U.S. patent application number 12/900206 was filed with the patent office on 2011-02-17 for method of manufacturing nickel zinc batteries.
This patent application is currently assigned to POWERGENIX SYSTEMS, INC.. Invention is credited to Jeffrey Phillips, Jason Zhao.
Application Number | 20110039139 12/900206 |
Document ID | / |
Family ID | 34215976 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110039139 |
Kind Code |
A1 |
Phillips; Jeffrey ; et
al. |
February 17, 2011 |
METHOD OF MANUFACTURING NICKEL ZINC BATTERIES
Abstract
Methods of manufacturing a rechargeable power cell are
described. Methods include providing a slurry or paste of negative
electrode materials having low toxicity and including dispersants
to prevent the agglomeration of particles that may adversely affect
the performance of power cells. The methods utilize semi-permeable
sheets to separate the electrodes and minimize formation of
dendrites; and further provide electrode specific electrolyte to
achieve efficient electrochemistry and to further discourage
dendritic growth in the cell. The negative electrode materials may
be comprised of zinc and zinc compounds. Zinc and zinc compounds
are notably less toxic than the cadmium used in NiCad batteries.
The described methods may utilize some production techniques
employed in existing NiCad production lines. Thus, the methods
described will find particular use in an already well-defined and
mature manufacturing base.
Inventors: |
Phillips; Jeffrey; (La
Jolla, CA) ; Zhao; Jason; (Suisun City, CA) |
Correspondence
Address: |
Weaver Austin Villeneuve & Sampson LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Assignee: |
POWERGENIX SYSTEMS, INC.
San Diego
CA
|
Family ID: |
34215976 |
Appl. No.: |
12/900206 |
Filed: |
October 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10921062 |
Aug 17, 2004 |
7833663 |
|
|
12900206 |
|
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60496208 |
Aug 18, 2003 |
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Current U.S.
Class: |
429/94 |
Current CPC
Class: |
H01M 4/244 20130101;
Y10T 29/4911 20150115; H01M 4/661 20130101; Y10T 29/49108 20150115;
H01M 10/44 20130101; H01M 10/30 20130101; Y02E 60/10 20130101; H01M
50/449 20210101 |
Class at
Publication: |
429/94 |
International
Class: |
H01M 10/36 20100101
H01M010/36 |
Claims
1-22. (canceled)
23. A rechargeable power cell comprising: a zinc negative electrode
material applied to an anode carrier sheet having first and second
opposing and substantially planar surfaces such that a first
electrode is formed, wherein the negative electrode material
includes a PTFE binder and little or no other organic dispersant; a
nickel positive electrode material applied to a cathode carrier
sheet having first and second opposing and substantially planar
surfaces such that a second electrode is formed; a first
semi-permeable dielectric sheet disposed along the first planar
surface of the first electrode, a second semi-permeable dielectric
sheet along the second planar surface of the first electrode, a
third semi-permeable dielectric sheet along the first planar
surface of the second electrode, and a fourth semi-permeable
dielectric sheet along the second planar surface of the second
electrode such that the first electrode and the second electrode
are separated by only two semi-permeable dielectric sheets and
rolled such that a cell assembly is formed; a first terminal
attached with a first end of the cell assembly such that the first
electrode is in electrical communication only with the first
terminal; a second terminal attached with a second end of the cell
assembly such that the second electrode is in electrical
communication only with the second terminal; a substantially
cylindrical retaining vessel; an electrolyte solution comprising
fluoride, and hydroxide; and a plurality of sealing terminals
disposed at both ends of the retaining vessel such that the
electrolyte and the cell assembly is substantially isolated from
the environment.
24. The rechargeable power cell of claim 23 wherein the plurality
of sealing terminals comprise a top terminal and a bottom
terminal.
25. The rechargeable power cell of claim 24 wherein the top
terminal is the positive terminal and the bottom terminal is the
negative terminal.
26. The rechargeable power cell of claim 24 wherein the top
terminal is the negative terminal and the bottom terminal is the
positive terminal.
27. The rechargeable power cell of claim 26, wherein the bottom
terminal comprises the substantially cylindrical retaining vessel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Utility application
Ser. No. 10/921,062, filed Aug. 17, 2004, which claims priority
under 35 USC .sctn.119(e) from U.S. Provisional Application No.
60/496,208, filed Aug. 18, 2003, the contents of which are
incorporated herein by reference for all purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the rechargeable battery
arts and, more particularly to the manufacture of nickel zinc
rechargeable battery cells.
[0004] 2. Description of the Related Art
[0005] The advent of portable communication and computation devices
that allow diverse mobile connectivity has fueled growth and
innovation in the rechargeable battery field. Increased capacity
and power has made possible the entry of rechargeable power sources
in various fields including the power tool arena. Because power
tools typically have large current demands, rechargeable power
sources have necessarily evolved to accommodate rapid discharge
characteristics. It can be appreciated that the present invention
will also find use in applications other than power tools such as
Uninterruptible Power Supplies (UPS), Electric Vehicles, and high
demand consumer electronics--all of which require high carrying
capacity and current discharging ability. Of course, the invention
also applies to relatively lower discharge rate applications such
as many mainstream consumer electronics applications.
[0006] Rechargeable power sources have various benefits over
non-rechargeable sources. For example, the use of non-rechargeable
power sources raises growing environmental concerns with respect to
hazardous waste disposal and remediation. In light of the
proliferation of portable devices, the number of non-rechargeable
power sources needed to use those devices would be staggering.
