Negative electrode slurry containing silicon-carbon composite, negative electrode, and lithium secondary battery

Abstract

The present invention relates to a negative electrode slurry, a negative electrode, a lithium secondary battery, a battery module, and a battery pack, the negative electrode slurry comprising: a silicon-carbon composite having a pH of 6 to 8 and a silicon content of 40 parts by weight to 60 parts by weight with respect to a total of 100 parts by weight of the silicon-carbon composite; a cellulose-based binder; and a conductive material.

Description

Technical Field

[0001] This application claims priority and benefit to Korean Patent Application No. 10-2024-0013211, filed with the Korean Intellectual Property Office on January 29, 2024, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to a negative electrode slurry containing a silicon-carbon composite, a negative electrode, a lithium secondary battery, a battery module, and a battery pack. Background Technology

[0003] As the use of fossil fuels increases rapidly, the demand for alternative or clean energy sources continues to grow, and as part of this, the most active area of ​​research is in the field of power generation and energy storage using electrochemical reactions.

[0004] Currently, secondary batteries are a representative example of electrochemical devices utilizing this electrochemical energy, and their applications are gradually expanding. In recent years, with the rapid proliferation of battery-powered electronic devices such as mobile phones, laptops, electric vehicles, electric power tools, and cleaning appliances, the demand for small, lightweight secondary batteries with relatively high capacity and / or high output has also increased rapidly. In particular, lithium secondary batteries have attracted considerable attention as a power source for electronic devices due to their lightweight nature and high energy density. Therefore, active research and development efforts are underway to improve the performance of lithium secondary batteries.

[0005] In a lithium secondary battery, electrical energy is generated through the oxidation / reduction reaction of lithium ions during insertion / extraction between the positive and negative electrodes, which are made of active materials capable of inserting and deintercalating lithium ions.

[0006] Graphite is primarily used as the negative electrode active material in lithium-ion batteries. However, graphite has a low capacity per unit mass, at 372 mAh / g, which makes it difficult to increase the capacity of lithium-ion batteries. Therefore, to improve the energy density of lithium-ion batteries, non-carbon-based negative electrode materials such as silicon, tin, and their oxides are being developed, which have higher energy densities than graphite. Although these non-carbon-based negative electrode materials have high capacity, they suffer from low initial efficiency. The resulting problems include high lithium consumption and significant irreversible capacity loss during the initial charge-discharge cycle.

[0007] Patent documents

[0008] (Patent Document 1) Korean Patent Application Publication No. 10-2022-0089687 A Summary of the Invention

[0009] [Technical Issues]

[0010] Compared to graphite or silicon oxide, silicon-carbon composites exhibit superior initial capacity and energy efficiency, but suffer from low phase stability during the manufacturing process of the negative electrode using the negative electrode slurry or during storage of the negative electrode slurry. Therefore, exemplary embodiments of the present invention provide a negative electrode slurry and a negative electrode, the negative electrode slurry comprising a silicon-carbon composite capable of improving the phase stability of the negative electrode slurry. Furthermore, exemplary embodiments of the present invention relate to lithium secondary batteries, battery modules, and battery packs comprising the aforementioned negative electrode.

[0011] [Technical Solution]

[0012] An exemplary embodiment of the present invention provides a negative electrode slurry comprising: a silicon-carbon composite having a pH of 6 to 8 and a silicon content of 40 to 60 parts by weight based on a total of 100 parts by weight of the silicon-carbon composite; a cellulose-based binder; and a conductive material.

[0013] An exemplary embodiment of the present invention provides a negative electrode slurry in which, when the intensity value of the maximum peak in the 284 to 285.5 eV band region in the XPS analysis of the surface of the silicon-carbon composite is set to 1, the intensity value of the maximum peak in the 288 to 289 eV band region is less than 0.1.

[0014] An exemplary embodiment of the present invention provides a negative electrode comprising: a current collector; and a negative electrode active material layer disposed on at least one surface of the current collector and comprising the negative electrode slurry.

[0015] An exemplary embodiment of the present invention provides a lithium secondary battery, the lithium secondary battery comprising: a negative electrode; a separator; and a positive electrode.

[0016] An exemplary embodiment of the present invention provides a battery module comprising the lithium secondary battery.

[0017] An exemplary embodiment of the present invention provides a battery pack comprising the lithium secondary battery.

[0018] An exemplary embodiment of the present invention provides a battery pack including the battery module.

[0019] [Beneficial Effects]

[0020] According to an exemplary embodiment of the present invention, the negative electrode slurry contains a silicon-carbon composite containing a specific amount of silicon and having a specific pH, such that the molecular weight change of the cellulose-based binder due to the interaction between the cellulose-based binder and the negative electrode active material is small, and the viscosity of the negative electrode slurry does not decrease, thereby obtaining a negative electrode slurry with excellent phase stability and enabling the manufacture of a uniform electrode. Detailed Implementation

[0021] This instruction manual will be described in more detail below.

[0022] It should be understood that the terms or words used throughout the specification should not be construed as limited to their usual or dictionary meanings, but rather as having meanings and concepts consistent with the technical ideas of the invention, based on the inventor's ability to appropriately define the concepts of the words or terms to best explain the principles of the invention.

