Negative electrode and lithium secondary battery including the same

Abstract

Disclosed are a negative electrode and a lithium secondary battery including the same. The negative electrode includes: a current collector; and a negative active material layer disposed on at least one surface of the current collector and including: 1) a negative active material including Mg-containing silicon oxide and a graphene coating layer surrounding a surface of the Mg-containing silicon oxide, 2) a conductive material including single-walled carbon nanotubes (SWCNTs), and 3) a binder, wherein graphene included in the graphene coating layer has a D / G band intensity ratio of 0.8 to 1.5, and the D / G band intensity ratio of the graphene is defined as an average value of a ratio of a maximum peak intensity of a D band at 1360±50 cm-1 to a maximum peak intensity of a G band at 1580±50 cm-1 determined from a Raman spectrum of the graphene. ‑1 ‑1 ​​

Description

Technical Field

[0001] This disclosure relates to a negative electrode with improved lifetime characteristics and a lithium secondary battery including the negative electrode.

[0002] This application claims priority to Korean Patent Application No. 10-2020-0121829, filed in Korea on September 21, 2020, the disclosure of which is incorporated herein by reference. Background Technology

[0003] Recently, with the development and widespread adoption of mobile devices, personal computers, electric motors, and modern capacitor devices, the demand for high-capacity energy has been continuously increasing. A typical example of such energy is the lithium-ion secondary battery. Since silicon's capacity (approximately 4200 mAh / g) is about 10 times or more than that of graphite-based materials conventionally used as anode materials (theoretical capacity: 372 mAh / g), silicon has gained considerable attention as anode material for next-generation types of non-aqueous electrolyte secondary batteries. Therefore, it has been proposed to use silicon, which alloys with lithium and exhibits high theoretical capacity, as a novel anode active material to replace carbonaceous materials.

[0004] However, silicon undergoes volume expansion during charging and volume contraction during discharging. As a result, when a secondary battery is repeatedly charged and discharged, the silicon used as the negative electrode active material becomes micronized and exhibits an increase in separated particles. These separated particles lose their conductive paths in the electrode, leading to capacity degradation of the secondary battery.

[0005] Attempts have been made to perform silicon micronization to improve cycling characteristics. As a result, improved cycling characteristics can be expected as micronization progresses. However, there are limitations in reducing the crystallite size of crystalline silicon. Therefore, it is difficult to adequately address the issue of silicon micronization during charge / discharge.

[0006] The use of silicon dioxide (SiO) has been proposed. x As another method to improve cycle characteristics. When silicon dioxide (SiO2) x When silicon dioxide (SiO2) decomposes into Si and SiO2 through disproportionation at high temperatures of 1,000°C or higher, it... xThis results in a structure in which silicon crystals of several nanometers in size are uniformly dispersed within silicon oxide. It is anticipated that when this silicon oxide is used as the negative electrode active material in secondary batteries, it will provide a low capacity, approximately half that of silicon negative electrode active materials, but exhibit a capacity approximately five times that of carbonaceous negative electrode active materials. Furthermore, silicon oxide exhibits small volume changes structurally during charge / discharge, providing excellent cycle life characteristics. However, during the initial charge, silicon oxide reacts with lithium to produce lithium silicide and lithium oxide (lithium oxide and lithium silicate). Specifically, lithium oxide cannot participate in subsequent electrochemical reactions, and a portion of the lithium transported to the negative electrode during the initial charge cannot return to the positive electrode, resulting in an irreversible reaction. In the case of silicon oxide, it exhibits a higher irreversible capacity compared to other silicon-based negative electrodes and provides a significantly lower initial charge efficiency (ICE, the ratio of initial discharge capacity to charge capacity) of 70%–75%. This low initial efficiency necessitates an excess positive electrode capacity during the manufacture of secondary batteries to offset the capacity per unit weight of the negative electrode.

[0007] Furthermore, when silicon dioxide is used as the negative electrode active material, carbon nanotubes (CNTs) are used as the conductive material to improve conductivity and suppress electrical short circuits. However, after undergoing volume shrinkage / expansion, the carbon nanotubes separate from the surface of the silicon dioxide, resulting in an electrical short circuit.

[0008] Therefore, when using silicon oxide as the negative electrode active material, it is still necessary to develop a silicon oxide-based material that reduces the generation of lithium oxide that causes this irreversibility, and thus meets the lifetime characteristics and initial capacity / efficiency requirements. Summary of the Invention

[0009] Technical issues

[0010] This disclosure aims to provide a negative electrode active material with excellent initial capacity / efficiency and lifetime characteristics, as well as a negative electrode and a lithium secondary battery including the negative electrode active material. These and other objects and advantages of this disclosure will be understood from the following detailed description and will become more fully apparent from exemplary embodiments of this disclosure. Furthermore, it will be readily understood that the objects and advantages of this disclosure can be achieved through the following technical solutions.

[0011] Technical solution

[0012] In one aspect of this disclosure, a negative electrode is provided according to any of the following embodiments.

[0013] According to a first embodiment of this disclosure, a negative electrode is provided, comprising:

[0014] Current collector; and

[0015] A negative electrode active material layer, the negative electrode active material layer being disposed on at least one surface of the current collector, and comprising:

[0016] 1) Anode active material, comprising Mg-containing silicon oxide and a graphene coating surrounding the surface of the Mg-containing silicon oxide; 2) Conductive material, comprising single-walled carbon nanotubes (SWCNTs); and 3) Binder.

[0017] The graphene contained in the graphene coating has a D / G band strength ratio of 0.8 to 1.5, and

[0018] The D / G band intensity ratio of the graphene is defined by the Raman spectrum of the graphene at 1360 ± 50 cm⁻¹. -1 The maximum peak intensity of the D band at 1580 ± 50 cm⁻¹ is related to the peak intensity at 1580 ± 50 cm⁻¹. -1 The average value of the ratio of the maximum peak intensity of the G-band at that location.

[0019] According to a second embodiment of this disclosure, a negative electrode as defined in the first embodiment is provided.

[0020] The graphene contained in the graphene coating has a D / G band strength ratio of 0.8 to 1.4.

[0021] According to a third embodiment of this disclosure, a negative electrode as defined in the first or second embodiment is provided.

