U.S. patent application number 12/877841 was filed with the patent office on 2011-03-10 for heat sealing separators for nickel-zinc cells.
This patent application is currently assigned to POWERGENIX SYSTEMS, INC.. Invention is credited to Bryan L. McKinney, Jeffrey Philips, Steve Salamon, Brian M. Schroeter, Todd F. Tatar, James Wu.
Application Number | 20110059343 12/877841 |
Document ID | / |
Family ID | 43016872 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110059343 |
Kind Code |
A1 |
McKinney; Bryan L. ; et
al. |
March 10, 2011 |
HEAT SEALING SEPARATORS FOR NICKEL-ZINC CELLS
Abstract
Embodiments are described in terms of selective methods of
sealing separators and jellyroll electrode assemblies and cells
made using such methods. More particularly, methods of selectively
heat sealing separators to encapsulate one of two electrodes for
nickel-zinc rechargeable cells having jellyroll assemblies are
described. Selective heat sealing may be applied to both ends of a
jellyroll electrode assembly in order to selectively seal one of
two electrodes on each end of the jellyroll.
Inventors: |
McKinney; Bryan L.; (San
Diego, CA) ; Salamon; Steve; (Poway, CA) ; Wu;
James; (Olmsted Township, OH) ; Tatar; Todd F.;
(Bonita, CA) ; Schroeter; Brian M.; (San Diego,
CA) ; Philips; Jeffrey; (La Jolla, CA) |
Assignee: |
POWERGENIX SYSTEMS, INC.
San Diego
CA
|
Family ID: |
43016872 |
Appl. No.: |
12/877841 |
Filed: |
September 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61240600 |
Sep 8, 2009 |
|
|
|
Current U.S.
Class: |
429/94 ;
29/623.4 |
Current CPC
Class: |
H01M 10/0431 20130101;
H01M 50/44 20210101; H01M 50/46 20210101; Y10T 29/4911 20150115;
Y02E 60/10 20130101; H01M 10/30 20130101; Y10T 29/49112 20150115;
H01M 50/411 20210101; H01M 50/449 20210101; Y10T 29/49114
20150115 |
Class at
Publication: |
429/94 ;
29/623.4 |
International
Class: |
H01M 10/02 20060101
H01M010/02; H01M 4/04 20060101 H01M004/04 |
Claims
1. A method of selectively sealing a first set of separator layers
disposed on both sides of and extending past an edge of a first
electrode of a jellyroll assembly comprising two electrodes, while
not sealing a second set of separator layers disposed on both sides
of and extending past an edge, parallel and proximate to the edge
of the first electrode, of a second electrode, both edges disposed
on the same end of the jellyroll assembly, while exposing said same
end of the jellyroll assembly to a heat source.
2. The method of claim 1, wherein selectively sealing the first set
of separator layers comprises: i) configuring the current
collecting substrate of the second electrode so that when the heat
source is applied to said 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 said same end of the jellyroll
assembly.
3. The method of claim 2, wherein configuring the current
collecting substrate of the second electrode comprises 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,
wherein the first set of separator layers and adjoining separator
layers from the second set of separator layers are disposed in the
substantially enclosed volume.
4. The method of claim 1, wherein selectively sealing the first set
of separator layers comprises: i) configuring the jellyroll
assembly such that the first set of separator layers comprises
layers that can seal to envelop the first electrode when the heat
source is applied, but the second set of separator layers comprises
layers that can not seal to envelop the second electrode when the
heat source is applied; and ii) applying the heat source to said
same end of the jellyroll assembly.
5. The method of claim 1, wherein the first set of separator layers
and the second set of separator layers each have different melting
points.
6. The method of claim 1, wherein the first set of separator layers
are polypropylene layers and the second set of separator layers are
cellulose-based layers.
7. The method of claim 6, wherein the second set of separator
layers are cellulose impregnated with polyvinyl alcohol.
8. The method of claim 7, wherein the heat source comprises at
least one of a convective heat source, an inductive heat source, a
conductive heat source and a radiative heat source.
9. The method of claim 8, wherein the heat source is a conductive
heat source.
10. The method of claim 9, wherein the conductive heat source is a
heated platen.
11. The method of claim 10, wherein said same end of the jellyroll
is contacted with the heated platen for between about 3 seconds and
about 10 seconds, wherein the platen temperature is between about
300.degree. C. and 600.degree. C.
12. The method of claim 11, wherein the jellyroll is contacted with
the heated platen with a force of between about 0.5 kg/cm.sup.2 and
about 5 kg/cm.sup.2.
13. The method of claim 1, wherein the first electrode is a zinc
electrode and the second electrode is a nickel electrode.
14. A jellyroll electrode assembly comprising: i) a first electrode
disposed between a first set of separator layers; and ii) a second
electrode disposed between a second set of separator layers;
wherein, at the same end of the jellyroll electrode assembly, one
of the first electrode and the second electrode is enveloped by its
respective set of separator layers and the other electrode is not
enveloped by its set of separator layers.
15. The jellyroll electrode assembly of claim 14, wherein the first
electrode is a zinc electrode and the second electrode is a nickel
electrode.
16. The jellyroll electrode assembly of claim 15, wherein the first
set of separator layers comprises polypropylene layers.
17. The jellyroll electrode assembly of claim 16, wherein the
second set of separator layers comprises polyvinyl alcohol
impregnated cellulose.
18. The jellyroll electrode assembly of claim 15, wherein at both
ends of the jellyroll electrode assembly, one of the first
electrode and the second electrode is enveloped by its respective
set of separator layers and the other electrode is not enveloped by
its set of separator layers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
Provisional Application Ser. No. 61/240,600 filed Sep. 8, 2009, the
contents of which are incorporated herein by reference in their
entirety and for all purposes.
BACKGROUND
[0002] This invention pertains generally to rechargeable batteries
and specifically to rechargeable nickel-zinc atteries. More
specifically, this invention pertains electrode assemblies used in
rechargeable nickel-zinc batteries and methods of manufacture.
