Note: Descriptions are shown in the official language in which they were submitted.
COIN CELL BATT~RY EMPLOYI~G
CONC~VE 8X~PED CONTAINER PIECE~
FIE~D OF THE INVENTION
The invention relates to the field of batteries.
In particular, it relates to coin cell ~ize batteries and
the hardware employed for the container of such batteries.
B~ACKGROUND OF THE INVENT ON
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Coin cell batterie~ are small, puck shaped
galvanic cells that are used in a variety of commercial
electronics applications. Typically, these are low drain
rate applications such as memcry back-up, watches, and the
like. However, since power requirements ar~ being reduced
for high drain rate electronics devices, the demand ~or
larger and/or higher drain rate coin cell batteries is
incr~asing. It is desirable in most applications that the
~:~power source have a slim pro~ile. Thusl in order to gek
~:high drain rate in a slim container, it is prP~erable to
use larger coin cell diameters and smaller coin call
thicknesses. A four digit convention is used to designate
coin cell size where the first two digits indicate the
diameter in mm and the last two digit~ indicate the overall
thickness times ten in mm. A typical commercial product is
~:a 2320 coin cell which is therefore 23 mm in diameter and
2.0 mm thick.
In many electronics applications, it is the
maximum thickness of the coin cell that determines whether
it will fit in the allocated space. Thus, it is preferable
that the faces of the coin cell be ~lat in order to
135 maximize the amount of active material contained in the
coin cell. Additionally, it is preferable to use the
minimum material thickness for the pieces comprising the
coin cell container, again to maximize the amount o~ active
~matexial contained therein.
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Generally, some pressure is applied to the coin
cell container by the contents. This can be unintended,
for example as a result of the generation of gaseous
products during operation of the battery. Pressure can
also be applied deliberatelyO For example, U.S. Patent No.
2,971,999 discusses a battery wher~in a spring is inserted
between a container wall and the cell electrodes in order
to maintain electrical contact between the cell electrodes
and the container wall even when internal pressure causes
the container wall to swell.
The presence of internal pressure in a coin cell
battery tends to deform the container. The extent to which
this deformation increases the maximum thickness of the
coin cell is a function of the coin cell diameter~ the
thickness of the pieces comprising the coin cell container,
and, of course, the internal pressure. Larger cell
diameters and thinner container pieces result in a battery
that is morP sensitive to deformation at a given internal
pressure.
SUMMARY QF THE INVENTION
The coin cell battery of this invention is one
that uses pieces for the container thereof that are concave
shaped prior to assembly. ~fter assembly of the pieces
into a coin cell battery, the container applies a
preloading pressure to the solid contents, hereinafter
referred to as the electrode stack, inside the cell.
Furthermore, the concave shape can be chosen such
that, after assembly, the pieces of the container are
deformed as a result of generation of this preloading
pressure such that the faces of the battery are ef~ectively
~lat.
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Hardware specific to one possible coin cell size
is disclosed that, if suitably shaped, allows the practical
application of preloading pr~ssures from about zero up to
about 40 psi. In addition, a specific coin cell battery
example using lithium ion type electrochemistry is
disclosed.
Lithium ion batteries ~enerally employ lithiated
transition metal oxides as the active cathode mat~rial,
carbonaceous compounds as the active anode material, and
lithium salts dissolved in one or more non-aqueous solven~s
as the electrolyte. ~he specific battery disclosed is a
2320 coin cell using LiNio2 as the active cathode material,
a coke-like carbon as the active anode material, and lM
LiN(CF3S02)2in ethylene carbonate (EC)/dimethoxyethane (DME)
solvants as the electrolyte. ;.
This invention pertains to a general method that
can be used to apply preloading pressure to the electrode
stack of a coin cell battery. The purpo~e for the
application of said preloading pressure can be to prevent
expansion of the cell electrodes, to maintain ePfective
electrical contact to the electrodes, to counteract gas ; .:
pressure generated a~ a result of operation of the cell, or
for any oth~x such reason.
In addition, a method is descri~ed that allows
th~ application of preloading pressure in a coin cell
battery such that the faces of the battery are effectively
~0 flat after assembly is complete. For a given cell size and
a given thickness o~ the container pieces, it is possible
to achieve preloading press~lres over a certain range. A
method for achieving a range from zero to about 40 psi
specific to a 2320 size coin cell battery with 300 ~m thick
~.ontainer pi~ces is also disclosed.
