|Title of Invention||
"AN ELECTRODE COMPRISING A SHAPED BODY WHICH IS FORMED OF HARDENED THERMOSET RESIN AND A METHOD THEREOF"
|Abstract||An electrode comprising a shaped body which is formed of hardened thermoset resin, the body having electrical paths defined by contacting conductive particles wherein (i) the conductive particles are titanium suboxides of the formula Tin02n-1 where n is 4 or greater, (ii) the particles have a size distribution with a standard deviation of less than about 50% of the mean particle size, (iii) the body has a density of 1.8 g/cc or above and (iv) the electrode is sufficiently pore-free such that the electrode has a current leakage of less than 1 A/m2.|
|Full Text||The present invention relates to an electrode comprising a shaped body which is formed of hardened thermoset resin and a method thereof.
The invention relates to electrodes for use in a battery, typically a bipolar lead-acid battery.
It is known to make bipolar plate electrodes for this purpose from lead and lead alloys. Ideally the electrodes are very thin to reduce the size, and weight of the battery but thin sheets of lead metal and lead alloys are difficult to seal around the edges. A reliable seal is required in bipolar batteries to prevent conductive paths of electrolyte being formed from one side of the bipolar plate to the other, which would cause self discharge of the battery. The plate electrodes are not entirely resistant to galvanic corrosion which generally results in through-plate porosity in the form of pinholes (and the electrodes are heavy if manufactured in greater thickness to overcome this problem). Proposals to reduce the effective weight of the lead include the use of porous ceramics with lead infiltrated into the pores (which need to be of fairly thick section to be mechanically robust, and are thereby still rather heavy); and the use of glass fibres and flakes coated with lead, lead alloy, or doped tin oxide, or lead oxides as conductive' particulate in a thermoplastic resin matrix but such electrodes are complex and expensive to produce. Carbon based materials have been tried, but most forms are susceptible to electrochemical oxidation.
Plates made exclusively of the Magneli phase suboxides of titanium (of the general formula Tin02n-1(where n is an integer greater than 4 or greater) satisfy many of the criteria above. However, they are expensive to make, are brittle, and do not easily accept surface features, for example to accept and retain the battery paste coating.
Tnis invention is based on the reaiisation that if the plates can be made from the Magneli titanium suboxide material in particuiate form in a suitable poiymeric matrix, most, if not all, of these weaknesses can be overcome.
According to the invention in one aspect there is provided an electrode cornprising a shaped substantially pore-free body of hardened resin, the body having electrica! paths defined by contacting particles of titanium suboxide of the formula Tln02n^ where n is 4 or greater.
The particuiate titanium suboxide is preferabiy selected to provide a high level of conductivity; TÎ407 and Ti509 are preferred. Some suboxides have low conductivity and poor corrosion resistance and preferabiy are avoided; an example is Ti305. Although the particles can be provided as a mixture of the Magneli phases it is important that the presence of lower oxides such as TiO, Ti203, Ti305 is minimised and preferabiy entirely avoided.
It is a preferred feature of the invention that the partide size distribution is selected so that the particles will contact each other intimately to create electrica) paths and provide conductivity. Preferabiy the partide size distribution is relatively narrow since this gives good electrical connectivity. Preferabiy the particles have a partide size distribution with a standard deviation of less than about 50% of the mean partide size. Polymodal mixtures can also be used but care must be taken to ensure that the populations of smailer particles do not reduce the electrical connectivity of the populations of larger particles.
We have found that specific partide sizes and partide size distributions are requfred for making eiectrodes of a specific thickness but a mean partide size (by volume) of around 100
to 150 micrometers îs suiiable for an eiectrode of 1 to 2 mm thickness. For making thinner electrodes which may be preferred, smaller particies are required of the plate is to be pore free. However, if the average partide size is small it is more drfficult to achieve a suitably narrow partide size disîribution to give a good conductivity.
