Negative electrode and secondary battery

Abstract

The present invention provides a negative electrode for a secondary battery and a secondary battery containing the same. [Solution] The negative electrode for a secondary battery includes a current collector; a first negative electrode active material layer provided on the current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer, wherein the first and second negative electrode active material layers contain a silicon-based active material and natural graphite, the natural graphite has an average particle size (D50) of 10 μm or less, and the OI (004 / 110) of the first and second negative electrode active material layers is 8 or less.

Description

[Technical Field]

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

[0002] This application claims the benefit as of the filing date of Korean Patent Application No. 10-2022-0125438, filed with the Korean Intellectual Property Office on September 30, 2022, and all its contents are incorporated herein by reference. [Background technology]

[0003] Rechargeable batteries are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs) that are powered by an electrical drive source.

[0004] Such rechargeable batteries offer not only the primary benefit of dramatically reducing the use of fossil fuels, but also the advantage of producing no by-products from energy use. Therefore, they are attracting attention as a new energy source that is environmentally friendly and improves energy efficiency.

[0005] Generally, a secondary battery includes a positive electrode, a negative electrode, a separator membrane interposed between the positive and negative electrodes, and an electrolyte. Furthermore, electrodes such as the positive and negative electrodes may have an electrode active material layer provided on a current collector.

[0006] As the use of secondary batteries increases, various battery performance characteristics are required. Attempts have been made to improve battery performance by adding additives to the active material layer. However, depending on the type of additive, some battery performance may improve, while others may deteriorate. Therefore, research is needed on the selection or combination of materials included in electrodes that can improve the required performance of secondary batteries. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] An object of the present invention is to provide a negative electrode for a secondary battery with improved rapid charging performance and a secondary battery including the same.

Means for Solving the Problems

[0008] One embodiment of the present invention is A current collector; A first negative electrode active material layer provided on the current collector; and A second negative electrode active material layer provided on the first negative electrode active material layer including the first negative electrode active material layer and the second negative electrode active material layer contain a silicon-based active material and natural graphite, and the average particle size (D50) of the natural graphite is 10 μm or less, and provides a negative electrode for a secondary battery in which the OI (004 / 110) of the first negative electrode active material layer and the second negative electrode active material layer is 8 or less.

[0009] Another embodiment of the present invention provides a secondary battery including the negative electrode for a secondary battery, a positive electrode, and a separator.

[0010] Another embodiment of the present invention is forming a first negative electrode active material layer on a current collector; and forming a second negative electrode active material layer on the first negative electrode active material layer including the first negative electrode active material layer and the second negative electrode active material layer contain a silicon-based active material and natural graphite, and the average particle size (D50) of the natural graphite is 10 μm or less, and further includes a step of magnetically aligning the first negative electrode active material and the second negative electrode active material layer to adjust the OI (004 / 110) to 8 or less, and provides a method for manufacturing a negative electrode for a secondary battery according to the above-described embodiment.

Effects of the Invention

[0011] According to the embodiments described herein, in a two-layer negative electrode active material layer containing a silicon-based active material and graphite, the rapid charging performance can be maximized by controlling the degree of graphite orientation in the negative electrode active material layer and optimizing the particle size of the natural graphite. Specifically, compared to forming the negative electrode active material layer as a single layer, forming it as a two-layer structure allows for improved lithium ion penetration and diffusion rates by making each layer thinner than a single layer. Furthermore, the two-layer structure can be configured with different materials for each layer as needed to improve battery characteristics. In addition, magnetic alignment of the graphite within the negative electrode reduces the tortuosity and degree of orientation of the negative electrode, allowing lithium movement within the negative electrode to proceed more smoothly. Compared to a single-layer structure, the durability, i.e., lifespan, of the battery after magnetic alignment can be improved in a two-layer structure. At the same time, by using natural graphite with a small particle size, the lithium diffusion distance within the particles is shortened, and filling can be accelerated. Furthermore, using natural graphite with a small particle size can improve the rapid filling performance of the negative electrode. [Modes for carrying out the invention]

[0012] The present invention will be described in further detail below to aid in understanding the present invention. The present invention can be embodied in a variety of different forms and is not limited to the embodiments described herein. In this context, terms or words used herein and in the claims shall not be interpreted to be limited to their ordinary or dictionary meanings, but rather to be interpreted in a sense and concept consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0013] In this specification, terms such as “includes,” “equip,” or “have” are intended to specify the presence of the described features, figures, stages, components, or combinations thereof, and are understood not to preemptively exclude the possibility of the presence or addition of one or more other features, figures, stages, components, or combinations thereof.

[0014] Furthermore, when a layer or other part is said to be "on top of" another part, this includes not only the case where it is "directly above" the other part, but also the case where there is another part in between. Conversely, when a part is said to be "directly above" another part, it means that there is no other part in between. Note that being "on top of" a reference part means being located above or below the reference part, and does not necessarily mean being located "up" in the opposite direction of gravity.

