An alkaline cell is disclosed having a cathode comprising manganese dioxide wherein the cathode is a semisolid during discharge of the cell. The cell has an anode comprising zinc and an electrolyte comprising potassium hydroxide. The semisolid cathode comprising manganese dioxide may be in the form of a putty or paste. The semisolid cathode reduces cathode polarization effects and results in increased manganese dioxide utilization (Amp-hr/g), particularly at high current drain, between about 0.5 and 2 Amp. The porosity of the cathode is between about 45% and 70%, and the volume ratio of electrolyte solution in the cathode to the solids in the cathode is at a value between about 0.7 and 2.3.
There is a growing need to make primary alkaline cells better suitable for high power application. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices require high current drain rates of between about 0.5 and 2 Amp, typically between about 0.5 and 1.5 Amp. Correspondingly, they require operation at power demands between about 0.5 and 2 Watt.
Conventional alkaline cells have solid cathodes comprising particulate manganese dioxide. Electrolytic MnO.sub.2 (EMD) is generally preferred because of its high density and since it is conveniently obtained at high purity by electrolytic methods. Other particulate MnO.sub.2, for example, MnO.sub.2 obtained by chemical methods referred to in the art as chemical MnO.sub.2 (CMD) may also be used. Such chemical MnO.sub.2 (CMD) is generally less dense than electrolytic MnO.sub.2 (EMD). In conventional alkaline cell cathodes the manganese dioxide composition is high--between about 70 and 87 percent by weight. Conventional alkaline cell cathode may typically be composed of electrolytic manganese dioxide (80-87 wt %), graphite (7-10 wt %), and a 7-11 Normal "aqueous KOH solution" (5-7 wt %). Such mixtures form a moist solid mix which is fully compacted into the cell casing using plungers or other such compacting devices forming a compacted solid cathode mass. The cathode material may be preformed into the shape of pellets or rings which are inserted into the cell in stacked arrangement, for example, as shown in U.S. Pat. No. 5,283,139, and then recompacted. The resulting compacted cathode in either case is a hard, solid material.
U.S. Pat. No. 5,501,924 discloses such conventional solid cathodes comprising MnO.sub.2 for alkaline cell. For example, this reference discloses a cathode for a D size alkaline cell wherein the cathode is preferably composed of 71.7 to 81.7 weight percent MnO.sub.2, about 8.5 weight percent graphite, and about 7.9 weight percent alkaline solution, such as 45% KOH solution, and about 0.4 weight percent deionized water, and about 1.5 weight percent binder solution containing tetrafluoroethylene in water. The cathode also contains about 0.1 to 10 weight percent SnO.sub.2 additive such that the combined weight percent of MnO.sub.2 and SnO.sub.2 is a constant of preferably about 81.8. (col. 2, lines 46 to 58). The reference discloses that such cathode material is placed into a steel can and molded into shape. The reference also discloses a cathode for AA size alkaline cell wherein the cathode is composed of 74.0 to 84.0 weight percent MnO.sub.2, about 7.3 weight percent graphite, and about 7.2 weight percent alkaline solution, such as 45% KOH solution, and about 1.5 weight percent deionized water and about 0.1 to 10 weight percent SnO.sub.2 additive such that the combined weight percent of the MnO.sub.2 and the SnO.sub.2 is preferably about 84.0. (col. 2, line 63 to col. 3, line 6) A person skilled in the art would recognize that the recited cathode material for either the D or AA cell would be a solid loosely packed mix of material even before it is placed in the cell because of the high solids content and low liquid content. Once molded in the cell this mass becomes even further compacted to form a compacted solid cathode mass because of the high level MnO2 and solids content in conjunction with the relatively low liquid content. Such mass has the property that it readily becomes compacted into a rigid solid structure upon application of compressive forces.
U.S. Pat. No. 5,489,493 discloses that the porosity of the cathode comprising MnO.sub.2 for alkaline cells may be adjusted by admixing a high porosity manganese dioxide (CMD) with a low porosity manganese dioxide (EMD). The reference discloses that when the materials are combined in a homogeneous mixture the preferred average porosity of the cathode is 15% to 35% and more preferably 20% to 25%. (col. 4, lines 25-29.) The reference discloses that the cathode material typically comprises between bout 80 to 85 weight percent of the total cathode and that an amount of electrolyte solution is added sufficient to wet the dry components. The mix is molded or compressed against the container, or premolded as rings and the rings pressed into the container. (col. 5, lines 28-30) A person skilled in the art would recognize that the referenced cathode material molded or compressed against the container results in a compacted solid mass of material because the reference indicates that the cells had a high (73.2 volume percent) solid packing in the cathode (col. 2, line 57-59).
