The manufacturing thereof electrode for lithium-sulfur secondary battery and electrode manufactured by the same

Abstract

The present invention relates to an electrode for a lithium-sulfur secondary battery and a method for manufacturing the same, and also to a lithium-sulfur secondary battery comprising said electrode. Specifically, a method for manufacturing an electrode is disclosed, comprising the steps of: (a) providing a multi-walled carbon nanotube-sulfur composite; and (b) mixing the composite prepared in step (a) with single-walled carbon nanotubes and dispersing them in a ball mill.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing an electrode for a lithium-sulfur secondary battery and an electrode manufactured thereby. Background Technology

[0003] With the development of flexible devices of various sizes and shapes, the development of high-energy flexible batteries is accelerating. While existing flexible batteries have been developed in thin-film, paper / fiber, and cable forms to ensure flexibility, they have suffered from issues such as low energy density, poor durability, and limited capacity.

[0004] In particular, in the case of lithium-sulfur secondary batteries, there was a problem in securing high performance because electron transfer during charging and discharging was not smooth due to sulfur-based cathode active materials, which have high theoretical capacity but low conductivity. Furthermore, when sulfur is used as the cathode active material, sulfur leaks into the electrolyte during oxidation-reduction reactions, not only degrading battery life but also causing lithium polysulfide, a reducing agent of sulfur, to leach out and no longer participate in electrochemical reactions.

[0005] Accordingly, attempts were made to improve electrochemical performance and conductivity by manufacturing composites with conductive carbon materials such as carbon nanotubes and graphene to compensate for the low electrical conductivity of sulfur itself; however, commercialization was hindered because the electrode surface was not smooth after fabrication and electrode material detached after flexibility tests for use in flexible batteries. Prior art literature

[0007] (Patent Document 0001) KR 10-2019-0118733 A The problem to be solved

[0008] To achieve the aforementioned technical objectives, the present invention aims to provide a method for manufacturing an electrode for a lithium-sulfur secondary battery that can be usefully used as a flexible battery, an electrode manufactured thereby, and a secondary battery including the same. means of solving the problem

[0010] To achieve the above technical objective, the present invention provides a method for manufacturing an electrode for a lithium-sulfur battery, comprising the steps of: (a) providing a multi-walled carbon nanotube-sulfur composite; and (b) mixing the composite provided in step (a) with a single-walled carbon nanotube and dispersing it in a ball mill.

[0011] In the method for manufacturing an electrode according to the present invention, the dispersion solvent of step (b) is preferably one or more selected from the group consisting of distilled water, ethanol, propanol, isopropyl alcohol (IPA), ethylene glycol (EG), and dimethylformamide (DMF).

[0012] In the electrode manufacturing method of the present invention, the dispersion in step (b) is preferably carried out at 400 to 450 rpm.

[0013] In the electrode manufacturing method of the present invention, it is preferable that step (b) additionally includes a step of vacuum filtration after ball mill dispersion.

[0014] In the method for manufacturing an electrode according to the present invention, the composite in step (a) is characterized by being prepared by ball-milling and mixing a multi-walled carbon nanotube and a sulfur precursor and then heat-treating them.

[0015] It is preferable that the multi-walled carbon nanotube and sulfur precursor of the present invention be mixed in a weight ratio of 10:90 to 40:60.

[0016] It is preferable that the multi-walled carbon nanotube-sulfur composite of the present invention and the single-walled carbon nanotube are mixed in a weight ratio of 65:35 to 85:15. Effects of the invention

[0018] A lithium-sulfur secondary battery comprising an electrode manufactured according to the present invention can be usefully used as a flexible battery because it has high dispersibility, flexible stability, and high battery capacity without using a current collector, binder, and dispersant. Brief explanation of the drawing