[0015] In this specification, the "weight loading ratio" of a particular layer means the weight percentage of that layer based on the total weight of all active material layers in the electrode.

[0016] In this specification, "SOC" means state of charge.

[0017] In this specification, particle size refers to the average particle size represented by D50. D50 can be defined as the particle size at 50% of the particle size distribution and can be measured using the laser diffraction method. For example, the method for measuring the average particle size (D50) involves dispersing particles in a dispersion medium, introducing them into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), irradiating them with ultrasound at approximately 28 kHz at an output of 60 W, and then calculating the average particle size (D50) corresponding to 50% of the cumulative volume in the measuring device.

[0018] In this specification, any description that refers only to the “active material layer” without the expressions “first” and “second” may apply to both the first and second active material layers.

[0019] A negative electrode for a secondary battery according to one embodiment of this specification includes a current collector; a first negative electrode active material layer provided on the current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer, wherein the first and second negative electrode active material layers include a silicon-based active material and natural graphite, the average particle size (D50) of the natural graphite is 10 μm or less, and the OI(004 / 110) of the first and second negative electrode active material layers is 8 or less. In other words, the negative electrode for a secondary battery can improve the rapid charging performance of the negative electrode by using small-particle natural graphite and adjusting the degree of graphite orientation of the negative electrode active material to the aforementioned range, thereby allowing lithium to move more actively within the negative electrode. When the OI(004 / 110) of the first and second negative electrode active material layers is 8 or less, the internal structure of the negative electrode active material layers is randomized, and the degree of disorder increases, which makes the movement and diffusion of lithium ions faster and easier, which is advantageous for rapid charging performance. The OI(004 / 110) of the first and second negative electrode active material layers may be 8 or less, 7 or less, or 6 or less.

[0020] The OI(004 / 110) of the first and second negative electrode active material layers is a value obtained through X-ray diffraction analysis, using a general method of X-ray diffraction analysis, such as JIS K 0131-1996, and can be obtained using a Bruker D4 Endeavor X-ray diffraction analyzer. In this case, the OI(004 / 110) is measured with the first and second negative electrode active material layers provided on the current collector. The OI is a value measured for the entire first and second negative electrode active material layers. The OI measured in this way includes not only the materials constituting the negative electrode active material layers but also the properties of the negative electrode active material layers imparted by manufacturing processes including coating and rolling, and is therefore distinguished from the OI value measured when the electrode material is in powder form.

[0021] The OI value of the negative electrode is I 004 / I 110 It can be expressed as, I 004 (004) represents the characteristic diffraction peak area of ​​the crystal plane, I 110The (110) crystal plane represents the characteristic diffraction peak area. The (004) crystal plane has a crystal structure in the parallel direction, and the (110) crystal plane has a crystal structure in the perpendicular direction. Such characteristics indicate the degree of orientation of the graphite crystals in the negative electrode active material layer. A larger OI value means that the orientation of the graphite is higher, and that the graphite is oriented more parallel to the measurement surface. In the above embodiment, by adjusting the OI value to 8 or less, the tortuosity and degree of orientation of the negative electrode can be reduced, and the lithium ions can be controlled to be more easily inserted into or removed from the crystal structure of the graphite. The OI value may be, for example, between 1 and 8, or between 3 and 8. The OI value can be measured at the electrode after negative electrode fabrication (fresh negative electrode or after charge / discharge including activation process).

[0022] In the aforementioned secondary battery negative electrode, when the weight loading ratio of the upper layer, i.e., the second negative electrode active material layer, is controlled to 20% to 35% of the total weight loading ratio of all negative electrode active material layers (including the first and second negative electrode active material layers), the thickness of the upper layer (second negative electrode active material layer) is set to 20% to 35% of the total thickness of the upper layer (second negative electrode active material layer) and the lower layer (first negative electrode active material layer). This reduces the distance lithium travels from the upper layer surface, allowing lithium to move more smoothly within the lower negative electrode through the aforementioned orientation. The weight loading ratio of the upper layer, the second negative electrode active material layer, may be 20% to 35%, for example, 25% to 35%, or 28% to 32%, relative to the entire upper and lower layers. Since the weight loading ratio of each layer is proportional to the thickness of each layer, the weight loading ratio can be confirmed from the thickness of each layer.

[0023] According to one embodiment, the thickness of the second negative electrode active material layer may be 40% to 60% of the thickness of the first negative electrode active material layer, for example, 45% to 55%, specifically 50%.

[0024] In this specification, the boundary between the first and second negative electrode active material layers can be confirmed by measuring the electrode cross-section. For example, the boundary between the two layers can be confirmed by cutting the electrode cross-section by ion milling and measuring the cross-sectional electrode with a scanning electron microscope.

