Negative electrode for secondary battery and secondary battery comprising same

Abstract

The present invention provides a negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, the negative electrode active material layer comprising a silicon-based active material and a conductive material comprising single-walled carbon nanotubes, wherein the Raman R value (D-band peak intensity (Id) / G-band peak intensity (Ig)) of the single-walled carbon nanotubes is 0.01 to 0.1.

Description

[0001] This application is a divisional application of a Chinese patent application filed on October 14, 2021, with Chinese patent application number 202111199312.6 and invention name “Negative electrode for secondary battery and secondary battery including the same”, and this application claims priority to the Korean application with application number 10-2020-0133786. Technical Field

[0002] The present invention relates to a negative electrode for a secondary battery and a secondary battery including the negative electrode, and more particularly, to a negative electrode for a secondary battery including a silicon-based negative electrode active material and a secondary battery including the negative electrode. Background Art

[0003] Secondary batteries are batteries that can be repeatedly charged and discharged. With the development of information communication and display industries, secondary batteries are widely used in portable electronic communication devices such as portable cameras, mobile phones and laptops. Examples of secondary batteries can include lithium secondary batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc. Among them, the operating voltage and energy density per unit weight of lithium secondary batteries are high, and they are beneficial in terms of charging speed and light weight. In this regard, lithium secondary batteries have been actively developed and used as power sources.

[0004] The lithium secondary battery may include an electrode assembly including a positive electrode, a negative electrode and a separator; and an electrolyte in which the electrode assembly is immersed. The lithium secondary battery may further include a case in a pouch shape, for example, in which the electrode assembly and the electrolyte are accommodated.

[0005] Recently, as the application fields of lithium secondary batteries have expanded from small electronic devices to large devices such as hybrid vehicles, sufficient capacity characteristics and output characteristics may not be achieved by existing lithium secondary batteries.

[0006] For example, in the case of an electric vehicle (EV) driven only by a battery, existing secondary batteries have limitations in ensuring a sufficient driving time due to a large power consumption rate and power consumption.

[0007] Therefore, there is a need to develop a lithium secondary battery that can ensure high rate characteristics and high capacity characteristics.

[0008] For example, Korean Laid-Open Patent No. 2017-0099748 discloses an electrode assembly for a lithium secondary battery and a lithium secondary battery including the same, but there are limitations in ensuring sufficient high-rate characteristics and high-capacity characteristics.

[0009] [Prior art literature]

[0010] [Patent Document]

[0011] Korean Patent Publication No. 10-2017-0099748 Summary of the invention

[0012] An object of the present invention is to provide a negative electrode for a secondary battery having stable electrical characteristics.

[0013] Another object of the present invention is to provide a secondary battery including a negative electrode having stable electrical characteristics.

[0014] In order to achieve the above-mentioned purpose, according to one aspect of the present invention, there is provided a negative electrode for a secondary battery, which comprises: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, the negative electrode active material layer comprising a silicon-based active material and a conductive material comprising single-walled carbon nanotubes, wherein the Raman R value (D band peak intensity (Id) / G band peak intensity (Ig)) of the single-walled carbon nanotubes is 0.01 to 0.1.

[0015] In an exemplary embodiment, the content of the silicon-based active material may be 5 wt % or more based on the total weight of the negative active material layer.

[0016] In exemplary embodiments, the single-walled carbon nanotube may be included in an amount of 0.02 wt % to 0.2 wt % based on the total weight of the negative active material layer.

[0017] In an exemplary embodiment, the length of the single-walled carbon nanotubes may be 5 μm or more.

[0018] In an exemplary embodiment, the diameter of the single-walled carbon nanotube may be 1.2 nm to 2 nm.

[0019] In an exemplary embodiment, the negative electrode active material layer may include: a first negative electrode active material layer formed on the negative electrode current collector, the first negative electrode active material layer including a first silicon-based active material and a first conductive material including single-walled carbon nanotubes; and a second negative electrode active material layer formed on the first negative electrode active material layer, the second negative electrode active material layer including a second silicon-based active material and a second conductive material including multi-walled carbon nanotubes.

[0020] In exemplary embodiments, a content of the first silicon-based active material based on the total weight of the first negative active material layer may be greater than a content of the second silicon-based active material based on the total weight of the second negative active material layer.