The powder îs manufactured by methods such as are taught in US-A-5173215. The manufacturing conditions are adjusted to ensure that the powder has a high proportion of the Ti4O7 and Ti5Og crystallography (to produce high conductivity) and effectively none of the non-Magneli Ti3O5 material (which causes poor corrosion resistance and low conductivity), The precursor TiO2 powder is chosen or treated to produce a Magneli phase suboxide powder with partide size distribution required for good conductivity.
The resin may be selected from a wide variety of materials. Preferred are thermoset resins. One suitabie resin to manufacture a corrosion resistant plate is an uncured epoxy such as Araldite® PY307-1, in conjunction with HY3203® hardener, both materials being avaiiable from Vantico Ltd. This has been found to be particulariy resistant to anodic corrosion and to make a pore free plate, although other resin systems will produce satisfactory produets. Thermoset resins are particulariy suitable for the manufacturing of good conductivity plates since they are handled in a hoţ press, which also presses the particies together for intimate electronic contact, and they also shrink somewhat on curing, further pushing the particies together. Other candidate thermoset resins include epoxyphenols, novolac resins, bisphenoi A based epoxy resins, bisphenol F epoxy resins; polyesters (saturated, unsaturated, isophthaiic, orthophthalic, neopentylglycol modified, modified vinylester; vinylester urethane and the like. Some grades of these polymers have been found to exhibit a relatively excessive amount of shrink'age on curing coupied with a relatively poor adhesicn
to the pariicies which allows interconnecting voids to appear around the surfaces of the particles which makes them unsuitabie for producing substantially -pore-free plates. However, low shrink and other additives may be included in commercial grades of these resins, provided that they do not have a detrimental effect on the chemical stability of the resin in the acid eiectrolyte. Some poiymers have been shown to be unstable in the polansed presence of an acid eiectrolyte. Some commercial resins have a mould release agent preblended in the mixture and these should be avoided in this application since they can adversely affect the adhesion of the active battery materials and potentially affect the corrosion stability of the plate and also the surface chemistry (surface tension etc.) of the battery acid eiectrolyte. The chosen resin will preferably be one which is resistant to the eiectrolyte acid, especially where the electrode is for bipolar batteries.
USP 5017446, discioses the inciusion of a wide range of conductive fillers in a thermoplastics resin. We have found that the high volume fraction of particles disciosed in USP 5017446 means that the finished electrode is very porous and unsuitabie for use as a bipolar electrode uniess great care is taken in ensuring that the partide size distribution of the particles is such as to engender a very close packing densrty, such as a bimodai or trimodai distribution. în additi'on, the matrix of 60% volume solids in a thermopiastic, which this source uses as an example has very poor fiow properties even at the high melt temperatures (370°C) cited, and would be unsuitabie for injection moulding - which is the preferred mass production technique for thermopiastic materials. In order to improve both the porosity and the fiow characten'stics of the melt, it is necessary to sianifîcantly reduce the fracticn of solid particles in.the mixture to less than about 35% voi. It is clearfrom Table III of USP 5017446 that the resulting material would have a resistivity which would be unsuitabie for use in a bipolar lead-acid battery where the threshold value of suitable resistivity is
generaily accepted to be lowerthan 1 Ohm.cm. In exampie 6, USP501 /446 indicates that a resistîvity of 9.2 Ohm.cm was achieved which îs unsuitabie for use as a bipolar electrode in a lead-acid battery. The present invention is of a material which has suitable resistivity and porosity, and can be made without the need for very careful partide size management and allows a well known industrial process to manufacture.
The conductivity of the titanium suboxide particies may be improved by contact with a gas such as helium or hydrogen for a period, say up to 24 hours before being incorporated in the resin composition in manufacture of the electrode.
The relative proportions of resin and suboxide powder and the partide size distribution of the suboxide powder wiil affect the properties of the electrode. For exampie an electrode will tend to have low conductivity if:
• too high a volume proportion of resin is used; and/or
• the plate or other body shape is pressed in manufacture with too little or with uneven
• the partide size distribution leads to low packing density; and/or
• the average partide size is too small; and/or
• îhe resin shrinks insufficiently on curing; and/or
• any excess resin is not ejected from the mould as flash due to either the resin curing
too quickly, the viscosity of the resin being too high (either intrinsically or by virtue of
the rnould temperature being too low), or by the mould clearances being too smail.
