PROCESS OF PURIFICATION OF RAW CARBON NANOTUBES

Abstract

The present invention provides a process for purifying raw carbon nanotubes. Implementations of the process include providing raw carbon nanotubes containing metallic impurities; and increasing a bulk density of the raw carbon nanotubes to produce compacted carbon nanotubes. The process further includes subjecting the compacted carbon nanotubes to a heat treatment under a sintering gas atmosphere to remove at least a portion of the impurities contained in the raw carbon nanotubes, thereby producing purified carbon nanotubes. The invention also relates to the use of the purified carbon nanotubes, obtained according to said process, as an electrical additive in an electrode material without any subsequent purification step.The invention also relates to an electrode for a lithium ion battery, comprising the purified carbon nanotubes obtained according to said process.

Description

TECHNICAL FIELD The technical background concerns carbon nanotubes in general and, more specifically, it concerns the purification of crude carbon nanotubes to obtain purified nanotubes. CONTEXT Carbon nanotubes, commonly referred to as CNTs, boast excellent electrical and thermal conductivity, mechanical flexibility, and a significantly large surface area. They are made of graphite sheets that are coiled and terminate in hemispheres, consisting of pentagons and hexagons with a structure similar to fullerene. The nanotubes are known to be composed of either a single sheet, also referred to as single-walled nanotubes (SWNTs); or multiple concentric sheets, also referred to as multi-walled nanotubes (MWNTs). CNTs are, therefore, ideal candidates for a wide range of applications, particularly as additives to enhance the electrical conductivity of both anode and cathode electrodes in lithium-ion batteries. However, CNTs are known to comprise impurities, such as metallic or mineral impurities mainly originating from their preparation process, in particular metallic impurities (including Fe, Mn, Cr, Co, Ni, Al, Mo, Va and Si), the total amount of non-carbon impurities being between 2 and 20% by weight, which can be detrimental to the performance of lithium-ion battery electrodes. Purification steps can be performed to reduce the level of impurities in CNTs. For example, chlorine gas or nitrogen gas can be used as a reagent to remove targeted metal impurities. Challenges still remain to design a complete purification process from crude CNTs, including impurities, to obtain purified CNTs with an adequate purity level for direct use of purified CNTs in various applications, such as lithium-ion batteries. SUMMARY The present invention addresses the above need by providing a process for purifying raw carbon nanotubes. Implementations of the process include providing raw carbon nanotubes containing metallic impurities; and increasing a bulk density of the raw carbon nanotubes to produce compacted carbon nanotubes. The process further includes subjecting the compacted carbon nanotubes to a heat treatment, under a sintering gas atmosphere, to remove at least a portion of the impurities contained in the raw carbon nanotubes, thereby producing purified carbon nanotubes. In some implementations of the process, increasing the bulk density of the raw carbon nanotubes includes agglomerating the raw carbon nanotubes to produce agglomerated carbon nanotubes. Optionally, increasing the bulk density of the raw carbon nanotubes further includes briquetting the raw carbon nanotubes to produce carbon nanotube briquettes. The produced compacted carbon nanotubes may be in the form of an agglomerated powder, granules, pellets, or briquettes. In some implementations of the process, the sintering gas atmosphere includes chlorine gas, and the impurities may be removed as chloride gas by a pulsed vacuum system. Alternatively, the sintering gas atmosphere includes nitrogen gas, and the impurities may be removed by vaporization. In some implementations of the process, the heat treatment is performed at a sintering temperature between 1200°C and 3000°C. Optionally, the Heat treatment is carried out in a continuous pushing furnace. Alternatively, the heat treatment can be carried out in a batch furnace. In some implementations of the process, the addition of an inorganic base in aqueous solution or the addition of water can be performed before, during or after step ii) of increasing the bulk density of the raw carbon nanotubes. Optionally, the process may further include at least one of the following steps: Drying, to remove moisture from the compacted carbon nanotubes, and a step of conditioning the purified carbon nanotubes. In some implementations of the process, the conditioning step includes dispersing the purified carbon nanotubes to reduce their bulk density. Optionally, the conditioning step includes packaging the purified carbon nanotubes. In some implementations of the process, the raw carbon nanotubes have a metal impurity