Patent Application
20110086252
This invention relates generally to rechargeable batteries and specifically to rechargeable nickel-zinc batteries. More specifically, the invention relates to improved rechargeable nickel-zinc batteries and methods of manufacture.
The popularity of cordless portable devices, such as power tools, has increased the needs and requirements for high energy density rechargeable batteries that can also deliver high power. As power and energy density requirements increase, the need for a high cycle life rechargeable electrodes also increases. The alkaline zinc electrode is known for its high voltage, low equivalent weight and low cost. The fast electrochemical kinetics associated with the charge and discharge process enables the zinc electrode to deliver both high power and high energy density. Nickel-zinc batteries can satisfy the need for higher power and higher energy density in e.g. batteries, suitable for electric vehicles (EV), plug-in hybrid electric vehicles (PHEV), consumer electronics and other applications.
Particularly important is not only the demand for higher power and higher energy density rechargeable batteries but also methods of manufacturing them more simply and at the same time making improvements in the battery performance. Sealed cylindrical nickel zinc cells have been proposed with both negative polarity cans and positive polarity cans, depending on consumer need. Each configuration has disadvantages that can result in significant reductions in the service life of the cell.
Conventional rechargeable alkaline batteries, including nickel-metal hydride and nickel cadmium batteries, have negative cans and positive caps. Cylindrical nickel-zinc cells may be beneficially designed with polarities reverse of that of conventional alkaline batteries. In the reverse polarity design, the battery vent cap is the negative terminal and the cylindrical case or can is the battery positive terminal. The reverse polarity design provides low impedance and low hydrogen evolution at the negative terminal. When employed in electricity powered portable devices, such as power tools, the reverse polarity design does not affect the consumer because the rechargeable battery may be built into the device or be separately wrapped or encased. However, when a reverse polarity cell is individually supplied, a consumer may possibly mishandle the cell by, e.g., incorrectly inserting it into an electronic device, and thereby damaging the cell or device. Further, there is a tendency for negative polarity vent caps to be more leakage prone due to a phenomenon known as electrolyte creep.
Unfortunately, negative polarity cans in contact with the rechargeable zinc electrode may promote excessive hydrogen evolution if the metal can surface is not protected by either impervious polymer coatings or by plating of the internal surface with metals, alloys or materials that exhibit high hydrogen evolution over-potentials.
Described are rechargeable nickel zinc cells, and methods of making them, of a configuration that utilizes a positive can with a vent cap at the positive pole of the battery. These improved cells, and methods of making them, take advantage of the stability imparted by a positive battery can, and avoid disadvantages associated with vent caps on the negative pole of the battery. Positive polarity cans, such as nickel plated steel cans, are relatively stable at the charge voltages of the alkaline nickel electrode. Even in the presence of poorly plated steel, the insolubility of the iron oxidation products limits the contamination of the zinc negative electrode such that gassing is maintained at a minimal level.
One aspect is a rechargeable nickel zinc cell, which in certain embodiments may include the following: i) a jellyroll electrode assembly including a nickel positive electrode, a zinc negative electrode, and at least one separator layer disposed between the nickel positive electrode and the zinc negative electrode; ii) a can in electrical communication with the nickel positive electrode, the can including an aperture at the base of the can; iii) a vent cap affixed to the base of the can and in electrical communication with the can, the vent cap configured to vent gas from the rechargeable nickel zinc cell via the aperture; and iv) a negative collector disc in electrical communication with the zinc negative electrode and electrically isolated from the can, the negative collector disc configured as a closure to the open end of the can.
Rechargeable nickel zinc cells as described may have nickel plated steel cans as well as negative current collector discs that are coated with a hydrogen evolution resistant material such as a metal, an alloy and/or a polymer. Electrical communication between the jellyroll and the negative current collector disk can be made via a welded metal tab, direct contact between the negative collector disc and the negative electrode or via a conductive spring configured to make contact with both the zinc negative electrode and the negative collector disc, the conductive spring compressed between the end of the jellyroll and the negative collector disc.
In some embodiments, a positive collector disc is interposed between, and in electrical communication with, the nickel positive electrode and the base of the can. In such embodiments, the base of the can may be punctured to form an aperture as described herein, before insertion of the positive collector disk or after. In one embodiment, the base of the can has a preformed aperture for venting as described in more detail below. In certain embodiments, the base of the can is thicker than a conventional can in order to resist bending or warpage from shape changes in the jellyroll. In other embodiments, the can base includes flutes, rings, ridges or other structures to aid in structural rigidity of the can base. The positive collector disk can be a metal foam that conforms to the aforementioned features at the bottom of the can so that undue volume is not sacrificed by the strengthening features.
Specific materials for the electrode substrates as well as formulations for the active materials and electrolyte are described. Rechargeable nickel zinc cells as described may be configured to non-commercial sizes or in some embodiments configured to a commercially available size, for example, AAA, AA, C, D and sub-C.
Other embodiments include methods of making rechargeable nickel zinc cells, including a method of making a rechargeable nickel zinc cell, the method including: i) sealing a jellyroll assembly, including a nickel positive electrode, a zinc negative electrode, and at least one separator layer disposed between said nickel positive electrode and zinc negative electrode, in a can such that the nickel positive electrode is in electrical communication with the base and the body of the can and the zinc negative electrode is in electrical communication with a negative current collecting disc at the other end of the can and electrically isolated from the can; the negative current collecting disc configured as a closure to the open end of the can; ii) puncturing the battery can at the base of the can, thereby making an aperture in the base of the can; and iii) affixing a vent cap at the base of the can, in electrical communication with the nickel positive electrode; the vent cap configured to vent gas from the rechargeable nickel zinc cell via the aperture. In some embodiments, the can has a preformed aperture and operation ii) is not necessary. The order of the process flow does not have to be as described, for example, the can may be punctured prior to sealing the jelly roll in the can. In embodiments where a preformed aperture is used, the vent cap can be affixed to the can prior to sealing the jellyroll in the can.
