Note: Descriptions are shown in the official language in which they were submitted.
~ 8894~
~60540 ::
S P E C I F I C A T I O N
_ _
Background of the Invention
.' '''
, This invention relates to flat alkaline cells
in general, and more particularly to a flat alkaline cell
construction wherein the active elements are enclosed
within a sealed, liquid-impervious plastic film envelope
and wherein the envelope is provided with both positive
and negative terminal connections and a third terminal
connection for an auxiliary electrode.
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Alkaline electrolytes are notoriously known for
their ability to readily wet most metal and plastic surfaces
and to creep past seals conventionally used in current~
; producing electrochemical cells. Our flat cell construc- .
,. . . .
tion as described above effectively solves this problem
! through the use of an adhesive sealant which is non-wettable
by the alkaline electrolyte. The thin layer of adhesive
sealant used to tightly bond the current collector to
the wall of the plastic film envelope actually resists
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.,
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106~D5 40 8894-1
creepage of the electrolyte past the sealing interfaces
3~ and out through the opening in the wall. Preferably, the
adhesive 8ealant is chosen from the class of compounds
known as "fatty polyamides", although other adhesive
sealants which are not readily wet by the alkaline ~ -
electrolyte can also be used.
Still another advantage of our 1at cell
construction resides in the provision of an extented
leakage path over which the electrolyte must travel in
order to escape from the cell. This extended leakage
path comprises the sealing interface between the current
collector and the inner surface of the wall of the sealed
envelope, l.e., the leakage path traverses the shortest
distance from the opening over the width or length of
; ~ the collector.
In a typical flat cell construction made in
accordance with our above-referred to copending applica-
~ , . ..
tion, the sealed envelope is advantageously formed by a
heat shrinkable p~ stic film tube which is heat shrunk
~ . ,
; 20 town arount the side walls of the electrode assembly andwhich overlaps the marginal borders of a pair of flat
current coilectors, one of each of which is disposed
adjaceot ;o an end of the electrode assembly. The pair
of current collectors and the portion of the heat shrunk
- tùbe overlapping the margLnal borders are~tightly adhered
together by a thin layer of the non-wetting adhesive
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1060S9UD 8894-1
sealant. The centermost portion of one of the pair of
-- current collectors which is left exposed~at one end of
the eleccrode assembly constitutes the positive terminal
conn~ction while the centermost portion of the othcr of
., s
the pair of collectors which is left exposed at the opposite
end of thP electrode assembly constitutes the negative
terminal connection of the cell. The arrangement of the
terminal connections is such that a plurality of the
individual flat cells may be stacked together with the
positive ~erminal connection of one cell making electrical
connection with the negative terminal connection of an
at3acent cell via a wire lead or the like attached thereto
to constltute a series-connected battery. Flat cells
1 with this arrangement of the terminal connections can
i also be assembled into parallel and series-parallel
connected batteries.
It has already been proposed in the prior art
to employ so-called "auxiliary electrodes" in current-
producing electrochemical cells. These auxiliary
electrodes are used in con3unction with the working
electrodes, i.e., positive and negative~electrodes, to
perform eertain valuable functions in operation of the
cells. Such auxiliary ~lectrodes have been used, for
instance, in rechargeable cells to detect or sense the
presence of excessive amounts of either oxygen or hydrogen
gas that may be generated under certain conditions such as
1060540
8894-l
. . .
when the cells are overcharged. -
- In U. S. Pat. No. 3,462,303 to H. Reber, there
is disclosed a sealed rechargeable cell wherein an auxiliary
electrode is maintained in contact with a gas space and a -~
liquid electrolyte. The auxiliary electrode will form
r'
; with the negative electrode of the cell a voltage
differentlal the value of which wlll be dependent on the
partial oxygen pressure in the gss space of the sealed
cell. When the cell is sub~ected to overcharging, the
partial oxygen pressure in the gas space will rise, a
change in the voltage differential will occur and this
change is utilLzet for actuating control devices for
~ termina~ng the charging current and thereby prohlbiting
¦ the build-up of an excessive gas pressure inslde the cell.
1 ~ Basically the same auxiliary electrode arrange-
I ment may be used in a rechargeable cell such as described
~ above to sense the presence of hydrogen gas in the cell. -
¦~ In this instance, the auxiliary electrode which will form
with the positive electrode of the cell a voltage
tifferential whose value will be dependent on the partial
:
hydrogen pressure in the gas space under conditions where
hydrogen-gas may be evolvéd during operation of the cell.
Auxiliary electrodes may also be incorporated in
; current-producing electrochemical cells as a voltage
.
