Note: Descriptions are shown in the official language in which they were submitted.
.'i
2099657
ELBCTROCHEMICAL CELL AND METHOD OF MANUFACT~RING 8AMB
FIELD OF THB lNV~ ON
The present invention pertains to an
electrochemical cell having current cutoff means for
preventing current flow therein and to a method of
manufacturing an electrochemical cell. More
particularly, the present invention relates to an
electrochemical cell having current cutoff means and
- burstable vent means for preventing a dangerous
explosion from occurring within the cell, and a method
of manufacturing same. Still more particularly, the
present invention relates to an electrochemical cell,
and a method of manufacturing such cell, having current
cutoff means operable upon the exertion of a gas
pressure in excess of a first predetermined pressure,
and burstable vent means operable upon the application
of gas pressure exerted directly on the diaphragm in
excess of a second predetermined pressure whereby the
second predetermined pressure exceeds the first
predetermined pressure.
BAC~GROUND OF THE lNV~ ON
The prior art is replete with electrochemical
cells including current cutoff switches for cutting off
current flow between the anode and the cathode of the
cell. Additionally, the prior art discloses numerous
electrochemical cells having vent means for permitting
gas trapped within a housing of the cell to escape
therethrough. However, the prior art attempts to design
and manufacture an electrochemical cell having current
- 30 cutoff means and vent valve means to prevent an
explosion within the cell, have been inadequate in their
efforts to develop a current cutoff means which will
ensure the occurrence of an open circuit condition upon
the application of a consistent internal gas pressure on
a diaphragm within the cell. Further, numerous prior
art devices provide no means for venting excessive gas
pressure from the interior of an electrochemical cell to
prevent an explosion in the event that the current
2099657
cutoff means fail to prevent chemical reactions within
the cell from producing additional gas.
Moreover, no known prior art discloses or
otherwise teaches a method for manufacturing an
s electrochemical cell comprising the steps of assembling
a ~header~ including current cutoff means in an ideal
environment away from an assembly line and testing the
header assembly under ideal conditions, and thereafter
assembling the header to a housing of an electrochemical
cell including an anode, a cathode, electrolyte solution
and an electrical lead extending from the cathode.
U.S. Patent No. 4,943,497 to Oishi et al.
discloses a battery having switch means for disrupting
current flow between the header and the anode or cathode
of the battery and vent means for permitting gas to be
expelled from the battery when the pressure therein
exceeds a predetermined limit. The Oishi et al . battery
includes a container having anode material, cathode
material and an electrolyte solution for forming an
electrochemical circuit system therein, and a header
electrically connected to the electrochemical circuit
system. An electrical lead extends from the
electrochemical circuit system and is directly welded to
a projection extending downwardly from an otherwise
generally flat conductive diaphragm to complete the
circuit between the header and the electrochemical
circuit system therebelow, all of which is shown in the
drawings of the Oishi et al. patent and is duplicated in
Fig. 1 of the present application.
The diaphragm is constructed and arranged to
pull away from the lead thereby breaking the welded
connection therebetween upon application of a sufficient
amount of pressure thereto. Once the diaphragm pulls
away from the electrical lead, the circuit is opened and
current can no longer flow from the header to the
electrochemical circuit system. The current cutoff
means of the Oishi et al. battery is undesirable because
the resulting separation between the diaphragm and the
2099657
--3--
electrical lead is small, thus allowing the diaphragm to
recede and cause a short circuit condition by forming a
bridge across the small gap if the battery is even
slightly leaky. Further, the structure and operation of
the diaphragm disclosed in the Oishi et al. battery is
such that the disconnect pressure is dependent almost
entirely on the weld strength, thereby permitting the
disconnect pressure to vary widely depending on the
tolerance of the weld strength itself. Finally, as
implied in Columns 6-8 of the Oish~ et al.
specification, the structure and operation of the
battery requires the weld between the diaphragm and the
electrical lead to be made on the assembly line. Thus,
the Oishi et al. battery cannot be separately tested,
checked and stored before use on the assembly line to
assure the highest degree of quality control.
