WO2018031064A1 - Collecteur de courant à film mince en feuille métallique lié à l'oxyde de graphène - Google Patents

Collecteur de courant à film mince en feuille métallique lié à l'oxyde de graphène Download PDF

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Publication number
WO2018031064A1
WO2018031064A1 PCT/US2017/018707 US2017018707W WO2018031064A1 WO 2018031064 A1 WO2018031064 A1 WO 2018031064A1 US 2017018707 W US2017018707 W US 2017018707W WO 2018031064 A1 WO2018031064 A1 WO 2018031064A1
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Prior art keywords
graphene oxide
graphene
current collector
less
gel
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PCT/US2017/018707
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English (en)
Inventor
Aruna Zhamu
Bor Z. Jang
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Nanotek Instruments, Inc.
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Priority claimed from US15/231,486 external-priority patent/US10586661B2/en
Priority claimed from US15/231,498 external-priority patent/US10158122B2/en
Application filed by Nanotek Instruments, Inc. filed Critical Nanotek Instruments, Inc.
Priority to KR1020197005445A priority Critical patent/KR20190039536A/ko
Priority to JP2019506509A priority patent/JP7126492B2/ja
Priority to CN201780047369.XA priority patent/CN109565053A/zh
Publication of WO2018031064A1 publication Critical patent/WO2018031064A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/665—Composites
    • H01M4/667—Composites in the form of layers, e.g. coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/66—Current collectors
    • H01G11/68—Current collectors characterised by their material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/66—Current collectors
    • H01G11/70—Current collectors characterised by their structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/661—Metal or alloys, e.g. alloy coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/663—Selection of materials containing carbon or carbonaceous materials as conductive part, e.g. graphite, carbon fibres

Definitions

  • the present invention provides a current collector for a lithium battery or supercapacitor.
  • the current collector is a metal foil bonded with a thin graphene oxide film produced from graphene oxide gel.
  • This graphene oxide-bonded thin metal foil is electrolyte-compatible, non- reactive, corrosion-protective, of low contact resistance, thermally and electrically conductive, ultra-thin, and light-weight, enabling a battery or capacitor to deliver a higher output voltage, higher energy density, high rate-capability, and much longer cycle life.
  • This patent application is directed at a current collector that works with an anode electrode (anode active material layer) or a cathode electrode (cathode active material layer) of a lithium cell (e.g. lithium-ion cell, lithium-metal cell, or lithium-ion capacitor), a supercapacitor, a non-lithium battery (such as the zinc-air cell, nickel metal hydride battery, sodium-ion cell, and magnesium-ion cell), and other electrochemical energy storage cells.
  • a lithium cell e.g. lithium-ion cell, lithium-metal cell, or lithium-ion capacitor
  • a supercapacitor e.g. lithium-ion cell, lithium-metal cell, or lithium-ion capacitor
  • a non-lithium battery such as the zinc-air cell, nickel metal hydride battery, sodium-ion cell, and magnesium-ion cell
  • This application is not directed at the anode active material layer or the cathode active material layer itself.
  • the lithium-metal cell includes the conventional lithium-metal rechargeable cell (e.g. using a lithium foil as the anode and Mn0 2 particles as the cathode active material), lithium-air cell (Li-Air), lithium-sulfur cell (Li-S), and the emerging lithium-graphene cell (Li-graphene, using graphene sheets as a cathode active material), lithium-carbon nanotube cell (Li-CNT, using CNTs as a cathode), and lithium-nano carbon cell (Li-C, using nano carbon fibers or other nano carbon materials as a cathode).
  • the anode and/or the cathode active material layer can contain some lithium, or can be prelithiated prior to or immediately after cell assembly.
  • Li-ion Rechargeable lithium-ion
  • Li-ion Li-ion
  • Li metal Li-sulfur, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones.
  • Lithium as a metal element has the highest lithium storage capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li 4 4 Si, which has a specific capacity of 4,200 mAh/g).
  • Li metal batteries having a lithium metal anode
  • conventional lithium-ion batteries having a graphite anode).
  • rechargeable lithium metal batteries were produced using non-lithiated compounds having relatively high specific capacities, such as TiS 2 , MoS 2 , Mn0 2 , Co0 2 , and
  • lithium-ion secondary batteries in which pure lithium metal sheet or film was replaced by carbonaceous materials (e.g. natural graphite particles) as the anode active material.
  • the carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium-ion battery operation.
  • the carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as Li x C 6 , where x is typically less than 1 (with graphite specific capacity ⁇ 372 mAh/g).
  • Li-ion batteries are promising energy storage devices for electric drive vehicles
  • state-of-the-art Li-ion batteries have yet to meet the cost, safety, and performance targets (such as 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 a positive electrode (cathode) that de/re-intercalates Li + at a high potential with respect to the carbon negative electrode (anode).
  • the specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-170 mAh/g.
  • the specific energy (gravimetric energy density) of commercially available Li-ion cells featuring a graphite anode and a lithium transition-metal oxide or phosphate based cathode is typically in the range of 120-220 Wh/kg, most typically 150-200 Wh/kg.
  • the corresponding typical range of energy density (volumetric energy density) is from 300 to 400 Wh/L.
  • a typical battery cell is composed of an anode current collector, an anode electrode (also referred to as the anode active material layer, typically including an anode active material, a conductive filler, and a binder resin component), an electrolyte/separator, a cathode electrode (also referred to as the cathode active material layer, typically including a cathode active material, a conductive filler, and a binder resin), a cathode current collector, metal tabs that are connected to external wiring, and casing that wraps around all other components except for the tabs.
  • the sum of the weights and the sum of the volumes of these components are the total cell weight and total cell volume, respectively.
  • the total amount of energy stored by a cell is governed by the amount of cathode active material and the corresponding amount of anode active material.
  • the specific energy and energy density of a cell is then defined as the total amount of energy stored by the total cell weight and cell volume, respectively. This implies that one way to maximize the specific energy and energy density of a cell is to maximize the amounts of active materials and to minimize the amounts of all other components (non-active materials), under the constraints of other battery design considerations.
  • the current collectors at the anode and the cathode in a battery cell are non-active materials, which must be reduced (in weight and volume) in order to increase the gravimetric and volumetric energy densities of the battery.
  • Current collectors typically aluminum foil (at the cathode) and copper foil (at the anode), account for about 15-20% by weight and 10-15% by cost of a lithium-ion battery. Therefore, thinner, lighter foils would be preferred.
  • state-of-the-art current collectors there are several major issues associated with state-of-the-art current collectors:
  • thinner than 6 ⁇ e.g. Cu
  • thinner than 12 ⁇ e.g. Al, Ni, stainless steel foil
  • the current collectors are crucially important for cost, weight, safety, and performance of a battery.
  • graphene or graphene-coated solid metal or plastic has been considered as a potential current collector material, as summarized in the references listed below:
  • graphene current collectors come in three different forms: graphene-coated substrate [Ref. 1-4], free-standing graphene paper [Ref. 5], and monolayer graphene film produced by transition metal (Ni, Cu)-catalyzed chemical vapor deposition (CVD) followed by metal etching [Ref. 6].
  • graphene-coated substrate small isolated sheets or platelets of graphene oxide (GO) or reduced graphene oxide (RGO) are spray-deposited onto a solid substrate (e.g. plastic film or Al foil).
  • the building blocks are separated graphene sheets/platelets (typically 0.5 - 5 ⁇ in length/width and 0.34-30 nm in thickness) that are typically bonded by a binder resin, such as PVDF [Refs. 1, 3, and 4].
  • a binder resin such as PVDF [Refs. 1, 3, and 4].
  • individual graphene sheets/platelets can have a relatively high electrical conductivity (within the confine of that 0.5 - 5 ⁇ )
  • the resulting graphene-binder resin composite layer is relatively poor in electrical conductivity (typically ⁇ 100 S/cm and more typically ⁇ 10 S/cm).
  • another purpose of using a binder resin is to bond the graphene-binder composite layer to the substrate (e.g.
  • a trace amount of H 2 0 in this electrolyte can trigger a series of chemical reactions that involve formation of HF (a highly corrosive acid) that readily breaks up the aluminum oxide layer and continues to corrode the Al foil and consume electrolyte.
  • HF a highly corrosive acid
  • the capacity decay typically becomes much apparent after 200-300 charge-discharge cycles.
  • Free-standing graphene paper is typically prepared by vacuum-assisted filtration of GO or RGO sheets/platelets suspended in water.
  • the building blocks are separated graphene sheets/platelets that are loosely overlapped together.
  • individual graphene sheets/platelets can have a relatively high electrical conductivity (within the confine of that 0.5 - 5 ⁇ )
  • the resulting graphene paper has a very low electrical conductivity; e.g. 8,000 S/m or 80 S/cm [Ref. 5], which is 4 orders of magnitude lower than the conductivity of Cu foil (8 x 10 5 S/cm).
  • the catalyzed CVD process involves introduction of a hydrocarbon gas into a vacuum chamber at a temperature of 500-800°C. Under these stringent conditions, the hydrocarbon gas gets decomposed with the decomposition reaction being catalyzed by the transition metal substrate (Ni or Cu). The Cu/Ni substrate is then chemically etched away using a strong acid, which is not an environmentally benign procedure. The whole process is slow, tedious, and energy-intensive, and the resulting graphene is typically a single layer graphene or few-layer graphene (up to 5 layers maximum since the underlying Cu/Ni layer loses its effectiveness as a catalyst).
  • Bhardwaj, et al [Ref. 6] suggested stacking multiple CVD-graphene films to a thickness of 1 ⁇ or a few ⁇ ; however, this would require hundreds or thousands of films stacked together (each film being typically 0.34 nm to 2 nm thick).
  • Bhardwaj, et al claimed that "The graphene may reduce the manufacturing cost and/or increase the energy density of a battery cell," no experimental data was presented to support their claim. Contrary to this claim, the CVD graphene is a notoriously expensive process and even a single-layer of CVD graphene film would be significantly more expensive than a sheet of Cu or Al foil given the same area (e.g. the same 5 cm x 5 cm).
  • a stack of hundreds or thousands of mono-layer or few-layer graphene films as suggested by Bhardwaj, et al would mean hundreds or thousands times more expensive than a Cu foil current collector. This cost would be prohibitively high. Further, the high contact resistance between hundreds of CVD graphene films in a stack and the relatively low conductivity of CVD graphene would lead to an overall high internal resistance, nullifying any potential benefit of using thinner films (1 ⁇ of graphene stack vs. 10 ⁇ of Cu foil) to reduce the overall cell weight and volume. It seems that the patent application of Bhardwaj, et al [Ref. 6], containing no data whatsoever, is nothing but a concept paper.
  • the present invention provides a graphene oxide-bonded metal foil (thin film) current collector in a battery or supercapacitor.
  • the current collector comprises: (a) a free-standing, nonsupported thin metal foil having a thickness from 1 ⁇ to 30 ⁇ and two opposed but substantially parallel primary surfaces; and (b) a thin film of graphene oxide sheets chemically bonded to at least one of the two opposed primary surfaces without using a binder or adhesive wherein the at least one primary surface does not contain a layer of passivating metal oxide (e.g.
