A negative electrode with single-walled carbon nanotubes and a secondary battery containing the same

Abstract

According to this embodiment, there are provided an anode for a secondary battery, in which an anode mixture is formed on at least one surface of an anode current collector, the anode mixture including an anode active material and a conductive material, the anode active material including a silicon-based active material, and the conductive material including single-walled carbon nanotubes (SWCNTs), a manufacturing method thereof, and a secondary battery including the same.

Description

[Technical Field]

[0001] [Cross-citation with related applications] This application claims priority rights under Korean Patent Application No. 10-2021-0148229 dated November 1, 2021, and Korean Patent Application No. 10-2022-0142922 dated October 31, 2022, and all content disclosed in the documents of said Korean patent applications is incorporated herein by reference.

[0002] This invention relates to a negative electrode to which single-walled carbon nanotubes are applied and a secondary battery containing the same. [Background technology]

[0003] The rapid increase in fossil fuel use has led to a growing demand for alternative and clean energy sources, and among the most actively researched areas in this field is electrochemical power generation and energy storage.

[0004] Currently, a typical example of an electrochemical element that uses this type of electrochemical energy is the secondary battery, and its range of applications is expanding day by day.

[0005] Recently, the demand for rechargeable batteries as an energy source has increased rapidly due to technological developments and growing demand for portable devices such as portable computers, mobile phones, and cameras. Among such rechargeable batteries, lithium-ion batteries, which exhibit high charge / discharge characteristics and lifespan, and are environmentally friendly, have been the subject of much research and are now commercially available and widely used.

[0006] Generally, lithium secondary batteries are manufactured by impregnating an electrode assembly consisting of a positive electrode, a negative electrode, and a porous separator membrane with a lithium non-aqueous electrolyte. Each electrode is manufactured by forming an electrode mixture containing an electrode active material, a conductive material, and a binder on a current collector.

[0007] The fundamental performance characteristics of such lithium secondary batteries are greatly influenced by the negative electrode material. To maximize battery performance, the negative electrode active material must have an electrochemical reaction potential close to that of the lithium metal, high reversibility of reaction with lithium ions, and a fast diffusion rate of lithium ions within the active material. Graphite has been widely used as a material that satisfies these requirements, and considering the excellent adhesive properties of natural graphite and the superior output characteristics and lifespan characteristics of artificial graphite, mixtures of natural and artificial graphite have been used to improve the performance of secondary batteries in various aspects.

[0008] However, with the recent growth of equipment sectors requiring high-capacity batteries, such as electric vehicles and hybrid electric vehicles, the energy density level required for lithium secondary batteries has been steadily increasing. As a result, attempts have been made to use negative electrodes containing Si, which has a high theoretical capacity, as the negative electrode active material.

[0009] On the other hand, when negative electrode active materials such as SiO containing Si are included, the volume expansion rate of the negative electrode due to charging and discharging is high, leading to problems such as desorption of the negative electrode mixture, which in turn reduces the lifespan characteristics.

[0010] Therefore, conventional methods involved using a porous matrix, dispersing small-sized active material in an inert matrix, reducing the size of the active material, or adjusting the type and amount of binder.

[0011] On the other hand, carbon nanotubes in a long shape were often used as conductive materials added to improve the conductivity of the active material, in order to enhance the conductivity between the active materials.

[0012] However, despite the use of carbon nanotubes, sufficient conductivity cannot be ensured, and there is a problem in that this conductivity decreases further as the cycle progresses, leading to a decline in lifespan characteristics.

[0013] Therefore, there is an urgent need for the development of secondary battery technology that can solve these problems and ensure sufficient conductivity.

Summary of the Invention

Problems to be Solved by the Invention

[0014] The object of the present invention is to solve the problems of the prior art as described above and the technical problems that have been demanded in the past.

[0015] Specifically, the problem to be solved by the present invention is to provide a secondary battery in which the active material ensures sufficient conductivity to reduce resistance and at the same time improves the life characteristics due to the progress of the cycle.

Means for Solving the Problems

[0016] According to an embodiment of the present invention for achieving such an object, a negative electrode for a secondary battery in which a negative electrode mixture is formed on at least one surface of a negative electrode current collector, the negative electrode mixture contains a negative electrode active material and a conductive material, the negative electrode active material contains a silicon-based active material, a negative electrode composed of single-walled carbon nanotubes (SWCNT, Single-walled Carbon nanotube) is provided as the conductive material.

