Method for producing 2h graphite from a carbon source

Abstract

The invention is concerned with a method for producing 2H graphite by graphitization. The method comprises heating a carbon source comprising a catalyst in a first heating step and in a second heating step. The first heating step is performed to a temperature that is below the evaporation temperature of the catalyst for a time sufficient to cause at least partial graphitization as a conversion of the carbon of the carbon source to an end product comprising 2H graphite. The second heating step is performed to a temperature to volatilize the catalyst and to complete the graphitization to a required level.

Description

[0001] METHOD FOR PRODUCING 2H GRAPHITE FROM A CARBON SOURCE

[0002] TECHNICAL FIELD

[0003] The invention is concerned with a method for producing 2H graphite.

[0004] BACKGROUND

[0005] Carbon structures including graphene and graphene like materials as well as graphite derived from natural resources by mining or by converting biomass or carbon-containing waste, has raised tremendous interest in the market due to highly potential high-tech applications. These include energy storage systems, clean air and water applications, electronics, etc. In addition, natural graphite is classified as a Critical Raw Material (CRM) by the European union (EU), and thus new resources of graphite materials are intensively sought after (https : / / eur-lex.europa.eu / legal-content / EN / TXT / ?uri=CELEX:52020DC0474).

[0006] The commercialization of the materials and the technologies for production of graphite materials have been slow. Conventional methods have been used for the purification of natural graphite as well. These methods, however, require several stages and thus a lot of energy is needed and they result in a significant amount of waste.

[0007] The three forms of graphite are amorphous graphite, flake graphite and crystalline vein graphite, and they each have unique properties that make them well-suited for certain applications. The crystalline form of graphite consists of stacked layers of graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice. Both hexagonal and rhombohedral polytypes of graphite are known depending on how the graphene sheets are staggered. The two main forms of graphite are called alpha (hexagonal) and beta (rhombohedral), sometimes also called the Bernal (2H) phase and the rhombohedral (3R) phase, respectively. In reality, the Bernal 2H graphite form, however, is a type of hexagonal graphite. Furthermore, turbostratic graphite exists, where layers present irregularity in interlayer distances.

[0008] Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large scale for uses in pencils, lubricants, and electrodes. Under high pressures and temperatures, it converts to diamond. It is a good but not excellent conductor of both heat and electricity.

[0009] Graphite, as an important material for lithium-ion battery anode and graphene preparation can exist in the above different forms. The alpha form (Li-ion) can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300 °C.

[0010] The use of graphite in batteries has increased since the 1970s. Natural and synthetic graphite are used as an anode material to construct electrodes in battery technologies. As an example, a Li-ion battery can contain high amounts of graphite.

[0011] There is an increasing interest also in highly crystalline graphitic materials or graphene-like materials. This is due to the increasing demand of graphite for Li-ion batteries and the application beyond Li- ion batteries but it is to be noted that crystalline graphitic materials or graphene-like materials differ from graphite in many aspects.

[0012] Even if graphitic carbon and graphite are two related substances and are sometimes used interchangeably, there are certain areas where these two are different, wherefor the terms should not be misused. One difference is that graphite is an allotrope of carbon made solely of pure carbon, whereas graphitic carbon is a type of carbon that is not pure, and thereby it is not suitable for use in e.g. Li-ion batteries.

[0013] Thus, the applications for Li-ion batteries are very demanding also with respect to a high purity requirement. Only graphite materials with a very narrow range of properties are capable of delivering the performance essential for these applications. Graphite has the advantage that it has more sites where Li-ions can attach with carbon as compared to hard carbon, and that is why synthetic-like graphite has a higher specific capacity.

[0014] Primarily, high-purity synthetic graphite has been produced by graphitization of graphitizable carbon forms, the most common basic raw material being the calcined petroleum coke and coal tar pitch, both of which are composed of highly graphitizable forms of carbon. Recently, methods for producing graphite from biocarbon have also been developed. US patent 11 , 518, 679 B2 is mentioned as prior art for a composition for the production of a graphite powder suitable for making high performance Li-ion battery anodes. The composition comprises a biochar that is derived from the pyrolysis of woody biomass.

[0015] Furthermore, attempts have been made for the graphitization of lignin. However, the methods of prior art for heating biochar (high carbon-content material obtained by pyrolysis of biomass) or lignin does not result in graphite that has properties similar to synthetic graphite, i.e. i.e. 2H / 3R graphite, and are therefore not suitable for use in e.g. Li-ion batteries.

[0016] Such prior art is e.g. presented in “Yan, Q., Li, J., Zhang, X. et al. Catalytic graphitization of kraft lignin to graphene-based structures with four different transitional metals. J Nanopart Res 20, 223 (2018). https: / / doi.org / 10.1007 / s1 1051 -018-4317-0”. The graphitization was performed over four transitional metal catalysts (Ni, Cu, Fe, and Mo) through a thermal treatment process under an argon flow at 1000 °C. The product using this process was hard carbon.

[0017] SUMMARY OF THE INVENTION

[0018] The method of the invention for producing 2H graphite by graphitization comprises heating a carbon source comprising a catalyst in a first heating step and a second heating step. The first heating step is performed to a temperature that is below the evaporation temperature of the catalyst for a time sufficient to cause at least partial graphitization as a conversion of the carbon of the carbon source to an end product comprising 2H graphite. The second heating step is performed to a temperature to volatilize the catalyst and to complete the graphitization to a required level.

[0019] The preferable embodiments of the invention presented in the following have the following characteristics.

[0020] In the method, carbon is provided from a carbon source, which can be a lignin composition, such a pyrolyzed lignin or solid carbon from natural gas or recycled graphite from black mass, such as acid leaked black mass from Li-ion batteries.

[0021] The carbon source comprising a catalyst is a dried or pyrolyzed lignin composition with added catalyst as mixed with the carbon source before or after the drying or pyrolysis. The carbon source can contain one or more catalysts if it is recycled graphite or solid carbon from natural gas, whereby adding a catalyst often is not necessary. Said first heating step is followed by a second heating step to complete the graphitization to an end product comprising 99,00 - 99,99 %, preferably at least 99,95 % of 2H graphite including possible traces of also 3R graphite.

[0022] The carbon source is a lignin composition or natural gas or recycled carbon from black mass, such as acid leached black mass from Li-ion batteries.