[0003] 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.
[0004] Particularly important is life cycle of rechargeable
batteries. Nickel-zinc batteries can suffer from electrical shorts
due to, e.g., dendrite formation from the negative (zinc) electrode
to the positive (nickel) electrode. Previous approaches to this
problem include, e.g., chemical modification of the electrodes to
reduce the propensity toward shorting, but these are not typically
optimal chemistries for high rate discharge and battery capacity.
Coating or taping edges of electrodes is difficult to implement on
a production scale and typically are not highly effective.
[0005] Separators are typically used to block dendrites from
creating shorts between the electrodes but dendrites can migrate
around separators unless they are sealed to envelop the electrodes.
Sealing separators to envelop individual electrodes effectively
blocks dendrite growth (or other particle migration) between
electrodes, which extends battery life. In prismatic cells
individual electrodes are enveloped prior to assembly of the
electrode stack.
[0006] In wound electrodes, enveloping individual electrodes prior
to winding is problematic due to wrinkling, binding and other
difficulties attributable to the physical characteristics of the
separator materials and the fact that many layers are wound
together in the jellyroll. Heat sealing separators post-winding is
known, but such methods only address sealing both electrodes on one
end of a wound jellyroll electrode assembly. These methods do not
allow for flexibility in internal cell design which is often
critical in ever changing uses for rechargeable nickel zinc
cells.
SUMMARY
[0007] The invention is most generally described in terms of
selective methods of sealing separators and jellyroll electrode
assemblies made using such methods. More particularly the invention
is described in terms of methods of selectively heat sealing
separators on one of two electrodes for nickel-zinc rechargeable
cells having jellyroll assemblies. Selective sealing can be
employed on one or both ends of a jellyroll assembly.
[0008] Thus, one aspect of the invention is a method of selectively
sealing a first set of separator layers disposed on both sides of
and extending past an edge of a first electrode of a jellyroll
assembly including two electrodes, while not sealing a second set
of separator layers disposed on both sides of and extending past an
edge, parallel and proximate to the edge of the first electrode, of
a second electrode, both edges disposed on the same end of the
jellyroll assembly, while exposing the same end of the jellyroll
assembly to a heat source. This method can be accomplished in a
number of ways in accord with the embodiments described herein.
[0009] 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 a
specific 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.
[0010] In another embodiment, selectively sealing the first set of
separator layers includes: i) configuring the jellyroll assembly
such that the first set of separator layers includes layers that
can seal to envelop the first electrode when the heat source is
applied, but the second set of separator layers includes layers
that can not seal to envelop the second electrode when the heat
source is applied; and ii) applying the heat source to the same end
of the jellyroll assembly.
[0011] In one embodiment, as applied to the embodiments described
above, the first set of separator layers and the second set of
separator layers each have different melting points. In another
embodiment, as applied to the embodiments above, the first set of
separator layers are polypropylene layers and the second set of
separator layers are cellulose-based layers. In one embodiment, the
cellulose-based layers are cellulose impregnated and/or coated with
polyvinyl alcohol (PVA).
[0012] In one embodiment, the heat source includes at least one of
a convective heat source, an inductive heat source, a conductive
heat source and a radiative heat source. In another embodiment the
heat source is a conductive heat source. In another embodiment the
conductive heat source is a heated platen. In one embodiment, the
end of the jellyroll that is heated, where the first electrode is
selectively enveloped via sealing the first set of separators, is
contacted with the heated platen for between about 1 second and
about 30 seconds, where the platen temperature is between about
130.degree. C. and 600.degree. C. In another embodiment, the
jellyroll is contacted with the heated platen for between about 3
seconds and about 10 seconds, where the platen temperature is
between about 300.degree. C. and 600.degree. C. In yet another
embodiment, the jellyroll is contacted with the heated platen for
between about 5 seconds and about 25 seconds, where the platen
temperature is between about 450.degree. C. and 550.degree. C.
[0013] In some embodiments, during contact with the heated platen,
the jellyroll is contacted with the heated platen with a force of
between about 0.5 kg/cm.sup.2 and about 5 kg/cm.sup.2. In other
embodiments, the jellyroll is contacted with the heated platen with
a force of between about 1 kg/cm.sup.2 and about 3 kg/cm.sup.2. In
other embodiments, the jellyroll is contacted with the heated
platen with a force of between about 1 kg/cm.sup.2 and about 2
kg/cm.sup.2. In other embodiments, the jellyroll is contacted with
the heated platen with a force of about 1.5 kg/cm.sup.2.
[0014] Methods of the invention can be practiced with any jellyroll
configured electrode assembly, and is particularly useful for
nickel zinc cells where dendrite formation from the zinc electrode
can short the electrodes.
[0015] Thus, another aspect of the invention is a jellyroll
electrode assembly including: i) a first electrode disposed between
a first set of separator layers; and ii) a second electrode
disposed between a second set of separator layers; where, at the
same end of the jellyroll electrode assembly, one of the first
electrode and the second electrode is enveloped by its respective
set of separator layers and the other electrode is not enveloped by
its set of separator layers. In one embodiment, the first electrode
is a zinc electrode and the second electrode is a nickel electrode.
In another embodiment, the first set of separator layers includes
polypropylene layers. In another embodiment, the second set of
separator layers includes polyvinyl alcohol impregnated cellulose.
Batteries which include the jellyroll electrode assemblies
described herein are another aspect of the invention.
[0016] These and other features and advantages are further
discussed below with reference to the associated drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1A, 1B and 1C are graphical representations of the
main components of cylindrical nickel zinc power cells of the
invention.
[0018] FIG. 2A is a perspective representation showing assembly of
electrodes and separator layers prior to winding into a
jellyroll.
[0019] FIG. 2B is a cross section of the assembly in FIG. 2A.
[0020] FIG. 2C is a cross section of a jellyroll assembly of the
invention.