The strategy in formulating conventional alkaline cell cathode is to employ high concentration of manganese dioxide in the mix in order to achieve high capacity (amp-hrs). In high power application, that is, at high current drain, deleterious effects such as cathode polarization can occur. Polarization results from limited mobility of ions within the cathode active material and within the electrolyte, which in turn reduces service life. The phenomenon of cathode polarization may be caused by the accumulation of hydroxide ions in the cathode during high rate discharge. The accumulation of hydroxide ions in the cathode prevents these ions from reaching the anode where they are required to sustain the oxidation of zinc, that is, to react with zinc ions to form zinc oxide by-product. In conventional solid MnO.sub.2 cathodes the polarization effect may be so serious at high current drain, for example, between about 1 and 1.5 Amp drain that only 20% or less of the cell's theoretical capacity (Amp-hr) may be obtainable. Thus, it is desirable to provide a way of reliably increasing the manganese dioxide utilization (actual specific capacity of the manganese dioxide, Amp-hr/g) at high drain without adversely affecting cell performance.
Also, conventional MnO.sub.2 cathodes for alkaline cells, because they are solid material, have the disadvantage that they are susceptible to breakage during compaction or accidental cell impact after the cathodes have been compacted into the cell. Also, tools are required to compact the solid cathodes into the cell. These tools need to be replaced frequently because of wear as they are constantly applied against the hard solid cathode during mass production cell assembly.
Thus, it is desirable to avoid cathode breakage and the need for replacement tools to compact the cathode in the cell.
SUMMARY OF THE INVENTION
In one aspect the invention is directed to an alkaline cell, particularly to a primary (non-rechargeable) alkaline cell having an anode comprising zinc and a cathode comprising manganese dioxide wherein the improvement resides in the cathode being in a semisolid state when the cell is fresh and preferably throughout the cell's useful service life.
The utilization of MnO.sub.2 (actual specific capacity, mAmp-hr/g) in the semisolid cathode of the invention can be much higher than with conventional compacted solid MnO.sub.2 cathodes, particularly under high current drain (0.5 to 2.0 Amp drain). Thus, the actual cell capacity (mAmp-hrs) of hermetically sealed alkaline cells employing the semisolid MnO.sub.2 cathode can be greater than that achieved in conventional alkaline cells employing compacted solid MnO.sub.2 cathodes. Typically the percent utilization of MnO.sub.2 in alkaline cells employing the conventional solid cathodes when discharged to 0.8Volts at 1 Amp is about 30%. By contrast the percent utilization of MnO.sub.2 in alkaline cells employing the semisolid cathode of the invention discharged to 0.8Volts at 1 Amp may be between about 40 and 70%. (Based on theoretical 100% utilization of MnO.sub.2 at 308 mAmp-hr/g.)
In another aspect the alkaline cells employing the cathode of the invention may exhibit an energy density of between about 0.10 and 0.30, more typically between about 0.12 and 0.25 Watt-hrs per gram MnO.sub.2 when the cell is drained at 1 Amp to a cut off voltage of about 0.8Volts, whereas the energy density of the same cell discharged at the same rate but employing conventional solid cathode may be about 0.1 Watt-hrs per gram MnO.sub.2.
In another aspect of the invention the semisolid cathode in the alkaline cell comprises manganese dioxide in amount which is less than 80 percent by weight of the cathode and preferably between about 40 and 80 percent by weight, typically between about 45 and 78, more typically between about 45 and 70 percent by weight of the cathode.
In one aspect of the invention the semisolid cathode comprising manganese dioxide may be in the form of a putty or paste or a highly viscous material having a measurable viscosity.
In another aspect the semisolid cathode of the invention comprises manganese dioxide and also carbon in the form of acetylene black. Advantageously, the acetylene black preferably comprises between about 5 and 15 percent by weight of the semisolid cathode. Desirably the semisolid cathode of the invention also comprises electrolyte solution comprising potassium hydroxide. The semisolid cathode may also comprise a binder such as polytetrafluroethylene, desirably comprising between about 0 and 2 percent by weight of the cathode. The acetylene black increases the conductivity of the semisolid cathode and provides an internal network to maintain the cathode in a semisolid state.
In another aspect the semis solid cathode material of the invention may have an porosity between about 30 and 70%, preferably between about 35 and 70%, more preferably between about 40 and 70%. The semisolid cathode material of the invention may desirably also have a higher electrolyte content as a weight percent of total cathode than conventionally employed in solid MnO.sub.2 containing cathodes for alkaline cells. The semisolid cathode material of the invention desirably may have a KOH content (pure) of between about 6 and 18 percent by weight of total cathode material and a total water content between about 9 and 27 percent by weight. The term KOH (pure) as used herein and in the examples with respect to KOH is the pure KOH content (i.e., without water) for calculation purposes. By contrast a conventional alkaline cell solid cathode may contain 2.7 percent by weight KOH (pure) and a total water content of about 4.3 percent by weight.
In another aspect the cathode material of the invention in the cell may have a volume ratio of electrolyte solution (including any additional water separately added the cathode mix) to the total solids of between about 0.4 and 2.3, preferably between about 0.7 and 2.3. The semisolid cathode advantageously comprises between about 15 and 45 percent by weight electrolyte solution (including any additional water separately added at any time to the cathode), preferably between about 20 and 45 percent by weight. The electrolyte solution may be an aqueous KOH solution desirably having a KOH concentration of between about 30 wt % and 51 wt % (including any additional water separately added at any time to the cathode).
