Abstract:
The invention provides improved paper-like electrodes and electrode active materials for use in flexible energy storage devices, and methods for preparing such electrodes and materials, as well as flexible energy storage devices fabricated from such electrodes and materials and methods of making such devices. The electrodes and electrode active materials comprise multi-layer high-quality thin carbon films, and the methods comprise the use of a repetitive laminar process to deposit such films directly on polymer separators or electrolyte membranes.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates broadly to free-standing and flexible energy storage devices, such as batteries and supercapacitors, and in particular, to electrode materials for such devices, and to methods for the preparation of the same. More specifically, this invention relates to batteries and supercapacitors which incorporate paper-like electrode materials constructed from thin carbon films such as graphene. 
       BACKGROUND OF THE INVENTION 
       [0002]    In order to cultivate interest in and promote the market penetration of sophisticated and multifunctional “smart” electronics with enhanced functions, such as rollup displays, electronic textiles, wearable gadgets, and printed circuits and devices that can be incorporated into curved objects, flexible energy storage systems with enhanced foldability and conformability must be developed. In recent years, significant progress has been made towards replacing the rigid metallic substrates and packages of conventional batteries and supercapacitors with ones that are light and flexible. However, because conventional battery and supercapacitor geometries are still too bulky and heavy, fully configurable, integratable and reliable energy storage systems are not yet widely available. 
         [0003]    The incorporation of carbonaceous nanomaterials such as carbon nanotubes, graphene, and conductive polymers into electronic components presents an appealing approach to enable flexible energy storage devices. In particular, graphene, a two-dimensional planar sheet or monolayer of conjugated carbon atoms, in which the carbon atoms are densely packed in a honeycomb crystal lattice comprising polycyclic aromatic rings with covalently bonded carbon atoms having sp 2  orbital hybridization, has been demonstrated as an attractive charge storage material and conductive additive in battery and supercapacitor electrodes. Graphene shows improved charge storage capability over other carbon allotropes, which is attributable to its extremely large surface area. In addition, the superior mechanical robustness and integrity of graphene eliminates the need for substrates and polymer binders, and the high electrical conductivity and stability of graphene allows the engineering of flexible, free-standing batteries and supercapacitors without sacrificing the charge/discharge rate capability and without reducing the life cycle of such devices. The removal of inactive substrates and additives further reduces the total weight and volume of the electrodes in such devices, which potentially enables thin and lightweight device designs with improved energy and power output. 
         [0004]    To fabricate graphene-based electrodes, previous efforts primarily involved the preparation of graphene-based films or conformal coatings by filtration or wet deposition of graphene nanoplatelets, graphene oxide powders or reduced graphene oxide nanosheets, followed by drying and/or post reduction conversion of the graphene oxide to graphene. The electrodes are then physically stacked with polymer separators into conventional battery or supercapacitor configurations. A recent study proceeded with dip coating of graphene ink onto the surface of macroporous fiber membranes or textiles, which facilitated the direct assembly of electrode materials to the separator membranes. This yielded an integratable and stretchable paper-like supercapacitor that could find applications in wearable electronics and energy harvesting. 
         [0005]    However, the discontinuous graphene sheets produced from reduction of graphene oxide precursors, as mentioned above, or even from exfoliation of graphite flakes, suffer from poor mechanical strength, low electrical conductivity, a strong tendency towards agglomeration, and an inability to control the quality and morphology of the graphene, all of which, in turn, hampers the overall charge storage and rate performance. Furthermore, the chemical reduction reactions do not always achieve complete reduction of the graphene oxide precursors, leaving “patches” of graphene oxide that lead to degraded electrical conductivity, thus reducing performance of the resulting electrode material. In addition, the “dip and drying” fabrication of a paper-like supercapacitor or battery electrodes necessitates the utilization of superabsorbent membrane materials, such as cotton sheets, that are prone to aging or oxidation, and hence are proscribed in practical electrochemical systems. Therefore, these device designs may not be applicable in practical circumstances to meet the omnipresent safety requirements. Accordingly, and for all of these reasons, a satisfactory alternative technique is needed for preparing graphene-based electrodes for use in flexible energy storage systems. 
