Negative electrode active material, negative electrode containing the same, secondary battery containing the same, and method for producing the negative electrode active material

Abstract

The present invention relates to a negative electrode active material, a negative electrode including the same, a secondary battery including the same, and a method for producing the negative electrode active material.

Description

Technical Field

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0165308, filed with the Korean Intellectual Property Office on November 26, 2021, and all of its contents are incorporated herein by reference.

[0002] The present invention relates to a negative electrode active material, a negative electrode including the same, a secondary battery including the same, and a method for manufacturing the negative electrode active material.

Background Art

[0003] In recent years, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries that are small and lightweight but relatively high in capacity has been rapidly increasing. In particular, lithium secondary batteries are lightweight and have a high energy density, and have been in the spotlight as a driving power source for portable devices. For this reason, research and development efforts to improve the performance of lithium secondary batteries have been actively carried out.

[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, and the like. In addition, an active material layer including a positive electrode active material and a negative electrode active material can be formed on a current collector for the positive electrode and the negative electrode. Generally, LiCoO 2 , LiMn 2 O 4 and other lithium-containing metal oxides are used as the positive electrode active material, and a carbon-based active material or a silicon-based active material that does not contain lithium is used as the negative electrode active material.

[0005] In the case of a silicon-based active material among negative electrode active materials, it is attracting attention in that it has a higher capacity than a carbon-based active material and excellent high-rate charging characteristics. However, the silicon-based active material has a disadvantage in that the degree of volume expansion / contraction due to charge / discharge is large and the irreversible capacity is large, so the initial efficiency is low.

[0006] On the one hand, among silicon-based active materials, silicon-based oxides, specifically SiO x (in the case of silicon-based oxides represented by (0 < x < 2)), it has an advantage in that the degree of volume expansion / contraction due to charge and discharge is lower compared to other silicon-based active materials such as silicon (Si). However, there is still a disadvantage that the initial efficiency decreases due to the presence of irreversible capacity in silicon-based oxides as well.

[0007] In relation to this, research has been continuously conducted to reduce the irreversible capacity and improve the initial efficiency by doping or inserting metals such as Li, Al, and Mg into silicon-based oxides. However, in the case of a negative electrode slurry containing a metal-doped silicon-based oxide as a negative electrode active material, there is a problem that the metal oxide formed by doping reacts with moisture to increase the pH of the negative electrode slurry and change the viscosity, resulting in a poor state of the manufactured negative electrode and a decrease in the charge and discharge efficiency of the negative electrode.

[0008] Therefore, there is a need to develop a negative electrode active material that can improve the phase stability of a negative electrode slurry containing a silicon-based oxide and thereby improve the charge and discharge efficiency of the manufactured negative electrode.

[0009] Korean Registered Patent No. 10-0794192 relates to a method for manufacturing a carbon-coated silicon-graphite composite negative electrode material for a lithium secondary battery and a method for manufacturing a secondary battery including the same, but there are limitations in solving the above-mentioned problems.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention relates to a negative electrode active material, a negative electrode containing the same, a secondary battery containing the same, and a method for manufacturing the negative electrode active material.

Means for Solving the Problems

[0012] One embodiment of the present invention is a silicon-based particle containing SiO x (0 < x < 2) and Li, and having a first carbon layer on at least a part of the surface; and a negative electrode active material including a second carbon layer provided on at least a part of the silicon-based particle, wherein when analyzed by Raman spectroscopy, the ratio of the peak intensity of the D band to the peak intensity of the G band (D / G band ratio) is 1.0 or more, and when analyzed by X-ray photoelectron spectroscopy (XPS), (content of C element) / (content of Li element + content of C element) is 0.6 or more.

[0013] One embodiment of the present invention is a step of forming a silicon-based particle containing SiO x (0 < x < 2) and Li, and having a first carbon layer on at least a part of the surface; and a step of forming a second carbon layer on at least a part of the silicon-based particle, and provides a method for manufacturing the negative electrode active material.

[0014] One embodiment of the present invention provides a negative electrode containing the negative electrode active material. One embodiment of the present invention provides a secondary battery containing the negative electrode.

Advantages of the Invention

[0015] A negative electrode containing a negative electrode active material according to one embodiment of the present invention and a secondary battery containing the negative electrode have an effect that the discharge capacity, initial efficiency, resistance performance, and / or life characteristics of the battery are improved.

Modes for Carrying Out the Invention

[0016] Hereinafter, the present specification will be described in more detail. In this specification, when a certain part "includes" a certain component, this means that, unless otherwise stated to the contrary, it does not exclude other components, but may further include other components.

[0017] In this specification, when a certain member is located "above" another member, this includes not only the case where a certain member is in contact with another member, but also the case where there are other members between the two members.

[0018] The terms and words used in this specification should not be construed as being limited to their ordinary or dictionary meanings. In accordance with the principle that the inventors can appropriately define the concepts of the terms in order to explain their inventions in the best way, they should be construed in meanings and concepts consistent with the technical idea of the present invention.

[0019] The singular expressions of the terms used in this specification include plural expressions unless otherwise clearly indicated in the context.

[0020] In this specification, the crystallinity of the structure contained in the negative electrode active material can be confirmed by X-ray diffraction analysis. The X-ray diffraction analysis can be performed using an XRD (X-ray diffraction) analyzer (product name: D4-endavor, manufacturer: bruker). In addition to this device, devices used in the industry can also be appropriately adopted.

[0021] In this specification, the presence and content of elements in the negative electrode active material can be confirmed by ICP analysis. The ICP analysis can be performed using an inductively coupled plasma optical emission spectrometer (ICPAES, Perkin-Elmer 7300).

