EP1060526A1

EP1060526A1 - Methods for preparing electrochemical cells using solvent extraction techniques

Info

Publication number: EP1060526A1
Application number: EP99911094A
Authority: EP
Inventors: Porter H. Mitchell
Original assignee: Valence Technology Inc
Current assignee: Valence Technology Inc
Priority date: 1998-03-04
Filing date: 1999-03-03
Publication date: 2000-12-20
Also published as: WO1999045604A1; AU2982199A; CA2318201A1; JP2002506277A

Abstract

Methods for preparing non-aqueous electrochemical cells and the components thereof using solvent extraction techniques to remove a carrier solvent from an anode, cathode and/or solid polymeric matrix mixture are disclosed. Electrodes and/or electrochemical cells prepared by such methods have improved performance properties.

Description

-1-

METHODS FOR PREPARING ELECTROCHEMICAL CELLS USING SOLVENT EXTRACTION TECHNIQUES

BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates to methods for preparing electrochemical cells and the components thereof. More particularly, this invention relates to methods for preparing non-aqueous electrochemical cells and the components thereof using solvent extraction techniques to remove a carrier solvent from an anode, cathode and/or solid polymeric matrix mixture.

State of the Art

Non-aqueous lithium electrochemical cells typically include a lithium anode, 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 cathode 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 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 L^C. During 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 further in U.S. Patent Nos. 4,472,487, 4,668,595 and 5,028,500, the disclosures of which are incorporated herein by reference in their entirety.

When fabricating such electrochemical cells, an anode film is typically prepared by forming an anode composition comprising an anodic material, a polymer and a volatile carrier solvent and then allowing the carrier solvent to evaporate or "air dry" from the composition to form the anode film. Typically, the anode film is formed directly on a current collector or, alternatively, on some other suitable substrate and then laminated onto a current collector. A cathode film is typically prepared in a similar manner from a cathode composition comprising a cathode active material, a polymer and a volatile carrier solvent.

Although it is common practice to allow the volatile carrier solvent to evaporate or air dry when forming an anode and/or cathode film for use in electrochemical cells, there are certain disadvantages associated with this procedure. In particular, when the anode and/or cathode compositions are allowed to air dry, the rate of evaporation of the carrier solvent is often difficult to control since it is dependent on a number of factors such as temperature, humidity, air flow, solvent volatility and the like. When the carrier solvent evaporates too quickly from such compositions, the porous structure of the resulting anode and/or cathode films has been found to be adversely affected as demonstrated by a decrease in the conductivity of the anode and/or cathode.

In addition, when the carrier solvent evaporates too rapidly from such compositions, the polymer component of the composition tends to migrate to the surface of the anode and/or cathode film thereby creating a polymer concentration gradient within the electrode. Since the polymer component functions, in part, to bind the anode and/or cathode film to the surface of the current collector, such a polymer gradient causes poor adhesion of the anode and/or cathode film to the current collector and a reduction in electrical contact. Moreover, the polymer gradient within the anode and/or cathode film reduces the cohesiveness of the electrode itself.

Similarly, the solid polymeric matrix which is interposed between the anode and the cathode in such electrochemical cells is typically prepared by allowing a carrier solvent to evaporate or air dry from a solid polymeric matrix composition comprising a polymer and a volatile carrier solvent. As with the anode and cathode films, the porous structure of the resulting solid polymeric matrix is adversely affected if the carrier solvent is allowed to evaporate too quickly from such compositions.

Accordingly, it would be particularly advantageous to be able to control the rate at which the carrier solvent evaporates or dries from anode, cathode and/or solid polymeric matrix compositions used to prepare electrochemical cells and the components thereof.

SUMMARY OF THE INVENTION

The present invention provides methods for preparing non-aqueous electrochemical cells and the components thereof using solvent extraction -4- techniques to remove a carrier solvent from an anode, cathode and/or solid polymeric matrix mixture. The carrier solvent is extracted from the anode, cathode and/or solid polymeric matrix mixture by contacting the mixture with a polymer non-solvent before the carrier solvent evaporates from the composition. Under such conditions, the carrier solvent diffuses from the anode, cathode and/or polymeric matrix mixture into the polymer non-solvent at a controlled rate thereby forming an anode film, a cathode film and/or solid polymeric matrix having improved performance properties. Following this solvent exchange process, whereby the polymer structure is formed, the polymer non-solvent is removed.

