Negative electrode and lithium secondary battery comprising same

Abstract

The present invention relates to a negative electrode and a lithium secondary battery comprising same. Specifically, the negative electrode comprises: a current collector; and a negative electrode active material layer disposed on at least one surface of the current collector, wherein the negative electrode active material layer comprises a carbon-based active material and a silicon-based active material, the carbon-based active material comprises artificial graphite particles in the form of secondary particles in which a plurality of primary particles are aggregated, the sphericity of the primary particles is 0.60 to 0.78, and the ratio of the average particle diameter (D50) of the silicon-based active material to the average particle diameter (D50) of the artificial graphite particles in the form of secondary particles is 0.4 to 1.

Description

Negative electrode, and lithium secondary battery including the same

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

[0002]

[0003] With growing interest in environmental issues, extensive research is being conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can replace fossil fuel-powered vehicles, such as gasoline and diesel cars, which are major causes of air pollution. Lithium-ion batteries, characterized by high energy density, high discharge voltage, and output stability, are primarily being researched and used as the power source for these EVs and HEVs.

[0004] Generally, a lithium secondary battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode or negative electrode is manufactured by mixing a positive active material or a negative active material with a binder, etc., dispersing it in a solvent to prepare a slurry, applying the slurry to the surface of an electrode current collector, and drying it to form an electrode active material layer.

[0005] At this time, a carbon-based active material capable of reversible lithium ion intercalation and extraction while maintaining structural and electrical properties may be used as the negative electrode active material. Various forms of carbon-based materials, such as artificial graphite, natural graphite, and hard carbon, have been applied as the carbon-based active material, and among them, graphite-based active materials, which can guarantee the lifespan characteristics of lithium secondary batteries due to their excellent reversibility, are the most widely used. Since the discharge voltage of the graphite-based active material is low at -0.2V compared to lithium, a battery using the graphite-based active material can exhibit a high discharge voltage of 3.6V, thereby providing many advantages in terms of energy density of the lithium battery.

[0006] Meanwhile, with the development of electric vehicles and mobile electronic devices, there have been attempts to improve rapid charging performance to meet the demands for convenience, efficiency, and economy of consumers. Efforts such as increasing the carbon coating layer content on graphite-based active materials and secondary particle formation of artificial graphite have been made, but there are problems such as a decrease in charge / discharge efficiency.

[0007] Accordingly, there is an urgent need to develop a cathode capable of simultaneously improving rapid charging performance and charge / discharge efficiency.

[0008]

[0009] One objective of the present invention is to provide a negative electrode that uses a carbon-based active material comprising artificial graphite particles in the form of secondary particles formed by the aggregation of a plurality of primary particles and a silicon-based active material, and designs the ratio of the average particle size of the silicon-based active material to the sphericity of the primary particles and the average particle size of the artificial graphite particles in the form of secondary particles to have a value within a specific range, thereby suppressing side reactions with the electrolyte and side reactions of the material itself, thereby preventing a decrease in discharge capacity and reducing battery resistance while simultaneously improving cycle life and rapid charging performance.

[0010] In addition, another objective of the present invention is to provide a lithium secondary battery comprising the aforementioned negative electrode.

[0011]

[0012] The present invention relates to a cathode, wherein the cathode comprises a current collector; and a cathode active material layer disposed on at least one surface of the current collector; wherein the cathode active material layer comprises a carbon-based active material and a silicon-based active material, and the carbon-based active material comprises artificial graphite particles in the form of secondary particles formed by the aggregation of a plurality of primary particles, wherein the degree of sphericity of the primary particles is 0.60 to 0.78, and the average particle size (D) of the artificial graphite particles in the form of secondary particles 50 The average particle size (D) of the above silicon-based active material for ) 50The ratio of ) provides a cathode of 0.4 to 1.

[0013] In addition, the present invention provides a lithium secondary battery comprising the aforementioned negative electrode.

[0014]

[0015] The cathode according to the present invention utilizes a carbon-based active material comprising artificial graphite in the form of secondary particles aggregated from a plurality of primary particles, and a silicon-based active material comprising a cathode active material. At this time, the degree of sphericity of the primary particles constituting the artificial graphite particles in the form of secondary particles is controlled to satisfy a specific range. Specifically, by adjusting the shape of the primary particles to be close to a sphere, particle breakage after the rolling process during cathode manufacturing and adverse reactions with the electrolyte caused by an increase in specific surface area can be prevented, and a diffusion path for lithium ions can be secured to form a stable film.

[0016] In addition, if the ratio of the average particle size of the silicon-based active material to the average particle size of the artificial graphite particles in the form of secondary particles is controlled to a specific range, lithium ions can be inserted into the interior of the silicon-based active material particles, thereby slowing down the degradation rate of the silicon-based active material and suppressing particle breakage of the silicon-based active material. Furthermore, the problem of reduced anode impregnation caused by byproducts generated from the breakdown of the SEI film formed on the surface of the silicon-based active material blocking the pores of the anode can be minimized. As a result, degradation of the battery's lifespan, capacity, and rapid charging performance can be prevented.

[0017] Accordingly, the cathode according to the present invention can simultaneously achieve the effect of increasing the capacity of the battery due to the use of a silicon-based active material, and the effect of reducing battery resistance and improving rapid charging performance due to controlling the ratio of the average particle size of the silicon-based active material to the average particle size of the artificial graphite particles in the form of primary particles and secondary particles.

[0018]

[0019] Terms and words used in this specification and claims shall not be interpreted as being limited to their ordinary or dictionary meanings, but shall be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0020] The terms used in this specification are used merely to describe exemplary embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise.

[0021] In this specification, terms such as “comprising,” “comprising,” or “having” are intended to specify the existence of the implemented features, numbers, steps, components, or combinations thereof, and should not be understood as precluding the existence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0022] In this specification, the average particle size (D 50 ) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The above average particle size (D 50 ) can be measured, for example, using the laser diffraction method. The laser diffraction method generally enables the measurement of particle sizes ranging from the submicron range to several millimeters, and can obtain results with high reproducibility and high resolution.

[0023] In this specification, 'primary particle' means a single particle, that is, a single particle, and 'secondary particle' means an aggregate formed by the primary particle being aggregated into multiple particles through an intentional assembly or bonding process.

[0024] The present invention will be described in detail below.

