Secondary structures of carbon nanotubes and sulfur-carbon composites containing them

Abstract

One aspect of the present invention relates to a carbon composite for use in a positive electrode of a lithium-sulfur battery, comprising a secondary structure formed by agglomerating carbon nanotubes, the secondary structure being characterized in that the secondary structure has a flower-like structure including a spherical core and at least one amorphous flake on a surface of the core.

Description

[Technical Field]

[0001] This invention relates to a secondary structure of carbon nanotubes that can be used as a porous carbon material for electrodes of electrochemical elements, and a method for producing the same.

[0002] In particular, this invention relates to a sulfur-carbon composite in which sulfur is supported on a secondary structure of carbon nanotubes, and to a lithium-sulfur battery containing the same.

[0003] This application claims priority based on Korean Patent Application No. 10-2022-0110402, filed on 31 August 2022, and all content disclosed in the specification and drawings of said application is incorporated herein. [Background technology]

[0004] A lithium-sulfur battery is a battery system that uses a sulfur-based material having a sulfur-sulfur bond (SS bond) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, the main material of the positive electrode active material, has the advantages of being abundant worldwide, non-toxic, and having a low atomic weight.

[0005] As the application areas of secondary batteries expand to electric vehicles (EVs), energy storage systems (ESS), etc., lithium-sulfur battery technology, which can theoretically achieve a higher "energy storage density per unit weight (~2,600 Wh / kg)" compared to lithium-ion secondary batteries with a relatively low "energy storage density per unit weight (~250 Wh / kg)," is attracting attention.

[0006] In a lithium-sulfur battery, during discharge, lithium, the negative electrode active material, releases electrons and is oxidized while ionizing into lithium cations, while the sulfur-based material, the positive electrode active material, is reduced while accepting electrons. Here, the reduction reaction of the sulfur-based material causes the SS bond to accept two electrons and is converted into a sulfur anion. The lithium cations produced by the oxidation reaction of lithium are transferred to the positive electrode by the electrolyte, and these combine with the sulfur anions produced by the reduction reaction of the sulfur-based compound to form a salt. Specifically, sulfur before discharge has a ring-shaped S8 structure, which is converted into lithium polysulfide (Li2Sx) by the reduction reaction, and is completely reduced to produce lithium sulfide (Li2S).

[0007] Thus, the sulfur used in the positive electrode active material is 5 × 10 -30 Because sulfur is a non-conductive material with an electrical conductivity of S / cm, research is being conducted on cathode materials in which sulfur is composited with a conductive material to impart reactivity to sulfur, so that it can be used as an active material and electrons can be transferred during electrochemical reactions. To improve the kinetic activity of electrochemical reactions during charging and discharging of lithium-sulfur secondary batteries, there is a continuous need for technological development of sulfur-carbon composites in which the cathode active material is supported on a porous carbon material as a cathode material.

[0008] For example, development is underway on materials using carbon nanotubes (CNTs) as porous carbon materials. CNTs have the advantage of being highly porous and allowing for increased sulfur load capacity, but they have the problem of low ion diffusion due to the inability to secure sufficient ion permeability pathways for electrode manufacturing. Furthermore, in order to develop high-energy-density lithium-sulfur secondary batteries, it is necessary to develop sulfur-carbon composites that can realize electrodes with low porosity.

[0009] For this reason, while porous carbon materials have a high specific surface area, it is necessary to ensure sufficient ion diffusion paths to facilitate the transfer of sulfur and electrolyte.

Summary of the Invention

Problems to be Solved by the Invention

[0010] The problems to be solved by the present invention are as follows: To provide a secondary structure of carbon nanotubes having a novel shape that can ensure a sufficient ion diffusion path in an electrode when used as a carrier for active material with a large specific surface area using carbon nanotubes.

[0011] In addition, it is intended to provide a sulfur-carbon composite in which sulfur (S8) is complexed as an active material using the secondary structure of carbon nanotubes having the novel shape as a carrier for the active material. The purpose is to provide a sulfur-carbon composite with increased kinetic activity of the oxidation / reduction reaction of sulfur.

[0012] Another object is to provide an electrode for an electrochemical element including the secondary structure of carbon nanotubes having the novel shape and an electrochemical element including the same.

[0013] In particular, an object is to provide a positive electrode for a lithium-sulfur battery including a sulfur-carbon composite including the secondary structure of carbon nanotubes as a positive electrode active material, and a lithium-sulfur battery including the same. The object is to provide a lithium-sulfur battery with a high energy density by implementing an electrode with a low porosity using the sulfur-carbon composite.

Means for Solving the Problems

[0014] In order to achieve the above problems, According to one aspect of the present invention, a secondary structure of the following embodiments is provided.

[0015] The secondary structure according to the first embodiment is It is a secondary structure in which carbon nanotubes are aggregated, and has a flower-like structure including a spherical core portion and at least one amorphous flake on the surface of the core portion.

[0016] According to the second embodiment, in the first embodiment, The specific surface area of the secondary structure can be 50 m 2 / g to 1,000 m 2 / g.

[0017] According to the third embodiment, in the first embodiment or the second embodiment, The average particle size D 50 of the secondary structure can be 1 μm to 100 μm.

[0018] According to the fourth embodiment, in any one of the first to third embodiments, The secondary structure has a spherical core portion with an average particle size D of 10 to 20 μm 50 and at least one amorphous flake with an average particle size D of 1 to 5 μm on the surface of the core portion, and may have a flower-like structure. 50

[0019] According to the fifth embodiment, in any one of the first to fourth embodiments, The sphericity of the secondary structure can be 0.5 to 0.99.

[0020] According to the sixth embodiment, in any one of the first to fifth embodiments, The volume of the total pores in the secondary structure can be 0.5 to 5 cm 3 / g.

[0021] According to the seventh embodiment, in any one of the first to sixth embodiments, The carbon nanotubes can have a diameter of 50 to 500 nm and a length of 0.1 to 5 μm.​​​​​The ratio of the diameter to the length of the carbon nanotube may be 1:1 to 1:30.

