Patent Application
20100239913
1. Technical Field
The present invention relates to the use of carbon foam in energy storage devices and, more particularly, to the external stabilization of carbon foam current collectors in an energy storage device.
2. Background
Electrochemical batteries, including, for example, lead acid batteries, rely upon chemical reactions to produce electrochemical potential differences. Certain types of these batteries are known to include at least one positive current collector, at least one negative current collector, and an electrolytic solution including, for example, sulfuric acid (H2SO4) and distilled water. Ordinarily, both the positive and negative current collectors in a lead acid battery are constructed from lead. The role of these lead current collectors is to transfer electric current to and from the battery terminals during the discharging and charging processes. Storage and release of electrical energy in lead acid batteries is enabled by chemical reactions that occur in a paste disposed on the current collectors. The positive and negative current collectors, once coated with this paste, are referred to as positive and negative plates, respectively.
While lead acid batteries have been widely used in various applications, a notable limitation on the durability and service life of lead acid batteries is corrosion of the lead current collector of the positive plate. For example, once the sulfuric acid electrolyte is added to the battery and the battery is charged, the current collector of each positive plate is continually subjected to corrosion due to its exposure to sulfuric acid and to the anodic potentials of the positive plate. As the lead current collector corrodes, lead dioxide is formed from the lead source metal of the current collector. An effect of this corrosion of the positive plate current collector is volume expansion, since lead dioxide has a greater volume than lead. Volume expansion induces mechanical stresses on the current collector that deform and stretch the current collector. At a total volume increase of the current collector of approximately 4% to 7%, the current collector may fracture. As a result, battery capacity drops, and eventually, the battery will reach the end of its service life. Additionally, at advanced stages of corrosion, internal shorting within the current collector and rupture of the cell case can occur. These corrosion effects may lead to failure of one or more of the cells within the battery.
One method of extending the service life of a lead acid battery is to increase the corrosion resistance of the current collectors and other electrically conductive components in the battery by including electrically conductive carbon in the current collectors and components. Because carbon does not oxidize at the temperatures at which lead acid batteries generally operate, some of these methods have involved using carbon in various forms to slow or prevent the detrimental corrosion process in lead acid batteries. For example, carbon foam has been proposed as a current collector material for use in lead acid batteries.
Use of carbon foam (e.g., graphite foam) as a current collector can increase the corrosion resistance and surface area of the current collector over lead current collector grids. This additional surface area of the current collectors may increase the specific energy and power of the battery, thereby enhancing its performance. However, among the network of pores formed in the foam, there may exist a plurality of defects that can allow intercalation of electrically charged ions of the electrolytic solution into the structure of the foam. The intercalation of the ions can cause internal damage such as separation and delamination between foam layers, and ultimately lead to reduced performance or premature failure of the current collector. The effects of intercalation may be particularly prevalent when the carbon foam structure includes graphite foam.
Thus, there is a need for a structure, such as a structural restraint system, that can improve the resistance of carbon foam to intercalation of ions and the harmful effects of this phenomenon. The presently disclosed embodiments are directed toward meeting this need.
According to one aspect, the present disclosure is directed toward an electrode plate for an energy storage device. The electrode plate may include a carbon foam current collector and an external restraint structure. A chemically active material may be disposed on the carbon foam current collector.
According to another aspect, the present disclosure is directed toward an energy storage device. The energy storage device may include a housing, a positive terminal, a negative terminal, and at least one cell disposed within the housing. Each cell may include an electrolytic solution, at least one positive plate, and at least one negative plate. The at least one positive plate may include a carbon foam current collector and an external restraint structure. A chemically active material may be disposed on the carbon foam current collector.
According to yet another aspect, the present disclosure is directed toward a method for making an electrode plate of an energy storage device. The method may include providing a carbon foam current collector, applying a polymer-based external restraint structure, and applying a chemically active material to the carbon foam current collector.
