HK1149371B - Flexible thin printed battery with gelled electrolyte and method of manufacturing same - Google Patents

Description

FIELD OF THE INVENTION

This invention relates to a flexible thin battery. More specifically, this invention relates to a flexible thin printed battery wherein one or more of the electrodes are printed onto a flexible substrate using a printable ink.

BACKGROUND OF THE INVENTION

Flexible planar thin batteries utilizing lithium-based chemistries are known wherein the electrodes are formulated by the deposition of an active material film onto a substrate using various deposition techniques such as pulsed laser deposition, spin coating and sputtering. These techniques tend to require relatively costly and complex equipment and do not lend themselves to a high throughput inexpensive manufacturing process. Further, many devices requiring a power supply, such as novelty packaging and greeting cards augmented with audio and/or visual outputs, are manufactured on high speed web-based printing lines. Lithium-based technologies are not an attractive power source for such low cost per unit applications. The ability to produce both the device and the power supply in a single process presents opportunities for cost savings. There is therefore a need to develop an inexpensive electrochemical power supply that can be produced in a web-based process by stenciling, screen printing or other thick film application processes. As used herein, "print" and "printing" and "printable" refer to any such thick film application process whereby the layer produced is between 10 and 250 µm thick and includes both stenciling and screen printing processes.

SUMMARY OF THE INVENTION

The present invention relates to a carbon zinc battery (1) comprising a cathode (5), an anode (3), a separator (9), an electrolyte and a printed cathode current collector (7), at least a portion of which is printed onto a flexible nonconductive substrate, wherein the printed cathode current collector (7) comprises:

• a metallic substrate or a metallic ink printed onto the flexible nonconductive substrate; and
• a protective carbon ink printed onto the metallic substrate or metallic ink, wherein said protective carbon ink comprises graphite or carbon black and further comprises a styrene-butadiene-styrene or styrene-ethylene-butylene-styrene block copolymer.

Preferred embodiments arc apparent from the dependent claims.

DESCRIPTION OF THE DRAWINGS

• FIGURE 1A is an electrochemical cell according to the invention.
• FIGURE 1B is a cross sectional view of FIGURE 1A as indicated
• FIGURE 2 is an electrochemical cell with cell contacts according to an embodiment of the invention.
• FIGURE 3 is an electrochemical cell with cell contacts according to an alternate embodiment of the invention.
• FIGURE 4 is an electrochemical cell with cell contacts according to another alternate embodiment of the invention.
• FIGURE 5 is an electrochemical cell with cell contacts according to another alternate embodiment of the invention.
• FIGURE 6A is an electrochemical cell with cell contacts according to another alternate embodiment of the invention.
• FIGURE 6B is an electrochemical cell with cell contacts according to another alternate embodiment of the invention.
• FIGURE 6C is an electrochemical cell with cell contacts according to another alternate embodiment of the invention. FIGURE 7 is a printed anode and zinc mesh tab according to the invention.
• FIGURE 8 is a printed cathode current collector and tab according to the invention.
• FIGURE 9 is a co-planar printed anode and cathode according to the invention.
• FIGURE 10 is a circular co-planar anode and cathode according to the invention.
• FIGURE 11 is a graph of internal resistance for cells using a gelled electrolyte versus a liquid electrolyte.
• FIGURE 12 is the printed circuitry for a sound card device powered by a printed cell according to the invention.
• FIGURE 13 is the final circuit for a sound card device powered by a printed cell according to the invention.
• FIGURE 14 is a graph comparing the thixotropic properties of a polymer electrolyte using polyethylene oxide versus fumed silica as a viscosifying agent.
• FIGURE 15 is a plot of required cathode area and discharge efficiency as a function of the weight percent of graphite for an aqueous based cathode ink at a given cathode thickness.
• FIGURE 16 is a plot of required cathode area and discharge efficiency as a function of the weight percent of graphite for a non-aqueous based cathode ink for the same cathode thickness.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Components of the thin flexible printed battery 1 of the invention include a printed anode 3, a printed cathode 5, a cathode current collector 7, a separator 9 and an aqueous electrolyte contained within a flexible thin battery package, housing or enclosure 11. See Fig. 1A and Fig. 1B.