[0023] In the embodiment, wherein the catalyst is mixed with the carbon source, it is performed by dry milling to a final particle size of the catalyst and the carbon source of 1 - 20 pm. The amount of catalyst is suitably 10 - 20 percent of weight of the total mass of carbon source and catalyst. The catalyst is removed after the graphitization by volatilization and collected on a particulate filter. The catalyst particles are diluted with air before collected on a particulate filter to convert the catalyst to an oxide and recycled back to the process by removing it from the particulate filter. After evaporation, the catalyst is in an elemental form and can be collected on a particulate filter in metallic form, but in practice this is an occupational safety risk, if there is any leakage, as the metallic particles can overheat or even explode.

[0024] The catalyst is for example a salt or oxide of Fe, such as Fe2Os, Fe(NOs)3 and / or FeSC , preferably a salt in dry form or in water solution. When the salt is used in a dry crystalline water formula e.g. as, the formula is in practice e.g. an iron nitrate (iron(lll)nitrate nonahydrate, [Fe(NO3)3-9H2O])catalyst, or an iron sulphate (iron(ll)sulphateheptahydrate [FeSO4-7H2O]) catalyst or an iron oxide (hematite) catalyst. The catalyst can also be a silicon (Si) catalyst, and / or a silicon carbide (SiC) catalyst. The catalyst can furthermore be an oxide or a salt of Cu, Al, Ni, Mn and / or Co or elemental Cu, Al, Ni, Mn and / or Co.

[0025] When the carbon source is pyrolyzed lignin, the pyrolysis of the carbon source or of the mixture of the carbon source and catalyst is performed in a pre-heating step at a pyrolysis temperature of 700°C -900°C.

[0026] After the pyrolysis, said first heating step is performed gradually to a temperature between 1500 - 2000°C, preferably ca 1800°C, as the second heating stage, and maintaining the reached temperature of the first heating step for 1 - 5 hours, preferably 1 hour, whereafter the second heating step is performed gradually to a temperature between 2400 - 3500°C, preferably ca 3000°C, depending on e.g. the existing catalyst, and maintaining the reached temperature of the second heating step for 1 - 5 hours, preferably 1 hour. Said heating steps are performed in an induction furnace or other heating furnace under Ar or N2 atmosphere. Other heating methods can be used but induction heating is the most efficient method in this context.

[0027] The invention is also concerned with a product prepared by the method of the invention comprising 99,00 - 99,99%, preferably at least 99,95%, 2H / 3R graphite. The id / ig ratio obtained from Raman spectroscopy is below 0.15.

[0028] The produced 2H / 3R graphite has a first cycle coulombic efficiency of at least 75% and a specific capacity of at least 300 mAh / g + 500 cycles of life observed in half cell tests of recharging Li batteries.

[0029] The inventive step relies in using two heating steps after a possible pyrolysis heating step, wherein the sample of the starting composition mixed with the used catalyst is kept in a temperature that is sufficiently high for causing at least partial graphitization but low enough not to volatilize the catalyst, which evaporates out with a carrier gas. The graphitization product is solid graphite having properties resembling 2H graphite or synthetic graphite and with a purity of at least 99.95 wt%. The resulting graphite material is suitable as anode material in Li-ion batteries.

[0030] Without using two heating steps after a possible pyrolysis heating step, the properties of the graphite formed do not correspond to those of 2H (or 3R) graphite or synthetic graphite since the catalyst evaporates too early.

[0031] With the inventive method, high-purity 2H / 3R graphite, can be synthetized from lignin and other carbon sources as mentioned above, from which most, but not all metals have been removed / or recovered. When the starting material is lignin, it can e.g. be kraft lignin. Kraft lignin is a kind of industrial lignin obtained from Kraft pulp being the main method for converting coniferous wood to pulp. The pulping yield is higher than other alkaline pulping methods. In the Kraft cooking process, about 90-95% of the lignin is dissolved into the cooking liquor containing sodium hydroxide and sodium sulfide. In the pulping process, the lignin macromolecules are fractured, the molecular weight is decreased, the lignin is dissolved in alkaline solution, making the solution turn dark brown. The kraft pulp method is also known as kraft pulping, kraft process or sulfate process. Instead of feeding kraft lignin into a recovery boiler as black liquor to produce heat and electricity, the sc. Ligno Boost method is an example of a method that can be used to separate lignin for various applications e.g. such as graphite for batteries and carbon fibres etc. In brief, LignoBoost involves leading a certain percentage of the black liquor away to a precipitation unit instead of incinerating it in the recovery boiler. In this unit, carbon dioxide is added to precipitate and filter out lignin, which is then washed in the next step.

[0032] In the method of the invention, the starting lignin or other carbon source material is, in the presence of and mixed with a catalyst, placed in a graphite crucible in an induction furnace and pyrolyzed and thereafter converted by graphitization into high purity graphite through a two-step heating process. Instead of an induction furnace, another type of furnace can be used, with which a heating up to the required temperatures is possible without damaging any components.

[0033] The method leaves no liquid or solid waste and the gas temperature at the furnace outlet is kept low enough for commonly used particle filters. Induction offers a very efficient heating by directly heating the starting material mixture of the carbon source and the catalyst in reducing conditions. Ar2 and N2 are used instead of air and it must not be air.

[0034] During heating, carrier gas (Ar or N2) is flowing over the graphite crucible being mixed with a sheet gas (Ar or N2), which is flowing near the induction furnace walls to prevent losses of the volatilized catalyst. The carrier gas flows over the graphite crucible resulting in condensable catalyst particles and other volatilized matter. The sheet gas is flowing along the induction furnace walls thus preventing fouling of the reactor chamber walls. The carrier gas and the sheat gases are mixed together before being exited from the furnace without any significant contamination at the walls.

