Negative electrode and secondary battery containing said negative electrode

Abstract

This invention relates to a negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises a silicon-based active material and a carbon-based active material, wherein the average particle size (D) of the carbon-based active material is... 50 The average particle size (D) of the silicon-based active material 50 The ratio of the negative electrode to the positive electrode is in the range of 2 to 8, and the porosity of the negative electrode is in the range of 48% to 62%.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2019-0147927, filed on November 18, 2019, the disclosure of which is incorporated herein by reference. Technical Field

[0004] The present invention relates to a negative electrode and a secondary battery comprising the negative electrode. Background Technology

[0005] Recently, with the rapid proliferation of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles, the demand for rechargeable batteries with relatively high capacity, small size, and light weight has increased rapidly. In particular, lithium-ion batteries have attracted significant attention as a power source for portable devices due to their light weight and high energy density. Therefore, research and development efforts to improve the performance of these lithium-ion batteries are actively underway.

[0006] Typically, a lithium-ion secondary battery comprises a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, an electrolyte, and an organic solvent. Furthermore, an active material layer comprising either a positive or negative electrode active material can be formed on the current collector for the positive and negative electrodes. Lithium-containing metal oxides such as LiCoO2 and LiMn2O4 are commonly used as the positive electrode active material, and correspondingly, lithium-free carbon-based or silicon-based negative electrode active materials are used as the negative electrode active material.

[0007] In particular, silicon-based anode active materials have attracted attention because their capacity is approximately 10 times higher than that of carbon-based anode active materials. Their advantage lies in the ability to achieve high energy density even with thin electrodes due to their high capacity. However, silicon-based anode active materials have not yet been widely used due to problems such as volume expansion caused by charging and discharging, resulting cracking / damage of active material particles, and consequently, degradation of lifetime characteristics.

[0008] Therefore, there is a need to develop a secondary battery that can improve lifetime characteristics while achieving high capacity and energy density of the silicon-based anode active material.

[0009] Korean Patent Application Publication No. 10-2017-0074030 relates to a negative electrode active material for a lithium secondary battery, a method for preparing the negative electrode active material, and a lithium secondary battery containing the negative electrode active material. It also discloses a negative electrode active material containing a porous silicon-carbon composite material, but it has limitations in solving the above-mentioned problems.

[0010] [Existing Technical Documents]

[0011] [Patent Literature]

[0012] Korean Patent Application Publication No. 10-2017-0074030 Summary of the Invention

[0013] Technical issues

[0014] One aspect of the present invention provides a hybrid negative electrode of silicon-based active material and carbon-based active material, which can prevent the degradation of lifetime characteristics due to volume expansion / contraction of silicon-based active material during charge and discharge, and can improve output performance and energy density.

[0015] Another aspect of the present invention provides a secondary battery comprising the aforementioned negative electrode.

[0016] Technical solution

[0017] According to one aspect of the present invention, a negative electrode is provided, the negative electrode comprising: a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises a silicon-based active material and a carbon-based active material, wherein the average particle size (D) of the carbon-based active material is... 50 The average particle size (D) of the silicon-based active material 50 The ratio of the negative electrode to the positive electrode is in the range of 2 to 8, and the porosity of the negative electrode is in the range of 48% to 62%.

[0018] According to another aspect of the present invention, a secondary battery is provided, the secondary battery comprising: the aforementioned negative electrode; a positive electrode facing the negative electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

[0019] Beneficial effects

[0020] The negative electrode according to the present invention is characterized in that the average particle size ratio of the silicon-based active material and the carbon-based active material, as well as the porosity of the negative electrode, are adjusted to a specific range. Since the negative electrode according to the present invention can adequately accommodate the volume expansion / contraction caused by the charging and discharging of the silicon-based active material by having a porosity within the aforementioned range, and the packing degree between particles can be improved by adjusting the particle size ratio of the large-particle-size carbon-based active material to the small-particle-size silicon-based active material, the negative electrode according to the present invention can also improve lifetime characteristics by improving the electrical contact between the active materials, and can achieve excellent output performance and energy density. Detailed Implementation

[0021] It should be understood that the words or terms used in the specification and claims should not be interpreted as having the meaning defined in a common dictionary, and it will be further understood that, based on the principle that the inventor can appropriately define the meaning of words or terms in order to best interpret the invention, the words or terms should be interpreted as having a meaning consistent with their meaning in the context of the related technology and the technical concept of the invention.

[0022] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, singular terms may include plural forms.