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BRIEF DESCRIPTION OF THE D~WINGS
.,
In drawings which illustrate specific embodiments
of the invention, but whi~h should not be construed as
restricting the spirit or scope of the invention in any
way:
Figure 1 shows a cross-section of a typical coin
cell battery.
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Figure 2 shows a cross-section of the coin cell
battery o~ Figure 1 as it would appear i~ deformed due to
a uniform increase in internal pressure.
15Figure 3 shows a cross-section of the invention
case and cap container hardware for a coin cell battery.
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Figure 4a shows a two stage die set suitably
modified for purposes of crimp closure of the invention
coin cell battery~
Figure 4b shows an enlarged cross-sectional view
o~ the first stage die set and coin cell assembly during
crimp closure.
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Figure 4c shows an enlarged cross-sectional view
of the second stage di~ set and coin cell assembly during
crimp closure. -~
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
OF TEIE INVENTION
A typical coin cell battery is depicted in Figure
1. The solids in the cell are inserted in a stack
consisting of a cathode 10, a separator 11, and an anods
12~ An electrolyte 13 permeates the pores o~ the separator
:~ 11 and, where applicable, the poras of the cathode 10 and
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anode 12. A housing conslsting of a metal case 14, a
gasket 15, and a metal cap 16 are conventionally used as a
container. The metal case 14 and metal cap 16 serve
additionally as external electrical contacts to the
electrodes ~ithin. Conventional cases 14 and caps 16 have
flat faces prior to assembly. The housing is sealed by
crimping the edge of the case 17 inwards, thereby deforming
the gasket and setting a psrmanent load that acts on the
edges of both the case 14 and the cap 16 as well as on the
gasket 15.
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If the coin cell as shown in Figure 1 is
subjected to a uniform increase in internal pressure,
deformation of the container is expected as shown in Figure
2. (The extent of elastic deformation has been exaggerated
for purposss of illustration.) Both the case 14 and cap 16
assume a convex shape that is a function of the diameter of
the cell, thickness of the case 1~ and cap 16, and the
materials of construction. Said deformation, if within the
elas~ic limits of the hardware/ is by definition reversible
upon removal of the applied pressure and can be calculated
by conventional stress/strain methods if the internal
pressure is known. The amount of deformation or bulge
measur~d at the centre o~ the case 14 and cap 16 has been
denoted by arrows X1 and X2 respectively.
The invention battery is on- that employs a case
14 and cap 16 that are concave shaped prior to assembly of
the battery. The specific shape employed for a given
preloading pressure and given hardware dimensions is the
inverse to the convex shape expected from ~lat hardware of
the same dimensions subjected to uniform application of the
same prelcading pre~sure. Figure 3 shows hardware with a
concave shape of the invention. (The extent of the concave
shaping has been exag~erated for purposes of illustration.~
The case 14 and cap 16 are shaped such that their centres
are an amoun~ indicated by arrows Yl and Y2 respectively
~,x :,. - - ,. - , ~ . : , ::
from the flat. Thus, the~ concave shaped pieces of the
invention are the inverse of the convex shaped pieces of
Figure 2 if the hardware dimensions are the same and if the
respective magnitudes Yl and Y2 are equal to Xl and X2. In
order to achieve the goals of the invention, the desired
preloading pressure must be small enough so as not to
inelastically deform the given hardware. In such a case,
the container can be crimped shut such that the desired
preload is applied with the resulting faces of the
container being effectively flat after assembly.
The invention battery thus differs significantly
~rom that described in U.S. Patent 3,928,077 wherein a
practically planar portion of the container is inwardly
o~fset in order to maintain efective contact to the
internal electrodes and thus to count~ract the effect of
internal pressure on the cell container. The planar
portion of the container in said patent does not apply
uniform pressure over the entire electrode ~tack nor does
: 20 it result in a cell with ~aces that are effeotively flat
after assembly.
Increased di~ficulty in cell assembly results
from the use of concave shapsd pieces for the container.
