Anode active material for lithium secondary battery, manufacturing method therefor, and lithium secondary battery comprising same

Abstract

An anode active material for a lithium secondary battery, according to the present invention, comprises a silicon-carbon composite including silicon nanoparticles and a carbon matrix, wherein, in an XPS survey analysis, the composition of a surface layer portion, which is a region within a depth of 2-3 nm from the surface thereof, includes Si, O, and C, and the value of equation (1) measured in the surface layer portion can satisfy 38-50. Equation (1): ([Si]+[O]) / [C] (In equation (1), [Si], [O], and [C] represent concentrations (atm %) of respective elements.)

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, general silicon-based cathode materials have the disadvantage of being difficult to apply to actual batteries because they involve a volume change of up to 300% during the cycle, and due to particle cracking and loss of electrical contact caused by continuous charging and discharging, they exhibit low discharge capacity ratio characteristics.

[0005] Therefore, there is a need to develop a negative electrode active material that possesses characteristics capable of improving the electrochemical performance of the battery while using silicon-based negative electrode materials.

[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 manufacturing a structurally stable negative electrode active material by controlling the oxygen content and crystal size of a silicon-carbon composite, a method for manufacturing the same, and a lithium secondary battery including the same.

[0007] The technical problems intended 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 and an amorphous carbon coating layer formed on the surface of the silicon-carbon composite, wherein the silicon-carbon composite comprises silicon nanoparticles; and a carbon matrix comprising crystalline carbon and amorphous carbon, wherein the composition of the surface layer, which is a region from the surface to 2 to 3 nm during XPS Survey analysis, comprises Si, O and C, and the following formula (1) measured in the surface layer may satisfy 38 or more and 50 or less.

[0009] Equation (1): ([Si]+[O]) / [C]

[0010] (In the above equation (1), [Si], [O] and [C] represent the concentration (atm%) of each element.)

[0011] The negative electrode active material according to one embodiment of the present invention may satisfy a value of Equation (1) of 39.0 or more and 49.0 or less.

[0012] The negative electrode active material according to one embodiment of the present invention may have an average crystallite size of 10 nm to 14 nm.

[0013] 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.

[0014] 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°.

[0015] According to one embodiment of the present invention, the silicon nanoparticles may be included in an amount of 50 to 70 weight percent based on the total weight of the silicon-carbon composite.

[0016] The crystalline carbon according to one embodiment of the present invention may include one or more selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

[0017] The amorphous carbon according to one embodiment of the present invention 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 250°C or lower.

[0018] According to one embodiment of the present invention, the carbon matrix may be included in an amount of 30% to 50% by weight with respect to the total weight of the silicon-carbon composite.

[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] A method for manufacturing a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention comprises: a step of manufacturing silicon nanoparticles by milling a silicon raw material; a step of manufacturing a mixture by mixing the silicon nanoparticles, crystalline carbon, and amorphous carbon; a step of manufacturing a molded body by spray-drying and compressing the mixture; a step of manufacturing a silicon-carbon composite by heat-treating the molded body; and a step of forming an amorphous carbon coating layer on the surface of the silicon-carbon composite; wherein the milling can be performed for 10 to 30 hours with 30 kWh to 60 kWh of energy.

[0021] According to one embodiment of the present invention, the cathode active material has a surface layer composition of 2 to 3 nm from the surface during XPS Survey analysis, which includes Si, O, and C, and the formula (1) measured in the surface layer may satisfy 38 or more and 50 or less.

[0022] The negative electrode active material according to one embodiment of the present invention may satisfy a value of Equation (1) of 39.0 or more and 49.0 or less.

[0023] The negative electrode active material according to one embodiment of the present invention may have an average crystallite size of 10 nm to 14 nm.

[0024] According to one embodiment of the present invention, the silicon raw material may have an average particle size (D50) of 2.5㎛ to 4.5㎛.

[0025] The heat treatment according to one embodiment of the present invention can be performed in an inert atmosphere of 1000°C or lower.

[0026] According to one embodiment of the present invention, the amorphous carbon coating layer may be formed with an average thickness of 10 nm or less.

[0027] A negative electrode according to one embodiment of the present invention may include the negative electrode active material for a lithium secondary battery.

[0028] A lithium secondary battery according to one embodiment of the present invention may include the negative electrode; the positive electrode; and the electrolyte.

