Anodes and secondary batteries

Abstract

The present invention relates to a negative electrode for a secondary battery, comprising: 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, the first and second negative electrode active material layers containing a silicon-based active material and natural graphite, the natural graphite having an average particle size (D50) of 10 μm or less, and the first and second negative electrode active material layers having an OI(004 / 110) of 8 or less, and a secondary battery including the same.

Description

[Technical Field]

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

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

[0003] Secondary batteries are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs), which are powered by electrical sources.

[0004] Such secondary batteries have the primary advantage of dramatically reducing the use of fossil fuels, as well as the advantage of not producing any by-products from energy use, making them environmentally friendly and drawing attention as a new energy source for improving energy efficiency.

[0005] In general, a secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. In addition, the electrodes such as the positive electrode and the negative electrode may have an electrode active material layer formed on a current collector.

[0006] As secondary batteries become more widely used, various battery performances are being demanded. 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 be improved, while other performance may be degraded. Therefore, research is needed on the selection or combination of materials to be included in the electrode that can improve the required performance of secondary batteries. Summary of the Invention [Problem to be solved by the invention]

[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 problem]

[0008] One embodiment of the present invention comprises: 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 natural graphite has an average particle size (D50) of 10 μm or less; The negative electrode for a secondary battery is provided, wherein the first negative electrode active material layer and the second negative electrode active material layer have an OI(004 / 110) of 8 or less.

[0009] Another embodiment of the present invention provides a secondary battery comprising the above-described 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 natural graphite has an average particle size (D50) of 10 μm or less; The method for manufacturing a negative electrode for a secondary battery according to the above embodiment further includes 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. [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, 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 a single-layer negative electrode active material layer, a two-layer negative electrode active material layer can improve the penetration and diffusion rate of lithium ions by forming each layer thinner than a single-layer negative electrode active material layer. Furthermore, a two-layer structure can be constructed with different materials for each layer as needed to improve battery performance. Furthermore, magnetic alignment of graphite within the negative electrode reduces the tortuosity and orientation of the negative electrode, facilitating smooth lithium migration within the negative electrode. Compared to a single-layer structure, the two-layer structure can improve the durability, i.e., lifespan, of a battery after magnetic alignment. At the same time, the use of natural graphite with a small particle size shortens the lithium diffusion distance within the particles, thereby accelerating charging. Furthermore, the use of natural graphite with a small particle size can improve the rapid charging performance of the negative electrode. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in more detail below to facilitate understanding of the present invention. The present invention can be embodied in various different forms and is not limited to the embodiments described herein. In this regard, the terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concepts of terms in order to best describe his or her invention.

[0013] In this specification, the terms "comprises," "comprises," or "having" are intended to specify the presence of stated features, numbers, steps, components, or combinations thereof, and are understood not to preclude the possible presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

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

[0015] As used herein, the "weight loading ratio" of a particular layer refers to the weight percent of that layer based on the total weight of all active material layers in the electrode.

[0016] As used herein, "SOC" means state of charge.

[0017] In this specification, particle size refers to the average particle size expressed as D50. D50 can be defined as the particle size at 50% of the particle size distribution and can be measured using a laser diffraction method. For example, the average particle size (D50) can be measured by dispersing particles in a dispersion medium, introducing the dispersed particles into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and irradiating the dispersed particles with ultrasonic waves of about 28 kHz at an output of 60 W. The average particle size (D50) corresponding to 50% of the cumulative volume in the analyzer can then be calculated.

[0018] In this specification, a description referring only to an "active material layer" without the terms "first" and "second" can apply to both the first and second active material layers.

