CA1082306A

CA1082306A - Binder for pressed nickel electrodes

Info

Publication number: CA1082306A
Application number: CA285,772A
Authority: CA
Inventors: Ronald G. Gunther; John M. Bilhorn; Harvey N. Seiger
Original assignee: Yardney Electric Corp
Current assignee: Yardney Electric Corp
Priority date: 1976-09-07
Filing date: 1977-08-30
Publication date: 1980-07-22
Also published as: DE2737799C3; FR2363903A1; DE2737799B2; AU2811077A; DE2737799A1; IL52838A; JPS5739022B2; IN145413B; IL52838A0; IT1091126B; AR222965A1; AU500786B2; GB1537218A; JPS5333333A; BR7705957A

Abstract

ABSTRACT OF THE DISCLOSURE
A pressed nickel electrode is made by combining an active nickel compound (and other electrode constituents such as conductive diluents) with a binder in the form of an emulsion or latex and pressing the resulting mixture into a current collector without having to use elevated temperatures. The binder utilized herein is an elastomeric material such as butyl rubber, chloro-butyl rubber or bromobutyl rubber.

Description

~ ~512306 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-' - 1-,,''"' ~
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~(~8Z306 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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- ~08Z306 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.) . ~ .
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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108Z3~

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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~O~Z306 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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~L082306 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-~' '~! ' rating butyl rubber as the binder was discharged at one ampere, 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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,:' ~j -6-, 1(~Ei 230~i 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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A pressed nickel electrode comprising:
a current collector; and a substantially homogeneous admixture comprising an active nickel compound, an electrically conductive diluent, and an elastomeric binder which is butyl rubber or halogenated butyl rubber, said binder being present in an amount between about 1% by weight and about 5% by weight of the weight of said admixtures, said admixture bring pressed into said current collector to form a cohesive electrode.

2. The pressed nickel electrode of claim 1 wherein said binder is present in latex particle size.

3. The pressed nickel electrode of claim 2 wherein said binder is present in a size range between about 0.1µm and about 0.8µm.

4. The pressed nickel electrode of claim 1 wherein said halogenated butyl rubber is a material selected from the group consisting of chlorobutyl and bromobutyl rubbers.

5. A pressed nickel electrode comprising:
an apertured current collector; and a substantially homogeneous admixture comprising a nickel compound which is Ni(OH)2 or NiOx where x is not an integer, an electrically conductive diluent, and an elastomeric binder which is butyl rubber, chlorobutyl rubber or bromobutyl rubber

Claim 5 continued:
and which is present in a size range between about 0.1µm and about 0.8µm, said binder being present in said admixture in an amount between about 1% and about 5% by weight of the weight of said admixture, said nickel compound and said conductive diluent being present in electrochemically effective amounts.

6. A method of making a pressed nickel electrode comprising;
admixing (1) an electrochemically active nickel compound, (2) an electrically conductive diluent, and (3) an aqueous dispersion of an elastomeric binder which is butyl rubber or halogenated butyl rubber to produce a substantially homogeneous electrode admixture, said binder being present in said admixture in an amount between about 1% and about 5% by weight of the weight of said admixture (solid basis); and pressing said admixture into a current collector at a pressure sufficient to form a cohesive electrode.

7. The method of claim 6 wherein said electrode is heated to remove any residual water therefrom, said heating occurring at a temperature at which said nickel compound is not adversely affected.

8. The method of claim 6 wherein said halogenated butyl rubber is chlorobutyl rubber or bromobutyl rubber.

9. The method of claim 6 wherein the dispersed particles of said elastomeric binder are within the size range between about 0.1µm and about 0.8µm.