Method for manufacturing coating solution for secondary battery negative electrode containing carbon-based nanocomposite

Abstract

To provide a coating solution for a secondary battery negative electrode containing a carbon-based nanocomposite, which is capable of increasing the capacity and energy density of a secondary battery by using a composite coating solution encapsulated with a carbon-based nanocomposite, and a method for manufacturing the same.SOLUTION: The present invention provides a method for manufacturing a coating solution for a secondary battery negative electrode, including the steps of a) preparing a carbon-based nanocomposite, b) preparing a composite dispersion containing carbon nanotubes and a conductive additive, and c) mixing the carbon-based nanocomposite with the composite dispersion to prepare a composite coating solution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a coating solution for a secondary battery negative electrode containing a carbon-based nanocomposite. [Background technology]

[0002] The use of artificial graphite instead of natural graphite as an anode active material in lithium-ion batteries has recently become popular due to its improved structural stability, but its higher price compared to natural graphite is a drawback. While graphite is expected to be used as an anode active material in the short term, as the theoretical storage capacity approaches 372 mAh / g, it has reached a limit at which the battery's charging capacity cannot be increased. To overcome this, active materials are being actively researched to develop new anode active materials. Because the anode active material determines the performance limit of lithium-ion batteries, it is important to note that the characteristics of the material itself and the morphology of the electrode significantly affect the battery's capacity and performance. One attempt to increase lithium storage capacity is to replace silicon materials with anode active materials. While existing graphite is in the form of LiC6, which has a structure in which six carbon atoms and one lithium atom are bonded, silicon-based anode active materials can be produced at 415°C with a structure in which up to 22 lithium atoms are bonded to five silicon atoms. 22 It has attracted much attention because it is made up of Si5, which theoretically has a capacity nearly 11 times that of graphite, i.e., 4,200mAh / g. However, the problem is that the volume expansion of silicon due to lithium insertion increases to 310% of the initial volume. This causes the battery to swell and the separator to break down, posing a safety risk to the battery. Of course, at room temperature, Li, which has a structure in which 15 lithium atoms are bonded to 4 silicon atoms, 15 It is composed of Si4, and although the volume expansion decreases slightly at high temperatures, there is still a volume expansion of 280%. Furthermore, as lithium is released, the volume shrinks, causing cracking and a rapid decrease in storage capacity. To solve this problem, various solutions have been proposed and researched, such as silicon oxide methods and methods that coat silicon with carbon, but these methods have not been able to solve the original problem of a decrease in the energy density of silicon. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Korean Patent No. 10-2572352 (registered August 24, 2023) [Patent Document 2] Korean Patent No. 10-1896103 (registered August 31, 2018) Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention aims to provide a coating solution for a secondary battery anode containing a carbon-based nanocomposite, which can increase the capacity and energy density of a secondary battery using a composite coating solution encapsulated in a carbon-based nanocomposite, and a method for producing the same. [Means for solving the problem]

[0005] The coating solution for a secondary battery anode containing a carbon-based nanocomposite of the present invention is characterized in that it is prepared by the steps of: a) preparing a carbon-based nanocomposite; b) preparing a composite dispersion containing carbon nanotubes and a conductive additive; and c) mixing the carbon-based nanocomposite with the composite dispersion to prepare a composite coating solution.

[0006] The step a) is characterized by comprising: a-1) a step of producing silicon nanoparticles; a-2) a step of producing surface-treated carbon nanostructures by adding a surface treatment agent to a carbon-based nanomaterial; and a-3) a step of adding the surface-treated carbon nanostructures and a binder to the silicon nanoparticles, followed by milling.

[0007] In the step a-3), conductive metal-expanded graphite composite particles are further added, followed by milling.

[0008] The silicon nanoparticles in step a-1) are characterized by having a particle size of 0.1 to 150 nm.

[0009] The carbon nanotube is at least one selected from a single-walled carbon nanotube (SWNT), a double-walled carbon nanotube (DWNT), a thin multi-walled carbon nanotube, and a multi-walled carbon nanotube (MWNT).

[0010] The conductive additive is characterized by comprising one selected from the group consisting of polyacrylic acid, polyacrylate, polymethacrylic acid, polymethyl methacrylate, polyacrylamide, polyvinyl acetate, polymaleic acid, polyethylene glycol, and polyimide.

[0011] The conductive metal is characterized by being aluminum.

