Battery separator membranes of high electrolytic conductivity comprising a cellulose ether and a compatible metallic salt of water soluble aliphatic acids and their hydroxy derivatives are described. It was found that methyl cellulose can be modified by another class of materials, nonpolymeric in nature, to form battery separator membranes of low electrolytic resistance but which have the flexibility of membranes made of unmodified methyl cellulose, and which in many cases enhance flexibility over membranes made with unmodified methyl cellulose. Separator membranes for electrochemical cells comprising a cellulose ether and a modified selected from the group consisting of metallic salts of water soluble alphatic acids and their hydroxy derivatives and to electrochemical cells utilizing said membranes are described

Hoyt, H. E.

Pfluger, H. L.

English

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(NASA-Case-GSC-10018-1) SEPAEATOE~.FOE
ALKALINE ELECTHIC BATTERIES AND METKOD CF
MAKING Patent (NASA) 3 p . , CSCL 10C
00/44-
N82-24644
Onclas
20223
https://ntrs.nasa.gov/search.jsp?R=19820016768 2020-03-21T16:46:01+00:00Z
3,519,483
Patented July 7, 1970
3,519,483
SEPARATOR FOR ALKALINE ELECTRIC
BATTERIES AND METHOD OF MAKING
Helmuth Louis Pfioger and Howard Eugene Hoyt, Hunt-'
* ingdon Valley, Pa*, assignors to Borden, IDC, New 5
York, N.Y., a corporation of New Jersey
- No Drawing. Ffkd Nov. 2, 1967, Set. No. 679,987
Int, CI. HOlm 35/00, 3/00
U-S.C1.13S—6 12 Claims
10
ABSTRACT OF THE DISCLOSURE
This invention relates to battery separator membranes
of high electrolytic conductivity comprising a cellulose
ether and a compatible metallic salt of water soluble ali- 15
phatic acids and their hydroxy derivatives.
20
25
ORIGIN OF THE INVENTION
The invention described herein was made in the per-
formance of work under a NASA contract and is sub-
, ject to the provisions of Section 305 of the National Aero-
nautics and Space Act of 1958, Public Law 85-568 (72
Stat 435; 42 U.S.C. 2457).
BACKGROUND OF THE INVENTION
The cycle life of rechargeable battery cells is known
to be limited by the tendency of the electrodes to short
circuit and this tendency can be slowed down by the use
 3fl
of a separator membrane between the electrodes. In the
particular case of alkaline silver-zinc cells, such a sepa-
rator must withstand both the oxidizing power of silver
oxide and the penetration of metallic zinc and must also
. have low resistance to the passage of an electrolytic cur- __
Films of cellulose ethers, particularly methyl cellu-
lose, have excellent withstanding powers against both
, silver oxidation and zinc meial penetration. In spite of
these advantages, the use of such cellulose ethers by
 40
•themselves has not been successful because their resist-
ance to passage of an electrolytic current is prohibitively
high in the 30 to 45% potassium hydroxide solutions cus-
tomarily required in battery cells. While ways of physical
modification cf methyl cellulose membranes in the wet
 45
state have been discovered by the present inventors, as
- covered in pending applications Ser. No; 658,999 filed
Aug. 8, 1967 and Ser. No. 680,048 filed Nov. 2, 1967, it is
desirable in some applications that the dry membrane
should be capable of developing high conductivity by go
•simple equilibration with the ceil electrolyte. In prior
art, such as shown in U.S. Pat. 2,858,353 (Mendelsohn)
this has been achieved by incorporation within methyl
cellulose films of certain highly polar, more conductive
polymeric material. However, films modified in this man- 55
ner are not as flexible in the dry state .is unmodified
methyl cellulose films and require special control of hu-
midity in order to insure freedom from damage during
handling.
SUMMARY OF THE INVENTION 60
We have now found that methyl cellulose can be modi-
fied by another class of materials, non-polymeric in na-
ture, to form battery separator-membranes of low elec-
trolytic resistance but which have the iiexibility of mem- Go
brahes made of unmodified methyl cellulose; and which
in many cases enhances flexibility over membranes made
with unmodified methyl cellulose.
Briefly slated, our invention is directed to separator
membranes for electrochemical cells comprising a cellu- 70
lose ether and a modified selected from the group con-
sisting of metallic sails of water-soluble aliphatic acids
and their hydroxy derivatives, and to electrochemical
cells utilizing said membranes.
DETAILED DESCRIPTION
This invention may be applied to any membrane made
of a water-soluble or water-swellable and alkali insolu-
ble cellulose ether, such as methyl cellulose, ethly cellu-
lose, propyl cellulose.