Rechargeable power sources allow a battery cell to be used
repeatedly thus reducing the introduction of hazardous waste
products into the environment. Further, rechargeable power sources
allow the conservation of the metal and chemical resources that
would otherwise be spent in non-rechargeable power sources.
Finally, the use of rechargeable power sources encourages and
expands continuing conservation efforts that are necessary for a
growing population to embrace.
[0007] Although the benefits of rechargeable power sources are
many, they are not without costs. In particular, the materials that
comprise rechargeable power sources frequently pose a significant
potential threat to the environment. Regional recycling agencies
like the Northeast Recycling Council (NERC) are actively addressing
the problems of disposal of rechargeable power sources. In a recent
report by NERC, nine of the ten member states banned the disposal
of lead acid batteries; six of the ten banned the disposal of
Nickel/Cadmium batteries; and four of the ten banned the disposal
of mercuric oxide batteries. The EPA has also weighed in and has
recently closed feedback on industry wide practices for disposal of
NiCad batteries stating, [0008] NiCad batteries, commonly used in
industrial and household appliances such as cordless phones, power
tools and laptop computers, are one of a number of products that
pose a potential environmental risk when disposed of incorrectly.
The batteries contain both Nickel and Cadmium, the most toxic
component, and can cause health problems when not disposed of
properly. They are both heavy metals and can also adversely affect
the environment during recycling and disposal.
[0009] Because of the hazardous nature of some of the commonly used
materials for conventional rechargeable power sources, it would be
desirable to manufacture a rechargeable power source that reduces
the quantity of any potentially hazardous materials. In particular,
it would be desirable to find a substitute for the widely used
nickel cadmium battery cell.
[0010] It has been found that rechargeable nickel zinc cells can
provide a power-to-weight ratio comparable to and even exceeding
nickel cadmium cells at a reasonable cost. However, nickel zinc
battery technology has not been widely deployed for at least two
reasons. First, it has been found to have a relatively limited
cycle life. In other words, a given nickel zinc cell can only
charge and discharge for a fraction of the cycles typically
attained with a comparable nickel cadmium cell. This is due to zinc
distribution and dendrite formation. Second, there has not been a
suitable high volume manufacturing process developed for nickel
zinc batteries.
[0011] It would be desirable to use, to the extent possible,
existing manufacturing techniques to produce environmentally safer
rechargeable power sources in order to leverage the existing
manufacturing infrastructure rather than require a wholly new
manufacturing base.
SUMMARY OF THE INVENTION
[0012] The present invention accomplishes the advantages described
above by employing a nickel-cadmium type manufacturing process with
certain important variations that allow for replacement of the
cadmium negative electrode with a less deleterious negative
electrode, such as an electrode fabricated from zinc or a zinc
compound such as zinc oxide or calcium zincate. Various methods of
manufacturing a cadmium-free power cell are described herein. The
methods employ high volume lines for nickel and zinc electrode
fabrication. As part of the manufacturing process, slurries or
pastes of negative and positive electrode materials are
continuously coated onto a carrier sheet.
[0013] The positive electrode material preferably has a composition
similar to that employed to fabricate the nickel electrode in a
conventional nickel-cadmium battery, although there may be some
important optimizations for the nickel zinc battery system. The
negative electrode preferably employs zinc oxide as an
electrochemically active material. In some embodiments, the
negative electrode includes other materials such as bismuth oxide
and/or aluminum oxide. The carrier for the negative electrode
(which serves as a current collector) should be electrochemically
compatible with the negative electrode materials. For a zinc
electrode, for example, the carrier material is preferably copper
or an alloy of copper in the form of a perforated sheet or an
expanded metal.
[0014] In one embodiment, the negative electrode materials include
dispersants to minimize agglomeration of zinc oxide particles, as
agglomeration has been found to adversely affect the performance of
nickel zinc cells. In further embodiments, the methods employ
multiple sheets (e.g., four) of separator material that separate
the electrodes and minimize formation of zinc dendrites. Examples
of suitable separator materials include nylon sheets and
microporous polyolefin sheets. Further, the fabrication methods of
this invention preferably make use of a high conductivity
electrolyte that discourages dendritic growth in the zinc
electrode.
[0015] Importantly, the fabrication methods produce cells having a
zinc negative electrode, yet they utilize certain production
techniques heretofore reserved for other cell types. In particular,
existing nickel cadmium production lines may be utilized with minor
modifications to accommodate the methods described herein. In one
example, the method employs the following sequence: coating
positive and electrode current collector sheets with pastes or
slurries of positive and negative electrode materials, drying and
compressing the nascent electrode sheets, cutting and cleaning the
sheets, and forming a "jelly roll" cell assembly from the cut
electrode sheets and interleaved microporous separator sheets. The
methods described will find particular use in an all ready
well-defined and mature manufacturing base.
[0016] Finally, methods are disclosed to increase the efficiency of
the rechargeable power cells by reversing the polarity of the cell
such that he terminals are reversed veer conventional methods of
manufacture.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention may be understood more fully by reference to
the following description taken in conjunction with the
accompanying drawings in which:
[0018] FIG. 1 is a diagrammatic representation of the process flow
of an embodiment of the present invention.
[0019] FIG. 2 is a further diagrammatic representation of the
process flow of an embodiment of the present invention.
[0020] FIG. 3 is a graphical cross-sectional representation of the
cathode and anode prior to winding.
[0021] FIG. 4a is a graphical representation of a reverse polarity
power cell sub assembly embodiment.
[0022] FIG. 4b is a graphical cross-sectional representation of a
reverse polarity power cell sub assembly embodiment.
