Abstract:
A method of fabricating an energy storage device with a large surface area electrode comprises: providing an electrically conductive substrate; depositing a semiconductor layer on the electrically conductive substrate, the semiconductor layer being a first electrode; anodizing the semiconductor layer, wherein the anodization forms pores in the semiconductor layer, increasing the surface area of the first electrode; after the anodization, providing an electrolyte and a second electrode to form the energy storage device. The substrate may be a continuous film and the electrode of the energy storage device may be fabricated using linear processing tools. The semiconductor may be silicon and the deposition tool may be a thermal spray tool. Furthermore, the semiconductor layer may be amorphous. The energy storage device may be rolled into a cylindrical shape. The energy storage device may be a battery, a capacitor or an ultracapacitor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. patent application Ser. No. 12/396,277, filed Mar. 2, 2009, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to energy storage devices, and more specifically to energy storage devices with porous electrodes. 
       BACKGROUND OF THE INVENTION 
       [0003]    All solid state Thin Film Batteries (TFB) are known to exhibit several advantages over conventional battery technology such as superior form factors, cycle life, power capability and safety. However, there is a need for cost effective and high-volume manufacturing (HVM) compatible fabrication technologies to enable broad market applicability of TFBs. Further, there is a need to improve the performance of TFBs. One approach for improving TFB performance is to increase battery electrode surface area without impacting the battery size. There is a need for methods for increasing TFB performance which are compatible with HVM and are low cost. 
         [0004]    An approach for increasing electrode surface area by anodizing a silicon wafer to produce a porous electrode is described by Shin et al., “Porous silicon negative electrodes for rechargeable lithium batteries,” Journal of Power Sources, vol. 138, no. 1-2, pp 314-320, 2005. However, the process and structure described by Shin et al. is based on processing of silicon wafers to make large area electrodes—this is too expensive, undesirable for HVM and is not sufficiently mechanically flexible to produce desired battery form factors. There is a need for lower cost, HVM compatible processes and structures. Furthermore, there is a need for flexible TFB cells which can readily be manipulated into desired form factors, such as rolled electrodes for cylindrical batteries. 
       SUMMARY OF THE INVENTION 
       [0005]    In general, embodiments of this invention contemplate providing a high-volume manufacturing solution for the fabrication of energy storage devices with large area porous electrodes. Embodiments of the present invention contemplate an alternative method of manufacturing energy storage devices using low cost, high-throughput processes. This approach includes the use of processes compatible with linear processing tools and continuous thin film substrates. Embodiments of the present invention contemplate porous electrodes made from a range of semiconductor materials, such as silicon, germanium, silicon-germanium, and other semiconductors and compound semiconductors. The semiconductor materials may be crystalline, polycrystalline or amorphous. More specifically, embodiments of the present invention may include processes combining: (1) deposition of a thin film semiconductor material; and (2) anodization of the thin film semiconductor, to produce a large surface area electrode. Furthermore, embodiments of this invention may provide flexible electrodes that permit a wide range of energy storage device form factors. For example, the energy storage device may be rolled to form a cylindrical battery or capacitor. Energy storage devices according to embodiments of the present invention may include batteries, thin film batteries (TFBs), capacitors and ultracapacitors. 
         [0006]    According to aspects of this invention, a method of fabricating an energy storage device with a large surface area electrode comprises: providing an electrically conductive substrate; depositing a semiconductor layer on said electrically conductive substrate, said semiconductor layer being a first electrode; anodizing said semiconductor layer, wherein said anodization forms pores in said semiconductor layer, increasing the surface area of said first electrode; after said anodization, providing an electrolyte and a second electrode to form said energy storage device. 
         [0007]    According to yet further aspects of this invention, an electrode of an energy storage device comprises: a thin film metal current collector; and a large surface area thin film semiconductor electrode having upper and lower surfaces, the lower surface being attached to the current collector, the thin film having pores extending from the upper surface into the thin film; wherein the semiconductor material between the pores is electrically conductive and electrically connected through the semiconductor electrode to the current collector. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0009]      FIG. 1  is a schematic representation of anodization of a silicon film, according to embodiments of the invention; 
           [0010]      FIG. 2  is a representation of a linear processing system for anodization of a continuous silicon film, according to embodiments of the invention; 
           [0011]      FIG. 3  shows a cross-section of an energy storage device, according to embodiments of the invention; 
           [0012]      FIG. 4  shows an energy storage device configured as a roll, according to embodiments of the invention; and 
           [0013]      FIG. 5  shows energy storage devices configured in a stack, according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration. 
