Non-sintered nickel electrode

Abstract

A non-sintered electrode includes a metal two-dimensional conductive support and a paste comprising an electrochemically active material containing nickel hydroxide and a binder which is a mixture of an elastomer consisting of a butadiene polymer and an ethylene/vinyl acetate copolymer.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a non-sintered nickel electrode, in particular a positive electrode for an alkaline electrolyte secondary storage cell.

[0004] It relates more precisely to a non-sintered electrode including a metal two-dimensional conductive support and a paste comprising an electrochemically active material containing nickel hydroxide and a binder which is a mixture of an elastomer comprising a butadiene polymer and a second polymer.

[0005] 2. Description of the Prior Art

[0006] There are several types of electrodes, including sintered electrodes and non-sintered electrodes, also referred to as paste electrodes or plasticized electrodes.

[0007] Non-sintered electrodes are the most widely used at present. Compared to other electrodes, a non-sintered electrode contains a greater quantity of active material so that its capacity per unit volume is increased and its fabrication cost is reduced.

[0008] A non-sintered electrode comprises 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. This is conventionally achieved by depositing the paste in a porous three-dimensional conductive support such as metal or carbon felt or foam.

[0009] For cost reasons, there is now a trend to using two-dimensional supports.

[0010] The document JP-3 165 469 proposes a nickel electrode comprising a two-dimensional porous conductive support, such as a grid, an expanded metal or a perforated metal, covered with a paste including nickel hydroxide, a conductive material and a thermoplastic binder, such as a butylene/ethylene/styrene copolymer To fix the active material to the support, a separator is hot-pressed onto each face of the electrode.

[0011] The document EP-0 750 358 describes a non-sintered nickel electrode whose support is a corrugated metal plate on which asperities are formed to attach a layer which is rough on a microscopic scale and is made up of powdered nickel and/or cobalt bound with butadiene-polyvinyl alcohol PVAI. Onto this layer is deposited a paste comprising carboxymethylcellulose CMC and a styrene/butadiene copolymer SBR.

[0012] Research has been conducted into improving the mechanical and chemical durability over the prior art, in particular with regard to electrochemical oxidation.

[0013] U.S. Pat. No. 6,335,120 proposes a non-sintered nickel electrode with a two-dimensional collector whose binder is a mixture of an elastomer and a crystalline polymer. The elastomer is chosen from a styrene/ethylene-butylene/styrene copolymer SEBS, a styrene/butadiene/vinylpyridine terpolymer SBVR, and a styrene/butadiene copolymer SBR, possibly carboxylated. The crystalline polymer is chosen from a polyolefin such as polyethylene PE and a fluoropolymer such as a fluorocopolymer of ethylene and propylene, polytetrafluoroethylene PTFE and hexafluoropropylene HFP.

[0014] It has been found that this kind of fluoropolymer binder does not achieve sufficient electrode adhesion and cohesion and leads to a loss of active material.

[0015] Moreover, a binder of this kind has a film-forming behavior leading to irregularities relating to the presence of large quantities of a crystalline polymer with a high melting point.

SUMMARY OF THE INVENTION

[0016] The invention solves this problem, and to achieve this, in accordance with the invention, the binder is a mixture of a butadiene copolymer and an ethylene/vinyl acetate copolymer EVA.

[0017] This composition of the binder ensures improved mechanical properties of the electrode, in particular correct elasticity, intergranular cohesion and adhesion to the conductive support.

[0018] The EVA is preferably an aqueous dispersion of an ethylene/vinyl acetate copolymer. Polyvinyl alcohol is preferably added to the aqueous solution to stabilize it. The proportion of vinyl acetate in the copolymer is preferably greater than 70 wt %.

[0019] In the preferred embodiment, the total amount of binder is from 1 wt % to 3 wt %.

[0020] The proportion of butadiene copolymer in the binder is advantageously from 1 0 wt % to 60 wt % and the proportion of ethylene/vinyl acetate copolymer in the binder is advantageously from 40 wt % to 90 wt %.

[0021] The butadiene copolymer is preferably a carboxylated styrene/butadiene copolymer SBR.

[0022] It is to be understood that the expression “electrochemically active material containing nickel hydroxide” used in this application can refer not only to a nickel hydroxide or a hydroxide containing mainly nickel but also to a nickel hydroxide containing at least one syncrystallized hydroxide of a particular element. A syncrystallized hydroxide contained in the nickel hydroxide is a hydroxide forming a solid solution with the nickel hydroxide, i.e. occupying, in continuously variable proportions, the atomic sites defined by the crystal lattice of the nickel hydroxide.

