy 
per unit volume of the electrode is increased to 4.0 g/cm3 or more. 
Moreover, in the electrode of this invention, a dried paste is not peeled 
down from a current collector, and charge/discharge efficiency is also 
high. In consequence, the electrode of this invention is useful as an 
electrode for a high energy density battery and is thus considered to have 
a great industrial value.