BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having an electrode formed by casting a slurry of electrode active material to a metal foil followed by pressing the laminate. The resulting electrode active componer.t is thinner than conventional pressed powder electrodes while maintaining acceptable flexibility characteristics. Thinner electrodes benefit electrode surface area which in turn results in increased discharge capacity.

2. Prior Art

Various patents disclose metals, metal oxides, mixed metal oxides and metal sulfides as active materials for use in electrochemical cells having increased energy density and which are capable of delivering the high current pulses required for powering implantable medical devices. Such materials include silver vanadium oxide and copper silver vanadium oxide and are described by U.S. Patent Nos. 4,310,609 to Liang et al.; 4,830,940 to Keister et al . ; 5,389,472 to Takeuchi et al . ; 5,472,810 to Takeuchi et al . and 5,498,494 to Takeuchi et al. These patents are all assigned to the assignee of the present invention and incorporated herein by reference.

The anhydrous, dry powder form of the active materials described in these prior art patents are typically mixed with a conductive diluent and a binder

material, and layers of the dry powder active mixture are pressed onto opposed sides of a conductive, current collector screen. It is believed that the resulting electrode maintains its mechanical integrity during manufacture into various cell configurations by the active mixture locking to itself through the screen perforations. However, pressed powder electrodes of the prior art have limited, if any, flexibility and are generally incapable of being formed at thickness less than approximately 0.013 inches.

To further enhance the energy density of cells comprising the above discussed active materials and others, there is a need for flexible electrodes having improved thickness characteristics i.e., electrodes which are thinner than those generally known by the prior art, but which have increased current capacity. Flexibility is important for shaping or forming the electrodes into various cell configurations such as the jellyroll configuration. However, flexibility must not be improved at the expense of mechanical integrity.

Improved electrode thickness characteristics result in an increased electrode surface area for a given volume casing which is believed to increase the current capacity of cells incorporating the electrodes.

SUMMARY OF THE INVENTION

The present invention is directed to a thin, flexible electrode comprising an electrode active material contacted to a conductive substrate serving as the current collector. The present invention is also directed to a method of manufacturing such an electrode. The thusly formed electrode is preferably a cathode for use in an alkali metal, organic electrolyte cell. The preferred conductive substrate is comprised of an aluminum or aluminum alloy in the form of a screen or a

foil. In the latter case, the foil can be roughened or etched using an acid solution, or the foil can be used in a cleaned but untreated condition. To provide an electrode, electrode active material preferably including a conductive diluent and a binder material is first formed into a slurry by mixing with a solvent, and then the active slurry is cast or otherwise contacted to the conductive substrate, followed by pressing.

These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and to the appended drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a photograph showing the surface condition of an untreated aluminum alloy substrate.

Fig. 2 is a photograph showing the surface condition of an aluminum alloy substrate similar to that shown in Fig. 1 but after treatment in the sulfuric acid solution according to the present invention.

Fig. 3 is a graph showing the voltage under background load and pi minima and p4 minima pulse train voltages versus amp hours of a cell having a cathode comprising a silver vanadium oxide admixture slurry cast and pressed to a surface treated aluminum substrate.

Fig. 4 is a graph showing the voltage versus amp hours of a cell having a cathode comprising a silver vanadium oxide admixture slurry cast and pressed to a surface treated conductive substrate and subjected to pulses 1.5 A in amplitude.

Fig. 5 is a graph showing the voltage versus amp hours of a cell having a cathode comprising a silver vanadium oxide admixture slurry cast and pressed to a surface treated conductive substrate and subjected to pulses 2.0 A in amplitude.

Fig. 6 is a graph showing the average voltage versus amp hours of five cells similar to those discharged to construct Figs. 4 and 5.