  • the thin film of graphene oxide has a thickness from 10 nm to 10 ⁇ , an oxygen content from 0.1% to 10% by weight, an inter- graphene plane spacing of 0.335 to 0.50 nm, a physical density from 1.3 to 2.2 g/cm 3 , all graphene oxide sheets being oriented substantially parallel to each other and parallel to the primary surfaces, exhibiting a thermal conductivity greater than 500 W/mK, and/or electrical conductivity greater than 1,500 S/cm when measured alone without the thin metal foil.
  • the thin metal foil e.g. Cu foil, Al foil, stainless steel foil, Ni foil, and Ti foil
  • the thin metal foil must be a free standing film (not supported on another piece of metal plate, for instance) in order to reduce the film thickness and, thus, the length of pathways that electrons collected from or transferred to an electrode active material have to travel.
  • the thin metal foil preferably has a thickness from 4 to 10 ⁇ .
  • the thin film of graphene oxide has a thickness from 20 nm to 2 ⁇ (further preferably ⁇ 1 ⁇ ).
  • both primary surfaces are each chemically bonded with a thin film of graphene oxide sheets without using a binder or adhesive; wherein said thin film of graphene oxide has a thickness from 10 nm to 10 ⁇ , an oxygen content from 0.1% to 10% by weight, an inter-graphene plane spacing of 0.335 to 0.50 nm, a physical density from 1.3 to 2.2 g/cm 3 , all graphene oxide sheets are oriented substantially parallel to each other and parallel to said primary surfaces, exhibiting a thermal conductivity greater than 500 W/mK and 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 conducted on Al foil in such a manner that the surfaces of the chemically etched Al foil have no passivating A1 2 0 3 film formed thereon prior to being bonded to the graphene oxide molecules.
  • graphene oxide gel (GO gel containing heavily oxidized graphene molecules in an acidic medium having a pH value of 5.0 or lower, preferably and typically ⁇ 3.0, and most typically ⁇ 2.0) is capable of removing the passivating A1 2 0 3 phase on Al foil surfaces.
  • These GO molecules in a GO gel have an oxygen content typically > 20% by wt, more typically > 30% by wt, and most typically > 40% by wt.
  • a simple suspension of discrete graphene or graphene oxide sheets in a liquid medium e.g. water or an organic solvent
  • the thin film of graphene oxide sheets has an oxygen content from 1% to 5% by weight.
  • the thin film of graphene oxide in the invented current collector has an oxygen content less than 1%, an inter-graphene spacing less than 0.345 nm, and an electrical conductivity no less than 3,000 S/cm.
  • the thin film of graphene oxide has an oxygen content less than 0.1%, an inter- graphene spacing less than 0.337 nm, and an electrical conductivity no less than 5,000 S/cm.
  • the thin film of graphene oxide has an oxygen content no greater than 0.05%, an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.7, and an electrical conductivity no less than 8,000 S/cm.
  • the thin film of graphene oxide has an inter-graphene spacing less than 0.336 nm, a mosaic spread value no greater than 0.4, and an electrical conductivity greater than 10,000 S/cm.
  • the thin film of graphene oxide exhibits an inter-graphene spacing less than 0.337 nm and a mosaic spread value less than 1.0.
  • the thin film of graphene oxide exhibits a degree of graphitization no less than 80% and/or a mosaic spread value no greater than 0.4.
  • the present invention also provides a process of producing the invented current collector.
  • the thin film of graphene oxide sheets is obtained by depositing a graphene oxide gel onto a primary surface or two primary surfaces of the metal foil under the influence of an orientation-controlling stress that aligns the GO molecules or sheets along the primary surface plane directions and then heat-treating the deposited graphene oxide gel at a heat treatment temperature from 80°C to 1,500°C.
  • the heat treatment temperature is from 80°C to 500°C.
  • the heat treatment temperature is from 80°C to 200°C.
  • 200°C is capable of facilitating edge-to-edge merger of highly oriented GO sheets (chemical linking, extending of sheet-like molecules, or "polymerizing or chain-growing" of heavily oxidized GO molecules (from a GO gel) that have been well-aligned.
  • the thin film of graphene oxide contains chemically bonded graphene molecules or chemically merged graphene planes that are parallel to one another.
  • the graphene oxide gel is obtained from a graphitic material having a maximum original graphite grain size (resulting in GO sheets having a maximum length) and the thin film of graphene oxide has a grain size larger than this maximum original grain size or maximum GO length.
  • the present invention also provides a process for producing a thin film graphene oxide- bonded metal foil current collector for a battery or supercapacitor.
  • the process comprises: (a) preparing a graphene oxide gel having graphene oxide molecules dissolved in a fluid medium wherein the graphene oxide molecules contain an oxygen content higher than 20% by weight; (b) dispensing and depositing a layer of graphene oxide gel onto at least one of two primary surfaces of a metal foil to form a layer of wet graphene oxide gel deposited thereon, wherein the dispensing and depositing procedure includes shear-induced thinning of graphene oxide gel; (c) partially or completely removing the fluid medium from the deposited wet layer of graphene oxide gel to form a dry film of graphene oxide having an inter-plane spacing d 0 02 of 0.4 nm to 1.2 nm as determined by X-ray diffraction and an oxygen content no less than 20% by weight; and (d) heat treating the dry film of graphene oxide
  • step (b) includes dispensing and depositing a layer of graphene oxide gel onto each of the two primary surfaces of the metal foil to form a layer of wet graphene oxide gel deposited on each of the two primary surfaces, wherein the metal foil has a thickness from 1 ⁇ to 30 ⁇ .
  • the metal foil may be selected from Cu, Ti, Ni, stainless steel, and chemically etched Al foil, wherein a surface of the chemically etched Al foil has no passivating A1 2 0 3 formed thereon prior to being bonded to said graphene oxide.
  • step (c) includes forming a graphene oxide layer having an inter- plane spacing d 0 02 of 0.4 nm to 0.7 nm and an oxygen content no less than 20% by weight; and step (d) includes heat-treating the graphene oxide layer to an extent that an inter-plane spacing doo2 is decreased to a value of from 0.3354 nm to 0.36 nm and the oxygen content is decreased to less than 2% by weight.
  • the graphene oxide gel has a viscosity greater than 2,000 centipoise when measured at 20°C prior to the shear-induced thinning and the viscosity is reduced to less than 2,000 centipoise during or after shear-induced thinning.
  • the graphene oxide gel has a viscosity from 500 centipoise to 500,000 centipoise when measured at 20°C prior to shear-induced thinning.
  • the graphene oxide gel has a viscosity no less than 5,000 centipoise when measured at 20°C prior to shear-induced thinning, and the viscosity is reduced to less than 2,000 centipoise during or after shear-induced thinning.
  • the graphene oxide gel has a viscosity that decreases by at least 10 times when a shear rate is increased at 20°C.
  • the graphene oxide gel has a pH value less than 5.0, preferably ⁇ 3.0, and more preferably ⁇ 2.0.
  • the shear-induced thinning may be conducted via a procedure selected from coating, casting, printing (e.g. inkjet printing, screen printing, etc.), air- assisted spraying, ultrasonic spraying, or extrusion.
  • step (d) includes heat treating said graphene oxide layer under a compressive stress.
  • the graphene oxide gel may be prepared by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid to form an initially optically opaque and dark suspension in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a graphene oxide gel that is a homogeneous solution and also optically transparent, translucent, or brown- colored, wherein the graphene oxide gel is composed of graphene oxide molecules dissolved in an acidic medium having a pH value of no higher than 5 and the graphene oxide molecules have an oxygen content no less than 20% by weight.
  • the graphitic material may be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro- bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
  • the process can be a roll-to-roll process wherein steps (b) and (c) include feeding a sheet of said metal foil from a roller to a deposition zone, depositing a layer of graphene oxide gel onto the at least one primary surface of the metal foil to form a wet layer of graphene oxide gel thereon, drying the wet layer of graphene oxide gel to form a dried graphene oxide layer deposited on a primary surface, and collecting dried graphene oxide layer-deposited metal foil on a collector roller.
  • the heat treatment temperature contains a temperature in a thermal reduction regime of 80°C-500°C and the film of graphene oxide has an oxygen content less than 5%, an inter-graphene spacing less than 0.4 nm, and/or a thermal conductivity of at least 100 W/mK. In certain embodiments, the heat treatment temperature contains a temperature in the range of 500°C-1,000°C and the unitary graphene material has an oxygen content less than 1%, an inter-graphene spacing less than 0.345 nm, a thermal conductivity of at least 1,300 W/mK, and/or an electrical conductivity no less than 3,000 S/cm.
  • the heat treatment temperature contains a temperature in the range of 1,000°C- 1,500°C and the graphene oxide film has an oxygen content less than 0.01%, an inter-graphene spacing less than 0.337 nm, a thermal conductivity of at least 1,500 W/mK, and/or an electrical conductivity no less than 5,000 S/cm.
  • the graphene oxide film exhibits an inter-graphene spacing less than 0.337 nm and a mosaic spread value less than 1.0. In certain embodiments, the graphene oxide film exhibits a degree of graphitization no less than 40% and/or a mosaic spread value less than 0.7. In certain embodiments, the graphene oxide film exhibits a degree of graphitization no less than 80% and/or a mosaic spread value no greater than 0.4.
  • the graphene oxide film contains chemically bonded graphene molecules or chemically merged graphene planes that are parallel to one another.
  • the graphene oxide gel is obtained from a graphitic material having multiple graphite crystallites exhibiting no preferred crystalline orientation as determined by an X-ray diffraction or electron diffraction method and wherein said graphene oxide film has a preferred crystalline orientation as determined by said X-ray diffraction or electron diffraction method.
  • the graphene oxide gel may be obtained by immersing a graphitic material in a powder or fibrous form in an oxidizing liquid medium in a reaction vessel at a reaction temperature for a length of time sufficient to obtain a homogeneous solution composed of graphene oxide molecules dissolved in the liquid medium, wherein the homogeneous solution is optically transparent, translucent, or brown colored and said graphene oxide molecules have an oxygen content no less than 20% by weight and a molecular weight less than 43,000 g/mole while in a gel state.
  • graphene oxide molecules have a molecular weight less than 4,000 g/mole while in a gel state.
  • graphene oxide molecules have a molecular weight between 200 g/mole and 4,000 g/mole while in a gel state.
  • the step of heat-treating induces chemical linking, merging, or chemical bonding of graphene oxide molecules, and/or re-graphitization or re-organization of a graphitic structure.
  • the process typically leads to a graphene oxide film having 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 tensile strength greater than 40 MPa. More typically, the graphene oxide film has an electrical conductivity greater than 5,000 S/cm, a thermal
  • 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/cm 3 , and/or a tensile strength greater than 80 MPa.
