CA2311876C - Lithium battery having improved current collecting means - Google Patents
Lithium battery having improved current collecting means Download PDFInfo
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- CA2311876C CA2311876C CA002311876A CA2311876A CA2311876C CA 2311876 C CA2311876 C CA 2311876C CA 002311876 A CA002311876 A CA 002311876A CA 2311876 A CA2311876 A CA 2311876A CA 2311876 C CA2311876 C CA 2311876C
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- current collector
- carbon
- lithium battery
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- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 title claims abstract description 54
- 229910052744 lithium Inorganic materials 0.000 title claims abstract description 54
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims abstract description 97
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 92
- 239000000203 mixture Substances 0.000 claims abstract description 59
- 239000002245 particle Substances 0.000 claims abstract description 47
- 239000011248 coating agent Substances 0.000 claims abstract description 32
- 238000000576 coating method Methods 0.000 claims abstract description 32
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 claims description 19
- 229910001416 lithium ion Inorganic materials 0.000 claims description 19
- 229910052751 metal Inorganic materials 0.000 claims description 16
- 239000002184 metal Substances 0.000 claims description 16
- 239000011230 binding agent Substances 0.000 claims description 15
- 229920002313 fluoropolymer Polymers 0.000 claims description 14
- 239000007774 positive electrode material Substances 0.000 claims description 14
- 150000001875 compounds Chemical class 0.000 claims description 11
- 239000003792 electrolyte Substances 0.000 claims description 11
- 238000009830 intercalation Methods 0.000 claims description 9
- 229920001577 copolymer Polymers 0.000 claims description 8
- 239000011888 foil Substances 0.000 claims description 8
- 229910052723 transition metal Inorganic materials 0.000 claims description 8
- 238000002844 melting Methods 0.000 claims description 7
- 230000008018 melting Effects 0.000 claims description 7
- 239000002131 composite material Substances 0.000 claims description 5
- -1 transition metal chalcogenide Chemical class 0.000 claims description 5
- 239000002033 PVDF binder Substances 0.000 claims description 4
- 229920002981 polyvinylidene fluoride Polymers 0.000 claims description 4
- 239000006104 solid solution Substances 0.000 claims description 3
- KRHYYFGTRYWZRS-UHFFFAOYSA-M Fluoride anion Chemical compound [F-] KRHYYFGTRYWZRS-UHFFFAOYSA-M 0.000 abstract description 2
- 239000002482 conductive additive Substances 0.000 abstract 2
- 239000010410 layer Substances 0.000 description 26
- 229920000642 polymer Polymers 0.000 description 16
- 238000000034 method Methods 0.000 description 8
- 229910052782 aluminium Inorganic materials 0.000 description 7
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 7
- 239000011263 electroactive material Substances 0.000 description 7
- 229910002804 graphite Inorganic materials 0.000 description 6
- 239000010439 graphite Substances 0.000 description 6
- 239000002904 solvent Substances 0.000 description 6
- CSCPPACGZOOCGX-UHFFFAOYSA-N Acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 description 5
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 4
- 239000006229 carbon black Substances 0.000 description 4
- 239000000835 fiber Substances 0.000 description 4
- 239000008187 granular material Substances 0.000 description 4
- 230000001965 increasing effect Effects 0.000 description 4
- 239000003273 ketjen black Substances 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 238000005259 measurement Methods 0.000 description 4
- 229920005569 poly(vinylidene fluoride-co-hexafluoropropylene) Polymers 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- 239000010936 titanium Substances 0.000 description 4
- 229910052719 titanium Inorganic materials 0.000 description 4
- 150000003624 transition metals Chemical class 0.000 description 4
- SECXISVLQFMRJM-UHFFFAOYSA-N N-Methylpyrrolidone Chemical compound CN1CCCC1=O SECXISVLQFMRJM-UHFFFAOYSA-N 0.000 description 3
- YXFVVABEGXRONW-UHFFFAOYSA-N Toluene Chemical compound CC1=CC=CC=C1 YXFVVABEGXRONW-UHFFFAOYSA-N 0.000 description 3
- 239000002003 electrode paste Substances 0.000 description 3
- 238000002156 mixing Methods 0.000 description 3
- 239000004033 plastic Substances 0.000 description 3
- 229920003023 plastic Polymers 0.000 description 3
- 229920000131 polyvinylidene Polymers 0.000 description 3
- 230000001737 promoting effect Effects 0.000 description 3
- 239000000523 sample Substances 0.000 description 3
- 239000000243 solution Substances 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- 239000002318 adhesion promoter Substances 0.000 description 2
- 230000000712 assembly Effects 0.000 description 2
- 238000000429 assembly Methods 0.000 description 2
- 238000009835 boiling Methods 0.000 description 2
- 239000006182 cathode active material Substances 0.000 description 2
- 150000004770 chalcogenides Chemical class 0.000 description 2
- CKFRRHLHAJZIIN-UHFFFAOYSA-N cobalt lithium Chemical compound [Li].[Co] CKFRRHLHAJZIIN-UHFFFAOYSA-N 0.000 description 2
- 230000001427 coherent effect Effects 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000001035 drying Methods 0.000 description 2
- 238000003487 electrochemical reaction Methods 0.000 description 2
- 230000002708 enhancing effect Effects 0.000 description 2
- 239000010954 inorganic particle Substances 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 229910000625 lithium cobalt oxide Inorganic materials 0.000 description 2
- BFZPBUKRYWOWDV-UHFFFAOYSA-N lithium;oxido(oxo)cobalt Chemical compound [Li+].[O-][Co]=O BFZPBUKRYWOWDV-UHFFFAOYSA-N 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 239000011255 nonaqueous electrolyte Substances 0.000 description 2
- 239000002491 polymer binding agent Substances 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- 229910000733 Li alloy Inorganic materials 0.000 description 1
- 229910001290 LiPF6 Inorganic materials 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- 229920003171 Poly (ethylene oxide) Polymers 0.000 description 1
- 150000001242 acetic acid derivatives Chemical class 0.000 description 1
- 229920006397 acrylic thermoplastic Polymers 0.000 description 1
- 239000000654 additive Substances 0.000 description 1
- 230000000996 additive effect Effects 0.000 description 1
- 229910052783 alkali metal Inorganic materials 0.000 description 1
- 229910001413 alkali metal ion Inorganic materials 0.000 description 1
- 150000001340 alkali metals Chemical class 0.000 description 1
- 239000006183 anode active material Substances 0.000 description 1
- 230000001680 brushing effect Effects 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011651 chromium Substances 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 239000011247 coating layer Substances 0.000 description 1
- 239000000571 coke Substances 0.000 description 1
- 239000002322 conducting polymer Substances 0.000 description 1
- 229920001940 conductive polymer Polymers 0.000 description 1
- 239000000356 contaminant Substances 0.000 description 1
- 239000012895 dilution Substances 0.000 description 1
- 238000010790 dilution Methods 0.000 description 1
- 230000003292 diminished effect Effects 0.000 description 1
- 238000007598 dipping method Methods 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 239000012799 electrically-conductive coating Substances 0.000 description 1
- 239000011883 electrode binding agent Substances 0.000 description 1
