Negative electrode and secondary battery

Abstract

The present invention relates to a secondary battery including 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 negative electrode active material layer and the second negative electrode active material layer contain a silicon-based active material and natural graphite, the natural graphite has an average particle size (D50) of 9 μm or less, and the difference in porosity between the first negative electrode active material layer and the second negative electrode active material layer is 6% or less.

Description

Technical Field

[0001] This specification claims the benefit of the filing dates of Korean Patent Application No. 10-2022-0125394 filed with the Korean Intellectual Property Office on September 30, 2022, and Korean Patent Application No. 10-2023-0128281 filed with the Korean Intellectual Property Office on September 25, 2023, and all the contents disclosed in the documents of the corresponding Korean patent applications are included in this specification.

[0002] The present invention relates to a negative electrode for a secondary battery and a secondary battery including the same.

Background Art

[0003] Secondary batteries are widely applied not only to portable devices but also to electric vehicles (EVs), hybrid electric vehicles (HEVs), etc., which are driven by an electric drive source.

[0004] Such secondary batteries have not only the primary advantage of significantly reducing the use of fossil fuels but also the advantage of generating no by-products due to energy use, and thus are attracting attention as an environmentally friendly new energy source for improving energy efficiency.

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

[0006] As the utilization rate of secondary batteries increases, various battery performances are required. In order to improve battery performance, attempts have been made to introduce additives into the active material layer. On the other hand, depending on the type of additive, some battery performances may be improved, but some performances may rather deteriorate. Therefore, research on the selection or combination of materials included in the electrode that can improve the performances required for secondary batteries is needed.

Summary of the Invention

Problems 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 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 natural graphite has an average particle size (D50) of 9 μm or less, and provides a negative electrode for a secondary battery in which the difference in porosity between the first negative electrode active material layer and the second negative electrode active material layer is 6% 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.

Advantages of the Invention

[0010] 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 adjusting the difference in porosity between the upper and lower layers and optimizing the particle size of natural graphite. Specifically, with the above configuration, the current density of the negative electrode can be made uniform to prevent local lithium precipitation and improve the rapid charge and discharge performance. When the difference in porosity between the upper and lower layers becomes large, the diffusion of lithium ions becomes non-uniform, and a phenomenon where lithium ions are locally rich may occur, which may cause lithium precipitation. Since small particle size natural graphite has a smaller particle size than large particle size natural graphite, the distance of lithium diffusion within the particles is short, it can move quickly, and filling can be performed promptly.

Modes for Carrying Out the Invention

[0011] Hereinafter, in order to assist the understanding of the present invention, the present invention will be described in more detail. The present invention can be embodied in various different forms and is not limited to the embodiments described herein. At this time, the terms or words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor should interpret them in a meaning and concept consistent with the technical idea of the present invention in accordance with the principle that the concept of the terms can be appropriately defined in order to explain his or her invention in the best way.

[0012] In this specification, terms such as "comprising", "including" or "having" are intended to specify that there are implemented features, numbers, steps, components, or combinations thereof, and it should be understood that they do not preclude in advance the presence or addition of one or other features, numbers, steps, components, or combinations thereof.

[0013] Also, when a certain part such as a layer is "on" 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 certain part is "directly above" another part, it means that there is no other part in between. Note that being "on" the reference part means being located above or below the reference part, and does not necessarily mean being located "upward" in the opposite direction of gravity.

[0014] In this specification, the particle size means the average particle size represented by D50. D50 can be defined as the particle size at the 50% criterion 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) of the positive electrode active material is to disperse the particles of the positive electrode active material in a dispersion medium and then introduce them into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), irradiate with ultrasonic waves of about 28 kHz at an output of 60 W, and then calculate the average particle size (D50) corresponding to 50% of the volume cumulative amount in the measuring device.

[0015] In this specification, the description that only refers to the "active material layer" without the first negative electrode active material layer and the second means can be applied to both the first active material layer and the second active material layer.

[0016] The negative electrode for a secondary battery according to an 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. The first negative electrode active material layer and the second negative electrode active material layer include a silicon-based active material and natural graphite. The natural graphite has an average particle size (D50) of 9 μm or less, and the difference in porosity between the first negative electrode active material layer and the second negative electrode active material layer is 6% or less. That is, the negative electrode for a secondary battery includes two layers of negative electrode active material layers, controls the difference in porosity between the two layers of negative electrode active material layers to be below a specific value, and uses natural graphite with an average particle size (D50) controlled to 9 μm or less together with a silicon-based active material as the active material. With such a configuration, the current density of the negative electrode can be made uniform, lithium precipitation can be prevented from a local position, and thereby the rapid charge and discharge performance can be improved.

