WO2001061772A1

WO2001061772A1 - Extraction of plasticizer from electrochemical cells

Info

Publication number: WO2001061772A1
Application number: PCT/US2001/003365
Authority: WO
Inventors: Tracy E. Kelley; Chariclea A. Scordilis-Kelley; Porter H. Mitchell; Jeremy Barker
Original assignee: Valence Technology, Inc.
Priority date: 2000-02-17
Filing date: 2001-02-01
Publication date: 2001-08-23
Also published as: AU2001233231A1

Abstract

A method for removing plasticizers such as dibutyl phthalate from an electrode / current collector assembly or from the anode, cathode, and separator components of an electrochemical cell precursor using an extraction solvent that includes an aliphatic alcohol selected from the group consisting of ethanol, butanol, pentanol and propanol and mixtures thereof. Most preferably the extraction solvent is isopropanol. Ultrasound waves can also be employed to significantly reduce the extraction time.

Description

EXTRACTION OF PLASTICIZER FROM ELECTROCHEMICAL CELLS

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to fabricating electrochemical (electrolytic) cells and, more particularly, to a method for removing plasticizer from an electrochemical cell (or a precursor thereof) by extraction.

State of the Art

Non-aqueous lithium electrochemical cells typically include an anode, a separator containing a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode.

More recently, the lithium metal anode has been replaced with a carbon anode such as coke or graphite intercalated with lithium ions to form Li_xC. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it reintercalates into the carbon. Because no metallic lithium is present in the cell, melting of the anode does not occur even under abuse conditions. Also, because lithium is reincorporated into the anode by intercalation rather than by plating, dendritic and spongy lithium growth does not occur. Non-aqueous lithium electrochemical cells are discussed in U.S. Patent Nos. 4,472,487, 4,668,595 and 5,028,500.

Various factors influence the performance of electrochemical cells. For instance, the morphology of the separator and of the polymeric binders in the anode and/or cathode can affect conductivity of the salts. Enhancement of conductivity has been demonstrated by forming porous polymeric matrices that form the separator and polymeric binders. One method of producing such porous structures comprises forming polymeric structures in the presence of a plasticizer and then removing the plasticizer to create pores in the polymer. Electrochemical cells typically include anode and cathode current collectors. A preferred material for the cathode current collector is aluminum. Unfortunately, it has been discovered that current extraction techniques that employ methanol to remove the plasticizer from electrochemical cell precursors also corrodes the aluminum current collector. This phenomenon is surprising since methanol and aluminum normally react very slowly at room temperature although the reaction rate does increase with temperature. The methanol reacts with alummum to form aluminum methoxide and hydrogen gas.

SUMMARY OF THE INVENTION

The present invention provides an efficient method for removing plasticizers from an electrochemical cell precursor by solvent extraction whereby the aluminum cathode current collector remains essentially unaffected. The invention is based in part on the recognition that the galvanic current that exists in the electrochemical cell precursor between either the carbon formulation of the grid coating of the current collector or the small potential difference between the cathode and anode of the precursor apparently increases the corrosion rate of the metal (e.g., aluminum) current collector.

Accordingly, in one aspect, the invention is directed to a method for removing plasticizer from an electrode/current collector assembly that comprises contacting an electrode/current collector assembly comprising (i) a current collector made from a metal that is selected from the group consisting of aluminum, copper, aluminum-copper alloys, stainless steel, and nickel, and (ii) an electrode composition comprising an electrode material, a polymer, and a plasticizer, with an alcohol extraction solvent that is selected from the group consisting of ethanol, propanol, butanol, and pentanol, and mixtures thereof.

In another aspect, the invention is directed to a method for removing plasticizer from an electrochemical cell precursor, that comprises contacting an electrochemical cell precursor comprising (i) an anode composition comprising an anodic material, a first polymer, and a first plasticizer; (ii) a cathode composition comprising a cathode active material, a second polymer, and a second plasticizer; and (iii) a separator comprising a third polymer and third plasitcizer, with an alcohol extraction solvent that is selected from the group consisting of ethanol, propanol, butanol, and pentanol, and mixtures thereof, wherein at least one of the anode composition or cathode composition is attached to a current collector.

In a further aspect, the invention is directed to a method of preparing an electrochemical cell, which method includes the steps of:

(a) forming an anode composition comprising an anodic material, a first polymer, and a first plasticizer; (b) forming a cathode composition comprising a cathode active material, a second polymer, and a second plasticizer;

(c) forming a separator comprising a third polymer and a third plasticizer; (d) attaching said separator to said anode composition and said cathode composition to form an electrochemical cell precursor wherein at least one of the anode composition or cathode composition is attached to a current collector that is made from a metal that is selected from the group consisting of aluminum, copper, aluminum-copper alloys, stainless steel and nickel;

(e) extracting at least a portion of said first, second and/or third plasticizer from said electrochemical cell precursor by contacting said electrochemical cell precursor with an extraction solvent while simultaneously applying ultrasound waves to said electrochemical cell precursor and said extraction solvent that is selected from the group consisting of ethanol, propanol, butanol, and pentanol, and mixtures thereof; and

(f) contacting said electrochemical cell precursor with an electrolyte solution comprising an electrolyte solvent and a salt to form an electrochemical cell.

Preferred solvents include the various isomeric forms of the alcohols including, for example, n-propanol, iso-propanol, n-butanol, sec-butanol, tert-butanol, isobutanol, n-pentanol, and iso-pentanol.

