SE547225C2

SE547225C2 - A method for producing a carbon enriched material

Info

Publication number: SE547225C2
Application number: SE2330479A
Authority: SE
Inventors: Abhishek Shetty; Mats Lindström; Vilhelm Olsson
Original assignee: Stora Enso Oyj
Priority date: 2023-10-26
Filing date: 2023-10-26
Publication date: 2025-06-10
Also published as: SE2330479A1; WO2025088440A1

Abstract

The present invention relates to a method for producing a carbon enriched material having a total metal content below 500 ppm and a pH value in the range of from 6.0 to 8.0. The method involves the steps of providing a biobased carbon precursor; subjecting the biobased carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of from 500 to 1500°C, wherein the heat treatment is carried out for a total time of from 0.5 to 10 hours, to obtain a carbon enriched material; providing the carbon enriched material with a carbon-based coating; subjecting the carbon enriched material to a de-mineralization treatment in an atmosphere comprising at least one halogen-containing gas, wherein the demineralization treatment is carried out at one or more temperatures in the range of from 800 to 1500 °C for a time period in the range of from 5 minutes to 10 hours, so as to obtain a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0; wherein the de-mineralization treatment is carried out after providing the carbon enriched material with a carbon-based coating.

Description

The present invention relates to a method for producing a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0. The present invention is also directed to a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0; a negative electrode for a non-aqueous secondary battery comprising said carbon enriched material as active material; and to use of said carbon enriched material as active material in a negative electrode of a non-aqueous secondary battery.

Background

Secondary batteries, such as lithium-ion batteries, are electrical batteries which can be charged and discharged many times, i.e. they are rechargeable batteries. For example, lithium-ion batteries are today commonly used for consumer electronic devices, electric vehicles and stationary energy storage systems. Lithium-ion batteries have high energy density, high operating voltage, low self-discharge and low maintenance requirements.

In lithium-ion batteries, lithium ions flow from the negative electrode through the electrolyte to the positive electrode during discharge, and back when charging. Today, typically a lithium compound, in particular a lithium transition metal oxide, such as lithium nickel manganese cobalt oxide, or alternatively a lithium iron phosphate, is utilized as material of the positive electrode and a carbonaceous material is utilized as material of the negative electrode.

Graphite (natural or synthetic graphite) is today utilized as material of the negative electrode in most lithium-ion batteries due to their high energy density and stable charge/discharge performance over time. An alternative to graphite is amorphous carbon materials, such as hard carbons (non-graphitizable amorphous carbons) and soft carbons (graphitizable amorphous carbons), which lack long-range graphitic order. Amorphous carbons can be used as sole active electrode materials or in mixtures with graphite.

Hard carbons often have good charge/discharge rate performance which is desired for fast charging devices and high-power systems. The electrochemical charge/discharge of hard carbons occurs between ca 1.3 V vs. Li+/Li and <0 V vs. Li+/li and, when plotting the electrode potential over capacity, comprises a steadily sloping potential region above approximately 0.1 V vs. Li+/Li and an extended potential plateau region below this value. The practical capacity of hard carbons can exceed that of graphite, reaching values of 500 mAh/g and beyond. The average electrode charge/discharge potential vs. Li+/Li is higher for hard carbons than for graphite.

Both graphite and amorphous carbons work at potential ranges outside the thermodynamic stability window of the electrolyte typically used in a lithium-ion battery. During the first charge the electrolyte is decomposed, and parts of the decomposition products form a protective layer at the electrode surface, the socalled “solid electrolyte interphase” (SEI). The formation of the SEI irreversibly consumes charge, mostly during the first charge, resulting in irreversible capacity in the first (few) cycle(s) and lowering the initial Coulombic efficiency. Once the SEI is fully formed, electrolyte decomposition comes to an end and reversible cycling becomes possible.

Various biobased materials have been used to prepare amorphous carbons for use in secondary batteries. For example, US2018162733A1 describes a carbonaceous material derived from coconut shell and US6143268A describes a carbonaceous material derived from carbohydrates such as cellulose or sucrose.

Amorphous carbons can also be derived from lignin, such as described in for example WO9746314A1. Lignin is an aromatic polymer, which is a major constituent in e.g. wood and one of the most abundant carbon sources on earth. In recent years, with development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry. Amorphous carbons derived from lignin are typically non-graphitizable, i.e. hard carbons. Today, the most commercially relevant source of lignin is kraft lignin. This lignin is obtained from hardwood or softwood through the kraft process. The lignin can be separated from alkaline black liquor using for example membrane- or ultrafiltration. LignoBoost is one common separation process and is described in for example WO2006031175A1. In this process lignin is precipitated from alkaline black liquor through reducing the pH level, usually by adding carbon dioxide, and then filtered off. The lignin filter cake is in the next step re-slurried under acidic conditions, commonly using sulfuric acid, and washed. The precipitated washed lignin can be used as it is or further dried.

There are stringent purity requirements for carbon enriched materials used in applications such as energy storage applications. Any metals impurities present, in particular transition metals, may for example interfere with the functionality of the carbon enriched material. For example, the electrochemical properties, the longterm stability, and the cycling performance may be impaired. It is believed that this is caused by precipitation of transition metals during charging and discharging, and also by decomposition of the electrolyte by chemical reactions in turn catalyzed by transition metals.

It is also important that the pH of the carbon enriched material to be used in energy storage applications is neutral. A neutral pH will facilitate further processing of an electrode comprising the carbon enriched material. For example, interactions between the carbon enriched material and a binder used in the electrode is improved when the pH of the carbon enriched material is neutral. The pH may also have an impact on the long-term corrosion of the electrode.

The metal content of carbon enriched materials obtained from biobased carbon precursors is typically too high since metal impurities present in the precursor will remain, and be enriched, after conversion to a carbon enriched material. In addition, carbon enriched materials obtained from biobased carbon precursors often have a high (i.e. alkaline) pH.

Carbon enriched materials intended for use in energy storage applications are often provided with an outer layer of a carbon material in order to reduce the specific surface area and reduce the porosity of the material. Such outer layers are typically provided by heat treatment in an atmosphere comprising a carbon-hydrogen gas.