         [0006]    It is therefore the principal object of the present invention to provide paper-like electrode materials constructed from thin carbon films such as graphene, and methods for preparing such materials, for use in flexible energy storage systems. 
         [0007]    It is another object of the present invention to provide paper-like electrode materials constructed from thin carbon films such as graphene, and methods for preparing such materials, which do not require filtration or wet deposition of graphene nanoplatelets, graphene oxide powders or reduced graphene oxide nanosheets, followed by drying and/or post reduction conversion of the graphene oxide to graphene, and which do not require dip coating of graphene ink onto the surface of macroporous fiber membranes or textiles. 
         [0008]    It is yet another object of the present invention to provide a versatile approach to the design of flexible and free-standing paper-like energy storage devices, including aqueous, non-aqueous and solid-state batteries and supercapacitors. 
       SUMMARY OF THE INVENTION 
       [0009]    These and other objects of the present invention are achieved by providing methods for constructing flexible energy storage systems which comprise the use of a repetitive laminar process to produce multi-layer high-quality thin carbon (i.e., graphene) films. Such films constitute electrode materials that are paper-like and, when directly integrated with polymer separators or electrolyte films, can function as electrodes that can be assembled into batteries and/or supercapacitors that are foldable and conformable. The objects of the present invention are also achieved by providing such paper-like electrode materials for use in flexible energy storage systems, which materials comprise pre-formed combinations of multi-layer graphene films with one or more polymer separators or electrolyte membranes. 
         [0010]    More specifically, the methods of the invention for forming paper-like thin carbon film electrodes comprise providing a first thin carbon film layer disposed on a first substrate, providing a second thin carbon film layer disposed on a second substrate, applying a resist composition to the second layer so as to substantially coat the second layer, drying the resist coating, releasing the second layer from the second substrate, positioning the second layer on top of and in contact relationship with the first layer so as to form a stack, removing the resist coating from the top of the stack, and then repeatedly adding further thin carbon film layers to the stack in the same manner until the stack reaches the desired thickness, followed by removing the first substrate from the bottom of the stack and then transferring the remainder of the stack to an isolator, thereby forming the electrode. 
         [0011]    Thus, one aspect of the present invention generally concerns improved electrodes and electrode materials for batteries and supercapacitors. One embodiment of this aspect provides the electrode material itself, while another embodiment provides an electrode employing such material, and yet another embodiment of this aspect of the invention provides a battery and/or supercapacitor employing one or more such electrodes. 
         [0012]    In still other embodiments of this aspect of the invention, improved flexible, paper-like electrodes for a supercapacitor, for a lithium-ion secondary battery, and for a lithium-air secondary battery are provided, and in still other embodiments of this aspect of the invention, an improved supercapacitor, an improved lithium-ion secondary battery, and an improved lithium-air secondary battery are provided. 
         [0013]    Another aspect of the invention generally concerns improved methods for manufacturing supercapacitors, lithium-ion secondary batteries and lithium-air secondary batteries. In one embodiment of this aspect of the invention, a method for preparing a bi- and/or multi-layer thin carbon film for use in electrode materials for such batteries and supercapacitors is provided. In another embodiment of this aspect of the invention, a method for manufacturing an electrode material for such batteries and supercapacitors is provided. 
         [0014]    It is a feature of the present invention that it can be used to fabricate a battery or supercapacitor that is fully bendable and stretchable. 
         [0015]    It is another feature of the present invention that the use of metal substrates as current collectors and as supports for the electrode material is completely eliminated, resulting in a lightweight device geometry with reduced complexity in packaging. 