.
refereuce device. It is possible for instance to electro-
chemically couple the positive or negative electrode of a
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1060540 8894-1
rechargeable nickel-cadmium cell to an auxiliary reference
electrode and study the discharge behavior of either
- electrode independently of the other. Auxiliary electrodes
in these applications are a valuable tool to the researcher
since he can carry out his studies without having to
dismantle the cell construction.
Provision must of course be made in the sealed
assembly of the current-producing electrochemical cell for
making external electrical connection with the auxiliary
electrode. Such means usually comprises a separate or
thirt terminal connection in the ~ealed assembly in
addition to both the positive and negative terminal
connectlons. Basically the 8ame type of construction is
used for the third terminal connection regardless of
whether the auxiliary electrode is employed as an oxygen
or hydrogen sensing electrode or voltage reference electrode.
- The principle object of this invention is the
provision of a flat alkaline cell construction of the
charac~er described in our above-referred to copending
application wherein there is provided both positive and
negative terminal connections in at least one wall of the
plastic fllm envelope together with a third terminal
connection for an auxiliary electrode~
This invention resides in a flat alkaline cell
construction wherein at least a pair of flat electrode
.
` 106~DS40 8894-1
.~
elements of opposite polarity having a porous separator
containing an alkaline electrolyte interposed therebetween
are arranged in the form of a conventional electrode stack
assembIy. A current collector which may be a flat metal
i plate or foil i8 disposed adjacent to and 'in electrical
'~ connection with one of the pair of electrode elements~ at
one end of the electrode assembly. The eIectrode assembly
!~ - ;
is enclosed within a se-led envelope made of a liquid
impervi~u8 plastic film and preferably a plastic film ''
l ~lO which is electrically non-conductive. The sealed envelope
¦'~ is formed with an opening in one wall thereof which exposes ;~
at lea8t a portion of the current collector for making
exeernal~electrical connec~ion'therewlth. A thin layer
of an adhè8ive sealant which is non-wettable by the alkaline ~'
' electro~yte, is interposed at the interface between the
current collector and the inner surface of the wall of
;the;sealed envelope surrounding the opening. The layer
; of adhesive sealan~ ~igh~ly-adheres and seals together
the~collector~and the en w lope wall and'prohibits leakage
of the alkaline electrolyte through the opening from
insit the cell. Preferably, the layer of adhesive sealant
covers s'ubstantially the entire face of the current
' coliector, except for a small area coinciding with the
openlng ln the wall. Alt~ough other non-wetting adhesive
sealants may be used in the practice of the inventlon,
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1060540
8894-1
the preferred adhesive sealant is a sealant formulated
from the general class of compounds known as "fatty
polyami.des". The fatty polyamides used in the sealant
preferab}y should have an amine number of above about 9.
The sealed collector assembly described above
; con8titutes a first terminal connection for the cell. A
second and third terminal connection are provided in the
sealed envelope in accordance with the invention, one or
both of which may constitute basically the same type of
~tructure as used in the first terminal connection. The
second tenminal connection may be made using another
current collector disposed adjacent to the other of the
pair of electrode elements of opposite polarity at the
opposite end of the electrode assembly. This current
collector is tightly adhered and sealed to the wall of
the envelope by a thin layer of the same non-wetting
adhesive sealant applied around another opening in the
wall which exposes a portion of the current collector.
; Suitable means may be provided for making external
electrical conr~ection with thé current collectors of both
the first and second terminal connections such as by
attaching a wire lead thereto. The third terminal connec-
tion for the cell may be made in basically the same manner
or alternatively by extending a wire lead out through a ;
leak-proof seam in the instance where the sealed envelope
.. 9
106054~ 8894~
is closed by sealing together edges or marginal borders of
the plæ.stic film such as by a heat seal. The first and
second terminal connections so formed are maintained in
electrical connection with the pair of electrode elements
of opposite polarity and constitute both the positive and
negative terminal connections for the cell. The third
terminal connection is maintained in electrical connection
with an auxiliary electrode inside the cell~which may be
an oxygen or hydrogen sensing electrode or an auxiliary
voltage reference electrode, for example.
Although the invent1on is widely applicable to
flat alkaline cell constructions in general, it will be
more fully described hereinafter with particular reference
to a rechargeable nickel-cadmium cell. Such a rechargeable
nlckel-cadmium cell comprises a positive electrode
contai~ing an electrochemicaily oxidizable active material
such as nickel hydroxide, a negative electrode containing
an electrochemically reducible active material such as
cadmium oxlde or cadmium hydroxide, and a porous bibulous
separator containing the alka~ine electrolyte interposed
between and in contact with both the positive and the
negative electrodes. The positive and negative electrodes
will be so balanced electrochemica~ly with the active
materials as to prohibit the generation of excessive gas
or gases upon overcharge of ~he cell. It will be under- ;
stood, of course, that the flat alkaline cell construction
.