[0023] It is to be understood that, in the drawings, like reference
numerals designate like structural elements. Also, it is understood
that the depictions in the figures are not necessarily to
scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The present invention relates generally to techniques for
the manufacture of nickel-zinc rechargeable battery cells using
techniques similar to generally accepted production techniques for
nickel-cadmium rechargeable power cells.
[0025] Embodiments of this aspect of the invention are discussed
below with reference to FIGS. 1-4b. However, those skilled in the
art will readily appreciate that the detailed description given
herein with respect to these figures is for explanatory purposes as
the invention extends beyond these limited embodiments.
[0026] Turning first to FIG. 1, FIG. 1 presents a diagrammatic
representation of a manufacturing process flow for an embodiment of
the present invention. Initially, the process includes two separate
paths, one for fabricating a negative electrode sheet and the other
for fabricating a positive electrode sheet. Eventually, in the
process described, these two paths converge when the separate
negative and positive electrodes are assembled into a single cell.
Typically, the process steps of these two paths are performed in
parallel, within a single plant, so that the electrodes can be
continuously formed as sheets that are then continuously combined
in the process of forming nickel zinc cells in accordance with this
invention.
[0027] Blocks 101 and 121 depict example starting materials for the
positive and negative electrodes. These blocks represent the fact
that the manufacturing process must begin by providing the
requisite starting materials for creating electrodes. In addition
to the raw electrode formulation materials, the process will also
require electrode carrier sheets (comprised of conductive material
that ultimately serves as the current collectors in the assembled
cells) as well as separator sheets for separating the positive and
negative electrodes in the assembled cells, electrolyte, water to
form electrode pastes or slurries, and cell packaging materials
(e.g., disk terminals, a cell can, etc.).
[0028] Considering the negative electrode first, the manufacturing
process begins by providing the negative electrode materials (101)
required to form the negative electrode. In the example embodiment
shown in FIG. 1, negative electrode materials (101) include ZnO,
Bi.sub.2O.sub.3, Al.sub.2O.sub.3, HEC, and dispersant. Indium oxide
is also included in some embodiments. Various other formulations
are possible, including those that employ other forms of zinc, such
as calcium zincate or precursors thereof (e.g., calcium oxide and
zinc oxide). Other electrode formulations include various inorganic
fluorides, inorganic fibers such as alumina-silica fibers, and
organic fibers such as cotton flock etc.
[0029] As noted above, the invention is generally directed to
methods of manufacturing nickel zinc batteries. As such, the
negative electrode materials are based on zinc and zinc compounds,
which are significantly less hazardous than the more commonly used
cadmium compounds. The EPA has classified zinc and zinc compounds
in Group D indicating inadequate evidence as to its carcinogenic
potential (U.S. EPA, 1995a). Further, the International Agency for
Research on Cancer (IARC) has not classified zinc as to its
carcinogenic potential (IARC, 1987a). In contrast, epidemiological
evidence strongly supports an association between cadmium exposure
and neoplasia, including respiratory and renal cancers (ARB,
1986c). Further, the EPA has classified cadmium in Group B1:
Probable human carcinogen, based on human and animal studies
showing an increase of lung cancer (U.S. EPA, 1994a). Further, the
IARC has classified cadmium and cadmium compounds in Group 1: Human
carcinogen based on epidemiological evidence of carcinogenicity in
humans and carcinogenic effects observed in animals (IARC,
1993b).
[0030] As indicated, zinc oxide is a suitable electrochemically
active material for use in the negative electrode. Other zinc
compounds having may be utilized as well. In particular, another
example embodiment, calcium zincate (CaZn(OH).sub.4), may be used
as a starting material in place of ZnO. It can be appreciated that
producing calcium zincate can comprise potentially damaging
exothermic reaction. The reaction is also strongly dehydrating and
can be difficult to control during generation of a slurry or paste
for manufacturing the negative electrode. Therefore, if calcium
zincate is to be used, it should at least be partially pre-formed
ex situ. Only then should it be added to the mixture of negative
electrode materials. A general procedure for using calcium zincate
in this manner is described in PCT Patent Application No.
CA02/00352 (International Publication No. WO 02/075825) by inventor
J. Phillips, filed Mar. 15, 2002, which is incorporated herein by
reference for all purposes.
[0031] It has been found that a high temperature "burn out"
procedure can improve the high rate performance of the resulting
zinc electrode. In a typical approach, zinc oxide is heated to a
temperature of between about 300 and 380.degree. C. (preferably
about 320.degree. C.) for a period of between about 0.5 and 2 hours
in an inert atmosphere or under vacuum (to limit oxidation of
underlying copper current collectors). The burn out procedure may
remove dispersion agents and other organic materials believed to
have a detrimental effect on high rate discharge of zinc
electrodes. Alternatively, or in addition, the burn out procedure
may remove carbonate, which may impede high rate discharge. It may
reduce the conductivity of the electrolyte by, possibly, depleting
hydroxide from the electrolyte and/or reducing the transport
capability of the electrolyte. Unfortunately, zinc oxide readily
reacts with carbon dioxide in the ambient to form zinc carbonate.
Hence the surface of zinc oxide particles exposed to the ambient
can gradually attain relatively high amounts of carbonate. Many
commercial sources of zinc oxide have significant carbonate
content. To mitigate this problem, it may be desirable to heat the
zinc oxide and drive off carbon dioxide prior to electrode
manufacture. In a preferred embodiment, the zinc oxide used to
manufacture negative electrodes contains not greater than about 1
percent by weight of carbonate.
