Abstract:
Irreversible first cycle capacity loss in lithium secondary cells having a cell electrode based on a powdered material and a binder may be significantly decreased or eliminated by using an aliphatic or cycloaliphatic polyimide binder. Compared to conventional polyimide binders prepared by reacting an aromatic dianhydride and a diamine, the disclosed polyimide binders have decreased aromatic carbonyl content, may be less likely to undergo electrochemical reduction, and may be less likely to consume electrons that might otherwise help lithiate the electrode.

Description:
STATEMENT OF PRIORITY 
     This application claims the priority of U.S. Provisional Application No. 60/625,931 filed Nov. 8, 2004, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electrode binders for rechargeable lithium-ion cells and to cells containing such electrodes or electrode binders. 
     BACKGROUND 
     Powdered alloys of main group elements and conductive powders such as carbon black have been used to make electrodes for lithium ion cells. Electrodes may be fabricated, for example, by mixing the powdered active ingredient with a polymeric binder such as polyvinylidene fluoride. The mixture may be prepared as a dispersion in a solvent for the polymeric binder, and coated onto a metal foil substrate. The resulting composite electrode may contain the powdered active ingredient in the binder, adhered to the metal substrate. 
     Lithium ion cell electrodes containing polyimide binders have been mentioned in U.S. Pat. Nos. 5,468,571 (Fujimoto et al.), 5,888,672 (Gustafson et al.), 6,001,507 (Ono et al.), 6,723,432 B2 (Sugo et al.), 6,759,164 B2 (Palazzo et al.), 6,790,561 B2 (Gan et al.) and 6,797,019 B2 (Takeuchi et al. &#39;019), U.S. Patent Application Publication Nos. US 2003/0049535 A1 (Ohta et al.), U.S. Pat. No. 6,946,223 (Kusumoto et al.), U.S. Pat. No. 6,797,019 (Takeuchi et al. &#39;868), U.S. Pat. No. 7,141,334 (Fukui et al.), US 2004/0062991 A1 (Fukui et al. &#39;991) and U.S. Pat. No. 7,153,611 (Minami et al.), and Japanese Published Patent Application No. JP 11-102708 (Asahi Glass Co. Ltd.). 
     SUMMARY OF THE INVENTION 
     Many conventional polyimides may be prepared via a condensation reaction between an aromatic dianhydride and a diamine in an aprotic polar solvent such as N-methylpyrrolidinone. This reaction leads to the formation of an aromatic polyamic acid, and subsequent chemical or thermal cyclization leads to the polyimide. The resulting polyimides may be used as binders for metal and graphite based lithium-ion cell electrodes, and can exhibit very good cycling properties. However, the first cycle irreversible capacity loss in the resulting cells can be unacceptably large, e.g., as large as 300 mAh/g or more for an electrode based on a powdered metal material. Without intending to be bound by theory, this irreversible capacity loss may be caused by electrochemical reduction of the polyimide, facilitated by the presence of electron-withdrawing carbonyl groups on the polyimide aromatic rings. The electrochemical reduction reaction may consume electrons that might otherwise help lithiate the electrode, and appears to be largely irreversible when using commonly-employed electrode and electrolyte formulations. 
     This irreversible first cycle capacity loss may be significantly decreased or eliminated by forming the electrode using an aliphatic or cycloaliphatic polyimide binder. Such a polyimide binder (which may be prepared by reacting an aliphatic or cycloaliphatic dianhydride and a diamine) has decreased aromatic carbonyl content compared to binders made from conventional polyimides (which typically are prepared by reacting an aromatic dianhydride and a diamine). The disclosed aliphatic or cycloaliphatic polyimide binders may be used to prepare electrodes and cells that experience decreased first cycle irreversible capacity loss compared to electrodes and cells made with conventional aromatic polyimides. 
     The invention provides, in one aspect, a secondary battery electrode composition comprising a powdered material capable of undergoing lithiation and delithiation and an aliphatic or cycloaliphatic polyimide binder. The polyimide binder comprises repeating units having the formula: 
     
       
                 
         
             
             
         
      
         
         
           
             where:
           R 1  is aliphatic or cycloaliphatic and   R 2  is aromatic, aliphatic, cycloaliphatic or a combination thereof.   
         