"he electrode wiil tend to have unacceptable through porosity if:
• too iow a voJume proportion of resin is used; and/or
• the partide size distribution provides a Iow packing density such that there is
more volume of inter-particle voids which needs to be filled with resin and so
the effective voiume proportion of resin becomes low and/or
• the average partide size is too large; and/or
• the resin shrinks excessively in manufactura of the eiectrode and by virtue of
poor adhesion to the particles forms cavities adjacent to and around the
particles on curing; and/or
• the resin cures too slowiy, is of low viscosity (either intrinsically or by virtue of
the mouid temperature) or the mould ciearances are too large that significant
amounts of resin are lost from the mouid.
When manufacturing the body it is preferred to have a slight excess of a thermoset resin. in press moulding the conducting particies are pressed together to form low resistance conductive paths. Any excess resin is ejected from the mould as "flash" before the final cure of the material, which occurs in the press, under pressure, thus locking in the electrical connectivity.
Particles with high (rods, fibres) or iow (flakes) aspect ratio of the titanium suboxide can also be present to increase connectivity between the electrically conductive suboxide particies in the eiectrode. High aspect particles are especially favoured because they provide longer unbroken electrical paths, so increasing conductivity.
A preferred eiectrode of the invention is a plate which nas the following combinaiion of features:
• is electronicafly conductive, i.e. an overaii electrica! conductivity greater than
Q.SS.crrT1 rnore specifically has an orthogonal conductivity of at least about
ÎS.cm"1 which is relatively uniform across the face of the piate;
• has essentially no through porosity (which would ailow ionic species to travel
through the pores causing seif discharge of the battery) as demonstrated by a
leakage current of less than 1A/m2;
> is resistant to chemical attack by the materials in a lead-acid battery (this is primarîly the acid, but aiso the oxidant Pb02 and the reductant Pb metal); is resistant to galvanic corrosion (especially at the oxidation potenţial which occurs during recharge of the positive side of the bipolar plate); provides an intimate and adherent surface to the active chemicals in the battery (such as PbO2, PbSO4, Pb, tri-basic lead sulphate, tetra-basic lead sulphate);
is mechanically robust in thin sections. Whilst the cured resin particulate electrode is generally sufficiently robust, the presence of a moulded-in grid on the surface cf an otherwise flat plate increases the stiffness of the thin plate; does not catalyse the production of oxygen or hydrogen at the potentials which occur during the recharge of the battery;
provides a surface to which adhesives and sealants and/or mechanical seals can be applied; .
ideally has some surface features, (such as a triangular, square, hexagonal or other tessellated pattern grid) which will ailow the active paste material to be easily and uniformly spread onto the cells thus formed, and to restrict the movement of the paste during the charge and discharge cycling of the battery, and
• ideaily is of low weight.
In another aspect the invention provides a method of making an electrode, the method comprising mixing an unhardened resin, a hardener therefor, and the particies of the Magneli îitanium suboxide and pouring the mix into a mould therefor to form the shaped body.
In one preferred method the resin and hardener are heated, the particies of titanium suboxide are added to form a dough, which is then added to a preheated mouid. In another preferred method the resin components and the suboxide particies are first formed into a sheet moulding compound which can be piaced uniformly in the mould because it can be handled easiiy.
The method preferably includes the step of placing the mould in a heated press and applying pressure. The pressure may be about 2000 Pa and the temperature at least 35°C, preferably at least 70° C (n one embodiment the method includes the further step of removing the shaped article from the mould and cieaning the surfaces by processes such as g rit blasting, applying corona discharge and piasmas, and other surface cieaning techniques.
The method further includes the step of applying a battery paste to the electrode. Different amounts of paste may be applied to different areas of the electrode.