[0021] In an exemplary embodiment, the content of the first silicon-based active material may be 5 wt % or more based on the total weight of the first negative active material layer, and the content of the second silicon-based active material may be less than 5 wt % based on the total weight of the second negative active material layer.

[0022] In exemplary embodiments, the single-walled carbon nanotubes may be contained in an amount of 0.02 to 0.2 wt % based on the total weight of the first negative active material layer, and the multi-walled carbon nanotubes may be contained in an amount of 0.2 to 0.5 wt % based on the total weight of the second negative active material layer.

[0023] According to another aspect of the present invention, there is provided a secondary battery including: a negative electrode for a secondary battery; a positive electrode; and a separator disposed between the negative electrode and the positive electrode.

[0024] According to one embodiment of the present invention, the negative electrode active material layer may include a silicon-based active material and a single-walled carbon nanotube having a Raman R value within a specific range. In this case, if an electrical short circuit occurs in the negative electrode due to expansion of the negative electrode during charging and discharging of the battery due to the inclusion of the silicon-based active material, the single-walled carbon nanotube may minimize the increase in resistance due to the electrical short circuit. Therefore, the life characteristics of the secondary battery may be improved by effectively preventing heat generation due to the increase in resistance.

[0025] According to one embodiment of the present invention, the negative electrode may include a first negative electrode active material layer containing single-walled carbon nanotubes and a second negative electrode active material layer containing multi-walled carbon nanotubes. In this case, the first negative electrode active material layer containing single-walled carbon nanotubes can effectively prevent the increase of negative electrode resistance, and the second negative electrode active material layer containing multi-walled carbon nanotubes can reduce interface resistance. Thus, the electrochemical safety of the secondary battery can be further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description in conjunction with the accompanying drawings, in which:

[0027] Figure 1 A schematic cross-sectional view showing a secondary battery according to one embodiment of the present invention; Figure 2 is a schematic cross-sectional view showing a negative electrode for a secondary battery according to another embodiment of the present invention. DETAILED DESCRIPTION

[0028] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, these embodiments are merely examples, and the present invention is not limited to the specific embodiments described as examples.

[0029] Figure 1 is a schematic cross-sectional view showing a secondary battery according to one embodiment of the present invention, Figure 2 is a schematic cross-sectional view showing a negative electrode for a secondary battery according to another embodiment of the present invention.

[0030] Reference Figure 1 The lithium secondary battery 10 may include an electrode assembly 150 and a case 160 accommodating the electrode assembly 150 .

[0031] The electrode assembly 150 may include a positive electrode 100 , a negative electrode 130 , and a separator 140 interposed between the positive electrode 100 and the negative electrode 130 .

[0032] The positive electrode 100 may include a positive electrode collector 105 and a positive electrode active material layer 110 disposed on at least one surface of the positive electrode collector 105 and including a positive electrode active material.

[0033] The positive active material may include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0034] In one embodiment, the positive active material layer 110 may be formed on both surfaces (eg, upper and lower surfaces) of the positive current collector 105. For example, the positive active material layer 110 may be coated on the upper and lower surfaces of the positive current collector 105, respectively, and may be directly coated on the surface of the positive current collector 105.

[0035] The positive electrode 100 may be prepared by coating the positive current collector 105 with a positive electrode slurry, followed by drying and rolling (or pressing) the same. The positive electrode slurry may be prepared by mixing a positive active material with a binder, a conductive material and / or a dispersant in a solvent, and then stirring them.

[0036] The positive electrode current collector 105 may include a metal substance that is non-reactive within the charge / discharge voltage range of the secondary battery 10 and is conducive to the coating and adhesion of the electrode active material. For example, the positive electrode current collector 105 may include stainless steel, nickel, aluminum, titanium, copper, zinc, or alloys thereof, and preferably includes aluminum or an aluminum alloy.

[0037] The positive active material layer 110 may include lithium metal oxide as a positive active material. For example, the positive active material may include lithium-transition metal composite oxide particles.

[0038] In some embodiments, the positive active material may include a lithium (Li)-nickel (Ni)-based oxide. For example, the lithium-transition metal composite oxide particles may include nickel, and may further include at least one of cobalt (Co) and manganese (Mn).