The accompanying drawings, which are incorporated in and constitute a part of this specification, provide diagrammatic representation of the disclosed embodiments and together with the description, serve to explain the principles of the invention. In the drawings:
As illustrated in
Energy storage device 10 may also include aqueous or solid electrolytic materials that at least partially fill a volume between positive plates 18 and negative plates 19. In a lead acid battery, for example, the electrolytic material may include an aqueous solution of sulfuric acid and water. Nickel-based batteries may include alkaline electrolyte solutions that include a base, such as potassium hydroxide, mixed with water. It should be noted that other acids and other bases may be used to form the electrolytic solutions of the disclosed batteries.
Each cell 16 may be electrically isolated from adjacent cells by a cell separator 22. Moreover, positive plates 18 may be separated from negative plates 19 by a plate isolator 23. Both cell separators 22 and plate isolators 23 provide electrical separation of plates, while allowing the flow of electrolyte and/or ions produced by electrochemical reactions in energy storage device 10. Therefore, cell separators 22 and plate isolators 23 may be made from electrically insulating yet porous materials or materials conducive to ionic transport, such as fiberglass, for example.
Depending on the chemistry of energy storage device 10, each cell 16 will have a characteristic electrochemical potential. For example, in a lead acid battery used in automotive and other applications, each cell may have a potential of about 2 volts. Cells 16 may be connected in series to provide the overall potential of the battery. As shown in
Once the total desired potential has been provided using an appropriate configuration of cells 16, this potential may be conveyed to terminals 14 on housing 12 using terminal leads 26. These terminal leads 26 may be electrically connected to any suitable electrically conductive components present in energy storage device 10. For example, as illustrated in
In addition, a chemically active material (not shown) may be disposed on carbon foam current collector 31. The composition of the chemically active material may depend on the chemistry of energy storage device 10. In a lead acid battery, for example, the active material may include an oxide or salt of lead. As additional examples, the anode plates (i.e., positive plates) of nickel cadmium (NiCd) batteries may include a cadmium hydroxide (Cd(OH)2) active material; nickel metal hydride batteries may include a lanthanum nickel (LaNi5) active material; nickel zinc (NiZn) batteries may include a zinc hydroxide (Zn(OH)2) active material; and nickel iron (NiFe) batteries may include an iron hydroxide (Fe(OH)2) active material. In all of the nickel-based batteries, the chemically active material on the cathode (i.e., negative) plate may be nickel hydroxide. As previously mentioned, the role of current collector 31 is to collect and transfer the electric current generated by the electrochemical reactions that, at least in some battery chemistries, occur in chemically active material during the discharging and charging processes. Because of the increased surface area of carbon foam current collector 31 due to the plurality of pores 32, chemically active material can effectively penetrate into the open pore structure of carbon foam current collector 31.
In one embodiment, carbon foam material used in current collector 31 may include from about 4 to about 50 pores per centimeter and an average pore size of at least about 200 micrometers. In other embodiments, however, the average pore size may be smaller. For example, in certain embodiments, the average pore size may be at least about 40 micrometers. In still other embodiments, the average pore size may be at least about 20 micrometers. While reducing the average pore size of the carbon foam material may have the effect of increasing the effective surface area of the material, average pore sizes below 20 micrometers may impede or prevent penetration of chemically active material into pores of carbon foam material.
Regardless of the average pore size, a total porosity value for carbon foam may be at least 60%. In other words, at least 60% of the volume of carbon foam structure may be included within pores 32. Carbon foam materials may also have total porosity values less than 60%. For example, in certain embodiments, carbon foam may have a total porosity value of at least 30%.
Moreover, carbon foam may have an open porosity value of at least 90%. Therefore, at least 90% of pores 32 are open to adjacent pores such that the network of pores 32 forms a substantially open network. This open network of pores 32 may allow the active material deposited on each current collector 31 to penetrate within the carbon foam structure. In addition to the network of pores 32, carbon foam includes a web of structural elements that provide support for carbon foam. In total, the network of pores 32 and the structural elements of the carbon foam may result in a density of less than about 0.6 g/cm3 for the carbon foam material.