[0023] According to another aspect of the present invention, a method for manufacturing a secondary structure of the following embodiment is provided.

[0024] The method for manufacturing a secondary structure according to the 9th embodiment is (S10) aggregating carbon nanotubes to obtain entangled macro carbon nanotubes; (S20) obtaining a secondary structure with a deformed structure from the entangled macro carbon nanotubes, and the secondary structure may have a flower-like structure including a spherical core part and at least one amorphous flake on the surface of the core part.

[0025] According to the 10th embodiment, in the 9th embodiment, the specific surface area of the secondary structure may be 50 m 2 / g to 1,000 m 2 / g.

[0026] According to the 11th embodiment, in the 9th or 10th embodiment, in the step S10, the dispersion liquid in which the carbon nanotubes are dispersed in the first dispersion medium may be spray-dried.

[0027] According to the 12th embodiment, in any one of the 9th to 11th embodiments, the step S20 may be a step of spray-drying the dispersion liquid in which the entangled macro carbon nanotubes are dispersed in the second dispersion medium.

[0028] According to the 13th embodiment, in any one of the 9th to 12th embodiments, the first dispersion medium or the second dispersion medium may include water, alcohol, benzene, toluene, pyridine, acetone, tetrahydrofuran (THF), dimethylformaldehyde (DMF), or two or more of these.

[0029] According to the 14th embodiment example, in any one of the 9th to 13th embodiment examples, The process may further include a step of carbonization after the spray drying described above.

[0030] According to the 15th example, in any one of the 9th to 14th examples, The carbonization can be carried out at a temperature of 500°C to 1,200°C.

[0031] According to the 16th example, in any one of the 9th to 15th examples, The process further includes a step of chopping the carbon nanotubes before step S10, in which a large, entangled carbon nanotube is obtained from the chopped carbon nanotubes.

[0032] According to the 17th example, in any one of the 9th to 16th examples, The aforementioned entangled giant carbon nanotubes may have a sphericity of 0.5 to 0.99.

[0033] According to the 18th example, in any one of the examples from the 9th to the 17th, The aforementioned entangled giant carbon nanotubes have an average particle size D of 20-200 μm. 50 It may have.

[0034] According to the 19th embodiment, in any one of the 9th to 18th embodiment examples, The aforementioned entangled giant carbon nanotubes may have a porosity of 60-90% by volume.

[0035] According to the 20th example, in any one of the examples from the 9th to the 19th, The aforementioned entangled giant carbon nanotubes are 200m 2 / g~400m 2 It may have a specific surface area of ​​ / g.

[0036] According to yet another aspect of the present invention, the following embodiment of a sulfur-carbon composite is provided.

[0037] The sulfur-carbon composite according to the 21st embodiment is, The invention may include a secondary structure described in any one of the first to eighth embodiments, and a sulfur-containing compound supported on at least a portion of the interior and exterior surfaces of the pores of the secondary structure.

[0038] According to the 22nd embodiment, in the 21st embodiment, The sulfur-containing compound may include inorganic sulfur (S8), lithium polysulfide (Li2Sn, 1≦n≦8), carbon sulfur polymer (C2Sx)m (2.5≦x≦50, 2≦m), or mixtures thereof.

[0039] According to the 23rd embodiment, in the 21st embodiment or the 22nd embodiment, The weight ratio of the secondary structure to the sulfur-containing compound may be between 10:90 and 40:60.

[0040] According to another aspect of the present invention, electrodes of the following embodiment are provided.

[0041] The electrode according to the 24th embodiment is, The electrode active material layer may include the secondary structure described in any one of the first to eighth embodiments.

[0042] According to yet another aspect of the present invention, an electrochemical element of the following embodiment is provided.

[0043] The electrochemical element according to the 25th embodiment is, The system includes a positive electrode, a negative electrode, a separation membrane interposed between the positive electrode and the negative electrode, and an electrolyte. At least one of the positive and negative electrodes may include a secondary structure described in any one of the first to eighth embodiments. [Effects of the Invention]

[0044] According to one embodiment of the present invention, This makes it possible to provide secondary structures of carbon nanotubes having a novel shape.

[0045] In particular, it is possible to provide a secondary structure of carbon nanotubes that has a high specific surface area and a controlled pore structure, thereby improving the ion diffusion pathway when used as an electrode and exhibiting high electrochemical activity, as well as a composite in which an active material is compounded therewith.

[0046] In particular, sulfur-carbon composites, in which sulfur (S8) is compounded into the secondary structure of the carbon nanotube, exhibit the effect of providing excellent kinetic activity in sulfur oxidation / reduction reactions. For example, the sulfur-carbon composite has a high specific surface area, which increases the number of active sites on which sulfur can participate in the reaction, thereby increasing the reactivity of sulfur. Furthermore, the sulfur-carbon composite has a large pore volume, which facilitates sulfur support and is advantageous in securing ion diffusion paths.

[0047] An electrode using the aforementioned sulfur-carbon composite as the positive electrode active material can realize an electrode with low porosity, thereby providing a lithium-sulfur secondary battery with high energy density.

[0048] The following drawings accompanying this specification illustrate preferred embodiments of the invention and, together with the detailed description of the invention, serve to further illustrate the technical idea of ​​the invention. Therefore, the invention should not be construed as being limited solely to what is shown in the drawings. [Brief explanation of the drawing]

[0049] [Figure 1] This is a scanning electron microscopy (SEM) image of the secondary structure of Comparative Example 1 in this specification. [Figure 2] This is an SEM image of the secondary structure of Example 1 in this specification. [Figure 3] This is a high-magnification SEM image of the sulfur-carbon composite of Comparative Example 2 described herein. [Figure 4] This is a low-magnification SEM image of the sulfur-carbon composite of Comparative Example 2 described herein. [Figure 5] This is a high-magnification SEM image of the sulfur-carbon composite of Example 2 described herein. [Figure 6] This is a low-magnification SEM image of the sulfur-carbon composite of Example 2 described herein. [Figure 7] This graph shows the charge-discharge evaluation results of lithium-sulfur coin cells manufactured using cathodes to which the sulfur-carbon composite of Comparative Example 2 and the sulfur-carbon composite of Example 2 were applied, respectively, according to one experimental example described herein. [Figure 8] This graph shows the lifetime evaluation results of lithium-sulfur coin cells manufactured using cathodes to which the sulfur-carbon composite of Comparative Example 2 and the sulfur-carbon composite of Example 2 were applied, respectively, according to one experimental example described herein. [Modes for carrying out the invention]

[0050] The present invention will be described in detail below. However, the present invention is not limited to the following, and each component may be modified in various ways or selectively mixed as needed. Therefore, it should be understood that all modifications, equivalents, or substitutes included in the spirit and technical scope of the present invention may be incorporated.