[0022] In this specification, the average particle size (D 50 ) can be defined as the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve (graph curve of the particle size distribution diagram). The average particle size (D 50) can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes in the range from the submicron region to about several millimeters, and can obtain highly reproducible and highly resolved results.

[0023] Hereinafter, preferred embodiments of the present invention will be described in detail. However, the embodiments of the present invention may be modified in various forms, and the scope of the present invention is not limited to the embodiments described below.

[0024] <Negative electrode active material> One embodiment of the present invention is a silicon-based particle containing SiO x (0 < x < 2) and Li, and having a first carbon layer provided on at least a part of the surface; and a negative electrode active material containing a second carbon layer provided on at least a part of the silicon-based particle, wherein the ratio of the peak intensity of the D band to the peak intensity of the G band (D / G band ratio) is 1.0 or more when analyzed by Raman spectroscopy, and (content of C element) / (content of Li element + content of C element) is 0.6 or more when analyzed by X-ray photoelectron spectroscopy (XPS). A negative electrode active material is provided.

[0025] The negative electrode active material according to one embodiment of the present invention contains silicon-based particles. The silicon-based particles may contain SiO x (0 < x < 2) and Li, and may have a first carbon layer provided on at least a part of the surface.

[0026] The SiO x (0 < x < 2) corresponds to a matrix in the silicon-based particles. The SiO x (0 < x < 2) may be in a form containing Si and SiO 2 , and the Si may form a phase. That is, the x corresponds to the number ratio of O to Si contained in the SiO x (0 < x < 2). When the silicon-based particles contain the SiO x (0 < x < 2), the discharge capacity of the secondary battery is improved.

[0027] In one embodiment of the present invention, the silicon-based particles may contain Li. The Li may be present in at least one form of lithium atom, lithium silicate, lithium silicide, and lithium oxide within the silicon-based particles, and preferably may be present in the form of lithium silicate. When the Li is present in the form of a Li compound, the Li compound may correspond to a matrix within the silicon-based particles. When the silicon-based particles contain a Li compound, there is an effect that the initial efficiency is improved.

[0028] The Li may be distributed on the surface and / or inside of the silicon-based particles in a doped form. The Li compound is distributed on the surface and / or inside of the silicon-based particles, and can control the expansion / contraction of the volume of the silicon-based particles to an appropriate level, and can play a role in preventing damage to the active material. Further, the Li may be included in terms of reducing the ratio of the irreversible phase (for example, SiO 2 ) of the silicon-based oxide particles and increasing the efficiency of the active material.

[0029] In one embodiment of the present invention, Li may be present in the form of lithium silicate. The lithium silicate is represented by Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and may be present in the form of at least one lithium silicate selected from the group consisting of Li 2 SiO 3 , Li 4 SiO 4 , and Li 2 Si 2 O 5 within the silicon-based particles.

[0030] In one embodiment of the present invention, Li contained in the negative electrode active material may be contained in an amount of 0.01 to 40 parts by weight, 0.01 to 20 parts by weight, 0.01 to 10 parts by weight, or 0.02 to 9 parts by weight based on 100 parts by weight in total of the negative electrode active material. Specifically, it may be contained in an amount of 0.1 to 9 parts by weight, 1 to 9 parts by weight, or 4 to 9 parts by weight. More specifically, it may be contained in an amount of 5 to 8 parts by weight. As the content of Li increases, although the initial efficiency increases, there is a problem that the discharge capacity decreases. Therefore, when the above range is satisfied, appropriate discharge capacity and initial efficiency can be realized.

[0031] The content of the Li element can be confirmed by ICP analysis. Specifically, after sampling a sample (about 0.01 g), it is transferred to a platinum crucible, nitric acid, hydrofluoric acid, and sulfuric acid are added, and it is completely decomposed on a hot plate. Then, using an inductively coupled plasma atomic emission spectrometer (ICPAES, Perkin-Elmer 7300), at the specific wavelength of the element to be analyzed, the intensity of a standard solution prepared using a standard solution (5 mg / kg) is measured to create a standard calibration curve. Then, the pretreated sample solution and blank sample are introduced into the instrument, the intensities of each are measured to calculate the actual intensity, and after calculating the concentration of each component by comparing with the above-prepared calibration curve, the content of the elements of the sample can be analyzed by converting so that the total sum becomes the theoretical value.

[0032] The silicon-based particles according to one embodiment of the present invention are provided with a first carbon layer on at least a part of the surface. At this time, the first carbon layer may be in a form that partially covers at least a part of the surface, that is, the surface of the particles, or covers the entire surface of the particles. The carbon layer imparts conductivity to the negative electrode active material, improving the initial efficiency, life characteristics, and battery capacity characteristics of the secondary battery.

[0033] Specifically, the first carbon layer may contain crystalline carbon or amorphous carbon. 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.

[0034] The amorphous carbon can appropriately maintain the strength of the first carbon layer and suppress the expansion of the silicon-based particles. The amorphous carbon may be at least one carbide selected from the group consisting of tar, pitch, and other organic substances, or a carbon-based substance formed using a hydrocarbon as a source in a chemical vapor deposition method.

[0035] The carbide of the other organic substances may be a carbide of an organic substance selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose and combinations thereof.

[0036] 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, or hexane, etc. Examples of the aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene, etc.

[0037] In one embodiment of the present invention, the first carbon layer may be included in an amount of 1 part by weight to 5 parts by weight, 2 parts by weight to 5 parts by weight, 2 parts by weight to 4.5 parts by weight, or 2 parts by weight to 4 parts by weight based on 100 parts by weight in total of the negative electrode active material. More specifically, it may be included in an amount of 3 parts by weight to 4 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.