[0022] The Mg-containing silicon oxide comprises 4% to 15% by weight of Mg.

[0023] According to a fourth embodiment of this disclosure, a negative electrode as defined in any of the first to third embodiments is provided.

[0024] The graphene coating content is from 0.5% to 10% by weight, based on the total weight of the negative electrode active material.

[0025] According to a fifth embodiment of this disclosure, a negative electrode as defined in any of the first to fourth embodiments is provided.

[0026] The content of the single-walled carbon nanotubes is 0.01% to 0.06% by weight, based on the total weight of the negative electrode active material layer.

[0027] According to a sixth embodiment of this disclosure, a negative electrode as defined in any of the first to fifth embodiments is provided.

[0028] The conductive material further includes carbon black, acetylene black, Ketjen black, carbon nanofibers, channel black, furnace black, lamp black, thermal black, carbon fiber, metal fiber, fluorocarbon, metal powder, conductive whiskers, conductive metal oxide, polyphenylene derivative, or two or more thereof.

[0029] According to the seventh embodiment of this disclosure, a negative electrode as defined in any of the first to sixth embodiments is provided.

[0030] The negative electrode active material layer further includes a carbonaceous active material.

[0031] According to the eighth embodiment of this disclosure, a negative electrode as defined in the seventh embodiment is provided.

[0032] The carbonaceous active material mentioned therein includes artificial graphite, natural graphite, graphitizable carbon fiber, graphitizable mesophase carbon microspheres, petroleum coke, baked resin, carbon fiber, pyrolytic carbon, or two or more of the above.

[0033] According to a ninth embodiment of this disclosure, a lithium secondary battery is provided, comprising a negative electrode as defined in any of the first to eighth embodiments.

[0034] Beneficial effects

[0035] In the negative electrode according to embodiments of this disclosure, instead of a conventional carbon coating, a graphene coating is introduced onto the surface of Mg-containing silicon oxide, and single-walled carbon nanotubes (SWCNTs) are used as the conductive material. Therefore, since the graphene coating exhibits excellent affinity with SWCNTs and demonstrates excellent flexibility, electrical short circuits can be prevented even when the silicon oxide undergoes shrinkage / expansion, thus achieving excellent results in improving lifetime characteristics.

[0036] In addition, since the graphene coating can be retained even under the volume expansion and contraction of Mg-containing silicon oxide, the direct exposure of Mg-containing silicon oxide to the electrolyte can be prevented, and thus the deterioration of Mg-containing silicon oxide under high-temperature storage conditions can be prevented. Attached Figure Description

[0037] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, serve to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure is not to be construed as limited to the drawings. Furthermore, the shape, size, scale, or ratio of some constituent elements in the drawings may be enlarged for the purpose of clearer description.

[0038] Figure 1This is a schematic diagram illustrating the lithiation and delithiation of conventional silicon oxide (a) with a carbon coating and a negative electrode active material (b) according to an embodiment of the present disclosure.

[0039] Figure 2 This is a schematic diagram illustrating the lithiation and delithiation of a conventional negative electrode (a) comprising silicon oxide with a carbon coating and a negative electrode (b) comprising a negative electrode active material according to an embodiment of the present disclosure. Detailed Implementation

[0040] The preferred embodiments of this disclosure will now be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terminology used in the specification and appended claims should not be construed as limited to its general and dictionary meanings, but rather should be interpreted based on the meanings and concepts corresponding to the technical aspects of this disclosure, on the basis of allowing the inventors to appropriately define the terms for best interpretation. Therefore, the description provided herein is merely a preferred example for illustrative purposes and is not intended to limit the scope of this disclosure; thus, it should be understood that other equivalents and modifications can be made to this disclosure without departing from its scope.

[0041] Throughout the specification, the expression 'a component includes an element' does not exclude the presence of any additional elements, but rather implies that the component may further include other elements.

[0042] In one aspect of this disclosure, a negative electrode is provided, comprising:

[0043] Current collector; and

[0044] A negative electrode active material layer, the negative electrode active material layer being disposed on at least one surface of the current collector, and comprising:

[0045] 1) Anode active material, comprising Mg-containing silicon oxide and a graphene coating surrounding the surface of the Mg-containing silicon oxide; 2) Conductive material, comprising single-walled carbon nanotubes (SWCNTs); and 3) Binder.

[0046] The graphene contained in the graphene coating has a D / G band strength ratio of 0.8 to 1.5, and

[0047] The D / G band intensity ratio of the graphene was defined by the Raman spectrum of the graphene at 1360 ± 50 cm⁻¹. -1 The maximum peak intensity of the D band at 1580 ± 50 cm⁻¹ is related to the peak intensity at 1580 ± 50 cm⁻¹. -1 The average value of the ratio of the maximum peak intensity of the G-band at that location.

[0048] The negative electrode active material includes Mg-containing silicon oxide corresponding to the core, and a graphene coating that partially or completely surrounds the outer side of the core and corresponds to the shell.

[0049] According to embodiments of this disclosure, Mg-containing silicon oxide may have a porous structure having one or more pores formed on its inner and outer surfaces. These pores may be open pores and / or closed pores, wherein the open pores may be interconnected, and components such as ions, gases, and liquids may permeate through the composite particles through the interconnected pores.

[0050] The graphene coating corresponding to the shell portion includes graphene, which can be bonded to, adhered to, or coated onto the surface of the Mg-containing silicon oxide core. When lithium ions are inserted into and extracted from the Mg-containing silicon oxide, the graphene coating remains on the surface of the Mg-containing silicon oxide even when the Mg-containing silicon oxide repeatedly undergoes volume expansion and contraction. Therefore, direct exposure of the silicon oxide to the electrolyte is avoided, thus preventing silicon oxide degradation even under high-temperature storage conditions. The reason the graphene coating remains on the surface of the Mg-containing silicon oxide even when it repeatedly undergoes volume expansion and contraction is that the graphene coating is flexible; therefore, even when the graphene coating expands and then contracts, it does not break but instead contracts again.

[0051] Here, graphene has a D / G band intensity ratio of 0.8 to 1.5, where the D / G band intensity ratio of graphene is defined by the Raman spectrum of graphene at 1360 ± 50 cm⁻¹. -1 The maximum peak intensity of the D band at 1580 ± 50 cm⁻¹ is related to the peak intensity at 1580 ± 50 cm⁻¹. -1 The average value of the ratio of the maximum peak intensity of the G-band at that location.

