WO1999063609A1 - Cross-linked polymeric components of rechargeable solid lithium batteries and methods for making same - Google Patents

Cross-linked polymeric components of rechargeable solid lithium batteries and methods for making same Download PDF

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Publication number
WO1999063609A1
WO1999063609A1 PCT/US1999/012096 US9912096W WO9963609A1 WO 1999063609 A1 WO1999063609 A1 WO 1999063609A1 US 9912096 W US9912096 W US 9912096W WO 9963609 A1 WO9963609 A1 WO 9963609A1
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Prior art keywords
cell
separator
polymer
plasticizer
electrolyte
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PCT/US1999/012096
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French (fr)
Inventor
David B. Swanson
Brendan Michael Coffey
Jeffrey A. Read
Stanley Lewin
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Ultralife Batteries, Inc.
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Priority to AU42257/99A priority Critical patent/AU4225799A/en
Publication of WO1999063609A1 publication Critical patent/WO1999063609A1/en

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    • 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
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/411—Organic material
    • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
    • H01M50/426—Fluorocarbon polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0565—Polymeric materials, e.g. gel-type or solid-type
    • 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
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • 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
    • H01M4/622—Binders being polymers
    • H01M4/623—Binders being polymers fluorinated polymers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • 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
    • H01M4/04—Processes of manufacture in general
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M6/00—Primary cells; Manufacture thereof
    • H01M6/14—Cells with non-aqueous electrolyte
    • H01M6/18—Cells with non-aqueous electrolyte with solid electrolyte
    • H01M6/188—Processes of manufacture
    • 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
    • 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
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • This invention relates to a solid electrolyte comprised of a polymeric matrix containing an encapsulated liquid electrolyte, and more particularly to ionically conducting components of batteries held together with a porous radiation-crosslinkable polymer, and to methods of producing these ionically conducting components.
  • Lithium ion batteries generally use two different insertion compounds for the active cathode and active anode materials. Insertion compounds are those that act as a host solid for the reversible insertion of guest ions (e.g. lithium ions, or other alkaline metal ions).
  • the electrode materials for lithium ion batteries generally are chosen such that the electrochemical potential of the inserted lithium within each material differs by about 3 to 4 volts; thus leading to a 3 or 4 volt battery.
  • the cathode active material is typically a lithiated metal oxide. Preferred components include a spinel lithium manganese oxide.
  • the anode or negative electrode is a form of carbon or graphite, which has high storage capacity for the intercalation of lithium.
  • lithium ions are extracted from the cathode while the lithium is concurrently inserted into the anode.
  • the reverse process occurs on discharging the battery while delivering electric current to power an external load (e.g. consumer electronics, etc.).
  • Lithium ion batteries may either be of the conventional type containing a liquid electrolyte that is free flowing within the cell package or alternatively the lithium ion battery may make use of some form of solid electrolyte to give a quasi solid state cell.
  • lithium is shuttled back and forth between the cathode and anode in the form of ions transported through the polymer electrolyte system.
  • the polymer electrolyte simultaneously acts as a separator to prevent electrical short- circuiting of the anode and cathode and as an ionically conducting membrane.
  • One class of polymer electrolytes consists of a mixed phase system of a polymeric matrix with a liquid electrolyte.
  • This class of polymer electrolyte is analogous to a gel, wherein the porous polymer phase provides the mechanical structure, while the encapsulated continuous liquid electrolyte phase provides the ionic conduction path.
  • the liquid electrolyte solution most typically consists of a lithium salt, such as lithium perchlorate, in a mixture of organic solvents, such as propylene carbonate, diethoxy ethane or dimethyl carbonate.
  • These voids are later filled with the liquid electrolyte during final assembly and packaging of the battery.
  • These separator elements include sufficiently large solution-containing voids so that continuous avenues may be established between the electrodes thereby leading to electrolyte interaction with the electrode materials and throughout the cell.
  • a similar method may be used to form the electrode components of the cell by replacing the inert filler with the appropriate active materials and an electronic conducting filler.
  • Suitable solvents include organic solvents such as acetone. Strong interaction between organic solvents and the polymer is required in order to provide these solubility characteristics as well as to favor the encapsulation of the organic solvents used in the liquid phase of the electrolyte.
  • Suitable polymers for lithium ion battery components must simultaneously satisfy several criteria. They must provide acceptable mechanical strength and creep resistance over a range of working temperatures above and below room temperature. They must be compatible with the electrolyte solution that is used and the active materials present. They must also be stable against decomposition in the electrical potential of the cell.
  • Known polymeric compositions for use in such batteries are composed of a homopolymer or a copolymers of PVDF, PVC and polypropylenes.
  • Certain fluorocarbon polymers particularly poly (vinylidene fluoride) (PVdF) copolymers with hexafluoropropylene (HFP), have been found to be suitable for use in solid lithium batteries in which ionic conductivity is enhanced by incorporating lithium salts and solvents which are compatible with the polymer (US Pat. No. 5,296,318). These copolymers perform satisfactorily even after heating up to 70°C. However, the plasticized copolymer is soluble in the liquid electrolyte at temperatures higher than 80°-95°C. Melting of the electrolyte film under stress may lead to internal shorting of the battery at these high temperatures.
  • PVdF poly (vinylidene fluoride) copolymers with hexafluoropropylene
  • Swelling and solubilization of the electrode materials can also occur in a polymer electrolyte lithium ion cell prepared as in the method described previously (USP 5,460,904). This can lead to degradation of the electrode and separator structures and degeneration of the cell resulting in cell failure and a reduced useful battery life. Thus while strong polymer solvent interactions are desirable during fabrication and assembly of the cell components, these types of interactions can lead to destabilizing of the assembled structure once it has been made and is filled with electrolyte.
