WO2000008704A1

WO2000008704A1 - Wound lead acid battery with non-circular cells

Info

Publication number: WO2000008704A1
Application number: PCT/US1999/017212
Authority: WO
Inventors: Frank Fleming
Original assignee: Hawker Energy Products, Inc.
Priority date: 1998-08-06
Filing date: 1999-08-05
Publication date: 2000-02-17
Also published as: AU5326699A

Abstract

A cell for a lead acid battery comprises: at least one cathode layer of lead-containing material; at least one anode layer of lead-containing material; at least one separator layer sandwiched between the cathode and anode layers that comprises glass fiber and polymeric material; and a casing. The cathode, separator and anode layers are arranged in overlying spirally wound relationship to form a cell element which takes a non-circular shape. The casing is also non-circular and snugly encases the cell element. In this configuration, the cell can be combined with other cells of the same configuration within a housing to form a lead acid battery that occupies less area than a lead acid battery of the same voltage and capacity that includes circular cells.

Description

WOUND LEAD ACID BATTERY WITH NON-CIRCULAR CELLS

Field of the Invention

The present invention relates generally to batteries and more particularly to lead acid batteries.

Background of the Invention A typical battery includes one or more electrochemical cells which are electrically connected within the battery and provide the source of electrical power for the battery. These cells generally comprise four basic components: a positive electrode, or anode, that receives electrons from an external circuit as the cell is discharged; a negative electrode, or cathode, that donates electrons to the external circuit as the cell is discharged; an electrolyte (often in a solution or paste) which provides a mechanism for electrical charge to flow between the positive and negative electrodes; and one or more separators which electrically isolate the positive and negative electrodes.

Battery performance can be measured by two basic parameters: voltage and capacity. Voltage is the electrical "force" that induces electrons to travel along a defined electrical path through the aforementioned external circuit. The magnitude of the voltage is determined by the cell chemistry and the quantity of electrochemical, separator and electrolyte materials included in each cell. As an example, lead acid batteries typically provide about 2 volts per cell; batteries having voltages higher than this figure typically include multiple interconnected cells.

A battery's capacity is the "number" of electrons that the battery can provide to the external circuit over the lifetime of the battery. As such, cell capacity is a measure of the magnitude of the electrical current supplied by a cell over time, and is often reported in ampere-hours. Not surprisingly, a cell's capacity depends upon the quantity of active materials the cell contains. As one might imagine, capacity is ordinarily more variable from cell to cell than is voltage. A third parameter often used to describe battery capability and performance is "energy density". Energy density is the total available energy stored within a cell (Le., the cell capacity) as a function of the weight or volume of the cell.

One type of battery that is popular when rechargeability is desired is a lead acid battery. These batteries are particularly desirable for rechargeable use due to their high tolerance for abuse and relatively low manufacturing cost, particularly when battery weight is not a great concern. As a result, lead acid batteries are often employed to power automobiles and trucks, as these environments can be quite harsh and present varied forms of maltreatment. Lead acid batteries are also often used in backup systems that provide power when an electrical power grid fails.

Most lead acid batteries generally rely on the same basic electrochemical reaction to produce power and typically employ the same basic active materials. The electrochemical reaction is shown below:

ANODE

PbO₂ + SO₄ ^"2 + 4H⁺ + 2e → PbSO₄ + 2H₂O

CATHODE

Pb + SO₄ ^"2 → PbSO + 2e^"

