Negative electrode active material for lithium secondary battery, preparation method thereof, and lithium secondary battery including same

Abstract

A negative electrode active material for a lithium secondary battery according to the present invention includes a silicon-carbon-based composite including: silicon nanoparticles; a carbon-based matrix; and a conductive additive, wherein the content of the conductive additive may be 0.2-10 wt% relative to the silicon nanoparticles in the composite.

Description

Negative electrode active material for a lithium secondary battery, method for manufacturing the same, and a lithium secondary battery including the same

[0001] The present invention relates to a negative electrode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same.

[0002] Lithium-ion batteries are currently the most widely used secondary battery systems in portable electronic communication devices, electric vehicles, and energy storage devices. These lithium-ion batteries are the focus of attention due to their advantages, such as high energy density, operating voltage, and a relatively low self-discharge rate, compared to commercial aqueous secondary batteries (Ni-Cd, Ni-MH, etc.). However, considering the need for more efficient usage time in portable devices and improved energy characteristics in electric vehicles, improvements in electrochemical properties remain technical challenges that need to be addressed. Consequently, extensive research and development are currently underway across the four major raw materials: the cathode, anode, electrolyte, and separator.

[0003] Among these raw materials, graphite-based materials exhibiting excellent capacity retention characteristics and efficiency are commercially available for the anode. However, the reality is that the relatively low theoretical capacity value (LiC6: 372 mAh / g) and low discharge capacity ratio of graphite-based materials fall somewhat short of meeting the high energy and high power density characteristics of batteries required by the market. Therefore, many researchers are interested in Group 4A elements (Si, Ge, Sn) of the periodic table, and among them, Si, in particular, has a very high theoretical capacity (Li 15 It is gaining attention as a very attractive material due to its characteristics of Si4 (3600mAh / g) and low operating voltage (~0.1V vs. Li / Li+).

[0004] However, conventional silicon-based anode materials have the disadvantage of being difficult to apply to actual batteries because they exhibit low discharge capacity ratio characteristics along with large volume changes compared to existing graphite anode materials.

[0005] Recently, active research has been conducted on silicon-carbon composite anode materials that improve reversibility by combining silicon, which is electrochemically reactive with lithium, with conductive materials (graphite or carbon). However, these composites present challenges in controlling silicon expansion and achieving long lifespan, making further research necessary.

[0006] One aspect of the present invention for solving the aforementioned problem is to provide a negative electrode active material for a lithium secondary battery capable of improving electrochemical properties such as initial efficiency, charge / discharge capacity, and lifespan characteristics by adding a carbon-based compound as a conductive additive to silicon, a method for manufacturing the same, and a lithium secondary battery including the same.

[0007] The technical problems to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0008] To achieve the above objective, a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention comprises a silicon-carbon composite comprising silicon nanoparticles, a carbon-based matrix, and a conductive additive, wherein the conductive additive may be included in an amount of 0.2% to 10% by weight relative to the silicon nanoparticles in the composite.

[0009] The conductive additive according to one embodiment of the present invention may be one or more carbon-based compounds selected from carbon nanotubes, activated carbon, and graphene.

[0010] The conductive additive according to one embodiment of the present invention may include carbon nanotubes having an average outer diameter of 1 nm to 500 nm.

[0011] According to one embodiment of the present invention, the silicon nanoparticles may have an average particle size (D50) of 30 nm to 200 nm.

[0012] According to one embodiment of the present invention, the silicon nanoparticle may have a half-width of X-ray diffraction angle (2theta) using CuKα rays in the (111) plane of 0.6° to 1.0°.

[0013] According to one embodiment of the present invention, the silicon nanoparticles may include needle-shaped silicon nanoparticles having an aspect ratio greater than 1.5 as 90% or more of the total weight of the silicon nanoparticles.

[0014] The carbon-based matrix according to one embodiment of the present invention may include one or more amorphous carbons selected from petroleum-based pitch, coal tar, PAA (poly(acrylic acid)), PVA (poly(vinyl alcohol)), soft carbon, hard carbon, and calcined coke, with a softening point of 300°C or lower.

[0015] According to one embodiment of the present invention, the carbon-based matrix may be included in an amount of 30% to 50% by weight based on the total weight of the silicon-carbon-based composite.

[0016] The negative electrode active material according to one embodiment of the present invention may further include one or more conductive materials selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

[0017] The average particle size (D50) of the conductive material according to one embodiment of the present invention may be 1 μm to 12 μm.

[0018] The silicon-carbon composite according to one embodiment of the present invention may further include an amorphous carbon coating layer on its surface.

[0019] The amorphous carbon coating layer according to one embodiment of the present invention may have an average thickness of 10 nm or less.

[0020] The negative electrode active material according to one embodiment of the present invention may have an average particle size (D50) of 2㎛ to 15㎛.