In particularly preferred embodiments, the extraction solvent is isopropanol and the plasticizers are dibutyl phthalate. In another preferred embodiment, the method includes the step of applying ultrasound waves to the electrode/current collector assembly or electrochemical cell precursor to facilitate the removal of the plasticizers. Preferably, the polymer(s) used in this invention is selected from the group consisting of copolymers of vinylidene difluoride and hexafluoropropylene, poly vinylidene difluoride, copolymers of ethylene and acrylic acid, copolymers of ethylene and vinyl acetate, and mixtures thereof. More preferably, the polymer is a copolymer of vinylidene difluoride and hexafluoropropylene .

Preferably, the plasticizer(s) employed in this invention is selected from the group consisting of dialkyl phthalates, wherein each alkyl group independently contains 1 to about 12 carbon atoms; trisbutoxyethyl

» phosphate; propylene carbonate; ethylene carbonate; trimethyl trimellitate; and mixtures thereof. More preferably, the plasticizer is a dialkyl phthalate selected from the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate and mixtures thereof. Still more preferably, the plasticizer is dibutyl phthalate.

The extraction solvent employed may also include solvent(s) selected from the group consisting of alkanes and cycloalkanes having 5 to about 12 carbon atoms; aliphatic alcohols having 5 to about 12 carbon atoms; haloalkanes and haloalkenes having 1 to about 6 carbon atoms; dialkyl ethers and cycloalkyl ethers having 4 to about 12 carbon atoms; dialkyl formates, wherein each alkyl group independently contains 1 to about 6 carbon atoms; diaikyl carbonates, wherein each alkyl group independently contains 1 to about 6 carbon atoms; and mixtures thereof.

In the methods of this invention, the electrolyte solvent preferably comprises one or more organic carbonates. More preferably, the electrolyte solvent comprises a mixture of ethylene carbonate and dimethyl carbonate. Additionally, the salt used in this invention is preferably an alkali metal salt of an anion selected from the group consisting of I^", Br^", SCN^", ClO₄", BF₄ ^", PF₆ ^", AsF₆\ CF₃COO\ CF₃SO₃-, and N(SO₂CF₃)₂-.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an improved method of extracting plasticizer from an electrochemical cell precursor whereby the structural integrity of the metal current collector(s) is not adversely affected as with some prior art techniques. The extraction solvent is an aliphatic alcohol selected from the group consisting of ethanol, propanol, butanol, and pentanol and mixtures thereof. Preferred solvents include the various isomeric forms of the alcohols including, for example, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, n-pentanol, and iso-pentanol. Preferably, the extraction solvent is isopropanol; the presence of methanol should be avoided. Upon removal of the plasticizer, the polymer network of these components has a stable porous structure. Thereafter, the precursor is activated by the addition of an electrolyte solution comprising electrolyte solvents and salts. Electrochemical cells so fabricated will demonstrate superior electrochemical performance.

In another aspect of the invention, ultrasound waves are applied to facilitate the extraction process.

Preferred electrochemical cells comprise a cathode comprising a cathode active material and an intercalation-based carbon anode, with each electrode capable of reversibly incorporating (e.g., intercalating) an alkali metal ion, and a separator containing an electrolyte solution comprising an organic electrolyte solvent and a salt of the alkali metal. Each electrode has a current collector. Particularly preferred electrochemical cells and batteries use lithium and salts thereof. Preferably, the anode comprises an anode film that is laminated onto one or both sides of a current collector which is a thin metal foil or grid. Typically, each anode film is from about 100 μm to about 250 μm in thickness, preferably about 110 μm to about 200 μm, and more preferably about 125 μm to about 175 μm.

Similarly, the cathode preferably comprises a cathode film that is laminated onto one or both sides of the current collector which is a thin foil or grid. Typically, each cathode film is from about 100 μm to about 200 μm in thickness, preferably about 130 μm to about 175 μm, and more preferably about 140 μm to about 165 μm.

The anode and cathode each also preferably include a current collector that comprises, for example, a screen, grid, expanded metal, woven or non-woven fabric formed from an electron conductive material such as metals or alloys. Preferably, the current collector has a thickness from about 25 μm to about 75 μm, preferably about 35 μm to about

65 μm, and more preferably about 45 μm to about 55 μm. Each current collector is also connected to a current collector tab which extends from the edge of the current collector. In batteries comprising multiple electrochemical cells, the anode tabs are preferably welded together and connected to a copper or nickel lead. The cathode tabs are similarly welded and connected to a lead. External loads can be electrically connected to the leads. Current collectors and tabs are described in U.S. Patent 4,925,752, 5,011,501, and 5,326,653, which are incorporated herein.

Prior to describing this invention in further detail, the following terms will be defined. The terms "ultrasound waves" or "ultrasonic waves" or "ultrasonic energy" refer to sound waves having a frequency above about 20,000 hertz.

The term "plasticizer" refers to an organic solvent, with limited solubility of polymers, that facilitates the formation of porous polymeric structures. By "porous structure" is meant that upon extraction of the plasticizer the polymer remains as a porous mass. Suitable plasticizers have high boiling points typically from about 100°C to about 350°C. A number of criteria are important in the choice of plasticizer including compatibility with the components of the electrochemical cell precursor, processability, low polymer solubility and extractability.