The level of metal impurities in the biobased carbon precursor can be reduced e.g. by washing the precursor prior to converting it to a carbon enriched material such as described in for example WO2018004447 A1. It is also possible to reduce the metal content by various treatments of the obtained carbon enriched material. This however does not solve the problem with a too high pH value of the carbon enriched material.

The pH of the carbon enriched material may be reduced by washing in water.

However, such processes are time-consuming and also challenging from a technical perspective and thus not desirable for industrial scale production.

Thus, there is a need for an improved method for producing a carbon enriched material from a biobased carbon precursor, where the obtained carbon enriched material has a low metal content and a neutral pH. In addition, the method should be suitable for large scale production and minimize the number of additional processing steps.

Summary of the invention

It is an object of the present invention to provide a carbon enriched material, obtained from a biobased carbon precursor, that has a sufficiently low total metal content and a sufficiently neutral pH so that it can be used in energy storage applications without any negative effect on the performance of the energy storage device.

It is a further object of the present invention to provide a method for providing a carbon enriched material having a sufficiently low total metal content and a sufficiently neutral pH, where the carbon enriched material is obtained from a biobased carbon precursor, and where the method is cost effective and compatible with large-scale manufacturing.

The above-mentioned objects, as well as other objects as will be realized by the person skilled in the art in light of the present invention, are achieved by the various aspects of the present invention.

According to a first aspect, the present invention relates to a method for producing a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0, wherein the method comprises:

- providing a biobased carbon precursor;

- subjecting the biobased carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of from 500 to 1500 °C, wherein the heat treatment is carried out br a total time in the range of from 0.5 to 10 hours, to obtain a carbon enriched material;

- providing the carbon enriched material with a carbon-based coating; - subjecting the carbon enriched material to a de-mineralization treatment in an atmosphere comprising at least one halogen-containing gas, wherein the de-mineralization treatment is carried out at one or more temperatures in the range of from 800 to 1500°C fora time period in the range of from 5 minutes to 10 hours, so as to obtain a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0; and

wherein the de-mineralization treatment is carried out after providing the carbon enriched material with a carbon-based coating.

The pH of the carbon enriched material obtained by conversion of the biobased carbon precursor is alkaline. The pH of the carbon enriched material is further increased when the carbon enriched material is provided with a coating. It is believed by the present inventors that the alkaline pH is due to reduction of any metal impurities present in the biobased carbon precursor (and thus also remaining during conversion to a carbon enriched material) such that reactive metal species are formed during heat treatment due to reactions between the metal impurities and e.g. carbon and/or hydrogen gas. In addition, the number of acidic oxygen functional groups present in the carbon enriched material is decreased. The described reactions are believed to occur at the elevated temperatures used at several steps during the method according to the present invention, and to be even more pronounced when a carbon-containing gas is present, such as during the coating step.

The inventive method according to the first aspect is based on the surprising realization that a carbon enriched material with a sufficiently low total metal content and neutral pH for use in energy storage applications can be obtained from a biobased carbon source by introducing only one additional process step involving subjecting the carbon enriched material to a de-mineralization treatment with at least one halogen-containing gas. Surprisingly, the present inventors noted that if the demineralization treatment was carried out after providing the carbon enriched material with a carbon-based coating, no additional neutralization treatment was required to obtain a neutral carbon enriched material. The de-mineralization treatment thus both reduced the amount of metal impurities and reduced the pH from the alkaline level reached after conversion to a carbon enriched material and the subsequent coating step. Thus, by utilizing the inventive method, there is no need to pre-treat the biobased carbon precursor in order to reduce the amount of metal impurities, and there is further no need to neutralize the obtained carbon enriched material.

Therefore, the inventive method has the advantage of being both cost-efficient and scalable, enabling its use in large-scale carbon production processes.

Importantly, as noted by the present inventors, if the step of providing the carbon enriched material with a carbon-based coating is instead performed after the demineralization treatment, the pH of the obtained carbon enriched material is not neutral. In such embodiments, an additional neutralization step is required to obtain a neutral carbon enriched material.

According to a second aspect, the present invention relates to a carbon enriched material derived from a biobased carbon precursor, wherein the carbon enriched material has a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0. The carbon enriched material of the second aspect may be obtained by the method of the first aspect.

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

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

The carbon enriched material of the present invention has a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0. The low metal content in combination with the neutral, or almost neutral, pH results in a carbon enriched material suitable for use as the active material in a negative electrode of a nonaqueous secondary battery.

Detailed description

The method according to the first aspect of the present invention involves a step of providing a biobased carbon precursor. The term “biobased carbon precursor”, as used herein, refers to any biobased material that can be converted to a carbon enriched material by a heat treatment. The biobased carbon precursor may for example be derived from biomass such as any type of wood, plants, shells, seeds, or bamboo. In some embodiments, the biobased carbon precursor is selected from lignin, sawdust, woodchips and/or coconut shells.

In some embodiments, the biobased carbon precursor further comprises an additive. Any suitable additives, such as binders or lubricants, may be added to facilitate the subsequent processing and to improve the density and mechanical properties of the biobased carbon precursor. In addition, additives having an influence on the properties of the final carbon enriched material may be added, such as functionalityenhancing additives. The total amount of additive(s) is preferably less than 5 wt%, such as less than 2 wt%, as based on the total dry weight of the additive and the biobased carbon precursor.

In preferred embodiments, the biobased carbon precursor is lignin, preferably kraft lignin. It is intended throughout the present disclosure that the term "lignin" refers to any kind of lignin which may be used as the carbon precursor for making a carbon enriched material. Examples of said lignin are, but are not limited to, lignin obtained from vegetable raw material such as wood, e.g. softwood lignin, hardwood lignin, and lignin from annular plants. Also, the lignin can be chemically modified.

Preferably, the lignin has been purified or isolated before being used in the method according to the present invention. The lignin may be isolated from black liquor and optionally be further purified before being used in the method according to the present invention. The purification is typically such that the purity of the lignin is at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin material. Thus, the lignin material used according to the method of the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% impurities, such as cellulose, carbohydrates and inorganic compounds, based on the dry weight of the lignin material.

