Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode including a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material, a binder, a conductive agent, and a dispersant, wherein the conductive agent includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded side by side, the average length of the carbon nanotube structure is 1 μm to 20 μm, and the QBR according to Equation 1 is between 1 and 1.75. [Equation 1] QBR = Bs / Bf where Bs is the average value of the amount of the binder measured in the range of the distance from the upper surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range of the distance from the lower surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2021-0056905, filed on Apr. 30, 2021, the disclosure of which is incorporated herein by reference. Technical Field

[0004] The present invention relates to a negative electrode and a secondary battery including the negative electrode, wherein the negative electrode refers to a negative electrode that can minimize binder migration. Background Art

[0005] In recent years, with the development of technologies and the increase in demand for mobile devices, the demand for batteries as an energy source has increased significantly, and accordingly, various studies have been conducted on batteries that can meet various demands. In particular, as a power source for these devices, lithium secondary batteries having excellent life and cycle characteristics and high energy density are being actively studied.

[0006] A lithium secondary battery refers to a battery in which an electrode assembly containing a non-aqueous electrolyte containing lithium ions includes a positive electrode containing a positive electrode active material capable of intercalating / deintercalating lithium ions, a negative electrode containing a negative electrode active material capable of intercalating / deintercalating lithium ions, and a microporous separator disposed between the positive electrode and the negative electrode. The negative electrode active material is contained in a negative electrode active material layer in the negative electrode. Graphite is mainly used as the negative electrode active material. Specifically, most negative electrode active materials may correspond to graphite, and more specifically, only graphite can be used as the negative electrode active material.

[0007] Since the binder present in the negative electrode active material layer during the preparation of the negative electrode moves to the upper part of the negative electrode active material layer, a relatively small amount of binder is present in the lower part of the negative electrode active material layer in contact with the current collector. Therefore, there is a problem that the adhesion force between the negative electrode active material layer and the current collector (hereinafter simply referred to as negative electrode adhesion force) is reduced.

[0008] To solve this problem, a technique such as forming the negative electrode active material layer into multiple layers has been introduced. However, since an additional process is required to form multiple layers, the process may be cumbersome. At the same time, conventionally, a binder having a stronger adhesion force or a technique for controlling the negative electrode drying conditions has also been used, but there are still disadvantages of increased manufacturing cost and reduced productivity.

[0009] Therefore, a new technique capable of solving the above binder migration problem is needed. Summary of the Invention

[0010] Technical Problem

[0011] One aspect of the present invention provides a negative electrode having improved negative electrode adhesion.

[0012] Another aspect of the present invention provides a secondary battery including the negative electrode.

[0013] Technical solution

[0014] According to one aspect of the present invention, there is provided a negative electrode including a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material, a binder, a conductive agent, and a dispersant, the conductive agent includes a carbon nanotube structure, in which a plurality of single-walled carbon nanotube units are joined side by side, the average length of the carbon nanotube structure is 1 μm to 20 μm, and the QBR according to the following Equation 1 is between 1 and 1.75.

[0015] [Equation 1]

[0016] QBR = Bs / Bf

[0017] wherein Bs is the average value of the amount of the binder measured in the range of the distance from the upper surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range of the distance from the lower surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer

[0018] According to another aspect of the present invention, there is provided a secondary battery including the negative electrode.

[0019] Advantageous effects

[0020] Regarding the negative electrode according to the present invention, rope-type (long fiber form) carbon nanotube structures are interconnected to form a network structure in the negative electrode, and two dispersants are used in the formation and dispersion of the carbon nanotube structures. The carbon nanotube structures are present in the conductive agent dispersion, and an electrode paste is formed through the conductive agent dispersion. In the conductive agent dispersion, the two dispersants form carbon nanotube structures with an appropriate diameter, and can also play a role in suppressing the aggregation of the carbon nanotube structures during the preparation of the electrode paste. Therefore, the carbon nanotube structures are uniformly dispersed in the negative electrode paste to form an effective network, while increasing the zero-shear viscosity of the negative electrode paste. Thus, the network caused by the carbon nanotube structures can suppress the migration of the binder. Additionally, even when the negative electrode paste is coated on the current collector, the migration of the binder in the negative electrode paste is further suppressed by the zero-shear viscosity. Therefore, a sufficient amount of binder can be present in the lower part of the negative electrode active material layer. As a result, since the adhesion of the negative electrode can be improved, the life performance of the battery can be enhanced. Moreover, since the adhesion of the negative electrode will not be reduced even when the coating speed of the negative electrode active material layer is increased, the productivity of the negative electrode can be improved. Description of the Drawings

[0021] Figure 1 It is a schematic diagram illustrating the measurement method of Formula 1 of the present invention. Detailed Description

[0022] It should be understood that the words or terms used in the specification and claims should not be construed as having the meanings defined in a common dictionary, and it should be further understood that these words or terms should be construed as having the following meanings: Based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the invention, they are consistent with their meanings in the context of the related art and the technical concept of the invention.

[0023] The terms used in this specification are only for describing exemplary embodiments and do not limit the present invention. In the specification, unless otherwise stated, the singular forms of the terms may include the plural forms.

[0024] It should be further understood that the term "comprising", "including" or "having" used in this specification indicates the presence of the described features, quantities, steps, elements or combinations thereof, but does not exclude the presence or addition of one or more other features, quantities, steps, elements or combinations thereof.

[0025] In this specification, the expression "specific surface area" is measured by the Brunauer-Emmett-Teller (BET) method. Specifically, the specific surface area can be calculated from the nitrogen adsorption amount at the liquid nitrogen temperature (77K) using the BELSORP-mini II of BEL Japan, Inc.

[0026] As used in this specification, the "average particle diameter (D 50 )" can be defined as the particle diameter at which the cumulative amount is 50% in the particle size distribution curve. For example, the average particle diameter (D 50 ) can be measured by using the laser diffraction method. The laser diffraction method can generally measure particle diameters from the submicron level to several millimeters and can obtain highly reproducible and high-resolution results.

[0027] As used in this specification, "QBR" can be confirmed by a scanning electron microscope (SEM) (Jeol7900f) of Zeol Corporation.

[0028] Hereinafter, the present invention will be described in detail.

[0029] Negative electrode

[0030] The negative electrode according to the present invention may be a negative electrode including a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material, a binder, a conductive agent, and a dispersant, wherein the conductive agent includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are joined side by side, the average length of the carbon nanotube structure is 1 μm to 20 μm, and the QBR according to the following Equation 1 is between 1 and 1.75.

