Current collectors for alkaline cells

An electrochemical cell having an alkaline electrolyte, a cathode comprised of a metal oxide and a zinc anode and wherein the anode current collector of the cell is made from the same type zinc material forming the anode.

FIELD OF THE INVENTION 
The invention relates to electrochemical cells having an alkaline 
electrolyte, a cathode comprised of a metal oxide, such as manganese 
dioxide, and a zinc anode and wherein said cells employ anode current 
collectors made from the same type of zinc material as the zinc anode. 
BACKGROUND OF THE INVENTION 
Alkaline cells are well known in the art and generally employ a zinc anode, 
a manganese dioxide cathode and an aqueous solution of potassium hydroxide 
for the electrolyte. These cells are readily available commercially for 
industrial and home applications. A detrimental characteristic of these 
cells is the formation of hydrogen gas. The gas can be formed by 
undesirable electrochemical reactions which occur at the surface of the 
anode current collector. As the quantity of gas increases, the internal 
pressure in the cell also increases and if not relieved, the cells could 
eventually leak. In order to reduce this gas buildup, mercury may be added 
to the anode. Unfortunately, as is well known, mercury may add to 
environmental pollution when cells are eventually discarded. To replace 
mercury, zinc corrosion inhibitors can be added to the zinc. Examples of 
these inhibitors include: lead, indium, cadmium, thallium, gold, silver, 
tin, gallium and compounds of these elements. 
A deficiency found in some of the mercury-free alkaline cells is that 
unexpected depression occurs in the cells' voltage during discharge. These 
deviations are generally temporary and the voltage generally recovers to 
provide the normal electrical power to the customer. This type of 
temporary depression and then recovery in the voltage is referred to as a 
"dip". The term "dud" describes a cell whose closed circuit voltage drops 
below an established voltage cutoff and is considered a failure. Cells 
plagued by temporary or permanent premature drops in the closed circuit 
voltage, described above as dips and duds, generally include a brass 
collector that was not been properly plated with zinc during the cell 
assembly process and/or cells in which the zinc's particle-to-particle 
contact is inadequate. 
Another problem with some mercury-free cells is that excessive gassing can 
develope in cells that have been partially discharged and then allowed to 
rest. This situation is referred to as "post partial discharge gassing" 
and generally occurs when the cell utilizes a brass current collector. 
Japanese Patent Application No. 61-58163 relates to an alkaline zinc 
battery which is an alkaline battery in which zinc is used as the anode 
active substance characterized in that the anode is comprised of a low 
amalgam of a low amalgamation ratio of less that 2% by weight or a 
unamalgamated zinc alloy powder and in that at least the outermost layer 
of the face of the anode current collector facing the anode is formed in 
advance by a zinc alloy of a composition that is the same as or similar to 
the zinc alloy powder that forms the anode. 
Japanese Patent Application No. 89-307160 relates to an alkaline cell that 
employs a negative electrode collector body, such as brass, that is coated 
with a zinc alloy. 
Japanese Patent Application No. 89-307, 161 relates to an alkaline cell 
which employs a negative electrode collector coated with indium and/or 
lead, and indium and/or lead substituted and deposited from aqueous 
solution on the surface of zinc powder as the negative electrode active 
material. 
U.S. Pat. No. 4,939,048 relates to an electrochemical battery having an 
alkaline electrolyte and a gelled negative electrode containing zinc 
powder free from mercury, cadmium, and lead, and 1 ppm to 1000 ppm of at 
least one organic stabilizer compound selected from polyfluorine compounds 
of the ethoxyl fluoroalcohol type and compounds of the alcoyl sulfide and 
polyethoxyl alcohol type, the electrode being provided with a negative 
current collector immersed in the gell and connected to the negative 
terminal, wherein the current collector comprises a bundle of conducting 
fibers which are fixed together at their ends closest to the negative 
terminal, the fibers having a diameter lying in the range 0.05 mm to 1 mm, 
and being constituted, at least superficially, by a metal selected from: 
indium; gallium; cadmium; and pure zinc. 