Accordingly, in one of its method aspects, the present invention is directed to a method for preparing an electrode film, said method comprising the steps of:

(a) forming an electrode mixture comprising an anodic material or a cathode active material, a polymer and a carrier solvent; and

(b) contacting the electrode mixture with a polymer non-solvent to extract at least a portion of the carrier solvent from the electrode mixture to form an electrode film.

Preferably, the anodic material employed in this invention comprises a carbon material selected from the group consisting of carbon black, coke, graphite, disordered carbon, hard carbon and mixtures thereof. The cathode active material employed is preferably selected from the group consisting of Li_xMn₂O₄, wherein 0<x<2, LiCoO₂, LiNiO₂, LiNi_yCθ]._yO₂, wherein 0 < y < 1 , and mixtures thereof.

In still another of its method aspects, the present invention is directed to a method for preparing a solid polymeric matrix, said method comprising the steps of: (a) forming a solid polymeric matrix mixture comprising a polymer and a carrier solvent; and -5-

(b) contacting the solid polymeric matrix mixture with a polymer non- solvent to extract at least a portion of the carrier solvent from the solid polymeric matrix mixture to form a solid polymeric matrix.

Preferably, the polymer(s) used in the methods of 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 .

In preferred embodiments of the above methods, the anode, cathode and solid polymeric matrix mixtures further comprise a plasticizer. Preferably, the plasticizer(s) employed in these methods 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 carrier solvent employed in the methods of this invention is preferably an inert organic liquid having a boiling of less than about 175 °C, more preferably less than about 100 °C, and still more preferably less than about 80 °C. Particularly preferred carrier solvents are organic liquids selected from the group consisting of aliphatic ketones having 3 to 6 carbon atoms, dialkyl carbonates having 3 to 7 carbon atoms, aliphatic and cycloaliphatic ethers having 2 to 6 carbon atoms, and N,N-dialkylcarboxamides having 3 to 5 carbon atoms.

Preferably, the carrier solvent is selected from the group consisting of acetone, dimethyl carbonate, and tetrahydro uran. More preferably, the carrier solvent is acetone.

Preferably, the polymer non-solvent employed in the methods of this invention is selected from the group consisting of water, aliphatic alcohols having 1 to about 6 carbon atoms, alkanes having 5 to about 12 carbon atoms and mixtures thereof. Particularly preferred polymer non-solvents include water, methanol, ethanol, isopropanol and mixtures thereof. When acetone is used as the carrier solvent, the use of alcohols as the polymer non-solvent is particularly preferred.

In yet another of its method aspects, the present invention is directed to a method for preparing an electrochemical cell, said method comprising the steps of:

(a) forming an anode mixture comprising an anodic material, a first polymer and an first carrier solvent;

(b) contacting the anode mixture with a first polymer non-solvent to extract at least a portion of the carrier solvent from the anode mixture to form an anode film and thereafter removing the first polymer non-solvent;

(c) forming a cathode mixture comprising a cathode active material, a second polymer and a second carrier solvent;

(d) contacting the cathode mixture with a second polymer non-solvent to extract at least a portion of the carrier solvent from the cathode mixture to form a cathode film and thereafter removing the second polymer non-solvent;

(e) forming an anode by laminating the anode film onto at least one side of an anode current collector;

(f) forming a cathode by laminating the cathode film onto at least one side of a cathode current collector;

(g) interposing a solid polymeric matrix between the anode and the cathode to form a electrochemical cell precursor; and -1-

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

In another embodiment of this invention, the anode and cathode mixtures are applied directly to an anode or cathode current collector. Accordingly, in another of its method aspects, this invention is directed to a method for preparing an electrochemical, said method comprising the steps of:

(a) forming an anode mixture comprising an anodic material, a first polymer and a first carrier solvent; (b) applying the anode mixture to an anode current collector to form a coated anode current collector;

(c) contacting the anode mixture on the coated anode current collector with a first polymer non-solvent to extract at least a portion of the carrier solvent from the anode mixture to form an anode and thereafter removing the first polymer non-solvent;