[0019] According to one embodiment of the present specification, a negative electrode for a secondary battery includes a current collector; a first negative electrode active material layer disposed on the current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer. The first and second negative electrode active material layers include a silicon-based active material and natural graphite, and the natural graphite has an average particle size (D50) of 10 μm or less and an OI (004 / 110) of 8 or less. In other words, by using small-particle-size natural graphite and adjusting the degree of graphite orientation of the negative electrode active material within the above range, the negative electrode for a secondary battery can more actively transport lithium within the negative electrode, thereby improving the fast charging performance of 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, increasing the degree of disorder, which facilitates faster and easier migration and diffusion of lithium ions, thereby improving fast charging performance. The first and second negative electrode active material layers may have an OI(004 / 110) of 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 by X-ray diffraction analysis, and can be obtained using a common X-ray diffraction analysis method, such as JIS K 0131-1996, and an X-ray diffraction analyzer such as a Bruker D4 Endeavor. The OI (004 / 110) is measured with the first and second negative electrode active material layers disposed on a current collector. The OI is a value measured for the entire first and second negative electrode active material layers. The OI measured in this manner includes not only the materials constituting the negative electrode active material layers but also the properties of the negative electrode active material layers imparted through manufacturing processes, including coating and rolling, and is therefore distinct from the OI value measured when the electrode material is in a powder state.

[0021] The OI value of the negative electrode is I 004 / I 110 and I 004 represents the characteristic diffraction peak area of ​​the (004) crystal plane, and I 110represents the characteristic diffraction peak area of ​​the (110) crystal plane. The (004) crystal plane has a parallel crystal structure, while the (110) crystal plane has a perpendicular crystal structure. These characteristics indicate the degree of orientation of the graphite crystals in the negative electrode active material layer. The larger the OI value, the higher the graphite orientation, meaning that the graphite is more parallel to the measurement surface. In this embodiment, adjusting the OI value to 8 or less reduces the tortuosity and orientation of the negative electrode, thereby enabling lithium ions to be more easily inserted into or extracted from the graphite crystal structure. The OI value may be, for example, 1 to 8, e.g., 3 to 8. The OI value can be measured on a fresh negative electrode or on a negative electrode after charge / discharge, including an activation process.

[0022] In the negative electrode for a secondary battery, when the weight loading ratio of the upper layer (i.e., the second negative electrode active material layer) of the two-layer 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 controlled 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 must travel to intercalate from the surface of the upper layer, thereby facilitating smooth lithium migration within the lower layer negative electrode through the aforementioned orientation. The weight loading ratio of the second negative electrode active material layer, which is the upper layer of the negative electrode active material layers, may be 20% to 35%, e.g., 25% to 35%, or 28% to 32%, based on the total weight of the upper and lower layers. Because the weight loading ratio of each layer is proportional to the thickness of each layer, the weight loading ratio can be determined 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%, for example 45% to 55%, specifically 50% of the thickness of the first negative electrode active material layer.

[0024] In this specification, the boundary between the first and second negative electrode active material layers can be confirmed by measuring the cross section of the electrode. For example, the electrode cross section can be cut by ion milling, and the cross sectioned electrode can be measured using a scanning electron microscope to confirm the boundary between the two layers.

[0025] According to one embodiment, the first and second negative electrode active material layers may have different silicon-based active material contents. According to one example, the second negative electrode active material layer may have a higher silicon-based active material content than the first negative electrode active material layer. Even when the second negative electrode active material layer contains a large amount of silicon-based active material, the thickness or weight loading ratio of the second negative electrode active material layer may be set to 20% or more to prevent the energy density from becoming too low. According to another embodiment, the first and second negative electrode active material layers may have the same silicon-based active material content.

[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) of more than 10 μm, in this embodiment, the smaller the average particle size (D50) is, i.e., 10 μm or less, the shorter the lithium ion diffusion distance, which is advantageous for accelerating lithium ion penetration and diffusion and improving fast charging characteristics. Specifically, the larger the particle size of natural graphite, the longer the lithium equivalent distance within the particle, resulting in poor fast charging. Conversely, the smaller the particle size, the shorter the lithium diffusion distance, which is more advantageous for improving fast charging. An exemplary method for adjusting the particle size of natural graphite is to spheroidize flake natural graphite and then coat the surface of the spheroidized natural graphite with carbon to reduce the specific surface area. The flake natural graphite can be spheroidized by air flow classification. The surface carbon coating treatment may be performed by disposing a precursor on the modified spheroidized natural graphite and then heat treating the precursor. The precursor is a precursor for carbon coating the surface of natural graphite, and a material such as pitch may be used as a non-limiting example.

