AU2012200586A1 - Electrode plate for an electrochemical cell - Google Patents

Abstract

-24 Abstract The invention relates to a structure for forming a current collector for an electrochemical cell. The structure includes a plurality of adjoined layers of electrically conductive sheet. Each sheet includes an array of apertures arranged to 5 partially overlap more than one aperture of each immediately adjoining sheet to form a network of voids for holding an active material. 17 16 M1 M2 1 7 -.... ... ... ... 20- a.......//.....2 F....G.2...

Description

P/00/011 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title: ELECTRODE PLATE FOR AN ELECTROCHEMICAL CELL Applicant: Subtrade of SB Pty Ltd The following statement is a full description of this invention, including the best method of performing it known to me: 1 -2 ELECTRODE PLATE FOR AN ELECTROCHEMICAL CELL Cross Reference This application is related to Australian Provisional Patent Application No. 5 2011900320 filed on 2 February 2011, the contents of which are to be taken as incorporated herein by this reference. Field of Invention The present invention relates to an improved electrode plate for electrochemical cells 10 and more particularly to an improved structure for forming a negative electrode plate. The electrode plate has particular, but not exclusive application, in lead acid batteries. Background of the Invention 15 Lead acid batteries include at least one positive current collector, at least one negative current collector and an electrolytic solution including, for example, sulphuric acid diluted with distilled water. Typically, both the positive current collector and the negative current collector are made from a grid manufactured from lead or copper and are covered in an active material in the form of a porous paste. Once 20 covered in this paste, the grid provides structural support for the active material. Once coated in active material the positive and negative current collectors are referred to respectively as positive and negative plates. The plates transfer electric current to and from the battery terminals during the charging and discharging 25 processes. The storage and release of electrical energy in lead acid batteries is enabled by chemical reactions that occur in the paste deposited on the current collectors. The paste allows the acid to react with the lead of the plates and thus increases the active 30 surface area. It is desirable to maximise the active surface area and thus it is important to maintain good contact between the paste and each of the current collectors. These chemical reactions also cause corrosion of the positive collector and sulfation on the negative plate. 35 Corrosion of the positive collector results in volume expansion of the positive plate as the lead dioxide corrosion product has a greater volume than the lead of the positive collector that is consumed to create the lead dioxide. The volume expansion induces -3 mechanical stresses on the positive current collector that cause deformation and stretching. Eventually, the amount of corrosion of the positive plate will reach a point where there is no longer an easy path for the electrons to flow and thus there is a drop in battery capacity and ultimately an end to the battery's service life. 5 Sulfation is the process by which lead sulphate formed in the discharge reaction forms a more stable crystalline form coating the negative plate. Crystalline lead sulphate does not conduct electricity and cannot be converted back into lead and lead oxide under normal charging conditions. The crystalline lead sulphate blocks 10 the conductive path needed for recharging and thus affects the charging cycle, resulting in longer charging times, less efficient and incomplete charging and excessive heat generation. Various attempts have been made to address the problems of corrosion and sulfation 15 in lead acid batteries and to also maintain good contact between the paste and the collectors. Australian patent 653862 describes a support matrix for negative electrodes of lead acid batteries. The support matrix is formed from an expanded metal grid produced 20 by an expanding machine. The patent teaches an arrangement whereby the plate thickness exceeds the grid thickness because of the amount of paste that can be retained on the grid. US patent 6,979,513 describes a current collector constructed from a carbon foam. 25 The carbon foam includes a network of pores into which an active material is disposed to create either a positive or negative plate for a battery. US patent 7,033,703 describes a composite material that may be used to form a current collector. The composite material includes a first carbon foam structure 30 including a network of fine pores and a second carbon foam structure including a network of fine pores. An intermediate structure is located between the first and second foam structures. Because current collectors made from foam materials hold the active material in a 35 network of small pores, they may provide a relatively short conduction path through the active material to the current collector. Furthermore, holding the active material in small pores may provide improved structural support of the active material, which -4 may permit the use of more porous active material which may in turn allow increased circulation of the electrolytic solution. Additionally, by holding the active material in small pores the growth of hard sulphates is mechanically limited by the size of the pores in the foam structures. 5 Unfortunately, current collectors manufactured from foam materials are typically very expensive to manufacture. Furthermore, foam based current collectors may be difficult to "paste" as they have a lower pastable volume. Hence, it may be difficult to thoroughly penetrate the paste into the foam composite structure. A uniform 10 distribution of active material throughout the plate is desirable in high performing current collectors to achieve optimum usage of the active material from all of the plate surface. This is particularly limiting in long plate designs. Foam current collectors are also difficult to use in long plate designs due to their lower mechanical strength. 15 US patent 5,543,250 describes an electrode made from a metal sheet having punched holes with burrs along their peripheries. The holes with burrs increases the thickness of the metal sheet thus allowing more paste to be held by the sheet and the burrs also increase the engagement or adhesion of the paste to the sheet. This 20 results in improved conductivity between the paste and the metal sheet and a reduction in shedding of the active material (i.e. the paste). A disadvantage of this approach is that there is a large disparity of sizes and shapes of each pocket of active material between the burrs and metal sheet, meaning that the active material utilisation is not controlled evenly. 