Note: Descriptions are shown in the official language in which they were submitted.
~2~32~
-1- T-2643-292
SE~LE~ NIC~ZINC E~
-BACKGROUND OF THE INVEIITION
1. Field Of The Invention
This invention relates to electroche~ical cells
and more particularly, to sealed, rechargeable nickel-
zinc cells.
2. Description Of The Prior Art
Many kinds of portable equipment, such as, for
example, tape and video recorders, power tools, calcu-
lators, radios, electric razors, television sets, tele-
phone pagers, microcomputers, and the like, have been
developed over the last several years; and their use
has become relatively widespread. In so~e situations,
a primary battery or cell has been utilized as the
power source. However, while primary cells offer the
advantage of relatively high enery densities, these are
relatively expensive because of the continual n~ed for
replacement.
For this reason, it has been useful to employ a
rechargeable cell as the power source. Various types
of lead-acid cells have been utilized, particularly
where cost is the primary consideration. However, such
cells have relatively low eneryy densities and cycle
life capabilities, as well as requiring relati~ely long
times for charging.
- ? - ~L~20~00
. .
The inadequacies of primary and lead-acid cells
have led to the use of nickel-cadmium cells in some
applications. Such cells, while xela~ively expensive
in comparison to lead-acid cellsr offer relatively high
energy densities and an extremely long cycle life.
Moreover, this type of cell is capable of being
employed at relatively high charge/discharge rates.
Despite the advantages provided by nickel cadmium
cells, there is a continuing demand for many applications
for a power source capable of achieviny even higher
energy densities and operating at higher working voltages.
This situation has led to the investigation of nickel-zinc
rechargeable cells for these applications. The nickel-
zinc system is well known and, at least potentially,
offers substantial advantages. In comparison to nickel-
cadmium cells, nickel-zinc cells have higher working or
operating voltages (viz. about 1.65 volts) and poten-
tially can provide significantly higher energy densities.
U.S. Patents 3,951,687 and 4,037,033 disclos~ configura-
2~ tions for nickel-zinc cells.
Despite this promise, the commercial use of nickel-
zinc cells for the portable applications described
herein has been extremely limited. This is principally
due to the inability to deal adequately with the rela-
tively high internal pressures inherent in this system.Thus, while the cadmium electrode in a nickel-cadmium
! cell is marginally kher~odynamically stable with respect
to reduction of aqueous alkaline battery electrolytes
with consequent evolution of hydrogen gas, the zinc
electrode in a nickel-zinc system is unstable. The
nickel-zinc system accordingly tends to evolve hydrogen
yas under all conditions of service, viz., charger
discharge, overcharge and open-circuit stand.
To be commercially useful f a sealed nickel-zinc
cell must therefore possess the ability to compensate
~21~9;2~
--3~
.
for the hydrogen gas evolved so that the cell under
conditions of use should not vent. If the cell vents,
decreased performance can result if enough electrolyte
is lost. Moreover, discharge of alkaline eletrolyte
into the environ~ent could be harmful to electronic or
other components in the area where the cell is employed.
The internal pressures which can be tolerated depend
upon the strength of the containers utilized. For
small cylindrical cells that typically use container
materials which will rupture at about 450 to 500
p.s.i.g. (at such pressures the container top typically
separates from the container), safety considerations
dictate that venting means be employed which will vent
at internal pressures of about 250 p.s.i.g. or so~ In
prismatic cells, the plastic container ~aterials typi-
cally used require that such cells vent at significantly
lower internal pressures, viz. - about 25 p.s.i.gO or
so .
The internal pressures developed in sealed nickel-
zinc cells under conditions of use can readily exceed
250 p.s.i.g. under various conditions. A considerable
amount of effort has been expended to develop a nickel-
zinc system capable of operation at satisfactorily low
internal pressures.
In addition, there ar~ a number of other problems
involved in developing commercially practical sealed
nickel-zinc cells. Thus, regardless of the means uti
lized to compensate for the hydrogen gas evolved, pro-
blems of undue pressure build-up have been found to
occur upon stand. It has thus been found thatt if
nickel-zinc cells are allowed to stand in a completely
discharged condition for an extended period of time,
the internal pressure build-up can reach a level where
venting of the cell occurs.