[0021] FIG. 2D is a cross section of a jellyroll assembly after a
current collecting substrate is folded in a particular
configuration and after selective heat sealing at one end of the
jellyroll.
[0022] FIG. 2E is a cross section of the jellyroll assembly of FIG.
2D incorporated into a reverse polarity battery.
[0023] FIG. 2F is a cross section of the jellyfoll assembly as
described in relation to FIG. 2D after the opposite end of the
jellyroll is subjected to selective heat sealing.
[0024] FIG. 2G is a cross section of a sealed separator from the
jellyroll described in relation to FIG. 2F.
[0025] FIG. 2H is a cross section of the jellyroll assembly of FIG.
2F incorporated into a reverse polarity battery.
[0026] FIG. 2I is a cross section of an electrode-separator
stack.
[0027] FIG. 2J is a cross section of a jellyroll assembly.
[0028] FIG. 2K is a cross section of the jellyroll assembly of FIG.
2J after heat sealing at one end of the jellyroll.
[0029] FIG. 2L is a cross section of the jellyroll assembly of FIG.
2K incorporated into a normal polarity battery.
[0030] FIG. 2M is a cross section of the jellyfoll assembly as
described in relation to FIG. 2K after the opposite end of the
jellyroll is subjected to selective heat sealing.
[0031] FIG. 2N is a cross section of the jellyroll assembly of FIG.
2M incorporated into a normal polarity battery.
[0032] FIG. 3 is a graph showing comparative results for nickel
zinc batteries manufactured using the heat sealing methods
described herein and for those not employing the heat sealing
methods.
DETAILED DESCRIPTION
A. Definitions
[0033] Some of the terms used herein are not commonly used in the
art. Other terms may have multiple connotations in the art.
Therefore, the following definitions are provided as an aid to
understanding the description herein. The invention as set forth in
the claims should not necessarily be limited by these
definitions.
[0034] "Heated Platen" refers to e.g. a heated stage, hotplate or
other hot surface upon which a work piece can be placed to expose
the work piece to heat.
[0035] "Conductive heat source" refers to a device that transfers
heat to a work piece via direct contact with the work piece and
thus heat is conducted from the heat source directly to the work
piece being heated. An example of a conductive heat source is a
heated platen, where the work piece is contacted with the heated
platen.
[0036] "Convective heat source" refers to a device that transfers
heat to a work piece via a gas or liquid by the circulation of
currents from one region to another. An example of a convective
heat source is a heat gun, which blows hot air onto a work piece to
heat the work piece.
[0037] "Inductive heat source" refers to a device that transfers
heat to a work piece via inducing electrical eddy currents in the
work piece by exposure to a magnetic field produced by an
electrical coil (typically using alternating current therethrough).
Heat is generated in the work piece via resistance (Joule heating)
or via magnetic hysteresis losses in material. An example of an
inductive heat source is a magnetic induction welder. For example,
plastics may be welded by induction, if they are either doped with
ferromagnetic ceramics (where magnetic hysteresis of the particles
provides the heat) or doped with metallic particles (where
electrical resistance within the metal particles provides the
heat).
[0038] "Radiative heat source" refers to a device that transfers
heat to a work piece via energy radiated to the work piece, and
once striking the work piece, the energy is transferred to the
molecules of the work piece, thus exciting the molecules to
increase molecular motion and heating the work piece due to the
molecular motion and/or friction. Examples of radiative heat
sources are lasers, microwave generators, infrared radiation
generators and the like.
[0039] "Envelop" is meant to mean that once separator layers are
sealed, they serve as a continuous wrapping or covering for an end
or edge of an electrode of a jellyroll electrode assembly.
"Envelop" is not necessarily meant to mean encapsulating the entire
electrode as in the traditional sense of the term. Thus "envelop"
can mean, for example, once separator layers are sealed together,
an electrode resides in a pouch of the separator material or a
bifold of the separator material. "Envelop" can also mean, for
example, closing, for example heating sealing together two edge
portions of, separator material over an otherwise exposed
electrode.
[0040] "Seal" means to join separator layers by fusing or melting
them together for example by applying heat to the layers at, about
or above the melting temperature of the separator layers or a
component of the separator layers so that the layers fuse together.
Typically, but not necessarily, sealing is done near the edges of
layers where the layers overlap or adjoin but are not yet attached.
In one example, the layers are polypropylene and the layers are
heated along substantially co-extensive edge regions so that they
melt together to make a continuous layer and thus are "sealed"
together.
B. Overview
[0041] Embodiments are most generally described in terms of
selective methods of sealing separators and jellyroll electrode
assemblies made using such methods. More particularly, methods of
selectively heat sealing separators so as to envelop only one of
two electrodes at the end of a jellyroll assembly are described.
These methods may be applied to one or both ends of the jellyroll.
In particular embodiments, the jellyroll assemblies are used for
nickel-zinc rechargeable cells.
[0042] 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 assembly.
Separator and electrode layer materials are configured so that,
once an end of the jellyroll assembly is subjected to heat sealing,
separator layers are sealed selectively enveloping only one of the
sandwiched electrode assemblies. As mentioned, selectively
enveloping a single electrode assembly avoids use of extra
separator material, e.g., used to unnecessarily envelop both
electrode assemblies, and thus saves costs and allows for greater
flexibility in internal cell design. Heat sealed separators as
described herein, and methods of heat sealing, produce cells with
greater cycle life.
[0043] 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
[0044] The charging process for a nickel hydroxide positive
electrode in an alkaline electrochemical cell is governed by the
following reaction:
Ni(OH).sub.2+OH--.fwdarw.NiOOH+H.sub.2O+e- (1)
[0045] 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).sub.4.sup.2-) 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+20H.sup.-+H.sub.2O.fwdarw.Zn(OH).sub.4.sup.2- (2)
and
Zn(OH).sub.4.sup.2-+2e.sup.-.fwdarw.Zn+4OH.sup.- (3)
[0046] Therefore, net electrode at the negative is
ZnO+H.sub.2O+2e-.fwdarw.Zn+2OH--+2e- (4)
[0047] Then, the overall Ni/Zn battery reaction can be expressed as
follows:
Zn+2NiOOH+H.sub.2O.dbd.ZnO+2Ni(OH).sub.2 (5)
[0048] 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.
