Method for manufacturing a coating solution for secondary battery negative electrodes containing carbon-based nanocomposites

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 liquid for a secondary battery negative electrode containing a carbon-based nanocomposite.

Background Art

[0002] When artificial graphite is used instead of natural graphite as the negative electrode active material of a lithium ion battery, the structural stability can be further improved, so it has been widely used recently. However, it has the disadvantage of being more expensive than natural graphite. In the short term, graphite is expected to be used as the negative electrode active material. However, when approaching the theoretical storage capacity of 372 mAh / g, the battery has reached a limit where the charging capacity cannot be increased any further. Therefore, in order to overcome this, extensive research is being conducted on new negative electrode active materials. Since the negative electrode active material has reached the limit of determining the performance limit value of the lithium ion battery, the fact that the characteristics of the material itself and the form when formed on the electrode also have a great influence on the capacity and performance of the battery is important. One attempt to increase the lithium storage capacity is to replace the silicon material with the negative electrode active material. Existing graphite is in the form of LiC6 with a structure in which six carbon atoms are bonded to one lithium atom. However, the negative electrode active material of the silicon material is Li 22 Si5, which is composed of five silicon atoms bonded to a maximum of 22 lithium atoms at 415°C. Therefore, theoretically, it can have a capacity nearly 11 times that of graphite, that is, 4,200 mAh / g, and thus has received much attention. However, the problem is that there is a serious problem that the volume expansion (swelling) of silicon due to lithium insertion increases to an initial ratio of 310%. This causes problems with the safety of the battery by swelling the battery and disassembling the separator. Of course, at room temperature, it is composed of Li 15 Si4 with a structure in which 15 lithium atoms are bonded to four silicon atoms, and the volume expansion decreases slightly with respect to high temperature, but there is still a volume expansion of 280%. Furthermore, a problem arises where the storage capacity rapidly decreases due to a phenomenon called fracture, caused by the volume contraction that occurs as lithium escapes. In order to solve this, various solutions such as the silicon oxide method and the method of coating silicon with carbon have been proposed and researched, but there was a problem that the original problem of the decrease in silicon's energy density could not be solved. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Korean Registered Patent No. 10-2572352 (Registered August 24, 2023) [Patent Document 2] Korean Registered Patent No. 10-1896103 (Registered August 31, 2018) [Overview of the project] [Problems that the invention aims to solve]

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

[0005] The coating solution for secondary battery negative electrodes containing the carbon-based nanocomposite of the present invention is characterized by being manufactured by a) a step of manufacturing a carbon-based nanocomposite; b) a step of manufacturing a composite dispersion containing carbon nanotubes and conductive additives; and c) a step of manufacturing a composite coating solution by mixing the carbon-based nanocomposite with the composite dispersion.

[0006] Step a) is characterized by comprising: a-1) a step of manufacturing silicon nanoparticles; a-2) a step of manufacturing a surface-treated carbon nanostructure by adding a surface treatment agent to a carbon-based nanomaterial; and a-3) a step of milling after adding the surface-treated carbon nanostructure and a binder to the silicon nanoparticles.

[0007] In step a-3) above, 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 characterized by being at least one selected from single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), thin multi-walled carbon nanotubes (thin multi-walled carbon nanotubes), or multi-walled carbon nanotubes (MWNTs).

[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 manufactured by any of the manufacturing methods described above. [Effects of the Invention]

[0013] This invention has the effect of increasing the capacity and energy density of secondary batteries by manufacturing a negative electrode by coating a target material with a composite coating solution encapsulated in carbon-based nanocomposites. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram illustrating a method for producing a coating solution for the negative electrode of a secondary battery according to an embodiment of the present invention. [Modes for carrying out the invention]

[0015] Specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. However, it should be understood that this does not limit the present invention to specific embodiments, but rather includes all transformations, equivalents, or substitutes that fall within the spirit and technical scope of the present invention. In describing the present invention, if it is determined that a specific description of related known technology may obscure the gist of the present invention, such detailed description will be omitted.

[0016] The coating solution for the negative electrode of a secondary battery of the present invention is characterized by being manufactured by a) a step of manufacturing a carbon-based nanocomposite; b) a step of manufacturing a composite dispersion containing carbon nanotubes and a conductive additive; and c) a step of manufacturing a composite coating solution by mixing the carbon-based nanocomposite with the composite dispersion.

[0017] By going through the processes described in steps a) to c), it is possible to increase the capacity and energy density of the secondary battery.

[0018] Step a) is characterized by comprising: a-1) a step of manufacturing silicon nanoparticles; a-2) a step of manufacturing a surface-treated carbon nanostructure by adding a surface treatment agent to a carbon-based nanomaterial; and a-3) a step of milling after adding the surface-treated carbon nanostructure and an additive to the silicon nanoparticles.

