Carbon nanotube material and method of manufacture

Abstract

A carbon nanotube material is described comprising a plurality of particles, each particle comprising an agglomeration of elongate carbon nanotube formations, each formation comprising a carbon nanotube or carbon nanotube bundle, the formations conformed to form an interconnected substantially planar structure. A method of manufacture of the same is described comprising the steps of: providing a precursor macroscopic carbon nanotube material selected from one or more of a yarn, strand, mat, fibre, aerogel; immersing the precursor material in an ion-containing electrolyte and applying a voltage thereto with respect to an additional immersed electrode to soften the macroscopic CNT material and cause the macroscopic CNT material to produce a plurality of carbon nanotube particulate agglomerates.

Description

[0001] CARBON NANOTUBE MATERIAL AND METHOD OF MANUFACTURE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a particulate form of carbon nanotube material and its manufacture. The particles are carbon nanotube agglomerates with a mesh-like structure of adhered nanotubes and nanotube bundles that interweave to form an interconnected, planar web. Products are disclosed comprising the nanotube particles in various formulations, for example as a dry power, or combined with a variety of liquid solvents or polymer solutions to form mixtures of varying viscosity and composition for incorporation into other material systems. Illustrative applications of the planar particles are demonstrated. For example they are shown to serve as an effective conductive additive in battery electrodes, and to form conductive films and coatings when cast in isolation or in combination with polymers. Methods for particle production and size refinement are also disclosed, based upon a discovered electrochemical exfoliation process that enables the division of macroscale nanotube material into micron-scale particles. Apparatus invented to manufacture the particles from macroscopic nanotube materials at industrial scales is described.

[0004] BACKGROUND OF THE INVENTION

[0005] Carbon nanotubes (CNTs) can be added to other material systems to enhance their properties and / or to endow them with additional functionality. Carbon nanotube walls have a Young's modulus on the order of 1 TPa, and their tensile strength can exceed 10 GPa. They are also electrically and thermally conductive, and inert especially when their walls are absent of defects. The art contains numerous examples of their application in other material systems, such as the endowment of ceramics and polymers with enhanced electrical and thermal conductivity, toughness, strength and stiffness. The work of failure of carbon nanotube composite fibres manufactured from nanotube-polymer dispersions exceeds that of dragline silk and Kevlar, and the strength of carbonised nanotube-polymer composite fibres can reach 6 GPa. Carbon nanotubes are also used within energy storages device such as a capacitors and batteries in which they may perform a variety of different roles. Upon forming a continuous, conductive scaffold of high specific surface area they may be coated with an active material to create a composite electrode, serve as an active material by hosting intercalated ions, or provide increased electrical conduction as a coating over electrode particles compared to widely used carbon black additives.

[0006] The prior art encompasses numerous patents and publications that employ carbon nanotubes as a component within a liquid-based system (variously described as suspensions, dispersions or solutions). Distinction between types of dispersion is achieved by the distinct form of carbon nanotube-based particles within the dispersion, by the dispersion behaviour, and its method of manufacture. Broadly, the existing art encompasses three types of nanotube particle systems as follows:

[0007] First, the well dispersed system contains separated carbon nanotubes within a liquid. Examples of such dispersions containing CNTs with length approximately 1 pm are presented by Coleman et al. in WO 2020 / 144289 Al [1], Upon evaporation of solvent, the nanotubes within spontaneously form an electrically conductive network within a battery electrode. Another example of a "well dispersed" carbon nanotube material may be found described in CN117342544 A [2], This looks to produce a dispersion that contains essentially fully separated carbon nanotubes. The second dispersed particle morphology is termed a carbon nanotube bundle, examples of which are described by Yoo et al. in EP 3 355 391 Bl [3], Nanotubes in such bundle particles are adhered to one another, follow along a single contour, from which they do not branch away: along the contour the orientation of the tubes is always parallel.

[0008] A third and final class of system comprises the other forms of agglomerates, whose microstructure and dimension (principally ID or 3D) varies across the art. Examples of ID agglomerates include the alewive described by Smalley et al. US 7,288,238 B2 [4], a needle shaped agglomerate of length between 10 pm and 50 pm and thickness 2 - 5 pm, in which carbon nanotubes are substantially aligned, densely packed, and free of tangles. Smalley et al. also describe another ID agglomerate, "super ropes", in which carbon nanotubes are long, entangled and enmeshed with one another. 3D nanotube agglomerates of the third class are described in EP 3 355 391 Bl [3] as spherical or potato shaped where nanotubes are entangled without directivity and bond at crossing points, in US 2007 / 0134496 Al [5] where nanotube particles possess a long chain morphology and entangle in the form of a cocoon, and in US 11,987,499 B2 [6] where weaker bonding between tubes absent significant bundling is employed to expedite their separation in a liquid. In JP 2011063458 A [7] particles comprise agglomerated carbon nanotubes oriented so that they extend parallel to the surface direction of the plate, and have portions that extend in a direction perpendicular to the orientation direction and are intertwined with one another. The high degree of alignment is a consequence of the parent material being grown as a vertically-aligned forest. US 9133031 B2 [8] also describes a material that derives from a nanotube forest.

[0009] Much existing art suggests that dispersions containing largely separate carbon nanotubes (the well dispersed systems) facilitate the most successful incorporation of carbon nanotubes into other material systems. For example, it has been argued that the strength of a nanotube-based composite may be improved by increasing the interfacial area and shear strength between its phases, and bonding area is maximised when nanotubes are fully separate within the matrix. For example CN117342544A [2] submits that "If the agglomerated multiwalled carbon nanotubes are used directly, the material performance improvement is small or poor, mainly because the agglomerated multiwall carbon nanotubes are not effectively dispersed or the dispersed carbon nanotubes are re-agglomerated together". Experiments of Coleman et al. [1] have shown well dispersed carbon nanotubes can form a conductive coating over storage particles within a battery electrode, capable of providing an electrical conductivity per mass of carbon that is several orders of magnitude above that offered by a carbon black conductive additive [1], Yet the final electrical conductivity or strength offered by an embedded nanotube network within a material system may also benefit from a greater degree of bonding between the constituent nanotubes that comprise the network.

