JPH0640490B2 - Plasticized cadmium electrode - Google Patents

Description

FIELD OF THE INVENTION The present invention relates to electrodes for electrochemical generators, and more particularly to cadmium oxide electrodes for storage batteries.

Description of the Prior Art Various and numerous routes have already been pioneered over the years for new electrodes leading to higher performance. In this field, research is mainly directed towards reducing the weight of collectors, enhancing retention of active substances, and increasing their electrochemical effect by establishing better charge accumulation. The use of porous collectors is substantially compared to the prior art by the use of a sintered metal structure first, and then recently by the use of a fibrous or reticulated metallization structure with open pores which can reach up to 90% or more. Progress has been made.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cadmium electrode in which the collector is a highly porous metal structure.

Preferably, the metallic substrate used for the purpose of forming both the charge collector and the matrix for holding the active substance forms a parallel array of cellular spaces and has a cellular structure with an open cell porosity of more than 95%. It has a reticulated structure.

This "metal foam" is formed by a surface layer of at least nickel. Since this structure is usually obtained by a metal deposit formed on a polyether or polyester foam, the thickness of said deposit is preferably of the order of 5 to 10 μm. The average pore diameter is preferably in the range 0.3 mm to 1 mm, and especially around 0.7 to 0.8 mm. The thickness of the substrate is chosen as a function of the intended use of the manufactured cadmium electrode. In particular, the thickness depends on whether it is spirally wound or mounted on a small or large sized accumulator. The thickness can also be determined in consideration of operating conditions and desired electrochemical performance. In most cases, the thickness of the substrate will be in the range 0.5 to 3 mm, preferably 1 to 2 mm.

The 2 mm thick collector defined above has an apparent surface area of about 50 mg / cm ² . The actual surface area to apparent surface area ratio is around 5.

The active substance of the electrodes, namely cadmium oxide, is introduced in the form of a paste into the collector, which also has the function of forming a holding matrix.

This paste is obtained by mixing the active substance with a plasticizer, to which a thickener may optionally be added.

According to the invention, it is proposed to add to the cadmium oxide a plasticizer, preferably consisting of Teflon (polytetrafluoroethylene). This plasticizer is preferably introduced in the form of an aqueous suspension having a concentration of about 60%. The ratio of Teflon to cadmium oxide ranges from 2 to 4% by weight, with 3% being generally employed. An alcohol diluent is then added to the mixture, which can be, for example, denatured ethanol. When making the paste, the ratio of alcohol to cadmium oxide is about 80 ml alcohol to 100 g CdO.
Is. The presence of a plasticizer that imparts a so-called plasticized electrode character to the finished electrode has the effect of creating a fibrous texture within the active material that imparts enhanced mechanical strength properties. This binder has the effect of reducing the brittleness resulting from the brittleness of the active substance, so that the latter gains some flexibility.

Finally, it is useful to introduce a thickener during the preparation of the paste so that the active substance can be easily loaded into the reticulated matrix. The function of this thickener is to obtain a smooth stickiness that satisfies the conditions of the pasting operation. The thickener considered is preferably methylcellulose, which is advantageously about 1% by weight with respect to cadmium oxide.
Can be incorporated into the active substance.

Pasting of the reticulated metal structure is then performed manually or by machine. Depending on the thickness of the support and the size of its pores, the pasting operation is preferably performed once or twice.
It will be carried out once. In the case of two paste forming operations, it is advantageous to introduce the paste from one surface at the first time and introduce the paste from the opposite side at the second time.

To ensure optimal formation of the electrodes according to the invention,
It is useful to perform additional work after pasting.

For this reason, it is advantageous to subject the electrodes to low-pressure calendering, for example by means of rubber-coated rollers, immediately after the pasting operation.

Drying is then preferred in order to remove the water and alcohol contained in the active substance. Care should be taken not to perform drying too rapidly, as rapid drying will cause cracks. This requirement requires that the drying step be carried out at low temperatures, preferably not higher than 100 ° C.

In the next step of the invention, it is particularly advantageous to carry out an operation which consists in doping the electrodes with nickel. This nickel doping operation is carried out by immersing the electrode in a nickel nitrate solution, and the optimum concentration is about 50 g / n. Immersion time is preferably 1
It is of the order of minutes, during which the impregnation of the electrodes takes place.

The purpose of the doping operation is to provide the cadmium electrode formed according to the present invention with electrochemical stability during cycling. As a practical matter, if this doping operation is not performed, an electrode voltage drop will be observed after several tens of cycles and the electrode capacitance will decrease gradually. When the electrode is doped with nickel, it can withstand thousands of charge / discharge cycles without loss of its capacity.

After this manufacturing step, it is useful to carry out a second drying under conditions similar to those already described for the first drying operation.

The final beneficial step according to the invention consists in subjecting the electrodes to a compacting operation. There are two basic reasons for this work. For one, by allowing a reduction in electrode thickness, one allows for an increase in ohmic capacity with simple mechanical work. In addition, the compactness has the effect of integrating the electrodes, so that high mechanical strength is obtained. This compacting operation is preferably carried out at a pressure of about 200 kg / cm ^{2 of} apparent surface area, so that the thickness of the electrode can be halved.

A cadmium electrode fabricated according to the present invention and subjected to a fabrication method including the process steps defined above has a resistivity of about 700 Ah.
It provides a volume capacity of / and a mass capacity of about 250 Ah / kg. This type of electrode has a C /
After 1000 cycles of charge / discharge in 2, no loss of capacity is shown.

As will be clear from the above, the present invention is not limited to the particular embodiments described above, but includes all alternatives.

Claims (6)

[Claims] 1. An active substance paste is prepared by mixing cadmium oxide as an active substance and 2 to 4% by weight of polytetrafluoroethylene of cadmium oxide as a plasticizer to prepare an active substance paste. The active material is filled over the entire thickness of the charge collector with a reticulated metal structure, the electrodes thus obtained are calendered, the electrodes are dried and then doped by dipping and impregnating with a nickel nitrate solution, and Next, a method for producing a plasticized cadmium electrode, characterized in that the electrode is dried and finally made compact to reduce its thickness. 2. The method according to claim 1, wherein the active substance paste is prepared by kneading cadmium oxide powder and polytetrafluoroethylene with alcohol. 3. The method of claim 2 wherein the alcohol is ethanol and about 8 ml is used per 100 g of cadmium oxide powder. 4. The method of claim 1 wherein said active agent pace comprises about 1% by weight of cadmium oxide methylcellulose as a thickening agent. 5. The method of claim 1 wherein the doping step of dipping and impregnating with the nickel nitrate solution is continued for about 1 minute. 6. The charge collector having a reticulated metal structure has an average pore size of 0.7 to 0.8 mm and a thickness of about 0.
The method of claim 1 wherein at least one nickel layer having a thickness of 5 to 3 mm is formed.