Method for removing sulfur from carbonaceous materials

Abstract

The present invention relates to a method for producing a carbon material having a sulfur content of less than 0.8% by weight, a BET specific surface area of less than 20 m 2 / g, and an average crystallite size (Lc) in the c-axis direction of less than 10 Å as determined using X-ray diffraction. The method includes providing a bio-based carbon precursor having a sulfur content in the range of 1.0 to 5.0% by weight; subjecting the bio-based carbon precursor to heat treatment in an inert atmosphere to obtain a carbon material; and subjecting the carbon material to a desulfurization treatment in an inert atmosphere containing hydrogen gas and / or at least one carbon-containing gas to remove sulfur from the carbon material and obtain a carbon material having a sulfur content of less than 0.8% by weight.

Description

Technical Field

[0001] The present invention relates to a method for producing a carbon material having a low sulfur content from a sulfur-containing biobased carbon precursor. The present invention also relates to a carbon material having a low sulfur content; a negative electrode for a non-aqueous secondary battery containing the carbon material as an active material; and the use of the carbon material as an active material in a negative electrode of a non-aqueous secondary battery.

Background Art

[0002] Secondary batteries such as lithium-ion batteries are rechargeable batteries, that is, rechargeable batteries. For example, lithium-ion batteries are currently commonly used in consumer electronic devices, electric vehicles, and stationary energy storage systems. Lithium-ion batteries have a high energy density, a high operating voltage, low self-discharge, and low maintenance requirements.

[0003] In a lithium-ion battery, lithium ions flow from the negative electrode through the electrolyte to the positive electrode during discharge and return during charging. Currently, typically lithium compounds, particularly lithium transition metal oxides, such as lithium nickel manganese cobalt oxide, or alternatively lithium iron phosphate, are used as the positive electrode material, and a carbonaceous material is used as the negative electrode material.

[0004] Graphite (natural or synthetic graphite) is currently used as a negative electrode material in most lithium-ion batteries because of its high energy density and stable charge-discharge performance over time. Substitutes for graphite are amorphous carbon materials such as hard carbon (poorly graphitizable amorphous carbon) and soft carbon (easily graphitizable amorphous carbon) that lack long-range graphite order. Amorphous carbon can be used alone as an active electrode material or mixed with graphite.

[0005] Hard carbon often has good charge-discharge rate performance required for rapid charging devices and high-power systems. The electrochemical charge-discharge of hard carbon occurs between approximately 1.3 V vs. Li+ / Li and <0 V vs. Li+ / Li. When the electrode potential is plotted against the capacity, it includes a steadily sloping potential region exceeding approximately 0.1 V vs. Li+ / Li and a wide plateau potential region lower than this value. The practical capacity of hard carbon exceeds that of graphite and reaches a value of 500 mAh / g or more. The average electrode charge-discharge potential vs. Li+ / Li is higher for hard carbon than for graphite.

[0006] Both graphite and amorphous carbon act in a potential range outside the thermodynamic stability window of the electrolyte typically used in lithium-ion batteries. During the first charge, the electrolyte decomposes, and some of the decomposition products form a protective layer, the so-called "solid electrolyte interface" (SEI), on the electrode surface. The formation of the SEI causes charge to be irreversibly consumed mainly during the first charge, resulting in irreversible capacity in the first (few) cycles and a decrease in the initial Coulombic efficiency. When the SEI is sufficiently formed, electrolyte decomposition ends, enabling reversible cycling.

[0007] To prepare amorphous carbon for use in secondary batteries, various bio-based materials are used. For example, US2018162733A1 discloses a carbonaceous material obtained from coconut shells, and US6143268A discloses a carbonaceous material obtained from carbohydrates such as cellulose or sucrose.

[0008] Amorphous carbon can also be obtained from lignin, as described, for example, in WO9746314A1. Lignin is an aromatic polymer and is, for example, a main component in wood and one of the most abundant carbon sources on Earth. In recent years, with the development and commercialization of technologies for extracting lignin in the form of highly purified solid fine particles from the pulp manufacturing process, lignin has attracted great attention as a potential renewable alternative to the mainly aromatic chemical precursors currently supplied by the petrochemical industry. The amorphous carbon obtained from lignin is typically non-graphitizable, i.e., hard carbon. However, the hard carbon obtained from lignin has hitherto shown the problem of insufficient Coulomb efficiency.

[0009] Today, the most commercially relevant lignin source is kraft lignin. This lignin is obtained from hardwood or softwood through the kraft process. Lignin can be separated from alkaline black liquor, for example, using membrane filtration or ultrafiltration. LignoBoost is one common separation process, as described, for example, in WO2006031175A1. In this process, lignin is usually precipitated from alkaline black liquor by adding carbon dioxide to lower the pH level and then filtered. The lignin filter cake is reslurried and washed in the next step, usually using sulfuric acid under acidic conditions. The precipitated and washed lignin can be used as it is or further dried.

[0010] To facilitate the cleavage of the chemical bond between cellulose and lignin, sulfur-based reagents such as sodium sulfide are used during the pulping process, so kraft lignin contains a high level of sulfur. Sulfur also remains in the carbon-enriched material obtained from kraft lignin, resulting in a high residual sulfur level in the carbon-enriched material, typically exceeding 1.0 wt%. When using the carbon-enriched material in secondary batteries, due to the high sulfur content, side reactions may occur during battery cycling, which may reduce the long-term performance and lifespan of the battery.

[0011] Removing sulfur from kraft lignin by washing is difficult because most of the sulfur in lignin, typically more than 70%, is covalently bonded to carbon via carbon-sulfur bonds. Therefore, to remove sulfur from lignin, severe chemical treatments such as with alkalis, oxidizing agents or reducing agents are required. These chemical treatments not only increase costs but also cause simultaneous decomposition of the lignin structure, which may lead to problems in the quality control of the resulting lignin. When such lignin is converted to carbon, the resulting carbon may also exhibit structural defects and such carbon is not suitable for use in batteries.

