Silicon-carbon composite, negative electrode active material, method for producing negative electrode active material, and negative electrode comprising negative electrode active material

Abstract

According to an exemplary embodiment of the present invention, a silicon-carbon composite that can improve the lifespan characteristics of a secondary battery is provided. The silicon-carbon composite is a silicon-carbon composite for use as a negative electrode active material, comprising a carbon matrix and silicon nanoparticles trapped in the carbon matrix, wherein the surface roughness (Rq) of the silicon-carbon composite can be 4-50 nm.

Description

Technical Field

[0001] This invention relates to a silicon-carbon composite, a negative electrode active material, a method for preparing the negative electrode active material, and a negative electrode containing the negative electrode active material. Background Technology

[0002] In recent years, the portability and wireless nature of electronic devices have been rapidly advancing, leading to an increasing demand for small, lightweight secondary batteries with high energy density (W) as power sources for these devices. Furthermore, high power characteristics, long-term durability, and safety are also required not only for small consumer applications but also for large secondary batteries used in energy storage devices and electric vehicles.

[0003] A secondary battery is a structure comprising an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, and an electrolyte contained within an outer casing. The electrode includes an electrode current collector and an electrode active material layer located on at least one side of the electrode current collector. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector.

[0004] Based on the types of negative and positive electrode active materials, secondary batteries can be classified into lead-acid batteries, nickel (Ni)-cadmium (Cd) batteries, nickel (Ni)-metal hydride (MH) batteries, and lithium batteries. Among them, the demand for lithium secondary batteries, which have superior energy density, power characteristics, and service life, is increasing.

[0005] The secondary battery reaction of a lithium secondary battery is mediated by lithium ions (Li... + It is carried out through chemical reactions that move between the positive and negative active materials.

[0006] In the field of rechargeable batteries, the lifespan characteristics of rechargeable batteries are directly related to the long-term performance of devices that use rechargeable batteries as a power source, and therefore a great deal of research is ongoing.

[0007] (Patent Document 1) Korean Patent Publication No. 10-1825919. Summary of the Invention

[0008] (a) Technical problems to be solved The technical problem to be solved by the present invention is to provide a silicon-carbon composite that can improve the lifespan characteristics of secondary batteries.

[0009] Another technical concept of the present invention is to provide a negative electrode active material that can improve the lifespan characteristics of secondary batteries.

[0010] Another technical concept of the present invention is to provide a method for preparing a negative electrode active material that can improve the lifespan characteristics of secondary batteries.

[0011] Another technical concept of the present invention is to provide a negative electrode containing a negative electrode active material that can improve the lifespan characteristics of a secondary battery.

[0012] The technical problem addressed by this invention is not limited to the above description. Those skilled in the art will readily understand the additional technical problems of this invention from the entirety of this specification.

[0013] (II) Technical Solution According to an exemplary embodiment of the present invention, a silicon-carbon composite is provided. The silicon-carbon composite is a silicon-carbon composite for negative electrode active materials, which may comprise a carbon matrix and silicon nanoparticles trapped in the carbon matrix, and the surface roughness (Rq) of the silicon-carbon composite may be 4-50 nm.

[0014] The D90 of the silicon nanoparticles can be below 180 nm.

[0015] The content of the silicon nanoparticles can be 35-60% by weight.

[0016] The content of the carbon matrix can be 35-55% by weight.

[0017] The carbon matrix may contain crystalline carbon and amorphous carbon.

[0018] The content of crystalline carbon can be 10-25% by weight. The content of amorphous carbon can be 25-40% by weight.

[0019] The crystalline carbon may include at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

[0020] The D50 of the crystalline carbon can be 5-10 μm.

[0021] The amorphous carbon may include any one of coal-based pitch, petroleum-based pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

[0022] According to other exemplary embodiments of the present invention, a negative electrode active material is provided. The negative electrode active material comprises: a silicon-carbon composite comprising silicon nanoparticles and a carbon matrix; and a coating distributed on the surface of the silicon-carbon composite, wherein the coating may comprise carbon nanotubes and residual binder distributed on the surface of the silicon-carbon composite.

[0023] Based on 100 parts by weight of silicon-carbon composite, the content of carbon nanotubes can be 0.1-0.5 parts by weight.

[0024] Based on 100 parts by weight of silicon-carbon composite, the content of the residual binder can be 0.1-1 parts by weight.

[0025] The water content of the negative electrode active material can be 0.6-2.0%.

[0026] The residual adhesive may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and combinations thereof.

[0027] According to other exemplary embodiments of the present invention, a method for preparing a negative electrode active material is provided. The method includes: pulverizing silicon raw materials to prepare silicon nanoparticles; mixing and drying the silicon nanoparticles and crystalline carbon to obtain a silicon-crystalline carbon precursor; combining the silicon-crystalline carbon precursor with an amorphous carbon precursor to obtain a mixture; heat-treating the mixture in an inert atmosphere at 700-1000°C to obtain a silicon-carbon composite; mixing a coating precursor solution containing carbon nanotubes and a binder with the silicon-carbon composite to obtain a slurry; and drying the slurry in an air atmosphere at 20-100°C.

[0028] In the step of obtaining the silicon nanoparticles, the silicon raw material can be pulverized by ball milling using beads, the diameter of which can be less than twice the D99 of the silicon raw material.

[0029] In the step of obtaining the silicon nanoparticles, the silicon raw material can be pulverized by wet pulverization using an organic solvent containing ethanol with a purity of 99.9%.

[0030] In the step of obtaining the silicon nanoparticles, the D90 of the silicon nanoparticles can be below 180 nm.

[0031] The D50 of the crystalline carbon can be 5-10 μm.