Preferably the method inciudes the step of first applying a thin layer of metai to the electrode before the paste is appiied." In one preferred technique the method includes appiying the metal layer by eiectroplating and adding dispersoids to the piating solution.
in another preferred feature the method includes the step of pressing a thin foii, say up to about 200 micron thick, of metal on to the surface of the electrode whilst in the mouiding press and the resin is curing. Other methods include plasma or flame spraying, sputtering, chemical vapour deposition and the like.
Low viscosity resins are preferred to wet the externai surface of the particles which will enhance low porosity say less than about 50 Pa.s at 20°C. These resins will also tend to infiltrate into the microscopic surface features of the particies to improve mechanical strength, The viscosity may be lowered by pre-heating or by selection of suitable resins. However extremely !ow viscosity resins shouid be avoided for the reasons stated above.
Coupling agents such as silanes to contact the surface of the particles may be used to improve the adhesion and wetting of the resin to the suboxide particles to enhance low porosity and high mechanical strength. The coupling and/or wetting agents (such as silanes and other surfactants) can be advantageousiy used on plates which do not have the rnetallic iayer imposed. The pasting of the piates is carried out in the usual way, with convenţional leady oxide paste or other lead containing pastes. The existence of the impressed surface features means that a controlled volume of paste is applied to the grid area-of the plates; pasting with thicker or thinner layers can be managed by having the grid higher or lower. It is aiso possible, by adjusting the shape of the mouid to have some areas with thick paste and other with thin paste in order to optimise the discharge characteristics of the battery. The paste on the electrode can be cured in the usual way.
In another aspect the invention provides a battery inciuding an eiectrode as defined herein or when made by a method as defined herein.
Preferabiy the battery comprises a pluraiity of electrodes and an acid eiectrolyte.
With pasted and cured plates, a battery is assembled using a number of bipolar plates, appropriately oriented, and a single positive monopole at one end and a single negative monopole at the other. Absorptive glass mats can be advantageousiy inserted between
each plate. Sealing of the plates is achieved in the laborator/ by the use of gaskets of appropriate thickness and made of say butyl or silicone rubber sheet. The entire assembly is held together by metal straps and bolts of suitable iength. In a cornmercia! battery, in a preferred feature of the invention, the plates are sealed into a pre-moulded plastic container, with siots for each plate. A certain amount of compression of the glass mat and of the paste can be engendered by correct dimensioning of the container. Such compression has been found to aid the adhesion of the paste to the bipolar electrode substrate. Low concentration sulphuric acid can be added followed by a lid having grooves which will seal onto the edges of each plate, placed on the top. The lid can advantageousiy also contain a suitable gas pressure regulating system.
The battery is then electrically formed in the usual way. As the formation takes place, then the acid increases in strength, by the conversion of the sulphate-containing paste to PbO2 on the positive plate and Pb metal on the negative. The iniţial strength of the sulphuric acid should be chosen to ensure that the final strength of the acid is in the range 30-40% by mass of sulphuric acid, or even higher.
Fhosphoric acid can alsc be advantageousiy added in part or total replacement of the more usua! sulphuric acid.
Batteries made by this method have high power and energy density (W/m3, Wh/m3), high specific power and energy (W/kg, Wh/kg.) They have high cycie life, even in deep discharge conditions, and can be manufactured cheaply with convenţional technology.
In a bipolar battery it is important for efficient discharge at high rates that the monopolar or end electrodes have excellent planar conductivity. By this invention monopolar plates can be made by substîtuting for one side of the mould a flat plate and then placing a metailic grid or mesh in the mould before the uncured resin and the suboxide materials are placed in the rnould. When the mould is closed and the resin is cured, the metal grid or mesh will be pressed into one side of the formed electrode, giving it excellent planar conductivity for the purposes of a monopolar or end plate. Of course, the metal grid or mesh should not be exposed to the electrolyte otherwise it will corrode. Preferably metal studs are eiectrically attached to the metal grid or mesh to provide terminal connections. Lead or lead alloy foiis can also be advantageously applied to the reverse face of the electrode in the mould instead of the metal grid or mesh to provide good planar conductivity for the monopolar or end electrodes.