[0017] At this time, the single-walled carbon nanotubes can be included in a first form that connects the surfaces of two or more negative electrode active material particles and a second form that covers the surfaces of the respective negative electrode active material particles. Here, the content of the conductive material formed in the first form can be 40% to 70% by weight of the total content of the conductive material.

[0018] Also, the diameter of the single-walled carbon nanotubes can be 1 nm to 2 nm, the length of the single-walled carbon nanotubes can be 2 μm to 5 μm, and the aspect ratio of the single-walled carbon nanotubes can be 1000 to 5000.

[0019] Furthermore, the single-walled carbon nanotubes can aggregate in a bundle shape and have a secondary shape.

[0020] On the other hand, the negative electrode mixture may further contain a binder and a dispersant.

[0021] According to yet another embodiment of the present invention, A method for manufacturing the negative electrode for the secondary battery, A method for manufacturing a negative electrode is provided in which, when producing a negative electrode active material slurry containing a conductive material composed of single-walled carbon nanotubes (SWCNTs), the conductive material of single-walled carbon nanotubes is introduced in a divided manner.

[0022] Specifically, the negative electrode active material slurry may further contain a binder, in which case the divided addition method may involve mixing 30% to 60% by weight of the conductive material composed of single-walled carbon nanotubes with the negative electrode active material first, and then adding the remaining 40% to 70% by weight of the conductive material composed of single-walled carbon nanotubes together with the binder and mixing them afterward.

[0023] More specifically, (a) A step of preparing a predispersion by mixing single-walled carbon nanotubes (SWCNTs) as a conductive material with a dispersant and a solvent; (b) A step of preparing an active material solution by adding a negative electrode active material to a portion of the pre-dispersion; (c) The step of adding the remaining pre-dispersion to the active material solution and adding a binder to produce an active material slurry; and (d) A step of manufacturing a negative electrode by applying the active material slurry to at least one surface of the negative electrode current collector, drying it, and rolling it; It may include.

[0024] At this time, in step (b), the negative electrode active material may be added to 30% to 60% by weight of the pre-dispersion based on the total weight of the pre-dispersion, and in step (c), the remaining 40% to 70% by weight of the pre-dispersion based on the total weight of the pre-dispersion may be added.

[0025] The present invention also, according to another embodiment, The aforementioned negative electrode; A positive electrode having a positive electrode mixture containing a positive electrode active material formed on at least one surface of the positive electrode current collector; A secondary battery in which an electrode assembly including a separator membrane interposed between the negative electrode and the positive electrode is built into a secondary battery case together with an electrolyte, The aforementioned secondary battery provides a secondary battery in which the increase rate of the DCIR value over 100 cycles relative to the initial DCIR value is 47% or less.

[0026] Furthermore, the secondary battery may maintain its discharge capacity for 100 cycles at a rate of 91% or more relative to its initial discharge capacity.

[0027] In this case, the positive electrode active material may include a lithium transition metal oxide represented by the following chemical formula 1.

[0028] Li 1+x Ni a Co b Mn c M 1-(a+b+c) O 2-y A y (1)

[0029] In the above formula, M is at least one selected from the group consisting of Cu, Ti, Mg, Al, Pt, and Zr. A is an oxygen-substituted halogen, The following conditions apply: 0≦x≦0.5, 0.8≦a≦1, 0≦b≦0.2, 0≦c≦0.2, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0030] Furthermore, by manufacturing the negative electrode using a method that involves dividing and inserting the conductive material, the conductive material enhances the connectivity between the active materials and also increases conductivity on the surface of the active materials, resulting in a significant reduction in resistance and a greater improvement in lifespan characteristics. [Brief explanation of the drawing]

[0031] [Figure 1] This is an SEM image of the surface of the negative electrode mixture in Example 1. [Figure 2]This is an SEM image of the surface of the negative electrode mixture in Comparative Example 1. [Figure 3] This is an SEM image of the surface of the negative electrode mixture in Comparative Example 2. [Figure 4] This is a graph showing the lifetime characteristics and resistance increase rate of Example 1 according to Experimental Example 2. [Figure 5] This graph shows the lifetime characteristics and resistance increase rate of Comparative Example 1 based on Experimental Example 2. [Figure 6] This graph shows the lifetime characteristics and resistance increase rate of Comparative Example 2 based on Experimental Example 2. [Figure 7] This graph shows the lifetime characteristics and resistance increase rate of Comparative Example 3 based on Experimental Example 2. [Modes for carrying out the invention]

[0032] The present invention will be described in more detail below to further enhance understanding of it.