[0010] Here, two or more nanotubes that adhere along a portion of their length are defined as bundling with one another. Experiments have shown that where two carbon nanotube bundles or two nanotubes cross and contact one another absent bundling then current flowing between them can encounter an appreciable electrical resistance on the order of 100 kQ: This is much above the resistance measured along a similar length of the axis of a conducting bundle or tube. Also, the measured mechanical strength of a bundled joint can far exceed that of the joint between two crossed tubes or bundles. For these the reasons the electrical conductivity, strength and toughness of a nanotube network may expected to benefit from a greater incidence of interwoven bundled joints between the nanotubes and nanotube bundles of the network as opposed to joints formed by crossing: This hypothesis forms our motivation for investigating the development and deployment of novel web- structured particles that are constructed from several well-bonded nanotubes.

[0011] Continuous agglomerates of well-bonded carbon nanotubes are expected to be manufactured at scale using a floating catalyst chemical vapour deposition process as previously detailed in US 7,323,157 B2 by Kinloch et al. [9], In this process, a macroscopic aerogel of carbon nanotubes is continuously assembled from nanotubes that are formed by the reaction of gases upon catalyst particles within a reactor. Formations comprising a single nanotube or nanotube bundle arise within the reactor, and these then assemble into the aerogel due to their mutual van der Waals attraction. The resultant substantial bonding between nanotubes within the aerogel endow it with sufficient strength to be continuously removed from the reactor via drawing and collected variously as aerogel, mat or as a fibre. A key feature of the microstructure of the extracted nanotube materials is an interwoven network of carbon nanotube bundles. Within this aerogel, the migration of long nanotubes (up to 1 mm in length) from one bundle to the next provides a high degree of electrical and mechanical interconnection, so that the condensed mats or fibres may possess a macroscopic electrical conductivity of over 10,000 S / m and a useful strength in the range of 10 MPa to several GPa. This well-bonded network avoids risks associated with handling short nanotubes in a fine particulate form, but raises a challenge of how to process such macroscopic CNT materials into a form of liquid-based dispersions or dry powders that are required for their incorporation into other material systems, for example as a mechanical reinforcement or as electrically conductive additives. This challenge and the opportunity to develop strong particles from a parent material of interwoven bundle microstructure endowed by the process described above motivates the development of new methods and particle morphologies such as disclosed hereinbelow. SUMMARY OF THE INVENTION

[0012] In accordance with the invention in a first aspect, there is disclosed a carbon nanotube material comprising a plurality of particles, wherein each particle comprises an agglomeration of elongate carbon nanotube formations, each formation comprising a carbon nanotube or carbon nanotube bundle. In the particles, the formations are conformed to form an interconnected substantially planar structure.

[0013] As used herein, a reference to a carbon nanotube, or CNT, formation is intended to encompass both single discrete elongate carbon nanotubes and bundles comprising two or more of such elongate carbon nanotubes aligned together for a substantial part of their respective lengths. Each particle comprises a substantially planar web of such carbon nanotube formations. The web is formed for example in that at least some of the carbon nanotube formations adhere together where two formations cross one another in the substantially planar structure, and / or in that at least some of the carbon nanotube formations interweave together, for example by exchange of a carbon nanotube between one carbon nanotube bundle and another carbon nanotube bundle.

[0014] In such an interconnected network or web structure, points of connection may be referred to herein as "nodes". Lengths of carbon nanotube formations, whether comprising single CNTs or bundles of two or more, extending between such nodes may be referred to herein as "struts". It will be understood that any strut formed by a single elongate carbon nanotube must join two nodes or terminate at a free end. Equivalent lengths of a carbon nanotube bundle may be continuous or partially continuous through such a node. In addition to an interwoven nodal connection formed by the branching of a bundle of one strut into two or more others, a weaker form of connection at a node also exists where formations cross one another.

[0015] The carbon nanotube material in preferred embodiments comprises particles having interwoven joints between the carbon nanotube formations that account for between 36% and 74% of the sum of interwoven joints, crossed joints and free ends within the particle. These relative proportions may be measured for example by imaging with an electron microscope.

[0016] The structure of each of the plurality of particles is substantially planar in the sense that the particles have a thickness that is mostly that of a single formation, although at the node where two formations cross, a local increase in thickness may occur up that of a few formations and particles are consequently referred to herein as 2D structures. The carbon nanotube material preferably comprises carbon nanotube formations having a thickness ranging from the diameter of an individual nanotube thickness up to 170 nm. The carbon nanotube material preferably comprises particles having a dimension at the widest point (in the planar dimension) in the range 1 pm to 200 pm. These dimensions may be measured for example by imaging with an optical microscope.

[0017] The invention disclosed here is a new nanotube agglomerate particle of the third general class discussed above that possesses a 2D web-like microstructure of interwoven carbon nanotubes and nanotube bundles. The particle morphology is distinct from other existing agglomerates in the art by virtue of its 2D nature (as opposed to the ID or 3D particles of prior art) and by the characteristics of its web structure in which nanotubes are predominantly bonded by the bundling and interweaving between nanotubes and bundles at joints. This intends to provide a particle whose structure offers a high strength and high electrical conductivity.

[0018] Various products and formulations may be considered to deliver the new nanotube agglomerate particles of the first aspect of the invention as a product for incorporation into other material systems.

[0019] For example, embodiments of the invention may include a formulation comprising particles of the carbon nanotube material of the above description in a liquid suspension, dispersion or slurry, optionally in combination with other species. In some embodiments of such a formulation, the liquid is selected from water, n-methyl-2-pryillodone, methyl-ether ketone, acetone, ethanol or dimethylacetamide and mixtures thereof.

[0020] For example, embodiments of the invention may include a formulation comprising particles of the carbon nanotube material in dry powder form, optionally in combination with other species. Some embodiements of such a formulation may further comprise additional polymeric material partly of wholly adsorbed on to the surface of the particles.