[0032] In addition to the zinc oxide or other electrochemically
active zinc source, the negative electrode slurry or paste may
include other materials that facilitate certain processes within
the electrode such as ion transport, electron transport, wetting,
porosity, structural integrity, active material solubility etc.
Other additives control the consistency for flow and other
process-relevant properties of the slurry or paste itself. In a
specific embodiment, the negative electrode slurry includes bismuth
oxide, aluminum oxide, hydroxyethyl cellulose (HEC), and a
dispersant.
[0033] Hydroxyethyl cellulose (HEC) may be used to control the
consistency of the slurry or paste of negative electrode materials.
HEC is a nonionic, water-soluble polymer that can thicken, suspend,
bind, emulsify, form films, stabilize, disperse, retain water, and
provide protective colloid action. It is readily soluble in hot or
cold water and can be used to prepare solutions with a wide range
of viscosities. Also, it has outstanding tolerance for dissolved
electrolytes. Hence, HEC or other material with related properties
is used in the present invention to force the negative electrode
materials (in its paste form) to retain water.
[0034] In one approach to forming a negative electrode slurry, two
separate mixtures are produced and then combined to form the
slurry. The first mixture comprises water and HEC (or other
suitable material) and the second mixture comprises water and
pre-sieved solids (e.g., ZnO, Al.sub.2O.sub.3, Bi.sub.2O.sub.3, and
dispersant).
[0035] Other negative electrode compositions 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 (J. Phillips), and U.S.
patent application Ser. No. 10/098,195 filed Mar. 15, 2002. Among
the negative electrode additives described in these references are
silica and fluorides of various alkaline earth metals, transition
metals, heavy metals, and noble metals.
[0036] As indicated, the manufacturing process preferably employs
low carbonate zinc oxide. Unfortunately it has been found that low
carbonate zinc oxide agglomerates in suspension much more readily
than the typical higher carbonate oxide. Hence it has been found
difficult to produce negative electrodes with well mixed, evenly
dispersed components, as the zinc oxide tends to form agglomerates.
To address this problem, the present invention employs a dispersant
to minimize agglomeration of low carbonate zinc oxide particles.
Generally, a dispersant modifies the surface properties of
particles to facilitate dispersal throughout a slurry or other
suspension. Many dispersants are conventional surfactants or
variations thereof tailored for the surface properties of
particular particles to be dispersed. As zinc oxide is widely used
in the paint industry, various dispersants for the oxide have been
developed. One such dispersant is commercially available as
NOPCOSPERSE 44, an anionic water soluble polymeric dispersant by
San Nopco Ltd. of Kyoto Japan. The dispersant, in sufficient
quantity, has been found to coat the surface of the zinc oxide
particles thus eliminating the agglomeration of zinc oxide
particles in manufacturing processes of this invention.
[0037] Returning to FIG. 1, the negative electrode materials (101)
are combined as described above to form a water-based slurry (103)
and continuously applied to a conductive carrier sheet (151) as a
slurry coat (105). It can be appreciated by one skilled in the art
that the material may also be made into a paste and continuously
applied via a paste head as in step 125 for the formation of the
positive electrode. In a specific example, the paste head applies
negative electrode paste mix at a pressure of approximately 3 psi
to one side of the carrier sheet. If slurry coating is employed, a
conventional slurry coating apparatus may be used. In such
apparatus, a continuous supply of slurry is provided to a chamber
through which the carrier sheet passes.
[0038] Note that conventional zinc electrodes are formed by vacuum
techniques that draw paste or slurry onto the carrier of the
negative electrode. In some cases, where calcium zincate has been
used as the active material, a petroleum-based additive is employed
to promote fibrillation of Teflon that facilitates the production
of an electrode sheet that may be pressed onto the carrier.
[0039] It can be appreciated that any of a number of carrier
materials may be used to form the positive electrode including, but
not limited to, nickel, nickel plated steel, silver, and the like.
One skilled in the art will understand that the carrier sheet
serves as the current collector for the negative electrode in the
finished nickel zinc cell. Copper and copper alloys are
particularly preferred materials for the carrier sheet given
copper's low resistivity, relatively low price, and electrochemical
compatibility with the zinc electrode. Notably, nickel plated steel
is the carrier of choice for the cadmium electrode in commercial
nickel cadmium cells.
[0040] The carrier sheet can be provided in various structural
forms including a perforated metal sheet, an expanded metal, and a
metal foam. Among the criteria employed to select a particular
structural form are cost, ease of coating, and ability to
facilitate electron transport between the electrochemically active
electrode material and the current collector. In a preferred
embodiment, the thickness of the carrier is between about 2 and 5
mils for perforated sheet but may be between 2 and 20 mils for
expanded metal. Metal foam substrates may be between 15 and 60
mils.
[0041] Once the negative electrode is coated, it is dried (107) to
drive off excess water used as a delivery medium for the electrode
material. A thermal drier employing flowing air or nitrogen may be
employed for this purpose.
[0042] The resulting negative electrode is then compressed (109)
using a roller or other appropriate compression mechanism. As can
be appreciated by one skilled in the art, compression brings the
electrode to a uniform thickness so that manufacturing tolerances
may be maintained. Compression also brings the negative electrode
to a desired porosity. It can be appreciated that porosity controls
the transport properties of ions between electrolyte and electrode.
Porosity also dictates the active surface area and hence the
current density of the negative electrode. In a typical example,
the post-compression thickness of the negative electrode is between
about 10 and 40 mils (e.g., about 20 mils) and the porosity is
between about 40 and 65%.