Preferred plasticizers are selected from the group consisting of dialkyl phthalates, wherein each alkyl group independently contains 1 to about 12 carbon atoms; trisbutoxyethyl phosphate; propylene carbonate; ethylene carbonate; and mixtures thereof. Particularly preferred plasticizers include, by way of example, dialkyl phthalates selected from the group consisting of dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate and mixtures thereof. Other preferred plasticizers include, for example, acetates, glymes and low molecular weight polymers.

Preferably the weight ratio of plasticizer to polymer is from about 1 to about 50, more preferably about 10 to about 30, and most preferably about 20 to about 25.

The term "extraction solvent" refers a low boiling organic solvent in which the plasticizer is soluble or miscible and the polymer is essentially insoluble (i.e., a non-solvent for the polymer). Extraction solvents used in the present invention include the alcohols described above. Other extraction solvents can also be included and these have boiling points of less than about 150°C, more preferably less than 100°C, and still more preferably less than 75 °C. Additionally, the extraction solvent preferably has a low viscosity, preferably less than about 2.0 cp at 25 °C, more preferably less than about 1.0 cp at 25 °C. Preferred solvents also have low toxicity and a low fire hazard risk.

Preferably, the other extraction solvents are selected from the group consisting of alkanes and cycloalkanes having 5 to about 12 carbon atoms; aliphatic alcohols having 5 to about 12 carbon atoms; haloalkanes and haloalkenes having 1 to about 6 carbon atoms; dialkyl ethers and cycloalkyl ethers having 4 to about 12 carbon atoms; dialkyl formates, wherein each alkyl group independently contains 1 to about 6 carbon atoms; dialkyl carbonates, wherein each alkyl group independently contains 1 to about 6 carbon atoms; and mixtures thereof.

Other preferred extraction solvents include diethyl ether, petroleum ether, propanes, butanes, pentanes, hexanes, cyclohexane, dichloromethane, trichloroethylene, fluorotrichloromethane, chlorotrifluoromethane, carbon tetrafluoride, dichlorofluromethane, chlorodifluoromethane, trifluoromethane, 1,2-dichlorotetrafluoroethane, hexafluoroethane, trichloroethane, and the like, and mixtures thereof.

The term "electrochemical cell precursor" or "electrolytic cell precursor" refers to the structure of the electrochemical cell prior to the addition of the electrolyte solution. The precμrsor typically comprises (each in precursor form) an anode, a cathode, and separator that typically comprises a polymeric matrix. The anode and/or cathode may each include a current collector. The term "activation" refers to the placement of an inorganic salt and electrolyte solvent into an electrochemical cell precursor. After activation, the electrochemical cell is charged by an external energy source prior to use.

The term "electrochemical cell" or "electrolytic cell" refers to a composite structure containing an anode, a cathode, and an ion-conducting electrolyte interposed therebetween.

The term "battery" refers to two or more electrochemical cells electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels.

The term "salt" refers to any salt, for example, an morgamc salt, which is suitable for use in a non-aqueous electrolyte. Representative examples of suitable inorganic ion salts are alkali metal salts of less mobile anions of weak bases having a large anionic radius. Examples of such anions are I^', Br, SCN^', C V, BF₄\ PF₆\ AsF₆ ^", CF₃COO\ CF₃SO₃\ N(SO₂CF₃)₂ ^", and the like. Specific examples of suitable inorganic ion salts include LiClO₄, LiSCN, LiBF₄, LiAsF₆, LiCF₃SO₃, LiPF₆, (CF₃SO₂)₂NLi, (CF₃SO₂)₃CLi, NaSCN, and the like. The inorganic ion salt preferably contains at least one cation selected from the group consisting of Li, Na, Cs, Rb, Ag, Cu, Mg and K.

The electrolyte typically comprises from about 5 to about 25 weight percent of the inorganic ion salt based on the total weight of the electrolyte; preferably, from about 10 to 20 weight percent; and even more preferably from about 10 to about 15 weight percent. The percentage of salt depends on the type of salt and electrolytic solvent employed. The term "compatible electrolyte solvent" or "electrolytic solvent," or in the context of components of the non-aqueous electrolyte, just "solvent," is a low molecular weight organic solvent added to the electrolyte and/or the cathode composition, which may also serve the purpose of solvating the inorganic ion salt. The solvent is any compatible, relatively non-volatile, aprotic, relatively polar, solvent. Preferably, these materials have boiling points greater than about 85 °C to simplify manufacture and increase the shelf life of the electrolyte/battery. Typical examples of solvent are mixtures of such materials as dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, methyl ethyl carbonate, gamma- butyrolactone, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and the like. When using propylene carbonate based electrolytes in an electrolytic cell with graphite anodes, a sequestering agent, such as a crown ether, is added in the electrolyte.

For electrochemical cells where (1) the cathode comprises lithiated cobalt oxides, lithiated manganese oxides, lithiated nickel oxides, Li_xNi,._yCo_yO₂, where x is preferably about 1 and y is preferably 0.1-0.9, LiNiVO₄, or LiCoVO₄, and (2) the anode comprises carbon, the electrolytic solvent preferably comprises a mixture of ethylene carbonate and dimethyl carbonate. For electrochemical cells where the cathode comprises vanadium oxides, e.g., V₆O₁₃ and the anode is lithium, the electrolytic solvent preferably comprises a mixture of propylene carbonate and triglyme.

