Patent Application
20060210877
This invention generally relates to electrochemical battery cells, with a negative electrode using metal in particulate form. More particularly the invention pertains to zinc electrodes in primary alkaline, secondary nickel/zinc, and secondary silver/zinc cells.
Small electrochemical cells are used by consumers to power a variety of devices including cameras, flashlights, toys, radios, timepieces, calculators, and other electronic devices. There is demand in the marketplace for both low-cost consumable electrochemical cells, such as primary alkaline cells which may be used, for example, in one-time use cameras, and secondary cells which may be recharged and reused.
Alkaline cells used in the consumer marketplace typically comprise a cylindrical cathode and a gelled anode inside the cylindrical cathode that includes zinc particles and an aqueous electrolyte absorbed by the gel dispensed on a current collector. Such a configuration is often referred to as a bobbin configuration or a bobbin cell. Alkaline cells comprising gelled anodes can be manufactured at a low cost relative to other battery types, are widely available and provide a low-cost and convenient energy source for many applications. While having these and other advantages, alkaline cells comprising gelled anodes have disadvantages. For example, zinc from gelled anodes can easily migrate within the battery cell, and migration of zinc species to the cathode can decrease the active life of the cell. The energy output of the cell is also limited by the anode to cathode interfacial surface area, which in the bobbin configuration is less than the external surface area of the cylinder and determined by the zinc content and microporosity of the gel. In addition, gelled anodes are typically formed within the cell during manufacture of the cell, rather than pre-manufactured and stored for future insertion in a cell. In the latter case such anodes would likely have a relatively short shelf life.
Pasted anodes can be mass-produced at a relatively low cost and stored for later inclusion in a manufactured cell. U.S. Pat. Nos. 6,207,326 (Kawakami, et al.); 5,888,666 (Kawakami); 5,837,402 (Araki, et al.); 5,728,482 (Kawakami, et al.); and U.S. application Ser. No. 2002/0164530 disclose a pasted zinc anode comprising zinc, zinc powder, and a binder rolled onto a current collector used in a secondary cell. However, pasted zinc anodes as currently used in the art also have disadvantages. Pasted anodes are typically manufactured in the discharged state with zinc in the form of Zn2+ (such as in zinc oxide (ZnO)) rather than in the charged state (as Zn0). Cells with pasted anodes manufactured in the discharged state must be charged after cell assembly and before use; thus pasted anodes are limited to secondary cells. Pasted anodes as currently known in the art are also rigid, which limits the configuration of the anode within the cell.
The invention seeks to provide a zinc anode that may be mass-produced prior to cell construction, that is appropriate for use in a primary cell, and that can be formed into various geometries in a cell. In accordance with the invention, this object is accomplished in a flexible pasted zinc anode comprising (a) a flexible current collector, and (b) a paste comprising (i) zinc particles and (ii) at least one block copolymer binder, wherein said flexible current collector and said paste form a unit. The invention also seeks to provide a primary cell with a higher discharge capacity than traditional gelled anodes. In accordance with the invention, this object is accomplished in a primary cell comprising (1) a flexible pasted zinc anode comprising (a) a flexible current collector, and (b) a paste comprising (i) zinc particles and (ii) at least one block copolymer binder, wherein said flexible current collector and said paste form a unit, (2) a cathode, and (3) a liquid electrolyte. The invention also seeks to provide a method of manufacturing cells and anodes as described above.
The present invention now will be described more fully hereinafter with reference to the accompanying drawing, in which one, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements.
Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
1. Paste
Anode pastes according to the invention comprise zinc particles and at least one block copolymer, optionally a gelling agent, and optionally zinc oxide. Zinc particles according to the invention include zinc granules, fibers, powders, pellets, flakes, and other suitably small solid forms of zinc. As with many other types of small solids, the size of zinc particles according to the invention may vary and may include a distribution of multiple sizes within a range. The sizes of the zinc particles may include any of the distributions of zinc particles as used in gelled or other pasted zinc electrodes. Preferred average particle diameter of the zinc particle distribution is 100-160 μm, particularly 130-150 μm. The zinc particles may also include additives including, but not limited to, bismuth, indium, aluminum, lead, and iron. An example of an acceptable zinc particle distribution with additives according to the invention is BIA 100 300 100 d140 zinc powder as marketed by Umicore.
Block copolymer binders according to the invention may include any block copolymer that is compatible with zinc, the electrolyte solution, and other components of the cell and that also will maintain flexibility without cracking when bent. As such, the block copolymers preferably have elastomeric qualities. Preferred elastomeric block copolymers include styrenic copolymers. Particularly preferred are styrene-ethylene/butylene-styrene (SEBS) copolymers, such as those manufactured by Kraton Polymers as Kraton™ G series polymers, including Kraton G 1651 and G 1901.
