This Application is related to electric storage batteries and more specifically to low-cost high-power batteries.
Batteries can be either primary or secondary type, with the secondary type having the capability to be charged and discharged repeatably. There are a number of different types of secondary battery technologies. One of the oldest and most widely used is the lead acid storage battery. Lead acid storage batteries can use either a monopolar, bipolar design (alternate configurations exist, such as a so-called quasi-bipolar design). Monopolar lead acid storage batteries generally have a structure in which spaces in lead grid plates are filled with a paste referred to as the active materials, containing a formulation of lead, acid and special additives in the form of a viscous clay, having a consistency akin to cement or concrete. In a monopolar battery, the grid plates are stacked in alternating arrangement with tabs on alternate plates connected to plus or minus electrodes with lead.
A battery with a bipolar configuration is known to be advantageous over the conventional monopolar configuration in terms of power output. In a conventional battery with a monopolar configuration, current generated by active materials travels to a current collector and through a lead interconnect (known as top lead) to reach the next cell. In a bipolar configuration, active materials of opposite polarities are placed on the two surfaces of a bipolar substrate. Current can thus flow through the substrate to the next cell. Because of a much shorter electrical path, power loss due to ohmic drop in the circuit is minimized. The volume of the battery is reduced due to elimination of the outer circuit materials such as straps, posts and tabs. In one bipolar battery design a porous bipolar plate is sandwiched between two acid glass mats (AGM). Each AGM is a glass fiber matt that absorbs the acid. This structure is in turn sandwiched between a positive active material (PAM) and a negative active material (NAM).
In a bipolar lead/acid battery, the role of the substrate is paramount. The substrate serves as an inter-cell connection and as a support to active materials. The substrate also provides seals between the individual cells and isolates the cells from each other. The substrate must retain its electrical conductivity in the corrosive lead/acid environment and break communication of electrolyte in adjacent cells through the service life of the battery. Furthermore, the substrate may not participate in or provide alternative routes to the battery reactions. These requirements call for a substrate that is electrically conductive, insoluble in sulfuric acid, stable in the potential window of the battery, having high oxygen and hydrogen overpotentials. Furthermore, the substrate should be inert to battery reactions, impervious to the electrolyte, have good adhesion to the battery active materials, and be easy to process and seal to the battery case.
British Patent Number 226,857 describes an early bipolar lead/acid battery that used lead sheet as the substrate. The active materials were formed on the substrate. The sheets were stacked between U-shaped rubber gaskets. This type of bipolar lead/acid battery had problems with sealing, substrate corrosion, and lack of capacity. Another bipolar battery configuration used a titanium sheet glued onto a lead sheet with conductive adhesives. Titanium tends to passivate at the potential of a lead-acid battery and attempts to protect it with a coating of gold or lead add weight or cost.
Other configurations have used gold-plated titanium or conductive plastic as the substrate material. This latter configuration was given up due to difficulties in making a pore-free gold plate and working the titanium into a usable configuration. Attempts to use commercially available conductive epoxides with conductive fillers (e.g., carbon, graphite, copper, or silver) were unsuccessful due to reaction with the cell environment. Attempts to use vitreous carbon powder as an epoxy filler were initially successfully. Unfortunately, attempts to paste and form active materials on this substrate were unsuccessful due to interfacial corrosion on the positive side of the substrate.
Attempts to find suitable ceramic materials for use as substrate indicated that very few of the ceramic materials screened possess desired properties to be applicable in a bipolar substrate for a lead/acid battery. Among 120 candidates, silicides of Ti, Nb and Ta were identified as acceptable for composite substrates. However, fabrication of a workable substrate using such materials would require texturing or lamination with a lead foil to interface the substrate and active materials is necessary to improve paste adhesion. See Weng-Hong Kao, in “Substrate materials for bipolar lead/acid batteries” in Journal of Power Sources, 1998, Elsevier Science S.A, pp 8-15.
Thus, there is a need in the art, for an improved bipolar lead-acid battery that overcomes the aforementioned drawbacks.
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention.
The main problems with both types of lead-acid battery designs are low materials utilization, high weight and corrosion of the components by the acid. These two problems are related. The acid tends to corrode the grid in a monopolar battery. Lead is used because it is corrosion resistant, which means that it corrodes relatively slowly. However, since lead does corrode, albeit slowly, relatively thick lead plates are used to provide sufficient lifetime for the battery. This increases the weight and cost of the battery.
