Patent Application
20100148128
1. Field of the Invention
The present invention generally relates to the conversion of chemical energy into electrical energy. More particularly, the present invention relates to pad printing processes for coating an electrode active suspension on a conductive current collector substrate. Preferably, the suspension is of an electrode active material, such as of silver vanadium oxide, for an electrochemical cell. The silver vanadium oxide is provided as a printable ink comprising an aqueous or non-aqueous carrier and a printing binder.
2. Prior Art
An implantable cardiac defibrillator is a device that requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. From time-to-time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected.
It is generally recognized that lithium cells containing silver vanadium oxide (SVO) and, in particular, ε-phase silver vanadium oxide (AgV2O5.5), are preferred for powering cardiac defibrillators. Silver vanadium oxide is preferred because it delivers high current pulses or high energy within a short period of time. However, it is believed that the discharge performance of this cell chemistry is further improved by contacting the high rate SVO material to the current collector in the form of printable ink. The ink promotes adhesion by more readily coating irregularities of the current collector including flowing into its openings to completely lock to the active material coated onto the opposite side thereof. This is regardless whether the active material mixtures on the opposite side of the current collector are of the same chemistry or different, for example SVO and CFx in an electrode configuration SVO/current collector/CFx. Coating an active ink suspension onto a conductive substrate using a pad printing technique does this.
Accordingly, the present invention describes the deposition of a metal-containing suspension onto a conductive current collector substrate by various pad-printing techniques. This results in consistent location of an electrode active coating on the current collector, especially when only a portion of the substrate is to be coated with other portions left uncoated. Other advantages include coating thickness uniformity and better adhesion.
In a pad-printing process, the printing ink contains the cathode active material and a printing binder well dispersed in a stable suspension comprising an aqueous or non-aqueous carrier. The ink is printed onto a conductive substrate serving as a current collector that is then heated to evaporate the solvent and decompose or otherwise at least partially remove the printing binder. The printing binder is a viscosity modifier that aids in processing the printing ink and in the pad printing process. Upon heating to evaporate the solvent, the printing binder burns off. However, it is not critical that the binder is completely decomposed, as residual carbon can serve as a conductivity enhancer or augment the conductive material that is typically added to the cathode active mixture.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by a reading of the following detailed description in conjunction with the appended drawings.
The present invention will be described with respect to various pad-printing techniques for depositing or coating an electrode active material suspension onto a conductive current collector substrate. The pad printing techniques include those performed by sealed ink cup pad printing, open inkwell pad printing and rotary gravure pad printing.
Turning now to the drawings,
Preferred preparation techniques for metal vanadium oxide nanoparticles comprise reacting a silver-containing compound and a vanadium-containing compound. For example, if silver vanadium oxide is the desired product, suitable silver starting materials include Ag, AgNO3, AgNO2, Ag2O2, AgVO3, Ag2CO3, Ag(CH3CO2), and mixtures thereof, while the vanadium-containing compound is selected from NH4VO3, AgVO3, VO, VO1.27, VO2, V2O4, V2O3, V3O5, V4O9, V6O13, V2O5, and mixtures thereof.
Suitable preparation techniques are more thoroughly discussed in U.S. application Ser. No. 10/391,885, filed Mar. 19, 2003. They include sol-gel synthesis (U.S. Pat. No. 5,555,680 to Takeuchi et al.), hydrothermal synthesis (U.S. application Ser. No. 10/894,305, filed Jul. 19, 2004), combustion chemical vapor deposition (CCVD) (U.S. Pat. No. 5,652,021 to Hunt et al.), spray pyrolysis, laser pyrolysis (U.S. Pat. No. 6,225,007 to Horne et al.), combination reaction (U.S. Pat. No. 5,221,453 to Crespi et al.), decomposition synthesis (U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al.), an amorphous SVO reaction (U.S. Pat. No. 5,498,494 to Takeuchi et al.), and a heat-treated SVO reaction (U.S. Pat. No. 5,955,218 to Crespi et al.). The above patents and applications are incorporated herein by reference.