[0049] 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. The invention addresses the efficiency
of the zinc negative electrode, for example, battery cells
employing the heat sealed separators of the invention allow for
many more charge-discharge cycles.
C. Embodiments
[0050] A more detailed description of nickel zinc batteries,
including description of electrode and components, particularly the
embodiments relating to selective methods of sealing separators and
jellyroll assemblies containing selectively sealed separators,
follows.
Nickel-Zinc Battery and Battery Components
[0051] FIGS. 1A and 1B are graphical representations of the main
components of a cylindrical power cell according to one embodiment,
with FIG. 1A showing an exploded view of the cell. Alternating
electrode and electrolyte layers are provided in a cylindrical
assembly 101 (also called a "jellyroll"). The cylindrical assembly
or jellyroll 101 is positioned inside a can 113 or other
containment vessel. The can may be plated on the inside with e.g
tin to aid in electrical conduction. A negative collector disk 103
(e.g. copper, optionally plated with e.g. tin) and a positive
collector disk 105 (e.g. nickel, e.g. in the form of a foam) are
attached to opposite ends of cylindrical assembly 101. The negative
and positive collector disks function as internal terminals, with
the negative collector disk electrically connected to the negative
electrode and the positive collector disk electrically connected to
the positive electrode. A cap 109 and the can 113 serve as external
terminals. In the depicted embodiment, negative collector disk 103
includes a tab 107 for connecting the negative collector disk 103
to cap 109. Positive collector disk 105 is welded or otherwise
electrically connected to can 113. In other embodiments, the
negative collector disk connects to the can and the positive
collector disk connects to the cap.
[0052] The negative and positive collector disks 103 and 105 are
shown with perforations, which may be employed to facilitate
bonding to the jellyroll and/or passage of electrolyte from one
portion of a cell to another. In other embodiments, the disks may
employ slots (radial or peripheral), grooves, or other structures
to facilitate bonding and/or electrolyte distribution. Negative
collector disks are typically copper, optionally coated with tin,
and positive collector disks typically are nickel or at least
include nickel in their composition.
[0053] A flexible gasket 111 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 cap 109 from can 113. In certain embodiments, the bead 115
on which gasket 111 rests is coated with a polymer coating. The
gasket may be any material that electrically isolates the cap from
the can. Preferably the material does not appreciably distort at
high temperatures; one such material is nylon. In other
embodiments, it may be desirable to use a relatively hydrophobic
material to reduce the driving force that causes the alkaline
electrolyte to creep and ultimately leak from the cell at seams or
other available egress points. An example of a less wettable
material is polypropylene.
[0054] 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. In certain embodiments, a sealing agent is used to
prevent leakage. Examples of suitable sealing agents include
bituminous sealing agents, tar and VERSAMID.TM. available from
Cognis of Cincinnati, Ohio.
[0055] 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. As
indicated, in certain embodiments the can may be either the
negative or positive terminal. In embodiments in which the can is
negative, the can material may be of a composition similar to that
employed in a conventional nickel cadmium battery, such as steel,
as long as the material is coated with another material compatible
with the potential of the zinc electrode. For example, a negative
can may be coated with a material such as copper to prevent
corrosion.
[0056] In embodiments where the can is positive and the cap
negative, the can may be a composition similar to that used in
convention nickel-cadmium cells, typically nickel-plated steel. In
some embodiments, the interior of the positive polarity can may be
coated with a material to aid hydrogen recombination. Any material
that catalyzes hydrogen recombination may be used. An example of
such a material is silver oxide. 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. 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.TM. (a trade name by E.I. Dupont de Nemours and
Company, of Wilmington Del., for polytetrafluoroethylene).
[0057] FIG. 1C depicts a more specific configuration of a jellyroll
nickel zinc cell. This cell is similar to that in FIGS. 1A and 1B,
having a jellyroll electrode assembly 101, a can 113, a cap 109, a
flexible gasket 111, etc., but in this example, the negative
collector disk, 103a, is slotted and there are vertical (decending)
tabs, or energy directors, 108 for forming electrical connection to
the wound negative current collector at the top of jellyroll 101.
When this cell is assembled tabs 108 are pressed against the
negative current collector and the topmost portion of negative
current collector disk 103a presses against cap 109 to complete the
electrical connection between the negative current collector and
cap 109. In one embodiment, tabs 108 are configured so as not to
rip or tear into the negative current collector (as depicted, tabs
108 have curved portions, e.g. in this depiction like skis, which
rest on the negative current collector). Negative current collector
disk 103a, also has a center hole for introducing electrolyte to
the jellyroll. The positive current collector disk can also be
configured as disk 103a, where the center hole is used to
facilitate electrolyte flow, e.g. where an electrolyte resovoir is
maintained at the lower portion of the cell, between the bottom of
jellyroll and the bottom of the can. In this embodiment however,
positive current collector disk 105a is perforated as described for
disk 105 in FIG. 1A, except that disk 105a also includes
protrusions 112 which make electrical contact with the wound
positive current collector at the bottom of the jellyroll 101. In
one embodiment, the wound positive current collector is folded over
against the bottom of jellyroll 101 and protrusions 112 pierce the
folded positive current collector to establish electrical
contact.
[0058] 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.
[0059] 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 nickel
zinc cell including a jellyroll with selectively sealed separators
as described herein. One embodiment is a battery pack including one
or more nickel-zinc battery cells described herein and appropriate
casing, contacts, and conductive lines to permit charge and
discharge in an electric device.
