Positive active electrode composition with graphite additive

Details Positive active electrode composition with graphite additive Positive active electrode composition with graphite additive

2001-11-27

2003-09-09

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S218200, C429S232000, C429S217000, C429S206000

Reexamination Certificate

active

06617072

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrodes for electrochemical devices such as batteries and fuel cells. In particular, the present invention relates to an active composition for use in electrodes of electrochemical devices.
BACKGROUND OF THE INVENTION
Electrochemical devices include both batteries and fuel cells. Rechargeable batteries may be classified as “nonaqueous” batteries or “aqueous” batteries. An example of a nonaqueous battery is a lithium-ion battery which uses intercalation compounds for both anode and cathode, and a liquid organic based or polymer electrolyte. Aqueous batteries may be classified as either “acidic” or “alkaline”. An example of an acidic battery is a lead-acid battery which uses sulfuric acid as the electrolyte, lead dioxide as the active material of the positive electrode, and metallic lead, in a high-surface area porous structure, as the negative active material.
Examples of alkaline batteries are “nickel-based” alkaline batteries. These batteries use an alkaline electrolyte (such a potassium hydroxide) and nickel hydroxide as the active material for the positive electrode. Nickel hydroxide has been used for years as an active material for the positive electrode of alkaline batteries. The reactions that take place at the positive electrode of a nickel-based rechargeable battery are reversible and include the following chemical reaction:
At the positive electrode, Ni(OH)
2
is oxidized to NiOOH during the charge operation. During discharge, the NiOOH is reduced to Ni(OH)
2
. Examples of such nickel-based alkaline batteries include nickel-metal hydride batteries (Ni—MH), nickel cadmium batteries (Ni—Cd), and nickel-zinc batteries (Ni—Zn). Ni—MH batteries comprise negative electrodes having a hydrogen storage alloy as the active material. The hydrogen storage alloy is capable of reversible electrochemical storage of hydrogen. In general, Ni—MH batteries utilize a negative electrode that is capable of reversible electrochemical storage of hydrogen, and a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical potential across a Ni—MH battery, the hydrogen storage alloy of the negative electrode is charged by the electrochemical discharge of hydrogen and the electrochemical generation of hydroxyl ions:
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron. (In a Ni—Cd cell, cadmium metal is the active material in the negative electrode).
The active electrode material for both the positive and negative electrodes is usually affixed to a conductive substrate to form the positive and negative battery electrodes. One way to affix the active material to the conductive substrates is to first make the active materials into a paste by adding a small amount of binder, and then applying this paste to the substrate. The present invention is directed to a new active electrode composition having improved electrochemical and mechanical properties.
SUMMARY OF THE INVENTION
Disclosed herein is an electrode for an electrochemical device, comprising: an active electrode material; a carbon material; and a elastomeric polymer.
Also disclosed herein is an electrode for an electrochemical device, comprising: an active electrode material; a carbon material; and a elastomeric polymer.
Also disclosed herein is an electrochemical device, comprising: an active electrode composition, comprising: active electrode material; a carbon material; and a elastomeric polymer.
DETAILED DESCRIPTION OF THE INVENTION
Disclosed herein is an active composition for an electrode of an electrochemical device. Preferably, the electrochemical device is a battery having one or more positive electrodes, one or more negative electrodes and an electrolyte. Generally, the active electrode composition comprises an active electrode material, a carbon material, and an elastomeric polymer. The elastomeric polymer is used as a binder for the active composition. Preferably, the active electrode composition is in the form of a physical mixture of the active electrode material, the carbon material and the elastomeric polymer. The mixture may be a dry mixture or a paste (a wet mixture). Preferably, the mixture is in the form of a paste.
Generally, the active electrode materials may be either a positive electrode material or a negative electrode material. Examples of positive electrode materials are powders of lead oxide, lithium cobalt dioxide, lithium nickel dioxide, lithium manganese oxide compounds and transition metal oxides, manganese dioxide, zinc oxide, nickel oxide, nickel hydroxide, manganese hydroxide, copper oxide, molybdenum oxide, carbon fluoride, etc.
Examples of negative electrode materials include metallic lithium and like alkali metals and alloys thereof, alkali metal absorbing carbon materials, zinc, cadmium hydroxide, hydrogen absorbing alloys, etc.
Preferably, the active electrode material is a positive electrode material. More preferably, the active electrode material is a nickel hydroxide material. It is within the spirit and scope of this invention that any nickel hydroxide material be used as the active material. Examples of nickel hydroxide materials are provided in U.S. Pat. Nos. 5,348,822, 5,637,423 and 6,177,213, the contents of which are incorporated by reference herein.
As described, the active composition comprises an active electrode material, a carbon material and an elastomeric polymer. Examples of possible carbon materials that may be used in the active composition include graphites. Other examples include carbon materials that contain graphitic carbons, such as graphitized cokes. Still other examples of possible carbon materials include non-graphitic carbons which are considered amorphous, non-crystalline, and disordered, such as petroleum cokes and carbon black. Preferably, the carbon material is a graphite.
The graphite (or other forms of the carbon material) is preferably in the form of a particulate (i.e., particles). The particles may have a variety of shapes. For example, they may be substantially spherical. Alternately, the particles may be elongated where one dimension is longer than another dimension. The particles may be in the form of threadlike fibers. In addition, the particles may be in the form of flakes.
The graphite used should be electrically conductive and is preferably present in sufficient amount to form an electrically conductive network of graphite particles within the active composition. In one embodiment of the invention the active composition preferably comprises at least 10 wt percent of the graphite, more preferably at least 13 wt percent of the graphite, and most preferably at least 15 wt percent of the graphite. In another embodiment of the invention, the active composition preferably comprises between about 10 wt percent to about 25 wt percent of the graphite, more preferably between about 13 wt percent to 20 wt percent of the graphite, and most preferably between about 13 wt percent to about 17 wt percent of the graphite.
It is noted that during the charging process of certain rechargeable batteries, such as a sealed nickel-metal hydride battery, the positive electrode reaches full charge before the negative and begins to evolve oxygen,
2OH
−
→H
2
O+1/2O
2
+2e
−
  (3)
The evolved oxygen can oxidize the positive electrode and cause its mechanical disintegration, thereby reducing the electrode's cycle life. In particular, the oxidation can reduce the adhesion and electrical conductivity between the active nickel hydroxide particles and the substrate, thereby increasing the electrode's resistance and reducing the amount of power available for output.
Furthermore, it is also believed that the evolved oxygen promotes the oxidation of the graphite by the reaction
C+O
2
→CO
2
  (4)
Oxidation of the graphite reduces the conductivity of the graphite and may also r

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