Structure of silicon-graphene composite and negative electrode active material for lithium secondary batteries containing the same

Abstract

The object of the present invention is to provide a dispersion method capable of dispersing a silicon-graphite fusion precursor in order to produce an optimal negative electrode active material for improving the charge / discharge capacity and initial efficiency of lithium secondary batteries and achieving optimal cycle characteristics, and to provide a silicon-graphene composite structure capable of producing an optimal negative electrode active material. [Solution] One embodiment of the present invention provides a method for dispersing silicon-graphene composites and silicon-graphite fusion precursors, characterized in that a cross-section including the center has a plurality of pair layers arranged in a unit circle, and the plurality of such unit circles are arranged concentrically, and the pair layers comprise a graphene layer and a particle layer containing a plurality of nanosilicon particles uniformly located on the graphene layer, wherein, according to the embodiment of the present invention, the silicon particles are uniformly dispersed on the graphite surface. The negative electrode active material and lithium secondary battery produced thereby can exhibit optimal charge / discharge capacity, initial efficiency, and cycle characteristics.

Description

Technical Field

[0001] The present invention relates to the structure of a silicon-graphene composite and a negative electrode active material for a lithium secondary battery containing the same. More specifically, a method for dispersing a silicon-graphite fusion precursor to form a state in which silicon particles are uniformly dispersed on graphite, optimizing the structure of the silicon-graphene composite into a concentric repeated array structure, and improving the charge-discharge capacity, initial efficiency, and cycle characteristics optimal as a negative electrode active material for a lithium secondary battery.

Background Art

[0002] Following the spread of portable electronic devices and the expansion of the electric vehicle market, research on rechargeable secondary batteries has been actively conducted. Along with conventional nickel-cadmium batteries, nickel-metal hydride batteries, nickel-hydrogen batteries, and lithium secondary batteries, the development of materials with improved stability, capacity, energy density, etc. compared to conventional secondary batteries has been accelerated.

[0003] Among various types of secondary batteries, a lithium secondary battery has a configuration having a positive electrode formed with a positive electrode active material layer, a negative electrode formed with a negative electrode active material layer, and a separator that is electrically insulated between the positive electrode and the negative electrode. If conductive agent particles are not sufficiently dispersed in the electrode active material slurry, the ionic conductivity of the electrode active material layer cannot be maintained uniformly.

[0004] However, when using a normal dispersion method, due to differences such as the difference in the size of electrode active material particles and the size of conductive agent particles, and specific gravity, it is difficult for the conductive agent particles to be uniformly dispersed in the slurry, which electrochemically reduces the charge-discharge characteristics, and stability, long-term reliability, etc. also decrease accordingly.

[0005] In the case of existing plasma-treated silicon-graphite fusion precursors, even if silicon particles are well dispersed on the graphite, some heterogeneity exists, and the particle morphology, where silicon particles exist on top of expanded graphite, has areas that are not easily sphericalized. Through my own research, I have revealed that there are problems with directly applying these precursors as negative electrode active materials.

[0006] While it was necessary to improve uniformity through dispersion, when dispersing silicon-graphite fusion precursors using a dry method, the exfoliation state of the graphite (plate-like graphite / graphene) is not uniform, making it difficult to control thickness and particle size. Some aggregation of silicon particles occurs, and when evaluated electrochemically, this results in a decrease in charge-discharge characteristics and initial efficiency, as well as insufficient long-term reliability. [Overview of the project] [Problems that the invention aims to solve]

[0007] The technical problem that this invention aims to solve is to provide a dispersion method capable of dispersing a silicon-graphite fusion precursor in order to produce an optimal negative electrode active material for improving the charge-discharge capacity and initial efficiency of lithium secondary batteries and realizing optimal cycle characteristics, and to provide a silicon-graphene composite structure that can produce an optimal negative electrode active material.

[0008] The technical problems that this invention aims to solve are not limited to those mentioned above, and other technical problems not mentioned should be clearly understood by a person with ordinary skill in the art to which this invention pertains from the following description. [Means for solving the problem]

[0009] To achieve the aforementioned technical objectives, one embodiment of the present invention provides a silicon-graphene composite characterized in that a cross-section including the center has a plurality of paired layers arranged in a unit circle, and the plurality of such unit circles are arranged concentrically, and the paired layers comprise a graphene layer and a particle layer containing a plurality of nanosilicon particles uniformly arranged on the graphene layer.