  • Also provided is a process for producing a thin film graphene oxide-bonded metal foil current collector for a battery or supercapacitor comprising: (a) preparing a bath of graphene oxide gel having graphene oxide molecules dissolved in a fluid medium wherein the graphene oxide molecules contain an oxygen content higher than 20% by weight and the graphene oxide gel has a pH value less than 5.0; (b) feeding a sheet of a metal foil into the GO gel bath and moving the sheet of metal foil out of the bath (creating a shear stress near the primary surfaces), enabling deposition of a wet layer of graphene oxide gel onto each of two primary surfaces of the metal foil; (c) partially or completely removing the fluid medium from the deposited wet layer of graphene oxide gel to form a dry film of graphene oxide having an inter-plane spacing d 0 02 of 0.4 nm to 1.2 nm as determined by X-ray diffraction and an oxygen content no less than 20% by weight; and (d) heat treating
  • the graphene oxide gel is produced from particles of a natural graphite or artificial graphite composed of graphite crystallites having an initial length L a in the crystallographic ⁇ -axis direction, an initial width L b in the ⁇ -axis direction, and a thickness L c in the c-axis direction, and the thin film of graphene oxide has a graphene domain or crystal length or width greater than the initial L a and L of the graphite crystallites.
  • the thin film of graphene oxide contains graphene planes having a combination of sp 2 and sp 3 electronic configurations.
  • the thin film of graphene oxide is a continuous length film having a length no less than 5 cm (preferably no less than 10 cm and further preferably no less than 20 cm) and a width no less than 1 cm (preferably no less than 10 cm).
  • the length and width of the continuous-length thin film of graphene oxide herein invented There are no practical limitations on the length and width of the continuous-length thin film of graphene oxide herein invented.
  • the thin film of graphene oxide when measured alone, has a physical density greater than 1.8 g/cm3, and/or a tensile strength greater than 40 MPa; preferably having a physical density greater than 1.9 g/cm3, and/or a tensile strength greater than 60 MPa and more preferably having a physical density greater than 2.0 g/cm 3 , and/or a tensile strength greater than 80 MPa.
  • the present invention also provides a rechargeable lithium battery or lithium -ion battery containing the presently invented current collector as an anode current collector and/or a cathode current collector.
  • the rechargeable lithium battery may be a lithium-sulfur cell, a lithium- selenium cell, a lithium sulfur/selenium cell, a lithium-air cell, a lithium-graphene cell, or a lithium-carbon cell.
  • the present invention also provides a capacitor containing the invented current collector as an anode current collector or a cathode current collector, which capacitor is a symmetric ultracapacitor, an asymmetric ultracapacitor cell, a hybrid supercapaci tor-battery cell, or a lithium-ion capacitor cell.
  • FIG.1(A) A flow chart illustrating various prior art processes of producing exfoliated graphite products (flexible graphite foils and flexible graphite composites) and pyrolytic graphite (bottom portion), along with a process for producing graphene oxide gel 21, and wet film of oriented GO 35, and thin film 37 of GO bonded to metal foil surface;
  • FIG.1 (B) Schematic drawing illustrating the conventional processes for producing paper, mat, and membrane of simply aggregated graphite or graphene (NGP) flakes/platelets. All processes begin with intercalation and/or oxidation treatment of graphitic materials (e.g. natural graphite particles).
  • graphitic materials e.g. natural graphite particles
  • FIG.1(C) Schematic drawing illustrating the prior art graphene -coated metal foil current
  • a binder resin layer (or passivating aluminum oxide layer) is present between the graphene layer and the metal foil, such as Cu foil (or Al foil).
  • FIG.1(D) Schematic drawing illustrating a preferred graphene-bonded metal foil current
  • FIG.2(A) A SEM image of a graphite worm sample after thermal exfoliation of graphite
  • GICs intercalation compounds
  • graphite oxide powders
  • FIG.2 An SEM image of a cross-section of a flexible graphite foil, showing many graphite flakes with orientations not parallel to the flexible graphite foil surface and also showing many defects, kinked or folded flakes.
  • FIG.3 A SEM image of a GO gel-derived graphene monolithic wherein multiple graphene planes (having an original length/width of 30 nm - 2 ⁇ ) in graphite particles, have been oxidized, exfoliated, re-oriented, and seamlessly merged into continuous-length graphene sheets or layers that can run for hundreds of centimeters wide or long.
  • FIG.3(B) A SEM image of a cross-section of a conventional graphene paper/film prepared from discrete graphene sheets/ platelets using a paper-making process (e.g. vacuum-assisted filtration).
  • the image shows many discrete graphene sheets being folded or interrupted (not integrated), with orientations not parallel to the film/paper surface and having many defects or imperfections;
  • FIG.3(C) Schematic drawing to illustrate the formation process of an integral GO entity
  • FIG.3(D) One plausible chemical linking mechanism (only 2 GO molecules are shown as an example; a large number of GO molecules can be chemically linked together to form a large graphene domain).
  • FIG.4(A) Thermal conductivity values of the GO gel-derived graphene layer, GO platelet paper, and FG foil plotted as a function of the final heat treatment temperature for
  • FIG.4(B) Thermal conductivity values of the GO gel-derived graphene layer, polyimide-derived pyrolytic graphite (PG), and CVD carbon-derived PG, all plotted as a function of the final graphitization or re-graphitization temperature;
  • FIG.4(C) Electric conductivity values of the GO gel-derived graphene layer, GO platelet paper, and FG foil, plotted as a function of the final graphitization or re-graphitization temperature;
  • FIG.5(A) X-ray diffraction curves of a GO film (dried GO gel);
  • FIG.5(B) X-ray diffraction curve of GO film thermally reduced at 150°C (partially reduced)
  • FIG.5(C) X-ray diffraction curve of highly reduced and re-graphitized GO film
  • FIG.5(D) X-ray diffraction curve of highly re-graphitized and re-crystallized GO crystal showing a high-intensity (004) peak
  • FIG.5(E) X-ray diffraction curve of a polyimide-derived HOPG with a HTT as high as 3,000°C.
  • FIG.6(B) the oxygen content in the GO gel-derived GO thin film
  • FIG.6(C) correlation between inter-graphene spacing and the oxygen content
  • FIG.7(A) Tensile strength of thin film of GO derived from GO gel, paper of discrete GO
  • FIG.7(B) Scratch resistance GO thin films plotted as a function of the heat treatment
  • FIG.8(A) Viscosity values (linear-linear scale) of graphene gel plotted as a function of
  • FIG.8 Viscosity values in log-linear scale
  • FIG.8 (C) Viscosity values log-log scale.
  • FIG.9(A) the discharge capacity values of three Li-S cells each as a function of the
  • first cell having GO-bonded Cu foil and GO-bonded Al foil as the anode and cathode current collectors, respectively; second cell having
  • GO/resin-coated Cu foil and GO-coated Al foil (no pre-etching) as the anode and cathode current collector, respectively (a prior art cell); third cell having a Cu foil anode current collector and Al foil cathode current collector (a prior art cell).
  • FIG.9(B) Ragone plots of the three cells: first cell having GO-bonded Cu foil and GO-bonded Al foil as the anode and cathode current collectors, respectively; second cell having
  • GO/resin-coated Cu foil and GO-coated Al foil (no pre-etching) as the anode and cathode current collector, respectively (a prior art cell); third cell having a Cu foil anode current collector and Al foil cathode current collector (a prior art cell).
  • FIG.10 The cell capacity values of three magnesium metal cells; first cell having GO-bonded Cu foil and GO-bonded Al foil as the anode and cathode current collectors, respectively; second cell having GO/resin-coated Cu foil and GO-coated Al foil (no pre-etching) as the anode and cathode current collector, respectively (a prior art cell); third cell having a Cu foil anode current collector and Al foil cathode current collector (a prior art cell).
  • the present invention provides a graphene oxide-bonded metal foil thin-film current collector (e.g. as schematically shown in FIG. 1(D)) in a battery or supercapacitor.
  • the current collector comprises: (a) a free-standing, non-supported thin metal foil (214 in FIG.1(D)) having a thickness from 1 ⁇ to 30 ⁇ and two opposed but substantially parallel primary surfaces; and (b) a thin film 212 of graphene oxide chemically bonded to at least one of the two opposed primary surfaces without using a binder or adhesive.
  • FIG. 1(D) only shows one primary surface of the metal foil 214 being bonded with a thin film 212 of graphene oxide.
  • the opposite primary surface is also bonded with a thin film of graphene oxide (not shown in FIG.1(D)).
  • a metal tab 218 is typically welded or soldered to the metal foil 214.
  • a preferred embodiment of the present invention is a graphene oxide-bonded metal foil current collector, wherein no binder resin layer or passivating aluminum oxide layer is present between the graphene oxide film and the Cu foil or Al foil.
  • the prior art graphene-coated metal foil current collector typically and necessarily requires a binder resin layer between the graphene layer (a graphene- resin composite) and the metal foil (e.g.
  • a very significant and unexpected advantage of bringing graphene oxide sheets in direct contact with the primary surfaces of a Cu, Ni, or Ti foil is the notion that graphene oxide molecules can be well -bonded to these metal foils under the presently invented processing conditions, without using an external resin binder or adhesive (hence, no dramatically increased contact resistance).
  • These processing conditions include well-aligning graphene oxide sheets on the metal foil surface and then heat-treating the two-layer structure at a temperature in the range of 80°C-1,500°C (more typically and desirably of 80°C-500°C, and most typically and desirably of 80°C-200°C).
  • the heat treatment temperature can be as high as 3,000°C.
  • These processing conditions include chemically etching off the passivating aluminum oxide layer prior to being coated with and bonded by graphene oxide, followed by a heat treatment under comparable temperature conditions described above.
  • the graphene oxide may be prepared in a GO gel state, which is characterized by having high oxygen contents, reflecting high amounts of -OH and - COOH groups and having a pH value less than 5.0 (preferably ⁇ 3.0 and even more preferably ⁇ 2.0).
  • the Al foil may be allowed to get immersed in a bath of GO gel, wherein the acidic environment naturally removes the passivating A1 2 0 3 layer.
  • the resulting thin film of graphene oxide in the presently invented graphene oxide-bonded metal foil has a thickness from 10 nm to 10 ⁇ , an oxygen content from 0.1% to 10% by weight, an inter-graphene plane spacing of 0.335 to 0.50 nm, a physical density from 1.3 to 2.2 g/cm 3 , all graphene oxide sheets being oriented substantially parallel to each other and parallel to the primary surfaces, exhibiting a thermal conductivity greater than 500 W/mK, and/or 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.
  • a graphene-coated current collector containing a binder or passivating metal oxide layer may be viewed as a three-layer structure (FIG. 1(C)) with the graphene film, interfacial binder resin layer (or passivating alumina layer), and metal foil layer electrically connected in series.
