Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode and a secondary battery including the same, and the negative electrode of the present invention includes a current collector, a first negative electrode active material layer located on at least one surface of the current collector, and a negative electrode active material layer located on the first negative electrode active material. Specifically, the second negative electrode active material layer includes lithium titanium oxide (LTO) and single-walled carbon nanotubes (SWCNTs), the first negative electrode active material layer includes a negative electrode active material other than the lithium titanium oxide (LTO), and the loading amount of the second negative electrode active material is 0.1 mAh / cm 2 The present invention is characterized in that:

Description

[Technical Field]

[0001] The present invention relates to a negative electrode and a secondary battery including the same.

[0002] This application claims priority based on Korean Patent Application No. 2021-0187852, filed on December 24, 2021, and the entire contents disclosed in the specification and drawings of that application are incorporated herein by reference. [Background technology]

[0003] As technological development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing, and research into batteries that can meet various requirements is being actively conducted. Among these, the development of rechargeable secondary batteries and lithium secondary batteries with high energy density has been the focus of attention, and recently, ensuring safety in the development of secondary batteries has been attracting much attention.

[0004] A lithium secondary battery has a structure in which an electrode assembly is formed by coating an active material on a current collector with a positive electrode and a negative electrode, and a porous separator is interposed between the positive electrode and the negative electrode, and an electrolyte containing a lithium salt is impregnated in the electrode assembly.

[0005] In lithium secondary batteries, lithium ions are released from the positive electrode active material during the initial charge and inserted into the negative electrode active material. Then, during discharge, they are released and shuttle between the two electrodes. This process of energy transfer enables charging and discharging. The negative electrode active material influences the basic performance characteristics of lithium secondary batteries, and conductive materials influence the electrical conductivity of the negative electrode active material. Therefore, research into negative electrode active material materials that can improve the basic performance of secondary batteries is progressing.

[0006] Meanwhile, lithium secondary batteries can explode or catch fire due to abnormal battery operation, such as short circuiting, overcharging exceeding the allowable current and voltage, exposure to high temperatures, or impact due to dropping. In response to this, research is actively underway to improve the safety of currently produced lithium secondary batteries through various methods, including improving the battery structure, electrolyte and separator performance, and ensuring the safety of battery components. However, while research is currently focused on separators to prevent short circuits between the positive and negative electrodes, further research is needed on negative electrode active materials to improve safety.

[0007] On the other hand, lithium titanium oxide (LTO) anodes are being actively researched as a next-generation anode material because they are more efficient during charging and discharging than conventional electrodes, but their low electrical conductivity makes commercialization difficult. Therefore, research is needed to improve the electrical conductivity of LTO anodes while also improving the stability of batteries. Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a negative electrode for a secondary battery that has excellent electrochemical performance and ensures stability.

[0009] Another object of the present invention is to provide an anode for a secondary battery, which has improved electrical conductivity and thereby improved charge / discharge efficiency of the battery, and a secondary battery using the same.

[0010] It is yet another object of the present invention to provide an LTO negative electrode with improved stability against internal short circuits and a secondary battery using the same. [Means for solving the problem]

[0011] In order to solve the above problems, according to one aspect of the present invention, there is provided a negative electrode according to the following embodiment.

[0012] According to a first embodiment, a battery includes a current collector, a first negative electrode active material layer disposed on at least one surface of the current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein the second negative electrode active material layer includes lithium titanium oxide (LTO) and single-walled carbon nanotubes (SWCNTs), the first negative electrode active material layer includes a negative electrode active material other than the lithium titanium oxide (LTO), and the second negative electrode active material layer has a load of 0.1 mAh / cm. 2 There is provided a negative electrode characterized in that:

[0013] According to the second embodiment, the load of the second negative electrode active material layer in the first embodiment is 0.01 mAh / cm 2 More than 0.1mAh / cm 2 It can be the following:

[0014] According to a third embodiment, in the first or second embodiment, the second negative electrode active material layer may have a thickness of 3 μm or less.

[0015] According to a fourth embodiment, in any one of the first to third embodiments, the thickness of the second negative electrode active material layer may be 1% to 5% of the total thickness of the first and second negative electrode active material layers.

[0016] According to a fifth embodiment, in any one of the first to fourth embodiments, the length of the single-walled carbon nanotube may be 1 μm to 30 μm.

[0017] According to a sixth embodiment, in any one of the first to fifth embodiments, the single-walled carbon nanotubes have an average cross-sectional diameter of 2 nm to 100 nm in a direction perpendicular to the longitudinal direction, and a BET specific surface area of ​​500 m 2 / g~1,600m 2 / g.

[0018] According to a seventh embodiment, in any one of the first to sixth embodiments, the content of the single-walled carbon nanotubes may be 0.01 wt % to 3 wt % based on the total weight of the second negative electrode active material.