At the anode, lead dioxide (PbO₂) reacts with sulfate ion (SO ^2") and is converted to lead sulfate (PbSO₄). At the cathode, metallic lead reacts with sulfate ion (SO₄ ^2" ) and is also converted to lead sulfate. Electrons are donated by the cathode and travel through the external circuit to be received by the anode. In practice, a typical lead acid battery includes overlying anode and cathode layers. Most often, these are arranged in one of two configurations: stacked plates or spirally wound elongate strips. In either instance, the anode and cathode layers are separated from each other by separator layers formed of an electrically- insulative material (typically a glass fiber mat or the like). A dilute sulfuric acid solution is typically used as the electrolyte to provide the sulfate ion. A conventional stacked plate lead acid battery 10 having anode plates 11, cathode plates 12 and separators 13 within a casing 14 is shown in Figures 1 and 2. Figure 3 shows a typical wound cell element 20' having in spirally wound relationship an anode layer 21 with an attached separator layer 22 and a cathode layer 23 with an attached separator layer 24; these are spirally wound as shown in Figure 3 and housed within a casing 25 to fomr a cell 20 (shown in Figure 5). A battery 26 that includes six spirally wound cells 20 is also shown in Figure 5. Lead acid batteries constructed from cells of stacked plates face some manufacturing and performance issues. First, achievement of a high volume production rate for stacked plate cells typically requires a high capital equipment cost. The high costs are due to the large number of discrete plates and separators that typically are produced and then accurately positioned for proper operation of each battery. Second, a stacked plate battery typically requires that the battery container end walls be relatively stiff so that plate-to-plate spacing and separator compression are maintained, as these spacing factors can be critical to the performance and life of lead acid batteries. However, a rectangular shape of a cell container that houses stacked plates is generally a disadvantageous configuration for a pressure-exerting housing. Thus, controlling the container wall deflection at elevated operating temperatures such that adequate compression of multiple numbers of plates and separators is maintained can be difficult, even under low loads.

These shortcomings can be reduced when lead acid batteries are constructed with wound cell elements. As is shown in Figures 3 and 5, these batteries normally consist of multiple wound cells that are connected in series to achieve a desired voltage. The wound cells comprise far fewer separate electrode and separator layers than the layered plates of a stacked plate cell, so the manufacturing cost of handling numerous layers of discrete plates is reduced or eliminated. Also, with wound cell elements, plate to plate spacing and separator compression can be more easily controlled during the winding process. The wound product can then be inserted with a snug fit into a cylindrical plastic container (a shape for a pressure vessel that is much preferred to that of the rectangular box of a stacked plate cell); thus, plate-to-plate spacing and separator compression at elevated temperatures can be more easily maintained. In addition, the wound cell can also possess superior shock and vibration characteristics as compared to a stacked cell. Unfortunately, the energy density of a battery constructed from multiple cylindrical cells is not favorable when compared to a battery of equivalent capacity formed of stacked plate cells. This is due to the greater "empty" volume between the cylindrical walls of individual cells once they are inserted into a battery container. As such, there is a need for lead acid batteries formed from spiral wound cells that can provide an improved energy density.

Summary of the Invention

In view of the foregoing, it is an object of the present invention is to improve the energy density of traditional wound lead acid cells and batteries without negatively impacting manufacturing and performance advantages that such cells and batteries can provide.

It is another object of the invention to provide a lead acid battery formed from wound cells with a higher energy density than presently possible without increasing the actual cell element area.

It is a further object of this invention to provide a lead acid battery formed of wound cells with a high energy density while decreasing the volume of the battery.

These and other objects are achieved by the present invention, which is directed to a wound lead acid battery formed of wound cells that include non- circular spiral wound cell elements. A cell for such a lead acid battery comprises: at least one cathode layer of lead-containing material; at least one anode layer of lead-containing material; at least one separator layer sandwiched between the cathode and anode layers that comprises glass fiber and polymeric material; and a casing. The cathode, separator and anode layers are arranged in overlying spirally wound relationship to form a cell element which takes a non-circular shape. The casing is also non-circular and snugly encases the cell element. In this configuration, the cell can be combined with other cells of the same configuration within a housing to form a lead acid battery that occupies less area than a lead acid battery of the same voltage and capacity that includes circular cells.