[0010] In embodiments of the present invention, the pair layers may be characterized by being circular in a connected or disconnected state.

[0011] In embodiments of the present invention, the thickness of one pair of layers can be characterized as being 50 nm to 500 nm.

[0012] In embodiments of the present invention, the thickness of the graphene layer can be characterized as being 1 nm to 300 nm.

[0013] To achieve the aforementioned technical objectives, one embodiment of the present invention provides a method for dispersing a silicon-graphite fusion precursor, comprising the steps of: including a silicon-graphite fusion precursor; plasma treating the silicon-graphite fusion precursor; and dispersing the plasma-treated silicon-graphite fusion precursor in a wet grinding step to form a silicon-graphene composite.

[0014] In embodiments of the present invention, the silicon-graphite fusion precursor may be characterized by having a structure in which a plurality of silicon particles are arranged on the upper and lower parts of a plate-shaped graphite.

[0015] In embodiments of the present invention, the silicon content is greater than 0 and less than or equal to 70% by wt%, but the silicon content can be characterized by being a composite obtained by applying the fusion precursor alone, applying a mixture of the fusion precursor and silicon, or applying a mixture of expanded graphite and silicon.

[0016] In embodiments of the present invention, the step of plasma treatment of the silicon-graphite fusion precursor is characterized by performing DC plasma treatment on the Si-expanded graphite mixed powder, which is the silicon-graphite fusion precursor, by purging it with nitrogen (N2) at a power output of 20 kW.

[0017] In embodiments of the present invention, the solvent used in the wet grinding process may be characterized by having a boiling point of 60°C to 160°C.

[0018] In embodiments of the present invention, the solvent used in the wet grinding step can be "any solvent".

[0019] In embodiments of the present invention, the solvent used in the wet grinding process may be characterized by having a Hansen solubility parameter (∂) of 17 MPa to 23 MPa 1 / 2 or 45 MPa to 50 MPa 1 / 2.

[0020] In one embodiment of the present invention, the wet grinding step may be characterized by adding a dispersant or coupling agent that can chemically activate the fusion precursor.

[0021] In one embodiment of the present invention, the wet grinding step is characterized by homogenizing the fusion precursor by adding shear stress and cavitation.

[0022] In embodiments of the present invention, the silicon-graphene composite may be characterized by a structure in which a plurality of silicon particles are arranged on the surface of a single exfoliated graphene.

[0023] In embodiments of the present invention, the silicon particles may be characterized in that they are uniformly distributed on the upper and lower parts of the single graphene.

[0024] To achieve the above technical problem, another embodiment of the present invention includes a step of including a silicon-graphite fusion precursor, a step of subjecting the silicon-graphite fusion precursor to plasma treatment, a step of dispersing the plasma-treated silicon-graphite fusion precursor in a wet grinding process to form a silicon-graphene composite in a exfoliated form, a step of binder-coating the exfoliated form of the silicon-graphene composite, and a step of granulating the binder-coated exfoliated form of the silicon-graphene composite to produce a granulated silicon-graphene composite, and provides a method for manufacturing a silicon-graphene composite.

[0025] In an embodiment of the present invention, it can be characterized in that the thickness of the graphene layer is 1 nm to 300 nm.

[0026] To achieve the above technical problem, another embodiment of the present invention provides a negative electrode active material for a lithium secondary battery including the silicon-graphene composite.

Advantages of the Invention

[0027] According to an embodiment of the present invention, in the structure of the silicon-graphene composite, it can be optimized to a structure in which the graphene layer and the silicon particle layer are repeatedly arranged concentrically.

[0028] According to an embodiment of the present invention, silicon particles can be uniformly dispersed on the graphite surface through the dispersion of the silicon-graphite fusion precursor.

[0029] The negative electrode active material and the lithium secondary battery manufactured thereby can exhibit optimal charge-discharge capacity, initial efficiency, and cycle characteristics.