[0019] According to an eighth embodiment, in any one of the first to seventh embodiments, the first negative electrode active material layer may include a carbon-based active material, a silicon-based active material, or a mixture thereof.

[0020] According to a ninth embodiment, there is provided a secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode is the negative electrode according to any one of the first to eighth embodiments.

[0021] According to a tenth embodiment, in the ninth embodiment, the secondary battery may be a lithium secondary battery. [Effects of the Invention]

[0022] The anode according to an embodiment of the present invention can significantly improve the electrochemical performance and stability of a secondary battery.

[0023] The anode according to an embodiment of the present invention may have improved electrical conductivity, thereby improving the charge / discharge efficiency of the battery.

[0024] The anode according to one embodiment of the present invention has an active material layer composed of at least two layers, and the active material layer containing lithium titanium oxide also contains single-walled carbon nanotubes as a conductive material, thereby providing sufficient electrical conductivity even with a very small amount of active material.

[0025] The negative electrode according to an embodiment of the present invention can significantly improve the stability of a secondary battery against internal short circuits.

[0026] In the anode according to an embodiment of the present invention, when a short circuit occurs in a fully charged state, the electrical conductivity rapidly decreases due to the desorption of lithium ions from the active material layer containing lithium titanium oxide, thereby significantly improving the stability of an internal short circuit caused by an increase in resistance in the anode.

[0027] The characteristics of the anode according to an embodiment of the present invention and the secondary battery using the same are not limited to those described above, and the effects of the present invention are not limited to those described above.

[0028] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters described in the drawings. [Brief explanation of the drawings]

[0029] [Figure 1] 1A and 1B are schematic cross-sectional views of a secondary battery including a negative electrode according to an embodiment of the present invention in a fully charged state and immediately after a short circuit; DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention will be described in detail below. However, the present invention is not limited to the following content, and each component may be variously modified or selectively mixed as necessary. Therefore, it should be understood that the present invention may include all modifications, equivalents, and alternatives included within the spirit and technical scope of the present invention.

[0031] Furthermore, throughout the specification, when a part is said to "include" or "comprise" a certain element, this does not mean that it excludes other elements, but that it may further include or be further comprised of other elements, unless otherwise specified.

[0032] In this specification, the expression "A and / or B" means "A or B or all of them."

[0033] According to one aspect of the present invention, there is provided a negative electrode comprising: a current collector; a first negative electrode active material layer disposed on at least one surface of the current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein the second negative electrode active material layer comprises lithium titanium oxide (LTO) and single-walled carbon nanotubes (SWCNTs), the first negative electrode active material layer comprises a negative electrode active material other than the lithium titanium oxide (LTO), and the second negative electrode active material layer has a load of 0.1 mAh / cm. 2 The following is the result.

[0034] According to an embodiment of the present invention, the current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or aluminum-cadmium alloy. The thickness of the current collector is not particularly limited, but may be a commonly used thickness of 3 to 500 μm.

[0035] According to an embodiment of the present invention, a plurality of negative electrode active material layers are provided on the current collector. In this specification, the plurality of negative electrode active material layers provided on the current collector are referred to as a first negative electrode active material layer, a second negative electrode active material layer, and the like, in order of their proximity to the current collector.

[0036] According to one embodiment of the present invention, at least one negative electrode active material layer including lithium titanium oxide (LTO) as a negative electrode active material and single-walled carbon nanotubes (SWCNTs) as a conductive material is disposed on the current collector. In this case, the negative electrode active material layer including LTO and SWCNTs is not in surface contact with the current collector, but is in surface contact with a negative electrode active material layer including a negative electrode active material other than LTO disposed on the current collector. Therefore, in this specification, the negative electrode active material layer including a negative electrode active material other than LTO may be referred to as a first negative electrode active material, and the negative electrode active material layer including LTO and SWCNTs may be referred to as a second negative electrode active material.

[0037] A secondary battery including a negative electrode according to one embodiment of the present invention is shown schematically in Figure 1. Figure 1 shows the structure of the secondary battery in a fully charged state and in the early stages of a short circuit.

[0038] Specifically, FIG. 1 shows a first anode active material layer 2, a second anode active material layer 3, a separator 4, a cathode active material layer 5, and a cathode current collector 6 stacked in this order on an anode current collector 1, and the second anode active material layer contains lithium titanium oxide 3a (LTO) and single-walled carbon nanotubes (SWCNT).

[0039] The lithium titanium oxide does not affect the performance of the battery under normal operating conditions, but when a short circuit occurs in a fully charged state, lithium ions are released.

[0040] For example, lithium titanium oxide 3a in the fully charged state is Li 7 / 3 Ti 5 / 3 O4, and in the early stage of a short circuit, lithium ions are released, causing Li 4 / 3 Ti 5 / 3 It can exist in the form O4.

[0041] Due to the desorption of lithium ions, the electrical conductivity of the negative electrode active material layer containing lithium titanium oxide decreases rapidly, and the resistance in the negative electrode increases, ensuring stability against internal short circuits.