[0035] When using lignin as a carbon source, catalyst was mixed either after or before a pyrolysis step. The resulting powder mixture was heated resulting in that the lignin was pyrolyzed at ca 700 °C, after which it was heated in an induction furnace under Ar or N2 atmosphere up to 2400 °C - 3500°C (such as 3000 °C ) after first having been heated to 1800 °C, with a dwell time of 1 - 5 hours, preferably 1 hour, and then heated with a controlled heating rate up to ca 3000 °C, with a dwell time of 1 - 5 hours, such as 1 hour. After this, the induction furnace was cooled down and the graphitized material was removed for analysis. Thus, the developed inventive method uses temperatures in the temperature treatment that are typically about or above 1800°C in the first heating step, and above 2400°C, preferably up to 3000°C or even 3500°C, in the second heating step. The temperature used partly depends on the catalyst used and can depend on other factors as well. To reach such high temperatures, special furnace set ups are usually required. Induction heating can be used, which is a heat- treating process that allows very targeted heating of metals by electromagnetic induction. The process relies on induced electrical currents within the material to produce heat and is the preferred method used to bond, harden or soften metals or other conductive materials.

[0036] The primary function of the carrier gases is to keep the oxygen away from the reaction environment, i.e. first the air in the reactor is substituted by the gases and then it has to be ensured that air does not enter into the reactor. The role of the sheath gas is to prevent particle matter or condensable gases to deposit on the reactor walls and they dilute gases that are released as a result of the graphitization reaction.

[0037] During the second heating process, the catalyst is volatilized diluted / cooled and collected on a particle filter. In the dilution and cooling process, either air can be used when the metal particles are oxidized and easy to circulate back and mixed with lignin or nitrogen gas can be used when metal particles are not oxidized, but are difficult to handle as they may burn when circulated back and mixed with lignin. It is, however, also possible to circulate back e.g. elemental metal particles of the catalyst by taking care of that they are not oxidized.

[0038] The catalysts can be recycled after volatilizing to be removed with the gas stream by collecting them on a particulate filter. Also, the carrier and sheath gases can be reused.

[0039] A high collection efficiency of 60-90% of the catalyst is reached due to the designed geometry of the furnace, whereby the gas flows so that losses on the reactor chamber and collection unit surfaces are minimized as was indicated before without fouling of the reactor chamber walls with substances ad with no contamination at the reactor walls.

[0040] The graphitized carbon obtained as the end product can be characterized by Raman spectroscopy and X-Ray Diffraction (XRD) (graphitization degree), Electron microscopy (SEM / TEM) to see the morphology / structure of the material, thermogravimetric analysis (TGA) to obtain the purity of produced 2H (and 3R) graphite. Coin cell tests or pouch cell tests can be performed to see the electrochemical properties of the graphite end product obtained.

[0041] The invention thus provides a high degree of graphitization of carbon structures derived from lignin and other carbon sources. The physico-chemical and electrochemical properties of our 2H / 3R graphite, mostly 2H graphite, match well with commercial Sigma-Aldrich battery grade synthetic graphite made from fossil resources.

[0042] Thus, the method of the invention produces a 2H graphite product with an increased crystallinity to a degree, which is suitable for demanding battery chemicals, meaning a purity at or above 99,5% and a high degree of crystallinity.

[0043] Thus, the end product of the invention contains 99,0% - 99,99% carbon consisting of mainly 2H graphite. As shown by Raman spectroscopy in the examples to follow, the end product mainly consisted of 2H graphite usually with traces of 3R graphite, both forms being useful for Li ion batteries.

[0044] The high purity and crystallinity (of the 2H graphite obtained) is achieved with the method of the invention. The evaporating catalyst is collected for recycling by means of a bag filter or particulate filter and is reused. The high purity of 99,95%, being mostly 2H graphite, is thus achieved partly by the evaporation of the catalysts and other impurities. The product has an ID / IG ratio obtained by Raman spectroscopy, which is below 0.15. An ID / IG ratio lower than 0.15 indicates the extensive highly graphitic structure.

[0045] The ID / IG ratio, also known as the D band to G band intensity ratio, is a crucial parameter obtained from Raman spectroscopy that provides information about the structural disorder and defects in carbon-based materials, particularly graphene and carbon nanotubes.

[0046] The intensity ratio of the 2D / G peak also has an importance, i.e., it can be used to estimate the number of the layers. The D peak is a defect peak, the G peak is the graphite peak and the 2D peak is the graphene peak.

[0047] Induction furnaces are preferably used in the invention. Furnaces based on induction heating are the most energy efficient ones for heating various materials like metals, metal oxides and carbon-based materials. There is a resistant graphite heater vessel or crucible in the induction furnace of the invention. As the lifetime of the resistant heater is very limited at a temperature of 2600 °C, it is preferred to use an inductive heating of a graphite tube or a graphite crucible with middle frequency i.e., a frequency of 8-10 kHz.

[0048] In the furnace to be used in the method, the gas flows should preferably be designed to carry the released catalyst metal vapours and particles out from the furnace chamber to (a) avoid fouling of the furnace chamber and graphite crucible and thus avoiding the process malfunction, and (b) to collect the volatilized catalyst particles by a bag or particulate filter and recycle them back to the process and (c) to avoid heavy metal emissions from the process.

[0049] In the following, the invention is illustrated by means of some background for clarity and some preferable embodiments and examples of the invention by referring to figures.

[0050] FIGURES

[0051] Figure 1 illustrates the method of the invention by using an induction furnace useful in the method of the invention

[0052] Figure 2 shows the heating rate used for heating the work piece of example 1 using lignin as the carbon source with or without catalyst

[0053] Figure 3 shows Raman spectra for the product of example 1 at different stages of the method

[0054] Figure 4 shows the Raman spectrum after a heat treatment of a pure Kraft lignin composition without any catalyst according to example 2

[0055] Figures 5 - 8 show the Raman spectra of the products of examples 3 - 6.

[0056] Figure 5 shows the Raman spectrum after a heat treatment of a pure Kraft lignin composition with an iron nitrate catalyst according to example 3

[0057] Figure 6 shows the Raman spectrum after a heat treatment of a pure Kraft lignin composition with an iron sulphate nitrate catalyst according to example 4 Figure 7 shows the Raman spectrum after a heat treatment of a pure Kraft lignin composition with a silicon catalyst according to example 5

[0058] Figure 8 shows the Raman spectrum after a heat treatment of a pure Kraft lignin composition with a silicon carbide catalyst according to example 6

[0059] Figure 9 shows the Raman spectrum of a pure Kraft lignin composition without any catalyst in accordance with example 7

[0060] Figure 10 shows the Raman spectrum of a pure kraft lignin composition with an iron nitrate catalyst in accordance with example 8.