[0060] Note that the embodiments shown in FIGS. 1A, 1B and 1C have
a polarity reverse of that in a conventional commercial cell, for
example a commercial nickel-cadmium cell, in that the cap is
negative and the can is positive. In conventional power cells, the
polarity of the cell is such that the cap is positive and the can
or vessel is negative. That is, internally, the positive electrode
of the cell assembly is electrically connected with the cap and the
negative electrode of the cell assembly is electrically connected
with the can that retains the cell assembly. In certain
embodiments, including that depicted in FIGS. 1A, 1B and 1C, the
polarity of the cell is opposite of that of a conventional cell.
Thus, the negative electrode is electrically connected with the cap
and the positive electrode is electrically connected to the can. It
should be understood that in certain embodiments of this invention,
the polarity remains the same as in conventional designs--with a
positive cap. At least one example of this embodiment is described
below.
[0061] More detailed description of specific "normal" and "reverse"
polarity cells as well as features of a venting cap, the positive
electrode, separator, electrolyte and negative electrodes
follows.
[0062] Venting Cap
[0063] 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 FIG. 1A, cap 109 is shown generically as a non-venting
cap, but typically is a venting cap. A typical nickel cadmium cell
vents gas at pressures of approximately 200 pounds per square inch
(psi). In some embodiments, a nickel zinc cell is designed to
operate at this pressure and even higher (e.g., up to about 300
psi) without the need to vent. This may encourage recombination of
any oxygen and hydrogen generated within the cell. In certain
embodiments, the cell is constructed to maintain an internal
pressure of up to about 450 psi and or even up to about 600 psi. In
other embodiments, a nickel zinc cell is designed to vent gas at
relatively lower pressures. This may be appropriate when the design
encourages controlled release of hydrogen and/or oxygen gases
without their recombination within the cell.
[0064] Some details of the structure of a vent cap and disk, as
well as the carrier substrate itself, 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).
[0065] The Positive Electrode
[0066] The nickel hydroxide electrode has been used as the positive
electrode in high power and high energy nickel-metal hydride
batteries, nickel-cadmium batteries and nickel-zinc batteries. 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. 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.TM.).
[0067] 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.
[0068] A nickel foam matrix may be used to support the
electro-active nickel oxide (e.g., Ni(OH).sub.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/m.sup.2 and thickness ranging from about 16-18 mils is
used.
[0069] In certain embodiments, the batteries include a non-nickel
positive electrode (e.g., a silver or air electrode). The
silver-zinc system employs silver-oxide as the positive electrode,
while the zinc-air system employs a gas-diffusion electrode
containing catalysis for oxygen reduction-production.
[0070] The Separator
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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
percent, 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.
[0076] 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.
[0077] 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.
Embodiments are directed toward selectively sealing separators.
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, 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.
[0078] 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."
[0079] The Electrolyte
[0080] 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.
[0081] Embodiments are 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.
[0082] 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.
[0083] According to various embodiments, the electrolyte may
include a liquid and a gel. The gel electrolyte may include a
thickening agent such as CARBOPOL.TM. 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.TM..
[0084] 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.
[0085] The Negative Electrode
[0086] 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, 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.
[0094] 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.
[0095] 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.
[0096] 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.
[0097] 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.
[0098] 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.
[0099] 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.
[0100] 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.
[0101] 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.
[0102] 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.
[0103] 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 (Bi.sub.2O.sub.3), 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 of the invention is a Nopcosperse.TM. (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.
[0104] The Electrodes and Separator Assembly--The Jellyroll
[0105] As mentioned, this invention is described in terms of
methods of selectively heat sealing separators so as to envelop
only one of two electrodes at the end of a jellyroll assembly. In
particular embodiments, the jellyroll assemblies are used for
nickel-zinc rechargeable cells.
[0106] 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. Particular to some embodiments described herein,
separator and electrode layer materials are configured so that,
once an end of the jellyroll assembly is subjected to heat sealing,
separator layers are sealed selectively enveloping only one of the
sandwiched electrode assemblies.
[0107] FIG. 2A is a perspective representation showing assembly of
electrodes and separator layers prior to winding into a jellyroll.
In the illustrated example, separators (200 and 208) are initially
folded over each of the negative electrode (conductive substrate
204 coated on each face with electrochemically active layer 206)
and the positive electrode (conductive substrate 210 coated on each
face with electrochemically active layer 212) along the electrode's
planar surface before being drawn or fed, with the electrode
sheets, into a winding apparatus. In this embodiment, each
separator sheet is a bifold, where each of the electrodes is
inserted (as indicated by the horizontal arrows) into the bifold
substantially to fold 202. 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. However, the bifold separators make
insertion and control of the stack easier when inserting into the
winding apparatus. 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, and when a wicking separator is used,
also aid in irrigation and ion current flow.
[0108] 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.
[0109] 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. In one 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 batter terminals at opposite ends of
the battery. In another embodiment, the exposed edges are on the
same side so that the electrical connections to the battery
terminals are made on the same end of the jellyroll.
[0110] FIG. 2B is a cross section (as indicated by cut A in FIG.
2A) of the assembly formed by stacking (as indicated by the heavy
double-headed arrow in FIG. 2A) the individual electrodes with
their respective separators in FIG. 2A. Separator 200 mechanically
and electrically separates the negative electrode (substrate 204
and electrochemically active layers 206) from the positive
electrode (substate 210 and electrochemically active layers 212)
while allowing ionic current to flow between the electrodes. In
this embodiment, separator 200 is microporous polypropylene, but
the invention is not so limited. As mentioned, the
electrochemically active layers 206 of the zinc negative electrode
typically include zinc oxide and/or zinc metal as the
electrochemically active material and may contain surfactant-coated
particles as described above. The layer 206 may also include other
additives or electrochemically active compounds such as calcium
zincate, bismuth oxide, aluminum oxide, indium oxide, hydroxyethyl
cellulose, and a dispersant.
[0111] 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.
[0112] 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 FIGS. 1A, 1B and 1C, structure 101.
[0113] As seen from FIG. 2B, conductive substrates 204 and 210 are
offset laterally so that once the jellyroll is wound, each of the
electrodes will be electrically connected to the battery terminals
at opposite ends of the jellyroll.