         [0016]    It is yet another feature of the present invention that since the fabrication procedure does not rely on specific polymer membranes as a device component, commercial polymer separators, gel-electrolyte or solid-electrolyte membranes with excellent mechanical tolerance and chemical sustainability can be incorporated readily, leading to more diverse device formats that can operate in relatively harsh thermal environments, and that are more resistant to tensile deformations and chemical attack. 
         [0017]    It is still another feature of the present invention that by using continuous graphene films with large lateral dimensions and long-range ordering, better electron conduction and structural homogeneity can be obtained, as compared with graphene nanoplatelets or reduced graphene oxide nanosheets, thus enhancing the rate capability and cyclability of the flexible energy storage devices produced. 
         [0018]    It is a further feature of the present invention that the electrochemical performance of the flexible energy storage devices produced can be further optimized in a well-controlled manner by engineering the structure, surface chemistry and the number of layers of graphene that are used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    These and other aspects, features, objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description of the presently most preferred embodiments thereof (which are given for the purposes of disclosure), when read in conjunction with the accompanying drawings (which form a part of the specification, but which are not to be considered as limiting its scope), wherein: 
           [0020]      FIG. 1  is a schematic view depicting a conventional prior art energy storage system, such as a battery or a supercapacitor; 
           [0021]      FIG. 2  is a sequential diagrammatic view depicting the process according to the invention by which a paper-like graphene bi-layer film may be formed on a metallic foil substrate such as copper; 
           [0022]      FIG. 3  is a sequential diagrammatic view depicting the process according to the invention by which a paper-like multi-layer graphene-based electrode may be fabricated; 
           [0023]      FIG. 4  is an enlarged fragmentary schematic view depicting a flexible symmetric supercapacitor formed in accordance with the invention, having two paper-like multi-layer graphene-based electrodes fabricated according to the process depicted in  FIG. 3 ; 
           [0024]      FIG. 5  is an enlarged fragmentary schematic view depicting a flexible lithium-air secondary battery, fabricated in accordance with the invention, having a paper-like multi-layer graphene-based cathode formed according to the process depicted in  FIG. 3  and attached to one surface of an isolator, and having a conventional lithium metal foil anode attached to the opposite surface of the isolator; 
           [0025]      FIG. 6  a sequential diagrammatic view depicting the process according to the invention by which a multi-layer, graphene-based hybrid electrode may be fabricated; 
           [0026]      FIG. 7  is an enlarged fragmentary schematic view depicting a flexible lithium-ion secondary battery fabricated, in accordance with the invention, having a paper-like multi-layer graphene-based hybrid cathode formed according to the process depicted in  FIG. 6  and attached to one surface of an isolator, and having a paper-like multi-layer graphene-based anode formed according to the process depicted in  FIG. 3  and attached to the opposite surface of the isolator; 
           [0027]      FIG. 8  is an enlarged fragmentary schematic view depicting a flexible lithium-ion secondary battery fabricated, in accordance with the invention, having a paper-like multi-layer graphene-based anode formed according to the process depicted in  FIG. 3  and attached to one surface of an isolator, and having a conventional cathode, consisting of an aluminum current collector coated with electrochemically active materials, attached to the opposite surface of the isolator; and 
           [0028]      FIG. 9  is an enlarged fragmentary schematic view depicting a flexible lithium-ion secondary battery fabricated, in accordance with the invention, having a paper-like multi-layer graphene-based hybrid cathode formed according to the process depicted in  FIG. 6  and attached to one surface of an isolator, and having a conventional anode, consisting of a copper current collector coated with electrode active materials, attached to the opposite surface of the isolator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    The preferred and other embodiments of each aspect of the present invention will now be further described. Although the invention will be illustratively described hereinafter with reference to the formation of a graphene film on a copper foil substrate, it should be understood that the invention is not limited to the specific case described, but extends also to the formation of graphene films utilizing other metallic foils (including nickel foils or aluminum foils) or other substrates. 