  • Crosslinking of polymers whereby adjacent polymer chains are caused to be covalently bonded to each after the polymer has been shaped into a part is one well known method of stabilizing a polymer structure and reducing its solubility.
  • Certain polymer compositions may be crosslinked through the introduction of chemical crosslinking additives during manufacture of the polymer. Chemical initiators or actinic radiation can induce Crosslinking of the monomer compositions with these additives.
  • Chemical initiators or actinic radiation can induce Crosslinking of the monomer compositions with these additives.
  • PVdF is an exception to the rule. It has been found (US Pat. No. 3,142,629) that irradiation of PVdF to a dose of 8-50 Mrad with a 4.5 MeV electron beam increases its mechanical properties at elevated temperatures by crosslinking the polymer.
  • the present invention provides a rechargeable solid polymer lithium ion battery cell assembly including a positive electrode, a negative electrode, and a separator membrane disposed there between, in which at least one of the positive electrode, the negative electrode and the separator comprises a crosslinkable polymer crosslinked by exposing the battery cell assembly to actinic radiation prior to providing an electrolyte to the battery cell assembly.
  • the copolymer is mixed with an organic plasticizer which is extracted from the cell component prior to irradiation.
  • a method for making a rechargeable solid polymer lithium ion battery cell comprising the steps of assembling a cell comprising a positive electrode, a negative electrode, and a separator membrane disposed there between, in which at least one of the positive electrode, the negative electrode and the separator comprises a crosslinkable polymer and an organic plasticizer; exposing the cell to actinic radiation prior to providing an electrolyte to the cell; and crosslinking the polymer.
  • a method for making a separator membrane for a rechargeable solid polymer lithium ion battery electrolytic cell including, providing a solution of a copolymer of poly(vinylidene fluoride) and hexafluoropropylene, and an organic plasticizer in an organic solvent; casting the solution onto a surface to form a membrane of the copolymer; exposing the membrane to actinic radiation prior to contacting the membrane with an electrolyte; and crosslinking the copolymer.
  • a method of making an electrode for a rechargeable solid polymer lithium ion battery electrolytic cell including the steps of providing an expanded metal current collector; coating the current collector with a coating comprising a copolymer of ⁇ oly( vinylidene fluoride) and hexafluoropropylene and an organic plasticizer; exposing the coated current collector to actinic radiation prior to contacting the coated current collector with an electrolyte; and crosslinking the copolymer.
  • crosslinkable polymer is free from crosslinking additives.
  • crosslinkable polymer is free from residual groups of active hydrogen compounds.
  • plasticizer of the polymeric cell component is extracted with an organic solvent prior to irradiation.
  • Figure 1 shows a schematic drawing of a polymer lithium ion battery.
  • the method of producing solid state lithium ion cells is well known in the prior art. See exemplary U.S. Patent Nos. 5,460,904; 5,456,000; 5,296,318; 5,478,668 and 5,429,891 for descriptions of rechargeable lithium ion batteries, the entire contents of which are incorporated by reference herein.
  • One particularly preferred type of polymer electrolyte cell uses a plasticized thermoplastic polymer as a binder for each of the three main battery components: anode, cathode and separator as shown in Fig. 1.
  • the individual layers are each produced from a slurry made by mixing substituent powders and organic solvents including plasticizers and volatile solvents. A single battery assembly can thus be made simply by applying heat and pressure to fuse the component layers together.
  • the plasticizer may then be extracted from the cell with a solvent to create microporous matrices or voids into which an ionically conductive liquid electrolyte can be backfilled.
  • the laminated cell can be stacked and connected either in series or parallel to achieve desired voltages.
  • the whole battery pack can be encapsulated in a polymer-compatible package.
  • the polymer of the electrolyte cell is preferably a thermoplastic polymer and may be any suitable copolymer, but is most preferably a copolymer of poly(vinylidene fluoride) - hexafluoropropylene (or PVdF-HFP). Since no free electrolyte exists in the solid state polymer lithium ion battery system, this solid state battery is considered to be safer than its liquid analog.
  • a cathode 10 and anode 12 are applied to opposite sides of a solid polymer electrolyte separator 14.
  • a positive electrode current collector 16 is placed adjacent the cathode 10
  • the negative electrode current collector 18 is placed adjacent the anode 12.
  • the anode, cathode, current collectors and separator together comprise the electrode assembly 20.
  • the electrode assembly 20 is set into an aluminum foil plastic laminate 22 and sealed under heat and pressure to form the final sealed cell 24.
  • the cell anode, cathode (electrodes) and separator elements preferably comprise the combination of a poly(vinylidene fluoride)-hexafluoropropylene copolymer matrix and a compatible organic plasticizer which maintains a homogeneous composition in the form of a self-supporting film.
  • the separator copolymer composition comprises from about 75% to about 92% by weight poly (vinylidene fluoride) (PVdF) and about 8 to about 25% by weight hexafluoropropylene (HFP), (both commercially available from Elf AtoChem North America as Kynar FLEXTTM), and an organic plasticizer.
  • the copolymer composition is also used in the manufacture of the electrodes to insure a compatible interface with the separator.