One preferred metal vanadium oxide has the general formula SMxV2Oy where SM is a metal selected from Groups IB to VIIB and VIII of the Periodic Table of Elements, wherein 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 metal vanadium oxide comprises silver vanadium oxide having the general formula AgxV2Oy in any one of its many phases, i.e., β-phase silver vanadium oxide having in the general formula x=0.35 and y=5.8, γ-phase silver vanadium oxide having in the general formula x=0.74 and y=5.37 and ε-phase silver vanadium oxide having in the general formula x=1.0 and y=5.5, and combination and mixtures of phases thereof. For a more detailed description of such cathode active materials reference is made to the previously discussed U.S. Pat. No. 4,310,609 to Liang et al. Another preferred metal vanadium oxide cathode material includes V2Oz wherein z≦5 combined with Ag2O with silver in either the silver(II), silver(I) or silver(0) oxidation state and CuO with copper in either the copper(II), copper(I) or copper(0) oxidation state to provide the mixed metal oxide having the general formula CuxAgyV2Oz, (CSVO) with 0.01≦z≦6.5. Typical forms of CSVO are Cu0.16Ag0.67V2Oz with z being about 5.5 and Cu0.5Ag0.5V2Oz 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. For a more detailed description of this cathode active material reference is made to U.S. Pat. Nos. 5,472,810 to Takeuchi et al. and 5,516,340 to Takeuchi et al., both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Metal vanadium oxide particles produced by the above-referenced techniques have an average particle size of less than about 500μ and, more preferably, an average diameter of from about 0.5μ to about 200μ for suspension in ink suitable for use with a pad printing process. Preferably, the active particles have a very narrow distribution of particle diameters without a tail. In other words, there are effectively no particles with a diameter an order of magnitude greater than the average diameter such that the particle size distribution rapidly drops to zero.
Suitable solvents include cyclohexanone (b.p.=155.6° C.), n-octyl alcohol (b.p.=171° C.), ethylene glycol (b.p.=197° C.), and water. These liquids do not readily evaporate at room temperature and maintain rheology or viscosity during printing. The printing ink 14 is preferably at a concentration of from about 150 to about 500 grams of the cathode active material per liter.
The printing ink 14 further includes a printing binder. Suitable printing binders include ethyl cellulose, acrylic resin, polyvinyl alcohol, polyvinyl butyral and a poly(alkylene carbonate) having the general formula R—O—C(═O)—O with R═C1 to C5. Poly(ethylene carbonate) and poly(propylene carbonate) are preferred as they burn out in virtually any atmosphere including nitrogen, air, hydrogen, argon and vacuum. Suitable poly(alkylene carbonate) binders are commercially available from Empower Materials, Inc., Newark, Del. under the designations QPAC 25 and QPAC 40.
If desired, the printing ink is further provided with an electrode binder comprising a powdered fluoro-polymer. The electrode binder is different than the printing binder, and does not decompose when subjected to the temperatures used to drive off or at least partially decompose the printing binder. Instead, it remains in the electrode active mixture to add structural integrity to the subsequently formed electrode so that the electrode active material does not crack, slough off or otherwise lose contact with the current collector during subsequent manufacturing steps and during cell discharge. More preferably, the fluoro-polymer electrode binder is powdered polytetrafluoroethylene or powdered polyvinylidene flouride present at about 1 to about 5 weight percent of the cathode mixture.
Further, up to about 10 weight percent of a conductive diluent is preferably added to the ink to improve conductivity of the product electrode active mixture. Suitable materials for this purpose include acetylene black, carbon black and/or graphite or a metallic powder such as powdered nickel, aluminum, titanium and stainless steel. The preferred cathode active mixture thus includes a powdered fluoro-polymer binder present at about 3 weight percent, a conductive diluent present at about 3 weight percent and about 94 weight percent of the metal vanadium oxide active material.
The current collector 16 preferably consists of a conductive metal such as titanium, molybdenum, tantalum, niobium, cobalt, nickel, stainless steel, tungsten, platinum, palladium, gold, silver, copper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron, and mixtures and alloys thereof, and comprises a major face serving as a contact surface 18 surrounded by a peripheral edge 20 (
Regardless of the material of the current collector 16, coating integrity relies mostly upon mechanical bonding to the contacted surface 18. It is, therefore, critical that the current collector 16 is properly prepared to ensure coating quality. For one, substrate surface cleanliness is very important in all coating systems. In that respect, it is required that the substrate 16 remain uncontaminated by lubricants from handling equipment or body oils from hands, and the like. Current collector cleaning includes chemical means such as conventional degreasing treatments using aqueous and non-aqueous solutions, as are well known to those skilled in the art. Plasma cleaning is also used.