[0012] Another option is to use lignins with a low initial sulfur content, such as organosolv lignin. However, such materials are typically more expensive than kraft lignin and are thus not very suitable for use as precursors for the industrial production of carbon materials.

[0013] EP2831182A2 describes the removal of sulfur by alkaline treatment of carbon black. The required alkaline chemicals are expensive and an additional neutralization step may be necessary because the pH of the carbon material is changed. In addition, the structure of the carbon material can be damaged.

[0014] Therefore, there is a need for an improved method for producing carbon materials from bio-based carbon precursors containing sulfur, and the resulting carbon should have a sufficiently low sulfur content and a sufficiently high initial Coulombic efficiency. SUMMARY OF THE INVENTION

[0015] An object of the present invention is to provide a carbon material obtained from a bio-based carbon precursor, which has a sufficiently low sulfur content to be used in energy storage applications without any adverse effects on the performance of the energy storage device.

[0016] A further object of the present invention is to provide a method for removing sulfur from a carbon material obtained from a bio-based carbon precursor.

[0017] A further object of the present invention is to provide a method for removing sulfur from carbon materials that is cost - effective and suitable for large - scale production.

[0018] The above object, as well as other objects recognized by those skilled in the art in view of the present invention, are achieved by various aspects of the present invention.

[0019] According to a first aspect, the present invention is a method for producing a carbon material having a sulfur content of less than 0.8 wt%, a BET specific surface area of less than 20 m 2 / g, and an average crystallite size (Lc) in the c - axis direction of less than 10 Å as quantified using X - ray diffraction, comprising: - providing a bio - based carbon precursor, wherein the bio - based carbon precursor has a sulfur content in the range of 1.0 to 5.0 wt%; - subjecting the bio - based carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of 500 °C to 1500 °C, wherein the heat treatment is carried out over a total time of 0.5 to 10 hours to obtain a carbon material; - subjecting the carbon material to desulfurization treatment in an inert atmosphere containing hydrogen gas and / or at least one carbon - containing gas, wherein the desulfurization treatment is carried out at one or more temperatures in the range of 800 °C to 1300 °C over a total time of 10 minutes to 5 hours, whereby sulfur is removed from the carbon material to obtain a carbon material having a sulfur content of less than 0.8 wt% and wherein the desulfurization treatment may be carried out at least partially during the heat treatment.

[0020] The method of the present invention according to the first aspect is based on the surprising recognition that sulfur can be removed from a carbon material obtained from a sulfur-containing bio-based carbon precursor by a desulfurization treatment carried out in an inert atmosphere containing hydrogen gas and / or at least one carbon-containing gas. The method of the present invention has the advantage of avoiding the need to remove sulfur from the bio-based carbon precursor, and thus is cost-effective, scalable, and enables its use in large-scale carbon manufacturing processes. In addition, the method of the present invention reduces the need for a desulfurization treatment that may damage the structure of the bio-based carbon precursor.

[0021] According to a second aspect, the present invention relates to a carbon material for a negative electrode of a non-aqueous secondary battery, which is derived from a bio-based carbon precursor having a sulfur content in the range of 1.0 to 5.0% by weight and has a sulfur content of less than 0.8% by weight; a BET specific surface area of less than 20 m 2 / g; and an average crystallite size (Lc) in the c-axis direction of less than 10 Å as determined by quantitative analysis using X-ray diffraction. The carbon material according to the second aspect can be obtained by the method according to the first aspect.

[0022] According to a third aspect, the present invention relates to a negative electrode for a non-aqueous secondary battery containing, as an active material, a carbon material obtainable by the method according to the first aspect or a carbon material according to the second aspect.

[0023] According to a fourth aspect, the present invention relates to the use of a carbon material obtainable by the method according to the first aspect or a carbon material according to the second aspect as an active material in a negative electrode of a non-aqueous secondary battery.

[0024] The carbon material according to the present invention has a low sulfur content of less than 0.8% by weight and a BET specific surface area of 20 m 2Less than / g, and the Lc value is less than 10 Å. By combining a low sulfur content with determined values for the BET specific surface area and Lc, a carbon material suitable for use as an active material in the negative electrode of a non-aqueous secondary battery can be obtained. The non-aqueous secondary battery containing the carbon material according to the present invention has desirable characteristics such as sufficiently high Coulomb efficiency and good long-term cycle performance.

Mode for Carrying Out the Invention

[0025] The method according to the first aspect of the present invention includes a step of providing a bio-based carbon precursor, wherein the bio-based carbon precursor has a sulfur content in the range of 1 to 5% by weight, preferably in the range of 1 to 3% by weight, or in the range of 1 to 2% by weight. In one embodiment, the bio-based carbon precursor has a sulfur content of at least 1% by weight. The term "bio-based carbon precursor" as used herein refers to any sulfur-containing bio-based material that can be converted into a carbon material by heat treatment. Examples of bio-based carbon precursors are tall pitch oil and sulfur-containing lignins such as kraft lignin and lignosulfonates.

[0026] The sulfur content in the bio-based carbon precursor and the carbon material can be quantified using elemental analysis such as inductively coupled plasma (ICP) spectrometry, organic elemental analysis (OEA), and X-ray fluorescence (XRF).

[0027] Preferably, the bio-based carbon precursor is kraft lignin, i.e., lignin obtained by the kraft process. Preferably, the kraft lignin is obtained from hardwood or softwood, and most preferably from softwood.

[0028] Kraft lignin can be obtained using the process disclosed in WO2006031175A1, generally referred to as the LignoBoost process. Typically, this process includes a step of precipitating lignin from alkaline black liquor by acidification; a step of separating the precipitated lignin; and a step of re-slurrying the lignin under acidic conditions at least once. The obtained lignin can be dried and pulverized, and thus provided as solid particles. The lignin may be further purified before being used in the method according to the present invention. The purification is typically carried out such that the purity of the lignin material is at least 90%, preferably at least 95%, more preferably at least 98% based on the dry weight of the lignin material. Accordingly, the lignin material used according to the method of the present invention preferably contains impurities such as cellulose and inorganic compounds in an amount of less than 10%, preferably less than 5%, more preferably less than 2% based on the dry weight of the lignin material. The sulfur content of the obtained lignin is typically in the range of 1 to 5% by weight. The major part of the sulfur present in the lignin is covalently bonded via sulfur-carbon bonds.

