Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode and a secondary battery including the same, wherein the negative electrode includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material and a conductive material, the negative electrode active material includes silicon particles, the conductive material includes single-walled carbon nanotubes and graphite-based particles, the graphite-based particles include first artificial graphite and second artificial graphite, the first artificial graphite is plate-shaped artificial graphite, and the second artificial graphite can be artificial graphite including a secondary particle structure in which a plurality of primary particles are bonded to each other via amorphous carbon.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0124080, filed on September 16, 2021, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a negative electrode including a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material and a conductive material, the negative electrode active material including silicon particles, the conductive material including single-walled carbon nanotubes and graphite-based particles, the graphite-based particles including first artificial graphite and second artificial graphite, the first artificial graphite being plate-like artificial graphite and the second artificial graphite being artificial graphite including a secondary particle structure in which a plurality of primary particles are bonded to each other via amorphous carbon, 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 been increasing rapidly, and accordingly, various researches have been conducted on batteries that can meet various needs. In particular, researches on lithium secondary batteries that have high energy density and excellent cycle characteristics as power sources for such devices have been actively conducted.

[0004] The lithium secondary battery refers to a battery that includes a non-aqueous electrolyte containing lithium ions in an electrode assembly including a positive electrode including a positive electrode active material capable of inserting / extracting lithium ions, a negative electrode including a negative electrode active material capable of inserting / extracting lithium ions, and a microporous separator interposed between the positive electrode and the negative electrode.

[0005] The negative electrode active material may be lithium metal, lithium alloy, crystalline or amorphous carbon, carbon composite, silicon-based active material, etc. Among these, the silicon-based active material is used alone or in combination with other negative electrode active materials to improve the capacity of the secondary battery.

[0006] In particular, silicon particles (pure silicon) among the silicon-based active materials have a very high capacity, and when used as an anode active material, they can significantly improve the capacity characteristics of secondary batteries. However, when silicon particles are used as an anode active material, the silicon particles in the upper portion of the anode (the region farthest from the current collector) are primarily responsible for battery operation, while the silicon particles in the lower portion of the anode (the region closest to the current collector) have relatively little effect on battery operation. This uneven reaction between the upper and lower portions of the anode can lead to problems such as insufficient battery capacity as planned, and rapid degradation of the silicon particles in the upper portion can result in low battery capacity retention.

[0007] On the other hand, conductive materials are used to improve conductivity within the anode. When small spherical conductive materials such as Super C are used as a conductive material in anodes that use silicon particles, the conductive material clogs the pores within the anode, reducing the directness of the lithium ion diffusion path. As a result, the silicon particles at the top of the anode react primarily, while the silicon particles at the bottom of the anode have relatively little effect on battery operation, further increasing the heterogeneous reaction. This results in problems such as a deterioration in the battery's capacity retention rate and fast charging performance. Summary of the Invention [Problem to be solved by the invention]

[0008] One problem to be solved by the present invention is to provide an anode that can minimize non-uniform reactions of silicon particles in an anode, thereby improving the capacity retention rate and fast charging performance of the battery, and a secondary battery including the same.

[0009] Another problem to be solved by the present invention is to improve the conductivity in the negative electrode and increase the amount of negative electrode active material participating in the charging and discharging processes of the battery, thereby improving the capacity of the battery. [Means for solving the problem]

[0010] According to one embodiment of the present invention, there is provided a negative electrode including a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material and a conductive material, the negative electrode active material including silicon particles, the conductive material including single-walled carbon nanotubes and graphite-based particles, the graphite-based particles including first artificial graphite and second artificial graphite, the first artificial graphite being plate-like artificial graphite, and the second artificial graphite being artificial graphite including a secondary particle structure in which a plurality of primary particles are bonded to each other via amorphous carbon.