           
         
       
    
     The invention provides, in another aspect, a rechargeable lithium-ion cell comprising a negative electrode comprising a powdered material capable of undergoing lithiation and delithiation and an aliphatic or cycloaliphatic polyimide binder, a positive electrode, and an electrolyte comprising a lithium salt and a charge carrying medium. 
     The disclosed polyimide binders can improve cycle life in rechargeable lithium ion cells employing electrodes based on small particle alloy powders. The disclosed polyimides may also allow fabrication of rechargeable lithium ion cells having improved capacities or employing novel or not previously employed alloy powders. The rechargeable lithium ion cells can be used, for example, in various electronic devices or in battery packs. 
     These and other aspects of the invention will be apparent from the detailed description below. In no event, however, should the above summaries be construed as limitations on the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an exploded perspective schematic view of an electrochemical cell. 
         FIG. 2  is a first lithiation (discharge) curve for cells employing the polyimide binders of Example 8 and Comparative Example 2. 
         FIG. 3  is a cyclic voltammogram for polyimides prepared from Preparatory Example 1 and Comparative Preparatory Example 1. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. The elements in the drawing are not to scale. 
     DETAILED DESCRIPTION 
     The words “a”, “an”, and “the” are used interchangeably with “at least one” to mean one or more of the elements being described. 
     The word “metal” refers to both metals and to metalloids such as silicon and germanium, whether in an elemental or ionic state. 
     The word “alloy” refers to a mixture of two or more metals. 
     The words “lithiate” and “lithiation” refer to a process for adding lithium to an electrode by reduction of lithium ions. 
     The words “delithiate” and “delithiation” refer to a process for removing lithium from an electrode by oxidation of lithium ions. 
     The words “charge” and “charging” refer to a process for providing electrochemical energy to a cell. 
     The words “discharge” and “discharging” refer to a process for removing electrochemical energy from a cell, e.g., when using the cell to do perform desired work. 
     The phrase “positive electrode” refers to an electrode (often called a cathode) where electrochemical reduction and lithiation occurs during a discharging process. 
     The phrase “negative electrode” refers to an electrode (often called an anode) where electrochemical oxidation and delithiation occurs during a discharging process. 
     Unless the context clearly requires otherwise, the words “aliphatic”, “cycloaliphatic” and “aromatic” include substituted and unsubstituted moieties containing only carbon and hydrogen, moieties that contain carbon, hydrogen and other atoms (e.g., nitrogen or oxygen ring atoms), and moieties that are substituted with atoms or groups that may contain carbon, hydrogen or other atoms (e.g., halogen atoms, alkyl groups, ester groups, ether groups, amide groups, hydroxyl groups or amine groups). 
     The phrase “aliphatic or cycloaliphatic polyimide binder” refers to a polyimide that may be derived from an aliphatic or cycloaliphatic polyanhydride. 
     The disclosed electrode compositions may be used in negative or positive electrodes. A variety of powdered materials may be employed to make the disclosed electrode compositions. Exemplary powdered materials may for example contain silicon, silver, lithium, tin, bismuth, lead, antimony, germanium, zinc, gold, platinum, palladium, arsenic, aluminum, gallium, indium, thallium, alloys containing any of the foregoing metals or metalloids and other powdered active metals and metalloids that will be familiar to those skilled in the art. Exemplary powders may have a maximum dimension that is no greater than 60 micrometers, no greater than 40 micrometers, or no greater than 20 micrometers. The powders may for example have a maximum particle diameter that is submicron, at least 1 micrometer, at least 2 micrometers, at least 5 micrometers, or at least 10 micrometers. For example, suitable powders often have a maximum dimension of 1 to 60 micrometers, 10 to 60 micrometers, 20 to 60 micrometers, 40 to 60 micrometers, 1 to 40 micrometers, 2 to 40 micrometers, 10 to 40 micrometers, 5 to 20 micrometers, or 10 to 20 micrometers. The powdered materials may contain optional matrix formers within powder particles. Each phase originally present in the particle (i.e., before a first lithiation) may be in contact with the other phases in the particle. For example, in particles based on a silicon:copper:silver alloy, a silicon phase may be in contact with both a copper silicide phase and a silver or silver alloy phase. Each phase in a particle may for example have a grain size less than 500 Angstroms, less than 400 Angstroms, less than 300 Angstroms, less than 200 Angstroms, or less than 150 Angstroms. 
     One exemplary silicon-containing powdered material is the silicon alloy Si 57 Al 28 Fe 15 . Other exemplary powdered materials include lithium intercalation compounds such as Li 4/3 Ti 5/3 O 4 ; the lithium alloy compositions described in U.S. Pat. Nos. 6,203,944 (Turner &#39;944), 6,255,017 B1 (Turner &#39;017), 6,436,578 B2 (Turner et al. &#39;578), 6,680,145 B2 (Obrovac et al.) and 6,699,336 B2 (Turner et al. &#39;336), U.S. Patent Application Publication 2003/0211390 A1 (Dahn et al.), U.S. Pat. No. 7,498,100 (Christensen et al.) (the disclosure of which is incorporated herein by reference) and U.S. Patent Publication 2006/028583 (Obrovac) (the disclosure of which is incorporated herein by reference); graphitic carbons, e.g., those having a spacing between (002) crystallographic planes, d 002 , of 3.45 ∪&gt;d 002 &gt;3.354 Å and existing in forms such as powders, flakes, fibers or spheres (e.g., mesocarbon microbeads); combinations thereof and other powdered materials that will be familiar to those skilled in the art. Some powdered alloy particles may include a conductive coating. For example, a particle that contains silicon, copper, and silver or a silver alloy may be coated with a layer of conducting material (e.g., with the alloy composition in the particle core and the conductive material in the particle shell). Suitable conductive materials include, for example, carbon, copper, silver, or nickel. 
     Exemplary powdered alloy materials may be prepared, for example, using a melt spinning process. Such processes are described generally, for example, in Amorphous Metallic Alloys, F. E. Luborsky, ed., Chapter 2, Butterworth &amp; Co., Ltd. (1983). According to this process, ingots containing the alloy composition can be melted in a radio frequency field and then ejected through a nozzle onto a surface of a rotating wheel (e.g., a copper wheel). Because the surface temperature of the rotating wheel is substantially lower than the temperature of the melt, contact with the surface of the rotating wheel quenches the melt. Quenching minimizes the formation of large crystallites that can be detrimental to electrode performance. When conductive coatings are employed, they may be formed using techniques such as electroplating, chemical vapor deposition, vacuum evaporation or sputtering. 
     A variety of aliphatic or cycloaliphatic polyimides may be employed as binders in the disclosed electrode compositions. The polyimide may have a plurality or even a majority of imide groups bonded to aliphatic or cycloaliphatic moieties. The polyimide may also include imide groups bonded to aromatic moieties. The aliphatic or cycloaliphatic polyimide binders comprises repeating units having the formula: 
     