U.S. Pat. No. 4,942,101 relates to an electrochemical cell having an 
alkaline electrolyte and a gelled negative electrode containing zinc 
powder free from mercury, from cadmium, and from lead, and having 1 ppm to 
1000 ppm of an organic stabilization compound selected from: polyfluoride 
compounds of the ethoxyl fluoroalcohol type; and compounds of the 
polyethoxyl alcohol and alcoyl sulfide type. The electrode is provided 
with a nail-shaped cylindrical negative current collector immersed in the 
gell and passing through a sealing plug and connected to the negative 
terminal. The current collector is constituted, at least superficially, by 
a substance selected from: pure zinc, pure cadmium, indium, and gallium; 
and the nail includes means assembled thereon without melting metal and 
serving to increase its developed surface area. 
It is an object of the present invention to provide mercury-free alkaline 
cells that are effectively free from premature voltage depressions during 
discharge of the cells. 
It is another object of the present invention to employ an alkaline cell 
with a current collector made of the same type of zinc material as the 
zinc material used as the anode so that unwanted voltage depressions can 
be effectively eliminated. 
It is an object of the present invention to provide an alkaline cell with a 
zinc alloy anode and an anode current collector made entirely 
homogeneously of the same type of zinc alloy as the anode so that the 
amount of zinc alloy in the cell will be greater than a similar cell using 
a brass or other type of current collector, thus providing a greater 
amount of power output or capacity for the cell of the invention. 
It is an object of the present invention to provide an alkaline cell with a 
current collector that will not form a gassing couple with the zinc alloy 
of the anode. 
The above and further objects will become apparent upon consideration of 
the following description and drawings thereof. 
SUMMARY OF THE INVENTION 
The invention relates to an electrochemical cell having an alkaline 
electrolyte, an anode comprising a zinc component, and a cathode contained 
within a housing, having a first terminal of one polarity and a second 
terminal of opposite polarity and wherein a conductive current collector 
is extended into the housing to make electrical contact with the anode and 
provide electrical contact with one of the terminals of the housing, said 
conductive current collector made entirely and solely of a zinc material, 
preferably a zinc alloy. Preferably, the anode is comprised of a zinc 
alloy and the conductor current collector is made of the same type of zinc 
alloy as is the anode. 
As used herein, the term, "made entirely and solely of a zinc material", is 
used to mean that at least the portion of the current collector inserted 
into the anode is entirely made of the zinc material. 
Preferably, the zinc material should be a zinc alloy containing lead in an 
amount from about 0.04 wt. % to 0.1 wt. % and more preferably in an amount 
from about 0.05 wt. % to 0.08 wt. %. The weight of the zinc material 
forming the conductor current collector could comprise 5% to 50% of the 
weight of the zinc material in the anode, and preferably be from 15% to 
20% of the weight of the zinc material in the anode. The volume percent 
zinc of the current collector should be greater than the volume percent 
percent zinc of the anode gel. 
The teaching of the invention can be used to eliminate dips and duds in 
alkaline cells and reduce the quantity of gas generated in both 
undischarged cells and partially discharged cells. In accordance with the 
present invention, the anode current collector should be made entirely 
from the same material as the cell's electrochemically active material, 
such as zinc. Thus the current collector that protrudes into the anode is 
made from the same type zinc or zinc alloy that is used to make the 
anode's powder zinc which serves as one of the cell's electrochemically 
active materials. By manufacturing the powdered zinc and the current 
collector from the same zinc material, there is no electrochemical couple 
established in undischarged or partially discharged cells which could then 
lead to gassing within the cell. Another advantage of manufacturing the 
current collector with the same type of zinc material as the anode is that 
the current collector is able to serve as the cell's anode until adequate 
electrical contact is established between the current collector and the 
zinc particles and/or among the zinc particles. By maintaining adequate 
electrical contact from the zinc particles to the cell's anode current 
collector, dips and duds can be effectively eliminated in zero mercury 
type alkaline cells. The use of the same zinc material for both the 
current collector and the anode, will assure no additional chemical 
elements beyond that specified for the zinc material will be used. In 
addition, by using the same zinc material for both the anode and the 
current collector, it will permit a greater amount of zinc material in a 
fixed volume cell than a similar cell using a current collector made of a 
material different than the zinc material. The cell with the greater 
amount of zinc material could provide more power capacity since the 
current collector would be homogeneously made of the zinc material in the 
anode. 
