Negative electrode active material, negative electrode, and lithium secondary battery

Abstract

The present invention relates to a negative electrode active material, a negative electrode comprising the same, and a lithium secondary battery, the negative electrode active material comprising: a core comprising a silicon carbon composite; an oxide layer disposed on at least a portion of the core and comprising a silicon oxide, and at least 50% of the oxide layer having a thickness greater than 5 nm; and a carbon layer disposed on at least a portion of the oxide layer.

Description

Technical Field

[0001] This application claims the priority and benefits 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, the entire contents of which are incorporated herein by reference.

[0002] This application relates to a negative electrode active material, a negative electrode, and a lithium secondary battery. Background Art

[0003] In recent years, with the rapid popularization of electronic devices using batteries, such as mobile phones, laptop computers, electric vehicles, power tools, and cleaners, the demand for small, lightweight, and relatively high-capacity and / or high-output secondary batteries is increasing rapidly. In particular, lithium secondary batteries have attracted much attention as a driving power source for electronic devices due to their light weight and high energy density. Therefore, research and development efforts have been actively made to improve the performance of lithium secondary batteries.

[0004] Generally, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, etc. In addition, a positive electrode and a negative electrode having active material layers each containing a positive electrode active material and a negative electrode active material can be formed on current collectors. Generally, for the positive electrode, lithium-containing metal oxides such as LiCoO2 and LiMn2O4 are used as the positive electrode active material, and for the negative electrode, a carbon-based active material or a silicon-based active material without lithium is used as the negative electrode active material.

[0005] Among negative electrode active materials, silicon-based active materials have attracted much attention because they have a higher capacity than carbon-based active materials and have excellent high-speed charging characteristics. However, the disadvantages of silicon-based active materials are that the degree of volume expansion / contraction during charge and discharge is high, and the irreversible capacity is high, resulting in low initial efficiency.

[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

[0007] Technical Problem

[0008] 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 negative electrode.

[0009] Technical Solution

[0010] An exemplary embodiment of the present invention provides a negative electrode active material, comprising: a core containing a silicon-carbon composite; an oxide layer provided on at least a part of the core and containing silicon oxide, with a thickness of more than 50% of the oxide layer being greater than 5 nm; and a carbon layer provided on at least a part of the oxide layer.

[0011] According to an exemplary embodiment of the present invention, the Si:C element ratio on the surface of the negative electrode active material of the above embodiment is from 1:1 to 1:4.

[0012] According to an exemplary embodiment of the present invention, the Si:C element ratio in the entire negative electrode active material of the above embodiment is from 0.9:1.1 to 1.1:0.9.

[0013] According to an exemplary embodiment of the present invention, the silicon oxide in the oxide layer of the above embodiment is SiO x (where x is 0.1 or more and less than 2).

[0014] According to an exemplary embodiment of the present invention, the negative electrode active material of the above embodiment contains silicon grains with a particle size of 8 μm or less.

[0015] An exemplary embodiment of the present invention provides a negative electrode, comprising the negative electrode active material, a conductive material, and a binder according to the above exemplary embodiment.

[0016] An exemplary embodiment of the present invention provides a lithium secondary battery, comprising the negative electrode, a positive electrode, and a separator according to the above exemplary embodiment.

[0017] An exemplary embodiment of the present invention provides a battery module, comprising the lithium secondary battery according to the above exemplary embodiment.

[0018] An exemplary embodiment of the present invention provides a battery pack, comprising the lithium secondary battery according to the above exemplary embodiment.

[0019] An exemplary embodiment of the present invention provides a battery pack, comprising the battery module according to the above exemplary embodiment.

[0020] Advantageous Effects

[0021] According to the exemplary embodiment of the present invention, by using a silicon-carbon composite as the core material of the negative electrode active material, high capacity and efficiency can be achieved. At the same time, due to the presence of the oxide layer and the carbon layer, the life characteristics of the battery are improved, and problems such as gas generation are prevented. Specifically, even when the silicon grains are small, the oxide layer and the carbon layer can prevent the reduction of life characteristics caused by the high reactivity of silicon, and can also prevent the problem of gas generation caused by the contact between silicon and water during the aqueous process. Detailed Implementation Modes

[0022] In the following, for a better understanding of the present invention, the present invention will be described in more detail. The present invention can be implemented in various different forms and is not limited to the exemplary implementation modes described herein. The terms or words used throughout the specification and claims should not be construed as limited to their ordinary or dictionary meanings, but should be construed as having meanings and concepts corresponding to the technical gist of the present invention based on the principle that the inventor can appropriately define the words or terms to best explain the present invention.

[0023] In addition, it should be understood that when used in this specification, the terms "comprising", "including" or "having" indicate the presence of the stated features, numbers, steps, components and / or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, components and / or combinations thereof.

