CA1079799A

CA1079799A - Ultraminiature high energy density cell

Info

Publication number: CA1079799A
Application number: CA272,514A
Authority: CA
Inventors: Arabinda N. Dey
Original assignee: PR Mallory and Co Inc
Current assignee: Duracell Inc USA
Priority date: 1976-03-08
Filing date: 1977-02-23
Publication date: 1980-06-17
Also published as: US4091188A; AU2285877A; AU506613B2; FR2344136B1; DK99677A; DE2709645A1; SE7702526L; FR2344136A1; CH616529A5; NL7702414A; US4080489A; JPS52128522A; IT1072485B; GB1518484A; BE852192A

Abstract

ULTRAMINIATURE HIGH ENERGY DENSITY CELL

ABSTRACT

A high energy density hermetic ultraminiature cylindrical cell containing a combination, electrolyte fill port and current collector and a method of construction of the cell.

Description

~9~9 ULTRP~IN~,TUF~E H;l;GH ENERGY DEN~IT~ CELI~

This invention relates to an ultramlniature, -~
cylindrical, electric cell utilizing a lithium anode and a high energy density catho~e material to obtain the benefits Qf the high energv content of the elements o~ ~ cell utilizing that construction.
In my U.S. P~tent No. 3,945,846 I describe an ultr~-miniature primary cell which is useful in many ~pplications where space limita~Qns ~ake such an ultramini~ture pxim~y cell especi~lly beneficial. The invention described therein utilizes the hi~h energy content that can be ~bt~ined with ~ lithium~
sul~ur dioxide system. That application waS directed to construction of miniature cells and to ~ method of m~kin~
such minl~ture cells havin~ dimensions on the order af ~ di~metex of 0.1 inch and a length of 0.75 inch. In the cells di~cl~sed therein~ adv~ntage iS taken of the high energy content th~t can be disposed, in this small volume, by using a lithium ~node and a sulfur dioxide depol~rizer electrolyte.
It has noW been ~ound that it is possible to ~xo~ide an ultraminiature prim~ry electxic cell With a volume up to 0,01 inch3 employing a lithium anode With a cathode ~atexial cap~ble o~ providing an even higher energy density than can be Qbtained by the uSe of sulfur dioxide~ and WhiCh ~ .,, . :-, :

", ~ ;' ~7~799 can be made within the same small dimensions defined by the cell described in my prior application.
In accordance with this invention there is provided a cell of these ultraminiature dimensions which provides the satis~actory kind of operation of a larger cell of the same elements, and which is made by a method that overcomes the usual difficulties oE working with materials especially volatile materials, in a space of such diminutive dimensions.
One of the maior problems iII manufacturing any closed electric cell is encountered in sealing the cell. In the construction of a miniature cell of the dimensions here involved, the sealing problem is especially difficult. Moreover, the presence of lithium as an element or component in the cell requires that all of the assembly wor~ be done in a dry atmos-phere since the presence of any water would introduce hazardous conditions becau~e oE the extreme activity of lithlum ln the presence O:e moisture.
The nece~tsity of assembling and sealing the cell in a dry atmosphere :Lntro-duces complications in the handling, the sealing and the filling operations.
The present invention provides both a design construction and a method of assembly and filling which assures the formation and mainten-ance of a hermetic seal, and which permits filling an already sealed cell container without destroying the seal. The resulting cell will maintain 379~

