JP7126492B2 - Graphene oxide bonded metal foil thin film current collector - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is directed to U.S. Patent Application Nos. 15/231,486 and 15/231,498, filed Aug. 8, 2016, the contents of which are hereby incorporated by reference. Claim priority.

The present invention provides current collectors for lithium batteries or supercapacitors. The current collector is a metal foil bonded to a thin graphene oxide film made from graphene oxide gel. This graphene oxide bonded thin metal foil is electrolyte compatible, non-reactive, anti-corrosion, low contact resistance, thermal and electrical conductivity, ultra-thin, and lightweight, enabling batteries or capacitors to achieve higher output voltages, more Allows for high energy density, high rate capability, and longer cycle life.

This patent application describes lithium cells (such as lithium-ion cells, lithium-metal cells, or lithium-ion capacitors), supercapacitors, non-lithium batteries (such as zinc-air cells, nickel-metal hydride batteries, sodium-ion cells, and magnesium-ion cells). cells), and other electrochemical energy storage cells, as current collectors that work with the anode electrode (anode active material layer) or cathode electrode (cathode active material layer). This application is not directed to the anode active material layer or the cathode active material layer per se.

Lithium-metal cells include conventional lithium-metal rechargeable cells (e.g., using lithium foil as anode and _MnO2 particles as cathode active material), lithium-air cells (Li-Air), lithium-sulphur cells ( Li-S), and advanced lithium-graphene cells (using graphene sheets as the cathode active material, Li-graphene), lithium-carbon nanotube cells (using CNT as the cathode, Li-CNT), and lithium-nanocarbon Included are cells (Li—C, using nanocarbon fibers or other nanocarbon materials as the cathode). The anode and/or cathode active material layers may contain some lithium or may be prelithiated prior to or immediately after assembly of the cell.

Rechargeable lithium-ion (Li-ion), lithium-metal, lithium-sulphur, and Li-metal-air batteries are widely used in electric vehicles (EV), hybrid electric vehicles (HEV), and portable electronic devices such as laptop computers and mobile phones. It is considered to be a promising power source for telephones. Lithium as the metallic element is the highest compared to any other metal or metal intercalated compound (except Li _4.4 Si, which has a specific capacity of 4,200 mAh/g) as the anode active material It has a lithium storage capacity (3,861 mAh/g). Thus, in general, Li metal batteries (with lithium metal anodes) have significantly higher energy densities than conventional lithium ion batteries (with graphite anodes).

Traditionally, rechargeable lithium metal batteries have used relatively high specific capacities such as TiS ₂ , MoS ₂ , MnO ₂ , CoO ₂ , and V ₂ O ₅ as cathode active materials combined with lithium metal anodes. was prepared using a non-lithiated compound with When the battery was discharged, lithium ions were transferred through the electrolyte from the lithium metal anode to the cathode, and the cathode was lithiated. Unfortunately, upon repeated charging and discharging, lithium metal formed dendrites at the anode that ultimately resulted in internal short circuits, thermal runaway, and explosion. As a result of a series of accidents related to this problem, production of these types of secondary batteries was discontinued in the early 1990's and replaced by lithium-ion batteries. Even now, cycle stability and safety issues remain major obstacles to further commercialization of Li-metal batteries (e.g., lithium-sulphur and lithium-transition metal oxide cells) for EV, HEV, and microelectronic device applications. It is the cause.

The aforementioned concerns about the safety of prior lithium metal secondary batteries have stimulated the development of lithium ion secondary batteries in which high purity lithium metal sheets or films are used as anode active materials with carbonaceous materials (e.g. natural graphite particles). The carbonaceous material absorbs lithium (eg, by intercalation of lithium ions or atoms between graphene planes) and desorbs lithium ions during each recharge and discharge phase of lithium ion battery operation. The carbonaceous material may firstly comprise graphite capable of intercalating lithium, and the resulting graphite intercalation compound may be represented as Li _x C ₆ , where x is typically less than 1 (graphite specific capacity is <372 mAh/g).

Lithium-ion (Li-ion) batteries are promising energy storage devices for electric-powered vehicles, but state-of-the-art Li-ion batteries meet cost, safety, and performance goals (e.g., high specific energy, high energy density, good cycle stability, and long cycle life). Li-ion cells typically use a lithium transition metal oxide or phosphate as the positive electrode (cathode) that de/reintercalates Li ⁺ at high potentials against a carbon negative electrode (anode). The specific capacity of lithium transition metal oxide or phosphate-based cathode active materials is typically in the range of 140-170 mAh/g. As a result, the specific energy (mass energy density) of commercial Li-ion cells featuring graphite anodes and lithium transition metal oxide or phosphate-based cathodes is typically 120-220 Wh/kg, most typically 150-200 Wh/kg. kg range. A corresponding typical range for energy density (volumetric energy density) is 300-400 Wh/L. These specific energy values are one-half to one-third the specific energy values required for widespread acceptance of battery-powered electric vehicles.

A typical battery cell comprises an anode current collector, an anode electrode (also referred to as an anode active material layer, typically containing an anode active material, a conductive filler, and a binder resin component), an electrolyte/ a separator, a cathode electrode (also referred to as a cathode active material layer typically containing a cathode active material, a conductive filler, and a binder resin), a cathode current collector, a metal tab connected to external wiring, and a casing that wraps around all other components except the tab. The total weight and volume of these components are the total cell weight and total cell volume, respectively. The total amount of energy stored by the cell depends on the amount of cathode active material and the corresponding amount of anode active material. In that case, the specific energy and energy density of the cell are defined as the total amount of energy stored by the total cell weight and cell volume respectively. This suggests that one way to maximize the specific energy and energy density of a cell is to maximize the amount of active material and all other constituents (non- active material) is to be minimized.

In other words, the anode-side and cathode-side current collectors in the battery cell are non-active materials that must be reduced (in weight and volume) to increase the mass and volumetric energy density of the battery. . Current collectors, typically aluminum foil (cathode side) and copper foil (anode side), account for about 15-20% by weight and 10-15% in cost of a lithium-ion battery. Therefore, thinner, lighter foils are preferred. However, there are some major problems associated with state-of-the-art current collectors:
(1) Thinner foils tend to be more expensive and more difficult to work with because they crease and tear easily.
(2) Due to technical constraints, it is not possible, if not impossible, to mass-produce metal foils thinner than 6 μm (e.g. Cu) or metal foils thinner than 12 μm (e.g. Al, Ni, stainless steel foils). difficult.
(3) The current collector must be electrochemically stable with respect to the cell constituents over the operating potential window of the electrode. In practice, continued corrosion of the current collector, primarily by the electrolyte, can lead to a gradual increase in the battery's internal resistance, resulting in a sustained decline in apparent capacity.
(4) Oxidation of metal current collectors is a strong exothermic reaction that can significantly contribute to thermal runaway in lithium batteries.

Thus, current collectors are extremely important to battery cost, weight, safety, and performance. Instead of metal, graphene or graphene-coated solid metals or plastics have been considered as promising current collector materials, as summarized in the references listed below:
1. Li Wang, Xiangming He, Jianjun Li, Jian Gao, Mou Fang, Guangyu Tian, Jianlong Wang, Shoushan Fan, “Graphene-coated plastic film as current collector for lithium/sulfur bat.” Power Source, 239 (2013) 623-627.
2. S. J. Ｒｉｃｈａｒｄ Ｐｒａｂａｋａｒ，Ｙｕｎ－Ｈｗａ Ｈｗａｎｇ，Ｅｕｎ Ｇｙｏｕｎｇ Ｂａｅ，Ｄｏｎｇ Ｋｙｕ Ｌｅｅ，Ｍｙｏｕｎｇｈｏ Ｐｙｏ，“Ｇｒａｐｈｅｎｅ ｏｘｉｄｅ ａｓ ａ ｃｏｒｒｏｓｉｏｎ ｉｎｈｉｂｉｔｏｒ ｆｏｒ ｔｈｅ ａｌｕｍｉｎｕｍ ｃｕｒｒｅｎｔ ｃｏｌｌｅｃｔｏｒ ｉｎ ｌｉｔｈｉｕｍ ｉｏｎ ｂａｔｔｅｒｉｅｓ”，Ｃａｒｂｏｎ，５２（２０１３）１２８－１３６。
3. Yang Li, et al. Chinese Patent Pub. No. CN 104600320A (2015, 05, 06).
4. Zhaoping Liu, et al (Ningbo Institute of Materials and Energy, China), WO 2012/151880 A1 (Nov. 15, 2012).
5. Gwon, H. Park, YC; Lee, YS; Ahn, BT; Kang, K "Flexible energy storage devices based on graphene paper", Energy and Environmental Science. 4 (2011) 1277-1283.
6. Ramesh C. Bhardwaj and Richard M. Mank, "Graphene current collectors in batteries for portable electronic devices", US 20130095389 A1, Apr 18, 2013.

Recently, graphene current collectors come in three different forms: graphene-coated substrates [refs 1-4], free-standing graphene papers [ref 5], and transition metal (Ni, Cu)-catalyzed Single-layer graphene films fabricated by chemical vapor deposition (CVD) followed by metal etching [ref. 6].

In the fabrication of graphene-coated substrates, small discrete sheets or platelets of graphene oxide (GO) or reduced graphene oxide (RGO) are spray deposited onto a solid substrate (eg, plastic film or Al foil). In the graphene layer, the constituents are separated graphene sheets/platelets (typically length/width 0.5 to 5 μm and thickness 0.34-30 nm). While a single graphene sheet/platelet can have relatively high electrical conductivity (within the range of 0.5-5 μm thereof), the resulting graphene-binder resin composite layer has relatively low electrical conductivity. Low (typically <100 S/cm and more typically <10 S/cm). Furthermore, another purpose of using a binder resin is to bond the graphene-binder composite layer to a substrate (eg, Cu foil), which is a It means that there is a binder resin (adhesive) layer. Unfortunately, this binder resin layer is electrically insulating and the resulting detrimental effects appear to have been completely overlooked by previous researchers.

Prabakar et al. [2] appear not to use a binder resin in forming discrete graphene oxide sheet-coated aluminum foils, but this graphene oxide-coated Al foil has its own problems. have It is known in the art that aluminum oxide (Al ₂ O ₃ ) forms quickly on the surface of aluminum foil and cleaning with acetone or alcohol cannot remove this passivation layer of aluminum oxide or alumina. is. This aluminum oxide layer is not only electrically and thermally insulating, but is practically impervious to certain types of electrolytes. For example, the most commonly used lithium-ion battery electrolyte is LiPF ₆ dissolved in an organic solvent. The traces of _H2O in this electrolyte trigger a series of chemical reactions with the formation of HF (highly corrosive acid) that quickly decomposes the aluminum oxide layer and corrodes the Al foil and continues to consume the electrolyte. be able to. A decrease in capacity typically becomes quite evident after 200-300 charge-discharge cycles.

Free-standing graphene papers are typically made by vacuum-assisted filtration of GO or RGO sheets/platelets suspended in water. In self-supporting paper, the constituents are separated graphene sheets/platelets that loosely overlap together. Also, while a single graphene sheet/platelet can have a relatively high electrical conductivity (in the range of 0.5-5 μm), the resulting graphene paper has a very low electrical conductivity, e.g. 000 S/m or 80 S/cm [5], which is four orders of magnitude lower than the conductivity of Cu foil (8×10 ⁵ S/cm).

Catalytic CVD methods require the introduction of hydrocarbon gases into a vacuum chamber at temperatures between 500-800°C. Under these harsh conditions, hydrocarbon gases are decomposed by decomposition reactions catalyzed by transition metal substrates (Ni or Cu). A strong acid is then used to chemically etch the Cu/Ni substrate, which is not an environmentally friendly procedure. The entire process is slow, tedious and energy intensive, and the resulting graphene is typically single-layer graphene or few-layer graphene (because the underlying Cu/Ni layer loses its effectiveness as a catalyst). , up to 5 layers).

Bhardwaj et al. [ref. 6] proposed stacking multiple CVD-graphene films to a thickness of 1 μm or a few μm. However, this requires hundreds or thousands of films stacked together (each film is typically 0.34 nm to 2 nm thick). Bhardwaj et al. claim that "graphene may reduce manufacturing costs and/or increase the energy density of battery cells," but provided no experimental data to support their claims. Contrary to this claim, CVD graphene is a notoriously expensive process, and even a single layer of CVD graphene film, given the same area (e.g. the same 5 cm x 5 cm) It would cost considerably more than a sheet of Cu or Al foil. Stacks of hundreds or thousands of single- or few-layer graphene films as proposed by Bhardwaj et al. mean hundreds or thousands of times more expensive than Cu foil current collectors. This cost is prohibitively high. In addition, the high contact resistance between the hundreds of CVD graphene films in the stack and the relatively low electrical conductivity of CVD graphene lead to high total internal resistance, making thinner films (1 μm graphene stacks versus 10 μm Cu foil All the potential benefits of using a body) to reduce the overall weight and volume of the cell are wasted. The Bhardwaj et al. patent application [Ref. 6], which contains no data, is considered nothing more than a concept paper.

The above discussion clearly shows that all three forms of graphene-enhanced or graphene-based current collectors do not meet the performance and cost demands for use in batteries or supercapacitors. There is a great need for different types of materials for use as current collectors.

The present invention provides graphene oxide bonded metal foil (thin film) current collectors in batteries or supercapacitors. The current collector consists of (a) a free-standing, unsupported thin metal foil having a thickness of 1 μm to 30 μm and two opposing substantially parallel major surfaces and (b) a and a thin film of graphene oxide sheets chemically bonded to at least one of two opposing major surfaces, wherein at least one major surface does not contain a passivating metal oxide layer (e.g., this major surface of Al foil). no alumina layer on the surface), and a thin film of graphene oxide with a thickness of 10 nm to 10 μm, an oxygen content of 0.1 wt % to 10 wt %, a graphene interplanar spacing of 0.335 to 0.50 nm, It has a physical density of 1.3-2.2 g/cm ³ , with all graphene oxide sheets oriented approximately parallel to each other and parallel to the major surfaces, and measured alone without the thin metal foil of 500 W/cm3. It exhibits thermal conductivity greater than mK and/or electrical conductivity greater than 1,500 S/cm. No binder resin or adhesive is used in the thin film GO layer itself and there is no binder resin/adhesive or passivating metal oxide layer between the thin film GO layer and the metal foil layer.

Thin metal foils (e.g., Cu foil, Al foil, stainless steel foil, Ni foil, and Ti foil) limit the thickness of the film and therefore the path electrons collected from or transferred to the electrode active material must travel. It must be a free-standing film (eg not supported on another piece of metal plate) to reduce length. The thin metal foil preferably has a thickness of 4-10 μm. Preferably, the thin film of graphene oxide has a thickness of 20 nm to 2 μm (more preferably <1 μm).

Preferably, both major surfaces are each chemically bonded with a thin film of graphene oxide sheet without the use of binders or adhesives, wherein said thin film of graphene oxide has a thickness of 10 nm to 10 μm, 0.5 μm. With an oxygen content of 1 wt% to 10 wt%, a graphene interplanar spacing of 0.335 to 0.50 nm, and a physical density of 1.3 to 2.2 g/ ^cm3 , all the graphene oxide sheets Oriented substantially parallel and parallel to said major surfaces, it exhibits a thermal conductivity greater than 500 W/mK and an electrical conductivity greater than 1,500 S/cm when measured alone without said thin metal foil.

The thin metal foil is preferably selected from Cu, Ti, Ni, stainless steel and chemically etched Al foil. Chemical etching is performed on the Al foil such that the surface of the chemically etched Al foil does not form a passivating Al ₂ O ₃ film on it before bonding to the graphene oxide molecules.

Graphene oxide gels (GO gels containing strongly oxidized graphene molecules in an acidic medium with a pH value of 5.0 or less, preferably and typically <3.0, most typically <2.0) are , we surprisingly observed that the passivating _Al2O3 phase _on the Al foil surface could be removed. These GO molecules in GO gels typically have an oxygen content of >20 wt%, more typically >30 wt%, and most typically >40 wt%. In contrast, a simple suspension of discrete graphene or graphene oxide sheets in a liquid medium (eg water or organic solvent) but not in the GO gel state does not possess this etching ability. Even strongly oxidized GO sheets can lose this etching ability when recovered from the gel state, dried and then redispersed in a liquid medium. A GO sheet could also lose this etching power even if only slightly reduced to reduced graphene oxide (RGO). These observations are really unexpected.

