Note: Descriptions are shown in the official language in which they were submitted.
NIC-7707
lS36
This invention relates to secondary alkaline battery
systems and, more particularly, to novel separator
materials for use therein~
Secondary alkaline batteries are particularly suited
for a wide variety of applications ranging from power
generation in air-borne and submersible systems to use
in portable tools and appliances to engine starting and,
importantly, to electrical vehicle propulsion, due to the
high energy densities which can be achieved. l'ypica'
electrode combinations include silver-zinc, silver-cadmium
and nickel-zinc.
Nickel-zinc batteries have shown particularly
outstanding potential. This potential has, however, not
been commercially realized. Thus, the use of zinc
electrodes in secondary batteries has been limited b~ their
-_ failure to withstand repeated cycling without an
irreversible loss of capacity upon repeated recharge. The
difficulty in achieving satisfactory cycle life becomes
more pronounced for applications requiring relatively deep
discharge cycles.
The decline in capacity as th~ cycle life of the
battery system progresses is associated largely with such
life limiting processes as sepaxator degradation and zinc
electrode shape changes. Of alL the technical difficul~ies
facing the economic commercial utilization of the
nickel-zinc system, the life limitation due to the
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degradation of the separators employed is perhaps the
most important problem.
Thus, as is known, in nickel-~inc hattery systems
using conventional aqueous solutions such as potassium
hydroxide as an electrolyte, the 7inc material is
soluble in the electrolyte to a significant extent during
discharge. Some of the active zinc material thus tends
to enter the electrolyte while the battery system is
being discharged and while the system stands in a
discharged condition. Upon recharging of the battery
system, these zinc specie in the electrolyte return to
the zinc electrode but not without altering the electrode
structure. Moreover, and importantly, the replating or
redeposition of zinc often occurs in the form of trees or
branched crystals having sharp points (dendrites) which
readily form a bridge between the plates of opposite
polarity, thereby causing short circuits and the
destruction of the cell.
Accordingly, a satisfactory material for a separator
in the nickel-zinc system must be capable of preventing
dendrite penetration yet allow electrolyte permeation
therethrough, desirably being wetted by the electrolyte.
Stated another way, the material used must possess
satisfactory ion transport characteristics. Also, the
material employed should possess satisfactory chemical
stability in the battery or cell environment. Still
further, as is known, in addition to the shape change of
the zinc electrodes, the nickel electrodes undergo
expansion to some extent during operation of the cell or
battery. Accordingly, an adequate separator material
must be capable of tolerating such change and expansion
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without significantly altering the other characteristics
required.
To satisfy these diverse and rigorous requirements,
a useful separator material for long cycle life
applications must either possess a relatively uniform,
and extremely small, pore size or be a semi-permeable
membrane, have low resistance to electrolyte transport,
have high bulk electrical resistivity and possess the
strength and flexibility characteristics required to
accommodate the shape change of the zinc anodes and ! he
expansion of the nickel cathodes. And, all of these
properties must be provided in as thin a layer as is
possible so as not to significantly lessen the volumetric
energy density. To further complicate the picture,
commercial requirements dictate that the material be
capable of ~eing economically formed into a thin separatox
layer within acceptable quality control tolerances.
An awesome amount of effort has been directed to
providing satisfactory separator materials for secondary
alkaline battery systems. This is perhaps a testimonial
to the difficulty which has been encountered in providing
a satisfactory material which possesses the many diverse
characteristics required for efficient functioning as a
separator. The proposed solutions have ranged from
~5 providing various organic microporous films or some
semi-permeable membranes to relatively rigid layers of
inorganic, often ceramic, particles bonded together in
some fashion. A still further solution involves combining
an organic material with inorganic particles to form what
is often termed an inorganic~organic separator or more
simply, an "I~O" separator. Yet o~her solutions involve
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either forming various types of laminates or utilizing
a plurality of layers of difrerent materials.
Some rather extraordinary claims have been made
for some of the proposed solutions, the separators
being said to be capable of providing cycle life up to
several hundred or even more cycles. However, the data
supporting these claims must be carefully reviewed. As
an example, cells subjected to shallow discharges do not
present ser~rice conditions that even test many of the
commercially available separator materials. On the
other hand, ihe conditions encountered with a battery
system of the type that would be used for electric
vehicles, together with the conditions involved in the
usage of such battery systems, provide an extremely
rigorous test of the capability of the separator
material.
At the present time, and despite the considerable
effort in this field, the development of a viable
separator material for secondary alkaline systems remains
as a principal obstacle to the wldespread utilization of
such battery systems. Suitable commercial separator
materials for alkaline battery systems are simply
unavailable, other than at extremely high prices.
