Negative active material, and negative electrode and secondary battery comprising the same

Abstract

The present invention provides a negative electrode active material comprising artificial graphite particles and having a coefficient of thermal expansion of 108 x 10 ‑6 / K to 150 x 10 ‑6 / K, measured by a specific method. The negative electrode active material of the present invention has excellent electrode adhesion, thereby having excellent processability and long cycle life characteristics, and a negative electrode comprising the negative electrode active material has high capacity and excellent initial efficiency.

Description

Technical Field

[0001] Cross-references to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2020-0120879, filed on September 18, 2020, the disclosure of which is incorporated herein by reference. Technical Field

[0004] This invention relates to negative electrode active materials, negative electrodes comprising said negative electrode active materials, and secondary batteries. Background Technology

[0005] As energy prices rise due to the depletion of fossil fuels and concerns about environmental pollution increase, environmentally friendly alternative energy sources are becoming an indispensable part of future life.

[0006] In particular, with the development and increasing demand for mobile device technology, the demand for secondary batteries as an environmentally friendly alternative energy source has increased significantly.

[0007] Furthermore, with growing concerns about environmental issues, there has been significant research into electric vehicles (EVs) and hybrid electric vehicles (HEVs), which could potentially replace fossil fuel-powered vehicles such as gasoline and diesel vehicles, a major contributor to air pollution. Lithium-ion batteries, characterized by high energy density, high discharge voltage, and high output stability, have been primarily studied and are being used as power sources for these EVs and HEVs.

[0008] In secondary batteries, lithium metal is usually used as the negative electrode, but the formation of dendrites has caused short circuits and the associated risk of explosion. Carbon-based active materials that can reversibly insert and deintercalate lithium ions while maintaining structural and electrical properties have begun to be used.

[0009] Various types of carbon-based materials, such as artificial graphite, natural graphite, and hard carbon, have been used as carbon-based active materials. Among them, graphite-based active materials have been the most widely used due to their excellent reversibility, which ensures the lifespan characteristics of lithium-ion batteries. Because graphite-based active materials have a low discharge voltage of -0.2V relative to lithium, batteries using these materials can exhibit a high discharge voltage of 3.6V, thus offering numerous advantages in terms of energy density for lithium-ion batteries.

[0010] Among the graphite-based active materials, artificial graphite has the advantage of better expansion suppression and superior high-temperature properties compared to natural graphite. However, since artificial graphite has lower output characteristics due to having fewer pores compared to natural graphite, it is known to use artificial graphite in the form of secondary particles obtained by the aggregation or bonding of primary particles to improve output characteristics and form pores in the particles.

[0011] However, the artificial graphite aggregated into secondary particles is likely to have an irregular and uneven shape depending on the shape of the primary particles and its aggregate. When used as a negative electrode, this artificial graphite suffers from poor electrode adhesion, resulting in poor processability, and long-term cycling performance degradation due to active material peeling during negative electrode operation.

[0012] Japanese Patent No. 4403327 discloses graphite powder for the negative electrode of lithium-ion secondary batteries, but does not provide an alternative to address the aforementioned problems.

[0013] [Existing technical documents]

[0014] [Patent Literature]

[0015] Japanese Patent No. 4403327 Summary of the Invention

[0016] Technical issues

[0017] One aspect of the present invention provides a negative electrode active material that exhibits excellent adhesion in a negative electrode active material comprising artificial graphite in the form of secondary particles, and has high capacity and initial efficiency.

[0018] Another aspect of the present invention provides a negative electrode comprising the above-described negative electrode active material.

[0019] Another aspect of the present invention provides a secondary battery comprising the aforementioned negative electrode.

[0020] Technical solution

[0021] According to one aspect of the present invention, a negative electrode active material is provided, comprising artificial graphite particles in the form of secondary particles obtained by combining a plurality of artificial graphite primary particles, wherein the coefficient of thermal expansion measured by a method comprising the following steps is within 10⁸ × 10⁻⁶. -6 / K to 150×10 -6 Within the range of / K.

[0022] (a) The negative electrode active material and the asphalt binder are mixed at a weight ratio of 90:10 to prepare a mixture granular material with a density of 1.5 g / cc to 2.0 g / cc; (b) The mixture granular material is subjected to thermomechanical analysis to obtain the coefficient of thermal expansion of the mixture granular material; (c) Asphalt binder granular material with a density of 1.5 g / cc to 2.0 g / cc is prepared from the asphalt binder, and thermomechanical analysis is performed to obtain the coefficient of thermal expansion of the asphalt binder granular material; and (d) The coefficient of thermal expansion of the negative electrode active material is obtained by the following formula 1:

[0023] [Formula 1]

[0024] A = {C - (B × 0.1)} / 0.9

[0025] In Formula 1, A is the coefficient of thermal expansion of the negative electrode active material, B is the coefficient of thermal expansion of the asphalt binder granules, and C is the coefficient of thermal expansion of the mixture granules.

[0026] According to another aspect of the present invention, a negative electrode is provided, comprising a negative electrode current collector; and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer comprises the aforementioned negative electrode active material.

[0027] According to another aspect of the present invention, a secondary battery is provided, comprising the aforementioned negative electrode; a positive electrode opposite to the negative electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

[0028] Beneficial effects

[0029] The negative electrode active material of the present invention is characterized by comprising artificial graphite particles in the form of secondary particles, wherein the coefficient of thermal expansion measured by the above method satisfies a specific range. The negative electrode active material satisfying the above-mentioned coefficient of thermal expansion range exhibits excellent adhesion in the negative electrode and can improve capacity and initial efficiency. Detailed Implementation

[0030] It should be understood that the words or terms used in this specification and claims should not be interpreted as having the meaning defined in a common dictionary, and should be further understood as meaning that is consistent with the meaning in the context of the relevant field and the technical concept of the invention, based on the principle that the inventor can appropriately define the meaning of the words or terms to best interpret the invention.

[0031] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to limit the invention. In this specification, unless otherwise stated, singular terms may include plural forms.

[0032] It should be further understood that, when used in this specification, the terms “comprising,” “including,” or “having” specify the presence of the said feature, quantity, step, element, or combination thereof, but do not exclude the presence or addition of one or more other features, quantities, steps, elements, or combinations thereof.

[0033] The term "average particle size (D)" in this specification is used to describe the particle size distribution. 50 The average particle size (D) can be defined as the particle size at which the cumulative volume in the particle size distribution curve is 50%. 50 For example, this can be determined using laser diffraction. Laser diffraction can typically determine particle sizes ranging from submicron to millimeters and can yield highly repeatable and high-resolution results.

[0034] The present invention will be described in detail below.

[0035] Negative electrode active materials

[0036] This invention relates to a negative electrode active material, and more particularly to a negative electrode active material for lithium secondary batteries.