~4~ ~209~
A further problem resides in the relatively high
impedance values of the prior nickel-zinc cells. For
whatever reason, prior nickel-zinc cells seem to be
characterized by impedance levels which restrict the
current levels that can be utilized.
Still further, prior cells of this type appear to
have less than an optimum tolerance to overcharge con-
ditions. While not fully understood, it is believed
that the less than optimum tolerance is due to the
separator configurations previously utilized. Also, in
this regard, it appears that the cycle life of nickel-
zinc cells, particularly at high discharge rates, are
less than optimum.
Lastly, prior cells of this type seem to result
in service in zinc passivation. This, of course, can
adversely affect the capacity.
Accordingly, despite the prior efforts to provide
a commercially viable sealed, nickel-zinc cell, such
cells still have made little, if any, inroad as re-
placements for the various power sources now e~ployedfor portable equipment applications. There certainly
exists the need to provide a com~ercially attractive
and cost-effective s~aled nickel-zinc cell capable of
obviating to a satisfactory degree the various problems
discussed herein.
OBJECTS OF T~E INVE~TION
It is accordingly a principal object of the present
invention to provide a sealed, nickel-zinc cell providing
improved cycle life and electrical performance in service.
Another object is to provide a cell of the foregoing
type that is simple in construction and which is capable
of being economically manufactured.
,
-5- ~2~Z~
A still further object of this invention lies in
the provision of a cell of the foregoing type having
the capabilit~ of operation at relatively high current
levels.
Yet another object of this invention is to provide
a cell of the foregoing type capable of being allowed
to stand for prolonged periods of time in a discharged
condition without undue internal pressure build-up.
Another object of the present invention is to
provide a cell of the ~oregoing type which minimizes
zinc passivation.
A still further object is to provide a cell of the
foregoing type which possesses improved tolerance to
overcharge conditions.
Other objects and advantages of the present inven-
tion will become apparent from the following detailed
description, and from the drawings in which:
FIGURE 1 is a side elevation of a nickel-zinc cell
embodying the present invention and partially cut-away
to show the internal configuration;
FIG ~ 2 is a cross-sectional view taken generally
along lines ~-2 of FIG~RE 1 and further illustrating
the internal configuration of a cell according to the
present inventicn;
~5 FIG. 3 is a graph illustrating the cycle life
performance of a cell of the present invention compared
to prior art!cells, and
FIG. 4 is a graph illustrating the hydrogen pressure
developed within a nickel-zinc cell with and without
30 the hydrogen recombination catalyst utilized according
to one aspect of the present invention.
~ hile the invention is susceptible to various
modifications and alternative ~orms, there is shown in
the drawings and will herein be described in detail,
~ Z092~0
the preferred embodiments. It is to be understood,
however, that it is not intended to li~it the invention
to the specific forms disclosed. On the contrary, it
is intended to cover all modifications and alternative
forms falling within the spirit and scope of the inven-
tion as expressed in the appended claims.
S~MMARY OF THE INVE~lTION
In general, the present invention is predicated on
the discovery that nickel-zinc cells having improved
electrical performance characteristics can be provided
by selection of the mixture utilized for the negative
electrode, the particular binder erilployed for the nega-
tive electrode, and the separator system utilized.
Each of ~hese parameters will individually impart im-
proved performance to the cell. Optimum performance isprovided by utilizing all of the features which will be
described herein.
In addition, for some applications, an auxiliary
feature of this invention provides a specific means for
dealing with the intexnal pressure build-up due to the
evolution of hydrogen.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to the illustrative embodiment, there
is shown in the drawings a rechargeable, sealed nick~l-
zinc cell incorporating the present in~ention, the cell
being generally designated at 10. The particular con-
figuration of the cell is only exemplaryJ and may be
~odified as desired. The cell 10 comprises an outer
housing 12 defining a cell 14. The cup-shaped housing
12 has an open end 16 which is closed by closure 18
sealingly Mounted upon open end 16 by an annular insu-
lator 20. A perforator disc 22 is secured to the open
end 16 of the outer housing 12 by an annular retainer
24 an~ is provided with a piercing tab 26 adapted ko
pierce closure 18 in the event the closure is urged
outwardly, as by internal pressure buildup within the
sealed battery. If desired, a resealable vent could
be employed, and many such vent constructions are known.