[0033] Terms and words used in this specification and in the claims should not be interpreted in a manner limited to their ordinary or lexicographical meanings, but rather in a manner and concept that is appropriate to the technical idea of ​​the present invention, in accordance with the principle that inventors themselves may define the concepts of terms as appropriate to best describe the invention.

[0034] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0035] In this specification, terms such as “includes,” “equip,” or “have” are used to specify the presence of implemented features, figures, stages, components, or combinations thereof, and should not be understood to preemptively exclude the possibility of the presence or addition of one or more other features, figures, stages, components, or combinations thereof.

[0036] According to one embodiment of the present invention, A negative electrode for a secondary battery, wherein a negative electrode mixture is formed on at least one surface of the negative electrode current collector, The negative electrode mixture comprises a negative electrode active material and a conductive material. The aforementioned negative electrode active material includes a silicon-based active material. The conductive material provided is a negative electrode composed of single-walled carbon nanotubes (SWCNTs).

[0037] Recently, research is underway to use silicon-based active materials with high theoretical capacity as negative electrode active materials in order to manufacture secondary batteries with high energy density.

[0038] However, when a silicon-based active material is used as the negative electrode active material, the volume expansion due to charging and discharging is so severe that it is difficult to ensure conductivity as the cycle progresses.

[0039] Therefore, to ensure such conductivity, carbon nanotubes with longer lengths are now used as conductive materials. However, conventionally, multi-walled carbon nanotubes (MWCNTs) with a large number of bonds forming the wall were used, but in this case, there was still the problem that their conductivity decreased as the cycle progressed. In particular, multi-walled carbon nanotubes are relatively short, which is disadvantageous for ensuring conductivity between active materials.

[0040] Therefore, the inventors of this application considered using single-walled carbon nanotubes that exhibit both metallic and semiconducting properties simultaneously, have a nano-rod shape, and use relatively long carbon nanotubes with a small number of individual nanotubes to demonstrate excellent conductivity between active materials. However, simply mixing conductive material and active material as in the conventional method has the problem that, although conductivity between the negative electrode current collector and the active material is ensured by the material located on the surface of the active material, the conductivity between the negative electrode active material decreases, making it impossible to achieve sufficient conductivity.

[0041] Therefore, the inventors have completed the present invention by confirming that when the single-walled carbon nanotubes are included in a first form that connects the surfaces of two or more negative electrode active material particles and in a second form that covers the surface of each negative electrode active material particle, sufficient conductivity can be maintained even as the cycle progresses, the rate of resistance increase is not large, and the lifetime characteristics are improved.

[0042] In this case, the content of the conductive material formed in the first embodiment may be 40% to 70% by weight of the total content of the conductive material.

[0043] If the content of the conductive material formed in the first embodiment is too small, outside the aforementioned range, sufficient conductivity between the active materials cannot be ensured, and the problem of decreased conductivity of the active materials due to volume expansion during cycle progression cannot be solved. If the content is too large, the content of the conductive material present on the surface of the active materials is relatively reduced, making it undesirable to ensure sufficient conductivity with the current collector.

[0044] The method for manufacturing a product that possesses both the first and second forms is described below.

[0045] On the other hand, the diameter of the single-walled carbon nanotube can be 1 nm to 3 nm, and more specifically, 1 nm to 2 nm.

[0046] If the average diameter is outside the aforementioned range and is excessively small, the dispersed carbon nanotubes become embedded between the negative electrode active material particles, making it difficult to form sufficient pores. If it is excessively large, it is undesirable because a good conductivity improvement effect cannot be obtained.

[0047] Furthermore, the length of the single-walled carbon nanotube is not particularly limited, or the length of the single-walled carbon nanotube is 1 μm to 7 μm, and more specifically, it may be 2 μm to 5 μm.