25 US 2009/0103242 describes a current collector having a plurality of raised and lowered portions with respect to a mean plane of the current collector. Slots are formed between the raised and lowered portions. The slots permit both electrical and fluid communication between regions where the paste is placed behind the raised 30 portions and the regions where the paste is placed behind the lowered portions. This assists in reducing the likelihood of flaking of the active material (i.e. the paste) during discharge and charge cycles as expansion and contraction of the current collector occurs. However, a disadvantage of this approach is that the surface area of the current collector bonded to active material is relatively high which tends to 35 restrict the uniform flow of ions from the electrolyte into the active material held in the slots. In other words, the total surface area of active material exposed to electrolyte is -5 less than the full plate area which severely restricts the flow of ions from electrolyte into the active material. The discussion of the background to the invention herein is included to explain the 5 context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of this application. Summary of the Invention 10 One aspect of the present invention provides a structure for forming a current collector for an electrochemical cell, the structure including a plurality of adjoined layers of electrically conductive sheet, each sheet including an array of apertures arranged to partially overlap more than one aperture of each immediately adjoining sheet to form a network of voids for holding an active material. 15 The structure is preferably configured to provide a generally rectangular plate having a width, height and thickness. In this form, the structure will include an upper edge, a lower edge, opposed side edges, and front and rear surfaces. For ease of explanation, the width of the structure will be described in terms of the spacing 20 between the opposed side edges, the height will be described in terms of the spacing between the upper and lower edge, and the thickness (or depth) will be described as the spacing between the front surface and rear surfaces of the structure. The front surface will typically include an outer facing surface of a front most layer and the rear surface will typically include an outer facing surface of a back most layer. One or 25 more additional layers of electrically conductive sheet may interpose the front most and back most layers. Thus the thickness of the structure will typically depend on the number of layers forming the structure and the thickness of each layer. The voids will preferably extend through the structure in a direction perpendicular to the notional plane of the plate so as to provide a passage therethrough. 30 It will be appreciated that spatial references, such as "side", "upper", "lower", "front", "rear", "downwardly", etc., are for the purpose of illustration only and do not limit the spatial orientation or location of the structure described. 35 The multiple layer structure serves a number of purposes. First, the multiple layers serve as structural support for the active material. Additionally, the multiple layers of electrically conductive sheets form a meshed network of electrical conductors -6 arranged to provide a relatively short and thus more direct current conduction path between active material held in the voids and the structure, which in turn reduces the resistance of the structure. The reduction in resistance may increase the utilisation of the collector plate in terms of the amount of active material that can be reacted 5 during use of the collector plate in the electrochemical cell. The structure will include at least two layers of electrically conductive sheet. Preferably the apertures and the layers are configured and arranged to form the network of voids such that each void forms a relatively small compartment or "pocket" 10 such that at any location within a void the length of the current conduction path to the structure is less than about 2mm. Each sheet is preferably generally flat or configured in a corrugated or other profile. Each sheet may include a metal sheet comprising aluminium, lead or copper. 15 Preferably, each layer has a "thickness" in the range of 0.5mm to 2.0mm. It is to be noted that the thickness of each layer need not correspond with the thickness of the sheet forming the layer. In other words, each layer may be formed to have a thickness which is greater than the thickness of the sheet forming the layer. Typically, each sheet will be of between 0.1mm and 0.5mm in thickness. 20 Each sheet may include a mesh of electrically conductive material, such as an expanded metal mesh or wire mesh including apertures in the form of mesh openings defined by intersecting strands forming the mesh. However, although expanded metal mesh is preferred, alternative embodiments may include a honeycomb type 25 metal mesh, a punched metal sheet, a pressed or formed metal sheet, or a perforated metal sheet. In embodiments in which each layer includes a mesh, such as an expanded metal or wire mesh, the mesh may be formed with strands forming the mesh sized between 0.75mm to 1.0mm in width. In this respect, in this context reference to the term the strand size is to be understood as the "width" of the strand 30 as opposed to the thickness of the strand which will correspond to the thickness of the sheet. Although an expanded metal mesh is preferred, it is also possible that each sheet of conductive material could include a grid or lattice type sheet. It is preferred that the apertures have substantially the same size and shape. In such 35 an arrangement, the partial overlapping relationship between apertures of immediately adjoining sheets may be formed by providing an offset in the position or angular orientation of the apertures of the adjacent sheets relative to each other. The -7 apertures of immediately adjoining sheets may thus be positionally offset relative to each other to thereby provide the partial overlapping relationship therebetween. Additionally or alternatively, the apertures of immediately adjoining sheets may be orientated at an angle with respect to each other to thereby form the partial 5 overlapping relationship. The apertures may include any suitable configuration. In one configuration the apertures are greater in length than in breadth. 