Patent Application
20070281213
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying Drawings in which:
Applicant has discovered a novel cathode design for a Li/CFx cell whose electrochemically active materials comprise only CFx, yet which produces a characteristic voltage that may be used to predict remaining energy capacity as the cell discharges during service. In particular, by constructing the cathode with a non-homogenous blend of CFx materials, the voltage discharge curve of the resultant cell will be altered from the characteristic shape associated with Li/CFx cells shown in
The inventive cathode comprises a blend of a first electrochemically active fluorinated carbon material and one or more additional electrochemically active fluorinated carbon materials. The preferred cathode blend comprises two fluorinated carbon materials. However, additional fluorinated carbon materials may be combined to create blends having three or more fluorinated carbon materials. The blended cathode materials comprise fluorinated carbons represented by the formula (CFx)n, where x is a number between 0 and 2 and n is an indefinite number referring to the number of monomer units, and is typically greater than 2. The abbreviation CFx as used throughout the present application refers to the formula (CFx)n as thus defined. The atomic weight of fluorine is 18.998 and the atomic weight of carbon is 12.011. The fluorination level of a given CFx material may be expressed as a percentage that represents the atomic weight contribution of the fluorine (18.998x) divided by the sum of the atomic weight contribution of the fluorine (18.998x) and the atomic weight contribution of the carbon (12.011). Thus, for a C1F1 stochiometry, the fluorination level would be 18.998/(18.998+12.011)=61.3%.
CFx is conventionally prepared from the reaction of fluorine gas with a crystalline or amorphous carbon. Graphite is an example of a crystalline form of carbon, while petroleum coke, coal coke, carbon black and activated carbon are examples of amorphous carbon. The reaction between fluorine and carbon is usually carried out at temperatures ranging from 250° C. to 600° C. in a controlled pressure environment. The reaction time is usually in the range of 1 to 24 hours. A variety of CFx materials are available from commercial sources, including materials derived from the fluorination of petroleum coke, carbon black and graphite.
Fluorinated carbons that may be used in forming a blended cathode as disclosed herein include fluorinated carbons that are based on different carbonaceous starting materials. For example, a blended cathode in accordance with the invention can be formed by blending fluorinated petroleum coke and fluorinated carbon black. Fluorinated petroleum coke is the most commonly used form of fluorinated carbon for Li/CFx cells and this material is described in numerous patents relating to battery construction and operation in the field of implantable medical use. The fluorinated petroleum coke for use in the present invention is preferably fully fluorinated to a fluorination level of approximately 60-62%. However, other fluorination levels could potentially also be used.
Fluorinated carbon black has been used as a cathode constituent in Li/CFx cells, as disclosed, for example, in U.S. Pat. No. 3,700,502 of Watanabe et al. and U.S. Pat. No. 4,271,242 of Toyoguchi et al. However, the Toyoguchi et al patent advises that an active carbon such as carbon black may be so amorphous as to have a discharge utility factor (60%) that is substantially less than that of fluorinated petroleum coke (90%), which is more crystallized. Fluorinated carbon black is also mentioned in the prior art as being combinable with fluorinated petroleum coke in order to improve the characteristic initial voltage discharge suppression typically associated with Li/CFx cells. See U.S. Pat. No. 4,765,968 of Shia et al., discussing Japanese Kokei No. 83 05,967 at column 3, lines 3-15. Shia et al. assert, however, that “ . . . fluorinated carbon blacks perform more poorly, particularly at high discharge rates, than do coke based fluorinated materials”, citing N. Watanabe et al., SOLID STATE IONICS, page 503 (1980). Shia et al. go on to caution that “[I]n light of this disclosure, it cannot be expected that fluorinated carbon black would be as effective in curing the voltage suppression phenomenon as is a coke-based fluorinated material.”Notwithstanding these teachings against the use of fluorinated carbon black in Li/CFx cells, applicant has determined that this material is effective in the context of the present invention, where the problem is one of providing a reliable elective replacement or end-of-service indication while maximizing energy density. The fluorinated carbon black for use in the present invention is preferably fully fluorinated to a fluorination level of approximately 60-65%. This range extends beyond the preferred fluorination range of the fluorinated petroleum coke material used in the inventive cathode disclosed herein. However, other fluorination levels could potentially also be used.