hermeticity and prevent leakage of volatile electrolyte thereby retaining the electrolyte needed for the proper performance of the cell.
In accordance with one embodiment of the in~ention, the ultra-minlature cell is constructed with a container casing formed from thln hollow tubing on whose inner surface a porous layer of an active cathode material such as silver chromate (Ag2CrO4) i6 Eormed. The cathode layer is formed by filling the container casing with dry powdered cathode material9 optionally containing suitable binder and conductive material, compacting the so disposed material, and drilling out the central core leaving behind the layer of cathode material on the inner wall of the container.
A small, elongated, cylindrical anode of lithium metal is formed around a linear, metallic hollow tube which is used as current collector, as support for the lithium, and as electrolyte Eill port. The anode thus formed ls enwrapped in one or more layers of thin, lnsulatin~, separator material, such as microporous polypropylene, and this assembly of anode and separator is then axially inserted into the axially disposed space within the surrounding cylindrical cathode layer.
Placement of the porous polyethylene separator inside the cathode can also be accomplished prior to anode insertion by winding the separator around a slotted mandrel, inserting the mandrel in the can, and then with-drawing the mandrel leaving the separator in place. The tension of the separator material is adequate to keep it in position on the cathode surface.

7g9 In order to facilitate manufacture of the ultraminiature cells of this invention, the outer extending end of the anode collector tube is anchored in a cell top, which may be an inert plastic septum or a glass to metal seal, prior to or during the preparation of the lithium anode. The plastic septum is essentially a ~rommet on the upper end of the anode tube;
and the septum is sized so that it will close the outer open end of a cell case as the entire anode assembly is axially inserted into operating position in the can. Alternatively the hollow tube or needle can be joined to a metal ring by an insulative glass member or ring. The metal ring can then in turn be hermetically sealed to the cell casing.
The hollow tube serves both as the filling conduit and as an anode collector pin which supports the body of the lithium anode material, and iB permanently disposed and sealed in the cell top. ~Eter the cell has been filled with the desired amount of electrolyte the input end of the hollow needle, outside of the cell, i9 closed off and welded at its outer end. This completes the seal for the cell.
In addition ~o silver chromate, other suitable active cathodic materials having high energy denslties include chromates of other metals such as copper, iton, cobalt, nickel, mercury, thallium, lead and bismuth, and their dichromates, basic chromates, vanadates, molybdates, permangan-ates, iodates, oxides, and carbonmonofluorides (CXF)n where x is equal to or less than ~ and n is a large but undetermined number of recurring CXF
groups.

~ ' . . ' ~, . .' .. '. ' ' ~' ' ~079~99 In a presently preferred embodiment, the cathodic m~terial is a silver chromate.
The present invention generally comprises a ~rimar~