[0027] According to one embodiment, the first and second negative electrode active material layers may further contain artificial graphite. The artificial graphite may have an average particle size (D50) of 15 μm to 50 μm. When the average particle size (D50) of the artificial graphite is 15 μm or more, the binder is uniformly coated on the surface of the active material, thereby improving the negative electrode adhesion to the current collector. When the average particle size (D50) of the artificial graphite is 50 μm or less, lithium ions are easily diffused, thereby improving the negative electrode rapid charging performance.

[0028] Because artificial graphite is an assembled secondary particle, it is difficult to adjust the particle size below 10 μm. Even if the size of assembled artificial graphite (primary particles assembled into secondary particles) is 10 μm, it does not have a favorable effect on adhesion to the current collector. This is because the small particles result in a greater amount of dead binder (a binder sandwiched between particles that cannot act as an adhesive), which adversely affects adhesion. Furthermore, the smaller the particle size of artificial graphite, the denser the particles are located within the electrode, resulting in a more densely packed structure after rolling, which is not beneficial for fast charging performance. A dense structure increases the tortuosity within the electrode, i.e., increases the distance that lithium ions can travel, which is detrimental to fast charging performance.

[0029] According to one embodiment, the first and second negative electrode active material layers may each contain 10 parts by weight to 50 parts by weight, for example, 15 parts by weight to 45 parts by weight, or 20 parts by weight to 45 parts by weight, of natural graphite based on 100 parts by weight of the 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, for example, 52 to 85 parts by weight, 55 to 80 parts by weight, or 58 to 72 parts by weight of artificial graphite, based on 100 parts by weight of the negative electrode active material. If the content of artificial graphite is higher than the content of natural graphite, this may be advantageous in improving fast charging performance.

[0031] According to one embodiment, the content of natural graphite in the second negative electrode active material layer is lower than the content of natural graphite in the first negative electrode active material layer. For example, the content of natural graphite in 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 content of natural graphite in 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 for improving fast charging performance compared to natural graphite. Meanwhile, natural graphite has a larger capacity than artificial graphite, which can increase energy density and improve adhesion to the negative electrode current collector. Therefore, while it is difficult to simultaneously obtain the fast charging performance of artificial graphite and the high energy density and adhesive strength to the electrode of natural graphite with a single-layer structure, the two-layer structure of the present invention relatively increases the natural graphite content in the first negative electrode active material layer disposed closer to the negative electrode current collector, and relatively increases the artificial graphite content in the second negative electrode active material layer, thereby improving the fast charging performance, energy density, and adhesive strength to the current collector.

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

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

[0036] The above SiO x (0<x<2) corresponds to a matrix within the silicon-based composite particles. The above SiO x (0<x<2) may be in a form containing Si and SiO2, and the Si may form a phase. That is, the above x corresponds to the number ratio of O to Si contained in the above SiO x (0<x<2). When the silicon-based composite particles contain the above 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 within 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 above 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 the present specification, the magnesium (Mg) element may be included in an amount of 0.1 wt % to 20 wt %, or 0.1 wt % to 10 wt %, based on a total of 100 wt % of the silicon-based active material. Specifically, the magnesium (Mg) element may be included in an amount of 0.5 wt % to 8 wt % or 0.8 wt % to 4 wt %. When this range is satisfied, the magnesium (Mg) compound can be contained in the silicon-based active material in an appropriate amount, which can easily suppress volumetric changes of the silicon-based active material during battery charge and discharge, thereby improving the discharge capacity and initial efficiency of the battery.