[0039] In some embodiments, the positive electrode active material or lithium-transition metal composite oxide particles may further include coating elements or doping elements. For example, the coating elements or doping elements may include Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La or their alloys or their oxides. These elements can be used alone or in combination of two or more of them. The positive electrode active material or lithium-transition metal composite oxide particles are passivated by the coating elements or doping elements, so that the stability and life span against external object penetration can be better improved.

[0040] In some embodiments, the lithium-transition metal composite oxide particles may be represented by Formula 1 below.

[0041] [Formula 1]

[0042] Li x Ni 1-y M y O 2+z

[0043] In Formula 1, x and y may be in the range of 0.9≤x≤1.2, 0≤y≤0.7, and z may be in the range of -0.1≤z≤0.1. M may represent at least one element selected from the group consisting of Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, and Zr.

[0044] For example, nickel (Ni) can be provided as a metal related to the capacity of a lithium secondary battery. The higher the nickel content, the better the capacity and output of the lithium secondary battery. However, if the nickel content is excessively increased, the lifespan may be shortened, and it may be disadvantageous in terms of mechanical stability and electrical stability. For example, cobalt (Co) can be a metal related to the conductivity or resistance of a lithium secondary battery. In one embodiment, M includes manganese (Mn), and Mn can be provided as a metal related to the mechanical stability and electrical stability of a lithium secondary battery. Through the interaction between the above-mentioned nickel, cobalt and manganese, the capacity, output, low resistance and life stability of the positive electrode active material layer 110 can be improved together.

[0045] In some embodiments, the content of nickel in the positive electrode active material may be 80 mol % or more, preferably 85 mol % or more, based on the total molar number of transition metal atoms. Accordingly, a high-capacity, high-output lithium secondary battery may be realized.

[0046] For example, the positive electrode slurry may be prepared by mixing the positive electrode active material with a binder, a conductive material and / or a dispersant in a solvent and then stirring them. The positive electrode slurry may be coated on the positive electrode collector 105 and then dried and pressed to prepare the positive electrode active material layer 110.

[0047] The binder may include, for example, an organic binder such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or a water-based binder such as styrene-butadiene rubber (SBR) which may be used with a thickener such as carboxymethyl cellulose (CMC).

[0048] For example, a PVDF-based binder may be used as the positive electrode binder. In this case, the amount of the binder used to form the positive active material layer 110 may be reduced, thereby increasing the output and capacity of the secondary battery.

[0049] Conductive materials may be included to facilitate electron migration between active material particles. For example, the conductive materials may include carbon-based conductive materials such as graphite, carbon black, graphene, or carbon nanotubes, and / or metal-based conductive materials such as tin, tin oxide, titanium oxide, perovskite such as LaSrCoO3 or LaSrMnO3, and the like.

[0050] In some embodiments, the electrode density of the positive electrode 100 may be 3.0 g / cc (grams per cubic centimeter) to 3.9 g / cc, and preferably 3.2 g / cc to 3.8 g / cc.

[0051] According to the exemplary embodiment, the positive active material layer 110 may have a multi-layer structure.

[0052] Reference Figure 1 and Figure 2 , the negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 formed on at least one surface of the negative electrode current collector 125. According to an exemplary embodiment, the negative electrode active material layer 120 may be formed on both surfaces (e.g., the upper surface and the lower surface) of the negative electrode current collector 125. The negative electrode active material layer 120 may be coated on the upper surface and the lower surface of the negative electrode current collector 125, respectively. For example, the negative electrode active material layer 120 may be in direct contact with the surface of the negative electrode current collector 125.

[0053] The negative electrode current collector 125 may include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and preferably includes copper or a copper alloy.

[0054] In one embodiment, the negative active material layer 120 may include a negative active material capable of intercalating and deintercalating lithium ions, a conductive material, and a binder. The negative active material may include a silicon-based active material, and the conductive material may include single-walled carbon nanotubes.

[0055] In one embodiment, the Raman R value of the single-walled carbon nanotube may be about 0.01 to 0.1. The Raman R value may be defined as the peak intensity ratio (Id / Ig), which is obtained by measuring the G band (about 1580 cm -1 ) and the peak intensity near the D band (about 1350cm -1 ) is represented by the peak intensity (Id) near .

