The present invention resides in an improved secondary rechargeable battery which comprises a sub¬ stantially hermetically sealed housing having at least two and preferably three or _. more cells electrically connected in series- Each cell contains at Least a pair of electrodes constructed of a carbonaceous material. At least one of said pair of electrodes is a commonly shared, unitary electrode, which extends between a pair of adjacent cells. Said commonly shared electrode is of a size sufficient to form an elec¬ trically effective electrode in each of said adjacent cell, thus forming a positive electrode in one cell and a negative electrode in the adjacent cell.

The first and the last cells in said cell series are generally defined as terminal cells and con¬ tain a terminal or end electrode which is provided with an electric current collector to provide a path for the charge of an electric current into and the dis¬ charge of an electric current out of said cell series.

The electric current collector is beneficially connected to the electrode, preferably at an edge portion of the electrode. An electric current conductor is preferably connected to the current collector and preferably extends to the outside of the housing to carry current to and from the cells of the battery.

Each cell is preferably provided with a foraminous, hydraulically permeable, separator posi- tioned between each pair of electrodes of opposite polarity in a cell to maintain the electrodes in a spaced apart and electrically insulated relation¬ ship.

Each cell consists of a compartment in the housing which is separated from an adjacent cell by a hydraulically impermeable interior wall or partition. The commonly shared, unitary electrodes extend through the cell walls to electrically connect each cell to an adjacent cell. Each cell is provided with an elec- trolyte comprising an ionizable salt in a non-aqueous liquid.

An intermediate portion of each commonly shared electrode is potted or embedded into a potting section provided in the cell wall or within the material of the cell wall itself to embed and hydraulically seal the intermediate electrode portion within the cell wall and to seal each cell from its adjacent cell to prevent wicking or leakage of electrolyte from one cell to the adjacent cell through the commonly shared electrode.

A primary advantage of the present invention resides in the feature of a unitary electrode which can be made of a single material, i.e. a carbonaceous material. The unitary electrode is a commonly shared electrode and consists of a current carrier per se which extends from one cell or compartment to an adjacent cell or compartment of a cell series. It is only the terminal or end electrodes of the battery of the invention that are provided with a current collector.

A further advantage of the battery of the invention resides in the simplicity of its design and the substantial space saving that is realized by the elimination of current collectors from the commonly shared electrodes.

Another advantage resides in the substantial cost advantages and savings that are realized by the elimination of a current collector from the commonly shared, intermediate, electrodes. Such current col- lector usually requires the application of a metal coating, e.g. copper, to at least one of the peripheral edges of the electrode, and internal or external con¬ nections to connect the electrodes in a series con¬ figuration. External connections, moreover, require the passage of the current conductors through a housing wall, adding to the overall problem of maintaining the battery housing hermetically sealed against the ingress of gases and vapors, particularly water vapor.

An additional advantage of the present cell design is that the cell is reversible, that is to say, the cell may be connected in a positive-negative manner

and on recharge can be reversed to a negative-positive manner without damage to the electrodes.

Accordingly, it is an object of the invention to provide for a secondary rechargeable battery compris- ing a substantially hermetically sealed housing, at least one hydraulically impermeable partition dividing the housing into at least a pair of compartments, each compartment forming a cell containing an electrolyte of an ionizable salt in a non-aqueous liquid and at least a pair of spaced electrodes electrically insulated from one another and made of an electrically conductive carbonaceous material, the first and the last cells of said battery containing a terminal electrode having a current collector associated therewith, characterized by at least one commonly shared, unitary electrode extending from one cell into an adjacent cell and having an intermediate portion which is hydraulica.lly sealed in the cell partition to prevent the transfer of electrolyte from one cell to an adjacent cell while permitting the flow of current through said commonly shared electrode between said cells.

The single Figure illustrates, in cross-section, a preferred embodiment of a secondary battery constructed in accordance with the invention.

