Negative electrode active material, method for manufacturing negative electrode active material, negative electrode slurry, negative electrode, and secondary battery

Abstract

The present invention relates to an anode active material comprising silicon-based oxide particles, the silicon-based oxide particles containing Li, and a first coating layer and a second coating layer provided on the surfaces of the silicon-based oxide particles, the first coating layer and the second coating layer being carbon layers, and either no irreversible substance formed by silicon-based oxide and Li is contained between the first coating layer and the second coating layer, or LiOH and Li2CO3 are contained in an amount of less than 1 wt % each, based on 100 wt % of the total anode active material, anodes comprising the same, secondary batteries comprising the anodes, and methods for producing the anode active material.

Description

[Technical Field]

[0001] This application claims the benefit as of the filing date of Korean Patent Application No. 10-2021-0164669, filed with the Korean Intellectual Property Office on November 25, 2021, and all contents disclosed in said Korean Patent Application are incorporated herein by reference.

[0002] The present invention relates to a negative electrode active material, a method for producing a negative electrode active material, a negative electrode slurry containing the negative electrode active material, a negative electrode, and a secondary battery containing the negative electrode. [Background technology]

[0003] The rapid increase in fossil fuel use has led to a growing demand for alternative and clean energy sources, and one of the most actively researched areas in this field is the generation and storage of electricity using electrochemical reactions.

[0004] Currently, a prime example of an electrochemical element that utilizes electrochemical energy is the secondary battery, and its range of applications is steadily expanding. In recent years, as technological development and demand for portable devices such as portable computers, mobile phones, and cameras have increased, the demand for secondary batteries as an energy source has risen sharply. Among these secondary batteries, much research has been conducted on high-energy-density, i.e., high-capacity lithium secondary batteries, and these have been commercialized and are widely used.

[0005] Generally, a secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material that inserts and removes lithium ions released from the positive electrode, and silicon-containing particles with a high discharge capacity can be used as the negative electrode active material. However, as the demands for performance of lithium secondary batteries continue to increase, continuous improvement is required for the materials of the battery, including the negative electrode active material. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The present invention aims to provide a negative electrode active material, a method for producing a negative electrode active material, a negative electrode slurry containing the negative electrode active material, and a negative electrode, as well as a secondary battery containing the negative electrode, which, when applied to a secondary battery, improve initial efficiency, minimize the reduction in discharge capacity, lower the viscosity of the slurry, and improve gas generation problems. [Means for solving the problem]

[0007] One embodiment of the present invention provides a negative electrode active material comprising silicon oxide particles, wherein the silicon oxide particles comprise Li, and the silicon oxide particles comprise a first coating layer and a second coating layer provided on the surface of the silicon oxide particles, the first coating layer and the second coating layer each being a carbon layer, and there is no irreversible substance formed by silicon oxide and Li between the first coating layer and the second coating layer, or LiOH and Li2CO3 are present in amounts of less than 1% by weight each, based on 100% by weight of the total negative electrode active material.

[0008] One embodiment of the present invention provides a method for producing a negative electrode active material containing silicon oxide particles, comprising the steps of: forming a first coating layer, which is a carbon layer, on silicon oxide particles; doping the silicon oxide particles on which the first coating layer is formed with Li; removing an irreversible substance formed by silicon oxide and Li on the silicon oxide particles on which the first coating layer is formed by etching; and forming a second coating layer, which is a carbon layer, on the etched silicon oxide particles on which the first coating layer is formed.

[0009] One embodiment of the present invention provides a negative electrode slurry containing the negative electrode active material.

[0010] One embodiment of the present invention provides a negative electrode containing the negative electrode active material.

[0011] One embodiment of the present invention provides a secondary battery including the negative electrode.

Advantages of the Invention

[0012] Since the negative electrode active material according to the embodiment of the present invention contains silicon-based oxide particles containing Li, it can be used as a high-efficiency material. Specifically, by containing lithium, the initial efficiency of the battery can be improved. In particular, lithium is a lighter substance compared to other materials, so there is an advantage that the decrease in discharge capacity due to an increase in lithium content is small. Further, in the step of incorporating the Li, lithium is doped in a state where a carbon layer is formed, and irreversible substances present on the carbon layer are removed by etching, thereby improving the problems of low viscosity of the negative electrode slurry and gas generation problems.

[0013] Also, after the etching, by further coating the carbon layer, the reaction between the surface of the active material and the slurry solvent (water) can be blocked, and the problems of low viscosity of the negative electrode slurry and gas generation can be improved.

Modes for Carrying Out the Invention

[0014] Hereinafter, in order to assist the understanding of the present invention, the present invention will be described in more detail.

[0015] The terms and words used in this specification and the claims should not be construed as being limited to the ordinary or dictionary meanings, and the inventors should interpret them in accordance with the principle that they can appropriately define the concepts of the terms in order to explain their invention in the best way, and should interpret them in meanings and concepts consistent with the technical idea of the present invention.