  • the thickness-direction resistivity pi of graphene layer 0.1 ohm-cm
  • the binder or alumina layer resistivity p 2 l x 10 14 ohm-cm
  • the total resistance of the three-layer structure would be 5 ⁇ 10 6 ohm and the overall conductivity would be as low as 1.4 ⁇ 10 "10 S/cm (see first data row in Table 1 below). If we assume that the binder resin layer is 10 nm thick, the total resistance of the three-layer structure would be 1 ⁇ 10 8 ohm and the overall conductivity would be as low as 7. Ox 10 "12 S/cm (see 4th data row in Table 1 below). Such a 3-layer composite structure would not be a good current collector for a battery or supercapacitor since a high internal resistance would mean a low output voltage and high amount of internal heat generated. Similar results are observed for Ni, Ti, and stainless steel foil-based current collectors (data rows 7-10 of Table 1).
  • the total resistance of a graphene oxide-bonded Cu foil has a value of l .Ox l O "5 ohm (vs. 1.0x l 0 +7 ohm of a 3-layer structure containing a l - ⁇ binder resin layer).
  • Table 2 This represents a difference by 12 orders of magnitude (not 12- fold)!
  • the conductivity would be 7. O x 10 +1 S/cm for the instant 2-layer structure, in contrast to 7.0x l 0 "u S/cm of the corresponding 3 -layer structure. Again, the difference is by 12 orders of magnitude.
  • lithium batteries and supercapacitors featuring the presently invented graphene oxide-bonded metal foil current collectors always exhibit a higher voltage output, higher energy density, higher power density, more stable chare- discharge cycling response, and last longer without capacity decay or corrosion issues as compared to prior art graphene-based current collectors
  • Bulk natural flake graphite is a 3-D graphitic material with each particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals.
  • Each grain is composed of multiple graphene planes that are oriented parallel to one another.
  • a graphene plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane).
  • the graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are different in orientation. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
  • a graphite single crystal (crystallite) per se is anisotropic with a property measured along a direction in the basal plane (crystallographic a- or b-axis direction) being dramatically different than the property measured along the crystallographic c-axis direction (thickness direction).
  • the thermal conductivity of a graphite single crystal can be up to approximately 1,920 W/mK (theoretical) or 1,800 W/mK (experimental) in the basal plane (crystallographic a- and b- axis directions), but that along the crystallographic c-axis direction is less than 10 W/mK
  • the multiple grains or crystallites in a graphite particle are typically all oriented along different directions. Consequently, a natural graphite particle composed of multiple grains of different orientations exhibits an average property between these two extremes (i.e. between5 W/mK and 1,800 W/mK, but typically ⁇ 100 W/mK).
  • the constituent graphene planes (typically 30 nm - 2 ⁇ wide/long) of a graphite crystallite can be exfoliated and extracted or isolated from the graphite crystallite to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome.
  • An isolated, individual graphene sheet of hexagonal carbon atoms is commonly referred to as single-layer graphene.
  • a stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of 0.3354 nm is commonly referred to as a multi-layer graphene.
  • a multi-layer graphene platelet has up to 300 layers of graphene planes ( ⁇ 100 nm in thickness), but more typically up to 30 graphene planes ( ⁇ 10 nm in thickness), even more typically up to 20 graphene planes ( ⁇ 7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community).
  • Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs).
  • NGPs nano graphene platelets
  • Graphene sheets/platelets or NGPs are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
  • NGPs are typically obtained by intercalating natural graphite particles with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(A) (process flow chart) and FIG. 1(B) (schematic drawing).
  • GIC graphite intercalation compound
  • GO graphite oxide
  • FIG. 1(A) process flow chart
  • FIG. 1(B) schematic drawing
  • the presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter-graphene spacing (i3 ⁇ 402, as determined by X-ray diffraction), thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction.
  • the GIC or GO is most often produced by immersing natural graphite powder (20 in FIG. 1(A) and 100 in FIG.
  • 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.
  • expandable graphite Upon exposure of expandable graphite to a temperature in the range of typically 800 - 1,050°C for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of 30-300 to form “graphite worms" (24 or 104), which are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected.
  • graphite worms A SEM image of graphite worms is presented in FIG. 2(A).
  • these graphite worms can be re-compressed to obtain flexible graphite sheets or foils (26 or 106) that typically have a thickness in the range of 0.1 mm (100 ⁇ ) - 0.5 mm (500 ⁇ ).
  • flexible graphite sheets or foils 26 or 106
  • Exfoliated graphite worms, expanded graphite flakes, and the recompressed mass of graphite worms are all 3-D graphitic materials that are fundamentally different and patently distinct from either the 1 -D nano carbon material (CNT or CNF) or the 2-D nano carbon material (graphene sheets or platelets, NGPs).
  • Flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically ⁇ 300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm.
  • the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets
  • high-intensity mechanical shearing e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill
  • NGPs (collectively called NGPs, 33 or 112), as disclosed in our US Application No. 10/858,814.
  • Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm, but more typically less than 20 nm.
  • Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation bas been increased from 0.3354 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets.
  • GO graphene oxide
  • RGO reduced graphene oxides
  • NGPs include discrete sheets/platelets of single-layer and multi-layer pristine graphene, graphene oxide, or reduced graphene oxide.
  • Pristine graphene has essentially 0% oxygen.
  • Graphene oxide (including RGO) can have 0.01%-50% by weight of oxygen.
  • RGO has an oxygen content of 0.01-10% by weight, more typically 0.01-5%) by weight, and most typically 0.01-2%) by weight.
  • the GO molecules in graphene oxide gel typically contains 20-47%) by weight oxygen (more typically 30-47%) immediately after removal of the liquid from the GO gel, but prior to a subsequent heat treatment.
  • the GO gel refers to a homogeneous solution of highly hydrophilic aromatic molecules (graphene oxide molecules bearing oxygen-containing groups, such as -OH, -COOH, and >0, on molecular planes or at the edges) that are dissolved (not just dispersed) in an acidic liquid (e.g. highly acidic water solution having a pH value lower than 5.0, more typically ⁇ 3.0, and often ⁇ 2.0).
  • the GO gel per se does not contain visibly discernible or discrete graphene or GO particles in the form of solid sheets or platelets.
  • These GO molecules and the dispersing liquid medium have comparable indices of refraction, making the resulting gel optically transparent or translucent (if the proportion of GO molecules are bot excessively high), or showing lightly brown color.
  • the simple mixture of graphite particles or discrete graphene or graphene oxide sheets/platelets with acids and/or water appears optically dark and totally opaque (even with only ⁇ 0.1% solid particles suspended in the liquid medium).
  • These particles or graphene or GO sheets/platelets are simply dispersed (not dissolved) in the fluid medium and they do not form a GO gel state.
  • GO molecules in a GO gel are highly reactive and may be considered as "living giant molecules".
  • the prior art solid sheets/platelets of graphene, GO, and RGO are essentially "dead” species.
  • the GO gel can be formed into a shape with a proper shearing or compression stress (e.g. via casting or molding) on a metal foil surface, dried (with liquid components partially or totally removed), and heat-treated under certain conditions to obtain a film of highly oriented graphene sheets chemically bonded to metal foil surface.
  • the heat treatment serves to chemically link these active or live GO molecules to form a 2-D or 3-D network of chemically bonded graphene molecules of huge molecular weights, and to drastically reduce the oxygen content of GO down to below 10% by weight (heat treatment temperature ⁇ 200°C), more typically ⁇ 5%, further more typically ⁇ 2% (heat treatment temperature ⁇ 500°C), and most typically « 1% (heat treatment temperature up to 1,500°C).
  • the GO gel per se does not contain visibly discernible/discrete graphene sheets/platelets or NGPs (including "dead” GO sheets/platelets), one can intentionally add discrete graphene sheets/platelets, expanded graphite flakes, and other type of solid filler in 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 merged GO sheets.
  • This graphene oxide gel-derived graphene material is now, reinforced with a filler phase (e.g. discrete NGPs, CNTs and carbon fibers).
  • Solid NGPs including discrete sheets/platelets of pristine graphene, GO, and GRO
  • a paper-like structure or mat made from platelets of discrete graphene, GO, or RGO e.g. those paper sheets prepared by vacuum-assisted filtration process
  • exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non-parallel platelets e.g. SEM image in FIG. 3(B)
  • FIG. 1(A) illustrates a typical process for producing pyrolytic graphitic films from a polymer.
  • the process begins with carbonizing a polymer film 46 (e.g. 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, which is followed by a graphitization treatment at 2,500-3,200°C under an ultrahigh pressure of 100-300 Kg/cm 2 for 1-24 hours to form a graphitic film 50.
  • a polymer film 46 e.g. polyimide
  • a carbonization temperature 400-1, 000°C under a typical pressure of 10-15 Kg/cm 2 for 2-10 hours
  • a graphitization treatment at 2,500-3,200°C under an ultrahigh pressure of 100-300 Kg/cm 2 for 1-24 hours to form a graphitic film 50.
  • polyacrylonitrile involves the emission of toxic species. Additionally, due to the difficulty in making the precursor polyimide films thinner than 30 ⁇ , it has not been possible to produce polyimide-derived pyrolytic film thinner than 15 ⁇ in a mass quantity. This does not meet the requirement of having a current collector 1-10 ⁇ thick.
  • a special type of graphene thin film ( ⁇ 2 nm) is prepared by catalytic CVD of hydrocarbon gas (e.g. C 2 H 4 ) on Ni or Cu surface.
  • hydrocarbon gas e.g. C 2 H 4
  • Ni or Cu being the catalyst, carbon atoms obtained via decomposition of hydrocarbon gas molecules at 800-1, 000°C are deposited onto Ni or Cu foil surface to form a sheet of single-layer or few-layer graphene that is poly-crystalline.
  • These graphene thin films being optically transparent and electrically conducting, are intended for use in applications such as the touch screen (to replace indium-tin oxide or ITO glass) or semiconductor (to replace silicon, Si).
  • these ultra-thin polycrystalline graphene films are not sufficiently conducting (too many grains or too much grain boundaries, and all grains being oriented in different directions) and not sufficiently thick for use as a current collector (most preferably from 1 ⁇ to 10 ⁇ ).
  • the Ni- or Cu-catalyzed CVD process does not lend itself to the deposition of more than 5-10 graphene planes (typically ⁇ 2-4 nm, more typically ⁇ 2 nm) beyond which the underlying Ni or Cu catalyst can no longer provide any catalytic effect.
  • CVD graphene layer thicker than 5 or 10 nm is possible, let alone 1 ⁇ (1,000 nm) to 10 ⁇ (10,000 nm).