In certain embodiments, the thin film of graphene oxide sheets has an oxygen content of 1% to 5% by weight. In certain other embodiments, the thin film of graphene oxide in the current collector of the present invention has an oxygen content of less than 1%, an inter-graphene spacing of less than 0.345 nm, and an electrical conductivity of 3,000 S/cm or greater. have Preferably, in the current collector of the present invention, the graphene oxide thin film has an oxygen content of less than 0.1%, an inter-graphene spacing of less than 0.337 nm, and an electrical conductivity of 5,000 S/cm or greater. . More preferably, the graphene oxide thin film has an oxygen content of 0.05% or less, an inter-graphene spacing of less than 0.336 nm, a mosaic spread value of 0.7 or less, and an electrical conductivity of 8,000 S/cm or more. have

In certain embodiments, the graphene oxide thin film has an inter-graphene spacing of less than 0.336 nm, a mosaic spread value of 0.4 or less, and an electrical conductivity of greater than 10,000 S/cm. In some of the current collectors of the present invention, the thin films of graphene oxide exhibit inter-graphene spacings of less than 0.337 nm and mosaic spread values of less than 1.0. Preferably and typically, the graphene oxide thin films exhibit a degree of graphitization of 80% or more and/or a mosaic spread value of 0.4 or less.

The present invention also provides a method of manufacturing the current collector of the present invention. In this method, a thin film of graphene oxide sheets is formed on a major surface or two major surfaces of a metal foil under an orientation-controlled stress that aligns the GO molecules or sheets along the plane direction of the major surfaces. It is obtained by a step of depositing and then heat treating the deposited graphene oxide gel at a heat treatment temperature of 80°C to 1,500°C. Preferably, the heat treatment temperature is between 80°C and 500°C. More preferably, the heat treatment temperature is 80°C to 200°C.

Unexpectedly, the heat treatment temperature, which is only 80 °C to 200 °C, leads to edge-to-edge assimilation (chemical bonding, elongation of sheet-like molecules, or strongly oxidized (from GO gels) of well-aligned highly oriented GO sheets. We have observed that it can promote the “polymerization or chain growth” of GO molecules.Thus, in certain embodiments, thin films of graphene oxide can be formed by chemically bonding graphene molecules or chemically parallel to each other. Contains assimilated graphene planes.

In some embodiments, a graphene oxide gel is obtained from a graphite material having the original maximum graphite grain size (resulting in a GO sheet having the maximum length), and a thin film of graphene oxide is formed from this original maximum crystallite size. It has a grain size larger than the grain size or the maximum GO length. This is the idea that highly oriented, strongly oxidized GO sheets or molecules from the gel state can undergo edge-to-edge assimilation or chemical bonding to form longer or wider graphene sheets or molecules. reflect.

The present invention also provides methods for fabricating thin film graphene oxide bonded metal foil current collectors for batteries or supercapacitors. The method comprises the steps of: (a) preparing a graphene oxide gel by dissolving the graphene oxide molecules in a fluid medium, wherein the graphene oxide molecules contain an oxygen content of more than 20% by weight; Distributing and depositing a layer onto at least one of the two major surfaces of the metal foil to form a layer of wet graphene oxide gel deposited thereon wherein the dispensing and depositing procedure is shear-induced thinning of the graphene oxide gel. (c) partially or completely removing the flow medium from the deposited wetting layer of graphene oxide gel, resulting in an area of 0.4 nm to 1.2 nm as measured by X-ray diffraction; forming a dry film of graphene oxide having an interplanar spacing d ₀₀₂ and an oxygen content of 20% by weight or more; and (d) reducing the interplanar spacing d ₀₀₂ to a value between 0.335 nm and 0.5 nm. and heat treating the dried film of graphene oxide at a heat treatment temperature of 80° C. to 2,500° C. to the extent that the oxygen content is reduced to less than 10% by weight to form a thin film graphene oxide bonded metal foil current collector. and a thin film of graphene oxide having a thickness of 10 nm to 10 μm and a physical density of 1.3 to 2.2 g/cm ³ , all graphene oxide sheets being substantially parallel to each other and having at least one major surface oriented parallel to the

In certain embodiments, step (b) comprises distributing and depositing a layer of graphene oxide gel onto each of the two major surfaces of the metal foil, and wet oxidation deposited onto each of the two major surfaces. Forming a layer of graphene gel, wherein the metal foil has a thickness of 1 μm to 30 μm. The metal foil may be selected from Cu, Ti, Ni, stainless steel, and chemically etched Al foil, wherein the surface of the chemically etched Al foil is No passivating Al ₂ O ₃ is formed thereon.

In certain embodiments, step (c) comprises forming a graphene oxide layer having an interplanar spacing d ₀₀₂ of 0.4 nm to 0.7 nm and an oxygen content of 20% by weight or more, and step (d) ) involves heat-treating the graphene oxide layer to such an extent that the interplanar spacing d ₀₀₂ is reduced to a value between 0.3354 nm and 0.36 nm and the oxygen content is reduced to less than 2% by weight.

In certain embodiments, the graphene oxide gel has a viscosity of greater than 2,000 centipoise when measured at 20° C. prior to shear-induced thinning, and a viscosity of 2,000 centipoise during or after shear-induced thinning. reduced to less than centipoise. Preferably, the graphene oxide gel has a viscosity of between 500 centipoise and 500,000 centipoise measured at 20° C. prior to shear-induced thinning. In some preferred embodiments, the graphene oxide gel has a viscosity of 5,000 centipoise or greater when measured at 20° C. prior to shear-induced thinning, and a viscosity of 2° C. during or after shear-induced thinning. ,000 centipoise. Typically, graphene oxide gels have a viscosity that decreases by at least 10-fold when the shear rate is increased at 20°C. The graphene oxide gel has a pH value below 5.0, preferably <3.0, more preferably <2.0. Shear-induced thinning may be performed by a procedure selected from coating, casting, printing (eg, inkjet printing, screen printing, etc.), air-assisted spraying, ultrasonic spraying, or extrusion. Preferably, step (d) includes heat-treating the graphene oxide layer under compressive stress.

Graphene oxide gels are obtained by immersing a graphite material in powder or fiber form in an oxidizing liquid for a sufficient time to obtain a homogeneous solution and an optically transparent, translucent, or brown graphene oxide gel. may be prepared by forming an initially optically opaque and darkened suspension in a reactor at the reaction temperature, where the graphene oxide gel is prepared in an acidic medium having a pH value of 5 or less. The graphene oxide molecules are composed of graphene oxide molecules dissolved in , and the graphene oxide molecules have an oxygen content of 20% by weight or more. Graphite materials may be selected from natural graphite, artificial graphite, mesophase carbon, mesophase pitch, mesocarbon microbeads, soft carbon, hard carbon, coke, carbon fibers, carbon nanofibers, carbon nanotubes, or combinations thereof.

The method can be a roll-to-roll method, wherein steps (b) and (c) comprise feeding a sheet of said metal foil from a roller to a deposition area and applying a layer of graphene oxide gel onto the metal foil. depositing on at least one major surface to form a wet layer of graphene oxide gel thereon; and drying the wet layer of graphene oxide gel to deposit a dried graphene oxide layer on the major surface. and collecting the metal foil with the dried graphene oxide layer deposited on a collector roller.

In certain embodiments, the heat treatment temperature comprises a temperature in the thermal reduction regime of 80° C. to 500° C., and the graphene oxide film has an oxygen content of less than 5%, an inter-graphene spacing of less than 0.4 nm, and/or It has a thermal conductivity of at least 100 W/mK. In certain embodiments, the heat treatment temperature comprises a temperature in the range of 500° C. to 1,000° C., and the unit graphene material has an oxygen content of less than 1%, an inter-graphene spacing of less than 0.345 nm, and at least 1,300 W /mK and/or an electrical conductivity of 3,000 S/cm or higher. In certain embodiments, the heat treatment temperature comprises a temperature in the range of 1,000° C. to 1,500° C., and the graphene oxide film has an oxygen content of less than 0.01%, an inter-graphene spacing of less than 0.337 nm, It has a thermal conductivity of at least 1,500 W/mK and/or an electrical conductivity of 5,000 S/cm or more.

In certain embodiments, the graphene oxide films exhibit inter-graphene spacings of less than 0.337 nm and mosaic spread values of less than 1.0. In certain embodiments, the graphene oxide film exhibits a degree of graphitization of 40% or greater and/or a mosaic spread value of less than 0.7. In certain embodiments, the graphene oxide film exhibits a degree of graphitization of 80% or greater and/or a mosaic spread value of 0.4 or less.

Typically, graphene oxide films contain chemically bonded graphene molecules or chemically assimilated graphene planes that are parallel to each other.

In certain embodiments, the graphene oxide gel is obtained from a graphite material having a plurality of graphite crystallites that do not exhibit a preferred crystallographic orientation as determined by X-ray diffraction or electron diffraction methods, and said graphene oxide film comprises said X It has a preferred crystal orientation determined by line diffraction or electron diffraction methods.

Graphene oxide gels are prepared by adding graphite material in powder or fibrous form to an oxidizing liquid medium in a reactor at a reaction temperature for a time sufficient to obtain a homogeneous solution composed of graphene oxide molecules dissolved in the liquid medium. may be obtained by immersion, in which a homogeneous solution is optically transparent, translucent, or brown, and said graphene oxide molecules have an oxygen content of 20% by weight or more and are in a gel state. It has a molecular weight of less than 43,000 g/mol for some time. In some GO gels, graphene oxide molecules have a molecular weight of less than 4,000 g/mol while in the gel state. In some other GO gels, the graphene oxide molecules have molecular weights between 200 g/mol and 4,000 g/mol while in the gel state.

In the method of the present invention, the step of heat treatment causes chemical ligation, assimilation, or chemical bonding of graphene oxide molecules and/or re-graphitization or reorganization of the graphitic structure.

This method typically uses an oxidized steel with an electrical conductivity greater than 3,000 S/cm, a thermal conductivity greater than 600 W/mK, a physical density greater than 1.8 g/cm, and/or a tensile strength greater than 40 MPa. result in a graphene film. More typically, graphene oxide films have an electrical conductivity greater than 5,000 S/cm, a thermal conductivity greater than 1,000 W/mK, a physical density greater than 1.9 g/cm, and/or a tensile Have strength. Graphene oxide films often have electrical conductivity greater than 15,000 S/cm, thermal conductivity greater than 1,500 W/mK, physical density greater than 2.0 g/cm ³ , and/or tensile strength greater than 80 MPa. Have strength.

Also provided is a method for fabricating a thin film graphene oxide bonded metal foil current collector for a battery or supercapacitor, the method comprising: (a) a graphene oxide gel with graphene oxide molecules dissolved in a fluid medium; (b) placing a sheet of metal foil in the GO gel bath; and move the sheet of metal foil out of the bath (creating shear stress near the major surfaces), allowing deposition of a wet layer of graphene oxide gel on each of the two major surfaces of the metal foil. and (c) partially or completely removing the flowing medium from the deposited wetting layer of graphene oxide gel to provide an interplanar spacing of 0.4 nm to 1.2 nm as measured by X-ray diffraction. (d) _{reducing the interplanar spacing d 002 to} a value between 0.335 nm and 0.5 nm and oxygen heat treating the dried film of graphene oxide at a heat treatment temperature of 80° C. to 2,500° C. to the extent that the content is reduced to less than 10% by weight to form a thin film graphene oxide bonded metal foil current collector; and a thin film of graphene oxide having a thickness of 10 nm to 10 μm, a physical density of 1.3 to 2.2 g/cm ³ , and all graphene oxide sheets being substantially parallel to each other and the at least one major surface oriented parallel to the This method is useful for the production of GO-coated aluminum foils because the GO gel itself can remove the passivated alumina layer on the Al foil surface and this method prevents reformation of the passivated alumina layer. particularly suitable for

In certain embodiments, the graphene oxide gel is composed of natural graphite crystallites having an initial length L _a along the a-axis crystallographic direction, an initial width L _b along the b-axis direction, and a thickness L _c along the c-axis direction. Thin films of graphene oxide, fabricated from particles of graphite or artificial graphite, have graphene domains or crystal lengths or _widths greater than the initial La and _Lb of the graphite crystallites.

Typically, thin films of graphene oxide contain graphene planes with a combination of sp ² and sp ³ electron configurations. Preferably, the thin film of graphene oxide is a continuous length film having a length of 5 cm or more (preferably 10 cm or more and more preferably 20 cm or more) and a width of 1 cm or more (preferably 10 cm or more). There is no practical limit to the length and width of the continuous length thin films of graphene oxide invented herein.

In certain embodiments, the graphene oxide thin film has a physical density greater than 1.8 g/cm and/or a tensile strength greater than 40 MPa, preferably 1.9 g/cm3, when measured alone. It has a physical density greater than cm3 and/or a tensile strength greater than 60 MPa, and more preferably has a physical density greater than 2.0 g/ ^cm3 and/or a tensile strength greater than 80 MPa.

The invention also provides a rechargeable lithium or lithium ion battery containing the current collector of the invention as an anode current collector and/or a cathode current collector. Rechargeable lithium batteries may be lithium-sulfur cells, lithium-selenium cells, lithium-sulfur/selenium cells, lithium-air cells, lithium-graphene cells, or lithium-carbon cells.

The present invention also provides capacitors containing the current collector of the present invention as an anode current collector or a cathode current collector, which capacitors include symmetric ultracapacitors, asymmetric ultracapacitor cells, hybrid supercapacitor-battery cells, Or a lithium ion capacitor cell.

FIG. 1(A) shows exfoliated graphite products (flexible graphite foil and 1 is a flow chart describing various prior art methods of making flexible graphite composites) and pyrolytic graphite (bottom part). FIG. 1(B) is a schematic diagram illustrating conventional methods for manufacturing monoglobulous graphite or graphene (NGP) flake/platelet papers, mats, and films. All methods start with an intercalation and/or oxidation treatment of graphite material (eg, natural graphite particles). FIG. 1(C) shows prior art graphene coated with a binder resin layer (or passivating aluminum oxide layer) between the graphene layer and a metal foil such as Cu foil (or Al foil). 1 is a schematic diagram illustrating a laminated metal foil current collector; FIG. 1(D) is a schematic diagram illustrating a preferred graphene-bonded metal foil current collector in which no binder resin layer or passivating aluminum oxide layer is present between the graphene oxide film and the Cu foil or Al foil. be. FIG. 2(A) is an SEM image of a graphite worm sample after thermal exfoliation of graphite intercalation compound (GIC) or graphite oxide powder. FIG. 2(B) is a cross section of a flexible graphite foil showing many graphite flakes with non-parallel orientation to the flexible graphite foil surface and also showing many flaws, twisted or broken flakes. is an SEM image of. FIG. 3(A) is an SEM image of GO gel-derived monolithic graphene, where multiple graphene planes (with original length/width of 30 nm-2 μm) in the graphite particles are oxidized, exfoliated, and reoriented. are seamlessly assimilated into continuous lengths of graphene sheets or layers that can span hundreds of centimeters in width or length. FIG. 3(B) is a cross-sectional SEM image of a conventional graphene paper/film made from discrete graphene sheets/plate lit using a papermaking process (eg, vacuum-assisted filtration). The image shows many discrete graphene sheets that are folded or interrupted (unintegrated), the orientation is not parallel to the film/paper surface, and has many defects or imperfections. FIG. 3(C) illustrates the formation process of an integrated GO entity composed of multiple graphene planes that are parallel to each other and chemically linked in the graphene plane direction and joined in the thickness direction or the c-axis crystallographic direction. It is a schematic diagram to do. FIG. 3(D) shows one plausible chemical conjugation mechanism (only two GO molecules are shown as an example; many GO molecules can be chemically linked together to form a large graphene domain). FIG. 4(A) is the thermal conductivity values of GO gel-derived graphene layers, GO platelet paper, and FG foil plotted as a function of final heat treatment temperature for graphitization. FIG. 4(B) is the thermal conductivity values of GO gel-derived graphene layers, polyimide-derived pyrolytic graphite (PG), and CVD carbon-derived PG, all plotted as a function of final graphitization or re-graphitization temperature. be done. FIG. 4(C) is the electrical conductivity values of GO gel-derived graphene layers, GO platelet paper, and FG foil plotted as a function of final graphitization or re-graphitization temperature. FIG. 5(A) is the X-ray diffraction curve of the GO film (dried GO gel). FIG. 5(B) is the X-ray diffraction curve of the GO film thermally reduced (partially reduced) at 150°C. FIG. 5(C) is the X-ray diffraction curves of the highly reduced and re-graphitized GO films. FIG. 5(D) is the X-ray diffraction curve of highly re-graphitized and re-crystallized GO crystals showing high intensity (004) peaks. FIG. 5(E) is an X-ray diffraction curve of polyimide-derived HOPG using HTT as high as 3,000°C. FIG. 6(A) is the spacing between graphene planes measured by X-ray diffraction. FIG. 6(B) is the oxygen content in the GO gel-derived GO thin film. FIG. 6(C) is the correlation between inter-graphene spacing and oxygen content. FIG. 6(D) is the thermal conductivity of GO gel-derived GO thin films and flexible graphite (FG) foils, all plotted as a function of final heat treatment temperature. FIG. 7(A) is the tensile strength of GO thin films from GO gel, discrete GO platelet paper (not from GO gel state), and flexible graphite foil for a wide range of heat treatment temperatures. FIG. 7(B) is the scratch resistance of GO thin films plotted as a function of heat treatment temperature. FIG. 8(A) is the graphene gel viscosity values (linear-linear scale) plotted as a function of viscometer spindle speed (proportional to shear rate). FIG. 8B shows viscosity values on a log-linear scale. FIG. 8C shows viscosity values on a logarithmic scale. FIG. 9(A) shows the discharge capacity values for each of the three Li—S cells as a function of the number of charge/discharge cycles. First cell with GO-bonded Cu foil and GO-bonded Al foil as anode and cathode current collectors, respectively; GO/resin-coated Cu foil and GO-coated as anode and cathode current collectors. a second cell (prior art cell) each having a pre-etched Al foil (no pre-etching); a third cell (prior art cell) having a Cu foil anode current collector and an Al foil cathode current collector. FIG. 9B is a three-cell Ragone plot. First cell with GO-bonded Cu foil and GO-bonded Al foil as anode and cathode current collectors, respectively, GO/resin-coated Cu foil and coated with GO as anode and cathode current collectors a second cell (prior art cell) each having a pre-etched Al foil (no pre-etching), a third cell (prior art cell) having a Cu foil anode current collector and an Al foil cathode current collector. FIG. 10 is the cell capacity values for three magnesium metal cells. First cell with GO-bonded Cu foil and GO-bonded Al foil as anode and cathode current collectors, respectively, GO/resin-coated Cu foil and coated with GO as anode and cathode current collectors a second cell (prior art cell) each having a pre-etched Al foil (no pre-etching), a third cell (prior art cell) having a Cu foil anode current collector and an Al foil cathode current collector.