- 239000007772 electrode material Substances 0.000 description 1
- 239000003623 enhancer Substances 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 125000002573 ethenylidene group Chemical group [*]=C=C([H])[H] 0.000 description 1
- 239000010419 fine particle Substances 0.000 description 1
- 239000012530 fluid Substances 0.000 description 1
- 150000002222 fluorine compounds Chemical group 0.000 description 1
- 230000005484 gravity Effects 0.000 description 1
- 239000004519 grease Substances 0.000 description 1
- 238000010348 incorporation Methods 0.000 description 1
- AMGQUBHHOARCQH-UHFFFAOYSA-N indium;oxotin Chemical compound [In].[Sn]=O AMGQUBHHOARCQH-UHFFFAOYSA-N 0.000 description 1
- 150000002642 lithium compounds Chemical class 0.000 description 1
- 229910002102 lithium manganese oxide Inorganic materials 0.000 description 1
- 229910003002 lithium salt Inorganic materials 0.000 description 1
- 159000000002 lithium salts Chemical class 0.000 description 1
- VLXXBCXTUVRROQ-UHFFFAOYSA-N lithium;oxido-oxo-(oxomanganiooxy)manganese Chemical compound [Li+].[O-][Mn](=O)O[Mn]=O VLXXBCXTUVRROQ-UHFFFAOYSA-N 0.000 description 1
- 239000013528 metallic particle Substances 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- 239000003960 organic solvent Substances 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 239000002006 petroleum coke Substances 0.000 description 1
- 229920003229 poly(methyl methacrylate) Polymers 0.000 description 1
- 229920005596 polymer binder Polymers 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 230000027756 respiratory electron transport chain Effects 0.000 description 1
- 238000007788 roughening Methods 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- ACXGJHCPFCFILV-UHFFFAOYSA-M sodium;2-(4-chloro-2-methylphenoxy)acetate;3,6-dichloro-2-methoxybenzoic acid Chemical compound [Na+].COC1=C(Cl)C=CC(Cl)=C1C(O)=O.CC1=CC(Cl)=CC=C1OCC([O-])=O ACXGJHCPFCFILV-UHFFFAOYSA-M 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 150000004763 sulfides Chemical class 0.000 description 1
- ISXSCDLOGDJUNJ-UHFFFAOYSA-N tert-butyl prop-2-enoate Chemical compound CC(C)(C)OC(=O)C=C ISXSCDLOGDJUNJ-UHFFFAOYSA-N 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 229910000314 transition metal oxide Inorganic materials 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/621—Binders
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/624—Electric conductive fillers
- H01M4/625—Carbon or graphite
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Landscapes
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Battery Electrode And Active Subsutance (AREA)
- Secondary Cells (AREA)
Abstract
Lithium electrochemical cells and batteries are described having electron conductive additives in the form of a mixture of carbon fibres and fine carbon particles. The electron conductive additives are provided in a polymeric fluoride coating between the electrode and the current collector. A
mixture of carbon fibres and fine carbon can also be admixed with the cathode-active component in the cathode.
mixture of carbon fibres and fine carbon can also be admixed with the cathode-active component in the cathode.
Description
Title: LITHIUM BATTERY HAVING IMPROVED CURRENT COLLECTING
MEANS
FIELD OF IHVSNTION
This invention is related to alkali metal electrochemical cells, in particular to lithium electrochemical cells or batteries.
BACKGROUND OF THE INVENTION
Electrochemical cells able to convert chemical to electrical energy are a well known source of energy.
Electrochemical cells which are based on alkali metal ions, more specifically on lithium ions, have been found to have very advantageous characteristics, such as high energy density per unit volume or unit weight. Lithium electrochemical cells can operate as primary cells, but more often utilization is in the form of secondary, rechargeable cells or rechargeable batteries. Lithium batteries or cells may be button shaped, cylindrically or prismatically wound, or flat, composed of layers, known as planar or laminar cells or planar batteries.
Lithium electrochemical cells or batteries include a negative electrode or anode, containing a substance capable of intercalating lithium, or lithium metal or an alloy of lithium, as the negative active component. The positive active component of the cathode is usually a chalcogenide of a transition metal and lithium, such as lithium-manganese oxide, lithium-cobalt oxide, or similar compounds which are stable under the conditions of operation and can incorporate lithium ions in their structure reversibly. A lithium cell and/or battery commonly has a non-aqueous electrolyte which may be a porous separator impregnated with an organic liquid or gel containing a dissolved lithium salt, or a solid polymer laminate containing a dissociable lithium compound, or granules of electrolyte mixed with particles containing one of the electrode active compounds. Lithium batteries are usually equipped with current collectors in close proximity of the electrodes, which can be a metal plate, rod, metal foil carried by a polymer laminate, or a similar electrically conducting structural element.
MEANS
FIELD OF IHVSNTION
This invention is related to alkali metal electrochemical cells, in particular to lithium electrochemical cells or batteries.
BACKGROUND OF THE INVENTION
Electrochemical cells able to convert chemical to electrical energy are a well known source of energy.
Electrochemical cells which are based on alkali metal ions, more specifically on lithium ions, have been found to have very advantageous characteristics, such as high energy density per unit volume or unit weight. Lithium electrochemical cells can operate as primary cells, but more often utilization is in the form of secondary, rechargeable cells or rechargeable batteries. Lithium batteries or cells may be button shaped, cylindrically or prismatically wound, or flat, composed of layers, known as planar or laminar cells or planar batteries.
Lithium electrochemical cells or batteries include a negative electrode or anode, containing a substance capable of intercalating lithium, or lithium metal or an alloy of lithium, as the negative active component. The positive active component of the cathode is usually a chalcogenide of a transition metal and lithium, such as lithium-manganese oxide, lithium-cobalt oxide, or similar compounds which are stable under the conditions of operation and can incorporate lithium ions in their structure reversibly. A lithium cell and/or battery commonly has a non-aqueous electrolyte which may be a porous separator impregnated with an organic liquid or gel containing a dissolved lithium salt, or a solid polymer laminate containing a dissociable lithium compound, or granules of electrolyte mixed with particles containing one of the electrode active compounds. Lithium batteries are usually equipped with current collectors in close proximity of the electrodes, which can be a metal plate, rod, metal foil carried by a polymer laminate, or a similar electrically conducting structural element.
The energy output of a cell or battery is strongly affected by the manner of collecting the current generated by the electrochemical reaction. There are known methods to improve contact between the metallic current collectors and the adjacent electrodes. Methods of etching, cleaning or micro-roughening the surface of the current collector in contact with the electrode have been described. Inserting a separate layer of an electronically conducting polymer or a polymer loaded with electrically conducting carbonaceous or inorganic particles, between the metallic current collector and the electrode of the electrochemical cell are known.