[0038] In one embodiment of the present invention, the thickness of the first carbon layer may be 1 nm to 500 nm, specifically, it may be 5 nm to 300 nm. When the above range is satisfied, the conductivity of the negative electrode active material is improved, the volume change of the negative electrode active material is easily suppressed, the side reaction between the electrolyte and the negative electrode active material is suppressed, and the initial efficiency and / or life of the battery are improved.

[0039] Specifically, the first carbon layer may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene.

[0040] In one embodiment of the present invention, the negative electrode active material includes a second carbon layer provided on at least a part of the silicon-based particles. Specifically, the second carbon layer may be in a form that partially covers the surface of the silicon-based particles or covers the entire surface of the particles.

[0041] In one embodiment of the present invention, the negative electrode active material includes a second carbon layer provided on at least a part of the first carbon layer. Specifically, the second carbon layer may be in a form that partially covers the surface of the first carbon layer or covers the entire surface of the particles.

[0042] Generally, in the process of doping silicon-based particles with Li, unreacted lithium by-products exist on the surface of the silicon-based particles, which will show basicity during the formation of the slurry, changing the rheological properties of the slurry, and there is a problem that the lithium by-products react with moisture to generate gas.

[0043] In the present invention, a second carbon layer is further coated on the silicon-based particles coated with the first carbon layer and containing Li, thereby suppressing side reactions of unreacted lithium by-products and effectively preventing the slurry from becoming basic. Further, since the second carbon layer can efficiently passivate the silicon-based particles, there is an effect of preventing the lithium by-products contained in the silicon-based particles from eluting and improving the aqueous system workability. Further, since the second carbon layer has hydrophobicity, the reaction between the negative electrode active material and water in the aqueous slurry can be minimized.

[0044] In one embodiment of the present invention, a lithium compound may be present between the silicon-based particles and the second carbon layer. At this time, the lithium compound means a lithium by-product formed by unreacted lithium.

[0045] In one embodiment of the present invention, the lithium compound present between the silicon-based particles and the second carbon layer may be contained in an amount of 0.1 part by weight to 3 parts by weight based on 100 parts by weight in total of the negative electrode active material.

[0046] In one embodiment of the present invention, a lithium compound may be present between the first carbon layer and the second carbon layer. At this time, the lithium compound means a lithium by-product formed by unreacted lithium.

[0047] Specifically, in the process of forming precursor particles containing SiO x (0 < x < 2), forming a first carbon layer on the precursor particles, and then doping with Li to produce the silicon-based particles described above, a lithium compound, that is, a lithium by-product formed by unreacted lithium, remains near the surface of the silicon-based particles.

[0048] In one embodiment of the present invention, the lithium compound is Li 2 O, LiOH, and Li 2 CO 3 and may include one or more selected from the group consisting of.

[0049] In one embodiment of the present invention, at least a part of the surface of the silicon-based particles provided with the first carbon layer may be provided with a lithium compound.

[0050] The lithium compound may be in a form that is located at least in part on the surface of the silicon-based particles, that is, is partially located on the surface of the particles or is located on the entire surface of the particles.

[0051] In one embodiment of the present invention, the second carbon layer may be provided on at least a part of the silicon-based particles provided with the lithium compound. Specifically, it may be in a form that partially covers the surface of the particles or covers the entire surface of the particles. More specifically, the second carbon layer may be present on at least a part of the lithium compound provided on the surface of the particles, or may be present on at least a part of the portion of the surface of the silicon-based particles where the lithium compound is not provided.

[0052] The second carbon layer may be provided on at least a part of the lithium compound. That is, the second carbon layer may be in a form that partially covers the surface of at least a part of the lithium compound, that is, the surface of the lithium compound or covers the entire surface of the lithium compound.

[0053] In one embodiment of the present invention, the second carbon layer contains amorphous carbon. The second carbon layer may further contain crystalline carbon.

[0054] The crystalline carbon can further improve the conductivity of the negative electrode active material. The crystalline carbon may contain at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0055] The amorphous carbon can appropriately maintain the strength of the second carbon layer and suppress the expansion of the silicon-based particles. The amorphous carbon may be at least one carbide selected from the group consisting of tar, pitch, and other organic substances, or a carbon-based substance formed using a hydrocarbon as a source in a chemical vapor deposition method.

[0056] The carbide of the other organic substances may be a carbide of an organic substance selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose carbide and combinations thereof.

[0057] 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, or hexane, etc. Examples of the aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene, etc.

[0058] In one embodiment of the present invention, the second carbon layer may be contained in an amount of 1 to 5 parts by weight, specifically 1.5 to 3 parts by weight, and more specifically 2 to 3 parts by weight based on 100 parts by weight of the total negative electrode active material. When the above range is satisfied, it is possible to prevent a decrease in the capacity and efficiency of the negative electrode active material and effectively passivate the negative electrode active material.

[0059] In one embodiment of the present invention, the thickness of the second carbon layer may be 1 nm to 500 nm, specifically, it may be 5 nm to 300 nm. When the above range is satisfied, the conductivity of the negative electrode active material is improved, the volume change of the negative electrode active material is easily suppressed, the side reaction between the electrolyte and the negative electrode active material is suppressed, and the initial efficiency and / or life of the battery are improved.

[0060] The second carbon layer may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, propane, and acetylene under temperature conditions of less than 800 °C.

[0061] In the present invention, the Raman spectroscopy is a method for analyzing the structure of the carbon layer. The peak existing in the region around the wavenumber 1,580 cm -1 in the Raman spectrum of the carbon layer is called the G band, which is a peak indicating the sp2 bond of the carbon layer and represents a carbon crystal without structural defects. On the other hand, the peak existing in the region around the wavenumber 1,360 cm -1 in the Raman spectrum is called the D band, which is a peak indicating the sp3 bond of the carbon layer and will increase when the atomic bond composed of sp2 bonds is broken and becomes an sp3 bond. Such a D band will increase when disorder or defects are generated in the carbon layer. The G band of the Raman spectrum for the carbon layer may be a peak existing in the region of 1,550 cm -1 ~1,620 cm -1 , and the D band may be a peak existing in the region of 1,330 cm -1 ~1,370 cm -1 . The wavenumber ranges for the G band and the D band correspond to the shiftable range according to the laser light source used in the Raman spectroscopy.