The present invention provides a graphene oxide bonded metal foil thin film current collector (eg, as shown schematically in FIG. 1(D)) in a battery or supercapacitor. The current collector consists of (a) a free-standing, unsupported thin metal foil (214 in FIG. 1(D)) having a thickness of 1 μm to 30 μm and two opposing substantially parallel major surfaces, and (b) a bond. and a thin film of graphene oxide 212 chemically bonded to at least one of the two opposing major surfaces without the use of an agent or adhesive. FIG. 1D simply shows one major surface of the metal foil 214 bonded to a thin film 212 of graphene oxide. Preferably, however, the opposite major surface is also bonded to a thin film of graphene oxide (not shown in FIG. 1(D)). Metal tabs 218 are typically welded or soldered to metal foil 214 as electrode terminals for electrical connection to external circuitry.

As illustrated in FIG. 1(D), a preferred embodiment of the present invention is a graphene oxide-bonded metal foil current collector, wherein a binder resin layer or passivated aluminum oxide layer is a graphene oxide film and a Cu foil. Or does not exist between Al foil. In contrast, as shown diagrammatically in FIG. (for example Cu foil) requires a binder resin layer. In the case of prior art graphene-coated Al foils [Prabakar et al.; ref. 2], a passivating aluminum oxide (alumina) layer naturally exists between the graphene layer and the Al metal foil. This is due to the known fact that aluminum foil always forms a passivating aluminum oxide layer on the surface of the Al foil during manufacture and exposure to room air. Simple cleaning with acetone or alcohol cannot remove this alumina layer. As explained in later paragraphs, the presence of a layer of binder resin or aluminum oxide, even with a thickness of only 1 nm, has a very large effect in increasing the contact resistance between the graphene layer and the metal foil. have. This surprising discovery by us has been completely overlooked by all prior art researchers, and thus prior art graphene-coated metal foils meet the performance and price requirements of lithium batteries or supercapacitor current collectors. not

A very significant and unexpected advantage of having graphene oxide sheets in direct contact with the major surfaces of Cu, Ni, or Ti foils is that without the use of external resin binders or adhesives (the contact resistance is therefore dramatically reduced to not increase), the notion that graphene oxide molecules can be well bonded to these metal foils under the processing conditions of the present invention. These processing conditions include the steps of well aligning the graphene oxide sheets onto the metal foil surface and then the bilayer structure at temperatures in the range of 80°C to 1,500°C (more typically and desirably 80°C). -500°C, and most typically and desirably in the range of 80°C to 200°C. Optionally, but not preferably, the heat treatment temperature can be as high as 3,000°C.

These processing conditions include, for aluminum foil-based current collectors, chemically etching away the passivating aluminum oxide layer before it is coated with graphene oxide and bonded, followed by the steps described above. and heat-treating under equivalent temperature conditions of Alternatively, graphene oxide may be prepared in the GO gel state, which has a high oxygen content, exhibits a high amount of —OH and —COOH groups, and has a pH value below 5.0 (preferably <3.0 and even more preferably <2.0). The Al foil may be left immersed in a bath of GO gel, where the acidic environment spontaneously removes the passivating Al ₂ O ₃ layer. When the Al foil emerges from the bath, the GO molecules or sheets spontaneously adhere to the clean, etched Al foil surface, effectively preventing exposure of the Al foil surface to open air (thus passivating Al ₂ O ₃ layers and no additional contact resistance between the Al foil surface and the GO layer). This method has never been disclosed or proposed before.

Besides the chemical bonding power of the GO layer of the present invention and the chemical etching power of the GO gel, the resulting thin film of graphene oxide in the graphene oxide bonded metal foil of the present invention has a thickness of 10 nm to 10 μm, a thickness of 0.1 With an oxygen content of ^wt . Oriented parallel and parallel to the major surfaces, it exhibits a thermal conductivity greater than 500 W/mK and/or an electrical conductivity greater than 1,500 S/cm when measured alone without the thin metal foil. This thin film of graphene oxide is chemically inert and provides a highly effective protective layer against corrosion of the underlying metal foil.

Now consider in more detail the magnitude of the total resistance (including the contact resistance) of the three-layer structure as illustrated in FIG. 1(C). Electrons in the graphene layer 202 (layer 1) move around within this layer, traverse the binder resin or passivated alumina layer 206 (layer 2), and then within the metal foil layer 204 (layer 3) the terminal tabs 208. must move towards For simplicity, only the total resistance to electrons traveling across the graphene layer thickness, the binder/passivation layer thickness, and the metal foil layer thickness is considered. The movement of electrons in the in-plane direction of both graphene or metal foil is rapid and of low resistance, so this resistance is neglected in familiar calculations.

The through-thickness resistance of a conductor sheet/film is given by R = (1/σ)(t/A), where A = conductor cross-section (length x width), t = conductor thickness σ = conductivity = 1/ρ, and ρ = resistivity, a material constant. A graphene-coated current collector containing a binder or passivated metal oxide layer has a graphene film, an interfacial binder resin layer (or a passivated alumina layer), and a metal foil layer electrically connected in series. may be regarded as a three-layer structure (FIG. 1(C)).

The total resistance is the sum of the resistance values of the _three layers: R= _R1 + _R2 +R3=ρ1 ₍ t1/A1) ₊ ρ2 ₍ t2 _{/ A2} ) _{+ ρ3 (} t3 _/ A3) =(1/σ ₁ )(t ₁ /A ₁ )+(1/σ ₂ )(t ₂ /A ₂ )+(1/σ ₃ )(t ₃ /A ₃ ), where ρ = resistivity , σ=conductivity, t=thickness, and A=area of the layer, and roughly A ₁ =A ₂ =A ₃ . Scanning electron microscopy shows that the binder resin or passivating alumina layer is typically 5-100 nm thick. The resistivity of the most commonly used binder resins (PVDF) and alumina (Al ₂ O ₃ ) is typically in the range of 10 ¹³ to 10 ¹⁵ ohm-cm. Considering A ₁ =A ₂ =A ₃ =1 cm ² , the through-thickness resistivity of the graphene layer ρ ₁ =0.1 ohm-cm, the resistivity of the binder or alumina layer ρ ₂ =1×10 ¹⁴ ohm-cm. cm and the resistivity of the metal foil layer is ρ ₃ =1.7×10 ⁻⁶ ohm-cm (Cu foil) or ρ ₃ =2.7×10 ⁻⁶ ohm-cm (Al foil). Also consider the optimal conditions where the thickness of the Cu or Al foil = 6 μm, the thickness of the graphene layer = 1 μm, and the thickness of the binder resin layer is only 0.5 nm (in practice it is between 5 nm and 100 nm). . In that case, the total resistance of the three-layer structure is 5×10 ⁶ ohms and the total conductivity is only 1.4×10 ⁻¹⁰ S/cm (see first row of data in Table 1 below). reference). If the binder resin layer is considered to be 10 nm thick, the total resistance of the three-layer structure is 1×10 ⁸ ohms and the total conductivity is only 7.0×10 ⁻¹² S/cm (see table below). 1, 4th data row). Such three-layer composite structures are not good current collectors for batteries or supercapacitors because high internal resistance means low output voltage and high amount of internal heat generated. Similar results were observed for current collectors based on Ni, Ti, and stainless steel foils (rows 7-10 of data in Table 1).

In contrast, when there is no binder resin or alumina layer (t ₂ =0), as in the case of the current collector of the present invention, the total resistance of the graphene oxide bonded Cu foil is It has a value of 1.0×10 ⁻⁵ ohms (compared to 1.0×10 ⁺⁷ ohms for the containing tri-layer structure). See Table 2 below. This represents a difference of 12 orders of magnitude (not 12 times)! The conductivity is 7.0×10 ⁺¹ S/cm for the two-layer structure in question, as opposed to 7.0×10 ⁻¹¹ S/cm for the corresponding three-layer structure. Also, the difference is 12 digits. In addition, lithium batteries and supercapacitors featuring the graphene oxide bonded metal foil current collectors of the present invention exhibit higher voltage output, higher voltage output, without capacity reduction or corrosion problems when compared to prior art graphene-based current collectors. We have found that they consistently exhibit higher energy density, higher power density, more stable charge-discharge cycle response, and last longer.

Since the graphene-enabled current collectors of the present invention are fabricated from graphene oxide gels, the terms graphene, graphene oxide (GO), prior art GO suspensions, and said GO gels are introduced herein. be considered.

Bulk natural flake graphite is characterized by the fact that each particle is composed of multiple grains (the grains are graphite single crystals or microcrystals), and the grain boundaries (amorphous or defect regions) are the boundaries of adjacent graphite single crystals. It is a three-dimensional graphite material that defines the boundary. Each grain is composed of multiple graphene planes oriented parallel to each other. The graphene planes within graphite crystallites are composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and joined by van der Waals forces in the c-direction (perpendicular to the graphene planes or basal planes). All the graphene planes of one grain are parallel to each other, but typically the graphene planes of one grain and the graphene planes of adjacent grains are oriented differently. In other words, the orientation of the various grains of graphite particles is typically different from grain to grain.

Graphite single crystals (microcrystals) themselves are anisotropic and properties measured along the direction of the basal plane (a-axis or b-axis crystallographic direction) are measured along the c-axis crystallographic direction (thickness direction). It is dramatically different from the stated properties. For example, the thermal conductivity of graphite single crystals can be up to about 1,920 W/mK (theoretical) or up to 1,800 W/mK (experimental) in the basal plane (a-axis and b-axis crystallographic directions), The thermal conductivity along the c-axis crystallographic direction is less than 10 W/mK (typically less than 5 W/mK). Furthermore, the multiple grains or crystallites of the graphite particles are typically all oriented in different directions. Natural graphite particles, composed of multiple grains of different orientations, therefore exhibit properties averaged between these two extremes (i.e., between 5 W/mK and 1,800 W/mK, but typically <100 W/mK).

It is highly desirable in many applications to produce bulk graphite bodies with sufficiently large dimensions and in which all graphene planes are essentially parallel to each other along one desired direction. For example, the c-axis directions of all graphene planes are approximately parallel to each other and have sufficiently large lengths and/or widths for certain applications (e.g., >10 ^cm2 and large for use as current collectors in small cells). >200 cm ² for use as current collectors in cells) and the desired thickness (eg, 10 nm to 10 μm) for the intended application (eg, as a thin layer bonded onto a metal foil surface). It is highly desirable to have large-sized graphite entities (eg, well-integrated layers of multiple graphene planes). So far, it has not been possible to fabricate this type of large-sized integrated graphene-metal foil entity from existing natural or synthetic graphite particles.

If the interplanar van der Waals forces can be overcome, the constituent graphene planes (typically 30 nm to 2 μm in width/length) of the graphite crystallites can be exfoliated and extracted or isolated from the graphite crystallites, leaving the carbon atoms. A single graphene sheet can be obtained. An isolated, single graphene sheet of hexagonal carbon atoms is commonly referred to as monolayer graphene. A laminate of multiple graphene planes joined by van der Waals forces in the thickness direction with an inter-plane spacing of 0.3354 nm is generally referred to as multi-layer graphene. Multilayer graphene platelets have up to 300 graphene planes (thickness <100 nm), but more typically up to 30 graphene planes (thickness <10 nm), even more typically up to 20 graphene planes. (thickness <7 nm), and most typically have up to 10 graphene planes (commonly referred to in the scientific community as few-layer graphene). Single-layer graphene and multi-layer graphene sheets are collectively referred to as “nanographene platelets” (NGP). Graphene sheets/platelets or NGPs are a novel class of carbon nanomaterials (two-dimensional nanocarbons) that are distinct from zero-dimensional fullerenes, one-dimensional CNTs, and three-dimensional graphite.

Our research group started developing graphene materials and related manufacturing processes as early as 2002: (1)B. Z. Jang and W. C. Huang, "Nano-scaled Graphene Plates", US Pat. No. 7,071,258 (07/04/2006), application filed Oct. 21, 2002; Z. Jang, et al. "Process for Producing Nano-scaled Graphene Plates," US patent application Ser. No. 10/858,814 (06/03/2004); and (3)B. Z. Jang, A. Zhamu, andJ. Guo, "Process for Producing Nano-scaled Platelets and Nanocomposites," US patent application Ser. No. 11/509,424 (08/25/2006).

As illustrated in FIG. 1(A) (process flow chart) and FIG. 1(B) (schematic diagram), NGP typically involves intercalating natural graphite particles with strong acids and/or oxidizing agents to form graphite intercalation compounds. (GIC) or graphite oxide (GO). The presence of chemical species or functional groups in the lattice space between the graphene planes serves to increase the inter-graphene spacing (d ₀₀₂ , measured by X-ray diffraction), thereby orienting the graphene planes along the c-axis. considerably reduces the van der Waals forces that hold along. GIC or GO very often combine natural graphite powder (20 in FIG. 1A and 100 in FIG. 1B) with sulfuric acid, nitric acid (oxidizing agents), and another oxidizing agent (e.g. sodium chlorate) by immersion in a mixture. The resulting GIC (22 or 102) is actually some type of graphite oxide (GO) particles. The GIC is then washed and rinsed repeatedly in water to remove excess acid, resulting in a graphite oxide suspension or dispersion that is discrete and visually identifiable dispersed in water. Contains graphite oxide particles. There are two processing paths that follow after this rinse step:

Route 1 involves removing water from the suspension to obtain "expandable graphite", which is essentially a mass of dried GIC or dried graphite oxide particles. When extensible graphite is exposed to temperatures typically in the range of 800-1,050° C. for about 30 seconds to 2 minutes, the GIC undergoes a 30- to 300-fold rapid expansion to form "graphite worms" (24 or 104 ), each of which is a collection of exfoliated but largely unseparated graphite flakes that remain interconnected. An SEM image of graphite worms is shown in FIG. 2(A).