In another aspect the aqueous electrolyte content in the cathode of the invention may comprise between about 15 and 45 percent by weight of said cathode and said aqueous electrolyte (including any additional water separately added to the cathode) has a potassium hydroxide concentration of between about 30 and 51 percent by weight.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood with reference to the drawings in which:
FIG. 1 is a cross sectional cut away view of an alkaline cell having a casing with inside surface contacting the cathode material of the invention.
FIG. 1A is a representative MnO.sub.2 utilization discharge profile, voltage vs. mAmp-hr per gram MnO.sub.2, comparing the cathode of the invention to conventional cathode.
FIG. 2A is pictorial view of a cylindrical cell casing.
FIG. 2B is a cross sectional view of the cell casing with the cathode material of the invention therein.
FIG. 2C is a cross sectional view of the cell casing with cathode material just prior to penetration of a mandrel (plunger) into the cathode material.
FIG. 2D is a cross sectional view of the cell casing showing the mandrel penetrating the cathode material of the invention.
FIG. 2E is a cross sectional view of the cell casing showing the cathode material being forced up into an annular region between the mandrel and the casing.
FIG. 2F is a cross sectional view of the cell casing showing a collar sliding up along the length of the mandrel as the mandrel is pushed into the cathode material.
FIG. 2G is a cross sectional view of the cell casing showing an anode slurry being injected into the casing through a central longitudinal aperture within the mandrel.
FIG. 2H is a cross sectional view of the cell casing showing an anode slurry filling the central core of the casing.
FIG. 2I is a cross sectional view of the cell casing showing the casing filled with anode material and the mandrel removed.
FIG. 2J is a cross sectional view of the mandrel with ion porous separator wrapped around it just prior to penetration into the cathode material of the invention.
FIG. 3A is a pictorial view of the cell casing.
FIG. 3B is a cross sectional view of the cathode material of the invention preformed into disk shaped pellets.
FIG. 3C is a pictorial view of the cell casing ready to receive the cathode pellets.
FIG. 3D is a cross sectional view of the cell casing with the disk shaped cathode pellets loaded therein.
FIG. 3E is a pictorial view of the ion porous separator ready for insertion into the core of the cell casing.
FIG. 3F is a cross sectional view of the cell casing ready for receiving the separator.
FIG. 3G is a cross sectional view of the cell casing with cathode pellets and separator loaded therein.
FIG. 3H is a cross sectional view of the cell casing with cathode, separator and anode slurry loaded therein.
DETAILED DESCRIPTION
It has been determined that the specific capacity (Amp-hr/g) of manganese dioxide which is used as active material in the cathode of an alkaline cell, particularly a primary (non-rechargeable) sealed alkaline cell, can be markedly increased if the cathode in the cell is a semisolid before the cell is discharged and preferably remains as a semisolid as the cell is discharged. The increase in specific capacity of the manganese dioxide, resulting in more effective utilization of the manganese dioxide in the semisolid cathode, is especially noticeable during cell discharge at high current drain, for example, between about 0.5 and 2 Amp, typically between about 0.5 and 1.5 Amp. This corresponds to a applications which have an average power demand between about 0.5 and 2 Watt, typically between about 0.5 and 1.5 Watt. The higher MnO.sub.2 utilization is also reflected by higher energy density (Watt-hrs per gram MnO.sub.2) resulting from use of the cathode of the invention in sealed alkaline cells applied to high drain applications, e.g., at 0.5 to 2.0 Amp drain.
The increase in specific capacity of the MnO.sub.2 in the semisolid cathode is obtainable irrespective of whether the manganese dioxide in the cathode is chemical manganese dioxide (CMD) or electrolytic manganese dioxide (EMD) or mixtures thereof.
The semisolid cathode of the invention can be expected to provide higher MnO.sub.2 utilization, (higher actual specific capacity of MnO.sub.2, Amp-hr/g) at high current drain (high power application) than obtained with conventional solid MnO.sub.2 cathodes even if the porosity of the two cathodes are the same. Therefore, porosity is not controlling of MnO.sub.2 utilization in high power application, rather the semisolid state of the cathode is believed to be the major factor leading to increased MnO.sub.2 utilization in high power application (0.5 to 2 Amp drain). It is conjectured that the semisolid cathode gives better utilization of MnO.sub.2 at high current drain primarily because the semisolid material provides for greater uniformity of pore and electrolyte distribution. The semisolid also allows for better mass transport at high current drain. Additionally the semisolid state allows for attainment of higher porosity cathodes, e.g. between about 40 and 70%, and higher electrolyte loading as a percent by weight and volume of the cathode than can be obtained with conventional solid mass cathodes. Thus, to the extent that increased porosity may be a contributing factor in attaining higher MnO.sub.2 utilization at high current drain, the semisolid state allows for the attainment of such increased porosity.
DEFINITION
The term semisolid as used herein is intended to broadly cover all physical states which are intermediate in property between true solid and true liquid. Thus, the term semisolid is intended to include (for example, but not by way of any limitation) physical states having the soft texture and rheological properties commonly associated with putties and pastes. The semisolid material as used herein includes (for example, but not by way of any limitation) materials that are not free flowing as is a liquid but generally requires an external force in order to cause the material to move through a conduit. The term semisolid is also intended (for example, but not by way of any limitation) to apply to materials which are extrudable and which can be deformed without fracturing upon exertion of external pressure. The term semisolid as used herein (for example, but not by way of any limitation) is also intended to include highly viscous substances which may slowly ooze through a conduit when placed in vertical position. In this regard the term semisolid is intended to include such highly viscous material, for example, at least as viscous as molasses and having a measurable kinematic viscosity at 20.degree. C.
