WO1994029916A1

WO1994029916A1 - Integral electrode structures having filler additives

Info

Publication number: WO1994029916A1
Application number: PCT/US1994/006452
Authority: WO
Inventors: Robert S. Feldstein
Original assignee: Derafe, Ltd.
Priority date: 1993-06-10
Filing date: 1994-06-09
Publication date: 1994-12-22
Also published as: AU7057094A

Abstract

An integral battery electrode structure is provided, and methods for fabricating the same. The electrode structure is a metal suchas lead, and has a filler additive such as chopped titanium or chrome steel wire; and may be used as a battery or an electrolysis electrode, for example. The filler additive material must have a small physical size and be substantially stable at the melt temperature of the electrode metal; as well as having a substantially similar thermal co-efficient of expansion as the electrode metal, and with good chemical stability in the presence of the electrolyte with which the electrode will be utilized. Moreover, the filler additive material must not corrode in the presence of the electrode metal, and must not cause corrosion of the electrode metal. Preferably, the filler additive material has good thermal conductivity and good electrical conductivity, and is such that it should be at least partially wetted by the molten electrode metal when it is added to the molten metal. The titanium or chrome steel wire is chopped into short lengths and has a fine diameter. Preferably, the titanium or chrome steel wire has been crimped.

Description

INTEGRAL ELECTRODE STRUCTURES HAVING FILLER ADDITIVES

FIELD OF THE INVENTION:

This invention relates to electrodes. In particular, the invention relates to electrode structures having an integral structure, and which may be self-supported electrodes such as those used in lead/acid batteries. The invention may also be applicable to other electrochemical systems where self-supported electrodes are used, for example fuel cells, or as a supported electrolysis electrode.

BACKGROUND OF THE INVENTION: Self-supporting battery electrodes, and particularly plates in conventional lead/acid batteries, are known to have very limited strength and durability. The problems as to strength and durability come particularly from the lead component in both negative plates and positive plates. Lead/acid batteries are widely used for a great variety of purposes.

Lead/acid batteries are found as batteries for starting, lighting and ignition in vehicles of all kinds (SLI batteries); they may be used as traction batteries in industrial fork lift trucks, golf carts and the like; and lead/acid batteries are used as stand-by or float batteries in communications systems, for uninterruptable power supply (UPS) and inverter systems, and the like.

Lead/acid batteries may particularly be used, in the future, in electric vehicles.

In use, the structural lead in the lead/acid batteries becomes progressively chemically involved in the electrochemical processes that are carried on in the battery. Lead/acid batteries may continually be charged and discharged, they may operate in float or stand-by service conditions, they may move from high temperature to low temperature regions, or they may be subjected to variations of electric load or charging current. In any event, because of the progressive involvement of the structural lead in the battery plates, there will be a reduction in the structural strength of the plates of the cell, as well as an adverse effect on the current and heat conductivity characteristics of the plates. Of course, especially in mobile applications such as in vehicles, lead/acid batteries may be subjected to considerable shock and vibration.

Some of the problems of wear and tear, degradation of characteristics and structural stability and strength of battery plates, may be overcome by alloying the lead. This is a common step, and is effected such as by the addition of up to 4% antimony, up to 1 % calcium, and up to 1 % tin, by weight as an alloying agent with the lead. Several co-pending applications filed of even date herewith in the name of the inventor Feldstein, herein, and assigned to Derafe, Ltd. address questions of ion implantation, the use of doping agents, and the like, to increase the strength of battery plates for lead/acid batteries. The first of those applications is entitled METHODS FOR

FABRICATING BATTERY PLATES FOR LEAD/ACID BATTERIES, and contemplates the production of improved lead/acid battery plates having a thin layer of lead which has been surface treated. The surface treating may also include oxidation, so that it is not necessary to paste lead oxide into a grid as occurs in conventional positive plates for lead/acid batteries as they are found at the time of filing this application.