ultra~iniature, electrical cell having a volume of less than about 0~01 cubic inch comprising an elongated, cylindrical metallic container open at one end thereof; a preformed, combination current collector, electrode, and containe:r sealing means .:
comprising a hollow metal needle having adhered thereto at its upper end means ~or sealing said open container end, and a layer of a first active solid electrode material adhered to s~id needle below said material for sealing; said container haviny an active solid electrode m~terial layer on its inner surface;
said electrode material providing a voltage diE.~erence there-between; said needle being closed at its uppex end and dis~osed axially within said container; an org~nic solvent disposed in said container; an alkali metal electrolyte salt dissolved in said organic solvent; and separating means between said ~irst and :.
second electrode ~ctive materials.
The present inYention further encompasseS a method of constructing an ultraminiature electrical cell comprising the steps of adhering a first solid active electrode material as a layer on the lower end of a hollow metal needle; adhering a second solid active electrode material as a ~yeron the inner : .
surface of an elongated cylindrical metallic container, .said .-.. .
container having a volume of less than about 0 01 cubic inch ~nd bein~ open at one end thexeof ~ith the inner surface of said second electrode material layer forming a cavity ha~ng a ~ -diameter gXeater than that of said needle With said layer of first ~
solid active electrode material thereon; securing me~ns for ~.
sealing the open end of said container on the upper end of said needle; thereafter axially placing a separator and said needler .1, .
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1~7g799 ~ lower end ~irst~ within said cavit~ With said ~ealing me~ns in p~sition to se~l said container; introducing an organic electrolyte solution through said needle; and closin~.the end of s~id needle above said sealing material. '~
- The construction o~ the cell, and the meth~d ~ ~Qxming~
sealin~ ~nd filling the:cell ~re expl~ined in more detail in the ~.
~ winc~ specif~cati4n and are illustrated in the'accomp~nying .' drawing~ in which;
Figuxe 1 is a vertical sectional view of a.cell showinc~
the genexal dis~osition of the components and elements~ and a c~nstruction for ~illincJ the cell in which the cell toP is a ~
: pl~stic septum; -. Figure 2 is a ve~tical cross ~ection o~ ~ sec'ond e~bodi~
ment o~ the cell utilizin~ ~ gl~ss to met~1 seal ~s the he~metic cl~su~e in pl~ce o~ the''pl~stic seal shown in ~iguxe 1. ~:
Figuxe 3 is a graph showing illustxatiYe cuxyeS of seYe . ~ the cells constxucted' in ~ccordance with the pxesent ~nyention ~nd includes ~ cQmparison with the Per~ormance ~ the cel'ls descxibed in m~ U S~ ~a~tent No 3,945,846 The general .features of this inYention invol~e the c~nst~ucti~n ~nd the''~illin~ opexation ~f ultxaminiatuxe c~lindxic~l cells having a siZe on the order o~ ~ di~meter of 0 1 inch ~nd ~ len~th of 0.75 inch. The ~dY~nt~ges of the cells of the invention Will be understood from the enexg~
output re~uixements which wexe set at t~enty-four (24~ milli~
watt-houx~ fox the cells de~cxibed in my U.S~ Patent No.
3,945,846 but which a~e'45 ~wHr for the pxesent application with identical ~iZe cell~ ~nd a Ag2CrO4 cathode ~ .' .

