Abstract:
A battery having a nanostructured battery electrode is disclosed wherein it is possible to reverse the contact of the electrolyte with the battery electrode and, thus, to return a battery to a reserve state after it has been used to generate current. In order to achieve this reversibility, the nanostructures on the battery electrode comprise a plurality of closed cells and the pressure within the enclosed cells is varied. In a first embodiment, the pressure is varied by varying the temperature of a fluid within the cells by, for example, applying a voltage to electrodes disposed within said cells. In a second illustrative embodiment, once the battery has been fully discharged, the battery is recharged and then the electrolyte fluid is expelled from the cells in a way such that it is no longer in contact with the battery electrode.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to batteries and, more particularly, to batteries having nanostructured surfaces. 
     BACKGROUND OF THE INVENTION 
     Many beneficial devices or structures in myriad applications rely on batteries as a power source. A typical liquid-cell battery, such as battery  101  in  FIG. 1 , is characterized by an electrolyte liquid  102  which provides a mechanism for an electrical charge to flow in direction  103  between a positive electrode  104  and a negative electrode  105 . When such a battery  101  is inserted into an electrical circuit  106  with illustrative load  108 , it completes a loop which allows electrons to flow in direction  107  around the circuit  106 . The positive electrode  104  thus receives electrons from the external circuit  106 . These electrons then react with the materials of the positive electrode  104  in reduction reactions that generate the flow of a charge to the negative electrode  105  via ions in the electrolyte liquid  102 . At the negative electrode  105 , oxidation reactions between the materials of the negative electrode  104  and the charge flowing through the electrolyte fluid  102  result in surplus electrons that are released to the external circuit  106 . 
     As the above process continues, the active materials of the positive and negative electrodes  104  and  105 , respectively, eventually become depleted and the reactions slow down until the battery is no longer capable of supplying electrons. At this point the battery is discharged. It is well known that, even when a liquid-cell battery is not inserted into an electrical circuit, there is often a low level reaction with the electrodes  104  and  105  that can eventually deplete the material of the electrodes. Thus, a battery can become depleted over a period of time even when it is not in active use in an electrical circuit. This period of time will vary depending on the electrolyte fluid used and the materials of the electrodes. 
     More recently, batteries having at least one nanostructured surface have been proposed wherein nanostructures are used to separate the electrolyte from the electrode until such a time that the battery is to be used. This is typically referred to as a reserve battery (as opposed to a primary battery that is manufactured with the electrolyte in contact with the electrodes of the battery). An example of the use of electrowetting principles applied to reserve batteries is described in copending U.S. patent application Ser. No. 10/716,084 filed Nov. 18, 2003 and entitled “Electrowetting Battery Having Nanostructured Surface,” which is hereby incorporated by reference herein in its entirety. As disclosed in the &#39;084 application, when it is desired that the battery generate a current, the electrolyte is caused to penetrate the nanostructured surface and to come into contact with the electrode of the battery, thus resulting in the above-discussed flow of electrons around a circuit. Such a penetration of nanostructures is achieved, for example, by applying a voltage to the nanostructures such that the contact angle of the electrolyte relative to the nanostructured surface is decreased. When the contact angle is decreased, the electrolyte penetrates the nanostructures and is brought into contact with the electrode. 
     SUMMARY OF THE INVENTION 
     The present inventors have realized that, while prior reserve and primary batteries were useful in many regards, they were limited in certain aspects. In particular, once the batteries were manufactured and activated (in the case of a reserve battery), it was typically impossible to return the batteries to a reserve state (i.e, to separate the electrolyte from the battery electrodes). 