THE ANODE

We have discovered that an effective, conductive, aqueous zinc ink can be formulated and printed directly onto the surface of a nonconductive without the necessity of first printing an anode current collector or otherwise supplying a conductive substrate to function as a discrete anode current collector. As used herein, the term "aqueous" means that water is utilized as at least one solvent in the anode ink formulation. We have discovered that the presence of excess zinc +2 cations enables a low resistance, high conductivity printable zinc ink. In a carbon zinc cell of the invention, that is a cell using an electrolyte comprising zinc chloride, the source of excess zinc +2 cations is an aqueous solution of zinc acetate (Zn(OOCCH₃)₂·2H₂O) such as is available from, for example, Fisher Scientific, product designation Z20. While not wanting to be bound by theory, it is believed that the source of excess zinc +2 cations changes the conformation and aggregation of the polymer binder used in the ink formulation so that the polymer is less likely to form an insulating layer on the zinc particles, thereby improving the zinc particle to particle contact. In such a carbon zinc embodiment of the invention, a polyvinylpyrrolidone (PVP) binder is preferred, preferably with a molecular weight of 2.2 to 2.8 million. Zinc nitrates and zinc sulfates are not appropriate sources for excess zinc cations since they are strong oxidants and will oxidize the zinc. Zinc chloride and other zinc halides are not appropriate sources of excess zinc cations in such a zinc ink formulation for use in a zinc chloride electrolyte since the PVP binder will not dissolve so as to form a uniform dispersion in a zinc chloride solution. Zinc acetate is therefore a preferred source of excess zinc cations in the zinc ink formulation for use in a zinc chloride electrolyte since the PVP binder does form a uniform dispersion in a zinc acetate solution.

The zinc powder of the zinc anode ink is commercially available from such sources as Big River Zinc, Union Miniere or Noranda, and is preferably alloyed with from 500 to 1600 ppm lead. Alternatively, the zinc is BIA zinc (a bismuth, indium and aluminum alloy) commercially available from zinc suppliers such as Noranda. The zinc anode ink of the within invention uses very fine zinc powder, or dust. The zinc dust preferably has a Microtrac particle size d(50) value of from 10 to 60 µm and is dimensioned such that the powder will pass through a 270 mesh (0.053 mm) sieve (USA standard). As a rule of thumb, the d(50) value should not exceed one half of the desired ink layer thickness. Thus, if a desired ink layer thickness is 50 µm, the d(50) value of the powder component should in general not exceed 25 µm,

Other components of the preferred zinc ink in the carbon zinc electrochemical cell embodiment of the invention include an appropriate binder that is compatible with the cell chemistry, including the cell electrolyte. In the carbon zinc embodiment of the invention, an aqueous solution of polyvinylpyrrolidone (PVP) having a molecular weight of 2.0 to 4.0 million is utilized in conjunction with a source of excess zinc +2 ions, such as zinc acetate, as disclosed above. PVP is soluble in a zinc acetate solution but not in traditional carbon zinc electrolytes such as zinc chloride and ammonium chloride. PVP is commercially available from ISP Technologies, Inc. Wayne, New Jersey, product designation PVP-K120.

One concern with a PVP binder in an aqueous solution is that the resulting ink may result in high surface tension, high polarity and fast drying, particularly in a low humidity environment, as well as the generation of hydrogen gas resulting from the zinc corrosion reaction with water. We have discovered that a co-solvent system employing an aprotic solvent miscible with water and having a higher boiling point than water will result in the reduction of the surface tension of the ink, a decrease in the polarity of the ink, a decrease in the ink drying rate and a decrease in gassing. The preferred co-solvent in the zinc acetate based zinc ink described herein is N-methyl pyrrolidone (NMP), available from Honeywell Burdick & Jackson, Muskegon, Michigan, catalog number 304-1. NMP is soluble in an aqueous solution of zinc acetate.