With particular reference to the drawing, there is provided a housing 10 which is constructed of a material to render it substantially hermetically impervious to the passage of gas therethrough, includ¬ ing particularily, water vapor. The housing 10 is provided with two internal cell walls or partitions 13a and 13b, defining within said housing a series of three

cells 14a, 14b and 14c, respectively. The cell walls are preferably constructed of the same hydraulically impervious material as the housing walls. However, the cell walls 13a and 13b may be constructed of a material which is of a somewhat lesser gas or vapor impervious nature, requiring only that ions from the electrolyte cannot pass therethrough. Each cell 14a-14c contains a pair of electrodes, made from a carbonaceous material having properties which are defined hereinafter and which are also more fully defined in copending pub¬ lished French Patent Publication No. 2,556,138 entitled Energy Storage Device, published June 7, 1985, inventors, Francis P. McCullough, Jr., et al. Cell 14a is an end or terminal cell and contains a terminal elec- trode 15a and a commonly shared electrode 16a which extends from cell 14a through the housing wall 13a into the adjacent intermediate cell 14b. Cell 14b also con¬ tains a second commonly shared electrode 16b which - extends from cell 14b through the housing wall 13b into an end or terminal cell 14c. The terminal cell also contains a terminal electrode 15b.

The commonly shared, unitary electrodes 16a and 16b are of a dimension such that they extend from one cell into an adjacent cell and such that the por- tions of each electrode extending into the adjacent cells having an opposite polarity. The electrodes 16a and 16b are shown as a unitary structural member or component which is bent or folded at an intermediate portion to form a pair of leg portions each of which extends into an adjacent cell. Further, while each commonly shared electrode is shown as a single, unitary sheet, film or cloth, one or both of the electrode leg

portions may be further folded or pleated in a serpen¬ tine manner (with their hydraulically permeable separator extending between the pleats) to increase the total active electrode area in each cell. The electrodes in each cell are preferably separated from electrical contact with each other by a foraminous and hydraul¬ ically permeable separator member 17 which is capable of passing the electrolyte and ions. Various forms of separator materials may be employed, e.g. a fiberglass mat, polypropylene scrim or webbing as well as membrane such as ion exclusion membranes, and the like. It will be understood that the electrodes can be electrically insulated from each other by merely spacing the elec¬ trodes apart to provide a distance between the elec- trodes sufficient to prevent electrical contact between the electrodes.

An electrolyte 18 is provided in each cell. The preferred electrolyte is a mixture of an ionizable salt dissolved in a non-aqueous, electrically non-conduc- tive liquid or paste. Alternatively, the electrolyte per se may be ionizable to some extent as well as any non-conducting solid through which ions will be trans¬ ported under the influence of an electrical charge or discharge, as more fully explained in the copending application referred to hereinabove.

Preferably, the electrically conductive carbonaceous material of the electrode should have the following physical properties:

(1) A Young's modulus of greater than 1,000,000 psi (6.9 GPa), preferably from 10,000,000 psi (69 GPa) to 55,000,000 psi (380 GPa), more preferably from 20,000,000 to 45,000,000 psi (138 GPa to 311 GPa).

(2) An aspect ratio of greater than 100:1. The aspect ratio is defined herein as the length to diameter (1/d) ratio of a fibrous or filament strand of the carbonaceous material, or as the length to depth ratio when the carbonaceous material is formed as a planar sheet.

(3) The structural and mechanical integrity of the carbonaceous material in whatever fabricated form it may be (sheet or film, woven or knitted cloth or non-woven sheeting made from continuous filament or staple fibers) must be such that it does' not require the presence of a support such as a pressure plate (face films or mesh) to maintain the carbonaceous material in the desired sheet or plate like shapes throughout at least 100 charge/discharge cycles.

(4) A surface area with respect to fibrous materials of at least 0.1 m ² /g but less than one asso¬ ciates with activated absorptive carbon. A surface area of less than 50 m ² /g, preferably less than 10 m ² /g, and more preferably less than a 5 m ² /g is suit¬ ably employed for the carbonatious material of the invention.

