Non-sintered PTFE-bound electrodes for alkaline storage batteries are made by coagulating polytetrafluoroethylene (i.e. PTFE) particles from an aqueous dispersion thereof to form a gelatinous coagulum, mechanically dispersing the coagulum in water such that the coagulum is subdivided into small, sticky clots of PTFE particles which are suspended in the water, mixing the suspension with comminuted electrochemically active material to form a slurry of active material and active-material-coated clots, filtering the slurry to separate the solids (i.e. as a filter cake) therefrom, integrating (e.g. sandwiching) an electrically conductive current collector with the filter cake, compressing the thusly formed composite, and drying the compress.

BACKGROUND OF THE INVENTION 
This invention relates to electrodes for alkaline storage batteries, and 
more specifically to a water-base process for making such electrodes 
whereby the electrode active material is held in a non-sintered binder of 
coagulated polytetrafluoroethylene (hereafter PTFE). The invention is 
useful for all of the popular alkaline storage battery electrodes (e.g. 
zinc, cadmium, nickel, silver etc.), but has particular advantage in 
reducing the weight and cost of nickel electrodes. 
One of the major drawbacks to the more extensive use of nickel alkaline 
batteries (e.g. Ni-Zn) is, for example, the high cost of the nickel 
electrodes. Originally such electrodes used sintered carbonyl nickel 
plaque current collector-supports which were impregnated with nickel salts 
and then converted into nickel hydroxide. Typically this was accomplished 
by filling the pores of the nickel plaque with an aqueous solution of a 
nickel salt and subsequently converting the salt to the hydroxide by 
chemical, electrochemical or thermal processes. The process normally 
required several repetitions to introduce the desired amount of nickel 
hydroxide into the plaque and utilized unnecessarily high amounts of 
nickel which added considerable cost and weight to the electrodes. 
More recent electrodes eliminate the plaque and bind the nickel hydroxide 
in a polymer binder. Some are made by milling (i.e. calendaring) nickel 
hydroxide, graphite, binder and a plasticizer together and then roll 
bonding it to a current collecting grid. Various techniques are used to 
make these electrodes porous. In one technique, a mixture of two 
immiscible thermoplastic resins is used as an initial binder, one of the 
resins later leached from the mass with a suitable solvent and the active 
material retained in a microporous matrix of the remaining insoluble 
resin. An additional sintering step removes any remaining soluble resin 
and coalesces the remaining resin. 
In another process (i.e. Strier et al. U.S. Pat. No. 3,706,601), an aqueous 
dispersion of active material particles (i.e. ZnO) is mixed with an 
aqueous dispersion of a latex type polymer (e.g. PTFE), and a film cast 
from the mixture, which is then dried and, sintered (e.g. 260.degree. 
C.-375.degree. C.). The sintered film is then rolled to form a fibrous 
polymer structure binding the electrode active material together. 
McBreen U.S. Pat. No. 4,000,005 discloses a process for making, compressed, 
non-sintered-binder nickel electrodes by forming a filter cake which is a 
coagulum of active material particles (i.e. nickel hydroxide, cobalt 
hydroxide and graphite) entrained in a three dimensional, reticulated, 
open cell polyvinylidene fluoride (i.e. Kynar) binder which was 
precipitated slowly out of solution in the presence of the active 
materials. As the polyvinylidene fluoride slowly precipitates and 
coagulates it entrains the active materials. 
Still other proposed techniques include: (1) precipitating nickel hydroxide 
as a slurry from a solution of a nickel salt and vacuum impregnating a 
porous nickel conductor with the slurry; (2) applying a layer of an 
aqueous paste of nickel hydroxide, nickel powder and a binder to a 
metallic substrate, compressing it to remove excess water, drying it and 
compressing it again to achieve intimate nickel hydroxidenickel metal 
interfacial contact; (3) mixing nickel hydroxide, graphite, 
dimethylformamide, polyvinylidene fluoride and dimethylacetaminde 
together, casting it into a thin film (e.g. 0.7-0.8 mm), drying it for a 
short while, immersing it in water to coagulate the polyvinylidene 
fluoride, and finally wrapping it with a current collector and fabric 
separator to form the electrode. Still another technique involves mixing 
dry powdered PTFE with the active materials in a nonaqueous lubricant to 
form a slurry, filtering the slurry to form a filter cake containing about 
25%-50% lubricant, calendering the filter cake to a self supporting 
condition, drying it and integrating it with a current collector. 