[0051] In a specification, when a configuration is described as "including" a certain component, unless otherwise specified, it means that it may include other components, rather than excluding them.

[0052] In this specification, the term "A and / or B" means A or B, or both.

[0053] According to one aspect of the present invention, a porous carbon material is provided that can be used as an electrode material for an electrochemical element. Specifically, a secondary structure of carbon nanotubes having a novel shape is provided. Such a secondary structure has a high specific surface area and a unique porosity structure. This provides an advantageous effect in realizing a low-porosity electrode when the secondary structure is applied to an electrode, and therefore, a high-energy-density electrochemical element can be provided.

[0054] In particular, the carbon nanotube secondary structure can be used as a support for the positive electrode active material of a lithium-sulfur battery, and can be mixed and compounded with inorganic sulfur (S8) to provide a sulfur-carbon composite that can be used as the positive electrode active material of a lithium-sulfur battery. However, the carbon nanotube secondary structure can be used not only as a support for the active material of a lithium-sulfur battery, but also as a conductive additive for electrodes of electrochemical elements, and the uses of the carbon nanotube secondary structure are not limited to these uses.

[0055] According to one aspect of the present invention, a secondary structure of carbon nanotubes is provided that can be used as a positive electrode material for a lithium-sulfur battery. Specifically, the secondary structure has a structure that includes a plurality of pores on its outer surface and internally. In this case, the secondary structure can be used as a porous carbon material for supporting a sulfur-containing compound as a positive electrode active material on at least one of the outer surface and / or the internal surface of the pores, but the applications of the present invention are not limited thereto.

[0056] A secondary structure according to one aspect of the present invention is a secondary structure formed by the aggregation of carbon nanotubes, and has a flower-like structure comprising a spherical core portion and at least one amorphous flake(s) on the surface of the core portion.

[0057] In one embodiment of the present invention, the secondary structure may be characterized by exhibiting a large specific surface area, thereby increasing the number of active sites on which sulfur can participate in oxidation / reduction reactions when a sulfur-containing compound, which is the positive electrode active material, is supported. However, the features of the present invention are not limited to this.

[0058] In one embodiment of the present invention, the secondary structure is characterized by having a large pore volume, which facilitates the support of sulfur-containing compounds as positive electrode active materials and is advantageous for securing ion diffusion paths. However, the features of the present invention are not limited to this.

[0059] As described later, a secondary structure according to one aspect of the present invention may be formed by aggregating and redispersing carbon nanotubes that are shorter in length than conventional carbon nanotubes, for example, conventional carbon nanotubes that have been shredded to reduce their length, that is, carbon nanotubes with a smaller ratio of length to diameter compared to conventional carbon nanotubes. This makes it possible to form a large secondary structure with a higher degree of sphericity and a larger specific surface area than when conventional carbon nanotubes with a very high ratio of length to diameter are aggregated. The carbon composite then redisperses the formed large secondary structure and deforms the structure into a flower-like shape, thereby achieving the effect of securing sufficient ion diffusion channels while having a large specific surface area.

[0060] In this specification, the flower-like structure is described as having a spherical core portion and at least one amorphous flake formed on the surface of the core portion.

[0061] Figures 1 and 2 are SEM images of secondary structures of carbon nanotubes with different shapes. Specifically, Figure 1 shows a secondary structure of carbon nanotubes formed to have a simple spherical shape, and Figure 2 shows a secondary structure of carbon nanotubes formed to have a flower-like structure with a spherical core and at least one amorphous flake formed on the surface of the core, according to one embodiment of the present invention.

[0062] Referring to Figure 1, it can be seen that carbon nanotubes aggregate to form a secondary structure with a spherical shape, and that the spherical surface of the secondary structure is formed relatively smoothly.

[0063] On the other hand, referring to Figure 2, it can be seen that the carbon composite has a flower-like structure in which a spherical core portion and at least one plate-like flake are formed on the surface of the core portion, thereby forming multiple recesses and protrusions on the spherical surface of the secondary structure.

[0064] In this specification, the term "spherical" for the core of the secondary structure refers not only to a perfect sphere but also to similar spherical shapes such as ellipses and donuts, which have slightly different radii.

[0065] In this specification, the "amorphous flakes" formed on the surface of the core portion of the secondary structure are formed by the spraying and drying of a carbon nanotube dispersion, followed by redispersion and secondary spraying and drying, and each flake is smaller in size than the core portion, and the structure formed on the surface of the core portion is collectively referred to as such.

[0066] In one embodiment of the present invention, the secondary structure may have the following physical properties. When the secondary structure has the following physical properties, it may have advantageous effects in improving the performance of the battery, but the present invention is not limited thereto.

[0067] In one embodiment of the present invention, the secondary structure is formed by the aggregation of carbon nanotubes and can exhibit a large specific surface area. For example, the specific surface area of ​​the secondary structure is 50 m². 2 / g~1,000m 2 / g, specifically 100m 2 / g~500m 2 / g, more specifically 150m 2 / g~200m 2 It could be / g

[0068] In this specification, the specific surface area refers to the BET specific surface area value measured by the BET method, and the BET specific surface area may be measured by known methods for measuring it. For example, the BET specific surface area may be a value calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-max from BEL Japan.