The bipolar design reduces the amount of lead by eliminating the grids and using a bipolar plate to separate the positive and negative sides of the battery. However, the bipolar plate is likely to corrode. To resist corrosion, the bipolar plate can be made thicker, but this defeats the benefit of low weight.
U.S. Pat. No. 4,658,499 to Rowlette describes a bipolar battery substrate in which conductive spherical elements are inserted into apertures in a thermoplastic resin. Heat and pressure are applied to the resin to deform the spherical elements into an approximately cylindrical shape and to melt, stretch and compress the surrounding plastic resin to provide a liquid impermeable sheath around the element. Because this substrate uses a thermoplastic resin, the apertures must be formed before inserting the spherical elements and the thermoplastic resin must be partially melted to seal the elements to the resin. Unfortunately, melting the plastic produces inadequate surface bonding of the elements to the plastic. This, in turn, can lead to permeation of liquid across the substrate, which can destroy the function of the battery.
Embodiments of the present invention overcome the problems with prior art bipolar batteries through the use of a novel bipolar battery substrate architecture in which conductive pellets are embedded into a thermoset polymer matrix. Electrical conduction takes place across the matrix through the pellets. Because pellets are spaced apart by the matrix there is little conduction parallel in the matrix (the active materials and lead foil transport current laterally to get to the spheres). The relatively large area of the pellets provides sufficient conductance (i.e., relatively low internal resistance). Use of a thermoset polymer formed from a liquid precursor provides good adhesion between the pellets and the matrix, thereby reducing permeation of active materials or electrolyte across the substrate.
As used herein the term “thermoset polymer” refers to a polymer that is irreversibly cured. Such polymers are sometimes known as thermosetting plastic, or simply as a thermoset. The cure may be done through application of heat (e.g., above 200° C. (392° F.)), through a chemical reaction (as in a two-part epoxy, for example), or through suitable irradiation such as ultraviolet light or electron beam. As is generally understood, thermoset polymers are can be formed from a liquid or malleable precursor prior to curing and designed to be molded into their final form, or used as adhesives. Other thermoset polymers are solids like that of the molding compound used in semiconductors and integrated circuits (IC's). The term “thermosetting polymer” or “thermosetting polymer precursor” is sometimes used to describe a polymer precursor used to form a thermoset polymer. Thermosetting polymer precursors may be in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing through application of heat, chemical reaction, or suitable radiation to the precursor.
Thermoset polymers and their precursors are distinguished from thermoplastics in that a thermoplastic, also known as a thermosoftening plastic, turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently.
As shown in
By way of example, and not by way of limitation, the foils 106 on either side of the matrix 102 can be about 50 microns thick and the matrix can be about 300 microns thick. The pellets 104 can be made of any suitable electrically conducting material, e.g., lead, or amorphous (glassy) carbon or amorphous metal. An advantage to the use of amorphous carbon pellets is that the amorphous carbon material would not have crystal domains and therefore lack of grain boundaries which are the weak points for the acid in a battery environment to corrode through the pellets. It is generally desirable for the material of the pellets to be resistant to corrosion in the acid environment of a storage battery. Resistance to corrosion is a relative term and can generally be understood in terms of the rate of corrosive reaction of the material (e.g., pure lead has a lower corrosion rate in sulfuric acid than lead alloys) and the overall dimensions of the pellets 104 (e.g., in a given corrosive environment, a thicker pellet takes longer to corrode than a thinner pellet made of the same material).
Amorphous carbon or vitreous pellets can be advantageous in spite of apparent disadvantages. Specifically, Vitreous carbon is both higher cost and has higher resistance than lead. Glassy carbon has a resistivity of 0.1 ohm-cm compared to 0.00002 ohm-cm for lead spheres. The cost of glass carbon is about 100 times higher than lead spheres on a per Kg basis. However, the disadvantage of higher resistivity can be overcome by using a sufficiently large number of amorphous carbon pellets of sufficiently large diameter so that the overall resistance of the composite substrate 100 is small. The larger diameter pellets would also take longer to corrode due to the lower reaction rate and larger size. These advantages can outweigh the cost if the resulting batter is both lighter and longer lasting.
To reduce weight, the pellets 104 may be sparsely distributed across the surface of the membrane 102. The weight can be greatly reduced since the pellets can take up less than 5% (e.g., about 1%) of the volume of the matrix. By way of example, and not by way of limitation, a volumetric ratio of the pellets to the membrane may be greater than 0% to about 10%, and preferably about 0.5% to about 1%. Even such a sparse distribution of the pellets 104 can offer a significant electrical conductivity advantage over an isotropic conductive film filled with powder fillers, such as vitreous carbon powder. The size of the conductive pellets 104 may range from about 0.1 millimeter (mm) to about 2 mm, preferably from about 0.3 mm to about 1 mm, depending on the corrosion resistance of the pellets.