Fig. 7 to 9 are graphs showing the voltage versus amp hours of cells having a cathode comprising a silver vanadium oxide admixture slurry cast and pressed to a surface treated conductive substrate and subjected to pulses 1.5 A, 1.75 A and 2.0 A in amplitude, respectively. Fig. 10 is a graph showing the voltage versus amp hours of a cell having a cathode comprising a silver vanadium oxide admixture slurry cast and pressed to an untreated aluminum substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrochemical cell of the present invention comprises an anode of a metal selected from Group IA of the Periodic Table of the Elements, including lithium, sodium, potassium, etc., and their alloys and intermetallic compounds including, for example, Li-Si, Li-Al, Li-B and Li-Si-B alloys and intermetallic compounds. The preferred anode comprises lithium, and the more preferred anode comprises a lithium alloy such as a lithium-aluminum alloy. However, the greater the amount of aluminum present by weight in the alloy the lower the energy density of the cell.

The form of the anode may vary, but preferably the anode is a thin metal sheet or foil of the anode metal, pressed or rolled on a metallic anode current collector, i.e., preferably comprising nickel, to form an anode component. In the exemplary cell of the present invention, the anode component has an extended tab or lead of the same material as the anode current collector, i.e., preferably nickel, integrally formed therewith such as by welding and contacted by a weld to

a cell case of conductive metal in a case-negative electrical configuration. Alternatively, the anode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to allow an alternate low surface cell design.

The electrochemical reaction at the cathode involves conversion of ions which migrate from the anode to the cathode in atomic or molecular forms. The solid cathode material may comprise a metal element, a metal oxide, a mixed metal oxide and a metal sulfide, and combinations thereof. The metal oxide or the mixed metal oxide can be formed by the chemical addition, reaction, or otherwise intimate contact of various metal oxides and/or metal elements, preferably during thermal treatment, sol-gel formation, chemical vapor deposition or hydrothermal synthesis in mixed states. The active materials thereby produced contain metals, oxides and sulfides of Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII which includes the noble metals and/or other oxide and sulfide compounds.

According to one aspect of the present invention, readily decomposable compounds consisting of metals from Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB, as well as similarly decomposable compounds from Group VIII, are thermally treated so as to effect the rapid preparation of the oxides or the respective metal elements themselves to be utilized further in the preparation of suitable cathode materials. Such readily decomposable materials include, but are not limited to, those classes of compounds known as nitrates, nitrites, carbonates and/or ammonium salts. The precursor materials (i.e., nitrates, nitrites, carbonates, ammonium compounds, etc.) may be decomposed in a combined state or individually decomposed and thereafter combined in an oxide/decomposable metal salt compound and subsequently decomposed to form the cathode active material

comprising a mixed metal oxide. Preferred mixed metal oxides are prepared by thermally decomposing a vanadium salt, suitably ammonium meta-vanadate to produce vanadium pentoxide. A decomposable metal salt, suitably the nitrates of a second metal and possibly a third metal, are then added to the vanadium pentoxide, thoroughly mixed therewith and thereafter thermally treated. The second and third metals are most preferably silver and copper. One preferred mixed metal oxide has the general formula SM_.V ₂ 0 _y wherein SM is a metal selected from the Groups IB to VIIB and VIII of the Periodic Table of elements and x is about 0.30 to 2.0 and y is about 4.5 to 6.0 in the general formula. By way of illustration, and in no way intended to be limiting, one exemplary cathode active material comprises silver vanadium oxide (SVO) having the general formula Ag _x V ₂ 0 _y in any one of its many phases, i.e., 3-phase silver vanadium oxide having in the general formula x = 0.35 and y = 5.8, γ- hase silver vanadium oxide having in the general formula x = 0.74 and y = 5.37 and e -phase silver vanadium oxide having in the general formula x = 1.0 and y = 5.5, and combinations and mixtures of phases thereof. Another preferred composite cathode material includes V ₂ 0 _z wherein z ≤ 5 combined with Ag ₂ 0 _z wherein z=0 to 1 and CuO _z wherein z=0 to 1 to provide the mixed metal oxide having the general formula Cu _x Ag _y V ₂ O _z , (CSVO) . Thus, this composite cathode active material may be described as a metal oxide-metal oxide-metal oxide, a metal-metal oxide-metal oxide, or a metal-metal-metal oxide and the range of material composition found for Cu _x Ag _y V ₂ Oi, is preferably about 0.01 ≤ x ≤ 1.0, about 0.01 ≤ y ≤ 1.0 and about 5.01 ≤ z ≤ 6.5. Typical forms of CSVO are Cu ₀ < ₆ Ag ₀₆₇ V ₂ O _z with z being about 5.5 and

Cu ₀₅ Ag ₀₅ V _j O _z with z being about 5.75. The oxygen content

is designated by z since the exact stoichiometric proportion of oxygen in CSVO can vary depending on whether the cathode material is prepared in an oxidizing atmosphere such as air or oxygen, or in an inert atmosphere such as argon, nitrogen and helium.