[0012] The present invention is characterized by providing a coating liquid produced by any of the above-mentioned production methods. [Effects of the Invention]

[0013] The present invention has the effect of increasing the capacity and energy density of a secondary battery by manufacturing an anode by coating a composite coating liquid encapsulated in a carbon-based nanocomposite onto a coating target material. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram illustrating a method for producing a coating liquid for a secondary battery negative electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, specific embodiments of the present invention will be described in more detail with reference to the accompanying drawings. However, it should be understood that this is not intended to limit the present invention to the specific embodiments, but rather to encompass all modifications, equivalents, or alternatives within the spirit and technical scope of the present invention. In describing the present invention, if it is determined that detailed descriptions of related known technologies may obscure the gist of the present invention, such detailed descriptions will be omitted.

[0016] The coating solution for a secondary battery anode of the present invention is characterized by being prepared by the steps of: a) preparing a carbon-based nanocomposite; b) preparing a composite dispersion containing carbon nanotubes and a conductive additive; and c) mixing the carbon-based nanocomposite with the composite dispersion to prepare a composite coating solution.

[0017] By going through the process of steps a) to c), it is possible to achieve the effect of increasing the capacity and energy density of the secondary battery.

[0018] The step a) is characterized by comprising: a-1) a step of producing silicon nanoparticles; a-2) a step of producing surface-treated carbon nanostructures by adding a surface treatment agent to a carbon-based nanomaterial; and a-3) a step of adding the surface-treated carbon nanostructures and additives to the silicon nanoparticles, followed by milling.

[0019] First, the silicon nanoparticles in step a-1) are produced by a ball milling process, which is a process aimed at minimizing the volume expansion of silicon due to lithium ion penetration.

[0020] . At this time, the silicon nanoparticles in step a-1) are characterized by having a particle size of 0.1 to 150 nm.

[0021] The silicon nanoparticles may have a particle size of 0.1 to 150 nm, preferably 0.5 to 120 nm, more preferably 0.8 to 100 nm, and most preferably 1 to 90 nm. If the particle size of the silicon nanoparticles is less than 0.1 nm, the particle size may be too small, resulting in reduced charge / discharge efficiency. If the particle size of the silicon nanoparticles exceeds 150 nm, a large number of lithium ions are inserted into each particle during charging of the secondary battery, resulting in an increased volume. During discharge, a large number of lithium ions are released, resulting in the formation of voids, which can lead to cracking. Furthermore, repeated charge / discharge cycles can cause lithium ions to split, forming an SEI layer, which prevents electron movement and can result in a rapid decrease in the capacity of the lithium-ion battery. This can also lead to a rapid decrease in electrode stability.

[0022] On the other hand, in the case of the step a-2) of preparing a surface-treated carbon nanostructure by adding a surface treatment agent to a carbon-based nanomaterial, the carbon nanotubes may be surface-modified by the surface treatment agent, resulting in the presence of functional groups such as hydroxyl groups (-OH), carboxyl groups (-COOH), or a combination thereof, on their surface.

[0023] In step a-2), the surface treatment agent may be at least one of sulfuric acid, nitric acid, phosphoric acid, hydrochloric acid, and hydrogen peroxide, and most preferably a mixed solution of sulfuric acid and nitric acid. Furthermore, to improve the dispersibility of the carbon nanotubes, an ultrasonic treatment process and a stirring process may be performed.

[0024] At this time, the stirring step is carried out at a speed of 1,000 rpm or less, so that the hydrophilized carbon fibers can be uniformly dispersed.

[0025] The stirring speed may be 1,000 rpm or less, 800 rpm or less, or 700 rpm or less.

[0026] Meanwhile, a-3) adds the surface-treated carbon nanostructures and a binder to the silicon nanoparticles, followed by milling to form a carbon-based nanocomposite. The binder allows the surface-treated carbon nanostructures from step a-2) to be uniformly distributed on the surface of the silicon particles, forming an excellent electron conduction network between the silicon particles and the current collector, which contributes to improving the output and life characteristics of the battery.

[0027] In this case, the binder may be selected from the group consisting of epoxy resin, styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, ethylene-propylene copolymer, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polytetrafluoroethylene, fluorinated polyvinylidene, polyvinylpyridine, chlorosulfonated polyethylene, Nafion, polyester resin, acrylic resin, phenol resin, polyvinyl alcohol resin, acrylate resin, polyaniline, polythiophene, polyacetylene, polypyrrole, and PEDOT, and is most preferably an epoxy resin.

[0028] That is, the carbon-based nanocomposite formed by steps a-1 to a-3) has a shape in which the carbon nanotube (carbon nanostructure) encases the silicon particle.

[0029] In addition, carbon-based nanocomposites can simultaneously achieve ionic conductivity, electronic conductivity, and stability, and electrochemical devices that use carbon-based nanocomposites as cathodes can ensure excellent charge capacity and high charge / discharge characteristics.