[0023] It should be further understood that the terms “comprising,” “including,” or “having” as used in this text indicate the presence of the stated feature, number, step, element, or combination thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

[0024] In this specification, the term "average particle size (D)" is used. 50 The average particle size (D) can be defined as the particle size at 50% of the cumulative volume in the particle size distribution curve. 50 This can be measured, for example, using laser diffraction. Laser diffraction can typically measure particle sizes from the submicron level to several millimeters, and can yield highly reproducible and high-resolution results.

[0025] The present invention will be described in detail below.

[0026] <Negative electrode>

[0027] This invention relates to negative electrodes, and more particularly, to negative electrodes for lithium secondary batteries.

[0028] Specifically, the negative electrode according to the present invention comprises a negative electrode current collector; and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises a silicon-based active material and a carbon-based active material, wherein the average particle size (D) of the carbon-based active material is... 50 The average particle size (D) of the silicon-based active material 50 The ratio of the negative electrode to the positive electrode is in the range of 2 to 8, and the porosity of the negative electrode is in the range of 48% to 62%.

[0029] Typically, silicon-based active materials are known to have a capacity approximately 10 times higher than that of carbon-based active materials. Therefore, when using silicon-based anode active materials as anodes, it is expected that thin-film electrodes with high energy density can be achieved even with small thicknesses. However, silicon-based active materials have the following limitation: during charge and discharge, lifetime degradation occurs due to volume expansion / contraction caused by lithium insertion / extraction. In particular, during the charging and discharging of silicon-based active materials, damage to the active material occurs due to rapid volume expansion / contraction, which accelerates lifetime degradation.

[0030] To overcome these limitations, by adjusting the porosity to the aforementioned level, the negative electrode of the present invention can adequately accommodate the volume changes of the silicon-based active material during rapid volume expansion / contraction, thereby significantly improving the lifetime characteristics of the negative electrode. Furthermore, since the negative electrode of the present invention can improve the filling between the particles of the silicon-based and carbon-based active materials by adjusting the average particle size ratio of the carbon-based active material to the silicon-based active material to a specific range, the contact between the particles can be improved, thus enhancing the lifetime characteristics of the negative electrode.

[0031] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause adverse chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used as negative electrode current collectors.

[0032] The thickness of the negative electrode current collector can typically range from 3 μm to 500 μm.

[0033] Micro-protrusions can be formed on the surface of the negative electrode current collector to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes, such as membranes, sheets, foils, meshes, porous bodies, foams, non-woven fabrics, etc.

[0034] The negative electrode active material layer is formed on the negative electrode current collector.

[0035] The negative electrode active material layer comprises silicon-based active materials and carbon-based active materials.

[0036] The silicon-based active material may contain silicon particles, which contain compounds represented by the following chemical formula 1.

[0037] [Chemical Formula 1]

[0038] SiO x

[0039] In Equation 1, 0 ≤ x < 2.

[0040] In Equation 1, SiO2 (when x = 2 in Equation 1) does not react with lithium ions and does not store lithium; therefore, x can be within the range mentioned above. Specifically, in terms of the structural stability of the active material, x can satisfy 0.5 ≤ x ≤ 1.5.

[0041] The silicon-based active material may also include metals distributed on the surface, inside, or on both the surface and inside of the silicon particles.

[0042] The metal may be contained in the silicon-based active material, and the distribution of the metal on the surface and / or inside the silicon-based active material reduces the ratio of the irreversible phase (e.g., SiO2) of the silicon-based active material, thereby improving the efficiency of the active material.

[0043] The metal may include at least one selected from the group consisting of lithium (Li), magnesium (Mg) and aluminum (Al), preferably at least one selected from the group consisting of Li and Mg, and more preferably Mg, in order to excellently achieve the above-mentioned effect of preventing damage to the silicon oxide particles and to further improve the lifetime characteristics of the negative electrode active material due to its low reactivity with moisture.

[0044] The metal may be included in the silicon-based active material in an amount of 0.1% to 25% by weight, for example, 3% to 15% by weight. When the amount of the metal is within the above range, the efficiency of the active material is improved without reducing the capacity, which is therefore preferred.

[0045] The silicon-based active material may also include a carbon coating formed on the silicon particles. The carbon coating can serve as a protective layer to suppress the volume expansion of the silicon particles and prevent side reactions with the electrolyte.

[0046] The carbon coating may be included in the silicon-based active material in an amount of 0.1% to 10% by weight, for example, 3% to 7% by weight. When the amount of the carbon coating is within the above range, the carbon coating can prevent side reactions with the electrolyte while controlling the volume expansion of the silicon particles at an excellent level, which is therefore preferred.