After current collector surface cleaning, surface roughness is the next most critical factor for coating adhesion. The contact surface 18 may be roughened by chemical means, for example, by contacting the current collector with hydrofluoric acid and/or hydrochloric acid containing ammonium bromide and methanol, and the like, by plasma etching, and by mechanical means such as scraping, machining, wire brushing, rough threading, grit blasting, a combination of rough threading then grit blasting and abrading such as by contacting the current collector with Scotch-Brite® abrasive sheets manufactured by 3M.
If desired, the electrical conductivity of the current collector 16 is improved prior to coating. Metal and metal alloys naturally have a native oxide on their exposed surfaces. This is a resistive layer and hence, if the material is to be used as a substrate for a cell electrode, the oxide is preferably removed or made electrically conductive prior to deposition of an active coating thereon. In order to improve the electrical conductivity of the current collector 16, various techniques can be employed. One is shown and described in U.S. Pat. No. 6,740,420 to Muffoletto et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
Returning to
The printing tampon 12 comprises a backing plate 32 detachably secured to a piston 34 at the distal end of a piston rod 36. The printing tampon 12 is more clearly shown in
The piston rod 36 resides in a closely spaced relationship in a cylinder 42 that precisely controls the axis of vertical movement of the piston 34 and attached printing tampon 12. A limit plate 44 is secured to the piston rod 36 adjacent to the piston 34. This ensures that the piston does not retract upwardly too far to be damaged by a collision with the C-shaped plate 30 and cantilevered arm.
The mainframe platform 24 supports a cliché 46 that actuates in a back and forth manner on a series of upper and lower bearings 48 and 50, respectively. The cliché 46 is a plate shaped metal member, such as of A2 tool steel coated with a diamond like carbon finish. The cliché has a chemically etched recess 52 sized to create the image or perimeter of the printing ink 14 to be deposited on the current collector 16. A cup 54 containing the printing ink 14 is supported on the cliché 46 by a magnetic sealing ring 56. The magnetic attraction between the cliché and ring provides a closely spaced tolerance that squeegees the printing ink 14 filled into the recess 52 to a precise depth. The printing ink 14 is now ready for subsequent transfer to the printing tampon 12 as the cliché 46 travels back and forth. This will be described in greater detail hereinafter.
As shown in
As shown in the drawing of
In
As shown in
In
During deposition of the printing ink 14 onto the current collector 16, the conductive block 58 and work stage 60 maintain the substrate at a temperature sufficient to evaporate or otherwise drive off the solvent from the deposited printing mixture. Thus, as the current collector 16 is being coated with the printing ink, the contact surface 18 is at a temperature sufficient to begin driving off or otherwise evaporating the solvent material. Preferably, the solvent is evaporated from the current collector 16 almost instantaneously with contact by the printing ink 14 resulting in deposition of a relatively thin film coating of the cathode active mixture. Heating the current collector to a first temperature of at least about 100° C. does this. In addition, printing can be done at ambient temperature with solvent removal performed in a subsequent process.
After deposition and solvent removal, the cathode active material coated current collector is heated to a second temperature of up to about 200° C. for at least about five minutes to about three hours. This heating protocol is sufficient to remove any residual solvent and at least partially burn off the printing binder from the cathode active coating. In another embodiment, the current collector 16 is maintained at a temperature sufficient to, for all intents and purposes, instantaneously evaporate the solvent and at least partially decompose the printing binder. A heating temperature for the current collector of up to about 200° C. is preferred for this. The decomposition temperature is about 220° C. for the previously described poly(ethylene carbonate) printing binder and about 250° C. for the poly(propylene carbonate) binder.
An important aspect of the present invention is that it is not critical that the printing binder be completely decomposed. Instead, residual carbon left by undecomposed printing binder serves as an electrically conductive material in the cathode active mixture. This means that less conductive diluent needs to be added to the ink to enhance discharge efficiency, as is well known by those skilled in the art. Also, as previously discussed the preferred cathode active mixture binder is a fluoro-polymer material, and preferably, powdered polytetrafluoroethylene. This material begins to decompose at about 230° C., so the final heating temperature must be less than that to prevent loss of functionality for the binder material.
After deposition and combustion of the printing binder, whether it is instantaneous or otherwise, the current collector 18 is ramped down or cooled to ambient temperature, maintained at the heated deposition temperature to enhanced bonding strength, or varied according to a specific profile. In general, it is preferred to conduct the heating steps while contacting the substrate with air or an oxygen-containing gas.
In the case of silver vanadium oxide, it is preferred that the resulting coating have a thickness of from about 1 micron to about 2 millimeters, or more.