In addition to a binder and solvent system, the zinc ink of the within invention can further include other cell additives to produce beneficial performance attributes. For example, the relatively fine particle size of the zinc employed in the zinc ink of the within invention results in increased gassing. A surfactant known to reduce gassing in alkaline cells has both a phosphate group and polyethylene oxide and/or polypropylene oxide chains. Such a surfactant is available commercially under the Union Carbide tradename Triton QS-44. We have discovered that surfactants of this type are even more beneficial in controlling gassing in acidic electrolytes such as LeClanche or zinc chloride electrolytes. As used herein, a "LeClanche electrolyte" is an electrolyte containing both zinc chloride and ammonium chloride.

Once the zinc ink has been formulated, it can then be screen printed or stenciled directly onto a flexible polymer substrate. The zinc ink of the within invention has sufficient conductivity so as to obviate the need for a distinct anode current collector to be printed or otherwise placed into contact with the anode formed from the zinc ink of the within invention. The anode formed from the zinc ink of the within invention maintains conductivity during the discharge even though the zinc is being consumed. The anode tab that forms the negative terminal external to the cell housing is directly connected with the zinc ink of the within invention, rather than being in electrical contact with a distinct anode collector.

The preferred substrate material is a flexible nonconductive polymer material that will be used to house the battery in a flexible package. Such a material is available as a laminate from Pharma Center Shelbyville, product designation 95014, with an ethylene acrylic acid heat sealable layer that forms the interior surface of the package. One of skill in the art will appreciate that the anode ink and the cathode current collector ink can also be printed directly onto other nonconductive materials that may or may not be flexible and may or may not form the battery package or housing. The surface upon which the anode ink and the cathode collector ink are applied will be the surface that ultimately is positioned within the battery package or housing. Such surfaces, in addition to providing a heat sealable surface can alternatively supply a pressure sealing surface, an epoxy sealing surface or other means of joining material together. Laminates which are constructed of a metal foil surrounded by a protective polymer on the outer side or surface and a heat or pressure sealable polyethylene or polypropylene on the opposing inner side are commonly available. Such laminates can be obtained from, for example, Pharma Center Shelbyville, Inc. of Shelbyville, Kentucky under the product designation 95014, Dai Nippon Printing Co., Ltd. of Tokyo, Japan under the product designation D-EL40E, and also, Sumitomo Electric Industries, Ltd. of Tokyo, Japan under the product designation L-NY-A1-TRPP-L. Alternatively, a laminate with an ethylene methacrylic or polyethylene methacrylic acid heat-sealable layer is made by Ludlow Coated Products of Homer, Louisiana. The appropriate laminate and associated sealing layer will be selected on the basis of, among other factors, the type of electrolyte to be used, as is known in the art. The impervious metallic foil layer can be any variety of metals such as, for example, aluminum, nickel, copper and stainless steel. The protective polymer layer is preferably a polyester or nylon, but other polymeric materials such as a polypropylene or a polyethylene could also be employed in this layer.

In the case of screen printing, it will be important to determine the optimum mesh opening for good printability of the ink, as is known in the art. Factors to consider include the particle size of the zinc, the ink viscosity and other flow properties under shear and the required thickness of the ink necessary to achieve sufficient capacity.

THE CATHODE ASSEMBLY

The cathode assembly of a carbon zinc cell according to the within invention (current collector and electrolytic manganese dioxide, or EMD, active material) is printed onto a flexible substrate to which the cathode current collector ink will adhere with minimal or no cracking, preferably onto the sealable surface of a flexible packaging material that will be used to house the battery. Such a flexible battery housing laminate material is available for example from Pharma Center Shelbyville, product designation number 95014, as described above.

First, a current collector is deposited onto the flexible polymer using a stencil, a screen or other suitable printing apparatus. The sealing surface of the laminate material is used as the printing surface, i.e. that surface of the material that will end up being positioned within the battery package or housing. The cathode current collector ink is preferably an ink formulated from materials sufficient to transfer electrons generated in the reduction of the cathode during discharge. The appropriate cathode current collector material will be selected based on the materials utilized in the cell, to maximize current transfer while minimizing undesirable reactions with other cell component materials, as is known in the art.

