Anode active material, anode, and lithium secondary battery

Abstract

The present invention relates to a negative electrode active material including a core containing a silicon carbon composite; an oxide layer containing silicon oxide and provided on at least a portion of the core, wherein 50% or more of the oxide layer has a thickness exceeding 5 nm; and a carbon layer provided on at least a portion of the oxide layer, as well as to a negative electrode and a lithium secondary battery including the same.

Description

[Technical Field]

[0001] This specification claims the benefit of Korean Patent Application No. 10-2023-0098692 filed with the Korean Intellectual Property Office on July 28, 2023, and Korean Patent Application No. 10-2024-0098727 filed with the Korean Intellectual Property Office on July 25, 2024, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present application relates to a negative electrode active material, a negative electrode, and a lithium secondary battery. [Background technology]

[0003] In recent years, with the rapid spread of battery-powered electronic devices, such as mobile phones, laptops, and electric vehicles, as well as power tools and vacuum cleaners, the demand for secondary batteries that are small, lightweight, and have relatively high capacity and / or high output has been rapidly increasing. In particular, lithium secondary batteries, which are lightweight and have high energy density, have attracted attention as power sources for electronic devices. As a result, active research and development efforts are being made to improve the performance of lithium secondary batteries.

[0004] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, an electrolyte, an organic solvent, etc. The positive electrode and the negative electrode may each have an active material layer formed on a current collector, the active material layer including a positive electrode active material and a negative electrode active material. Generally, the positive electrode uses a lithium-containing metal oxide such as LiCoO2 or LiMn2O4 as the positive electrode active material, and the negative electrode uses a lithium-free carbon-based active material or a silicon-based active material as the negative electrode active material.

[0005] Silicon-based negative electrode active materials have attracted attention due to their higher capacity and superior fast charging characteristics compared to carbon-based active materials. However, silicon-based active materials have the disadvantage of low initial efficiency due to their large irreversible capacity caused by large volume expansion / contraction during charging / discharging.

[0006] Therefore, there is a need to develop a negative electrode active material that can improve the performance of lithium secondary batteries. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention relates to a negative electrode active material capable of improving the life characteristics of a lithium secondary battery, a negative electrode including the negative electrode active material, and a secondary battery including the same. [Means for solving the problem]

[0008] One embodiment of the present invention provides a negative electrode active material comprising: a core comprising a silicon carbon composite; an oxide layer comprising a silicon oxide and disposed on at least a portion of the core, wherein 50% or more of the oxide layer has a thickness exceeding 5 nm; and a carbon layer disposed on at least a portion of the oxide layer.

[0009] According to one embodiment of the present invention, the Si:C element ratio on the surface of the negative electrode active material of the above-mentioned embodiment is 1:1 to 1:4.

[0010] According to one embodiment of the present invention, the Si:C element ratio in the entire negative electrode active material of the above-mentioned embodiment is 0.9:1.1 to 1.1:0.9.

[0011] According to one embodiment of the present invention, the silicon oxide of the oxide layer in the above-described embodiment is SiOx (x is 0.1 or more and less than 2).

[0012] According to one embodiment of the present invention, the negative electrode active material of the above-described embodiment contains silicon crystal grains having a grain size of 8 μm or less.

[0013] One embodiment of the present invention provides a negative electrode comprising a negative electrode active material according to the above-described embodiment, a conductive material, and a binder.

[0014] One embodiment of the present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, and a separator according to the above-described embodiment.

[0015] One embodiment of the present invention provides a battery module including the lithium secondary battery according to the above-described embodiment.

[0016] One embodiment of the present invention provides a battery pack including the lithium secondary battery according to the above-described embodiment.

[0017] One embodiment of the present invention provides a battery pack including a battery module according to the above-described embodiment. [Effects of the Invention]

[0018] According to an embodiment of the present invention, by using a silicon carbon composite as the core material of a negative electrode active material, high capacity and efficiency can be achieved, and the presence of an oxide layer and a carbon layer can improve the battery's life characteristics and prevent problems such as gas generation. Specifically, the oxide layer and the carbon layer can prevent a decrease in life characteristics due to the high reactivity of silicon even when the silicon crystal grains are small, and can prevent the problem of gas generation when silicon comes into contact with water in an aqueous process. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in more detail below to facilitate understanding of the present invention. The present invention may be realized in various different forms and is not limited to the embodiments described herein. In this regard, the terms and words used in the specification and claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted in a manner that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best describe their invention.