     Therefore, the present inventors have invented a small battery having a nanostructured battery electrode wherein it is possible to reverse the contact of the electrolyte with the battery electrode and, thus, to return a battery to a reserve state after it has been used to generate current. In order to achieve this reversibility, the nanostructures on the battery electrode comprise a plurality of closed cells and the pressure within the enclosed cells is varied. In a first embodiment, the pressure is varied by varying the temperature of a fluid within the cells by, for example, applying a voltage to electrodes disposed within said cells. In a second illustrative embodiment, once the battery has been fully discharged, the battery is recharged and then the electrolyte fluid is expelled from the cells in a way such that it is no longer in contact with the battery electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a prior art liquid-cell battery as used in an electrical circuit; 
         FIG. 2  shows a prior art nanopost surface; 
         FIGS. 3A ,  3 B,  3 C,  3 D and  3 E show various prior art nanostructure feature patterns of predefined nanostructures that are suitable for use in the present invention; 
         FIG. 4  shows a more detailed view of the prior art nanostructure feature pattern of  FIG. 3C ; 
         FIGS. 5A and 5B  show a device in accordance with the principles of the present invention whereby electrowetting principles are used to cause a liquid droplet to penetrate a nanostructure feature pattern; 
         FIG. 6  shows the detail of an illustrative nanopost of the nanostructure feature pattern of  FIGS. 5A and 5B ; 
         FIG. 7  shows an illustrative liquid-cell battery in accordance with the principles of the present invention wherein the electrolyte in the battery is separated from the negative electrode by nanostructures; 
         FIG. 8  shows the illustrative battery of  FIG. 7  wherein the electrolyte in the battery is caused to penetrate the nanostructures and to thus contact the negative electrode; and 
         FIGS. 9A ,  9 B and  9 C show a battery with the principles of the present invention wherein a droplet of electrolyte is disposed in an initial position suspended on top of a nanostructured feature pattern ( FIG. 9A ), is caused to penetrate the feature pattern ( FIG. 9B ), and is then caused to return to a position suspended on top of the feature pattern ( FIG. 9C ); 
         FIGS. 10A and 10B  show an illustrative closed-cell structure in accordance with the principles of the present invention; 
         FIGS. 11A and 11B  show the detail of one cell in the illustrative structure of  FIGS. 10A and 10B ; and 
         FIGS. 12A ,  12 B and  12 C show a battery in accordance with the principles of the present invention wherein a droplet of electrolyte is disposed in an initial position suspended on top of a nanostructured feature pattern ( FIG. 12A ), is caused to penetrate the feature pattern ( FIG. 12B ), and is then caused to return to a position suspended on top of the feature pattern ( FIG. 12C ). 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an illustrative nanopost pattern  201  with each nanopost  209  having a diameter of less than 1 micrometer. While  FIG. 2  shows nanoposts  209  formed in a somewhat conical shape, other shapes and sizes are also achievable. In fact, cylindrical nanopost arrays have been produced with each nanopost having a diameter of less than 10 nm. Specifically,  FIGS. 3A-3E  show different illustrative arrangements of nanoposts produced using various methods and further show that such various diameter nanoposts can be fashioned with different degrees of regularity. Moreover, these figures show that it is possible to produce nanoposts having various diameters separated by various distances. An illustrative method of producing nanoposts, found in U.S. Pat. No. 6,185,961, titled “Nanopost arrays and process for making same,” issued Feb. 13, 2001 to Tonucci, et al, is hereby incorporated by reference herein in its entirety. Nanoposts have been manufactured by various methods, such as by using a template to form the posts, by various means of lithography, and by various methods of etching. 
       FIG. 4  shows the illustrative known surface  401  of  FIG. 3C  with a nanostructure feature pattern of nanoposts  402  disposed on a substrate. Throughout the description herein, one skilled in the art will recognize that the same principles applied to the use of nanoposts or nanostructures can be equally applied to microposts or other larger features in a feature pattern. The surface  401  and the nanoposts  402  of  FIG. 4  are, illustratively, made from silicon. The nanoposts  402  of  FIG. 4  are illustratively approximately 350 nm in diameter, approximately 6 μm high and are spaced approximately 4 μm apart, center to center. It will be obvious to one skilled in the art that such arrays may be produced with regular spacing or, alternatively, with irregular spacing. 
     As typically defined a “nanostructure” is a predefined structure having at least one dimension of less than one micrometer and a “microstructure” is a predefined structure having at least one dimension of less than one millimeter. However, although the disclosed embodiments refer to nanostructures and nanostructured surfaces, it is intended by the present inventors, and will be clear to those skilled in the art, that microstructures may be substituted in many cases. Accordingly, the present inventors hereby define nanostructures to include both structures that have at least one dimension of less than one micrometer as well as those structures having at least one dimension less than one millimeter. The term “feature pattern” refers to either a pattern of microstructures or a pattern of nanostructures. Further, the terms “liquid,” “droplet,” and “liquid droplet” are used herein interchangeably. Each of those terms refers to a liquid or a portion of liquid, whether in droplet form or not. 