[0023] As used herein, unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well.

[0024] In this specification, unless explicitly stated otherwise, when a portion is referred to as "containing" a particular constituent element, it means that the portion may also contain other constituent elements, without excluding other constituent elements.

[0025] Throughout this specification, when a component is referred to as being "on" another component, the component may be in direct contact with the other component, or there may be an intermediate component present.

[0026] In this specification, the carbon (C) content in the negative electrode active material particles can be analyzed using a CS analyzer (Bruker, G8 Galileo), and the oxygen (O) content can be analyzed using an ONH analyzer (Bruker, G-4 ICARUS Series II).

[0027] In this specification, the average particle size D50 can be defined as the particle size corresponding to 50% of the cumulative volume on the particle size distribution curve (a graphical curve of particle size distribution). The average particle size can be measured using, for example, laser diffraction. In laser diffraction, it is generally possible to measure particle sizes ranging from the submicron region to several millimeters, and high reproducibility and high resolution results can be obtained.

[0028] Average particle size can be measured using water and Triton-X100 dispersant with a Microtrac instrument (manufacturer: Microtrac, model name: S3500). Specifically, the average particle size of the positive electrode active material can be measured at a refractive index of 1.5 to 1.7, and the average particle size of the negative electrode active material can be measured at a refractive index of 1.97 or 2.42. For example, after dispersing the particles in a dispersion medium, the resulting dispersion is introduced into a commercially available laser diffraction particle size analyzer and irradiated with ultrasound at an output of 60 W and approximately 28 kHz. Subsequently, a cumulative volume particle size distribution map is obtained, and the average particle size is determined by obtaining the particle size corresponding to 50% of the cumulative volume.

[0029] In this specification, the specific surface area of ​​the negative electrode active material particles can be measured using the BET (Brunauer-Emmett-Teller) method. For example, it can be measured using a porosity analyzer (Bell Japan Inc., Belserp-mini II) via nitrogen adsorption distribution using the BET six-point method.

[0030] <Negative Electrode Slurry>

[0031] A negative electrode slurry according to an exemplary embodiment of the present invention comprises: a silicon-carbon composite having a pH of 6 to 8, and having a silicon content of 40 to 60 parts by weight based on a total of 100 parts by weight of the silicon-carbon composite; a cellulose-based binder; and a conductive material.

[0032] The pH of the silicon-carbon composite is between 6 and 8. For example, the lower limit of the pH of the silicon-carbon composite is 6, 6.1, 6.2, 6.3, 6.4, or 6.5, and the upper limit of the pH of the silicon-carbon composite is 8, 7.9, 7.8, 7.7, or 7.6. Therefore, when the pH of the silicon-carbon composite is between 6 and 8, the change in molecular weight due to the interaction between the binder and the negative electrode active material is small, and the viscosity of the negative electrode slurry does not decrease, making it possible to produce a stable slurry. The pH of the silicon-carbon composite can be measured by dispersing 10 g of the silicon-carbon composite in 100 g of water.

[0033] The process of manufacturing the silicon-carbon composite can be carried out using various methods. For example, the silicon-carbon composite can be manufactured by the following steps: manufacturing a carbon scaffold; heat-treating silane gas to grow silicon within the carbon; and treating the outer surface as needed. Furthermore, the pH of the silicon-carbon composite can vary depending on the raw materials used in each step, the heat treatment atmosphere, or the temperature. For example, a method can be used that involves heat treatment while mixing butane gas and CO2, or a method can be used that involves heat treatment while mixing butane gas and ammonia.

[0034] In the case of vapor deposition, a conventional method for manufacturing silicon-carbon composites, no solution is used, and even in processes where a solution is used, pH changes can occur due to oxidation or reduction reactions of the silicon-carbon composite itself. Therefore, it is important to distinguish between the pH of the process solution and the pH of the silicon-carbon composite during its manufacturing process. The pH of the silicon-carbon composite can be measured by dispersing it in water.

[0035] The pH of the silicon-carbon composite varies with the degree of oxidation of the silicon-carbon composite, i.e., the degree of oxidation of the surface functional groups of carbon and silicon. For example, when the pH of the silicon-carbon composite is 6, the amount of acidic functional groups is small. As an example, the degree of carbon oxidation is determined according to the manufacturing process conditions of the silicon-carbon composite. Specifically, when the polymer resin is carbonized at high temperatures, graphite without functional groups exhibits a neutral or higher pH, while at lower temperatures, some oxygen remains in some of the carbon. Alternatively, when the polymer resin is carbonized at high temperatures in a CO2 or steam atmosphere (rather than in an inert atmosphere), the amount of oxygen in the carbon increases. The degree of silicon oxidation varies with the temperature and atmosphere of the carbonization process. When the polymer resin is carbonized at high temperatures in a CO2 or steam atmosphere (rather than in an inert atmosphere), some silicon is also oxidized.