In the semisolid cathode of the invention the MnO.sub.2 content as a percent by weight or percent by volume of the cathode may characteristically be less than that achieved with conventional compacted solid cathodes containing MnO.sub.2. For example, with conventional compacted solid MnO.sub.2 containing cathodes for alkaline cells the MnO.sub.2 content is typically between about 80 to 87 percent by weight. When the cathode is formulated so that it is in a semisolid state in accordance with the invention, the content of MnO.sub.2 may be reduced, for example, to a level desirably less than about 80 percent by weight of the total cathode. The MnO.sub.2 content in the semisolid cathode of the invention may desirably be between about 40 and 80 percent by weight, desirably between about 45 and 78 percent by weight, typically between about 45 and 70 percent by weight.
Since the total amount of MnO.sub.2 in the semisolid cathode is typically less than a conventional compacted solid cathode for the same size cell, the theoretical capacity (Amp-hrs) of the semisolid cathode will be less than the theoretical capacity of the conventional compacted solid MnO.sub.2 cathode. However, during high current drain (between about 0.5 and 2 Amp) a greater percent of the MnO.sub.2 in the semisolid cathode will actually be reduced (discharged) to the normal discharge by-product MnO(OH) than in the conventional compacted cathode. That is, cathode polarization is reduced and the MnO.sub.2 material in the semisolid cathode is better utilized, thus resulting in a higher specific capacity of MnO.sub.2 (Amp-hr/g) in the semisolid cathode than in conventional compacted solid cathodes. For example, at high current drain of about 1 Amp the actual specific capacity of MnO.sub.2 in a conventional Zn/MnO.sub.2 alkaline cell employing compacted solid cathode comprising between about 80 to 87 percent by weight MnO.sub.2 may only be about 96 mAmp-hr/g. Since the theoretical capacity of MnO.sub.2 (EMD) in an alkaline cell is about 308 mAmp-hr per gram MnO.sub.2 (reduction of MnO.sub.2 to MnO(OH)), the actual utilization of MnO.sub.2 at 1 Amp drain in the conventional cell is only about 31%. However, in the same alkaline cell employing the semisolid MnO.sub.2 cathode of the invention the actual MnO.sub.2 specific capacity at 1 Amp drain may be about 195 mAmp-hr/g, resulting in a MnO.sub.2 utilization of about 63%. Thus, the semisolid cathode of the invention surprisingly reduces the level of deleterious polarization effects and results in much higher actual utilization of MnO.sub.2. The higher utilization of MnO.sub.2 in the semisolid cathode of the invention also results in a less expensive cell, since less MnO.sub.2 active material need be supplied to the cell to achieve the same or better overall actual capacity for high power application. The percent utilization of MnO.sub.2 in alkaline cells employing the semisolid cathode of the invention discharged to 0.8Volts at 1 Amp may typically be between about 40 and 70%, more typically between about 44 and 68%. (Based on theoretical 100% utilization of MnO.sub.2 at 308 mAmp-hr/g.) This represents a considerable improvement over the representative conventional cell (cathode formulation A) which had a percent MnO.sub.2 utilization of 31% when the cell was discharged to 0.8Volts at 1 Amp.
Because of the higher MnO.sub.2 utilization the cells employing the cathode of the invention also exhibit a high energy density between, for example, about 0.14 and 0.30 Watt-hrs per gram MnO.sub.2 when the cell is drained at 1 Amp to a cut off voltage of about 0.8Volts, whereas the energy density of the same cell discharged at the same rate but employing conventional solid cathode may be about 0.1 Watt-hrs per gram MnO.sub.2.
It has been determined that the semisolid cathode material of the invention may desirably also have a higher KOH content (measured as pure KOH, that is, without water, in weight percent of total cathode) than conventionally employed in solid MnO.sub.2 containing cathodes for alkaline cells. The semisolid cathode material of the invention desirably may have a KOH content (pure) of between about 6 and 18 percent by weight of total cathode material. The total water content of the semisolid cathode may desirably be between about 9 and 27 percent by weight during cell discharge. The porosity of the semisolid cathode material of the invention may desirably be between about 30% and 70%, desirably between about 35% and 70%, preferably between about 40% and 70%, for example, between about 45 and 70%.
The cathode porosity may be calculated by determining the volume occupied by electrolyte, other liquids and entrapped air (including volume of liquid and air entrapped within the micropores of solids) and dividing that volume by the apparent volume of the cathode and multiplying by 100. (The apparent volume is the overall volume of a sample as contained within the external boundary of the sample.) The cathode porosity may be conveniently calculated by first obtaining the real density of each of the solids by conventional helium replacement method, before the solids are mixed into the cathode. (The real density of each solid is the solid sample weight divided by its real volume, that is, the solids sample apparent volume reduced by the volume occupied by entrapped air.) The respective weight of solids to be mixed into the cathode is then divided by their respective real densities to obtain the real volume of the solids in the cathode. The real volume of the solids is subtracted from the apparent volume of the cathode as a whole and this difference is divided by the cathode apparent volume.times.100 to yield porosity, percent. In essence the porosity of the cathode is the percent of the apparent volume of the cathode which is occupied by everything therein (liquids and entrapped air) except solids. The porosity of the cathode formulations presented and discussed herein including comparative formulation A were determined in the above manner.
