Negative electrode and secondary battery

Abstract

The invention relates to a negative electrode and a secondary battery. Disclosed are a negative electrode for a secondary battery and a secondary battery including the same, the negative electrode including: a current collector; the first negative electrode active material layer is arranged on the current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer, in which the first negative electrode active material layer and the second negative electrode active material layer comprise a lithium-substituted carboxymethyl cellulose, in which the first negative electrode active material layer comprises a carbon-based active material, and the second negative electrode active material layer comprises a lithium-substituted carboxymethyl cellulose. And only the second negative electrode active material layer contains a silicon-based active material.

Description

[0001] This application is a divisional application. The international application number of the original application is PCT / KR2023 / 014944, the application date is September 27, 2023, the application number of the Chinese national phase is 202380046441.2, and the invention title is "Negative Electrode and Secondary Battery". Technical Field

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

[0003] This application claims priority and benefit to Korean Patent Application No. 10-2022-0125384, filed on September 30, 2022, with the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference. Background Technology

[0004] Secondary batteries are not only widely used in portable devices, but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs) powered by electric drive sources.

[0005] Because these secondary batteries not only have the major advantage of significantly reducing the use of fossil fuels, but also the advantage of not producing any byproducts caused by the use of energy, they have attracted much attention as an environmentally friendly and energy-efficient new energy source.

[0006] Typically, a secondary battery includes a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte. Furthermore, electrodes such as the positive and negative electrodes may have layers of electrode active material disposed on a current collector.

[0007] As the utilization rate of secondary batteries increases, various battery performance requirements arise. Although attempts have been made to add additives to the active material layers to improve battery performance, depending on the type of additive, some battery performance may be improved, while others may be degraded. Summary of the Invention

[0008] [Technical Issues]

[0009] The present invention aims to provide a negative electrode for a secondary battery that can provide a secondary battery with improved charging performance and lifespan, as well as a secondary battery including the same.

[0010] [Technical Solution]

[0011] One aspect of the present invention provides a negative electrode for a secondary battery, comprising:

[0012] current collector;

[0013] A first negative electrode active material layer disposed on the current collector; and

[0014] A second negative electrode active material layer disposed on the first negative electrode active material layer.

[0015] Wherein, at least one of the first negative electrode active material layer and the second negative electrode active material layer contains lithium-substituted carboxymethyl cellulose.

[0016] The first negative electrode active material layer contains carbon-based active materials, and only the second negative electrode active material layer contains silicon-based active materials.

[0017] Another aspect of the present invention provides a secondary battery comprising a negative electrode, a positive electrode, and a separator for the secondary battery.

[0018] [Beneficial Effects]

[0019] According to the aspects described in this specification, the two-layer structure of the negative electrode active material layer and the addition of lithium-substituted carboxymethyl cellulose can exhibit a synergistic effect to improve battery charging performance and ensure battery life performance. Furthermore, since silicon-based active materials may have lower conductivity compared to carbon-based active materials, the silicon-based active material in the thickness direction of the negative electrode active material layer (…) Figure 1 The uniform distribution of silicon-based active material on the electrode (T) can potentially induce non-uniformity in conductivity across the entire electrode, thus negatively impacting charging performance (fast charging). However, when silicon-based active material is only contained in the second polar electrode active material layer of the bilayer structure (T), Figure 1 In the case of lithium-substituted carboxymethyl cellulose (202), the conductivity increases, and then the contact resistance between the layers decreases. This effect is maximized by also applying lithium-substituted carboxymethyl cellulose, which has a higher electrical / ionic mobility for Li. When lithium-substituted carboxymethyl cellulose is used, the mobility of Li ions increases, thus also ensuring the slightly degraded lifetime durability that occurs when silicon-based active materials are only contained in the second anode active material layer.