Methods may also include interposing a positive collector disc between, and in electrical communication with, the nickel positive electrode and the base of the can, prior to sealing the jellyroll in the can. Methods also include introducing an alkaline electrolyte into the can, either prior to sealing the jellyroll in the can or via the aperture in the can after the jellyroll is sealed in the can.
Methods as described are used to make cells as described above and in more detail herein.
These and other features and advantages are further discussed below with reference to the associated drawings.
The invention is most generally described in terms of rechargeable nickel zinc cells and methods of making them. More specifically the invention is described in terms of rechargeable nickel zinc cells of a configuration that utilizes a positive can with a vent cap at the positive pole of the battery.
Below is a brief discussion of nickel zinc battery chemistry as it relates to the invention, followed by more detailed discussion of battery design with focus on specific features of the present invention.
Electrochemical Reactions of Nickel Zinc Batteries
The charging process for a nickel hydroxide positive electrode in an alkaline electrochemical cell is governed by the following reaction:
Ni(OH)2+OH−→NiOOH+H2O+e− (1)
Alkaline electrolyte acts as ion carrier in the Zn electrode. In the rechargeable Zn electrode, the starting active material is the ZnO powder or a mixture of zinc and zinc oxide powder. The ZnO powder dissolves in the KOH solution, as in reaction (2), to form the zincate (Zn(OH)42−) that is reduced to zinc metal during the charging process, as in reaction (3). The reaction at the Zn electrode can be written as follows:
ZnO+2OH−+H2O→Zn(OH)42− (2)
and
Zn(OH)42−+2e−→Zn+4OH− (3)
Therefore, net electrode at the negative is
ZnO+H2O+2e−→Zn+2OH−+2e− (4)
Then, the overall Ni/Zn battery reaction can be expressed as follows:
Zn+2NiOOH+H2O═ZnO+2Ni(OH)2 (5)
In the discharging process of the zinc electrode, the zinc metal donates electrons to form zincate. At the same time, the concentration of the zincate in the KOH solution increases.
Upon recharge, reactions (1)-(5) are repeated. During the life of a nickel zinc battery, these charge-discharge cycles are repeated a number of times.
A more detailed description of nickel zinc batteries of the invention, including description of electrode and components, particularly the embodiments relating to methods of manufacture, follows.
Nickel-Zinc Battery and Battery Components
A portion of flexible gasket 111 rests atop the negative collector disk and a portion also rests on a circumferential bead 115 provided along the perimeter in the upper portion of can 113, proximate to the cap 109. The gasket 111 serves to electrically isolate negative collector disc 103 from can 113.
After the can or other containment vessel is filled with electrolyte, the vessel is sealed to isolate the electrodes and electrolyte from the environment typically by a crimping process using the portion of the can above bead 115 and crimping that annular portion of can 113 inward and over the top portion of gasket 111 and a circumferential portion of negative collector disc 103, sealing the can shut.
Battery can 113 is the vessel serving as the outer housing or casing of the final cell. In conventional cells, where the can is the negative terminal, it is typically nickel-plated steel. In conventional cells, the can may be either the negative or positive terminal. When the can is positive, the vent cap is on the negative pole; when the can is negative, the vent cap is on the positive pole, i.e. a normal polarity cell. That is, in conventional cells the vent cap is part of the component that seals the open end of the can.
The present invention utilizes a positive can and a venting cap at the positive pole, thus a normal polarity cell with a positive can. An aperture in the base of the can is sufficiently aligned with an aperture in a vent cap that is attached to the base of the can. This configuration maintains the vent on the positive terminal for maximum resistance to electrolyte creep which is more prevalent at the negative pole. As mentioned, the jelly roll is inserted into the can and the negative terminal of the cell is connected to a current collector disc that is crimped, with an intervening gasket to electrically isolate the disc from the can, during cell closure. The disc can be readily plated or coated with materials that inhibit the evolution of hydrogen without the difficulty that is associated with the uniform plating of the can interior with such materials. This cell configuration and methods of manufacture thereof, provides at least the following advantages: 1) less tendency for the negative electrode to gas as a result of less surface area contact with plated materials such as the can interior, 2) less tendency for the electrolyte to leak through the vent via a creepage mechanism by locating the vent on the positive terminal, 3) there is no need to plate the can interior with hydrogen inhibiting materials, 4) there is no need to use polymeric sleeves around the jellyroll, 5) vent operation will be more reproducible because the vent assembly is not subject to the stress of the crimping operation, 6) cost savings due to less materials used (as explained in more detail below), and 7) cost savings due to simpler design and thus manufacturing demands are less.
One aspect is a rechargeable nickel zinc cell, including, i) a jellyroll electrode assembly including a nickel positive electrode, a zinc negative electrode, and at least one separator layer disposed between the nickel positive electrode and the zinc negative electrode; ii) a can in electrical communication with the nickel positive electrode, the can including an aperture at the base of the can; iii) a vent cap affixed to the base of the can and in electrical communication with the can, the vent cap configured to vent gas from the rechargeable nickel zinc cell via the aperture; and iv) a negative collector disc in electrical communication with the zinc negative electrode and electrically isolated from the can, the negative collector disc configured as a closure to the open end of the can. More detailed description of jellyroll assemblies suitable for the invention are described below in the section devoted to jellyroll description.
In this application, “can” refers to a battery can, generally but not necessarily, a metal can, e.g. steel or stainless steel. Typically, but not necessarily, the can is plated with nickel. Other cans would suffice, e.g., a polymer based can that is coated with an electrically conductive material would be appropriate in some embodiments of the invention. Also, the term “base of the can” refers to the closed end (or vented end when it includes an aperture) or the battery can's “bottom” (although the invention is not limited to any such orientational constraints). In one embodiment, the can is nickel plated steel.
Again referring to
Can 113 has an aperture 108 at the base of the can (depicted here as the bottom of the can). The jellyroll is inserted into the can, and negative current collector 103 is placed atop the jellyroll in the can. In one embodiment, the negative collector disc is a metal disc, e.g. copper or brass, coated with a hydrogen evolution resistant material. In one embodiment, the hydrogen resistant material includes at least one of a metal, an alloy and a polymer. In another embodiment, the hydrogen resistant material includes at least one of tin, silver, bismuth, brass and lead. In yet another embodiment, the hydrogen resistant material includes an optionally perfluorinated polyolefin, in a more specific embodiment, Teflon™ (a trade name by E.I. Dupont de Nemours and Company, of Wilmington Del., for polytetrafluoroethylene).
