Chemical-Free Production of Graphene-Wrapped Electrode Active Material Particles for Battery Applications

Abstract

Provided is a simple, fast, scalable, and environmentally benign method of producing graphene-embraced or encapsulated particles of a battery electrode active material directly from a graphitic material, the method comprising: a) mixing graphitic material particles, multiple particles of an electrode active material, and non-polymeric particles of milling media to form a mixture in an impacting chamber, wherein the graphitic material has never been intercalated, oxidized, or exfoliated and the chamber contains therein no previously produced graphene sheets; b) operating the energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from the graphitic material and transferring graphene sheets to surfaces of electrode active material particles to produce graphene-embraced active material particles; and c) recovering the graphene-embraced particles from the impacting chamber. Also provided is a mass of the graphene-embraced particles, electrode containing such particles, and battery containing this electrode.

Claims

1. An impact-transfer method of producing a graphene-embraced or graphene-encapsulated electrode active material directly from a graphitic material, said method comprising: a) mixing multiple particles of a graphitic material, multiple particles of a solid electrode active material, and non-polymeric particles of ball-milling media to form a mixture in an impacting chamber of an energy impacting apparatus, wherein said graphitic material has never been previously intercalated, oxidized, or exfoliated and said impacting chamber contains therein no previously produced isolated graphene sheets; b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said particles of graphitic material and transferring said peeled graphene sheets to surfaces of said solid electrode active material particles and fully embrace or encapsulate said particles to produce particles of graphene-embraced or graphene-encapsulated electrode active material inside said impacting chamber; and c) recovering said particles of graphene-embraced or graphene-encapsulated electrode active material from said impacting chamber and separating said non-polymeric particles of ball-milling media from said particles of graphene-embraced or graphene-encapsulated electrode active material.

2. The method of claim 1, wherein said non-polymeric particles of ball-milling media contain milling balls selected from ceramic particles, including ZrO.sub.2 and non-ZrO.sub.2 metal oxide particles, metal particles, glass particles, or a combination thereof.

3. The method of claim 1, further comprising a step of incorporating said graphene-embraced electrode active material into a battery electrode.

4. The method of claim 1, wherein an amount of residual graphitic material remains after said step b) and said method further comprises a step of incorporating said graphene-embraced electrode active material and said residual graphitic material into a battery electrode wherein said residual graphitic material is used as a conductive additive in said battery electrode.

5. The method of claim 1, wherein an amount of residual graphitic material remains after said step b), and said step c) includes a step of partially or completely separating said residual amount of said graphitic material from said graphene-embraced electrode active material.

6. The method of claim 1, wherein said particles of solid electrode active material contain prelithiated or pre-sodiated particles having 0.1% to 54.7% by weight of lithium or sodium ions preloaded into said particles prior to step (a) of mixing.

7. The method of claim 1, wherein said particles of solid electrode active material contain particles pre-coated with a layer of conductive material selected from a carbon, pitch, carbonized resin, conductive polymer, conductive organic material, metal coating, metal oxide shell, or a combination thereof

8. The method of claim 1, wherein said particles of solid electrode active material contain particles pre-coated with a carbon precursor material prior to step (a), wherein said carbon precursor material is selected from a coal tar pitch, petroleum pitch, mesophase pitch, polymer, organic material, or a combination thereof so that said carbon precursor material resides between surfaces of said particles of solid electrode active material and said graphene sheets, and said method further contains a step of heat-treating said graphene-embraced electrode active material to convert said carbon precursor material to a carbon material and pores, wherein said pores form empty spaces between surfaces of said particles of solid electrode active material and said graphene sheets and said carbon material is coated on said surfaces of solid electrode active material particles and/or chemically bonds said graphene sheets together.

9. The method of claim 1, wherein said particles of solid electrode active material contain particles pre-coated with a sacrificial material selected from a metal, pitch, polymer, organic material, or a combination thereof so that said sacrificial material resides between surfaces of said particles of solid electrode active material and said graphene sheets, and said method further contains a step of partially or completely removing said sacrificial material to form empty spaces between surfaces of said solid electrode active material particles and said graphene sheets.

10. The method of claim 1, further comprising a step of exposing said graphene-embraced electrode active material to a liquid or vapor of a conductive material that is conductive to electrons and/or ions of lithium, sodium, magnesium, aluminum, or zinc.

11. The method of claim 1, wherein said particles of electrode active material are an anode active material selected from the group consisting of: (A) lithiated and un-lithiated silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (B) lithiated and un-lithiated alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (C) lithiated and un-lithiated oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, or Cd, and their mixtures, composites, or lithium-containing composites; (D) lithiated and un-lithiated salts and hydroxides of Sn; (E) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; and combinations thereof.

12. The method of claim 1, wherein said electrode active material is a cathode active material selected from an inorganic material, an organic or polymeric material, a metal oxide/phosphate/sulfide, or a combination thereof.

13. The method of claim 12, wherein said metal oxide/phosphate/sulfide is selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, sodium cobalt oxide sodium nickel oxide, sodium manganese oxide, sodium vanadium oxide, sodium-mixed metal oxide, sodium iron phosphate, sodium manganese phosphate, sodium vanadium phosphate, sodium mixed metal phosphate, transition metal sulfide, lithium polysulfide, sodium polysulfide, magnesium polysulfide, or a combination thereof.

14. The method of claim 1, wherein said electrode active material is a cathode active material selected from sulfur, sulfur compound, sulfur-carbon composite, sulfur-polymer composite, lithium polysulfide, transition metal dichalcogenide, a transition metal trichalcogenide, or a combination thereof.

15. The method of claim 12, wherein said inorganic material is selected from TiS.sub.2, TaS.sub.2, MoS.sub.2, NbSe.sub.3, MnO.sub.2, CoO.sub.2, an iron oxide, a vanadium oxide , or a combination thereof.

16. The method of claim 12, wherein said metal oxide/phosphate/sulfide contains a vanadium oxide selected from the group consisting of VO.sub.2, Li.sub.xVO.sub.2, V.sub.2O.sub.5, Li.sub.xV.sub.2O.sub.5, V.sub.3O.sub.8, Li.sub.xV.sub.3O.sub.8, Li.sub.xV.sub.3O.sub.7, V.sub.4O.sub.9, Li.sub.xV.sub.4O.sub.9, V.sub.6O.sub.13, Li.sub.xV.sub.6O.sub.13, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.

17. The method of claim 12, wherein said metal oxide/phosphate/sulfide is selected from a layered compound LiMO.sub.2, spinel compound LiM.sub.2O.sub.4, olivine compound LiMPO.sub.4, silicate compound Li.sub.2MSiO.sub.4, Tavorite compound LiMPO.sub.4F, borate compound LiMBO.sub.3, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

18. The method of claim 12, wherein said inorganic material is selected from: (a) bismuth selenide or bismuth telluride, (b) transition metal dichalcogenide or trichalcogenide, (c) sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt, manganese, iron, nickel, or a transition metal; (d) boron nitride, or (e) a combination thereof

19. The method of claim 12, wherein said organic material or polymeric material is selected from poly(anthraquinonyl sulfide) (PAQS), a lithium oxocarbon, 3,4,9,10-peryl enetetracarboxylic dianhydride (PTCDA), poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT), polymer-bound PYT, quino(triazene), redox-active organic material, tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxy anthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS.sub.2).sub.3]n), lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer, hexaazatrinaphtylene (HATN), hexaazatriphenylene hexacarbonitrile (HAT(CN).sub.6), 5-benzylidene hydantoin, isatine lithium salt, pyromellitic diimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi.sub.4), N,N-diphenyl-2,3,5,6-tetraketopiperazine (PHP), N,N-diallyl-2,3,5,6-tetraketopiperazine (AP), N,N-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, a quinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT), 5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ), 5-amino-1,4-dyhydroxy anthraquinone (ADAQ), calixquinone, Li.sub.4C.sub.6O.sub.6, Li.sub.2C.sub.6O.sub.6, Li.sub.6C.sub.6O.sub.6, or a combination thereof.