[0020] In particular, when charged with high current (e.g., fast charging), the lithium-ion flux is relatively large compared to normal charging (normal cycling). When the lithium-ion flux is large, the fast-charging performance of the cell can be improved if the negative electrode active material can rapidly accept lithium ions. Since silicon-based active materials accept lithium ions through alloying and their charging starts from a lower potential, their fast-charging performance is improved compared to graphite-based negative electrode active materials that accept lithium ions through intercalation. More advantageously, by placing this silicon-based active material close to the positive electrode, which is the direction in which lithium-ion flux is first accepted—that is, by placing it on top (the second negative electrode active material layer)—the silicon-based active material reacts rapidly with lithium ions, thereby significantly improving fast-charging performance. Attached Figure Description

[0021] Figure 1An example of the negative electrode of one aspect of the present invention is shown. Detailed Implementation

[0022] The invention will be described in more detail below to aid in understanding. The invention can be implemented in various different forms and is not limited to the aspects described herein. In this context, the terms or words used in the specification and claims should not be construed as limited to their typical or dictionary meanings, but should be interpreted with meanings and concepts consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concepts of the terms in order to best describe his / her own invention.

[0023] In this invention, the terms “comprising,” “including,” or “having” are intended to indicate the presence of the implemented features, numbers, steps, constituent elements, or any combination thereof, and should be understood to not exclude the possibility of the presence or addition of one or more other features or numbers, steps, constituent elements, or any combination thereof.

[0024] The situation where a part, such as a layer, exists "above" or "on" another part includes not only the case where the part exists "directly" "above" another part, but also the case where another part exists in between. Conversely, the case where a part exists "directly" "above" another part means that there are no other parts in between. In addition, the case of being "above" or "on" a reference part means being above or below the reference part, and does not necessarily mean being "above" or "on" in the opposite direction of gravity.

[0025] In this specification, only the term "negative electrode active material layer" is mentioned without specifying whether the description of "first" and "second" can be applied to both the first negative electrode active material layer and the second negative electrode active material layer.

[0026] One aspect of this specification describes a negative electrode for a secondary battery comprising: a current collector; a first negative electrode active material layer disposed on the current collector; and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein at least one of the first and second negative electrode active material layers comprises lithium-substituted carboxymethyl cellulose, wherein the first negative electrode active material layer comprises a carbon-based active material, and only the second negative electrode active material layer comprises a silicon-based active material. In other words, the negative electrode for a secondary battery is characterized by comprising lithium-substituted carboxymethyl cellulose and having two layers of negative electrode active material. The distribution of the silicon-based active material can be confirmed by SEM images of the electrode cross-section. Therefore, the region where the silicon-based active material is present can be defined as the second negative electrode active material layer.

[0027] Figure 1 An example of a negative electrode is shown, comprising a first negative electrode active material layer 201, a second negative electrode active material layer 202, and a current collector 101.

[0028] The inventors have discovered that lithium-substituted carboxymethyl cellulose, compared to sodium-substituted carboxymethyl cellulose, is advantageous in improving battery charging performance and ensuring lifespan performance. In particular, when lithium-substituted carboxymethyl cellulose is used in conjunction with a multilayer structure containing a silicon-based active material layer of negative electrode material, the charging performance and lifespan of the battery can be further maximized through a synergistic effect, thus completing the present invention.

[0029] Lithium-substituted carboxymethyl cellulose can also be contained in either the first negative electrode active material layer or the second negative electrode active material layer, or it can be contained in one layer of either the first negative electrode active material layer or the second negative electrode active material layer.

[0030] For example, the content of lithium-substituted carboxymethyl cellulose in the second negative electrode active material layer may be greater than that in the first negative electrode active material layer, or it may be contained only in the second negative electrode active material layer. In one aspect, one or both of the first and second negative electrode active material layers contain lithium-substituted carboxymethyl cellulose, but do not contain sodium-substituted carboxymethyl cellulose at all. However, in other aspects, sodium-substituted carboxymethyl cellulose may be further included. For example, one of the first and second negative electrode active material layers contains lithium-substituted carboxymethyl cellulose, and the other may contain sodium-substituted carboxymethyl cellulose. In some embodiments, lithium-substituted carboxymethyl cellulose and sodium-substituted carboxymethyl cellulose are the only carboxymethyl cellulose salts contained in the electrode.

[0031] In one aspect of this specification, silicon-based active materials include SiO2. x (0≤x<2), SiM y (M is a metal, 1≤y≤4) and at least one of Si / C. It may include only one silicon-based active material, or it may include two or more together.