In a preferred embodiment, the separator layers are formed of material that comprises between about 5 and 20 percent coarse glass fiber, 60 and 90 percent fine glass fiber, and 5 and 40 percent polymeric material. It is also preferred that the separator layers have a puncture strength of at least 700 to 1 ,000 grams (per Battery Council International approved test methods) to enable them to be deformed into a non-circular shape within damaging the cathode and anode layers. The cells and batteries of the present invention can be produced by a method comprising the steps of: positioning a lead-containing cathode layer, a separator layer, and a lead-containing anode layer in overlying relationship, wherein the separator layer comprises glass fibers and polymeric material; spirally winding the cathode, separator and anode layers into a circular cell element; forming the circular cell element into a non-circular configuration; and inserting the non-circular cell element into a casing such that the cathode, separator and anode layers fit snugly therewithin. Preferably, the forming step is carried out by pressing opposite sides of the cell element such that the cell element takes the aforementioned non-circular shape.

Brief Description of the Drawings

Figure 1 is a cross-sectional view of a prior art single-cell sealed-lead acid battery.

Figure 2 is a cut-away isometric view of the stacked plate cell assembly found in the battery of Figure 1. Figure 3 is a top section view of a prior art circular spiral wound cell element.

Figure 4 is a top section view of a wound cell element for a battery of the present invention.

Figure 5 is a top section view of the prior art wound cell battery of Figure 3. Figure 6 is a top section view of a battery including a plurality of wound cell elements of Figure 4.

Detailed Description of Preferred Embodiments

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art

Referring now to the drawings, a cell element of the present invention, designated broadly at 40', is lllustiated in Figure 4 The cell element 40' includes a positive electrode layer 41 and a negative electrode layer 42, each of which is attached in overlying contacting relationship to a respective separator layer 43a, 43b As is shown m Figure 4, these layers are arranged in overlying, non-circular, spirally wound relationship such that one of the separator layers 43a, 43b is positioned between the positive and negative electrodes 41, 42 at all points within the spiral to prevent contact between the positive and negative electrode layers 41, 42

The cell element 40' is placed within a casing 45 (typically formed of a suitable polymeric mateπal such as polyethylene) that encases the cell element 40', the combination of the cell element 40' and the casing 45 forms a cell 40 (see Figure 6) The casing 45 includes upπght walls 46 rising from a non-circular floor 47 Once the cell element 40' is positioned within the casing 45, the casing 45 (Figure 6) is filled with an electrolyte solution that enables the cell 40 to operate

As used herein, "overlying" relationship means that a layer is in contact with and extends m all directions along substantially the same path as an immediately adjacent layer, it is not intended that the term be limited to a horizontal orientation of the layers, vertical orientations and orientations between hoπzontal and vertical are also encompassed by this term The term "spirally wound" in reference to a layer means that the layer defines a path about a central point in which, for a given angle relative to an imaginary baseline that passes through the central point, subsequent layers increase in distance from the central point The term is intended to include "non-circular" spiral paths, such as those m which the path formed by a layer is generally elliptical, oblong or oval in shape as well as paths in which a circular, elliptical or oval shape is "flattened" somewhat, such as by the application of pressure from opposite sides The term "non- circular" can also include paths in which the layers are generally square or rectangular, but with rounded corners (e_g_, for a cell having a width of 2 0 inches and a depth of 3 7 inches, the radius of curvature of the "rounded" corner may be between about 0 25 and 0 5 inches) Those skilled m this art will recognize that other non-circular configurations may also exist and be suitable for use with the present invention.

Both the positive and negative electrode layers 41, 42 are formed of lead- containing materials. As used herein, "lead-containing material" means that the material contains at least 50 percent lead by weight. Although each of the electrodes 41, 42 contains lead, they should be formed of different lead-containing materials. As an example, in one embodiment both electrode layers 41, 42 comprise an open-meshed lead grid covered with a lead oxide/lead sulfate paste; however, both the grids and the paste differ for the positive and negative electrodes. More specifically, in this embodiment the positive electrode layer 41 comprises a lead or lead alloy grid covered with a lead compound paste, and the negative electrode layer 42 comprises a lead or lead alloy grid covered with a lead compound paste that includes surface area-enhancing additives. The materials and thickness of the electrode layers 41, 42 should be selected such that the electrode layers 41, 42 have sufficient malleability to be able to be formed into the desired spirally wound configuration, including withstanding both the spiral winding of the electrodes 41, 42 and any post-winding reshaping that may occur. Also, the electrode layers 41, 42 should be of a thickness (preferably between about 0.5 and 2.0 mm, and typically about 1 mm) that enables them to provide the desired electrochemical and physical properties for operation. Those skilled in this art will recognize that other lead-containing materials may also be employed as the electrode layers in accordance with the present invention.

