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

Abstract

1. A negative electrode active material comprising a silicon-based active material, the silicon-based active material comprising silicon-based crystal grains, wherein a high-angle grain boundary ratio within the silicon-based crystal grains is 30% or more, and the silicon-based active material has a chemical structure that satisfies the following formulas 1 and 2: [Formula 1] 1 μm≦particle size of silicon-based active material (D50)≦10 μm [Formula 2] 2 nm≦crystal grain size of silicon-based active material≦1 μm.

Description

Technical Field

[0001] This application was filed with the Korean Intellectual Property Office on April 20, 2023, and claims priority based on Korean Patent Application No. 10-2023-0052006, the entire contents of which are incorporated herein by reference.

[0002] This application relates to a negative electrode active material, a method for manufacturing the negative electrode active material, a negative electrode composition, a negative electrode for a lithium secondary battery including the same, and a lithium secondary battery including the negative electrode.

Background Art

[0003] Due to the rapid increase in the use of fossil fuels, the demand for alternative and clean energy has been increasing, and as part of this, the fields of power generation and power storage using electrochemical reactions are the most actively studied.

[0004] Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is showing a trend of increasing more and more.

[0005] With the development of technologies related to mobile devices and the increase in demand, the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, lithium secondary batteries having a high energy density, a high voltage, a long cycle life, and a low self-discharge rate have been commercialized and widely used. In addition, research on methods for manufacturing high-density electrodes with a higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries has been actively conducted.

[0006] [[ID=z8]]Generally, a secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material that inserts and desorbs lithium ions emitted from the positive electrode, and silicon-based particles having a large discharge capacity may be used as the negative electrode active material. Therefore, research on improving the performance in the negative electrode active material using silicon-based particles has been actively conducted.

Prior Art Documents

[0007] [Patent Document 1] Japanese Patent Publication No. 2009-080971 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Research aimed at reducing grain size has confirmed that, prior to the grinding of conventional MG (Metallurgical Grade)-Si, adjusting the cooling rate in the rapid cooling process to form a plate-like silicon precursor, and then grinding this precursor, can reduce the desired grain size and adjust the high-angle boundary ratio.

[0009] This application relates to a negative electrode active material, a method for producing a negative electrode active material, a negative electrode composition, a negative electrode for a lithium secondary battery containing the same, and a lithium secondary battery containing the negative electrode. [Means for solving the problem]

[0010] One embodiment of this specification provides a negative electrode active material comprising a silicon-based active material, wherein the silicon-based active material comprises silicon-based crystal grains, the high-angle grain boundary ratio within the silicon-based crystal grains is 30% or more, and the silicon-based active material satisfies the following formulas 1 and 2. [Equation 1] Approximately 1 μm ≤ Particle size of silicon-based active material (D50) ≤ Approximately 10 μm [Equation 2] Approximately 2 nm ≤ Crystal grain size of silicon-based active material ≤ Approximately 1 μm

[0011] In another embodiment, the present invention provides a method for producing a negative electrode active material, comprising the steps of: rapidly cooling metallic silicon to form a silicon precursor; and pulverizing the silicon precursor to form a silicon-based active material, wherein the rapid cooling step includes melt spinning, suction casting, or injection casting.

[0012] Another embodiment provides a negative electrode composition comprising a negative electrode active material, a negative electrode conductive material, and a negative electrode binder according to this application.

[0013] In another embodiment, a negative electrode for a lithium secondary battery is provided, comprising a negative electrode current collector layer and a negative electrode active material layer provided on one or both sides of the negative electrode current collector layer, wherein the negative electrode active material layer comprises the negative electrode composition according to this application or a cured product thereof.

[0014] Furthermore, the present invention provides a lithium secondary battery comprising a positive electrode; a negative electrode for a lithium secondary battery according to this application; a separator provided between the positive electrode and the negative electrode; and an electrolyte. [Effects of the Invention]

[0015] In the case of a negative electrode active material according to one embodiment of the present invention, unlike conventional pulverization methods, a rapid cooling step is performed prior to MG (Metallurgical Grade)-Si pulverization to form a plate-shaped silicon precursor, and the crystal grain size and high-angle grain boundary ratio of the generated silicon-based active material can be optimized by adjusting the cooling rate.

[0016] As described above, silicon-based active materials having nano-sized crystal grains can be obtained by pulverizing a plate-shaped silicon precursor having nano-sized crystal grains, and when this is used to manufacture a negative electrode, the cycle capacity retention rate and initial capacity efficiency are increased.

[0017] The negative electrode active material of the present invention is a silicon-based active material, SiO x (x=0) and SiOx It contains one or more selected from the group consisting of (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO x It contains about 70 parts by weight or more of (x = 0). For example, while having a pure silicon (Pure Si) active material, the problem of volume expansion due to charge and discharge, which is a problem caused by this, is solved by adjusting the grain size.

Brief Description of Drawings

[0018] [Figure 1] It is a flowchart of the manufacturing process of the silicon-based active material according to an embodiment of the present application. [Figure 2] It is a figure showing an enlarged view of the silicon-based active material according to an embodiment of the present application. [Figure 3] It is a figure showing the laminated structure of the negative electrode for a lithium secondary battery according to an embodiment of the present application. [Figure 4] It is a figure showing the laminated structure of the lithium secondary battery according to an embodiment of the present application. [Figure 5] It is a figure showing a method for calculating the grain size. [Figure 6] It is a figure showing the misorientation angle.

Modes for Carrying Out the Invention

[0019] In a part of the accompanying drawings, the same reference numerals are given to corresponding components. Those skilled in the art should understand that the drawings clearly show the elements simply and are not necessarily drawn to scale. For example, for the purpose of assisting the understanding of various embodiments, the dimensions of some elements shown in the drawings may be exaggerated compared to other elements. Also, elements of known technology that are useful or essential in commercially feasible embodiments may not be depicted so as not to obscure the gist of various embodiments of the present invention.

[0020] Before explaining the present invention, first, some terms will be defined.

[0021] In this specification, when a part "includes" a component, this means that, unless otherwise stated, it may include other components rather than excluding them.

[0022] In this specification, "p~q" means "greater than or equal to p and less than or equal to q".

[0023] In this specification, "specific surface area" is measured by the BET method, specifically calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using a BELSORP-mini II manufactured by BEL Japan. In other words, in this application, BET specific surface area can mean the specific surface area measured by the above measurement method.

[0024] In this specification, "Dn" refers to the particle size distribution, and means the particle size at the n% point of the cumulative particle number distribution by particle size. That is, D50 is the particle size (average particle size) at the 50% point of the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution by particle size. On the other hand, the average particle size may be measured using the laser diffraction method. For example, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500), and the particle size distribution is calculated by measuring the difference in diffraction patterns due to particle size as the particles pass through the laser beam.

[0025] In one embodiment of this application, particle size or particle size may refer to the average particle size or representative particle size of each individual particle constituting the metal powder.

[0026] In this specification, the statement that a polymer contains a monomer as a monomer unit means that the monomer participates in the polymerization reaction and is included as a repeating unit within the polymer. In this specification, when a polymer is said to contain a monomer, this is interpreted as being equivalent to the polymer containing a monomer as a monomer unit.

[0027] In this specification, the term "polymer" is understood to be used in a broad sense, including copolymers, unless otherwise explicitly stated as "homopolymer."

[0028] In this specification, weight-average molecular weight (Mw) and number-average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers of various degrees of polymerization (standard samples) commercially available for molecular weight measurement. In this specification, molecular weight refers to weight-average molecular weight unless otherwise specified.

[0029] As used herein, “approximately,” “abstractly,” and “substantially” are used to mean a range of numerical values ​​or degrees or close to them, taking into account inherent manufacturing and material tolerances, and are used to prevent infringers from unfairly exploiting disclosures that refer to precise or absolute numerical values ​​provided to aid in understanding the invention.