[0025]

[0026] cathode

[0027] The present invention relates to a negative electrode, more specifically, to a negative electrode active material for a lithium secondary battery.

[0028] Specifically, the cathode according to the present invention comprises a current collector; and a cathode active material layer disposed on at least one surface of the current collector; wherein the cathode active material layer comprises a carbon-based active material and a silicon-based active material, and the carbon-based active material comprises artificial graphite particles in the form of secondary particles formed by the aggregation of a plurality of primary particles, the degree of sphericity of the primary particles is 0.60 to 0.78, and the average particle size (D) of the artificial graphite particles in the form of secondary particles 50 The average particle size (D) of the above silicon-based active material for ) 50 The ratio of ) is characterized by being 0.4 to 1.

[0029]

[0030] The above current collector can be any current collector commonly used in the field without limitation, and is not particularly limited as long as it has high conductivity without causing chemical changes in the lithium secondary battery, for example. For example, the above negative current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, preferably copper.

[0031] The above current collector may form fine irregularities on its surface to strengthen the bonding force of the negative active material, and can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

[0032] The above-mentioned cathode current collector can generally have a thickness of 3㎛ to 500㎛.

[0033]

[0034] The above-mentioned negative electrode active material layer is disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector.

[0035] The above cathode active material layer includes a cathode active material.

[0036] The above-mentioned cathode active material includes a carbon-based active material and a silicon-based active material.

[0037] The above carbon-based active material and silicon-based active material may each be in the form of particles.

[0038] The above carbon-based active material includes artificial graphite particles in the form of secondary particles formed by the aggregation of multiple primary particles.

[0039] The aforementioned silicon-based active material has the advantage of possessing a higher capacity and energy density compared to carbon-based active materials such as graphite, but it has the disadvantage of undergoing significant volume changes during charging and discharging. Such volume expansion and contraction of the silicon-based active material lead to the breakdown of conductive connections within the anode, causing increased resistance and reduced lifespan performance. Furthermore, the SEI film formed on the anode during the activation process of lithium-ion batteries can be damaged by the volume changes of the silicon-based active material. This promotes side reactions in the electrolyte, causing problems such as increased resistance due to the increased thickness of the SEI film and electrolyte depletion; consequently, this can lead to a degradation in lifespan and storage characteristics.

[0040] Although methods involving the combined use of carbon-based active materials are being studied to resolve the aforementioned problems, there are currently additional issues such as the formation of non-uniform films due to particle breakage caused by various factors—including the degree of sphericity of particles constituting the carbon-based active material, the difference in mechanical strength between carbon-based and silicon-based active materials, and the ratio of the average particle size and content of silicon-based active materials to carbon-based active materials—and the degradation of cycle life characteristics due to side reactions of the electrolyte.

[0041] The present invention solves the above problems by designing the cathode active material to have a specific range of values ​​when the cathode active material comprises a silicon-based active material and a carbon-based active material comprising artificial graphite particles in the form of secondary particles in which a plurality of primary particles are aggregated.

[0042] The artificial graphite above may be in the form of secondary particles in which a plurality of primary particles are aggregated.

[0043] The above secondary particles refer to particles formed by the aggregation of multiple, for example, dozens to hundreds of primary particles. When the above artificial graphite particles are in the form of secondary particles, voids may be formed within the artificial graphite particles, and the voids may be empty spaces formed between the primary particles, may be amorphous, and may exist in two or more places.

[0044] The carbon-based active material containing the above-mentioned secondary particle-shaped artificial graphite particles has the advantage of being able to increase charging and discharging speeds by facilitating the movement of lithium ions through the voids formed between the secondary particles, thereby promoting the electrochemical reaction of lithium ions and improving current conductivity.

[0045] The degree of sphericity of the primary particles constituting the artificial graphite particles in the form of secondary particles may be 0.60 to 0.78. When a negative electrode active material comprising a carbon-based active material and a silicon-based active material is used, and the primary particles satisfy the above range, it is desirable that the carbon-based active material does not break easily during the rolling process in the negative electrode manufacturing process, a pathway for the movement of lithium ions within the secondary particles is secured, and a conductive network with the silicon-based active material can be maintained.

[0046] Specifically, if the degree of sphericity of the primary particles is less than 0.60, the BET specific surface area may increase as the carbon coating on the surface of the artificial graphite in the form of secondary particles is broken by silicon, which has relatively high particle strength, during the additional rolling process due to low tap density and curvature. An increase in the BET specific surface area leads to increased adverse reactions between the broken areas of the artificial graphite carbon coating and the electrolyte, which may result in the excessive formation of a film on the electrode surface. This reduces the porosity at the electrode end, making it difficult for lithium ions to move to the bottom of the electrode and can lead to a problem where charging and discharging are concentrated on the silicon material at the top of the electrode. Consequently, the expansion and contraction of the silicon-based active material present at the top of the electrode may be intensified. If this process is repeated, the silicon-based active material located at the top of the electrode comes into direct contact with the electrolyte, causing additional side reactions that may electrochemically deactivate a portion of the surface of the silicon-based active material. Furthermore, lithium ions may irreversibly adhere to the surface of the silicon-based active material, leading to a decrease in battery capacity due to electrolyte decomposition and reduction of the silicon active material. Additionally, accelerated degradation of the top of the electrode due to the deterioration of the silicon-based active material can result in reduced cycle life and increased battery resistance.

[0047] Conversely, if the degree of sphericity of the primary particles exceeds 0.78, breakage caused by silicon-based active material particles during the rolling process is minimal, but the high degree of sphericity may increase the degree of exposure of the graphite edge surfaces. The edge surfaces of the graphite are the ends of the graphite crystal structure and possess more active bonds compared to the internal basal surfaces. Since lithium ion insertion into the graphite occurs through insertion into the edge surfaces rather than the basal surfaces, a larger edge / basal value results in a shape that is advantageous for rapid charging. However, if the edge / basal value becomes excessively large due to the high sphericity of the primary particles, the material may react easily not only with lithium ions but also with electrolyte solvents or lithium salts. Specifically, during charging, side reactions with the electrolyte may occur at the edge surfaces of the graphite, causing the electrolyte to undergo electrochemical decomposition. The film formed through such electrochemical decomposition is thick and unstable, which can lead to a decrease in battery performance and a shortened lifespan.