level of between 6,000 ppm and 10,000 ppm. Preferably, the purified carbon nanotubes have a metal impurity level of between 5 ppm and 200 ppm, further preferably between 5 ppm and 50 ppm. Preferably, the metal impurities include iron. In some implementations of the process, purified carbon nanotubes are directly usable as an electrical additive in an electrode material, without any subsequent purification step. In another aspect, a use of the purified carbon nanotubes, obtained according to the process as defined herein, is provided for producing an electrode material. In another aspect, a method of producing an electrode material from raw carbon nanotubes is provided, comprising the process for purifying raw carbon nanotubes as defined herein. In another aspect, an electrode comprising the electrode material obtained according to the above method is provided. Optionally, the electrode is a positive or negative electrode for a lithium ion battery. While the invention will be described in conjunction with exemplary embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments. Furthermore, it is intended to cover all alternatives, combinations, modifications, and equivalents that may be included, as defined by this description. The objects, advantages, and other features of the process, method, use, and related electrode will become more apparent and better understood upon reading the following non-limiting description of the invention, given with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a process diagram of a purification process according to one embodiment of the present invention. Figure 2 is a process diagram of a purification process according to another embodiment of the present invention. Figure 3 is a diagram of the total purification process showing the successive steps of agglomeration, briquetting and sintering according to another embodiment of the present invention. Figure 4 is a partial flow diagram showing successive agglomeration and sintering steps according to another embodiment of the present invention. Figure 5 is a partial flow diagram showing successive steps of briquetting and sintering according to another embodiment of the present invention. Figure 6 is a partial flow diagram showing successive steps of dispersing and packaging according to another embodiment of the present invention. DETAILED DESCRIPTION CNTs can be synthesized using various processes known to those skilled in the art. They can be divided into two families: “High Temperature” (HT) and “Medium Temperature” (MT) processes. HT processes, such as “laser ablation” or the “electric arc process”, can lead to CNTs with a very low level of metallic contamination. However, they also lead to many carbon impurities and are not really used on an industrial scale. MT processes are mainly based on C-CVD (catalytic chemical vapor deposition). This process is very selective in terms of CNT synthesis and consists of breaking down a carbon source, such as a hydrocarbon which can be saturated, unsaturated, acyclic, cyclic or aromatic (ethylene, methane, ethane, acetylene, benzene, ...), carbon monoxide or an alcohol such as ethanol, on a metal catalyst at temperatures generally between 500°C and 1,500°C.The catalyst is thus trapped in the carbon structure and the carbon purity of the CNTs is, therefore, directly linked to the quantity of catalyst used and the yield of the synthesis. For example, the synthesis may be carried out by contacting an iron-containing catalyst (e.g., Fe3O4, Fe on a carbon support, Fe on an alumina vehicle, or Fe on a carbon fibril support) with a gaseous carbon-containing compound (CO or hydrocarbons), in the presence of a compound capable of reacting with carbon to generate gaseous products (e.g., CO, H2, or H2O). It should be understood that the term “raw” here refers to a state of CNTs coming directly from the synthesis process and without any subsequent purification step. The crude CNTs which are purified, according to the invention, can be of single-walled or multi-walled type, preferably of multi-walled type (MWNT). The crude CNTs which are purified, according to the invention, can be of renewable origin. MWNTs can be produced, for example, according to the process defined in US patent 7,799,246 or in US patent 8,771,627, corresponding to the raw CNTs produced by Arkema. MWNTs can be characterized as follows: they generally have an average diameter in the range of 0.1 to 100 nm, preferably 0.4 to 50 nm and, more preferably, 0.1 to 30 nm or even 10 to 15 nm and, advantageously, they have a length ranging from 0.1 to 10 pm. Their length / diameter ratio is preferably greater than 10 and usually greater than 100. Their specific surface area is, for example, between 100 and 300 m2 / g, advantageously between 200 and 300 m2 / g, and their apparent density may be in particular between 0.05 and 0.5 g / cm3 and, more preferably, between 0.1 and 0.2 g / cm3. They consist of 5 to 15 leaflets (or walls) and, more specifically, 