[0030] The effects of the present invention are not limited to the above effects, and it should be understood to include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention. [Brief explanation of the drawing]

[0031] [Figure 1] This is a schematic diagram showing a cross-section of a silicon-graphene composite. [Figure 2] This is an SEM image of a silicon-graphene composite cross-section with improper dispersion. [Figure 3] This is an SEM image of a silicon-graphene composite cross-section with improper dispersion. [Figure 4] These are SEM and EDS mapping images of the most dispersed silicon-graphene composite cross-section. [Figure 5] These are SEM and EDS mapping images of a cross-section of a highly dispersed silicon-graphene composite. [Figure 6] These are SEM and EDS mapping images of a cross-section of a low-dispersion silicon-graphene composite. [Figure 7] These are SEM and EDS mapping images of a cross-section of a low-dispersion silicon-graphene composite. [Figure 8] This is an image of the electrode surface of a mixed electrode manufactured using a highly dispersed silicon-graphene composite after approximately 40 cycles. [Figure 9] This is an image of the electrode surface of a mixed electrode manufactured using a low-dispersion silicon-graphene composite after approximately 40 cycles. [Figure 10] This graph shows the change in cycle capacity of mixed electrodes manufactured using silicon-graphene composites with different dispersion levels. (Mixed electrode capacity: 450 mAh / g) [Figure 11] This graph shows the cycle capacity retention rate of mixed electrodes manufactured using silicon-graphene composites with varying degrees of dispersion. [Figure 12] This is a graph of the cycle Coulomb efficiency of mixed electrodes manufactured using silicon-graphene composites with different degrees of dispersion. [Figure 13]This is a schematic diagram showing the state of the silicon-graphite fusion precursor before and after dispersion. [Figure 14] This is an SEM image of a silicon-graphite fusion precursor. [Figure 15] (a) SEM image of silicon particles aggregated on the graphene surface instead of being uniformly dispersed, (b) EDS mapping images of silicon and (c) graphene. [Figure 16] (a) SEM image of silicon particles aggregated on the graphene surface instead of being uniformly dispersed, (b) EDS mapping images of silicon and (c) graphene. [Figure 17] (a) SEM image of silicon particles uniformly dispersed on a graphene surface, (b) EDS mapping images of silicon and (c) graphene. [Figure 18] Figures 15 and 16 show cross-sectional views of the negative electrode active material fabricated using the silicon-graphene composite. [Figure 19] Figure 17 shows a cross-sectional view of the negative electrode active material fabricated using the silicon-graphene composite. [Figure 20] This flowchart shows the steps from the plasma treatment stage to the final physical property evaluation. [Figure 21] This graph shows the electrochemical evaluation results of the comparative example. [Figure 22] This graph shows the electrochemical evaluation results for manufacturing example 1. [Figure 23] This graph shows the electrochemical evaluation results for manufacturing example 2. [Figure 24] This graph shows the cycle retention for the comparative example, manufacturing example 1, and manufacturing example 2. [Figure 25] This is the result of observing the surface condition after cleaning the cathode electrode surface with DMC solution from the charged state after cycle retention. [Figure 26]This is the result of observing the surface condition after cleaning the cathode electrode surface with DMC solution from the charged state after cycle retention. [Modes for carrying out the invention]

[0032] The present invention will be described below with reference to the accompanying drawings. However, the present invention may be carried out in several different forms and is therefore not limited to the embodiments described herein. In order to clearly illustrate the present invention, parts unrelated to the description have been omitted from the drawings, and similar parts are denoted by similar reference numerals throughout the specification.

[0033] Throughout this specification, when a part is described as being "connected (connected, in contact with, joined)" to another part, this includes not only cases where it is "directly connected" but also cases where it is "indirectly connected" with another member in between. Furthermore, when a part is described as "containing" a certain component, this means that, unless otherwise stated, it may further contain other components rather than excluding them.

[0034] The terms used herein are used solely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as “includes” or “having” are intended to specify the existence of features, figures, stages, operations, components, parts, or combinations thereof described in the specification, and should not be understood to preemptively exclude the possibility of the existence or addition of one or more other features, figures, stages, operations, components, parts, or combinations thereof.

[0035] Embodiments of the present invention will be described in detail below with reference to the attached drawings.