(5) Sufficent integrity of the form of the carbonaceous material to enable the carbonaceous mater- ial to retain its plate or sheet-like shape when of a size greater than 1 in ² (6.45 cm ² ) to greater than 144 in ² (930 cm ² ) without any support, i.e. other than a metallic current collector frame along the peripheral edge portions of the terminal electrode.

Performance Criteria

(6) The carbonaceous material of an electrode should be capable of sustaining more than 100 electrical charge and discharge cycles without any appreciable damage due to flaking of the carbonaceous material. Preferably, no appreciable damage should occur after more than 500 electrical charge and discharge cycles, at a discharge capacity of greater than 150 coulombs per gram of carbonaceous material.

_ (7) The coulometric (coulombic) efficiency of the carbonaceous material of the electrode should be greater than 70 percent, preferably greater than 80 percent and most preferably greater than about 90 percent.

(8) The carbonaceous material of the elec¬ trode should be capable of sustaining deep electrical discharges of greater than 70 percent of its electrical charg capacity for at least 100 cycles of electrical charge and discharge, and preferably greater than 80 percent for more than 500 electrical charge and dis¬ charge cycles.

(9) The battery in which the electrodes of this invention are employed should be substantially free of water to the extent of less than 100 ppm. Preferably, the water content should be less than 20 ppm and most preferably less than 10 ppm. The battery of the invention is capable of operating with a water content of up to 300 ppm but will have a somewhat reduced cycle life. Further, it is to be understood that should the water content level become onorous, the battery may be disassembled, dried and reassembled in

such dry state without substantial damage to its con¬ tinued operability.

Accordingly, the carbonaceous material of an electrode having the physical properties hereinbefore described preferably should be capable of sustaining electrical discharge and recharge of more than 100 cycles at a discharge capacity of greater than 150 coulombs per gram of carbonaceous material in an electrode and at a coulometric efficiency of greater than 70 percent without any substantial irreversible change in dimensions (dimensional change of less than about 5 percent) .

Usually, the carbonaceous material will be obtained by heating a precursor material to a temper- ature above 850°C until electrically conductive.

Carbonaceous precursor starting materials capable of forming the electrically conductive carbonaceous material portion of the electrode may be formed from pitch (petroleum or coal tar), polyacetylene, poly- acrylonitrile, polyphenylene, SARAN (Trade Mark) ^' , and the like. The carbonaceous precursor starting material should have some degree of skeletal orientation, i.e., the material should either have substantial concentra¬ tions of oriented benzenoid structural moieties or moieties which are capable of conversion, on heating, to benzenoid or equivalent skeletal orientation at or near the surface thereof (because of the skeletal orientation of the starting material) .

Exemplary of preferred carbonaceous precursor materials which exhibit such skeletal orientation on heating are assemblies of multi- or monofilament strands

or fibers prepared from petroleum pitch or polyacrylo- nitrile. Such multi- or monofilament strands or fibers are readily converted into threads or yarns which can then be fabricated into a sheet-like cloth or fabric. One technique for producing suitable monofilament fibers is disclosed in U.S. Patent No. 4,005,183 where the fibers are made into a yarn which is then woven into a cloth. The cloth is then subjected to a tem¬ perature, usually above 1000°C, sufficient to carbonize the cloth to make the carbonaceous material electri¬ cally conductive and so as to provide the material with the physical property characteristics hereinbefore described under paragraphs (1) through (6). Such a cloth, in conjunction with an electron collector, is ^' particularly suitable for use as the terminal elec¬ trodes in the bipolar battery of the present invention. The bipolar commonly shared electrode of the invention may require a sizing or coating on the edges thereof to prevent fraying during assembly. Suitable plastic coating materials are, for example, polyethylene or Derakane brand curable vinyl ester epoxy resin compo¬ sitions.