The same cathode porosity (%) may also be determined by calculating the difference between the cathode solids real density and the cathode solids apparent density in the cathode and dividing the difference by the solids real density and multiplying by 100. The solids real density is the cathode sample weight excluding electrolyte and any other liquid present and any entrapped air and the sample volume reduced by the volume occupied by electrolyte, liquid and entrapped air. The cathode solids apparent density is the cathode sample weight excluding electrolyte and other liquid and entrapped air, but sample overall volume with electrolyte, other liquid and entrapped air therein. This calculation will give the porosity, that is, the percent of the apparent volume of the sample which is occupied by electrolyte, other liquid and entrapped air, as above.
The semisolid cathode material of the invention may have a volume ratio of electrolyte solution (including any additional water separately added) to the total solids of between about 0.4 and 2.3, preferably between about 0.5 and 2.3, more preferably between about 0.7 and 2.3. The volume ratios of said components are after they have been admixed to form the cathode mixture. The semisolid cathode advantageously comprises between about 15 and 45 percent by weight electrolyte solution (including any additional water separately added), preferably between about 20 and 45 percent by weight. The electrolyte solution may be an aqueous KOH solution desirably having a KOH concentration of between about 30 wt % and 51 wt % (including any additional water separately added the cathode mix).
A representative cell 10 employing the semisolid MnO.sub.2 cathode 12 of the invention is shown in FIG. 1. The cell depicted in FIG. 1 is an AAA or AA size alkaline cell. However, the cell chemistry employing the semisolid cathode material of the invention is not intended to be restricted to any particular size cell. The cathode material of the invention, for example, may be employed in AAAA, AAA, AA, C and D size alkaline cells. Thus cell 10 may contain conventional alkaline cell anode chemistries including those which contain zero added mercury (less than 50 parts mercury per million parts total cell weight, preferably less than 10 parts mercury per million parts total cell weight) and modifications thereof. Such representative anode chemistries, for example, are disclosed in U.S. Pat. No. 5,401,590, herein incorporated by reference.
The zinc/MnO.sub.2 cell 10 may represent a conventional primary alkaline cell manufactured in conventional manner, except for the MnO.sub.2 cathode 12. The representative alkaline cell 10 may employ a conventional anode 50 comprising particulate zinc, conventional potassium hydroxide electrolyte and conventional separator. It is preferred that the zinc anode has no "added mercury". Zero-added mercury cells contain no added amounts of mercury, but may contain only the residual amount of mercury present in commercially available pure zinc, including the residual amount of mercury, if any, present in the other cell components. The total mercury content in such cells is less than 50 parts mercury per million parts total cell weight, typically less than 10 parts mercury per million parts total cell weight. However, the present invention is also applicable to zinc/MnO.sub.2 primary alkaline cells containing mercury. All the components in the cell including the anode active material, electrolyte and separator, except for cathode 12, may be composed of materials conventionally employed in such cells. Cathode 12 comprises the semisolid cathode material of the invention. Cathode 12 is a semisolid, which may typically have the consistency of a putty or paste, for example, the consistency of a sealing putty or adhesive paste. Cathode 12 of the invention could also have the flow properties of a highly viscous material such as molasses, having a measurable viscosity, for example, having a kinematic viscosity of at least 100 centistokes. Cathode 12 desirably comprises less than about 80 percent by weight MnO.sub.2, for example, between about 40 and 80, typically between about 45 and 78 percent by weight MnO.sub.2, more typically between about 45 and 70 percent by weight MnO.sub.2. In the representative alkaline cell 10 the anode material 50 forms the central core of the cell and the cathode material 12 is located around the anode material with the separator 60 therebetween. The cathode material contacts the inside surface of the cell casing which is typically of nickel plated steel or stainless steel.
AA size alkaline cells 10 were prepared with the same conventional gelled zinc anode composition, separator and KOH electrolyte. The following cathode semisolid paste formulations were made and used as cathode material in the alkaline cell. A same size AA cell was prepared but conventional compacted solid MnO.sub.2 cathode was employed (formulation A) as cathode material instead of the semisolid paste. The cells were discharged at 1 Amp continuous rate to 0.8V cut off and the actual specific capacity (utilization) of the MnO2 in the cells were determined. The results are presented in Table 1.
Formulation A (Conventional Compacted Solid Cathode)
Wt % Vol %
MnO2 (EMD) 86.0 65.2
graphite 7.0 10.8
entrapped air negligible 6.5
electrolyte: 7.0 17.5
38 wt % KOH (9 Normal)
100.0 100.0
Porosity: 24 percent.