[0062] In the present invention, the crystallinity of the carbon layer can be confirmed by calculating the D / G band ratio using Raman spectroscopy. Specifically, it can be measured using a Renishaw 2000 Raman microscope system and a 532 nm laser excitation, with a low laser output density and an exposure time of 30 seconds to avoid the thermal effect of the laser, and using a 100-fold optical lens. To reduce the deviation by position, a total of 25 points are measured for a 5 μm × 5 μm region, and after fitting using a Lorentzian function, the average values of the D band and the G band can be calculated and used for the calculation.

[0063] In one embodiment of the present invention, when analyzing the negative electrode active material by Raman spectroscopy, the ratio of the peak intensity of the D band to the peak intensity of the G band (D / G band ratio) may be 1.0 or more. Specifically, the D / G band ratio may be 1.01 or more. The upper limit of the D / G band ratio may be 3 or less, 2.5 or less, 2 or less, 1.5 or less, 1.3 or less, or 1.2 or less.

[0064] When the D / G band ratio satisfies 1.0 or more, the degree of amorphization of all the carbon contained in the first carbon layer and the second carbon layer included in the negative electrode active material increases. That is, since the carbon layer contained in the negative electrode active material contains a large amount of amorphous carbon and can effectively coat the residual lithium by-products, there is an effect of minimizing the side reaction with moisture. On the other hand, when the D / G band ratio is less than 1.0, the degree of amorphization of all the carbon contained in the first carbon layer and the second carbon layer decreases, and the crystallinity of the carbon layer increases. As described above, the carbon layer with high crystallinity is formed at a relatively high temperature, and conversely, there is a disadvantage that the grain size of silicon (Si) increases and the life performance deteriorates.

[0065] In one embodiment of the present invention, the content of the total carbon layers (the first carbon layer and the second carbon layer) contained in the negative electrode active material may be 2 parts by weight to 10 parts by weight, 2 parts by weight to 9 parts by weight, 2 parts by weight to 8 parts by weight, or 3 parts by weight to 8 parts by weight based on 100 parts by weight of the total negative electrode active material. More specifically, it may be contained in an amount of 4 parts by weight to 7 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.

[0066] On the surface of the negative electrode active material, there may be almost no lithium compound or no lithium compound at all. By including two carbon layers as described above, since the content of lithium by-products located on the surface of the negative electrode active material is small or does not exist, side reactions caused by lithium by-products during the formation of the slurry can be controlled, and the slurry can be effectively prevented from becoming basic. On the contrary, when only one carbon layer is formed, due to lithium by-products on the surface of the negative electrode active material generated during the process of doping Li after the formation of one carbon layer, the slurry will show basicity during the formation of the slurry, resulting in a change in the rheological properties of the slurry, and there is a problem that lithium by-products react with moisture to generate gas.

[0067] In one embodiment of the present invention, when analyzing the negative electrode active material by X-ray photoelectron spectroscopy (XPS), (content of C element) / (content of Li element + content of C element) is 0.6 or more.

[0068] Specifically, (content of C element) / (content of Li element + content of C element) may be 0.6 or more, 0.63 or more, or 0.65 or more. The upper limit of (content of C element) / (content of Li element + content of C element) may be 1 or less, 0.95 or less, 0.9 or less, 0.85 or less, 0.8 or less, 0.75 or less, or 0.73 or less.

[0069] When the relationship between the contents (atomic %) of the C and Li elements satisfies the above range, the content of lithium by-products located on the surface of the negative electrode active material is extremely small or non-existent. Therefore, side reactions caused by lithium compounds during the formation of the slurry can be controlled, and the slurry can be effectively prevented from becoming basic. On the contrary, when (content of C element) / (content of Li element + content of C element) is less than 0.6, since there are many lithium by-products present on the surface of the negative electrode active material, the slurry will exhibit basicity during the formation of the slurry, resulting in a change in the rheological properties of the slurry. There is also a problem that lithium by-products react with moisture to generate gas.

[0070] The relationship between the contents of the C and Li elements can be confirmed by analyzing the negative electrode active material by X-ray photoelectron spectroscopy (XPS). Specifically, it can be confirmed by the XPS (Nexsa ESCA System, Thermo Fisher Scientific (NEXSA 1)) depth profile. The depth profile can be calculated using the content (atomic %) of the element at 10 seconds of the depth profile after performing it up to 3000 seconds using monatomic Ar (low current).

[0071] In one embodiment of the present invention, the total content of Li contained in the negative electrode active material may be 0.1 part by weight to 10 parts by weight, 1 part by weight to 10 parts by weight, 5 parts by weight to 10 parts by weight, or 6 parts by weight to 9 parts by weight based on 100 total parts by weight of the negative electrode active material. Specifically, it may be 6 parts by weight to 8 parts by weight, and more specifically, it may be 7 parts by weight to 8 parts by weight. Since there is a problem that the initial efficiency increases but the discharge capacity decreases as the content of Li increases, when the above range is satisfied, appropriate discharge capacity and initial efficiency can be realized.

[0072] The BET specific surface area of the negative electrode active material is 1 m 2 / g to 10 m 2 / g, and specifically, it may be 1 m 2 / g to 5 m 2It may also be / g. When the above range is satisfied, the side reaction between the electrolyte and the negative electrode active material during charging and discharging of the battery can be reduced, so that the life characteristics of the battery are improved.