[0036] "High degree of dispersion" means that dispersion is effectively and well performed, and that silicon particles are evenly or uniformly distributed on the graphene layer.

[0037] "Low dispersion" means that dispersion is not effectively achieved, and the silicon particles are not evenly or uniformly distributed on the graphene layer, but are partially aggregated.

[0038] A "silicon-graphene composite" refers to a composite formed when graphene, exfoliated from graphite, fuses with silicon particles to form multiple layers.

[0039] Figure 1 is a schematic diagram showing a cross-section of a silicon-graphene composite.

[0040] The structure of the silicon-graphene composite according to an embodiment of the present invention will be described with reference to Figure 1.

[0041] A silicon-graphene composite according to one embodiment of the present invention is characterized in that, in a cross-section including the center, a plurality of paired layers are arranged in a unit circle, and the plurality of unit circles are arranged concentrically, and the paired layers comprise a graphene layer and a particle layer containing a plurality of nanosilicon particles uniformly arranged on the graphene layer.

[0042] When silicon particles are uniformly distributed on the graphene surface and dispersion is improved, the negative electrode active material and secondary battery produced therefrom can exhibit high charge-discharge characteristics and stable cycle characteristics.

[0043] If silicon particles are not uniformly dispersed, pores may be observed within the composite, and the aggregation of such silicon particles or the presence of pores within the composite can reduce the electrical capacity and initial efficiency during the electrochemical evaluation of the secondary battery.

[0044] The paired layers can form a circle in a connected or disconnected state. A single graphene layer may not be a connected circle, but can form a circle in several disconnected states.

[0045] The thickness of one pair layer may be 50 nm to 500 nm. This may be equivalent to having 2 to 20 pair layers per μm, but is not limited to this. The thickness of the pair layer may be 50 nm to 500 nm, but is not limited to this.

[0046] Figure 13 is a schematic diagram showing the state of the silicon-graphite fusion precursor before and after dispersion.

[0047] Referring to Figure 13, a method for dispersing a silicon-graphite fusion precursor according to an embodiment of the present invention will be described.

[0048] A method for dispersing a silicon-graphite fusion precursor according to one embodiment of the present invention comprises the steps of: including a silicon-graphite fusion precursor; plasma treating the silicon-graphite fusion precursor; and dispersing the plasma-treated silicon-graphite fusion precursor in a wet grinding step to form a silicon-graphene composite.

[0049] First, there is a step that includes a silicon-graphite fusion precursor.

[0050] For silicon, we can use either μm-class amorphous Si or submicron-class silicon sludge, which is a by-product of solar cell ingot manufacturing. For graphite, we used expanded graphite obtained by intercalating flaked graphite with a strong acid. A Si-expanded graphite mixed powder is produced by mixing Si powder (over 0% but less than 70 wt%) and expanded graphite (99.9-30 wt%) in a mixer.

[0051] The silicone content is greater than 0 and less than or equal to 70 wt%, and the silicone content can be characterized in cases where the fusion precursor is applied alone, the fusion precursor and silicon are applied together, or a composite is applied together with expanded graphite and silicon. Here, if the Si is mixed in at a level higher than 70 wt%, instead of condensing into nanosilicon due to excess Si, a large number of nano-Si inter-grain growths occur in the fusion precursor, and the possibility of forming μm-class Si becomes very high.

[0052] The next step is to plasma-treat the silicon-graphite fusion precursor.

[0053] In the step of plasma treatment of the silicon-graphite fusion precursor, the previously prepared silicon-graphite fusion precursor, a Si-expanded graphite mixed powder, is purged with N2 at 20 kW and treated with DC plasma to generate vaporization of Si. At the same time, as the expanded graphite expands, nano-sized Si is condensed between the layers of expanded graphite, and finally, a fusion precursor in which plasma-treated silicon and graphite are bonded can be obtained.

[0054] The silicon-graphite fusion precursor can be characterized by a structure in which multiple silicon particles are arranged on the upper and lower parts of a plate-shaped graphite. This may be a state in which nanosilicon particles are unevenly arranged between the expanded graphite.

[0055] The next step is to disperse the plasma-treated silicon-graphite fusion precursor in a grinding process to form a silicon-graphene composite in a detached form.