7 to 10 leaflets. However, any implementation of the process described here can be used and adapted to any raw NTC, unless there is an obvious incompatibility. CNTs are generally in powder form, particularly in a range of particle sizes in which the average particle diameter is about 400 microns. CNTs can be passed reliably from one receptacle to another using a double valve device such as, for example, a "Buck®" or "Glatt®" type device, as described in patent EP 2085312. CNTs may contain impurities derived from the catalyzed synthesis of CNTs. The purity of CNTs may be characterized by the ratio of (the amount of CNTs formed) / (the amount of CNTs formed + the amount of catalyst introduced), the catalyst being a metal supported on an inorganic solid. Impurities may also be quantified by the number of ppm contained in the synthesized CNTs. For example, a raw CNT material (or raw CNTs) may comprise between 6,000 ppm and 50,000 ppm of a metal such as iron, optionally between 7,000 ppm and 9,000 ppm of a metal such as iron, optionally furthermore between 7,500 ppm and 8,000 ppm of a metal such as iron, and optionally further between 7,800 ppm and 25,000 ppm of a metal such as iron. As mentioned above, the presence of impurities in CNTs can be detrimental to the performance of systems in which CNTs are implemented, for example in lithium or lithium-ion battery electrodes. In one aspect, a purification process is provided comprising steps and operating conditions designed to provide further purification to crude CNTs and to produce purified CNTs having a degree of purity adequate for direct commercial use. The process comprises compacting the CNTs, to increase a bulk density thereof, prior to thermally treating the compacted CNTs. It should be noted that the purification process can reduce the level of impurities by at least 90%, preferably at least 95%, further preferably about 99.9%. For example, the design of the equipment and the operating conditions of the process can be varied, optimized and designed to purify crude CNTs having a metal level of between 6,000 ppm and 50,000 ppm, and to produce ready-to-use purified CNTs having a significantly reduced metal level of between 5 ppm and 200 ppm, preferably between 10 ppm and 100 ppm and further preferably less than 50 ppm. The metal preferably comprises iron. Increasing the bulk density, according to the process of the invention, can allow to obtain an enhanced production rate, to improve the heat exchanges during the heat treatment, to maximize the product density of the purified CNTs. For example, the compaction step, which allows to increase the bulk density of the raw material, can thereby reduce the number of operation lines for the subsequent sintering step and further reduce the cost of the purification process. It should be noted that a CNT powder can be made up of aggregates of CNT particles, of a quasi-spherical shape of different sizes, from from a few pm up to 1 mm. The void volume between the particles contributes significantly to a low apparent density of the raw CNT powder. It should also be understood that an increase in the bulk density of the raw CNT powder to be purified refers to a reduction in the void volume between particles (or “tube-to-tube” void volume) of the raw CNTs. Compaction to increase bulk density may include agglomeration, briquetting, or a combination thereof. In addition, compaction may produce raw CNTs in the form of an agglomerated powder, granules, pellets, or briquettes, preferably in the form of briquettes. As a result of this compaction, the aggregates of CNT particles may be partially overlapped or overfused. If compaction is successful, the bulk density of the “agglomerated” CNTs will be greater than the bulk density of the raw CNT powder. The purification process also includes a sintering step, which is primarily a heat treatment of the raw CNTs in the presence of a gaseous medium, during which the metallic impurities are vaporized and / or chemically converted to remove them from the CNTs. Purified CNTs are thus produced. When considering removal of metallic impurities such as iron, chlorine gas or nitrogen gas may be used as the gaseous medium in the sintering step, as discussed above. The compacting step may preferably be performed before the heat treatment. The heat treatment may be carried out in a continuous pusher furnace or in a batch furnace. In some implementations, the process also includes a conditioning step to place the purified CNTs in a suitable condition for subsequent commercialization. Conditioning may include dispersion and packaging or only packaging of the purified CNTs. In some implementations, the process may include adding a small amount of an inorganic base, such as LiOH, KOH, NaOH, or a combination thereof, to the raw CNT powder. It has been found that adding an inorganic base to the raw CNT powder can enhance the outgassing of impurities during the subsequent heat treatment at elevated temperatures. Optionally, water, preferably