The lignin used in method according to the present invention may be obtained through different extraction methods such as an organosolv process or a kraft process. The lignin may also be obtained from processes such as steam explosion or acidic pre-treatment followed by enzymatic hydrolysis. Preferably, the lignin used in the method according to the present invention is kraft lignin, i.e. lignin obtained through the kraft process. The kraft lignin may be obtained from hardwood or softwood. The lignin may be obtained by the process disclosed in WO2006031175 A1 commonly referred to as the LignoBoost process. Typically, this process involves the steps of precipitation of lignin from alkaline black liquor by acidification; separation of the precipitated lignin; and re-slurrying the lignin under acidic conditions at least once. The obtained lignin may be dried and pulverized and thus provided as solid particles.

In embodiments where the biobased carbon precursor is lignin, the lignin may be provided in the form of agglomerated lignin having an average particle size in the range of from 0.8 to 2.0 mm.

In the context of the present invention, the average particle size is defined as the volume average particle size (DV5o). This value refers to the maximum particle size below which 50% of the volume of the sample exists. The particle size is in the context of the present invention taken to be the diameter of the particle. The average particle size may be determined using for example laser diffraction. In the context of the present invention, the diameter of a particle is the equivalent spherical diameter of the particle, if the particle is not spherical. The equivalent spherical diameter is the diameter of a sphere of equivalent volume.

The term “agglomerated lignin” as used herein refers to macroscopic particles in turn comprising clustered smaller particles of lignin. By providing the lignin in agglomerated form, a more compact and hard material is achieved. Hard agglomerates are advantageous during subsequent processing as they can resist physical impact during processing.

The agglomerated lignin may be obtained by the method described in WO2021250604A1. Briefly the method comprises compacting lignin powder and crushing the compacted lignin to obtain agglomerated lignin.

The compaction of the lignin is preferably carried out by roll compaction. Lignin powder is usually fed through a hopper and conveyed by means of a horizontal or vertical feeding screw into a compaction zone where the material is compacted into flakes by compaction rollers with a defined gap. By controlling the feeding screw speed and the pressure development in the compaction zone, flakes with uniform density can be obtained. As the powder is dragged between the rollers, it enters what is termed as the nip area where the density of the material is increased, and the powder is converted into a flake or ribbon. The specific press force exerted during the compaction may vary depending on the equipment used for compaction, but may be in the range of from 1 to 100 kN/cm. Equipment suitable for carrying out the compaction is known in the art.

In the crushing step, the compacted lignin is subjected to crushing or grinding, such as by means of rotary granulator, cage mill, beater mill, hammer mill or crusher mill and/or combinations thereof. After crushing, the crushed material is preferably subjected to a sieving step, to remove fine material. In addition, large material may be removed and/or recirculated back to the crushing step.

Providing the lignin in the form of agglomerated lignin is advantageous as the lignin is less prone to melting/swelling and dimensional changes during the subsequent heat treatment. In addition, dusting during handling of lignin powders is decreased by compacting lignin powder to agglomerates, thus avoiding problems such as explosions that can be caused by lignin dust during the processing. It is particularly advantageous to provide kraft lignin in the form of agglomerated lignin.

The method according to the first aspect of the present invention may comprise a further step of pre-heating the biobased carbon precursor at one or more temperatures in the range of from 140 to 300 °C fora time period of at least 30 minutes in an oxidative atmosphere. The pre-heating step is carried out prior to the heat treatment.

The pre-heating is carried out such that the biobased carbon precursor is heated at one or more temperatures in the range of from 140 to 300 °C, preferably from 180 to 250 °C. The pre-heating is carried out for at Ieast30 minutes, i.e. the residence time of the biobased carbon precursor inside the equipment used for the pre-heating is at least 30 minutes. In one embodiment, the pre-heating is carried out for at least 1 hour, or at least 1.5 hours. Preferably, the pre-heating is carried for less than 12 hours, or less than 6 hours. The pre-heating may be carried out at the same temperature throughout the entire pre-heating stage or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. More preferably, the pre-heating is carried out such that the biobased carbon precursor is first heated at a temperature in the range of from 140 to 175°C for a period of at least 15 minutes and subsequently heated at a temperature in the range of from 175 to 250 °C for at least 15 minutes.

The step of pre-heating the biobased carbon precursor can be carried out continuously or in batch mode. The pre-heating can be carried out using methods and equipment as known in the art. For example, the pre-heating may be carried out in a rotary kiln, moving bed furnace or rotary hearth furnace.

Oxidative species that can react so as to cross-link lignin are present in the oxidative atmosphere. The pre-heating may be carried out for example in the presence of oxygen, iodine, ozone, nitrogen dioxide, nitrobenzene, hydrogen peroxide and peracetic acid. Preferably, the pre-heating is carried out in air. Alternatively, any suitable oxidative species may be supplied in a nitrogen atmosphere. The carbon yield is typically improved by carrying out a pre-heating step. The biobased carbon precursor may be cross-linked by pre-heating in an oxidative atmosphere. This increases the stability of the biobased carbon precursor during the subsequent heat treatment. In embodiments where the biobased carbon precursor is lignin, such as kraft lignin, the thermal stability of the lignin is improved by carrying out a preheating step, such that a thermally stabilized (i.e. cross-linked) lignin is obtained. In particular, the processability of the lignin in terms of avoiding melting/swelling and retaining shape and dimension during the subsequent heat treatment is improved by the pre-heating step.

The method according to the first aspect of the present invention involves a step of subjecting the biobased carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of from 500 to 1500°C, wherein the heat treatment is carried out for a total time of from 0.5 to 10 hours, to obtain a carbon enriched material.

The term “carbon enriched material” as used herein refers to a carbon material obtained from a biobased carbon precursor. The carbon enriched material consists essentially of carbon, such that it comprises at least 90 wt% carbon, or at least 95% carbon, or at least 99% carbon. The carbon enriched material of the present invention is also provided with a carbon-based coating. The term “carbon enriched material” as used herein thus encompasses both the carbon enriched material prior to coating, and the carbon-coated carbon enriched material obtained after the step of providing the carbon enriched material with a carbon-based coating. Depending on the carbon materials and also on the analytical technique used, it may not always be possible to distinguish the carbon-based coating from the carbon enriched material on which it was deposited.

The term “heat treatment” as used herein, refers to a process of heating the biobased carbon precursor at one or more temperatures and for a sufficient time so that the carbon content of the biobased carbon precursor is increased, and so that the biobased carbon precursor is converted to a carbon enriched material.