[0056] The aforementioned grinding process can be characterized as a wet grinding process. When a silicon-graphite fusion precursor is dispersed in a conventional dry grinding process, the exfoliation state of the graphite (plate-like graphite / graphene) is not uniform, making it difficult to control the thickness and particle size, and causing aggregation of silicon particles. When electrochemically evaluated, this can lead to a decrease in charge-discharge characteristics and initial efficiency, as well as insufficient long-term reliability.

[0057] The solvent used in the wet grinding process may be a single component from the aqueous and organic solvent systems, or a combination of miscible solvents. These include alcohol systems such as water, ethanol, isopropanol, N-butanol-amyl alcohol, and cyclohexanol; ester systems such as acetone, MEK, MIBK, ketones like cyclohexanone, ethyl acetate, isopropanol acetate, N-butyl acetate, and amyl acetate; hydrocarbon systems such as mineral spirit, heptane, cyclohexane, toluene, and xylene; and glycol ether acetates such as butyl cellosolve, ethyl cellosolve, acetate, butyl carbitol, and butyl carbitol acetate. Solvents used include acetate-based solvents, halogenated hydrocarbon-based solvents such as 1,1,1-trichloroethane (1,1,1-TCE), TCE, and EDC (1,2-dichloroethane), furan-based solvents such as tetrahydrofuran, and lactam-based solvents such as NMP (N-Methyl-2-pyrrolidone).

[0058] The optimal solvent conditions are preferably a boiling point between 60 and 160°C, and a Hansen solubility parameter (∂) of 17 to 23 MPa. 1 / 2 Between 45 and 50 MPa 1 / 2 A range between 18 and 22 MPa is preferred. More preferably, 18 to 22 MPa is preferred. 1 / 2 Between 47 and 48 MPa 1 / 2 It could be between these two points.

[0059] Suitable solvents that satisfy this requirement include acetaldoxime, acetetic acid, acetetic anhydride, acetonecyanhydrin, N-acetyl caprolactam, acetylacetone, acetylbromide, allyl acetate, arryl acetoacetate, allyl alcohol, amyl acetate, Benzene, N-benzyl pyrrolidone, 4-bromo-1-butene, 1-butanethiol, 2-buanol, 1-butene, carbon tetrachloride, chloroacetaldehyde, cyclohexanone, cyclohexanol, 2-chloro allyl alcohol, 4-chloro-1, 2-butadiene, 1-chloro-2-butene, ethanool, isoamyl acetate, methyl ethyl ketone, isoamyl alcohol, xylene, tetrahydrofuran, toluene, and water. It is preferable to use these solvents individually or in combination.

[0060] The Hansen Solubility Parameter, developed by Charles M. Hansen in 1966, is a solvent-specific value that can be composed of three parameters: dispersion force, hydrogen bonding, and terminal polarity. The Total Solubility Parameter can then be calculated using the following formula.

[0061] HANSEN SOLUBILITY PARAMETERS(HSP) d 2 =dD 2 +dP 2 +dH 2 d = square root of cohesive energy density δD: Energy derived from dispersion forces δP: Energy derived from polarity δH: Energy derived from hydrogen bonding force

[0062] The wet grinding step can be characterized by chemically activating the fusion precursor obtained from the plasma.

[0063] Titanate coupling agents such as isopropyl di(dioctylphosphite) titanate, tetraoctyl bis(ditridecylphosphite)titanate, and isopropyl triisostearoyl titanate, which can be dispersed with inorganic material on the surface for activation; silane coupling agents such as vinyltrimethoxysilane, 3-Aminopropyltriethoxysilane, and 3-(Trimethoxysilyl)propylsuccinic anhydride; fatty acids such as stearic acid, palmitic acid, and oleic acid; acrylate copolymers; copolymers having pigment affinity groups; sodium alkylnaphthalene sulfonate, sodium polyacrylate, olefin-sodium maleate copolymer, carboxymethylcellulose, alkylbenzene (naphthalene) sulfonate, fatty acid amides, polyoxyethylene alkylamines, alkylamines (acetates, fatty acid salts), alkyl secondary (tertiary) amines (amides), and alkylimidazolines; Emulsifiers such as xanthan gum, sugar fatty acid esters, glycerin fatty acid esters, propylene glycol fatty acid esters, polyvinylphyridone, and polyethylene-polypropylene glycol, as well as polymeric dispersants such as alkylphenols, fatty acids, and higher fatty acid amines, can be added to activate the inorganic Si / Graphite so that it can be dispersed in the solvent.