         [0030]    Referring first to  FIG. 1 , the structure and configuration of a conventional prior art energy storage system  1  (i.e., a battery or a supercapacitor) is depicted schematically, in which a conventional anode material  2  positioned adjacent an associated conventional anode current collector  3 , as well as a conventional cathode material  4  positioned adjacent an associated conventional cathode current collector  5 , are physically stacked in a package  6 , with the electrolyte  7  and a polymer separator  8  positioned in between the electrodes (i.e., between anode material  2  and cathode material  4 ). Although current collectors  3  and  5 , which are generally formed from metallic substrates, and package  6  itself are usually thin, none of them is bendable or stretchable to any great degree, and therefore the resulting battery or supercapacitor  1  is not particularly flexible and is ill-suited for use in flexible electronic devices. 
         [0031]    Referring now to  FIG. 2  in addition to the aforementioned  FIG. 1 , the preferred embodiments of the present invention will now be described. One aspect of the invention relates to a multi-layer thin carbon film, and in particular, a bi-layer graphene film, that may be fabricated in accordance with the invention by the process depicted in  FIG. 2 . That process initially comprises either providing, or forming, two instances of a first precursor material  9  (these two instances are respectively designated  9 A and  9 B in  FIG. 2 ). First precursor material  9  comprises a graphene film  10  comprised of either a unitary monolayer of graphene, or no more than about five monolayers of graphene, supported on a surface of a generally flat copper foil substrate  20 ; the copper foil substrate  20  is shown schematically at step  201  in  FIG. 2 , prior to the formation of first precursor material  9 . 
         [0032]    The graphene film  10  may be formed, as shown at step  202  in  FIG. 2 , by any known process, such as, for example, via chemical vapor deposition in a conventional CVD furnace (not shown) at a temperature in the range of 500-1,200 degrees C., and preferably at about 1,000 degrees C., in the manner described generally in prior art U.S. Patent Application Publication No. 2011/0091647, although alternative vapor deposition processes such as PECVD or ALD may be used. For the purposes of the present invention, the area of the copper foil substrate  20  on which the graphene film  10  is formed preferably may range from approximately 1 cm×1 cm to approximately 10 m×100 m, and the thickness of the copper foil substrate  20  preferably may range from approximately 1 μm to approximately 1,000 μm. As can be seen in  FIG. 2 , within first precursor material  9 , graphene film  10  has a first surface  30  that is positioned adjacent the surface of copper foil substrate  20 , and a second surface  35  that is not adjacent substrate  20  and is exposed. 
         [0033]    In the next step in the fabrication process, substantially the entire exposed second surface  35  of one instance (instance B) of first precursor material  9  is then coated with a layer of a polymeric photo-resist  40 , thus creating a second precursor material  45 , as shown at step  203  in  FIG. 2 . Preferably, the polymeric photo-resist layer  40  is composed of polymethylmethacrylate (“PMMA”), which is available commercially, dissolved in anisole in a variety of concentrations, from a number of manufacturers, such as MicroChem. Corp. of Newton, Mass., U.S.A., which markets this material in bottle form. For the purposes of the present invention, any concentration of PMMA up to about 60% may be used, although the concentration that is preferred ranges from about 0.5% to about 20%. The PMMA may be coated onto the second surface  35  of first precursor material  9  either by spray coating, or by spin-coating, or even by dipping (i.e., immersing) the graphene film directly into the PMMA solution (each of these methods is well known in the art, and therefore none of them is illustrated in the drawings). Regardless of which coating method is used, however, the polymeric photo-resist layer  40  is thereafter either dried by baking, or by allowing it to air dry. The purpose of the polymeric coating  40  is to provide additional support for the graphene film  10  during subsequent steps in the fabrication process. 
         [0034]    Thereafter, as shown at step  204  in  FIG. 2 , the copper foil substrate  20  is removed from the second precursor material  45  preferably via etching, using conventional copper etching materials which are commercially available. For example, an acidic solution or an oxidizing agent may be used to etch away the copper foil substrate  20 , thereby releasing the graphene film  10  which, with the polymeric photo-resist layer  40  still attached, forms a third precursor material  50 . Etching techniques by which to remove the copper foil substrates on which graphene films have been deposited are well known in the art, and therefore will not be further described herein. 