Current collector 103 is configured to make electrical communication with the zinc negative electrode, typically via the negative substrate. In one embodiment, which can be employed with respect to any of the embodiments above, electrical communication between the negative collector disc and the zinc negative electrode is made via a welded metal tab. In a specific embodiment, the metal tab is welded to the negative substrate and to the negative current collector. In one embodiment, which can be employed with respect to any of the embodiments above, electrical communication between the negative collector disc and the zinc negative electrode is made via direct contact between the negative collector disc and the negative electrode. In a specific embodiment, the negative substrate, without any welded tab, comes in direct contact with the negative current collector. In embodiments where the negative current collector is coated with a non-electrically conductive material, e.g. the hydrogen evolution resistant material, the substrate is configured to pierce the non-electrically conductive material upon assembly of the cell so as to establish electrical communication. In one embodiment, which can be employed with respect to any of the embodiments above, electrical communication between the negative collector disc and the zinc negative electrode is made via a conductive spring configured to make contact with both the zinc negative electrode and the negative collector disc, the conductive spring is compressed between the end of the jellyroll and the negative collector disc. In a specific embodiment, the conductive spring has protrusions on either or both sides so as to make better electrical communication with the negative substrate and/or the negative current collector. In embodiments where the negative current collector is coated with a non-electrically conductive material, e.g. the hydrogen evolution resistant material, the conductive spring is configured to pierce, e.g. via protrusions emanating from the spring, the non-electrically conductive material upon assembly of the cell so as to establish electrical communication with the negative current collector. In one embodiment, the conductive spring, e.g. metal, is configured to pierce a separator interposed between the spring and the negative substrate of the jellyroll, thereby making electrical communication with the negative substrate. The conductive spring can be a metal spring, or a plastic or other polymeric material coated with a conductive material. Negative current collector 103 serves as a closure element for can 113 once the jellyroll is sealed in the can. In order to electrically isolate the negative current collector from the can (which is positive due to electrical communication (in this example via direct contact) with the positive substrate of the jellyroll)) gasket 111 is placed between the can and current collector prior to crimping the can shut to seal the jellyroll in the can.
As mentioned, in this example the positive substrate of the jellyroll makes direct contact with the end of the can with aperture 108. Electrolyte can be introduced into the can prior to sealing the jellyroll in the can or after the can is sealed, electrolyte can be introduced to the can via aperture 108. In one embodiment, which can be employed with respect to any of the embodiments above, a positive collector disc, for example a nickel foam disc, can be interposed between the nickel positive electrode and the base of the can having aperture 108, thus establishing electrical communication between the positive electrode (substrate) and the can. In one embodiment the positive collector disc includes nickel foam.
In one embodiment, which can be employed with respect to any of the embodiments above, the nickel positive electrode includes a nickel foam substrate impregnated with a mixture of: (a) cobalt oxide, in the range of between about 1% to about 10% by weight in the positive electrode, contained within a nickel oxide matrix; and (b) cobalt metal in the range of about 1% to 10% by weight in the positive electrode. More detailed description of positive electrodes is included in the section specific to the nickel positive electrode below.
In one embodiment, which can be employed with respect to any of the embodiments above, the zinc negative electrode includes: (a) a copper or brass substrate plated with tin or tin/zinc having a thickness of about 40-80 μIn; and, (b) a zinc oxide based electrochemically active layer. In one embodiment, the zinc oxide based electrochemically active layer includes: (a) zinc metal particles coated with at least one of lead and tin; (b) zinc oxide; (c) bismuth oxide; (d) a dispersing agent; and (e) a binding agent. In another embodiment, the zinc oxide based electrochemically active layer includes inorganic fiber, the inorganic fiber including silica and alumina. In yet another embodiment, the zinc oxide based electrochemically active layer includes carbon fiber, with or without surfactant coating. More detailed description of negative electrodes is included in the section specific to the zinc negative electrode below.
In one embodiment, which can be employed with respect to any of the embodiments above, the rechargeable nickel zinc cell contains an alkaline electrolyte, the alkaline electrolyte including: (a) between about 0.025 M and 0.25 M phosphate; (b) between about 4 M and about 9 M free alkalinity; and (c) up to about 1M borate. More detailed description of electrolytes suitable for the invention are included in the section specific to electrolytes below.
Vent cap 109 is attached, e.g. welded, to the end of the can having aperture 108. Aperture 108 is aligned sufficiently with aperture 112 in the vent cap to allow gas to vent through the adjoining apertures. More detailed description of vent caps suitable for the invention are included in the section specific to vent caps below.
In certain embodiments, the cell is configured to operate in an electrolyte “starved” condition. Further, in certain embodiments, nickel-zinc cells of this invention employ a starved electrolyte format. Such cells have relatively low quantities electrolyte in relation to the amount of active electrode material. They can be easily distinguished from flooded cells, which have free liquid electrolyte in interior regions of the cell. Starved format cells are discussed in U.S. patent application Ser. No. 11/116,113, filed Apr. 26, 2005, titled “Nickel Zinc Battery Design,” published as US 2006-0240317 A1, which is hereby incorporated by reference for all purposes. It may be desirable to operate a cell at starved conditions for a variety of reasons. A starved cell is generally understood to be one in which the total void volume within the cell electrode stack is not fully occupied by electrolyte. In a typical example, the void volume of a starved cell after electrolyte fill may be at least about 10% of the total void volume before fill. Specifically, one embodiment includes aspects of any one embodiment described above, where the cell is configured in an electrolyte starved condition.