Metal piates, grids or meshes may be advantageously incorporated into the bipolar plates in order to increase the planar conductivity and ensure good current distribution over the ful! area of the electrodes, Cooling channeis can be introduced into the bipolar plates in like manner.
in another aspect the inventicn includes a method of testing to confirm the absence of învisibie micropores which lead to though porosity in an electrode before pasting, comprising placing the siectrode in a simuiated battery and measuring the flow of current over time.
A satisfactory electrode wiH have a current leakage of less than 1A/m2 over 28 days when tested in the apparatus of Exampie 2.
In order that the invention may be well understood it wil! now be described with references to the following Examples.
24g of ARALDITE PY307+1 resin and 8.8 g of the HY3203 hardenerwere weighed out into separate containers and pre-warmed in an oven at 50°C for a minimum of 7 minutes. These materials are available from Vantico Ltd. They were then thoroughly mixed together and 65g of the Magneii suboxide powder as beiow is added and mixed in thoroughly to form a dough. The phase analysis of the Magneii suboxide powder was measured by X-ray
The partide size distribution was measured on a Malvern Mastersizer to be:
100 voi % below 300 micrometres 95 voi % below 150 micrometres 90 voi % below 125 micrometres 50 voi % below 85 micrometers 10 vo! % beiow 40 micrometres
The dough was evenly spread into a rnould that has been pre-warmed to 75°C. Even spreading is important to achieve uniform conductivity across the face of the plate. The laboratory mouid is of a "window trame" type and consists of two platens and a frame. The mouid cavity has an area of 149 x 109mm (O.Q1624m2) and will therefore produce plates of this size. The volume of dough was sufficient to produce a plate about 1.5mm thick at the base of the grid cells. Two locating pins at diagonal corners are used to locate îhe various parts of the mouid. Spacer levers are available to re-open the mouid to eject the manufactured part after moulding is completed. Both platens can be fitted with plates which have machined slots 1 mm deep in the face, so that the moulded part can have a raised grid on either surface. In the example, this grid covers the central 136 x 96 mm. The grid of the laboratory plates did not extend to the perimeter of the plate to provide a flange for sealing. The dimensions of the grid can be changed by altering the shape of the mouid, and thus different volumes of active paste material will be applied to the plates in a controiled manner.
The mouid can be advantageousfy treated with an appropriate mouid reiease agent such as Frekote 770NC®. The rnould was closed and placed in a heated press at 75°C. The mouid was initially pressed at 70kN (1137Pa) for 5 seconds and then 100kN (1625Pa) for_25 minutes. The mouid \s opened and the resulting plate is extracted. Any flashing is removed with a metal spatula.
The conductivity of the plate was then tested and was found to be in the range 1-2 S.cm"1. In this example, the density of the final plate was around 2.2 g/cc. Higher pressing pressures produce higher levels of conductivity. Thus the preferred range of densities for :he final product is in the range of 1.8 to 2.4 g/cc or above
The surface of the plate was cleaned by g rit blasting, in a biast chamber such as a Qyson Formula F1200®. The biast gun was supplied with air at a pressure of O.SMPa. Alumina was used for the biast medium, although other biast conditions and other cleaning methods wifl undoubtedly produce satisfactory results. The blasting was carried out manually until the
entire surface was uniformly matt grey in colour. Tests with surface impedance scanning techniques have shown that this blasting in this fashion produces a plate with very uniform surface impedance. The surface of the plate may also be further modified by techniques such as corona discharge or by the applicati'on of plasmas.