  • the present invention also provides a process for producing a graphene oxide-bonded metal foil current collector, the process comprising: (a) preparing a graphene oxide gel having graphene oxide molecules dispersed and dissolved in a fluid medium wherein the graphene oxide molecules contain an oxygen content higher than 20% by weight (typically higher than 30% and more typically between 30% and 47% by weight); (b) dispensing and depositing a layer of graphene oxide gel onto a primary surface of a supporting metal foil to form a deposited graphene oxide gel thereon, wherein the dispensing and depositing procedure includes shear- induced thinning of the graphene oxide gel (resulting in graphene oxide molecules well -packed and well -aligned in desired direction(s), conducive to merging and integration of GO molecules during a subsequent heat treatment); (c) partially or completely removing the fluid medium from the deposited graphene oxide gel layer to form a dry graphene oxide layer having an inter-plane spacing d 0 02 of 0.4 nm to
  • step (c) includes forming a graphene oxide layer having an inter-plane spacing doo 2 of 0.4 nm to 0.7 nm and an oxygen content no less than 20% by weight; and step (d) includes heat-treating the graphene oxide layer to an extent that an inter- plane spacing d 0 02 is decreased to a value of from 0.3354 nm to 0.36 nm and the oxygen content is decreased to less than 2% by weight.
  • the thin film of graphene oxide obtained from heat-treating a graphene oxide gel at a temperature contains chemically bonded graphene molecules. These planar aromatic molecules or graphene planes (hexagonal structured carbon atoms) are parallel to one another. The lateral dimensions (length or width) of these planes are huge, typically several times or even orders of magnitude larger than the maximum crystallite dimension (or maximum constituent graphene plane dimension) of the starting graphite particles.
  • the graphene oxide gel is a very unique and novel class of material that surprisingly has great cohesion power (self-bonding, self-polymerizing, and self-crosslinking capability) and adhesive power (capable of chemically bonding to a wide variety of solid surfaces). These characteristics have not been taught or hinted in the prior art.
  • the GO gel is obtained by immersing powders or filaments of a starting graphitic material in an oxidizing liquid medium (e.g. a mixture of sulfuric acid, nitric acid, and potassium permanganate) in a reaction vessel.
  • an oxidizing liquid medium e.g. a mixture of sulfuric acid, nitric acid, and potassium permanganate
  • the starting graphitic material may be selected from natural graphite, artificial graphite, meso- phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
  • the resulting slurry (heterogeneous suspension) initially appears completely dark and opaque.
  • the reacting mass can eventually become a
  • the solution can be optically translucent or transparent or brown-colored, which also looks and behaves like a polymer gel.
  • This heavy oxidation-induced graphene oxide gel is composed of graphene oxide molecules dissolved in the liquid medium.
  • the graphene oxide molecules prior to any subsequent heat treatment, have an oxygen content no less than 20% by weight (typically from 30-50%) by weight) and their molecular weights are typically less than 43,000 g/mole (often less than 4,000 g/mole, but typically greater than 200 g/mole) while in a gel state.
  • the graphene oxide gel is composed of graphene oxide molecules dissolved in an acidic medium having a pH value of typically no higher than 5.0 (more typically ⁇ 3.0 and most typically ⁇ 2.0).
  • the graphene oxide gel has a typical viscosity from 500 centipoise (cP) to 500,000 cP when measured at 20°C prior to shear-induced thinning.
  • the 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.
  • the viscosity of the GO gel as a precursor to unitary graphene material is in the range of 2,000 - 50,000 cP.
  • the GO gel is subjected to a shear stress field that the viscosity is reduced to below 2,000 cP (or even below 1,000 cP) during or after shear-induced thinning.
  • the graphene oxide gel has a viscosity greater than 5,000 cP when measured at 20°C prior to shear-induced thinning, but is reduced to below 5,000 cP (preferably and typically below 2,000 cP or even below 1,000 cP) during or after shear- induced thinning.
  • the straight line of the data when plotted in a log-log scale indicates a shear thinning fluid flow behavior.
  • step (b) the GO gel is formed into a shape preferably under the influence of a shear stress.
  • a shearing procedure is casting or coating a thin film of GO gel (gellike fluid) using a coating machine. This procedure is similar to a layer of varnish, paint, coating, or ink being coated onto a solid substrate.
  • the roller, "doctor's blade", or wiper creates a shear stress when the film is shaped, or when a relative motion is imposed between the
  • such a shearing action reduces the effective viscosity of the GO gel and enables the planar graphene oxide (GO) molecules to well align along, for instance, a shearing direction.
  • a molecular alignment state or preferred orientation is not disrupted when the liquid components in the GO gel are subsequently removed to form a well-packed GO mass that is at least partially dried.
  • the dried GO mass has a high birefringence coefficient between an in-plane direction and the normal -to-plane direction.
  • Another example of such a procedure is injecting or die-casting a GO mass into a mold cavity or shaping die/tooling under the influence of a shearing stress.
  • the liquid component of the sheared GO mass in a mold cavity is then partially or completely removed to obtain a partially or totally dried GO mass containing well-packed and well-aligned "live" GO molecules.
  • This dried GO mass is then subjected to a properly programmed heat treatment.
  • the GO mass primarily undergoes chemical reactions with metal foil surfaces, sustains some chemical merging between GO molecules, and thermally reduces oxygen content from typically 30-50% (as dried) to 5-6%.
  • This treatment results in a reduction of inter-graphene spacing from approximately 0.6-1.0 nm (as dried) to approximately 0.4 nm and an increase in in-plane thermal conductivity from approximately 100 W/mK to 500 W/mK and electrical conductivity from 800 S/cm to > 2,000 S/cm. Even with such a low temperature range, some chemical linking occurs.
  • the GO molecules remain well-aligned, but the inter-GO spacing remains relative large (0.4 nm or larger). Many O-containing functional groups survive.
  • the graphene oxide film or lightly oxidized graphite crystalline material having a d 002 higher than 0.3440 nm reflects the presence of oxygen-containing functional groups (such as -OH, >0, and -COOH on graphene molecular plane surfaces) that act as a spacer to increase the inter-graphene spacing.
  • oxygen-containing functional groups such as -OH, >0, and -COOH on graphene molecular plane surfaces
  • a nearly perfect single crystal of graphite is characterized by having a mosaic spread value of 0.2-0.4. Most of our unitary graphene materials have a mosaic spread value in this range of 0.2-0.4 (with a heat treatment temperature no less than 2,000°C). However, some values are in the range of 0.4-0.7 if the highest heat treatment temperature (HTT) is between 1,000 and 1,500°C, and in the range of 0.7-1.0 if the HTT is between 500 and 1,000°C.
  • HTT highest heat treatment temperature
  • the heat treatment temperature conditions for GO are such that the thin film of graphene oxide coated on a metal foil is relatively pore-free having a physical density of at least 1.5 g/cm 3 or a porosity level lower than 20%. Under more typical processing conditions, the thin film has a physical density of at least 1.7 g/cm 3 or a porosity level lower than 10%. In most cases, the film has a physical density greater than 1.8 g/cm 3 or a porosity level less than 5%.
  • the chemically bonded graphene planes in the film typically contain a combination of sp 2 and sp 3 electronic configurations (particularly for those films prepared with the maximum treatment temperature lower than 1,500°C).
  • the graphene oxide (GO) gel -derived thin film of graphene oxide-bonded on a metal foil has the following characteristics (separately or in combination):
  • the thin film of graphene oxide is an integrated graphene phase if formed under a desired shearing stress field condition, followed by a proper heat treatment.
  • the film has wide/long chemically bonded graphene planes that are essentially oriented parallel to one another. In other words, the crystallographic c-axis directions of all grains and all their constituent graphene planes are essentially pointing in the same direction.
  • the paper-like sheets of exfoliated graphite worms i.e., flexible graphite foils
  • mats of expanded graphite flakes 100 nm in thickness
  • paper or membrane of graphene or GO platelets are a simple, un-bonded aggregate/stack of multiple discrete graphite flakes or discrete platelets of graphene, GO, or RGO.
  • the presently invented thin film of graphene oxide from GO gel is a fully integrated, single graphene entity or monolith containing no discrete flakes or platelets.
  • the preparation of the presently invented thin film of GO involves heavily oxidizing the original graphite particles, to the extent that practically every one of the original graphene planes has been oxidized and isolated from one another to become individual molecules that possess highly reactive functional groups (e.g. -OH, >0, and - COOH) at the edge and, mostly, on graphene planes as well.
  • highly reactive functional groups e.g. -OH, >0, and - COOH
  • These individual hydrocarbon molecules containing elements such as O and H, in addition to carbon atoms
  • the reaction medium e.g. mixture of water and acids
  • This gel is then cast onto a smooth substrate surface or injected into a mold cavity, typically under shear stress field conditions, and the liquid components are then removed to form a dried GO layer.
  • these highly reactive molecules react and chemically join with one another mostly in lateral directions (length or width directions) along graphene planes (in an edge-to-edge manner) and, in some cases, between graphene planes as well.
  • FIG. 3(D) Illustrated in FIG. 3(D) is a plausible chemical linking mechanism where only 2 aligned GO molecules are shown as an example, although a large number of GO molecules can be chemically linked together to form a unitary graphene layer. Further, chemical linking could also occur face-to-face, not just edge-to-edge. These linking and merging reactions proceed in such a manner that the molecules are chemically merged, linked, and integrated into one single entity or monolith. The molecules completely lose their own original identity and they no longer are discrete sheets/platelets/flakes. There is only one single layer-like structure (unitary graphene entity) that is one huge molecule or just a network of interconnected giant molecules with an essentially infinite molecular weight.
  • All the constituent graphene planes are very large in lateral dimensions (length and width) and, if produced under shear stress conditions (particularly into thin films, ⁇ 20 ⁇ in thickness) and heat-treated at a higher temperature (e.g. > 1,000°C or much higher), these graphene planes are essentially parallel to one another.
  • This integrated graphene entity is not made by gluing or bonding discrete flakes/platelets together with a resin binder or adhesive. Instead, GO molecules in the GO gel are merged through joining or forming of covalent bonds with one another, into an integrated graphene entity, without using any externally added binder molecules or polymers.
  • This monolithic graphene entity typically has the crystallographic c-axis in all grains being essentially parallel to each other.
  • This entity is derived from a GO gel, which is in turn obtained from natural graphite or artificial graphite particles originally having multiple graphite crystallites. Prior to being chemically oxidized, these starting graphite crystallites have an initial length (L a in the crystallographic a-axis direction), initial width (J b in the b- axis direction), and thickness (L c in the c-axis direction). Upon heavy oxidation, these initially discrete graphite particles are chemically transformed into highly aromatic graphene oxide molecules having a significant concentration of edge- or surface-borne functional groups (e.g.
  • the graphene oxide gel-derived monolithic film of graphene oxide has a unique combination of outstanding thermal conductivity, electrical conductivity, mechanical strength, and scratch resistance. This film is also well-bonded to the underlying metal foil without using a binder or adhesive.
  • a graphite particle (e.g. 100) is typically composed of multiple graphite crystallites or grains.
  • natural graphite can be treated so that the spacing between the graphene layers can be appreciably opened up so as to provide a marked expansion in the c-axis direction, and thus form an expanded graphite structure in which the laminar character of the carbon layers is substantially retained.
  • flakes of natural graphite e.g. 100 in FIG. 1(B)
  • the GICs are washed, dried, and then exfoliated by exposure to a high temperature for a short period of time.
  • the exfoliated graphite flakes are vermiform in appearance and, hence, are commonly referred to as worms 104.