[0021] The negative electrode active material according to one embodiment of the present invention may have a BET specific surface area of ​​5 m² / g or less.

[0022] A method for manufacturing a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention may include: a step of manufacturing silicon nanoparticles by grinding a silicon raw material; a step of manufacturing spherical assembled particles by mixing and sphericalizing the silicon nanoparticles and a conductive additive; a step of manufacturing a silicon-carbon composite precursor by mixing the spherical assembled particles with a carbon-based matrix; and a step of manufacturing a silicon-carbon composite by heat-treating the silicon-carbon composite precursor.

[0023] The conductive additive according to one embodiment of the present invention may be a carbon nanotube having an average outer diameter of 1 nm to 500 nm.

[0024] According to one embodiment of the present invention, the silicon nanoparticles may have an average particle size (D50) of 30 nm to 200 nm.

[0025] According to one embodiment of the present invention, the silicon nanoparticles may include needle-shaped silicon nanoparticles having an aspect ratio greater than 1.5 as 90% or more of the total weight of the silicon nanoparticles.

[0026] In the step of manufacturing the spherical assembled particles according to one embodiment of the present invention, a conductive material having an average particle size (D50) of 1 μm to 12 μm may be further included.

[0027] The step of manufacturing the silicon-carbon-based composite precursor according to one embodiment of the present invention may be performed by mixing and binding spherical assembled particles and a carbon-based matrix using a dry method or a wet method.

[0028] The carbon-based matrix according to one embodiment of the present invention may include one or more amorphous carbons selected from petroleum-based pitch, coal tar, PAA (poly(acrylic acid)), PVA (poly(vinyl alcohol)), soft carbon, hard carbon, and calcined coke, with a softening point of 300°C or lower.

[0029] In the step of manufacturing the silicon-carbon composite according to one embodiment of the present invention, the heat treatment may be performed for 0.5 to 2 hours in an inert atmosphere of 1000°C or lower.

[0030] After the step of manufacturing the silicon-carbon composite according to one embodiment of the present invention, the method may further include the step of forming an amorphous carbon coating layer on the surface of the silicon-carbon composite.

[0031] The amorphous carbon coating layer according to one embodiment of the present invention may have an average thickness of 10 nm or less.

[0032] A negative electrode according to one embodiment of the present invention may include the negative electrode active material for a lithium secondary battery.

[0033] A lithium secondary battery according to one embodiment of the present invention may include the negative electrode; the positive electrode; and the electrolyte.

[0034] According to the present invention, by adding a carbon-based compound as a conductive additive to silicon, electrochemical properties such as initial efficiency, charge / discharge capacity, and lifespan characteristics can be improved. Furthermore, a negative electrode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same can be provided.

[0035] The effects obtainable from the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0036] Figure 1 is a dQ / dV graph obtained by differentiating the charge / discharge curve of Example 1 for 50 cycles according to one embodiment of the present invention.

[0037] Figure 2 is a dQ / dV graph obtained by differentiating the charge / discharge curve of Comparative Example 1 after 50 cycles.

[0038] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0039] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0040] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense. For instance, singular expressions in this specification include plural expressions unless the context clearly indicates an exception.

[0041] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0042] In this invention, by combining silicon nanoparticles with a conductive material, a conductive additive, and a carbon-based matrix, the expansion of the silicon nanoparticles is absorbed by the other materials, thereby suppressing cracking of the cathode material or delamination from the electrode caused by volume expansion; additionally, the use of a conductive additive facilitates the bonding of conductive carbon particles, thereby providing higher conductivity.

[0043] A negative electrode active material for a lithium secondary battery according to one embodiment of the present invention comprises a silicon-carbon composite comprising silicon nanoparticles, a carbon-based matrix, and a conductive additive, wherein the conductive additive may be included in an amount of 0.2% to 10% by weight relative to the silicon nanoparticles in the composite.

[0044] The above silicon nanoparticles are not particularly limited as long as they are made of a silicon-containing material, and, for example, powders such as crystalline silicon or amorphous silicon may be used. In addition, the surface of the silicon nanoparticles may be oxidized.

[0045] It is desirable for the above silicon nanoparticles to have a small particle size to suppress pulverization (cracking or destruction of particles) during charging and discharging due to volume expansion.

[0046] The silicon nanoparticles may have an average particle size (D50) of 30 nm to 200 nm, preferably 50 nm to 150 nm or 60 nm to 140 nm. When the average particle size of the silicon nanoparticles falls within the above range, the volume expansion of the silicon nanoparticles is reduced, thereby suppressing pulverization during repeated charging and discharging, which can improve the initial efficiency and lifespan characteristics of the battery.

[0047] The silicon nanoparticles may have a full width at half maximum of 0.6° to 1.0° for the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane, and preferably 0.65° to 0.75° and 0.70° to 0.73°.