[0032] In one aspect of this specification, the second negative electrode active material layer comprising a silicon-based active material may further comprise a carbon-based active material. In this case, based on a total of 100 parts by weight of active material contained in the second negative electrode active material layer comprising silicon-based active material, the content of silicon-based active material may be from 1 part by weight to 40 parts by weight, for example, 2 parts by weight to 35 parts by weight, 3 parts by weight to 30 parts by weight, 5 parts by weight to 25 parts by weight, or 7 parts by weight to 15 parts by weight.

[0033] In one aspect of this specification, the first negative electrode active material layer comprises a carbon-based active material, and only the second negative electrode active material layer comprises a silicon-based active material. In this case, the first negative electrode active material layer does not contain a silicon-based active material.

[0034] Contains SiO x(0 ≤ x < 2) As the active material of the silicon-based active material, it can be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0035] According to the present disclosure, the composite particles or composites refer to two or more materials that are physically aggregated but not chemically bonded.

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

[0037] The silicon-based composite particles can further include at least one of a Mg compound and a Li compound. The Mg compound and the Li compound can correspond to the matrix in the silicon-based composite particles.

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

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

[0040] In one aspect of the present specification, based on a total of 100% by weight of the silicon-based active material, the content of the Mg element can be 0.1% to 20% by weight, or 0.1% to 10% by weight. Specifically, the content of the Mg element can be 0.5% to 8% by weight or 0.8% to 4% by weight. When the above range is satisfied, the Mg compound can be contained in the silicon-based active material in an appropriate content, so that the volume change of the silicon-based active material during the charge and discharge process of the battery can be easily suppressed, and the discharge capacity and initial efficiency of the battery can be improved.

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

[0042] In one aspect of the present invention, the Li compound may include a form of lithium silicate. The lithium silicate is represented by Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and can be divided into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate may exist in the silicon-based composite particles in the form of at least one lithium silicate selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5, and 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 these forms.

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

[0044] The content of the Mg element or the Li element can be confirmed by ICP analysis. For ICP analysis, a predetermined amount (about 0.01 g) of the negative electrode active material is accurately aliquoted, and then the negative electrode active material is completely decomposed on a hot plate by transferring the aliquot to a platinum crucible and adding nitric acid, hydrofluoric acid, or sulfuric acid thereto. Thereafter, a reference calibration curve is prepared by measuring the intensity of a standard liquid prepared using a standard solution (5 mg / kg) at the inherent wavelength of the Mg element or the Li element using an inductively coupled plasma atomic emission spectrometer (ICP AES, Perkin-Elmer 7300). Thereafter, the pretreated sample solution and the blank sample are each introduced into the device, the actual intensity is calculated by measuring each intensity, the concentration of each component is calculated relative to the prepared calibration curve, and then the content of the Mg element or the Li element of the prepared silicon-based active material can be analyzed by converting the sum to a theoretical value.

[0045] In one aspect of the present specification, a carbon layer may be provided on the surface and / or inside the pores of the silicon-based composite particles. Through the carbon layer, conductivity is imparted to the silicon-based composite particles, and 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 can be improved. Based on a total of 100% by weight of the silicon-based composite particles, the total weight of the carbon layer may be included in an amount of 5% to 40% by weight.

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

[0047] The average particle size (D) of silicon composite particles 50 The particle size can range from 2μm to 15μm, specifically from 3μm to 12μm, and more specifically from 4μm to 10μm. When the above range is met, the side reactions between silicon composite particles and electrolyte can be controlled, and the discharge capacity and initial efficiency of the battery can be effectively achieved.

[0048] In this specification, the average particle size (D) 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 Particle sizes can be measured using methods such as laser diffraction. Laser diffraction can typically measure particle sizes from the submicron region to several millimeters, and can yield results with high reproducibility and high resolution.

[0049] Silicon-based active materials containing Si / C are composites of silicon (Si) and carbon (C), and are distinct from silicon carbide represented as SiC. Silicon-carbon composites can be composites of silicon and graphite, or they can form structures in which the core of a silicon-graphite composite is surrounded by graphene or amorphous carbon. The silicon dispersed in the silicon-carbon composite can be nano-silicon. The average particle size (D) of active materials containing Si / C is... 50 The thickness can range from 2 μm to 15 μm, specifically from 3 μm to 12 μm, and more specifically from 4 μm to 10 μm. The carbon layer can be disposed on the surface of an active material containing Si / C.

