Method for purifying graphite material

Abstract

Provided herein are methods of purifying graphite comprising: electrochemically treating graphite comprising an impurity selected from the group consisting of metals, metal oxides, and combinations thereof in the presence of an electrolyte; a portion of the impurities is removed by electrochemical treatment and purified graphite is provided. Also provided are purified graphite provided by such a method, as well as a negative electrode and a battery each comprising the purified graphite.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for purifying graphite containing impurities. Specifically, the present invention relates to a method for purifying graphite materials containing impurities by removing impurities such as metals, metal oxides, and combinations thereof. Appropriately purified graphite materials have a wide range of applications, including carbon brushes, refractory materials, material composites, electrodes, lubricants, coatings, transportation vehicles, mechanical components, textiles, and household consumer applications.

[0002] In its most preferred form, the present invention specifically relates to a method for purifying graphite produced via a process, which can produce iron-containing graphite having a specific and optional morphology. However, it should be recognized that the present invention is not limited to this specific most preferred field of application.

[0003] Another form of the present invention relates to purified graphite having a reduced concentration of metal and / or metal oxide impurities that can be produced by the method of the present invention. BACKGROUND ART

[0004] Carbon, especially graphite, is considered a key material in the emerging green technology market. It has been demonstrated that it can be used in energy storage, conductive devices, catalyst supports, lubricant additives, and modern electronic devices.

[0005] Carbon has several different allotropes (i.e., different physical forms). Different carbon allotropes can have different physical properties. For example, diamond is the hardest naturally occurring substance, while graphite is extremely soft, can be split with very light pressure, and has a low specific gravity. Diamond is transparent, is an excellent abrasive, can be an electrical insulator and a thermal conductor, while graphite is opaque, is a very good lubricant, is a good electrical conductor, and is also an effective thermal insulator. The allotropes of carbon are not limited to diamond and graphite, and also include graphene (two-dimensional crystalline carbon layers), amorphous carbon, glassy carbon, carbon nanofoam, and other carbon allotropes that are particularly relevant to a process, namely carbon nanotubes (CNT), carbon nano-onions (CNO), carbon microspheres (CMS), etc.

[0006] Graphite has a wide range of applications, including carbon brushes, refractory materials, material composites, electrodes, lubricants, coatings, transportation vehicles, mechanical components, textiles, and household consumer applications. In recent years, specialty markets for graphite have emerged, including electric vehicle (EV) batteries; with the growth of EV sales, the demand for battery-grade graphite is expected to surge. Although battery chemistries are evolving, graphite is expected to remain a key element in EV batteries for at least the next decade. Both synthetic graphite and natural graphite in the form of the intermediate product spherical graphite are used in the anodes of lithium-ion batteries.

[0007] For certain applications, high-purity graphite may be required. In addition, those skilled in the art should understand that graphite with a higher quality grade (i.e., purity) is suitable for special applications such as those described above. The value of high-purity graphite (99.9%) is 5-10 times that of ordinary purified graphite (94%-98%).

[0008] The concept of purifying graphite is not new. In fact, graphite beneficiation has been widely implemented at the industrial level. There are two methods to obtain commercial concentrates or products: (1) flotation: repeated regrinding and flotation (up to seven times) to purify the concentrate, or (2) acid leaching (dissolving) of gangue with hydrofluoric acid (for silicate gangue) or hydrochloric acid (for carbonate gangue). Both methods rely on releasing or exposing impurities to a certain extent from any graphite inclusions, which limits their applicability to graphite containing extremely small (nanoscale) impurities. Acid leaching can be a dangerous and environmentally polluting method. In addition, it should be recognized that crushing and flotation will damage the morphology of the purified graphite.

[0009] US2,787,528 discloses a method for purifying graphite with an ash content of less than 3%. US 2,787,528 discloses a method of first treating impure graphite with a dilute sulfuric acid solution and then with a mild caustic. The main impurity in most high-carbon natural graphite is mica.

[0010] US1,600,730 discloses a method of using an electrolytic cell to purify and treat naturally occurring graphite from mines to remove a portion of the natural impurities. The natural impurities in the naturally occurring graphite taken from a schist geological formation are silica, alumina, iron oxide, calcium oxide, magnesium oxide, sulfur anhydride, and various alkalis after mechanical purification. Notably, US1,600,730 does not disclose the purity of the graphite after treatment with the electrolytic cell.

[0011] It has been confirmed that the process directly produces graphite carbon with a total graphite carbon (TGC) of 70-97% w / w from the reactor. To attract high-end graphite application markets such as battery manufacturing, a graphite purity of 99.9% or higher is required.

[0012] Table 1 below lists the prices of graphite of different purities in 2013. Obviously, increasing the purity of graphite materials will increase the value of the product.

[0013] Table 1: Graphite prices in January 2013 (USD / ton)

[0014]

[0015]

[0016] International Patent Publication WO 2016 / 154666 describes the production of graphite carbon via a process. In addition to graphite carbon, the process also produces hydrogen. The feedstock is a hydrocarbon gas, most preferably a methane source, and the method involves contacting a low-grade iron oxide catalyst with the hydrocarbon gas at a temperature of 600 °C to 1000 °C, thereby catalytically converting at least a portion of the hydrocarbon gas into hydrogen and graphite carbon. The graphite carbon deposits on the low-grade iron oxide catalyst, inevitably resulting in a certain degree of iron-based contamination in the obtained graphite.

[0017] International Patent Publication WO 2017 / 031529 describes subsequent improvements to the process to selectively obtain various forms of graphite species. Since different forms of graphite exhibit different properties, the commercial uses of graphite also highly depend on the form. According to the method described in WO 2017 / 031529, four different forms of graphite can be selectively produced.

[0018] First is graphite fiber, which is a fibrous carbon structure typically 100 nm to 100 microns in length; the most well-known graphite fiber is the carbon nanotube (CNT), which is a cylindrical nanostructure containing single or multiple concentrically arranged or perpendicular graphite sheets around a central axis.

[0019] Second is carbon nano-onion (CNO), which is a structure composed of multiple concentrically layered spherical graphite sheets from a central core (usually a catalyst particle or void); the diameter of these carbon structures is typically 50 - 500 nm.

[0020] Third is carbon microsphere (CMS), which is a hollow spherical graphite structure, usually larger than 500 nm in size. They are spherical and may also be chain-like. Naturally occurring CMS are found in meteorites.

[0021] Finally, the temperature and pressure parameters can be controlled in the Hazer process to produce graphene, which is characterized by single or few-layer graphite sheets.

[0022] Regarding the earlier version of the technology described in WO 2016 / 154666, it should be recognized that when using low-grade iron oxides as catalysts (the most preferred forms being Fe2O3, Fe3O4, hematite ore, or goethite ore), the produced graphite, regardless of its form, will contain a certain amount of iron-based impurities, thus requiring subsequent purification for downstream uses in many emerging and established industries.

[0023] Previously, techniques including high-temperature thermal purification and microwave-assisted acid digestion have been successfully used to purify Hazer graphite to greater than 99.9% w / w. However, these methods may be difficult to scale up and are costly.

[0024] An object of the present invention is to overcome or ameliorate one or more disadvantages of the prior art, or at least to provide a useful alternative.

[0025] An object of a preferred form of the present invention is to provide a method for purifying graphite to a high purity (i.e., greater than 90%, even greater than 99%) by weight.

[0026] An object of at least one preferred form of the present invention is to provide a method for purifying graphite without affecting the graphite morphology or degrading the graphite.

[0027] Although the present invention will be described with reference to specific embodiments, those skilled in the art should recognize that the present invention can also be embodied in many other forms. Summary of the Invention

[0028] The present invention generally relates to a method for electrochemically extracting iron impurities to increase the purity of Hazer graphite from 50% to 99.9% by weight, preferably from 80% to 99.7%. Another advantage is that the iron impurities can be recovered as a high-purity catalyst for the process. Experiments conducted by the inventors have shown that the electrochemical purification (ECP) method can purify graphite in both solid and slurry forms. However, the convenience associated with treating Hazer graphite as a slurry may come at the expense of reaction rate and / or power consumption. The results further demonstrate the potential for purifying Hazer graphite on a gram scale and scaling up.

[0029] High-purity graphite has a wide range of applications, including carbon brushes, refractory materials, batteries, material composites, electrodes, lubricants, coatings, transportation vehicles, mechanical components, textiles, and household consumer applications.

[0030] Chemical purification of naturally occurring mined graphite has been utilized to remove natural impurities in geological formations. However, regardless of the source (i.e., naturally occurring graphite or synthetically produced graphite), obtaining high-purity graphite via an economically viable method has been problematic. However, the impurities in some graphite samples (especially those produced via a process) are firmly encapsulated in the graphite material and are thus more difficult to remove. Fine grinding is typically employed to release the entrapped impurities. However, this is not always practical or possible for nano-scale impurities (such as process graphite). The technology proposed by the present invention provides a solution to this problem.

[0031] According to a first aspect, the present invention provides a method for purifying a graphite material, the method comprising:

[0032] electrochemically treating a crude graphite material containing impurities selected from metals, metal oxides, and combinations thereof;

[0033] using a predetermined or regenerated electrolyte;

[0034] within a predetermined time period;

[0035] within a predetermined voltage range;

[0036] within a predetermined temperature range;

[0037] using a predetermined anode composition;

[0038] using a predetermined cathode composition;

[0039] thereby removing a part of the impurities by electrochemical treatment and providing a purified graphite material.

[0040] In an embodiment, the method employs a predetermined permeable membrane that inhibits contact between the graphite and the cathode, which may cause a short circuit.

[0041] In an embodiment, the impurities are selected from metals, metal-containing impurities, non-metals, non-metal-containing impurities, organic substances, inorganic substances, and combinations thereof. In a preferred embodiment, the impurities are non-carbon impurities. In a particularly preferred embodiment, the impurities are metals. Most preferably, the metal is iron or iron carbide substances, such as ferrite, austenite, and cementite.

[0042] In another embodiment, the raw graphite material is compressed before being used in the method.

[0043] In another embodiment, the method does not include stirring and agitation.

[0044] In another embodiment, the metal-containing impurities are selected from metal oxides, metal hydroxides, metal nitrates, carbonates, carboxylic acids, salts, and the like.

[0045] In an embodiment, the morphology of the purified graphite material is substantially the same as that of the raw graphite material. Preferably, the morphology of the graphite material is selected from graphite fibers (including carbon nanotubes), carbon nano-onions, and carbon microspheres.

[0046] In an embodiment, the electrolyte is selected from metals or transition metals and sulfates, sulfites (including bisulfates), phosphates, carbonates, bicarbonates, hydroxides, permanganates, chromates, dichromates, oxalates, formates, acetates, benzoates, halides, chlorites, perchlorates, and hypochlorites, etc. (such as perfluorates, hypobromites, etc.), acids, etc. and mixtures thereof, such as sulfates and sulfuric acid. In a particularly preferred embodiment, the electrolyte is ammonium sulfate, ferrous sulfate, or a mixture thereof. In another embodiment, the electrolyte is sulfuric acid or nitric acid. Preferably, the electrolyte is ferrous sulfate or ammonium sulfate. Those skilled in the art should recognize that any suitable salt can be used as long as the selected salt forms soluble compounds and / or complexes with iron or metal impurities. In other embodiments, the electrolyte is selected from sulfides, phosphides, phenolates, superoxides, peroxides, oxides, silicates, sulfones, thiocyanates, thiosulfates, selenates, triiodides, azides, cyanides, cyanates, borates, fulminates, arsenates, vanadates, antimonates, etc.

[0047] In an embodiment, the iron impurities precipitate on the cathode in the form of iron dendrites. Generally, the iron dendrites are collected and applied in industrial fields. In some embodiments, the iron dendrites are used as catalysts for the thermal catalytic decomposition of methane into hydrogen and iron-contaminated graphite materials. In other embodiments, the iron can be in the form of sludge. In certain embodiments, depending on the post-treatment method, the sludge can be recycled and refined and / or adjusted to be a catalyst, such as a low-efficiency or high-efficiency catalyst. In a preferred embodiment, once the sludge is refined to produce an iron-containing catalyst, it can be used in the Hazer process to produce hydrogen and graphite from hydrocarbon compounds (preferably methane).

[0048] In an embodiment, the iron impurities remain in the solution. In an embodiment, the iron impurities are precipitated from the solution and used as a catalyst for the thermal catalytic decomposition of methane into hydrogen and iron-contaminated graphite materials, such as process.

[0049] The precipitated iron can have a specific shape, size, purity, or composition (pure iron or complex). The precipitation can be achieved as part of an electrolytic cell or as a separate process.

[0050] In an embodiment, the iron impurities are precipitated from the solution and used as a catalyst for the thermal catalytic decomposition of methane into hydrogen and iron-contaminated graphite materials. In an embodiment, once the iron impurities are precipitated, they are dried, crushed, and / or filtered.

[0051] In an embodiment, the iron-contaminated graphite material is subsequently purified by the method defined in the first aspect of the present invention.

[0052] In an embodiment, the method is carried out in a continuous, substantially continuous, or batch manner.

[0053] In certain embodiments, the method is carried out in batches, with each batch containing from about 1 g to 3 kg of graphite material. For example, each batch contains from about 1 g to about 10 g, or from about 10 g to about 20 g, or from about 20 g to about 30 g, or from about 30 g to about 40 g, or from about 40 g to about 50 g, or from about 50 g to about 60 g, or from about 60 g to about 70 g, or from about 70 g to about 80 g, or from about 80 g to about 90 g, or from about 90 g to about 100 g, or from about 100 g to about 200 g, or from about 200 g to about 300 g, or from about 300 g to about 400 g, or from about 400 g to about 500 g, or from about 500 g to about 600 g, or from about 600 g to about 700 g, or from about 700 g to about 800 g, or from about 800 g to about 900 g, or from about 900 g to about 1 kg, or from about 1 kg to about 2 kg, or from about 2 kg to about 3 kg of graphite material. In an embodiment, the method does not lose efficiency or lose the minimum efficiency due to an increase in the amount of graphite material in each batch.

[0054] In an embodiment, the voltage range is from about 1 V to 300 V. Preferably, the voltage range is from about 5 - 300 V. In a specific embodiment, the voltage is about 20 V. It has been found that, compared with other voltage ranges, a voltage of 20 V improves purification within 24 hours and increases the reaction kinetics. However, it does not improve the final purity of the graphite material.

[0055] In an embodiment, the current is constant in the method. In other embodiments, the current varies in the method. In certain embodiments, the current can be increased to increase the reaction rate of the method.

[0056] In an embodiment, the time period is from about 30 minutes to about 2 weeks. In another embodiment, the time period is from about 2 h to about 96 h. Preferably, the time period is from about 24 h to about 48 h.

[0057] In an embodiment, the temperature range is from about 5 °C to about 100 °C. In an embodiment, the temperature range is from about 20 °C to about 80 °C. In an embodiment, the temperature range is from about 40 °C to about 60 °C.

[0058] In an embodiment, the anode includes one or more structures. Preferably, the structures include rods, plates, filaments, etc. In an embodiment, the structure is crystalline or amorphous. In an embodiment, the structure is a graphite structure. In an embodiment, the structure includes platinum or titanium coated with platinum, preferably platinum. In an embodiment, the anode includes graphite, lead, lead alloy, platinum, titanium coated with platinum, and combinations thereof. In a particularly preferred embodiment, the structure is titanium coated with platinum.

[0059] In an embodiment, the cathode comprises graphite, lead, lead alloy, platinum, titanium coated with platinum, and combinations thereof. In an embodiment, the cathode comprises platinum, titanium coated with platinum, and graphite electrodes, various grades of stainless steel, ferroalloys, other transition metal alloys, and the like. In a preferred embodiment, the cathode comprises titanium coated with platinum.

[0060] In an embodiment, the method further comprises covering at least a portion of the anode, the cathode, or both with a permeable membrane. Preferably, the permeable membrane is an electrically neutral permeable membrane, an anion exchange membrane, or a cation exchange membrane. More preferably, the permeable membrane is selected from asbestos cloth, cellulose, glass cloth, filter cloth, glass cloth impregnated with silica gel, porous sintered stainless steel, vinyl chloride acrylonitrile, polysulfone, polyethersulfone (PES), polycarbonate, polytetrafluoroethylene, polyethylene terephthalate (PET), and combinations thereof. Alternatively, the membrane may comprise sintered metal and non-metal materials, such as ceramics.