  • These worms of graphite flakes which have been greatly expanded can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as "flexible graphite" 106) having a typical density of about 0.04-2.0 g/cm 3 for most applications.
  • FIG. 1(A) shows a flow chart that illustrates the prior art processes used to fabricate flexible graphite foils and the resin-impregnated flexible graphite composite.
  • the processes typically begin with intercalating graphite particles 20 (e.g., natural graphite or synthetic graphite) with an intercalant (typically a strong acid or acid mixture) to obtain a graphite intercalation compound 22 (GIC).
  • intercalant typically a strong acid or acid mixture
  • the GIC After rinsing in water to remove excess acid, the GIC becomes “expandable graphite.”
  • the GIC or expandable graphite is then exposed to a high temperature environment (e.g., in a tube furnace preset at a temperature in the range of 800- 1,050°C) for a short duration of time (typically from 15 seconds to 2 minutes).
  • This thermal treatment allows the graphite to expand in its c-axis direction by a factor of 30 to several hundreds to obtain a worm-like vermicular structure 24 (graphite worm), which contains exfoliated, but un-separated graphite flakes with large pores interposed between these interconnected flakes.
  • An example of graphite worms is presented in FIG. 2(A).
  • the exfoliated graphite (or mass of graphite worms) is re- compressed by using a calendering or roll-pressing technique to obtain flexible graphite foils (26 in FIG. 1(A) or 106 in FIG. 1(B)), which are typically much thicker than 100 ⁇ .
  • An SEM image of a cross-section of a flexible graphite foil is presented in FIG. 2(B), which shows many graphite flakes with orientations not parallel to the flexible graphite foil surface and there are many defects and imperfections.
  • the exfoliated graphite worm 24 may be impregnated with a resin and then compressed and cured to form a flexible graphite composite 28, which is normally of low strength as well.
  • the electrical and thermal conductivity of the graphite worms could be reduced by two orders of magnitude.
  • the exfoliated graphite may be subjected to high -intensity mechanical shearing/separation treatments using a high-intensity air jet mill, high-intensity ball mill, or ultrasonic device to produce separated nano graphene platelets 33 (NGPs) with all the graphene platelets thinner than 100 nm, mostly thinner than 10 nm, and, in many cases, being single-layer graphene (also illustrated as 112 in FIG. 1(B).
  • An NGP is composed of a graphene sheet or a plurality of graphene sheets with each sheet being a two-dimensional, hexagonal structure of carbon atoms.
  • graphite worms tend to be separated into the so-called expanded graphite flakes (108 in FIG. 1(B) having a thickness > 100 nm.
  • These flakes can be formed into graphite paper or mat 106 using a paper- or mat-making process.
  • This expanded graphite paper or mat 106 is just a simple aggregate or stack of discrete flakes having defects, interruptions, and mis-orientations between these discrete flakes.
  • a mass of multiple NGPs may be made into a graphene paper (34 in FIG. 1(A) or 114 in FIG. 1(B)) using a paper-making process.
  • FIG. 3(B) shows a SEM image of a cross-section of a graphene paper/film prepared from discrete graphene sheets using a paper-making process. The image shows the presence of many discrete graphene sheets being folded or interrupted (not integrated), most of platelet orientations being not parallel to the film/paper surface, the existence of many defects or imperfections. These NGP aggregates, even when being closely packed, exhibit a relatively low electrical conductivity.
  • the precursor to the thin film of graphene oxide-bonded on metal foil is graphene oxide gel 21 (FIG. 1(A)).
  • This GO gel is obtained by immersing a graphitic material 20 in a powder or fibrous form in a strong oxidizing liquid in a reaction vessel to form a suspension or slurry, which initially is optically opaque and dark.
  • This optical opacity reflects the fact that, at the outset of the oxidizing reaction, the discrete graphite flakes and, at a later stage, the discrete graphene oxide flakes scatter and/or absorb visible wavelengths, resulting in an opaque and generally dark fluid mass.
  • this 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.
  • the GO gel the GO molecules are uniformly dissolved in an acidic liquid medium.
  • conventional suspension of discrete graphene sheets, graphene oxide sheets, and expanded graphite flakes in a fluid look dark, black or heavy brown in color with individual graphene or graphene oxide sheets or expanded graphite flakes discernible or recognizable even with naked eyes or a low- magnification light microscope (100X-1,000X).
  • the graphene oxide molecules dissolved in the liquid medium of a graphene oxide gel are aromatic chains that have an average number of benzene rings in the chain typically less than 1,000, more typically less than 500, and many less than 100. Most of the molecules have more than 5 or 6 benzene rings (mostly > 10 benzene rings) from combined atomic force microscopy, high-resolution TEM, and molecular weight measurements. Based on our elemental analysis, these benzene-ring type of aromatic molecules are heavily oxidized, containing a high concentration of functional groups, such as -COOH and -OH and, therefore, are "soluble" (not just dispersible) in polar solvents, such as water.
  • the estimated molecular weight of these graphene oxide polymers in the gel state is 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 4,000 g/mole.
  • the typical viscosity values of GO gels are shown in FIG. 8(A) to FIG.8(C).
  • These soluble molecules behave like polymers and are surprisingly capable of reacting and getting chemically connected with one another (during the subsequent heat treatment or re- graphitization treatment) to form a graphene layer of good structural integrity and high thermal conductivity.
  • Conventional discrete graphene sheets, graphene oxide sheets, or graphite flakes do not have any self-reacting or cohesive bonding capability.
  • these soluble molecules in the GO gel are capable of chemically bonding metal foil surfaces.
  • these graphene oxide molecules present in a GO gel state are capable of chemically bonding, linking, or merging with one another and getting integrated into extremely long and wide graphene layers (e.g. FIG.
  • FIG. 3(A) depicts an example of such a huge unitary body.
  • the formation process for such a graphene entity is further illustrated in FIG. 3(C).
  • the starting graphitic material to be heavily oxidized for the purpose of forming graphene oxide gel may be selected from natural graphite, artificial graphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.
  • the graphitic material is preferably in a powder or short filament form having a dimension lower than 20 ⁇ , more preferably lower than 10 ⁇ , further preferably smaller than 5 ⁇ , and most preferably smaller than 1 ⁇ .
  • a typical procedure involves dispersing graphite particles in an oxidizer mixture of sulfuric acid, nitric acid, and potassium permanganate (at a weight ratio of 3 : 1 :0.05) at a temperature of typically 0- 60°C for typically at least 3 days, preferably 5 days, and more preferably 7 days or longer.
  • the average molecular weight of the resulting graphene oxide molecules in a gel is approximately 20,000-40,000 g/mole if the treatment time is 3 days, ⁇ 10,000 g/mole if 5 days, and ⁇ 4,000 g/mole if longer than 7 days.
  • the required gel formation time is dependent upon the particle size of the original graphitic material, a smaller size requiring a shorter time. It is of fundamental significance to note that if the critical gel formation time is not reached, the suspension of graphite powder and oxidizer (graphite particles dispersed in the oxidizer liquid) appears completely opaque and heterogeneous, meaning that discrete graphite particles or flakes remain suspended (but not dissolved) in the liquid medium.
  • the whole suspension becomes optically translucent or transparent (if sufficiently low GO contents) and brown colored, meaning that the heavily oxidized graphite completely loses its original graphite identity and the resulting graphene oxide molecules are completely dissolved in the oxidizer liquid, forming a homogeneous solution (no longer just a suspension or slurry).
  • the resulting thin film of graphene oxide is a graphene structure having a grain size significantly larger than this original grain size.
  • the film does not have any grain that can be identified to be associated with any particular particle of the starting graphitic material.
  • Original particles have already completely lost their identity when they are converted into graphite oxide molecules that are chemically linked up and merged or integrated into a network of graphene chains essentially infinite in molecular weight.
  • graphene oxide gel is obtained from a graphitic material having multiple graphite crystallites exhibiting no preferred crystalline orientation (e.g.
  • the resulting thin film bonded on a metal foil typically exhibits a very high degree of preferred crystalline orientation as determined by the same X-ray diffraction or electron diffraction method.
  • the present invention also provides a rechargeable battery that contains a presently invented graphene oxide thin film-bonded metal foil as an anode current collector and/or a cathode current collector.
  • This can be 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, just to name a few.
  • This invented battery can be a rechargeable lithium battery containing the unitary graphene layer as an anode current collector or a cathode current collector, which lithium battery can be a lithium-sulfur cell, a lithium-selenium cell, a lithium
  • Another embodiment of the invention is a capacitor containing the current collector of the present invention as an anode current collector or a cathode current collector, which capacitor is a symmetric ultracapacitor, an asymmetric ultracapacitor cell, a hybrid
  • the present invention provides a rechargeable lithium-metal cell composed of a current collector at the anode, a lithium film or foil as the anode, a porous separator/electrolyte layer, a cathode containing a cathode active material (e.g. lithium -free V 2 O 5 and Mn0 2 ), and a current collector.
  • a current collector at the anode, a lithium film or foil as the anode, a porous separator/electrolyte layer, a cathode containing a cathode active material (e.g. lithium -free V 2 O 5 and Mn0 2 ), and a current collector.
  • a cathode active material e.g. lithium -free V 2 O 5 and Mn0 2
  • Either or both the anode current collector and cathode current collector can be a graphene-based current collector of the present invention.
  • Another example of the present invention is a lithium-ion capacitor (or hybrid
  • supercapacitor composed of a current collector at the anode, a graphite or lithium titanate anode, a porous separator soaked with liquid or gel electrolyte, a cathode containing a cathode active material (e.g. activated carbon having a high specific surface area), and a current collector.
  • a cathode active material e.g. activated carbon having a high specific surface area
  • either or both the anode current collector and cathode current collector can be a graphene- based current collector of the present invention.
  • Yet another example of the present invention is another lithium-ion capacitor or hybrid supercapacitor, which is composed of a current collector at the anode, a graphite anode (and a sheet of lithium foil as part of the anode), a porous separator soaked with liquid electrolyte, a cathode containing a cathode active material (e.g. activated carbon having a high specific surface area), and a current collector.
  • a cathode active material e.g. activated carbon having a high specific surface area
  • the anode current collector and cathode current collector can be a graphene-based current collector of the present invention.
  • Still another example of the present invention is a lithium-graphene cell composed of a current collector at the anode, a porous, nano- structured anode (e.g.
  • Example 1 Preparation of discrete graphene sheets (NGPs) and expanded graphite flakes
  • Chopped graphite fibers with an average diameter of 12 ⁇ and natural graphite particles were separately used as a starting material, which was immersed in a mixture of concentrated sulfuric acid, nitric acid, and potassium permanganate (as the chemical intercalate and oxidizer) to prepare graphite intercalation compounds (GICs).
  • the starting material was first dried in a vacuum oven for 24 h at 80°C. Then, a mixture of concentrated sulfuric acid, fuming nitric acid, and potassium permanganate (at a weight ratio of 4: 1 :0.05) was slowly added, under appropriate cooling and stirring, to a three-neck flask containing fiber segments.