[0069] In one embodiment of the present invention, the secondary structure may be formed by the aggregation of carbon nanotubes, exhibiting a large specific surface area, while having a particle size smaller than 100 μm, specifically smaller than large particles exceeding 50 μm, in order to realize an electrode with low porosity. For example, the average particle size D of the secondary structure 50 This can be 1-100 μm, specifically 1-50 μm, 5-50 μm, 10-100 μm, 10-50 μm, 12-100 μm, 12-50 μm, or more specifically 10-30 μm or 10-20 μm.

[0070] In this specification, the particle size may be measured by scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM), or laser diffraction. Measurement using laser diffraction may, for example, be performed using a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000).

[0071] In this specification, the average particle size D 50 This represents the particle size at the 50% point of the cumulative distribution of particle counts by diameter.

[0072] In one embodiment of the present invention, the secondary structure having the flower-like structure has an average particle size D of 10 to 20 μm. 50 A spherical core having a surface with an average particle size D of 1 to 5 μm, specifically 1 to 3 μm. 50It may be in a form comprising at least one amorphous flake having a particle size of 10-20 μm. Specifically, the average particle size D 50 The surface of the spherical core portion has an average particle size D of 1 to 5 μm, specifically 1 to 3 μm. 50 It may be a form that includes multiple amorphous flakes having [a certain characteristic].

[0073] In this case, the size of the spherical core can be measured using the laser diffraction particle size analyzer described above, and the size of the amorphous flakes indicates the length of the longest axis in a single flake. When the secondary structure is in the form described above, it can have a beneficial effect on improving the performance of electrodes and batteries to which it is applied, but the present invention is not limited thereto.

[0074] In one embodiment of the present invention, the secondary structure may have a roundness of 0.5 to 0.99, for example. Specifically, the roundness of the secondary structure may be 0.6 to 0.95, and more specifically, 0.7 to 0.9. The secondary structure is formed by redispersing entangled giant carbon nanotubes that have aggregated and become spherical, as described later, and by maintaining a high degree of roundness at the level of the entangled giant carbon nanotubes, it is possible to impart uniform reactivity in the electrode, but the present invention is not limited thereto.

[0075] In this specification, the sphericity of the secondary structure is calculated by measuring the lengths of the minor axis and major axis from the overall shape of one secondary structure, and then dividing the length of the major axis by the length of the minor axis. In this case, the minor axis and major axis can be measured by the same method as the method for measuring particle size.

[0076] Sphericity = [(Length of the major axis of the secondary structure) / (Shortening length of the secondary structure)]

[0077] In one embodiment of the present invention, the secondary structure is formed by the aggregation of carbon nanotubes, and contains a plurality of pores inside the secondary structure, for example, the total volume of the pores in the secondary structure is 0.5 to 5 cm³. 3 / g, specifically 0.5-3cm3 / g, more specifically 0.8-2cm 3 It may be / g. When the total volume of pores in the secondary structure is within the range described above, it becomes possible to provide an electrode that has an excellent sulfur load in the sulfur-carbon composite using it, as well as an excellent packing density, which can have a favorable effect in providing a battery with high energy density, but the present invention is not limited thereto.

[0078] In this specification, the total volume of pores in the secondary structure may be measured, for example, by the Brunauer-Emmett-Teller (BET) method using nitrogen gas or by a mercury porosimeter (Hg porosimeter) and ASTM D-2873.

[0079] In one embodiment of the present invention, the carbon nanotubes, which are the primary particles constituting the secondary structure, may have a diameter of, for example, 50 to 500 nm, more specifically 75 to 350 nm, and more specifically 100 to 200 nm. The diameter of the carbon nanotubes can serve as a pathway for the movement of sulfur and electrolyte in the sulfur-carbon composite, and therefore, if the carbon nanotubes have a diameter within the aforementioned range, it may have a favorable effect on sulfur loading and electrolyte mass transfer, but the present invention is not limited thereto.

[0080] In this specification, unless otherwise specified, the diameter of the carbon nanotube may be measured using a transmission electron microscope (TEM), and the length of the carbon nanotube may be measured using a scanning electron microscope (SEM), but the measurement method is not limited to these.

[0081] In one embodiment of the present invention, the carbon nanotube may have a length of, for example, 0.1 to 5 μm, more specifically 0.2 to 4 μm, and more specifically 0.3 to 3 μm. Not only the diameter of the carbon nanotube, but also its length affects the accessibility of material to the interior of the secondary structure, and furthermore, the length of the carbon nanotube can affect the external shape of the secondary structure. Specifically, when the length of the carbon nanotube is within the aforementioned range, the sphericity and porosity of the final secondary structure are increased, thereby ensuring sufficient passages for the movement of sulfur and electrolyte within the final secondary structure, which is advantageous for sulfur loading and electrolyte material transfer.

[0082] In one embodiment of the present invention, as described above, not only the diameter and length of the carbon nanotubes constituting the secondary structure, but also the ratio of diameter to length may be important. In one specific example of the present invention, the ratio of diameter to length of the carbon nanotubes may be 1:1 to 1:30, more specifically 1:3 to 1:25, and more specifically 1:5 to 1:20.

[0083] The following describes a method for producing secondary structures by aggregating the carbon nanotubes mentioned above.

[0084] A method for producing a secondary structure according to another aspect of the present invention includes the steps of (S10) obtaining entangled giant carbon nanotubes from carbon nanotubes, and (S20) obtaining a structurally deformed secondary structure from the entangled giant carbon nanotubes. In this case, the secondary structure is the secondary structure described above and has a flower-like structure including a spherical core and at least one amorphous flake on the surface of the core.

[0085] In one embodiment of the present invention, the secondary structure has a specific surface area of ​​50 m² as described above. 2 / g~1,000m 2 It can be / g. Furthermore, it possesses the physical properties of the secondary structure described above.

[0086] To obtain the secondary structure described above, first, (S10) carbon nanotubes are granulated as primary particles to obtain large, intertwined carbon nanotubes.