Other additives or auxiliaries may optionally be added to the solid phase of the anode paste according to the invention. A gelling agent may be added to increase the absorbency of the electrolyte by the anode. Examples of common gelling agents include crosslinked acrylic acid carbomers (such as Carbopol™ 940), polyethylene oxide, polyacrylic acid, and various forms of cellulose. Gelling agents according to the invention must be compatible with zinc, the electrolyte solution, the block copolymer, and other components of the anode and electrolyte solution during cell use. Preferred gelling agents are crosslinked acrylic acid carbomers (such as Carbopol™ 940).
The anode paste according to the invention may also include zinc oxide. Zinc oxide is a reaction product of zinc with hydroxide solutions, and is included in an anode paste and electrolyte solution to maintain an equilibrium of zinc oxide and potassium hydroxide in the cell to prevent zinc depletion through the formation of zinc oxide. For anodes used in primary cells according to the invention, the preferred quantity of zinc oxide is less than 5% of the dry weight of the anode paste, particularly preferably 0.5 to 2.5% of the dry weight, most preferably about 0.5% by weight. The preferred ratio of zinc metal (Zn0) to zinc oxide (ZnO) is preferably from 35:1 to 220:1 by weight, most preferably about 190:1 to 220:1 by weight.
In addition to the above-mentioned components, the anode paste at the time of application to the current collector may comprise at least one solvent. The solvent may be used to obtain a paste-like consistency with the dry ingredients and may be used to lower the viscosity to ease application of the paste. The solvent is selected to be compatible with the other paste components and to promote defect-free and uniform drying of the anode. Organic solvents, particularly petroleum distillates such as Stoddard solvent or other aliphatic or aromatic hydrocarbons may be used, and such solvents are readily available. Mixtures of different organic solvents may also be used. Preferred solvents include Stoddard solvent and VM&P naphtha. After application to the current collector, the majority of the solvent is removed from the paste via an evaporation process. However, some residual solvent may remain in the paste following the evaporation process.
A typical paste at the time of application to a current collector comprises 75-80% zinc (Zn0) particles, 0-0.5% zinc oxide, 10-20% solvent, 0.2-2.5% block copolymer binders, and up to 5% gelling agents. The viscosity range of the paste at the time of application is preferably 25,000-45,000 cps.
2. Anode
Pasted zinc anodes according to the current invention comprise anode paste described herein and a flexible current collector. Materials for current collectors for anodes according to the present invention may include any material that is electrochemically conductive, that is flexible, and that is not electrochemically reactive with zinc and reduces hydrogen gassing in an alkaline. Suitable materials may include tin plated steel, copper, or brass. The current collector may be in a form suitable for applying a paste, including but not limited to screen or mesh, perforated metal, and expanded metal (such as that available under the trade name Exmet®). The paste may be applied to or “pasted” with the current collector to form a pasted zinc anode. Following the pasting process, the pasted zinc anode should form a unit that can be deformed without separation of the paste from the current collector.
Pasted zinc anodes according to the invention may be produced in batch, continuous, or semi-continuous processes. One preferred process comprises combining dry paste ingredients including zinc particles, optionally zinc oxide particles, optionally one or more gelling agents, and optionally one or more auxiliaries to form a dry particulate mixture. An elastomeric block copolymer and a solvent are combined to form a solution. The solution may be heated to reach a desired viscosity, then the dry particulate mixture is added to the solution to form a zinc anode paste. Heating after addition of dry particles is alternatively attempted.
A particularly preferred process comprises combining zinc powder and up to 2.5% zinc oxide (based on the combined weight of the zinc powder and zinc oxide) to thoroughly distribute the zinc oxide in the zinc powder to form a dry zinc mixture. In a separate container, SEBS block copolymer (about 2.5% by weight, based on the total weight of the polymer solution) and Stoddard solvent are combined and heated to 40-50° C. to dissolve the polymer and form a polymer solution. The dry zinc mixture and polymer solution is combined at a ratio of about 5 parts dry zinc mixture to 1 part polymer solution to form a viscous slurry/paste, and additional Stoddard solvent (about 0.25 parts) is added to reduce the viscosity and form a paste for application to the current collector.
After manufacture, the pasted strips may be cut into anodes and immediately fabricated into batteries. Alternately, the pasted strips may be cut into anode portions and stored for inclusion in cells to be manufactured at a later date. Prior to storage, the pasted anodes may be wrapped in a separator material, such as flexible nonwoven separator material made from a polyolefin, such as nonwovens (such as FS 2203) manufactured under the trade name Viledon® by Freudenberg Nonwovens, or other suitable separator materials, such as separators manufactured by Advanced Membrane Systems, Inc under the trade name FAS™, or the like.
3. Process for anode manufacture
One process for manufacturing flexible zinc anode according to the invention is illustrated in
The resultant paste is deposited to a current collector, preferably by delivering the paste at a constant volumetric flow rate to a coating or extruding device including coating dies, roll coaters, and doctor blades. The current collector is preferably a continuous roll of perforated brass foil. The thickness of the coated sheet anode web may be adjusted using settings on the coating device or by using shims on the device. After deposition of the paste, the thickness of the anode may be further adjusted by calendaring, a doctor blade, or other suitable apparatus. The preferred thickness of the pasted sheet anode is 0.5 mm to 2.5 mm, with the paste evenly distributed on each side of the current collector.