[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 a form of lithium silicate. 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% to 20% by weight, or may be contained at 0.1% to 10% by weight based on 100% by weight of the silicon-based active material. Specifically, the Li element may be contained at 0.5% to 8% by weight, and more specifically, may be contained at 0.5% to 4% by weight. 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 during charging and discharging of the battery can be easily suppressed, 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 sampling a certain amount (about 0.01 g) of the negative electrode active material for the ICP analysis, it is transferred to a platinum crucible, and nitric acid, hydrofluoric acid, and sulfuric acid are added and completely decomposed on a hot plate. Then, using an inductively coupled plasma optical emission spectrometer (ICPAES, Perkin-Elmer 7300), the intensity of a standard solution (5 mg / kg) prepared using a standard solution at the characteristic wavelength of the Mg element or Li element is measured to create a 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 conversion so that the total sum becomes the theoretical value can be analyzed.

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

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

[0047] The silicon-based active material may have an average particle size (D50) of 2 μm to 15 μm, specifically 3 μm to 12 μm, and more specifically 4 μm to 10 μm. When the silicon-based active material satisfies this range, 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 achieved.

[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 ) can be measured using, for example, the laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0049] The silicon-based active material containing Si / C is a composite of Si and C and is distinguished from silicon carbide, also referred to as SiC. The silicon carbon composite may be a composite of silicon and graphite, or may have 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 the present specification, the negative electrode active material may be contained 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, in 100 parts by weight of each negative electrode active material layer.

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

[0052] The negative electrode binder may improve adhesion between negative electrode active material particles and between the negative electrode active material particles and the negative electrode current collector. The negative electrode binder may be any binder known in the art, and non-limiting examples thereof may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, 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 hydrogen is substituted with Li, Na, or Ca, or various copolymers thereof.

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

[0054] The negative electrode active material layer may not contain a conductive material, but may further contain a conductive material if necessary. The conductive material contained in the negative electrode active material layer is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; 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. 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 purposes of the present invention, when the carbon-based active material is graphite, such as natural graphite or artificial graphite, the weight parts of the graphite used as the carbon-based active material are not taken into account when defining the total weight parts of the conductive material. Similarly, when the conductive material selected for the negative electrode is graphite, the weight parts of the conductive material described when defining the total weight parts of the carbon-based negative electrode active material are not included. Therefore, when graphite is selected as both the carbon-based negative electrode active material and the negative electrode conductive material, the total weight parts of graphite correspond to the sum of the weight parts of the graphite used as the carbon-based negative electrode active material and the weight parts of the graphite used as the negative electrode conductive material.

[0056] According to an 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 fiber such as carbon fiber or metal fiber; or conductive tube such as carbon nanotube.

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

[0058] In one embodiment of the present specification, the negative electrode 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 can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that effectively adsorbs carbon, such as copper or nickel, can be used as the current collector. The thickness of the current collector can be 1 μm to 500 μm, but is not limited thereto.

[0059] A further embodiment herein provides a secondary battery comprising an anode, a cathode and a separator according to the previous embodiment.

[0060] In one embodiment of the present 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 positive electrode active material layer may have a thickness of 20 μm to 500 μm.

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

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

[0063] In one embodiment, the positive electrode active material may be contained 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, in 100 parts by weight of the positive electrode active material layer.

[0064] According to a further embodiment of the present specification, the positive electrode active material layer according to the aforementioned embodiment may further include a positive electrode binder and a conductive material.

[0065] The positive electrode binder may serve to improve adhesion between positive electrode active material particles and between the positive electrode active material particles and the positive electrode current collector. The positive electrode binder may be any binder known in the art, and non-limiting examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination.

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

[0067] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode, and can be any material that does not undergo chemical changes in the battery and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination of two or more.

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

[0069] The positive and negative electrodes can be fabricated according to conventional methods for fabricating positive and negative electrodes, except for the use of the positive and negative electrode active materials. Specifically, they can be fabricated by coating a composition for forming an active material layer containing the active material and, optionally, a binder and a conductive material, on a current collector, followed by drying and rolling. 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 may be used alone or in combination. The amount of solvent used should be sufficient to dissolve or disperse the active material, conductive material, and binder, taking into account the coating thickness and manufacturing yield of the slurry, and to provide a viscosity that allows excellent thickness uniformity when applied to fabricate positive and negative electrodes. Alternatively, the positive electrode and the negative electrode may be produced by casting the active material layer-forming composition on a separate support, peeling the composition from the support, and laminating the resulting film on a current collector.

