A method for deep purification of carbon nanotubes and carbon nanotubes obtained thereby

Abstract

The application provides a deep purification method of carbon nanotubes and the obtained carbon nanotubes, the deep purification method avoids the limitation of single-stage acid washing through gradient fractionation treatment, and by means of the synergistic effect of a small-molecule polar inner salt additive and a multidentate chelating agent, deep penetration and removal can be achieved, the deep removal of metal impurities can be achieved by using a milder oxidation condition than the traditional process, the oxidation temperature is reduced or the oxidation time is shortened, so as to protect the intrinsic structure and performance of the carbon nanotubes, the process is simple and suitable for mass production, and has good industrial application value, and the metal impurity residual amount of the carbon nanotubes obtained by deep purification is less than or equal to 4500 ppm, more preferably less than or equal to 2000 ppm, and the Raman spectrum IG / ID is greater than 85.

Description

Technical Field

[0001] This invention relates to the field of nanomaterial post-processing technology, specifically to a method for deep purification of carbon nanotubes and the resulting carbon nanotubes. Background Technology

[0002] Since the 1990s, carbon nanotubes (CNTs) have been considered one of the most promising nanomaterials of the 21st century due to their excellent physicochemical properties. Their unique tubular structure endows them with comprehensive properties such as super strength, superconductivity, ultralightness, and ultrastability, showing broad application prospects in composite materials, lithium batteries, catalysis, and other fields. Among them, single-walled carbon nanotubes (SWCNTs), due to their larger aspect ratio, better crystallinity, and superior conductivity, have become a research hotspot in academia and industry, and have shown excellent performance in applications such as functionalized toughened / conductive composite materials, positive and negative electrode materials, and transparent thin films.

[0003] Chemical vapor deposition (CVD) is currently the mainstream technology for the large-scale, low-cost preparation of single-walled carbon nanotubes (SWCNTs) worldwide. However, the CVD synthesis process leaves behind a large amount of metal catalyst debris, metal ions, and amorphous carbon impurities. High impurity content has become a key bottleneck limiting the application of SWCNTs in high-end electronics, high-purity conductive materials, and semiconductor supporting fields.

[0004] Crude CVD single-walled carbon nanotubes possess ultra-high specific surface area, abundant inter-tube gaps, and a multi-level micro / nano-channel structure, resulting in a significant strong adsorption effect at the nanoscale. Metal impurity particles and metal cations can be firmly adsorbed onto the outer wall of the carbon nanotubes, the gaps between the interwoven tubes, and the internal pore structure through van der Waals forces, electrostatic interactions, and pore trapping. Conventional single acid washing can only remove free metal impurities on the material surface, failing to disrupt the strong adsorption binding, and deeply adsorbed metals cannot be effectively removed. Furthermore, metal ions dissolved by acid washing are prone to secondary adsorption and redeposition in the solid-liquid system, further compressing the purification limit and resulting in persistently high levels of impurities.

[0005] Traditional purification methods often employ high-temperature oxidation combined with prolonged acid rinsing using a single high-concentration strong acid. The inner walls of carbon-based nanotubes are highly hydrophobic and have low polarity, resulting in poor wettability at the interface with acidic aqueous media. Furthermore, hydrated ions face significant diffusion resistance and low mass transfer efficiency within the confined nanoscale space, making it difficult for reagents to penetrate deep into adsorption sites and further reduce the content of metal impurities. To overcome this plateau, existing techniques often resort to extreme enhancements to process parameters, such as increasing oxidation temperatures (500-600℃), significantly increasing hydrochloric acid concentrations (15-30%), raising acid rinsing temperatures (80-100℃), or indefinitely extending treatment times (>10h). However, high-intensity oxidation and strong acid treatments easily cause irreversible etching and damage to the walls of carbon nanotubes, leading to a surge in defect density and disrupting their lattice integrity. This severely degrades the electrical properties and mechanical strength of carbon nanotubes, making them unsuitable for demanding downstream applications such as semiconductor devices and high-end conductive composite materials.

[0006] CN115403034A discloses a method for purifying high-purity, high-conversion-rate single-walled carbon nanotubes, comprising the following steps: Step 1, pretreatment of crude single-walled carbon nanotubes obtained by CVD method; Step 2, ultrasonic dispersion of the single-walled carbon nanotubes treated in Step 1 with surfactant and organic solvent, followed by washing to neutrality, filtration, and finally freeze-drying; Step 3, high-temperature oxidation treatment of the single-walled carbon nanotubes treated in Step 2; Step 4, placing the high-temperature oxidized single-walled carbon nanotubes in a mixed acid solution and heating while stirring; Step 5, washing the single-walled carbon nanotubes after the reaction in Step 4 to neutrality, filtering, and drying to obtain purified single-walled carbon nanotubes. CN120887412A discloses a method for purifying carbon nanotubes. The method involves adding crude carbon nanotubes and a surfactant to deionized water for ultrasonic dispersion, performing gradient centrifugation on the dispersion, collecting the supernatant and mixing it with hydrochloric acid and nitric acid to treat the carbon nanotubes, then gently oxidizing the dilute acid-treated nanotubes with a low-concentration hydrogen peroxide solution, placing them in a magnetic field to adsorb magnetic impurities, performing cross-flow filtration using a polycarbonate microfiltration membrane, collecting the treated carbon nanotubes, annealing them under inert gas, and freeze-drying them to obtain purified carbon nanotubes. However, the above technical solutions all suffer from cumbersome operation and cannot break the diffusion barrier within the nanoscale confinement space, making it difficult for the cleaning solution to penetrate deep and remove impurities.