A metallic current collector will be more conductive than a carbon current collector, but will react with the manganese dioxide in a zinc chloride electrolyte. We have discovered that by coating a conductive metal or metallic ink such as silver, silver ink or aluminum with a protective conductive carbon film, the benefits of a metallic current collector can be achieved without the disadvantages of reactivity in a zinc chloride or a Leclanche electrolyte. The protective carbon coating preferably consists of a mixture of graphite such as KS6 (20-25 weight percent), an SEBS block copolymer such as Kraton G1650 (15-18 weight percent) and toluene (56-62 weight percent). Alternatively, the protective conductive carbon coating formulation can utilize carbon black (5-10 weight percent) with Kraton G1650 (15-18 weight percent) and toluene (72-75 weight percent). Other block co-polymers in the Kraton line are also suitable as binders for this protective coating ink, including styrene-butadiene -styrene materials.

Cells were evaluated using anode current collectors and cathode current collectors of silver ink with a printed protective conductive carbon ink. A silver ink was applied to the sealing surface of the flexible laminate packaging material such as described above and was cured at 70 °C for one to two hours to a thickness of around 30 to 40 microns. The protective coating ink formulation was 18 weight percent Kraton 1650, 22 weight percent KS6 and 60 weight percent toluene, and was stenciled onto the silver and cured to a thickness of about 100 to 120 µm. The entire exposed surface of the silver ink was covered. Zinc anode inks as described in Table I and electrolytic manganese dioxide cathode inks as described in Table II were then stenciled onto these protected silver collectors. The cells were assembled with a resulting interfacial surface area of 39 millimeters X 37 millimeters using a suitable separator and a 28 weight percent ammonium chloride + 12 weight percent zinc chloride electrolyte and the housing was heat sealed. TABLE I (ANODE DRY FORMULATIONS):

		BINDER (weight percent)
FORMULATION #1	99.0	1.0 methyl cellulose
FORMULATION #2	98.0	2.0 PVDF

TABLE I (ANODE DRY FORMULATIONS):

seived through a 270 mesh (10.053 mm) screen, 500 ppm leaded zinc

TABLE II (CATHODE DRY FORMULATIONS):

	MnO2 (weight percent)	BINDER (weight percent)
(FORMULATION #1)	90	2.0 methyl cellulose	8.0
(FORMULATION #2	90	2.0 Carbopol 940	8.0

TABLE II (CATHODE DRY FORMULATIONS):

KS6

The cells were cathode limited and had stable open circuit voltages as demonstrated in Table III below and discharged at 10 mA continuous to a cutoff voltage of .9 volts with about 35 to 40 percent cathode efficiency. TABLE III (STABILITY TESTS):

	OCV (volts)	OCV (volts)	OCV (volts)
	1 day	32 days	62 days
Cell 1	1.750	1.681	1.665
Cell 2	1.755	1.674	1.650
Cell 3	1.766	1.673	1.653

Once an appropriate collector is printed onto the substrate, the cathode ink is then printed onto the printed current collector. The cathode ink formulation is a mixture of EMD, binder and conductor in an aqueous or a non-aqueous solvent. The EMD powder utilized will depend on the targeted electrode thickness, desired discharge efficiency and intended application for the cell. Non-milled EMD with a d(50) of around 40 µm is unsuitable for a printed cathode with a targeted thickness of 50 µm or 40 µm is unsuitable for a printed cathode with a targeted thickness of 50 µm, or less. EMD with ad(50) measurement of around 1 µm can be obtained by jet-milling the EMD. Such a process is available from, for example, Sturtevant, Inc. in Hanover Massachusetts.

However the relatively poor rate capability of jet-milled EMD can require an excessively large electrode area or thickness or both for a given application. We have discovered that for a given cathode ink formulation and thickness and a desired discharge current there is a relationship between the amount of graphite used in the ink formulation, the discharge efficiency of the electrode and the required electrode area. Thus, for example, if we were to target a 50 µm thick electrode using an aqueous cathode ink comprising jet-milled EMD and a PVP binder in a cell with a targeted discharge current of 8 mA, a graphite content of between 12 weight percent and 49 weight percent (dry formula) results in a printed cathode with an optimum area and discharge efficiency. Where concerns of electrode area dominate discharge efficiency concerns, a graphite content of from about 19 weight percent to 35 weight percent (dry formula) should be utilized. See Fig. 15 (data predicted by model developed from actual cells). The preferred conductive graphite is KS6 synthetic graphite as is available from Timeal America, product designation Timrex LB 1099. For the same cell thickness using a nonaqueous cathode ink formulation with a PVDF binder and the same targeted discharge current, a graphite content of from about 12 weight percent to 70 weight percent (dry formula) is preferred. Where concerns of electrode area dominate discharge efficiency concerns, a graphite content of from, about 28 weight percent to about 49 weight percent should be utilized. See Fig. 16 (data predicted by model developed from actual cells).

