CA1060540A - Flat alkaline cell with positive and negative terminal connections and a third terminal connection for an auxiliary electrode - Google Patents

Abstract

FLAT ALKALINE CELL WITH POSITIVE AND NEGATIVE
TERMINAL CONNECTIONS AND A THIRD TERMINAL
CONNECTION FOR AN AUXILIARY ELECTRODE
Abstract of the Disclosure A flat alkaline cell is described wherein an electrode assembly including at least a pair of flat electrode elements of opposite polarity having a porous separator containing an alkaline electrolyte interposed therebetween and a current collector disposed adjacent to and in electrical connection with one of the pair of electrode elements at one end of the electrode assembly, are enclosed within a sealed, liquid impervious plastic film envelope having an opening in one wall thereof which exposes at least a portion of the current collector for making external electrical connection therewith, and wherein a layer of an adhesive sealant which is non-wettable by the alkaline electrolyte, tightly adheres and seals together the current collector and the wall of the envelope at least around the periphery of the opening. The exposed portion of the current collector constitutes a first terminal connection to which a wire lead or the like may be attached. A second and third terminal connection are provided in the sealed envelope, the former being in electrical connection with the other of the pair of electrode elements of opposite polarity and the latter being in electrical connection with an auxiliary electrode.

Description

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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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1l~6VS4~) 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 - - " , ~ . . ,. .; ~ , ., . ~ :
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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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 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 .
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refereuce device. It is possible for instance to electro-chemically couple the positive or negative electrode of a .~ .
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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 .

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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, 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 \:

~0~540 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 :

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 ;' - ."

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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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, 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.
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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 ' l~ -18-: ., - . . .... :
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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.

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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 .
', ' 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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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 `
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~ leads 68, 69 pass through the openings 65, 67 respectively . .
and are sccured such as by welding to the center of each .. ' , , , .

' 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 ~' ' .. . . . , ~ .

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 ~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 -. . ~. .: - , ::
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 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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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 , `'`.' 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 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 ~-.,~" ~' .

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.

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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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Claims (20)

WHAT IS CLAIMED IS:

1. A flat alkaline cell comprising an electrode assembly including at least a pair of flat electrode elements of opposite polarity, an auxiliary electrode and a porous separator containing an alkaline electrolyte interposed between and in contact with the pair of electrode elements, said electrode assembly being enclosed within a sealed, liquid impervious plastic film envelope having a first, second and third terminal connection, and at least one current collector, said first terminal connection com-prising a current collector disposed adjacent to one end of said electrode assembly in electrical connection with one of either said pair of electrode elements or said auxiliary electrode and having at least a portion thereof exposed through an opening in one wall of said envelope for making external electrical connection, and a layer of adhesive sealant which is non-wettable by the alkaline electrolyte, tightly adhering and sealing together said current collector and said wall of said plastic film envelope around at least the periphery of said opening, said second and third terminal connections being in electrical connection respectively with the other of said pair of electrode elements and the auxiliary electrode.

2. The flat alkaline cell as defined by claim 1, wherein at least one of said second and third terminal connections comprises a separate current collector disposed adjacent to one end of said electrode assembly in electrical connection with at least one of the other of said pair of electrode elements and said auxiliary electrode and having at least a portion thereof exposed through another opening in one wall of said envelope, and wherein a layer of adhesive sealant which is non-wettable by the alkaline electrolyte, tightly adheres and seals together said separate collector ant said wall of said plastic film envelope around at least the periphery of said other opening.

3. The flat alkaline cell as defined by claim 1, wherein at least one of said second and third terminal connections comprises a terminal lead affixed at one end to at least one of the other of said pair of electrode elements and said auxiliary electrode, said terminal lead extending outwardly through a liquid tight seam formed by sealing together portions of said plastic film envelope.

4. The flat alkaline cell as defined by claim 1, wherein said layer of adhesive sealant is applied to substantially one entire face of said current collector except for the part thereof which is exposed through said opening for making external electrical connection.

5. The flat alkaline cell as defined by claim 1, wherein said adhesive sealant comprises-a fatty polyamide.

6. The flat alkaline cell as defined by claim 1, wherein said sealed envelope is made of a plastic film material selected from the group consisting of polyvinyl chloride, polyvinyl chloride copolymers, polypropylene and polyethylene.

