Anode and secondary battery including said anode

Abstract

The present invention discloses a negative electrode and a secondary battery, the negative electrode comprising a negative electrode active material layer including a negative electrode active material and a conductive material, the negative electrode active material comprising a silicon-based negative electrode active material, the conductive material comprising a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are aligned and bonded to one another, the carbon nanotube structures having an average length of 2 μm to 20 μm, and 60 carbon nanotube structures having an average length of 2 μm to 20 μm as observed when a surface of the negative electrode active material layer is observed through a scanning electron microscope (SEM) at a magnification of 20,000 are selected, and an A value defined by the following [Equation 1] is measured for each of the selected carbon nanotube structures, and then an average A value calculated by arithmetically averaging the remaining 50 A values ​​other than the top 5 values ​​and the bottom 5 values ​​among the measured A values ​​is 70 to 100. [Formula 1] A = {straight-line distance between both ends of the carbon nanotube structure P / total length of the carbon nanotube structure Q} x 100

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0023961, filed on February 23, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a negative electrode and a secondary battery including the negative electrode, and more particularly to a negative electrode in which a conductive network can be effectively maintained during battery operation, and a secondary battery including the same. [Background technology]

[0003] Recently, with the rapid development of technologies and increasing demand for mobile devices, the demand for batteries as energy sources has increased dramatically, and as a result, extensive research has been conducted on batteries that can meet various needs. In particular, research has been actively conducted on lithium secondary batteries, which have high energy density and excellent life and cycle characteristics as power sources for such devices.

[0004] A lithium secondary battery is a battery including a positive electrode including a positive electrode active material capable of intercalating / deintercalating lithium ions, a negative electrode including a negative electrode active material capable of intercalating / deintercalating lithium ions, an electrode assembly in which a microporous separator is interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte containing lithium ions.

[0005] Recently, lithium secondary batteries using silicon-based active materials have been developed to improve the capacity of the negative electrode. However, silicon-based active materials undergo significant volume changes during battery operation, which reduces the conductive network in the negative electrode with continued battery operation, resulting in a decrease in the battery's lifespan.

[0006] To address these issues, efforts have been made to improve the conductivity of anodes that use silicon-based active materials by forming a carbon coating layer on the silicon-based active material or by using multi-walled carbon nanotubes as a conductive material. However, while these methods can improve the short conductive network, they fail to maintain a long conductive network during battery operation. Therefore, recent attempts have been made to form a long conductive network by using long single-walled carbon nanotubes as a conductive material together with the silicon-based active material.

[0007] However, there is a problem that the single-walled carbon nanotubes in the manufactured negative electrode are entangled with each other or bent excessively, making it difficult to easily form the desired long conductive network. To solve this problem, attempts have been made to adjust the diameter of the single-walled carbon nanotubes or to dry-mill the single-walled carbon nanotubes to disperse them in the negative electrode, but these techniques have not solved the above problem.

[0008] Therefore, there is an increasing need for a negative electrode in which single-walled carbon nanotubes smoothly form a long conductive network. Summary of the Invention [Problem to be solved by the invention]

[0009] One problem to be solved by the present invention is to provide a negative electrode in which a conductive network can be effectively maintained even during operation of a battery.

[0010] Another problem to be solved by the present invention is to provide a secondary battery including the above negative electrode and having improved life characteristics. [Means for solving the problem]

[0011] According to one embodiment of the present invention, there is provided a negative electrode comprising a negative electrode active material layer including a negative electrode active material and a conductive material, wherein the negative electrode active material includes a silicon-based negative electrode active material, and the conductive material includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded to one another in a line, and the carbon nanotube structures have an average length of 2 μm to 20 μm. Sixty carbon nanotube structures having an average length of 2 μm to 20 μm are selected from the carbon nanotube structures observed when the surface of the negative electrode active material layer is observed under a scanning electron microscope (SEM) at 20,000 magnifications, and an A value, defined by the following Equation 1, is measured for each of the selected carbon nanotube structures. Then, an arithmetic average of the remaining 50 A values, excluding the top 5 and bottom 5 A values, is calculated, resulting in an average A value of 70 to 100. [Formula 1] A = {linear distance P between both ends of the carbon nanotube structure / total length Q of the carbon nanotube structure} × 100

[0012] On the other hand, in this specification, "average" means the arithmetic mean.

[0013] According to yet another embodiment of the present invention, there is provided a secondary battery including the negative electrode. [Effects of the Invention]

[0014] When the negative electrode contains a carbon nanotube structure that satisfies a specific average A value, as in the present invention, the carbon nanotube structures having a specific length are present in a long, aligned form within the negative electrode active material layer. Therefore, even if the volume of the silicon-based active material changes suddenly during battery operation, the conductive network of the negative electrode can be effectively maintained, thereby improving the life characteristics of the battery. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic diagram for explaining the A value of the present invention. [Figure 2] 1 is a SEM photograph of a negative electrode prepared according to Example 1 of the present invention. [Figure 3] 1 is a SEM photograph of a negative electrode prepared according to Example 1 of the present invention. [Figure 4] 1 is a SEM photograph of a negative electrode prepared according to Example 1 of the present invention. [Figure 5] 1 is a SEM photograph of a negative electrode prepared according to Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0017] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0018] It should be understood that in this specification, the terms "comprises," "includes," "has," and the like specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possible presence or addition of one or more different features, numbers, steps, components, or combinations thereof.

[0019] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0020] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0021] In this specification, the average particle size (D50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0022] In this specification, the A value is an index showing the degree of entanglement of the carbon nanotube structure in the negative electrode active material layer, and is a value defined by the following formula 1.

[0023] [Formula 1] A = (linear distance P between both ends of the carbon nanotube structure / total length Q of the carbon nanotube structure) × 100

[0024] Meanwhile, in the present invention, the average A value refers to a value calculated by selecting 60 carbon nanotube structures having an average length of 2 μm to 20 μm when the surface of the negative electrode active material layer is observed through a scanning electron microscope (SEM) at 20,000 magnifications, measuring the A value defined by the above [Equation 1] for each of the selected carbon nanotube structures, and then arithmetically averaging the remaining 50 A values ​​excluding the top 5 and bottom 5 A values. Here, the linear distance P between both ends of the carbon nanotube structure and the total length Q of the carbon nanotube structure in the above [Equation 1] can be measured by SEM image analysis.