[0047] The average particle size (D) of the silicon-based active material 50 The particle size can be in the range of 0.1 μm to 20 μm, preferably 1 μm to 12 μm, more preferably 4 μm to 8 μm, to ensure the structural stability of the active material during charging and discharging, prevent the problem of increased volume expansion / contraction due to excessive particle size, and prevent the problem of reduced initial efficiency due to excessively small particle size.

[0048] Since the carbon-based active material exhibits a relatively low degree of volume expansion due to charging and discharging compared to the silicon-based active material, the carbon-based active material can reduce the overall volume expansion of the negative electrode.

[0049] The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon, and may preferably include at least one selected from the group consisting of artificial graphite and natural graphite.

[0050] The average particle size (D) of the carbon-based active material 50 The size can be in the range of 5μm to 35μm, preferably 11μm to 25μm, more preferably 12μm to 18μm, to ensure structural stability during charge and discharge and reduce side reactions with the electrolyte.

[0051] The average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50 The ratio is in the range of 2 to 8.

[0052] In this specification, the average particle size (D) of carbon-based active materials is described. 50 The average particle size (D) of silicon-based active materials 50 The ratio of the average particle size (D) of the carbon-based active material is used to express the ratio of the average particle size (D) of the carbon-based active material. 50 Divide by the average particle size (D) of the silicon-based active material 50 The value obtained is ).

[0053] According to the present invention, the negative electrode active material layer comprises the small-particle-size silicon-based active material and the large-particle-size carbon-based active material. Since the ratio of the average particle size can be adjusted to the aforementioned ratio, the contact and filling between the particles of the silicon-based active material and the carbon-based active material can be improved. Therefore, the output characteristics and energy density of the active material can be improved. Furthermore, although the volume of the silicon-based active material varies, the electrical contact between the active materials can be maintained, thus improving lifetime characteristics.

[0054] The average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50 When the ratio of the particles to the carbon-based active material is less than 2, the energy density decreases and electrical contact is reduced due to poor filling between the particles. Therefore, it is difficult to improve lifetime characteristics, and the average particle size (D) of the carbon-based active material also decreases. 50 The average particle size (D) of the silicon-based active material 50When the ratio of ) is greater than 8, the resistance increases due to the decrease in output performance, which is undesirable in terms of long-term cycle life characteristics.

[0055] The average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50 The ratio of ) can preferably be in the range of 2.2 to 5, in which case the output performance and energy density can be further improved while improving the electrical contact between the active materials.

[0056] The silicon-based active material and the carbon-based active material can be included in the negative electrode active material layer in a weight ratio of 1:99 to 50:50, for example, 3:97 to 20:80, to further improve the above-mentioned lifetime performance and capacity characteristics of the active material.

[0057] The silicon-based and carbon-based active materials may be included in the negative electrode active material layer in an amount of 80% to 99% by weight, for example, 90% to 98.5% by weight.

[0058] The negative electrode active material layer may include an adhesive.

[0059] The adhesive may comprise at least one selected from the group consisting of styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and polyacrylamide (PAM) to further improve electrode adhesion and provide sufficient resistance to volume expansion / contraction relative to the active material. Preferably, the adhesive may comprise styrene-butadiene rubber, thereby possessing high strength, excellent resistance to volume expansion / contraction of the silicon-based negative electrode active material, and providing excellent flexibility to prevent electrode deformation or bending.

[0060] The adhesive can be included in the negative electrode active material layer in an amount of 0.5% to 10% by weight. When the amount of the adhesive is within the above range, the volume expansion of the active material can be more effectively controlled, which is therefore desirable.

[0061] The negative electrode active material layer may further include a conductive agent. The conductive agent can be used to improve the conductivity of the negative electrode, and preferably a conductive agent that is conductive and does not cause adverse chemical changes.

[0062] Specifically, the conductive agent may include at least one selected from the group consisting of linear conductive agents and dot conductive agents.

[0063] Preferably, the conductive agent may comprise a linear conductive agent and a point-type conductive agent. The linear conductive agent exists in the negative electrode in the form of a line to improve the electrical contact between the active materials, and, for example, maintains a conductive network between the active materials even if the volume of the silicon-based active material expands due to charge and discharge. Furthermore, the linear conductive agent prevents aggregation between conductive agents and pore blockage of the active materials that may occur when only a point-type conductive agent is used. However, since the linear conductive agent has a higher specific surface area than the point-type conductive agent, there is a risk of electrolyte side reactions during high-temperature storage; therefore, a mixture of the linear and point-type conductive agents is preferred.

[0064] The dot-type conductive agent may comprise at least one selected from the group consisting of conductive materials such as: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives, and may specifically comprise at least one selected from the group consisting of graphite (e.g., natural or artificial graphite) and carbon black.