A preferred nonaqueous wet cathode ink formulation is 1.0 to 2.0 weight percent PVDF, 4.0 to 45.0 weight percent graphite and 17.0 to 66.0 weight percent EMD and 28.0 to 37.0 weight percent NMP solvent An even more preferred formulation is 1.0 to 2.0 weight percent PVDF, 12.0 to 31.0 weight percent graphite and 31.0 to 51.0 weight percent EMD and 34.0 to 35.0 weight percent NMP solvent. A preferred aqueous wet cathode ink formulation is 1.0 to 4.0 weight percent PVP, 6.0 to 25.0 weight percent graphite and 25.0 to 43.0 weight percent EMD, balanced with water. Even more preferred is 1.5 to 2.0 weight percent PVP, 11.0-16.0 weight percent graphite and 33.0 to 38.0 weight percent EMD, balanced with water.

The cathode ink is prepared by pre-dissolving the binder in water, grinding the solid components together (EMD and conductive additive) and adding the solids to the binder solution. The mixture is stirred and then is printed onto the existing current collector. The cathode is then cured at a slightly elevated temperature for a time sufficient to dry the ink and drive off the solvents.

SEPARATOR AND ELECTROLYTE

For co-facial electrode assemblies, a separator is necessary to electrically isolate the electrodes while still enabling the flow of ions, as is known in the art. The separator can be a paper separator, a gelled separator or a printed separator. In a carbon zinc embodiment of the invention using an electrode assembly with a co-facial arrangement, a coated kraft paper separator can be utilized as a separator. As an example, a suitable separator base paper is available commercially from Munksjo #300542 (57 g/m²) and is preferably coated to a level of 20 grams per square meter (gsm) (dry) with a mixture having a dry coating composition of starch (preferably 83.6 weight percent, commercially available from, for example, Roquette LAB2469), gel (preferably 7.9 weight percent, commercially available from, for example, Courtaulds B1209), PVP (preferably 2.1 weight percent), and surfactant additive (preferably 1.4 weight percent ethyl tallow amine known commercially as Crodamet) and water (5.0 weight percent). Appropriate coated kraft paper separators are described, for example, in EP 0832502 B1 , WO 96/38869 , WO 98/07204 , US Pat. No. 6221532 and WO 99/35700 . Other suitable separator materials can be used in cells according to the invention.

For a carbon zinc cell according to the invention, the electrolyte is preferably an aqueous solution of zinc chloride, as is known in the art. Additives to prevent or reduce gassing and to encourage other performance attributes can be used, such as cetyltrimethylammonium bromide (available commercially as Cetrimide) and lead chloride. Cetyltrimethylammonium bromide is available from Aldrich, product number 855820. Cetrimide can also be introduced into the cell in a variety of ways, such as in the electrode ink formulations or as a component of a separate coating printed or otherwise applied to an electrode or separator paper surface.

We have further discovered an alternative gelled electrolyte for use in a carbon zinc printed cell of the invention that is particularly beneficial in reducing the internal resistance of cells having coplanar electrodes. We have discovered that the addition of nonionic or anionic derivatives with natural guar gum to an aqueous zinc chloride solution produces such a gelled electrolyte. The preferred additive is Galactasol A4 available commercially from Aqualon Company in Wilmington Delaware.