[0024] In addition, it should be understood that when a part such as a layer is referred to as being "on" another part, the part can be "directly on" the other part, or there can also be an intermediate part. In contrast, when a part is referred to as being "directly on" another part, there is no intermediate part. In addition, when a part is referred to as being "above" or "on" a reference part, the part is located above or below the reference part and does not necessarily mean that the part is located "above" or "on" in the direction opposite to gravity.

[0025] It should be understood that the terms or words used throughout the specification should not be construed as limited to their ordinary or dictionary meanings, but should be construed as having meanings and concepts corresponding to the technical gist of the present invention based on the principle that the inventor can appropriately define the words or terms to best explain the present invention.

[0026] As used herein, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are also intended to include the plural forms.

[0027] In this specification, the "average particle size (D50)" can be defined as the particle size corresponding to 50% cumulative volume in the particle size distribution curve of the particles. The average particle size (D50) can be measured using, for example, the laser diffraction method. In the laser diffraction method, generally, the particle size ranging from the submicron region to several millimeters can be measured, and results with high reproducibility and high resolution can be obtained.

[0028] The measurement of the average particle size (D50) can be confirmed using water and Triton-X100 dispersant with a Microtrac device (manufacturer: Microtrac, model name: S3500). Specifically, the average particle size (D50) of the positive electrode active material can be measured within a refractive index range of 1.5 to 1.7, and the average particle size (D50) of the negative electrode active material can be measured under refractive index conditions of 1.97 or 2.42. For example, after dispersing the particles in a dispersion medium, the resulting dispersion is introduced into a commercially available laser diffraction particle size measuring device and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. Thereafter, a volume cumulative particle size distribution graph is obtained, and then the average particle size can be measured by obtaining the particle size corresponding to 50% of the volume cumulative amount.

[0029] In an exemplary embodiment of the present application, the grain size can be calculated as the value of the full width at half maximum (FWHM) by XRD analysis. Specifically, the grain size can be measured using the FWHM obtained by XRD analysis and the Debye-Scherrer formula shown in Equation 1-1 below.

[0030] [Equation 1-1]

[0031] FWHM = (Kλ) / (LCOSθ)

[0032] In Equation 1-1,

[0033] L is the grain size, K is a constant, θ is the Bragg angle, and λ refers to the wavelength of the X-ray.

[0034] Furthermore, the shapes of the grains are diverse and can be measured in three dimensions. Generally, the size of the grains can be measured by the commonly used circular method or diameter measurement method, but the present invention is not limited thereto.

[0035] In the diameter measurement method, the size of the grains can be measured by drawing 5 to 10 parallel lines each with a length of L mm on a micrograph of the target particles, counting the number of grains z on each line, and taking their average. In this case, only the grains completely contained within the line are counted, and the grains partially placed (straddling) on the line are excluded. When the number of lines is P and the magnification is V, the average particle size can be calculated by Equation 1-2 below.

[0036] [Equation 1-2]

[0037] Dm = (L×P×10 3 ) / (zV) (μm)

[0038] In addition, the circular method is a method in which a circle with a predetermined diameter is drawn on a micrograph of a target particle, and then the average area of the crystal grains is calculated from the number of crystal grains within the circle and the number of crystal grains across the boundary line, and the average area can be calculated by the following formula 1-3.

[0039] [Formula 1-3]

[0040] Fm = (Fk×10 6 ) / ((0.67n + z) V 2 ) (μm 2 )

[0041] In Formula 1-3, Fm is the average particle area, Fk is the measured area on the photograph, z is the number of particles in the circle, n is the number of particles across the circle, and V is the magnification of the microscope.

[0042] Hereinafter, preferred embodiments of the present invention will be described in detail. However, it should be understood that the exemplary embodiments of the present invention can be modified in various different forms, and the scope of the present invention is not limited to the exemplary embodiments described below.

[0043] <Negative electrode active material>

[0044] An exemplary embodiment of the present invention provides a negative electrode active material, comprising: a core containing a silicon-carbon composite; an oxide layer provided on at least a part of the core and containing silicon oxide, wherein the thickness of more than 50% of the oxide layer is greater than 5 nm; and a carbon layer provided on at least a part of the oxide layer. Here, more than 50% of the oxide layer means more than 50% of the area covered by the oxide layer.

[0045] According to an exemplary embodiment, the silicon-carbon composite contained in the core may be a Si / C-based active material.

[0046] In the present specification, the silicon-carbon composite is a composite of Si and C, and is different from silicon carbide represented as SiC. Silicon carbide does not undergo an electrochemical reaction with lithium, so all properties including lifespan can be measured as 0.

[0047] The silicon-carbon composite may be a composite of silicon, graphite, etc., and may form a structure in which a core composite of silicon, graphite, etc. is surrounded by graphene, amorphous carbon, etc. In the silicon-carbon composite, the silicon may be silicon nanoparticles.

[0048] According to an exemplary embodiment, the silicon-carbon composite includes porous carbon-based particles and silicon particles located on the surface or in the internal pores of the porous carbon-based particles.