(d) forming a cathode mixture comprising a cathode active material, a second polymer and a second carrier solvent;

(e) applying the cathode mixture to a cathode current collector to form a coated cathode current collector; (f) contacting the cathode mixture on the coated cathode current collector with a second polymer non-solvent to extract at least a portion of the carrier solvent from the cathode mixture to form a cathode and thereafter removing the second polymer non-solvent;

(g) interposing a solid polymeric matrix between the anode and the cathode to form a electrochemical cell precursor; and

(h) contacting the electrochemical cell precursor with an electrolyte solution comprising an electrolyte solvent and a salt to form an electrochemical cell. -8-

In preferred embodiments of this invention, the anode mixture, cathode mixture and solid polymeric matrix employed in the above methods each further comprises a plasticizer and the above methods further comprises the step of removing the plasticizer(s) from the electrochemical cell precursor prior to contacting the precursor with an electrolyte solution.

In further preferred embodiments, the solid polymeric matrix employed in the above methods is prepared by a method which comprises the steps of:

(a) forming a solid polymeric matrix mixture comprising a third polymer and a third carrier solvent; and (b) contacting the solid polymeric matrix mixture with a third polymer non-solvent to extract at least a portion of the third carrier solvent from the solid polymeric matrix mixture to form a solid polymeric matrix.

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 is based in part on the discovery that when an anode, cathode and/or solid polymeric matrix mixture containing a carrier solvent is contacted with a polymer non-solvent, the carrier solvent is extracted from the mixture at a controlled rate thereby forming an anode film, cathode film and/or solid polymeric matrix having improved performance properties. 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 polymeric matrix 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 -10- 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 term "carrier solvent" refers to refers to a liquid organic solvent which serves to solubilize and/or suspend the components of the anode, cathode and solid polymeric matrix mixtures. Preferably, the carrier solvent is an inert organic liquid having a low molecular weight (e.g., less than about 200) and a boiling (at atmospheric pressure) of less than about 175 °C, more preferably less than about 100°C, still more preferably less than about 80°C, and even more preferably less than 70 °C. Suitable carrier solvents include, by way of example, aliphatic ketones having 3 to 6 carbon atoms, such as acetone, methyl ethyl ketone, and the like; dialkyl carbonates having 3 to 7 carbon atoms, such as dimethyl carbonate, diethyl carbonate; aliphatic and cycloaliphatic ethers having 2 to 6 carbon atoms, such as diethyl ether, methyl tert-butyl ether, tetrahydrofuran and the like; N,N- dialkylcarboxamides having 3 to 5 carbon atoms, such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DM AC) and the like; and mixtures thereof. Preferred carrier solvents are selected from the group consisting of acetone, dimethyl carbonate, and tetrahydrofuran. A particularly preferred carrier solvent is acetone.

The term "polymer non-solvent" refers to a solvent in which the polymer component of the anode, cathode and/or solid polymer matrix mixture is essentially insoluble, but which solubilizes or is miscible with the carrier solvent used in such mixtures. The particular polymer non-solvent employed in the methods of this invention will depend upon the polymer and carrier solvent used in the anode, cathode and/or solid polymeric matrix mixture. One skilled in the art -11- can readily determine a suitable polymer non-solvent for use with a particular polymer and carrier solvent by determining the solubility of the polymer and the carrier solvent in the polymer non-solvent using procedures well known to those of ordinary skill in the art. Examples of suitable polymer non-solvents include, but are not limited to, water; aliphatic alcohols having 1 to about 6 carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol and the like; alkanes having 5 to about 12 carbon atoms, such as pentanes, hexanes, heptanes, octanes and the like; and mixtures thereof. When acetone is used as the carrier solvent, the use of alcohols as the polymer non-solvent is particularly preferred.

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 greater than about 200°C, preferably from about 250°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. -12-

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 "electrochemical cell precursor" or "electrolytic cell precursor" refers to the structure of the electrochemical cell prior to the addition of the electrolyte solution. The precursor typically comprises (each in precursor form) an anode, a cathode, and polymeric matrix. The anode and/or cathode may each include a current collector. The polymeric matrix can function as a separator between the anode and cathode.