[0061] Figure 11 shows the XRD spectrum of the product obtained in example 8

[0062] Figure 12 shows TEM Overview images of heat treated lignin mixed with Fe(NOs)3 catalyst in accordance with example 8.

[0063] Figure 13 shows the same as figure 12 in a high resolution TEM image.

[0064] Figure 14 shows SEM Overview images of heat treated lignin mixed with Fe(NOs)3 in accordance with example 8.

[0065] Figure 15 shows the same as figure 13 as a high resolution SEM image.

[0066] Figure 16 shows a Thermogravimetric (TGA) analysis of heat treated lignin mixed with Fe(NOs)3 catalyst in accordance with example 8.

[0067] Figure 17 consisting of figures 17a - 17c shows electrochemical results of heat treated lignin mixed with Fe(NOs)3 in accordance with example 8 in comparison with Sigma Aldrich commercial synthetic graphite.

[0068] Figure 18 shows a Raman spectrum of a carbon product obtained from a black mass with metal catalysts in accordance with example 11 . Figure 19 shows a SEM overview image of a carbon product obtained from a black mass with the metal catalysts Li, Ni, Co, Mn and Cu in accordance with example 11 .

[0069] Figure 20 shows a high resolution SEM image of a carbon product obtained from a black mass with the metal catalysts Li, Ni, Co, Mn and Cu in accordance with example 11 .

[0070] Figure 21 shows a Thermogravimetric (TGA) analysis result of a carbon product obtained from a black mass with the metal catalysts Li, Ni, Co, Mn and Cu in accordance with example 11.

[0071] Figure 22 is a table showing contamination elements in a carbon product obtained from a black mass with the metal catalysts Li, Ni, Co, Mn and Cu in accordance with example 11 . (DL is the detection limit in the ICP-MS analysis method used).

[0072] Figure 23 shows volatilized compounds collected on a particle filter after a dilutor in accordance with example 11 .

[0073] Figure 24 shows electrochemical results of a graphite product obtained from a black mass with the metal catalysts Li, Ni, Co, Mn and Cu in accordance with example 11 . (C relates to carbon coated graphite sample).

[0074] Figure 25 shows a Raman spectrum of a carbon product obtained from a natural gas mass with metal catalysts in accordance with example 10.

[0075] Figure 26 shows a XRD spectrum for the product of example 11 .

[0076] DETAILED DESCRIPTION

[0077] Figure 1 illustrates the method of the invention by using an induction furnace useful in the method of the invention.

[0078] In the method, carbon is provided from a carbon source, which can be a lignin composition, such as pyrolyzed lignin or natural gas or recycled carbon or recycled graphite from black mass, such as acid leaked black mass from Li-ion batteries. When using a lignin produced e.g. by the Kraft process is dry milled together with a catalyst, such as Fe(NOs)3 and / or FeSCU, ( see the complete molecular forms later in this text) to a final particle size of the catalyst and the lignin of 1 - 20 pm. A suitable amount of catalyst is 10 -15 percent of weight of the total mass of lignin and catalyst.

[0079] Some carbon sources already have catalysts needed, so e.g. when using recycled carbon or recycled graphite or natural gas, it is usually not necessary to mix any further catalysts with the carbon source.

[0080] The mixture containing lignin or other carbon source and catalyst is gradually heated in nitrogen atmosphere to a temperature of ca 700°C in order to pyrolyze the lignin in a preheating step. As earlier was mentioned, the catalyst can be mixed after the pyrolysis. The pyrolysis step is usually omitted when the carbon source is other than lignin.

[0081] The carbon source with catalyst is then placed in a graphite crucible 3 (surrounded by a graphite sphere or a graphite felt heat shield) of an induction furnace and heated gradually to a temperature of ca 1800°C as a first heating stage, and maintaining the reached temperature of the first heating step for 1 - 5 hours, preferably 1 hour, whereafter a second heating stage is performed gradually to a temperature between 2400 - 3500°C, preferably ca 3000°C, and maintaining the reached temperature of the second heating step for 1 - 5 hours, preferably 1 hour. Such a temperature sufficient for the conversion of the carbon of the carbon source to and end product comprising 2H / 3R graphite, mainly 2H graphite.

[0082] Said heating is performed under an Ar or N2 carrier gas atmosphere by introducing the gas to the induction furnace from below through entrance 2.

[0083] A sheath flow feed of the same gas, Ar or N2, is introduced through entrance 1 in order to keep the walls of the reactor free of fouling.

[0084] Reference number 4 illustrates a vessel tube formed as a cylindrical cone and reference number 5 illustrates a support tube (= diluter using air in order to form an oxide with minimal catalyst losses).

[0085] The gases are released from a particulate filter 6 on top of the induction furnace through opening 7.

[0086] Reference number 8 is the place for a pyrometer that measures the temperature of the sample (graphite powder temperature). The end product is collected from the crucible 3.

[0087] The catalyst is removed after the graphitization by volatilization in a temperature below 3 000° C and collected on a particulate filter 6 or a bag filter outside the induction furnace (not shown) through opening 7.

[0088] The method of the invention is further described in the following by means of examples. The examples described herein are provided for the purpose of illustrating specific embodiments of the invention and are not intended to limit the invention in any way. Although the examples described herein have been used to describe a method, it is understood that such detail is solely for this purpose and variations may be made therein by those skilled in the art without departing from the spirit and scope of the overall process.

[0089] EXAMPLES

[0090] General for examples 1 - 8:

[0091] The examples described herein are provided for the purpose of illustrating specific embodiments of the invention and are not intended to limit the invention in any way. It is understood that variations may be made by those skilled in the art without departing from the scope of the overall process.

[0092] Generally, the method mixing the starting material comprising a carbon source (with a catalyst when there is a catalyst is used, and in some embodiments pyrolyzing the resulting mixture. The pyrolyzed mixture is heated to a pyrolysis temperature that is below the evaporation temperature of the catalyst followed by further heating steps (first and second heating step) sufficient for the conversion of the lignin to and end product comprising 2H graphite.

