Note: Descriptions are shown in the official language in which they were submitted.
10586~8 9571-1
Field of the Invention
This invention relates to alkaline silver oxide-
zinc cell~, and specifically to such cells wherein the
positive electrode comprises divalent silver oxide having
electrical contact means, and wherein a discontinuous
oxidizable metal material is interposed between the
positive electrode and the electrical contact means
and/or a discontinuous oxidizable metal material between
the positive electrode and the cell separator or a dis-
continuous oxidizable metal material between the positive
electrode and electrical contact means and a discontinuous
oxidizable metal material between the positive electrode
and the separator.
BackRround of the Invention
Miniature button alkaline silver oxide-zinc
cells have gainet wide acceptance in the battery industry
for many applications because they are characterized as
being high capacity, small volume electric cells. In
other words, the have a high power output and energy
density per unit weight and unit volume of active cathode
material. However, one of the ma~or disadvantages of
silver oxide-zinc cells is that they discharge at two
different potentials. This is due to the fact that the
active cathode materials of such cells are ususlly both
divalent silver oxide (AgO) and monovalent silver oxide
(Ag20). Silver oxide-zinc cells using monovalent silver
oxide as the only active cathode material will have a
2.
~058698 9571-1
theoretical unipotential discharge at about 1.57 volts
but the capacity in milliampere hours per gram of mono-
valent silver oxide is substantially lower than the
capacity of divalent silver oxide. On the other hand,
silver oxide-zinc button cells using divalent silver
oxide as the only active cathode material will discharge
at a first potential at about 1.7 volts acro~s a 300-ohm
resistor for 40 hours, for example, and then drop to
approximately 1.5 volts for an additional period of time
of about 70 hours. Thus monovalent silver oxide cells
have the advantage of discharging at a unipotential
plateau with the disadvantage of having a rather low
capacity while divalent silver oxide cells have the
advantage of having a rather high capacity bue the dis-
advantage of discharging at two distinct voltage plateaus.
Divalent ~ilver oxide has about 1.9 times more capacity
per gram than monovalent silver oxide and about 2 times
more capacity per unit volume than monovalent silver oxide.
Many cell or battery applications, particularly
transistorized devices such as hearing aids, watches
and the like, require a substantial unipotential discharge
source for proper operation and, therefore, cannot use
the dual voltage level discharge which i8 characteristic
of divalent silver oxide cell6.
Consequently, many methods have been proposed
for obtaining a unipotential discharge from a divalent
silver oxide cell. One method disclosed in U. S.
3.
1058698 9571-1
Patents 3,615,858, and 3,655,450, entalls providing a
continuous layer of monovalent silver oxide in physical
and electrical contact with a divalent silver oxide
pellet. During assembly of the cell the cathode pellet
is disposed against the inner surface of a cathode cup
or collector whereupon the layer of monovalent silver
oxide physically isolates the divalent silver oxide from
contact with the cathode cup 80 that the sole electronic
path for discharge of the divalent silver oxide is
through the monovalent silver oxide layer.
In U. S. Patent 3,476,610 a silver oxide battery
is disclosed which employs a positive electrode com-
prised mainly of divalent silver oxide with the addition
of monovalent silver oxide present in part as an electrolyte-
impermeable masking layer. This layer isolates the
divalent silver oxide from contact with the electrolyte
of the battery until discharge begins whereupon the mono-
valent silver oxide becomes electrolyte-permeable. When
this occurs, the electrolyte then begins to contact the
divalent silver oxide. In addition, the monovalent silver
oxide is also present as an interposed layer between
the divalent silver oxide and the inner surface of the
cathode cup or collector 80 as to isolate the divalent
silver oxide from physical contact with said cathode cup
which i8 the positive terminal of the cell.
In U. S. Patent 3,484,295 a silver oxide battery
is disclosed which utilizes a positive silver oxide
10 5 869 8 9571-1
electrode comprising divalent silver oxide and mono-
valent silver oxide. The latter oxide is employed as
an electrolyte-impermeable layer which is interposed
between the divalent silver oxide and the battery CO~t-
ponents containing the electrolyte so as to isolate
the divalent silver oxide from contact with the electro-
lyte until the monovalent silver material is discharged.