[0025] According to one embodiment, the silicon-based active material content in the first negative electrode active material layer and the second negative electrode active material layer may differ. For example, the silicon-based active material content in the second negative electrode active material layer may be greater than the silicon-based active material content in the first negative electrode active material layer. In this way, even if the silicon-based active material content in the second negative electrode active material layer is large, the energy density can be prevented from becoming too low by setting the thickness or weight loading ratio of the second negative electrode active material layer to 20% or more. According to another embodiment, the silicon-based active material content in the first negative electrode active material layer and the second negative electrode active material layer may be the same.

[0026] According to one embodiment, the average particle size (D50) of the natural graphite may be 10 μm or less, 9 μm or less, or 6 μm to 9 μm. While typical natural graphite has an average particle size (D50) greater than 10 μm, in the above embodiment, the smaller the average particle size (D50), the shorter the diffusion distance of lithium ions becomes, which is advantageous for improving rapid charging characteristics by accelerating the penetration and diffusion of lithium ions. Specifically, the larger the particle size of the natural graphite, the longer the equivalent distance of lithium within the particle, resulting in inferior rapid charging. Conversely, the smaller the particle size, the shorter the lithium diffusion distance, which is more advantageous for improving rapid charging. As an exemplary method for adjusting the particle size of natural graphite, a method can be used in which flaky natural graphite is spheroidized and the surface of the spheroidized natural graphite is coated with carbon to reduce its large specific surface area. Flaky natural graphite can be spheroidized by airflow classification. The carbon coating process on the surface may be carried out by a heat treatment method after placing a precursor on the modified spheroidized natural graphite. The precursor is a precursor for carbon coating the surface of the natural graphite, and non-limiting examples include materials such as pitch.

[0027] According to one embodiment, the first and second negative electrode active material layers may further contain artificial graphite. The average particle size (D50) of the artificial graphite may be 15 μm to 50 μm. When the average particle size (D50) of the artificial graphite is 15 μm or more, the binder is uniformly applied to the surface of the active material, which can improve the adhesion of the negative electrode to the current collector. When it is 50 μm or less, lithium ions can diffuse easily, which can improve the rapid charging performance of the negative electrode.

[0028] Because artificial graphite consists of assembled secondary particles, it is difficult to adjust the particle size to less than 10 μm. Furthermore, even if the assembled artificial graphite (assembled from primary particles to form secondary particles) is 10 μm in size, it does not have a positive effect on the adhesion to the current collector. This is because the smaller particle size results in a greater amount of dead binder (binder sandwiched between particles that cannot contribute to adhesion), which negatively affects the adhesion. Moreover, the smaller the particle size of the artificial graphite, the more densely the particles are located within the electrode, resulting in a denser structure in the electrode after rolling, which is not advantageous for rapid charging performance. A dense structure contributes to the tortuosity within the electrode, i.e., the longer the distance lithium ions can travel, which is disadvantageous for rapid charging performance.

[0029] According to one embodiment, the first and second negative electrode active material layers may each contain 10 to 50 parts by weight, for example 15 to 45 parts by weight, or 20 to 45 parts by weight, of natural graphite based on 100 parts by weight of negative electrode active material.

[0030] According to one embodiment, the first and second negative electrode active material layers may each contain 50 to 89 parts by weight of artificial graphite, for example, 52 to 85 parts by weight, 55 to 80 parts by weight, or 58 to 72 parts by weight, based on 100 parts by weight of negative electrode active material. When the artificial graphite content is higher than the natural graphite content, it may be advantageous for improving rapid charging performance.

[0031] According to one embodiment, the natural graphite content of the second negative electrode active material layer is less than the natural graphite content of the first negative electrode active material layer. For example, the natural graphite content of the second negative electrode active material layer may be 25 to 35 parts by weight per 100 parts by weight of graphite in the second negative electrode active material layer, and the natural graphite content of the first negative electrode active material layer may be 35 to 45 parts by weight per 100 parts by weight of graphite in the first negative electrode active material layer. Artificial graphite is advantageous compared to natural graphite for improving rapid charging performance. On the other hand, natural graphite has a larger capacity than artificial graphite, can increase energy density, and can increase adhesion to the negative electrode current collector. Therefore, in a single-layer structure, it is difficult to simultaneously ensure rapid charging performance due to artificial graphite and high energy density and adhesive strength to the electrodes due to natural graphite. However, in the two-layer structure of the present invention, by relatively increasing the content of natural graphite in the first negative electrode active material layer located close to the negative electrode current collector, and relatively increasing the content of artificial graphite in the second negative electrode active material layer, it is possible to improve rapid lamination, energy density, and adhesive strength to the current collector.

[0032] In one embodiment of this specification, the silicon-based active material is SiO x (0≦x<2), SiM y The silicon-based active material contains at least one of the following: (M is a metal, 1≦y≦4) and Si / C. The silicon-based active material may contain only one type, or two or more types together. When both of the two negative electrode active material layers contain a silicon-based active material, the same type of silicon-based active material may be used for the two active material layers, or different types or different combinations of silicon-based active materials may be used.