[0037] Specifically, the negative electrode active material according to the present invention comprises artificial graphite in the form of secondary particles obtained by combining a plurality of primary artificial graphite particles, wherein the coefficient of thermal expansion measured by a method including the following steps is within 108 × 10⁻⁶. -6 / K to 150×10 -6 Within the range of / K.

[0038] (a) The negative electrode active material and the asphalt binder are mixed at a weight ratio of 90:10 to prepare a mixture granular material with a density of 1.5 g / cc to 2.0 g / cc;

[0039] (b) Perform thermomechanical analysis on the mixture granules to obtain the coefficient of thermal expansion of the mixture granules;

[0040] (c) Preparing asphalt binder granules with a density of 1.5 g / cc to 2.0 g / cc from the asphalt binder, and performing thermomechanical analysis to obtain the coefficient of thermal expansion of the asphalt binder granules; and

[0041] (d) The coefficient of thermal expansion of the negative electrode active material is obtained by Equation 1 below.

[0042] [Formula 1]

[0043] A = {C - (B × 0.1)} / 0.9

[0044] (In Equation 1, A is the coefficient of thermal expansion of the negative electrode active material, B is the coefficient of thermal expansion of the asphalt binder granules, and C is the coefficient of thermal expansion of the mixture granules.)

[0045] Previously, it was known that artificial graphite exhibits lower expansion and superior storage properties compared to natural graphite, but poorer output characteristics. To improve the output characteristics of artificial graphite, methods are being investigated to prepare secondary particle form artificial graphite by aggregating or combining multiple primary particles and creating gaps between the primary particles. However, artificial graphite aggregated in the form of secondary particles suffers from significantly reduced electrode adhesion due to its irregular and complex shape. This leads to reduced processability and deterioration of long-term lifespan characteristics due to active material peeling during negative electrode operation.

[0046] To address these issues, the negative electrode active material of the present invention is characterized by adjusting the coefficient of thermal expansion, measured by the above method, to a specific range. A negative electrode active material satisfying the above-mentioned coefficient of thermal expansion range is evaluated as having a random structure of primary particles in the active material. Furthermore, since the secondary particles formed by combining these random primary particles have a smooth surface and reduced shape irregularity, electrode adhesion can be improved. Therefore, this negative electrode active material can exhibit high processability and lifetime characteristics. In addition, a negative electrode active material satisfying the above-mentioned coefficient of thermal expansion range can have improved electrode adhesion while also exhibiting excellent capacity and initial efficiency.

[0047] The negative electrode active material contains artificial graphite particles.

[0048] The artificial graphite particles can be artificial graphite particles in the form of secondary particles obtained by combining multiple primary artificial graphite particles. Specifically, the artificial graphite particles can be a combination of multiple primary artificial graphite particles. When the artificial graphite particles are in the form of secondary particles, since gaps are formed between the primary artificial graphite particles, the output characteristics of the artificial graphite particles can be further improved by ensuring these gaps.

[0049] For the artificial graphite particles in the form of secondary particles, the secondary particles can be an aggregate of multiple artificial graphite primary particles. Specifically, in the artificial graphite particles in the form of secondary particles, the artificial graphite primary particles are not bonded to each other by van der Waals forces, but rather the multiple artificial graphite primary particles can be bonded or aggregated by resin adhesives such as bitumen to form secondary particles.

[0050] The artificial graphite primary particles can be formed after pulverizing a carbon precursor. The carbon precursor can be at least one selected from the group consisting of coal-based heavy oil, fibrous heavy oil, tar, pitch, and coke. Since the artificial graphite primary particles formed from powdered carbon precursors can have improved cohesion, artificial graphite primary particles with high hardness can be formed.

[0051] The artificial graphite particles in the secondary particle form can be formed by adding a carbon precursor in powder form to a reactor, operating the reactor to bind the powder together by centrifugal force to form secondary particles obtained by binding the primary particles, and graphitizing at a temperature of 2,500°C to 3,500°C, for example, 2,700°C to 3,200°C. In the graphitization process, the graphitization of the primary and secondary particles can be carried out simultaneously. In the powder binding process, a resin binder such as pitch can be added to the reactor together, and heat treatment can be performed at a temperature of approximately 400°C to 800°C.

[0052] The average particle size (D) of the artificial graphite primary particles 50 The particle size can be in the range of 5 μm to 15 μm, preferably 8 μm to 12 μm, and more preferably 8.5 μm to 9.5 μm. When the average particle size (D) of the primary particles of the artificial graphite is... 50 Within the above range, the problems of increased specific surface area and decreased adhesion caused by excessively large particle size can be prevented, as well as the problems of increased orientation and decreased output characteristics caused by excessively large particle size. Therefore, the electrode adhesion, output characteristics and capacity characteristics of the negative electrode active material can be easily achieved.

[0053] The negative electrode active material may contain sulfur (S) distributed in the primary particles of artificial graphite.

[0054] Sulfur is typically treated as an impurity and can be removed during the graphitization and iron removal processes in the preparation of artificial graphite. For example, impurity removal can be achieved by heat treatment at a high temperature of 1000°C to 1500°C before grinding the raw material (such as coke) during the preparation of artificial graphite. However, when the raw material contains an appropriate amount of sulfur, it can cause the crystal structure of the primary particles of artificial graphite prepared by, for example, grinding the raw material to become randomized. Therefore, the randomness of the shape of the primary particles of artificial graphite obtained by grinding can be improved. Thus, if the primary particles of artificial graphite containing an appropriate amount of sulfur combine with each other to form secondary particles, artificial graphite particles in the form of secondary particles with smooth surfaces can be prepared, enabling the development of a negative electrode active material with the aforementioned coefficient of thermal expansion and improving electrode adhesion.

[0055] The sulfur content in the negative electrode active material can be from 15 ppm to 40 ppm, preferably from 18 ppm to 35 ppm, more preferably from 23 ppm to 25 ppm, and it is ideal when the amount of sulfur is within the above range, because it is easy to achieve the coefficient of thermal expansion of the negative electrode active material of the present invention, and while the electrode adhesion and life characteristics are improved, the increase of electrolyte side reactions, the decrease in initial efficiency and the decrease in capacity caused by excessive sulfur are prevented.

[0056] The sulfur content according to the present invention can be achieved by controlling the heat treatment conditions of the graphitization and iron removal processes during the preparation of artificial graphite particles; omitting the calcination process typically performed during the preparation of artificial graphite; controlling the calcination conditions; or appropriately selecting the artificial graphite raw materials. Specifically, the sulfur content according to the present invention can be achieved by omitting the calcination process prior to grinding the artificial graphite raw materials (coke, etc.) during the preparation of artificial graphite particles, or by performing the calcination process at a low temperature of 500°C or below, preferably 300°C or below.