20. The method of claim 19, wherein said thioether polymer is selected from poly[methanetetryl-tetra(thiomethylene)] (PMTTM), poly(2,4-dithiopentanylene) (PDTP), a polymer containing poly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioether polymers, a side-chain thioether polymer having a main-chain consisting of conjugating aromatic moieties, and having a thioether side chain as a pendant, poly(2-phenyl-1,3-dithiolane) (PPDT), poly(l,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB), poly(tetrahydrobenzodithiophene) (PTHBDT), poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, or poly[3,4(ethylenedithio)thiophene] (PEDTT).

21. The method of claim 12, wherein said organic material contains a phthalocyanine compound selected from copper phthalocyanine, zinc phthalocyanine, tin phthalocyanine, iron phthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadyl phthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine, manganous phthalocyanine, dilithium phthalocyanine, aluminum phthalocyanine chloride, cadmium phthalocyanine, chlorogallium phthalocyanine, cobalt phthalocyanine, silver phthalocyanine, a metal-free phthalocyanine, a chemical derivative thereof, or a combination thereof

22. The method of claim 1, wherein said electrode active material is a cathode active material containing a mixture of an organic material and an inorganic material or a metal oxide/phosphate/sulfide.

23. The method of claim 1, wherein said electrode active material particles include powder, flakes, beads, pellets, spheres, wires, fibers, filaments, discs, ribbons, or rods, having a diameter or thickness from 10 nm to 20 m.

24. The method of claim 23, wherein said diameter or thickness is from 1 m to 10 m.

25. The method of claim 1, wherein said graphitic material is selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nanofiber, graphite fluoride, chemically modified graphite, mesocarbon micro-bead, partially crystalline graphite, or a combination thereof.

26. The method of claim 1, wherein the energy impacting apparatus is a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, cryogenic ball mill, micro ball mill, tumbler ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, plasma-assisted ball mill, freezer mill, vibratory sieve, bead mill, nano bead mill, ultrasonic homogenizer mill, centrifugal planetary mixer, vacuum ball mill, or resonant acoustic mixer.

27. The method of claim 1 wherein said graphene sheets contain single-layer graphene sheets.

28. The method of claim 1 wherein said graphene sheets contain at least 80% single-layer graphene or at least 80% few-layer graphene having no greater than 10 graphene planes.

29. The method of claim 1, wherein said procedure of operating said energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device.

30. A mass of graphene-embraced particles of solid active material produced by the method of claim 1, wherein a graphene proportion is from 0.01% to 20% by weight based on the total weight of graphene and solid active material particles combined.

31. A battery electrode containing said graphene-embraced or graphene-encapsulated electrode active material produced in claim 1.

32. A battery containing the battery electrode of claim 31.

33. A battery electrode containing said graphene-embraced or graphene-encapsulated electrode active material produced in claim 1 as an electrode active material, wherein said battery is a lithium-ion battery, lithium metal secondary battery, lithium-sulfur battery, lithium-air battery, lithium-selenium battery, sodium-ion battery, sodium metal secondary battery, sodium-sulfur battery, sodium-air battery, magnesium-ion battery, magnesium metal battery, aluminum-ion battery, aluminum metal secondary battery, zinc-ion battery, zinc metal battery, or zinc-air battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0091] FIG. 1 A flow chart showing the most commonly used prior art process of producing highly oxidized graphene sheets (or nano graphene platelets, NGPs) that entails tedious chemical oxidation/intercalation, rinsing, and high-temperature exfoliation procedures.

[0092] FIG. 2 A diagram showing the presently invented process for producing graphene-embraced or encapsulated electrode active material particles via an energy impacting apparatus.

[0093] FIG. 3 A diagram showing the presently invented process for producing graphene-embraced electrode active material particles via a continuous ball mill.

[0094] FIG. 4 Charge-discharge cycling behaviors of 3 lithium cells featuring Co.sub.3O.sub.4 particle-based anodes: a) containing un-protected Co.sub.3O.sub.4 particles, b) reduced graphene oxide (RGO)-embraced Co.sub.3O.sub.4 particles (a prior art approach), and c) graphene-encapsulated Co.sub.3O.sub.4 particles produced by the instant impact transfer method.

[0095] FIG. 5 Charge-discharge cycling behaviors of 2 lithium cells featuring SnO.sub.2 particle-based anodes: one containing un-protected SnO.sub.2 particles and the other containing graphene-encapsulated SnO.sub.2 particles produced by the instant impact transfer method (stainless steel balls).

[0096] FIG. 6 Charge-discharge cycling behaviors of 3 lithium cells featuring micron-scaled (3 m) Si particle-based anodes: a) one cell containing un-protected Si particles, b) a second cell containing graphene-embraced Si particles produced by the indirect transfer method (with externally added milling media, ZrO.sub.2 balls), and c) a third cell containing graphene-encapsulated Si particles produced by the direct transfer method (Si particles themselves being the graphene-peeling agent).

[0097] FIG. 7 Charge-discharge cycling behaviors of 2 lithium cells featuring lithium iron phosphate (LFP) particle-based cathodes: one containing un-protected LFO particles (mixed with 12% by weight carbon) and the other containing graphene-encapsulated carbon-added LFP particles produced by the instant impact transfer method (4% graphene+8% C).

[0098] FIG. 8 Charge-discharge cycling behaviors of 3 lithium cells featuring LiV.sub.2O.sub.5 nanorod-based cathodes: a) containing protected LiV.sub.2O.sub.5 nanorods (mixed with 10% by weight carbon black particles, b) graphene-embraced LiV.sub.2O.sub.5 nanorods produced by the impact transfer method using MoO.sub.2 balls (approximately 5% graphene +7% C), and c) LiV.sub.2O.sub.5 nanorods protected by a carbon matrix.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0099] Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite), or a whole range of intermediate structures that are characterized in that various proportions and sizes of graphite crystallites and defects are dispersed in an amorphous matrix. Typically, a graphite crystallite is composed of a number of graphene sheets or basal planes that are bonded together through van der Waals forces in the c-axis direction, the direction perpendicular to the basal plane. These graphite crystallites are typically micron- or nanometer-sized. The graphite crystallites are dispersed in or connected by crystal defects or an amorphous phase in a graphite particle, which can be a graphite flake, carbon/graphite fiber segment, carbon/graphite whisker, or carbon/graphite nanofiber. In other words, graphene planes (hexagonal lattice structure of carbon atoms) constitute a significant portion of a graphite particle.

[0100] One preferred specific embodiment of the present invention is a method of peeling off graphene planes of carbon atoms (1-10 planes of atoms that constitute single-layer or few-layer graphene sheets) that are directly transferred to surfaces of electrode active material particles. A graphene sheet or nano graphene platelet (NGP) is essentially composed of a sheet of graphene plane or multiple sheets of graphene plane stacked and bonded together (typically, on an average, less than 10 sheets per multi-layer platelet). Each graphene plane, also referred to as a graphene sheet or a hexagonal basal plane, comprises a two-dimensional hexagonal structure of carbon atoms. Each platelet has a length and a width parallel to the graphite plane and a thickness orthogonal to the graphite plane. By definition, the thickness of an NGP is 100 nanometers (nm) or smaller, with a single-sheet NGP being as thin as 0.34 nm. However, the NGPs produced with the instant methods are mostly single-layer graphene and some few-layer graphene sheets (<10 layers). The length and width of a NGP are typically between 200 nm and 20 um, but could be longer or shorter, depending upon the sizes of source graphite material particles.

[0101] The present invention provides a strikingly simple, fast, scalable, environmentally benign, and cost-effective process that avoids essentially all of the drawbacks associated with prior art processes of producing graphene sheets and obviates the need to execute a separate (additional) process to combine the produced graphene sheets and particles of an electrode active material together to form a composite or hybrid electrode active material.