[0030] Meanwhile, in the step b) of preparing a composite dispersion containing carbon nanotubes and a conductive additive, the carbon nanotubes are at least one selected from single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), thin multi-walled carbon nanotubes, and multi-walled carbon nanotubes (MWNTs). More preferably, single-walled carbon nanotubes (SWNTs) are used to form a stronger carbon nanotube layer on the surface of silicon particles, thereby preventing the carbon nanotube layer from easily detaching despite the contraction and expansion of silicon during battery operation. More preferably, to achieve the intended capacity and energy density of the present invention, the weight ratio of carbon-based nanocomposite to carbon nanotubes is preferably 0.1:10 to 5:10. This weight ratio has the effect of improving dispersibility and electrical conductivity.

[0031] The dispersed particle size of the single-walled carbon nanotubes is not particularly limited, but is preferably 1 to 15 μm. This is because if the dispersed particle size of the single-walled carbon nanotubes is smaller than 1 μm, it is difficult to uniformly coat the entire surface of the silicon particles or carbon-based nanocomposite, resulting in a problem of excessively high contact resistance. If the dispersed particle size is larger than 15 μm, the dispersion stability of the composite coating liquid decreases, resulting in a problem of reduced reproducibility of resistance during coating. In particular, to significantly reduce powder resistance, it is more preferable that the dispersed particle size of the single-walled carbon nanotubes be 3 to 10 μm.

[0032] The conductive additive is characterized by including one selected from the group consisting of polyacrylic acid, polyacrylate, polymethacrylic acid, polymethyl methacrylate, polyacrylamide, polyvinyl acetate, polymaleic acid, polyethylene glycol, and polyimide.

[0033] The content of the conductive additive is not particularly limited, but in order to minimize an increase in electrode resistance, it is preferably 10 wt % or less, more preferably 0.1 to 5 wt %, based on the total weight of the composite coating solution.

[0034] Meanwhile, in the step a-3), conductive metal-expanded graphite composite particles are further added and then milled.

[0035] The conductive metal is characterized by being aluminum.

[0036] More specifically, the conductive metal-expanded graphite composite is formed by carbon nanostructures expanding graphite, and ultrathin expanded graphite nanoplatelets are inserted into aluminum particles. This composite not only has a high aspect ratio and a large surface area, but also has good dispersibility and excellent mechanical properties, which are advantageous for increasing the charge capacity of secondary batteries.

[0037] The carbon nanostructures are uniformly distributed on the surface of the silicon particles, and the conductive metal-expanded graphite composite is distributed below, inside, or above the carbon nanostructure coating, or at a combination of these locations, and can be integrated with the carbon nanostructures. In other words, the carbon nanostructures and the conductive metal-expanded graphite composite are heterogeneously doped on the surface of the silicon particles, which can contribute to improved electrochemical stability and increased charge capacity.

[0038] According to the method, step b) may further include a graphite composite to produce a composite dispersion.

[0039] In this case, the graphite composite corresponds to a composite obtained by mixing natural graphite, artificial graphite, and an adhesive and then heat-treating the mixture. Natural graphite exhibits swelling and poor fast charging performance due to its high degree of orientation, and has poor high-temperature characteristics compared to artificial graphite due to a relatively large number of functional groups on its surface. Therefore, by mixing natural graphite, artificial graphite, and an adhesive and then heat-treating the mixture, the artificial graphite precursor is converted into artificial graphite, and the functional groups of the natural graphite are effectively removed to obtain a graphite composite. This can be used to improve the capacity and lifespan of secondary batteries.

[0040] The present invention may provide a coating solution prepared by any of the above-described methods. The coating solution may be used after being coated on a surface of a coating target material and then dried.

[0041] Hereinafter, a coating solution for a secondary battery negative electrode according to a preferred embodiment of the present invention will be described in more detail, but the scope of the present invention is not limited to the following examples. [Example]

[0042] Silicon nanoparticle production

[0043] (a-1) Silicon powder was ball milled to obtain silicon nanoparticles of 150 nm or less.

[0044] Fabrication of surface-treated carbon nanostructures

[0045] (a-2) 1 wt% of carbon nanotubes (MWCNTs) was added to a mixed solution prepared by mixing 8M sulfuric acid and nitric acid in a volume ratio of 3:1 (75 / 25, v / v), and after ultrasonic treatment for 20 minutes, the mixture was stirred at 80°C and 600 rpm using an overhead stirrer for 2 hours. After filtration and washing, the mixture was dried in a vacuum oven at 80°C for 48 hours to obtain carbon nanostructures.