[0007] In summary, existing single-walled carbon nanotube purification technologies suffer from drawbacks such as the difficulty in breaking the strong adsorption of nanoparticles through single-stage acid washing, the difficulty in removing deep metal impurities, the tendency to reach purification bottlenecks, and the tendency of strong corrosion processes to damage the carbon nanotube structure and degrade material properties. Therefore, it is necessary to develop a novel deep purification method for carbon nanotubes. Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention provides a method for deep purification of carbon nanotubes and the resulting carbon nanotubes. The deep purification method avoids the limitations of single-stage acid washing by gradient step-by-step treatment. With the synergistic effect of small molecule polar internal salt auxiliaries and multidentate chelating agents, deep penetration and removal can be achieved. Milder oxidation conditions than traditional processes can be used to achieve deep removal of metal impurities, reducing oxidation temperature or shortening oxidation time to protect the intrinsic structure and properties of carbon nanotubes. The process is simple, suitable for mass production, and has good industrial application value. Moreover, the residual metal impurities in the carbon nanotubes obtained by deep purification are ≤4500ppm, and even better, ≤2000ppm, with Raman spectra IG / ID>85.

[0009] To achieve this objective, the present invention adopts the following technical solution: One objective of this invention is to provide a method for the deep purification of carbon nanotubes, the method comprising the following steps: Prepare crude carbon nanotubes and sequentially perform oxidation treatment, first acid washing, first water washing, second acid washing, second water washing, and drying to obtain deeply purified carbon nanotubes. The first pickling stage uses a pickling solution with a concentration of 1-8 wt%; the second pickling stage uses a second pickling solution with a pH of 3.5-6.5 and includes a small molecule polar internal salt auxiliary agent and a multidentate chelating agent.

[0010] The deep purification method described in this invention employs a segmented impurity removal approach, removing impurities in stages based on differences in their adsorption strength. First, oxidation treatment is used to form uniform open channels from the dense, graphite-like carbon shell on the outer surface of the residual catalyst particles, without causing excessive etching or structural damage to the carbon nanotube walls. Then, a first-stage acid wash is performed using a high-concentration first-stage acid wash solution, preferentially removing surface-free and weakly adsorbed metal impurities. Subsequently, a second-stage acid wash is performed using a lower-concentration second-stage acid wash solution containing a small-molecule polar internal salt auxiliary agent and a multi-toothed chelating agent. This binary composite auxiliary system establishes an interface regulation and mass transfer enhancement mechanism, fundamentally improving the solid-liquid interface properties of the pores, enhancing the penetration efficiency and diffusion capacity of the reaction reagents, weakening strong interfacial adsorption, improving pore wetting and ion mass transfer efficiency, and efficiently removing residual metals bound in pores and deeply adsorbed. This invention avoids the limitations of single-stage pickling through gradient-stage processing. By leveraging the synergistic effect of small-molecule polar internal salt additives and multi-toothed chelating agents, deep penetration and removal can be achieved. Milder oxidation conditions than traditional processes can be used to achieve deep removal of metal impurities, reducing oxidation temperature or shortening oxidation time to protect the intrinsic structure and properties of carbon nanotubes. The process is simple, suitable for mass production, and has good industrial application value.

[0011] It should be noted that the concentration of the pickling solution described in this invention is 1-8 wt%, such as 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, or 8 wt%, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0012] It should be noted that the pH of the second-stage pickling solution described in this invention is 3.5-6.5, such as 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, or 6.5, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable. Moreover, the pH of the second-stage pickling solution is limited to 3.5-6.5, preferably 4.0-6.0, for the following reasons: (i) Metal cations, such as iron ions, generated after the first-stage pickling are prone to forming hydroxide precipitates, such as flocculent ferric hydroxide precipitates, in neutral and alkaline environments, making it more difficult to remove metal impurities. Even in near-neutral environments, iron ions are prone to hydrolysis to form ferric hydroxide. Therefore, to avoid the formation of precipitates, a weakly acidic second-stage pickling solution is more suitable; (ii) Chelating agents are protonated under strong acid, resulting in a decrease in chelation efficiency. Therefore, the second-stage pickling solution needs to be weakly acidic.

[0013] It should be noted that the first and second pickling stages of this invention are both conventional overwashing processes, and there are no excessive restrictions on the amount of pickling solution used.