~he electrode stack is conventionally created by stacking
one component at a time on top of either a gasket 15 - case
14 subassembly or a gasket 15 - cap 16 subassembly. The
concave shape of the container does not allow the electrode
stack to lie flat and thus the stack may be more easily
disturbed during assembly. In addition, the case 14 and
cap 16 must be manipulated into their final respective
positions during the crimping closure process, rather than
prior to the crimping process. To accomplish this, only
minor modifications to conventional coin cell crimping dies
¦ 35 are required.
Figure 4a shows a conventional two stage coin
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cell crimping die set 20. The first stage die 21 consists
of a base 22 and a precrimper 23. Figure 4b shows an
enlarged cross~sectional view of the first stage die and
coin cell assembly during crimping. The bottom surface 24
of the precrimper 23 is shaped to effect a partial crimp
closure of the coin cell. An uncrimped coin cell 2S is
placed in the base 22, oriented with the cap 16 upwards as
shown. The precrimper 23 is then brought down vertically~
The crimping begins when the shaped bottom surface 24 of
the precrimper 23 contacts the perimeter of the case 14.
The design of the shaped bottom surface 24 is such that
some limited loading is applied to perimeter of the cap
face 26 during this process. The first stage crimping i~
complete once the faces of the precrimper 23 and base 22
contact each other. The components of the partially
crimped coin cell (not shown) are now reasonably fixed in
place.
Final crimping of the partially crimped coin cell
takes place in the second stage die 29. The construction
and operation of the second stage die 29 are similar to
that of the first stage die 21. Figure 4c shows an
enlarged cross-sectional view of the second stage die and
partially crimped coin cell 25a during crimping. Similar
items are identified with a number corresponding to those
used in the ~irst stage with a suffix "a" (ie. second stage
base is 22a). The surface 24a of the cximper 23a is shaped
to effect the final desired shape of the assembled coin
cell. The design o~ the surface 24a is such that increased
loading is applied to the cap face ~7a as the crimper 23a
travels downwards. Thus, the cap 16 is manipulated into
its final position concurrent with the final crimping of
the perimeter of the case 14.
The invention has been employed succ ssfully in
the construction of certain coin cell batteries employing
lithium ion type electrochemistry. In this instance, a
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signi~icant internal pressure existed during the normal
operation of said batteries. The examples to follow are
presented to illustrate the application of the invention in
this specific case but should not be construed as limiting
the scope or spirit of the invention in any way.
COMPARATIVE EX~MPL~ -
It was considered desirable to fabricate 2320
size coin cell batteries as shown in Figure 1 using a
speci~ic lithium ion type electrochemistry comprising use
of LiNio2 as the active cathode material, ~oke like carbon
as the active anode material and a lM solution of
LiN(CF3So2)2 salt dissolved in equal volumes of ethylene
carbonate ~EC) and dimethoxyethane (DME) solvents as the
electrolyte.
Cathodes were fabricated in the ~ollowing manner.
A cathoda slurry was prepared using LiNio2 powder as the
active material, Super S carbon black (trade-mark of
Ensagri) as a conductive dilutant, ethylene propylene diene
monomer (EPDM) rubber as a bind4r, and cyclohexane as a
solvent for the binder. After blending, the cyclohexane
was evaporated away to create a cathode powder blend in
which the ratio of the components LiNio2- Super S: EPDM by
weight was 100:10:2. Us.ing this dry powder blend, tablets
were formed in a press in a size close to that desired for
2320 cells.
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In a similar manner, anode tablets were prepared
using Conoco XP coke (product of Conoco Inc.) as the active
material in which the ratio of the components coke: Super
S: EPDM by weight was 100~5:2.
In the ~atteries of this example, Celgard 2502
~trade-mark of Hoechst-Celanese) was used as a separator
material~ Conventional 2320 cap and case hardware with
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g
flat faces prior to assembly were used. The cap material
was annealed 304 stainless steel and the case material was
a special corrosion resistant grade of stainless steel
known as Shomac 30-2 (trade~mark). The thickness of the
material in both instances was 300 ~m. Coin cells were
fabricated as described in a conventional manner without a
significant intended preloading pressure. Some slight
inter~erence (of order of 50 ~m or less) between the
container pieces and the electrode stack was intended to
ensure electrical contact was maintained with the cathode
and anode tablets. ~fter assembly to this point, the coin
cell faces are effectively flat.