[0036] Cellulose-based binders such as CMC are used as thickeners in negative electrode slurries, and are therefore important for controlling the phase stability, i.e., viscosity, of the slurry. Due to the acidic functional groups in silicon-carbon composites, the molecular weight of cellulose-based binders such as CMC tends to decrease, which may lead to a decrease in the viscosity of the negative electrode slurry. When this decrease in viscosity occurs, it can cause quality and production defects during coating, potentially significantly affecting the overall yield. Therefore, to control this situation, a method can be chosen to cover the acidic portions of the silicon-carbon composite (e.g., the portions containing acidic functional groups) with a carbon coating. That is, this phenomenon can be controlled based on the amount or quality of the carbon coating, which can be confirmed by the pH of the negative electrode slurry, the initial capacity and efficiency of the electrode, XPS analysis, etc.

[0037] Based on 100 parts by weight of the silicon-carbon composite, the silicon content in the silicon-carbon composite is 40 to 60 parts by weight, for example, 45 to 55 parts by weight. When the silicon content is 40 parts by weight or more, the initial capacity is excellent, which is beneficial for improving the electrode capacity; while when the silicon content is 60 parts by weight or less, a composite with uniform silicon distribution can be manufactured. The silicon content in the silicon-carbon composite is determined by back-calculating the silicon content after obtaining the carbon (C) content and oxygen (O) content in the silicon-carbon composite. The carbon (C) content can be analyzed using a CS analyzer (Bruker, G8 Galileo), and the oxygen (O) content can be analyzed using an ONH analyzer (Bruker, G-4 ICARUS Series II).

[0038] According to an exemplary embodiment of the present invention, based on a total of 100 parts by weight of the silicon-carbon composite, the oxygen (O) content in the silicon-carbon composite is 10 parts by weight or less. Specifically, the oxygen content in the silicon-carbon composite is 10 parts by weight or less, 9 parts by weight or less, 8 parts by weight or less, 7 parts by weight or less, 6 parts by weight or less, or 5 parts by weight or less, and is 0 parts by weight or more, 1 part by weight or more, 1.5 parts by weight or more, 1.6 parts by weight or more, 1.7 parts by weight or more, or 1.8 parts by weight or more. When the oxygen content is 10 parts by weight or less based on 100 parts by weight of the silicon-carbon composite, the capacity efficiency of the silicon-carbon composite is excellent.

[0039] In one example, based on 100 parts by weight of the silicon-carbon composite, the total amount of carbon and silicon is 90 to 100 parts by weight.

[0040] According to one example, the silicon-carbon composite may have a structure including a porous carbon structure body and silicon provided inside and / or outside the porous carbon structure body, or may have a structure including a porous silicon structure body and carbon provided inside and / or outside the porous silicon structure body. The silicon-carbon composite does not need to be manufactured by a specific manufacturing method, but can be manufactured by methods known in the art. For example, the manufacture of the silicon-carbon composite can be carried out by a method including the following steps: manufacturing a porous carbon structure body, such as a step of thermally decomposing a polymer to manufacture a porous carbon structure body; and a step of flowing silane gas to the porous carbon structure body and thermally decomposing it at a high temperature to grow silicon inside and / or outside the porous carbon structure body. In addition, according to the purpose, a step of modifying the surface layer with carbon or other components can also be carried out. In addition, as another example, Si and SiO2 can be deposited simultaneously to manufacture a Si-containing SiO x oxide (0 < x < 2), the oxide can be chemically etched to manufacture a porous silicon structure body, and a heat treatment process can be carried out on the porous silicon structure body with carbon gas to manufacture a silicon-carbon composite.

[0041] According to an exemplary embodiment of the present invention, when the intensity value of the maximum peak in the 284 to 285.5 eV band region in the XPS analysis of the surface of the silicon-carbon composite is set to 1, the intensity value of the maximum peak in the 288 to 289 eV band region is 0.1 or less. In XPS analysis, the 284 to 289 eV band region includes the -OH, C=O, and -COOH regions, and these regions may affect the decomposition reaction caused by the interaction with the cellulose-based binder.

[0042] In XPS analysis, the intensity value of the maximum peak refers to the maximum height of the peak and represents the intensity value of the maximum peak within a specific range, regardless of peak overlap.

[0043] Specifically, when the intensity value of the maximum peak in the 284 to 285.5 eV band region of the silicon-carbon composite surface is set to 1 in XPS analysis, the intensity value of the maximum peak in the 288 to 289 eV band region can be below 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, or 0.03. When the intensity value of the maximum peak in the 288 to 289 eV band region is within a specific range, the decrease in viscosity of the cellulose-based adhesive can be suppressed because the decomposition reaction is inhibited through the interaction between the surface functional groups of the silicon-carbon composite and the cellulose-based adhesive. Here, XPS analysis can be performed under the following conditions.

[0044] - Equipment used: NEXSA G2, Thermo Fisher Scientific (equipment name ESCA-03)

[0045] - Sample preparation: Place the powdered sample into the powder holder and apply pressure, and flatten the measurement surface.

