Negative electrode active material for lithium secondary battery, manufacturing method therefor, 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 composite including silicon nanoparticles and a carbon-based matrix, wherein the silicon-carbon composite includes 0.5-7 parts by weight of polyvinyl butyral (PVB) based on 100 parts by weight of the silicon nanoparticles.

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, a method for manufacturing the same, and a lithium secondary battery including the same, which can improve the dispersibility of silicon nanoparticles and secure structural stability of the silicon-carbon composite by forming crosslinks with pitch by adding polyvinyl butyral (PVB) during the preparation of a silicon-carbon composite, and improve electrochemical properties such as charge / discharge capacity and lifespan characteristics.

[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 and a carbon-based matrix, wherein the silicon-carbon composite may comprise 0.5 to 7 parts by weight of polyvinyl butyral (PVB) per 100 parts by weight of silicon nanoparticles.

[0009] The polyvinyl butyral according to one embodiment of the present invention may have a weight-average molecular weight of 20,000 to 70,000.

[0010] The negative electrode active material according to one embodiment of the present invention may have an average crystal size of 10 nm to 14 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 0.8°.

[0013] 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.

[0014] The carbon-based matrix according to one embodiment of the present invention may include one or more crystalline carbons selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

[0015] The carbon-based matrix according to one embodiment of the present invention 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, having a softening point of 300°C or lower.

[0016] 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 composite.

[0017] The silicon-carbon composite according to one embodiment of the present invention may further include an amorphous carbon coating layer on its surface.

[0018] The amorphous carbon coating layer according to one embodiment of the present invention may have an average thickness of 10 nm or less.

[0019] A method for manufacturing a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention may include the step of preparing a mixture by mixing silicon nanoparticles, a carbon matrix, and polyvinyl butyral (PVB); the step of preparing a molded body by spray-drying and compressing the mixture; and the step of preparing a silicon-carbon composite by heat-treating the molded body.

[0020] According to one embodiment of the present invention, the polyvinyl butyral may be included in an amount of 0.5 to 7 parts by weight per 100 parts by weight of the silicone raw material.

[0021] The heat treatment according to one embodiment of the present invention may be performed for 0.5 to 2 hours in an inert atmosphere of 1000°C or lower.

[0022] 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.

[0023] A negative electrode according to one embodiment of the present invention may include the negative electrode active material for a lithium secondary battery.

[0024] A lithium secondary battery according to one embodiment of the present invention may include the negative electrode; the positive electrode; and the electrolyte.

[0025] According to the present invention, by adding polyvinyl butyral (PVB) during the preparation of a silicon-carbon composite, the dispersibility of silicon nanoparticles can be improved, and structural stability of the silicon-carbon composite can be secured by forming cross-links with pitch, and electrochemical properties such as charge / discharge capacity and lifespan can be improved. In this way, a negative electrode active material for a lithium secondary battery, a method for preparing the same, and a lithium secondary battery including the same can be provided.

[0026] 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.

[0027] FIG. 1 is a diagram showing the initial efficiency after a long-term cycle of a cathode active material according to one embodiment of the present invention.

[0028] FIG. 2 is a diagram showing the lifespan characteristics after a long-term cycle of a negative electrode active material according to one embodiment of the present invention.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The present invention aims to provide a negative electrode active material for a lithium secondary battery with improved electrochemical properties by adding polyvinyl butyral (PVB) during the preparation of a silicon-carbon composite to improve the dispersibility of silicon nanoparticles and forming cross-links with pitch to secure the structural stability of the silicon-carbon composite.

[0034] 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 and a carbon-based matrix, wherein the silicon-carbon composite may comprise 0.5 to 7 parts by weight of polyvinyl butyral (PVB) per 100 parts by weight of silicon nanoparticles.

[0035] In the case of silicon-carbon composite cathode materials manufactured by combining silicon and a carbon matrix, the non-uniform dispersion of silicon within the carbon matrix can lead to a decrease in the structural stability of the cathode material.

[0036] Accordingly, in the present invention, by adding polyvinyl butyral to a silicon-carbon composite, silicon nanoparticles within the carbon matrix are uniformly dispersed and stabilized, thereby preventing the aggregation of silicon nanoparticles. Furthermore, by forming crosslinking with the amorphous carbon that serves as the carbon matrix, structural stability of the cathode material is promoted, which can improve electrochemical properties such as lifespan characteristics.

[0037] That is, the above polyvinyl butyral effectively disperses silicon nanoparticles within a carbon matrix while simultaneously forming cross-links with the carbon matrix, thereby preventing aggregation between silicon nanoparticles and enabling the production of a structurally stable cathode material.