The term "organic carbonate" refers to hydrocarbyl carbonate compounds of no more than about 12 carbon atoms and which do not contain any hydroxyl groups. Preferably, the organic carbonate is an aliphatic carbonate and more preferably a cyclic aliphatic carbonate. Suitable cyclic aliphatic carbonates for use in this invention include l,3-dioxolan-2-one (ethylene carbonate); 4-methyl-l,3-dioxolan-2-one (propylene carbonate); 4,5-dimethyl-l,3-dioxolan-2-one; 4- ethyl-1 ,3-dioxolan-2-one; 4,4-dimethyl-l ,3-dioxolan-2-one; 4-methyl-5- ethyl- l,3-dioxolan-2-one; 4,5-diethyl-l,3-dioxolan-2-one; 4,4-diethyl-l,3- dioxolan-2-one; l,3-dioxan-2-one; 4,4-dimethyl-l, 3-dioxan-2-one; 5,5-di- methyl-1 ,3-dioxan-2-one; 5-methyl-l ,3-dioxan-2-one; 4-methyl-l ,3-dioxan- 2-one; 5,5-diethyl-l,3-dioxan-2-one; 4,6-dimethyl-l,3-dioxan-2-one; 4,4,6- trimethyl-l,3-dioxan-2-one; and spiro (1 , 3-oxa-2-cy clohexanone-5 ' , 5 ' , 1 ' , 3 ' -oxa-2 ' -cyclohexanone) .

Several of these cyclic aliphatic carbonates are commercially available such as propylene carbonate and ethylene carbonate. Alternatively, the cyclic aliphatic carbonates can be readily prepared by well known reactions. For example, reaction of phosgene with a suitable alkane-α,β-diol (dihydroxy alkanes having hydroxyl substituents on adjacent carbon atoms) or an alkane-c ,γ-diol (dihydroxy alkanes having hydroxyl substituents on carbon atoms in a 1,3 relationship) yields an a cyclic aliphatic carbonate for use within the scope of this invention. See, for instance, U.S. Patent No. 4,115,206, which is incorporated herein by reference in its entirety.

Likewise, the cyclic aliphatic carbonates useful for this invention may be prepared by transesterification of a suitable alkane-α,β-diol or an alkane- α,γ-diol with, e.g., diethyl carbonate under transesterification conditions. See, for instance, U.S. Patent Nos. 4,384,115 and 4,423,205 which are incorporated herein by reference in their entirety. Additional suitable cyclic aliphatic carbonates are disclosed in U.S. Patent No. 4,747,850 which is also incorporated herein by reference in its entirety. The anode typically comprises a compatible anodic material which is any material which functions as an anode in an electrolytic cell. Such compatible anodic materials are well known in the art and include, by way of example, lithium, lithium alloys, such as alloys of lithium with aluminum, mercury, manganese, iron, zinc, intercalation based anodes such as those employing carbon, tungsten oxides, and the like. Preferred anodes include lithium intercalation anodes employing carbon materials such as graphite, cokes, mesocarbons, disordered carbon, hard carbon and the like. The anode may also include an electron conducting material such as carbon black.

Such carbon intercalation based anodes typically include a polymeric binder and extractable plasticizer suitable for forming a bound porous composite having a molecular weight of from about 1,000 to 5,000,000. Preferred polymeric binders include, but are not limited to, copolymers of vinylidene difluoride and hexafluoropropylene, polyvinylidene difluoride, copolymers of ethylene and acrylic acid, copolymers of ethylene and vinyl acetate, and mixtures thereof. Examples of other suitable polymeric binders include EPDM (ethylene propylene diamine termonomer) and the like. Especially preferred polymers are copolymers of vinylidenedifluoride and hexafluoropropylene. Preferably, the polymer or copolymer employed has a high average molecular weight. Preferably, the average molecular weight is between 50,000 to 750,000, more preferably 50,000 to 200,000, and most preferably 50,000 to 120,000. Furthermore, it is preferred that polymer or copolymer has a narrow molecular weight have range.

Polyvinylidene difluoride (PVDF) and hexafluoropropylene (HFP) copolymers are common binder materials, and are the binder materials which are generally useful in the subject invention. The copolymer generally comprises about 75 to 92% (by weight) of the PVDF, and about 8 to 25 % HFP. Preferably, the copolymer comprises about 85 to 90% of the PVDF, and about 10 to 15% HFP. One especially preferred, commercially available copolymer material is KYNAR. Flex 2801 (Elf Atochem North America, Philadelphia, PA), which provides an 88:12 ratio of PVDF:HFP. Inorganic filler adjuncts, such as fumed alumina or fumed silica, are added as desired to provide structural stability to the binder and provide a film having desirable qualities.

The cathode typically comprises a compatible cathodic material (i.e., insertion compounds) which is any material which functions as a positive pole in an electrolytic cell. Such compatible cathodic materials are well known in the art and include, by way of example, transition metal oxides, sulfides, and selenides, including lithiated compounds thereof. Representative materials include cobalt oxides, manganese oxides, molybdenum oxides, vanadium oxides, sulfides of titanium, molybdenum and niobium, the various chromium oxides, copper oxides, lithiated cobalt oxides, e.g., LiCoO₂ and LiCoVO₄, lithiated manganese oxides, e.g., LiMn₂O₄, lithiated nickel oxides, e.g., LiNiO₂ and LiNiVO_4> and mixtures thereof. Cathode-active material blends of Li_xMn₂O₄ (spinel) is described in U.S. Patent 5,429,890 which is incorporated herein. The blends can include Li_xMn₂O₄ (spinel) and at least one lithiated metal oxide selected from

Li_xNiO₂ and Li_xCoO₂ wherein 0 <x<2. Blends can also include Li_y-α-MnO₂ (0<y< 1) which has a hollandite-type structure and is described in U.S. Patent 5,561,007 which is incorporated herein by reference.