[0020] It should be understood that in this specification, terms such as "comprises," "provides," or "has" specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0021] Furthermore, when a part such as a layer exists "on" or "above" another part, it does not only mean that it is "directly above" that part, but also includes the case where there is another part between them. In contrast, when a part exists "directly above" another part, it means that there is no other part between them. Furthermore, when a part exists "on" or "above" a reference part, it does not necessarily mean that it is located above or below the reference part, and is not necessarily located "above" or "above" the direction opposite to gravity.

[0022] The terms and words used in this specification should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0023] As used herein, the singular expression of a term includes the plural expression unless the context clearly indicates otherwise.

[0024] In this specification, the "average particle size (D50)" can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve of the particles. The average particle size (D50) can be measured, for example, using a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0025] The average particle size (D50) can be measured using a Microtrac device (manufacturer: Microtrac, model name: S3500) with water and Triton-X100 dispersant. Specifically, the average particle size (D50) of the positive electrode active material can be measured at a refractive index of 1.5 to 1.7, and that of the negative electrode active material can be measured at a refractive index of 1.97 or 2.42. For example, the particles can be dispersed in a dispersion medium, introduced into a commercially available laser diffraction particle size analyzer, and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. A volume cumulative particle size distribution graph can then be obtained, and the particle size corresponding to 50% of the volume cumulative amount can be determined.

[0026] In one embodiment of the present application, the crystal grain size can be calculated from the FWHM (Full Width at Half Maximum) value by XRD analysis. Specifically, the crystal grain size can be measured using the FWHM obtained by XRD analysis and the Debey-Scherrer equation shown in Equation 1-1 below.

[0027] [Formula 1-1] FWHM=(Kλ) / (LCOSθ)

[0028] In the formula 1-1, L is the crystal grain size, K is a constant, θ is the Bragg angle, and λ is the wavelength of the X-ray.

[0029] Furthermore, the crystal grains have various shapes and can be measured three-dimensionally. Generally, the crystal grain size can be measured by commonly used circle method and diameter measurement method, but is not limited thereto.

[0030] The diameter measurement method involves drawing 5 to 10 equilibrium lines, each with a length of L mm, on a micrograph of the target particle, counting the number of crystal grains z on the lines, and averaging them. Only those grains that fit completely are counted, and those that do not fit are excluded. If the number of lines is P and the magnification is V, the average grain size can be calculated using the following formula 1-2.

[0031] [Formula 1-2] Dm=(L*P*10 3 ) / (zV)(μm)

[0032] The circle method is a method in which a circle of a specified diameter is drawn on a micrograph of the target particle, and then the average area of ​​the crystal grains is calculated from the number of crystal grains that fit within the circle and the number of crystal grains that cross the boundary line, and can be calculated using the following formula 1-3.

[0033] [Formula 1-3] Fm=(Fk*10 6 ) / ((0.67n+z)V 2 )(μm 2 )

[0034] In the above formulas 1-3, Fm is the average particle area, Fk is the measured area on the photograph, z is the number of particles that fit within the circle, n is the number of particles that span the arc, and V is the magnification of the microscope.

[0035] Preferred embodiments of the present invention will be described in detail below. However, the embodiments of the present invention may be modified in various ways, and the scope of the present invention is not limited to the embodiments described below.

[0036] <Negative electrode active material> One embodiment of the present invention provides an anode active material comprising: a core comprising a silicon carbon composite; an oxide layer comprising silicon oxide and disposed on at least a portion of the core, the oxide layer having a thickness exceeding 5 nm and covering at least 50% of the oxide layer; and a carbon layer disposed on at least a portion of the oxide layer, where "at least 50% of the oxide layer" means at least 50% of the area covered by the oxide layer.

[0037] According to one embodiment, the silicon carbon composite contained in the core may be a Si / C based active material.

[0038] In this specification, the silicon carbon composite is a composite of Si and C and is distinguished from silicon carbide, which is represented as SiC. Silicon carbide does not electrochemically react with lithium and all performances, including lifespan, can be measured as zero.

[0039] The silicon carbon composite may be a composite of silicon and graphite, or may have a structure in which a core of the composite of silicon and graphite is surrounded by graphene or amorphous carbon, etc. The silicon in the silicon carbon composite may be silicon nanoparticles.

[0040] According to one embodiment, the silicon carbon composite comprises porous carbon-based particles and silicon particles located on the surface or in the internal pores of the porous carbon-based particles.