[0048] The longer the length of the single-walled carbon nanotube, the better the conductivity of the negative electrode, as well as its strength and the storability of the electrolyte. However, if the length is excessively long outside the specified range, the dispersibility may decrease, so the specified range is the most appropriate.

[0049] Here, the diameter and length can be measured by AFM (Atomic Force Microscopy).

[0050] Thus, the aspect ratio (length / diameter) of the single-walled carbon nanotube defined by the ratio of the diameter to the length of the single-walled carbon nanotube can be 500 to 7000, and specifically can be 1000 to 5000.

[0051] On the other hand, the single-walled carbon nanotubes can generally exist in a form in which a plurality of such single-walled carbon nanotubes are aggregated, and depending on the form, they can be entangled or bundled to have a secondary shape, and specifically can have a secondary shape of being bundled.

[0052] Specifically, the "bundle type" refers to a secondary shape of a bundle or rope in which a plurality of carbon nanotubes are arranged side by side or twisted spirally, unless otherwise specified. The "entangled type" means a shape in which a plurality of carbon nanotubes are entangled without being limited to a specific orientation.

[0053] In the chemical vapor deposition method, the shape can be produced by varying the temperature in order to produce carbon nanotubes of a desired form. At this time, since the carbon nanotubes having the entangled structure have an aggregated structure and are similar to an intermediate shape between the punctate conductive material and the carbon nanotubes having the bundle structure, it is disadvantageous for the formation of a network structure. On the other hand, since the carbon atoms in the bundle structure are present one by one at a predetermined distance from each other, it is easier in electron transfer. Therefore, when it is necessary to ensure conductivity according to the present invention, it is more preferable to have a bundle structure.

[0054] The specific surface area of the single-walled carbon nanotubes having such a secondary shape is 800 to 1400 m 2 / g, and specifically can be 1000 to 1200 m 2 / g.

[0055] If it is outside the above range and has an excessively small specific surface area, it is difficult to ensure sufficient conductivity. If it is excessively large, the dispersibility may decrease, which is not preferable.

[0056] Such a specific surface area indicates the BET specific surface area and is measured by the BET method. Specifically, it can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II manufactured by BEL Japan.

[0057] Such a conductive material may be contained in an amount of 0.01 to 10% by weight, specifically 0.05 to 1% by weight, and more specifically 0.05 to 0.2% by weight based on the total weight of the negative electrode binder.

[0058] On the other hand, in addition to the silicon-based active material, the negative electrode active material may contain other conventionally known active materials.

[0059] For example, carbon such as graphitizable carbon and graphite-based carbon; Li x Fe2O3 (0 ≦ x ≦ 1), Li x WO2 (0 ≦ x ≦ 1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), etc. metal composite oxides; lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials, etc. can be used.

[0060] In particular, graphite-based carbon among the active materials can be mixed.

[0061] In this case, the silicon-based active material may be included in an amount of 1% to 20% by weight, more specifically 5% to 10% by weight, based on the total weight of the negative electrode active material.

[0062] Furthermore, the negative electrode mixture may further contain a binder and a dispersant.

[0063] The binder is not limited to any component that assists in the bonding of the active material to the conductive material and to the current collector, and can be selected from, for example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-dientelpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0064] In this case, the binder may be included in an amount of 0.1 to 30% by weight, more specifically 0.5 to 10% by weight, and more specifically 1 to 5% by weight, based on the total weight of the negative electrode mixture.

[0065] The aforementioned dispersant is not limited to those conventionally known in the industry, but may be one or more substances selected from the group consisting of carboxymethylcellulose (CMC), polyvinylpyrrolidone, polyvinyl alcohol, polyacrylic acid, acrylonitrile-butadiene rubber systems, and styrene-ethylene oxide systems, and more specifically, it may be a mixture of acrylonitrile-butadiene rubber systems and styrene-ethylene oxide systems.

[0066] Here, the dispersant may be included in an amount of 0.01 to 5% by weight, more specifically 0.05 to 1% by weight, and more specifically 0.05 to 0.5% by weight, based on the total weight of the negative electrode mixture.