Two other applications are entitled CORED BATTERY PLATES FOR LEAD/ ACID BATTERIES and BIPOLAR LEAD/ ACID BATTERIES. The first of those other applications relates to the preparation of improved plates having a core of titanium or other metal, and where another metal is utilized it is plated or ion bombarded or neutral atom embedded with titanium before lead is bonded to the titanium. The latter of those other applications relates to bipolar lead/acid batteries, embodying the cored battery plates of the former application but having lead and lead oxide active surfaces on opposite sides of the bipolar plate.

Plates according to either of those inventions may incorporate the present invention, as will be evidence hereafter, by including a filler additive in the lead surface so as to provide for better strength and corrosion characteristics.

Yet a further application filed of even date herewith is entitled INTEGRAL BATTERY ELECTRODE STRUCTURE FOR LEAD/ ACID

BATTERIES. That application relates to improved lead/acid battery plates of the conventional type found at the time of filing of this application, but having another ion implanted or atomic embedded atomic species or more than one species for purposes of improved performance. Indeed, the teachings of that further application may also be applied to the remaining applications, and indeed to the present application, where it may be appropriate to include an atomic species in the electrode where the atomic species is one which is not generally ecologically acceptable but where its presence may be restricted to such low densities that the improved performance is more desirable than any potential undesirability surrounding that atomic species.

However, it may be desirable for there to be a further enhancement of the integral structure of the plate and to its strength characteristics, no matter whether the battery plate has been cast, or expanded, or extruded, or even if it is a cored plate structure as discussed in the co-pending applications mentioned above. Of course, additional mechanical enhancement of the plate structure is desirable whether or not the active material of the plate has been added to it such as by pasting, or has been formed from the plate such as by the formation of lead oxide in situ, as disclosed in the aforementioned co-pending applications.

The present invention finds greatest utility in respect of cast or molded battery plates, the form of battery plates most commonly used in present day lead/acid batteries. The present invention will find its utility even in respect of positive plates which are electrochemically formed, as is common in the industry, or otherwise. Still further, in its broader sense the present invention relates to electrode structures in general. Such electrode structures are usually found as battery plates, but they may be electrolysis electrodes, fuel cell electrodes, and the like. Moreover, while generally such electrodes as are discussed herein are manufactured by casting, extrusion, or other molding techniques, they may also be formed from sheet metals which are subsequently perforated or expanded.

OBJECTS OF THE INVENTION: It is a principal purpose of the present invention to provide an integral electrode structure which has enhanced mechanical properties, and which therefore leads to longer cyclic or stand-by service life, with increased capability of withstanding heat cycling, shock and vibration. A further object of the present invention is to provide methods whereby such electrodes as described immediately above may be fabricated.

In its simplest form, the present invention provides electrodes or plates by proposing the addition of a filler additive material to the metal from which the plate is cast or molded, while that metal is in its molten state, so as to form a reinforced composite. When the battery electrode or plate is fabricated, the structure that results is a composite matrix material having chemical, mechanical, electrical and thermal properties that have been modified and enhanced by the composite nature of the matrix due to the presence of the reinforcement or additive filler material.

Of course, care must be taken that the choice of additive or filler materials be wisely made, because it is possible that the wrong choice of additive filler materials could degrade the chemical, mechanical, electrical or thermal properties of the plate.

For example, if a polished fine powdered alumina (aluminum oxide) were added to molten lead, all of those characteristics noted above would be moderately to severely degraded. Alumina is not easily wetted by molten lead, therefore every interface of lead to alumina would provide a potential starting site for lead conversion chemistry in a lead/acid battery, when in use. Thus, for example, invasion or intrusion of sulphuric acid electrolyte into the lead oxide positive plate structure could result in chemical involvement of the lead adjacent to the alumina, creating what would be essentially a "conversion wedge" within the material. The structural integrity and the electrical characteristics of the plate would thereby degrade.

Similarly, the mechanical weakness of the lead/alumina interface within the plate structure could result in a structure which is essentially porous. Such structure has a reduced effective cross-section and would produce a number of fault initiation sites. This might be especially true in conditions where the plate is being flexed, or in vibration or shock conditions.