We have further discovered that a low molecular weight polyethylene glycol (PEG) based polymer dissolved in a zinc chloride solution and crosslinked via UV exposure leads to a gelled material that can be printed directly onto a printed electrode in a carbon zinc cell according to the invention. The preferred polymer is a polyethylene glycol diacrylate as is available from, for example, Sartomer Company, Exton PA. The PEG diacrylate material of the within invention has the following structure: where n is greater than 3 and less than 100. The preferred molecular weight range of the PEG diacrylate material is greater than 300 and less than 4500 and still more preferably has a molecular weight of between 700 and 800. In one gel formulation according to the within invention, SR610 available from Sartomer, with a molecular weight of 742, is used. The gel formulation preferably further includes a photoinitiator, a viscosifier and a surfactant in the following weight percent ranges: PEG diacrylate - 5.0 to 25.0 percent, polymer binder viscosifier -- 1.0 to 10.0 percent; photoinitiator -- .10 to 2.0 percent; surfactant -- .01 to 2.0 percent, combined in a 28.0 weight percent zinc chloride solution..

High molecular weight (600,000 Daltons) PEO was added to a PEG diacrylate/ZnCl₂ aqueous solution according to the following formulation where the solvent was 28wt.% ZnCl₂ (wt. % listed below)

• 10% SR610 PEG diacrylate
• 6% 600,000MW PEO
• 0.5% Irgacure 184
• 0.1 % Triton QS 44
• 83% electrolyte (28% ZnCl2)

This formulation was mixed by slow rotation to avoid degradation of the high MW PEO with high shear mixing. The solution viscosity of this formulation at 5rpm was 10.8 Pa·s (10,800cP). After curing in air with UV black light for 15 seconds, the ionic conductivity of the film in 28% ZnCl₂ was 30mS/cm, compared to 35-40mS/cm for the traditional carbon zinc separator paper soaked in ZnCl₂. The electrolyte content of the traditional separator paper upon equilibrium was 75%.

Uncured solution was placed on a thin printed cathode provided via stencil printing. The solution was cured as above and the cell impedance of a co-facial cell with this separator was 48mΩ. This value is in line with cells made with the traditional carbon zinc coated separator paper.

An alternate formulation improves the screen printing characteristics of the resulting solution by using a viscosifier with a low extensional viscosity, so that the solution will break cleanly away from the screen. Replacing the polymer binder with a non-polymeric thixotropic gelant such as fumed silica produces such solution and obviates the need for a surfactant. The preferred formulation in weight percent for this low extensional viscosity formula is: PEG diacrylate -- 3.0 to 25.0 percent, fumed silica viscosifier - 1.0 to 10.0 percent; photoinitiator -- .10 to 2.0 percent, combined in a 28.0 percent zinc chloride solution. When fumed silica is added to 28% ZnCl₂ at 5% by weight, a thick gel is formed that has very good screen printing "break away" properties. The formulation below (in 28% ZnCl₂) produced films after 15 seconds of UV curing with black light in air.

• 5% Aerosil 200VS fumed silica
• 10% SR 610 PEG diacrylate
• 0.5% Irgacure 184
• 84% electrolyte (28% ZnCl2)

The ionic conductivity of these films was 105mS/cm. The fumed silica requires high energy mixing to gel due to the compacted nature of the VS Aerosil.

Fig. 14 shows a comparison of the thixotropic behavior of the PEO and fumed silica gelled polymer electrolytes. The fumed silica is a much better thixotrope than PEO. The viscosity of the fumed silica shows a much larger dependence on shear rate than does PEO. This behavior results in the clean breakaway properties of the fumed silica formulation, advantageous for screen printing. Further, a surfactant was not required with the fumed silica formulation.

We have discovered that alternative electrolyte solutions can also be utilized with printed zinc anodes and printed EMD cathodes that will enable more robust and conductive cathode current collectors to be utilized. For example, a 1.4 to 3.0 molar concentrated solution of zinc acetate with a pH of about 6.5 to 7 can be utilized as an electrolyte according to the invention. In this pH range, silver will not react with the manganese dioxide cathode, enabling the use of a silver ink as a cathode current collector and an anode current collector. Silver is a very conductive metal and therefore highly desirable as a current collector material. Such a silver ink is available from, for example, Ercon, Inc., Waltham, Massachusetts, product designation E1660-136. While zinc acetate is the preferred acetate, ammonium acetate may also be used as an electrolyte in this system. When using a zinc acetate or ammonium acetate electrolyte, the preferred EMD cathode ink binder is polyvinylidene fluoride (PVDF) such as is available from Kureha product number 1100, to enhance the integrity of the cathode in the electrolyte.