While not shown in the drawings, the inked current collector 16 is removed from the conductive block 58 and heated work stage 60 for further processing into an electrical energy storage device, such as an electrochemical cell. A second current collector is then positioned on the conductive block and the cycle is repeated.
As particularly shown in
In
As particularly shown in
A cliché 116 is connected to the main frame 112 and serves as a stage for backward and forward movement of the ink cup 54 there along. The ink cup 54 is sealed to the cliché 116 by a squeegee ring 56. The cliché 116 includes a recess 118 so that as the ink cup 54 travels back and forth along the cliché 116, the printing ink 14 is precisely filled into the recess 118 (
As shown in
In that manner, a cycle of the pad printing apparatus 110 is not complete until the ink cup 54 has traveled back and forth across the cliché 116, filling the recess 118 each time. This benefits cycle time as each movement of the ink cup 54 across the cliché 116 results in an inked current collector.
A horizontal beam 224 is connected to the depending beam 214 with the doctor blade 212 pivotably supported at the distal end of the horizontal beam 224. An actuatable arm 226 connects between the support beam 208 and the secondary arm 218 for precise pivotable movement of the doctor blade 212 into and out of contact with the cliché 202.
As shown in
In
In
Cathode electrodes prepared by one of the previously described pad printing processes are particularly useful in a novel electrode construction having a relatively high rate capability metal vanadium oxide, for example, SVO, contacted to one side of a current collector while a relatively high energy density material, for example, CFx, is contacted to the other side of the current collector. This design has the separate SVO and CFx materials short-circuited to each other through the current collector. Providing the active materials in a short circuit relationship means that their respective attributes of high rate and high energy density benefit overall cell discharge performance.
Accordingly, SVO cathode material, which provides a relatively high power or rate capability but a relatively low energy density or volumetric capability and CFx cathode material, which has a relatively high energy density but a relatively low rate capability, are individually contacted to current collector screens, preferably by one of the previously described pad printing processes. This provides both materials in direct contact with the current collector. Therefore, one exemplary cathode plate for a primary cell has the following configurations:
An important aspect of the present invention is that the high rate cathode material (in this case the SVO material) maintains direct contact with the current collector. Another embodiment of the present invention has the high capacity/low rate material sandwiched between the high rate cathode materials, in which the low rate/high capacity material is in direct contact with the high rate material. This cathode design has the following configuration:
SVO/current collector/SVO/CFx/SVO/current collector/SVO
Another important aspect of the present invention is that the high capacity material having the low rate capability is preferably positioned between two layers of high rate cathode material (either high or low capacities). In other words, the exemplary CFx material never directly faces the lithium anode. In addition, the low rate cathode material must be short circuited with the high rate material, either by direct contact as demonstrated above in the second embodiment, or by parallel connection through the current collectors as in the first illustrated embodiment above. This electrode construction is described in U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present invention and incorporated herein by reference.
In a preferred construction method, CFx is first contacted to one side of the current collector by any suitable means including one of the present pad printing techniques. A barrier is provided on the opposite side of the current collector to prevent CFx from moving through the current collector openings to the other side thereof. Then, the SVO material is contacted to the opposite side of the current collector by one of the pad printing processes.
As particularly shown in
This is in contrast to contacting the active materials, whether they are the same or different, to the opposite sides of the current collector in a conventional manner. This can be as a pressed powder mixture as described in U.S. Pat. Nos. 4,830,940 and 4,964,877 to Keister et al. or as a freestanding sheet as described in U.S. Pat. Nos. 5,435,874 and 5,571,640 to Takeuschi et al., or by other well known techniques. These patents are assigned to the assignee of the present invention and incorporated herein by reference.
A conventional electrode construction is shown in
In addition to silver vanadium oxide, copper silver vanadium oxide, V2O5, MnO2, LiCoO2, LiNiO2, LiMn2O4, TiS2, Cu2S, FeS, FeS2, copper oxide, copper vanadium oxide, and mixtures thereof are useful as the first active material. And, in addition to fluorinated carbon, Ag2O, Ag2O2, CuF, Ag2CrO4, MnO2, and even SVO itself, are useful as the second active material. The theoretical volumetric capacity (Ah/ml) of CFx is 2.42, Ag2O2 is 3.24, Ag2O is 1.65 and AgV2O5.5 is 1.37. Thus, CFx, Ag2O2, Ag2O, all have higher theoretical volumetric capacities than that of SVO.