PTFE-bound electrodes are highly resistant to the chemical and 
electrochemical environment of alkaline cells and, do not lose active 
material (i.e. graphite and nickel oxide) during formation. Hence they 
offer advantages over other polymer binders. However they cannot be made 
by the McBreen process as no acceptable solvent system is known, and even 
if there were the slow precipitation and cost of materials and the 
handling thereof in a plant make the McBreen process undesirable on a 
commercial scale. Moreover the sintered PTFE processes cannot be used with 
all active materials as the sintering temperatures (i.e. 260.degree. 
C.-375.degree. C.) tend to destroy the electrochemical activity of some of 
them notably nickel hydroxide (Ni(OH).sub.2). 
Accordingly objects of the present invention are to provide an inexpensive, 
water-base process for manufacturing, non-sintered, PTFE-bound electrodes 
as well as the electrodes themselves. These and other objects of this 
invention will become more apparent from the description thereof which 
follows. 
BRIEF SUMMARY OF THE INVENTION 
The subject invention comprehends coagulating PTFE particles from an 
aqueous dispersion thereof to form a gelatinous PTFE coagulum, 
mechanically dispersing the coagulum in water such that the coagulum is 
subdivided into small, sticky clots of PTFE particles which are suspended 
in the water, mixing the thusly formed suspension with comminuted 
electrode active material to form a slurry of active material and PTFE 
clots coated with active material, filtering the slurry to separate the 
solids therefrom as a filter cake, integrating an electrically conductive 
current collector with the filter cake, compressing the thusly-formed 
collector-cake composite to densify the cake to enhance the electrical 
contact among and between the active material and the current collector 
and to interlock the particles together into a coherent mass, and drying 
the thusly formed compress.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, commercially available aqueous 
dispersions of PTFE particles (e.g., Dupont's "TEFLON" 30B 
TFE-Fluorocarbon resin) are coagulated to form a gelatinous mass or 
coagulum of the PTFE. Coagulation may be achieved by any of the known 
techniques for coagulating PTFE dispersion (i.e. shear forces, PH 
reduction, organic coagulants etc.) so long as there is sufficient 
coagulation thereof that there is no substantial loss through the filter 
medium by uncoagulated PTFE during the filtration step. In accordance with 
a preferred embodiment of the invention, coagulation of the PTFE 
dispersion is achieved by adding a small amount of coagulant (i.e. 
preferably isopropyl alcohol) thereto to form the initial gelatinous 
coagulum. Other suitable coagulants would include PH reducing acids, low 
molecular weight polar alcohols, such as methanol, ethanol, 1-propanol, 
2-propanol, 1-butanol, 2-butanol, 1,1,1,-trimethylmethanol, and certain 
trivalent or tervalent cations such as aluminum or zirconium salt 
solutions. Only small amounts of coagulant need be added to the 
concentrated PTFE dispersion to effect substantially complete coagulation 
and formation of the gelatinous coagulum. 
The size of the dispersed PTFE particles (i.e. prior to coagulation) can 
vary anywhere from a fraction of a micron to several microns in diameter. 
Successful electrodes have been made from a material known as "Teflon" 30B 
TFE-Fluorocarbon resin aqueous dispersion. This material is a negatively 
charged hydrophobic colloid containing about sixty (60) percent by weight 
PTFE particles (i.e. ranging in size from about 0.05 microns to about 0.5 
microns) stabilized with about six percent (i.e. based on weight of PTFE) 
non-ionic wetting agent. 
The gelatinous coagulum is then added to water and rapidly stirred therein 
to subdivide the coagulum into small clots of PTFE particles and suspend 
them in the water. These PTFE clots are small, highly irregularly shaped, 
sticky masses which take on and become coated with active material 
particles during the mixing/slurrying step. They then become even more 
irregularly shaped and grow to sufficient size to preclude their passing 
through or plugging the filter medium during the filtration step. 
Comminuted electrode active material is added to the suspension of clots in 
the blender and intimately mixed therewith to form a slurry. The active 
material is preferably added dry, but may optionally be preslurried in 
water and then poured into the blender. During mixing, much of the active 
material adheres to the surface of the sticky PTFE clots and thereby 
increases their size. Following thorough mixing of the active material 
with the PTFE clots and before any significant settling occurs, the thus 
formed slurry is quickly filtered as by pouring into a mold on a vacuum 
table with filter paper therebetween and drawing off the water so as to 
leave the PTFE and active material as a filter cake atop the filter paper. 