Still further, alumina has high electrical insulating properties, and therefore its use could result in a net reduction of electrical conductivity of the plate. On the other hand, alumina has good thermal conductivity, so that the thermal characteristics of the plate might not be significantly degraded.

It should also be mentioned that the choice of the filler additive material may depend, to some extent, on which of the critical parameters or characteristics of the plate is specifically to be enhanced, depending on its intended use. Likewise, there may be some acceptable degree of possible degradation of other physical characteristics. Such might be the circumstance if a polymer or ceramic additive which is electrically non- conductive is used, or a moderately conductive inorganic fibre such as boron filament or pyrolytic graphite, is used. In that case, however, either the filler additive material must be used in very modest quantities, or there must be an additional amount of the electrode material added to compensate for the otherwise degraded conductivity. The above problems surrounding the choice of additive material may be easily overcome provided that the filler additive material that is chosen meets several criteria: specifically, the filler additive material must be stable at the melt temperature of the electrode metal ~ say, for example, lead or lead/antimony alloy; it should have small physical size when employed as a filler additive; it should have substantially similar thermal co-efficient of expansion as the electrode metal; and it should have a good chemical stability in the presence of the electrolyte with which the self- supported battery electrode will be utilized. Moreover, the filler additive material must be such that it is not corrodible in the presence of the electrode material, nor should it be such that it will corrode the electrode metal. Preferably, the filler additive material has good thermal conductivity and good electrical conductivity; and moreover, when lead or a lead alloy is the electrode metal, the filler additive material should be such that it may be at least partially wetted by the lead. It should be noted that, in circumstances where significant masses of metals are being compared, and substantially equal masses of metal are being compared, then the thermal coefficient of expansion of lead and of titanium will be found to be different. However, as described hereafter, the use of crimped wire as the filler material, and the crimping of the wire, provide that the crimped wire will expand and contract in much the same way as an accordion, so that when it is present in small quantities and pieces having a small physical size, as described above, there will be an acceptable thermal match to the lead alloy matrix and thereby the necessary strain relief is provided and accommodated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Accordingly, the present invention provides an integral electrode structure having a filler additive. The integral electrode is of a metal which is suitable for electrode use either in its elemental or alloyed state, and generally the electrode is prepared in its integral structure by casting, extrusion, molding, perforating, expanding, etc. The filler additive is preferably present in the range of from about 0.1 % to about 2% by weight of the electrode; and comprises a material which has a small physical size and which is substantially stable at the melt temperature of the electrode metal. The filler additive material has a substantially similar thermal co¬ efficient of expansion as the electrode material of the integral, self- supported battery electrode, and has good chemical stability in the presence of a predetermined electrode with which the integral self-supported battery electrode will be utilized. The filler additive material is also such that it is not corrodible in the presence of the electrode material and that it will not corrode the electrode material. The present invention also provides a method of fabricating such an electrode, by utilizing the steps of:

(a) melting the electrode metal;

(b) admixing from about 0.1 % to about 2.0% by weight of the filler additive material to the molten electrode material; and

(c) casting, molding, extruding, or forming sheet material for further perforating or expanding, so as to form the integral electrode from the admixed molten electrode metal and filler additive material.

A particular embodiment of the present invention which is contemplated, is the addition of crimped and chopped titanium or chrome steel wire to the lead or lead with antimony and/or calcium and/or lead alloy which will be used as the electrode structure. As noted above, the additive material may be preferably present in an amount of from about

0.1% to about 2.0% by weight of the electrode, generally in the amount of about 0.5% to 1.5% by weight. An amount of 1 % is quite typical.

The amount of additive material is high enough that it enhances several characteristics as noted above, notably the chemical and mechanical properties of the electrode. At the same time, especially when chopped titanium or chrome steel wire is used, the amount of filler additive material is sufficiently low that the increase in costs using such material is tolerable, especially considering the enhanced characteristics that are achieved.