[0033] In one embodiment of this specification, the silicon-based active material may be present in an amount of 1 to 40 parts by weight, for example, 1 to 20 parts by weight, based on a total of 100 parts by weight of active material contained in the negative electrode active material layer containing the silicon-based active material. The silicon-based active material in the first and second negative electrode active material layers may be present in the same amount or in different amounts.

[0034] As the silicon-based active material, SiO xThe active material containing (0≦x<2) is SiO x It may be silicon-based composite particles containing (0<x<2) and pores.

[0035] In this specification, a composite particle or a composite means that two or more materials or substances are physically aggregated without a chemical bond.

[0036] The SiO x (0<x<2) corresponds to a matrix in the silicon-based composite particles. The SiO x (0<x<2) may be in a form containing Si and SiO2, and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiO x (0<x<2). When the silicon-based composite particles contain the SiO x (0<x<2), the discharge capacity of the secondary battery can be improved.

[0037] The silicon-based composite particles may further contain at least one of a magnesium (Mg) compound and a lithium (Li) compound. The magnesium (Mg) compound and the lithium (Li) compound can correspond to a matrix in the silicon-based composite particles.

[0038] The magnesium (Mg) compound and / or the lithium (Li) compound may be present inside and / or on the surface of the SiO x (0<x<2). The initial efficiency of the battery can be improved by the magnesium (Mg) compound and / or the lithium (Li) compound.

[0039] The Mg compound may include at least one selected from the group consisting of magnesium (Mg) silicate, magnesium (Mg) silicide, and magnesium (Mg) oxide. The magnesium (Mg) silicate may include at least one of Mg2SiO4 and MgSiO3. The magnesium (Mg) silicide may include Mg2Si. The Mg oxide may include MgO.

[0040] In one embodiment of this specification, the magnesium (Mg) element may be present in an amount of 0.1% to 20% by weight, or 0.1% to 10% by weight, based on 100% by weight of the total silicon-based active material. Specifically, the magnesium (Mg) element may be present in an amount of 0.5% to 8% by weight or 0.8% to 4% by weight. When these ranges are met, the magnesium (Mg) compound can be included in the silicon-based active material in an appropriate amount, so that volume changes of the silicon-based active material can be easily suppressed during charging and discharging of the battery, and the discharge capacity and initial efficiency of the battery can be improved.

[0041] The lithium(Li) compound may include at least one selected from the group consisting of lithium(Li) silicate, lithium(Li) silicide, and lithium(Li) oxide. The lithium(Li) silicate may include at least one of Li2SiO3, Li4SiO4, and Li2Si2O5. The lithium(Li) silicide may include Li7Si2. The lithium(Li) oxide may include Li2O.

[0042] In one embodiment of the present invention, the lithium (Li) compound may include the form of a lithium silicate. The lithium silicate is Li a Si b O c(2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and can be classified into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate can exist in the form of at least one lithium silicate selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5 within the silicon-based composite particles, and the amorphous lithium silicate is Li a Si b O c It may be in the form of (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and is not limited to the above form.

[0043] In one embodiment of the present specification, the Li element may be contained at 0.1 wt% to 20 wt%, or may be contained at 0.1 wt% to 10 wt% based on 100 wt% of the total silicon-based active material. Specifically, the Li element may be contained at 0.5 wt% to 8 wt%, and more specifically may be contained at 0.5 wt% to 4 wt%. When the above range is satisfied, the Li compound can be contained in an appropriate content within the silicon-based active material, so that the change in the volume of the negative electrode active material can be easily suppressed during charging and discharging of the battery, and the discharge capacity and initial efficiency of the battery can be improved.

[0044] The content of the Mg element or Li element can be confirmed by ICP (Inductively Coupled Plasma) analysis. After accurately collecting a certain amount (about 0.01 g) of the negative electrode active material for the ICP analysis, it is transferred to a platinum crucible, nitric acid, hydrofluoric acid, and sulfuric acid are added, and it is completely decomposed on a hot plate. Then, using an inductively coupled plasma optical emission spectrometer (ICPAES, Perkin-Elmer 7300), the intensity of the standard solution (5 mg / kg) prepared with a standard solution at the specific wavelength of the Mg element or Li element is measured to create a standard calibration curve. Then, the pretreated sample solution and the background sample are introduced into the instrument, the intensities of each are measured to calculate the actual intensity, and after calculating the concentration of each component by comparing with the created calibration curve, the content of the Mg element or Li element of the silicon-based active material manufactured by converting so that the overall sum becomes the theoretical value can be analyzed.

[0045] In one embodiment of this specification, a carbon layer may be provided on the surface and / or inside the pores of the silicon-based composite particles. The carbon layer imparts conductivity to the silicon-based composite particles, thereby improving the initial efficiency, lifespan, and battery capacity characteristics of a secondary battery containing a negative electrode active material that includes the silicon-based composite particles. The total weight of the carbon layer may be 5% to 40% by weight, based on 100% by weight of the silicon-based composite particles.