Preferred material is methyl cellulose, particularly
methyl cellulose in which 1.6 to 1.9 out of every 3.0
available hydroxyl units of cellulose have been etherified.
Methods for etherifying cellulose to different degrees and
using a variety of ether groups are known to the art and
are not a subject of this patent application.
The additive may be any water-soluble salt formable
by the reaction between a hydroxide of a metal of Group
I and Group II of the Periodic Table and an acid selected
from the group consisting of monobasic aliphatic acids
with from one to four carbon atoms, dibasic aliphatic
acids with from two to five carbon atoms, and tribask
aliphatic acids with from four to six carbon atoms. The
acid may have one, two or three carboxy groups and
may be hydroxylated. Thus, for example, the acid may
be formic, acetic, propionic, butyric, isobutyric, succinic,
glutaric, fumaric, maleic citric, tartaric, glycolic or lactic
acid. The cation of the salt may be sodium, potassium,
lithium, calcium, strontium, ammonium or substituted
ammonium, as for example tetramethyl amonium. Pre-
ferred are the acids of lower equivalent weight, less than
2-3 C's per carboxy group, and, specifically, potassium
lactate and potassium acetate.
The amount of salt may vary from 3% to 40% by
•veight with respect to the cellulose ether in the formula-
tion. Preferred amounts are in the range of 20 to 35%
by weight, the choice being determined by the degree of
resistance-lowering desired and by the necessity of avoid-
ing frosting out of the salt from the film which occurs
at varying threshold levels depending upon the salt used.
In addition to the cellulose ether, the polymeric com-
position of the film may also include minor amounts of
more polar polymeric materials such as polyacrylic acid
or the 1:1 copolymer of vinyl methyl ether and maleic
anhydride. The polar polymeric substance may vary in
concentration up to 30% by weight, but it is generally
preferred not to exceed 10% because of the stiffening and
non-flexibilizing effects which such components impart
The membrane may be prepared by combining aque-
ous solutions respectively of methyl cellulose and of the
other additives. The resulting composition may be cast in
sheet form and evaporated to dryness. Other forms may
be made as for example plates, slabs, "buttons," films
and the like. Variations in the procedure include the use
of aqueous solutions of the salts or adding alkali to the
aqueous solution of the corresponding acids either before
or after admixing with the methy! cellulose solution. So-
lutions of the components should not be so concentrated
as to precipilate the components before drying.
Good flexibility in the film is of particular importance
in the construction of the individual battery cells dur-
ing the operation of wrapping the separators around the
electrodes. In one common technique a so-called U wrap
is made whereby two positive electrodes are placed butt
end to butt end on a sheet of separator and after, for
example, six wraps of the pair the wrapped assembly is
folded at the junction in the form of the letter U. When
separators of poorer flexibility are used cracks tend to
occur in the separator at the base of the U, particularly
in the outer wraps, thus destroying the utility of the sep-
arator at these points. We have found experimentally
that this is particularly likely to occur below a certain
critical threshold of flexibility, corresponding to with-
standing at least about 800 flex cycles as measured by
the ASTM Folding Endurance Test D643-43 with 200
grams tension on the specimen.
The invention will be further illustrated by description
in connection with the following specific examples of
the practice of it; the proportions here and elsewhere
herein are expressed as parts by weight unless specifically
stated to the contrary.
EXAMPLE i
3,519,483
4
EXAMPLE?
The procedure of Example I was used with 24.6 parts
of potassium acetate to 75.4 parts methyl celluose. The
film had 1.6 mil thickness, MIT flex of 7311 cycles and
5 electrolytic resistance in 45% potassium hydroxide of
34 milliohms-in.2:
EXAMPLES
10200 grams of a 10% solution of meihyl cellulose (Meth-
ocel 15) in water and 20.0 grams of a 14.3% solution
of potassium lactate in water were completely mixed at
5° C. Films were cast on levelled glass plates by use of
the drawdown procedure, using a 23 mil clearance on the jg
doctor blade. A clear homogeneous, one-phase film of
1.4 mil thickness was obtained. The flexibility of the
film was evaluated in 'terms of the MIT flex test, using
the ASTM Specifications D643-43 for Folding Endur-
ance, with 200 g. tension. Another portion of the film was 20
equilibrated with 45% potassium hydroxide solution and
its electrolytic resistance measured in an appropriate
conductivity celL
The same tests were made on a 1.5 mil straight methyl
The procedure of Example 1 was used with 14.9 parts
potassium acetate to 85.1 parts of methyl cellulose. The
film had 1.6 mil thickness, MTT flex of 5987 and elec-
trolytic resistance of 137 milEohms-in.2 in 45% potas-
sium hydroxide.