The separator layers 43a, 43b comprise a blend of glass fibers and polymeric materials formed into a sheet. The glass fibers are typically microfibers, and preferably are a blend of coarse microfibers (i.e.. fibers having a diameter of greater than 1.5 microns) and fine microfibers (i.e.. fibers having a diameter of less then about 1.5 microns). The coarse and fine glass fibers are preferably blended in a ratio of about 5 to 40 percent to one another by weight i^, a ratio of coarse fiber to fine fiber of between about 1 :20 to 2:3). It is also preferred that the surface area of the glass in the composite separator be between about 0.5 and 5 m /g. The polymeric material of the separator layers 43a, 43b can be any polymeric material that serves to bind the glass fibers of the separator layers 43a, 43b and, by doing so, improve the tensile strength, resilience and puncture strength of the separator layers 43a, 43b. These properties can be important in enabling the separators 43a, 43b to withstand the winding and subsequent shaping into a non- circular configuration and to provide increased service life to the battery. Preferably, the polymeric material of the separtors 43a, 43b is in fiber form. Exemplary polymeric materials include polyester, polyolefms such as polyethylene and polypropylene, and derivatives, blends and copolymers thereof. The entire polymeric fiber may be formed of a single material, or it may comprise a bi- component structure, such as a core/sheath structure. In one preferred embodiment, the polymeric fiber comprises a polyester or polyolefin core and a polyester sheath. The polymeric fiber should comprise between about 5 and 40 percent of the separator layers 43a, 43b, and more preferably between about 5 and 25 percent, by weight.

It should be noted that the polymeric material of the separator may also take the form of sheets that sandwich a layer of glass fiber. In this configuration, the polymeric layers can prevent the glass fibers from protruding into and damaging the cathode and anode layers 41, 42.

Exemplary separator materials include HOVOSORB® IIP-15 (grades BG180GB117 and BG200GB117) which are proprietary materials available from Hollingsworth & Vose Co. (West Groton, Massachusetts) that comprise about 15 percent polyester fiber. This material has a puncture strength of about 900 g and a tensile strength of about 6.5 lb/in (per Battery Council International testing procedures). Those skilled in this art will recognize that other separator materials may also be suitable for use with the present invention. Preferably, the selected separator material has a puncture strength of at least 700 to 1,000 g and and a tensile strength of at least 5 lb/in.

The separators 43a, 43b may be formed in a wet-laid process as a mat using a method similar to that used to make paper. In this method, a fibrous, water-based slurry, containing both the glass microfibers and the polymeric fibers, is released onto a moving belt. The belt allows water to drain from the slurry, resulting in a wet mat similar to wet paper. This mat is heat treated at a temperature sufficient that the polymeric fiber melts and fuses with and binds the glass microfibers, resulting in a much stronger mat. Typically, this temperature is sufficient to melt the polymer, and cause the surface of the mat to become tacky. The tackiness of the mat assists in creating and maintaining the attachment of the separators 43a, 43b to the positive and negative electrode layers 41, 42 during subsequent processing.