As best seen in FIG. 2, a cell element shown
generally at 28 is contained in cell 14 in the form
of a wound roll comprising a negative electrode layer
30, a positive electrode layer 32, and a separator
shown generally at 34, intermediate the electrode
layers.
Pursuant to one aspect of the present inven-
tion, a wicking layer for absorbing electrolyte is
preferably provided. It has thus been found that the
inclusion of a wicking layer on the side of the
separator adjacent the positive electrode layer serves
- to impart to the cell longer cycle life, particularly
when the service regime involves relatively high dis-
charge rates (e.g. - about 2C or higher)~
Any alkali-resistant material capable of
absorbing electrolyte can be utilized. In general, a
non-woven fabric of a synthetic resin, such as poly-
propylene, may be employed. One illustrative example
2S of a suitable polypropylene wicking sheet is WE~RIL M
1488 non-woven fabric (Kendall Company) having a thick-
ness of about 3 mils.
Accordingly, as best seen in FIG. 2, a wicking
layer 36 for absorbing electrolyte is provided on the
side of the separator 34 adjacent the positive electrode
layer 32. A wicking layer could be likewise provided
adjacent the negative electrode layer 30, i~ desired.
However, and while the use of a wicking layer adjacent
the negative electrode may offer advantage in relation
to the inclusion of no wicking layer ad3acent either
lZ0~20~
electrode, it has been found that superior perfo~mance
is achieved when a wicking layer is present only on
this side of the separator adjacent the positive elec-
trode la~er.
As best seen in FIGURE 1, perforator disc 22
cooperates with closure 18 in defining the negative
terminal of the housing. More specifically, a first
connecting means tab 38 is electrically connected to
the negative electrode layer 30, extending outwardly
from the roll into electrically connected association
with closure 18.
Outer housing 12 suitably comprises a metal
can which defines the positive terminal of the battery.
Thus, as is illustrated in FIGURE 1, a second con-
necting tab means 40 is electrically connected withpositive electrode layer 32 and housing 12.
When utilized in a cylindrical cell, as is
shown in the illustrative embodiment, the positive
and negative electrode layers and the separator should
be sufficiently flexible so that a wound element can
be provided. The manufacturing techniques to provide
suitable positive and negative electrode layers of
adequate flexibility are well known.
The negative zinc electrodes may thus be
made by conventional techniques. As one exarnple, a
powdered mixture of the desired materials and a binder
can be rolled onto a suitable current collector, such
as, for example, a copper screen.
While such mixtures have previously utilized
both zinc oxide and zinc, it has been found that the pres-
ence of zinc metal tends to result in undesired pressure
buildup if the cell is allowed to stand in a completely
-9- ~20~
discharged condition for an extended time period. This
pressure build-u~ could reach a level causing the cell
to vent. For this reason, pursuant to a principal
aspect of the present invention, the mixture utilized
incorporates little or no zinc metal.
A variety of binder materials for fabricating zinc
electrodes is known. Typically, the binder ~a~erial
used is inert in the cell environment and is incorporated
in an amount just sufficient to hold the mixture together,
providing a positive bond as well to the current collector.