  • the plasticizer may be one of the various organic compounds used as solvents for electrolyte salts such as, for example, propylene carbonate or ethylene carbonate or mixtures thereof. Particularly preferred are the higher-boiling point plasticizers such as dibutyl phthalate, dimethyl phthalate, diethyl phthalate and tris-butoxyethyl phosphate.
  • inorganic fillers may be added to enhance the physical strength and melt viscosity of the separator membrane, and to increase the electrolyte solution absorption level. Preferred fillers include fumed alumina and silanized fumed silica.
  • any common procedure for casting or forming films or membranes of polymer compositions may be used to make the present membrane materials.
  • a readily evaporated casting solvent may be added to obtain desired viscosity during casting.
  • Particularly preferred casting solvents include tetrahydrofuran (THF) and acetone.
  • THF tetrahydrofuran
  • Such coatings preferably are first air-dried at room temperature to yield self-supporting films of homogenous, plasticized copolymer compositions.
  • the membrane material to be used as the separator may also be formed by allowing the copolymer in commercial form (i.e. beads, powder, etc.) to swell in a proportionate amount of plasticizer solvent and then press the swollen mass between heated (e.g. 130°C) plates or rollers, or extruding the mixture.
  • the plasticizer solvent is extracted by an organic solvent. In this way the electrolyte is more completely absorbed into the separator as would be readily understood by one skilled in the field.
  • the lamination of assembled cell structures may be accomplished according to well-known methods such as those set forth in U.S. Patent No. 5,460,904 which is incorporated herein by reference. It is contemplated that the batteries of the present invention can be made into any shape necessary for the desired end use, e.g. cellular phones, video camcorders, computers, etc.
  • the preferred anodes of the present invention are made by preparing a slurry of PVdF-HFP and acetone along with the dibutylphthalate (DBP). To this mixture is added the active material.
  • the active material is preferably a graphitic composition, most preferably mesocarbon micro beads (MCMB, Osaka Gas, Osaka, Japan). An amount of carbon black is preferably added to this mixture to facilitate electrical conductivity.
  • the preferred separator is prepared in the same manner as the anode, except that no MCMBs or carbon black is provided. In their place, an electrically insulating but ionically conducting material may be provided, preferably silanized fused silica.
  • the cathode preferably is prepared in a fashion similar to the anode. However, an amount of lithium manganese oxide (Li 1+X Mn 2 O 4 ) is added to the mix in place of the graphite.
  • the DBP is then extracted leaving voids in the electrode (anode, cathode, and separator) materials.
  • the electrolyt is then added to the cell assembly; a solution of a soluble lithium salt in ethylene carbonate and dimethyl carbonate (EC-DMC) which is a non-aqueous solvent mixture.
  • EC-DMC ethylene carbonate and dimethyl carbonate
  • Preferred soluble lithium salts include LiPF 6 , LiBF 4 , LiClO 4 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 , and LiN(C 2 F 5 SO 2 ) 2 , with LiPF 6 being particularly preferred.
  • the current collectors which are assembled to be in intimate contact with the cathode and the anode are preferably made from aluminum and copper, respectively, and may be foil or grid-like in configuration.
  • the rechargeable solid polymer lithium ion battery polymeric components are exposed to actinic radiation to crosslink the polymeric compositions of the components without affecting the battery active materials, for example, lithium manganese oxide, carbon black, graphite, silicon dioxide, copper and aluminum.
  • the resulting crosslinking causes structural changes in the polymers which affects the physical and chemical characteristics of the cell.
  • One such change is superior adherence of, for example, electrode material to the expanded metal grid current collectors.
  • Another change is that crosslinking the electrode materials reduces the solubility of the electrode compositions in the electrolyte medium. Irradiation and crosslinking of the electrode material is preferably carried out prior to charging the cell with electrolyte. This is done to insure that the electrolyte does not interact with the polymer or is itself altered as a result of the irradiation, which could reduce the conductive performance of the electrolyte and adversely impact the power capacity of the cell.
  • the irradiation supplied to the cell components may be any suitable type of irradiation to cause the desired level of polymer crosslinking.
  • Preferred radiation includes actinic radiation, including ultraviolet and electron beam radiation, with electron beam radiation being particularly preferred.
  • Crosslinking is carried out with an electron beam of from about 100 kV to about 10 MeV preferably at about 4.5 MeV, at a dosage that will vary depending upon the desired level of crosslinking, but will generally be from about 2 to about 50 Mrad, with a dosage of from about 5 to about 20 Mrads being particularly preferred.
  • the crosslinking of the components according to the present invention preferably takes place in the temperature range of from about 15°C to about 70°C, and more preferably from about 20°C to about 45°C.
  • the components of the present invention are made from polymers that will crosslink without requiring substantial hydrogen fluoride (HF) scavengers that could interfere with the desired electrolytic activity.
  • Suitable polymers include copolymers of PVdF and HFP in which the PVdF component is preferably present at from about 75% to about 92% by weight.
  • the preferred polymers of the invention are free from crosslinking additives.
  • selected crosslinking agents may be added to affect crosslinking levels.
  • Preferred crosslinking agents are free from functional groups, such as residual groups of active hydrogen compounds, which might react with other electrode compositions.
  • Cell components were manufactured according to conventional procedures described above from Kynar 2801 resin, a copolymer of poly(vinylidene fluoride) and hexafluoropropylene with dibutyl phthalate as the plasticizer. Cells and cell components were manufactured and vacuum-sealed in laminate packaging material, such as Mylar Irradiation was performed both on cells and components that have been extracted to remove plasticizer and those that have not been extracted. Assembled cells and individual cell components were exposed to a 4.5 MeV E-beam radiation and irradiated at six different electron beam dosage levels; 2.5, 5.0, 7.5, 10, 15 and 20.0 (Mrads).