[0061] In an embodiment, the molecular weight cut-off (MWCO) of the permeable membrane is less than about 1 million Da. Preferably, the molecular weight cut-off (MWCO) of the permeable membrane is from about 10 kDa to about 0.5 kDa. More preferably, the molecular weight cut-off (MWCO) of the permeable membrane is about 3.5 kDa. In one embodiment, the gas permeability of the permeable membrane at 200 Pa is 0.1 - 100 L / min / dm 2 . Preferably, the gas permeability of the permeable membrane at 200 Pa is 1 - 50 L / min / dm 2 . More preferably, the gas permeability of the permeable membrane at 200 Pa is 2 - 30 L / min / dm 2 .

[0062] In an embodiment, the purity of the purified graphite material is greater than about 95% w / w. Preferably, the purity of the purified graphite material is greater than about 99% w / w. More preferably, the purity of the purified graphite material is greater than about 99.5% w / w. Most preferably, the purity of the purified graphite material is greater than about 99.9% w / w.

[0063] In an embodiment, the purified graphite material is used as the crude graphite material in the method to provide iterative purification.

[0064] According to a second aspect of the present invention, there is provided a purified graphite material purified by the method defined in the first aspect of the present invention.

[0065] The inventors have unexpectedly found a novel and simple method for purifying graphite at a relatively low cost. The electrochemical method (electrolytic cell or electrolytic refining) employed provides a surprisingly efficient and relatively low-cost method for purifying graphite due to its simple operation.

[0066] In some embodiments, impurities undergo redox reactions during the electrochemical treatment, thereby removing some of the impurities from the graphite. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment form salts, such as metal salts. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment form water-soluble salts. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment form water-insoluble salts.

[0067] In some embodiments, the impurity is iron. The iron can be synthetic (e.g., from Fe3O4), or naturally occurring (e.g., from hematite), or elemental iron or iron carbide, etc. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment form cationic species. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment form iron cationic species, such as Fe 3+ 。

[0068] In some embodiments, the electrochemical treatment does not substantially affect or damage the morphology of the graphite. Advantageously, purification without damaging or affecting the morphology of the treated graphite can increase the yield and value of the purified graphite.

[0069] In some embodiments, the method includes using a permeable membrane between the cathode and the anode during the electrochemical treatment. The main function of the membrane is to prevent the graphite from contacting the cathode and short-circuiting the electrochemical cell.

[0070] In some embodiments, the salts formed by the redox reaction also undergo metathesis reactions. In certain embodiments, the metathesis reactions form insoluble salts. In some embodiments, the insoluble salts formed by the metathesis reaction are iron hydroxides or iron complexes, such as jarosite. Advantageously, insoluble salts such as iron hydroxide can be valuable by-products formed during the purification method of the present invention. For example, iron hydroxide can be used as a catalyst, such as for decomposing methane to form graphite and hydrogen, most conveniently via process. As will be appreciated by those skilled in the art, various insoluble iron species can be used directly or indirectly as catalysts for the method.

[0071] In some embodiments, the electrolyte is a sulfate. In some embodiments, the electrolyte is selected from ammonium sulfate, ferrous sulfate, sulfuric acid, and combinations thereof. Advantageously, when ferrous sulfate is used as the electrolyte, elemental iron is formed on the cathode during the electrochemical treatment of impure graphite. The elemental iron can be in the form of dendrites and is formed instead of insoluble iron hydroxide. However, those skilled in the art will understand that the electrolytic cell can be adjusted such that the deposited metal does not form dendrites but forms a uniform metal coating around the cathode (which may be preferred); dendrites are generally not preferred because they can cause short circuits. Similarly, elemental iron can be a valuable by-product formed during the purification method of the present invention. For example, iron can also be used as a catalyst, such as for decomposing methane to form graphite and hydrogen. Similarly, most conveniently via method.

[0072] In some embodiments, it is preferred that the iron is in solution form or the elemental iron is precipitated as an insoluble substance, rather than depositing the elemental iron on the cathode.

[0073] In some embodiments, the impurities are naturally derived, e.g., natural impurities from geological formations (e.g., mined graphite). In some embodiments, the impurities are synthetically derived, e.g., the impurities are introduced due to the synthetic preparation of graphite. For example, as described above, the method described in WO 2016 / 000115. This disclosure, together with WO 2017 / 031529, is hereby incorporated by reference in its entirety.

[0074] In some embodiments, the purity of the purified graphite is greater than 90% (by weight), preferably greater than 95%, more preferably greater than 98%, and even more preferably greater than 99%. In some embodiments, the purity of the purified graphite is greater than about 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99%. In some embodiments, the purity of the purified graphite is greater than about 98.1, 98.2, 98.3, 98.4, 98.5, 98.6, 98.7, 98.8, 98.9, 99.0, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, or 99.9%. In some embodiments, the purity of the purified graphite is greater than about 99.95%.

[0075] According to another form, the present invention provides a method for purifying graphite, comprising: electrochemically treating graphite containing impurities selected from iron, iron oxides, and combinations thereof in the presence of an electrolyte containing a sulfate; thereby removing a portion of the impurities by the electrochemical treatment and providing purified graphite.

[0076] According to a third aspect, the present invention provides purified graphite containing impurities selected from metals, metal oxides, and combinations thereof, wherein the impurity concentration of the purified graphite is less than 20% w / w.

[0077] In some embodiments, the impurity concentration of the purified graphite is less than 15% w / w, preferably less than 10%, preferably less than 5%, more preferably less than 3%, more preferably less than 2%, and even more preferably less than 1%. In some embodiments, the impurity concentration of the purified graphite is less than 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% w / w. In some embodiments, the impurity concentration of the purified graphite is less than 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, or 0.05% w / w.

[0078] According to a fourth aspect of the present invention, there is provided a negative electrode material comprising a coating on a substrate, the coating comprising: MnO2 (electrolytic manganese dioxide (EMD)), a carbon conductive additive in the form of a purified graphite material according to the second aspect, and a binder.

[0079] In some embodiments, the EMD is one or more of α-, β-, γ-, δ-, or λ-MnO2.

[0080] In some embodiments, the EMD consists mainly of α-, β-, γ-, δ-, or λ-MnO2. In some embodiments, the EMD is γ-MnO2.

[0081] In some embodiments, the EMD, the carbon conductive additive, and the binder are mixed together in a weight ratio of 4-9:2:1.

[0082] In some embodiments, the EMD, the carbon conductive additive, and the binder are mixed together in a weight ratio of 7:0.1-3:1.

[0083] In some embodiments, the EMD, the carbon conductive additive, and the binder are mixed together in a weight ratio of 7:2:0.1-3.

[0084] In some embodiments, the EMD, the carbon conductive additive, and the binder are mixed together in a weight ratio of 4-9:0.1-3:0.1-3.

[0085] In one embodiment, the EMD, the carbon conductive additive, and the binder are mixed together in a weight ratio of 7:2:1.

[0086] In some embodiments, the binder comprises a fluoropolymer selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy polymer (PFA), and polyvinyl fluoride (PVF).

[0087] In some embodiments, the binder is selected from carboxymethyl cellulose (CMC), sodium alginate, starch, styrene-butadiene rubber (SBR), xanthan gum, polyvinyl chloride (PVC), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyamideimide (PAI).

[0088] In one embodiment, the binder comprises polyvinylidene fluoride (PVDF).

[0089] In some embodiments, the substrate comprises a metal foil.

[0090] In some embodiments, the metal foil is made of a conductive metal.

[0091] In some embodiments, the conductive metal is copper, zinc, aluminum, iron, or any mixture thereof.

[0092] In some embodiments, the coating at least partially surrounds the substrate.

[0093] In some embodiments, the coating surrounds the substrate.

[0094] In some embodiments, the thickness of the coating is from about 1 micron to about 25 microns.

[0095] In some embodiments, the thickness of the coating is about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or about 25 microns.

[0096] In some embodiments, the thickness of the coating is from about 5 microns to about 20 microns.

[0097] In some embodiments, the thickness of the coating is from about 7 microns to about 15 microns.

[0098] In one embodiment, the thickness of the coating is about 10 microns.

[0099] In some embodiments, the conductivity of the negative electrode is from about 70 S / m -1 to about 100 S / m -1 .

[0100] In some embodiments, the conductivity of the negative electrode is about 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or about 100 S / m -1 .

[0101] In some embodiments, the conductivity of the negative electrode is about 80 S / m -1 to about 95 S / m -1 .

[0102] In one embodiment, the conductivity of the negative electrode is about 90 S / m -1 .

[0103] According to a fifth aspect of the present invention, there is provided a battery comprising:

[0104] a positive electrode

[0105] a negative electrode; and

[0106] an electrolyte in contact with the positive electrode and the negative electrode,

[0107] wherein the negative electrode comprises a coating on a substrate, and wherein the coating comprises MnO₂ (electrolytic manganese dioxide (EMD)), a carbon conductive additive in the form of a purified graphite material according to the second aspect, and a binder.

[0108] In some embodiments, the EMD is one or more of α-, β-, γ-, δ- or λ-MnO₂.

[0109] In some embodiments, the EMD consists mainly of α-, β-, γ-, δ- or λ-MnO₂.

[0110] In some embodiments, the EMD is γ-MnO₂.

[0111] In some embodiments, the EMD, the carbon conductive additive and the binder are mixed together in a weight ratio of 4-9:2:1.

[0112] In some embodiments, the EMD, the carbon conductive additive and the binder are mixed together in a weight ratio of 7:0.1-3:1.

[0113] In some embodiments, the EMD, the carbon conductive additive and the binder are mixed together in a weight ratio of 7:2:0.1-3.

[0114] In some embodiments, the EMD, the carbon conductive additive and the binder are mixed together in a weight ratio of 4-9:0.1-3:0.1-3.

[0115] In one embodiment, the EMD, carbon conductive additive, and binder are mixed together at a weight ratio of 7:2:1.

[0116] In some embodiments, the binder comprises a fluoropolymer selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy polymer (PFA), and polyvinyl fluoride (PVF).

[0117] In some embodiments, the binder is selected from carboxymethyl cellulose (CMC), sodium alginate, starch, styrene-butadiene rubber (SBR), xanthan gum, polyvinyl chloride (PVC), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyamideimide (PAI).

[0118] In one embodiment, the binder comprises polyvinylidene fluoride (PVDF).

[0119] In some embodiments, the substrate comprises a metal foil.

[0120] In some embodiments, the metal foil is made of a conductive metal.

[0121] In some embodiments, the conductive metal is copper, zinc, aluminum, iron, or any mixture thereof.

[0122] In some embodiments, the coating at least partially surrounds the substrate.

[0123] In some embodiments, the coating surrounds the substrate.

[0124] In some embodiments, the thickness of the coating is from about 1 micron to about 25 microns.

[0125] In some embodiments, the thickness of the coating is about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or about 25 microns.

[0126] In some embodiments, the thickness of the coating is from about 5 microns to about 20 microns.

[0127] In some embodiments, the thickness of the coating is from about 7 microns to about 15 microns.

[0128] In one embodiment, the thickness of the coating is about 10 microns.

[0129] In some embodiments, the conductivity of the negative electrode is about 70 S / m -1 to about 100 S / m -1 .

[0130] In some embodiments, the conductivity of the negative electrode is about 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 or about 100 S / m -1 .

[0131] In some embodiments, the conductivity of the negative electrode is about 80 S / m -1 to about 95 S / m -1 .

[0132] In one embodiment, the conductivity of the negative electrode is about 90 S / m -1 .

[0133] In some embodiments, the electrolyte is an aqueous electrolyte.

[0134] In some embodiments, the electrolyte is an aqueous electrolyte having a concentration of about 0.01 M to about 10 M.

[0135] In some embodiments, the electrolyte is an aqueous electrolyte having a concentration of about 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.95, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5 or about 10 M.

[0136] In some embodiments, the electrolyte is an aqueous electrolyte having a concentration of about 0.01 M to about 5 M.

[0137] In some embodiments, the electrolyte is an aqueous electrolyte having a concentration of about 0.01 M to about 2 M.

[0138] In some embodiments, the electrolyte is an aqueous electrolyte selected from sulfates, nitrates, chlorides, and combinations thereof.

[0139] In some embodiments, the electrolyte is an aqueous electrolyte selected from ammonium sulfate, sodium sulfate, magnesium sulfate, ferrous sulfate, copper sulfate, zinc sulfate (ZnSO4), cadmium nitrate, cobalt nitrate, sodium nitrate, sodium chloride, nickel chloride, potassium chloride, ammonium chloride, calcium chloride, sulfuric acid, and combinations thereof.

[0140] In some embodiments, the electrolyte is zinc sulfate (ZnSO4), zinc chloride (ZnCl2), ammonium chloride (NH4Cl), and zinc trifluoromethanesulfonate (Zn(CF3SO3)2) or a mixture thereof.

[0141] In some embodiments, the electrolyte is an ionic liquid comprising cations selected from 1-alkyl-3-methylimidazolium, N-alkylpyridinium, tetraalkylammonium, tetraalkylphosphonium, and combinations thereof.

[0142] In some embodiments, the alkyl is selected from C2-C 12 alkyl.

[0143] In some embodiments, the ionic liquid comprises cations selected from 1-ethyl-3-methyl-1H-imidazolium, 1-butyl-3-methyl-1H-imidazolium, 1-butylpyridinium, and combinations thereof.

[0144] In some embodiments, the electrolyte is ZnSO4.

[0145] In some embodiments, the concentration of the ZnSO4 electrolyte is from about 0.01 M to about 10 M.

[0146] In one embodiment, the concentration of the ZnSO4 electrolyte is 1.0 M.

[0147] In some embodiments, the positive electrode is a zinc metal electrode.

[0148] In some embodiments, when measured between 1.5 V and 0.7 V at a current density of 0.05 Ag -1 the discharge specific capacity of the negative electrode is from about 50 mAh g -1 to about 200 mAh g -1 .

[0149] In some embodiments, when measured between 1.5 V and 0.7 V at a current density of 0.05 Ag -1 the discharge specific capacity of the negative electrode is from about 70 mAh g -1 to about 120 mAh g -1 .

[0150] In one embodiment, when measured between 1.5 V and 0.7 V at a current density of 0.05 Ag -1 the discharge specific capacity of the negative electrode is at least about 109 mAh g -1 .

[0151] Definitions

[0152] In describing and defining the present invention, the following terms will be used according to the definitions described below. It should also be understood that the terms used herein are only for describing specific embodiments of the present invention and are not intended to be limiting. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0153] For the purposes of the present invention, the terms "graphite" and "graphite material" are generally regarded as synonyms. This definition includes carbon materials with crystalline structure (short-range and long-range crystallinity), and most preferably includes various forms as described above in relation to the applicant's previously published WO 2017 / 031529.

[0154] Unless the context clearly requires otherwise, throughout the specification and claims, the words "comprise", "comprising", etc. shall be construed in an inclusive sense, rather than an exclusive or exhaustive sense; in other words, they shall be construed to mean "including but not limited to".

[0155] As used herein, the phrase "consisting of" excludes any element, step or ingredient not specified in the claim. When the phrase "consisting of" (or its variants) appears in a clause of the claim body, rather than immediately following the preamble, it only limits the elements specified in that clause; it does not exclude other elements from the entire claim. As used herein, the phrase "consisting essentially of" limits the scope of the claim to the specified element or method step, and those elements or method steps that do not materially affect the basic and novel features of the claimed subject matter.

[0156] Regarding the terms "comprising", "consisting of" and "consisting essentially of", when one of these three terms is used herein, the currently disclosed and claimed subject matter may include the use of any of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of "comprising" may be replaced by "consisting of" or "consisting essentially of".

[0157] Except for operating examples, or where otherwise indicated, all numbers representing component amounts or reaction conditions used herein shall in all cases be understood to be modified by the term "about" and taking into account the normal tolerances in the art. The examples are not intended to limit the scope of the present invention. Hereinafter, or where otherwise indicated, "%" means "% by weight" (or "% w / w"), "ratio" means "weight ratio", and "parts" means "parts by weight".

[0158] Unless otherwise indicated, the term "substantially" as used herein shall mean comprising more than 50% by weight (where applicable).

[0159] The recitation of a numerical range using endpoints includes all values within that range (e.g., 1-5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

[0160] The terms "preferred" and "preferably" mean that certain embodiments of the present invention may bring certain benefits in certain circumstances. However, in the same or other circumstances, other embodiments may also be preferred. In addition, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, nor is it intended to exclude other embodiments from the scope of the present invention.

[0161] It should also be noted that, unless the context clearly dictates otherwise, the singular forms "a", "an", and "the" used in the specification and the appended claims include plural referents.

[0162] The prior art owned by Hazer Group Limited mentioned herein (i.e., WO 2016 / 000115 and WO2017 / 031529) is incorporated herein by reference in its entirety.

[0163] Any discussion of prior art in the entire specification should not be regarded as an admission that such prior art is well known or constitutes a part of the common general knowledge in the art.