[0030] Numerical ranges used herein (e.g., ≤, parts by weight, thickness, greater than or equal to, less than or equal to, etc.) are used to take into account inherent manufacturing and material tolerances, to mean a range of values ​​or degrees or close to them, and are used to prevent infringers from unfairly exploiting disclosures that refer to precise or absolute numerical values ​​provided to aid in understanding the invention.

[0031] Recently, with the increasing demand for high-density energy batteries, Si / C and SiO2, which have more than 10 times the capacity of graphite-based materials, have become popular as negative electrode active materials. xResearch is being conducted on methods to increase battery capacity by using silicon-based compounds such as the one shown. On the other hand, while silicon-based compounds are high-capacity materials and have a larger capacity than conventionally used graphite, they have the problem of rapidly expanding in volume during the charging process, disrupting the conductive path and degrading battery performance.

[0032] Therefore, various studies are being conducted to resolve the problems that arise when using silicon-based compounds as negative electrode active materials. For example, various methods are being discussed for using silicon-based compounds as negative electrode active materials, such as methods to adjust the driving potential, methods to further coat a thin film on the active material layer, methods to suppress volume expansion itself such as adjusting the particle size of the silicon-based compound, or various methods to prevent the conduction path from being interrupted. However, each method has its own advantages and disadvantages.

[0033] On the other hand, in the case of Si negative electrode active materials, research has revealed that the wider the grain boundary, i.e., the smaller the grain size, the more advantageous it is for the performance characteristics of secondary batteries, because the grain boundary acts as a diffusion pathway for lithium. However, when manufacturing negative electrode active materials by crushing metallic silicon (MG-Si), the typical grain size of MG-Si is several hundred mm to several mm, so there is a limit to how much the grain size of the Si active material can be reduced, and thus there is still a limit to how much the performance characteristics of secondary batteries can be improved.

[0034] In order to improve the capacity performance of secondary batteries, the present invention provides a technique that allows for the control of the size of crystal grains within Si particles, for example, by reducing the size of the crystal grains within Si particles to a size of several nanometers to tens of nanometers, so as to prevent damage to the conductive path due to volume expansion of the silicon compound when a silicon-based active material is used as the negative electrode active material.

[0035] Hereinafter, in order for those with ordinary knowledge in the technical field to which the present invention pertains to easily implement the present invention, it will be described in detail in conjunction with the drawings. However, the present invention can be embodied in various different forms and is not limited to the following description.

[0036] One embodiment of the present specification is a negative electrode active material containing a silicon-based active material, wherein the silicon-based active material contains silicon-based crystal grains, and the high angle grain boundary ratio in the silicon-based crystal grains is 30% or more, and the silicon-based active material provides a negative electrode active material that satisfies the following formulas 1 and 2. [Formula 1] Approximately 1 μm ≤ particle size (D50) of the silicon-based active material ≤ approximately 10 μm [Formula 2] Approximately 2 nm ≤ crystal grain size of the silicon-based active material ≤ approximately 1 μm

[0037] In the case of the negative electrode active material according to one embodiment of the present invention, different from the conventional pulverization process, prior to pulverizing MG-Si, a rapid cooling process is carried out first to form a plate-like silicon precursor, and as a result, the crystal grain size and high angle grain boundary (HAGB) ratio of the silicon-based active material produced by pulverizing the plate-like silicon precursor formed by adjusting the cooling rate can be optimized.

[0038] As described above, by pulverizing the plate-like silicon precursor having nano-sized crystal grains, a silicon-based active material having nano-sized crystal grains can be ensured. When using this to manufacture the negative electrode of a lithium secondary battery, the cycle capacity retention rate and initial capacity efficiency of the lithium secondary battery are increased.

[0039] In one embodiment of the present application, the silicon-based active material contains one or more selected from the group consisting of SiO x (x = 0) and SiO x (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may be contained in an amount of about 70 parts by weight or more. <s

[0040] In one embodiment of the present application, the silicon-based active material contains SiO x (x = 0), and based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may be contained in an amount of about 70 parts by weight or more.

[0041] In another embodiment, based on 100 parts by weight of the silicon-based active material, the SiO x (x = 0) may be contained in an amount of about 70 parts by weight or more, for example, about 80 parts by weight or more, or about 90 parts by weight or more, and may be contained in an amount of about 100 parts by weight or less, for example, about 99 parts by weight or less, or about 95 parts by weight or less.

[0042] In one embodiment of the present application, pure silicon (Si) particles may be used as the silicon-based active material. Using pure silicon (Si) particles as the silicon-based active material means that, as described above, when based on 100 parts by weight of the entire silicon-based active material, pure Si particles (SiO x (x = 0)) not combined with other particles or elements are contained within the above range.

[0043] In one embodiment of the present application, the silicon-based active material may be composed of silicon-based particles having 100 parts by weight of SiO x (x = 0) based on 100 parts by weight of the silicon-based active material.

[0044] In one embodiment of the present application, the silicon-based active material may contain metal impurities. In this case, the impurities are metals that may generally be contained in the silicon-based active material. For example, based on 100 parts by weight of the silicon-based active material, they may be contained in an amount of about 0.1 part by weight or less.

[0045] In the case of a silicon-based active material used as the negative electrode active material of a lithium secondary battery, when compared with the conventionally used graphite-based active material, the capacity is significantly higher and the attempt to apply it has increased. However, the volume expansion rate during charge and discharge is high, and such attempts have stalled, such as when a trace amount of the silicon-based active material is mixed with the graphite-based active material and used.

[0046] Therefore, in the present invention, in order to improve capacity performance, while using a silicon-based active material as the negative electrode active material, the problems of the silicon-based active material described above were solved by adjusting the size of the crystal grains of the silicon-based active material itself, rather than by adjusting the composition of the conductive material and binder.

[0047] This application describes a method for reducing the size of crystal grains by performing a rapid cooling step prior to MG-Si grinding in the manufacturing process of the negative electrode active material to produce a plate-shaped silicon precursor, which is then ground to produce a silicon-based active material.

[0048] In one embodiment of this application, the silicon precursor satisfies the following formulas 3 and 4. [Equation 3] Approximately 10 μm ≤ Silicon precursor plate thickness ≤ Approximately 2 mm [Equation 4] Approximately 2 nm ≤ Size of crystal grains in silicon precursor ≤ Approximately 1 μm

[0049] In one embodiment of this application, formula 3 may satisfy approximately 10 μm ≤ silicon precursor plate thickness ≤ 2 mm, or approximately 13 μm ≤ silicon precursor plate thickness ≤ 1.5 mm, or approximately 15 μm ≤ silicon precursor plate thickness ≤ 1 mm.

[0050] In one embodiment of this application, formula 4 satisfies approximately 2 nm ≤ size of crystal grains in the silicon precursor ≤ 1 μm, and can satisfy, for example, approximately 10 nm ≤ size of crystal grains in the silicon precursor ≤ 1 μm, or approximately 40 nm ≤ size of crystal grains in the silicon precursor ≤ 1 μm.

[0051] Referring to Figure 1, in one embodiment of the present invention, when forming a plate-shaped silicon precursor using melt spinning, first, MG silicon is placed in a graphite crucible and a current of approximately 10 kA is passed through a copper coil to produce molten silicon at approximately 2000°C (S1). Next, the molten silicon is injected at a pressure of 1.2 kPa onto a 25 cm diameter copper wheel rotating at a speed of 3000 rpm to produce a plate-shaped silicon precursor (S2). Meanwhile, it is checked whether the produced precursor satisfies equations 3 and 4 (S3). If it does (S3, YES), the produced silicon precursor is pulverized by methods such as a ball mill, pin mill, disk mill, or jet mill to form a silicon-based active material (S4). If the precursor produced in S3 does not satisfy equations 3 and 4, the process is repeated by returning to step S1 to produce molten metallic silicon, or by returning to step S2 to produce plate-shaped metallic silicon by rapid cooling of the molten metallic silicon. On the other hand, the active material formed in step S5 is checked to see if it satisfies equations 1 and 2. If it does (YES, S5), the silicon-based active material is secured in step S6 and the process is terminated. After this, the secured silicon-based active material can be used to manufacture, for example, the negative electrode of a lithium secondary battery. On the other hand, if the active material formed in step S5 does not satisfy equations 1 and 2 (NO, S5), the process is repeated back to step S4.