[0048] Furthermore, if the sphericity of the primary particles exceeds 0.78, the tortuosity may also increase. Tortuosity is an indicator of how curved a path lithium ions must follow when passing through an electrode. For example, a low tortuosity value means there are fewer factors hindering the passage of lithium ions. When the particle sphericity is high, the contact surface between primary particles increases, and the spacing between primary particles within secondary particles becomes relatively narrow, lengthening the diffusion path of lithium ions. Consequently, the overall tortuosity of the electrode increases, forming a structure where it is difficult for lithium ions to diffuse toward the bottom of the electrode. If the lithium diffusion path is obstructed, it can lead to a decrease in battery capacity and an increase in resistance due to increased side reactions caused by accelerated degradation at the top of the electrode.

[0049] In the present invention, the degree of sphericity of the primary particle may be 0.60 or higher, 0.62 or higher, 0.64 or higher, or 0.66 or higher. The degree of sphericity of the primary particle may be 0.78 or lower, 0.76 or lower, 0.74 or lower, or 0.72 or lower.

[0050] More specifically, the degree of sphericity of the primary particles may be 0.60 to 0.78, specifically 0.62 to 0.76, more specifically 0.64 to 0.74, and even more specifically 0.66 to 0.72. When the degree of sphericity of the primary particles satisfies the above range, side reactions with the electrolyte are reduced, and while preventing a decrease in discharge capacity, the effect of reducing battery resistance can be achieved, and cycle life and rapid charging performance can be improved simultaneously.

[0051] Meanwhile, the sphericity mentioned above can be measured by analyzing images using an optical microscope (Keyence VHX-7000), reflection mode, and ring illumination. Specifically, 4 mg of primary particles are dispersed in 2 ml of NMP, and sonication treatment is performed for 1 minute. Afterward, the solution is dropped onto a slide glass and covered with a cover glass to prepare a sample. After placing the sample on the stage of the optical microscope and focusing, if the magnification is fixed at 300x, 100 to 200 particle images can be acquired in that area. Furthermore, the sphericity of each particle can be calculated by utilizing the automatic area measurement function in the software of the equipment, and the sphericity of the primary particles can be quantified by calculating the average of the values. Meanwhile, the sphericity of each particle is calculated by the following Equation 1.

[0052] [Equation 1]

[0053] Circularity = 4πA / P 2

[0054] (P: Perimeter of the particle boundary, A: Area of ​​the particle)

[0055] Meanwhile, the effect of improving lifespan characteristics and suppressing the increase in battery resistance, which can be achieved by designing the sphericity of the primary particles to satisfy the value within the range described above, may not appear when a carbon-based active material is used alone. That is, the effect of the present invention through the control of the sphericity value of the primary particles may appear when the negative electrode active material includes both a carbon-based active material and a silicon-based active material.

[0056] In the present invention, the average particle size (D) of the artificial graphite particles in the form of secondary particles 50 The average particle size (D) of the above silicon-based active material for ) 50 The ratio of ) satisfies 0.4 to 1. When a carbon-based active material comprising artificial graphite particles in the form of secondary particles formed by the aggregation of a plurality of primary particles and a negative electrode active material comprising a silicon-based active material are used, the average particle size (D) of the artificial graphite particles in the form of secondary particles is 50 The average particle size (D) of the above silicon-based active material for ) 50 If the ratio of ) satisfies the above range, it is desirable in that both the capacity characteristics and lifespan characteristics of the finally manufactured lithium secondary battery can be secured.

[0057] Specifically, the average particle size (D) of the artificial graphite particles in the form of secondary particles mentioned above 50 The average particle size (D) of the above silicon-based active material for ) 50 If the ratio of ) is less than 0.4, the specific surface area of ​​the silicon-based active material increases, leading to an increase in the lithium used as a component of the SEI film formed on the surface of the silicon-based active material. Furthermore, the SEI film formed on the surface of the silicon-based active material is easily destroyed due to the volume expansion of the silicon-based active material during battery operation, and byproducts from the destroyed SEI film can block the pores of the negative electrode, thereby reducing the electrolyte impregnation properties of the negative electrode. Consequently, the battery's capacity performance and rapid charging performance may be degraded. Conversely, the average particle size (D) of the artificial graphite particles in the form of secondary particles mentioned above 50The average particle size (D) of the above silicon-based active material particles for ) 50 If the ratio of ) exceeds 1, the significant difference in the degree of volume expansion between the interior and surface of the silicon-based active material particles can lead to intensified degradation and particle breakage of the silicon-based active material. Specifically, when silicon-based and carbon-based active materials are used in combination, the lithium insertion potential of the silicon-based active material is higher than that of the carbon-based active material; therefore, the reaction of lithium ions being inserted into the surface of the silicon-based active material particles occurs preferentially. However, as the average particle size of the silicon-based active material increases, the energy required for lithium ions to diffuse into the interior of the silicon-based active material particles increases; consequently, the reaction of lithium ions being inserted into the carbon-based active material occurs preferentially over the reaction of lithium ions diffusing into the interior of the silicon-based active material particles. In other words, a situation arises where lithium ions cannot be inserted into the interior of the silicon-based active material particles. If the lithium ion insertion / extraction reaction occurs only at the surface of the silicon-based active material particles in this manner, the degradation rate of the silicon-based active material accelerates, leading to particle breakage problems.

[0058] Average particle size (D) of the artificial graphite particles in the form of secondary particles above 50 The average particle size (D) of the above silicon-based active material for ) 50 The ratio of ) may be specifically 0.40 to 0.95, more specifically 0.45 to 0.90, and even more specifically 0.5 to 0.8. The average particle size (D) of the average particle size of the artificial graphite particles in the form of secondary particles is 50 The average particle size (D) of the above silicon-based active material for ) 50 When the ratio of ) satisfies the above range, the occurrence of the aforementioned irreversible loss of available lithium and the particle breakage problem of silicon-based active material can be suppressed, so the capacity and lifespan performance of the battery are excellent.

[0059] In addition, the ratio of the average particle size of the silicon-based active material to the average particle size of the primary particle within the artificial graphite particles in the form of secondary particles may be 0.65 to 1.15, specifically 0.70 to 1.10, more specifically 0.75 to 1.10, and even more specifically 0.80 to 1.05. When the above range is satisfied, the difference in degradation rates between the carbon-based active material and the silicon-based active material can be minimized by preventing the movement path of lithium ions within the primary particle from becoming excessively long compared to the silicon-based active material, thereby improving lifespan performance.