[0052] Specifically, at 1360 ± 50 cm -1 The D band at 1580 ± 50 cm indicates the presence of carbon particles and the characteristics of incomplete and disordered walls. -1 The G-band at the point represents a continuous type of carbon-carbon (CC) bond, which is characteristic of the crystalline layers of graphene.

[0053] The degree of disorder or defect in graphene can be assessed by the intensity ratio of the D-band peak to the G-band peak (D / G intensity ratio). A high intensity ratio indicates that the graphene is highly disordered or defective. A low intensity ratio indicates that the graphene has low defects and high crystallinity. Here, the term 'defect' refers to an incomplete portion of the graphene array, such as a lattice defect, which is caused by the insertion of unwanted atoms as impurities, a lack of required carbon atoms, or the formation of dislocations in the carbon-carbon bonds that form graphene. Therefore, defective portions can be easily cleaved by external stimuli.

[0054] For example, the intensities of the D-band and G-band peaks can be defined as the height of the average value along the X-axis in the Raman spectrum or the area below the peak. For ease of determination, the height of the average value along the X-axis can be used.

[0055] The D / G band strength ratio of graphene is 0.8 to 1.5, and according to embodiments of the present disclosure, the D / G band strength ratio may be 0.8 to 1.4, 1 to 1.4, 0.8 to 1.31, 1 to 1.31, 1 to 1.3, 1.2 to 1.31, or 1.3 to 1.31.

[0056] When the D / G band intensity ratio meets the above-defined range, the graphene is oxidized to a certain extent and has defects, thus exhibiting increased hydrophilicity, which allows it to be better adsorbed onto Mg-containing silicon oxide to advantageously increase the coverage of the Mg-containing silicon oxide graphene.

[0057] Furthermore, when the D / G band strength ratio of graphene is less than 0.8, graphene exhibits a reduced degree of oxidation, resulting in decreased adsorption. When the D / G band strength ratio is greater than 1.5, graphene exhibits high adsorption, but provides reduced conductivity and increased side reaction sites, leading to an undesirable decrease in efficiency.

[0058] According to embodiments of this disclosure, the graphene coating content can be 0.5-10% by weight, 1-10% by weight, 0.7-7% by weight, 1-5% by weight, 2-4% by weight, or 3-4% by weight, based on the total weight of the negative electrode active material. When the graphene coating content meets the above-defined range, it can sufficiently cover the Mg-containing silicon oxide surface without causing a reduction in capacity and efficiency.

[0059] Mg-containing silicon oxide includes magnesium silicate (Mg silicate) comprising Si and Mg, and may further include Si and SiO2. x(0 < x ≤ 2) represents silicon dioxide. Mg silicate includes MgSiO3 and Mg2SiO4. As a result, as determined by X-ray diffraction, the negative electrode active material according to this disclosure shows peaks for both Mg2SiO4 and MgSiO3, but no peak for MgO. When a peak for MgO is observed, gas generation may occur because MgO reacts with water when the slurry is mixed in an aqueous system. Furthermore, since MgO exists in an unbonded state with SiO2, irreversibility occurs, thus failing to adequately improve the initial efficiency. In addition, there is no effect on suppressing expansion during Li insertion / extraction, leading to degradation of battery performance.

[0060] In addition, the peak intensity ratio I(Mg2SiO4) / I(MgSiO3) is the ratio of the intensity I(Mg2SiO4) of the peak belonging to Mg2SiO4 to the intensity I(MgSiO3) of the peak belonging to MgSiO3. This peak intensity ratio I(Mg2SiO4) / I(MgSiO3) is less than 1, wherein the peak belonging to Mg2SiO4 is observed at 2θ = 32.2 ± 0.2° and the peak belonging to MgSiO3 is observed at 2θ = 30.9 ± 0.2°.

[0061] Specifically, the strength ratio I(Mg₂SiO₄) / I(MgSiO₃) can be from 0.1 to 0.9, more specifically from 0.2 to 0.7. The reason for using magnesium silicate obtained through the reaction of SiO with Mg instead of using SiO alone is to improve initial efficiency. SiO exhibits higher capacity compared to graphite but offers a lower initial efficiency. Therefore, it is necessary to increase the initial efficiency of SiO to maximize the capacity of the actual battery. The effectiveness of improving initial efficiency can vary with the amount of SiO. x The amount of Mg bonded varies (0 < x < 2). When the peak intensity ratio I(Mg2SiO4) / I(MgSiO3) meets the above-defined range, a large amount of MgSiO3 can be formed through the reaction of SiO with the same amount of Mg, thus providing a higher initial efficiency improvement effect compared to the formation of Mg2SiO4.

[0062] The peak belonging to Mg₂SiO₄ was observed at 2θ = 32.2 ± 0.2°, and the peak belonging to MgSiO₃ was observed at 2θ = 30.9 ± 0.2°. These peaks were observed by X-ray diffraction (XRD) using a Cu (Kα ray) source (wavelength: 1.54 Å).

[0063] In Mg-containing silicon oxide, Mg, magnesium silicate, and silicon oxide exist in the following state: the elements of each phase diffuse, causing the boundary surface of one phase to bond to the boundary surface of another phase (i.e., these phases are bonded to each other at the atomic level), thus undergoing a small volume change during lithium-ion insertion / extraction, so that the silicon oxide-based composite particles do not crack even after repeated charging / discharging.

[0064] Furthermore, according to embodiments of this disclosure, Mg-containing silicon oxide may include 4 to 15% by weight of Mg, specifically 4 to 10% by weight. When the Mg content meets the above-defined range, efficiency can be improved while minimizing capacity reduction. It also prevents the formation of MgO byproducts and reduces pores in the internal structure, thus contributing to improved lifetime characteristics.