[0164] Although the exemplary embodiments of the disclosed technology are explained in detail herein, it should be understood that other embodiments are also contemplated. Therefore, the scope of the disclosed technology is not limited to the details of the structure and arrangement of the components set forth in the following specification or illustrated in the drawings. The disclosed technology may have other embodiments and can be implemented or carried out in various ways.

[0165] "Alkyl", as a group or part of a group, refers to a straight-chain or branched-chain aliphatic hydrocarbon group, preferably C1-C 12 alkyl, more preferably C1-C 10 alkyl, most preferably C1-C6 alkyl, unless otherwise specified. Examples of suitable straight-chain and branched-chain C1-C6 alkyl substituents include methyl, ethyl, n-propyl, 2-propyl, n-butyl, sec-butyl, tert-butyl, hexyl, etc. This group can be a terminal group or a bridging group.

[0166] "Aryl", as a group or part of a group, refers to (i) an optionally substituted monocyclic or fused polycyclic aromatic carbocyclic ring (a ring structure in which all ring atoms are carbon), each ring preferably having 5-12 atoms. Examples of aryl include phenyl, naphthyl, etc.; (ii) an optionally substituted partially saturated polycyclic aromatic carbocyclic moiety, in which a phenyl group and one or more C 5-7 cycloalkyl and / or C 5-7 cycloalkenyl groups are fused together to form a ring structure, such as tetrahydronaphthyl, indenyl or indanyl. This group can be a terminal group or a bridging group. Generally, aryl is C6-C 18 aryl.

[0167] Throughout this specification, unless the context requires otherwise, terms such as "synthetic", "synthetically derived", etc. shall be understood to mean that the material has been synthesized by chemical techniques.

[0168] Throughout this specification, unless the context requires otherwise, terms such as "natural", "naturally derived", etc. shall be understood to mean that the material has not been synthesized by chemical techniques, for example, it has been mined from geological formations. Although the terms "natural", "naturally derived", etc. do include naturally occurring materials, they shall not be understood to exclude materials that have undergone physical beneficiation such as crushing, screening, or sizing. BRIEF DESCRIPTION OF THE DRAWINGS

[0170] The preferred embodiments selected for the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

[0171] Figure 1 is a schematic embodiment of an electrochemical electrolytic cell for purifying graphite containing impurities. Specifically, Figure 1a is a 2D side view; Figure 1b is a 3D schematic showing spaced-apart electrode plates.

[0172] Figure 2 The embodiment of the present invention for an electrochemical purification apparatus is shown in block diagram form.

[0173] Figure 3 is a flowchart representing an embodiment of the configuration of the electrochemical purification apparatus.

[0174] Figure 4 Shows a method flowchart representing an embodiment of the configuration of the electrochemical purification apparatus.

[0175] Figure 5a is a photograph of a laboratory-scale electrochemical electrolytic cell taken during testing; Figure 5b is at 3.5 kDa Figure 5a is a schematic diagram of the embodiment shown in the photograph.

[0176] Figure 6 is a thermogravimetric analysis (TGA) curve of electrochemically purified graphite after 24 hours of treatment.

[0177] Figure 7 Shows photographs of the electrochemical purification of graphite over a period of up to 48 hours and shows the removal of iron in the form of Fe(OH)3 or iron complexes from the graphite core (e.g., photograph at 24 h on day 1).

[0178] Figure 8 is a thermogravimetric analysis (TGA) curve of electrochemically purified graphite after 48 hours of treatment.

[0179] Figure 9 shows comparative scanning electron microscope (SEM) micrographs of as-received graphite and purified graphite samples. Figure 9a shows as-received graphite containing impurities prior to electrochemical treatment; Figure 9b shows electrochemically purified graphite.

[0180] Figure 10 shows a backscattered SEM image of purified graphite after 48 h of treatment. Graphite with any remaining residual iron core is circled.

[0181] Figure 11 is a photograph of elemental iron dendrites, rather than iron oxide, formed on the cathode when using ferrous sulfate as the aqueous electrolyte.

[0182] Figure 12a is a schematic diagram of the ECP method for Hazer graphite slurry. The schematic shows that the experimental beaker contains a 16% v / v Hazer graphite / 1 M (NH4)2SO4 slurry and is stirred with a magnetic stir bar. The dialysis bag contains 1 M (NH4)2SO4 and a carbon cathode. The experiment was conducted for 24 h.

[0183] Figure 12b shows a test of the graphite slurry. Hazer graphite at a concentration of 16% v / v was mixed with the electrolyte to produce the graphite slurry. The pure electrolyte was placed in the dialysis bag together with the cathode, while the anode was placed in the slurry. The image shows the 24-h method.

[0184] Figure 13 shows the TGA of the red sludge collected by ECP purification. The thermal decomposition of the iron by-products in the TGA generally occurs in four stages. These decompositions are similar to those found in the literature, indicating 1) loss of water; 2) dehydroxylation, followed by the loss of OH - ; 3) loss of ammonia and water; 4) loss of sulfate in iron sulfate. The ramp rate of the TGA was 15 °C / min.

[0185] Figure 14 shows the catalytic performance of the iron by-products (red sludge) using a TGA-EG reactor system under 100% CH4 flow, at 900 °C, 1 atm, and 8 h of operation.

[0186] Figure 15 is a TGA comparison of laboratory sample "PFBR-019-FL01" purified with different electrolytes. Each electrolyte used was able to purify the pilot-plant graphite from approximately 80% to approximately 93%. However, it is notable that the thermal decomposition of each sample was different, which may imply a change in the graphite structure.

[0187] Figure 16 shows the FESEM image of the raw graphite material prior to the ECP method.

[0188] Figure 17 shows the FESEM images of the purified graphite material from Figure 16 .

[0189] Figure 18 shows the relative pressure and pores of the pristine sample and the purified graphite material. (a) N2 physical adsorption isotherm, and (b) pore size distributions of the pristine sample, investigated samples 1b, 3b, 4a, and 6.

[0190] Figure 19 shows the XRD patterns of the pristine sample and the treated graphite material: (a) XRD pattern of the pristine sample (PFBR-26-FL01); (b) standard patterns of Fe (00-006-0696), C (01-089-8487), and Fe3C (01-089-7271); (c) and (d) comparison of the XRD patterns of the samples collected before and after ECP operation.

[0191] Figure 20 shows the scanning electron microscope (SEM) images of the carbon materials used as conductive additives in this study. (a) Super P, (b) Carbon-O, (c) Carbon-T, and (d) Carbon-E.

[0192] Figure 21 shows the particle size distribution graphs of the carbon materials (Super P, Carbon-O, Carbon-T, and Carbon-E) used as conductive additives in this study.

[0193] Figure 22 shows the (a) N2 physical adsorption isotherms, (b) pore size distributions, (c) Raman spectra, and (d) TGA curves of different carbon materials (Super P, Carbon-O, Carbon-T, and Carbon-E).

[0194] Figure 23 shows the derivative thermogravimetry (DT) curves of different carbon materials (Super P, Carbon-O, Carbon-T, and Carbon-E).

[0195] Figure 24 shows the electrolyte absorption capacity over time of the EMD electrodes fabricated using different carbon conductive additives.

[0196] Figure 25 shows at discharge current densities of (a) 1.0 Ag -1 , (b) 0.5 Ag -1 , (c) 0.1 A g -1 and (d) 0.05 Ag -1The galvanostatic discharge curves of Zn-C batteries fabricated using different carbon conductive additives. (e) Comparison of the specific capacities of Zn-C batteries at different discharge current densities. The inset shows an enlarged view of the low discharge current density region. (f) Nyquist plot of the Zn-C battery. The inset shows the intercept of the impedance curve with the real axis.

[0197] Figure 26 Show the (a) GITT curves of Zn-C batteries assembled using different carbon additives; (b) enlarged view of the GITT segment during the third test cycle; (c) battery resistance of the Zn-C battery during 30 GITT test cycles; and (d) OCV plots of the Zn-C battery in the long-term stability test over one month.

[0198] Figure 27 Show the galvanostatic discharge curves of the Zn-C battery at 0.1 Ag -1 after the long-term stability test. Detailed Description

[0199] Those skilled in the art will understand that the present invention includes the embodiments and features disclosed herein, as well as all combinations and / or permutations of the disclosed embodiments and features.

[0200] Method

[0201] As described above, one form of the present invention provides a method for purifying graphite materials, the method comprising electrochemically treating a crude graphite material containing impurities in metallic form; using a predetermined electrolyte; for a predetermined period of time; within a predetermined voltage range; within a predetermined temperature range; using a predetermined anode composition; using a predetermined cathode composition; thereby removing a portion of the impurities by electrochemical treatment and providing a purified graphite material.

[0202] In another form, the present invention provides a method for purifying graphite, comprising: electrochemically treating graphite containing impurities selected from iron, iron oxides, and combinations thereof in the presence of an electrolyte containing sulfate or a mixed composition; thereby removing a portion of the impurities by electrochemical treatment and providing a purified graphite.

[0203] The inventors unexpectedly discovered a novel and simple method for purifying graphite at relatively low cost. The electrochemical method employed (electrolytic cell or electrolytic refining), due to its simple operation, provides an unexpectedly efficient and relatively low-cost method for graphite purification.

[0204] In some embodiments, the method steps of the present invention can be repeated. In certain embodiments, the method steps of the present invention can be repeated one, two, three, four, five, six, seven, eight, nine, or ten (or more) times.

[0205] As will be understood by those skilled in the art, the impurities can be any suitable metals with a size range from nanometers to micrometers. Suitable metals can be selected from alkali metals, alkaline earth metals, transition metals, rare earth elements, and combinations thereof. In certain embodiments, the metal is selected from gold, aluminum, copper, iron, lead, silver, platinum, tin, cobalt, nickel, zinc, and combinations thereof. In a preferred embodiment, the metal is iron. In other embodiments, the metal is selected from titanium, sodium, potassium, magnesium, manganese, calcium, or phosphorus. In other embodiments, the impurities can include metalloids such as silicon or non-metals such as sulfur. It should be recognized that there are many impurities that can be found in graphite, including any impurities visible in typical iron ores.

[0206] Suitable metal oxides can be selected from oxides of alkali metals, alkaline earth metals, transition metals, rare earth elements, and combinations thereof. In certain embodiments, the metal oxide is aluminum oxide, copper oxide, iron oxide, silver oxide, tin oxide, cobalt oxide, nickel oxide, zinc oxide, and combinations thereof. In a preferred embodiment, the metal oxide is iron oxide. In other embodiments, the oxide includes silicon dioxide.

[0207] In some embodiments, the impurities undergo redox reactions during the electrochemical treatment process, and the electrochemical treatment removes a portion of the impurities from the graphite. In a preferred embodiment, the impurities are oxidized during the electrochemical treatment process, and the electrochemical treatment removes a portion of the impurities from the graphite. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment process form salts, such as metal salts. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment process form water-soluble salts. In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment process form water-insoluble salts.

[0208] In some embodiments, the impurities that undergo redox reactions during the electrochemical treatment process form cationic species. In some embodiments, the cationic species are selected from gold cations, aluminum cations, copper cations, iron cations, lead cations, silver cations, platinum cations, tin cations, cobalt cations, nickel cations, zinc cations, and combinations thereof. In a preferred embodiment, the impurities that undergo redox reactions during the electrochemical treatment process form iron cationic species such as Fe 2+ or Fe 3+ species.

[0209] Without wishing to be bound by theory, it is believed that the anions of the electrolyte in the solution (preferably sulfate) react with impurities through redox reactions when transported to the positive electrode (anode) during the electrochemical treatment, causing the impurities to form salts (i.e., metal salts). The metal salts are soluble in the solution (separated into cations and anions) and can then diffuse out of the graphite, thereby removing the impurities from the graphite. The water-soluble metal salts can then undergo subsequent reactions through metathesis reactions to produce water-insoluble salts. Depending on the electrolyte used, the elemental metal derived from the impurities can be deposited on the cathode during the electrochemical treatment. For example, sulfate ions diffuse into the graphite and form iron(III) sulfate (or iron(II) sulfate), which then diffuses out in the form of a dissolved salt, or the iron impurities are oxidized to form Fe 3+ , which diffuses out of the graphite enclosure and then forms iron(III) sulfate.

[0210] For example, if ammonium sulfate ((NH4)2SO4) is used as the electrolyte, it is believed that ammonium sulfate in water dissolves according to the equation shown below, producing ammonium cations (NH4 + ) and sulfate anions (SO4 2- ) in the solution. Then, the negatively charged sulfate ions are electrostatically attracted to the positive electrode (anode, such as graphite containing impurities), where the anions penetrate the graphite "core" to contact the impurities. In the case of an iron core, the iron is converted to water-soluble iron(III) sulfate (Fe2(SO4)3) through a redox reaction.

[0211] Then, the iron(III) sulfate can diffuse into the bulk electrolyte solution and optionally through a semipermeable membrane (such as a dialysis bag). NH 4+ is attracted to the negative electrode (cathode, such as a graphite rod), producing ammonium hydroxide (NH4OH) in the electrolytic solution. The hydrogen ions in the solution are attracted to the cathode, where they produce hydrogen gas. The water-soluble iron(III) sulfate can then undergo a metathesis reaction with ammonium hydroxide in the solution to produce insoluble iron(III) hydroxide (Fe(OH)3) in the form of a reddish-brown precipitate. The reactions in this embodiment can be summarized as follows:

[0212] (NH4)2SO4 →2NH4 + +SO4 2-

[0213] H2O →H + +OH -

[0214] 2H + +2e - →H2 (cathode)

[0215] NH4 + +OH - →NH4OH

[0216] Fe → Fe 3+ + 3e - (Anode)

[0217] 2Fe III + 3SO4 2- → Fe2(SO4)3 (water-soluble) (Anode)

[0218] NH4OH + Fe2(SO4)3 → 3(NH4)2SO4 + 2Fe(OH)3 (water-insoluble)

[0219] In other embodiments, iron precipitates in the form of an iron complex, such as jarosite, e.g., NH4[Fe(OH)2]3(SO4)2.

[0220] In some embodiments, the salts formed by the redox reaction further include salt metathesis reactions, such as the reaction in the above equation. In certain embodiments, the salt metathesis reaction forms an insoluble salt. In some embodiments, the insoluble salt formed by the salt metathesis reaction is iron hydroxide. Advantageously, the insoluble salt, such as iron hydroxide, can be a valuable by-product formed during the purification method of the present invention. As previously mentioned, iron hydroxide can be used as a catalyst, e.g., for decomposing methane to form graphite and hydrogen, e.g., via Process.

[0221] In some embodiments, the electrochemical treatment does not affect or damage the morphology of the graphite in any way. Other graphite purification techniques, such as using hydrofluoric acid and microwave purification, will damage or change the morphology of the graphite after purification. In some embodiments, at least about 70 w / w%, at least about 80 w / w%, at least about 90 w / w%, at least about 95 w / w%, at least about 98 w / w%, at least about 99 w / w% of the purified graphite, relative to the total graphite containing impurities, maintains the original morphology before the purification method and / or is not damaged.

[0222] In some embodiments, the method includes using a permeable membrane between the cathode and the anode during the electrochemical treatment. In certain embodiments, the anode is surrounded by a permeable membrane. In a preferred embodiment, the cathode is surrounded by a permeable membrane. In certain embodiments, both the cathode and the anode are surrounded by a permeable membrane.

[0223] Alternatively, an unpartitioned electrolytic cell (i.e., without using a permeable membrane) can also be used for electrochemical treatment to remove impurities in the graphite. Due to the conductivity of the graphite and / or the electrolyte, the most obvious obstacle to using an unpartitioned electrolytic cell may be the tendency to short-circuit.

[0224] As will be understood by those skilled in the art, any suitable type of permeable membrane can be used in the method of the present invention. In some embodiments, the permeable membrane is a neutral permeable membrane. In some embodiments, the permeable membrane is an anion exchange membrane. In some embodiments, the permeable membrane is a cation exchange membrane.