[0057] The amount of sulfur can be determined by inductively coupled plasma (ICP) analysis.

[0058] The negative electrode active material may also include a carbon coating disposed on the artificial graphite particles. The carbon coating can help improve the structural stability of the artificial graphite particles and prevent side reactions between the negative electrode active material and the electrolyte.

[0059] The carbon coating content in the negative electrode active material can be from 0.1% to 5% by weight, for example, from 1% to 4% by weight. The presence of the carbon coating can improve the structural stability of the negative electrode active material. However, due to concerns that excessive carbon coating formation may lead to a decrease in initial efficiency and deterioration in high-temperature storage performance caused by the increase in specific surface area during negative electrode calendering, it is ideal to form the carbon coating within the above-mentioned amount range.

[0060] The carbon coating may comprise amorphous carbon. For example, the carbon coating may be formed by heat-treating the carbon coating precursor after providing it to artificial graphite particles with at least one carbon coating precursor selected from the group consisting of coal-based heavy oil, fibrous heavy oil, tar, pitch, and coke. The heat treatment process for forming the carbon coating may be performed at 1,000°C to 1,500°C to promote uniform formation of the carbon coating.

[0061] The negative electrode active material may have an average particle size (D) of 10 μm to 30 μm, preferably 14 μm to 25 μm, more preferably 15 μm to 20 μm, for example 16 μm to 17 μm. 50In particular, when the negative electrode active material comprises artificial graphite particles in the form of secondary particles and has an average particle size within the above-mentioned range, it can be evaluated that the smooth aggregation of the secondary particles can further improve electrode adhesion and improve processability in negative electrode preparation.

[0062] The negative electrode active material can have a density of 0.3m. 2 / g to 2.5m 2 / g, for example 0.5m 2 / g to 1m 2 The Brunauer-Emmett-Teller (BET) specific surface area is / g, and when the BET specific surface area is within the above range, it is ideal in terms of further improving the initial efficiency by preventing side reactions with the electrolyte.

[0063] The interplanar spacing d002, as measured by X-ray diffraction (XRD) analysis of artificial graphite particles in the form of secondary particles, can be in the range of 0.3357 nm to 0.3361 nm, for example, 0.33575 nm to 0.33585 nm. When the spacing d002 is within the above range, it is ideal because the above range of thermal expansion coefficients can be easily achieved to improve electrode adhesion, and the crystallization of the graphite layer and its laminated structure can be well achieved to ensure the capacity of the negative electrode active material and improve the initial efficiency and energy density.

[0064] The crystallite size Lc of the artificial graphite particles in the secondary particle form can be in the range of 45 nm to 75 nm, for example, 60 nm to 70 nm, along the c-axis. When the crystallite size Lc is within this range, it is ideal because the aforementioned range of thermal expansion coefficients can be easily achieved to improve electrode adhesion, and the crystallization of the graphite layer and its laminated structure can be well achieved to ensure the capacity of the negative electrode active material and improve initial efficiency and energy density.

[0065] The negative electrode active material can have a tap density of 0.88 g / cc to 1.20 g / cc, preferably 0.92 g / cc to 1.15 g / cc, and more preferably 1.04 g / cc to 1.10 g / cc. It is ideal to satisfy the tap density range and the coefficient of thermal expansion range because it can achieve a negative electrode active material with a smooth and uniform surface, thereby achieving high electrode adhesion.

[0066] The coefficient of thermal expansion of the negative electrode active material, measured by a method including the following steps (steps (a) to (d)), is 108 × 10⁻⁶. -6 / K to 150×10 -6 Within the range of / K.

[0067] (a) The negative electrode active material and the asphalt binder are mixed at a weight ratio of 90:10 to prepare a mixture granular material with a density of 1.5 g / cc to 2.0 g / cc;

[0068] (b) Perform thermomechanical analysis on the mixture granules to obtain the coefficient of thermal expansion of the mixture granules;

[0069] (c) Preparing asphalt binder granules with a density of 1.5 g / cc to 2.0 g / cc from the asphalt binder, and performing thermomechanical analysis to obtain the coefficient of thermal expansion of the asphalt binder granules; and

[0070] (d) Obtain the coefficient of thermal expansion of the negative electrode active material using the following formula 1:

[0071] [Formula 1]

[0072] A = {C - (B × 0.1)} / 0.9

[0073] (In Equation 1, A is the coefficient of thermal expansion of the negative electrode active material, B is the coefficient of thermal expansion of the asphalt binder granules, and C is the coefficient of thermal expansion of the mixture granules).

[0074] For step (a) of the method for determining the coefficient of thermal expansion, it is a step of preparing a mixture granule by mixing the negative electrode active material and a viscous asphalt binder. Because the negative electrode active material of the present invention does not contain viscous and volatile substances in the artificial graphite particles contained in the negative electrode active material, it is difficult to prepare it into granule form. As will be described later, after determining the coefficient of thermal expansion of the mixture granule, the coefficient of thermal expansion of the negative electrode active material can be evaluated according to Equation 1.

[0075] To ensure the objectivity of the evaluation, the mixture granules can be made to have a density of 1.5 g / cc to 2.0 g / cc, for example 1.8 g / cc.

[0076] The mixture particles may have a diameter of 10 mm to 15 mm, for example, 13 mm.

[0077] Step (b) in the method for determining the coefficient of thermal expansion is a step of determining the coefficient of thermal expansion of the mixture particles by thermomechanical analysis. The thermomechanical analysis and the determination of the coefficient of thermal expansion can be performed using a thermomechanical analyzer (TMA), specifically, a thermomechanical analyzer (TMA) with a heating rate set to 10°C / min and measuring the coefficient of thermal expansion within a temperature range of 30°C to 100°C.

[0078] Step (c) in the method for determining the coefficient of thermal expansion is a step of determining the coefficient of thermal expansion of the asphalt binder itself by preparing asphalt binder granules. The asphalt binder granules can be prepared to have the same shape, density, and diameter as the mixture granules.

[0079] The asphalt binder granules can be formulated to have a density of 1.5 g / cc to 2.0 g / cc, for example 1.8 g / cc, to ensure the objectivity of the evaluation. Specifically, they can be formulated to have the same density, shape, and diameter as the mixture granules prepared in step (a).

[0080] The asphalt binder granules can be made into a diameter of 10 mm to 15 mm, for example, 13 mm.

[0081] Step (d) in the method for determining the coefficient of thermal expansion is a step of obtaining the coefficient of thermal expansion of the negative electrode active material itself by taking into account the mixing weight ratio (90:10) of the negative electrode active material to the asphalt binder in step (a). Specifically, in the mixed granules, the negative electrode active material and the asphalt binder have their respective coefficients of thermal expansion and contribute to the coefficient of thermal expansion of the mixed granules according to their weight ratio. When this is taken into account, the relationship between the coefficient of thermal expansion (C) of the mixed granules, the coefficient of thermal expansion (B) of the asphalt binder granules, and the coefficient of thermal expansion (A) of the negative electrode active material can be expressed by the following Equation 2.