[0102] As schematically illustrated in FIG. 2, one preferred embodiment of this method entails placing particles of a source graphitic material, particles of a solid electrode active material, and impacting balls (particles of ball-milling media) in an impacting chamber. After loading, the resulting mixture is exposed to impacting energy, which is accomplished, for instance, by rotating the chamber to enable the impacting of the milling balls against graphite particles. These repeated impacting events (occurring in high frequencies and high intensity) act to peel off graphene sheets from the surfaces of graphitic material particles and tentatively transferred to the surfaces of these impacting balls first. When the graphene-coated impacting balls subsequently impinge upon the solid electrode active material particles, the graphene sheets are transferred to surfaces of the electrode active material particles to form graphene-coated active material particles. Typically, the entire particle is covered by graphene sheets (fully wrapped around, embraced or encapsulated). Subsequently, the externally added impacting balls (e.g. ball-milling media) are separated from the graphene-embraced particles.

[0103] The non-polymeric particles of ball-milling media may contain milling balls selected from ceramic particles (e.g. ZrO.sub.2 or non-ZrO.sub.2-based metal oxide particles), metal particles, glass particles, or a combination thereof.

[0104] In less than two hours (often less than 1 hour) of operating the direct transfer process, most of the constituent graphene sheets of source graphite particles are peeled off, forming mostly single-layer graphene and few-layer graphene (mostly less than 5 layers or 5 graphene planes). Following the transfer process (graphene sheets wrapped around active material particles), the residual graphite particles (if present) are separated from the graphene-embraced (graphene-encapsulated) particles using a broad array of methods. Separation or classification of graphene-embraced (graphene-encapsulated) particles from residual graphite particles (if any) can be readily accomplished based on their differences in weight or density, particle sizes, magnetic properties, etc. The resulting graphene-embraced particles are already a two-component material; i.e. they are already mixed and there is no need to have a separate process of mixing isolated graphene sheets with electrode active material particles.

[0105] In other words, production of graphene sheets and mixing of graphene sheets with an electrode active material are essentially accomplished concurrently in one operation. This is in stark contrast to the traditional processes of producing graphene sheets first and then subsequently mixing the graphene sheets with an active material. Traditional dry mixing typically does not result in homogeneous mixing or dispersion of two or multiple components. It is also challenging to properly disperse nanomaterials in a solvent to form a battery slurry mass for coating on a current collector.

[0106] As shown in FIG. 1, the prior art chemical processes for producing graphene sheets or platelets alone typically involve immersing graphite powder in a mixture of concentrated sulfuric acid, nitric acid, and an oxidizer, such as potassium permanganate or sodium perchlorate, forming a reacting mass that requires typically 5-120 hours to complete the chemical intercalation/oxidation reaction. Once the reaction is completed, the slurry is subjected to repeated steps of rinsing and washing with water and then subjected to drying treatments to remove water. The dried powder, referred to as graphite intercalation compound (GIC) or graphite oxide (GO), is then subjected to a thermal shock treatment. This can be accomplished by placing GIC in a furnace pre-set at a temperature of typically 800-1100 C. (more typically 950-1050 C.). The resulting products are typically highly oxidized graphene (i.e. graphene oxide with a high oxygen content), which must be chemically or thermal reduced to obtain reduced graphene oxide (RGO). RGO is found to contain a high defect population and, hence, is not as conducting as pristine graphene. We have observed that that the pristine graphene paper (prepared by vacuum-assisted filtration of pristine graphene sheets, as herein prepared) exhibit electrical conductivity values in the range of 1,500-4,500 S/cm. In contrast, the RGO paper prepared by the same paper-making procedure typically exhibits electrical conductivity values in the range of 100-1,000 S/cm.

[0107] In the most common implementation of prior art ball mill mixing, previously produced graphene sheets or platelets are added to electrode active material powders. Impact energy is applied via ball mill for a period of time to disperse graphene platelets or sheets in the powder. This is often carried out in a liquid (solvent) solution. The disadvantages of this graphene/active material mixing process are obviouspreviously made graphene is a costly input material, solvent recovery is required, and most significantly, the graphene input into the process has been damaged by oxidation during prior processing. This reduces desirable end properties, such as thermal conductivity and electrical conductivity.

[0108] Another prior art process is coating of CVD onto metal nanoparticles. This is the most limited of all prior art methods, being possible only on certain metals that are suitable catalysts for facilitating decomposition of hydrocarbon gas to form carbon atoms and as templates for graphene to grow on. As a bottom up graphene production method, it requires costly capital equipment and costly input materials.

[0109] In all these prior art processes for producing graphene-coated electrode active material particles, isolated graphene sheets and particles of the active material are dispersed in a solvent (e.g. NMP) to form a slurry. The slurry is then dried (e.g. using spray drying) to form graphene-active material composite particles. These composites do not necessarily have the morphology or structure of active material particles being fully wrapped around or embraced.

[0110] In contrast, the presently invented impacting process entails combining graphene production, functionalization (if desired), and mixing of graphene with electrode active material particles in a single operation. This fast and environmentally benign process not only avoids significant chemical usage, but also produces embracing graphene sheets of higher qualitypristine graphene as opposed to thermally reduced graphene oxide produced by the prior art process. Pristine graphene enables the creation of embraced particles with higher electrical and thermal conductivity.

[0111] Although the mechanisms remain incompletely understood, this revolutionary process of the present invention has essentially eliminated the conventionally required functions of graphene plane expansion, intercalant penetration, exfoliation, and separation of graphene sheets and replace it with a single, entirely mechanical peeling process. The whole process can take less than 2 hours (typically 10 minutes to 1 hour), and can be done with no added chemicals. This is absolutely stunning, a shocking surprise to even those top scientists and engineers or those of extraordinary ability in the art.

[0112] Another surprising result of the present study is the observation that a wide variety of carbonaceous and graphitic materials can be directly processed without any particle size reduction or pre-treatment. The particle size of graphite can be smaller than, comparable to, or larger than the particle size of the electrode active material. The graphitic material may be selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, mesocarbon micro-bead, graphite fiber, graphitic nanofiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, or a combination thereof. It may be noted that the graphitic material used for the prior art chemical production and reduction of graphene oxide requires size reduction to 75 um or less in average particle size. This process requires size reduction equipment (for example hammer mills or screening mills), energy input, and dust mitigation. By contrast, the energy impacting device method can accept almost any size of graphitic material. A starting graphitic material of several mm or cm in size or larger or a starting material as small as nanoscaled has been successfully processed to create graphene-coated or graphene-embedded particles of cathode or anode active materials. The only size limitation is the chamber capacity of the energy impacting device; but this chamber can be very large (industry-scaled).

[0113] The presently invented process is capable of producing single-layer graphene sheets that completely wrap around the particles of an electrode active material. In many examples, the graphene sheets produced contain at least 80% single-layer graphene sheets. The graphene produced can contain pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride, graphene oxide with less than 5% fluorine by weight, graphene with a carbon content of no less than 95% by weight, or functionalized graphene.

[0114] The presently invented process does not involve the production of GIC and, hence, does not require the exfoliation of GIC at a high exfoliation temperature (e.g. 800-1,100 C.). This is another major advantage from environmental protection perspective. The prior art processes require the preparation of dried GICs containing sulfuric acid and nitric acid intentionally implemented in the inter-graphene spaces and, hence, necessarily involve the decomposition of H.sub.2SO.sub.4 and HNO.sub.3 to produce volatile gases (e.g. NO.sub.x and SO.sub.X) that are highly regulated environmental hazards. The presently invented process completely obviates the need to decompose H.sub.2SO.sub.4 and HNO.sub.3 and, hence, is environmentally benign. No undesirable gases are released into the atmosphere during the combined graphite expansion/exfoliation/separation process of the present invention.