[0046] - Measurement conditions: X-ray source: Monochromatic Al Kα (1486.6 eV) X-ray spot size: 400 μm Etching conditions (sputtering gun): single-atom Ar (energy: 1,000 eV, current: low, grating width: 2 mm), etching rate 0.13 nm / s (based on Ta2O5, which may vary with the sample). Charge compensation (diffuse gun): 0.3 V, 150 μA Full scan: 200 eV pass energy, 1 eV energy level Narrow scan: pass energy 50 eV, energy level 0.1 eV Sensitivity Factor (SF): Al THERMO1, Energy Correction Factor (ECF): TPP-2M background subtraction "Smart" The plasma loss peak of Si was not included in the quantitative analysis.

[0047] According to an exemplary embodiment of the present invention, the silicon-carbon composite may include a carbon layer disposed on at least a portion of its surface.

[0048] According to an exemplary embodiment of the present invention, the carbon layer content is 0.1 to 50 parts by weight based on a total of 100 parts by weight of the silicon-carbon composite. Specifically, the carbon layer content can be 0.1 to 30 parts by weight, 0.1 to 20 parts by weight, 0.5 to 15 parts by weight, or 1 to 10 parts by weight based on a total of 100 parts by weight of the silicon-carbon composite.

[0049] When this specific range is met, the conductivity of the silicon-carbon composite is improved, and the volume change of the silicon-carbon composite during battery charging and discharging is easily suppressed, thereby improving the battery's lifespan characteristics.

[0050] The carbon layer can be formed using at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, butane, and acetylene via chemical vapor deposition (CVD).

[0051] The carbon layer may comprise at least one selected from the group consisting of amorphous carbon and crystalline carbon.

[0052] The crystalline carbon can further improve the conductivity of the silicon-carbon composite. The crystalline carbon may contain at least one selected from the group consisting of fullerenes, carbon nanotubes, and graphene.

[0053] The amorphous carbon can adequately maintain the strength of the carbon layer to suppress the expansion of the silicon-carbon composite. The amorphous carbon can be a carbide selected from at least one of the group consisting of tar, pitch, and other organic materials, or it can be a carbon-based material formed by using hydrocarbons as a source for chemical vapor deposition.

[0054] The other organic materials may be carbonides of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldose, hexulose, and organic materials selected from combinations thereof.

[0055] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, hexane, etc. The substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, phenanthrene, etc.

[0056] According to an exemplary embodiment of the present invention, the average particle size D50 of the silicon-carbon composite can be from 1 μm to 20 μm, for example, from 1.5 μm to 15 μm or from 3 μm to 10 μm. When the average particle size of the silicon-carbon composite falls within this specific range, the dispersibility of the active material during slurry preparation is appropriate, ensuring the structural stability of the active material during charge and discharge, and minimizing coating defects caused by large particles during electrode coating. Furthermore, it can prevent problems such as increased volume expansion and shrinkage due to excessively large particle size, and can prevent problems such as decreased initial efficiency due to excessively small particle size.

[0057] According to an exemplary embodiment of the present invention, the specific surface area of ​​the silicon-carbon composite can be 20 m².2 / g or less, 15 m 2 / g or less, 10 m 2 / g or less or 5 m 2 The specific surface area of ​​the above silicon-carbon composite can be less than 0.1 m² / g. 2 / g or more, 1 m 2 / g or more, 1.5 m 2 / g or more, 2 m 2 / g or more or 3 m 2 / g or more. The specific surface area refers to the total specific surface area of ​​the substance measured by BET technology.

[0058] In this field, BET (Brunauer / Emmett / Teller) techniques are commonly used to determine the accessible surface area of ​​a material by measuring the amount of gas adsorbed on it using an inert gas such as nitrogen. For example, the measurement can be performed using a BET measuring device (BEL-SORP-MAX, Nippon Bell) by degassing at 200°C for 8 hours, followed by nitrogen adsorption / desorption at 77 K.

[0059] The negative electrode adhesive can improve the adhesion between negative electrode active material particles and the adhesion strength between negative electrode active material particles and negative electrode current collector.

[0060] According to an exemplary embodiment of the present invention, the negative electrode binder is a cellulose-based binder and may contain at least one selected from the group consisting of carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose, and may also contain various copolymers thereof or polymers having derived structures. When the cellulose-based binder is used with a silicon-carbon composite having a pH of 6 to 8 and a silicon content of 40 to 60 parts by weight based on a total of 100 parts by weight of the silicon-carbon composite, the phase stability of the slurry can be maintained, and a high-capacity electrode can be manufactured.

[0061] Based on a total solid content of 100 parts by weight of the negative electrode slurry, the content of the negative electrode binder can be from 0.5 to 20 parts by weight, specifically from 1 to 15 parts by weight.

[0062] There are no particular limitations on the conductive material, as long as it is conductive and does not cause chemical changes in the battery. Examples of suitable materials include graphite such as natural or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbon compounds; metal powders such as aluminum and nickel powders; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxides; and conductive materials such as polyphenylene derivatives.

[0063] Based on the total solid content of the negative electrode slurry being 100 parts by weight, the content of the conductive material can be from 0.5 to 25 parts by weight, specifically from 1 to 20 parts by weight.

[0064] The negative electrode slurry may also contain additional negative electrode active materials.