In one possible subsequent step, these graphite worms (exfoliated graphite or “network of interconnected/unseparated graphite flakes”) are recompressed to 0.1 mm (100 μm) to 0.5 mm A flexible graphite sheet or foil (26 or 106) having a thickness typically in the range (500 μm) can be obtained. Alternatively, low intensity air mills or shears are used for the purpose of producing so-called "expanded graphite flakes" (108) containing mainly graphite flakes or platelets thicker than 100 nm (thus not nanomaterials by definition). You may choose to simply break the graphite worms using.

Exfoliated graphite worms, expanded graphite flakes, as well as recompressed masses of graphite worms (commonly referred to as flexible graphite sheets or flexible graphite foils) are all made from one-dimensional nanocarbon materials (CNTs or CNFs) or A clearly distinguishable three-dimensional graphitic material that is fundamentally different from either two-dimensional nanocarbon material (graphene sheets or platelets, NGP). Flexible graphite (FG) foil can be used as a heat spreader material, but typically has a maximum in-plane thermal conductivity of less than 500 W/mK (more typically <300 W/mK) and an in-plane thermal conductivity of 1,500 S/cm or less. Indicate electrical conductivity. These low conductivity values are a direct result of many defects, wrinkled or broken graphite flakes, interruptions or gaps between graphite flakes, and non-parallel flakes (e.g. SEM image of FIG. 2(B)). is. Many flakes are tilted with respect to each other at very large angles (eg 20-40 degrees misorientation).

In another possible subsequent step, the exfoliated graphite is subjected to high intensity mechanical shear (e.g., using an ultrasonicator, high shear mixer, high intensity air jet mill, or high energy ball mill) to achieve our U.S. patent application Separated single-layer and multi-layer graphene sheets (collectively referred to as NGP, 33 or 112) are formed as disclosed in 10/858,814. Single-layer graphene can be as little as 0.34 nm, while multi-layer graphene can have a thickness of up to 100 nm, but is more typically less than 20 nm.

Route 2 involves sonicating the graphite oxide suspension for the purpose of separating/isolating the single graphene oxide sheets from the graphite oxide particles. This indicates that the graphene interplanar separation is increased from 0.3354 nm in natural graphite to 0.6–1.1 nm in highly oxidized graphite oxide, and the van der Waals forces holding adjacent faces together It is based on the idea of significantly weakening The ultrasonic power may be sufficient to further separate the graphene plane sheets to form separate, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets are then chemically or thermally reduced to give a "Reduced graphene oxide" (RGO) can be obtained that typically has an oxygen content of less than

For the purposes of defining the claims of this application, NGP includes discrete sheets/platelets of single- and multi-layer pure graphene, graphene oxide, or reduced graphene oxide. Pure graphene has essentially 0% oxygen. Graphene oxide (including RGO) can have 0.01% to 50% oxygen by weight. RGO has an oxygen content of 0.01 to 10 wt%, more typically 0.01 to 5 wt%, and most typically 0.01 to 2 wt%.

GO molecules in graphene oxide gels (GO gels), which are described in detail later, are typically enriched with 20-47 wt.% oxygen (more than typically 30-47%). GO gels are dissolved (not just dispersed) in acidic liquids (e.g., highly acidic aqueous solutions with pH values below 5.0, more typically <3.0, often <2.0). It refers to a homogeneous solution of highly hydrophilic aromatic molecules (eg, graphene oxide molecules carrying oxygen-containing groups such as —OH, —COOH, and >O on their molecular faces or at their edges). The GO gel itself does not contain clearly discernable or discrete graphene or GO particles in the form of solid sheets or platelets. These GO molecules and the dispersed liquid medium have comparable refractive indices, making the resulting gel optically transparent or translucent (if the proportion of GO molecules is not too high) or exhibiting a light brown color. .

In contrast, simple mixtures of graphite particles or discrete graphene or graphene oxide sheets/platelets with acid and/or water (if they have less than 0.1% solid particles suspended in the liquid medium But) it looks optically dark and totally opaque. These particles or graphene or GO sheets/platelets are simply dispersed (not dissolved) in the flowing medium and they do not form a GO gel state.

These GO molecules in the GO gel are highly reactive and may be considered "living macromolecules". In contrast, prior art solid sheets/platelets of graphene, GO, and RGO are essentially "dead" species. Using appropriate shear or compressive stress (e.g., by casting or molding), the GO gel is formed into a shape on the metal foil surface, dried (the liquid component is partially or completely removed), and subjected to a specific It can be heat treated under conditions to obtain a film of highly oriented graphene sheets chemically bonded to the metal foil surface. Heat treatment chemically links these active or live GO molecules to form a two- or three-dimensional network of chemically bonded graphene molecules of large molecular weight, reducing the oxygen content of GO to less than 10 wt% (heat treatment temperature <200 °C), more typically <5%, even more typically <2% (heat treatment temperatures <500 °C), and most typically dramatically to <<1% (heat treatment temperatures up to 1,500 °C). help reduce

GO gels themselves do not contain clearly discernible/discrete graphene sheets/platelits or NGPs (including “dead” GO sheets/platelits), but discrete graphene sheets/platelits, expanded graphite flakes, and other types of solid fillers can be intentionally added into the GO gel to form a mixture gel. This mixture gel may be dried and subjected to the same heat treatment to convert the live GO molecules into a film of highly oriented and chemically assimilated GO sheets. This graphene oxide gel-derived graphene material is reinforced with filler phases such as discrete NGP, CNT and carbon fibers.

Solid NGPs (such as discrete sheets/platelets of pure graphene, GO, and GRO) typically do not exhibit high electrical conductivity when packed into films, membranes, or paper sheets (34 or 114). In general, paper-like structures or mats made from platelets of discrete graphene, GO, or RGO (e.g., those paper sheets made by vacuum-assisted filtration process) have many defects, wrinkles, or show broken graphene sheets, interruptions or gaps between platelits, and non-parallel platelits (eg, SEM image in FIG. 3(B)), resulting in relatively low electrical conductivity and low structural strength.

The lower portion of FIG. 1(A) illustrates a typical method for producing pyrolytic graphite films from polymers. The method begins by carbonizing a polymer film 46 (eg, polyimide) at a carbonization temperature of 400-1,000° C. under a typical pressure of 10-15 Kg/cm ² for 2-10 hours to obtain a carbonized material. 48 is obtained, which is then subjected to graphitization treatment at 2,500-3,200° C. under ultra-high pressure of 100-300 Kg/cm ² for 1-24 hours to form a graphite film 50 . It is technically extremely difficult to maintain such ultra-high pressures at such ultra-high temperatures. This is a difficult, slow, tedious, energy intensive and extremely expensive process. Furthermore, charring of certain polymers (eg polyacrylonitrile) is accompanied by the release of toxic species. Furthermore, it has not been possible to mass-produce polyimide-derived pyrolytic films thinner than 15 μm due to the difficulty in producing precursor polyimide films thinner than 30 μm. This does not meet the requirement for current collectors to be 1-10 μm thick.

A special type of graphene thin films (< ₂ nm) is fabricated by catalytic CVD of hydrocarbon gases (eg _C2H4 ) on Ni or Cu surfaces. When Ni or Cu is the catalyst, the carbon atoms obtained by the decomposition of hydrocarbon gas molecules at 800-1,000° C. are deposited on the Ni or Cu foil surface and are polycrystalline in single or few layers. Form a sheet of graphene. These graphene thin films are optically transparent and electrically conductive, for use in applications such as touch screens (replacing indium tin oxide or ITO glass) or semiconductors (replacing silicon, Si). is intended. However, these ultrathin polycrystalline graphene films are not sufficiently conductive (too many grains or too many grain boundaries and all grains are oriented in different directions) and current collecting. Not thick enough for use as a body (most preferably 1 μm to 10 μm). Furthermore, CVD processes with Ni or Cu catalysts are limited to more than 5-10 graphene planes (typically <2-4 nm, more typically typically <2 nm) deposition. There was no experimental evidence that CVD graphene layers thicker than 5 or 10 nm, let alone 1 μm (1,000 nm) to 10 μm (10,000 nm) are possible.

The present invention also provides a method for producing a graphene oxide-bonded metal foil current collector, the method comprising the steps of (a) preparing a graphene oxide gel in which graphene oxide molecules are dispersed and dissolved in a fluid medium; wherein the graphene oxide molecules contain an oxygen content greater than 20 wt% (typically greater than 30 wt% and more typically between 30 wt% and 47 wt%); and (b) a layer of graphene oxide gel. onto a major surface of a supporting metal foil to form a graphene oxide gel deposited thereon, wherein the dispensing and deposition procedure promotes assimilation and integration of GO molecules during subsequent heat treatment ( (c) removing a flowing medium from the deposited graphene oxide gel layer, resulting in well-packed and well-aligned graphene oxide molecules in a desired direction; partially or completely removing a dry graphene oxide layer having an interplanar spacing d ₀₀₂ of 0.4 nm to 1.2 nm as measured by X-ray diffraction and an oxygen content of 20% by weight or more; (d) forming 80 graphene oxide layers such that the interplanar spacing d ₀₀₂ of the graphene oxide film layer is reduced to a value between 0.3354 nm and 0.5 nm and the oxygen content is reduced to less than 10% by weight; and heat-treating at a heat-treating temperature of ~1,500°C.

In a more preferred embodiment, step (c) comprises forming a graphene oxide layer having an interplanar spacing d ₀₀₂ of 0.4 nm to 0.7 nm and an oxygen content of 20% by weight or more; ) involves heat-treating the graphene oxide layer to such an extent that the interplanar spacing d ₀₀₂ is reduced to a value between 0.3354 nm and 0.36 nm and the oxygen content is reduced to less than 2% by weight.

A thin film of graphene oxide obtained by heat-treating a graphene oxide gel at temperature contains chemically bonded graphene molecules. These planar aromatic molecules or graphene planes (hexagonal carbon atoms) are parallel to each other. The lateral dimensions (length or width) of these planes are very large, typically several times or even orders of magnitude larger than the largest crystallite size (or largest constituent graphene plane size) of the starting graphite particles. A thin film composed of many 'giant graphene oxide domains' with all constituent graphene planes essentially parallel to each other, with a thin layer of GO molecules chemically bonded to the underlying metal foil. This is a unique and novel class of materials not previously discovered, developed, or possibly suggested to exist.

Graphene oxide gels are a very unique and novel class that surprisingly possess great cohesive (self-bonding, self-polymerizing and self-crosslinking ability) and adhesive (capable of chemically bonding to a wide variety of solid surfaces) properties. is the material of These properties are neither taught nor suggested in the prior art. GO gels are obtained by immersing powders or filaments of the starting graphite material in an oxidizing liquid medium (eg a mixture of sulfuric acid, nitric acid and potassium permanganate) in a reactor. The starting graphite material may be selected from natural graphite, artificial graphite, mesophase carbon, mesophase pitch, mesocarbon microbeads, soft carbon, hard carbon, coke, carbon fibers, carbon nanofibers, carbon nanotubes, or combinations thereof. .

When the starting graphite powder or filaments are mixed into an oxidizing liquid medium, the resulting slurry (heterogeneous suspension) initially appears quite dark and opaque. When the oxidation of graphite proceeds at the reaction temperature for a sufficient period of time under controlled pH conditions, the reaction mass eventually becomes (as opposed to an initially heterogeneous suspension containing identifiable solid particles can be a homogeneous solution with no discernible or visibly identifiable solid particles dispersed therein. The solution can be optically translucent or transparent or brown, but it also looks and behaves like a polymer gel. This heavy oxidation-induced graphene oxide gel is composed of graphene oxide molecules dissolved in a liquid medium. Graphene oxide molecules have an oxygen content of 20% by weight or more (typically 30-50% by weight) before any subsequent heat treatment, and their molecular weight is typically 43 while in the gel state. ,000 g/mol (often less than 4,000 g/mol, but typically greater than 200 g/mol). Graphene oxide gels are typically composed of graphene oxide molecules dissolved in an acidic medium having a pH value of 5.0 or less (more typically <3.0 and most typically <2.0).

Graphene oxide gels have typical viscosities of 500 centipoise (cP) to 500,000 cP when measured at 20° C. prior to shear-induced thinning. Viscosity is more typically greater than 2,000 cP and less than 300,000 cP when measured at 20° C. prior to the shear-induced thinning procedure. Preferably, the viscosity of GO gel as a precursor of unit graphene material is in the range of 2,000-50,000 cP. Preferably, the GO gel is subjected to a shear stress field in which the viscosity is reduced to less than 2,000 cP (or even less than 1,000 cP) during or after shear-induced thinning. In embodiments, the graphene oxide gel has a viscosity greater than 5,000 cP when measured at 20° C. prior to shear-induced thinning, but less than 5,000 cP (preferably and typically typically less than 2,000 cP or even less than 1,000 cP). Viscosity data measured at 20° C., shown as examples in FIGS. 8(A), 8(B), and 8(C), clearly show can also be reduced to 1,000-2,000 cP at sufficiently high shear rates. This is a reduction of over two orders of magnitude and a highly unexpected measurement. A straight line in the data when plotted on a log-log scale indicates the flow properties of the shear thinning fluid.

In step (b), the GO gel is preferably formed into a shape under shear stress. An example of such a shearing procedure is casting or coating a thin film of GO gel (gel-like fluid) using a coating machine. This procedure is analogous to a layer of varnish, paint, coating, or ink being coated onto a solid substrate. Rollers, "doctor blades", or wipers create shear stress when the film is shaped or when relative motion is applied between the roller/blade/wiper and the supporting substrate. Quite unexpectedly and importantly, such shearing action can reduce the effective viscosity of the GO gel and allow planar graphene oxide (GO) molecules to be well aligned, for example, along the shear direction. Even more surprisingly, such molecular alignment or preferred orientation is not destroyed when forming a fully packed GO mass that is subsequently removed to at least partially dry the liquid component in the GO gel. The dried GO mass has a high birefringence coefficient between the in-plane and normal-to-plane directions. Another example of such a procedure is pouring or die casting a GO mass under shear stress into a mold cavity or shaping die/tool. The liquid component of the sheared GO mass in the mold cavity is then partially or completely removed to partially or completely remove the GO molecules containing well-packed and well-aligned “live” GO molecules. to obtain a GO mass dried to

This dried GO mass is then subjected to an appropriately programmed heat treatment. For the temperature range of 80°C to 500°C, the GO mass primarily undergoes chemical reactions with the metal foil surface, sustaining a certain chemical assimilation between the GO molecules, reducing the oxygen content to typically 30-50%. (when dried) to 5-6% thermal reduction. This treatment reduces the inter-graphene spacing from about 0.6-1.0 nm (when dry) to about 0.4 nm, and the in-plane thermal conductivity from about 100 W/mK to 500 W/mK, and the electrical The result is an increase in conductivity from 800 S/cm to >2,000 S/cm. Even in such a low temperature range some chemical bonding occurs. Although the GO molecules remain well-aligned, the inter-GO spacing remains relatively large (>0.4 nm). Many O-containing functional groups remain present.

For the heat treatment temperature range of 500° C. to 1,500° C., extensive compounding, polymerization, and cross-linking occur between adjacent GO molecules. The oxygen content is typically reduced to <2.0% (more typically <1.0%), resulting in a reduction of the spacing between graphenes to about 0.345 nm. Some initial graphitization can occur at such low temperatures, in marked contrast to conventional graphitizable materials (such as carbonized polyimide films), which typically require temperatures as high as 2,500° C. to initiate graphitization. This means that it has already started at This is another distinct feature of the graphene film-bonded metal foil of the present invention and its manufacturing process. These chemical ligation reactions result in an increase in the in-plane thermal conductivity of graphene thin films to 1,400-1,500 W/mK and/or the in-plane electrical conductivity to >5,000 S/cm. .

X-ray diffraction patterns were obtained using an X-ray diffractometer equipped with CuKcv radiation. Diffraction peak shifts and broadenings were calibrated using silicon powder standards. The degree of graphitization, g, uses Mering's formula d ₀₀₂ =0.3354g+0.344(1−g), where d ₀₀₂ is the interlayer spacing of graphite or graphene crystals in nm. and calculated from the X-ray pattern. This formula is valid only when d ₀₀₂ is less than or equal to about 0.3440 nm. Graphene oxide films or lightly oxidized graphite crystalline materials with d ₀₀₂ higher than 0.3440 nm have oxygen-containing functional groups (e.g. − OH, >O, and —COOH, etc.).