[0042] In the present invention, the load of the second negative electrode active material layer is 0.1 mAh / cm 2 The present invention is characterized by the following:

[0043] According to an embodiment of the present invention, the load of the second negative electrode active material layer is, for example, 0.099 mAh / cm 2 Below, 0.095mAh / cm 2 Below, 0.09mAh / cm 2 Below, 0.090mAh / cm 2 Below, 0.085mAh / cm 2 Below, 0.08mAh / cm 2Below, 0.080mAh / cm 2 Below, 0.075mAh / cm 2 Below, 0.07mAh / cm 2 Below, 0.070mAh / cm 2 Below, 0.065mAh / cm 2 Below, 0.06mAh / cm 2 Below, 0.060mAh / cm 2 Below, 0.055mAh / cm 2 Below, 0.05mAh / cm 2 Less than or equal to 0.050mAh / cm 2 More specifically, the loading amount of the second negative electrode active material may be 0.01 mAh / cm or less. 2 More than 0.015mAh / cm 2 Over 0.02mAh / cm 2 More than 0.025mAh / cm 2 More than 0.03mAh / cm 2 Over 0.099mAh / cm 2 Below, 0.095mAh / cm 2 Below, 0.09mAh / cm 2 Below, 0.090mAh / cm 2 Below, 0.085mAh / cm 2 Below, 0.08mAh / cm 2 Below, 0.080mAh / cm 2 Below, 0.075mAh / cm 2 Below, 0.07mAh / cm 2 Below, 0.070mAh / cm 2 Below, 0.065mAh / cm 2 Below, 0.06mAh / cm 2 Below, 0.060mAh / cm 2 Below, 0.055mAh / cm 2 Below, 0.05mAh / cm 2 For example, the loading of the second negative electrode active material may be 0.01 mAh / cm or less. 2 Over 0.1mAh / cm 2 Below, 0.01mAh / cm 2 or greater than 0.099mAh / cm 2 Less than or equal to 0.01mAh / cm 2Over 0.09mAh / cm 2 When the loading amount of the second negative electrode active material layer satisfies the above range, the thickness of the second negative electrode active material layer can be thin, thereby improving the stability of the electrode against internal short circuits and maintaining excellent cell capacity and basic performance of the negative electrode, but the present invention is not limited thereto.

[0044] According to another embodiment of the present invention, the second negative electrode active material layer may have a thickness of 3 μm or less. Specifically, the thickness of the second negative electrode active material layer may be, for example, 2.99 μm or less, 2.95 μm or less, 2.9 μm or less, 2.85 μm or less, 2.8 μm or less, 2.75 μm or less, 2.7 μm or less, 2.65 μm or less, 2.6 μm or less, 2.55 μm or less, 2.5 μm or less, 2.45 μm or less, 2.4 μm or less, 2.35 μm or less, 2.3 μm or less, 2.25 μm or less, 2.2 μm or less, 2.15 μm or less, 2.1 μm or less, 2.05 μm or less, 2.0 μm or less, or 2 μm or less. More specifically, the thickness of the second negative electrode active material layer may be 0.01 μm or more, 0.05 μm or more, 0.1 μm or more, 0.5 μm or more, 1 μm or more, 2.99 μm or less, 2.95 μm or less, 2.9 μm or less, 2.85 μm or less, 2.8 μm or less, 2.75 μm or less, 2.7 μm or less, 2.65 μm or less, 2.6 μm or less, 2.55 μm or less, 2.5 μm or less, 2.45 μm or less, 2.4 μm or less, 2.35 μm or less, 2.3 μm or less, 2.25 μm or less, 2.2 μm or less, 2.15 μm or less, 2.1 μm or less, 2.05 μm or less, 2.0 μm or less, or 2 μm or less. For example, the thickness of the second negative electrode active material layer may be 0.01 μm to 2.99 μm, 0.5 μm to 2.5 μm, or 1 μm to 2 μm. When the thickness of the second negative electrode active material layer satisfies the above range, the thin thickness of the second negative electrode active material layer may have the effect of improving the stability of the electrode against internal short circuits, but the present invention is not limited thereto.

[0045] According to one embodiment of the present invention, the thickness of the second negative electrode active material layer may be 1% to 5% of the total thickness of the first and second negative electrode active material layers. Specifically, the thickness of the second negative electrode active material layer may be 1% to 3%, 2% to 3%, 2% to 2.5%, or 2.5% to 3% of the total thickness of the first and second negative electrode active material layers. When the thickness of the second negative electrode active material layer satisfies the above ranges with respect to the total thickness of the first and second negative electrode active material layers, it may be possible to improve the charge / discharge efficiency of the electrode and the stability of the electrode against internal short circuits, but the present invention is not limited thereto.