Methods are also known to increase adhesion between the electrode and the current collector. The adhesion promoting layer can be coated onto the current collector as a polymer coating carrying conductive particles, which may be subsequently cured by known methods or the solvent in the polymer solution is evaporated. The electrically conducting particles in the adhesion promoting coating are most frequently fine carbon or carbon black, but may be fine particles of metal or electrically conducting oxides, or such like. Adding conductive binders or admixing carbon or metallic particles or metal coated carbon fibres to increase the conductivity of the electrode mixture itself are known.
Examples of lithium batteries incorporating some of the above discussed features are provided in United States patents 5,464,707, issued to Moulton et al. on November 7, 1995, in U.S. 5,589,297, issued to Koga et al. on December 31, 1996, and in U.S. patent 5,554,459, issued to Gozdz et al. on September 10, 1996. U.S. patent 5,824,120, issued to Mitchell et al. on October 20, 1998, describes an electrically conductive adhesion promoter layer located between the current collector surface and the cathode. The adhesion promoter layer of Mitchell et al. is made of a polymeric material, such as acrylics or acetates, containing conductive particles which may be graphite particles, hollow carbon fibres, ultrafine metal powders, metal coated carbon fibres, metal coated or conductive polymeric fibres or inorganic particles, such as indium tin oxide.
Methods are also known to increase adhesion between the electrode and the current collector. The adhesion promoting layer can be coated onto the current collector as a polymer coating carrying conductive particles, which may be subsequently cured by known methods or the solvent in the polymer solution is evaporated. The electrically conducting particles in the adhesion promoting coating are most frequently fine carbon or carbon black, but may be fine particles of metal or electrically conducting oxides, or such like. Adding conductive binders or admixing carbon or metallic particles or metal coated carbon fibres to increase the conductivity of the electrode mixture itself are known.
Examples of lithium batteries incorporating some of the above discussed features are provided in United States patents 5,464,707, issued to Moulton et al. on November 7, 1995, in U.S. 5,589,297, issued to Koga et al. on December 31, 1996, and in U.S. patent 5,554,459, issued to Gozdz et al. on September 10, 1996. U.S. patent 5,824,120, issued to Mitchell et al. on October 20, 1998, describes an electrically conductive adhesion promoter layer located between the current collector surface and the cathode. The adhesion promoter layer of Mitchell et al. is made of a polymeric material, such as acrylics or acetates, containing conductive particles which may be graphite particles, hollow carbon fibres, ultrafine metal powders, metal coated carbon fibres, metal coated or conductive polymeric fibres or inorganic particles, such as indium tin oxide.
Carbon or Qraphitic particles capable o! intercalating lithium ions or carbon tibres are frequently utilized in the anode mixes! into a paste with an sonically conducting binder.
ohsaki et al. in V.S. 5,856,043 teach a lithium cell anode made of 87 - 95 ~ vapour-grown graphitized carbon fibres mixed with a binder. The conductivity of the cathode is adjusted by the addition of carbon black.
~s discussed above, fine carbon particles have been deployed as a means of increasing electronic conductivity between the current collector and the positive active material, either in a coating or layer directly in contact with the current collector, or mixed with the oxidic or sulphidic electrode-active material. The individual particles of carbon, however, may be isolated trom one another or fro' the current collector by other components in the mixture forming the electrode coating or the electrode paste, thus reducing the effectiveness of electron transfer. Moreover, the resistance of current collector surface in touch with the electrode or the conductivity enhancing layer may have local ZO variations, hence increase in lateral conductivity can be a desirable feature.
StJI~RY OF TH8 II~IVBliTIOI~
There ie a need for improving electrical contact between the current collector and the positive active material o! the Zs cathode in a lithiu~ battery. For the best energy utilization of lithium cells and batteries easy electron passage is required both in the shortest distance between the electrode and the current collector as well as in.interal directions.
By one aspect o! the invention a mixture of tine carbon 3D and carbon fibres are added to a fluorinated polymer having melting point higher than 90'C, to form a mixture which is coated on the current collector surface in contact with the positive electrode of a lithium electrochemical cell.
Hy another aspect o! the invention a mixture of fine 35 carbon and carbon fibres, is blended with the positive active material and optionally with an sonically conductive binder compound, to provide a cathode aaterial o! a lithium battery.
' - 4 -BRIEF DESCRIPTION OF THE DRAWINGS
Fig.i shows schematically conductivity measurements taken in the lateral direction on a conductive particles containing coating.
The preferred embodiments of the invention illustrated by working examples will be described below.
DETAILED DESCRIPTION OF THE PREFERRED EMHODINENTS
The essential component of an electrode of an electrochemical cell is the electroactive material. The electroactive material of the positive electrodes in lithium electrochemical cells is usually a compound which is capable of reversibly incorporating lithium ions in its structure.
Frequently used electroactive materials include transition metal oxides, mixed oxides of transition metals, sulphides of transition metals, or solid solutions of chalcogenides of transition metals with other metals, or any compound which is capable of reversibly intercalating lithium ions and is stable at the temperature of operation of the electrochemical cell.
The above compounds are most often utilized as cathode active materials, but can be used as anode active materials as well, under certain circumstances.
The electroactive materials, in particular the positive active materials, usually in the form of small particles are mixed with a binder compound to form a paste. A frequently used binder is a fluoride containing polymer, such as a vinylidene fluoride copolymer, in particular a vinylidene fluoride - hexafluoropropylene copolymer. The binder is usually admixed in less than 12 wt.%, more conveniently in about 3-8 wt.%.
The electroactive material bearing mixture is subsequently brought in contact with an oxidation and corrosion resistant metal current collector. Commonly used current collectors for positive active material containing electrodes are sheets of aluminum, stainless steel high in molybdenum and chromium, titanium, graphite sheet, foils of aluminum, grid or woven mesh of filaments made of the above metals, or aluminum foil carried on a tough, preferably moisture impermeable, polymeric laminate. The surface of the 1f -currant collector is usually cleaned of corrosion products or grease, or similar contaminants, before contacting the electrode. The current collectors conveniently have an extension, such as a tab or lead, which provide means for the lithium battery or electrochemical cell to be connected to electrical equipment for charging or discharging.
As discussed hereinabove, most electrochemical cells or batteries provide a path for the electrons generated by the electrochemical reaction to proceed to and from the current collectors. 8lectrically conductive particles, most frequently tine carbon particles are mixed with the electrode components. It has, however, been observed that the binder in the electroactive material-binder mixture may coat some of the fine carbon partioles. Commonly used binder compounds are insulntore or sonically aonduoting materials, thus the binder coat may reduce the conductivity of the line carbon particles added specifically to diminish the impedance of the mixture.