Previously utilized binder materials, such as
polytetrafluoroethylene, require relatively large amounts
to be employed in order to achieve the desired coherent
structure for the negative electrode, amounts on the
order of 10% by weight based upon the weight of the
mixture often being used. Such relatively large amounts
of binder result in the cell having relatively high
impedance values. This restricts the current level
which the cell can utilize in service.
j 20 Accordingly, a further principal aspect of the
I present invention comprises utilizing an elastomeric,
self-cured carboxylated styrene-butadiene latex as the
binder material. It has been found satisfactory to
utilize this binder in an amount preferably in the
range of about 3.8% to about 5%, based upon the total
weight of the negative electrode mixture. Amounts in
this level have been found to achie~e an adequate co-
herent structure for the negative electrodes. Moreover,
and importantly, this results in cel 15 characterized by
relatively low impedance in comparison to prior cells
and may thus allow significantly higher current levels
in service. Amounts in excess of 5% by weight may
certainly be utilized, but such amounts offer little
advantage and tend to provide increased impedance.
æ~
.
-- 10 --
Specific illustrative examples of suitable binders are
AMSCOTM R~S 4150 and 4816, manufactured by th~ AMSCO
Division of Union Oil Company.
If desired, the negative electrode may contain
other ingredients, some of which are known. Moreover,
pursuant to a further aspect of the present invention,
it has been found useful to include a minor but signifi-
cant amount of cadmium. This is believed to act to
stabilize the negative electrode against shape change
as well a.s reducing the rate of evolution of hydrogen.
Indeed, the cadmium present is electrochemically inert
until the operating cell voltage has decreased to about
1.30 volts or so. Under these conditions,1 the inclu-
sion of cadmium appears to serve to minimize zinc
passivation that would otherwise occur.
The amount of cadmium utilized should be such
as to provide 20% of the ampere-hour capacity of the
positive active material. Amounts above this minimum
level may certainly be utilized, -the upper limit likely
being constrained by economic considerations. Based
upon the total weight of the negative electrode mixture,
the amount of cadmium in the range of about 5 to 6%
or so should be suitable to provide such minimum.
The cadmium component may be utilized in the
mixture as cadmium oxide. However, it is preferred
to utilize cadmium metal. The use of cadmium oxide
may accordingly result in some loss in capacity.
It has also been found useful to include in
the negative electrode mixture bismuth oxide, Bi2O3,
in an amount of about 7 to 8% based upon the weight
of the negative electrode mixture. This is believed
to function as a corrosion inhibitor.
.2~2~
Prior techniques have utilized calcium hydroxide
as a further component of the negative electrode mixture.
The negative electrode mixtures described herein provide
satisfactory performance; and, accordingly, there is no
necessity for incluAing calcium hydroxide~ It is accor-
dinyly preferred that the negative electrode mixture be
essentially free of calcium.
As is well known, in nickel-zinc systems using
conventional aqueous solutions, such as potassiu~ hydro-
xide, as an electrolyte, the zinc specie(s) formedduring discharge is soluble in the electrolyte to a
siynficant extent. Some of the active zinc material
thus tends to enter the electrolyte while the system is
being discharged, as well as while the system stands in
a discharged condition. Upon recharging of the battery
system, the zinc specie(s) in the electrolyte returns
to the zinc electrode but can alter the electrode struc-
ture. The active zinc material can thus migrate from
the edges or periphery of the electrode structure and
collect in the central regions of electrode, resulting
in an irreversible loss of capacity. This pheno~enon
has been often termed "shape change".
Because of this phenomenon, the cell element uti-
lized in the present invention should be positioned in
the cell in a fashion which will at least minimize
shape change. It has been found satisfactory~ when a
cylindrical cell is involved, simply to wind the ele~ent
such that the element is under compression while in
position within the cell. This assists in minimizing
shape change as a problem.
As is likewise well known, the replating or re-
deposition of zinc often occurs in the form of ~reed or
branched crystals having sharp points (dendrites~ which
- 12 -
can readily bridge the gap between the plates or elec-
trodes of opposite polarity, thereby causing ~hort
circuits and the destruction of the cell. Accordingly,
the material used for the separator should be a mem-
brane having a relatively fine, uniformly sized porestructure which allows electrolyte permeation there-
through while preventing dendrite penetration. Still
further, the material employed should possess chemical
stability in the cell environment. Additionally,
suitable materials should possess sufficient flexi-
bility and strength characteristics to endure
adequately any shape change and/or electrode expansion
that might take place during service. A large number
of materials have been proposed for use and are well
known, as are their methods of manufacture.
