FR2796495A1 - Lead-acid battery electrode, comprises dispersion of conductive, chemically-inert ceramic powder - Google Patents

Abstract

The active mass is a mixture including a dispersion of conductive, chemically-inert ceramic powder.

Description

Lead acid batteries. The lead accumulator remains the most widely used electrochemical generator. Considerable progress has been made since its discovery by <i> Gaston Planté, </ I> but one of the major obstacles to even greater use remains its low specific energy. This is related to the specific mass characteristics of the active elements (lead, lead dioxide), but also to the practical mass capacities of the active ingredients which are between 35 and 50% of the theoretical value. This yield defect is particularly marked for positive electrodes. The efficiency is all the more affected as the accumulator operates at a high discharge rate.

To alleviate these problems, various solutions have been proposed, in particular the improvement of the network for collecting charges thanks to grids with complex structures, or the use of very thin active layers deposited on a collector.

Other solutions, alternative or complementary, have been proposed, such as the addition of conductive fillers. These charges must meet several criteria chemical and electrochemical stabilities, compatibility with the production process, oxygen and hydrogen evolution surge voltages high enough not to affect the charge of the positive and negative electrodes, low cost.

 Among the proposed additives are carbons, electronically conductive polymers and, more recently, conductive titanium suboxides in the form of fibers.

Increases in capacity of 10 to 20% of the positive electrode have been observed, but the stability of some of these materials remains problematic under operating conditions (very anodic potential during charging, and high acidity). The cost and manufacturing constraints of some also constitute a severe limitation to their use as lead acid batteries, which must remain an economic system.

 The influence of the addition of these additives to the active mass is not limited only to effects related to electronic conduction. The preferential appearance of the P-form of the lead dioxide, which is more active than the α-form but is less able to cycle, is also sometimes observed in the positive mass, in conjunction with their presence. In order to solve the difficulties related to the use of unstable additives (carbons, conductive polymers) or else difficult to obtain and expensive (under titanium oxides), the present invention seeks to highlight the interest that can to present, under specified conditions, the use of ceramic additives, resistant to oxidation in a concentrated sulfuric medium, and capable of providing various significant advantages. The objective of the present invention has several aspects to promote the progress of the so-called step of maturing electrodes, to improve the distribution of the electric field within the electrodes, in charge and discharge, in particular for the positive electrode.

To fulfill these functions, the materials preferably used according to the present invention, in a nonlimiting manner of said invention, are conducting ceramics.

These may in particular be chosen from borides, carbides, nitrides and silicides of various metals such as hafnium, magnesium, niobium, titanium and vanadium. It may thus advantageously be nitride and / or hafnium carbide, and / or carbide and / or nitride and / or magnesium silicide, and / or carbide and I or niobium nitride, and / or carbide and / or nitride. and / or titanium silicide, and 1 or vanadium nitride. It may also be nitrides, carbides, borides or double silicides of any two of the metals mentioned above; and carbonitrides of any two of the same metals.

These ceramics may be retained for use in the context of the present invention, insofar as they meet the essential characteristics already indicated # to be conductive, # to be chemically inert in the accumulator, # and to have a strong oxygen surge and / or hydrogen, depending on the polarity of the electrode.

For proven effectiveness in the various roles assigned to them, it should be added that the conductive powders used are fine and dispersed as homogeneously as possible in the active mass.

The choice of the size of the particles, and in particular the relative size of the conductive ceramic particles relative to that of the grains constituting the active material of the electrode is a parameter significantly affecting the effectiveness of the ceramic additive.

As indicated above, the contribution of the chemically inert conductive ceramic powders according to the invention is twofold.

It appears from the initial stage called "ripening" of the electrodes. After placing the active mass (positive or negative) on or within its support-collector, and before mounting the electrode in the accumulator, it should be allowed to ripen the electrode. The latter, which is characteristic of lead electrodes, consists in the gradual completion of reactions through the active mass, by the action of the sulfuric acid present, the oxygen of the air, the water contained in it. the course of these reactions being influenced by the temperature. On the one hand, ripening will allow the partial conversion of lead and lead oxide to sulphate and, on the other hand, help to ensure the cohesion of the active ingredient and the bonding of the latter with its support.

It has been shown that the addition of chemically inert conductive ceramic powders according to the invention makes it possible to improve the curing operation, by shortening the duration thereof, which is usually several days or even several weeks. The advantage obtained translates very directly in terms of manufacturing cost. This shortening of the ripening stage is obtained thanks to the catalytic effect caused by the conductive ceramic particles according to the invention.

It is therefore understood that it is advantageous to retain firstly a ceramic powder dispersion that is as homogeneous as possible in the active mass, on the other hand a fine particle size of the ceramic particles. This will preferably be chosen of the order of a few microns maximum and may be submicron. Finally, and also without limitation of the invention, it appears advantageous that the size of the ceramic particles is close to that of the particles of active material, and preferably less than or equal to.