Additional cathode active materials include manganese dioxide, lithium cobalt oxide, lithium nickel oxide, copper oxide, titanium disulfide, copper sulfide, iron sulfide, iron disulfide, copper vanadium oxide, and mixtures thereof. The cathode active material is preferably present in the cathode at about 80 to 99 weight percent.

The above described active materials are formed into an electrode for incorporation into an electrochemical cell by mixing one or more of them with up to about 5 weight percent of a conductive additive such as acetylene black, carbon black and/or graphite. A preferred carbonaceous diluent is Ketjenblack carbon. Metallic powders such as nickel, aluminum, titanium and stainless steel in powder form are also useful as conductive diluents when mixed with the above listed active materials. The electrode further comprises up to about 10 weight percent of a binder material which is preferably a fluoro-resin powder such as powdered polytetrafluoroethylene (PTFE) or powdered polyvinylidene fluoride (PVDF) . More specifically, a preferred cathode active admixture comprises, by weight, about 94% SVO in any one of its many phases, or mixtures thereof, and/or CSVO mixed with about 3 weight percent of a binder material and about 3 weight percent of a conductive diluent.

According to the present invention, the thusly formed electrode active admixture is converted into a slurry by mixing with a high permittivity solvent such as a cyclic amide, a cyclic carbonate or a cyclic ester. Suitable high permittivity solvents include propylene

carbonate, ethylene carbonate, γ-butyrolactone, N- methyl-pyrrolidinone, dimethyl sulfoxide, acetonitrile, dimethyl formamide and dimethyl acetamide, and mixtures thereof. The active slurry is then cast or otherwise applied or contacted to a conductive foil substrate which serves as the current collector for the electrode. Suitable materials for the current collector substrate include nickel, aluminum, stainless steel, mild steel and titanium with aluminum being preferred. According to one embodiment of the present invention, prior to being contacted with the active slurry the foil is roughened to increase its surface area by immersion in an acidic etchant solution such as a sulfuric acid solution. A preferred etchant solution is designated a P2 solution and comprises sulfuric acid (H ₂ S0 ₄ ) having ferrous sulfate (Fe ₂ (S0 ₄ ) ₃ -5H ₂ 0) dissolved therein. If surface roughening is desired, the foil is immersed in the sulfuric acid solution for about 8 to 12 minutes at about 50°C to 70°C to provide the substrate with a pitted or honeycombed surface texture. On the other hand, if the surface roughening is not desired, the foil substrate is preferably cleaned of surface contamination such as dirt, grease, oil and the like and then used in its cleaned but untreated condition. After drying to remove all residual water from the slurry, contacted to the conductive substrate, the resulting anhydrous active admixture is calendared under a pressure of about 40 tons/inch ² to laminate the active admixture to either the etched or unetched current collector foil, as the case may be, to thereby form the electrode according to the present invention. Furthermore, if desired the substrate can be a screen comprised of one or more of the enumerated conductive materials. Cathodes prepared as described above are flexible and may be in the form of one or more plates operatively

associated with at least one or more plates of anode material, or in the form of a strip wound with a corresponding strip of anode material in a structure similar to a "jellyroll" . As previously discussed in the Prior Art section, conventional electrodes fabricated from dry powder active mixtures pressed onto a perforated conductive screen generally are limited to total thicknesses not less than 0.013 inches (measured electrode thickness includes the first and second active layers or plates pressed onto the opposed sides of the intermediate conductive screen) . In the present invention, the electrode comprising the active admixture cast and pressed onto the opposed sides of the conductive foil ranges between about 0.002 to 0.009 inches. The conductive foil itself typically is 0.001 inches thick which means that castings as thin as 0.0005 inches can be applied or contacted to the foil without compromising mechanical integrity. Decreasing electrode thickness beneficially increases electrode surface area which in turn benefits discharge current capacity.