[0029] In one embodiment, the kraft lignin is provided in the form of agglomerated lignin having a particle size distribution such that at least 80% by weight of the agglomerates have a diameter in the range of 0.2 to 5.0 mm.

[0030] In the context of the present invention, when the particles are not spherical, the diameter of the particles is the equivalent spherical diameter of the particles. The equivalent spherical diameter is the equivalent volume spherical diameter.

[0031] Agglomerated lignin can be obtained by the method described in WO2021250604A1. Briefly, this method includes compressing lignin powder and pulverizing the compressed lignin to obtain agglomerated lignin. The agglomerated lignin can have a bulk density in the range of 0.5 g / cm 3 to 0.7 g / cm 3 and preferably in the range of 0.5 g / cm 3 to 0.6 g / cm 3 .

[0032] The compression of lignin is preferably carried out by roll compression. In the compression process, an intermediate product is produced. Here, lignin fine powder is usually supplied through a hopper and conveyed to the compression zone by a horizontal or vertical supply screw, where the material is compressed into flakes by compression rollers having a defined gap. By controlling the supply screw speed and the pressure generation in the compression zone, flakes of uniform density can be obtained. The pressure generation in the compression zone is preferably monitored and can be controlled by the rotational speed of the compression rolls. When the powder is dragged between the rollers, it enters a region called the nip region where the density of the material increases, and the powder is converted into flakes or ribbons. The rolls used have cavities. The depth of each cavity used in roll compression is from 0.1 mm to 10 mm, preferably from 1 mm to 8 mm, more preferably from 1 mm to 5 mm or from 1 mm to 3 mm. The specific pressing force exerted during compression can vary depending on the apparatus used for compression, but can be in the range of 1 kN / cm to 100 kN / cm. Apparatus suitable for carrying out the compression are known in the art.

[0033] In the grinding process, the intermediate product from the compression process is subjected to grinding or crushing by a rotary granulator, cage mill, beater mill, hammer mill, or crusher mill, and / or combinations thereof. During this process, a further intermediate product is produced.

[0034] After grinding, the ground material is preferably subjected to a screening process to remove fine materials. In addition, large materials such as aggregates with a diameter exceeding 5.0 mm can be removed and / or recycled to the grinding process.

[0035] Providing kraft lignin in the form of agglomerated lignin is advantageous because the lignin is less likely to melt / expand and change dimensions during subsequent heat treatment. In addition, by compressing the lignin powder into agglomerates, the dust during the handling of the lignin powder is reduced, thus avoiding problems such as explosions that can be caused by dust during processing.

[0036] In one embodiment, the bio-based carbon precursor further includes an additive. Any suitable additive such as a binder or a lubricant can be added to facilitate subsequent processing and improve the density and mechanical properties of the bio-based carbon precursor. In addition, additives that affect the properties of the final carbon material, such as additives that enhance functionality, may be added. The total amount of the additive is preferably less than 5 wt%, for example less than 2 wt%, based on the total dry weight of the additive and the bio-based carbon precursor.

[0037] The method according to the first aspect of the present invention includes subjecting a bio-based carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of 500 °C to 1500 °C, and the heat treatment is carried out over a total time of 0.5 to 10 hours to obtain a carbon material.

[0038] As used herein, the term "carbon material" refers to a non-graphitizable hard carbon material obtained from a bio-based carbon precursor. The carbon material consists essentially of carbon and contains at least 90 wt% carbon, or at least 95% carbon, or at least 99% carbon.

[0039] As used herein, the term "heat treatment" refers to a process of heating a bio-based carbon precursor at one or more temperatures for a sufficient time to increase the carbon content of the bio-based carbon precursor and convert the bio-based carbon precursor into a carbon material. Depending on the temperature during the heat treatment, different types of carbon materials such as charcoal or hard carbon can be obtained. To obtain hard carbon, a sufficiently high temperature (e.g., at least 1000 °C) must be applied at some point during the heat treatment.

[0040] The heat treatment is carried out such that the bio-based carbon precursor is heated to a temperature in the range of 500 °C to 1500 °C, preferably 600 °C to 1300 °C. The heat treatment is carried out over a time in the range of 0.5 to 10 hours. That is, the residence time of the bio-based carbon precursor in the apparatus used for the heat treatment is in the range of 0.5 to 10 hours.

[0041] The heat treatment may be carried out at the same temperature throughout the heat treatment, or it may be carried out with the temperature changing, for example, by increasing the temperature stepwise or using a temperature gradient. The heat treatment may include a temperature gradient from the starting temperature to the target temperature. The heating rate can be from 1 to 100 °C / min. For example, the heat treatment may include several intermediate temperatures with a temperature gradient before reaching the target temperature required for the carbonization of the bio-based carbon precursor. The heat treatment can be carried out as a batch process or a continuous process. Any suitable reactor can be used. The heat treatment is carried out under an inert atmosphere, preferably a nitrogen atmosphere.

[0042] Preferably, the heat treatment includes an initial heating step and preferably a subsequent final heating step. The initial heating step is preferably carried out at one or more temperatures in the range of 500 °C to 900 °C, for example, between 500 °C and 700 °C, over a time of 0.5 to 5 hours. The BET specific surface area of the product obtained after the initial heating step is typically in the range of 100 to 700 m 2 / g. The BET specific surface area is measured using nitrogen gas.

[0043] The final heating step is preferably carried out at one or more temperatures in the range of 900 °C to 1500 °C, for example, between 1000 °C and 1300 °C, over a time of 10 minutes to 3 hours. After the final heating step carried out at 1000 °C or higher, the BET specific surface area of the product obtained is typically less than 50 m 2 / g, preferably less than 20 m 2 / g. The BET specific surface area is measured using nitrogen gas.

