CA1121536A - Inorganic/organic separator materials for secondary alkaline battery systems - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An inorganic/organic separator material, capable of relatively long cycle life in deep discharge conditions in secondary alkaline systems, particularly when used with other separator materials, is provided which comprises micrometer-sized particles of an inorganic material such as titanium dioxide dispersed in a matrix of an organic polymer such as a styrene-butadiene polymer. In a preferred embodiment, two types of inorganic particles are utilized, one being leachable in the alkaline electrolyte used and the other being non-leachable.

Description

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 . ' ~ .

53~

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 53~

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 ~153f~

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 3L1;~153~

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 53~

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 11~153~

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 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 L53~

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 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 153~i 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 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 5~
-:L4-(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 ii3~

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.

Claims (10)

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

1. A composite separator material for secondary alkaline battery systems which comprises a matrix of a polymer selected from the group consisting of styrene-butadiene copolymers and copolymers of acrylic acid with olefins and micron-sized inorganic particles embedded therein, the inorganic particles being present in an amount of from about 30 to about 80%
based upon the total weight of the particles and the polymer matrix, said inorganic particles being of two types, one being leachable by an alkaline electrolyte and the other being non-leachable.

2. The separator material of claim 1 wherein,said inorganic particles are selected from the group selected from the group consisting of titanium dioxide, cerium hydrate, cerium oxide, lead peroxide, magneslum titanate, calcium zirconate, aluminum silicate, silica and mixtures thereof.

3. The separator material of claim 1 or 2, wherein said leachable inorganic particles are present in an amount of from about 0.25 to about 4% based upon the total weight of the inorganic particles.

4. The separator material of claim 1 or 2, wherein the leachable inorganic particles are silica and the non-leachable particles are titanium dioxide.

5. The separator material of claim 1 or 2, wherein the leachable inorganic particles have an average particle size of less than about 100 Angstroms.

6. The separator material of claim 1 or 2, wherein the material is a film having a thickness of from about 2 to about 5 mils.

7. The separator material of claim 1, wherein the polymer is a styrene-butadiene copolymer with a styrene content of from about 50 to about 60% based upon the total copolymer weight.

8. The separator material of claim 7, wherein the copolymer is carboxylated.

9. A composite battery separator for secondary alkaline batteries, which is formed from a battery comprising a matrix of styrene-butadiene copolymer or an olefin/acrylic acid copolymer, the matrix having dispersed therein inorganic particles having a particle size of not more than 1 micron, the inorganic particles being a mixture of two kinds, the first kind being leachable by an alkaline electrolyte and the second kind being non-leachable by the electrolyte, and being together present in an amount of 30 to 80%, based on the total weight of the matrix and inorganic particles.

10. A composite separator material for secondary alkaline battery systems which comprises a thin fabric substrate, said substrate having a coating consisting of a matrix of a polymer selected frorn the group consisting of styrene-butadiene copolymers and copolymers of acrylic acid with olefins and micron-sized inorganic particles embedded therein, the in-organic particles being present in an amount of from about 30 to about 80% based upon the total weight of the particles and the polymer matrix.