Another structure index that can be used to characterize the degree of ordering of the unitary graphene materials of the present invention and conventional graphite crystals is the full width at half maximum of the rocking curve (X-ray diffraction intensity) of the (002) or (004) reflection is represented as a "mosaic spread". This degree of ordering characterizes the graphite or graphene crystal size (or grain size), the amount of grain boundaries and other defects, and the preferred degree of grain orientation. A nearly perfect single crystal of graphite is characterized by having a mosaic spread value of 0.2 to 0.4. Most of our unitary graphene materials have mosaic spread values in this range of 0.2-0.4 (using heat treatment temperatures of 2,000° C. and above). However, some values range from 0.4 to 0.7 when the highest heat treatment temperature (HTT) is between 1,000 and 1,500°C, and when HTT is between 500 and 1,000°C. In some cases it ranges from 0.7 to 1.0.

The heat treatment temperature conditions for GO are such that the thin film of graphene oxide coated on the metal foil is relatively pore-free and has a physical density of at ^least 1.5 g/cm or a porosity level lower than 20%. is like having Under more typical processing conditions, thin films have a physical density of at least 1.7 g/cm ³ or a porosity level of less than 10%. Often the films have a physical density greater than 1.8 g/cm ³ or a porosity level less than 5%. (Particularly for those films made with maximum processing temperatures below 1,500° C.) chemically bonded graphene planes within the films typically contain a combination of sp ² and sp ³ electron configurations.

Graphene oxide (GO) gel-derived thin films of graphene oxide bonded onto metal foils have the following properties (separately or in combination):
(1) When formed under desired shear stress field conditions, followed by appropriate heat treatment, thin films of graphene oxide are integrated graphene phases. The film has wide/long chemically bonded graphene planes that are essentially oriented parallel to each other. In other words, the c-axis crystallographic directions of all grains and all their constituent graphene planes are essentially oriented in the same direction. This conclusion was made after detailed investigation using a combination of SEM, TEM, selected area diffraction (using TEM), X-ray diffraction, atomic force microscopy (AFM), Raman spectroscopy, and FTIR.

(2) paper-like sheets of exfoliated graphite worms (i.e., flexible graphite foils), mats of expanded graphite flakes (100 nm thick), and papers or films of graphene or GO platelits are composed of a plurality of discrete graphite flakes or graphene , GO, or RGO discrete platelit aggregates/stacks. In contrast, the present thin films of graphene oxide from GO gels are well-integrated single graphene entities or monoliths containing no discrete flakes or platelets.

(3) In the prior art methods, discrete graphene sheets (thickness <<100 nm) or expanded graphite flakes (>100 nm) that constitute the original structure of graphite particles can be obtained by expansion, exfoliation and separation processes. . By simply mixing and recompressing these discrete sheets/flakes into a thin film, one can attempt to orient the sheets/flakes, preferably along one direction. However, using these conventional methods, the resulting constituent flakes or sheets of film (aggregate, paper, film, or matte) can be visualized with the naked eye or using a low power optical microscope (100x-1000x). remains discrete flakes/sheets/platelets that can be easily identified or clearly observed even though

In contrast, the fabrication of the present invention thin films of GO requires that virtually every single pristine graphene plane is oxidized and isolated from one another, creating highly reactive functional groups such as —OH, >O, and —COOH ) at the edges and similarly, strongly oxidizing the pristine graphite particles to the extent that they become single molecules with primarily graphene planes. These single hydrocarbon molecules (containing atoms such as O and H in addition to carbon atoms) are dissolved in a reaction medium (e.g., a mixture of water and acid) and are referred to herein as GO gels. , to form a gel-like mass. This gel is then typically cast under shear stress field conditions onto a smooth substrate surface or injected into a mold cavity, followed by removal of the liquid component to form a dried GO layer. . When heated, these highly reactive molecules react with each other primarily laterally (length or width) along the graphene planes (edge-to-edge) and, in some cases, also between graphene planes. and chemically bond.

Shown in FIG. 3(D) is a plausible chemical ligation mechanism where only two aligned GO molecules are shown as an example, but a large number of GO molecules can be chemically ligated together to form a unitary graphene layer. can do. In addition, chemical linkages can also occur face-to-face as well as edge-to-edge. These ligation and anabolic reactions proceed as molecules are chemically assimilated, linked and integrated into a single entity or monolith. The molecules have completely lost their own original essence and they are no longer discrete sheets/platelets/flakes. There is only one single layered structure (a unitary graphene entity) that is simply a network of one macromolecules or interconnected macromolecules with essentially infinite molecular weight. All constituent graphene planes are very large in lateral dimensions (length and width) and can be produced under shear stress conditions (especially in thin films of thickness <20 μm) at higher temperatures (e.g. >1,000 °C). above), these graphene planes are essentially parallel to each other.

In-depth studies using a combination of SEM, TEM, selected area diffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIR show that graphene monoliths consist of several very large graphene planes (typically >>100 μm, larger than typically >>1 mm, and most typically >>1 cm). These giant graphene planes are often stacked and bonded along the thickness direction (c-axis crystallographic direction) not only by van der Waals forces (as in conventional graphite crystallites) but also by covalent bonds. While not to be limited by theory, Raman and FTIR spectroscopy studies indicate coexistence of sp ² (dominant) and sp ³ (weak but present) electron configurations in graphite as well as conventional sp ² I think that the.

(4) The integrated graphene entities are not produced by gluing or bonding together discrete flakes/platelets using resin binders or adhesives. Instead, the GO molecules in the GO gel are assimilated by covalent bonding or formation of covalent bonds with each other into integrated graphene entities without the use of any externally added binder molecules or polymers.

(5) This monolithic graphene entity typically has the c-axis crystal axes of all grains essentially parallel to each other. This entity is obtained from GO gel, which in turn is obtained from natural or artificial graphite particles with a plurality of graphite crystallites. Before being chemically oxidized, these starting graphite crystallites have an initial length (L _a along the a-axis crystallographic direction), an initial width (L _b along the b-axis), and a thickness (L _c ). When strongly oxidized, these initially discrete graphite particles are chemically transformed into higher aromatic graphene oxide molecules with significant concentrations of edge-supported or surface-supported functional groups (such as —OH, —COOH, etc.). . These aromatic GO molecules in the GO gel have lost their original identity being part of graphite particles or flakes. When the liquid component is removed from the GO gel, the resulting GO molecules form an essentially amorphous structure. Upon heat treatment, these GO molecules are chemically assimilated and linked into highly ordered unitary or monolithic graphene entities.

The resulting thin films of graphene oxide typically have lengths or widths significantly greater than the L _a and L _b of the original crystallites. The length/width of this thin film is typically larger than the L _a and L _b of the original crystallites. Even a single grain in this graphene entity has a length or width significantly greater than the L _a and L _b of the original crystallites.

(6) their unique chemical composition (including oxygen content), morphology, crystal structure (including spacing between graphene), and structural features (e.g., few defects, good chemical bonding, and spacing between graphene sheets; Graphene oxide gel-derived monolithic films of graphene oxide exhibit a unique combination of remarkable thermal conductivity, electrical conductivity, mechanical strength, and scratch resistance due to the absence of graphene planes and interruption of the graphene planes. have. Also, the film bonds well to the underlying metal foil without the use of binders or adhesives.

The aforementioned features are further described and explained in detail as follows: As shown in FIG. be done.

Expanded graphite in which due to the weak van der Waals forces that hold the graphene layers parallel, the spacing between the graphene layers widens considerably leading to significant expansion along the c-axis, thus substantially preserving the layered properties of the carbon layers. Natural graphite can be processed to form structures. Generally, flakes of natural graphite (eg, 100 in FIG. 1B) are intercalated in an acid solution to produce a graphite intercalation compound (GIC, 102). The GIC is washed, dried, and then stripped by exposure to high temperature for a short period of time. This causes the flakes to expand or exfoliate up to 80-300 times their original dimensions in the c-axis direction of the graphite. Exfoliated graphite flakes are elongated in appearance and are therefore commonly referred to as worms 104 . These worms of highly expanded graphite flakes are spun into expanded graphite, such as webs, with typical densities of about 0.04-2.0 g/cm ³ for most applications, without the use of binders. It can be formed into agglomerated or consolidated sheets of paper, strips, tapes, foils, mats, etc. (typically referred to as "flexible graphite" 106).

The upper left portion of FIG. 1(A) shows a flow chart describing a prior art method used to make flexible graphite foils and resin impregnated flexible graphite composites. The method typically begins by intercalating graphite particles 20 (eg, natural or synthetic graphite) with an intercalant (typically a strong acid or acid mixture) to obtain a graphite intercalation compound 22 (GIC). . After washing in water to remove excess acid, the GIC becomes "expandable graphite". The GIC or expandable graphite is then exposed for a short period of time (typically 15 seconds to 2 minutes) to a high temperature environment (eg, in a tube furnace preset at temperatures in the range of 800-1,050° C.). do. This heat treatment expands the graphite 30 to several hundred times in its c-axis direction to achieve worm-like worm-eaten structures 24 (graphite worms), in which large pores are sandwiched between these interconnecting flakes. It contains exfoliated but not separated graphite flakes. An example of a graphite worm is shown in FIG. 2(A).

In one prior art method, exfoliated graphite (or a mass of graphite worms) was recompacted to form a flexible graphite foil (26 in FIG. 1A or FIG. 1) by using calendering or roll pressing techniques. (B) 106), but they are typically much thicker than 100 μm. A cross-sectional SEM image of the flexible graphite foil is shown in FIG. There is

Due to the presence of these misorientations and defects in graphite flakes in general, commercially available flexible graphite foils typically have an in-plane electrical conductivity of about 1,500 S/cm and a through-plane (thickness) of 15-30 S/cm. or Z-direction) electrical conductivity, in-plane thermal conductivity of 140-300 W/mK, and through-plane thermal conductivity of about 10-30 W/mK. These defects and misorientations are also responsible for low mechanical strength (eg defects are potential stress concentration sites where cracks are preferentially initiated). These properties are insufficient for many thermal management applications, and the present invention is implemented to address these issues.

In another prior art method, exfoliated graphite worms 24 may be impregnated with resin and then compressed and cured to form a flexible graphite composite 28, which is also typically of low strength. In addition, the electrical and thermal conductivity of graphite worms can be reduced by two orders of magnitude when resin impregnated.

Alternatively, the exfoliated graphite was subjected to a high intensity mechanical shear/separation process using a high intensity air jet mill, a high intensity ball mill, or an ultrasonic device to produce separated nanographene platelets 33 (NGP). Well, all graphene platelets are thinner than 100 nm, mostly thinner than 10 nm, and often single-layer graphene (also shown as 112 in FIG. 1(B)). NGPs are composed of a graphene sheet or multiple graphene sheets, each sheet being a two-dimensional, hexagonal structure of carbon atoms.

Further alternatively, by low intensity shear, graphite worms tend to separate into so-called expanded graphite flakes (108 in FIG. 1(B)) with a thickness >100 nm. These flakes can be formed into graphite paper or mat 106 using paper-making or mat-making methods. This expanded graphite paper or mat 106 is a very simple assembly or stack of discrete flakes with imperfections, interruptions, and misorientations between these discrete flakes.

A mass of multiple NGPs (33 in FIG. 1(A), comprising discrete sheets/platelets of single-layer and/or few-layer graphene) may be fabricated into graphene paper using a papermaking process (FIG. 1 ( 34 in A) or 114 in FIG. 1(B)). FIG. 3(B) shows a cross-sectional SEM image of a graphene paper/film made from discrete graphene sheets using a papermaking process. The image has many discrete graphene sheets that are folded or interrupted (unintegrated), most of the platelet orientations are not parallel to the film/paper surface, and many defects or imperfections are present. indicate. These NGP assemblies exhibit relatively low electrical conductivity even when closely packed.

The precursor of the thin film of graphene oxide bonded onto the metal foil is the graphene oxide gel 21 (FIG. 1(A)). This GO gel is obtained by immersing graphite material 20 in powder or fibrous form in a strongly oxidizing liquid in a reactor to form a suspension or slurry, which is initially optically opaque and blackened. I'm in. This optical opacity is attributed to the fact that at the onset of the oxidation reaction, the discrete graphite flakes and, at a later stage, the discrete graphene oxide flakes disperse and/or absorb visible wavelengths, resulting in an opaque and generally dark fluid mass. reflect the facts. When the reaction between the graphite powder and the oxidizing agent is allowed to proceed for a sufficient time and at a sufficiently high reaction temperature, this opaque suspension is transformed into a brown, typically translucent or transparent solution, It is now a homogeneous fluid called "graphene oxide gel" containing no discernible discrete graphite flakes or graphite oxide platelets (21 in Fig. 1(A)). When dispensed and deposited under a shear stress field, the GO gel acquires viscosity reduction and molecular orientation to form "oriented GO" 35, which is heat treated to form a monolithic thin film entity 37 bonded to a metal foil. can become

Also, the graphene oxide gel is typically optically transparent or translucent and visually homogeneous, with no discernible discrete flakes/platelets of graphite, graphene, or graphene oxide dispersed therein. In GO gels, GO molecules are uniformly dispersed in an acidic liquid medium. In contrast, conventional suspensions of discrete graphene sheets, oxidized graphene sheets, and expanded graphite flakes in fluids (e.g., water, organic acids, or solvents) appear dark, black, or dark brown, compared to graphene alone or oxidized. Graphene sheets or expanded graphite flakes are discernible or recognizable with the naked eye or even with a low power optical microscope (100×-1,000×).

The graphene oxide molecules dissolved in the liquid medium of the graphene oxide gel are aromatic chains with an average number of benzene rings in the chain typically less than 1,000, more typically less than 500, and often less than 100. . The majority of the molecules have more than 5 or 6 benzene rings (mainly >10 benzene rings) by a combination of atomic force microscopy, high-resolution TEM, and molecular weight measurements. Based on our elemental analysis, these benzene ring-type aromatic molecules are strongly oxidized, contain high concentrations of functional groups such as —COOH and —OH, and are therefore “soluble” in polar solvents such as water. (not just dispersive). The estimated molecular weights of these graphene oxide polymers in the gel state are typically between 200 g/mole and 43,000 g/mole, more typically between 400 g/mole and 21,500 g/mole, and most typically between 400 g/mole and 400 g/mole. between mol and 4,000 g/mol. Typical viscosity values for GO gels are shown in FIGS. 8(A)-8(C).

These soluble molecules behave like polymers and surprisingly (during subsequent heat treatment or re-graphitization) react and chemically connect with each other to provide good structural integrity and high thermal conductivity. of graphene layers can be formed. Conventional discrete graphene sheets, graphene oxide sheets, or graphite flakes have no self-reaction or cohesive bonding ability. Also very surprisingly, these soluble molecules in the GO gel are able to chemically bond the metal foil surface during the subsequent heat treatment or re-graphitization treatment.

Also, specifically and most importantly, these graphene oxide molecules present in the GO gel state are chemically bonded to each other when heat treated to dry the gel at a sufficiently high temperature for a sufficiently long time. , linked, or assimilated into very long and wide graphene layers (eg, FIG. 3(A)). There are no discernible individual graphene platelets or sheets. They are chemically transformed into chemically active or live GO molecules that are fully linked to each other and chemically integrated to form layered monomers along the graphene planes, and these monomers are aligned along the thickness (or It is believed that they are chemically bonded to each other along the Z direction). X-ray diffraction studies showed that the d-spacing (distance between graphene planes) returned from about 0.3354 nm (with 0.01 wt% oxygen) to 0.40 nm (with about 5.0-10% oxygen). backed up that. There appear to be no gaps between these graphene layers, so the layers are essentially assimilated into one large monolith. FIG. 3A represents an example of such a giant simplex. The formation process of such graphene entities is further illustrated in FIG. 3(C).

Starting graphite materials that are strongly oxidized to form graphene oxide gels include natural graphite, artificial graphite, mesophase carbon, mesophase pitch, mesocarbon microbeads, soft carbon, hard carbon, coke, carbon fiber, carbon nanofiber, carbon Nanotubes, or combinations thereof. The graphite material is preferably in the form of powder or short filaments having dimensions of less than 20 μm, more preferably less than 10 μm, even more preferably less than 5 μm, most preferably less than 1 μm.