[0093] An induction furnace of figure 1 was used for the heating steps of the method in the form of a laboratory setup. The induction heating furnace has 50kW heating power, the input current ca be up to 95A and the working frequency is about ~9kHz. The temperature was followed with a Kleiber 730-LO infrared pyrometer with a range from 350 to 3000°C. A part of the electronics, the inductor coil and the reactor chamber quartz tube were water cooled. In the generator, the three phase mains supply is transformed to DC in a generator and in converted to 9kHz AC in a converter. The control pulses for the converter are frequency modulated and because of this, the output power of the converter is regulated by adjusting the oscillating frequency. The temperature of a heated sample of starting material to be graphitized and catalyst was measured to be linearly dependent on the output power (oscillating frequency of the converter) and repeatable between experiments.

[0094] The dimensions of the induction furnace used were designed for a crucible with at least 20 cm3capacity and a 5 mm wall thickness. The heating crucible, wherein the material to be graphitized is to be placed, was made of graphite.

[0095] The crucible is encased in graphite felt thermal insulation laying on top of an AI2O3 tube. A quartz glass tube around the insulations and ALOstube holds them centered. A Carrier gas, which is Ar or N2 flows (with a flow of ca 5 l / min (STP) at maximum) inside the AI2O3 tube and is directed towards the crucible bottom inside the graphite felt heat shield. A sheath gas of N2 flows (with a flow of ca 50 l / min (STP) at maximum) outside the quartz glass tube. The sheath gas rapidly cools the carrier gas outside the core and stops the growth of the catalyst particles, which are heated mixed together with the material to be graphitized.

[0096] The catalysts (in those examples were added or existed) are evaporated in a second heating step and the exhaust gas containing the catalysts are diluted with air in a porous tube diluter causing oxidation of the catalysts. They are then collected on particle filters. Dilution controlled catalyst oxidation is needed to avoid the collected metal catalyst particles to undergo uncontrolled oxidation and burning to cause hazardous conditions and to protect the workers occupation health.

[0097] The facility of figure 1 used for the method can be scaled up to graphitise large quantities of carbon-based materials mixed with various metal catalysts. High quality 2H / 3R graphite, (mainly 2H graphite) has been produced that has similar electrochemical properties as synthetic graphite.

[0098] Example 1 : Lignin example without catalyst

[0099] A lignin composition containing 61 wt% C, 6 wt% H, 0.9 wt% N, and <0.15 wt% S was used as starting material. The lignin content was 80 wt% and the ash content <0.5 wt% calculated from the dry mass. Said lignin composition was first ball milled to a particle size of about 10 microns. The lignin composition was then heated in a pre-heating step to a temperature of 700°C for its pyrolysis.

[0100] After that the lignin composition was heated with a heating rate of about 50 K / min as shown in figure 2 in an induction furnace of figure 1 without any catalyst. (In this example no second step heating to 1800°C was performed.) Figure 2 shows the nominal and actual power of the induction furnace and the measured temperature of the sample as measured by said pyrometer.

[0101] When a temperature of 3 000 °C was reached as measured by the pyrometer, the sample was held at 3 000 °C with a dwell time of one (1 ) hour. About 60 wt% of the lignin mass was lost when heated up to 3 000 °C. Ar gas was used as carrier gas.

[0102] A Raman spectrum analysis was performed for the starting material and for the heated resulting product. Figure 3 shows the Raman spectra, wherein the uppermost curve is for the lignin feedstock (said lignin composition used as starting lignin raw material composition), the second curve just below the uppermost curve is for the same material after being pyrolyzed at 700°C and the lower curves are for the same material after heat treatment at 3 000°C for one hour.

[0103] From the Raman spectra it can clearly be concluded that the resulting product of the starting material that was heat treated at 3000°C corresponds to hard carbon as the ratio of ID / IG was over 0.5 measured from five spots at different places of the sample.

[0104] Thus, example 1 shows that 2H / 3R graphite was not synthetised in this embodiment, wherein the heating of the lignin raw material up to 3000°C was performed without any catalyst and without any first heating step to 1800°C which is relevant when there are catalysts used.

[0105] Examples 2 - 6: Kraft lignin examples without catalyst (example 2) and with catalyst (examples 3 - 6)

[0106] A dry ash free kraft lignin composition containing 65 wt% C, 6 wt% H, 27wt% O, 0.15 wt% N, 2.25 wt% S, and 0.01 wt% Cl was used as starting material. The lignin content was 96.5 wt%, the ash content <1 wt%, Na content 175g / kg, and K content was 52.5 g / kg calculated from the dry mass of the kraft lignin composition used as starting material.

[0107] All materials were dried, thoroughly ground and mixed with a ball mill using agate balls and an agate bowl. The milling was done with 300 rpm for one hour and the obtained particle diameter was about 10 microns.

[0108] In example 2, which was performed without catalyst, said lignin composition was ball milled to a particle size of about 10 microns.

[0109] Also, in examples 3 - 6, wherein iron nitrate, iron sulphate, silicon and silicon carbide catalysts were used and mixed with the kraft lignin composition, a similar ball milling was performed and the ball milling also included mixing of the catalysts with the kraft lignin composition.

[0110] For the samples of the pure kraft lignin composition, and also for the mixed samples of kraft lignin composition and catalyst, respectively, pyrolysis at 700 °C in a pre-heating step was done for 30 minutes in nitrogen atmosphere.

[0111] After the pyrolysis, the samples were heated in an induction furnace of the above type in a first heating step in examples 3 - 6 to 1800°C for one hour to cause graphitization before the catalyst evaporates.

[0112] As said, the sample in example 2 was without catalyst. In examples 3 - 6, the samples contained different catalysts. When a temperature of 2400 °C was reached in a second heating step in examples 3 - 6, as measured by a pyrometer, the samples were held at 2400 °C with a dwell time of 20 minutes. Ar gas was used as carrier gas.

[0113] Figure 4 shows the Raman spectrum after this heat treatment of a pure Kraft lignin composition without any catalyst in example 2 at 2 400 °C, from which a peak intensity ratio ID / IG of 0.32 can be calculated, which indicates a low level of ordered graphite. The peak intensity ratio ID / IG should be below 0.15 to fill the goal of the invention with respect to produce 2H / 3R graphite. In examples 3 - 6, mixtures of a pure kraft lignin composition and iron nitrate, (iron(lll)nitrate iron sulphate, silicon (Si) catalyst, and silicon carbide (SiC) catalyst, respectively, were as said above heated first by pyrolysis in a pre-heating step at 700°C and then in a first heating step up to 2 400 °C with a dwell time of 20 minutes.

