Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-02-26
2001-04-03
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231800
Reexamination Certificate
active
06210833
ABSTRACT:
The present invention relates to a nickel electrode used in particular as a positive electrode in a storage cell having an alkaline electrolyte. The invention also extends to the method of manufacturing the electrode.
Several types of electrode are in existence that are suitable for use in an alkaline electrolyte storage cell, such as an electrode of the pocket type or a sintered electrode. To satisfy the increasing requirements of users, these electrodes can nowadays be made by the paste method. Compared with other types of electrode, a pasted electrode contains a larger quantity of active material, thereby increasing its capacity per unit volume, and it is cheaper to manufacture.
A pasted nickel electrode is made by depositing a paste on a two-dimensional conductive support such as expanded metal, a grid, a cloth, or a solid or perforated sheet, or indeed on a porous three-dimensional conductive support such as a metal or carbon felt or foam. A paste comprising as its main components the active material, usually in powder form, and a polymer binder, usually in association with a conductive material is then inserted into such a structure. While the electrode is being made, a volatile solvent is added to the paste to adjust its viscosity and thus make it easier to shape. Once the paste has been deposited on or in the support, the paste-and-support assembly is compressed and dried to obtain an electrode of desired density and thickness.
In a pasted nickel electrode, the active material is constituted by a nickel-based hydroxide. Nickel hydroxide is a compound that conducts electricity poorly and that requires a material suitable for ensuring good percolation of electricity to be added in the electrode. The paste thus generally contains a conductive material which may, for example, be a cobalt compound such as metallic cobalt Co, a cobalt hydroxide Co(OH)
2
, and/or a cobalt oxide CoO. On the first occasion that the alkaline storage cell is charged, these compounds are oxidized into cobalt oxyhydroxide CoOOH in which the oxidation number of the cobalt is raised 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.
When stored in the fully discharged state, an alkaline storage cell having a pasted nickel positive electrode suffers from a decrease in voltage over time. When the duration of storage exceeds a few months, its voltage tends towards 0 V. Under such conditions, the cobalt oxyhydroxide reduces slowly. The oxidation number of the cobalt goes firstly to +2.66 in Co
3
O
4
, followed by oxidation number +2 in Co(OH)
2
, and finally reaching oxidation number 0 in Co.
Unfortunately, cobalt hydroxide Co(OH)
2
is a compound that is highly soluble in the electrolyte. Consequently, after a storage period of several months, a loss of conductivity is observed due to the percolation array of the pasted electrode being partially dissolved. That phenomenon leads to the cobalt present in the positive electrode being redistributed in non-uniform manner, with preferential increase of cobalt particles of large dimensions to the detriment of small particles. This gives rise to an irreversible loss of capacity that can exceed 20%. This irreversible loss of capacity takes place regardless of the particular cobalt compound that was initially included in the paste.
European patent application EP 0 634 804 proposes increasing the conductivity of the positive electrode by including in the paste a carbon powder having a lattice constant d002 such that 0.335 nm<d002<0.345 nm.
European patent EP-A-0 658 978 proposes using graphite having crystallite size of 2000 Å and with an ash content of less than 0.5%. Nevertheless, it is well known that graphites have particles of very large sizes, in excess of one micron or even ten microns, thus making them unsuitable for use in a storage cell of high energy density. To obtain good electrode conductivity, it would be necessary to add an unacceptable amount of conductor.
An object of the present invention is to propose a pasted electrode which does not suffer irreversible loss of capacity in storage, when compared with presently known electrodes.
The present invention provides a pasted nickel electrode for a storage cell having an alkaline electrolyte, the electrode comprising a current collector and an active mass based on nickel hydroxide in powder form together with a carbon-based conductor, the electrode being characterized by the fact that said carbon-based conductor is made up of particles of carbon that withstand electrochemical oxidation and that satisfy the following relationship:
W>0.025 in units of 10
9
g/m, where
W=TC002/S×G, where
TC002 is the [002] crystallite size on the X-ray diffraction pattern expressed in nanometers;
S is the specific surface area of the particles expressed in m
2
/g;
G is the graphitization coefficient of the carbon defined as follows:
G
=(
d
002−0.3354)/(0.3450−0.3354)
d002 is the lattice constant in the 002 direction in nanometers.
The X-ray diffraction pattern was made under the following conditions:
tube voltage: 40 kV
tube current: 30 mA
scanning speed: 0.9°/min
copper K&agr; line: 0.154 nm.
Preferably, said carbon particles constitute a fraction lying in the range 4% to 15% by weight of the active material of the nickel positive electrode. At higher values, the capacity of the electrode per unit volume decreases because of the high proportion of conductor in the electrode.
The term “carbon particles” covers particles of any shape, going from particles that are more or less spherical in shape to particles that are completely irregular, and including fibers.
When the particles are substantially spherical in shape, their mean diameter D
1
is less than or equal to D/20 where D is the mean diameter of the powder grains of said active material, with D advantageously lying in the range 5 &mgr;m to 15 &mgr;m.
D
1
is preferably less than or equal to D/100, and in particular less than or equal to 0.1 &mgr;m.
When the particles are in the form of fibers, their mean diameter D
2
is less than or equal to D and their mean length L
2
is greater than or equal to 25 times the value of D
2
.
L
2
is preferably greater than or equal to 75 times the value of D
2
.
Naturally, the term “nickel hydroxide” as used in the present application covers not only a nickel hydroxide or a hydroxide containing mainly nickel, but also at least one hydroxide syncrystallized with an element selected from cobalt, manganese, and at least one hydroxide syncrystallized from an element selected from cadmium, zinc, magnesium, calcium, yttrium, copper, and aluminum.
In an embodiment of the electrode of the invention, the current collector is a nickel foam, and said paste contains said active material, said conductors, a first binder based on polytetrafluoroethylene (PTFE), and a second binder selected from carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), polyvinylidene fluoride (PVDF), and styrene butadiene rubber (SBR).
The invention will be better understood and other advantages and features thereof will appear on reading the following examples given by way of non-limiting illustration.
REFERENCES:
patent: 4367208 (1983-01-01), Glasstetter et al.
patent: 0 587 974 (1994-03-01), None
patent: 0 634 804 (1995-01-01), None
Yunchang D et al: “A Study of the Performance of a Past-Type Nickel Cathode” Journal of Power Sources, vol. 56, No. 2, Aug. 1, 1995, pp. 201-204, XP000589879.
Chemical Abstracts, vol. 84, No. 22, May 31, 1976, Columbus, Ohio, US; abstract No. 153085 Mukunoki, Junji et al: “Cathode for alkaline batteries” XP002081989 & JP 50 092427 (Matsushita Electric Industrial Co., Ltd, Japan).
Database WPI Section Ch, Week 9515 Derwent Publications Ltd., London GB; AN 95-110329 XP 002081848 & JP 07 033420 (Toho Rayan KK), Feb. 3, 1995-& Patent Abstracts of
Audry Claudette
Bernard Patrick
Alcatel
Kalafut Stephen
Sughrue Mion Zinn Macpeak & Seas, PLLC
Wills M.
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