[0067] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, copper, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc., can be used. The current collector can also have fine irregularities formed on its surface to enhance the adhesion of the negative electrode active material, and can take various forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0068] On the other hand, according to another embodiment of the present invention, A method for manufacturing the negative electrode for the secondary battery, A method for manufacturing a negative electrode is provided in which, when producing a negative electrode active material slurry containing a conductive material composed of single-walled carbon nanotubes (SWCNTs), the conductive material of single-walled carbon nanotubes is introduced in a divided manner.

[0069] Specifically, the negative electrode active material slurry may further contain a binder, and the divided addition may involve mixing 30% to 60% by weight of the conductive material composed of single-walled carbon nanotubes with the negative electrode active material first, and then adding the remaining 40% to 70% by weight of the conductive material composed of single-walled carbon nanotubes together with the binder and mixing them afterward.

[0070] More specifically, The method for manufacturing the negative electrode is as follows: (a) A step of preparing a predispersion by mixing single-walled carbon nanotubes (SWCNTs) as a conductive material with a dispersant and a solvent; (b) A step of preparing an active material solution by adding a negative electrode active material to a portion of the pre-dispersion; (c) The step of adding the remaining pre-dispersion to the active material solution and adding a binder to produce an active material slurry; and (d) Steps to manufacture a negative electrode by applying the active material slurry to at least one surface of the negative electrode current collector, drying it, and rolling it. It may include.

[0071] First, a pre-dispersion is prepared by mixing the conductive material, which consists of single-walled carbon nanotubes, with a dispersant in a solvent before mixing it with the negative electrode active material.

[0072] Here, the types of conductive material and dispersant for the single-walled carbon nanotubes are as described above.

[0073] At this time, the ratio of conductive material to dispersant can be 2:1 to 100:1 by weight, and more specifically, 5:1 to 20:1.

[0074] The solvent is a solvent for manufacturing the negative electrode active material and can be used without particular limitations as long as it is commonly used in the art. Specifically, examples include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these can be used in mixtures of two or more. The amount of solvent used is sufficient to dissolve or disperse the negative electrode active material, conductive material, dispersant, and binder, taking into consideration the thickness of the slurry coating and the manufacturing yield, and to have a viscosity that can exhibit excellent thickness uniformity when subsequently coated for positive electrode manufacturing.

[0075] Subsequently, a portion of the pre-dispersion is added to produce an active material solution.

[0076] At this time, the pre-dispersion used can be a pre-dispersion that is 30% to 60% by weight, based on the total weight of the manufactured pre-dispersion.

[0077] Upon introduction of the negative electrode active material in this manner, the conductive material of the pre-dispersion is positioned to cover the surface of the negative electrode active material.

[0078] Subsequently, the remaining pre-dispersion is added to the mixed active material solution, and then the binder is added and mixed to produce an active material slurry.

[0079] At this time, the pre-dispersion added is the remainder after removing the pre-dispersion already added, and may be 40% to 70% by weight based on the total weight of the pre-dispersion.

[0080] The pre-dispersion added in step (c) is formed in a manner that connects the surfaces of the active material particles to create conductivity between the active materials.

[0081] In other words, according to the present invention, if a pre-dispersion of single-walled carbon nanotubes mixed with a dispersant is added before and after the mixing of the active material, the single-walled carbon nanotubes are present in an appropriate amount to include both a first form that connects the surfaces of two or more negative electrode active material particles and a second form that covers the surface of each negative electrode active material particle. As a result, excellent conductivity can be ensured even as the cycle progresses, the increase in resistance is reduced, and the effect of improving life characteristics can be achieved.

[0082] On the other hand, according to another embodiment of the present invention, A secondary battery in which an electrode assembly comprising the negative electrode; a positive electrode having a positive electrode mixture containing a positive electrode active material formed on at least one surface of the positive electrode current collector; and a separation membrane interposed between the negative electrode and the positive electrode is housed together with an electrolyte in a secondary battery case. The aforementioned secondary battery is provided in which the increase rate of the DCIR value over 100 cycles from the initial DCIR value is 47% or less.