       
                 
         
             
             
         
      
         
         
           
             where:
           R 1  is aliphatic or cycloaliphatic and   R 2  is aromatic, aliphatic, cycloaliphatic, or a combination thereof.
 
The R 1  and R 2  moieties in Formula I may if desired be further substituted with groups that do not interfere with the use of the polyimide binder in a lithium ion cell. For example, when substituents are present on R 1 , the substituents preferably are electron-donating rather than electron-withdrawing groups. Preferably the polyimide is capable of electrochemical charge transport when evaluated as described in L. J. Krause, P. S. Lugg, and T. A. Speckhard,  Electronic Conduction in Polyimides , J. E. Electrochem. Soc., Vol. 136, No. 5, May 1989. Preferably the polyimide also can not be electrochemically reduced within the electrochemical window and temperature range (e.g. from about −30° C. to about 70° C.) within which the cell electrodes may operate.
   
         
           
         
       
    
     The disclosed aliphatic or cycloaliphatic polyimide binders may be formed, for example, via a condensation reaction between an aliphatic or cycloaliphatic polyanhydride (e.g., a dianhydride) and an aromatic, aliphatic or cycloaliphatic polyamine (e.g., a diamine or triamine) to form a polyamic acid, followed by chemical or thermal cyclization to form the polyimide. The disclosed polyimide binders may also be formed using reaction mixtures additionally containing aromatic polyanhydrides (e.g., aromatic dianhydrides), or from reaction mixtures containing copolymers derived from aromatic polyanhydrides (e.g., aromatic dianhydrides) and aliphatic or cycloaliphatic polyanhydrides (e.g., aliphatic or cycloaliphatic dianhydrides). For example, about 5 to about 95 mole percent of the imide groups in the polyimide may be bonded to aliphatic or cycloaliphatic moieties and about 95 to about 5 mole percent of the imide groups may be bonded to aromatic moieties. As a further example, about 10 to about 90 mole percent of the imide groups may be bonded to aliphatic or cycloaliphatic moieties and about 90 to about 10 mole percent of the imide groups may be bonded to aromatic moieties. As still further examples, about 20 to about 80 mole percent of the imide groups may be bonded to aliphatic or cycloaliphatic moieties and about 80 to about 20 mole percent of the imide groups may be bonded to aromatic moieties, or about 40 to about 60 mole percent of the imide groups may be bonded to aliphatic or cycloaliphatic moieties and about 60 to about 40 mole percent of the imide groups may be bonded to aromatic moieties. Representative aromatic polyanhydrides are described, for example, in U.S. Pat. No. 5,504,128 (Mizutani et al.). 
     Representative R 1  moieties include radicals derivable by removing the carboxylic acid anhydride groups from tetracarboxylic acid dianhydrides such as meso-1,2,3,4-butanetetracarboxylic dianhydride, dl-1,2,3,4-butanetetracarboxylic dianhydride, cyclobutane tetracarboxylic dianhydride, 1,2,3,4-cyclopentane tetracarboxylic dianhydride, cyclohexane tetracarboxylic dianhydride, bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic 2,3:5,6-dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, bicyclo[2.2.1]-heptane-2,3,5,6-tetracarboxylic 2,3:5,6-dianhydride, (4arH,8acH)-decahydro-1,t, 4t:5c, 4-cyclohexene-1,1,2,2-tetracarboxylic 1,2:1,2-dianhydride, 4,5-dimethyl-(8CI), bicyclo[2.2.1]heptane-2-exo-3-exo-5-exo-tricarboxyl-5-endo-acetic dianhydride, bicyclo[4.2.0]oxetane-1,6,7,8-tetracarboxylic acid intramolecular dianhydride (6CI), combinations thereof and other polyanhydrides that will be familiar to those skilled in the art. Expressed somewhat differently, representative R 1  moieties include tetrafunctional radicals derivable from materials including butane, cyclobutane, cyclopentane, cyclohexane, cyclohexene, bicyclooctane, bicyclooctene, bicycloheptane, bicyclooxetane and other materials that will be familiar to those skilled in the art. 
     Representative R 2  moieties include radicals derivable from polyamines such as diamines and triamines. These include diamines such as 4,4′-oxydianiline, m-phenylenediamine, p-phenylenediamine, benzidene, 3,5-diaminobenzoic acid, o-dianisidine, 4,4′-diaminodiphenyl methane, 4,4′-methylenebis(2,6-dimethylaniline), 1,4-diaminobutane, 1,6-diaminohexane, 1,7-diaminoheptane, 1,9-diaminononane, 1,10-diaminodecane, 1,12-diaminododecane, 5-amino-1,3,3-trimethylcyclohexanemethylamine, 2,5-bis(aminomethyl)bicyclo[2.2.1]heptane, 2,6-bis(aminomethyl)bicyclo[2.2.1]heptane, 2,4-diaminotoluene, 1,4-diamino-2-methoxybenzene, 1,4-diamino-2-phenylbenzene and 1,3-diamino-4-chlorobenzene, 4,4′-diaminobiphenyl, 2,2-bis(4-aminophenyl)propane, 2,2-bis(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 4,4′-diaminodiphenyl ether, 3,4-diaminodiphenyl ether, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)biphenyl, 