[0060] In the present invention, the average particle size (D) of the primary particle. 50 The ) may be 1㎛ to 30㎛, specifically 3㎛ to 25㎛, more specifically 5㎛ to 18㎛, and even more specifically 7㎛ to 13㎛. When the above range is satisfied, the density of primary particles within the artificial graphite particles in the form of secondary particles is not too small, so the tap density is excellent. If the tap density is excellent, the degree of thickness change is not large during the rolling process to the desired electrode thickness, so the occurrence of particle breakage can be suppressed.

[0061]

[0062] The artificial graphite in the form of secondary particles described above can be manufactured by a method of mixing artificial graphite particles in the form of primary particles with a binder material (e.g., pitch), spherizing them, and heat-treating them to aggregate the artificial graphite particles in the form of primary particles into secondary particles. Alternatively, if the artificial graphite particles are in the form of secondary particles, the artificial graphite particles can be manufactured by mixing a carbon precursor and a binder material, performing spherization and aggregation processes to produce an intermediate in the form of secondary particles, and heat-treating the intermediate at a temperature of 3,000°C or higher to graphitize it. In this case, the carbon precursor may be coal-based heavy oil, petroleum-based heavy oil, tar, pitch, coke, etc., and specifically, may be at least one selected from the group consisting of needle coke, mosaic coke, and coal tar pitch.

[0063]

[0064] The carbon-based active material may further include an amorphous carbon coating layer located on the surface of the artificial graphite particles in the form of secondary particles. The amorphous carbon coating layer can contribute to improving the structural stability of the artificial graphite particles and preventing adverse reactions between the cathode active material and the electrolyte.

[0065] The above amorphous carbon coating layer may be formed in an amount of 0.1% to 10% by weight, preferably 1% to 5% by weight, based on the total weight of the carbon-based active material. Although the presence of the amorphous carbon coating layer can improve the structural stability of the carbon-based active material, excessive formation of the amorphous carbon coating layer may cause a decrease in initial efficiency due to an increase in the specific surface area during cathode rolling and may lead to a deterioration in high-temperature storage performance; therefore, it is desirable to form the carbon coating layer with an amount within the above-mentioned range.

[0066] The above amorphous carbon coating layer can be formed by providing a carbon coating layer precursor to artificial graphite particles and then heat treating them.

[0067] The carbon coating layer precursor may include at least one selected from polymer resin and pitch. Specifically, the polymer resin may include at least one selected from the group consisting of sucrose, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, polyacrylonitrile resin, polyamide resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin, vinyl chloride resin, and polyvinyl chloride. The pitch may include at least one selected from the group consisting of coal-based pitch, petroleum-based pitch, and mesophase pitch. The heat treatment process for forming the above amorphous carbon coating layer can be carried out at 1,000°C to 1,500°C in order to promote the uniform formation of the amorphous carbon coating layer.

[0068] In the present invention, the average particle size (D) of the artificial graphite particles in the form of secondary particles 50 ) may be 5㎛ to 60㎛, specifically 6㎛ to 35㎛, more specifically 8㎛ to 20㎛, and even more specifically 9㎛ to 14㎛.

[0069] In the present invention, the degree of sphericity of the artificial graphite particles in the form of secondary particles may be 0.30 to 0.95, specifically 0.5 to 0.9, more specifically 0.60 to 0.85, and even more specifically 0.65 to 0.85.

[0070] In the present invention, the BET specific surface area of ​​the carbon-based active material is 0.05 m² 2 / g to 100m 2 / g, specifically 0.2m 2 / g to 10m 2 / g, more specifically 0.5m 2 / g to 1.5m 2 It may be / g. When the BET specific surface area of ​​the carbon-based active material satisfies the above range, side reactions and non-uniform reactions with the electrolyte on the cathode surface can be prevented.

[0071] The above BET specific surface area can be measured using a BEL Sorption instrument (BEL Japan Co., Ltd.).

[0072] In the present invention, the tap density of the carbon-based active material may be 0.3 g / cc to 2.0 g / cc, specifically 0.7 g / cc to 1.3 g / cc, and more specifically 0.95 g / cc to 1.10 g / cc. When the tap density of the carbon-based active material of the present invention satisfies the above range, it is preferable in that it has an advantage in mixing coating processability because the space between particles is reduced during secondary particle assembly, thereby increasing the solid content of the slurry.

[0073]

[0074] In the present invention, the silicon-based active material is silicon (Si), a Si-Me alloy (wherein Me is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), and silicon oxide (SiO₂). x , 0 <x<2), 실리콘-탄소 복합체(Si / C 복합체) 등과 같은 실리콘계 음극 활물질을 포함할 수 있고, 가장 바람직하게는 실리콘(Si), 실리콘 산화물(SiO x , 0 <x<2) 및 실리콘-탄소 복합체(Si / C 복합체)로 이루어지는 군으로부터 선택된 적어도 1종을 포함할 수 있다.

[0075] In the present invention, the average particle size (D) of the silicon-based active material is 50 ) may be 0.5㎛ to 60㎛, specifically 1㎛ to 35㎛, more specifically 3㎛ to 20㎛, even more specifically 5㎛ to 10㎛, and even more specifically 7㎛ to 9㎛.

[0076] In the present invention, the weight ratio of the carbon-based active material and the silicon-based active material may be 50:50 to 99:1, specifically 70:30 to 97:3, more specifically 80:20 to 97:3, and even more specifically 85:15 to 97:3. When the weight ratio satisfies the above range, the capacity of the lithium secondary battery can be effectively improved while minimizing side reactions caused by volume expansion of the silicon-based active material.

[0077] The above negative electrode active material may be included in the negative electrode active material layer in an amount of 60% to 99% by weight, preferably 75% to 98% by weight. When the content of the negative electrode active material is within the above range, the desired battery capacity can be achieved.

[0078] The above-mentioned negative electrode active material layer may further include a binder, a conductive material, and / or a thickener in addition to the aforementioned negative electrode active material.

[0079] The above binder is a component that assists in bonding between the active material and / or the current collector, and can typically be included in the negative active material layer in an amount of 0.1% to 30% by weight, preferably 1% to 10% by weight.

[0080] The binder may include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, and fluororubber, preferably at least one selected from polyvinylidene fluoride and styrene-butadiene rubber.