In order to prevent internal short circuit conditions, the cathode is separated from the Group IA anode material by a suitable separator material . The separator is of electrically insulative material, and the separator material also is chemically unreactive with the anode and cathode active materials and both chemically unreactive with and insoluble in the electrolyte. In addition, the separator material has a degree of porosity sufficient to allow flow therethrough of the electrolyte during the electrochemical reaction of the electrochemical cell. Illustrative separator materials include fabrics woven from fluoropolymeric fibers including polyvinylidene fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene used either alone or

laminated with a fluoropolymeric microporous film, non- woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.) , polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a member commercially available under the designation DEXIGLAS (C.H. Dexter, Div. , Dexter Corp.) . The electrochemical cell of the present invention further includes a nonaqueous, ionically conductive electrolyte which serves as a medium for migration of ions between the anode and the cathode electrodes during the electrochemical reactions of the cell. The electrochemical reaction at the electrodes involves conversion of ions in atomic or molecular forms which migrate from the anode to the cathode. Thus, nonaqueous electrolytes suitable for the present invention are substantially inert to the anode and cathode materials, and they exhibit those physical properties necessary for ionic transport, namely, low viscosity, low surface tension and wettability.

A suitable electrolyte has an inorganic, ionically conductive salt dissolved in a nonaqueous solvent, and more preferably, the electrolyte includes an ionizable alkali metal salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent and a high permittivity solvent. The inorganic, ionically conductive salt serves as the vehicle for migration of the anode ions to intercalate into the cathode active material, and has the general formula MM'F ₆ or MM' F ₄ wherein M is an alkali metal similar to the alkali metal comprising the anode and M' is an element selected from the group consisting of phosphorous, arsenic, antimony and boron. Examples of salts yielding M'F ₆ are: hexafluorophosphate (PF ₆ ) , hexafluoroarsenate (AsF ₆ ) and

hexafluoroantimonate (SbF ₆ ) while tetrafluoroborate (BF ₄ ) is exemplary of salts yielding M'F ₄ . Thus, in the case of the anode comprising lithium, the alkali metal salt comprises lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium hexafluoroantimonate or lithium tetrafluoroborate dissolved in a suitable solvent mixture. Alternatively, the corresponding sodium or potassium salts may be used. Other inorganic salts useful with the present invention include LiC10 ₄ , LiAlCl ₄ , LiGaCl ₄ , LiC(S0 ₂ CF ₃ ) ₃ , LiN(S0 ₂ CF ₃ ) ₂ , LiS0 ₃ F, LiB(C ₆ H ₅ ) ₄ and LiCF ₃ S0 ₃ , and mixtures thereof.

Low viscosity solvents include tetrahydrofuran (THF) , methyl acetate (MA) , diglyme, triglyme, tetraglyme, dimethyl carbonate (DMC) , diethyl carbonate, 1, 2-dimethoxyethane (DME) and mixtures thereof, and high permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC) , ethylene carbonate (EC) , acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-butyrolactone (GBL) and N-methyl-pyrrolidinone (NMP) and mixtures thereof. In the present invention, the anode is preferably lithium metal and the preferred electrolyte is 1.0M to 1.4M LiAsF ₆ dissolved in an aprotic solvent mixture comprising a 50/50 mixture (by volume) of propylene carbonate (PC) and dimethoxyethane (DME) .

The preferred form of the electrochemical cell is a case-negative design wherein the anode/cathode couple is inserted into a conductive metal casing such that the casing is connected to the anode current collector in a case-negative configuration, as is well known to those skilled in the art. A preferred material for the casing is titanium although stainless steel, mild steel, nickel-plated mild steel and aluminum are also suitable. The casing header comprises a metallic lid having an opening to accommodate the glass-to-metal seal/terminal

pin feedthrough for the cathode electrode. The anode electrode is preferably connected to the case or the lid. An additional opening is provided for electrolyte filling. The casing header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cell is thereafter filled with the electrolyte solution described hereinabσve and hermetically sealed such as by close-welding a stainless steel plug over the fill hole, but not limited thereto. The cell of the present invention can also be constructed in a case-positive design.