[0025] In this specification, I G / I D is a spectrum of 1360 ± 50 cm obtained by Raman spectroscopy using a 532 nm laser with a Raman spectrometer (DXR3xi manufactured by Thermo Fisher Scientific). -1 Maximum peak intensity of D band at I D 1580±50cm -1 Maximum G-band peak intensity at I GThis means the ratio of

[0026] The present invention will be specifically described below.

[0027] negative electrode The negative electrode according to the present invention includes a negative electrode active material layer including a negative electrode active material and a conductive material, wherein the negative electrode active material includes a silicon-based negative electrode active material, and the conductive material includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded to one another in a line, and the carbon nanotube structure may have an average length of 2 μm to 20 μm, and an average A value measured by observing the surface of the negative electrode active material layer with a scanning electron microscope may be 70 to 100, preferably 90 to 100. When the average A value satisfies this range, the carbon nanotube structures present in the negative electrode active material layer are less likely to be entangled or bent, and a long conductive network may be formed. As a result, even if the silicon-based negative electrode active material expands in volume during battery operation, the conductive network may not be broken and may be maintained.

[0028] The average A value is calculated by selecting 60 carbon nanotube structures having an average length of 2 μm to 20 μm when the surface of the negative electrode active material layer is observed through a scanning electron microscope (SEM) at 20,000 magnifications, measuring the A value defined by the following [Equation 1] for each of the selected carbon nanotube structures, and then arithmetically averaging the remaining 50 A values ​​excluding the top 5 and bottom 5 A values ​​among the measured A values.

[0029] [Formula 1] A = {linear distance P between both ends of the carbon nanotube structure / total length Q of the carbon nanotube structure} × 100

[0030] The A value is an index showing the degree of entanglement of the carbon nanotube structures contained in the negative electrode active material, and the closer the A value is to 100, the less entanglement or bending of the carbon nanotube structures.

[0031] In conventional negative electrodes that use silicon-based negative electrode active materials and carbon nanotubes as conductive materials, the carbon nanotubes are typically present in a twisted or entangled form within the negative electrode active material layer. When the carbon nanotubes are present in a twisted or entangled form within the negative electrode active material layer, the length of the conductive material is shortened, making it difficult to form a long conductive network within the negative electrode active material layer, and only a short conductive network is formed. Therefore, during battery operation, excessive volume change of the silicon-based negative electrode active material makes it difficult to maintain the conductive network within the negative electrode active material layer, resulting in a problem of the conductive network being easily broken.

[0032] In contrast, the negative electrode using the silicon-based negative electrode active material according to the present invention uses carbon nanotube structures of a specific length as a conductive material, and has a large average A value of 70 to 100, which is an index of the degree of entanglement of the carbon nanotube structures. This allows the carbon nanotube structures to exist in a long, aligned shape within the negative electrode active material layer. As a result, even if there is an excessive volume change of the silicon-based negative electrode active material during battery operation, disconnection of the conductive network within the negative electrode active material layer is minimized, and the improved conductivity due to the long conductive network can be maintained.

[0033] The negative electrode may include a negative electrode active material layer, more specifically, the negative electrode may include a current collector and a negative electrode active material layer disposed on the current collector, although this does not exclude a so-called "free-standing negative electrode" in which the negative electrode is composed only of the negative electrode active material layer without a current collector.

[0034] The current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. Specifically, transition metals that adsorb carbon well, such as copper and nickel, may be used as the current collector.

[0035] The negative electrode active material layer may be disposed on one or both sides of the current collector. In the case of a free-standing negative electrode, the negative electrode active material layer may function as a negative electrode by itself without a current collector.

[0036] The negative electrode active material layer may include a negative electrode active material, a binder, and a conductive material.

[0037] (1)Negative electrode active material The negative electrode active material may include a silicon-based negative electrode active material, which has a large theoretical capacity and thus allows the negative electrode to have a high capacity.

[0038] Preferably, the silicon-based negative electrode active material may contain silicon particles. The silicon particles may be so-called pure silicon silicon particles (particles made of silicon). While silicon particles have the advantage of effectively improving the capacity of the negative electrode, they undergo rapid volume changes during the charge and discharge process of the battery, which can cause the conductive network to break and reduce the lifespan of the negative electrode. However, as in the present invention, when the average A value of the carbon nanotube structures in the negative electrode active material layer satisfies a specific range, a long conductive network can be formed in the negative electrode active material layer. As a result, even when the volume of the silicon particles expands during battery operation, the conductive network in the negative electrode active material layer can be effectively maintained, thereby improving the lifespan of the battery.

[0039] The average particle size D of the silicon particles 50 The thickness of the carbon nanotube structure may be 1 μm to 15 μm, specifically 2 μm to 12 μm, and more specifically 3 μm to 10 μm. When the thickness of the carbon nanotube structure is within this range, the conductive connection of the silicon particles by the carbon nanotube structure may be effective.

[0040] The specific surface area of ​​the silicon particles is 0.5 m 2 / g~5.0m 2 / g, specifically 0.7m 2 / g~4.0m 2 / g, more specifically, 1.0m 2 / g~2.5m 2 When this range is satisfied, side reactions in the electrolyte can be minimized and the conductive connection of silicon particles by the carbon nanotube structure can be effective.

[0041] The silicon particles may be contained in the negative electrode active material layer in an amount of 2 wt % to 95 wt %, specifically 3 wt % to 93 wt %, more specifically 5 wt % to 90 wt %. When this range is satisfied, the capacity of the negative electrode can be effectively improved.

[0042] The negative electrode active material may be composed of silicon particles. In this case, the silicon particles may be contained in the negative electrode active material layer in an amount of 70 to 99% by weight, specifically 80 to 97% by weight. When this range is satisfied, the capacity of the battery can be effectively improved.

[0043] Alternatively, the negative electrode active material may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may include graphite. The graphite may include at least one of artificial graphite and natural graphite.