Examples of electrochemical cells suited for use with this invention are 
zinc/manganese dioxide, zinc/silver oxide, zinc/nickel oxide and zinc/air 
type cells. Preferably, cells of this invention are zero-added mercury 
alkaline cells comprising an alkaline electrolyte, a metal oxide such as 
manganese dioxide and a zinc anode. By "zero-added" is meant that no 
mercury is added to any of the cell components. Typically, a residual 
amount of mercury can be present in many natural products. Thus, for the 
preferred zero-added mercury cells of this invention the only amount of 
mercury present in the cell is the residual amount that is naturally 
present in the cell components. Thus, when the cells are disposed of, no 
additional amount of mercury is added to the environment that is greater 
than the amount that was present before the cells were assembled. For 
example, commercially available "pure zinc" cans contain about 20 parts of 
mercury per billion parts by weight of zinc, and often contain much less 
than 20 parts per billion. For the zero-added mercury cells of this 
invention, the amount of mercury present will preferably be less than the 
level that can be detected analytically. Typically, the amount is less 
than 10 parts per million by total weight of the cell preferably less than 
5 parts per million, more preferably less than 2 parts per million, and 
most preferably less than 1 part per million. 
As stated above, the current collector of this invention is entirely and 
homogeneously made from the same type of zinc material as is used in the 
anode. The current collector is generally an elongated member such as a 
blade, a flat stick or other shape member. The current collector could 
have a cross-sectional area shape selected from the group consisting of a 
square shape, rectangular shape, polygonal shape, annular shape and a star 
shape with three or more radial fins extending from the center. The 
current collector would be projected in the anode to provide good surface 
contact there between. Preferably, the current collector is solid while 
the anode is a gel that is plastic and displaceable so that the current 
collector can be easily inserted into the anode gel. This will enable the 
current collector to be easily projected into the anode. In another 
embodiment, the current collector is in the form of a tube with spaced 
apart annular segments, or any other arrangement, that provides a cavity 
which can be used for accommodating any gas formed by the anode and then 
directing the gas to a void space generally provided in the cell. 
The electrochemical cells of the invention can comprise an alkaline 
electrolyte, a cathode and an anode arranged in a sealed container in a 
manner effective to provide electrochemical energy, i.e. when the cell is 
placed in a circuit, electrochemical energy is provided to the circuit. 
The cells have terminals of opposite polarity. One terminal is in contact 
with the cathode and the other is in contact with the anode. The cell is 
sealed in a manner effective to contain the cell components in the 
container under conditions of transport and use. The cell construction can 
include a cupped metallic can, suitably constructed of steel or other 
metal and can be nickel plated in whole or in part. A tubular cathode 
containing the active cathode material, conductor, and in some cases, a 
binder, can be lined on the inner surface of the can, and a separator 
suitably made of a non-woven cellulosic or polymer fiber or microporous 
plastic, or cellophane film can be lined on the inner surface of the 
tubular cathode. In this construction, the can is in contact with the 
cathode and thus is the cathodic terminal. 
An anode made of a mixture of active anode material, electrolyte, 
optionally an electrolyte swellable binder such as a polyacrylic acid can 
be enclosed with the separator. An anode current collector member is 
inserted into the anode. The cell is closed with a cover and sealed. The 
cover is in contact with the anode current collector member and is the 
anodic terminal of the cell. Any conventional seal can be employed. It is 
desired that the cell construction not have a venting means that would 
vent due to the normal pressure generated in the cell during a normal 
discharge. 
The electrolyte used in this invention is an aqueous alkaline solution, 
such as potassium hydroxide or sodium hydroxide. The concentration of the 
solution can be any concentration that provides for ionic conductivity. 
Typically, in the assembled cell the concentration preferably ranges from 
about 30% to about 42%. 
The cathode used in this invention is comprised of a metal oxide as the 
active cathode component. Examples include manganese dioxide and silver 
oxide. The preferred cathode comprises manganese dioxide, and more 
preferred is electrolytic manganese dioxide (EMD). (EMD, a widely 
available commercial product, is prepared by plating manganese dioxide 
from a manganese sulphate plating solution onto an electrode. The 
deposited manganese dioxide is subsequently removed from the electrode and 
recovered. 