  • the lithium rechargeable cells treated according to the methods of the present invention display superior properties such as reduced degradation of the electrodes, even at high or low temperatures known to reduce rechargeable cell performance.

Abstract

A rechargeable solid polymer lithium ion battery cell assembly including a positive electrode, a negative electrode, and a separator membrane in which at least one of the positive electrode, the negative electrode and the separator includes a cross-linkable polymer free from cross-linking additives and cross-linked by exposing the assembly to actinic radiation prior to providing an electrolyte to the assembly is provided. A method is provided for making the solid polymer lithium ion battery cell assembly and the individual cell components by providing a cross-linkable polymer to at least one of the cell components, exposing the component to actinic radiation, and cross-linking the polymer. This invention can prevent degradation of the cell electrode and separator structures in a polymer electrolyte lithium ion cell and reduces cell problems related to high temperature failure and reduced useful battery life.

Description

CROSSLINKED POLYMERIC COMPONENTS OF RECHARGEABLE SOLID LITHIUM BATTERIES AND METHODS
FOR MAKING SAME Field of the Invention This invention relates to a solid electrolyte comprised of a polymeric matrix containing an encapsulated liquid electrolyte, and more particularly to ionically conducting components of batteries held together with a porous radiation-crosslinkable polymer, and to methods of producing these ionically conducting components. Background of the Invention
Lithium ion batteries generally use two different insertion compounds for the active cathode and active anode materials. Insertion compounds are those that act as a host solid for the reversible insertion of guest ions (e.g. lithium ions, or other alkaline metal ions). The electrode materials for lithium ion batteries generally are chosen such that the electrochemical potential of the inserted lithium within each material differs by about 3 to 4 volts; thus leading to a 3 or 4 volt battery. The cathode active material is typically a lithiated metal oxide. Preferred components include a spinel lithium manganese oxide. The anode or negative electrode, is a form of carbon or graphite, which has high storage capacity for the intercalation of lithium. Upon charging of a lithium ion battery, lithium ions are extracted from the cathode while the lithium is concurrently inserted into the anode. The reverse process occurs on discharging the battery while delivering electric current to power an external load (e.g. consumer electronics, etc.).
Lithium ion batteries may either be of the conventional type containing a liquid electrolyte that is free flowing within the cell package or alternatively the lithium ion battery may make use of some form of solid electrolyte to give a quasi solid state cell. In a solid state rechargeable or secondary lithium ion battery system, lithium is shuttled back and forth between the cathode and anode in the form of ions transported through the polymer electrolyte system. The polymer electrolyte simultaneously acts as a separator to prevent electrical short- circuiting of the anode and cathode and as an ionically conducting membrane. One class of polymer electrolytes consists of a mixed phase system of a polymeric matrix with a liquid electrolyte. This class of polymer electrolyte is analogous to a gel, wherein the porous polymer phase provides the mechanical structure, while the encapsulated continuous liquid electrolyte phase provides the ionic conduction path. The liquid electrolyte solution most typically consists of a lithium salt, such as lithium perchlorate, in a mixture of organic solvents, such as propylene carbonate, diethoxy ethane or dimethyl carbonate.
A method of forming such a two-phase polymer electrolyte with suitable conduction properties has been reported in the prior art (USP 5,460,904). In this method the separator is formed in a multi-stage process beginning with the dissolution of a PVDF-HFP polymeric resin along with a plasticizer and an inert inorganic filler in an organic solvent. The plasticizer and filler are uniformly distributed throughout the solution of polymer. The solution is coated onto a substrate, while allowing the solvent to evaporate. This leaves a freestanding polymeric film containing an even distribution of filler and plasticizer that may be removed from the substrate for assembly into the battery. Extraction of the plasticizer from the polymer by suitable organic solvents creates microscopic voids in the polymer. These voids are later filled with the liquid electrolyte during final assembly and packaging of the battery. These separator elements include sufficiently large solution-containing voids so that continuous avenues may be established between the electrodes thereby leading to electrolyte interaction with the electrode materials and throughout the cell. A similar method may be used to form the electrode components of the cell by replacing the inert filler with the appropriate active materials and an electronic conducting filler.
Note that the polymer electrolyte formation method described above requires solubility of the polymer in some high vapor pressure solvent given the preferred method of assembly. Suitable solvents include organic solvents such as acetone. Strong interaction between organic solvents and the polymer is required in order to provide these solubility characteristics as well as to favor the encapsulation of the organic solvents used in the liquid phase of the electrolyte.
Suitable polymers for lithium ion battery components must simultaneously satisfy several criteria. They must provide acceptable mechanical strength and creep resistance over a range of working temperatures above and below room temperature. They must be compatible with the electrolyte solution that is used and the active materials present. They must also be stable against decomposition in the electrical potential of the cell. Known polymeric compositions for use in such batteries are composed of a homopolymer or a copolymers of PVDF, PVC and polypropylenes.
Certain fluorocarbon polymers, particularly poly (vinylidene fluoride) (PVdF) copolymers with hexafluoropropylene (HFP), have been found to be suitable for use in solid lithium batteries in which ionic conductivity is enhanced by incorporating lithium salts and solvents which are compatible with the polymer (US Pat. No. 5,296,318). These copolymers perform satisfactorily even after heating up to 70°C. However, the plasticized copolymer is soluble in the liquid electrolyte at temperatures higher than 80°-95°C. Melting of the electrolyte film under stress may lead to internal shorting of the battery at these high temperatures.