[0029] According to the present invention, by controlling the oxygen content and crystal size of a silicon-carbon composite to produce a structurally stable negative electrode active material, it is possible 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, a method for manufacturing the same, and a lithium secondary battery including the same.

[0030] Furthermore, the silicon-carbon composite according to the present invention has excellent structural stability, allowing for reversible charging and discharging without cracking, and thus possesses long lifespan and low expansion characteristics. Additionally, since carbon can be completely captured within the silicon-carbon composite, it can provide a very high-capacity negative electrode active material.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Silicon-based anode materials have the disadvantage of being difficult to apply to actual batteries due to their low discharge capacity ratio characteristics. Accordingly, there have been attempts to improve the electrochemical properties of lithium-ion batteries by utilizing silicon-carbon composites formed by controlling silicon particle size to the nanoscale and combining them with a carbon matrix.

[0037] In silicon-carbon composites, silicon nanoparticles must be perfectly captured by the carbon matrix. However, to enable reversible volume expansion and contraction of silicon during charging and discharging, it is necessary to reduce the crystal grain size of the silicon nanoparticles; consequently, this leads to an increased number of silicon nanoparticles, making capture by the carbon matrix difficult. This results in an increase in the specific surface area of ​​the negative electrode active material, which in turn causes a significant deterioration in lifespan characteristics.

[0038] Furthermore, while it is desirable to reduce the grain size of silicon nanoparticles for reversible volume expansion and contraction, reducing the grain size requires a longer grinding time for the silicon raw material during manufacturing. During this process, silicon oxidizes easily, increasing the oxygen content within the silicon nanoparticles and consequently leading to an increased degree of oxidation in the cathode active material.

[0039] As the oxygen content in these silicon nanoparticles increases, the weight of the negative electrode active material increases, and the weight of the battery may also increase; consequently, this can lead to irreversible lithium consumption due to oxidized silicon, resulting in a high initial irreversible capacity.

[0040] Accordingly, the present invention aims to provide a silicon-carbon composite that facilitates reversible volume expansion and contraction of silicon nanoparticles by appropriately controlling the grain size and oxygen content during the manufacture of silicon nanoparticles, while simultaneously securing excellent electrochemical characteristics when applied to lithium secondary batteries.

[0041] In the present invention, "carbon matrix" means "any one selected from crystalline carbon, amorphous carbon, and mixtures thereof," and even if not otherwise specified, "carbon matrix" means any one selected from crystalline carbon, amorphous carbon, and mixtures thereof.

[0042] Hereinafter, a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention will be described.

[0043] A negative electrode active material for a lithium secondary battery according to one embodiment of the present invention comprises a silicon-carbon composite and an amorphous carbon coating layer formed on the surface of the silicon-carbon composite, wherein the silicon-carbon composite comprises silicon nanoparticles; and a carbon matrix comprising crystalline carbon and amorphous carbon, wherein the composition of the surface layer, which is a region from the surface to 2 to 3 nm during XPS Survey analysis, comprises Si, O and C, and the following formula (1) measured in the surface layer may satisfy 38 or more and 50 or less.

[0044] Equation (1): ([Si]+[O]) / [C]

[0045] (In the above equation (1), [Si], [O] and [C] represent the concentration (atm%) of each element.)

[0046] The value of the following equation (1) is a numerical value obtained by analyzing the elements contained in the surface layer, which is a region from the surface of the cathode active material to 2 to 3 nm, by the XPS Survey method to confirm that they are Si, O, and C, and by calculating the Si concentration (atm%), O concentration (atm%), and C concentration (atm%) contained in the surface layer. The following equation (1) measured in the surface layer can satisfy 38 or more and 50 or less.

[0047] If the value of the above equation (1) is 38 or less, the reversible volume expansion and contraction of silicon nanoparticles during the charging and discharging process may not be smooth, and if it exceeds 50, the weight of the silicon-carbon composite may increase significantly, and the initial irreversible capacity may increase.

[0048] Therefore, the value of the above formula (1) is preferably 38 or more and 50 or less, and more preferably 39.0 or more and 49.0 or less.

[0049] In addition, in the present invention, by controlling the average crystal size along with the value of Equation (1) of the negative electrode active material, it is possible to improve the initial efficiency and lifespan characteristics of the battery, such as preventing volume expansion during the charge-discharge process while using a high-content silicon-carbon composite, thereby improving capacity performance.