         [0035]    Following the release of the graphene film  10  from the copper foil substrate  20  (i.e., after the etching step is complete), the resulting third precursor material  50  is physically stacked, as shown at step  205  in  FIG. 2 , upon the other instance (instance A) of first precursor material  9 , such that the released side of graphene film  10  in third precursor material  50  is positioned adjacent to the exposed surface  35  of the graphene film in instance A of first precursor material  9 . After the stacking step, the polymeric coating  40  is then removed or eliminated, preferably by rinsing in an organic solvent such as acetone, yielding a resulting first composite material  55 , as shown at step  206  in  FIG. 2 , which comprises a graphene bi-layer film  60  attached to a copper foil substrate. This first composite material  55  may then be used directly, with the copper foil substrate still attached, or the graphene bi-layer film  60  may be separated or transferred from the substrate in a known manner (not shown), preferably via etching, and then the separated graphene bi-layer  60  may be utilized in a graphene application or otherwise further processed for ultimate use. 
         [0036]    Referring now to  FIG. 3  in addition to the aforementioned  FIGS. 1 and 2 , another aspect of the present invention relates to paper-like multi-layer thin carbon film electrodes for use in flexible energy storage systems. In a preferred embodiment, a multi-layer graphene film electrode may be fabricated in accordance with the invention by a process which comprises initially preparing the composite material  55 , in accordance with the procedure described above in connection with  FIG. 2  for formation of a bi-layer graphene film, which thereafter serves as a base during subsequent steps in the fabrication process, as shown at step  301  in  FIG. 3 . Then, further graphene film layers are added to that base in a laminar fashion, first by repeating stacking step  205 , utilizing in each repetition a new instance of third precursor material  50  (which, as will be evident to those skilled in the art, may itself be prepared separately from yet another new instance of first precursor material  9 , using the coating step  203  and the etching step  204  described above), and then by repeating the removal step  206  to eliminate the polymeric coating  40  from the resulting composite via rinsing (these repetitions of steps  205  and  206  are not shown in  FIG. 3 ), until the desired number of graphene film layers is reached, thereby forming a second composite material  65 , as shown at step  302  in  FIG. 3 . 
         [0037]    As will be apparent to those skilled in the art, the number of repetitions used will be determined by the desired properties or the field of application of the electrode and/or energy storage device being fabricated. It should be understood, however, that the resulting second composite material  65  will include a copper foil substrate  20  coated with a multi-layer graphene film comprising up to as many as about 1 million monolayers graphene sheets). After the desired number of layers has been produced, the copper foil substrate underlying the base is then removed from the second composite material  65 , again preferably via etching, as shown at step  303  in  FIG. 3 , leaving the multi-layer graphene film  70  which also constitutes an electrode active material that can be utilized as a graphene-based electrode. 
         [0038]    In order to use it in a supercapacitor or a battery, electrode  70  may then be transferred, as shown at step  304  in  FIG. 3 , to one side of a flexible isolator  75 , which comprises either a polymer separator or a solid-state polymer electrolyte film. The polymer separator can be a porous membrane that may preferably be made from any one of several materials, including but not limited to polyethylene, polypropylene and glass fiber. Such separators are commercially available from a wide variety of sources such as Celgard, LLC of Charlotte, N.C., U.S.A. and Membrana GmbH of Wuppertal, Germany. The solid-state polymer electrolyte film may be, for example, a gel polymer electrolyte, which preferably may be formed from any one of several polymer film materials, including, but not limited to, and most preferably selected from, the group consisting of poly(vinylidene fluoride-co-hexafluoropropene), poly(ethylene oxide), poly(propylene oxide) and poly(acrylonitrile) films, in which one or more of the lithium salts is dissolved, the lithium salt(s) preferably being selected from the group consisting of LiPF 6 , LiBF 4 , LiClO 4 , LiAlO 2  and LiCF 3 SO 3 . Such polymer film materials and lithium salts are commercially available from a variety of sources including Kureha Corporation of Tokyo, Japan, from which the polymer film materials may be obtained, and Honeywell International Inc. of Danbury, Conn., U.S.A., from which the lithium salts may be obtained. 