  • 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 being dried at 100°C overnight, the resulting graphite intercalation compound (GIC) was subjected to a thermal shock at 1050°C for 45 seconds in a tube furnace to form exfoliated graphite (or graphite worms).
  • GIC graphite intercalation compound
  • MCMBs Meso-carbon microbeads
  • This material has a density of about 2.24 g/cm 3 with a median particle size of about 16 ⁇ .
  • MCMB (10 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4: 1 :0.05) for 72 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HC1 to remove most of the sulphate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral.
  • Graphene oxide gel was prepared by oxidation of graphite flakes with an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4: 1 :0.05 at 30°C.
  • an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4: 1 :0.05 at 30°C.
  • this gel by casting this gel on a metal foil surface (Cu, Al, Ni, Ti, or stainless steel) and removing the liquid medium from the cast film we obtain a thin film of graphene oxide that is optically transparent.
  • This thin film looks like, feels like, and behaves like a regular polymer film.
  • this GO film upon heat treatments at a temperature (from 80°C to 1,500°C) for typically 1-3 hours, this GO film is transformed into a monolithic thin film entity comprising large-size graphene domains. This GO film is well bonded to the underlying metal foil.
  • the GO film exhibits the formation of a hump centered at 22° (FIG.
  • the doo 2 spacing is decreased to approximately to 0.3354 nm, identical to that of a graphite single crystal.
  • the (004) peak intensity relative to the (002) intensity on the same diffraction curve, or the 7(004)//(002) ratio, is a good indication of the degree of crystal perfection and preferred orientation of graphene planes.
  • the (004) peak is either non-existing or relatively weak, with the 7(004)//(002) ratio ⁇ 0.1, for all graphitic materials heat treated at a temperature lower than 2,800°C.
  • the 7(004)//(002) ratio for the graphitic materials heat treated at 3,000-3,250°C is in the range of 0.2-0.5.
  • FIG. 5(E) One example is presented in FIG. 5(E) for a polyimide-derived PG with a HTT of 3,000°C for two hours, which exhibits a 7(004)//(002) ratio of about 0.41.
  • a thin film of GO bonded on a metal foil prepared with a HTT of 1,500°C for 4 hours exhibits a 7(004)//(002) ratio of 0.78 and a Mosaic spread value of 0.21, indicating a practically perfect graphene single crystal with an exceptional degree of preferred orientation.
  • the "mosaic spread" value obtained from the full width at half maximum of the (002) reflection in an X-ray diffraction intensity curve.
  • This index for the degree of ordering characterizes the graphite or graphene crystal size (or grain size), amounts of grain boundaries and other defects, and the degree of preferred grain orientation.
  • a nearly perfect single crystal of graphite is characterized by having a mosaic spread value of 0.2-0.4. Most of our unitary graphene materials have a mosaic spread value in this range of 0.2-0.4 (with a heat treatment temperature no less than 1,500°C).
  • the 7(004)//(002) ratio for all tens of flexible graphite samples investigated are all « 0.05, practically non-existing in most cases.
  • the 7(004)//(002) ratio for all graphene paper/membrane samples is ⁇ 0.1 even after a heat treatment at 3,000°C for 2 hours. Attempts to graphitize the ultra-thin films ( ⁇ 2 nm in thickness) prepared by Cu-catalyzed CVD led to the breaking up of the film and the formation of graphite particles instead.
  • GO film-bonded metal foil is a new and distinct class of material that is fundamental different from any pyrolytic graphite (PG), flexible graphite (FG), and paper/film/membrane of conventional graphene/GO/RGO sheets/platelets (NGPs) that are free-standing or coated on a metal foil.
  • PG pyrolytic graphite
  • FG flexible graphite
  • NGPs paper/film/membrane of conventional graphene/GO/RGO sheets/platelets
  • FIG. 6(B) Corresponding oxygen content values in the GO gel-derived GO films are shown in FIG. 6(B).
  • FIG. 6(C) Corresponding oxygen content values in the GO gel-derived GO films are shown in FIG. 6(B).
  • FIG. 6(C) A close scrutiny of FIG. 6(A) to FIG.6(C) indicate that there are 4 HTT ranges (80-200°C; 200-500°C; 500-l,250°C; and 1,250- 1,500°C) that lead to 4 respective oxygen content ranges and inter-graphene spacing range.
  • the thermal conductivity of GO gel-derived GO film and corresponding flexible graphite (FG) foil, also plotted as a function of the same final heat treatment temperature range is summarized in FIG. 6(D).
  • planar GO molecules are derived from the graphene planes that constitute the original structure of starting natural graphite particles (used in the procedure of graphite oxidation to form the GO gel). The original natural graphite particles, when randomly packed into an aggregate or
  • graphite compact would have their constituent graphene planes randomly oriented, exhibiting relatively low thermal conductivity and having essentially zero strength (no structural integrity). In contrast, the strength of the unitary graphene layer (even without an added reinforcement) is typically already in the range of 40-140 MPa.
  • the resulting unitary graphene layer exhibits a thermal conductivity of 1,148 W/mK, in contrast to the observed 244 W/mK of the flexible graphite foil with an identical heat treatment temperature.
  • the flexible graphite foil only shows a thermal conductivity lower than 600 W/mK.
  • the presently invented unitary graphene layer delivers a thermal conductivity of 1,807 W/mK (FIG. 4(A) and FIG. 6(D)) even though the metal foil has been melted at such a high temperature.
  • SEM scanning electron microscopy
  • TEM transmission electron microscopy
  • SAD selected-area electron diffraction
  • BF bright field
  • DF dark-field
  • FIG. 2(A), FIG. 3(A), and FIG. 3(B) A close scrutiny and comparison of FIG. 2(A), FIG. 3(A), and FIG. 3(B) indicates that the graphene layers in a monolithic GO thin film are substantially oriented parallel to one another; but this is not the case for flexible graphite foils and graphene oxide paper.
  • the inclination angles between two identifiable layers in the GO thin film entity are mostly less than 5 degrees.
  • there are so many folded graphite flakes, kinks, and mis-orientations in flexible graphite that many of the angles between two graphite flakes are greater than 10 degrees, some as high as 45 degrees (FIG. 2(B)).
  • the mis-orientations between graphene platelets in NGP paper (FIG. 3(B)) are also high and there are many gaps between platelets.
  • the unitary graphene entity is essentially gap-free.
  • FIG.4 (A) shows the thermal conductivity values of the GO gel-derived GO film, GO platelet paper prepared by vacuum-assisted filtration of RGO, and FG foil, respectively, all plotted as a function of the final HTT.
  • FIG. 4(B) shows the thermal conductivity values of the GO-derived thin film and the CVD carbon film-derived PG heat-treated for 3 hours, all plotted as a function of the final graphitization or re-graphitization temperature.
  • the thermal conductivity is always lower than that of a GO gel-derived GO film.
  • the thin film of GO- bonded on metal foil is fundamentally different and patently distinct from the flexible graphite (FG) foil, graphene/GO/RGO paper/membrane, and pyrolytic graphite (PG) in terms of chemical composition, crystal and defect structure, crystal orientation, morphology, process of production, and properties.
  • Example 5 Electrical and thermal conductivity measurements of various graphene oxide gel- derived thin films
  • the thermal conductivity of the flexible graphite foil alone is less than 237 W/mK if the FG foil is not heat-treated at or above 700°C.
  • the thermal conductivity of the FG foil increases from 237 to 582 W/mK, indicating some but limited re-organization of the graphitic structure induced by the heat treatment.
  • the thermal conductivity of the GO gel- derived thin graphene layer alone increases from 983 to 1,807 W/mK.
  • This thin film is obtained 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. This indicates a significant or dramatic re-organization of the graphitic structure induced by the heat treatment, with all GO molecules linked or merged edge-to-edge and face-to-face into an integrated graphene body of fully and orderly bonded graphene planes.
  • FIG. 7(A) A series of GO gel-derived thin-film graphene layers, GO platelet paper, and FG foil were prepared. A universal testing machine was used to determine the tensile strength of these materials. The tensile strength values of the thin film of graphene oxide, GO platelet paper, and FG paper are plotted as a function of the re-graphitization temperature, FIG. 7(A). These data have demonstrated that the tensile strength of the flexible graphite foil remains relatively constant (all ⁇ 20 MPa) and that of the GO paper increases slightly (from 22 to 43 MPa) when the heat treatment temperature increases from 700 to 2,800°C.
  • the tensile strength of the GO-derived unitary graphene layer increases dramatically from 32 to > 100 MPa over the same range of heat treatment temperatures.
  • This result is quite striking and further reflects the notion that the GO gel-derived GO layer contains highly live and active molecules during the heat treatment, while the graphene platelets in the conventional GO paper and the graphite flakes in the FG foil are essentially dead molecules.
  • the GO-derived unitary graphene entity is a class of material by itself.
  • the scratch test was conducted using the so-called Ford Lab Test Method (FLTM) BN108-13. This apparatus consists of a movable platform connected to five beams with 250 mm in length. 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 a weight that exerts a force of 7N, 6N, 3N, 2N, and 0.6N, respectively.
  • the beams draw the pins across the specimen surface and generate scratches.
  • the scratch is made at a sliding velocity of approximately 100 mm/s. All tests were performed at room temperature. Although the test method requires that grained surfaces be evaluated, only the smooth surfaces of the specimens were tested in this study.
  • the specimen plaques were scratched, they were evaluated with a reflected light polarizing microscope incorporating a Xenon light source.
  • An image analyzer with Image Analysis Software was used to measure the "gray scale mass," which is the total gray scale value of the object.
  • the camera objective lens is positioned at an angle of 90° from the scratch.
  • the objective lens registers a portion of the scratch about 1 mm long.
  • the electron signal for each scratch line is then integrated and recorded.
  • the optical mass of an object is the sum of the gray level values, GL, of all pixels in the object.
  • the depth of the scratch was measured using an interferometer. The magnification was set at 5X. Depth measurements were made from the depth histogram of the scanned area. The scratch test results are shown in FIG. 7(B). The scratches were also examined using a scanning electron microscope (SEM).
  • Example 7 Li-S Cell Containing a Graphene Oxide-Bonded Metal Foil Current Collector at the Anode and at the Cathode
  • Li-S cells Three (3) Li-S cells were prepared and tested, each one having a lithium foil as the anode active material, a sulfur/expanded graphite composite (75/25 wt. ratio) as the cathode active material, 1M of LiN(CF 3 S0 2 )2 in DOL as the electrolyte, and a Celgard 2400 as the separator.
  • the first cell (a baseline cell for comparison) contains a 10- ⁇ thick Cu foil as the anode current collector and a 20- ⁇ thick Al foil as the cathode current collector.
  • the second cell (another baseline cell for comparison) has a 10- ⁇ thick GO-resin layer as the anode current collector and a sheet of 14- ⁇ RGO-coated Al foil as the cathode current collector.