[0048] The full width at half maximum of the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane of the above silicon nanoparticles can be controlled by adjusting the size of the silicon particles or by changing the silicon nanoparticle manufacturing process. When the full width at half maximum of the X-ray diffraction angle (2theta) falls within the above range, reversible charging and discharging is possible without cracking of the silicon nanoparticles, thereby improving the lifespan characteristics of the battery.

[0049] In addition, the silicon nanoparticles may include needle-shaped silicon nanoparticles with an aspect ratio greater than 1.5 as more than 90% of the total weight of the silicon nanoparticles.

[0050] It is preferable that the silicon nanoparticles be included in an amount of 50% to 70% by weight relative to the total weight of the silicon-carbon composite, and more preferably in an amount of 50% to 65% by weight. When the content of silicon nanoparticles falls within the above range, the structural stability of the silicon-carbon composite and the battery capacity and lifespan characteristics can be further improved.

[0051] The aforementioned conductive additive can sufficiently accommodate volume changes during the expansion of silicon nanomaterials due to repeated charging and discharging, thereby suppressing phenomena such as cracking of the negative active material or delamination from the electrode caused by volume expansion. Furthermore, by complementing the conductive material and the conductive role within the composite, it enhances the reversibility of the composite, which can improve expansion suppression and lifespan characteristics.

[0052] The conductive additive may include one or more carbon-based compounds selected from carbon nanotubes, activated carbon, and graphene, and preferably may be one or more carbon nanotubes selected from SWCNT (Single-wall carbon nanotube), TWCNT (Thin-wall carbon nanotube), and MWCNT (Multi-wall carbon nanotube). In particular, TWCNT may be used considering cost aspects.

[0053] The average outer diameter of the conductive additive is preferably 1 nm to 500 nm to accommodate silicon nanoparticles of various particle diameters, and more preferably 5 nm to 50 nm. In addition, the average length of the conductive additive is preferably 0.1 μm to 100 μm to accommodate volume changes when silicon nanoparticles expand, and more preferably 1 μm to 10 μm.

[0054] The conductive additive is preferably included in an amount of 0.2% to 10% by weight relative to the silicon nanoparticles in the composite, more preferably in an amount of 0.2% to 0.8% by weight, and most preferably in an amount of 0.2% to 0.5% by weight. If the content of the conductive additive is less than 0.2% by weight, it may not be sufficient to accommodate the volume change when the silicon nanoparticles expand, and if it exceeds 10% by weight, the initial capacity may be lowered, and the cost may increase due to the excessive use of expensive ingredients.

[0055] When applied to a battery, the carbon-based matrix enables silicon nanoparticles to maintain electrical contact with other materials despite volume expansion and contraction due to cycling. In other words, the carbon-based matrix can serve as a matrix that enhances the structural stability of the silicon-carbon composite by controlling the expansion of silicon nanoparticles.

[0056] The above carbon-based matrix may include one or more selected from petroleum pitch, coal tar, PAA (poly(acrylic acid)), PVA (poly(vinyl alcohol)), soft carbon, hard carbon, and calcined coke, with a softening point of 300°C or lower.

[0057] The carbon-based matrix is ​​preferably included in an amount of 30% to 50% by weight relative to the total weight of the silicon-carbon-based composite to further improve charge / discharge capacity and cycle characteristics while maintaining Coulomb efficiency, and more preferably in an amount of 15% to 25% by weight.

[0058] As described above, the silicon-carbon composite comprising silicon nanoparticles, a carbon-based matrix, and a conductive additive may further include a conductive material.

[0059] The above conductive material can improve the electrochemical reversibility and conductivity within the cathode active material by providing a surface contact function that maintains the conductivity of the cluster of nanoparticles gathered within the composite.

[0060] The conductive material may include one or more selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene, and preferably is flake graphite.

[0061] The average particle size (D50) of the conductive material may be 1 μm to 12 μm, and preferably 3 μm to 11 μm. If the average particle size of the conductive material is less than 1 μm, the conductivity may be inferior, and if it exceeds 12 μm, it may be difficult to incorporate it into the composite.

[0062] The conductive material is preferably included in an amount of 15% to 30% by weight relative to the total weight of the silicon-carbon composite, and more preferably in an amount of 15% to 25% by weight. When the content of the conductive material falls within the above range, reversible volume expansion and contraction of silicon during charging and discharging can be secured, thereby improving the initial efficiency and lifespan characteristics of the battery, such as improving capacity performance.

[0063] An amorphous carbon layer can be formed on the surface of the silicon-carbon composite as described above.

[0064] The above amorphous carbon coating layer may include one or more amorphous carbons selected from petroleum pitch, coal tar, PAA (poly(acrylic acid)), PVA (poly(vinyl alcohol)), soft carbon, hard carbon, and calcined coke, with a softening point of 250°C or lower.