[0082] [Equation 2]

[0083] C = {(A×90) + (B×10)} / 100

[0084] If Equation 2 is converted into an equation related to the coefficient of thermal expansion (A) of the negative electrode active material, Equation 1 can be obtained, and the coefficient of thermal expansion of the negative electrode active material itself can be predicted and evaluated.

[0085] When the coefficient of thermal expansion of the negative electrode active material is within the above-mentioned range, not only can high electrode adhesion and the resulting improvement in processability and lifespan characteristics be obtained, but also a negative electrode active material with high capacity and initial efficiency can be realized.

[0086] If the coefficient of thermal expansion of the negative electrode active material is less than 108 × 10⁸ -6In the case of / K, because it is difficult to see that the primary particles of the artificial graphite have a random structure and the surface of the aggregates of primary particles, i.e., the secondary particles, is not smooth, it is difficult to improve electrode adhesion, processability is reduced, and the possibility of delamination of the negative electrode active material during negative electrode operation is high. Therefore, long-term cycle life characteristics may deteriorate. If the coefficient of thermal expansion of the negative electrode active material is greater than 150 × 10⁻⁶, -6 In the case of / K, the low degree of graphitization, excessive capacity reduction, and potential decrease in initial efficiency are due to the excessively random structure of primary particles in artificial graphite, which is undesirable.

[0087] The coefficient of thermal expansion of the negative electrode active material is preferably 110 × 10⁻⁶. -6 / K to 140×10 -6 / K, for example, 112×10 -6 / K to 120×10 -6 Within the range of / K, when the coefficient of thermal expansion of the negative electrode active material is within the above range, it is preferable to simultaneously improve the electrode adhesion, lifetime characteristics, capacity and initial efficiency.

[0088] The coefficient of thermal expansion of the negative electrode active material can be achieved by appropriately adjusting the randomness of the primary artificial graphite particles and the shape of the secondary artificial graphite particles. Specifically, the coefficient of thermal expansion of the negative electrode active material can be achieved by appropriately adjusting the amount of impurities such as sulfur in the primary artificial graphite particles, the size of the primary artificial graphite particles, the size of the secondary artificial graphite particles, or the degree of granulation, but the present invention is not limited thereto.

[0089] negative electrode

[0090] Furthermore, the present invention provides a negative electrode comprising the above-mentioned negative electrode active material, and more particularly, provides a negative electrode for lithium secondary batteries.

[0091] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer includes the aforementioned negative electrode active material.

[0092] As the negative electrode current collector, negative electrode current collectors commonly used in the art can be used without limitation. For example, the negative electrode current collector is not particularly limited, as long as it has high conductivity and will not cause adverse chemical changes in the lithium secondary battery. For example, the negative electrode current collector may contain at least one selected from copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloys, preferably copper.

[0093] The negative electrode current collector may have fine surface irregularities to improve the bonding strength with the negative electrode active material, and the negative electrode current collector may be used in various shapes, such as membrane, sheet, foil, mesh, porous body, foam, non-woven fabric, etc.

[0094] The negative electrode current collector can typically have a thickness of 3μm to 500μm.

[0095] The negative electrode active material layer is stacked on the negative electrode current collector and includes the aforementioned negative electrode active material.

[0096] The content of the negative electrode active material in the negative electrode active material layer can be from 80% to 99% by weight, for example, from 93% to 98% by weight.

[0097] Based on the above-mentioned negative electrode active material, the negative electrode active material layer may also contain a binder, a conductive agent, and / or a thickener.

[0098] The adhesive is a component that facilitates adhesion between the active material and / or the current collector, wherein the content of the adhesive in the negative electrode active material layer can typically be from 1% to 30% by weight, for example, from 1% to 10% by weight.

[0099] The adhesive may contain at least one selected from the group consisting of: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, 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.

[0100] Any thickener used in conventional lithium secondary batteries can be used as the thickener, and an example of such thickener is carboxymethyl cellulose (CMC).

[0101] The conductive agent is a component used to further improve the conductivity of the negative electrode active material, wherein the content of the conductive agent in the negative electrode active material layer can be from 1% to 30% by weight, for example from 1% to 10% by weight.

[0102] Any conductive agent can be used without particular restriction, as long as it is conductive and will not cause adverse chemical changes in the battery. For example, conductive materials can be used, such as: graphite (natural or synthetic); carbon black (acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black); conductive fibers (carbon fiber or metal fiber); fluorocarbons; metal powders (aluminum and nickel powder); conductive whiskers (zinc oxide and potassium titanate whiskers); conductive metal oxides (titanium oxide); or polyphenylene derivatives. Specific examples of commercial conductive agents may include acetylene black-based products (Chevron Chemicals), Danka Black (Danka Singapore Pte Ltd or Gulf Oil), Ketjen black, ethylene carbonate (EC)-based products (Armak), Vulcan XC-72 (Cabot), and Super P (Timcal Graphite & Carbon).

[0103] The negative electrode slurry is prepared by mixing the aforementioned negative electrode active material and at least one selected from binders, conductive agents and thickeners in a solvent, and the negative electrode active material layer can be prepared by coating a negative electrode current collector with the negative electrode slurry and calendering and drying the coated negative electrode current collector.

[0104] The solvent may comprise water or an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that the desired viscosity is obtained when the negative electrode active material and optionally a binder and a conductive agent are included. For example, the solvent content may be such that the concentration of the solid component comprising the negative electrode active material and optionally at least one selected from binders, thickeners, and conductive agents is in the range of 50% to 95% by weight, for example, 70% to 90% by weight.

[0105] During X-ray diffraction analysis of the negative electrode, the area ratio I(004) / I(110) (orientation index) can be in the range of 7 to 14, for example, 7.5 to 9.5. When the area ratio I(004) / I(110) is within the above range, the active material particles can be arranged to minimize the diffusion path of lithium ions, thus reducing the lithium ion diffusion resistance. The orientation index can be achieved by using the above-described negative electrode active material in the negative electrode.

[0106] Secondary batteries

[0107] Furthermore, the present invention provides a secondary battery comprising the above-mentioned negative electrode, and more particularly, provides a lithium secondary battery.

[0108] The secondary battery may include the aforementioned negative electrode, a positive electrode opposite to the negative electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte.

[0109] The positive electrode may include a positive electrode current collector; and a positive electrode active material layer provided on the positive electrode current collector.

[0110] As the positive electrode current collector, a positive electrode current collector commonly used in the art can be used without limitation. For example, the positive electrode current collector is not particularly limited as long as it has high conductivity and does not cause adverse chemical changes in the secondary battery. For example, the positive electrode current collector may include at least one selected from copper, stainless steel, aluminum, nickel, titanium, fired carbon, and an aluminum-cadmium alloy, preferably aluminum.