10 In an embodiment, the apertures are generally polyhedron shaped apertures. In preference, the apertures are generally diamond-shaped with a major dimension in the range of 5mm to 10mm and a minor dimension in the range of 3mm to 5mm. In an embodiment which includes diamond-shaped apertures, the major and minor dimensions may correspond with respective diagonals of the diamond shaped 15 apertures. It is possible that other shaped apertures may be used, such as, rectangular, square or circular shaped apertures. It is also possible that the apertures of an array may differ in shape and size. Preferably, immediately adjoining sheets are orientated perpendicularly with each 20 other with respect to the orientation of the major axis of the apertures, although this arrangement is not limitative. Arranging the layered sheets so that the major axis of the apertures of immediately adjoining sheets are orientated perpendicularly may improve the rigidity of the structure, which may in turn render the structure less susceptible to shedding due to vibration, or provide other advantages. The major 25 axis of the apertures of immediately adjoining sheets may be orientated relative to each other to reduce the pliability of the structure. The network of voids preferably provides a greater density of smaller sized apertures compared with the apertures of the sheets forming the structure. In other words, the 30 structure will preferably provide a network of voids forming an increased number of apertures per unit area with each void forming a passage extending through the structure having a narrower extent than the apertures of the layer. It is preferred that the voids are distributed relatively uniformly throughout the structure. 35 In order to provide structural support for the active material, the voids must not be excessively large so as to not be able to hold the active material during normal use of the current collector. However, the voids must be large enough to permit active -8 material to permeate through the network of voids. The network of voids formed by the overlapping apertures should thus be configured to allow the active material to permeate throughout the structure when the active material is applied to the structure by a suitable process, such, as pasting, to form a body of active material held within 5 the network of voids. Preferably the partially overlapping apertures are configured and arranged not only to allow the active material to permeate the structure, but also to provide structural support to normally retain the active material therein. Thus, the network of voids may be arranged to mechanically contain the active material, even when the current collector expands during a discharge reaction in order to reduce 10 shedding of the active material. The configuration of the layers and the network of voids provided by the overlapping relationship between the apertures of immediately adjoining sheets preferably provides a relatively large contact surface area between the active material and the 15 structure. The active material may include Lead Oxide. One or more additives may be added to the active material to modify properties of the active material. Suitable additives may include graphite (or carbon black) to increase conductivity, glass floc to assist 20 acid diffusion, and Indulin AT (Lignosulphate) and/or Barium Sulphate to act as nucleation sites for fine needle like sulphates crystals to thereby inhibit growth of larger sulphates. The layered arrangement of electrically conductive sheets preferably supports 25 current collection throughout the mass of active material held in the structure when active material is held in the network of voids. In accordance with a preferred embodiment, adjoined sheets are arranged and/or configured to provide a distribution of areas or nodes in mechanical and electrical contact to allow current conduction between the layered sheets, and thus throughout the layered arrangement. In one 30 embodiment, adjoined electrically conductive sheets electrically contact each other interfacially to form the distribution of areas or nodes in mechanical and electrical contact. Preferably the areas or nodes are distributed throughout the structure to provide a substantially uniform average length of current paths from the active material to the structure. 35 In an embodiment in which the sheets comprise expanded metal mesh, the areas or nodes in mechanical and electrical contact are formed by positioning the strands -9 forming the mesh of adjoined sheets layers in mechanical and electrical contact. Alternatively, the areas or nodes in mechanical and electrical contact may comprise interfacing regions of the sheets in which the apertures have been formed to form the layers. 5 By arranging the layers of sheets to form the network of voids, the structure may dimensionally limit the growth of sulphates that may form during a discharge reaction of the electrochemical cell. The structure may also increase the chargeability of the plate by reducing the length of the current conduction paths in the active material. 10 More specifically, since the conductivity of the layers will be substantially higher than the conductivity of the active material, reducing the length of the current conduction path (in other words, the distances that electrons travel through the active material) reduces voltage drops across the plate which thus provides a more uniform total voltage across the plate, and improved charging and discharging performance. 15 The layers of sheets may be supported or held in the layered arrangement by a suitable support structure, such as an electrically conductive frame or a non conductive frame. In one embodiment, the upper, lower and opposite side edges of the structure are each received within an electrically conductive frame, such as a 20 metal extrusion. The electrically conductive frame may include, for example, a metal frame providing a channel for receiving the edges of the plural layers. A suitable frame (conductive or non-conductive) may include an I-section or C-section channel. The edges of the layered sheets may be joined or otherwise secured to or within the 25 frame using a suitable process, such as mechanical crimping, ultrasonically welding, or soldering. In one embodiment the edges of each sheet are plated or coated with a compatible fusible metal alloy (such as an alloy of tin and lead), such as by dipping the edges into a molten bath of the fusible metal alloy. The edges of the layered sheets may then be located in the channel of the frame and heated until the fusible 30 metal alloy bonds the layers to the frame. In an alternative embodiment, joining the edges of the layers to the frame may involve applying a layer of compatible fusible metal alloy to the channel of the frame and heating the channel to melt the fusible metal and bond the edges of the layers to the frame. It will be appreciated that the references to "compatible fusible metal alloy" refer to a fusible metal alloy which is 35 capable of fusing with the metals to be joined to form the joint therebetween.