[0041] According to one embodiment, the silicon carbon composite is produced by the BET method to a surface area of ​​0.5 m 2 / g~10m 2 / g, with a pore volume of 0.005 cm 3 / g~0.03cm 3 The silicon carbon composite may have a pore volume of 0.005 cm3 measured by mercury penetration spectroscopy, and the pore size measured by BET method may be 10 nm to 20 nm. 3 / g~0.03cm 3 / g.

[0042] According to one embodiment, the silicon carbon composite may have a D90 particle size of 15 μm to 25 μm, a D50 particle size of 2 μm to 10 μm, and a D10 particle size of 0.1 μm to 1 μm.

[0043] The silicon particles formed on the surface and in the internal pores of the carbon-based particles may be silicon nanoparticles, and may be crystalline, quasi-crystalline, amorphous, or a combination thereof.

[0044] According to one embodiment of the present invention, an oxide layer comprising silicon oxide is provided on at least a portion of the core, and 50% or more of the oxide layer has a thickness greater than 5 nm, 7 nm or more, 8 nm or more, 9 nm or more, or 10 nm or more.

[0045] According to one embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a portion of the core, and 50% or more of the oxide layer has a thickness of 10 nm or more.

[0046] According to one embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a portion of the core, and 50% or more of the oxide layer has a thickness of 100 nm or less, 80 nm or less, 50 nm or less, 40 nm or less, or 30 nm or less.

[0047] If the thickness of 50% or more, preferably 70% or more, and more preferably 90% or more of the oxide layer exceeds 5 nm, it is possible to prevent deterioration of life characteristics and gas generation due to contact with water during aqueous processes by blocking silicon exposure or reducing the reactivity of silicon.If the thickness of 50% or more of the oxide layer containing silicon oxide is 5 nm or less, internal silicon particles may be exposed to the outside, which may deteriorate aqueous processability.In this case, the state of the electrode may be poor, and the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery may be reduced.

[0048] According to this embodiment, the oxide layer includes silicon oxide.

[0049] By including silicon oxide in the oxide layer, contact between water and silicon is prevented during the production of the aqueous slurry, improving the condition of the electrode and improving the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0050] In particular, when the oxide layer contains silicon oxide and the thickness of more than 50% of the oxide layer exceeds 5 nm, the probability that most of the silicon is passivated by the oxide layer is high, and the aqueous processability is improved, so that the charge-discharge capacity, initial efficiency, and / or life characteristics of the secondary battery are improved.

[0051] According to one embodiment, the silicon oxide contains SiOx (0 < x ≤ 2). The SiOx (0 < x ≤ 2) may be in a form containing various oxidation states such as Si and SiO2. That is, the x corresponds to the number ratio of O to Si contained in the SiOx (0 < x ≤ 2). In other words, the SiOx (0 < x ≤ 2) is not in a single state but in a state where multiple oxidation states are mixed, and in XPS, while etching, the average value of x can be obtained by the content of oxygen (O).

[0052] According to an example, the silicon oxide is SiOx (0.1 ≤ x < 2). The x may be 1 or more and less than 2, for example, 1.5 or more and less than 2.

[0053] According to one embodiment, the silicon oxide is in an amorphous state. The SiOx (0 < x ≤ 2) may exist in the form of an island type or a thin film type layer, and is not limited thereto, and may exist in various forms.

[0054] According to one embodiment, the thickness of the oxide layer may be 200 nm or less, for example, 100 nm or less, 50 nm or less, 30 nm or less, or 20 nm or less. When the thickness of the oxide layer is excessively thin, the effect of blocking contact with water cannot be expected, and when it is excessively thick, the resistance increases and the discharge capacity is not exhibited. The content of the oxide layer may be 1 to 5 parts by weight based on 100 parts by weight of the negative electrode active material. Such a content range is advantageous for providing the effect of blocking contact with water by the oxide layer and providing an appropriate discharge capacity.

[0055] The negative electrode active material according to one embodiment includes a carbon layer provided on at least a portion of the oxide layer.

[0056] Specifically, the carbon layer, together with the oxide layer, can prevent the exposure of silicon or reduce the reactivity of silicon, thereby preventing deterioration of life characteristics and gas generation due to contact with water during aqueous processes. Furthermore, the carbon layer imparts conductivity to the negative electrode active material, thereby improving the initial efficiency, life characteristics, and battery capacity characteristics of the secondary battery.