[0111] The positive electrode current collector may have fine surface irregularities to improve the bonding strength with the positive electrode active material, and the positive electrode current collector can be used in various shapes, such as a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0112] The positive electrode current collector generally may have a thickness of 3 μm to 500 μm.

[0113] The positive electrode active material layer may include a positive electrode active material.

[0114] The positive electrode active material is a compound capable of reversibly inserting and extracting lithium, and the positive electrode active material may specifically include a lithium composite metal oxide, which contains lithium and at least one metal such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium composite metal oxide may include lithium manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (such as LiCoO2, etc.), lithium nickel-based oxides (such as LiNiO2, etc.), lithium nickel manganese oxides (such as, LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni z O4 (where 0 < Z < 2), etc.), lithium nickel cobalt-based oxides (such as, LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium manganese cobalt-based oxides (such as, LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co z1 O4 (where 0 < Z1 < 2), etc.), lithium nickel manganese cobalt oxides (such as Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1 and p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2)O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2 and p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O2 (where M is selected from the group consisting of aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg) and molybdenum (Mo), and p2, q2, r3 and s2 are atomic fractions of the respective independent elements, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < S2 < 1 and p2 + q2 + r3 + S2 = 1), etc.), and may include any one or a mixture of two or more thereof. Among these materials, in terms of improving the capacity characteristics and stability of the battery, the lithium composite metal oxide may include LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2), or lithium nickel cobalt aluminum oxide (e.g., LiNi 0.8 Co 0.15 Al 0.05 O2, etc.). Considering the significant improvement due to the control of the type and content ratio of the elements constituting the lithium composite metal oxide, the lithium composite metal oxide may include Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, and any one or a mixture of two or more thereof may be used.

[0115] The content of the positive electrode active material in the positive electrode active material layer may be 80% to 99% by weight.

[0116] The positive electrode active material layer may further include at least one selected from a binder and a conductive agent on the basis of containing the positive electrode active material.

[0117] The adhesive is a component that facilitates adhesion between the active material and the conductive agent, as well as adhesion to the current collector, wherein the amount of adhesive added is typically from 1% to 30% by weight based on the total weight of the cathode material mixture. Examples of the adhesive may be at least one selected from the group consisting of: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber.

[0118] The content of the adhesive in the positive electrode active material layer can be from 1% to 30% by weight.

[0119] Any conductive agent can be used without particular restriction, as long as it is conductive and will not cause adverse chemical changes in the battery. For example, conductive materials such as graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked black; conductive fibers such as carbon fiber or metal fiber; fluorocarbons; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or polyphenylene derivatives. Specific examples of commercial conductive agents may include acetylene black products (Chevron Chemicals), Danka Black (Danka Singapore Pte Ltd or Gulf Oil), Ketjen black, ethylene carbonate (EC) products (Armak), Vulcan XC-72 (Cabot), and Super P (Timcal Graphite & Carbon).

[0120] The amount of the conductive agent added to the positive electrode active material layer can be from 1% to 30% by weight.

[0121] The separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is typically used in secondary batteries. Specifically, separators with high electrolyte retention capacity and low resistance to electrolyte ion transfer can be used. In particular, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. In addition, to ensure heat resistance or mechanical strength, coated separators containing ceramic or polymer components can be used, and separators with single-layer or multi-layer structures can be selectively used.

[0122] Furthermore, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used to prepare lithium secondary batteries, but this invention is not limited thereto.

[0123] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0124] Any organic solvent can be used without particular limitation, as long as it serves as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; or carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitrile solvents such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group, and may contain double-bonded aromatic rings or ether bonds); amide solvents such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane can be used as the organic solvent. Among these solvents, carbonate-based solvents are preferred. For example, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant and a linear carbonate compound with low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred, as this mixture can improve the charge / discharge performance of the battery. In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, excellent electrolyte performance can be obtained.

[0125] The lithium salt can be used without particular restriction, as long as it is a compound capable of providing lithium ions for use in lithium secondary batteries. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used as the lithium salt. The lithium salt can be used in a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, excellent electrolyte performance can be obtained and lithium ions can move efficiently because the electrolyte can have appropriate conductivity and viscosity.

[0126] As described above, since the lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and lifespan characteristics, it is suitable for portable devices such as mobile phones, laptops, and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs). In particular, it is preferably used as a constituent battery in medium-to-large-sized battery modules. Therefore, the present invention also provides medium-to-large-sized battery modules comprising the above-described secondary battery as a unit cell.

[0127] The medium-to-large battery modules are preferably used in power sources that require high output and large capacity, such as electric vehicles, hybrid electric vehicles, and energy storage systems.

[0128] In the following, embodiments of the invention will be described in detail in a manner that can be readily practiced by those skilled in the art. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0129] Example

[0130] 1. Preparation of negative electrode active materials

[0131] Example 1: Preparation of negative electrode active material

[0132] <Preparation of Negative Electrode Active Materials>

[0133] The average particle size (D) of petroleum-based coke with a sulfur content of 2,916 ppm was obtained by grinding it using an impact mill. 50 The powder is 10 μm in size. No separate calcination process was performed during the grinding of the coke. Secondary particles, which are the combination of multiple primary particles, were prepared by heat-treating the powder at 550°C in an inert gas (N2) atmosphere using a vertical granulator.

[0134] Next, the secondary particles are graphitized by heat treatment at 3,000°C for more than 20 hours in an inert gas atmosphere to prepare artificial graphite particles in the form of secondary particles.

[0135] The artificial graphite particles in the form of secondary particles are mixed with petroleum-based bitumen and heat-treated in a roller hearth kiln at 1,300°C to form an amorphous carbon coating on the secondary particles.

[0136] The sulfur content of the negative electrode active material prepared in this way is 29.4 ppm, and the average particle size of the primary particles of the artificial graphite is (D 50 The average particle size (D) of the negative electrode active material is 11 μm. 50The crystallite size is 19.4 μm, the d002 measured by XRD is 0.3360 nm, the crystallite size Lc along the c-axis is 63.9 nm, and the BET specific surface area is 0.7 m². 2 / g, tap density is 0.95g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 3% by weight.

[0137] In this case, the tap density is obtained by measuring the apparent density, which is obtained by measuring the final volume obtained by placing 40g of negative electrode active material into a container and vibrating it up and down 1,000 times.

[0138] The BET specific surface area of ​​the negative electrode active material was determined by the BET (Brunor-Emmett-Teller) measurement method using the BELSORP (BET instrument) from BEL Japan, wherein the negative electrode active material was pretreated at 130°C and nitrogen was used.