[0073] The average particle size (D 50 ) of the negative electrode active material may be 0.1 μm to 30 μm, specifically may be 1 μm to 20 μm, and more specifically may be 1 μm to 15 μm. When the above range is satisfied, the structural stability of the active material during charge and discharge can be achieved, preventing the problem that the volume expansion / contraction level increases due to the excessive increase in particle size, and preventing the problem that the initial efficiency decreases due to the excessive decrease in particle size.

[0074] <Method for manufacturing negative electrode active material> One embodiment of the present invention is to form silicon-based particles containing SiO x (0 <x <2) and Li, and having a first carbon layer on at least a part of the surface; and forming a second carbon layer on at least a part of the silicon-based particles, and providing a method for manufacturing the negative electrode active material.

[0075] The silicon-based particles provided with the first carbon layer are formed by heating and vaporizing a mixed powder of Si powder and SiO 2 powder, then depositing the vaporized mixed gas to form preliminary particles; forming a first carbon layer on the preliminary particles; and heat-treating after mixing the preliminary particles with the first carbon layer formed and Li powder.

[0076] Specifically, the mixed powder of the Si powder and SiO 2 powder may be heat-treated under vacuum at 1400 °C to 1800 °C, or 1400 °C to 1600 °C.

[0077] The first carbon layer may be formed by using chemical vapor deposition (CVD) using a hydrocarbon gas or by a method of carbonizing a substance serving as a carbon source.

[0078] Specifically, after the formed preliminary particles are introduced into the reactor, hydrocarbon gas may be deposited by chemical vapor deposition (CVD) at 600 to 1200 °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 900 °C to 1000 °C.

[0079] The step of heat-treating after mixing the preliminary particles with the first carbon layer formed and Li powder may be performed at 700 °C to 900 °C for 4 to 6 hours, and specifically, it may be performed at 800 °C for 5 hours.

[0080] The silicon-based particles may contain the lithium silicate described above. The particle size of the particles can be adjusted by methods such as a ball mill, a jet mill, or air classification, but is not limited thereto.

[0081] At least a part of the surface of the silicon-based particles provided with the first carbon layer as described above is provided with a lithium compound. Specifically, the SiO x (0 < x < 2) is formed to form preliminary particles, and after forming the first carbon layer on the preliminary particles and doping with Li to produce the silicon-based particles described above, a lithium compound, that is, a lithium by-product formed by unreacted lithium, remains near the surface of the silicon-based particles.

[0082] In order to suppress side reactions caused by the unreacted lithium compound, a step of forming a second carbon layer on at least a part of the silicon-based particles may be performed.

[0083] The second carbon layer provided on at least a part of the silicon-based particles may be formed by using a chemical vapor deposition method (CVD) using hydrocarbon gas or by a method of carbonizing a substance serving as a carbon source.

[0084] Specifically, after the formed preliminary particles are introduced into the reactor, hydrocarbon gas may be formed by chemical vapor deposition (CVD) at a temperature of less than 800°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 a temperature of 600 to less than 800°C. For example, it may be carried out under heat treatment conditions of 700°C for 4 hours in an atmosphere of acetylene or propane gas.

[0085] By heat-treating at less than 800°C as described above, the degree of amorphousness of the carbon layer can be increased and formed. By increasing the degree of amorphousness of the carbon layer, residual lithium by-products can be effectively coated, so there is an effect of minimizing side reactions with moisture and an effect of improving rapid charging performance. On the other hand, when the degree of amorphousness of the carbon layer is low, that is, when the crystallinity is high, the carbon coating is performed at a relatively high temperature, so there is a disadvantage that the size of the Si crystal grains increases and the life performance deteriorates.

[0086] When analyzing the negative electrode active material manufactured as described above by Raman spectroscopy, the ratio of the peak intensity of the D band to the peak intensity of the G band (D / G band ratio) may be 1.0 or more.

[0087] <Negative electrode> The negative electrode according to an embodiment of the present invention may include the negative electrode active material described above.

[0088] 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. Further, the negative electrode active material layer may further include a binder and / or a conductive material.

[0089] 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 the current collector, followed by drying and rolling.

[0090] The negative electrode slurry contains the negative electrode active material, a binder, and / or a conductive material. The negative electrode slurry may further contain an additional negative electrode active material.

[0091] As the additional negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. 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, or Al alloys; SiO β (0 < β < 2), SnO 2 , metal oxides capable of doping and undoping lithium such as vanadium oxides, lithium titanate oxides, and lithium vanadate oxides; or composites containing the metallic compound and the carbonaceous material such as Si-C composites or Sn-C composites, and mixtures of any one or more of these may be used. Further, a thin film of metallic lithium may be used as the negative electrode active material. Also, as the carbon material, either low-crystalline carbon or high-crystalline carbon may be used. Representative examples of low-crystalline carbon are soft carbon and hard carbon, and representative examples of high-crystalline carbon are amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0092] The additional negative electrode active material may be a carbon-based negative electrode active material. In one embodiment of the present invention, the weight ratio of the negative electrode active material and the additional negative electrode active material contained in the negative electrode slurry may be 10:90 to 90:10, and specifically may be 10:90 to 50:50.

[0093] The negative electrode current collector only needs to be one that does not induce a chemical change in the battery and has conductivity, and is not particularly limited. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or one obtained by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Specifically, a transition metal that adsorbs carbon well, 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 the thickness of the current collector is not limited thereto.

[0094] 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, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, poly acrylic acid, and substances in which hydrogen thereof is substituted with Li, Na, or Ca, etc., and may also include various copolymers thereof.

[0095] The conductive material is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. 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 fluorocarbons, aluminum, 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 may be used.

