Composite negative electrode active material, method for producing the same, negative electrode including the same, and secondary battery

Abstract

The present invention relates to a composite negative electrode active material comprising silicon-based oxide particles and a metal distributed on, inside, or on and inside the silicon-based oxide particles, the composite negative electrode active material having an average diameter of 65 nm or less obtained by a specific method. The composite negative electrode active material has a uniform and small diameter of the aggregates, which can minimize the deterioration of the life performance of the composite negative electrode active material caused by the expansion and contraction of the volume of silicon during charging and discharging.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0088020, filed with the Korean Intellectual Property Office on July 5, 2021, the entire contents of which are incorporated herein by reference.

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

[0003] In recent years, with the rapid spread of battery-powered electronic devices such as mobile phones, laptop computers, and electric vehicles, the demand for secondary batteries that are small, lightweight, and have relatively high capacity has been rapidly increasing. In particular, lithium secondary batteries, which are lightweight and have high energy density, have attracted attention as power sources for portable devices. For this reason, active research and development efforts are being made to improve the performance of lithium secondary batteries.

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

[0005] Among negative electrode active materials, silicon-based active materials have attracted attention because they have a capacity approximately 10 times higher than carbon-based active materials, and their high capacity allows them to achieve high energy density even with thin electrodes. However, silicon-based active materials are not widely used due to the problem of volume expansion during charging and discharging, which causes cracks and damage to the active material particles and reduces lifespan characteristics.

[0006] Therefore, there is a demand for the development of secondary batteries that can improve the life characteristics while realizing high capacity and energy density of silicon-based active materials.

[0007] Korean Patent Publication No. 10-2017-0074030 relates to a negative electrode active material for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same, and discloses a negative electrode active material including a porous silicon-carbon composite, but there are limitations in solving the above-mentioned problems. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Korean Patent Publication No. 10-2017-0074030 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a composite negative electrode active material that minimizes the effect of volume expansion and contraction during charge and discharge when using a silicon-based active material, prevents damage to the active material, and has excellent life performance.

[0010] Another object of the present invention is to provide a method for producing the above-mentioned composite negative electrode active material.

[0011] It is still another object of the present invention to provide a negative electrode and a secondary battery including the above-mentioned composite negative electrode active material. [Means for solving the problem]

[0012] The present invention provides a composite negative electrode active material comprising silicon-based oxide particles and a metal distributed on the surface, inside, or on the surface and inside of the silicon-based oxide particles, wherein the composite negative electrode active material contains one or more aggregates containing one or more selected from silicon, oxygen, and a metal, the metal including at least one selected from the group consisting of Li, Mg, and Al, and the average diameter of the aggregates obtained by a method comprising the following steps (a) to (e) provides a composite negative electrode active material having an average diameter of 65 nm or less. (a) photographing the composite negative electrode active material with a scanning electron microscope to obtain a square scanning electron microscope photograph; (b) selecting two different sides in the scanning electron micrograph, selecting one point from each of the two selected sides, and obtaining a straight line connecting the two points selected from the two sides; (c) using a digital image analysis program to obtain a gray profile graph of the line, the vertical axis being the gray value and the horizontal axis being the distance of the line, and then calculating an average gray value; (d) obtaining the number of intersections between the reference line having the average gray value and parallel to the horizontal axis and the gray profile in the linear gray profile graph, and then defining the value calculated by the following mathematical formula 1 as the diameter of the aggregate; and (e) performing steps (a) to (d) two or more times to obtain an average value of the diameters of the aggregates; [Mathematical formula 1] Diameter of the cluster = length of the line / (number of intersections between the reference line and the density profile + 1).

[0013] The present invention also provides a method for producing the aforementioned composite negative electrode active material, comprising the steps of: subjecting a silicon-based oxide containing a compound represented by the following chemical formula 2 to a first heat treatment to generate a first vapor; subjecting a metal containing at least one selected from the group consisting of Li, Mg, and Al to a second heat treatment to generate a second vapor; mixing the first vapor and the second vapor to cause a gas-phase reaction; and cooling the gas-phase reaction to obtain a silicon-based oxide-metal composite; wherein the silicon-based oxide contains a compound represented by the following chemical formula 2, and the difference between the temperature during the first heat treatment and the temperature during the cooling is 400°C to 550°C. [Chemical formula 2] SiO a (0 <a<2)。

[0014] The present invention also provides a negative electrode, comprising: a negative electrode current collector; and a negative electrode active material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the composite negative electrode active material described above.

[0015] The present invention also provides a secondary battery comprising the above-mentioned negative electrode; a positive electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte. [Effects of the Invention]

[0016] The composite anode active material of the present invention includes silicon-based oxide particles and a metal distributed on, within, or both the surface and the interior of the silicon-based oxide particles. The composite anode active material has an average aggregate diameter of 65 nm or less, as measured by a specific method. The aggregates are present within the composite anode active material and include at least one selected from silicon, oxygen, and a metal. The average aggregate diameter of the composite anode active material is controlled to a small and uniform level within the above-mentioned range, thereby minimizing the effects of volume expansion and contraction during charge and discharge of the composite anode active material, significantly reducing the possibility of damage during charge and discharge, and improving initial efficiency to an excellent level. Therefore, the life performance of anodes and secondary batteries containing the composite anode active material can be improved. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 1. [Figure 2] FIG. 10 is a diagram for explaining the process of calculating the average diameter of aggregates according to the present invention. [Figure 3] FIG. 10 is a diagram for explaining the process of calculating the average diameter of aggregates according to the present invention. [Figure 4] FIG. 10 is a diagram for explaining the process of calculating the average diameter of aggregates according to the present invention. [Figure 5] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 2. [Figure 6] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 3. [Figure 7] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 4. [Figure 8] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 5. [Figure 9] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 6. [Figure 10] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 7. [Figure 11] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Example 8. [Figure 12] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Comparative Example 1. [Figure 13] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Comparative Example 2. [Figure 14] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Comparative Example 3. [Figure 15] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Comparative Example 4. [Figure 16]10 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Comparative Example 5. [Figure 17] 1 is a scanning electron microscope (SEM) photograph of the composite negative electrode active material of Comparative Example 6. DETAILED DESCRIPTION OF THE INVENTION

[0018] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0019] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless otherwise clearly indicated in the context.

[0020] It should be understood that in this specification, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but do not preclude the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0021] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured, for example, by using the laser diffraction method. The laser diffraction method is generally capable of measuring particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0022] The present invention will be specifically described below.

[0023] <Composite negative electrode active material> The present invention relates to a composite negative electrode active material, which can be preferably used in a negative electrode for a lithium secondary battery.

[0024] The composite negative electrode active material of the present invention is a composite negative electrode active material comprising silicon-based oxide particles; and a metal distributed on the surface, inside, or on the surface and inside of the silicon-based oxide particles; wherein the composite negative electrode active material contains one or more aggregates containing one or more selected from silicon, oxygen, and a metal, and the metal includes at least one selected from the group consisting of Li, Mg, and Al, and the average diameter of the aggregates obtained by a method comprising the following steps (a) to (e) is 65 nm or less: (a) photographing the composite negative electrode active material with a scanning electron microscope to obtain a square scanning electron microscope photograph; (b) selecting two different sides in the scanning electron micrograph, selecting one point from each of the two selected sides, and obtaining a straight line connecting the two points selected from the two sides; (c) using a digital image analysis program to obtain a gray profile graph of the line, the vertical axis being the gray value and the horizontal axis being the distance of the line, and then calculating an average gray value; (d) obtaining the number of intersections between the reference line having the average gray value and parallel to the horizontal axis and the gray profile in the linear gray profile graph, and then defining the value calculated by the following mathematical formula 1 as the diameter of the aggregate; and (e) performing steps (a) to (d) two or more times to obtain an average diameter of the aggregates. [Mathematical formula 1] Diameter of the cluster = length of the line / (number of intersections between the reference line and the gray profile + 1)

[0025] In general, silicon-based active materials are known to have a capacity approximately 10 times higher than carbon-based active materials. Therefore, when silicon-based active materials are used in negative electrodes, it is expected that thin-film electrodes with high energy density can be realized even with a small thickness.