[0031] [Equation 1]

[0032] QBR = Bs / Bf

[0033] wherein Bs is the average value of the amount of the binder measured in the range of the distance from the upper surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range of the distance from the lower surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer.

[0034] The negative electrode includes a negative electrode active material layer. More specifically, it includes a current collector and a negative electrode active material layer provided on the current collector. However, the negative electrode does not exclude the so-called "free-standing negative electrode" in which the negative electrode consists only of the negative electrode active material layer without a current collector.

[0035] There is no particular limitation on the current collector as long as it has electrical conductivity without causing adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver or similar materials can be used as the current collector. Specifically, transition metals that can well absorb carbon, such as copper and nickel, can be used as the current collector.

[0036] The negative electrode active material layer can be provided on one surface or both surfaces of the current collector. Of course, in the case of an independent negative electrode, the negative electrode active material layer can become the negative electrode by itself without a current collector.

[0037] The negative electrode active material layer can include a negative electrode active material, a binder, a conductive agent, and a dispersant.

[0038] The negative electrode active material layer can include graphite. More specifically, the negative electrode active material can be graphite. That is, only graphite can be used as the negative electrode active material. In this case, the initial efficiency and life characteristics of the battery can be improved. The graphite can include at least one of artificial graphite and natural graphite.

[0039] Graphite can be included in the negative electrode active material layer in an amount of 80% to 99% by weight, and specifically in an amount of 90% to 98% by weight. When graphite constitutes most of the negative electrode active material layer, the phenomenon of a decrease in the adhesion force of the negative electrode due to binder migration is usually relatively serious. The present invention shows that even when graphite constitutes most of the negative electrode active material layer, binder migration can be suppressed.

[0040] The binder is used to ensure the adhesion force between the negative electrode active materials or the adhesion force between the negative electrode active material and the current collector. Commonly used binders in the art can be used, and their types are not particularly limited. For example, the binder can include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber or various copolymers thereof, and any one or a mixture of two or more of them can be used.

[0041] Based on the total weight of the electrode active material layer, the binder can be included in an amount of 0.1% to 10% by weight, and specifically in an amount of 0.5% to 5% by weight. When the amount of the binder satisfies the above range, excellent negative electrode adhesion can be achieved while minimizing the increase in the negative electrode resistance.

[0042] The conductive agent may include a carbon nanotube structure.

[0043] The carbon nanotube structure may include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure may be a structure in which a plurality of single-walled carbon nanotube units are bonded side by side. The single-walled carbon nanotube units may be arranged side by side and bonded in the carbon nanotube structure (an elastic cylindrical structure in which the units are bonded together so that the long axes of the units are parallel to each other) to form the carbon nanotube structure. The carbon nanotube structures are interconnected in the electrode to form a network structure.

[0044] The carbon nanotube structure may be a carbon nanotube structure in which 2 to 5000 single-walled carbon nanotube units are bonded side by side. More specifically, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 4500 single-walled carbon nanotube units are bonded to each other. For example, considering the dispersibility of the carbon nanotube structure and the durability of the electrode, the carbon nanotube structure is most preferably a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded to each other.

[0045] Conventional electrodes including carbon nanotubes are generally prepared by dispersing a bundle type or entangled type of carbon nanotubes (a form in which single-walled carbon nanotube units or multi-walled carbon nanotube units are interconnected or entangled) in a dispersion medium, and then using the conductive agent dispersion. In this case, the carbon nanotubes are completely dispersed in the conventional conductive agent dispersion and exist as the conductive agent dispersion, in which the carbon nanotube units are dispersed in a single-strand form. In the conventional conductive agent dispersion, the carbon nanotube units are easily cut by an excessive dispersion process, so that the length of the carbon nanotube units is shorter than the initial length. In addition, the carbon nanotube units may be easily cut during the rolling process of the negative electrode, and during the operation of the battery, the carbon nanotube units (especially single-walled carbon nanotube units) are cut by the excessive volume change of the negative electrode active material, which is an additional limitation. Therefore, the conductivity of the negative electrode may be reduced, thereby reducing the life characteristics of the battery. In addition, in the case of multi-walled carbon nanotube units, due to the node growth mechanism (not a smooth linear shape, but having nodes due to defects generated during the growth process), the structural defects are very high. Therefore, during the dispersion process, the multi-walled carbon nanotube units are more likely to be cut, and the short-cut multi-walled carbon nanotube units are likely to aggregate with each other through π-π stacking of carbon in the units. Therefore, it is difficult for the multi-walled carbon nanotube units to be more uniformly dispersed and present in the negative electrode slurry.

[0046] Alternatively, regarding the carbon nanotube structure included in the negative electrode of the present invention, since the carbon nanotube structure is in the form of a rope, in which a plurality of single-walled carbon nanotube units maintaining relatively high crystallinity and free of structural defects are joined side by side, even if the volume of the negative electrode active material changes excessively, the single-walled carbon nanotube units will not be cut, and their length can be maintained smoothly. Therefore, the conductivity of the negative electrode can be maintained. Moreover, due to the high conductivity of the single-walled carbon nanotube units with high crystallinity, the conductivity of the electrode is increased, and the input characteristics, output characteristics, and life characteristics of the battery can be significantly improved. In addition, since the carbon nanotube structures can be interconnected to have a network structure in the electrode, the generation of cracks can be prevented by suppressing the excessive change in the volume of the negative electrode active material, and at the same time, a strong conductive network can be ensured.

[0047] In addition, since the carbon nanotube structure is not easily damaged and can maintain its long shape, the conductive network can be strengthened over the entire negative electrode active material layer. Moreover, by suppressing the exfoliation of the negative electrode active material, the adhesion of the negative electrode can be significantly improved.

[0048] The carbon nanotube structure can play a role in suppressing the migration of the binder. Specifically, since the carbon nanotube structure is in the form of a long rope and can form a single network with the binder, the zero-shear viscosity of the negative electrode slurry is increased. Therefore, since the movement of the binder in the negative electrode slurry can be suppressed, the migration of the binder can be suppressed during the drying process of the negative electrode slurry, and the adhesion of the negative electrode can be improved.