The plates were pasted with active material and assembled into batteries as below. They satisfy all the criteria above. Better results were obtained if a thin metallic layer is first applied to the grid area of the plates. This layer can be of pure lead, or of lead ailoys (with, for instance, antimony, barium, bismuth, calcium, siiver, ţin, tellurium) and be applied in a number of ways such as eiectroplating, sputtering, thermal evaporation and deposition, chemical vapour deposition, lead and lead alloy shot blasting, plasma orthermai spraying or by direct application of thin metal foiis in the pressing mould. It is an advantage of the invention that a wider variety of alloys can be considered than has previousiy been available to the lead-acid battery engineer, where the alloys must not only satisfy corrosion conditions, but also strength criteria and an ability to be fabricated into metallic grids. One convenient way of applying the interlayer in the laboratory is by eiectroplating as follows:
One sia'e of the flanges were painted with a stopping-off lacquer such as Lacomit® from HS Waish & Sons Ltd. The plate was then sealed with a rubber O-ring onto the bottom of a piastic platina tank with the stopped-off fîange uppermost A lead metal strip was pressed against the other side of the rlange to provide an electrica! connection. VVhen plating the side which wiil be used as a pcsitive, abcut SCOmI of a plating soluţie n such as 27% lead/tin
methane-suiphonic acid, containing a starter additive such as Circamac HS ST6703 (both maten'als are suppiied by MacDermid Canning Ltd.) was poured into the pfating tank. A large pure !ead anode was used as the counter electrode. On the plates of the laboratory size, a current of 0.5A is appiied for 7 hours, which deposited approximateiy 10g of an alloy whose composition is approximateiy 6:94 tiniiead.
Plating the negative side was similar except the plating soiution is lead methane-suiphonic acid (Circamac HS ST6703). A current of 0.5A was appiied for approximateiy 3 hours which deposits about 5g of lead metal.
Other plating solutions such as those based of fluoroboric acid can be used. The plating process can also involve the use inter alia of "dispersoids" such as titania, to produce a rougher surface finish for better keying with the paste subsequentiy appiied.
Adjusiments to the plating current and other additives can also advantageousiy affect the surface morphology of the layer.
After electroplating, the plates are removed from the plating bath and washed thoroughiy in deionised water. The stopping-off lacquer is removed with acetone.
Another convenient way is by direct application of thin metallic foils in the pressing mould. For instance, a foii of lead with two percent ţin alloy, 50 micron thick, is placed in the bottom cf the preheated mcuid and the resin and the powder mixture spread therecn. A second foii is piaced over the spread material before the mould is cicsed and the resin is cured as above. At tnis stage, the metallic layer, wheîher appiied by electropfate, direct foii pressing,
plasma or flame spraying. sputtering, chemicai vapour deposition, or any other method can be activated by washing it in concentrated suiphuric acid immediately prior to pasting.
In another embodiment of the invention, a lead dioxide layer or a ţin dioxide (suitably doped with for instance antimony to increase the conductivity) can be appiied on to the substrate by methods such as anodic electroplating, sputtering, chemicai vapour deposition and like processes, either directly or after the metailic layer is appiied. Such a layer is preferabiy appiied on the positive side of a bipolar electrode.
It is weil understood in the lead-acid battery industry that a certain low level of corrosion of a lead or lead ailoy electrode improves the adhesion of the active paste (particularly the positive paste) to the electrode. However, in the case of an interlayer of the present invention, if the corrosion rate is too high, the interlayer can be completely consumed, especiady under deep discharge or high overcharge conditions of a lead-acid battery. One aspect of the invention is to provide an interlayer with different areas, some of which are highly corrodible (which give good paste adhesion) and other areas are more corrosion resistant (which gives long life).
The method described above produces plates which are nominaily flat. However, plates with simple and compound curvature and different perimeter shapes can be made by appropriate modification of the shape of the mould. When assembled into batteries, such plates will engender an appropriate shape on the finished battery to enable it to be installed more conveniently in (for insiance) a body panel of a vehicle.
Piate eiectrodes of the invention were tested before the application of any metallic layer or active battery paste to confirm the absence of any invisible micropores through the piate which wouid allow ionic species (such as H+, OH" SO42") to migrate through the plate. A suitabie testing cell which simufates very closely the processes which occur in a battery is shown in the accompanying Figure 1. The plate was assembled as if it is a bipole in a 4V cell which also contams a fully pasted, cured and charged positive mcnopole and a similar negative monopole. These are preferably of the convenţional lead grid type. 30% sulphuric acid was placed between the plate and the monopoles in the convenţional manner. A potentiostat was applîed across the monopoles to hold the voltage across the test plate (measured by two identica! reference electrodes in the acid either side of the test plate) to be 2.6V - which is chosen as the maximum that will be applied across a lead acid battery bipole in normal operation. The current flowing is noted.