As previously discussed, before fabrication into an electrode structure, the first and second cathode active materials are preferably mixed with a binder material such as a powdered fluoro-polymer at about 1 to about 5 weight percent of the cathode mixture and up to about 10 weight percent of a conductive diluent. The preferred first cathode active mixture thus includes a powdered fluoro-polymer electrode binder present at about 3 weight percent, a conductive diluent present at about 3 weight percent and about 94 weight percent of the metal vanadium oxide active material. A preferred second active mixture is, by weight, 91% CFx, 4% PTFE and 5% carbon black.
The thusly produced cathode electrodes, preferably containing silver vanadium oxide contacted to one side of the current collector and CFx contacted to the other side are incorporated into primary electrochemical cells that possess sufficient energy density and discharge capacity required to meet the rigorous requirements of implantable medical devices, such as cardiac defibrillators. In that manner, the cathode is coupled to an anode of a metal selected from Groups IA, IIA and IIIB of the Periodic Table of the Elements. Such anode active materials include 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. An alternate anode comprises a lithium alloy such as a lithium-aluminum alloy. The greater the amounts of aluminum present by weight in the alloy, however, the lower the energy density of the cell.
The form of the anode may vary, but preferably it comprises a thin metal sheet or foil of the anode metal, pressed or rolled on a metallic anode current collector of titanium, titanium alloy, nickel, copper, tungsten or tantalum. The anode has an extended tab that is subsequently welded 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.
A separator structure of electrically insulative material is provided between the anode and the cathode to prevent an internal electrical short circuit between the electrodes. 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 capacitor. Illustrative separator materials include woven and non-woven fabrics of polyolefinic fibers including polypropylene and polyethylene or fluoropolymeric fibers including polyvinylidene fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene laminated or superposed with a polyolefinic or fluoropolymeric microporous film, non-woven glass, glass fiber materials and ceramic materials. Suitable microporous films include a polyethylene membrane commercially available under the designation SOLUPOR (DMS Solutech), a polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), a polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).
A secondary cell according to the present invention takes advantage of active materials that are typically used as cathode active materials in primary cells, but which cannot normally be used in conventional secondary cells. The current art in rechargeable cells is to use the positive electrode as the source of alkali metal ions. This prohibits the use of metal-containing active materials that do not contain alkali metal ions. Examples of such metal-containing materials include V2O5, V6O13, silver vanadium oxide (SVO), copper silver vanadium oxide (CSVO), MnO2, TiS2, MOS2, NbSe3, CuO2, Cu2S, FeS, FeS2, CFx, Ag2O, Ag2O2, CuF, Ag2CrO4, copper oxide, copper vanadium oxide, and mixtures thereof.
However, the positive electrode of the present secondary cells is built in a double current collector configuration having a “sacrificial” piece of alkali metal, preferably lithium, sandwiched between the current collectors. A pad printed cathode active material capable of intercalation and de-intercalation the alkali metal contacts the opposite side of at least one, and preferably both, of the current collectors. The purpose of the sacrificial alkali metal is to react with the cathode active material upon the cell being activated with an electrolyte. The reaction results in a lithiated cathode active material.
Suitable current collectors are selected from copper, stainless steel, titanium, tantalum, platinum, gold, aluminum, nickel, cobalt nickel alloy, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium-, and molybdenum-containing alloys. Preferably the current collector is a perforated foil or screen, such as an expanded screen.
Preferred embodiments for a secondary cell include the following positive electrode configurations:
vanadium oxide/current collector/lithium/current collector/vanadium oxide, or
vanadium oxide/current collector/vanadium oxide/lithium/vanadium oxide/current collector/vanadium oxide, or
vanadium oxide/current collector/lithium, with the vanadium oxide facing the negative electrode.
By the term “vanadium oxide” is meant V2O5, V6O13, silver vanadium oxide, and copper silver vanadium oxide in a nanoparticle form.
With this double current collector electrode design, the amount of lithium metal is adjusted to fully lithiate the cathode active material. Upon activating the cell with an ion-conductive electrolyte, the alkali metal migrates into the cathode active material resulting in complete consumption of the alkali metal. The absence of the alkali metal in the cell preserves the desirable safety and cycling properties of the intercalation negative and positive electrodes.