[0026] However, silicon-based active materials have the problem of shortened lifespan due to volume expansion / contraction caused by lithium insertion / extraction during charging / discharging.In addition, the presence of irreversible sites in silicon-based active materials causes an irreversible reaction in which some of the lithium that moves to the negative electrode during initial charging does not return to the positive electrode during discharge.

[0027] Previously, research has been conducted to dope and react metals into silicon-based active materials to reduce the irreversible phase of the silicon-based active material and increase the initial efficiency, but there are still limitations in solving the problem of reduced lifespan due to the expansion / contraction of the volume of the silicon-based active material.

[0028] To address these issues, the present invention provides a composite anode active material comprising silicon-based oxide particles and a metal distributed on, within, or both the surface and the interior of the silicon-based oxide particles, characterized in that the average diameter of the aggregates measured by a specific method is 65 nm or less. The aggregates are present within the composite anode active material and include at least one selected from silicon, oxygen, and metal. The composite anode active material of the present invention minimizes the effects of volume expansion and contraction during charge and discharge, significantly reduces the risk of damage during charge and discharge, and improves initial efficiency to an excellent level. Therefore, the lifespan of anodes and secondary batteries containing the composite anode active material can be improved.

[0029] The composite negative electrode active material of the present invention includes silicon-based oxide particles; and a metal distributed on the surface, inside, or both the surface and inside of the silicon-based oxide particles.

[0030] The silicon-based oxide particles can insert / desorb lithium and can function as core particles of the composite negative electrode active material.

[0031] The silicon-based oxide particles may contain a compound represented by the following Chemical Formula 1. [Chemical Formula 1] SiO x In Chemical Formula 1, 0 < x < 2 may be satisfied.

[0032] In Chemical Formula 1, in the case of SiO2 (when x = 2 in Chemical Formula 1), since it does not react with lithium ions and cannot store lithium, x is preferably within the above range. Specifically, in Chemical Formula 1, in terms of the structural stability of the composite negative electrode active material, x may be 0.5 ≤ x ≤ 1.5.

[0033] The x corresponds to the number ratio of O to Si contained in the SiO x (0 < x < 2). When the composite negative electrode active material contains the SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0034] The average particle size (D 50 ) of the silicon-based oxide particles is preferably 1 μm to 15 μm, more preferably from 2 μm to 8 μm, in terms of achieving the structural stability of the composite negative electrode active material during charge and discharge, preventing the problem that the volume expansion / shrinkage level increases due to an excessively large particle size, and preventing the problem that the initial efficiency decreases due to an excessively low particle size.

[0035] The silicon-based oxide particles may be contained in the composite negative electrode active material at 75% to 99% by weight, preferably 80% to 97% by weight. When in the above range, since the content of the metal can be improved to an appropriate level, it is preferable in terms of improving the capacity of the negative electrode.

[0036] The silicon-based oxide particles may include Si crystal grains having a crystal grain size of 3 nm to 20 nm, specifically 6 nm to 15 nm, and more specifically 8 nm to 11 nm. In the present invention, "crystal grain" refers to a single-crystal particle unit having a regular atomic arrangement. When the active material includes Si crystal grains having a crystal grain size within this range, changes in the internal structure of the active material during charge and discharge are minimized, thereby further improving the lifespan.

[0037] The size of the Si crystal grains can be obtained by performing XRD (X-ray diffraction) analysis on the silicon-based oxide particles using an XRD analyzer, determining the full width at half maximum and angle (θ) of the Si (220) plane peak, and then substituting the values ​​into the Scherrer equation.

[0038] [Scherrer's formula] Si crystal grain size (nm) = (K × λ) / (FWHM × Cosθ)

[0039] In the above formula, K is the Scherrer constant, λ is the wavelength of the light source, FWHM is the full width at half maximum of the Si (220) plane peak in XRD analysis, and Cosθ is the cosine value of the angle θ corresponding to the Si (220) plane peak.

[0040] The metal may be distributed on the surface, inside, or both the surface and the inside of the silicon-based oxide particles, and may be distributed on the surface and / or inside of the silicon-based oxide particles in a doped form.

[0041] The metal may be distributed on the surface and / or inside of the silicon-based oxide particles to control the volume expansion / contraction of the silicon-based oxide particles to an appropriate level and prevent damage to the composite anode active material. The metal may also be included to reduce the proportion of irreversible phases (e.g., SiO2) in the silicon-based oxide particles, thereby increasing the efficiency of the composite anode active material.

[0042] The metal may include at least one selected from the group consisting of Li, Mg, and Al. Specifically, at least one selected from the group consisting of Li and Mg may be included in order to achieve excellent effects such as controlling the volume expansion of silicon-based oxide particles, preventing damage, and improving initial efficiency.

[0043] The metal atoms may be present in the form of a metal silicate, which can be classified into crystalline metal silicates and amorphous metal silicates.

[0044] When the metal atom is Li, Li may be present in the core in the form of at least one lithium silicate selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5.

[0045] When the metal atom is Mg, Mg may be present in the core in the form of at least one magnesium silicate of Mg2SiO4 and MgSiO3.

[0046] The metal may be contained in the composite negative electrode active material in an amount of 1 to 20% by weight based on the total weight of the silicon-based oxide particles and the metal. When the amount is within this range, the damage prevention effect and life characteristics of the silicon-based oxide particles are more preferably realized, and the excellent capacity characteristics of the silicon-based oxide particles are not impaired.

[0047] Specifically, when the metal includes Mg, the metal may be included in the composite negative electrode active material in an amount of 5 wt% to 15 wt%, more specifically 8 wt% to 13 wt%, based on the total weight of the silicon-based oxide particles and the metal. When the metal content is within this range, the composite negative electrode active material can achieve excellent improvements in capacity characteristics and initial efficiency. Furthermore, even when the metal content of the composite negative electrode active material increases to improve the initial efficiency of the composite negative electrode active material, the average diameter of the aggregates (described below) is controlled to be small and uniform, thereby improving the structural stability of the composite negative electrode active material and preventing a decrease in lifespan performance.

[0048] On the other hand, when the metal includes Li, the metal may be contained in the composite negative electrode active material in an amount of 3 wt% to 12 wt%, more specifically 4 wt% to 8 wt%, based on the total weight of the silicon-based oxide particles and the metal. When the amount is within this range, the capacity characteristics and initial efficiency of the composite negative electrode active material can be improved to an excellent level.

[0049] The composite negative electrode active material of the present invention includes one or more aggregates containing therein at least one selected from silicon, oxygen, and a metal.

[0050] The aggregates are formed by the aggregation and bonding of silicon and oxygen of the silicon-based oxide particles and the metal, and one or more aggregates, specifically two or more aggregates, may be present inside the composite negative electrode active material.

[0051] The aggregates may exist in the form of, for example, aggregates consisting of silicon or containing silicon as a primary component (first aggregates), or aggregates containing silicon, oxygen, and a metal (second aggregates). Specifically, referring to FIG. 1 , a scanning electron micrograph of an exemplary composite negative electrode active material, the compartmentalization of aggregates within the composite negative electrode active material can be seen with the naked eye, and the existence of two main types of aggregates can be confirmed. More specifically, in FIG. 1 , the first aggregates are relatively dark because they consist of silicon or contain silicon as a primary component, and the second aggregates are relatively brighter than the first aggregates due to the presence of oxygen.