A second type of favorable intervention ceramics according to the invention is obtained during the operation of the accumulator electrodes. It is here especially the character of electronic conductor of the ceramics used which is highlighted. The objective is to constitute within the active masses of electrodes a secondary collector network of electrical charges which makes it possible to optimize the distribution of the electric field. This secondary network must complete that which constitutes the support-electrode collector. It is particularly useful when the active materials do not exhibit good electrical conduction characteristics, either permanently or in only one of the two charged or discharged states. In the case of the lead accumulator with acid electrolyte, the problem is relatively weak for the negative electrode, whose active ingredient evolves between lead and lead sulfate. It is more critical for the positive electrode for which the active ingredient (PbS04 - Pb0z) does not have the same level of conductivity.

In all cases, it is important, in order to obtain an optimal yield of the active ingredients, to use for each electrode, charge collection modes that make it possible to achieve a distribution of the electric field which is the most homogeneous possible. The present invention is particularly attached to it, as will be shown in the following descriptions.

The positive and negative electrodes according to the invention, containing within their respective active masses the dispersion of inert conductive powder, can use the most diverse types of supports, such as in particular cast grids, expanded lead, ...

However, it appears particularly advantageous, through a complementary approach proposed in the context of the present invention, and proceeding from the same logic, to implement a support of a nature to effectively supplement the effect provided during operation of the accumulator, by the use of conductive powders dispersed in the active mass.

According to the invention, it is proposed to produce the positive and negative electrodes by pasting, coating, or filling by any means, in the liquid phase or in the form of a paste, with a high porosity metal or metallized three-dimensional support, with a paste containing in particular lead oxide and metal lead, the dispersion of conductive particles chemically inert in the medium, sulfuric acid and additives known to those skilled in the art.

Three-dimensional high porosity metal or metallized structures are here understood to mean large-area developed structures of foam, felt or woven type with a high level of open porosity, offering the appearance of a dense network of fibers or meshes. three-dimensional framework, defining a plurality of open spaces in communication with each other and with the outside of the structures. Foams are crosslinked alveolar structures of high porosity (greater than 80%, and up to about 98%) and open porosity by uncapping, in which the meshes of the network communicate with each other in all, or at least in significant proportions. .

The felts are random entanglements of non-woven fibers (most of them, however, positioned substantially in the plane of the formed web), defining between them inter-fiber spaces of varying shapes and sizes, communicating with one another. Their fibers may or may not be bonded with a binding agent.

Woven fabrics are structures formed by assembling interlaced textile yarns or fibers, either woven or meshed. They can be in the form of thick and complex structures, especially when they consist of two outer woven faces connected by knitting son which keep them spaced and interconnected at a time, as allow for example to achieve the looms Raschel type.

These various three-dimensional structures can - subject to adequacy of the relative dimensional characteristics of electrode thickness and size of the internal openings of the support -, constitute dense networks of electrical collection within the active mass. This filling the open interior space of their architectural organization, and therefore the entire internal porosity of the support, instead of being, more conventionally, deposited on the surface of a substantially plane support, it is possible to to increase the direct contact surface between the main collector and the active material, due to the increase of the developed surface of the support. This configuration also makes it possible, within the entire electrode, to reduce the maximum distance separating any particle of active material from the point closest to the mesh or fiber adjacent to the metal collector. In this way, an "intimate" contact is made between any point of the active mass and the main support or collector. This embodiment makes it possible to increase the efficiency of use of the active ingredient, and to multiply the recharging sites for the formation of active materials throughout the volume of the electrode.

It will be added that the use of a three-dimensional structure with a high porosity to constitute the main support and charge collector of the electrode makes it possible to optimize the retention capacity of the active ingredients, and thus also to participate by this characteristic in the increased life of the electrodes. The two approaches described in the context of the invention, namely use of a dispersion of conductive powders acting as a secondary collector within the active mass on the one hand, possible additional use of a three-dimensional support-collector to high porosity and large developed surface on the other hand, provide their full effect through their combined implementation. As has been said above, it is however possible to combine the dispersion of conductive powders with any type of electrode support.

The implementation of the invention will be better understood by means of the following nonlimiting descriptions of the invention.

For the production of a lead-acid accumulator of prismatic or cylindrical configuration, the preferred and non-limiting embodiment of the invention is used as support and main collector of negative electrode, cross-linked cellular foam or felt (non-woven). .

These structures may be made of any metal or alloy, especially electrochemically depositable, and preferably but not limited to the invention, lead, nickel or copper. It is necessary to coat nickel or copper with a protective layer of lead or lead alloy, making it possible to avoid direct contact between, for example, nickel or copper on the one hand and electrolyte on the other hand, in order to avoid the risk of corrosion of nickel or copper especially in contact with the electrolyte. The coating lead or lead alloy can be advantageously performed by electrolytic deposition.