[0046] In one embodiment of this specification, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0047] The average particle size (D50) of the silicon-based active material is 2 μm to 15 μm, specifically 3 μm to 12 μm, and more specifically 4 μm to 10 μm. When this range is met, the side reactions between the silicon-based composite particles and the electrolyte are controlled, and the discharge capacity and initial efficiency of the battery can be effectively realized.

[0048] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 The particle size can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can obtain highly reproducible and high-resolution results.

[0049] The silicon-based active material containing Si / C is a composite of Si and C, and is distinguished from silicon carbide, which is denoted as SiC. The silicon-carbon composite may be a composite of silicon and graphite, or it may form a structure in which a core of silicon and graphite is surrounded by graphene or amorphous carbon. The silicon dispersed in the silicon-carbon composite may be nanosilicon.

[0050] In one embodiment of this specification, the negative electrode active material in each 100 parts by weight of the negative electrode active material layer may be present in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0051] According to further embodiments of this specification, the negative electrode active material layer may further include a negative electrode binder in addition to the silicon-based active material and the carbon-based active material.

[0052] The negative electrode binder can serve to improve adhesion between negative electrode active material particles and adhesion between negative electrode active material particles and negative electrode current collector. The negative electrode binder can be any known in the art, and non-limiting examples include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which their hydrogen atoms are substituted with Li, Na, or Ca, and may also include various copolymers thereof.

[0053] The negative electrode binder may be included in an amount of 0.1 parts by weight or more and 20 parts by weight or less based on 100 parts by weight of the negative electrode active material layer, for example, preferably 0.3 parts by weight or more and 20 parts by weight or less, and more preferably 0.5 parts by weight or more and 10 parts by weight or less.

[0054] The negative electrode active material layer does not necessarily have to contain a conductive material, but may further contain a conductive material as needed. The conductive material contained in the negative electrode active material layer is not particularly limited as long as it does not induce a chemical change in the battery and is conductive, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; 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 can be used. The content of the conductive material in the negative electrode active material layer may be 0.01 to 20 parts by weight, preferably 0.03 to 18 parts by weight, per 100 parts by weight of the negative electrode active material layer.

[0055] For the purposes of this invention, if the carbon-based active material is graphite such as natural graphite or artificial graphite, the weight of the graphite used as the carbon-based active material is not considered when defining the total weight of the conductive material. Similarly, if the conductive material selected according to the negative electrode is graphite, the weight of the conductive material described when defining the total weight of the carbon-based negative electrode active material is not included. Therefore, if graphite is selected for both the carbon-based negative electrode active material and the negative electrode conductive material, the total weight of the graphite corresponds to the sum of the weight of the graphite used as the carbon-based negative electrode active material and the weight of the graphite used as the negative electrode conductive material.

[0056] For example, the conductive material contained in the negative electrode active material layer may be carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; or conductive tubes such as carbon nanotubes.

[0057] In one embodiment of this specification, the thickness of the first negative electrode active material layer may be 1 μm or more and 50 μm or less, for example, 2 μm or more and 40 μm or less, 5 μm or more and 30 μm or 10 μm or more and 25 μm or less; the thickness of the second negative electrode active material layer may be 5 μm or more and 100 μm or less, for example, 10 μm or more and 80 μm or less, 20 μm or more and 60 μm or less, or 25 μm or more and 50 μm or less. The sum of the thicknesses of the first negative electrode active material layer and the second negative electrode active material layer may be 6 μm or more and 150 μm or less, 50 μm or more and 140 μm or less, or 80 μm or more and 120 μm or less.

[0058] In one embodiment of this specification, the negative electrode current collector is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, the current collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Specifically, transition metals that effectively adsorb carbon, such as copper and nickel, can be used as the current collector. The thickness of the current collector may be 1 μm to 500 μm, but is not limited thereto.

[0059] Further embodiments of this specification provide a secondary battery comprising a negative electrode, a positive electrode, and a separator membrane according to the embodiments described above.

[0060] In one embodiment of this specification, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material. The thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less.

[0061] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., can be used. The positive electrode current collector may also have a thickness of 1 to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0062] In one embodiment of this specification, the positive electrode may include a lithium-compound transition metal compound containing nickel (Ni) and cobalt (Co) as an active material. The lithium-compound transition metal compound may further include at least one of manganese and aluminum. The lithium-compound transition metal compound may contain 80 mol% or more, for example, 80 mol% or more but less than 100 mol%, of nickel among the metals excluding lithium.

[0063] In one embodiment, the positive electrode active material in the 100 parts by weight of the positive electrode active material layer may be present in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0064] According to further embodiments of this specification, the positive electrode active material layer according to the above-described embodiment may further include a positive electrode binder and a conductive material.

[0065] The positive electrode binder can play a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material particles and the positive electrode current collector. As the positive electrode binder, any known material in the art can be used, and non-limiting examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used.