[0017] According to one embodiment, the difference in porosity between the first negative electrode active material layer and the second negative electrode active material layer may be 6% or less, or 5% or less. In order to control the difference in porosity in this way, the type of components, particle size, content of each component, or rolling conditions included in each layer can be adjusted. In terms of rolling conditions, the rolling temperature, rolling speed, or heating timing of the rolling roll can be adjusted. For example, a method of heating the rolling roll before rolling to increase the roll temperature can be used. At this time, the temperature of the rolling roll can be controlled between 30°C and 60°C. When rolling the two-layer negative electrode active material layer using the heated rolling roll, the difference in porosity within the upper and lower negative electrode active material layers can be reduced.

[0018] According to one embodiment, the porosity of the first negative electrode active material layer and the second negative electrode active material layer may be 1% to 40%, for example, 5% to 7%, respectively. If the difference in porosity between the upper layer and the lower layer is large, a concentration difference of lithium ions may occur, which may cause local deviation of lithium ions and lithium precipitation.

[0019] In this specification, the boundary between the first negative electrode active material layer and the second negative electrode active material layer can be confirmed through measurement of the electrode cross-section. For example, the electrode cross-section can be cut by ion milling treatment of the electrode cross-section, and the boundary between the two layers can be confirmed by measuring the cross-sectioned electrode with a scanning electron microscope.

[0020] According to one embodiment, the average particle size (D50) of the natural graphite may be 9 μm or less, or may be 6 μm to 9 μm. The larger the particle size of the natural graphite, the longer the conversion distance of lithium inside the particles, resulting in inferior rapid charging. Conversely, the smaller the particle size, the shorter the lithium diffusion distance, which can act more favorably on the improvement of 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 then carbon-coated on the surface of the spheroidized natural graphite to reduce the large specific surface area. The flaky natural graphite can be spheroidized by an air classification method. The treatment of carbon-coating the surface may be performed by a heat treatment method after arranging a precursor on the modified spheroidized natural graphite. The precursor is for carbon-coating the surface of natural graphite, and materials such as pitch may be used as non-limiting examples.

[0021] According to one embodiment, the first negative electrode active material layer and the second negative electrode active material layer 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 negative electrode adhesion can be improved. When it is 50 μm or less, the particle size can be effectively controlled to improve the negative electrode rapid charging performance.

[0022] According to one embodiment, the first negative electrode active material layer and the second negative electrode active material layer may each contain 10 parts by weight to 50 parts by weight, for example, 10 parts by weight to 45 parts by weight of natural graphite based on 100 parts by weight of the negative electrode active material.

[0023] According to one embodiment, the first negative electrode active material layer and the second negative electrode active material layer may each contain 50 parts by weight to 99 parts by weight, for example, 60 parts by weight to 90 parts by weight, or 50 parts by weight to 85 parts by weight of artificial graphite based on 100 parts by weight of the negative electrode active material. Making the content of artificial graphite higher than the content of natural graphite may be advantageous for improving the rapid charging performance.

[0024] In one embodiment of this specification, the silicon-based active material is SiO x (0 ≦ x < 2), SiMy It contains at least one of (M is a metal, 1 ≤ y ≤ 4) and Si / C. The silicon-based active material may contain only one kind or two or more kinds may be contained together. When all of the two-layer anode active material layers contain a silicon-based active material, the same kind of silicon-based active material may be used for the two-layer active material layers, or different types or different combinations of silicon-based active materials may be used.

[0025] In one embodiment of the present specification, the anode active material layer containing the silicon-based active material may further contain a carbon-based active material. At this time, based on 100 parts by weight of the total active materials contained in the anode active material layer containing the silicon-based active material, the silicon-based active material may be contained in an amount of 1 part by weight to 40 parts by weight, for example, 1 part by weight to 20 parts by weight.

[0026] As the silicon-based active material, SiO x (0 ≤ x < 2) The active material containing may be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0027] The SiO x (0 < x < 2) corresponds to the matrix within the silicon-based composite particles. The SiO x (0 < x < 2) may be in a form containing Si and SiO 2 , 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.