4,4′-bis(3-aminophenoxy)biphenyl, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, 4,4′-diaminodiphenyl thioether, 4,4′-diaminodiphenyl sulfone, 2,2′-diaminobenzophenone, and 3,3′-diaminobenzophenone; and naphthalene diamines such as 1,8- and 1,5-diaminonaphthalene. Other useful polyamines include polycyclic triamines such as naphthalene triamines; heterocyclic aromatic diamines such as 2,6-diaminopyridine, 2,4-diaminopyrimidine, and 2,4-diamino-s-triazine; 1,8-diamino-4-(aminomethyl)octane, combinations of the above-mentioned polyamines and other polyamines that will be familiar to those skilled in the art. Expressed somewhat differently, representative R 2  moieties include polyfunctional radicals derivable from materials including benzene, dibenzene, diphenyl ether, diphenylmethane, diphenylpropane, naphthalene, pyridine, pyrimidine, triazine, propane, butane, heptane, hexane, cyclohexane, heptane, octane, nonane, decane, dodecane and other materials that will be familiar to those skilled in the art. Polyoxyethylated polyamines (such as those available from Huntsman Petrochemical Corporation, Houston, Tex. under the tradename JEFFAMINE) may also be useful for forming the disclosed polyimides. 
     Exemplary polyimides are also described in D. F. Loncrini and J. M. Witzel,  Polyaryleneimides of meso - and dl -1,2,3,4- Butanetetracarboxylic Acid Dianhydrides , Journal of Polymer Science, Part A-1, Vol. 7, 2185-2193 (1969); Jong-Young Jeon and Tae-Moon Tak,  Synthesis of Aliphatic - Aromatic Polyimides by Two - Step Polymerization of Aliphatic Dianhydride and Aromatic Diamine , Journal of Applied Polymer Science, Vol. 60, 1921-1926 (1995); Hiroshi Seino et al.,  Synthesis of Aliphatic Polyimides Containing Adamantyl Units , Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 3584-3590 (1999); Hiroshi Seino et al.,  High Performance Polymers , Vol. 11, 255-262 (1999), T. Matsumoto,  High Performance Polymers , Vol. 13 (2001), E. Schab-Balcerzak et al.,  Synthesis and characterization of organosoluble aliphatic - aromatic copolyimides based on cycloaliphatic dianhydride , European Polymer Journal, Vol. 38, 423-430 (2002); Amy E. Eichstadt et al.,  Structure - Property Relationships for a Series of Amorphous Partially Aliphatic Polyimides , Journal of Polymer Science: Part B: Polymer Physics, Vol. 40, 1503-1512 (2002) and Xingzhong Fang et al.,  Synthesis and properties of polyimides derived from cis - and trans -1,2,3,4- cyclohexanetetracarboxylic dianhydrides , Polymer, Vol. 45, 2539-2549 (2004). 
     The disclosed aliphatic or cycloaliphatic polyimide binders may provide important advantages over other binders such as fluorochemical elastomers or semi-crystalline materials, including high adhesion to metals (e.g. metal substrates) and metal alloys (e.g., powdered alloys). The disclosed aliphatic or cycloaliphatic polyimide binders may also offer especially good performance with smaller size powdered alloy particles despite the volumetric changes such alloy particles may experience during cell cycling. 
     The electrode composition may contain additives such as will be familiar to those skilled in the art. For example, the electrode composition may include an electrically conductive diluent to facilitate electron transfer from the powdered material to a current collector. Exemplary electrically conductive diluents include, but are not limited to, carbon (e.g., carbon black for negative electrodes and carbon black, flake graphite and the like for positive electrodes), metal, metal nitrides, metal carbides, metal silicides, and metal borides. For example, representative electrically conductive carbon diluents include carbon blacks such as SUPER P™ and SUPER S™ carbon blacks (both commercially available from MMM Carbon, Belgium), SHAWANIGAN BLACK™ (commercially available from Chevron Chemical Co., Houston, Tex.), acetylene black, furnace black, lamp black, graphite, carbon fibers and combinations thereof. The electrode composition may include an adhesion promoter that promotes adhesion of the powdered material or electrically conductive diluent to the polyimide binder. The combination of an adhesion promoter and an aliphatic or cycloaliphatic polyimide binder may help the binder better accommodate volume changes that may occur in the powdered material during repeated lithiation/delithiation cycles. The disclosed aliphatic or cycloaliphatic polyimide binders may however offer sufficiently good adhesion to metals and alloys so that addition of an adhesion promoter may not provide an appreciable advantage. If used, an adhesion promoter may be made a part of the polyimide binder (e.g., in the form of an added functional group), may be a coating on the powdered material, may be added to the electrically conductive diluent, or may be a combination of such measures. Examples of adhesion promoters include, but are not limited to, silanes, titanates, and phosphonates as described in U.S. Pat. No. 7,341,804 (Christensen), the disclosure of which is incorporated herein by reference. 