[0052] In order to reduce the size of the crystal grains, this application describes a method in which, in the manufacturing process of the negative electrode active material, a rapid cooling step is performed on molten MG-Si prior to MG-Si grinding to produce a plate-shaped silicon precursor, and this adjustment produces a silicon precursor that satisfies the above formulas 3 and 4.

[0053] By crushing the aforementioned plate-shaped silicon precursor having nanoscale crystal grains, a silicon-based active material having nanoscale crystal grains can be secured, and by using this to manufacture a negative electrode, for example, the cycle capacity retention rate and initial capacity efficiency of a secondary battery can be increased.

[0054] In one embodiment of this application, a silicon-based active material can be formed by pulverizing the plate-shaped silicon precursor described above, and the silicon-based active material according to this application satisfies the following formulas 1 and 2. [Equation 1] Approximately 1 μm ≤ Particle size of silicon-based active material (D50) ≤ 10 μm [Equation 2] Approximately 2 nm ≤ size of crystal grains in silicon-based active material ≤ 1 μm

[0055] For example, Equation 1 can satisfy the range of approximately 3 μm ≤ particle size (D50) of silicon-based active material ≤ 9 μm, or approximately 3 μm ≤ particle size (D50) of silicon-based active material ≤ 7 μm.

[0056] Furthermore, Equation 2 can satisfy, for example, approximately 5 nm ≤ crystal grain size of silicon-based active material ≤ 700 nm, or approximately 20 nm ≤ crystal grain size of silicon-based active material ≤ 500 nm.

[0057] As described above, by adjusting the size of the crystal grains during the formation of the silicon precursor, and further by ultimately satisfying the crystal grain size within the range of Equation 2 through the grinding process, the cycle capacity retention rate and initial capacity efficiency of the secondary battery are increased.

[0058] In one embodiment of this application, the grain size can be calculated using the FWHM (Full Width at Half Maximum) value by XRD analysis. For example, Figure 5 illustrates how to calculate the grain size. In Figure 5, the remaining values, excluding L, are measured by XRD analysis of the silicon-based active material, and the grain size can be measured based on the fact that FWHM and grain size are inversely proportional, as shown in the Debye-Scherrer equation. In this case, the Debye-Scherrer equation is as shown in Equation 1-1 below.

[0059] [Formula 1-1] FWHM = Kλ / LCosθ In the above formula 1-1, L represents the size of the crystal grain, K is a constant, θ is the Bragg angle, and λ represents the wavelength of the X-ray.

[0060] Furthermore, the shape of the crystal grains can be varied and measured three-dimensionally, and the size of the crystal grains can be measured using the commonly used circle method or diameter measurement method, but is not limited to these.

[0061] The aforementioned diameter measurement method involves drawing 5 to 10 parallel lines, each with a length of L mm, on a microscope image of the target particle. The number of crystal grains z along each line is then counted and averaged to determine the average grain size. In this process, only grains that completely fit within the line are counted, while those overlapping are excluded. If the number of lines is P and the magnification is V, the average grain size can be calculated using the following formula 1-2.

[0062] [Formula 1-2] Dm=(L*P*10 3 ) / (zV)(μm)

[0063] Furthermore, the circle method involves drawing a circle of a predetermined diameter on a micrograph of the target particles, and then determining the average area of ​​the crystal grains by the number of crystal grains that fall within the circle and the number of crystal grains that fall on the boundary. This can be calculated using the following equations 1-3.

[0064] [Formula 1-3] Fm=(Fk*10 6 ) / ((0.67n+z)V 2 )(μm 2 ) In equations 1-3 above, Fm represents the average particle area, Fk represents the measurement area on the photograph, z represents the number of particles inside the circle, n represents the number of particles along the arc, and V represents the microscope magnification.

[0065] In one embodiment of this application, a negative electrode active material is provided in which the silicon-based active material includes silicon-based crystal grains, and the ratio of high-angle boundaries within the silicon-based crystal grains is about 30% or more.

[0066] In another embodiment, the silicon-based active material includes silicon-based crystal grains, and the ratio of high-angle boundaries within the silicon-based crystal grains may be approximately 30% or more, 35% or more, 40% or more, or approximately 80% or less, 75% or less, or 70% or less.

[0067] In this application, a silicon-based crystal grain can mean each individual particle that makes up a silicon-based active material, and these silicon-based crystal grains come together to form a silicon-based active material. In this case, a grain boundary can be defined as a gap between the crystal grains.

[0068] When manufactured using the aforementioned method, many high-angle grain boundaries (HAGBs) with high grain boundary energy are formed. Lithium ions diffuse more easily at high-angle boundaries compared to low-angle grain boundaries (LAGBs).

[0069] In this application, the high-angle boundary ratio can mean the ratio of grain boundaries with a boundary angle of approximately 15° or more in the entire silicon-based active material.

[0070] In this application, the high-angle grain boundary can mean a misorientation angle > approximately 15°, and the low-angle grain boundary can mean approximately 2° < misorientation angle < 15°.

[0071] Figure 6 shows the misorientation angle. This misorientation angle 4 can be defined as the angle with respect to the difference in crystal orientation between two adjacent silicon-based crystal grains 2. In this case, the grain boundary can be defined as the crystal defect between different silicon-based crystal grains 2.

[0072] The silicon-based active material according to this application can form a relatively high proportion of high-angle boundaries at the overall grain boundary, thereby improving the performance of the battery.

[0073] In one embodiment of this application, a negative electrode active material is provided in which the average misorientation angle of the silicon-based crystal grains is approximately 5° or more.

[0074] Figure 2 shows an enlarged view of a silicon-based active material according to one embodiment of this application. For example, the silicon-based active material 1 consists of a plurality of silicon-based crystal grains 2, in which case the crystal grains have the size of crystal grains as shown in formula 2 above. Furthermore, crystal defects between silicon-based crystal grains can be defined as grain boundaries.

[0075] In one embodiment of this application, the silicon-based active material may include silicon-based particles having a particle size distribution of about 0.01 μm to 30 μm.

[0076] The fact that the silicon-based active material contains silicon-based particles having a particle size distribution of approximately 0.01 μm to 30 μm means that it contains a large number of individual silicon-based particles having particle sizes within the range, and the number of silicon-based particles included is not limited.

[0077] The particle size of the silicon-based particles can be expressed by their diameter if they are spherical, but even if they have other shapes that are not spherical, the particle size can be measured in a way that is more efficient than in the case of spherical particles, and the particle size of individual silicon-based particles can be measured using methods commonly used in this industry.

[0078] In one embodiment of this application, the silicon-based active material may exist in a crystalline or amorphous form, for example, and may not be porous. The silicon particles may be spherical or multi-piece particles, for example. In one embodiment, the silicon particles may also have a fibrous structure, or may exist in the form of a silicon-containing thin film or coating, although this is less preferred.

[0079] One embodiment of this application provides a negative electrode composition comprising a negative electrode active material; a negative electrode conductive material; and a negative electrode binder.

[0080] In one embodiment of this application, the negative electrode composition is provided, wherein the negative electrode active material is approximately 40 parts by weight or more, based on 100 parts by weight of the negative electrode composition.

[0081] In another embodiment, the negative electrode active material may be present in an amount of about 40 parts by weight or more, for example, 60 parts by weight or more, or 65 parts by weight or more, or 70 parts by weight or more, based on 100 parts by weight of the negative electrode composition, or it may be 95 parts by weight or less, or 90 parts by weight or less, or 85 parts by weight or less.