[0081] As the above-mentioned thickener, any thickener conventionally used in lithium secondary batteries can be used, and an example is carboxymethylcellulose (CMC).

[0082] The above conductive material is a component for further improving the conductivity of the negative electrode active material, and may be included in the negative electrode active material layer in an amount of 0.1% to 30% by weight, preferably 1% to 10% by weight.

[0083] The above conductive material is not particularly limited as long as it possesses conductivity without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powder such as aluminum or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives may be used. Specific examples of commercially available conductive materials include acetylene black series products such as those from Chevron Chemical Company, Denka Black (Singapore Private Limited), and Gulf Oil Company, Ketjenblack, EC series (Armak Company products), Vulcan XC-72 (Cabot Company products), and Super P (Timcal products).

[0084] The thickness of the above-mentioned negative electrode active material layer may be 10㎛ to 300㎛, specifically 50㎛ to 200㎛, but is not limited thereto.

[0085] The above-mentioned negative electrode active material layer may be prepared by applying, rolling, and drying a negative electrode slurry, prepared by adding a negative electrode active material, optionally a binder, a thickener, and / or a conductive material to a solvent, onto the above-mentioned negative electrode current collector. At this time, the solvent may include water or an organic solvent such as NMP (N-methyl-2-pyrrolidone), and more specifically, may be water.

[0086] The above cathode can be manufactured by preparing a cathode slurry by mixing a cathode active material and optionally a binder and a conductive material in a solvent such as NMP (N-methyl-2-pyrrolidone) and water, then coating the cathode slurry onto a cathode current collector, and then drying and rolling.

[0087] The cathode according to the present invention controls the sphericity of primary particles constituting the carbon-based active material and the content with the silicon-based active material, thereby preventing loss of the carbon-based active material and ensuring the mobility of lithium ions. As such, it has superior capacity characteristics, rapid charging performance, and cycle life characteristics compared to conventional cathodes, and the rate of increase in resistance of the battery is controlled.

[0088] Meanwhile, the BET specific surface area of ​​the above cathode is 0.1 m² 2 / g to 30m 2 / g, specifically 0.5m 2 / g to 10m 2 / g, more specifically 1m 2 / g to 6m 2 / g, more specifically 2m 2 / g to 4m 2 / g, more specifically 2.1 m 2 / g to 3.0 m 2 / g, and if the BET specific surface area of ​​the above cathode satisfies the above range, it is desirable in that it can prevent breakage of artificial graphite particles in the form of secondary particles and maximize the mobility of lithium ions.

[0089] The porosity of the above cathode may be 15% to 40%, specifically 25% to 35%.

[0090] The porosity of the above cathode can be calculated using the following Equation 2.

[0091] [Equation 2]

[0092] Porosity (%) = {1 - (Cathode electrode density / True density of the cathode)} × 100

[0093] In the above Equation 2, the true density of the cathode is calculated according to the mixing ratio within the cathode active material layer using the true densities of each of the cathode active material, conductive material, and binder included in the cathode active material layer, and the electrode density of the cathode is the density of the cathode active material layer measured by taking a cathode of a certain size.

[0094] Meanwhile, the true density of each of the above-mentioned cathode active material, conductive material, and binder is a value measured using a Gas Pycnometer (Quantachrone).

[0095]

[0096] lithium secondary battery

[0097] Next, a lithium secondary battery according to the present invention will be described.

[0098] The lithium secondary battery according to the present invention comprises a negative electrode, a positive electrode, and an electrolyte, and may further comprise a separator as needed. In this case, since the negative electrode is identical to the negative electrode according to the present invention described above, a detailed description is omitted, and the remaining components excluding the negative electrode will be described below.

[0099]

[0100] anode

[0101] The anode according to the present invention comprises an anode composite layer comprising an anode active material, an anode conductive material, and an anode binder. The anode can be manufactured by a method of forming an anode composite layer by coating an anode slurry comprising an anode active material, an anode binder, an anode conductive material, and a solvent, etc., onto an anode current collector and then rolling it.

[0102] The above positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used.

[0103] The above-mentioned cathode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium metal oxide may be a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiNiO2, etc.), or a lithium-nickel-manganese-based oxide (e.g., LiNi1-Y Mn Y O2(here, 0 <Y<1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2(here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2(here, 0 <Y2<1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mnr )O2(where, 0<p<1, 0<q<1, 0<r<1, p+q+r=1) or Li(Ni p1 Co q1 Mn r1 )O4 (where 0<p1<2, 0<q1<2, 0<r1<2, p1+q1+r1=2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 Examples include )O2(wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r2 and s2 are each atomic fractions of independent elements, such that 0<p2<1, 0<q2<1, 0<r2<1, 0<s2<1, p2+q2+r2+s2=1), etc., and any one or more of these compounds may be included.

[0104] Specifically, the above positive active material may include a lithium transition metal oxide represented by the following [Chemical Formula 1].

[0105] [Chemical Formula 1]

[0106] Li x Ni a Co b M 1 c M 2 d O2

[0107] In the above chemical formula 1, the M 1 It is one or more selected from Mn and Al, and preferably, for durability, it may be Mn or a combination of Mn and Al.

[0108] M 2 It may be one or more selected from the group consisting of Zr, Y, B, W, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, F, P, and S.

[0109] The above x represents the atomic fraction of lithium in the lithium transition metal oxide, and may be 0.90≤x≤1.1, preferably 0.95≤x≤1.08, and more preferably 1.0≤x≤1.08.

[0110] The above a represents the atomic fraction of nickel among metal elements excluding lithium in the lithium transition metal oxide, and may be 0.50≤a<1.0, 0.60≤a≤0.95, 0.65≤a≤0.95, or 0.80≤a≤0.95. When the nickel content satisfies the above range, high capacity characteristics can be achieved.

[0111] The above b represents the atomic fraction of cobalt among the metal elements excluding lithium in the lithium transition metal oxide, where 0 <b<0.5, 0<b<0.4, 또는 0.01≤b≤0.3일 수 있다.

[0112] The above c is M among the metal elements excluding lithium in the lithium transition metal oxide. 1 Representing the atomic fraction of, 0 <c<0.5, 0<c<0.4, 또는 0.01≤c≤0.3일 수 있다.

[0113] The above d is M among the metal elements excluding lithium in the lithium transition metal oxide. 2 It represents the atomic fraction of , which can be 0≤d≤0.1 or 0≤d≤0.05.