[0046] In the present invention, the lithium titanium oxide (LTO) may be included in the second negative electrode active material layer as a negative electrode active material, and the single-walled carbon nanotubes (SWCNTs) may be included in the second negative electrode active material layer as a conductive material.

[0047] Although lithium titanium oxide (LTO) has a drawback of having lower electrical conductivity than carbon-based materials, which are typical negative electrode active materials, when used together with the single-walled carbon nanotubes, it is possible to improve the electrical conductivity of the second negative electrode active material layer to the level of carbon-based materials. Furthermore, the second negative electrode active material layer may function as a resistive layer within the electrode, thereby significantly improving the stability of internal short circuits. However, the mechanism of the present invention is not limited thereto.

[0048] According to an embodiment of the present invention, the lithium titanium oxide may be represented by the following Chemical Formula 1:

[0049] [Chemical formula 1] Li a Ti b O4 In the above Chemical Formula 1, 0.5≦a≦3 and 1≦b≦2.5.

[0050] As a specific example, Li 0.8 Ti 2.2 O4, Li 2.67 Ti 1.33 O4, LiTi2O4, Li 1.33 Ti1.67 O4, Li 1.14 Ti 1.71 O4, Li2Ti5O 12 , Li4Ti5O 12 It can be, but is not limited to,

[0051] The lithium titanium oxide may be in the form of primary particles or secondary particles formed by aggregation of a plurality of the primary particles. 50 The average particle size D may be, for example, about 0.1 to 3 μm. 50 The average particle size may refer to the particle size at the 50% point of the cumulative particle number distribution by particle size. The average particle size can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The particle size distribution can be calculated by measuring the difference in diffraction patterns depending on the particle size when the particles pass through a laser beam.

[0052] The content of the lithium titanium oxide may be, for example, 95 wt % to 100 wt %, for example, 96 wt % to 98 wt % or 97 wt % to 98 wt %, based on the total weight of the second negative electrode active material layer. When the content of the lithium titanium oxide is within the above range, an advantageous effect in terms of stability can be achieved.

[0053] The term "single-walled carbon nanotube (SWCNT)" refers to a single tubular carbon nanotube in which hexagons formed by bonding six carbon atoms are interconnected, and the number of walls (graphite planes) is one. Such single-walled carbon nanotubes exhibit excellent electrical properties due to their one-dimensional structure, and they exhibit various electrical properties depending on the chirality structure of their hexagonal honeycomb structure and diameter.

[0054] According to one embodiment of the present invention, the single-walled carbon nanotubes may be pre-dispersed in the form of a dispersion containing a dispersant and / or a dispersion medium to facilitate smooth dispersion within the negative electrode active material layer, specifically, the second negative electrode active material. When pre-dispersed in the form of a dispersion, uniform electrical conductivity can be achieved, and as described later in this specification, even when the single-walled carbon nanotubes are used in very small amounts, it is easy to add a specific amount. The dispersant and / or dispersion medium may be any material capable of dispersing carbon nanotubes. For example, the dispersant may be hydrogenated nitrile butadiene rubber (H-NBR), polyvinylpyrrolidone (PVP), or carboxymethyl cellulose (CMC), and the dispersion medium may be N-methyl-2-pyrrolidone (NMP) or water.

[0055] According to one embodiment of the present invention, the single-walled carbon nanotubes may have a length of, for example, 1 μm to 30 μm. Specifically, the length of the single-walled carbon nanotubes may be 1 μm to 15 μm or 15 μm to 30 μm. When the aspect ratio satisfies the above range, advantageous effects in terms of electrical conductivity may be achieved, but the present invention is not limited thereto. The aspect ratio can be determined by averaging the aspect ratios of 15 single-walled carbon nanotubes with a large aspect ratio and 15 single-walled carbon nanotubes with a small aspect ratio when observing the single-walled carbon nanotube powder using an SEM.

[0056] According to one embodiment of the present invention, the single-walled carbon nanotubes may have an average cross-sectional diameter of 2 nm to 100 nm in a direction perpendicular to the longitudinal direction. Specifically, the average diameter of the single-walled carbon nanotubes may be 2 nm to 70 nm or 70 nm to 100 nm. When the average diameter of the single-walled carbon nanotubes satisfies this range, an advantageous effect can be achieved in terms of maintaining excellent electrical conductivity of the negative electrode even when the single-walled carbon nanotubes are contained in a very small amount, but the present invention is not limited thereto. The average diameter of the single-walled carbon nanotubes may be measured from a transmission electron microscope (TEM) image.

[0057] According to one embodiment of the present invention, the single-walled carbon nanotubes have a BET specific surface area of ​​500 m 2 / g~1,600m 2 Specifically, the specific surface area of ​​the single-walled carbon nanotubes may be 500 m / g. 2 / g~1,500m 2 / g or 700m 2 / g~1,500m 2 / g. When this range is satisfied, a conductive material dispersion having a desired solid content can be obtained. The BET specific surface area can be measured by a nitrogen adsorption BET method.

