Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This iDvention relates to electrochemical cells having compacted
cathodes and more particularly to alkaline electrolyte cells having extruded
cathodes.
Cathodes for cells su~h as alkaline electrolyte cells have been
constructed by various compaction procedures whereby active materials such
as mercuric oxide (BgO), silver oxide (Ag20) and manganese dioxide (MnO2)
are pelletized or extruded into desired configurations. Cathode pellets
are generally constructed under relatively high pressures (10,000 p5i or more)
to form independent structures. In the extrusion procedure a cathode
active mater$al together with conductive additives such as graphite or
carbon and the cell electrolyte are admixed and plaeed in a cell container.
The mixture i5 then ex~ruded against the cell container walls by a punch at
pressures of about 1200 psi. Sinee the extruded cathode is not structurally
independent lower compactlng pressures are adequate.
In bo~h the pelletizing and extrusion procedures the cathode active
n~tterials are admixed with graphite or carbon for both conductivity and for
lubrication during ~he compacting. However, despite the presence of the
graphite or carbon as a lubricant in relatively large amoun~s of generally
about 6% or more of the cathode weight several problems have been encountered
in the co~pacting procedures particularly with "wet" extrusions (compaction
of a cathode mixture which includes fluid electrolyte). Additionally,
cells m~de in accordance with such procedures have suffered from reduced
dPlivered capaci~y because of such problems. The extruded cathodes tend to
have large residual bottom thicknesses with such material being generally
u~available for di~charge therefore resulting in reduced cell capacity.
FurthPrmore, desplte the presenc~ of the lubricaeing ~raphite or carbon,
the cathodes do not have the entirely de6ired uniformity of density an~ as
- a result cell efficiency is detrimentally affected.
It i~ an obje~t of the present invention to-provide a means wh~reby
, . . .
the dischar~e ch~racter!stic6 of cell6 having eompactecl cathodes nre enhanced.
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It is a further object of the present lnvention to provide such means
whereby the procedure of cathDde compaction for said cells is also ~acilitated.
These and other objects, features and ad~antages of the present
invention will become more evldent from the following discussion.
Generally, the present invention comprises an electrochemical cell
having a compacted cathode wherein, prior to the compaction of the cathode
such as by extrusion, a small amount of a finely powdered polyolefin such
as polypropylene or polyethylene ~s added to the cathode ma~erials. It has
been discovered that ~mall amounts of the powdered polyolefin, on the order
of less than about 3% by weight of the cathode, unexpectedly m~rkedly
improve the structure of the compacted cathode and the capacity o~ the cell
containin~ such cathode. Additionally, particularly in the extrusion
procedure, substantially reduced extrusion pressure (15% or more reduction)
can be employed to Eorm the requlsite extruded cathode e.g. an extrusion
pressure of about lOQ0 psi or less is sufficlent. Concomitantly, wear of
the extrusion punch is also minimized.
The polyolefin powder additives, ~hich are useful in the practice of
the present invention have particle sizes in the fini6hed cell ranging from
about 30 microns to about 300 microns. Larger particle 6ized may, however,
be initially added to the cathode mix prior to compaction provided that
precompaction procedures (such as mixing with an intensifier bar) com-
minutes the particles to ~he requisite dimenslons prior to the actual
compaction (e.g. extrusion). The amount of the polyolefin powder added to
the cathode should be minimal slnce it is non conductive and, unlike the
conductive graphite or carbon, itB presence in larger amounts would impair
conductlvity and capacity of the cathode. The amount of polyolefin powder
added to the cathode materials prior to compaction i6 between about 0.1'~ to
a maximum of about 3% by weight It iB preferred ~hat the amount of poly~-
~lefin powder be present in an amount ranging from about n.2~ to ~bout 1%.
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The polyolefin additives of the pre6ent invention are chemically
stable and may be utilized with generally any powdered and compacted cathode
material such as the aforementioned HgO, Ag20 and MnO2 active cathode
~aterials. Because of the ~tability of the polyolefin additives they may
be utiliæed ln various cell environments such as alkaline cells having zlnc
anodes and sodium or potassium hydroxide electrolyte solutions or non-
aqueous electrolyte cells such as those having li~hium or other alkali or
alkaline earth metal anodes.
In order to more fully illustrate the efficacy of the present invention
the following examples are pre6ented. I~ is understood, however, that such
examples are ~or illustrative purposes only and the invention is not
limi~ed to specifics contained therein. ~nless otherwise indicated all
parts are parts by weight.
Example 1 (PRIOR ART)
A cathode mix comprised of 76% MnO2, 13.5% graphite (partlcle size of
less than 10 microns), and 10.5% 7.2N KOH electrolyte solution was prepared
and a 7.8 gram portiDn thereof was placed in a cell container (0.55" (1.4
cm) ID x 1.96" 5.0 cm) HT) and ex~ruded therein by a punch at 1200 psi
against the cell container wall to form a cathode with an ID of O.366"
(0.93 cm). The cell was completed with the insertlon of a closed tubular
separator into said cathode, the further insertion of about 3 grams of a
gelled amalgamated zinc anode (93% Zn, 7% Hg) into the separ~tor and the
addi~ion of about 2cc of 40% KOH electrolyte. After sealing of the cell ~t
was discharged at a continuous discharge rate of 4 ohms with the results
given in Table I.
Example 2
A cell was made in accordance with the procedure and with the materials
in Example 1 b~t with 75.5% MnO2 and 0.5% powdered polypropylene (about 150
~30 micron particle size). The cell was discharged as above with the results
given in Table I.
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TABLE I
Hour6 Hours
VoltageExample 1 Cell Example 2 Cell % Improvement
1.10 l.58 1.77 12
1.00 2.82 2.9B 5.7
0.90 3.45 3.73 8.1
0.80 3.~9 4.38 1~.6
0.75 4.10 4.~0 12.2
Example 3 ~PRIOR ART)
A cell ~as made as in Example 1 but from a cathode ~ix 74.5% MnO2,
14.5 % graphite (about 35 microns particle size) and 10.5% 7.2N KOH and
discharged at a continuous rate of 4 ohms. The results are given in Table
II.
Example 4
A cell was made as in Example 3 but with 74.5% MnO2 and 0.5~ powdered
polypropylene ~about 150 micron particle size in the cathode mix). The cell
was discharged as above and the results are given in Table II.
TABLE II
Hours Hours
VoltageExample 3 Cell Example 4 Cell ~ Improvement
1.10 1.52 2.n5 34.9
1.00 2.6D 3.26 25.4
0.90 3.42 4.05 18.4
0.80 3.92 4.63 18.1
0.75 3.~2 4.78 21.9
: It is readily apparent from the above examples that despite the reduction
of active cathode materials in the cathodes, the cells of Example.s 2 and 4
(havin~ the additive of the present invention) are markedly superior at all
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cutoff voltages. It is further evident from Examples 3 and 4 that despite
changes in amount and type of ~raphite and the reductlon cf active cathode
material which causes reduction in capacity of ~he cell in Example 3, the
~capacity of the cell in Example 4 actually exhibits improved capacity.
It is understood that the above examples are for illustrative purposes
only and that further changes in cell and cathode compositions and construc-
tions may be made ~ithout departing from the scope of the present invention
as defined in the following clai~s.