water at a temperature between room temperature and 80°C, can be used to perform such a step. Indeed, water can contain inorganic bases or carbonates or hydrocarbons (LiOH, NaOH, KOH, Na2CO3, etc.) or alkali organic salts (acetates, maleates, CMC salts, etc.). These compounds can further react with impurities in a catalyst and further accelerate the purification process during the heat treatment. The addition of an inorganic base in aqueous solution or the addition of water can be performed before, during or after compaction. In this case, a drying step can be implemented to remove moisture from the compacted CNTs. For example, water can be added, optionally, during an agglomeration step of the raw CNT powder before a sintering step. Figures 1, 2, and 3 illustrate the general process steps for purifying crude CNTs. Figures 4 and 5 provide examples of the implementation of the agglomeration, briquetting, and sintering steps in Figure 1. Figure 6 provides examples of the implementation of the dispersion and packaging steps in Figure 1. Figures 4 to 6 will be detailed in the experimentation section. Implementations of bulk density increase Agglomeration In some implementations, compacting may include agglomeration of the raw CNT powder to produce an agglomerated raw CNT powder prior to subjecting the raw CNTs to heat treatment. It should be noted that CNTs tend to agglomerate naturally and the agglomeration step is used to amplify their agglomeration. Referring to Figure 3 for the purpose of increasing the bulk density, the raw material (A) is treated as follows: the raw material is deaerated in a vacuum deaerated system (B) in order to remove dissolved corrosive gases (O2, CO2). Then it is transferred to a mixer (C) where the material is mixed in the presence of hot water. Before briquetting and sintering, the moistened powder is transferred to a dryer (D) to remove moisture and produce the agglomerated CNTs (E). The target bulk density of the agglomerated material is approximately 200 to 300 kg / cm3. Bricklaying In some implementations, the compaction may further comprise briquetting the agglomerated raw CNT powder to produce raw CNT briquettes before subjecting the raw CNTs to heat treatment. An addition of an inorganic base in aqueous solution or an addition of water may be performed during the briquetting step. In this case, a drying step may be implemented to remove moisture before producing the briquetted material. Referring to Figure 3, in order to increase the bulk density to a higher value, the agglomerated material (E) is processed as follows: the agglomerated material is deaerated in a vacuum deaerated system (F) for the same reason cited above. Then it is transferred to a mixer (G) where the material is mixed in two stages, in the presence of hot water for a time much longer than that of the agglomeration stage. The moistened agglomerated material is then sent to a compactor (H) to increase the bulk density. Briquettes are prepared by cutting the compacted material, using a nibbler (I). Another drying stage is required before sintering, where the moistened powder is transferred to a dryer (J) in order to remove moisture from the briquettes and produce briquetted material (K). The target apparent density of briquetted material is approximately 400 to 600 kg / cm3. It should be noted that the briquetting step may be optional and agglomeration may be the only step to be performed to increase a bulk density of raw CNTs. Depending on the initial bulk density of raw CNTs, i.e., if the initial bulk density is sufficiently high, the compaction involves agglomeration only to reach a threshold of apparent density. Implementations of sintering In some implementations, the purification process may include a sintering step in which the compacted raw CNTs are heated above the evaporation temperature of the metal to be removed. Impurities such as metallic impurities, particularly iron, may be vaporized as a result to improve the purity of the CNTs. The sintering step may be performed, for example, under a nitrogen or chlorine atmosphere. In some implementations, the sintering temperature may be between 1200°C and 3000°C, optionally between 1500°C and 2800°C, optionally further between 2000°C and 2500°C, and optionally further it may be 2200°C. The residence time may be between 1 hour and 8 hours, optionally between 2 hours and 6 hours, optionally further between 2 hours and 4 hours, and optionally further it may be 3 hours. Referring to Figures 1 to 5, the sintering step may comprise feeding the compacted CNTs, in the form of agglomerated or briquetted CNTs, to a firing furnace, which may be a continuous furnace or a batch furnace. The sintering gas atmosphere may advantageously be a chlorine or nitrogen atmosphere. Advantageously, the firing furnace operates in a continuous mode and the at least one sintering line may be designed to a length required to process the raw material input. In some implementations, the