[0087] In one embodiment of the present invention, step S10 may include a step of producing a dispersion of carbon nanotubes. Specifically, it may include a step of spray-drying the dispersion containing the dispersed carbon nanotubes.

[0088] In this specification, the dispersion medium for producing the carbon nanotube dispersion is referred to as the first dispersion medium to distinguish it from the dispersion medium in step S20 described later. The first dispersion medium may, but is not limited to, water, alcohol, benzene, toluene, pyridine, acetone, tetrahydrofuran (THF), dimethylformaldehyde (DMF), or two or more of these.

[0089] In one embodiment of the present invention, the dispersion may further contain a known dispersant to improve the dispersibility of the carbon nanotubes. The dispersant may be, but is not limited to, polystyrene sulfonate, polyacrylic acid, polyvinylpyrrolidone, polyvinyl alcohol, carboxymethylcellulose, polyethylene oxide, polyvinyl butyral, ethylcellulose, or a mixture of two or more of these. The dispersant may not remain in the secondary structure obtained by the carbonization step in a subsequent step to obtain the secondary structure.

[0090] In one embodiment of the present invention, the carbon nanotubes may be present in an amount of 0.5 to 5% by weight based on the total weight of the carbon nanotube dispersion. Furthermore, if the dispersion also contains the dispersant, the weight ratio of the carbon nanotubes to the dispersant may be, for example, 1:10 to 1:20, but the present invention is not limited thereto.

[0091] In one embodiment of the present invention, step S10 may include a spray-drying step in order to aggregate the carbon nanotubes and granulate them into a large secondary structure. The spray-drying method can be carried out by supplying a dispersion of the carbon nanotubes into a spraying device, forming droplets by spraying, and then drying the droplets. In this case, the spray-drying device may include a spraying device (droplet generator), a reactor, and a collection unit, but the present invention is not limited thereto.

[0092] In one embodiment of the present invention, the spray drying in step S10 may be carried out by spraying, pressurized spraying, or electrostatic spraying at room temperature and atmospheric pressure, for example, and is not particularly limited in the present invention. For example, pressurized spraying is a method in which a dispersion liquid is sprayed under pressure from a pressurized sprayer to form droplets, and then particles are produced using a diffusion dryer. Alternatively, electrostatic spraying is a method in which droplets are formed by an electrostatic spray nozzle using a high-voltage generator, and then particles are produced using a diffusion dryer.

[0093] In one embodiment of the present invention, in step S10, in order to use shorter carbon nanotubes by shredding the length of conventional carbon nanotubes, shredded carbon nanotubes may be used. For this purpose, a step of shredding (chopping) the carbon nanotubes is further included before step S10, and in step S10, the shredded carbon nanotubes may be used as primary particles.

[0094] In one embodiment of the present invention, the shredding of the carbon nanotubes can be performed, for example, by a ball mill.

[0095] In an embodiment of the present invention, the shredded carbon nanotubes may, for example, have a diameter of 50 to 500 nm and a length of 0.1 to 5 μm. Furthermore, the shredded carbon nanotubes may have a diameter-to-length ratio of, for example, 1:1 to 1:30.

[0096] The gigantic secondary structure obtained by step S10 is called a giant, entangled carbon nanotube because its particle size is larger than that of the carbon composite that will ultimately be produced as a secondary structure.

[0097] In one embodiment of the present invention, the entangled carbon nanotube (entangle CNT) is formed by the aggregation of carbon nanotubes that constitute it, and has advantages such as high sphericity, specific surface area, and porosity.

[0098] In one embodiment of the present invention, the sphericity of the entangled giant carbon nanotubes may be, for example, 0.5 to 0.99, more specifically 0.6 to 0.95, and more specifically 0.7 to 0.9. The sphericity of the entangled giant carbon nanotubes can be measured with respect to the carbon composite by the method described above.

[0099] In one embodiment of the present invention, the entangled giant carbon nanotubes have an average particle size D of, for example, 20-200 μm, 20-150 μm, 20-100 μm, 20-50 μm, 20-40 μm, specifically 25-35 μm. 50 It may have the following characteristics. In one embodiment of the present invention, a high-load cathode is required to realize a high-energy-density lithium-sulfur battery, and it may be advantageous to increase the particle size of the sulfur-carbon composite and reduce the amount of binder and conductive material. In this respect, the size of the porous carbon material (i.e., the secondary structure of carbon nanotubes) that is ultimately formed may be important. Such an intertwined giant carbon nanotube as an intermediate structure for forming the secondary structure of carbon nanotubes may be a structure made up of short strands of carbon nanotubes intertwined like a bird's nest. The size of the intertwined giant carbon nanotube may be measured with respect to the secondary structure by the method described above.

[0100] In one embodiment of the present invention, the porosity of the entangled giant carbon nanotubes may be, for example, 60 to 90 volume%, more specifically 65 to 88 volume%, and more specifically 70 to 85 volume%. The porosity may be measured by the method of ISO 15901:2019, which is known in the art. While the present invention is not limited to the above, having a porosity of the entangled giant carbon nanotubes within the aforementioned range may have a favorable effect in terms of the transferability of internal material in the final carbon composite produced.

[0101] In one embodiment of the present invention, the specific surface area of ​​the entangled giant carbon nanotubes is, for example, 200 to 400 m². 2 / g, specifically 210-350m 2 / g, more specifically 250-300m 2 It may be / g. The specific surface area of ​​the entangled giant carbon nanotubes can be measured with respect to the secondary structure by the method described above.

[0102] In one embodiment of the present invention, the entangled giant carbon nanotubes have the characteristics of a large specific surface area, high sphericity, and excellent porosity. However, when they are used as electrodes with a sulfur-containing compound supported as an active material, they may have the problem of not being able to adequately secure ion diffusion channels. Therefore, the present invention aims to use a secondary structure obtained by deforming the shape of the obtained entangled giant carbon nanotube particles as a porous carbon material.