It is accordingly a principal object of the present
invention to provide a separator material for secondary
alkaline battery systems capable of achieving a
relatively long cycle life under conditions of deep
discharge, particularly when employed with other
separator materials.
Another object provid~s for such systems a
separator material possessing satisfactory
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characteristics for use with cells of the si~e useful
in electric vehicle applications.
A further object of this invention lies in the
provision of a separator material naving suitable
characteristics in a relatively tnin layer.
Yet another object is to provide a separator
material capable of being economically produced. A
related anZ more specific object provides a separator
material capable of being reproducibly made within
acceptable tolerances.
Other objects and advantages of the present
invention will become apparent from the ensuing
description.
While the invention is susceptible of various
modifications and alternative forms, specific
embodiments thereof are described in detail herein.
It should be understoocl, however, that it is not intended
to limit the invention to the particular forms disclosed,
but, on the contrary, the intention is to cover all
modifications, equivalents, and alternatives falling
within the spirit and scope of the invention as expressed
in the appended claims. Thus, for example, while the
present invention will be principally described in
connection with a nickel-zinc secondary rechargeable
battery system, it should be appreciated that the present
invention may likewise be employed alone or in
combination with other separator materials and with
other electrode combinations requiring a separator
material having some or all of the characteristics
described herein.
53~i
The present invention is, in general, predicated
on the development of an I/O separator material
comprising micron or micrometer-sized (the terms being
used interchangeably herein) inorganic particles such as,
for example, titanium dioxide, bonded together with a
polymer matrix such as a styrene-butadiene ~viz.-SBR)
copolymer. In a preferred embodiment, two different
types of inorganic particles are employed, one type
being leachable from the composite separator material by
the alkaline electrolyte employed and the other being
non-leachable. ~nsupported films of the desired
thickness, typically 2~5 mils, can be readily cast
after addition of the inorganic particles to a latex of
the polymer used. To provide increased dimensional
strength and more facile formation, a fibrous substrate
can be used to provide an integral composite. The
separator materials provided may be employed in
nickel-zinc battery systems and possess characteristics
allowing systems having relatively long cycle lives under
deep discharge conditions, even in cells of the size
required for electric vehicle applications, particularly
when employed with other separator materials.
With regard to the polymer component used, any of
several commercially available materials may be utilized.
The principal requirements are that the polymer be
capable of being provided in a latex or water emulsion
form and be relatively chemically stable in the
environment of the alkaline electrolyte being utilized.
As representative examples, ethylene-acrylic acid and
styrene-~utadiene copolymers and copolymers of acrylic
acid with ~ther olefins are useful and are commercially
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available. ~t has been found suitable, for example, to
employ a copolyrner in which the styrene content ranges
from about 5n-60 percent by weight and ihe butadiene
content ranges from about 40-50 percent by weight.
Importantly, such polymers allow ready processability
since the need to employ organic solvent recovery
equipment is obviated. To improve 'che stability of a
SBR latex suspension, the SBR material may be
carboxyiated. Suitable carboxylated materials are
commercial'y available.
~ eyardiny the inorganic component, a variety of
materials may be employed. The principal re~uirements
are that the material be capable of being provided in a
relatively uniform, small particle size on the order of
about 1 micron or less and be sufficiently wettable to
decrease the resistance to electrolyte transport of the
resulting I/O composite to the desired level.
Representative examples of suitable inorganic
materials include TiO2, cerium hydrate (CeO2.xH2O),
cerium oxide, lead peroxide, magnesium titanate, calcium
zirconate. ~or example, naturally occurring minerals
such as kaolin (aluminum silicate)may be utilized in
certain circumstances.
The inorganic component should be present, in a
functional sense, in an amount at least sufficient to
decrease the tackiness of the resulting composite
sufficiently to allow ready usage in assembling cells as
well as to provide the desired low resistance to
electrolyte ~ransport, physical stability of pores, and
thermal stability of the system. On the other hand, the
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maximum amolmt vf the inorganic component is limited
by the amouni o~ polymer needed to satisfactorily bind
the inorganic particles to form a physically stable
composite. When too little polyrner is used, the effects
can be seen upon handling of the composite~ inorganic
par~icles tending to be removed, presenting a chalky feel
to the touch. ~t has been found satisfactory, for
example, to use the inorganic component in an amount of
from about 30 to about 80 percent based upon the total
weight of the inorganic and organic materials.
Of the inorganic cGmponents used, at least one com-
s
ponentlnon-leachable from the separator in an alkaline
electrolyte environment. The particle size of the non-
leachable inGrganic component or components should be
less than about 1 micrometer and preferably significantly
less~ Particles of such small particle size provide
homogeneous resistance to the electrolyte (in the case of
a leachable type) and satisfactorily prevents dendritic
penetration tin the case of a non-leachable type). In
both cases, enhanced ion transport of electrolyte is pro~
~ided. ~lithin this particle size range, as may be appre-
ciated, use o~ particles approaching 1 micrometer will re-
quire less polymer for the composite than will be the case
with smaller particles due to the lesser total surface
area of the larger particles.