[0139] <Determination of the coefficient of thermal expansion>

[0140] The negative electrode active material prepared above and the asphalt binder were mixed at a weight ratio of 90:10, and a mixture granule with a density of 1.8 g / cc and a diameter of 13 mm was prepared.

[0141] 1 g of the mixture granules was placed in a TMA instrument (manufacturer: Mettler Toledo, instrument name: SDTA840), the heating rate was set to 10 °C / min, and the coefficient of thermal expansion of the mixture granules was measured in the temperature range of 30 °C to 100 °C (128 × 10⁻⁶). -6 / K).

[0142] After preparing the same asphalt binder as used above, asphalt binder granules with a density of 1.8 g / cc, a diameter of 13 mm, and the same shape as the mixture granules were prepared. 1 g of the asphalt binder granules was placed in a TMA instrument, and the coefficient of thermal expansion (56 × 10⁻⁶) of the asphalt binder granules was measured under the same conditions as the coefficient of thermal expansion measurement of the mixture granules. -6 / K).

[0143] Next, the coefficient of thermal expansion of the negative electrode active material (136 × 10⁻⁶) is calculated using Equation 1. -6 / K).

[0144] [Formula 1]

[0145] A = {C - (B × 0.1)} / 0.9

[0146] (A: Coefficient of thermal expansion of the negative electrode active material; B: Coefficient of thermal expansion of the asphalt binder granules; C: Coefficient of thermal expansion of the mixed granules)

[0147] Example 2: Preparation of negative electrode active material

[0148] <Preparation of Negative Electrode Active Materials>

[0149] The negative electrode active material was prepared in the same manner as in Example 1, except that the average particle size (D) was obtained by grinding petroleum-based coke with a sulfur content of 2,531 ppm using an impact mill. 50 The powder is 10 μm in size and secondary particles are prepared from the powder using a horizontal granulator.

[0150] The sulfur content of the negative electrode active material prepared above is 22.4 ppm, and the average particle size of the primary particles of the artificial graphite is (D 50 The average particle size (D) of the negative electrode active material is 10 μm. 50 The crystallite size is 18.1 μm, the d002 measured by XRD is 0.3359 nm, the crystallite size Lc along the c-axis is 66.7 nm, and the BET specific surface area is 0.7 m². 2 / g, tap density is 1.03g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 3% by weight.

[0151] <Determination of the coefficient of thermal expansion>

[0152] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0153] The coefficient of thermal expansion of the granular mixture is 119 × 10⁻⁶. -6 / K

[0154] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0155] The coefficient of thermal expansion of the negative electrode active material is 126 × 10⁻⁶. -6 / K

[0156] Example 3: Preparation of negative electrode active material

[0157] <Preparation of Negative Electrode Active Materials>

[0158] The negative electrode active material was prepared in the same manner as in Example 1, except that the average particle size (D) was obtained by grinding petroleum-based coke with a sulfur content of 2,008 ppm using an impact mill. 50The powder is 10 μm in size. Secondary particles are prepared from the powder using a horizontal granulator. The mixing weight ratio of artificial graphite particles in the form of secondary particles to petroleum-based asphalt is adjusted so that the amount of amorphous carbon coating in the negative electrode active material is 2 by weight.

[0159] The sulfur content of the negative electrode active material prepared above is 23.6 ppm, and the average particle size of the primary particles of the artificial graphite is (D 50 The average particle size (D) of the negative electrode active material is 9 μm. 50 The crystallite size is 16.5 μm, the d002 measured by XRD is 0.3358 nm, the crystallite size Lc along the c-axis is 73.1 nm, and the BET specific surface area is 0.6 m². 2 / g, tap density is 1.05g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 2% by weight.

[0160] <Determination of the coefficient of thermal expansion>

[0161] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0162] The coefficient of thermal expansion of the granular mixture is 110×10. -6 / K

[0163] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0164] The coefficient of thermal expansion of the negative electrode active material is 116 × 10⁻⁶. -6 / K

[0165] Example 4: Preparation of negative electrode active material

[0166] <Preparation of Negative Electrode Active Materials>

[0167] The negative electrode active material was prepared in the same manner as in Example 1, except that when preparing secondary particles from multiple primary particles by combining them using powder, a horizontal granulator was used instead of a vertical granulator.

[0168] The sulfur content of the negative electrode active material prepared above is 26.7 ppm, and the average particle size of the primary particles of the artificial graphite is (D 50 The average particle size (D) of the negative electrode active material is 11 μm. 50 The crystallite size is 22.4 μm, the d002 measured by XRD is 0.3360 nm, the crystallite size Lc along the c-axis is 64.1 nm, and the BET specific surface area is 0.7 m². 2 / g, tap density is 0.89g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 3% by weight.

[0169] <Determination of the coefficient of thermal expansion>

[0170] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0171] The coefficient of thermal expansion of the granular mixture is 123 × 10⁻⁶. -6 / K

[0172] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0173] The coefficient of thermal expansion of the negative electrode active material is approximately 130 × 10⁻⁶. -6 / K

[0174] Comparative Example 1: Preparation of Negative Electrode Active Materials

[0175] <Preparation of Negative Electrode Active Materials>

[0176] The average particle size (D) was obtained by heat-treating petroleum-based needle coke with a sulfur content of 1,671 ppm to 1,000 °C at a heating rate of 25 °C / min and then grinding the petroleum-based needle coke using an impact mill. 50 The powder has a particle size of 10 μm. The powder and petroleum-based asphalt were mixed at a weight ratio of 87:13, and secondary particles (with an average particle size of D) were prepared by heat treatment at 550°C for 10 hours in an inert gas (N2) atmosphere using a vertical granulator. 50 (22.8μm).

[0177] Next, the secondary particles were graphitized by heat treatment at 3,000°C for more than 20 hours in an inert gas atmosphere to prepare artificial graphite particles in the form of secondary particles.

[0178] The artificial graphite particles in the form of secondary particles are mixed with petroleum-based bitumen and heat-treated in a roller hearth kiln at 1,300°C to form an amorphous carbon coating on the secondary particles.

[0179] The negative electrode active material prepared in this way has a sulfur content of less than 10 ppm and an average particle size (D) of artificial graphite primary particles. 50 The average particle size (D) of the negative electrode active material is 10 μm. 50 The crystallite size is 21.8 μm, the d002 measured by XRD is 0.3361 nm, the crystallite size Lc along the c-axis is 67.7 nm, and the BET specific surface area is 0.7 m². 2 / g, tap density is 0.86g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 3% by weight.