The battery cells described herein can have any of a number of different shapes and sizes. For example, cylindrical cells of this invention may have the diameter and length of conventional AAA cells, AA cells, D cells, C cells, etc. Custom cell designs are appropriate in some applications. In a specific embodiment, the cell size is a sub-C cell size of diameter 22 mm and length 43 mm. Note that the present invention also may be employed in relatively small cell formats, as well as various larger format cells employed for various non-portable applications. Often the profile of a battery pack for, e.g., a power tool or lawn tool will dictate the size and shape of the battery cells. One embodiment is a battery pack including one or more nickel-zinc battery cells and appropriate casing, contacts, and conductive lines to permit charge and discharge in an electric device. In a specific embodiment, rechargeable cells are configured to a commercially available size of AAA, AA, C, D or sub-C.
More detailed description of specific cells as well as features of a venting cap, the positive electrode, separator, electrolyte, negative electrode and jellyroll configurations follows.
Venting Cap
Although the cell is generally sealed from the environment, the cell may be permitted to vent gases from the battery that are generated during charge and discharge. Thus in reference for example to
Some details of the structure of the vent cap are found in the following patent applications which are incorporated herein by reference for all purposes: PCT/US2006/015807 filed Apr. 25, 2006 and PCT/US2004/026859 filed Aug. 17, 2004 (publication WO 2005/020353 A3. As well, the vent cap is described in more detail in relation to
The Positive Electrode
The nickel positive electrode generally includes electrochemically active nickel oxide or hydroxide or oxyhydroxide and one or more additives to facilitate manufacturing, electron transport, wetting, mechanical properties, etc. In this application, “nickel oxide” is meant to include active nickel in the form of oxide, oxyhydroxide and/or hydroxide. For example, a positive electrode formulation may include nickel hydroxide particles, zinc oxide, cobalt oxide (CoO), cobalt metal, nickel metal, and a thixotropic agent such as carboxymethyl cellulose (CMC). Note that the metallic nickel and cobalt may be provided as chemically pure metals or alloys thereof. The positive electrode may be made from paste containing these materials and a binder such as a polymeric fluorocarbon (e.g., Teflon™).
In certain embodiments, the nickel hydroxide electrode includes nickel hydroxide (and/or nickel oxyhydroxide), cobalt/cobalt compound powder, nickel powder and binding materials. The cobalt compound is included to increase the conductivity of the nickel electrode. In one embodiment, the nickel positive electrode includes at least one of cobalt oxide, cobalt hydroxide, and/or cobalt oxyhydroxide; optionally coated on nickel hydroxide (or oxyhydroxide) particles.
A nickel foam matrix may be used to support the electro-active nickel oxide (e.g., Ni(OH)2) electrode material. The foam substrate thickness may be may be between 15 and 60 mils. The thickness of the positive electrode, which includes nickel foam filled with the electrochemically active and other electrode materials, ranges from about 16-24 mils, preferably about 20 mils thick. In one embodiment, a nickel foam density of about 350 g/m2 and thickness ranging from about 16-18 mils is used.
The Separator
Typically, a separator will have small pores. In certain embodiments the separator includes multiple layers. The pores and/or laminate structure may provide a tortuous path for zinc dendrites and therefore effectively bar penetration and shorting by dendrites. Preferably, the porous separator has a tortuosity of between about 1.5 and 10, more preferably between about 2 and 5. The average pore diameter is preferably at most about 0.2 microns, and more preferably between about 0.02 and 0.1 microns. Also, the pore size is preferably fairly uniform in the separator. In a specific embodiment, the separator has a porosity of between about 35 and 55% with one preferred material having 45% porosity and a pore size of 0.1 micron.
In a certain embodiments, the separator includes at least two layers (and preferably exactly two layers)—a barrier layer to block zinc penetration and a wetting layer to keep the cell wet with electrolyte, allowing ionic current to flow. This is generally not the case with nickel cadmium cells, which employ only a single separator material between adjacent electrode layers.
Performance of the cell may be aided by keeping the positive electrode wet and the negative electrode relatively dry. Thus, in some embodiments, the barrier layer is located adjacent to the negative electrode and the wetting layer is located adjacent to the positive electrode. This arrangement improves performance of the cell by maintaining electrolyte in intimate contact with the positive electrode.
In other embodiments, the wetting layer is placed adjacent to the negative electrode and the barrier layer is placed adjacent to the positive electrode. This arrangement aids recombination of oxygen at the negative electrode by facilitating oxygen transport to the negative electrode via the electrolyte.
The barrier layer is typically a microporous membrane. Any microporous membrane that is ionically conductive may be used. Often a polyolefin having a porosity of between about 30 and 80 per cent, and an average pore size of between about 0.005 and 0.3 micron will be suitable. In a preferred embodiment, the barrier layer is a microporous polypropylene. The barrier layer is typically about 0.5-4 mils thick, more preferably between about 1.5 and 4 mils thick.
The wetting (or wicking) layer may be made of any suitable wettable separator material. Typically the wetting layer has a relatively high porosity e.g., between about 50 and 85% porosity. Examples include polyamide materials such as nylon-based as well as wettable polyethylene, polypropylene and cellulose-based materials. One particular material is cellulose impregnated and/or coated with polyvinylalcohol. In certain embodiments, the wetting layer is between about 1 and 10 mils thick, more preferably between about 3 and 6 mils thick. Examples of separate materials that may be employed as the wetting material include NKK VL100 (NKK Corporation, Tokyo, Japan), Freudenberg FS2213E, Scimat 650/45 (SciMAT Limited, Swindon, UK), and Vilene FV4365.
Other separator materials known in the art may be employed. As indicated, nylon-based materials and microporous polyolefins (e.g., polyethylenes and polypropylenes) are very often suitable. In one embodiment, separators are selectively sealed so that the electrodes are further isolated from one another. Virtually any separator material will work so long as it can be sealed via application of one of the heat sources described herein. In some embodiments of the invention, separator materials of differing melting points are employed, in other embodiments separators that seal are employed in conjunction with those that do not seal under the conditions to which one or both ends of the jellyroll are exposed. These specific embodiments will be described in more detail below in relation to
Another consideration in the electrode/separator design is whether to provide the separator as simple sheets of approximately the same width as the electrode and current collector sheet or to encase one or both electrodes in separator layers. In the latter example, the separator serves as a “bag” for one of the electrode sheets, effectively encapsulating an electrode layer. In some embodiments, enveloping the negative electrode in a separator layer will aid in preventing dendrite formation. Specific heat sealing embodiments are described in more detail below in relation to the section entitled, “Electrodes and Separator Assembly—The Jellyroll.”