[0049] According to an exemplary embodiment, the silicon-carbon composite may have a specific surface area of 0.5 to 10 m 2 / g, a pore volume of 0.005 to 0.03 cm 3 / g, and a pore size of 10 to 20 nm as measured by the BET method. The silicon-carbon composite may have a pore volume of 0.005 to 0.03 cm 3 / g as measured by mercury intrusion porosimetry (Hg porosimeter).

[0050] According to an exemplary 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.

[0051] 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, semi-crystalline, amorphous, or a combination thereof.

[0052] According to an exemplary embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a part of the core, and the thickness of more than 50% of the oxide layer is greater than 5 nm, 7 nm or more, 8 nm or more, 9 nm or more, or 10 nm or more.

[0053] According to an exemplary embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a part of the core, and the thickness of more than 50% of the oxide layer is 10 nm or more.

[0054] According to an exemplary embodiment of the present invention, an oxide layer containing silicon oxide is provided on at least a part of the core, and the thickness of more than 50% of the oxide layer is 100 nm or less, 80 nm or less, 50 nm or less, 40 nm or less, or 30 nm or less.

[0055] When the thickness of more than 50%, preferably more than 70%, more preferably more than 90% of the oxide layer is greater than 5 nm, it is possible to prevent the exposure of silicon or reduce the reactivity of silicon, thereby preventing a decrease in life characteristics and problems of gas generation due to the contact of silicon with water during the aqueous process. If the thickness of more than 50% of the oxide layer containing silicon oxide is 5 nm or less, there is a concern that the internal silicon particles may be exposed to the outside, which may deteriorate the aqueous processability. In this case, the electrode conditions are poor, which may reduce the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0056] According to an exemplary embodiment, the oxide layer contains silicon oxide.

[0057] During the manufacture of the aqueous slurry, contact between water and silicon is prevented by the oxide layer containing silicon oxide, thereby improving the electrode conditions and the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0058] In particular, when the oxide layer contains silicon oxide and the thickness of more than 50% of the oxide layer exceeds 5 nm, there is a high possibility that most of the silicon is passivated by the oxide layer, thereby improving the aqueous processability, and thus improving the charge / discharge capacity, initial efficiency, and / or life characteristics of the secondary battery.

[0059] According to an exemplary embodiment, the silicon oxide contains SiO x (0 < x ≤ 2). The SiO x (0 < x ≤ 2) may be in a form containing various oxidation states, such as Si and SiO2. That is, x corresponds to the number ratio of O to Si contained in the SiO x (0 < x ≤ 2). In other words, the SiO x (0 < x ≤ 2) is not in a single state, but in a state in which various states are mixed, and the average value of x can be obtained by performing XPS while etching and analyzing the content of oxygen (O).

[0060] In one example, the silicon oxide is SiO x (0.1 ≤ x < 2). x may be 1 or more and less than 2, for example 1.5 or more and less than 2.

[0061] According to an exemplary embodiment, the silicon oxide is amorphous. The SiO x (0 < x ≤ 2) may exist in the form of island type or thin film type layer, but may exist in various forms not limited thereto.

[0062] In an exemplary 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. If the thickness of the oxide layer is too thin, it may be difficult to expect the effect of blocking contact with water. If the thickness is too thick, the resistance may increase and the discharge capacity may not be exhibited. Based on 100 parts by weight of the negative electrode active material, the amount of the oxide layer may be 1 to 5 parts by weight. This content range is advantageous for providing the effect of blocking contact with water through the oxide layer while providing an appropriate discharge capacity.

[0063] The negative electrode active material according to an exemplary embodiment includes a carbon layer provided on at least a part of the oxide layer.

[0064] Specifically, the carbon layer, together with the oxide layer, blocks the exposure of silicon or reduces the reactivity of silicon, so that a decrease in life characteristics or gas generation due to contact with water during the aqueous process can be prevented. In addition, the carbon layer imparts conductivity to the negative electrode active material, which can improve the initial efficiency, life characteristics, and battery capacity characteristics of the secondary battery.

[0065] In one exemplary embodiment, the carbon layer may include at least one of amorphous carbon and crystalline carbon.

[0066] In one exemplary embodiment, the carbon layer may be an amorphous carbon layer. The amorphous carbon can appropriately maintain the strength of the carbon layer to inhibit the expansion of the silicon-carbon composite.

[0067] In addition, the carbon layer may further include or not include crystalline carbon.

[0068] The crystalline carbon can 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.

[0069] The amorphous carbon can appropriately maintain the strength of the carbon layer to inhibit the expansion of the silicon-carbon composite. The amorphous carbon may be a carbide of at least one substance selected from the group consisting of tar, pitch, and other organic substances, or a carbon-based material formed by using a hydrocarbon as a source for chemical vapor deposition.

[0070] The carbide of the other organic substance may be an organic carbide selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, hexose or hexulose carbide, and combinations thereof.