[0114] The above positive active material may be included in an amount of 60 to 99 weight%, preferably 70 to 99 weight%, and more preferably 80 to 98 weight% based on the total weight of the positive composite layer.

[0115] The above-mentioned anode binder is a component that assists in the bonding of the anode active material and the anode conductive material, as well as the bonding to the current collector.

[0116] Examples of such anode binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers.

[0117] Typically, the anode binder may be included in an amount of 1 to 20 weight%, preferably 1 to 15 weight%, and more preferably 1 to 10 weight% based on the total weight of the anode composite layer.

[0118] The above-mentioned positive electrode conductive material is a component intended to further enhance the conductivity of the positive electrode active material, and is not particularly limited as long as it is conductive without causing chemical changes in the battery. For example, carbon powders such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers or metal fibers; fluorocarbon powder; conductive powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0119] Typically, the anode conductive material may be included in an amount of 1 to 20 weight%, preferably 1 to 15 weight%, and more preferably 1 to 10 weight% based on the total weight of the anode composite layer.

[0120] The solvent for the anode slurry may include organic solvents such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), and acetone, and may be used in an amount that results in a desirable viscosity when including the anode active material, anode binder, and anode conductive material. For example, the concentration of the solid component, which includes the anode active material and optionally the anode binder and anode conductive material, may be 50 to 95 weight%, preferably 70 to 95 weight%, and more preferably 70 to 90 weight%.

[0121]

[0122] electrolytes

[0123] The electrolyte used in the present invention may be any of the various electrolytes usable in lithium secondary batteries, such as organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., and the types thereof are not particularly limited.

[0124] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0125] The above organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having C2 to C20 structures and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.

[0126] The above lithium salt can be used without special limitations as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, or combinations thereof. The concentration of the lithium salt is preferably used within the range of 0.1 to 5.0 M, more preferably 0.1 to 3.0 M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and allow lithium ions to move effectively.

[0127] Meanwhile, in addition to the above components, the electrolyte may additionally include additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery. For example, the electrolyte may include at least one additive selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sulfone compounds, sulfate compounds, phosphate compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0128] Examples of the above-mentioned cyclic carbonate compounds include vinylene carbonate (VC) or vinylethylene carbonate.

[0129] Examples of the above-mentioned halogen-substituted carbonate compounds include fluoroethylene carbonate (FEC).

[0130] Examples of the above sulfone-based compounds include at least one compound selected from the group consisting of 1,3-propane sulfone (PS), 1,4-butane sulfone, ethen sulfone, 1,3-propene sulfone (PRS), 1,4-butene sulfone, and 1-methyl-1,3-propene sulfone.

[0131] Examples of the above sulfate compounds include ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0132] Examples of the above-mentioned phosphate compounds include one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyl trimethylsilyl phosphate, trimethylsilyl phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0133] Examples of the above borate compounds include tetraphenylborate, lithium oxalyl difluoroborate (LiODFB), and lithium bisoxalate toborate (LiB(C2O4)2, LiBOB).

[0134] Examples of the above nitrile compounds include at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0135] Examples of the above benzene-based compounds include fluorobenzene, examples of the above amine-based compounds include triethanolamine or ethylenediamine, and examples of the above silane-based compounds include tetravinylsilane.

[0136] The above lithium salt-based compound is a compound different from the lithium salt included in the above-mentioned non-aqueous electrolyte, and examples include lithium difluorophosphate (LiDFP), LiPO2F2, or LiBF4.

[0137] The above additive may be included in an amount of 0.1 to 10 weight%, preferably 0.1 to 5 weight%, based on the total weight of the electrolyte.

[0138]

[0139] Separator

[0140] The above separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions; any separator typically used in lithium secondary batteries can be used without any special restrictions. Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength.

[0141]

[0142] A lithium secondary battery according to the present invention as described above can be used to manufacture a battery pack. The battery pack comprises an assembly of lithium secondary batteries electrically connected according to the present invention and a pack housing that accommodates the same, wherein the pack housing may include a busbar for electrically connecting the lithium secondary batteries, a cooling unit, an external terminal, etc. The battery pack may be mounted in a vehicle. The vehicle may be, for example, an electric vehicle, a hybrid vehicle, or a plug-in hybrid vehicle. The vehicle includes a four-wheeled vehicle or a two-wheeled vehicle. In particular, the lithium secondary battery according to the present invention has high energy density and excellent rapid charging performance, so it can be usefully used as a battery for an electric vehicle.

[0143]

[0144] The present invention will be explained in more detail below through specific embodiments. However, the following embodiments are intended only to aid in understanding the present invention, and the scope of the present invention is not limited to these embodiments.

[0145]

[0146] Examples and Comparative Examples

[0147] Example 1

[0148] A cathode slurry was prepared by adding cathode active material : conductive material (carbon black) : binder (SBR-CMC) to distilled water in a weight ratio of 95.6 : 1.0 : 3.4.

[0149] The above-mentioned negative electrode active material was a mixture of a carbon-based active material and a silicon-based active material in a weight ratio of 90:10. The above-mentioned carbon-based active material consists of artificial graphite particles in the form of secondary particles formed by the aggregation of multiple primary particles, on which an amorphous carbon coating layer is formed. The average particle size (D) of the above-mentioned primary particles 50 ) was 9㎛, and the degree of sphericity was 0.75. In addition, the carbon coating layer was included at 2.5 wt% relative to the carbon-based active material. In addition, the average particle size (D) of the artificial graphite particles 50The ) was 12㎛, the degree of sphericity was 0.82, and the specific surface area was 0.8m 2 / g. In addition, the silicon-based active material is Si / C, and the average particle size (D 50 ) was 8㎛.

[0150] After applying the above cathode slurry onto a copper current collector, drying and rolling, and vacuum drying were performed to form a cathode active material layer (thickness: 70 μm) to manufacture a cathode. The porosity of the cathode was 28%.

[0151]

[0152] Example 2

[0153] As a carbon-based active material, it is an artificial graphite particle in the form of a secondary particle formed by the aggregation of a plurality of primary particles, and the average particle size (D) of the primary particles 50 ) is 7.9㎛, the degree of sphericity is 0.65, and the average particle size (D) of the artificial graphite particles is 50 ) is 10.5㎛, sphericity is 0.76, and specific surface area is 0.9m 2 A cathode was prepared in the same manner as in Example 1, except that a / g material was used. The porosity of the cathode was 28%.