[0057] In one embodiment, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0058] In one embodiment, the carbon layer may be an amorphous carbon layer, which can maintain the strength of the carbon layer appropriately and suppress the expansion of the silicon carbon composite.

[0059] The carbon layer may or may not further contain crystalline carbon.

[0060] The crystalline carbon may further improve the conductivity of the negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0061] The amorphous carbon can appropriately maintain the strength of the carbon layer and suppress expansion of the silicon carbon composite. The amorphous carbon may be a carbon-based material formed by using at least one carbide selected from the group consisting of tar, pitch, and other organic substances, or a hydrocarbon as a source in a chemical vapor deposition process.

[0062] The carbonized organic material may be a carbonized organic material selected from carbonized sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose, and combinations thereof.

[0063] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, hexane, or the like. The aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, phenanthrene, or the like.

[0064] In one embodiment, the carbon layer may be included in an amount of 0.1 to 50 parts by weight, 0.1 to 30 parts by weight, or 0.1 to 20 parts by weight, based on a total of 100 parts by weight of the negative electrode active material. More specifically, the carbon layer may be included in an amount of 0.5 to 15 parts by weight, or 1 to 10 parts by weight. When the amount is within the above range, the conductivity can be improved and a decrease in the capacity and efficiency of the negative electrode active material can be prevented.

[0065] In one embodiment, the thickness of the carbon layer may be 1 nm to 500 nm, specifically 5 nm to 300 nm, and more specifically 5 nm to 100 nm. When the thickness is within this range, the conductivity of the negative electrode active material is improved, volume change of the negative electrode active material is easily suppressed, and side reactions between the electrolyte and the negative electrode active material are suppressed, resulting in improved initial efficiency and / or lifespan of the battery.

[0066] Specifically, the carbon layer may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

[0067] The carbon layer is formed after the formation of the oxide layer, and therefore may be formed on the oxide layer containing silicon oxide or on the surface of the core where the oxide layer is not formed. Furthermore, the carbon layer may completely surround the oxide layer, or may be formed only on a portion of the oxide layer, so that both the oxide layer and the carbon layer may be observed on the surface of the negative electrode active material.

[0068] According to one embodiment of the present invention, the negative electrode active material may include silicon crystal grains having a particle size of 8 μm or less. The silicon crystal grains may be contained in a core containing the silicon carbon composite described above. The inclusion of silicon crystal grains having a particle size of 8 μm or less is advantageous not only for improving life performance, but also for preventing gas generation in aqueous processes due to the oxide layer and carbon layer described above. According to one example, the negative electrode active material may include silicon crystal grains having a particle size of 8 μm or less, e.g., 5 μm or less, 1,000 nm or less, or on the order of several tens of nanometers.

[0069] According to one embodiment of the present invention, the Si:C elemental ratio on the surface of the negative electrode active material of the above-described embodiment is 1:1 to 1:4, for example, 1:2 to 1:4, or 1:3 to 1:4. This means that the carbon content is higher in the surface region because the negative electrode active material layer has a carbon layer on the surface. This elemental ratio indicates that the surface of the negative electrode active material is well coated with carbon, which can be measured by XPS. The elemental ratio can be adjusted by adjusting the amount of carbonized material during the carbon layer introduction process or by changing the heat treatment time.

[0070] According to one embodiment of the present invention, the Si:C element ratio in the entire negative electrode active material of the above-described embodiment is 0.9:1.1 to 1.1:0.9, for example, 1:0.95 to 1:1.1. A higher Si content is advantageous for increasing capacity, but can lead to particle growth due to Si being deposited on the surface rather than within the carbon pores, which can adversely affect lifespan, processability, and other aspects. The above-described Si:C element ratio range is advantageous for satisfying all of the requirements for capacity, lifespan, and processability. The negative electrode active material according to the above-described embodiment is primarily composed of three elements: Si, C, and O. The Si content can be calculated by subtracting the percentage of C from the total, while eliminating the influence of the remaining elements, by measuring C using a CS analyzer and O using an ONH analyzer.

[0071] According to one embodiment, the average particle size (D50) of the negative electrode active material may be 0.1 μm to 30 μm, specifically 1 μm to 20 μm, more specifically 1 μm to 10 μm, or 2 μm to 10 μm. When the average particle size is within this range, the active material is structurally stable during charge and discharge, the problem of an excessively large particle size resulting in an increased level of volume expansion / contraction is prevented, and the problem of an excessively small particle size resulting in a decrease in initial efficiency is prevented.