[0044] The initial heating step and the final heating step can be carried out as individual steps or as a single directly continuous step. The initial heating step and the final heating step can include heating at one or more temperatures, as discussed above for the heat treatment. For example, the initial heating includes raising the temperature to about 700 °C. The final heating step is preferably carried out between 1000 °C and 1300 °C, such as about 1000 °C.

[0045] The heat treatment can be carried out continuously or in batch form. Heating can be carried out using methods known in the art and can be carried out in an inert atmosphere such as a nitrogen atmosphere. Preferably, the heating is carried out in a rotary kiln, a moving bed furnace, a pusher furnace, or a rotary hearth furnace. When carrying out the first heating step and the final heating step, these can be carried out in the same furnace or in different furnaces.

[0046] After the heat treatment, the obtained carbon material has a BET specific surface area of less than 20 m 2 / g, preferably less than 15 m 2 / g, and even more preferably less than 10 m 2 / g. The surface area is measured using the BET method with nitrogen gas. In the case of a carbon material to be used in an energy storage device such as a non-aqueous secondary battery, it is important that the surface area is about 20 m 2 / g or less; otherwise, the performance of the battery may be impaired. For example, a decomposition reaction with the electrolyte may occur, or the retention of the charge-discharge capacity may decrease. One way to obtain a low surface area is to carry out at least one stage of the heat treatment at a temperature of at least 1000 °C.

[0047] When the carbon material obtained after the heat treatment is quantified using X-ray diffraction, it has an average crystallite size (Lc) in the c-axis direction of less than 10 Å, preferably less than 9.5 Å. The Lc value corresponds to the average total thickness of the stacked carbon layers in the graphite-like region of carbon. It is important that the obtained carbon material has an Lc value of less than 10 Å, and as a result, a non-aqueous secondary battery using the carbon material as the negative electrode has good output characteristics such as charge-discharge performance and low resistance. In one embodiment, the obtained carbon material has an Lc value in the range of 8 Å to 10 Å, or 8 Å to 9.5 Å. As is known to those skilled in the art, by selecting an appropriate temperature during the heat treatment, a carbon material having an Lc value within the specified range can be obtained.

[0048] The obtained carbon material has a density of 1.4 to 2.1 g / cm 3It may have a true density of helium within the range. As is known to those skilled in the art, the true density of helium can be quantified using a pycnometer. It is important that the true density of helium is in the range of 1.4 to 2.1 g / cm 3 ; otherwise, when used as the active material of the negative electrode of a non-aqueous secondary battery, the doping and undoping capacities of the carbon material may decrease, and the irreversible capacity of the battery may increase. If the density of the carbon material is too low, the energy density of the electrode may also decrease.

[0049] The obtained carbon material may have an atomic ratio of hydrogen to carbon (H / C) of less than 0.01, preferably less than 0.005, as quantified by elemental analysis. If the H / C ratio is higher than 0.01, the irreversible capacity of the non-aqueous secondary battery in which the carbon material is used may increase. A low H / C ratio is also desirable in order to increase the aromaticity of the carbon structure and enable low electrical resistance. The H / C ratio can be controlled using the temperature used during the heat treatment.

[0050] The obtained carbon material may have an atomic ratio of oxygen to carbon (O / C) of less than 0.04, preferably less than 0.035, as quantified by elemental analysis. If the O / C ratio is higher than 0.04, the irreversible capacity of the non-aqueous secondary battery in which the carbon material is used may increase. A low O / C ratio is also desirable in order to enable low electrical resistance and good initial Coulombic efficiency.

[0051] Generally, it is desirable that both the ratios of H / C and O / C are low, because this indicates that most of the bio-based carbon precursor has been converted to carbon. When the obtained carbon material is used as the active material in the negative electrode of a non-aqueous secondary battery, residual hydrogen and oxygen may cause undesirable reactions and reduce the performance of the battery. Also, the presence of heteroatoms such as hydrogen and oxygen may interfere with the graphite planes within the graphite-like regions of the carbon material, causing defects that affect the electrochemical properties of the carbon material.

[0052] The H / C ratio and the O / C ratio can be quantified using various elemental analysis methods such as organic elemental analysis (OEA).

[0053] The obtained carbon material is quantified by X-ray diffraction and has an average lattice spacing (d 002 ) in the range of 3.5 Å to 4.0 Å, preferably in the range of 3.6 Å to 3.9 Å. The d 002 value corresponds to the distance between carbon layers within the graphite-like region of the carbon material. By having the d 002 value within the specified range, the carbon material can be used for constructing a battery having good performance, particularly with respect to capacity. To insert ions such as lithium or larger ions such as sodium, a sufficiently large d 002 value is required.

[0054] The method according to the first aspect further includes a step of subjecting the obtained carbon material to a desulfurization treatment in an inert atmosphere containing hydrogen gas and / or at least one carbon-containing gas. The desulfurization treatment removes sulfur from the carbon material and is carried out at one or more temperatures in the range of 800 °C to 1200 °C, preferably 900 °C to 1100 °C, for a total time of 10 minutes to 5 hours, preferably 30 minutes to 3 hours, so as to obtain a carbon material having a sulfur content of less than 0.8% by weight.

[0055] As used herein, the term "desulfurization treatment" refers to a gas treatment that removes sulfur from the carbon material and thus reduces the total amount of sulfur present in the carbon material.

[0056] As used herein, the term "inert atmosphere" refers to an atmosphere depleted of oxygen. Preferably, the oxygen content in the inert atmosphere is less than 1% by volume, for example less than 0.1% by volume. In one embodiment, the inert atmosphere during the desulfurization treatment is composed of nitrogen and hydrogen gas and / or at least one carbon-containing gas.

[0057] The desulfurization treatment may be carried out at the same temperature throughout the entire desulfurization treatment, or it may be carried out with the temperature changed, such as increasing the temperature step by step or using a temperature gradient. The desulfurization treatment may include a temperature gradient from the starting temperature to the target temperature. The heating rate may be 1 to 100 °C / min. For example, the desulfurization treatment may include several intermediate temperatures with a temperature gradient therebetween before reaching the target temperature required for sulfur removal.

