Electrode-forming compositions and electrode members

Details Electrode-forming compositions and electrode members Electrode-forming compositions and electrode members

2002-03-26

2004-04-20

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Composite

Of epoxy ether

C428S413000, C428S450000, C428S458000, C523S457000, C524S778000, C528S028000, C528S038000

Reexamination Certificate

active

06723432

ABSTRACT:

This invention relates to electrode-forming compositions having improved heat resistant adhesion and electrode members, and more particularly, to electrode-forming compositions and electrode members suitable for use in lithium ion batteries and electrical double-layer capacitors.
BACKGROUND OF THE INVENTION
Electrode-forming compositions or adhesive compositions and electrode members are generally constructed of an electrically conductive component such as carbonaceous materials (e.g., activated carbon or carbon black) and fine particles of conductive metal oxides and a binder component capable of bonding to or forming a surface layer on a current collector of aluminum or the like. They are utilized as electrically conductive electrodes in rechargeable lithium ion batteries and electrical double-layer capacitors.
The binder components known in the art include resinous binders such as cellulose, and inorganic binders such as water glass. The resinous binders are less resistant to heat and organic electrolyte solutions whereas the inorganic binders are less adhesive to current collectors. JP-A 9-270370 proposes the use of polyimide resins and polyamide imide resins as the binder component capable of overcoming these drawbacks. However, the polyimide resins and polyamide imide resins are generally diluted with high-boiling solvents such as N-methyl-2-pyrrolidone, which requires high-temperature treatment in order to evaporate off the solvent and form a resin coating. Even when polyamic acid which is a polyimide precursor is used, high-temperature treatment is still needed in order to form a polyimide resin by way of dehydration and ring closure. Another drawback of the polyimide resins and polyamide imide resins is that they are less adhesive to metals and less flexible.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an electrode-forming composition of the heat curing type which can form an electrically conductive electrode at relatively low temperatures and which has improved chemical resistance, heat resistant adhesion to current collectors, and flexibility. Another object of the present invention is to provide an electrode member obtained using the electrode-forming composition.
Making a study on an electrically conductive composition comprising an electrically conductive material and a binder component, we have discovered that use of a solvent-soluble polyimide silicone and an epoxy compound as the binder component leads to an electrode-forming composition of the heat curing type from which an electrically conductive electrode can be formed at relatively low temperatures and which has improved chemical resistance, heat resistant adhesion to current collectors, and flexibility.
Accordingly, the present invention provides an electrode-forming composition comprising a binder component containing a solvent-soluble polyimide silicone and an epoxy compound, and an electrically conductive material; and an electrode member obtained by applying the composition to a current collector and heat curing.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The binder component used in the electrode-forming composition and the electrode member according to the invention includes a solvent-soluble polyimide silicone and an epoxy compound.
The polyimide silicone used herein preferably has a structure represented by the following general formula (1).
In formula (1), X is at least one tetravalent organic group of the following formula (2), (3) or (4).
W is a divalent organic group of the following general formula (5):
wherein B is a group of the following formula (6), (7) or (8).
Y is a divalent organic group of the following general formula (9):
wherein D is CH
2
, (CH
3
)
2
C or (CF
3
)
2
C, and “a” is 0 or 1.
Z is a divalent siloxane residue of the following general formula (10):
wherein R is independently a monovalent hydrocarbon group of 1 to 8 carbon atoms, and “b” is a positive number of 0 to 30.
In formula (1), k is a number corresponding to 3 to 40 mol %, m is a number corresponding to 40 to 90 mol %, and n is a number corresponding to 1 to 40 mol %.
In formula (10), R is independently selected from monovalent hydrocarbon groups of 1 to 8 carbon atoms, preferably 1 to 6 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, pentyl and hexyl, cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl, and aryl groups such as phenyl. Of these, methyl, phenyl and vinyl are preferred from the standpoint of availability of a corresponding reactant. The R groups may be the same or different. The letter “b” representative of the number of recurring siloxane units is preferably in the range of 1 to 30, and more preferably 5 to 20.
The units with subscript k account for 3 to 40 mol %, and preferably 10 to 30 mol % in formula (1). With k below the range, solvent solubility may be lost. Beyond the range, chemical resistance may become poor.
The units with subscript m account for 40 to 90 mol %, and preferably 60 to 85 mol % in formula (1). With m below the range, chemical resistance may sometimes be lost. Beyond the range, the compound may become solvent insoluble.
The units with subscript n account for 1 to 40 mol %, and preferably 5 to 20 mol % in formula (1). With n below the range, the resulting electrode member may not be expected flexible. Beyond the range, the compound may have a lower glass transition temperature and poor heat resistance.
It is acceptable to incorporate recurring polyimide units other than the aforementioned units in the polyimide silicone according to the invention.
In preparing the polyimide silicone according to the invention, any well-known method may be used. For example, a tetracarboxylic dianhydride, an aromatic diamine, a diamine having a phenolic hydroxyl group, and a diaminosiloxane are dissolved in a suitable solvent such as cyclohexanone, and reacted at a low temperature of about 20 to 50° C. to synthesize a polyamic acid which is a polyimide silicone precursor. The total amount of diamine components combined relative to the tetracarboxylic dianhydride component is determined as appropriate depending on the molecular weight of the polyimide silicone to be adjusted, and is usually in a molar ratio of from 0.95 to 1.05, preferably from 0.98 to 1.02. It is noted that a monofunctional reactant such as phthalic anhydride or aniline can be added for the purpose of adjusting the molecular weight of the polyimide silicone. When added, the amount of the monofunctional reactant is up to 2 mol % based on the amount of the polyimide silicone. Subsequently, the polyamic acid solution is heated at a temperature usually in the range of 80 to 200° C., preferably 140 to 180° C., to induce dehydration/ring closure reaction in the acid amide moiety of the polyamic acid. The polyimide silicone is then obtainable in solution form. Alternatively, an acetic anhydride/pyridine mixture is added to the polyamic acid solution, and the resulting solution is heated at about 50° C. for imidization.
Examples of the tetracarboxylic dianhydride include 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride, 4,4′-hexafluoropropylidene bisphthalic dianhydride, and 1,3-bis(3,4-dicarboxyphenyl)-1,1,3,3-tetramethyldisiloxane.
Examples of the aromatic diamine include 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis(4-aminophenoxy-phenyl)hexafluoropropane, and bis[4-(4-aminophenoxy)-phenyl]sulfone.
Examples of the phenolic hydroxyl-bearing diamine include 3,3′-dihydroxy-4,4′-diaminobiphenyl, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, and 2,2-bis(3-hydroxy-4-aminophenyl)propane.
The diaminosiloxane is preferably selected from those of the following general formula (11).
Herein, R is independently a monovalent hydrocarbon group of 1 to 8 carbon atoms, and “b” is an integer of 0 to 30. The definition of R and b in formula (11) is the same as in formula (10), with their preferred examples or range being also the same. Typical examples of the diaminosiloxane of formula (11) are given below.
The polyim

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