Note: Descriptions are shown in the official language in which they were submitted.
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A METI-IOD FOR TREAT)<NG CONTACT LEADS ON ELECTROC)EIEMICAL CELLS
SO AS TO ACHIEVE IMPROVED HEItMETICITY AND HERMETI1CALLY SEALED
ELECTROCHEMICAL CELLS DERIVED THEREFROM
This applicatiozi claims priority to U.S. Serial No. 60/125,638, sled March
22, 1999, and
entitled "A Ivlethod to Treat Tabs for Hermetic Sealing of Elcctrocheznicai
Cells in Plastic Bags"
FIELD OF THE Il\'VENTrON
The presea~.t invention is directed to an economical method of preparing
contact leads for
electrochemical cells and the cells derived therefrom. 1tz particular, the
presezzt invention relates to
a method of treating metal contact tabs ozz electrochemical cells and
thereafter packagizzg said cells
in a final closure sealing member so as to achieve an inexpensively
manufactured, compact cell
heaving optimal hermeticity.
BACKGROUND OF THE INVENTION
Electrochezoical cells represent the state-of the-art in rechargeable battery
technology. Such
batteries are often packaged in a rechargeable polymer battery pack. This pack
utilizes ion
conductors havinc a predetermined fornzula that is lazown in the art to
pror°ide high energy density,
high performance reliability and a prolonged shelf life. Sigtti~cant weight
savings are realized with
electrocl~en~ical cells which meet the need for reduced battery size and shape
in diminutive portable
electronic devices. These cells further demonstrate superior results rwhen
subjected to high-rate
cycling, znechanieal shock, thermal shock, vibration, oyez voltage, under
voltage, short circuits,
automobile battery charging, nail penetration and high pressure tests. These
properties make
electrochemical cells increasingly desirable fox use in a wide range of
applications, including
mobile phones, pagers, laptop computers, hand held terminals and wireless
communication devices.
W the course o.f the discussion herein, the terms "battery" and "cell" will be
used interchangeably.
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I-Iermeticity is also a highly desirable property in an electrochemical cell.
The prior an
recognizes that it is extremely izxzportant for a hezmetically sealed cell to
retain its zi~anufactured,
gas tight condition, even after prolonged periods of storage or use.
Conventional battery
teclu~ologies often use metallic casings to achieve sufficient rigidity and
herzneticity. In
electrochemical cells having a metal casing, one means of providing a hermetic
seal is by welding a
seal member in the casing. The casing adjacent the weld, however, conducts
heat to the electrolyte
contained therein, invariably inducing electrolyte evaporation_ When these
gases escape from the
cell casing, they leave pinholes izi the weld that compromise the hermeticity
of the entire cell
stnteture. Simply welding a seal in a rrzetal casing, therefore, does not
necessarily assure gas tight
henneticity. In addition, metal casings, although demonstrating an inherently
ntgged constructiozi,
suffer from sigzaificant drawbacks, including excessive weight and limitation
in construction (i.e.
being capable of limited fotznation. in only simple geometric shapes such as
cylinders and
rectangles).
These and related disadvantages can be overcome, tzowever, by instead
employing fnal
closure sealing zn,en~bers such as multi-layered plastic sealing pouches as
cell casings. The bonding
of the electrodes to the separator in an electrochemical cell eliminates the
need for rigid cell
casings, thereby reducing the weight of the cell significantly. Accordingly,
electrochemical cells
may be packaged in thin, flexible znulti-layered pouches that exhibit reduced
weight as well as
formability in a pletkzora of geometries having mi.ninmm thickness. The use of
such sealing
members has been popularized in recent years for use with electrochemical
cells such as polymer
electrolyte-based lithium or lithium ion battezies like those described in
commonly assigned U.S.
Patent Application No. 09/405,200, incorporated by reference herein.
Mufti-layered packaging for electrochemical cells typically includes at last
one layer of a
thin metal foil, such as aluminum, to provide an oxygen and moisture barrier.
The foil layer is then
typically encapsulated between a sheet of a polymer matezial having superior
mechanical integrity,
such as nylon or polyester, and a layer of a lo~~ temperature polyolefin, such
as polyethylene, for
heat sealing to form a laminate_ This combination of polymer layers provides a
seal that, when
given sufficient width, is hermetic over a prolonged period of ti.z~~e_
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Achievement o.f herzoetieity in this manner is limited by the need for an
electrical
connection from the cell inside the pouch to external contacts via contact
leads, often in the form of
metal contact tabs. The contact tabs transport cun-ent to and from the
negative and positive
terminals of the cell, In order to achieve a strong bond, sealing of plastic
sealing pouches is
typically carried out at an elevated temperature and pressure. Tab thickness
is therefore restricted
to allow the heat seal layer to flow around the tabs to produce a hermetic
seal. V~Then this is done
for an. electrochemical cell, the elevated temperature and pressure can cause
deformation of the
packaging material, as well as penetration of the metallic tabs tlu-ough the
adhesive layer. The tabs
must therefore extend beyond the package and past the seal area, consequently
weakening the
strong and homogeneous adhesive/adhesive bond that typically characterizes the
seal. Thus,
unintentional electrical shorting between individual tabs and between the tabs
and the metal foil
laminates remains a prominent concern.