[0064] The aforementioned wet grinding process can be characterized by homogenizing the fusion precursor using a device that applies shear stress and cavitation to the activated slurry.

[0065] A silicon-graphene composite is manufactured by a method comprising the steps of: including a silicon-graphite fusion precursor; plasma treating the silicon-graphite fusion precursor; dispersing the plasma-treated silicon-graphite fusion precursor in a wet grinding step to form a silicon-graphene composite in a detached form; binder coating the detached silicon-graphene composite; and granulating the binder-coated and detached silicon-graphene composite to produce a granulated silicon-graphene composite.

[0066] The steps of including a silicon-graphite fusion precursor, plasma-treating the silicon-graphite fusion precursor, and dispersing the plasma-treated silicon-graphite fusion precursor in a pulverization step to form a silicon-graphene composite in an exfoliated form are the same as described above.

[0067] The next step is to binder-coat the exfoliated silicon-graphene composite. Binder coating is a coating process for granulation. The binder is a resin dissolved in a solvent and is used to coat and granulate the exfoliated particles. Thermoplastic resins, thermosetting resins, pitch, and hydrocarbon resins are used as the binding resin. Thermoplastic binders may consist of one or more mixtures of acrylic, ethyl cellulose, polyester, polysulfone, phenoxy, and polyamide, while thermosetting resin binders may consist of one or more mixtures of amino, epoxy, and phenol. For pitch, coal-based or petroleum-based pitch can be used. In this embodiment, coal tar pitch was added at a ratio of 10 wt% to the composite and mixed using a mixer.

[0068] The final step involves granulating the binder-coated, exfoliated silicon-graphene composite to produce the granulated silicon-graphene composite. Equipment is used to obtain the desired rotational force to sphere the binder-coated particles, and sphere formation is possible by controlling the working RPM and process time of equipment such as ball mills, attrition mills, paste mixers, and ultrafine grinders with adjustable rotational force, or mechano fusions that can solidify particles with rotational force.

[0069] The following describes negative electrode active materials for lithium secondary batteries according to other embodiments of the present invention.

[0070] An anode active material for a lithium secondary battery according to one embodiment of the present invention may include the silicon-graphene composite.

[0071] The negative electrode active material, consisting of the silicon-graphene composite, exhibits a uniform distribution without pores or aggregation through dispersion, enabling the achievement of optimal charge / discharge capacity and initial efficiency in lithium secondary batteries, and realizing excellent cycle characteristics.

[0072] The following sections provide detailed descriptions of manufacturing and experimental examples.

[0073] <Manufacturing Examples and Comparative Examples>

[0074] Dispersion is carried out by selecting from a high shear mixer, ball mill, attrition mill, high pressure homogenizer, three roll mill, basket mill, apex mill, paste mixer, planetary mixer, spike mill, and ultrasonic. In this invention, primary dispersion is carried out using a basket mill at 3000 rpm for 2 hours with 0.4 mm zirconia balls, and then graphite and silicon are further detached using a 4000 watt class ultrasonic disperser. Subsequently, a greater degree of detachment can be obtained by adding equipment that can add high shear.

[0075] Table 1 shows the process conditions for existing comparative examples and manufacturing examples 1 and 2 based on the manufacturing process. A pigment affinity copolymer dispersant was used for plasma treatment and activation, with a boiling point between 60 and 160°C, and coin half cells were manufactured with differences in the Hansen solubility parameter.

[0076] [Table 1]

[0077] <Experimental Example 1> Cross-section of a silicon-graphene composite obtained by dispersion

[0078] Figures 2 and 3 are SEM images of silicon-graphene composite cross-sections where dispersion was unsuccessful.

[0079] Referring to Figures 2 and 3, aggregated silicon particles are observed on the graphene layer, as well as graphene layers without silicon particles, confirming the presence of large pores within the composite. Furthermore, it can be confirmed that the distribution of silicon particles is non-uniform.