[0065] As the additional negative electrode active material, compounds capable of reversibly inserting and de-intercalating lithium can be used. Specific examples may include: carbon-based active materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metallic substances capable of forming alloys with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; and metal oxides capable of doping and de-doping lithium, such as SiO₂. β (0<β<2), SnO2, vanadium oxide, lithium titanium oxide and lithium vanadium oxide; or a composite containing the said metallic substance and carbonaceous material such as Si-C composite or Sn-C composite, and any one of them or a mixture of two or more thereof may be used.

[0066] In addition, low-crystallinity carbon and high-crystallinity carbon can also be used as carbonaceous materials. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, while representative examples of high-crystallinity carbon include irregular, plate-like, flake-like, spherical or fibrous natural or artificial graphite, floating graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch, and high-temperature calcined carbon such as coke derived from petroleum or coal tar pitch.

[0067] According to an exemplary embodiment of the present invention, a negative electrode slurry is provided, the negative electrode slurry further comprising a carbon-based active material. That is, the additional negative electrode active material can be a carbon-based active material, and can include, for example, graphite, specifically at least one selected from the group consisting of artificial graphite and natural graphite.

[0068] The weight ratio of the negative electrode active material contained in the negative electrode slurry to the additional negative electrode active material can be 1:99 to 99:1, specifically 10:90 to 90:10, for example 5:95 to 50:50 or 10:90 to 30:70.

[0069] Based on the total solid content of the negative electrode slurry being 100 parts by weight, the amount of all negative electrode active materials contained in the negative electrode slurry can be 60 to 99 parts by weight, specifically 70 to 98 parts by weight.

[0070] The negative electrode slurry may also contain a thickener. The thickener may be carboxymethyl cellulose (CMC), but is not limited thereto; any thickener used in the art may be suitable.

[0071] Based on a total solid content of 100 parts by weight of the negative electrode slurry, the content of the thickener can be from 0.5 to 20 parts by weight, specifically from 0.5 to 15 parts by weight.

[0072] The negative electrode slurry may further include a solvent for forming the negative electrode slurry. Specifically, in order to facilitate component dispersion, the solvent for forming the negative electrode slurry may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropanol, specifically distilled water.

[0073] In one exemplary embodiment of the present invention, based on a total of 100 parts by weight of the negative electrode slurry, the solid content of the negative electrode slurry may be 20 to 75 parts by weight, specifically 30 to 70 parts by weight.

[0074] <Negative electrode>

[0075] Furthermore, according to an exemplary embodiment of the present invention, the negative electrode comprises: a current collector; and a negative electrode active material layer disposed on at least one surface of the current collector and comprising the negative electrode slurry.

[0076] There are no particular limitations on the negative electrode current collector, as long as it is conductive and will not cause chemical changes in the battery. For example, materials such as copper, stainless steel, aluminum, nickel, titanium, and sintered carbon can be used as the current collector; aluminum or stainless steel can also be surface-treated with carbon, nickel, titanium, silver, etc. Specifically, transition metals such as copper and nickel, which adsorb carbon well, can be used as the current collector. The thickness of the current collector can be from 6 μm to 20 μm. However, the thickness of the current collector is not limited to this.

[0077] The negative electrode active material layer can be formed by coating a negative electrode slurry containing a negative electrode active material, a binder, a conductive material, and a thickener as needed onto at least one surface of the current collector, followed by drying and calendering.

[0078] <Lithium secondary batteries>

[0079] A lithium secondary battery according to an exemplary embodiment of the present invention may include a negative electrode according to the above exemplary embodiment. Specifically, the lithium secondary battery may include a negative electrode, a positive electrode, and a separator interposed between the positive electrode and the negative electrode, and may also include an electrolyte. Since the negative electrode has already been described above, its detailed description is omitted.

[0080] The positive electrode may include a positive electrode current collector and a positive electrode active material layer, wherein the positive electrode active material layer is formed on the positive electrode current collector and contains positive electrode active material.

[0081] There are no particular limitations on the positive electrode current collector, as long as it is conductive and will not cause chemical changes in the battery. For example, it can be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. Furthermore, the positive electrode current collector can typically have a thickness of 3 to 500 μm, and its surface can be formed with fine irregularities to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0082] The positive electrode active material can be a commonly used positive electrode active material. Specifically, the positive electrode active material can be: layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds replaced by one or more transition metals; lithium iron oxide such as LiFe3O4; lithium manganese oxide such as Li 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5 and Cu2V2O7; and LiNi 1-c2 M c2 O2 (where M is at least one selected from the group consisting of: Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01 ≤ c2 ≤ 0.5) represents a Ni-site type lithium nickel oxide; LiMn 2-c3 M c3 Lithium-manganese composite oxides represented by O2 (where M is at least one selected from the group consisting of: Co, Ni, Fe, Cr, Zn, and Ta, and satisfying 0.01≤c3≤0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of: Fe, Co, Ni, Cu, and Zn); LiMn2O4, etc., but not limited to these, in which a portion of the Li in the chemical formula is replaced by alkaline earth metal ions. The positive electrode can be Li metal.