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~Ot~D5 40 8894-1
of the invention may utilize other electrodé systems such
as the zinc/manganese dioxide system as will readily occur
to those skilled in the art.
The in~ention will be hereinfurther described
in detail by reference to the specific embodiments of the
flat cell construction illustrated in the accompanying
drawings.
Fig. 1 is a perspective view of one embodiment
of the flat alkaline cell construction of the invention;
Fig. LA is a cross-sectional view of the flat
cell illustrated in Fig. 1, taken along the line lA-lA;
Fig. 2 is a perspective view of another
embodiment of the flat alkaline cell construction;
Fig. 2A is a cross-sectional view of the flat
cell illustrated in Fig. 2, taken along the line 2A~2A;
Fig. 3 is a perspective view of another embodi-
ment of the flat alkaline cell construction;
Fig. 3A is a cross-sectional view of the flat
cell illustrated in Fig. 3, taken along the line 3A-3A;
- Fig. 4 is a perspective view of s~ill another
embodiment of the flat alkaline cell construction;
Fig. 4A is a cross-sectional view of the flat
cell illustrated in Fig. 4, taken along the line 4A-4A;
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1~6~D540 8894-1
Fig. 5 is a perspective view of a further embodi- ~
` ment of the flat alkaline cell construction; and .
Fig. 5A is a cross-sectional view of ~he flat ~.
cell illustrated in Fig.. 5, taken along the line SA-5A.
Figs. 1 and lA illustrate a rechargeable flat
nickel-cadmium cell made in accordance with the invention.
The ce~l comprises a positive electrode plate 10, a pair
of negative electrode plates 11, 12, one of each of which
is disposed on each side of the positive electrode plate
10, porous separators 13, 14 containing an alkaline
electrolyte sandwiched between and in facial contact
with the positive electrode plate 10 and each of the pair
of negative electrode plates 11, 12, the.arrangement of
the electrodes and separators forming a conventional
3 electrode stack assembly. An auxiliary electrode lS which
is smaiier in size than the positive electrode plate 10
and the negative electrode plates 11,.12, is positioned :
over the left hand side of the electrode assembly adjace~t
to the negative electrode plate 11 but separated therefrom
.by a porous separator 16. The positive electrode plate :-
10, negative electrode plates 11, 12 and the separators
13, 14 and 16 are rectangular in shape and are of substan-
tially the same size such that the electrode plates and
separators are stacked congruently. Both the posltive
electrode plate 10 and negative electrode plates 11, 12
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~0~540
8894-1
may be sintered type electrodes fabricated fro~ a sintered
metal plaque which may be made, for example, by sintering
a layer of metal powder, e.g., nickel, onto both sides of
an open or porous substrate such as a nickel screen, which
serves as a mechanical support and electri~l path. The
sintered metal plaque is impregnated with the electro-
chemically active material in accordance with conventional
methods well known in the art. The porous separators 13,
14 and 16 containing the alk~line electrolyte may be made
from a conventional separator material such as a non-woven
organic fiber matte. A preferred type is made from nylon
fiber under the trademark "Pellon". The alkaline
electrolyte used in the cell may be, for example, a 30
percent by weight solution of potassium hydroxide.
At each end of the electrode assembly is provided
Z
one of a pair of current collectors 17, 18. The current
collector 17 is also smaller in size than the positive
electrode plate lO and negative electrode plates ll, 12 -~
but i5 larger than the auxiliary electrode 15 and overlies
substantially more than half of the electrode assembly. A ~;
third current collector 19 is positioned over and in
. .
contact with the auxiliary electrode 15. Auxiliary
electrode 15 and the current collector 19 which i8
substantially the same size as the auxiliary electrode 15,
are spaced apart from the current collector 17 at one end
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~060540 8~94-1
of the electrode assembly. The current collector 18 is
; positioned in contact with the negative electrode plate
. 12 at the opposite end of the electrode assembly. The
pair of negative electrode plates 11, 12 are e~ectrically
interconnected by an insulated metal conductor 20 and the
positive electrode plate 10 is electrically interconnected
to the current collector 17 also by an insulated metal
conductor 21. The current collectors 17, 18 and 19 are
made from an electrically conductive metal, preferably in
the form of thin metal foil, whlch is inert to the alkaline
electrolyte such as nickel or nickel plated steel.