Indeed, the use of chopped and crimped titanium wire or chrome steel wire in lead or lead alloy battery plate structures will provide sites at which crack propagation within the electrode structure will stop, but no initiation sites at which a conversion wedge may start — for example, where lead oxide will be converted to lead sulphate in the presence of sulphuric acid electrolyte ~ because even slight titanium diffusion into the surrounding lead will reduce lead conversion significantly. Titanium wire or chrome steel wire is not particularly a ductile material. It may not be provided in diameters which are very thin ~ in the range of 0.005 inches, which is comparable to the scale of typical lead oxide crystallites in charged positive plates ~ but it may be up to about 0.015 inches. A typical wire is 0.01 inches in diameter. The wire may be chopped into lengths which range from about 0.01 inches up to about 0.10 inches. Typically, the wire is chopped into lengths of about 0.03 inches to about 0.07 inches, with an average length of about 0.05 inches. Indeed, that may be a typical length, in that the distribution of lengths within the range noted above is centered at 0.05 inches in a typical bell curve distribution.

Before being chopped, the titanium or chrome steel wire may be crimped such as by passing it through crimping rollers. The stiffness of the wire is such that it will hold a 30° set of the crimps with a repeat spacing of the crimps at about 2 or 3 wire diameters, without tearing. The wire is then chopped into "pseudo random" lengths as discussed above, using non-uniformly spaced cutters.

It should be noted that chopped titanium or chrome steel wire having dimensions within the ranges noted above is small enough in each instance so that each particle of filler additive material is small enough that it may be used in substantially every conventional plate making facility and apparatus. Usually, lead battery plates are cast; and all casting machines utilizing a melt having a filler additive as described herein can be used without substantial modification. The average length to diameter ratio of 5: 1 is such that there is reasonable utilization of the expensive lead titanium or chrome steel wire material, without incurring inordinate wire fabrication costs.

Moreover, because small diameter wire is used, the contribution of the wire filler additive material to the formation of pores at the surface of the cast battery plate or electrode is insubstantial. Such pores as may be formed are in the same general range as typical pore dimensions between crystals within the structure. Accordingly, there will be no aggressive conversion sites presented where lead may be unduly converted, such as to a lead sulphate.

Titanium has a high gas threshold overvoltage, and its use is generally preferable to the use of chrome steels which have a lower gas threshold. Therefore, particularly at end-of-charge conditions of a lead/acid battery, there should be no mechanical disruption of the local environment at the surface of lead plate structures if titanium wire is used as the additive material. It should also be noted that titanium is a hard refractory-like material, and has virtually no mobility at lead casting temperatures. However, lead will diffuse, at least to a slight extent, into the surface of the titanium under casting conditions. The lead then acts as a reducing agent, depassifying the titanium surface; whereby adsorbed oxygen is essentially dissolved, and will then diffuse out slowly over a few hours at low and rapidly declining concentrations. Such circumstances will occur at depths of only a few dozen atomic layers. Accordingly, a very shallow graded junction of lead and titanium is created. This promotes a greater wetability of the titanium wire when placed in the lead melt. Moreover, even low concentrations of lead within the shallow sub-surface regions of the titanium wire will inhibit passivation and thereby improve wetability. Corrosion of the plate will therefore be significantly reduced, due to the lowered conversion efficiency of lead/titanium mixed systems. Because titanium fibres and chrome steel fibres are quite strong mechanically, they provide a limitation to crack propagation. In general, that phenomenon would be expected to be limited to the extent that the fibres bond to the lead. However, the mechanical strength of the bond between the titanium or chrome steel fibres and the lead will be greatly enhanced by pre-crimping the fibres before they are admixed to the lead or lead alloy melt.

As noted above, without the use of filler additive materials as mechanical reinforcement within a lead plate structure, the lead or lead with antimony and/or calcium and/or lead alloy is generally too soft for many desired plate or cell geometries. Some improvement can be obtained by the proper alloy, but care must be taken that electrical and thermal characteristics are not unduly affected, or that the structure does not become too brittle. However, by introducing a composite filler additive such as crimped titanium or chrome steel wire, it will be noted that even though the ductility of the lead will slightly decline, the improved engagement between the titanium or chrome steel fibres and the matrix structure of the plate will actually improve crack resistance. The structure is therefore mechanically stronger.