~s has been mentioned above, in some of the known lithium cell assemblies a carbon particle bearing layer is placed between the electrode, usually the cathode, and the current collector surface for increasing or promoting adhesion. The film forming component of the layer fs frequently a poorly conducting polymeric material. in such instances some of the carbon particles can be isolated from one another by the Z5 polymer, thus the available paths for electron conduction may be notably diminished. It is known that there may be local variations in the ability of a current collector surface to conduct electrons. Hence lateral conductivity within an electron conductive coating ae wail as in an electrode layer can beneficially affect the impedance of an electrode -current collector assembly. It has now been found that it carbon fibres are added to the electroactive material containing mixture, and to the material forming a layer or coating between the current collector and the electrode layer, the conductivity within the electrochemical cell or battery is increased.
~s briefly noted above, lithiua based electrochemicsl cells have lithium ion conducting non-aqueous electrolytes placed between the r_athod~~ and the anode of the cell. The electrolyte can be a conventional porous or microporous polymer which is impregnated with a lithium sat containing non-aqueous liquid, or it can be a so=i.id or gelled polymer containing a dissociable lithium <:ompc.~und. Alt:ernat_ively, the particles of electroactive mate~:ial a.re mixed with granules of a mobile lithium ion c~~ntaining p,:>lymer and the mixture is loaded on a l~ metal current col.lector_, thus providing a composite current collector-electrode--ele~t~:vo:!~rte layer. Such composites are usually separated by ~ porous separator from the other electrode. There are several known arrangements for assembling lithium ion based electrochemical cells having an anode, a IS cathode, lithium ior~ conducting ei.ectrol~rte and current collectors. The present invent.i.on is direct:ed to the utilization of mixtures o~ carbon fir>rres and fine carbon, in enhancing the passage o~: elect:rons between the electroactive component, in particular tr~e~ positive a~t ive component, and the current collector, in planaa or laminar lithium cells. However, such carbonaceous conductive particles may be used in other types of batte:ri es a:cs wel l .
In one embodiment oa the present: invention the internal conductivity, in ocher wo.rcis, the elec_ron transfer between the ZS electrode of the lithium bat:tery and the current collector is enhanced by p:Lacincx a mixt:~ire of carbc~r_ fibres and fine carbon along the interface between the electrode and the current collector. The mixture a:f carbon fibres and fine carbon are admixed in a fluor mated pc; lyzner and coated on the face of the current collector which i_s in conr_act with the positive electrode of the ~ ithium cell or bat::ery. The type of fine carbon in thi:~ utilization :n<3y be any f~_re carbon such as carbon black, Shawini.gan bla<~k, ac:et.yl.ene black or any carbon product having similar siz~:e range. A preferred carbon black contains ~5 predominantly elon<:tated cr-,~stalline r-:~3rbon particles, such as marketed by Akzo Nobe~1 ~...':hemicals Lnc. under the tradename of "Ketjen Black"*. The convenient carbon fibre for this application has less than LG ~m diamet:er arid average length, preferably, longer than 3r: um, t:hat is, the aspect ratio of the fibres is in excess o', 5, and prreferabl_y in * Trade-mark eXCess'of T0.' The carbon fibres or mixture of carbon fibres ani~ f ink carbarr 'are 'blended with a fluorinated polymer, most converiie.ntly with a vinylidene~fluor'ide containing copolymer, referred to hereinafter as polyvinylidene fluoride copolymer.
It is preferred that the fluorinated polymer has melting point higher than 70°C. The blended fluorinated polymer - carbon particle bearing coating mixture preferably contains higher than 60 wt.% Carbonaceous particles. The upper limit of the wcarbon content is dictated by coriveriience only. The mixture of carbonaceous particles and fluorinated polymer is coated 'on the face of the current collector designed to be~ in contact with the 'positive electrode in a 'thickness ~o yield a coherent layer of less than 25 /gym, preferably less than l5~em. To enhance the ease of application of the mixture to the current ~ collector face, it~may be convenient to add a few percents of commercially available, low boiling point solvent, such as toluene, acetone, n-methyl pyrrolidinone also known as NMP, and/or similar compatible organic solvent. The coating is applied by brushing or dipping, and is allowed to dry before contact is made with the electrode. The coated current collector may be heated to accelerate drying. The~preferred composition of the carbonaceous particles in the coating is 5-15 wt.% carbon fibres, the balance being fine carbon. The carbon fibre and fine carbon mixture is blended with the fluorinated polymer~solution in~a conventional manner.
' In another embodiment of the invention the electron conductivity of the electrode~of a lithium cell or battery is enhanced by admixing a mixture of carbon fibres and fine carbon in the electrode components. As discussed above, the electrode commonly comprises electroactive particles, an electron conducting component and a binder, preferably an ionically conducting binder compound, in an amount sufficient to provide a coherent paste. The electron conducting component is added in less than 12 wt.%, and usually higher than 3 wt.%, however, this is dictated by particle size and specific gravity of the electrode components.
It i~s noted that the object in assembling a lithium electrochemical cell is to avoid unnecessary dilution of the electroactive materials present in order'to maximize energy density per volume or weight of the cell. In other word, the carbonaceous particles are added to the electrode, and~or to the coating on the current collector surface in amounts sufficient to reduce the impedance of the cell to a desirable . level. It is convenient to utilize the same type of carbon fibres and fine carbon in the electrode as in the conductive coating between the electrode and current collector of the cell,_however, this is not a requirement for the working of the invention. The mixture of carbon fibres and fine carbon, is. applied as a conductivity enhancer ir1 the positive electrodes of a lithium cell, but may be used in a negative electrode as well, depending on the nature of the .,electroactive component of the latter. The cbrbohaceous .particles. in the electrode paste preferably contain carbon ..fibres ~in 5-15 wt. %,. ' based ' on . the total ' weight of the carbonaceous particles added. ~ ~ - .
The cathode paste is loaded on the current collector by doctor s blade method but any convenient commercially available method will do. In one form of utilization the cathode-active material, the carbon fibre - fine carbon mixture and a fluoride containing polymer binder are blended .together for a relatively short period and the obtained paste .is then. loaded onto the current collector to form a~layer which .is usually notably thicker than 50 Vim, bt~t the electrode layex thickness is determined by convenience only. Care is to. be taken that the carbon fibres are got unduly broken ..during blending. The fluorinated polymer having' melting point greater than 70'C, is conveniently dissolved in~a suitable, -low , boiling vpoint solvent.' The' preaerice of ' th~r solvent ' .increases.fluidity in the mixture~and allows blending Without breaking the.fibres. The electrode layer is~allowed to dry, or can be heated to accelerate drying, before it is brought into contact. with a lithium ion-bearing polymer serving as electrolyte. The preferred fluorinated polymer binder in this ' .invention-~is~.a copolymer of polyvinylidene fluoride, n compound.capable of allowing th~ passage of lithium ions. The same .copolymer .of. polyvinylidene fhioride bay be~ uBed iri ~trie - g -electrically conductive carbonaceous particle bearing coating on the current collector, however, this is a matter of choice.
It is noted also that it is not essential that the polymer in the coating which carries the conductive particles be an sonically conductive substance but it may be convenient.