[0016] The terms used in this specification are merely used to explain exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0017] In this specification, terms such as “includes,” “equip,” or “have” are intended to indicate the existence of implemented features, figures, steps, components, or combinations thereof, and should be understood not to preemptively exclude the existence or possibility of adding one or more other features, figures, steps, components, or combinations thereof.

[0018] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size at 50% of the cumulative volume in the particle size distribution curve. 50 The particle size can be measured, for example, using the laser diffraction method. This laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can obtain highly reproducible and high-resolution results.

[0019] In this specification, the specific surface area of ​​the silicon oxide particles can be measured by the BET (Brunauer-Emmett-Teller; BET) method. For example, it can be measured using a porosimetry analyzer (Bell Japan Inc., Belsorp-II mini) with a nitrogen gas adsorption flow method using the BET 6-point method.

[0020] In this specification, the Li content in each particle of silicon oxide can be determined by ICP analysis. For example, for the ICP analysis, a fixed amount (approximately 0.01 g) of silicon oxide particles is accurately separated, transferred to a platinum crucible, and completely decomposed on a hot plate with the addition of nitric acid, hydrofluoric acid, and sulfuric acid. Then, using an inductively coupled plasma atomic emission spectrometer (ICPAES, Perkin-Elmer 7300), the intensity of a standard solution prepared using a standard solution (5 mg / kg) is measured at the wavelength specific to the element Li to create a reference calibration curve. Subsequently, the pre-treated sample solution and a blank sample are introduced into the instrument, their respective intensities are measured to calculate the actual intensities, and the concentrations of each component are calculated by comparing them with the calibration curve created above. The total sum is then converted to a theoretical value, and the Li concentration in the particles can be analyzed.

[0021] In this specification, after etching, the content of Li by-products (LiOH, Li2CO3) on the surfaces of the first and second coating layers can be measured using pH titration. For example, a 20% SiO (solvent: water) solution can be washed with water for 10 minutes (using a stirrer), filtered for 30 minutes to extract the solution, and then pH titration can be performed to analyze the content of Li by-products (LiOH, Li2CO3).

[0022] <Negative electrode active material> A negative electrode active material according to one embodiment of the present invention comprises silicon oxide particles, the silicon oxide particles comprising Li, and comprising a first coating layer and a second coating layer provided on the surface of the silicon oxide particles, the first coating layer and the second coating layer being carbon layers, and characterized in that there is no irreversible substance formed by silicon oxide and Li between the first coating layer and the second coating layer, or LiOH and Li2CO3 are contained in amounts of less than 1% by weight each, based on 100% by weight of the total negative electrode active material.

[0023] In the above embodiment, a first coating layer and a second coating layer are provided on the surface of silicon oxide particles containing Li. This can be achieved by doping with Li after the formation of the first coating layer by a manufacturing method described later, and then removing the irreversible substance formed between the first coating layer and the second coating layer by etching such as acid treatment before the formation of the second coating layer.

[0024] If a first coating layer is not formed and lithium is directly doped into silicon oxide particles and etching is performed, the physical properties of the negative electrode material may be impaired, potentially degrading battery performance. Specifically, since lithium-doped silicon oxide materials are very vulnerable to moisture and oxidation, if lithium is directly doped into the material and etching is performed without forming a first coating layer, the physical properties of the material will inevitably deteriorate. Furthermore, if lithium doping and etching are performed without forming a first coating layer, there is a high possibility that not only the irreversible substance to be removed but also the Li-SiO structure present inside the particles will be etched together, leading to excessive etching.

[0025] In the above embodiment of the present invention, after forming a first coating layer which is a carbon layer, irreversible substances on the first coating layer can be removed without impairing the physical properties of the material by doping with Li and etching, thereby contributing to surface stabilization. Furthermore, by further forming a second coating layer on the first coating layer, stabilization of the inside of the particles can be contributed to. As described above, by removing irreversible substances, the amount of such irreversible substances in the first and second coating layers is reduced, and the problems of low viscosity and gas generation in a negative electrode slurry containing such particles can be improved.

[0026] According to one embodiment, the content of Li may be 1% by weight or more and 15% by weight or less, based on 100% by weight of the total negative electrode active material. More preferably, it is 1.5% by weight or more and 13% by weight or less, for example, 3% by weight or more and 12% by weight or less. When the Li content is 1% by weight or more, it is advantageous for increasing the initial efficiency of the battery. When the Li content is 1% by weight or more, it is advantageous for increasing the initial efficiency of the battery, and when the Li content is 15% by weight or less, it is advantageous in terms of slurry processability.