-10 The frame may set the desired thickness of the active material to be applied to the structure and form a guide for a pasting belt to paste the active material to the structure. 5 The frame may act to retain the active material and may further incorporate an electrically insulating coating for the purpose of avoiding short circuits that may occur from plate edges. The electrically insulating coating may include Ethylene Chlorotrifluoroethlyene or another insulating fluorocarbon based polymers such as Polytetrafluoroethylene or Polyvinylidene fluoride. Other suitable electrically 10 insulating coating would be known to a skilled reader. The frame may form a current conductor edging for providing a current conduction path to a lug or terminal mounted on, or formed integrally with, the frame. 15 It is envisaged that embodiment the structure may be particularly suitable for current collectors requiring long plates, such as plates having a length in the range of 600mm to 1200mm, where a greater length of current collectors may be required, and extra stiffness is needed for handle-ability. 20 The present invention also provides a structure for forming a current collector for an electrochemical cell, the structure being formed from plural overlaid sheets of expanded metal mesh, each sheet including a uniform arrangement of apertures of substantially the same size and shape, each aperture having a major axis and a minor axis extending perpendicular to the major axis, wherein adjoined layers are 25 orientated generally perpendicularly with each other with respect to the orientation of the major axis of the apertures to improve the rigidity of the structure, and wherein the apertures are arranged to partially overlap more than one aperture of each adjoined layer to form a network of voids for holding an active material. 30 According to another aspect of the invention there is provided an electrode plate made from a structure according to an earlier aspect of the invention. The electrode plate may be suitable for use as a negative electrode plate in an electrochemical cell, such as a lead-acid battery. An electrode plate may be formed from a single structure or plural structures. 35 - 11 Brief Description of the Accompanying Drawings The following description refers in more detail to the various features and steps of the present invention. To facilitate an understanding of the invention, reference is made in the description to the accompanying drawing where the invention is illustrated in a 5 preferred embodiment. It is to be understood however that the invention is not limited to the preferred embodiment illustrated in the drawing. In the drawings: 10 Fig. 1A is an exploded view of a structure according to an embodiment of the invention; Fig. 1B is a front view of the structure shown in Fig. 1A; 15 Fig. 2 is a close-up view of a part of the structure shown in Fig. 1A and Fig.1B; Figs. 3A to 3D show close up views of sections of a structure in accordance with other embodiments of the invention; 20 Figs. 4A to 4D show close up views of sections of a structure in accordance with another embodiment of the invention; Fig. 5A is an exploded view of a structure according to an another embodiment of the invention; 25 Fig. 5B is a front view of the structure shown in Fig. 5A; Fig. 6 is a close-up view of a part of the structure shown in Fig. 5A and Fig.5B; 30 Fig. 7A illustrates a negative electrode plate including the structure shown in Fig.1; Fig. 7B shows a close-up view of an edge section of the negative electrode plate shown in Fig. 7A; 35 Fig. 7C shows a close-up sectional view of the edge section shown in Fig. 7A; -12 Fig. 8A illustrates another embodiment of a negative electrode plate including the structure shown in Fig. 1; Fig. 8B shows a close-up view of an edge section of the negative electrode plate 5 shown in Fig. 8A; Fig. 8C shows a close-up sectional view of the edge section shown in Fig. 8A; and Fig. 9 illustrates a portion of another structure made from two overlapped sheets with 10 diamond shaped apertures. Detailed Description of Embodiments of the Invention Figs. 1A and 1B illustrate a structure 10 in accordance with an embodiment of the invention suitable for forming a current collector for a negative collector plate or 15 electrode of an electrochemical cell, such as a lead-acid battery. The structure 10 includes a pair of adjoined layers 12, 14 of electrically conductive sheets. The layers 12, 14 are formed from metal sheet, such as for example, lead, aluminium or copper. The layers 12, 14 may be manufactured by cutting, punching, stamping, expansion, 20 or other processing from a thin sheet of metal, although in alternative embodiments the layers 12, 14 may be formed as a woven mesh sheet of metal strands or wires. The processing may create a layer as a generally rectangular layer having a generally planar form with a notional primary plane. 25 In the present example, and as is best illustrated in Fig. 2, each layer 12, 14 is formed as a sheet of expanded metal mesh comprising electrically conductive strands 18, 18', where strands 18 are for layer 12, and strands 18' are for layer 14. The expanded metal mesh is formed by expanding a copper sheet having a sheet 30 thickness of 0.1mm to form an expanded metal sheet having a thickness of 1.6mm and diamond-shaped apertures 16 as of about 9.5mm by 5mm. It should be understood that the size of the complete structure 10 may depend on a number of factors such as the size of the mesh from which the structure 10 is formed, and the number of layers. Another factor may be the size of the current collector to be 35 formed using the structure 10. A formation process for forming a current collector from the structure 10 will be described later.