[0225] In some embodiments, the molecular weight cut-off (MWCO) of the permeable membrane (e.g., dialysis tubing) is less than about 1,000,000 Da, less than about 900,000 Da, less than about 800,000 Da, less than about 700,000 Da, less than about 600,000 Da, less than about 500,000 Da, less than about 400,000 Da, less than about 300,000 Da, less than about 200,000 Da, less than about 100,000 Da, less than about 900 kDa, less than about 800 kDa, less than about 700 kDa, less than about 600 kDa, less than about 500 kDa, less than about 400 kDa, less than about 300 kDa, less than about 200 kDa, less than about 100 kDa, less than about 90 kDa, less than about 80 kDa, less than about 70 kDa, less than about 60 kDa, less than about 50 kDa, less than about 40 kDa, less than about 35 kDa, less than about 30 kDa, less than about 25 kDa, less than about 20 kDa, less than about 15 kDa, less than about 10 kDa, less than about 7.5 kDa, less than about 5 kDa, less than about 4.5 kDa, less than about 4 kDa, less than about 3.5 kDa, less than about 3 kDa, less than about 2.5 kDa, less than about 2 kDa, less than about 1.5 kDa or less than about 0.5 kDa. In one embodiment, the gas permeability of the permeable membrane at 200 Pa is 0.1 - 100 L / min / dm 2 . Preferably, the gas permeability of the permeable membrane at 200 Pa is 1 - 50 L / min / dm 2 . More preferably, the gas permeability of the permeable membrane at 200 Pa is 2 - 30 L / min / dm 2 .

[0226] In some embodiments, the anion exchange membrane contains a positive charge, such as a phosphonium cation (i.e., PR3 + ), a sulfonium cation (i.e., SR2 + ), an ammonium cation (NH3 + ), etc., where according to the respective definitions provided above, R is independently H, alkyl, aryl, or halide.

[0227] In some embodiments, the cation exchange membrane contains a negative charge, such as a phosphate anion (i.e., PO3 - ), a sulfonate anion (i.e., SO3 - ), a carboxylate anion (i.e., COO –)、C6H4O – ) etc.

[0228] In certain embodiments, the permeable membrane is selected from asbestos cloth, cellulose, glass cloth, filter cloth, glass cloth impregnated with silica gel, porous sintered stainless steel, vinyl chloride acrylonitrile, polyethylene terephthalate (PET), and combinations thereof. In an embodiment, the membrane can be any sintered medium that does not react with the electrolyte, such as sintered PTFE.

[0229] As will be understood by those skilled in the art, any suitable electrolyte can be used in the present invention. In some embodiments, the electrolyte is sulfate, nitrate, chloride, and combinations thereof.

[0230] In some embodiments, the electrolyte is selected from ammonium sulfate, sodium sulfate, magnesium sulfate, ferrous sulfate, copper sulfate, zinc sulfate (ZnSO4), zinc chloride (ZnCl2), zinc trifluoromethanesulfonate (Zn(CF3SO3)2), cadmium nitrate, cobalt nitrate, sodium nitrate, sodium chloride, nickel chloride, potassium chloride, ammonium chloride (NH4Cl), calcium chloride, sulfuric acid, and combinations thereof. In other embodiments, the electrolyte can be nitric acid or sulfuric acid.

[0231] In a preferred embodiment, the electrolyte is selected from ammonium sulfate, ferrous sulfate, and combinations thereof (e.g., ammonium sulfate), and sulfuric acid. Advantageously, when ferrous sulfate is used as the electrolyte in the method of the present invention, elemental iron is formed on the cathode during the electrochemical treatment of graphite. The elemental iron is in the form of dendrites and forms instead of insoluble iron hydroxide. Similarly, elemental iron can be a valuable byproduct formed during the purification method of the present invention. For example, iron can also be used as a catalyst, such as decomposing methane to form graphite and hydrogen, such as via process, as described above.

[0232] Based on preliminary data, using ferrous sulfate as the electrolyte can provide higher purity graphite compared to using ammonium sulfate.

[0233] In certain embodiments, the electrolyte is an ionic liquid. In certain embodiments, the ionic liquid contains cations selected from 1-alkyl-3-methylimidazolium, N-alkylpyridinium, tetraalkylammonium, tetraalkylphosphonium, and combinations thereof. In some embodiments, the alkyl is selected from C2-C 12 alkyl. In some embodiments, the ionic liquid contains cations selected from 1-ethyl-3-methyl-1H-imidazolium, 1-butyl-3-methyl-1H-imidazolium, 1-butylpyridinium, and combinations thereof.

[0234] In certain embodiments, the method of the present invention includes partially or completely replacing the used electrolyte with fresh electrolyte (i.e., new electrolyte not used in the method). In certain embodiments, the partial or complete replacement of the electrolyte can be carried out via a batch or continuous method.

[0235] In certain embodiments, the electrolyte solution has any suitable pH. In some embodiments, the pH of the electrolyte solution is about 1 - 10, about 6 - 8, about 8 - 10, preferably about 1 - 6, about 1 - 5, and more preferably about 1 - 3. In a preferred embodiment, the pH of the electrolyte solution is less than about 6, less than about 5, and preferably less than about 3.

[0236] In certain embodiments, the concentration of the electrolyte solution is about 0.01 - 10 M. For example, the concentration of the electrolyte solution is about 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.95, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or about 10 M.

[0237] In some embodiments, the concentration of the electrolyte solution is preferably about 0.01 - 5 M, about 0.01 - 3 M, about 0.01 - 2 M, about 0.01 - 1 M, about 0.01 - 0.5 M, about 0.05 - 1 M, about 0.05 - 0.5 M, about 0.05 - 0.3 M, and preferably about 0.05 - 0.15 M. Most preferably, the concentration of the electrolyte solution is about 0.1 M.

[0238] In the method of the present invention, any suitable solvent can be used to dissolve the electrolyte. Generally, the electrolyte is an aqueous solution.

[0239] In certain embodiments, the solvent is water, an organic solvent, an inorganic non - aqueous solvent, and combinations thereof. In other embodiments, the solvent can be polar. In a preferred embodiment, the dispersion medium is selected from water, glycerol, glycerin, cellulose ethers, and combinations thereof.

[0240] Suitable organic solvents can be selected from pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, 1,4 - dioxane, chloroform, diethyl ether, dichloromethane, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, acetonitrile, dimethyl sulfoxide, nitromethane, propylene carbonate, n - butanol, isopropanol, n - propanol, ethanol, methanol, formic acid, acetic acid, hexafluoroisopropanol, trifluoroacetic acid, and combinations thereof.

[0241] Suitable inorganic solvents may be selected from liquid ammonia, liquid sulfur dioxide, thionyl chloride, sulfuryl chloride fluoride, phosphoryl chloride, dinitrogen tetroxide, antimony trichloride, bromine pentafluoride, hydrogen fluoride, pure sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid, hydrofluoric acid, hydrobromic acid, perchloric acid, hydroiodic acid, and combinations thereof.

[0242] In certain embodiments, the electrolyte may be dissolved in a mixture of two or more miscible solvents, such as a mixture of water and a water-soluble solvent or a mixture of an organic solvent and a water-soluble solvent.

[0243] The method of the present invention can be carried out at any suitable temperature. In some embodiments, the method is carried out at a temperature of about 5°C to about 200°C, about 5°C to about 100°C, about 5°C to 80°C, about 5°C to 50°C, about 50°C to 100°C, about 60°C to 90°C, about 70°C to 80°C, about 5°C to 30°C. In some embodiments, the method is carried out at a temperature less than about 100°C, less than about 80°C, less than about 50°C, preferably less than about 30°C. In a preferred embodiment, the method is carried out at about 25°C or 70 - 80°C. In some embodiments, the method is carried out at a temperature less than about 100, 90, 80, 70, 60, 50, 40, 30, 20 or 10°C.

[0244] In some embodiments, the predetermined temperature is about 5°C to about 200°C, about 5°C to about 100°C, about 5 - 80°C, about 5 - 50°C, about 50 - 100°C, about 60 - 90°C, about 70 - 80°C, about 5 - 30°C. In some embodiments, the predetermined temperature is less than about 100°C, less than about 80°C, less than about 50°C, preferably less than about 30°C. In a preferred embodiment, the predetermined temperature is about 25°C or 70 - 80°C. In some embodiments, the predetermined temperature is less than about 100, 90, 80, 70, 60, 50, 40, 30, 20 or 10°C.

[0245] In an embodiment, a pressure of about 1 bar (gauge) to about 100 bar (gauge), about 5 - 90 bar (gauge), about 10 - 80 bar (gauge), about 15 - 70 bar (gauge), about 20 - 60 bar (gauge), about 25 - 50 bar (gauge), about 30 - 40 bar (gauge) or 35 bar (gauge) may be applied to the electrolytic cell.

[0246] In some embodiments, the impurities are naturally derived, e.g., natural impurities from geological formations (such as mined graphite). In some embodiments, the impurities are synthetically derived, e.g., impurities are introduced due to the synthesis of graphite.

[0247] As will be understood by those skilled in the art, any suitable graphite containing impurities selected from metals, metal oxides, and combinations thereof can be used in the purification method of the present invention. In certain embodiments, the graphite is naturally derived. In a preferred embodiment, the graphite is synthetically derived, such as the graphite produced via the process described above.

[0248] As will be understood by those skilled in the art, graphite can exist in various forms. For example, graphite fibers are fibrous carbon structures with a length typically ranging from 100 nm to 100 micrometers, such as anisotropic nanofibers (ANF). Carbon nanotubes (CNT), which are cylindrical nanostructures containing single or multiple concentrically arranged or perpendicular-to-the-center-axis graphite sheets, also fall within the category of graphite fibers; carbon nano-onions (CNO), which are structures composed of multiple spherical graphite sheets concentrically layered from a central core, and the central core is usually a catalyst particle or void. The diameters of these carbon structures are typically 50 - 500 nm; carbon microspheres (CMS), which can be hollow spherical graphite structures or concentrically layered around a central core, with a size usually greater than 500 nm. They are spherical and may also be chain-like. As described above, the morphology of synthetic graphite can be controlled as described in the method of WO 2017 / 031529.

[0249] In certain embodiments, the shape of the graphite can be flaky, spherical, needle-like, plate-like, linear, tubular, whisker-like, spherulitic, nano-graphite, tube, wire, and combinations thereof.

[0250] The average particle size (d 50) It can be from about 10 nm to 350 μm, such as 10 nm to 1 μm, or 1 μm - 65 μm, or 100 nm - 10 μm. For example, the average particle size of the graphite used can be from about 10 nm to 1000 nm. For example, the average particle size of the graphite used is 10 nm to 100 nm, or 50 nm to 250 nm, or 200 to 500 nm, or 500 to 1000 nm or 400 - 750 nm, such as 10 nm, 50 nm, 100 nm, 500 nm or 1000 nm. The average particle size of the graphite used can also be from about 1 μm to 350 μm. For example, the average particle size of the graphite used can be 1 μm to 45 μm, or 40 μm to 60 μm, or 20 μm to 40 μm, or 30 μm to 50 μm, or 40 μm to 50 μm, or 40 μm to 60 μm, or 50 μm to 100 μm, or 100 μm to 250 μm, or 200 μm to 350 μm, or less than 300 μm, less than 200 μm, less than 100 μm, less than 65 μm, less than 60 μm, less than 55 μm, less than 50 μm, less than 45 μm, less than 40 μm, less than 35 μm, less than 30 μm, less than 25 μm, less than 20 μm, less than 15 μm, less than 10 μm or less than 5 μm, 350, 300, 250, 200, 150, 100, 85, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10, 5, 4, 3, 2 or 1 μm.

[0251] The surface area of the graphite used in the method of the present invention can be about 1 m 2 / g to about 1000 m 2 / g, about 1 - 700 m 2 / g, about 1 m 2 / g to about 400 m 2 / g. For example, the graphite can be low surface area graphite with a surface area of 1 - 5 or 1 - 10 or 5 - 20 or 20 - 30 or 15 - 25 or 1 - 30 m 2 / g, such as 1, 2, 3, 4, 5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5 or 30 m 2 / g. Alternatively, the graphite can be high surface area graphite with a surface area of 50 - 400 m 2 / g, such as 100 - 150 or 100 - 200 or 50 - 200 or 150 - 250 or 200 - 375 or 250 - 350 or 300 - 400 or 100 - 400 m 2 / g, such as 50, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380 or 400 m 2 / g. In some embodiments, the surface area of the graphite is less than 300 m 2 / g, or less than 200 m 2 / g, or less than 100 m 2 / g. The surface area can be the N2 (NSA) BET surface area. Based on the Brunauer, Emmett, and Teller (BET) theory of multilayer gas adsorption (see also ASTM method D6556-04), in this article, nitrogen adsorption measurements at liquid nitrogen temperature can be used to characterize the total surface area of the graphite.

[0252] The crystallinity of the graphite used in the method of the present invention is preferably about 60% to 99.9%. For example, the crystallinity of the graphite is preferably 60% to 80%, or 75% to 90%, or 85% to 99%, or 90% to 99%, or 95% to 99%, for example, the crystallinity is at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99% or at least 99.9%, for example, the crystallinity is 60%, 70%, 80%, 85%, 90%, 95%, 99% or 99.9%.

[0253] The resistivity of the graphite used in the method of the present invention is preferably less than about 1.0 Ω·cm, less than about 0.8 Ω·cm, less than about 0.5 Ω·cm, less than about 0.1 Ω·cm, or less than about 0.05 Ω·cm. For example, the resistivity is 0.01 - 0.05 Ω·cm, or 0.05 - 0.10 Ω·cm, or 0.05 - 0.15 Ω·cm, or 0.10 - 0.20 Ω·cm, or 0.15 - 0.25 Ω·cm, or 0.25 - 0.4 Ω·cm, or 0.20 - 0.50 Ω·cm, or about 0.4 - 0.65 Ω·cm, or 0.50 - 0.75 Ω·cm, or 0.75 - 1.0 Ω·cm, or 0.01 - 1 Ω·cm. For example, the resistivity of the graphite can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1.0 Ω·cm.

[0254] In some embodiments, the graphite can be natural graphite, synthetic graphite, amorphous graphite, calcined petroleum coke, crystalline flake graphite, natural flake graphite, surface-enhanced flake graphite, expandable graphite, purified flake graphite, purified crystalline flake graphite, purified petroleum coke, purified synthetic graphite, purified vein graphite, synthetic graphite, primary artificial graphite, secondary artificial graphite, spherical natural graphite, vein graphite, and combinations thereof.

[0255] The carbon content of the graphite used herein is preferably from about 5% to 99.9%. For example, the carbon content of the graphite is preferably from 60% to 80%, or from 75% to 90%, or from 85% to 99%, or from 90% to 99%, or from 95% to 99%, such as a carbon content of at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99% or at least 99.9%, such as a carbon content of 60%, 70%, 80%, 85%, 90%, 95%, 99% or 99.9%.

[0256] In some embodiments, the purity of the purified graphite is greater than about 90%, preferably greater than 95%, more preferably greater than 98%, and even more preferably greater than 99%.

[0257] In some embodiments, the impurity concentration of the purified graphite is less than 15% w / w, preferably less than 10%, preferably less than 5%, more preferably less than 3%, more preferably less than 2%, and even more preferably less than 1%.

[0258] It should be recognized that the electrodes to be used as anodes or cathodes can be made of any suitable conductive material. In some embodiments, the electrodes are made of a metal or a metal alloy. In some embodiments, the electrodes are made of a material selected from the group consisting of electroceramics, copper, aluminum, platinum, titanium, gold, silver, iron, steel, stainless steel, brass, bronze, nickel, lead, lead alloys, conductive rubber, conductive carbon (such as graphite, graphene, and reduced graphene oxide), and combinations thereof. In some embodiments, the electrodes can include a coating of another conductive material. In other embodiments, the electrodes can be non-metal electrodes, such as graphite, conductive polymers, conductive ceramics, and other conductive materials (or similar materials) suitable for electrode construction or coating.

[0259] It should also be recognized that the electrodes to be used as anodes or cathodes can have any suitable thickness. In some embodiments, the thickness of the electrodes is from about 1 mm to 30 mm, from about 1 mm to about 20 mm, or from about 5 mm to about 15 mm.

[0260] In some embodiments, the method of the present invention uses multiple cathodes. In some embodiments, the method includes using two, three, four, five, six, seven, eight, nine, ten (or more) cathodes. In some embodiments, the method includes using 2 - 50 cathodes, 2 - 40 cathodes, 2 - 30 cathodes, 2 - 20 cathodes, and 2 - 10 cathodes.

[0261] In some embodiments, the method of the present invention uses multiple anodes. In some embodiments, the method of the present invention includes using two, three, four, five, six, seven, eight, nine, ten (or more) anodes. In some embodiments, the method includes using 2 - 50 anodes, 2 - 40 anodes, 2 - 30 anodes, 2 - 20 anodes, and 2 - 10 anodes.

[0262] As understood by those skilled in the art, the method of the present invention can be carried out using any suitable voltage. In certain embodiments, the method is carried out at the following voltages: about 5 - 300 V, about 5 - 240 V, about 5 - 220 V, about 5 - 200 V, about 5 - 150 V, about 5 - 100 V, about 5 - 50 V, about 5 - 30 V, about 5 - 24 V, about 5 - 12 V.

[0263] In a preferred embodiment, the method of the present invention is carried out using direct current. In a preferred embodiment, the method of the present invention is carried out using alternating current.