Any suitable mixing ratio may be used to blend the fluorinated CFx materials for use in the inventive cathode. For example, in a two-material blend comprising fluorinated petroleum coke and fluorinated carbon black, the weight ratio of the fluorinated petroleum coke to the fluorinated carbon black may be approximately 2:1. As described by way of example below, a cathode blended in this matter produced an Li/CFx cell whose voltage discharge curve exhibited a gradual negative slope. The use of other weight ratios for the cathode materials will no doubt produce different voltage discharge curves, as will the use of cathode material blends other than a mixture of fluorinated petroleum coke and fluorinated carbon black, or which include more than two fluorinated carbon materials.
Cathodes in accordance with the present invention will preferably include the usual non-electrochemically active materials, such as a conductivity enhancer and a binder. A preferred conductivity enhancer is acetylene black, although other materials such as carbon black, graphite or mixtures thereof may also be used. Metals such as nickel, aluminum, titanium and stainless steel in powder form may likewise be used. The binder is preferably an aqueous dispersion of a fluorinated resin material, such as a polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) emulsion. For the present invention, an inert PTFE emulsion is the preferred binder material. Any suitable mixing ratio of the fluorinated carbon blend, the conductivity enhancer and the binder may be used. For example, as described by way of example below, a cathode in accordance with the present invention may comprise 86% of the fluorinated carbon material, 8.6% conductivity enhancer and 5.2% binder.
During fabrication of the CFx cathode, the fluorinated carbon materials, which come in powder form, are mixed with the conductive additive. The CFx and conductive filler are then combined with the binder and blended together by hand (e.g., mortar and pestle) or by using a mixer or ball mill. During this processing, the blend may be wetted with a suitable liquid phase pore-former that is readily removable from the blend following processing. An exemplary pore-former comprises a 1:1 ratio of isopropyl alcohol and water that can be volatilized by conventional means at temperatures between 50° C. and 250° C. to convert the mixture into a desired cathode structure. The pore-former can be added to the mixture in a quantity representing 10% by weight of the total admixture. The wetted cathode mixture is intimately blended and then pressed into a homogeneous cathode sheet or otherwise shaped. If pressed into sheet form, the dried material can be processed into pellets that may then be converted by conventional polymer processing techniques into desired cathode shapes. The pore-forming liquid can be volatilized following pressing or during subsequent processing. After fabrication of the shaped cathode, the cathode may then be pressed or otherwise affixed onto a suitable positive current collector selected from the group consisting of stainless steel, titanium, tantalum, platinum and gold.
An Li/CFx cell can be fabricated using the inventive cathode in accordance with conventionally known battery construction techniques. As is well known in the art, such cells make use of an electrochemically active anode coupled to the electrochemically active cathode by way of a non-aqueous electrolyte. The electrolyte serves as a medium for the migration of ions in atomic or molecular form between the anode and the cathode during the cell electrochemical reactions, resulting in a negative charge at the anode and a positive charge at the cathode.
The active anode material of an Li/CFx cell may comprise lithium, potassium, sodium, calcium, magnesium, aluminum, or other light metals found in Groups IA, IIA and IIB of the Periodic Table of Elements, together with alloys and intermetallic compounds thereof. Lithium is preferred for use in the present invention because of its ductility and ease of assembly, and because it possesses a high energy-to-weight ratio. The anode, which is typically formed as a thin sheet or foil comprising the anode material, is generally mounted to a metallic backing element that acts as a negative anode current collector. Materials that are commonly used for the anode current collector in Li/CFx cells include titanium, titanium alloy or nickel.
Li/CFx cells utilize non-aqueous electrolytes. As used herein, the term “non-aqueous” allows for the presence of minor amounts of water as an impurity. Such minor amounts of water can be present in the electrolyte because of the insolubility of fluorinated carbons, even with respect to water. The non-aqueous electrolyte used in Li/CFx cells typically comprises an inorganic, ionically conductive salt dissolved in a non-aqueous solvent to produce an ionically conductive solution. Lithium tetrafluoroborate (LiBF4) is one exemplary salt that may be used as the electrolyte solute. Other lithium salts may also be used, including LiPF6, LiAsF6, LiSbF6, LiClO4, LiO2, LiAlCl4, LiGaCl4, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF3, LiC6F5SO3, LiO2CCF3, LiSO6F, LiB(C6H5)4, LiCF3SO3, and mixtures of the foregoing. The electrolyte non-aqueous solvent may comprise compounds such as lactones, alykylene carbonates, lactams, polyethers, cyclic ethers, cyclic sulfones, dialkylsulfites, monocarboxylic acid esters, and alkylnitriles. Typical preferred solvents include γ-butyrolactone (GBL), tetrahydrofuran (THF), methyl tetrahydrofuran, sulfolane, ethyl acetate, methyl acetate (MA), diglyme, triglyme, tetraglyme, dimethyl carbonate (DMC), 1,2-dimethyloxyethane (DME), 1,1- and 1,2-diethoxyethane (DEE), 1-ethoxy, 2-methoxyethane (EME), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, N-methyl-pyrrolidinone (NMP), and mixtures of the foregoing. For the present invention, the preferred inorganic, ionically conductive salt is 1.0-1.4 molar lithium tetrafluoroborate and the preferred non-aqueous solvent is GBL.