[0115] In a desired embodiment, the presently invented method is carried out in an automated and/or continuous manner. For instance, as illustrated in FIG. 3, the mixture of graphite particles 1 and electrode active material particles 2 (along with non-polymeric milling balls) is delivered by a conveyer belt 3 and fed into a continuous ball mill 4. After ball milling to form graphene-embraced particles, the product mixture (possibly also containing some residual graphite particles) is discharged from the ball mill apparatus 4 into a screening device (e.g. a rotary drum 5) to separate graphene-embraced particles from residual graphite particles (if any). The graphene-embraced particles may be delivered into a powder classifier, a cyclone, and or an electrostatic separator. The particles may be further processed, if so desired, by melting 7, pressing 8, or grinding/pelletizing apparatus 9. These procedures can be fully automated. The process may include characterization or classification of the output material and recycling of insufficiently processed material into the continuous energy impacting device. The process may include weight monitoring of the load in the continuous energy impacting device to optimize material properties and throughput.

[0116] The separation of the milling balls from the final products may be assisted by a magnetic separator 6 if the milling balls are ferromagnetic (e.g. containing Fe, Co, Ni, or Mn-based metal in some desired electronic configuration).

[0117] The electrode active materials that are placed into the impacting chamber can be an anode active material or a cathode active material. For the anode active materials, those materials capable of storing lithium ions greater than 372 mAh/g (theoretical capacity of natural graphite) are particularly desirable. Examples of these high-capacity anode active materials are Si, Ge, Sn, SnO.sub.2, Co.sub.3O.sub.4, etc. As discussed earlier, these materials, if implemented in the anode, have the tendency to expand and contract when the battery is charged and discharged. At the electrode level, the expansion and contraction of the anode active material can lead to expansion and contraction of the anode, causing mechanical instability of the battery cell. At the anode active material level, repeated expansion/contraction of particles of Si, Ge, Sn, SnO.sub.2, Co.sub.3O.sub.4, etc. quickly leads to pulverization of these particles and rapid capacity decay of the electrode.

[0118] Thus, for the purpose of addressing these problems, the particles of solid electrode active material may contain prelithiated or pre-sodiated particles. In other words, before the electrode active material particles (such as Si, Ge, Sn, SnO.sub.2, Co.sub.3O.sub.4, etc.) are embraced by graphene sheets, these particles have already been previously intercalated with Li or Na ions (e.g. via electrochemical charging). This is a highly innovative and unique approach based on the following considerations. The intercalation of these particles with Li or Na would allow these particles to expand to a large volume or to its full capacity (potentially up to 380% of its original volume). If these prelithiated or pre-sodiated particles are then wrapped around or fully embraced by graphene sheets and incorporated into an electrode (e.g. anode containing graphene-embraced Si or SnO.sub.2 particles), the electrode would no longer have any issues of electrode expansion and expansion-induced failure during subsequent charge-discharge cycles of the lithium- or sodium-ion battery. In other words, the Si or SnO.sub.2 particles have been expanded to their maximum volume (during battery charging) and they can only shrink (during subsequent battery discharge). These contracted particles have been previously provided with expansion space between these particles and the embracing graphene sheets. Our experimental data have shown that this strategy surprisingly leads to significantly longer battery cycle life and better utilization of the electrode active material capacity.

[0119] In some embodiments, prior to the instant graphene production, impact transfer and embracing process, the particles of solid electrode active material contain particles that are pre-coated with a coating of a conductive material selected from carbon, pitch, carbonized resin, a conductive polymer, a conductive organic material, a metal coating, a metal oxide shell, or a combination thereof. The coating layer thickness is preferably in the range from 1 nm to 10 m, preferably from 10 nm to 1 m, and further preferably from 20 nm to 200 nm. This coating is implemented for the purpose of establishing a solid-electrolyte interface (SEI) to increase the useful cycle life of a lithium-ion or sodium-ion battery.

[0120] In some embodiments, the particles of solid electrode active material contain particles that are pre-coated with a carbon precursor material selected from a coal tar pitch, petroleum pitch, mesophase pitch, polymer, organic material, or a combination thereof so that the carbon precursor material resides between surfaces of the solid electrode active material particles and the graphene sheets, and the method further contains a step of heat-treating the graphene-embraced electrode active material to convert the carbon precursor material to a carbon material and pores, wherein the pores form empty spaces between surfaces of the solid electrode active material particles and the graphene sheets and the carbon material is coated on the surfaces of solid electrode active material particles and/or chemically bonds the graphene sheets together. The carbon material helps to completely seal off the embracing graphene sheets to prevent direct contact of the embraced anode active material with liquid electrolyte, which otherwise continues to form additional SEI via continuously consuming the lithium ions or solvent in the electrolyte, leading to rapid capacity decay.

[0121] In some embodiments, the particles of solid electrode active material contain particles pre-coated with a sacrificial material selected from a metal, pitch, polymer, organic material, or a combination thereof in such a manner that the sacrificial material resides between surfaces of solid electrode active material particles and the graphene sheets, and the method further contains a step of partially or completely removing the sacrificial material to form empty spaces between surfaces of the solid electrode active material particles and the graphene sheets. The empty spaces can accommodate the expansion of embraced active material particles without breaking the embraced particles.

[0122] In some embodiments, the method further comprises a step of exposing the graphene-embraced electrode active material to a liquid or vapor of a conductive material that is conductive to electrons and/or ions of lithium, sodium, magnesium, aluminum, or zinc. This procedure serves to provide a stable SEI or to make the SEI more stable.

[0123] The particles of electrode active material may be an anode active material selected from the group consisting of: (A) lithiated and un-lithiated silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (B) lithiated and un-lithiated alloys or intermetallic compounds of Si,

[0124] Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (C) lithiated and un-lithiated oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, or Cd, and their mixtures, composites, or lithium-containing composites; (D) lithiated and un-lithiated salts and hydroxides of Sn; (E) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; and combinations thereof. Both sodiated and un-sodiated versions of the materials in the above list are also anode active materials for sodium-ion batteries.

[0125] The electrode active material may be a cathode active material selected from an inorganic material, an organic material, an intrinsically conducting polymer (known to be capable of string lithium ions), a metal oxide/phosphate/sulfide, or a combination thereof. The metal oxide/phosphate/sulfide may be selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, sodium cobalt oxide sodium nickel oxide, sodium manganese oxide, sodium vanadium oxide, sodium-mixed metal oxide, sodium iron phosphate, sodium manganese phosphate, sodium vanadium phosphate, sodium mixed metal phosphate, transition metal sulfide, lithium polysulfide, sodium polysulfide, magnesium polysulfide, or a combination thereof.

[0126] In some embodiments, the electrode active material may be a cathode active material selected from sulfur, sulfur compound, sulfur-carbon composite, sulfur-polymer composite, lithium polysulfide, transition metal dichalcogenide, a transition metal trichalcogenide, or a combination thereof. The inorganic material may be selected from TiS.sub.2, TaS.sub.2, MoS.sub.2, NbSe.sub.3, MnO.sub.2, CoO.sub.2, an iron oxide, a vanadium oxide, or a combination thereof. This group of materials is particularly suitable for use as a cathode active material of a lithium metal battery.

[0127] The metal oxide/phosphate/sulfide contains a vanadium oxide selected from the group consisting of VO.sub.2, Li.sub.xVO.sub.2, V.sub.2O.sub.5, Li.sub.xV.sub.2O.sub.5, V.sub.3O.sub.8, Li.sub.xV.sub.3O.sub.8, Li.sub.xV.sub.3O.sub.7, V.sub.4O.sub.9, Li.sub.xV.sub.4O.sub.9, V.sub.6O.sub.13, Li.sub.xV.sub.6O.sub.13, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5. In some embodiments, the metal oxide/phosphate/sulfide is selected from a layered compound LiMO.sub.2, spinel compound LiM.sub.2O.sub.4, olivine compound LiMPO.sub.4, silicate compound Li.sub.2MSiO.sub.4, Tavorite compound LiMPO.sub.4F, borate compound LiMBO.sub.3, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

[0128] The inorganic material may be selected from: (a) bismuth selenide or bismuth telluride, (b) transition metal dichalcogenide or trichalcogenide, (c) sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt, manganese, iron, nickel, or a transition metal; (d) boron nitride, or (e) a combination thereof.