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

[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 can 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. At this time, if the particle size of the silicon nanoparticles is less than 0.1 nm, the particle size is excessively small and there is a risk that the charge-discharge efficiency will decrease. If the particle size of the silicon nanoparticles exceeds 150 nm, when the secondary battery is charged, a large amount of lithium ions are inserted into one particle, so there is a problem that the volume increases. When discharging, a large amount of lithium ions escape and voids are generated, so there is a problem that a crack phenomenon is induced. Moreover, due to the lithium ions being broken and the SEI layer being formed by repeated charge and discharge, there is a risk that no electron movement will occur and the capacity of the lithium ion battery will rapidly decrease. That is, there is a risk that the electrode stability will rapidly decrease.

[0022] On the other hand, in the case of the step of manufacturing a surface-treated carbon nanostructure by adding a surface treatment agent to the a-2) carbon-based nanomaterial, when the carbon nanotube is surface-modified by the surface treatment agent, there may be a functional group on its surface that is a hydroxyl group (-OH), a carboxyl group (-COOH), or a combination thereof.

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

[0024] In this process, stirring at a speed of 1,000 rpm or less allows for uniform dispersion of the hydrophilized carbon fibers.

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

[0026] On the other hand, a-3) carbon-based nanocomposites are formed by adding the surface-treated carbon nanostructures and binder to the silicon nanoparticles and then milling them. The binder ensures that the carbon nanostructures surface-treated in step a-2) are uniformly distributed on the surface of the silicon particles, thereby forming an excellent electron conduction network between the silicon particles and the current collector. This has the effect of improving the output characteristics and lifespan characteristics of the battery.

[0027] In this case, the binder is selected from the group consisting of epoxy resin, styrene-butadiene rubber, acrylic 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, phenolic resin, polyvinyl alcohol resin, acrylate resin, polyaniline, polythiophene, polyacetylene, polypyrrole, and PEDOT, and is most preferably an epoxy resin.

[0028] In other words, the carbon-based nanocomposite formed by steps a-1 to a-3) has a structure in which carbon nanotubes (carbon nanostructures) encase silicon particles.

[0029] Furthermore, carbon-based nanocomposites can simultaneously pursue ionic conductivity, electronic conductivity, and stability, and electrochemical devices that apply carbon-based nanocomposites to the cathode can ensure excellent charge capacity and high charge / discharge characteristics.

[0030] On the other hand, in step b) producing a composite dispersion containing carbon nanotubes and a conductive additive, the carbon nanotubes are characterized by being at least one selected from single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), thin multi-walled carbon nanotubes (thin multi-walled carbon nanotubes), or multi-walled carbon nanotubes (MWNTs). More preferably, single-walled carbon nanotubes (SWNTs) are used to more firmly form a carbon nanotube layer on the surface of silicon particles, so that the carbon nanotube layer does not easily detach despite the contraction and expansion of silicon when the battery is operated. More preferably, for the capacity expansion and energy density increase targeted by the present invention, the weight ratio of carbon-based nanocomposites to carbon nanotubes is preferably 0.1:10 to 5:10. When the above weight ratio is present, the dispersibility and electrical conductivity can be improved.

[0031] The dispersion particle size of the single-walled carbon nanotubes is not particularly limited, but is preferably 1 to 15 μm. This is because if the dispersion particle size of the single-walled carbon nanotubes is smaller than 1 μm, it is difficult to uniformly coat the surface of the silicon particles or carbon-based nanocomposite, resulting in a problem of excessively high contact resistance. If the dispersion particle size exceeds 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 dispersion particle size of the single-walled carbon nanotubes is 3 to 10 μm.

[0032] The conductive additive is characterized by containing 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 the increase in electrode resistance, it is preferably 10% by weight or less of the total weight of the composite coating liquid, and more preferably 0.1 to 5% by weight.

[0034] On the other hand, in step a-3), the conductive metal-expanded graphite composite particles are further added, followed by milling.

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

[0036] More specifically, the conductive metal-expanded graphite composite is a form in which carbon nanostructures expand graphite, and expanded graphite nanoplatelets with ultrathin plate thickness are inserted into aluminum particles. This has the advantage of increasing the charge capacity of secondary batteries because it not only has a high aspect ratio and a large surface area, but also good dispersibility and excellent mechanical properties.

[0037] With the carbon nanostructures uniformly distributed on the surface of the silicon particles, the conductive metal-expanded graphite composite can be distributed at the bottom, inside, top, or multiple locations below the carbon nanostructure film and can integrate with the carbon nanostructures. In other words, since the silicon particles are doped with heterogeneous carbon nanostructures and conductive metal-expanded graphite composites, this has the effect of improving electrochemical stability and increasing charge capacity.

[0038] According to the method, in step b), a composite dispersion can be produced by further including a graphite composite.