If the discharge product of the monovalent silver oxide
material is oxidized by the divalent silver oxide material
in the presence of the battery electrolyte, then it is
possible that the battery will yield a unipotential
discharge.
Although it is theoretically possible to protuce
a unipotential discharge from a divalent silver oxide
cell u6ing the above teachings, it requires a high degree
of quality control to insure that the necessary layer
of monovslent silver oxide is disposed in its proper
location 80 as to prevent any of the divalent silver oxide
from directly contacting the cathode or positive terminal
in one cell arrange~tent and/or the electrolyte of the
cell in another arrang~nent.
U. S. Patent 3,925,102 discloses another approach
to producing divalent ~ilver oxide-zinc cells having a
unipotential tischarge level on low drain conditions.
The cells use a po~itive electrode comprising divalent
silver oxide housed in a positive electrode container
having an upstanding wall and a closed end. Interposed
between the positive electrode and the inner upstanding
1058698 9571-1
wall of the cathode container i8 anoxidizable zinc ring
which functions to isolate a portion of the positive
electrode from the container 80 as to produce a uni-
potential discharge on low drain conditions.
A procesæ is also known in the art whereby
silver oxide-zinc cells having a positive electrode com-
prising divalent silver oxide housed in a cathode con-
tainer are given a predischarge on a high current drain
such that a substantial silver layer is formed at the
interface between the positive electrode and the cathode
container with the concentration of silver decreasing
from a maximum at the interface to a minimum within the
center portion of the poæitive electrode.
Accordingly, it is the primsry ob3ect of this
invention to provide a silver oxide-zinc cell which
employs a positive electrode compri~ing divalent silver
oxide and which has a substantially unipotential dis-
charge plateau over the useful life of the cell.
Another ob~ect of this invention is to provide
a silver oxide-zinc cell which employs a positive elec-
trode comprising divalent ~ilver oxide and which has a
pretictable discharge potential curve.
Another ob3ect of this invention is to provide
a silver oxide-zinc cell which employs a discontinuous
oxidizable metal material disposed between a portion of
the cell's positive electrode and the inner surface of
the cell's cathode container, said positive electrode
~058698 9571-1
comprising divalent silver oxide and said cathode con-
tainer being the positive terminal of the cell.
Another object of this invention is to provide
a ~ilver oxide-zinc cell which employs an oxidizable
metal screen interposed between a portion of the cell's
positive electrode and the inner surface of the cell's
cathode container and between a portion of the cell's
positive electrode and the cell's separator.
Another ob~ect of this invention is to provide
a silver oxide-zinc cell which employs an oxidizable
metal screen between a portion of the cell's positive
electrode and the separator.
SummarY of the Invention
The invention relates to an alkaline silver
oxide cell having a negative electrode, e.g., zinc,
a positive electrode housed in a conductive container
havlng a bottom surface and side wall, a separator dis-
po~ed between said negative electrode and said positive
electrode, said positive electrode comprising divalent
~ilver oxide and wherein a discontinuous oxidizable
metal material is interposed between, and electrically
and physically in contact with, said positive electrode
and the inner surface of the conductive container and/or
a discontinuous oxidizable metal material between said
positive electrode and ~aid ~eparator 80 that the elec-
trochemical reaction of the oxidizable metal ~aterial
with the positive electrode in the presence of the
7.
1058698 9571-1
electrolyte will effectively produce a substantially
unipotential discharge plateau over the useful life of
the cell.
As used herein, a discontinuous metal material
shall mean a metal ficreen, a metal strip, or a plurality
of discrete metal particles which when disposed on a flat
surface in a substantially dispersed arrangement will
define a plurality of openings or spaces between ad~acent
particles.
Whether using the metal screen embodiment, the
metsl strip embodiment, or the discrete metal particle
embodiment, it is preferable that when either is inter-
posed between the positive (cathode) silver oxide electrode
and the inner ~urface of the cathode contsiner (terminal)
or between the positive electrode and the separator, the
metal msterial ant, where applicable, the openings or
spaces disposet or defined within or by the metal material
be substantially uniformly tispersed therebetween. This
feature is desirable to insure a more rapit and uniform
electrolyte cont~ct between the oxidizable metal material
ant the active silver oxite cathote of the cell after
assembly 80 that the reaction between these msterials
will occur substantially uniformly therebetween. This
reaction between the oxidizable metal material and the
active cathote material of divalent silver oxide in
the presence of the electrolyte of the cell is believed
to result in a portion of the divalent silver oxide being
1058698 9571-1
reduced to silver with possibly a minor amount of mono-
valent silver oxide with or without the oxide of the
oxidizable metal.