[0038] The above polyvinyl butyral preferably has a weight-average molecular weight of 20,000 to 70,000, and more preferably 25,000 to 65,000. If the weight-average molecular weight of the above polyvinyl butyral is less than 20,000, the effect of preventing aggregation of silicon nanoparticles may be reduced, and if it exceeds 70,000, aggregation or precipitation between polyvinyl butyral may occur.

[0039] The above polyvinyl butyral may be included in an amount of 0.5 to 7 parts by weight per 100 parts by weight of silicon nanoparticles, more preferably in an amount of 0.55 to 6.5 parts by weight, and most preferably in an amount of 0.6 to 6 parts by weight. If the content is less than 0.5 parts by weight, aggregation between silicon nanoparticles may occur, and precipitation of aggregated silicon nanoparticles may occur; if it exceeds 7 parts by weight, the physical properties as a cathode material may be degraded due to the excessive addition of polyvinyl butyral.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] The silicon nanoparticles may 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, and preferably 0.65° to 0.75° and 0.70° to 0.73°.

[0044] 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.

[0045] 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.

[0046] The above carbon matrix may include crystalline carbon and amorphous carbon.

[0047] The above crystalline carbon may include one or more selected from natural graphite, artificial graphite, flake graphite, earth graphite, expanded graphite, and graphene.

[0048] 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.

[0049] 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.

[0050] 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 acting as a binder, it improves 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.

[0051] The carbon matrix 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.

[0052] As described above, a silicon-carbon composite comprising silicon nanoparticles, a carbon matrix, and polyvinyl butyral can have an amorphous carbon layer formed on its surface.

[0053] 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.

[0054] The above amorphous carbon coating layer may have an average thickness of 10 nm or less, and 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.

[0055] The average crystal size of the negative electrode active material of the present invention as described above is preferably 10 nm to 14 nm, and more preferably 10.5 nm to 13.5 nm.

[0056] 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.

[0057] Hereinafter, a method for manufacturing a negative electrode active material for a lithium secondary battery 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 the silicon-carbon composite described above, and the present embodiment is not limited to the following method.

[0058] A method for manufacturing a negative electrode active material for a lithium secondary battery according to another embodiment of the present invention may include the steps of: mixing silicon nanoparticles, a carbon matrix, and polyvinyl butyral (PVB) to prepare a mixture; spray-drying and compressing the mixture to produce a molded body; and heat-treating the molded body to produce a silicon-carbon composite.

[0059] First, a mixture is prepared by mixing silicon nanoparticles, a carbon matrix, and polyvinyl butyral.

[0060] The above carbon matrix is ​​as described above, so it will be omitted here.

[0061] The above silicon nanoparticles can be manufactured by grinding silicon raw materials.

[0062] 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㎛.

[0063] In addition, the 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°. The full width at half maximum range for the (111) plane of the silicon nanoparticles can be controlled by increasing the time in the grinding process using the silicon raw material, increasing the BPR (ball per ratio), or controlling the solid content to increase the probability of collision with zirconium balls.

[0064] Silicon nanoparticles can be manufactured by preparing the above silicon raw material and then grinding it using a top-down milling method.

[0065] Specifically, silicon raw materials can be ground by milling them together with zirconia balls using an organic solvent.

[0066] 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.

[0067] 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%.

[0068] 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.

[0069] The silicon nanomaterial prepared as described above is a mixture prepared by mixing a carbon matrix and polyvinyl butyral. Subsequently, the mixture is spray-dried and compressed to produce a molded body.

[0070] Next, the molded body manufactured above is heat-treated to produce a silicon-carbon composite.

[0071] 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.

[0072] The above heat treatment can be performed in a furnace in an inert atmosphere, specifically a nitrogen (N2) atmosphere.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] A cathode according to another embodiment of the present invention may include the aforementioned cathode active material.

[0079] 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.

[0080] 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.

[0081] The above-mentioned negative electrode active material is the same as that of the aforementioned embodiment of the present invention, so it is omitted.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] A lithium secondary battery according to another embodiment of the present invention may include the aforementioned negative electrode, positive electrode, and electrolyte.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] The above electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0092] The above-mentioned non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] Examples

[0098] (1) Manufacturing of silicon nanoparticles

[0099] 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.