[0043] After compressing the electrode sheet by an appropriate
degree, it may be cut and cleaned at a step 111. Note that the
carrier sheet typically has a width significantly greater than that
required for a single battery cell; e.g., the width may be on the
order of one yard. Therefore, the negative electrode is cut to a
width conforming to end-product specific tolerances (e.g.,
approximately 1.25 inches for a sub C size cell for example). A
portion of the cut negative electrode may be "cleaned" prior to
further assembly. In particular, a strip of the negative electrode
material along the edgewise length of the negative electrode is
removed. The cleaned strip facilitates the attachment of a terminal
to the negative electrode current collector at a further step 203
by exposing the underlying current collector metal. While other
methods of terminal attachment may be accomplished, cleaning leaves
a surface that is particularly well suited to soldering, spot
welding, or any other type of electro-conductive bonding known in
the art. As can be appreciated by one skilled in the art, cleaning
the electrode may be accomplished by any of the following methods
without limitation: scraping, scouring, grinding, washing, and
wiping. This is typically achieved in combination with a vacuum
clean up to eliminate particulate matter.
[0044] In like manner, as discussed above for the negative
electrode, the positive electrode may be formed. Beginning at an
initial step 121, positive electrode materials are provided to form
the positive electrode. In an example embodiment and as shown in
FIG. 1, the positive electrode materials (121) comprising nickel
hydroxide (Ni(OH).sub.2), zinc oxide, cobalt oxide (CoO), nickel
metal, optionally cobalt metal, and a flow control agent such as
carboxymethyl cellulose (CMC) are provided. In a preferred
embodiment, at least some of the zinc oxide and cobalt oxide are
provided with the nickel hydroxide in a chemical mixture, whereby
individual particles contain nickel hydroxide, zinc oxide and
cobalt oxide. Such premixed materials may be prepared by
co-precipitation of the individual components and may be acquired
in a commercially available formula from commonly known vendors
such as International Nickel Corporation, and Tanaka. These
materials prevent leaching by locking the oxides into the insoluble
nickel matrix. Co-precipitation also apparently helps charge
transfer efficiency by creating conductive channels through the
positive electrode materials. In a preferred embodiment, the zinc
oxide and cobalt oxide are each present in the co-precipitated
material in concentrations of about 2-3% by weight for the zinc and
about 2-6% for the cobalt oxide. Further, the positive electrode
materials may additionally include chemically pure cobalt and
nickel metal.
[0045] If cobalt metal is employed in the positive electrode, it is
preferably present in a concentration of between about 1% to 10% by
weight. This concentration range is appropriate for a wide range of
discharge rates (e.g., about 0.001 to 0.4 Amperes/cm.sup.2 of zinc
electrode surface area). In a typical high rate application (e.g.,
discharge is conducted at about 0.01 to 0.4 Amperes/cm.sup.2 of
zinc electrode surface area), the concentration of cobalt metal is
between about 4-10% by weight in the positive electrode. In a
typical low rate application, the concentration of cobalt metal is
between about 1-5% by weight, and the discharge is conducted at
about 0.001 to 0.01 Amperes/cm.sup.2 of zinc electrode surface
area.
[0046] In alternate embodiments, cobalt oxide may be added to the
material to enhance conductivity at operation 121. However, it is
generally preferred that the starting materials include little or
no additional cobalt oxide. Note that in commercial nickel cadmium
cells, free cobalt oxide is commonly employed in the positive
electrode mixture.
[0047] In one approach, the positive electrode paste is formed from
two separate mixtures: one that includes CMC
(carboxymethylcellulose) and water and another that includes water
and the co-precipitated nickel hydroxide--cobalt oxide--zinc oxide,
nickel metal, and relatively pure cobalt oxide. These to mixtures
are then combined to form the positive electrode paste. Note that
the CMC is included to improve flow characteristics for the
resulting paste at step 123.
[0048] A few positive electrode compositions are described in the
following documents, each of which is incorporated herein by
reference: PCT Publication No. WO 02/039534 (J. Phillips)
(co-precipitated Ni(OH).sub.2, CoO and finely divided cobalt metal)
and U.S. patent application Ser. No. 10/098,194 filed Mar. 15, 2002
(fluoride additives).
[0049] At block 125, the paste is continuously applied to a
positive electrode carrier sheet via a paste head. It can be
appreciated that any of a number of carrier metals may be used to
form the negative electrode so long as they meet appropriate design
criteria such as low cost, high conductivity, electrochemical
compatibility with the positive electrode, and good contact with
the electrochemically active material. Examples include, but not
limited to, nickel and nickel-plated stainless steel. The carrier
metal may possess one of various structural forms including
perforated metal, expanded metal, sintered metal, metal foam and
metal-coated polymer materials. In a preferred embodiment, the
carrier is a nickel metal foam formed by, for example, pyrolyzing a
urethane foam on which nickel was electrolytically deposited. The
thickness of the foam positive electrode carrier after undergoing a
prepress to ensure thickness uniformity is between about 15 and 60
mils, for example.
[0050] Once the carrier sheet is coated with positive electrode
material, the resulting sheet is dried (127) to drive off excess
water and then compressed (129). These process operations can be
performed in much the same manner as operations 107 and 109 for the
negative electrode. As can be appreciated by one skilled in the art
and as noted above for the negative electrode, compression brings
the electrode to a uniform desired thickness and porosity as
described above for the negative electrode path. Preferably, the
resulting positive electrode sheet has a thickness of between about
15 and 40 mils (e.g., about 25 mils) and has a porosity of between
about 30 and 45%.