It is preferred to utilize at least one inorganic compon-
ent which is relatively insoluble in an alkaline en~ironment.
Satisfactory stability can be determined by exposing
the I/O material to an alkaline solution for a period up
536
g
to 2~ hours at elevated temperatures. For example,
exposure to a 31 percent by weight KOH solution at
80C. for 24 hours will satisfactorily determine
stability. When a sin~le type is employed, the use of
a non-leachable inorganic component ls preferred since
satisfactorily low resistance to ion electrolyte
transport is provided by the inherent microporosity of
the composite created during formation, while ~he increased
resistance to ion transport that can potentially occur
due to the loss of dimensional stability and concomitant
lower microporosity upon leaching of inorganic particles
is obviated.
More preferably, in accordance with this invention,
the I/O composite employs both a leachable and a
non-leachable component, the non-leachable component
imparting the requisite dimensional stability to the
composite and the leachable component enhancing the ion
transport characteristics of the composite after its
removal therefrom. The criteria previously discussed
for the inorganic component are generally applicable.
Howeverl the particle size of the leachable inorganic
particles present is desirably in the range of from
about 70 to 500 Angstroms, preferably from about 70 to
100 Angstroms. Also, and importantly, to achieve an
optimum combination of dimensional stability and
minimum resistance to ion transport, the inorganic
leachable component is desirably present in an amount
of lrom about 0.25 to 4O0 percent, based upon the total
weight of the inorganic components. In this connection,
the maximum amount of the leachable type is also dependent
upon the ratio of inorganic to the polymer used~ As the
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amount of the p~ meric component is increased, the
amount of the l~achable inorganic component can
generally be increased somewhat.
The preferred inorganic components are titanium
dioxide (non-leachable), available as an aqueous dispen~
sion from Gulf and Western ~ew Jersey 3MC and silicon
dioxide (leachable), such as Ludox (Dupont). Suitable
materials in micron or less size are commercially
available. A significant advantage of the preferred
components is their availability in aqueous suspensions,
allowing homogeneous mixtures with polymer latexes to
be prepared. This, in turn, simplifies the task of
forming a homogeneous I/O composite with little likeli-
hood of agglomerated or clustered inorganic particles.
An I/O composite in accordance with this invention
may be made by adding the micron-sized inorganic ~omponent,
suitably diluted in a dispersing medium, preferably water,
to a SBR or other polymer latices to form a slurry which
is suitably agitated to insure homogeneity. Minor amounts
of conventional functional additives such as antioxidants,
defoam~rs or the like may be addedr if desired. The
slurry should be sufficiently concentrated to provide a
defect-free film upon casting. ~ concentration of from
about 30 to about 60 percen by volume has been found
suitable. The particular concentrations employed for a
specific polymer will vary depending upon the rheolo~y
(e.g., viscosity and thixotropy) of the system, and
useful volume percents for the polymer could possibly
be above or below the range previous set forth under
some circumstances. The separator material may then be
readily formed by casting a film onto a supporting
substrate in the thickness desired, typically from
11;Z1536
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about 2-5 mils, and the water in the system removed as
may be accomplished with typical dryiIIg equipment.
To provide moxe facile preparation, the composite
I/O separator may be prepared by in~orporating a fabric
material as a substrate onto which the I/O comyonents
are coated. Lhe use of a fabric substrate also may
enhance dimensional stability and is useful where the
polymer being employed may be subject to cold flow or
creep. This may be accomp]ished by simply dipping the
fibrous support into an a~itated SBR-inorganic component
slurry. Alternatively, this supported separator may be
formed in a continuous fashion. To this end, in one
illustrative example, a fibrous material may be unwound
from a roll passed over a guide roll and run through a
vessel containing suitable quantities of the S~-inorganic
component slurry. Suitably, additional slurry can be
supplied from a reservoir with an outlet returning to
the reservoir being provided so as to insure
homogeneity of the slurry. The concentration of the
-~ 20 slurry should, of course/ be sufficient to provide a
defect-free surface but not so great as to cause an
uneven coating to be formed.
The thus-coated fabric, may then be passed through
a conventional oven using a residence time adequate to
insure that the coating has been satisfactorily dried.
The resulting composite may then be wound onto a roll.
If desired, the dried composite can make additional
passes through the system to provide the desired thickness.