[0014] As a preferred technical solution of the present invention, the oxidation treatment includes gas-phase chemical oxidation and / or liquid-phase chemical oxidation. The gas-phase chemical oxidation uses an oxidizing gas as an oxidant, which includes air or other mixed gases containing oxygen, and may even be selected as a gas such as carbon dioxide as needed. The liquid-phase chemical oxidation uses persulfate solution and / or hydrogen peroxide as an oxidant.

[0015] Preferably, before and / or during the oxidation treatment, microwave-assisted oxidation and / or mechanical ball milling-assisted oxidation are also included.

[0016] As a preferred technical solution of the present invention, the temperature of the gas-phase chemical oxidation is 350-500℃, such as 350℃, 380℃, 400℃, 420℃, 440℃, 460℃, 480℃ or 500℃, preferably 380-440℃, but not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0017] Preferably, the temperature of the liquid-phase chemical oxidation is 50-70°C, such as 50°C, 55°C, 60°C, 65°C or 70°C, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0018] It should be noted that the oxidation treatment described in this invention has a significant impact on the amount of residual metal impurities in the carbon nanotubes obtained from deep purification. If a more vigorous gas-phase chemical oxidation is used, the amount of residual metal impurities is lower; if a more gentle liquid-phase chemical oxidation is used, the amount of residual metal impurities is higher. However, regardless of the oxidation treatment method, it can form a uniform open channel on the dense graphite-like carbon shell on the outside of the residual catalyst particles, without causing excessive etching and structural damage to the carbon nanotube wall. Moreover, the holding time for the oxidation treatment described in this invention is 30-120 min, such as 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, or 120 min, preferably 40-90 min, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0019] As a preferred embodiment of the present invention, the solutes of the first-stage pickling solution and the second-stage pickling solution independently include any one or a combination of at least two of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, phosphoric acid, or acetic acid.

[0020] Preferably, the concentration of the pickling solution is 1-5 wt%, such as 1 wt%, 2 wt%, 3 wt%, 4 wt%, or 5 wt%, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0021] Preferably, the target temperature for the first pickling stage is 40-100℃, such as 40℃, 50℃, 55℃, 60℃, 65℃, 70℃, 80℃, 90℃ or 100℃, etc., preferably 50-70℃, and the holding time is 1-4h, such as 1h, 2h, 2.2h, 2.5h, 2.6h, 2.8h, 3h, 3.5h or 4h, etc., preferably 2-3h, but not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0022] Preferably, the first stage of washing includes washing with pure water until the pH of the washing filtrate is 6-7, such as 6, 6.1, 6.3, 6.5, 6.6, 6.8 or 7, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0023] As a preferred technical solution of the present invention, in the two-stage pickling solution, the mass ratio of the small molecule polar internal salt auxiliary agent to the multidentate chelating agent is 1:(0.2-5.0), for example, 1:0.2, 1:0.5, 1:0.8, 1:1.0, 1:1.2, 1:1.5, 1:1.8, 1:2.0, 1:3.0, 1:4.0 or 1:5.0, etc., preferably 1:(0.5-2.0), but not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0024] Preferably, the total amount of small molecule polar internal salt additive and multidentate chelating agent added in the two-stage pickling solution is 0.5-5.0 wt%, such as 0.5 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.7 wt%, 2.0 wt%, 3.0 wt%, 4.0 wt%, or 5.0 wt%, etc., preferably 1.0-3.0 wt%, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0025] Experimental verification shows that the addition of a small-molecule polar internal salt auxiliary agent and a multi-toothed chelating agent to the two-stage pickling solution to form a binary composite surface-interface mass transfer enhancement agent is not necessarily better the higher the addition ratio, but rather there is an optimal performance range. Within a certain range, increasing the proportion of the auxiliary agent can enhance pore penetration and metal chelation ability, and improve the depth of impurity removal; however, when the total addition amount is too high, it will lead to increased difficulty in solid-liquid separation, increased water washing load, easy generation of surface residue, and increased raw material costs, which is not conducive to industrial mass production. Therefore, the total addition amount of the small-molecule polar internal salt auxiliary agent and the multi-toothed chelating agent is controlled at 0.5-5.0%, preferably 1.0-3.0%, to achieve the best balance between purification effect, processability, and economy.

[0026] Preferably, the pH of the second-stage pickling solution is 4.0-6.0, such as 4.0, 4.3, 4.5, 4.7, 5.0, 5.2, 5.5, 5.8 or 6.0, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0027] As a preferred embodiment of the present invention, the molecular weight of the small molecule polar internal salt auxiliary agent is ≤1000 Da and does not contain C8 or higher long-chain alkyl groups.

[0028] Preferably, the small molecule polar internal salt adjuvant includes any one or a combination of at least two of the following: trimethylammonium ethanolate (betaine), 2-(N-morpholine) ethanesulfonate (MES), 3-(N-morpholine) propanesulfonate (MOPS), or 1,4-piperazine diethanesulfonic acid (PIPES).

[0029] It should be noted that the small molecule polar internal salt auxiliaries described in this invention are electrically neutral small molecule internal salts with balanced positive and negative charges within the molecule, a molecular weight ≤1000Da, do not contain C8 or higher long-chain alkyl groups, do not have surface activity, do not undergo micellization in solution, and do not significantly reduce the surface tension of the system.