[0032] The surface roughness (Rq) of the silicon-carbon composite can be 4-50 nm.

[0033] The preparation method of the negative electrode active material may further include a pressure molding step of pressurizing the mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor.

[0034] The adhesive may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and combinations thereof.

[0035] (III) Beneficial Effects The silicon-carbon composite according to an exemplary embodiment of the present invention can improve the lifespan characteristics of a secondary battery due to the inclusion of nanoscale silicon particles, while the inclusion of a carbon matrix can improve the characteristics of the secondary battery.

[0036] According to other exemplary embodiments of the present invention, the negative electrode active material has a structure in which carbon nanotubes and residual binder are distributed on the surface of the aforementioned silicon-carbon composite. This can improve the lifespan characteristics of the secondary battery.

[0037] The method for preparing the negative electrode active material according to other exemplary embodiments of the present invention can prepare the negative electrode active material at a relatively low temperature, thereby improving the preparation efficiency of the negative electrode active material.

[0038] The various and beneficial advantages and effects of the present invention are not limited to the above description, and can be more easily understood in the process of describing specific embodiments of the present invention. Attached Figure Description

[0039] Figure 1 It is a SEM image of the surface of the negative electrode active material according to Example 2.

[0040] Figure 2 It is a SEM image of the surface of the negative electrode active material according to Comparative Example 3.

[0041] Figure 2 This is a graph illustrating the lifespan characteristics of secondary batteries according to embodiments and comparative examples. Best practice

[0042] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Prior to this, the terms or words used in this specification and claims should not be limited to their ordinary or dictionary meanings, but should be interpreted according to the principle that the inventor can appropriately define the concepts of terms to best illustrate their invention, and should be interpreted as conforming to the technical idea and concepts of the present invention.

[0043] Hereinafter, when describing with reference to the accompanying drawings, the same or corresponding constituent elements will be given the same reference numerals, and repeated descriptions of them will be omitted.

[0044] In the following implementation scheme, the terms "first," "second," etc., are not limiting in meaning, but are used to distinguish one constituent element from other constituent elements.

[0045] In the following implementation, unless the context clearly indicates a different meaning, the singular expression includes the plural expression.

[0046] In the following implementation, terms such as include, comprise, or have refer to the presence of the features or constituent elements described in the specification, without pre-excluding the possibility of additional features or constituent elements.

[0047] In the accompanying drawings, the dimensions of the constituent elements may be enlarged or reduced for ease of illustration. For example, the dimensions and thicknesses of the various components shown in the drawings are arbitrarily illustrated for ease of explanation, and therefore the invention is not necessarily limited to what is shown.

[0048] Where a particular implementation scheme can be carried out differently, the specific process sequence may be performed in a different order than that described. For example, two processes described consecutively may be performed substantially simultaneously, or they may be performed in the reverse order of the description.

[0049] Furthermore, when describing the present invention, detailed descriptions of related known structures or functions will be omitted if it is determined that such detailed descriptions may obscure the essential points of the present invention.

[0050] In this invention, the nanoscale size is defined based on the D90 of the particle size distribution. That is, it refers to the case where the D90 of the particles is less than 1.0 μm.

[0051] In this invention, D50, D10, and D90 can be defined as the particle sizes corresponding to a volume accumulation of 50%, 10%, and 90%, respectively, in the particle size distribution curve. D50, D10, and D90 can be measured, for example, using laser diffraction. Laser diffraction can typically measure particle sizes from the submicron region to several millimeters, yielding highly reproducible and resolvable results.

[0052] The following provides a detailed description of the negative electrode, the negative electrode active material, and the method for manufacturing the negative electrode according to an exemplary embodiment of the present invention.

[0053] [negative electrode] According to an exemplary embodiment, the negative electrode may include a negative electrode current collector and a negative electrode active material layer.

[0054] There are no particular restrictions on the negative electrode current collector, as long as it does not cause a chemical change in the battery and has high conductivity. As a non-limiting example, the negative electrode current collector may include any one or more of the following: copper, stainless steel, aluminum, nickel, titanium, calcined carbon, substances that have been surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, and aluminum-cadmium alloys.

[0055] The thickness of the negative electrode current collector can be 3-500μm.

[0056] The negative electrode active material layer can be disposed on one or both sides of the negative electrode current collector. The negative electrode active material layer can be obtained by coating a negative electrode slurry containing a negative electrode binder, conductive material and negative electrode active material onto the negative electrode current collector and then drying it.

[0057] A negative electrode binder is a component that facilitates the bonding between the negative electrode active material and the current collector. As a non-limiting example, the negative electrode binder may contain one or more of the following: polyvinyl alcohol, carboxymethyl cellulose / styrene-butadiene rubber, hydroxypropyl cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene.

[0058] There is no particular limitation on the content of the negative electrode binder. As a non-limiting example, the negative electrode binder may be added to the negative electrode active material layer at a rate of 30% or less (including 0%).

[0059] Conductive materials are components used to further improve the conductivity of negative electrode active materials. As non-limiting examples, conductive materials may include: graphite such as carbon nanotubes, natural graphite, and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorinated carbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and one or more conductive materials such as polyphenylene derivatives.

[0060] There are no particular restrictions on the content of conductive material. As a non-limiting example, conductive material may be added to the negative electrode active material layer at a content of 20% or less (including 0%).

[0061] [Negative Electrode Active Material] According to an exemplary embodiment, the negative electrode active material may comprise a silicon-carbon composite and a coating.