[0065] The linear conductive agent may comprise at least one selected from the group consisting of conductive fibers, such as carbon fibers, carbon nanofibers (CNFs), and metal fibers; conductive tubes, such as carbon nanotubes (CNTs); and conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers. Specifically, it may comprise at least one selected from the group consisting of carbon nanofibers and carbon nanotubes, and more specifically, it may be carbon nanotubes, which have excellent strength and can further improve the electrical contact of the active material.

[0066] The conductive agent may comprise the linear conductive agent and the dot conductive agent in a weight ratio of 0.01:99.9 to 50:50 (e.g., 10:90 to 40:60). When the weight ratio is within the above range, it is more preferable to achieve the effect of improving electrical contact through the linear conductive agent and the effect of improving high-temperature storage performance through the dot conductive agent.

[0067] The conductive agent may be included in the negative electrode active material layer in an amount of 0.5% to 10% by weight, for example, 1% to 5% by weight.

[0068] In terms of increasing electrical contact with the negative electrode material components, the thickness of the negative electrode active material layer can be in the range of 30 μm to 150 μm, for example, in the range of 40 μm to 120 μm.

[0069] The porosity of the negative electrode is in the range of 48% to 62%.

[0070] Because the negative electrode according to the present invention has a relatively high porosity, it can ideally accommodate the volume expansion / contraction caused by the charging and discharging of the silicon-based active material and / or the carbon-based active material. Therefore, it can prevent the degradation of lifetime characteristics due to the volume expansion of the active material, thereby improving the lifetime characteristics of the negative electrode. Furthermore, when the porosity of the negative electrode is within the aforementioned range, there is a concern that the energy density and electrical contact of the negative electrode may decrease. However, in the present invention, since the average particle size ratio of the carbon-based active material to the silicon-based active material is adjusted to the aforementioned range, the electrical contact of the active material is maintained while improving the energy density of the negative electrode. Thus, the lifetime characteristics can be improved to a more ideal level.

[0071] If the porosity of the negative electrode is less than 48%, there is a concern that the lifetime characteristics may be significantly reduced because the pores in the negative electrode cannot smoothly accommodate the volume expansion of the silicon-based active material. If the porosity of the negative electrode is greater than 62%, there is a concern that the lifetime characteristics may be reduced because the electrical contact between the active materials is reduced.

[0072] The porosity of the negative electrode is preferably in the range of 52% to 58%. Under these conditions, the volume change of the silicon-based active material can be smoothly accommodated, and the lifetime characteristics can be improved to an ideal level by maintaining the electrical contact between the active materials.

[0073] In this specification, the porosity of the negative electrode can be calculated using the following formula 1.

[0074] [Formula 1]

[0075] Porosity (%) of negative electrode = {1 - (electrode density of negative electrode / true density of negative electrode)} × 100 In Equation 1, the true density of negative electrode is the density of the negative electrode active material layer measured when a negative electrode of a predetermined size is collected and pressed with a press until the thickness of negative electrode remains unchanged, and the electrode density of negative electrode is the density of the negative electrode active material layer measured by collecting a negative electrode of a predetermined size.

[0076] The porosity of the negative electrode can be achieved by appropriately adjusting the calendering conditions during the preparation of the negative electrode.

[0077] The negative electrode slurry is prepared by dispersing negative electrode material, binder and conductive agent in a solvent for forming negative electrode slurry, and the negative electrode can be prepared by coating the negative electrode slurry onto the negative electrode current collector, drying and calendering the coated negative electrode current collector.

[0078] In terms of promoting the dispersion of components, the solvent for forming the negative electrode slurry may contain at least one selected from the group consisting of distilled water, ethanol, methanol and isopropanol, and may preferably contain distilled water.

[0079] <Secondary battery>

[0080] The present invention provides a secondary battery including the above-described negative electrode, specifically a lithium secondary battery.

[0081] Specifically, the secondary battery according to the present invention includes the above-described negative electrode; a positive electrode facing the negative electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

[0082] The positive electrode may include a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector.

[0083] The positive electrode current collector is not particularly limited as long as it has high conductivity and does not cause adverse chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium or silver, or an aluminum cadmium alloy can be used as the positive electrode current collector.

[0084] The thickness of the positive electrode current collector can generally be 3 μm to 500 μm.

[0085] The surface of the positive electrode current collector may have fine irregularities to improve the adhesion to the positive electrode active material. For example, the positive electrode current collector can be used in various shapes, such as a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0086] The positive electrode active material layer may include a positive electrode active material.