[0030] As a preferred technical solution of the present invention, the multidentate chelating agent includes any one or a combination of at least two of the following: citric acid, malic acid, glycolic acid, oxalic acid, tartaric acid, gluconic acid, disodium ethylenediaminetetraacetate (EDTA 2Na), tetrasodium ethylenediaminetetraacetate (EDTA 4Na), diethylenetriaminepentaacetate (DTPA), sodium gluconate, sodium citrate, or potassium sodium tartrate.

[0031] It should be noted that the pH of the two-stage pickling solution described in this invention is 3.5-6.5 and includes a small molecule polar internal salt additive and a multidentate chelating agent as a binary composite additive. The small molecule polar internal salt reduces the diffusion barrier in the nano-confined space by adjusting the polarity and dielectric environment, while the multidentate chelating agent accelerates the dissolution and removal of metal ions through chelation. The two work synergistically to improve the pickling depth and catalyst removal rate.

[0032] Specifically, this invention employs an intramolecularly neutral small-molecule internal salt with balanced positive and negative charges as a surface-to-interface mass transfer enhancing agent, with a molecular weight ≤1000 Da. The polar internal salt used in this invention possesses zwitterionic characteristics with self-balancing intramolecular charges, containing both cations and anions within the molecule, achieving charge neutralization. No free sodium ions, chloride ions, or other inorganic impurities are introduced into the system, and no inorganic salt impurities remain. This type of internal salt possesses a strongly polar multifunctional structure, exhibiting significantly superior molecular polarity compared to conventional surfactants. Carbon nanotube defect sites, edge vacancies, and the carbon-coated iron interface itself exhibit characteristics of charge inhomogeneity, polar site enrichment, dangling bonds, and unsaturated coordination, compared to intact carbon nanotube sp. 2Conjugate planes have higher surface energy and polar interaction potential energy. It can selectively adsorb onto carbon nanotube surface defects, interlaminar gaps, carbon-iron interfaces, and agglomeration gaps through electrostatic interactions, hydrogen bonding, and polar interface interactions. It also possesses excellent dispersing ability and metal coordination complexing properties, effectively dissociating carbon nanotube agglomerates and improving system dispersion uniformity. Furthermore, it exhibits weak coordination and capture of ferrous and ferric ions, forming a synergistic deep impurity removal effect with multidentate chelating agents. Its small, well-defined molecular structure prevents the formation of large micelles, allowing for strong penetration into the micropores and tightly packed aggregates of carbon nanotubes, targeting hidden embedded metal impurity sites. The internal salt is electrically neutral, exhibiting good compatibility with acidic liquid environments and chelating agent systems, without flocculation, precipitation, or phase separation, resulting in excellent system stability. Moreover, this type of polar internal salt adsorbs moderately on the carbon nanotube surface in a monolayer manner, without over-coating, clogging pores, or masking surface active sites, thus preserving the intrinsic structure and performance of the carbon nanotubes. It features rapid diffusion, strong penetration, and good interfacial compatibility. This type of additive generally does not contain C8 or higher long-chain alkyl groups, and its molecular structure lacks the hydrophobic long-chain structure found in traditional surfactants. Therefore, it does not possess typical surface activity, does not undergo micellization in solution, and does not significantly reduce the surface tension of the system. This avoids problems such as foaming, emulsification, interfacial contamination, residual adsorption, and mass transfer interference caused by the introduction of surfactants.

[0033] As a preferred technical solution of the present invention, the target temperature of the second pickling stage is 50-65℃, such as 50℃, 53℃, 55℃, 57℃, 60℃, 63℃ or 65℃, and the holding time is 3-6h, such as 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0034] Preferably, the second stage of washing includes washing with pure water until the pH of the washing filtrate is 6-7, such as 6, 6.1, 6.3, 6.5, 6.6, 6.8 or 7, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0035] As a preferred technical solution of the present invention, the drying includes: vacuum drying at 60-100℃ for 8-12 hours, which can fully remove residual moisture inside the material and finally obtain a purified carbon nanotube product with high purity, low defects and low metal residue.

[0036] It should be noted that the vacuum drying temperature is 60-100℃, such as 60℃, 70℃, 80℃, 90℃ or 100℃, and the vacuum drying time is 8-12h, such as 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h or 12h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0037] As a preferred technical solution of the present invention, the deep purification method includes the following steps: (1) Oxidation treatment: Prepare the crude carbon nanotubes and carry out oxidation treatment. Control the oxidation temperature of the oxidation treatment to 350-500℃ and the holding time to 30-120min, so that the dense graphite-like carbon shell on the outside of the catalyst particles forms a uniform open channel, and does not cause excessive etching of the carbon nanotube wall and structural damage.