[0050] In one aspect of this specification, the carbon-based active material may be graphite, and the graphite may be natural graphite, artificial graphite, or a mixture thereof. Based on a total weight of 100 parts by weight of the first negative electrode active material layer, the first negative electrode active material layer may include 80 parts by weight and 99.8 parts by weight or less, for example, 90 to 99 parts by weight or 93 to 97 parts by weight of carbon-based negative electrode active material. Based on a total weight of 100 parts by weight of active material contained in the second negative electrode active material layer, the second negative electrode active material layer may include 60 parts by weight and 99 parts by weight or less, for example, 75 parts by weight and 98 parts by weight or 85 parts by weight and 95 parts by weight of carbon-based negative electrode active material.

[0051] In one aspect of this specification, the content of the negative electrode active material in 100 parts by weight of the negative electrode active material layer may be 80 parts by weight or more and 99.8 parts by weight or less, preferably 90 parts by weight or more and 99.5 parts by weight or less, more preferably 95 parts by weight or more and 99 parts by weight or less.

[0052] In one aspect of this specification, one or both of the first negative electrode active material layer and the second negative electrode active material layer contain lithium-substituted carboxymethyl cellulose.

[0053] In lithium-substituted carboxymethyl cellulose, the degree to which the hydroxyl group (-OH) is substituted by lithium carboxymethyl (-CH2COOLi) can be 0.1 or more, for example 0.5 or more, specifically 0.7 to 1.3, or 0.8 to 1.2, more specifically 0.8 to 1.0.

[0054] The molecular weight (Mn) of lithium-substituted carboxymethyl cellulose can be from 300,000 to 1,000,000, for example from 350,000 to 900,000, particularly from 500,000 to 900,000.

[0055] Lithium-substituted carboxymethyl cellulose can be included in the negative electrode active material layer containing silicon-based active materials, and it can also be included in the negative electrode active material layer containing only carbon-based active materials without silicon-based active materials. Lithium-substituted carboxymethyl cellulose can also be included in a second negative electrode active material layer containing both silicon-based and carbon-based active materials.

[0056] Lithium-substituted carboxymethyl cellulose may be contained only in the first negative electrode active material layer, or only in the second negative electrode active material layer, or in both the first negative electrode active material layer and the second negative electrode active material layer.

[0057] Based on a total weight of 100 parts by weight of the first and second negative electrode active material layers, the content of lithium-substituted carboxymethyl cellulose can be from 0.1 parts by weight to 5 parts by weight, for example, 0.1 parts by weight to 3 parts by weight, 0.2 parts by weight to 3 parts by weight, 0.3 parts by weight to 2 parts by weight, or 0.5 parts by weight to 1 part by weight. Based on a total weight of 100 parts by weight of the first or second negative electrode active material layer containing lithium-substituted carboxymethyl cellulose, the content of lithium-substituted carboxymethyl cellulose can be from 0.1 parts by weight to 5 parts by weight, for example, 0.1 parts by weight to 3 parts by weight, 0.2 parts by weight to 3 parts by weight, 0.3 parts by weight to 2 parts by weight, or 0.5 parts by weight to 1 part by weight.

[0058] According to another aspect of this specification, in addition to silicon-based and carbon-based active materials, the negative electrode active material layer may further include a negative electrode binder.

[0059] Anode binders can be used to improve the bonding between anode active material particles and the adhesion between anode active material particles and anode current collector. As anode binders, those known in the art can be used, and non-limiting examples may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials whose hydrogen is substituted by Li, Na, or Ca, etc., and may also include various copolymers thereof.

[0060] Based on 100 parts by weight of the negative electrode active material layer, the content of the negative electrode binder can be more than 0.1 parts by weight and less than 20 parts by weight, for example, preferably more than 0.3 parts by weight and less than 20 parts by weight, more preferably more than 0.5 parts by weight and less than 10 parts by weight.

[0061] The negative electrode active material layer may not contain conductive material, but may further contain conductive material if necessary. There are no particular limitations on the conductive material included in the negative electrode active material layer, as long as the conductive material is conductive and does not cause chemical changes in the battery. Examples include: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon 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; conductive metal oxides, such as titanium oxide; and conductive materials such as polyphenylene derivatives. Based on 100 parts by weight of the negative electrode active material layer, the content of conductive material in the negative electrode active material layer can be from 0.01 parts by weight to 20 parts by weight, preferably from 0.03 parts by weight to 18 parts by weight.