[0087] Specifically, the positive electrode active material is a compound capable of reversibly inserting and extracting lithium, and the positive electrode active material may include a lithium transition metal composite oxide, the lithium transition metal composite oxide including lithium and at least one transition metal composed of nickel, cobalt, manganese and aluminum. For example, the lithium transition metal composite oxide includes lithium and a transition metal containing nickel, cobalt and manganese.

[0088] More specifically, the lithium transition metal composite oxide may include lithium manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (such as LiCoO2, etc.), lithium nickel-based oxides (such as LiNiO2, etc.), lithium nickel manganese-based oxides (such as, LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2)), lithium nickel cobalt-based oxides (such as, LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1)), lithium manganese cobalt-based oxides (such as, LiCo 1-Y2 Mn Y2O2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1 O4 (where 0 < Z1 < 2)), lithium nickel manganese cobalt oxide (e.g., Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1 and p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2 and p1 + q1 + r2 = 2)), or lithium nickel cobalt transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O2 (where M is selected from the group consisting of aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and p2, q2, r3, and s2 are atomic fractions of the respective independent elements, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1 and p2 + q2 + r3 + s2 = 1)), where any one or a mixture of two or more thereof may be included. Among these materials, in terms of improving the capacity characteristics and stability of the battery, the lithium transition metal composite oxide may include LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni<00000​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​0.7 Mn 0.15 Co 0.15 O2, or Li(Ni) 0.8 Mn 0.1 Co 0.1 O2, etc., wherein any one or a mixture of two or more thereof may be used.

[0089] Considering that the positive electrode active material exhibits sufficient capacity, the positive electrode active material may be included in the positive electrode active material layer in an amount of 80% to 99% by weight, for example, 92% to 98.5% by weight.

[0090] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may also include a binder and / or a conductive agent.

[0091] The adhesive is a component that facilitates adhesion between the active material and the conductive agent, as well as adhesion to the current collector. Specifically, the adhesive may contain at least one selected from the group consisting of: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0092] To ensure sufficient adhesion between components (e.g., positive electrode active material), the adhesive may be included in the positive electrode active material layer in an amount of 1% to 20% by weight, for example, 1.2% to 10% by weight.

[0093] The conductive agent can be used to assist and improve the conductivity of the secondary battery, and is not particularly limited, as long as the conductive agent is conductive without causing adverse chemical changes. Specifically, the conductive agent may contain at least one selected from the group consisting of: graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives, and for the purpose of improving conductivity, the conductive agent may preferably contain carbon black.

[0094] To ensure sufficient conductivity, the conductive agent may be included in the positive electrode active material layer in an amount of 1% to 20% by weight, for example, 1.2% to 10% by weight.

[0095] The thickness of the positive electrode active material layer can be in the range of 30 μm to 400 μm, for example, 50 μm to 110 μm.

[0096] The positive electrode can be manufactured by coating a positive electrode slurry onto a positive electrode current collector, and then drying and calendering the coated positive electrode current collector. The positive electrode slurry contains the positive electrode active material and selective binders, conductive agents and solvents for forming the positive electrode slurry.

[0097] The solvent for forming the positive electrode slurry may comprise an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that a desired viscosity is obtained when the positive electrode active material and selective binders and conductive agents are included. For example, the solvent for forming the positive electrode slurry may be included in an amount such that the concentration of the solid components comprising the positive electrode active material and selective binders and conductive agents is in the range of 50% to 95% by weight, for example, 70% to 90% by weight.

[0098] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is generally used in lithium secondary batteries. In particular, separators with high moisture-holding capacity for the electrolyte and low resistance to electrolyte ion transfer can be used. Specifically, porous polymer membranes can be used, for example, porous polymer membranes prepared from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or using their two- or more-layered laminated structures. Furthermore, typical porous nonwoven fabrics can be used, for example, nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. In addition, separators coated with ceramic or polymer components can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can be selectively used.

[0099] Furthermore, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used in the preparation of secondary batteries, but this invention is not limited thereto.

[0100] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0101] Any organic solvent can be used without particular limitation, as long as it serves as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as organic solvents, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; or carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group, which may contain double-bonded aromatic rings or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane, etc. Among these solvents, carbonate-based solvents can be used. For example, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) that can increase the charge-discharge performance of the battery can be used. In this case, the electrolyte performance may be excellent when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

[0102] Lithium salts can be used without particular restriction, as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used. These lithium salts can be used in concentrations ranging from 0.1 M to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte exhibits suitable conductivity and viscosity, thus achieving excellent electrolyte performance and allowing for efficient lithium ion movement.