-13 The apertures 16 are arranged uniformly across the primary plane of each layer 12, 14 both in terms of positioning and alignment. The apertures 16 are generally diamond shaped. In the present case, each diamond shaped aperture is formed by intersecting strands defining the edges of the aperture. 5 Although the illustrated embodiment includes diamond shaped apertures, it will be appreciated that other shaped apertures may be used. For example, Fig. 3A to Fig. 3D show, in close-up view, alternative aperture configurations for a layered arrangement 301 of sheets 300, 300' in which the apertures 302 are generally 10 rectangular in shape. In this arrangement, the adjoined layers form a network of generally square voids 304. Similarly, Fig. 4A to Fig. 4D show, in close-up view, alternative aperture configurations for a layered arrangement 401 of sheets 400, 400' in which the apertures 402 are generally circular in shape. In this arrangement, the adjoined layers 400, 400' form a network of generally voids 404 having curved walls. 15 Returning now to Fig. 2, each aperture 16 includes vertices 17 formed by the intersection of the strands 18 of the expanded metal mesh. The size of the apertures 16 may be described with reference to the distances between the vertices 17. As shown in Fig. 2, the distances can be represented as a major dimension "A" 20 extending along a major axis "M1" of the aperture 16 and a minor dimension "B" extending along a minor axis "M2" of the aperture. In the present case, each aperture 16 has a major dimension of about 7mm and a minor dimension of about 3mm. In this instance the major and minor dimensions correspond with respective diagonals of the apertures 16. Examples 1 to 4 below provide further examples of alternative 25 expanded metal mesh configurations. 7= - Strand Width Knuckle Thickness ra Knuckle U) Long Way Measurement