[0071] 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, etc. 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, etc.

[0072] In one exemplary embodiment, based on a total of 100 parts by weight of the negative electrode active material, the content of the carbon layer may be 0.1 part by weight to 50 parts by weight, 0.1 part by weight to 30 parts by weight, or 0.1 part by weight to 20 parts by weight. More specifically, the content of the carbon layer may be 0.5 part by weight to 15 parts by weight or 1 part by weight to 10 parts by weight. When the above range is satisfied, a decrease in the capacity and efficiency of the negative electrode active material can be prevented while improving conductivity.

[0073] In an exemplary embodiment, the thickness of the carbon layer may be from 1 nm to 500 nm, specifically from 5 nm to 300 nm, and more specifically from 5 nm to 100 nm. When within the above ranges, the conductivity of the negative electrode active material is improved, the 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, thereby improving the initial efficiency and / or lifespan of the battery.

[0074] 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.

[0075] Since the carbon layer is formed after the formation of the above-mentioned oxide layer, the carbon layer can be provided on the oxide layer containing silicon oxide and can also be provided on the surface of the above-mentioned core where the oxide layer is not provided. In addition, since the carbon layer can completely cover the oxide layer or can be formed only on a part of the oxide layer, both the oxide layer and the carbon layer can be observed on the surface of the negative electrode active material.

[0076] According to an exemplary embodiment of the present invention, the negative electrode active material may include silicon grains having a particle size of 8 μm or less. The silicon grains may be included in the core containing the silicon-carbon composite. In an exemplary embodiment of the present invention, by including silicon grains having a particle size of 8 μm or less, it is not only beneficial to improve the lifespan performance, but also the above-mentioned oxide layer and carbon layer can prevent gas generation during the aqueous process. According to an example, the negative electrode active material may include silicon grains having a particle size of 8 μm or less, such as 5 μm or less, 1,000 nm or less, or in the range of several tens of nanometers.

[0077] According to an exemplary embodiment of the present invention, the Si:C element ratio on the surface of the negative electrode active material of the above exemplary embodiment is from 1:1 to 1:4, such as from 1:2 to 1:4 or from 1:3 to 1:4. Since the negative electrode active material layer has the carbon layer on its surface, the above element ratio means that the carbon proportion on the surface is higher. Having such an element ratio means that the surface of the negative electrode active material is well coated with carbon, which can be measured via XPS. By adjusting the amount of carbonized material or changing the heat treatment time during the carbon layer introduction process, control of the element ratio can be achieved.

[0078] According to an exemplary embodiment of the present invention, the Si:C element ratio in all the negative electrode active materials of the above embodiment is 0.9:1.1 to 1.1:0.9, and may be, for example, 1:0.95 to 1:1.1. If the Si content increases, it is beneficial to increase the capacity, but it may cause problems such as significant particle growth due to deposition on the surface rather than in the carbon pores, which may have a negative impact on the lifespan, processability, etc. The above Si:C element ratio range is beneficial to meet the capacity, lifespan, and processability. The negative electrode active material according to an exemplary embodiment mainly consists of three elements Si, C, and O. Excluding the influence of other elements, the C content is measured using a CS analyzer, and the O content is measured using an ONH analyzer. Then, the value obtained by subtracting the measured values from the total can be calculated as the Si content.

[0079] According to an exemplary 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 above range is satisfied, the structural stability of the active material is ensured during charge and discharge, the problem that the level of volume expansion / shrinkage increases as the particle size becomes too large can be prevented, and the problem of reduced initial efficiency when the particle size is too small can also be prevented.

[0080] According to an exemplary embodiment, the BET specific surface area of the negative electrode active material is 20 m 2 / g or less, and preferably, for example, 10 m 2 / g or less. For example, the BET specific surface area of the negative electrode active material may be 0.11 m 2 / g or more and 10 m 2 / g or less, for example, 4 m 2 / g to 6 m 2 / g. The specific surface area can be measured by the Brunauer-Emmett-Teller (BET) method. For example, the specific surface area can be measured by the BET six-point method using a porosity analyzer (Bell Japan Inc., Belsorp-II mini) by the nitrogen adsorption method.

[0081] <Method for manufacturing negative electrode active material>

[0082] The present invention provides a method for manufacturing a negative electrode active material, and more specifically, provides a method for manufacturing a negative electrode active material according to the above exemplary embodiment.

[0083] An exemplary embodiment of the present invention provides a method for manufacturing the negative electrode active material of the present invention, the method comprising the following steps: forming a core comprising a silicon-carbon composite; forming an oxide layer disposed on at least a part of the core and comprising silicon oxide; and forming a carbon layer on at least a part of the oxide layer.

[0084] According to an exemplary embodiment, the step of forming the core comprising the silicon-carbon composite may include the step of etching carbon-based particles comprising internal pores to enlarge the internal pores of the carbon-based particles; and forming silicon particles on the surface and in the internal pores of the carbon-based particles having the enlarged internal pores.