Swelling and solubilization of the electrode materials can also occur in a polymer electrolyte lithium ion cell prepared as in the method described previously (USP 5,460,904). This can lead to degradation of the electrode and separator structures and degeneration of the cell resulting in cell failure and a reduced useful battery life. Thus while strong polymer solvent interactions are desirable during fabrication and assembly of the cell components, these types of interactions can lead to destabilizing of the assembled structure once it has been made and is filled with electrolyte.
Crosslinking of polymers whereby adjacent polymer chains are caused to be covalently bonded to each after the polymer has been shaped into a part, is one well known method of stabilizing a polymer structure and reducing its solubility. Certain polymer compositions may be crosslinked through the introduction of chemical crosslinking additives during manufacture of the polymer. Chemical initiators or actinic radiation can induce Crosslinking of the monomer compositions with these additives. It is well known that, while the majority of fluorocarbon polymers lacking specific functional groups undergo degradation by scission rather than crosslinking when exposed to ionizing radiation, PVdF is an exception to the rule. It has been found (US Pat. No. 3,142,629) that irradiation of PVdF to a dose of 8-50 Mrad with a 4.5 MeV electron beam increases its mechanical properties at elevated temperatures by crosslinking the polymer.
US Pat. No. 5,429,891 discloses that elastomeric copolymers of PVdF and HFP are chemically crosslinkable with chemical additives and then used in some battery systems. Crosslinking additives, such as diamines and bis-phenols, were found to be unsuitable for use in Li-ion batteries due to side reactions with the functional groups, such as active hydrogens of the amine or phenol groups. It is further disclosed that certain copolymers of PVdF and 8-15% HFP which are crosslinked by actinic radiation in the presence of chemical crosslinking additives selected from the group consisting of an acrylate ester, a di- or triallyl ester and a di- or triglycidyl ether and a plasticizer are suitable for manufacturing Li-ion battery components. The battery components are then manufactured using the crosslinked polymer compositions. The use of such crosslinked components decreased the capacity of the battery slightly while increasing the complexity and cost of manufacturing lithium batteries.
There remains a need for a polymeric material suitable for use in polymer electrolyte lithium ion batteries which prevents degradation of the electrodes and reduces cell problems related to high temperature failure; and for a method of manufacturing the batteries which does not add substantially to the cost of manufacturing. Summary of the Invention
The present invention provides a rechargeable solid polymer lithium ion battery cell assembly including a positive electrode, a negative electrode, and a separator membrane disposed there between, in which at least one of the positive electrode, the negative electrode and the separator comprises a crosslinkable polymer crosslinked by exposing the battery cell assembly to actinic radiation prior to providing an electrolyte to the battery cell assembly. In a preferred embodiment of the invention the copolymer is mixed with an organic plasticizer which is extracted from the cell component prior to irradiation.
In another aspect of the invention, there is provided a method for making a rechargeable solid polymer lithium ion battery cell comprising the steps of assembling a cell comprising a positive electrode, a negative electrode, and a separator membrane disposed there between, in which at least one of the positive electrode, the negative electrode and the separator comprises a crosslinkable polymer and an organic plasticizer; exposing the cell to actinic radiation prior to providing an electrolyte to the cell; and crosslinking the polymer.
In another aspect of the invention there is provided a method for making a separator membrane for a rechargeable solid polymer lithium ion battery electrolytic cell including, providing a solution of a copolymer of poly(vinylidene fluoride) and hexafluoropropylene, and an organic plasticizer in an organic solvent; casting the solution onto a surface to form a membrane of the copolymer; exposing the membrane to actinic radiation prior to contacting the membrane with an electrolyte; and crosslinking the copolymer.
In another aspect of the invention there is provided a method of making an electrode for a rechargeable solid polymer lithium ion battery electrolytic cell including the steps of providing an expanded metal current collector; coating the current collector with a coating comprising a copolymer of ρoly( vinylidene fluoride) and hexafluoropropylene and an organic plasticizer; exposing the coated current collector to actinic radiation prior to contacting the coated current collector with an electrolyte; and crosslinking the copolymer.
In a preferred aspect of the invention the crosslinkable polymer is free from crosslinking additives. In another aspect of the invention the crosslinkable polymer is free from residual groups of active hydrogen compounds. In another preferred aspect of the invention the plasticizer of the polymeric cell component is extracted with an organic solvent prior to irradiation.
The novel aspects of this invention are set forth with particularity in the appended claims. The invention itself, together with further objects and advantages thereof may be more fully comprehended by reference to the following detailed description of a presently preferred embodiment of the invention taken in conjunction with the accompanying drawings. Brief Description of the Drawing
Figure 1 shows a schematic drawing of a polymer lithium ion battery.
Detailed Description of the Invention
The method of producing solid state lithium ion cells is well known in the prior art. See exemplary U.S. Patent Nos. 5,460,904; 5,456,000; 5,296,318; 5,478,668 and 5,429,891 for descriptions of rechargeable lithium ion batteries, the entire contents of which are incorporated by reference herein. One particularly preferred type of polymer electrolyte cell uses a plasticized thermoplastic polymer as a binder for each of the three main battery components: anode, cathode and separator as shown in Fig. 1. The individual layers are each produced from a slurry made by mixing substituent powders and organic solvents including plasticizers and volatile solvents. A single battery assembly can thus be made simply by applying heat and pressure to fuse the component layers together. The plasticizer may then be extracted from the cell with a solvent to create microporous matrices or voids into which an ionically conductive liquid electrolyte can be backfilled. The laminated cell can be stacked and connected either in series or parallel to achieve desired voltages. The whole battery pack can be encapsulated in a polymer-compatible package. The polymer of the electrolyte cell is preferably a thermoplastic polymer and may be any suitable copolymer, but is most preferably a copolymer of poly(vinylidene fluoride) - hexafluoropropylene (or PVdF-HFP). Since no free electrolyte exists in the solid state polymer lithium ion battery system, this solid state battery is considered to be safer than its liquid analog.