Using artificial graphite having an average particle size of 9.7 μm as an example, a typical procedure typically involves exfoliating graphite particles for at least 3 days, preferably 5 days, and more preferably 7 days or more. It requires dispersing in an oxidant mixture of sulfuric acid, nitric acid and potassium permanganate (in a weight ratio of 3:1:0.05) at a temperature of 0-60°C. The average molecular weight of the resulting graphene oxide molecules in the gel is about 20,000-40,000 g/mol when the treatment time is 3 days, <10,000 g/mol when the treatment time is 5 days, and less than 7 days. <4,000 g/mol in the long case. The gel formation time required is dependent on the particle size of the original graphite material, with smaller particle sizes requiring shorter times. It is very important to point out that if the critical gel formation time has not been reached, the graphite powder and oxidant suspension (graphite particles dispersed in the oxidant liquid) will appear quite opaque and inhomogeneous. , means that the discrete graphite particles or flakes remain suspended (but not dissolved) in the liquid medium. As soon as this critical time is exceeded, the entire suspension becomes optically translucent or transparent (for sufficiently low GO contents) and brown, indicating that the strongly oxidized graphite has completely lost its original graphite nature. The resulting graphene oxide molecules are completely dissolved in the oxidant liquid to form a homogeneous solution (no longer just a suspension or slurry).

If the suspension or slurry is rinsed and dried after a treatment time shorter than the required gel formation time, the graphite oxide powder or graphite intercalation compound (GIC) powder is simply recovered, which is then exfoliated and dried. It should be further pointed out that discrete nanographene platelets (NGPs) can be produced separately. Without a sufficient amount of strong oxidant and a sufficient oxidation time, the graphite or graphite oxide particles will not convert to the GO gel state.

When a graphene oxide gel is obtained from a graphite material having an original graphite grain size (e.g., average grain size, D _g ), the resulting thin film of graphene oxide has grains much larger than the original grain size. A graphene structure having a diameter. The film has no identifiable grains associated with any particular grain of the starting graphite material. The original particles have already lost all their nature when converted to graphitic oxide molecules that are chemically linked and assimilated or integrated into a network of graphene chains whose molecular weight is essentially infinite.

Furthermore, even if the graphene oxide gel is obtained from a graphite material (e.g. powder of natural graphite) having a plurality of graphite crystallites that do not exhibit a preferred crystallographic orientation as measured by X-ray diffraction or electron diffraction methods, The resulting bonded thin films typically exhibit a very high degree of preferred crystalline orientation as measured by the same X-ray diffraction or electron diffraction methods. This is because the constituent graphene planes of the hexagonal carbon atoms that make up the particles of the original or starting graphite material are chemically modified, transformed, realigned, reoriented, linked or crosslinked, assimilated and integrated. , re-graphitized and further re-crystallized.

The present invention also provides rechargeable batteries containing the graphene oxide thin film-bonded metal foils of the present invention as anode current collectors and/or cathode current collectors. This can be, for example, any rechargeable battery such as a zinc-air cell, a nickel metal hydride cell, a sodium ion cell, a sodium metal cell, a magnesium ion cell, or a magnesium metal cell, to name a few. can. This invented battery can be a rechargeable lithium battery containing a unit graphene layer as an anode current collector or a cathode current collector, and the lithium battery can be a lithium-sulfur cell, a lithium-selenium cell, a lithium-sulphur /selenium cells, lithium ion cells, lithium-air cells, lithium-graphene cells, or lithium-carbon cells. Another embodiment of the invention is a capacitor containing the current collector of the invention as an anode current collector or a cathode current collector, the capacitor being a symmetric ultracapacitor, an asymmetric ultracapacitor cell, a hybrid supercapacitor-battery cells, or lithium-ion capacitor cells.

By way of example, the present invention provides a cathode containing a current collector on the anode side, a lithium film or foil as the anode, a porous separator/electrolyte layer, cathode active materials such as lithium _- free _V2O5 and _MnO2 . , and a current collector. Either or both of the anode and cathode current collectors can be graphene-based current collectors of the present invention.

Another example of the present invention is a current collector on the anode side, a graphite or lithium titanate anode, a porous separator soaked with a liquid or gel electrolyte, a cathode containing a cathode active material such as activated carbon with a high specific surface area. , and a current collector. Also, either or both of the anode current collector and the cathode current collector can be graphene-based current collectors of the present invention.

Yet another example of the present invention is another lithium-ion capacitor or hybrid supercapacitor, which comprises a current collector on the anode side, a graphite anode (and a sheet of lithium foil as part of the anode), immersed in a liquid electrolyte. It consists of a porous separator, a cathode containing a cathode active material (eg activated carbon with a high specific surface area), and a current collector. Also, either or both of the anode current collector and the cathode current collector can be graphene-based current collectors of the present invention.

Yet another example of the present invention is a current collector on the anode side, a porous nanostructured anode (e.g., a surface stabilized lithium powder on which returning lithium ions can be deposited during cell recharge). (including high surface area graphene sheets with sheets of lithium foil mixed with particles or attached to nanostructures), porous separators soaked with liquid electrolyte, graphene-based cathode active materials (e.g., cell A lithium-graphene cell composed of a cathode containing graphene, graphene oxide, or graphene fluoride sheets with a high specific surface area that traps lithium ions during discharge, and a cathode current collector. Also, either or both of the anode current collector and the cathode current collector can be graphene-based current collectors of the present invention.

Example 1 Preparation of Discrete Graphene Sheets (NGP) and Expanded Graphite Flakes Chopped graphite fibers with an average diameter of 12 μm and natural graphite particles were separately used as starting materials, which were treated with (chemical intercalators and oxidants As) a graphite intercalation compound (GIC) was prepared by immersion in a mixture of concentrated sulfuric acid, nitric acid, and potassium permanganate. The starting material was first dried in a vacuum oven at 80° C. for 24 hours. A mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (in a weight ratio of 4:1:0.05) is then slowly added to the three-necked flask containing the fiber segments under appropriate cooling and stirring. did. After 16 hours of reaction, the acid-treated graphite fibers or natural graphite particles were filtered and washed thoroughly with deionized water until the pH level of the solution reached 6. After drying overnight at 100° C., the resulting graphite intercalation compound (GIC) was subjected to thermal shock at 1050° C. for 45 seconds in a tube furnace to form exfoliated graphite (or graphite worms).

Five grams of the resulting exfoliated graphite (graphite worms) was mixed with 2,000 ml of an alcoholic solution consisting of alcohol and distilled water at a ratio of 65:35 for 12 hours to obtain a suspension. The mixture or suspension was then subjected to ultrasonic irradiation at a power of 200 W for various times. After 2 hours of sonication, the EG particles were effectively broken into thin NGP. The suspension was then filtered and dried at 80° C. to remove residual solvent. As-prepared NGP has an average thickness of about 9.7 nm.

Another 5 grams of the resulting exfoliated graphite (EG) was subjected to low intensity air jet milling to fracture the graphite worms and form expanded graphite flakes (having an average thickness of 139 nm). Samples of both expanded graphite flakes and graphene sheets were mixed with a binder resin (PVDF) and then coated onto the major surfaces of Cu and Al foils to form expanded graphite-coated current collectors and graphene oxide. A current collector was formed. Additionally, sheets of Al foil were cleaned with acetone and then spray coated with both GO and RGO sheets. The resulting current collectors were evaluated in both lithium batteries and supercapacitors.

Example 2: Preparation of graphene from mesocarbon microbeads (MCMB) Mesocarbon microbeads (MCMB) were obtained from China Steel Chemical Co., Ltd. supplied from. This material has a density of about 2.24 g/cm ³ and a median particle size of about 16 μm. MCMB (10 grams) was intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate in a 4:1:0.05 ratio) for 72 hours. At the end of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMB was washed repeatedly in a 5% solution of HCl to remove most of the sulfate ions. The samples were then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was dried and stored in a vacuum oven at 60°C for 24 hours. The dried powder sample was placed in a quartz tube and inserted into a horizontal tubular furnace preset at the desired temperature of 1,080° C. for 45 seconds to obtain the graphene material. TEM and atomic force microscopy show that most of the NGP was single-layer graphene. These graphene sheets were either fabricated into freestanding graphene paper or deposited (with a resin binder) onto thin metal foils. In the case of Al foil, samples of Al foil coated with graphene (without binder) were also made.

Example 3 Preparation of Pure Graphene Sheets/Platelits In a typical procedure, 5 grams of graphite flakes pulverized to a size of about 20 μm or less were added to 1,000 mL of deionized water (Zonyl® from DuPont). FSO, containing 0.1% by weight of dispersing aid) to give a suspension. An ultrasonic energy level of 85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation, and crushing of the graphene sheets for 15 minutes to 2 hours.

Example 4 Preparation of Graphene Oxide (GO) Gel Graphene oxide is oxidized with an oxidant liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate in a ratio of 4:1:0.05 at 30° C. to graphite flakes. Prepared by When natural graphite flakes (14 μm particle size) were immersed and dispersed in the oxidant mixture liquid, the suspension or slurry appeared optically opaque and dark. The suspension remains opaque during the first 52 hours of reaction. However, the suspension gradually becomes optically translucent (slightly cloudy) when the reaction time exceeds 52 hours, and the color of the suspension changes from black to dark brown. After 96 hours, the suspension suddenly becomes an optically clear solution with a light brown color. The solution appears very uniform in color and clear, indicating the absence of any dispersed discrete objects. The whole solution behaves like a gel, much like a typical polymer gel.

Surprisingly, by casting this gel onto a metal foil surface (Cu, Al, Ni, Ti, or stainless steel) and removing the liquid medium from the cast film, thin films of optically transparent graphene oxide were obtained. is obtained. This thin film looks, feels and behaves like an ordered polymer film. However, when heat-treated at temperatures (between 80° C. and 1,500° C.), typically for 1-3 hours, the GO films are transformed into monolithic thin-film entities containing large-sized graphene domains. This GO film bonds well to the underlying metal foil.

GO film before heat treatment (GO gel coated on glass surface, liquid medium removed), GO film thermally reduced at 150 °C for 1 hour, and highly reduced and re-graphitized GO film (unit The X-ray diffraction curves of the graphene layer) are shown in FIGS. 5(A), 5(B) and 5(C), respectively. The peak at about 2θ=12° in the dried GO film (Fig. 5(A)) corresponds to a spacing (d ₀₀₂ ) between graphenes of about 0.7 nm. With some heat treatment at 150 °C, the GO film exhibits the formation of humps centered at 22° (Fig. 5(B)), reducing the spacing between graphenes, indicating the initiation of chemical ligation and ordering processes. Indicates the process it started. A heat treatment temperature of 1,250° C. for 3 hours reduced the d ₀₀₂ spacing to about 0.34, which is close to 0.3354 nm for graphite single crystals.

A heat treatment temperature of 1,500° C. for 3 hours reduces the d _{002 spacing to about 0.3354 nm, which is equal to the d 002 spacing} of graphite single crystals. Furthermore, a second diffraction peak with high intensity appears at 2θ=55° corresponding to X-ray diffraction from the (004) plane (FIG. 5(D)). The (004) peak intensity relative to the (002) intensity on the same diffraction curve, or the I(004)/I(002) ratio, is a good indication of the crystalline perfection and preferred orientation of the graphene planes. The (004) peak is either absent or relatively weak for all graphite materials heat treated at temperatures below 2,800° C., and the I(004)/I(002) ratio <0. 1. The I(004)/I(002) ratio of graphite materials heat-treated at 3,000-3,250° C. (eg, highly oriented pyrolytic graphite, HOPG) is in the range of 0.2-0.5. An example is shown in FIG. 5(E) for polyimide-derived PG using HTT at 3,000° C. for 2 hours, which exhibits an I(004)/I(002) ratio of about 0.41. In contrast, a thin film of GO bonded onto a metal foil made using HTT at 1,500° C. for 4 hours exhibited an I(004)/I(002) ratio of 0.78 and 0 It exhibits a mosaic spread value of 0.21, indicating a nearly perfect graphene single crystal with a particular degree of preferred orientation. There is a synergy between thin GO layers (<1 μm) made from GO gels and underlying metal foils (Cu, Ni, Ti, and steel).

The "mosaic spread" value is obtained from the full width at half maximum of the (002) reflection of the X-ray diffraction intensity curve. This index for the degree of ordering characterizes the graphite or graphene crystal size (or grain size), the amount of grain boundaries and other defects, and the preferred degree of grain orientation. A nearly perfect single crystal of graphite is characterized by having a mosaic spread value of 0.2 to 0.4. Most of our unitary graphene materials have mosaic spread values in this range of 0.2-0.4 (using heat treatment temperatures of 1,500° C. and above).

It should be noted that the I(004)/I(002) ratios of all the dozens of flexible graphite samples investigated are all <<0.05, and in many cases are virtually non-existent. would be good too. The I(004)/I(002) ratios of all graphene paper/film samples are <0.1 even after heat treatment at 3,000° C. for 2 hours. Attempts to graphitize ultra-thin films (<2 nm thick) produced by Cu-catalyzed CVD lead to rupture of the film and instead the formation of graphite particles. The GO film-bonded metal foils of the present invention can be any pyrolytic graphite (PG), flexible graphite (FG), as well as conventional graphene/GO/RGO sheets/free-standing or coated onto metal foils. These observations further supported and confirmed the already established fact that platelit (NGP) is a novel and distinct class of materials fundamentally different from paper/film/membrane.

The inter-graphene spacing values of GO gel-derived GO films obtained by heat treatment at various temperatures over a wide temperature range are summarized in Fig. 6(A). The corresponding oxygen content values for GO gel-derived GO films are shown in FIG. 6(B). The data of FIGS. 6(A) and 6(B) are plotted again in FIG. 6(C) to show the correlation between inter-graphene spacing and oxygen content. Close inspection of FIGS. 6(A)-6(C) reveals four HTT ranges (80-200° C.; 200-500° C.; 500° C.; 500° C.; ~1,250°C; and 1,250-1,500°C). The thermal conductivity of the GO gel-derived GO film and the corresponding flexible graphite (FG) foil, also plotted as a function of the same final heat treatment temperature range, are summarized in FIG. 6(D).

We found that the heat treatment temperature, which is only 500 °C, is sufficient to make the average inter-graphene spacing of GO less than 0.4 nm, increasingly approaching the average inter-graphene spacing of natural graphite or the average inter-graphene spacing of graphite single crystals. should point out. The advantage of this method is that the GO gel method reorganizes, reorients, and chemically assimilates planar graphene oxide molecules from originally dissimilar graphite particles or graphene sheets such that all graphene planes now have lateral dimensions. , we were able to form graphene monoliths that were larger at (significantly larger than the length and width of the original graphene planes) and essentially parallel to each other. This already yields thermal conductivities of >420 W/mK (using 500° C. HTT) and >950 W/mK (using 700° C. HTT), which are comparable to those of flexible graphite foils. (200 W/mK), 2 to more than 4 times greater. These planar GO molecules originate from the graphene planes that make up the original structure of the starting natural graphite particles (used in the graphite oxidation procedure to form the GO gel). Pristine natural graphite particles, when randomly packed into aggregates or "graphite compacts", exhibit relatively low thermal conductivity with their constituent graphene planes randomly oriented and exhibit essentially has zero strength (has no structural integrity). In contrast, the strength of unit graphene layers (even without additional reinforcement) is typically already in the range of 40-140 Mpa.

Using HTT, which is only 800° C., the resulting unitary graphene layer exhibits a thermal conductivity of 1,148 W/mK, in contrast to the observed 244 W/mK of flexible graphite foil using the same heat treatment temperature. rate. In fact, no matter how high the HTT (eg, as high as 2,800° C.), flexible graphite foils exhibit thermal conductivities of less than 600 W/mK. At 2,800 °C HTT, the unit graphene layer of the present invention gives a thermal conductivity of 1,807 W/mK even when the metal foil is melted at such high temperature (Figs. 4(A) and 6(D). )).

Scanning electron microscopy (SEM), transmission electron microscopy (TEM) photographs, and selected area electron diffraction (SAD), bright field (BF), and dark field (DF) images of lattice imaging of graphene layers were also performed. to characterize the structure of unit graphene materials. For cross-sectional measurements of films, samples were embedded in a polymer matrix, sliced using an ultramicrotome, and etched with Ar plasma.