The amount of oxidizable metal used, as based
on the electrochemical capacity of the total active
cathode material, should be at least about 0.5~/.. The
use of le~s than the lower limit of 0.5% would provide
insufficient oxidizable metal to effectively react with
the cathode to produce the unipotential discharge.
In the preferred embodiment, a metal screen is
employed wherein the opening area in the oxidizable metal
body should be greater than about 20/o of the surface
of the oxidizable metal which contacts the cathode con-
tainer. The 20% openlng requirement of the oxidizable
metal i8 important, since it provides a greater surface area
of the ox~dizable metal which can react with the divalent
silver oxide upon introduction of the cell electrolyte.
Thus, the unipotential discharge level would be reached
more rapitly either on discharge or on storage.
When using a metal strip, the plane area of the
strip should be no greater than the plane area of the
inner bottom of the cathode container, i.e., exclusive
of the site wall(s).
The amount of oxidizable metal to be disposed
between the silver oxide cathode and the cathode container
or the cathode and the separator can vary somewhat depend-
ing on the thickness of the screen, strip, or on the
~058698 9571-1
size of the discrete particles, whichever is used.
However, the oxidizable metal should be disposed between
the positive silver oxide electrode and cathode collector
or between the positive electrode and the separator
80 that between about 10% and about 80h of the normally
common contacting area of these components ha~ the metal
material interposed. Preferably, the oxidizable metal
should be interposed between about 20% and about 60Z
of the common contacting area.
The upper limit on the total amount of oxidizable
metal employed should be less than that which would com-
pletely reduce the divalent silver oxide to the mono-
valent level. For example, in a cell having an all-
divalent silver oxide cathode, if one were to use an
amount equivalent to 50h of the divalent silver oxide
capacity, the capacity output of this cell would be no
greater than if an all-monovalent silver oxide cathode
were used. A practical range of oxitizable metal of
between about 2% and about 10% of the divalent silver
oxide capacity is preferred.
As used herein, the term screen shall mean any
sheet of material having a plurality of openings or
apertures produced, for example, by either perforating a
solid sheet of material; by interlocking wire strands
or link8 in a conventional arrangement to form a mesh
or net construction; or by imparting a plurality of
small slits in an expandable sheet of material 80 that
10.
10586~8 9571-1
upon being expanded in a direction perpendicular to
the slits, diamond-shaped openings will be disposed in
the expanded sheet.
As used herein, a metal strip shall mean any
geometrically shaped essentially solid metsl member
such as a metal disk, rectangular-shaped strip, square,
diamond, circular cross-sectional body, annulus, or
the like.
As is apparent, the use of a metal screen or
metal strip rather than the discrete metal particles
would facilitate the assembling of the cell components
and also provide for a more exact distribution of the
metal and, where applicable, the openings or spaces
within or defined by the metal, between the silver oxide
cathode and the cathote terminsl.
As used in thi~ invention, oxidizable metal
shall mean a metal that will electrochemically react
with divalent silver oxite in the pre~ence of the elec-
trolyte of the cell during storage or during the initial
discharge of the cell to produce metallic silver with
possibly a minor amount of monovalent silver oxide with
or without the oxide of the oxidizable metal which will
effectively isolate a portion of the divalent silver
oxide from the inner surface of the positive electrode
terminal. A suitable metal for use in this invention
can be selected from the group consisting of zinc, copper,
silver, tin, cadmium, and lead. Of the above metals,
1058698 9571-1
zinc is preferable in a zinc cell system because it
introduces no foreign ions into the cell and will easily
form zinc oxide in the presence of an alkaline electrolyte.
Furthermore, since zinc oxide has a low electrical
resistance, it will provide a good electrical path between
the silver oxide and the cathode container. Similarly,
cadmium would be ideally suited for this use when a
cadmium anode is employed.