[0044] (2) Conductive material The conductive material may include a carbon nanotube structure. The carbon nanotube structure may include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure may include a plurality of single-walled carbon nanotube units connected in parallel to one another. For example, the carbon nanotube structure may be formed into a flexible cylindrical or rope-like structure in which the single-walled carbon nanotube units are arranged and connected in parallel to one another, with the long axes of the units connected in parallel to one another. In the negative electrode active material layer, the carbon nanotube structures may be connected to one another to form a network structure.

[0045] The carbon nanotube structure may be a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded to each other in a row, specifically, 2 to 5,000 single-walled carbon nanotube units are bonded to each other in a row, more specifically, 2 to 4,500 single-walled carbon nanotube units are bonded to each other. For example, in consideration of the dispersibility of the carbon nanotube structure and the durability of the negative electrode, it is most preferable that the carbon nanotube structure be a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded to each other.

[0046] Conventional electrodes containing carbon nanotubes are generally fabricated by dispersing bundle-type or entangled-type carbon nanotubes (single-walled carbon nanotube units or multi-walled carbon nanotube units attached to or entangled with each other) in a dispersion medium to prepare a conductive material dispersion, and then using the conductive material dispersion. Here, the carbon nanotubes are completely dispersed in the conventional conductive material dispersion, resulting in a conductive material dispersion in which single-walled carbon nanotube units are dispersed. In the conventional conductive material dispersion, the carbon nanotube units are easily broken due to an excessive dispersion process, resulting in a shorter length than the initial length. Furthermore, the carbon nanotube units can easily break during the rolling process of the negative electrode. Another problem occurs when the carbon nanotube units (especially single-walled carbon nanotube units) are broken due to excessive volume change of the negative electrode active material during battery operation. This can result in a decrease in the conductivity of the negative electrode and a decrease in the battery's lifespan. Furthermore, multi-walled carbon nanotube units have many structural defects due to the mechanism of nodal growth (i.e., nodes exist due to defects that occur during the growth process, rather than a smooth linear chain). Therefore, the multi-walled carbon nanotube units are more easily cut during the dispersion process, and the cut multi-walled carbon nanotube units are more likely to aggregate with each other due to π-π stacking of the carbon atoms in the units. Therefore, it is difficult for them to be more uniformly dispersed in the anode slurry.

[0047] In contrast, the carbon nanotube structure included in the negative electrode of the present invention has a rope-like structure in which multiple single-walled carbon nanotube units are connected to one another in a row, and therefore is resistant to breakage even when excessive volumetric changes occur in the negative electrode active material, and can smoothly maintain its length. Furthermore, the carbon nanotube structures can be interconnected to form a network structure within the electrode, which can suppress excessive volumetric changes in the negative electrode active material, preventing cracks and ensuring a strong conductive network. Furthermore, because the carbon nanotube structure is resistant to breakage and can maintain its long shape, the conductive network can be strengthened throughout the negative electrode active material layer. Furthermore, detachment of the negative electrode active material is suppressed, significantly improving negative electrode adhesion.

[0048] The average length of the carbon nanotube structures may be 2 μm to 20 μm, specifically 3 μm to 15 μm, and more specifically 4 μm to 9 μm, for example, 5.5 μm to 8.5 μm. If the average length of the carbon nanotube structures is less than 2 μm, a conductive network structure cannot be effectively formed in the negative electrode, resulting in increased negative electrode resistance and easy disconnection of the conductive network during battery operation, resulting in reduced battery life. Conversely, if the average length of the carbon nanotube structures exceeds 20 μm, the number of carbon nanotube structures decreases when the same content of carbon nanotube structures is used, making it difficult to form a wide and uniform network in the negative electrode. This results in a decrease in negative electrode adhesion, which in turn reduces the uniformity of negative electrode adhesion and resistance. The average length refers to the average length of the remaining 50 carbon nanotube structures, excluding the top 5 longest carbon nanotube structures and the bottom 5 longest carbon nanotube structures, out of 60 carbon nanotube structures observed when the surface of the negative electrode active material layer is observed through an SEM at 20,000 magnifications when the manufactured electrode is observed through an SEM.

[0049] The average diameter of the carbon nanotube structures may be 10 nm to 100 nm, specifically 10 nm to 60 nm, more specifically 15 nm to 45 nm, for example, 15 nm to 28 nm. When the average diameter of the carbon nanotube structures satisfies this range, a conductive network is effectively formed, improving negative electrode adhesion and reducing negative electrode resistance and battery resistance. The average diameter refers to the average value of the diameters of the remaining 50 carbon nanotube structures, excluding the top 5 and bottom 5 carbon nanotube structures, among 60 carbon nanotube structures observed when the surface of the negative electrode active material layer is observed through SEM at 20,000 magnification when the fabricated electrode is observed through SEM.

[0050] Meanwhile, the single-walled carbon nanotube unit exhibits I G / I D can be 30 to 200, specifically 40 to 180, more specifically 50 to 170, for example 110 to 150. When this range is satisfied, the graphitization degree of the single-walled carbon nanotube units can be at an appropriate level, the conductivity and dispersibility of the carbon nanotube structure can be improved, and the carbon nanotube structure can be present in a long, aligned shape in the negative electrode active material layer, thereby improving the lifespan characteristics of the battery. In other words, in order for the carbon nanotube structure to satisfy the average A value to improve the lifespan characteristics of the battery, the I G / I D must satisfy the range.

[0051] The average A value of the carbon nanotube structures in the negative electrode active material layer may be 70 to 100, specifically 75 to 100, more specifically 80 to 100, for example, 90 to 100. The average A value is calculated by selecting 60 carbon nanotube structures having an average length of 2 μm to 20 μm when observed on the surface of the negative electrode active material layer at 20,000 magnifications through a SEM, measuring the A value according to the following formula 1, and then arithmetically averaging the remaining 50 A values ​​excluding the top 5 A values ​​and the bottom 5 A values ​​among the measured A values.