process includes a sintering step consisting of subjecting the compacted raw CNTs to a heat treatment, under a chlorine gas atmosphere, in a pulsed vacuum system, so as to produce removable chloride gas and purified CNTs, for example in the form of briquettes. The exhaust chloride gas can be safely removed from the furnace. Referring to Figures 2 to 5, the sintering step can be carried out in a baking furnace, which is a batch furnace. Several sintering units can be designed and operated in parallel, so as to adapt to the quantity of raw CNTs to be purified. For example, at least 33 units are required if the bulk density after briquetting is approximately 600 kg / m3. Chlorine gas is used to remove metallic impurities (Fe, Cr, Co, Ni, Mn, etc.) efficiently with the formation of gaseous chloride compounds, by thermal reaction at high temperature. The reactions with iron and chromium are as follows: 2Fe + 3Cl2 -> 2FeCl3 2Cr + 3Cl2 -* 2CrCl3 According to the above, metallic impurities react with chlorine at high temperature and are chemically converted into gaseous chloride compounds. The use of chlorine gas during sintering can provide substantial advantages, including the reduction of damage to the CNT powder, compared to other available treatments in acidic solution or at extremely high temperature (2800 to 3000°C). Referring to Figure 3 for the purpose of producing the purified CNTs, the briquetted material (K) is processed as follows: the material (E) is subjected to a sintering step (L) (batch or continuous), optionally followed by a dispersion step (M) and, after packaging (N), the purified CNTs (O) are ready for use. Packaging Implementations In some implementations, the purification process may include packaging the purified CNTs into a form suitable for future application. Referring to Figure 6, the process may include a dispersion step intended to disperse the purified CNT agglomerates or briquettes and reduce their apparent density. The equipment may include a tank discharger, a feeding device with a screw and an oscillating agitator (1 to 4), and a high-performance powder processing machine (1 to 4). The screw and oscillating agitator are used to feed the high-performance powder processing machine, after sintering, with the materials. Typical mixers are geared towards one of the 3 mixing mechanisms and either lack the ability to form, requiring an additional machine to compound, or lack the ability to disperse nanoparticles, resulting in the product with agglomerates. With the high-performance powder processing machine, the combinations of compression, shear, and impact forces are applied to the particle leading to nanoparticles that are formed efficiently without binder. The peak rotor speed is 20 to 40 m / sec, with a capacity of approximately 20 to 40 Kg / h per machine unit. Optionally, the target bulk density can be between 70 and 130 Kg / m3. Referring again to Figure 6, the process may include a packaging step to package the purified CNTs so that they are ready for use in various applications that require a high level of purity (less than 50 ppm of a metal such as iron, for example). Purified CNTs are usually packaged in containers, which avoid any further contamination by metallic impurities, preferably using a device of the "Buck®" or "Glati®" type. For example, purified CNTs can be packaged in a plastic drum with a lid and handles, for easy storage and transportation. It should be understood that various operating parameters (including sintering temperature, residence time in the furnace) play a critical role and should be optimized and well chosen depending on the amount of impurities to be removed or the level of purity to be achieved for a specific application. It should be noted that each of the above-mentioned aspects of the purification process can be applied to single-walled carbon nanotubes and / or multi-walled carbon nanotubes without departing from the scope of the present invention. Lithium-ion battery implementations It is worth noting that purified CNTs can be used as an electrical conduction additive in anode and cathode materials in lithium-ion batteries, but they are not limited to such application. As such, also contemplated for electrodes, are purified CNTs, as defined herein or as produced by the present process, i.e., an electrode material comprising the present purified CNTs on a current collector. The electrode material may comprise the CNTs as the sole electrochemically active material or in combination with other elements. The electrode material may also comprise the CNTs as a conductive material. For example, the electrode material may further comprise at least one electrochemically active material in addition to the CNTs. Examples of electrochemically active materials (EAMs) include, but are not limited to, titanate particles, lithium titanates, lithium metal phosphates, vanadium oxides, lithium metal oxides, and combinations thereof. For example, the electrochemically active