[0062] According to exemplary embodiments, the surface roughness (Rq) of the silicon-carbon composite can be 4-100 nm. More specifically, the surface roughness (Rq) of the silicon-carbon composite can be 4-90 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-80 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-60 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-50 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-30 nm. The surface roughness (Rq) of the silicon-carbon composite can be 6-30 nm. The surface roughness (Rq) of the silicon-carbon composite can be 9-30 nm.

[0063] When the surface roughness (Rq) of the silicon-carbon composite is less than 4 nm, the coating may not be properly adhered to the surface of the silicon-carbon composite in the coating process described later. Therefore, a higher surface roughness (Rq) of the silicon-carbon composite is more beneficial to the coating process efficiency. However, when the surface roughness (Rq) of the silicon-carbon composite is too high, the coating cannot form uniformly and may form a thicker coating in localized areas. As a result, the resistance of the negative electrode active material may increase. Alternatively, an increase in the specific surface area of ​​the negative electrode active material may cause side reactions with the electrolyte during the use of the secondary battery.

[0064] Silicon-carbon composites can include silicon nanoparticles, a carbon matrix, and unavoidable impurities introduced during the fabrication process. More specifically, silicon-carbon composites can include structures in which silicon nanoparticles are trapped within a carbon matrix. That is, silicon-carbon composites can include structures in which silicon nanoparticles are mechanically bonded together in contact with a carbon matrix.

[0065] Previously, graphite-based materials, with their excellent capacity retention and efficiency, were used as anode active materials. However, graphite-based materials suffer from relatively low theoretical capacity values ​​and low discharge capacity ratios. Therefore, research on silicon (Si)-based materials has been ongoing as an alternative to graphite-based anode active materials.

[0066] Silicon-based materials, due to their high theoretical capacity and low operating voltage, can improve the characteristics of secondary batteries when used as negative electrode active materials. However, silicon-based materials suffer from particle cracking caused by the continuous charge-discharge reaction of the secondary battery, thus reducing the battery's lifespan.

[0067] According to an exemplary embodiment of the present invention, nanoscale silicon-based materials (silicon nanoparticles) can be used as the negative electrode active material of a secondary battery. As described above, by using nanoscale silicon-based materials, the occurrence of particle cracking can be minimized. This improves the lifespan characteristics of the secondary battery.

[0068] More specifically, the D90 of silicon nanoparticles can be below 180 nm. More specifically, the D90 of silicon nanoparticles can be below 150 nm. The D90 of silicon nanoparticles can be 100-180 nm. The D90 of silicon nanoparticles can be 100-150 nm.

[0069] According to an exemplary embodiment, the silicon-carbon composite may contain 35-60% by weight of silicon nanoparticles based on the total weight of the composite.

[0070] To improve the capacity of secondary batteries, it is necessary to increase the content of silicon nanoparticles. Furthermore, the higher the silicon nanoparticle content, the more completely the silicon nanoparticles can be trapped within the carbon matrix. This can promote the structural stability of the silicon-carbon composite. However, when the silicon nanoparticle content is too high, the silicon nanoparticles may not be sufficiently trapped in the carbon matrix, and the mechanical and physical properties of the silicon-carbon composite may deteriorate. In addition, the silicon nanoparticles may detach from the carbon matrix.

[0071] According to an exemplary embodiment, the silicon-carbon composite may contain 35-55% by weight of carbon matrix, based on the total weight of the silicon-carbon composite.

[0072] A carbon matrix can be configured to support silicon nanoparticles. More specifically, the silicon nanoparticles can be in direct contact with and mechanically bonded to the carbon matrix. Thus, the carbon matrix can support the silicon nanoparticles while maintaining contact with them. Furthermore, even with the volume expansion of the silicon nanoparticles due to the charge-discharge reaction of the secondary battery, electrical contact with other internal components of the secondary battery (e.g., the negative electrode current collector) can be maintained through the carbon matrix.

[0073] The carbon matrix can contain crystalline carbon and amorphous carbon.

[0074] Crystalline carbon can complement the electrical properties of silicon nanoparticles, thereby improving the charge-discharge reversibility of secondary batteries.

[0075] According to an exemplary embodiment, the silicon-carbon composite may contain 10-25% by weight of crystalline carbon. When the content of crystalline carbon is less than 10% by weight, the effect of silicon nanoparticles in supplementing the electrical properties may be negligible. However, when the content of crystalline carbon is greater than 25% by weight, although electrical properties can be improved, the silicon nanoparticles may not be trapped in the carbon matrix. This may excessively increase the specific surface area of ​​the silicon-carbon composite. In this case, excessive side reactions between the silicon-carbon composite and the electrolyte may occur during the charge and discharge of the secondary battery, which may reduce the lifespan characteristics of the secondary battery. Furthermore, a reduction in the content of silicon nanoparticles may lead to a decrease in the capacity of the secondary battery.

[0076] According to an exemplary embodiment, the D50 of crystalline carbon can be 5-10 μm.

[0077] As a non-limiting example, crystalline carbon may include at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

[0078] According to an exemplary embodiment, the silicon-carbon composite may contain 25-40% by weight of amorphous carbon based on the total weight of the composite.

[0079] Amorphous carbon acts as a binder, stably supporting crystalline carbon and silicon nanoparticles. As mentioned above, crystalline carbon and silicon nanoparticles can be physically and electrically connected through amorphous carbon. Therefore, even with repeated charge-discharge cycles of a secondary battery, structural collapse of the silicon-carbon composite can be prevented, and the low conductivity of silicon nanoparticles can be more effectively compensated for.

[0080] According to an exemplary implementation, amorphous carbon may include any one of coal-based pitch, petroleum-based pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

[0081] More specifically, amorphous carbon may comprise coal-based pitch with a fixed carbon content of 50% by weight or more. As a non-limiting example, the fixed carbon content of coal-based pitch may be 60-80% by weight.