[0049] The average length of the carbon nanotube structure can be 1 μm to 20 μm, particularly 1 μm to 10 μm, more particularly 2 μm to 7 μm, for example, 3 μm to 7 μm. In the case where the average length of the carbon nanotube structure is less than 1 μm, since the conductive network structure may not be effectively formed in the negative electrode and the migration of the binder may not be suppressed, the adhesion of the negative electrode is reduced and the resistance of the negative electrode increases. On the contrary, in the case where the average length of the carbon nanotube structure is greater than 20 μm, since the number of carbon nanotube structures decreases when the same amount of carbon nanotube structure is used, it is difficult to form a wide and uniform network in the negative electrode. Therefore, the adhesion of the negative electrode is reduced, and there is a problem of reduced uniformity of the adhesion and resistance of the negative electrode. When observing the prepared electrode with a scanning electron microscope (SEM), the average length corresponds to the average of the lengths of the top 100 carbon nanotube structures and the bottom 100 carbon nanotube structures with a relatively large average length.

[0050] The average diameter of the carbon nanotube structure can be from 5 nm to 100 nm, particularly from 5 nm to 30 nm, and more particularly from 5 nm to 20 nm. When the average diameter of the carbon nanotube structure satisfies the above range, a conductive network can be effectively formed, the adhesion of the negative electrode can be improved, and the effects of reducing the negative electrode resistance and the battery resistance can be achieved. When observing the prepared negative electrode with SEM, the average diameter corresponds to the average value of the diameters of the top 100 carbon nanotube structures and the bottom 100 carbon nanotube structures having a relatively large average diameter.

[0051] The carbon nanotube structure can be included in the negative electrode active material layer in an amount of 0.005 wt% to 0.2 wt%, particularly 0.01 wt% to 0.1 wt%, and more particularly 0.015 wt% to 0.075 wt%. When the above range is satisfied, since the conductive path of the negative electrode can be ensured, the life characteristics of the battery can be improved while maintaining a low level of the electrode resistance. In the process of preparing the conductive agent dispersion, in the case where the bundled carbon nanotubes are completely dispersed (as a general dispersion method, when dispersing, the single strands of the carbon nanotube units should be separated from each other as much as possible), no carbon nanotube structure is generated, or even if inadvertently generated, the amount generated is very small (for example, 0.0005 wt%). That is, under normal circumstances, the above range of amounts may never be achieved.

[0052] For the related art in which the negative electrode includes multi-walled carbon nanotube units, a high amount (for example, greater than 0.5 wt%) of multi-walled carbon nanotube units must be used to compensate for the low conductivity of the multi-walled carbon nanotube units. In addition, even in the case of preparing a negative electrode using a conductive agent dispersion, even if the single-walled carbon nanotube units are completely dispersed therein, due to the problem of the single-walled carbon nanotube units being cut, the single-walled carbon nanotube units cannot be used in a low content.

[0053] On the contrary, the carbon nanotube structure included in the negative electrode of the present invention is in a form in which a plurality of single-walled carbon nanotube units are joined side by side. Therefore, even if the volume of the negative electrode active material changes excessively, the carbon nanotube structure will not be cut, and its length can be smoothly maintained. Therefore, the conductivity of the negative electrode can be maintained, and due to the high conductivity of the single-walled carbon nanotube units, the conductivity of the negative electrode can be smoothly ensured. Therefore, even if the number of carbon nanotube structures in the negative electrode is very low, the life characteristics of the battery can be excellent.

[0054] In some cases, the single-walled carbon nanotube units can be surface-treated by an oxidation treatment or a nitridation treatment to improve the affinity with the dispersant.

[0055] The conductive agent may further include a dot-shaped conductive agent. The dot-shaped conductive agent may include carbon black. Since carbon black has high dispersibility and conductivity and can fill the large pores between the negative electrode active materials, when carbon black is used in combination with the carbon nanotube structure, the migration of the binder can be more effectively suppressed. The carbon black may be at least one selected from the group consisting of acetylene black, Ketjen black, channel black, and furnace black, but is not limited thereto.

[0056] The weight ratio of the carbon nanotube structure to the dot-shaped conductive agent may be in the range of 1:5 to 1:70, for example, 1:5 to 1:35. When the above range is satisfied, a long conductive network can be formed by the carbon nanotube structure, and since the dot-shaped conductive agent plays a pivotal role in the conductive network, the uniformity of the conductive network can be further improved. In addition, since the pores between the negative electrode active materials can be filled with the dot-shaped conductive agent, the adhesion of the negative electrode can be improved, and the output characteristics, storage characteristics, and life characteristics of the battery can be improved.

[0057] The dispersant may include a polymer dispersant containing an amine and a phenolic compound containing two or more aromatic rings, and specifically consists of a polymer dispersant containing an amine and a phenolic compound containing two or more aromatic rings. In the case of using these two dispersants, since the carbon nanotube structure is effectively dispersed and present in the negative electrode and a long network can be formed, the migration of the binder can be effectively suppressed.

[0058] Specifically, the carbon nanotube structure is present in the conductive agent dispersion, and an electrode paste is formed through the conductive agent dispersion. In the conductive agent dispersion, the two dispersants form a carbon nanotube structure with an appropriate diameter, and can also play a role in suppressing the aggregation of the carbon nanotube structure when preparing the electrode paste. Therefore, the carbon nanotube structure is uniformly dispersed in the negative electrode paste to form an effective network, while increasing the zero-shear viscosity of the negative electrode paste. Thus, the network formed by the carbon nanotube structure can suppress the migration of the binder. In addition, even when the negative electrode paste is coated on the current collector, the zero-shear viscosity can further suppress the migration of the binder in the negative electrode paste. Therefore, a sufficient amount of the binder can be present in the lower part of the negative electrode active material layer. Therefore, since the adhesion of the negative electrode can be improved, the life performance of the battery can be improved. In addition, since the adhesion of the negative electrode will not be reduced even when the coating speed of the negative electrode active material layer is increased, the productivity of the negative electrode can be improved.

[0059] For example, the amine-containing polymer dispersant may be at least one selected from the group consisting of polyvinylpyrrolidone, polyacrylic acid hydrazide, poly-N-vinyl-5-methoxazolidon, N-alkyl poly imine, N-acetyl poly imine, polyacrylamide, poly-L-lysine hydrobromide, benzyl-dodecyl-dimethylammonium chloride, and polyethylenhnine. In the case of using a specific polymer (wherein the polymer structure contains an amine) dispersant as described above, the carbon nanotube structure can be effectively dispersed in the negative electrode. More specifically, the amine-containing polymer dispersant may be polyvinylpyrrolidone.