[0083] The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder in addition to the above-mentioned positive electrode active material.

[0084] In this context, the positive electrode conductive material is used to impart conductivity to the electrode, and it can be used without particular restriction as long as the positive electrode conductive material has electronic conductivity without causing chemical changes in the battery to be constructed. Specific examples may include: graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc., and any one of them or a mixture of two or more thereof can be used.

[0085] Furthermore, the positive electrode adhesive is used to improve the bonding between particles of the positive electrode active material and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used.

[0086] Besides using the aforementioned positive and negative active materials, the positive and negative electrodes can be manufactured using conventional methods for manufacturing positive and negative electrodes. Specifically, each can be manufactured by coating an active material layer containing active materials, optional binders, and conductive materials onto a current collector, followed by drying and calendering. In this case, the types and amounts of the positive and negative active materials, binders, and conductive materials are as described above. The solvent can be a solvent commonly used in the relevant art, including dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water, and one of these solvents can be used alone, or a mixture of two or more can be used. Considering the coating thickness and manufacturing yield of the slurry, the amount of solvent used is sufficient as long as it can dissolve or disperse the active materials, conductive materials, and binders, and then, when used to manufacture the positive and negative electrodes, achieve a viscosity that exhibits excellent thickness uniformity. In addition, as another method, the positive and negative electrodes can also be manufactured by casting an active material layer forming composition onto a separate support, and then pressing the film layer obtained by peeling it from the support onto the current collector layer.

[0087] The separator is used to separate the negative and positive electrodes and provide a migration path for lithium ions. Any separator can be used without particular limitation, as long as it is typically used in secondary batteries. In particular, separators with high electrolyte retention capacity and low resistance to ion migration in the electrolyte are preferred. Specifically, porous polymer membranes can be used, such as those made from polyolefin polymers like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer; or stacked structures of two or more layers thereof. Alternatively, common porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can be selectively used.

[0088] The electrolyte may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used to manufacture lithium secondary batteries.

[0089] Specifically, the electrolyte may contain non-aqueous organic solvents and metal salts.

[0090] As the aforementioned non-aqueous organic solvent, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, or ethyl propionate can be used.

[0091] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, as cyclic carbonates, are high-viscosity organic solvents and are preferred because they have high dielectric constants and thus readily dissociate lithium salts. When this cyclic carbonate is mixed in a suitable proportion with linear carbonates such as dimethyl carbonate or diethyl carbonate, which have low viscosity and low dielectric constants, an electrolyte with high conductivity can be prepared, and therefore it is even more preferred to use it.

[0092] Lithium salts can be used as the metal salts, and lithium salts are materials that are readily soluble in non-aqueous electrolytes, wherein, for example, one or more of the following can be used as the anion of the lithium salt: F - Cl- I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - .

[0093] In order to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, the electrolyte may contain one or more additives such as the following, in addition to the above-mentioned electrolyte components: alkylene carbonate halide compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted thiazolidinones, N,N-substituted imidazolidinanes, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride.

[0094] According to an exemplary embodiment of the present invention, a battery module comprising the lithium secondary battery as a unit cell, a battery pack comprising the lithium secondary battery, and a battery pack comprising the battery module are provided. Because the battery module and battery pack comprise secondary batteries with high capacity, high rate capability, and high cycle characteristics, the battery module and battery pack can be used as a power source for medium to large-sized devices selected from the group consisting of: electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0095] Preferred embodiments will be provided below to better understand the invention. It will be apparent to those skilled in the art that the embodiments are provided merely to illustrate the invention, and various modifications and changes can be made within the scope and spirit of the invention. Such modifications and changes naturally fall within the scope of the claims included herein.

[0096] Model for implementing invention

[0097] <Preparation of Silicon-Carbon Composites>

[0098] Example 1-1

[0099] 1.0 g has a specific surface area of ​​1850 to 1900 m² 2 / g and pore volume of 0.80 to 0.85 cm³ 3 An amorphous porous carbon scaffold of / g was placed in a ceramic crucible, which was then placed in the center of a horizontal tube furnace. After sealing the furnace and purging it with nitrogen, the furnace temperature was raised to 400 to 550°C at a rate of 10°C / min. Then, silane gas and hydrogen gas were introduced while varying their flow rates and held for 30 to 90 minutes, resulting in a silicon content of 48 parts by weight based on 100 parts by weight of the silicon-carbon composite. After further raising the furnace temperature to 780 to 900°C at a rate of 10°C / min, butane gas was introduced while varying its flow rate and held for 30 to 60 minutes, thereby preparing a silicon-carbon composite with the properties shown in Table 1.

[0100] Examples 1-2

[0101] Except for increasing the flow rate of silane gas to make the silicon content in the silicon-carbon composite 51 parts by weight, the same process as in Example 1-1 was performed.

[0102] Examples 1-3

[0103] Except for increasing the flow rate of silane gas during the preparation of the silicon-carbon composite to make the silicon content 49 parts by weight, and mixing butane gas and CO2 and heat-treating them in the final stage while maintaining butane gas, the same procedures as in Examples 1-1 were performed.