[0027] According to one embodiment, the silicon-based oxide particles may contain SiOx (0 < x < 2) as a silicon-based oxide component. The SiOx (0 < x < 2) can correspond to a matrix within the silicon-based oxide particles. The SiOx (0 < x < 2) may be in a form containing Si and SiO2, and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiOx (0 < x < 2). When the silicon-based oxide particles contain the SiOx (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0028] According to another embodiment of the present invention, the Li may exist as a Li compound phase. The Li compound phase may exist in at least one form of lithium silicate, lithium silicide, and lithium oxide. The Li compound phase may be lithium silicate (Li-silicate), for example, Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and more specifically, it may contain one or more selected from the group consisting of Li2SiO3, Li2Si2O5, Li3SiO3, Li4SiO4, LiOH, and Li2CO3.

[0029] According to one embodiment, the irreversible substance formed between the first coating layer and the second coating layer by silicon oxide and Li may be lithium silicate (Li-silicate), and specifically may include one or more selected from the group consisting of Li2SiO3, Li2Si2O5, Li3SiO3, Li4SiO4, LiOH, and Li2CO3, for example, LiOH or Li2CO3.

[0030] The lithium silicate (Li-silicate) may have different phases depending on its Li content, and may mainly produce Li2SiO3 or Li2Si2O5.

[0031] In this embodiment of the present invention, the LiOH and Li2CO3 may be produced by the reaction of Li with H2O and CO2 before doping with Li and etching after forming the first coating layer, which is a carbon layer. These are removed by the etching, and the LiOH and Li2CO3 may be present in amounts of less than 1% by weight each, i.e., small amounts, based on 100% by weight of the total negative electrode active material.

[0032] The Li compound may be distributed on the surface and / or inside the silicon oxide particles in a doped form. The Li compound, distributed on the surface and / or inside the silicon oxide particles, can control the volume expansion / contraction of the silicon oxide particles to an appropriate level and can play a role in preventing damage to the active material. The Li compound may also be included in a way that reduces the proportion of the irreversible phase (e.g., SiO2) of the silicon oxide particles and increases the efficiency of the active material.

[0033] According to one embodiment, the first coating layer may cover the entire surface of the silicon oxide particles, or it may cover only a portion of the surface. The second coating layer may be provided on at least a portion of the surface of the silicon oxide particles to which the first coating layer is provided.

[0034] According to one embodiment, a portion of the second coating layer may be provided in direct contact with the surface of silicon oxide particles, while a portion or all of the second coating layer may be provided in contact with the first coating layer.

[0035] According to one embodiment, the first coating layer and the second coating layer may each be a carbon layer.

[0036] The carbon layer of the first coating layer and the carbon layer of the second coating layer impart conductivity to the silicon oxide particles, effectively suppressing volume changes in the negative electrode active material containing the silicon oxide particles, thereby improving the battery's lifespan characteristics.

[0037] In one embodiment of this specification, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0038] The crystalline carbon can further improve the conductivity of the silicon oxide particles. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotubes, and graphene.

[0039] The amorphous carbon can appropriately maintain the strength of the carbon layer and suppress the expansion of the silicon oxide particles. The amorphous carbon may be a carbon-based material formed by using at least one carbide or hydrocarbon selected from the group consisting of tar, pitch, and other organic substances as a source in chemical vapor deposition.

[0040] The aforementioned carbonized organic substances may be carbonized organic substances selected from sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose, and combinations thereof.

[0041] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. Examples of the aliphatic or alicyclic hydrocarbon include methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, or hexane. Examples of the substituted or unsubstituted aromatic hydrocarbon include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene.

[0042] In one embodiment, the carbon layer may be present in an amount of 1% to 50% by weight based on 100% by weight of the total anode active material, more specifically, in an amount of 1% to 40% by weight, 1% to 30% by weight, or 1% to 20% by weight, and more specifically, in an amount of 1% to 15% by weight. For example, if both the first and second coating layers are carbon layers, when only the first coating layer is coated and measured, the carbon layer may be present in an amount of 1% to 7% by weight based on 100% by weight of the total anode active material, and when measured after coating up to the second coating layer, the carbon layer may be present in an amount of 1% to 15% by weight based on 100% by weight of the total anode active material.

[0043] When the above range is met, the conductivity of the negative electrode active material is improved, the volume change of the negative electrode active material during battery charging and discharging is easily suppressed, and the battery life characteristics can be improved.

[0044] In one embodiment, the thickness of the carbon layer may be 1 nm to 500 nm, and more specifically, 5 nm to 300 nm. When the above range is met, volume changes of the negative electrode active material are easily suppressed, side reactions between the electrolyte and the negative electrode active material are suppressed, and the battery life characteristics can be improved.

[0045] In one embodiment, the carbon layer may contain one of the elements Li, F, and O. The elements may also include additional constituent elements in the carbon layer depending on the composition of the electrolyte.