(LWM)

-14 Example 1: Material Thickness: 0.10mm Strand Width: 0.75mm 5 Long Way Measurement (LWM): 6.7 mm Short Way Measurement (SWM): 3.3 mm Layer Thickness: 0.75 Example 2: 10 Material Thickness: 0.10mm Strand Width: 1.0mm LWM: 7.0 mm SWM: 3.6 mm Layer Thickness: 1.0 15 Example 3: Material Thickness: 0.15mm Strand Width: 1.0mm LWM: 7.0 mm 20 SWM = 3.6 mm Layer Thickness: 1.0 Example 4: Material Thickness: 0.9mm 25 Strand Width: 1.0mm LWM: 9.0 mm SWM: 4.4 mm Layer Thickness: 1.5 30 Referring now to Fig. 2, each aperture 16 of the layers 12, 14 partially overlaps more than one aperture 16 of the adjoined layer 14, 12 to form a network of voids 20 (shown as shaded regions) for holding active material. The network of voids 20 allows active material (not shown) to be positioned in contact with electrically conductive strands 18, 18' of the adjoined layers 12, 14 to provide a relatively large 35 contact surface area between the active material and the structure 10. The adjoined layers 12, 14 are orientated perpendicularly with each other with respect to the orientation of the major axis "M1" of the apertures 16. It is envisaged that arranging the layers 12, 14 so that the major axis "M1" of the apertures of -15 adjoined layers 12, 14 are orientated perpendicularly will improve the rigidity of the structure 10, which may in turn render a collector plate formed which incorporates the structure 10 less susceptible to shedding due to vibration, or provide other advantages. If shedding of the collector plate can be minimised then the negative 5 plate capacity of the battery remains strong as the active material will be attached to the current collector plate as opposed to either floating within or sitting on the base of the battery housing. As mentioned above, the network of voids 20 formed by the overlapping apertures 16 10 are arranged to receive therein active material which is typically in the form of a paste. The paste is applied to the structure 10 so as to "fill" the voids 20 and thus "load" the structure 10 with the active material. The adjoined layers 12, 14 of electrically conductive sheets are adjoined in 15 mechanical and electrical contact with each other interfacially. When arranged in this way, each section of strand 18, 18' extending between opposite vertices 17 crosses, and is in electrical contact with, at least one section of a strand 18, 18' of the other layer 12, 14. This arrangement means that each strand 18, 18' of a layer 12, 14 forms electrical contact with strands 18', 18 of the adjoined layer 14, 12 at plural 20 points along its length to provide a distribution of locations of mechanical and electrical contact. This is important because it provides a shorter and thus more direct current conduction path between active material held in the voids and the edges of the current collector by reducing the distance between the active material and the structure 10, which in turn reduces the resistance of the structure 10. The 25 reduction in resistance will tend to increase the utilisation of the collector plate (i.e. the amount of active material that can be reacted during use of the collector plate in a battery). Although the above described structure 10 includes two adjoined layers 12, 14, it is 30 possible that more than two adjoined layers may be used to form the structure. If additional layers are used, it may be necessary to increase the size of the apertures to ensure that active material is able to penetrate the structure 10. For fine gauge options where upwards of four layers are adjoined, the sheets may be 35 manufactured from "half hard" copper. If a softer material is used, the individual sheets may compress from stressed applied when the paste is loaded. This then -16 significantly reduces the thickness of the sheet and ultimately the performance of the structure when used in a current collector. Fig. 5A and Fig. 5B show views of a structure 500 according to another embodiment 5 of the invention. The structure 500 incorporates adjoined layers of electrically conductive sheets 502, 504, 506 including an array of apertures 508 having the same configuration as the layers 12, 14 forming the structure 10 described earlier. However, structure 500 includes three adjoined layers comprising a front layer 502, a rear layer 506, and an intervening layer 504 located between the front layer 502 and 10 the rear layer 506. The adjoined layers 502, 504, 506 are in mechanical and electrical contact with each other interfacially. As is shown more clearly in Fig. 6, each aperture 508 of the layers 502, 504, 506 partially overlaps more than one aperture 508 of the adjoined layer to form a network 15 of voids 510 (shown as shaded) for holding active material. The network of voids 510 allows active material (not shown) to be positioned in contact with electrically conductive surfaces of the layers 502, 504, 506 to provide a relatively large contact surface area between the active material and the structure. In the present case the electrically conductive surfaces includes surfaces of strands 512 forming the mesh of 20 the layers 502, 504, 506. Referring again to Fig. 5A, the front most layer 502 and rearmost layer 506 are orientated parallel with each other with respect to the orientation of the major axis of the apertures 508. However, the intervening layer 504 is orientated perpendicularly 25 with the front layer 502 and the rear layer 506 with respect to the orientation of the major axis of the apertures 508. It is envisaged that arranging the adjoined layers 502, 504, 506 so that the major axis of the apertures of adjoined layers are orientated perpendicularly will improve the rigidity of the structure 500, which may in turn render a collector plate formed which incorporates the structure 500 less susceptible to 30 shedding due to vibration, or provide other advantages. If shedding of the collector plate can be minimised then the negative plate capacity of the battery remains strong as the active material will be attached to the current collector plate as opposed to either floating within or sitting on the base of the battery housing. The structure 500 illustrated in Fig. 5A and Fig. 5B thus includes an arrangement of layers in which 35 immediately adjoined layers are orientated perpendicularly with each other with respect to the orientation of the major axis of the apertures.