[0264] In certain embodiments, the method of the present invention can be carried out for about 1 hour to 2 weeks, about 1 hour to 1 week, about 1 hour to 5 days, about 1 hour to 4 days, about 1 hour to 3 days, about 1 hour to 96 hours, about 1 hour to 48 hours, about 6 hours to 48 hours, about 12 hours to 48 hours, about 24 hours to 48 hours.

[0265] In some embodiments, the method of the present invention is a batch method. In some embodiments, the method of the present invention is a continuous method. In other preferred embodiments, the method of the present invention can be carried out continuously by periodically or continuously circulating the electrolyte, adding crude graphite, and extracting purified graphite. In some embodiments, the crude graphite can be recycled into the electrochemical electrolytic cell for further purification.

[0266] In certain embodiments, the method of the present invention further includes steps of washing and / or sonication to remove more impurities. In certain embodiments, the method of the present invention further includes a filtration step to remove impurities, such as using a 0.45 μm filter or centrifugation.

[0267] As understood by those skilled in the art, the method of the present invention may further include one or more additional purification steps. For example, the purified graphite can be further pickled, alkali-washed, heat-treated, or a combination thereof to increase the carbon purity to exceed the purity obtained via the method of the present invention. In some embodiments, the purity of the graphite material pretreated by pickling and sonication steps is about 96.3%. In some embodiments, the purity is further increased by another ECP method.

[0268] In certain embodiments, the methods of the present invention include a plurality of electrochemical cells. In some embodiments, the method includes using two, three, four, five, six, seven, eight, nine, ten (or more) electrochemical cells. In these embodiments, the electrochemical cells can be in series or in parallel. Each electrochemical cell includes at least one cathode and at least one anode.

[0269] Graphite slurry solution

[0270] Prior to the refinement of the present invention, the inventors' experiments were conducted by filling dialysis bags with graphite. However, there are significant difficulties in handling large quantities of filled graphite, and these difficulties can be alleviated by using graphite slurries. Therefore, the feasibility of using graphite slurries in the ECP method was explored. The experimental setup was designed similar to the above experiment, but with the contents reversed: 1.0 M (NH4)2SO4 electrolyte and a graphite rod (cathode) were placed in a dialysis bag. The Hazer graphite was made into a 16 vol% slurry solution with 1.0 M (NH4)2SO4 electrolyte. A platinum wire was inserted directly into the graphite solution, and the ECP method was run for 24 h. The setup is depicted in Figure 12. The slurry experiments revealed several interesting findings, as discussed below, thus providing several parameters for Hazer to consider in the design of large-scale purification.

[0271] Figure 12b The slurry experimental setup after 1 h and 24 h is shown. During this method, a red sludge was seen to form in the graphite solution and inside the dialysis bag. During the ECP of the graphite slurry, a significant decrease in current was also observed. This indicates that the resistivity of the graphite slurry is greater than that of the filled graphite, because resistivity is inversely proportional to current at a constant voltage. This reduces the reaction rate, such that a longer reaction time is required to achieve the same purity level.

[0272] In the ECP method, the ionization of iron, i.e., Fe to Fe 3+ + 3e - , is crucial for purification. The power required for this reaction can be simulated with the conversion of 1 kg of iron. 1 kg of iron is equivalent to 17.86 moles, and each mole of iron requires 3e - , equivalent to 53.57 moles of e - . Therefore, for 1 kg of Fe or 5 kg of 80% pure Hazer graphite, the required power is:

[0273]

[0274] Based on these results, tests were conducted to determine other required parameters. The resistivity tests showed that adding an electrolyte to the filled graphite significantly reduces the conductivity. The tests were conducted in a three-electrolyzer system, with a glass frit separating the other two chambers.

[0275] Using a three - electrolyzer system, current tests were conducted with a multimeter to study the effect of electrode distance. As the electrodes were moved further apart, the current decreased significantly, from 0.3 mA to 0.03 mA.

[0276] In the three - electrolyzer system, the effect of increasing voltage was studied. By increasing the voltage while maintaining a set electrode distance, a gradual increase in current was observed. By increasing the voltage from 10 V to 20 V, the current increased from 0.064 A to 0.145 A. Notably, this was a 27% increase over the current increase expected from the voltage increase, meaning the resistance of the system decreased. The overall increase in current provided more power to the electrolyzer, thus increasing the reaction rate.

[0277] Iron by - product (red sludge)

[0278] During the development and scale - up of ECP, understanding the state of iron is required to remove the iron by - product (red sludge). To this end, the iron by - product generated during the ECP process was studied using two different analytical techniques: TGA and Raman spectroscopy. Both techniques showed that the iron by - product was more complex than Fe(OH)3.

[0279] Figure 13 The TGA spectrum in... showed that there were multiple stages of weight loss in the iron sludge, similar to that seen in other research papers on jarosite ((NH4)Fe3(SO4)2(OH)6). As described by Frost et al. [Thermal decomposition of ammonium jarosite (NH4Fe3(SO4)2(OH)6); J Therm Anal Calorim 84, 489–496 (2006). DOI: 10.1007 / s10973 - 005 - 6953 - 8], for the graphite TGA spectrum, the thermal decomposition stages observed in jarosite were seen.

[0280] In addition, the Raman spectrum of the dried iron sludge (dried at 50 °C; not shown) indicated that the sample consisted of multiple iron oxide species. The peaks observed indicated the presence of Fe2O3, Fe3O4, and graphite. The presence of Fe3O4 has never been observed in graphite before. This may be due to the presence of iron sulfate leading to an unexpected thermal liquid - phase transition at low temperature, as no SO4 2- peak (around 1005 cm -1 and 1094 cm -1 ) was observed.

[0281] These test results indicate that the red sludge is an iron complex, probably jarosite. According to TGA, iron oxides can be recovered from graphite by heat treatment at >550 °C. Therefore, the iron species recovered from graphite can be used as a high-purity catalyst.

[0282] It was observed that most of the red sludge or jarosite would accumulate outside the dialysis bag and then settle as sediment. However, this kind of accumulation is also expected to occur inside the dialysis bag. Therefore, in order to obtain high-purity graphite, it may be necessary to separate and remove jarosite from carbon. Physical separation via centrifugation was not successful, but loose particles attached to the surface of graphite can be removed by vacuum filtration washing, thus obtaining a purity of 99.5%. The method for removing jarosite (alkaline hydrous sulfate material) from graphite is to dissolve the material in an acid such as H2SO4.

[0283] To determine whether H2SO4 washing can replace the time-consuming vacuum filtration washing, 0.1 M H2SO4 was added to the iron sludge collected from the electrolyte at a ratio of 1:4. The mixture of H2SO4 and Fe sludge is more transparent compared to the control added with DI water. After 15 h (overnight), the Fe sludge mixed with H2SO4 is transparent, while different from the control, in which the Fe sludge has settled.

[0284] This experiment proves that H2SO4 can digest iron-based sludge, making the washing step milder and less time-consuming. According to the preferred design for scaling up this technology, this result provides directional information for the downstream cleaning method of the scaled-up design. Optimizing the addition of H2SO4 to remove excessive jarosite and determining the removal rate and concentration required to remove jarosite can better clarify the results of this study.

[0285] The iron by-product (red sludge) collected from the above experiment was used as a potential catalyst for the thermal decomposition method. This test was carried out using TGA-EG with a 100% methane flow at 900 °C, 1 atm, and a reaction time of 8 h ( Figure 14 ). Then, the performance was compared with the previous two catalysts: sample XG (synthetic iron oxide) and sample E (iron ore), and the performance comparison is shown in Table 2 below.

[0286] Table 2: Comparison of kinetic parameters of sample "XG", sample "E" and red sludge

[0287]

[0288] As shown in Table 2, at The complete reduction of the iron by-product (red sludge) in the process is significantly longer than that of other catalysts. This can be attributed to the complex structure of the iron by-product detailed above. Interestingly, the average reaction rate of the iron by-product is the same as that of Sample E. It should be noted that the tested iron by-product was collected during the purification process of the sample initially synthesized from Sample E. Compared with the other two catalysts, this catalyst exhibits excellent catalytic activity in the fast region. Theoretically, this may be related to the size, surface area of the catalyst / crystals, and extremely high iron purity in the completely reduced state of the red sludge, as the purification method significantly altered Sample E.

[0289] Using different electrolytes in the ECP method can improve the efficiency of the reaction kinetics. The choice of electrolyte depends on its ability to react with iron during the ECP process and form iron sulfate or other iron-soluble structures, as well as the convenience of removal from the purified graphite. Sulfuric acid is commonly used as an electrolyte because of its high conductivity, which can provide a large proton density during the electrochemical reaction, thus increasing the reaction rate. To determine the efficiency of H2SO4 as an electrolyte in the ECP method, experiments were conducted as described above. However, the electrolyte was replaced with 0.1 M H2SO4.

[0290] The initial observations included the generation of a large number of bubbles, which may be due to the electrochemical reaction producing H2 and O2.

[0291] Iron(II) sulfate is the second test candidate. FeSO4 has many advantages, such as producing the by-product elemental iron (Fe), which can be reused as a catalyst, and the side reactions are relatively simple; the electrolyte used is 0.1 M FeSO4. The experiment was conducted for more than 24 h:

[0292]

[0293] The FeSO4 electrolyte was initially a turbid yellow, but after 1 h of ECP, the solution turned into a transparent yellow. Small gray particles could be observed at the bottom of the beaker, and dendritic growth was observed on the carbon rod. After 3 h, the electrolyte became transparent, and more dendritic growth was observed on the cathode surface of the carbon rod. These dendrites may be Fe, which can be collected and optionally reused as a catalyst in the process.

[0294] When comparing the TGA results of different electrolytes, the efficiency of each electrolyte is very similar. All electrolytes can purify graphite from 80% to over 93% within 24 h. The difference between the final purities of graphite is small enough to be considered within the experimental error. Therefore, the purification efficacy is not affected by the choice of electrolyte. Instead, when considering electrolytes, commercial feasibility, including the ability to recover the catalyst, increase the reaction rate, and / or the cost of the electrolyte, becomes particularly important. This requires an appropriate techno-economic analysis to determine the preferred electrolyte.

[0295] Purified graphite

[0296] In another form, the present invention provides purified graphite containing impurities selected from metals, metal oxides, and combinations thereof, wherein the impurity concentration of the purified graphite is less than 20% w / w.

[0297] In some embodiments, the impurity concentration of the purified graphite is less than 15% w / w, preferably less than 10%, preferably less than 5%, more preferably less than 3%, more preferably less than 2%, more preferably less than 1%, more preferably less than 0.1%, and most preferably less than 0.05% w / w.

[0298] In some embodiments, the purity of the purified graphite is greater than about 90%, preferably greater than 95%, more preferably greater than 98%, and even more preferably greater than 99%.

[0299] In some embodiments, the carbon purity of the purified graphite is greater than about 90%, preferably greater than 95%, more preferably greater than 98%, and even more preferably greater than 99%.

[0300] In some embodiments, the shape of the purified graphite can be, for example, flaky, spherical, needle-shaped, plate-shaped, linear, tubular, whisker-shaped, ball-shaped, nano-graphite, tube, wire, and combinations thereof.

[0301] In some embodiments, the average particle size (d 50) It can be about 10 nm to 350 μm, such as 10 nm to 1 μm, or 1 μm to 65 μm, or 100 nm to 10 μm. For example, the average particle size of the purified graphite of the present invention is about 10 nm to 1000 nm. For example, the average particle size of the purified graphite is 10 nm to 100 nm, or 50 nm to 250 nm, or 200 - 500 nm, or 500 - 1000 nm, or 400 - 750 nm, such as 10 nm, 50 nm, 100 nm, 500 nm or 1000 nm. The average particle size of the purified graphite of the present invention is about 1 μm to 350 μm. For example, the average particle size of the purified graphite is 1 μm to 45 μm, or 40 μm to 60 μm, or 20 μm to 40 μm, or 30 μm to 50 μm, or 40 μm to 50 μm, or 40 μm to 60 μm, or 50 μm to 100 μm, or 100 μm to 250 μm, or 200 μm to 350 μm, or less than 300 or less than 200 or less than 100 or less than 65, less than 60, less than 55, less than 50, less than 45, less than 40, less than 35, less than 30, less than 25, less than 20, less than 15, less than 10 or less than 5 μm, or 350, 300, 250, 200, 150, 100, 85, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10, 5, 4, 3, 2 or 1 μm.

[0302] The surface area of the purified graphite of the present invention can be about 1 m 2 / g to about 1000 m 2 / g, about 1 - 700 m 2 / g, about 1 - 400 m 2 / g. For example, the purified graphite can be low - surface - area graphite with a surface area of 1 - 5 or 1 - 10 or 5 - 20 or 20 - 30 or 15 - 25 or 1 - 30 m 2 / g, such as 1, 2, 3, 4, 5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5 or 30 m 2 / g. Alternatively, the purified graphite can be high - surface - area graphite with a surface area of 50 - 400 m 2 / g, such as 100 - 150 or 100 - 200 or 50 - 200 or 150 - 250 or 200 - 375 or 250 - 350 or 300 - 400 or 100 - 400 m 2 / g, such as 50, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360, 380 or 400 m 2 / g. In some embodiments, the surface area of the purified graphite is less than 300 m 2 / g, or less than 200 m2 / g, or less than 100 m 2 / g. The surface area can be the N2 (NSA) BET surface area.

[0303] The crystallinity of the purified graphite of the present invention is preferably from about 60% to 99.9%. For example, the crystallinity of the purified graphite is preferably 60 - 80%, or 75 - 90%, or 85 - 99%, or 90 - 99%, or 95 - 99%, for example, the crystallinity is at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99% or at least 99.9%, for example, the crystallinity is 60%, 70%, 80%, 85%, 90%, 95%, 99% or 99.9%.

[0304] The resistivity of the purified graphite of the present invention is preferably less than about 1.0 Ω.cm, less than about 0.8 Ω.cm, less than about 0.5 Ω.cm, less than about 0.1 Ω.cm or less than about 0.05 Ω.cm. For example, the resistivity is 0.01 - 0.05 or 0.05 - 0.10 or 0.05 - 0.15 or 0.10 - 0.20 or 0.15 - 0.25 or 0.25 - 0.4 or 0.20 - 0.50 or about 0.4 - 0.65 or 0.50 - 0.75 or 0.75 - 1.0 or 0.01 - 1 Ω.cm. For example, the resistivity of the purified graphite can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1.0 Ω.cm.

[0305] The carbon content of the purified graphite of the present invention is preferably from about 60% to 99.9%. For example, the carbon content of the purified graphite is preferably 60 - 80% or 75 - 90% or 85 - 99% or 90 - 99% or 95 - 99%. For example, the carbon content is at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99% or at least 99.9%, for example, the carbon content is 60%, 70%, 80%, 85%, 90%, 95%, 99%, 99.9% or 99.95%.

[0306] As will be understood by those skilled in the art, before and after treatment, any suitable technique known in the art can be used to determine the purity of the graphite for the method of the present invention. For example, a suitable technique for measuring carbon purity is thermogravimetric analysis (TGA) or atomic emission spectroscopy (e.g., inductively coupled plasma atomic emission spectroscopy, which can also determine the amount of impurities).

[0307] Example 1: Producing Synthetic Graphite Containing Impurities

[0308] Iron oxides were used as catalysts for the decomposition of methane to produce graphite. Two high-grade iron oxides were used: hematite (99%, <5 μm, Sigma-Aldrich) and magnetite (95%, <5 μm, Sigma-Aldrich); and two iron ore samples: hematite ore (Pilbara ore) and goethite ore (Yandi ore). The ore samples were milled to less than 150 μm but were not otherwise treated. The "as received" composition data, particle size distribution, and surface area of all samples are shown in Table 3.

[0309] Table 3: Composition, particle size, and surface area data of iron oxide samples

[0310]

[0311]

[0312] Each sample was placed in a separate single-stage reactor. The reactor was a vertical stainless steel (SS316 Swagelok) tube with a diameter of 1 / 2" (1.27 cm), lined with a 3 / 8" (0.95 cm) quartz tube. The quartz tube lining reduced the catalytic effect of the stainless steel reactor wall by limiting contact with the reactive methane gas. The catalyst sample (20 g) was contained in a 3 / 8" "test tube-shaped" quartz chamber.

[0313] Each sample was reacted in the temperature range of 750 - 950 °C, using pure methane (UHP) at 10 cm 3 / min, and the reaction pressure was 1 - 9 bar (absolute pressure). After complete deactivation (about 19 h), the reaction was terminated, and the sample was cooled with pure nitrogen (UHP) at 20 cm 3 / min. The resulting carbon in the form of graphite (as well as the embedded catalyst particles as impurities) was weighed to determine the total carbon yield per gram of iron catalyst used.