[0062] For the purposes of this disclosure, when the carbon-based active material is graphite such as natural or artificial graphite, the weight percentage of graphite used as the carbon-based active material is not considered when defining the total weight percentage of the conductive material. Similarly, when the conductive material selected for the negative electrode is graphite, the weight percentage of said conductive material is not included when defining the total weight percentage of the carbon-based negative electrode active material. Therefore, when graphite is selected as both the carbon-based negative electrode active material and the negative electrode conductive material, the total weight percentage of graphite corresponds to the sum of the weight percentages of graphite used as the carbon-based negative electrode active material and the weight percentages of graphite used as the negative electrode conductive material.

[0063] The conductive materials included in the negative electrode active material layer are, for example: carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal cracking carbon black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes.

[0064] In one aspect of this specification, the thickness of each of the first negative electrode active material layer and the second negative electrode active material layer may be 30 μm or more and 100 μm or less, for example, 45 μm or more and 75 μm or less. The sum of the thicknesses of the first negative electrode active material layer and the second negative electrode active material layer may be 90 μm or more and 150 μm or less.

[0065] In one aspect of this application, a negative electrode current collector is sufficient and not particularly limited, as long as it is conductive without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel with a surface treated with carbon, nickel, titanium, or silver can be used as the current collector. Specifically, transition metals that readily adsorb carbon (such as copper or nickel) can be used as the current collector. Although the thickness of the current collector can range from 1 μm to 500 μm, the thickness is not limited to this.

[0066] Another aspect of this specification provides a secondary battery comprising the negative electrode, positive electrode, and separator described above.

[0067] In one aspect of this specification, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and comprising a positive electrode active material. The thickness of the positive electrode active material layer can be more than 20 μm and less than 500 μm.

[0068] There are no particular restrictions on the positive electrode current collector, as long as it is conductive and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surfaces treated with carbon, nickel, titanium, and silver can be used. Furthermore, the thickness of the positive electrode current collector can typically range from 1 μm to 500 μm, and the adhesion of the positive electrode active material can be enhanced by forming fine irregularities on the surface of the current collector. For example, the positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0069] In one aspect of this specification, the positive electrode may include a lithium complex transition metal compound comprising nickel (Ni) and cobalt (Co) as an active material. The lithium complex transition metal compound may further comprise at least one of manganese and aluminum. Among metals other than lithium, the lithium complex transition metal compound may contain more than 80 mol%, for example, more than 80 mol% and less than 100 mol% nickel.

[0070] In one aspect, the content of the positive electrode active material in 100 parts by weight of the positive electrode active material layer may be 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 99.8 parts by weight or less.

[0071] According to another aspect of this specification, the positive electrode active material layer described above may further comprise a positive electrode binder and a conductive material.

[0072] Positive electrode binders can be used to improve the bonding between positive electrode active material particles and the adhesion between positive electrode active material particles and positive electrode current collectors. As positive electrode binders, those known in the art can be used, non-limiting examples of which include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more of them can be used.

[0073] Based on 100 parts by weight of the positive electrode active material layer, the content of the positive electrode binder can be more than 0.1 parts by weight and less than 20 parts by weight, for example, preferably more than 0.3 parts by weight and less than 18 parts by weight, more preferably more than 0.5 parts by weight and less than 15 parts by weight.

[0074] 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 restrictions, as long as the conductive material has electronic conductivity without causing chemical changes in the battery. Specific examples include: graphite, such as natural or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, carbon fibers, and carbon nanotubes; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, any one or a mixture of two or more of these can be used. Specifically, in one aspect, the conductive material may include one or more of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).

[0075] Based on 100 parts by weight of the composition for the positive electrode active material layer, the content of the conductive material can be 0.1 parts by weight or more and 10 parts by weight or less, for example, preferably 0.2 parts by weight or more and 7 parts by weight or less, more preferably 0.3 parts by weight or more and 5 parts by weight or less.

[0076] According to one aspect, the sum of the weights of the first and second negative electrode active material layers can be 170 mg / 25 cm³. 2 Up to 280mg / 25cm 2 Here, the weight is based on the weight of the dried product (solids) excluding the solvent. This range is advantageous for high energy density and fast charging characteristics.