[0085] The step of enlarging the internal pores of the carbon-based particles may be carried out in a nitrogen (N2) atmosphere, an oxygen (O2) atmosphere or an air atmosphere. Specifically, the flow rate of oxygen (O2) or oxygen-containing air may be controlled to 0.1 L / min to 10 L / min.

[0086] The step of enlarging the internal pores of the carbon-based particles may be carried out at a temperature in the range of 400 °C to 1200 °C for 30 minutes to 4 hours.

[0087] The pore characteristics of the obtained porous carbon-based particles may vary according to the conditions used to enlarge the internal pores of the carbon-based particles. [[ID=X]]

[0088] An etchant may be used to enlarge the internal pores of the carbon-based particles. For example, an alkaline material such as KOH may be used. For example, the carbon-based particles and KOH may be mixed at a weight ratio of 1:1 to 1:5 to enlarge the internal pores of the carbon-based particles.

[0089] The step of forming the silicon particles may be carried out using a chemical vapor deposition (CVD) method. In this case, silicon nanoparticles are deposited on the surface and / or in the internal pores of the carbon-based particles having the enlarged internal pores, whereby a silicon coating in the form of a film, islands or a combination thereof may be formed.

[0090] The step of forming the silicon particles may include forming the silicon-carbon composite by flowing SiH4 / H2 gas to the carbon-based particles at 500 °C to 900 °C using a chemical vapor deposition (CVD) apparatus.

[0091] The step of forming the oxide layer disposed on at least a part of the core and comprising silicon oxide may include forming the oxide layer by partially or completely oxidizing the surface of the core comprising the above silicon-carbon composite.

[0092] According to an exemplary embodiment of the present invention, the step of forming the oxide layer disposed on at least a part of the core and comprising silicon oxide may include a heat treatment step carried out in an atmosphere containing oxygen.

[0093] Specifically, the heat treatment step can be carried out at 500 °C to 700 °C, for example 600 °C to 700 °C, to form the oxide layer.

[0094] If the heat treatment step is carried out at a temperature exceeding 700 °C, the growth of silicon carbide composite (SiC) is caused, which is not desired because it instead results in a reduction in the capacity and efficiency of the material.

[0095] The oxygen-containing atmosphere can contain more than 0 vol% and 10 vol% or less of oxygen, for example 3 vol% to 7 vol% of oxygen. The oxygen-containing atmosphere can further contain an inert gas such as argon on the basis of containing oxygen. The heat treatment time is not particularly limited, but can be, for example, 1 hour to 12 hours, 1 hour to 8 hours, or 2 hours to 5 hours.

[0096] The thickness of the oxide layer can be controlled by the heat treatment temperature, time, etc.

[0097] The step of forming a carbon layer on at least a part of the oxide layer can be carried out by using a chemical vapor deposition (CVD) method in which a carbon-based material such as a hydrocarbon gas is utilized, or can be carried out by carbonizing a material used as a carbon source.

[0098] Specifically, after placing a silicon carbide composite having an oxide layer containing silicon oxide into a reactor, a hydrocarbon gas can be chemically vapor deposited (CVD) at a temperature of 600 °C to 700 °C to form the carbon layer. The hydrocarbon gas can be at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene, and the heat treatment can be carried out at 600 °C to 700 °C.

[0099] <Negative electrode>

[0100] An exemplary embodiment of the present invention provides a negative electrode comprising a negative electrode active material, a conductive material, and a binder according to the above exemplary embodiment.

[0101] Specifically, the negative electrode can 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 can contain the negative electrode active material. In addition, the negative electrode active material layer can further contain a binder and / or a conductive material.

[0102] The negative electrode active material layer can be formed by applying a negative electrode paste containing a negative electrode active material, a binder, and / or a conductive material to at least one surface of the negative electrode current collector and drying and calendering the negative electrode current collector.

[0103] The negative electrode paste contains the negative electrode active material, the binder, and / or the conductive material.

[0104] The negative electrode paste may further contain an additional negative electrode active material.

[0105] As the additional negative electrode active material, a compound capable of reversibly inserting and extracting lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds capable of forming an alloy with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides capable of doping and undoping lithium such as SiO β (0 < β < 2), SnO2, vanadium oxides, lithium titanium oxides, and lithium vanadium oxides; or a composite containing a metal compound and a carbonaceous material such as a Si-C composite or a Sn-C composite, and any one or a mixture of two or more thereof can be used. In addition, as the negative electrode active material, a thin film of metallic lithium can be used. In addition, as the carbon material, both low-crystalline carbon and high-crystalline carbon can be used. Representative examples of the low-crystalline carbon include soft carbon and hard carbon, and representative examples of the high-crystalline carbon include amorphous, plate-like, scaly, spherical, or fibrous natural graphite or artificial graphite, floating graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, mesophase pitch, and high-temperature calcined carbon such as coke derived from petroleum or coal tar pitch.