[0065] According to embodiments of this disclosure, the Mg-containing silicon oxide powder may have an average particle size (D) of 0.1 to 20 μm, specifically 0.5 to 10 μm. 50 ), average particle size (D 50 This refers to the particle size at 50% of the volumetric cumulative particle size distribution. Furthermore, the particle size (D) of Mg-containing silica powder at 90% of the volumetric cumulative particle size distribution... 90 The particle size can be 30 μm or smaller, specifically 15 μm or smaller, and more specifically 10 μm or smaller. Additionally, the maximum particle size of the Mg-containing silica powder in the volumetric cumulative particle size distribution can be 35 μm or smaller, specifically 25 μm or smaller. For example, as determined using a currently used laser diffraction particle size distribution analyzer, the 50% particle size, 90% particle size, and maximum particle size in the volumetric cumulative particle size distribution can be obtained from the cumulative frequency.

[0066] Reference Figure 1 Part (a) shows a schematic diagram illustrating the lithiation and delithiation of a conventional negative electrode active material having a carbon coating 20 on a surface containing Mg silicon oxide 10, and part (b) shows a schematic diagram illustrating the lithiation and delithiation of a negative electrode active material having a graphene coating 30 on a surface containing Mg silicon oxide 10 according to an embodiment of the present disclosure.

[0067] Reference Figure 1 In part (a), during charging, lithium insertion into the conventional negative electrode active material (i.e., the negative electrode active material is lithiated) causes the Mg-containing silicon oxide 10 to expand. After delithiation (lithium deintercalation), the Mg-containing silicon oxide 10 contracts, allowing it to return to its original size, while the carbon coating 20 does not recover its original size. As a result, the Mg-containing silicon oxide surface A, which is not coated by the carbon coating but is directly exposed, reacts with the electrolyte, which can cause degradation of the silicon oxide.

[0068] On the contrary, Figure 1In portion (b) of the negative electrode active material provided with a graphene coating 30 on the surface of Mg-containing silicon oxide 10 according to an embodiment of the present disclosure, the Mg-containing silicon oxide 10 expands after lithium insertion (i.e., the negative electrode active material is lithiated). However, even when lithium deintercalation (i.e., the negative electrode active material is delithiated) and the Mg-containing silicon oxide 10 shrinks back to its original size, the graphene coating 30 remains unchanged on the surface of the Mg-containing silicon oxide 10. Therefore, the problem of silicon oxide degradation due to exposure to the electrolyte can be prevented.

[0069] The method for preparing the negative electrode active material according to the embodiments of this disclosure will be described in more detail below.

[0070] A method for preparing a negative electrode active material according to embodiments of the present disclosure includes the following steps:

[0071] Execute SiO x (0 < x < 2) The gas reacts with Mg gas and the reaction mixture is cooled at 400°C to 900°C to deposit Mg-containing silicon oxide;

[0072] The deposited Mg-containing silicon dioxide was pulverized; and

[0073] The pulverized Mg-containing silica was mixed with an aqueous graphene dispersion and spray-dried to form a graphene coating containing graphene on the surface of the Mg-containing silica.

[0074] According to embodiments of this disclosure, SiO2 can be prepared by evaporating Si and SiO2 at 1,000°C to 1,800°C. x (0 < x < 2) gas, and Mg gas can be prepared by evaporating Mg at 800°C to 1,600°C.

[0075] SiO x (0 < x < 2) The reaction of the gas with Mg gas can be carried out at 800°C to 1800°C. Then, quenching can be performed to reach a target cooling temperature of 400°C to 900°C, specifically 500°C to 800°C, within 1 to 6 hours. When in SiO... x (0 < x < 2) After the gas-phase reaction between the gas and Mg gas, if the quenching time meets the above-defined range, this method of quenching to a lower temperature in a shorter time can solve the reaction between Mg and SiO. x The problem of incomplete reaction, Mg and SiO x Incomplete reaction can lead to the failure of silica formation and the presence of unwanted residual phases, such as MgO. Therefore, it can significantly improve initial efficiency and prevent expansion, thus providing a significantly improved battery life.

[0076] After cooling, further heat treatment can be performed, in which the size of the Si crystallites and the proportion of Mg silicate can be controlled according to the heat treatment temperature. For example, when additional heat treatment is performed at a higher temperature, the Mg2SiO4 phase may be increased and the size of the Si crystallites may be increased.

[0077] According to embodiments of the present disclosure, the deposited Mg-containing silicon oxide may include a crystalline silicon phase and a matrix in which the silicon phase is dispersed, wherein the matrix includes Mg silicate and silicon oxide.

[0078] Next, Mg-containing silicon oxide can be pulverized using mechanical grinding or similar processes to obtain a core with a particle size (D) of 0.1 to 20 μm. 50 The process involves using Mg-containing silica powder. This powder is then mixed with a graphene dispersion, and the resulting mixture is spray-dried to form a graphene coating serving as the shell.

[0079] Spray drying can be performed at 150°C to 250°C, 175°C to 225°C, or 200°C.

[0080] The negative electrode according to embodiments of this disclosure can be obtained by applying a mixture of a negative electrode active material, a conductive material including single-walled carbon nanotubes (SWCNTs), and a binder to a negative electrode current collector and then drying it. If desired, the mixture may further include fillers. The negative electrode active material includes the negative electrode active material described above comprising Mg-containing silicon oxide and a graphene coating surrounding the Mg-containing silicon oxide surface.

[0081] According to this disclosure, the current collector is formed to have a thickness of 3 μm to 500 μm. The current collector is not particularly limited, as long as it does not cause chemical changes in the corresponding battery and has high conductivity. Specific examples of current collectors may include stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel with a surface treated with carbon, nickel, titanium, or silver, or similar materials. A suitable current collector can be selected according to the polarity of the positive or negative electrode.

[0082] Adhesives are components that facilitate the adhesion of electrode active materials and conductive materials to current collectors. Typically, adhesives are added in amounts ranging from 1 to 50% by weight based on the total weight of the electrode mixture. Specific examples of adhesives include polyacrylonitrile-co-acrylate, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), polyacrylic acid, polyacrylic acid substituted with alkali cations or ammonium ions, poly(alkyl-co-maleic anhydride) substituted with alkali cations or ammonium ions, poly(alkyl-co-maleic acid) substituted with alkali cations or ammonium ions, polyethylene oxide, fluororubber, or two or more of these. More specifically, examples of polyacrylic acid substituted with basic cations or ammonium ions can be lithium polyacrylate (Li-PAA, lithium-substituted polyacrylic acid), and examples of poly(alkyl-co-maleic anhydride) substituted with basic cations or ammonium ions can be lithium-substituted polyisobutylene-co-maleic anhydride.