[0065] The above amorphous carbon coating layer may have an average thickness of 10 nm or less, preferably 1 nm to 10 nm. By forming an amorphous carbon coating layer with the above thickness on the surface of the silicon-carbon composite, the surface of the negative electrode active material can be uniformly coated to reduce the specific surface area, and by controlling the penetration of the electrolyte into the negative electrode active material, side reactions between the electrolyte and the negative electrode active material can be minimized, thereby improving the battery life characteristics and further enhancing electrochemical performance, such as improving capacity characteristics.

[0066] As described above, the negative electrode active material of the present invention may have an average particle size (D50) of 2 μm to 15 μm, preferably 3 μm to 13 μm or 5 μm to 10 μm. When the average particle size of the negative electrode active material falls within the above range, lithium ions can easily diffuse into the negative electrode active material, and electrical resistance and rate characteristics can be improved. In addition, by suppressing an excessive increase in the specific surface area of ​​the negative electrode active material, side reactions with the electrolyte can be reduced.

[0067] The above-described negative electrode active material may have a BET specific surface area of ​​5 m² / g or less. When the above-described BET specific surface area is within the above range, side reactions with the electrolyte can be suppressed, thereby improving the efficiency characteristics of the battery. In addition, the negative electrode active material of the present invention may have a maximum peak height width of 0.5 to 1.5 at a voltage of 0.2 to 0.3 V based on 50 cycles, and a maximum peak height width of 1.5 to 3 at a voltage of 0.45 to 0.55 V based on 50 cycles. That is, the negative electrode active material according to the present invention has excellent conductivity and excellent battery performance, as the peak is well preserved even after 50 cycles.

[0068] Hereinafter, a method for manufacturing a negative electrode active material according to another embodiment of the present invention will be described. The following manufacturing method is an example of a method for manufacturing a negative electrode active material for a lithium secondary battery comprising a silicon-carbon composite, and the present embodiment is not limited to the following method.

[0069] A method for manufacturing a negative electrode active material according to another embodiment of the present invention may include: a step of grinding a silicon raw material to produce silicon nanoparticles; a step of mixing and sphericalizing the silicon nanoparticles and a conductive additive to produce spherical assembled particles; a step of mixing the spherical assembled particles with a carbon-based matrix to produce a silicon-carbon composite precursor; and a step of heat-treating the silicon-carbon composite precursor to produce a silicon-carbon composite.

[0070] First, silicon nanoparticles are manufactured by grinding silicon raw materials.

[0071] The above silicon raw material may have an average particle size (D50) of 2.5㎛ to 4.5㎛. Specifically, the D1 particle size of the silicon raw material may be 0.1㎛ to 0.6㎛, the D10 particle size may be 0.7㎛ to 1.3㎛, the D50 particle size may be 2.5㎛ to 4.5㎛, the D90 particle size may be 5.8㎛ to 7㎛, and the D99 particle size may be 7.5㎛ to 8.5㎛.

[0072] In addition, the above silicon raw material may have a full width at half maximum of 0.2° or more of the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane, and preferably 0.2° to 0.4°.

[0073] The half-width range for the (111) plane of the above silicon nanoparticles can be controlled by increasing the time in the grinding process using the above silicon raw material, increasing the BPR (ball per ratio), or controlling the solid content to increase the probability of collision with zirconium balls.

[0074] Silicon nanoparticles can be manufactured by preparing the above silicon raw material and then grinding it using a top-down milling method.

[0075] Specifically, silicon raw materials can be ground by milling them together with zirconia balls using an organic solvent.

[0076] If the zirconia balls are too small, the nano-sizing efficiency of the silicon particles decreases excessively, which may lead to side effects such as increased processing time or oxidation of the silicon. Therefore, the zirconia balls can be used with a size less than twice the D99 particle size of the silicon raw material.

[0077] The above organic solvent is used to prevent oxidation of silicon nanoparticles during silicon grinding, and organic solvents such as ethanol or IPA (isopropyl alcohol) can be used, and specifically, ethanol with a purity of 99.9% can be used. For effective silicon nano-synthesis, grinding can be carried out in a solid content ratio range of 8% to 15%.

[0078] The BPR of the silicon raw material and the zirconia ball can be 5:1, and the rotational speed of the internal rotor of the grinder can be controlled to 2500 rpm.

[0079] Next, the silicon nanoparticles prepared above are mixed with a conductive additive and spheroidized to produce spherical assembled particles. When mixing the silicon nanoparticles and the conductive additive, a conductive material may be further included.

[0080] The above silicon nanoparticles, conductive material, and conductive additive are as described above, so they will be omitted here.

[0081] Spherical aggregated particles can be produced by adding a conductive material to the above silicon nanoparticles and conductive additive as needed, uniformly mixing and dispersing them, and then performing a wet spray drying process at 50°C to 150°C.