The Electrolyte
In certain embodiments pertaining to nickel-zinc cells, the electrolyte composition limits dendrite formation and other forms of material redistribution in the zinc electrode. Examples of suitable electrolytes are described in U.S. Pat. No. 5,215,836 issued to M. Eisenberg on Jun. 1, 1993, which is hereby incorporated by reference. In some cases, the electrolyte includes (1) an alkali or earth alkali hydroxide, (2) a soluble alkali or earth alkali fluoride, and (3) a borate, arsenate, and/or phosphate salt (e.g., potassium borate, potassium metaborate, sodium borate, sodium metaborate, and/or a sodium or potassium phosphate). In one specific embodiment, the electrolyte includes about 4.5 to 10 equiv/liter of potassium hydroxide, from about 2 to 6 equiv/liter boric acid or sodium metaborate and from about 0.01 to 1 equivalents of potassium fluoride. A specific preferred electrolyte for high rate applications includes about 8.5 equiv/liter of hydroxide, about 4.5 equivalents of boric acid and about 0.2 equivalents of potassium fluoride.
The invention is not limited to the electrolyte compositions presented in the Eisenberg patent. Generally, any electrolyte composition meeting the criteria specified for the applications of interest will suffice. Assuming that high power applications are desired, then the electrolyte should have very good conductivity. Assuming that long cycle life is desired, then the electrolyte should resist dendrite formation. In the present invention, the use of borate and/or fluoride containing KOH electrolyte along with appropriate separator layers reduces the formation of dendrites thus achieving a more robust and long-lived power cell.
In a specific embodiment, the electrolyte composition includes an excess of between about 3 and 5 equiv/liter hydroxide (e.g., KOH, NaOH, and/or LiOH). This assumes that the negative electrode is a zinc oxide based electrode. For calcium zincate negative electrodes, alternate electrolyte formulations may be appropriate. In one example, an appropriate electrolyte for calcium zincate has the following composition: about 15 to 25% by weight KOH, about 0.5 to 5.0% by weight LiOH.
According to various embodiments, the electrolyte may include a liquid and a gel. The gel electrolyte may include a thickening agent such as CARBOPOL™ available from Noveon of Cleveland, Ohio. In a preferred embodiment, a fraction of the active electrolyte material is in gel form. In a specific embodiment, about 5-25% by weight of the electrolyte is provided as gel and the gel component includes about 1-2% by weight CARBOPOL™.
In some cases, the electrolyte may contain a relatively high concentration of phosphate ion as discussed in U.S. Pat. No. 7,550,230, entitled “Electrolyte Composition for Nickel Zinc Batteries,” filed Feb. 1, 2006, by J. Phillips and S. Mohanta, which is incorporated herein by reference for all purposes.
The Negative Electrode
As applied to nickel-zinc cells, the negative electrode includes one or more electroactive sources of zinc or zincate ions optionally in combination with one or more additional materials such as surfactant-coated particles of the invention, corrosion inhibitors, wetting agents, etc. as described below. When the electrode is fabricated it will be characterized by certain physical, chemical, and morphological features such as coulombic capacity, chemical composition of the active zinc, porosity, tortuosity, etc.
In certain embodiments, the electrochemically active zinc source may include one or more of the following components: zinc oxide, calcium zincate, zinc metal, and various zinc alloys. Any of these materials may be provided during fabrication and/or be created during normal cell cycling. As a particular example, consider calcium zincate, which may be produced from a paste or slurry containing, e.g., calcium oxide and zinc oxide.
Active material for a negative electrode of a rechargeable zinc alkaline electrochemical cell may include zinc metal (or zinc alloy) particles. If a zinc alloy is employed, it may in certain embodiments include bismuth and/or indium. In certain embodiments, it may include up to about 20 parts per million lead. A commercially available source of zinc alloy meeting this composition requirement is PG101 provided by Noranda Corporation of Canada. In one embodiment, the electrochemically active zinc metal component of nickel zinc cells contains less than about 0.05% by weight of lead. Tin may also be used in the zinc negative electrode.
In certain embodiments, the zinc metal particles may be coated with tin and/or lead. The zinc particles may be coated by adding lead and tin salts to a mixture containing zinc particles, a thickening agent and water. The zinc metal can be coated while in the presence of zinc oxide and other constituents of the electrode. A zinc electrode containing lead or tin coated zinc particles is generally less prone to gassing when cobalt is present in the electrolyte. The cycle life and shelf life of the cells is also enhanced, as the zinc conductive matrix remains intact and shelf discharge is reduced. Exemplary active material compositions suitable for negative electrodes of this invention are further described in U.S. patent application Ser. No. 12/467,993, entitled “Pasted Zinc Electrode for Rechargeable Nickel-Zinc Batteries,” by J. Phillips et. al., filed May 18, 2009, which is hereby incorporated by reference for all purposes.
The zinc active material may exist in the form of a powder, a granular composition, fibers, etc. Preferably, each of the components employed in a zinc electrode paste formulation has a relatively small particle size. This is to reduce the likelihood that a particle may penetrate or otherwise damage the separator between the positive and negative electrodes.
Considering the electrochemically active zinc components in particular (and other particulate electrode components as well), such components preferably have a particle size that is no greater than about 40 or 50 micrometers. In one embodiment the particle size is less than about 40 microns, i.e. the average diameter is less than about 40 microns. This size regime includes lead coated zinc or zinc oxide particles. In certain embodiments, the material may be characterized as having no more than about 1% of its particles with a principal dimension (e.g., diameter or major axis) of greater than about 50 micrometers. Such compositions can be produced by, for example, sieving or otherwise treating the zinc particles to remove larger particles. Note that the particle size regimes recited here apply to zinc oxides and zinc alloys as well as zinc metal powders.