[0083] Conductive materials essentially include single-walled carbon nanotubes (SWCNTs). Compared to carbon coatings used for conventional silica active materials, graphene exhibits a higher affinity for SWCNTs. Therefore, when using SWCNTs as conductive materials, the electrical network between the active and conductive materials is advantageously maintained during charging / discharging processes involving lithium-ion intercalation / deintercalation (i.e., lithiation / delithiation). As a result, lifetime and high-temperature storage properties can be significantly improved.

[0084] Single-walled carbon nanotubes are materials consisting of carbon atoms arranged in a hexagonal shape to form a tubular shape. They exhibit properties such as non-conductors, conductors, or semiconductors due to their unique chirality. They provide tensile strength approximately 100 times that of steel by means of carbon atoms linked by strong covalent bonds, achieving excellent flexibility and elasticity, and are chemically stable.

[0085] Single-walled carbon nanotubes can have an average diameter of 3 to 10 nm, specifically 5 to 8 nm. Meeting these ranges allows for optimal levels of viscosity and solids content when preparing conductive material dispersions. Single-walled carbon nanotubes can entangle themselves to form aggregates within the conductive material dispersion. Therefore, the diameter of these optionally entangled single-walled carbon nanotube aggregates extracted from the conductive material dispersion can be determined using scanning electron microscopy (SEM) or transmission electron microscopy (TEM), and the average diameter can be calculated by dividing the aggregate diameter by the number of single-walled carbon nanotubes forming the aggregate.

[0086] Single-walled carbon nanotubes can have a diameter of 200 to 400 m. 2 / g, specifically 250 to 330m 2 The BET specific surface area is measured in g. When the above-defined range is met, a conductive material dispersion with the desired solids content is obtained, and excessive increase in the viscosity of the negative electrode slurry is avoided. The BET specific surface area can be determined by the nitrogen adsorption BET method.

[0087] Single-walled carbon nanotubes can have aspect ratios ranging from 500 to 3,000, specifically 1,000 to 2,000. When these ranges are met, single-walled carbon nanotubes possess a high specific surface area, thus enabling them to be adsorbed onto active material particles in the negative electrode with a strong attractive force. Therefore, the conductive network can be smoothly maintained even when the volume of the active material in the negative electrode expands. The aspect ratio can be determined by calculating the average of the aspect ratios of 15 single-walled carbon nanotubes with a large aspect ratio and 15 single-walled carbon nanotubes with a small aspect ratio.

[0088] Single-walled carbon nanotubes have a larger aspect ratio, a larger length, and a larger volume compared to multi-walled or double-walled carbon nanotubes, making them advantageous for constructing electrical networks with smaller quantities.

[0089] In addition to single-walled carbon nanotubes, conductive materials may further include components that do not cause chemical changes in the corresponding battery. Specific examples of such components include: carbon black, such as carbon black, acetylene black, Ketjen black (trade name), carbon nanofibers, channel black, furnace black, lamp black, or thermal black; conductive fibers, such as carbon fibers or metal fibers; metal powders, such as fluorocarbons, 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.

[0090] Based on the total weight of the negative electrode active material layer, the content of single-walled carbon nanotubes can be 0.01 to 0.06 wt%, 0.01 to 0.05 wt%, 0.01 to 0.04 wt%, or 0.04 to 0.06 wt%. When the content of single-walled carbon nanotubes meets the above-defined range, the electrical network can be sufficiently constructed without causing a degradation in the initial efficiency of Mg-containing silicon oxide.

[0091] According to embodiments of this disclosure, the negative electrode active material layer may further include a carbonaceous active material as the negative electrode active material. This carbonaceous active material may include any one of the following materials: artificial graphite, natural graphite, graphitizable carbon fibers, graphitizable mesophase carbon microspheres, petroleum coke, baked resin, carbon fibers, and pyrolytic carbon, or two or more of these. The carbonaceous material may have an average particle size of 25 μm or less, 5 to 25 μm, or 8 to 20 μm. When the carbonaceous material has an average particle size of 25 μm or less, room temperature and low temperature output characteristics can be improved, and high-rate charging can be promoted.

[0092] Based on the total weight of the negative electrode active material layer, carbonaceous active materials can be used in amounts of 70 to 97% by weight, 75 to 95% by weight, or 80 to 93% by weight.

[0093] Furthermore, according to embodiments of this disclosure, the weight ratio of the negative electrode active material, including Mg-containing silicon oxide and a graphene coating surrounding the surface of the Mg-containing silicon oxide (i.e., Mg-containing silicon oxide with a graphene coating), to the carbonaceous active material can be 1:2 to 1:33, 1:3 to 1:32, 1:4 to 1:30, or 1:5.7 to 1:20.

[0094] When carbonaceous active materials within the above-defined range are used in the negative electrode active material layer, the carbonaceous active materials can be used as a matrix for the negative electrode active material and contribute to the realization of capacity.

[0095] Reference Figure 2 Part (a) shows a schematic diagram illustrating a composite material including graphite 110 and single-walled carbon nanotubes 120 as conductive materials. Figure 1 (a) shows the lithiation and delithiation of the negative electrode active material having a carbon coating on a Mg-containing silicon oxide surface, and (b) shows a schematic diagram illustrating a combination of graphite 110 and single-walled carbon nanotubes 120 as conductive materials, such as... Figure 1 (b) shows the lithiation and delithiation of a negative electrode of a negative electrode active material having a graphene coating on a surface containing Mg-containing silicon oxide, according to an embodiment of the present disclosure.

[0096] Reference Figure 2In part (a), when lithium is inserted during charging (lithiation occurs), the Mg-containing silicon oxide in the conventional negative electrode active material expands and contracts and returns to its original size after delithiation (lithium deintercalation). However, the carbon coating does not return to its original size. As a result, a gap is created between the Mg-containing silicon oxide and the single-walled carbon nanotubes 120 and graphite 110 bonded to the carbon coating, leading to an electrical short circuit.