The amount of PTFE needed for binding can vary considerably and depends 
largely on the composition of the active material, whether or not there 
are any conductive diluents added to the active material and the 
electrochemical capacity needed from the electrode. As a general rule, no 
more PTFE is used than is necessary to effect adequate binding of the 
active materials into a coherent mass sufficient to withstand the rigors 
of repeated cycling. In this regard nickel electrodes, for example, will 
contain PTFE in amounts of about 6% to about 18% by weight of the dried 
filter cake, with about 7%-8% being preferred. Zinc electrodes on the 
other hand require no more than about 8% by weight PTFE for successful 
binding with only about 3%-4% by weight being preferred. 
The filter cake is then integrated with an appropriate current collector, 
and this may be accomplished in a variety of ways. In one such way, the 
current collector is positioned in the mold and the slurry poured over the 
collector such that the filter cake builds up on and around the collector. 
A preferred technique involves preparing two separate filter cakes or 
layers, sandwiching the current collector between the two layers and then 
proceeding to the compression and drying steps. The current collector is 
essentially a macro-porous metal network extending substantially 
throughout the planar extent of the electrode for effectively gathering 
and conducting current to a site for its removal from the electrode. 
Acceptable current collectors include metal screen, expanded metal, metal 
foam, etc. Regardless of how the current collector and filter cake are 
integrated, the composite thereof is compressed which squeezes out much of 
the remaining water, enhances the electrical contact between and among the 
active material particles and the current collector and mechanically 
keys/interlocks the irregularly-shaped, coated clots together to form a 
strong coherent mass. The wet pressing of nickel electrodes is performed 
at pressures of about 9 MPa to about 31 MPa. Following compression, the 
electrode is dried in an oven at about 65.degree. C.-100.degree. C. for 
about five to fifteen minutes and again pressed at about the same 
pressures to ensure uniform thickness of the electrode. 
Conductive diluents may be added to those electrodes whose active materials 
are not inherently conductive (e.g., Ni(OH).sub.2). Graphite is typically 
used for this purpose, but other materials, e.g. silicon carbide, nickel, 
etc. might also be used depending on the particular type of electrode 
being made. Graphite is preferred since it is inert to the cell 
environment, lightweight, conductive, inexpensive and is readily available 
in both powdered and fiber form. The total graphite content of a nickel 
electrode, for example, advantageously comprises up to about 30% by weight 
of the dry filter cake with about 23%-30% being preferred. The graphite 
therein is preferably in both the powdered and fibrous form (i.e. about 
0.5 mm long), there being about half again as much powdered graphite (i.e. 
by weight) as there is fibrous graphite, though this can vary 
considerably. Graphite particles greater than about 5 microns in diameter 
appear to have relatively poor conductivity while particles less than 
about 0.5 microns in diameter seem to produce only short-lived electrodes. 
Particular success has been achieved in nickel electrodes using airspun 
graphite having an average particle size of about 2.5 microns and which is 
commercially available under the name of Dixon KS-2. Preferably, the 
nickel hydroxide (i.e. normally less than about 200 mesh) and powdered 
graphite are intimately mixed, as by dry ball-milling, to smear the 
graphite over the surface of the nickel hydroxide powder and thereby 
effect intimate contact therebetween. When graphite fibers are used they 
need not be premixed with the nickel hydroxide, but rather can be added 
separately to the slurry in the blender. In the case of nickel electrodes 
using ball-milled graphite-nickel hydroxide mixes, short blending times 
(i.e. less than about sixty seconds) are preferred because excessive 
blending (e.g. such as employed by McBreen) tends to wipe the graphite off 
the nickel hydroxide and reduce its effectiveness. 
Accelerated filtration aided by vacuum or centrifugal force speeds up the 
process. It is preferred to use conventional paper-making techniques 
wherein the water is removed by drawing it off with a vacuum through an 
appropriate filter medium such as filter paper. Newsprint has proven quite 
acceptable for this purpose. The newsprint is positioned on a vacuum 
table, and a frame-like mold placed atop the paper. The mold cavity 
receives and contains the slurry from the blender while the water is drawn 
off through the filter paper. 