In addition to the electrochemically active zinc component(s), the negative electrode may include one or more additional materials that facilitate or otherwise impact certain processes within the electrode such as ion transport, electron transport (e.g., enhance conductivity), wetting, porosity, structural integrity (e.g., binding), gassing, active material solubility, barrier properties (e.g., reducing the amount of zinc leaving the electrode), corrosion inhibition etc.
Various organic materials may be added to the negative electrode for the purpose of binding, dispersion, and/or as surrogates for separators. Examples include hydroxylethyl cellulose (HEC), carboxymethyl cellulose (CMC), the free acid form of carboxymethyl cellulose (HCMC), polytetrafluoroethylene (PTFE), polystyrene sulfonate (PSS), polyvinyl alcohol (PVA), nopcosperse dispersants (available from San Nopco Ltd. of Kyoto Japan), etc.
In certain embodiments, polymeric materials such as PSS and PVA may be mixed with the paste formation (as opposed to coating) for the purpose of burying sharp or large particles in the electrode that might otherwise pose a danger to the separator.
When defining an electrode composition herein, it is generally understood as being applicable to the composition as produced at the time of fabrication (e.g., the composition of a paste, slurry, or dry fabrication formulation), as well as compositions that might result during or after formation cycling or during or after one or more charge-discharge cycles while the cell is in use such as while powering a portable tool.
Various negative electrode compositions within the scope of this invention are described in the following documents, each of which is incorporated herein by reference: PCT Publication No. WO 02/39517 (J. Phillips), PCT Publication No. WO 02/039520 (J. Phillips), PCT Publication No. WO 02/39521, PCT Publication No. WO 02/039534 and (J. Phillips), US Patent Publication No. 2002182501. Negative electrode additives in the above references include, for example, silica and fluorides of various alkaline earth metals, transition metals, heavy metals, and noble metals.
Finally, it should be noted that while a number of materials may be added to the negative electrode to impart particular properties, some of those materials or properties may be introduced via battery components other than the negative electrode. For example, certain materials for reducing the solubility of zinc in the electrolyte may be provided in the electrolyte or separator (with or without also being provided to the negative electrode). Examples of such materials include phosphate, fluoride, borate, zincate, silicate, stearate. Other electrode additives identified above that might be provided in the electrolyte and/or separator include surfactants, ions of indium, bismuth, lead, tin, calcium, etc.
For example, in some embodiments, the negative electrode includes an oxide such as bismuth oxide, indium oxide, and/or aluminum oxide. Bismuth oxide and indium oxide may interact with zinc and reduce gassing at the electrode. Bismuth oxide may be provided in a concentration of between about 1 and 10% by weight of a dry negative electrode formulation. It may facilitate recombination of oxygen. Indium oxide may be present in a concentration of between about 0.05 and 1% by weight of a dry negative electrode formulation. Aluminum oxide may be provided in a concentration of between about 1 and 5% by weight of a dry negative electrode formulation.
In certain embodiments, one or more additives may be included to improve corrosion resistance of the zinc electroactive material and thereby facilitate long shelf life. The shelf life can be critical to the commercial success or failure of a battery cell. Recognizing that batteries are intrinsically chemically unstable devices, steps may be taken to preserve battery components, including the negative electrode, in their chemically useful form. When electrode materials corrode or otherwise degrade to a significant extent over weeks or months without use, their value becomes limited by short shelf life.
Specific examples of anions that may be included to reduce the solubility of zinc in the electrolyte include phosphate, fluoride, borate, zincate, silicate, stearate, etc. Generally, these anions may be present in a negative electrode in concentrations of up to about 5% by weight of a dry negative electrode formulation. It is believed that at least certain of these anions go into solution during cell cycling and there they reduce the solubility of zinc. Examples of electrode formulations including these materials are included in the following patents and patent applications, each of which is incorporated herein by reference for all purposes: U.S. Pat. No. 6,797,433, issued Sep. 28, 2004, titled, “Negative Electrode Formulation for a Low Toxicity Zinc Electrode Having Additives with Redox Potentials Negative to Zinc Potential,” by Jeffrey Phillips; U.S. Pat. No. 6,835,499, issued Dec. 28, 2004, titled, “Negative Electrode Formulation for a Low Toxicity Zinc Electrode Having Additives with Redox Potentials Positive to Zinc Potential,” by Jeffrey Phillips; U.S. Pat. No. 6,818,350, issued Nov. 16, 2004, titled, “Alkaline Cells Having Low Toxicity Rechargeable Zinc Electrodes,” by Jeffrey Phillips; and PCT/NZ02/00036 (publication no. WO 02/075830) filed Mar. 15, 2002 by Hall et al.
Conductive fibers added to the negative electrode may also serve the purpose of irrigating or wetting the electrode. Surfactant coated carbon fibers are one example of such material. However, it should be understood that other materials may be included to facilitate wetting. Examples of such materials include titanium oxides, alumina, silica, alumina and silica together, etc. Generally, when present, these materials are provided in concentrations of up to about 10% by weight of a dry negative electrode formulation. A further discussion of such materials may be found in U.S. Pat. No. 6,811,926, issued Nov. 2, 2004, titled, “Formulation of Zinc Negative Electrode for Rechargeable Cells Having an Alkaline Electrolyte,” by Jeffrey Phillips, which is incorporated herein by reference for all purposes.
Zinc negative electrodes contain materials that establish conductive communication between the electrochemically active component of the zinc negative electrode and the nickel positive electrode. The inventors have found that introduction of surfactant-coated particles into the negative electrode increases the overall current carrying capability of the electrode, particularly surfactant coated carbon particles, as described in U.S. patent application Ser. No. 12/852,345, filed Aug. 6, 2010, titled, “Carbon Fiber Zinc Negative Electrode,” by Jeffrey Phillips, which is incorporated herein by reference for all purposes.