[0103] The secondary battery can be prepared according to conventional methods for preparing secondary batteries, by placing the separator between the negative electrode and the positive electrode and then injecting electrolyte.

[0104] The secondary battery according to the invention is suitable for portable devices, such as mobile phones, laptops, and digital cameras, as well as electric vehicles, such as hybrid electric vehicles (HEVs). In particular, it can be preferably used as a constituent battery in medium-to-large-sized battery modules. Therefore, the invention also provides a medium-to-large-sized battery module comprising the aforementioned secondary battery as a unit cell.

[0105] The medium-to-large battery modules are preferably used as power sources requiring high power and large capacity, such as electric vehicles, hybrid electric vehicles, or energy storage devices.

[0106] In the following, embodiments of the invention will be described in detail in a manner readily achievable by those skilled in the art. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments described herein.

[0107] Example

[0108] Example 1: Preparation of the negative electrode

[0109] <Preparation of Silicon-Based Active Materials>

[0110] SiO particles are prepared as silicon-based particles. The SiO particles are mixed with magnesium (Mg) as a metallic material, and the mixture is heat-treated at 1200°C for 3 hours to prepare SiO particles with Mg distributed on its surface and / or inside.

[0111] Silicon-based active materials (average particle size (D) 50 The 6μm carbon coating was prepared by chemical vapor deposition (CVD) of methane as a hydrocarbon gas on SiO particles with Mg distributed on the surface and / or inside, at 950 °C.

[0112] In the silicon-based active material, the weight ratio of the silicon particles to the metal (Mg) to the carbon coating is 85:10:5.

[0113] <Preparation of the negative electrode>

[0114] The silicon-based active material prepared above and natural graphite (average particle size (D) as a carbon-based active material were combined. 50 The mixture of 16μm and 16μm was used as the negative electrode active material.

[0115] The negative electrode active material, styrene-butadiene rubber (SBR) as a binder, carbon black as a dot-type conductive agent, carbon nanotubes (CNT) as a linear conductive agent, and carboxymethyl cellulose (CMC) as a thickener are mixed in a weight ratio of 95.5:2:1:0.5:1, and the mixture is added to distilled water as a solvent for forming the negative electrode slurry to prepare the negative electrode slurry.

[0116] The negative electrode slurry was coated onto one surface of a copper current collector (thickness: 15 μm) serving as the negative electrode current collector, with a loading capacity of 3.6 mAh / cm². 2The coated negative electrode current collector was then rolled and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer (thickness: 93 μm), which was used as the negative electrode according to Example 1 (thickness of the negative electrode: 108 μm).

[0117] The rolling process is performed by forming the negative electrode active material layer on the negative electrode current collector, placing it between two rollers of a press, and applying extrusion pressure. The gap between the two rollers is set to 108 μm.

[0118] In Example 1, the average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50 The ratio of ) is approximately 2.6, and the porosity of the prepared negative electrode is 54%.

[0119] Example 2: Preparation of the negative electrode

[0120] The negative electrode of Example 2 was prepared in the same manner as in Example 1, except that the rolling was performed by setting the gap between the two rollers to 101 μm.

[0121] In Example 2, the average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50 The ratio of ) is approximately 2.6, and the porosity of the prepared negative electrode is 50%.

[0122] Example 3: Preparation of the negative electrode

[0123] The negative electrode of Example 3 was prepared in the same manner as in Example 1, except that the rolling was performed by setting the gap between the two rollers to 122 μm.

[0124] In Example 3, the average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50 The ratio of ) is approximately 2.6, and the porosity of the prepared negative electrode is 60%.

[0125] Example 4: Preparation of the negative electrode

[0126] The negative electrode of Example 4 was prepared in the same manner as in Example 1, except that SiO particles (average particle size (D)) were used. 50 (6μm) is used as a silicon-based active material.

[0127] In Example 4, the average particle size (D) of the carbon-based active material 50 The average particle size (D) of the silicon-based active material 50The ratio of ) is approximately 2.6, and the porosity of the prepared negative electrode is 55%.

[0128] Comparative Example 1: Preparation of the negative electrode

[0129] The negative electrode of Comparative Example 1 was prepared in the same manner as in Example 1, except that the rolling was performed by setting the gap between the two rollers to 93 μm.

[0130] In Comparative Example 1, the average particle size (D) of the carbon-based active material was... 50 The average particle size (D) of the silicon-based active material 50 The ratio of ) is approximately 2.6, and the porosity of the prepared negative electrode is 45%.

[0131] Comparative Example 2: Preparation of the Negative Electrode

[0132] The negative electrode of Comparative Example 2 was prepared in the same manner as in Example 1, except that the rolling was performed by setting the gap between the two rollers to 137 μm.