[0082] The negative electrode composition according to this application uses a negative electrode active material that satisfies a specific surface area size that allows the volume expansion rate to be controlled during the charge and discharge process, even when using a silicon-based active material with significantly high capacity within the aforementioned range. This prevents a decrease in negative electrode performance even when including the aforementioned range, and results in improved output characteristics during charging and discharging compared to conventional secondary batteries.

[0083] Traditionally, graphite-based compounds were commonly used as negative electrode active materials. However, with the increasing demand for high-capacity batteries, there has been a growing trend to mix in silicon-based active materials to increase capacity. However, as mentioned above, even if the properties of silicon-based active materials themselves are adjusted, a problem can arise where their volume rapidly expands during the charge / discharge process, damaging the conductive paths formed within the negative electrode active material layer.

[0084] In one embodiment of this application, the negative electrode conductive material may include one or more selected from the group consisting of point conductive materials, planar conductive materials, and linear conductive materials.

[0085] In one embodiment of this application, the point-shaped conductive material can be used to improve conductivity in the negative electrode and means a point-shaped or spherical conductive material that is conductive without inducing a chemical change. For example, the point-shaped 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, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and may include carbon black in that it embodies high conductivity and improves dispersibility in one embodiment.

[0086] In one embodiment of this application, the point-shaped conductive material has a BET specific surface area of ​​approximately 40 m². 2 / g or more 70m 2 It may be less than / g, for example, 45m 2 / g or more 65m 2 Less than / g, or 50m 2 / g or more 60m 2 It may be less than / g.

[0087] In one embodiment of this application, the point-shaped conductive material may have a volatile matter content of about 0.01% or more and about 1% or less, for example, about 0.01% or more and about 0.3% or less, or about 0.01% or more and about 0.1% or less.

[0088] When the functional group content of the dot-shaped conductive material satisfies the above range, functional groups are present on the surface of the dot-shaped conductive material, and when water is used as the solvent, the dot-shaped conductive material can be smoothly dispersed in the solvent. For example, in the present invention, by using a specific silicon-based active material, the functional group content of the dot-shaped conductive material can be reduced, thereby improving dispersibility.

[0089] In one embodiment of this application, a silicon-based active material is included along with a point-type conductive material having a functional group content within the aforementioned range, wherein the functional group content can be adjusted by the degree of heat treatment of the point-type conductive material.

[0090] In one embodiment of this application, the particle size of the dot-like conductive material may be about 10 nm to about 100 nm, for example, about 20 nm to 90 nm, or about 20 nm to 60 nm.

[0091] In one embodiment of this application, the conductive material may include a planar conductive material.

[0092] The planar conductive material can improve conductivity by increasing surface contact between silicon particles within the negative electrode, and at the same time suppress the disruption of the conductive path due to volume expansion. The planar conductive material may also be described as a plate-type conductive material or a bulk-type conductive material.

[0093] In one embodiment of this application, the planar conductive material may include at least one selected from the group consisting of plate-type graphite, graphene, graphene oxide, and graphite flakes, and in one embodiment, it may be plate-type graphite.

[0094] In one embodiment of this application, the average particle size (D50) of the planar conductive material may be about 2 μm to 7 μm, for example, about 3 μm to 6 μm, or about 3.5 μm to 5 μm. When the above range is satisfied, the particle size is sufficient, making dispersion easier while preventing the viscosity of the negative electrode slurry from increasing too much. Therefore, the dispersion effect is improved when dispersion is performed using the same apparatus and time.

[0095] In one embodiment of this application, a negative electrode composition is provided in which the planar conductive material has a D10 of about 0.5 μm or more and 2.0 μm or less, a D50 of about 2.5 μm or more and 3.5 μm or less, and a D90 of about 6.5 μm or more and 15.0 μm or less.

[0096] In one embodiment of this application, the planar conductive material may be a planar conductive material with a high BET specific surface area; or a planar conductive material with a low specific surface area.

[0097] In one embodiment of this application, the planar conductive material may be any planar conductive material with a high specific surface area or a planar conductive material with a low specific surface area without limitation. However, the planar conductive material according to one embodiment may be affected to some extent by dispersion in terms of electrode performance, so a planar conductive material with a low specific surface area that does not cause dispersion problems may be used.

[0098] In one embodiment of this application, the planar conductive material has a BET specific surface area of ​​approximately 0.25 m². 2 It may be more than / g.

[0099] In another embodiment, the planar conductive material has a BET specific surface area of ​​approximately 1 m². 2 / g or more 500m 2 It may be less than / g, for example, about 5m 2 / g or more 300m 2 / g or less, or 5m 2 / g or more 250m 2 / g is also acceptable.

[0100] The planar conductive material relating to this application may be a high specific surface area planar conductive material or a low specific surface area planar conductive material.

[0101] In another embodiment, the planar conductive material is a planar conductive material with a high specific surface area, and its BET specific surface area is approximately 50 m². 2 / g or more 500m 2 Less than / g, for example, about 80m 2 / g or more 300m 2 / g or less, or 100m 2 / g or more 300m 2 The range of / g or less may also be satisfied.

[0102] In another embodiment, the planar conductive material is a planar conductive material with a low specific surface area, and its BET specific surface area is approximately 1 m². 2 / g or more 40m 2 Less than / g, for example, about 5m 2 / g or more 30m 2 / g or less, or 5m 2 / g or more 25m 2 The range of / g or less may also be satisfied.

[0103] Other conductive materials may include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may contain multiple carbon nanotube units. For example, here, "bundle type" refers to a secondary shape in which multiple carbon nanotube units are arranged in parallel with substantially the same orientation along their longitudinal axes, or twisted into a bundle or rope. The carbon nanotube units have a graphite sheet that is cylindrical with a nanoscale diameter and has an sp2 bond structure. In this case, the properties of a conductor or semiconductor can be exhibited depending on the angle and structure in which the graphite sheet is wound. Compared to entangled-type carbon nanotubes, the bundle-type carbon nanotubes can be uniformly dispersed during the manufacture of the negative electrode, smoothly forming a conductive network within the negative electrode and improving the conductivity of the negative electrode.

[0104] In one embodiment of this application, the negative electrode conductive material is provided in an amount of about 10 parts by weight or more and 40 parts by weight or less, based on 100 parts by weight of the negative electrode composition.

[0105] In another embodiment, the negative electrode conductive material may contain approximately 0.1 parts by weight or more and 40 parts by weight or less, based on 100 parts by weight of the negative electrode composition, for example, approximately 0.2 parts by weight or more and 30 parts by weight or approximately 0.4 parts by weight or more and 25 parts by weight or approximately 0.4 parts by weight or more and 10 parts by weight or less.

[0106] In one embodiment of this application, a negative electrode composition is provided in which the negative electrode conductive material includes a planar conductive material and a linear conductive material.

[0107] In one embodiment of this application, the negative electrode conductive material is provided as a negative electrode composition comprising, based on 100 parts by weight of the negative electrode conductive material, about 80 parts by weight or more and 99.9 parts by weight of the planar conductive material; and about 0.1 parts by weight or more and 20 parts by weight of the linear conductive material.

[0108] In another embodiment, the negative electrode conductive material may contain approximately 80 to 99.9 parts by weight of the planar conductive material, based on 100 parts by weight of the negative electrode conductive material, for example, approximately 85 to 99.9 parts by weight, or 95 to 98 parts by weight.

[0109] In another embodiment, the negative electrode conductive material may contain approximately 0.1 parts by weight to 20 parts by weight of the linear conductive material, based on 100 parts by weight of the negative electrode conductive material, for example, 0.1 parts by weight to 15 parts by weight, or 0.2 parts by weight to 5 parts by weight.

[0110] In one embodiment of this application, the negative electrode conductive material includes a planar conductive material and a linear conductive material, each satisfying the aforementioned composition and proportion, without significantly affecting the life characteristics of a conventional lithium secondary battery. For example, when a planar conductive material and a linear conductive material are included, the number of charge and discharge points increases, improving the output characteristics of the secondary battery at a high C-rate and reducing the amount of high-temperature gas generated.