[0060] Next, phenolic compounds containing two or more aromatic rings can reduce the viscosity of the carbon nanotube dispersion, particularly the water-based carbon nanotube dispersion, and can significantly improve the increase in viscosity over time due to the bulky structure generated by two or more aromatic rings and the influence of the hydroxyl groups contained in the phenolic groups. In the case of using phenolic compounds containing only one aromatic ring (such as dopamine, gallic acid, pyrogallol, catechol, etc.), the effect of increasing the viscosity of the dispersion and the effect of suppressing the change in viscosity over time are insufficient.

[0061] Preferably, the phenolic compound may include at least one aromatic ring structure selected from the group consisting of a phenolic structure, a catechol structure, a gallol structure, and a naphthol structure, and specifically includes at least one aromatic ring structure selected from the group consisting of a catechol structure and a gallol structure. The phenolic structure is a structure in which one hydroxyl group is bonded to a benzene ring, the catechol structure is a structure in which two hydroxyl groups are bonded to a benzene ring, the gallol structure is a structure in which three hydroxyl groups are bonded to a benzene ring, and the naphthol structure is a structure in which one hydroxyl group is bonded to a naphthalene.

[0062] In the case where the phenolic compound containing two or more aromatic rings includes the above structure, due to the interaction between the aromatic ring and the carbon nanotube and the interaction between the -OH of the phenolic compound and the polymer dispersant through hydrogen bonds being properly balanced in the carbon nanotube dispersion, the effect of reducing the viscosity of the conductive agent dispersion and suppressing the increase in viscosity over time can be exhibited.

[0063] Specific examples of the phenolic compound containing two or more aromatic rings may be at least one selected from the group consisting of baicalin, luteolin, taxifolin, myricetin, quercetin, rutin, catechin, epigallocatechin gallate, butein, piceatannol, and tannic acid, and may preferably be tannic acid, quercetin, epigallocatechin gallate, or a combination thereof.

[0064] In one example of the present invention, the aromatic ring contained in the phenolic compound containing two or more aromatic rings may have a structure in which one aromatic ring is not fused with another aromatic ring, or two aromatic rings are fused with each other, and does not include a structure in which three or more aromatic rings are fused with each other.

[0065] That is, a structure in which three or more aromatic rings are fused in the molecular structure can be excluded from the scope of the phenolic compound containing two or more aromatic rings.

[0066] In the case where the phenolic compound containing two or more aromatic rings includes a structure in which three or more aromatic rings are fused in the molecular structure, since the structure in which three or more aromatic rings are fused may induce aggregation between carbon nanotubes by exerting an excessively strong binding force on the carbon nanotubes in the carbon nanotube dispersion, it may not be applicable to improving the dispersion of carbon nanotubes. In addition, since the balance between the interaction between the aromatic ring and the carbon nanotube in the carbon nanotube dispersion and the interaction between the -OH of the phenolic compound and the polymer dispersant through hydrogen bonds is broken, it may be difficult to correctly exhibit the effect of reducing the viscosity of the carbon nanotube dispersion and the effect of suppressing the increase in viscosity over time.

[0067] Specifically, the phenolic compound containing two or more aromatic rings may be tannic acid. In this case, in the process of preparing the conductive agent dispersion, the bundled carbon nanotubes can be smoothly dispersed.

[0068] The weight ratio of the polymer dispersant containing an amine to the phenolic compound containing two or more aromatic rings may be in the range of 5:1 to 1:1, particularly 4:1 to 2:1, more particularly 3.5:1 to 2.5:1. When the above range is satisfied, the dispersibility of the carbon nanotube structure is improved, and the effect of reducing the viscosity of the carbon nanotube dispersion is achieved.

[0069] The dispersant may include polyvinylpyrrolidone as an amine-containing polymer dispersant and tannic acid as a phenolic compound containing two or more aromatic rings. When polyvinylpyrrolidone is used in combination with tannic acid, the effects of reducing the dispersibility and dispersion stability of the carbon nanotube structure and the viscosity of the carbon nanotube structure can be obtained simultaneously.

[0070] The dispersant may be included in the negative electrode active material layer in an amount of 0.005 wt% to 0.5 wt%, and specifically in an amount of 0.02 wt% to 0.3 wt%. When the above range is satisfied, the carbon nanotube structure can be smoothly dispersed, so that the conductive network can be smoothly developed, and the adhesion of the negative electrode and the battery life performance can be improved because the migration of the binder can be suppressed.

[0071] Based on 100 parts by weight of the carbon nanotube structure in the negative electrode, the dispersant may be included in an amount of 50 parts by weight to 200 parts by weight, and specifically in an amount of 80 parts by weight to 170 parts by weight. When the above range is satisfied, the dispersibility of the carbon nanotube structure can be improved, the viscosity of the dispersion can be reduced, and the change over time can be improved.

[0072] In the negative electrode, the QBR according to the following Equation 1 may be in the range of 1 to 1.75, particularly 1 to 1.65, more particularly 1 to 1.60.

[0073] [Equation 1]

[0074] QBR = Bs / Bf

[0075] Where Bs is the average value of the amount of the binder measured in the range (upper limit range) of the distance from the upper surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range (lower limit range) of the distance from the lower surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer. Refer to Figure 1, the negative electrode active material layer 200 is disposed on the current collector 100. The expression "direction D" is a direction perpendicular to the upper surface 200a or the lower surface 200b of the negative electrode active material layer 200, which refers to the thickness direction of the negative electrode active material layer 200. Specifically, each range refers to the product of the thickness corresponding to 15% of the total thickness of the negative electrode active material layer 200 and the horizontal length at a corresponding magnification (e.g., 400 times), where the horizontal length can be in the range of 200 μm to 500 μm, and specifically can be 300 μm. In addition, the amount of the binder can be confirmed by staining the binder in the negative electrode active material layer with OsO4, performing SEM-EDS mapping analysis on the cross-section of the negative electrode active material layer, and counting the signals of OsO4 confirmed in each upper limit range and lower limit range. The so-called "average value" refers to the value obtained by adding the amounts of the binder measured 100 times by the above measurement method and then dividing by 100.

[0076] In the case where the QBR is less than 1, since the binder is only concentrated in the lower part (closer to the current collector) of the negative electrode active material layer, the interfacial resistance increases, and due to the non-uniform resistance of the negative electrode active material layer, the battery resistance increases and the life characteristics deteriorate. On the contrary, in the case where the QBR is greater than 1.75, since an excessive amount of the binder is distributed in the upper part of the negative electrode active material layer, the negative electrode adhesion force decreases, and the life characteristics of the battery are poor.