The following examples describe the manner and process of manufacturing an electrochemical cell according to the present invention, and they set forth the best mode contemplated by the inventors of carrying out the invention, but they are not to be construed as limiting.

EXAMPLE I

Silver vanadium oxide (SVO) was sieved to remove particles larger than 0.007 inches. 4.25 grams of the resulting SVO, 0.15 grams of Ketjenblack carbon and 0.10 grams of graphite #38 powder were mixed using a mortar and pestle. The resulting admixture was added to 2.87 grams of a 17.3 wt% solution of Kynar 711 polyvinylidene fluoride in dimethyl formamide (DMF) and made less viscous with 14 ml of additional DMF.

Aluminum alloy foil designated as 1145-0 was etched using a sulfuric acid solution designated as a P2 solution which was made by bringing 210 ml of concentrated sulfuric acid (H ₂ S0 ₄ ) and 183.76 grams of ferrous sulfate (Fe ₂ (S0 ₄ ) ₃ 5H ₂ 0) to 1 liter with water.

The aluminum was immersed for 10 minutes in the etchant solution held at 60°C, then washed in deionized water and air dried. This treatment produced a honeycombed or pitted pattern on the aluminum surface. Figs. 1 and 2 show lOOOx magnification scanning electron microscope photos of untreated and P2 etched 1145-0 aluminum alloy foil, respectively.

The SVO slurry was shaken and transferred using a scoopula to the etched aluminum foil. The slurry was spread using a doctor blade with a gap of 0.008 inches to provide a uniform coating thereon. The casting was allowed to dry in a flowing air hood overnight before a coupon of 5.2cm ² was cut from the laminate. Drying generally reduces the thickness of the active mixture by about one-half. In this example, the dried active casting had a thickness of about 0.004 inches. A 0.125 x 0.125 inch area was cleared from one corner of the coupon before it was pressed at 6.15 tons/cm ² . Pressing further reduced the thickness of the active casting to about 0.0015 to 0.002 inches.

A 0.125 x 6 inch aluminum lead was resistance spot welded to the cleared area of the coupon. The cathode laminate was wrapped in two layers of CELGARD 4560 polypropylene separator material, and a piece of lithium metal of a similar dimension was placed against the SVO coated side of the cathode followed by a screen with a lead to serve as the current collector for the lithium anode. The electrode assembly was held together between glass slides using a metal clip and positioned in a glass vessel of suitable dimension with the leads allowed to hang over the rim of the vessel. The vessel was filled with 1M LiAsF ₆ in 1:1, by volume, propylene carbonate and dimethoxyethane. A rubber stopper provided a lid for the vessel. The cell was discharged under semi-hourly applied pulse trains comprising four, 10 second, 0.094 A (18 mA/cm ² ) pulses with 15 seconds rest between each pulse. A plot of the open circuit voltage prior to each pulse train application, the minimum voltage during the first pulse (pi minima) of each train and the minimum voltage during the fourth pulse (p4 minima) of each train is presented in Fig. 3. Specifically, Fig. 3 is a graph showing the discharge of the above described cell constructed in this example according to the present invention, wherein curve 30 was constructed from the prepulse voltage and curves 32 and 34 were constructed from the PI Min. and P4 Min. discharge voltages versus delivered capacity, respectively.

The cell delivered 0.028 Ah to a 2.0 V cutoff as defined by a pulse four minimum at or below 2.0 V.

EXAMPLE II

Silver vanadium oxide was ball milled for 2 hours using 1/2 x 1/2 inch cylindrical ceramic media in a polytetrafluoroethylene jar. This material was then

baked at 375°C for 65 hours prior to being sieved to remove particles larger than 0.0041 inches. 47 grams of the resulting sample of SVO was mixed with 1.5 grams of Ketjenblack carbon (3% by weight) using a mortar and pestle. The resulting admixture was added to 7.5 grams of a 17.3 wt% solution of Kynar 711 polyvinylidene fluoride in dimethyl formamide and made less viscous with about 45 ml of additional DMF.