Ratio of Vol % Electrolyte to Vol % Total Solids: 0.23
Note: KOH (pure) 2.7 wt %; total H.sub.2 O content 4.3 wt %.
Formulation 1 (Semisolid Cathode Putty/Paste)
Wt % Vol. %
MnO.sub.2 (EMD) 48.44 22.6
Carbon Black 8.94 9.6
(Shawinigan
Acetylene Black)
Polytetrafluorethylene 0.17 0.2
(PTFE TE5211)
Clay (Laponite RDS) 0.42 0.9
Water 3.80 7.9
Electrolyte: 38.23 58.8
38 wt % KOH (9 Normal)
100.0 100.0
Porosity: 66.7 percent.
Ratio of Vol % Electrolyte (including 7.9 vol % added water) to Vol % Total Solids: 2.00
Note: KOH (pure) 14.5 wt %; total H.sub.2 O content 23.7 wt %.
Formulation 2 (Semisolid Cathode Putty/Paste)
Wt % Vol %
MnO.sub.2 (EMD) 50.7 24.9
Carbon Black 9.3 10.5
(Shawinigan Acetylene
Black)
Electrolyte: 40.0 64.6
38 wt % KOH (9 Normal)
100.0 100.0
Porosity: 64.6 percent.
Ratio of Vol % Electrolyte to Vol % Total Solids: 1.82
Note: KOH (pure) 15.2 wt %; total H.sub.2 O content 24.8 wt %.
Formulation 3 (Semisolid Cathode Putty/Paste)
Wt % Vol %
MnO.sub.2 (EMD) 56.1 28.9
Carbon Black 6.8 8.0
(Shawinigan Acetylene
Black)
Electrolyte: 37.1 63.1
38 wt % KOH (9 Normal)
100.0 100.0
Porosity: 63.1 percent.
Ratio of Vol % Electrolyte to Vol % Total Solids: 1.71
Note: KOH (pure) 14.1 wt %; total H.sub.2 O content 23.0 wt %.
Formulation 4 (Semisolid Cathode Putty/Paste)
Wt % Vol %
MnO.sub.2 (EMD) 52.3 26.3
Carbon Black 10.6 12.3
(Shawinigan Acetylene
Black)
Electrolyte: 37.1 61.4
38 wt % KOH (9 Normal)
100.0 100.0
Porosity: 61.4 percent.
Ratio of Vol % Electrolyte to Vol % Total Solids: 1.59
Note: KOH (pure) 14.1 wt %; total H.sub.2 O content 23.0 wt %.
Formulation 5 (Semisolid Cathode Putty/Paste)
Wt % Vol. %
MnO.sub.2 (EMD) 69.4 42.0
Carbon Black 5.2 7.2
(Shawinigan Acetylene
Black)
Graphite 2.6 3.2
Polytetrafluoroethylene 0.4 0.5
(PTFE T30)
Electrolyte: 22.4 47.1
30 wt % KOH
100.0 100.0
Porosity: 47.1 percent.
Ratio of Vol % Electrolyte to Vol % Total Solids: 0.89
Note: KOH (pure) 6.7 wt %; total H.sub.2 O content 15.7 wt %.
Formulation 6 (Semisolid Cathode Putty/Paste)
Wt % Vol %
MnO.sub.2 (EMD) 73.0 49.2
Carbon Black 7.0 10.8
(Shawinigan Acetylene
Black)
Electrolyte: 20.0 40.0
(50 wt % KOH solution)
100.0 100.0
Porosity: 40.0 percent.
Ratio of Vol % Electrolyte to Vol % Total Solids: 0.67
Note: KOH (pure) 10.0 wt %; total H.sub.2 O content 10.0 wt %.
General Notes:
1. Laponite RDS Clay from Southern Clay Products Company PTFE T30 and TE5211 are aqueous suspensions of TEFLON resin (40 to 60% concentration) from E.I. Dupont de Nemours & Co.
2. Vol % of solids based on solids real density.
3. Real densities: MnO.sub.2 (EMD)=4.48 g/cc; graphite=2.2 g/cc; Shawinigan acetylene black=1.95 g/cc; PTFE=2.1 g/cc; Laponite clay=1.0 g/cc; KOH electrolyte (38 wt %)=1.36 g/cc; KOH electrolyte (50 wt %)=1.51 g/cc; KOH electrolyte (30 wt %)=1.29 g/cc.
4. The porosity of the above semisolid cathode formulations 1-5 of the invention were essentially equal to the vol % of liquids in the cathode, since the volume contribution of entrapped air in the semisolid cathode is negligible. The porosity of the solid cathode of comparative formulation A was higher than the vol% of liquids in the cathode because of the entrapped air within the solid cathode.
The above formulation A was prepared by first mixing the particulate MnO.sub.2 and graphite in a V shaped blender for about 10 minutes or until a homogenous mixture was obtained. The KOH electrolyte solution was then added and mixing continued for about another 5 minutes until a homogenous mixture obtained. When formulation A is compacted into a cell it forms a hard compacted solid mass. Such solid mass cannot be penetrated at 20.degree. C. by tapping of a nail (1/16 inch diameter) at a 90.degree. contact angle formed between the central longitudinal axis of the nail and the surface of the mass, without shattering the mass. Generally, conventional solid cathodes comprising MnO.sub.2 physically fail, e.g., fracture or shatter, if a 1/16 inch diameter nail as above described is tapped into the mass of such cathodes.