     A variety of electrolytes may be employed in the disclosed lithium ion cell. Representative electrolytes contain one or more lithium salts and a charge carrying medium in the form of a solid, liquid or gel. Exemplary lithium salts are stable in the electrochemical window and temperature range (e.g. from about −30° C. to about 70° C.) within which the cell electrodes may operate, are soluble in the chosen charge-carrying media, and perform well in the chosen lithium-ion cell. Exemplary lithium salts include LiPF 6 , LiBF 4 , LiClO 4 , lithium bis(oxalato)borate, LiN(CF 3  SO 2 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiAsF 6 , LiC(CF 3 SO 2 ) 3 , combinations thereof and other lithium salts that will be familiar to those skilled in the art. Exemplary charge carrying media are stable without freezing or boiling in the electrochemical window and temperature range within which the cell electrodes may operate, are capable of solubilizing sufficient quantities of the lithium salt so that a suitable quantity of charge can be transported from the positive electrode to the negative electrode, and perform well in the chosen lithium-ion cell. Exemplary solid charge carrying media include polymeric media such as polyethylene oxide, polytetrafluoroethylene, polyvinylidene fluoride, fluorine-containing copolymers, polyacrylonitrile, combinations thereof and other solid media that will be familiar to those skilled in the art. Exemplary liquid charge carrying media include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate, γ-butylrolactone, methyl difluoroacetate, ethyl difluoroacetate, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), tetrahydrofuran, dioxolane, combinations thereof and other media that will be familiar to those skilled in the art. Exemplary charge carrying media gels include those described in U.S. Pat. Nos. 6,387,570 (Nakamura et al.) and 6,780,544 (Noh). The charge carrying media solubilizing power may be improved through addition of a suitable cosolvent. Exemplary cosolvents include aromatic materials compatible with Li-ion cells containing the chosen electrolyte. Representative cosolvents include toluene, sulfolane, dimethoxyethane, combinations thereof and other cosolvents that will be familiar to those skilled in the art. The electrolyte may include other additives that will familiar to those skilled in the art. For example, the electrolyte may contain a redox chemical shuttle such as those described in U.S. Pat. Nos. 5,709,968 (Shimizu), 5,763,119 (Adachi), 5,536,599 (Alamgir et al.), 5,858,573 (Abraham et al.), 5,882,812 (Visco et al.), 6,004,698 (Richardson et al. &#39;698), 6,045,952 (Kerr et al.), and 6,387,571 B1 (Lain et al.); in U.S. Patent Publication 2005/0221196 (Dahn et al.) (the disclosure of which is incorporated herein by reference) and in PCT Published Patent Application No. WO 01/29920 A1 (Richardson et al. &#39;920). 
     The disclosed cells may be used in a variety of devices, including portable computers, tablet displays, personal digital assistants, mobile telephones, motorized devices (e.g, personal or household appliances and vehicles), instruments, illumination devices (e.g., flashlights) and heating devices. Multiple cells can be combined to provide battery packs. Further details regarding the construction and use of rechargeable lithium-ion cells and battery packs will be familiar to those skilled in the art. 
     The invention is further illustrated in the following illustrative examples, in which all parts and percentages are by mole unless otherwise indicated. 
     Preparatory Example 1 
     Preparation of Aliphatic Polyimide Precursor BuTDA-ODA from 1,2,3,4-butanetetracarboxylic Dianhydride and 4,4′-oxydianiline 
     3 Grams (0.015 moles) 4,4′-oxydianiline were added to 33.8 grams NMP in a round-bottomed vessel. 2.97 Grams (0.015 moles) 1,2,3,4-butanetetracarboxylic dianhydride (“BuTDA”, prepared from 1,2,3,4-butanetetracarboxylic acid by refluxing in acetic anhydride) were added to the reaction mixture. The solution was allowed to stir under dry nitrogen overnight to form a 20 wt. % aliphatic polyimide precursor solution. 
     Preparatory Examples 2-6 
     Preparation of Aromatic/Aliphatic Polyimide Copolymer Precursors BTDA/BuTDA-ODA from Mixtures of 3,3′4,4′-benzophenonetetracarboxylic Dianhydride, 1,2,3,4-butane-tetracarboxylic Dianhydride and 4,4′-oxydianiline 
     Using the method of Preparatory Example 1, some of the BuTDA was replaced with 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (“BTDA”, available from Aldrich Chemicals, Milwaukee, Wis.) to form mixed aromatic/aliphatic polyimide copolymer precursor solutions based on BTDA/BuTDA mixtures as shown below in Table I: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Preparatory 
                   