[0106] The additional negative electrode active material may be a carbon-based negative electrode active material.

[0107] In an exemplary embodiment of the present invention, the weight ratio of the negative electrode active material contained in the negative electrode paste to the additional negative electrode active material may be 10:90 to 90:10, specifically 10:90 to 50:50.

[0108] The negative electrode current collector is not particularly limited as long as it has conductivity and does not cause a chemical change in the battery. For example, for the current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. Specifically, transition metals such as copper and nickel that can well adsorb carbon can be used for the current collector. The thickness of the current collector may be 6 μm to 20 μm. However, the thickness of the current collector is not limited thereto.

[0109] The binder may include at least one selected from the group consisting of polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene - propylene - diene monomer (EPDM), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, polyacrylic acid, and the above materials in which hydrogen is substituted by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0110] The conductive material is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, graphite such as natural graphite or artificial graphite can be used; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal - cracked carbon black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc.

[0111] The negative electrode paste may contain a solvent for forming the negative electrode paste. Specifically, the solvent for forming the negative electrode paste may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropanol. Specifically, distilled water is used in terms of promoting the dispersion of components.

[0112] <Secondary battery>

[0113] An exemplary embodiment of the present invention provides a lithium secondary battery including a negative electrode, a positive electrode, and a separator according to the above - mentioned exemplary embodiments.

[0114] 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 a positive electrode active material.

[0115] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, aluminum or stainless steel surface - treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode current collector generally may have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and non - woven fabric bodies.

[0116] 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) and lithium nickel oxide (LiNiO2) or a compound substituted with one or more transition metals; lithium iron oxide, such as LiFe3O4; lithium manganese oxide, such as the chemical formula Li 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxide, such as LiV3O8, V2O5, and Cu2V2O7; Ni-site type lithium nickel oxide, represented by the chemical formula LiNi 1-c2 M c2 O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤c2≤0.3); lithium manganese composite oxide, represented by the chemical formula LiMn 2-c3 M c3 O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 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 a part of Li in the chemical formula is replaced by an alkaline earth metal ion; etc., but not limited thereto. The positive electrode may be Li metal.

[0117] On the basis of containing the above positive electrode active material, the positive electrode active material layer may further contain a positive electrode conductive material and a positive electrode binder.

[0118] In this case, the positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitation as long as the positive electrode conductive material has electron conductivity and does not cause chemical changes in the formed battery. Specific examples may include graphite, such as natural graphite and artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon 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; or conductive polymers, such as polyphenylene derivatives, etc., and any one of them or a mixture of two or more of them can be used.

[0119] In addition, the positive electrode binder is used to improve the binding between the particles of the positive electrode active material and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc., and any one of them or a mixture of two or more thereof can be used.

[0120] The separator is used to separate the negative electrode and the positive electrode and provide a migration path for lithium ions. Any separator can be used as the separator without particular limitation as long as it is commonly used in secondary batteries. In particular, a separator with high moisture retention capacity for the electrolyte and low resistance to the movement of electrolyte ions can be preferably used. Specifically, a porous polymer membrane can be used, such as a porous polymer membrane made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer; or a laminated structure having two or more layers thereof. In addition, ordinary porous non-woven fabrics can be used, such as non-woven fabrics formed of high melting point glass fibers, polyethylene terephthalate fibers, etc. In addition, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material can be used, and a separator having a single-layer or multi-layer structure can be selectively used.

[0121] The lithium secondary battery may further include an electrolyte. Examples of the electrolyte may include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, or molten-type inorganic electrolytes that can be used in the manufacture of the lithium secondary battery.

[0122] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0123] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, 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, triphosphate ester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, methyl propionate, or ethyl propionate can be used.

[0124] In particular, in carbonate-based organic solvents, ethylene carbonate and propylene carbonate as cyclic carbonates are high-viscosity organic solvents, and since they have a high dielectric constant to dissociate lithium salts well, they can be preferably used. When the cyclic carbonate is mixed and used with linear carbonates such as dimethyl carbonate and diethyl carbonate having low viscosity and low dielectric constant in an appropriate ratio, an electrolyte having high conductivity can be prepared, and thus such a combined use can be more preferable.

[0125] As the metal salt, a lithium salt can be used, and the lithium salt is a material that is easily soluble in the non-aqueous electrolyte. As the anion of the lithium salt, for example, at least one selected from the group consisting of the following can be used: 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 - .

[0126] For the purposes of improving the life characteristics of the battery, suppressing the reduction of the battery capacity, improving the discharge capacity of the battery, etc., the electrolyte may further contain, on the basis of the above electrolyte components, one or more of the following additives, for example: halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, diethylene glycol dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride.