[0100] (2) Preparation of cathode active material

[0101] Based on 100% by weight of the silicon-carbon composite, 65% by weight of a slurry containing silicon nanoparticles prepared in (1) above, 15% by weight of graphite particles with an average particle size (D50) of 5 to 10 μm, and 0.5% by weight of polyvinyl butyral per 100% by weight of the silicon nanoparticles were introduced into a high-speed mixer and dispersed to produce a silicon-graphite precursor with 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. Subsequently, 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.

[0102] 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.

[0103] Examples 2~5

[0104] The above Example 1 was carried out in the same manner as Example 1, except that the content of polyvinyl butyral was used differently as shown in Table 1 below.

[0105] Comparative Examples 1-3

[0106] The above Example 1 was carried out in the same manner as Example 1, except that silicon nanoparticles, graphite, and pitch were used in weight percent as shown in Table 1 below, and polyvinyl butyral was not used.

[0107] Classification Si:Pitch:Graphite (weight%) Polyvinyl Butyral (weight parts) Example 1 65:20:150.5 parts Example 2 65:20:151 parts Example 3 65:20:153 parts Example 4 65:20:155 parts Example 5 65:20:157 parts Comparative Example 157:30:130 parts Comparative Example 255:30:150 parts Comparative Example 353:29:180 parts

[0108] Experimental Example 1. Electrochemical Evaluation

[0109] (1) Coin-type half-battery manufacturing

[0110] After manufacturing a CR2032 coin cell using the cathode active material prepared as described above, an electrochemical evaluation was conducted.

[0111] 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.

[0112] 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 ~1.5 g / cc.

[0113] 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.

[0114] (2) Evaluation of charge / discharge characteristics

[0115] After aging the coin-type half battery manufactured in (1) above at room temperature (25℃) for 24 hours, a charge / discharge test was performed.

[0116] 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 1.5 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 Figures 1 and 2 and Table 2 below.

[0117] Classification Capacity (mAh / g) Initial Efficiency (%) Lifespan (%, 50 cycles) Example 1 1,587 89.27 6.4 Example 2 1,716 89.58 5.6 Example 3 1,513 90.59 5.7 Example 4 1,667 90.89 8.5 Example 5 1,508 89.36 4.1 Comparative Example 1 1,503 89.15 6.7 Comparative Example 21,498 88.56 7.7 Comparative Example 31,586 89.34 6.5

[0118] As shown in Table 2 above, in the case of Examples 1 to 5, in which polyvinyl butyral was added in an amount of 0.5 to 7 parts by weight per 100 parts by weight of silicon nanoparticles according to the present invention, the initial capacity was found to be somewhat lower compared to Comparative Examples 1 to 3, in which polyvinyl butyral was not added; however, it was confirmed that the initial efficiency (see FIG. 1) and lifespan characteristics (see FIG. 2) were excellent after 50 or more long-term cycles. 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 gist of the claims.

Claims

1. A silicon-carbon composite comprising silicon nanoparticles and a carbon-based matrix, and The above silicon-carbon composite is a negative electrode active material for a lithium secondary battery comprising 0.5 to 7 parts by weight of polyvinyl butyral (PVB) per 100 parts by weight of silicon nanoparticles.

2. In Paragraph 1, The above polyvinyl butyral is a negative electrode active material for a lithium secondary battery having a weight-average molecular weight of 20,000 to 70,000.

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 silicon nanoparticles are negative electrode active materials for lithium secondary batteries having an average particle size (D50) of 30 nm to 200 nm.

5. In Paragraph 1, The above silicon nanoparticles are a negative electrode active material for a lithium secondary battery having a half-width of X-ray diffraction angle (2theta) using CuKα rays in the (111) plane of 0.6° to 0.8°.

6. 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.

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

8. 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, having a softening point of 300°C or lower.

9. 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 composite.

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

11. In Paragraph 10, 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 preparing a mixture by mixing silicon nanoparticles, a carbon matrix, and polyvinyl butyral; A step of manufacturing a molded body by spray-drying and compressing the above mixture; and A method for manufacturing a negative electrode active material for a lithium secondary battery, comprising the step of heat-treating the above-mentioned molded body to produce a silicon-carbon composite.

13. In Paragraph 12, A method for manufacturing a negative electrode active material for a lithium secondary battery, wherein the above polyvinyl butyral is included in an amount of 0.5 to 7 parts by weight per 100 parts by weight of the above silicon nanoparticles.

14. In Paragraph 12, A method for manufacturing a negative electrode active material for a lithium secondary battery, wherein the above heat treatment is performed for 0.5 to 2 hours in an inert atmosphere of 1000℃ or lower.

15. In Paragraph 12, 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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