[0072] According to one embodiment, the BET specific surface area of ​​the negative electrode active material is 20 m 2 / g or less, e.g., 10m 2 For example, the BET specific surface area of ​​the negative electrode active material is preferably 0.11 m / g or less. 2 / g or more 10m 2 / g or less, e.g., 4m 2 / g~6m 2 The specific surface area may be 1 / g. The specific surface area can be measured by the Brunauer-Emmett-Teller (BET) method. For example, it can be measured by the BET 6-point method using a porosimetry analyzer (Belsorp-II mini, Bell Japan Inc.) and a nitrogen gas adsorption / flow method.

[0073] <Method of manufacturing negative electrode active material> The present invention provides a method for manufacturing a negative electrode active material, specifically, a method for manufacturing a negative electrode active material according to the above-described embodiment.

[0074] One embodiment of the present invention provides a method for producing an anode active material of the present invention, comprising the steps of: forming a core containing a silicon carbon composite; forming an oxide layer comprising a silicon oxide on at least a portion of the core; and forming a carbon layer on at least a portion of the oxide layer.

[0075] According to one embodiment, the step of forming the core comprising the silicon carbon composite may be performed by a method including the steps of: etching carbon-based particles having internal pores to expand the internal pores of the carbon-based particles; and forming silicon particles on the surfaces and in the internal pores of the carbon-based particles whose internal pores have been expanded.

[0076] The step of expanding the internal pores of the carbon-based particles may be performed in a nitrogen (N2), oxygen (O2), or air atmosphere, and the flow rate of the oxygen (O2) or oxygen-containing air may be controlled to 0.1 L / min to 10 L / min.

[0077] The step of expanding the internal pores of the carbon-based particles may be performed at a temperature ranging from 400° C. to 1200° C. for 30 minutes to 4 hours.

[0078] The pore characteristics of the resulting porous carbon-based particles may vary depending on the conditions for expanding the internal pores of the carbon-based particles.

[0079] An etching agent, such as a basic material such as KOH, may be used to expand the internal pores of the carbon-based particles. For example, the carbon-based particles and KOH may be mixed in a weight ratio of 1:1 to 1:5 to expand the internal pores of the carbon-based particles.

[0080] The step of forming the silicon particles may be performed using a chemical vapor deposition (CVD) method, in which silicon nanoparticles are deposited on the surfaces and / or in the pores of the carbon-based particles with expanded pores, forming a silicon coating layer in the form of a film, islands, or a mixture thereof.

[0081] The step of forming the silicon particles may be performed by flowing SiH4 / H2 gas through the carbon-based particles at 500°C to 900°C using a chemical vapor deposition (CVD) apparatus to form a silicon carbon composite.

[0082] The step of forming an oxidation layer including silicon oxide on at least a portion of the core may be performed by partially or entirely oxidizing the surface of the core including the silicon carbon composite.

[0083] According to one embodiment of the present invention, the step of forming an oxide layer containing silicon oxide on at least a portion of the core may include a heat treatment step performed in an atmosphere containing oxygen.

[0084] Specifically, the heat treatment may be performed at a temperature of 500 to 700° C., for example, 600 to 700° C., to form an oxide layer.

[0085] If the heat treatment process is carried out at a temperature exceeding 700°C, silicon carbon composite (SiC) will grow, which is undesirable as it will reduce the capacity and efficiency of the material.

[0086] The oxygen-containing atmosphere may contain oxygen in an amount of more than 0 vol% and 10 vol% or less, for example, 3 vol% to 7 vol%. The oxygen-containing atmosphere may contain an inert gas such as argon in addition to oxygen. The heat treatment time is not particularly limited, but may be, for example, 1 hour to 12 hours, 1 hour to 8 hours, or 2 hours to 5 hours.

[0087] The thickness of the oxide layer may be adjusted by the heat treatment temperature, time, etc.

[0088] The step of forming the carbon layer on at least a portion of the oxide layer may be performed by chemical vapor deposition (CVD) using a carbon-based material, for example, a hydrocarbon gas, or by carbonizing a material that serves as a carbon source.

[0089] Specifically, the silicon carbon composite having an oxide layer containing silicon oxide formed thereon may be placed in a reactor, and then subjected to chemical vapor deposition (CVD) with a hydrocarbon gas at 600°C to 700°C. The hydrocarbon gas may be at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene, and may be heat-treated at 600°C to 700°C.