[0111] In one embodiment of this application, the negative electrode conductive material may be a linear conductive material. When a linear conductive material is used alone, the electrode tortuosity, which is a problem with silicon-based negative electrodes, can be simplified, the electrode structure can be improved, and the resistance to lithium ion movement within the electrode can be reduced.

[0112] In one embodiment of this application, when the negative electrode conductive material includes a linear conductive material alone, the negative electrode conductive material may be included in an amount of about 0.1 parts by weight or more and 5 parts by weight or less, for example, about 0.2 parts by weight or more and 3 parts by weight or about 0.4 parts by weight or more and 1 part by weight or less, based on 100 parts by weight of the negative electrode composition.

[0113] The negative electrode conductive material according to this application has a substantially different configuration from the positive electrode conductive material applied to the positive electrode. Specifically, the negative electrode conductive material according to this application plays a role in controlling the contact points between silicon-based active materials, which experience very large volume expansion of the electrodes due to charging and discharging, while the positive electrode conductive material, when rolled, acts as a buffer while partially imparting conductivity, and thus its configuration and role are substantially different from the negative electrode conductive material of the present invention.

[0114] Furthermore, the negative electrode conductive material described in this application is applied to silicon-based active materials and has a substantially different structure from conductive materials applied to graphite-based active materials. For example, conductive materials used in electrodes with graphite-based active materials simply have particles that are smaller than the active material, and therefore have the properties of improving output characteristics and imparting some conductivity. Thus, their structure and role are substantially different from negative electrode conductive materials applied together with silicon-based active materials, as in the present invention.

[0115] In one embodiment of this application, the planar conductive material used as the negative electrode conductive material has a different structure and role from the carbon-based active material typically used as the negative electrode active material. For example, the carbon-based active material used as the negative electrode active material may be artificial graphite or natural graphite, and refers to a material that is processed into a spherical or point-like form to facilitate the storage and release of lithium ions.

[0116] On the other hand, planar conductive materials used as negative electrode conductive materials are substances having a planar or plate-like form, and can be represented as plate-type graphite. For example, a substance included to maintain conductive pathways within the negative electrode active material layer, and not a substance that plays a role in lithium storage and release, but rather a substance that secures conductive pathways in a planar manner within the negative electrode active material layer.

[0117] In other words, in this application, the use of plate-shaped graphite as a conductive material means that it is processed into a planar or plate-shaped form and used as a material to secure a conductive path that does not serve the role of storing or releasing lithium. In this case, the negative electrode active material included together has high capacity characteristics for lithium storage and release and plays a role in storing and releasing all lithium ions transmitted from the positive electrode.

[0118] On the other hand, in this application, the use of a carbon-based active material as the active material means that it was processed into a point-like or spherical shape and used as a substance that plays a role in storing or releasing lithium.

[0119] For example, in one embodiment of this application, the carbon-based active material, artificial graphite or natural graphite, is punctate and has a BET specific surface area of ​​0.1 m². 2 / g or more 4.5m 2 The range of less than / g may also be satisfied. In addition, plate-type graphite, which is a planar conductive material, is planar and has a BET specific surface area of ​​approximately 5 m². 2 It may be more than / g.

[0120] In one embodiment of this application, the negative electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these are substituted with Li, Na, or Ca, or may contain a variety of copolymers thereof.

[0121] The negative electrode binder according to one embodiment of this application plays a role in controlling the active material and conductive material in order to prevent twisting and structural deformation of the negative electrode structure during the volume expansion and relaxation of the silicon-based active material. Any binder can be applied as long as it fulfills the above role, for example, an aqueous binder or a PAM-based binder may be used.

[0122] In one embodiment of this application, the negative electrode binder may be about 30 parts by weight or less, for example, about 25 parts by weight or less, or about 20 parts by weight or less, based on 100 parts by weight of the negative electrode composition, or it may be about 5 parts by weight or more, or about 10 parts by weight or more.

[0123] One embodiment of this application provides a method for producing a negative electrode active material, comprising the steps of: rapidly cooling metallic silicon to form a silicon precursor; and pulverizing the silicon precursor to form a silicon-based active material, wherein the rapid cooling step includes melt spinning, suction casting, or injection casting.

[0124] As described above, instead of immediately pulverizing the metallic silicon itself, the size of the crystal grains of the silicon precursor formed by adjusting the cooling rate through a rapid cooling process is adjusted, and the precursor is then pulverized to produce a silicon-based active material.

[0125] In other words, when casting molten silicon, the shorter the solidification time (Ts), the faster the cooling rate. The shorter the solidification time, the faster the cooling rate can be adjusted, as the plate thickness of the silicon-based precursor satisfies the range of Equation 3 (for example, approximately 10 μm to 2 mm). Furthermore, as described above, a faster cooling rate allows for a shorter time for crystal grain growth, enabling the crystal grain size to meet the aforementioned requirements (for example, approximately 2 nm to 11 μm).

[0126] One embodiment of this application provides a method for producing a negative electrode active material, further comprising the step of heating metallic silicon in an induction furnace before the step of rapidly cooling metallic silicon to form a silicon precursor.

[0127] As described above, the process involves heating metallic silicon in an induction furnace to create a molten state, followed by a rapid cooling step, which includes melt spinning, suction casting, or injection casting. Any method that can rapidly cool molten metallic silicon in accordance with the spirit of the present invention to form a plate-shaped silicon precursor is applicable.

[0128] Generally, melt spinning can be defined as the process of rapidly cooling molten material by radiating it onto a rotating copper wheel (Cu wheel). Suction casting refers to the process of casting by sucking molten material into a mold under vacuum, while injection casting refers to the process of casting by injecting molten material into a mold using high-pressure gas.

[0129] In the rapid cooling stage described above, the melt spinning, suction casting, or injection casting processes differ and are diverse, making it difficult to generally specify the conditions. However, as a result, all silicon-based precursors can be manufactured in plate form.

[0130] In this application, during rapid cooling, the cooling rate adjustment differs for each process. Generally, in melt spinning, the faster the wheel rotation speed, the faster the cooling rate. In suction casting or injection casting, the thinner the mold, the faster the cooling rate. The cooling rate can also be adjusted by adjusting the external temperature, such as by flowing cooling water through the mold or cooling the wheel and mold with liquid nitrogen. Rapid cooling, on the other hand, is a type of cooling method that rapidly cools MG silicon to form a plate-shaped silicon precursor. This rapid cooling means a type of non-equilibrium solidification, as it is a cooling method that suppresses reactions such as phase transformation and grain coarsening that occur in equilibrium. For example, when rapidly cooling metallic silicon using melt spinning, the cooling rate can be adjusted by adjusting the rotation speed of the copper wheel (Cu wheel). It cools from approximately 1,500°C, which is the temperature of the molten state, to approximately 25°C, which is room temperature, but this takes less than 1 second.

[0131] In one embodiment of this application, a method for producing a negative electrode active material is provided, wherein, after the step of rapidly cooling metallic silicon to form a silicon precursor, the silicon precursor satisfies the aforementioned formulas 3 and 4.

[0132] A silicon precursor is formed through a rapid cooling process, which can be interpreted as satisfying equations 3 and 4.

[0133] In this application, the present invention provides a method for producing a negative electrode active material, comprising the step of pulverizing the silicon precursor to form a silicon-based active material; thereafter, the silicon-based active material satisfies the aforementioned formulas 1 and 2.

[0134] By rapidly cooling the silicon precursor that satisfies specific equations 3 and 4, and then grinding it, a silicon-based active material that satisfies the range of equations 1 and 2 described above can be produced.

[0135] In one embodiment of this application, the grinding step may include one or more steps selected from the group consisting of a ball mill, a pin mill, a disk mill, and a jet mill, but is not limited thereto. Any grinding step that can grind a plate-shaped metallic silicon precursor to produce a silicon-based active material satisfying the ranges of Formulas 1 and 2 described above is applicable.