U.S. Patent Nos. 4,025,696 to Tuchoski et al.
and 4,028,478 to Tuchos~i disclose sealed cells having
an irreversible switch means for cutting off current
flow within an electrochemical cell. The switch means
is actuable upon expansion of a cell container above a
predetermined limit. The cell expansion may result from
gaseous by-products caused by chemical reactions in an
electrochemical circuit system within the cell. The
switch means comprises an active electrically conductive
disk-shaped spring member defining a central opening
therethrough and a passive insulating member. The
insulating member also includes a centrally disposed
opening wherein the opening has a larger diameter than
the opening of the active conductive spring member. The
passive member is disposed between the active switch
member and a conductive plate, and is further arranged
with its centrally disposed opening surrounding the
central opening of the active switch member. When the
cell is in its normal operating state, the active switch
member is in electrical conduct with the conductive
plate in the vicinity of its centrally disposed opening,
and is also in electrical contact with a cell cover at
_4 20~99657
its outer periphery. When an overload condition arises,
the cell container bulges upwardly in the vicinity of
the header which includes the switch means. In response
to such upward bulge, the central portion of the
conductive active switch member also deflects upwardly
toward the cover while the outer perimeter of the disk
- member snaps downwardly and engages an insulating
passive switch. Thus, the current which would
ordinarily flow from the header to the electrochemical
circuit system therebelow, during charging of the cell,
is cut off.
The aforementioned Tuchoski references do not
cause the switch member to open the circuit as a result
of gas pressure applied directly to the switch member
itself. Further, the Tuchoski references do not
disclose any vent means whatsoever to permit excess gas
pressure within the cell to flow therefrom should such
venting become necessary to prevent an explosion.
U.S. Patent Nos. 4,690,879 to Huhndorff et al.
and 4,871,553 to Huhndorff disclose galvanic cells
having switch means for disrupting current flow between
a conductive cell cover and an electrochemical circuit
system within the cell container. The switch means
disclosed in both of these references are not actuable
due to the direct application of excessive gas pressure
thereto. - Instead, the operation of the batteries
disclosed in the Huhndorff references are similar to the
operation of the foregoing Tuchoski references wherein
the switch means is actuable as a result of the degree
of bulging of a container therebelow. Also like the
Tuchoski references, the Huhndorff references do not
disclose any vent means whatsoever.
U.S. Patent No. 4,788,112 to ~ung discloses a
a battery including current cutoff means and vent means.
The current cutoff means of the Rung references is
undesirable in that it does not permanéntly create an
open circuit condition. Further, the Rung reference
does not disclose a burstable vent means.
~5~ 20g9657
Thus, the prior art devices for cutting off
current between the anode and the cathode within a
battery have several shortcomings which make the need
for an improved battery and a method of manufacturing
same evident.
8UMMARY OF TH~ lNv~ lON
The present invention solves the
aforementioned shortcomings of the prior art, and thus
fulfills the needs of the industry by providing an
improved electrochemical cell which includes improved
current cutoff means, both with and without vent valve
means. Further, the present invention provides a new
and improved method for manufacturing such an
electrochemical cell.
In accordance with a broad aspect of the
present invention there is provided an electrochemical
cell comprising a housing having a top portion and a
bottom portion which includes an electrochemical circuit
system therein. The electrochemical circuit system
includes an anode, a cathode and an electrolyte solution
which permeates the anode and the cathode. A conductive
diaphragm means is arranged in the top portion of the
housing for cutting off current flow between the anode
and the cathode. The diaphragm means is adapted for
quick movement between a first stable conducting
position and a second stable non-conducting position
upon direct application of gas pressure generated in the
electrochemical circuit system in excess of a first
predetermined pressure, and is unstable between said
first and second stable positions. A conductive lead
means is provided for electrically connecting the
electrochemical circuit system to the diaphragm means
when the diaphragm means is in said first stable
conducting position, and is remote from the diaphragm
means when said diaphragm means is arranged in said
second stable non-conducting position.