[0020] Figure 1 is a photograph of an electrode manufactured according to one embodiment of the present invention. Figure 2 is an SEM image of an electrode manufactured according to one embodiment of the present invention. Figure 3 is an SEM-EDS analysis image of an electrode manufactured according to one embodiment of the present invention. FIG. 4 is a photograph (Fig. 4 (a)) and an SEM image (Fig. 4 (b)) taken after folding an electrode manufactured according to one embodiment of the present invention. Figure 5 shows a photograph of an electrode manufactured according to a comparative example (Figure 5(a)) and a photograph of the electrode after folding (Figure 5(b)). Figure 6 shows the results of measuring the initial capacity of electrodes manufactured according to one embodiment (Figure 6(a)) and a comparative example (Figure 6(b)) of the present invention. Specific details for implementing the invention

[0021] Hereinafter, the present invention is described in detail with reference to the attached description so that those skilled in the art can easily implement it. In describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the present invention, such detailed description is omitted.

[0022] Furthermore, in this application, terms such as “comprising” or “having” are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof. In this application, singular expressions may include plural expressions unless the context clearly indicates otherwise.

[0023] In the present invention, carbon nanotubes are included to compensate for the low electrical conductivity of sulfur material alone, as sulfur alone does not have electrical conductivity. Specifically, a composite can be prepared by coating sulfur with carbon nanotubes, which are a conductive material, or by mixing sulfur with carbon nanotubes.

[0024] Carbon nanotubes are macromolecules that have a shape in which hexagonal honeycomb-shaped graphite planes, in which one carbon atom is bonded to three other carbon atoms, are rolled into a nanoscale diameter and possess unique physical properties depending on their size or shape. They are hollow, lightweight, have electrical conductivity as good as copper, thermal conductivity as excellent as diamond, and tensile strength comparable to steel. Depending on their rolled shape, they are classified into single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), and rope carbon nanotubes.

[0025] The multi-walled carbon nanotube used in the present invention is in the form of a tube having multiple layers of walls composed of carbon atoms in a single tube, and the single-walled carbon nanotube is in the form of a tube having one wall composed of carbon atoms.

[0027] The multi-walled carbon nanotube-sulfur composite of the present invention can be prepared by coating multi-walled carbon nanotubes onto sulfur or by mixing carbon nanotubes with sulfur. Preferably, sulfur can be melted at 80°C or higher to coat the multi-walled carbon nanotubes.

[0029] The sulfur precursor included to prepare the multi-walled carbon nanotube-sulfur composite of the present invention may include not only elemental sulfur but also various types of sulfur compounds or mixtures thereof. Specifically, Li2S n (n≥1), organic sulfur compounds or carbon-sulfur polymers ((C2S x ) n , x=2.5~50, n≥2), etc., may be used, and also includes the use of mono- and / or di-sulfides.

[0031] The present invention relates to a method for manufacturing an electrode for a lithium-sulfur battery and an electrode manufactured thereby, characterized by providing an electrode that can be usefully used as a flexible battery by manufacturing an electrode having high dispersibility, flexible stability, and a high loading value without using a dispersant.

[0033] The method for manufacturing an electrode for a lithium-sulfur battery according to the present invention comprises: (a) providing a multi-walled carbon nanotube-sulfur composite; and (b) mixing the composite prepared in step (a) with single-walled carbon nanotubes and dispersing them in a ball mill.

[0034] The present invention is characterized by producing an electrode with a constant loading value by mixing a multi-walled carbon nanotube-sulfur composite and a single-walled carbon nanotube as in step (b) above and ball-milling the mixture, thereby homogeneously dispersing the mixture through a simple method without using a dispersant.

[0035] In the present invention, dispersion refers to the mixture of the multi-walled carbon nanotube-sulfur composite and single-walled carbon nanotubes being dispersed in a solvent, such that each particle is scattered.

[0036] In the present invention, the dispersion solvent may be one or more selected from the group consisting of distilled water, ethanol, propanol, isopropyl alcohol (IPA), ethylene glycol (EG), and dimethylformamide (DMF). To minimize the loss of the circulating material or damage to the electrode due to temperature during electrode drying, it is preferable to use ethanol, which dries quickly due to its low vaporization point.