[0114] Figures 5 - 8 show the Raman spectra of the products of examples 3 - 6, from which the ratios ID / IG of 0.09 (for the iron nitrate catalyst in example 3, figure 5), 0.17 (for the iron sulphate catalyst in example 4, figure 6), 0.15 (for the silicon catalyst in example 5, figure 7), and 0.22 (for the silicon carbide catalyst in example 6, figure 8) can be calculated.

[0115] Thus, the method of example 3 resulted in the highest end product quality of 2H / 3R graphite followed by that of example 5, example 4, and example 6 in said order.

[0116] From the Raman spectra of figures 5 - 8, it can be concluded that when a kraft lignin composition mixed with iron( 11 l)nitrate was used as starting material as was done in example 3, the lowest peak intensity ratio ID / IG=0.09 was achieved, and it was therefore chosen for the tests of example 7 to be carried out at a very high temperature of 3 000°C in the second heating step.

[0117] Examples 7 - 8

[0118] In example 7, a pure kraft lignin composition was used as starting material as in example 2.

[0119] In example 8, a sample of the same Kraft lignin composition as in examples 2 - 6 was first mixed with iron(lll)nitrate catalyst (10 wt% elemental iron (Fe) of the total mass) as in example 3 and used as starting material.

[0120] In example 7, which was performed without catalyst, said kraft lignin composition was ball milled to a particle size of about 10 microns as was done in example 2.

[0121] Also, in example 8, wherein said iron nitrate catalyst was used and mixed with the kraft lignin composition, a similar ball milling was performed and the ball milling also resulted in mixing of the catalyst with the Kraft lignin composition. For the sample of the pure kraft lignin composition of example 7, and also for the mixed sample of kraft lignin composition and catalyst of example 8, respectively, pyrolysis at 700 °C was done for 30 minutes in nitrogen atmosphere. In practice, Fe(NOs) is decomposed in the pyrolysis already at about 250°C to hematite Fe2Os or the iron can react directly with carbon.

[0122] After the pyrolysis, the sample was heated in an induction furnace of the above type in a first heating step in example 8 to 1800°C for one hour so that the catalyst caused graphitization before it evaporates.

[0123] Thereafter, the sample was heated up in a second heating step to a temperature of 3 000 °C, with a heating rate of 50 K / min and dwell time of 60 minutes, to volatilize the catalyst to reach a 99.95% carbon content.

[0124] Figures 9 and 10 show the Raman spectra of the pure kraft lignin composition (without catalyst in figure 9) and the above mixture of Kraft lignin and catalyst (in figure 10), respectively.

[0125] From the Raman spectra of Figures 9 and 10, it can be concluded that kraft lignin with iron( 11 l)nitrate used in example 8 had a lower peak intensity ratio ID / IG=0.11 compared to that of the pure lignin composition of example 7 (figure 9), which was 0.61 . The product obtained with the method in example 8 thus represented very good ordered 2H / 3H graphite, since a value of 0.1 1 . for the peak intensity ratio indicates a very good quality for 2H / 3R synthetic graphite.

[0126] The X-ray diffractogram of the product of example 8 presented as figure 11 further shows that the crystal structure of the product corresponds to that of 2H graphite. The d-spacing of 3.363 A (based on the (002)-peak) is in good agreement to that of ideal graphite (3.354 A). Furthermore, the so-called 3D-peaks (e.g. near 45° 20) indicate relatively large well-ordered crystalline regions.

[0127] In the transmission electron microscopy image (HR-TEM) shown in figure 12, the graphite layers can be clearly seen. Figure 11 shows TEM Overview images of heat treated lignin mixed with Fe(NOs)3 in accordance with example 8. The interlayer distance between single graphite sheets was calculated from the TEM image and on average it is 0.35 nm. For 2H graphite, the interlayer distance is typically 0.34 nm and for multilayer graphene 0.37-0.38 nm. Thus, the interlayer distance calculated from figure 12 shows that the product obtained by the method of example 8 is 2H graphite.

[0128] Figure 13 shows the same as figure 12 in a high resolution TEM image. The graphite layers can be seen, and they are straight indicating being 2H graphite.

[0129] Figure 14 shows SEM Overview images of heat treated lignin mixed with Fe(NOs)3 in accordance with example 8. From these SEM images, it can be seen that there are several spheric graphite particles being 2H graphite and then separate fine particles being contamination at the handling of the end product as analyzed or because of handling before pyrolysis.

[0130] Figure 15 shows the same as figure 13 as a high resolution SEM image. The graphite flakes can clearly be seen.

[0131] Figure 16 shows a Thermogravimetric (TGA) analysis of heat treated lignin mixed with Fe(NOs)3 in accordance with example 8.

[0132] Based on the TGA analysis in air (Figure 16), the purity of the graphite is 99.26%. The x- axis indicates the temperature, and the y-axis is the product (amount of total mass). However, as the tests were made in a non-optimal condition i.e. not in a clean room condition, and also packing and transport can cause contamination, some minor contamination could be observed by Inductively Coupled Plasma Mass Spectrometry (ICP- MS) analysis, mainly Na, Al and Cu. The iron (Fe) content was 153.6 mg / kg.

[0133] The Iron nitrate used as catalyst decomposes to an oxide, i.e. Fe2Os, in the pyrolysis step and tests show that Fe2Os can also be used directly but it is preferable to use the salt form. In the second heating step, the oxygen from the oxide reacts with the carbon (of the starting material) leaving pure iron (Fe) left (FeO -> Fe), which is volatilized. So Fe, used as a component in the catalyst, should be the only element to observe after the heat treatment of the third step. When only the Fe content is considered as an impurity, the graphite purity is 99.985%, i.e. much higher than the industrial standard of 99.95% for graphite to be used in an anode.

[0134] The product of example 8 was used to make up a negative electrode, whose electrochemical performances were investigated in the form of a half-cell configuration of Li-ion batteries.