Advantageously, the carbonaceous precursor material is in the form of a continuous filament or fiber, thread(s) constituted of continuous filament(s) or non-continuous fiber tow (yarn) which can be made into assemblies such as woven, non-woven, or knitted assemblies, or the staple fibers per se layered to form a cloth, paper-like or felt-like planar member. However, acceptable results are obtained when yarns made from short fibers (about 1 to 10 cm long) are woven or knitted into a cloth-like product (provided such short

fibers still have, when heat treated, the required physical properties hereinbefore mentioned under (1) through (6)). It is, of course, to be understood that while it is advantageous to form the precursor material, preferably in a stabilized state (such as is obtained by oxidation), into the desired form (knit, woven or felt) prior to carbonization, such construction may be done after carbonization if the Young's modulus is below about 55,000,000 psi (380 GPa) and preferably below about 39,000,000 psi (269 GPa) for machine fabri¬ cation. It is to be understood that the carbonaceous material may also be formed from a film precursor.

It is also contemplated that the final form or shape of the carbonaceous material may be produced, e.g. a roll of a tubular or spirally shaped or pleated sheet material, and the material is subsequently car¬ bonized. This technique is particularily advantageous if a woven, vis-a-vis a knitted fabric is to be employed, the latter being more flexible after carbonization than a woven material and is thus readily capable of being rolled up (as in a spiral jelly roll).

The degree of carbonization and/or graphiti- zation does not appear to be a controlling factor in the performance of the material as an electrode except that it must be enough to render the material suffi¬ ciently electrically conductive. The degree of car¬ bonization and/or graphitization should also be suf¬ ficient to provide the aforementioned physical and mechanical properties under the designated use con- ditions. Carbonaceous materials having about 90 percent carbonization are referred to in the literature

as partially carbonized, while carbonaceous materials having from 91 to 98 percent carbonization are referred to in the literature as a carbonized material. Mater¬ ials having a carbonization of greater than 98 percent are referred to as graphitized. It has surprisingly been found that carbonaceous materials having a degree of carbonization of from 90 to 99 percent have failed as an electrode material unless the carbonaceous mater¬ ial has the required dimensional stability during electrical charge and discharge cycling. For example, RPG graphite and GRAFOIL, while having the requisite degree of carbonization, electrical conductivity and surface area, do not have the required physical proper¬ ties of Young's Modulus and aspect ratio and thus have failed.

In accordance with the invention, a recharg- able and polarity reversible battery can be prepared by aligning at least two pairs of electrodes, each elec¬ trode made from the aforedescribed carbonaceous material, the intermediate electrodes being bipolar as hereinbefore described and shared with adjacent cells, and the terminal electrodes being associated with an electron collector (which is electrically conductive), in a housing. The housing has an interior surface which is substantially impervious to moisture or gases. The electrodes are immersed in an electrolyte comprising an ionizable salt in a non-aqueous liquid which is contained in said housing. The liquid itself must be capable of forming, or contains dissolved therein, at least one ionizable metal salt. The ter¬ minal electrodes are provided with an electron col¬ lector which is preferably insulated against contact with the electrolyte.

In the construction of the battery of the invention, separators such as, for example, separators made of fiberglass, polymeric materials, or composites of polymeric materials, may be employed to separate the positive and negative electrodes from each other. Pre¬ ferably, a non-woven porous polypropylene sheet or a functionalized membrane is employed as the separator since it has the desired degree of porosity and yet has a sufficient tortuous flow path to prevent carbonaceous fibers from penetrating through it, thus preventing electrical shorting. The porous separators also bene¬ ficially act as stiffeners or supports for the elec¬ trodes during assembly.

Energy storage devices which are contained in fluid-tight housings are generally known in the art. Such housings may be suitably employed in the present invention as long as the housing material is electri¬ cally non-conductive or at least insulated from contact with one electrode; and is impervious to gases and/or moisture (water or water vapor).

Materials which are compatible as a housing material include, for example, polyvinylchloride, poly¬ ethylene, polypropylene, polytrifluoroethylene and related perfluorinated polymers, instant set polymer (ISP), a rapidly solidifying reactive urethane mixture, the aramids, a metal sheet clad or coated with a non-conductive polymeric resinous material such as an epoxy e.g. DER* 331 or with DERAKANE* a curable vinyl ester epoxy resin, ZETABON* a plastic-metal-

^Trademark of The Dow Chemical Company

-plastic laminate and/or glass or a metal oxide, fluoride, or the like. Housing materials found not to be suitable in the preferred propylene carbonate electrolyte system include acrylics, polycarbonate and nylon. Acrylics and polycarbonates both craze and become extremely brittle, while nylon (except for the aramids) is chemically reactive.