In one preferred embodiment of the present
invention, an insulating means is provided in the top
-6- 2~-~9~57
portion of the housing for retaining the electrochemical
circuit system in the bottom portion of the housing. An
electrically conductive plate having a bottom surface
and a top surface and at least one vented area defining
vent holes extending therethrough is also preferably
arranged in the top portion of the housing above the
insulating means so that gaseous by-products resulting
from chemical reactions in the electrochemical circuit
system can flow through the electrically conductive
plate. In this embodiment, the conductive lead means
extending from the electrochemical circuit system is
directly connected to the electrically conductive plate
to create a current flow path between the anode and the
cathode of the electrochemical circuit system
therebelow. The conductive diaphragm means is
releasably and electrically connected directly to the
top surface of the electrically conductive plate and
completes the circuit between the anode and the cathode
of the electrochemical circuit system during normal
operating conditions of the electrochemical cell. The
conductive diaphragm means is constructed and arranged
so that same will invert from a concave shape to a
convex shape, thus moving from a conducting position to
a non-conducting position, when the pressure directly
exerted on the diaphragm by the gas by-products exceeds
a first predetermined pressure thereby permanently
opening the circuit to cut off current flow between the
anode and the cathode of the electrochemical circuit
system
In another preferred embodiment of the present
invention, the cell includes gasket means disposed
between the housing and the diaphragm means for forming
a hermetic seal and electrically insulating the
diaphragm means from the housing. Further, in this
preferred arrangement, the gasket means is at least
partially disposed between the outer periphery of the
electrically conductive plate and the diaphragm means to
prevent electrical contact from recurring therebetween
~7~ 209~fi57
after the diaphragm means has inverted, e.g., moved from
its conducting position of its non-conducting position.
It is also preferable for the cell of the
present invention to comprise a cover arranged above the
diaphragm means and connected along its outer perimeter
to the outer perimeter of the diaphragm means within the
- gasket means in an appropriately sized and shaped area
of the top portion of the housing. Most preferably, the
diaphragm means according to this aspect of the present
invention comprises a truncated-cone shape and includes
a central portion being releasably and electrically
connected to a centrally located area at the top surface
of the electrically conductive plate.
A cell according to this aspect of the present
invention will effectively create an open circuit
condition to cut off the current flow between the anode
and the cathode as the gas by-products formed from
chemical reactions in the electrochemical circuit system
exer~ a pressure in excess of a first predetermined
pressure directly on the diaphragm. The truncated-cone
shape of the diaphragm of the present invention ensures
that the open circuit condition will consistently occur
within the desired predetermined pressure range.
Another aspect of the present invention
provides a cell having current cutoff means as described
above in combination with burstable vent means for
permitting gas to flow out of the cell upon exertion of
a second predetermined pressure in excess of the first
predetermined pressure range after the current cutoff
means has been actuated to create an open circuit
condition between the anode and the cathode. The cell
according to this aspect of the present invention
includes a diaphragm means as described above further
comprising at least one area on the diaphragm means
which has been weakened so that the weakened area will
~ ,
burst upon the application of gas pressure in excess of
a second predetermined pressure range applied directly
to the diaphragm. The second predetermined pressure
-8- 20996~7
range being greater than the first predetermined
pressure range so that the at least one weakened area on
the diaphragm means will burst to create a passageway
for the gas to flow therethrough only if the internal
pressure continues to rise after the diaphragm means has
created an open circuit condition in the electrochemical
circuit system.
It is also desirable for the cover of the cell
according to this aspect of the present invention to
include at least one vented area defining a gas flow
path to permit gas to flow therethrough after the gas
has caused the weakened area of the diaphragm to burst.
As can be appreciated, a cell according to
this aspect of the present invention will ensure that an
open circuit condition will result when the diaphragm
inverts, thus moving away from its conductive position
against the top surface of the conductive plate, and
will ensure that the gas can safely be vented out of the
cell to---avoid an explosion if the gas pressure should
continue to build up within the cell after the open
circuit condition has been created.
Another aspect -of the present invention
provides a method of manufacturing an electrochemical
cell. More particularly, the present method facilitates
the assembly of a ~header~ at a location remote from an
assembly line so that the header can be assembled and
tested in an ideal environment and thereafter, it can be
stored prior to being electrically connected to an
electrochemical circuit system within the cell.
Preferably, a method according to this aspect
of the present invention includes the step of connecting
the outer peripheries of a conductive cover and a
conductive diaphragm. The connected outer peripheries
of the cover and the diaphragm are then press-fitted
into an insulating gasket. Most preferably, the central
portion of the diaphragm is then electrically connected
to a conductive plate. The conductive plate is then
attached to a flexible lead which extends from a cathode
~ -9- ~099657
through the open top of the housing of the
electrochemical cell. The header, which includes the
cover, the diaphragm, the conductive plate and the
insulating gasket, is then placed within the open top of
the housing by folding the electrical lead and press-
fitting the gasket of the header within the top portion
of the housing.