         [0039]    Following transfer to isolator  75 , the polymeric coating  40  is removed, again preferably by rinsing in an organic solvent such as acetone, yielding a product  80 , as shown at step  305  in  FIG. 3 , which may advantageously be incorporated into, and actually form a part of, an energy storage device such as a supercapacitor or a lithium-ion battery or lithium-air battery, as described below. 
         [0040]    Referring now to  FIGS. 4 and 5  in addition to the aforementioned  FIGS. 1-3 , a flexible symmetric supercapacitor device, which constitutes yet another aspect of the invention, may be formed by attaching a graphene-based electrode  70 , fabricated in accordance with the invention, to both surfaces of either a polymer separator or a solid-state polymer electrolyte film, as shown in  FIG. 4 . In other words, after the graphene-based electrode  70  is formed and is attached to one surface of isolator  75  as described above (at steps  304  and  305  in  FIG. 3 ), thereby forming the product  80 , a second, separate multi-layer graphene film electrode  70  may be produced in the same manner (this second production process is not shown in the drawings), after which the second multi-layer graphene film electrode may be transferred to the opposite surface of isolator  75  of product  80 , resulting in the isolator  75  being “sandwiched” between two graphene-based electrodes. This flexible laminar sandwich  85  functions as a supercapacitor, i.e., it may be charged by connecting the electrodes to a source of electrical current, and it will retain that charge until it is discharged. 
         [0041]    Similarly, as shown in  FIG. 5 , a flexible lithium-air secondary battery, which constitutes still another aspect of the invention, may be formed by attaching a graphene-based electrode  70 , fabricated in accordance with the invention, to one surface of an isolator, first by attaching the electrode to an isolator  75  (thereby forming the product  80 ), as described above, and then by attaching a lithium metal foil  90  to the opposite surface of the isolator. Such lithium foils are commercially available from a variety of sources such as Sigma-Aldrich Co. LLC of St. Louis, Mo., U.S.A. and Alfa Aesar of Ward Hill, Mass., U.S.A. In this configuration, the graphene-based electrode acts as a catalytic cathode for oxygen reduction and evolution reactions, while the lithium metal foil  90  functions as an anode. 
         [0042]    The morphology and composition of the graphene-based cathode can be engineered so as to achieve the optimum capacity and rate performance for such a lithium-air secondary battery. For example, in order to accelerate oxygen diffusion (which is depicted by the arrows A in  FIG. 5 ), in-plane pores can be introduced into the graphene layers by physical irradiation with energetic molecules, such as electron or ion beams, or by chemical etching with potassium hydroxide or by acid activation. As another example, in order to enhance the catalytic properties of the graphene-based cathode, heteroatoms such as nitrogen and/or boron can be introduced into the graphene layers by heat treatment of the graphene in nitrogen- and/or boron-containing gases, such as ammonia (NH 3 ) and/or boron chloride (BCl 3 ). 
         [0043]    Referring now to  FIG. 6  in addition to the aforementioned  FIGS. 1-5 , another aspect of the invention relates to the assembly of a multi-layer graphene-based hybrid electrode, which can be assembled in a laminar manner similar to that shown in  FIG. 3  for the assembly of a multi-layer graphene-based (non-hybrid) electrode. As shown at step  601  in  FIG. 6 , the fabrication process initially comprises either providing, or forming, two instances of first precursor material  9  (for ease of illustration, only one instance is shown in  FIG. 6 ). Thereafter, nanoparticles  95  of one or more electrochemically active materials preferably comprised of lithium metal salts, and more preferably selected from among lithium metal oxides and lithium metal phosphates (such as LiMn 2 O 4 , LiCoO 2 , or LiFePO 4 ), the particles ranging in diameter from approximately 1 nm to approximately 1 mm, are deposited onto the exposed second surface  35  of graphene film  10  of first precursor material  9 . Such electrochemically active materials are commercially available from a number of sources such as Umicore Group of Brussels, Belgium and Tronox Limited of Stamford, Conn., U.S.A., but can alternatively be fabricated directly via chemical routes, such as solid state calcination, solution phase precipitation and/or sol-gel methods. 
