Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-01-12
2001-08-14
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S232000, C429S233000
Reexamination Certificate
active
06274270
ABSTRACT:
The present invention relates to a non-sintered nickel electrode as used in particular as the positive electrode in an alkaline electrolyte secondary electro-chemical cell, for example nickel-cadmium, nickel-iron, nickel-hydrogen, or nickel-hydridable metal storage cells, and to a cell containing such an electrode. The method further relates to a method of preparing the electrode.
BACKGROUND OF THE INVENTION
Many types of electrode can be used in an alkaline electrolyte storage cell, such as sintered electrodes and non-sintered electrodes, also referred to as paste electrodes or plasticized electrodes. Compared to other types of electrode, a non-sintered electrode contains a greater quantity of material and its capacity per unit volume is therefore increased while its manufacturing cost is reduced.
A non-sintered nickel electrode includes a support serving as a current collector coated with a paste containing both the active material and a binder, to which paste a conductive material is usually added. Said support is a porous three-dimensional conductive support such as felt or foam based on metal or carbon; it can also be a two-dimensional conductive support such as expanded metal, mesh, woven fabric, or solid or perforated strip. While the electrode is being manufactured, a volatile solvent is added to the paste to adjust its viscosity to facilitate shaping it. When the paste has been deposited on or in the support, the result is compressed and dried to obtain an electrode of the required density and thickness.
In a non-sintered nickel electrode, the active material is a nickel-based hydroxide. Nickel hydroxide is a poor conductor and requires a material enabling good electrical percolation to be added to the electrode. The paste therefore generally contains a conductive material, for example a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH)
2
, and/or cobalt oxide CoO. The first time the alkaline storage cell is charged, these compounds are oxidized to form cobalt oxyhydroxide CoOOH in which the degree of oxidation of the cobalt is greater than or equal to +3. Cobalt oxyhydroxide is stable in the normal range of operation of the nickel positive electrode and is insoluble in the alkaline electrolyte. It provides the electrical percolation of the electrode.
If an alkaline storage cell having a non-sintered nickel-positive electrode is stored in a completely discharged state, its voltage decreases with time. If the battery is stored for more than a few months, its voltage tends to 0 V. Under those conditions, the cobalt oxyhydroxide is slowly reduced. The degree of oxidation of the cobalt initially falls to +2.66 (Co
3
O
4
) and then to +2 (Co(OH)
2
).
Cobalt hydroxide Co(OH)
2
is highly soluble in the electrolyte. After several months in storage there is therefore a loss of conductivity due to partial dissolution of the percolating structure of the non-sintered electrode. This causes an irreversible loss of capacity, which can exceed 15%, and which occurs regardless of the cobalt compound in the paste.
European Patent Application EP-A-0 798 801 and U.S. Pat. No. 5,672,447 propose the use of nickel hydroxide powder coated with a disordered cobalt compound having a valency greater than +2. European Patent Application EP-A-0 789 408 specifies a cobalt compound of that kind containing from 0.1% to 10% by weight sodium. Such coatings are likewise not stable during storage at a low voltage.
OBJECT AND SUMMARY OF THE INVENTION
The object of the present invention is to propose a non-sintered nickel electrode in which irreversible loss of capacity in storage is greatly reduced compared to prior are electrodes.
The present invention consists in a non-sintered nickel electrode for alkaline electrolyte storage cells including a current collector and a paste containing an active material based on nickel hydroxide and a conductive material, wherein said conductive material is a lithium-containing cobalt and nickel oxide whose degree of oxidation obtained after electrochemical conditioning is stable after prolonged storage in the alkaline electrolyte.
The rate of reduction of the lithium-containing oxide in accordance with the invention is much lower than that of a standard prior art substance having a degree of oxidation of +3 after electrochemical conditioning. Consequently, for the same storage period, the irreversible loss of capacity is very greatly reduced compared to that observed in prior art cells.
For the positive electrode to have an optimum utilization ratio, the conductivity of said conductive material after electrochemical conditioning is greater than 10
−2
Siemens.cm
−1
.
In a preferred embodiment of the invention said lithium-containing oxide has the formula Li
x
Ni
y
Co
1−y
O
2
with x in the range from 0.1 to 1 and y in the range from 0.02 to 0.9.
In a first variant said lithium-containing oxide is in the form of powder mixed with the nickel hydroxide powder and the grains of said lithium-containing oxide powder have a mean particle size less than 0.7 &mgr;m and preferably less that 0.5 &mgr;m; the quantity of said lithium-containing oxide in the form of powder relative to said active material is preferably in the range from 6% to 12% by weight.
In a second variant said lithium-containing oxide coats the nickel hydroxide grains; the quantity of said lithium-containing oxide relative to said active material is then preferably in the range from 3% to 9% by weight.
In a third variant said lithium-containing oxide is both in the form of powder mixed with the nickel hydroxide powder and in the form of a coating on the nickel hydroxide grains.
In another embodiment of the invention said lithium-containing oxide has the formula Li
x
Na
z
Ni
y
Co
1−y
O
2
with x+z in the range from 0.1 to 1 and z in the range from 0 to 0.5.
In the foregoing description, the expression “nickel hydroxide” means a hydroxide containing principally nickel and also at least one syncrystallized hydroxide of an element chosen from zinc, cadmium, magnesium, and aluminum; it can also contain at least one syn-crystallized hydroxide of an element chosen from cobalt, manganese, yttrium, calcium, and zirconium.
Said electrode support is advantageously nickel foam.
The paste containing the active material also includes a first binder based on polytetrafluoroethylene (PTFE) and a second binder chosen from carboxymethyl cellulose (CMC), hydroxypropylmethyl cellulose (HPMC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), polyvinylidene fluoride (PVDF) and styrene buta-diene rubber (SBR).
REFERENCES:
patent: 4943497 (1990-07-01), Oishi et al.
patent: 5409786 (1995-04-01), Bailey
patent: 5672447 (1997-09-01), Yamawaki et al.
patent: 6017655 (2000-01-01), Ovshinsky et al.
patent: 6045771 (2000-04-01), Matsubara et al.
patent: 6114063 (2000-09-01), Katsumoto et al.
patent: 0 866 510 A2 (1998-09-01), None
patent: 10 326 616 (1998-12-01), None
Audry Claudette
Baudry Michelle
Bernard Patrick
Jan Olivier
Alcatel
Sughrue Mion Zinn Macpeak & Seas, PLLC
Weiner Laura
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