We have found that a typical current observed initially to be about 0.3A/m2. This holds very constant over long periods (months) when the plate is manufactured as above with. the preferred epoxy resin With other resins, it is possible that although the current measured starts off low, it rises over a few days or weeks by several orders of magnitude. This implies that some resins are being corroded or otherwise degraded by the acid at high oxidation and reduction potenţiala and that ionic porosity is being created. Such a piate formulation is unsuitable for bipolar battery electrodes and means that by using the test outlined, the perscn skiiled in the ar: will be ab!e to determine which resins are best used in this invention.
The invention is not iimited to the examples. The piate electrode may have a flange moulded of resin which is free of the suboxide powder. This will reduce the cost of the plate but stiîl
provide effective sealing. The invention îs applicabie to eiectrochemicai cells in generai, inciuding bipolar lead acid batteries, to other types of batteries and to fuel cells, redox energy storage ceils and the like.
This invention is not restricted to conductive particles such as the titanium suboxides although these are known to be very highly corrosion resistant, when manufactured according to the teachings of US-A-5173215 which is required for lead-acid battery electrode appiications. Other conductive particles can also be used such as niobium doped titanium oxides, tungsten oxides, niobium oxides, vanadium oxides, molybdenum oxides and other transition metal oxides in both stoichiometric and non stoichiometric forms. It is an advantage of the invention that good conductivity electrodes can be made from relatively low conductivity particuiate materials, or by a smaller proportion of relatively expensive particulate materials.
1. An electrode comprising a shaped body which is formed of hardened thermoset resin, the body having electrical paths defined by contacting conductive particles wherein (i) the conductive particles are titanium suboxides of the formula Tin02n-1 where n is 4 or greater, (ii) the particles have a size distribution with a standard deviation of less than 50% of the mean particle size, (iii) the body has a density of 1.8 g/cc or above and (iv) the electrode is sufficiently pore-free such that the electrode has a current leakage of less than 1 A/m2.
2. An electrode as claimed in claim 1, wherein the titanium suboxides are suboxides selected from the group consisting of Ti4O7, Ti5O9 and Ti6O11.
3. An electrode as claimed in claim 1, wherein the particles have a mean particle size in a range of 50 to 300 micron.
4. An electrode as claimed in claim 1, wherein there is a bimodal distribution of substantially uniform large particles and a proportion of smaller particles dimensioned to fit in interstices formed between the large particles.
5. An electrode as claimed in claim 1, wherein there is a polymodal distribution of a range of particle sizes ranging from large particles to successively smaller particles dimensioned to fit in interstices formed between larger particles.
6. An electrode as claimed in claim 1, having high aspect and/or low aspect particles of the titanium suboxide to increase connectivity.
7. An electrode as claimed in claim 1, having an overall electrical conductivity greater than 0.5S.cm-1.
8. An electrode as claimed in claim 7, having an orthogonal conductivity greater than l.S.cm1.
9. An electrode as claimed in claim 1, in the form of a plate which is flat or has curvature.
10. An electrode as claimed in claim 1, which has a metallic layer applied to a surface thereof.
11. An electrode as claimed in claim 10, where the metallic layer has areas of differing corrosion rates.
12. An electrode as claimed in claim 1, having a lead dioxide or doped tin dioxide layer applied to a surface thereof.
13. An electrode as claimed in claim 9, wherein the plate has a flange adapted to secure the electrode to a casing of a cell.
14. An electrode as claimed in claim 13, wherein the electrode is sealed in a casing which is secured to the flange by adhesive or welding.
15. An electrode as claimed in claim 13, wherein the flange is free of the particles of titanium suboxide.
16. An electrode as claimed in claim 9, wherein the plate is received in a slot in a casing therefor.
17. An electrode as claimed in claim 1, comprising a plate having a metal grid or mesh or sheet in the body thereof.
18. An electrode as claimed in claim 1, comprising a plate having cooling channels in the body thereof.
19. An electrode as claimed in claim 1, wherein a surface of the electrode has surface deformations for receiving and retaining active paste material.