[0046] According to another embodiment of the present invention, the silicon oxide particles may include a phosphate layer on their surface comprising at least one of aluminum phosphate and lithium phosphate, wherein the phosphate layer may be an aluminum phosphate layer, a lithium phosphate layer, or an aluminum lithium phosphate layer.

[0047] A phosphate layer may be formed by dry mixing the silicon oxide particles and phosphate and then heat-treating them to perform a coating treatment; mixing the phosphate with the silicon oxide particles and phosphate in a solvent and then reacting them while evaporating the solvent to perform a coating treatment; dry mixing an aluminum or lithium precursor, such as aluminum oxide or lithium oxide, with a phosphorus precursor, such as phosphorus oxide, and then heat-treating them to perform a coating treatment; or mixing an aluminum or lithium precursor, such as aluminum oxide or lithium oxide, with a phosphorus precursor, such as phosphorus oxide, in a solvent and then reacting them while evaporating the solvent to perform a coating treatment.

[0048] For example, the following methods can be used: a dry mixing of the above-mentioned silicon oxide particles and AlyPzOw (aluminum phosphate) followed by heat treatment to coat; a mixing of AlyPzOw and silicon oxide particles in a solvent followed by a coating treatment while the solvent is evaporated; a dry mixing of AlxOy (aluminum precursor) and PzOw (P precursor) followed by a coating treatment while heat treatment; and a mixing of AlxOy (aluminum precursor) and PzOw (P precursor) in a solvent followed by a coating treatment while the solvent is evaporated and the reaction is carried out.

[0049] As another example, a method of dry-mixing the above silicon-based oxide particles and LixAlyPzOw (Li-Al-P-O system) and performing a coating treatment by heat treatment, or a method of mixing LixAlyPzOw and silicon-based oxide particles in a solvent and then reacting them while evaporating the solvent to perform a coating treatment can be used.

[0050] Here, x, y, z, and w may satisfy 0 < x ≦ 10, 0 < y ≦ 10, 0 < z ≦ 10, and 0 < w ≦ 10, and represent the atomic number ratios.

[0051] According to one embodiment of the present invention, x may satisfy 0 < x ≦ 3.

[0052] According to one embodiment of the present invention, y may satisfy 0 < y ≦ 1.

[0053] According to one embodiment of the present invention, z may satisfy 0.5 ≦ z ≦ 3.

[0054] According to one embodiment of the present invention, w may satisfy 4 < w ≦ 12.

[0055] As an example, AlPO4 or Al(PO3)3 may be used as a surface treatment substance for forming a phosphate layer, and the substances formed on the particle surface may be Li3PO4 or AlPO4.

[0056] In the phosphate layer, Al may be contained in an amount of 0.01% to 0.5% by weight based on 100% by weight of the negative electrode active material, and P may be contained in an amount of 立0.01% to 1.5% by weight based on 100% by weight of the total silicon-based oxide particles. When the above range is satisfied, there is an advantage that the reaction between the silicon-based oxide particles and water in the aqueous mixing step is suppressed and the slurry processability is improved.

[0057] According to another embodiment of the present invention, the average particle size (D50) of the silicon-based oxide particles may be 1 μm to 30 μm. Specifically, the average particle size (D50) of the silicon-based oxide particles may be 3 μm to 20 μm, and more specifically, may be 5 μm to 10 μm. When the above range is satisfied, the side reaction between the negative electrode active material and the electrolyte can be controlled, and the discharge capacity and initial efficiency of the battery can be effectively realized.

[0058] According to another embodiment of the present invention, the BET specific surface area of the silicon-based oxide particles is 0.5 m 2 / g to 60 m 2 / g. Specifically, the BET specific surface area of the silicon-based oxide particles may be 0.6 m 2 / g to 20 m 2 / g, and more specifically, may be 0.8 m 2 / g to 12 m 2 / g. When the above range is satisfied, during charging and discharging of the battery, the side reaction between the electrolyte and the negative electrode active material can be reduced, and the life characteristics of the battery can be improved.

[0059] According to another embodiment of the present invention, the silicon-based oxide particles may further contain Si crystal grains. The Si crystal grains may have a particle size of 1 nm to 15 nm. For example, the particle size of the Si crystal grains may be 1 nm to 14 nm, or 1 nm to 13 nm. The particle size of the Si crystal grains can be calculated by XRD analysis.

[0060] <Method for manufacturing negative electrode active material> A method for producing a negative electrode active material containing silicon oxide particles according to one embodiment includes the steps of: forming a first coating layer, which is a carbon layer, on silicon oxide particles; doping the silicon oxide particles on which the first coating layer is formed with Li; removing an irreversible substance formed by silicon oxide and Li on the silicon oxide particles on which the first coating layer is formed by etching; and forming a second coating layer, which is a carbon layer, on the etched silicon oxide particles on which the first coating layer is formed.

[0061] Before or after forming the first coating layer, which is the carbon layer, the silicon oxide particles can be adjusted in particle size by grinding methods such as mechanical milling, if necessary.