[0056] When the single-walled carbon nanotubes satisfy the above Raman R value range, the negative electrode active material layer 120 including the single-walled carbon nanotubes has fewer defects and high crystallinity in the structure, so that the electrical conductivity can be further improved. In addition, when the single-walled carbon nanotubes satisfy the length and diameter to be described below, the above effect can be further enhanced, thereby realizing a secondary battery with excellent electrochemical performance.

[0057] In some embodiments, the single-walled carbon nanotubes may have a length of 5 μm or more, 20 μm or more, or 50 μm or more. In this case, a conductive material network formed by the single-walled carbon nanotubes can be easily formed, and the increase in resistance caused by electrical short circuits can be more effectively prevented. Thus, a secondary battery with excellent electrochemical performance can be achieved.

[0058] In some embodiments, the single-walled carbon nanotube may have a length of 200 μm or less, or 100 μm or less. When its length meets the above range, the aggregation of the single-walled carbon nanotube in the negative electrode slurry can be prevented, and better dispersing power can be ensured. However, it is not necessary to be limited thereto, and the upper limit of the length of the single-walled carbon nanotube can be appropriately adjusted according to the type of the negative electrode active material, the binder and the dispersion medium, which are mixed during the preparation of the negative electrode slurry.

[0059] In some embodiments, the content of the single-walled carbon nanotubes may be about 0.02 wt % to 0.2 wt % based on the total weight of the negative active material layer 120. More preferably, the content of the single-walled carbon nanotubes may be about 0.05 wt % to 0.15 wt % based on the total weight of the negative active material layer 120.

[0060] For example, when the content of single-walled carbon nanotubes satisfies the above range, an increase in resistance caused by including a large amount of silicon-based active material can be effectively prevented. Therefore, the life characteristics of the secondary battery can be better improved.

[0061] In one embodiment, the diameter of the single-walled carbon nanotubes can be from 0.5 nm to 10 nm, preferably from 1 nm to 5 nm, and more preferably from 1.2 nm to 2 nm. In this case, an increase in resistance caused by including a large amount of silicon-based active material can be more effectively prevented.

[0062] In one embodiment, as the conductive material, a material that is substantially the same as or similar to the material used to form the positive electrode 100 can be further included.

[0063] The silicon-based active material can include SiO x (0 < x < 2) or SiO containing a lithium compound x (0 < x < 2). SiO containing a lithium compound x can be SiO containing lithium silicate x . Lithium silicate can be present in at least a part of the SiO x (0 < x < 2) particles. For example, it can be present inside and / or on the surface of the SiO x (0 < x < 2) particles. In one embodiment, the lithium silicate can include Li2SiO3, Li2Si2O5, Li4SiO4, Li4Si3O8, etc.

[0064] The silicon-based active material can further include a silicon-carbon composite compound, such as silicon carbide (SiC), a silicon oxide-carbon composite, or a silicon-silicon oxide-carbon composite.

[0065] In one embodiment, based on the total weight of the negative electrode active material layer, the content of the silicon-based active material can be 5% by weight or more. For example, based on the total weight of the negative electrode active material layer, the content of the silicon-based active material can be from 5% by weight to 40% by weight. When the content of the silicon-based active material satisfies the above range, the output and capacity of the lithium-ion battery can be further improved, and a high energy cell can be more easily achieved.

[0066] For example, during the charge and discharge process of the battery, due to the repeated deintercalation and intercalation of lithium ions, the expansion of the silicon-based active material increases, causing an electrical short circuit between the negative electrode active materials, which may reduce the stability of the battery. However, in the case of the present invention, the stable conductive material network formed by single-walled carbon nanotubes can effectively reduce the increase in resistance caused by the electrical short circuit, and the life characteristics of the battery can be better improved.

[0067] In one embodiment, the negative electrode active material may further include at least one of a carbon-based active material and a lithium composite active material.

[0068] The carbon-based active material may include a graphite-based active material and a non-graphite-based active material. The graphite-based active material may include at least one of natural graphite and artificial graphite.

[0069] The non-graphite-based active material may include at least one of hard and soft carbon, carbon nanotubes, carbon fibers, coke, and pitch.

[0070] For example, synthetic graphite and hard carbon may include primary particles and / or secondary particles.

[0071] Artificial graphite has relatively superior life characteristics compared to natural graphite, and since natural graphite has a larger specific surface area than artificial graphite, its resistance is relatively low, which may be advantageous in terms of increasing output. For example, artificial graphite and natural graphite may be mixed and used.