     In many applications, it is desirable to be able to control the penetration of a given liquid into a given nanostructured or microstructured surface and, thus, control the contact of the liquid with the underlying substrate supporting the nanostructures or microstructures.  FIGS. 5A and 5B  show one embodiment where electrowetting is used to control the penetration of a liquid into a nanostructured surface. Electrowetting principles and controlling the movement of a liquid across a nanostructured or microstructured surface are generally described in U.S. patent application Ser. No. 10/403,159 filed Mar. 31, 2003 and titled “Method And Apparatus For Variably Controlling The Movement Of A Liquid On A Nanostructured Surface,” which is hereby incorporated by reference herein in its entirety. As discussed previously, the general use of electrowetting principles in batteries is described in above-referenced copending U.S. patent application Ser. No. 10/716,084. 
     Referring to  FIG. 5A , a droplet  501  of conducting liquid (such as an electrolyte solution in a liquid-cell battery) is disposed on nanostructure feature pattern of cylindrical nanoposts  502 , as described above, such that the surface tension of the droplet  501  results in the droplet being suspended on the upper portion of the nanoposts  502 . In this arrangement, the droplet only covers surface area f 1  of each nanopost and has a contact angle with each nanopost of, for example, θ 0 . The nanoposts  502  are supported by the surface of a conducting substrate  503 . Droplet  501  is illustratively electrically connected to substrate  503  via lead  504  having voltage source  505 . An illustrative nanopost is shown in greater detail in  FIG. 6 . In that figure, nanopost  502  is electrically insulated from the liquid ( 501  in  FIG. 5A ) by material  601 , such as an insulating layer of dielectric material. The nanopost is further separated from the liquid by a low surface energy material  602 , such as a well-known fluoro-polymer. Such a low surface energy material allows one to obtain an appropriate initial contact angle (i.e., θ 0 ) between the liquid and the surface of the nanopost. It will be obvious to one skilled in the art that, instead of using two separate layers of different material, a single layer of material that possesses sufficiently low surface energy and sufficiently high insulating properties could be used. 
       FIG. 5B  shows that, by applying a low voltage (e.g., 10-20 volts) to the conducting droplet of liquid  501 , a voltage difference results between the liquid  501  and the nanoposts  502 . The contact angle between the liquid and the surface of the nanopost decreases and, at a sufficiently low contact angle, the droplet  501  moves down in the y-direction along the surface of the nanoposts  502  and penetrates the nanostructure feature pattern until it completely surrounds each of the nanoposts  502  and comes into contact with the upper surface of substrate  503 . In this configuration, the droplet covers surface area f 2  of each nanopost. Since f 2 &gt;&gt;f 1 , the overall contact area between the droplet  501  and the nanoposts  502  is relatively high such that the droplet  501  contacts the substrate  503 . One skilled in the art will recognize that other methods of causing the electrolyte to penetrate the nanostructures, such as decreasing the temperature of the electrodes, can be used. The present invention is intended to encompass any such method of causing such penetration. 
       FIG. 7  shows an illustrative battery  701  whereby an electrolyte fluid  702  is contained within a housing having containment walls  703 . The electrolyte fluid  702  is in contact with positive electrode  704 , but is separated from negative electrode  708  by nanostructured surface  707 . Nanostructured surface  707  may be the surface of the negative electrode or, alternatively, may be a surface bonded to the negative electrode. One skilled in the art will recognize that the nanostructured surface could also be used in association with the positive electrode with similarly advantageous results. In  FIG. 7 , the electrolyte fluid is suspended on the tops of the nanoposts of the surface, similar to the droplet of  FIG. 5A . The battery  701  is inserted, for example, into electrical circuit  705  having load  706 . When the electrolyte liquid  702  is not in contact with the negative electrode, there is substantially no reaction between the electrolyte and the electrodes  704  and  708  of the battery  701 . Accordingly, there is no depletion of the materials of the electrodes. Thus, it is possible to store the battery  701  for relatively long periods of time without the battery becoming discharged. 