[0058] According to another embodiment of the present invention, the single-walled carbon nanotubes have a length of 1 μm to 30 μm, an average cross-sectional diameter in a direction perpendicular to the longitudinal direction of 2 nm to 100 nm, and a BET specific surface area of ​​500 m. 2 / g~1,600m 2 Specifically, when the single-walled carbon nanotubes satisfy the above-mentioned physical properties, even if only a small amount is used, an advantageous effect can be obtained in that the electrical network can be eliminated, but the present invention is not limited thereto.

[0059] According to one embodiment of the present invention, the content of the single-walled carbon nanotubes may be, for example, 0.01% to 3% by weight based on the total weight of the second negative electrode active material. Specifically, the content of the single-walled carbon nanotubes may be 0.01% to 3% by weight, 0.01% to 1% by weight, or 1% to 3% by weight based on the total weight of the second negative electrode active material.

[0060] According to another embodiment of the present invention, the content of the single-walled carbon nanotubes may be, for example, 0.01 to 5 parts by weight, 0.01 to 4 parts by weight, or 0.05 to 3.3 parts by weight based on 100 parts by weight of the lithium titanate (LTO) in the second negative electrode active material.

[0061] When the content of the single-walled carbon nanotubes satisfies the above-described range, it is possible to sufficiently drive an electrical network in the negative electrode active material layer with a very small amount, and thus an advantageous effect in terms of preserving the energy density can be achieved, but the present invention is not limited thereto.

[0062] In the present invention, the first negative electrode active material layer contains a negative electrode active material other than lithium titanate (LTO).

[0063] The negative electrode active material contained in the first negative electrode active material layer may be any commonly used negative electrode active material, and all are applicable except for lithium titanate. For example, the first negative electrode active material layer may contain a carbon-based active material, a silicon-based active material, or a mixture thereof.

[0064] Examples of the carbon-based active material include, but are not limited to, artificial graphite, natural graphite, hard carbon, soft carbon, graphitized carbon fiber, graphitized mesocarbon microbeads, petroleum coke, resin fired body, carbon fiber, pyrolytic carbon, or a mixture thereof. Examples of the silicon-based active material include, but are not limited to, Si, SiO x Silicon oxide represented by (0 < x ≦ 2) or a mixture thereof.

[0065] Artificial graphite is typically produced by carbonizing raw materials such as coal tar, coal tar pitch, and heavy petroleum oil at temperatures above 2,500°C. After graphitization, the artificial graphite is used as an anode active material through particle size adjustment, such as pulverization and secondary particle formation. Artificial graphite has randomly distributed crystals within the particles, resulting in lower sphericity and a slightly sharper shape than natural graphite. Commonly used artificial graphites include mesophase carbon microbeads (MCMB), mesophase pitch-based carbon fiber (MPCF), block-type graphitized artificial graphite, and powder-type graphitized artificial graphite. The sphericity of artificial graphite can be 0.91 or less, 0.6-0.91, or 0.7-0.9. The artificial graphite can have a particle size of 5-30 μm or 10-25 μm.

[0066] The natural graphite is generally in the form of plate-like agglomerates before processing, and the plate-like particles are processed into spheres with smooth surfaces by post-processing such as particle crushing and re-granulation to be used as an active material for manufacturing electrodes. The natural graphite may have a sphericity of more than 0.91 and not more than 0.97, or 0.93 to 0.97, or 0.94 to 0.96. The natural graphite may have a particle size of 5 to 30 μm, or 10 to 25 μm.

[0067] The first negative electrode active material layer may contain two or more active materials. In this case, different active materials may be distributed in the surface direction near the current collector of the active material layer, or two or more active materials of the same type but with different average particle sizes or morphologies may be included. Of course, the active material layer may contain two or more active materials of different types, morphologies, or average particle sizes.

[0068] According to an embodiment of the present invention, the first negative electrode active material layer may further include a conductive material within a range that does not impair the object of the present invention. The conductive material may be any material that does not induce chemical changes in the battery and has conductivity. For example, graphite such as natural graphite or artificial graphite; carbon nanotubes; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0069] In another embodiment, the first negative electrode active material layer may include a predetermined negative electrode active material, a conductive material, and a binder polymer.

[0070] The binder polymer may be any of a variety of polymers, such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene-butylene rubber (SBR), fluororubber, and various copolymers. Some of the binder polymers may also function as thickeners, increasing the viscosity of the active material layer slurry and improving the dispersion characteristics of the active material and conductive material.

[0071] In one embodiment of the present invention, the load of the first negative electrode active material layer is 1 mAh / cm 2 ~10mAh / cm 2 , specifically 3mAh / cm 2 ~3.6mAh / cm2 Furthermore, the thickness of the first negative electrode active material layer is not particularly limited as long as it satisfies the thickness ratio to the second negative electrode active material layer, and may be, for example, 3.0 μm to 3.6 μm or 3.3 μm to 3.5 μm.