[0136] Through the aforementioned grinding process, a silicon-based active material satisfying the ranges of Equations 1 and 2 can be produced.

[0137] One embodiment of this application provides a negative electrode for a lithium secondary battery, comprising a negative electrode current collector layer; and a negative electrode active material layer formed on one or both sides of the negative electrode current collector layer, the negative electrode composition according to this application or a cured product thereof.

[0138] Figure 3 shows a laminated structure of a negative electrode for a lithium secondary battery according to one embodiment of the present application. For example, a negative electrode 100 for a lithium secondary battery can be seen in which a negative electrode active material layer 20 is included on one surface of the negative electrode current collector layer 10. Figure 3 shows that the negative electrode active material layer 20 is formed on one surface, but it may also be included on both sides of the negative electrode current collector layer.

[0139] In one embodiment of this application, the negative electrode for the lithium secondary battery may be formed by applying and drying a negative electrode slurry containing the negative electrode composition to one or both sides of a negative electrode current collector layer.

[0140] In this case, the negative electrode slurry may contain the aforementioned negative electrode composition and slurry solvent.

[0141] In one embodiment of this application, the solid content of the negative electrode slurry may be in the range of approximately 5% to 40%.

[0142] In another embodiment, the solid content of the negative electrode slurry may be in the range of approximately 5% to 40%, for example, approximately 7% to 35%, or approximately 10% to 30%.

[0143] The solid content of the negative electrode slurry may mean the content of the negative electrode composition contained in the negative electrode slurry, or it may mean the content of the negative electrode composition based on 100 parts by weight of the negative electrode slurry.

[0144] When the solid content of the negative electrode slurry satisfies the above range, the viscosity is appropriate during the formation of the negative electrode active material layer, minimizing the caking phenomenon of the particles of the negative electrode composition, and enabling efficient formation of the negative electrode active material layer.

[0145] In one embodiment of this application, the slurry solvent can be any solvent that can dissolve the negative electrode composition, and may be, for example, water or NMP.

[0146] In one embodiment of this application, the negative electrode current collector layer typically has a thickness of about 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy may be used. Furthermore, fine irregularities may be formed on the surface to strengthen the bonding force of the negative electrode active material, and it may be used in various forms such as film, sheet, foil, net, porous body, foam, nonwoven fabric.

[0147] In one embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the thickness of the negative electrode current collector layer is approximately 1 μm or more and 100 μm or less, and the thickness of the negative electrode active material layer is approximately 5 μm or more and 500 μm or less.

[0148] However, the thickness may vary depending on the type and application of the negative electrode used.

[0149] In one embodiment of this application, the porosity of the negative electrode active material layer may be in the range of approximately 10% to 60%.

[0150] In another embodiment, the porosity of the negative electrode active material layer may be in the range of approximately 10% to 60%, for example, approximately 20% to 50%, or approximately 30% to 45%.

[0151] The aforementioned porosity is varied by the composition and content of the silicon-based active material, conductive material, and binder contained in the negative electrode active material layer. For example, by including the silicon-based active material and conductive material according to this application in specific compositions and content amounts, the aforementioned range is satisfied, thereby ensuring that the electrical conductivity and resistance of the electrode are within an appropriate range.

[0152] One embodiment of this application provides a lithium secondary battery comprising a positive electrode; a negative electrode for a lithium secondary battery according to this application; a separation membrane provided between the positive electrode and the negative electrode; and an electrolyte.

[0153] Figure 4 shows a stacked structure of a lithium secondary battery according to one embodiment of the present application. For example, a negative electrode 100 for a lithium secondary battery containing a negative electrode active material layer 20 can be seen on one side of a negative electrode current collector layer 10, and a positive electrode 200 for a lithium secondary battery containing a positive electrode active material layer 40 can be seen on one side of a positive electrode current collector layer 50, showing that the negative electrode 100 and the positive electrode 200 for a lithium secondary battery are formed in a stacked structure with a separation membrane 30 in between.

[0154] A secondary battery according to one embodiment of this specification may include the negative electrode for lithium secondary batteries described above. For example, the secondary battery may include a negative electrode, a positive electrode, a separator membrane interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the same as the negative electrode described above. Since the negative electrode has been described above, a detailed explanation will be omitted.

[0155] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

[0156] In the positive electrode, the positive electrode current collector is not particularly limited as long as it is conductive without inducing a chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surfaces treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0157] The positive electrode active material may be a commonly used positive electrode active material. For example, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4 (0 ≤ c1 ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, satisfying 0.01 ≤ c2 ≤ 0.3); chemical formula LiMn 2-c3 M c3Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion, etc., can be mentioned, but are not limited to these. The positive electrode may be Li metal (Li-metal).

[0158] The positive electrode active material layer may also include a positive electrode conductive material and a positive electrode binder, along with the positive electrode active material described above.

[0159] The positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations as long as it has electronic conductivity in the battery without causing chemical changes. Examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. One of these alone or a mixture of two or more may be used.

[0160] Furthermore, the positive electrode binder plays a role in improving adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used.

[0161] The separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is a membrane typically used in secondary batteries. For example, it can be used if it has low resistance to ion movement of the electrolyte and excellent moisture-retaining capacity for the electrolyte. For example, porous polymer films, such as those made from polyolefin polymers like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof, may be used. Alternatively, ordinary porous nonwoven fabrics, such as those made from high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, coated separation membranes containing ceramic components or polymeric substances to ensure heat resistance or mechanical strength may be used, and they may be selectively used in single-layer or multi-layer structures.

[0162] Examples of the aforementioned electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0163] For example, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0164] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate may be used.

[0165] Among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be used because they are high-viscosity organic solvents with high dielectric constants that effectively dissociate lithium salts. By mixing such cyclic carbonates with low-viscosity, low-dielectric-constant chain carbonates such as dimethyl carbonate and diethyl carbonate in appropriate ratios, electrolytes with high electrical conductivity can be produced, and therefore they can be used.

[0166] The metal salt may be a lithium salt, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte, for example, the anion of the lithium salt may be F - Cl - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 -(CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - You may use one or more selected from the group consisting of the following:

[0167] In addition to the components of the electrolyte, the electrolyte may further contain one or more additives for the purpose of improving the battery's lifespan, suppressing the decrease in battery capacity, and improving the battery's discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexalic acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0168] One embodiment of the present invention provides a battery module including the secondary battery as a unit cell, and a battery pack including the same. Because the battery module and battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, they may be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems. [Examples]

[0169] The following examples are provided to aid in understanding the present invention. However, these examples are merely illustrative of the description, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the description and the technical concept, and such variations and modifications naturally fall within the scope of the claims.

[0170] [Manufacturing example] <Manufacturing of silicon-based precursors> <Example 1> A plate-shaped silicon precursor was produced by placing MG silicon in a graphite crucible, melting it using an induction furnace, and then melt-spinning the resulting material.

[0171] For example, silicon in a molten state at 2000°C was produced by passing a current of 10kA through a copper coil, and the molten silicon was injected at a pressure of 1.2kPa onto a 25cm diameter copper wheel rotating at a speed of 3000rpm to obtain a plate-shaped silicon precursor. In this case, the thickness of the silicon-based precursor plate was approximately 20μm, and the size of the crystal grains was approximately 50nm, satisfying equations 3 and 4 mentioned above, respectively.

[0172] Except for changing the rotation speed to the conditions shown in Table 1 below in order to adjust the cooling rate during rapid cooling by melt spinning as in Example 1, the silicon-based precursors of Examples 2, 3 and Comparative Example 3 were produced in the same manner as in Example 1.

[0173] [Table 1]

[0174] In Example 2, when the rotation speed of the copper wheel in the melt spinning process was 4000 rpm, which is higher than the 3000 rpm in Example 1, the plate thickness of the silicon-based precursor was approximately 15 μm, and the crystal grain size was approximately 40 nm, satisfying equations 3 and 4 as described above. In Example 3, when the rotation speed of the copper wheel in the melt spinning process was 1000 rpm, the plate thickness of the silicon-based precursor was approximately 100 μm, and the crystal grain size was approximately 300 nm, satisfying equations 3 and 4 as described above.