[0070] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without limitation. It is particularly preferable that the separator has low resistance to electrolyte ion movement and excellent electrolyte humidification capability. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and it can be used in a single-layer or multi-layer structure.

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

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

[0073] Examples of the non-aqueous organic solvent that may be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0074] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because they are high-viscosity organic solvents with high dielectric constants and can dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte with high electrical conductivity can be produced, and therefore, these cyclic carbonates are more preferably used.

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

[0076] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0077] A further embodiment of the present specification provides a method for manufacturing a negative electrode for a secondary battery according to the above embodiment, the method including: 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, the first and second negative electrode active material layers comprising a silicon-based active material and natural graphite, the natural graphite having an average particle size (D50) of 10 μm or less, and 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 achieved 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 in a magnetic state. 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 may be coated and dried, and the second negative electrode active material layer may be coated and dried in a magnetic state.

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

[0079] This embodiment further includes 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 the magnetic alignment intensity or time, and the stronger the intensity or the longer the exposure time, the better the magnetic alignment and the lower the OI value. Therefore, magnetism can be strategically applied during the above-described method to adjust the OI value 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 includes an assembly including a positive electrode, a negative electrode, a separator, and an electrolyte, and may be a lithium secondary battery. The lithium secondary battery has excellent fast charging performance, and for example, when a lithium coin half cell is fabricated using the negative electrode included in the secondary battery, the 3C Li plating SOC may be 27% or more, preferably 28% or more or 29% or more, as measured by an experimental method in the Examples described below.

[0081] A further embodiment of the present invention provides a battery module including the aforementioned secondary battery as a unit cell, and a battery pack including the same. The battery module and battery pack include the secondary battery having high capacity, excellent rate-limiting characteristics, and excellent cycle characteristics, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0082] The secondary battery according to the embodiment of the present invention stably exhibits excellent discharge capacity, output characteristics, and cycle performance, and can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, as well as portable devices such as mobile phones, laptops, and digital cameras. For example, the battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0083] In the following, preferred embodiments are presented to aid in understanding the present invention, but these embodiments are merely illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the present invention and the technical spirit of the present invention. Such changes and modifications are naturally intended to fall within the scope of the appended claims.

[0084] Example 1 A first negative electrode active material layer-forming composition containing artificial graphite with a D50 of 21 μm, SiO, and natural graphite with a D50 of 9 μm as negative electrode active materials was coated on a 15 μm-thick copper foil and then dried to form a first negative electrode active material layer. A second negative electrode active material layer-forming composition containing artificial graphite with a D50 of 21 μm, SiO, and natural graphite with a D50 of 9 μm as negative electrode active materials was coated on the first negative electrode active material layer and then dried to form a second negative electrode active material layer, thereby fabricating a negative electrode. The coating of the first and second negative electrode active material layer-forming compositions was performed using magnet equipment, and magnetic orientation was performed by 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 above-mentioned negative electrode active material, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener in a weight ratio 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 then adding water.

[0086] The loading amount of the composition for forming the second negative electrode active material layer was 30 wt % based on the total amount of the compositions for forming the first and second negative electrode active material layers, and the OI of the produced first and second negative electrode active material layers was 6.

[0087] A lithium coin half cell was fabricated by dissolving 0.5 wt% vinylene carbonate in a mixed solution of methyl ethyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 7:3, and injecting an electrolyte solution in which 1M LiPF6 was dissolved.

[0088] OI was measured in the following manner.