[0133] In Comparative Example 2, the average particle size (D) of the carbon-based active material was... 50 The average particle size (D) of the silicon-based active material 50 The ratio of ) is approximately 2.6, and the porosity of the prepared negative electrode is 65%.

[0134] Comparative Example 3: Preparation of the Negative Electrode

[0135] <Preparation of Silicon-Based Active Materials>

[0136] SiO particles are prepared as silicon-based particles. The SiO particles are mixed with Mg as a metallic material, and the mixture is heat-treated at 1200°C for 3 hours to prepare SiO particles with Mg distributed on its surface and / or inside.

[0137] Silicon-based active materials (average particle size (D) 50 The 10 μm carbon coating was prepared by chemical vapor deposition (CVD) of methane as a hydrocarbon gas on SiO particles with Mg distributed on the surface and / or inside, at 950 °C.

[0138] In the silicon-based active material, the weight ratio of the silicon particles to the metal (Mg) to the carbon coating is 85:10:5.

[0139] <Preparation of the negative electrode>

[0140] The negative electrode was prepared in the same manner as in Example 1, except that the silicon-based active material prepared above was used.

[0141] Comparative Example 4: Preparation of the Negative Electrode

[0142] <Preparation of Silicon-Based Active Materials>

[0143] SiO particles are prepared as silicon-based particles. The SiO particles are mixed with Mg as a metallic material, and the mixture is heat-treated at 1200°C for 3 hours to prepare SiO particles with Mg distributed on its surface and / or inside.

[0144] Silicon-based active materials (average particle size (D) 50 The 2μm carbon coating was prepared by chemical vapor deposition (CVD) of methane as a hydrocarbon gas on SiO particles with Mg distributed on the surface and / or inside, at 950 °C.

[0145] In the silicon-based active material, the weight ratio of the silicon particles to the metal (Mg) to the carbon coating is 85:10:5.

[0146] <Preparation of the negative electrode>

[0147] The negative electrode was prepared in the same manner as in Example 1, except that the silicon-based active material prepared above and natural graphite (average particle size (D)) was used as the carbon-based active material. 50 ): 20μm).

[0148] Table 1

[0149]

[0150] Experimental Example

[0151] <Preparation of Secondary Batteries>

[0152] LiMn2O4 as the positive electrode active material, Danka Black as the conductive agent, and polyvinylidene fluoride (PVdF) as the binder were added to N-methylpyrrolidone (NMP) in a weight ratio of 96.5:2.0:1.5 to prepare a positive electrode slurry. The positive electrode slurry was coated onto an aluminum current collector, rolled, and dried to prepare the positive electrode.

[0153] A porous polyethylene membrane was disposed between the positive electrode described above and each negative electrode prepared in Examples 1 to 4 and Comparative Examples 1 to 4, and an electrolyte was injected to prepare a pouch-type secondary battery.

[0154] The electrolyte used was prepared by dissolving 1.5% by weight vinylene carbonate (VC) in a solution in which ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were mixed in a volume ratio of 7:3, and LiPF6 was dissolved to a concentration of 1M.

[0155] Experimental Example 1: Lifetime Characteristics Evaluation

[0156] <Capacity Retention Rate Evaluation>

[0157] The cycle capacity retention of the secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 4 was evaluated using an electrochemical charge-discharge apparatus.

[0158] Regarding the cycle capacity retention, charging and discharging were performed at 0.1C in the first and second cycles, and at 0.5C from the third cycle onwards (charging conditions: CC (constant current) / CV (constant voltage), 5mV / 0.005C cutoff; discharging conditions: CC, 1.5V cutoff).

[0159] The capacity retention rate is calculated as follows.

[0160] Capacity retention (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the first cycle)} × 100

[0161] (Where N is an integer greater than or equal to 1.)

[0162] The capacity retention rate (%) after the 100th cycle is shown in Table 2 below.

[0163] Table 2

[0164] Capacity retention rate (%) after 100 cycles Example 1 98 Example 2 96 Example 3 97 Example 4 93 Comparative Example 1 71 Comparative Example 2 78 Comparative Example 3 79 Comparative Example 4 81

[0165] As can be seen from Table 2, compared with the comparative examples, the negative electrodes of Examples 1 to 4, in which the average particle size ratio of carbon-based active materials to silicon-based active materials and the porosity of the negative electrode meet the scope of the present invention, have better lifetime performance.

[0166] Experiment Example 2: Output Performance Evaluation

[0167] The secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to 1) resistance evaluation and 2) fast charging performance evaluation.