Referring to the cell construction of FIG. 1, a cathode 10 and anode 12 are applied to opposite sides of a solid polymer electrolyte separator 14. A positive electrode current collector 16 is placed adjacent the cathode 10, and the negative electrode current collector 18 is placed adjacent the anode 12. The anode, cathode, current collectors and separator together comprise the electrode assembly 20. The electrode assembly 20 is set into an aluminum foil plastic laminate 22 and sealed under heat and pressure to form the final sealed cell 24.
The cell anode, cathode (electrodes) and separator elements preferably comprise the combination of a poly(vinylidene fluoride)-hexafluoropropylene copolymer matrix and a compatible organic plasticizer which maintains a homogeneous composition in the form of a self-supporting film. The separator copolymer composition comprises from about 75% to about 92% by weight poly (vinylidene fluoride) (PVdF) and about 8 to about 25% by weight hexafluoropropylene (HFP), (both commercially available from Elf AtoChem North America as Kynar FLEXT™), and an organic plasticizer. The copolymer composition is also used in the manufacture of the electrodes to insure a compatible interface with the separator. The plasticizer may be one of the various organic compounds used as solvents for electrolyte salts such as, for example, propylene carbonate or ethylene carbonate or mixtures thereof. Particularly preferred are the higher-boiling point plasticizers such as dibutyl phthalate, dimethyl phthalate, diethyl phthalate and tris-butoxyethyl phosphate. In addition, inorganic fillers may be added to enhance the physical strength and melt viscosity of the separator membrane, and to increase the electrolyte solution absorption level. Preferred fillers include fumed alumina and silanized fumed silica.
Any common procedure for casting or forming films or membranes of polymer compositions may be used to make the present membrane materials. A readily evaporated casting solvent may be added to obtain desired viscosity during casting. Particularly preferred casting solvents include tetrahydrofuran (THF) and acetone. Such coatings preferably are first air-dried at room temperature to yield self-supporting films of homogenous, plasticized copolymer compositions. The membrane material to be used as the separator may also be formed by allowing the copolymer in commercial form (i.e. beads, powder, etc.) to swell in a proportionate amount of plasticizer solvent and then press the swollen mass between heated (e.g. 130°C) plates or rollers, or extruding the mixture. To facilitate the entry of an electrolyte into the polymer matrix, particularly of the separator, the plasticizer solvent is extracted by an organic solvent. In this way the electrolyte is more completely absorbed into the separator as would be readily understood by one skilled in the field.
The lamination of assembled cell structures may be accomplished according to well-known methods such as those set forth in U.S. Patent No. 5,460,904 which is incorporated herein by reference. It is contemplated that the batteries of the present invention can be made into any shape necessary for the desired end use, e.g. cellular phones, video camcorders, computers, etc.
The preferred anodes of the present invention are made by preparing a slurry of PVdF-HFP and acetone along with the dibutylphthalate (DBP). To this mixture is added the active material. In the case of the anode, the active material is preferably a graphitic composition, most preferably mesocarbon micro beads (MCMB, Osaka Gas, Osaka, Japan). An amount of carbon black is preferably added to this mixture to facilitate electrical conductivity.
The preferred separator is prepared in the same manner as the anode, except that no MCMBs or carbon black is provided. In their place, an electrically insulating but ionically conducting material may be provided, preferably silanized fused silica.
The cathode preferably is prepared in a fashion similar to the anode. However, an amount of lithium manganese oxide (Li1+X Mn2O4) is added to the mix in place of the graphite.
The DBP is then extracted leaving voids in the electrode (anode, cathode, and separator) materials. The electrolyt is then added to the cell assembly; a solution of a soluble lithium salt in ethylene carbonate and dimethyl carbonate (EC-DMC) which is a non-aqueous solvent mixture. Preferred soluble lithium salts include LiPF6, LiBF4, LiClO4, LiCF3SO3, LiN(CF3SO2) 2, and LiN(C2F5SO2) 2, with LiPF6 being particularly preferred.
The current collectors which are assembled to be in intimate contact with the cathode and the anode are preferably made from aluminum and copper, respectively, and may be foil or grid-like in configuration.
In the present invention the rechargeable solid polymer lithium ion battery polymeric components are exposed to actinic radiation to crosslink the polymeric compositions of the components without affecting the battery active materials, for example, lithium manganese oxide, carbon black, graphite, silicon dioxide, copper and aluminum. The resulting crosslinking causes structural changes in the polymers which affects the physical and chemical characteristics of the cell. One such change is superior adherence of, for example, electrode material to the expanded metal grid current collectors. Another change is that crosslinking the electrode materials reduces the solubility of the electrode compositions in the electrolyte medium. Irradiation and crosslinking of the electrode material is preferably carried out prior to charging the cell with electrolyte. This is done to insure that the electrolyte does not interact with the polymer or is itself altered as a result of the irradiation, which could reduce the conductive performance of the electrolyte and adversely impact the power capacity of the cell.