[0052] In this specification, the term "aggregate" is defined differently from the term "Si crystal grains." Specifically, the first aggregate, which is made of silicon or contains silicon as a major component, may contain both crystalline Si and amorphous Si. The second aggregate may be made of metal, silicon, and oxygen and may include crystalline metal silicate, amorphous metal silicate, crystalline SiO2, and amorphous SiO2.

[0053] By controlling the size of these aggregates to be small and uniform, the present invention can minimize the effect of volume expansion of silicon, thereby significantly improving the capacity characteristics and initial efficiency of the composite anode active material.

[0054] In the composite negative electrode active material of the present invention, the average diameter of the aggregates obtained by the method including the following steps (a) to (e) is 65 nm or less. (a) photographing the composite negative electrode active material with a scanning electron microscope to obtain a square scanning electron microscope photograph; (b) selecting two different sides in the scanning electron micrograph, selecting one point from each of the two selected sides, and obtaining a straight line connecting the two points selected from the two sides; (c) using a digital image analysis program to obtain a gray profile graph of the line, the vertical axis being the gray value and the horizontal axis being the distance of the line, and then calculating an average gray value; (d) obtaining the number of intersections between the reference line having the average gray value and parallel to the horizontal axis and the gray profile in the linear gray profile graph, and then defining the value calculated by the following mathematical formula 1 as the diameter of the aggregate; and (e) performing steps (a) to (d) two or more times to obtain an average value of the diameters of the aggregates; [Mathematical formula 1] Diameter of the cluster = length of the line / (number of intersections between the reference line and the density profile + 1).

[0055] The method for measuring the diameter of an aggregate (steps (a) to (e)) of the present invention will be described in detail below with reference to Figures 1 to 4. Specifically, Figure 1 is a scanning electron micrograph of an exemplary composite negative electrode active material (Example 1), Figure 2 is a graph of an arbitrary line drawn on the scanning electron micrograph of Figure 1, Figure 3 is a grayscale profile graph of the line drawn using a digital image analysis program (Image J), ​​and Figure 4 is a diagram illustrating a process for determining the intersection of the average grayscale value reference line of the grayscale profile graph with the grayscale profile graph.

[0056] <(a) Step of Obtaining a Scanning Electron Microscope Photograph of the Composite Negative Electrode Active Material> The method for calculating the average diameter of the aggregates includes the step of (a) photographing the composite negative electrode active material with a scanning electron microscope to obtain a rectangular scanning electron microscope photograph.

[0057] A scanning electron microscope is a type of electron microscope that scans the surface of a sample with an electron beam to create an image. As shown in Figure 1, by photographing a composite anode active material through a scanning electron microscope, two-dimensional image information of the composite anode active material can be obtained.

[0058] The scanning electron microscope that can be used in the method for measuring the diameter of the aggregates or the average diameter of the aggregates is not particularly limited, and may be, for example, S-4800 manufactured by Hitachi Corporation.

[0059] Furthermore, the magnification of a scanning electron microscope applicable to the method for measuring the average diameter of aggregates is not particularly limited as long as it does not interfere with the setting of compartments such as aggregates, and may be, for example, 50,000 times to 120,000 times, specifically 100,000 times.

[0060] (b) Step of drawing a straight line on a scanning electron microscope photograph As shown in FIG. 2, the method for calculating the average diameter of the aggregates includes the steps of (b) selecting two different sides in the scanning electron micrograph, selecting one point from each of the two selected sides, and obtaining a straight line connecting the two points selected from the two sides.

[0061] The line is plotted to obtain a shade profile graph, which allows the shade variation of the aggregate to be identified via a digital image analysis program.

[0062] As shown in Figure 2, two different sides of a rectangular scanning electron microscope photograph can be selected, one point on each side can be selected, and a straight line can be obtained by connecting the selected points. For example, the two selected sides of the rectangular scanning electron microscope photograph can be opposite sides that face each other.

[0063] <(c) Step of Obtaining a Gray Profile Graph and an Average Gray Value> The method for calculating the average diameter of the aggregates may include (c) using a digital image analysis program to obtain a gray profile graph of the line, where the vertical axis is the gray value and the horizontal axis is the distance of the line, and then calculating the average gray value.

[0064] The digital image analysis program can generate a gray profile graph showing the gray level of the line. As shown in Figure 3, in the gray profile graph, the vertical axis represents gray value and the horizontal axis represents the distance of the line. The gray value is a relative value representing the gray level of the line and may not have a unit.

[0065] The digital image analysis program is not particularly limited as long as it can obtain the linear grayscale profile, and may be, for example, the Image J program. Specifically, the grayscale profile graph in Figure 3 is drawn using the Image J program.

[0066] In one example, using the digital image analysis program (Image J), ​​the pixel scale is set using the scale bar on the previously taken scanning electron microscope photograph, and then a plot profile is run to obtain a gray profile graph of the line, with the vertical axis representing the gray value and the horizontal axis representing the distance of the line.

[0067] The average gray value means the average value of each gray value according to the distance of the straight line in the gray profile graph.

[0068] <(d) Step for calculating the diameter of the aggregate> The method for calculating the average diameter of the aggregates includes (d) obtaining the number of intersections between the grayscale profile and a reference line having the average grayscale value and parallel to the horizontal axis in the linear grayscale profile graph, and then defining the value calculated using the following mathematical formula 1 as the diameter of the aggregates. [Mathematical formula 1] Diameter of the cluster = length of the line / (number of intersections between the reference line and the gray profile + 1)

[0069] As shown in FIG. 4, a reference line having the average gray value obtained above and parallel to the horizontal axis is plotted on a gray profile graph, and the number of intersections between the gray profile graph and the reference line can be obtained.

[0070] In the present invention, the diameter of the aggregate is defined as the value calculated using the length of the straight line obtained in step (b) and the number of intersections according to Equation 1. This method makes it possible to predict the size of the aggregates present inside the composite negative electrode active material.

[0071] (e) Calculating the average diameter of the aggregates The method for calculating the average diameter of the aggregate may include (e) performing steps (a) to (d) two or more times to determine the average diameter of the aggregate. In order to improve the accuracy of measuring the diameter of the aggregate, the present invention is characterized in that the average diameter of the aggregate is determined by performing steps (a) to (d) two or more times.

[0072] Specifically, steps (a) to (d) may be repeated 10 or more times, specifically 50 times, and the average diameter of the aggregates obtained by repeating these steps can be obtained.

[0073] The step of performing step (a) two or more times may involve photographing different regions of the composite negative electrode active material, or the same region of the composite negative electrode active material. When photographing the same region of the composite negative electrode active material, performing steps (a) to (d) two or more times may involve performing steps (b) to (d) two or more times. That is, two or more different straight lines can be obtained from a scanning electron microscope photograph taken of the same region of the composite negative electrode active material to measure the average diameter of the aggregates.

[0074] The composite negative electrode active material of the present invention may include a region in which the average diameter of the aggregates obtained by the method including steps (a) to (e) is 65 nm or less.

[0075] The method including steps (a) to (e) may be performed on any region of the composite negative electrode active material. That is, step (a) of photographing the composite negative electrode active material with a scanning electron microscope to obtain a rectangular scanning electron microscope photograph may be performed on any region of the composite negative electrode active material. When the average diameter of the aggregates obtained by performing steps (a) to (e) on the any region satisfies 65 nm or less, the composite negative electrode active material is the composite negative electrode active material of the present invention.

