Note: Descriptions are shown in the official language in which they were submitted.
1056904
BA KGROUND
Field of the Invention
.
The pre~ent invention generally relates to improved
separators and methods of making the same and more particularly -
to improve inorganic separators for alkaline electrochemical
cells having zinc negative electrodes.
Prior Art
The usual types of alkaline cells and batteries com-
prise a positive electrode of, for example, silver oxide, nickel
hydroxide or the like and a negative electrode of zinc. The
electrolyte may be, for example, concentrated aqueous potassium
hydroxide or the like. When such cells are operated, the zinc
electrode gradually loses zinc ions into the electrolyte and
becomes physically deformed. Certain electrical characteristics
of the cell or battery are dependent upon the physical configur-
`ation of the electrodes. Any shape change in either electrode
results in a change in the electrical properties of the cell,
obviously an undesirable development. Moreover, reduction in the
size of one of the electrodes, for example the negative electrode
places a limit on the durability of the cell. Inasmuch as such
cells are relatively expensive, it would be highly desirable to
provide means for stabilizing the shape of the electrodes and
thus stabilizing the electrical properties of the cell while
increasing the usable life of the cell.
SUMMARY OF THE INVENTION
The foregoing needs have now been satisfied by
the present invention. In this regard, an improved separator
for alkaline electrochemical cells having zinc negative elec-
trodes which comprises rare earth inorganic hydroxide in finely
7F
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1056904
1 divided solid particulate form dispersed uniformly within a
shape-retaining non-reactive matrix is provided. The matrix may
be, for example, polytetrafluoroethylene or the like and the
separator is insoluble and chemically inert in the cell electro-
lyte. The separator traps or chemically combines with soluble
zinc ions so as to keep the ions within the compartment contain-
ing the zinc electrode. This reduces the solubility of the zinc
electrode. The resultant reduction in shape change reduces the
shift in electrical properties which occurs during cycling of
the cell. An improved method of making the novel separator is
also provided wherein a mixture of the hydroxide and the matrix
material is extruded into a thin sheet.
The separator prevents the migration of zinc
ions from the compartment containing the
-la-
1056904
1 zinc negative electrode in an alkaline electrochemical cell.
The separator accomplishes this by either trapping or chemically
combining with those zinc ions. Accordingly, the rate of
removal of zinc from the electrode in the form of zinc ions
solubilized in the electrolyte is depressed. The net effect is
that the zinc electrode retains its shape over a substantially
greater period of time during cycling of the cell than is the case
with conventional cells and exhibits greater stability of
electrical properties than do conventional cells.
The separator may employ any one or a mixture of rare
earth inorganic hydroxides such as cerium hydrate, yttrium
hydroxide and hydroxides of neodymium, samarium, gadolinum, terbium
and the like. The hydroxide is disposed in finely divided
particulate form within a non-reactive matrix such as polytetra-
fluoroethylene, vinyl acetate and ethylene copolymer resin, poly-
vinyl chloride polymer or the like to form the desired separator
sheet, which sheet is then placed around the negative electrode
in each cell. Further details of the present invention are set
forth in the following detailed description and the accompanying
drawings.
DRAWINGS
Figure 1 is a schematic perspective view of a
preferred embodiment of an electrochemical battery employing
the improved inorganic separator of the invention, portions
being broken away to illustrate certain internal features of the
battery.
DETAILED DESCRIPTION
An improved inorganic separator is made in accordance
with the present invention utilizing rare earth inorganic
hydroxide in finely divided solid particulate form. The
-` ~056904
1 hydroxide may be any one or more of the rare earth hydroxides.
Normally, the average particle diameter of the hydroxide is no
greater than about 2 microns and the average density may vary,
but for cerium hydrate, for example, is usually no greater than
about 0.9 gm/cm3.
This fine powder is mixed intimately with a suitable
binder-forming inert matrix material which is non-reactive with
the usual alkaline cell electrolytes and which may be, for example,
polytetrafluoroethylene emulsion, a low molecular weight poly-
olefin resin, a mixture of vinyl acetate and a copolymer ofethylene and vinyl acetate, polyvinyl fluoride resin, fluorinated .
polyethylene or polyvinyl chloride polymer.
The resulting mixture has the rare earth hydroxide
uniformly dispersed therein; for example, when polytetrafluoroethy-
lene emulsion is used, drying can take place in an oven at 100C
for four hours. If the matrix material is in solid form initially,
it is utilized as a finely divided powder and is merely mixed
uniformly with the hydroxide. The resulting dry mixture is then
ready for formation into a separator configuration of desired
average pore diameter and porosity.