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~07~799 In both types of cells the energy output is determined at two to three volts, for currents of two to three milliamperes.
The two modifications of cell design are illustrated. The first modification 10 of the cell is shown in Figure 1, and consists of hollow tube 12 which acts as current collector for a concentric, axially disposed lithium anode 14. Tube 12 extends beyond both ends of the anode. A con-centric Ag2Cro4 cathode 16 is supported on the inner wall surface of an enclosing cylindrical, stainless steel can 18, with the lithium anode 14 and the cathode 16 being separated by an insulating separator 19. The hollow tube 12 extends through plastic septum 22 which is used to seal the can before the electrolyte is introduced. The electrolyte in liquid phase is introduced into the cell by in~ection, as with a syringe, through the hollow ~ube.
The tube 12 is of greater length than anode 14 and ~x~ends com-pletely therethrough so that a predetermined amount of electrolyte fluid can be introduced through the hollow tube 12 to exit from the bottom of said tube 12 into the operating space between the lithium anode 14 and the cathode 16. After the filling operation, the combination tube and anode collector is separated from the filling source, closed above septum 22, and sealed as shown in the operation described in my parent application.

. , . . ~ . . .. .

7~9 As shown in Figure 1, the upper, initially open end of the stain-less steel container can 18 is radially peened inwardly at region 24 to tightly compress the upper portion of septum 22 so as to impose a downward clamping pressure on the upper sur~ace of the septum rhis downward pres-sure is counteracted by an upward pressure maintained by the bead 23 formed in the can, upon which bead the septum is seated. A polytetra~luoroethylene disc 20 insulates the bottom of anode 14 from the metal can 18.
Figure 2 shows another embodiment of the invention in which a glass to metal seal is used to hermetically seal cell 110 in place of the plastic septum 22 of Figure 1. In this embodiment the upper end of the hollow tube 112 is sealed to a glass ring section 121 which is, in turn, surrounded by and sealed to metal outer ring 122. The glass to metal bonds between the glass ring 121 and both the metal tube 112 and the metal ring 122 are hermetic and do not allow any electrolyte seepage to the exterior of the cell.
The glass to metal seal is formed prior to the application of the lithium anode 114 to the tube 112. The application of the lithium to the tube is accomplished in the hereinabove described manner and the entire sub-assembly comprising tube 112 having rings 121, 122 and anode 114 secured thereto is then fitted into the cell cavity formed in the cathode, also as described above. The sub-assembly is welded to the can 118 at its upper periphery by suitable means such as electron beam or laser welding, with the weld being effected between the steel container can 118 and the metal ring 122. Filling oE the cell and sealing of the tube are accomplished by the same means used for the cell in Figure 1.
Although the cells in Figuras 1 and 2 depict the anode as the central electrode with the lithium being secured to the hollow tube and the cathode being disposed as an inner layer on the container the positions can be reversed according to the following procedure.
A special device was designed and fabricated for molding a layer of approximately 0.005" thick lithium on the inner wall of a can. It com-prises a turntable with a mandrel and appropriate guides to insert the mandrel into the can. The container is securely placed in a nest and a slug containing an exactly predetermined amount oP lithium inserted into the can. The cell contalner is then posltioned dlrectly under the mandrel which is lowered lnto the can. The mandrel is forced lnto the can wlth sufficient force to extrude the lithium against the inside walls of the can. The mandrel is then retracted, allowing the lithium to remain on the inner wall of the can.
A pressed cathode mix, consisting of powdered material, is nor-mally extremely friable. Two types of useful dies were designed and fabricated for molding the cathodes.

~L~7~799 A split die can be used for the cathode molding. In this case a layer of cathode mix is first placed on one-half of the die. The hollow tube, guided by guide pins, is then placed in the die and additional cathode mix is placed on top. The die is then closed and the cathode mix is compressed around the tube. Cathodes made in this manner are fragile but it is possible to improve the adherence of the cathode to the needle by preliminarily winding a fine titanium wire or a fine~mesh titanium screen around the needle and spot welding it at several places for elec-trical contact. Cathodes made in this manner are able to withstand handling during cell assembly.
The second, and presently preferred, type of cathode molding die for forming a central cathode member comprises a tubular cavity. The tube i9 positioned at the center oÉ the cavity by means of guides~-land the cathode mix is extruded into the die cavity by means of a plunger. Cathodes made with this type of die are more compact and amenable to handling.
Unlike the Li/S02 cell system of my copending appl~cation Serial No. 314,316, a cell system such as the above mentioned Li/Ag2CrO4 system has a solid depolarizer ~Ag2CrO4) and does not cause an increase in internal pressure as the temperature is raised. The lithium anode is identical in both systems, and the electrolyte consists of a suitable salt such as LiC104 dissolved in one or a mixture of organic solvents. the postulated cell reactions are:

.~'': ' ' ., ~ - . '. : ' , , . .. '. . . , ~Lli79799 4Li 4 Li + 4e (anodic) Ag2Cr4 + 4Li + 4e 2Ag ~ 2Li20`+ GrO2 (cathodic) g2 4 2Ag ~ 2Li20 ~ CrO2 On discharge, approximately 70% of the theoretical cathode capacity is realized above 2 volts at a current density of 1 ma/cm2. The ràte capa-bility of the Li/Ag2CrO4 system is lower than that of the Li/S02 system.
However, the theoretical volumetric energy density (55 whr/in3) of the Li/Ag2CrO4 system is at least twice that of the Li/S02 system (22 whr/in ).
Materials that are compatible with the various cell components include the following:
~ e~ aterials Anode (eg. lithium) current collector Steel, Ni, Cu, W. Ta, and Ti Cathode (eg. Ag2CrQ4) current collector Stainless Steel, Ta, Ti, Pt, and Au Insulating and solvent resistant Polyole~ins (eg. polypropylene, separator and sealant materials polyethylene) and polyhalogenated olefins (eg. polytetrafluoroethylene) Some sealant materials which are stable in the Li/S02 system may be unstable in the electrolyte of the Li/Ag2CrO4 system. Therefore, thè sealing charac-teristics of the Li/Ag2CrO4 cell can be improved by the use of a polyethy-lene or polypropylene grommet instead of the rubber top disclosed in my earlier application.
The present system does not develop an excess pressure at ambient temperature, neither does it generate any gas on storage as do alkaline systems having zinc electrodes. Both of these features facilitate the construction of truly hermetic cells. In one specific embodiment of the invention, Li/Ag2CrO4 cells are made in stainless steel cans with an O.D. of 0.0955", an I.D. of 0.0885" and a height of Q.75" ~Figure 1).
Cathodes are formed on the inner walls of the steel cans by filling the cans with dry depolarizer mix comprlsing 86% Ag2CrO4, 9% graphite and 5%
colloidal polytetrafluoroethylene by weight, and subse~uently drilling out a central core in each can, leaving behind a lyer of the depolarizer mix on the inner wall of each can. A polytetrafluoroethylene disc is forced to the bottom of the central core to electrically isolate the can bottoms. Lithium anodes are formed on stainless steel, hollow tube, current collectors cum electrolyte fill ports. In forming the anodes the needles are each forced through a polyethylene septum and the portion of the needles to be covered with llthium are placed In a Eorming tool between two layers of 0.01 inch thick lithium ribbons. These two layers are then pressed around each tube. Thereafter a layer of microporous polypropylene, as separator, is wrapped around each anode.
The assemblies are then transferred to a t'dry box" and inserted into the central core of the steel cans containing the depolarizer mix.
The cells are then crimped on a lathe with appropriate tools.
Organic electrolyte, comprising a lM LiC104 solution in a mix-ture of propylene carbonate and tatrahydrofuran in equal volume, is introduced into the cell through the . ' ' '' '' : ' ' ':

~!L079~

hollow tube-anode collector by means of a syringe. ~fter the electrol,~te filling, the needle terminal is sealed by tungsten inert gas (TIG) weldi,ng in the mannex descxibed in mv U,S. Patent No. 3,945,846 in which a welder with a tungsten cathode is positioned just above the hollo~ tube end to be sealed, a cJ~s is passed through said electrode, and, on tric~gering the welder, heated gas me~ts the tip of the tube to form a xound be~d o~ metal thus sealing the tube.
The cells formecl as described above are dischaxged ~t const~nt cuXrents o~ 0.5, l, 1.5 and 2,0 ma. at room tempeX~tuxe.
The open circuit vQltage o~ the cell i5 appxoximatel~ 3.4 volts. The aver~ge ope,rating voltage Varies Exom 2,4 volts to 2.7 volts depending upon the cu~xent. The cell volt,~ge re~ins rel~tively st~bl~ ~bove 2 volts ~nd dro,ps sh~ l,y ~t the end of di~ch~rge, In Fig. 3 typical disch~xge curves of the pxesent cel'ls are shown in comparison to ~,he best per~prming Li~S02 cells made in accoxdance With the proceduxe descxibed in m,y U,S. Patent No. 3~945~846. Discharge dat~ for the pXeSent cell$ are shown in Table I. ~ver~ge ener~y outputS of 59 ~Whr at 1 mar 5~ mWhr ~t 1,5 ma and 45 mWhr ~t 2.0 ma are'~chieved, ThiS corxesponds to ~ twofold improYement oyer the Li~so2 mini~ture cell~.
The volu~etric energy densityof the Li~A~2cro4 miniatuxe cell is found to be lOWHr/in3 at l ma dxaln~
The Li/~g2CrO~ system exhibited the ~ollo~ing '~
-:. .
~d~ant~ges over the Li'/SO:2 system~