         [0044]    Preferably, and as shown at step  602  in  FIG. 6  (but illustratively for only one instance of first precursor material  9 ), nanoparticles  95  are deposited onto the exposed surface of the graphene film via spray coating through a nozzle  100 , followed by air drying, although other application methods may be used, such as spin-coating or even dip-coating (i.e., immersing) the graphene film directly into the particles, and thereafter allowing it to air dry (these alternative methods are not shown in the drawings, as they are well known in the art). The application of the electrochemically active materials to surface  35  of both instances of first precursor material  9  creates two instances of a tri-layer first hybrid precursor material  105 , each comprising a hybrid graphene-nanoparticle film  110  supported on a copper foil substrate  20  (these instances are respectively designated  105 A and  105 B in  FIG. 6 ). 
         [0045]    The next step in the fabrication process is to coat the hybrid graphene film  110  in one instance (instance B) of tri-layer first hybrid precursor material  105  with a layer of a polymeric photo-resist  40 , as shown at step  603  in  FIG. 6 , in the same manner as described above in connection with  FIG. 2 , thus creating a second hybrid precursor material  115 . 
         [0046]    Thereafter, as shown at step  604  in  FIG. 6 , the copper foil substrate  20  is removed from second hybrid precursor material  115  via etching, thereby releasing hybrid graphene film  110  which, with the polymeric photo-resist layer  40  still attached, forms a third hybrid precursor material  120 . Following the release of hybrid graphene film  110  from the copper foil substrate  20  (i.e., after the etching step is complete), the resulting third hybrid precursor material  120  is physically stacked, as shown at step  605  in  FIG. 6 , upon the other instance (instance A) of tri-layer first hybrid precursor material  105 , such that the released side of hybrid graphene film  110  in third hybrid precursor material  120  is positioned adjacent to the exposed surface of the hybrid graphene film in instance A of first hybrid precursor material  105 . After this stacking step, the polymeric photo-resist layer  40  is then removed by rinsing (this step is not shown in the drawings), yielding a product which thereafter serves as a base during subsequent steps in the fabrication process. 
         [0047]    Then, further hybrid graphene film layers are added to that base in a laminar fashion, first by repeating stacking step  605 , utilizing in each repetition a new instance of third hybrid precursor material  120  (which, as will be evident to those skilled in the art, may itself be prepared separately from a new instance of first hybrid precursor material  105 , using the coating step  603  and the etching step  604  described above), and then by repeating the removal step (not shown in the drawings) to eliminate the polymeric photo-resist layer  40  from the resulting composite via rinsing (these repetitions of step  605  and the removal step are not shown in  FIG. 6 ), until the desired number of hybrid graphene film layers is reached, thereby forming a hybrid composite material  125  which includes a copper foil substrate coated with a multi-layer hybrid graphene film  130  comprising up to as many as about 1 million graphene monolayers with embedded electrochemically active nanoparticles, as shown at step  606  in  FIG. 6 . After the desired number of layers has been produced, the copper foil substrate  20  underlying the base is then removed, as shown at step  607  in  FIG. 6 , from hybrid composite material  125 , again preferably via etching, leaving the multi-layer hybrid graphene film  130  which also constitutes an electrode active material that can be utilized as a multi-layer graphene-based hybrid electrode. 