[0127] Other exemplary embodiments of the present invention provide 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. Since the battery module and the battery pack include a secondary battery having high capacity, high rate capacity, and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0128] Mode of the Invention

[0129] Hereinafter, examples will be described in detail to specifically describe this specification. However, the examples according to this specification can be modified in other forms, and the scope of this application should not be construed as being limited to the following examples. The examples of this application are provided to more completely explain this specification to those skilled in the art.

[0130] Example 1

[0131] Put a 0.5 M sucrose aqueous solution into an autoclave and react at 180 °C for 24 hours to synthesize spherical particles. Wash the obtained carbon-based particles with ethanol 2 to 3 times. Mix the carbon-based particles dried at 100 °C for more than 12 hours with KOH at a ratio of 1:3 by weight, and heat the resulting mixture in a nitrogen atmosphere at 800 °C for 2 hours to expand the pores. Thereafter, wash the carbon-based particles with distilled water and dry them at 100 °C for more than 12 hours. Then place the carbon-based particles with expanded pores in the hot zone of a CVD apparatus. Subsequently, flow a gas mixture of SiH4 / H2 (= 5 / 95) at a flow rate of 50 ml / min at 600 °C for 2 hours to prepare a silicon / carbon composite. Then, perform a heat treatment at 700 °C for 3 hours in an atmosphere of O2 / Ar (= 5 / 95) to form an oxide layer. Place the silicon / carbon composite in the hot zone of a CVD apparatus, and introduce methane into the hot zone at 700 °C using Ar as a carrier gas and react for 1 hour to form a carbon layer on the surface of the silicon / carbon composite, thereby preparing a negative electrode active material containing an oxide layer and a carbon layer on the surface of the silicon / carbon composite.

[0132] Example 2

[0133] A negative electrode active material was prepared in the same manner as in Example 1, except that the ratio of the carbon-based particles to KOH was changed to 1:5 and the heat treatment temperature for expanding the pores was changed to 900 °C.

[0134] Example 3

[0135] A negative electrode active material was prepared in the same manner as in Example 1, except that the oxide layer formation conditions were changed to an atmosphere of O2 / Ar (= 10 / 90), 800 °C, and 3 hours.

[0136] Example 4

[0137] 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.

[0138] Example 5

[0139] A negative electrode active material was prepared in the same manner as in Example 1, except that the SiH4 / H2 (= 5 / 95) gas was flowed at a flow rate of 50 ml / min at 600 °C for 1 hour.

[0140] Comparative Example 1

[0141] 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 excluded.

[0142] Comparative Example 2

[0143] The negative electrode active material was prepared in the same manner as in Example 1, except that the step of forming a carbon layer was excluded.

[0144] Comparative Example 3

[0145] The negative electrode active material was prepared in the same manner as in Example 1, except that the step of forming an oxide layer was carried out after the step of forming a carbon layer.

[0146] Comparative Example 4

[0147] The negative electrode active material was prepared in the same manner as in Example 1, except that an Al2O3 coating with a thickness of about 1 nm was formed as the oxide layer.

[0148] The particle size (D50) of the negative electrode active material and the particle size of the silicon grains were analyzed by laser diffraction particle size analysis using a Microtrac S3500 device.

[0149] The specific surface area of the negative electrode active material was measured by the BET six-point method using the nitrogen adsorption method with a porosimeter (Belsorp-II mini, Bell Japan Co., Ltd.).

[0150] The thickness and surface Si:C ratio of the oxide layer were measured by XPS analysis. The XPS apparatus used was a Nexsa 2 from Thermo Fisher Scientific Inc. Since the XPS apparatus only analyzes the sample within the x-ray spot size (400 μm) region, about 2 to 3 points were measured while etching the sample surface during the analysis. In this case, when the elemental ratio varies according to the position, the measurement results are expressed as a range having a measurement maximum value and a measurement minimum value corresponding to the upper limit and the lower limit, respectively, based on the deviation at different positions.

[0151] The overall Si:C ratio in the negative electrode active material and the composition of the oxide layer were determined by measuring the C content using a CS analyzer and the O content using an ONH analyzer, and then calculating the value obtained by subtracting the measured values from the sum as the Si content.

[0152]

[0153] <Experimental Example: Evaluation of Discharge Capacity, Initial Efficiency, and Life (Capacity Retention Rate) Characteristics>

[0154] The negative electrodes and batteries were prepared using the negative electrode active materials of the examples and comparative examples, respectively.

[0155] A mixture was prepared by mixing the negative electrode active material, carbon black as a conductive material, and polyacrylic acid (PAA) as a binder at a weight ratio of 80:10:10. Thereafter, 7.8 g of distilled water was added to 5 g of this mixture, and then it was stirred to prepare a negative electrode slurry. The negative electrode slurry was applied to a copper (Cu) metal thin film with a thickness of 20 μm as a negative electrode current collector and dried. In this case, the temperature of the circulating air was 60°C. Then, the thin film was calendered and dried in a vacuum oven at 130°C for 12 hours to prepare a negative electrode.