EXAMPLES
The procedure of Example 1 -^as used with 15.2 parts
potassium propionate on 100 parts methyl celiulose. The
film had 1.8 mil thickness, MIT flex of 5015 cycles and
electrolytic resistance in 45% potassium hydroxide of
EXAMPLE 10
The procedure of Example 1 was used with 14.3 parts
potassium butyrate on 100 parts, methyl cellulose. The
cellulose film. These tests showed that the potassium lac- 25 film had 1.4 mil thickness, MIT flex of 5154 cycles, and
tale addition had resulted in a lowering in electrolytic electrolytic resistance in 45% potassium hydroxide of
resistance from 6080 to 295 milliohms-in.2. The flex re-
sistance had simultaneously been improved from 3798
225 milliohms-in.2.
cycles to 5672 cycles.
EXAMPLE 11
EXAMPLE 2
30 The procedure of Example 1 was used with 28.6 parts
potassium butyrale on 100 parts of "methyl cellulose. The
film had 1.4 mil thickness, MFC flex of 7591 cycles and
electrolytic resistance in 45% potassium hydroxide of
110 milliohms-in.2.
EXAMPLE 1235
EXAMPLE 3
The procedure of Example 1 was used with 7.80 parts
The procedure of Example 1 was used with 4.10 parts
potassium lactate to 95.9 parts methyl cellulose. The film
had 1.4 mil thickness, MIT flex of 5024 cycles and electro-
lytic resistance in 45% KOH of 2225 milliohms-in.2.
The procedure of Example i was; nsed with 90 parts
methyl cellulose, 10 parts of the 1:1- copolymer of poly-
vinyl methyl ether and maleic anhydride and 28 parts
potassium lactate to 92.2 parts methyl cellulose. The film
 40 potassium lactate. The film had 1.7 mil thickness, MIT
had 1 5 mils thickness, MIT flex of 4172 cycles and 875 «ex of 7926 cycles and electrolytic resistance in 45%
miHiohms-in.2 resistance in 45% KOH. potassium hydroxide of 69 miffiofams-in.2.
EXAMPLE 4 EXAMPLE 13
Representative films of cellophane an dfilms of this
The procedure of Example 1 was used with 27.2 parts 45 jnvernion were stirred for 72 hours in 30% potassium by-
potassium lactate to 77.8 parts methyl cellulose. The film droxide saturated with silver oxide. At the end of this
had 1.6 mil thickness, MIT flex of 5840 cycles and elec-
trolytic resistance of only 170 milliohms-4n.2 in 45%
KOH.
EXAMPLES 50
A film was cast using the procedure of Example 1
from a solution containing 70 parts methyl cellulose and
50 parts of the 1:1 copolymer of vinyl methyl ether
and maleic anhydride. The film was 1.6 mils thick, had 55
an electrolytic resistance of 376 milliohms-in.2 in 45%
potassium hydroxide. However, its flexibility was so low
that it withstood only 518 cycles under the MIT Flex Test.
The film, under the designation "C3" has shown promis-
ing performance in several cycling tests in commercial-
type cells at Crane Ammunition Depot and elsewhere,
but its low flexibility makes special handling necessary
to insure freedom from mechanical damage during as-
sembly of cells.
EXAMPLE 6 .
The procedure of Example 1 was used •with 8.9 parts of
potassium acetate to 91.1 parts methyl cellulose. The
film had 1.6 mil thickness, MIT flex of 4731 cycles and
period the saturated films were broken in a tensile testing
machine. As controls, representative samples of the same
films were stirred for 72 hours in. 30% potassium hy-
droxide in the absence of silver oxide and likewise broken
in a tensile testing machine. The stability of the films to
the silver oxide was measured in terms of the ratio of the
strength of the oxidized film to that of the film exposed to
potassium hydroxide only.
The stability ratio was found to be well above 80% for
the films of this invention whereas cellophane had zero
strength after the exposure to aftaline silver oxide. For
example, a film composed of 77.8 parts methyl cellulose
and 22.2 parts potassium lactate had'a stability factor of
83%; and a film composed of 75.4 parts methyl cellulose
and 24.6 parts potassium acetate had a stability factor of
.88%. Thus the films of this invention are superior to the
customarily used cellophane in withstanding the oxidation
power of silver oxide which is a animal constituent of the
65 ele;trolyte in silver-zinc, silver-cadmhun battery cells, and
other silver cells.