20. An electrode as claimed in claim 19, wherein the deformations are moulded in the surface.
21. An electrode as claimed in claim 19, wherein the deformations are formed in the surface after the electrode has been moulded.
22. An electrode as claimed in claim 19, wherein the deformations are dimensioned and shaped to receive different thicknesses of paste in different areas.
23. A method of making an electrode as claimed in claim 1, the method comprising:
mixing the conductive particles, an unhardened thermosettable resin and a hardener therefor to form a mix,
pouring the mix into a mould therefor and
moulding the mix to form a shaped body and thereby provide the electrode.
24. A method as claimed in claim 23, wherein the mix has a bimodal distribution of substantially uniform large particles and a proportion of smaller particles dimensioned to fit in interstices formed between the large particles.
25. A method as claimed in claim 23, wherein the mix has a polymodal distribution of a range of particle sizes ranging from large particles to successively smaller particles dimensioned to fit in interstices formed between larger particles.
26. A method as claimed in claim 23, wherein the titanium suboxide comprises Ti4O7 and/or Ti5O9.
27. A method as claimed in claim 23, wherein the particles of the titanium suboxide are first contacted with a gas for a period to extend the conductivity thereof.
28. A method as claimed in claim 27, wherein the gas is helium or hydrogen.
29. A method as claimed in claim 23, wherein the resin has a viscosity of less than 50Pa.s at 25°C.
30. A method as claimed in claim 23, wherein the resin, hardener and particles are first formed into sheet moulding compound which is added to the mould.
31. A method as claimed in claim 30, having the step of applying foils of metal to one or both surfaces of the sheet moulding compound.
32. A method as claimed in claim 23, having the step of removing the shaped body from the mould and cleaning its surfaces.
33. A method as claimed in claim 32, wherein the method of cleaning includes grit blasting.
34. A method as claimed in claim 23, wherein excess resin is ejected from the mould during pressing.
35. A method as claimed in claim 23, having the step of applying a thin layer of metal to the electrode before a paste is applied.
36. A method as claimed in claim 23, having the step of pressing a metal foil on to a surface of the electrode whilst in the moulding press while the resin is curing.
37. A method as claimed in claim 36, wherein the metal foil is up to 200 micron thick.
38. A method as claimed in claim 35, including applying the metal layer by electroplating using a plating solution and optionally adding dispersoids to the plating solution.
39. A method as claimed in claim 35, including treating a surface of the metal layer with a corona discharge or plasma.
40. A method as claimed in claim 35, including adding a coupling and/or wetting agent to the paste.
41. A method of testing an electrode to confirm the absence of invisible micropores through the electrode before pasting, the method comprising placing a test electrode in a simulated battery and measuring the flow of current over time.
42. A method as claimed in claim 41, including using an external power supply to adjust the electrical potential across the electrodes.
43. An electrode having a current leakage of less than lA/m2 for 28 days when tested by a method as claimed in claim 41.
44. A battery including plurality of electrode as claimed in claim 1, and an acid electrolyte
45. An electrode as claimed in claim 1, wherein the hardened thermoset resin comprises an epoxy.
46. An electrode as claimed in claim 1, having a conductivity in the range of 1 to 2 Scm-1.
47. An electrode as claimed in claim 1, wherein the conductive particles comprise particles of Ti4O7, Ti5O9 and Ti6O11.
|Indian Patent Application Number||01293/DELNP/2003|
|PG Journal Number||48/2008|
|Date of Filing||14-Aug-2003|
|Name of Patentee||ATRAVERDA LIMITED|
|Applicant Address||UNITS A AND B, ROSEHEYWORTH BUSINESS PARK, ABERTILLERY NP 13 1SX, GREAT BRITAIN.|
|PCT International Classification Number||H01M 4/66|
|PCT International Application Number||PCT/GB02/00230|
|PCT International Filing date||2002-01-21|