[0052] [Formula 1] A = (linear distance P between both ends of the carbon nanotube structure / total length Q of the carbon nanotube structure) × 100

[0053] The A value will be explained with reference to FIG. 1 as follows. In FIG. 1, length P refers to the linear distance between both ends of the carbon nanotube structure (represented by the black solid line). Length Q (white dotted line) refers to the total length of the carbon nanotube, that is, its actual length. Therefore, if the carbon nanotube structure is perfectly linear, the A value is 100, and the closer the A value is to 100, the more linear the carbon nanotube structure becomes, indicating that the carbon nanotube structure is arranged in a long line.

[0054] When the average A value of the carbon nanotube structures is less than 70, it means that a large number of carbon nanotube structures are present in the negative electrode active material layer in a highly bent form, and therefore, the carbon nanotube structures are unlikely to form a long conductive network, and the conductive network may be broken or deformed due to repeated volume changes of the silicon-based negative electrode active material, which may reduce the lifespan of the battery.

[0055] Meanwhile, in the negative electrode of the present invention, the carbon nanotube structure has an average A value of 70 to 100, which means that the carbon nanotube structure has a long, aligned configuration within the negative electrode active material layer. Therefore, the carbon nanotube structure can form a long conductive network within the negative electrode active material layer, and the conductive network can be prevented from being broken or deformed due to repeated volume changes of the silicon-based negative electrode active material. As a result, the battery life characteristics can be improved.

[0056] The carbon nanotube structure is formed from bundled single-walled carbon nanotubes. However, the average A value is not derived simply on the assumption that bundled single-walled carbon nanotubes are used. G / I D When preparing a conductive material dispersion in which the carbon nanotube structure is dispersed, the conditions must be controlled to ensure that the carbon nanotube structure has an appropriate length and diameter. When preparing a negative electrode slurry, various efforts are required, such as adjusting mixing conditions.

[0057] The carbon nanotube structure may be included in the negative electrode active material layer in an amount of 0.01 wt % to 5 wt %, specifically 0.02 wt % to 3 wt %, and more specifically 0.05 wt % to 1 wt %. When the range is satisfied, the conductive path of the negative electrode is secured, the electrode resistance is maintained at a low level, and the battery life characteristics are improved.

[0058] Meanwhile, depending on the case, the single-walled carbon nanotube unit may be surface-treated through oxidation or nitridation to improve its affinity with the dispersant.

[0059] (3) Binder The binder ensures adhesion between negative electrode active materials or between the negative electrode active material and the current collector, and may be any binder commonly used in the art, without any particular limitation. Examples of the binder include vinylidene 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, and various copolymers thereof. These binders may be used alone or in combination.

[0060] The binder may be included in an amount of 0.5 wt % to 15 wt %, specifically 1 wt % to 12 wt %, based on the total weight of the electrode active material layer. When the binder content is within this range, an increase in the negative electrode resistance can be minimized and excellent negative electrode adhesion can be achieved.

[0061] (4) Dispersant The negative electrode active material layer may further include a dispersant, which allows the carbon nanotube structures to be uniformly dispersed within the negative electrode active material layer.

[0062] The dispersant may comprise an amine-containing polymer dispersant and a phenolic compound having two or more aromatic rings, specifically, an amine-containing polymer dispersant and a phenolic compound having two or more aromatic rings. When the two dispersants are used, the carbon nanotube structure can be effectively dispersed and present in the anode, forming a long network, thereby improving the battery life.

[0063] Specifically, the carbon nanotube structures are present in a conductive material dispersion liquid, and the conductive material dispersion liquid is used to form a negative electrode slurry. The two dispersants allow carbon nanotube structures with appropriate diameters and lengths to be formed in the conductive material dispersion liquid, and also prevent re-aggregation of the carbon nanotube structures during the negative electrode slurry preparation process. As a result, the carbon nanotube structures are uniformly dispersed in the negative electrode slurry to form an effective network, while simultaneously increasing the zero-shear viscosity of the negative electrode slurry. Therefore, the network formed by the carbon nanotube structures can suppress binder migration. Furthermore, even when the negative electrode slurry is coated on a current collector, the zero-shear viscosity further suppresses binder migration within the negative electrode slurry. As a result, a sufficient amount of binder can be present at the bottom of the negative electrode active material layer. This can improve negative electrode adhesion and further improve battery life. Furthermore, since the negative electrode adhesion does not decrease even when the coating speed of the negative electrode active material layer is increased, negative electrode productivity can be improved.

[0064] The amine-containing polymer dispersant may be, for example, one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylic acid hydrazide, poly-N-vinyl-5-methoxazolidone, N-alkyl polyimine, N-acetyl polyimine, polyacrylamide, poly-L-lysine hydrobromide, benzyl-dodecyl-dimethylammonium chloride, and polyethyleneimine. When a specific polymer dispersant containing an amine in its polymer structure is used, the carbon nanotube structure can be effectively dispersed in the negative electrode. More specifically, the amine-containing polymer dispersant may be polyvinylpyrrolidone.

[0065] Next, the phenolic compound containing two or more aromatic rings can reduce the viscosity of a conductive material dispersion (carbon nanotube structure dispersion), particularly a water-based conductive material dispersion, and significantly improve the increase in viscosity over time due to the bulky structure generated by the two or more aromatic rings and the hydroxyl group contained in the phenol group. When a phenolic compound containing only one aromatic ring (e.g., dopamine, gallic acid, pyrogallol, catechol, etc.) is used, the effect of improving the viscosity of the dispersion and the effect of suppressing the change in viscosity over time are insufficient.

[0066] Preferably, the phenolic compound may contain one or more structures selected from the group consisting of a phenol structure, a catechol structure, a gallol structure, and a naphthol structure in one or more of the aromatic rings, and specifically, may contain one or more structures selected from the group consisting of a catechol structure and a gallol structure in one or more of the aromatic rings. The phenol structure is a structure in which one hydroxy group is bonded to a benzene ring, the catechol structure is a structure in which two hydroxy groups are bonded to a benzene ring, the gallol structure is a structure in which three hydroxy groups are bonded to a benzene ring, and the naphthol structure is a structure in which one hydroxy group is bonded to a naphthalene.