[0011] According to another embodiment of the present invention, there is provided a secondary battery including the negative electrode. [Effects of the Invention]

[0012] According to the present invention, single-walled carbon nanotubes and graphite-based particles are used as the conductive material. The graphite-based particles include plate-shaped artificial graphite and artificial graphite with a secondary particle structure in which a plurality of primary particles are bonded to each other. This ensures a long, straight diffusion path for lithium ions within the anode, allowing for more uniform diffusion of lithium ions within the anode. This reduces the difference in the degree of deterioration between the upper and lower portions of the anode, improving the cycle characteristics and fast charging performance of the battery. Furthermore, the use of the conductive material ensures smooth conductive paths within the anode, increasing the amount of anode active material participating in the charge and discharge processes of the battery, thereby improving battery capacity. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in more detail below to facilitate understanding of the present invention. The terms and words used in the specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms to best describe their inventions.

[0014] In this specification, the average particle size (D 50The 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.

[0015] 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.

[0016] As used herein, a single-walled carbon nanotube is a tubular carbon allotrope with a single wall made up of carbon atoms.

[0017] <Negative electrode> An anode according to one embodiment of the present invention includes a negative electrode active material layer, the negative electrode active material layer including a negative electrode active material and a conductive material, the negative electrode active material including silicon particles, the conductive material including single-walled carbon nanotubes and graphite-based particles, the graphite-based particles including first artificial graphite and second artificial graphite, the first artificial graphite being plate-like artificial graphite, and the second artificial graphite being artificial graphite including a secondary particle structure in which a plurality of primary particles are bonded to each other via amorphous carbon.

[0018] The negative electrode active material layer may be disposed on a current collector, or alternatively, the negative electrode active material layer may be a free-standing negative electrode without a current collector.

[0019] 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, can be used as the current collector. The thickness of the current collector may be 6 μm to 20 μm, but is not limited thereto.

[0020] The negative electrode active material layer may include a negative electrode active material and a conductive material.

[0021] The negative electrode active material may include silicon particles. The silicon particles may be composed entirely of silicon (Si), specifically, pure silicon. The silicon particles have a higher capacity than other negative electrode active materials, such as carbon-based active material particles, SiO, and Si / C. Therefore, when the silicon particles are used as the negative electrode active material, the capacity of the battery can be significantly improved.

[0022] The silicon particles may be contained in the negative electrode in an amount of 50 to 90% by weight, specifically 70 to 90% by weight. When this range is satisfied, the cycle characteristics (capacity retention rate) and rapid charging performance of the battery are maintained, and the energy density per unit volume of the battery can be improved.

[0023] The average particle size (D 50) can be 0.1 μm to 100 μm, specifically 1 μm to 10 μm, and more specifically 4 μm to 6 μm. When silicon particles having this average particle size are used, the phenomenon of the silicon particles being crushed during the charging and discharging process of the battery can be minimized. In addition, since the silicon particles have an appropriate contact area with the plate-shaped artificial graphite, contact can be maintained even with large volume changes of the silicon particles during battery operation, allowing a larger amount of silicon particles to participate in the electrochemical reaction. This can improve the battery capacity. In addition, when the above range is satisfied, damage to the single-walled carbon nanotubes during battery operation can be minimized and the single-walled carbon nanotubes can be in sufficient contact with the surfaces of the silicon particles.

[0024] The conductive material may include single-walled carbon nanotubes and graphite-based particles.

[0025] The single-walled carbon nanotubes provide electrical conductivity to the negative electrode, allowing the battery to charge and discharge efficiently.

[0026] The average length of the single-walled carbon nanotubes may be 1 μm to 1,000 μm, specifically 2 μm to 100 μm, and more specifically 10 μm to 50 μm. When this range is satisfied, a conductive network is effectively formed within the negative electrode, thereby enabling efficient charging and discharging of the battery. The average length of the single-walled carbon nanotubes is determined by cutting a cross section of the negative electrode using an ion milling device or similar device and observing it with a cross-section SEM. Specifically, it can be determined by averaging the lengths of the top 100 longest single-walled carbon nanotubes and the bottom 100 longest single-walled carbon nanotubes in an image magnified 3000 times using the SEM. This allows the average length of the single-walled carbon nanotubes to be determined.