[0097] On the contrary, Figure 2 In section (b), the Mg-containing silicon oxide in the negative electrode active material according to an embodiment of the present disclosure expands after lithium insertion (lithiation). Then, even when lithium deintercalation (delithiation) occurs and the Mg-containing silicon oxide shrinks and returns to its original size, the graphene coating remains intact and retains the electrical network formed between the Mg-containing silicon oxide and the single-walled carbon nanotubes 120 and graphite 110 bonded to the graphene coating. Therefore, in the negative electrode according to an embodiment of the present disclosure, the surface of the Mg-containing silicon oxide is provided with a graphene coating that has excellent flexibility and high affinity for single-walled carbon nanotubes, thus enabling secondary batteries using this negative electrode to provide significantly improved lifetime characteristics and high-temperature storage characteristics.

[0098] According to embodiments of this disclosure, when a negative electrode is manufactured by applying a mixture of a negative electrode active material, a conductive material, and a binder to a negative electrode current collector, the negative electrode can be obtained through a dry process by directly applying a solid mixture comprising the negative electrode active material, the conductive material, and the binder. Alternatively, the negative electrode can be obtained through a wet process by adding the negative electrode active material, the conductive material, and the binder to a dispersion medium, then stirring, applying the resulting mixture in slurry form, and removing the dispersion medium by drying or a similar method. Specific examples of the dispersion medium used in the wet process may include: an aqueous medium, such as water (deionized water or the like); or an organic medium, such as N-methyl-2-pyrrolidone (NMP) or acetone.

[0099] In another aspect, a lithium secondary battery is provided, the lithium secondary battery including a positive electrode, a negative electrode and a separator inserted between the negative electrode and the positive electrode, wherein the negative electrode includes a negative electrode according to an embodiment of the present disclosure.

[0100] A positive electrode can be obtained by applying a mixture of positive electrode active material, conductive material, and binder to a positive electrode current collector and then drying it. If desired, the mixture may further include filler. Specific examples of positive electrode active materials include, but are not limited to: layered compounds, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or those substituted with one or more transition metals; lithium manganese oxide, such as those with the chemical formula Li... 1+x Mn 2-xLithium manganese oxides represented by O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxides (Li2CuO2); vanadium oxides, such as LiV3O8, LiV3O4, V2O5, or Cu2V2O7; and those represented by the chemical formula LiNi. 1-x M x Ni-site type lithium nickel oxide represented by O2 (where M is Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x is 0.01 to 0.3); by chemical formula LiMn 2-x M x Lithium manganese composite oxides represented by O2 (where M is Co, Ni, Fe, Cr, Zn or Ta, and x is 0.01 to 0.1) or Li2Mn3MO8 (where M is Fe, Co, Ni, Cu or Zn); LiMn2O4 wherein Li is partially replaced by alkaline earth metal ions; disulfide compounds; Fe2(MoO4)3; or similar materials.

[0101] For the conductive materials, current collectors, and adhesives used for the positive electrode, please refer to the description above for the negative electrode.

[0102] A separator is inserted between the positive and negative electrodes and can be an insulating film with high ion permeability and mechanical strength. Typically, the separator may have a pore size and thickness of 0.01 to 10 μm and 5 to 300 μm, respectively. Specific examples of separators include: chemically resistant and hydrophobic olefin polymers, such as polypropylene; sheets or nonwoven meshes made of glass fiber or polyethylene; or the like. The separator may further include a porous layer on its outermost surface, comprising a mixture of inorganic particles and a binder resin.

[0103] According to this disclosure, the electrolyte comprises an organic solvent and a predetermined amount of lithium salt. Specific examples of the organic solvent include propylene carbonate (PC), ethylene carbonate (EC), butenyl carbonate (BC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propionate (MP), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), γ-butyrolactone (GBL), fluoroethylene carbonate (FEC), methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, pentyl acetate, methyl propionate, ethyl propionate, butyl propionate, or combinations thereof. Furthermore, halogen derivatives and straight-chain ester compounds of the organic solvent may also be used. The lithium salt is a component readily soluble in non-aqueous electrolytes, and specific examples include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB. 10 Cl 10LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborate, lithium lower aliphatic carboxylic acids, lithium tetraphenylborate, imide, or similar substances.

[0104] A secondary battery according to this disclosure can be obtained by housing an electrode assembly together with an electrolyte in a casing material (such as a battery casing), said electrode assembly comprising alternating stacked positive and negative electrodes with separators interposed therebetween. Any conventional method for manufacturing secondary batteries can be used without particular limitation.

[0105] In another aspect, a battery module comprising a secondary battery as a unit cell is provided, as well as a battery pack comprising the battery module. Since the battery module and battery pack comprise a secondary battery exhibiting excellent fast-charging characteristics under high load, they can be used as a power source for electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems. Among these secondary batteries, lithium secondary batteries, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries, are preferred.

[0106] Meanwhile, for battery components not described in this article, such as conductive materials, reference will be made to the descriptions of components commonly used in the battery field (specifically, the field of lithium secondary batteries).

[0107] The present disclosure will now be described in more detail with reference to embodiments. However, the following embodiments may be implemented in many different forms and should not be construed as limiting to the exemplary implementations set forth herein. Rather, these exemplary implementations are provided so that the present disclosure will be complete and comprehensive, and will fully convey the scope of the present disclosure to those skilled in the art.

[0108] Example 1

[0109] (1) Preparation of negative electrode active material

[0110] Silicon powder and silicon dioxide (SiO2) powder were uniformly mixed in a 1:1 molar ratio, and the resulting mixture was heat-treated at 1,400 °C under a reduced pressure atmosphere of 1 Torr to prepare SiO2. x (0 < x < 2) gas, and Mg was heat-treated at 900℃ to prepare Mg gas.

[0111] The obtained SiO x(0 < x < 2) gas and Mg gas were reacted at 1,300 °C for 3 hours, and then cooled to 800 °C over 4 hours to deposit the product. The resulting product was then pulverized using a jet mill to recover particles with an average particle size (D) of 5 μm. 50 ) Mg-containing silicon oxide powder.

[0112] The recovered Mg-containing silicon oxide powder was stirred together with an aqueous graphene dispersion using a wet mixer, and the resulting mixture was spray-dried at 200°C to obtain Mg-containing silicon oxide with a graphene coating as a negative electrode active material.