[0028] The silicon-based composite particles may further contain at least one of a Mg compound and a Li compound. The Mg compound and the Li compound can correspond to the matrix within the silicon-based composite particles.

[0029] The Mg compound and / or the Li compound is the SiOx It may be present inside and / or on the surface of (0 < x < 2). The initial efficiency of the battery can be improved by the Mg compound and / or Li compound.

[0030] The Mg compound may include at least one selected from the group consisting of Mg silicate, Mg silicide, and Mg oxide. The Mg silicate is Mg 2 SiO 4 and MgSiO 3 and may include at least one of them. The Mg silicide may include Mg 2 Si. The Mg oxide may include MgO.

[0031] In one embodiment of the present specification, the Mg element may be contained in an amount of 0.1% by weight to 20% by weight, or may be contained in an amount of 0.1% by weight to 10% by weight, based on 100% by weight of the total silicon-based active material. Specifically, the Mg element may be contained in an amount of 0.5% by weight to 8% by weight or 0.8% by weight to 4% by weight. When the above range is satisfied, the Mg compound can be contained in an appropriate content in the silicon-based active material, so that the volume change 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.

[0032] The Li compound may include at least one selected from the group consisting of lithium (Li) silicate, lithium (Li) silicide, and Li oxide. The Li silicate is Li 2 SiO 3 and, Li 4 SiO 4 and, Li 2 Si 2 O 5 and may include at least one of them. The Li silicide may include Li 7 Si 2 The Li oxide may include Li 2 O.

[0033] In one embodiment of the present invention, the Li compound may include a lithium silicate form. 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 Li 2 SiO 3 and Li 4 SiO 4 and Li 2 Si 2 O 5 in the silicon-based composite particles. The amorphous lithium silicate may be in the form of Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and is not limited to this form.

[0034] In one embodiment of the present specification, the Li element may be contained in an amount of 0.1 wt% to 20 wt%, or may be contained in an amount of 0.1 wt% to 10 wt%, based on 100 wt% of the total silicon-based active material. Specifically, the Li element may be contained in an amount of 0.5 wt% to 8 wt%, and more specifically, may be contained in an amount of 0.5 wt% to 4 wt%. When the above range is satisfied, the Li compound can be contained in an appropriate content in 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.

[0035] The content of the Mg element or Li element can be confirmed by ICP analysis. For the ICP analysis, after accurately sampling a certain amount (about 0.01 g) of the negative electrode active material, 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 spectrometry (ICPAES, Perkin-Elmer 7300), the intensity of a standard solution (5 mg / kg) prepared using the characteristic wavelength of the Mg element or Li element is measured to create a standard calibration curve. Then, the pretreated sample solution and the substrate sample are introduced into the instrument, the intensity of each is measured to calculate the actual intensity, and after calculating the concentration of each component with respect to the created calibration curve, the content of the Mg element or Li element in the silicon-based active material manufactured by conversion so that the total sum becomes the theoretical value can be analyzed.

[0036] 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 imparts conductivity to the silicon-based composite particles, and can improve the initial efficiency, life characteristics, and battery capacity characteristics of a secondary battery including the negative electrode active material containing the silicon-based composite particles. The total weight of the contained carbon layer may be 5 wt% to 40 wt% based on 100 wt% in total of the silicon-based composite particles.

[0037] In one embodiment of the present specification, the carbon layer may contain at least one of amorphous carbon and crystalline carbon.

[0038] 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 may be 4 μm to 10 μm. When the above range is satisfied, the side reaction between the silicon-based composite particles and the electrolyte can be controlled, and the discharge capacity and initial efficiency of the battery can be effectively realized.

[0039] In this specification, the average particle diameter (D50) can be defined as the particle diameter corresponding to 50% of the volume cumulative amount in the particle diameter distribution curve of the particles. The average particle diameter (D50) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle diameters in the range from the submicron region to about several millimeters, and can obtain results with high reproducibility and high resolution.

[0040] The active material containing Si / C as the silicon-based active material is a composite of Si and C, and is distinguished from silicon carbide denoted as SiC. The silicon-carbon composite may be a composite of silicon and graphite or the like, or may form a structure in which a core in which silicon and graphite or the like are compositeized is surrounded by graphene or amorphous carbon or the like. In the silicon-carbon composite, the silicon may be nanosilicon.