A method according to this aspect of the
present invention permits the manufacture of the
electrochemical cell in a new and improved manner which
has not heretofore been possible. Particularly, the
present method permits the ~header~ to be assembled and
tested off of the assembly line under ideal conditions.
Thereafter, the header can be electrically and
physically connected into a housing to complete an
electrochemical cell such as the electrochemical cell of
the present invention.
These and other aspects of the present
invention will be more clearly understood when read in
conjunction with the detailed descriptio~ and the
accompanying drawings which follow.
BRIEF DE8CRIPTION OF T~E DRAWING8
FIG. 1 is an enlarged partial cross-sectional
view of a prior art cell with the diaphragm directly
connected to the electrical lead in its conducting
position.
FIG. 2 is an enlarged partial cross-sectional
view of a prior art cell with the diaphragm disconnected
from the conductive lead in its non-conducting position.
FIG. 3 is an enlarged partial cross-sectional
view of one embodiment of the cell of the present
invention with the diaphragm connected to the conductive
plate in its conductive position.
FIG. 4 is an enlarged partial cross-sectional
view of the cell shown in FIG. 3 showing internal forces
exerted by gas pressure and counteracting forces due to
the diaphragm structure and the weld joint between the
diaphragm and the conductive plate.
' 20996S7
--10--
FIG. 5 is an enlarged partial cross-sectional
view of the cell shown in FIG. 3 with the diaphragm
disconnected from the conductive plate in its non-
conducting position.
5FIG. 6 shows the qualitative characteristics
of displacement of the prior art diaphragm shown in
FIGS. 1 and 2 as a function of pressure applied thereto.
FIG. 7 shows the qualitative characteristics
of the displacement of the present diaphragm shown in
10FIGS. 3 and 5 as a function of pressure applied thereto.
FIG. 8 is an enlarged partial cross-sectional
view of one embodiment of the header isolated from the
electrochemical cell of the present invention.
FIG. 9 illustrates the step of electrically
15connecting the header to the electrical lead according
to one aspect of the present method.
DE8CRIPTION OF THE DETAILED EMBODIMENT
An electrochemical cell generally designated
10 in accordance with a preferred embodiment of the
20present invention includes a generally cylindrical
housing having a top portion 12 and a bottom portion 14,
including an electrochemical circuit system 60 therein.
The electrochemical circuit system includes anode
material (not shown), cathode material (not shown) and
25an electrolyte solution (not shown) in contact with the
anode material and the cathode material. The anode is
preferably made of a carbon based material such as
Spherical Carbon or Carbon Black, while the cathode
preferably comprises lithium ions. A conductive lead 54
30extends from the cathode of the electrochemical circuit
system through a centrally disposed passageway 52 in an
insulating plate 50. Insulating plate 50 is
manufactured from any non-conductive and non-reactive
material such as polypropylene, and is arranged in the
35electrochemical cell to retain the electrochemical
circuit system 60 within the bottom portion 14 of the
housing.
-11- 2099657
Conductive lead 54 includes a lead plate 56 at
its uppermost portion and is eleatrically and
permanently connected to the bottom surface 48 of a weld
plate 36 at a weld joint 58. Weld plate 36 is
preferably constructed of 1145 Aluminum or 4000 Series
Aluminum, such as 4047 Aluminum; however, it can be made
of any suitable metal or alloy compatible with the lead
plate 56 and the diaphragm 30 which will be described in
detail below.
It is also preferable for the weld plate 36 to
include four symmetrically-spaced vent holes arranged
about a centrally located area 42 to define a
circumferentially arranged vent area 40 to permit gases
formed due to chemical reactions in the electrochemical
circuit system 60, such as the C02 formed during
decomposition of the cathode material and other gases
formed due to the reaction between oxygen and lithium
ions of the cathode, to flow therethrough and exert an
upwardly directed force on the diaphragm 30.
The structure and operation of a preferred
embodiment of the present invention will now be
discussed with reference to Figs. 3-5. When the
electrochemical cell 10 is in its assembled position, a
Diaphragm 30 is arranged above the top surface 44 of the
weld plate 36 and is desirably laser welded to the
centrally located area 42 thereon at weld joint 46.