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 "solid polymeric matrix" refers to an electrolyte compatible material formed by polymerizing an inorganic or organic monomer (or partial polymer thereof) and which, when used in combination with the other components of the electrolyte, renders the electrolyte solid. Suitable solid polymeric matrices are well known in the art and include solid matrices formed from organic polymers, inorganic polymers or a mixture of organic polymers with inorganic non- -13- 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 carrier 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 carrier 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 poly vinylidene difluoride and hexafluoropropylene dissolved in sufficient amount of acetone or other suitable carrier solvent to form a slurry. Upon casting of the slurry, the carrier solvent is removed, preferably by contacting the slurry with a polymer non-solvent, to form the solid polymeric matrix. The slurry may be casted directly onto a current -14- 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 SiOj 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 "salt" refers to any salt, for example, an inorganic 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^", ClO₄ , 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 -15- 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_xNi₁._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., N₆O₁₃ 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-l ,3-dioxolan-2-one; 4,4-dimethyl-l ,3-dioxolan-2-one; 4-methy 1-5 -ethyl- 1 ,3-dioxolan-2-one; 4,5- -16- diethyl-1 ,3-dioxolan-2-one; 4,4-diethyl-l ,3-dioxolan-2-one; 1 ,3-dioxan-2-one; 4,4-dimethyl-l ,3-dioxan-2-one; 5 ,5-dimethyl-l ,3-dioxan-2-one; 5-methyl-l,3-dioxan-2-one; 4-methyl-l,3-dioxan-2-one; 5,5-di- ethyl-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-cyclohexanone-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- ,β-diol (dihydroxy alkanes having hydroxyl substituents on adjacent carbon atoms) or an alkane-α,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- ,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 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, -17- 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 anode typically comprises a compatible anodic material which is any material which functions as an anode in a solid 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, manga- nese, 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, poly vinylidene 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. -18-

The cathode typically comprises a compatible cathodic material (i.e., insertion compounds) which is any material which functions as a positive pole in a solid 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 LiCoNO₄, lithiated manganese oxides, e.g. , LiMn₂O₄, lithiated nickel oxides, e.g., LiΝiO₂ and LiNiNO₄ 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- -MnO₂ (O≤y < 1) which has a hollandite-type structure and is described in U.S. Patent 5,561,007, which is incorporated herein.

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.

Suitable binders for use in the cathode have 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, poly vinylidene 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 -19- 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.

In one preferred embodiment, the cathode is prepared from a cathode paste which comprises from about 35 to 65 weight percent of a compatible cathodic material; from about 1 to 20 weight percent of an electroconductive agent; from about 1 to 20 weight percent of suitable polymeric binders that may include EPDM (ethylene propylene diene termonomer), PNDF (poly vinylidene difluoride), EAA (ethylene acrylic acid copolymer), EVA (ethylene vinyl acetate copolymer), EAA/EVA copolymers, and the like; from about 0 to about 20 weight percent of polyethylene oxide having a number average molecular weight of at least 100,000; from about 10 to 50 weight percent of solvent comprising a 10:1 to 1:4 (w/w) mixture of an organic carbonate and a glyme; and from about 5 weight percent to about 25 weight percent of a solid matrix forming monomer or partial polymer thereof. Also included is an ion conducting amount of an inorganic ion salt. Generally, the amount of the salt is from about 1 to about 25 weight percent. (All weight percents are based on the total weight of the cathode.)

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. -20-

The electrolyte composition typically comprises from 0 to about 80 weight percent electrolyte solvent (e.g., organic carbonate/glyme 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 well known in the art. See, for example, U.S. Patent Nos. 5,300,373, 5,316,556, 5,346,385, 5,262,253, 4,472,487, 4,668,595, and 5,028,500, each of which is incorporated herein by reference in its entirety. The methods provided by the present invention can be adapted to known procedures to form anode films, cathode films, solid polymeric matrices and/or electrochemical cells having improved performance properties.

Preferably, the methods of this invention are conducted by first forming an anode, cathode or polymeric matrix mixture using any art recognized procedure. Typically, this step is conducted by combining the components of mixture (i.e. , an -21- anodic material, a polymer, a carrier solvent, and optionally a plasticizer, for the anode mixture) and blending the mixture until the components are homogeneous distributed. Preferably, when preparing the anode and cathode mixtures, the polymer and carrier solvent are first mixed together. The anode or cathode components are then mixed separately with the carrier solvent and this mixture is then added to the polymer/carrier solvent mixture. The components are then thoroughly blended. Preferably, the components are mixed under low shear conditions to minimize degradation of the polymer.