[0096] The negative electrode slurry may contain a thickening agent. The thickening agent may be carboxymethyl cellulose (CMC), but is not limited thereto, and thickening agents used in the technical field may be appropriately employed.

[0097] The negative electrode slurry may contain a solvent for forming the negative electrode slurry. Specifically, the solvent for forming the negative electrode slurry is at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol in terms of facilitating the dispersion of components. Specifically, it may contain distilled water.

[0098] <Secondary battery> The secondary battery according to an embodiment of the present invention may include a negative electrode according to the above-described embodiment. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode is the same as the above-described negative electrode. Since the negative electrode has been described above, a specific description thereof will be omitted.

[0099] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0100] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface-treating the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Further, the positive electrode current collector may usually have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. For example, it may be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0101] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ); a compound substituted with one or more transition metals; lithium iron oxide such as LiFe 3 O 4 ; lithium manganese oxides represented by the chemical formula Li 1+c1 Mn 2-c1 O 4 (0 ≦ c1 ≦ 0.33), LiMnO 3 , LiMn 2 O 3 , LiMnO 2 ; lithium copper oxide (Li 2 CuO 2 ); vanadium oxides such as LiV 3 O 8 , V 2 O 5 , Cu 2 V 2 O 7 ; Ni-site type lithium nickel oxides represented by the chemical formula LiNi 1-c2 M c2 O 2 (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 oxides represented by the chemical formula LiMn 2-c3 M c3 O 2(Here, 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 Li 2 Mn 3 MO 8 (Here, M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn) lithium manganese composite oxide represented by; or a part of Li in the chemical formula is substituted with an alkaline earth metal ion LiMn 2 O 4 and the like, but not limited thereto. The positive electrode may be a lithium metal (Li-metal).

[0102] The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder together with the above-described positive electrode active material.

[0103] At this time, the positive electrode conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it can be used without particular limitation as long as it has electron conductivity without causing a chemical change. 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 powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one kind alone or a mixture of two or more kinds thereof may be used.

[0104] In addition, the positive electrode binder serves to improve the adhesion between the positive electrode active material particles and the adhesive force between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc. One kind alone or a mixture of two or more kinds thereof may be used.

[0105] The separator separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Usually, it can be used without particular limitation as long as it is used as a separator in a secondary battery. In particular, it preferably has a low resistance to the ion migration of the electrolyte and excellent electrolyte moisture retention ability. Specifically, a porous polymer film, for example, a porous polymer film 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 of two or more layers thereof may be used. Also, a normal porous non-woven fabric, for example, a non-woven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc. may be used. Further, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer substance may be used, and it may be selectively used as a single layer or a multi-layer structure.

[0106] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used during the manufacture of lithium secondary batteries. Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0107] Examples of the non-aqueous organic solvent 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, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, etc.

[0108] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, can be preferably used as high-viscosity organic solvents because they have a high dielectric constant and can well dissociate lithium salts. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constant such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, an electrolyte having high electrical conductivity can be produced, so it can be more preferably used.

[0109] A lithium salt may be used as the metal salt. The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte. For example, as an anion of the lithium salt, F - , Cl - , I - , NO 3 - , N(CN) 2 - , BF 4 - , ClO 4 - , PF 6 - , (CF3 ) 2 PF 4 - 、(CF 3 ) 3 PF 3 - 、(CF 3 ) 4 PF 2 - 、(CF 3 ) 5 PF - 、(CF 3 ) 6 P - 、CF 3 SO 3 - 、CF 3 CF 2 SO 3 - 、(CF 3 SO 2 ) 2 N - 、(FSO 2 ) 2 N - 、CF 3 CF 2 (CF 3 ) 2 CO - 、(CF 3 SO 2 ) 2 CH - 、(SF 5 ) 3 C - 、(CF 3 SO 2 ) 3 C - 、CF 3 (CF 2 ) 7 SO 3 - 、CF 3 CO 2 - 、CH 3 CO 2 - 、SCN - 、および(CF 3 CF 2 SO 2 ) 2 N - One or more selected from the group consisting of may be used.

[0110] In addition to the constituent components of the electrolyte, for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, improving the discharge capacity of the battery, etc., the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, triamide hexaline, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0111] According to another embodiment of the present invention, there are provided a battery module including the secondary battery as a unit cell and a battery pack including the same. Since the battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and cycle characteristics, they 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.

Examples

[0112] Hereinafter, preferred examples are presented to assist in the understanding of the present invention. However, it is obvious to those skilled in the art that the said examples are merely illustrative of the description, and various changes and modifications are possible within the scope of the description and the scope of the technical idea. Needless to say, such variations and modifications belong to the scope of the appended claims.

[0113] <Examples and Comparative Examples> Example 1 Si and SiO 2 After mixing 94 g of a powder obtained by mixing Si and SiO in a molar ratio of 1:1 in a reactor, it was vacuum-heated at a sublimation temperature of 1,400°C. Then, the vaporized Si and SiO 2The mixed gas was reacted in a cooling zone under vacuum with a cooling temperature of 800 °C and condensed into a solid phase. The preliminary silicon-based negative electrode active material was pulverized using a ball mill for 3 hours to produce silicon-based particles with a size of 6 μm. Then, while flowing Ar gas to maintain an inert atmosphere, the silicon-based negative electrode active material was placed in the hot zone of a CVD apparatus, and using Ar as a carrier gas, the methane was blown into the hot zone at 900 °C and reacted for 10 -1 torr for 5 hours to form a first carbon layer on the surface of the silicon-based negative electrode active material. Then, 6 g of Li metal powder was added, and additional heat treatment was performed at a temperature of 800 °C in an inert atmosphere to produce silicon-based particles.