[0104] Comparative Example 1-1

[0105] Except for omitting the final stage of maintaining butane gas during the preparation of the silicon-carbon composite, the same procedures as in Examples 1-1 were performed.

[0106] Comparative Examples 1-2

[0107] Except for mixing butane gas and ammonia gas and heat treatment during the final stage of preparing the silicon-carbon composite, the same procedures as in Example 1-1 were performed.

[0108] Comparative Examples 1-3

[0109] Except for lowering the first heating temperature and reducing the flow rate and residence time of the silane gas, so that the silicon content in the silicon-carbon composite is 37 parts by weight, the same process as in Example 1-1 was performed.

[0110] Comparative Examples 1-4

[0111] Except for lowering the first heating temperature and increasing the flow rate and residence time of the silane gas to make the silicon content in the silicon-carbon composite 62 parts by weight, the same process as in Example 1-1 was performed.

[0112] <Evaluation 1: Measurement of the content of each component in the silicon-carbon composite>

[0113] The pH, silicon content, and oxygen (O) content of the silicon-carbon composites prepared in the examples and comparative examples were measured using the methods described below, and the results are shown in Table 1.

[0114] - pH: The pH of the dispersion was measured after 10 g of silicon-carbon composite was added to 100 g of aqueous solution and mixed with a homogenizer for 1 hour.

[0115] - Silicon content: The silicon content was calculated by estimating the weight excluding the carbon (C) and oxygen (O) content, which was analyzed using a CS analyzer (Bruker, G8 Galileo) and the oxygen (O) content was analyzed using an ONH analyzer (Bruker, G-4 ICARUS Series II).

[0116] - Oxygen (O) content: Oxygen (O) content was analyzed using an ONH analyzer (Bruker, G-4 ICARUS Series II).

[0117] <Evaluation 2: XPS Analysis of the Surface of the Silicon-Carbon Composite>

[0118] XPS analysis can be performed under the following conditions.

[0119] - Equipment used: NEXSA G2, Thermo Fisher Scientific (equipment name ESCA-03)

[0120] - Sample preparation: Place the powdered sample into the powder holder and apply pressure, and flatten the measurement surface.

[0121] - Measurement conditions: X-ray source: Monochromatic Al Kα (1486.6 eV) Etching conditions (sputtering gun): single-atom Ar (energy: 1,000 eV, current: low, grating width: 2 mm), etching rate 0.13 nm / s (based on Ta2O5, which may vary with the sample). Charge compensation (diffuse gun): 0.3 V, 150 μA Full scan: 200 eV pass energy, 1 eV energy level Narrow scan: pass energy 50 eV, energy level 0.1 eV Sensitivity Factor (SF): Al THERMO1, Energy Correction Factor (ECF): TPP-2M background subtraction "Smart" When the intensity value of the maximum peak in the 284 to 285.5 eV band region in the XPS analysis of the surface of the silicon-carbon composite is set to 1, the intensity values ​​of the maximum peak in the 288 to 289 eV band region (the ratio of the intensity values ​​of the maximum peak) are shown in Table 1 below.

[0122] [Table 1]

[0123] <Preparation of Negative Electrode Slurry>

[0124] Example 2-1

[0125] A negative electrode composition was prepared by using the silicon-carbon composite prepared in Example 1-1 as the negative electrode active material, two particulate conductive materials (SFG6L) and SWCNT (product name: Tuball, OCSiAl) as conductive materials, and an aqueous binder (product name: Daicel 2200) as a cellulose-based binder. The negative electrode active material, particulate conductive material, SWCNT and cellulose-based binder were mixed in a ratio of 81:8.6:0.8:9.6 (based on weight ratio) to prepare a negative electrode composition. Water was added as a solvent to prepare a negative electrode slurry.

[0126] Example 2-2

[0127] Except that the silicon-carbon composite of Examples 1-2 was used instead of the silicon-carbon composite of Example 1-1, the same procedures as in Example 2-1 were performed.

[0128] Example 2-3

[0129] Except that the silicon-carbon composites of Examples 1-3 were used instead of the silicon-carbon composites of Examples 1-1, the same procedures as in Examples 2-1 were performed.

[0130] Comparative Examples 2-1 to 2-4

[0131] Except that the silicon-carbon composites of Comparative Examples 1-1 to 1-4 were used instead of the silicon-carbon composite of Example 1-1, the same procedures as in Example 2-1 were performed.

[0132] <Evaluation 3: Measurement of Phase Stability (Viscosity Change) of Negative Electrode Slurry>

[0133] Using a TA rheometer, the shear viscosity was set at a shear rate of 2.5, and the viscosity of the negative electrode slurry was measured after preparation and after standing for 24 hours to determine the viscosity decrease rate relative to the initial viscosity.

[0134] <Evaluation 4: Battery fabrication and evaluation of battery characteristics>

[0135] A negative electrode slurry was coated onto a 18 μm thick copper foil and dried to form a 50 μm thick electrode active material layer on one surface of the copper foil. This was then stamped into a circular shape with a diameter of 14Φ (mm) to prepare the test electrode (negative electrode). A 0.3 mm thick lithium metal foil was used as the positive electrode. A 0.1 mm thick porous polyethylene sheet was used as the separator. Furthermore, as the electrolyte, a solution obtained by dissolving LiPF6 (as a lithium salt) at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio was used.