In addition to being chemically compatible, a housing material must also offer an absolute barrier against the transmission of water vapor from the external environment of the housing (A transmission of less than 0.02 gram of H ₂ 0/yr/m ² is preferred). No presently known thermoplastic material alone offers this absolute barrier against moisture at a thickness which would be useful for a battery housing. At present only metals, such as for example, aluminum or mild steel, offer an absolute barrier against moisture at foil thicknesses. Aluminum foil having a thickness of greater than 0.0015 in. (0.038 mm) has been shown to be impervious to water vapor transmission. It has also been shown that when laminated to other materials, aluminum foil as thin as 0.00035 in. (0.009 mm) can provide adequate protection against water vapor trans¬ mission. Suitable housings made of a metal-plastic laminate, CED-epoxy-coated metal (cathodic electro deposited), or metal with an internal liner of a plastic material or glass presently satisfies the requirements for both chemical compatability and moisture barrier ability. Most of the cells and batteries built to date have been tested in either a dry box having a H ₂ 0 level of >5 ppm, a glass cell or a double walled housing with the space between the walls filled with an activated molecular sieve, e.g. 5A zeolite.

The electrolyte preferably consists of an electrically non-conductive, chemically stable, non-aqueous solvent for an ionizable salt or salts wherein the ionizable salt is dissolved in the solvent. One can employ as the solvent those compounds that are generally known in the art such as, for example, com¬ pounds having oxygen, sulfur, and/or nitrogen atoms bound to carbon atoms in an electrochemically non- -reactive state. Preferably, one can employ nitriles such as acetonitrile; amides such as dimethyl form- amide; ethers, such as tetrahydrofuran; sulfur com¬ pounds, such as dimethyl sulfite; and other compounds such as propylene carbonate. It is, of course, to be understood that the solvent itself may be ionizable under conditions of use sufficient to provide the necessary ions in the solvent. Thus, the ionizable salt must be at least partially soluble and ionizable either when it is dissolved and goes into solution into the solvent or upon liquefaction. While it is to be understood that slightly soluble salts are operable, it will be recognized that the rate of electrical charging and discharging may be adversely affected by the low concentration of such salts in solution.

Ionizable salts which may be employed in the practice of the invention are those taught in the prior art and include salts of the more active metals, such as, for example, the alkali metal salts, preferably lithium, sodium or potassium, or mixtures thereof con¬ taining stable anions such as perchlorate (C10 ₄ =), tetrafluoroborate (BF ₄ =), hexafluoroarsenate (AsF ₆ =), hexafluoroanti onate (SbF ₆ =) or hexafluorophosphate (PF ₆ =).

The electrolyte (solvent and salt) must be substantially water-free, that is, it should contain •less than 100 ppm, preferably less than 20 ppm, most preferably less than 10 ppm of water. Of course, the electrolyte can be made up having more than the desired amount of water and dried as for example, over activated zeolite 5A molecular sieves. Such agents may also be combined into the finished battery to ensure that the low level water requirement is maintained. The elec- trolyte should also be such as to permit ions (anions and cations) of the ionizable salt to move freely through the solvent as the electrical potential of charge and discharge move the ions to and from their respective poles (electrodes).

The terminal electrodes, when constructed as a cloth or sheet, include an electron collector conduc- tively associated with the carbonaceous fibers or sheet. The current collector, when made of a metal, is further protected by a material to insulate the collector and to substantially protect the electron collector from contact with the electrolyte. The protective material must, of course, be unaffected by the elec¬ trolyte. As more particularly shown in the drawing, the peripheral edge(s) of the terminal electrode(s) (15a) and (15b) is plated with a continuous bead (20) a metal or metal alloy such as copper, silver, or the like. A wire mesh (22) is folded over the initially plated peripheral bead and the plating continued until the wire mesh, preferably a copper wire mesh, is co - pletely embedded within the bead. A current conductor (24) is connected to the edge of the terminal electrode and embedded within the metal bead. The plated edge of

the electrode is then provided with an insulating coating (26) of a synthetic resinous material such as, for example, a Derakane resin. The conductor 24 extends through the housing wall and is provided with an electric current conducting terminal (28) externally of the housing.