[0038] In step (b) above, the dispersion is characterized by proceeding at 400 to 450 rpm. Below 400 rpm, the dispersion effect of the multi-walled carbon nanotube-sulfur composite and the single-walled carbon nanotube is not significant, and above 450 rpm, the single-walled carbon nanotube decomposes excessively, making it difficult to expect sufficient flexibility, tensile strength, and shape retention when creating the electrode.

[0039] In addition, the dispersion is characterized by being 1 to 2 times. Although the dispersion effect increases as the number of dispersions increases, it is difficult to maintain the shape of the single-walled carbon nanotube, so problems with flexibility and tensile strength may occur as described above.

[0041] However, when the manufacturing method of the present invention is implemented in an industrial setting, the use of a dispersant must not be completely excluded. That is, although the present invention is characterized by achieving high dispersibility through a simple physical dispersion process without the use of a dispersant, the use of a conventional dispersant recognizable by a person skilled in the art may be considered to enjoy additional ease of dispersion. However, it should be noted that the use of such a dispersant is solely to further maximize the advantages of the manufacturing method of the present invention, and that the manufacturing method of the present invention does not necessarily require the use of a dispersant.

[0043] The present invention also provides a method for manufacturing an electrode, which additionally includes a step of vacuum filtration after ball mill dispersion in step (b). The vacuum filtration is intended to filter ethanol, which is a dispersion solution, and can be performed by various methods, but the ethanol can be filtered by reducing the pressure using a vacuum pump, etc., after placing filter paper, etc.

[0045] In the electrode manufacturing method of the present invention, the step of (a) providing a multi-walled carbon nanotube-sulfur composite may be prepared by ball-milling and mixing a multi-walled carbon nanotube and a sulfur precursor, and then heat-treating the mixture. When ball-milling is additionally performed during the preparation of the multi-walled carbon nanotube-sulfur composite, the multi-walled carbon nanotube and the sulfur material are well mixed, allowing the sulfur to be evenly coated on the multi-walled carbon nanotube during subsequent heat treatment. By additionally performing the above process, the dispersibility can be improved when mixing the composite with a single-walled carbon nanotube in the subsequent step (b).

[0047] To manufacture the above multi-walled carbon nanotube-sulfur composite, a sulfur precursor and a multi-walled carbon nanotube can be mixed. As described above, the sulfur precursor may include not only elemental sulfur but also various types of sulfur compounds or mixtures thereof. The multi-walled carbon nanotube and the sulfur precursor may be mixed in a weight ratio of 10:90 to 40:60. If the proportion of sulfur in the above weight ratio is less than 10, it may be difficult to achieve a high-loading electrode, high energy density of the electrode, and sufficient capacity; if it exceeds 40, the conductivity of sulfur, which is an insulator, may not be sufficiently secured, resulting in insufficient discharge capacity and making it difficult to operate the battery at the expected C-rate.

[0048] In addition, the multi-walled carbon nanotube-sulfur composite and the single-walled carbon nanotube may be mixed in a weight ratio of 65:35 to 85:15. Preferably, they may be mixed in a weight ratio of 80:20. If the content of single-walled carbon nanotubes is lower than the above weight ratio, it is difficult to expect sufficient tensile strength and flexibility when creating the electrode, and there is a problem that the likelihood of electrode failure and active material detachment increases during flexibility testing. Furthermore, if the content of single-walled carbon nanotubes exceeds the above weight ratio, the energy density may decrease due to an excessive increase in the volume and thickness of the electrode, and it does not contribute significantly to the capacity or conductivity of the electrode.

[0050] The present invention may also provide an electrode manufactured according to the above method. The electrode of the present invention may be used as an anode.

[0052] In addition, the present invention can provide a lithium-sulfur battery comprising an electrode manufactured according to the above method; an electrolyte; and a lithium negative electrode.

[0053] In the present invention, a lithium-sulfur battery refers to a battery that uses a material containing sulfur as the positive electrode material and lithium metal as the negative electrode material. During discharge, the lithium-sulfur battery may undergo an oxidation reaction of lithium at the negative electrode and a reduction reaction of sulfur at the positive electrode.