[0154]

[0155] Comparative Example 1

[0156] As a carbon-based active material, it is an artificial graphite particle in the form of a secondary particle formed by the aggregation of a plurality of primary particles, and the average particle size (D) of the primary particles 50 ) is 5㎛, the degree of sphericity is 0.57, and the average particle size (D) of the artificial graphite particles is 50 The particle size is 7.2㎛, the degree of sphericity is 0.63, and the specific surface area is 0.4m 2 A cathode was prepared in the same manner as in Example 1, except that a / g material was used. The porosity of the cathode was 28%.

[0157]

[0158] Comparative Example 2

[0159] As a carbon-based active material, it is an artificial graphite particle in the form of a secondary particle formed by the aggregation of a plurality of primary particles, and the average particle size (D) of the primary particles 50 ) is 13.3㎛, the degree of sphericity is 0.91, and the average particle size (D) of the artificial graphite particles is 50 The particle size is 16.5㎛, the degree of sphericity is 0.89, and the specific surface area is 1.3m 2 A cathode was prepared in the same manner as in Example 1, except that a / g material was used. The porosity of the cathode was 28%.

[0160]

[0161] Comparative Example 3

[0162] As a silicon-based active material, the average particle size (D 50 A cathode was prepared in the same manner as Comparative Example 1, except that Si / C with a porosity of 6㎛ was used. The porosity of the cathode was 28%.

[0163]

[0164] Comparative Example 4

[0165] As a silicon-based active material, the average particle size (D 50 A cathode was prepared in the same manner as in Example 1, except that Si / C with a porosity of 14 μm was used. The porosity of the cathode was 28%.

[0166]

[0167] Comparative Example 5

[0168] As a silicon-based active material, the average particle size (D 50 A cathode was prepared in the same manner as in Example 1, except that Si / C with a porosity of 3㎛ was used. The porosity of the cathode was 28%.

[0169]

[0170] 1) Sphericity

[0171] Measurements were taken by analyzing images using an optical microscope (Keyence VHX-7000), reflection mode, and ring illumination. Specifically, 4 mg of particles containing the cathode active material used in Examples 1–2 and Comparative Examples 1–5 were dispersed in 2 ml of NMP, and sonication treatment was performed for 1 minute. Subsequently, the solution was dropped onto a slide glass and covered with a cover glass to prepare a sample. Afterward, the sample was placed on the stage of the optical microscope, focused, and the magnification fixed at 300x, allowing for the acquisition of 100 to 200 particle images in the corresponding area. The degree of sphericity of each particle was calculated according to Equation 1 below using the automatic area measurement function of the optical microscope software, and the average of these values ​​is shown in Table 1 above.

[0172] [Equation 1]

[0173] Circularity = 4πA / P 2

[0174] (P: Perimeter of the particle boundary, A: Area of ​​the particle)

[0175]

[0176] 2) Tap density

[0177] 40g of the cathode active material powder included in Examples 1 and 2 and Comparative Examples 1 to 5 was placed in a container and tapped 1,000 times to compress the powder, and then the volume and mass were measured to calculate the tip density. The tap densities calculated from the artificial graphite particles of Examples 1 and 2 and Comparative Examples 1 to 5 are shown in Table 1 above.

[0178]

[0179] 3) BET specific surface area of ​​the cathode

[0180] The specific surface area of ​​the cathodes of Examples 1 and 2 and Comparative Examples 1 to 5 was measured by using a BEL Sorption instrument (BEL Japan Co., Ltd.) after punching 300 rolled electrodes using a 6φ diameter electrode puncher and placing them in a BET measuring glass. The results of the measurements for the carbon-based active materials of Examples 1 and 2 and Comparative Examples 1 to 5 and the cathodes of Examples 1 and 2 and Comparative Examples 1 to 5 are shown in Table 1 above.

[0181]

[0182] 4) Tortuosity Assessment

[0183] An electrode assembly was prepared by assembling two cathodes with a separator interposed between them using the cathodes of Examples 1-2 and Comparative Examples 1-5, and the electrode assembly was placed in a battery case. An electrolyte containing 1M lithium salt (LiPF6) (ethylene carbonate (EC) : ethylmethyl carbonate (EMC) = 3 : 7 v / v%) was injected, and the mixture was aged for 12 to 24 hours to prepare a symmetric coin cell. Then, a current of 0.1 to 1,000,000 Hz and 10 Mv was applied to the prepared symmetric coin cell, and the electrode pore resistance was measured using a graph obtained by Electrochemical Impedance Spectroscopy (EIS) to calculate the average movement distance Ls of lithium ions inside the cathode active material layer. The Tortuosity (Ls / L0) was calculated by dividing this by the thickness L0 of the cathode active material layer.

[0184] The tortuosity calculated using this method is shown in Table 1 above.

[0185]

[0186] Experimental Example

[0187] Experimental Example 1: Evaluation of Initial Efficiency and Discharge Capacity

[0188] (Manufacturing of coin half-cells)

[0189] An electrode assembly was manufactured by interposing a separator between the negative electrode and the lithium counter electrode of Examples 1-2 and Comparative Examples 1-5, and a coin-half cell was manufactured by placing the electrode assembly in a battery case and then injecting an electrolyte.

[0190] (Initial efficiency and discharge capacity evaluation)

[0191] The coin-half cells according to Examples 1 and 2 and Comparative Examples 1 to 5 were each charged at room temperature (25℃) in CCCV mode at 0.1C with 0.005V and 0.005C cut-off conditions, and discharged in CC mode at 0.1C to 1.5V to measure the discharge capacity and initial efficiency.

[0192] The measurement results are shown in Table 2 below.

[0193]

[0194] Experimental Example 2: Evaluation of Dose Retention Rate

[0195] A coin half-cell was fabricated in the same manner as in Experimental Example 1 above. In addition, when the above charging and discharging were performed as one cycle, 300 cycles were carried out at 25°C. Afterwards, the discharge capacity after 300 cycles (capacity retention rate) was measured based on the discharge capacity after one cycle as 100% and is shown in Table 2 below.