[0058] The desulfurization treatment is carried out over a total time of 10 minutes to 5 hours. The total time in this case refers to the total time during which the carbon material is in contact with hydrogen gas and / or at least one carbon-containing gas in the reactor, or the total time during which hydrogen gas and / or at least one carbon-containing gas is supplied to the reactor.

[0059] The desulfurization treatment can be carried out using both hydrogen gas and at least one carbon-containing gas, or using only hydrogen gas, or using only at least one carbon-containing gas. In one embodiment, the desulfurization is carried out using hydrogen gas and at least one carbon-containing gas at least partially simultaneously. In one embodiment, the desulfurization treatment is carried out using hydrogen gas, and then at least one carbon-containing gas is added. In one embodiment, the desulfurization treatment is carried out using hydrogen gas first and then at least one carbon-containing gas in separate steps.

[0060] In one embodiment, the total amount of hydrogen gas and / or at least one carbon-containing gas during the desulfurization treatment is in the range of 5 to 30% by volume. The term "total amount" as used herein refers to the sum of the amount of hydrogen gas and the amount of at least one carbon-containing gas present in the reactor used for the desulfurization treatment. In one embodiment, the desulfurization treatment is carried out in an inert atmosphere containing nitrogen in the range of 70 to 95% by volume and hydrogen gas and / or at least one carbon-containing gas in the range of 5 to 30% by volume. In one embodiment, the desulfurization treatment is carried out in an inert atmosphere containing nitrogen in the range of 70 to 95% by volume and hydrogen gas in the range of 5 to 30% by volume. In one embodiment, the desulfurization treatment is carried out in an inert atmosphere containing nitrogen in the range of 80 to 95% by volume and at least one carbon-containing gas in the range of 5 to 20% by volume. In one embodiment, the concentration of hydrogen gas and / or at least one carbon-containing gas remains the same throughout the desulfurization treatment. In one embodiment, the concentration of hydrogen gas and / or at least one carbon-containing gas changes during the desulfurization treatment such that the concentration increases or decreases during the desulfurization treatment. Hydrogen gas or at least one carbon-containing gas may be supplied to the reactor together with an inert gas or as a separate stream. Preferably, hydrogen gas or at least one carbon-containing gas is supplied as a separate stream to improve the control of the supply. When hydrogen gas or at least one carbon-containing gas is supplied as a separate stream, it is also preferably mixed with an inert gas such as nitrogen.

[0061] In an alternative embodiment, the total amount of hydrogen gas and / or at least one carbon-containing gas during the desulfurization treatment is in the range of 5 to 60% by volume, such as in the range of 20 to 45% by volume, or in the range of 30 to 40% by volume. For example, the desulfurization treatment can be carried out in an inert atmosphere containing nitrogen in the range of 60 to 70% by volume and hydrogen gas in the range of 30 to 40% by volume.

[0062] The term "carbon-containing gas" as used herein refers to any carbon-containing compound that is in a gaseous state or can be volatilized into a gas. In one embodiment, at least one carbon-containing gas has the general structure C x H yhaving, preferably the number of carbon atoms ranges from 1 to 4, and the number of hydrogen atoms ranges from 2 to 8. In one embodiment, at least one carbon-containing gas is selected from at least one of methane, ethane, propane, methylene, ethylene, propylene, acetylene and methylacetylene. In a preferred embodiment, at least one carbon-containing gas is selected from acetylene and ethylene. As will be understood by those skilled in the art, the temperature during the desulfurization process may have to be adjusted depending on the type of carbon-containing gas used.

[0063] In some embodiments, only one carbon-containing gas is used for the desulfurization treatment. In a preferred embodiment, the carbon-containing gas is selected from acetylene or ethylene.

[0064] In some embodiments, more than one carbon-containing gas is used for the desulfurization treatment. In a preferred embodiment where two carbon-containing gases are used, the carbon-containing gases are acetylene and ethylene.

[0065] In some embodiments, hydrogen gas and one carbon-containing gas are used for the desulfurization treatment. In a preferred embodiment where both hydrogen gas and a carbon-containing gas are used, the carbon-containing gas is selected from methane or ethane.

[0066] In embodiments where at least one carbon-containing gas is used for the desulfurization treatment, a carbon material coated with a carbon coating may result from the desulfurization treatment. Depending on parameters such as the concentration and temperature of at least one carbon-containing gas, the coating may or may not be obtained. Typically, a high concentration of at least one carbon-containing gas and a high temperature are required to deposit a carbon coating on the carbon material. If a carbon-coated carbon material is desired, the desulfurization treatment using a carbon-containing gas can be carried out at least partially during a coating process such as during a chemical vapor deposition (CVD) process.

[0067] In one embodiment, the desulfurization treatment is carried out at least partially during the heat treatment. In one embodiment, the entire desulfurization treatment is carried out during the heat treatment. In one embodiment, the desulfurization treatment is carried out at least partially during the final heating step. By carrying out at least a part of the desulfurization treatment during the heat treatment, the required heating steps are reduced, thus achieving a more efficient process.

[0068] If the desulfurization treatment is carried out completely during the heat treatment, the total time of the heat treatment may be longer than the total time of desulfurization. Preferably, the desulfurization treatment is started (by supplying hydrogen gas or at least one carbon-containing gas) when the temperature reaches at least 800°C. The heat treatment can continue even after the desulfurization treatment is stopped.

[0069] In one embodiment, the entire desulfurization treatment is carried out after the heat treatment. In this embodiment, the desulfurization treatment starts by heating carbon in an inert atmosphere to a temperature of at least 800°C for a time of 0.5 to 2 hours before applying hydrogen gas and / or at least one carbon-containing gas. Heating ensures that the carbon material has a sufficient temperature at the start of the treatment with hydrogen gas and / or at least one carbon-containing gas. The carbon material may also be cooled in an inert atmosphere after the desulfurization treatment.