[0082] The higher the fixed carbon content of coal-based tar pitch, the more conductive paths can be formed that connect with silicon nanoparticles with low conductivity, thereby inducing an increase in capacity and efficiency. Furthermore, when the fixed carbon content of coal-based tar pitch meets the aforementioned range, the internal porosity of the carbon matrix can be reduced, thus decreasing the specific surface area. Therefore, when using secondary batteries, side reactions between the negative electrode active material and the electrolyte can be reduced, which can contribute to an increase in the initial efficiency of the secondary battery.

[0083] As a non-limiting example, the β-resin value of coal-based tar pitch can be 20 or higher. More specifically, the β-resin value of coal-based tar pitch can be from 25 to 40. The β-resin value of coal-based tar pitch can be from 25 to 35.

[0084] The β-resin value refers to the value of removing quinoline-insoluble substances from benzene-insoluble matter. This β-resin value is proportional to the adhesion. According to an exemplary embodiment, when the β-resin value meets the above-mentioned range, the internal porosity of the silicon-carbon composite can be reduced, thereby stably maintaining the structure of contact (capture) between the carbon matrix and silicon nanoparticles. Therefore, using the negative electrode active material according to the exemplary embodiment, a secondary battery with excellent lifetime characteristics and plate expansion characteristics can be achieved.

[0085] As a non-limiting example, the silicon-carbon composite may contain impurities unavoidable during the preparation process. As an example, an unavoidable impurity could be an oxide film formed on the surface of silicon nanoparticles. The remainder of the silicon-carbon composite besides the silicon nanoparticles and the carbon matrix can be such unavoidable impurities.

[0086] The coating can be applied to the surface of the silicon-carbon composite. The coating can suppress the volume expansion of the silicon-carbon composite during the use of the secondary battery. This improves the battery's lifespan characteristics. Furthermore, it can improve the conductivity of the negative electrode active material, thereby enhancing the characteristics of the secondary battery utilizing the negative electrode active material.

[0087] A coating can refer to a layer applied to the surface of a silicon-carbon composite with a specified thickness. Alternatively, all substances distributed on the surface of a silicon-carbon composite can be collectively referred to as a coating. Importantly, by distributing the components of the coating described below on the surface of the silicon-carbon composite, the lifespan characteristics of the secondary battery can be improved.

[0088] According to an exemplary embodiment, the coating may comprise carbon nanotubes (CNTs) and residual binder.

[0089] Carbon nanotubes can be distributed on the surface of the silicon-carbon composite. Based on 100 parts by weight of silicon-carbon composite, 0.1-0.5 parts by weight of carbon nanotubes according to the exemplary embodiments can be included.

[0090] Carbon nanotubes can improve the electrical properties of silicon-carbon composites. Therefore, when using the negative electrode active material according to the exemplary embodiment, the resistance of the negative electrode can be reduced, thereby improving the characteristics of the secondary battery. For this purpose, the carbon nanotube content can be 0.1 parts by weight or more. However, when the carbon nanotube content is greater than 0.5 parts by weight, the carbon nanotubes may aggregate during the preparation process. In this case, the coating efficiency of the silicon-carbon composite may be reduced. Furthermore, the volume expansion of the silicon-carbon composite may not be sufficiently suppressed, and the lifespan characteristics of the secondary battery may be reduced. In addition, due to the aggregated carbon nanotubes, the production efficiency during the fractionation process of the negative electrode active material deteriorates, thereby potentially increasing the unit price of the negative electrode active material.

[0091] As a non-limiting example, carbon nanotubes can be any of single-walled carbon nanotubes (CNTs), thin-walled carbon nanotubes (CNTs), multi-walled carbon nanotubes (CNTs), and combinations thereof. More specifically, the thin-walled carbon nanotubes can be carbon nanotubes with 5 or fewer walls. The multi-walled carbon nanotubes can be carbon nanotubes with more than 5 walls.

[0092] Residual binder may be distributed on the surface of the silicon-carbon composite. Based on 100 parts by weight of silicon-carbon composite, 0.1-1 parts by weight of residual binder according to the exemplary embodiment may be included.

[0093] Residual binder can further improve the contact efficiency between carbon nanotubes and silicon-carbon composites. This can further enhance the conductivity improvement effect of the negative electrode active material brought about by carbon nanotubes. However, excessive residual binder distribution may cause carbon nanotubes to aggregate, thereby reducing coating efficiency. Furthermore, it may increase the resistance of the negative electrode active material, potentially degrading its electrical properties.

[0094] According to an exemplary embodiment, the residual adhesive may comprise a shear-thinning water-based adhesive. As a non-limiting example, the residual adhesive may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and combinations thereof.

[0095] According to an exemplary embodiment, the moisture content of the negative electrode active material can be 0.6-2.0%. The moisture content of the negative electrode active material refers to its water content. This moisture content can be calculated by drying the negative electrode active material and comparing its weight before and after drying.

[0096] When the water content of the negative electrode active material is less than 0.6%, it may be due to insufficient distribution of residual binder on the surface of the silicon-carbon composite. In this case, the effects brought about by the residual binder may not be fully realized. However, when the water content of the negative electrode active material is 2.0%, it may be due to excessive distribution of residual binder on the surface of the silicon-carbon composite. In this case, carbon nanotubes may aggregate due to the residual binder, and the coating quality of the silicon-carbon composite may deteriorate.

[0097] [Preparation method of negative electrode active material] The following describes in detail the preparation method of the negative electrode active material according to an exemplary embodiment.