[0046] Fabrication of carbon-based nanocomposites

[0047] (a-3) Carbon nanostructures and epoxy resin were added to silicon nanoparticles, followed by ball milling to produce carbon nanocomposites in which the carbon nanostructures encase the silicon particles. The silicon nanoparticles, carbon nanostructures, and epoxy resin were mixed in a weight ratio of 10:5:1.

[0048] b) A composite dispersion was prepared by mixing 2 wt % of 10 μm single-walled carbon nanotubes, 5 wt % of polyacrylic acid, 10 wt % of graphite, and the remaining amount of water.

[0049] c) The carbon-based nanocomposite prepared in step a) was mixed with the composite dispersion to prepare a composite coating liquid, where the carbon-based nanocomposite was added in an amount of 10 wt % based on the composite dispersion composition. [Example]

[0050] The procedure was the same as in Example 1, except that aluminum particles (97 vol%) and expanded graphite nanoplatelets (3 vol%) were dried at 100°C and then ball milled at 400 rpm to form composite particles in which expanded graphite was intercalated into aluminum particles, and the composite particles were further added in step a-3). At this time, the silicon nanoparticles, carbon nanostructures, composite particles, and epoxy resin were mixed in a weight ratio of 10:5:2:1. [Example]

[0051] In Example 2, step b) further includes 5 wt % of a graphite composite. The graphite composite was prepared by mixing spherical natural graphite, coke, and an adhesive in a weight ratio of 1:1:0.1 to prepare a mixture, and then heat-treating the mixture at 200°C to 300°C. The procedure was the same as in Example 2, except that the graphite composite was obtained by heat-treating the mixture at 200°C to 300°C.

[0052] <Comparative Example> 97 wt% of silicon-based negative electrode active material SiOx, 1 wt% of single-walled carbon nanotubes, and 2 wt% of binder (styrene butadiene rubber) were mixed and stirred to prepare a coating solution for a secondary battery negative electrode.

[0053] <Experimental Example> The coating solutions prepared in Examples 1 to 3 and Comparative Example 1 were coated onto copper current collectors to a thickness of 60 μm to prepare thin negative electrode films. The negative electrodes on which the final thin films were formed were compressed using a heating roll press to control the electrode density to 1.6 g / cc. Lithium secondary batteries were fabricated using each of the thus-prepared negative electrodes, and the characteristics of the lithium secondary batteries were evaluated. Table 1 below shows the characteristics of lithium secondary batteries using the negative electrode materials of Examples 1 to 3 and Comparative Example 1.

[0054] [Table 1]

[0055] As shown in Table 1, when a carbon-based nanocomposite is used as in Examples 1 to 3, a higher charge-discharge capacity is exhibited compared to when unmodified silicon is used, and a higher capacity per electrode volume is obtained.

[0056] In addition, when aluminum-expanded graphite composite particles are used, it is possible to maintain significantly lower charge / discharge capacity characteristics, capacity retention rate, volume expansion rate, etc. for a longer period of time than when only carbon-based nanocomposites are used.

[0057] Although the present invention has been described above with reference to limited embodiments, it should be understood that the present invention is not limited thereto, and that various modifications and variations may be made by those skilled in the art within the scope of the technical concept of the present invention and the scope of the following claims.

Claims

1. a) preparing a carbon-based nanocomposite; b) preparing a composite dispersion containing carbon nanotubes and a conductive additive; c) mixing the carbon-based nanocomposite with the composite dispersion to prepare a composite coating liquid; The step a) A method for producing a coating solution for a secondary battery negative electrode, comprising: a-1) a step of producing silicon nanoparticles; a-2) a step of adding a surface treatment agent to a carbon-based nanomaterial to produce a surface-treated carbon nanostructure; and a-3) a step of adding the surface-treated carbon nanostructure and a binder to the silicon nanoparticles, followed by milling.

2. 2. The method for preparing a coating solution for a secondary battery negative electrode according to claim 1, wherein in step a-3), conductive metal-expanded graphite composite particles are further added and then milled.

3. 2. The method for producing a coating solution for a secondary battery negative electrode according to claim 1, wherein the silicon nanoparticles in step a-1) have a particle size of 0.1 to 150 nm.

4. 10. The method of claim 1, wherein the carbon nanotubes are at least one selected from the group consisting of single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT), thin multi-walled carbon nanotubes, and multi-walled carbon nanotubes (MWNT).

5. 2. The method of claim 1, wherein the conductive additive comprises polyacrylic acid, graphite, and water.

6. 3. The method for producing a coating solution for a secondary battery negative electrode according to claim 2, wherein the conductive metal is aluminum.

7. A coating liquid produced by the method according to any one of claims 1 to 6.

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