[0039] In this case, the graphite composite refers to a composite obtained by mixing natural graphite, artificial graphite, and an adhesive, and then subjecting it to heat treatment. Natural graphite exhibits problems such as swelling due to its high degree of orientation and inferior rapid charging performance, and has poor high-temperature characteristics because it has a relatively large number of functional groups on its surface compared to artificial graphite. Therefore, by mixing natural graphite, artificial graphite, and an adhesive and then subjecting it to heat treatment, it is possible to obtain a graphite composite in which the artificial graphite precursor is converted into artificial graphite and the functional groups of natural graphite are effectively removed, and this can be used to improve the capacity and life characteristics of secondary batteries.

[0040] The present invention can provide a coating liquid manufactured by any of the manufacturing methods described above. The coating liquid can be used after being coated onto the surface of a material to be coated and then undergoing a drying process.

[0041] The following describes in more detail a coating solution for the negative electrode of a secondary battery according to a preferred embodiment of the present invention. However, the scope of the present invention is not limited to the following examples. [Examples]

[0042] Manufacturing of silicon nanoparticles

[0043] (a-1) Silicon nanoparticles of 150 nm or less were obtained by ball milling of silicon powder.

[0044] Fabrication of surface-treated carbon nanostructures

[0045] (a-2) A mixed solution of 8M sulfuric acid and nitric acid prepared in a volume ratio of 3:1 (75 / 25, v / v) was mixed with 1% by weight of carbon nanotubes (MWCNTs), and after a sonication process for 20 minutes, the mixture was stirred for 2 hours at 80°C and 600 rpm using an overhead stirrer. 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, and then ball milling was performed to produce carbon-based nanocomposites in which the carbon nanostructures encase the silicon particles. At this time, 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 remainder of water.

[0049] c) A composite coating solution was prepared by mixing the carbon-based nanocomposite produced in step a) with the composite dispersion. At this time, the carbon-based nanocomposite was added in a manner that its content was 10% by weight relative to the composite dispersion composition. [Examples]

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

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

[0052] <Comparative Example> A coating solution for secondary battery anodes was prepared by mixing 97% by weight of silicon-based anode active material SiOx, 1% by weight of single-walled carbon nanotubes, and 2% by weight of binder (styrene-butadiene rubber), and then stirring.

[0053] <Example of experiment> The coating solutions produced in Examples 1-3 and Comparative Example 1 were coated onto a copper current collector to a thickness of 60 μm to produce a negative electrode thin film. The negative electrode with the final thin film formed was compressed via a heating roll press to control the electrode density to 1.6 g / cc. Lithium secondary batteries were fabricated using each of the negative electrodes produced in this way, 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 1-3 and Comparative Example 1.

[0054] [Table 1]

[0055] As shown in Table 1 above, when using carbon-based nanocomposites, as in Examples 1 to 3, it can be seen that a higher charge-discharge capacity is observed and a higher capacity per electrode volume is achieved compared to when using unmodified silicon.

[0056] Furthermore, it has been found that when using aluminum-expanded graphite composite particles, the characteristics of significantly reduced charge / discharge capacity, capacity retention rate, and volume expansion rate can be maintained for a longer period compared to when using only carbon-based nanocomposites.

[0057] Although the present invention has been described above in terms of limited embodiments, it goes without saying that the present invention is not limited thereto, and various modifications and variations can be made by persons with ordinary skill in the art to which the present invention pertains, within the equivalent scope of the technical concept of the present invention and the following claims.

Claims

1. a) A step of manufacturing a carbon-based nanocomposite, b) A step of preparing a composite dispersion containing carbon nanotubes and additives, c) The step of mixing the carbon-based nanocomposite with the composite dispersion to produce a composite coating solution, Step a) is, a-1) Steps for manufacturing silicon nanoparticles, a-2) A step of adding a surface treatment agent to a carbon-based nanomaterial to produce a surface-treated carbon nanostructure, a-3) A method for producing a coating liquid for a secondary battery negative electrode, comprising the steps of adding the surface-treated carbon nanostructure and binder to the silicon nanoparticles and then milling them.

2. A method for producing a coating liquid for a secondary battery negative electrode according to claim 1, characterized in that, in step a-3), aluminum particles (97 vol%) and expanded graphite nanoplatelets (3 vol%) are dried at 100°C, then ball-milled at 400 rpm, and conductive metal-expanded graphite composite particles in which expanded graphite is inserted into the aluminum particles are further added and then milled.

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

4. The method for producing a coating liquid for a secondary battery negative electrode according to claim 1, characterized in that the carbon nanotube is at least one selected from single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT), thin multi-walled carbon nanotubes (thin multi-walled carbon nanotubes), or multi-walled carbon nanotubes (MWNT).

5. The method for producing a coating liquid for a secondary battery negative electrode according to Claim 1, characterized in that the composite dispersion liquid contains graphite, water, and polyacrylic acid as the additive.

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