Electrode conductive material

Details Electrode conductive material Electrode conductive material

2000-10-27

2002-09-10

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Metal compound containing

C423S138000, C423S140000, C429S209000, C429S218200

Reexamination Certificate

active

06447699

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to an electrode conductive material and more particularly to the conductive material used in the positive electrode of an alkaline electrolyte secondary storage cell.
2. Description of the prior art
A non-sintered nickel electrode (also referred to as a pasted or plasticized electrode) includes a support serving as a current collector and coated with a paste containing the active material and a binder, to which a conductive material is usually added. During fabrication of the electrode a volatile solvent is added to the paste to modify its viscosity in order to facilitate shaping. When the paste has been deposited on or in the support, the combination is compressed and dried to obtain an electrode with the required density and of the required thickness.
The active material in an alkaline electrolyte storage cell positive electrode is usually a nickel-based hydroxide. Nickel hydroxide is a poor conductor and requires the addition to the electrode of a material enabling good electrical percolation. The paste therefore usually contains a conductive material, which can in particular be a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH)
2
and/or a cobalt oxide such as cobalt monoxide CoO. For example, it has been proposed to use as the conductive material a non-stoichiometric cobalt oxide with the formula Co
x
O where 0.93≦×≦0.97, a cobalt oxide with the formula CoO
x
where 0<×<1 or a cobalt monoxide CoO in which the surface of the particles is covered with hydroxyl groups or a layer of cobalt oxide with a valency at least equal to 2, such as Co
2
O
3
or Co
3
O
4
.
When an alkaline storage cell is charged for the first time, the above substances are oxidized to form cobalt oxyhydride CoOOH in which the degree of oxidation of the cobalt is +3 or greater. Cobalt oxyhydride is stable in the normal range of operating conditions of the nickel positive electrode and insoluble in the alkaline electrolyte. It assures the electrical percolation of the electrode.
When stored in a completely discharged state, the voltage of an alkaline storage cell having a non-sintered nickel positive electrode decreases with time. If the storage time exceeds a few months, the voltage tends towards 0 V. Under these conditions the cobalt oxyhydride is reduced slowly. The degree of oxidation of the cobalt first falls to +2.66, in Co
3
O
4
, and then +2, in Co(OH)
2
. Cobalt oxyhydride Co(OH)
2
is highly soluble in the electrolyte. Consequently, after a storage period of several months, a loss of conductivity is observed due to partial dissolution of the percolating network of the non-sintered electrode. This causes an irreversible loss of capacity, which can exceed 15%. This irreversible loss of capacity occurs whichever cobalt compound known in the art is introduced into the paste.
The object of the present invention is to propose a conductive material for a non-sintered nickel electrode such that the irreversible loss of capacity of the electrode in storage is greatly reduced compared to the electrodes known in the art, in combination with a high yield.
SUMMARY OF THE INVENTION
The present invention provides an electrode conductive material including a non-stoichiometric oxidized cobalt compound whose electrical conductivity is greater than 5×10
−3
S/cm and which has a direct spinel structure and formula Co
2+
x
Co
3+
y
O
4
, with 0.73≦x≦0.80 and 1.87 ≦y≦1.95.
The above substance is isomorphic with spinel (magnesium aluminate MgAl
2
O
4
). Its structure is based on a cubic unit cell with eight face-centered oxygen lattice units. In this instance the divalent Co
2+
ions are in the tetrahedral sites of the cubic lattice, the trivalent Co
3+
ions are in the octahedral sites of the lattice, and the oxygen ions are at the apices of the tetrahedrons and the octahedrons. This structure is referred to as the “normal” or “direct” structure, as compared with the inverse structure in which the divalent and trivalent ions are disposed differently. In a structure of this type corresponding to the general formula AB
2
O
4
, there are usually twice as many trivalent ions as divalent ions. One important feature of the conductive material according to the present invention is that the cobalt oxide is non-stoichiometric, i.e. there is a cobalt deficit.
The method of fabricating an electrode conductive material according to the present invention, as previously described, includes the following steps: a basic solution made up of a mixture of potassium hydroxide KOH and sodium hydroxide NaOH is introduced into a reactor and stirred mechanically, a constant flow of oxygen is established in said solution, an initial powder containing cobalt hydroxide is introduced into said solution and left in contact with said solution, a final powder is separated from said solution, and said final powder is washed and dried.
Said solution is preferably a mixture of 6N potassium hydroxide KOH and 4N sodium hydroxide NaOH. The purpose of mechanical stirring is to take the powder up into suspension in the solution. Stirring can be obtained by means of a paddle-wheel, for example. For the reaction to be homogeneous, the flow of oxygen preferably enters the reactor from the bottom to improve the distribution of oxygen in the solution.
For example, the temperature of the solution can be from 80° C. to 120° C. and the time for which the powder is in contact with the solution can be from 5 hours to 48 hours.
In a first variant, the initial powder is cobalt hydroxide.
In another variant, the initial powder consists of particles based on nickel hydroxide covered with a layer of cobalt hydroxide.
When the reaction has taken place, the powder finally obtained is separated from the solution. The final powder is then washed, preferably with water, and then dried at moderate temperature, for example in a vacuum.
The present invention also provides an alkaline electrolyte secondary storage cell positive electrode including a current collector and a layer containing a binder, particles of an electrochemically active material and a conductive material including a non-stoichiometric oxidized cobalt compound whose electrical conductivity is greater than 5×10
+3
S/cm and which has a direct spinel structure and formula Co
2+
x
Co
3+
y
O
4
, with 0.73≦x<0.80 and 1.87≦y≦1.95, the proportion by weight of said conductive material in said layer being from 4% to 10% of said active material.
In a first variant, the conductive material consists of particles. In the layer containing a binder, particles of the powder conductive material are mixed with particles of said active material and the binder. Preferably the average diameter of the particles of the conductive material is less than 2 &mgr;m.
In a second variant, the conductive material consists of a coating covering the particles of the active material. The active material particles are therefore coated with a layer of the conductive material.
In a third variant, the conductive material consists of particles mixed with the particles of the active material and of a coating covering the particles of the active material.
The current collector can be a two-dimensional conductive support, such as solid or perforated strip, expanded metal, grid or woven material, or a porous three-dimensional conductive support, such as felt or foam. The support can be based on metal or carbon.
The binder includes at least one substance chosen from carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), a copolymer of styrene, ethylene, butylene and styrene (SEBS), a terpolymer of styrene, butadiene and vinylpyridine (SBVR), a copolymer of styrene and butadiene (SBR), possibly carboxylated, a copolymer of acrylonitrile and butadiene (NBR), polytetrafluoroethylene (PTFE), a fluorinated copolymer of ethylene and propylene (FEP), polyhexafluoropropylen

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