Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-08-11
2002-09-17
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C029S623500
Reexamination Certificate
active
06451480
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to lithium ion batteries. In particular, it is related to lithium ion batteries containing a liquid electrolyte and at least one ionically conductive and electrochemically active polyimide-based electrode.
BACKGROUND OF THE INVENTION
Typical rechargeable lithium cells use lithium metal electrodes as an ion source in conjunction with positive electrodes. These positive electrodes comprise compounds capable of intercalating the lithium ions within their structure during discharge of the cell. These cells rely on separator structures or membranes that physically contain a measure of fluid electrolyte, usually in the form of a solution of a lithium compound. The separator structure also provides a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric have been saturated with solutions of a lithium compound, such as LiClO
4
, LiPF
6
, or LiBF
4
, in an organic solvent such as propylene carbonate, diethoxyethane, or dimethyl carbonate, to form an electrolyte/separator element. A fluid electrolyte bridge is thus established between the electrodes and effectively provides the necessary Li
++
ion mobility at conductivities in the range of about 10
−3
S/cm.
Gozdz et al. (U.S. Pat. No. 5,460,904) point out that these separator elements unfortunately comprise sufficiently large solution-containing voids establishing continuous avenues between the electrodes. In turn, lithium dendrite formation is enabled during charging cycles and eventually internal cell short-circuiting occurs. To combat this problem, lithium-ion cells have been made where both electrodes comprise intercalation materials, such as lithiated metal oxides, graphites, and carbons. This eliminates the lithium metal which promotes the deleterious dendrite growth. However, these cells do not attain the capacity provided by lithium metal electrodes.
Gozdz et al. proposed an electrolytic cell electrode and separator elements that employ a combination of poly(vinylidene fluoride)copolymer matrix and a compatible organic solvent plasticizer which maintains a homogeneous composition in the form of a flexible, self-supporting film. The copolymer comprises about 75 to 92% by weight vinylidene fluoride (VdF) and 8 to 25% hexafluoropropylene (HFP). The HFP limits the crystallinity of the final copolymer to a degree such that it ensures good film strength while enabling the retention of about 40 to 60% of preferred solvents for lithium electrolyte salts. Within this range of solvent content, the 5 to 7.5% salt ultimately comprising a hybrid electrolyte membrane yields an effective room temperature ionic conductivity of about 10
−4
to 10
−3
S/cm, yet the membrane exhibits no evidence of solvent exudation which might lead to cell leakage or loss of conductivity.
Each electrode is typically prepared from a collector foil in the form of an open mesh, upon which is laid either a positive or a negative electrode membrane. This membrane comprises an intercalatable material dispersed in a polymeric binder matrix such as poly(vinylidene fluoride) or poly(tetrafluoroethylene). The binder matrix provides no electrochemical benefit to the electrode and functions strictly to hold the intercalatable materials to the collector foil while the electrodes are exposed to the liquid electrolyte. Typically, these binders are fluorinated polymers.
The use of fluorinated polymers proves to be destructive to the cell because lithium has a tendency to react with the fluorine in the polymer to form lithium fluoride. This reduction leads to degradation of performance since the lithium ions are removed from the charge/discharge reaction. In addition, the fluorinated polymers may decompose to generate hydrogen fluoride which reacts vigorously and exothermically with the lithium salt to degrade or halt the operation of the battery. Although the currently used binders have good cohesive properties for holding or consolidating particles, they are poor adherents for binding particles to the metal current collectors. Some of these binders also contain moisture which reacts with the lithium salts and degrades performance. Lastly, some of the binders cannot withstand exposure to high temperatures. Therefore, the useful temperature range for the battery is limited.
Fujimoto et al. (U.S. Pat. No. 5,468,571) addressed the temperature problem by providing a secondary battery wherein the negative electrode is prepared with a polyimide binder. However, the polyimides used by Fujimoto et al. are condensation type polyimides which require a dehydration condensation reaction. If the dehydration condensation reaction has not been driven to completion, water may be released when the battery temperature becomes abnormally high. This residual water reacts vigorously with lithium. Although polyimides exhibit good binding and adhesion properties, Fujimoto et al. observed that use of polyimides in excess of 2 parts by weight caused a decrease in capacity.
Gan et al. (“The Effect of Binder Type on Li-Ion Electrode Performance”, 15
th
International Seminar and Exhibit on Primary and Secondary Batteries,
Mar. 3, 1998, pp. 1-12.) studied the use of polyimides as binders for both anodes and cathodes. They observed that graphite (anodes) electrodes with polyimide binder exhibited high irreversible capacities and the higher the polyimide concentration, the larger the irreversible capacity. However, they also noted that although graphite anodes containing polyimide binder showed reasonably good adhesion to the foil substrate, they were much more brittle and prone to cracking than the PVDF-type electrodes. For the cathode, it was found that test cells having a polyimide (≧3.6%) binder had practically no charge capacities and could not be cycled. In addition, when the binder content was reduced, the test cells continued to not cycle well. It was concluded that cathodes using polyimide binders were more brittle than the other cathodes and suffered from cracking.
Gustafson et al. (U.S. Pat. No. 5,888,672) disclose a battery where the anode, the cathode, and the electrolyte each comprise a soluble, amorphous, thermoplastic polyimide. Since the polyimides are pre-imidized prior to the fabrication of the battery, there is no need to further cure them at high temperatures, thus reducing the risk of damaging the battery. Nor is there a chance of incidental condensation as the battery temperature rises. In addition, since no further polymerization will occur, there are no byproducts of the condensation reaction (water) to interact with the lithium salts. The battery of Gustafson et al. is a dry cell.
In fabricating the battery, a minimal amount of pressure or an adhesive is applied to the laminate to allow for intimate lateral contact to be made between the layers. Ultimately, a uniform assembly is formed that is self-bonded and exhibits adhesion between the layers. Since the polyimides used are amorphous, there is an unobstructed pathway for ionic mobility. However, the battery of Gustafson et al. requires bonding or application of an adhesive (prepared from the electrolyte solution) between the layers to promote an unobstructed pathway for ionic mobility. If there are any gaps or defects between the layers, the ionic pathway is upset and the battery function is impaired.
An object of the present invention is to provide a polyimide-based battery wherein the ionic conductivity is insured through the use of a solid electrolyte polyimide binding material.
Another object of the present invention is to provide a polyimide-based battery having at least one ionically conductive and electrochemically active electrode.
Another object of the present invention is to provide a polyimide-based battery that has excellent high temperature stability.
Another object of the present invention is to provide a polyimide-based battery that has a low (less than 1%) initial fade rate over 50 cycles.
A
Antonucci Joseph T.
Gustafson Scott D.
Nelson Loel R.
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