[0021] The invention in a second aspect is a method of manufacture of a carbon nanotube material comprising the steps of: providing a precursor macroscopic carbon nanotube material selected from one or more of a yarn, strand, mat, fibre, aerogel; immersing the precursor material in an ion-containing electrolyte and applying a voltage thereto with respect to an additional immersed electrode to soften the macroscopic CNT material and cause the macroscopic CNT material to produce a plurality of carbon nanotube particulate agglomerates. The method is in particular a method of producing carbon nanotube material according to the first aspect of the invention, and the carbon nanotube particulate agglomerates produced by the immersing and electrolysing process step are in particular the carbon nanotube particles of such a material described herein, and preferred features of the same will be inferred accordingly by analogy. For example, the particles may thus each comprise a substantially planar agglomeration of elongate carbon nanotube formations, each formation comprising a carbon nanotube or carbon nanotube bundle, the formations conformed to form an interconnected substantially planar structure.

[0022] The immersing and electrolysing process step is itself found and described below to be inherently capable of producing carbon nanotube particles in accordance with the principles of the first aspect of the invention, and subject to appropriate control of process parameters to be a necessary and sufficient step to produce useful particulate material product. Although the invention should not be seen to be limited to any particular mechanism by which the immersing and electrolysing process step achieves this, various examples are discussed below which will enable the skilled person to perform the step to achieve the required result.

[0023] Optionally however, for example to control the morphology of the carbon nanotube particles, the method may further comprise mechanically dividing the electrolysed CNT material, for example wherein the mechanical division step is performed by one or more of mechanical grinding, blending, homogenization, sonication.

[0024] Further steps of the method of manufacture of a carbon nanotube material comprising particles with desired properties may include one or more of:

[0025] (i) removing the electrolyte, for example: (a) by rinsing in water, and optionally further removing the rinsing water by evaporation;

[0026] (b) using a filter press, that is, remove electrolyte via separating material and electrolyte species by passing through a porous membrane;

[0027] (c) mechanically, for example by pressing of material between plates or rollers so as to expunge electrolyte-containing water as in a mangle, by centrifuge, or

[0028] (d) other method;

[0029] (ii) optionally further replacing with a further liquid excipient.

[0030] The properties and morphology of the carbon nanotube particles may be further influenced by selection of precursor material. In preferred embodiments, the precursor macroscopic carbon nanotube material is an as-manufactured or condensed aerogel. In preferred embodiments, the precursor macroscopic carbon nanotube material is fabricated by a floating catalyst chemical vapour deposition process (for example as previously detailed in US 7,323,157 B2). In preferred embodiments, the precursor macroscopic carbon nanotube material is an as-manufactured direct-spun carbon nanotube material.

[0031] Thus, in accordance with the method of the invention, a route is provided for particle manufacture that preserves elements of the interwoven bundle microstructure that results from the manufacturing of carbon nanotube aerogels, in particular such as might be produced in a floating catalyst chemical vapour deposition reactor. Contrary to an expectation that a well-dispersed system should offer the greatest performance when incorporated into other material systems, and that the high toughness of a carbon nanotube parent such as that which might be manufactured via a floating catalyst chemical vapour deposition process should render the energy required for their dispersion impractically high, the usefulness of the particles and a practical, efficient means invented for their manufacture are both identified here by way of examples below. The 2D form disclosed here is intended to aid their successful incorporation into material systems such as by the efficient covering of internal surfaces - for example those of electrode particles or ceramic grains prior to sintering. A high interfacial area is offered for bonding with other phases, and a high loading of carbon nanotubes within a composite is assisted by a planar particle morphology that assists packing and incorporation over a wide range of volume fraction. Illustrations of the application of the planar particles within an electrode and as a coating are provided by way of examples below. A series of methods to manufacture these planar particles from precursor macroscopic nanotube materials, which for example are manufactured in a floating catalyst chemical vapour deposition are also disclosed to offer an industrially scalable route for their production.

[0032] The methods disclosed in the examples below will assist the skilled person in optimising the performance of the method of the invention to produce the particles of the invention, and in particular to determine suitable precursor materials such as the preferred precursor macroscopic nanotube material manufactured in floating catalyst chemical vapour deposition processes, and to determine reagents and parameters for the electrolysis process.

[0033] For example, in embodiments, the applied voltage is at least 6V and for example up to 28V.

[0034] For example, in embodiments, the voltage is applied for at least 10 minutes and for example up to 90 minutes.

[0035] In another example embodiment, parent nanotube material is continuously fed into an electrolyte under the application of a voltage to effect the desired softening and exfoliation. In some embodiments, the electrolyte is a potassium sulfate solution in waterthat has a salt content of between 1% and 5 % by weight.

[0036] In some embodiments, the surface of the additional immersed electrode exposed to the electrolyte is substantially inert.

[0037] In some embodiments, the electrolysis step and where applicable the further mechanical division step is performed in such manner as to produce particles having a dimension at the widest point in the range 1 pm to 200 pm. This may be measured by imaging with an optical microscope.

[0038] In some embodiments, the electrolysis step and where applicable the further mechanical division step is performed in such manner as to produce particles exhibiting interwoven joints between the carbon nanotube formations that are observed to account for between 36% and 74% of the sum of interwoven joints, crossed joints and free ends within the particle. This may be measured by imaging with an electron microscope.

[0039] The method may further comprise the removal of liquid from the plurality of carbon nanotube particles to create a dry powder, optionally in combination with other species.

[0040] The method may further comprise the removal of liquid from the plurality of carbon nanotube particles and the substitution of an alternative liquid, for example selected from water, n-methyl-2-pryillodone, methyl-ether ketone, acetone, ethanol or dimethylacetamide and mixtures thereof, to create a liquid suspension, dispersion or slurry of the particles, optionally in combination with other species.