7. The flat alkaline cell as defined by claim 1, wherein said electrode assembly includes at least one flat electrode element containing nickel hydroxide as the positive active material and at least one flat electrode element containing cadmium oxide or cadmium hydroxide as the negative active material.

8. The flat alkaline cell as defined by claim 2, wherein said plastic film envelope is made from a heat shrinkable plastic film tube which is heat shrunk tightly around both ends of said electrode assembly and wherein said opening is located in one wall of said envelope formed by said heat shrunk tube over one end of said electrode assembly.

9. The flat alkaline cell as defined by claim 8, wherein a terminal lead is secured in electrical connection with the portion of said current collector constituting said first terminal connection which is exposed through said opening.

10. The flat alkaline cell as defined by claim 9, wherein a terminal lead is secured in electrical connec-tion with the portion of said separate current collector constituting said second terminal connection which is exposed through said other opening.

11. The flat alkaline cell as defined by claim 9, wherein said third terminal connection comprises a separate current collector disposed adjacent to one end of said electrode assembly in electrical connection with at least one of the other of said pair of electrode elements and said auxiliary electrode and having a portion thereof exposed through still another opening in one wall of said envelope, and wherein a layer of said adhesive sealant tightly adheres and seals together said separate collector and said wall of said envelope at least around the periphery of said other opening.

12. The flat alkaline cell as defined by claim 11, wherein a terminal lead is secured in electrical connection with the portion of said current collector constituting said third terminal connection which is exposed through said other opening.

13. The flat alkaline cell as defined by claim 10, wherein a terminal lead is secured in electrical connection with at least one of the other of said pair of electrode elements and said auxiliary electrode, and wherein said terminal lead extends outwardly through a liquid-tight seam formed by sealing together portions of said envelope.

14. The flat alkaline cell as defined by claim 13, wherein said terminal lead is coated with a layer of adhesive sealant at the point where said lead extends through said liquid-tight seam.

15. A flat alkaline cell comprising, in combination, an electrode assembly including at least a pair of flat electrode elements of opposite polarity and a porous separator containing an alkaline electrolyte inter-posed between and in contact with the pair of electrode elements, a pair of current collectors for said pair of flat electrode elements, one of which is disposed adjacent to one end of said electrode assembly and the other of which is disposed adjacent to the opposite end of said elec-trode assembly, each one of said pair of current collectors being in electrical connection with one of the pair of electrode elements of opposite polarity, an auxiliary electrode and a separate current collector positioned in electrical connection with said auxiliary electrode, wherein said electrode assembly is enclosed within a sealed, liquid impervious plastic film envelope having openings within the walls thereof which expose a part of each one of said current collectors for making external electrical connection, and a layer of adhesive sealant which is non-wettable by the alkaline electrolyte, tightly adhering and sealing together each one of said current collectors and the walls of said plastic film envelope.

16. The flat alkaline cell as defined by claim 15, wherein said plastic film envelope is made from a heat shrinkable plastic film tube which is heat shrunk tightly around the side walls of said electrode assembly with the open ends of said tube tightly overlapping each one of said pair of current collectors forming said walls of said envelope and wherein said openings are formed in said overlapping ends of said tube forming said side walls.

17. The flat alkaline cell as defined by claim 16, wherein a terminal lead is secured in electrical connection with the exposed portion of each one of said current collectors.

18. The flat alkaline cell as defined by claim 15, wherein said plastic film envelope is made from a heat shrinkable plastic film tube having one closed end and one open end, said plastic film tube being heat shrunk tightly around both ends of said electrode assembly with the open end of said tube being heat sealed to form a liquid-tight seam and wherein said openings are located in the walls of said envelope formed by said heat shrunk tube over the ends of said electrode assembly.

19. The flat alkaline cell as defined by claim 15, wherein said auxiliary electrode is positioned along side of said electrode assembly.

20. The flat alkaline cell as defined by claim 19, wherein a portion of said separate current collector for said auxiliary electrode is exposed through an opening in the side wall of said plastic film envelope and wherein a layer of said adhesive sealant tightly adheres and seals together said side wall of said envelope and said separate collector at least around the periphery of said opening.