All of the cell elements as described above are
sealed within a liquid-impervious, electrically non-
condu~tive, plastic film envelope 22. The envelope 22
fits tlghtly around the side walls of the electrode
assembl~ and also around the opposite ends thereof
forming a pair of end walls 23, 24. The end wall 23
compleS~ely covers the current collector 19 but does not
completely overlap the current collector 17 but rather
20 - leaves an opening 25 which exposes a portion of the current
collector 17. Similarly, the end wall 24 does not completely
overlap the current collector 18 but leaves the center
thereof exposed forming an opening 26. As shown in both
Figs. 1 and lA, a metal terminal lead 27 is secured such
as by welding to the exposed portion of the current
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1060540 8894_1 i
collector 17 which constitutes the positive terminal for
the cell. A metal terminal lead 28 is secured to the
exposed portion of the current collector 18 which
const~tutes the negative terminal for the cell.
Substantially the entire outer surface of each
of the pair of current collectors 17, 18, except for the
exposed portion théreof, are coated with a thin layer 29,
30, respectively, of an adhesive sealant in accordance
with the invention. Similarly, the entire outer surface
of the current collector 19, except for a portion at the
center which is to be left exposed, is coated with a layer
31 of the same adhesive sealant. The layers 29, 30 and 31
of adhe8ive sealant tightly seal the interfaces between
each of he pair of end walls 23, 24 of plastic film and
; the current collectors 17, 18 and 19 against leakage of
. ~ . . ...
alkaline electrolyte. Suitably, the adhesive sealant should
~;~ be an organic resin which will adhesively bond to both the ~-
pla3tic-ilm and metal collectors. Preferably the
adhesive sealant employs a fatty polyamide which is
chemically resistant to and not readily wet by the alkaline
.
electrolyte. The layers of adhesive sealant are first
applied as a thin layer over the outer surface of each of ~-
the collectors 17, 18 and 19.
~;~ The side wall 23 is formed with an opening 32
which coincides with the exposed center of the current
:
-15-
10~D540 8894-1
collector 19. A third terminal lead 33 passes through the
opening 32 and is attached as by welding to the current
collector 19 which constitutes a third terminal connection
for the auxiliary electrode 15.
The envelope 22 is made from a tubular heat
shrinkable.plastic film such as a vinyl film. In assembly
of the cell, the positive electrode plate 10 and the pair
of negative plates 11, 12, the separators.13, 14 and 16,
auxiliary electrode 15 and current collectors 17, 18 and
: . .
19 are first stacked together in the manner as described
above and then inserted inside the heat shrLnkable tube
w.ith the outer ends of the tube protruding beyond the
coatet current collectors. The plastic fLlm tube is then
: hested and is caused to shrink down tightly around the :
side wail~.of the electrode stack and at the same time,
the protruding ends of the tube shrink down forming the
~: pair of end walls 23, 24. Application of heat and
. . pressure to the end walls 23, 24 establLsh the final
adhesive bond.
Figs. 2-5 and the corresponding FLgs. 2A-5A, . .
; incIusive, illustrate a number of other embodiments of the
~: flat c-li construction in accordance with the invention.
In all of these embodiments, the cell elements may be
basically the same as those just described, that is, the
same sintered type positive and negative electrodes may
;'
- -16-
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- ~06~D59UD
8894-1
be used together with the same porous, bibulous separators
containing the alkaline electrolyte, interposed bebween ~
the electrodes to form the electrode assembly. For the ''
sake of convenience, the same reference numerals will ;
denote identical cell elements where applicable in the "
,
following description. -
In the embodiment of the invention illustrated
in Figs.-Z and 2A, the arrangement of the electrode ' '
assembly'and the auxiliary electrode 15 and current ''~
collector 19 are the same as in the flat cell ~ust
describctd. The cell elements are again encloset within a
sealed, liquid ~mpervious, electrically non-conductive,
; ~ plastic iilm e~nvelope 34. This envelope 34'is also made
from a heat shrinkable plastic film tube but in this case
the tube has one closed and one open end. The tube is
. . .
heat shrunk tightly around the side walls of the electrode
assembly, except ehat corresponding with the open end of
,. :
~' the tube, and also over both ends of the electrode assembly
fonmlng t'ne end walls 35, 36. The open end of the tube ~ .
is then heat sealed together to form the liquid-tight seam
37. The'end wall 35 i8 provided with an opening 38 which
exposes d portion of the current collector 17 and also '~
with an opening 39 which exposes the center of the
;~ collect'or 19. In similar fashion, the other end wall
36 is provided with an opening 40 which exposes the center ' '
" ' ' ' '~",""''.