         [0015]    In general, embodiments of this invention provide a high-volume manufacturing solution, at low cost and with high throughput for the fabrication of energy storage devices with large area porous electrodes. The following description provides examples of large area electrodes made of porous silicon. However, the present invention also contemplates porous electrodes made from a range of semiconductor materials, such as germanium, silicon-germanium, and other semiconducting elements and compounds. The semiconductor materials may be crystalline, polycrystalline or amorphous. The approach of the present invention includes, but is not limited to, the use of processes compatible with linear processing tools and continuous thin film substrates. Embodiments of the present invention may include processes combining: (1) deposition of a thin film semiconductor material; and (2) anodization of the thin film semiconductor, to produce a large surface area electrode. 
         [0016]    Energy storage devices are described generally herein, and specific examples of TFB devices are provided. However, embodiments of the present invention are not limited to TFBs, but are applicable to energy storage devices generally, including batteries, TFBs, capacitors and ultracapacitors. 
         [0017]      FIG. 1  shows an electrochemical processing system  100  configured for anodization of a semiconductor film  110 . The system  100  includes a processing tank  102  which contains an electrolyte  106 , a cathode  104  and an anode comprised of the semiconductor film  110  on a metal substrate  112 . The metal substrate  112  and the cathode  104  are connected to a power supply and controller  108 . The controller  108  is operated in a constant current mode in the particular configuration shown in  FIG. 1 , although anodization may also be achieved in a constant voltage mode, as is familiar to those skilled in the art. The anodization process results in pores  111  being formed in the semiconductor film  110 . The metal substrate  112  may need to be protected from the electrolyte, in which case a protective coating may be applied to the substrate or a special holder may be utilized. 
         [0018]    Although not shown, the electrochemical processing system  100  of  FIG. 1  may also include a means for circulating the electrolyte  106  within the tank  102 , for example using a stirrer or a circulation pump. Furthermore, the system  100  may include a light source. The specific configuration of the processing system  100  is shown for purposes of illustration; there are many other configurations and methods for anodization of semiconductors that are known to those skilled in the art that may be utilized with the present invention. 
         [0019]    The electrolyte  106  may comprise a mixture of hydrofluoric acid (HF), water and glacial acetic acid (CH 3 COOH). A mixture of HF (49%-w) and glacial acetic acid in a volumetric ratio of 1:1 was found to provide uniform etching of lightly-doped p-type (100) crystalline silicon at a constant current of 100 mA cm −2  in the dark. This mixture was found to provide a more macroscopically uniform porous layer than when using ethanol in place of the glacial acetic acid, with an electrolyte comprising, by volume, 70% of HF (49%-w) and 30% ethanol. 
         [0020]    Higher volumetric fractions of glacial acetic acid in the above electrolyte provide for more uniform etching of silicon. This is due to high volume fractions of glacial acetic acid resulting in more electrically resistive electrolytes. However, the HF concentration needs to be sufficient to support a high enough rate of pore formation. On the other hand, HF is usually sourced from a 49%-w solution. Therefore, when the HF concentration gets too high, the water concentration gets too high, since 51%-w of the commonly used HF source solution is water. Hence it is preferable to use 30-70%-volume glacial acetic acid, with the balance being HF 49%-w solution. More preferably, a 40-60%-volume solution is used of glacial acetic acid, with the balance being HF 49%-w solution. 
         [0021]    The objective of the anodization process is to increase the surface area of the semiconductor film  110  which can act as a battery cell electrode. Consequently, the anodization process must be controlled to form a porous structure and avoid electropolishing of the semiconductor film. Further, it is preferred that the semiconductor material remaining between the pores  111  remains electrically conductive, such that there is a current path from the surface of the porous electrode, through the porous layer and to the metal substrate  112  (current collector). Furthermore, the pore size and spacing is dependent on the anodization conditions and the doping level of the semiconductor material. The dopant type and level and the anodization conditions are chosen to meet a desired porosity and maintain electrical conductivity of the porous semiconductor. The anodization may be controlled so that pores  111  extend part way through or completely through the semiconductor film  110 . 