After forming an anode, cathode or polymeric matrix mixture, the mixture is applied or cast onto a substrate, such as a carrier web and the like, to form a coated substrate. Typically, the mixtures are cast onto the substrate by conventional means such as by extrusion as described in U.S. Patent 5,316,556 which is incorporated herein.

The anode, cathode or polymeric matrix mixture on the coated substrate is then contacted with a polymer non-solvent to remove the carrier solvent from the mixture by diffusion of the carrier solvent from the mixture into the polymer non- solvent. Removal of the carrier solvent forms the corresponding anode, cathode or polymeric matrix film. Preferably, the anode, cathode or polymeric matrix mixture on the substrate is immersed in the polymer non-solvent for a time sufficient to remove substantially all the carrier solvent and preferably at least about 90 percent of the carrier solvent from the mixture based on the total amount of carrier solvent present in the anode, cathode or polymeric matrix mixture on the substrate. Preferably, the polymer non-solvent is mixed during the contact period either by mechanical means, such as stirring, or by bubbling air or nitrogen through the vessel containing the polymer non-solvent. Following removal of the carrier solvent from the anode, cathode or polymer matrix, the polymer non- solvent is in turn removed from the component by conventional means, e.g., -22- evaporation. In removing the polymer non-solvent, heat can be applied to the component (e.g., anode) to facilitate evaporation. Any remaining residue of carrier solvent would also be removed at this stage. The polymer non-solvent can be removed any time prior to addition of the electrolyte solvent.

The time period necessary for the carrier solvent to diffuse into the polymer non-solvent will vary depending on the thickness of the mixture on the substrate, the solubility of the carrier solvent in the polymer non-solvent and the temperature of the polymer non-solvent. Typically, the mixture will be contacted with polymer non-solvent for about 1 to about 5 minutes at ambient temperature.

The anode film formed by the inventive method is then laminated onto at least one side of an anode current collector using procedures well known to those skilled in the art to form an anode. In a preferred embodiment, the anode comprises an anode film that is laminated onto one or both sides of the current collector. Typically, each anode film is from about lOOμm to about 250μm in thickness, preferably about HOμm to about 200μm, and more preferably about 125μm to about 175μm.

Similarly, the cathode film formed by the inventive method is then laminated onto at least one side of a cathode current collector using procedures well known to those skilled in the art to form a cathode. In a preferred embodiment, the cathode comprises a cathode film that is laminated onto one or both sides of the current collector. Typically, each cathode film ranges in thickness from about lOOμm to about 200μm, preferably about 130μm to about 175μm, and more preferably about 140μm to about 165μm.

Alternatively, in another embodiment of this invention, the anode or cathode can be prepared by applying the anode or cathode mixture onto at least one -23- side of an anode or cathode current collector to form a coated current collector. The coated current collector is then contacted with a polymer non-solvent as described above to form an anode or cathode.

The anode, cathode and polymeric matrix films prepared by the above procedures are then assembled into an electrochemical cell using art recognized procedures. Such procedures are further described by the following representative examples.

EXAMPLES

The following examples illustrate how an electrolytic cell could be fabricated using the methods of this invention. Examples 1 and 2 describe the methods for preparing the anode and cathodes, respectively. Example 3 describes the method for fabricating a solid electrolytic cell.

These examples describe anode and cathode structures wherein the electrode materials (or films) are laminated onto both sides of the current collectors, however, it is understood 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 A polymer mixture comprising a copolymer of vinylidenedifluoride (VDF) and hexafluoropropylene (HFP) is prepared by mixing 6.8 grams of the copolymer in 20 grams of acetone. The copolymer (average molecular weight 125,000) is Kynar Flex 2801™ from Elf Atochem North America, in Philadelphia, PA. The mixture is stirred for about 24 hours in a milling jar available from VWR -24-

Scientific, in San Francisco, CA, model H-04172-00. The copolymer functions as a binder for the carbon in the anode.

