Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-01-10
2002-02-19
Le, Hoa Van (Department: 1752)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S206000, C429S232000
Reexamination Certificate
active
06348284
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a non-sintered nickel electrode used in particular as a positive electrode in a secondary electrochemical cell having an alkaline electrolyte, such as nickel metal hydride, nickel cadmium, nickel iron, nickel zinc, or nickel hydrogen storage batteries. The invention also relates to a cell containing the electrode, and to a method of preparing the electrode.
A non-sintered nickel electrode is made up of a two-dimensional support such as a continuous or perforated foil, an expanded metal, a grid, or a cloth, or indeed a three-dimensional support such as a foam or a felt, the support serving as a current collector. A paste containing the active material which is constituted by a nickel-based hydroxide and a binder, usually associated with a conductive material, is coated on the collector. Nickel hydroxide is a poorly conductive compound which requires conductive material to be included in the electrode to enable electricity to percolate well. As a general rule, the conductive material is a cobalt compound such as metallic cobalt Co, cobalt hydroxide Co(OH)
2
, and/or cobalt oxide CoO.
The first time the storage battery is charged, these compounds are oxidized into cobalt oxyhydroxide CoOOH in which the cobalt is taken to a degree of oxidation greater than or equal to +3. Cobalt oxyhydroxide is stable in the normal operating range of the nickel positive electrode and it is insoluble in the alkaline electrolyte. It enables electricity to percolate in the electrode.
For example, in order to accelerate the formation of cobalt oxyhydroxide CoOOH, document U.S. Pat. No. 5,405,714 proposes using metallic cobalt powder Co in the electrode together with nickel oxyhydroxide powder NiOOH which is the active material, at a concentration that is less than 60% by weight of the cobalt. The storage battery is left at rest until the potential of the positive electrode reaches that of the Co/Co(OH)
2
couple, after which it is charged and discharged. The particles of cobalt Co are covered in a layer of cobalt oxyhydroxide CoOOH while the nickel oxyhydroxide NiOOH is reduced to the hydroxide state Ni(OH)
2
.
On initial charging, the oxidation of the cobalt compounds corresponds to equal quantities of electricity on the positive and negative electrodes. In addition, during subsequent discharges, the positive electrode is not fully discharged (oxidation degree 2) but is discharged only to the nickel being oxidized to degree 2.2. As a result, the non-discharged capacity or “precharge” of the negative electrode increases on each cycle, thereby progressively decreasing the effective capacity of said electrode and contributing to shortening the lifetime of the storage battery.
When stored in a fully discharged state, an alkaline storage battery possessing a non-sintered nickel positive electrode sees its voltage decrease over time. When the duration of storage exceeds a few months, its voltage tends towards 0 V. Under such conditions, cobalt oxy-hydroxide CoOOH reduces slowly. The cobalt is taken initially to oxidation degree +2.66 in Co
3
O
4
, and then it reaches oxidation degree +2 in Co(OH)
2
.
Unfortunately, cobalt hydroxide Co(OH)
2
is a compound that is highly soluble in the electrolyte. After being stored for a period of several months, a loss of conductivity is observed due to part of the percolation network in the non-sintered electrode dissolving. This gives rise to an irreversible loss of capacity.
Document EP-0 789 408 proposes using nickel hydroxide powder having grains coated in a cobalt compound containing 0.1% to 10% by weight of sodium. Documents U.S. Pat. No. 5,672,447 and EP-0 798 801 propose covering a nickel hydroxide powder in a disordered cobalt compound of valency greater than +2. Such coatings are likewise not stable during storage at low potential.
In order to remedy that problem, European patent application EP-0 866 510 proposes an electrode containing nickel hydroxide as the main component with a conductive material constituted by an oxide of lithium and cobalt represented by the formula Li
x
CoO
2
, where x lies in the range 0.2 to 0.9. The active material of the paste can also contain a mixture of nickel hydroxide powder and of nickel hydroxide powder in which the surface of the particles is coated in a layer of lithium and cobalt oxide, with lithium and cobalt oxide being added thereto as the conductive material. During storage of the cell, the observed irreversible loss of capacity is still too high.
SUMMARY OF THE INVENTION
An object of the present invention is to propose a non-sintered nickel electrode in which irreversible loss of capacity during storage in the discharged state is smaller than that of presently known electrodes.
Another object of the invention is to provide a nickel storage battery whose precharge is reduced by using a novel positive electrode.
The present invention provides a non-sintered nickel electrode for a secondary cell having an alkaline electrolyte, the electrode comprising a current collector and a paste comprising an active material in powder form based on nickel hydroxide, a conductive material containing lithium and cobalt, and a binder, the electrode being characterized in that said active material is constituted by particles of a hydroxide containing a majority of nickel that is at least partially oxidized into &bgr; structure oxyhydroxide, said particles being at least partially coated in said conductive material which is a lithiated oxide of nickel and cobalt.
In order to ensure that the positive electrode has an optimum usage ratio, the conductivity of said conductive material is greater than 10
−2
Siemens.cm
−1
after at least one charge/discharge cycle, referred to as “electrochemical forming”.
In a preferred embodiment, said lithiated oxide of nickel and cobalt has the formula Li
x
Ni
y
CO
1-y
O
2
where 0.1≦x≦1 and 0≦y≦0.9, and preferably 0.02≦y≦0.9.
The degree of oxidation of the cobalt in said lithiated oxide is not less than 3, and preferably equal to or greater than 3.2.
In another embodiment, the lithiated oxide contains sodium. Preferably, the lithiated oxide has the formula Li
x
Na
z
Ni
y
Co
1-y
O
2
where x+z lies in the range 0.1 to 1 and z lies in the range 0 to 0.5, i.e. 0.1≦x+z≦1 and 0≦z≦0.5.
Preferably, the quantity of lithiated oxide lies in the range 3% to 9% by weight relative to said active material.
In a variant, said paste also contains the powder form of the lithiated oxide of nickel and cobalt.
The conductive material occupies at least part of the micropores in the surface of the nickel-based hydroxide particle. This is the microporosity that is accessible to the electrolyte and that contributes to the electrochemically active surface of the hydroxide.
Without modifying the invention, the nickel-based hydroxide particles can be of various shapes, going from a more or less spherical shape to an irregular shape.
In a preferred implementation of the invention, 5% to 35% by weight of said nickel hydroxide Ni(OH)
2
is oxidized into a &bgr; structure oxyhydroxide NiOOH, and preferably 5% to 20% by weight, and more preferably still 10% to 20%.
It is important that the &ggr;-NiOOH oxyhydroxide does not form since the &ggr; phase has lattice parameters that are larger than those of the &bgr; phase. This characteristic of the &ggr; phase gives rise to breaks and to partial destruction in the coating which harms the performance of the electrode, and in particular harms its ability to conserve storage capacity.
It should naturally be understood that the term “electrochemically active material containing nickel hydroxide” as used in the present application can mean nickel hydroxide, a hydroxide that contains mainly nickel, and also a nickel hydroxide containing at least one syncrystallized hydroxide of an element selected from zinc (Zn), cadmium (Cd), magnesium (Mg), and aluminum (Al), and at least one syncrystallized hydroxide of an element selected from cobalt (Co), manganese (Mn), alum
Bernard Patrick
Senyarich Stephane
Alcatel
Le Hoa Van
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