[0041] The invention in a further aspect comprises the use of the method as herein described to produce a material or formulation as herein described. Thus, the present invention provides a form of dispersed carbon nanotube particle that has characteristic dimension in the range 5 pm to over 200 pm. The particles are planar in shape and their microstructure is a web structure of interwoven carbon nanotubes and carbon nanotube bundles that forms a 2D mesh. This particle form readily facilitates incorporation into other material systems such as battery electrodes, electrically-conductive coatings, and ceramic systems in which they may provide electrical conductivity and mechanical toughening. Also invented is an electrochemical treatment, the necessary apparatus and other processing steps that together facilitate the division of as-manufactured nanotube aerogel or condensed aerogel materials into planar particles of agglomerated carbon nanotubes with mesh-like structure, and that control the subsequent tailoring of their size distribution. The invented method and the invented associated apparatus are of merit in that they facilitate production of the particles at an industrial scale.

[0042] LIST OF FIGURES

[0043] The invention is now described in more detail by way of example only, and in aid of this a series of illustrations and plots of experimental data are included. They are listed as follows:

[0044] Figure 1: Sketch of the morphology of carbon nanotube particles, illustrating their planar structure of interconnected CNTs and CNT bundles (CNT formations) that give rise to the network of struts. Nodes are formed at strut crossings and where the struts interweave with one another. Struts also terminate at free ends. The characteristic dimension D of the particles is annotated as measured at their widest point.

[0045] Figure 2: Cumulative frequency plot of the length of struts as measured within the planar particles and within the parent material.

[0046] Figure 3: Images of exfoliated CNT particles in liquid obtained with an optical microscope. All scale bars are of length 20 pm.

[0047] Figure 4: TEM images of the carbon nanotube particles revealing their planar structure of interconnected CNT struts and CNT bundle struts.

[0048] Figure 5: SEM images of the example parent CNT source material that is processed into particles: (a) macroscopic view of carbon nanotube fibres and (b) their microstructure of interconnected CNT and CNT bundle struts.

[0049] Figure 6: Composition of (a) parent CNT fibres as measured with thermogravimetric analysis, and (b) of exfoliated CNT particles via energy-dispersive X-ray analysis, obtained in a transmission electron microscope.

[0050] Figure 7: Raman spectra obtained for the parent CNT fibre, and for parent CNT fibres exfoliated at the voltages 6 V, 14 V and 28 V.

[0051] Figure 8: (a) Images and measured dimensions of dispersed CNT particles obtained with an optical microscope, and (b) the evolution of the cumulative probability distribution of CNT particle size after grinding with a pestle and mortar and upon blending with increasing time.

[0052] Figure 9: Refining of the size distribution of dispersed CNT particles due to mechanical grinding and sonication as measured with an optical microscope. Figure 10: The fraction of interwoven nodes, crossed nodes and free ends measured from the inspection of images of the CNT particles and the parent CNT fibre obtained with a scanning electron microscope and a transmission electron microscope.

[0053] Figure 11: Illustrations of the softening, exfoliating process and associated apparatus employed: (a) CNT material is immersed in the electrolyte affixed to a submerged, inert, conductive platinum rod, and voltage applied between the CNT material and an opposing platinum-coated counter electrode, (b) The CNT material is fed from the surface into the liquid, and a voltage is applied across the mat and an immersed counter electrode from some distance above the surface of the liquid, much above the rising front associated with intercalated wicking electrolyte.

[0054] Figure 12: Illustration of a dispersion apparatus in which CNT material is fed into a liquid electrolyte, sealed against the external atmosphere air against a compliant rubber roller that also acts to limit wicking of electrolyte upward from the electrolyte surface.

[0055] Figure 13: Rinsing process for removing electrolyte ions from a particle dispersion following exfoliation.

[0056] Figure 14: Drying of the CNT particle dispersion into a dry powder and subsequent redispersion of CNT particles into a replacement liquid.

[0057] Figure 15: (a) The microstructure of an LTO anode incorporating CNT particles of planar, weblike structure as the conductive additive, and (b) the specific capacity of anodes with CNTs as the conductive additive at a series of different charge rates, compared with that of anodes that employ carbon black. DETAILED DESCRIPTION OF THE INVENTION

[0058] Invented and discussed below in the form of example embodiments is a form of carbon nanotube particles of characteristic structure illustrated in Figure 1. The particles comprise a planar web of bonded formations that forms a network of struts, where the crosssection of each strut comprises a single or two or more bundled carbon nanotubes. The struts branch and interweave with one another by the exchange of nanotubes from one strut into the next, and joints between the struts can also form when two nanotubes or nanotube bundles adhere where they cross one another. Specifically shown in the schematic of Figure 1 are examples of individual CNT struts 1, strut crossings 2, interwoven struts 3 and CNT bundle struts 4. The struts terminate at nodes or free ends 5. The particles have a characteristic dimension denoted D, which is the dimension of the particle at its widest point. D may be tailored within a range of 5 pm to >200 pm via suitable particle preparation.

[0059] A typical cumulative frequency plot of the length of struts as measured within the planar particles and within the parent material is presented in Figure 2. As will be seen, in the planar particles substantially all strut lengths may be found to fall in the range 100 nm to 10 pm. In embodiments of the material of the invention, at least most of the strut lengths and optionally substantially all of the strut lengths may be above 300 nm and / or below 2 pm and for example between 300 nm and 2 pm.

[0060] A series of images of the particles obtained with an optical microscope are shown in Figure 3. The smallest particles in Figure 3 have D < 10 pm and the largest have D exceeding 200 pm. In embodiments of the material of the invention, at least most of the particle sizes and optionally substantially all of the particle sizes may be above 10 pm and / or below 200 pm and for example between D of 10 pm and 200 pm. Their planar structure is evident, and a varying degree of transparency can occur across the particles depending on their local thickness. The particles have a thickness that is mostly that of a single formation, although where two formations cross, a local increase in thickness may occur up that of a few formations. In embodiments of the material of the invention, at least most of the particles, and optionally substantially all of the particles, have a thickness perpendicular to the planar direction ranging from the diameter of an individual nanotube thickness up to 170 nm.

[0061] Their microstructure is viewed at greater magnification in images obtained with a transmission electron microscope, see Figure 4. The particles were deposited upon a TEM grid as an aqueous dispersion and imaged after subsequent evaporation of the liquid. The interwoven connections by which the nanotube and nanotube bundle struts interconnect to form a planar network structure is evident, e.g. Figure 4(d). Catalyst nanoparticles may also be adhered to the network struts, see Figure 4(f).