[0098] According to an exemplary embodiment, the negative electrode active material can be prepared in the order of [preparation of silicon nanoparticles - formation of silicon-carbon composite - coating].

[0099] Silicon nanoparticles can be prepared by pulverizing silicon raw materials. More specifically, the silicon raw materials can be mechanically pulverized. Silicon nanoparticles can then be prepared.

[0100] The silicon raw material can be pulverized using a ball mill with beads. More specifically, the silicon raw material can be pulverized by rotating the beads together with a pulverizer. The diameter of the obtained silicon nanoparticles can then be controlled by the diameter of the beads.

[0101] According to an exemplary embodiment, the diameter of the beads can be less than twice the D99 of the silicon raw material. If the bead diameter is too large compared to the silicon raw material, the silicon pulverization time will increase, which may cause oxidation of the silicon particles.

[0102] The silicon raw material can be pulverized using a wet pulverization method with organic solvents. More specifically, a wet pulverization method using an organic solvent containing 99.9% pure ethanol can be used. As mentioned above, during the pulverization process of the silicon raw material, the silicon particles may be oxidized as their surface area increases. To prevent this side reaction, the oxidation of silicon nanoparticles can be prevented by mixing the silicon raw material in anhydrous ethanol. In this case, the solids ratio can be in the range of 8-15%. The solids during silicon pulverization can refer to the silicon raw material itself.

[0103] As a non-limiting example, in silicon pulverization, the ball-to-bead ratio (BPR) of the silicon raw material can be 5:1. The rotational speed of the pulverizer can be 2000-5000 rpm. More specifically, the rotational speed of the pulverizer can be 2000-3000 rpm.

[0104] The formation of silicon-carbon composites may include: obtaining a silicon-crystalline carbon precursor; bonding amorphous carbon; and heat treatment.

[0105] The process of obtaining silicon-crystalline carbon precursors can be achieved by mixing silicon nanoparticles with crystalline carbon particles and then drying them.

[0106] Silicon nanoparticles and crystalline carbon can be mixed in an organic solvent and provided in the form of a silicon nanoparticle slurry.

[0107] As a non-limiting example, the obtained silicon nanoparticle powder can be mixed with crystalline carbon particle powder in a separate organic solvent to provide a silicon nanoparticle slurry. Alternatively, when using a wet milling method to pulverize the silicon raw material, the crystalline carbon particles can be mixed in an organic solvent containing the pulverized silicon raw material (silicon nanoparticles) to provide a silicon nanoparticle slurry.

[0108] According to an exemplary embodiment, the D50 of crystalline carbon can be 5-10 μm. This can compensate for the insufficient electrical properties of silicon nanoparticles, thereby improving the charge-discharge reversibility of the secondary battery.

[0109] As a non-limiting example, crystalline carbon may include at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

[0110] Silicon-crystalline carbon precursor particles can be obtained by spray drying silicon nanoparticle slurry using a spray dryer.

[0111] The amorphous carbon bonding step can be achieved by mixing silicon-crystalline carbon precursor particles and amorphous carbon precursors.

[0112] The mixing of silicon-crystalline carbon precursor particles with amorphous carbon precursors can be achieved through a mechanical polishing process. As a non-limiting example, the mechanical polishing process can be carried out in any of the following ways: mechanical fusion, ball milling, and combinations thereof.

[0113] As a non-limiting example, amorphous carbon precursors may include any one of coal-based pitch, petroleum-based pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

[0114] According to an exemplary embodiment, a pressure molding step may be further included, which involves pressurizing the mixture of silicon-crystalline carbon precursor and amorphous carbon precursor. This allows for a more compact bonding between the amorphous carbon precursor and the silicon nanoparticles and crystalline nanoparticles. Consequently, by minimizing the internal porosity of the carbon matrix structure in a subsequent heat treatment process, the mixture of silicon-crystalline carbon precursor and amorphous carbon precursor can be made more dense. As a result, the electrochemical properties of the mixture of silicon-carbon composite and amorphous carbon precursor can be improved.

[0115] As a non-limiting example, the pressure applied to a mixture of silicon-crystalline carbon precursor and amorphous carbon precursor can be 1 ton / cm³. 2 the following.

[0116] As a non-limiting example, after pressure molding, the mixture of silicon-crystalline carbon precursor and amorphous carbon precursor can be provided in block form such as pellets. In this case, it can be easily transported within the preparation process, and the side reactions of surface oxidation of the mixture of silicon-crystalline carbon precursor and amorphous carbon precursor can be minimized by minimizing the surface area.

[0117] The heat treatment step can be achieved by heat-treating a mixture of silicon-crystalline carbon precursor and amorphous carbon precursor in an inert atmosphere at 700-1000°C. More specifically, the mixture of silicon-crystalline carbon precursor and amorphous carbon precursor can be heat-treated in an inert atmosphere at 700-900°C. This allows amorphous carbon to be supplied from the amorphous carbon precursor to the silicon-crystalline carbon precursor. The supplied amorphous carbon then physically and electrically links the silicon nanoparticles and the crystalline carbon, thereby obtaining a silicon-carbon composite with a structure in which silicon nanoparticles are trapped within a carbon matrix.

[0118] After heat treatment, the silicon-carbon composite can be dry-milled and classified. As a non-limiting example, dry milling can be performed using any of a jet mill, a pin mill, or a combination thereof.

[0119] According to exemplary embodiments, the surface roughness (Rq) of the silicon-carbon composite can be 4-100 nm. More specifically, the surface roughness (Rq) of the silicon-carbon composite can be 4-90 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-80 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-60 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-50 nm. The surface roughness (Rq) of the silicon-carbon composite can be 4-30 nm. The surface roughness (Rq) of the silicon-carbon composite can be 6-30 nm. The surface roughness (Rq) of the silicon-carbon composite can be 9-30 nm.