[0089] The orientation index I(004) / I(110) indicates the degree to which the crystalline structure within the negative electrode active material layer is aligned in a certain direction, and can be measured by X-ray diffraction (XRD) to evaluate the orientation of the crystals within the negative electrode active material layer. More specifically, the orientation index is the area ratio ((004) / (110)) obtained by measuring the (110) and (004) planes of the negative electrode active material included in the negative electrode active material layer by XRD and then integrating the peak intensities of the (110) and (004) planes. More specifically, the XRD measurement conditions are as follows:

[0090] - Target: Cu (Kα line) graphite monochromator -Slit: Divergence 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 same procedures as in Example 1 were carried out, 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 composition for forming the second negative electrode active material was 30 wt % based on the total amount of the compositions for forming the first and second negative electrode active materials, and the OI of the prepared first and second negative electrode active material layers was 3. Compared to Example 1, the exposure time to the magnetic equipment was increased, thereby reducing the tortuosity and orientation of the negative electrode due to the magnetic alignment of the graphite within the negative electrode.

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

[0093] Comparative Example 1 The same procedure as in Example 1 was carried out, except that the first and second negative electrode active material layer-forming compositions were not coated using a magnet. The loading amount of the second negative electrode active material composition was 30 wt % based on the total amount of the first and second negative electrode active material layers, and the OI of the prepared first and second negative electrode active material layers was 14.

[0094] Comparative Example 2 The same procedures as in Example 1 were carried out, 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 composition for forming the second negative electrode active material was 30 wt % based on the total amount of the compositions for forming the first and second negative electrode active material, and the exposure time to the magnet equipment was reduced compared to Example 1. The OI of the prepared first and second negative electrode active material layers was 10.

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

[0096] Comparative Example 4 The same procedures as in Example 1 were carried out, 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 composition for forming the second negative electrode active material was 30 wt % based on the total amount of the compositions for forming the first and second negative electrode active material, and the OI of the first and second negative electrode active material layers prepared without exposure to a magnet was 22.

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

[0098] The fabricated half cell was charged and discharged three times at 0.1C, and then charged in CC mode (3C) for 15 minutes, with the discharge capacity of the third cycle at 1C as the reference. The output voltage due to the change in SOC (state of charge) was differentiated against the capacity to find the gradient change point, and the Li plating SOC was determined to evaluate the fast charge performance. The Li plating SOC is the SOC point at which lithium is deposited.

[0099] [Table 1]

[0100] As shown in Table 1, it was confirmed that the half cells manufactured in the examples had superior fast charging performance compared to 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 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, natural graphite, and artificial graphite, and the natural graphite has an average particle size (D50) of 10 μm or less; the OI(004 / 110) of the first and second negative electrode active material layers is 1 or more and 8 or less; a content of natural graphite in the second negative electrode active material layer is less than a content of natural graphite in the first negative electrode active material layer, and the content of natural graphite in the first negative electrode active material layer is 35 parts by weight to 45 parts by weight per 100 parts by weight of graphite in the first negative electrode active material layer.

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

3. 2. 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. 2. The negative electrode for a secondary battery according to claim 1, wherein the natural graphite has an average particle size (D50) of 6 μm to 9 μm.

5. 2. The negative electrode for a secondary battery according to claim 1, wherein the artificial graphite has an average particle size (D50) of 15 μm to 50 μm.

6. 2. 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 part by weight to 40 parts by weight of a silicon-based active material based on 100 parts by weight of the negative electrode active material.

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

8. A negative electrode for a secondary battery as described in claim 1, wherein the content of natural graphite in the second negative electrode active material layer is 25 to 35 parts by weight per 100 parts by weight of graphite in the second negative electrode active material layer.

9. A secondary battery comprising the negative electrode for a secondary battery according to any one of claims 1 to 8, a positive electrode, and a separator.

10. The secondary battery according to claim 9 , wherein the positive electrode contains a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as an active material.

11. The secondary battery according to claim 10 , wherein the lithium composite transition metal compound further contains at least one of manganese and aluminum.

12. A method for producing the negative electrode for a secondary battery according to any one of claims 1 to 8, comprising: 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, natural graphite, and artificial graphite, and the natural graphite has an average particle size (D50) of 10 μm or less; The method further includes magnetically aligning the first and second negative electrode active material layers to adjust OI(004 / 110) to 1 or more and 8 or less.

13. The method of claim 12 , wherein a weight loading ratio of the second negative electrode active material layer is 20% to 35% of a weight loading ratio of the first negative electrode active material layer and the second negative electrode active material layer.

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