[0023] In this sense, in accordance with the invention, the electrochemically active material containing the nickel hydroxide preferably contains an element chosen from zinc, cadmium and magnesium.

[0024] The electrochemically active material containing nickel hydroxide advantageous also contains an element chosen from cobalt, manganese, aluminum, yttrium, calcium, strontium, zirconium, copper, lithium, and sodium.

[0025] The nickel hydroxide is preferably spheroidal and has a range of particle sizes from 7 microns to 20 microns.

[0026] In the preferred embodiment, the nickel hydroxide is covered with a coating based on cobalt hydroxide, possibly partly oxidized. The coating can further contain elements chosen from nickel, zinc, aluminum and/or magnesium.

[0027] The paste can further contain a powdered yttrium compound, preferably an yttrium oxide such as Y2O3 or an yttrium hydroxide such as Y(OH)3.

[0028] Nickel hydroxide is a poor conductor and this necessitates the addition of a conductive material to allow good electrical percolation.

[0029] The paste advantageously further contains a conductive substance consisting essentially of a cobalt compound, preferably a cobalt oxide such as CoO, metallic cobalt Co or a cobalt hydroxide such as Co(OH)2.

[0030] The paste can equally further contain a powder chosen from zinc oxide and zinc hydroxide.

[0031] The expression “two-dimensional support” means a plane support on which a layer of paste is deposited. The properties of the binder are therefore essential for retaining the layer to the support, especially if the finished electrode is rolled up.

[0032] The two-dimensional support can be a solid or perforated tape, an expanded metal, a grid or a woven material. For example, it is an expanded nickel support or a nickel-plated steel tape with a thickness from 20 microns to 100 microns.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0033] One embodiment of an electrode according to the invention is described hereinafter.

[0034] An AA size nickel-metal hydride (Ni-MH) sealed secondary storage cell whose nominal capacity Cn is 1 200 mAh was manufactured in the following manner.

[0035] The positive electrode used a paste having the following composition (expressed in wt % with respect to the weight of the paste):

electrochemically active material 92.7%
CoO conductive material          8%
EVA binder                       1.5%
SBR binder                       0.5%
thickener                        0.3%

[0036] The powdered electrochemically active material was a nickel-based hydroxide. The thickener was the sodium salt of hydroxypropylmethylcellulose (HPMC). The viscosity of the paste was adjusted with water. The paste was deposited simultaneously on both sides of a two-dimensional metal support (perforated nickel-plate steel 70 microns thick) in a homogeneous manner. The resulting assembly was then dried for 30 minutes at 80° C. to eliminate water and then rolled to the required thickness for the electrode.

[0037] A comparative efficiency test is described hereinafter. The test compared the specific embodiment of the invention described above (cell B), a prior art electrode with a three-dimensional collector and a binder consisting of PTFE (cell A), an electrode with a binder consisting of EVA and SBR in which the proportion of binder is less than 1 wt % (cell C), and an electrode with a binder comprising EVA and SBR in which the proportion of binder is greater than 3 wt % (cell D).

[0038] The first reference positive electrode (cell A) was made using a paste having the following composition (expressed in wt % with respect to the weight of the paste):

electrochemically active material 92.7%
CoO conductive material          8%
PTFE binder                      1%
thickener                        0.3%

[0039] The powdered electrochemically active material was a nickel-based hydroxide. The binder was polytetrafluoroethylene (PTFE). The thickener was the sodium salt of hydroxypropylmethylcellulose (HPMC). The viscosity of the paste was adjusted with water. The paste was introduced into a conductive support serving as a current collector in the form of a nickel foam with a porosity of approximately 95%. When the paste had been introduced into the support, the resulting combination was dried to eliminate water and then rolled to obtain the thickness required for the electrode.

[0040] The second positive electrode (cell C) was made with a paste having the following composition (expressed in wt % with respect to the weight of the paste):

electrochemically reactive material 92.7%
CoO conductive material          8%
EVA binder                       0.5%
SBR binder                       0.2%
thickener                        0.3%

[0041] The powdered electrochemically active material was a nickel-based hydroxide. The thickener was the sodium salt of hydroxypropylmethylcellulose (HPMC). The viscosity of the paste was adjusted with water. The paste was deposited simultaneously on both sides of a two-dimensional metal support (perforated nickel-plated steel 70 microns thick) in a homogeneous manner. The resulting assembly was then dried for 30 minutes at 80° C. to eliminate the water and then rolled to the thickness required for the electrode.