[0113] Here, the content of Mg-containing silicon oxide and the content of graphene coating, based on the total weight of the negative electrode active material, are shown in Table 1 below. Furthermore, the D / G band strength ratio of the graphene coating is also shown in Table 1.

[0114] Analysis of the negative electrode active material by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) revealed that the negative electrode active material has a Mg concentration of 8 wt%.

[0115] (2) Manufacturing of secondary batteries

[0116] The obtained negative electrode active material: artificial graphite: conductive material (carbon black): conductive material (single-walled carbon nanotubes (SWCNTs): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) was introduced into water as a dispersion medium in a weight ratio of 14.3: 81: 0.96: 0.04: 1.2: 2.5 to prepare a negative electrode mixture slurry. Here, the single-walled carbon nanotubes have an average diameter of 20 nm and a thickness of 580 μm. 2 Specific surface area of ​​ / g and aspect ratio of 250.

[0117] The negative electrode mixture slurry was uniformly coated onto both surfaces of a 20 μm thick copper foil. Coating was performed at a drying temperature of 70 °C and a coating rate of 0.2 m / min. The negative electrode mixture layer was then pressed to a porosity of 28% using a roller press to achieve the target thickness. Finally, it was dried in a vacuum furnace at 130 °C for 8 hours to obtain the negative electrode.

[0118] Next, 96.7 parts by weight of Li[Ni] was used as the positive electrode active material. 0.6 Mn 0.2 Co 0.2A cathode mixture slurry was prepared by dispersing O2, 1.3 parts by weight of graphite as a conductive material, and 2.0 parts by weight of polyvinylidene fluoride (PVdF) as a binder in 1-methyl-2-pyrrolidone as a dispersion medium. This slurry was coated onto both surfaces of an aluminum foil with a thickness of 20 μm. Coating was carried out at a drying temperature of 80 °C and a coating rate of 0.2 m / min. The cathode mixture layer was then pressed to a porosity of 24% using a roller press to achieve the target thickness. Finally, it was dried in a vacuum furnace at 130 °C for 8 hours to obtain the cathode.

[0119] A porous membrane (30 μm, Celgard) made of polypropylene was inserted between the resulting negative and positive electrodes to form an electrode assembly. Electrolyte was injected into the assembly, and then the electrode assembly was allowed to stand for 30 hours to allow the electrolyte to fully penetrate the electrode. The electrolyte was prepared by dissolving LiPF6 in an organic solvent containing a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:7 (volume ratio) to a concentration of 1.0 M, and then adding vinylene carbonate (VC) at a concentration of 2% by weight.

[0120] Example 2

[0121] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0122] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0123] Example 3

[0124] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0125] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0126] Example 4

[0127] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0128] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0129] Example 5

[0130] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0131] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0132] Example 6

[0133] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0134] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0135] Example 7

[0136] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0137] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material obtained as described above was used.

[0138] Comparative Example 1

[0139] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0140] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material: artificial graphite: conductive material (carbon black): conductive material (single-walled carbon nanotube) (SWCNT): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) were introduced into water as a dispersion medium in a weight ratio of 14.3: 81: 1: 0: 1.2: 2.5 to prepare the negative electrode mixture slurry.

[0141] Comparative Example 2

[0142] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0143] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that the negative electrode active material: artificial graphite: conductive material (carbon black): conductive material (single-walled carbon nanotube) (SWCNT): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) were introduced into water as a dispersion medium in a weight ratio of 14.3: 81: 1: 0: 1.2: 2.5 to prepare the negative electrode mixture slurry.

[0144] Comparative Example 3

[0145] Silicon powder and silicon dioxide (SiO2) powder were uniformly mixed in a 1:1 molar ratio, and the resulting mixture was heat-treated at 1,400 °C under a reduced pressure atmosphere of 1 Torr to prepare SiO2. x (0 < x < 2) gas, and Mg was heat-treated at 900℃ to prepare Mg gas.

[0146] The obtained SiO x (0 < x < 2) gas and Mg gas were reacted at 1,300 °C for 3 hours, and then cooled to 800 °C over 4 hours to deposit the product. The resulting product was then pulverized using a jet mill to recover particles with an average particle size (D) of 5 μm. 50 ) Mg-containing silicon oxide composite powder.

[0147] The recovered silica composite powder was heated in a tube furnace at a rate of 5 °C / min, and then subjected to chemical vapor deposition (CVD) at 950 °C for 2 hours in the presence of a mixed gas of argon (Ar) and methane (CH4) to obtain a negative electrode active material comprising a Mg-containing silica composite with a carbon coating. Here, the carbon coating content was 5 parts by weight per 100 parts by weight of the Mg-containing silica composite.

[0148] The negative electrode active material was analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The analysis revealed a Mg concentration of 8 wt% in the negative electrode active material. After X-ray diffraction (CuKα), the crystallite size was found to be 9 nm.

[0149] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that: the negative electrode active material: artificial graphite: conductive material (carbon black): conductive material (single-walled carbon nanotube) (SWCNT): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) was introduced into water as a dispersion medium in a weight ratio of 14.3: 81: 0.96: 0.04: 1.2: 2.5 to prepare the negative electrode mixture slurry.

[0150] Comparative Example 4

[0151] The negative electrode active material was obtained in the same manner as in Comparative Example 3.

[0152] The negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1, except that: the negative electrode active material: artificial graphite: conductive material (carbon black): conductive material (single-walled carbon nanotube) (SWCNT): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) was introduced into water as a dispersion medium in a weight ratio of 14.3: 81: 0.92: 0.08: 1.2: 2.5 to prepare the negative electrode mixture slurry.

[0153] Comparative Example 5

[0154] The negative electrode active material was obtained in the same manner as in Example 1, except that graphene with a D / G band strength ratio of 0.7 was used.

[0155] Then, the negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1.

[0156] Comparative Example 6

[0157] The negative electrode active material was obtained in the same manner as in Example 1, except that graphene with a D / G band strength ratio of 1.6 was used.

[0158] Then, the negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1.

[0159] Comparative Example 7

[0160] The negative electrode active material was obtained in the same manner as in Example 1, except that the content of Mg-containing silicon oxide, the content of graphene coating, and the D / G band strength ratio of graphene coating were changed as shown in Table 1.