[0050] The average crystal size of the above-mentioned negative electrode active material is preferably 10 nm to 14 nm, and more preferably 10.5 nm to 13.5 nm.

[0051] The average crystal size of the above-mentioned cathode active material can be quantitatively analyzed using X-ray diffraction analysis (XRD) by CuKα X-rays (Xrα). Specifically, the average crystal size of the cathode active material can be quantitatively analyzed by placing cathode active material particles in a holder, irradiating the particles with X-rays, and analyzing the resulting diffraction grating.

[0052] The above silicon-carbon composite may include silicon nanoparticles and a carbon matrix.

[0053] In the above silicon-carbon composite, the silicon nanoparticles must have a sufficiently low crystal size so that the silicon-carbon composite has long lifespan and low expansion characteristics, and the silicon nanoparticles must be captured in a carbon matrix containing crystalline carbon and amorphous carbon.

[0054] The silicon nanoparticles may have an average particle size (D50) of 50 nm to 150 nm, preferably 60 nm to 140 nm or 70 nm to 130 nm. Additionally, the average particle size (D90) of the silicon nanoparticles may be 120 nm to 200 nm, preferably 130 nm to 190 nm or 135 nm to 180 nm. When the average particle size of the silicon nanoparticles falls within the above range, the silicon nanoparticles can be effectively captured in the carbon matrix, and the expansion of the silicon nanoparticles is reduced, thereby improving the initial efficiency and lifespan characteristics of the battery.

[0055] 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 0.6° to 0.73° and 0.62° to 0.70°.

[0056] 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.

[0057] When the full width at half maximum of the above 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.

[0058] As the silicon nanoparticle content increases, the silicon nanoparticles within the carbon matrix can be completely captured. However, if the silicon nanoparticle content becomes too high, the structure of the silicon-carbon composite may collapse. Accordingly, in the present invention, by reducing the oxygen content and average crystal size of the silicon-carbon composite, a silicon-carbon composite with improved structural stability can be synthesized even with an increase in silicon content.

[0059] Accordingly, in order to secure the structural stability of the silicon-carbon composite and improve battery capacity, 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.

[0060] In the above silicon-carbon composite, the carbon matrix may include crystalline carbon and amorphous carbon.

[0061] The above crystalline carbon may include one or more selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

[0062] The crystalline carbon may have an average particle size (D50) of 8 µm to 10 µm, preferably 4 µm to 8 µm. When the average particle size of the crystalline carbon falls within the above range, it can compensate for the insufficient conductivity of silicon nanoparticles and further improve the charge-discharge reversibility of the lithium secondary battery.

[0063] The above amorphous carbon 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 250°C or lower.

[0064] The amorphous carbon can suppress side reactions with the electrolyte by reducing the pore volume of the silicon-carbon composite. In addition, the amorphous carbon can suppress battery expansion by buffering the expansion of silicon nanoparticles within the silicon-carbon composite, and by performing a beandering role, it can improve the binding strength between the raw materials contained within the silicon-carbon composite, thereby preventing the breakage of the composite particles and maintaining their shape well.

[0065] The carbon matrix containing the crystalline carbon and amorphous carbon is preferably included in an amount of 30% to 50% by weight relative to the total weight of the silicon-carbon composite, and more preferably in an amount of 35% to 50% by weight. When the carbon matrix content falls within the above range, a negative electrode active material with excellent conductivity and flammability and excellent structural stability can be manufactured.

[0066] An amorphous carbon coating layer can be formed on the surface of the silicon-carbon composite as described above.

[0067] 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.

[0068] The 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.

[0069] As described above, the negative active material of the present invention, comprising a silicon-carbon composite and an amorphous carbon coating layer formed on the surface of the silicon-carbon composite, may have an average crystal size of 10 nm to 14 nm and may be 10.5 nm to 13.5 nm.

[0070] In addition, the above-mentioned negative electrode active material may have an average particle size (D50) of 2㎛ to 15㎛, preferably 3㎛ to 13㎛ or 5㎛ to 10㎛. When the average particle size of the above-mentioned 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.

[0071] The above-mentioned negative electrode active material may have a BET specific surface area of ​​5 m² / g or less. When the above-mentioned 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.

[0072] 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.