[0062] The step of forming a first coating layer, which is a carbon layer, on the surface of the silicon oxide particles may, for example, be carried out by injecting a carbon-based raw material gas such as methane gas and performing heat treatment in a rotary tubular furnace. Specifically, the silicon oxide particles are placed in a rotary tubular furnace, the temperature is raised at a rate of 3 to 10°C / min or about 5°C / min to 800°C to 1,150°C, 900°C to 1,050°C, or 950°C to 1,000°C, and heat treatment is performed for 30 minutes to 8 hours while argon gas and a carbon-based raw material gas are flowed through the rotary tubular furnace to form a carbon layer.

[0063] The step of doping the silicon oxide particles on which the first coating layer is formed with Li may be carried out by mixing the silicon oxide particles on which the carbon layer is formed with lithium metal powder or lithium precursor, such as lithium metal powder (Li metal powder), LiOH, or Li2O, and heat-treating the mixture at 400°C to 1200°C if necessary. Alternatively, the step may be carried out using an electrochemical method.

[0064] The step of removing the irreversible material formed by silicon oxide and Li on the silicon oxide particles on which the first coating layer is formed may be carried out by etching.

[0065] At this time, the etching may be carried out using an acid or a base, the acid may be at least one of hydrofluoric acid (HF), nitric acid (HNO3), sulfuric acid (H2SO4), and hydrochloric acid (HCl), and the base may be at least one of sodium hydroxide (NaOH) and potassium hydroxide (KOH). Specifically, a mixed solution of hydrofluoric acid and ethanol may be used during the etching. The etching may be carried out for 1 to 3 hours, specifically, 1 to 2.5 hours.

[0066] In one embodiment, the etching may be carried out using NaOH, HF, or KOH.

[0067] The etching process described above removes irreversible material from the first coating layer. This irreversible material may be, for example, LiOH, Li2CO3, or lithium silicate.

[0068] The process involves forming a second coating layer, which is at least one of an aluminum phosphate layer and a carbon layer, on silicon oxide particles on which the first coating layer has been etched as described above. As the carbon layer for the second coating layer, the method exemplified for forming the first coating layer may be used.

[0069] According to one embodiment, the second coating layer may further include an aluminum phosphate layer, which may be formed by the same method as the phosphate layer formation method described above.

[0070] The step of doping the silicon oxide particles on which the first coating layer is formed with Li may be carried out by mixing the silicon oxide particles on which the carbon layer is formed with lithium metal powder or lithium precursor, such as lithium metal powder, LiOH, or Li2O, and heat-treating at 400°C to 1200°C if necessary. Alternatively, the step may be carried out using an electrochemical method.

[0071] <Negative electrode> A negative electrode according to yet another embodiment of the present invention may include a negative electrode active material, wherein the negative electrode active material is the same as the negative electrode active material of the above embodiment. Specifically, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer may contain the negative electrode active material. Furthermore, the negative electrode active material layer may further include a binder and / or a conductive material.

[0072] One embodiment of the present invention provides a negative electrode slurry containing the above-mentioned negative electrode active material.

[0073] The negative electrode slurry according to one embodiment may further contain a solvent in the material of the negative electrode active material layer described above. For example, the viscosity of the negative electrode slurry can be measured with a graphite mixed composition, for example, with a composition of 80% artificial graphite and 20% silicon oxide, with a total negative electrode active material of 96.5%, a conductive material of 1%, a binder of 1.5%, and a thickener of 1%. The viscosity of the negative electrode slurry produced above may be 10,000 cps to 25,000 cps when measured at room temperature.

[0074] According to one embodiment, after placing 20 g of the negative electrode slurry in a pouch and sealing it, the gas generation rate measured by the change in volume after storage in a 40°C chamber can be 10 mL to 200 mL after 7 days of storage in the 40°C chamber. The above change in volume can be determined by placing the sealed pouch in water and measuring the change in the volume of water. Specifically, the gas generation rate may be 10 mL to 150 mL, 10 mL to 100 mL, or 10 mL to 70 mL.

[0075] The negative electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and is conductive. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. Specifically, transition metals that readily adsorb carbon, such as copper and nickel, may be used as the current collector. The thickness of the current collector may be 6 μm to 20 μm, but is not limited thereto.

[0076] The binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these substances are substituted with Li, Na, or Ca, and may also contain a variety of copolymers thereof.

[0077] The conductive material is not particularly limited as long as it does not induce a chemical change in the battery and is conductive. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

[0078] <Secondary battery> A secondary battery according to yet another embodiment of the present invention may include the negative electrode of the embodiment described above. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the same as the negative electrode described above.

[0079] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0080] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, mesh, porous material, foam, nonwoven fabric.

[0081] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, 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 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions, etc., are examples, but are not limited to these. The positive electrode may be Li-metal.