It will be understood that it is intended to cover all
changes and modifications of the examples of the inven-
60
tion herein chosen for the purpose of illustration which
electrolytic resistance in 45% KOH of 399 milliohms- 70 do not constitute departures from the spirit and scope
m
-
2
- of the invention.
Thus by this method a film is obtained which has an What is claimed is:
elechnlytic resistance comparable with the product of 1. A battery separator membrane far alkaline electro-
Example 5, but also has the advantage of high flexibility chemical cells cotnrrising a cellulose ether and a con-
which makes it easier to handle during cell assembly. 75 ductance-modifying :.oilt selected from the group con-
3,519,483
sisting of water-soluble meta! salts of aliphatic acids and
their hydroxy derivatives, said salt being present in a-
proportion greater than about 3% by weight of the cellu-
lose ether, but below that proportion at which it will frost
out from (he membrane. '
 g
2. The membrane of claim 1 wherein the cellulose
ether is methyl cellulose and the salt is a potassium salt
of a monobasic aliphatic acid having from 1 to 4 carbon
atoms.
3. The membrane of claim 2 wherein the ether is methyl JQ
cellulose with a degree of substitution in the range of
1.6 to 1.9 and the salt is potassium acetate in a propor-
tion of from about 3% to about 40% by weight of trie
methyl cellulose.
4. The membrane of claim 2 wherein the ether is methyl j$
cellulose with a degree of substitution in the range of
1.6 to 1.9 and the salt is potassium lactate in a propor-
tion of from about 3% to about 40% by weight of the,
methyl cellulose.
5. An alkaline electrochemical cell comprising eke- 20
trodes, a concentrated aqueous alkali fluid, and the battery -
separator of claim 1 interposed between the electrodes.
6. An alkaline silver cell comprising electrodes, a con- .
centrated aqueous alkali fluid, and the battery separator
of claim 1 interposed between the electrodes.
7. An alkaline silver-zinc cell comprising a silver elec-
trode, a zinc electrode, a concentrated aqueous alkali
fluid, and the battery separator of claim 3 inten>osed be-
tween the electrodes.
8. An alkaline silver-zinc cell comprising a siiver elec- 30
trode, a zinc electrode, a concentrated aqueous alkali fluid,
and the battery separator of claim 4 interposed between
the electrodes.
9. A battery separator membrane for alkaline electro-
chemical cells comprising methyl cellulose with a degree 35
of substitution in the range of 1.6 to 1.9 and a con-
ductance-modifier dispersed therein selected from the
group consisting of potassium acetate and potassium lac-
tats, said modifier being present in a proportion of from
about 3% to about 40% by weight of the methyl cellulose, 40
10. An alkaline silver-zinc celt comprising a silver
electrode, a zinc electrode, a concentrated alkali fluid and,
interposed between the electrodes, a separator membrane
comprising methyl cellulose with a degree of substitution'
in the range of 1.6 to 1.9 and a conductance-modifier dis-
persed therein selected from the group consisting of potas-
sium acetate and potassium lactate, said modifier being
present in a'proportion of from about 3% to about 40%
by weight of the methyl cellulose.
11. An alkaline electrochemical cell comprising elec-
trodes, a concentrated aqueous alkali fluid and a battery
separator comprising methyl cellulose with a desree of
substitution in the range of 1.6 to 1.9 and a conductance-
modifier dispersed therein selected from the group con-
sisting of potassium acetate and potassium lactate, said
modifier being present in a proportion of about 3% to
about 40% by weight of the methyl cellulose.
12. A method for increasing the electrolytic conductance
which a methyl cellulose membrane develops after being
swollen in concentrated aqueous alkali;, which method
comprises
(a) making an aqueous solution comprising methyl
cellulose and a salt selected from the group consist-
ing of water-soluble metal salts of aliphatic acids and
their hydroxy derivatives,
(b) casting said solution in sheet form, and
(c) evaporating said sheet to dryness, •
25 said salt being present in a proportion greater than about
3% by weight of the cellulose ether, but below that
proportion at which it will frost out from the membrane.
References Cited
UNITED STATES PATENTS
2,534,336 12/1950 Cahoon.
2,858,353 10/1958 Mendelsohn 136—146
2,900,433 8/1959 Cahoon etal 136—146
3,013,099 12/1961 Mendelsohn 136—146
3,347,689 10/1967 Futami et al 106—178
WINSTON A. DOUGLAS, Primary Examiner
D. L. WALTON, Assistant Examiner
U.S.CI.X.R.
106—178; 136—146; 260—232


Separator for alkaline electric batteries and method of making

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