[0062] Secondly invented and discussed below in the form of example embodiments are manufacturing routes for the creation of planar particles, that take as input CNT fibres and CNT mats. In the described embodiments these are manufactured by a floating catalyst chemical vapour deposition process as has been detailed elsewhere, for example US 7,323,157 B2 by Kinloch et al. [9], An example of parent material in the form of a condensed fibre is imaged in Figure 5(a), in which the parent fibres are of diameter between 120 pm and 200 pm. The parent material microstructure is imaged in Figure 5(b), and comprises an interconnected network of nanotubes and nanotube bundles. The concentration of metal catalyst present in the material after manufacture is a function of processing parameters and typically lies in the range 5% to 19% [9], Treatments, using a combination of oxidation and subsequent acid immersion were employed to control the concentration of residual catalyst present in the parent material down to a level below 1%. This low concentration was evident from thermogravimetric analysis of the treated parent material, see Figure 6(a). Energy dispersive X-ray spectra of the particles after the division process confirmed that chemicals employed as electrolyte species in the exfoliation process that aids division of the patent material were also removed. The impurity concentration of the resultant planar particles is varied by the pre-treatment of the parent material, in particular the temperature of heating in air prior to acid immersion, see Figure 6(a). The mass content of carbon after manufacture and treatment can exceed 99%.

[0063] Raman spectra measured of divided parent material in the form of planar particles are plotted in Figure 7 and compared with that of the parent material. All spectra are normalised by the value of their peak present at ~1580 cm1that is attributed to the vibration of carbon atoms within the plane of the nanotube wall. The ratio of the relative intensity of the so- called D-band peak located at ~1350 cm1to that of the G-peak increased from ~0.35 for the parent material to ~1.37 after division into particles. This increase in intensity ratio between the two bands is attributed to defects in the atomic structure of the nanotube walls that result from the exfoliation methods employed to aid division of the parent material.

[0064] Characteristic features of the planar web-structured particles are now explored in greater depth. The characteristic dimension D of the particles as defined earlier and illustrated in Figure 1 is assessed by imaging small volumes of a diluted particle dispersion between glass slides, as pictured in Figure 8(a). The size of particles may be evolved over a wide range between 1 pm and >200 pm by employing different methods for particle refinement such as mechanical grinding, blending and sonication, see the range of size distribution plotted in Figure 8(b) and Figure 9. The web-like microstructure of the particles is also characterised in terms of its degree of interconnectivity due to bundled joints. Images of the particle microstructure obtained with a transmission electron microscope and scanning electron microscope allow for a determination of the relative occurrence of the different joint types and free strut ends. For clarity, interwoven joints connect three network struts, whereas crossed joints interconnect four struts, or two tubes / bundles that are continuous through the node. The relative frequency of these features in the parent material and in a series of different planar particles is charted in Figure 10. The fraction of free ends approaches null in the parent material, but increases to between 15% and 62% of the total sum of free ends and nodes within the divided particles. Nodes are 46% interwoven within the parent material (remainder crossed and the percentage of free ends is negligible) but this percentage is varied in the planar web-structured particles to between 36% and 74% of the total number of nodes and free ends. The fraction of interwoven nodes increases as aerogel layers separate, and then thereafter decreases with increasing incidence of free ends: In all cases the percentage of crossed nodes is reduced relative to the parent material to comprise only a few percent of the total sum of nodes and free ends that are counted. The width of struts is observed to vary between a lower limit that is the diameter of an individual carbon nanotube up to 170 nm.

[0065] Example methods and apparatus for electrochemical treatment employed to soften direct-spun carbon nanotube materials and aid their division into planar particles of web-like structure are now described. The following set of specific embodiments for apparatus employed to achieve softening differ in their specific design, but all three exploit a common defining feature that was discovered unexpectedly in experiment to aid division of macroscopic carbon nanotube materials and is claimed herein: Macroscopic parent CNT materials are softened and weakened sufficiently for division into fine particles by the application of sufficiently large positive voltage (6V and above) with respect to a counter electrode when both are immersed within an electrolyte and voltage applied for a sufficient length of time. Three embodiments of experimental apparatus are now described for the voltage-driven exfoliation stage of processing, with the caveat that a discussion of such specific embodiments sufficient for description of an example process and associated apparatus do not limit the invention which is defined by the claims appended hereto and encompasses other designs that would similarly employ the defining features of the invention claimed herein and disclosed above.

[0066] Embodiment 1: This embodiment is sketched in Figure 11(a). Macroscopic CNT material is affixed in the electrolyte ion solution to a submerged, inert and conductive rod. The rod is partially submerged so that the portion outside the electrolyte can be connected to the positive terminal of a power supply to serve as a positive electrode. A conductive rod to serve as a negative electrode is connected to the negative terminal of a power supply and similarly immersed in the ion solution. Exfoliated product particles are created at the positive electrode. In another instance of this embodiment, macroscopic CNT materials are held in place upon a mesh of inert metal that is connected, above the liquid, to the power supply. The inert metal may take the form of a retaining basket, alternatively nanotube material may be pressed against it by a suitable flow of electrolyte through the mesh.

[0067] Embodiment 2: This embodiment is sketched in Figure 11(b). Macroscopic parent CNT is fed into the electrolyte by passing it through the contact between rotating conductive rollers. The voltage from a power supply is connected to the rollers and the other terminal to a counter electrode (not sketched) so that CNT material above the surface of the electrolyte a conveys current to the immersed CNT materials that then undergo exfoliation. Embodiment 3: The embodiment is illustrated in Figure 12. CNT materials in the form of random fibers or mats are fed into the liquid between an elastically soft roller whose outer surface is electrically insulated (e.g. a rubber or rubber-coated roller may be employed), and a second electrically-insulating roller with which it may seal, e.g. of nylon. The point of contact between the rollers is in close proximity to (or even below) the free surface of the electrolyte. As the two non-conductive rollers press against one another and grip the CNT material, they can limit the upward wicking of electrolyte within the parent CNT material as it is passes into the electrolyte and undergoes exfoliation. Voltage is applied to the parent CNT material via contact with a third, electrically conductive roller that is connected to a power supply. Suitable brushes, scrapers or fluid jets are used to retain exfoliated CNT materials within the electrolyte by removing them from the rollers if carried above the free surface of the electrolyte.