[0076] When the average diameter of the aggregates is adjusted to 65 nm or less, the composite anode active material of the present invention can be evaluated as having aggregate diameters or sizes adjusted to a small and uniform level, thereby minimizing the effects of volume expansion and contraction during charge and discharge of the composite anode active material, significantly reducing the possibility of damage during charge and discharge, and improving initial efficiency to an excellent level.

[0077] If the average diameter of the aggregates exceeds 65 nm, the aggregates may have large and inconsistent sizes, making it difficult to control the volumetric expansion and contraction of the composite negative electrode active material, resulting in a significant decrease in lifespan.

[0078] Specifically, the average diameter of the aggregates may be 5 nm to 65 nm, more specifically, 8 nm to 60 nm. When the average diameter is within this range, the influence of volume expansion and contraction during charge and discharge of the composite negative electrode active material is minimized, and the life performance of the secondary battery can be further improved.

[0079] The standard deviation of the diameter of the aggregates obtained in step (e) may be 10 nm or less, specifically 8 nm or less, and within this range, the size of the aggregates can be controlled uniformly, minimizing the effects of volume expansion and contraction during charge and discharge of the composite negative electrode active material.

[0080] The composite anode active material of the present invention may further include a carbon coating layer located on the surface thereof. The carbon coating layer may function as a protective layer to suppress volume expansion of the silicon-based oxide particles and prevent side reactions with the electrolyte.

[0081] The carbon coating layer may cover at least a portion of the surface of the composite negative electrode active material. That is, the carbon coating layer may cover a portion of the surface of the composite negative electrode active material or the entire surface of the composite negative electrode active material. The carbon coating layer may impart conductivity to the composite negative electrode active material, thereby improving the initial efficiency, life characteristics, and battery capacity characteristics of the secondary battery.

[0082] The carbon coating layer may be included in the composite negative electrode active material in an amount of 0.1 wt % to 10 wt %, preferably 3 wt % to 7 wt %, and when included in the above range, the carbon coating layer is preferred in that it can effectively control the volume expansion of the silicon-based oxide particles while preventing side reactions with the electrolyte.

[0083] The carbon coating layer may include at least one of amorphous carbon and crystalline carbon.

[0084] The crystalline carbon may further improve the conductivity of the composite negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0085] The amorphous carbon may be a carbon-based material formed by using at least one carbide or hydrocarbon selected from the group consisting of tar, pitch, and other organic materials as a source in a chemical vapor deposition process.

[0086] The carbon coating layer may be an amorphous carbon coating layer, and may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

[0087] The thickness of the carbon coating layer may be 1 nm to 500 nm, specifically 5 nm to 300 nm, and when the thickness is within this range, the conductivity of the composite negative electrode active material is improved, resulting in improved initial efficiency and lifespan of the battery.

[0088] The average particle size (D 50 ) may be 1 μm to 15 μm, more preferably 2 μm to 8 μm, in order to achieve structural stability of the active material during charge and discharge, to prevent the problem of an excessively large particle size resulting in an increased level of volume expansion / contraction, and to prevent the problem of an excessively small particle size resulting in a decrease in initial efficiency.

[0089] <Method of manufacturing the composite negative electrode active material> The present invention also provides a method for manufacturing a composite negative electrode active material. Specifically, the method for manufacturing the composite negative electrode active material may be the method for manufacturing the composite negative electrode active material described above.

[0090] Specifically, the method for producing a composite negative electrode active material of the present invention includes the steps of: subjecting a silicon-based oxide containing a compound represented by the following chemical formula 2 to a first heat treatment to generate a first vapor; subjecting a metal containing at least one selected from the group consisting of Li, Mg, and Al to a second heat treatment to generate a second vapor; mixing the first vapor and the second vapor to cause a gas-phase reaction; and cooling the gas-phase reaction to obtain a silicon-based oxide-metal composite; wherein the difference between the temperature during the first heat treatment and the temperature during the cooling is 400°C to 550°C. [Chemical formula 2] SiO a (0 <a<2)

[0091] According to the method for producing the composite negative electrode active material of the present invention, in a method of heating a mixture of a silicon-based oxide and a metal and reacting them in the gas phase and then cooling to obtain a silicon-based oxide-metal composite, the difference between the heat treatment temperature of the silicon-based oxide and the cooling temperature is adjusted to the range described above. When adjusting the difference between the heat treatment temperature of the silicon-based oxide and the cooling temperature to the range described above, the cooling rate of the silicon-based oxide-metal composite is adjusted to a preferable level, and it is possible to prevent the problem that the size of the aggregates in the composite negative electrode active material becomes excessively large or becomes non-uniform due to an excessive cooling rate. Therefore, when using the method for producing the composite negative electrode active material described above, it is possible to minimize the problem of deterioration of the life performance exerted on the composite negative electrode active material due to the expansion and contraction of the volume of silicon.

[0092] The method for producing the composite negative electrode active material of the present invention includes a step of subjecting a silicon-based oxide containing a compound represented by the following Chemical Formula 2 to a first heat treatment to generate a first vapor. [Chemical Formula 2] SiO a In Chemical Formula 2, 0 < a < 2 may be satisfied. In Chemical Formula 2, in the case of SiO2 (when a = 2 in Chemical Formula 2), since it does not react with lithium ions and cannot store lithium, a is preferably within the above range.

[0093] a may be 0.8 or more and 1.2 or less, or may be 1.

[0094] The method for manufacturing a composite anode active material according to the present invention is characterized in that a silicon-based oxide containing a compound represented by Formula 2 is used as a source material for the first vapor used to manufacture a silicon-based oxide-metal composite or a composite anode active material. This is distinct from using a mixture of silicon (Si) and silicon dioxide (SiO2) as a source material for forming the silicon-based oxide vapor in the composite anode active material. Silicon dioxide is known to vaporize at temperatures exceeding approximately 2,000°C, and its vaporization temperature decreases upon contact and reaction with silicon. Therefore, when a mixture of silicon and silicon dioxide is used as a source material for forming silicon-based oxide particles, the presence and speed of a vapor-phase reaction may vary depending on whether silicon dioxide and silicon come into contact with each other, making it difficult to ensure uniformity of the vapor-phase reaction. Therefore, according to the method for manufacturing a composite negative electrode active material of the present invention, the uniformity of the gas phase reaction is improved by using a silicon-based oxide containing a compound represented by Chemical Formula 2 as a raw material for forming the first vapor, and in particular, even if the difference between the heating temperature and the cooling temperature is adjusted to 400°C to 550°C, a smooth and uniform reaction can occur.

[0095] The temperature during the first heat treatment may be 1,300°C to 1,500°C, specifically 1,350°C to 1,450°C. When the temperature is within this range, a silicon-based oxide-containing vapor can be sufficiently generated while the difference with the cooling temperature described below can be adjusted to a preferred level, thereby enabling the production of a desired composite negative electrode active material.

[0096] The first heat treatment may be performed in a vacuum atmosphere or a reduced pressure atmosphere of −50 torr to −200 torr (reduced pressure of 50 torr to 200 torr).

[0097] The method for producing a composite negative electrode active material of the present invention includes a step of subjecting a metal containing at least one selected from the group consisting of Li, Mg, and Al to a second heat treatment to generate second vapor.

[0098] The metal may include at least one selected from the group consisting of Li, Mg, and Al, and specifically, may include at least one selected from the group consisting of Li and Mg, which can achieve excellent levels of control of volume expansion of silicon-based oxide particles, prevention of damage, and improvement of initial efficiency.