As one illustrative e~ample, the separator
may comprise a commercially available CELGARDTM poly-
propylene film (Celanese Fiber Company). It has been
found particularly desirable to utilize two layers of
such material (each layer about one mil thick being
adequ~te) to form,the separator layer 34, the indiv-
idual layers bein~ shown generally at 42 and 44 (FIG.
23. T~e use of two layers allows the large pores or
holes, due to imperfections produced during manu-
facture or subsequently, in each layer to be non-
aligned with respect to each other to minimize
problems with dendrites. Of course, a single layer
or more than two layers may likewise be employed if
de,sired.
Prior techniques have utiliYed either films
such as polyvinyl alcohol on the separator or as cements
to bond the individual layers together to form a com-
posite, integral structure. However, it has been found
that such techniques substantially decrease the tolerance
of the cell to overcharge conditions. Pursuant to yet
s~
-13- ~ æ ~ ~ 2 00
another aspect of the present invention, it is preferred
to utilize a separator construction free of any film on
the separator layer or layers utilized and with no
cement or other bond beiny em~loyed where multiple
separator layers are utilized. In this ~ashion, the
resulting cell has been found to possess superior
tolerance to overcharge conditions~
Any conventional alkaline electrolyte used with a
nickel-zinc system ~ay be employed~ As one exar~ple, it
is satisfactory to utilize an aqueous potassium hydroxide
solution containing about 25% by weight potassium hydro-
xide. It is desirable to utilize initially an electrolyte
saturated with Zn(OH)2 so as to prevent initïal dissolu-
tion of zinc oxide into the electrolyte. As is known
in the sealed cell art, the amount of electrolyte used
should be restricted sufficiently so that an effective
oxygen recombination reaction will be provided. In the
illustrative e~bodiment, the necessary electrolyte can
be added to the open space in the core of the wound
cell element 28 prior to the sealing of the cell.
With respect to the first connecting tab 38, this
should be made of a conductive material having an over-
voltage for hydrogen evolution at least approximately
as high as that of zinc. An illustrative example is a
nickel element, plated with copper and then overplated
with silver. The closure 18 may suitably coMprise a
steel sheet plated with nickel which is, in turn, covered
with copper plating, and then covered with silver plating.
The second connecting tab 40 may comprise, for example,
a nickel ele~ent which is electrically connected to the
nickel plating 46 of outer housing 12.
A wide variety of materials are known *or the
connecting tabs and the housing for nickel-zinc systems,
and such materials may be utilized in the nickel-zinc
-14- ~ 2 ~ ~ 2 ~ ~
cell of the present invention. The particular ~aterials
of construction may accordingly vary rather widely.
Further, if desired, a water sealant coating, as
is known, may be applied to the r~etal or other surfaces
in the ce]l. A suitable sealant is the styrene-butadiene
material described herein as the binder for the negative
electrode mixture. As shown in FIG~RE 1, a coating 48
has been applied to the exposed surfaces of the closure
18 and the first connecting tab 38. This may be applied
by brushing on to a thickness, for examplel of about 1
mil.
In addition, to insure that adequate insulation is
provided between the cell element 28 and the ter~inals,
insulators 50, 50' may be included, if desired. ~hile
shown as spatially re~oved from the cell element 28 for
simplicity of illustration, insulator 50 may suitably
rest upon separator layers 42 r 44 which desirably
terminate somewhat above the upper end of the electrodes.
The nickel-zinc cell of the present invention may
be utilized in either a prismatic or cylindrical design,
as is desired for the particular application. Likewise,
the capacity of the cell may vary within wide limits,
the size being dictated by the requirements of the
particular end use application. As one example, a
cylindrical sub-C size cell for use in cordless or
portable power tools may suitably have a capacity of,
for example, 1.2 Ampere-~iours.
The cells of the present invention ~ust likewise
incorporate a means for oxidizing the hydrogen evolved
in service to maintain a satisfactorily low internal
pressure within the cell. A variety of catalytic means
are known and may be employed.