In the case of the use of a foam-type collector, it is advantageously located among the standards 40 to 110 ppl (pores per inch), that is to say having between approximately 15 and 45 surface pores per centimeter. linear. The most open products are easier to use when filling the active mass structure; in contrast, the smaller pore products offer a denser collection network and therefore a greater surface area for direct contact with the active ingredient on the one hand, and a smaller maximum distance with the lead oxide particles. not benefiting from this direct contact, on the other hand, finally, a power of reinforced mechanical retention of the active mass.

In the case of the use of a metallic nonwoven as the main collector, it is advantageous to employ a product having a porosity greater than or equal to 95%, and having inter-fiber openings essentially between 50 and 300 microns.

The collectors used as supports for the negative electrode may consist of copper, nickel, or alloy of these metals, having densities of 150 to 650 grams per square meter of apparent surface, and preferably 200 to 450 g / m2; they can also be made of lead or lead alloy, having densities of 150 to 2000 g / m2, and preferably 400 to 900 g / m2. Their thickness is usefully between 0.9 and 5.0 millimeters, and preferably between 1.3 and 3.0 millimeters.

When the collectors are made of nickel, copper, or alloy of these metals, they are preferably coated with a protective layer of lead or lead alloy. The thickness selected for this advantageously corresponds to the minimum threshold necessary to obtain a total effective protective covering. Beyond this threshold, any extra thickness is a penalty in terms of mass capacity and incidentally volume.

The collectors used as support for the positive electrode are preferably made of pure lead or lead alloy, to avoid any risk of corrosion of the substrate. The density, in this case, will be increased from 1,000 to 3,000 g / m2, their thickness being usefully between 0.9 and 5.0 millimeters, and preferably between 1.3 and 3.0 millimeters.

The active mass is advantageously prepared in the form of a paste comprising the following elements: # lead powder and lead oxide <B>; # conductive particles forming the secondary collector <B>; </ B> # sulfuric acid; # specific additives for the positive or negative electrode.

After filling of the support-collector with the active mass, it is possible to proceed by rolling, compression or any other means, to a reduction of the thickness of the electrode, by first selecting, in a suitable way, the thickness of the support-collector, its density, and the amount of active mass introduced into the structure. This operation is particularly useful in the context of the production of cylindrical accumulators, the electrodes of which must be wound around a mandrel.

The conductive particles according to the invention may advantageously be chosen, in whole or in part, from conducting ceramics, and in particular consist of nitrides of titanium and / or hafnium, or of a double nitride of titanium-hafnium, which meet fully to the desired characteristics, in terms of conductivity, level of oxygen or hydrogen overvoltage, density, neutrality with respect to the electrochemical system, and cost. Titanium carbides and / or hafnium can also be used. For these unitary particles, it is advantageous to use a particle size substantially less than about 10 microns, and in particular submicron, and a weight concentration relative to lead oxide, advantageously between 1 and 25%, and preferably between 5 and 18%. . In most cases, lower concentrations are insufficient to obtain the desired effect; higher concentration levels do not usually bring much more significant benefit, while they penalize the electrode in terms of mass and volume capacities, and consequently the accumulator in terms of mass energies and volume.

It should be noted that, in a logical manner, for the same ability to cycle the electrode, the concentration of conductive particles must be even greater than the developed surface of the main collector is less. It is thus superior, for the same electrode thickness, with a collector of the type deployed for example, compared to that with a metal foam, and in the same way superior with a foam having 15 pores per linear centimeter. than with a 45 pore foam per linear centimeter.

Naturally, and as is moreover largely the result of the foregoing, the invention is not limited to the particular embodiments which have been described by way of examples.

The invention is not limited to the examples that have been given, but embraces all variants.

Claims (3)

<B> CLAIMS </ B> 1. lead accumulator electrode characterized in that the active mass comprises a conductive ceramic powder dispersion chemically inert in the medium. Lead-acid storage electrode according to claim 1, characterized in that the inert conductive ceramic is selected from single or double borides, carbides, nitrides and silicides of hafnium, magnesium, niobium, titanium and vanadium, or from carbonitrides of the same metals. 3. lead accumulator electrode according to claim 1, characterized in that the unit particles of inert conductive ceramic are essentially less than ten micrometers in size. <B> 4. </ B> Lead-acid accumulator electrode according to Claim 1, characterized in that the dispersion of the inert conductive ceramic powder is present within the active electrode mass in a proportion of 1 to 25% by weight relative to the electrochemically active material. 5. Lead-acid accumulator electrode according to claims 1 and 2, characterized in that the inert conductive ceramic is a double nitride of titanium-hafnium. 6. lead accumulator electrode according to claim 1, characterized in that it is carried out by introduction of the active mass containing the inert conductive ceramic powder dispersion, within a three-dimensional high-grade metal or metallized support-collector. porosity, cross-linked cellular foam, felt or woven. Lead - acid accumulator electrode according to claims 1 and 6, characterized in that the collector support is made of lead or lead alloy, copper, nickel, or copper - nickel alloy, and coated, if is not made of lead or lead alloy, a protective layer of lead or lead alloy.