[0082] The positive electrode active material layer may also include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0083] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without particular limitations as long as it has electronic conductivity in the battery without causing a chemical change. Specific examples include graphite such as natural graphite or artificial graphite; carbon-containing materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; 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. One of these alone or a mixture of two or more may be used.

[0084] Furthermore, the positive electrode binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used.

[0085] The separator separates the negative and positive electrodes and provides a passage for lithium ions to move. Generally, any separator used in secondary batteries can be used without particular limitations, but it is especially preferable that it has low resistance to ion movement in the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymeric substances may be used, and they may be selectively used as single-layer or multi-layer structures.

[0086] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0087] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0088] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene 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, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, and ethyl propionate may be used.

[0089] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents because they have high dielectric constants and dissociate lithium salts well. Furthermore, when such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in appropriate proportions, an electrolyte with high conductivity can be produced, and therefore they can be used even more preferably.

[0090] As the metal salt, a lithium salt may be used, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte. For example, 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 - You may use one or more selected from the group consisting of the following:

[0091] In addition to the components of the electrolyte, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing the decrease in battery capacity, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0092] Another embodiment of the present invention provides a battery module and a battery pack including the secondary battery as a unit cell. The battery module and battery pack include the secondary battery having high capacity, high rate characteristics, and cycle characteristics. The secondary battery can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0093] The following are preferred embodiments to aid in understanding the present invention. These embodiments are merely illustrative examples of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the present description and the technical concept, and that such variations and modifications fall within the scope of the appended claims.

[0094] <Examples> [Example 1: Manufacturing of negative electrode active material] (1) Production of preliminary silicon oxide particles (formation of the first coating layer) A mixture of Si powder and SiO2 powder in a 1:1 molar ratio was heat-treated at 1400°C under reduced pressure to recover the SiO powder. After cooling and precipitating the reacted powder, it was pulverized with a jet mill to recover particles with an average particle size (D50) of approximately 6 μm.

[0095] The recovered particles were fed into a tubular furnace and subjected to CVD (Chemical Vapor Deposition) treatment under a mixed gas of argon (Ar) and methane (CH4) to produce preliminary silicon oxide particles with a carbon coating layer.

[0096] (2) Production of silicon oxide particles (Li doping) Li-containing particles were produced by heat-treating a preliminary silicon oxide powder and Li metal powder in an inactive atmosphere at a temperature of 800°C for 2 hours.

[0097] (3) Manufacturing of preliminary negative electrode active material (etching process) A mixed solution of hydrofluoric acid and ethanol was used as the etching solution. Silicon oxide particles were added to the etching solution in a weight ratio of 20:1, mixed for about 1 hour, and then filtered, washed, and dried to form preliminary negative electrode active material particles.

[0098] (4) Manufacturing of negative electrode active material (formation of second coating layer) The aforementioned preliminary negative electrode active material particles were placed in a tubular furnace and subjected to CVD (Chemical Vapor Deposition) treatment under a mixed gas of argon (Ar) and methane (CH4) to produce negative electrode active material particles in which the second coating layer was a carbon coating layer.

[0099] ICP analysis revealed that the Li content of the manufactured negative electrode active material was 4.8% by weight, based on 100% by weight of negative electrode active material particles.

[0100] CS analysis revealed that the carbon content of the manufactured negative electrode active material was 7% by weight, based on 100 wt% of negative electrode active material particles.

[0101] The negative electrode active material particles were mixed with water as a solvent at a ratio of approximately 20% for 10 minutes, followed by a filtration process, and then the pH of the solution was titrated. The concentrations of LiOH and Li2CO3 were measured to be 0.23% by weight and 0.11% by weight, respectively, based on 100% by weight of the total negative electrode active material.

[0102] [Example 2: Manufacturing of negative electrode active material] The anode active material was manufactured in the same manner as in Example 1, except that the etching time was extended to 30 minutes during the preparation of the preliminary anode active material.

[0103] [Example 3: Manufacturing of negative electrode active material] The negative electrode active material was manufactured in the same manner as in Example 1, except that the ratio of the preliminary silicon oxide powder to the Li metal powder was different during the production of the silicon oxide particles.

[0104] [Example 4: Manufacturing of negative electrode active material] The anode active material was manufactured in the same manner as in Example 3, except that the etching time was extended to 30 minutes during the preparation of the preliminary anode active material.

[0105] [Comparative Example 1: Manufacturing of Negative Electrode Active Material] The negative electrode active material was manufactured in the same manner as in Example 1, except that a surface coating layer composed of AlPO4 was formed instead of a carbon coating layer during the manufacturing of the negative electrode active material.

[0106] The surface coating layer, composed of AlPO4, is formed by mixing the manufactured pre-negative electrode active material particles with Al(PO3)3 and then heat-treating it at a temperature of 600°C.