[0090] <Negative electrode> One embodiment of the present invention provides a negative electrode comprising a negative electrode active material according to the above-described embodiment, a conductive material, and a binder.

[0091] 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 include the negative electrode active material. Furthermore, the negative electrode active material layer may further include a binder and / or a conductive material.

[0092] The negative electrode active material layer may be formed by applying a negative electrode slurry containing a negative electrode active material, a binder, and / or a conductive material to at least one surface of a negative electrode current collector, followed by drying and rolling.

[0093] The negative electrode slurry contains the negative electrode active material, a binder, and / or a conductive material.

[0094] The negative electrode slurry may further include an additional negative electrode active material.

[0095] The additional negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, lithium titanium oxide, and lithium vanadium oxide (0<β<2); or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. A mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. The carbon material may be either low-crystalline carbon or high-crystalline carbon. Typical low-crystalline carbons are soft carbon and hard carbon, while typical high-crystalline carbons are amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbons such as petroleum or coal tar pitch-derived cokes.

[0096] The additional negative electrode active material may be a carbon-based negative electrode active material.

[0097] In one embodiment of the present invention, the weight ratio of the negative electrode active material to the additional negative electrode active material contained in the negative electrode slurry may be 10:90 to 90:10, specifically 10:90 to 50:50.

[0098] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, the current collector may be made of copper, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that easily adsorbs carbon, such as copper or 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 to this.

[0099] The binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, 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 hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0100] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. For example, graphite such as natural graphite and 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; metal powders such as fluorocarbon, 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.

[0101] The negative electrode slurry may include a solvent for forming a negative electrode slurry. Specifically, the solvent for forming a negative electrode slurry may include at least one solvent selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, specifically distilled water, in order to facilitate dispersion of components.

[0102] <Secondary battery> One embodiment of the present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, and a separator according to the above-described embodiment.

[0103] 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 including the positive electrode active material.

[0104] The positive electrode current collector in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used. The positive electrode current collector may typically have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0105] 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; a lithium iron oxide such as LiFe3O4; 1+c1 Mn 2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦c2≦0.3 is satisfied); 2-c3 M c3 Examples of the lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (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); and LiMn2O4, in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may be Li metal.

[0106] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the above-described positive electrode active material.

[0107] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be any material that has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples 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 black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination of two or more.

[0108] The positive electrode binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used singly or in combination.

[0109] The separator separates the negative electrode and positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitation. It is particularly preferable that the separator exhibits low resistance to ion migration and has excellent electrolyte humidifying ability. Specifically, porous polymer films, such as those made from polyolefin polymers 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. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric material may be used, and it may be selectively used as a single-layer or multi-layer structure.

[0110] The lithium secondary battery may further include an electrolyte, which may be used in manufacturing a lithium secondary battery, such as, but not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte.

[0111] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0112] Examples of the non-aqueous organic solvent that can be used include 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, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0113] In particular, among the carbonate organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents, because they have a high dielectric constant and dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be prepared, and therefore such cyclic carbonates can be more preferably used.

[0114] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be 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 - One or more selected from the group consisting of:

[0115] In addition to the constituent components of the electrolyte, the electrolyte may further include one or more additives, such as a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, in order to improve the life characteristics of the battery, suppress a decrease in battery capacity, and improve the discharge capacity of the battery.

[0116] According to another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, a battery pack including the battery module, or a battery pack including the secondary battery. The battery module and battery pack include the secondary battery having high capacity, high rate characteristics, and high cycle characteristics, and 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 power storage systems. [Example]

[0117] Hereinafter, the present specification will be described in detail with reference to examples. However, the examples according to the present specification may be modified into various other forms, and the scope of the present application should not be interpreted as being limited to the examples described below. The examples of the present application are provided to more completely explain the present specification to those skilled in the art.

[0118] Example 1 Spherical particles were synthesized by placing a 0.5M sucrose solution in an autoclave and reacting at 180°C for 24 hours. After the reaction, the resulting carbon-based particles were washed two to three times with ethanol. The carbon-based particles, which had been dried at 100°C for over 12 hours, were mixed with KOH in a 1:3 ratio by weight and heated at 800°C for two hours in a nitrogen atmosphere to expand the pores. After washing with distilled water, the particles were dried at 100°C for over 12 hours. The carbon-based particles with the oxide layer were placed in the hot zone of a CVD system and subjected to a flow of SiH4 / H2 = 5 / 95 gas at a flow rate of 50 mL / min at 600°C for two hours to produce a silicon / carbon composite. The oxide layer was then formed by heat treatment at 700°C for three hours in an O2 / Ar = 5 / 95 atmosphere. The silicon / carbon composite was placed in the hot zone of a CVD apparatus, and methane was blown into the hot zone at 700°C using Ar as a carrier gas for a reaction for 1 hour to form a carbon layer on the surface, producing an anode active material including an oxide layer and a carbon layer on the surface of the silicon / carbon composite.