[0114] 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 FIG. 1A.
[0115] FIG. 2C is a cross-section (cut B as shown in FIG. 1A) of
jellyroll 101a, similar to jellyroll 101 as depicted in FIG. 1A,
and specifically where the jellyroll is made by winding the stack
structure as described in FIG. 2B. The cross sections of jellyrolls
depicted herein are essentially "slices;" that is, some depth
detail is avoided in order to simplify the figures. Void 201 is
formed when the mandrel of the winding device is removed after the
jellyroll is wound. Void 201 serves as an electrolyte reservoir. As
mentioned, one embodiment is a method of selectively sealing a
first set of separator layers disposed on both sides of and
extending past an edge of a first electrode of a jellyroll assembly
including two electrodes, while not sealing a second set of
separator layers disposed on both sides of and extending past an
edge, parallel and proximate to the edge of the first electrode, of
a second electrode, both edges disposed on the same end of the
jellyroll assembly, while exposing the same end of the jellyroll
assembly to a heat source. The FIG. 2C cross section of jellyroll
101a shows that there are alternating layers of
separator-sandwiched electrodes as described in relation to FIG.
2B. Importantly, the separator materials protrude past the
electrochemically active materials on each electrode, and each of
the conductive substrates protrude from the end of the jellyroll,
on one end, further than the separator material so that electrical
connection can be made to the battery terminals. In this example, a
jellyroll for a reverse polarity battery, the negative current
collecting substrate 204 protrudes past the electroactive and
separator materials at the top of the jellyroll, while the positive
current collecting substrate 210 protrudes past the electroactive
and separator materials at the bottom of the jellyroll. Negative
collector 204 will connect to the vent cap terminal, and positive
collector 210 will connect to the battery can, when the battery is
assembled as depicted in FIGS. 1A and 1B. Methods described herein
selectively seal only one electrode, of two, at either or both ends
of a jellyroll. Note that separators, in this example,
polypropylene separator 200 and wicking separator 208 are adjoining
except for on the outside of the jellyroll, and in the interior of
void 201. Note also that at the bottom of the jellyroll separator
200 does not extend as far down as separator 208--in embodiments
were both separators 200 and 208 were to be sealed over the
negative electrode, this configuration would allow enough of 208 to
melt over or combine with 200 when it is sealed. Also having 208
longer at the bottom of the jellyroll is done because electrode
substrate 210 extends further down as well, so if sealing is not
complete, 210 is further protected by 208. Analogously, at the top
of the jellyroll separator 200 extends further upward than 208,
because substrate 204 extends further than substrate 210 and thus
204 is further protected by separator 200.
[0116] 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 bottom of the jellyroll where
current collector substrate 210 protrudes beyond the separator
layers.
[0117] 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. FIG. 2D depicts a cross section of
jellyroll 101a after current collecting substrate 210 has been
folded over and heat applied to that end of the jellyroll to heat
seal the negative electrode (which includes current collector 204
and electrochemically active material 206). Folding can be done
manually or with, e.g., a rolling machine that grasps the jellyroll
assembly and applies a roller (from outer edge of jellyroll towards
inner edge in this example) to fold the current collector over as
depicted.
[0118] Referring again to FIG. 2D, after collector 210 is folded
over, a volume 211 is formed (as indicated by the heavy dotted
circle) where the separator materials at the end of the assembly
are surrounded by the positive current collector 210 on three
sides, the vertical walls and the bent over portion of collector
210. When configured in this way, and when heat is applied to the
bottom end of the jellyroll (as indicated by the heavy upward
arrow) on the folded over outer surfaces of current collector 210,
the polypropylene separator melts and fuses to form a continuous
layer as indicated at fusion point 200a. The configuration of
current collector 210 serves at least three purposes in this
example. The foldover aids transmission of heat to volume 211
(essentially a small oven). The extension of 210 beyond the
separator materials physically blocks separator material 208 from
sealing over (if it were sealable, embodiments include dual
separators where both are heat sealable) or folding over current
collector 210. Finally, the extension past the separators also
allows electrical communication of the current collector with the
can (e.g. via current collecting disk 105) and the foldover
maximizes electrical contact with the can or current collector
disk.
[0119] 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
volumes 211 protect the positive from contamination).
[0120] FIG. 2E depicts the selectively heat sealed jellyroll
assembly 101a incorporated into a final battery assembly analogous
to that described in FIGS. 1A and 1B. Current collecting disk 105
makes contact with the folded over surface of positive current
collector 210 for improved current transfer. Current collector
substrate 204 makes contact with, and thus is in electrical
communication with, current collector disk 103. While not wishing
to be bound to theory, it is believed that 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, in this example, the negative electrode
prevents particles causing shorts between the electrodes. At the
top of jellyroll 101a, where the negative substrate 204 make
electrical contact with a negative current collector disk 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 configuring the electrodes at the top of
jellyroll is done is such a way that the electrodes are not in
direct line of sight with each other and the difference in height,
C, 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 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. Can
113, tab 107, gasket 111 and cap 109 are analogous to those
described in relation to FIGS. 1A and 1B.
[0121] FIG. 2F depicts a cross section of jellyroll 101a, as
depicted in FIG. 2D, where heat has been applied to the top (as
depicted) of the jellyroll. Here, both ends of the jellyroll have
been subjected to selective sealing. The bottom (as depicted) is
sealed as described in relation to FIG. 2D. At the top of the
jellyroll, selective sealing is achieved by virtue of the
arrangement of the separators and the electrodes at this end of the
jellyroll. When heat is appropriately applied, for example pressing
the top of the jellyroll onto a hot platen as described herein,
layers of separator 200 are fused at points 200b, in between
neighboring layers of the negative substrate 204. Separator layers
208 do not fuse (supra) but are encapsulated by fusions 200b, at
least in internal layers of the jellyroll. On the outermost layer
and innermost layer, separator 200 is melted, but being the
outermost and innermost layers, each has no complimentary layer of
separator 200 to make a corresponding fusion 200b. Still, fusion of
the interior layers of 200 at this end of the jellyroll
encapsulates the positive electrode. Also, by virtue of the
outermost and innermost layers of separator 200 deforming due to
exposure to heating, there is at least some additional protection
(partial enclosure) of the outermost and innermost positive
electrodes at the top end of the jellyroll. Essentially, separator
layers 200 have been fused into a single sheet of separator formed
into concentric tubes that have open portions at the top and the
bottom. FIG. 2G depicts separator layers 200, now fused into a
single separator 200, by virtue of fusions 200a and 200b. In FIG.