[0072] A secondary battery according to one aspect of the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the negative electrode is the negative electrode according to one embodiment of the present invention described above.

[0073] According to an embodiment of the present invention, the secondary battery may be a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0074] According to an embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector.

[0075] The positive electrode current collector may typically have a thickness of about 3 to 500 μm. There are no particular limitations on the positive electrode current collector, so long as it does not induce chemical changes in the battery and has high conductivity. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector may have fine irregularities on its surface to enhance the adhesive strength of the positive electrode active material, and may be in a variety of forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0076] According to an embodiment of the present invention, the positive electrode active material layer may include a positive electrode active material, a positive electrode conductive material, and a positive electrode binder polymer.

[0077] The positive electrode active material may include, but is not limited to, lithium transition metal oxides, lithium metal iron phosphate compounds, lithium nickel-manganese-cobalt oxides, oxides in which a portion of lithium nickel-manganese-cobalt oxide is substituted with another transition metal, or two or more of these. Specifically, the positive electrode active material may include, but is not limited to, layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; 1+x Mn 2-x O4 (where x is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x M x Ni-site lithium nickel oxide represented by the chemical formula LiMnO2 (where M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x=0.01 to 0.3). 2-x M x Lithium manganese composite oxides represented by Li2Mn3MO8 (where M is Co, Ni, Fe, Cr, Zn, or Ta, and x is 0.01 to 0.1) or Li2Mn3MO8 (where M is Fe, Co, Ni, Cu, or Zn); lithium metal phosphate compounds LiMPO4 (where M is Fe, CO, Ni, or Mn); lithium nickel-manganese-cobalt oxide Li 1+x (Ni a Co b Mn c ) 1-x O2 (x=0~0.03, a=0.3~0.95, b=0.01~0.35, c=0.01~0.5, a+b+c=1); Lithium nickel-manganese-cobalt oxide with some aluminum substitution a [Ni b Co c Mn d Al e ] 1-f M1 fO2 (M1 is one or more selected from the group consisting of Zr, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S; 0.8 ≦ a ≦ 1.2, 0.5 ≦ b ≦ 0.99, 0 < c < 0.5, 0 < d < 0.5, 0.01 ≦ e ≦ 0.1, 0 ≦ f ≦ 0.1); an oxide in which part of lithium nickel-manganese-cobalt oxide is substituted with other transition metals, Li 1+x (Ni a Co b Mn c M d ) 1-x O2 (x = 0 to 0.03, a = 0.3 to 0.95, b = 0.01 to 0.35, c = 0.01 to 0.5, d = 0.001 to 0.03, a + b + c + d = 1, M is any one selected from the group consisting of Fe, V, Cr, Ti, W, Ta, Mg, and Mo). Disulfide compounds; Fe2(MoO4)3, etc. are mentioned, but are not limited thereto.

[0078] The positive electrode conductive material is usually added in an amount of 1% to 50% by weight based on the total weight of the mixture containing the positive electrode active material. Such a positive electrode conductive material is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives can be used.

[0079] The positive electrode binder polymer is a component that aids in bonding between the positive electrode active material and the positive electrode conductive material, and bonding to the current collector, and is typically added in an amount of 0.5 wt% to 50 wt% based on the total weight of the slurry containing the positive electrode active material. Examples of such positive electrode binder polymers include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butylene rubber, fluororubber, and various copolymers.

[0080] The separator used with the electrode of the present invention is not particularly limited. The separator is a thin insulating membrane interposed between the positive and negative electrodes to separate them, and has high ion permeability and mechanical strength. Any material typically used as a separator in secondary batteries can be used without particular limitations. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminate structures of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used.

[0081] In addition, in order to ensure heat resistance or mechanical strength, a separator coated with inorganic particles, a binder polymer, or a mixture of inorganic particles and a binder polymer may be used, and may be selectively used in a single-layer or multi-layer structure. In addition, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as the separator.

[0082] The present invention will be described in detail below with reference to examples. However, the following examples are for the purpose of illustrating the present invention, and the scope of the present invention is not limited thereto.

[0083] [Secondary battery manufacturing] The secondary batteries of the examples and comparative examples were manufactured by the following method.

[0084] Example 1 <Anode manufacturing> Two parts by weight of SBR as a binder polymer and one part by weight of CMC were dispersed in distilled water as a solvent, and then one part by weight of carbon black (Super C65) as a conductive material and 96 parts by weight of flake-type artificial graphite (maximum length 20 μm, average aspect ratio 3.5) as a negative electrode active material were added to obtain a uniform first negative electrode active material slurry. The first negative electrode active material slurry was applied to a copper current collector with a thickness of 15 μm and dried at 70°C using a comma coater at a speed of 0.2 m / min, resulting in a load of 3.6 mAh / cm. 2 The first negative electrode active material layer was formed to a thickness of 112 μm.