EXAKpLE 1 8g of carbon fibres having 7.5 ~m average diameter and 60 hem average length, or aspect ratio 8, were mixed by hand with 52g petroleum coke. 20 g of polyvinylidene fluoride -hexafluoro propylene copolymer, marketed under the name of Rynar, was made fluid by adding 200 ml NMP-acetone solvent to it. The prepared mixture of carbon fibres and coke was added to the fluidized polymer and blended by hand. The solution of carbon particle containing polymer was painted on one face of an aluminum foil supported on a polymer laminate, to provide a thin, continuous layer. The layer was dried under a heat lamp. A lithium-cobalt oxide, 5 wt.% polyvinylidene fluoride - hexafluoro propylene copolymer and 10 wt.% fine carbon containing conventional positive electrode mixture was applied to the dry, coated surface of the aluminum foil by doctor's blade method to provide a 190 ~m thick electrode-current collector composite for use in a lithium cell.
A positive electrode material made of lithium-cobalt oxide-polyvinylidene copolymer mixture as in Example 1, and a blended mixture of 7 parts of Shawinigan black and 1 part of carbon fibres , the latter carbon particle mixture being added in 10 wt.% based on the weight of the electrode-binder mixture, were combined with NMP-acetone solvent in sufficient amount to permit good mixing of the components. The resulting electrode paste was coated by means of doctor's blade onto another segment of the coated aluminum current collector prepared in Example 1, to yield a 190 pm thick layer. The electrode layer located on current collector was allowed to dry under a heat lamp.
In a comparative test C, a lithium-cobalt oxide-polyvfnylidene fluoride copolymer binder and 10 wt.%
Shawinigan black containing mixture, that is the same basic cathode composition as in Examples 1 and 2, but without the carbon fibres added in the positive electrode in Example 2, was prepared to provide a conventional positive electrode of 190 ~cm thickness, and subsequently was positioned to be in contact with an aluminum foil bearing polymer laminate, for incorporation in a conventional laminar lithium cell.
EX~I~IPLE 3 The positive electrode-current collector assemblies for use in a rechargeable laminar lithium cell, prepared in Examples 1, 2 and comparative test C, and having similar thickness, were each subjected to resistance measurements by known methods, between the surface to be brought in contact with an electrolyte layer, and the current collector.
The average of ten resistance measurements of each electrode-current collector assembly is shown below:
Example 1: 290.10'' ohm. cm Example 2: 180.10'' ohm.cm Test C: 2190.10'' ohm.cm It can be seen that the average resistance of the electrode-current collector assembly having carbon fibres in the electrode mixture and in the coating layer between the electrode and the current collector, is the lowest. The average resistance of a conventional electrode supported on a current collector coated with a carbon fibre-fine carbon mixture bearing polymer is somewhat higher. The average resistance of a conventional electrode-current collector assembly prepared to have the same thickness but containing no carbon fibre in the electrode and having no conductive coating between the current collector and the adjacent electrode, is nearly an order of magnitude higher.
EXAI~IPLg 4 Lateral resistance of an electrically conductive coating carried on an inert non-conductive layer replacing the current collector, and prepared in accordance with this invention, was measured by inserting measuring probes in the coating as shown on Fig.i. Assembly l0 represents schematically an inert plastic strip 12, coated with a carbon fibre-fine graphite-polyvinylidene fluoride-hexafluoro propylene copolymer mixture bearing layer 14, prepared using the same carbon fibre-fine graphite ratio as described in Example 1. The inert non-conducting plastic strip was 100mm wide and long enough to be coated on one of its faces with a 100mm wide and 420mm long layer 14. The thickness of the coating was 40~m. Probes 16' and 16" were inserted below the coating surface close to the edges, as shown. The probes were connected to a high sensitivity Hewlitt Packard 4-point multimeter. 6 readings were taken, giving an average lateral resistance value of 21.48 ohms.
A similar inert plastic strip was coated with a similar conductive particles containing layer in the same length, width and depth as above, but the coating contained only fine graphite. The composition of the coating was 75 wt.% fine graphite mixed into 25 wt. % polyvinylidene f luoride-hexaf luoro propylene copolymer. The average of 6 lateral resistance measurements was found to be 332 ohms.
It can be seen that the presence of the graphite fibres increased the conductivity of the coating by about one order of magnitude.
It is noted that in the above examples polyvinylidene fluoride-hexafluoro propylene copolymer was used as the carrier of the carbon fibres-fine carbon mixtures, however, another fluorinated polymer having melting point higher than 70°C, or its chemical equivalent could equally well have been used.
EX7~MPLE 5 A thin titanium sheet was coated on both faces with a composition made of a mixture of elongated carbon crystallites marketed as 'Ketjen Black', and carbon fibres blended with polyvinylidene fluoride copolymer. The carbon particles were present in 65 wt.%. The carbon particles were a blend of 7 parts of Ket jen Black, and 1 part carbon f ibres having average dimensions as stated in Example 1. The coating was prepared as described in Example 1. A positive electrode mixture was made of components as detailed in Example 2, but having admixed carbon particles composed of Ketjen Black and carbon fibres in the same ratio as in the coating on the titanium current collector. The electrode mixture was blended with granules of polyethylene oxide containing 6% LiPF6. The electrolyte granules had average particle size of 50 Vim. The blended mixture was deposited in a layer over the coated faces of the titanium sheet. The composite current collector-electrode-electrolyte was incorporated in a rechargeable lithium battery. The obtained lithium battery exhibited lower impedance than a similar lithium battery having conventional carbon particles admixed as electron conducting additive.
As was described in the foregoing paragraphs and shown in the examples, carbon fibres added as electron conductive components in lithium cells improve the performance of lithium batteries.
The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. Nowever, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.
ohsaki et al. in V.S. 5,856,043 teach a lithium cell anode made of 87 - 95 ~ vapour-grown graphitized carbon fibres mixed with a binder. The conductivity of the cathode is adjusted by the addition of carbon black.
~s discussed above, fine carbon particles have been deployed as a means of increasing electronic conductivity between the current collector and the positive active material, either in a coating or layer directly in contact with the current collector, or mixed with the oxidic or sulphidic electrode-active material. The individual particles of carbon, however, may be isolated trom one another or fro' the current collector by other components in the mixture forming the electrode coating or the electrode paste, thus reducing the effectiveness of electron transfer. Moreover, the resistance of current collector surface in touch with the electrode or the conductivity enhancing layer may have local ZO variations, hence increase in lateral conductivity can be a desirable feature.
StJI~RY OF TH8 II~IVBliTIOI~
There ie a need for improving electrical contact between the current collector and the positive active material o! the Zs cathode in a lithiu~ battery. For the best energy utilization of lithium cells and batteries easy electron passage is required both in the shortest distance between the electrode and the current collector as well as in.interal directions.
By one aspect o! the invention a mixture of tine carbon 3D and carbon fibres are added to a fluorinated polymer having melting point higher than 90'C, to form a mixture which is coated on the current collector surface in contact with the positive electrode of a lithium electrochemical cell.