[0107] ICP analysis revealed that the Al and P content of the manufactured negative electrode active material was 0.18% by weight and 0.54% by weight, respectively, based on 100% by weight of the total negative electrode active material.

[0108] [Comparative Example 2: Manufacturing of Negative Electrode Active Material] The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that the etching time was extended to 30 minutes during the manufacturing of the preliminary negative electrode active material.

[0109] [Comparative Example 3: Manufacturing of Negative Electrode Active Material] The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that the ratio of the pre-silicon oxide powder to the Li metal powder was different during the production of the silicon oxide particles.

[0110] [Comparative Example 4: Manufacturing of Negative Electrode Active Material] The anode active material was manufactured in the same manner as in Comparative Example 3, except that the etching time was extended to 30 minutes during the preparation of the preliminary anode active material.

[0111] [Comparative Example 5: Manufacturing of Negative Electrode Active Material] The negative electrode active material was manufactured in the same manner as in Example 1, except that a carbon coating layer was not formed during the production of the preliminary silicon oxide particles.

[0112] [Comparative Example 6: Manufacturing of Negative Electrode Active Material] The negative electrode active material was manufactured in the same manner as in Comparative Example 1, except that a carbon coating layer was not formed during the production of the preliminary silicon oxide particles.

[0113] [Comparative Example 7: Manufacturing of Negative Electrode Active Material] The negative electrode active material was manufactured in the same manner as in Example 1, except that a carbon coating layer was not formed during the manufacturing of the negative electrode active material.

[0114] [Comparative Example 8: Manufacturing of Negative Electrode Active Material] The anode active material was manufactured in the same manner as in Example 1, except that the etching process, which is a manufacturing step for the preliminary anode active material, was omitted.

[0115] [Comparative Example 9: Manufacturing of Negative Electrode Active Material] The anode active material was manufactured in the same manner as in Comparative Example 1, except that the etching process, which is part of the manufacturing process for the preliminary anode active material, was omitted.

[0116] [Table 1]

[0117] <Measurement of slurry viscosity and gas generation amount> [Examples 1A-4A] The negative electrode slurry of Example 1A was prepared by mixing the negative electrode active material produced in Example 1 with artificial graphite in a weight ratio of 2:8, carbon black as a conductive material, carboxymethylcellulose (CMC) as a binder, and styrene-butadiene rubber (SBR) with water as a solvent in a weight ratio of negative electrode active material:conductive material:binder (CMC):binder (styrene-butadiene rubber) = 96.5:1:1:1.5.

[0118] The negative electrode slurries of Examples 2A to 4A were manufactured in the same manner as in Example 1A, except that the negative electrode active materials of Examples 2 to 4 were used as the negative electrode active materials.

[0119] The manufactured slurry was stored at room temperature for two days, and then measured at 23°C using a rheometer (TA Corporation, HR20). The relative viscosity is shown in Table 2 below, with the 1Hz viscosity value of Example 1 as the reference.

[0120] Gas generation can be confirmed by placing 20g of the slurry mentioned above into a pouch, sealing it, storing it in a 40°C chamber for 7 days, and then measuring the volume of the pouch after it has changed. At this time, the amount of gas generated can be confirmed by placing the sealed pouch in water and measuring the volume of the water after it has changed. The amount of gas generated is shown in Table 2 below.

[0121] [Comparative Examples 1A to 9A] The negative electrode slurries of Comparative Examples 1A to 9A were manufactured in the same manner as in Example 1A, except that the negative electrode active materials of Comparative Examples 1 to 9 were used as the negative electrode active materials.

[0122] [Table 2]

[0123] In Comparative Examples 1A to 9A, it was confirmed that Li doping resulted in a decrease in viscosity in the Li-containing slurry, leading to the generation of gas.

[0124] On the other hand, Examples 1A to 4A are slurries in which silicon oxide particles contain Li, a first coating layer which is a carbon layer, and a second coating layer which is a carbon layer provided on the surface of the silicon oxide particles, and there is no irreversible substance formed by silicon oxide and Li between the first coating layer and the second coating layer, or LiOH and Li2CO3 are contained in amounts of less than 1% by weight each based on 100% by weight of the total negative electrode active material. Compared to Comparative Examples 1A to 9A, it was confirmed that the viscosity was higher and the problem of gas generation was reduced.

[0125] <Manufacturing of negative electrodes and lithium secondary batteries> [Examples 1B-4B] A uniform negative electrode slurry was prepared by mixing the negative electrode active material produced in Example 1 with carbon black as a conductive material and Li-PAA as a binder in a weight ratio of 80:10:10 with water (H2O) as a solvent. The prepared negative electrode slurry was coated onto one surface of a copper current collector, dried and rolled, and then punched to a predetermined size to produce a negative electrode.