The cell element 40' can be formed into its non-circular, spirally wound configuration by first forming sandwich units of layers, in which the positive and negative electrode layers 41, 42 are attached to respective separator layers 43 a, 43b. The sandwich units can then be spirally wound into the cylindrical cell element 40' illustrated in Figure 3. This process can be carried out on a circular arbor or by other methods known to those skilled in this art. Once the spirally wound cell element 40' has been formed, it can be pressed (for example, with a pair of reciprocating platens) such that it deforms into a desired non-circular shape. The malleability of the electrode layers 41, 42 enables the cell element 40' to maintain its non-circular shape even after it is removed from the platens. The pliability of the separators 43a, 43b resulting from the presence of the polymeric fiber enables them to undergo this deformation without puncturing or otherwise damaging the electrodes. From this point, a cell 40 is formed by inserting the spirally wound cell element 40' into the casing 45, which preferably is sized and configured to substantially match the outer surface of the spirally wound assembly and, more preferably, is sized slightly smaller than the outer surface of the spirally wound assembly so that the walls of the casing exert pressure on the layers of the assembly, as such a configuration typically improves cell performance. Electrolyte is then added, and the casing 45 is sealed (typically with a vented structure to allow for the safe release of excessive cell pressure).

Although the cell element 40' illustrated in Figure 4 includes two sandwich units therein, those skilled in this art will recognize that additional sandwich units of electrode and separator layers can be included. Alternatively, one or more sandwich units comprising in sequence an anode layer, a separator layer, a cathode layer, and another separator may also be used. Also, manufacturing and assembly methods other than that described hereinabove may also be used. For example, the sandwich assembly of layers may be wound around an oval, elliptical, rectangular, or other oblong-shaped mandrel to achieve a desired non-circular configuration without the pressing step noted above. After this step, there may or may not be further shaping of the spirally wound layers in order to produce a desired final shape. In addition, the spirally wound assembly may be pressed in multiple directions; for example, the spirally wound assembly may be pressed in orthogonal directions to form a "rectangle" with rounded corners as described hereinabove. Other manufacturing techniques suitable for use with the present invention should be known to those skilled in this art.

A plurality of cells 40 can be employed in a battery 50 (see Figure 6). As shown, the battery 50 includes a housing 51 within which the cells 40 reside. Illustratively and preferably, the housing 51 has a rectangular footprint. Of course, the cathode layers 41 of each of the cells 40 are electrically interconnected in series (as are the anode layers); these in turn are interconnected to terminals to provide power to an external source.

Notably, the cells 40 can be packed into the rectangular housing 51 more tightly (Le_^, in greater density) than prior art circular cells. This can be demonstrated by reference to Figures 5 and 6. The length L' of the cell 40 of the present invention, as shown in Figure 6, is greater than the length L of the prior art cells shown in Figure 5. Correspondingly, the width W' of the cell 40 of the present invention is less than the width W of the prior art cell. However, because there is less "unoccupied" space in the battery 50, the same number of cells 40 (which corresponds to the same power output) can be included with the battery 50 as with the prior art battery in less total housing area.

For comparative purposes, a circular cell 20 having a diameter of 1.93 inches can be compressed such that it has a length of 1.82 inches and a width of 1.76 inches. With these dimensions, the six prior art cells 20 shown in Figure 5 can fit in a 2 by 3 pattern within a housing having dimensions of 3.86 inches by 5.79 inches (Le., an overall area housing area of 22.35 sq. inches). In contrast, six cells 40 of the present invention can fit into a housing having dimensions of 3.52 inches by 5.46 inches (i.e.. an overall housing area of 19.22 sq. inches). Thus, the overall area (and, for cells of similar height, the overall volume) occupied by the battery 50 would be about 15 percent less than that of a prior art battery of similar power (primarily due to the reduced amount of unoccupied space within the housing ), even though each provides the same voltage. The difference in area may be greater in other configurations, as the cell 40 may be shaped into non- circular shapes that have even greater space advantage. Also, if a compressing step is employed to form the cell into a non-circular shape, the compression itself may also contribute to reducing the area occupied by each cell.

The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein. In the claims, means- plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.