[0083] Specifically, the initial DCIR value is obtained by performing CC / CV charging of the manufactured secondary battery at 0.5C, 4.2V (cut-off condition) and CC discharge at 2.0C, 2.5V (cut-off condition), then recharging it and discharging it at 2.0C for 10 seconds with a SOC of 50%, and measuring its resistance. The 100 DCIR values ​​are obtained by performing the above charge / discharge conditions 100 times, then recharging it and discharging it at 2.0C for 10 seconds with a SOC of 50%, and measuring its resistance.

[0084] At this point, the growth rate in the values ​​compared is 47% or less.

[0085] Furthermore, the secondary battery may maintain its discharge capacity for 100 cycles at a rate of 91% or more relative to its initial discharge capacity.

[0086] The initial discharge capacity is specifically the discharge capacity when the manufactured secondary battery is subjected to CC / CV charging at 0.5C, 4.2V (cut-off conditions) and CC discharge at 2.0C, 2.5V (cut-off conditions). The 100-cycle discharge capacity is the discharge capacity after 100 cycles under the aforementioned charge / discharge conditions. Therefore, the capacity retention rate can be calculated using the value of (100th discharge capacity / 1st discharge capacity) × 100.

[0087] In this case, as the requirement for high energy density in secondary batteries increases, a positive electrode active material with a high Ni content (Ni excess) can be used.

[0088] The positive electrode active material may include a lithium transition metal oxide represented by the following chemical formula 1.

[0089] Li 1+x Ni a Co b Mn c M 1-(a+b+c) O 2-y A y (1)

[0090] In the above formula, M is at least one selected from the group consisting of Cu, Ti, Mg, Al, Pt, and Zr. A is an oxygen-substituted halogen, The following conditions apply: 0≦x≦0.5, 0.8≦a≦1, 0≦b≦0.2, 0≦c≦0.2, 0.9≦a+b+c≦1, and 0≦y≦0.001.

[0091] More specifically, a can be 0.88 ≤ a < 1.

[0092] Furthermore, the lithium transition metal oxide may be a bimodal mixture of large and small particles.

[0093] In this case, the large particles may have an average diameter (D50) of 10 to 20 μm, and the small particles may have an average diameter (D50) of 1 to 7 μm.

[0094] As described above, having a bimodal structure is preferable because it can improve performance by increasing the packing density and reducing the resistance inside the electrode, and by maximizing the contact between the positive electrode active material and the electrolyte.

[0095] The aforementioned bimodal structure refers to a structure in which two distribution peaks appear based on the average diameter distribution diagram of lithium transition metal oxides.

[0096] On the other hand, the positive electrode active material may also include, in addition to the lithium transition metal oxide with excess Ni, layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x Lithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented as O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3); chemical formula LiMn 2-x M x Lithium manganese composite oxide represented as O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn); LiNi x Mn 2-xThis can further include lithium manganese complex oxides with a spinel structure represented as O4; LiMn2O4 in which part of the Li in the chemical formula is replaced by an alkaline earth metal ion; lithium iron phosphorus oxide represented as LiFePO4; disulfide compounds; Fe2(MoO4)3, etc.

[0097] In this case, the lithium transition metal oxide may be present in an amount of 80% to 100% by weight, and more specifically, 100% by weight, based on the total weight of the positive electrode active material.

[0098] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, or silver can be used. The positive electrode current collector can also typically have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it can be used in a variety of forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0099] Since the other components included in secondary batteries are well known in the industry, a detailed explanation of them is omitted in this invention, and the conventional configuration is included in this invention.

[0100] The following describes preferred embodiments of the present invention, comparative examples for comparison therewith, and experimental examples for evaluating them. However, the above embodiments are merely illustrative of this description, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of this description and the technical concept, and it goes without saying that such variations and modifications fall within the scope of the attached claims.

[0101] <Manufacturing Example 1> SWCNTs (Single-walled carbon nanotubes, ANP Corporation) were prepared as the conductive material.

[0102] Pre-dispersion A was prepared by mixing the conductive material and carboxymethylcellulose (CMC) as a dispersant in an N-methylpyrrolidone solvent in a weight ratio of 1:2.

[0103] <Manufacturing Example 2> Pre-dispersion B was prepared by mixing carboxymethylcellulose (CMC) as a dispersant in N-methylpyrrolidone solvent.