The irradiation supplied to the cell components may be any suitable type of irradiation to cause the desired level of polymer crosslinking. Preferred radiation includes actinic radiation, including ultraviolet and electron beam radiation, with electron beam radiation being particularly preferred. Crosslinking is carried out with an electron beam of from about 100 kV to about 10 MeV preferably at about 4.5 MeV, at a dosage that will vary depending upon the desired level of crosslinking, but will generally be from about 2 to about 50 Mrad, with a dosage of from about 5 to about 20 Mrads being particularly preferred. The crosslinking of the components according to the present invention preferably takes place in the temperature range of from about 15°C to about 70°C, and more preferably from about 20°C to about 45°C.
Preferably, the components of the present invention are made from polymers that will crosslink without requiring substantial hydrogen fluoride (HF) scavengers that could interfere with the desired electrolytic activity. Suitable polymers include copolymers of PVdF and HFP in which the PVdF component is preferably present at from about 75% to about 92% by weight. The preferred polymers of the invention are free from crosslinking additives. However, selected crosslinking agents may be added to affect crosslinking levels. Preferred crosslinking agents are free from functional groups, such as residual groups of active hydrogen compounds, which might react with other electrode compositions.
Cell components were manufactured according to conventional procedures described above from Kynar 2801 resin, a copolymer of poly(vinylidene fluoride) and hexafluoropropylene with dibutyl phthalate as the plasticizer. Cells and cell components were manufactured and vacuum-sealed in laminate packaging material, such as Mylar Irradiation was performed both on cells and components that have been extracted to remove plasticizer and those that have not been extracted. Assembled cells and individual cell components were exposed to a 4.5 MeV E-beam radiation and irradiated at six different electron beam dosage levels; 2.5, 5.0, 7.5, 10, 15 and 20.0 (Mrads).
The lithium rechargeable cells treated according to the methods of the present invention display superior properties such as reduced degradation of the electrodes, even at high or low temperatures known to reduce rechargeable cell performance.
While the invention has been described in connection with a presently preferred embodiment thereof, those skilled in the art will recognize that many modifications and changes may be made therein without departing from the true spirit and scope of the invention, which accordingly is intended to be defined solely by the appended claims.

Claims

In the Claims
1. A rechargeable solid polymer lithium ion battery cell assembly, comprising:
(a) a positive electrode;
(b) a negative electrode; and (c) a separator membrane disposed between the positive electrode and the negative electrode; and
(d) in which at least one of the positive electrode, the negative electrode and the separator comprises a crosslinkable polymer free from crosslinking additives and crosslinked by exposing the at least one of the positive electrode, the negative electrode and the separator to actinic radiation prior to providing an electrolyte to the cell assembly.
2. The cell assembly of Claim 1, in which the polymer comprises a copolymer of poly(vinylidene fluoride) and hexafluoropropylene.
3. The cell assembly of Claim 2, in which the copolymer comprises from about 75 to about 92% poly(vinylidene fluoride) and from about 8 to about 25% of hexafluoropropylene by weight.
4. The cell assembly of Claim 1, in which the polymer is free from residual groups of active hydrogen compounds.
5. The cell assembly of Claim 1, in which the at least one of the positive electrode, the negative electrode and the separator further comprises an organic plasticizer selected from the group consisting of propylene carbonate, ethylene carbonate, dibutyl phthalate, dimethyl phthalate, diethyl phthalate, tris-butoxyethyl phosphate and mixtures thereof.
6. The cell assembly of Claim 1, further comprising a lithium salt dissolved in a non-aqueous solvent.
7. The cell assembly of Claim 1, in which the actinic radiation is an electron beam radiation.
8. The cell assembly of Claim 7, in which the electron beam radiation is administered in dosages levels of from about 3 Mrad to about 20 Mrad.
9. The cell assembly of Claim 1, in which the positive electrode and the negative electrode further comprise one of lithium manganese oxide, carbon, carbon black, graphite, copper, aluminum, silica or a combination thereof.
10. A method for making a rechargeable solid polymer lithium ion battery cell, the method comprising:
(a) assembling a cell comprising a positive electrode, a negative electrode, and a separator membrane disposed there between, in which at least one of the positive electrode, the negative electrode and the separator comprises an organic plasticizer and a crosslinkable polymer free from crosslinking additives;
(b) exposing the cell to actinic radiation prior to providing an electrolyte to the cell; and (c) crosslinking the polymer.
11. The method of Claim 10, in which the polymer comprises a copolymer of poly(vinylidene fluoride) and hexafluoropropylene.
12. The method of Claim 11, in which the copolymer comprises from about 75 to about 92% poly(vinylidene fluoride) and from about 8 to about 25% of hexafluoropropylene by weight.
13. The method of Claim 10, in which the polymer is free from residual groups of active hydrogen compounds.
14. The method of Claim 10, in which the actinic radiation is electron beam radiation.
15. The method of Claim 10, further comprising;
(a) extracting the at least one of the positive electrode, the negative electrode and the separator with an organic solvent to remove a portion of the plasticizer; and (b) providing an electrolyte to the cell.
16. The method of Claim 10, further comprising;
(a) extracting the at least one of the positive electrode, the negative electrode and the separator with an organic solvent to remove a portion of the plasticizer prior to exposing the cell to actinic radiation; and
(b) providing an electrolyte to the cell.