This cathode active mixture was spread on one side of a P2 etched 1145-0 aluminum foil using a doctor blade having a gap of about 0.008 inches, and allowed to dry to a thickness of about 0.004 inches, as described in Example I. The second side of the aluminum foil was then coated in a similar manner to provide samples comprising the SVO cathode active material contacted to both sides of the P2 surface treated foil collector. The samples were about 5 x 12 inches. From these large samples, smaller ribbons were cut to 0.695 inches in width. Cathodes of 13.25 inches in length were fabricated from smaller pieces by clearing areas of

0.125 inches from the ribbon ends, pressing the pieces at 6.15 tons/cm ² and resistance spot welding the pieces together. Pressing compacted the active material to a thickness of about 0.0015 to 0.002 inches on each side of the aluminum foil. Leads were welded to the cathodes in the cleared areas. Lead placement was at positions about one-quarter of the total length of the cathode from each of its ends.

To test the mechanical integrity of the pressed ribbons, several ribbons were manipulated into S-shaped configurations. Those folds forming a trough placed the pressed active material under compression forces while the folds forming ridges subjected the active press to tension forces. In either case, it was noted that the active material remained contacted to the conductive foil and that no sloughing or cracking was present.

Anodes of 14.25 x 0.605 x 0.008 inches were fabricated by applying 0.0012 thick lithium to a 0.580 inch thick expanded nickel metal screen. The anodes had leads at one end and at a position about 3/4 along their lengths.

Both the anodes and the cathodes were wrapped in one layer of CELGARD 2400 polypropylene separator before being wound together in a jellyroll configuration and placed in a stainless steel vessel of 0.551 x 0.708 x 0.998 inch dimension. The anode leads were welded to the side of the vessel and the cathode leads were welded to a glass- o-metal hermetic feedthrough in the vessel lid. The cells were then activated with an electrolyte of 1 M LiAsF ₆ in 1:1, by volume, propylene carbonate and dimethoxyethane. Open circuit voltages of the cells were 3.45 to 3.47 V. Thirty cells were made.

All testing was performed at 37°C. The cells were discharged under loads of 20kQ for 20 hours followed by one week at open circuit storage. Pulse trains comprising four, 10 second, 1.5 A (13 mA/cm ² ) pulses with 15 seconds rest between each pulse were applied to each cell. The average last load voltage under the 20kΩ load, open circuit voltage prior to the pulse train application, the minimum voltage during the first pulse of each train, the voltage at the end of the first pulse of each train and the minimum voltage during the fourth pulse of the train for the thirty cells constructed in this example are presented in Table 1.

TABLE 1 last load, V 3.203 prepulse, V 3.258 pi min, V 2.311 pi last 2.318 p4 min 2.358

EXAMPLE III Nine of the cells constructed in Example II were pulse discharged by applying to each cell a pulse train comprising four, 10 second 1.5 A (14 mA/cm ² ) pulses with 15 seconds rest between each pulse every thirty minutes. A typical discharge curve is shown in Fig. 4. Particularly, Fig. 4 is a graph showing the discharge of a typical one of these nine cells wherein curve 40 was constructed from the prepulse voltage and curves 42 and 44 were constructed from the Pi Min. and P4 Min. discharge voltages versus delivered capacity, respectively.

Nine additional cells of those constructed in Example II were pulse discharged by applying to each cell a pulse train comprising four, 10 second, 2.0 A (18 mA/cm ² ) pulses with 15 seconds rest between each pulse every thirty minutes. A typical discharge curve is shown in Fig. 5. Specifically, Fig. 5 is a graph showing the discharge of a typical one of these nine cells wherein curve 50 was constructed from the prepulse voltage and curves 52 and 54 were constructed from the PI Min. and P4 Min. discharge voltages versus delivered capacity, respectively.

The average capacity delivered by each group of nine cells to specified cutoffs is presented in Table 2.