The current collector intimately contacts the carbonaceous material of the electrode, preferably at least along one peripheral edge portion and more pre- ferably on all peripheral edge portions thereof, i.e when the carbonaceous electrode is in the form of an assembly such as a planar sheet, film, or felt. It is also envisioned that the electrode may be constructed in other shapes such as in the form of a cylindrical or tubular bundle of fibers, threads or yarns in which the ends of the bundle are provided with a current col¬ lector. It is also apparent that an electrode in the form of a planar, sheet-like body, e.g. a woven, or knitted cloth, can be rolled up with a porous separator between the layers of the planar, sheet-like body and with the opposed edges of the rolled up body connected to a current collector. While copper has been used as a current collector, any electro-conductive metal or alloy may be employed, such as, for example, silver, gold, platinum, cobalt, palladium, and alloys thereof. Likewise, while electrodeposition has been used in bonding a metal or metal alloy to the carbonaceous material, other coating techniques (including melt applications) or electroless deposition methods may be employed as long as the edges or ends of the elec¬ trode, including a majority of the fiber ends at the edges of the carbonaceous material are wetted by the metal to an extent sufficient to provide a low-resistant electrical contact and current path.

Collectors made from a non-noble metal, such as copper, nickel, silver or alloys of such metals, must be protected from the electrolyte and therefore are preferably coated with a synthetic resinous mater- ial or an oxide, fluoride or the like which will not be attacked by the electrolyte or undergo any significant degradation at the operating conditions of a cell.

In the present cell it is also possible to use inductive coils imbedded in the housing material connected to the end electrodes. Inductive coils may be used to charge/discharge the battery when no external connections are desired.

Example 1

In a representative example a 2.5 cm by 12.5 cm strip of Panex cloth (PWB-6) was sealed within a "Cellgard " pocket. A pair of 5.cm square panels of Panex cloth were copper plated along each edge, a copper mesh screen was pressed over the copper, plate on all four sides of each panel and the resulting struc- ture again electroplated to embed the mesh in a copper plate coating. The copper plating was then coated with a Derakane resin, an epoxy-vinyl ester resin, and cured. Each of the panels (electrodes) was placed in a polyethylene bag, the 12.5 cm strip was doubled over and an end placed in each bag. A 15 percent solution of LiC10 ₄ - in propylene carbonate was added to each bag. The assembly was done in a dry box with all components having been dried prior to assembly. The entire assem¬ bly was left in the dry box over night, then placed on electrical charge at a voltage of 10.5 volts. The charge was 28.246 coulombs. Upon discharge the voltage

was discharged down to 0.10 V through a 100 ohm resis¬ tance during a 45 minute period yielding 15 coulombs. This represents a 53 percent efficiency.

The low coulometric efficiency was attributed to contact of the electrolyte of one cell with the electrolyte of another cell due to wickiήg of the electrolyte over the commonly shared electrode extend¬ ing between the two cells. This was prevented in later batteries by potting the cloth connection of the bipolar electrodes extending into adjacent cells with a bead of Derakane. Additional cycles were run. The results are set forth below:

TABLE

Cycle Couloumbs Load eff.

% Charge Discharge (ohms) charge

1 28 1000

15 100 53%

2 51 500

35 500 70%

3 46 500

33 500 73% 4 113 1000 65 1000 57% 5 80 500

35 400 ^' 45

6 76 150 33 150 57%

Example 2

A bipolar cell was constructed as shown in Figure 1 except four compartments were employed.