[0077] On the contrary, in the present invention, due to using a carbon nanotube structure having specific physical properties as a conductive agent at an appropriate level, using two dispersants, and adopting a specific preparation method, the migration phenomenon of the binder is minimized, and accordingly, the QBR can satisfy the range of 1 to 1.75.

[0078] Method for preparing negative electrode

[0079] Next, a method for preparing the electrode of the present invention will be described.

[0080] The method for preparing the negative electrode of the present invention includes the following steps: preparing a conductive agent dispersion (S1); and forming a negative electrode paste (S2) including the conductive agent dispersion, a negative electrode active material, a binder, and a dispersant, wherein the preparing the conductive agent dispersion (S1) includes the following steps: preparing a mixed solution (S1-1) containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes, and dispersing the bundled single-walled carbon nanotubes by applying a shearing force to the mixed solution by a high-pressure homogenizer to form a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are arranged side by side (S1-2), wherein the average length of the carbon nanotube structure is 1 μm to 20 μm, and the QBR according to Equation 1 is in the range of 1 to 1.75.

[0081] [Equation 1]

[0082] QBR = Bs / Bf

[0083] The negative electrode includes a negative electrode active material layer formed from the negative electrode paste,

[0084] wherein Bs is the average value of the amount of the binder measured in the range of a distance of 15% of the total thickness corresponding to the negative electrode active material layer from the upper surface of the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range of a distance of 15% of the total thickness corresponding to the negative electrode active material layer from the lower surface of the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer.

[0085] The negative electrode in the above embodiment can be prepared by the above method. The negative electrode active material, binder, dispersant, and carbon nanotube structure are the same as those of the negative electrode described in the above embodiment.

[0086] (1) Preparation of the conductive agent dispersion (S1)

[0087] The preparation of the conductive agent dispersion (S1) may include the following steps: preparing a mixed solution containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (S1-1), and applying a shearing force to the mixed solution by a high-pressure homogenizer to disperse the bundled single-walled carbon nanotubes to form a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are joined side by side (S1-2).

[0088] In step S1-1, the mixed solution can be prepared by adding the bundled single-walled carbon nanotubes and the dispersant to the dispersion medium. The bundled single-walled carbon nanotubes exist in a bundled form, in which the above single-walled carbon nanotube units are joined, and the bundled carbon nanotubes include, for example, 5000 or more single-walled carbon nanotube units.

[0089] The bundled single-walled carbon nanotubes may be included in the mixed solution in an amount of 0.01 wt% to 0.1 wt%. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, the carbon nanotube structure can be formed at an appropriate level, and the dispersion stability can be improved.

[0090] For example, the dispersion medium may include amide-based polar organic solvents (such as water (H2O), dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP)), monoalcohols (such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol), diols (such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol), polyols (such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol), ethylene glycol ethers (such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether), ketones (such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone), and esters (such as ethyl acetate, γ-butyrolactone, and ε-caprolactone), and any one or a mixture of two or more thereof, but the present invention is not limited thereto. Specifically, the dispersion medium may be N-methylpyrrolidone (NMP).

[0091] The dispersant is the same as the dispersant in the above embodiments. Therefore, the same description will be omitted. Due to the use of this dispersant, the dispersibility of the carbon nanotube structure can be improved, the viscosity of the dispersion for preparing the negative electrode paste can be reduced, and the stability of the dispersion can be improved. Therefore, the migration of the binder can be effectively inhibited.

[0092] The weight ratio of the bundled carbon nanotubes to the dispersant in the conductive agent dispersion can be in the range of 1:0.1 to 1:10, for example, 1:1 to 1:10. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure can be formed at an appropriate level, and the stability of the dispersion can be improved.

[0093] The solid content in the mixed solution can be in the range of 0.1 wt% to 20 wt%, for example, 1 wt% to 10 wt%. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure at an appropriate level can be formed, and the dispersion stability can be improved. At the same time, the negative electrode paste can have a viscosity and elasticity suitable for the electrode preparation process, which is also beneficial to increasing the solid content of the negative electrode paste.

[0094] In step S1-2, the mixed solution can be stirred by a high-pressure homogenizer, and during this process, the bundled single-walled carbon nanotubes can be dispersed to form a carbon nanotube structure. This carbon nanotube structure is a structure in which multiple single-walled carbon nanotube units are arranged side by side and is the same as the carbon nanotube structure of the negative electrode in the above-described embodiment.

[0095] The high-pressure homogenizer may include a primary nozzle and a secondary nozzle. The mixed solution passes through the primary nozzle and the secondary nozzle in sequence while pressure is applied to the mixed solution. Since the diameter of the secondary nozzle is smaller than that of the primary nozzle, the mixed solution is subjected to a shear force when passing through the nozzle, and in this case, the bundled single-walled carbon nanotubes are dispersed.

[0096] The primary nozzle may have a diameter of 100 mm to 500 mm, particularly 150 mm to 300 mm, and more particularly 150 nm to 250 mm. The diameter of the primary nozzle may be 100 μm to 1000 μm, particularly 200 μm to 800 μm, and more particularly 200 μm to 650 μm. Additionally, the pressure may be in the range of 500 Bar to 1800 Bar, specifically in the range of 5000 Bar to 1600 Bar, and more specifically in the range of 800 Bar to 1600 Bar. If the pressure is 1,800 Bar or higher, since the bundled single-walled carbon nanotubes are completely dispersed, the carbon nanotube structure may not be formed smoothly.

[0097] The mixed solution can pass through the high-pressure homogenizer 5 to 10 times, and correspondingly, the diameter of the carbon nanotube structure can be in the range of 1 nm to 30 nm.

[0098] In step S1, different from the conventional method of completely dispersing the bundled single-walled carbon nanotubes, the bundled single-walled carbon nanotubes are not completely dispersed, but are dispersed to an appropriate level by appropriately combining conditions such as the conditions of the high-pressure homogenizer used (nozzle size, pressure, etc.), the physical properties of the bundled single-walled carbon nanotubes used, and the dispersant used. In the conductive agent dispersion thus formed, few or no single-walled carbon nanotube units exist independently in a single strand, and most can exist in the above-described carbon nanotube structure.