2G, the wicking separator layers are not depicted and the
electrodes are depicted only as series of "+" and "-". By virtue of
seals 200b and 200a, the positive material is protected from the
negative at the top (as depicted) end of the jellyroll, and the
negative material is protected from the positive material at the
bottom of the jellyroll, respectively. Thus, selective sealing in
this example at both ends of the jellyroll, encapsulates the
negative electrode at one end of the jellyroll and encapsulates the
positive electrode at the other end of the jellyroll. Using
selective sealing after winding allows formation of a unique
unitary separator structure, 200. FIG. 2H depicts the jellyroll of
FIG. 2F incorporated into a reverse polarity battery, where the
components are analogous to those described in relation to FIG. 2E,
for example negative cap 109, positive current collector 105,
etc.
[0122] In one embodiment, selectively sealing the first set of
separator layers includes: i) configuring the jellyroll assembly
such that the first set of separator layers includes layers that
can seal to envelop the first electrode when the heat source is
applied, but the second set of separator layers includes layers
that can not seal to envelop the second electrode when the heat
source is applied; and ii) applying the heat source to the same end
of the jellyroll assembly. As depicted in, but not limited to, the
example described in relation to FIGS. 2F and 2G (and, for example,
FIGS. 2M and 2N below), in one embodiment, the method further
includes configuring the jellyroll assembly such that the first set
of separator layers includes layers that can seal to envelop the
second electrode at the other end of the jellyroll when the heat
source is applied to that end of the jellyroll, and applying the
heat source to the other end of the jellyroll. In one embodiment,
as applied to the embodiments described above, the first set of
separator layers and the second set of separator layers each have
different melting points. In another embodiment, as applied to the
embodiments above, the first set of separator layers are made of
materials that can melt and fuse when the sealing heat is applied
and the second set of separator layers are materials that can not
melt and fuse when the same sealing heat is applied. An example of
the latter embodiment is where the first set of separator layers
are polypropylene layers and the second set of separator layers are
cellulose-based layers. In one embodiment, the cellulose-based
layers are cellulose impregnated with polyvinyl alcohol (PVA).
[0123] FIG. 2I shows another stack assembly, like that in FIG. 2B,
except the separator materials and electrodes are laterally offset
differently than in FIG. 2B. Here, positive substrate 210 does not
protrude past the separator materials, while negative substrate 204
does so. This stack is an example of one used for a normal polarity
battery.
[0124] FIG. 2J is a cross-section (cut B as shown in FIG. 1A) of a
jellyroll 101b, similar to jellyroll 101 as depicted in FIG. 1A,
and specifically where the jellyroll is made by winding the stack
structure as described in FIG. 2I. In this example the reference
numbers are the same as those used in reference to separators,
electrodes, and electrochemically active materials. At the bottom
of the jellyroll the relative distance, C, between the ends of the
electrodes is the same as those at the top of the jellyroll in the
previous embodiment. However, in this example, the relative
distance, D, between the ends of the electrodes at the top of the
jellyroll is not as great as that in the previous embodiment. This
configuration is desirable to employ selective sealing at one or
both ends of the jellyroll (infra). Here separators 200 and 208 are
staggered at the bottom of the jellyroll consistent with those at
the top of jellyroll 101a of the previous embodiment, but the
separators at the top of the jellyroll are staggered consistent
with those at the bottom of jellyroll 101a of the previous
embodiment. In one example, selective sealing of one of the
electrodes at the top (as depicted) of jellyroll 101b is depicted
in FIG. 2K.
[0125] Since separator layers 200 are polypropylene layers and
separator layers 208 are cellulose-based layers, when heat is
applied to the top of jellyroll 101b sufficient to melt and seal
polypropylene separator layers 200, while separator layers 208 are
not sealed. Thus, the negative electrode is sealed, while the
positive electrode is not sealed. In most embodiments, because heat
is applied quickly, it is substantially localized to the end of the
jellyroll where applied, and thus heat damage (for example melting
shut separator pore structure) to the separator proximate to the
electrochemically active material is minimized. FIG. 2K shows the
result of applying heat (to the top of jellyroll 101b as indicated
by the heavy downward arrow) sufficient to seal, e.g., a
polypropylene separator layer 200 while the cellulose-based layer
208 is not sealed. Analogous to the relative relation described
with respect heat sealed jellyroll 101a, separator layers 200 are
melted and fused at point 200a while separator layers 208 are not
melted and fused, as indicated at 208a.
[0126] In one embodiment employing jellyroll 101b, a tab, 214, is
welded to positive current collector substrate 210 near the top of
jellyroll 101b. Tab 214 can be welded to the positive substrate
before or after heat treatment. In one embodiment tab 214 is
attached prior to heat sealing. In this embodiment, tab 214 is
folded over, substantially parallel to the end of the jellyroll,
during heat sealing so that the entire end of the jellyroll is
heated. In this embodiment, heat is transfered to the separator
materials under folded over tab 214 via the folded over portion of
tab 214. After heat sealing, tab 214 is unfolded, as depicted in
FIG. 2K, so that it can be welded to the battery cap or current
collector.