[0051] The positive electrode is then tabbed and slit at a step
131. The slitting of the electrodes to width may be done before or
after tabbing, however most automated lines slit the electrode
stock after an in-line tabbing process. Tabbing of the coined edges
facilitates the final welding of the current collector arrangement
after winding of the jelly-roll. The tab consists of a strip of
nickel or nickel-plated steel that is seam welded, resistance
welded or ultrasonically welded to a coined unpasted edge of the
electrode. This creates a solid strip along the length of the
electrode that promotes a strong bond to the current collection
disc.
[0052] At this point in the process, the two paths (negative
electrode fabrication and positive electrode fabrication) converge
to provide a layered arrangement of sheets that remain with the
cell to completion. At the convergence point (represented as "A" in
FIG. 1), multiple sheets of material are brought together and wound
or otherwise assembled into a cell structure. In a particular
example four sheets of separator material (depicted as sources 113
and 133) are interleaved with the positive electrode sheet and the
negative electrode sheet. Thus, once the electrodes are fabricated,
they are sandwiched between semi-permeable separator sheets (113,
133).
[0053] In an example embodiment, the separator for the positive
electrode (113) comprises a plurality of sheets of a microporous
polyolefin that may be commonly acquired as CELGARD.TM. line of
separators from Celgard Inc. (Charlotville, N.C.) or SOLUPORE
products from Solutech. The separator serves to mechanically
isolate the positive and negative electrodes, while allowing ionic
exchange to occur between the electrodes and the electrolyte. Thus,
good permeability to the electrolyte is desirable. In an example
embodiment, the separator for the negative electrode (133)
comprises a nylon sheet. Other separator materials known in the art
may be employed. As indicated, nylon-based materials and
microporous polyolefins (e.g., polyethylenes and polypropylenes)
are suitable.
[0054] Turning briefly to FIG. 3, FIG. 3 is a cross-sectional
representation of the positive and negative electrodes prior to
winding. In the illustrated example, separators (301, 305) are
initially folded over the electrodes (303, 307) along the
electrode's planar surface before being drawn or fed, with the
electrode sheets, into a winding apparatus 309. In this approach
two sources of separator are employed. In an alternative
embodiment, each electrode sheet is straddled by two separate
sources of separator sheet so that four sources of separator,
rather than two are employed. Thus, initially, a separator sheet is
not folded over the leading edge of an electrode. However, the
resulting layered structure is the same. Both approaches produce a
structure in which two layers of separator separate each electrode
layer from the next adjacent electrode layer. This is generally not
the case with nickel cadmium cells, which employ only a single
layer of separator between adjacent electrode layers. The
additional layers employed in the nickel zinc cell help to prevent
shorting that could result from zinc dendrite formation.
[0055] 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.
[0056] Note that the separator sheets generally do not entirely
cover the full widths of the electrode sheets. Specifically, one
edge of each electrode sheet remains exposed for attaching
terminals at step 203. This is the edge that was wiped clean of
electrode material at steps 111 and 131. Further, and for the same
reason, the electrodes are offset laterally by approximately the
width of the cleaned strip that was discussed above. This presents
one lateral edge having only exposed negative current collector and
the opposite lateral edge having only exposed positive current
collector to accommodate terminal attachment in step 203.
[0057] The winding apparatus draws the various sheets in at the
same time and rolls them into a jellyroll-like structure. After a
cylinder of sufficient thickness is produced, the apparatus cuts
the layers of separator and electrodes to produce the finished cell
assembly.
[0058] Turning to FIG. 2, a continuation of the process flow of
FIG. 1 is shown. The process of FIG. 2 begins with a cell assembly
201 resulting from the assembling operation "A." For cylindrical
cells, this assembly is a jellyroll-like structure. In an
alternative embodiment, it is a rectangular or prismatic layered
structure. It can be appreciated by one skilled in the art that the
form factor of the cell assembly is user dependent and may take any
of a number of forms well known in the art. If a prismatic form is
used, it may be desirable to pre-wrap the positive and negative
electrode sheets in their own separators prior to assembly.
[0059] Once winding or other assembly of electrode layers and
interleaved separator layers is complete, separate internal
terminals are conductively attached (203) to each of the negative
electrode and the positive electrode. More specifically, a positive
terminal is conductively attached to the exposed positive current
collector at one axial end of the cell assembly and a negative
terminal is conductively attached to the exposed negative current
collector at the other axial end of the cell assembly. Attachment
of the internal terminal may be accomplished by any method well
known in the art such as spot welding, ultrasonic welding, laser
welding, soldering, or any other type of electro-conductive bonding
appropriate for the terminal and current collector materials. In
one embodiment of the present invention, the internal terminals
comprise disks, which may or may not be perforated or slotted. In
another embodiment, the internal terminals comprise H-shaped
structures. Regardless of their actual structure, the internal
terminals may be conductively attached to the electrodes without
requiring tabs to be present on the electrode with which an
internal terminal may attach.
[0060] After the internal terminals are attached, the cell assembly
is inserted into a retaining vessel (205) such as a can in the case
of a cylindrical cell assembly. The can or other vessel serves as
the outer housing or casing of the final cell. With the cell
assembly in the can, the terminal disks (or other internal
terminals) can be conductively attached to the cell's can and lid
or other external terminals at step 207.
[0061] The external terminals provide direct electrical access to
the power cell for a powered device. As such, the external
terminals, in some embodiments, are preferably plated with a
non-corrosive metal such as nickel plate. Further, the external
terminals may function to isolate the electrodes from mechanical
shock. Given the portable nature of electronic devices, the
likelihood of mechanical shock is reasonably high. If the external
terminals were directly attached with the electrode current
collectors, significant failure of the welded joint or direct
damage to the electrodes might occur.