[0067] When the phenolic compound containing two or more aromatic rings contains the above structure, the interaction between the aromatic rings and the carbon nanotubes and the interaction due to hydrogen bonding between the -OH of the phenolic compound and the polymer dispersant are appropriately balanced in the carbon nanotube dispersion, thereby reducing the viscosity of the conductive material dispersion and suppressing the increase in viscosity over time.

[0068] Specific examples of the phenolic compound containing two or more aromatic rings include one or more selected from the group consisting of baicalin, luteolin, taxifolin, myricetin, quercetin, rutin, catechin, epigallocatechin gallate, butein, piceatannol, and tannic acid, and preferably tannic acid, quercetin, epigallocatechin gallate, or a combination thereof.

[0069] In one embodiment of the present invention, the aromatic ring contained in the phenolic compound containing two or more aromatic rings may be a single aromatic ring that is not fused to another aromatic ring or a structure in which two aromatic rings are fused to each other, and may not include a structure in which three or more aromatic rings are fused to each other.

[0070] That is, the scope of the phenolic compounds containing two or more aromatic rings excludes those containing a structure in which three or more aromatic rings are fused within the molecular structure.

[0071] When the phenolic compound containing two or more aromatic rings has a structure in which three or more aromatic rings are fused within its molecular structure, the structure in which three or more aromatic rings are fused may exert a stronger than appropriate bonding force with carbon nanotubes in the conductive material dispersion, which may induce aggregation between the carbon nanotubes, making it unsuitable for improving the dispersibility of carbon nanotubes. Furthermore, the balance between the interaction between the aromatic rings and carbon nanotubes in the conductive material dispersion and the interaction due to hydrogen bonding between the -OH of the phenolic compound and the polymer dispersant may be lost, making it difficult to achieve the appropriate effect of reducing the viscosity of the conductive material dispersion and the effect of suppressing viscosity increase over time.

[0072] Specifically, the phenolic compound containing two or more aromatic rings may be tannic acid, which allows the bundled carbon nanotubes to be dispersed smoothly during the production of the conductive material dispersion liquid.

[0073] The weight ratio of the amine-containing polymer dispersant to the phenolic compound having two or more aromatic rings may be 5:1 to 1:1, specifically 4:1 to 2:1, and more specifically 3.5:1 to 2.5:1. When the ratio is within this range, the dispersibility of the carbon nanotube structure is improved, and the viscosity of the conductive material dispersion is reduced.

[0074] The dispersant may contain polyvinylpyrrolidone as the polymer dispersant containing the amine, and tannic acid as the phenolic compound containing two or more aromatic rings. When polyvinylpyrrolidone and tannic acid are used in combination, the dispersibility and dispersion stability of the carbon nanotube structure and the viscosity reduction of the carbon nanotube structure can be simultaneously achieved.

[0075] The dispersant may be included in the negative electrode active material layer in an amount of 0.005 wt % to 0.5 wt %, specifically 0.02 wt % to 0.3 wt %. When the amount is within this range, the carbon nanotube structures can be smoothly dispersed, a conductive network can be smoothly developed, binder migration can be suppressed, and negative electrode adhesion and cell life performance can be improved.

[0076] The dispersant may be contained in the negative electrode in an amount of 50 to 200 parts by weight, specifically 80 to 170 parts by weight, relative to 100 parts by weight of the carbon nanotube structure. When the range is satisfied, the dispersibility of the carbon nanotube structure is improved, the viscosity of the dispersion liquid is reduced, and deterioration over time can be improved.

[0077] Negative electrode manufacturing method Next, the method for producing the negative electrode of the present invention will be described.

[0078] The method for manufacturing a negative electrode of the present invention can include the steps of (S1) preparing a conductive material dispersion liquid and a negative electrode active material dispersion liquid, and (S2) mixing and stirring the conductive material dispersion liquid and the negative electrode active material dispersion liquid to form a negative electrode slurry composition. The negative electrode is the same as the negative electrode of the above-described embodiment. Specifically, the negative electrode active material (e.g., a silicon-based negative electrode active material) and the conductive material (e.g., a carbon nanotube structure, Formula 1) are the same as the negative electrode active material and conductive material of the above-described embodiment.

[0079] (1) Step (S1) of preparing a conductive material dispersion liquid and a negative electrode active material dispersion liquid 1) Preparation of conductive material dispersion In the step (S1) of preparing a conductive material dispersion and a negative active material dispersion, the conductive material dispersion may include a step (S1-1) of preparing a mixture containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes, and a step (S1-2) of applying shear force to the mixture using a high-pressure homogenizer to disperse the bundled single-walled carbon nanotubes and form a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bound together in a row.

[0080] In step S1-1, the mixture may be prepared by adding the bundled carbon nanotubes and the dispersant to a dispersion medium. The bundled carbon nanotubes are composed of the above-mentioned single-walled carbon nanotube units bonded together to form a bundle, for example, containing 5,000 or more single-walled carbon nanotube units.

[0081] When Raman spectroscopy is performed on the single-walled carbon nanotube units in the bundled carbon nanotubes, the I G / I D can be 30 to 200, specifically 40 to 180, and more specifically 50 to 170. When this range is satisfied, one factor that allows the carbon nanotube structure to exist in the negative electrode in a form that satisfies the formula 1, i.e., in a long, aligned form, can be satisfied. This can improve the lifespan of the battery. In other words, in order for the carbon nanotube structure to satisfy the average A value in order to improve the lifespan of the battery, the I G / I D must satisfy the range.

[0082] The bundled single-walled carbon nanotubes may be contained in the mixture in an amount of 0.01 wt % to 0.1 wt %. When this range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, carbon nanotube structures with appropriate diameters and lengths can be formed, and dispersion stability can be improved.

[0083] Examples of the dispersion medium include water (HO), amide polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and various solvents such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of suitable dispersion media include, but are not limited to, hydric alcohols; 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, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone. These may be used singly or in combination. More specifically, the dispersion medium may be N-methylpyrrolidone (NMP).

[0084] The dispersant may be the same as the dispersant in the above embodiment, and therefore the same description will be omitted. The use of the dispersant may improve the dispersibility of the carbon nanotube structures, reduce the viscosity of the dispersion liquid for preparing the anode slurry, and improve the dispersion stability.