[0068] After undergoing the electrochemical softening process, electrolyte species may be removed via a rinsing process, see Figure 13, in which the successive removal and replenishment of fresh water dilutes remaining electrolyte species to a tolerably low level. Settling of the particles allows the removal of electrolyte absent particles. A dispersion of CNT particles in non-aqueous liquids may be subsequently obtained by the evaporation of water followed by replacement with another liquid of choice, as illustrated in Figure 14.

[0069] EXAMPLES

[0070] A series of examples are listed below to further illustrate examples of practice of manufacturing web-structured carbon nanotube particles or of possible process steps of that manufacturing process, and of the properties of the resultant products, and to highlight their successful use within other material systems. These examples are nonlimiting in that their description here does not restrict the properties, features or applications of the web-like particles that are disclosed nor the choice of methods or method parameters employed in their manufacture.

[0071] Example 1: This example describes how the content of metal catalyst impurities present in the web-structured CNT particles can be varied by modifying the metal content present within the parent material prior to exfoliation and subsequent division. Masses of CNT fibre (between 1.0 g and 1.2 g) were heated for a period of 3 hours in air at a series of temperatures in the range 350°C to 500°C. They were then immersed overnight in 80 ml volumes of concentrated hydrochloric acid (37 wt%). They were then removed from the acid and rinsed eight times or more successively in fresh 100 ml volumes of di-ionised water, until the pH of the water after rinsing was close to neutral. Thermogravimetric analysis was performed on samples of as-received random CNT fibre and CNT fibre that underwent these described steps of heating, acid immersion, washing and drying to measure the effect upon the content of metallic impurities. After holding at 150°C until residual moisture was removed, the temperature was increased at a rate of 5°C / min and the remaining fraction of mass recorded as a function of temperature, see Figure 6(a). The remaining fraction of mass at 1000°C is identified as oxidised metal catalyst, and this decreases with increasing temperature of heating employed prior to acid immersion from 7% at 350°C to 1% for 500°C. The mass concentration of metal catalyst impurity in the dispersed CNT particles is similarly modulated by suitable treatment of the as-received CNT material prior to dispersion. By way of example, the spectra presented in Figure 6(b) obtained from energy-dispersive X-ray spectroscopy is demonstrative of a very low content of sulphur and iron that are employed as metal catalysts for the process of CNT production and so are present in the as-manufactured parent CNT materials used herein. Example 2: This example describes exfoliation processes that were employed to soften macroscopic CNT mats and fibres sufficiently for division into fine particles.

[0072] (a) Samples of parent CNT material (dry mass 250 mg) were affixed to one end of a platinum rod and immersed in a 200 ml solution of potassium sulphate in distilled water of concentration 0.3 mol / L. The platinum rod was partially immersed in the electrolyte, as sketched in Figure 11(a). The exposed length of platinum rod was connected to the positive terminal of a power supply. A platinum-plated counter-electrode was connected to the negative terminal of the power supply. In four separate experiments, voltages of 2 V, 6 V, 14 V and 28 V were applied across the two electrodes for a period of 10 minutes; the voltages 6 V, 14 V and 28 V induced currents in the range 6 A to 15 A. Swelling and exfoliation of the immersed CNT material was observed and was found to be increasingly rapid and greater in its extent with increasing voltage. Evolution of gas was observed at both electrodes. For voltages in the range 6 V to 28 V, exfoliated CNT fibres could subsequently be ground in a pestle and mortar to produce small particles. In contrast for fibres exfoliated at potential below 2 V, subsequent grinding with a pestle and mortar did not yield division into particles of dimension less than 1 mm. This was also found to be the case for fibres exfoliated at 2 V for longer time periods of up to 90 minutes.

[0073] (b) A length of CNT mat, with nominal thickness 100 pm, length 100 mm and width 20 mm and mass 50 mg, and 100 mg mass of random CNT fibre were dispersed in separate experiments employing the sealed-roller apparatus sketched in Figure 12. A voltage of 28 V was applied between conductive roller and counter electrode, and the molar concentration of potassium sulphate within the electrolyte bath was 0.3 mol / L . The feed rate of CNT materials through the rollers into the electrolyte bath was 0.2 mm / s. Both mat and fibre were divided into large particles (size 1 mm to 5 mm) upon their entry into the electrolyte. Subsequent grinding of wet particles collected from the electrolyte bath with a pestle and mortar or within a blender rapidly reduced their size below 1 mm.

[0074] Example 3: This example describes a blending method by which exfoliated CNT materials (such as manufactured in Example 2) are divided into particles, and decreased in size to much below 100 pm. A 1 g sample of parent CNT material resulted from the combination of four exfoliated 250 mg samples whose preparation is detailed previously in example 2(a). The wet 1 g sample was ground in an agate pestle and mortar for a period of 5 minutes. The size distribution of the resultant planar particles was characterised by diluting a small quantity of the particles with water and imaging them between glass slides with an optical microscope. The resultant size distribution is given in the plot of Figure 8(b). Then, the size distribution of the planar particles was further refined by combining them with 200 ml of water and blending them for increasing periods of 2.5 minutes, 5 minutes, 10 minutes and 20 minutes. The measured size distribution after each blending step are plotted in Figure 7(b). Evident is the tailoring of size distribution by selection of blending time within a range 8 pm to 200 pm.

[0075] Example 4: This example describes a sonication method by which exfoliated CNT materials (such as are manufactured in example 2) are divided into particles. An wet, exfoliated sample of parent CNT material obtained via exfoliation of a dry 250 mg mass (see example 2(a)) was ground using an agate pestle and mortar by hand for a period of 30 minutes. The resulting particles were examined using an optical microscope to characterise their size distribution, which is plotted in Figure 9. The ground CNT particles were combined with 80 ml of distilled water, and sonicated with a 5 mm diameter sonication horn (obtained from Sonic Systems Ltd) with power 500 W and frequency of 20 kHz for a time period of 3 hours. The resulting distribution of particle size is also plotted in Figure 9(b). Sonication decreases the median size of particle by more than a factor of 2, and the smallest particles obtained are of size ~1 pm.