Close inspection and comparison of FIGS. 2(A), 3(A), and 3(B) show that the graphene layers in the monolithic GO thin films are oriented substantially parallel to each other. However, this is not the case for flexible graphite foil and graphene oxide paper. The tilt angle between two identifiable layers within the GO thin film entity is predominantly less than 5 degrees. In contrast, there are so many broken graphite flakes, kinks, and misorientations within flexible graphite that many of the angles between two graphite flakes are greater than 10 degrees, and some are as high as 45 degrees. (Fig. 2(B)). Although not bad at all, the misorientation between graphene platelets in the NGP paper (Fig. 3(B)) is also high, with many gaps between the platelets. A unitary graphene entity is essentially void-free.

FIG. 4(A) shows the thermal conductivity values of the GO gel-derived GO film, GO platelet paper made by vacuum-assisted filtration of RGO and FG foils, respectively, all plotted as a function of final HTT. These data clearly demonstrated the superiority of GO thin films with the thermal conductivity achievable at the given heat treatment temperature. All prior art studies on the preparation of papers or films from pure graphene or graphene oxide sheets/platelits clearly follow different processing routes, leading to simple assemblies or stacks of discrete graphene/GO/RGO platelits. Bring. These simple assemblies or laminates exhibit many broken graphite flakes, kinks, voids, and misorientation, resulting in poor thermal conductivity, low electrical conductivity, and weak mechanical strength. As shown in Fig. 4(A), even at heat treatment temperatures as high as 2,800 °C, the GO platelet paper yields a 1,000 W/mK exhibit a thermal conductivity of less than

For comparison, a polyimide film was carbonized in an inert atmosphere at 500° C. for 1 hour and 1,000° C. for 3 hours, then the film was subjected to temperatures ranging from 2,500 to 3,000° C. for 1 to 5 hours. to form a conventional pyrolytic graphite (PG) film. FIG. 4(B) shows the thermal conductivity values of GO-derived thin films and CVD carbon film-derived PG heat-treated for 3 h, all plotted as a function of final graphitization or re-graphitization temperature. These data indicate that conventional PG produced by carbonizing polyimide (PI) and then graphitizing the carbonized PI is less than GO gel, given the same HTT for the same length of heat treatment time. It shows consistently lower thermal conductivity compared to the derived GO thin films alone. For example, PG from PI exhibits a thermal conductivity of 820 W/mK after graphitization at 2,000° C. for 1 hour and 1,242 W/mK at 2,000° C. for 3 hours. These observations demonstrated a clear and significant advantage of using the GO gel method to produce thin films of GO bonded onto metal foils over the conventional PG method to produce oriented graphite crystals. In fact, no matter how long the graphitization time for PG, the thermal conductivity is always lower than that of GO gel-derived GO films. In other words, thin films of GO bonded on metal foils are comparable to flexible graphite (FG) foils, graphene/GO/RGO, in terms of chemical composition, crystal and defect structure, crystal orientation, morphology, manufacturing process, and properties. It is fundamentally different and clearly distinguishable from paper/membrane and pyrolytic graphite (PG).

The above conclusions are further supported by the data in Fig. 4(C), in which the electrical conductivity values of the GO-derived GO films are similar to those of the GO papers from the RGO platelets and FG foils over the entire range of final HTT investigated. , which is much superior to the electrical conductivity values of

Example 5 Measurement of Electrical and Thermal Conductivities of Various Graphene Oxide Gel-Derived Thin Films A four-point probe test was performed on thin films of the GO layer alone (coated onto a metal foil surface and then peeled off and heat treated). ), GO/RGO paper, and FG foil alone to measure their in-plane electrical conductivity. Their in-plane thermal conductivities were measured using the laser flash method (Netzsch thermal diffusion device).

The in-plane thermal and electrical conductivity and tensile properties of various films were investigated. For a thickness of about 75 μm, the thermal conductivity of the flexible graphite foil alone is less than 237 W/mK if the FG foil is not heat treated above 700°C. When the heat treatment temperature after recompression is increased from 700° C. to 2,800° C. (for 1 h of graphitization treatment in each case), the thermal conductivity of the FG foil increases from 237 to 582 W/mK, and the heat treatment shows some limited reorganization of graphite structures caused by In contrast, the thermal conductivity of the GO gel-derived thin graphene layers alone increases from 983 to 1,807 W/mK. This thin film was prepared by shearing and depositing a layer of GO gel on a Cu foil surface, removing the liquid from the GO layer in vacuum for 1 hour, and heat-treating the dried solid GO layer. obtained by This indicates a marked or dramatic reorganization of the graphite structure caused by the heat treatment, with all GO molecules linked edge-to-edge and inter-facet as integrated graphene bodies of well- and orderedly joined graphene planes. or assimilated.

Also corresponding flexible graphite foils (FG made by roll pressing exfoliated graphite worms) and expanded graphite flake foils or mats (graphite worms were broken into graphite flakes as described in Example 1). , then made by packing and roll-pressing them into thin foils/mats) were obtained. The highest thermal conductivity values achievable with expanded graphite foil are <800 W/mK and those achievable with FG are <600 W/mK, both for unit graphene matrix and graphene matrix composites. is dramatically lower than the value of

Example 6: Tensile Strength of Various Graphene Oxide-Derived Unit Graphene Matrix Composites A series of GO gel-derived thin-film graphene layers, GO platelet paper, and FG foils were fabricated. A universal testing machine was used to measure the tensile strength of these materials. The tensile strength values of graphene oxide thin films, GO platelet paper, and FG paper are plotted as a function of re-graphitization temperature, FIG. 7(A). The tensile strength of the flexible graphite foil remained relatively constant (all <20 MPa) and the tensile strength of the GO paper (from 22 MPa to 43 MPa) when the heat treatment temperature was increased from 700 to 2,800 °C. These data demonstrated a slight increase. In contrast, the tensile strength of GO-derived unit graphene layers increases dramatically from 32 to >100 Mpa over the same range of heat treatment temperatures. This result is very impressive, GO gel-derived GO layers contain highly live and active molecules during heat treatment, whereas graphene platelets in conventional GO paper and graphite flakes in FG foil are essentially further reflects the idea of being a dead molecule. The GO-derived unit graphene entity is the material class itself.

Scratch tests were performed using the so-called Ford Lab test method (FLTM) BN108-13. The apparatus consists of a movable platform connected to five beams with a length of 250 mm. A scratch pin is attached to one end of each beam. A highly polished hardened steel ball (1.0±0.1 mm diameter) is placed on the tip of each pin. Each pin is loaded with weights exerting forces of 7N, 6N, 3N, 2N, and 0.6N respectively. Moved by compressed air, the beam pulls the pin across the surface of the specimen, creating a scratch. Scratching is performed at a sliding speed of about 100 mm/s. All tests were performed at room temperature. Only the smooth side of the sample was tested in this study, although the test method requires that the grainy surface be evaluated.

After scratching the sample plaques, they were evaluated using a reflected light polarization microscope incorporating a xenon light source. An image analyzer with image analysis software was used to measure the total grayscale value of the object, the "grayscale mass." The camera objective was placed at a 90° angle from the scratch. And then the objective lens records a part of the scratch about 1 mm in length. The electronic signal for each scratch line is then integrated and recorded. The optical mass M of an object is the sum of the gray level values, GL, of all pixels of the object. Single gray level values are assigned by the image analysis program in unit steps ranging from 0 to 255 (0=black and 255=white). The optical mass M can be calculated from M=ΣGL _i (sum from i to n), where n is the number of pixels. The brightness of the object, B, is B=M/A (A represents the area of the object). The percentage change in brightness between the scratch and the background is the visibility ΔB of the scratch, given by ΔB=[(B _scratch −B _background )/(B _background )]×100%. The scratch depth was measured using an interferometer. Magnification was set at 5x. Depth measurements were made from depth histograms of the scanned area. The scratch test results are shown in FIG. 7(B). Scratches were also examined using a scanning electron microscope (SEM).

Example 7 Li—S Cells Containing Anode Side and Cathode Side Graphene Oxide Bonded Metal Foil Current Collectors Three (3) Li—S cells were fabricated and tested, each with a lithium foil active material as the anode. , sulfur/expanded graphite composite (75/25 weight ratio) as cathode active material, 1 M LiN(CF ₃ SO ₂ ) ₂ in DOL as electrolyte, and Celgard 2400 as separator. The first cell (reference cell for comparison) contains a 10 μm thick Cu foil as the anode current collector and a 20 μm thick Al foil as the cathode current collector. A second cell (another reference cell for comparison) has a 10 μm thick GO-resin layer as the anode current collector and a 14 μm RGO-coated Al foil sheet as the cathode current collector. The third cell has a GO-bonded Cu foil of the invention (12 μm total thickness) as the anode current collector and a 20 μm thick sheet of GO-coated Al foil as the cathode current collector.

Charge storage capacity was measured and recorded periodically as a function of cycle number. The specific discharge capacity referred to herein is per unit mass of cathode composite (counting the weight of the cathode active material, conductive additive or support, and binder, but excluding the current collector): It is the total charge put into the cathode during discharge. Specific energy and specific power values given in this section are based on total cell weight (including anode and cathode, separator and electrolyte, current collectors, and packing materials). Morphological or microstructural changes in selected samples after a desired number of repeated charging and recharging cycles are observed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). was done.

FIG. 9(A) shows the discharge capacity values for each of the three cells as a function of the number of charge/discharge cycles. Each cell was designed to have an initial cell capacity of 100mAh for ease of comparison. It is clear that the Li—S cell featuring the GO junction current collector of the present invention on both the anode and cathode sides exhibits the most stable cycling behavior, undergoing 6% capacity loss after 50 cycles. Cells with GO/resin-coated Cu and GO-coated Al current collectors experience a 23% capacity loss after 50 cycles. A cell with a Cu foil anode current collector and an Al foil cathode current collector undergoes a 26% capacity loss after 50 cycles. Post-cycle inspection of the cell shows that the Al foil in all prior art electrodes suffers from severe corrosion problems. In contrast, the graphene oxide-bonded Al current collector of the present invention remains intact.

FIG. 9(B) shows the Ragone plot (mass power density versus mass energy density) of the three cells. Our GO-bonded metal foil current collectors surprisingly consisted of prior art graphene/resin coated current collectors on the anode side (using GO-coated Al foil on the cathode side) and Cu/Al Note that it gives both a higher energy density and a higher power output density for the Li—S cell compared to the current collector. It is quite unexpected that Cu foil has an electrical conductivity that is more than an order of magnitude higher than that of graphene films. The difference between the energy density and power density values exceeds the calculated value by the physical density difference between the Cu foil and the graphene film on the anode side.

Example 8: Magnesium Ion Cell Containing _Current Collectors Enabled by Graphene on the Anode Side and Cathode Side For the Preparation of the Cathode Active Material ₍ Magnesium Manganese Silicate, _{Mg1.03Mn0.97SiO4} ) , reagent grade KCl (melting point = 780°C) was used as flux after drying under vacuum at 150°C for 3 hours. Starting materials were magnesium oxide (MgO), manganese(II) carbonate (MnCO ₃ ) and silicon dioxide (SiO ₂ , 15-20 nm) powders. The theoretical amounts of precursor compounds were controlled at a molar ratio of 1.03:0.97:1 for Mg:Mn:Si. The mixture (flux/reactant molar ratio=4) was manually pulverized with a pestle in a mortar for 10 minutes and then poured into a corundum crucible. The powder mixture was then dried in vacuum at 120° C. for 5 hours to minimize water content in the mixture. The mixture was then immediately transferred to a tube furnace and heated in a reducing atmosphere (Ar + 5 wt% H2) at 350°C for 2 hours to remove carbonate groups. This was followed by a final firing at various temperatures at a rate of 2°C/min for 6 hours, then allowed to cool naturally to room temperature. Finally, the product (magnesium manganese silicate, _{Mg1.03Mn0.97SiO4} ) was washed _three times with deionized water to dissolve any remaining salts, separated by centrifugation and allowed to _cool for 2 hours. It was dried under vacuum at 100°C.

The electrode (either anode or cathode) is typically made of 85 wt% electrode active material (eg Mg _1.03 Mn _0.97 SiO ₄ particles, 7 wt% acetylene black (Super-P), and 8 wt% polyfluoride). was made by mixing a vinylidene chloride binder (PVDF dissolved in N-methyl-2-pyrrolidinoe (NMP), 5 wt% solids) to form a slurry mixture. After coating the slurry on the current collector, the resulting electrode was dried in vacuum at 120° C. for 2 hours to remove the solvent before pressing.Three cells with different current collectors were prepared. investigated: first cell with GO-bonded Cu foil and GO-bonded Al foil as anode and cathode current collectors, respectively; GO/resin coated Cu foil as anode and cathode current collectors; A second cell (prior art cell) each having a GO coated Al foil (no pre-etching); a third cell (prior art cell) having a Cu foil anode current collector and an Al foil cathode current collector. ).

The electrodes were then cut into discs (diameter=12 mm) for use as cathodes. A thin sheet of magnesium foil was attached to the anode current collector surface and then a piece of porous separator (eg Celgard 2400 membrane) was stacked on top of the magnesium foil. A single cathode disk coated onto the cathode current collector was used as the cathode and stacked on top of the separator layer to form a CR2032 coin cell. The electrolyte used was 1 M Mg(AlCl ₂ EtBu) ₂ in THF. Assembly of the cell was performed inside an argon-filled glovebox. CV measurements were performed using a CHI-6 electrochemical workstation with a scan rate of 1 mV/s. The electrochemical performance of the cells was also tested galvanostatically at current densities from 50 mA/g to 10 A/g (up to 100 A/g for some cells) using Arbin and/or LAND electrochemical workstations. / Evaluated by discharge cycle progress.

FIG. 10 shows the cell discharge specific capacity values for each of the three cells as a function of charge/discharge cycle number. It is clear that the Mg-ion cell featuring current collectors of the invention on both the anode and cathode sides exhibits the most stable cycling behavior, undergoing a 2.5% capacity loss after 25 cycles. A cell with a GO/resin coated Cu foil current collector and a GO coated Al foil current collector undergoes a 17% capacity loss after 25 cycles. A cell with a Cu foil anode current collector and an Al foil cathode current collector undergoes a 30% capacity loss after 25 cycles. Post-cycle inspection of the cells showed that the GO/resin-coated Cu foil current collector and the GO-coated Al foil current collector were swollen, indicating some delamination from the cathode layer, and Al foil shows that it suffers from severe corrosion problems. In contrast, the GO-bonded metal foil current collector of the present invention remains intact.

Example 9: Chemical and mechanical compatibility testing of various current collectors for various intended batteries or supercapacitors Battery or supercapacitor electrolytes, as demonstrated in Examples 8 and 9 above The long term stability of the current collector is a major concern. To understand the chemical stability of various current collectors, a major effort was initiated exposing the current collectors in several representative electrolytes. After an extended period of time (eg, 30 days), the current collectors were removed from the electrolyte solution and observed using optical and scanning electron microscopy (SEM). A list of the results is set forth in Table 3 below, and they demonstrate that the GO-bonded metal foil current collectors of the present invention are highly compatible with all types of liquid electrolytes commonly used in batteries and supercapacitors. consistently demonstrate that The materials of the invention are resistant to any chemical attack. These GO protective current collectors are essentially electrochemically inert over the voltage range of 0-5.5 volts versus Li/Li ⁺ and are suitable for use with nearly any battery/capacitor electrolyte. Are suitable.

It may be pointed out that each current collector must be connected to a tab and then to an external circuit line. The current collector must be mechanically compatible with the tab and must be readily or easily secured or bonded thereto. We have found that whenever the CVD graphene film is just mechanically fixed to the tab, it breaks or bursts easily. Even with the aid of adhesives, CVD films are easily ruptured during the procedure of connecting to tabs or battery cell packaging.

In conclusion, we have successfully developed an absolutely new, novel, unexpected, and distinct class of highly conductive materials: graphene oxide gel-derived thin films of GO bonded on metal foils. The chemical composition, structure (crystalline perfection, grain size, defect clusters, etc.), crystallographic orientation, morphology, manufacturing method, and properties of this new class of materials have been characterized by flexible graphite foil, polymer-derived pyrolytic graphite, CVD-derived PG (HOPG, etc.) as well as catalytic CVD graphene thin films free-standing or coated on metal foils are fundamentally different and distinct. The thermal conductivity, electrical conductivity, scratch resistance, surface hardness, and tensile strength exhibited by the materials of the present invention are comparable to prior art flexible graphite sheets, discrete graphene/GO/RGO platelet paper, or Much higher than other graphite films can possibly achieve. These GO gel-derived thin film structures have the best combination of excellent electrical conductivity, thermal conductivity, mechanical strength, surface scratch resistance, hardness, and no tendency to flake off.