[0070] In one embodiment, the desulfurization treatment includes a first desulfurization step and a second desulfurization step. One or both of the first or second desulfurization steps can be carried out at least partially during the heat treatment. Parameters such as temperature, time, and the concentration and composition of the gas used during the desulfurization treatment may be the same or different between different desulfurization steps. By optimizing such parameters, more efficient removal of sulfur from the carbon material can be achieved. By carrying out the desulfurization treatment in two steps, more efficient removal of sulfur can be achieved.

[0071] The first desulfurization step and the second desulfurization step can be carried out as individual steps or as a single directly continuous step. The temperatures during the first heating step and the final heating step may be the same or different as described above for the desulfurization treatment.

[0072] In one embodiment, the first desulfurization step is carried out during the heat treatment. In one embodiment, the first desulfurization step is carried out in an inert atmosphere containing hydrogen gas. The amount of hydrogen gas in the first desulfurization step can be in the range of 5 to 30% by volume. The first desulfurization step can be carried out at a temperature in the range of 800°C to 1300°C for a total time in the range of 10 minutes to 3 hours. In one embodiment, the first desulfurization step is carried out during the final heating step. By carrying out the first desulfurization step during the final heating step, the number of process steps is reduced, so a more efficient process is achieved. As discussed above for the desulfurization treatment, the total time of the final heating step may be longer than the total time of the first desulfurization step.

[0073] In one embodiment, the second desulfurization step is carried out in a separate step after the heat treatment. In this embodiment, the second desulfurization step can be started by heating carbon to a temperature of at least 800°C for a time of 0.5 to 2 hours in an inert atmosphere before applying hydrogen gas and / or at least one carbon-containing gas. Heating ensures that the carbon material has a sufficient temperature at the start of the treatment with hydrogen gas and / or at least one carbon-containing gas. After the second desulfurization step, the carbon material can also be cooled in an inert atmosphere.

[0074] In one embodiment, the second desulfurization step is carried out in an inert atmosphere containing at least one carbon-containing gas. The amount of at least one carbon-containing gas can be in the range of 5 to 20% by volume. The second desulfurization step can be carried out at a temperature in the range of 800°C to 1100°C for a total time in the range of 10 minutes to 3 hours. The second desulfurization can be carried out during the CVD coating of the carbon material. By carrying out the two desulfurization steps, more efficient removal of sulfur can be achieved.

[0075] In one embodiment, the desulfurization treatment is carried out in an inert atmosphere containing hydrogen gas and is a first desulfurization step carried out during the final heating step; and a second desulfurization step carried out in an inert atmosphere containing at least one carbon-containing gas and preferably during CVD coating after the heat treatment. By carrying out the first desulfurization step in an atmosphere containing hydrogen gas, the temperature and time can be optimized so that sulfur removal becomes more efficient. Since the characteristics of hydrogen gas and at least one carbon-containing gas are different, different temperatures and times are appropriate for optimal sulfur removal in different atmospheres. Therefore, in some embodiments, it is advantageous to carry out the desulfurization treatment in two steps, using hydrogen gas in the first step and at least one carbon-containing gas in the second step. In addition, by simply supplying a carbon-containing gas in the second step, the formation of excessive soot is prevented.

[0076] The desulfurization treatment can be carried out continuously or in batch form. Any suitable reactor can be used. Preferably, the desulfurization treatment is carried out in a rotary kiln, a moving bed furnace, a fluidized bed, a pusher furnace, or a rotary hearth furnace. When the first desulfurization step and the second desulfurization step are carried out, they may be carried out in the same furnace or in different furnaces.

[0077] Sulfur is removed from the carbon material by desulfurization treatment. As a result of the desulfurization treatment, hydrogen sulfide, carbon disulfide, or other volatile sulfur compounds are considered to be formed. Due to their volatility, these compounds do not remain in the carbon. After the desulfurization treatment, the obtained carbon has a sulfur content of less than 0.8% by weight, for example less than 0.7% by weight, or less than 0.5% by weight, or less than 0.3% by weight. In one embodiment, the obtained carbon has a sulfur content in the range of 0 to 0.8% by weight, for example 0 to 0.7% by weight, 0 to 0.5% by weight, or 0 to 0.3% by weight. In one embodiment, the obtained carbon has a sulfur content of 0.01 to 0.8% by weight, for example 0.01 to 0.7% by weight, or 0.01 to 0.5% by weight, or 0.01 to 0.3% by weight. In one embodiment, the obtained carbon has a sulfur content of 0.1 to 0.8% by weight, for example 0.1 to 0.7% by weight, or 0.1 to 0.5% by weight, or 0.1 to 0.3% by weight.

[0078] In one embodiment, when the desulfurization treatment is carried out in an inert atmosphere containing at least one carbon-containing gas, the carbon material obtained after the desulfurization treatment has a BET specific surface area (measured with nitrogen gas) of less than 5 m 2 / g and a helium true density in the range of 1.4 to 1.8 g / cm 3 . In one embodiment, no carbon-containing gas is used during the desulfurization treatment. Instead, the carbon material has a BET specific surface area (measured with nitrogen gas) of less than 20 m 2 / g and a helium true density in the range of 1.4 to 2.1 g / cm 3 . The desulfurization treatment typically has no significant effect on the Lc value, d 002 value, or H / C ratio, regardless of which gas is used during the desulfurization treatment. Therefore, after the desulfurization treatment, these parameters remain within the same range as defined for the carbon material obtained after the heat treatment. The O / C ratio typically decreases somewhat due to the desulfurization treatment, especially when at least one carbon-containing gas is used in the desulfurization treatment.

[0079] In one embodiment, the method according to the first aspect of the present invention includes an additional step of preheating a bio-based carbon precursor in an oxidizing atmosphere at a temperature in the range of 180°C to 250°C for at least 0.5 hours, and the preheating is carried out before the heat treatment.