Any of a variety of substrates may be
satisfactorily used. Illustrative examples include
nonwoven fabrics such as nylon, polypropylene and
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polypropylene-polyethylene ma-terials, which may be
co~.mercially available. In addition, various
cellulosic materials may be utilized but are not
particularly preferred when used alone, as ~here is
some tendency to swell. The substrate should be
sufficientlv thin to allcw the desired composite
thichness to be provided. To insure uniformity of the
resulting supported composite, the fabric substrate
should be as homogeneous as possible. Likewise, fabrics
should be desirably selected which will not be leached
from the composite by the alkaline electrolyte to be
used.
The resulting I/O separator material of the
present invention has a typical mean pore size of from
about 0.010-0.013 micrometer (as dete~mined by water
penetration) and possesses electrical resistivities
(ohm-cm.) in the range of from 20 to 60 for a thickness
of 2 to 5 mils.
The following Examples are intended to be merely
illustrative of the use of the present invention and
are not in limitation thereof.
EXAMPLE_1
This Example illustrates the use of a kaolin/SBR
I/O separator material made in accordance with the
present invention and the performance thereof in
relatively large cells when exposed to conditions of
- deep discharge.
An initial coating was prepared by ball milling,
for about l hour, 645 grams of kaolin in lO00 mil~ of
water, after which 500 ml. of a carboxylated styrene
(55)/butadiene (45) copolymer latex (50~ by volume
1536
solids) was added to the kaolin-water mixture, and the
resultant mixture ~as again ball m;lled for about
5 minutes, with cessation before agglomeration of the
latex took place. Fifty ml. of a polyoxyethylene (20)
sorbitan monolaurate, specific gravity 1.1 gm./ml.,
was then added to the slurry using a magnetic stirrer.
The resulting slurry contained, based on total volume,
50~ kaolin and 50% SBR.
A second coating was thereafter prepared in the
same fashion, except that the amounts of the
constituents were varied to provide a slurry
containing, based on total volume, 45~ kaolin and
55% SBR.
The resulting initial coating was then applied
to a "Lyonel" nonwoven nylon fabric ~Howard Textile
Mllls) by dip coating the fabric at a rate of about
6 inches/mlnute, after which drying was carried out in
a convective drier at a temperature of a~out 60C.
The second coating was thereafter applied and dried
in the same fashion.
Two nickel-zinc cells having a nominal 300
ampere-hour rating were constructed with 8 nickel
cathodes and 9 ~inc anodes. An overall ratio of 8:1
of zinc/nickel active material was used. The anodes
were wrapped with a conventional "Celgard"
polypropylene separator material (Celanese Corp.),
and the cathodes were wrapped as follows: (]). a
"Pellon" polypropylene conventional separator
material, ~2). an overlying layer of "Celgard"
polypropylene separator material, (3). the I/O
separator material previously described herein, and
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(4). a second layer of the "Celgard" polyprGpylene
separator material. The electrolytes used comprised
aqueous KO~ solutions of slightly varying
compositions.
The cells were then subjected to differing depths
of discharge, and the capacity determined. One cell
was subjected to a 100~ depth of discharge at a
60 Amp. discharge rate with 1 cycle/day being carried
out. The other cell was ]ikewise subjected to a 60 Amp.
discharge rate to a 75% depth of discharge, with 2
cycles/day being carried out.
The initial cell retained over 85% of its rated
capacity for about 17C cycles in spite of the 100~
depth of discharge. The other cell at a lesser depth
of discharge, but still quite substantial, retained over
90% of its rated capaclty for about 190 cycles.
EX~MPLE 2
This Example illustrates the preparation of a
further I/O separator material in accordance with the
present invention.
A coatin~ formulation comprising, by weight, 28.6%
SBR, 71% TiO2 and 0.4~ SiO2 was prepared using generally
the procedures set forth in Example 1 and a cellulosic
nonwoven fabric was dip coated with the coating
formulation at a speed of about 7 inches/min.; and the
water from the system was thereafter removed by
infrared drying.
A sample of the dried composite was soaked for
about 30 minutes in a 30~ aqueous KOH solution at a
temperature of 100C. The electrical resistivity after
this KOH soak was 45 ohm-cm., and the composite had
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pore size of 0.03 micron as determined by water
penetration.
Thus, as has been seenj the present invention
provides separator materials that may be readily formed
and which are capable of being used in secondary
alkaline systems to provide cells having relatively
long cycle lives even under deep discharge conditions,
particularly when used with other separator materials.
In the preferred embodiment, utilization of two
different types of inorganic components allows the
separator material being formed to be tailored to the
end use contemplated to maximize the performance.
Multiple layers of the separator materials may be used,
and the separator materials sf this invention may be
employed in conjunction with other separator materials
to achieve the desired performan~e characteristics.