[0027] The average diameter of the single-walled carbon nanotubes may be 0.5 nm to 10 nm, specifically 0.5 nm to 5 nm, and more specifically 1 nm to 2 nm. When the diameter satisfies this range, the single-walled carbon nanotubes have sufficient conductivity and flexibility, thereby improving the conductivity within the negative electrode. The average diameter of the single-walled carbon nanotubes is determined by cutting a cross section of the negative electrode using an ion milling device or similar device and observing the cross section through a SEM. Specifically, it can be determined by averaging the diameters of the top 100 single-walled carbon nanotubes and the bottom 100 single-walled carbon nanotubes in diameter on a 3000x magnification screen through the SEM. This allows the average diameter of the single-walled carbon nanotubes to be determined.

[0028] The single-walled carbon nanotubes may be included in the negative electrode at 0.01 wt % to 10 wt %, specifically 0.1 wt % to 1 wt %, more specifically 0.2 wt % to 0.5 wt %. When the single-walled carbon nanotubes are included in the negative electrode at this range, side reactions caused by the single-walled carbon nanotubes can be minimized and aggregation between the single-walled carbon nanotubes can be prevented, thereby improving the cycle characteristics and fast charging performance of the battery.

[0029] The graphite-based particles may include a first artificial graphite and a second artificial graphite.

[0030] The first artificial graphite may be plate-shaped artificial graphite. The term "plate-shaped" refers to a shape having a predetermined thickness and a wide surface perpendicular to the thickness, and the thickness may be 500 nm to 3,000 nm, and the longest length of the wide surface may be 1 μm to 30 μm. Because the plate-shaped artificial graphite is used as the first artificial graphite, conductive contact between the silicon particles and the first artificial graphite can be maintained even if the silicon particles undergo significant volumetric changes and particle crushing during battery operation. This can improve the capacity retention rate of the battery. Furthermore, when the first artificial graphite is used in combination with single-walled carbon nanotubes, the first artificial graphite can provide a surface that can attach to the single-walled carbon nanotubes and form a conductive network in the negative electrode, thereby more effectively forming a conductive network in the negative electrode.

[0031] The first artificial graphite may be in a single particle form. The term "single particle form" means that the small particles are not intentionally bonded together to form a single large secondary particle, but exist in the negative electrode as a single small particle. The single particle form of the first artificial graphite means that a large surface area of ​​plate-like artificial graphite is secured. Therefore, the silicon particles and the first artificial graphite can be attached with a large contact area, and the contact between the silicon particles and the first artificial graphite can be effectively maintained.

[0032] The average longest length of the first artificial graphite may be 1 μm to 20 μm, specifically 3 μm to 10 μm, and more specifically 4 μm to 6 μm. When this range is satisfied, the first artificial graphite can wrap the silicon particles at an appropriate level, thereby improving the charge and discharge performance of the battery. The "longest length" refers to the longest length when a line is assumed to connect one point on the first artificial graphite to another. The average longest length of the first artificial graphite can be confirmed by cutting a cross section of the negative electrode using a device such as ion milling and then examining the cross section using a SEM. Specifically, it can be determined by averaging the longest lengths of the top 100 first artificial graphite particles and the bottom 100 first artificial graphite particles in terms of particle size on a 3000x magnification screen using an SEM. This allows the average longest length of the first artificial graphite to be confirmed.

[0033] The specific surface area of ​​the first artificial graphite is 10 m 2 / g~60m 2 / g, specifically 15m 2 / g~30m 2 / g, specifically 17m 2 / g~18m 2 When this range is satisfied, side reactions caused by the first artificial graphite can be minimized and the silicon particles can be smoothly contacted with the first artificial graphite, thereby improving the cycle characteristics of the battery.

[0034] The first artificial graphite may be contained in the negative electrode in an amount of 1 wt % to 50 wt %, specifically 5 wt % to 30 wt %, more specifically 10 wt % to 15 wt %. When this range is satisfied, the capacity of the negative electrode is ensured and the negative electrode conductivity is improved, thereby improving the energy density and capacity retention rate of the battery.