[0080] The preheating is carried out such that the bio-based carbon precursor is heated to a temperature in the range of 180°C to 250°C, preferably 180°C to 230°C. The preheating is carried out for at least 30 minutes, that is, the residence time of the bio-based carbon precursor in the apparatus used for preheating is at least 30 minutes. In one embodiment, the preheating is carried out for at least 1 hour, or at least 1.5 hours. Preferably, the preheating is carried out for less than 12 hours, or less than 6 hours. The preheating may be carried out at the same temperature throughout the preheating stage, or may be carried out with a stepwise increase in temperature or by using a temperature gradient to change the temperature. More preferably, the preheating is carried out such that the bio-based carbon precursor is first heated to a temperature in the range of 140°C to 175°C for at least 15 minutes, and then heated to a temperature in the range of 175°C to 250°C for at least 15 minutes.

[0081] The step of preheating the bio-based carbon precursor can be carried out continuously or in batch form. The preheating can be carried out using methods known in the art and is carried out in an oxidizing atmosphere such as air. Since the temperature used during preheating is low, oxygen is required. Preferably, the preheating is carried out in a rotary kiln, a moving bed furnace, or a rotating hearth furnace.

[0082] In an embodiment where the bio-based carbon precursor is lignin, the preheating step improves the thermal stability of lignin and a thermally stabilized lignin is obtained. In particular, the preheating step improves the processability of lignin in terms of avoiding melting / swelling during subsequent heat treatment and maintaining the shape and dimensions.

[0083] In one embodiment, the method pulverizes the carbon material to have an average particle size (D) in the range of 5 μm to 25 μm, preferably in the range of 7 to 15 μm. V50Further includes the step of obtaining a pulverized carbon material having ( ). When the carbon material is used as an active material in the negative electrode of a non-aqueous secondary battery, it is important that the average particle size is not too large, otherwise the charge and discharge may become too slow. On the other hand, if the average particle size is too small, the surface area increases. This has an adverse effect on battery characteristics such as capacity and reactivity with the electrolyte. The average particle size (D V50 ) can be quantified, for example, using laser diffraction.

[0084] Pulverization can be carried out by any method known in the art. Some examples include cutting mills, blade mixers, ball mills, impact mills, hammer mills, and / or jet mills. Optionally, after pulverization, sorting of fine / coarse particles by classification and / or sieving may be carried out.

[0085] In an embodiment where the heat treatment includes the first heating step and the final heating step, the pulverization step can be carried out after the first heating step or after the final heating step. Pulverization can be carried out either before or after the desulfurization treatment.

[0086] After heat treatment and desulfurization, the obtained carbon material may be further treated, for example, carbon coating by chemical vapor deposition (CVD), pitch coating, heat and / or chemical purification (other than desulfurization), further heat treatment, particle size adjustment, and mixing with other electrode materials, etc., in order to further improve its electrochemical performance.

[0087] The carbon material obtained by the method according to the first aspect is preferably used as an active material in the negative electrode of a non-aqueous secondary battery such as a lithium-ion battery. Since the sulfur content in the carbon material used as the active material of the negative electrode is low, the long-term performance and life of the non-aqueous secondary battery are improved. In addition, the Coulomb efficiency is sufficiently high.

[0088] When using the carbon material obtained by the method according to the first aspect for the production of a negative electrode, any suitable method for forming such a negative electrode can be utilized. In the formation of the negative electrode, the carbon-enriched material can be processed together with further components. Such further components can include, for example, one or more binders for forming the carbon material into an electrode, conductive materials such as carbon black, carbon nanotubes or metal powders, and / or further Li storage materials such as graphite or lithium. For example, the binder can be selected from, but not limited to, poly(vinylidene fluoride), poly(tetrafluoroethylene), carboxymethyl cellulose, natural butadiene rubber, synthetic butadiene rubber, polyacrylate, poly(acrylic acid), alginate, etc., or combinations thereof. Optionally, a solvent such as, for example, 1-methyl-2-pyrrolidone, 1-ethyl-2-pyrrolidone, water, or acetone is used during the processing.

[0089] According to a second aspect, the present invention relates to a carbon material for a negative electrode of a non-aqueous secondary battery, which is derived from a bio-based carbon precursor having a sulfur content in the range of 1.0 to 5.0% by weight, with a sulfur content of less than 0.8% by weight; a BET specific surface area of less than 20 m 2 / g; and an average crystallite size (Lc) in the c-axis direction of less than 10 Å as quantified using X-ray diffraction. The carbon material according to the second aspect can be obtained by the method according to the first aspect. The carbon material according to the second aspect is suitable for use as an active material in the negative electrode of a non-aqueous secondary battery due to its low sulfur content. The carbon material according to the second aspect can be further defined as described above with reference to the first aspect.

[0090] According to a third aspect, the present invention relates to a negative electrode for a non-aqueous secondary battery containing, as an active material, the carbon material obtainable by the method according to the first aspect or the carbon material according to the second aspect. The negative electrode according to the third aspect can be further defined as described above with reference to the first aspect.

[0091] According to a fourth aspect, the present invention relates to the use of a carbon material obtainable by the method according to the first aspect or a carbon material according to the second aspect as an active material in the negative electrode of a non-aqueous secondary battery. The negative electrode according to the fourth aspect can be further defined as described above with reference to the first aspect.

Examples

[0092] Example 1 A kraft lignin powder obtained from the LignoBoost process and having a sulfur content of 2% by weight was heat-treated to obtain a carbon material. The heat treatment was carried out in a nitrogen atmosphere by first heating to 500 °C and then raising the temperature to a final temperature of 1000 °C. After the obtained carbon material was cooled to room temperature, it was pulverized using a jet mill until it had an average particle size of 15 μm. The desulfurization treatment was carried out in a rotary kiln by first heating at 1050 °C for 1 hour in a nitrogen atmosphere, then lowering the temperature to 900 °C, and changing the atmosphere to 25% by volume of acetylene (and 75% by volume of nitrogen). After maintaining the heating for 2 hours, the gas atmosphere was switched to nitrogen and the carbon material was cooled to room temperature.