[0196] Discharge Capacity (mAh / g) Initial Efficiency (%) Capacity Retention Rate (%) After 300 Cycles Example 1 52589.584.3 Example 2 52388.981.1 Comparative Example 1 51987.275.2 Comparative Example 2 52188.179.3 Comparative Example 3 51685.976.7 Comparative Example 4 51887.476.2 Comparative Example 5 52087.677.1

[0197] Referring to Table 2 above, it can be seen that the coin half-cells according to Examples 1 and 2, which use a negative electrode active material that falls within the range of sphericity of the primary particles according to the present invention and includes a carbon-based active material and a silicon-based active material, and in which the ratio of the average particle size of the silicon-based active material to the average particle size of the artificial graphite particles in the form of secondary particles satisfies the range of the present invention, have superior discharge capacity, initial efficiency, and capacity retention rate after 300 cycles compared to the coin half-cells of Comparative Examples 1 to 3, which fall outside the range of sphericity of the primary particles according to the present invention.

[0198] In addition, it can be seen that the coin half-cells according to Examples 1 and 2 satisfy the sphericity range of the primary particles according to the present invention, but are superior to the coin half-cells of Comparative Examples 4 and 5, which deviate from the ratio of the average particle size of the silicon-based active material to the average particle size of the artificial graphite particles in the form of secondary particles according to the present invention, in terms of discharge capacity, initial efficiency, and capacity retention rate after 300 cycles.

[0199]

[0200] Reference Example

[0201] Reference Example 1

[0202] A negative electrode was prepared in the same manner as in Example 1, except that the negative electrode active material was composed of a carbon-based active material alone without mixing a silicon-based active material.

[0203]

[0204] Reference Example 2

[0205] A negative electrode was prepared in the same manner as in Example 2, except that the negative electrode active material was composed of a carbon-based active material alone without mixing a silicon-based active material.

[0206]

[0207] Reference Example 3

[0208] A cathode was prepared in the same manner as Comparative Example 1, except that the cathode active material was composed of a carbon-based active material alone without mixing a silicon-based active material.

[0209]

[0210] Reference Example 4

[0211] A cathode was prepared in the same manner as Comparative Example 2, except that the cathode active material was composed of a carbon-based active material alone without mixing a silicon-based active material.

[0212]

[0213] Reference Experimental Example 1: Evaluation of Initial Efficiency and Discharge Capacity

[0214] (Manufacturing of coin half-cells)

[0215] An electrode assembly was manufactured by interposing a separator between the negative electrode and the lithium counter electrode manufactured according to Reference Examples 1 to 3, and a coin-half cell was manufactured by placing the electrode assembly in a battery case and then injecting an electrolyte.

[0216] (Initial efficiency and discharge capacity evaluation)

[0217] The coin-half cells according to Reference Examples 1 to 3 were each charged at room temperature (25℃) in CCCV mode at 0.1C with cut-off conditions of 0.005V and 0.005C, and discharged in CC mode at 0.1C to 1.5V to measure the discharge capacity and initial efficiency.

[0218] The measurement results are shown in Table 3 below.

[0219]

[0220] Reference Experiment Example 2: Evaluation of Lifespan Retention Rate

[0221] A coin half cell was fabricated in the same manner as in Reference Experiment 1 above. In addition, when the above charging and discharging were performed as one cycle, 300 cycles were carried out at 25°C. Afterwards, the discharge capacity after 300 cycles (capacity retention rate) was measured based on the discharge capacity after one cycle as 100% and is shown in Table 3 below.

[0222] Discharge Capacity (mAh / g) Initial Efficiency (%) Capacity Retention Rate After 300 Cycles (%) Reference Example 1 35293.286.7 Reference Example 2 34993.684.9 Reference Example 3 35794.583.2 Reference Example 4 34591.88.5

[0223] Referring to Table 3 above, it can be seen that at least one of the discharge capacity, initial efficiency, and capacity retention rate after 300 cycles of Reference Example 1 is not superior to Reference Examples 2 to 4, which fall outside the sphericity range of the present invention.

[0224] This means that even if a carbon-based active material containing artificial graphite particles in the form of secondary particles aggregated from a plurality of primary particles included in the sphericity range according to the present invention is used as a negative electrode active material, the effect of improving initial efficiency and cycle life characteristics does not appear when it is not mixed with a silicon-based active material.

[0225] Therefore, it can be clearly understood that a significant effect can be achieved only when the negative active material includes not only carbon-based active materials but also silicon-based active materials, in a specific range configuration of the sphericity of the primary particles according to the present invention.

Claims

1. The entire house; and A negative active material layer disposed on at least one surface of the above-mentioned current collector; comprising The above negative electrode active material layer comprises a carbon-based active material and a silicon-based active material, and The above carbon-based active material comprises artificial graphite particles in the form of secondary particles formed by the aggregation of a plurality of primary particles, and The degree of sphericity of the above primary particles is 0.60 to 0.78, and Average particle size (D) of the artificial graphite particles in the form of secondary particles above 50 The average particle size (D) of the above silicon-based active material for ) 50 A cathode with a ratio of ) of 0.4 to 1.

2. In Claim 1, The average particle size (D) of the above primary particles 50 ) is a cathode with a size of 1㎛ to 30㎛.

3. In Claim 1, Average particle size (D) of the artificial graphite particles in the form of secondary particles above 50 ) is a cathode with a size of 5㎛ to 60㎛.

4. In Claim 1, A cathode having a degree of sphericity of 0.30 to 0.95 of the artificial graphite particles in the form of secondary particles above.

5. In Claim 1, The BET specific surface area of ​​the above carbon-based active material is 0.05 m² 2 / g to 100m 2 / g cathode.

6. In Claim 1, A cathode having a tap density of the carbon-based active material of 0.3 g / cc to 2.0 g / cc.

7. In Claim 1, A cathode having an average particle size of the silicon-based active material of the above-mentioned amount of 0.5㎛ to 60㎛.

8. In Claim 1, The above silicon-based active material is silicon (Si) and silicon oxide (SiO₂). x , 0 <x<2) 및 실리콘-탄소 복합체로 이루어지는 군에서 선택된 적어도 1종을 포함하는 음극.

9. In Claim 1, A cathode in which the weight ratio of the carbon-based active material and the silicon-based active material is 50:50 to 99:

1.

10. In Claim 1, The BET specific surface area of ​​the above cathode is 0.1 m² 2 / g to 30m 2 / g cathode.

11. A cathode according to claim 1; An anode facing the above cathode; A separator interposed between the above cathode and the above anode; and A lithium secondary battery containing a non-aqueous electrolyte.

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