After deposition of the anode paste, solvent is driven from the sheet anode. Methods for removal of solvent include but are not limited to passive air drying, forced air ovens, and infrared ovens. After the solvent removal process, selected or residual amounts of solvent may remain in the sheet anode. After solvent removal, the thickness of the pasted anodes is further adjusted by calendaring or other processes.
A second process for manufacturing flexible zinc anode according to the invention is illustrated in
The sheet anode may then be cut to size to fit cells as desired. The sheet anode is flexible and may then be formed or folded into various configurations, such as spiral, prismatic, arcuate, single fold, partial fold and multiple fold configurations.
4. Cells
Primary cells according to the present invention comprise pasted zinc anodes as described herein, a cathode, and an electrolyte. Suitable cathodes for a primary alkaline cell include various conventional types. Aqueous potassium hydroxide is the preferred electrolyte, although other known electrolytes may be used. In an alkaline battery using a zinc anode and potassium hydroxide electrolyte according to the present invention, the following reaction occurs at the anode:
Zn(s)+2 OH−(aq)→Zn(OH)2(s)+2e−
For primary cells according to the present invention comprising a manganese dioxide cathode, the following reaction occurs at the cathode:
2 MnO2(s)+H2O(l)+2e−→Mn2O3(s)+2 OH−(aq)
Cells according to the present invention may include, rigid, flexible, or deposited (filled) cathodes.
As shown in the embodiment depicted in
As shown in the embodiment depicted in
The following ingredients were dry blended in a beaker:
A mixture of SEBS block copolymer (5.2 g of Kraton G1654x) and solvent naphtha (102 g of Shell Sol 340 HT) was heated until the mixture liquefied. 45 g of this liquefied solution was added to 297 g of the dry mixture above, and the mixture paste was kept hot on a hotplate. A sheet of polytetrafluoroethylene (PTFE) was placed on a piece of polyvinylchloride (PVC), and a strip of tin-coated substrate measuring 6 inches by 3 inches was placed over the PTFE. Some of the mixture paste was poured onto the tin-coated substrate and covered with another sheet of PTFE to form a “sandwich.” The mixture paste was spread over the substrate on the one side of the sandwich with a rolling pin, and the sandwich was flipped, material was added to the other side of the tin substrate and rolled with a rolling pin. The sandwich was covered with 0.040 inch shims and passed through a roll mill twice, rotating the sandwich each time. The PTFE sheets were removed and excess paste was removed from the edges of the substrate. The pasted substrate was dried on a screen overnight. The pasted substrate was then cut into four electrodes, each with a paste weight of approximately 4.6 to 4.7 grams and a thickness of approximately 0.040 inch.
The following ingredients were dry blended in a beaker to form a dry mixture:
10 grams of 1% SEBS copolymer in solvent naphtha was added to the dry mixture under heat to form a paste, and the paste was rolled out onto a current collector as described in Example 1. The pasted substrate was dried and cut into 4 electrodes.
A base solvent mixture of 2.51 grams (2.5%) SEBS copolymer and 98.34 grams (97.5%) Stoddard solvent were combined and heated to 45° C. in a water bath until the copolymer dissolved in the solvent. In a separate container, 250 grams of a commercial zinc powder (d50=140 μm) and 1.25 grams of zinc oxide powder were combined to form a zinc mixture. An aliquot of 50.91 grams of the base solvent mixture was removed and added to a clean container, and the zinc mixture was added to the solvent mixture and stirred to combine. An additional 12.45 grams of Stoddard solvent was added to the mixture, forming a paste. The paste was then cooled to approximately 35° C.
A vertical coating die was set up so that a perforated metal strip current collector substrate could be pulled through the die. The die opening was set to approximately 0.075 inches. The die was then heated to maintain a temperature of approximately 30° C. The coating die was then filled with the paste. The substrate was pulled through the die to coat the paste onto each side of the current collector substrate. After the current collector substrate was coated with the paste, it was air dried to evaporate the solvent. After drying, the resulting pasted strip was approximately 0.16 mm thick with a porosity of approximately 16%.
The pasted strip was cut into electrodes that were 31 mm by 41 mm. The electrodes had a non-coated strip approximately 2 mm wide at one end of the electrode. The electrodes were then wrapped with two layers of a flexible separator material with the inner layer made from a nonwoven polyolefin material and the outer layer made from a microporous membrane material.
Several AA zinc-manganese dioxide cells were fabricated using the electrodes in the configuration as described by
Test Method for Cells:
Results from the testing of these cells are shown in
This application claims priority to U.S. provisional application Ser. No. 60/662,085, filed Oct. 25, 2004, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60662085 | Mar 2005 | US |
