Supercapacitor and electrode having cellulose nanofiber-spaced graphene sheets and production process

1. A supercapacitor comprising an anode, a cathode, an ion-permeable separator disposed between said anode and said cathode, and an electrolyte in ionic contact with said anode and said cathode, wherein at least one of the anode and the cathode contains multiple graphene sheets spaced by cellulosic nanofibers and has a specific surface area from 50 to 3,300 m 2 /g and wherein said at least one of the anode and the cathode has a physical density from 0.5 to 1.7 g/cm 3 , and wherein said at least one of the anode and the cathode is capable of delivering a gravimetric specific capacitance greater than 210 F/g.

2. The supercapacitor of claim 1 , wherein said graphene sheets are selected from a pristine graphene or a non-pristine graphene material, having a content of non-carbon elements from 2% to 50% by weight, selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, doped graphene, or a combination thereof.

3. The supercapacitor of claim 1 , wherein said cellulosic nanofibers have a diameter from 1 nm to 100 nm.

4. The supercapacitor of claim 1 , wherein said cellulosic nanofibers have a diameter from 2 nm to 10 nm.

5. The supercapacitor of claim 1 , wherein said multiple graphene sheets are substantially aligned along a desired direction.

6. The supercapacitor of claim 1 , wherein said at least one of the anode and the cathode has a physical density from 0.7 to 1.3 g/cm 3 .

7. The supercapacitor of claim 1 , wherein said graphene sheets are deposited with a nanoscaled coating or particles of a redox pair partner selected from an intrinsically conductive polymer, a transition metal oxide, and/or an organic molecule, wherein said redox pair partner and said graphene sheets form a redox pair for pseudo-capacitance.

8. The supercapacitor of claim 7 , wherein said intrinsically conducting polymer is selected from polyaniline, polypyrrole, polythiophene, polyfuran, sulfonated polyaniline, sulfonated polypyrrole, sulfonated polythiophene, sulfonated polyfuran, sulfonated polyacetylene, or a combination thereof.

9. The supercapacitor of claim 1 , wherein said electrolyte contains an aqueous electrolyte, an organic electrolyte, an inorganic electrolyte, an ionic liquid electrolyte, or a mixture of an organic and an ionic electrolyte.

10. The supercapacitor of claim 1 , further comprising an anode current collector in electronic contact with said anode or a cathode current collector in electronic contact with said cathode.

11. The supercapacitor of claim 1 , wherein both the anode and the cathode contain graphene sheets spaced by cellulosic nanofibers and have a specific surface area from 50 to 3,300 m 2 /g.

12. The supercapacitor of claim 1 , which is a lithium-ion capacitor or sodium-ion capacitor, wherein said cathode contains said cellulosic nanofiber-spaced graphene sheets and said anode contains a pre-lithiated anode active material or a pre-sodiated anode active material.

13. A supercapacitor electrode containing an electrolyte-impregnated laminar structure containing multiple graphene sheets that are spaced by cellulosic nanofibers and having a specific surface area from 50 to 3,300 m 2 /g, wherein said supercapacitor electrode contains a liquid or gel electrolyte residing in a space between graphene sheets, wherein said electrode provides a specific capacitance greater than 210 F/g.

14. The supercapacitor electrode of claim 13 , wherein said graphene sheets are selected from a pristine graphene or a non-pristine graphene material, having a content of non-carbon elements from 2% to 50% by weight, selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, doped graphene, or a combination thereof.

15. The supercapacitor electrode of claim 13 , wherein said cellulosic nanofibers have a diameter from 1 nm to 100 nm.

16. The supercapacitor electrode of claim 13 , wherein said cellulosic nanofibers have a diameter from 2 nm to 10 nm.

17. The supercapacitor electrode of claim 13 , wherein said graphene sheets are deposited with a nanoscaled coating or particles of a redox pair partner selected from an intrinsically conductive polymer, a transition metal oxide, and/or an organic molecule, wherein said redox pair partner and said graphene sheets form a redox pair for pseudo-capacitance.

18. The supercapacitor electrode of claim 17 , wherein said intrinsically conducting polymer is selected from polyaniline, polypyrrole, polythiophene, polyfuran, sulfonated polyaniline, sulfonated polypyrrole, sulfonated polythiophene, sulfonated polyfuran, sulfonated polyacetylene, or a combination thereof.

19. The supercapacitor electrode of claim 13 , wherein said multiple graphene sheets are substantially aligned along a desired direction, and wherein said electrode has a physical density from 0.5 to 1.7 g/cm 3 .

20. The supercapacitor of claim 13 , wherein said electrode has a physical density from 0.7 to 1.3 g/cm 3 .

21. A process of producing the supercapacitor electrode of claim 13 , said process comprising a) dispersing said multiple graphene sheets, said cellulosic nanofibers, an optional conductive additive, and an optional resin binder in a liquid medium to form a graphene slurry; b) dispensing and depositing said graphene slurry onto a surface of a solid substrate or a current collector and forming a wet graphene layer thereon which is optionally subjected to a compression treatment to align graphene sheets along a desired direction; c) at least partially removing said liquid medium from said wet graphene layer to form a dry graphene layer wherein multiple graphene sheets are spaced by said cellulosic nanofibers to form said supercapacitor electrode, and d) an optional compression treatment to increase a density of said supercapacitor electrode.