[0072] In one embodiment, the carbon-based material may include a non-graphite-based active material or minute carbon particles. The minute carbon particles may include carbon black, conductive carbon black (Super P), etc. The carbon-based material may improve the high rate characteristics of the negative electrode active material.

[0073] In one embodiment, the negative electrode slurry can be prepared by mixing and stirring the negative electrode active material with a binder, a conductive material and / or a dispersant in a solvent. The negative electrode slurry can be applied (coated) to the negative electrode collector 125, and then dried and pressed to prepare the negative electrode active material layer 120.

[0074] A material substantially the same as or similar to that used to form the positive electrode 100 may be used as a binder to form the negative electrode.

[0075] In some embodiments, the binder used to form the negative electrode 130 may include, for example, styrene-butadiene rubber (SBR) or an acrylic binder to maintain consistency with the carbon-based active material, and may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0076] In one embodiment, the density of the negative active material layer 120 may be 1.4 g / cc to 1.9 g / cc.

[0077] In some embodiments, the negative electrode 130 may have a larger area (e.g., contact area with the separator 140) and / or volume than the positive electrode 100. Therefore, lithium ions generated from the positive electrode 100 can smoothly move to the negative electrode 130 without precipitation in the middle, thereby further improving the output and capacity characteristics.

[0078] Reference Figure 2 , the negative electrode active material layer 120 may have a multi-layer structure.

[0079] In one embodiment, the negative electrode active material layer 120 may include a first negative electrode active material layer 122 formed on the negative electrode current collector 125 and including a first silicon-based active material and a first conductive material including single-walled carbon nanotubes, and a second negative electrode active material layer 124 formed on the first negative electrode active material layer 122 and including a second silicon-based active material and a second conductive material including multi-walled carbon nanotubes.

[0080] For example, when the negative electrode active material layer 120 includes a first negative electrode active material layer 122 and a second negative electrode active material layer 124 including different types of carbon nanotubes, the resistance of the negative electrode 130 can be effectively prevented from increasing due to repeated charging by the single-walled carbon nanotubes included in the first negative electrode active material layer 122, and the interface resistance between the separator 140 and the negative electrode 130 can be reduced by the multi-walled carbon nanotubes included in the second negative electrode active material layer 124. Therefore, the internal resistance of the secondary battery can be reduced, so that the fast charging characteristics and stability can be further improved.

[0081] In some exemplary embodiments, the content of the first silicon-based active material based on the total weight of the first negative active material layer 122 may be greater than the content of the second silicon-based active material based on the total weight of the second negative active material layer 124 .

[0082] According to some exemplary embodiments, the content of the first silicon-based active material may be 5 wt % or more based on the total weight of the first negative active material layer 122 , and the content of the second silicon-based active material may be less than 5 wt % based on the total weight of the second negative active material layer 124 .

[0083] For example, when the content range of the first silicon-based active material and the second silicon-based active material satisfies the above conditions or the above range, as the content of the first silicon-based active material increases, even if the expansion of the first negative electrode active material layer 122 increases, the increase in resistance due to electrical short circuit can be effectively prevented by the single-walled carbon nanotubes included in the first negative electrode active material layer 122.

[0084] In addition, since the content of the second silicon-based active material included in the second negative electrode active material layer 124 is less than the content of the first silicon-based active material, even when multi-walled carbon nanotubes are included, electrical short circuits caused by expansion of the second negative electrode active material layer 124 can be effectively prevented, and the interface resistance with the separator 140 can be reduced by the multi-walled carbon nanotubes.

[0085] In one embodiment, the single-walled carbon nanotubes may be present in an amount of 0.02 wt % to 0.2 wt % based on the total weight of the first negative active material layer 122 , and the multi-walled carbon nanotubes may be present in an amount of 0.2 wt % to 0.5 wt % based on the total weight of the second negative active material layer 124 .

[0086] When the content of the single-walled carbon nanotubes and the multi-walled carbon nanotubes satisfies the above range, the resistance increase due to expansion of the negative electrode 130 and the increase in interface resistance between the negative electrode 130 and the separator 140 are prevented, thereby further improving the fast charging characteristics and life characteristics of the secondary battery.

[0087] In one embodiment, single-walled carbon nanotubes of the same length and diameter as described above may be used as they are.