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10 60 5 40 8894-1
of the collector 18 at the opposite end of the electrode
assembly. The collectors 17, 18 and 19 are also coated
with the layers 29, 30 and 31 of the same non-wetting
ad~esive sealant described hereinabove, except for those
portions of the collectors which are to be left exposed
through the re8pective openings in the end walls 35, 36.
Terminal leads 41, 42 pass through the openings 38, 40
and are secured such as by welding to the exposed portion
~ of each collector 17, 18 which constitutes the positive
¦ ~ 10 and negative terminal connections for the cell. Similarly,
I a third terminal lead 43 passes through the opening 39 and
is attached to the collector 19 which constitutes a third
i3 tenminal connection for the auxiliary electrode 15. After
; the tube is heat shrunk tightly around the electrode assembly,
the end walls 35, 36 are sealed directly to the current
; collectors 17, 18 and 19 by application of heat and ;
pressure.
~ , . .
In the embodiment of the invention illustrated
in Figs. 3 and 3A, the arrangement of the electrode
,
assembly is again the same as in the flat cells described
hereinabove. However, in this flat cell the auxiliary
~ . .
j~ electrode 44 is the same size as the positive electrode
¦~ plate 10 and neg~tive e}ectrode plates 11, 12. The auxiliary
electrode 44 is positioned at one end of the electrode
assembly adjacent to the negative electrode plate 11 but
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l~ -18-
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1060S40 8894-1
is separated therefrom by a porous sepa~ator 45. Placed
over the auxiliary electrode 44 and in electrical contact
therewith is a current collector 46 which may also be a
metal pla.e or foil. Both of the negative electrode
plates 11, 12 are interconnected by an insulated metal
conductor 47. A terminal lead 48 is secured at one end
to the positive electrode piate 10 and another terminal
lead 49 is secured to the negative electrode plate 11,
such as by welding, the terminal leads extending outwardly
from oppooite sides of the electrode assembly. The terminal
lead 48 constitutes the positive terminal connection and
the te~ninal lead 49 constitutes the negative terminal
connection for the cell.
.
The cell elements are enclosed within a sealed,
li~uid-impervious, electrically non-conductive, plastic
film envelope 50. This envelope 50 is also made from a
tubular heat shrinkable plastic film material. In this
; case, ho~ever, the tube is heat shrunk tightly over both
ends of ~he electrode assembly. The envelope 50 is thus -
$ormed with side walls 51, 52 and open énds which are
heat sealed to one another to form liquid tight seams 53,
. .
54. The open ends of the tube are also sealed around the
protruding ter~inal leads 4~, 49.
The envelope 50 is provided with an opening 55
in the side wall 51 which exposes the center of the current
.
',
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10 60 5 40 8894-1
collector 46. A metal terminal lead 56 passes through
the opening 55 and i9 secured such as by welding to the
collector 46 which constitutes a third terminal connection
for the auxiliary electrode 44. A layer 57 of adhesive -
sealant is applied on the outer surface of the collector
46, except for the small area left exposed through the
opening 55, and also around the terminal leads 48, 49 at
the poin~ where they protrude through the liquid-tight
8eams 53, 54. Preferably, the adhesive sealant used is
.
the same fatty polyamide sealant which is non-wettable by
the alkaline electrolyte. After heat shrinking the
plastic tube as above describedS the application of heat
and pressure tightly bonds the plastic film to both the
oollector 46 and the terminal leads 47, 48.
Figs. 4 and 4A illustrate another embodiment of
the inver,tion wherein the cell elements are sealed within
a liquid impervLous, electrically non-conductive, plastic
film envelope or housing of a somewhat different construc-
tion.~The electrode assembly in this embodiment is
: , . ,;. ~ .
basically the same as that illustrated in Figs. lA and
2A. However, in this flat cell the auxiliary electrode
15, and the current collectors 17 and 19 are each less
than half the size of the positive electrode plate 10
and the negative electrode plates 11, 12. The auxiliary
electrode 15 and the collector 19 are positioned over the
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: ~ :.
1060540 8894-1
~ ':
left hand side of the electrode assembly while the collector
17 is positioned over the right hand side of the assembly. -
A thin layer 58, 59 of the adhesive sealant, preferably a
fatty pclyamide sealant, is applied to the outer surface
of the collectors 17, 18, except for a small area left
exposed at the center of each collector.~ In similar
fashion, a thin layer 60 of the adhesive sealant is
.. . .