[0103] For this reason, step S20 is a step in which a structurally deformed secondary structure is obtained from the entangled giant carbon nanotubes obtained above.

[0104] In one embodiment of the present invention, step S20 may include a step of obtaining a dispersion in which the intertwined giant carbon nanotubes obtained above are dispersed in a dispersion medium (referred to as a second dispersion medium), and spray-drying the dispersion. In this case, the dispersion may further contain a dispersant, and the dispersion medium and dispersant described above shall be used as referenced.

[0105] In one embodiment of the present invention, the entangled giant carbon nanotubes may be present in an amount of 0.1 to 5% by weight, 0.1 to 2% by weight, or 0.5 to 1% by weight, based on the total weight of the dispersion of entangled giant carbon nanotubes. When the weight of the entangled giant carbon nanotubes is within the aforementioned range, it may have advantageous effects in terms of particle size and particle size uniformity of the manufactured secondary structure, but the present invention is not limited thereto.

[0106] In one embodiment of the present invention, the spray drying method is the one described above.

[0107] In one embodiment of the present invention, step S20 may further include a step of carbonization after spray drying. For example, the carbonization may be carried out by heating the particles formed by spray drying in an inactive gas atmosphere. The carbonization may be carried out at a temperature of, for example, 500°C to 1,200°C, specifically 500°C to 1,000°C or 500°C to 800°C for 30 minutes to 5 hours or 1 hour to 2 hours.

[0108] The secondary structure obtained by the method described above is a secondary structure formed by the aggregation of carbon nanotubes described above, and is characterized by having a flower-like structure that includes a spherical core and at least one amorphous flake on the surface of the core.

[0109] According to yet another aspect of the present invention, a sulfur-carbon composite is provided in which the aforementioned secondary structure of carbon nanotubes is used as a support for a sulfur-containing compound.

[0110] The sulfur-carbon composite comprises the aforementioned secondary structure and a sulfur-containing compound supported on at least a portion of the interior and exterior surfaces of the pores of the secondary structure.

[0111] The sulfur-containing compound can be used without limitation as long as it is suitable for use as a positive electrode active material in a lithium-sulfur secondary battery. For example, the sulfur-containing compound may, but is not limited to, inorganic sulfur (S8), lithium polysulfide (Li2Sn, 1≦n≦8), carbon-sulfur polymer (C2Sx)m (2.5≦x≦50, 2≦m), or mixtures thereof.

[0112] Within the sulfur-carbon composite, the sulfur-containing compound may be included by physical adsorption with the secondary structure, or by chemical bonding such as covalent bonding or van der Waals bonding between the sulfur element and carbon in the secondary structure.

[0113] In one embodiment of the present invention, the secondary structure and the sulfur-containing compound in the sulfur-carbon composite may be present in a weight ratio of, for example, 1:99 to 99:1. Specifically, they may be present in weight ratios of 10:90 to 90:10, 10:90 to 40:60, 10:90 to 50:50, 10:90 to 25:75, 10:90 to 30:70, 15:85 to 30:70, 15:85 to 25:75, or 10:90 to 15:85. When the weight ratio of the secondary structure and the sulfur-containing compound in the sulfur-carbon composite is within the aforementioned range, a high content of the sulfur-containing compound can enhance the kinetic activity of the sulfur-carbon composite and provide advantageous effects in terms of improving conductivity due to the carbon composite, but the present invention is not limited thereto.

[0114] The sulfur-carbon composite of the present invention, as described above, has a high specific surface area while also containing a carbon composite that ensures sufficient ion diffusion passages through multiple internal pores. This not only increases the amount of sulfur that can be supported but also provides multiple active sites for sulfur oxidation / reduction reactions. As a result, it can be used in the positive electrode of a lithium-sulfur battery to improve battery efficiency and energy density, but the mechanism of the present invention is not limited to this.

[0115] According to another aspect of the present invention, an electrode comprising the aforementioned carbon nanotube secondary structure is provided.

[0116] In one embodiment of the present invention, the secondary structure of carbon nanotubes can be included as a conductive additive in the electrode active material layer of the electrode to improve the electrochemical performance of the electrode, but the present invention is not limited thereto.

[0117] In one embodiment of the present invention, the electrode may have a structure that includes an electrode active material layer on a current collector and at least one surface of the current collector. Alternatively, the electrode may be a current collector-free electrode.

[0118] According to another aspect of the present invention, a positive electrode is provided that contains the aforementioned sulfur-carbon composite as a positive electrode active material.

[0119] In one embodiment of the present invention, the positive electrode may further include, as necessary, a binder, a conductive material, an additive, etc., in addition to the positive electrode active material. In this case, the specific types of the binder, conductive material, and additive can be ordinary ones, so their description is omitted in this specification.

[0120] In other embodiments of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer in which the positive electrode active material is coated on one or both sides of the current collector together with a binder, but the positive electrode structure is not limited thereto. The positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and has high conductivity.

[0121] In one embodiment of the present invention, the positive electrode containing the sulfur-carbon composite can exhibit excellent effects in terms of sulfur loading and the activity of the sulfur oxidation / reduction reaction, but the effects of the present invention are not limited to this.

[0122] According to yet another aspect of the present invention, an electrochemical element is provided comprising a positive electrode, a negative electrode, a separation membrane interposed between the positive electrode and the negative electrode, and an electrolyte, wherein at least one of the positive electrode and the negative electrode comprises the aforementioned carbon nanotube secondary structure.

[0123] According to yet another aspect of the present invention, an electrochemical element is provided comprising a positive electrode, a negative electrode, a separation membrane interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode contains the aforementioned sulfur-carbon composite as an active material.

[0124] According to one embodiment of the present invention, the electrochemical element may be a lithium-sulfur battery.

[0125] In one embodiment of the present invention, the negative electrode, separation membrane, and electrolyte can be used without particular limitations as long as they are suitable for use in an electrochemical element, particularly a lithium-sulfur battery, as long as they do not hinder the objectives of the present invention; therefore, a description of specific types will be omitted.