[0054] As described above, the electrode of the present invention can be manufactured according to a manufacturing method comprising: (a) providing a multi-walled carbon nanotube-sulfur composite; and (b) mixing the composite prepared in step (a) with single-walled carbon nanotubes and dispersing them in a ball mill.

[0055] In addition, the dispersion solvent of step (b) above may be one or more selected from the group consisting of distilled water, ethanol, propanol, isopropyl alcohol (IPA), ethylene glycol (EG), and dimethylformamide (DMF), and it is preferable to use ethanol.

[0056] In addition, the above step (b) may additionally include a step of vacuum filtration after ball mill dispersion.

[0057] In step (a) above, the composite may be prepared by ball-milling and mixing multi-walled carbon nanotubes and a sulfur precursor, and then heat-treating the mixture.

[0059] In the present invention, the lithium anode is a material capable of absorbing and releasing lithium ions, and is not limited to any material containing lithium. For example, it may be metallic lithium, lithium alloy, metal oxide, metal sulfide, etc.

[0061] The electrolyte used in the lithium-sulfur battery of the present invention may be composed of conventional materials that can be used in lithium-sulfur batteries. As the electrolyte serves as a medium for transferring ions, both solid and liquid electrolytes may be used as the electrolyte of the present invention; however, using a liquid electrolyte is preferred in that it facilitates the realization of improved battery performance.

[0063] The present invention will be explained in more detail below through examples. However, the following examples are intended only to specifically illustrate the present invention and do not limit the scope of the present invention. That is, simple variations or modifications of the present invention can be easily implemented by a person skilled in the art to which the present invention pertains, and all such variations or modifications are considered to be included within the scope of the present invention.

[0065] < Examples1>

[0066] Multiwalled carbon nanotubes and sulfur powder were mixed in a weight ratio of 36:64 and then dispersed in an ethanol solution using a ball mill (10 mm ball size, 300 rpm, 6 cycles). Afterward, the ethanol was removed by vacuum filtration, followed by drying in a vacuum oven at 60 ℃ for 6 hours and heat treatment in a vacuum oven at 155 ℃ for 2 hours to prepare a multiwalled carbon nanotube-sulfur composite.

[0067] Single-walled carbon nanotubes were mixed with the prepared multi-walled carbon nanotube-sulfur composite in an 8:2 weight ratio, then placed in an ethanol solution and dispersed using a ball mill (450 rpm, 2 cycles). Afterward, the ethanol was removed by vacuum filtration, and then dried in a vacuum oven at 60°C for 12 hours to prepare the multi-walled carbon nanotube-sulfur composite and the single-walled carbon nanotube electrode.

[0069] Figure 1 is a photograph of an electrode prepared according to Example 1 above. Figure 1 shows that the apparent surface is homogeneous and clean. SEM images are shown in Figure 2 to observe this in detail. Figure 2(a) shows that the multi-walled carbon nanotube-sulfur composite and the single-walled carbon nanotube are well bonded. Figure 2(b) confirms that the surface of the electrode is formed in an ideal shape.

[0070] Figure 3 is an SEM-EDS analysis image of the electrode prepared according to Embodiment 1 above. As a result of the analysis, it can be seen that sulfur and carbon materials are very evenly dispersed and that the ratio of sulfur and carbon materials is appropriate.

[0071] Figure 4 is a photograph taken after performing a flexible test on the electrode manufactured according to Example 1. The photograph (Figure 4(a)) and SEM image (Figure 4(b)) taken after folding the electrode manufactured for the flexible test more than 120° are shown. Even looking at the SEM image, it can be seen that there are no surface marks or cracks at all after folding.

[0073] < Comparative example 1>

[0074] A multi-walled carbon nanotube-sulfur composite was prepared by mixing multi-walled carbon nanotubes and sulfur powder in a weight ratio of 36:64, drying in a vacuum oven at 60°C for 6 hours, and heat-treating in a vacuum oven at 155°C for 2 hours.