~l applied to the outer surface of the collector 19. The
,
! electrode assembly is placed inside a rectangular, open
endet container 61 which is made of the lLquid impervious,
electrically non-contuctive plastic film. The container 61
includes a bottom wall 62 and may be made, for example,
by vacuum forming a flat sheet of the plastic film
material. The electrode assembly fits snugly inside the
container 61 whose open end is sealed off by a cover 63
also made of the same plastic film material. The cover 63
has its outer peripheral edges heat sealed to the peripheral
; edges of the container 61 forming a continuous liquid-tight
` seam 64.~The cover 63 is provided with a pair of openings
` 20 65, 66 through which the centers of current
collectors 17, 19 are exposed. The container bottom wall
62 is provided with an opening 67 through which the
center-or the current collector 18 is exposed. Terminal `
:: . ............................... .
~ leads 68, 69 pass through the openings 65, 67 respectively
. .
and are sccured such as by welding to the center of each
-21-
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1060540 8894-1
' of the collectors 17, 18 which constitute the positive
and negstlve terminal connections for the cell. A third
terminal lead 70 passes through the opening 66 in the
cover 63 and is secured to the center of the current
collecto; 19 constituting a third terminal connection for
..
the auxiliary electrode 15. In the final assembly of
the cell~ both the cover 63 and the bottom wall 62 are
heated,and pressure is applied thereto in order to assure
thst the thin layers 58-60 of adhesive sealant tightly
bond the film to each of the current collectors.
The embodiment of the invention illustrated in
Figs. 5,and 5A incorporates still another type of
constructlon for the auxiliary electrode. The electrode
assembly is again basically the same as that illustrated in
Figs. ]A and 2A except that the current collector 17 is
replaced-by a collector 71 which is the same size as the
positive electrode plate 10 and the negative electrode plates
Il, 12. The arrangement of the auxiliary electrode is how-
ever dlfferent in this cons'truction. As shown in Fig. 5A,
, .
the auxiliary electrode 72 is- positloned along side of
, the electrode assembly. As in the previous flat cells,
a thin layer 73, 74 of the adhesive sealant is applied
to the outer surface of each of the collectors 71, 18,
excep~ for a small area which is left exposed at the
~'
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106~S40 8894-1
.
center thereof. The pair of negative electrode plates 11,
12 are interconnected by an insulated wire lead 75 while
the positive electrode plate 10 is connected to the ',
collector 71 via the insulated wire lead 76. All of the
cell elements are enclosed within a sealed envelope 77
1, which is ~ade from a tube of a heat shrinkable plastic
,,, film material. The tube is heat shrunk down over the
electrode assembly in the same manner as described for
' the cell illustrated in Figs. 1 and lA forming both side
; 10 walls 78 and end walls 79, 80. Both end walls 79, 80
I' overlap the current collectors 71, 18 but leave openings ,,
81, 82, respectively, corresponding with the exposed
center o each collector. After the tube has been heat
shrunk around the electrode assembly, heat and pressurè
are applied to the end walls 79, 80, tightly sealing the
plastic f1m to the current collectors 71, 18 through
the layers 73, 74 of adhesive sealant. Terminal leads 83,
84 pass through the openings 81, 82 and are secured as by
weLding to the exposed center of the c,ollectors 71, 18
which constitute the positive and negative terminal
connections for the cell.
Auxiliary electrode 72 is separated from the
electrode assembly by a porous separator 85 containing
the alkaline electrolyte and is secured such as by welding
to a current collector 86 which is disposed adjacent to
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~06~D540 8894
the side wall 78 formed by the plastic tube. This side
wall 78 has an opening 87 which exposes the center of the
collector 86. Collector 86 is also tightly bonded to the
plastic film forming the side wall 78 by a thin layer 88
of the adhesive sealant. A third terminal lead 89 is
then attached as by welding to the center of the collector
86.
In cells utilizing the rechargeable nickel-
cadmium electrode system, gas generation and the consequent
build up of substantia1 gas pressure inside the cell can
occur particularly if the cell is placed on overcharge
for long periods of time. On overcharge, oxygen gas
initis11y may be liberated at the positive electrode at
a faster rate than it can be recombined at the negative
electrode leading to a build up of high Lnternal gas
pre~sure. Hydrogen gas can for example be evolvet when
the cell is subjected to deep discharge. The evoiution
of hydrogen gas further increases the gas pressure inside
the cell since it does not normally recombine within the
cell as does the oxygen.