[0120] When the surface roughness (Rq) of the silicon-carbon composite is less than 4 nm, the coating may not be properly adhered to the surface of the silicon-carbon composite in the coating process described later. Therefore, a higher surface roughness (Rq) of the silicon-carbon composite is more beneficial to the coating process efficiency. However, when the surface roughness (Rq) of the silicon-carbon composite is too high, the coating cannot form uniformly and may form a thicker coating in localized areas. As a result, the resistance of the negative electrode active material may increase. Alternatively, an increase in the specific surface area of ​​the negative electrode active material may cause side reactions with the electrolyte during the use of the secondary battery.

[0121] Coating can be achieved by mixing a silicon-carbon composite and a coating precursor solution to provide a slurry, and then drying the slurry. This yields a negative electrode active material according to an exemplary embodiment.

[0122] The coating precursor solution may contain carbon nanotubes and binders.

[0123] As a non-limiting example, carbon nanotubes can be any of thin-walled carbon nanotubes, single-walled carbon nanotubes, multi-walled carbon nanotubes, and combinations thereof.

[0124] According to an exemplary embodiment, based on 100 parts by weight of silicon-carbon composite, it may contain 0.1-0.5 parts by weight of carbon nanotubes.

[0125] According to an exemplary embodiment, the adhesive may comprise a shear-thinning water-based adhesive. A shear-thinning adhesive is a non-Newtonian fluid whose viscosity decreases as the applied shear stress increases.

[0126] More specifically, the solids ratio of the coating precursor solution of the mixed silicon-carbon composite can be in the range of 50-70%. This allows the coating precursor solution to be subjected to sufficient shear stress during coating due to its relatively high viscosity. However, if the viscosity of the coating precursor solution remains too high during the coating process of the silicon-carbon composite, an uneven coating may form. In this case, the coating effect will be reduced, and the capacity per weight of the negative electrode active material may decrease. Consequently, the performance of the secondary battery may be degraded. The solids may contain the silicon-carbon composite.

[0127] Water-based adhesives exhibit excellent adhesion due to their point contact with silicon-carbon composites. Therefore, carbon nanotubes can be fixed onto silicon-carbon composites even in relatively small quantities.

[0128] Therefore, when using the adhesive according to the exemplary embodiment, the coating thickness can be made thinner, and the coating efficiency of carbon nanotubes can be improved.

[0129] According to an exemplary embodiment, the adhesive may include any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and combinations thereof.

[0130] According to an exemplary embodiment, based on 100 parts by weight of the silicon-carbon composite, the binder content can be 0.1-1 parts by weight. More specifically, the binder content can be approximately 1.5 to 3 times the content of carbon nanotubes. By controlling the binder content in the coating precursor solution within the above range, the resistance of the negative electrode active material can be prevented from increasing beyond a certain level.

[0131] According to an exemplary embodiment, the drying of the slurry can be carried out at a relatively low temperature.

[0132] Previously, a coating was formed on the surface of the negative electrode active material by heat-treating the slurry at temperatures above 800°C. However, this process can cause thermal deformation of the coating precursor material and the particles constituting the negative electrode active material, resulting in unpredictable changes in their physical, chemical, and electrical properties.

[0133] However, according to an exemplary embodiment of the present invention, by controlling the composition of the coating precursor solution as described above, slurry drying can be performed even at relatively low temperatures. This prevents changes in the properties of the negative electrode active material caused by a high-temperature atmosphere. Furthermore, energy used for heating to high temperatures can be saved, and the method for preparing the negative electrode active material according to the exemplary embodiment can be easily applied to large-scale mass production processes.

[0134] Furthermore, by drying the slurry at a relatively low temperature, the binder contained in the coating precursor solution remains on the surface of the silicon-carbon composite. Thus, as described above, the electrode conductivity and electrode adhesion of the negative electrode active material can be further improved.

[0135] According to exemplary embodiments, the drying of the slurry can be carried out in an air atmosphere at 20-100°C. More specifically, the drying of the slurry can be carried out in an air atmosphere at 20-95°C. The drying of the slurry can be carried out in an air atmosphere at 20-80°C. The drying of the slurry can be carried out in an air atmosphere at 20-70°C. The drying of the slurry can be carried out in an air atmosphere at 20-60°C. The drying of the slurry can be carried out in an air atmosphere at 30-100°C.

[0136] When the drying temperature of the slurry is below 20°C, the drying process may take too long. When the drying temperature of the slurry is above 100°C, silicon oxidation or binder carbonization or cyclization may occur, which may alter the electrical properties of the coating or negative electrode active material. Detailed Implementation

[0137] The embodiments of the present invention will now be described in detail so that those skilled in the art can readily implement them. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

[0138] (Example 1) Polycrystalline silicon (Poly-Si) was used as the silicon raw material. The silicon raw material was pulverized using a 99.9% pure anhydrous ethanol solution and beads. More specifically, the silicon raw material to bead ratio was 5:1, and the pulverizer speed was maintained at 2500 rpm. The size of the pulverized silicon raw material was measured using a nanoparticle size analyzer (Beckmann Coulter) to obtain a silicon nanoparticle slurry containing silicon nanoparticles with a D90 of less than 180 nm.

[0139] Flake graphite, acting as crystalline carbon, is mixed into a silicon nanoparticle slurry and spray-dried to obtain a silicon-crystalline carbon precursor. The silicon-crystalline carbon precursor and coal-based pitch are then mixed by grinding. The mixture of the silicon-crystalline carbon precursor and amorphous carbon is placed in a mold and uniaxially pressurized at approximately 50 tons. Subsequently, the molded body is carbonized in an inert atmosphere at 700–1000°C, followed by pulverization and classification to obtain a silicon-carbon composite.