Although it is theoretically possible that nickel
can be consideret an oxitizable material, it has been
fount that when nickel or nickel alloy cathode containers
are employed to house a positive divalent silver oxide
electrode, the output voltage is characterized as having
a distinct two-step plateau, wherein the higher plateau
is tisplayet for an untesirably long portion of the
tischarge periot.
The active cathote material of this invention
can be 100% tivalent silver oxite or a mixture of divalent
silver oxite ant monovalent silver oxite. When using
mixtures of the silver oxites, preferably at least 50%
by weight of the mixture shoult be divalent silver oxite
because of its high capacity characteristics. The silver
oxite electrote can be formet in a number of ways as,
for example, finely divided divalent silver oxite powder,
mixed with or without monovalent ~ilver oxide, can be
pelletized into a desired size pellet using a conventional
die. Regardless of how the electrode i~ made, it has
12.
~ 0 5~ 69 8
to be sufficiently porous to permit the electrolyte of
the cell to diffuse through the electrode. However,
the pellet also has to be sufficiently dense so that
it can occupy a relatively small space when used in
miniature type cell housings if it is to provide the
required capacity of such cells.
The use of an oxidizable metal material disposed
in electrical and physical contact between the positive
electrote and the inner surface of the positive container
or between the positive electrode and the separator of
a silver oxide-zinc cell according to this invention will
permit a more rapid electrolyte contact and better
electrolyte distribution between the metal material and
the ad~acent silver oxide electrode on addition of the
cell's electrolyte during assembly than would otherwise
be obtainable when using a continuous coating or liner
of the oxidizable metal. In the preferred embodiment,
the metal screen is employed because it is believed that
the porous metal structure will more effectively act as
a wicking means to promote better electrolyte distribution.
With the addition of an oxidizable metal material
to a 100% divalent silver oxide cell of a 220 nominal
milliampere-hour rated capacity according to this invention,
the discharge voltage of the cell on a 16 microampere
drain (96 K-ohm load) will initially be that of the
divalent silver oxide but within a period of one hour or
less, and usually only 15 minutes, the voltage will drop
1058698 9571-1
to that of the monovalent silver oxide level where
it will remain until the cell is fully di~charged.
Contrary to this, an identical cell, but without the
oxidizable metal material, will discharge on a 16
microampere drain at the voltage of the divalent silver
oxide level for over 500 hours before dropping to the
voltage level of the monovalent silver oxide.
After partial discharge of a lOOV/o AgO cell
employing an oxidizable metal material, the open circuit
voltsge will return to the AgO level on shelf storage.
~n subsequent discharge, however, the closed circuit
voltage of the cell will return to the Ag2O level within
15 to 60 minutes.
With the addition of an oxidizsble metal material
to a 50% divalent-50% monovalent silver oxide cell of
a 250 nominal milliampere-hour rated capacity according
to this invention, the discharge voltage of the cell on
a 25 microampere drain (62 R-ohm load) will be that of
the monovalent silver oxide level within five minutes
and remain at this level throughout discharge.
Brief Description of the Drawin~s
Figure 1 is a cross-sectional view of a silver
oxide-zinc cell having an oxidizable metal screen disposed
between the positive silver oxide electrode and the
inner wall of the cathode container in accordance with
the pre~ent invention.
Figure 2 is a top view of an oxidizable metal
14.
1058698 9571-1
sheet having apertures which is suitable for use in a
silver oxide-zinc cell as generally shown in Figure 1.
Figure 3 is a top view of an oxidizable metal
mesh suitable for use in a silver oxide-zinc cell as
generally shown in Figure lo
Figure 4 is a top view of an oxidizable expanded
metal suitable for use in a silver oxide-zinc cell as
generally shown in Figure 1.
Figure 5 i8 a top view of a cut-away cathode
container showing discrete oxidizable metal particles
uniformly dispersed on the inner bottom surface of the
cathode container.
Figure 6 is a top view of an oxidizable rectangular
: metal strip suitable for use in a silver oxide-zinc cell
as generally shown in Figure 1.
Figure 7 is a top view of an oxidizable metal
di8k 8uitable for use in a silver oxide-zinc cell as
generally shown in Figure 1.