[0038] (2) First stage of acid washing and first stage of water washing: The oxidized carbon nanotubes are added to the prepared first stage of acid washing solution and dispersed by mechanical stirring or low-power ultrasonication to form a uniform and stable solid-liquid dispersion system. The dispersion system is placed at 40-100℃ for constant temperature acid washing reaction, and the reaction time is controlled to be 1-4h. After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration and pure water washing is used until the pH of the washing filtrate is 6-7.

[0039] (3) Second-stage acid washing and second-stage water washing: The carbon nanotubes after the first-stage water washing are placed in the second-stage acid washing solution; the pH of the second-stage acid washing solution is 3.5-6.5 and includes small molecule polar internal salt additive and multi-toothed chelating agent; in the second-stage acid washing solution, the mass ratio of small molecule polar internal salt additive to multi-toothed chelating agent is 1:(0.2-5.0), and the total amount of small molecule polar internal salt additive and multi-toothed chelating agent added is 0.5-5.0wt%; the temperature is raised to 50-65℃, stirred and kept warm for 3-6h, and the second-stage acid washing is carried out; after the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration, and the filter cake is circulated and replaced with deionized water until the pH of the washing filtrate is 6-7, to ensure that residual metal ions, acid radical ions and additives in the system are completely removed.

[0040] (4) Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 60-100℃ for 8-12 hours to fully remove residual moisture from the material and finally obtain a purified carbon nanotube product with high purity, low defects and low metal residue.

[0041] The solutes of the first-stage pickling solution and the second-stage pickling solution each independently include any one or a combination of at least two of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, phosphoric acid, or acetic acid; the preparation method of the first-stage pickling solution includes: slowly adding concentrated acid to deionized water, continuously stirring until the mixture is uniform, and preparing a first-stage pickling solution with a concentration of 1-8 wt%.

[0042] Since the second stage of acid washing in step (3) targets carbon nanotubes with residual water introduced from the first stage of water washing, in order to ensure that the multidentate chelating agent can work in an environment with a pH of 3.5-6.5, the following two methods can be referred to, or other methods can be selected according to the actual situation. Method I: Replace the residual water in the carbon nanotubes after the first stage of water washing with an acid solution with a pH of 3.5-6.5 until the pH of the washing solution rises to 3.5-6.5, thereby forming a suspension with a pH of 3.5-6.5. Add a small molecule polar internal salt auxiliary agent and a multidentate chelating agent with a mass ratio of 1:(0.2-5.0), so that the total amount of small molecule polar internal salt auxiliary agent and multidentate chelating agent added is 0.5-5.0wt%. Method II: Add the first-stage washed carbon nanotubes to a certain amount of water, and then add a high-concentration acid solution, such as a first-stage acid washing solution with a concentration of 1-8 wt%, until the pH reaches the target value of 3.5-6.5, forming a suspension with a pH of 3.5-6.5. Add a small molecule polar internal salt auxiliary agent and a multi-toothed chelating agent at a mass ratio of 1:(0.2-5.0), so that the total amount of small molecule polar internal salt auxiliary agent and multi-toothed chelating agent added is 0.5-5.0 wt%.

[0043] The second objective of this invention is to provide a carbon nanotube obtained by the deep purification method described in the first objective, wherein the residual amount of metal impurities in the carbon nanotube is ≤4500ppm and the Raman spectrum IG / ID>85. That is, the carbon nanotube obtained by the deep purification method of this invention has the advantages of low defects, high purity and no long-chain alkyl sulfonates on the surface.

[0044] Compared with existing technical solutions, the present invention has at least the following beneficial effects: (1) The deep purification method described in this invention employs a two-stage acid washing process, breaking through the bottleneck of mass transfer within the nanoscale confinement and achieving deep removal of catalyst impurities. Specifically, this invention adopts a two-stage graded purification mode, first conventional acid washing followed by weak acid fine washing, with a clear division of labor. The first stage rapidly removes surface free metals and weakly adsorbed impurities, significantly reducing the impurity base. The second stage uses a weakly acidic system to specifically weaken the strong adsorption effect of the ultra-high specific surface area of ​​single-walled carbon nanotubes. By introducing small-molecule polar internal salt additives, the interfacial polarity mismatch between the graphite-like hydrophobic channels and the strongly polar aqueous phase is effectively improved, optimizing the dielectric environment inside the channels and reducing its diffusion resistance and transport barrier within the nanopores. Combined with the stable chelation and rapid removal of metal ions by multi-toothed chelating agents, the problem of metal ion retention and re-precipitation in traditional acid washing is solved, breaking through the plateau period of purification effect and achieving stronger purification stability.

[0045] (2) The deep purification method described in this invention uses a binary composite auxiliary agent system, which is clean and interference-free, and does not introduce interface pollution or residue. Specifically, the binary composite auxiliary agents used in this invention are all small molecule polar compounds, do not contain C8 or higher long-chain alkyl groups, do not have traditional surface activity, do not form micelles in solution, do not produce emulsions, do not significantly reduce the surface tension of the system, do not form stubborn adsorption on the surface and pores of carbon nanotubes, and have good water solubility, are easy to wash, and leave no residue, and will not cause secondary pollution to the materials. It is suitable for high-end application scenarios with strict requirements for purity and cleanliness.