[0104] <Example 1> An active material solution was prepared by adding a negative electrode active material, which was a mixture of SiO:graphite in a weight ratio of 5:95, to 50% by weight of the aforementioned pre-dispersion A.

[0105] Subsequently, the remaining pre-dispersion A was added to the active material solution, and styrene-butadiene rubber (SBR) was added as a binder and carboxymethylcellulose-sodium (CMC) as a thickener. The mixture was then prepared to produce an active material slurry.

[0106] At this time, the amount of negative electrode active material and binder added was such that the weight ratio of the negative electrode active material, conductive material, dispersant, binder, and thickener in the final active material slurry was 97.2:0.2:0.6:1:1.

[0107] The manufactured active material slurry is loaded onto a 10 μm copper current collector with a loading amount of 10 mg / cm². 2 The negative electrode was manufactured by applying the material in this manner.

[0108] <Comparative Example 1> In Example 1, the anode was manufactured in the same manner as in Example 1, except that the anode active material was added to the entire 100% by weight of pre-dispersion A to produce an active material solution, and then pre-dispersion B was mixed in.

[0109] <Comparative Example 2> In this example, the anode was manufactured in the same manner as in Example 1, except that the anode active material was added to the pre-dispersion B, and then 100% by weight of the pre-dispersion A was added and mixed in after the anode active material was added.

[0110] <Comparative Example 3> SWCNTs (Single-walled carbon nanotubes, ANP Corporation) and MWCNTs (Multi-walled carbon nanotubes, ANP Corporation) were prepared as conductive materials.

[0111] A conductive material was prepared by mixing the aforementioned SWCNTs and MWCNTs in a weight ratio of 1:10, and a pre-dispersion C was prepared by mixing this with carboxymethylcellulose (CMC) as a dispersant in an N-methylpyrrolidone solvent in a weight ratio of 1:1.5.

[0112] The anode was manufactured in the same manner as in Example 1, except that the anode active material was added to 100% by weight of the pre-dispersion C to produce an active material solution, and then the pre-dispersion B was mixed in.

[0113] <Experimental Example 1> SEM image Planar SEM images of the negative electrodes manufactured in Example 1 and Comparative Examples 1-2 were taken, and the results are shown in Figures 1-3.

[0114] Referring to Figures 1 to 3, it can be confirmed that in the negative electrode according to the present invention, the conductive material connects the active materials and is distributed across the entire surface of the active materials. In contrast, in the negative electrode according to Comparative Example 1, the conductive material is distributed on the surface of the active materials but does not effectively connect the active materials, and in the negative electrode according to Comparative Example 2, the conductive material connects the active materials, but the amount distributed on the surface is extremely small.

[0115] <Experimental Example 2> Measurement of the DCIR increase rate LiNi 0.88 Co 0.07 Mn 0.04 Al 0.01A composition for forming a positive electrode was prepared by mixing O2, SWCNTs as a conductive material, PVdF as a binder, and hematoporphyrin derivative (HPD) as a dispersant in N-methylpyrrolidone solvent in a weight ratio of 98.5:0.05:1.37:0.08, and loading this composition onto an aluminum current collector at a rate of 18 mg / cm². 2 The positive electrode was manufactured by coating it in this manner.

[0116] A polyethylene separation membrane (thickness: 15 μm) was interposed between the negative electrode and the positive electrode manufactured in Example 1 and Comparative Examples 1-3. Then, an electrolyte containing a solvent mixed with ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:10:70, and containing 1 M of 1.3 M LiPF6 in the total electrolyte volume, was injected to manufacture a secondary battery.

[0117] Subsequently, the manufactured secondary battery was subjected to CC / CV charging at 0.5C, 4.2V (cut-off condition) and CC discharge at 2.0C, 2.5V (cut-off condition). It was then recharged and discharged at 2.0C for 10 seconds at SOC 50%, and its resistance was measured. The resistance value due to the cycle was calculated as the rate of increase compared to the resistance value of the initial cycle, and the results are shown in Figures 4 to 7 below.

[0118] Referring to Figures 4 to 7, it can be confirmed that in Example 1 according to the present invention, the resistance increase rate is approximately 43.4%, while in Comparative Examples 1 and 2, the resistance increase rate is over 47%. Furthermore, even when using MWCNTs, if the process is carried out by adding them only once, sufficient conductivity between the active materials cannot be ensured, so the resistance increase rate is similar to that of Comparative Example 1.