Depending on the temperature during the heat treatment, different types of carbon enriched materials, such as charcoal or hard carbon, can be obtained. Hard carbon may be obtained for example by applying a sufficiently high temperature (e.g. at least 1000°C) at some point during the heat treatment.

The heat treatment is carried out such that the biobased carbon precursor is heated to at least one temperature in the range of from 500 to 1500°C, preferably from 600 to 1300°C. Preferably, the temperature exceeds 1000°C during at least one stage of the heat treatment. Thus, the biobased carbon precursor is preferably subjected to a temperature in the range of from 1000 to 1500°C duiing the heat treatment. The heat treatment is carried out for a time period in the range of from 0.5 to 10 hours, i.e. the residence time of the biobased carbon precursor inside the equipment used for the heat treatment is in the range of from 0.5 to 10 hours.

The heat treatment may be carried out at the same temperature throughout the entire heat treatment or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. The heat treatment may comprise a temperature ramp from a starting temperature to a target temperature. The heating rate may be 1-100 °C/min.For example, the heat treatment may involve several intermediate temperatures, with temperature ramps in between them, before reaching the target temperature needed for conversion of the biobased carbon precursor to a carbon enriched material. The heat treatment is carried out under inert atmosphere, preferably nitrogen atmosphere.

The term “inert atmosphere” as used herein, refers to an atmosphere that is depleted of oxygen. Preferably, the content of oxygen in the inert atmosphere is less than 1 vol%, such as less than 0.1 vol%. Preferably, a nitrogen atmosphere is used in all steps of the method according to the present invention where an inert atmosphere is required.

Preferably, the heat treatment comprises a first heating step, preferably followed by a final heating step. The first heating step is preferably carried out at one or more temperatures in the range of from 500 to 900 °C, such as between 500 and 700 °C, for a time period of from 0.5 to 5 hours. The BET specific surface area of the product obtained after the first heating step is typically in the range of from 100 to 700 m<2>/g. The BET specific surface area is measured using nitrogen gas.

The final heating step is preferably carried out at one or more temperatures in the range of from 900 to 1500 °C, such as between 1000 aid 1500 °C or between 1000 and 1300°C, for a time period of from 10 minutes to3 hours. After a final heating step carried out at 1000 °C or higher, the BET specfic surface area of the product obtained is typically less than 20 m<2>/g, preferably less than 15 m<2>/g, and more preferably less than 10 m<2>/g.

The first and final heating steps may be carried out as discrete steps or as one single step in direct sequence. The first and final heating steps may involve heating at one or more temperatures, as discussed above for the heat treatment. For example, the first heating step involves increasing the temperature to about 700 °C. The final heating step is preferably carried out between 1000 and 1300°C, such as at about 1000°C.

The heat treatment can be carried out continuously or in batch mode. The heating can be carried out using methods known in the art. Preferably, the heating is carried out in a rotary kiln, moving bed furnace, pusher furnace or rotary hearth furnace. If a first heating step and a final heating step is carried out, these may be carried out in the same reactor or in different reactors.

After the heat treatment, the obtained carbon enriched material typically has a BET specific surface area of less than 20 m<2>/g, preferably less than 15 m<2>/g and more preferably less than 10 m<2>/g. For a carbon enriched material that is to be used in an energy storage device, such as in a non-aqueous secondary battery, it is important that the surface area is not higher than about 20 m<2>/g, as the performance of the battery may otherwise be impaired. One way of obtaining a low surface area is to carry out at least one stage of the heat treatment at a temperature of at least 1000°C.

The obtained carbon enriched material typically has an alkaline pH value. For example, the pH may be higher than 10.0, or higher than 11.0. It is believed by the present inventors that the high pH may be explained by metal impurities, in particular alkali metals and alkali earth metals, such as Na, Ca and K, present in the biobased carbon precursors being reduced by reactions with e.g. carbon at the high temperatures used during the heat treatment. This in turn results in the formation of reactive metal species. Simultaneously, the number of acidic oxygen functional groups on the precursor/carbon enriched material is decreased during the heat treatment.

The obtained carbon enriched material is preferably a hard carbon material. A hard carbon material may for example be obtained by carrying out at least one stage of the heat treatment of the biobased carbon precursors at a temperature of about 1000°C.

The obtained carbon enriched material may have a helium true density in the range of from 1.4 to 2.1 g/cm<3>. The helium true density may be determined using a pycnometer, as known by a person skilled in the art. It is important to have a helium true density in the range of from 1.4 to 2.1 g/cm<3 >as the doping and de-doping capacity of the carbon enriched material when used as the active material in the negative electrode of a non-aqueous secondary battery may otherwise be reduced, and the irreversible capacity of the battery may become large. If the true density of the carbon enriched material is too low, the energy density of the electrode may also be decreased.

The obtained carbon enriched material may have a hydrogen atom to carbon atom (H/C) ratio of less than 0.01, preferably less than 0.005, as determined by elemental analysis. If the H/C ratio is higher than 0.01, the irreversible capacity of a nonaqueous secondary battery in which the carbon enriched material is used, can become large. A low H/C ratio is desired also in order to enable a low electrical resistance, due to increased aromaticity of the carbon structure. The temperature used during heat treatment can be used to control the H/C ratio.

The obtained carbon enriched material may have an oxygen atom to carbon atom (O/C) ratio of less than 0.04, preferably less than 0.035, as determined by elemental analysis. If the O/C ratio is higher than 0.04, the irreversible capacity of a nonaqueous secondary battery in which the carbon enriched material is used, can become large. A low O/C ratio is desired also in order to enable a low electrical resistance and a good initial Coulombic efficiency.

In general, low ratios of both H/C and O/C are desired, as this indicates that a large portion of the biobased carbon precursor has been converted to carbon. Remaining hydrogen and oxygen can contribute to unwanted reactions when the obtained carbon enriched material is used as active material in the negative electrode of a non-aqueous secondary battery, thus reducing the performance of the battery. The presence of heteroatoms like hydrogen and oxygen may also interfere with the graphitic planes in graphitic-like areas of the carbon enriched material and thus cause defects which will have an impact on the electrochemical properties of the carbon enriched material.

Different methods of elemental analysis, such as organic elemental analysis (OEA), may be used to determine the H/C ratio and the O/C ratio.