Diaphragm 30 is preferably constructed of a flexible
conductive material such as aluminum; however, it can
also be made of any suitable metal or alloy which has
sufficient resistance and flexibility characteristics to
serve its intended purpose as discussed
hereinbelow. Most preferably, the diaphragm 30
comprises a truncated-cone shape and includes a
downwardly extending central portion 28 when it is in
its conducting position. The truncated-cone shape of
diaphragm 30 is a significant improvement over prior art
diaphragms, which are often completely flat or generally
flat with a centrally located pro;ection extending
- 12 - 209965~
downwardly therefrom, in that its current cutoff charac-
teristics are greatly enhanced as will be described in
detail below. It is also preferable for diaphragm 30 to
include a thin-walled grooved area 34 designed to burst
upon application of sufficient force thereto. The
operation of this "burstable vent" function of the
grooved area 34 is described more fully hereinbelow
where the operation of the cell 10 is set forth.
A gasket 16 is desirably arranged between the
top portion 12 of the housing and the outer periphery 32
of conductive diaphragm 30. The gasket 16 preferably
provides a hermetic seal between the electrochemical
circuit system 60 beneath the header 10 and the outside
environment. Additionally, the gasket 16 electrically
insulates the diaphragm 30 from the top portion 12 of
the housing. To effectively provide its insulating and
sealing functions, the gasket 16 is made from a non-
conductive material such as polypropylene and preferably
comprises a rubber or tar coating. Suitable coatings
have been found to be VISTANEX, Trade-mark, Exxon (a
commercially sold rubber) or bitumen (a commercially
sold tar). The gasket 16 includes a bottom portion 20
disposed between the outer periphery 38 of the weld
plate 36 and a portion of the diaphragm 30 to ensure
that no electrical contact occurs at that location.
An electrically conductive cover 22 con-
structed of stainless steel or other suitable metal
alloy, is preferably included in the cell 10 to complete
the construction thereof. The cover 22 includes an
outer periphery 26 mounted within the outer periphery 32
of the diaphragm 30 and is electrically insulated from
the top portion 12 of the housing by the non-conductive
gasket 16. The top portion 18 of the gasket 16 is
arranged to wrap around the outer periphery 32 of the
diaphragm 30 and the outer periphery 26 of the cover 22,
thus effectively insulating the diaphragm 30 and the
cover 22 from the top portion 12 of the conductive
k'~
-13- 20g96~7
housing. It is preferable for the cover 22 to include a
vent hole 24 arranged between the central portion of the
cover and the outer periphery 26 thereof. The foregoing
description of the cell 10 is clearly illustrated in
FIG. 3.
In operation, the electrochemical cell 10 is
arranged to receive a charging current from an outside
source to recharge the electrochemical circuit system 60
therein. Thus, during charging operations, current is
permitted to flow through the passageway -created by
cover 22, diaphragm 30, weld plate 36 and conductive
lead 54 of header 10 and into the electrochemical cell
therebelow. If an overcharged condition should result,
the cathode material in the electrochemical circuit
system 60 would begin to decompose, thus emitting
gaseous by-products, such as CO2, therefrom. So long as
the anode and the cathode are electrically connected,
the generation of gas will continue. Thus, the internal
gas pr_ ~ure within the housing will continuously
increase. Under such circumstances, it is most
desirable to cut off ~ current flow to the
electrochemical circuit s~stem so that the chemical
reactions therein will stop forming gaseous by-products
before an explosion occurs.
As shown in FIG. 4, as the internal gas
pressure builds up, a force is exerted upwardly through
the circumferentially arranged vented area 40 in the
weld plate 36 directly upon the diaphragm 30 and a
counter force is exerted downwardly due to the
combination of the inherent strength of the truncated-
cone shape of the diaphragm 30 and the weld location 46
to retain the diaphragm in contact with the top surface
44 of the weld plate 36 as illustrated in FIG. 3.
However, when the upwardly exerted gas pressure exceeds
a pressure within a predetermined range, e.g.,
approximately 150-200 psi, it overbears the
counteracting forces shown in FIG. 4 and the diaphragm
becomes ~inverted~ as it quickly snaps away from its
' -14- 2099657
conducting position on the weld plate 36 thereby
breaking the electrical connection at the weld joint 46,
all of which is clearly shown in FIG. 5. As will be
discussed more fully below, the truncated-cone shape of
the diaphragm 30 ensures that the pressure required to
invert it is substantially constant because any
variation in the disconnect pressure is due to
variations in the weld strength, which is a small
percentage of the overall disconnect pressure.