[0099] In the method for manufacturing a composite anode active material according to the present invention, the metal content of the second vapor may be 1 wt % to 20 wt % based on the total weight of the silicon-based oxide contained in the first vapor and the metal contained in the second vapor. This range is preferable in that it more effectively prevents damage to the silicon-based oxide particles and improves their lifespan, while not impairing the excellent capacity characteristics of the silicon-based oxide particles. The metal content of the composite anode active material may be measured using an inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0100] Specifically, when the metal includes Mg, the metal content may be 5 wt% to 15 wt%, more specifically 8 wt% to 13 wt%, based on the total weight of the silicon-based oxide included in the first vapor and the metal included in the second vapor. When the metal content is within this range, the resulting composite anode active material can exhibit excellent improvements in both capacity and initial efficiency. When the metal includes Li, the metal content may be 3 wt% to 12 wt%, more specifically 4 wt% to 8 wt%, based on the total weight of the silicon-based oxide included in the first vapor and the metal included in the second vapor. This can result in excellent improvements in both capacity and initial efficiency of the resulting composite anode active material.

[0101] The temperature during the second heat treatment may be appropriately selected taking into consideration the sublimation temperature of the metal used, and may be, for example, 500° C. to 1,000° C. Specifically, when the metal contains Mg, the temperature during the second heat treatment may be 750° C. to 1,000° C., and when the metal contains Li, the temperature during the second heat treatment may be 500° C. to 750° C.

[0102] The second heat treatment may be performed in a vacuum atmosphere or a reduced pressure atmosphere of −50 torr to −200 torr.

[0103] On the other hand, in this specification, the terms "first heat treatment" and "second heat treatment" are used to specify that the heat treatment steps are carried out independently, and are not intended to limit the order of the heat treatment steps.

[0104] The method for manufacturing a composite negative electrode active material of the present invention includes a step of mixing the first vapor and the second vapor to cause a gas-phase reaction, specifically, the first vapor and the second vapor may be mixed in a single reactor to cause a gas-phase reaction.

[0105] The gas phase reaction may be carried out for 3 hours to 12 hours, specifically 5 hours to 10 hours, in order to allow the first vapor and the second vapor to react sufficiently.

[0106] The method for producing a composite negative electrode active material of the present invention includes a step of obtaining a silicon-based oxide-metal composite by cooling after the gas-phase reaction. After the first vapor and the second vapor are reacted, the reaction mixture can be cooled to precipitate a silicon-based oxide-metal composite.

[0107] The cooling temperature may be 750°C to 1,100°C, specifically 800°C to 950°C. When the cooling temperature is within this range, the cooling or precipitation is prevented from occurring at an excessively fast rate, thereby allowing small and uniform aggregates to be formed in the composite negative electrode active material.

[0108] The cooling may be carried out for 3 to 12 hours, specifically 5 to 10 hours, in order to carry out a sufficient reaction.

[0109] According to the method for producing the composite negative electrode active material of the present invention, the difference between the temperature during the first heat treatment and the temperature during the cooling is 400°C to 550°C.

[0110] When the difference between the heating temperature and the cooling temperature is adjusted within the above-described range, the cooling rate of the silicon-based oxide-metal composite can be adjusted to a desirable level, thereby preventing the problem of excessively large or inconsistent aggregate size in the composite negative electrode active material due to an excessive cooling rate. Therefore, the above-described method for manufacturing a composite negative electrode active material can minimize the problem of reduced life performance caused by volumetric expansion and contraction of silicon.

[0111] If the difference between the heating temperature and the cooling temperature exceeds 550°C, the silicon-based oxide-metal composite is cooled rapidly, making it difficult to ensure uniformity in the diameter of the aggregates and causing the aggregates to grow excessively large, making it difficult to control the volumetric expansion of the silicon. Furthermore, if the difference between the heating temperature and the cooling temperature is less than 400°C, excessively slow cooling can cause the aggregates to grow excessively. Therefore, in order to reduce the diameter or size of the aggregates and ensure uniformity, it is necessary to adjust the difference between the heating temperature and the cooling temperature within the above-mentioned temperature range.

[0112] Specifically, the difference between the heating temperature and the cooling temperature may be 420°C to 520°C. In this case, the diameter or size of the aggregates can be controlled to be uniform and small, thereby minimizing the problem of reduced life performance of the composite negative electrode active material due to volumetric expansion and contraction of silicon.

[0113] The method for producing a composite negative electrode active material of the present invention may further include a step of pulverizing the silicon-based oxide-metal composite.

[0114] The average particle size (D 50 ) may be preferably 1 μm to 15 μm, more preferably 2 μm to 8 μm, in order to achieve structural stability of the composite negative electrode active material during charge and discharge, to prevent the problem of an excessively large particle size resulting in an increased level of volume expansion / contraction, and to prevent the problem of an excessively small particle size resulting in a decrease in initial efficiency.

[0115] The method for manufacturing a composite anode active material of the present invention may further include forming a carbon coating layer on the surface of the pulverized silicon-based oxide-metal composite. The carbon coating layer may function as a protective layer that appropriately controls volume expansion of the composite anode active material in response to charge and discharge and prevents side reactions with the electrolyte. The composite anode active material of the present invention may include the silicon-based oxide-metal composite described above or a silicon-based oxide-metal composite having a carbon coating layer formed thereon.

[0116] The step of forming the carbon coating layer may be performed by chemical vapor deposition (CVD), specifically, by using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene. This method allows the carbon coating layer to be formed uniformly on the surface of the pulverized silicon-based oxide-metal composite, thereby smoothly controlling the volume expansion of the composite negative electrode active material and preventing side reactions with the electrolyte.

[0117] The step of forming the carbon coating layer may be performed at a temperature of 800 to 1,300°C, preferably 900 to 1,000°C.

[0118] The other carbon coating layers have been described above.

[0119] <Negative electrode> The present invention also provides a negative electrode containing the above-described negative electrode active material.

[0120] Specifically, the negative electrode of the present invention includes a negative electrode current collector; and a negative electrode active material layer located on at least one surface of the negative electrode current collector, the negative electrode active material layer including the composite negative electrode active material described above.

[0121] The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. Specifically, the negative electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, and an aluminum-cadmium alloy, and may specifically include copper.

[0122] The negative electrode current collector may generally have a thickness of 3 μm to 500 μm.

[0123] The negative electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0124] The negative electrode active material layer may be located on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be located on one or both surfaces of the negative electrode current collector.

[0125] The negative electrode active material layer includes the composite negative electrode active material described above. The composite negative electrode active material can exhibit excellent capacity and life characteristics by being contained in the negative electrode active material layer. The composite negative electrode active material has been described above.

[0126] The negative electrode active material layer may further include a carbon-based active material in addition to the composite negative electrode active material, whereby the carbon-based active material, which has a low degree of volumetric expansion during charging and discharging, can reduce the degree of overall volumetric expansion of the negative electrode active material layer.

[0127] The carbon-based active material may include at least one selected from the group consisting of graphite, artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and preferably includes at least one selected from the group consisting of artificial graphite and natural graphite.

[0128] The average particle size (D 50) may be 5 μm to 35 μm, preferably 10 μm to 20 μm, in order to ensure structural stability during charge and discharge and reduce side reactions with the electrolyte.

[0129] Specifically, the negative electrode active material layer preferably uses both the composite negative electrode active material and the carbon-based active material in terms of simultaneously improving capacity characteristics and cycle characteristics. Specifically, the negative electrode active material layer may contain the composite negative electrode active material and the carbon-based active material in a weight ratio of 1:99 to 50:50, preferably 3:97 to 40:60.

[0130] The negative electrode active material layer may contain the composite negative electrode active material and the carbon-based active material in an amount of 80% to 99% by weight, preferably 90% to 98.5% by weight.

[0131] The negative electrode active material layer may contain a binder.

[0132] The binder may include at least one selected from the group consisting of polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), acrylonitrile butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and polyacryl amide (PAM), in order to further improve electrode adhesive strength and provide sufficient resistance to volume expansion / contraction of the active material.