[0127] FIG. 2L depicts jellyroll 101b incorporated into a normal
polarity battery. Here, tab 214 is welded to cap 109, e.g. the
vented cap as described above. This configuration allows the
electrode assemblies in jellyroll 101b to be longer, saving space
without a current collector disk and providing for more
electrochemically active material in the battery. In an alternative
embodiment, tab 214 is in electrical communication with, either
welded to or e.g. under spring contact pressure, with positive
current collector disk 105 (not shown). Can 113 and gasket 111 are
analogous to those described in relation to FIGS. 1A and 1B.
Negative current collecting substrate 204 is in electrical
communication with negative current collector 103 now at the bottom
of can 113. In this example, cap 109 is positive.
[0128] FIG. 2M depicts a cross section of jellyroll 101b, as
depicted in FIG. 2K, where heat has been applied to the bottom (as
depicted) of the jellyroll. Here, both ends of the jellyroll have
been subjected to selective sealing. The top (as depicted) is
sealed as described in relation to FIG. 2K. At the bottom of the
jellyroll, selective sealing is achieved by virtue of the
arrangement of the separators and the electrodes at this end of the
jellyroll. When heat is appropriately applied, for example pressing
the top of the jellyroll onto a hot platen as described herein,
layers of separator 200 are fused at points 200b, in between
neighboring layers of the negative substrate 204. Separator layers
208 do not fuse (supra) but are encapsulated by fusions 200b, at
least in internal layers of the jellyroll. On the outermost layer
and innermost layer, separator 200 is melted, but being the
outermost and innermost layers, each has no complimentary layer of
separator 200 to make a corresponding fusion 200b. Fusion of the
interior layers of 200 at this end of the jellyroll encapsulates
the positive electrode, analogous to the jellyroll and process
described in relation to FIG. 2F-G. FIG. 2N depicts the jellyroll
of FIG. 2M incorporated into a normal polarity battery, where the
components are analogous to those described in relation to FIG. 2L,
for example positive cap 109, negative current collector 103,
etc.
[0129] In each of the embodiments above, the heat source used to
seal separators includes at least one of a convective heat source,
an inductive heat source, a conductive heat source and a radiative
heat source. In one embodiment the heat source is a conductive heat
source. In another embodiment the conductive heat source is a
heated platen. In some embodiments, although e.g. about 5 seconds
may be sufficient to seal a polypropylene separator, if there are
additional layers and/or layers that may insulate (e.g.
cellulose-based layers) more time may be needed to transfer
sufficient heat to the ends of the separators to seal them. In one
embodiment, the end of the jellyroll that is heated, where the
first electrode is selectively enveloped via sealing the first set
of separators, is contacted with the heated platen for between
about 1 second and about 30 seconds, where the platen temperature
is between about 130.degree. C. and 600.degree. C. In another
embodiment, the jellyroll is contacted with the heated platen for
between about 3 seconds and about 10 seconds, where the platen
temperature is between about 300.degree. C. and 600.degree. C. In
yet another embodiment, the jellyroll is contacted with the heated
platen for between about 5 seconds and about 25 seconds, where the
platen temperature is between about 450.degree. C. and 550.degree.
C.
[0130] In some embodiments, during contact with the heated platen,
the jellyroll is contacted with the heated platen with a force of
between about 0.5 kg/cm.sup.2 and 5 kg/cm.sup.2. In other
embodiments, the jellyroll is contacted with the heated platen with
a force of between about 1 kg/cm.sup.2 and 3 kg/cm.sup.2. In still
other embodiments, the jellyroll is contacted with the heated
platen with a force of between about 1 kg/cm.sup.2 and about 2
kg/cm.sup.2. In still other embodiments, the jellyroll is contacted
with the heated platen with a force of about 1.5 kg/cm.sup.2. In
some embodiments, for example those described in relation to
jellyrolls 101a and 101b, this force is used to aid heating of the
end of the jellyroll where selective heat sealing takes place. In
embodiments were folded substrates are employed, applied force may
also serve to flatten the folds of the conductive substrate for
more uniform heating.
[0131] As mentioned, methods described herein can be practiced with
any jellyroll configured electrode assembly, and is particularly
useful for nickel zinc cells where dendrite formation from the zinc
electrode can short the electrodes.
[0132] Thus, given the detailed description of various embodiments,
another aspect of the invention is a jellyroll electrode assembly
including: i) a first electrode disposed between a first set of
separator layers; and ii) a second electrode disposed between a
second set of separator layers; where, at the same end of the
jellyroll electrode assembly, one of the first electrode and the
second electrode is enveloped by its respective set of separator
layers and the other electrode is not enveloped by its set of
separator layers. Either the nickel positive or the zinc negative
electrode can be the one selectively sealed. In one embodiment, the
first electrode is a zinc electrode and the second electrode is a
nickel electrode. In another embodiment, the first set of separator
layers includes polypropylene layers. In another embodiment, the
second set of separator layers includes polyvinyl alcohol
impregnated cellulose. Batteries which include the jellyroll
electrode assemblies described herein are another aspect of the
invention, batteries of normal and reverse polarity as described
above.
EXPERIMENTAL
[0133] FIG. 3 shows test results of cells incorporating heat-sealed
separators in accord with embodiments described in relation to
jellyroll 101a, where positive current collector substrate 210 was
folded over after the jellyroll was wound and then the end of the
jellyroll was exposed to a hot plate within the times, temperature
ranges and applied forces described above in (3) Test cells vs. a
set of (2) Control cells where no heat sealing was performed. The
cells were tested under a rate of 5 C discharge. These curves
indicate when the cell has gone into an overcharge condition, e.g.,
up to or greater than 105% overcharge (on the Y-axis, 0.9=90%,
1=100%, 1.1=110%, etc.). When the curves have a steady rise, this
indicates a short within the cell. Control cells lasted from 100 to
250 cycles before shorting as indicated by the rising curves. Heat
sealing as described herein allows the 3 test cells to operate past
500 and up to 650 cycles before any general degradation of the cell
occurs (all three curves substantially overlap). Since the
implementation of heat-sealing in over 100 cells, no cells have
failed from a negative migration short.
[0134] 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.
* * * * *