       FIG. 8  shows the battery  701  of  FIG. 7  inserted into electrical circuit  705  wherein, utilizing the electrowetting principles described above, a voltage is applied to the nanostructured surface  707  thus causing the electrolyte fluid  702  to penetrate the surface  707  and to come into electrical contact with the negative electrode  708 . One skilled in the art will recognize that this voltage can be generated from any number of sources such as, for example, by passing one or more pulses of RF energy through the battery. When the penetration of the electrolyte into the nanostructures occurs, electrons begin flowing in direction  801  through the circuit  705 , as described above, and the load  706  is powered. Thus, the embodiment of  FIGS. 7 and 8  show how a battery can be stored without depletion for a relatively long period of time and can then be “turned on” at a desired point in time to power one or more electrical loads in an electrical circuit. 
     The battery described in  FIGS. 7 and 8  is referred to as a reserve battery or, in other words, a battery that is manufactured with the electrolyte separated from at least one of the electrodes in the battery. Primary batteries, on the other hand, are batteries that are manufactured with the electrolyte in contact with the electrodes of the battery. As such, primary batteries are always undergoing oxidation reactions, even when not inserted in an electrical circuit. Therefore, primary batteries typically have a relatively short shelf-life relative to reserve batteries. 
     The present inventors have recognized that it would be desirable to be able to selectively turn on and off the generation of current in a battery. Such a capability would have many novel uses. For example, the battery could be turned on only when it was needed, thus preventing excess oxidation that could lead to premature discharge of the battery. Additionally, such a capability could lead to a new category of reserve rechargeable batteries that, once recharged, can be turned off. As is well-known, rechargeable batteries (also referred to herein as secondary batteries) are batteries in which the electrodes can be regenerated by reversing the current flow to and within the battery. While it is possible to recharge the reserve nanostructured batteries described previously, no effective methods have yet been realized for returning the recharged battery to a reserve state once it is recharged. 
     The present inventors have further realized that, in the nanostructured batteries discussed above herein, it would be desirable to reverse the penetration of the electrolyte in a way such that it is restored to its original reserve position suspended on the nanostructures above the electrode. Reversible penetration of nanostructured or microstructured surfaces by a droplet of liquid is the subject of copending U.S. patent application Ser. No. 10/674,448, filed Sep. 30, 2003 and entitled “Reversible Transitions On Dynamically Tunable Nanostructured Or Microstructured Surfaces,” which is hereby incorporated by reference herein in its entirety. 
       FIGS. 9A ,  9 B and  9 C illustrate a selective/reversible penetration of droplet  901 , which is illustratively a droplet of electrolyte such as electrolyte  702  in  FIG. 7  into nanostructure pattern  904 . Specifically,  FIG. 9A  shows electrolyte droplet  901  disposed on a nanostructure or microstructure feature pattern  904  that is supported by substrate  905  (which is, illustratively, the electrode  503  in  FIG. 5A ). Next, as shown in  FIG. 9B  and discussed above, droplet  901  is caused to penetrate the feature pattern  904 . Finally, as shown in  FIG. 9C , it is desirable to reverse the penetration of droplet  902 .  FIGS. 10A and 10B  show, respectively, a three-dimensional view and a top cross-sectional view of an illustrative feature pattern in accordance with the principles of the present invention that is capable of accomplishing the reversible penetration shown in  FIGS. 9A-9C . Specifically, in the present illustrative embodiment represented by  FIGS. 10A and 10B , the feature pattern does not comprise a number of posts spaced a distance away from each other. Instead, a number of closed cells  1001 , here illustrative cells of a hexagonal cross section, are used. Each cell  1001  has an electrode  1002  disposed along the inner wall of the cell. As used herein, the term closed cell is defined as a cell that is enclosed on all sides except for the side upon which a liquid, such as an electrolyte liquid, is intended to be disposed. One skilled in the art will recognize that other, equally advantageous cell configurations and geometries are possible to achieve equally effective closed-cell arrangements.  FIGS. 11A and 11B  show a top cross-sectional view and a side view of an illustrative individual cell of the feature pattern of  FIGS. 10A and 10B . Specifically, referring to  FIG. 11A , each individual cell  1101  is characterized by a maximum width  1102  of width d, an individual side length  1103  of length d/2 and a wall thickness  1104  of thickness t. Referring to  FIG. 11B , the height  1105  of cell  1101  is height h. 