[0085] Next, 1.5 parts by weight of a binder polymer (polyvinylidene fluoride, PVdF), 1.5 parts by weight of single-walled carbon nanotubes (SWCNTs) with an average diameter of 10 nm as a conductive material, and lithium titanium oxide (Li4Ti5O 12 , average particle size D 50 97 parts by weight of the powder (0.9 μm) was dispersed in N-methylpyrrolidone as a dispersion medium to obtain a uniform second negative electrode active material slurry.

[0086] The second negative electrode active material slurry was coated on the surface of the first negative electrode active material layer and then dried at 120°C at a speed of 0.2 m / min using a comma coater to form a second negative electrode active material. At this time, the LTO loading in the second negative electrode active material was 0.06 mAh / cm. 2 The thickness was 2.5 μm.

[0087] <Production of positive electrodes> Li(Ni) as the positive electrode active material 0.69 Mn 0.2 Co 0.2A cathode active material slurry was prepared by adding N-methylpyrrolidone (NMP) as a solvent to prepare a cathode active material slurry containing 102 (NCM-622), carbon black as a conductive material, and PVdF as a binder polymer in a weight ratio of 96:2:2. The cathode active material slurry was coated onto one side of an aluminum current collector with a thickness of 15 μm, and then dried and rolled at 120°C using a comma coater at a speed of 0.1 m / min to prepare a cathode. Based on the dry weight of the cathode active material layer, the load was 3.26 mAh / cm. 2 It was.

[0088] <Manufacturing lithium secondary batteries> An electrolyte solution was prepared by dissolving LiPF6 to a concentration of 1.0 M in an organic solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3:7, and dissolving 0.5 wt% vinylene carbonate (VC) as an additive.

[0089] A porous polypropylene separator was interposed between the negative electrode and the positive electrode, and the electrolyte was injected into the separator to prepare a lithium secondary battery.

[0090] Example 2 The second negative electrode active material layer was charged with LTO at a loading of 0.065 mAh / cm 2 A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode was formed to have the following structure. Then, a secondary battery was manufactured using the negative electrode.

[0091] Example 3 The second negative electrode active material layer was charged with LTO at a loading of 0.07 mAh / cm 2 A negative electrode was manufactured in the same manner as in Example 1, except that the negative electrode was formed to have the following structure. Then, a secondary battery was manufactured using the negative electrode.

[0092] Example 4 A negative electrode was manufactured in the same manner as in Example 1, except that the first negative electrode active material layer was formed to a thickness of 136 μm, and then a secondary battery was manufactured using the negative electrode.

[0093] Comparative Example 1 Two parts by weight of SBR as a binder and one part by weight of CMC were dispersed in distilled water as a solvent, and then one part by weight of carbon black (Super C65) as a conductive material and 96 parts by weight of flake-type artificial graphite (maximum length 20 μm, average aspect ratio 1:2.5) as a negative electrode active material were added to obtain a negative electrode active material slurry. The negative electrode active material slurry was applied to a copper current collector with a thickness of 15 μm and dried at 70°C using a comma coater at a speed of 0.2 m / min, resulting in a load of 3.6 mAh / cm. 2 The negative electrode was formed so that

[0094] A secondary battery was manufactured in the same manner as in Example 1, except that the negative electrode was used.

[0095] Comparative Example 2 Unlike Comparative Example 1, which used only artificial graphite as the negative electrode active material, artificial graphite and lithium titanium oxide (Li4Ti5O 12 A secondary battery was manufactured in the same manner as in Comparative Example 1, except that the mixture contained 100% cellulose nitrate (0.9 μm, average particle size 0.9 μm) in a weight ratio of 97:3.

[0096] Comparative Example 3 <Production of negative electrodes> A first negative electrode active material layer was formed in the same manner as in Example 1.

[0097] Next, 1.5 parts by weight of a binder polymer (polyvinyllidene fluoride; PVdF), 1.5 parts by weight of multi-walled carbon nanotubes (MWCNT) with an average diameter of 30 nm as a conductive material, and lithium titanium oxide (Li4Ti5O 12 , average particle size D 50 97 parts by weight of the powder (0.9 μm) was dispersed in N-methylpyrrolidone as a dispersion medium to obtain a uniform second negative electrode active material slurry.

[0098] The second negative electrode active material slurry was applied to the surface of the first negative electrode active material layer and then dried at 120°C at a speed of 0.2 m / min using a comma coater to form a second negative electrode active material layer. At this time, the LTO loading in the second negative electrode active material layer was 0.06 mAh / cm.2 The thickness was 2.5 μm.

[0099] Thereafter, a secondary battery was produced in the same manner as in Example 1.

[0100] Comparative Example 4 <Production of negative electrodes> A secondary battery was manufactured in the same manner as in Comparative Example 3, except that 3 parts by weight of MWCNT was used.