TABLE 2

Ah to 2.0 V Ah to 1.7 V Ah to 1.5 V 1.5 A trains 224 253 264

2.0 A trains 224 255 268

The remaining twelve cells constructed in Example II were discharged under loads of 73.2 kΩ. Every eight weeks these cells were subjected to a pulse train comprising four, 10 second, 1.5 A (14 mA/cm ² ) pulses with

15 seconds rest between each pulse. The average voltage prior to the pulse train application, the minimum voltage during the first pulse of each train, and the minimum voltage during the fourth pulse of the train are presented graphically in Fig. 6 for a sampling of five of these cells. Particularly, Fig. 6 is a graph showing the average discharge of five of these cells wherein curve 60 was constructed from the average prepulse voltage and curves 62 and 64 were constructed from the average PI Min. and P4 Min. discharge voltages versus delivered capacity, respectively.

EXAMPLE IV

Silver vanadium oxide was attrited for 1 hour using 1/4 inch spherical steel media to reduce its average particle size. 9 grams of the resulting ground SVO was mixed with 0.5 grams of Ketjenblack carbon (5% by weight) using a mortar and pestle. The resulting admixture was added to 2.86 grams of a 17.3 wt % solution of Kynar 711 polyvinylidene fluoride in dimethyl formamide and made less viscous by the addition of more DMF.

This cathode active admixture was spread on one side of a P2 etched 1145-0 aluminum foil (0.008 inches in thickness) and dried to a thickness of about 0.004 inches thick, as described in Example I. The second side of the aluminum foil was then coated in a similar manner so that a sample having both sides coated resulted. The sample was about 5 x 12 inches. From this large sample, smaller ribbons were cut to 0.695 inches in width. Cathodes of 15.5 inches in length were fabricated from smaller pieces by clearing areas of 0.125 inches from the ribbon ends, pressing the pieces at 6.15 tons/cm ² to a thickness of about 0.0015 to 0.002 inches, and resistance spot welding the pieces together.

Leads were welded to the cathodes in the cleared areas . Lead placement was at positions about was one-quarter of the total length of the cathode from each of its ends. Anodes of 16.5 x 0.605 x 0.008 inches were fabricated by applying 0.0012 thick lithium to a 0.580 inch thick expanded nickel metal screen. The anodes had leads at one end and at a position about 3/4 along their lengths.

Both the anodes and the cathodes were wrapped in one layer of CELGARD 2400 separator before being wound together in a jellyroll configuration and placed in a stainless steel vessel of 0.551 x 0.708 x 0.998 inch dimension. The anode leads were welded to the side of the vessel and the cathode leads were welded to a glass- to-metal hermetic feedthrough in the vessel lid.

Nineteen cells were constructed in this manner and activated with an electrolyte of 1M LiAsF ₆ in 1:1, by volume, propylene carbonate and dimethoxyethane. Open circuit voltages of the cells were 3.45 to 3.47 V. All testing was performed at 37°C. Each of the cells was burned-in by discharge under a load of 20 kΩ for 20 hours. Three of the cells were then pulse discharged by applying to each cell a pulse train comprising four, 10 second pulses with 15 seconds rest between each pulse every thirty minutes. Pulse amplitudes of either 1.5 A, 1.75 A or 2.0 A were used. Discharge curves are shown in Figs. 7 to 9 , respectively. Specifically, Fig. 7 is a graph showing the discharge of the cell subjected to 1.5 A pulses wherein curve 70 was constructed from the prepulse voltage and curves 72 and 74 were constructed from the PI Min. and P4 Min. discharge voltages versus delivered capacity, respectively. Fig. 8 is a graph showing the discharge of the cell subjected to 1.75 A pulses wherein curve 80 was constructed from the prepulse voltage and curves 82 and 84 were constructed from the Pi Min. and

P4 Min. discharge voltages versus delivered capacity, respectively. Finally, Fig. 9 is a graph showing the discharge of the cell subjected to the 2.0 A pulses wherein curve 90 was constructed from the prepulse voltage and curves 92 and 94 were constructed from the PI Min. and P4 Min. discharge voltages versus delivered capacity, respectively.