[0035] The second artificial graphite may have a secondary particle structure in which a plurality of primary particles are bonded to each other via amorphous carbon. Specifically, the second artificial graphite may have a spherical secondary particle structure in which a plurality of artificial graphite particles are bonded to each other as primary particles. The high electrical conductivity of the amorphous carbon in the second artificial graphite facilitates electrical contact between the single-walled carbon nanotubes and the second artificial graphite, improving the conductivity within the anode and improving the fast charging performance of the battery. Furthermore, the second artificial graphite effectively maintains its internal pore structure even through the rolling process performed during the manufacture of the anode. This ensures long, linear lithium ion diffusion paths within the anode, allowing for more uniform diffusion of lithium ions within the anode. This reduces the difference in the degree of deterioration between the upper and lower portions of the anode, thereby improving the cycle characteristics and fast charging performance of the battery. In addition, since the second artificial graphite is in the form of secondary particles rather than single particles, the irreversible amount of lithium can be reduced during battery operation, thereby lowering the electrical resistance of the negative electrode.

[0036] The average particle size of the second artificial graphite may be 10 μm to 40 μm, specifically 12 μm to 20 μm, and more specifically 14 μm to 19 μm. The average particle size of the second artificial graphite can be determined by cutting a cross section of the negative electrode using an ion milling device or similar device and then examining the cross section using a SEM. Specifically, the average particle size can be determined by averaging the particle sizes of the top 100 and bottom 100 particles of the second artificial graphite in an image magnified 3000 times using an SEM. This allows the average particle size of the second artificial graphite to be determined. When the above range is satisfied, damage to the current collector caused by the second artificial graphite during the rolling process of the negative electrode can be minimized, and the silicon particles and the second artificial graphite can smoothly contact each other within the negative electrode, improving the diffusion of lithium ions and improving the charge and discharge performance of the battery.

[0037] The specific surface area of ​​the second artificial graphite is 0.1 m 2 / g~10m2 / g, specifically 0.5m 2 / g~1m 2 / g, more specifically 0.7m 2 / g~0.8m 2 When this range is satisfied, side reactions caused by the second artificial graphite are minimized, thereby improving the capacity of the battery.

[0038] The weight ratio of the first artificial graphite to the second artificial graphite may be 2.5:7.5 to 7.5:2.5, specifically 4:6 to 6:4. When this range is satisfied, the cycle characteristics of the battery can be more effectively improved.

[0039] The graphite-based particles may be included in an amount of 500 to 25,000 parts by weight, specifically 5,000 to 15,000 parts by weight, more specifically 7,500 to 12,500 parts by weight, based on 100 parts by weight of the single-walled carbon nanotubes. When the amount is within this range, a conductive network can be effectively formed in the negative electrode, thereby further improving the charge and discharge performance of the battery.

[0040] The graphite particles may be included in the negative electrode active material layer at 1 wt % to 50 wt %, specifically 10 wt % to 25 wt %, more specifically 15 wt % to 20 wt %. When the content is within this range, the conductivity of the negative electrode active material and the diffusibility of lithium ions can be maintained at a high level, thereby improving the cycle characteristics and fast charging performance of the battery.

[0041] The conductive material may be contained in the negative electrode active material layer in an amount of 1 wt % to 50 wt %, specifically 10 wt % to 30 wt %, more specifically 15 wt % to 25 wt %. When this range is satisfied, the capacity of the negative electrode can be maintained at a high level and the life of the negative electrode can be improved.

[0042] The negative electrode active material layer may further include a binder. The binder secures adhesion between electrode active materials or between the electrode active material and the current collector. A binder commonly used in the art may be used, and the type of binder is not particularly limited. Examples of binders 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.

[0043] <Secondary battery> A secondary battery according to another embodiment of the present invention may include the negative electrode of the above-described embodiment. Specifically, the secondary battery may be a lithium secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator, and the negative electrode may be the same as the negative electrode of the above-described embodiment.

[0044] The secondary battery can be manufactured by a conventional method known in the art, for example, by inserting a separator between a positive electrode and a negative electrode and then introducing an electrolyte solution containing a lithium salt dissolved therein.