In flat cells of this invention, the plastic
film envelope which is made, for example, of a poly-
propylene or vinyl film is flexible and fairly weak. The
cell can rupture, although without danger? if the internal
gas pressure is allowet to built up to any significant
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106054~ 8894-1 ~
level such as 160 psi, for example. In order to avoid
this problem, it has been found desirable to provide a
mechanis.n for preventing the generation of hydrogen at
the negative èlectrode while at the same time facilitating
the reca.obination of oxygen. Accordingly, in the preferred
embodiment of a rechargeable nickel-cadmlum flat cell,
the positive and negat~ve electrodes sre balanced electro-
chemically with re6pect to one another such that the
capaciey of the negative electrode is greater than that
of the positive electrode. Preferably, the capacLty of
tho negative electrode is at least one and one-half times
greater ~han that of the positive electrode and may be as
great as three times the positive capacity.
As a further deterrent against cell rupture due
to the build up of excessive internal gas pressure, flat
cells of ~he construction disclosed and claimed in our
above-referred to copending application may incorporate
an auxiliary oxygen or hytrogen sensing electrode together
with a thLrd terminal connection in accordance with the
invention. In the instance where the auxiliary electrode
is an oxygen sensing electrode, the auxiliary electrode
forms with the negative electrode a voltage differential
whose~vaIue will depend on the partial oxygen pressure
that is developed inside the cell. Conversely, in the
case where the auxiliary electrode is a hydrogen sensing
.
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:
:
. - 106~D540 8894-1
~ electrode, the auxiliary electrode forms with the positive
S~ electrode a voltage differential whose value will depend
on the partial hydrogen pressure inside the cell. Under
~ condit~ons where the cell evolves copious quantities of
¦ oxygen gas on overcharge, for instance, the partial
axygen pressure will rise inside the cell, the voltage
differentlal will change and this change can be utilized
as a signal for actuating a control device in the charge
circuit to cut off the charging current and thereby
I 10 prohibit the further build up of gas pressure inside the
I cell. Charging circuits utilizing such control devices
are of course well known to those skilled in the art.
In assemblin~ batteries of flat cells of the
con~truction tiscloset and claimed in our above-referred
to copending application, only one cell incorporates a
third terminal connection an~ an auxiliary oxygen or
.
hydrogen sensing electrode and this cell is utilized as
a so-called "control cell", it being understood that the
remaining battery cells will be maintained in approximately
~1 20 the same state of charge or discharge. Such batteries
may be assembled with the flat cells interconnected in
series, parallel or series parallel arrangement. In
assembling a series-connected battery, for instance, the
-~ flat cells may be stacked one on top of another with the
positive terminal connection of each cell making
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1060540 8894-1
electrical contact with-the negative terminal connection
of the next cell via the terminal leads. The control
cell may then be placed at either end of the battery stack
of cells in order to more readily facilitate the electrical
connection of the control cell into the charging circuit.
Ot~ler arrangements of the battery assembly and control
cell are of course possible as will readily occur to those
skilled in the art.
The auxiliary electrode used in the embodiments
of the flat cell iLlustrated in the drawing may be either
an oxygen or hydrogen sensing electrode. Preferably,
the auxillary electrode is positioned in close proximity
to the negative electrode in the instance where the
auxlliary electrode is an oxygen sensing electrode in
orter to more effectively sense oxygen gas that is
llberated at the negative electrode on overcharge.
Conversely, the auxillary electrode is positioned in close
proximity to the positive electrode in the instance where
the;~auxiliary electrode is used as a hydrogen sensing
electrode. When employed as an oxygen sensing electrode,
the auxiliary electrode may be composed of nickel as the
active material. The auxiliary electrode when used as a
hydrogen sensing electrode may contain a platinum group
metal as the active material, such as platinum, palladium
or rhodium.
.
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,
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10 60 540 8894-1
Flat cells utilizing a third terminal connection
in accordance with the invention may also incorporate an
auxiliary electrode which functions as a voltage reference
device. Such applications are particularly useful to the
researcher since the auxiliary electrode can be utilized
to study the behavior of either the positive or negative
electrode without the necessity of disassembling the cell
structure. In the embodiments of the flat cell illustrated
in the drawing, the auxiliary electrode can be readily
..
ut~lized as a voltage reference device. It is only
necessary to make the proper external electrical
connect.ions between the positive or negative and the third
tenminal connections of the cell. The auxiliary voltage
reference electrode in this instance may be composed of
a sintered nickel plaque impregnated with metal ~ydroxite
as the active material.