[0129] The organic material or polymeric material may be selected from Poly(anthraquinonyl sulfide) (PAQS), a lithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT), polymer-bound PYT, Quino(triazene), redox-active organic material, Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxy anthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS.sub.2).sub.3]n), lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer, Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile (HAT(CN).sub.6), 5-Benzylidene hydantoin, Isatine lithium salt, Pyromellitic diimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi.sub.4), N,N-diphenyl-2,3,5,6-tetraketopiperazine (PHP), N,N-diallyl-2,3,5,6-tetraketopiperazine (AP), N,N-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, a quinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT), 5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ), 5-amino-1,4-dyhydroxy anthraquinone (ADAM), calixquinone, Li.sub.4C.sub.6O.sub.6, Li.sub.2C.sub.6O.sub.6, Li.sub.6C.sub.6O.sub.6, or a combination thereof. These compounds are preferably mixed with a conducting material to improve their electrical conductivity, rigidity and strength so as to enable the peeling-off of graphene sheets from the graphitic material particles.

[0130] The thioether polymer in the above list may be selected from Poly[methanetetryl-tetra(thiomethylene)] (PMTTM), Poly(2,4-dithiopentanylene) (PDTP), a polymer containing Poly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioether polymers, a side-chain thioether polymer having a main-chain consisting of conjugating aromatic moieties, and having a thioether side chain as a pendant, Poly(2-phenyl-1,3-dithiolane) (PPDT), Poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB), poly(tetrahydrobenzodithiophene) (PTHBDT), poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, or poly[3,4(ethylenedithio)thiophene] (PEDTT).

[0131] In some embodiments, the organic material contains a phthalocyanine compound selected from copper phthalocyanine, zinc phthalocyanine, tin phthalocyanine, iron phthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadyl phthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine, manganous phthalocyanine, dilithium phthalocyanine, aluminum phthalocyanine chloride, cadmium phthalocyanine, chlorogallium phthalocyanine, cobalt phthalocyanine, silver phthalocyanine, a metal-free phthalocyanine, a chemical derivative thereof, or a combination thereof. These compounds are preferably mixed with a conducting material to improve their electrical conductivity and rigidity so as to enable the peeling-off of graphene sheets from the graphitic material particles.

[0132] In some embodiments, the electrode active material particles include powder, flakes, beads, pellets, spheres, wires, fibers, filaments, discs, ribbons, or rods, having a diameter or thickness from 10 nm to 20 m. Preferably, the diameter or thickness is from 1 m to 100 m.

[0133] In the invented method, the graphitic material may be selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nanofiber, graphite fluoride, chemically modified graphite, mesocarbon micro-bead, partially crystalline graphite, or a combination thereof.

[0134] The energy impacting apparatus may be a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, cryogenic ball mill, micro ball mill, tumbler ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, plasma-assisted ball mill, freezer mill, vibratory sieve, bead mill, nano bead mill, ultrasonic homogenizer mill, centrifugal planetary mixer, vacuum ball mill, or resonant acoustic mixer. The procedure of operating the energy impacting apparatus may be conducted in a continuous manner using a continuous energy impacting device

[0135] Graphene sheets transferred to electrode active material surfaces have a significant proportion of surfaces that correspond to the edge planes of graphite crystals. The carbon atoms at the edge planes are reactive and must contain some heteroatom or group to satisfy carbon valency. There are many types of functional groups (e.g. hydroxyl and carboxylic) that are naturally present at the edge or surface of graphene nanoplatelets produced through transfer to a solid carrier particle. The impact-induced kinetic energy is of sufficient energy and intensity to chemically activate the edges and even surfaces of graphene sheets embraced around active material particles (e.g. creating highly active sites or free radicals). Provided that certain chemical species containing desired chemical function groups (e.g. OH, COOH, NH.sub.2, Br, etc.) are included in the impacting chamber, these functional groups can be imparted to graphene edges and/or surfaces. In other words, production and chemical functionalization of graphene sheets can be accomplished concurrently by including appropriate chemical compounds in the impacting chamber. In summary, a major advantage of the present invention over other processes is the simplicity of simultaneous production and modification of graphene surface chemistry for improved battery performance.

[0136] Graphene sheets derived by this process may be functionalized through the inclusion of various chemical species in the impacting chamber. In each group of chemical species discussed below, we selected 2 or 3 chemical species for functionalization studies.

[0137] In one preferred group of chemical agents, the resulting functionalized NGP may broadly have the following formula(e): [NGP]R.sub.m, wherein m is the number of different functional group types (typically between 1 and 5), R is selected from SO.sub.3H, COOH, NH.sub.2, OH, RCHOH, CHO, CN, COCl, halide, COSH, SH, COOR', SR', SiR'.sub.3, Si(OR).sub.yR.sub.3-y, Si(OSiR.sub.2)OR, R, Li, AlR.sub.2, HgX, TlZ.sub.2 and MgX; wherein y is an integer equal to or less than 3, R is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate.

[0138] Graphene-embraced electrode active material particles may be used to improve the mechanical properties, electrical conductivity and thermal conductivity of an electrode. For enhanced lithium-capturing and storing capability, the functional group NH.sub.2 and OH are of particular interest. For example, diethylenetriamine (DETA) has three NH.sub.2 groups. If DETA is included in the impacting chamber, one of the three NH.sub.2 groups may be bonded to the edge or surface of a graphene sheet and the remaining two un-reacted NH.sub.2 groups will be available for reversibly capturing a lithium or sodium atom and forming a redox pair therewith. Such an arrangement provides an additional mechanism for storing lithium or sodium ions in a battery electrode.

[0139] Other useful chemical functional groups or reactive molecules may be selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), hexamethylenetetramine, polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof. These functional groups are multi-functional, with the capability of reacting with at least two chemical species from at least two ends. Most importantly, they are capable of bonding to the edge or surface of graphene using one of their ends and, during subsequent epoxy curing stage, are able to react with epoxide or epoxy resin material at one or two other ends.

[0140] The above-described [NGP]-R.sub.m may be further functionalized. This can be conducted by opening up the lid of an impacting chamber after the R.sub.m groups have been attached to graphene sheets and then adding the new functionalizing agents to the impacting chamber and resuming the impacting operation. The resulting graphene sheets or platelets include compositions of the formula: [NGP]-A., where A is selected from OY, NHY, OCOY, PCNRY, OCSY, OCY, CR1OY, NY or CY, and Y is an appropriate functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from ROH, RNR.sub.2, RSH, RCHO, RCN, RX, RN.sup.+(R).sub.3X.sup., RSiR.sub.3, RSi(OR).sub.yR.sub.3-y, RSi(OSiR.sub.2)OR, RR, RNCO, (C.sub.2H.sub.4O).sub.wH, (C.sub.2H.sub.4O).sub.wR, (C.sub.3H.sub.6O).sub.wR, R, and w is an integer greater than one and less than 200.

[0141] The NGPs may also be functionalized to produce compositions having the formula: [NGP]-[RA].sub.m, where m, R and A are as defined above. The compositions of the invention also include NGPs upon which certain cyclic compounds are adsorbed. These include compositions of matter of the formula: [NGP]-[XR.sub.a].sub.m, where a is zero or a number less than 10, X is a polynuclear aromatic, polyheteronuclear aromatic or metallopolyheteronuclear aromatic moiety and R is as defined above. Preferred cyclic compounds are planar. More preferred cyclic compounds for adsorption are porphyrins and phthalocyanines. The adsorbed cyclic compounds may be functionalized. Such compositions include compounds of the formula, [NGP]-[XA.sub.a].sub.m, where m, a, X and A are as defined above.