By way of example, nickel electrode stock material is prepared which, 
excluding the current collector, comprises 62.1 percent by weight hydrated 
nickel hydroxide (Ni(OH).sub.2), 3.5 percent by weight cobalt hydroxide, 
16.4 percent by weight graphite particles (2.5 micron), 10.5% by weight 
graphite fibers (i.e. Thornel grade VMA-Union Carbide), and 7.5% by weight 
PTFE. Green nickel hydrate (Ni(OH).sub.2) and cobalt hydrate (C.sub.o 
(OH).sub.2) powders (i.e., less than 200 mesh) are dry ball-milled for 
four hours along with the powdered graphite particles. In a separate 
operation two milliliters of TEFLON 30B TFE Fluorocarbon dispersion are 
drawn into a hypodermic syringe and one milliliter of isopropyl alcohol 
added thereto in the syringe to coagulate the PTFE particles into a 
gelatinous coagulum. One half of this coagulum is then injected from the 
syringe into 150 ml of water which is being rapidly stirred in a Waring 
blender. The blender action breaks up the coagulum into small clots of 
PTFE particles and suspends them in the water. 9.4 grams of the 
Ni(OH).sub.2 -graphite-C.sub.o (OH).sub.2 mix and 1.2 grams of graphite 
fibers are then added to the suspension in the blender and blending 
continued for one (1) minute. The blender is stopped and the slurry 
therein quickly poured onto a sheet of newsprint on a vacuum table. A mold 
on top of the newsprint confines the slurry to a 12 cm.times.12 cm.times.2 
cm mold cavity. A vacuum of about 25 in. Hg draws off the water in about 
thirty seconds and leaves the solids as a filter cake in the cavity. A 
second batch is made up the same way to form a second filter cake 
identical to the first. A 5 mil thick expanded nickel screen (i.e., 20 
mesh) is then sandwiched between the two filter cakes and the sandwich 
compressed at about 73 Kg/Cm.sub.2 followed by drying for three minutes at 
100.degree. C. A second pressing at the same pressure finishes the 
electrodes stock material to a thickness of about 0.104 cm. 
Electrodes so made were tested. In one test electrodes (i.e., 47.6 
mm.times.79.4 mm) were cut from the stock material to have a theoretical 
capacity of 1.16 A.h, wrapped in non-woven polyamide felt and heat sealed 
around the edges. The felt had a thickness of about 0.010 cm and was 
supplied by the Pellon Corp. as their material 2504K4. Each electrode was 
assembled into cells between two like-sized inert sintered Ni placque 
electrodes heat sealed in polyamide felt the same as the test electrodes. 
A room temperature electrolyte comprising 37.5% by st KOH and 12 g/e L.OH 
was used and the electrodes formed with three charge-discharge cycles 
which included twenty hours charge at a current density of about 1.3 
mA/Cm.sup.2 and discharged at about 4.4 mA/Cm.sup.2. In the third cycle, 
the active material utilization was tested by discharging the electrodes 
from an initial 1.73 volts to a cut off voltage of 1.0 volt as measured 
against a zinc reference electrode. These electrodes displayed a nickel 
hydrate utilization of about 0.30- 0.31 A.h/g or about 106% of theoretical 
capacity and about 5.91-5.93 g/A.b of total electrode weight (i.e., 
including collector, graphite and PTFE). 
In another test, electrodes (i.e., 38.1 mm.times.57.2 mm) were cut from the 
stock material to have a theoretical capacity of about 0.67 A.h. These 
electrodes were heat-sealed as before in polyamide felt and assembled into 
cells between two sintered nickel oxide electrodes each of which was 
wrapped in a single "U"-fold of radiation-grafted polyethylene supplied by 
Radiation Applications Inc. as their material 2291. The same electrolyte 
was used and the electrodes formed with two cycles each comprising 
charging for 16 hours at about 2mA/Cm.sup.2 and discharging at 0.335 
amperes to -0.85 volts/cell (i.e., equivalent to 1.0 V as against a Zn 
reference). The cells were connected to a regulated, filtered DC supply 
and constant current cycled as follows: charge for 6 hours at about 2.8 
mA/Cm.sup.2 ; and discharge at about 7.7 mA/Cm.sup.2 to a cut off voltage 
of -0.85 volts. This cycle was repeated until the electrode's capacity 
fell to about 75% of theoretical. One electrode so tested survived 167 
such cycles while another electrode remained at 93% of its theoretical 
capacity after 278 cycles. 
while this invention has been disclosed primarily in terms of specific 
embodiments thereof, it is not intended to be limited thereto but rather 
only to the extent set forth hereinafter in the claims which follow.