[0088] In one embodiment, the length of the multi-walled carbon nanotubes may be from 1 μm to 100 μm, more preferably from 10 μm to 50 μm.

[0089] In one embodiment, the length of the multi-walled carbon nanotubes may be 5 nm to 50 nm, preferably 5 nm to 20 nm, and more preferably 8 nm to 17 nm.

[0090] When the length and the diameter of the multi-walled carbon nanotube satisfy the above ranges, an increase in interface resistance with the separator 140 may be more effectively prevented.

[0091] In some embodiments, the length of the single-walled carbon nanotube may be greater than that of the multi-walled carbon nanotube. In this case, the electrochemical performance of the secondary battery can be more stably ensured.

[0092] In one embodiment, the first negative active material layer 122 may be prepared by coating the negative current collector 125 with a first negative electrode slurry including a first silicon-based active material, a first conductive material including single-walled carbon nanotubes, a binder, and a solvent, followed by drying and rolling. In addition, the second negative active material layer 124 may be prepared by coating the first negative active material layer 122 with a second negative electrode slurry including a second silicon-based active material, a second conductive material including multi-walled carbon nanotubes, a binder, and a solvent, followed by drying and rolling.

[0093] In one embodiment, the first negative electrode slurry is applied to the negative electrode collector 125, and the second negative electrode slurry may be further applied to the applied first negative electrode slurry, and then the applied first and second negative electrode slurries may be dried and roll-pressed to prepare the first and second negative electrode active material layers 122 and 124.

[0094] In some embodiments, a loading weight ratio of the second negative electrode slurry to the first negative electrode slurry may be 0.25 to 1.25, preferably 0.5 to 1.

[0095] Reference Figure 1 , the separator 140 may be interposed between the positive electrode 100 and the negative electrode 130. The separator 140 may include a porous polymer film made of a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer.

[0096] The separator 140 may include a non-woven fabric made of glass fiber having a high melting point, polyethylene terephthalate fiber, or the like.

[0097] The separator 140 may extend in the second direction between the positive electrode 100 and the negative electrode 130 and may be folded and wound in the thickness direction of the lithium secondary battery. A plurality of positive electrodes 100 and negative electrodes 130 may be laminated in the thickness direction with the separator 140 interposed therebetween.

[0098] For example, an electrode unit is formed by the positive electrode 100, the negative electrode 130, and the separator 140, and a plurality of electrode units are stacked to form the electrode assembly 150. The electrode assembly 150 may be formed by winding, stacking, or folding (eg, z-folding) the separator 140, or the like.

[0099] The electrode assembly 150 is accommodated in the case 160, and an electrolyte may be injected together into the case 160. The case 160 may include, for example, a shape of a pouch, a can, or the like.

[0100] According to the exemplary embodiment, a nonaqueous electrolyte may be used as the electrolyte.

[0101] The non-aqueous electrolyte includes a lithium salt electrolyte and an organic solvent. The lithium salt is composed of, for example, Li + X - Represents, and as the anion of lithium salt (X - ), we can exemplify F - , Cl - Br - ,I - 、NO3 - 、N(CN)2 - 、BF4- 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - 、(CF3CF2SO2)2N - wait.

[0102] As the organic solvent, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, cyclopentane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, etc. can be used. These compounds can be used alone or in combination of two or more thereof.

[0103] The tabs (positive tabs and negative tabs) protrude from the positive current collector 105 and the negative current collector 125 belonging to each electrode battery, respectively, and may extend to one side of the outer shell 160. The tabs may be fused with one side of the outer shell 170 to form electrode leads (positive lead and negative lead) extending or exposed outside the outer shell 170.

[0104] The positive electrode lead and the negative electrode lead may be formed on the same side of the lithium secondary battery or the case 160 , or may be formed on sides opposite to each other.

[0105] For example, the positive electrode lead may be formed at one end of the case 160 , and the negative electrode lead may be formed at the other end of the case 160 opposite to the end.

[0106] The lithium secondary battery can be manufactured in, for example, a cylindrical shape (using a can), a square shape, a pouch shape, or a coin shape.

[0107] Hereinafter, specific experimental examples are proposed to facilitate understanding of the present invention. However, the following examples are given only to illustrate the present invention, and it will be clearly understood by those skilled in the art that various changes and modifications can be made within the scope and spirit of the present invention. Such changes and modifications are appropriately included in the appended claims.