The end electrodes having a copper current collector protected by a Derakane resin coating were 10 cm by 11.25 cm active/carbonaceous material in contact with the electrolyte. The three center elec- trodes were made from 10 cm by 25 cm pieces of graphite cloth which were enclosed in glass cloth bags. Each cloth bag was folded over to provide leg portions to be insertable into adjacent cells. Each pair of elec¬ trodes was inserted into a polyethylene bag to hold an electrolyte. The thickness of the entire assembly was 1.8 ^' cm.

The electrolyte consisting of 30 cm ³ of a 15 percent LiC10 ₄ in propylene carbonate solution was added into each bag. This amount of electrolyte is equivalent to 1140 ma-hour of capacity. The end electrodes weighed 5.5 grams each, which, at a capacity of 250 coulombs per gram equals a capacity of 382 ma-hours.

This battery of 4 bipolar .cells was charged and discharged repeatedly. Charging was conducted at 18.2 volts. Discharge was to battery terminal voltages as low as 7.6 volts. Battery resistance was about 12 ohms. Current densities as calculated for a terminal electrode having a dimension of 10 cm by 10 cm varied from 0.6 to 2.3 mA per cm ² .

Example 3

A bipolar cell was constructed as in example 2 except that a Celgard ® (a non-woven polypropylene separator) polymer separator was used instead of glass cloth. The total thickness across all four cells was

approximately 1.0 cm. This cell was charged and dis¬ charged repeatedly. Charging was done at 20.1 volts. Open circuit voltage on full charge was 19.4 volts. Battery resistance is a minimum at full charge and was approximately 5 ohms. Discharge was to an open circuit voltage of about 10 volts. This cell used a Derakane ® resin bead on the area of electrode fabric of the bipolar electrodes, at the point of juncture of the two adjacent cells. The results were improved over example 1 _# especially in coulometric efficiency which was typically over 85 percent.

Example 4

A two cell bipolar battery was prepared by sealing a pair of polypropylene sheets on their four edges to form a pocket and by cutting a slit through one side sheet midway thereof so that when folded in half along the slit, two pockets are formed. Two pieces of a Thornel woven cloth having a dimension of 13.75 cm by 13.75 cm were each electroplated with copper on each of the four edges thereof. A five mesh copper wire screen was folded over each copper plated edge and the electroplating continued until the screen was completely embedded in copper. A wire was attached to each electrode and the copper plated edges and the

® wire were embedded in a Derakane resin. These elec¬ trodes were inserted, one in each pocket. A Celgard separator sheet having a dimension of 12.5 cm by 12.5 cm was attached about a 12.5 cm square of each end of a

® . . .

Panex cloth having a dimension of 30 cm by 15 cm. The cloth was folded in half and potted in the area of the fold by a Derakane ® potting resin leaving an active area of 11.25 cm by 11.25 cm. One end of the active area was placed in each cell so that the Celgard ®

separator sheet was positioned between the electrode having the collector frame and the adjacent electrode of the Panex cloth. The Panex cloth was provided

® . . with a bead of Derakane potting resin along an lnter- mediate portion of the cloth extending from one edge to an opposite edge of the cloth.

This assembly was placed in a double wall polyethylene bag and then in a plexiglass holder and set in a dry box for 48 hours.

A 15 percent LiC10 ₄ solution in propylene carbonate, which had been dried in a container filled with a dry, highly activated 5A molecular sieves for 48 hours, was used to fill the cells and the battery connected to a charger and alternatively to a variable resistance for discharge.

The battery was then operated over a varying time schedule of charge/discharge using different resistances and discharge cut-off voltages. After 55 cycles with a 2 ohm charge measuring resistance and a 52 ohm discharge load to a 3.43 volt cut-off, the coulo- metric efficiency was calculated to be 85.7 percent. After 65 cycles through the same 2 ohm charge resis¬ tance using a 102 ohm discharge resistance and a cutoff voltage of 3.02 the efficiency was 90 percent. After 74 cycles using a 2 ohm charge and 202 ohm discharge resistance to a cutoff voltage of 3.7 volts, the efficiency was calculated 80.3 percent. The-above data illustrates the uniqueness of the construction of the a rechargeable battery of the invention.