[0077] In addition to using the aforementioned positive and negative electrode active materials, the positive and negative electrodes can be manufactured using existing methods for manufacturing positive and negative electrodes. Specifically, after coating an active material layer forming composition comprising the aforementioned active materials, along with optional binders and conductive materials, onto a current collector, the positive and negative electrodes can be manufactured by drying and rolling the current collector. In this case, the types and amounts of the positive and negative electrode active materials, binders, and conductive materials are as described above. The solvent can be a solvent commonly used in the art, examples of which include dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water, wherein any one or a mixture of two or more of these can be used. Considering the coating thickness and preparation yield of the slurry, the amount of solvent used is sufficient as long as the amount dissolves or disperses the active materials, conductive materials, and binders and has a viscosity capable of exhibiting excellent thickness uniformity during subsequent coating processes in the manufacture of the positive and negative electrodes. Alternatively, the positive and negative electrodes can be manufactured by casting an active material layer forming composition onto a separate support, and then pressing the resulting membrane layer obtained by peeling it off from the support onto a current collector.

[0078] The separator separates the negative and positive electrodes and provides a pathway for lithium ion movement. It can be used without particular limitations, as long as it is typically used in secondary batteries. In particular, separators with excellent ability to retain electrolyte moisture and low resistance to ion movement in the electrolyte are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes formed from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminates of two or more layers thereof. Alternatively, typical porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers and polyethylene terephthalate fibers. Furthermore, coated separators including ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used as single-layer or multi-layer structures.

[0079] Examples of electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used to prepare lithium secondary batteries.

[0080] Specifically, electrolytes can include non-aqueous organic solvents and metal salts.

[0081] As non-aqueous organic solvents, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butyl 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, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate can be used.

[0082] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferred among carbonate organic solvents because cyclic carbonates, as high-viscosity organic solvents, have high dielectric constants and thus readily dissociate lithium salts. Furthermore, since cyclic carbonates can be mixed in appropriate proportions with low-viscosity and low-dielectric-constant linear carbonates (such as dimethyl carbonate and diethyl carbonate) to prepare electrolytes with high conductivity, such cyclic carbonates are even more preferred.

[0083] Lithium salts can be used as metal salts, as they are readily soluble in non-aqueous electrolytes. For example, F-type anions can be used as the lithium salt anions. - Cl - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 -CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - One or more of the groups formed.

[0084] In order to improve the battery's lifespan, suppress the reduction of battery capacity, and increase the battery's discharge capacity, in addition to the above-mentioned electrolyte components, the electrolyte may further include one or more additives, such as halogenated alkyl carbonate compounds such as difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol ethers, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride.

[0085] One aspect of the present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, and may be a lithium secondary battery.

[0086] Another aspect of the present invention provides a battery module comprising the aforementioned secondary battery as a unit cell, and a battery pack comprising the battery module. The battery module and battery pack include secondary batteries with high capacity, high rate capability, and high cycle performance, and therefore can be used as a power source for medium to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0087] Because the secondary battery of this invention consistently exhibits excellent discharge capacity, output characteristics, and cycle performance, it can be used as a power source for portable devices such as mobile phones, laptops, and digital cameras, as well as for medium to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems. For example, the battery module or battery pack can be used as a power source for one or more of the following medium to large-sized devices: power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and energy storage systems.

[0088] Example

[0089] Preferred embodiments will be presented below to facilitate understanding of the invention; however, the embodiments are provided for illustrative purposes only, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the invention, and such changes and modifications obviously fall within the scope of the appended claims.

[0090] Examples 1 to 3 and Comparative Examples 1 to 6

[0091] <Battery Cell Manufacturing>

[0092] Example 1

[0093] Manufacturing of the positive electrode

[0094] A lithium composite transition metal compound containing nickel (Ni), cobalt (Co), and manganese (Mn) in an atomic ratio of 84:8:8 and doped with aluminum (Al), a conductive material (CNT), and a binder (PVDF) were placed in a methylpyrrolidone (NMP) solvent at a weight ratio of 97:1:2 to prepare a positive electrode slurry (based on the total positive electrode slurry, the solid content of the positive electrode slurry was 70 parts by weight).