CELL CONTACTS

Cell contacts present a design challenge for a number of reasons. It is desirable to be able to select the external contact materials without regard to the potential for unfavorable reactions between the external contacts and the materials utilized within the cell (such as electrolyte). Constraints imposed by the internal cell environment can interfere with the development of external tabs with the desired strength and current carrying properties. Further, leakage of electrolyte can be a problem with cells using aqueous electrolytes and metal structures for carrying current from the electrode to the external cell terminal. This leakage is a result at least in part of the propensity of electrolyte to travel, or "creep" along the metal surface of a current carrying structure that extends from the interior of the cell housing or package and through the sealed cell perimeter out to the external cell environment. We have discovered that an advantageous cell design employs a "discontinuous" current carrying system. As used herein, a "discontinuous" current carrying system exists where two distinct structures are employed for the purpose of carrying current between an electrode and the external cell terminal. One structure, referred to herein as a current collector, extends from the interior of the cell into the seal area and has a terminal end within the seal area or at the seal outer perimeter. A second external structure, referred to herein as the external terminal, extends from the external environment into the seal area, or contacts a conductive adhesive or epoxy that is positioned within the seal area. A conductive bridge, formed of direct contact between the two structures, or a conductive adhesive or epoxy that extends between the two structures, provides a pathway for current flow within the seal perimeter area. In this way, the cells of the within invention do not have a single metallic pathway for electrolyte creepage. The "seal area" as used herein includes the area of the cell packaging or housing material that is joined together using a pressure seal or heat seal or epoxy or other means of joining two sections together.

In one embodiment depicted in Fig. 2, the current collector 13 for an electrode extends into the seal area 15, while a second metallic external terminal 17 extends into the sealing area and contacts the current collector 13 within the sealing area. In this embodiment, electrical conductivity for current flow is provided by the physical contact between the internal current collector and the external terminal. In a second embodiment depicted in Fig. 3, the current collector 13 and the external terminal 17 are not in physical contact. Electrical conductivity is provided by an electrically conductive adhesive or epoxy 19 located at least in part within the seal area 15 and bridging the two structures. In a third embodiment depicted in Fig. 4, the conductive adhesive or epoxy 19 extends to the area external to the cell and forms the external cell contact. In Fig. 5, the adhesive or epoxy 19 extends to the area external to the cell and contacts an external metallic tab or terminal 21. Figures 6A-6C illustrate further alternate embodiments of the external cell contact or terminal 17. In these embodiments, at least one of the external contacts has an increased surface area external to the cell packaging or housing. We have found that increasing the external contact surface area improves the discharge efficiency of the cell.

The anode and cathode external terminals or contacts are preferably printed onto a flexible nonconductive polymer substrate with a silver based conductive polymer ink such as Electrodag 479SS available from Acheson Colloids, Port Huron, Michigan. The cathode collector is then printed onto the external cathode contact so that the collector and the external contact overlap in at least the seal area of the cell package or container. In the same manner, the anode ink is printed onto the external anode contact so that the anode and the external contact overlap in at least the seal area of the cell package or container.

At least a portion of the seal area includes an adhesive or epoxy for joining together two surfaces of packaging material to form the cell package or housing. The adhesive can be activated by heat or pressure or other means as is known in the art. Alternatively, the seal area can compromise an epoxy that forms a seal by a polymerization reaction initiated chemically, thermally or using photoinitiation or encapsulation as is known in the art. Use of a two part conductive epoxy can accommodate delays in manufacturing by avoiding epoxy curing during the delay.

CELL ASSEMBLY AND PACKAGING

Initially, the external tabs for the electrodes are printed onto a flexible nonconductive polymer substrate that preferably forms the battery package. The zinc ink is formulated and applied directly to the substrate surface. The shape of the electrode is selected according to the cell design for the given application, as is known in the art. In a carbon zinc cell of the invention, the zinc ink is printed onto the substrate using a silk screen, stencil or other suitable printing apparatus with a pattern that allows the ink to form an area that will interface with a cathode, and an area that will overlap a portion of the tab in the area that will be sealed to form the package. A suitable drying and/or curing protocol is engaged, depending on the ink formulation.