[0041] In one embodiment of this specification, the negative electrode active material in 100 parts by weight of each negative electrode active material layer 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.

[0042] According to a further embodiment of this 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.

[0043] The negative electrode binder can play a role in improving the adhesion between negative electrode active material particles and the adhesion force between the negative electrode active material particles and the negative electrode current collector. As the negative electrode binder, those known in the art can be used. Non-limiting examples include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, poly acrylic acid, and at least one selected from the group consisting of substances in which their hydrogens are substituted with Li, Na, Ca, etc., and various copolymers thereof may also be included.

[0044] The negative electrode binder may be contained in an amount of 0.1 part 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, it may be contained in an amount of 0.3 part by weight or more and 20 parts by weight or less, and more preferably 0.5 part by weight or more and 10 parts by weight or less.

[0045] 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 has conductivity without inducing a chemical change in the battery. For example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbons, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. 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, based on 100 parts by weight of the negative electrode active material layer.

[0046] In one embodiment of the present specification, the thicknesses of the first negative electrode active material layer and the second negative electrode active material layer may each be 5 μm or more and 500 μm or less. The thickness ratio of the first negative electrode active material layer and the second negative electrode active material layer may be between 1:2 and 2:1.

[0047] In one embodiment of the present specification, the negative electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used. Specifically, transition metals such as copper and nickel that can adsorb carbon well can be used as the current collector. The thickness of the current collector can be 1 μm to 500 μm, but the thickness of the current collector is not limited thereto.

[0048] A further embodiment of the present specification provides a secondary battery including a negative electrode, a positive electrode, and a separator according to the foregoing embodiments.

[0049] 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 thickness of the positive electrode active material layer may be 20 μm or more and 500 μm or less.

[0050] The positive electrode current collector is not particularly limited as long as it has conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel can be used. Further, the positive electrode current collector may usually have a thickness of 1 to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesive force of the positive electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabric bodies, etc.

[0051] 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 active materials. The lithium composite transition metal compound may further include at least one of manganese and aluminum. The lithium composite transition metal compound may contain 80 mol% or more, for example, 80 mol% or more and less than 100 mol% of nickel among the metals excluding lithium.

[0052] In one embodiment, the positive electrode active material in 100 parts by weight of the positive electrode active material layer 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 still more preferably 98 parts by weight or more and 99.9 parts by weight or less.

[0053] According to a further embodiment of the present specification, the positive electrode active material layer according to the foregoing embodiment may further include a positive electrode binder and a conductive material.

[0054] The positive electrode binder can play a role in improving the adhesion between the positive electrode active material particles and the adhesion force between the positive electrode active material particles and the positive electrode current collector. As the positive electrode binder, those known in the art can be used. Non-limiting examples 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, or various copolymers thereof, etc. Among these, one kind alone or a mixture of two or more kinds may be used.

[0055] The positive electrode binder may be contained in an amount of 0.1 part by weight or more and 50 parts by weight or less based on 100 parts by weight of the positive electrode active material layer. For example, preferably, it may be contained in an amount of 0.3 part by weight or more and 35 parts by weight or less, and more preferably, 0.5 part by weight or more and 20 parts by weight or less.

[0056] 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 limitation as long as it has electron conductivity without undergoing chemical changes in the battery. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, silver; conductive whiskers such as zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc. Among these, one kind alone or a mixture of two or more kinds may be used.

[0057] Specifically, in one embodiment, the conductive material may include one or more of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). The conductive material may be included in an amount of 0.1 part by weight or more and 2 parts by weight or less based on 100 parts by weight of the composition for the positive electrode active material layer. For example, preferably, it may be included in an amount of 0.3 part by weight or more and 1.5 parts by weight or less, and more preferably 0.5 part by weight or more and 1.2 parts by weight or less.

[0058] The positive electrode and the negative electrode can be manufactured according to the usual manufacturing methods of positive and negative electrodes, except that the above-mentioned positive and negative electrode active materials are used. Specifically, after applying a composition for forming an active material layer, which includes the above-mentioned active material and optionally a binder and a conductive material, onto a current collector, it can be manufactured by drying and rolling. At this time, the types and contents of the positive and negative electrode active materials, the binder, and the conductive material are as described above. The solvent may be a solvent generally 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 thereof may be used. The amount of the solvent used is such that, considering the coating thickness of the slurry and the production yield, it can dissolve or disperse the active material, the conductive material, and the binder, and then has a viscosity that can exhibit excellent thickness uniformity during coating for manufacturing the positive and negative electrodes. Also, as another method, the positive and negative electrodes may be manufactured by casting the composition for forming the active material layer onto another support, and then laminating a film obtained by peeling from the support onto the current collector.