Descr$Pt$on of the Preferred Embotiment
The preferred embodiment of this invention can
be described in con~unction with Figures 1 to 4. Referring
to Figure 1, there is shown a sectional elevation of a
silver oxide-zinc cell having a negative electrode 2,
separator 3, and positive electrode 4 housed within a
two-part container comprising a cathode container 5
and anode cup 6. As shown, cathode container 5 has a
flange 7 which is crimped inwardly against a U-shaped
105~3698 9571-1
flange 11 on anode cup 6 via grommet 8 during assembly
to seal the cell as disclosed, for example, in U. S.
Patent 3,069,489. The cathode container may be of
nickel-plated steel, nickel, stainless steel, or the
like, while the anode cup 6 may be made of tin-plated
steel, copper-clad stainle~s steel, gold-plated copper-
clad stainless steel, or the likeO The grommet 8 may
be made of a suitable resilient electrolyte-resistant
material such as neoprene, nylon, or the like.
The separator 3 may be a three-layer laminate
consisting of two outer layers of radiation-grafted
polyethylene and an inner cellophane* layer or the like.
Disposed between anode 2 ant separator 3 is a layer of
electrolyte-absorbent material 12 which may consist of
various cellulosic fibers.
The anode (negative) electrode can comprise a
lightly compressed pellet 2 of finely divided amalgamated
zinc containing, if desired, a gelling agent. Cadmium
may also be used as the anode material. The cathode
(positive) electrode can comprise a rather densely com-
pressed pellet 4 of divalent silver oxide powder or a
mixture of divalent silver oxide powter and monovalent
silver oxide powder.
The cell electrolyte may be an aqueous solution
of potas~ium hydroxide, sodium hydroxide, or mixtures
~hereof.
*Trademark of British Cellophane Ltd. for sheets of
transparent cellulose and cellulose wrappings.
16.
1058698
9571-1 -
As shown in Figure 1, an oxidizable metal
screen 9 i8 interposed between positive electrode 4 and
the inner bottom surface 10 of cathode container 5
and an oxidizable metal screen 9 is interposed between
the positive electrode 4 and the separator 3. Preferably
a metal screen 9 should be placed in only one of the
above two positions, although separate ~creens could
be placed in both positions as shown in Figure 1. How-
ever, when using two separate oxidizable metal members,
the overall amount of said metal and the total plane area
of said metal should be within the limits specified
above. Metal screens 9 are of the type shown in Figure 2
which comprises a metal sheet 20 having apertures 21.
Figure 3 shows another preferred embodiment of
a discontinuous metal member for use in this invention
which comprises a metal mesh 30 having apertures 31.
Figure 4 shows a further preferred embodiment
of a discontinuous metal member for use in this invention
which comprises expanded metal 40 having apertures 41.
An alternate embodiment of a discontinuous metal
arrangement for use in this invention is shown in
Figure 5 wherein metal particles 50, such as powder, are
~hown uniformly dispersed about the inner bottom surface 51
of a cathode container 52. The metal particles are
arranged in such a manner that ad~acent particles define
openings or spaces 53. These spaces 53 perform the same
function as the apertures shown in Figures 2 to 4, that
105~3698 9571-1
being to permit more rapid electrolyte contact between
the oxidizable metal and the positive electrode on
addition of the electrolyte to the cell during assembly.
Figure 6 shows a discontinuous rectangular metal
strip for use in this invention which strip i8 identified
as 60. Metal strip 60 would be disposed within the
silver oxide-zinc cell in place of the metal screen 9
discussed above and shown in Figure 1.
Figure 7 shows a discontinuous metal disk for
use in this invention which tisk is identified as 70.
Metal disk 70 would be disposed within the silver oxide-
zinc cell in place of the metal screen 9 discussed above
and shown in Figure 1.
It is also within the scope of this invention
to connect two or more cells in series or parallel by
conventional mean~ and then place them in a housing to
produce a battery which can be used in variou~ battery-
operated devices.
EXAMPLE 1
Three miniature button cells of the type shown
in Figure 1, diameter 0.450 inch and overall height
approximate~y 0.210 inch, were produced using a gelled
zinc powder anode, a pellet of active cathode material
of 50/50 by weight AgO/Ag20 molded at about a 2-ton
pressure and a 3-layer separator consisting of two outer
layers of radiation-grafted polyethylene and an inner
18O
~058698 9571-1
cellophane* layer. An additional electrolyte-ab~orbent
separator layer was employed at~acent to the anode.