[0133] The binder may be contained in the negative electrode active material layer in an amount of 0.5% by weight to 10% by weight, and when it is in this range, it is preferable in that the volume expansion of the active material can be more effectively controlled.

[0134] The negative electrode active material layer may further include a conductive material. The conductive material may be used to improve the conductivity of the negative electrode, and preferably does not induce chemical changes and is conductive. Specifically, the conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, carbon nanotubes (CNTs), fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives. Preferably, the conductive material may include carbon black to achieve high conductivity.

[0135] The conductive material may be contained in the negative electrode active material layer in an amount of 0.5% by weight to 10% by weight.

[0136] The negative electrode active material layer may have a thickness of 30 μm to 100 μm, preferably 40 μm to 80 μm, in order to improve electrical contact with the components of the negative electrode active material layer.

[0137] The negative electrode may be fabricated by dispersing a negative electrode active material, a binder, a conductive material, and the like in a solvent for forming a negative electrode slurry on the negative electrode current collector, coating the negative electrode slurry on the negative electrode current collector, and then drying and rolling the coating.

[0138] The solvent for forming the negative electrode slurry may contain at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, preferably distilled water, in order to facilitate dispersion of the components.

[0139] <Secondary battery> The present invention provides a secondary battery, specifically a lithium secondary battery, including the above-described negative electrode.

[0140] Specifically, the secondary battery according to the present invention includes the above-mentioned negative electrode; a positive electrode; a separator interposed between the negative electrode and the positive electrode; and an electrolyte.

[0141] The positive electrode can face the negative electrode.

[0142] The positive electrode may include a positive electrode current collector; and a positive electrode active material layer located on at least one surface of the positive electrode current collector.

[0143] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. Specifically, the positive electrode current collector may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, and an aluminum-cadmium alloy, and may specifically include aluminum.

[0144] The positive electrode current collector may generally have a thickness of 3 to 500 μm.

[0145] The positive electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the negative electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0146] The positive electrode active material layer may include a positive electrode active material.

[0147] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically may include a lithium transition metal composite oxide containing lithium and at least one transition metal selected from nickel, cobalt, manganese, and aluminum, preferably a lithium transition metal composite oxide containing lithium and a transition metal including nickel, cobalt, and manganese.

[0148] More specifically, examples of the lithium transition metal composite oxide include lithium-manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (e.g., LiCoO2, etc.), lithium-nickel-based oxides (e.g., LiNiO2, etc.), lithium-nickel-manganese-based oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1), or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r3 M S2)O2 (Here, M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are atomic fractions of independent elements, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1.) and the like, and one or more of these compounds may be included. Among them, in terms of improving the capacity characteristics and safety of the battery, the lithium transition metal composite oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel-manganese-cobalt oxide (for example, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), or lithium nickel cobalt aluminum oxide (for example, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), and considering the significance of the improvement effect according to the control of the types and content ratios of the constituent elements forming the lithium transition metal composite oxide, the lithium transition metal composite oxide is Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc., and one or more of these mixtures may be used.

[0149] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 80% by weight to 99% by weight, preferably 92% by weight to 98.5% by weight, in consideration of sufficient capacity of the positive electrode active material.

[0150] The positive electrode active material layer may further include a binder and / or a conductive material in addition to the positive electrode active material.

[0151] The binder is a component that aids in binding the active material and conductive material, etc., and in binding them to the current collector, and specifically may include at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, and preferably polyvinylidene fluoride.

[0152] The binder may be contained in the positive electrode active material layer in an amount of 1 to 20% by weight, preferably 1.2 to 10% by weight, in order to ensure sufficient binding strength between components such as the positive electrode active material.

[0153] The conductive material may be used to supplement and improve the conductivity of the secondary battery, and is not particularly limited as long as it does not induce chemical changes and has conductivity. Specifically, the conductive material may include at least one selected from the group consisting of graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives. Preferably, the conductive material may include carbon black in order to improve conductivity.

[0154] In order to ensure sufficient electrical conductivity, the conductive material may be contained in the positive electrode active material layer in an amount of 1 to 20% by weight, preferably 1.2 to 10% by weight.

[0155] The thickness of the positive electrode active material layer may be 30 μm to 400 μm, preferably 50 μm to 110 μm.

[0156] The positive electrode may be fabricated by coating a positive electrode slurry containing a positive electrode active material and, optionally, a binder, a conductive material, and a solvent for forming the positive electrode slurry on the positive electrode current collector, followed by drying and rolling.

[0157] The solvent for forming the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a preferred viscosity when containing the positive electrode active material, and optionally a binder and a conductive material. For example, the solvent for forming the positive electrode slurry may be included in the positive electrode slurry so that the concentration of the solids, including the positive electrode active material, and optionally a binder and a conductive material, is 50 wt % to 95 wt %, preferably 70 wt % to 90 wt %.

[0158] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in lithium secondary batteries can be used without particular limitations. It is particularly preferable that the separator exhibits low resistance to electrolyte ion migration and has excellent electrolyte humidifying ability. Specifically, porous polymer films, such as those made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminates of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, separators coated with ceramic components or polymeric materials may be used, and they may be selectively used as single-layer or multi-layer structures.

[0159] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of secondary batteries, but are not limited to these.

[0160] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0161] The organic solvent may be used without any particular limitation as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specific examples of the organic solvent that may be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes. Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of a battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, which can provide excellent electrolyte performance.

[0162] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt is preferably used at a concentration in the range of 0.1 M to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has suitable conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0163] The secondary battery can be manufactured by a normal method for manufacturing a secondary battery, by interposing a separator between the negative electrode and the positive electrode, and then injecting an electrolytic solution (electrolyte).

[0164] The secondary battery according to the present invention is useful in portable devices such as mobile phones, laptop computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs), and is particularly suitable as a component battery of a medium- to large-sized battery module. Accordingly, the present invention provides a medium- to large-sized battery module including the above-described secondary battery as a unit cell.

[0165] Such a medium- to large-sized battery module can be suitably applied to power sources that require high output and large capacity, such as electric vehicles, hybrid electric vehicles, and power storage devices. [Example]

[0166] The present invention may be embodied in various different forms and should not be construed as being limited to the embodiments set forth herein, although the present invention may be embodied in various different forms and should not be construed as being limited to the embodiments set forth herein.

[0167] Example Example 1: Preparation of composite negative electrode active material (1) Manufacturing of composite negative electrode active material SiO was placed in the first tray as a silicon-based oxide and subjected to a first heat treatment at 1,400°C in a reduced pressure atmosphere of -100 torr to generate a first vapor. Mg was placed in the second tray as a metal and subjected to a second heat treatment at 900°C in a reduced pressure atmosphere of -100 torr to generate a second vapor. The first and second heat treatments for each tray were carried out independently. The weight ratio of SiO in the first tray to the metal in the second tray was 88:12.

[0168] The first vapor and the second vapor were mixed in one reactor and subjected to a gas phase reaction for 8 hours.

[0169] After the gas phase reaction was completed, the mixture was cooled to 900° C. to form a silicon-based oxide particle-metal composite. The cooling was carried out for 8 hours.

[0170] The silicon-based oxide particle-metal composite was pulverized in a jet mill to an average particle size (D 50 ) produced 6 μm silicon-based oxide particle-metal composites.

[0171] The crushed silicon-based oxide particle-metal composite was subjected to CVD treatment in a mixed gas of argon (Ar) and methane (CH4) at 950°C for 4 hours to form a carbon coating layer on the surface of the silicon-based oxide particle-metal composite, which was used as the composite negative electrode active material of Example 1. The composite negative electrode active material had an average particle diameter (D 50 ) is 6 μm and contains a carbon coating layer at 4 wt %.