       FIGS. 12A ,  12 B and  12 C show how an illustrative closed-cell feature pattern similar to the feature pattern of  FIGS. 10A and 10B , here shown in cross-section, may be used illustratively to cause a droplet  1201  of liquid to reversibly penetrate the feature pattern. Specifically, each cell within feature pattern  1204 , such as cell  1101  having a hexagonal cross-section, is a completely closed cell once the droplet of liquid covers the opening of that cell. Thus, referring to  FIG. 12A , each such closed cell over which the droplet is disposed contains a fluid having an initial temperature T=T 0  and an initial pressure P=P 0 . As used herein, the term fluid is intended to encompass both gases (such as, illustratively, air) and liquids that could be disposed within the cells of the feature pattern. The present inventors have recognized that, by changing the pressure within the individual cells, such as cell  1101 , the liquid droplet  1201  can be either drawn into the cells or, alternatively, repelled out of the cell. Specifically, referring to  FIG. 12B , if the pressure within the cell  1101  is caused to be below the initial pressure (i.e., P&lt;P 0 ), then the droplet above that cell will be drawn into the cell a distance related to the magnitude in reduction of the pressure P. Such a reduction in pressure may be achieved, illustratively, by reducing the temperature of the fluid within the cells such that T&lt;T 0 . Such a temperature reduction may be achieved, illustratively, by reducing the temperature of the substrate  1205  and/or the feature pattern  1204 . One skilled in the art will recognize that any method of reducing the pressure within the cells, including any other method of reducing the temperature of the fluid within the cells, will have similar results. For example, each of the cells could be connected either in series or in parallel to one or more remote ballast gas reservoirs. The pressure of the gas in this reservoir could be changed, thus raising or lowering the pressure in the cells. Similarly, the pressure within the cells could be changed by moving a diaphragm disposed within each of the cells, thus displacing a fluid within the cell and varying the pressure within that cell. Additionally, as discussed more fully in the aforementioned copending patent applications, electrowetting may be used instead of pressure reduction to draw the liquid into the cells of the feature pattern  1204 . Specifically, by applying a voltage to the conducting drople  1201 , a voltage difference results between the liquid and the cells in the feature pattern  1204 . Hence, as discussed herein above, the droplet  1201  moves down and penetrates the nanostructure feature pattern  1204  until it comes into contact with the upper surface of substrate  1205 . Other methods of changing the pressure within the cells will be readily apparent to one skilled in the art in light of the teachings herein. 
       FIG. 12C  shows how, by increasing the pressure to or above the initial pressure P 0 , it is possible to reverse the penetration of the droplet  1201 , whether that penetration was initiated by pressure reduction or by electrowetting. Once again, such a pressure increase may be achieved by changing the temperature of the fluid within the cells, illustratively in  FIG. 12C  to a temperature greater than the initial temperature T 0 . One illustrative method if increasing this temperature is to apply a voltage to electrodes  1002  in  FIG. 10  in a way such they heat the insides of the cells. The increased temperature will increase the pressure within the cells above the initial pressure P 0 . The contact angle between the droplet and the elements of the feature pattern will thus change to θ 3 , which is smaller than θ 1  and the liquid will move out of the cells, thus returning droplet  1201  to a very low flow resistance contact with feature pattern  1204 . Once again, one skilled in the art will recognize that any method of increasing the pressure within the cells to reverse the penetration of the droplet  1201 , including any other method of increasing the temperature of the fluid within the cells, will have similar results. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are within its spirit and scope. For example, one skilled in the art, in light of the descriptions of the various embodiments herein, will recognize that the principles of the present invention may be utilized in widely disparate fields and applications. For example, while the embodiment disclosed herein is a battery having nanostructured surfaces, one skilled in the art will appreciate that such nanostructured surfaces may be used for other uses, such as in use as a thermostat. In such a case, the characteristics of the pattern of nanostructures and the liquid in contact with the nanostructures can be chosen in a way such that, upon a temperature increase of known amount, the liquid will penetrate the surface, thus achieving a desired result. One skilled in the art will be able to devise many similar uses of the underlying principles associated with the present invention, all of which are intended to be encompassed herein. All examples and conditional language recited herein are intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting aspects and embodiments of the invention, as well as specific examples thereof, are intended to encompass functional equivalents thereof.