[0135] Thus, the half-cell tests were conducted for an electrode made up of the heat-treated mixture of lignin with Fe(NOs)3 produced according to the method of example 8. Said heat-treated mixture of lignin with Fe(NOs)3, which is further called as an active material, was sieved with a 45 pm sieve (Retsch). A slurry contained 80 wt.% of the active material, 10 wt.% of a conductive agent (carbon black SUPER C65), and 10 wt.% of a binder (CMC:SBR, 1 :1 ) was prepared and spread on a Cu foil (12 pm thickness) with an automatic vacuum film applicator (Qualtech). The slurry was and dried at 80QC for 12 h in a vacuum oven (Thermo Scientific™). The amount of the active material was approximately 1 .5 mg cm'2for the analysed sample and 1 .7 mg cm'2for commercial graphite. CR2016 coin cells were built by using metallic Li as a counter electrode. The 1 M LiPFe dissolved in EC / DMC in the ratio of 1 / 1 vol% was used as an electrolyte and the glass microfiber as a separator.

[0136] The electrochemical characterization of the cells consists of the analysis of galvanostatic performance during charging / discharging and cycling voltammetry (CV) curves. The galvanostatic performance of the coin cells was examined between 0.01 and 2.0 V vs. Li+ / Li at various current rates (C-rates) of 0.05, 0.1 , 0.2, 0.5, 1 , and 2 C. CV curves for each electrode were measured in a voltage range of 0.01 -2.0 V vs. Li+ / Li, at a scan rate of 0.1 mV s'1to determine the lithium de / -intercalation mechanisms at equilibrium conditions.

[0137] The electrochemical properties of the selected sample were compared to the electrochemical properties of commercial synthetic graphite (< 20 pm particle size, quality level 200, Merck) used as reference material.

[0138] The electrochemical results, using 2H graphite obtained with the method of example 8, are shown in figure 17.

[0139] Figure 17 consisting of figures (a), (b), and (c) shows electrochemical results of heat treated lignin mixed with Fe(NOs)3 in accordance with example 8 in comparison with Sigma Aldrich commercial synthetic graphite used as reference material, (a) shows first charging and discharging voltage curves, (b) shows CV curves, and (c) rate capability test.

[0140] The results showed that the obtained graphitic sample from lignin using said iron nitrate catalyst has a layered graphitic structure like synthetic graphite structure. The three voltage plateaus observed on the voltage curve of the analysed sample are like the voltage plateaus observed for the reference material. The analysed material has slightly lower specific capacity (312.46 mAh / g) and initial coulombic efficiency (78%) than the reference material (363.1 mAh / g and 82%, respectively). However, the specific capacity retention in percentages of the sample obtained from lignin are slightly better than the reference material, which can be seen from Fig. 17 (b) and (c), especially at high values of C-rate current.

[0141] EXAMPLE 10 - another carbon source

[0142] As a second raw material, solid carbon obtained from a process, where hydrogen is separated from natural gas, is used in the method of the invention. In this separation process iron (Fe) is used as catalyst to get H2 and C (solid carbon).

[0143] The temperature in the last heating step in the methods of this example tests up to 3000 °C in this example, but the amount of iron (Fe) of the Fe2Os catalyst used was 20 wt%.

[0144] Test 1 : The mixture was heated up to 1 800 °C as the first heating step with a holding time of one hour and then the mixture was stepwise heated up to 3000 °C as the second heating step with a holding time of one hour.

[0145] Test 2: The mixture was heated up to 3000 °C by omitting the first heating step (contrary to the method of the invention) with a holding time of one hour.

[0146] Test 3: The mixture was heated up to 1 800 °C as the first heating step with a holding time of one hour and then the mixture was stepwise heated up to 3000 °C as the second heating step with a holding time of five hours.

[0147] The resulted end product was analyzed with Raman spectroscopy. 11

[0148] Figure 25 shows the Raman spectra obtained, wherein the calculated average ratios of the peak intensities ID / IG were:

[0149] Test 1 : 0,14

[0150] Test 2. 0.25

[0151] Test 3. 0.15

[0152] The results show that a heating directly up to 3 000 °C without any first heating step does not result in a graphite that contains mostly 2H graphite. The results of Tests 1 and 3 show that holding the mixture for one hour at 1800 °C in a first heating step increases the amount of 2H graphite significantly.

[0153] EXAMPLE 11 - still another carbon source

[0154] As a third raw material, black mass from a Li-ion battery was used as the raw material in the method of the invention.

[0155] The recycled black mass used was first cleaned by acid leaching to recover metals like Li, Ni, Co, Mn and Cu. The concentration of these elements is described below in the table of figure 22. After the acid leaching, the purity of the black mass was 91 .3 wt%.

[0156] As the industrial standard for graphite anode material in Li-ion battery cells is 99.95 wt%, it is clear that the acid leached black mass is not suitable for Li-ion batteries as such and further purification and graphitization is needed.

[0157] However, a significant amount of these metals still existed in the material heat treated according to the method of the invention, and no extra catalyst was needed to get 2H graphite by a heat treatment up to 3 000 °C in the second heating step. During the heat treatment up to very high temperature 3 000 °C, the impurities volatilized and a high purity 2H graphite was reached. Three different heating methods were tested for the mixture of said raw material and said catalyst after having performed the heating step at 700°C consisted of the pyrolysis:

[0158] Test 1 : The mixture was heated up to 1 800 °C as the first heating step with a holding time of one hour and then the mixture was stepwise heated up to 3000 °C as the second heating step with a holding time of one hour.

[0159] Test 2: The mixture was heated up to 1 800 °C as the first heating step with a holding time of one hour and then the mixture was stepwise heated up to 3000 °C as the second heating step with a holding time of two hours.

[0160] Test 3: The mixture was heated up to 1 800 °C as the first heating step with a holding time of one hour and then the mixture was stepwise heated up to 3000 °C as the second heating step with a holding time of five hours.

[0161] The resulted end product was analyzed with Raman spectroscopy.

[0162] In the Raman spectrum obtained the calculated average ratios of the peak intensities ID / IG were:

[0163] Test 1 : 0.06

[0164] Test 2. 0.14 Test 3. 0.10

[0165] From the Raman spectrum of figure 18 obtained for test 1 , it can be concluded that the recyclable black mass had the lowest peak intensity ratio ID / IG=0.06 for a one hour dwell time, which is below our criteria for 2H / 3R synthetic graphite i.e. below 0.15. This corresponds to a commercial synthetic graphite Raman result.