[0046] (3) The acid washing system of the deep purification method described in this invention is single-phase stable, the process is simple and controllable, and it has wide applicability. Specifically, this invention adopts a uniform single-phase aqueous washing system, which is stable, easy to operate, has mild conditions, and strong equipment compatibility. It can be directly upgraded and modified based on the existing purification process without significant changes to the production process. It is suitable for the industrial-scale high-efficiency purification of different types of single-walled carbon nanotubes and has good prospects for large-scale application. Detailed Implementation

[0047] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows: In the specific embodiments of the present invention, the crude carbon nanotubes are all single-walled carbon nanotubes prepared by chemical vapor deposition, with a purity of 85%, an Fe content of 14200ppm, a Raman spectral IG / ID value of 35, and a BET value of 403.

[0048] The following examples and comparative examples use hydrochloric acid pickling system and air oxidation method: Example 1 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0049] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0050] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0051] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0052] S5, Second stage pickling: Add 5g MOPS and 5g EDTA 2Na to the system, heat to 55℃, and stir for 3 hours.

[0053] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0054] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0055] Example 2 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0056] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0057] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0058] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0059] S5, Second stage pickling: Add 3.3g MOPS and 6.7g EDTA 2Na to the system, heat to 55℃, and stir for 3h.

[0060] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0061] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0062] Example 3 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0063] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0064] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0065] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0066] S5, Second stage pickling: Add 6.7g MOPS and 3.3g EDTA 2Na to the system, heat to 55℃, and stir for 3h.

[0067] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0068] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0069] Example 4 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0070] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0071] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0072] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0073] S5, Second stage pickling: Add 2.5g MOPS and 2.5g EDTA 2Na to the system, heat to 55℃, and stir for 3h.

[0074] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0075] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0076] Example 5 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0077] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0078] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0079] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0080] S5, Second stage pickling: Add 1.25g MOPS and 1.25g EDTA 2Na to the system, heat to 55℃, and stir for 3h.

[0081] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0082] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0083] Example 6 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0084] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0085] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0086] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0087] S5, Second stage pickling: Add 12.5g MOPS and 12.5g EDTA 2Na to the system, heat to 55℃, and stir for 3 hours.

[0088] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0089] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0090] Example 7 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0091] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0092] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0093] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 3.5 is formed using hydrochloric acid solution.

[0094] S5, Second stage pickling: Add 5g MOPS and 5g EDTA 2Na to the system, heat to 55℃, and stir for 3 hours.

[0095] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0096] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0097] Example 8 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0098] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0099] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0100] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 6.5 is formed using hydrochloric acid solution.

[0101] S5, Second stage pickling: Add 5g MOPS and 5g EDTA 2Na to the system, heat to 55℃, and stir for 3 hours.

[0102] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0103] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0104] Comparative Example 1 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0105] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0106] S3. Pickling: The carbon nanotubes that have undergone mild oxidation treatment are added to a prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system. The dispersion system is placed at 80°C for constant temperature pickling reaction, and the reaction time is controlled to be 3 hours.

[0107] S4. Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7.

[0108] S5. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0109] Comparative Example 2 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 510℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0110] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a hydrochloric acid solution with a mass fraction of 20%.

[0111] S3. Add the carbon nanotubes that have undergone mild oxidation treatment to the prepared hydrochloric acid solution with a mass fraction of 20% and stir mechanically to form a uniform and stable solid-liquid dispersion system; place the dispersion system at 80℃ for constant temperature acid washing reaction, and control the reaction time to 6h.

[0112] S4. Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7.

[0113] S5. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0114] Comparative Example 3 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0115] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0116] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0117] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0118] S5. Second stage pickling: Add 10g MOPS to the system, heat to 55℃, and stir for 3h.

[0119] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7.

[0120] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0121] Comparative Example 4 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0122] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0123] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0124] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0125] S5. Second stage of pickling: Add 10g EDTA 2Na to the system, heat to 55℃, and stir for 3 hours.

[0126] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7.

[0127] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0128] Comparative Example 5 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0129] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0130] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0131] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0132] S5, Second stage pickling: Add 5g PSBMA (polymethyl methacrylate sulfonate betaine) and 5g EDTA2Na to the system, heat to 55℃, and stir for 3 hours.

[0133] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7.

[0134] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0135] Comparative Example 6 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in a tubular oxidation device and subjected to low-temperature and mild oxidation treatment in air; the system was heated to 420℃ at a heating rate of 5℃ / min and kept at a constant temperature for 60min. After oxidation, the system was naturally cooled to room temperature with the furnace.

[0136] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0137] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0138] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 3.0 is formed using hydrochloric acid solution.

[0139] S5, Second stage pickling: Add 5g MOPS and 5g EDTA 2Na to the system, heat to 55℃, and stir for 3 hours.