In one preferred embodiment, the compatible cathodic material is mixed with an electroconductive material including, by way of example, graphite, powdered carbon, powdered nickel, metal particles, conductive polymers (i.e., characterized by a conjugated network of double bonds like polypyrrole and poly acetylene), and the like, and a polymeric binder to form under pressure a positive cathodic plate. The polymeric binder for the cathode can be the same as those used in the anode.

The term "separator" refers to the ionic medium between the anode and cathode. The electrolyte/separator is typically a solid electrolyte, or separator and liquid electrolyte. Solid electrolytes typically comprise polymeric matrices which contain an ionic conductive medium. Liquid electrolytes typically comprise a solvent and an alkali metal salt which form an ionically conducting liquid. In this latter case, the separation between the anode and cathode is maintained, for example, by a relatively inert layer of material.

A separator membrane may be a commercially available separator made of glass fiber, porous polypropylene or porous polyethylene. Such separators include Type A/E glass fiber filter (Gelman Sciences, Ann Arbor, MI), and Celgard (Hoechst-Celanese Corp., NY, NY).

Preferred polymeric (or electrolyte membranes) are produced using a casting process in which a carrier liquid is removed to form a flexible sheet. An alternate preferred method produces polymeric electrolyte membranes by extrusion processes. Suitable polymeric electrolyte membranes provide a porous structure, permeated with a plasticizer, upon casting or curing.

Suitable solid polymeric matrices include solid matrices formed from organic polymers, inorganic polymers or a mixture of organic polymers with inorganic non-polymeric materials. Preferably, the solid polymeric matrix is an organic matrix derived from a solid matrix forming monomer and from partial polymers of a solid matrix forming monomer. See, for example, U.S. Patents 5,501,921, 5,498,491, 5,491,039, 5,489,491, 5,482,795, 5,463,179, 5,419,984, 5,393,621, 5,358,620, 5,262,253, 5,346,787, 5,340,669, 5,300,375, 5,294,501, 5,262,253, and 4,908,283, which are incorporated herein. Inorganic monomers are disclosed in U.S. Patents 4,247,499, 4,388,385, 4,414,607, 4,394,280, 4,432,891, 4,539,276, and 4,557,985, which are incorporated herein.

The solid matrix forming monomer or partial polymer can be cured or further cured prior to or after addition of the salt, solvent and, optionally, a viscosifier. For example, a composition comprising requisite amounts of the monomer or partial polymer, salt, organic carbonate solvent and viscosifier can be applied to a substrate and then cured. Alternatively, the

» monomer or partial polymer can be first cured and then dissolved in a suitable volatile solvent. Requisite amounts of the salt, organic carbonate solvent and viscosifier can then be added. The mixture is then placed on a substrate and removal of the volatile solvent would result in the formation of a solid electrolyte. In either case, the resulting solid electrolyte would be a homogeneous, single phase product which is maintained upon curing, and does not readily separate upon cooling to temperatures below room temperature.

Preferably, the solid polymeric matrix can be formed by a casting process which does not require the use of monomers or prepolymers, that is, no curing is required. A preferred method employs a copolymer of polyvinylidene difluoride and hexafluoropropylene dissolved in acetone or other suitable solvent. Upon casting the solution, the solvent is evaporated to form the solid polymeric matrix. The solution may be casted directly onto a current collector. Alternatively, the solution is casted onto a substrate, such as a carrier web, and after the solvent (e.g., acetone) is removed, an electrode film is formed thereon. Preferably, the solid polymeric matrix further comprises a silanized fumed SiO₂. The SiO₂ is a filler which impart toughness and strength to the polymeric matrix. In addition, it is believed that the SiO₂ assists the activation process by creating physico-chemical conditions such that the electrolyte solution quickly and completely fills the pores created in the polymeric matrix by the extraction of the plasticizer.

The term "viscosifier" refers to a suitable viscosifier for solid electrolytes. Viscosifiers include conventional viscosifiers such as those known to one of ordinary skill in the art. Suitable viscosifiers include film forming agents well known in the art which include, by way of example, polyethylene oxide, polypropylene oxide, copolymers thereof, and the like, having a number average molecular weight of at least about 100,000, polyvinylpyrrolidone, carboxymethylcellulose, and the like. Preferably, the viscosifier is employed in an amount of about 1 to about 10 weight percent and more preferably at about 2.5 weight percent based on the total weight of the electrolyte composition.

The electrolyte composition typically comprises from about 5 to about 25 weight percent of the inorganic ion salt based on the total weight of the electrolyte; preferably, from about 10 to 20 weight percent; and even more preferably from about 10 to about 15 weight percent. The percentage of salt depends on the type of salt and electrolytic solvent employed.

The electrolyte composition typically comprises from 0 to about 80 weight percent electrolyte solvent (e.g., organic carbonate/giyme mixture) based on the total weight of the electrolyte; preferably from about 60 to about 80 weight percent; and even more preferably about 70 weight percent. The electrolyte composition typically comprises from about 5 to about 30 weight percent of the solid polymeric matrix based on the total weight of the electrolyte; preferably from about 15 to about 25 weight percent.

In a preferred embodiment, the electrolyte composition further comprises a small amount of a film forming agent. Suitable film forming agents are well known in the art and include, by way of example, polyethylene oxide, polypropylene oxide, copolymers thereof, and the like, having a numbered average molecular weight of at least about 100,000. Preferably, the film forming agent is employed in an amount of about 1 to about 10 weight percent and more preferably at about 2.5 weight percent based on the total weight of the electrolyte composition.