It is therefore desirable to provide an improved process for ensuring
hermeticity arid thereby
prevent degradation of the cell to the detriment of cell performance and user
safety.
S Uh'llV;(,ARY OF THE INVENTION
It is an obj ect of the present invention to provide an improved method of
manufacturing
electrochemical cells.
It is another object of the present invention to provide an improved seal for
electrochez~~ical
cells so as to achieve hermeticity of the electrolyte arid contact tabs
thereby.
It is still another object of the present invention to facilitate flexibility
irx electrochemical
cell design.
It is a further object of the present invention to augment the he~.~neticity
of an
electrochemical cell while providing a compact cell configuration.
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Tt is yet another object of the present invention to pzovxde a low prof le
hermetic seal for an
electrochemical cell that accommodates zedueed-dimension receptacles and
improves the
appearance of the cell.
It is still a further object of the present invention to pzovide a method by
which a metal-to-
adhesive bond can be strengthened foz hermetic sealing of a plastic cell
casing.
It is yet another object ofthis invention to develop a rapid anal inexpensive
sealing method
that is amenable to high-speed automation.
T.n the efficient attainment of these and other objects, the present invention
provides a
method of hermetically sealing an electrochezrtical cell. In the disclosed
method, a coating of an
adhesive composition is applied to an external metal tab that serves as a
contact lead from the cell
to a device within which the cell is implanted. The adhesive coats and
heremetically seals the tab
surface thereby. The coatiatg process can be executed by directly heat-sealing
an adhesive film
onto the tab surface. Alternatively, an adueous suspension of the adhesive
composition can be
sprayd onto the tab surface, or the tab may be roll coated in the composition
and thereafter
subjected to heat treatment, causing the adhesive to melt and flow. The coated
tab is thereafter
welded to a corresponding current collector in the form of a metal foil. The
complete cell assembly
is heat-sealed in a final closure sealing member such that the sealed tabs
extend theretbrough.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a schematic representation of an electrochemical cell of the
present
inventive pI'oCe55 pt~or to sealing thereof in a final closure sealing member.
Figure 2 shows a n~.etal tab of the electrochemical cell of Figure 1 after
sealing of the final
closure sealitag member therearound.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to a hermetically sealed electrochemical
cell assembly and
a method of manufacture thereof Such an assembly is xhade up of a plurality of
cells having
contact leads in the form of metal tabs extending therefrom for the
transmission of electzical current
bet<vcen the cell and a device within which it is emplvyed_ Each metal tab is
de.fzned by opposed
planar surfaces, one or both of which receive a coating of an adhesive
composition thereon for the
purpose of hermetically sealing the tab. The cell along with the treated tabs
is thereafter encased in
a final closure sealing member such as a plastic cell casing that is heat
sealed so as to encapsulate
the electrochemical cell therein.
Now referring to the Figures 1 and 2, wherein like elements are identically
numbered, an
electrochemical cell assembly 10 of the present invention is provided which
zncludes an
electrochemical cell 12 having a plurality of bi-cells 14 therewithin. Bi-
cells 14 may be
manufactured in separate, contiguous compartments wherein a plurality of
anode, cathode and
separator members are subjected tv heat lamination and cutting. After cutting
of the members into
individual cells, the cells are inserted into tl~e cozz~partn~ents and the
compartments are .laernaetically
sealed thereafter. Cell 12 may alternatively be az~ranged in a Z-shaped
manifold wherein each
bi-cell 14 includes an anode of predetermined shape and size interposed
between a pair of planar
separator films acting as a substrate. On a face of each separator film not in
contact with an anode,
a cathode is positioned thereon so as to be in overlying registry with a
corresponding anode. Folds
are thereafter effected at bi-cell interfaces deftned by spaces between the bi-
cells where the
separator film connects adjacent bi-cells in overlying registry with one
another. This latter
embodiment is further described in commonly assigned U.S. Patent Application
No. 09/405,200
and incorporated by reference herein.
Electrochemical cell assembly 10 further includes a final closure sealing
member, shown
herein as a plastic cell casing 16. Cell casing 16 includes an upper sealing
layer 16a and a lower
sealing layer 16b in overlying registry with one another. As further shown in
Figure 2, sealing
member 16 incorporates a laminate-type stntcture for sealing electrochemical
cell 12 along sealing
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rims 17. As specifically shown, each of upper sealing layer 16a and lower
sealing layer 16b
incorporates a minimum of three layers. Polymer covers 18 and 18' are made of
a polymer having
superior mechanical integrity, such as polyester or nylon, for insulative
protection and hermetic
sealing of electrochemical cell asseza~bly 10. Metal layers 20 and 20' are
fabricated from a
lightrvcight material such as aluminum so as to provide an additional hez-
metic moisture barrier
thereby. Adhesive layers 22 and 22'are provided for additional hermeticity and
heat sealed to one
another to a predeternlined width. The material composition of adhesive layers
22 and 22' varies
based upon manufacturing and performance requireznezzts, however, examples of
desirable
materials for use herein include polyolefins such as polyethylene, ionomer
resins of poly(ethylen.e
znethacrylate) such as Surlyn (a registered trademark of DuPont Company) ox
laminating adhesives
such as Adcote (a registered trademark of Morlon International). Although
three distinct layers are
shown as comprising each of layers 16a and 16b, it is understood that this
configuration of sealing
member 16 is merely illustrative and that the total number of layers may vazy
depending on the
needs of the battery manufacturer and the end use of the battery. For example,
there could be
layers between the adhesive and metal layers, as well as between metal and
polymer layers, so as to
improve interlayer bonding.