Hy another aspect o! the invention a mixture of fine 35 carbon and carbon fibres, is blended with the positive active material and optionally with an sonically conductive binder compound, to provide a cathode aaterial o! a lithium battery.
' - 4 -BRIEF DESCRIPTION OF THE DRAWINGS
Fig.i shows schematically conductivity measurements taken in the lateral direction on a conductive particles containing coating.
The preferred embodiments of the invention illustrated by working examples will be described below.
DETAILED DESCRIPTION OF THE PREFERRED EMHODINENTS
The essential component of an electrode of an electrochemical cell is the electroactive material. The electroactive material of the positive electrodes in lithium electrochemical cells is usually a compound which is capable of reversibly incorporating lithium ions in its structure.
Frequently used electroactive materials include transition metal oxides, mixed oxides of transition metals, sulphides of transition metals, or solid solutions of chalcogenides of transition metals with other metals, or any compound which is capable of reversibly intercalating lithium ions and is stable at the temperature of operation of the electrochemical cell.
The above compounds are most often utilized as cathode active materials, but can be used as anode active materials as well, under certain circumstances.
The electroactive materials, in particular the positive active materials, usually in the form of small particles are mixed with a binder compound to form a paste. A frequently used binder is a fluoride containing polymer, such as a vinylidene fluoride copolymer, in particular a vinylidene fluoride - hexafluoropropylene copolymer. The binder is usually admixed in less than 12 wt.%, more conveniently in about 3-8 wt.%.
The electroactive material bearing mixture is subsequently brought in contact with an oxidation and corrosion resistant metal current collector. Commonly used current collectors for positive active material containing electrodes are sheets of aluminum, stainless steel high in molybdenum and chromium, titanium, graphite sheet, foils of aluminum, grid or woven mesh of filaments made of the above metals, or aluminum foil carried on a tough, preferably moisture impermeable, polymeric laminate. The surface of the 1f -currant collector is usually cleaned of corrosion products or grease, or similar contaminants, before contacting the electrode. The current collectors conveniently have an extension, such as a tab or lead, which provide means for the lithium battery or electrochemical cell to be connected to electrical equipment for charging or discharging.
As discussed hereinabove, most electrochemical cells or batteries provide a path for the electrons generated by the electrochemical reaction to proceed to and from the current collectors. 8lectrically conductive particles, most frequently tine carbon particles are mixed with the electrode components. It has, however, been observed that the binder in the electroactive material-binder mixture may coat some of the fine carbon partioles. Commonly used binder compounds are insulntore or sonically aonduoting materials, thus the binder coat may reduce the conductivity of the line carbon particles added specifically to diminish the impedance of the mixture.
~s has been mentioned above, in some of the known lithium cell assemblies a carbon particle bearing layer is placed between the electrode, usually the cathode, and the current collector surface for increasing or promoting adhesion. The film forming component of the layer fs frequently a poorly conducting polymeric material. in such instances some of the carbon particles can be isolated from one another by the Z5 polymer, thus the available paths for electron conduction may be notably diminished. It is known that there may be local variations in the ability of a current collector surface to conduct electrons. Hence lateral conductivity within an electron conductive coating ae wail as in an electrode layer can beneficially affect the impedance of an electrode -current collector assembly. It has now been found that it carbon fibres are added to the electroactive material containing mixture, and to the material forming a layer or coating between the current collector and the electrode layer, the conductivity within the electrochemical cell or battery is increased.
~s briefly noted above, lithiua based electrochemicsl cells have lithium ion conducting non-aqueous electrolytes placed between the r_athod~~ and the anode of the cell. The electrolyte can be a conventional porous or microporous polymer which is impregnated with a lithium sat containing non-aqueous liquid, or it can be a so=i.id or gelled polymer containing a dissociable lithium <:ompc.~und. Alt:ernat_ively, the particles of electroactive mate~:ial a.re mixed with granules of a mobile lithium ion c~~ntaining p,:>lymer and the mixture is loaded on a l~ metal current col.lector_, thus providing a composite current collector-electrode--ele~t~:vo:!~rte layer. Such composites are usually separated by ~ porous separator from the other electrode. There are several known arrangements for assembling lithium ion based electrochemical cells having an anode, a IS cathode, lithium ior~ conducting ei.ectrol~rte and current collectors. The present invent.i.on is direct:ed to the utilization of mixtures o~ carbon fir>rres and fine carbon, in enhancing the passage o~: elect:rons between the electroactive component, in particular tr~e~ positive a~t ive component, and the current collector, in planaa or laminar lithium cells. However, such carbonaceous conductive particles may be used in other types of batte:ri es a:cs wel l .
In one embodiment oa the present: invention the internal conductivity, in ocher wo.rcis, the elec_ron transfer between the ZS electrode of the lithium bat:tery and the current collector is enhanced by p:Lacincx a mixt:~ire of carbc~r_ fibres and fine carbon along the interface between the electrode and the current collector. The mixture a:f carbon fibres and fine carbon are admixed in a fluor mated pc; lyzner and coated on the face of the current collector which i_s in conr_act with the positive electrode of the ~ ithium cell or bat::ery. The type of fine carbon in thi:~ utilization :n<3y be any f~_re carbon such as carbon black, Shawini.gan bla<~k, ac:et.yl.ene black or any carbon product having similar siz~:e range. A preferred carbon black contains ~5 predominantly elon<:tated cr-,~stalline r-:~3rbon particles, such as marketed by Akzo Nobe~1 ~...':hemicals Lnc. under the tradename of "Ketjen Black"*. The convenient carbon fibre for this application has less than LG ~m diamet:er arid average length, preferably, longer than 3r: um, t:hat is, the aspect ratio of the fibres is in excess o', 5, and prreferabl_y in * Trade-mark eXCess'of T0.' The carbon fibres or mixture of carbon fibres ani~ f ink carbarr 'are 'blended with a fluorinated polymer, most converiie.ntly with a vinylidene~fluor'ide containing copolymer, referred to hereinafter as polyvinylidene fluoride copolymer.
It is preferred that the fluorinated polymer has melting point higher than 70°C. The blended fluorinated polymer - carbon particle bearing coating mixture preferably contains higher than 60 wt.% Carbonaceous particles. The upper limit of the wcarbon content is dictated by coriveriience only. The mixture of carbonaceous particles and fluorinated polymer is coated 'on the face of the current collector designed to be~ in contact with the 'positive electrode in a 'thickness ~o yield a coherent layer of less than 25 /gym, preferably less than l5~em. To enhance the ease of application of the mixture to the current ~ collector face, it~may be convenient to add a few percents of commercially available, low boiling point solvent, such as toluene, acetone, n-methyl pyrrolidinone also known as NMP, and/or similar compatible organic solvent. The coating is applied by brushing or dipping, and is allowed to dry before contact is made with the electrode. The coated current collector may be heated to accelerate drying. The~preferred composition of the carbonaceous particles in the coating is 5-15 wt.% carbon fibres, the balance being fine carbon. The carbon fibre and fine carbon mixture is blended with the fluorinated polymer~solution in~a conventional manner.