[0082] Subsequently, the above-mentioned spherical assembled particles are mixed with a carbon-based matrix to produce a silicon-carbon-based composite precursor.

[0083] The above spherical assembled particles and carbon-based mattress can be mixed and compounded using a planetary mixer, 3D-mixer, V-mixer, meccano-fusion, hybridizer, Nobilta, homo mixer, Henschel mixer, inline mixer, spray dryer, etc.

[0084] Specifically, when a negative electrode active material is obtained by attaching and binding spherical assembled particles containing silicon nanoparticles to a carbon-based matrix and then heat-treating it, the silicon nanoparticles can maintain electrical contact with other materials despite volume expansion and contraction phenomena caused by cycling when applied to a battery. In other words, the carbon-based matrix can control the expansion of the silicon nanoparticles.

[0085] The silicon-carbon composite precursor manufactured above may further include a step of compressing and molding prior to heat treatment.

[0086] The above compression can be performed at a pressure of 50 MPa to 150 MPa, preferably 75 MPa to 150 MPa, or 75 MPa to 125 MPa. When compressing within the above pressure range, the spacing between silicon nanoparticles is appropriately maintained and the pore volume formed inside the silicon-carbon composite is controlled, thereby suppressing side reactions between the electrolyte and silicon nanoparticles and improving initial efficiency and lifespan characteristics.

[0087] Next, the molded body manufactured above is heat-treated to produce a silicon-carbon composite.

[0088] The above heat treatment can be performed at a temperature of less than 1000°C, preferably between 700°C and 1000°C or between 800°C and 1000°C. When heat treatment is performed in the above temperature range, the strength of the silicon-carbon composite can be strengthened as the amorphous carbon is carbonized, the conductivity of the negative electrode active material can be improved, and the initial efficiency of the battery can be improved.

[0089] The above heat treatment can be performed in a furnace in an inert atmosphere, specifically a nitrogen (N2) atmosphere.

[0090] Subsequently, a silicon-carbon composite can be obtained by undergoing a dry grinding process such as a JET mill or a fin mill and then classifying it.

[0091] In addition, the silicon-carbon composite manufactured as described above can form an amorphous carbon coating layer of 10 nm or less by coating a carbon-based matrix on its surface.

[0092] The above carbon-based matrix may use one or more amorphous carbons selected from petroleum pitch, coal tar, PAA (poly(acrylic acid)), PVA (poly(vinyl alcohol)), soft carbon, hard carbon, and calcined coke with a softening point of 300°C or lower.

[0093] The above coating can be performed using a twist blade mixer, and process variables include time and rotational speed, but these are not described as they are not critical components that need to be controlled in the present invention.

[0094] After forming the above amorphous carbon coating layer, heat treatment is performed at a temperature of less than 1000°C in an inert atmosphere, and the final cathode active material can be obtained through sieving.

[0095] The negative electrode active material of the present invention manufactured as described above may have an average particle size (D50) of 2 μm to 15 μm and a BET specific surface area of ​​5 m² / g or less. In addition, the negative electrode active material of the present invention may have a maximum peak height width of 0.5 to 1.5 at a voltage of 0.2 to 0.3 V based on 50 cycles, and a maximum peak height width of 1.5 to 3 at a voltage of 0.45 to 0.55 V.

[0096] A cathode according to another embodiment of the present invention may include the aforementioned cathode active material.

[0097] Specifically, the above cathode can be manufactured by mixing a cathode active material, a binder, and optionally a conductive material to prepare a composition for forming a cathode active material layer, and then applying the composition to a cathode current collector.

[0098] The above-mentioned negative current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and may be, for example, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

[0099] The above-mentioned negative electrode active material is the same as that of the aforementioned embodiment of the present invention, so it is omitted.

[0100] The above-mentioned negative electrode active material may be included in an amount of 1% to 90% by weight, for example, 1% to 80% by weight, 1% to 70% by weight, or 1% to 60% by weight, based on the total weight of the composition for forming the negative electrode active material layer.

[0101] The above binder serves to adhere the negative electrode active material particles well to each other and also to adhere the negative electrode active material well to the negative electrode current collector. For example, the above binder may include polyvinyl alcohol, carboxymethylcellulose / styrene-butadiene rubber, hydroxypropylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene, but is not limited thereto. The above binder may be mixed in an amount of 1% to 30% by weight relative to the total weight of the composition for forming the negative electrode active material layer.

[0102] The above conductive material is used to impart conductivity to the electrode and is not particularly limited as long as it is a material that is conductive without causing chemical changes in the battery being formed. Specifically, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fiber or metal fiber; metal powders such as carbon fluoride, aluminum, or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives may be used. The above conductive material may be mixed in an amount of 0.1% to 30% by weight relative to the total weight of the composition for forming the negative electrode active material layer.