[0045] The positive electrode may include a positive electrode active material. The positive electrode active material may preferably be a lithium transition metal oxide, for example, Li x1 CoO2(0.5 <x1<1.3)、Li x2 NiO2(0.5 <x2<1.3)、Li x3 MnO2(0.5 <x3<1.3)、Li x4Mn2O4(0.5 < x4 < 1.3), Li x5 (Ni a1 Co b1 Mn c1 )O2(0.5 < x5 < 1.3, 0 < a1 < 1, 0 < b1 < 1, 0 < c1 < 1, a1 + b1 + c1 = 1), Li x6 Ni 1-y1 Co y1 O2(0.5 < x6 < 1.3, 0 < y1 < 1), Li x7 Co 1-y2 Mn y2 O2(0.5 < x7 < 1.3, 0 ≦ y2 < 1), Li x8 Ni 1-y3 Mn y3 O2(0.5 < x8 < 1.3, O ≦ y3 < 1), Li x9 (Ni a2 Co b2 Mn c2 )O4(0.5 < x9 < 1.3, 0 < a2 < 2, 0 < b2 < 2, 0 < c2 < 2, a2 + b2 + c2 = 2), Li x10 Mn 2-z1 Ni z1 O4(0.5 < x10 < 1.3, 0 < z1 < 2), Li x11 Mn 2-z2 Co z2 O4(0.5 < x11 < 1.3, 0 < z2 < 2), Li x12 CoPO4(0.5 < x12 < 1.3) and Li x13 FePO4(0.5 < x13 < 1.3) can be one or more mixtures selected from the group consisting of.

[0046] As the separator included in the lithium secondary battery according to the present invention, a normal porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer can be used alone or by laminating these, or a normal porous non-woven fabric, for example, a non-woven fabric made of high melting point glass fibers, polyethylene terephthalate fibers, etc. can be used, but it is not limited thereto.

[0047] The electrolyte solution contained in the lithium secondary battery according to the present invention may be one or more mixed organic solvents selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma-butyrolactone (GBL), fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, pentyl acetate, methyl propionate, ethyl propionate, ethyl propionate, and butyl propionate.

[0048] In addition, the electrolyte solution according to the present invention may further contain a lithium salt, and the anion of the lithium salt may be F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , F3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C-, (CF3SO2)3C-, CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , and (CF3CF2SO2)2N - It can be one or more selected from the group consisting of:

[0049] The lithium secondary battery according to the present invention may be a cylindrical, square, or pouch-type secondary battery, but is not limited thereto as long as it is a charge / discharge device.

[0050] The present invention also provides a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0051] The battery pack can be used as a power source for one or more medium- to large-sized devices selected from the group consisting of power tools; electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0052] Although the present invention may be embodied in various different forms, it is not intended to be limited to the embodiments set forth herein, and the present invention may be practiced in various different forms, without departing from the spirit or scope of the present invention.

[0053] The following conductive materials were prepared:

[0054] A-1: Single-wall carbon nanotube A-2: Multi-wall carbon nanotubes B-1: Plate-shaped artificial graphite (specific surface area: 18 m 2 / g) B-2: Spherical artificial graphite (D 50 :11μm, specific surface area: 14m 2 / g) C-1: Secondary particulate artificial graphite (specific surface area: 0.7 m) in which multiple primary particles of artificial graphite are bonded together via amorphous carbon. 2 / g) C-2: Single particle spherical artificial graphite (specific surface area: 10 m 2 / g)

[0055] Example 1: Production of negative electrode Average particle size (D 50 A negative electrode slurry was prepared containing silicon particles (pure silicon) with a particle size of 5 μm, single-walled carbon nanotubes (A-1), plate-shaped artificial graphite (first artificial graphite, B-1), secondary particle-like artificial graphite in which multiple primary particles of artificial graphite are bonded via amorphous carbon (second artificial graphite, C-1), a polyacrylamide polymer as a binder, and water as a solvent.