-17 When a structure according to an embodiment of the present invention is used to form a plate, such as a negative electrode plate, for a battery, the structure is formed to the correct size (i.e. the size of the required plate or a part thereof). The edges of the sheets forming the structure are then joined or secured using a suitable process 5 to position and hold the sheets in the layered arrangement. Fig. 7A to Fig. 7C shows views of a negative electrode plate 700 according to an embodiment of the invention incorporating the structure 10 described earlier. As shown, the negative plate 700 includes an electrically conductive current collector 10 edging 702 extending entirely about the periphery of the structure 10 to form a conductive frame thereabouts. The current collector edging 702 comprises four metal extrusions or sections, such as lead plated copper extrusions, disposed about the periphery of the structure 10, though in other embodiments may be formed using other arrangements, such as a one-piece rectangular frame. As is more clearly 15 shown in Fig. 7B and Fig. 7C, each extrusion includes a channel 704, such as a C section channel, having a width (W) and depth (D) for receiving an edge of the layers 12, 14. It is to be noted that in other embodiments other forms or configurations of current collector edging 702 may be used. For example, the current collector edging may be moulded onto the edges of the layers 12, 14. 20 When received with the channel, each edge of the structure 10 is retained therein to form a mechanical and electrical termination therewith which extends substantially entirely around the length of the respective edge. The mechanical and electrical termination between the edges of the layers 12, 14 and the current collector edging 25 may be formed by any suitable process. One suitable process may include plating the edges of the layers 12, 14 with a fusible metal alloy (such as a 60/40 lead tin mix) and locating the plated edges into the channel 704 of the current collector edging 702. 30 To locate the edges of the layers 12, 14 within the channel it may be necessary to temporarily apply a compressive force to the layers 12, 14 to compress the edges to form a thickness which can be accommodated within the channel 704. Compressing the layers 12, 14 to locate the edges within the channel 704 may involve securing or holding the layers 12, 14 in place using a holding force applied to the layers using a 35 plate or press and then further compressing the edges using a compressive force to reduce the thickness of the edges to allow easier fitting within the channel 704. Whilst the edges of the layers 12, 14 are compressed, the current collector edging -18 702 is fitted over the edges of the layers 12, 14. After fitment, the compressive force is removed from the edges of the layers 12, 14 which may allow the edges to then expand against the inside surfaces of the channel 704. 5 In the present case, the channel 704 has an approximate wall thickness of 0.4mm. As shown in Fig. 7C, a separate electrically conductive member 706, such as a copper bar, may be located within the channel and disposed between an end wall of the channel and the edges of the layers 12, 14 to extend entirely around the current collector edging 702. The conductive member 706 preferably forms electrical contact 10 with one or more interior surfaces of the channel 704. This electrical contact may be formed by sizing the conductive member 706 to fit tightly within the channel 704 so that opposite upper and lower interior surfaces of the channel 704 urge against the conductive member 706 to thereby form an interference type fit. Alternatively, the conductive member 706 may be bonded to one or more interior surfaces of the 15 channel 704 using a suitable boding process to thereby form the electrical contact. In either case, the conductive member 706 will have a cross section area which contributes to increase the electrical conductivity of the current collector edging 702. In the embodiment illustrated, the conductive member 706 has a cross sectional area of about 3mm by 4mm and the conductive member 706 is bonded to interior surfaces 20 of the channel 704. Bonding the channel 704 to the conductive member 706 may involve lead dipping the channel 704 and the conductive member 706 and then applying heat to form a bond. It is to be noted that it is not essential that a separate conductive member 706 be 25 included since a similar result may be achieved by increasing the thickness of the end wall 708 of the channel 704. Once an edge of the structure 10 is located within a respective channel 704, a compressive force may be applied, by a press or the like, to compress the current 30 collector edging 702 to thereby form a mechanical joint between the channel 704 and the edge of the structure 10 located therein. Heat may then be applied to the current collector edging 702 at sufficient temperature to melt the 60/40 lead tin mix applied to the edges of the layers 12, 14 earlier to bond the edge of the structure 10 to the current collector edging 702. A supplementary or localised heat source (such as a 35 welding unit) may also be used to ensure that sufficient heat is provided to melt the 60/40 lead tin mix.

-19 Another embodiment of a negative electrode plate 800 is shown in Fig. 8A, Fig. 8B, and Fig. 8C. The configuration of the electrode plate 800 is generally similar to the electrode plate 700 depicted in Fig. 7A with the exception that the current collector edging 802 shown here is formed using separate frame members or straps 804, 804' 5 which are disposed on the edges of the structure 10 so that the structure 10 interposes the frame members, as is best shown in Fig. 8C. The frames members or straps 804, 804 may be bonded or joined to the edges of the structure 10 using a suitable bonding or joining process. 10 Fig. 9 illustrates a portion of a structure 900 in accordance with another embodiment of the invention. The structure 900 incorporates two electrically conductive sheets 902, 904. Each sheet 902, 904 includes an array of diamond shaped apertures having a major axis that extends vertically (when orientated as depicted) and a minor axis that extends horizontally (when orientated as depicted). The sheets 902, 904 15 are overlapped so that the minor axis of the diamonds in a particular overlapped row of the overlapped sheets extends coaxially and the major axis of the overlapped diamonds is offset horizontally. Such an arrangement creates an increased density of the structure without significantly increasing the thickness of the electrode plate as the two sheets 902, 904. 20 An advantage of a structure in accordance with an embodiment of the present invention is that it provides performance comparable to a foam structure, but at a reduced manufacturing cost and with improved mechanical strength and durability. In addition, embodiments of the present invention may provide an increased pastable 25 volume as compared to foam structures which may improve the ability of the paste to thoroughly penetrate the structure during a pasting process.