Commercial plastic films which may be util~zed
in forming the sealed énvelope in flat cells of the
inventlon include those made of the following materials:
vinyl polymérs and copolymers, polyvinylidene chloride,
polyethylene, polypropylene, nylon, polysulfone, polystyrene,
and fluorocarbon polymers. For use with the preferret
fatty polyamide adhesive, films made of polyethylene,
polypropylene, and vinyl polymers and copolymers are
preferred. Regular and shrink-type films are available
,
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`'`.'
1060540
8894-1
in these mate~ als. Desired film characteristics include
the following: low cost, flexibility, tear and puncture
resistance, chemical stability and resistance to alkaline
battery 1ectrolyte, hot-formability, low oxygen gas and
water vapor transmission rates, and of course strong
surface adherence with fatty polyamide or equivalent
adhesive. To reduce the gas and water vapor transmission
rate of the plastic film, it may be vacuum metallized or ~ ;~
otherwise given a surface metallic coating on one or
both sides providing of course the film is not made
electr cally conductive enough to put a parasitic current ;;
drain on the cell.
Although there are probably a number of organic
compounds which exhibit a non-wetting characteristic when
in contact with an alkaline electrolyte, the most
preferred adhesive sealant for use in the practice of the
invention are the fatty polyamides. Such fatty polyamide
sealants are disclosed in U.S. Patent No. 3,922,178 issued
to J. Winger on November 25, 1975 and assigned to the common
assignee hereof. As disclosed in this patent application,
fatty polyamides are produced by reacting a polybasic acid with ^
a polyfunctional amine. Generally, the fatty polyamides useful
in the practice of the invention are those having an amine
8894-1
11)6~S~ :
number of above about 9. The amine number is the number
of milligrsms of KOH equivalent to one gram of fstty
polyam~e and is determined by procedures well known in
the art. The fatty polyamide sealants can be mixed with
extenders and modifiers in order to modify the physical
properties of the fatty polyamide. In constructing flat -
cells in accord with the invention, the fatty polyamide
sealant can be applied as a hot-melt or from solution in
a solvent ~uch as an alcohol/aromatic hydrocarbon mixture.
Among the specific commercially available fatty polyamides
that are particularly useful in constructing flat cells
of the invention are those produced under the trademarks ;
of VERS.~LON snd GENBOMD, General Mills, Inc. and Swift's
Z-610, Swift and Company.
It has been found that during the assembly of
flat cells of the invention the preferred adhesive sealants,
i.e., fatty polyamides, do not readily wet some types of
plastic f~lm and accordingly a liquid tight seal between
the plastic film forming the sealed envelope and the metal
collectors is sometimes difficult to attain. This
difficulty may be overcome and the bond between the
plastic film and the collectors substantially improved
if the plastic film is first subjected to successive heat
and corona discharge trea~ments. The process for ~reating -~
the plastic film is disclosed and claimed in ~-
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.,~" ~'
.
106054~ 8894-1
V.S. Patent No. 3,914,521, issued on November 21, 1975.
In this process, the plastic film is heated to an elevated
temperature, about 120C in the case of a polypropylene
film, for a period of about one half minute, optionally
cooling tne plastic film to ambient temperature and then
subjectin~ the film to a high intensity corona discharge
in the area of the film where the collectors are to be
adhered.
Although the invention has been described herein
}0 with particular reference to rechsrgeable flat cell systems
such as the nickel-cadmium cell wherein an auxiliary
electrode is used as an oxygen or hydrogen sensing electrode,
it will be understood that the invention is not so limited
and i9 applicable as well to primary flat cells wherein
the auxiliary electrode is utilized as a voltage reference
electrode.
Primary flat alkaline cells which are believed
to be readily adaptable to the flat cell construction
herein described include the following: alkaline manganese
dioxide-zinc, silver oxide-zinc, and mercuric oxide-zinc.
Electrodes and other materials employed would
of course be those required by and compatible with the
cell system employed. For exemple, the collector plates -
used wlth the alkaline manganese dioxide-zinc system would
preferably be of copper or brass rather than steel.
. ! ~ . . ' . . , , , , ' ., '
' ' ' . . ' . . .. ' ~ . :
10~540 8894-1
Similarly the flat electrodes employed could be of the
well known pressed powder type on an open or expanded
conductive substrate rather thsn of the impregnated porous
sinter type.
~ It is obvious that the flat cell construction
of the invention need not be limited to the rectangular
format. Other shapes of cell such as square, circular,
elliptical, hexagonal, and various irregular figures could
be encased in plastic film and provided with a positive,
:
negative and a third terminal connection by the technique
herein disclosed.
-.
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-32- ~ ~
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