[0114] The silicon-based particles were placed in a reactor, and using Ar as a carrier gas, the acetylene was blown into the hot zone at 700 °C and reacted for 10 -1 torr for 3 hours to produce a negative electrode active material with a second carbon layer formed on the surface of the silicon-based particles.

[0115] Example 2 A negative electrode active material was produced in the same manner as in Example 1, except that the reaction was carried out for 3 hours during the formation of the first carbon layer.

[0116] Example 3 A negative electrode active material was produced in the same manner as in Example 1, except that the reaction was carried out for 4 hours during the formation of the first carbon layer and for 4 hours during the formation of the second carbon layer.

[0117] Example 4 A negative electrode active material was produced in the same manner as in Example 1, except that the reaction was carried out for 4 hours during the formation of the second carbon layer.

[0118] Comparative Example 1 A negative electrode active material was produced in the same manner as in Example 1, except that the second carbon layer was not formed.

[0119] Comparative Example 2 The temperature for forming the second carbon layer was 900 °C, and the negative electrode active material was produced in the same manner as in Example 1 except that the reaction was carried out for 3 hours.

[0120] Comparative Example 3 The negative electrode active material was produced in the same manner as in Example 1 except that the second carbon layer was not formed and the silicon-based particles were pickled with 0.1 M concentration of HCl. The compositions of the negative electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 3 were measured as follows and are shown in Table 1 below.

[0121]

Table 1

[0122] <Measurement of the content of the carbon layer> The contents of the first carbon layer and the second carbon layer were analyzed using a CS-analyzer (CS-800, Eltra), and the content of the second carbon layer was confirmed by calculating “the content of the total carbon layer - the content of the first carbon layer”.

[0123] <D of the negative electrode active material 50 measurement> The average particle size (D 50 ) of the negative electrode active material particles was measured by the laser scattering method of Microtrac.

[0124] <Measurement of the BET specific surface area of the negative electrode active material> The BET specific surface area of the negative electrode active material was measured using a BET measuring device (BEL-SORP-MAX, Nippon Bell) by removing gas (degassing) at 200 °C for 8 hours and performing N 2 adsorption / desorption at 77K.

[0125] <Measurement of the content of Li contained in the negative electrode active material> The content of Li atoms contained in the negative electrode active material was confirmed by ICP analysis using an inductively coupled plasma optical emission spectrometer (ICP-OES manufactured by Perkin-Elmer 7300, AVIO 500).

[0126] <Measurement of D / G band ratio by Raman analysis> The crystallinity of the entire carbon layer contained in the negative electrode active material was confirmed by calculating the D / G band ratio through Raman analysis.

[0127] Specifically, it was measured using a Renishaw 2000 Raman microscope system and a laser excitation of 532 nm, and in order to avoid the thermal effect of the laser, it was measured with a low laser output density and an exposure time of 30 seconds using a 100-fold optical lens. To reduce the deviation by position, a total of 25 points were measured for a region of 5 μm × 5 μm, and after fitting using a Lorentzian function, the average values of the D band and the G band were calculated and computed.

[0128] <Relationship between C and Li by X-ray photoelectron spectroscopy (XPS)> The C / Li ratio of the negative electrode active material was confirmed by the XPS (Nexsa ESCA System, Thermo Fisher Scientific (NEXSA 1)) depth profile. Specifically, the depth profile was performed up to 3,000 seconds using monatomic Ar (low current), and (content of C element) / (content of Li element + content of C element) was calculated using the elemental content (atomic %) at 10 seconds of the depth profile.

[0129] The measurement and data processing conditions are as follows. -X-ray source:Monochromated Al Kα(1486.6eV) -X-ray spot size:400μm - Sputtering gun: Monatomic Ar (energy: 1000 eV, current: low, raster width: 2 mm) - Etching rate: 0.09 nm / s for Ta 2 O 5 - Operation Mode: CAE (Constant Analyzer Energy) mode - Survey scan: pass energy 200 eV, energy step 1 eV - Narrow scan: scanned mode, pass energy 50 eV, energy step 0.1 eV - Charge compensation: flood gun off - SF: Al THERMO1 - ECF: TPP - 2M - BG subtraction: Shirley

[0130] <Experimental Example: Evaluation of Discharge Capacity, Initial Efficiency, and Life (Capacity Retention) Characteristics> Anode and battery were manufactured using the anode active materials of the examples and comparative examples, respectively.

[0131] Manufacture of negative electrode As the anode material, a mixture of the anode active material produced in Example 1 and graphite (average particle size (D 50 ): 20 μm) with a weight ratio of 15:85 was used.

[0132] The anode material, styrene - butadiene rubber (SBR) as a binder, Super C65 as a conductive material, and carboxymethyl cellulose (CMC) as a thickener were mixed at a weight ratio of 96:2:1:1, and this was added to distilled water as a solvent for forming the anode slurry to produce the anode slurry.

[0133] On one side of a copper current collector (thickness: 15 μm) as the anode current collector, the anode slurry was applied at 3.6 mAh / cm 2Coated with the loading amount of , roll-pressed, and dried in a vacuum oven at 130 °C for 10 hours to form a negative electrode active material layer (thickness: 50 μm) to produce a negative electrode (negative electrode thickness: 65 μm).

[0134] In addition, except that the negative electrode active materials of Examples 2 to 4 and Comparative Examples 1 to 3 were used instead of the negative electrode active material of Example 1, the negative electrodes of Examples 1 to 4 and Comparative Examples 1 to 3 were produced in the same manner as in Example 1.

[0135] Manufacture of secondary battery A lithium metal foil was prepared as the positive electrode. A porous polyethylene separator was interposed between the negative electrodes and the positive electrodes of Examples 1 to 4 and Comparative Examples 1 to 3 manufactured above, an electrolytic solution was injected, and coin-type half cells were produced respectively.