Claims

That Which is Claimed is:

1. A cell for a lead acid battery, comprising: at least one cathode layer of lead-containing material; at least one anode layer of lead-containing material; at least one separator layer sandwiched between said cathode and anode layers, said separator layer comprising glass fiber and polymeric material; said cathode, separator and anode layers being arranged in overlying spirally wound relationship to form a cell element, said cell element taking a non- circular shape; and a non-circular casing snugly encasing said cell element.

2. The cell defined in Claim 1 , wherein said separator layer comprises between about 60 and 95 percent glass fiber by weight and between about 5 and 40 percent polymeric fiber by weight.

3. The cell defined in Claim 1, wherein said polymeric material is polymeric fiber and is selected from the group consisting of: polyolefms, polyesters and co-polymers and blends thereof.

4. The cell defined in Claim 1 , wherein said polymeric material is a polymeric fiber and comprises an internal core and an external sheath, and wherein said core comprises a first polymeric material and said sheath comprises a second polymeric material.

5. The cell defined in Claim 4, wherein said first polymeric material comprises polyester, and said second material is selected from the group consisting of polyester and polyolefms.

6. The cell defined in Claim 1 , wherein said cell element takes an oblong shape.

7. The cell defined in Claim 1, wherein said casing has a floor and side walls extending upwardly therefrom, and said floor is oblong.

8. The cell defined in Claim 1 , wherein said floor is oval.

9. The cell defined in Claim 1 , wherein said separator layer has a puncture strength of at least 700 g.

10. A method of forming a cell for a battery, comprising the steps of: positioning a lead-containing cathode layer, a separator layer, and a lead- containing anode layer in overlying relationship, wherein said separator layer comprises glass fibers and polymeric material; spirally winding said cathode, separator and anode layers into a circular cell element; forming said circular cell element into a non-circular configuration; and inserting said non-circular cell element into a casing such that said cathode, separator and anode layers fit snugly therewithin.

11. The method defined in Claim 10, further comprising the step of adding electrolyte to said casing.

12. The method defined in Claim 10, further comprising the step of heating said casing and said cathode, separator and anode layers to a temperature sufficient to soften said polymeric fibers of said separator such that said polymeric material serves to bind said glass fibers within said separator.

13. The method defined in Claim 10, wherein said separator has a puncture strength of at least 700 g.

14. The method defined in Claim 10, wherein said forming step comprises pressing on a first set of opposite sides of said cell element.

15. The method defined in Claim 14, wherein said forming step comprises repeating said pressing on a second set of opposite sides of said cell element that is orthogonal to said first set of opposite sides.

16. A lead acid battery including a plurality of cells positioned within a housing, each of said cells comprising: at least one cathode layer of lead-containing material; at least one anode layer of lead-containing material; at least one separator layer sandwiched between said cathode and anode layers, said separator layer comprising glass fiber and polymeric material; said cathode, separator and anode layers being arranged in overlying spirally wound relationship to form a cell element, said cell element taking a non- circular shape; and a non-circular casing snugly encasing said cell element.

17. The battery defined in Claim 16, wherein said separator layer comprises between about 60 and 95 percent glass fiber by weight and between about 5 and 40 percent polymeric material by weight.

18. The battery defined in Claim 16, wherein said polymeric material is polymeric fiber selected from the group consisting of: polyolefms, polyesters and co-polymers and blends thereof.

19. The battery defined in Claim 16, wherein said polymeric material is polymeric fiber comprising an internal core and an external sheath, and wherein said core comprises a first polymeric material and said sheath comprises a second polymeric material.

20. The battery defined in Claim 19, wherein said first polymeric material comprises polyester, and said second material is selected from the group consisting of polyester and polyolefins.

21. The battery defined in Claim 16, wherein said cell element takes an oblong shape.

22. The battery defined in Claim 16, wherein said casing has a floor and side walls extending upwardly therefrom, and said floor is oblong.

23. The battery defined in Claim 16, wherein said floor is oval.

24. The battery defined in Claim 16, wherein said separator layer has a puncture strength of at least 700 g.