[0166] In the SEM images in figures 19 and 20 taken for the same product as figure 18, it can be seen that there are several micron round graphite particles indicating the existence of mostly 2H graphite and traces of 3R graphite. The 2H graphite flakes are clearly seen in the high resolution SEM image of figure 20. A 2H graphite specific surface area range of 0.2 to 50 m2 / g or less is desirable for achieving an acceptable first Cycle Coulombic efficiency ICE"(US 11 s518s679 B) for use in a Li ion battery. The Specific Surface Area (SSA) was after the heat treatment in tests 1 - 5 was 4.6-5.3 m2 / g, which is a good value and related to ICE as said above.

[0167] Based on the TGA analysis in air (Figure 21 ), the purity of the graphite is 99.26%. However, as the tests were made in non-optimal conditions, i.e. not in a clean room conditions during handling and packing and shipping, the samples can get some contamination. The main contamination elements were Al, Mn, Co, Ni, Li, S and Cu for untreated material detected by ICP-MS (Li and S cannot be detected from ICP-MS) and ICP-OES analysis. For the heat treated product of test 1 , the concentration of these elements were very low (see Table in figure 22). Thus, it can be concluded that a very high purity was achieved already at one hour heat treatment at 3 000 °C in the second heating step.

[0168] In a XRD spectrum taken for the product of example 11 presented as figure 26, a 2H (112) peak was obtained at around 85 degree and 002 peak below 30 degree, which clearly shows that the obtained product is 2H / 3R graphite that corresponds to Sigma Aldrich synthetic battery grade graphite.

[0169] The volatilized compounds were collected on a particle filter after dilutor and valuable metals were recovered. The result of collected metals are shown in figure 23. The recovery amount was about 60 wt%. The measured concentrations of these various elements from the recovered particle mass were analysed by the ICP-MS method.

[0170] The half-cell tests were conducted for the black mass heat treated at 3 000 °C in the second heating step according to example 11. The half-cell tests were conducted similarly as described for the heat-treated lignin electrochemical tests described in figure 17 example 9 for Lignin wherein an iron catalyst was used. The results of these electrochemical tests are shown in figure 24.

[0171] Figure 24 shows electrochemical results of heat-treated black mass heat treated at 3000 °C in the second heating step according to example 1 1 . A comparison with Sigma Aldrich commercial synthetic graphite CV curves (C means carbon coated) is shown in the dotted line. The specific capacity of this product in accordance with tests 1 - 3 in a 1 -5 hours dwell time was 360-370 mAh / g and the specific capacity of the commercial synthetic graphite was 370 mAh / g. Thus, the products obtained in the test methods 1 - 3 were very close to commercial Sigma Aldrich synthetic graphite.

Claims

CLAIMS1 . A method of producing 2H graphite by graphitization comprising heating a carbon source comprising a catalyst in a first heating step and a second heating step, wherein the first heating step is performed to a temperature that is below the evaporation temperature of the catalyst for a time sufficient to cause at least partial graphitization as a conversion of the carbon of the carbon source to an end product comprising 2H graphite, and the second heating step is performed to a temperature to volatilize the catalyst and to complete the graphitization to a required level.

2. Method of claim 1 , wherein the carbon source is a lignin composition, such as pyrolyzed lignin or solid carbon from natural gas or recycled graphite from black mass, such as acid leached black mass from Li-ion batteries.

3. Method of claim 1 or 2, wherein the carbon source comprising a catalyst is recycled graphite from black mass or solid carbon from natural gas, whereby the comprised catalyst is contained in the carbon source.

4. Method of claim 1 or 2, wherein the carbon source comprising a catalyst is a dried or pyrolyzed lignin composition, whereby the comprised catalyst is added catalyst as mixed with the carbon source before or after the drying or pyrolysis.

5. Method of any of claims 1 , 2 or 4, wherein when the carbon source is pyrolyzed lignin, the pyrolysis of the carbon source or of the mixture of the carbon source and catalyst being performed at a pyrolysis temperature of 700°C -900°C before the first heating step.

6. Method of any of claims 1 - 5, wherein said first heating step is followed by the second heating step to complete the graphitization to an end product comprising at least 99,00 - 99,99 %, preferably at least 99,95 % of 2H graphite including possible traces of 3R graphite.

7. The method of any of claims 2 and 4 - 5, wherein the catalyst is mixed with the carbon source by dry milling to a final particle size of the catalyst and the lignin of 1 - 20 pm.

8. The method of any of claims 1 - 7, wherein the amount of catalyst is 10 - 20 percent of weight of the total mass of carbon source and catalyst.

9. The method of any of claims 1 - 8, wherein the catalyst is an oxide of Fe, such as Fe2Os, or a salt of Fe, such as Fe(NOs)3 and / or FeSCU in dry form or in water solution, a silicon (Si) catalyst, a silicon carbide (SiC) catalyst and / or an oxide or a salt of or an elemental form of Cu, Al, Ni, Mn and / or Co.

10. The method of any of claims 1 - 9, wherein said first heating step is performed gradually to a temperature between 1500 - 2000°C, preferably ca 1800°C, and maintaining the reached temperature of the first heating step for 1 - 5 hours, preferably 1 hour, whereafter the second heating step is performed gradually to a temperature between 2400 - 3500°C, preferably ca 3000°C, depending on e.g. the existing catalyst, and maintaining the reached temperature of the second heating step for 1 - 5 hours, preferably 1 hour.

11. The method of any of claims 1 - 10, wherein said heating steps are performed in an induction furnace under Ar or N2 atmosphere.

12. The method of any of claims 1 - 1 1 , where the catalyst is removed after the graphitization by volatilization and collected on a particulate filter.

13. The method of claim 12 wherein the catalyst particles thereafter are diluted with air to convert the catalyst to an oxide collected on a particulate filter and then recycled back to the process by removing it from the particulate filter.

14. A product prepared by a method of any of claims 1 - 13 comprising, 99,00 - 99,99%, preferably at least 99,95% 2H / 3R graphite.

15. The product of claim 14, wherein the ld / lgratio obtained from Raman spectroscopy is below 0.15.

16. The product of claim 14 or 15, wherein the produced 2H / 3R graphite has a first cycle coulombic efficiency of at least 75% and a specific capacity of at least 300 mAh / g + 500 cycles of life in half cell tests of recharging Li batteries.

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