' In another embodiment of the invention the electron conductivity of the electrode~of a lithium cell or battery is enhanced by admixing a mixture of carbon fibres and fine carbon in the electrode components. As discussed above, the electrode commonly comprises electroactive particles, an electron conducting component and a binder, preferably an ionically conducting binder compound, in an amount sufficient to provide a coherent paste. The electron conducting component is added in less than 12 wt.%, and usually higher than 3 wt.%, however, this is dictated by particle size and specific gravity of the electrode components.
It i~s noted that the object in assembling a lithium electrochemical cell is to avoid unnecessary dilution of the electroactive materials present in order'to maximize energy density per volume or weight of the cell. In other word, the carbonaceous particles are added to the electrode, and~or to the coating on the current collector surface in amounts sufficient to reduce the impedance of the cell to a desirable . level. It is convenient to utilize the same type of carbon fibres and fine carbon in the electrode as in the conductive coating between the electrode and current collector of the cell,_however, this is not a requirement for the working of the invention. The mixture of carbon fibres and fine carbon, is. applied as a conductivity enhancer ir1 the positive electrodes of a lithium cell, but may be used in a negative electrode as well, depending on the nature of the .,electroactive component of the latter. The cbrbohaceous .particles. in the electrode paste preferably contain carbon ..fibres ~in 5-15 wt. %,. ' based ' on . the total ' weight of the carbonaceous particles added. ~ ~ - .
The cathode paste is loaded on the current collector by doctor s blade method but any convenient commercially available method will do. In one form of utilization the cathode-active material, the carbon fibre - fine carbon mixture and a fluoride containing polymer binder are blended .together for a relatively short period and the obtained paste .is then. loaded onto the current collector to form a~layer which .is usually notably thicker than 50 Vim, bt~t the electrode layex thickness is determined by convenience only. Care is to. be taken that the carbon fibres are got unduly broken ..during blending. The fluorinated polymer having' melting point greater than 70'C, is conveniently dissolved in~a suitable, -low , boiling vpoint solvent.' The' preaerice of ' th~r solvent ' .increases.fluidity in the mixture~and allows blending Without breaking the.fibres. The electrode layer is~allowed to dry, or can be heated to accelerate drying, before it is brought into contact. with a lithium ion-bearing polymer serving as electrolyte. The preferred fluorinated polymer binder in this ' .invention-~is~.a copolymer of polyvinylidene fluoride, n compound.capable of allowing th~ passage of lithium ions. The same .copolymer .of. polyvinylidene fhioride bay be~ uBed iri ~trie - g -electrically conductive carbonaceous particle bearing coating on the current collector, however, this is a matter of choice.
It is noted also that it is not essential that the polymer in the coating which carries the conductive particles be an sonically conductive substance but it may be convenient.
EXAKpLE 1 8g of carbon fibres having 7.5 ~m average diameter and 60 hem average length, or aspect ratio 8, were mixed by hand with 52g petroleum coke. 20 g of polyvinylidene fluoride -hexafluoro propylene copolymer, marketed under the name of Rynar, was made fluid by adding 200 ml NMP-acetone solvent to it. The prepared mixture of carbon fibres and coke was added to the fluidized polymer and blended by hand. The solution of carbon particle containing polymer was painted on one face of an aluminum foil supported on a polymer laminate, to provide a thin, continuous layer. The layer was dried under a heat lamp. A lithium-cobalt oxide, 5 wt.% polyvinylidene fluoride - hexafluoro propylene copolymer and 10 wt.% fine carbon containing conventional positive electrode mixture was applied to the dry, coated surface of the aluminum foil by doctor's blade method to provide a 190 ~m thick electrode-current collector composite for use in a lithium cell.
A positive electrode material made of lithium-cobalt oxide-polyvinylidene copolymer mixture as in Example 1, and a blended mixture of 7 parts of Shawinigan black and 1 part of carbon fibres , the latter carbon particle mixture being added in 10 wt.% based on the weight of the electrode-binder mixture, were combined with NMP-acetone solvent in sufficient amount to permit good mixing of the components. The resulting electrode paste was coated by means of doctor's blade onto another segment of the coated aluminum current collector prepared in Example 1, to yield a 190 pm thick layer. The electrode layer located on current collector was allowed to dry under a heat lamp.
In a comparative test C, a lithium-cobalt oxide-polyvfnylidene fluoride copolymer binder and 10 wt.%
Shawinigan black containing mixture, that is the same basic cathode composition as in Examples 1 and 2, but without the carbon fibres added in the positive electrode in Example 2, was prepared to provide a conventional positive electrode of 190 ~cm thickness, and subsequently was positioned to be in contact with an aluminum foil bearing polymer laminate, for incorporation in a conventional laminar lithium cell.
EX~I~IPLE 3 The positive electrode-current collector assemblies for use in a rechargeable laminar lithium cell, prepared in Examples 1, 2 and comparative test C, and having similar thickness, were each subjected to resistance measurements by known methods, between the surface to be brought in contact with an electrolyte layer, and the current collector.
The average of ten resistance measurements of each electrode-current collector assembly is shown below:
Example 1: 290.10'' ohm. cm Example 2: 180.10'' ohm.cm Test C: 2190.10'' ohm.cm It can be seen that the average resistance of the electrode-current collector assembly having carbon fibres in the electrode mixture and in the coating layer between the electrode and the current collector, is the lowest. The average resistance of a conventional electrode supported on a current collector coated with a carbon fibre-fine carbon mixture bearing polymer is somewhat higher. The average resistance of a conventional electrode-current collector assembly prepared to have the same thickness but containing no carbon fibre in the electrode and having no conductive coating between the current collector and the adjacent electrode, is nearly an order of magnitude higher.
EXAI~IPLg 4 Lateral resistance of an electrically conductive coating carried on an inert non-conductive layer replacing the current collector, and prepared in accordance with this invention, was measured by inserting measuring probes in the coating as shown on Fig.i. Assembly l0 represents schematically an inert plastic strip 12, coated with a carbon fibre-fine graphite-polyvinylidene fluoride-hexafluoro propylene copolymer mixture bearing layer 14, prepared using the same carbon fibre-fine graphite ratio as described in Example 1. The inert non-conducting plastic strip was 100mm wide and long enough to be coated on one of its faces with a 100mm wide and 420mm long layer 14. The thickness of the coating was 40~m. Probes 16' and 16" were inserted below the coating surface close to the edges, as shown. The probes were connected to a high sensitivity Hewlitt Packard 4-point multimeter. 6 readings were taken, giving an average lateral resistance value of 21.48 ohms.
A similar inert plastic strip was coated with a similar conductive particles containing layer in the same length, width and depth as above, but the coating contained only fine graphite. The composition of the coating was 75 wt.% fine graphite mixed into 25 wt. % polyvinylidene f luoride-hexaf luoro propylene copolymer. The average of 6 lateral resistance measurements was found to be 332 ohms.
It can be seen that the presence of the graphite fibres increased the conductivity of the coating by about one order of magnitude.