[0180] <Determination of the coefficient of thermal expansion>

[0181] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0182] The coefficient of thermal expansion of the mixed granules: 97×10 -6 / K

[0183] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0184] The coefficient of thermal expansion of the negative electrode active material is approximately 10² × 10⁻⁶. -6 / K

[0185] Comparative Example 2: Preparation of Negative Electrode Active Materials

[0186] <Preparation of Negative Electrode Active Materials>

[0187] The negative electrode active material was prepared in the same manner as in Comparative Example 1, except that the average particle size (D) was obtained by grinding petroleum-based needle coke with a sulfur content of 1,513 ppm using an impact mill. 50 The powder, which has a particle size of 9 μm, is mixed with petroleum-based asphalt at a weight ratio of 90:10 during the grinding of coke without a separate calcination process, and the secondary particles are granulated using a vertical granulator.

[0188] The sulfur content of the negative electrode active material prepared above is 12.2 ppm, and the average particle size of the primary particles of the artificial graphite is (D 50 The average particle size (D) of the negative electrode active material is 9 μm. 50 The crystallite size is 13.4 μm, the d002 measured by XRD is 0.3359 nm, the crystallite size Lc along the c-axis is 69.4 nm, and the BET specific surface area is 0.9 m². 2 / g, tap density is 1.00g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 3% by weight.

[0189] <Determination of the coefficient of thermal expansion>

[0190] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0191] The coefficient of thermal expansion of the granular mixture is 98 × 10⁻⁶. -6 / K

[0192] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0193] The coefficient of thermal expansion of the negative electrode active material is approximately 10³ × 10⁻⁶. -6 / K

[0194] Comparative Example 3: Preparation of Negative Electrode Active Materials

[0195] <Preparation of Negative Electrode Active Materials>

[0196] The negative electrode active material was prepared in the same manner as in Comparative Example 2, except that petroleum-based needle coke with a sulfur content of 1,236 ppm was used.

[0197] The sulfur content of the negative electrode active material prepared above is 10.8 ppm, and the average particle size of the primary particles of the artificial graphite is (D 50 The average particle size (D) of the negative electrode active material is 10 μm. 50 The crystallite size is 15.6 μm, the d002 measured by XRD is 0.3357 nm, the crystallite size Lc along the c-axis is 75.7 nm, and the BET specific surface area is 0.8 m². 2 / g, tap density is 1.05g / cc, and the amount of amorphous carbon coating in the negative electrode active material is 3% by weight.

[0198] <Determination of the coefficient of thermal expansion>

[0199] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0200] The coefficient of thermal expansion of the granular mixture is 94 × 10⁻⁶. -6 / K

[0201] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0202] The coefficient of thermal expansion of the negative electrode active material is approximately 98 × 10⁻⁶. -6 / K

[0203] Comparative Example 4: Preparation of Negative Electrode Active Materials

[0204] <Preparation of Negative Electrode Active Materials>

[0205] The negative electrode active material was prepared in the same manner as in Example 1, except that the average particle size (D) was obtained by grinding petroleum-based coke with a sulfur content of 4,000 ppm or higher using an impact mill. 50 The powder is 10 μm in size and no amorphous carbon coating is formed on the artificial graphite in the form of secondary particles.

[0206] The sulfur content of the negative electrode active material prepared above is 64.0 ppm, and the average particle size of the primary particles of the artificial graphite is (D50 The average particle size (D) of the negative electrode active material is 10 μm. 50 The crystallite size is 15.2 μm, the d002 measured by XRD is 0.3371 nm, the crystallite size Lc along the c-axis is 29 nm, and the BET specific surface area is 0.9 m². 2 / g, and the tap density is 0.86g / cc.

[0207] <Determination of the coefficient of thermal expansion>

[0208] The coefficient of thermal expansion of the negative electrode active material was determined in the same manner as in Example 1, except that the negative electrode active material prepared as described above was used.

[0209] The coefficient of thermal expansion of the granular mixture is 158 × 10⁻⁶. -6 / K

[0210] The coefficient of thermal expansion of asphalt adhesives: 56 × 10⁻⁶ -6 / K

[0211] The coefficient of thermal expansion of the negative electrode active material is approximately 169 × 10⁻⁶. -6 / K

[0212] 2. Preparation of the negative electrode

[0213] The negative electrode active material prepared in Example 1, carbon black as a conductive agent, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 95.3:1.0:1.2:2.5, and water was added to prepare a negative electrode slurry.

[0214] The negative electrode slurry was coated onto a copper negative electrode current collector (thickness: 15 μm), vacuum dried at approximately 130°C for 8 hours, and then calendered to form a negative electrode active material layer (thickness: 84 μm) to prepare the negative electrode of Example 1. In this case, the negative electrode was prepared such that the negative electrode loading was 3.6 mAh / cm³. 2 .

[0215] The negative electrodes of Examples 2 to 4 and Comparative Examples 1 to 4 were prepared in the same manner as in Example 1, except that the negative electrode active materials prepared in Examples 2 to 4 and Comparative Examples 1 to 4 were used respectively.

[0216] The orientation index of each negative electrode in the examples and comparative examples was obtained as the area ratio I(004) / I(110), which was obtained by measuring the (004) and (110) surfaces using XRD and integrating the measured XRD peaks.

[0217] [Table 1]

[0218]

[0219] Experimental Example

[0220] Experimental Example 1: Adhesion Evaluation

[0221] The negative electrode of Example 1 was cut into a size of 20mm × 150mm and fixed to the center of a 25mm × 75mm glass slide using double-sided tape. The 90-degree peel strength was then measured using a universal testing machine (UTM) (manufacturer: LLOYD Instruments, machine name: LFPlus) while simultaneously peeling the negative electrode active material layer from the negative electrode current collector. Five identical negative electrodes of Example 1 were prepared, and the 90-degree peel strength was measured five times in the same manner. The average value was taken as the adhesion force (unit: gf / 10mm) of the negative electrode of Example 1.

[0222] The 90-degree peel strength of Examples 2 to 4 and Comparative Examples 1 to 4 was measured in the same manner as in Example 1.

[0223] Experimental Example 2: Evaluation of Discharge Capacity and Initial Efficiency

[0224] <Preparation of Secondary Batteries>

[0225] Prepare a lithium metal counter electrode as the positive electrode.

[0226] After placing a polyethylene separator between the negative and positive electrodes prepared in Examples 1 to 4 and Comparative Examples 1 to 4, an electrolyte was injected to prepare the secondary batteries of the Examples and Comparative Examples. The electrolyte was a solution in which vinylene carbonate (VC) was added at 0.5% by weight to a non-aqueous electrolyte solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 2:8, and 1M LiPF6 was dissolved.

[0227] <Evaluation of Initial Discharge Capacity and Initial Efficiency>

[0228] The charging and discharging capacities of the secondary batteries prepared as described above for the examples and comparative examples were measured, and the initial efficiency was calculated using the following formula. The results are shown in Table 2. The charging and discharging conditions are as follows.

[0229] Charging conditions: CCCV (constant current constant voltage) mode, 0.1C charging, 5mV and 1 / 200C cutoff.