[0172] (2) Measurement and calculation of the average diameter of the aggregates Step (a): The composite negative electrode active material prepared above was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). Figure 1 shows a scanning electron microscope photograph of the composite negative electrode active material of Example 1.

[0173] Step (b): As shown in FIG. 2, two opposing sides in FIG. 1 were selected, arbitrary points were selected from the two sides, and a straight line was drawn by connecting the two selected points.

[0174] Step (c): Using a digital image analysis program (Image J), ​​the pixel scale was set using the scale bar on the scanning electron micrograph, and a Plot Profile was run to obtain a gray profile graph of the line, with the vertical axis representing the gray value and the horizontal axis representing the distance along the line. The average gray value was calculated based on the gray value over the distance along the line in the gray profile graph. As shown in Figure 3, the average gray value over the distance along the line was 106.

[0175] Step (d): As shown in Figure 4, a reference line having the average gray value and parallel to the horizontal axis was plotted on the linear gray profile graph, and the number of intersections between the gray profile graph and the reference line was obtained. Next, the value calculated using the following mathematical formula 1 was defined as the diameter of the aggregate. [Mathematical formula 1] Diameter of the cluster = length of the line / (number of intersections between the reference line and the gray profile + 1)

[0176] Step (e): Steps (a) to (d) were repeated 50 times to obtain 50 aggregates, and the average diameter was calculated. The average aggregate diameter was 39.6 nm, with a standard deviation of 4.2 nm.

[0177] Example 2: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the cooling temperature was set to 950°C. The prepared composite negative electrode active material was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in FIG. 5. The average diameter of the aggregates measured in the same manner as in Example 1 was 34.1 nm, with a standard deviation of 3.8 nm.

[0178] Example 3: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the temperature during the first heat treatment was 1,450°C and the temperature during the second heat treatment was 920°C. The composite negative electrode active material prepared above was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in FIG. 6.

[0179] The average diameter of the aggregates measured in the same manner as in Example 1 was 55.5 nm, with a standard deviation of 4.8 nm.

[0180] Example 4: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the weight ratio of SiO in the first tray to the metal in the second tray was 95:5. The composite negative electrode active material was photographed at 100,000x magnification using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in Figure 7.

[0181] The average diameter of the aggregates measured in the same manner as in Example 1 was 37.6 nm, with a standard deviation of 5.9 nm.

[0182] Example 5: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the weight ratio of SiO in the first tray to the metal in the second tray was 85:15. The composite negative electrode active material was photographed at 100,000x magnification using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in Figure 8.

[0183] The average diameter of the aggregates measured in the same manner as in Example 1 was 42.7 nm, with a standard deviation of 4.7 nm.

[0184] Example 6: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that Li was used instead of Mg as the metal, the weight ratio of SiO in the first tray to the metal in the second tray was 94:6, and the temperature during the second heat treatment was 600°C. The composite negative electrode active material prepared above was photographed at 100,000x magnification using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in Figure 9.

[0185] The average diameter of the aggregates measured in the same manner as in Example 1 was 27.7 nm, with a standard deviation of 5.9 nm.

[0186] Example 7: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 6, except that the weight ratio of SiO in the first tray to the metal in the second tray was 97:3 and the temperature during the second heat treatment was 550°C. The composite negative electrode active material was photographed at 100,000x magnification using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in Figure 10.

[0187] The average diameter of the aggregates measured in the same manner as in Example 1 was 25.3 nm, with a standard deviation of 2.4 nm.

[0188] Example 8: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 6, except that the weight ratio of SiO in the first tray to the metal in the second tray was 90:10 and the temperature during the second heat treatment was 700°C. The composite negative electrode active material was photographed at 100,000x magnification using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in Figure 11.

[0189] The average diameter of the aggregates measured in the same manner as in Example 1 was 32.5 nm, with a standard deviation of 5.5 nm.

[0190] Comparative Example 1: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the cooling temperature was 500°C. The composite negative electrode active material was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in FIG.

[0191] The average diameter of the aggregates measured in the same manner as in Example 1 was 74.2 nm, with a standard deviation of 14.5 nm.

[0192] Comparative Example 2: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the cooling temperature was 1,050°C. The composite negative electrode active material was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in FIG.

[0193] The average diameter of the aggregates measured in the same manner as in Example 1 was 78.4 nm, with a standard deviation of 15.5 nm.

[0194] Comparative Example 3: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that a 1:1 molar mixture of Si and SiO2 was placed in the first tray instead of silicon-based oxide SiO, and the weight ratio of the total weight of Si and SiO2 placed in the first tray to the metal placed in the second tray was 88:12. The composite negative electrode active material was photographed at 100,000x magnification using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope image of the composite negative electrode active material is shown in Figure 14.

[0195] The average diameter of the aggregates measured in the same manner as in Example 1 was 83.2 nm, with a standard deviation of 26.1 nm.

[0196] Comparative Example 4: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the cooling temperature was 800°C. The composite negative electrode active material was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in FIG.

[0197] The average diameter of the aggregates measured in the same manner as in Example 1 was 72.2 nm, with a standard deviation of 16.7 nm.

[0198] Comparative Example 5: Preparation of composite negative electrode active material A composite negative electrode active material was prepared in the same manner as in Example 1, except that the cooling temperature was 1,200°C. The composite negative electrode active material was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). A scanning electron microscope photograph of the composite negative electrode active material is shown in FIG.

[0199] The average diameter of the aggregates measured in the same manner as in Example 1 was 143.4 nm, with a standard deviation of 74.1 nm.

[0200] Comparative Example 6: Preparation of composite negative electrode active material As silicon oxide, Si(D 50 = 20 μm) 20 kg and SiO2 (D 50 50 kg of ZnO (=50 nm) was added to water, stirred for 12 hours, and the resulting mixture was dried at 1,200°C to form a mixture. The mixture was placed in a first tray and heat-treated at 1,600°C in a reduced pressure atmosphere of 0.1 torr to generate first steam. 11 kg of Mg metal was placed in a second tray and heat-treated at 900°C in a reduced pressure atmosphere of 0.1 torr to generate second steam. The first and second heat treatments on each tray were performed independently.

[0201] The first vapor and the second vapor were mixed in one reactor and subjected to a gas phase reaction for 3 hours.

[0202] After the gas phase reaction was completed, the silicon-based composite oxide deposited on the substrate inside the reactor was quickly cooled to room temperature to form a silicon-based oxide particle-metal composite.

[0203] The silicon-based oxide particle-metal composite was pulverized in a jet mill to an average particle size (D 50 ) produced 6 μm silicon-based oxide particle-metal composites.

[0204] 50 g of the silicon-based oxide particle-metal composite was placed in a tubular electric furnace, and Ar and methane gases were flowed at 1 L / min each while maintaining the furnace at 1000°C for 1 hour to form a carbon coating layer on the surface of the silicon-based oxide particle-metal composite, which was used as the composite negative electrode active material of Comparative Example 6.

[0205] The composite negative electrode active material prepared above was photographed at a magnification of 100,000 times using a scanning electron microscope (instrument name: S-4800, manufacturer: HITACHI). The scanning electron microscope photograph of the composite negative electrode active material is shown in FIG.

[0206] The average diameter of the aggregates measured in the same manner as in Example 1 was 75 nm, with a standard deviation of 23.5 nm.

[0207] The composite negative electrode active materials prepared in Examples 1 to 8 and Comparative Examples 1 to 6 are shown in Table 1 below.