[0073] A method for manufacturing a negative electrode active material according to another embodiment of the present invention may include: a step of milling and grinding a silicon raw material to produce silicon nanoparticles; a step of mixing the silicon nanoparticles, crystalline carbon, and amorphous carbon to produce a mixture; a step of spray-drying and compressing the mixture to produce a molded body; a step of heat-treating the molded body to produce a silicon-carbon composite; and a step of forming an amorphous carbon coating layer on the surface of the silicon-carbon composite.

[0074] First, silicon nanoparticles are manufactured by milling silicon raw materials.

[0075] 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㎛.

[0076] 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°.

[0077] The half-width range for the (111) plane of the above silicon nanoparticles can be controlled by increasing the milling time in the milling 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.

[0078] After preparing the above silicon raw material, it is milled to a nanoscale.

[0079] Specifically, silicon raw materials can be milled together with zirconia balls using an organic solvent.

[0080] 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.

[0081] The above organic solvent is used to prevent oxidation of silicon nanoparticles during silicon milling, 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, milling can be performed within a solid content ratio range of 8% to 15%.

[0082] 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.

[0083] In addition, the above milling can be performed for 10 to 30 hours with milling energy of 50 kWh / kg to 100 kWh / kg.

[0084] If the above milling energy is too low, the time required for grinding becomes too long, resulting in a high silicon oxidation rate, and thus the value of Equation (1) intended in the present invention cannot be achieved. If it is too high, excessive impact is applied to the silicon particles, which may cause the shape of the ground particles to become irregular or break, thereby reducing the uniformity of the particles.

[0085] Therefore, it is desirable to control the milling energy during grinding to 50 kWh / kg to 100 kWh / kg, and more preferably to 30 kWh / kg to 60 kWh / kg, or 35 kWh / kg to 50 kWh / kg.

[0086] In addition, if the milling time is too short, the silicon raw material is not sufficiently ground, making it impossible to obtain silicon nanoparticles of the desired size; conversely, if the milling time is too long, the grinding efficiency may be reduced due to excessive particle size reduction or mechanical damage and wear.

[0087] Therefore, it is desirable to control the milling time during grinding to 10 to 30 hours, and more preferably to 12 to 28 hours, or 15 to 25 hours.

[0088] When milling with the milling energy and milling time as described above, the average particle size (D50) of the silicon nanoparticles can be controlled to 50 nm to 150 nm and D90 to 120 nm to 200 nm as intended in the present invention, and the oxidation degree of the silicon-carbon composite can be controlled to 10% to 13%.

[0089] Next, a mixture is prepared by mixing the silicon nanoparticles prepared above with a carbon matrix containing crystalline carbon and amorphous carbon.

[0090] The above silicon nanoparticles, crystalline carbon, and amorphous carbon are as described above, so they will be omitted here.

[0091] According to one embodiment of the present invention, the step of mixing the silicon nanoparticles and the carbon matrix may be performed by: a step of preparing a silicon-crystalline carbon precursor by mixing the silicon nanoparticles and crystalline carbon; and a step of mixing the silicon-crystalline carbon precursor with amorphous carbon.

[0092] Specifically, to impart conductivity and reversibility, crystalline carbon is mixed with silicon nanoparticles using a high-speed disperser and spray-dried to produce a silicon-crystalline carbon precursor powder. At this time, one or more selected from natural graphite, artificial graphite, flake graphite, earthy graphite, expanded graphite, and graphene may be used as the crystalline carbon, and preferably, graphite may be used. The graphite may be smaller than the average particle size (D50) of the precursor based on the average particle size (D50), and specifically, the average particle size (D50) of the graphite may be 5㎛ to 10㎛.

[0093] The silicon-crystalline carbon precursor prepared in this way can be mixed with amorphous carbon to form a carbon support layer in the final product and bonded. The bonding process is not limited to any specific method that minimizes the independent flow of the silicon-graphite precursor, but processes involving contact with powder, such as mechano fusion or ball milling, may be applied.

[0094] Next, the above-mentioned mixture is spray-dried and compressed to produce a molded body.

[0095] The above drying can be performed at 50°C to 150°C using a spray dryer.

[0096] 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.

[0097] Next, the molded body manufactured above is heat-treated to produce a silicon-carbon composite.

[0098] 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.

[0099] The above heat treatment can be performed in a furnace in an inert atmosphere, specifically a nitrogen (N2) atmosphere.