[0099] (2) Forming a negative electrode paste (S2) including a conductive agent dispersion, a negative electrode active material, a binder, and a dispersant

[0100] Step S2 includes stirring an active material solution containing an active material, a binder, and a solvent (S2-1) and adding a conductive agent dispersion to the stirred active material solution (S2-2).

[0101] The active material solution includes a negative electrode active material, a binder, and a solvent.

[0102] In this case, the negative electrode active material in the above-described embodiment can be used as the negative electrode active material. The binder in the above-described embodiment can be used as the binder. The solvent may include amide-based polar organic solvents (e.g., dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP)), monoalcohols (e.g., methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol), diols (e.g., ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol), polyols (e.g., glycerol, trimethylolpropane, pentaerythritol, or sorbitol), ethylene glycol ethers (e.g., ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether), ketones (e.g., acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone), and esters (e.g., ethyl acetate, γ-butyrolactone, and ε-propiolactone), and any one or a mixture of two or more thereof, but the present invention is not limited thereto. The solvent may be the same as or different from the dispersion medium used in the predispersion, and preferably the solvent is N-methylpyrrolidone (NMP).

[0103] By effectively dispersing the negative electrode active material in step S2-1, a conductive network caused by the carbon nanotube structure can be smoothly formed. That is, step S2-1 is one of the main factors, from which a configuration with a QBR according to Equation 1 in the range of 1 to 1.75 can be obtained.

[0104] In step S2-1, the stirring includes applying a strong shear force to the active material solution. In this case, the shear force can be applied by a HIVIS MIX planetary mixer / kneader of PRIMIX B.V.

[0105] The binder used in step S2-1 may be carboxymethyl cellulose. In this case, the weight of the carboxymethyl cellulose used in step S2-1 may be in the range of 30% to 70%, for example, in the range of 40% to 60% of the total weight of the carboxymethyl cellulose contained in the negative electrode active material layer. In this case, since the shear force can be effectively applied to the active material solution, the negative electrode active material can be more effectively dispersed in the active material solution, and the adhesion of the negative electrode active material layer can be improved.

[0106] In step S2-2, the conductive agent dispersion prepared in step S1 can be added to the stirred active material solution. In addition, the negative electrode paste can be prepared by adding the conductive agent dispersion to the active material solution and then stirring. Further, a binder can be additionally added in step S2-2.

[0107] The negative electrode active material includes graphite, and based on the total solid content in the negative electrode paste, graphite can be included in an amount of 80 wt% to 99 wt%, for example, 90 wt% to 98 wt%.

[0108] The solid content in the negative electrode paste can be in the range of 40 wt% to 80 wt%, specifically, 40 wt% to 60 wt%. When the above range is satisfied, during the drying process after coating the negative electrode paste, migration of the conductive agent and the binder due to evaporation of the solvent can be suppressed, and a negative electrode having excellent electrode adhesion and conductivity can be prepared. In addition, a high-quality negative electrode with little deformation of the negative electrode during the rolling process can also be prepared.

[0109] The carbon nanotube structure can be included in the solid content of the negative electrode paste in an amount of 0.005 wt% to 0.2 wt%, particularly in an amount of 0.01 wt% to 0.1 wt%, and more particularly in an amount of 0.015 wt% to 0.075 wt%. When the above range is satisfied, since the conduction path of the electrode is ensured, the life characteristics of the battery can be improved while maintaining a low level of electrode resistance.

[0110] Next, the negative electrode paste prepared above is dried to form a negative electrode active material layer. Specifically, the negative electrode active material layer can be prepared by coating the negative electrode paste on the electrode current collector and drying the coated electrode current collector, or can be prepared by casting the negative electrode paste on a separate carrier and then laminating a film separated from the carrier on the negative electrode current collector. If necessary, the negative electrode active material layer can be formed by the above method, and then a rolling process can be further performed. In this case, considering the physical properties of the finally prepared electrode, drying and rolling can be carried out under appropriate conditions without particular limitation. The carbon nanotube structure can be included in the negative electrode active material layer in an amount of 0.005 wt% to 0.2 wt%.

[0111] Secondary battery

[0112] Next, a secondary battery according to another embodiment of the present invention will be described.

[0113] The secondary battery according to another embodiment of the present invention may include the negative electrode of the above embodiment.

[0114] Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the same as the negative electrode of the above-described embodiment. Since the negative electrode has been described above, a detailed description thereof will be omitted.

[0115] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material.

[0116] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, or silver may be used. In addition, the thickness of the positive electrode current collector is generally 3 μm to 500 μm, and its surface may have fine roughness to improve the adhesion to the positive electrode active material. The positive electrode current collector may have various shapes, for example, a film, a sheet, a foil, a net, a porous body, a foam body, a non-woven fabric body, etc.

[0117] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may include a layered compound (such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2)) or a compound substituted with one or more transition metals, lithium iron oxide (such as LiFe3O4), lithium manganese oxide (such as Li 1+c1 Mn 2-c 1O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, and LiMnO2), lithium copper oxide (Li2CuO2), vanadium oxide (such as LiV3O8, V2O5, and Cu2V2O7), nickel (Ni)-site type lithium nickel oxide represented by the chemical formula LiNi 1- c2 M c2 O2 (where M is at least one selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), and gallium (Ga), and c2 satisfies 0.01 ≤ c2 ≤ 0.3), lithium manganese composite oxide represented by the chemical formula LiMn 2-c3 M c3 O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, chromium (Cr), zinc (Zn), and tantalum (Ta), and c3 satisfies 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn), and LiMn2O4 (a part of lithium (Li) of which is substituted with an alkaline earth metal ion), but the positive electrode active material is not limited thereto. The positive electrode may be lithium metal.

[0118] The positive electrode active material layer may include a positive electrode conductive agent, a positive electrode binder, and the above-mentioned positive electrode active material.

[0119] In this case, the positive electrode conductive agent is used to provide conductivity to the electrode. Among them, any conductive agent can be used without particular limitation as long as it has electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of the positive electrode conductive agent may be graphite, such as natural graphite and artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders, such as copper powder, nickel powder, aluminum powder, and silver powder, or metal fibers; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and one of them can be used alone or a mixture of two or more of them can be used.

[0120] In addition, the function of the positive electrode binder is to improve the binding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder may be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of them can be used alone or a mixture of two or more of them can be used.