Forming of the separator is usually accomplished by
extruding the dry mixture of the rare earth hydroxide and matrix
forming material through a suitable shape nozzle to form a sheet
of material approximately, for example, 0.010" thick. Other
thicknesses such as 0.002", 0.005", 0.015" and 0.030" are also
suitable. Such extrusion may, for example, be carried out under
the following conditions utilizing the following equipment:
The previously described powder mix is mixed and
compounded with an extrusion aid (lubricant) such as propylene
glycol, methyl alcohol, isopropyl alcohol, ethyl alcohol, acetone
1056904
i or the like in a mixer such as Patterson Kneadmaster mixer Model
CK453. The resultant dough is extruded using, for example, a
Patterson Extruder having an auger diameter of three (3) inches J
and an extrusion nozzle opening of 6" wide x 0.010" thick. The
lubricant can be present in the extruded dough in a weight
concentration of, for example, about 10 - 20 percent. Following
extrusion, the sheet is passed through a curing oven at a tempera-
ture of 3600F for about 1 - 10 minutes so as to drive off the
lubricant and cure the matrix material.
The sheet material thus formed by extrusion is then
cut to size and normally used directly as a separator in an
alkaline cell of the type described. Preferably, the extruded
sheet material has a very low average pore diameter, for example,
below about 200 angstroms and a high volume porosity, preferably
above about 50%. It will be understood that for special applica-
tions the separator material may be formed having other ranges
of physical characteristics. Generally, however, a very low pore
diameter with high porosity is desired so that efficient contain-
ment of solubilized zinc ions can be achieved. The improved
separator may be utilized in any suitable way, for example, in the
containment surrounding the zinc electrode in the cell or as
follows:
The sheet may be placed directly on the zinc electrode
so as to act not only as a negative interseparator but also as
a support structure for the zinc active material. In this
case, the improved inorganic separator material which has been
cut to dimensions larger than that of the negative electrode....
is pressed on and into the zinc electrode structure during the
latter's fabrication.
3~ Preferably, the improved separator is, in use, disposed
- 1056904
1 between protective layers of conventional separator material,
such as polypropylene film having the following characteristics:
average pore diameter = 400 - 600 microns; average thickness =
5.0 mils. Such films can be, if desired, of such materials
as the following: cellophane, porous irradiated polyethylene,
rayon, dynel,* nylon, etc.
Now referring more particularly to Fig. 1 of the
accompanying drawings, a typical alkaline battery having
zinc negative electrodes and the improved separator material of
tO the invention is schematically illustrated. Thus, a battery 10
is shown which includes a container 12 having a hollow interior
14 and a closed top 16 through which a filler and vent tube 18
and the positive and negative terminals 20 and 22, respectively,
of the battery extend.
A pair of negative electrodes 24 of zinc in the form
of plates are disposed in spaced relation within interior 14 of
container 12 and are connected to the negative terminal 22 by a
pair of leads 26. A sheet 28 of the improved separator 30 of
the invention is wrapped around each negative electrode 24 in
container 12 and is disposed between two sheets 32 and 34 of poly-
propylene separator material to form therewith containment bag 36
for each negative electrode. Bag 36 has an open upper end 38
through which the leads 26 protrude.
The negative electrodes 24 in their containment bags
36 are sandwiched between three positive electrodes 40 in the
form of plates connected by leads 42 to the positive terminal 20.
An electrolyte 44 in the form of concentrated (about 40%) aqueous
potassium hydroxide (solution) is also disposed in interior 14 of
container 12 below the upper end 38 of each containment bag 36. It
will be understood that other arrangements of the improved
* Trade mark for a copolymer of vinyl chloride and acrylonitrile
** Registered trade mark for a film produced from wood pulp by
the viscose process
~05~;904
1 separator within alkaline electrochemical cells and batteries
can be provided. The following specific Examples further illus-
trate certain features of the present invention:
EXAMPLE I
Cerium hydrate is passed through a 325 mesh screen
so that it has an average particle size of less than 2 microns.
This fine powder is then mixed with sufficient polytetrafluoro-
ethylene emulsion so that the solids by weight ratio of the
polymer to the hydroxide is about 1:4. The mixing is accomplished
for a sufficient period of time until the mixture is uniform.
The resulting product is then dried in an air convection oven
at 100C for 4 hours. Four parts (by weight) of the dried
mixture are then mixed with one part of propylene glycol and the
resulting dough is extruded through a nozzle under 1200 psi at
about 85C to obtain a sheet which, after curing in an oven at
about 360F for about 5 minutes, has an average thickness of
about 0.010". This finished sheet has an average pore diameter
of less than 200 angstroms and a volume porosity of about 55%.