Due to the conductivity of the carbon foam of the present disclosure, current collectors 31 can efficiently transfer current to and from battery terminals 14, or any other conductive elements providing access to the electrical potential of battery 10. In certain forms, carbon foam may offer sheet resistivity values of less than about 1 ohm-cm. In other forms, carbon foam may have sheet resistivity values of less than about 0.75 ohm-cm.
In certain disclosed embodiments, the carbon foam may include graphite foam. Density and pore structure of graphite foam may be similar to carbon foam. A primary difference between graphite foam and carbon foam is the orientation of carbon atoms that make up the structural elements. For example, in carbon foam, carbon may be at least partially amorphous. In graphite foam, however, the carbon tends to be ordered into a layered structure. Because of the ordered nature of the graphite structure, graphite foam may offer higher conductivity than carbon foam. Graphite foam may exhibit electrical resistivity values of between about 100 micro-ohm-cm and about 2,500 micro-ohm-cm.
Within the carbon foam structure, particularly in the graphite foam structure, there may exist a plurality of layers. When the carbon foam is exposed to the electrically charged ions in an electrolytic solution, the ions may intercalate between the layers of the foam structure through surface defects and discontinuities that may exist among the network of open pores. The ions may act like a wedge being driven into the carbon foam structure, pulling the layers apart and causing internal damage. Intercalation of the ions may eventually cause separation of the foam layers within the carbon foam structure, which can lead to cracking and, ultimately, failure of the carbon foam as a current collector. In order to prevent or minimize intercalation of electrically charged ions of the electrolytic solution into the structure of carbon foam, an external restraint 33 may be disposed on the outer surface of carbon foam current collector 31. The external restraint may physically hold the layers of the foam structure together, particularly in layers adjacent to the restraint structure, and stabilize the carbon foam against occurrences of intercalation. Depending on its configuration, the external restraint may be effective in stabilizing carbon foam of varying thicknesses. In one embodiment, external restraint 33 may stabilize carbon foam layers having thickness of up to 1 to 2 mm. Stabilization of carbon foam of thicknesses greater than 2 mm, however, may also be accomplished by, for example, adjusting the thickness and/or material properties of external restraint 33.
One such graphite foam, under the trade name PocoFoam™, is available from Poco Graphite, Inc. PocoFoam™ is very anisotropic due to the ordered layers of carbon atoms. In preparing a bulk PocoFoam™ material for use in energy storage device, the bulk PocoFoam™ material may be cut into sheets or plates having two large primary surfaces and four edge surfaces. As the bulk foam is cut in a direction that is perpendicular to a plane of the ordered layers of carbon atoms in the foam, the primary surfaces of the PocoFoam™ sheets may contain a majority of the surface defects present, and the edge surfaces may contain fewer surface defects. Application of external restraint 33 to the primary surfaces of the carbon foam current collector can maximize the effectiveness of the restraint in minimizing intercalation of ions into the foam through surface defects and discontinuities existing on the primary surfaces.
The external restraint 33 disposed on the carbon foam current collector 31 may be porous to allow transport of various substances, ions, etc. through external restraint 33. For example, external restraint 33 may allow ions from the electrolytic solution of a battery to pass through and interact with the active material disposed on current collector 31.
A variety of materials may be used to produce external restraint 33. Any acid resistant material that is chemically stable in a battery environment can be used to form external restraint 33. For example, external restraint 33 may be produced from a variety of non-conductive materials including polymers, such as styrene, PVC, ABS, polyethylene, polypropylene, among others. In other embodiments, conductive materials such as metals can be used. The external restraint structure may be physically bonded to the surface of the current collector using an adhesive. Alternatively, the external restraint may be secured onto the current collector by sewing or any other suitable bonding or attaching technique. The external restraint may be configured in many different ways, such as a web structure, a mesh, grids, etc.