[0095] The prepared positive electrode slurry was coated onto the Al current collector, dried, and then rolled at room temperature to manufacture the positive electrode.

[0096] Manufacturing of negative electrode

[0097] For the first negative electrode active material layer, carbon-based active materials (including artificial graphite and natural graphite in a weight ratio of 8:2), conductive materials (carbon black), binders (SBR), and thickeners (Li-CMC) are placed in distilled water solvent in a weight ratio of 96:1:2:1 to prepare a negative electrode slurry (the content of solids in the negative electrode slurry is 50 parts by weight of the total negative electrode slurry).

[0098] For the second negative electrode active material layer, a negative electrode active material containing Mg-doped SiO active material and carbon-based active material (including artificial graphite and natural graphite in a weight ratio of 8:2) (based on 100 parts by weight of all negative electrode active material, the content of Mg-doped SiO active material is 5 parts by weight (based on 100 parts by weight of negative electrode active material in the second negative electrode active material layer, it is 10 parts by weight)), conductive material (carbon black), binder (SBR) and thickener (Li-CMC) are placed in distilled water solvent in a weight ratio of 96:1:2:1 to prepare a negative electrode slurry (the content of solids in the negative electrode slurry is 50 parts by weight of all negative electrode slurry).

[0099] After coating the first negative electrode active material layer slurry prepared above onto the Cu current collector, the second negative electrode active material layer slurry is sequentially coated onto the first negative electrode active material layer. At the same time, the first negative electrode active material layer and the second negative electrode active material layer are dried, and then rolled at room temperature to manufacture the negative electrode.

[0100] Battery cell manufacturing

[0101] The separator is placed between the positive and negative electrodes manufactured as described above and assembled, electrolyte is injected, and then activated to manufacture the battery cell.

[0102] - Electrolyte composition: 1M LiPF6, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (volume ratio 3 / 7), vinylene carbonate (VC) / propane sulpholactone (PS) (included in the electrolyte in amounts of 3 parts by weight and 1.5 parts by weight, respectively)

[0103] - Activation: 0.1C, 3 hours. Degassing after high-temperature / room-temperature aging following charging.

[0104] Example 2

[0105] Except for using Na-CMC instead of Li-CMC as the slurry thickener for the first negative electrode active material layer, the electrodes and cells were manufactured in the same manner as in Example 1.

[0106] Example 3

[0107] Except for using Na-CMC instead of Li-CMC as the slurry thickener for the second negative electrode active material layer, the electrodes and cells were manufactured in the same manner as in Example 1.

[0108] Comparative Example 1

[0109] Except for using Na-CMC instead of Li-CMC as the slurry thickener for the first and second negative electrode active material layers, the electrodes and cells were manufactured in the same manner as in Example 1.

[0110] Comparative Example 2

[0111] Except that only carbon-based active materials are used as negative electrode active materials in both the first negative electrode active material layer and the second negative electrode active material layer slurry to manufacture the negative electrode, the electrode and the cell are manufactured in the same manner as in Comparative Example 1.

[0112] Comparative Example 3

[0113] Except that the first negative electrode active material layer slurry from Comparative Example 6 is coated onto the Cu current collector as a single layer (the thickness of the single layer is the same as the thickness of the first and second negative electrode active material layers in Comparative Example 6), and no second negative electrode active material layer is formed, the electrode and the cell are manufactured in the same manner as in Comparative Example 6.

[0114] Comparative Example 4

[0115] Except that Na-CMC is used instead of Li-CMC as a thickener in the first negative electrode active material layer slurry in Comparative Example 6, the slurry is coated onto the Cu current collector as a single layer and no second negative electrode active material layer is formed, the electrode and the cell are manufactured in the same manner as in Comparative Example 6.

[0116] Comparative Example 5

[0117] Except that the slurry of the second negative electrode active material layer in Example 1 is used as the slurry of the first negative electrode active material layer and first coated onto the Cu current collector, and the slurry of the first negative electrode active material layer in Example 1 is used as the slurry of the second negative electrode active material layer and sequentially coated onto the first negative electrode active material layer, the electrode and the cell are manufactured in the same manner as in Example 1.