17. A method for making a separator membrane for a rechargeable solid polymer lithium ion battery cell, comprising: (a) providing a solution of an organic plasticizer and a copolymer of poly (vinylidene fluoride), and hexafluoropropylene free from crosslinking additives in an organic solvent;
(b) casting the solution onto a surface to form a membrane;
(c) exposing the membrane to actinic radiation prior to contacting the membrane with an electrolyte; and
(d) crosslinking the copolymer.
18. The method of Claim 17, further comprising extracting the separator membrane with an organic solvent to remove a portion of the plasticizer.
19. The method of Claim 17, further comprising extracting the separator membrane with an organic solvent to remove a portion of the plasticizer prior to exposing the membrane to actinic radiation.
20. The method of Claim 17, in which the copolymer comprises from about 75 to about 92% poly(vinylidene fluoride) and from about 8 to about 25% of hexafluoropropylene by weight.
21. The method of Claim 17, in which the actinic radiation is electron beam radiation.
22. A method of making an electrode for a rechargeable solid polymer lithium ion battery cell, comprising:
(a) providing an expanded metal current collector;
(b) coating the current collector with a coating comprising an organic plasticizer and a copolymer of poly(vinylidene fluoride) and hexafluoropropylene free from crosslinking additives;
(c) exposing the coated current collector to actinic radiation prior to contacting the coated current collector with an electrolyte; and
(d) crosslinking the copolymer.
23. The method of Claim 22, further comprising extracting the coated current collector with an organic solvent to remove a portion of the plasticizer.
24. The method of Claim 22, further comprising extracting the coated current collector with an organic solvent to remove a portion of the plasticizer prior to exposing the coated current collector to radiation.
25. The method of Claim 22, in which the copolymer comprises from about 75 to about 92% poly(vinylidene fluoride) and from about 8 to about 25% of hexafluoropropylene by weight.
26. The method of Claim 22, in which the actinic radiation is electron beam radiation.
27. A rechargeable solid polymer lithium ion battery cell produced according to the method of Claim 10.
28. A separator membrane for a rechargeable solid polymer lithium ion battery cell produced according to the method of Claim 17.
29. An electrode for a rechargeable solid polymer lithium ion battery cell produced according to the method of Claim 22.
PCT/US1999/012096 1998-06-02 1999-06-01 Cross-linked polymeric components of rechargeable solid lithium batteries and methods for making same WO1999063609A1 (en)

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EP1014465A1 (en) * 1998-12-16 2000-06-28 Hughes Electronics Corporation Lithium ion battery cell having an etched-sheet current collector
WO2001067535A1 (en) * 2000-03-06 2001-09-13 Koninklijke Philips Electronics N.V. Method of manufacturing a lithium battery
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US8323815B2 (en) 2006-06-16 2012-12-04 Porous Power Technology, LLC Optimized microporous structure of electrochemical cells
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US9257721B2 (en) 2010-12-27 2016-02-09 Mamoru Baba Method for manufacturing all solid-state lithium-ion rechargeable battery, and method for testing all solid-state lithium-ion rechargeable battery
US9276246B2 (en) 2009-05-20 2016-03-01 Samsung Electronics Co., Ltd. Treatment and adhesive for microporous membranes
US10102979B2 (en) 2013-05-17 2018-10-16 Miltec Corporation Actinic and electron beam radiation curable water based electrode binders and electrodes incorporating same

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Cited By (14)

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Publication number Priority date Publication date Assignee Title
EP1014465A1 (en) * 1998-12-16 2000-06-28 Hughes Electronics Corporation Lithium ion battery cell having an etched-sheet current collector
WO2001067535A1 (en) * 2000-03-06 2001-09-13 Koninklijke Philips Electronics N.V. Method of manufacturing a lithium battery
US8323815B2 (en) 2006-06-16 2012-12-04 Porous Power Technology, LLC Optimized microporous structure of electrochemical cells
WO2009103082A2 (en) * 2008-02-17 2009-08-20 Porous Power Technologies, Llc Lamination configurations for battery applications using pvdf highly porous film
WO2009103082A3 (en) * 2008-02-17 2009-12-10 Porous Power Technologies, Llc Lamination configurations for battery applications using pvdf highly porous film
US9276246B2 (en) 2009-05-20 2016-03-01 Samsung Electronics Co., Ltd. Treatment and adhesive for microporous membranes
US9752063B2 (en) 2009-05-20 2017-09-05 Samsung Electronics Co., Ltd. Treatment and adhesive for microporous membranes
US8906548B2 (en) 2009-10-07 2014-12-09 Miltec Corporation Actinic and electron beam radiation curable electrode binders and electrodes incorporating same
WO2011044310A1 (en) * 2009-10-07 2011-04-14 Miltec Corporation Actinic and electron beam radiation curable electrode binders and electrodes incorporating same
US9543565B2 (en) 2009-10-07 2017-01-10 Miltec Corporation Actinic and electron beam radiation curable electrode binders and electrodes incorporating same
US9257721B2 (en) 2010-12-27 2016-02-09 Mamoru Baba Method for manufacturing all solid-state lithium-ion rechargeable battery, and method for testing all solid-state lithium-ion rechargeable battery
US10102979B2 (en) 2013-05-17 2018-10-16 Miltec Corporation Actinic and electron beam radiation curable water based electrode binders and electrodes incorporating same
US11043336B2 (en) 2013-05-17 2021-06-22 Miltec Corporation Actinic and electron beam radiation curable water based electrode binders and electrodes incorporating same
WO2015124406A1 (en) * 2014-02-24 2015-08-27 Robert Bosch Gmbh Sei-formation-inhibiting anode material additive

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