[0059] The separation membrane separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Generally, any material that can be used as a separation membrane in a secondary battery can be used without particular limitation. In particular, it is preferably low in resistance to the ion migration of the electrolyte while having excellent electrolyte moisture retention ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. Also, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used. Further, in order to ensure heat resistance or mechanical strength, a coated separation membrane containing a ceramic component or a polymer substance can also be used, and it may be selectively used in a single-layer or multilayer structure.

[0060] 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 the manufacture of lithium secondary batteries.

[0061] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0062] Examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, etc. may be used.

[0063] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are organic solvents with high viscosity and high dielectric constant, which can dissociate lithium salts well, so they are preferably used. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate in an appropriate ratio and used, an electrolyte having high electrical conductivity can be made and can be more preferably used.

[0064] As the metal salt, a lithium salt can be used. The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte. For example, as the anion of the lithium salt, F - , Cl - , I - , NO 3 - , N(CN) 2 - , BF 4 - , ClO 4 - , PF 6 - , (CF 3 ) 2 PF 4 - , (CF 3 )3 PF 3 - ,(CF 3 ) 4 PF 2 - ,(CF 3 ) 5 PF - ,(CF 3 ) 6 P - ,CF 3 SO 3 - ,CF 3 CF 2 SO 3 - ,(CF 3 SO 2 ) 2 N - ,(FSO 2 ) 2 N - ,CF 3 CF 2 (CF 3 ) 2 CO - ,(CF 3 SO 2 ) 2 CH - ,(SF 5 ) 3 C - ,(CF 3 SO 2 ) 3 C - ,CF 3 (CF 2 ) 7 SO 3 - ,CF 3 CO 2 - ,CH 3 CO 2 - ,SCN - and (CF 3 CF 2 SO 2 ) 2 N - One or more selected from the group consisting of can be used.

[0065] In addition to the electrolyte constituent components, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, triamide hexaline, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, improving the discharge capacity of the battery, etc.

[0066] The secondary battery according to an embodiment of the present invention includes an assembly including a positive electrode, a negative electrode, a separator and an electrolyte, and this may be a lithium secondary battery.

[0067] 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. Since the battery module and the battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, they can be used as a power source 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.

[0068] The secondary battery according to an embodiment of the present invention can be used as a power source for not only portable devices such as mobile phones, notebook computers, and digital cameras, but also 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, in order to stably exhibit excellent discharge capacity, output characteristics, and cycle performance. For example, the battery module or battery pack can be used as a power source for one or more medium- and large-sized devices among power tools; electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); or power storage systems.

Example

[0069] Hereinafter, preferred examples are presented to assist in the understanding of the present invention. However, it is obvious to those skilled in the art that the examples are illustrative of the description and various changes and modifications can be made within the scope of the description and the scope of the technical idea. It is natural that such variations and modifications belong to the scope of the appended claims.

[0070] (Example 1) A composition for forming a first negative electrode active material layer containing artificial graphite with a D50 of 20 μm, SiO, and natural graphite with a D50 of 9 μm as a negative electrode active material was coated on a copper foil with a thickness of 15 μm so as to have a dry thickness of about 33 μm, and then dried to form a first negative electrode active material layer. A composition for forming a second negative electrode active material layer containing artificial graphite with a D50 of 20 μm, SiO, and natural graphite with a D50 of 9 μm as a negative electrode active material was coated on the first negative electrode active material layer so as to have a dry thickness of about 33 μm, and then dried to form a second negative electrode active material layer to manufacture a negative electrode.

[0071] The compositions for forming the first negative electrode active material layer and the second negative electrode active material layer were prepared by mixing the aforementioned negative electrode active material, Super C65 as a conductive material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener at weight ratios of 96 (artificial graphite / natural graphite / SiO 50 / 49 / 1):1:2:1 (for the first negative electrode active material layer) and 96 (artificial graphite / natural graphite / SiO 70 / 27 / 3):1:2:1 (for the second negative electrode active material layer), respectively. Water was added to the mixtures to prepare the compositions, which were then coated onto copper foils.