As mentioned, a slurry/paste having a stable viscosity and that is easy to work with during manufacture of the zinc electrode may be used to make the zinc negative electrode. Such slurry/pastes have zinc particles optionally coated by adding lead and tin salts to a mixture containing the zinc particles, a thickening agent and a liquid, e.g. water. Constituents such as zinc oxide (ZnO), bismuth oxide (Bi2O3), a dispersing agent, and a binding agent such as Teflon™ are also added. Binding agents suitable for this aspect include, but are not limited to, P.T.F.E., styrene butadiene rubber, polystyrene, and HEC. Dispersing agents suitable for this aspect include, but are not limited to, a soap, an organic dispersant, an ammonium salt dispersant, a wax dispersant. An example of commercially available dispersants in accord with this aspect is a Nopcosperse™ (trade name for a liquid series of dispersants available from Nopco Paper Technology Australia Pty. Ltd.). Liquids suitable for this aspect include, but are not limited to, water, alcohols, ethers and mixtures thereof.
The Electrodes and Separator Assembly—The Jellyroll
To make a jellyroll, individual electrode layer assemblies are sandwiched between one or more layers of separator materials. The sandwiched electrode assemblies are stacked and then wound into a jellyroll.
Dendrites are crystalline structures having a skeletal or tree-like growth pattern (“dendritic growth”) in metal deposition. In practice, dendrites form in the conductive media of a power cell during the lifetime of the cell and effectively bridge the negative and positive electrodes causing shorts and subsequent loss of battery function.
Note that the separator sheets generally do not entirely cover the full widths of the electrode sheets. Specifically, one edge (the conductive substrate) of each electrode sheet remains exposed for attaching terminals or otherwise establishing electrical communication with a terminal of the battery once complete. In this particular embodiment, these exposed edges are on opposite sides so that once the jellyroll is wound, each of the positive and the negative electrodes will make electrical contact with the battery terminals at opposite ends of the battery. The exposed edges can be on the same side so that the electrical connections to the battery terminals are both made via the same end of the jellyroll without exceeding the scope of the invention, however, it is more convenient to have the exposed substrate on opposite ends of the jellyroll. In the embodiment where the exposed substrate edges are on opposite ends of the jellyroll, all of each of the respective edges need not be exposed. That is, a portion of the edges may be used to make contact with, for example, a current collector disk or metal tab, while the remaining portion is unexposed or otherwise protected. In a particular embodiment, substantially all of each edge of each electrode substrate is exposed in order to maximize contact with a current collector or the can for efficient current collection.
The negative electrode substrate 204 should be electrochemically compatible with the negative electrode materials 206. As described above, the electrode substrate may have the structure of a perforated metal sheet, an expanded metal, a metal foam, or a patterned continuous metal sheet. In some embodiments, the substrate is simply a metal layer such as a metal foil. In a particular embodiment, the negative substrate is copper plated with tin and/or lead.
Opposite from the negative electrode on the other side of separator 200 is the positive electrode and separator 208. In this embodiment, separator 208 is a cellulose-based material, more specifically cellulose impregnated and/or coated with polyvinylalcohol, but the invention is not so limited. This layer is a wicking layer (e.g. from NKK, as is discussed in more detail in the separator section above). The positive electrode also includes electrochemically active layers 212 and an electrode substrate 210. The layers 212 of the positive electrode may include nickel hydroxide, nickel oxide, and/or nickel oxyhydroxide as electrochemically active materials and various additives, all of which are described herein. The electrode substrate 210 may be, for example, a nickel metal foam matrix or nickel metal sheets. Note that if a nickel foam matrix is used, then layers 212 would form one continuous electrode because they fill the voids in the metal foam and pass through the foam. The layered zinc negative electrode and nickel positive electrode structure is wound into a jellyroll as depicted in
As seen from
A winding apparatus draws the various sheets in at the same time and rolls them into a jellyroll assembly. After a cylinder of sufficient thickness is produced, the apparatus cuts the layers of separator and electrodes to produce the finished jellyroll assembly 101, as in
In one embodiment, selectively sealing the first set of separator layers includes: i) configuring the current collecting substrate of the second electrode so that when the heat source is applied to the same end of the jellyroll assembly, the first set of separator layers can seal to envelop the first electrode, but the second set of separator layers are physically obstructed from sealing and enveloping the second electrode; and ii) applying the heat source to the same end of the jellyroll assembly. In this example, heat sealing is done at the end (the bottom as depicted) of the jellyroll where current collector substrate 210 protrudes beyond the separator layers.
In one embodiment, configuring the current collecting substrate of the second electrode includes folding the current collecting substrate of the second electrode substantially over, but not touching, the current collecting substrate of the first electrode, so that a substantially enclosed volume is formed, where the first set of separator layers and adjoining separator layers from the second set of separator layers are disposed in the substantially enclosed volume.
Referring again to
Once this seal is formed, a small volume, 203, can be formed which, along with the foldover, saves valuable space in the battery assembly so that more electroactive material can be used (because effectively the electrodes can be taller). In this example, as indicated by 208a, wicking layers from the next nearest positive wind do not fuse because it is a cellulose based material and does not melt (although it may deform as depicted). Heat sealing used for cells described herein are not limited in this way. In some embodiments, both separators (or in some embodiments more than two separator layers) are made of material that can fuse to form a double seal over one of the electrodes. That is, if the two different separator materials are compatible to melt together they may form a single layer fused end, but double thick. If the two different separator materials are not compatible to melt together, a bilayer seal is formed. In this embodiment, the current collectors are configured so that when a sealing heat is applied, only one of the electrodes can be encapsulated because there is a physical barrier preventing the other electrode, in this example the positive, from being sealed under the separator (although seals 200a in volumes 211 protect the positive from contamination).
The jellyroll, once wound, has the positive electrode layer (along with separator) on the final wind on the outside of the jelly roll. Although the invention is not limited in this way, in this configuration the nickel plated can interior is further isolated from the negative electrode and thus there is no need to coat the interior of the can with hydrogen evolution resistant materials, for example, negative current collector 103 can alone be coated.