[0161] Then, the negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1.

[0162] Example 8

[0163] The negative electrode active material was obtained in the same manner as in Example 1, except that graphene was not used and the composition of the negative electrode active material and conductive material as shown in Table 1 was used.

[0164] Then, the negative electrode, positive electrode, and secondary battery were obtained in the same manner as in Example 1.

[0165] Test case

[0166] Test Example 1: Determination of the D / G band intensity ratio of graphene

[0167] The D / G band integral values ​​of each sample were measured at 25-point intervals using Raman spectroscopy with a laser wavelength of 532 nm, and the D / G band intensity ratio was calculated from these values ​​to determine the D / G band intensity ratio of the graphene coating provided in each of the negative electrode active materials according to Examples 1 to 7 and Comparative Examples 1 to 8.

[0168] Here, the D / G band intensity ratio of graphene is defined by the Raman spectrum of graphene at 1360 ± 50 cm⁻¹. -1 The maximum peak intensity of the D band at 1580 ± 50 cm⁻¹ is related to the peak intensity at 1580 ± 50 cm⁻¹. -1 The average value of the ratio of the maximum peak intensity of the G-band at that location.

[0169] The D / G band intensity ratio of graphene is shown in Table 1 below.

[0170] Test Example 2: Capacity retention after storage at 60°C for 8 weeks

[0171] The capacity retention of each of the secondary batteries according to Examples 1 to 7 and Comparative Examples 1 to 8 after storage at 60°C for 8 weeks was evaluated below.

[0172] The capacity of the first charge / discharge cycle was determined and used as a standard. Each battery was fully charged, stored in a high-temperature chamber at 60°C for 8 hours, and then discharged. The capacity retention rate of the discharge capacity obtained by repeating one charge / discharge cycle was then calculated.

[0173] Charging conditions: Constant current (CC) / constant voltage (CV), 0.3 C, 4.25 V, 0.05 C cut-off.

[0174] Discharge conditions: CC 0.3 C, 2.5 V cut-off

[0175] The test results are shown in Table 1.

[0176] Test Example 3: High Temperature (45℃) Capacity Retention (300th Cycle)

[0177] The high-temperature (45°C) capacity retention of each of the secondary batteries according to Examples 1 to 7 and Comparative Examples 1 to 8 at the 300th cycle was evaluated below.

[0178] Charging conditions: CC / CV, 1 C, 4.25 V, 0.05C cut-off.

[0179] Discharge conditions: CC 1 C, 2.5 V cut-off

[0180] Capacity retention is defined by the following formula.

[0181] Capacity retention (%) = [Discharge capacity at 300th cycle / Discharge capacity at 2nd cycle] x 100

[0182] [Table 1]

[0183]

[0184] In Table 1, the content of Mg-containing silicon oxide and the content of graphene coating are calculated based on the total weight of the Mg-containing silicon oxide on which the graphene coating is formed, and the content of SW-CNT is calculated based on the total weight of the negative electrode active material. Referring to Table 1, compared with the secondary batteries according to Comparative Examples 1 to 8, in the case of the secondary batteries according to Examples 1 to 7, which include single-walled carbon nanotubes as conductive materials and use negative electrode active materials that meet the conditions of a D / G band strength ratio of 0.8 to 1.5 for graphene contained in the graphene coating, it can be seen that each secondary battery exhibits a higher capacity retention rate at 60°C (8 weeks) and a higher capacity retention rate at 45°C (300th cycle) corresponding to 88% or higher.

Claims

1. A negative electrode, comprising: Current collector; and A negative electrode active material layer is disposed on at least one surface of the current collector and comprises: 1) a negative electrode active material comprising Mg-containing silicon oxide and a graphene coating surrounding the Mg-containing silicon oxide surface; 2) a conductive material comprising single-walled carbon nanotubes (SWCNTs); and 3) a binder. The graphene contained in the graphene coating has a D / G band strength ratio of 0.8 to 1.5, and The D / G band intensity ratio of the graphene was defined by the Raman spectrum of the graphene at 1360 ± 50 cm⁻¹. -1 The maximum peak intensity of the D band at 1580 ± 50 cm⁻¹ is related to the peak intensity at 1580 ± 50 cm⁻¹. -1 The average ratio of the maximum peak intensity of the G-band at that location; The content of the single-walled carbon nanotubes is 0.01% to 0.06% by weight, based on the total weight of the negative electrode active material layer. The graphene coating content is from 0.5% to 10% by weight, based on the total weight of the negative electrode active material.

2. The negative electrode according to claim 1, wherein the graphene included in the graphene coating has a D / G band strength ratio of 0.8 to 1.

4.

3. The negative electrode according to claim 1, wherein the Mg-containing silicon oxide comprises 4% to 15% Mg by weight.

4. The negative electrode according to claim 1, wherein the content of the graphene coating is from 1% to 7% by weight based on the total weight of the negative electrode active material.

5. The negative electrode according to claim 1, wherein the content of the single-walled carbon nanotubes is from 0.01% to 0.04% by weight, based on the total weight of the negative electrode active material layer.

6. The negative electrode according to claim 1, wherein the conductive material further comprises carbon black, carbon fiber, metal fiber, fluorocarbon, metal powder, conductive whisker, conductive metal oxide, polyphenylene derivative, or a mixture of two or more thereof.

7. The negative electrode according to claim 6, wherein the carbon black comprises acetylene black, Ketjen black, channel black, furnace black, lampblack, thermal black, or a mixture of two or more thereof.

8. The negative electrode according to claim 6, wherein the carbon fiber comprises carbon nanofibers.

9. The negative electrode according to claim 1, wherein the negative electrode active material layer further comprises a carbonaceous active material.

10. The negative electrode according to claim 9, wherein the carbonaceous active material comprises artificial graphite, natural graphite, graphitizable mesophase carbon microspheres, petroleum coke, carbon fiber, pyrolytic carbon, or a mixture of two or more thereof.

11. The negative electrode according to claim 10, wherein the carbon fiber comprises graphitizable carbon fiber.

12. The negative electrode according to claim 9, wherein the carbonaceous active material comprises a baked resin.

13. A lithium secondary battery, the lithium secondary battery comprising a negative electrode as defined in any one of claims 1 to 12.

Images