When the diaphragm 30 i8 in its ~inverted~
position as shown in FIG. 5, an open circuit condition
exists; thus, the charging current between the diaphragm
30 and the electrochemical circuit system 60 is cut off.
Preferably this open circuit condition will prevent the
formation of significant amounts of additional gases due
to chemical reactions in the electrochemical circuit
system. The now increased internal gas pressure
continues to exert an upwardly directed pressure through
the vent area 40 against the undersurface of the
diaphragm 30. The top surface 44 of the weld plate 36
remains flush against the bottom portion 20 of the
gasket 16 at all times. Thus, as shown in FIG. 5, the
combination of the gasket 16 and the weld joint 58
between the lead plate 56 and the bottom surface 48 of
the weld plate 36 effectively prevents any possibility
of electrical contact from occurring between the top
surface 44 of the weld plate 36 and the diaphragm 30
when the diaphragm 30 is in its inverted position.
Also, as clearly shown in FIG. 5, the distance between
the diaphragm 30 and the top surface 44 of the weld
plate 36 is relatively large compared to prior art
~disconnect distances~, and is preferably between about
.50 and 1.0 mm, and typically is about .75 mm. Thus,
the large ~disconnect distance~ of the present invention
eliminates or at least greatly minimizes the possibility
of a short circuit condition from occurring between the
diaphragm 30 and the top surface 44 of the weld plate 36
- 15 - 2 0 ~ 9 6 5 7
after the diaphragm 30 has snapped to its inverted
position, even if the cell becomes leaky.
Thus, as evident from the aforementioned
current cutoff aspect of the present invention, the
5 charging current can be disconnected before the internal
gas pressure rises to such a level as to cause an
explosion. Ideally, the range of disconnect pressures
of about 150-200 psi and more preferably of about 170
psi, in the foregoing example should be sufficient to
create an open circuit condition as described above
before the venting aspect of the present invention is
actuated. However, under certain extreme overcharge
conditions, the internal gas pressure will continue to
rise after the diaphragm 30 has separated from the weld
15 plate 36. Under these circumstances, weakened areas
such as a circumferentially arranged groove (not shown)
in diaphragm 30 are designed to burst when the internal
gas pressure reaches between about 250-500 psi and
preferably between about 350-450 psi, so that the gas
20 by-products can flow out of the newly created vent area
at the groove 34 and can continue to flow out of header
10 through a vent hole 24 in the cover 22 and into the
outside environment. Thus, the burstable venting
feature of the present invention permits gas pressure to
25 be let out of the electrochemical cell before a danger-
ous explosion results. As can be appreciated, the
present invention is not limited to any particular range
of disconnect pressures. Indeed, the disconnect pres-
sure can be varied for use in various types of cells by
30 changing the thickness of the diaphragm 30. Further-
more, the disconnect pressure can be changed by changing
the configuration of the truncated cone-shaped diaphragm
30. For example, if a higher disconnect pressure range
is required, such as 240-260 psi, the cone configuration
35 of the diaphragm 30 can be exaggerated.
According to one preferred embodiment, the
truncated-cone shape of the diaphragm 30 will permit the
,
' ~ -16- 20'996S7
diaphragm to remain in its normal downwardly extending
state against the top surface 44 of the weld plate 36
until about 115 psi in the absence of the weld joint 46.
The presence of the center weld 46 raises the pressure
required to invert the diaphragm, as shown in FIG. 5, to
approximately 170 psi. FIG. 6 qualitatively illustrates
an example of the displacement of the center portion of
the diaphragm 30 as a function of the internal pressure
exerted thereon, both with and without the presence of
the weld joint 46. Significantly, the truncated-cone
design of the diaphragm 30 contributes greater than two-
thirds of the overall resistance strength against
displacement of the diaphragm. This is due to the novel
shape of the burstable diaphragm 30 which includes
inherent forces acting downwardly against the internal
gas pressure to resist displacement and inversion of the
diaphragm. As can be appreciated, the anti-displacement
and inversion characteristics of the present truncated-
cone shaped diaphragm 30 are far superior to the
generally flat prior art diaphragms, such as the
diaphragm disclosed in the Oishi et al. header, which
rely almost entirely upon the strength of a weld joint,
such as weld joint 46, to prevent displacement of the
diaphragm. As discussed above, the presence of the weld
joint 46 between the downwardly extending central
portion 28 of the conical diaphragm 30 and the top
surface 44 of the weld plate 36 increases the
displacement strength of the diaphragm 30 by only about
30%.