[0121] The separator separates the negative electrode and the positive electrode and provides a movement path for lithium ions. Among them, any separator can be used as the separator without particular limitation as long as it is commonly used in secondary batteries. In particular, a separator with high moisture retention capacity for the electrolyte and low resistance to the transfer of electrolyte ions can be used. Specifically, a porous polymer film can be used, for example, a porous polymer film prepared from polyolefin-based polymers, such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a layered structure having two or more layers thereof. In addition, typical porous non-woven fabrics can also be used, for example, non-woven fabrics formed from high melting point glass fibers or polyethylene terephthalate fibers. In addition, a coated separator including a ceramic component or a polymer component can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be selectively used.

[0122] The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte, and these electrolytes can be used to prepare lithium ion secondary batteries, but the present invention is not limited thereto.

[0123] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0124] Examples of the non-aqueous organic solvent may be aprotic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0125] In particular, ethylene carbonate and propylene carbonate are cyclic carbonates among carbonate organic solvents. Due to their high dielectric constant, they can dissociate lithium salts well as high-viscosity organic solvents in the electrolyte solution. Therefore, cyclic carbonates are preferably used. Since a high-conductivity electrolyte can be obtained by mixing cyclic carbonates with low-viscosity and low-dielectric-constant chain carbonates such as dimethyl carbonate and diethyl carbonate in an appropriate proportion, cyclic carbonates can be more preferably used.

[0126] A lithium salt can be used as the metal salt, and the lithium salt is a material that is easily soluble in the non-aqueous electrolyte solution. Among them, for example, a material selected from F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 -, CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - at least one selected from the group consisting of as an anion of the lithium salt.

[0127] For the purpose of improving the life characteristics of the battery, preventing the battery capacity from decreasing, and improving the discharge ability of the battery, in addition to the above electrolyte components, the electrolyte may further include at least one additive, for example, compounds such as halogenated alkylene carbonates such as vinylene difluoride carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride.

[0128] According to another embodiment of the present invention, there is provided a battery module including a secondary battery as a unit cell and a battery pack including the battery module. Since the battery module and the battery pack include secondary batteries having high capacity, high rate capability and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

[0129] Hereinafter, the present invention will be described in more detail based on specific examples.

[0130] Preparation Example 1-1: Preparation of Conductive Agent Dispersion

[0131] 0.8 parts by weight of a bundle-type carbon nanotube composed of single-walled carbon nanotube units having an average diameter of 1.5 nm and an average length of 5 μm (specific surface area of 650 m 2 / g) and 1.2 parts by weight of a dispersant were mixed in 98 parts by weight of N-methylpyrrolidone (NMP) as a dispersion medium to prepare a mixed solution having a solid content of 1.0% by weight. After the mixed solution was put into a high-pressure homogenizer, a pressure of 1000 Bar was applied, and then the mixed solution was passed through a primary nozzle having a diameter of 200 mm and a secondary nozzle having a diameter of 500 μm a total of 6 times.

[0132] As the dispersant, polyvinylpyrrolidone and tannic acid were used in a weight ratio of 3:1.

[0133] Preparation Example 2-1: Preparation of Conductive Agent Dispersion

[0134] 7.0 parts by weight of a bundle-type carbon nanotube composed of multi-walled carbon nanotube units with an average diameter of 10 nm and an average length of 1 μm (specific surface area: 185 m 2 / g) and 1.4 parts by weight of a dispersant were mixed in 91.6 parts by weight of N-methylpyrrolidone (NMP) as a dispersion medium to prepare a mixed solution with a solid content of 8.4% by weight. After the mixed solution was put into a high-pressure homogenizer, a pressure of 1000 Bar was applied, and then the mixed solution was passed through a primary nozzle with a diameter of 200 mm and a secondary nozzle with a diameter of 500 μm a total of 7 times.

[0135] As the dispersant, polyvinylpyrrolidone and tannic acid were used in a weight ratio of 3:1.

[0136] Preparation Example 2-2: Preparation of Conductive Agent Dispersion

[0137] A conductive agent dispersion was prepared in the same manner as in Preparation Example 1-1, except that only polyvinylpyrrolidone was used as the dispersant.

[0138] Preparation Example 2-3: Preparation of Conductive Agent Dispersion

[0139] A conductive agent dispersion was prepared in the same manner as in Preparation Example 1-1, except that only tannic acid was used as the dispersant.

[0140] Preparation Example 2-4: Preparation of Conductive Agent Dispersion

[0141] A dispersion was prepared in the same manner as in Preparation Example 1-1, except that the mixed solution was passed through the high-pressure homogenizer a total of 4 times instead of 7 times.

[0142] Preparation Example 2-5: Preparation of Conductive Agent Dispersion

[0143] A dispersion was prepared in the same method as in Preparation Example 1-1, except that the mixed solution was passed through the high-pressure homogenizer a total of 13 times instead of 7 times.

[0144] Examples and comparative examples

[0145] Example 1: Preparation of Negative Electrode

[0146] (1) Preparation and Stirring of Active Material Solution

[0147] Using an average particle size (D 50) Artificial graphite with a particle size of 15 μm is used as the negative electrode active material. The negative electrode active material is mixed with an aqueous solution of carboxymethyl cellulose. In this case, 1 wt% of carboxymethyl cellulose is dissolved in the aqueous solution of carboxymethyl cellulose, and the weight-average molecular weight of carboxymethyl cellulose (CMC) is 100,000 g / mol, and the degree of substitution is 1.0. The weight of the carboxymethyl cellulose used is 50% of the weight of the carboxymethyl cellulose finally included in the negative electrode active material layer.

[0148] The mixture is stirred (kneaded) by a planetary mixer while applying a strong shearing force to prepare an active material solution.

[0149] (2) Preparation of negative electrode paste

[0150] The active material solution, styrene-butadiene rubber (SBR) as an adhesive, and carboxymethyl cellulose (CMC) (weight-average molecular weight: 100,000 g / mol, degree of substitution: 1.0) are mixed with the conductive agent dispersion of Preparation Example 1 to prepare a negative electrode paste (solvent: H2O). The weight ratio of the negative electrode active material, adhesive, carbon nanotube structure, and dispersant is 96.925: 3.0: 0.03: 0.045. The weight ratio of SBR to CMC is 2.0: 1.0.

[0151] Each negative electrode paste is applied at a loading amount of 300 mg / 25 cm 2 onto a 20-μm-thick copper (Cu) metal film as a negative electrode current collector and dried. In this case, the temperature of the circulating air is 70 °C. Subsequently, the negative electrode current collector coated with the paste and dried is roll-pressed and dried in a vacuum furnace at 130 °C for 8 hours to prepare a negative electrode including a negative electrode active material layer.