[0108] Examples 1 to 6 and Comparative Examples 1 to 3

[0109] (1) Preparation of positive electrode

[0110] The positive electrode active material Li[Ni 0.88 Co 0.09 Mn 0.03 ]O2, carbon black as a conductive material and polyvinylidene fluoride (PVDF) as a binder are mixed to prepare a positive electrode slurry.

[0111] The prepared positive electrode slurry was uniformly coated onto an aluminum foil, followed by a drying and roll-pressing process to prepare a positive electrode.

[0112] (2) Preparation of negative electrode

[0113] The negative electrode active material, the conductive material, and the binder were mixed according to the composition and weight ratio of Table 1 below, and dispersed in water to prepare a negative electrode slurry.

[0114] As the conductive substance, single-walled carbon nanotubes (OCSiAl Corporation, SW-CNT, with a length of more than 5 µm and a diameter of 1.2 nm to 2 nm) or multi-walled carbon nanotubes (OCSiAl Corporation, MW-CNT, with a length of 10 µm to 50 µm and a diameter of 8 nm to 17 nm) are used.

[0115] The Raman R values ​​of SW-CNT and MW-CNT shown in Table 1 below represent the Raman spectrum obtained by measuring the G band (about 1580 cm -1 ) peak intensity (Ig) and D band (about 1350cm -1 ) The peak intensity (Id) is expressed as the peak intensity ratio (Id / Ig). Raman R value was measured using Renishaw's inVia.

[0116] The negative electrode slurry was coated on a copper thin film (thickness: 6 μm), followed by drying and roll pressing to a density of 1.7 g / cc, thereby preparing a negative electrode with a thickness of 133 μm.

[0117] [Table 1]

[0118]

[0119] (3) Manufacturing of secondary batteries

[0120] The positive electrode and the negative electrode are arranged such that a polyethylene (PE) separator (13 µm) is interposed therebetween to form an electrode cell, and the electrode cells are stacked to prepare an electrode assembly. The electrode assembly 150 is accommodated in a soft package, and the tab portions are fused. Thereafter, an electrolytic solution is injected and then sealed to manufacture a secondary battery.

[0121] After preparing a 1 M LiPF6 solution using a mixed solvent of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (EC / EMC / DEC, 25 / 45 / 30; volume ratio), 1 wt% of vinylene carbonate (VC), 0.5 wt% of 1,3 - propanesultone (PRS), and 0.5 wt% of lithium bis(oxalate)borate (LiBOB) based on the total weight of the electrolytic solution are added thereto to be used as the electrolytic solution.

[0122] Examples 7 to 10

[0123] Artificial graphite, SiO x (0 < x < 2), single - walled carbon nanotubes (SW - CNT), and SBR / CMC are mixed at a weight ratio of 89.1:8:0.1:2.8 and dispersed in water to prepare a first negative electrode paste.

[0124] Then, the negative electrode active material, the conductive material, and the binder are mixed according to the composition and weight ratio in Table 2 below and dispersed in water to prepare a second negative electrode paste.

[0125] The first negative electrode paste is coated on a copper thin film (thickness: 6 µm), and then the second negative electrode paste is further coated on the coated first negative electrode paste. The loading weight ratio of the first negative electrode paste to the second negative electrode paste is 1:1.

[0126] Thereafter, the coated first negative electrode paste and second negative electrode paste are dried and then roll - pressed to have a density of 1.7 g / cc, thereby preparing a negative electrode with a thickness of 133 µm.

[0127] [Table 2]

[0128]

[0129] Experimental Example

[0130] (1) Electrode resistance (Ωcm)

[0131] The electrode resistance (Ωcm) of the negative electrodes of the examples and comparative examples is measured.

[0132] A Hioki XF057 Probe unit from Hioki was used as the measurement device. The measurement conditions were 100 µA current and 0.5 V voltage range, and the number of pin contacts was 500.

[0133] (2) Evaluation of room temperature rapid cycling

[0134] The lithium secondary batteries according to the embodiments and comparative examples were charged / discharged 200 times by the following steps: at room temperature (25°C), charged with a step-by-step current value (2C, 1.75C, 1.5C, 1.25C, 1C, 0.75C and 0.5C) from 2C to 0.5C at intervals of 0.25C, cut off at 4.2V, and then left for 10 minutes, and discharged with a constant current of 0.5C and cut off at 2.5V, and then left for 10 minutes.