FIG. 1 illustrates a prior out battery having
a generally flat diaphragm 70 having a center projection
72 extending downwardly therefrom, in contrast to the
truncated-cone shaped diaphragm of the present
invention. The projection 72 of the generally flat
diaphragm 70 is welded to a lead plate 74 at a weld
joint 76. When internal pressure caused by gaseous by-
products as described above, is exerted directly upon
the diaphragm 70, the projection 72 is forced upwardly
s, -17- 2099657
and pulls away from the weld joint 76 thereby creating
an open circuit between the diaphragm 70 and the lead
plate 74, as shown in FIG. 2. As can be appreciated,
the distance between the lowermost portion of the
projection 72 and the lead plate 74, after the internal
gas pressure has caused a separation therebetween, is
very small in comparison to the large permanent gap
between the conical diaphragm 30 and the weld plate 36
of the present invention, when the diaphragm 30 is in
its inverted position, as shown in FIG. 5. If the
electrochemical cell becomes leaky (i.e. gases are
permitted to escape from the housing), as is often the
case when the cell is greatly overcharged, the al~eady
small disconnect distance between the projection 72 of
the diaphragm 70 and the lead plate 74 may diminish
until an electrical connection is again created
therebetween. The small disconnect distance as
described above is largely a result of the generally
flat construction of the prior art diaphragms.
Furthermore, an additional shortcoming of this prior art
design is due to "burrs", i.e., rough edges, which may
form on the projection 72 and the lead plate 74 after
they have become separated, as discussed above. The
burrs can facilitate the reoccurrence of the electrical
connection between the projection 72 of the diaphragm 70
and the lead plate 74 and are thus, a drawback of the
prior art diaphragms.
From a pressure-strength standpoint, the
generally flat prior art diaphragm functions essentially
as a flat plate regardless of the presence of a central
projection extending therefrom. As discussed above, the
pressure required to raise the prior art diaphragm,
shown in FIGS. 1 and 2, away from its connection with
the lead plate 74 is minimal without the presence of the
weld joint 76. Thus, in prior art diaphragms, the weld
joint 76 provides an extremely large percentage of the
-overall disconnect strength as compared with the weld
joint 44 between the truncated-cone shaped diaphragm 30
'~ -18- 2099657
and the top surface 44 of the weld plate 36 of the
present invention. The qualitative relationship between
the displacement of projection 72 of the prior art
diaphragm 70 from the weld plate 74 as a function of
s internal gas pressure applied thereto is illustrated in
FIG. 7. An analysis of the qualitative relationship
illustrated in FIG. 7 clearly shows that the structure
of the prior art diaphragm contributes very little to
the forces which counteract the internal gas pressure to
keep electrical contact between the diaphragm and the
electrical lead. The inadequacy of the prior art design
is more evident when the prlor art diaphragm
displacement versus internal pressure relationship, as
shown in FIG. 7, is directly compared with the same
qualitative relationship of the truncated-cone shaped
diaphragm of the present invention as shown in FIG. 6.
According to the present method of
manufacturing an electrochemical cell, the diaphragm 30
can be laser welded, or electrically connected according
to other known methods, to the top surface 44 of the
weld plate 36 in an environment away from the general
assembly line which permits the welded parts to be
tested, checked and stored before insertion into the
electrochemical cell 10 on the assembly line.
The present - method of manufacturing an
electrochemical cell is a significant improvement over
prior art methods. The improvement is clear when viewed
in light of the method of manufacturing batteries as
taught in the prior art. In particular, prior art cap
assemblies, such as the cap assembly comprising a
weldable diaphragm in the Oishi et al. reference
discussed above, required the cap assembly, and in
particular the ~critical weld~ between the diaphragm and
an electrical lead to be made on an assembly line under
less than ideal conditions. Additionally, all testing
of the ~disconnect pressure~ in prior art batteries
(e.g., the pressure required to separate the diaphragm
from the electrical lead) must be conducted with the cap
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assembly integrally formed as part of the
electrochemical cell. Thus, the cap assembly of the
Oishi et al. reference cannot be manufactured or tested
in an ideal environment off of the assembly line.
s Additionally, the prior art cap assemblies cannot be
stored as an integral unit prior to assembly into the
battery.