[0056] The negative electrode slurry was applied to a copper (Cu) metal thin film, which was a negative electrode current collector, with a thickness of 20 μm, and dried. The temperature of the circulating air was 60°C. The negative electrode was then rolled and dried in a vacuum oven at 130°C for 12 hours to prepare a negative electrode (loading: 8.55 mAh / cm). 2 ).

[0057] In the manufactured negative electrode, the weight ratio of the silicon particles, the single-walled carbon nanotubes, the first artificial graphite, the second artificial graphite, and the binder was 70:0.21:10:10:9.79.

[0058] [Examples 2 to 7 and Comparative Examples 1 to 5: Production of Negative Electrodes] A negative electrode was produced in the same manner as in Example 1, except that the composition was changed as shown in Table 1 below.

[0059] [Table 1]

[0060] The average length of the single-walled carbon nanotubes was 10 μm, and the average diameter was 2 nm. The average length and average diameter were determined by cutting the anode using an ion milling device or similar device and then examining the cross-section using a SEM. Specifically, the average length of the single-walled carbon nanotubes was determined by averaging the lengths of the top 100 longest single-walled carbon nanotubes and the bottom 100 longest single-walled carbon nanotubes in an image magnified 3000 times using the SEM. The average diameter was also determined using the same method. The average longest length of the plate-shaped artificial graphite (B-1) was 6 μm. The average particle size of the secondary particle-like artificial graphite (C-1), in which multiple artificial graphite primary particles were bonded via amorphous carbon, was 18 μm.

[0061] The average particle size of the single-particle spherical artificial graphite (C-2) was 9 μm.

[0062] The average longest length of the plate-like artificial graphite (B-1), the average particle size of the secondary particle-like artificial graphite (C-1) in which a plurality of primary particles of artificial graphite are bonded via amorphous carbon, and the average particle size of the single particle-like spherical artificial graphite (C-2) were determined by the same method as used to determine the average length of the single-walled carbon nanotubes.

[0063] [Experimental Example 1: Evaluation of capacity retention rate] The cycle characteristics of the negative electrodes of Examples 1 to 7 and Comparative Examples 1 to 5 were evaluated as follows, and the results are shown in Table 2.

[0064] (1) Coin cell manufacturing In NMP, Li[Ni 0.8 Mn 0.1 Co 0.1 ]O2, PVdF as a binder, and Super P as a conductive material were added and stirred to prepare a positive electrode slurry, which was then coated onto an aluminum current collector to form a positive electrode (loading: 4.5 mAh / cm 2 ) The positive electrode is placed at 1.76715 cm 2 It was cut into circles.

[0065] A battery was assembled by interposing a porous polyethylene separator between the positive electrode and the prepared negative electrode, and an electrolyte (dimethyl carbonate (DMC) / fluoroethylene carbonate (FEC) = 7 / 3 (volume ratio), 3 wt% vinylene carbonate, and 1M concentration lithium hexafluorophosphate (LiPF6)) was injected into the assembled battery to prepare a lithium coin cell.

[0066] (2) Evaluation Each coin cell was charged and discharged under the following conditions.

[0067] 1 cycle: Charging was performed at a constant current of 0.1 C, and when the voltage reached 0.005 C and 0.05 V, constant voltage charging (0.05 V) was performed. Discharging was performed at a constant current (0.1 C) until the voltage reached 1.5 V. 2 cycles: Charging was performed at a constant current of 0.1 C, and when the voltage reached 0.005 C and 0.05 V, constant voltage charging (0.05 V) was performed. Discharging was performed at a constant current (0.1 C) until the voltage reached 1.0 V. 3 to 200 cycles: Charging was performed at a constant current of 0.5 C, and when the voltage reached 0.005 C and 0.05 V, constant voltage charging (0.05 V) was performed. Discharging was performed at a constant current (0.5 C) until the voltage reached 1.0 V.