The finished sheet is cut into two appropriately
sized sheets and installed in a battery having the configuration
shown in Figure 1. The battery has two zinc negative electrodes
in the form of plates, each plate having the following dimensions:
1.625" wide x 1.50" high x 0.042" thick. Three silver oxide
positive electrodes are spaced between and outside the two zinc
electrodes. The positive electrodes are each in the form of a
plate having the following dimensions: 1.625" wide x 1.50 " high x
0.034" thick. The electrodes are disposed in a casing of styrene-
acrylonitrile plastic sold under the trademark Lustran 400 of
Monsanto Co., St. Louis, Mo.
The negative electrodes are each enclosed in an open
-- 6 --
1056904
1 topped containment bag comprising an inner and an outer layer
of polypropylene, each about 5 mils thick and having an average
pore diameter of about 500 microns,and a middle layer comprising
a sheet of the improved separator of the invention. An electro-
lyte comprising a 40% aqueous solution of potassium hydroxide is
disposed in the battery below the open top of each containment
bag. The battery (A) exhibits great durability and stability of
electrical properties in contrast to a battery (B) fabricated in
the same way utilizing the same materials except for the absence
of the cerium hydrate-containing separator of the invention,
as shown in the following table:
TABLE
Characteristics Battery A Battery B
life at ngrmal load 160 102
(at 140 F) days
maximum number of cycles 56 22
percent negative electrode 20 70
shape change
EXAMPLE II
An improved separator in accordance with the present
invention is fabricated by intimately and uniformly mixing
together 85 parts by weight of yttrium hydroxide with 15 parts
by weight of powdered polyethylene resin having an average
particle diameter of about 0.2 microns. The yttrium hydroxide
is in particulate form having an average particle diameter of
about 1 micron and a bulk density of about 1.6 gm/cm3. The
mixing is continued until a spongy mixture is obtained, whereupon
the mixture is blended into a dough with methyl alcohol (6 parts
of mixture to 1 part of alcohol, by weight) and the dough is
extruded at 85C and 1000 psi into a sheet which, after curing
in an oven at about 350F for about 10 minutes, has a thickness of
1056904
1 ~bout 10 mils. The finished sheet has an average pore diameter
of 20~ - 300 and a volume porosity of about 40%.
The separator sheet is cut into two sheets and utilized
in a battery (C) identical to battery A of Example I in
place of the cerium hydrate-containing separator sheets thereof.
Battery C is co~lpared in electrical characteristics with a
battery (B) which is identical except for the absence of a rare
earth hydroxide-containing separator. The results of the
comparison are set forth in the table below:
TABLE
Electrical CharacteristicsBattery C Battery B
battery life (at 140Fo) days 102 35
maximum cycles (at 70 F) 40 22
percent negative electrode 55 70
shape change
As in Example I, comparative tests show that improved
electrical stability and durability are achieved by using
the rare earth hydroxide-containing separator of the invention.
When parallel tests are conducted substituting equivalent
amounts of other rare earth hydroxides for the cerium hydrate
of Example I and the yttrium hydroxide of this Example in the
separator of the invention, comparable improved results are
obtained. Substitution in separate tests of polypropylene,
vinyl acetate-ethylene copolymers, polyvinylidene fluoride,
fluorinated ethylene-propylene resin and polyvinyl chloride for
the polytetrafluoroethylene emulsion and the polyethylene resin
of Examples I and II, respectively, is made without varying the
results, when the separator average pore diameter and total pore
volume, as well as the weight ratio of such matrix material to
the rare earth hydroxide are substantially as set forth above.
Accordingly, an improved separator is economically
formed in accordance with the present method, utilizing effective
-- 8 --
56904
concentrations of rare earth hydroxides in finely divided
particle form uniformly dispersed in a suitable matrix. Such
separators when used in containment around the zinc negative
electrodes of the cell, effectively reduce the loss of soluble
zinc ions from the contained area and thus reduce the loss of
zinc from the zinc electrode during operation of the electro-
chemical cell. Such loss reduction results in improved longevity
for the cell and more stable electrical characteristics, as
demonstrated in the tables of Examples I and II. Accordingly,
improved alkaline cells and batteries having zinc negative elec-
trodes can be provided in an economical manner through the use
of the improved separator of the invention.
Various modifications, changes, alterations and
additions can be made in the present separator, its components,
and in the present method and its steps and parameters. All
such modifications, changes, alterations and additions as are
within the scope of the appended claims form part of the present
invention.
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