         [0022]      FIG. 2  shows a schematic of a high throughput linear electrochemical processing system  200 . System  200  includes a tank  202  which contains an electrolyte  206 , a cathode  204 , and a continuous thin film  220 . System  200  is configured for electrochemical processing of the continuous thin film  220  which is directed through the processing tank  202  by a plurality of rollers  222 . A controller  208  is connected between the cathode  204  and the continuous thin film  220 , which is held at earth potential. The controller  208  is operated as described above for controller  108 . The continuous thin film  220  may be comprised of a semiconductor film on a thin flexible metal substrate. 
         [0023]    Further to the configuration shown in  FIG. 2 , anodization may be carried out using a spray tool, rather than requiring complete immersion in an electrolyte. 
         [0024]      FIG. 3  shows a cross section of an energy storage device, which in this example is a battery cell  300 . The battery cell  300  comprises an anode current collector  312 , a porous anode  310 , a separator  314 , a battery electrolyte  315 , a cathode  316  and a cathode current collector  318 . The anode current collector  312  may be a metal such as copper, chosen for its good electrical conductivity, mechanical stability and flexibility. The porous anode  310  may be a porous semiconductor material such as porous silicon, porous germanium, etc. The semiconductor material is chosen for its suitability for forming a porous structure using electrochemical anodization, where the semiconductor thin film is rendered porous by anodization, without compromising the electrical conductivity of the remaining semiconductor material—in other words, the semiconductor material between the pores is electrically conductive and electrically connected through the semiconductor anode  310  to the anode current collector  312 . The battery electrolyte  315  may be a chemical such as propylene carbonate, ethylene carbonate, LiPF 6 , etc. The separator  314  may be porous polyethylene, porous polypropylene, etc. The cathode  316  may be a metal foil, such as lithium foil, or a material such as LiCoO 2 . The cathode current collector may be aluminum. Note that the electrolytes, separators and electrodes must be matched to provide desirable battery performance. 
         [0025]      FIG. 4  shows a cylindrical energy storage device, which in this example is a cylindrical battery  400 . Flexible thin battery cell  440  includes an isolation layer—such as an insulating layer covering one surface of the cell  440 —which prevents shorting of the battery electrodes when the battery cell is rolled up. Electrical contacts  442  and  444  are made to the top and bottom surfaces, respectively, of the battery cell  440 .  FIG. 5  shows an alternative configuration of the battery cells  440 , forming a battery stack  500 . The battery cells  440  within the battery stack  500  may be electrically connected together either in series or in parallel. (The electrical connections are not shown.) 
         [0026]    Referring again to  FIG. 3 , a method for fabricating an embodiment of the battery cell  300  is described. A metal film is provided for the anode current collector (ACC)  312 . A thin film  310  of semiconductor material is deposited on the ACC  312 . Suitable deposition processes may include processes such as chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma enhanced chemical vapor deposition (PECVD), and thermal spray in an inert environment. The ACC  312  may be a continuous thin metal film and may be moved linearly through the semiconductor deposition tool. A reel-to-reel system may be utilized for linear movement of the ACC  312 . The semiconductor thin film  310  is anodized to increase the electrode surface area. In the case of a continuous thin film, the film may be moved through the anodization tool during the anodization process. Again, a reel-to-reel system may be used. A separator film  314  is applied to the surface of the anodized semiconductor electrode  310 . A cathode  316  and cathode current collector (CCC)  318  are applied to the top surface of the separator  314 . The cathode  316  and CCC  318  are most conveniently prepared by depositing the cathode material on the CCC  318 . The stack may then be covered by an insulating layer  319  and then rolled to form a cylindrical battery  400 , as shown in  FIG. 4 , or stacked to form a rectangular format battery, as shown in  FIG. 5 . The battery cells  300 ,  440  are then injected with battery electrolyte  315  and are sealed. 
         [0027]    The methods of the present invention may also be applicable to forming electrodes for energy storage devices using porous germanium. Germanium thin films may be deposited using HVM compatible processes, as described above for silicon film deposition, and the germanium may be rendered porous following the general anodization methods described above for silicon. Furthermore, the methods of the present invention may also be applicable to forming electrodes for energy storage devices using porous compound semiconductors such as SiGe, GaAs, etc. 
         [0028]    Although the present invention has been particularly described with reference to embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications. The following claims define the present invention.