[0076] Example 5: This example describes the production of a dry powder of CNT particles from a manufactured liquid suspension of low impurity concentration suitable for subsequent refinement as a dry state. Aqueous CNT particle suspensions of 0.3 wt% in distilled water were obtained according to example 2(b) and washed as illustrated in Figure 13. They were placed in an oven at a temperature of 120°C for time periods of between 3 hours and 6 hours in a shallow dish. The resulting evaporation of water formed a dry, weak and low-density agglomerate of CNT particles. Grinding of this agglomerate in a pestle and mortar in the dry state swiftly led to its division into small particles of dimension D « 100 pm.

[0077] Example 6: This example illustrates the manufacture of a CNT particle suspension in two nonaqueous fluids. Two samples of dried CNT particles were prepared according to Example 5, each of mass 1 g. One was combined with 200 ml of methyl ether ketone, and another with 200 ml of cyclohexane. Both were blended at 20,000 RPM for eight periods of 2 minutes. Between each 2 minute period of blending the samples were cooled by placing the blending container inside an ice bath so that its temperature was decreased below 10°C. After blending, small samples of the dispersions were diluted in their respective fluids so that particles could be imaged separately from one another with an optical microscope when deposited upon a glass slide and their size distribution determined. In both cases the size of particles was found to be below 100 pm.

[0078] Example 7: This example describes the manufacture of a CNT-particle and polymer suspension within a liquid. A dry mass of 0.1 g of carboxymethyl cellulose (CMC) of molecular weight Mwin the range 300,000-380,000 (Nippon Paper Industries Co. Ltd.) was mixed with 20 ml of distilled water using a magnetic stirrer. Once the CMC was fully dissolved the polymer-water solution was combined with a CNT particle suspension that contained 0.3 g of CNT particles in 30 ml of distilled water that had been rinsed as illustrated in Figure 13 so as to remove electrolyte species. The resulting 50 ml mixture of CNTs and CMC was thoroughly mixed using an IKA T 25 500 W homogeniser at 8000 rpm for a period of 20 minutes. The viscosity of the resultant CMC-CNT suspension was subsequently evolved over the range 850 mPa s1to 11,000 mPa s1(as measured with an Anton Paar parallel plate rheometer) via the evaporation or addition of water. Evaporation yielded a thick dope or gel that capable of remaining adhered to the surface of an inverted container, whereas addition of water gave a thinner suspension that could be poured between beakers. In all cases the mixtures were found to conduct electricity, and this was verified by metering the resistance between the inserted metal prongs of an Ohmmeter.

[0079] Example 8: This example describes the manufacture of freestanding CNT films via the evaporation of liquid from deposited films of CNT-particle suspensions. In one instance an aqueous CNT suspension prepared according to example 3 was rinsed of electrolyte and poured onto a glass slide within a flat petri dish, and residual water left to evaporate at room temperature. The film was then peeled from the filter paper. The in-plane conductivity of the film was measured by four-point probe, and was found to be approximately 11 kS / m. In a second instance a mixture prepared according to example 7 was cast and dried and was found to exhibit an in-plane conductivity of approximately 8 kS / m.

[0080] Example 9: This example describes the manufacture and characterisation of electrodes for use as the anodes in lithium ion batteries that employ CNT particles of the form disclosed here as a conductive additive. A CNT-CMC suspension of CNT weight fraction 1.6% and CMC:CNT mass ratio 1:3 was manufactured according to examples 1, 2(a), 3, 4 and 7 (impurity removal, exfoliation, blending, sonication, rinsing, combination with polymer and subsequent evaporation). The suspension took the form of a thick, viscous and smooth dope. It was combined with lithium titanium oxide (LTO) powder (NEI Corporation) and styrene-butadiene rubber (SBR) using a planetary mixer (Thinky ARE-310), in which it was mixed at 20,000 RPM for three periods of 15 minutes, with cooling time between each mixing period of 5 minutes. The mass of ratio of carbon nanotube conductive additive to LTO was varied between 0.5% and 6%. Additional anode slurries in which carbon black (MEI Corporation) was used as the conductive additive at equivalent mass loadings were also prepared so that the performance of the CNT conductive additive could be compared. Anode slurries were cast upon glass slides using a doctor blade to form layers of 80 pm nominal thickness, so that their in-plane electrical conductivity could be measured using a 4-point probe technique. Anode slurries containing carbon nanotubes as their conductive additive possessed a substantially greater in-plane conductivity than those employing carbon black as the conductive additive: for example, at 5% conductive additive by weight the anode with planar CNT particles as the conductive additive was found to have an in-plane conductivity greater than 20x that of the electrode employing an equivalent mass-loading of carbon black. The microstructure of an anode with CNT conductive additive was imaged with a scanning electron microscope, see Figure 11(a). The planar web-like structures of the CNT particles are visible, and form conductive bridge structures that electrically connect the active particles of LTO. Coin cells were assembled in an argon glovebox that employed a lithium metal foil as the counter electrode, and were cycled between cut-off voltages of 1 V and 2.5 V vs Li+at increasing rates from 0.1 C to 10 C, where C is the theoretical capacity of LTO (taken as 175 mAhg-1). The measured capacity of the cells is plotted against cycle number in Figure 11(b). It is evident that the electrodes employing CNTs as a conductive additive offer approximately 5% greater capacity per unit mass of anode material at higher discharge rates when compared to electrodes that employ a carbon black additive.

[0081] REFERENCES

[0082] [1] Jonathan Coleman, Valeria Nicolosi, Sang Hoon Park, Paul King. "Highly efficient electrodes enabled by segregated networks". International Patent WO 2020 / 144298 Al.

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[0084] [3] Houngsik Yoo, Jongheon Seol, Jon Won Lee, Dong Hyun Kim, Gyemin Kwon, Yelin Kim, Jungkeun Yoo, Dong Hhun Cho, Sang Hook Choy, Hyeon Choi, Kyungyeon Kang, Jihee Woo. "Carbon nanotube dispersion and method for producing same". EP 3 355 391 Bl (2019).