[0156] As the positive electrode, a lithium (Li) metal thin film obtained by cutting a lithium metal sheet into a circular shape with a diameter of 1.7671 cm 2 was used. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, and an electrolytic solution was injected to prepare a lithium coin half-cell. The electrolytic solution was obtained by dissolving 0.5 parts by weight of vinylene carbonate in a mixed solution in which ethylene methyl carbonate (EMC) and ethylene carbonate (EC) were mixed at a volume ratio of 7:3, and dissolving LiPF6 to a concentration of 1 M.

[0157] The prepared batteries were charged and discharged to evaluate the discharge capacity, initial efficiency, and capacity retention rate, and the evaluation results are shown in Table 2 below.

[0158] For the first and second cycles, charging and discharging were performed at 0.1C, and from the third cycle to the 49th cycle, charging and discharging were performed at 0.5C. At the 50th cycle, charging and discharging were terminated in the charged state (lithium was contained in the negative electrode).

[0159] Charging conditions: CC (constant current) / CV (constant voltage) (5 mV / 0.005C current cut-off)

[0160] Discharging conditions: CC (constant current) condition 1.5 V

[0161] The discharge capacity (mAh / g) and initial efficiency (%) were obtained from the results after one charge and discharge. Specifically, the initial efficiency (%) was calculated as follows.

[0162] Initial efficiency (%) = (discharge capacity of the first time / charge capacity of the first time) × 100%

[0163] The capacity retention rate was calculated as follows.

[0164] Capacity retention rate (%) = (discharge capacity of the 49th time / discharge capacity of the first time) × 100%

[0165] <Example of Experiment: Evaluation of Processability (Shear Viscosity) Characteristics>

[0166] As part of the evaluation of processability, the change in shear viscosity at a shear rate (= 1 Hz) was measured for a slurry prepared by mixing graphite, the negative electrode active material, carbon black, CMC, and PAA at a weight ratio of 77:20:1:1:1 and is shown in Table 2 below. Specifically, the change amount (%) of shear viscosity was obtained by the following calculation formula.

[0167] Change amount (%) of shear viscosity = ((Shear viscosity of the slurry after 48 hours - Shear viscosity of the slurry immediately after mixing) / Shear viscosity of the slurry immediately after mixing) × 100%

[0168] Time point of gas generation: 20 g of the slurry was placed in a 10×15 cm aluminum bag and then vacuum-sealed. Thereafter, the volume change was measured using Archimedes' principle, and the time point at which the volume change exceeded 4 mL was defined as the time point of gas generation.

[0169] The characteristics evaluated as described above are shown in Table 2 below.

[0170]

[0171] Examples 1 to 5 using a silicon-carbon composite with an oxide layer and a carbon layer of appropriate thickness as the negative electrode active material showed that battery performance such as discharge capacity, initial efficiency, and capacity retention rate was superior to Comparative Example 1 in which a fine oxide layer was formed only by natural oxidation without an oxide layer formation process, Comparative Example 2 in which no carbon layer was formed, Comparative Example 3 in which the oxide layer and the carbon layer were formed in the reverse order, and Comparative Example 4 in which an Al2O3 coating with a thickness of 1 nm was formed. In addition, in Examples 1 to 5, due to the presence of the oxide layer and the carbon layer, by controlling gas generation during processing, the change amount of shear viscosity was significantly small, and the time point of gas generation could be significantly delayed. That is, in the examples, it was confirmed that not only the battery performance was significantly improved, but also the processability could be significantly improved by preventing gas generation.

Claims

1. A negative electrode active material, comprising: A core containing a silicon-carbon composite; An oxide layer provided on at least a part of the core and containing silicon oxide, wherein the thickness of more than 50% of the oxide layer is greater than 5 nm; and A carbon layer provided on at least a part of the oxide layer.

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

4.

3. The negative electrode active material according to claim 1, wherein the Si:C element ratio in all of the negative electrode active material is from 0.9:1.1 to 1.1:0.

9.

4. The negative electrode active material according to claim 1, wherein the silicon oxide in the oxide layer is SiO x (where 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 grains having a particle size of 8 μm or less.

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

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

8. The negative electrode active material according to claim 1, wherein the silicon-carbon composite contains porous carbon-based particles and silicon particles located on the surface or in the internal pores of the porous carbon-based particles.

9. A negative electrode, comprising: The negative electrode active material according to any one of claims 1 to 8; A conductive material; and An adhesive.

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. A method for manufacturing the negative electrode active material according to any one of claims 1 to 8, the method comprising: Forming a core containing a silicon-carbon composite; Forming an oxide layer provided on at least a part of the core and containing silicon oxide; And Forming a carbon layer on at least a part of the oxide layer.

15. The method according to claim 14, wherein the forming of the oxide layer provided on at least a part of the core and containing silicon oxide includes performing heat treatment in an atmosphere containing oxygen, and wherein the heat treatment is performed at 600°C to 700°C.

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