Note: Descriptions are shown in the official language in which they were submitted.
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BAC~;G~UND OF THE INV13NTION
This invention relates to pressed nickel electrodes
and, more particularly, it relates to a binder for use in such
electrodes.
Heretofore, pressed nickel electrodes have usually been
made by combinnng an active nickel compound with a binder and
other components such as conductive diluents and pressing the
resulting mixture into an apertured current collector at pressures
:; and temperatures required to make a cohesive integrated electrode.
Presen~ly-employed binders for use in pressed nickel electrodes
inc~ude polytetrafluoroethylene, polystyrene and polyethylene.
Use of presently-utilized binders is accompanied by one
or more of the following problems and disadvantages. One problem
~ results from the fact that the active nickel compounds can be
adversely affected by heating them to the temperatures re~uired to
sinter the binder during ~le electrode-formlng procedure. For
example, if nickel hydroxide, i.e., Ni(oH)2, is used as the active
electrode compound, it is converted to electrochemically inactive
~ bunsenite (Nio) at approximately 125C. Therefore, in order to
20 prevent the foregoing conversion from taking place, it.is
~: necessary to restrict the binder to a material which does not
require sintering at temperatures on the order of 125C or higher.
Another problem with binders presently utilized in
pressed nickel electrodes is that many of them bind the active
electrode material by coating the particles of the latter. Such
coating provides an electrically insulative layer around the active
electrode material thereby further reducing the internal electrical
conductivity of the electrode. Polytetrafluoroethylene is
~ exemplary of this type of binder.
: 30 Still another problem arises from the degradation of some :
binders such as silicone rubber in the chemical and electro-
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1 chemical environments to which ~le nickel electrode constituents
are subjected. Such degrad~tion of the binder results in
weake~ed electrode s-tructures.
SUM~RY OF T~IE Ii~VENTION
A pressed nickel electroae is made by combining an
electrochemically active nickel compound with an elastomeric binder
in the form of an emulsion or dispersion and pressing the re~ulting
admixture, ~hich may include other electrode constituents such
as conductive diluents, into a nickel current collector at a
1~ pressure sufficient to provide a cohesive electrode structure.
Preferably, the binder is butyl rubber, chlorobutyl rubber or
bromobutyl rubber. The amount o~ binder utilized varies between
about 1~ and 5~ by weight of the weight of the electrode mix
(grid and tabs on electrode excluded)~
Several benefits are realized from the use o~ the
herein-described binder. First, because this binder binds with-
out covering the active electrode material with an electrically
insulative layer, the internal electrical conductance of an
electrode is not adversely affected. Secondly, this binder can
~ be combined with the active electrode material without using
elevated temperatures so that the effective amount o~ the latter
material is not reduced by heating. Thirdly,~he elastomeric binder
described herein is relatively stable in the chemical and
electrochemical environment that is the pressed nickel electrode.
The foregoing benefits manifest themselves in improve-
; ments in the charge/discharge and energy density characteristics
; o~ electrochemical cells utilizing pressed nickel electrodes
incorporating the described binder materials as compared with
cells utilizing pressed nickel ~lectrodes incorporating presently-
emplQyed binder materials~
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D~SCRIPTION OF rr'HE DRAWINGS
T~le Figure is a graph of cell voltage vs. percent of
discharge for electrochemical cells incorpora-ting pressed nickel
electrodes made as described herein and incorporating prior art
pressed nickel electrodes, and illustrates the improvement in
cell performance obtained from cells incorporating electrodes
made as descri~ed herein.
DESCRIPTION OF THE PREFERRE:D EMBODIMENT
Pressed nickel electrodes are made by admixing an
1o effective amount of an electrochemically active nickel compound,
a binder of the type and in the form as described hereinafter, and
other desired electrode constituents to form a substantially
homogeneous admixture. That admixtu~e is pressed into an apertured
; current collector to form an integral, cohesive electrode. Except
for the binder, the other electrode constituents and their relative
proportions are well known.
The nickel compound may be nickel hydroxide or the -
berthollide ~ix where x is not an integer. A particularly useful
form o the latter is characterized in that it contains about
55~ by weight of nickel and has an x value between about 1.65 and
~ 1.8. The nickel electrode may also include, and preferably does
; include, as an active electrode materialr a cobalt-containing com-
pound such as cobalt hydroxide or the berthollide C0x which is
analogous to the ~iOx. Typically, the cobalt-con~aining compound
is present in an amount sufficient to provide a nickel/cobalt (wt.)
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ratio of about 9:1 to 9.8:0.2.
Other desirable electrode constituents which are
usually present in pressed nickel electrodes include conductive
`; diluents and pore formers. The conductive diluent is typically
nickel or graphite powder and the pore former is typically an
easily removed chemical compound such as ammonium carbonate.
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1 T;le current collectors may be, for example, perforated
nickel sheets, woven wire mesh, or expanded nickel metal.
The binder is an elastomeric material which is stable
in the chemical and electrochemical environments to which pressed
nickel electrodes are subjected and which are represented by
such electrodes themselves. In particular, the elastomeric
material must be resistant to oxidative degradation and to attack
by strong alkali. It has now been discovered that butyl (isobuty- -
lene-isoprene) rubber, and halogenated butyl rubber such as
chlorobutyl rubber and bromobutyl rubber satisfy these requirements.