The anode or negative electrode for the secondary cell comprises an anode material capable of intercalating and de-intercalating lithium. Typically, the anode material of the negative electrode comprises any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glassy carbon, etc.) that are capable of reversibly retaining the lithium species. Graphite is particularly preferred in conventional secondary cells. “Hairy carbon” is another particularly preferred conventional material due to its relatively high lithium-retention capacity. “Hairy carbon” is a material described in U.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
The negative electrode for a secondary cell is fabricated by mixing about 90 to 97 weight percent of the carbonaceous anode material with about 3 to 10 weight percent of an electrode binder material, which is preferably a fluoro-resin powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE), polyamides, polyimides, and mixtures thereof. This negative electrode admixture is provided on a current collector selected from copper, stainless steel, titanium, tantalum, platinum, gold, aluminum, nickel, cobalt nickel alloy, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium-, and molybdenum-containing alloys. The current collector is a foil or screen and contact is by casting, pressing, or rolling the admixture thereto.
Another type of pad printable anode material useful with the present invention is a metal that reversibly alloys with alkali metals. Such metals include, but are not limited to, Sn, Si, Al, Pb, Zn, Ag, SnO, SnO2, SiO, and SnO(B2O3)x(P2O5)y. For a more detailed description of the use of these materials in the negative electrode of a secondary cell, reference is made to U.S. application Ser. No. 10/008,977, filed Nov. 8, 2001, which is assigned to the assignee of the present invention and incorporated herein by reference.
The primary electrochemical cell further includes a nonaqueous electrolyte that exhibits those physical properties necessary for ionic transport, namely, low viscosity, low surface tension and wettability. The electrolyte has an inorganic, ionically conductive salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent and a high permittivity solvent. In the case of an anode comprising lithium, preferred lithium salts that are useful as a vehicle for transport of lithium ions from the anode to the cathode include LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4, LiO2, LiAlCl4, LiGaCl4, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF3, LiC6F5SO3, Li2CCF3, LiSO6F, LiB(C6H5)4 and LiCF3SO3, and mixtures thereof.
Low viscosity solvents useful with the present invention include esters, linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran, methyl acetate, diglyme, trigylme, tetragylme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1-ethoxy, 2-methoxyethane, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate, dipropyl carbonate, and mixtures thereof, and high permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone, N-methyl-pyrrolidone, and mixtures thereof. In a primary cell, the preferred anode is lithium metal and the preferred electrolyte is 0.8M to 1.5M LiAsF6 or LiPF6 dissolved in a 50:50 mixture, by volume, of propylene carbonate and 1,2-dimethoxyethane.
A preferred electrolyte for a secondary cell comprises a solvent mixture of EC:DMC:EMC:DEC. Most preferred volume percent ranges for the various carbonate solvents include EC in the range of about 20% to about 50%; DMC in the range of about 12% to about 75%; EMC in the range of about 5% to about 45%; and DEC in the range of about 3% to about 45%. In a preferred form, the electrolyte is at equilibrium with respect to the molar ratio of DMC:EMC:DEC. This electrolyte is described in detail in U.S. Pat. No. 6,746,804 to Gan et al., which is assigned to the assignee of the present invention and incorporated herein by reference.
The above described primary and secondary cells are in the form of one or more cathode plates operatively associated with one or more anode plates. Alternatively, the negative and positive electrodes, both in strip form, are provided with an intermediate separator and wound together in a “jellyroll” type configuration or “wound element cell stack” such that the negative electrode is on the outside of the roll to make electrical contact with the caseing in a case-negative configuration. Using suitable top and bottom insulators, the wound cell stack is inserted into a metallic case of a suitable size dimension. The metallic case may comprise materials such as stainless steel, mild steel, nickel-plated mild steel, titanium, tantalum or aluminum, but not limited thereto, so long as the metallic material is compatible for use with the other cell components.
The cell header comprises a metallic disc-shaped body with a first hole to accommodate a glass-to-metal seal/terminal pin feedthrough and a second hole for electrolyte filling. The glass used is of a corrosion resistant type having up to about 50% by weight silicon such as CABAL 12, TA 23, FUSITE 425 or FUSITE 435. The positive terminal pin feedthrough preferably comprises titanium although molybdenum, aluminum, nickel alloy, or stainless steel can also be used. The cell header is typically of a material similar to that of the case. The positive terminal pin supported in the glass-to-metal seal is, in turn, supported by the header, which is welded to the casing. The cell is thereafter filled with the electrolyte solution described hereinabove and hermetically sealed such as by close-welding a stainless steel ball over the fill hole, but not limited thereto.
While a case-negative design is preferred, it is well known to those skilled in the art that the present electrochemical systems can also be constructed in case-positive configurations.
It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.