[0103] A lithium secondary battery according to another embodiment of the present invention may include the aforementioned negative electrode, positive electrode, and electrolyte.

[0104] The above-mentioned anode can be manufactured by mixing an anode active material, a binder, and optionally a conductive material to prepare a composition for forming an anode active material layer, and then applying this composition to an anode current collector. In this case, the binder and the conductive material can be used in the same manner as in the case of the aforementioned cathode.

[0105] The above-mentioned positive current collector may be, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

[0106] The above-mentioned positive electrode active material may use a compound capable of reversible intercalation and deintercalation of lithium (a lithated intercalation compound). Specifically, one or more of complex oxides of lithium and a metal of cobalt, manganese, nickel, or a combination thereof may be used, and specific examples thereof may include compounds represented by any one of the following chemical formulas.

[0107] Lia AM 1-b R b D2(diameter, 0.90≤a≤1.8 µ 0≤b≤0.5 µ); Li a E 1-b R b YOU ARE 2-c D c (.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a E 2-b R b YOU ARE 4-c D c (0≤b≤0.5, 0≤c≤0.05); Li a Ni 1-b-c Co. Co b R c D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α≤2); Li a Ni 1-b-c Co. Co b R c YOU ARE 2-α Z α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-b-c Co. Co b R c YOU ARE 2-α Z2(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<); Li a Ni 1-b-c Mn b R c D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α≤2); Li a Ni 1-b-c Mn b R c YOU ARE 2-α Z α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-bc Mn b R c YOU ARE 2-α Z α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Lia Ni b E c G d O2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1); Li a Ni b Co c Mn d G e O2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li a NiG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a CoG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a MnG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn2G b O4(0.90≤a≤1.8, 0.001≤b≤0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiTO2; LiNiVO4; Li (3-f) J2(PO4)3(0≤f≤2); Li (3-f) Fe2(PO4)3(0≤f≤2); and LiFePO4.

[0108] In the above chemical formula, A is Ni, Co, Mn or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; Z is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; T is Cr, V, Fe, Sc, Y or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu or a combination thereof.

[0109] The above electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0110] The above-mentioned non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0111] The above lithium salt is a material that is dissolved in an organic solvent and acts as a source of lithium ions within the battery, enabling the basic operation of a lithium secondary battery and facilitating the movement of lithium ions between the positive and negative electrodes.

[0112] Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. As such a separator, polyethylene, polypropylene, polyvinylidene fluoride, or multilayer films of two or more layers thereof may be used, and of course, mixed multilayer films such as polyethylene / polypropylene two-layer separators, polyethylene / polypropylene / polyethylene three-layer separators, and polypropylene / polyethylene / polypropylene three-layer separators may be used.

[0113] Lithium-ion batteries can be classified into lithium-ion batteries, lithium-ion polymer batteries, and lithium-polymer batteries depending on the type of separator and electrolyte used; they can be classified by shape into cylindrical, prismatic, coin, and pouch types; and they can be divided into bulk and thin-film types depending on size. As the structures and manufacturing methods of these batteries are widely known in this field, a detailed description is omitted.

[0114] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

[0115] Example 1

[0116] (1) Manufacturing of silicon nanoparticles

[0117] After preparing the polysilicon raw material, a slurry containing silicon nanoparticles with a viscosity of 5530 cps was prepared by milling with zirconia balls in an ethanol atmosphere.

[0118] (2) Preparation of cathode active material

[0119] Based on 100 wt% of a silicon-carbon composite, 65 wt% of a slurry containing silicon nanoparticles prepared in (1) and 0.5 wt% of thin-wall carbon nanotubes (TWCNT) were mixed and dispersed, and then wet spray-dried at 50°C to 150°C to produce spherical aggregate particles. 35 wt% of pitch powder was mixed and compounded with the spherical aggregate particles using a planetary mixer to produce a silicon-carbon composite precursor. Subsequently, the silicon-carbon composite precursor was heat-treated at less than 1000°C in an inert atmosphere and then ground with a jet mill to produce a silicon-carbon composite.

[0120] Example 2

[0121] The above Example 1 was carried out in the same manner as Example 1, except that spherical assembled particles were prepared by adding 5.0 wt% of carbon nanotubes (TWCNT) in the above Example 1.

[0122] Example 3

[0123] In the above Example 1, 60 wt% of a slurry containing silicon nanoparticles, 10 wt% of flake graphite having an average particle size (D50) of 7 to 9 μm, and 5.0 wt% of carbon nanotubes (TWCNT) were mixed and dispersed, and then spherical aggregate particles were prepared by wet spray drying at 50°C to 150°C, and then 30 wt% of pitch powder was mixed with the spherical aggregate particles; except for this, the procedure was carried out in the same manner as in Example 1.