While not wishing to be bound to theory, it is believed that electrical shorts due to particle contamination are more likely when current collecting substrates are folded over and thus, in this example, positive substrate 210 is in direct line of sight with negative current collecting substrate 204. Sealing e.g. in this case the negative electrode prevents particles causing shorts between the electrodes at the end of the jellyroll with such heat sealing. At the end of jellyroll 101, where the negative substrate 204 makes electrical contact with negative current collector disc 103, substrates 204 and 210 are not in direct line of sight and therefore for any dendrite growth would have to migrate from electrochemically active material 206, up and over both separator layers 200 and 208, and down again to substrate 210 in order to cause a short. Thus the electrodes are configured is such a way that the electrodes are not in direct line of sight with each other and the difference in height, D, between the electrodes is sufficiently different, coupled with the separators forming a traversal barrier obviates the need to seal separators at this end of the jellyroll. The invention is not so limited however. In some embodiments, the electrodes and separators of the jellyroll are configured so that selective sealing of one of the two electrodes is done on both ends of the jellyroll, for example where it is desirable to minimize the relative distance between the positive and negative electrodes at both ends of the jellyroll. A more detailed description of selective sealing of separator layers is contained in U.S. non-provisional patent application entitled, “Heat Sealing Separators for Nickel Zinc Cells, Ser. No. 12/877,841, filed Sep. 8, 2010, by McKinney et. al., which is herein incorporated by reference for all purposes. One embodiment is a cell as described herein including a positive can and vent cap, including a jellyroll as described in U.S. patent application Ser. No. 12/877,841.
Thus for this invention, “a vent cap” is meant to encompass a vent mechanism that is formed using the base of the can, e.g. as described in relation to
One of ordinary skill in the art would appreciate that although the cell depicted in
In some embodiments the battery can is reinforced to provide additional rigidity as against shape change and other forces the cell encounters. In one embodiment, the can is thicker at the base than at the sidewall, or of sufficient thickness, to withstand forces exerted on the can, for example, shape change and/or gas pressure. In another embodiment, the base of the can is reinforced.
In one embodiment, ridges as described in relation to
One of ordinary skill in the art would appreciate that the ridge configurations in
In one embodiment, a vent mechanism uses these trenches as a passage for venting. In this embodiment, a vent cap similar to that described in relation to
In other embodiments, a can with a flat bottom is used but a strengthening member is attached to the bottom (inside) of the can.
Methods of Making Rechargeable Cells
In relation to
Also, it is desirable, although not necessary to practice the invention, to plate the interior of the can with, e.g., nickel. If the jellyroll is sealed in the can and the can subsequently punctured to form an aperture as described in the previous embodiment, there may be a small portion of the can at the site of the puncture that is not protected with nickel. Also, in some cases is it difficult to plate the interior of the can effectively. In some embodiments, as mentioned, the positive electrode layer is on the outside of the jellyroll, and therefore, for example when the can is steel, iron degradation products from the can do not significantly interfere with the positive electrode function, and therefore this configuration constitutes an improved nickel zinc cell for this reason, among others described, especially when the negative electrode is selectively sealed as described in relation to
In some embodiments however, it is desirable to start with a preformed aperture in the can, then plate the can with a protective agent, for example nickel. In this way there is no portion of aperture 108, and hopefully the interior of the can, that is not protected with nickel. In these embodiments, process operation 504 would be absent from the process flow. Thus another embodiment is a method of making a rechargeable nickel zinc cell, the method including: 502) sealing a jellyroll assembly, including a nickel positive electrode, a zinc negative electrode, and at least one separator layer disposed between said nickel positive electrode and zinc negative electrode, in a can such that the nickel positive electrode is in electrical communication with the base and the body of the can and the zinc negative electrode is in electrical communication with a negative current collecting disc at the other end of the can and electrically isolated from the can; the can including an aperture in the base of the can; the negative current collecting disc configured as a closure to the open end of the can; and 506) affixing a vent cap at the base of the can; the vent cap configured to vent gas from the rechargeable nickel zinc cell via the aperture. Again, process flow operations 502 and 506 can be performed in reverse order as well.
In one embodiment, which can be employed with respect to any of the embodiments above, the can is nickel plated steel. In another embodiment, the negative collector disc is a metal disc coated with a hydrogen evolution resistant material, e.g., at least one of a metal, an alloy and a polymer. Specific examples of these materials are described above an included in embodiments of methods of the invention. The negative disc, for example, can be a steel, brass or copper disk coated with at least one of tin, silver, bismuth, brass, zinc and lead. In one example the disc is brass or copper coated with tin and/or silver. In one embodiment, at least a portion of the disc is coated with a polymer, for example, Teflon.
In method embodiments, which can be employed with respect to any of the embodiments above, electrical communication between the negative collector disc and the zinc negative electrode is made via a welded metal tab, direct contact between the negative collector disc and the negative electrode, and/or a conductive spring configured to make contact with both the zinc negative electrode and the negative collector disc, the conductive spring, for example a spring or reversibly compressive material such as a metal sponge, compressed between the end of the jellyroll and the negative collector disc.
In one embodiment, which can be employed with respect to any of the embodiments above, the methods of making rechargeable nickel zinc cells further include interposing a positive collector disc between, and in electrical communication with, the nickel positive electrode and the base of the can, prior to sealing the jellyroll in the can. In one embodiment, the positive collector disc includes nickel foam.
The materials used to construct the positive electrode and the negative electrode are the same as included in the description above in relation to cells of the invention.
In one embodiment, which can be employed with respect to any of the embodiments above, the methods of making rechargeable nickel zinc cells further includes introducing an alkaline electrolyte into the can, either prior to sealing the jellyroll in the can or via the aperture after the jellyroll is sealed in the can, the alkaline electrolyte including: (a) between about 0.025 M and 0.25 M phosphate; (b) between about 4 M and about 9 M free alkalinity; and (c) up to about 1M borate.
Another embodiment, which can be employed with respect to any of the embodiments above, is a method of making a rechargeable nickel zinc cell as described herein, where the rechargeable nickel zinc cell is configured to a commercially available size selected from the group consisting of AAA, AA, C, D and sub-C.
Although the foregoing invention has been described in some detail to facilitate understanding, the described embodiments are to be considered illustrative and not limiting. It will be apparent to one of ordinary skill in the art that certain changes and modifications can be practiced within the scope of the appended claims.
This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 61/251,222 filed Oct. 13, 2009, the contents of which are incorporated herein by reference in their entirety and for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 61251222 | Oct 2009 | US |