These components along with a 22% NaOH electrolyte
(5.5 M NaOH) were assembled in a nickel-plated cathode
container and a gold-plated copper-clad stainless steel
anode cup and then the cell was sealed by crimping the
top annular section of the cathode container inwardly
against the anode cup as described in U. S. Patent
3,069,489 via a grommet of nylonO
Each cell, when discharged across a 62 K-ohm
resistor at room temperature on a drain of 25 microamperes,
exhibited the higher AgO voltage level. On a continuous
discharge, it took the two cells an average of 8 hours
before the higher divalent oxide voltage dropped to the
monovalent voltage level.
EXAMPLE 2
Three cells, identical to the miniature button
cells of Example 1, were produced, except that a 0.003
inch thick expanded zinc mesh was interposed between the
50/50 by weight AgO/Ag20 cathode and the cathode container,
as generally shown in Figures 1 and 4 except that Figure 1
also shows a ~creen interposed between the cathode and
the separator. The expanded zinc mesh had a ~trand width
of 0.03 inch and mesh dimensions of 0.31 inch by 0.22 inch.
After storage at room temperature for 456 hours,
each cell was discharged across a 62 K-ohm resistor at
*Trademark of British Cellophane Ltd. for sheets of
transparent cellulose and cellulose wrappings.
19.
9571-
~058698
95F., on a drain of 25 microamperes. ~oth the open
circuit voltage and the discharge voltage of each cell
over a continuous period of 24 hours were at the lower
Ag20 voltage plateau of 1.60 and 1.59, respectively.
This demonstrates the effectiveness of having a discon-
tinuous oxidizable metal material interposed between
the silver oxide cathode and the cathode container to
produce a unipotential discharge even though the cathode
was 50% by weight divalent silver oxide.
EXAMPLE 3
Two cells, identical to the miniature button
cells of Example 2, were produced, except that the cathode
consisted of 100% AgO.
After storage at room temperature for four
months, each cell was then discharged across a 96 K-ohm
resistor and after a period ranging from 15 to 60 minutes
on discharge, the voltage of each cell dropped from the
divalent plateau of 1.78 to the monovalent plateau of
1.57. This demonstrates that the monovalent voltage level
can be obtained from a silver oxide cell using a 100%
divalent silver oxide cathode by employing the teachings
of this invention. The discharge of this type cell on a
continuous load of 96 K-ohms i8 felt to represent a severe
test condition for obtaining the lower voltage level since,
under this condition, the monovalent silver oxide voltage
level i8 least likely to be obtained or displayed.
20.
1058698 9571-1
EXAMPLE 4
Three cells, identical to the miniature button
cells of Example 1, were produced, except that a zinc
strip measuring 0.070 inch wide, 0.442 inch long, and
0.007 inch thic~ was dispo~ed between the cathode and
the inner bottom of the cathode container.
After storage at room temperature for 30 days~
each cell was discharged acro~s a 6~ K-ohm resistor at
95F., on a drain of 25 microamperes. Both the open
circuit voltage and the discharge voltage of each cell
over a continuous period of 24 hours were at the lower
Ag20 voltage plateau of l.S9 and 1.57, respectively.
This demonstrates the effectiveness of having a discon-
tinuous oxidizable metal strip interposed between the
silver oxide cathode and the cathode container to produce
a unipotential discharge even though the cathode contained
a 50/50 mixture of divalent silver oxide and monovalent
silver oxide.
From the above examples, we see that using the
teaching of this invention a silver oxide cell using a
50/50AgO/Ag20 cathode can exhibit a monovalent voltage
level (open circuit) of about 1.57 volts being stored
at room temperature for a period of 456 hours. It also
demonstrates that a silver oxide cell using a 1007. AgO
cathode can exhibit a monovalent voltage level of about
1.57 volts after being discharged on a low drain
(96 K-ohm loat) for a periot of only 15 to 60 minutes.
21.
105~3698 9571-1
It has also benn observed that, when using a zinc
screen, the separator showed less degradation due to
oxidation than the separators in the cells that did
not contain the zinc screen.
It is to be understood that other modifications
and changes in the preferred embodiments of the invention
herein shown and described can also be made without
departing from the spirit and scope of the invention.