As mentioned hereinabove, a problem itzherent in sealizag an electrochemical
cell within a
flexible plastic ce31 easing such as sealing member 16 is the requirezxzent
for contact leads that
establish electzical communication between cell 12 and a device within which
the cell is employed.
Such contact leads often take the form of metal tabs such as aJuzninuzxz tab
30 and copper tab 30'.
Tabs 30 ad 30' are fabricated frozxz metal conducive to electrical
communication with the anodes
and cathodes of cell 12. Tabs 30 and 30' thus act as barriers between adhesive
layers 22 and 22'
and prevent tl.~e formation of an optimally l~ez~z~etic seal therebetween. To
mitigate this problem, the
present invention incozporates a pretreatment of metal tabs 30 and 30' pnor to
sealing of sealing
member 16.
Now the manufacture of cell assembly 10 can be described.
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The preparation of metal tabs 30 and 30' for sealing purposes is carried out
in the following
mamier. Tabs 30 and 30' are washed in a preselected solvent such as acetone so
as to remove and
processing oils or debris from the tab surfaces. The tabs are dried and
thereafter coated with a heat-
seaJable, adhesive material such as polyethylene or other polyolefin, Surlyn,
Adeote or other
material that is sealingly compatible with the materials used to fabricate
sealing member 16. The
coating is applied by dip-coating, spray coating, roll coating, brushing or
any other commonly used
coating procedure. Tabs 30 and 30' are then dried and then placed in an oven
to melt the adhesive.
The adhesive flows at~d uniformly wets and coats the surface of the metal
tabs, forming a strong
metal-adhesive bond. Each tab is then welded to a corresponding cun-ent
collector and heat sealed
within the sealing member 16. The seal provides cell 12 with a reduced,
aesthetic prof le and does
not hinder the insertion of cell 12 into a receptacle therefor wherein
conservation of space is at a
premium.
Illustrations of the execution of the above-described process are provided in
the following
examples. It is understood that these examples arc merely illustrative of the
types of material
combinations anticipated by this disclosure and in no way limit the
application thereof in the
manufacture of hermetically sealed electrochemical cells.
)EXAMPLE 1
An aluminum foil current collector having a thic)Ct~ess of 50 m is washed with
acetone or
similar solvent 2o remove any processing oil from its surface and then dried.
Using a spray gun and
an aqueous suspension of EAA adhesive, an aluminum tab is coated with a S 111
layer of the
adhesive. The aluminum tab is then placed in an oven. at approximately
250°C and sintered for
about 5 minutes. This heat-treatment causes the EA,A to melt and flow, thereby
evenly coveting the
aluminum tab surface. The aluminum tab is then welded to the aluminum grid of
the cell. A copper
tab is treated in the same matu~er and welded to a copper foil current
collector of the cell, thereby
fonoi~.~g an electrode stack. The electrode stack is then placed inside a
plastic sealing pouch. The
pouch is heat sealed around the stack at a temperature of about 130°C
and a pressure of about 10
psi. ~fl~e seal is hermetic and can withstand elevated temperature storage
without leakage.
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EXAMpL)E 2
An aluminum foil current collector having a thickness of 50 m is washed with
acetone to
rerzaove any processing oil from its surface and then dried. An aluminum tab
is then roll-coated
with a mixture of 50% of. Adcote {a registered trademark of Morton
Iztternationai) and 50% ethanol..
The aluz~~i~~.uoa tab is then placed in an oven at approximately 250°C
and sintered for about 5
minutes. This heat-treatment causes the adhesive to melt and flow, thereby
evenly covering the
aluminum tab surface. The tab is ihexl welded to the aluminum foil of the
cell. A. copper tab is
likeu-ise treated and welded to a copper foil current collector of the cell,
thereby forming an
electrode stack. The electrode stack is then placed inside a plastic sealing
pouch and heat sealed
therewithin at a temperature of approximately 130°C and a pressure of
about 10 psi.
The seal e~z~ployed in the current inventive process achieves henneticity
while being able to
~~~ithstand elevated temperature storage without leakage. Because many
applications require the
battery of the present invention to be frequently recharged, the battezy
remains easily manipulable
bertveen a battery-powered device and a recharges. The battery is therefore
compatible with
various cotmector configurations, sucJt That the same battery can potentially
be used with numerous
types of devices.
Various changes to the foregoing described and shown structures would now be
evident to
those skilled in the art. accordingly, the particularly disclosed scope of the
invention is set forth in
the following claims.
S