[0093] Example 2 A kraft lignin powder obtained from the LignoBoost process and having a sulfur content of 2% by weight was heat-treated to obtain a carbon material. The heat treatment was carried out first at 500 °C and then raised to a final temperature of 1000 °C in a nitrogen atmosphere. After the obtained carbon material was cooled to room temperature, it was pulverized using a jet mill until it had an average particle size of 10 μm. The desulfurization treatment was carried out in a fluidized bed reactor at 900 °C for 2 hours in a nitrogen atmosphere containing 5% hydrogen gas. After desulfurization, the carbon material was cooled to room temperature under a nitrogen atmosphere.

[0094] Example 3 The kraft lignin powder obtained from the LignoBoost process and having a sulfur content of 2% by weight was heat-treated to obtain a carbon material. The heat treatment was first carried out at 500 °C. After the obtained carbon material was cooled to room temperature, it was pulverized using a jet mill until it had an average particle size of 10 μm. The desulfurization treatment was carried out in a rotary kiln reactor at 1050 °C for 2 hours in a nitrogen atmosphere containing 40% hydrogen gas. After desulfurization, the carbon material was cooled to room temperature under a nitrogen atmosphere.

[0095] Example 4 - Comparative Example A carbon material was produced as outlined in Example 1 above, except that the desulfurization treatment was omitted.

[0096] The carbon materials obtained in Examples 1 to 4 were analyzed, and the results are shown in Table 1. The BET specific surface area was quantified using nitrogen. The true density was measured with helium using a pycnometer. The sulfur content, H / C ratio, and O / C ratio were quantified using organic elemental analysis using flash combustion. The Lc value and d 002 value were quantified using wide-angle X-ray diffraction.

[0097] Example 5 In this example, electrodes were fabricated from the carbon material powders obtained in Examples 1, 2, and 4. The electrodes were fabricated as follows: 82% by weight of carbon powder was mixed with 8% by weight of poly(vinylidene fluoride) binder dissolved in 1-methyl-2-pyrrolidone, and this mixture was coated on a Cu foil by the doctor blade method and dried. A laboratory-type three-electrode cell in which each cell included a carbon electrode, a Li metal counter electrode, and a Li metal reference electrode was constructed using a glass fiber separator and 1 M LiPF 6 dissolved in ethylene carbonate:dimethyl carbonate (weight ratio 1:1) as the electrolyte. The cell was charged and discharged galvanostatically between 0 mV vs. Li / Li+ and 1.5 V vs. Li / Li+ using a specific current of 74.4 mA / g (AM). Here, g(AM) indicates the mass of the active material in the electrode. The charge capacity, discharge capacity, and initial Coulombic efficiency were evaluated. An overview of the results of the carbon materials obtained in Examples 1, 2, and 4 is shown in Table 1. TIFF2025516515000001.tif45170

[0098] In view of the foregoing detailed description of the present invention, other modifications and variations will be apparent to those skilled in the art. However, it should be apparent that such other modifications and variations can be made without departing from the spirit and scope of the present invention.

Claims

1. Sulfur content less than 0.8% by weight, 20m ２ A method for producing a carbon material having a BET specific surface area of ​​less than / g and a c-axis mean crystallite size (Lc) of less than 10 Å, which can be quantified using X-ray diffraction, - A step of providing a bio-based carbon precursor, wherein the bio-based carbon precursor has a sulfur content in the range of 1.0 to 5.0% by weight; - A step of subjecting a bio-based carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of 500°C to 1500°C, wherein the heat treatment is carried out for a total time of 0.5 to 10 hours to obtain a carbon material; - A process of subjecting a carbon material to a desulfurization treatment in an inert atmosphere containing hydrogen gas and / or at least one carbon-containing gas, wherein the desulfurization treatment is carried out at one or more temperatures in the range of 800°C to 1300°C for a total time of 10 minutes to 5 hours, thereby removing sulfur from the carbon material and obtaining a carbon material having a sulfur content of less than 0.8% by weight. A method comprising a desulfurization treatment which may be carried out at least partially during the heat treatment.

2. The method according to claim 1, wherein the bio-based carbon precursor is Kraft Lignin.

3. The method according to claim 2, wherein Kraft lignin is provided in the form of aggregated lignin having a particle size distribution in which at least 80% by weight of the aggregates are in the range of 0.2 to 5.0 mm in diameter.

4. - A step of preheating a bio-based carbon precursor to a temperature in the range of 180°C to 250°C in an oxidizing atmosphere for at least 30 minutes. The method according to claim 1, further comprising the preheating step being performed before the heat treatment.

5. The carbon material is crushed to obtain an average particle size (D) in the range of 5 μm to 25 μm. ｖ５０ A process to obtain a crushed carbon material having ) The method according to claim 1, further comprising:

6. The method according to claim 1, wherein the desulfurization treatment comprises a first desulfurization step and a second desulfurization step.

7. The method according to claim 1, wherein the heat treatment comprises an initial heating step and a subsequent final heating step.

8. A carbon material for the negative electrode of a non-aqueous secondary battery, derived from a bio-based carbon precursor having a sulfur content in the range of 1.0 to 5.0 wt% and a sulfur content of less than 0.8 wt%; 20m ２ A carbon material having a BET specific surface area of ​​less than 10 Å / g, and a c-axis mean crystallite size (Lc) of less than 10 Å as quantified using X-ray diffraction.

9. Average particle size (D) in the range of 5 μm to 25 μm ｖ５０ The carbon material according to claim 8, having ).

10. 1.4 to 2.1 g / cm³ ３ The carbon material according to claim 8, having a helium true density in the range.

11. The carbon material according to claim 8, wherein, as determined by elemental analysis, it has a hydrogen atom to carbon atom (H / C) ratio of less than 0.

01.

12. The carbon material according to claim 8, wherein, as determined by elemental analysis, it has an oxygen atom to carbon atom (O / C) ratio of less than 0.

04.

13. The average lattice spacing (d) in the range of 3.5 Å to 4.0 Å is quantified by X-ray diffraction. ００２ The carbon material according to claim 8, having ).

14. The carbon material according to claim 8, wherein the bio-based carbon precursor is kraft lignin.

15. A negative electrode for a non-aqueous secondary battery, comprising a carbon material obtainable by the method described in claim 1 or the carbon material described in claim 8 as an active material.