[0208] [Table 1]

[0209] *Measurement of the size of Si crystal grains in silicon-based oxide particles The size of the Si crystallites of the silicon-based oxide particles was measured by performing XRD analysis using an XRD (X-ray diffraction) analyzer (product name: D4-endavor, manufacturer: bruker), measuring the peak of the (220) plane of Si, and then using the Scherrer equation. Specifically, the XRD analysis was performed under the following conditions.

[0210] <XRD analysis conditions> 1) Type and wavelength of the light source: The X-ray wavelength generated by Cu Kα was used, and the wavelength (λ) of the light source was 0.15406 nm. 2) Method for preparing the sample: 0.3 g of silicon-based oxide particles were placed in a cylindrical holder with a diameter of 2.5 cm and a height of 2.5 mm, and a flattening operation was performed with a slide glass so that the height of the sample in the holder was constant, preparing a sample for XRD analysis. 3) Setting conditions of the XRD analyzer: The SCAN TIME was set to 1 hour and 15 minutes, the measurement range was set to the range where 2θ was 10° to 90°, and the STEP TIME and STEP SIZE were set so that 2θ was scanned at 0.02° per second. At this time, in order to measure the peak of the (220) plane of Si, the peak in the range where 2θ was 45° to 50° was measured.

[0211] After that, the size of the silicon crystallites of the silicon-based oxide particles was calculated using the following Scherrer equation.

[0212] [Scherrer equation] Size of Si crystallites (nm) = (K × λ) / (FWHM × Cosθ)

[0213] In the above formula, K is 0.89 as the Scherrer constant, λ is 0.15406 nm as the wavelength of the light source, FWHM is calculated using the Lorentz function as the full width at half maximum of the peak of the (220) plane of Si during XRD analysis, and Cosθ is the cosine value of the angle θ corresponding to the peak of the (220) plane of Si.

[0214] Experimental example Experimental example 1: Evaluation of life characteristics <Production of negative electrodes> The composite negative electrode active material prepared in Example 1 was used as the negative electrode active material, and graphite (average particle size (D 50 ):20 μm) in a weight ratio of 15:85 was used.

[0215] The negative electrode active material, styrene-butadiene rubber (SBR) as a binder, Super C65 as a conductive material, and carboxymethyl cellulose (CMC) as a thickener were mixed in a weight ratio of 96:2:1:1, and the mixture was added to distilled water as a solvent for forming a negative electrode slurry to prepare a negative electrode slurry.

[0216] The negative electrode slurry was applied to one surface of a copper current collector (thickness: 15 μm) as a negative electrode current collector at 3.6 mAh / cm 2 The coated layer was rolled and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer (thickness: 50 μm), which was used as the negative electrode of Example 1 (thickness of negative electrode: 65 μm).

[0217] In addition, negative electrodes of Examples 2 to 8 and Comparative Examples 1 to 6 were manufactured in the same manner as in Example 1, except that the composite negative electrode active material of Examples 2 to 8 and Comparative Examples 1 to 6 were used instead of the composite negative electrode active material of Example 1.

[0218] <Secondary battery manufacturing> A lithium metal foil was prepared as the positive electrode.

[0219] A porous polyethylene separator was interposed between the negative electrode and positive electrode of Examples 1 to 8 and Comparative Examples 1 to 6 produced above, and an electrolyte solution was injected to produce coin-shaped half cells of Examples 1 to 8 and Comparative Examples 1 to 6.

[0220] The electrolyte used was a solution in which ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were mixed in a volume ratio of 7:3, vinylene carbonate (VC) was dissolved at 0.5 wt %, and LiPF6 was dissolved at a concentration of 1M.

[0221] <Evaluation of capacity retention rate> The secondary batteries produced in Examples 1 to 8 and Comparative Examples 1 to 6 were evaluated for cycle capacity retention using an electrochemical charger / discharger.

[0222] The cycle capacity retention rate was measured at 25°C. The first and second cycles were charged and discharged at 0.1C, and from the third cycle onwards, the charge and discharge were carried out at 0.5C (charge conditions: CC / CV, 5mV / 0.005C cut-off, discharge conditions: CC, 1.5V cut-off).

[0223] The capacity retention rate was calculated as follows. Capacity retention rate (%) = {(discharge capacity at Nth cycle) / (discharge capacity at 1st cycle)} × 100 (In the above formula, N is an integer of 1 or more.)

[0224] The capacity retention rate (%) at the 50th cycle is shown in Table 2 below.

[0225] [Table 2]

[0226] Referring to Table 2, it can be seen that the composite negative electrode active materials of Examples 1 to 8, in which the average diameter of the aggregates was adjusted to a preferred level, exhibited excellent improvements in battery life characteristics compared to the composite negative electrode active materials of Comparative Examples 1 to 6.

Claims

1. generating first vapor by subjecting a silicon-based oxide containing a compound represented by the following Chemical Formula 2 to a first heat treatment; a step of subjecting a metal including at least one selected from the group consisting of Li, Mg, and Al to a second heat treatment to generate second vapor; mixing the first vapor and the second vapor to cause a gas phase reaction; and a step of cooling the gas phase reaction to obtain a silicon-based oxide-metal composite; the difference between the temperature during the first heat treatment and the temperature during the cooling is 400°C to 550°C; The first heat treatment is performed in a reduced pressure atmosphere of −50 torr to −200 torr, The second heat treatment is performed in a reduced pressure atmosphere of −50 torr to −200 torr, The temperature during the first heat treatment is 1,300°C to 1,500°C. Method for producing the composite negative electrode active material: [Chemical formula 2] SiO ａ (0<a<2) Here, the composite negative electrode active material is silicon-based oxide particles; and a metal distributed on, within, or on and within the silicon-based oxide particles; the composite negative electrode active material includes one or more aggregates containing at least one selected from silicon, oxygen, and a metal therein; the metal includes at least one selected from the group consisting of Li, Mg, and Al; The average diameter of the aggregates obtained by the method comprising the following steps (a) to (e) is 5 nm to 65 nm; The standard deviation of the diameter of the aggregates obtained by step (e) is 10 nm or less: (a) photographing the composite negative electrode active material with a scanning electron microscope to obtain a square scanning electron microscope photograph; (b) selecting two different sides in the scanning electron micrograph, selecting one point from each of the two selected sides, and obtaining a straight line connecting the two points selected from the two sides; (c) using a digital image analysis program to obtain a gray profile graph of the line, the vertical axis being gray value and the horizontal axis being distance of the line, and then calculating an average gray value; (d) obtaining the number of intersections between the reference line having the average gray value and parallel to the horizontal axis and the gray profile in the linear gray profile graph, and then defining the value calculated by the following mathematical formula 1 as the diameter of the aggregate; and (e) performing steps (a) to (d) two or more times to obtain an average diameter of the aggregates; [Mathematical formula 1] Diameter of the aggregate = length of the straight line / (number of intersections between the reference line and the gray profile + 1).

2. 2. The method of claim 1, wherein the cooling temperature is 750 to 1,100°C.

3. The method for producing a composite negative electrode active material according to claim 1, further comprising the step of pulverizing the silicon-based oxide-metal composite.

4. 2. The method of claim 1, wherein the metal is contained in an amount of 1 wt % to 20 wt % based on the total weight of the silicon-based oxide particles and the metal.

5. the metal includes Mg; 2. The method of claim 1, wherein the metal is contained in an amount of 5 wt % to 15 wt % based on the total weight of the silicon-based oxide particles and the metal.

6. the metal includes Li; 2. The method of claim 1, wherein the metal is contained in an amount of 3% by weight to 12% by weight based on the total weight of the silicon-based oxide particles and the metal.

7. A method for manufacturing a composite negative electrode active material as described in claim 1, wherein the composite negative electrode active material further includes a carbon coating layer located on the surface of the composite negative electrode active material.

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