The formulations 2,3, 4, and 6 were prepared in a similar manner by first mixing the particulate MnO.sub.2 and carbon black in a V shaped blender for about 10 minutes or until a homogenous mixture was obtained. The KOH electrolyte solution was then added and mixing continued for about another 5 minutes until a homogenous mixture was obtained. The resulting mixture formulations 2, 3, and 4 had the consistency of a putty or paste. Mixture formulations 2, 3, and 4 were thus soft to the touch and a sample of each could be readily penetrated at a temperature of 20.degree. C. by tapping of a nail (1/16 inch diameter) at a 90.degree. contact angle formed between the central longitudinal axis of the nail and the surface of the mixture, without shattering the mixture. The term "shattering" as used herein is intended to be the equivalent of "fracturing". Such "nail" test as used herein is used to illustrate a typical physical property of the cathode of the invention and the term "semisolid" as given in the above definition is not intended to be limited to or by such test. The consistency does not noticeably change when the mixture formulations 2,3, and 4 are injected into the cell to form the cell's cathode. Mixture formulation 6 had a higher concentration of MnO.sub.2 and upon mixing formed a moist pasty powder mix. This mix may be injected (extruded) under pressure into the cell casing whereupon it forms a semisolid having the consistency of a soft putty. The soft putty is readily penetrated at a temperature of 20.degree. C. by tapping of a nail (1/16 inch diameter) at a 90.degree. contact angle formed between the central longitudinal axis of the nail and the surface of the putty, without shattering the putty.
The formulations 1 and 5 were prepared in the same manner as above described except that the additional components Laponite RDS (formulation 1) was made ahead as a 10% solution. That is, a solution of 10 wt % Laponite RDS in water was prepared and this solution was added to the blend along with the electrolyte and the mixture blended for additional time if needed (at about 40 rpm) to obtain a homogenous putty or paste. (The above compositions separate the water content from the Laponite RDS). In the case of formulation 1 and 5 polytetrafluoroethylene (PTFE TE5211 or T30) was also added to the blend along with electrolyte and the mixture blended for additional time (about 10 minutes at 40 rpm) until a homogenous semisolid mixture was obtained. Mixture formulation 1 was putty or paste-like in consistency. It was soft to the touch and a sample of each could be readily penetrated at a temperature of 20.degree. C. by tapping of a nail (1/16 inch diameter) at a 90.degree. contact angle formed between the central longitudinal axis of the nail and the surface of the sample, without shattering the sample. The consistency does not noticeably change when the mixture formulation 1 is injected into the cell to from the cell's cathode. Mixture formulations 5 had a higher concentration of MnO2 and less liquid and upon mixing formed a moist pasty powder mix. This mix may be injected (extruded) under pressure into the cell casing whereupon it forms a semisolid having the consistency of a soft putty. The soft putty is readily penetrated at a temperature of 20.degree. C. by tapping of a nail (1/16 inch diameter) at a 90.degree. contact angle formed between the central longitudinal axis of the nail and the surface of the putty, without shattering the putty.
The same AA size test cells were prepared with each cell having a cathode composition corresponding to the above formulations. The cell components in each case were otherwise the same, including essentially the same anode, separator, current collector and KOH electrolyte, all of which cell components (except for the semisolid cathode paste of the invention) being conventionally employed in commercial alkaline cells. The anode material in each case had the formulation: zinc alloy powder 62 to 69 wt % (99.9 wt % zinc containing indium), aqueous KOH solution 38 wt % (9 Normal containing about 2 wt % ZnO); CARBOPOL C940 (B.F. Goodrich) gelling agent (about 0.5 wt %). The separator in each case was a conventional cellulosic ion porous separator. The zinc alloy powder within above range does not significantly effect cathode performance. The cells tested were balanced in conventional manner so that the mAmp-hr capacity of MnO.sub.2 (based on 370 mAmp-hr per gram MnO.sub.2) divided by the mAmp-hr capacity of zinc alloy (based on 822 mAmp-hr per gram zinc alloy) is about 1.
The comparative AA cell contained the conventional solid cathode (formulation A) above referenced containing about 86 wt % MnO2 (EMD) and 7 wt % graphite. This conventional cathode (formulation A) was compacted against the inside surface of the cell casing in conventional manner by applying external force to the mixture to compact it against the cell casing surface. The resulting compacted cathode was a compacted solid material containing about 86 percent by weight MnO.sub.2.
Similar AA size cells were prepared but with each having a semisolid cathode corresponding to formulations 1-4 of the invention. The cathode semisolid paste (formulations 1-4) in each case may be injected into an empty cylindrical cell casing using a syringe or forcing it under pressure through an injection nozzle. A cylindrical plunger or the like may then be inserted into the cell and forced against a surface of the paste. A preferred method of loading the cathode paste of the invention into the cell and forcing it against the cell casing surface is described below. The resulting cathode material of the invention (formulations 1-4) pressed against the inside surface of the cell casing is a semisolid having the consistency of a putty or paste.