[0135] The percentage value of the discharge capacity A2 measured after performing the charge / discharge cycle 200 times to the discharge capacity A1 measured after performing one charge / discharge cycle was calculated (A2 / A1×100%), and the results are shown in Table 3 below.

[0136] (3) Evaluation of high temperature (45°C) cycle

[0137] The lithium secondary batteries of the embodiments and comparative examples were charged / discharged 200 times by the following steps: charging at a constant current of 0.3C and cutoff at 4.2V and 0.05C, followed by standing for 10 minutes, discharging at a constant current of 0.5C and cutoff at 2.5V, followed by standing for 10 minutes.

[0138] The percentage value of the discharge capacity B2 measured after performing the charge / discharge cycle 200 times to the discharge capacity B1 measured after performing the charge / discharge cycle once was calculated (B2 / B1×100%), and the results are shown in Table 3 below.

[0139] (4) Evaluation of high temperature (60°C) storage

[0140] The lithium secondary batteries of the examples and comparative examples were charged to 100% state of charge (SOC) under the following conditions: charged at a constant current of 0.3C and cut off at 4.2V and 0.05C, and then stored at a high temperature of 60°C for 8 weeks.

[0141] The percentage value of the discharge capacity C2 measured after high-temperature storage to the discharge capacity C1 measured before high-temperature storage (C2 / C1×100%) was calculated, and the results are shown in Table 3 below.

[0142] [Table 3]

[0143]

[0144] Referring to Table 3 above, it can be confirmed that the lithium secondary battery according to the embodiment exhibits excellent effects in room temperature rapid cycle characteristics, high temperature cycle characteristics, and high temperature storage characteristics.

[0145] [Explanation of Reference Numerals]

[0146] 100: Positive

[0147] 105: Positive electrode current collector

[0148] 110: Positive electrode active material layer

[0149] 130: Negative electrode

[0150] 125: Negative electrode current collector

[0151] 120: Negative electrode active material layer

[0152] 122: First negative electrode active material layer

[0153] 124: Second negative electrode active material layer

[0154] 140: Diaphragm

[0155] 150: Electrode assembly

[0156] 160: Shell

Claims

1. A negative electrode for a secondary battery, comprising: Anode current collector; a first negative electrode active material layer formed on the negative electrode current collector, the first negative electrode active material layer comprising a first silicon-based active material and a first conductive material comprising single-walled carbon nanotubes; as well as a second negative electrode active material layer formed on the first negative electrode active material layer, the second negative electrode active material layer comprising a second silicon-based active material and a second conductive material comprising multi-walled carbon nanotubes, The Raman R value of the single-walled carbon nanotube, i.e., D band peak intensity (Id) / G band peak intensity (Ig), is 0.01 to 0.1, Wherein, based on the total weight of the first negative electrode active material layer, the content of the single-walled carbon nanotubes is 0.02 wt % to 0.2 wt %, and The content of the multi-walled carbon nanotubes is 0.2 wt % to 0.5 wt % based on the total weight of the second negative electrode active material layer. 2 . The negative electrode for a secondary battery according to claim 1 , wherein a content of the first silicon-based active material is 5 wt % or more based on the total weight of the first negative electrode active material layer. 3 . The negative electrode for a secondary battery according to claim 1 , wherein the length of the single-walled carbon nanotube is 5 μm or more. 4 . The negative electrode for a secondary battery according to claim 1 , wherein the single-walled carbon nanotube has a diameter of 1.2 nm to 2 nm. 5 . The secondary battery negative electrode according to claim 1 , wherein a content of the first silicon-based active material based on the total weight of the first negative electrode active material layer is greater than a content of the second silicon-based active material based on the total weight of the second negative electrode active material layer.

6. The negative electrode for a secondary battery according to claim 1, wherein the content of the first silicon-based active material is 5 wt % or more based on the total weight of the first negative electrode active material layer, and The content of the second silicon-based active material is less than 5 wt % based on the total weight of the second negative electrode active material layer.

7. A secondary battery comprising: The negative electrode for a secondary battery according to claim 1; A positive electrode; and A separator is disposed between the negative electrode and the positive electrode.

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