  • the third cell has a GO-bonded Cu foil (totally 12- ⁇ thick) of the present invention as the anode current collector and a sheet of a 20- ⁇ thick GO-coated Al foil as the cath
  • the specific discharge capacity herein referred to is the total charge inserted into the cathode during the discharge, per unit mass of the composite cathode (counting the weights of cathode active material, conductive additive or support, and the binder, but excluding the current collectors).
  • the specific energy and specific power values presented in this section are based on the total cell weight (including anode and cathode, separator and electrolyte, current collectors, and packaging materials).
  • the morphological or micro-structural changes of selected samples after a desired number of repeated charging and recharging cycles were observed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
  • FIG. 9(A) shows the discharge capacity values of the three cells each as a function of the charge/discharge cycle number.
  • Each cell was designed to have an initial cell capacity of 100 mAh to facilitate comparison. It is clear that the Li-S cell featuring the presently invented GO- bonded current collector at both the anode and the cathode exhibits the most stable cycling behavior, experiencing a capacity loss of 6% after 50 cycles.
  • the cell containing GO/resin- coated Cu and GO-coated Al current collector suffers from a 23% capacity decay after 50 cycles.
  • the cell containing a Cu foil anode current collector and an Al foil cathode current collector suffers from a 26% capacity decay after 50 cycles. Post-cycling inspection of the cells indicate that Al foil in all prior art electrodes suffered a severe corrosion problem. In contrast, the presently invented graphene oxide-bonded Al current collectors remain intact.
  • FIG. 9(B) shows the Ragone plots (gravimetric power density vs. gravimetric energy density) of the three cells. It is of interest to note that our GO-bonded metal foil current collectors surprisingly impart both higher energy density and higher power power density to the Li-S cell compared to prior art graphene/resin-coated current collector at the anode (with GO- coated Al foil at the cathode), and Cu/Al current collectors. This is quite unexpected considering that Cu foil has an electrical conductivity that is more than one order of magnitude higher than that of the graphene film. The difference in the energy density and power density values are more than what can be accounted for by the physical density difference between Cu foil and graphene film at the anode.
  • Example 8 Magnesium-Ion Cell Containing a Graphene-enabled Current Collector at the Anode and at the Cathode
  • a cathode active material Magnetic Manganese Silicate, Mgi.03Mn 0. 7SiO 4
  • the starting materials were magnesium oxide (MgO), manganese (II) carbonate (MnC0 3 ) and silicon dioxide (Si0 2 , 15-20 nm) powder.
  • the stoichiometric amounts for the precursor compounds were controlled with the molar ratio of 1.03 :0.97: 1 for Mg:Mn:Si.
  • the electrodes were typically prepared by mixing 85 wt% of an electrode active material (e.g. Mg 1.03 Mn 0 .9 7 SiO 4 particles, 7 wt% acetylene black (Super- P), and 8 wt% polyvinylidene fluoride binder (PVDF, 5 wt % solid content dissolved in N- methyl-2-pyrrolidinoe ( MP)) to form a slurry-like mixture. After coating the slurry on an intended current collector, the resulting electrode was dried at 120°C in vacuum for 2 h to remove the solvent before pressing.
  • an electrode active material e.g. Mg 1.03 Mn 0 .9 7 SiO 4 particles, 7 wt% acetylene black (Super- P), and 8 wt% polyvinylidene fluoride binder (PVDF, 5 wt % solid content dissolved in N- methyl-2-pyrrolidinoe ( MP)
  • PVDF polyvinylidene flu
  • first cell having GO-bonded Cu foil and GO-bonded Al foil as the anode and cathode current collectors, respectively; second cell having GO/resin-coated Cu foil and GO- coated Al foil (no pre-etching) as the anode and cathode current collector, respectively (a prior art cell); third cell having a Cu foil anode current collector and Al foil cathode current collector (a prior art cell).
  • a thin sheet of magnesium foil was attached to the anode current collector surface, and a piece of porous separator (e.g., Celgard 2400 membrane) was, in turn, stacked on top of the magnesium foil.
  • a piece of cathode disc coated on a cathode current collector was used as a cathode and stacked over the separator layer to form a CR2032 coin-type cell.
  • the electrolyte used was 1 M of Mg(AlCl 2 EtBu) 2 in THF.
  • the cell assembly was performed in an argon-filled glove-box.
  • the CV measurements were carried out using a CHI-6 electrochemical workstation at a scanning rate of 1 mV/s.
  • the electrochemical performance of the cells was also evaluated by galvanostatic charge/discharge cycling at a current density of from 50 mA/g to 10 A/g (up to 100 A/g for some cells), using an Arbin and/or a LAND electrochemical workstation.
  • FIG. 10 shows the cell discharge specific capacity values of the three cells each as a function of the charge/discharge cycle number. It is clear that the Mg-ion cell featuring the presently invented current collectors at both the anode and the cathode exhibits the most stable cycling behavior, experiencing a capacity loss of 2.5% after 25 cycles.
  • GO/resin-coated Cu foil and GO-coated Al foil current collectors suffers from a 17% capacity decay after 25 cycles.
  • the cell containing a Cu foil anode current collector and an Al foil cathode current collector suffers from a 30% capacity decay after 25 cycles.
  • Post-cycling inspection of the cells indicate that GO/resin-coated Cu foil and GO-coated Al foil current collectors got swollen and showed some delamination from the cathode layer and that Al foil suffered a severe corrosion problem.
  • inventive GO-bonded metal foil current collectors remain intact.
  • Example 9 Chemical and Mechanical Compatibility Testing of Various Current Collectors for Various Intended Batteries or Supercapacitors
  • each current collector must be connected to a tab that is, in turn, connected to an external circuit wire.
  • the current collector must be mechanically compatible with the tab, being readily or easily fastened or bonded thereto.
  • CVD graphene films just cannot be mechanically fastened to the tab without being easily broken or fractured. Even with the assistance of adhesive, the CVD film is easily fractured during the procedures of connecting to a tab or battery cell packaging.
  • thermal conductivity, electrical conductivity, scratch resistance, surface hardness, and tensile strength exhibited by the presently invented materials are much higher than what prior art flexible graphite sheets, paper of discrete graphene/GO/RGO platelets, or other graphitic films could 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.

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  • Electric Double-Layer Capacitors Or The Like (AREA)

Abstract

L'invention concerne également un procédé de production d'un collecteur de courant à feuille métallique lié à l'oxyde de graphène à film mince pour une batterie ou un super-condensateur, comprenant : (a) préparer un gel d'oxyde de graphène ayant des molécules d'oxyde de graphène (GO) dissoutes dans un milieu fluide; (b) déposer une couche de gel de GO sur au moins une des deux surfaces primaires d'une feuille métallique pour former une couche de gel d'oxyde de graphène humide, la procédure de dépôt comprenant l'amincissement induit par cisaillement du gel de GO; (c) l'élimination partielle ou complète dudit milieu fluide de la couche humide déposée pour former un film sec de GO ayant un espacement inter-plan d? 002 # 191 comprise entre 0,4 nm et 1,2 nm, telle que déterminée par diffraction des rayons X; et (d) traitement thermique du le film sec d'oxyde de graphène pour former le collecteur de courant à feuille métallique lié à l'oxyde de graphène à film mince à une température de traitement thermique comprise entre 80 °C et 2500°C.
PCT/US2017/018707 2016-08-08 2017-02-21 Collecteur de courant à film mince en feuille métallique lié à l'oxyde de graphène WO2018031064A1 (fr)

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KR1020197005445A KR20190039536A (ko) 2016-08-08 2017-02-21 그래핀 산화물 결합 금속 포일 박막 집전체
JP2019506509A JP7126492B2 (ja) 2016-08-08 2017-02-21 酸化グラフェン接合金属箔薄フィルム集電体
CN201780047369.XA CN109565053A (zh) 2016-08-08 2017-02-21 结合氧化石墨烯的金属箔薄膜集流体

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US15/231,486 US10586661B2 (en) 2016-08-08 2016-08-08 Process for producing graphene oxide-bonded metal foil thin film current collector for a battery or supercapacitor
US15/231,486 2016-08-08
US15/231,498 2016-08-08
US15/231,498 US10158122B2 (en) 2016-08-08 2016-08-08 Graphene oxide-bonded metal foil thin film current collector and battery and supercapacitor containing same

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WO2018031064A1 true WO2018031064A1 (fr) 2018-02-15

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CN111009663A (zh) * 2018-10-08 2020-04-14 南京鼎腾石墨烯研究院有限公司 电池电极的制备方法及包含电池电极的充电电池
CN111661838A (zh) * 2020-04-03 2020-09-15 武汉汉烯科技有限公司 轻质高导电柔性锂电池集流体材料及其制备方法和应用
CN114730888A (zh) * 2020-05-08 2022-07-08 株式会社Lg新能源 无锂电池用负极集电器、包含该无锂电池用负极集电器的电极组件、以及无锂电池
CN114957897A (zh) * 2022-06-27 2022-08-30 苏福（深圳）科技有限公司 一种高性能石墨烯膜及其制备方法
CN117023569A (zh) * 2023-08-17 2023-11-10 小米汽车科技有限公司 石墨烯电热膜及其制备方法、负极、锂金属电池及其去除枝晶的方法
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CN110265666A (zh) * 2019-06-11 2019-09-20 东旭光电科技股份有限公司 石墨烯复合微孔铜箔及其制备方法、锂离子电池
CN110212153A (zh) * 2019-06-24 2019-09-06 珠海格力电器股份有限公司 一种集流体及其制备方法
WO2023056591A1 (fr) * 2021-10-08 2023-04-13 宁德时代新能源科技股份有限公司 Collecteur de courant d'électrode positive composite, pièce polaire et batterie secondaire
CN118425556B (zh) * 2024-07-04 2024-08-30 深圳市晶扬电子有限公司 一种具有滞回特征的气流传感器

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CN111009663A (zh) * 2018-10-08 2020-04-14 南京鼎腾石墨烯研究院有限公司 电池电极的制备方法及包含电池电极的充电电池
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CN110350201A (zh) * 2019-06-29 2019-10-18 华南理工大学 一种水系电池用轻质高导电石墨烯集流体及其制备方法
CN111661838A (zh) * 2020-04-03 2020-09-15 武汉汉烯科技有限公司 轻质高导电柔性锂电池集流体材料及其制备方法和应用
CN114730888A (zh) * 2020-05-08 2022-07-08 株式会社Lg新能源 无锂电池用负极集电器、包含该无锂电池用负极集电器的电极组件、以及无锂电池
CN114957897A (zh) * 2022-06-27 2022-08-30 苏福（深圳）科技有限公司 一种高性能石墨烯膜及其制备方法
CN117023569A (zh) * 2023-08-17 2023-11-10 小米汽车科技有限公司 石墨烯电热膜及其制备方法、负极、锂金属电池及其去除枝晶的方法
CN117023569B (zh) * 2023-08-17 2024-05-07 小米汽车科技有限公司 石墨烯电热膜及其制备方法、负极、锂金属电池及其去除枝晶的方法

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