METHODOLOGY

Electrochemical cells are known in the art. See, for example, U.S. Patents 5,300,373, 5,316,556, 5,346,385, 5,262,253, 4,472,487,

4,668,595, and 5,028,500, all of which are incorporated herein. The methods of this invention can be adapted to form porous anode, cathode, and/or polymeric matrix structures in prior art electrochemical cells.

In a preferred embodiment, the extraction method of the present invention is conducted by first placing an electrochemical cell precursor comprising (i) an anode composition comprising an anodic material, a first polymer, and a first plasticizer; (ii) a cathode composition comprising a cathode active material, a second polymer, and a second plasticizer; and (iii) a separator comprising a third polymer and a third plasticizer, in a vessel. The precursor includes current collector(s) for the anode and/or cathode. An extraction solvent, e.g. isopropanol, is then introduced into the vessel so that the electrochemical cell precursor is contacted with, and preferably immersed in, the extraction solvent. The extraction solvent is removed after a sufficient time to permit the plasticizers to be removed. Thereafter, the precursor is removed from the extraction solvent. The extraction process can be repeated with fresh extraction solvent until the desired level of plasticizer is removed.

In another preferred embodiment, ultrasound waves are applied to the electrochemical cell precursor and the extraction solvent in the vessel so that the first, second and/or third plasticizer in the electrochemical cell precursor is solubilized into the extraction solvent. The extraction solvent containing the plasticizer is then removed from contact with the electrochemical cell precursor.

The extraction process can be performed using either batch processes or continuous extraction methods. In a preferred embodiment, the extraction solvent flows or is circulated through the extraction vessel so that extraction solvent containing extracted plasticizer is continually removed from the extraction vessel while pure or fresh extraction solvent is added. Preferably, the extraction solvent is vigorously mixed during the extraction process either by mechanical means, such as stirring, or by bubbling air or nitrogen through the extraction vessel.

Preferably, the ultrasound waves employed have a frequency ranging from about 30 kHz to about 45 kHz. More preferably, the ultrasound waves have a frequency ranging from about 38 kHz to about 42 kHz. In a preferred embodiment, a sweeping frequency, i.e. a frequency continuously increasing and decreasing from about 30 kHz to about 45 kHz, preferably from 38 kHz to about 42 kHz, is employed to eliminate standing waves. Any art-recognized apparatus or vessel may be used to conduct the ultrasound extraction process. A preferred apparatus for conducting the extraction is a Bransonic Ultrasonic Cleaner (Model 5210-MTH), available from VWR Scientific Products. One of the benefits of employing ultrasound waves is that the time required to complete extraction is significantly reduced without adversely affecting the electrochemical performance of electrode cells made.

The extraction process is preferably carried out at a temperature ranging from ambient to the boiling point of the extraction solvent. In one preferred embodiment, the extraction is carried out at a temperature of from about 0° to about 50 °C, still more preferably, from about 20 °C to. about 30°C. In another preferred embodiment, the extraction is carried out at a temperature near but less than the boiling point of the extraction solvent, preferably less than about 10 °C below the boiling point of the extraction solvent.

The ultrasonic extraction process of this invention extracts at least a portion of the plasticizer from the electrochemical ceil precursor. Preferably, at least 50% of the total amount of plasticizer in the electrochemical cell precursor is extracted by the ultrasonic extraction process. More preferably, at least 75% and still more preferably at least 90% of the plasticizer is removed by the extraction process. Removal of at least 95% of the plasticizer is especially preferred.

The extraction solvent used in the methods of this invention can be recovered and recycled by, for example, conventional distillation techniques, such as flash distillation. Similarly, the plasticizer can be recovered and reused after removal of the extraction solvent. EXAMPLES

The following examples are offered to illustrate this invention and are not to be construed in any way as limiting the scope of this invention. In these examples, the invention will be described using anode and cathode structures in which electrode materials (or films) are laminated onto both sides of the current collectors. It is to be understood, however, that the invention is applicable to other configurations, for example, where one side of the anode and/or cathode current collector is laminated.

Example 1

Preparation of an Anode Composition A polymer mixture comprising a copolymer of vinylidenedifluoride (VDF) and hexafluoropropylene (HFP) was prepared by mixing 6.8 grams of the copolymer in 20 grams of acetone. The copolymer (average molecular weight 125,000) was Kynar Flex 2801™ from Elf Atochem North America, in Philadelphia, Pennsylvania. The mixture was stirred for about 24 hours in a milling jar available from VWR Scientific, in San Francisco, California, model H-04172-00. The copolymer functions as a binder for the carbon in the anode.

A graphite mixture was prepared separately by first adding 23.4 grams of graphite into 0.9 grams of carbon black into a solution containing 60 grams acetone, 10.5 grams dibutyl phthalate, and 0.5 grams of a surfactant. A preferred graphite is available under the designation BG35 graphite from Superior Graphite Co., Chicago, Illinois. A preferred carbon black is available under the designation Super P™ from M.M.M. Carbon, Willebroek, Belgium. The graphite mixture was then vigorously mixed in a high shear mixer until a substantially homogeneous blend was formed. A suitable mixer is available from Ross, Model ME100DLX, Hauppauge, New York, operating at its highest setting (about 10,000 RPM) for 30 minutes. An anode slurry was then prepared by mixing the polymer mixture and the graphite mixture together under low shear conditions to form an anode slurry wherein the components are well mixed. A portion of the acetone was allowed to evaporate from the slurry and it was then laminated onto each side of a current collector. The anode current collector employed was a sheet of expanded copper metal that is about 50 μm thick. It is available under the designation 2Cu5-125 (flatten) from Delker, in Branford, Connecticut. Anode films formed when the remaining portion of the acetone evaporated.