[0100] Subsequently, a silicon-carbon composite can be obtained by classifying the material after undergoing dry grinding processes such as a JET mill or a fin mill.

[0101] Next, amorphous carbon is coated on the surface of the silicon-carbon composite prepared above to form an amorphous carbon coating layer of 10 nm or less.

[0102] 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.

[0103] 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.

[0104] The cathode active material of the present invention manufactured as described above may have a surface layer composition of 2 to 3 nm from the surface during XPS Survey analysis, which includes Si, O and C, and the following formula (1) measured in the surface layer satisfies 38 or more and 50 or less, an average crystal size of 10 nm to 14 nm, and a BET specific surface area of ​​5 m² / g or less.

[0105] Equation (1): ([Si]+[O]) / [C]

[0106] (In the above equation (1), [Si], [O] and [C] represent the concentration (atm%) of each element.)

[0107] A cathode according to another embodiment of the present invention may include the aforementioned cathode active material.

[0108] 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.

[0109] 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.

[0110] The above-mentioned negative electrode active material is the same as that of the aforementioned embodiment of the present invention, so it is omitted.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] A lithium secondary battery according to another embodiment of the present invention may include the aforementioned negative electrode, positive electrode, and electrolyte.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] The above electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0121] The above-mentioned non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] Examples 1–4 and Comparative Examples 1–3

[0127] (1) Manufacturing of silicon nanoparticles

[0128] After preparing the polysilicon raw material, milling was performed with zirconia balls in an ethanol atmosphere as shown in Table 1 below to prepare a slurry containing silicon nanoparticles with a viscosity of 5530 cps.

[0129] (2) Preparation of cathode active material

[0130] Based on 100% by weight of the silicon-carbon composite, 65% by weight of a slurry containing silicon nanoparticles prepared in (1) and 15% by weight of graphite particles having an average particle size (D50) of 5 to 10 μm were fed into a high-speed mixer and dispersed to produce a silicon-graphite precursor having an average particle size (D50) of 15 to 20 μm. 20% by weight of pitch powder was mixed with the silicon-graphite precursor to produce a mixed powder. To form a carbon support, the mixed powder was loaded into a mold of a certain size and uniaxial pressure molding was performed at a pressure of about 50 tons. Subsequently, to prevent oxidation of the silicon nanoparticles, heat treatment was performed at less than 1000°C in an inert atmosphere, and then the silicon-carbon composite was produced by grinding with a jet mill.

[0131] Next, the silicon-carbon composite prepared above and coal tar were fed into a twisted blade mixer and stirred for about 30 minutes, then heat-treated in an inert atmosphere below 1000°C and sieved to prepare a negative electrode active material with an amorphous carbon coating layer 4 to 5 nm thick. At this time, coal tar was added in an amount of 3 to 10 wt% based on the total weight of the silicon-carbon composite.

[0132] The average particle size of silicon nanoparticles according to milling energy during silicon nanoparticle manufacturing is shown in Table 1 below.

[0133] The average particle size of silicon nanoparticles was measured using X-ray diffraction analysis (XRD) by CuKα X-rays (Xrα) and quantitatively analyzed using the formula (D-0.9λ / t cosθ) (t: full width at half maximum, 0.6°–1.0°).

[0134] Classification Milling Energy (kWh / kg) Average Particle Size of Silicon Nanoparticles (nm) D50 D90 Example 1 35 10 7 16 2 Example 2 40 10 2 15 6 Example 3 45 96 14 8 Example 4 50 92 13 9 Comparative Example 1 20 130 19 8 Comparative Example 2 25 12 4 19 2 Comparative Example 3 65 90 12 5

[0135] Experimental Example 1. Evaluation of Cathode Active Material Characteristics

[0136] For the negative electrode active materials prepared in the above examples and comparative examples, the half-width of the X-ray diffraction angle (2theta) using CuKα rays at the (111) plane of the silicon nanoparticles, the value of Equation (1) of the negative electrode active material, and the specific surface area were measured, and the results are shown in Table 2 below.

[0137] The full width at half maximum of the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane of the silicon nanoparticle is the value obtained by measuring the full width at half maximum of the diffraction peak (111) in the (111) plane using CuKα rays for the silicon nanoparticle.

[0138] The average crystal size of the cathode active material was expressed as the average value of measurements taken using XRD.