22. The process of claim 21 , further comprising combining said supercapacitor electrode and a second electrode to form a supercapacitor cell.

23. A process of producing the supercapacitor electrode of claim 13 , said process comprising (a) preparing a graphene dispersion having multiple isolated graphene sheets and cellulosic nanofibers dispersed in a liquid or gel electrolyte; and (b) subjecting said graphene dispersion to a forced assembly procedure, forcing said multiple graphene sheets and cellulosic nanofibers to assemble into a electrolyte-impregnated laminar structure, wherein said multiple graphene sheets are alternately spaced by thin electrolyte layers having a thickness from 0.4 nm to 10 nm and having cellulosic nanofibers dispersed in said thin electrolyte layers and said multiple graphene sheets are substantially aligned along a desired direction, and wherein said laminar graphene structure has a physical density from 0.5 to 1.7 g/cm 3 and a specific surface area from 50 to 3,300 m 2 /g, when measured in a dried state of said laminar structure with said electrolyte removed.

24. The process of claim 23 , wherein said forced assembly procedure is conducted in the presence of a current collector, which current collector is embedded in said electrolyte-impregnated laminar graphene structure or bonded to said electrolyte-impregnated laminar graphene structure to form said supercapacitor electrode.

25. The process of claim 23 , wherein said forced assembly procedure includes introducing said graphene dispersion, having a initial volume V 1 , in a mold cavity cell and driving a piston into said mold cavity cell to reduce the graphene dispersion volume to a smaller value V 2 , allowing excess electrolyte to flow out of said cavity cell and aligning said multiple graphene sheets along a desired direction.

26. The process of claim 23 , wherein said forced assembly procedure includes introducing said graphene dispersion in a mold cavity cell having an initial volume V 1 , and applying a suction pressure through a porous wall of said mold cavity to reduce the graphene dispersion volume to a smaller value V 2 , allowing excess electrolyte to flow out of said cavity cell through said porous wall and aligning said multiple graphene sheets along a desired direction.

27. The process of claim 23 , wherein said forced assembly procedure includes introducing a first layer of said graphene dispersion onto a surface of a supporting conveyor and driving said layer of graphene suspension supported on said conveyor through at least a pair of pressing rollers to reduce a thickness of said graphene dispersion layer and align said multiple graphene sheets along a direction parallel to said conveyor surface for forming a layer of electrolyte-impregnated laminar graphene structure.

28. The process of claim 27 , further including a step of introducing a second layer of said graphene dispersion onto a surface of said layer of electrolyte-impregnated laminar graphene structure to form a two layer laminar structure, and driving said two-laver laminar structure through at least a pair of pressing rollers to reduce a thickness of said second layer of graphene dispersion and align said multiple graphene sheets along a direction parallel to said conveyor surface for forming a layer of electrolyte-impregnated laminar graphene structure.

29. The process of claim 23 , further including a step of compressing or roll-pressing said electrolyte-impregnated laminar structure to reduce a thin electrolyte layer thickness in said impregnated laminar structure, improving orientation of graphene sheets, and squeezing excess electrolyte out of said impregnated laminar graphene structure for forming said supercapacitor electrode.

30. The process of claim 27 , which is a roll-to-roll process wherein said forced assembly procedure includes feeding said supporting conveyor, in a continuous film form, from a feeder roller to a deposition zone, continuously or intermittently depositing said graphene dispersion onto a surface of said supporting conveyor film to form said layer of graphene dispersion thereon, and collecting said layer of electrolyte-impregnated laminar graphene structure supported on conveyor film on a collector roller.

31. The process of claim 23 , further comprising a step of cutting said electrolyte-impregnated laminar graphene structure into multiple sheets and stacking said multiple sheets to form a supercapacitor electrode.

32. The process of claim 23 , further comprising a stern of attaching said electrolyte-impregnated laminar graphene structure to a current collector, wherein said graphene sheets are aligned parallel to a primary surface of said current collector.

33. The process of claim 23 , further comprising a step of attaching said electrolyte-impregnated laminar graphene structure to a current collector, wherein said graphene sheets are aligned perpendicular to a primary surface of said current collector.

34. A process of producing a supercapacitor electrode, comprising stacking a current collector with at least a layer of said electrolyte-impregnated laminar graphene structure of claim 13 to form a multiple-layer structure and further comprising a step of compressing and consolidating said multi-layer structure to increase a physical density and decrease a thickness of said multi-layer structure to form said supercapacitor electrode.

35. The process of claim 34 , wherein at least one layer of said electrolyte-impregnated laminar graphene structure is attached to one surface of said current collector and at least one layer of said electrolyte-impregnated laminar graphene structure is attached to the opposing surface of said current collector prior to said step of compressing and consolidating.

36. The process of claim 23 , wherein said graphene dispersion contains a graphene oxide dispersion prepared by immersing a graphitic material in a powder or fibrous form in an oxidizing fluid in a reaction vessel at a reaction temperature for a length of time sufficient to obtain said graphene dispersion wherein said graphitic material is selected from natural graphite, article graphite, mesophase carbon, mesophase pitch, mesocarbon micro-bead, soft carbon, hard carbon, coke, carbon fiber, carbon nanofiber, carbon nanotube, or a combination thereof and wherein said graphene oxide has an oxygen content no less than 5% by weight.