[0068] The capacity retention rates were calculated as follows:

[0069] Capacity retention rate (%) = (200th discharge capacity / 1st discharge capacity) x 100

[0070] [Experimental Example 2: Evaluation of rapid charging performance] (1) Manufacturing of coin half cells 1.76715cm 2A lithium metal thin film cut into a circle of the specified size was used as the positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode of the Example or Comparative Example to assemble a battery. An electrolyte (dimethyl carbonate (DMC) / fluoroethylene carbonate (FEC) = 7 / 3 (volume ratio), 3 wt% vinylene carbonate, and 1M lithium hexafluorophosphate (LiPF6) concentration) was injected into the assembled battery to prepare a lithium coin half-cell.

[0071] (2) Evaluation Each coin half-cell was charged and discharged under the following conditions.

[0072] To activate the coin half-cell, three charge-discharge cycles were performed. Specifically, charging was performed at a constant current of 0.1 C, and when the voltage reached 0.005 C and 0.05 V, a constant voltage charge (0.05 V) was performed. Discharging was performed at a constant current (0.1 C) down to 1.0 V. The porosity of the negative electrode used was 40%, and the loading capacity was 8.55 mAh / cm. 2 The evaluation was carried out at room temperature (25°C). Next, the battery was charged at a constant current of 6C, and the SOC corresponding to the discharge capacity at 6C for three cycles was used as the X-axis variable, and the voltage was used as the Y-axis variable. The graph was then differentiated twice on the X-axis to confirm the inflection point, which was evaluated as the lithium deposition SOC.

[0073] [Table 2]

[0074] It can be seen that Comparative Example 1, which did not use the second artificial graphite, and Comparative Example 2, which did not use the first artificial graphite, were inferior in both capacity retention and fast charge performance compared to Example 1, which used both the first and second artificial graphite. Also, it can be seen that Comparative Example 3, which used multi-walled carbon nanotubes instead of single-walled carbon nanotubes, was inferior in both capacity retention and fast charge performance.

[0075] Furthermore, in Comparative Example 4, in which spherical artificial graphite was used as the first artificial graphite instead of plate-like artificial graphite, and in Comparative Example 5, in which single-particle artificial graphite was used as the second artificial graphite instead of secondary particle artificial graphite, both the capacity retention rate and the fast charging performance were found to be inferior.

Claims

1. a negative electrode active material layer, the negative electrode active material layer contains a negative electrode active material and a conductive material, the negative electrode active material contains silicon particles, the conductive material includes single-walled carbon nanotubes and graphite-based particles; The graphite-based particles include a first artificial graphite and a second artificial graphite, The first artificial graphite is plate-shaped artificial graphite, the second artificial graphite is artificial graphite having a secondary particle structure in which a plurality of primary particles are bonded to each other via amorphous carbon, The silicon particles have an average particle size (D 50 ) of 0.1 μm to 100 μm; The average longest length of the first artificial graphite is 1 μm to 20 μm, The second artificial graphite has an average particle size (D 50 ) of 10 μm to 40 μm; The average length of the single-walled carbon nanotubes is 1 μm to 1,000 μm; The single-walled carbon nanotubes have an average diameter of 0.5 nm to 10 nm.

2. 2. The negative electrode according to claim 1, wherein the silicon particles are contained in the negative electrode active material layer in an amount of 50% by weight to 90% by weight.

3. The negative electrode according to claim 1 , wherein the first artificial graphite is in the form of a single particle.

4. The specific surface area of ​​the first artificial graphite is 10 m ２ / g~60m ２ The negative electrode according to claim 1 , wherein the anode has a Cr content of 1.0 / g.

5. The specific surface area of ​​the second artificial graphite is 0.1 m ２ / g to 10m ２ The negative electrode according to claim 1 , wherein the anode has a Cr content of 1.0 / g.

6. 2. The negative electrode according to claim 1, wherein a weight ratio of the first artificial graphite to the second artificial graphite is 2.5:7.5 to 7.5:2.

5.

7. 2. The negative electrode according to claim 1, wherein the graphite-based particles are contained in an amount of 500 to 25,000 parts by weight based on 100 parts by weight of the single-walled carbon nanotubes.

8. 2. The negative electrode according to claim 1, wherein the graphite-based particles are contained in the negative electrode active material layer in an amount of 1% by weight to 50% by weight.

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

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