                 BuTDA, 
                   
                 BTDA:BuTDA 
               
               
                 Example No. 
                 BTDA, Grams 
                 Grams 
                 ODA, Grams 
                 Molar Ratio 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Preparatory 
                 0.742 
                 3.62 
                 3 
                 25:75 
               
               
                 Example 2 
               
               
                 Preparatory 
                 1.485 
                 2.415 
                 3 
                 50:50 
               
               
                 Example 3 
               
               
                 Preparatory 
                 2.28 
                 1.21 
                 3 
                 75:25 
               
               
                 Example 4 
               
               
                 Preparatory 
                 2.67 
                 0.483 
                 3 
                 90:10 
               
               
                 Example 5 
               
               
                 Preparatory 
                 2.82 
                 0.241 
                 3 
                 95:5  
               
               
                 Example 6 
               
               
                   
               
             
          
         
       
     
     Comparative Preparatory Example 1 
     Preparation of Aromatic Polyimide Precursor BTDA-ODA from 3,3′,4,4′-benzophenonetetracarboxylic Dianhydride and 4,4′-oxydianiline 
     3 Grams (0.015 moles) 4,4′-oxydianiline (“ODA”, available from Aldrich Chemicals, Milwaukee, Wis.) were added to 33.8 grams N-methylpyrrolidinone (“NMP”, freshly distilled from CaH 2 ) in a round-bottomed vessel. 4.83 Grams BTDA were added to the reaction mixture. The solution was allowed to stir under dry nitrogen overnight to form an aromatic polyimide precursor solution. 
     Preparatory Example 7 
     Preparation of Si 57 Al 28 Fe 15  Alloy 
     A negative electrode powdered alloy material was prepared as described in the above-mentioned U.S. Pat. No. 7,498,100 (Christensen et al.). 6.12 Grams aluminum shot, 12.54 grams silicon flakes, and 6.56 grams iron flakes (all 99.9% or better purity) were weighed and placed in an arc furnace. The mixture was melted in an Ar atmosphere in the presence of a Ti pool oxygen getter to yield 25 grams of an ingot having the composition Si 57 Al 28 Fe 15 , where all amounts are in atomic percent. 
     The ingot was broken into pieces less than 15 mm in diameter. 10 Grams of the pieces were placed inside a quartz tube ending in a 0.89 μm diameter nozzle. A thin carbon sleeve was also inserted in the tube as a radio frequency coupler to initiate melting of the ingot. The tube was placed in the chamber of a melt spinner above a 200 mm diameter copper wheel such that the distance from the nozzle orifice to the wheel surface was 10 mm. The chamber was evacuated to 80 millitorr and backfilled with He to 200 Torr. The ingot was melted in a radio frequency field. When the melt had reached 1150° C., the molten liquid was ejected at 80 Torr He overpressure onto the copper wheel while the wheel rotated at a 35 m/sec surface speed. This quenched the melt and formed approximately 9 grams of ribbon fragments. 
     Examples 8-12 and Comparative Example 2 
     Preparation of Negative Electrodes Containing Powdered Si 57 Al 28 Fe 15  Alloy and Polyimide Binders 
     A powdered alloy electrode composition was prepared by combining, in a PULVERISETTE 7™ planetary micro mill (available from Fritsch GmbH) equipped with a 40 ml tungsten carbide vessel, 1.7 grams of the Preparatory Example 7 Si 57 Al 28 Fe 15  alloy, 0.1 grams SUPER P™ carbon powder (available from MMM Carbon, Belgium), 1.33 grams of the Preparatory Example 1 aliphatic polyimide precursor solution, and an additional 2.5 grams NMP. Two 14.25 mm diameter and ten 6 mm diameter tungsten carbide balls were added to the vessel for milling purposes. The vessel and its contents were milled for one hour. The resulting slurry was cast onto a nickel foil using a 0.125 mm notch bar coater. The coating was dried at 80° C. for 30 minutes in air and then placed in an air oven at 150° C. The oven temperature was ramped up to 250° C. and held for one hour, yielding a finished negative electrode with an aliphatic polyimide binder. This electrode was used to form a lithium coin cell identified as Example 8, as further described below. Using a similar procedure, the Preparatory Example 1 aliphatic polyimide precursor solution was replaced with 1.33 grams of the Preparatory Example 3 through Preparatory Example 6 aromatic/aliphatic polyimide precursor solutions to form negative electrodes containing aromatic/aliphatic polyimide binders, and used to form lithium coin cells identified as Examples 9-12. Using a similar procedure, the Preparatory Example 1 aliphatic polyimide precursor solution was replaced with 1.33 grams of the Comparative Preparatory Example 1 aromatic polyimide precursor solution to form a negative electrode containing an aromatic polyimide binder and used to form a lithium coin cell identified as Comparative Example 2. 
     Coin-type test cells were built in 2325 coin cell hardware as described in A. M. Wilson and J. R. Dahn,  J. Electrochem. Soc.,  142, 326-332 (1995). An exploded perspective schematic view of a 2325 coin cell  10  is shown in  FIG. 1 . Stainless steel cap  24  and oxidation resistant case  26  contain the cell and serve as the negative and positive terminals respectively. The negative electrode  14  was formed on nickel foil current collector  18  as described above. The positive electrode  12  was formed from metallic lithium on a copper current collector  16 . Separator  20  was formed from two layers of CELGARD™ 2400 microporous material (available from Celgard, Inc., Charlotte, N.C.), and wetted with an electrolyte prepared from 1 M LiPF 6  in a 1:2 (by volume) mixture of ethylene carbonate and diethyl carbonate. Gasket  27  provided a seal and separated the two terminals. A tightly squeezed stack was formed when the cell was crimped closed. The assembled cells were cycled at room temperature using a MACCOR™ current cycler (available from Maccor, Inc., Tulsa, Okla.) operated at a constant current of 0.125 mA between 0.9V and 0.025V for the first cycle, and at a constant current of 0.5 mA between 0.9V and either 0.050V or 0.005V for all additional cycles. The % irreversible capacity loss during the first cycle was measured and is reported in Table II below along with the irreversible capacity loss per gram of electrode material (minus the weight of the current collector: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Example No. 
                   