[0075] Single-walled carbon nanotubes were mixed with the prepared multi-walled carbon nanotube-sulfur composite in a weight ratio of 8:2, then placed in an ethanol solution and sonicated at 750 W for 100 minutes. Afterward, the ethanol was removed by vacuum filtration, and then dried in a vacuum oven at 60 ℃ for 12 hours to prepare the multi-walled carbon nanotube-sulfur composite and the single-walled carbon nanotube electrode.

[0077] Figure 5 shows a photograph (Figure 5(a)) of an electrode prepared according to Comparative Example 1 and a photograph (Figure 5(b)) of a flexible test. Looking at Figure 5, it can be seen that the surface of the electrode prepared according to Comparative Example 1 is very non-uniform and cracks occur after folding. Additionally, it was separately confirmed that the active material detaches from the electrode prepared according to Comparative Example 1 after folding. That is, in the case of Comparative Example 1, the single-walled carbon nanotubes are not properly dispersed, causing a clumping phenomenon, and consequently, there is a problem in that it is difficult to form an electrode with a constant loading value. To solve this problem, a separate dispersant is required, but if a dispersant is used, resistance and side reactions caused by the residual dispersant within the electrode may be induced.

[0079] The present invention is characterized by the ability to form a uniform electrode through a simple process without using a dispersant. In addition, as can be seen in FIG. 6, the electrode prepared according to Example 1 (Fig. 6(a)) exhibits superior initial capacity at similar loading values ​​compared to the electrode prepared according to Comparative Example 1 (Fig. 6(b)). Specifically, FIG. 6 shows the results of a battery performance test conducted at 25°C with a cut-off voltage set to 1.5–3.3 V and a rate of 0.1C. In the case of Example 1, the average charge capacity was 1267.62 mAh / g and the discharge capacity was 1153.08 mAh / g. On the other hand, in the case of Comparative Example 1, the average charge capacity was 893.52 mAh / g and the discharge capacity was 961.47 mAh / g, indicating a significant difference in initial capacity.

[0080] That is, the electrode manufacturing method of the present invention can produce an electrode having a constant loading value, exhibiting uniform electrode volume expansion, and having a high initial capacity by increasing the dispersibility of the electrode material, particularly single-walled carbon nanotubes.

[0082] The foregoing description is merely an illustrative explanation of the technical concept of the present invention, and those skilled in the art to which the present invention pertains will be able to make various modifications and variations within the scope of the essential characteristics of the present invention. Accordingly, the embodiments described in the present invention are intended to explain the technical concept of the present invention, and the technical concept of the present invention is not limited by such embodiments. The scope of protection of the present invention shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention.

Claims

Claim 1 A method for manufacturing an electrode for a lithium-sulfur battery, comprising: (a) a step of mixing multi-walled carbon nanotube-sulfur powder in an ethanol solution, dispersing it in a ball mill, and then heat-treating it in a vacuum oven after vacuum filtration to produce a multi-walled carbon nanotube-sulfur composite; A method for manufacturing an electrode comprising: (b) mixing the multiwalled carbon nanotube-sulfur composite and singlewalled carbon nanotube prepared in step (a) in ethanol, dispersing them in a ball mill, filtering under reduced pressure, and drying them in a vacuum oven; wherein the dispersion solvent in step (b) is one or more selected from the group consisting of distilled water, ethanol, propanol, isopropyl alcohol (IPA), ethylene glycol (EG), and dimethylformamide (DMF); wherein the dispersion in step (b) is carried out at 400 to 450 rpm; wherein in step (a), the multiwalled carbon nanotube and the sulfur precursor are mixed in a weight ratio of 36:64; and in step (b), the multiwalled carbon nanotube-sulfur composite and the singlewalled carbon nanotube are mixed in a weight ratio of 65:35 to 85:

15. Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 delete Claim 7 delete Claim 8 Electrode for a lithium-sulfur battery manufactured according to the method of claim 1. Claim 9 A lithium-sulfur battery comprising the electrode of claim 8; an electrolyte; and a lithium negative electrode.

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