[0118] Comparative Example 6

[0119] Except that the anode active material layer containing Mg-doped SiO active material and carbon-based active material (comprising artificial graphite and natural graphite in a weight ratio of 8:2) is used instead of the carbon-based active material in the first anode active material layer slurry, and the content of Mg-doped SiO active material in both the first and second anode active material layers is 5 parts by weight based on 100 parts by weight of all anode active material, the electrode and cell are manufactured in the same manner as in Example 1.

[0120] Experimental Example 1. Cell Resistance Performance

[0121] The third discharge capacity was measured by charging the fabricated cell three times with a constant current / constant voltage (CC / CV) at 0.33C to 4.2V (0.05C cutoff) and discharging it three times with a constant current (CC) at 0.33C (2.5V cutoff). Subsequently, after charging as described above, the SOC was set to 50% by discharging at 0.33C and then pulsed at 2.5C for 10 seconds to measure the resistance (initial resistance). The results are shown in Table 2 below.

[0122] Experimental Example 2. Cell Charging Rate Performance (C Rate)

[0123] The charging capacity according to the C-rate was measured by charging the manufactured cells at various C-rates (0.1C / 2.0C) with constant current / constant voltage (CC / CV) to 4.2V (0.05C cutoff) and discharging at a constant current (CC) of 0.33C (2.5V cutoff). Based on the measured charging capacity, the cell charging C-rate performance (2.0C charging capacity / 0.1C charging capacity × 100) was calculated and is shown in Table 2.

[0124] Experimental Example 3. High-Temperature Cyclic Life Performance of Battery Cells

[0125] The fabricated cells were cycled at a high temperature (45°C) by charging at a constant current / constant voltage (CC / CV) of 0.33C to 4.2V (0.05C cutoff) and discharging at a constant current (CC) of 0.33C (2.5V cutoff). After repeating the experiment 200 times, the capacity was measured in the same manner as in Example 1 to determine the capacity retention rate (capacity after 400 cycles / initial capacity × 100%). The high-temperature cycle life performance of the cells in Examples 1 to 3 was measured to be 96.0%, 95.5%, and 94.9%, respectively.

[0126] [Table 1]

[0127]

[0128] [Table 2]

[0129]

[0130] As shown in Table 2, it can be confirmed that the embodiments exhibit superior performance in terms of cell resistance and cell charging rate (C-rate) compared to the comparative examples. Furthermore, the cells manufactured according to the embodiments also maintain excellent lifespan performance.

Claims

1. A negative electrode for a secondary battery, comprising: current collector; A first negative electrode active material layer is disposed on the current collector; as well as A second negative electrode active material layer disposed 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 lithium-substituted carboxymethyl cellulose, and The first negative electrode active material layer contains carbon-based active materials, and only the second negative electrode active material layer contains silicon-based active materials.

2. The negative electrode as described in claim 1, wherein, The silicon-based active material includes SiO₂. x (0≤x<2), SiM y (M is a metal, 1≤y≤4) and at least one of Si / C.

3. The negative electrode as described in claim 1, wherein, The carbon-based active material includes at least one of artificial graphite and natural graphite.

4. The negative electrode as described in claim 1, wherein, The second negative electrode active material layer also contains carbon-based active materials.

5. The negative electrode as described in claim 4, wherein, The carbon-based active material includes at least one of artificial graphite and natural graphite.

6. The negative electrode as described in claim 4, wherein, Based on 100 parts by weight of all active materials contained in the negative electrode active material layer containing silicon-based active materials, wherein the content of silicon-based active materials is from 1 part by weight to 40 parts by weight.

7. The negative electrode as described in claim 1, wherein, The sum of the weights of the first negative electrode active material layer and the second negative electrode active material layer is 170 mg / 25 cm². 2 Up to 280mg / 25cm 2 .

8. A secondary battery comprising a negative electrode, a positive electrode, and a separator as described in any one of claims 1 to 7.

9. The secondary battery as described in claim 8, wherein, The positive electrode comprises a lithium complex transition metal compound containing nickel (Ni) and cobalt (Co) as the active material.

10. The secondary battery as claimed in claim 9, wherein, The lithium complex transition metal compound also contains at least one of manganese and aluminum.

11. The secondary battery as claimed in claim 8, wherein, The cell resistance is below 1.85Ω.

12. The secondary battery as described in claim 8, wherein, The battery cell's charging rate performance (C-rate) is over 97%.

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