[0142] The functionalized NGPs of the instant invention can be prepared by sulfonation, electrophilic addition to deoxygenated platelet surfaces, or metallation. The graphitic platelets can be processed prior to being contacted with a functionalizing agent. Such processing may include dispersing the platelets in a solvent. In some instances the platelets may then be filtered and dried prior to contact. One particularly useful type of functional group is the carboxylic acid moieties, which naturally exist on the surfaces of NGPs if they are prepared from the acid intercalation route discussed earlier. If carboxylic acid functionalization is needed, the NGPs may be subjected to chlorate, nitric acid, or ammonium persulfate oxidation.

[0143] Carboxylic acid functionalized graphitic platelets are particularly useful because they can serve as the starting point for preparing other types of functionalized NGPs. For example, alcohols or amides can be easily linked to the acid to give stable esters or amides. If the alcohol or amine is part of a di- or poly-functional molecule, then linkage through the O- or NH-leaves the other functionalities as pendant groups. These reactions can be carried out using any of the methods developed for esterifying or aminating carboxylic acids with alcohols or amines as known in the art. Examples of these methods can be found in G. W. Anderson, et al., J. Amer. Chem. Soc. 96, 1839 (1965), which is hereby incorporated by reference in its entirety. Amino groups can be introduced directly onto graphitic platelets by treating the platelets with nitric acid and sulfuric acid to obtain nitrated platelets, then chemically reducing the nitrated form with a reducing agent, such as sodium dithionite, to obtain amino-functionalized platelets. Functionalization of the graphene-coated inorganic particles may be used as a method to introduce dopants into the electrode active material.

[0144] The following examples serve to provide the best modes of practice for the present invention and should not be construed as limiting the scope of the invention:

Example 1

Graphene Embraced Particles of Electrode Active Materials

[0145] Several types of electrode active materials (both anode and cathode active materials) in a fine powder form were investigated. These include Co.sub.3O.sub.4, Si, LiCoO.sub.2, LiMn.sub.2O.sub.4, lithium iron phosphate, etc., which are used as examples to illustrate the best mode of practice. These active materials were either prepared in house or purchased from commercial sources.

[0146] In a typical experiment, 1 kg of electrode active material powder and 100 grams of natural flake graphite, 50 mesh (average particle size 0.18 mm; Asbury Carbons, Asbury N.J.), and milling balls (stainless steel balls, ZrO.sub.2 balls, glass balls, and MoO.sub.2 balls, etc.) were placed in a high-energy ball mill container. The ball mill was operated at 300 rpm for 0.5 to 4 hours. The container lid was then removed and particles of the active materials were found to be fully coated (embraced or encapsulated) with a dark layer, which was verified to be graphene by Raman spectroscopy. The mass of processed material was placed over a 50 mesh sieve and, in some cases, a small amount of unprocessed flake graphite was removed.

Example 2

Functionalized Graphene-Encapsulated Sn Particles

[0147] The process of example 1 was replicated with the inclusion of 50 grams of urea as a nitrogen source. The coated powder created was functionalized graphene-encapsulated Sn particles for use as an anode active material in a lithium-ion battery. It may be noted that chemical functionalization is used to improve wettability of the electrode active material by the electrolyte or the compatibility between the electrode active material and the electrolyte in a battery.

Example 3

Graphene-Embraced SnO.SUB.2 .Particles

[0148] In an experiment, 2 grams of 99.9% purity tin oxide powder (90 nm diameter), 0.25 grams highly oriented pyrolytic graphite (HOPG), and 1 gram of ZrO.sub.2 balls were placed in a resonant acoustic mill and processed for 5 minutes. For comparison, the same experiment was conducted, but without the presence of zirconia milling beads. The direct transfer process (tin oxide particles serving as the milling media per se without the externally added zirconia milling beads) led to mostly single-particle particulate (each particulate contains one particle encapsulated by graphene sheets). In contrast, with the presence of externally added milling beads, a graphene-embraced particulate tends to contain some multiple tin oxide particles (typically 3-50) wrapped around by graphene sheets. These same results were also observed for most of metal oxide-based electrode active materials (both anode and cathode).

Example 4

Graphene-Encapsulated Si Micron Particles

[0149] In a first experiment, 500g of Si powder (particle diameter 3 m), 50 grams of highly oriented pyrolytic graphite (HOPG), and 100 grams of ZrO.sub.2 balls were placed in a high-intensity ball mill. The mill was operated for 20 minutes, after which the container lid was opened and un-processed HOPG was removed by a 50 mesh sieve. The Si powder was coated with a dark layer, which was verified to be graphene by Raman spectroscopy.

[0150] In a second experiment, micron-scaled Si particles from the same batch were pre-coated with a layer of polyethylene (PE) using a micro-encapsulation method that includes preparing solution of PE dissolved in toluene, dispersing Si particles in this solution to form a slurry, and spry-drying the slurry to form PE-encapsulated Si particles. Then, 500 g of PE-encapsulated Si particles and 50 grams of HOPG were placed in a high-intensity ball mill. The mill was operated for 20 minutes, after which the container lid was opened and un-processed HOPG was removed by a 50 mesh sieve. The PE-encapsulated Si particles (PE layer varied from 0.3 to 2.0 m) were now also embraced with graphene sheets. These graphene-embraced PE-encapsulated particles were then subjected to a heat treatment (up to 600 C.) that converted PE to carbon. The converted carbon was mostly deposited on the exterior surface of the Si particles, leaving behind a gap or pores between the Si particle surface and the encapsulating graphene shell. This gap provides room to accommodate the volume expansion of the Si particle when the lithium-ion battery is charged. Such a strategy leads to significantly improved battery cycle life.

[0151] In a third experiment, the Si particles were subjected to electrochemical prelithiation to prepare several samples containing from 5% to 54% Li. Prelithiation of an electrode active material means the material is intercalated or loaded with lithium before a battery cell is made. Various prelithiated Si particles were then subjected to the presently invented graphene encapsulation treatment. The resulting graphene-encapsulated prelithiated Si particles were incorporated as an anode active material in several lithium-ion cells.

Example 5

Graphene-Embraced Ge Particles (Using Mesocarbon Micro Beads or MCMBs as The Graphene Source)

[0152] In one example, 500 grams of B-doped Ge powder (anode active material) and 10 grams of MCMBs (China Steel Chemical Co., Taiwan) were placed in a ball mill (with or without milling balls), and processed for 3 hours. In separate experiments, un-processed MCMB was removed by sieving, air classification, and settling in a solvent solution. The graphene loading of the coated particles was estimated to be 1.4 weight %.

Example 6

Graphene Encapsulation Via Indirect Direct Transfer vs. Chemical Production of Graphene Sheets Plus Freezer Milling

[0153] A sample of graphene-embraced lithium titanate particles was prepared via the presently invented indirect transfer method (using silicon carbide balls as the milling media and natural graphite as the graphene source).

[0154] In a separate experiment, 10 grams of lithium titanate powder and 1 gram of reduced graphene oxide sheets (produced with the Hummer's method explained below) were placed in a freezer mill (Spex Mill, Spex Sample Prep, Metuchen N.J.) and processed for 10 minutes. In this experiment, graphite oxide as prepared by oxidation of graphite flakes with sulfuric acid, nitrate, and permanganate according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The graphite oxide was repeatedly washed in a 5% solution of HCl to remove the majority of the sulfate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was spray-dried and placed in a vacuum oven at 60 C. for 24 hours. The interlayer spacing of the resulting laminar graphite oxide was determined by the Debey-Scherrer X-ray technique to be approximately 0.73 nm (7.3 A). A sample of this material was subsequently transferred to a furnace pre-set at 650 C. for 4 minutes for exfoliation and heated in an inert atmosphere furnace at 1200 C. for 4 hours to create a low density powder comprised of few-layer reduced graphene oxide (RGO). Surface area was measured via nitrogen adsorption BET.