As constructed, this type of lithium ion cell is
in the discharged statel Upon recharging, there is a net
expansion in the total volume occupied by the solids
inside. Additionally, gas can be generated inside as a
result o~ certain chemical reactions that take place during
this process. For these reasons, additional internal
pressure is applied to the cell container on recharge and
consequently an increase in overall cell thickness is
expected.
Several coin cell batteries were fabricated and
~5 recharged as described in the preceding. The overall
thickn~ss at the centre of several cells was measured after
final crimp closure. Each cell was then recharged fully
and measured again. The increase in overall thickness of
each cell was roughly ~00 ~m. This corresponded to a
calculated total internal pressure increase of about 35
psi. The coin cell at this point had pronounced convex
shaped or domed faces.
This example demonstrates that the internal
¦35 pressure o~ this specific coin cell is significantly higher
¦during operation than it is immediately after initial crimp
IclosureO The increase in pressure during operation created
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significant distortion of the coin cell container.
INVENTIVE EXAMPLE
Further testing on cells similar to those of the
Comparative Example demonstrated that the maximum increase
in coin cell th.ickness after repeated charge-discharge
cycles was approximately 250 ~m. This increase
corresponded to a calculated total pressure increase of
about 45 psi. Additionally, in special laboratory cells
(described in Effects of Physical Constraints on Lithium
Cyclability by D.P. Wilkinson et al, 5th International
Meeting on Lithium Batteries, Beijing, China, 1990), it was
demonstrated that the increase in thickness of such coin
cells could be arrested by preloading the el~ctrode stack
to a similar pressure value.
Coin cells were then constructed usiny concave
shaped cases and caps which were designed to apply a
preloading pressure of 40 psi. With reference to Figure 3,
the magnitudes of Yl and Y2 llsed for the concave case and
cap respectively were determined by calcl71ating the bulges
Xl and X2 in the eguivalent pieces of Figure 2 assuming the
internal pressure applied to the equivalent pieces was 40
~j25 pSin Thu~, after crimp closure, the container of this
:inventive ex~mple was expected to interfere with the
electrode stack an amount Yl + Y2 more than that of the
container in the Comparative example. This interference
was expected to create a preloading pressure of 40 psi in
such a case.
It was additionally necessary to tailor the
components of the cell such that the thicknesses of the
electrodes under the desired preload pressure were suitable
for the final as~embled cell thickness. To achieve this
result, the cathode used was about 310 mg by weight,
approximately 19 mm in diameter and 570 ~m thick. The
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anode used was about 230 mg by weight, approximately 19 mm
in diameter and 680 ~m thick. :
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Coin cells were constructed and recharged as
describad in the foregoing disclosure. The overall
thicknesses of the cells increased only slightly (of order
of 50 ~m or less~ after recharging. Upon repeated charge-
discharge cycling, no significant increa es in overall
thicknass occurred.
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Calculations also indicated that greater values
for Y1 could be employed without exceeding the elastic
limit of the case material. The maximum equivalent
preloading pressure achievable within the elastic limit for
the case was determined to be 70 psi. For the cap however,
the maximum equi~alent preloading pressure was determined
to be only 41 p~i as a consequence of using annealed
material for construction. Use of a harder grade of
stainless steel for the cap would increase the maximum
possible preloading pre~sure achievable. Thus, if
reguired, greater preloading pressure could be achieved in
a slightly modiPied constructionO
This example illustrates the effectiveness of the
~ 25 invention and additionally indicates a possible method for
: its practical use.
As will be apparent to those skilled in the art
in light of the foregoing disclosure, many alterati.ons and
modi~ications are possible in the practice of this
invention without departing from the spirit or scope
t,hereof. While the examples in the foregoing disclosure
applied only to lithium ion coin cell batteries and the
need for preloading to prevent electrode expansion, ths
hardware and methods described equally can be used ~or
. other battery chemistries with other requirements for
preloading such as a need to counteract the ePfects of gas
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generation. Accordingly, the scope of the invention is to
be construed in accordance with the ~ubstance defined by
the following claims.
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