[0314] The low-grade iron ore samples performed almost as well as the high-grade oxides in producing graphite, with a carbon yield of 9.2 - 8.9 g per gram of iron, corresponding to carbon purities of 90 wt% to 89 wt% respectively.

[0315] Example 2: Apparatus for Purifying Graphite

[0316] An electrochemical electrolytic cell as shown in Figure 1, for example, is used. The electrolytic cell includes a permeable membrane (e.g., a belt filter cloth) that separates the anode and the cathode. Fresh catholyte (i.e., catholyte that has not been used in the purification process) is continuously pumped through the cathode made of stainless steel. A mixture of fresh anolyte and graphite slurry containing impurities is pumped through the anode made of lead alloy, and the mixture is continuously stirred using a stirrer. The catholyte and the anolyte can be the same or different.

[0317] The cathode consists of three stainless-steel plates, and the anode consists of four lead-alloy plates. The graphite slurry containing impurities is restricted from contacting the cathode by the permeable membrane. The catholyte and the anolyte use a ferrous sulfate solution (0.1 M). During the electrochemical treatment process, the iron ions formed through redox reactions can permeate through the permeable membrane and form insoluble iron hydroxide particles with the elemental iron on the cathode. To prevent the formation of "dead" zones on the cathode and the anode, the electrolytic cell is equipped with an inlet diverter and an outlet diverter.

[0318] A water bath is used to maintain the temperature of the catholyte and the anolyte at about 70 - 80 °C. A positive voltage of +10 V is applied to the anode. Then, as shown in Figure 1, the purified graphite is collected, and the anolyte and the iron particles are collected together with the catholyte.

[0319] Figure 2 The block diagram of an electrochemical purification device is shown. This device can process commercial production capacities as needed, such as 1 ton, 5 tons, 10 tons, 15 tons, or 20 tons of graphite products per day, with a target purity greater than 99.5%. As described above, the graphite is transported and processed in slurry form, and the system alternately uses two electrochemical electrolytic cells for batch processing. In this system, the two electrolytic cells share filtration equipment. The precipitated or removed iron does not remain on the graphite.

[0320] Figure 3 An alternative configuration, i.e., the flowchart of a commercial-scale graphite purification device, is shown. For example, this system is designed to process up to 1 ton of graphite products per day.

[0321] Figure 4 Another configuration is shown. A description of the main unit operations of the ECP method (ECPP) is provided below.

[0322] Reagent Preparation

[0323] Solid ammonium sulfate is unloaded from bulk bags into a 10 m 3 stirred mixing tank that contains fresh Perth Tap Water (PTW) for dissolving the ammonium sulfate.

[0324] 15 m 3The ammonium sulfate tank will be filled from the mixing tank as needed and will fill the reagent adjustment tank as required. Then, the reagent adjustment tank will feed the pre-leaching tank 1 at the required ammonium sulfate feeding rate.

[0325] Graphite feeding and pre-leaching unit operations

[0326] It is expected that graphite will be transported from the Hazer process to a hopper of 10 m 3 which contains a safety / barrier grid of 20 mm x 20 mm and is designed to remove any waste that may be transferred from the Hazer process. The hopper of 10 m 3 is covered to minimize the entry of moisture and contaminants.

[0327] Then, the graphite is screw-fed into the feed vortex mixer, which is designed to fully wet the graphite with recycled process water before feeding the graphite into the first pre-leaching tank.

[0328] The ammonium sulfate solution is passed from the ammonium sulfate tank of 15 m 3 through an in-line solution heater to increase the temperature to about 70 °C and then fed into the pre-leaching tank 1. During the pre-leaching circuit test, due to insufficient heat from the acid addition reaction, heat needs to be added to the circuit to keep the slurry at the target temperature.

[0329] Nitric acid is also fed into the pre-leaching tank 1 at the required flow rate. Nitric acid is needed to prevent graphite from foaming. Graphite foaming causes significant material handling problems, so without the addition of nitric acid, the graphite slurry will not flow effectively through the pre-leaching tank.

[0330] The slurry will flow from the pre-leaching tank 1 to the pre-leaching tank 3 by gravity and then be pumped into the ECPP circuit. The temperature of the pre-leaching circuit will be maintained by an immersion heater (designed with titanium to withstand the acidic environment and set at a set point of 70 °C). Each pre-leaching tank will be stirred using an overhead stirrer to keep the slurry evenly suspended.

[0331] Each tank will be covered and provided with an outlet to the top of the building to remove the fumes generated by the process.

[0332] ECPP electrolyzer operation

[0333] Each ECPP electrolyzer contains a single anode in the central region, separated by a filter cloth that acts as a membrane allowing only solution and ions to pass between the inner region and the two outer regions. Each outer region contains a cathode. The slurry in the central anode region is suspended by two overhead stirrers (one on each side of the anode) to keep the slurry in this region homogeneous. A positive charge is applied to the central anode region, while a negative charge is applied to each cathode. Each electrolyzer is covered and provided with an outlet leading to the top of the building to remove the fumes generated by the process.

[0334] The slurry from the pre-leaching circuit is discharged into the central anode region of ECPP electrolyzer 1, and the liquid containing precipitated iron (iron slurry) from ECPP electrolyzer 2 is fed into the two outer regions of ECPP electrolyzer 1. Sulfuric acid is also pumped into the anode region of ECPP electrolyzer 1 at a controlled rate.

[0335] The iron slurry at the cathode of ECPP electrolyzer 1 is pumped to an iron precipitate thickener, while the slurry in the inner anode region is pumped to ECPP electrolyzer 2 at a controlled rate (the same as the pre-leaching discharge rate).

[0336] The slurry in the anode region is again pumped to the subsequent ECPP electrolyzer at a controlled rate until it is pumped from ECPP electrolyzer 4 into the graphite thickener. The purpose of pumping the slurry in the anode region to the back of the circuit is to reduce the iron concentration in the downstream graphite filter and subsequent iron washing requirements.

[0337] Ammonium sulfate solution, sulfuric acid, and nitric acid are introduced into the recycle conditioning tank together with recycled process water. The recycled process water is pumped into the two cathode regions of ECPP electrolyzer 4. Due to the increase in pH in the cathode region, the vast majority of the iron expected to be dissolved in the solution precipitates in the form of ammonium iron sulfate. ECPP tests have shown that due to hydrogen evolution at the cathode, the pH in the outer cathode region increases, subsequently causing iron to precipitate. The iron slurry from each cathode region is pumped to the previous ECPP cathode region until, as described above, the iron slurry is discharged into the iron precipitate thickener.

[0338] Graphite thickener

[0339] The anode discharge slurry from ECPP electrolyzer 4 is fed into the graphite thickener. The graphite thickener introduces a predetermined amount of flocculant to cause the graphite slurry to settle and produce a slurry with an increased solids content, which is pumped from the bottom of the thickener as a thickened slurry (underflow) to the graphite filter. The thickener overflow liquid, which will contain a low total suspended solids concentration, is discharged into the process liquid collection tank.

[0340] Graphite filter

[0341] The underflow of the graphite thickener will be pumped into the agitation buffer tank and then into the plate and frame vacuum filter, where it is filtered into a filter cake. After filtration, it is washed with a 5% sulfuric acid solution to remove any dissolved iron inherent in the liquid contained in the filter cake. Then it is washed with PTW to increase the pH of the liquid within the filter cake and remove any inherent sulfuric acid. The initial filtrate and the two washing solutions are fed into a water collection tank for recycling, with most of it being recycled for use in the Hazer ECPP circuit.

[0342] The washed filter cake is fed by screw into the graphite flash dryer and then discharged into a hopper.

[0343] Graphite flash dryer and bag filter

[0344] Graphite will be fed into the graphite flash dryer to produce a dried product, which will be discharged into the bag filter. The bag filter will pack the dried graphite into 1m 3 big bags, and then the big bags will be transported to the bag filter storage area.

[0345] Iron precipitate thickener

[0346] The iron slurry discharged from the anode of ECPP electrolyzer 1 will be fed into the iron precipitate thickener. Iron precipitate will be added together with a predetermined amount of flocculant to cause the iron slurry to settle and produce a slurry with increased solid content, which will be pumped as a thickened slurry (underflow) from the bottom of the thickener to the iron precipitate filter. The overflow liquid of the thickener will contain a low total suspended solids concentration and will be discharged into the process liquid water collection tank.

[0347] Iron precipitate filter

[0348] The underflow of the iron precipitate thickener will be pumped into the agitation buffer tank and then into the plate and frame vacuum filter, where it is filtered into a filter cake. The filtrate will enter the water collection tank for recycling, with most of it being recycled for use in the Hazer ECPP circuit.

[0349] The filter cake will be fed by screw into the flash dryer and then discharged into a hopper.

[0350] Iron precipitate flash dryer and bag filter

[0351] The iron precipitate filter cake will be fed into the flash dryer to produce a dried product, which will be discharged into the bag filter. The bag filter will pack the dried iron precipitate into 1m 3 big bags, and then the big bags will be transported to the bag filter storage area.

[0352] Example 3: Determining Carbon Purity by Thermogravimetric Analysis

[0353] The carbon purity was determined using thermogravimetric analysis (TGA) of purified graphite powder. The TGA analysis of purified graphite was carried out in two steps. In the first step, the temperature was raised to 100 °C and held for 15 min to remove moisture, and then the temperature was gradually increased to 900 °C in air to ablate any carbon. The weight loss occurred in two stages: removal of functional groups and then weight loss due to carbon ablation. The relative weight of the remaining material (equivalent to impurities) can be used to determine the carbon purity.

[0354] Example 4: Electrochemical Purification of Graphite

[0355] Impure graphite (5.0 g) (e.g., produced as in Example 1) was compacted in a dialysis bag (3.5 kDa MWCO), and a platinum wire was inserted, which served as a current collector. The dialysis bag was sealed at both ends with plastic clips. The graphite in the dialysis bag served as the working electrode (anode), and a graphite rod served as the counter electrode (cathode).

[0356] The working electrode and the counter electrode were immersed in 2 L of an aqueous 0.1 M (NH4)2SO4 solution. A positive voltage (+10 V DC) was applied to the working electrode for 24 hours. A photograph of the electrochemical device is shown in Figure 5a as shown, and a schematic diagram of the device is shown in Figure 5b as shown.

[0357] The initial purity of the impure graphite used was 80.4%. The electrochemically purified graphite was washed and centrifuged several times (at least three times) with deionized water to remove any residual salts from the electrolytic solution. The subsequently dried powder was used for characterization.

[0358] The carbon purity of the purified graphite was determined using thermogravimetric analysis (TGA). Figure 6 The weight loss curve shown indicates that approximately 2% of the weight loss is attributed to the moisture content of the electrochemically treated graphite powder. There is no significant weight loss before the temperature reaches approximately 500 °C, which may indicate a very low degree of functionalization during the electrochemical process. The rapid weight loss around 600 °C is attributed to carbon ablation, and the remaining weight is residual iron present in oxide form. After 24 hours of electrochemical treatment, the carbon purity of this batch of electrochemically treated graphite was calculated to be 93.2% (by weight).

[0359] As described above, the TGA analysis before and after electrochemical treatment shows that after 24 hours of electrochemical treatment, the carbon purity increased from 80.4% (by weight) of the impure unprocessed graphite to 93.2% of the purified graphite.

[0360] Another batch of electrochemical treatment was also carried out over a 48-hour period. In this batch, the electrolyte was replaced every 24 hours. Figure 7Shows the electrochemical treatment of graphite with impurities over a 48 - hour period. As shown in the photo "Day 1; Start", the beaker initially contained a clear solution of (NH4)2SO4, and its color turned yellow after two hours ("Day 1; 2h later").

[0361] At the end of the first day ("Day 1; 24h later"), the reaction bath containing the electrolyte was filled with a red / orange sludge, which began to accumulate and settle at the bottom of the beaker. Similar results were observed during Day 2, and the reaction bath again began to produce aggregated red sludge.

[0362] As described above regarding the electrochemical reaction of aqueous ammonium sulfate solution, the inventors believe that water - soluble iron(III) sulfate reacts with ammonium hydroxide in the solution to produce insoluble iron(III) hydroxide (Fe(OH)3), which is the reddish - brown precipitate.

[0363] After 48 hours of electrochemical treatment, the graphite was analyzed by TGA to determine the graphite purity, and the results are as Figure 8 shown. The graphite purity increased from 80.2% of the as - received graphite with impurities to 93.2% after the first day ( Figure 6 ), and then to 96% after the second day ( Figure 8 ), which demonstrates the effectiveness of the method of the present invention for purifying graphite.

[0364] Surprisingly, the inventors also found that the method of the present invention does not affect the structure or morphology of the graphite. SEM micrographs of the graphite before and after electrochemical treatment are shown in Figure 8 a and Figure 8 b respectively, where the intact CNO and ANF structures can be seen after treatment.

[0365] Back - scattered SEM micrographs of the treated graphite were also taken to check for iron removal from the graphite core; the results are as Figure 10 shown. The image reveals many regions of CMS and CNO morphology without an iron core, while some CNOs still have residual iron (circled). Even after 48 hours of electrochemical treatment, the structure of most of the purified graphite remains undamaged, which is an unexpected result.

[0366] In another test batch, it was found that the purity of the purified graphite increased from an initial graphite purity of 90.2% to 99.6% after 48 hours of electrochemical treatment.

[0367] In yet another test batch, based on an initial graphite purity of 85%, it was found that the purity of the purified graphite was 99.5% after 96 hours of electrochemical treatment, where the impurity was iron.

[0368] Other experimental data are shown in Table 4 below.

[0369] Table 4: Purity of purified graphite after electrochemical treatment

[0370]

[0371]

[0372] The results shown in Table 4 above confirm that a graphite purity of 99.5% can be obtained using the purification method of the present invention.

[0373] Example 5: Electrochemical Purification Using Alternative Electrolytes

[0374] In Example 5, graphite was purified using the method of Example 1, and an aqueous solution of ferrous sulfate was used as the electrolyte.

[0375] The inventors unexpectedly found that when using an aqueous solution of ferrous sulfate (FeSO4), the insoluble iron species Fe(OH)3 does not contaminate the purified graphite, which would otherwise affect the purity of the graphite (or require expensive treatment, such as acid digestion after electrochemical treatment).

[0376] The inventors observed that due to the use of ferrous sulfate electrolyte, the graphite purity was increased. In addition, there was no obvious precipitated iron in the graphite.

[0377] As Figure 11 shown, using ferrous sulfate as the electrolyte forms elemental iron dendrites on the cathode, rather than iron oxides as expected. The iron dendrites are valuable by-products and can be used as catalysts, for example, in the method of forming graphite, such as the method described in Example 1.

[0378] Example 6: Electrochemical Purification Compared with Standard High-Temperature Heat Treatment

[0379] To compare the purity of graphite materials purified via the electrochemical method (ECP) of the present invention, graphite carbon materials were purified by standard high-temperature heat treatment and the electrochemical method (ECP), which were produced via catalytic decomposition of methane (CDM) using Fe ore as a catalyst.

[0380] Material synthesis

[0381] Specifically, the graphite carbon material was synthesized in a fluidized bed reactor that was operated with methane at 900 °C and 8 bar (gauge pressure). Here, methane was decomposed via "dusting" into graphite and hydrogen on the surface of an iron ore catalyst. The term "dusting" is an industrial term used to describe the reaction in a carburizing environment that disintegrates metallic materials (usually iron-based materials) into fragments and graphite. This effect begins with the adsorption and dissociation of methane molecules (or other carbon-containing gases) on the surface of the metal-containing catalyst, and the resulting carbon diffuses into the surface of the bulk metal. Once the outer layer of carbon is saturated, metal carbides are formed and then precipitate in the form of graphite carbon from the metal grain boundaries. Over time, this generates intergranular pressure, separating the metal carbide particles from the parent bulk metal, leading to the disintegration of the metal structure due to "dusting".

[0382] During this period, the catalyst separated and fragmented into nano-sized pieces, which were encapsulated by carbon / graphite. The resulting graphite carbon material encapsulating Fe particles is hereinafter referred to as "Carbon-O". According to the standard thermal purification method, Carbon-O was purified in a vacuum furnace under a high-temperature treatment of up to 2800 °C to remove the encapsulated Fe particles. The resulting material is denoted as "Carbon-T". For comparison, Carbon-O was also purified according to the electrochemical method (ECP) of the present invention in an electrochemical cell (as Figure 5b shown). Here, Carbon-O was compressed into carbon rods and subsequently used as the positive working electrode in the electrochemical cell. In a 0.1 M ammonium sulfate ((NH4)2SO4) electrolyte, a platinum foil was used as the negative counter electrode, and a direct current of 10 V was applied to both electrodes. The charged ions in the electrolyte intercalated between the graphene layer carbon materials in the carbon rods, and the encapsulated Fe particles were slowly leached out within 20 h. The resulting carbon material in the carbon rods was collected and denoted as "Carbon-E".