[0085] The weight ratio of the bundled carbon nanotubes to the dispersant in the conductive material dispersion may be 1:0.1 to 1:10, specifically 1:1 to 1:10. When the weight ratio satisfies this range, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure of an appropriate level is formed, and dispersion stability is improved.

[0086] The solid content of the mixture may be 0.1 wt% to 20 wt%, specifically 1 wt% to 10 wt%. When this range is satisfied, the bundled single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure of an appropriate level is formed, and dispersion stability is improved. Furthermore, the negative electrode slurry may have viscosity and elasticity suitable for the electrode manufacturing process, which also helps to increase the solid content of the negative electrode slurry.

[0087] In step S1-2, the mixture may be agitated using a high-pressure homogenizer, and during this process, the bundled single-walled carbon nanotubes may be dispersed to form a carbon nanotube structure. The carbon nanotube structure has a plurality of single-walled carbon nanotube units bonded together in a row, and is the same as the carbon nanotube structure described in connection with the anode of the above embodiment.

[0088] The high-pressure homogenizer may include a primary nozzle and a secondary nozzle. Pressure is applied to the mixture, causing the mixture to pass through the primary nozzle and then the secondary nozzle. Because the diameter of the secondary nozzle is smaller than that of the primary nozzle, the mixture is subjected to shear force while passing through the nozzle, dispersing the bundled single-walled carbon nanotubes.

[0089] The diameter of the primary nozzle may be 100 mm to 500 mm, specifically 150 mm to 300 mm, and more specifically 150 mm The diameter of the secondary nozzle may be 100 μm to 1000 μm, specifically 200 μm to 800 μm, more specifically 200 μm to 650 μm. The pressure may be 500 Bar to 1800 Bar, specifically 50 0B The pressure may be 1 to 1600 Bar, more specifically, 800 to 1600 Bar. If the pressure is 1800 Bar or higher, the bundled single-walled carbon nanotubes are not completely dispersed, and the carbon nanotube structure cannot be smoothly formed. On the other hand, if this condition is met, the average diameter and average length of the carbon nanotube structure can be at an appropriate level, and one of the factors that allows the carbon nanotube structure to exist in the negative electrode in a form that satisfies Equation 1, i.e., in a long, aligned form, can be satisfied.

[0090] The mixture may be passed through the high-pressure homogenizer 5 to 10 times, and the diameter of the carbon nanotube structure may be 1 nm to 30 nm.

[0091] In step S1, unlike the conventional method of completely dispersing bundled single-walled carbon nanotubes, the bundled single-walled carbon nanotubes are not completely dispersed but dispersed to an appropriate level by appropriately combining conditions such as the conditions for applying the high-pressure homogenizer (nozzle size, pressure, etc.), the properties of the bundled single-walled carbon nanotubes used, and the dispersant used, etc. In the conductive material dispersion thus formed, there are almost no or almost no single-walled carbon nanotube units existing independently as single strands, and most of the single-walled carbon nanotubes are present as the above-mentioned carbon nanotube structures.

[0092] 2) Preparation of negative electrode active material dispersion The preparation of the negative electrode active material dispersion liquid may include adding the negative electrode active material (same as the negative electrode active material in the above embodiment) to a solvent and stirring.

[0093] The solvent may be water (HO), dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), or other amide-based polar organic solvents; alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, or sorbitol; Examples of suitable solvents include, but are not limited to, alcohols; 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, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone. These solvents may be used singly or in combination. More specifically, the solvent may be N-methylpyrrolidone (NMP).

[0094] In the negative electrode active material dispersion liquid, the negative electrode active material may be included in an amount of 50 parts by weight to 100 parts by weight, specifically 60 parts by weight to 100 parts by weight, and more specifically 70 parts by weight to 100 parts by weight, relative to 100 parts by weight of the solvent.

[0095] When preparing the negative electrode active material dispersion, a thickener may be added in addition to the negative electrode active material. The thickener increases the viscosity of the negative electrode active material dispersion, causing the negative electrode active material dispersion to have a paste-like structure. This allows a higher shear force to be applied to the negative electrode active material dispersion when the negative electrode active material dispersion is mixed using a mixing device. This allows the negative electrode active material to be more effectively dispersed in the negative electrode active material dispersion, and subsequently, when the negative electrode active material dispersion and the conductive material dispersion are mixed, the carbon nanotube structures can be present in a long, aligned form.

[0096] The thickener may be at least one of carboxymethyl cellulose and carbon nanofiber.

[0097] (2) Step (S2) of mixing and stirring the conductive material dispersion liquid and the negative electrode active material dispersion liquid to form a negative electrode slurry composition. The conductive material dispersion and the negative electrode active material dispersion can be mixed and stirred to form a negative electrode slurry composition.

[0098] In a conventional process for preparing a negative electrode slurry composition, a powdered negative electrode active material is added to the conductive material dispersion. Since powdered negative electrode active material is difficult to apply sufficient shear force during the mixing step with the conductive material dispersion, the negative electrode active material cannot be effectively dispersed. Furthermore, this phenomenon prevents the carbon nanotube structures from being uniformly mixed with the negative electrode active material, causing the carbon nanotube structures to become entangled with each other. This makes it difficult for the carbon nanotube structures to form a long, aligned structure within the negative electrode. Instead, the carbon nanotube structures generally bend significantly or wrap around the negative electrode active material. Therefore, it is difficult to achieve the average A value of 70 to 100.

[0099] Meanwhile, when manufacturing the negative electrode of the present invention, a negative electrode active material dispersion is used instead of a negative electrode active material powder, which allows for uniform mixing of the negative electrode active material and the carbon nanotube structures, minimizes entanglement of the carbon nanotube structures, and is one factor that enables the carbon nanotube structures to satisfy the average A value.

[0100] The negative electrode slurry composition may further include a binder and a solvent, if necessary. The binder may be the same as that described in the above embodiment. Examples of the solvent include amide polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of suitable solvents include, but are not limited to, glycols 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, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone, and any one or a mixture of two or more thereof can be used. The solvent can be the same as or different from the dispersion medium used in the dispersion liquid described above, and is preferably N-methylpyrrolidone (NMP).