[0124] Comparative Example 1

[0125] The above Example 1 was carried out in the same manner as Example 1, except that spherical assembled particles were prepared by adding 0.1 wt% of carbon nanotubes (TWCNT) in the above Example 1.

[0126] Comparative Example 2

[0127] The above Example 1 was carried out in the same manner as Example 1, except that spherical assembled particles were prepared by adding 13.0 wt% of carbon nanotubes (TWCNT) in the above Example 1.

[0128] Comparative Example 3

[0129] The above Example 1 was carried out in the same manner as Example 1, except that carbon nanotubes were not used.

[0130] Experimental Example 1. Electrochemical Evaluation

[0131] (1) Coin-type half-battery manufacturing

[0132] After manufacturing a CR2032 coin cell using the cathode active material prepared as described above, an electrochemical evaluation was conducted.

[0133] Specifically, a mixture was prepared by mixing 96.1 wt% of a negative electrode active material, 1 wt% of a conductive material (super C65), 1.7 wt% of CMC (carboxymethyl cellulose), and 1.2 wt% of styrene-butadiene rubber (SBR). 8 wt% of the above mixture was mixed with commercial natural graphite having a capacity of 360 mAh / g to prepare a negative electrode active material slurry having a capacity of approximately 440 mAh / g.

[0134] Next, the above slurry was coated onto a Cu current collector, dried, and then rolled to produce a cathode. The loading amount of the cathode was ~8.6 mg / ㎠, and the electrode density was ~1.55 g / cc.

[0135] A 2032 coin-type half-cell was manufactured by a conventional method using the above-mentioned cathode, a lithium metal cathode (thickness 300 μm, MTI), an electrolyte, and a polypropylene separator. The electrolyte was prepared by dissolving 1M LiPF6 in a mixed solvent of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (mixing ratio EC:EMC = 3:7 volume%) to prepare a mixed solution, and then adding 1.5 wt% vinylene carbonate (VC) and 10 wt% fluoroethylene carbonate (FEC) to it.

[0136] (2) Evaluation of charge / discharge characteristics

[0137] After aging the coin-type half battery manufactured in (1) above at room temperature (25℃) for 24 hours, a charge / discharge test was performed.

[0138] The capacity evaluation was based on a reference capacity of 440 mAh / g. Charge-discharge tests were conducted in the operating voltage range of 0.005 V to 1.0 V, and the current during charge-discharge was measured at 0.1 C during the initial cycle. Additionally, based on the first 1 C capacity, a current of 0.5 C was applied during charge-discharge to measure the lifespan over 50 cycles. At this time, the charge cut-off current was set to 0.005 C. The results are shown in Table 1 below.

[0139] In Table 1 below, the converted discharge capacity and converted initial efficiency are values ​​calculated by converting the capacity of pure Si-C excluding the capacity of graphite added during electrode fabrication, and were calculated as shown in the following equations (1) and (2).

[0140] Equation (1): Theoretical graphite capacity × Graphite content = Graphite capacity

[0141] Equation (2): Pure Si-C capacity = {(Finished product capacity)-(Graphite capacity)} / Si-C content

[0142] Discharge Capacity (mAh / g) Initial Efficiency (%) Converted Discharge Capacity (mAh / g) Converted Initial Efficiency (%) Lifespan (%, 50 cycles) Example 1: 455.39 0.2 1,563 85.59 7.9 Example 2: 459.99 0.0 1,615 85.09 5.2 Example 3: 454.88 9.6 1,550 83.48 7.6 Comparative Example 1: 456.68 9.8 1,573 84.38 5.6 Comparative Example 2: 450.69 1.1 1,523 88.28 2.6 Comparative Example 3: 455.39 1.3 1,557 89.38 3.5

[0143] As shown in Table 1 above, in the case of Examples 1 to 3 containing carbon nanotubes in an amount of 0.2 wt% to 10 wt% relative to silicon nanoparticles according to the present invention, it was found that the discharge capacity, initial efficiency characteristics, and long-term life characteristics were all superior compared to Comparative Example 1, in which the carbon nanotube content was less than 0.2 wt%, Comparative Example 2, in which the carbon nanotube content exceeded 10 wt%, and Comparative Example 3, in which carbon nanotubes were not added. (3) Conductivity evaluation through dQ / dV graph

[0144] The coin-type half-cell manufactured in (1) above was aged at room temperature (25℃) for 24 hours, and then a charge-discharge test was performed. The charge-discharge test was performed in the operating voltage range of 0.005V to 1.0V, and the current during charge-discharge was measured as 0.1C in the initial cycle. In addition, based on the first 1C capacity, a current of 0.5C was applied during charge-discharge to measure the lifespan of 50 cycles.