The obtained carbon enriched material may have an average lattice spacing (d002) in the range of from 3.5 Å to 4.0 Å, preferably in the range of from 3.6 Å to 3.9 Å, as determined by X-ray diffraction. The d002 value corresponds to the distance between carbon layers in a graphite-like area of the carbon enriched material. By having a doo2 value in the defined range, the carbon enriched material can be used to construct a battery with good performance, in particular with regards to capacity. A sufficiently large d002 value is required for intercalation of ions, like lithium, and also larger ions, like sodium.

In some embodiments, the method according to the first aspect of the present invention further comprises a step of pulverizing the carbon enriched material so as to obtain a pulverized carbon enriched material having an average particle size (DV5O) in the range of from 1 to 25 μm, wherein the step of pulverizing is carried out prior to providing the carbon enriched material with a carbon-based coating. When using the carbon material as active material in a negative electrode of a nonaqueous secondary battery, it is important that the average particle size is not too large as charging and discharging may otherwise be too slow. If on the other hand the average particle size is too small, the specific surface area will increase. This will have a negative impact on battery properties such as capacity and reactivity with the electrolyte. In some embodiments, it is therefore advantageous that the carbon enriched material has an average particle size in the range of from 5 to 25 μm. In some embodiments, it is instead desirable to have a small particle size, such that the average particle size of the carbon enriched material is in the range of from 1 to 10 μm. For example, charging and discharging will be rapid when a small particle size is used. In addition, in some applications a small average particle size is desirable in order to facilitate production of electrodes.

Pulverization may be carried out by any method as known in the art. Some examples include a cutting mill, blade mixer, ball-mill, impact mill, hammer mill and/or jet-mill. Optionally, fine/coarse particle selection by classification and/or sieving may be performed subsequent to the pulverization.

In embodiments where the heat treatment comprises a first heating step and a final heating step, the step of pulverization may be carried out after the first heating step or after the final heating step. Preferably, the step of pulverization is carried out after the first heating step and prior to the final heating step.

The method according to the first aspect of the present invention further involves a step of providing the carbon enriched material with a carbon-based coating. By providing the carbon enriched material with a carbon-based coating, the specific surface area of the carbon enriched material is further reduced. This provides the carbon enriched material with improved properties when used as active material in an anode of a secondary battery. In particular, the irreversible capacity of the material is reduced, and the initial coulombic efficiency is increased. The carbonbased coating forms an outer layer of the carbon enriched material. This outer carbon layer is not always distinguishable from the carbon enriched material it was deposited on.

In preferred embodiments, the step of providing the carbon enriched material with a carbon-based coating involves heating the carbon enriched material in an inert atmosphere comprising at least one carbon-containing gas at one or more temperatures in the range of from 800 to 1200°C fora time period in the range of from 10 minutes to 3 hours. The carbon-based coating is in such embodiments deposited on the carbon enriched material by means of chemical vapour deposition (CVD). In such embodiments, a carbon-containing gas is injected into a reactor, reacts and is deposited on the surface of the carbon enriched material. Due to the high temperatures used during the coating step, the deposited carbon-based coating is converted to a carbon enriched material. Depending on the type of carboncontaining gas and the temperature used, a hard carbon or soft carbon coating is obtained on the carbon enriched material.

The step of heating the carbon enriched material in an inert atmosphere comprising at least one carbon-containing gas may be carried out at the same temperature throughout the entire step or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. The heating may comprise a temperature ramp from a starting temperature to a target temperature. The heating rate may be 1-100°C/min. For example, the heating may involve several intermediate temperatures, with temperature ramps in between them, before reaching the target temperature needed for deposition of carbon.

The heating in inert atmosphere comprising at least one carbon-containing gas is carried out for a total time period of from 10 minutes to 3 hours. The total time period in this case refers to the total time that the carbon enriched material is in contact with the at least one carbon-containing gas inside a reactor, or the total time during which the at least one carbon-containing gas is supplied to the reactor.

The term “carbon-containing gas” as used herein refers to any carbon-containing compound that is in gas form, or that can be volatilized to a gas. In one embodiment, the at least one carbon containing gas has the general structure CxHy and preferably the number of carbon atoms is in the range of from 1 to 4, and the number of hydrogen atoms is in the range of from 2 to 8. In one embodiment, the at least one carbon-containing gas is selected from at least one of methane, ethane, propane, ethylene, propylene, acetylene and methylacetylene. In a preferred embodiment, the at least one carbon-containing gas is selected from acetylene and ethylene. As realized by a person skilled in the art, the temperature during the coating step may have to be adjusted depending on the type of carbon-containing gas used.

The carbon-containing gas may be injected into the reactor such that it is mixed with an inert gas in the reactor. Alternatively, the carbon-containing gas may be mixed with an inert gas prior to injection into the reactor. The amount of carbon-containing gas may be in the range of from 5 to 90 vol%. The remaining volume is made up of inert gas, preferably nitrogen.

Any suitable equipment may be used for providing the carbon enriched material with a carbon-based coating, and the process can be carried out continuously or in batch mode. For example, the coating step may be carried out in a rotary kiln, pusher or roller kiln, fluidized bed or rotary hearth furnace.

In an alternative embodiment, the step of providing the carbon enriched material with a carbon-based coating is carried out using pitch-coating. As known by a person skilled in the art, there are several different ways of providing a carbon coating using pitch. For example, the pitch may be provided as a powder coating on the carbon enriched material. When heated, the powder will melt and cover the carbon enriched material. The pitch may also be dissolved in a solution also comprising the carbon enriched material. Another example involves melt-mixing where pitch is melted prior to applying it to the carbon enriched material, by means of e.g. kneading, pressing or extrusion, as a coating. The pitch coating is then converted to carbon by a heat treatment carried out at one or more temperatures in the range of from 800 to 1500 °C.

In embodiments where the method according to the first aspect comprises a step of pulverizing the carbon enriched material, the step of providing the carbon enriched material with a carbon-based coating is carried out after the step of pulverization. This is advantageous since the coating will constitute the outermost layer of the carbon enriched material.

In embodiments where the heat treatment comprises a first heating step and a final heating step, the step of providing the carbon enriched material with a carbon-based coating is preferably carried out after the final heating step.