         [0048]    In order to use it in a lithium-ion secondary battery, electrode  130  may then be transferred, as shown at step  608  in  FIG. 6 , to one side of a flexible isolator  75 , which again comprises either a polymer separator or a solid-state polymer electrolyte film, as mentioned above. Following this transfer to isolator  75 , the polymeric coating  40  is removed, again preferably by rinsing in an organic solvent such as acetone (this removal step is not shown in the drawings). 
         [0049]    Referring now to  FIGS. 7-9  in addition to the aforementioned  FIGS. 1-6 , several different flexible lithium-ion secondary batteries with paper-like electrodes may also be formed in accordance with, and constitute still further aspects of, the invention. As shown in  FIG. 7 , a flexible lithium-ion secondary battery with a pair of paper-like electrodes may be formed by attaching a multi-layer graphene-based hybrid electrode  130 , fabricated in accordance with the invention, to one surface of an isolator  75 , thus forming the cathode, and by attaching to the opposite surface of isolator  75  a multi-layer graphene-based (non-hybrid) electrode  70 , which functions as the anode for lithium ion storage. As shown at step  608  in  FIG. 6 , this assembly can best be accomplished by attaching (via an evaporation transfer procedure) electrode  130 , formed as described above in connection with  FIG. 6 , to one side of an isolator  75 , which already carries a multi-layer graphene-based (non-hybrid) electrode  70 , formed as described above in connection with  FIG. 3 , on its opposite side. The graphene-based hybrid electrode  130  may comprise only a single hybrid graphene-nanoparticle layer, or a vast number of such layers (e.g., up to as many as about 1 million layers), resulting in a total thickness ranging from about 1 nm to about 1 mm, and as will be apparent to those skilled in the art, the number of layers (i.e., the film thickness), as well as the amount of the electrochemically active material introduced and the assembly conditions such as the coating parameters (e.g., the coating velocity and drying temperature), can each be varied in order to maximize the homogeneity and utility of electrode  130 , and thereby optimize its performance. 
         [0050]    Furthermore, as shown in  FIGS. 8 and 9 , flexible lithium-ion batteries with a single paper-like electrode are also within the scope of the invention. In one embodiment, as illustrated in  FIG. 8 , the anode can comprise a multi-layer graphene-based electrode  70  supported on an isolator  75  and formed in accordance with the invention, while the cathode can conventionally comprise an aluminum current collector  135  coated with electrochemically active materials  140  (such as LiMn 2 O 4 , LiCoO 2 , or LiFePO 4 , as mentioned above) in powder form. In another alternative, as depicted in  FIG. 9 , the cathode can comprise a multi-layer graphene-based hybrid electrode  130  supported on an isolator  75  and formed in accordance with the invention, while the anode can conventionally comprise a copper current collector  145  coated with electrode active materials  150  such as, for example, intercalation carbon materials (e.g., graphite, carbon nanotubes or carbon nanospheres), metals (e.g., silicon [Si], germanium [Ge] or tin [Sn]), transition metal oxides (e.g., tin dioxide [SnO 2 ], iron oxide [Fe x O y ] or manganese dioxide [MnO 2 ]), electrically conducting polymeric materials (e.g., polyaniline [“PANi”], polypyrrole [“PPy”] or poly(3,4-ethylenedioxythiophene) [“PEDOT”]), or alloy powders (e.g., silicon-germanium [Si—Ge] alloy or silicon-iron [Si—Fe] alloys). As those skilled in the art will understand, all of these materials are conventional electrode active materials which are available commercially or can be fabricated by conventional chemical methods. 
         [0051]    While there has been described what are at present considered to be the preferred embodiments of the present invention, it will be apparent to those skilled in the art that the embodiments described herein are by way of illustration and not of limitation. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. Therefore, it is to be understood that various changes and modifications may be made in the embodiments disclosed herein without departing from the true spirit and scope of the present invention, as set forth in the appended claims, and it is contemplated that the appended claims will cover any such modifications or embodiments.