[0062] In a next step 209, an appropriate electrolyte is provided.
Of particular relevance to the present invention, the electrolyte
should possess a composition that limits dendrite formation and
other forms of material redistribution in the zinc electrode. Such
electrolytes have generally eluded the art. But one that appears to
meet the criterion is described in U.S. Pat. No. 5,215,836 issued
to M. Eisenberg on Jun. 1, 1993, which is hereby incorporated by
reference. A particularly preferred electrolyte includes (1) an
alkali or earth alkali hydroxide present in an amount to produce a
stoichiometric, excess of hydroxide to acid in the range of 2.5 to
11.0 equivalents per liter, (2) a soluble alkali or earth alkali
fluoride in an amount corresponding to a concentration range of
0.01 to 1.0 equivalents per liter of total solution, and (3) a
borate, arsenate, and/or phosphate salt (preferably potassium
borate, potassium metaborate, sodium borate, and/or sodium
metaborate). In one specific embodiment, the electrolyte comprises
4.5 to 10 equiv/liter of potassium hydroxide, from 2.0 to 6.0
equiv/liter boric acid or sodium metaborate and from 0.01 to 1.00
equivalents of potassium fluoride. A currently preferred
electrolyte for high rate applications comprises 8 equiv/liter of
hydroxide, 4.5 equivalents of boric acid and 0.2 equivalents of
potassium fluoride.
[0063] 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.
[0064] In a specific embodiment, the electrolyte composition
includes an excess of between about 3 and 5 equiv/liter hydroxide
(e.g., KOH), NaOH, 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.
[0065] Various techniques may be employed to fill the vessel with
electrolyte. In one example, the electrolyte is introduced via an
injection process in which electrolyte enters the cell via a fill
port. In other cases the electrolyte may be added prior to lid
application and the cell is spun to distribute the fluid.
[0066] After the can or other containment vessel is filled with
electrolyte, the vessel is sealed to isolate the electrodes and
electrolyte from the environment. See block 211. As can be
appreciated by one skilled in the art, any of a number of sealing
methods may be utilized including, but not limited to crimping,
welding, stamping, or gluing. Note that in a cylindrical cell, the
lid is typically seated on a gasket residing on a circumferential
bead in the upper portion of the can. To effect sealing, the top
edge of the can is then crimped down toward the lid without making
electrical contact.
[0067] Although the cell is generally sealed from the environment,
the cell may be, in some embodiments, permitted to vent gases from
the battery that are generated during charge and discharge. A
typical nickel cadmium cell vents gas at approximately 200 PSI. In
some embodiments, a nickel zinc cell is designed to operate at this
pressure and even higher (e.g., up to 300 PSI) without the need to
vent. This may encourage recombination of any oxygen and hydrogen
generated within the cell. Preferably the cell is constructed to
maintain an internal pressure of not greater than about 600 PSI and
more preferably not greater than about 450 PSI. In other
embodiments, a nickel zinc cell is designed to vent gas at
relatively lower pressures. This may be appropriate to release
hydrogen and/or oxygen gases rather than encourage their
recombination within the nickel zinc cell. A vent mechanism is
preferably designed to allow gas escape but not allow electrolyte
passage that may interfere with the reproducible function of the
vent. The use of hydrophobic membranes can be effective for this
purpose (see e.g., U.S. patent application Ser. No. 10/098,193,
"Leak Proof Pressure Relief Valve for Secondary Batteries," filed
Mar. 15, 2002 for J. Phillips, which is incorporated herein by
reference for all purposes) and these may be used alone or in
conjunction with a tortuous gas escape route. Many other battery
venting mechanisms are known in the art and are suitable for use
with this invention.
[0068] To prepare a cell for use typically requires one or more
"formation" cycles to modify the electrode structure. In FIG. 2,
formation of the cell is accomplished at step 213. A formation
cycle follows a specific voltage-current-time curve that accounts
for factors such as electrode composition and cell capacity. In a
typical case, formation is accomplished using a large power supply
that charges numerous cells at once over a period of, for example,
about 24 to 74 hours.
[0069] In a specific example, formation and related operations may
be performed as follows. The cells are vacuum filled with
electrolyte and soaked for less than 2 hours (for comparison,
nickel cadmium cells are generally soaked for 24 hours but are
usually drip filled). Before the 2 hour period is up the cells are
placed on formation charge where a net 100%-150% of the theoretical
capacity is input over 24-60 hours. The formation protocol includes
a discharge and recharge step that is believed to help distribute
electrolyte, but this example can be practiced without this
operation. In another example, the mid-formation discharge is
eliminated. Cells may be charged singly or with two in series, but
in a production environment it is more usual to charge the cells in
larger series strings. During formation, one may monitor the
voltage behavior over the complete formation process and collect
weight loss, impedance values and open circuit voltages to classify
and identify possible weak cells.
[0070] Once formation is complete, every cell is discharged during
a quality control step 215 to determine the particular capacity of
each cell. Cells having similar capacities are grouped for use in
the same battery packs. In this way, each cell in a battery pack
becomes fully charged after receiving substantially the same
quantity of charge. Thus, when charge is complete no cell in the
pack is significantly under utilized and no cell in the pack is
significantly overcharged. Groupings of batteries may be restricted
to two or more groupings depending on the nature and sensitivity of
the application.