The step of providing the carbon enriched material with a carbon-based coating may increase the pH value of the carbon enriched material further as compared to the pH of the carbon enriched material obtained after conversion of the biobased carbon precursor to a carbon enriched material. This is in particular the case when a carbon-containing gas is used in the coating step, since the carbon and hydrogen gases contribute to the reductive reactions with metals in the carbon enriched material, as well as in reactions with acidic oxygen functional groups.

The method according to the first aspect of the present invention further involves a step of subjecting the carbon enriched material to a de-mineralization treatment in an atmosphere comprising at least one halogen-containing gas, wherein the demineralization treatment is carried out at one or more temperatures in the range of from 800 to 1500 °C for a time period in the range of from 5 minutes to 10 hours, so as to obtain a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0.

The term “de-mineralization treatment” as used herein refers to a treatment of removing inorganic impurities, such as metals, from the carbon enriched material. Without being bound by any theory, it is believed by the present inventors that the metal impurities present in the carbon enriched material will react with the halogen compounds in the halogen-containing gas. The formed metal halides will be removed from the carbon enriched materials at the high temperatures used during the de-mineralization treatment. Therefore, the total metal content of the carbon enriched material will be lower after the de-mineralization treatment compared to before. All metals will typically not be removed by the de-mineralization treatment. It is believed by the present inventors that the remaining metal impurities have however also reacted with the halogen-containing gas such that the metals are no longer in an alkaline oxide form but in a neutral form. Therefore, the pH of the carbon enriched material is in a range of from 6.0 to 8.0, such as about 7, after the de-mineralization treatment.

The de-mineralization treatment may be carried out at the same temperature throughout the entire treatment or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. The demineralization treatment may comprise a temperature ramp from a starting temperature to a target temperature. The heating rate may be 1-100°C/min. For example, the heating may involve several intermediate temperatures, with temperature ramps in between them, before reaching the target temperature needed for de-mineralization. The de-mineralization treatment may be more efficient at a higher temperature.

The de-mineralization treatment is carried out for a total time period of from 5 minutes to 10 hours, such as from 5 minutes to 5 hours, or from 5 minutes to 2 hours. The total time period in this case refers to the total time that the carbon enriched material is in contact with the at least one halogen-containing gas inside a reactor, or the total time during which the at least one halogen-containing gas is supplied to the reactor. To ensure a sufficient de-mineralization, it is sometimes advantageous to carry out the de-mineralization treatment for a time period in the range of from 4 to 10 hours, such as from 5 to 8 hours. In particular, a longer time period is required if a large batch of material is to be subjected to the demineralization treatment. However, from a cost perspective it is often preferable to carry out the de-mineralization for a shorter period of time, such as from 5 minutes to 3 hours, or from 5 minutes to 2 hours, while ensuring that a sufficient demineralization is enabled.

The term “halogen-containing gas” as used herein refers to any gas comprising at least one halogen-containing compound in gas form. The halogen may be selected from at least one of chlorine, fluorine, bromine or iodine. Preferably, the halogen is selected from at least one of chlorine or fluorine, more preferably the halogen is chlorine. Thus, the at least one halogen-containing gas may be selected from a chlorine-containing gas and a fluorine-containing gas, more preferably a chlorinecontaining gas, such as Cl2 or HCl. In some embodiments, one halogen-containing gas is used during the de-mineralization treatment. In other embodiments, a mixture of two or more halogen-containing gases is used during the de-mineralization treatment.

The amount of the at least one halogen-containing gas used during the demineralization step may be in the range of from 1 to 100 vol%, such as from 1 to 20 vol%, or from 10 to 90 vol%, or from 10 to 50 vol%, or from 50 to 100 vol%, or from 90 to 100 vol%. The de-mineralization may be more efficient when high amounts of halogen-containing gas is used. However, handling of halogen-containing gases is typically difficult, and using large amounts of halogen-containing gases is also less beneficial from a cost perspective.

In embodiments where the amount of the at least one halogen-containing gas used during the de-mineralization step is 100 vol%, only the at least one halogencontaining gas is present in the reactor during the de-mineralization step. The at least one halogen-containing gas may be injected into the reactor. Alternatively, at least one halogen-containing compound in solid form, such as NH4Cl and/or polyvinyl chloride, may be added to the reactor. At least one halogen-containing gas will be evaporated from the solid compound(s) at the high temperatures using the de-mineralization treatment. The reactor may be purged with an inert gas, such as nitrogen, after the de-mineralization treatment.

In embodiments where the amount of the at least one halogen-containing gas used during the demineralization step is less than 100 vol%, the at least one halogencontaining gas is mixed with an inert gas, preferably nitrogen, during the demineralization step. The at least one halogen-containing gas may be injected into the reactor such that it is mixed with an inert gas in the reactor. Alternatively, the at least one halogen-containing gas may be mixed with an inert gas prior to injection into the reactor. In alternative embodiments, at least one halogen-containing compound in solid form, such as NH4Cl and/or polyvinyl chloride, may be added to the reactor. At least one halogen-containing gas will be evaporated from the solid compound at the high temperatures using the de-mineralization treatment and mixed with an inert gas present in the reactor.

In some embodiments, the de-mineralization step involves reducing the pressure in the reactor. This may improve the efficiency of the de-mineralization.

Parameters such as time, temperature, pressure, type of reactor, amount of the at least one halogen-containing gas and gas flow all influence the efficiency of the demineralization treatment as outlined above. As realized by a person skilled in the art, the selection of parameters is based on both a sufficient de-mineralization and on process economy. For example, increasing the amount of the at least one halogencontaining gas while maintaining the time and temperature may improve the demineralization efficiency. The de-mineralization efficiency may also be improved by increasing the time while maintaining the amount of at least one halogen-containing gas and the temperature, or by increasing the temperature and maintaining the amount of at least one halogen-containing gas and the time.

After the de-mineralization step, the carbon enriched material has a total metal content below 500 ppm, or below 400 ppm, or below 300 ppm, or below 200 ppm, or below 100 ppm. In some embodiments, the carbon enriched material after the demineralization treatment has a total metal content in the range of from 0 to 500 ppm, or from 0.1 to 500 ppm, or from 0 to 400 ppm, or from 0.1 to 400 ppm, or from 0 to 300 ppm, or from 0.1 to 300 ppm, or from 0 to 200 ppm, or from 0.1 to 200 ppm, or from 0 to 100 ppm, or from 0.1 to 100 ppm.