[0066] The positive electrode binder may be included in an amount of 0.1 parts by weight or more and 20 parts by weight or less based on 100 parts by weight of the positive electrode active material layer, for example, preferably 0.3 parts by weight or more and 35 parts by weight or less, and more preferably 0.5 parts by weight or more and 20 parts by weight or less.

[0067] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode and can be used without particular limitations as long as it has electronic conductivity without undergoing chemical changes within the battery. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0068] Specifically, in one embodiment, the conductive material may include one or more single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The conductive material may be included in an amount of 0.1 parts by weight or more and 2 parts by weight or less per 100 parts by weight of the positive electrode active material layer composition, for example, preferably 0.3 parts by weight or more and 1.5 parts by weight or less, and more preferably 0.5 parts by weight or more and 1.2 parts by weight or less.

[0069] The positive and negative electrodes can be manufactured according to conventional methods for manufacturing positive and negative electrodes, except that the positive and negative electrode active materials are used. Specifically, they can be manufactured by coating a composition for forming an active material layer, which includes the active material and, selectively, a binder and a conductive material, onto a current collector, followed by drying and rolling. In this case, the types and contents of the positive and negative electrode active materials, binder, and conductive material are as described above. The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of solvent used should be sufficient to dissolve or disperse the active material, conductive material, and binder, taking into consideration the coating thickness of the slurry and the manufacturing yield, and to have a viscosity that allows for excellent thickness uniformity when coated for manufacturing the positive and negative electrodes. Alternatively, the positive and negative electrodes may be manufactured by casting the active material layer forming composition onto another support, peeling it off the support, and then laminating the resulting film onto the current collector.

[0070] The separation membrane separates the negative and positive electrodes and provides a pathway for lithium ions to move. It can be used without particular limitations as long as it is the type typically used as a separation membrane in secondary batteries. Particularly preferred is a membrane that exhibits low resistance to electrolyte ion movement while maintaining excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separation membranes containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used in single-layer or multi-layer structures.

[0071] Examples of the aforementioned electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0072] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0073] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0074] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferred because they are high-viscosity organic solvents with high dielectric constants that effectively dissociate lithium salts. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high electrical conductivity can be created, and this mixture is even more preferable.

[0075] The metal salt can be a lithium salt, and the lithium salt is a substance that dissolves easily in the non-aqueous electrolyte. For example, the anion of the lithium salt is F - Cl - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3- , (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - You can use one or more selected from the group consisting of the following.

[0076] In addition to the electrolyte components, the electrolyte may further contain one or more additives for purposes such as improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0077] Further embodiments of this specification provide a method for manufacturing a negative electrode for a secondary battery according to the embodiments described above, comprising the steps of forming a first negative electrode active material layer on a current collector; and forming a second negative electrode active material layer on the first negative electrode active material layer, wherein the first and second negative electrode active material layers comprise a silicon-based active material and natural graphite, the natural graphite having an average particle size (D50) of 10 μm or less, and further comprising the step of magnetically aligning the first and second negative electrode active material layers to adjust the OI(004 / 110) to 8 or less. The magnetic alignment may be performed by coating the first negative electrode active material layer on the current collector; and coating the second negative electrode active material layer on the first negative electrode active material layer, both under magnetic conditions. If necessary, the steps of coating the first and second negative electrode active material layers may each further include a drying step. That is, the first negative electrode active material layer can be coated and dried, and the second negative electrode active material layer can be coated and dried under magnetic conditions.

[0078] According to one embodiment, the weight loading ratio of the second anode active material layer may be 20% to 35% of the weight loading ratio of the first and second anode active material layers. When the weight loading ratio of the second anode active material layer is 20% or more, the energy density is high, which is advantageous for matching pseudo-grade cell specifications.

[0079] The above embodiment further includes a step of magnetically aligning the first and second negative electrode active material layers to adjust the OI to 8 or less. The degree of alignment can be controlled by magnetic alignment intensity or time, with stronger intensity or relatively longer exposure time resulting in better magnetic alignment and a lower OI value. Therefore, magnetism can be strategically applied during the above method to adjust the OI values ​​of the first and second negative electrode active material layers to 8 or less.

[0080] A secondary battery according to one embodiment of the present invention comprises an assembly including a positive electrode, a negative electrode, a separator membrane, and an electrolyte, which may be a lithium secondary battery. The lithium secondary battery has excellent rapid charging performance, and for example, as measured by the experimental method in the examples described later, the negative electrode included in the secondary battery may have a 3C Li plating (SOC) of 27% or more, preferably 28% or more or 29% or more when a lithium coin half cell is fabricated.