Moreover, the electrical and thermal characteristics of the complex matrix are not significantly altered, so that the functioning operation of the improved electrode will be as expected. Still further, low concentrations of titanium or chrome steel will not materially affect the conductivity of the electrode structure. Therefore, the displacement volume of the integral electrode structure may be significantly reduced for the same mechanical integrity and strength of the structure, as compared to ordinary prior art cast electrode structures.

In keeping with the above, a lighter and more compact cell, which in general should be less expensive, can be constructed than has been possible using prior art structures. This is especially so in the circumstances where the corrosion life, which is a function of the charge and discharge cycle service or the float or standby service of a cell or battery are governing factors; or even where the mechanical ruggedness of the cell or battery is a critical parameter. Even where high peak power capabilities of the battery are critical, a benefit from the present invention is derived because thinner plate support structures are permitted, thereby permitting additional plates to be placed in a cell, and thereby providing a greater active surface area.

Even negative lead/acid battery plates may be advantageously produced according to the present invention; although since there is less likelihood of electrochemical action to occur at the surfaces of the negative plates, there may be somewhat less incentive to employ the present invention unless factors such as mechanical strength and ruggedness are also considered. In any event, just saving weight and space is an advantage; and particularly if the lead/acid battery is to be used in electric vehicle circumstances, savings of weight and space by using thinner and stronger grids will be important.

It follows, of course, that the present invention is best utilized in circumstances where relatively thin plates are required, such as where low cost cells and batteries are required, and generally where moderate power delivery characteristics are sufficient. On the other hand, in all applications, even those that have the most demanding performance requirements, there can be found a significant contribution if all factors of economy, weight, and space, are taken into account. It is also to be noted that the present invention also finds utility in the preparation of other electrode structures. For example, fuel cell electrodes may be advantageously manufactured in keeping with the teachings herein, and be lighter in weight, stronger, and perhaps even less expensive. Such supported electrode structures as electrolysis electrodes, which ideally are quite thin so as to function efficiently, may be made according to this invention. Moreover, the metal of the electrode, and the additive material, may be other than lead or lead with antimony and/or calcium and/or lead alloy, and titanium or chrome steel wire. It is only important that the electrode metal and the additive materials meet the criteria outlined above as to the physical size and the temperature stability of the additive material; its thermal co-efficient of expansion, its chemical stability in the presence of the intended electrolyte to which the electrode will be exposed, and the fact that it will not corrode in the presence of the electrode metal nor cause corrosion of the electrode metal. Teachings of the co-pending applications noted above may also be incorporated into plates that are fabricated in keeping with the present invention. For example, cast plates might be provided in keeping with this invention which are then subjected to ion bombardment of oxygen so as to produce positive plates, or surface embedment modifications, doping, or sintered negative plate surface fabrication, are employed.

The scope of the present invention is defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An integral electrode structure having a filler additive, wherein said electrode structure is made of electrode metal which is suitable for electrode use in a predetermined circumstance and may function as an electrode in such predetermined circumstance in the elemental state of said metal or in an alloyed state of said metal, and wherein said integral electrode structure is a reinforced composite structure; wherein said filler additive is present in said integral electrode structure in an amount of about 0.1% to about 2.0% by weight of said electrode; wherein said filler additive is a material having small physical size and which is substantially stable at the melt temperature of said electrode metal, and has a substantially similar thermal coefficient of expansion as the electrode material of said integral electrode, and has good chemical stability in the presence of a predetermined electrolyte with which said electrode will be utilized in said predetermined circumstance; and wherein said filler additive material is such that it is not corrodible in the presence of said electrode metal and will not corrode said electrode metal.

2. The integral electrode structure of claim 1, wherein said filler additive material has good thermal conductivity and good electrical conductivity.

3. The integral electrode structure of claim 2, wherein said filler additive material is such that it may be at least partially wetted by said electrode metal when admixed to molten electrode metal.