[0101] Comparative Example 5 <Production of negative electrodes> The loading of LTO in the second negative electrode active material was 1.0 mAh / cm 2 A secondary battery was manufactured in the same manner as in Example 1, except that the thickness was set to 6.0 μm.

[0102] [Physical property evaluation method] Thickness evaluation of the second negative electrode active material layer The thickness of the second negative electrode active material was measured from the cross section of the electrode using a TEM image.

[0103] battery performance Mono-cell capacity evaluation method The negative and positive electrodes and porous polypropylene separators prepared in the above examples and comparative examples were assembled and then injected with an electrolyte to prepare a mono cell. After wetting for 24 hours, the cells were activated by charging to 30% SOC at 0.1 C current. Three cycles of CC / CV mode charging at 0.33 C, 4.2 V, and 0.05 C cutoff and CC mode discharging at 0.33 C, 3.0 V cutoff were performed, and the third discharge capacity was measured. The measurement results are shown in Table 1 below.

[0104] Monocell resistance evaluation method After fully charging in CC / CV mode at 0.33C, 4.2V, and 0.05C cutoff charge mode, the resistance was measured at each state of discharge (DOD). Resistance was measured by applying a pulse discharge current for 10 seconds, then no current for 30 seconds, and then applying a discharge current in the charge direction for 10 seconds, and calculating the resistance from the voltage change during discharge.

[0105] Method for evaluating the resistance of electrode interfaces The interfacial resistance of the electrode was measured using a four-point probe. A constant current was applied from the measurement probe and the potential change that occurred on the surface was measured. This makes it possible to confirm changes in resistance / interfacial resistance due to the electrode material, composition, and manufacturing conditions.

[0106] Internal short circuit evaluation method To fabricate a monocell, a 1cm x 1cm hole was drilled in the separator, and then covered with a 1.5cm x 6cm rectangular separator. After soaking in electrolyte for 24 hours, the cell was activated by charging to 30% SOC at a current of 0.1C, and then charged in CC / CV mode at 0.33C, 4.2V, and a 0.5C cutoff. The rectangular separator that had been covering the cell was removed to allow direct contact between the positive and negative electrodes. The temperature and voltage changes that occurred at the separator where the hole was drilled were measured and recorded.

[0107] [Evaluation results] The physical properties of the negative electrode active material layers and the performance evaluation results of the batteries using the examples and comparative examples, which were performed by the above-described method, are shown in Tables 1 and 2 below.

[0108] [Table 1]

[0109] [Table 2]

[0110] From the evaluation results obtained by the above method, it was found that when single-walled carbon nanotubes are used in the second negative electrode active material layer and the thickness is less than 3 μm, the cell capacity and resistance are excellent, and the temperature and voltage changes are reduced when an internal short circuit occurs, thereby ensuring the stability of the battery.

[0111] The present invention has been described above with reference to the embodiments and drawings. However, a person having ordinary knowledge in the field to which the present invention belongs can make various applications and modifications within the scope of the present invention based on the above content.

Claims

1. A current collector; a first negative electrode active material layer located on at least one surface of the current collector; a second negative electrode active material layer located on the first negative electrode active material layer, the second negative electrode active material layer includes lithium titanium oxide (LTO) and single-walled carbon nanotubes (SWCNTs); the first negative electrode active material layer contains a negative electrode active material other than lithium titanium oxide (LTO), The load of the second negative electrode active material layer is 0.1 mAh / cm ２ is as follows: the thickness of the second negative electrode active material layer is 1% to 5% of the total thickness of the first negative electrode active material layer and the second negative electrode active material layer; a negative electrode, wherein the second negative electrode active material layer has a thickness of 2.5 μm or less.

2. The load of the second negative electrode active material layer is 0.01 mAh / cm ２ 0.1mAh / cm or more ２ 2. The negative electrode of claim 1, wherein:

3. 2. The negative electrode according to claim 1, wherein the single-walled carbon nanotubes have a length of 1 μm to 30 μm.

4. The single-walled carbon nanotubes have an average cross-sectional diameter in a direction perpendicular to the longitudinal direction of 2 nm to 100 nm and a BET specific surface area of ​​500 m ２ / g~1,600m ２ The negative electrode according to claim 1 , wherein the anode has a Cr content of 1.0 / g.

5. 2. The negative electrode of claim 1, wherein the content of the single-walled carbon nanotubes is 0.01 wt % to 3 wt % based on the total weight of the second negative electrode active material layer.

6. 10. The negative electrode of claim 1, wherein the first negative electrode active material layer comprises a carbon-based active material, a silicon-based active material, or a mixture thereof.

7. The battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, A secondary battery, wherein the negative electrode is the negative electrode according to claim 1 .

8. The secondary battery according to claim 7 , wherein the secondary battery is a lithium secondary battery.

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