The term “total metal content” as used herein, refers to the total amount of metals present in the carbon enriched material. Typically, these metals are sodium, potassium, magnesium, calcium, aluminium, iron and manganese. Trace amounts of other metals may also be present. The metal content may be determined by inorganic elemental analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES). The total metal content is determined by summarizing the amounts of all individual metals.

After the de-mineralization treatment, the carbon enriched material has a pH in the range of from 6.0 to 8.0, or from 6.5 to 7.5, or from 6.7 to 7.2. In some embodiments, the pH is around 7. The pH of the carbon enriched material is thus neutral, or almost neutral. The pH of the carbon enriched material can be measured by dispersing the carbon enriched material in water according to ASTM D1512-15b.

The de-mineralization treatment can be carried out using any suitable equipment, such as rotary kiln, pusher or roller kiln, fluidized bed and rotary hearth furnace.

Preferably, the method according to the first aspect comprises a further step involving subjecting the carbon enriched material to a halogen-removal treatment in an inert atmosphere at one or more temperatures in the range of from 800 to 1200°C for a time period in the range of from 1 to 120 minutes, wherein the step of halogen-removal is carried out after the de-mineralization treatment. After the demineralization treatment, some halogen-containing compounds may remain in the carbon enriched material. Such remaining halogen-containing compounds may be removed by the halogen-removal treatment. Importantly, the pH of the carbon enriched material is not increased by the elevated temperatures used during the halogen-removal treatment. Likely this is due to the fact that the alkaline elements present in the carbon enriched material has already been neutralized during the demineralization treatment.

The halogen-removal treatment may be carried out at the same temperature throughout the entire treatment or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. The halogen-removal treatment may comprise a temperature ramp from a starting temperature to a target temperature. The heating rate may be 1-100°C/min. For example, the heating may involve several intermediate temperatures, with temperature ramps in between them, before reaching the target temperature needed for halogen-removal.

The halogen-removal treatment is carried out for a total time period of from 1 to 120 minutes, i.e. the residence time of the carbon enriched material inside the equipment used for the halogen removal treatment is in the range of from 1 to 120 minutes.

Preferably, the halogen-removal treatment is carried out in direct sequence with the de-mineralization treatment. At the end of the de-mineralization, the supply of the at least one halogen-containing gas is stopped and instead only inert gas, such as nitrogen, is supplied to the reactor during the halogen-removal treatment. Thus, the de-mineralization treatment and the halogen-removal treatment may be carried out in the same reactor. The halogen-removal treatment may be carried out at the same temperature as the de-mineralization step, or the temperature may be adjusted after the supply of the at least one halogen-containing gas has been stopped.

In some embodiments, the heat treatment, the de-mineralization treatment and the step of providing the carbon enriched material with a carbon-based coating are all carried out as discrete steps in different reactors. In some embodiments, some of the steps of the method according to the first aspect may be carried out in direct sequence in the same reactor, as realized by a person skilled in the art.

Importantly, the de-mineralization treatment is carried out after the step of providing the carbon enriched material with a carbon-based coating. As discussed above, the pH of the carbon enriched material is increased during the coating step, while after the de-mineralization treatment the pH is reduced to a neutral value at the same time as the metal content of the carbon enriched material is reduced. Thus, demineralization and neutralization are combined in one process step, enabling a more efficient process suitable for large-scale production. The combination in one process step is based on the realization that the de-mineralization treatment not only reduces the metal content but also lowers the pH of the carbon enriched material.

If instead carrying out the de-mineralization treatment before the step of providing the carbon enriched material with a carbon-based coating, the metal content is reduced by the de-mineralization treatment and the pH is also reduced. However, the pH will be increased again during the coating step such that the pH of the obtained carbon enriched material will be alkaline.

The properties of the carbon enriched material after coating and de-mineralization remain essentially the same as before those processes, except for the reduction in total metal content. Thus, parameters such as specific surface area, true density, H/C ratio, O/C ratio, Lc and d002 are the same, or essentially the same, as described above for the carbon enriched material obtained after heat treatment. The H/C ratio and O/C ratio may be slightly altered by the de-mineralization treatment, but will remain within the ranges defined above for the carbon enriched material obtained after heat treatment.

The carbon enriched material obtained by the method according to the first aspect is preferably used as an active material in a negative electrode of a non-aqueous secondary battery, such as a lithium-ion battery. Due to the low metal content and the neutral pH, the carbon enriched material is particularly suitable for such use.

When the carbon enriched material obtained by the method according to the first aspect is used for producing a negative electrode, any suitable method to form such a negative electrode may be utilized. In the formation of the negative electrode, the carbon enriched material may be processed together with further components. Such further components may include, for example, one or more binders to form the carbon enriched 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 binders may be selected from, but are not limited to, poly(vinylidene fluoride), poly(tetrafluoroethylene), carboxymethylcellulose, natural butadiene rubber, synthetic butadiene rubber, polyacrylate, poly(acrylic acid), alginate, etc., or from combinations thereof.

Optionally, a solvent such as e.g. 1-methyl-2-pyrrolidone, 1 -ethyl-2-pyrrolidone, water, or acetone is utilized during the processing.

According to a second aspect, the present invention relates to a carbon enriched material derived from a biobased carbon precursor, wherein the carbon enriched material has a metal content below 500 ppm and a pH in the range of from 6.0 to 8.0.

The carbon enriched material according to the second aspect may be obtained by the method according to the first aspect. The carbon enriched material according to the second aspect is suitable for use as active material in a negative electrode of a non-aqueous secondary battery since it has a low metal content and a neutral pH. The carbon material according to the second aspect may be further defined as set out above with reference to the first aspect.

According to a third aspect, the present invention relates to a negative electrode for a non-aqueous secondary battery comprising the carbon enriched material obtainable by the method according to the first aspect or the carbon enriched material according to the second aspect as active material. The negative electrode according to the third aspect may be further defined as set out above with reference to the first aspect.

According to a fourth aspect, the present invention relates to use of the carbon enriched material obtainable by the method according to the first aspect or the carbon enriched material according to the second aspect as active material in a negative electrode of a non-aqueous secondary battery. The negative electrode according to the fourth aspect may be further defined as set out above with reference to the first aspect.

