Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-09-15
2001-09-18
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S217000, C429S231100, C429S332000
Reexamination Certificate
active
06291100
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electrode composition having a doped tungsten (IV) active material, which electrode is suitable for use in an electrochemical cell. The invention also relates to an electrochemical cell which employs the electrode composition, as well as to a method of preparing the electrode composition. The invention has particular applicability to the manufacture of primary and rechargeable power sources.
2. Description of the Related Art
Metal oxides have been extensively used as electrode active materials in electrochemical systems, for example in batteries and capacitors. A lithium ion cell can be formed using metal oxides for both anode and cathode. Such a cell is desirable if a sufficiently high voltage and high capacity can be achieved.
Recently, lithium ion technology with metal oxide cathodes has been shown to outperform most of the existing rechargeable batteries. Such metal oxides arc special materials in that they can act as host materials in accepting guest atoms and ions into their crystal lattice. Commonly used cathode metal oxides include, for example, lithiated cobalt oxides (LiCoO
2
), lithiated nickel oxides (LiNiO
2
) and lithiated manganese oxides (LiMn
2
O
4
). Typical metal oxides used in the anode include, for example, iron oxide (Fe
2
O
3
), tin oxide (SnO
2
) and tungsten oxide (WO
2
). Because of the instability of these oxides and their solubility in electrolyte solutions, lithium ion cells employing these oxides in the electrode active material crystal structure lose charge-discharge capacity during long term storage and/or after cycling.
Most of the commercially available lithium ion cells employ carbon as the anode active material. While carbon is capable of accepting lithium atoms into its crystal lattice and of performing well at ambient temperatures, the anode performance degrades at higher temperatures (e.g., greater than 45° C.) due to exfoliation caused by mechanical stress after repeated lithium intercalation. In addition, at high charge rates during intercalation, lithium metal tends to deposit on the surface of the carbon electrode. Such metal deposition on the carbon surface creates safety concerns due to dendrite formation as well as causing premature cell failure. Moreover, lithium ion cells can catch fire when operated under abuse conditions. In such cases, carbon can enhance the magnitude of the fire, thereby raising major safety concerns.
The use of tungsten (IV) oxide (WO
2
) as an mode material is known. See, e.g., Aubom et al, “Lithium Intercalation of Cells Without Metallic Lithium,” J. Electrochem. Soc.: Electrochemical Science and Technology, March 1987, pp. 638-641. The present inventors have also explored the use of tungsten (IV) oxide as an electrode material and found that it can provide significant benefits over other electrode materials such as carbon. For example, unlike carbon, tungsten oxide is not flammable and no dendrites are formed even at very high charge rates of, for example, greater than 20 mA/cm
2
. These features are particularly important for high rate applications, for example, in electric vehicle anal power tool applications.
The electrochemical performance of a Li/WO
2
cell is shown in FIG.
1
. As can be seen from that graph, the capacity of the cell increases with a decrease in cell voltage, as more lithium is being intercalated into the tungsten oxide host crystal structure. As the lithium intercalation proceeds, the crystal stricture of the resultant tungsten oxide changes and becomes electrochemically inactive. For example, when this cell was charged after discharging to 0.2V vs. Li, only a small fraction of the capacity was obtained. This is due to deterioration of the oxide crystal structure. It is, therefore, preferable to cycle the cell between 0.7 and 3.0V to obtain several hundred of charge-discharge processes.
To overcome or conspicuously ameliorate the disadvantages of the related art, it is an object of the present invention to provide an electrode composition which makes the electrode particularly suitable for use in an electrochemical cell. The electrode composition exhibits improved charge-discharge capacity per unit weight of the electrode active material, and is additionally chemically and electrochemically stable.
Other objects, advantages and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a novel electrode composition suitable for use in an electrochemical cell is provided. The composition comprises a polymeric binder material and a doped tungsten (IV) oxide active material. The active material comprises a tungsten (IV) oxide host material and a metal dopant in the host material effective to increase the charge-discharge capacity per unit weight of the active material when used in an electrochemical cell.
According to a further aspect of the invention, a method of forming an electrode composition suitable for use in an electrochemical cell is provided. The method comprises the steps of:
(a) forming an electrode paste or slurry from components comprising a first solvent, a polymeric binder material and a doped tungsten (IV) oxide active material, wherein the active material is formed by a process comprising the steps of:
(i) mixing a tungsten (VI) oxide or a tungsten salt with a salt of the metal dopart and a second solvent, thereby forming a mixture;
(ii) heating the mixture to substantially remove the solvent therefrom;
(iii) performing a further heat treatment of the mixture; and
(iv) cooling the resulting product to ambient temperature;
(b) forming a coating of the electrode slurry; and
(c) evaporating the solvent.
According to yet a further aspect of the invention, an electrochemical cell is provided. The cell comprises an anode, a cathode and an electrolyte providing a conducting medium between said anode and the cathode. The anode or the cathode comprises an electrode composition comprising a polymeric binder material and a doped tungsten (IV) oxide active material. The active material comprises a tungsten (IV) oxide host material and a metal dopant in the host material effective to increase the charge-discharge capacity per unit weight of the active material.
REFERENCES:
patent: 3915740 (1975-10-01), Elsenberg
patent: 4476204 (1984-10-01), Auborn
patent: 5187033 (1993-02-01), Koshiba
patent: 5558961 (1996-09-01), Doeff et al.
patent: 5567401 (1996-10-01), Doddapaneni et al.
patent: 5783333 (1998-07-01), Mayer
patent: 6-215770 (1994-08-01), None
patent: 11-111294 (1999-04-01), None
patent: 2000277111 (2000-10-01), None
Auborn et al, “Lithium Intercalation Cells Without Metallic Lithium,”J. Electrochem. Soc.: Electrochemical Science and Technology,Mar. 1987, pp. 638-641.
Search Report issued in International Application No. PCT/US00/25091, Nov. 2000.
Denzumi Shigenobu
Doddapaneni Narayan
Hu Zhendong
Burns Doane , Swecker, Mathis LLP
IRMA America, Inc.
Maples John S.
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