[0101] The binder may be added to a mixture of the conductive material dispersion and the negative electrode active material dispersion.

[0102] The solid content of the negative electrode slurry composition may be 20 wt % to 80 wt %, specifically, 30 wt % to 60 wt %.

[0103] The negative electrode slurry composition thus prepared is then dried to form a negative electrode active material layer. Specifically, the negative electrode active material layer can be formed by coating the negative electrode slurry composition on an electrode current collector and then drying, or by coating the negative electrode slurry composition on a separate substrate and then peeling it off from the substrate, resulting in a film that can be laminated on the negative electrode current collector. If necessary, a rolling process can be further performed after the negative electrode active material layer is formed using the above method. The drying and rolling processes can be performed under appropriate conditions, taking into account the physical properties of the final electrode to be manufactured, and are not particularly limited.

[0104] secondary battery Next, a secondary battery according to still another embodiment of the present invention will be described.

[0105] A secondary battery according to still another embodiment of the present invention may include the negative electrode of the above-described embodiment.

[0106] Specifically, the secondary battery may include the negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode may be the same as the negative electrode in the above-described embodiment. Since the negative electrode has been described above, detailed description thereof will be omitted.

[0107] 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 the positive electrode active material.

[0108] The positive electrode current collector in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. It can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0109] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; or a lithium ion battery having the chemical formula Li 1+c1 Mn 2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦c2≦0.3 is satisfied); 2-c3 M c3 Examples of the lithium manganese composite oxide include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and 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 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may be Li metal.

[0110] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the above-mentioned positive electrode active material.

[0111] The positive electrode conductive material is used to impart conductivity to the electrode and may be any material that does not cause chemical changes in the battery and has electronic conductivity. Specific examples include 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 powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These materials may be used alone or in combination.

[0112] The positive electrode binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder include polyvinylidene fluoride (PVDF), vinylidene 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, and various copolymers thereof. These binders may be used alone or in combination.

[0113] The separator separates the negative electrode and positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitations. In particular, a separator with low resistance to electrolyte ion movement and excellent electrolyte humidification capacity is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and can be used in a single-layer or multi-layer structure.

[0114] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries.

[0115] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0116] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0117] In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferably used because they are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be produced, and thus such cyclic carbonates are more preferably used.

[0118] The metal salt may be a lithium salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be 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 - , (SF3)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of:

[0119] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in the capacity of the battery, and improving the discharge capacity of the battery.

[0120] According to yet another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, excellent rate-limiting characteristics, and excellent cycle characteristics, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0121] The present invention will now be described in more detail with reference to specific examples.

[0122] Example 1: Preparation of negative electrode (1) Manufacturing of conductive material dispersion The average diameter is 1.5 nm, the average length is 5 μm, and the Raman spectrum is measured using I G / I D The bundled carbon nanotubes (specific surface area 650 m) consist of single-walled carbon nanotube units with a specific surface area of ​​143. 2 0.8 parts by weight of cellulose acetate / g) and 1.2 parts by weight of a dispersant were mixed with 98 parts by weight of N-methylpyrrolidone (NMP) as a dispersion medium to prepare a mixture with a solid content of 2.0% by weight. The mixture was placed in a high-pressure homogenizer, and a pressure of 1500 Bar was applied, and the mixture was passed through a primary nozzle with a diameter of 200 mm and a secondary nozzle with a diameter of 500 μm, in sequence, for a total of five times.

[0123] The dispersant used was a mixture of polyvinylpyrrolidone and tannic acid in a weight ratio of 3:1.

[0124] (2) Preparation of negative electrode active material dispersion Average particle size (D 50 Si particles with a particle size of 5 μm and carboxymethyl cellulose (a content equivalent to half of the carboxymethyl cellulose content in the final negative electrode slurry) were added to NMP as a solvent and stirred to prepare a negative electrode active material dispersion. In the dispersion, the Si particles were included in an amount of 80 parts by weight and the carboxymethyl cellulose was included in an amount of 3 parts by weight, based on 100 parts by weight of the NMP.

[0125] (3) Preparation of negative electrode slurry composition The conductive material dispersion and the negative electrode active material dispersion were mixed, and then carboxymethyl cellulose, styrene butadiene rubber, and a solvent (water) were added and stirred to prepare a negative electrode slurry composition.

[0126] (4) Manufacturing of negative electrodes The negative electrode slurry composition was applied to a copper (Cu) metal thin film, which was a 20 μm thick negative electrode current collector, in an amount of 300 mg / 25 cm 2The negative electrode current collector was coated with the negative electrode slurry composition and dried at a loading rate of 1000 kJ / cm. The temperature of the circulating air was 70°C. The negative electrode current collector coated with the negative electrode slurry composition and dried was then rolled and dried in a vacuum oven at 130°C for 8 hours to prepare a negative electrode including a negative electrode active material layer. In the negative electrode active material layer, the weight ratio of the negative electrode active material, the carbon nanotube structure, the binder, and the dispersant was 90:0.5:9:0.5. The weight ratio of the SBR and CMC was 60:40.

[0127] Example 2: Preparation of negative electrode When preparing the conductive material dispersion, instead of the bundled carbon nanotubes used in Example 1, a conductive material dispersion having an average diameter of 1.5 nm and an average length of 5 μm was used. G / I D A negative electrode was prepared in the same manner as in Example 1, except that bundled carbon nanotubes consisting of single-walled carbon nanotube units with a carbon content of 100 were used.

[0128] Comparative Example 1: Production of negative electrode When preparing the conductive material dispersion, instead of the bundled carbon nanotubes used in Example 1, a conductive material dispersion having an average diameter of 1.5 nm and an average length of 5 μm was used. G / I D A negative electrode was prepared in the same manner as in Example 1, except that bundled carbon nanotubes composed of single-walled carbon nanotube units having a .DELTA. value of 190 were used.

[0129] Comparative Example 2: Production of negative electrode When preparing the conductive material dispersion, instead of the bundled carbon nanotubes used in Example 1, a conductive material dispersion having an average diameter of 1.5 nm and an average length of 5 μm was used. G / I D A negative electrode was prepared in the same manner as in Example 1, except that bundled carbon nanotubes composed of single-walled carbon nanotube units having a carbon content of 25 were used.