4. The integral electrode structure of claim 3, wherein said electrode metal is primarily lead, and said filler additive material is metallic titanium.

5. The integral electrode structure of claim 3, wherein said electrode is a self-supported battery electrode.

6. The integral electrode structure of claim 4, wherein said electrode metal is lead having from 0% to 4% of antimony, 0% to 2% of calcium, and 0% to 1 % of tin, alloyed therewith, and said filler additive material is titanium wire or chrome steel wire which is present in an amount of about 0.5% to 1.5% by weight.

7. The integral electrode structure of claim 6, wherein said titanium wire or chrome steel wire is finely chopped into lengths of about

0.01 inches to about 0.10 inches.

8. The integral electrode structure of claim 7, wherein said titanium wire or chrome steel wire is finely chopped into lengths of about 0.03 inches to about 0.07 inches, and has a diameter of about 0.01 inches.

9. The integral electrode structure of claim 8, wherein said titanium wire or chrome steel wire is finely chopped into lengths of about 0.01 inches to about 0.10 inches, having an average of about 0.05 inches; and is present in an amount of about 1.0% by weight.

10. The integral electrode structure of claim 8, wherein said titanium wire or chrome steel wire has been crimped; and is present in an amount of about 1.0% by weight.

11. The integral electrode structure of claim 10, wherein said crimped titanium wire or chrome steel wire has crimps of about 30° spaced at spacings of about two or three wire diameters.

12. The integral electrode structure of claim 3, wherein said electrode is an integral electrode structure that has been cast, extruded, molded, perforated from sheet material or expanded from sheet material.

13. A method of fabricating an integral electrode structure having a filler additive, wherein said electrode structure is made of electrode metal which is suitable for electrode use in a predetermined circumstance and may function as an electrode in such predetermined circumstance in the elemental state of said metal or in an alloyed state of said metal, and wherein said integral electrode structure is a reinforced composite structure; wherein said filler additive is a material is a material having small physical size and which is substantially stable at the melt temperature of said electrode metal, and has a substantially similar thermal coefficient of expansion as the electrode material of said integral electrode, and has good chemical stability in the presence of a predetermined electrolyte with which said integral electrode will be utilized in said predetermined circumstance; and wherein said filler additive material is such that is such that it is not corrodible in the presence of said electrode metal and will not corrode said electrode metal; said method comprising the steps of:

(a) melting said electrode metal;

(b) admixing from about 0.1 % to about 2% by weight of said filler additive metal to the molten electrode material; and (c) casting, molding, extruding, or forming sheet material for further perforating or expanding, so as to form said integral electrode structure from said admixed molten electrode metal and filler additive material.

14. The method of claim 13, wherein said filler additive material has good thermal conductivity and good electrical conductivity.

15. The method of claim 14, wherein said filler additive material is such that it may be at least partially wetted by said electrode metal when admixed to molten electrode metal.

16. The method of claim 15, wherein said electrode metal is primarily lead, and said filler additive material is metallic titanium.

17. The method of claim 16, wherein said electrode is intended for use as a self-supported battery electrode.

18. The method of claim 17, wherein said electrode metal is lead having from 0% to 4% of antimony, 0% to 2% of calcium, and 0% to 1% of tin, alloyed therewith, and said filler additive material is titanium wire or chrome steel wire which is present in an amount of about 0.5% to 1.5% by weight.

19. The method of claim 18, wherein said titanium wire or chrome steel wire is finely chopped into lengths of about 0.03 inches to about 0.07 inches, and has a diameter of about 0.01 inches.

20. The method of claim 19, wherein said titanium wire or chrome steel wire is finely chopped into lengths of about 0.01 inches to about 0.10 inches, having an average of about 0.05 inches; and is present in an amount of about 1.0% by weight.

21. The method of claim 20, wherein said titanium wire or chrome steel wire has been crimped; and is present in an amount of about 1.0% by weight.

22. The method of claim 21, wherein said crimped titanium wire or chrome steel wire has crimps of about 30° spaced at spacings of about two or three wire diameters.