According to embodiments of the invention, the bipolar substrate 100 can be used in a bipolar battery, e.g., a bipolar lead-acid battery 200 as shown in
As seen in
Embodiments of the present invention include a method for fabricating a substrate for a bipolar battery of the type described above. An example of such a method is illustrated in
Once the pellets are embedded in the precursor layer 302, heat and/or pressure may be applied to the precursor as shown in
By way of example and not by way of limitation, the pellets may be lead particles and the polymer precursor may be a thermosetting polymer precursor solution for ABS polymer.
Conductive foils 306 may be attached to opposite surfaces of the thermoset polymer matrix 302′ such that the foils make electrical contact with the conductive pellets 304. By way of example, and not by way of limitation, As shown in
According to an alternative embodiment of the present invention, corrosion resistance of a bipolar batter substrate may be enhanced without significantly increasing weight. Corrosion resistance can generally be enhanced by increasing the path length for corrosion or by use of corrosion resistant materials. Typically, the path length for corrosion is increased by increasing the thickness of the corrodable components used in bipolar batteries. In the case of the substrate, the conventional approach to corrosion resistance is to increase the thickness of the foils (e.g., foils 106). This increases the corrosion path length in proportion to the increase in the foil thickness. However, increasing the foil thickness also increases the weight for the foils, so there is a limit to the amount of increase in the foil thickness.
In an alternative embodiment of the present invention, the corrosion path length can be increased by an amount that is significantly greater than is possible by simply increasing the thickness of the foil. The concept behind this embodiment is illustrated in
The planar conductive member 405 is sandwiched between the first and second matrices 402A, 402B such that it is in electrical contact with the conductive pellets 404A, 404B. The planar conductive member may be a conductive foil made of a suitable material, e.g., lead. However, the planar conductive member may alternatively be a mesh or grid made of conductive material. By way of example, and not by way of limitation, the mesh or grid may be characterized by a grid spacing that is smaller than a diameter of the conductive pellets 404A, 404B to ensure good electrical contact between the pellets and the mesh or grid. The first and second sets of conductive pellets are laterally offset with respect to each other by an offset distance do thereby increasing a corrosion path length for the substrate by an amount of the offset distance. This offset distance can be made significantly greater than the thickness of the planar conductive member 405 and polymer matrices 402A, 402B. Consequently, the corrosion path length can be increased by an amount that is substantially greater than the increase in thickness of the substrate 400.
There are a number of different possible configurations for the offset between the two sets of conductive pellets 404A, 404B. For example, as shown in
There are a number of other different possible configurations for a substrate for a bipolar battery. For example, in one alternative configuration, depicted in
According to alternative embodiments of the invention, the bipolar substrate 400 can be used in a bipolar battery, e.g., a bipolar lead-acid battery 500 as shown in
A plurality of cells 420 can make up the battery 500 as seen in
Embodiments of the present invention include a method for fabricating a substrate for a bipolar battery of the type described above with respect to
Alternatively, one or more of the polymer matrices 602A, 602B may be formed from a sheet of thermoplastic polymer. Holes may be formed in the thermoplastic polymer, e.g., by molding or punching, and the pellets may be pressed into the holes. Heat and pressure may then be applied to partially melt the polymer around the pellets. The pellets may be securely embedded when the polymer re-solidifies around the pellets.
The planar conductive member 605 may be disposed between the polymer matrices with embedded conductive pellets as shown in
It is noted that in some embodiments, the sandwiching of the conductive member 605 between the polymer matrices 604A, 604B may be combined with the curing of the polymer precursor. In such a case, the adhesive might not be necessary if the cured polymer makes a sufficiently good mechanical bond to the foils.
In some embodiments, it may be desirable to add outer electrodes to the substrate 600. The substrate can be disposed between sheets of conductive material 606, e.g., lead foil, as shown in
Embodiments of the invention provide for a highly corrosion resistant and low weight substrate for a bipolar battery that can be manufactured at relatively low cost. It is believed that the use of a thermoset polymer in conjunction with conductive pellets in a bipolar substrate for a storage battery can provide a long-lasting battery that develops no pinhole permeability in the substrate after 350 or more 80% depth-of-discharge cycles.
The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents incorporated herein by reference.
All the features disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.” Any feature described herein, whether preferred or not, may be combined with any other feature, whether preferred or not.