[0140] A silicon-carbon composite was mixed in a coating precursor solution, and a binder and carbon nanotubes were coated onto the silicon-carbon composite. The coating precursor solution used contained approximately 0.125 parts by weight of carbon nanotubes and approximately 0.375 parts by weight of binder per 100 parts by weight of the silicon-carbon composite. Carboxymethyl cellulose was used as the binder. Single-walled carbon nanotubes were used as the carbon nanotubes. The coating process involved mixing at 25 rpm for 5 minutes using a twisted blade mixer, followed by mixing at 100 rpm for 30 minutes. Finally, the coating precursor solution containing the silicon-carbon composite was dried in air at 100°C to obtain the negative electrode active material.

[0141] (Example 2) The negative electrode active material was prepared by the same method as in Example 1, except that a coating precursor solution containing 0.25 parts by weight of carbon nanotubes and 0.375 parts by weight of binder was used as a basis for 100 parts by weight of silicon-carbon composite.

[0142] (Example 3) The negative electrode active material was prepared by the same method as in Example 1, except that a coating precursor solution containing 0.5 parts by weight of carbon nanotubes and 0.75 parts by weight of binder was used as a basis for 100 parts by weight of silicon-carbon composite.

[0143] (Comparative Example 1) The preparation was carried out in the same manner as in Example 1, except that the silicon-carbon composite was coated with coal tar and then heat-treated in an inert atmosphere at a temperature above 800°C.

[0144] (Comparative Example 2) Without coating, the silicon-carbon composite was used as the negative electrode active material.

[0145] Experimental Example 1: Confirming the degree of coating based on surface roughness The surface roughness (Rq) of the silicon-carbon composite according to Example 2 was measured three times using atomic force microscopy (AFM), and the results are shown in Table 1 below. Furthermore, as Comparative Example 3, a silicon-carbon composite having the surface roughness (Rq) shown in Table 1 below was prepared.

[0146] [Table 1] Subsequently, the silicon-carbon composite according to Example 2 and the silicon-carbon composite according to Comparative Example 3 were coated under the same coating conditions as described in Example 2.

[0147] After coating, the final sample was obtained by sieving, and then the surface of the negative electrode active material according to Example 2 and the negative electrode active material according to Example 3 was observed using a scanning electron microscope (SEM).

[0148] Figure 1 The above refers to SEM images of the surface of the negative electrode active material according to Example 2 and Comparative Example 3.

[0149] Reference Figure 1 It can be confirmed that carbon nanotubes were not properly coated on the surface of Comparative Example 3. This is determined because the surface roughness of Comparative Example 3 did not meet the range proposed by the present invention. In contrast, it can be confirmed that carbon nanotubes and residual binder are distributed on the negative electrode active material according to Example 2. This is determined because the surface roughness of the silicon-carbon composite according to Example 2 meets the range of the exemplary embodiments of the present invention.

[0150] Experimental Example 2: Confirmation of the water content of the negative electrode active material The negative electrode active material according to Example 2 and the negative electrode active material according to Comparative Example 1 were dried in a drying oven at 120°C. The moisture content was then calculated by comparing the mass difference before and after drying, and is shown in Table 2 below.

[0151] The moisture content is calculated as follows.

[0152] [Relation 1] Moisture content = (Weight before drying - Weight after drying) / Weight before drying [Table 2] As described above, it can be confirmed that the moisture content of Comparative Example 1, which underwent carbonization treatment in a high-temperature atmosphere, was outside the range proposed by the present invention. This can be interpreted as the adhesive residue on the surface of the silicon-carbon composite, based on Comparative Example 1, not being properly distributed. However, it was confirmed that Example 2, which underwent drying treatment at a low temperature, met the moisture content range proposed by the present invention. This can be interpreted as the residual adhesive being properly distributed on the surface of the silicon-carbon composite.

[0153] Experiment Example 3: Confirmation of the Lifetime Characteristics of Secondary Batteries Secondary batteries were manufactured using Examples 1 to 3, Comparative Example 1, and Comparative Example 2 described above.

[0154] More specifically, an active material, a negative electrode binder, and a conductive material are mixed to prepare a negative electrode slurry. As the active material, a mixture of the negative electrode active material according to Example 1 and commercially available natural graphite is used. As the negative electrode binder, carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) are used. The prepared negative electrode slurry is coated onto a copper thin film and dried to obtain a negative electrode. Subsequently, a negative electrode, a separator, and a positive electrode are sequentially stacked, and the stacked electrodes are immersed in an electrolyte to manufacture a CR2032 type secondary battery (coin half cell).

[0155] Using the above method, CR2032 type coin-shaped half-cells were manufactured using the negative electrode active materials according to Examples 2, 3, Comparative Example 1 and Comparative Example 2.

[0156] Subsequently, charge-discharge tests were conducted on the manufactured secondary batteries within an operating voltage range of 0.005V to 1.0V. The charge / discharge current was based on the initial 1C capacity, with a 0.5C current applied, and the cycle life was measured for 50 cycles. The cut-off current was set to 0.005C. In each cycle (charge-discharge), the charge and discharge capacity of the secondary battery were measured to confirm the charge-discharge efficiency.