Examples

Example 1

Lignin powder (softwood kraft lignin obtained from LignoBoost process) was subjected to a first heating step at 600 °C for 2 hcurs. After cooling down to room temperature, the obtained carbon enriched material was pulverized using a jet-mill to an average particle size of 10 pm. The pulverized material was heated in a final heating step to a temperature of 1050°C in a nitrocpn atmosphere. Next, the pulverized material was coated inside a rotary kiln at 900 °C in an atmosphere comprising nitrogen and 10 vol% acetylene. The temperature was maintained for two hours, whereafter the gas atmosphere was shifted to nitrogen only and cooled down to room temperature. The pH of the carbon material was determined to be alkaline (11.8) and the metal content was high (>5000 ppm). The pH was measured on a water dispersion according to ASTM D1512-15b. The metal content was determined using ICP.

Example 2

A carbon enriched material was produced as described in example 1. After coating, the carbon enriched material was subjected to a combined de-mineralization and neutralization step. The material was treated in hydrogen chloride gas (5 vol%) in a rotary kiln reactor at 1050°C for 30 minutes. The pH of the carbon material was determined to be neutral (6.7) and the metal content was low (<400 ppm). The pH was measured on a water dispersion according to ASTM D1512-15b. The metal content was determined using ICP.

Example 3 - comparative

Lignin powder (softwood kraft lignin obtained from LignoBoost process) was subjected to a first heating step at 600 °C for 2 hcurs. After cooling down to room temperature, the obtained carbon enriched material was pulverized using a jet-mill to an average particle size of 10 pm. The pulverized material was heated in a final heating step to a temperature of 1050 °C in a nitrocpn atmosphere. After the final heating step, the material was treated in hydrogen chloride gas (5 vol%) in a rotary kiln reactor at 1050°C for 30 minutes. After the demineralization treatment, the carbon material was coated inside a rotary kiln at 900 °C in an atmosphere comprising nitrogen and 10 vol% ethylene. The temperature was maintained for two hours, whereafter the gas atmosphere was shifted to nitrogen only and cooled down to room temperature. After carbon coating, the pH of the carbon material was alkaline (10.2) and the metal content was 550 ppm. The pH was measured on a water dispersion according to ASTM D1512-15b. The metal content was determined using ICP.

In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art.

However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

1. Claims A method for producing a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0, wherein the method comprises: - providing a biobased carbon precursor;\g¿s; subjecting the biobased carbon precursor to heat treatment in an inert atmosphere at one or more temperatures in the range of from 500 to 1500°C, wherein the heat treatment is carried out for a total time in the range of from 0.5 to 10 hours, to obtain a carbon enriched material; _...\ \ t. :Rxäz-ä. - providing the carbon enriched material with a carbon-based coating; - subjecting the carbon enriched material to a de-mineralization treatment in an atmosphere comprising at least one halogen-containing gas, wherein the de-mineralization treatment is carried out at one or more temperatures in the range of from 800 to 1500°C for a time period in the range of from 5 minutes to 10 hours, so as to obtain a carbon enriched material having a total metal content below 500 ppm and a pH in the range of from 6.0 to 8.0, wherein the pH is measured by dispersing the carbon enriched material in water according to ASTM D1512-15b; and wherein the de-mineralization treatment is carried out after providing the carbon enriched material with a carbon-based coating. The method according to claim 1, wherein the biobased carbon precursor is lignin. The method according to claim 2, wherein the lignin is provided in the form of agglomerated lignin having an average particle size in the range of from 0.8 to 2.0 mm. The method according to any one of the preceding claims, wherein the method further comprises a step of: - pre-heating the biobased carbon precursor at one or more temperatures in the range of from 140 to 300°C for a time period of at least 30 minutes in an oxidative atmosphere; and wherein the pre-heating step is carried out prior to the heat treatment. The method according to any one of the preceding claims, wherein the method further comprises a step of: - pulverizing the carbon enriched material so as to obtain a pulverized carbon enriched material having an average particle size in the range of from 5 to 25 um; and wherein the step of pulverizing is carried out prior to providing the carbon enriched material with a carbon-based coating. The method according to any one of the preceding claims, wherein the step of providing the carbon enriched material with a carbon-based coating involves heating the carbon enriched material in an inert atmosphere comprising at least one carbon-containing gas at one or more temperatures in the range of from 800 to 1200°C for a time period in the range of fromminutes to 3 hours. The method according to claim 6, wherein the at least one carbon-containing gas is selected from at least one of: methane, ethane, propane, ethylene, propylene, acetylene and methylacetylene. The method according to any one of the preceding claims, wherein the at least one halogen-containing gas is selected from at least one of a chlorine- containing gas and a fluorine-containing gas. The method according to any one of the preceding claims, wherein the method further comprises a step of: - subjecting the carbon enriched material to a halogen-removal treatment in an inert atmosphere at one or more temperatures in the range of from 800 to 1200°C for a time period in the range of from 1 to 120 minutes; and wherein the halogen-removal treatment is carried out after the de- mineralization treatment. The method according to any one of the preceding claims, wherein the heat treatment comprises a first heating step, followed by a final heating step. The method according to claim 10, wherein the first heating step is carried out at one or more temperatures in the range of from 500 to 900°C for a time period in the range of from 0.5 to 5 hours. The method according to any one of claims 10 or 11, wherein the final heating step is carried out at one or more temperatures in the range of from 900 to 1500°C for a time period in the range of from 10 minutes to 3 hours. The method according to any one of claims 10-12, wherein the step of pulverization is carried out after the first heating step and prior to the final heating step. The method according to any one of claims 10-13, wherein the step of providing the carbon enriched material with a carbon-based coating is carried out after the first heating step and prior to the final heating step. A carbon enriched material derived from a biobased carbon precursor, wherein the carbon enriched material has a metal content below 500 ppm and a pH in the range of from 6.0 to 8. A negative electrode for a non-aqueous secondary battery comprising the carbon enriched material obtainable by the method according to any one of claims 1-14 or the carbon enriched material according to claim 15 as active material. Use of the carbon enriched material obtainable by the method according to any one of claims 1-14 or the carbon enriched material according to claim 15 as active material in a negative electrode of a non-aqueous secondary battery.