In this regard, an auxiliary aspect of the present
invention provides as the hydrogen oxidation source a
26~0
hydrogen recombination catalyst located in the cell
which is free of electrical connection -to the cell
elements. Any fuel cell cathode may suitably be
employed. As an illustrative example, the recombina-
tion catalyst may suitably comprise carbon clothhaving about 1% by weight platinum catalyst on carbon
particles bonded to the cloth by a hydrophobic binder,
such as polytetrafluoroethylene. Suitable recom-
bination catalysts such as the illustrative embodi-
ment are commercially available. As is seen in thedrawings, a hydrogen recombination catalyst 52 is
positioned in the axial core space of the wound role
cell element and is free of electrical connection
to the cell electrodes. In this fashion, the assem-
bly of the cell is facilitated.
Utilization of the hydrogen recombinationcatalyst has been found to substantially reduce the
internal pressure developed under typical cycling
conditions. However, the performance upon prolonged
stand and high rate charge/discharge conditions can
certainly be improved. Under either of these condi-
tions, internal pressure can develop to the point
where the cells may well ve~t.
For these reasons, it is preferred to
utilize, as the means for dealing with the hydrogen
evolution a positive electrode which contains a
catalyst such as silver for the oxidation of hydro-
genO Sealed nickel-zinc cells utilizing this approach
are characterized by relatively low internal pressures
in use and on stand and are capable of being operated
under relatively high rate charge and discharge with-
out building up pressures that would cause venting.
-16- ~ 20~
FIG. 3 demonstrates the extended cycle life of
cells pursuant to the present invention at high dis-
charge rates. Curves A and C of FIG. 3 represent dis-
cllarge curves of cells accordiny to the present inven-
tion at two hour and one-half hour rates, respectively.
Curves B and D are discharge curves for previous state-
of-the-art nickel-zinc cells at 2.5-hour and 1 hour
discharge rates, respectively. These latter rates
should be substantially less stressful than the cor-
responding rates used for the cells of this invention.As ~ay he seen in FIG. 3, the discharge capacity of the
cells of this invention is maintained substantially
higher than the discharge capacity of a conventional
nickel-zinc cell up to two hundred cycles or more.
15 - FIG. 4 illustrates the perfor~ance of a cell which
is achieved using the hydrogen reco~bination catalyst
52. Curve ~ illustrates the hydrogen pressure developed
in the absence of the recombination catalyst 52, and
the Curve F illustrates the hydrogen pressure developed
2~ in the cell where the recombination electrode 52 is
provided. The substantial decrease in the developed
pressure due to the inclusion of the recombination
catalyst is apparent~
One exa~ple of suitable parameters to provide a
1.2 Ampere-Hour, sub-C size, cell pursuant to the pre-
sent invention is as follows. The negative electrode
! layer comprises a first ~ixture of zinc, zinc oxide,
cadmium oxide, bismuth oxide and a styrene-butadiene
binder rolled onto a copper screen. Based upon the
weight of the ~ixture, zinc was present in an a~ount of
4~5~, cadmium oxide in an amount of 5.8%, bismuth oxide
in an amount of 7.5~ and binder in an amount of about
4%, the balance being ~inc oxide. The hydrogen recom-
bination catalyst comprised a 0.1 inch x 1 inch carbon
-17~
cloth strip containing 1% by weight platinum. The
negative electrode dimensions w~re 0.016 inch x 1.31
inch x 9 inch and the positive lectrode dimensions
were 0.028 inch x 1.2 inch x 7 inch.
The negative electrode may initially contain a
charyed zinc mass in the amount of about 35% of the
total theoretical Ampere-Hour capacity; however, this
should be converted to zinc oxide by the reaction of
the zinc with the added cadmium oxide and bis~uth oxide.
The resulting cadmiuM should represent about 25% of the
1.2 Ampere-Hour battery capacity. The amount of zinc
oxide initially present is roughly 425% of the Ampere-
~our capacity of the cell, the actual capacity being
limited by the positive electrode.