                 BTDA: 
                   
                 Irreversible 
               
               
                 or 
                 Polyimide 
                 BuTDA 
                 % Irreversible 
                 Capacity Loss, 
               
               
                 Comparative 
                 Precursor 
                 Molar 
                 Capacity Loss 
                 mAh/g 
               
               
                 Example No. 
                 Solution 
                 Ratio 
                 (1 st  Cycle) 
                 Electrode 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 8 
                 Preparatory 
                  0:100 
                 19.4 
                 155 
               
               
                   
                 Example 1 
               
               
                 Example 9 
                 Preparatory 
                 50:50 
                 25.6 
                 205 
               
               
                   
                 Example 3 
               
               
                 Example 10 
                 Preparatory 
                 75:25 
                 22 
                 182 
               
               
                   
                 Example 4 
               
               
                 Example 11 
                 Preparatory 
                 90:10 
                 20 
                 163 
               
               
                   
                 Example 5 
               
               
                 Example 12 
                 Preparatory 
                 95:5  
                 19.7 
                 158 
               
               
                   
                 Example 6 
               
               
                 Comparative 
                 Comparative 
                 100:0  
                 34 
                 285 
               
               
                 Example 2 
                 Preparatory 
               
               
                   
                 Example 1 
               
               
                   
               
             
          
         
       
     