[0157] Figure 2 This is a graph illustrating the lifespan characteristics of the secondary batteries according to the embodiments and comparative examples. Figure 2 In the diagrams, Example 1 shows data for a secondary battery containing the negative electrode active material according to Example 1. Example 2 shows data for a secondary battery containing the negative electrode active material according to Example 2. Example 3 shows data for a secondary battery containing the negative electrode active material according to Example 3. Comparative Example 1 shows data for a secondary battery containing the negative electrode active material according to Comparative Example 1. Comparative Example 2 shows data for a secondary battery containing the negative electrode active material according to Comparative Example 2.

[0158] Reference Figure 2 It can be confirmed that the secondary batteries containing the negative electrode active materials according to Examples 1, 2, and 3 maintain their charge-discharge efficiency even after repeated charge-discharge cycles. That is, it can be confirmed that using negative electrode active materials containing silicon-carbon composites and coatings containing carbon nanotubes and residual binders can improve the lifespan characteristics of the secondary batteries. In contrast, the secondary batteries containing the negative electrode active materials according to Comparative Examples 1 and 2 show a decrease in charge-discharge capacity starting from the initial stage of cycling (less than 10 cycles), thus confirming that they cannot maintain their charge-discharge capacity. That is, their lifespan characteristics are poor. In particular, Comparative Example 2, which underwent heat treatment at a high temperature, exhibited the worst lifespan characteristics.

[0159] The present invention has been described in more detail above with reference to the accompanying drawings and embodiments. However, the configurations described in the drawings or embodiments in this specification are merely one embodiment of the present invention and do not represent all the technical ideas of the present invention. Therefore, it should be understood that at the time of this application, there are various equivalents and modifications that can replace them.

Claims

1. A silicon-carbon composite, which is a silicon-carbon composite for use as a negative electrode active material, comprising a carbon matrix and silicon nanoparticles trapped in said carbon matrix. in, The surface roughness (Rq) of the silicon-carbon composite is 4-50 nm.

2. The silicon-carbon composite according to claim 1, wherein, The silicon nanoparticles have a D90 of less than 180 nm.

3. The silicon-carbon composite according to claim 1, wherein, The silicon-carbon composite contains 35-60% by weight of the silicon nanoparticles.

4. The silicon-carbon composite according to claim 1, wherein, The silicon-carbon composite contains 35-55% by weight of the carbon matrix.

5. The silicon-carbon composite according to claim 1, wherein, The carbon matrix comprises crystalline carbon and amorphous carbon.

6. The silicon-carbon composite according to claim 5, wherein, The content of crystalline carbon is 10-25% by weight, and the content of amorphous carbon is 25-40% by weight.

7. The silicon-carbon composite according to claim 5, wherein, The crystalline carbon includes at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, graphene, and combinations thereof.

8. The silicon-carbon composite according to claim 5, wherein, The D50 of the crystalline carbon is 5-10 μm.

9. The silicon-carbon composite according to claim 5, wherein, The amorphous carbon includes any one of coal-based pitch, petroleum-based pitch, coal tar, polyacrylic acid (PAA), polyvinyl alcohol (PVA), and combinations thereof.

10. A negative electrode active material, comprising: A silicon-carbon composite comprising silicon nanoparticles and a carbon matrix; and A coating, the coating being distributed on the surface of the silicon-carbon composite, in, The coating comprises carbon nanotubes and residual binder distributed on the surface of the silicon-carbon composite.

11. The negative electrode active material according to claim 10, wherein, Based on 100 parts by weight of the silicon-carbon composite, the negative electrode active material contains 0.1-0.5 parts by weight of the carbon nanotubes.

12. The negative electrode active material according to claim 10, wherein, Based on 100 parts by weight of the silicon-carbon composite, the negative electrode active material contains 0.1-1 parts by weight of the residual binder.

13. The negative electrode active material according to claim 10, wherein, The residual adhesive includes any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and combinations thereof.

14. The negative electrode active material according to claim 10, wherein, The water content of the negative electrode active material is 0.6-2.0%.

15. A method for preparing a negative electrode active material, comprising: The step of crushing silicon raw materials to prepare silicon nanoparticles; The step of mixing and drying the silicon nanoparticles and crystalline carbon to obtain a silicon-crystalline carbon precursor; The step of combining the silicon-crystalline carbon precursor with the amorphous carbon precursor to obtain a mixture; The step of heat-treating the mixture in an inert atmosphere at 700-1000°C to obtain a silicon-carbon composite. The step of mixing a coating precursor solution containing carbon nanotubes and a binder with the silicon-carbon composite to obtain a slurry; as well as The drying step involves drying the slurry in an air atmosphere at 20-100°C.

16. The method for preparing the negative electrode active material according to claim 15, wherein, In the step of obtaining the silicon nanoparticles, the silicon raw material is pulverized by ball milling using beads, the diameter of which is less than twice the D99 of the silicon raw material.

17. The method for preparing the negative electrode active material according to claim 15, wherein, In the step of obtaining the silicon nanoparticles, the silicon raw material is pulverized by wet pulverization using an organic solvent containing ethanol with a purity of 99.9%.

18. The method for preparing the negative electrode active material according to claim 15, wherein, In the step of obtaining the silicon nanoparticles, the D90 of the silicon nanoparticles is below 180 nm.

19. The method for preparing the negative electrode active material according to claim 15, wherein, The D50 of the crystalline carbon is 5-10 μm.

20. The method for preparing the negative electrode active material according to claim 15, wherein, The surface roughness (Rq) of the silicon-carbon composite is 4-50 nm.

21. The method for preparing the negative electrode active material according to claim 15, wherein, The method for preparing the negative electrode active material further includes a pressure molding step of pressurizing the mixture of the silicon-crystalline carbon precursor and the amorphous carbon precursor.

22. The method for preparing the negative electrode active material according to claim 15, wherein, The adhesive includes any one of carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and combinations thereof.

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