(i) The s~stem is non-pressurized, The ~apor '~

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107~799 :-pressure of the organ$c solvents is below 1 atm. at temperatures oE
below 80C.
(ii) The above feature results in easier cell construction and greater reproducibility.
(iii)The packaged capcity per unit volume of the Li/Ag2CrO4 cells is greater than that of the Li/~02 cells.
(iv) The delivered energy of the miniature Li/Ag2CrO4 cells is on the order oE twice that of the Li/S02 cells.
It may be concluded that the Ll/Ag2CrO~ system is better suited for ultraminiature cell applications at current drains of 2 ma or less. ~ ;
While the above disclosure has described the invention with reference to lithium, it will be obvious that other anode materials, such as the active metals oE Groups IA and IIA, can also be used. In addition it will be recognlzed by those skllled in the art that many organic elec-trolyte solvents may be uaed. For example organic aolventa that may be used include tetrahydrofuran, propylene carbonate, dimethyl aulfite, di-methyl sulfoxide, N-nitrosodimethylamine, gamma-butyrolactone, dimethyl carbonate, methyl formate, butyl formate, dimethoxyethane, acetonitrile and N:N dimethyl formamide, and electrolyte aalts for such cells include light metal salts such aa perchlorates, te~rachloroaluminates, tetrafluo-boratea, halides, hexafluophosphates and hexafluoarsenatea. Additionally, though the described structure and method of construction re~er to primary cells they are equally applicable to aecondary cells.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PRO
PERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A primary ultraminiature, electrical cell having a volume of less than about 0.01 cubic inch comprising an elongated, cylindrical metallic container open at one end thereof; a preformed, combination current collector, electrode, and container sealing means comprising a hollow metal needle having adhered thereto at its upper end means for sealing said open container end, and a layer of a first active solid electrode material adhered to said needle below said material for sealing; said container having a solid active solid electrode material layer on its inner sur-face; said electrode material providing a voltage difference therebetween; said needle being closed at its upper end and disposed axially within said container; an organic solvent disposed in said container; an alkali metal electrolyte salt dissolved in said organic solvent; and separating means between said first and second electrode active materials.

2. A method of constructing an ultraminiature electrical cell comprising the steps of adhering a first solid active electrode material as a layer on the lower end of a hollow metal needle; adhering a second solid active electrode material as a layer on the inner surface of an elongated cylindrical metallic container, said container having a volume of less than about 0.01 cubic inch and being open at one end thereof with the inner surface of said second electrode material layer forming a cavity having a diameter greater than that of said needle with said layer of first solid active electrode material thereon; securing means for sealing the open end of said container on the upper end of said needle; thereafter axially placing a separator and said needle, lower end first, within said cavity with said sealing means in position to seal said container; introducing an organic electrolyte solution through said needle; and closing the end of said needle above said sealing material.

3. A method as in claim 2 wherein said first active electrode layer is lithium.

4. A method as in claim 3 wherein said second active material is a silver chromate.

5. A method as in claim 2 wherein said second active electrode layer comprises an electrode active material formed by filling said container with said electrode active material in powder form; compacting the so disposed powder;
and drilling out the central core of said powder, thereby simultaneously forming said cavity and said layer on the inner surface of said can.

6. A method as in claim 2 wherein said first active electrode material comprises an electrode active material placed on said needle by the steps of: placing a first por-tion of said first electrode active material as a powder in one half of split die; placing said needle upon the so disposed powder;placing a second portion of said powder upon said needle; and closing said die to compress said powder around said needle.

7. A method as in claim 6 wherein the outer surface of said needle is covered, prior to application of said powder, by a fine mesh screen of a metal which is chemically com-patible with said electrode powder.

8. A method as in claim 2 wherein said second active electrode material is lithium and said layer of lithium is formed by inserting a slug of lithium into said container and forcing a mandrel into said container with sufficient force to extrude the lithium as a layer along said inner surface.

9. A method as in claim 2 wherein said sealing means comprises concentric rings of glass and metal, with said glass ring interposed between and sealed to said needle and said metal ring, and said metal ring, which is sized to fit the open end of said container, is welded to the container at its open end thereby providing an hermetic closure.

10. A method as in claim 2 wherein said organic electrolyte solution is introduced into said container can by a syringe inserted into said needle.