[0126] Li metal was used as the counter electrode, and a polyolefin separator was interposed between the negative electrode and the Li metal. Then, an electrolyte in which 1M LiPF6 was dissolved in a solvent prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 30:70 was injected to produce the negative electrode and coin-type half-cell of Example 1B.

[0127] The negative electrodes and coin-type half-cells of Examples 2B to 4B were manufactured in the same manner as in Example 1B, except that the negative electrode active materials of Examples 2 to 4 were used as the negative electrode active materials.

[0128] [Comparative Examples 1B to 9B] The negative electrodes and coin-type half-cells of Comparative Examples 1B to 9B were manufactured in the same manner as in Example 1B, except that the negative electrode active materials of Comparative Examples 1 to 9 were used as the negative electrode active materials.

[0129] <Evaluation of discharge capacity, initial efficiency, and cycle characteristics of secondary batteries> The secondary batteries of Examples 1B to 4B and Comparative Examples 1B to 9B were subjected to charging and discharging, and their discharge capacity, initial efficiency, and cycle characteristics were evaluated and are shown in Table 3 below.

[0130] The batteries manufactured in Examples 1B-4B and Comparative Examples 1B-9B were charged at 25°C with a constant current (CC) of 0.1C until they reached 5mV. Then, they were charged with a constant voltage (CV) until the charging current reached 0.005C (cut-off current) for the first charge. After leaving them for 20 minutes, they were discharged with a constant current (CC) of 0.1C until they reached 1.5V, and the initial efficiency was confirmed.

[0131] Subsequently, the capacity retention rate was measured and the cycle characteristics were evaluated by repeatedly charging and discharging at 0.5C for up to 40 cycles.

[0132] The initial efficiency (%) was derived from the results of one charge-discharge cycle using the following formula.

[0133] Initial efficiency (%) = {Discharge capacity of negative electrode active material (mAh / g) / Charge capacity of negative electrode active material (mAh / g)} × 100

[0134] The capacity retention rate was derived using the following formula.

[0135] Capacity retention rate (%) = (40 discharge capacity / 1 discharge capacity) × 100

[0136] [Table 3]

[0137] Examples 1B to 4B describe a battery whose initial efficiency and lifespan can be improved by ensuring that the silicon oxide particles contain Li, and that the silicon oxide particles are provided with a first coating layer which is a carbon layer and a second coating layer which is a carbon layer on the surface of the silicon oxide particles, and that there is no irreversible substance formed by silicon oxide and Li between the first coating layer and the second coating layer, or that LiOH and Li2CO3 are contained in amounts of less than 1% by weight each, based on 100% by weight of the total negative electrode active material.

[0138] On the other hand, in the case of Comparative Examples 1B to 4B, the second coating layer contained an aluminum phosphate layer, and when compared to Examples 1B to 4B, it was confirmed that it could not contribute to the stabilization of the inside of the particles, resulting in a decrease in discharge capacity, initial efficiency, and capacity retention rate.

[0139] In the case of Comparative Examples 5B and 6B, since the carbon layer, which is the first coating layer, is not included, and Li is directly doped into the silicon oxide particles and etching is performed, it was confirmed that the physical properties as a negative electrode material are impaired, the battery performance deteriorates, and the discharge capacity, initial efficiency, and capacity retention rate decrease.

[0140] In the case of Comparative Example 7B, the absence of a carbon layer, which is the second coating layer, prevented it from contributing to the stabilization of the inside of the particles, and it was confirmed that the discharge capacity, initial efficiency, and capacity retention rate decreased.

[0141] In Comparative Examples 8B and 9B, it was confirmed that the presence of an irreversible substance formed of silicon oxide and Li between the first and second coating layers resulted in problems of reduced viscosity of the negative electrode slurry and gas generation, leading to a decrease in discharge capacity, initial efficiency, and capacity retention rate.

Claims

1. A method for producing a negative electrode active material containing silicon oxide particles, The silicon oxide particles contain Li and include a first coating layer and a second coating layer provided on the surface of the silicon oxide particles, the first coating layer and the second coating layer each being a carbon layer. Either there is no lithium silicate formed of silicon oxide and Li between the first coating layer and the second coating layer, or LiOH and Li₂CO₃ are included in amounts of less than 1% by weight each, based on 100% by weight of the total negative electrode active material. The Li content is 1% by weight or more and 15% by weight or less, based on 100% by weight of the total negative electrode active material. Steps include forming a first coating layer, which is a carbon layer, on silicon oxide particles; A step of doping silicon oxide particles on which the first coating layer is formed with Li metal powder; A step of removing lithium silicate formed by silicon oxide and Li on silicon oxide particles on which the first coating layer is formed by etching; and The step of forming a second coating layer, which is a carbon layer, on silicon oxide particles on which the etched first coating layer is formed. A method for producing a negative electrode active material, including the above.

2. The method for producing a negative electrode active material according to claim 1, wherein the etching is performed using NaOH, HF, or KOH.

Images