In addition to the metal oxide, the cathode further comprises a compound 
that is electrically conductive. This compound is called a conductor and 
of the many types of known conductors, synthetic or natural graphite, are 
preferably used in the cells of this invention synthetic and natural 
graphite are readily commercially available. One source is Lonza Ltd., a 
Swiss company. The cathode can further comprise a binder. Examples of 
suitable binders include polytetrafluoroethylene and polyethylene. 
The cathodes comprise a major amount of the metal oxide, a conductive mount 
of the graphite and often an effective amount of binder. Typically, the 
metal oxide will comprise between about 80 to 85 weight percent of the 
total cathode weight. When a binder is employed, the binder will comprise 
less than about 5.0% by weight. The remainder of the cathode will be 
comprised of graphite and electrolyte solution. The amount of the 
electrolyte solution is sufficient to wet the dry components, and to 
provide a mixture that can be molded. The cathodes are prepared by mixing 
the components together and dispensing the mix into the container. The mix 
is then molded or compressed against the inside of the container, or 
premolded as rings and the rings pressed into the container. 
Zinc is used as the active anode material in the cells of the invention. 
Preferably, the zinc is low gassing zinc, and is in powder form. The 
powdered zinc is combined with a binder, optional components, and an 
amount of the electrolyte solution to form a gel. The anode gel generally 
expands when it is discharged. The zinc material is the same type of zinc 
material that is used to make the anode current collector. A suitable zinc 
alloy would contain zinc and a minimum amount of lead. 
The cells of this invention preferably employ additives that inhibit the 
corrosion of zinc. One beneficial component that can be added to the cell 
to inhibit the corrosion of zinc is an ethylene oxide polymer and 
derivatives thereof. Some examples of materials that can be added to the 
anode would include lead, indium, cadmium, bismuth, thallium, tin, 
aluminum, and compounds thereof. An indium-containing compound can be 
added to the anode mix as a zinc corrosion inhibitor. Suitable compounds 
include indium hydroxide, indium oxide, indium metal and the like.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, the alkaline cell 2 is assembled in a conventional 
conductive steel container 4 which also forms an external terminal for the 
cell. The cathode 6 for cell 2 is a porous mixture of graphite, manganese 
dioxide, electrolyte and sometimes a binder. 
After the cathode 6 is formed in the container 4 a separator 8 is added to 
physically isolate the anode material 10 from the cathode 6 and the 
container 4 while still permitting ion transport between the electrodes. 
The separator 8 could be made of two strips of separator material arranged 
perpendicular to one another and inserted into the cathode's tubular shape 
6 forming a separator basket with a central opening. The anode mix 10 is 
then added to the separator lined cavity of the cell. An open area 12 is 
left in the cell to provide room for any expansion of the anode mix 10. 
To complete assembly of the cell an external bottom cover 30 is placed into 
the steel container 4 and is also insulated from contact with the 
container 4 by the peripheral wall 32 of seal member 34. The bottom cover 
30 makes electrical contacts with current collector 22, enabling the 
bottom cover 30 to become an external terminal for cell 2. The edge of the 
steel container 4 is rolled to hold the upturned portion 36 of the bottom 
cover 30 locked in position in the bottom of the cell 2. Top cover 40 can 
be fastened to the container by welds 42 after the cathode is reed into 
place. 
The anode current collector 22 shown in FIG. 1 is a rectangular blade or 
slat that is made entirely of the same type zinc material that is used to 
make the anode. FIG. 2 shows another embodiment of an anode current 
collector 50 formed in a shape of a cylinder. FIG. 3 shows an anode 
current collector 52 formed of two spaced apart annular segments defining 
a cavity 53. FIG. 4 shows an anode current collector 54 formed as a star 
cross-sectional shape member with three fins equally spaced and projected 
from the longitudinal axis of the current collector 54. FIG. 5 shows an 
anode current collector 56 formed as a star cross-sectional shape member 
with four fins equally spaced and projected from the longitudinal axis of 
the current collector 54. 
Though the invention has been described with respect to preferred 
embodiments thereof, many variations and modifications will become 
apparent to those skilled in the art. It is therefore the intention that 
the appended claims be interpreted as broadly as possible in view of the 
prior art including all such variation and modifications.