[0126] In one embodiment of the present invention, the external shape of the electrochemical element may be, for example, coin-shaped, cylindrical, pouch-shaped, or rectangular, and its external shape is not particularly limited. Furthermore, the electrochemical element can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in medium- and large-sized battery modules containing multiple battery cells, and its usage is not particularly limited.

[0127] In one embodiment of the present invention, a lithium-sulfur battery using a positive electrode containing the aforementioned sulfur-carbon composite can also achieve excellent results in terms of the battery's energy density, but the effects of the present invention are not limited thereto.

[0128] In one embodiment of the present invention, the lithium-sulfur battery can achieve a significant improvement in energy density by increasing the amount of sulfur loaded in the electrodes and reducing the amount of electrolyte, but the effects of the present invention are not limited to this.

[0129] In the following, a lithium-sulfur battery using a carbon composite according to one embodiment of the present invention will be described in detail by example. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited to these examples.

[0130] [Fabrication of secondary structures using carbon nanotubes] Comparative Example 1 Manufacturing of entangled giant carbon nanotubes Carbon nanotubes with an average diameter of 150 nm (manufacturer: Pyrograf Products, Inc.) were shredded to an average length of 2 μm using zirconium oxide (ZrO2) balls in a ball mill for 6 hours. The shredded carbon nanotubes were dispersed in a 1 wt% polyacrylic acid solution (weight-average molecular weight (Mw) of polyacrylic acid: 450,000 g / mol). The amount of carbon nanotubes added was adjusted so that the weight ratio of carbon nanotubes to polyacrylic acid solution was 1:10.

[0131] A dispersion of carbon nanotubes was spray-dried (conditions: Buchi B-290 mini spray-dryer, inlet temperature 180℃, aspirator 95%, feeding rate 12) to form particles, obtaining large, intertwined carbon nanotubes. The obtained large, intertwined carbon nanotubes had an average particle size of D 50 The diameter is 30 μm, the sphericity is 0.8, and the specific surface area is 250 m². 2 It was / g.

[0132] Figure 1 is an SEM image of the secondary structure according to Comparative Example 1. Referring to Figure 1, it was confirmed that the secondary structure is a secondary particle formed by the aggregation and spheroidization of carbon nanotubes.

[0133] Example 1 Manufacturing of secondary structures The entangled giant carbon nanotubes (secondary particles) obtained in Comparative Example 1 were first dispersed in an 18 wt% polystyrene sulfonate solution, and then diluted five times with distilled water to produce a dispersion. The dispersion was prepared so that the weight ratio of entangled giant carbon nanotubes:polystyrene sulfonate:distilled water in the prepared dispersion was 18:50:2000.

[0134] The manufactured dispersion of entangled giant carbon nanotubes was pre-treated (HSH (8k / 90 min) followed by tip sonication for 90 min) and supplied to a sprayer. Spray-drying (conditions: Buchi B-290 mini spray-dryer, inlet temperature 200℃, aspirator 95%, N2 flow 50, feeding rate 15) was performed to atomize the material, and then it was heated and carbonized at 600℃ for 2 hours to obtain secondary carbon nanotube structures. The obtained secondary structures had an average particle size of D 50 The thickness is 15 μm, and the specific surface area is 160 m². 2 It was / g.

[0135] Figure 2 is an SEM image of the secondary structure according to Example 1. Referring to Figure 2, it was confirmed that the secondary structure has a flower shape including a spherical core and a plurality of amorphous flakes, specifically plate-like flakes, formed on the surface of the spherical core.

[0136] [Manufacturing of lithium-sulfur coin cells] Example 2 Preparation of sulfur-carbon complexes As a porous carbon material, the secondary structure of Example 1 and sulfur (S8) were mixed in a weight ratio of 25:75 and reacted at a temperature of 155°C for 35 minutes to prepare a sulfur-carbon composite in which sulfur was supported in the internal pores and on the surface of the secondary structure.

[0137] Figures 5 and 6 are SEM images of the sulfur-carbon composite produced by Example 2 immediately after production. Figure 5 is a 9,000x magnification image, and Figure 6 is a 4,000x magnification image.

[0138] Manufacturing of positive electrodes A positive electrode slurry composition was prepared by mixing 90% by weight of the sulfur-carbon composite prepared above, 5% by weight of Denka Black as a conductive material, and 5% by weight of styrene-butadiene rubber / carboxymethylcellulose (SBR:CMC weight ratio = 7:3) as a binder.

[0139] The manufactured cathode slurry composition was applied to a 20 μm thick aluminum foil, then coated to a thickness of 150 μm using a blade coater, and finally dried in a 50°C oven to produce the cathode (load capacity 4 mAh / cm²). 2 ).

[0140] Manufacturing of lithium sulfur coin cells After positioning the prepared positive electrode and a lithium metal (200 μm thick) as the negative electrode opposite each other, a separation membrane (20 μm thick, polyethylene with 45 vol% porosity) was interposed between them, and then an ether-based electrolyte containing 1.0 M LiFSI and 2 wt% LiNO3 was injected to manufacture a lithium-sulfur battery.

[0141] Comparative Example 2 Lithium sulfur coin cells were manufactured in the same manner as in Example 2, except that the secondary structure produced in Comparative Example 1 was used as the porous carbon material.

[0142] Figures 3 and 4 are SEM images of the sulfur-carbon composite produced by Comparative Example 2 immediately after production. Figure 3 is a 5,000x magnification image, and Figure 4 is a 2,000x magnification image.

[0143] [Performance evaluation of sulfur-carbon composites] From the SEM images of the sulfur-carbon composites in Figures 3 and 4 (Comparative Example 2) and Figures 5 and 6 (Example 2), it was confirmed that the sulfur-carbon composite according to the present invention can form a pore structure that is more advantageous for ion / electron transfer. Along with the experimental results below, it can be inferred that the flower-like appearance including amorphous flakes may show performance differences in the high-efficiency range.