[0383] In addition to the above three types of graphite carbon materials, a commercial carbon conductive additive (Super P, ThermoFisher) was also used as a reference.

[0384] The morphology of the carbon materials was examined by scanning electron microscopy (SEM, Zeiss, Gemini Ultra Plus). The average particle size of the carbon materials dispersed in water was analyzed using a particle size analyzer (Malvern Mastersizer 3000). The surface area and pore structure were characterized by N2 physisorption using a pore size analyzer (Quantachrome Autosorb iQ). The pore size distribution was calculated based on their N2 physisorption isotherms using the density functional theory (DFT) method. Their chemical properties were characterized by Raman spectroscopy (Renishaw Raman inVia Reflex) with a 532 nm excitation laser. Under flowing gas conditions, the chemical composition was studied by thermogravimetric analysis (TGA) on a thermogravimetric analyzer (TA Instruments Q500) with a heating rate of 15 °C min -1 from 25 °C to 900 °C. The elemental composition of the ash obtained after TGA was characterized by XRF using a wavelength-dispersive X-ray fluorescence spectroscopy (XRF) spectrometer (PANalytical AXIOS, PW2400) equipped with a 4-kW X-ray source.

[0385] Electrode fabrication and characterization

[0386] First, MnO2 (electrolytic manganese dioxide (EMD)) was homogenized in a planetary ball mill (Changsha Tianchuang PowerTechnology XQM-0.4) at 400 rpm for 5 h.

[0387] EMD is one or more of α-, β-, γ-, δ-, or λ-MnO2.

[0388] In some embodiments, EMD consists mainly of α-, β-, γ-, δ-, or λ-MnO2.

[0389] Good results were obtained when γ-MnO2 was used as EMD.

[0390] Then, γ-MnO2, a carbon conductive additive, and a binder were mixed together in a suitable solvent.

[0391] In some embodiments, the binder is in the form of a fluoropolymer binder, which may be selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy polymer (PFA), and polyvinyl fluoride (PVF). In other embodiments, the binder may be selected from carboxymethyl cellulose (CMC), sodium alginate, starch, styrene-butadiene rubber (SBR), xanthan gum, polyvinyl chloride (PVC), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyamideimide (PAI).

[0392] In some embodiments, the EMD, carbon conductive additive, and binder are mixed together at a weight ratio of 4 - 9:2:1, preferably at a weight ratio of 7:0.1 - 3:1, more preferably at a weight ratio of 4 - 9:0.1 - 3:0.1 - 3.

[0393] In one embodiment, the EMD, carbon conductive additive, and binder are mixed together at a weight ratio of 7:2:1.

[0394] Good results were obtained by using polyvinylidene fluoride (PVDF) as the binder and mixing γ-MnO2, carbon conductive additive, and polyvinylidene fluoride (PVDF) binder at a weight ratio of 7:2:1 in N-methyl-2-pyrrolidone (NMP).

[0395] Four carbon materials (Super P, Carbon-O, Carbon-T, and Carbon-E) were used as the conductive additive respectively. Then the obtained slurry was cast onto a conductive metal foil using a doctor blade method to form at least a partial coating on the conductive metal foil, and the thickness of the coating was about 1 micron to about 25 microns. For example, the thickness of the coating can be about 1, 2, 3, 4, 5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, or 30m 2 / g, about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or about 25 microns.

[0396] In some embodiments, it is preferred to cast the obtained slurry onto a conductive metal foil to form a coating around the conductive metal foil, and the thickness of the coating is about 5 microns to about 20 microns, more preferably about 7 microns to about 15 microns.

[0397] Suitable conductive metal foils may be selected from the group of conductive metal foils consisting of: titanium, copper, zinc, aluminum, iron, or any mixture thereof.

[0398] Good results were obtained when the slurry was cast onto a titanium foil with a thickness of about 10 μm and then dried in a vacuum furnace at 80 °C for 12 h.

[0399] The mass loading of the active electrode material on the Ti foil was approximately 2.5 mg cm -2 . Finally, the Ti foil coated with the electrode material slurry was cut into small pieces with a size of 1.1 cm 2 by a stamping machine.

[0400] The in-plane conductivity of the EMD cathodes fabricated using different carbon materials was examined by a four-point probe sheet resistance meter (Guardian SRM-232). The conductivity (σ, S m S ) was calculated by σ = 1 / R -1 t, where R S (Ω / sq) is the sheet resistance and t is the thickness of the electrode material. The electrolyte absorption capacity of the carbon conductive additive was measured by immersing a carbon electrode (without EMD) fabricated using a carbon material (90 wt.%) and PVDF (10 wt.%) into the same volume of 1 M zinc sulfate (ZnSO4) solution and recording the weight gain as a function of time.

[0401] Zn-C battery assembly and characterization

[0402] Using the prepared EMD cathode, Zn metal foil (1.1 cm 2 ) as the anode, and 1 M ZnSO4 solution as the electrolyte, a button cell (type 2032) was assembled to evaluate its electrochemical performance. Galvanostatic charge / discharge (GCD) in the voltage range of 1.5 - 0.7 V and electrochemical impedance spectroscopy (EIS) measurements in the frequency range of 10 6 –0.008 Hz were collected using an electrochemical workstation (CHI 760D). The specific capacity of the battery was calculated based on the mass of the EMD in the cathode. Using a battery test system (LANHE), the assembled battery was tested by galvanostatic intermittent titration technique (GITT), where a series of galvanostatic discharge pulses of 120 s were performed at 50 mA g -1 followed by a 4-h rest. During a one-month rest period, the long-term stability of the battery was tested using a battery test system by continuously recording the open-circuit voltage (OCV). All electrochemical tests were carried out at room temperature.

[0403] Example 7: Summary of ECP Results

[0404] Table 5: Summary of ECP results

[0405]

[0406]

[0407]

[0408] Example 8: Further Study of the ECP Method

[0409] To explore the possible influence of different operating conditions on the results of the ECP method, further research was conducted. The research was carried out for two days, using 20 g of graphite (purity 66.54%, original sample), 0.1 M (unless otherwise specified) (NH4)2SO4 as the electrolyte, a black cloth filter bag, two Pt-coated Ti electrodes, and 10 V (unless otherwise specified).

[0410] · Electrolyte stirring

[0411] The graphite purity of the ECP method with electrolyte stirring (Study 1a) was 96.2% compared to the ECP method without stirring (Study 1b).

[0412] · Bag compression

[0413] The graphite purity of the ECP method with compression of the black cloth filter bag was 96.2% compared to the ECP method without compression (Study 2).

[0414] · Voltage

[0415] The graphite purity of the ECP method operating at 7 V (Study 3a) was 94.2%. The purities of the ECP methods operating at 10 V and 20 V (Study 3b) were 96.2% and 98.8% respectively.

[0416] · Electrolyte concentration

[0417] For an electrolyte concentration of 0.1 M, the graphite purity was 96.2%. For the methods operating at 0.5 M (Study 4a) and 1.0 M (Study 4b), the graphite purities were 99.13% and 99.26% respectively.

[0418] · Electrode distance

[0419] When the electrode distance was 5 cm (Study 5), the graphite purity was 99.13%. When the electrode distance was 10 cm, the graphite purity was also 99.13%.

[0420] · Electrode size

[0421] For an electrode size of 1.5 cm x 10 cm x 25 μm, the graphite purity was 99.13%. For an electrode size of 2.5 cm x 10 cm x 25 μm (Study 6), the graphite purity was 99.27%.

[0422] · Reaction duration

[0423] Samples were collected regularly and analyzed. The results are shown in Table 6 below. This study found that for this particular original sample, the vast majority of purification occurred within the first 6 hours.

[0424] Table 6 : Purity of graphite at different reaction times

[0425]

[0426] · Influence of operating conditions on graphite properties.

[0427] The morphology of the samples was examined by scanning electron microscopy (SEM, Zeiss, HD). From Figure 16 it can be seen that the raw carbon material (purity 66.54%) is a mixture of micron-scale structures with various irregularities on the surface, including carbon nano-onions (CNO), carbon nanotubes (CNT), and micro-carbon shells (MCS). The bright spots observed in the FESEM images are Fe particles, most of which are coated with graphite carbon from the catalytic decomposition reaction of methane.

[0428] The morphology of the samples obtained from the above study was investigated by FESEM. As Figure 17 shown, compared with the original morphology, the obtained morphology did not show any obvious changes, except that the bright spots representing Fe particles were sparser.

[0429] According to the wet PSD operating instructions (Malvern Mastersizer 3000), the average particle size of the carbon materials dispersed in ethanol (5 mg / mL) was analyzed using a particle size analyzer. As shown in Table 7, the average particle size distribution of all ECP-treated samples was relatively smaller compared to the original samples.

[0430] Table 7 : Particle size distribution obtained from all samples using wet PSD via Malvern Mastersizer 3000

[0431]

[0432] The surface area and pore structure of the raw samples and ECP-treated samples were characterized by N2 physical adsorption using a pore size analyzer (Quantachrome Autosorb iQ). In the ECP-treated samples, changes in the operating parameters showed significant changes in the final purity level. Their pore size distributions were calculated using the density functional theory (DFT) method based on the N2 physical adsorption isotherm. The results of all tested samples are as Figure 18 a and Figure 18 b shown. Their BET specific surface areas ranged from 15.47 - 34 m 2 g -1 varying, following the original <Study 6 <Study 4b <Study 3b <Study 1b. The pore size distributions determined by the DFT method are as Figure 18As shown, the range is from micropores (<2 nm) to macropores (>10 nm). It is worth noting that after electrochemical purification, the surface area increased due to the expansion and exfoliation of the graphite layers.

[0433] Finally, under Cu-Kα radiation, the effect of ECP treatment on the structural changes of graphite was investigated using X-ray diffraction (XRD) patterns via a diffractometer (Pananalytical Flat Plate powder XRD). The XRD pattern of the original sample is as Figure 19a shown, with multiple diffraction peaks. The strongest peaks at 2θ = 26.38° and 2θ = 44.72° are from graphite carbon and metallic iron. In addition, several other diffraction peaks from metallic Fe and Fe3C were observed. The strongest Fe peaks are from metallic Fe (2θ = 44.67, 65.02, and 82.33°, JCPDS No. 06–0696) and Fe3C (2θ = 37.64, 37.76, 42.89, 44.57, and 45.00°, according to JCPDS No. 89–7271). Figure 19c and Figure 19d The XRD pattern of the ECP-treated sample shown in

[0434] Results and discussion

[0435] Properties of carbon materials

[0436] Figure 20 The SEM images in Figure 21 show the morphologies of four carbon materials (Super P, Carbon-O, Carbon-T, and Carbon-E). Super P exhibits a powdery, fine-grained morphology with a nanoscale particle size, different from the other three graphite carbon materials synthesized by CDM. These carbon materials are mainly composed of micron-sized agglomerate structures with many irregularities on their surfaces. Carbon-O shows a relatively rough surface morphology, while Carbon-T and Carbon-E have a relatively smooth surface formed by irregularly shaped particles composed of carbon flakes. Their average particle sizes were analyzed using a particle size analyzer. As 50 shown, when dispersed in an aqueous solution, Super P, Carbon-O, and Carbon-T all have similar median diameters (D 50 ), ranging from 9.1 to 12.4 μm. In contrast, Carbon-E is smaller in size, at approximately 1.9 μm.

[0437] The specific surface area and pore structure of the carbon materials were characterized by N2 physical adsorption. Figure 22 Figure a shows their N2 physical adsorption isotherms. Their BET specific surface areas range from 19 to 37 m 2 g -1Not equal, following Carbon-T < Carbon-O < Carbon-E. The specific surface area of Super P is much larger, being 78 m 2 g -1 . The pore size distribution determined by the DFT method is as shown in Figure 22 b. Super P has the largest pore volume, which is 1.83 cm 3 g -1 , with an average pore size of 36 nm, indicating its mesoporous nature. Carbon-O and Carbon-E exhibit a wide pore size distribution from micropores to macropores. The pore volumes and average pore sizes of Carbon-O and Carbon-E are similar, being 0.13 cm 3 g -1 , 3.8 nm and 0.15 cm 3 g -1 , 2.9 nm respectively, indicating that the electrochemical purification method did not significantly change the porous structure of Carbon-O. In contrast, the pore size distribution of Carbon-T is concentrated at approximately 30.1 nm and it has a relatively large pore volume of 0.30 cm 3 g -1 . The disappearance of micropores and the increase in pore volume in Carbon-T may be related to the complete removal of metal residues encapsulated in the carbon and the recombination and closure of micropores at high temperatures.

[0438] First, the chemical structures of the carbon materials were analyzed by Raman spectroscopy. Figure 22 c shows their Raman spectra, which have three typical characteristics of carbon materials: the G band corresponds to the zone center vibration of carbon atoms in the graphene plane, the D band is related to the disorder of the graphite lattice, and the G' band is related to the number of graphene layers and their stacking order. Super P only shows the D band and the G band, indicating the lack of a long-range graphite structure. The intensity ratio of the D band and the G band (I D / I G ) may be related to the defects, structural vacancies, and surface functional groups of the carbon materials. The I D / I G of Super P is 1.18, higher than 0.73 of Carbon-O, 0.84 of Carbon-E, and 0.29 of Carbon-T. These results indicate that due to the high-temperature treatment conditions, Carbon-T has the lowest defect level. In contrast, due to the exfoliation of graphene layers during the purification process, Carbon-E has a relatively high defect concentration. Super-P has the most defective structure.

[0439] Further examinations of their chemical structures and metal residues were carried out by TGA. Figure 22 d and its inset show their weight loss curves in air when the temperature is increased from 25 °C to 900 °C (for Carbon-T, from 25 °C to 1040 °C due to its higher thermal stability). The derivative thermogravimetry (DT) curves are as shown in Figure 23As shown, the main weight loss of Carbon-T occurs at 882 °C, the highest temperature, indicating the highest level of graphitization. The main weight loss temperature of Super P is 776 °C, which is also higher than that of Carbon-E and Carbon-O. The main weight loss temperature of Carbon-O is the lowest, at 632 °C, which is attributed to the catalytic carbon oxidation decomposition by Fe residues. In addition, Carbon-E has some weight loss starting from 48 °C, which is due to the formation of volatile components during its electrochemical purification process. Carbon-T has the lowest ash content, at 0.18 wt.%, indicating the highest carbon purity, at 99.82 wt.%. The purities of Carbon-E and Super P are also high, at 99.59 wt.% and 99.47 wt.% respectively. Carbon-O has the highest ash content, at 31.10 wt.%. The chemical composition of the ash residue obtained after TGA was analyzed using XRF.

[0440] As shown in Table 8, Fe2O3 accounts for 95.32 wt.% of the ash, formed by the oxidation of iron residues encapsulated in the carbon material. The purity of Carbon-O is estimated to be 78.25%.

[0441] Table 8 : XRF analysis of the ash composition of Carbon-O used for TGA testing after combustion at 900 °C (left), and XRF analysis of the Fe ore catalyst used for the CDM method (right).

[0442]

[0443] Properties of the fabricated electrodes

[0444] Different carbon materials were used as conductive additives to fabricate EMD electrodes at the same mass ratio. The conductivity of the fabricated electrodes can be affected by various factors, such as the graphitic structure of the carbon additive, its particle size, surface area, and porosity.

[0445] The inventors observed that the in-plane conductivity of the EMD electrodes fabricated using these materials is approximately 70 S / m -1 to approximately 100 S / m -1 . For example, the in-plane conductivity of these specific EMD electrodes can be approximately 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or approximately 100 S / m -1 . In some embodiments, the in-plane conductivity of these specific EMD electrodes is preferably approximately 80 S / m -1 to approximately 95 S / m -1 , more preferably approximately 90 S / m -1 .

[0446] For example, as shown in Table 9, the in-plane conductivity of the EMD electrode fabricated using Super P is the lowest, at 72 S / m -1 , which may be attributed to its porous and defective structure. In contrast, the conductivities of the EMD electrodes fabricated using two purified carbon materials (i.e., Carbon-T and Carbon-E) are similar, at 98 S / m -1 and 90 S / m -1 , respectively. As shown by the Raman results, the higher conductivity is associated with its more graphite characteristics. The pore size difference between Carbon-T and Carbon-E does not seem to significantly affect the conductivity. The larger surface area of Super P also does not bring a favorable impact on the conductivity of the electrode. The conductivity of the electrode fabricated using Carbon-O is the highest, at 143 S / m -1 , given that the I D / I G of Carbon-O is relatively high, and the specific surface area and average pore size are similar to those of Carbon-E, which may be related to its large amount of Fe residues.