material may be selected from TiO2, Li2TiO3, Li4Ti50i2, H2Ti5On and H2Ti4O9 or a combination thereof, LiM'PO4 in which M' is Fe, Ni, Mn, Co or a combination thereof, LiV3O8, V2O5, LiMn2O4, LiM”O2 in which M” is Mn, Co, Ni or a combination thereof, Li(NiM'”)O2 in which M'” is Mn, Co, Al, Fe, Cr, Ti or Zr, as well as combinations thereof. Such MAE particles may be further coated, for example with carbon. The electrode materials may also include additional components such as an additional conductive material (e.g., graphene, graphite, carbon black, Ketjen® black, carbon black, etc.) Denka®, VGCF® type carbon fibers, a non-powdered carbon coating derived from the pyrolysis of an organic precursor or a combination thereof), inorganic particles, salts and / or one or more binders. Examples of binders include, but are not limited to, SBR (styrene butadiene rubber), NBR (acrylonitrile butadiene rubber), HNBR (hydrogenated NBR), CHR (epichlorohydrin rubber), ACM (acrylate rubber), CMC (carboxymethyl cellulose), PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene) and the like. The electrode material can be prepared as a dry mix or as a slurry in a solvent and coated onto the current collector, for example by the Dr. Blade method or the die coating method. When a solvent is present, a drying step can be included in addition. Electrochemical cells, comprising an electrode, as defined herein, an electrolyte and a counter electrode, are also contemplated as well as their use, for example, in electric or hybrid vehicles or in information technology devices. EXPERIMENTATION AND RESULTS In some implementations, each step of the purification process can be scaled and designed to accommodate the amount of CNTs to be purified as well as the amount of purified CNTs to be produced. For example, various operations in a process step can be performed repeatedly on a single batch process line, or a plurality of equivalent process lines can be designed in parallel or in series. Examples of implementations of a purification process under nitrogen atmosphere, according to Figure 1, are described as follows. Referring to Figure 4, a T1 deaeration screw feeder is used to deaerate the raw material. The screw tip speed is between 0.5 and 1.5 m / sec, the deaeration pressure is about -20 KPa (total vacuum). Its capacity is between 60 and 150 kg / h. The bulk density of the raw material is 40 to 110 kg / m3. Then the deaerated material is transferred to a continuous type T3 mixer. The 1st step for agglomeration is by pumping hot water at 70°C, with a ratio of approximately 20 to 40% by weight of raw CNTs for 60 to 80% by weight of hot water, as binder. The 2nd and 3rd steps will swell this mixture for agglomeration, with a few seconds of retention time (3 to 30 seconds) per machine unit. The tip speed of the blade is 20 to 40 m / sec and the capacity is between 50 and 100 kg / h per machine unit. The target bulk density is approximately 200 to 300 kg / m3. The agglomeration may further comprise feeding the raw CNT turbulation powder to a fluidized bed dryer T6, wherein heat is supplied through at least one heat exchanger T9 to remove moisture from the CNT powder and produce agglomerated CNTs. The air inlet temperature is 100 to 180°C and the outlet temperature is 50 to 120°C. The air velocity in the fluidized bed is 0.5 to 3 m / sec. Referring to Figure 5, the same conditions as those for the deaeration stage C1 are used here. Here, the deaerated material is transferred to a vertical-type conical mixer in a batch manner. The retention time is much longer than in the agglomeration stage and is about 3 to 6 min per machine unit. The working volume is 0.5 to 1.5 m3 per machine unit. The screw tip speed is 0.5 to 1.5 m / sec. The target bulk density is about 400 to 600 kg / m3. The agglomerated material is then briquetted by the briquetting machine C7, where the roller pressure is 50 to 150 tons (the linear roller pressure is 2.2 to 15 tons / cm). The roller diameter is 300 to 600 mm and the roller width is 100 to 500 mm. Then, the mixed material is sent to a compactor C11, by means of another screw conveyor C10, so that the material is compacted before being sent to a crusher C13 to be cut into briquettes. The agglomerated material is dried by the fluidized bed dryer C14, with a temperature air inlet temperature from 100 to 180°C and an outlet temperature from 50 to 120°C. The air velocity is 0.5 to 3 m / sec. A T11 collector is connected to the C14 fluidized bed dryer to collect all fine particles, so as to reduce dust and meet environmental requirements. 5 Referring to Figure 6, the process may include a dispersion step for dispersing the purified agglomerates or briquettes of CNTs. The rotor peak speed is 20 to 40 m / sec, with a capacity of approximately 20 to 40 Kg / h per machine unit. The target bulk density is 70 to 130 Kg / m3. For example, four or more operation lines may be configured in parallel to appropriately disperse a quantity of purified CNTs.