[0119] Example 2 A negative electrode active material was prepared in the same manner as in Example 1, except that the ratio of carbonaceous particles to KOH was changed to 1:5 and the heat treatment temperature was changed to 900°C.

[0120] Example 3 A negative electrode active material was produced in the same manner as in Example 1, except that the conditions for forming the oxide layer were changed to an atmosphere of O2 / Ar=10 / 90, 800°C, and 3 hours.

[0121] Example 4 A negative electrode active material was prepared in the same manner as in Example 1, except that the heat treatment time for forming the carbon layer was increased to 3 hours.

[0122] Example 5 A negative electrode active material was prepared in the same manner as in Example 1, except that SiH4 / H2=5 / 95 gas was flowed at a flow rate of 50 ml / min at 600° C. for 1 hour.

[0123] Comparative Example 1 A negative electrode active material was prepared in the same manner as in Example 1, except that the step of forming the oxide layer was omitted.

[0124] Comparative Example 2 A negative electrode active material was prepared in the same manner as in Example 1, except that the carbon layer formation step was omitted.

[0125] Comparative Example 3 A negative electrode active material was prepared in the same manner as in Example 1, except that the oxide layer was formed after the carbon layer was formed.

[0126] Comparative Example 4 A negative active material was prepared in the same manner as in Example 1, except that an Al2O3 coating layer with a thickness of about 1 nm was formed as the oxide layer.

[0127] The particle size (D50) of the negative electrode active material and the particle size of the silicon crystal grains were analyzed by laser diffraction particle size analysis using a Microtrac S3500 device.

[0128] The specific surface area of ​​the negative electrode active material was measured by the BET 6-point method using a porosimetry analyzer (Belsorp-II mini, Bell Japan Inc.) via a nitrogen gas adsorption flow method.

[0129] The thickness of the oxide layer and the surface Si:C ratio were measured by XPS. The XPS instrument used was a Nexsa 2 manufactured by Thermo Fisher Scientific. Since the XPS instrument only analyzes samples within an area of ​​the x-ray spot size (400 μm), measurements were taken at approximately 2 to 3 points, and the sample surface was etched while the measurements were taken. In this case, if the element ratio differed depending on the position, the maximum and minimum values ​​measured according to the deviation at each position were used as the upper and lower limits of the range.

[0130] The total Si:C ratio and the composition of the oxide layer of the negative active material were determined by measuring C using a CS analyzer and O using an ONH analyzer, and then subtracting the respective ratios from the total to calculate the Si content.

[0131] [Table 1]

[0132] <Experimental example: Evaluation of discharge capacity, initial efficiency, and life (capacity retention rate) characteristics> Negative electrodes and batteries were manufactured using the negative electrode active materials of the examples and comparative examples, respectively.

[0133] The negative electrode active material, carbon black as a conductive material, and PAA (polyacrylic acid) as a binder were mixed in a weight ratio of 80:10:10 to prepare a mixture. 7.8 g of distilled water was then added to 5 g of the mixture and stirred to prepare a negative electrode slurry. The negative electrode slurry was applied to a copper (Cu) metal thin film as a negative electrode current collector with a thickness of 20 μm and dried. The circulating air temperature was 60°C. The mixture was then rolled and dried in a vacuum oven at 130°C for 12 hours to prepare a negative electrode.

[0134] The manufactured negative electrode was placed in a 1.7671 cm 2A lithium (Li) metal thin film cut into a circular shape was used as the positive electrode. A porous polyethylene separator was placed between the positive electrode and the negative electrode, and an electrolyte solution prepared by dissolving 0.5 parts by weight of vinylene carbonate in a mixed solution of ethyl methyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 7:3 and dissolving LiPF6 at a concentration of 1M was injected to prepare a lithium coin half-cell.

[0135] The manufactured batteries were charged and discharged to evaluate the discharge capacity, initial efficiency, and capacity retention rate, and the results are shown in Table 2 below.