[0139] The O concentration and the value of Equation (1) are values ​​obtained by calculating the Si concentration (atm%), O concentration (atm%), and C concentration (atm%) contained in the surface layer, which is the region from the surface of the cathode active material to 2 to 3 nm, by the XPS Survey method, and then calculating the values ​​according to Equation (1).

[0140] The specific surface area is a value measured using a BET measuring instrument (Micromeritics TriStar II 3020).

[0141] In Table 2 below, Comparative Examples 4 to 6 are silicon-carbon composites that did not form an amorphous carbon coating layer in Examples 1 to 3 above.

[0142] Classification Full width at half maximum of silicon nanoparticles (°) Average crystal size of cathode active material (nm) O concentration (atm%) Value of Equation (1) Specific surface area (m² / g) Example 1 0.60 13.5 216.5 38.2 2.2 Example 20.62 13.29 17.2 39.5 2.4 Example 30.68 11.9 118.3 41.6 2.4 Example 40.73 11.25 20.1 49.8 2.5 Comparative Example 10.45 18.2 214.2 36.3 2.3 Comparative Example 20.50 16.4 14.9 36.9 2.2 Comparative Example 31.05 7.8 123.68 7.0 2.7 Comparative Example 40.47 17.4 417.3 46.2 5.7 Comparative Example 50.4617.8217.451.16.8 Comparative Example 60.5815.7720.865.77.5

[0143] As shown in Table 2 above, in the case of the silicon-carbon composites of Examples 1 to 3 prepared using silicon nanoparticles that were milled with a milling energy of 50 kWh to 100 kWh according to the present invention and had a full width at half maximum of 0.6° to 1.0° of the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane, it was confirmed that the value of Equation (1) was 38 or higher and 50 or lower, and the specific surface area was 5 m² / g or lower. On the other hand, in Comparative Examples 1 to 2, where the milling energy was less than 30 kWh / kg, the full width at half maximum of the silicon nanoparticles was found to be less than 0.6°, and in Comparative Example 3, where the milling energy exceeded 60 kWh / kg, the full width at half maximum of the silicon nanoparticles exceeded 1.0°, and the value of Equation (1) was also found to be high at 87.0, so it was confirmed that the oxidation degree and specific surface area intended in the present invention could not be achieved.

[0144] In addition, in the case of Comparative Examples 4 to 6, which did not form an amorphous carbon coating layer, the half-width of the silicon nanoparticles was less than 0.6°, and the value of Equation (1) was also higher than that of the example, and it was confirmed that the specific surface area was wide at 5.7 m² / g or more.

[0145] Experimental Example 2. Electrochemical Evaluation

[0146] (1) Coin-type half-battery manufacturing

[0147] After manufacturing a CR2032 coin cell using the cathode active material prepared as described above, an electrochemical evaluation was conducted.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] (2) Evaluation of charge / discharge characteristics

[0152] After aging the coin-type half battery manufactured in (1) above at room temperature (25℃) for 24 hours, a charge / discharge test was performed.

[0153] 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 3 below.

[0154] Classification Capacity (mAh / g) Initial Efficiency (%) Lifespan (%, 50 cycles) Example 1 1.68 590.190.8 Example 2 1.6748 9.992.5 Example 3 1.6638 9.798.0 Example 4 1.6328 9.298.2 Comparative Example 1 1.7128 8.965.0 Comparative Example 21.6988 8.574.7 Comparative Example 31.5328 8.582.5 Comparative Example 41.6788 4.980.5 Comparative Example 5 1.6658 4.382.3 Comparative Example 6 1.6368 3.581.6

[0155] As shown in Table 3 above, in the case of the cathode active materials of Examples 1 to 3, which are ground with a milling energy of 50 kWh to 100 kWh according to the present invention, have a full width at half maximum of 0.6° to 0.8° for the X-ray diffraction angle (2theta) using CuKα rays in the (111) plane, an oxidation degree of 10 to 13%, and a specific surface area of ​​5 m² / g or less, it was confirmed that the capacity, initial efficiency, and lifespan characteristics were superior compared to Comparative Examples 1 to 6.

[0156] Experimental Example 3. Electrochemical evaluation according to silicon and carbon ratio

[0157] When preparing the negative electrode active material of Example 1 above, the ratio of silicon nanoparticles to carbon (graphite + pitch) was varied as follows, and charge / discharge characteristics according to the silicon and carbon ratio were evaluated, and the results are shown in Table 4 below.