Table 3 shows the capacity delivered by each cell to the specified cutoffs:

TABLE 3

Ah to 2 V Ah to 1.7 V Ah to 1.5 V

1.5 A trains 280 377 426

1.75 A trains 247 359 410 2 A trains 228 339 392

It was shown that cathodes prepared according to the present invention from a slurry of an electrode active material, exemplified by silver vanadium oxide, contacted to a surface-treated foil collector can be wound about a 1/8 inch metal rod while maintaining their mechanical integrity, even after several windings and unwindings . Thus, the electrodes comprising a cathode active material contacted to a surface treated conductive foil according to the present invention exhibit improved structural integrity even when tightly wound with an anode in a jellyroll configuration while maintaining high current densities typically associated with an alkali metal/mixed metal oxide electrical couple. It is noted that dry cathode active materials pressed to untreated conductive substrates according to the prior art have been known to experience noticeable separation or sloughing off from the current collector during manufacture, particularly while being wound with the anode into the jellyroll configuration. While acceptable, the etched surface of the conductive foil

according to the present invention helps prevent even slight separation or sloughing from occurring without deterring from the energy density of the cell. In fact, cells incorporating electrodes fabricated according to the present invention having thickness in the range of about 0.002 to about 0.009 inches exhibit comparable to increased discharge capacities with respect to prior art cells of similar chemistries. It is believed that the increased capacity is at least partially attributable to increased contact surface area between the electrode active material and the current collector.

EXAMPLE V

Silver vanadium oxide was sieved to less than 140 mesh and, 4.25 grams of the sieved material was combined with 10 grams of graphite #38 and 15 grams of Ketjenblack carbon using a mortar and pestle. The resulting mixture was added to a solution of 50 grams Kynar 461 polyvinylidene fluoride in 800 grams of dimethyl formamide and 10 grams of propylene carbonate. The resulting slurry was milled/mixed and transfered to untreated Reynolds, heavy duty aluminum foil of 0.001 inches in thickness using a scoopula. The slurry was spread using a doctor blade with a gap of 0.015 inches such that a uniform coating resulted. The casting was allowed to dry to a thickness of about 0.008 inches in a heated chamber before a coupon of 5.2 cm ² was cut from the laminate. A 0.125 x 0.125 inch area was cleared from one corner of the coupon before it was pressed at 6.15 tons/cm ² to provide the coupon having an active material thickness of about 0.004 inches. A 0.125 x 6 inch aluminum lead was resistance spot welded to the cleared area of the cathode active coupon laminate.

The cathode laminate was wrapped in two layers of CELGARD 4560 polypropylene separator material, and a coupon of lithium metal of the same dimension was placed against the SVO coated side of the cathode as was a screen with a lead to serve as a current collector for the lithium anode. The electrode assembly was held together using a metal clip and positioned in a glass vessel of suitable dimension with the leads allowed to hang over the rim of the vessel. The vessel was filled by 1M LiAsF ₆ in 1:1, by volume, propylene carbonate and 1, 2-dimethoxyethane. A rubber stopper provided a lid for the vessel.

The cell was discharged using semi-hourly applied pulse trains comprising four, 10 second, 0.094 A (0.018A/cm ² ) pulses with 15 seconds rest between each pulse. A plot of the discharge curves are present in Fig. 10. Specifically, curve 100 was constructed from the prepulse voltage of the cell constructed according to this example, and curves 102 and 104 were constructed from the PI Min. and P4 Min. discharge voltages versus delivered capacity, respectively of this cell. The cell delivered 0.0075 Ah to a 2 V cutoff as defined by a pulse minimum at or below 2 V.

The mechanical integrity of the coupon laminates was tested by manipulating them into the S-shaped configuration described in Example II. It was noted that even with an untreated foil, the cast and pressed active mixture did not exhibit mechanical failure as no sloughing off or crack of the active material was noted. Furthermore, it was an unexpected advantage of the present invention that the application of a slurry of cathode active material to a cleaned but untreated conductive substrate had sufficient mechanical integrity for incorporation into a cell while exhibiting comparable to increased discharge capacities with

respect to capacities known for prior art press powder cells.

It is appreciated that various modifications to the inventive concepts described herein may be apparent to those skilled in the art without departing from the spirit and the scope of the present invention defined by the hereinafter appended claims.