[0136] The first and second cycles were charged and discharged at 0.1 C, and the third to 49th cycles were charged and discharged at 0.5 C. The 50th cycle was completed in a charged state (with lithium in the anode). Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off) Discharge condition: CC (constant current) condition 1.5V

[0137] The discharge capacity (mAh / g) and initial efficiency (%) were calculated from the results of one charge / discharge. Specifically, the initial efficiency (%) was calculated as follows: Initial efficiency (%) = (single discharge capacity / single charge capacity) x 100

[0138] The capacity retention rate was calculated as follows. Capacity retention rate (%) = (49 discharge capacity / 1 discharge capacity) × 100

[0139] <Experimental example: Evaluation of processability (Shear viscosity) characteristics> As part of the processability evaluation, the amount of change in shear viscosity at a shear rate of 1 Hz of a slurry prepared by mixing graphite, the negative electrode active material, carbon black, CMC, and PAA in a weight ratio of 77:20:1:1:1 was measured, and the results are shown in Table 2. Specifically, the amount of change in shear viscosity (%) was calculated using the following formula:

[0140] Change in shear viscosity (%) = ((shear viscosity of slurry after 48 hours - shear viscosity of slurry immediately after mixing) / shear viscosity of slurry immediately after mixing) × 100

[0141] Gas generation time: 20 g of slurry was placed in a 10 x 15 cm aluminum pouch and vacuum sealed. The volume change was measured using Archimedes' principle, and the time when the volume change reached 4 mL or more was defined as the time when gas generation occurred.

[0142] The properties evaluated as described above are shown in Table 2 below.

[0143] [Table 2]

[0144] In Examples 1 to 5, silicon-carbon composites with oxide and carbon layers of appropriate thicknesses were used as anode active materials, and the battery performance, including discharge capacity, initial efficiency, and capacity retention, was superior to that of Comparative Example 1, which did not undergo an oxide layer formation process and only had a fine oxide layer formed by natural oxidation; Comparative Example 2, which did not have a carbon layer; Comparative Example 3, which had the oxide and carbon layers formed in a different order; and Comparative Example 4, which had a 1-nm Al2O3 coating layer. Furthermore, in Examples 1 to 5, the presence of the oxide and carbon layers controlled gas generation during processing, resulting in extremely low shear viscosity change and significantly delayed gas generation. In other words, the Examples confirmed that not only battery performance but also processability could be significantly improved by preventing gas generation.

Claims

1. a core comprising a silicon carbon composite; an oxide layer comprising silicon oxide disposed on at least a portion of the core, wherein 50% or more of the oxide layer has a thickness greater than 5 nm; and a carbon layer provided on at least a portion of the oxide layer; A negative electrode active material comprising:

2. 2. The negative electrode active material according to claim 1, wherein the Si:C atomic ratio on the surface of the negative electrode active material is 1:1 to 1:

4.

3. 2. The negative electrode active material according to claim 1, wherein the Si:C atomic ratio in the entire negative electrode active material is 0.9:1.1 to 1.1:0.

9.

4. 2. The negative electrode active material according to claim 1, wherein the silicon oxide of the oxide layer is SiOx (x is 0.1 or more and less than 2).

5. The negative electrode active material according to claim 1 , wherein the negative electrode active material contains silicon crystal grains having a grain size of 8 μm or less.

6. The negative electrode active material according to claim 1 , wherein the oxide layer has a thickness of 200 nm or less.

7. The negative electrode active material according to claim 1 , wherein 50% or more of the oxide layer has a thickness of 10 nm or more.

8. The negative electrode active material of claim 1 , wherein the silicon carbon composite comprises porous carbon particles and silicon particles located on the surfaces or in the internal pores of the porous carbon particles.

9. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 8, a conductive material, and a binder.

10. A lithium secondary battery comprising the negative electrode according to claim 9, a positive electrode, and a separator.

11. A battery module comprising the lithium secondary battery according to claim 10.

12. A battery pack comprising the lithium secondary battery according to claim 10.

13. A battery pack comprising the battery module according to claim 11.

14. forming a core comprising a silicon carbon composite; forming an oxide layer at least partially on the core, the oxide layer comprising silicon oxide; and forming a carbon layer on at least a portion of the oxide layer; The method for producing the negative electrode active material according to any one of claims 1 to 8, comprising:

15. the step of forming an oxide layer including silicon oxide on at least a portion of the core includes a step of performing a heat treatment in an atmosphere including oxygen; The method for producing a negative electrode active material according to claim 14, wherein the heat treatment is performed at 600°C to 700°C.

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