[0072] After coating the compositions, the rolling roll temperature was set before rolling, and then the negative electrode was placed between the rolls to proceed with rolling. At this time, the rolling roll temperature was set to about 40°C to 45°C.

[0073] The difference in porosity between the manufactured first negative electrode active material layer and the second negative electrode active material layer was 5%. The porosity difference was calculated by measuring the cross-sectional image of the negative electrode, separating the upper and lower negative electrode active material layers, excluding the compositions in each layer, finding the voids, calculating the porosity of each layer, and then calculating the difference.

[0074] Vinylene carbonate dissolved at 0.5 wt% was dissolved in a mixed solution of methyl ethyl carbonate (EMC) and ethylene carbonate (EC) with a mixed volume ratio of 7:3, and 1 M concentration of LiPF 6 was dissolved to prepare an electrolyte solution, and a lithium coin half-cell was manufactured.

[0075] (Example 2) The same procedure as in Example 1 was carried out, except that natural graphite with a D50 of 9 μm was used and the rolling roll temperature was 40°C to 45°C. The difference in porosity between the manufactured first negative electrode active material layer and the second negative electrode active material layer was 3%.

[0076] (Comparative Example 1) The same procedure as in Example 1 was carried out, except that natural graphite with a D50 of 18 μm was used. The difference in porosity between the manufactured first negative electrode active material layer and the second negative electrode active material layer was 5%.

[0077] (Comparative Example 2) The procedure was the same as in Example 1, except that rolling was carried out at a low temperature (room temperature 25°C). The porosity difference between the produced first negative electrode active material layer and the second negative electrode active material layer was 15%.

[0078] (Comparative Example 3) The procedure was the same as in Example 2, except that natural graphite with a D50 of 12 μm was used. The porosity difference between the produced first negative electrode active material layer and the second negative electrode active material layer was 7%.

[0079] (Comparative Example 4) The procedure was the same as in Example 1, except that natural graphite with a D50 of 10 μm was used. The porosity difference between the produced first negative electrode active material layer and the second negative electrode active material layer was 5%.

[0080] (Comparative Example 5) The procedure was the same as in Example 1, except that natural graphite with a D50 of 10 μm was used and the rolling roll temperature was about 30°C. The porosity difference between the produced first negative electrode active material layer and the second negative electrode active material layer was 10%.

[0081] After the produced half cell was charged and discharged 3 times at 0.1C, based on the discharge capacity at 1C in the third cycle, while charging in CC mode (3C) for 15 minutes, the output voltage due to the change in SOC (State of Charge) was found by taking the derivative with respect to the capacity to find the gradient change point, and the rapid charging performance was evaluated by the method of determining the Li plating SOC.

[0082]

Table 1

[0083] As shown in Table 1 above, the batteries fabricated in Examples 1 and 2 showed higher 3C Li plating SOC (%) values compared to Comparative Examples 1 to 5. The higher the 3C Li plating SOC value, the faster the charging can be achieved up to a high SOC without lithium deposition. Therefore, it was confirmed that Examples 1 and 2 are superior to Comparative Examples 1 to 5 in terms of the rapid charging performance of the half cell.

Claims

1. 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 comprising wherein the first and second negative electrode active material layers contain a silicon-based active material and natural graphite, and the natural graphite has an average particle size (D50) of 9 μm or less, a negative electrode for a secondary battery, wherein the difference in porosity between the first negative electrode active material layer and the second negative electrode active material layer is 6% or less.

2. 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.

3. The negative electrode for a secondary battery according to Claim 1, wherein the first and second negative electrode active material layers further contain artificial graphite.

4. The negative electrode for a secondary battery according to Claim 3, wherein the average particle size (D50) of the artificial graphite is 15 μm to 50 μm.

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

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

7. The negative electrode for a secondary battery according to Claim 3, wherein the first and second negative electrode active material layers each contain 50 to 99 parts by weight of artificial graphite based on 100 parts by weight of the negative electrode active material.

8. A secondary battery comprising the negative electrode according to any one of Claims 1 to 7, a positive electrode, and a separator.

9. The secondary battery according to Claim 8, wherein the positive electrode contains a lithium composite transition metal compound containing nickel (Ni) and cobalt (Co) as active materials.

10. The secondary battery according to Claim 9, wherein the lithium composite transition metal compound further contains at least one of manganese and aluminum.

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