[0119] <Experimental Example 3> Life characteristics The secondary batteries manufactured in Example 1 and Comparative Examples 1-3 were subjected to CC / CV charging at 0.5C, 4.2V (cut-off conditions) and CC discharge at 2.0C, 2.5V (cut-off conditions) while their discharge capacity was measured. The discharge capacity maintenance rate for each cycle was calculated as (discharge capacity at 100th cycle / discharge capacity at 1st cycle) × 100 relative to the initial discharge capacity, and the results are shown in Figures 4-7 below.

[0120] Referring to Figures 4 to 7, it can be seen that Example 1 of the present invention exhibits an excellent capacity retention rate of 91% or more, while Comparative Examples 1 to 3 have inferior capacity retention rates compared to the present invention, and it can be expected that this difference will increase as the cycle progresses.

[0121] Anyone with ordinary skill in the field to which this invention belongs will be able to make various applications and modifications within the scope of this invention based on the above. [Industrial applicability]

[0122] As described above, the negative electrode according to one embodiment of the present invention, by including single-walled carbon nanotubes (SWCNTs) as a conductive material, ensures excellent conductivity even as the cycle progresses, reduces the increase in resistance of the secondary battery containing it, and has the effect of improving its lifespan characteristics.

Claims

1. A method for manufacturing a negative electrode for a secondary battery, wherein a negative electrode mixture is formed on at least one surface of a negative electrode current collector, The negative electrode mixture comprises a negative electrode active material and a conductive material. The aforementioned negative electrode active material includes a silicon-based active material. The conductive material is composed of single-walled carbon nanotubes (SWCNTs), The negative electrode composite material contains a conductive material in an amount of 0.05% to 1% by weight, based on the total weight of the negative electrode composite material. The single-walled carbon nanotubes are included in a first form that connects the surfaces of two or more negative electrode active material particles, and in a second form that covers the surface of each negative electrode active material particle. When manufacturing a negative electrode active material slurry containing an active material and a conductive material composed of single-walled carbon nanotubes (SWCNTs), the conductive material composed of single-walled carbon nanotubes is added in segments. A method for manufacturing a negative electrode, wherein the negative electrode active material slurry further contains a binder, and 30% to 60% by weight of the conductive material composed of single-walled carbon nanotubes is first mixed with the negative electrode active material, and the remaining 40% to 70% by weight of the conductive material composed of single-walled carbon nanotubes is added together with the binder and then mixed.

2. A method for manufacturing a negative electrode for a secondary battery, wherein a negative electrode mixture is formed on at least one surface of a negative electrode current collector, The negative electrode mixture comprises a negative electrode active material and a conductive material. The aforementioned negative electrode active material includes a silicon-based active material. The conductive material is composed of single-walled carbon nanotubes (SWCNTs), The negative electrode composite material contains a conductive material in an amount of 0.05% to 1% by weight, based on the total weight of the negative electrode composite material. The single-walled carbon nanotubes are included in a first form that connects the surfaces of two or more negative electrode active material particles, and in a second form that covers the surface of each negative electrode active material particle. When manufacturing a negative electrode active material slurry containing an active material and a conductive material composed of single-walled carbon nanotubes (SWCNTs), the conductive material composed of single-walled carbon nanotubes is added in segments. The method for manufacturing the negative electrode is as follows: (a) A step of preparing a predispersion by mixing a conductive material composed of single-walled carbon nanotubes (SWCNTs) with a dispersant and a solvent; (b) A step of adding a negative electrode active material to a portion of the pre-dispersion to produce an active material solution; (c) The step of adding the remaining pre-dispersion to the active material solution and adding a binder to produce an active material slurry; and (d) A step of manufacturing a negative electrode by applying the active material slurry to at least one surface of the negative electrode current collector, drying it, and rolling it; A method for manufacturing a negative electrode, including the method described above.

3. The method for producing a negative electrode according to claim 2, wherein in step (b), a negative electrode active material is added to 30% to 60% by weight of the predispersion based on the total weight of the predispersion, and in step (c), the remaining 40% to 70% by weight of the predispersion is added based on the total weight of the predispersion.

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