[0081] Further embodiments of the present invention provide a battery module and a battery pack containing the aforementioned secondary battery as a unit cell. Since the battery module and battery pack include the secondary battery having high capacity, high rate-limiting characteristics and cycle characteristics, they can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

[0082] The secondary battery according to the embodiment of the present invention exhibits excellent discharge capacity, output characteristics, and cycle performance stably, and can therefore be used as a power source for not only portable devices such as mobile phones, laptop computers, and digital cameras, but also for medium- and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems. For example, the battery module or battery pack can be used as a power source for one or more medium- and large-sized devices from among power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0083] Preferred embodiments are presented below to aid in understanding the present invention, but these embodiments are illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the present description and the technical concept. Such variations and modifications naturally fall within the scope of the appended claims.

[0084] Example 1 A composition for forming a first negative electrode active material layer, containing artificial graphite with a D50 of 21 μm and natural graphite with SiO and D50 of 9 μm, was coated onto a 15 μm thick copper foil and then dried to form the first negative electrode active material layer. A composition for forming a second negative electrode active material layer, containing artificial graphite with a D50 of 21 μm and natural graphite with SiO and D50 of 9 μm, was coated onto the first negative electrode active material layer and then dried to form the second negative electrode active material layer, thereby fabricating the negative electrode. Magnetic orientation was performed during the coating of the first and second negative electrode active material layer forming compositions by passing them through a magnetic facility and controlling the magnet strength and total exposure time.

[0085] The compositions for forming the first and second negative electrode active material layers were prepared by mixing the aforementioned negative electrode active material, Super C65 as a conductive material, styrene-butadiene rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener in the weight ratios of 96 (artificial graphite / natural graphite / SiO 60 / 39 / 1):1:2:1 (first negative electrode active material layer) and 96 (artificial graphite / natural graphite / SiO 70 / 29 / 1):1:2:1 (second negative electrode active material layer), respectively, and adding water.

[0086] The loading amount of the second negative electrode active material layer forming composition was set to 30% by weight of the total amount of the first and second negative electrode active material layer forming compositions, and the OI of the produced first and second negative electrode active material layers was 6.

[0087] A lithium coin half cell was manufactured by dissolving vinylene carbonate at a volume ratio of 7:3 in a mixed solution of methyl ethyl carbonate (EMC) and ethylene carbonate (EC), dissolving it at a concentration of 0.5% by weight, and then injecting an electrolyte solution containing 1M LiPF6.

[0088] OI was measured using the following method.

[0089] The orientation index I(004) / I(110) indicates the degree to which the crystal structure within the negative electrode active material layer is aligned in a specific direction, allowing for evaluation of the orientation of the crystals within the negative electrode active material layer, and can be measured by X-ray diffraction (XRD). More specifically, the orientation index is the area ratio ((004) / (110)) obtained by integrating the peak intensities of the (110) plane and the (004) plane after measuring the (110) plane and the (004) plane of the negative electrode active material contained in the negative electrode active material layer by XRD, and more specifically, the XRD measurement conditions are as follows.

[0090] - Target: Cu (Kα line) graphite monochromator - Slit: Divergent slit = 1 degree, Receiving slit = 0.1 mm, Scattering slit = 1 degree - Measurement area and step angle / measurement time: (110) plane: 76.5 degrees < 2θ < 78.5 degrees, 0.01 degrees / 3 seconds (004) plane: 53.5 degrees < 2θ < 56.0 degrees, 0.01 degrees / 3 seconds, In the above, 2θ represents the diffraction angle.

[0091] Example 2 The procedure was carried out in the same manner as in Example 1, except that the D50 of the natural graphite contained in the first and second negative electrode active materials was 7 μm. The loading amount of the second negative electrode active material forming composition was 30% by weight of the total amount of the first and second negative electrode active material forming composition, and the OI of the manufactured first and second negative electrode active material layers was 3. Compared to Example 1, the exposure time to the magnetic equipment was increased, which reduced the torsion and orientation of the negative electrode due to the magnetic alignment of the graphite within the negative electrode.

[0092] Example 3 The procedure was carried out in the same manner as in Example 1, except that natural graphite with a D50 of 10 μm was used instead of natural graphite with a D50 of 9 μm during the production of the first and second negative electrode active material layers, and the OI of the first and second negative electrode active material layers was adjusted to 8.

[0093] Comparative Example 1 The procedure was carried out in the same manner as in Example 1, except that the coating of the first and second negative electrode active material layer forming compositions did not involve passing through magnetic equipment. The loading amount of the second negative electrode active material forming composition was 30% by weight of the total amount of the first and second negative electrode active material forming compositions, and the OI of the manufactured first and second negative electrode active material layers was 14.

[0094] Comparative Example 2 The procedure was carried out in the same manner as in Example 1, except that the natural graphite contained in the first and second negative electrode active materials had a D50 of 18 μm. The loading amount of the second negative electrode active material forming composition was 30% by weight of the total amount of the first and second negative electrode active material forming compositions, and the exposure time to the magnetic equipment was reduced compared to Example 1, resulting in an OI of 10 for the first and second negative electrode active material layers.