[0140] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0141] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0142] The following examples and comparative examples use hydrochloric acid pickling system and hydrogen peroxide oxidation method: Example 9 This embodiment provides a method for deep purification of carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in 500ml of 8wt% hydrogen peroxide solution and ultrasonically treated at 60℃ for 1h. After repeated filtration and washing with water, the wet material was dried.

[0143] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0144] S3, First stage of acid washing: The carbon nanotubes that have undergone mild oxidation treatment are added to the prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system; the dispersion system is placed at 80°C for constant temperature acid washing reaction, and the reaction time is controlled to be 3 hours.

[0145] S4. Preparation of low-acid suspension: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. In this process, the liquid is first washed with pure water until the pH is 6-7, and then 500g of suspension with pH 5 is formed using hydrochloric acid solution.

[0146] S5, Second stage pickling: Add 3.3g MES and 6.7g citric acid to the system, heat to 55℃, and stir for 3 hours.

[0147] S6. Final Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7, ensuring that residual metal ions, acid radicals and additives in the system are completely removed.

[0148] S7. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0149] Comparative Example 7 This comparative example provides a method for purifying carbon nanotubes, including the following steps: S1. Oxidation treatment: 10g of crude carbon nanotubes prepared by chemical vapor deposition were placed in 500ml of 8wt% hydrogen peroxide solution and ultrasonically treated at 60℃ for 1h. After repeated filtration and washing with water, the wet material was dried.

[0150] S2. Preparation of acid solution: Slowly add acid solution to deionized water and stir continuously until the mixture is uniform to prepare a 4% hydrochloric acid solution.

[0151] S3. Pickling: The carbon nanotubes that have undergone mild oxidation treatment are added to a prepared hydrochloric acid solution with a mass fraction of 4% and mechanically stirred to form a uniform and stable solid-liquid dispersion system. The dispersion system is placed at 80°C for constant temperature pickling reaction, and the reaction time is controlled to be 3 hours.

[0152] S4. Washing: After the acid washing reaction is completed, solid-liquid separation is achieved by vacuum filtration. The filter cake is then circulated and rinsed with deionized water until the pH of the washing filtrate is 6-7.

[0153] S5. Drying: Transfer the washed carbon nanotubes to a drying device and vacuum dry them at 90°C for 10 hours to fully remove residual moisture from the material and finally obtain purified carbon nanotube products.

[0154] The key parameters of the above embodiments and comparative examples, as well as the quality test data of purified carbon nanotubes, are summarized in Table 1.

[0155] Table 1 The following points can be seen from Table 1: (1) Based on the hydrochloric acid pickling system and the air oxidation method for deep purification, comparing Examples 1-6 and Comparative Example 1, it can be seen that the two-stage pickling method reduces the metal content by 65-85%. Examples 1-3 are different ratios of internal salt: chelating agent, and Examples 1, 4.0-6.0 are different total addition amounts, indicating that the internal salt: chelating agent ratio of 1:1 and the total addition amount of 2% are the optimal experimental conditions. Under these conditions, the residual iron content is low and the defect ratio is low (IG / ID=90).

[0156] (2) Comparing Example 1, Comparative Example 1 and 2, Comparative Example 1 did not undergo two-stage pickling and had a significantly higher iron content. Although the traditional pickling mode (Comparative Example 2) had a low iron content, the structure was damaged (IG / ID=41) and the capillary was lost (BET decreased by 10%), which was not conducive to the subsequent slurry and battery performance.

[0157] (3) Comparing Example 1 with Comparative Examples 3 and 4, if a single auxiliary agent or chelating agent (Comparative Examples 3 and 4) is used, the process of "desorption-complexation-anti-deposition" cannot be achieved. The amount of metal residue after purification is higher than that in Example 1 with the optimal ratio, which fully demonstrates that a specific compound ratio and reasonable total amount control are the key to achieving deep purification of single-walled carbon nanotubes.

[0158] (4) The selection of additives is also very important. PSBMA (polymethyl methacrylate sulfobetaine) used in Comparative Example 5 is a high molecular weight internal salt with a large molecular weight. Due to its significant steric hindrance, large molecular weight, and long branched chain, it cannot enter the deep layer of the pores to complete desorption. At the same time, the high molecular weight chain is easy to adsorb on the surface of carbon materials, and conventional water washing is difficult to completely remove it, resulting in residue (BET reduced by 7%).