A graphite mixture is 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, IL. A preferred carbon black is available under the designation Super P™ from M.M.M. Carbon, Willebroek, Belgium. The graphite mixture is then vigorously mixed in a high shear mixer until a substantially homogeneous blend is formed. A suitable mixer is available from Ross, Model ME100DLX, Hauppauge, NY, operating at its highest setting (about 10,000 RPM) for 30 minutes.

An anode slurry is 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 is allowed to evaporate from the slurry and it is 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, CT.

The laminated current collector is then submerged in a vessel containing a polymer non-solvent for about a minute to remove the acetone. The plasticizer will also be removed by the polymer non-solvent; otherwise an additional step that uses another solvent (e.g., diethyl ether or a dense fluid such as supercritical CQ) is employed. Thereafter, the laminated current collector is removed from the vessel and the polymer non-solvent is allowed to evaporate. -25-

Example 2 Preparation of a Cathode A polymer mixture comprising a copolymer of vinylidenedifluoride (VDF) and hexafluoropropylene (HFP) is prepared by mixing 4.4 grams of the copolymer in 15 ml of acetone. The copolymer is Kynar Flex 2801™. The mixture is 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 LiMrijO₄ 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.

The laminated current collector is submerged in polymer non-solvent to remove the acetone and plasticizer.

Example 3 Preparation of a Polymeric Matrix A polymeric matrix is formed by casting a polymeric slurry comprising acetone, dibutyl phthalate, silanized fumed SiO₂, and the VDF/HFP copolymer on -26- a suitable substrate or carrier web and removing the acetone by submerging the polymeric slurry in a polymer non-solvent. 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 Preparation of Electrochemical Cells

Solid electrochemical cells are prepared by first positioning a polymeric matrix prepared as described in Example 3 above between an anode and a cathode prepared as described in Examples 1 and 2 above, respectively. The components are then fused under moderate pressure and temperature (e.g., 130°C) to form an electrochemical cell precursor.

The extracted electrochemical cell precursors are then activated by immersion under a substantially moisture-free atmosphere in a 1 M electrolyte solution of LiPF₆ in ethylene carbonate/dimethyl carbonate (2:1 by weight) for about 50 minutes to provide electrochemical cells.

While the invention has been described in terms of various preferred embodiments, the skilled artisan will appreciate the various modifications, substitutions, and changes which may be made without departing from the spirit hereof. The descriptions of the subject matter in this disclosure are illustrative of the invention and are not intended to be construed as limitations upon the scope of the invention.

Claims

-27-Claims:

1. A method for preparing an electrode film, said method comprising the steps of:

(a) forming an electrode mixture comprising either an anodic material or cathode active material, a polymer and a carrier solvent; and

2. The method according to Claim 1 wherein the electrode mixture comprises an anodic material that is a carbon material that is selected from the group consisting of carbon black, coke, graphite, disordered carbon, hard carbon and mixtures thereof.

3. The method according to Claim 2 wherein the polymer 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.

4. The method according to Claim 1 wherein the carrier solvent is selected from the group consisting of aliphatic ketones having 3 to 6 carbon atoms, dialkyl carbonates having 3 to 7 carbon atoms, aliphatic and cycloaliphatic ethers having 2 to 6 carbon atoms, and N,N-dialkylcarboxamides having 3 to 5 carbon atoms.

5. The method according to Claim 1 wherein the polymer non-solvent is selected from the group consisting of water, aliphatic alcohols having 1 to about 6 carbon atoms, alkanes having 5 to about 12 carbon atoms and mixtures thereof. -28-

6. The method according to Claim 1 wherein the electrode mixture comprises a cathode active material that is selected from the group consisting of Li_xMn₂O₄, wherein 0<x<2, LiCoO₂, LiNiO₂, LiNi_yC╬╕_!._yO₂, wherein 0<y < 1, and mixtures thereof.