[0144] [Performance evaluation of lithium-sulfur coin cells] Capacity evaluation The lithium-sulfur batteries of Example 2 and Comparative Example 2 manufactured above were subjected to a charge / discharge measurement device (P&E Solution Co., Ltd.) at 25°C with a current density of 0.1C, measuring 1.7~3.0V (vs. Li / Li + The battery's capacity characteristics were evaluated by performing three charge and discharge cycles within the specified voltage range. The results of the battery's capacity characteristics evaluation are shown in Figure 7.

[0145] As shown in Figure 7, while Comparative Example 2 was able to achieve capacitance at 0.1C, it was confirmed to be inferior to Example 2 in terms of overvoltage generation.

[0146] Lifespan evaluation The lithium-sulfur batteries of Example 2 and Comparative Example 2 manufactured above were subjected to a charge-discharge measurement device (P&E Solution Co., Ltd.) and charged and discharged six times at 25°C with a current density of 0.1C. After that, the battery life characteristics were evaluated by charging at a current density of 0.2C and discharging at a current density of 0.3C. The results of the evaluation of the battery life characteristics are shown in Figure 8.

[0147] As shown in Figure 8, it was confirmed that the high-rate capacity of Example 2 was far superior to that of Comparative Example 2 under the high-rate discharge condition of 0.3C.

Claims

1. A secondary structure formed by the aggregation of carbon nanotubes, It has a flower-like structure comprising a spherical core and at least one amorphous flake on the surface of the core, A secondary structure having a specific surface area of ​​150 m² / g to 200 m² / g.

2. The average particle size D of the secondary structure ５０ The secondary structure according to claim 1, wherein the particle size is 1 μm to 100 μm.

3. The aforementioned secondary structure has an average particle size D of 10 to 20 μm. ５０ A spherical core having a surface with an average particle size D of 1 to 5 μm ５０ The secondary structure according to claim 1, having a flower-like structure comprising at least one amorphous flake having

4. The secondary structure according to claim 1, wherein the sphericity of the secondary structure is 0.5 to 0.

99.

5. The total volume of pores in the secondary structure is 0.5 to 5 cm³. ３ The secondary structure according to claim 1, wherein the value is / g.

6. The carbon nanotube has a diameter of 50 to 500 nm and a length of 0.1 to 5 μm, and is a secondary structure according to claim 1.

7. The secondary structure according to claim 1, wherein the ratio of the diameter to the length of the carbon nanotubes is 1:1 to 1:

30.

8. (S10) Step S10 involves aggregating carbon nanotubes to obtain a large, entangled carbon nanotube. (S20) The process includes step S20, in which a structurally deformed secondary structure is obtained from the entangled giant carbon nanotubes, Step S10 includes spray-drying the dispersion in which the carbon nanotubes are dispersed in the first dispersion medium. Step S20 includes spray-drying the dispersion in which the intertwined giant carbon nanotubes are dispersed in the second dispersion medium, The secondary structure has a flower-like structure comprising a spherical core and at least one amorphous flake on the surface of the core. The specific surface area of ​​the secondary structure is 150 m² / g to 200 m² / g. The carbon nanotube has a length of 0.1 to 5 μm, and the method for producing a secondary structure.

9. The method for producing a secondary structure according to claim 8, wherein the first dispersion medium comprises water, alcohol, benzene, toluene, pyridine, acetone, tetrahydrofuran (THF), dimethylformaldehyde (DMF), or two or more of these.

10. The method for producing a secondary structure according to claim 8, wherein step S20 includes spray-drying a dispersion in which the entangled giant carbon nanotubes are dispersed in a second dispersion medium.

11. The method for producing a secondary structure according to claim 10, wherein the second dispersion medium comprises water, alcohol, benzene, toluene, pyridine, acetone, tetrahydrofuran (THF), dimethylformaldehyde (DMF), or two or more of these.

12. The method for producing a secondary structure according to claim 10, further comprising the step of carbonizing after spray drying.

13. The method for producing a secondary structure according to claim 12, wherein the carbonization is carried out at a temperature of 500°C to 1,200°C.

14. The process further includes a step of shredding the carbon nanotubes prior to step S10, The method for producing a secondary structure according to claim 8, wherein in step S10, a giant entangled carbon nanotube is obtained from the shredded carbon nanotube.

15. The method for manufacturing a secondary structure according to claim 8, wherein the intertwined giant carbon nanotubes have a sphericity of 0.5 to 0.

99.

16. The aforementioned entangled giant carbon nanotubes have an average particle size D of 20 to 200 μm. ５０ A method for manufacturing a secondary structure according to claim 8, comprising having

17. The method for producing a secondary structure according to claim 8, wherein the intertwined giant carbon nanotubes have a porosity of 60 to 90 volume percent.

18. The aforementioned entangled giant carbon nanotubes, 200m ２ / g to 400m ２ A method for manufacturing a secondary structure according to claim 8, characterized in that it has a specific surface area of ​​ / g.

19. A sulfur-carbon composite comprising a secondary structure according to any one of claims 1 to 7, and a sulfur-containing compound supported on at least a portion of the interior and exterior surfaces of the pores of the secondary structure.

20. The sulfur-containing compound is inorganic sulfur (S ８ ), lithium polysulfide (Li ２ Sn, 1 ≤ n ≤ 8), carbon sulfur polymer (C ２ Sx)m, 2.5 ≤ x ≤ 50, 2 ≤ m) or a mixture thereof, and the sulfur-carbon composite according to claim 19 is characterized in that it contains the above substances.

21. The sulfur-carbon composite according to claim 19, wherein the weight ratio of the secondary structure and the sulfur-containing compound is 10:90 to 40:

60.

22. An electrode comprising a secondary structure according to any one of claims 1 to 7 in the electrode active material layer.

23. The system includes a positive electrode, a negative electrode, a separation membrane interposed between the positive electrode and the negative electrode, and an electrolyte. An electrochemical element characterized in that at least one of the positive electrode and the negative electrode includes a secondary structure as described in any one of claims 1 to 7.

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