In contrast to the method of manufacturing
electrochemical cells, including a ~critical weld~, as
taught in the prior art, the present method of
manufacturing an electrochemical cell permits the
separate components of a header 11 to be assembled ~off-
line~ under ideal working conditions. As clearly shown
in FIG. 8, the components of the header 11 of the
present invention include the cover 22, the diaphragm
30, the gasket 16 and the electrically conductive
plate 36.
Thus, according to a preferred method of the
present invention, the sequence of steps during the
assembly of an electrochemical cell, such as
electrochemical cell 10 described above, includes the
step of placing the outer periphery 26 of the conductive
cover 22 within the outer periphery 32 of the conductive
diaphragm 30. The outer periphery 32 of the diaphragm
30 is then crimped to secure the outer periphery 26 of
the cover 22 therein. Most preferably, the diaphragm 30
and the cover 22 will be arranged so that the central
portion of the diaphragm 30 comprises a concave shape
with respect to the cover 22.
The now unitary cover 22 and diaphragm 30 is
then press-fitted into the insulating gasket 16 between
the top portion 18 and the bottom portion 20 thereof.
The friction-fit between the crimped cover 22 and the
diaphragm 30 and the gasket 16 is sufficiently secure to
retain the cover 22 and the diaphragm 30 therein.
Preferably, the now secured cover 22 and
diaphragm 30 are arranged so that the central portion of
the diaphragm 30 abuts the top surface 44 of the weld
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plate 36 at the centrally located area 42. As shown in
FIG. 8, it is preferable for the diaphragm 30 to include
a convex shape with respect to the top surface 44 of the
weld plate 36. The diaphragm 30 is then welded thereto
by any suitable welding technique, such as laser
welding, to form the weld joint 46. The weld formed
during this welding operation is a ~critical weld,~
since it must break upon application of a predetermined
pressure, and can be performed according to the method
of the present invention in an ideal environment to
assure that the weld is precisely formed within a
predetermined tolerance.
After the foregoing welding operation is
completed, the header 11 can be tested apart from the
rest of the electrochemical cell 10 under ideal
conditions. Additionally, the header 11 comprising the
cover 22, the diaphragm 30, the weld plate 36 and the
gasket 16 can now be stored until it is desirable to
electrically connect same to the top portion 56 of the
electrical lead 54 which extends through the open top 12
of an electrochemical cell as shown in FIG. 9.
It is desirable, according to the method of
the present invention, for the bottom surface 48 of the
conductive plate 36 to be welded to the electrical lead
on the assembly line. The weld joint 58 formed between
bottom surface 48 of the conductive plate 36 and top
portion 56 of the electrical lead 54 is a ~non-criticaln
weld joint. The non-critical weld joint 58 need not be
made under ideal conditions because the weld joint 58 is
not designed to break at a predetermined pressure, as is
the critical weld joint 46. After this non-critical
welding operation occurs, it is preferable for
electrical lead 54 to be folded by a specially designed
folding apparatus (not shown) so that the lead plate
portion 56 will be arranged between the electrochemical
circuit system (i.e., the anode, the cathode and the
electrolyte solution) and the electrically conductive
plate 36 when the header 11 is placed within the open
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top portion 12 of the electrochemical cell 10. Finally,
the top portion 12 of the cell is crimped into position
to secure the cell into assembled position.
It is most desirable to place the header 11
into the top portion 12 of the housing in a manner such
that the gasket 16 forms an insulating seal between the
header 11 and the top portion of the housing 12 as
clearly shown in FIG. 1.
As can be appreciated, the structure of the
components, and especially diaphragm 30, of the present
electrochemical cell 10 facilitates the foregoing
advantageous sequence of assembly steps resulting in the
manufacturing advantage as discussed above. Such a
manufacturing advantage (i.e., the ~off-line~ assembly,
testing and storage of the components of the header 11
of the present invention) has not heretofore been
achieved.
While the foregoing description and figures
are directed toward the preferred embodiment and method
of the present invention, it should be appreciated that
numerous modifications can be made to each of the
individual components of the entire apparatus and the
steps in the method as discussed above, and are indeed
encouraged to be made in the materials, structure and
arrangement of the disclosed method and embodiment
without departing from the spirit and scope of the
present invention. Thus, the foregoing description of
the preferred embodiments should be taken by way of
illustration rather than by way of limitation of the
present invention as described by the claims set forth
below.