[0145] Charge and discharge tests were conducted in the operating voltage range of 0.005V to 1.0V, and the current during charge and discharge was measured at 0.1C in the initial cycle. In addition, based on the first 1C capacity, a current of 0.5C was applied during charge and discharge, and 50 cycles were performed.

[0146] Figures 1 and 2 show dQ / dV graphs obtained by differentiating the charge / discharge curve for 50 cycles to obtain a first derivative curve (dQ / dV) by differentiating the initial charge / discharge curve obtained by performing the first charge and first discharge.

[0147] Figure 1 is a dQ / dV graph obtained by differentiating the charge / discharge curve of Example 1 after 50 cycles, and Figure 2 is a dQ / dV graph obtained by differentiating the charge / discharge curve of Comparative Example 1 after 50 cycles.

[0148] Silicon basically undergoes a delithiation process in which it stores 3.5 Li atoms and gradually releases 2.0 Li atoms. As shown in Fig. 1, in the case of Example 1 containing 0.5 wt% of carbon nanotubes according to the present invention, it was confirmed that peaks at 0.25 V and 0.5 V were well preserved up to 50 cycles on the dQ / dV graph. This indicates that the conductivity within the material is excellent, and it was found that the polarization effect is reduced and battery performance is improved with the addition of carbon nanotubes.

[0149] On the other hand, in the case of Comparative Example 1, which did not have carbon nanotubes added, the peak on the dQ / dV graph did not appear clearly from 30 cycles onwards, and from these results, it was found that the conductivity of the material was inferior and the battery performance was inferior.

[0150] Although embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be made by those skilled in the art to which the disclosed invention belongs without departing from the essence claimed in the claims.

Claims

1. A silicon-carbon composite comprising silicon nanoparticles, a carbon-based matrix, and a conductive additive, and The above conductive additive is included in an amount of 0.2% to 10% by weight relative to silicon nanoparticles in the composite, for a negative electrode active material for a lithium secondary battery.

2. In Paragraph 1, The above conductive additive is a negative electrode active material for a lithium secondary battery, which is one or more carbon-based compounds selected from carbon nanotubes, activated carbon, and graphene.

3. In Paragraph 2, The above conductive additive is a negative electrode active material for a lithium secondary battery comprising carbon nanotubes having an average outer diameter of 1 nm to 500 nm.

4. In Paragraph 1, The above silicon nanoparticles are a negative electrode active material for a lithium secondary battery having an average particle size (D50) of 30 nm to 200 nm.

5. In Paragraph 1, The above silicon nanoparticles are negative electrode active materials for a lithium secondary battery, wherein the half-width of the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane is 0.6° to 1.0°.

6. In Paragraph 1, The above silicon nanoparticles are a negative electrode active material for a lithium secondary battery comprising needle-shaped silicon nanoparticles with an aspect ratio greater than 1.5 as 90% or more of the total weight of the silicon nanoparticles.

7. In Paragraph 1, The above carbon-based matrix is ​​a negative electrode active material for a lithium secondary battery comprising one or more amorphous carbons selected from petroleum pitch, coal tar, PAA (poly(acrylic acid)), PVA (poly(vinyl alcohol)), soft carbon, hard carbon, and calcined coke, with a softening point of 300°C or lower.

8. In Paragraph 1, The above carbon-based matrix is ​​a negative electrode active material for a lithium secondary battery, comprising 30% to 50% by weight of the total weight of the silicon-carbon-based composite.

9. In Paragraph 1, The above negative electrode active material is a negative electrode active material for a lithium secondary battery that further comprises one or more conductive materials selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

10. In Paragraph 9, A negative electrode active material for a lithium secondary battery, wherein the average particle size (D50) of the conductive material is 1 μm to 12 μm.

11. In Paragraph 1, The above silicon-carbon-based composite is a negative electrode active material for a lithium secondary battery that further includes an amorphous carbon coating layer on its surface.

12. In Paragraph 11, The above amorphous carbon coating layer is a negative electrode active material for a lithium secondary battery having an average thickness of 10 nm or less.

13. In Paragraph 1, The above negative electrode active material is a negative electrode active material for a lithium secondary battery having an average particle size (D50) of 2㎛ to 15㎛.

14. A step of manufacturing silicon nanoparticles by grinding silicon raw materials; A step of preparing spherical assembled particles by mixing and spheroidizing the above silicon nanoparticles and conductive additives; A step of preparing a silicon-carbon composite precursor by mixing the above-mentioned spherical assembled particles with a carbon-based matrix; and A step of manufacturing a silicon-carbon composite by heat-treating the above silicon-carbon composite precursor; A method for manufacturing a negative electrode active material for a lithium secondary battery comprising 15. In Paragraph 14, A method for manufacturing a negative electrode active material for a lithium secondary battery, further comprising the step of forming an amorphous carbon coating layer on the surface of the silicon-carbon composite after the step of manufacturing the silicon-carbon composite.

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