[0175] In Comparative Examples 1 and 2, MG silicon was not placed in a graphite crucible, melted using an induction furnace, and then melt-spinned to produce a plate-shaped silicon precursor. Instead, the MG silicon itself was used directly in the milling process described later. On the other hand, in Comparative Example 3, the copper wheel rotation speed in the melt-spinning process was set to 100 rpm. However, the plate thickness of the silicon-based precursor was about 3 mm thicker than in the examples of the present invention, and the crystal grain size was 2.5 μm, which is larger than in the examples of the present invention. Thus, it can be seen that the above-mentioned equations 3 and 4 are not satisfied.

[0176] <Example 4> In Example 4, unlike in Examples 1, 2, and 3 described above, MG silicon was melted into a molten state through arc melting, and then plate-shaped silicon precursors were produced through a suction casting process.

[0177] For example, a vacuum of 10 -6 By filling the Torr chamber with argon at a pressure of 0.5 kPa, a high-purity argon atmosphere was created, and molten silicon at 2000°C was produced using arc plasma. Subsequently, the vacuum level at the bottom of the mold was reduced to 10 -5 After holding the temperature above torr, the valve between the bottom of the mold and the chamber was opened to draw the molten silicon into the mold, thereby producing a plate-shaped silicon precursor. A mold with a thickness of 0.5 mm, a width of 10 mm, and a length of 50 mm was used, and the crystal grain size was 50 nm.

[0178] Except for changing the mold thickness, which affects the cooling rate, to the conditions shown in Table 2 below during rapid cooling by suction casting as in Example 4, a silicon-based precursor was produced in the same manner as in Example 4.

[0179] [Table 2]

[0180] In Examples 4, 5, and 6, the plate thicknesses of the silicon precursors were 500 μm, 400 μm, and 100 μm, respectively, satisfying the conditions of Equation 3. The crystal grain sizes within the silicon precursors were 400 nm, 500 nm, and 1,000 nm, respectively, satisfying Equation 4.

[0181] In the cases of Comparative Examples 4 and 5, instead of moltening the MG silicon through arc melting and then producing plate-shaped silicon precursors by suction casting, the MG silicon itself was used directly in the milling process described later.

[0182] On the other hand, in Comparative Example 6, where the plate-shaped mold thickness was 3 mm, the silicon precursor plate thickness was 3 mm, which could not satisfy the conditions of Equation 3, and the size of the crystal grains in the silicon precursor was 2.510 μm, which could not satisfy Equation 4.

[0183] Based on the values ​​in Tables 1 and 2, it was confirmed that, in general, the melt spinning process facilitates the production of thinner silicon precursors compared to the suction casting process.

[0184] <Manufacturing of silicon-based active materials> A silicon-based active material was produced from the silicon-based precursor prepared as described above using a ball mill.

[0185] For example, a wet grinding method using n-hexane as a medium was employed, and zirconia (ZrO2) was used as the ball. In this case, the mass ratio of the silicon-based precursor to the ball was set to approximately 1:40, and the grinding was carried out for 30 minutes. The particle size of the produced negative electrode active material was 5 μm, the crystal grain size was 50 nm, and it satisfied equations 1 and 2, respectively.

[0186] The silicon precursors of the aforementioned examples and comparative examples were pulverized using the same method as described above, and the results are shown in Table 3.

[0187] [Table 3]

[0188] According to Table 3, the particle size (D50) of the silicon-based active material in Examples 1 to 6 satisfies Equation 1, while Comparative Examples 5 and 6 do not. Furthermore, the crystal grain size of the silicon-based active material in Examples 1 to 6 satisfies Equation 2, while Comparative Examples 1 to 6 do not. In addition, the high-angle boundary ratio in Examples 1 to 6 is 30% or more, while in Comparative Examples 4 to 6, the high-angle boundary ratio is less than 30%.

[0189] <Manufacturing of negative electrodes> In one embodiment, a negative electrode slurry was prepared by adding a negative electrode active material containing the silicon-based active material shown in Table 3, a first conductive material, a second conductive material, and polyacrylamide as a binder in a weight ratio of 80:9.6:0.4:10 to distilled water used as a solvent for forming the negative electrode slurry (solid content concentration 25% by weight).

[0190] For example, the first conductive material is a plate-shaped graphite (specific surface area: 17 m²). 2 The second conductive material was SWCNT (semi-carbon nanotube), with a density of approximately 3.5 μm (average particle size D50).

[0191] As a mixing method, the first conductive material, the second conductive material, the binder, and water were dispersed using a homomixer at approximately 2500 rpm for 30 minutes, and then the silicon-based active material was added and dispersed at approximately 2500 rpm for another 30 minutes to produce a negative electrode slurry.

[0192] As the negative electrode current collector layer, approximately 85 mg / 25 cm of the negative electrode slurry is applied to both sides of the copper current collector (thickness: 8 μm). 2 The material was coated with the specified load, rolled (roll press), and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer (thickness: 33 μm), which was then used as the negative electrode (negative electrode thickness: 41 μm, negative electrode porosity: 40.0%).

[0193] <Manufacturing of secondary batteries> In one embodiment, LiNi 0.6 Co 0.2 Mn 0.2 A cathode slurry was prepared by adding O2 (average particle size (D50): 15 μm), carbon black (product name: Super C65, manufacturer: Timcal) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of approximately 97:1.5:1.5 to N-methyl-2-pyrrolidone (NMP) as a solvent for cathode slurry formation (solid content concentration approximately 78% by weight).

[0194] As the positive electrode current collector, approximately 537 mg / 25 cm of the positive electrode slurry was applied to both sides of an aluminum current collector (thickness: 12 μm). 2 The cathode was manufactured by coating with the specified load, rolling (roll press), and drying in a vacuum oven at 130°C for 10 hours to form a cathode active material layer (thickness: 65 μm) (cathode thickness: approximately 77 μm, porosity: approximately 26%).

[0195] A polyethylene separation membrane was interposed between the positive electrode and the negative electrodes of the above-mentioned examples and comparative examples, and an electrolyte was injected to manufacture a lithium secondary battery.

[0196] The aforementioned electrolyte was prepared by adding vinylene carbonate at approximately 3% by weight relative to the total weight of the electrolyte to an organic solvent mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DMC) in a volume ratio of 10:90, and adding LiPF6 as a lithium salt at a concentration of approximately 1M.

[0197] <Example of experiment> Experimental Example 1: Monocell Life Performance Results The secondary batteries containing the negative electrodes manufactured in the above examples and comparative examples were evaluated for their lifespan using an electrochemical charger / discharger, and their capacity retention rate was assessed. The secondary batteries underwent in-situ cycle testing at 4.2-3.0V and 1C / 0.5C, and during the test, they were charged / discharged at 0.33C / 0.33C (4.2-3.0V) every 50 cycles, and the capacity retention rate was measured.

[0198] Life retention rate (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the first cycle)} × 100

[0199] [Table 4]

[0200] As can be seen in Table 4, when the capacity retention rate by cycle was examined for Examples 1-6 and Comparative Examples 1-6 to which the present invention was applied, it was confirmed that the capacity retention rate of the Comparative Examples to which the present invention was not applied was lower than that of the Examples. This corresponds to the fact that when the crystal grain size of the silicon-based active material satisfies the range related to the present invention, the lithium insertion and desorption reactions during charging and discharging can react uniformly, reducing the stress on the silicon-based active material and mitigating particle cracking, thereby improving the life retention rate of the electrode.

[0201] Experimental Example 2: Monocell Resistance Change In Experimental Example 1, during the test, the battery was charged / discharged at 0.33C / 0.33C (4.2-3.0V) every 50 cycles to measure the capacity retention rate. Then, the battery was discharged with a 2.5C pulse at SOC50, the resistance was measured, and the resistance increase rate was compared and analyzed.