The cathode formulations 5 and 6 result in a moist pasty powder mixture when all of the components are admixed. The formulations 5 and 6 are preferably compressed into disk shaped pellets 12, 12b, 12c and 12d each having a height of about 10 mm (FIG. 3B). As the mixture is compressed into pellets the mixture transforms into a soft putty-like consistency. The pellets are then separately inserted into casing 20 in a stacked arrangement (FIG. 3D). The pellets are inserted so that their outer surface contacts the inside surface of casing 20 (FIG. 3D). After insertion into the casing 20, the pellets may again be slightly recompressed. The pellets, even if recompressed, still retain a putty-like consistency and are soft to the touch. Each pellet 12a-12d can be readily penetrated at a temperature of 20.degree. C. by a 1/16 inch diameter nail at 90.degree. contact angle formed between the central longitudinal axis of the nail and surface of the pellet, without shattering the pellet. The pellets made with formulations 5 and 6 retain their soft putty-like consistency during cell discharge at 1 amp. Alternatively, cathode semisolid formulation 5 and 6 may be injected into casing 20 without preforming the mixture into pellets. As the mix is injected (extruded) under pressure it transforms into a semisolid having a soft putty-like consistency. However, it is preferable to preform semisolid formulations 5 and 6 into pellets before insertion into casing 20 to more readily achieve a uniformity of density of the cathode.
The above described cathode formulations 1-5 retain their semisolid characteristic under normal ambient cell operating conditions.
TABLE 1
DISCHARGE PERFORMANCE.sup.1
Specific
Capacity Grams Capacity
Cathode (mAmp- MnO.sub.2 (mAmp-hr per Utiliza-
Formula hr) Watt-Hrs in Cell gram MnO.sub.2) tion.sup.2
A (solid) 914 0.93 9.5 96 (31.2%)
1 (semisolid) 556 0.52 3.7 151 (49.0%)
2 (semisolid) 762 0.79 3.7 195 (63.3%)
3 (semisolid) 633 0.62 4.5 138 (44.8%)
4 (semisolid) 697 0.68 4.2 166 (53.9%)
5 (semisolid) 1380 1.45 6.6 209 (67.9%)
6 (semisolid) 990 0.93 6.4 155 (50.3%)
Notes:
.sup.1 Discharge at 1 Amp continuous drain to 0.8 Volt cut-off.
.sup.2 Values in parentheses are percent utilization MnO.sub.2 (Basis: 100%
utilization taken at 308 mAmp-Hr per gram MnO.sub.2 based on reduction of
MnO.sub.2 to MnO(OH).)
As may be seen from Table 1 all of the cells employing the semisolid MnO.sub.2 cathode of the invention show increased MnO.sub.2 utilization, that is, increase in actual specific capacity of MnO2 for cell discharge at high rate of 1 Amp to 0.8V cutoff. The most significant improvement in MnO2 utilization is obtained employing cathode paste formulations 2, 4, 5, and 6. These formulations have MnO.sub.2 content between about 40 and 80 wt %, carbon black or acetylene black between about 5 and 15 wt % and electrolyte (9 Normal KOH) between about 15 and 45 wt %.
As may be seen from Table 1 a preferred formulation 2 provides a MnO.sub.2 utilization (mAmp-Hr/g MnO.sub.2) which is about twice that obtained with the conventional compacted solid cathode (formulation A). Other preferred formulations 5 and 6 provide MnO2 utilization (mAmp-Hr/g MnO.sub.2) respectively about 2.2 and 1.6 times that obtained with the conventional compacted solid cathode (formulation A). The actual total capacity of the AA cell using cathode formulation 2 (762 mAmp-Hr) approaches that of the AA cell employing a conventional cathode formulation A (914 mAmp-Hr) when the cells are discharged at a high rate of 1 Amp to 0.8V cutoff. Also the total energy (0.79 Watt-hrs) output of the AA cell using cathode formulation 2 approaches that of the same cell employing conventional cathode formulation A (0.93 Watt-hrs). The actual total capacity of the AA cell using cathode formulation 5 (1380 mAmp-Hr) exceeds that of the AA cell employing a conventional formulation A (914 mAmp-Hr). Also the total energy (1.45 Watt-hrs) output of the AA cell using cathode formulation 5 exceeds that of the same cell employing conventional cathode formulation A (0.93 Watt-hrs) at the 1 Amp high rate of discharge. An AA cell employing cathode formulation 5 was also discharged at a low constant current drain of 0.1 Amp (100 milliAmp). The actual capacity of the cell at this low current drain (100 milliAmp) was 2250 mAmp-hr which compared favorably with the discharge capacity 2359 mAmp-hr of the same cell using comparative formulation A.
The actual total capacity of the AA cell using cathode formulation 6 (990 mAmp-Hr) exceeds that of the AA cell employing a conventional formulation A (914 mAmp-Hr). Also the total energy (0.93 Watt-hrs) output of the AA cell using cathode formulation 6 is the same as the AA cell employing conventional cathode formulation A (0.93 Watt-hrs).
Such results represent a significant advance in view of the fact that the cell containing cathode formul