[0130] Comparative Example 3: Production of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the pressure of the high-pressure homogenizer was changed from 1500 Bar to 2000 Bar when preparing the conductive material dispersion.

[0131] Comparative Example 4: Production of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the pressure of the high-pressure homogenizer was changed from 1500 Bar to 300 Bar when preparing the conductive material dispersion.

[0132] Comparative Example 5: Production of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the number of times the high-pressure homogenizer was applied during the preparation of the conductive material dispersion was changed from 5 times to 13 times.

[0133] Comparative Example 6: Production of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the number of times the high-pressure homogenizer was used during the preparation of the conductive material dispersion was changed from five times to one time.

[0134] Comparative Example 7: Production of negative electrode A negative electrode was produced in the same manner as in Example 1, except that a negative electrode active material dispersion was not prepared during the preparation of the negative electrode slurry composition, and the negative electrode active material was mixed in a powder state with a conductive material dispersion.

[0135] Experimental Example 1 The surface of the negative electrode active material layer of each of the negative electrodes prepared in Examples 1 and 2 and Comparative Examples 1 to 7 was observed using a scanning electron microscope to measure the average A value. Specifically, the surface of the negative electrode active material layer of each negative electrode was measured using a scanning electron microscope (SEM) at 20,000 magnification to obtain an SEM photograph. Using the obtained SEM photograph, 60 carbon nanotube structures with an average length of 2 μm to 20 μm were selected. The linear distance P between the two ends of each selected carbon nanotube structure was then determined by SEM image analysis, and the total length Q was substituted into Equation 1 to measure the A value. The average A value was then calculated by arithmetically averaging the A values ​​of the remaining 50 A values, excluding the top 5 and bottom 5 A values. The measurement results are shown in Table 1 below.

[0136] 2 to 5 show SEM photographs obtained by observing the surface of the negative electrode active material layer of Example 1 with a scanning electron microscope.

[0137] Experimental Example 2: Evaluation of negative electrode adhesive strength A negative electrode punched to a width of 20 mm and a length of 15 cm was attached to a glass slide using double-sided tape and pressed with a predetermined pressure. A 90° peel test was then performed to measure the negative electrode adhesive strength (unit: gf / 20 mm). The results are shown in Table 1.

[0138] Experimental Example 3: Capacity Protection Ownership evaluation Using the negative electrodes of the examples and comparative examples, batteries were fabricated as follows.

[0139] As the positive electrode active material, Li[Ni 0.6 Mn 0.2 Co 0.2 The positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 94:4:2 with N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode slurry.

[0140] The prepared positive electrode slurry was applied to a 15 μm thick aluminum metal thin film, which was a positive electrode current collector, and dried under circulating air at 110° C. The coated layer was then rolled and dried in a vacuum oven at 130° C. for 2 hours to form a positive electrode active material layer.

[0141] The negative electrodes of the Examples and Comparative Examples, the positive electrodes prepared above, and porous polyethylene separators were stacked together, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio)) and lithium hexafluorophosphate (LiPF 61 mol) were injected into the assembled batteries to prepare lithium secondary batteries.

[0142] Each lithium secondary battery was charged and discharged under the following conditions.

[0143] Charging conditions: 0.5C constant current charging up to 4.25V, then constant voltage charging at 4.2V until the current rate drops to 0.1C Discharge conditions: Constant current discharge at a current rate of 0.5C to 2.8V

[0144] The above charge and discharge cycle was counted as one cycle, and the battery was subjected to 200 cycles at 45° C. Next, the discharge capacity retention rate after 200 cycles relative to the discharge capacity after one cycle (100%) was measured and shown in Table 1.

[0145] [Table 1]

[0146] Referring to Table 1, it can be seen that the negative electrodes of Examples 1 and 2, which have an average A value of 70 to 100, have better electrode adhesion than the negative electrodes of Comparative Examples 1 to 7, which have an average A value of less than 70, and exhibit better life characteristics when applied to batteries.

Claims

1. a negative electrode active material layer including a negative electrode active material and a conductive material, the negative electrode active material includes a silicon-based negative electrode active material, the conductive material includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded to each other in parallel, The carbon nanotube structure has an average length of 2 μm to 15 μm, The carbon nanotube structure has an average diameter of 15 nm to 45 nm, The negative electrode has a carbon nanotube structure having an average length of 2 μm to 15 μm, which is observed when the surface of the negative electrode active material layer is observed through a scanning electron microscope (SEM) at 20,000 magnifications. Sixty carbon nanotube structures are selected from these carbon nanotube structures, and an A value defined by the following [Equation 1] is measured for each of the selected carbon nanotube structures. The A value is then calculated by arithmetically averaging the remaining 50 A values ​​other than the top 5 and bottom 5 A values, resulting in an average A value of 70 to 100. [Formula 1] A={linear distance P between both ends of the carbon nanotube structure / total length Q of the carbon nanotube structure}×100

2. 2. The negative electrode according to claim 1, wherein the average A value is 90 to 100.

3. The average particle size D of the silicon-based negative electrode active material ５０ The negative electrode according to claim 1, wherein the thickness is 1 μm to 15 μm.

4. 2. The negative electrode according to claim 1, wherein the carbon nanotube structure has an average diameter of 15 nm to 28 nm.

5. 2. The negative electrode according to claim 1, wherein the carbon nanotube structure is contained in the negative electrode active material layer in an amount of 0.01% by weight to 5% by weight.

6. The single-walled carbon nanotube unit was measured by Raman spectroscopy. Ｇ / I Ｄ 2. The negative electrode according to claim 1, wherein the .sigma.

7. The negative electrode active material layer further contains a dispersant, The negative electrode according to claim 1 , wherein the dispersant comprises an amine-containing polymer dispersant and a phenolic compound containing two or more aromatic rings.

8. 8. The negative electrode according to claim 7, wherein the amine-containing polymer dispersant and the phenolic compound having two or more aromatic rings are contained in a weight ratio of 5:1 to 1:

1.

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

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