It is noted that in the above examples polyvinylidene fluoride-hexafluoro propylene copolymer was used as the carrier of the carbon fibres-fine carbon mixtures, however, another fluorinated polymer having melting point higher than 70°C, or its chemical equivalent could equally well have been used.
EX7~MPLE 5 A thin titanium sheet was coated on both faces with a composition made of a mixture of elongated carbon crystallites marketed as 'Ketjen Black', and carbon fibres blended with polyvinylidene fluoride copolymer. The carbon particles were present in 65 wt.%. The carbon particles were a blend of 7 parts of Ket jen Black, and 1 part carbon f ibres having average dimensions as stated in Example 1. The coating was prepared as described in Example 1. A positive electrode mixture was made of components as detailed in Example 2, but having admixed carbon particles composed of Ketjen Black and carbon fibres in the same ratio as in the coating on the titanium current collector. The electrode mixture was blended with granules of polyethylene oxide containing 6% LiPF6. The electrolyte granules had average particle size of 50 Vim. The blended mixture was deposited in a layer over the coated faces of the titanium sheet. The composite current collector-electrode-electrolyte was incorporated in a rechargeable lithium battery. The obtained lithium battery exhibited lower impedance than a similar lithium battery having conventional carbon particles admixed as electron conducting additive.
As was described in the foregoing paragraphs and shown in the examples, carbon fibres added as electron conductive components in lithium cells improve the performance of lithium batteries.
The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. Nowever, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.
Claims (15)
1. In a lithium battery having an anode and anode current collector, lithium ion conducting electrolyte, a cathode containing a positive active material and a cathode current collector, the improvement comprising having an electrically conducting coating between said cathode current collector and said cathode containing positive active material, wherein said electrically conducting coating is comprising a fluorinated polymer having melting point greater than 70°C, admixed with a mixture of fine carbon and carbon fibres.
2. An improved lithium battery as claimed in claim 1, wherein said fluorinated polymer is a copolymer comprising polyvinylidene fluoride.
3. An improved lithium battery as claimed in claim 1, wherein said electrically conducting coating contains 40 wt.% or less of said fluorinated polymer.
4. An improved lithium battery as claimed in claim 1, wherein said mixture in said electrically conducting coating is a mixture of fine carbon and 5-15 wt% carbon fibres.
5. An improved lithium battery as claimed in claim 4, wherein said fine carbon in said mixture comprises elongated crystallites of fine carbonaceous particles.
6. An improved lithium battery as claimed in claim 1, wherein said positive active material is a transition metal chalcogenide or a solid solution of transition metal chalcogenides.
7, An improved lithium battery as claimed in claim 1, wherein said cathode current collector is selected from the group consisting of metal foil, metal plate, metal grid, metal mesh and metal foil supported by a polymeric laminate.
8. A rechargeable laminar lithium battery comprising, an anode capable of reversibly intercalating lithium ions and an anode current collector, a lithium ion conducting electrolyte, and a cathode layer comprising a positive active material capable of reversibly intercalating lithium, an adjacent cathode current collector, and an electrically conducting coating between said cathode layer and said adjacent cathode current collector, said electrically conducting coating comprising a fluorinated polymer having melting point greater than 70°C, admixed with a mixture of fine carbon and carbon fibres.
9. In a lithium battery having an anode and anode current collector, a lithium ion conducting electrolyte, a cathode containing a first mixture of a positive active material and electrically conducting particles, and a cathode current collector, the improvement comprising that said electrically conducting particles contained in said first mixture are carbonaceous particles composed of a second mixture of fine carbon and carbon fibres.
10. An improved lithium battery as claimed in claim 9, wherein said first mixture also contains a binder compound capable of conducting lithium ions.
11. An improved lithium battery as claimed in claim 9, wherein said carbonaceous particles in said second mixture are composed of fine carbon and 5-15 wt.% carbon fibres.
12. An improved lithium battery as claimed in claim 11, wherein said fine carbon in said second mixture is comprising elongated crystallites of fine carbonaceous particles.
13. An improved lithium battery as claimed in claim 9, wherein said positive active material is a transition metal chalcogenide or a solid solution of transition metal chalcogenides.
14. A rechargeable laminar lithium battery comprising, an anode capable of reversibly intercalating lithium ions and an anode current collector, a lithium ion conducting electrolyte, a cathode layer comprising a first mixture of a positive active material capable of reversibly intercalating lithium, and electrically conducting carbonaceous particles composed of a second mixture of fine carbon and carbon fibres, and a cathode current collector adjacent said cathode layer.
15. A rechargeable laminar lithium battery comprising, an anode capable of reversibly intercalating lithium ions and an anode current collector, a lithium ion conducting electrolyte, a composite cathode having a cathode layer comprising a first mixture of a positive active material capable of reversibly intercalating lithium, and electrically conducting carbonaceous particles composed of a second mixture of fine carbon and carbon fibres, a cathode current collector adjacent said cathode layer, and an electrically conducting coating between said cathode layer and said cathode current collector, said electrically conducting coating comprising a fluorinated polymer having melting point greater than 70°C, and carbonaceous particles composed of a third mixture of fine carbon and carbon fibres.
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| EP1077498A1 (en) * | 1999-03-04 | 2001-02-21 | Japan Storage Battery Co., Ltd. | Composite active material and method for preparing active material, electrode and method for preparing electrode, and non-aqueous electrolyte cell |
| US6767670B2 (en) * | 2001-11-14 | 2004-07-27 | Wilson Greatbatch Technologies, Inc. | Carbon-coated titanium current collectors for use in alkali metal electrochemical cells |
| US6852449B2 (en) * | 2002-08-29 | 2005-02-08 | Quallion Llc | Negative electrode including a carbonaceous material for a nonaqueous battery |
| US6998192B1 (en) | 2002-08-29 | 2006-02-14 | Quallion Llc | Negative electrode for a nonaqueous battery |
| TWI459616B (en) * | 2004-08-16 | 2014-11-01 | Showa Denko Kk | Lithium batteries with positive and the use of its lithium batteries |
| US7174207B2 (en) * | 2004-09-23 | 2007-02-06 | Quallion Llc | Implantable defibrillator having reduced battery volume |
| US7582273B1 (en) * | 2004-10-25 | 2009-09-01 | Los Almos National Security, Llc | Preparation of carbon nanoparticles and carbon nitride from high nitrogen compound |
| US20060115738A1 (en) * | 2004-12-01 | 2006-06-01 | Sergiy Sazhin | Lithium-fluorinated carbon cells |
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| US6046268A (en) * | 1998-08-02 | 2000-04-04 | Motorola, Inc. | Electrode with enhanced adhesion to substrates |
-
1999
- 1999-07-28 US US09/361,977 patent/US6261722B1/en not_active Expired - Lifetime
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2000
- 2000-06-27 CA CA002311876A patent/CA2311876C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2311876A1 (en) | 2001-01-28 |
| US6261722B1 (en) | 2001-07-17 |
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