Claims

CLAIMS 1. A process for purifying raw carbon nanotubes, the process comprising the steps of: (i) provide raw carbon nanotubes containing metallic impurities; ii) compacting the raw carbon nanotubes to increase their apparent density; and (iii) subjecting the compacted carbon nanotubes to a heat treatment under a sintering gas atmosphere, in order to remove at least part of the impurities contained in the raw carbon nanotubes and to produce purified carbon nanotubes.

2. The process of claim 1, wherein compacting the raw carbon nanotubes comprises agglomerating the raw carbon nanotubes to produce agglomerated carbon nanotubes.

3. The process of claim 1 or 2, wherein compacting the raw carbon nanotubes comprises briquetting the raw carbon nanotubes to produce carbon nanotube briquettes.

4. A process according to any one of claims 1 to 3, wherein the compacted carbon nanotubes are in the form of an agglomerated powder, granules, pellets or briquettes.

5. A process according to any one of claims 1 to 4, wherein the sintering gas atmosphere comprises chlorine gas and wherein impurities are removed in the form of chloride gas by a pulsed vacuum system.

6. A process according to any one of claims 1 to 4, wherein the sintering gas atmosphere comprises nitrogen gas and wherein impurities are removed by vaporization.

7. A process according to any one of claims 1 to 6, wherein the heat treatment is carried out at a sintering temperature between 1200°C and 3000°C.

8. Process according to any one of claims 1 to 7, wherein the heat treatment is carried out in a continuous pushing furnace.

9. Process according to any one of claims 1 to 7, wherein the heat treatment is carried out in a batch furnace.

10. Process according to any one of claims 1 to 9, comprising the addition of an inorganic base in aqueous solution or the addition of water before, during or after step ii) of compacting the raw carbon nanotubes.

11. The process of claim 10, further comprising a drying step to remove moisture from the compacted carbon nanotubes.

12. Process according to any one of claims 1 to 11, further comprising a conditioning step.

13. The process of claim 12, wherein the conditioning step comprises dispersing the purified carbon nanotubes to reduce their apparent density.

14. The process of claim 12 or 13, wherein the conditioning step comprises packaging the purified carbon nanotubes.

15. Process according to any one of claims 1 to 14, in which the raw carbon nanotubes are of the multi-wall type having a length / diameter ratio greater than 10, a specific surface area of between 100 and 300 m2 / g, and an apparent density of between 0.05 and 0.5 g / m3. PURIFICATION PROCESS OF RAW CARBON NANOTUBE ABRIDGED The present invention provides a process for purifying raw carbon nanotubes. Implementations of the process include providing raw carbon nanotubes containing metallic impurities; and increasing a bulk density of the raw carbon nanotubes to produce compacted carbon nanotubes. The process further includes subjecting the compacted carbon nanotubes to a heat treatment under a sintering gas atmosphere to remove at least a portion of the impurities contained in the raw carbon nanotubes, thereby producing purified carbon nanotubes. The invention also relates to the use of the purified carbon nanotubes, obtained according to said process, as an electrical additive in an electrode material without any subsequent purification step. The invention also relates to an electrode for a lithium ion battery, comprising the purified carbon nanotubes obtained according to said process. Raw material H Purified NTC Fig. 3 AGGLOMERATION FIG. 5 BRIQUETTING PACKAGING DISPERSION

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