[0136] The negative electrode, positive electrode, separator, and electrolyte were sealed in a stainless steel container to prepare an evaluation coin battery with a thickness of 2 mm and a diameter of 32 mm.

[0137] The coin cell battery was charged at a constant current of 0.05C until the voltage reached 0.01 V, and then discharged at a constant current of 0.05C until the voltage reached 1.0 V to confirm the discharge capacity and initial efficiency. The results are shown in Table 2 below.

[0138] Based on the results of a single charge-discharge cycle, the initial efficiency (%) was calculated using the following formula.

[0139] Initial efficiency (%) = {Discharge capacity of negative electrode active material (mAh / g) / Charge capacity of negative electrode active material (mAh / g)} × 100%

[0140] [Table 2]

[0141] According to Table 2, in the case of Comparative Example 2-1, in which a silicon-carbon composite (Comparative Example 1-1) without a carbon layer and with a pH less than 6 (pH 5.3) was used to prepare the negative electrode slurry, it was confirmed that the molecular weight of the cellulose-based binder changed significantly due to the interaction between the negative electrode binder and the negative electrode active material, and the viscosity of the negative electrode composition was greatly reduced due to the decrease in the viscosity of the binder.

[0142] In Comparative Example 2-2, where a silicon-carbon composite with a pH greater than 8 (pH 10.3) (Comparative Example 1-2) was used to prepare the negative electrode slurry, it was confirmed that the molecular weight of the cellulose-based binder changed significantly due to the interaction between the negative electrode binder and the negative electrode active material, and similarly, the viscosity of the negative electrode slurry was greatly reduced due to the decrease in the viscosity of the binder.

[0143] In Comparative Examples 2-3, where a silicon-carbon composite with a silicon content of less than 40 parts by weight (37 parts by weight) (Comparative Examples 1-3) was used to prepare the negative electrode slurry, it was confirmed that the capacity of the electrode was not easily increased due to insufficient initial capacity.

[0144] In Comparative Examples 2-4, where a silicon-carbon composite with a silicon content exceeding 60 parts by weight (62 parts by weight) (Comparative Examples 1-4) was used to prepare the negative electrode slurry, it was confirmed that the molecular weight of the cellulose-based binder changed significantly due to the interaction between the negative electrode binder and the negative electrode active material. During the preparation of the negative electrode slurry, severe gas generation occurred due to the oxidation reaction of silicon, the viscosity of the negative electrode slurry decreased significantly, and the electrode capacity did not easily increase despite the increase in silicon content.

[0145] In Examples 2-3, where a silicon-carbon composite (Examples 1-3) with a pH of 6 to 8 (pH 6.6) and a silicon content of 40 to 60 parts by weight (51 parts by weight) was used to prepare the negative electrode slurry, it was confirmed that the molecular weight change of the cellulose-based binder due to the interaction between the negative electrode binder and the negative electrode active material was small, and the viscosity decrease rate of the negative electrode slurry was small. However, it was confirmed that based on a total of 100 parts by weight of the silicon-carbon composite, the oxygen (O) content exceeded 10 parts by weight (12 parts by weight), thereby increasing the loss of lithium availability and leading to a decrease in the initial efficiency of the electrode.

Claims

1. A negative electrode slurry, the negative electrode slurry comprising: A silicon-carbon composite having a pH of 6 to 8, and having a silicon content of 40 to 60 parts by weight based on a total of 100 parts by weight of the silicon-carbon composite. Cellulose-based adhesives; and Conductive materials.

2. The negative electrode slurry according to claim 1, wherein when the intensity value of the maximum peak in the 284 to 285.5 eV band region in the XPS analysis of the surface of the silicon-carbon composite is set to 1, the intensity value of the maximum peak in the 288 to 289 eV band region is less than 0.

1.

3. The negative electrode slurry according to claim 1, wherein, based on a total of 100 parts by weight of the silicon-carbon composite, the oxygen (O) content in the silicon-carbon composite is less than 10 parts by weight.

4. The negative electrode slurry according to claim 1, wherein the silicon-carbon composite comprises a carbon layer disposed on at least a portion of its surface.

5. The negative electrode slurry according to claim 1, wherein the average particle size D50 of the silicon-carbon composite is 1 μm to 20 μm.

6. The negative electrode slurry according to claim 1, wherein the specific surface area of ​​the silicon-carbon composite is 20 m². 2 / g or less.

7. The negative electrode slurry according to claim 1, wherein the negative electrode slurry further comprises a carbon-based active material.

8. A negative electrode, said negative electrode comprising: Current collector; and 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 the negative electrode slurry according to any one of claims 1 to 7.

9. A lithium secondary battery, the lithium secondary battery comprising: The negative electrode as described in claim 8; Diaphragm; and positive electrode.

10. A battery module comprising the lithium secondary battery of claim 9.

11. A battery pack comprising the lithium secondary battery of claim 9.

12. A battery pack comprising the battery module of claim 10.

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