[0202] For the measurement and evaluation of the resistance increase rate mentioned above, data was calculated for 200 cycles, and the results are shown in Table 5 below.

[0203] [Table 5]

[0204] Tables 4 and 5 confirm that the life characteristics and resistance changes of batteries using silicon produced in Examples 1 to 6 as the negative electrode active material are excellent. In the case of the negative electrode active material according to one embodiment of the present invention, unlike the conventional pulverization method, a rapid cooling step is performed prior to MG-Si pulverization to form a plate-shaped silicon precursor, and the size of the crystal grains is optimized by adjusting the cooling rate. As in the examples, by pulverizing a plate-shaped silicon precursor having nano-sized crystal grains, it is possible to secure a silicon-based active material that satisfies nano-sized crystal grains and a specific high-angle boundary ratio. It was confirmed that when a negative electrode is manufactured using this, the cycle capacity retention rate and initial capacity efficiency are increased.

[0205] For reference, Comparative Examples 1 to 3 correspond to cases where the particle size (Equation 1) and crystal grain size (Equation 2) of the silicon-based active material are the same, that is, cases where there are no grain boundaries inside the particles of the silicon-based active material in the form of a single crystal, and high-angle boundaries cannot exist. Comparative Examples 4 to 6 correspond to cases where the value of Equation 1 is greater than the value of Equation 2, and grain boundaries exist inside the particles, but the ratio of high-angle boundaries does not fall within the scope of this application. In the case of Examples 1 to 6 of this application, manufactured by the manufacturing method described above, high-angle grain boundaries (HAGB) with high grain boundary energy are formed in the proportions shown in Table 3, and it was confirmed that high-angle boundaries are superior to comparative examples in terms of lifetime characteristics and resistance increase rate because lithium ion diffusion is easier compared to low-angle grain boundaries (LAGB).

[0206] Although the present invention has been described above with reference to embodiments, a person skilled in the art or a person with ordinary knowledge in the art should understand that the present invention can be modified and altered in various ways without departing from the spirit and technical domain of the invention as described in the claims. Therefore, the technical scope of the present invention should not be limited to what is described in the detailed description of the specification, but should be defined by the claims. [Explanation of Symbols]

[0207] 1. Silicon-based active material 2. Silicon-based crystal grains 3...grain boundary 4...misorientation angle 10 ···Negative electrode current collector layer 20...Negative electrode active material layer 30...Separation membrane 40...Cathode active material layer 50 ···Positive electrode current collector layer 100 ···Negative electrode for lithium secondary batteries 200 ···Positive electrode for lithium secondary batteries

Claims

1. A negative electrode active material containing a silicon-based active material, The silicon-based active material includes silicon-based crystal grains, The high-angle grain boundary ratio within the silicon-based crystal grain is 30% or more. The silicon-based active material is a negative electrode active material having a chemical configuration that satisfies the following formulas 1 and 2: [Equation 1] 1 μm ≤ Particle size (D50) of silicon-based active material ≤ 10 μm [Equation 2] 2 nm ≤ size of crystal grains of silicon-based active material ≤ 1 μm.

2. The silicon-based active material includes pulverized silicon precursor, The silicon precursor comprises a chemical configuration satisfying the following formulas 3 and 4, wherein the negative electrode active material is as described in claim 1: [Equation 3] 10 μm ≤ silicon precursor plate thickness ≤ 2 mm [Equation 4] 2 nm ≤ size of crystal grains in silicon precursor ≤ 1 μm

3. The negative electrode active material according to claim 1, wherein the average grain boundary misorientation angle of the silicon-based crystal grains is 5° or more.

4. The aforementioned silicon-based active material is SiO ｘ (x=0) and SiO ｘ It comprises one or more selected from the group consisting of (0 < x < 2), and based on 100 parts by weight of the silicon-based active material, the SiO ｘ The negative electrode active material according to claim 1, comprising 70 parts by weight or more of (x=0).

5. A negative electrode active material comprising a silicon-based active material, The silicon-based active material includes pulverized silicon precursor, The silicon precursor comprises a negative electrode active material having a chemical configuration satisfying the following formulas 3 and 4: [Equation 3] 10 μm ≤ silicon precursor plate thickness ≤ 2 mm [Equation 4] 2 nm ≤ size of crystal grains in silicon precursor ≤ 1 μm

6. A step of rapidly cooling metallic silicon to form a silicon precursor; and A step of crushing the silicon precursor to form a silicon-based active material; A method for producing a negative electrode active material, comprising: The rapid cooling process is carried out by one of the following: melt spinning, suction casting, or injection casting. After the step of rapidly cooling metallic silicon to form a silicon precursor, The silicon precursor comprises a chemical configuration satisfying the following formulas 3 and 4, and is used to produce a negative electrode active material: [Equation 3] 10 μm ≤ silicon precursor plate thickness ≤ 2 mm [Equation 4] 2 nm ≤ size of crystal grains in silicon precursor ≤ 1 μm

7. Before the rapid cooling stage of metallic silicon, The method for producing a negative electrode active material according to claim 6, further comprising the step of heating the metallic silicon in an induction furnace.

8. The step of crushing the silicon precursor to form a silicon-based active material; thereafter, The method for producing the negative electrode active material according to claim 6, wherein the silicon-based active material includes a chemical configuration that satisfies the following formulas 1 and 2: [Equation 1] 1 μm ≤ Particle size (D50) of silicon-based active material ≤ 10 μm [Equation 2] 2 nm ≤ size of crystal grains of silicon-based active material ≤ 100 nm.

9. The negative electrode active material according to any one of claims 1 to 4; Negative electrode conductive material; and Negative electrode binder; A negative electrode composition containing the following:

10. The negative electrode composition according to claim 9, wherein the negative electrode active material is 40 parts by weight or more, based on 100 parts by weight of the negative electrode composition.

11. The negative electrode composition according to claim 9, wherein the negative electrode conductive material includes a planar conductive material and a linear conductive material.

12. The material includes a negative electrode current collector layer and a negative electrode active material layer provided on one or both sides of the negative electrode current collector layer. The negative electrode active material layer comprises the negative electrode composition according to claim 9 or a cured product thereof, for use as a negative electrode for a lithium secondary battery.

13. The thickness of the negative electrode current collector layer is 1 μm or more and 100 μm or less. The negative electrode for a lithium secondary battery according to claim 12, wherein the thickness of the negative electrode active material layer is 5 μm or more and 500 μm or less.

14. positive electrode; A negative electrode for a lithium secondary battery according to claim 12; A separation membrane provided between the positive electrode and the negative electrode; and Electrolyte; Lithium-ion batteries, including lithium-ion batteries.

15. The stage of producing molten metallic silicon; A step of rapidly cooling the molten metal silicon to form a plate-shaped metal silicon precursor; A step of determining whether the plate thickness of the formed metallic silicon precursor is 10 μm or more and 2 mm or less, and whether the size of the crystal grains within the metallic silicon precursor is 2 nm or more and 1 μm or less; If the plate thickness and grain size of the metallic silicon precursor are determined to be 10 μm or more and 2 mm or less, and 2 nm or more and 1 μm or less, respectively, the metallic silicon precursor is pulverized to produce a silicon-based active material; A step of determining whether the particle size (D50) of the manufactured silicon-based active material is 1 μm or more and 10 μm or less, and whether the crystal grain size of the manufactured silicon-based active material is 2 nm or more and 1 μm or less; and When the particle size (D50) and crystal grain size of the manufactured silicon-based active material are 1 μm or more and 10 μm or less, and 2 nm or more and 1 μm or less, respectively, the silicon-based active material is classified as an active material for manufacturing the negative electrode of a lithium secondary battery; A method for producing a negative electrode active material, including the material itself.

16. The method for producing a negative electrode active material according to claim 15, wherein the rapid cooling step is carried out by melt spinning, suction casting, or injection casting.

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