[0159] (5) Comparing Examples 1, 7, and 8 with Comparative Example 6, it can be seen that the pH range of the two-stage pickling system affects the efficiency of removing metal impurities during the two-stage pickling process. Compared to Example 1, in Example 7, the pH was lowered to 3.5, making the system more acidic. The chelating agent was prone to protonation and lost its ability to chelate with iron ions, resulting in an increase in the residual Fe impurity in the carbon nanotube product to 1877 ppm, which was less effective than Example 1 (962 ppm). Compared to Example 1, in Example 8, the pH was raised to 6.5, making the system more neutral. Free iron ions were prone to hydrolysis, generating insoluble iron hydroxide flocculent precipitate, which was difficult to remove by pickling. This resulted in an increase in the residual Fe impurity in the carbon nanotube product to 1392 ppm, which was also slightly less effective than Example 1 (962 ppm). Compared to Example 1, in Comparative Example 6, the pH was lowered to 3, further increasing the acidity of the system and deepening the protonation of the chelating agent. Furthermore, the ability to chelate with iron ions is further lost, resulting in an increase in the residual Fe impurity in the carbon nanotube product to 2537 ppm, which no longer meets the requirement of ≤2000 ppm for residual metal impurities. Moreover, those skilled in the art will readily recognize that if the pH of the second-stage pickling system rises to neutral or even alkaline, free iron ions are more likely to hydrolyze, further generating more insoluble ferric hydroxide flocculent precipitates that are difficult to remove by pickling, leading to a significant increase in the residual Fe impurities in the carbon nanotube product, failing to meet the requirement of ≤2000 ppm for residual metal impurities. Therefore, the second-stage pickling needs to lock in a suitable pH window, balancing the activity of the chelating agent and the stable complexation and dissolution of iron ions, avoiding the problems of protonation failure and ferric hydroxide precipitation.

[0160] (6) Example 9 and Comparative Example 7 are hydrogen peroxide oxidation systems, which show that two-stage pickling also has the effect of reducing iron content (reducing by 47%).

[0161] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0162] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0163] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0164] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for deep purification of carbon nanotubes, characterized in that, The deep purification method includes the following steps: Prepare crude carbon nanotubes and sequentially perform oxidation treatment, first acid washing, first water washing, second acid washing, second water washing, and drying to obtain deeply purified carbon nanotubes. The first pickling stage uses a pickling solution with a concentration of 1-8 wt%; the second pickling stage uses a second pickling solution with a pH of 3.5-6.5 and includes a small molecule polar internal salt auxiliary agent and a multidentate chelating agent.

2. The deep purification method according to claim 1, characterized in that, The oxidation treatment includes gas-phase chemical oxidation and / or liquid-phase chemical oxidation, wherein the gas-phase chemical oxidation uses an oxidizing gas as an oxidant, and the liquid-phase chemical oxidation uses a persulfate solution and / or hydrogen peroxide as an oxidant. Preferably, before and / or during the oxidation treatment, microwave-assisted oxidation and / or mechanical ball milling-assisted oxidation are also included.

3. The deep purification method according to claim 2, characterized in that, The temperature of the gas-phase chemical oxidation is 350-500℃, preferably 380-440℃; Preferably, the temperature of the liquid-phase chemical oxidation is 50-70°C.

4. The deep purification method according to any one of claims 1-3, characterized in that, The solutes in the first-stage pickling solution and the second-stage pickling solution each independently include any one or a combination of at least two of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, phosphoric acid, or acetic acid. Preferably, the concentration of the pickling solution is 1-5 wt%; Preferably, the target temperature for the first pickling stage is 40-100℃, more preferably 50-70℃, and the holding time is 1-4h, more preferably 2-3h; Preferably, the first stage of washing includes washing with pure water until the pH of the washing filtrate is 6-7.

5. The deep purification method according to any one of claims 1-4, characterized in that, In the two-stage pickling solution, the mass ratio of small molecule polar internal salt auxiliaries to multidentate chelating agents is 1:(0.2-5.0), preferably 1:(0.5-2.0); Preferably, in the two-stage pickling solution, the total amount of small-molecule polar internal salt additive and multidentate chelating agent added is 0.5-5.0 wt%, more preferably 1.0-3.0 wt%. Preferably, the pH of the second-stage pickling solution is 4.0-6.

0.

6. The deep purification method according to any one of claims 1-5, characterized in that, The molecular weight of the small molecule polar internal salt auxiliary agent is ≤1000 Da, and it does not contain C8 or higher long-chain alkyl groups; Preferably, the small molecule polar internal salt auxiliaries include any one or a combination of at least two of trimethylammonium ethanolate, 2-(N-morpholine)ethanesulfonate, 3-(N-morpholine)propanesulfonate, or 1,4-piperazine diethanesulfonic acid.

7. The deep purification method according to any one of claims 1-6, characterized in that, The multidentate chelating agent includes any one or a combination of at least two of the following: citric acid, malic acid, glycolic acid, oxalic acid, tartaric acid, gluconic acid, disodium ethylenediaminetetraacetate, tetrasodium ethylenediaminetetraacetate, diethylenetriaminepentaacetate, sodium gluconate, sodium citrate, or potassium sodium tartrate.

8. The deep purification method according to any one of claims 1-7, characterized in that, The target temperature for the second stage of pickling is 50-65℃, and the holding time is 3-6 hours. Preferably, the second stage of washing includes washing with pure water until the pH of the washing filtrate is 6-7.

9. The deep purification method according to any one of claims 1-8, characterized in that, The drying process includes vacuum drying at 60-100℃ for 8-12 hours.

10. A carbon nanotube obtained by the deep purification method according to any one of claims 1-9, characterized in that, The residual amount of metal impurities in the carbon nanotubes is ≤4500ppm, and the Raman spectrum IG / ID>85.

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