- 20 2 10 Partial State of Charge - 4 Layer Mesh Vs Single Layer Mesh 2050 #Single Layer Mesh LR 4 Layer Mesh LR 20 M M Single Layer Mesh HR _ A 4 Layer Mesh HR '1950 o190D 1850 A 18W 1750 0 20 40 60 80 100 120 140 160 180 Number of Cydes Graph I Graph 1 shows the result of a test in which the cells started from fully charged and 5 were discharged at a defined rate for a specified period of time. The cells were then recharged to 2.4V. This cycle was then repeated until the cell reached 1.8V. The test was performed on a standard single mesh plate and on an equivalent 'multi mesh' plate (4 layers of a fine expanded mesh grid). The test was completed at two different discharge rates (HR= high rate, LR= low rate) and the results show a significant 10 increase in the number of cycles that were completed before the voltage was reduced to 1.8V, particularly at low rates of charge where the cell did not reach 1.8V in over 150 cycles. It will be appreciated by persons skilled in the art that numerous variations and/or 15 modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 20

Claims (18)

1. A structure for forming a current collector for an electrochemical cell, the structure including a plurality of adjoined layers of electrically conductive sheet, each sheet including an array of apertures arranged to partially overlap more 5 than one aperture of each immediately adjoining sheet to form a network of voids for holding an active material.

2. A structure according to claim 1 wherein the network of voids provides a greater density of apertures having a smaller size relative to the apertures of 10 the sheets.

3. A structure according to claim 1 or 2 wherein adjoined electrically conductive sheets electrically contact each other interfacially. 15 4. A structure according to any one of claims 1 to 3 wherein the apertures of each array have substantially the same size and shape.

5. A structure according to claim 4 wherein the partially overlapping apertures of immediately adjoining sheets are laterally offset relative to each other to form 20 the partial overlapping relationship therebetween.

6. A structure according to claim 4 wherein the partially overlapping apertures of immediately adjoining sheets are orientated at an angle with respect to each other to form the partial overlapping relationship therebetween. 25

7. A structure according to any one of claims 1 to 4 wherein the partial overlapping relationship between the apertures of immediately adjoining sheets is established by providing a difference in position and/or orientation between the arrays of apertures of the adjoining sheets relative to each other. 30

8. A structure according to any one of claims 1 to 7 wherein each electrically conductive sheet comprises an expanded mesh.

9. A structure according to any one of claims 1 to 7 wherein each electrically 35 conductive sheet comprises a honeycomb structure. - 22 10. A structure according to any one of claims 1 to 8 wherein the apertures are generally diamond shaped. 5 11. A structure according to claim 10 wherein the diamond shaped apertures have a major axis and a minor axis, the sheets being arranged so that the diamonds of adjacent sheets overlap with the minor axis of the overlapped diamonds extending coaxially and the major axis being offset. 10 12. A structure according to claim 10 wherein each diamond shaped aperture include diagonals of different lengths.

13. A structure according to claim 1 or claim 12 wherein each array includes a uniform distribution of apertures having a major axis and a minor axis, and 15 wherein immediately adjoining sheets are orientated generally perpendicularly with each other with respect to the orientation of the major axis of the apertures to improve the rigidity of the structure.

14. A structure according to any one of claims 1 to 13 wherein at any location 20 within a void the length of a current conduction path to the structure when active material is held in the void is less than about 2mm.

15. A structure according to any one of claims 1 to 14 wherein each electrically conductive sheet includes a metal sheet having a thickness of between 0.1mm 25 and 0.5mm.

16. A structure according to claim 15 wherein the metal sheet is an expanded metal mesh. 30 17. A structure for forming a current collector for an electrochemical cell, the structure being formed from plural overlaid sheets of expanded metal mesh, each sheet including a uniform arrangement of apertures of substantially the same size and shape, each aperture having a major axis and a minor axis extending perpendicular to the major axis, wherein adjoining overlaid sheets 35 are orientated generally perpendicularly with each other with respect to the orientation of the major axis of the apertures to improve the rigidity of the structure, and wherein the apertures are arranged to partially overlap more - 23 than one aperture of each adjoining layer to form a network of voids for holding an active material.

18. A structure according to claim 17 wherein the apertures are generally diamond 5 shaped.

19. A structure according to claim 18 wherein each diamond shaped aperture include diagonals of different lengths. 10 20. A structure according to any one of claims 17 to 19 wherein at any location within a void the length of a current conduction path to the structure when active material is held in the void is less than about 2mm.

21. A structure according to any one of claims 17 to 20 wherein each sheet of 15 expanded metal mesh has a thickness of between 0.1mm and 0.5mm.

22. A structure substantially as hereinbefore described with reference to any of the accompanying drawings. 20 23. An electrode plate including one or more structures according to any one of claims 1 to 22.

24. A current collector including a structure according to any one of the preceding claims 25

25. A battery including at least one electrode plate according to claim 23.