Furt~ermore, these elastomers strongly bind the active electrode
materials together without significantly coating them. This means
that the electrode internal conductivity is not adversely a~fected
by these binder materials (other than the reduction in internal
conductivity reauLting from the presence, per se, of any
organic binder).
The elastomeric binder material is utili~ed as a
dispersion or latex. In this form, the small particles commonly
found in latexes (typically 0.1_um to 0.8~um) facilitate binding
the o~her electrode constituents together using small amounts of
the binder. On the other hand, i~ it is attempted to use the
elastome!ric material in powdered form, it will be found that it
; is di~ficult to obtain the binder paxticles in the desired small
size and that the small particles in poweder form tend to
agglomerate so that it is difficult to disperse the powder
uniformly throughout the electrode mixture.
~ latex o~ the identified elastomers typically comprises
about 61~ to 65~ by weight of the elastomer dispersed in water with
approximately 400ppm of formaldehyde as a preservative.
The binder described herein is employed in essentially
the same proportions as are used with prior binding materials.
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1 That is, the binder is preferably presen-t in amounts between
about 1~ and about 5% by weight of the total weigh-t (dry ~asis)
of active electrode material, conductive diluent and binder.
Most preferably, the binder is used in amounts between about 1%
and about 3~ by weight. Below about 1% by weight, there is
insufficient binding agent to produce a mechanically strong
electrode whereas above about 5~ by weightt the lit-tle electrode
strength that is gained is offset by a marked decrease in the
internal conductivity of the electrode.
Electrochemically effective amounts of active electrode
material and conductive diluent are used. Typically, the electrode
material and conductive diluent are utilized in amounts between
about 80% - 90% (by wt.) and about 5% - 15~ (by wt.), respectively.
A pressed nickel electrode incorporating the herein-
; described binder is made as follows. The active electrode
material, conductive diluent and any other electrode constituents
are mi~ed together with a dispersion of the binder until a
substantially homogeneous admixture is produced. The admixture is
then pressed into a current collector at pressures, e.g., 8000 psi
- 20,000 psi, to produce a cohesive electrode. No heating of
the electrode to~a temperature at which the Ni(OH)2 would be
adversely affected is required since sintering of the binder is
not required. However, the electrode may be heated to 50C - 60C
to remove any residual water or to decompose any ammonium
carbonate pore former without adversely affecting the electrode.
The pressed nickel electrodes described herein find
utili~y in high energy density nickel batteries such as nickel/
iron, nickel/zing, nickel/hydrogen and nickel/cadmium batteries.
Such batteries utilize alkaline electrolytes such as lithium
hydroxide, sodium hydroxide and potassium hydroxide.
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1 As -thus described, this invention comprises the
utilization in pressed nic~el electrodes of an elastomeric binder
which is substan-tially homogeneously dispersed throughout the
electrode. Such dispersion is obtained by incorporating the
elastomeric binder in the electrode mix as a latex or equivalent
dispersion~ By incorporating the elastomeric ~inder in the
electrode in this manner, not only is the desired homogeneity
obtained, but the resulting small particle size of the binder
ensures effective binding without electrically insulating
the active electrode materials.
This invention will be further described by the
following Examples.
EXA~lPLE I
Two sets of electrodes were made utilizing an admixture
having the following composition: 77 wt. ~ Ni0X, 9 wt. % CoOx,
5 wt. % nickel flake, 7 wt. % graphite, and 2 wt. ~ binder. One
set of electrodes incorporated polytetrafluoroethylene (PTFE) as
the binder, whereas the other set included butyl rubber as the
binder. In both cases, the binder was added to the other
chemical constituents as an aqueous dispersion. The admixture
for each set of electrodes was pressed into expanded nickel
current collectors to an admixture density (dry) a~ 2.1-2.3 gm./cc.
The resulting pressed nickel electrode~ were coupled
~;~ with zlnc electrodes in nickel/zinc batteries having the same
j~ number of positive and negative electrodes and having approximately
the same total electrode area so that each battery had approxi-
mately the same theoretical capacity.
The battery (A) containing nickel electrodes incorpo-
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whereas the battery incorporating PTFE as the binder was discharged
at 0.6 amperes. Both batteries were discharged to a cell
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1 voltage level of one volt. ~uring discharge o~ each battery,
the cell voltage was moni-tored as a function of the percent dis-
charge to provide the data shown in the Figure.
As shown in the Figure, the battery utilizing a pressed
nickel electrode incorporating the herein-described binder
(battery A) is significan-tly superior to a battery utilizing
pressed nickel electrodes incorporating a representative prior
art binder (kattery B).The improvement Frovided by the herein-
described binder would be shown to be even more significant
in the context of this Example if battery B had been discharged
at the same one ampere level at which battery A was discharged.
EXAMPLE II
An electrode incorporating butyl rubber as the binder
was made as described in Example I except that the amount of
binder was 2.5 wt. % and except that the total weight o~ MioX
and Cx was 85.5% ~Ni:Co = 9.1). That electrode gave substan-
tially the same performance as tne electrode wit'n 2 wt. %
butyl rubber.
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