[0071] Note that the operations of FIG. 2 are generally the same as
or very similar to corresponding operations employed in the
manufacture of nickel cadmium cells. Hence, the apparatus employed
for these operations in nickel cadmium cell fabrication generally
can also be used in nickel zinc cell fabrication. Note, however,
that the electrode, current collector, and electrolyte differences
may require customized apparatus or processing. For example, the
zinc electrode may employ a copper current collector, which cannot
be employed with a cadmium electrode. While copper has better
electronic conductivity than steel, it can present fabrication
issues. For example, attaching a current collecting copper disk to
the copper sheet may require specific laser welding settings and
the appropriate jigs to provide continuous pressure during the
weld.
[0072] Nickel zinc cells prepared in accordance with this invention
may have particularly useful properties for power tool and UPS
applications. For example, energy densities will exceed about 60
Wh/kg and continuous power densities are commonly in excess of
about 500 W/kg.
[0073] In another embodiment of the present invention, the polarity
of the cell's terminals is reversed in comparison to conventional
cells for consumer electronics. Many of the same methods of
manufacture described herein are utilized to accomplish this
embodiment with minor variations that are herein described. In
conventional power cell manufacturing, the polarity of the cell is
established wherein the lid is positive and the can or vessel is
negative. That is, the positive electrode of the cell assembly is
electrically connected with the lid and the negative electrode of
the cell assembly is electrically connected with the can that
retains the cell assembly. Notably, the lid is electrically
isolated from the can through the use of a flexible gasket and a
bituminous sealing agent.
[0074] Turning to FIG. 4a, FIG. 4a is a graphical representation of
a reverse polarity power cell sub assembly embodiment. A cell
assembly (407) is manufactured using the techniques provided
herein. A negative internal terminal (405) is electrically
connected with the negative electrode of the cell assembly. Because
of the unique properties of Nickel/Zinc electrochemistry, the
negative internal terminal (405) is preferably comprised of copper.
Typically, conventional power cells exhibit an overall impedance of
approximately 3 to 5 milliohms. Approximately 0.8 milli-Ohm may be
attributed to the positive current collector and the resistance
weld to the cap. This is due in part to the compositional
requirements of the terminal given the electrochemistry of
conventional power cells. In the present embodiment, the use of
copper in the manner described results in a significant impedance
reduction of approximately 0.5 milli-Ohm at the now negative
terminal thus achieving a more power efficient cell. One skilled in
the art will appreciate that the foregoing advantages may be
achieved by coating or plating the interior surface of the
retaining vessel (411) with copper, although this method would
necessitate alternative manufacturing steps.
[0075] Importantly, copper, in this and other embodiments, requires
particular attention when used as an internal terminal. In
particular, copper, as is well known in the art, is particularly
effective, not only in carrying current, but also in conducting
heat. As such, the electrical attachment of the internal terminal
with the negative electrode requires specific manufacturing
techniques. In an example embodiment, the copper internal terminal
is perforated and attached at multiple points along the negative
electrode so that the electrode is attached to the current
collector at multiple points along its length. By activating a
greater area, the charge efficiency is further enhanced. The
perforations also serve to locate the electrode and allow
electrolyte to penetrate the stack uniformly during the electrolyte
fill operation. In another embodiment, the internal terminal is
slotted to achieve the named advantages. Further, as noted above,
copper is particularly efficient in conducting heat requiring novel
techniques of attaching the internal terminal with the negative
electrode to avoid damaging the negative electrode. Notably, these
novel techniques may be used in conjunction with other techniques
well known in the art such as spot welding, ultrasonic welding,
laser welding, soldering, or any other type of electro-conductive
bonding appropriate for the terminal and current collector
materials.
[0076] In one preferred embodiment, laser welding is employed to
form a low impedance connection between the internal terminal and
the copper current collector. In a specific approach, the laser
welding is performed in cross fashion or other tracking method that
maximizes the number of weldments, in which the laser is moved
across the surface of the internal terminal to catch each point. It
has been found that good welds can be achieved using laser pulses
with a 600 micron beam diameter having a power of between about 0.2
and 5 kW, pulse widths of between about 0.5 and 4 milliseconds and
pulse frequencies of between about 1 and 20 Hz. These power levels
and duty cycles can be achieved with a pulsed Nd:YAG laser such as
model number LW70A from Unitek Miyachi Corporation. In one
approach, wound nickel--zinc jelly rolls are inserted into a jig
where the copper current collection disc is pressed onto the
exposed copper edges of the negative electrode. The laser beam is
then programmed to lap weld the connections as it traverses the end
of the jelly-roll. The required power levels are dependent on the
thickness of the copper electrode substrate and the thickness of
the current collection disc. The former is normally between
0.002-0.005 inches and the latter is between 0.002-0.01 inches
depending on the cell size and current carrying capability.
[0077] Returning to FIG. 4a, the positive internal terminal (409)
is electrically connected with the positive electrode using
techniques well known in the art. The power cell fabrication then
proceeds using the manufacturing techniques described herein. For
illustrative purposes, FIG. 4b is presented to further clarify the
embodiment described above. In particular, FIG. 4b is a graphical
cross-sectional representation of a reverse polarity power cell sub
assembly embodiment. Notably, FIG. 4b illustrates the position of
the insulating gasket (403) that serves to electrically isolate the
negative external terminal (i.e. the lid (401)) from the positive
external terminal (i.e. the can (409). It is contemplated that a
variety of materials may be used to achieve electrical isolation in
the present embodiment.
[0078] The foregoing describes the instant invention and its
presently preferred embodiments. Numerous modifications and
variations in the practice of this invention are expected to occur
to those skilled in the art. Such modifications and variations are
encompassed within the following claims.
[0079] The entire disclosures of all references cited herein are
incorporated by reference for all purposes.
* * * * *