Example 2

Preparation of a Cathode Composition A polymer mixture comprising a copolymer of vinylidenedifluoride (VDF) and hexafluoropropylene (HFP) was prepared by mixing 4.4 grams of the copolymer in 15 ml of acetone. The copolymer was Kynar Flex 2801™. The mixture was stirred for about 24 hours in a milling jar.

A cathode mixture was prepared separately by mixing 28.9 grams of LiMn₂O₄, 2.4 grams of carbon black (Super P™) into a solution containing 60 grams acetone and 8.7 grams dibutyl phthalate. The mixture was then vigorously mixed in the a high shear mixer until a substantially homogeneous blend was formed. The amount of cathode-active material LiMn₂O₄ employed can be varied to provide different cathode to anode mass ratios.

A cathode slurry was prepared by mixing the polymer mixture and the cathode mixture together under low shear conditions to form the cathode slurry wherein the components are well mixed. A portion of the acetone was allowed to evaporate from the slurry and it was then laminated onto each side of a cathode current collector. The cathode current collector employed was a sheet of expanded aluminum that is about 50 μm thick. The aluminum grid is available under the designation 2AL5-077 from Delker, in Branford, Connecticut. Cathode films formed when the remaining portion of the acetone evaporated.

The above anode and cathode films were formed directly on the current collector by laminating the slurry mixtures onto the current collector surfaces. Alternatively, each film can be prepared by first casting a slurry onto a substrate or carrier web and allowing the solvent to evaporate thus leaving the film. Thereafter, the films can be laminated onto each side of the current collector.

Example 3

Preparation of a Separator A polymeric matrix was formed by casting a polymeric slurry comprising acetone, dibutyl phthalate, silanized fumed SiO₂, and the VDF/HFP copolymer on a suitable substrate or carrier web and allowing the acetone to evaporate. No curing by radiation is required. The SiO₂ is a filler which imparts toughness and strength to the film. In addition, it is believed that the SiO₂ assists the activation process by creating physico- chemical conditions such that the electrolyte solution quickly and completely fills the pores created by the extraction of the dibutyl phthalate. Preferably, the polymeric slurry is mixed under low shear conditions as not to degrade the copolymer.

Example 4 Extraction Center with Methanol and Isopropyl

Electrochemical cell precursors were generally prepared employing the components set forth in Example 1-3. Each precursor comprised a "bicell" structure that has a cathode disposed between two anodes. The cathode includes an aluminum current collector grid with cathode material laminated on both sides of the grid; similarly the anode includes a copper current collector grid with anode material laminated on both sides. In forming the bicell, a polymeric layer is disposed between each of the two anode/cathode interfaces. In the first set of experiments, the bicell was cut into individual laminate structures or "coupons" which were subject to solvent extraction to remove the plasticizer, i.e., dibutyl phthalate.

The plasticizers in the coupons were extracted with an extraction solvent that contained methanol and varying amounts of water. The coupons were submerged in the extraction solvent within an enclosed vessel for 2 hours; thereafter, the amount of hydrogen in the air vessel was measured. Table 1 sets forth the results.

TABLE 1

* Weight of each coupon

* * DMAC is N,N-dimethylacetamide As is apparent, the presence of water in the extraction solvent proportionally decreases the amount of hydrogen generated; this suggests that aluminum corrosion increases with the dryness of the methanol. Therefore, ah aqueous methanol mixture containing, for example, 10% (wt) water is a suitable extraction system for removing plasticizers. In contrast, as further described herein, bicells that were placed in an extraction solvent consisting of isopropyl alcohol showed no hydrogen generation at all.

Samples J and K demonstrate that methanol is relatively unreactive towards aluminum but methanol is very reactive toward pieces of aluπiinum and copper placed ^"in physical contact with each other. It is believed that coupled metals creates galvanic corrosion of the aluminum. Specifically, in the electrochemical cell the aluminum becomes "anodic" when coupled with the copper, which becomes "cathodic", with the methanol acting as the electrolyte.

In a second set of experiments, bicells were immersed in 15 ml of methanol (with 250 ppm H₂O) for two hours and the amount of hydrogen gas was measured. The copper anode current collectors were either uncoated or coated with one of two types of adhesives: (1) the "PC" coating which is a mixture of polymer and graphite or (2) the "CA" which is a polymeric material that is cured. The aluminum cathode current collector was coated with PC or CA. In Examples L, M, N, and O the bicell coupons were subject to extraction to remove the plasticizer prior to being immersed in the methanol.

The results of the test are set forth in Table 2. As is apparent, even with the coatings of the current collectors, significant aluminum corrosion occurred. TABLE 2

In a third set of experiments, bicells were immersed in 15 ml of isopropyl alcohol (with 850 ppm H₂O) for two hours and the amount of hydrogen gas was measured. The current collectors were either uncoated or coated as described above. In each case, the bicell coupons were subject to extraction to remove the plasticizer prior to being immersed in the alcohol.

The results of the test are set forth in Table 3. As is apparent, no aluminum corrosion occurred in any of the examples.