[0085] [4] Richard E. Smalley, Rajesh Kumar Saini, Ramesh Sivarajan, Robert H. Hauge, Virginia Angelica Davis, Matteo Pasquali, Lars Martin Ericson, Satish Kumar, Sreekumar Thaliyil Veedu. "Single-wall carbon nanotube alewives, process for making, and compositions thereof". United States Patent US 7,288,238 B2 (2007).

[0086] [5] Kazuaki Katagiri, Atsushi Kakitsuji. "Carbon nanotube-dispersed composite material, method for producing same and article same is applied to". United States Patent Application Publication US 2007 / 0134496 Al (2007).

[0087] [6] Sung Jin Kim, Dong Hyun Cho, Jae Keun Yoon, Tae Hyung Kim, Og Sin Kim. "Entangled-type carbon nanotubes and method for preparing the same". United States Patent US 11,987,499

[0088] B2 (2024). [7] Goto Toshiki et al, "Carbon nanotube powder, auxiliary conductive agent for electrode, electrode using the same, and electric storage device using the electrode", JP 2011063458A (2011).

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Claims

CLAIMS1. A carbon nanotube material comprising a plurality of particles, each particle comprising an agglomeration of elongate carbon nanotube formations, each formation comprising a carbon nanotube or carbon nanotube bundle, the formations conformed to form an interconnected substantially planar structure.

2. The carbon nanotube material of claim 1 wherein at least some of the carbon nanotube formations adhere together where two formations cross one another in the substantially planar structure.

3. The carbon nanotube material of any preceding claim wherein at least some of the carbon nanotube formations interweave together by exchange of a carbon nanotube between one carbon nanotube bundle and another carbon nanotube bundle.

4. The carbon nanotube material of any preceding claim comprising particles having interwoven joints between the carbon nanotube formations that account for between 36% and 74% of the sum of interwoven joints, crossed joints and free ends within the particle.

5. The carbon nanotube material of any preceding claim comprising particles having a dimension at the widest point in the range 1 pm to 200 pm.

6. The carbon nanotube material of any preceding claim comprising carbon nanotube formations having a thickness ranging from the diameter of an individual nanotube thickness up to 170 nm.

7. A formulation comprising particles of the carbon nanotube material of any preceding claim in a liquid suspension, dispersion or slurry, optionally in combination with other species.

8. The formulation of claim 7, wherein the liquid is selected from water, n-methyl-2- pryillodone, methyl-ether ketone, acetone, ethanol or dimethylacetamide and mixtures thereof.

9. A formulation comprising particles of the carbon nanotube material of any one of claims 1 to 6 in dry powder form, optionally in combination with other species.

10. The formulation of any one of claims 7 to 9 further comprising additional polymeric material partly of wholly adsorbed on to the surface of the particles.

11. A method of manufacture of a carbon nanotube material comprising the steps of: providing a precursor macroscopic carbon nanotube material selected from one or more of a yarn, strand, mat, fibre, aerogel; immersing the precursor material in an ion-containing electrolyte and applying a voltage thereto with respect to an additional immersed electrode to soften the macroscopic CNT material and cause the macroscopic CNT material to produce a plurality of carbon nanotube particulate agglomerates.

12. The method of claim 11, further comprising mechanically dividing the electrolysed CNT material, for example wherein the mechanical division step is performed by one or more of mechanical grinding, blending, homogenization, sonication.

13. The method of claim 11 or claim 12, further comprising one or more of:(i) removing the electrolyte, for example:(a) by rinsing in water, and optionally further removing the rinsing water by evaporation;(b) using a filter press, that is, remove electrolyte via separating material and electrolyte species by passing through a porous membrane;(c) mechanically, for example by pressing of material between plates or rollers so as to expunge electrolyte-containing water as in a mangle, by centrifuge, or(d) other method;(ii) optionally further replacing with a further liquid excipient.

14. The method of any one of claims 11 to 13, wherein the precursor macroscopic carbon nanotube material is an as-manufactured or condensed aerogel, and preferably wherein the precursor macroscopic carbon nanotube material is fabricated by a floating catalyst chemical vapour deposition process.

15. The method of any one of claims 11 to 14, wherein the precursor macroscopic carbon nanotube material is an as-manufactured direct-spun carbon nanotube material.

16. The method of any one of claims 11 to 15, wherein the applied voltage is at least 6V and for example up to 28V.

17. The method of any one of claims 11 to 16, wherein the voltage is applied for at least 10 minutes and for example up to 90 minutes.

18. The method of any one of claims 11 to 17, wherein the electrolyte is a potassium sulfate solution in water that has a salt content of between 1% and 5 % by weight.

19. The method of any one of claims 11 to 18, wherein the surface of the additional immersed electrode exposed to the electrolyte is substantially inert.

20. The method of any one of claims 11 to 19, wherein mechanical division step is performed by one or more of mechanical grinding, blending, homogenization, sonication.

21. The method of any one of claims 11 to 20, performed in such manner as to produce particles having a dimension at the widest point in the range 1 pm to 200 pm.

22. The method of any one of claims 11 to 21, performed in such manner as to produce particles exhibiting interwoven joints between the carbon nanotube formations that are observed to account for between 36% and 74% of the sum of interwoven joints, crossed joints and free ends within the particle.

23. The method of any one of claims 11 to 22, further comprising the removal of liquid from the plurality of carbon nanotube particles to create a dry powder, optionally in combination with other species.

24. The method of any one of claims 11 to 23, further comprising the removal of liquid from the plurality of carbon nanotube particles and the substitution of an alternative liquid, for example selected from water, n-methyl-2-pryillodone, methyl-ether ketone, acetone, ethanol or dimethylacetamide and mixtures thereof, to create a liquid suspension, dispersion or slurry of the particles, optionally in combination with other species.

25. The use of the method of any one of claims 11 to 24 to produce a material or formulation in accordance with any one of claims 1 to 10.

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