[0042] The third positive electrode (cell D) was made with a paste having the following composition (expressed in wt % with respect to the weight of the paste):

electrochemically active material 92.7%
CoO conductive material          8%
EVA binder                       3%
SBR binder                       1%
thickener                        0.3%

[0043] The powdered electrochemically active material was a nickel-based hydroxide. The thickener was the sodium salt of hydroxypropylmethylcellulose (HPMC). The viscosity of the paste was adjusted with water. The paste was deposited simultaneously on both sides of a two-dimensional metal support (perforated nickel-plated steel 70 microns thick) in a homogeneous manner. The resulting assembly was then dried for 30 minutes at 80° C. to eliminate water and then rolled to the thickness required for the electrode.

[0044] The prior art negative electrode used for the electrochemically active material includes an intermetallic compound capable of forming a hydride when charged. Its capacity was higher than that of the positive electrode. Each positive electrode was placed back-to-back with a negative electrode from which it was isolated by a non-woven polypropylene separator to form the electrode assembly. The coiled electrode assembly was inserted into a metal container and impregnated with an alkaline electrolyte in the form of an aqueous alkaline solution comprising a mixture of 7.5N potassium hydroxide KOH, 0.5N lithium hydroxide LiOH, and 0.4N sodium hydroxide NaOH to form the cells A, B, C, D.

[0045] After resting for 48 hours, the storage cells were electrically formed under the following conditions:

[0046] Rest for 2 hours at 85° C.

[0047] Cycle 1:

[0048] Charge for 8 hours at 0.1 Ic and 85° C., where Ic is the current necessary to discharge the nominal capacity Cn of the cell in 1 hour,

[0049] Rest for 4 hours at 20° C.,

[0050] Charge for 3 hours at 0.33 Ic,

[0051] Discharge for 1 hour at 0.66 Ic,

[0052] Charge for 1 hour at Ic and 1 hour 1 2 minutes at 0.5 Ic,

[0053] Discharge at 0.2 Ic to a stopping voltage of 0.9 volt;

[0054] Cycles 2 to 10

[0055] Charge for 16 hours at 0.1 Ic and 20° C.,

[0056] Discharge at 0.2 Ic to a stopping voltage of 0.9 volt.

[0057] The table below sets out the electrical performance of the various cells:

TABLE
Cell:	A	B	C	D
Electrochemical efficiency of the	255	250	235	200
positive electrodes during cycle 10
(mAh/g)

[0058] The above results show that the cell B according to the invention had performance equivalent to the reference cell A. When the total amount of binder was 0.7 wt % (cell C), the electrical performance was worse, probably because of imperfect mechanical strength of the electrode. When the total amount of binder was 4 wt % (cell D), the electrical performance was worse, probably because of a high level of covering of the active material particles.

[0059] The electrode of cell B according to the invention thus had electrical performance comparable to that of a three-dimensional collector PTFE electrode, combined with the cost advantages of a two-dimensional collector and improved cohesion and adhesion compared to fluoropolymer binder electrodes.

Claims

1. A non-sintered electrode including a metal two-dimensional conductive support and a paste comprising an electrochemically active material containing nickel hydroxide and a binder which is a mixture of an elastomer consisting of a butadiene polymer and an ethylene/vinyl acetate copolymer.

2. The electrode claimed in claim 1 wherein the total proportion of binder is from 1 wt % to 3 wt %.

3. The electrode claimed in claim 1 wherein the proportion of butadiene copolymer in said binder is from 10 wt % to 60 wt % and the proportion of ethylene/vinyl acetate copolymer in said binder is from 40 wt % to 90 wt %.

4. The electrode claimed in claim 1 wherein said butadiene copolymer is a carboxylated styrene/butadiene copolymer.

5. The electrode claimed in claim 1 wherein said electrochemically active material containing nickel hydroxide contains an element chosen from zinc, cadmium and magnesium.

6. The electrode claimed in claim 1 wherein said electrochemically active material containing nickel hydroxide contains an element chosen from cobalt, manganese, aluminum, yttrium, calcium, strontium, zirconium, copper, lithium and sodium.

7. The electrode claimed in claim 1 wherein said nickel hydroxide is spheroidal and has a particle size range from 7 microns to 20 microns.

8. The electrode claimed in claim 1 wherein said paste further contains a powdered yttrium compound.

9. The electrode claimed in claim 8 wherein said powder is yttrium oxide or yttrium hydroxide.

10. The electrode claimed in claim 1 wherein said paste further contains a conductive material consisting essentially of a cobalt compound.

11. The electrode claimed in claim 10 wherein said compound is chosen from cobalt oxide, metallic cobalt and cobalt hydroxide.

12. The electrode claimed in claim 1 further including a powder chosen from zinc oxide and zinc hydroxide.

13. An alkaline electrolyte secondary storage cell including an electrode as claimed in any preceding claim.