[0152] Example 2 and Comparative Examples 1 to 5: Preparation of negative electrode

[0153] The negative electrode is prepared in the same manner as in Example 1, except that the conditions are changed as shown in Table 1 below.

[0154] Comparative Example 6: Preparation of negative electrode

[0155] Using an average particle size (D 50) Artificial graphite with a particle size of 15 μm was used as the negative electrode active material. The negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0), the conductive agent dispersion of Preparation Example 1, and water as a solvent were put into a stirrer together, and the negative electrode active material, binder, and conductive agent dispersion were mixed at one time to prepare a negative electrode slurry. The weight ratio of the negative electrode active material, binder, carbon nanotube structure, and dispersant was 96.925:3.0:0.03:0.045. The weight ratio of SBR to CMC was 2.0:1.0.

[0156] Each negative electrode slurry was coated on a 20-μm-thick metallic copper (Cu) film as a negative electrode current collector at a loading amount of 300 mg / 25 cm 2 and dried. In this case, the temperature of the circulating air was 70 °C. Subsequently, the negative electrode current collector coated with the slurry and dried was roll-pressed and dried in a vacuum furnace at 130 °C for 8 hours to prepare a negative electrode including a negative electrode active material layer.

[0157] [Table 1]

[0158]

[0159] The average length and average diameter were confirmed by SEM. Specifically, the average length (or average diameter) was evaluated as the average of the top 100 carbon nanotube structures (or multi-walled carbon nanotube units) and the bottom 100 carbon nanotube structures (or multi-walled carbon nanotube units) having a larger average length (or average diameter).

[0160] Experimental example 1: Evaluation of binder migration degree (QBR)

[0161] The QBR value of Equation 1 was confirmed for each negative electrode of the examples and comparative examples.

[0162] [Equation 1]

[0163] QBR = Bs / Bf

[0164] Bs is the average value of the amount of the binder measured in the range (upper limit range) of a distance of 15% of the total thickness of the corresponding negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range (lower limit range) of a distance of 15% of the total thickness of the corresponding negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer. This range refers to the product of the thickness corresponding to 15% of the total thickness of the negative electrode active material layer and a length of 300 μm. Further, by staining the binder in the negative electrode active material layer with OsO4, SEM-EDS mapping analysis (magnification: 400 times) is performed on the cross section of the negative electrode active material layer, and then the signals of OsO4 confirmed in each of the upper limit range and the lower limit range are counted to confirm the amount of the binder. Such measurement is performed 100 times, and the QBR value is determined as the average value thereof.

[0165] Experimental example 2: Evaluation of negative electrode adhesion

[0166] After attaching the negative electrode punched out to a width of 20 mm and a length of 15 cm to a glass slide by double-sided tape, the negative electrode is pressed with a constant pressure. Specifically, a 90° peel test is performed in units of gf / 20 mm to confirm the negative electrode adhesion force.

[0167] [Table 2]

[0168]

[0169]

[0170] Referring to Table 2, for Examples 1 and 2, since the QBR values are lower than the QBR values of the Comparative Examples, it can be understood that the binder migration phenomenon occurs less, and accordingly, a higher negative electrode adhesion force can be confirmed.

Claims

1. A negative electrode including a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material, a binder, a conductive agent, and a dispersant, wherein the conductive agent includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded side by side, the average length of the carbon nanotube structure is 1 μm to 20 μm, and the QBR according to Equation 1 is in the range of 1 to 1.75: [Equation 1] QBR = Bs / Bf wherein Bs is the average value of the amount of the binder measured in the range of the distance from the upper surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range of the distance from the lower surface of the negative electrode active material layer to 15% of the total thickness of the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer.

2. The negative electrode according to claim 1, wherein the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.005 wt% to 0.2 wt%.

3. The negative electrode according to claim 1, wherein the average diameter of the carbon nanotube structure is 5 nm to 100 nm.

4. The negative electrode according to claim 1, wherein the dispersant includes an amine-containing polymer dispersant and a phenolic compound containing two or more aromatic rings.

5. The negative electrode according to claim 4, wherein the amine-containing polymer dispersant includes polyvinylpyrrolidone.

6. The negative electrode according to claim 4, wherein the phenolic compound containing two or more aromatic rings includes tannic acid.

7. The negative electrode according to claim 4, wherein the weight ratio of the amine-containing polymer dispersant to the phenolic compound containing two or more aromatic rings is in the range of 5:1 to 1:

1.

8. The negative electrode according to claim 1, wherein based on 100 parts by weight of the carbon nanotube structure in the negative electrode, the dispersant is included in an amount of 50 parts by weight to 200 parts by weight.

9. The negative electrode according to claim 1, wherein the conductive agent further includes a dot-type conductive agent.

10. The negative electrode according to claim 9, wherein the weight ratio of the carbon nanotube structure to the dot-type conductive agent is between 1:5 and 1:

70.

11. The negative electrode according to claim 1, wherein the negative electrode active material includes graphite.

12. A method for preparing a negative electrode, the method including the following steps: preparing a conductive agent dispersion (S1); and forming a negative electrode slurry including the conductive agent dispersion, a negative electrode active material, a binder, and a dispersant (S2), wherein the preparing the conductive agent dispersion (S1) includes the following steps: preparing a mixed solution containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (S1-1), and dispersing the bundled single-walled carbon nanotubes by applying a shear force to the mixed solution through a high-pressure homogenizer to form a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded side by side (S1-2), The formation of the negative electrode paste (S2) includes the steps of stirring an active material solution (S2-1) containing an active material, a binder, and a solvent, and adding a conductive agent dispersion (S2-2) to the stirred active material solution. Wherein the average length of the carbon nanotube structure is from 1 μm to 20 μm, and the QBR according to Equation 1 is in the range of 1 to 1.

75. [Equation 1] QBR = Bs / Bf The negative electrode includes a negative electrode active material layer formed from the negative electrode paste. Wherein Bs is the average value of the amount of the binder measured in the range of the distance from the upper surface of the negative electrode active material layer to 15% of the total thickness corresponding to the negative electrode active material layer in the direction toward the lower surface of the negative electrode active material layer, and Bf is the average value of the amount of the binder measured in the range of the distance from the lower surface of the negative electrode active material layer to 15% of the total thickness corresponding to the negative electrode active material layer in the direction toward the upper surface of the negative electrode active material layer.

13. A secondary battery comprising the negative electrode according to claim 1.

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