     As shown above by the Comparative Example 2 cell test results, a cell whose negative electrode employed an aromatic polyimide binder suffered a high first cycle irreversible capacity loss. As shown above by the Example 8 cell test results, a cell whose negative electrode employed an aliphatic polyimide binder suffered a much less severe first cycle irreversible capacity loss. As shown above by the Example 9-12 test results, cells whose negative electrode employed an aromatic/aliphatic polyimide binder also suffered less severe first cycle irreversible capacity losses. 
     First lithiation (discharge) voltage curves were measured for the cells of Example 8 and Comparative Example 2. The curves are shown in  FIG. 2 . Curve A was obtained using the Example 8 cell and Curve B was obtained using the Comparative Example 2 cell. The Example 8 cell clearly shows extra irreversible capacity amounting to about 120 mAh/g as evidenced by the horizontal distance in  FIG. 2  between point C (the x-axis intercept for the curve A initial slope shown as a dashed oblique line in  FIG. 2 ) and point D (the x-axis value for the curve A/curve B intersection shown as a dashed vertical line in  FIG. 2 ). 
     Examples 13-15 and Comparative Examples 3-4 
     Preparation of MCMB (Synthetic Graphite) Carbon Electrodes with Polyimide or Polyvinylidene Fluoride Binders 
     Electrodes containing MCMB™ mesocarbon microbeads (available from MER Corporation, Tucson, Ariz.) were prepared and evaluated using the general method of Examples 8-12 and Comparative Example 2, with the following exceptions. In each instance, 1.7 grams mesocarbon microbeads were used in place of the Si 57 Al 28 Fe 15  alloy, the mesocarbon microbeads were mixed rather than milled with the binder, copper foil was used as the current collector and electrode curing was carried out at 120° C. in air. Additionally, Example 13 employed 1.33 grams of the Preparatory Example 1 composition as the polyimide precursor solution; Example 14 employed 1.33 grams of the Preparatory Example 2 composition as the polyimide precursor solution; Example 15 employed 1.33 grams of the Preparatory Example 4 solution as the polyimide precursor; Comparative Example 3 employed 1.33 grams PYRALIN™ PI2525 polyamic acid solution (a BTDA-based aromatic polyamic acid solution of benzophenone tetracarboxylic dianhydride/4,4-oxydianiline/m-phenylenediamine in NMP available from HD Microsystems, Wilmington, Del.) as the polyimide precursor solution; and Comparative Example 4 employed 1.33 grams KYNAR™ 461 polyvinylidene fluoride (“PVDF”, available from Boedeker Plastics, Inc., Shiner, Tex.) in place of the polyimide precursor solution. The performance test results for cells employing the resulting electrodes are shown below in Table III. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                   
                   
                   
                 % 
                   
               
               
                 Example No. 
                   
                 BTDA: 
                 Irreversible 
                 Irreversible 
               
               
                 or 
                 Binder 
                 BuTDA 
                 Capacity 
                 Capacity Loss, 
               
               
                 Comparative 
                 Precursor 
                 Molar 
                 Loss 
                 mAh/g 
               
               
                 Example No. 
                 Solution 
                 Ratio 
                 (1 st  Cycle) 
                 Electrode 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 13 
                 Preparatory 
                  0:100 
                 13.5 
                 39 
               
               
                   
                 Example 1 
               
               
                 Example 14 
                 Preparatory 
                 25:75 
                 23 
                 80 
               
               
                   
                 Example 2 
               
               
                 Example 15 
                 Preparatory 
                 75:25 
                 15.4 
                 55 
               
               
                   
                 Example 4 
               
               
                 Comparative 
                 PYRALIN 
                 100:0  
                 37.5 
                 173 
               
               
                 Example 3 
                 PI2525 
               
               
                   
                 Polyamic Acid 
               
               
                 Comparative 
                 KYNAR 461 
                 Not 
                 13.6 
                 36 
               
               
                 Example 4 
                 PVDF 
                 Applicable 
               
               
                   
               
             
          
         
       
     
     As shown above in Table III, cells whose negative electrode employed an aromatic/aliphatic polyimide binder suffered less first cycle irreversible capacity loss than a cell whose negative electrode employed an aromatic polyimide binder. The irreversible capacity loss results for the Example 13 cell were similar to those obtained using PVDF as the binder, and offered other important advantages. For example, polyimide binders usually have higher density and better metal adhesion than PVDF, thus allowing fabrication of thinner electrodes having improved adhesion to metal foil current collectors. Example 13 was repeated using commercially-supplied BuTDA from TIC Americas, Inc. to prepare the polyimide precursor, resulting in a cell having a 12.6% first cycle irreversible capacity loss, or 45 m Ah/g of electrode. 
     Example 16 
     Cyclic Voltammetry 
     The Preparatory Example 1 aliphatic polyimide precursor solution was diluted using NMP to form an approximately 2.5 wt. % solution. The solution was dip coated onto a titanium wire, dried at 80° C. for about 30 minutes and cured at 250° C. for 1 hour. A similar coated wire was prepared using the Comparative Preparatory Example 1 aromatic polyimide precursor solution. A cyclic voltammetry measurement was performed in a standard 3-electrode cell using a large (4 cm 2 ) platinum flag as the counter electrode, an aqueous Ag/AgCl reference electrode 0.1M aqueous NaClO 4  electrolyte and a 5 mV/s sweep rate. The electrolyte was de-aerated using a nitrogen gas bubble for several minutes prior to applying the voltage sweep. The resulting cyclic voltammetry curves are shown in  FIG. 3 . Curve A was obtained using the aliphatic polyimide and Curve B was obtained using the aromatic polyimide. The aromatic polyimide clearly undergoes an irreversible electrochemical reduction (as evidenced by the current peak at about −1.6 volts in Curve B) whereas Curve A exhibits no current peak and no indication of electrochemical reduction. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.