[0447] Table 9 : Summary of the physicochemical properties of carbon materials, the conductivities of EMD / carbon electrodes, and the electrolyte absorption capacity of carbon electrodes.

[0448]

[0449] Another essential function of the carbon conductive additive is to absorb and retain the electrolyte to maintain a stable solid-liquid interface in the electrode. However, an extremely high electrolyte absorption capacity will consume more electrolyte, thus increasing the battery manufacturing cost. The electrolyte absorption capacity is affected by the pore volume, pore size, and surface area of the carbon additive. Figure 24 shows the absorption of electrolyte by different types of carbon materials (unit: mg electrolyte / mg carbon) within 5 min when the carbon electrode is immersed in 1 M ZnSO4 solution. The specific surface area and pore volume of Super P are much larger, resulting in faster electrolyte absorption and the highest absorption capacity, at 10.94, which is approximately 6 times that of Carbon-O (1.84 after 5 min). The electrolyte absorption capacities of Carbon-T (2.14) and Carbon-E (4.20) are both higher than that of Carbon-O.

[0450] Characteristics of assembled Zn / C batteries

[0451] Next, in a 1 M ZnSO4 aqueous electrolyte, Zn / C batteries were assembled using EMD cathodes and zinc metal anodes fabricated with different carbon conductive additives. Figure 25a -d shows at current densities of 1.0, 0.5, 0.1, and 0.05 A / g -1Under the conditions, the galvanostatic discharge curves of the battery within the voltage window of 1.5 - 0.7 V are shown. All discharge curves are characterized by typical Zn-C battery discharge behavior, i.e., an ohmic voltage drop (IR drop) appears at the beginning of discharge, reaching a flat discharge plateau, and then the voltage drops to the cut-off voltage. At high current densities of 1.0 and 0.5 Ag -1 the discharge specific capacity of the Zn-C battery containing the graphite carbon material synthesized by CDM is significantly higher than that of the battery containing Super P. The carbon-T battery provides a specific capacity 1.4 times that of the Super P battery at 1.0 Ag -1 . In contrast, at a lower current density of 0.1 Ag -1 the observed difference is small. The specific capacity ranges from 99 to 10 4 mAh g -1 and follows the order of carbon-O battery < carbon-T battery < carbon-E battery < Super P battery. At the lowest current density of 0.05 Ag -1 the specific capacity of the carbon-E battery is typically about 50 mAh g -1 to about 200 mAh g -1 , preferably about 70 mAh g -1 to about 120 mAh g -1 , and more preferably about 109 mAh g -1 when measured between 1.5 V and 0.7 V at a current density of 0.05 Ag -1 . This result was compared with the specific capacities of the carbon-O and carbon-T batteries, which were measured to be 122 mAh g -1 and 114 mAh g -1 respectively, and are comparable to 124 mAh g -1 of the Super P battery.

[0452] Figure 25e The specific capacities at different discharge current densities were compared. As the discharge current density increases from 0.05 Ag -1 to 1.0 Ag -1 the capacity of all batteries decreases, especially the most significant decrease shown by the Super P battery. Figure 25f The Nyquist plot of the EIS spectrum of the Zn-C battery is shown. The intercept of the impedance curve with the real axis represents the ohmic resistance.

[0453] As Figure 25fAs shown in the illustration, the Super P battery exhibits a relatively large ohmic resistance, which is consistent with the lower in-plane conductivity measured for the EMD electrode. The difference in ohmic resistance among the carbon-T, carbon-E, and carbon-O batteries is negligible. In addition, based on the semicircles in the high-frequency region, the charge transfer resistance of the carbon-T, carbon-E, and carbon-O batteries is much lower than that of the Super P battery. Previous studies have reported that due to the aggregation of Super P particles, the carbon conductive additive network may not fully cover all EMD particles. In contrast, the other three types of carbon particles seem to fill the voids between EMD particles more effectively. The charge transfer resistance of the carbon-O battery is close to that of the carbon-T and carbon-E batteries, indicating that although the in-plane conductivity of the EMD electrode fabricated using carbon-O is high, the Fe residues in carbon-O have a negative effect on promoting electron transfer at the electrode / electrolyte interface.

[0454] Overall, the Zn-C battery fabricated using graphite carbon materials synthesized by CDM exhibits better performance at high discharge current densities than the battery fabricated using Super P, but comparable performance at low discharge current densities. The improvement in performance at high discharge rates is related to the higher conductivity of the carbon materials synthesized by CDM and the effective electron network formed in the battery.

[0455] We also used the GITT technique to study the voltage response of the Zn-C battery under intermittent operating conditions, which is more relevant to its actual working conditions. As Figure 26 shown in -1 a, all batteries were discharged at 0.05 A g Figure 26 for 120 s and then allowed to rest for 4 h, and this test cycle was repeated for 30 segments.

[0456] As Figure 26 shown in

[0457] Figure 26 d compared the long-term stability of Zn-C batteries fabricated using different carbon conductive additives over one month. The carbon-O battery exhibited the largest OCV drop of 0.037 V, which was attributed to the iron residues in carbon-O causing self-discharge or secondary reactions. In contrast, the carbon-T and carbon-E batteries showed OCV drops of 0.001 and 0.020 V, respectively, which were negligible and better than that of the Super-P battery at 0.014 V. The improvement in their stability indicated that both purification methods successfully removed Fe residues and avoided their adverse effects on the long-term performance of the batteries. After the stability test, all Zn-C batteries were discharged at 0.1 Ag -1 at a constant current to evaluate their discharge characteristics after long-term storage. Figure 27 showed that the discharge curves of the Zn-C batteries were similar, and when discharged to 0.7 V, the specific capacity was approximately 95 mAh g -1 .

[0458] Conclusion

[0459] Graphite carbon materials (carbon-O) were synthesized by CDM using iron ore as a catalyst, with H2 as a byproduct. Through purification by standard high-temperature heat treatment at 2800 °C (carbon-T) and an alternative electrochemical method (carbon-E), their carbon purity was increased from 78.25% to 99.82% and 99.59%, respectively. They were evaluated as conductive carbon additives for Zn-C batteries. The MnO2 cathodes fabricated using carbon-T or carbon-E at a mass ratio of 7:2 exhibited conductivities of 98 S cm -1 and 90 S cm -1 , and the electrolyte (1 M ZnSO4) absorption capacities of their carbon electrodes were 2.14 mg mg -1 and 4.20 mg mg -1 . The specific capacities of the Zn-C batteries assembled using carbon-T or carbon-E were 114 mAh g -1 and 109 mAh g -1 , respectively, which were comparable to those when using a commercial carbon conductive additive (Super P). Importantly, when the current density increased from 0.05 Ag -1 to 1.0 Ag -1When, they exhibit better rate performance because their graphite structure endows them with high electrical conductivity. They also show excellent performance under intermittent operating conditions and long-term stability tests because the effective removal of Fe residues prevents self-discharge behavior. There is no obvious difference between Carbon-T and Carbon-E, indicating that both purification methods are effective. The graphite carbon materials purified by CDM are expected to be effective carbon conductive additives for batteries, which in turn converts the solid waste of CDM into high-value-added commodities, thus improving the economic feasibility of the CDM-based H2 production method.

[0460] Main features of the present invention

[0461] Several important findings were obtained in this study and can be summarized as follows:

[0462] The ECP method is applicable to graphite slurries; the electrical conductivity of graphite slurries is lower than that of filled graphite; thus, the reaction rate is reduced; the electrode spacing of the ECP method affects the reaction rate and is therefore the main design consideration; the increase in voltage is roughly proportional to the increase in the reaction rate.

[0463] Regarding the feasibility of separating iron by-products: The iron by-products are iron salt complexes such as jarosite; the iron by-products are not easily separated from graphite by physical separation techniques such as centrifugation; as an alternative washing step, the iron by-products can be dissolved in H2SO4.

[0464] Regarding electrolytes: H2SO4 and FeSO4 were tested as alternative electrolytes; the performance of both alternative electrolytes is comparable to that of (NH4)2SO4; all electrolytes can purify the graphite in the pilot plant, with the initial purity being purified from 80% to over 93%; H2SO4 does not produce observable solid by-products, but it generates a large amount of H2 and O2 at the electrodes; when FeSO4 is used as the electrolyte, the iron in the electrolyte and graphite can be recovered at the cathode.

[0465] Finally, when measured between 1.5 V and 0.7 V at a current density of 0.05 Ag -1 , the discharge specific capacity of the Zn / C battery fabricated with the negative electrode including a metal foil substrate coated with the purified graphite material produced by the ECP method is at least about 109 mAh g -1 .

[0466] Those skilled in the art should recognize that, in addition to the specifically described content, other changes and modifications can be made to the present invention as described above. It should be understood that the present invention includes all such changes and modifications that fall within the spirit and scope of the present invention.

[0467] Future patent applications may be filed in Australia or overseas based on or claiming priority from this application. It should be understood that the following provisional claims are provided by way of example only and are not intended to limit the scope of the claims of any such future applications. Features may be added to or omitted from the provisional claims at a later date to further define or redefine the invention.

Claims

1. A method for purifying a graphite material, the method comprising: electrochemically treating a crude graphite material containing impurities selected from metals, metal oxides, and combinations thereof; using a predetermined electrolyte; for a predetermined period of time; within a predetermined voltage range; within a predetermined temperature range; using a predetermined anode composition; using a predetermined cathode composition; thereby removing a portion of the impurities by the electrochemically treatment and providing a purified graphite material.

2. The method according to claim 1, further comprising a predetermined semipermeable membrane to protect the cathode from short circuiting when contacting the graphite material.

3. The method according to claim 1 or 2, wherein the morphology of the purified graphite material is substantially the same as that of the crude graphite material, and wherein the morphology of the graphite material is selected from graphite fibers (including carbon nanotubes), carbon nano-onions, carbon microspheres, and graphene.

4. The method according to any one of the preceding claims, wherein the impurities are selected from metals, metal oxides, and combinations thereof.

5. The method according to claim 4, wherein the impurities are selected from iron, elemental iron, iron oxides, iron carbide species, and combinations thereof, and the iron carbide species are, for example, ferrite, austenite, or cementite.

6. The method according to any one of the preceding claims, wherein the electrolyte is ammonium sulfate, ferrous sulfate, nitric acid, sulfuric acid, or a mixture thereof.

7. The method according to claim 5 or 6, wherein the iron impurities are deposited on the cathode in the form of elemental iron, precipitated in the form of iron hydroxide or iron complexes, or retained in the solution.

8. The method according to any one of the preceding claims, wherein the voltage range is about 1 V to 300 V, preferably about 5 V to 300 V.

9. The method according to any one of the preceding claims, wherein the period of time is about 2 h to about 2 weeks, preferably about 2 h to 96 h.

10. The method according to any one of the preceding claims, wherein the temperature range is about 5 °C to about 100 °C.

11. The method according to any one of the preceding claims, wherein the cathode comprises one or more of metals, metal alloy plates, platinum, or titanium coated with platinum, and wherein the anode comprises graphite, lead, lead alloy, platinum, titanium coated with platinum, and combinations thereof.

12. The method according to claim 11, further comprising covering at least a portion of the anode, cathode, or both with a permeable membrane, wherein the permeable membrane is a neutral permeable membrane, an anion exchange membrane, or a cation exchange membrane, and wherein the molecular weight cut-off (MWCO) of the permeable membrane is less than about 1 million Da, preferably about 10 kDa to about 0.5 kDa.

13. The method according to claim 12, wherein the gas permeability of the permeable membrane at 200 Pa is 0.1-100 L / min / dm 2 .

14. The method according to any one of the preceding claims, wherein the purity of the purified graphite material is greater than about 95% w / w, preferably greater than about 99% w / w, more preferably greater than about 99.5% w / w, and even more preferably greater than about 99.95% w / w.

15. The method according to any one of the preceding claims, wherein the crude graphite material is slurried before the electrochemically treatment method.

16. The method according to any one of the preceding claims, wherein the rate of the purification reaction is proportional to the applied voltage.

17. A purified graphite material obtainable or obtained by the method defined in any one of the preceding claims.

18. A negative electrode material comprising a coating on a substrate, the coating comprising: MnO2 (electrolytic manganese dioxide (EMD)), a carbon conductive additive in the form of the purified graphite material according to claim 17, and a binder.

19. The electrode according to claim 18, wherein the EMD is one or more of α-MnO2, β-MnO2, γ-MnO2, δ-MnO2 or λ-MnO2.

20. The electrode according to claim 18 or 19, wherein the EMD, the carbon conductive additive and the binder are mixed together in a weight ratio of 4 - 9:2:

1.

21. The electrode according to any one of claims 18 - 20, wherein the binder is selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy polymer (PFA), polyvinyl fluoride (PVF), carboxymethyl cellulose (CMC), sodium alginate, starch, styrene butadiene rubber (SBR), xanthan gum, polyvinyl chloride (PVC), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and polyamideimide (PAI).

22. The electrode according to any one of claims 18 - 21, wherein the substrate comprises a metal foil made of a conductive foil selected from copper, zinc, aluminum, iron or any mixture thereof.

23. The electrode according to any one of claims 18 - 22, wherein the coating at least partially surrounds the substrate, and wherein the thickness of the coating is from about 1 micron to about 25 microns.

24. The electrode according to any one of claims 18-23, wherein the conductivity of the negative electrode is about 70 S / m -1 to about 100 S / m -1 .

25. A battery comprising a positive electrode; a negative electrode; and an electrolyte in contact with the positive electrode and the negative electrode, wherein the negative electrode comprises a coating on a substrate, and wherein the coating comprises MnO2 (electrolytic manganese dioxide (EMD)), a carbon conductive additive in the form of the purified graphite material according to claim 17, and a binder.

26. The battery according to claim 25, wherein the EMD is one or more of α-MnO2, β-MnO2, γ-MnO2, δ-MnO2 or λ-MnO2.

27. The battery according to claim 25 or 26, wherein the EMD, the carbon conductive additive and the binder are mixed together in a weight ratio of 4 - 9:2:

1.

28. The battery according to any one of claims 25 - 27, wherein the binder is selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy polymer (PFA), polyvinyl fluoride (PVF), carboxymethyl cellulose (CMC), sodium alginate, starch, styrene - butadiene rubber (SBR), xanthan gum, polyvinyl chloride (PVC), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyamideimide (PAI).

29. The battery according to any one of claims 25 - 28, wherein the substrate comprises a metal foil made of a conductive metal selected from copper, zinc, aluminum, iron, or any mixture thereof.

30. The battery according to any one of claims 25 - 29, wherein the coating at least partially surrounds the substrate, and wherein the thickness of the coating is from about 1 micron to about 25 microns.

31. The battery according to any one of claims 25 - 30, wherein the conductivity of the negative electrode is from about 70 S / m -1 to about 100 S / m -1 .

32. The battery according to any one of claims 25 - 31, wherein the electrolyte is an aqueous electrolyte having a concentration in the range of about 0.01 M to about 10.0 M.

33. The battery according to any one of claims 25 - 32, wherein the electrolyte is an aqueous electrolyte selected from: ammonium sulfate, sodium sulfate, magnesium sulfate, ferrous sulfate, copper sulfate, zinc sulfate (ZnSO4), zinc chloride (ZnCl2), zinc trifluoromethanesulfonate (Zn(CF3SO3)2), cadmium nitrate, cobalt nitrate, sodium nitrate, sodium chloride, nickel chloride, potassium chloride, ammonium chloride (NH4Cl), calcium chloride, sulfuric acid, and combinations thereof.

34. The battery according to any one of claims 25 - 33, wherein the electrolyte is an ionic liquid comprising a cation selected from the group consisting of 1-alkyl-3-methylimidazolium, N-alkylpyridinium, tetraalkylammonium, tetraalkylphosphonium, 1-ethyl-3-methyl-1H-imidazolium, 1-butyl-3-methyl-1H-imidazolium, 1-butylpyridinium, and combinations thereof, wherein the alkyl is selected from C2-C 12 alkyl.

35. The battery according to any one of claims 25 - 34, wherein the electrolyte is ZnSO4 having a concentration in the range of about 0.01 M to about 10 M.

36. The battery according to any one of claims 25 - 35, wherein the positive electrode is a zinc metal electrode.

37. The battery according to any one of claims 25-36, wherein when measured between 1.5 V and 0.7 V at a current density of 0.05 Ag -1 , the discharge specific capacity of the negative electrode is in the range of about 50 mAh g -1 to about 200 mAh g -1 .

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