[0158] Classification Silicon:Carbon Ratio Capacity (mAh / g) Initial Efficiency (%) Lifetime (%, 50 cycles) Example 365: 351.66389.798.0 Example 550: 501.41390.198.2 Example 670: 301.75688.896.2 Comparative Example 745: 551.27888.796.3 Comparative Example 875: 251.82085.172.8

[0159] As shown in Table 4 above, in the case of Examples 3, 5, and 6 containing silicon and carbon in a ratio of 50~70:30~50 according to the present invention, it was confirmed that the capacity, initial efficiency, and lifespan characteristics were superior compared to Comparative Examples 7 and 8. These results are inferred to be obtained by reducing the degree of oxidation to 10~13% and the average crystal size to 10~14 nm by performing the milling energy and time during the manufacture of silicon nanoparticles within the range specified in the present invention. It was found that according to the present invention, a structurally stable negative electrode active material for a lithium secondary battery can be manufactured while completely capturing silicon nanoparticles within the carbon matrix even with an increase in silicon content.

[0160] 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 and an amorphous carbon coating layer formed on the surface of the silicon-carbon composite, and The above silicon-carbon composite comprises silicon nanoparticles; and a carbon matrix comprising crystalline carbon and amorphous carbon, and A negative electrode active material for a lithium secondary battery, wherein the composition of the surface layer, which is a region from the surface up to 2 to 3 nm during XPS Survey analysis, includes Si, O and C, and the following formula (1) measured in the surface layer satisfies 38 or more and 50 or less. Equation (1): ([Si]+[O]) / [C] (In the above equation (1), [Si], [O] and [C] represent the concentration (atm%) of each element.) 2. In Paragraph 1, The above negative electrode active material is a negative electrode active material for a lithium secondary battery that satisfies the value of Equation (1) being 39.0 or higher and 49.0 or lower.

3. In Paragraph 1, The above negative electrode active material is a negative electrode active material for a lithium secondary battery having an average crystal size of 10 nm to 14 nm.

4. In Paragraph 1, The above negative electrode active material is a negative electrode active material for a lithium secondary battery having a BET specific surface area of ​​5 m² / g or less.

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, A negative electrode active material for a lithium secondary battery, wherein the average particle size (D50) of the silicon nanoparticles is 50 nm to 150 nm and the average particle size (D59) is 120 nm to 200 nm.

7. In Paragraph 1, The above silicon nanoparticles are included in an amount of 50 to 70 weight% based on the total weight of the silicon-carbon composite, and are a negative electrode active material for a lithium secondary battery.

8. In Paragraph 1, The above crystalline carbon is a negative electrode active material for a lithium secondary battery comprising one or more selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

9. In Paragraph 1, The above amorphous carbon is a negative electrode active material for a lithium secondary battery comprising 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 250°C or lower.

10. In Paragraph 1, The above carbon 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 composite.

11. In Paragraph 1, 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.

12. A step of manufacturing silicon nanoparticles by milling and grinding silicon raw materials; A step of preparing a mixture by mixing the above silicon nanoparticles, crystalline carbon, and amorphous carbon; A step of manufacturing a molded body by spray-drying and compressing the above mixture; A step of manufacturing a silicon-carbon composite by heat-treating the above-mentioned molded body; and The method includes the step of forming an amorphous carbon coating layer on the surface of the silicon-carbon composite; A method for manufacturing a negative electrode active material for a lithium secondary battery, wherein the above-mentioned milling is performed for 10 to 30 hours with a milling energy of 30 kWh / kg to 60 kWh / kg.

13. In Paragraph 12, A method for manufacturing a negative electrode active material for a lithium secondary battery, wherein the negative electrode active material has a surface layer composition of 2 to 3 nm from the surface when analyzed by XPS Survey, and the following formula (1) measured in the surface layer satisfies 38 or more and 50 or less. Equation (1): ([Si]+[O]) / [C] (In the above equation (1), [Si], [O] and [C] represent the concentration (atm%) of each element.) 14. In Paragraph 12, A method for manufacturing a negative electrode active material for a lithium secondary battery, wherein the value of Equation (1) satisfies 39.0 or more and 49.0 or less.

15. In Paragraph 12, A method for manufacturing a negative electrode active material for a lithium secondary battery, wherein the above heat treatment is performed in an inert atmosphere of 1000℃ or lower.