Any amount of polymer can be added to the solvent to achieve a desired consistency of the mixture. For example, the polymer can be added to the solvent until the mixture reaches a syrup-like consistency. When an appropriate amount of polymer has been added to the solvent and the mixture of solvent and dissolved polymer reaches a desired consistency, the mixture may be rolled onto an applicator (e.g., a glass plate) in preparation for application onto the carbon foam surface. An ink roller may be used in rolling out the mixture. The mixture of dissolved polymer and solvent on the glass substrate creates a thin film of dissolved polymer. The polymer film spread on the glass plate can have any appropriate thickness for providing a desired restraint thickness. In one embodiment, the thickness of the film may be up to about 5 micrometers to maximize the probability that the restraint is disposed only on the ridges and not significantly in the voids of the carbon foam outer surface.
Next, as shown in step 52, the prepared film may be applied to one or more surfaces of the carbon foam. The film may be applied to one primary surface, or alternatively to two opposite primary surfaces. In certain embodiments, one or more edge surfaces of the carbon foam may also receive a coating of the prepared film. To coat the ridges of the carbon foam, a layer of carbon foam may be placed on the glass plate and in contact with the prepared film formed thereon. The film mixture may wet the surface ridges 41 of the foam without significantly filling the surface voids 42 on the carbon foam.
In step 54, the carbon foam coated with the prepared film of restraint material solution can be dried to allow the solvent to evaporate. The coated carbon foam can be air-dried or placed in a furnace for removal of the solvent. As the solvent is removed, the remaining polymer hardens on the outer surface of the carbon foam (e.g., on the ridges 41 of the outer surface) and forms a polymer web-like structure providing restraint on the carbon foam current collector.
The thickness of the polymer disposed on the outer surface of the carbon foam may be chosen to provide a desired level of rigidity and structural restraint to the carbon foam. For example, in one embodiment, the thickness of the polymer coated on the foam (i.e., restraint 33) may be up to about 100 micrometers. In certain embodiments, the desired thickness of the polymer may between about 20 micrometers and 50 micrometers. Multiple applications of the polymer are also permissible.
A second method consistent with
Melting the polymer and application of the melted polymer according to step 52 may be accomplished by any suitable method. In one embodiment, a sheet of polymer can be placed on a heated plank surface and melted. In another embodiment, a polymer may be melted first in a heating plate or a furnace and then spread onto a surface of, for example, a plank, which may be heated to maintain the melted polymer in its viscous state. Application of the restraint material in step 52 may proceed by exposing the carbon foam to the melted polymer, wherein a portion of the melted polymer is deposited onto one or more surfaces of the carbon foam surface. As in the embodiment described above, the melted polymer of this embodiment may be applied to the surface ridges 41 of the foam, leaving voids 42 substantially free of the melted polymer. At step 54, the melted polymer on the surface of the carbon foam may be cured by, for example, allowing the melted polymer to cool and harden on the surface of the carbon foam to form a web-like structure.
While the embodiments described above include a restraint material 33 formed on one or more surfaces of the carbon foam in a web-like structure, many other suitable configurations of external restraint 33 are possible. For example, external restraint 33 may include a mesh, as diagrammatically illustrated in
In yet another exemplary embodiment, external restraint 33 may include two grids (e.g., metal or polymer) placed on opposite sides of a carbon foam layer and sewn together or attached by any other suitable means. Such an arrangement is diagrammatically illustrated in
In yet another exemplary embodiment, external restraint 33 may include a three-dimensional interlocking structure, as diagrammatically illustrated in
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed materials and processes without departing from the scope of the invention. Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the present disclosure. It is intended that the specification and examples be considered as exemplary only, with a true scope of the present disclosure being indicated by the following claims and their equivalents.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/34161 | 8/31/2006 | WO | 00 | 2/18/2009 |