7. The method according to Claim 6 wherein the polymer 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.

8. A method for preparing a solid polymeric matrix, said method comprising the steps of:

(a) forming a solid polymeric matrix mixture comprising a polymer and a carrier solvent; and

9. The method according to Claim 8 wherein the polymer 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.

10. The method according to Claim 8 wherein the carrier solvent is selected from the group consisting of aliphatic ketones having 3 to 6 carbon atoms, dialkyl carbonates having 3 to 7 carbon atoms, aliphatic and cycloaliphatic ethers having 2 to 6 carbon atoms, and N,N-dialkylcarboxamides having 3 to 5 carbon atoms. -29-

11. The method according to Claim 8 wherein the polymer non-solvent is selected from the group consisting of water, aliphatic alcohols having 1 to about 6 carbon atoms, alkanes having 5 to about 12 carbon atoms and mixtures thereof.

12. A method for preparing an electrochemical cell, said method comprising the steps of:

(f) forming a cathode by laminating the cathode film onto at least one side of a cathode current collector; (g) interposing a solid polymeric matrix between the anode and the cathode to form a electrochemical cell precursor; and

13. The method according to Claim 12 wherein the solid polymeric matrix is prepared by a method which comprises the steps of: -30-

(a) forming a solid polymeric matrix mixture comprising a third polymer and a third carrier solvent; and

(b) contacting the solid polymeric matrix mixture with a third polymer non-solvent to extract at least a portion of the third carrier solvent from the solid polymeric matrix mixture to form a solid polymeric matrix.

14. The method according to Claim 12 wherein the anodic material comprises a carbon material selected from the group consisting of carbon black, coke, graphite, disordered carbon, hard carbon and mixtures thereof.

15. The method according to Claim 12 wherein the cathode active material is selected from the group consisting of Li_xMn₂O₄, wherein 0<x<2,

LiCoO₂, LiNiO₂, LiNi_yC╬╕_╬╣__yO₂, wherein 0 < y < 1 , and mixtures thereof.

16. The method according to Claim 13 wherein the first, second and third polymers are independently 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.

17. The method according to Claim 13 wherein the first, second and third carrier solvents are independently selected from the group consisting of aliphatic ketones having 3 to 6 carbon atoms, dialkyl carbonates having 3 to 7 carbon atoms, aliphatic and cycloaliphatic ethers having 2 to 6 carbon atoms, and N,N-dialkylcarboxamides having 3 to 5 carbon atoms.

18. A method for preparing an electrochemical, said method comprising the steps of: -31-

(a) forming an anode mixture comprising an anodic material, a first polymer and a first carrier solvent;

(b) applying the anode mixture to an anode current collector to form a coated anode current collector; (c) contacting the anode mixture on the coated anode current collector with a first polymer non-solvent to extract at least a portion of the carrier solvent from the anode mixture to form an anode and thereafter removing the first polymer non-solvent;

(e) applying the cathode mixture to a cathode current collector to form a coated cathode current collector;

(f) contacting the cathode mixture on the coated cathode current collector with a second polymer non-solvent to extract at least a portion of the carrier solvent from the cathode mixture to form a cathode and thereafter removing the second polymer non-solvent;

19. The method according to Claim 18 wherein the solid polymeric matrix is prepared by a method which comprises the steps of:

(b) contacting the solid polymeric matrix mixture with a third polymer non-solvent to extract at least a portion of the third carrier solvent from the solid -32- polymeric matrix mixture to form a solid polymeric matrix and thereafter removing the third polymer non-solvent.

20. The method according to Claim 18 wherein the anodic material comprises a carbon material selected from the group consisting of carbon black, coke, graphite, disordered carbon, hard carbon and mixtures thereof.

21. The method according to Claim 18 wherein the cathode active material is selected from the group consisting of Li_xMn₂O₄, wherein 0 <x< 2, LiCoO₂, LiNiO₂, LiNi_yCo^O^ wherein 0 < y < 1 , and mixtures thereof.

22. The method according to Claim 18 wherein the first, second and third polymers are independently 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.

23. The method according to Claim 18 wherein the first, second and third carrier solvents are independently selected from the group consisting of aliphatic ketones having 3 to 6 carbon atoms, dialkyl carbonates having 3 to 7 carbon atoms, aliphatic and cycloaliphatic ethers having 2 to 6 carbon atoms, and N,N-dialkylcarboxamides having 3 to 5 carbon atoms.

24. The method according to Claim 18 wherein the first, second and third polymer non-solvent are independently selected from the group consisting of water, aliphatic alcohols having 1 to about 6 carbon atoms, alkanes having 5 to about 12 carbon atoms and mixtures thereof.