TABLE 3

Although only preferred embodiments of the invention are specifically disclosed and described above, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for removing plasticizer from an electrode/current collector assembly that comprises contacting an electrode/current collector assembly comprising (i) a current collector made from a metal that is selected from the group consisting of aluminum, copper, aluminum-copper alloys, stainless steel, and nickel; and (ii) an electrode composition comprising an electrode material, a polymer, and a plasticizer, with an alcohol extraction solvent that is selected from the group consisting of ethanol, propanol, butanol, pentanol and mixture thereof.

2. The method of claim 1 wherein the current collector is made of aluminum.

3. The method of claim 2 wherein the extraction solvent is isopropanol.

4. The method of claim 1 wherein the polymer is selected from the group consisting of copolymers of vinylidene difluoride and hexafluoropropylene, polyvinylidene difluoride, copolymers of ethylene and acrylic acid, copolymers of ethylene and vinyl acetate, and mixtures thereof.

5. The method of claim 1 wherein the plasticizer is selected from the group consisting of dialkyl phthalates, wherein each alkyl group independently contains 1 to about 12 carbon atoms, trisbutoxyethyl phosphate, propylene carbonate, ethylene carbonate, and mixtures thereof.

6. The method of claim 1 wherein the extraction solvent consists essentially of propanol.

7. The method of claim 1 wherein the extraction solvent consists essentially of isopropanol.

8. The method of claim 1 further comprising applying ultrasound waves to the electrode/current collector assembly and the extraction solvent to extract at least a portion of the plasticizer from the electrode/current collector assembly.

9. The method of claim 1 further comprising removing the extraction solvent from contact with the electrode/current collector assembly.

10. A method for removing plasticizer from an electrochemical cell precursor, that comprises contacting an electrochemical cell precursor comprising (i) an anode composition comprising an anodic material, a first polymer, and a first plasticizer; (ii) a cathode composition comprising a cathode active material, a second polymer, and a second plasticizer; and (iii) a separator comprising a third polymer and a third plasticizer, with an alcohol extraction solvent that is selected from the group consisting of ethanol, propanol, butanol, and pentanol and mixtures thereof, wherein at least one of the anode composition or cathode composition is attached to a current collector.

11. The method of claim 10 wherein the current collector is made of aluminum.

12. The method of claim 11 wherein the extraction solvent is isopropanol.

13. The method of claim 10 wherein said first, second and third polymers are each independently selected from the group consisting of copolymers of vinylidene difluoride and hexafluoropropylene, polyvinylidene difluoride, copolymers of ethylene and acrylic acid, copolymers of ethylene and vinyl acetate, and mixtures thereof.

14. The method of claim 10 wherein said first, second and third plasticizers are each independently selected from the group consisting of dialkyl phthalates, wherein each alkyl group independently contains 1 to about 12 carbon atoms, trisbutoxyethyl phosphate, propylene carbonate, ethylene carbonate, and mixtures thereof.

15. The method of claim 10 wherein the extraction solvent consists essentially of propanol.

16. The method of claim 10 wherein the extraction solvent consists essentially of isopropanol.

17. The method of claim 10 further comprising applying ultrasound waves to the electrochemical cell precursor and the extraction solvent to extract at least a portion of the plasticizer from the electrochemical cell precursor.

18. The method of claim 10 further comprising removing the extraction solvent from contact with the electrochemical cell precursor.

19. A method of preparing an electrochemical cell, which method comprises the steps of:

(a) forming an anode composition comprising an anodic material, a first polymer, and a first plasticizer;

(b) forming a cathode composition comprising a cathode active . material, a second polymer, and a second plasticizer; (c) forming a separator comprising a third polymer and a third plasticizer;

(d) attaching said separator to said anode composition and said cathode composition to form an electrochemical cell precursor wherein at least one of the anode composition or cathode composition is attached to a current collector that is made from a metal that is selected from the group consisting of aluminum, copper, aluminum-copper alloys, stainless steel, and nickel;

(e) extracting at least a portion of said first, second and/or third plasticizer from said electrochemical cell precursor by contacting said electrochemical cell precursor with an extraction solvent that is selected from the group consisting of ethanol, propanol, butanol, pentanol and mixtures thereof, while simultaneously applying ultrasound waves to said electrochemical cell precursor and said extraction solvent; and

20. The method of claim 19 wherein the current collector is made of aluminum.

21. The method of claim 20 wherein the extraction solvent is isopropanol.

22. The method of claim 19 wherein said first, second and third polymers are each independently selected from the group consisting of copolymers of vinylidene difluoride and hexafluoropropylene, polyvinylidene difluoride, copolymers of ethylene and acrylic acid, copolymers of ethylene and vinyl acetate, and mixtures thereof.

23. The method of claim 19 wherein said first, second and third plasticizers are each independently selected from the group consisting of dialkyl phthalates, wherein each alkyl group independently contains 1 to about 12 carbon atoms, trisbutoxyethyl phosphate, propylene carbonate, ethylene carbonate, and mixtures thereof.

24. The method of claim 19 wherein the extraction solvent consists essentially of propanol.

25. The method of claim 19 wherein the extraction solvent consists essentially of isopropanol.

26. The method of claim 19 step (e) further comprising applying ultrasound waves to the electrochemical cell precursor and the extraction solvent to extract at least a portion of the plasticizer from the electrochemical cell precursor.

27. The method of claim 19 step (e) further comprising removing the extraction solvent from contact with the electrochemical cell precursor.