[0095] Comparative Example 3 The procedure was carried out in the same manner as in Example 1, except that the loading amount of the second negative electrode active material forming composition was set to 70% by weight relative to the total amount of the first and second negative electrode active material forming compositions. The OI of the first and second negative electrode active material layers produced by reducing the exposure time to the magnetic equipment compared to Example 1 was 10.

[0096] Comparative Example 4 The procedure was carried out in the same manner as in Example 1, except that the natural graphite contained in the first and second negative electrode active materials had a D50 of 20 μm. The loading amount of the second negative electrode active material forming composition was 30% by weight of the total amount of the first and second negative electrode active material forming composition, and the OI of the first and second negative electrode active material layers manufactured without exposure to the magnetic equipment was 22.

[0097] Comparative Example 5 The procedure was carried out in the same manner as in Example 1, except that natural graphite with a D50 of 20 μm was used in the production of the first negative electrode active material layer, the degree of negative electrode orientation was adjusted to 24, and the second negative electrode active material layer was not formed. Due to the material properties, when there was a large amount of small fine powder in the particle size distribution, the electrode orientation after electrode rolling increased, and the OI value increased.

[0098] After the manufactured half-cells were charged and discharged three times at 0.1C, the discharge capacity of the third cycle (1C) was used as the reference. The rapid charging performance was evaluated by determining the Li-plating SOC by finding the point of gradient change in the output voltage due to the change in SOC (state of charge) through differentiation with respect to capacity while charging in CC mode (3C) for 15 minutes, and determining the Li-plating SOC. The Li-plating SOC is the SOC point where lithium is deposited.

[0099] [Table 1]

[0100] As shown in Table 1 above, it was confirmed that the rapid charging performance of the half-cells manufactured in the examples was superior to that of comparative examples 1 to 5.

Claims

1. Current collector, A first negative electrode active material layer provided on the current collector, and A second negative electrode active layer provided on the first negative electrode active layer Includes, The first negative electrode active material layer and the second negative electrode active material layer contain a silicon-based active material and natural graphite, and the average particle size (D50) of the natural graphite is 10 μm or less. A negative electrode for a secondary battery, wherein the OI(004 / 110) of the first and second negative electrode active material layers is 8 or less.

2. The negative electrode for a secondary battery according to claim 1, wherein the thickness of the second negative electrode active material layer is 20% to 35% of the total thickness of the first negative electrode active material layer and the second negative electrode active material layer.

3. The negative electrode for a secondary battery according to claim 1, wherein the thickness of the second negative electrode active material layer is 40% to 60% of the thickness of the first negative electrode active material layer.

4. The negative electrode for a secondary battery according to claim 1, wherein the average particle size (D50) of the natural graphite is 6 μm to 9 μm.

5. The negative electrode for a secondary battery according to claim 1, wherein the first negative electrode active material layer and the second negative electrode active material layer further comprise artificial graphite.

6. The negative electrode for a secondary battery according to claim 5, wherein the average particle size (D50) of the artificial graphite is 15 μm to 50 μm.

7. The negative electrode for a secondary battery according to claim 1, wherein the first negative electrode active material layer and the second negative electrode active material layer each contain 1 to 40 parts by weight of silicon-based active material based on 100 parts by weight of negative electrode active material.

8. The aforementioned silicon-based active material is SiO ｘ (0≦x<2), SiM ｙ The negative electrode for a secondary battery according to claim 1, comprising (M is a metal, 1 ≤ y ≤ 4), and at least one of Si / C.

9. The negative electrode for a secondary battery according to claim 1, wherein the first negative electrode active material layer and the second negative electrode active material layer each contain 10 to 50 parts by weight of natural graphite based on 100 parts by weight of negative electrode active material.

10. The negative electrode for a secondary battery according to claim 1, wherein the first negative electrode active material layer and the second negative electrode active material layer each contain 50 to 99 parts by weight of artificial graphite based on 100 parts by weight of negative electrode active material.

11. A secondary battery comprising a negative electrode, a positive electrode, and a separator membrane according to any one of claims 1 to 10.

12. The secondary battery according to claim 11, wherein the positive electrode contains a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as an active material.

13. The secondary battery according to claim 12, wherein the lithium composite transition metal compound further comprises at least one of manganese and aluminum.

14. A method for manufacturing a negative electrode for a secondary battery according to any one of claims 1 to 10, The steps include forming a first negative electrode active material layer on the current collector, and Step of forming a second negative electrode active material layer on the first negative electrode active material layer. Includes, The first negative electrode active material layer and the second negative electrode active material layer contain a silicon-based active material and natural graphite, and the average particle size (D50) of the natural graphite is 10 μm or less. A method further comprising the step of magnetically aligning the first negative electrode active material layer and the second negative electrode active material layer to adjust OI(004 / 110) to 8 or less.

15. The method according to claim 14, wherein the weight loading ratio of the second anode active material layer is 20% to 35% of the weight loading ratio of the first anode active material layer and the second anode active material layer.

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