[0136] As the electrolytic solution, vinylene carbonate (VC) was dissolved at 0.5 wt% in a solution obtained by mixing ethyl methyl carbonate (EMC) and ethylene carbonate (EC) at a volume ratio of 7:3, and LiPF 6 dissolved at a concentration of 1 M was used.

[0137] Evaluation of discharge capacity, initial efficiency, and capacity retention rate For the secondary batteries manufactured in Examples 1 to 4 and Comparative Examples 1 to 3, the discharge capacity, initial efficiency, and cycle capacity retention rate were evaluated using an electrochemical charge and discharge device.

[0138] The evaluation of the cycle capacity retention rate was carried out at a temperature of 25 °C. The first cycle and the second cycle were charged and discharged at 0.1 C, and from the third cycle, the charge and discharge were carried out at 0.5 C (charge condition: CC / CV, 5 mV / 0.005 C cut-off, discharge condition: CC, 1.5 V cut off).

[0139] From the results of the first charge and discharge, the discharge capacity (mAh / g) and the initial efficiency (%) were derived. The capacity retention rate was calculated as follows. Capacity retention rate (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the first cycle)} × 100 (In the above formula, N is an integer of 1 or more.) The capacity retention rate (%) in the 50th cycle is shown in Table 2 below.

[0140]

Table 2

[0141] The negative electrode active material according to the present invention is characterized by including a first carbon layer and a second carbon layer. Since the second carbon layer effectively coats lithium by-products and there are no lithium by-products on the surface of the negative electrode active material, it is possible to prevent the phenomenon that lithium by-products react with the moisture in the slurry and deteriorate the physical properties of the slurry. Further, by increasing the degree of amorphization of the carbon layer, residual lithium by-products can be effectively coated, the size of the Si crystal grains can be effectively controlled, and the stability of the electrode state and / or the charge-discharge capacity can be improved.

[0142] In Table 2 above, in Examples 1 to 4, (Content of C element) / (Content of Li element + Content of C element) is 60% or more, there are fewer lithium by-products on the surface of the negative electrode active material than in the comparative examples, the D / G band ratio is 1.0 or more, and the degree of amorphization of the carbon layer is higher than that of the comparative examples. Therefore, it can be confirmed that all of the discharge capacity, initial efficiency, and capacity retention rate of the battery are excellent.

[0143] On the other hand, Comparative Example 1 did not include the second carbon layer and had a low (Content of C element) / (Content of Li element + Content of C element). From this, it can be seen that the content of lithium by-products present on the surface of the negative electrode active material is high due to the absence of the second carbon layer. From this, it was confirmed that a side reaction occurred between the negative electrode active material and the moisture in the slurry, resulting in a decrease in the initial efficiency and the capacity retention rate.

[0144] In Comparative Example 2, since the coating of the second carbon layer is performed at a relatively high temperature, the crystallinity of the carbon layer is high, which can be confirmed from the fact that the D / band ratio is 0.91. Therefore, it was confirmed that the carbon coating at a high temperature increased the grain size of Si crystal grains in the negative electrode active material and significantly reduced the life performance.

[0145] Comparative Example 3 is obtained by acid-washing lithium by-products on silicon-based particles after the coating of the first carbon layer. During the acid-washing, Li elutes from the Li compound (lithium silicate) contained in the silicon-based particles in the negative electrode active material, and the surface structure of the negative electrode active material collapses. Therefore, it was confirmed that the efficiency and the capacity retention rate decrease.

Claims

1. SiO ｘ Silicon-based particles containing Si (0 < x < 2) and Li and having a first carbon layer provided on at least a part of the surface, and A negative electrode active material comprising a second carbon layer provided on at least a part of the silicon-based particles, wherein a lithium compound is present between the silicon-based particles and the second carbon layer, when analyzed by Raman spectroscopy, the D / G band ratio, which is the ratio of the peak intensity of the D band to the peak intensity of the G band, is 1.0 or more, when analyzed by X-ray photoelectron spectroscopy (XPS), (content of C element) / (content of Li element + content of C element) is 0.6 or more, the negative electrode active material.

2. The negative electrode active material according to claim 1, wherein the Li exists in the form of lithium silicate.

3. The lithium compound is Li ２ O, LiOH, and Li ２ CO ３ The negative electrode active material according to claim 1, comprising one or more selected from the group consisting of

4. The total content of Li contained in the negative electrode active material is 5 parts by weight to 10 parts by weight based on 100 parts by weight in total of the negative electrode active material, the negative electrode active material according to claim 1.

5. The first carbon layer is contained in an amount of 1 part by weight to 5 parts by weight based on 100 parts by weight in total of the negative electrode active material, the negative electrode active material according to claim 1.

6. The lithium compound is contained in an amount of 0.1 part by weight to 3 parts by weight based on 100 parts by weight in total of the negative electrode active material, the negative electrode active material according to claim 1.

7. The second carbon layer is contained in an amount of 1 part by weight to 5 parts by weight based on 100 parts by weight in total of the negative electrode active material, the negative electrode active material according to claim 1.

8. The content of the total carbon layers (the first carbon layer and the second carbon layer) contained in the negative electrode active material is 2 parts by weight to 10 parts by weight based on 100 parts by weight in total of the negative electrode active material, the negative electrode active material according to claim 1.

9. SiO ｘ forming silicon-based particles containing Si (0 < x < 2) and Li and having a first carbon layer provided on at least a part of the surface, and A step of forming a second carbon layer on at least a part of the silicon-based particles The method for producing a negative electrode active material according to any one of claims 1 to 8.

10. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 8.

11. A secondary battery comprising the negative electrode according to claim 10.

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