[0168] Specifically, regarding 1) resistance evaluation, the voltage change was measured by discharging each secondary battery at 3C for 30 seconds at 50% negative electrode state of charge (SOC), and the resistance value was calculated using this voltage change. When Example 1 was set to 100%, the relative resistance values ​​of the Example and the Comparative Example are shown in Table 3. Furthermore, regarding 2) fast charging performance evaluation, each secondary battery was charged at 3C to measure the SOC at which lithium deposition occurred, and the results are shown in Table 3.

[0169] Table 3

[0170] Resistance evaluation (%) 3C lithium plating SOC (%) Example 1 100 32 Example 2 106 30 Example 3 97 34 Example 4 99 31 Comparative Example 1 107 27 Comparative Example 2 88 34 Comparative Example 3 87 30 Comparative Example 4 86 29

[0171] Referring to Table 3, it can be seen that the anodes of Examples 1 to 4, whose average particle size ratio of carbon-based active materials to silicon-based active materials and whose anode porosity meet the scope of this invention, exhibit relatively high output performance levels. Therefore, it can be confirmed that the anode according to the present invention can simultaneously improve lifetime characteristics and output performance to the desired level.

[0172] Regarding Comparative Example 1, due to the increased resistance and the earlier lithium deposition time during high-current charging at 3C, it can be confirmed that the fast charging performance is significantly reduced.

[0173] Regarding Comparative Example 2, the output performance appears to be slightly better than that of Example 1, but as mentioned above, the lifespan characteristics are significantly reduced, and therefore, the overall battery performance is poor.

[0174] Regarding Comparative Examples 3 and 4, the fast charging performance was reduced compared to Example 1, and as mentioned above, the lifespan characteristics were significantly reduced, resulting in poor overall battery performance.

Claims

1. A negative electrode, said negative electrode comprising: Negative current collector; and The negative electrode active material layer formed on the negative electrode current collector in, The negative electrode active material layer comprises silicon-based active materials and carbon-based active materials. Wherein, the average particle size D of the carbon-based active material 50 The average particle size D of the silicon-based active material 50 The ratio is in the range of 2 to 5, and The porosity of the negative electrode is between 48% and 62%, wherein The porosity of the negative electrode is calculated using the following formula: Porosity (%) of the negative electrode = {1 - (electrode density of the negative electrode / true density of the negative electrode)} × 100 In the above formula, the true density of the negative electrode is the density of the negative electrode active material layer measured when a negative electrode of a predetermined size is collected and pressed with a press until the thickness of the negative electrode remains unchanged, and the electrode density of the negative electrode is the density of the negative electrode active material layer measured by collecting a negative electrode of a predetermined size.

2. The negative electrode according to claim 1, wherein the porosity of the negative electrode is in the range of 52% to 58%.

3. The negative electrode according to claim 1, wherein the average particle size D of the carbon-based active material is... 50 The average particle size D of the silicon-based active material 50 The ratio is in the range of 2.2 to 5.

0.

4. The negative electrode according to claim 1, wherein the negative electrode active material layer comprises the silicon-based active material and the carbon-based active material in a weight ratio of 1:99 to 50:

50.

5. The negative electrode according to claim 1, wherein the average particle size D of the carbon-based active material is... 50 Within the range of 5μm to 35μm.

6. The negative electrode according to claim 1, wherein the average particle size D of the silicon-based active material is... 50 Within the range of 0.1 μm to 20 μm.

7. The negative electrode according to claim 1, wherein the silicon-based active material comprises silicon particles, and the silicon particles comprise a compound represented by chemical formula 1: [Chemical Formula 1] SiO x , where 0≤x<2.

8. The negative electrode according to claim 7, wherein the silicon-based active material further comprises a metal distributed on the surface, inside, or on the surface and inside of the silicon particles, and The metal comprises at least one selected from the group consisting of lithium, magnesium, and aluminum.

9. The negative electrode according to claim 8, wherein the metal is included in the silicon-based active material in an amount of 0.1% to 25% by weight.

10. The negative electrode according to claim 1, wherein the carbon-based active material comprises at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, and graphene.

11. The negative electrode according to claim 1, wherein the negative electrode active material layer further comprises a conductive agent, and The conductive agent comprises at least one selected from the group consisting of linear conductive agents and point conductive agents.

12. The negative electrode according to claim 11, wherein the conductive agent comprises the linear conductive agent and the point conductive agent in a weight ratio of 0.01:99.9 to 50:

50.

13. A secondary battery, the secondary battery comprising: The negative electrode according to claim 1; Facing the positive terminal of the aforementioned negative terminal; A diaphragm disposed between the negative electrode and the positive electrode; and Electrolytes.

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