[0155] As discussed in the Background section, there are seven (7) major problems associated with the chemical method of graphene production. In addition, the graphene sheets, once produced, tend to result in the formation of multiple-particle particulates that each contains a plurality of electrode active material particles embraced or encapsulated by graphene sheets. They appear to be incapable of encapsulating a single particle.

Example 7

Graphene-Encapsulated Lithium Iron Phosphate (LFP) as a Cathode Active Material for a Lithium Metal Battery

[0156] LFP powder, un-coated or carbon-coated, is commercially available from several sources. The carbon-coated LFP powder and un-coated LFP powder samples were separately mixed with natural graphite particles in ball mill pots of a high-intensity ball mill apparatus. The apparatus was operated for 0.5 to 4 hours for each LFP material to produce graphene-encapsulated LFP particles. Zirconia beads were added as milling media in the ball milling apparatus.

Example 8

Graphene-encapsulated V.SUB.2.O.SUB.5 .as an example of a Transition Metal Oxide Cathode Active Material of a Lithium Battery

[0157] V.sub.2O.sub.5 powder is commercially available. A mixture of V.sub.2O.sub.5 powder and natural graphite (10/1 weight ratio) was sealed in each of 4 ball milling pots symmetrically positioned in a high-intensity ball mill containing MoO.sub.2 balls as the milling media. The mill was operated for 1 hour to produce particulates of graphene-encapsulated V.sub.2O.sub.5 particles, which were implemented as the cathode active material in a lithium metal battery.

Example 9

LiCoO.SUB.2 .as an Example of Lithium Transition Metal Oxide Cathode Active Material for a Lithium-Ion Battery

[0158] In a set of experiments, a mixture of LiCoO.sub.2 powder and natural graphite (100/1-10/1 weight ratio) was sealed in each of 4 ball milling pots symmetrically positioned in a high-intensity ball mill containing ZrO.sub.2 balls as the milling media. The mill was operated for 0.5-4 hours to produce particulates of graphene-encapsulated LiCoO.sub.2 particles.

Example 10

Organic Material (Li.SUB.2.C.SUB.6.O.SUB.6.) as a Cathode Active Material of a Lithium Metal Battery

[0159] The experiments associated with this example were conducted to determine if organic materials, such as Li.sub.2C.sub.6O.sub.6, can be encapsulated in graphene sheets using the presently invented impact transfer method. The result is that organic active materials alone (without particles of an inorganic material) are typically incapable of peeling off graphene sheets from graphite particles. Preferably, rigid milling balls (e.g. ZrO.sub.2) are larger than 50 m to prevent the balls from being packed with the organic active material into graphene-wrapped particles.

[0160] In order to synthesize dilithium rhodizonate (Li.sub.2C.sub.6O.sub.6), the rhodizonic acid dihydrate (species 1 in the following scheme) was used as a precursor. A basic lithium salt, Li.sub.2CO.sub.3 can be used in aqueous media to neutralize both enediolic acid functions. Strictly stoichiometric quantities of both reactants, rhodizonic acid and lithium carbonate, were allowed to react for 10 hours to achieve a yield of 90%. Dilithium rhodizonate (species 2) was readily soluble even in a small amount of water, implying that water molecules are present in species 2. Water was removed in a vacuum at 180 C. for 3 hours to obtain the anhydrous version (species 3).

##STR00001##

A mixture of an organic cathode active material (Li.sub.2C.sub.6O.sub.6) and an inorganic cathode active material (V.sub.2O.sub.5 and MoS.sub.2, separately) was ball-milled for 0.5-2.0 hours to obtain a mixture of graphene-encapsulated particles. It may be noted that the two Li atoms in the formula Li.sub.2C.sub.6O.sub.6 are part of the fixed structure and they do not participate in reversible lithium ion storing and releasing. This implies that lithium ions must come from the anode side. Hence, there must be a lithium source (e.g. lithium metal or lithium metal alloy) at the anode. In one battery cell herein tested, the anode current collector (Cu foil) is deposited with a layer of lithium (via sputtering). The resulting cell is a lithium metal cell.

Example 11

Graphene-Encapsulated Na.SUB.3.V.SUB.2.(PO.SUB.4.).SUB.3./C and Na.SUB.3.V.SUB.2.(PO.SUB.4.).SUB.3 .Cathodes for Sodium Metal Batteries

[0161] The Na.sub.3V.sub.2(PO.sub.4).sub.3/C sample was synthesized by a solid state reaction according to the following procedure: a stoichiometric mixture of NaH.sub.2PO.sub.4.2H.sub.2O (99.9%, Alpha) and V.sub.2O.sub.3 (99.9%, Alpha) powders was put in an agate jar as a precursor and then the precursor was ball-milled in a planetary ball mill at 400 rpm in a stainless steel vessel for 8 h. During ball milling, for the carbon coated sample, sugar (99.9%, Alpha) was also added as the carbon precursor and the reductive agent, which prevents the oxidation of V3.sup.+. After ball milling, the mixture was heated at 900 C. for 24 h in Ar atmosphere. Separately, the Na.sub.3V.sub.2(PO.sub.4).sub.3 powder was prepared in a similar manner, but without sugar. Samples of both powders were then subjected to ball milling in the presence of natural graphite particles to prepare graphene-encapsulated Na.sub.3V.sub.2(PO.sub.4).sub.3 particles and graphene-encapsulated carbon-coated Na.sub.3V.sub.2(PO.sub.4).sub.3 particles. The cathode active materials were used in several Na metal cells containing 1 M of NaPF.sub.6 salt in PC+DOL as the electrolyte. It was discovered that graphene encapsulation significantly improved the cycle stability of all Na metal cells studied. In terms of cycle life, the following sequence was observed: graphene-encapsulated Na.sub.3V.sub.2(PO.sub.4).sub.3/C>graphene-encapsulated Na.sub.3V.sub.2(PO.sub.4).sub.3>Na.sub.3V.sub.2(PO.sub.4).sub.3/C>Na.sub.3V.sub.2(PO.sub.4).sub.3.

Example 12

Preparation of Graphene-Encapsulated MoS.SUB.2 .Particles as a Cathode Active Material of a Na Metal Battery

[0162] A wide variety of inorganic materials were investigated in this example. For instance, an ultra-thin MoS.sub.2 material was synthesized by a one-step solvothermal reaction of (NH.sub.4).sub.2MS.sub.4 and hydrazine in N, N-dimethylformamide (DMF) at 200 C. In a typical procedure, 22 mg of (NH.sub.4).sub.2MoS.sub.4was added to 10 ml of DMF. The mixture was sonicated at room temperature for approximately 10 min until a clear and homogeneous solution was obtained. After that, 0.1 ml of N.sub.2H.sub.4.H.sub.2O was added. The reaction solution was further sonicated for 30 min before being transferred to a 40 mL Teflon-lined autoclave. The system was heated in an oven at 200 C. for 10 h. Product was collected by centrifugation at 8000 rpm for 5 min, washed with DI water and recollected by centrifugation. The washing step was repeated for 5 times to ensure that most DMF was removed. Finally, MoS.sub.2 particles were dried and subjected to graphene encapsulation by high-intensity ball milling (with MoO.sub.2 balls) in the presence of natural graphite particles.

Example 13

Preparation of Two-Dimensional (2D) Layered Bi.SUB.2.Se.SUB.3 .Chalcogenide Nanoribbons

[0163] The preparation of (2D) layered Bi.sub.2Se.sub.3 chalcogenide nanoribbons is well-known in the art. In the present study, Bi.sub.2Se.sub.3 nanoribbons were grown using the vapor-liquid-solid (VLS) method. Nanoribbons herein produced are, on average, 30-55 nm thick with widths and lengths ranging from hundreds of nanometers to several micrometers. Larger nanoribbons were subjected to ball-milling for reducing the lateral dimensions (length and width) to below 200 nm. Nanoribbons prepared by these procedures were subjected to graphene encapsulation using the presently invented impact transfer method. The graphene-encapsulated Bi.sub.2Se.sub.3 nanoribbons were used as a cathode active material for Na battery. Surprisingly, Bi.sub.2Se.sub.3 chalcogenide nanoribbons are capable of storing Na ions on their surfaces.