The electrolyte used in Li/CFx cells is carried in a porous separator disposed between the anode and the cathode. The separator is electrically insulative, chemically unreactive with the electrochemically active materials of the anode and cathode, and insoluble in the electrolyte. The porosity of the separator is sufficient to allow the electrolyte to flow therethrough during the cell's electrochemical reactions. Exemplary separator materials include fabrics woven from polypropylene and fluropolymeric fibers including polyvinylidene fluoride, polyethylenetetrafluorethylene, and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film or membrane, non-woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, and polytetrafluoroethylene (PTFE) or polypropylene films and membranes. A preferred separator for use in the present invention comprises polypropylene non-woven fabric or cloth and a superimposed microporous polypropylene film. Preferably, the non-woven fabric faces the cathode and the poypropylene microporous film faces the anode. In this orientation, the non-woven fabric will act as a wicking material to more effectively wet the cathode. It will also serve as a barrier to protect against puncture of the polypropylene film due to loose carbon particles.
A CFx cathode was constructed using a non-homogenous blend of fluorinated petroleum coke and fluorinated carbon black as described above. The fluorinated petroleum coke was produced through the direct fluorination of petroleum coke to a yield a material with a fluorine content of 60% to 62%. This material is available from Lodestar Company of Howell, N.J. under the product designation “PC/10.” The fluorinated carbon black was produced through the direct fluorination of carbon black to yield a material with a fluorination level of 60% to 65%. This material is available from Lodestar Company under the product designation “CB65.” The two fluorinated carbon materials were blended at a weight ratio of two parts fluorinated petroleum coke to one part fluorinated carbon black to yield a homogeneous, electrochemically active cathode material. The blended material was uniformly mixed with acetylene black carbon to increase the electrical conductivity of the final cathode mixture. A PTFE emulsion (where the PTFE content is 60% in an aqueous suspension) was added to the cathode admixture to act as an inert binder. The composition (by weight) of the resulting cathode mixture was 86.2% cathode active material, 8.6% acetylene black and 5.2% solid PTFE. An additional liquid phase comprising a 1:1 ratio of isopropyl alcohol and water was added to the mixture in a quantity representing 10% by weight of the total admixture. This cathode mixture was intimately blended by means of a mortar and pestle and pressed into a homogeneous cathode sheet.
The resulting paste was pressed to a thickness of 0.010″ and then dried at 80° C. for a period of 16 hours. Individual cathode components were cut from the sheet. A lithium metal anode was cut to a similar size. The thickness of the anode was selected so that the mass of the anode had an equivalent electrochemical capacity in excess of that of the corresponding cathode, thus achieving a cathode limited design. Disposed between the anode and cathode was a micro-porous polypropylene separator that maintained a physical separation between the electrodes.
An electrolyte comprising a 1 molar solution of lithium tetrafluoroborate in a γ-butyrolactone solvent was added to the separator 16 in a quantity not less than ten times the weight of the cathode component. The rods 18 and 24 were pressed together until the spring 22 was at 50% of its free length to ensure intimate contact between the active cell materials during the cell discharge. Both of the rods 18 and 24 were sealed to the union 26 by means of a compression seal (not shown).
The assembled cell was allowed to stabilize at 37° C. for 16 hours following assembly. Constant resistance discharge was then achieved through the connection of the positive polarity rod 18, which is in electrical contact with the CFx cathode 12, to the negative polarity rod 24, with is in electrical contact with the lithium anode 14, through a 100,000 Ω resistor to achieve a discharge rate of 0.027 milliamperes.
The resulting discharge curve is displayed as curve 40 in
The discharge curve 50 of
Turning now to
Turning now to
Accordingly, the use of two or more fluorinated carbon materials to create a blended carbonaceous cathode for a lithium primary cell has been disclosed. The blended cathode results in a cell with a discharge voltage characteristic that allows the prediction of remaining cell capacity by way of a simple voltage measurement. It should, of course, be understood that the description and the drawings herein are merely illustrative, and it will be apparent that the various modifications, combinations and changes can be made of these materials disclosed in accordance with the invention. It should be understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.