The ink for the cathode current collector is formulated and applied to a second section of flexible polymer substrate material, by stencil, screen or other suitable printing apparatus, followed by a suitable drying protocol. The second section of flexible polymer substrate material upon which the cathode current collector is printed may either be a section that allows for a co-planar arrangement between anode and cathode or a section that allows for a co-facial arrangement of anode and cathode. As used herein, "co-facial" electrodes share an interfacial area between a major anode surface and a major cathode surface. Co-facial electrodes are to be distinguished from "co-planar" electrodes, where a major anode assembly (anode + collector, if any) surface and a major cathode assembly (cathode + collector, if any) surface lie approximately in the same plane and are printed directly or indirectly onto a single piece of substrate material. The cathode current collector shape is selected so as to allow for sufficient contact with the cathode ink, and preferably also forms an area that will overlap a portion of the cathode tab in the seal area. The current collector ink is dried and then the cathode ink is printed onto the current collector and dried.

A separator is disposed between the anode and cathode in the case of electrodes in a co-facial arrangement Electrolyte is introduced into the cell by way of separator paper soak up of free electrolyte or by way of a gel formulation that incorporates electrolyte or by way of electrode soak up of free electrolyte, or a combination thereof as is known in the art. The cell package or housing is then sealed together. In a preferred embodiment, the external contacts for the cell are discontinuous, as defined herein.

Although the foregoing discussion of a thin preferably flexible battery focuses on a single anode coupled to a single cathode, the skilled artisan will appreciate that any number of anode and cathode couples, either in parallel or in series or both, can be incorporated within the battery.

Claims (13)

1. A carbon zinc battery (1) comprising a cathode (5), an anode (3), a separator (9), an electrolyte and a printed cathode current collector (7), at least a portion of which is printed onto a flexible nonconductive substrate, wherein the printed cathode current collector (7) comprises:

   a metallic substrate or a metallic ink printed onto the flexible nonconductive substrate; and

   a protective carbon ink printed onto the metallic substrate or metallic ink, wherein said protective carbon ink comprises graphite or carbon black and further comprises a styrene-butadiene-styrene or styrene-ethylene-butylene-styrene block copolymer.
2. The battery (1) of claim 1, wherein the metallic substrate comprises silver foil or aluminum foil.
3. The battery (1) of claim 1, wherein the metallic ink is selected from the group consisting of silver ink and aluminum ink.
4. The battery (1) of claim 1, wherein the metallic ink is printed directly onto the flexible nonconductive substrate.
5. The battery (1) of claim 1, wherein the protective carbon ink is also printed onto the flexible nonconductive substrate.
6. The battery (1) of claim 1, wherein the cathode (5) is a printed cathode comprising a cathode ink.
7. The battery (1) of claim 6, wherein the cathode ink is printed onto the flexible nonconductive substrate and the cathode current collector (7).
8. The battery (1) of claim 6, wherein the cathode ink comprises a mixture of electrolytic manganese dioxide, a binder and a conductor.
9. The battery (1) of claim 1, wherein the anode is selected from the group consisting of a zinc foil, a zinc mesh and a zinc ink.
10. The battery (1) of claim 1, wherein the separator (9) is a paper separator, a gelled separator or a printed separator, preferably wherein the separator (9) is a gelled separator comprising a gelled electrolyte, and the gelled electrolyte comprises a gellant selected from the group consisting of a polyethylene glycol based polymer, nonionic or anionic derivatives with natural guar gum, a copolymer comprising acrylic or methacrylic acid and styrene sulphonate, and non-polymeric thixotropic gellant.
11. The battery (1) of claim 1, wherein the cell comprises a material selected from the group consisting of cetyltrimethylammonium bromide and a polymer having a phosphate group and chains comprising polyethylene oxide, polypropylene oxide or polyethylene oxide and polypropylene oxide.
12. The battery (1) of claim 1, wherein the electrolyte is an acidic electrolyte comprising zinc chloride or ammonium chloride.
13. The battery (1) of claim 1, wherein the cathode and the anode are coplanar electrodes.