[0230] Discharge conditions: CC mode, 0.1C discharge, 1.5V cutoff.

[0231] Initial efficiency = (Discharge capacity in the first cycle / Charge capacity) × 100

[0232] [Table 2]

[0233]

[0234] Referring to Table 2, for the negative electrode and secondary battery containing the negative electrode active materials of Examples 1 to 4, it can be confirmed that while the electrode adhesion is excellent, the initial efficiency and initial discharge capacity are both improved.

[0235] For the negative electrode active materials of Comparative Examples 1 to 3, it can be confirmed that the coefficient of thermal expansion of the negative electrode active materials is low, therefore the surface of the artificial graphite particles in the form of secondary particles is not smooth and the adhesion is very low.

[0236] Furthermore, for Comparative Example 4, due to the excessively large coefficient of thermal expansion of the negative electrode active material, it can be confirmed that the discharge capacity is very low and the initial efficiency is reduced.

[0237] Experimental Example 3: Lifetime Characteristics Evaluation

[0238] <Preparation of Secondary Batteries>

[0239] By using Li[Ni] as the positive electrode active material 0.6 Mn 0.2 Co 0.2 O2, carbon black as a conductive agent, and PVdF as a binder are mixed in a weight ratio of 94:4:2, and N-methylpyrrolidone as a solvent is added to prepare a positive electrode slurry. The positive electrode slurry is then coated onto aluminum foil, vacuum dried at approximately 130°C for 8 hours, and calendered to prepare the positive electrode. In this case, the positive electrode is prepared such that the positive electrode loading is 3.34 mAh / cm³. 2 .

[0240] After a polyethylene separator was placed between the negative electrode prepared in Examples 1 to 4 and Comparative Examples 1 and 2 and the aforementioned positive electrode, an electrolyte was injected to prepare the secondary batteries of the Examples and Comparative Examples. The electrolyte was prepared using a solution in which vinylene carbonate (VC) was added at 0.5% by weight to a non-aqueous electrolyte solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 2:8 and 1M LiPF6 was dissolved.

[0241] <Cyclic Capacity Retention Rate Evaluation>

[0242] The capacity retention rate at 45°C after 300 cycles was evaluated for the secondary batteries of Examples 1 to 4 and Comparative Examples 1 and 2.

[0243] Specifically, the secondary batteries of Examples 1 to 4 and Comparative Examples 1 and 2 were charged and discharged under charging (CC / CV mode, 1.0C charging, 4.2V, 0.05C cutoff) and discharging (CC mode, 1.0C discharging, 3.0V cutoff) conditions up to 300 cycles.

[0244] The capacity retention rate after 300 cycles was evaluated using the following formula, and the results are shown in Table 3.

[0245] 300-cycle capacity retention (%) = {(Discharge capacity in the 300th cycle) / (Discharge capacity in the 1st cycle)} × 100

[0246] [Table 3]

[0247] Capacity retention rate after 300 cycles (%) Example 1 90.0 Example 2 89.8 Example 3 90.1 Example 4 89.5 Comparative Example 1 85.7 Comparative Example 2 79.9

[0248] Referring to Table 3, it can be confirmed that the secondary batteries of Examples 1 to 4, in which the coefficient of thermal expansion of the negative electrode active material meets the scope of the present invention, exhibit significantly improved capacity retention compared to the secondary batteries of Comparative Examples 1 and 2, in which the coefficient of thermal expansion of the negative electrode active material does not meet the scope of the present invention.

Claims

1. A negative electrode active material, said negative electrode active material comprising: Artificial graphite particles are formed by the combination of multiple primary artificial graphite particles, resulting in secondary particle forms. The coefficient of thermal expansion measured by the method including the following steps is 10⁸ × 10⁻⁶. -6 / K to 150 × 10 -6 Within the range of / K: (a) The negative electrode active material and the asphalt binder are mixed at a weight ratio of 90:10 to prepare a mixture granular material with a density of 1.5 g / cc to 2.0 g / cc; (b) Perform thermomechanical analysis on the mixture granules to obtain the coefficient of thermal expansion of the mixture granules; (c) Preparing asphalt binder granules with a density of 1.5 g / cc to 2.0 g / cc from the asphalt binder, and performing thermomechanical analysis to obtain the coefficient of thermal expansion of the asphalt binder granules; and (d) Obtain the coefficient of thermal expansion of the negative electrode active material using Equation 1: [Formula 1] A = {C - (B × 0.1)} / 0.9 in, In Formula 1, A is the coefficient of thermal expansion of the negative electrode active material, B is the coefficient of thermal expansion of the asphalt binder granules, and C is the coefficient of thermal expansion of the mixture granules.

2. The negative electrode active material according to claim 1, wherein the artificial graphite primary particles have an average particle size D of 5 μm to 15 μm. 50 .

3. The negative electrode active material according to claim 1, wherein the spacing d002 of the (002) planes of the artificial graphite particles in the form of secondary particles, as obtained by X-ray diffraction, is in the range of 0.3357 nm to 0.3361 nm.

4. The negative electrode active material as described in claim 1, wherein the Brunol-Emmett-Teller (BET) specific surface area of ​​the negative electrode active material is 0.3 m². 2 / g to 2.5 m 2 Within the range of / g.

5. The negative electrode active material according to claim 1, wherein the crystallite size Lc of the artificial graphite particles in the c-axis direction is in the range of 45 nm to 75 nm.

6. The negative electrode active material according to claim 1, wherein the negative electrode active material further comprises sulfur distributed in the primary particles of the artificial graphite in an amount of 15 ppm to 40 ppm.

7. The negative electrode active material according to claim 1, wherein the tap density of the negative electrode active material is in the range of 0.88 g / cc to 1.20 g / cc.

8. The negative electrode active material according to claim 1, wherein the average particle size D of the negative electrode active material is... 50 Within the range of 10 μm to 30 μm.

9. The negative electrode active material according to claim 1, wherein the negative electrode active material further comprises a carbon coating disposed on the artificial graphite particles.

10. The negative electrode active material of claim 9, wherein the carbon coating content in the negative electrode active material is from 0.1% to 5% by weight.

11. The negative electrode active material of claim 9, wherein the carbon coating comprises amorphous carbon.

12. A negative electrode, said negative electrode comprising: Negative current collector; and A layer of negative electrode active material disposed on the negative electrode current collector. The negative electrode active material layer comprises the negative electrode active material according to claim 1.

13. The negative electrode of claim 12, wherein the area ratio I(004) / I(110) of the negative electrode during X-ray diffraction analysis is in the range of 7 to 14.

14. A secondary battery, the secondary battery comprising: The negative electrode as described in claim 12; The positive electrode opposite to the negative electrode; A diaphragm disposed between the negative electrode and the positive electrode; and Electrolytes.

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