Example 14

Preparation of Graphene-Encapsulated MnO.SUB.2 .and NaMnO.SUB.2 .Cathode Active Material for Na Metal Cells and Zn Metal Cells

[0164] For the preparation of the MnO.sub.2 powder, a 0.1 mol/L KMnO.sub.4 aqueous solution was prepared by dissolving potassium permanganate in deionized water. Meanwhile, 13.32 g surfactant of high purity sodium bis(2-ethylhexyl) sulfosuccinate was added in 300 mL iso-octane (oil) and stirred well to get an optically transparent solution. Then, 32.4 mL of 0.1 mol/L KMnO.sub.4 solution was added into the solution, which was ultrasonicated for 30 min to prepare a dark brown precipitate. The product was separated, washed several times with distilled water and ethanol, and dried at 80 C. for 12 h. Some amount of the MnO.sub.2 powder was then subjected to the impact transfer treatment to obtain graphene-encapsulated MnO.sub.2 particles.

[0165] Additionally, NaMnO.sub.2 particles were synthesized by ball-milling a mixture of Na.sub.2CO.sub.3 and MnO.sub.2 (at a molar ratio of 1:2) for 12 h followed by heating at 870 C. for 10 h. The resulting NaMnO.sub.2 particles were then subjected to ball-milling in the presence of MCMB particles and ZrO.sub.2 balls to prepare graphene encapsulated NaMnO.sub.2 particles.

[0166] The MnO.sub.2 particles, with or without graphene encapsulation, are also incorporated in alkaline Zn/MnO.sub.2 cells. Graphene encapsulation was found to dramatically increase the cycle life of this type of cell. The Zn-graphene/MnO.sub.2 battery is composed of a graphene/MnO.sub.2-based cathode (with an optional cathode current collector and an optional conductive filler), a Zn metal or alloy-based anode (with an optional anode current collector), and an aqueous electrolyte (e.g. a mixture of a mild ZnSO.sub.4 or Zn(NO.sub.3).sub.2 with MnSO.sub.4 in water).

Example 15

Layered Zinc Hydroxide Salts Encapsulated by Graphene Sheets as the Hybrid Cathode Material

[0167] The structural arrangements of dodecyl sulfate (DS) anions in the interlayer space of layered zinc hydroxide salts (LZH-DS) and of the structure of zinc hydroxide layers were investigated. As-prepared, highly crystalline LZH-DS has a basal spacing of 31.5 (3.15 nm). After treatment with methanol at room temperature, zinc hydroxide layers shrank to form two new layered phases with basal spacings of 26.4 and 24.7 . The shrinking was accompanied by a decrease in the content of DS anions in the interlayer space, indicating a change in the alignment of the intercalated anions and a decrease in the charge density of the zinc hydroxide layers. This study indicates that tetrahedra Zn ions can be reversibly removed from the hydroxide layers, with the octahedrally coordinated Zn ions left unaffected. This result suggests that layered zinc hydroxide can be used as a Zn intercalation compound. In the present investigation, layered zinc hydroxide particles were also subjected to ball milling in the presence of natural graphite particles and ZrO.sub.2 balls, resulting in the formation of graphene-encapsulated zinc hydroxide particles. It was discovered that graphene encapsulation imparts high-rate capability to the layered zinc hydroxide when used as a cathode active material of a Zn metal cell.

Example 16

Preparation and Electrochemical Testing of Various Battery Cells

[0168] For most of the anode and cathode active materials investigated, we prepared lithium-ion cells or lithium metal cells using the conventional slurry coating method. A typical anode composition includes 85 wt. % active material (e.g., graphene-encapsulated Si or Co.sub.3O.sub.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 (NMP). After coating the slurries on Cu foil, the electrodes were dried at 120 C. in vacuum for 2 h to remove the solvent. With the instant method, typically no binder resin is needed or used, saving 8% weight (reduced amount of non-active materials). Cathode layers are made in a similar manner (using Al foil as the cathode current collector) using the conventional slurry coating and drying procedures. An anode layer, separator layer (e.g. Celgard 2400 membrane), and a cathode layer are then laminated together and housed in a plastic-Al envelop. The cell is then injected with 1 M LiPF.sub.6 electrolyte solution dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). In some cells, ionic liquids were used as the liquid electrolyte. The cell assemblies were made in an argon-filled glove-box.

[0169] The cyclic voltammetry (CV) measurements were carried out using an Arbin electrochemical workstation at a typical scanning rate of 1 mV/s. In addition, the electrochemical performances of various cells were also evaluated by galvanostatic charge/discharge cycling at a current density of from 50 mA/g to 10 A/g. For long-term cycling tests, multi-channel battery testers manufactured by LAND were used.

[0170] In lithium-ion battery industry, it is a common practice to define the cycle life of a battery as the number of charge-discharge cycles that the battery suffers 20% decay in capacity based on the initial capacity measured after the required electrochemical formation.

[0171] FIG. 4 shows the charge-discharge cycling behaviors of 3 lithium cells featuring Co.sub.3O.sub.4 particle-based anodes: one cell containing un-protected Co.sub.3O.sub.4 particles, a second cell containing reduced graphene oxide (RGO)-embraced Co.sub.3O.sub.4 particles (a prior art approach), and a third cell containing graphene-encapsulated Co.sub.3O.sub.4 particles produced by the instant impact transfer method. It is clear that the presently invented chemical-free production method leads to graphene-encapsulated Co.sub.3O.sub.4 particles that exhibit the most stable battery cycle life. The cell containing unprotected Co.sub.3O.sub.4 particles has a cycle life of approximately 150 cycles. In contrast, the cell featuring the graphene-protected Co.sub.3O.sub.4 particles prepared according to the instant invention experiences only a 5.7% reduction in capacity after 300 cycles.

[0172] Shown in FIG. 5 are the charge-discharge cycling behaviors of 2 lithium cells featuring SnO.sub.2 particle-based anodes: one containing un-protected SnO.sub.2 particles and the other containing graphene-encapsulated SnO.sub.2 particles produced by the instant impact transfer method using stainless steel balls. The present invention imparts a much stable cycle life to a lithium-ion battery.

[0173] Shown in FIG. 6 are the charge-discharge cycling behaviors of 3 lithium cells featuring micron-scaled (3 m) Si particle-based anodes: one cell containing un-protected Si particles, a second cell containing graphene-embraced Si particles produced by the indirect transfer method (with externally added milling media, ZrO.sub.2 balls), and a third cell containing graphene-encapsulated Si particles produced by the direct transfer method (Si particles themselves being the graphene-peeling agent). Again, the invented graphene-embraced Si particles lead to very stable cycling behavior.

[0174] Example of the graphene-protected cathode active materials are presented in FIG. 7 and FIG. 8. FIG. 7 has summarized the charge-discharge cycling behaviors of 2 lithium cells featuring lithium iron phosphate (LFP) particle-based cathodes: one containing un-protected LFO particles (mixed with 12% by weight carbon) and the other containing graphene-encapsulated carbon-added LFP particles produced by the instant impact transfer method (4% graphene +8% C). Again, the presently invented method offers a better charge-discharge cycling behavior. FIG. 8 shows the charge-discharge cycling behaviors of 3 lithium cells featuring LiV.sub.2O.sub.5 nanorod-based cathodes: one containing protected LiV.sub.2O.sub.5 nanorods (mixed with 10% by weight carbon black particles, one containing graphene-embraced LiV.sub.2O.sub.5nanorods produced by the impact transfer method using MoO.sub.2 balls (approximately 5% graphene +7% C), and one containing LiV.sub.2O.sub.5 nanorods protected by a carbon matrix. The invention provides the best approach to protecting the cathode active materials.