Claims (59)

In graphene oxide bonded metal foil current collectors in batteries or supercapacitors,
(a) a free-standing, unsupported thin metal foil having a thickness of 1 μm to 30 μm and two opposing parallel major surfaces;
(b) a thin film of graphene oxide sheet chemically bonded to at least one of said two opposing major surfaces of said metal foil without the use of a binder or adhesive, said at least one major surface being non-uniform; Said thin film of graphene oxide does not contain a layer of mobilized metal oxide and has a thickness of 10 nm to 10 μm, an oxygen content of 0.1 wt % to 10 wt %, and a graphene surface of 0.335 to 0.50 nm. spacing, with a physical density of 1.3-2.2 g/cm ³ , all graphene oxide sheets oriented approximately parallel to each other and parallel to the major surfaces, measured alone without the thin metal foil. A graphene oxide-bonded metal foil current collector, characterized in that it exhibits a thermal conductivity greater than 500 W/mK and/or an electrical conductivity greater than 1,500 S/cm when charged. 2. The current collector of claim 1, wherein each of the two opposing major surfaces is chemically bonded to a thin film of graphene oxide sheet without the use of a binder or adhesive, and wherein the thin film of graphene oxide is has a thickness of 10 nm to 10 μm, an oxygen content of 0.1% to 10% by weight, a spacing between graphene planes of 0.335 to 0.50 nm, and a physical density of 1.3 to 2.2 g/cm ³ . wherein all graphene oxide sheets are oriented substantially parallel to each other and parallel to the major surfaces, and have a thermal conductivity greater than 500 W/mK and greater than 1,500 S/cm when measured alone without the thin metal foil A current collector characterized by exhibiting high electrical conductivity. 2. The current collector according to claim 1, wherein said thin metal foil has a thickness of 4-10 μm. 2. The current collector of claim 1, wherein said thin film of graphene oxide sheet has a thickness of 20 nm to 2 μm. 2. The current collector of claim 1, wherein the metal foil is selected from Cu, Ti, Ni, stainless steel, and chemically etched Al foil, and the surface of the chemically etched Al foil is , a current collector, wherein no passivating Al ₂ O ₃ is formed thereon before being bonded to said graphene oxide sheet. The current collector of claim 1, wherein the thin film of graphene oxide sheet has an oxygen content of 1 wt% to 5 wt%. The current collector of claim 1, wherein the thin film of graphene oxide has an oxygen content of less than 1%, an inter-graphene spacing of less than 0.345 nm, and an electrical conductivity of 3,000 S/cm or greater. A current collector characterized by: 2. The current collector of claim 1, wherein the thin film of graphene oxide has an inter-graphene spacing of less than 0.337 nm and an electrical conductivity of 5,000 S/cm or greater. 2. The current collector of claim 1, wherein the thin film of graphene oxide has an inter-graphene spacing of less than 0.336 nm, a mosaic spread value of 0.7 or less, and an electrical conductivity of 8,000 S/cm or greater. A current collector characterized by: The current collector of claim 1, wherein the thin film of graphene oxide has an inter-graphene spacing of less than 0.336 nm, a mosaic spread value of 0.4 or less, and an electrical conductivity of greater than 10,000 S/cm. A current collector characterized by: 2. The current collector of claim 1, wherein said thin film of graphene oxide exhibits inter-graphene spacing of less than 0.337 nm and a mosaic spread value of less than 1.0. 2. The current collector of claim 1, wherein the thin film of graphene oxide exhibits a degree of graphitization of 80% or more and/or a mosaic spread value of 0.4 or less. 2. A method of manufacturing a current collector according to claim 1, wherein said thin film of graphene oxide sheets deposits graphene oxide gel on said at least major surface under the influence of an orientation-controlled stress; , and heat-treating the graphene oxide gel at a heat treatment temperature of 80°C to 1,500°C. A method according to claim 13, characterized in that the heat treatment temperature is between 80°C and 500°C. A method according to claim 13, characterized in that the heat treatment temperature is between 80°C and 200°C. 2. The current collector of claim 1, wherein the thin film of graphene oxide contains chemically bonded graphene molecules or chemically assimilated graphene planes that are parallel to each other. 14. The method of claim 13, wherein the graphene oxide gel is obtained from a graphite material having an original maximum graphite grain size, and the thin film of graphene oxide has grains larger than the original maximum grain size. A method characterized by having a diameter. 14. The method of claim 13, wherein the graphene oxide gel is formed from graphite crystallites having an initial length L _a in the a-axis crystallographic direction, an initial width L _b in the b-axis direction, and a thickness L _c in the c-axis direction. made from composed natural or artificial graphite particles, and characterized in that said thin film of graphene oxide has graphene domains or crystal lengths or _widths greater than said initial La and _Lb of said graphite crystallites. and how. 2. The current collector of claim 1, wherein the thin film of graphene oxide contains graphene planes having a combination of sp ² and sp ³ electron configurations. 2. The current collector of claim 1, wherein said thin film of graphene oxide is a continuous length film having a length of 5 cm or more and a width of 1 cm or more. The current collector of claim 1, wherein said thin film of graphene oxide has a physical density greater than 1.8 g/cm ³ and/or a tensile strength greater than 40 MPa when measured alone. A current collector characterized by: The current collector of claim 1, wherein said thin film of graphene oxide has a physical density greater than 1.9 g/cm ³ and/or a tensile strength greater than 60 MPa when measured alone. A current collector characterized by: The current collector of claim 1, wherein said thin film of graphene oxide has a physical density greater than 2.0 g/cm ³ and/or a tensile strength greater than 80 MPa when measured alone. A current collector characterized by: A rechargeable lithium or lithium ion battery, characterized in that it contains the current collector according to claim 1 as anode current collector and/or cathode current collector. A rechargeable lithium battery containing the current collector according to claim 1 as anode current collector or cathode current collector, comprising a lithium-sulfur cell, a lithium-selenium cell, a lithium-sulfur/selenium cell, a lithium-air cell, a lithium- A rechargeable lithium battery characterized by being a graphene cell or a lithium-carbon cell. A capacitor containing the current collector of claim 1 as an anode current collector or a cathode current collector, said capacitor being a symmetric ultracapacitor, an asymmetric ultracapacitor cell, a hybrid supercapacitor-battery cell, or a lithium ion capacitor cell. A capacitor characterized by: A method for fabricating a thin film graphene oxide bonded metal foil current collector for a battery or supercapacitor, said method comprising the steps of: (a) preparing a graphene oxide gel in which graphene oxide molecules are dissolved in a fluid medium; wherein said graphene oxide molecules contain an oxygen content greater than 20% by weight; and (b) distributing and depositing a layer of said graphene oxide gel on at least one of the two major surfaces of a metal foil to forming a layer of wet graphene oxide gel deposited thereon, wherein said dispensing and deposition procedure includes shear-induced thinning of said graphene oxide gel; graphene oxide partially or completely removed from the wetted layer and having an interplanar spacing d ₀₀₂ of 0.4 nm to 1.2 nm and an oxygen content of 20% by weight or more as measured by X-ray diffraction and (d) said drying of graphene oxide to such an extent that said interplanar spacing d ₀₀₂ is reduced to a value between 0.335 nm and 0.5 nm and said oxygen content is reduced to less than 10% by weight. heat treating the film at a heat treatment temperature of 80° C. to 2,500° C. to form said thin film graphene oxide bonded metal foil current collector, and said thin film of graphene oxide having a thickness of 10 nm to 10 μm. , a physical density of 1.3-2.2 g/cm ³ , and wherein all graphene oxide sheets are oriented substantially parallel to each other and parallel to said at least one major surface. 28. The method of claim 27, wherein step (b) distributes and deposits a layer of the graphene oxide gel on each of the two major surfaces of the metal foil so that each of the two major surfaces A method comprising forming a layer of wet graphene oxide gel deposited thereon, wherein the metal foil has a thickness of 1 μm to 30 μm. 28. The method of claim 27, wherein the metal foil is selected from Cu, Ti, Ni, stainless steel, and chemically etched Al foil, the surface of the chemically etched Al foil being the A method characterized in that no passivating Al ₂ O ₃ is formed on the graphene oxide before it is bonded. 28. The method of claim 27, wherein step (c) comprises forming a graphene oxide layer having an interplanar spacing d ₀₀₂ of 0.4 nm to 0.7 nm and an oxygen content of 20% by weight or more; Step (d) includes heat-treating the graphene oxide layer to such an extent that the interplanar spacing d ₀₀₂ is reduced to a value between 0.3354 nm and 0.36 nm and the oxygen content is reduced to less than 2% by weight. A method characterized by: 28. The method of claim 27, wherein the graphene oxide gel has a viscosity greater than 2,000 centipoise when measured at 20°C prior to the shear-induced thinning, and wherein the viscosity is is reduced to less than 2,000 centipoise during or after. 28. The method of claim 27, wherein the graphene oxide gel has a viscosity of 500 centipoise to 500,000 centipoise measured at 20° C. prior to said shear-induced thinning. 28. The method of claim 27, wherein said graphene oxide gel has a viscosity greater than or equal to 5,000 centipoise when measured at 20°C prior to said shear-induced thinning, and said viscosity is equal to or greater than shear-induced thinning. is reduced to less than 2,000 centipoise during or after. 28. The method of claim 27, wherein said graphene oxide gel has a pH value of less than 5.0. 28. The method of claim 27, wherein said graphene oxide gel has a pH value of less than 3.0. 28. The method of claim 27, wherein shear-induced thinning is performed by a procedure selected from coating, casting, printing, air-assisted spraying, ultrasonic spraying, or extrusion. 28. The method of claim 27, wherein step (d) comprises thermally treating the graphene oxide layer under compressive stress. 28. The method of claim 27, wherein the graphene oxide gel is in powder or fibrous form for a time sufficient to obtain a homogenous solution and an optically transparent, translucent, or brown graphene oxide gel. prepared by immersing a graphite material in an oxidizing liquid to form an initially optically opaque and darkened suspension in a reactor at reaction temperature, wherein the graphene oxide gel is A method comprising graphene oxide molecules dissolved in an acidic medium having a pH value, and wherein said graphene oxide molecules have an oxygen content of 20% by weight or more. 28. The method of claim 27, wherein the graphene oxide gel is homogenously and optically and allowing an oxidation reaction to proceed until a clear, translucent, or brown solution is formed, wherein the graphite material comprises natural graphite, artificial graphite, mesophase carbon, mesophase pitch, mesocarbon microbeads, soft carbon, selected from hard carbon, coke, carbon fibers, carbon nanofibers, carbon nanotubes, or combinations thereof. 28. The method of claim 27, wherein steps (b) and (c) are a roll-to-roll method, wherein the steps (b) and (c) include feeding a sheet of the metal foil from rollers to a deposition area; depositing on said at least one major surface of a foil to form a wet layer of graphene oxide gel thereon; and drying said wet layer of graphene oxide gel to dry deposited on said major surface. and collecting the metal foil having the dried graphene oxide layer deposited thereon on a collector roller. 28. The method of claim 27, wherein the heat treatment temperature comprises a temperature in the thermal reduction regime of 80°C to 500°C, and the thin film graphene oxide bonded metal foil current collector has an oxygen content of less than 5%, 0 and/or having a thermal conductivity of at least 100 W/mK. 28. The method of claim 27, wherein the heat treatment temperature comprises a temperature in the range of 500° C. to 1,000° C., and the thin film of graphene oxide has an oxygen content of less than 1%, graphene of less than 0.345 nm. spacing, a thermal conductivity of at least 1,300 W/mK, and/or an electrical conductivity of 3,000 S/cm or more. 28. The method of claim 27, wherein the heat treatment temperature comprises a temperature in the range of 1,000° C. to 1,500° C., and the graphene oxide film has an oxygen content of less than 0.01%, less than 0.337 nm. , a thermal conductivity of at least 1,500 W/mK, and/or an electrical conductivity of 5,000 S/cm or greater. 28. The method of claim 27, wherein the graphene oxide film exhibits an inter-graphene spacing of less than 0.337 nm and a mosaic spread value of less than 1.0. 28. The method of claim 27, wherein the graphene oxide film exhibits a degree of graphitization of 40% or more and/or a mosaic spread value of less than 0.7. 28. The method of claim 27, wherein the graphene oxide film exhibits a graphitization degree of 80% or more and/or a mosaic spread value of 0.4 or less. 28. The method of claim 27, wherein the graphene oxide film contains chemically bonded graphene molecules or chemically assimilated graphene planes that are parallel to each other. 28. The method of claim 27, wherein the graphene oxide gel is obtained from a graphite material having an original maximum graphite grain size, and wherein the graphene oxide thin film has grains larger than the original maximum graphite grain size. A method characterized by having a diameter. 28. The method of claim 27, wherein the graphene oxide gel is in an oxidizing liquid state in the reactor at the reaction temperature for a time sufficient to obtain a homogeneous solution composed of graphene oxide molecules dissolved in the liquid medium. Obtained by immersing a graphite material in powder or fiber form in a medium, said homogeneous solution being optically transparent, translucent or brown, and said graphene oxide molecules having an oxygen content of 20% by weight or more. and having a molecular weight of less than 43,000 g/mole while in the gel state. 50. The method of claim 49, wherein the graphene oxide molecules have a molecular weight of less than 4,000 g/mole while in the gel state. 50. The method of claim 49, wherein the graphene oxide molecules have a molecular weight between 200 g/mol and 4,000 g/mol while in the gel state. 28. The method of claim 27, wherein the step of heat treatment causes chemical ligation, assimilation, or chemical bonding of graphene oxide molecules and/or regraphitization or reorganization of graphitic structures. . 28. The method of claim 27, wherein the graphene oxide film has an electrical conductivity greater than 3,000 S/cm, a thermal conductivity greater than 600 W/mK, a physical density greater than 1.8 g/ ^cm3 , and/or A method characterized by having a tensile strength greater than 40 MPa. 28. The method of claim 27, wherein the graphene oxide film has an electrical conductivity greater than 5,000 S/cm, a thermal conductivity greater than 1,000 W/mK, a physical density greater than 1.9 g/ ^cm3 , and / or having a tensile strength greater than 60 MPa. 28. The method of claim 27, wherein the graphene oxide film has an electrical conductivity greater than 15,000 S/cm, a thermal conductivity greater than 1,500 W/mK, a physical density greater than 2.0 g/ ^cm3 , and / or having a tensile strength greater than 80 MPa. A method according to claim 27, characterized in that said metal foil has a thickness of 4-10 µm. 28. The method of claim 27, wherein the graphene oxide film has a thickness between 20 nm and 2 μm. A method for fabricating a thin film graphene oxide bonded metal foil current collector for a battery or supercapacitor, the method comprising the steps of (a) preparing a graphene oxide gel bath in which graphene oxide molecules are dissolved in a fluid medium. wherein said graphene oxide molecules contain an oxygen content of greater than 20% by weight and said graphene oxide gel has a pH value of less than 5.0; moving the sheet of metal foil out of the bath to allow deposition of a wet layer of graphene oxide gel on each of the two major surfaces of the metal foil; Partially or completely removing from the deposited wet layer of graphene oxide gel to form a dry film of graphene oxide having an interplanar spacing d ₀₀₂ of 0.4 nm to 1.2 nm as measured by X-ray diffraction (d) heating said dried film of graphene oxide at 80° C. to such an extent that said interplanar spacing d ₀₀₂ is reduced to a value between 0.335 nm and 0.5 nm and said oxygen content is reduced to less than 10% by weight. and heat treating at a heat treatment temperature of 2,500° C. to form said thin film graphene oxide bonded metal foil current collector, and said thin film of graphene oxide having a thickness of 10 nm to 10 μm and a thickness of 1.3 to 1.3 μm. A method having a physical density of 2.2 g/cm ³ and wherein all graphene oxide sheets are oriented substantially parallel to each other and parallel to said at least one major surface. 59. The method of claim 58, wherein said metal foil comprises aluminum foil.

Images