[0080] Figure 4 shows SEM and EDS mapping images of the most dispersed silicon-graphene composite cross-section.

[0081] The SEM image in Figure 4a shows a uniformly distributed silicon particle layer on the graphene layer, most similar to the schematic diagram in Figure 1, confirming that it is arranged in a repeating concentric circular structure.

[0082] This can also be confirmed through the EDS mapping images in Figures 4b to 4d.

[0083] Figure 5 shows SEM and EDS mapping images of a highly dispersed silicon-graphene composite cross-section.

[0084] As seen in the SEM image in Figure 5a, similar to the schematic diagram in Figure 1, a uniformly distributed layer of silicon particles is observed on the graphene layer, confirming that they are repeatedly arranged in a concentric circular structure.

[0085] This can also be confirmed through the EDS mapping images in Figures 5b to 5d.

[0086] Figures 6 and 7 show SEM and EDS mapping images of a low-dispersion silicon-graphene composite cross-section.

[0087] As seen in the SEM images of Figures 6a and 7a, unlike the schematic diagram in Figure 1, the silicon particle layer is not uniformly distributed on the graphene layer, but rather partially aggregated, and large pores are also observed within it, making it impossible to confirm that it is arranged in a repeating concentric circular structure.

[0088] This can also be confirmed through the EDS mapping images in Figures 6b-6d and 7b-7d.

[0089] <Experimental Example 2> Electrode surface before and after cycle by dispersion

[0090] Figure 8 shows an image of the electrode surface of a mixed electrode manufactured using a highly dispersed silicon-graphene composite after approximately 40 cycles.

[0091] Figure 9 shows an image of the electrode surface of a mixed electrode fabricated using a low-dispersion silicon-graphene composite after approximately 40 cycles.

[0092] Referring to Figure 8, it is possible to observe some particle cracking by comparing the electrode surface 40 cycles before (Figure 8a) with the electrode surface 40 cycles after (Figure 8b).

[0093] However, referring to Figure 9, we can observe numerous particle cracks by comparing the electrode surface 40 cycles before (Figure 9a) with the electrode surface 40 cycles after (Figure 9b).

[0094] Figure 10 shows a graph of the change in cycle capacity of mixed electrodes manufactured using silicon-graphene composites with different degrees of dispersion. (Mixed electrode capacity: 450 mAh / g)

[0095] Figure 11 is a graph showing the cycle capacity retention rate of mixed electrodes manufactured using silicon-graphene composites with different degrees of dispersion.

[0096] Figure 12 shows the cycle Coulomb efficiency graph of mixed electrodes manufactured using silicon-graphene composites with different degrees of dispersion.

[0097] Referring to Figures 10 and 11, it can be seen that the cycle capacity change and capacity retention rate are improved when using a silicon-graphene composite with a high degree of dispersion compared to a silicon-graphene composite with a low degree of dispersion.

[0098] Referring to Figure 12, we can observe the change in Coulomb efficiency for each cycle. It can be seen that when a mixed electrode is manufactured using a silicon-graphene composite with low dispersibility, the cycle Coulomb efficiency is low, while when a mixed electrode is manufactured using a silicon-graphene composite with high dispersibility, the cycle Coulomb efficiency is improved.

[0099] This confirms that the cycle characteristics of lithium secondary batteries are improved through a silicon-graphene composite negative electrode active material with a high degree of dispersion.

[0100] <Experimental Example 3> Electrode expansion rate before and after cycling

[0101] Table 2 shows the electrode expansion rate before and after cycling due to the structure of the silicon-graphene composite. The electrode expansion rate was calculated by the difference between the electrode thickness at cycle 40 and the electrode thickness before cycling, relative to the electrode thickness before cycling.

[0102] [Table 2]

[0103] When the degree of dispersion is high, nanosilicon particles are uniformly distributed on the graphene layer, and at the same time, they exist within the composite as primary particle nanoparticles without aggregation. As a result, the graphene layer, composed of paired particles, suppresses the expansion of the nanosilicon, causing the nanosilicon to exhibit low volume expansion.

[0104] On the other hand, if the degree of dispersion is low, the nanosilicon particles will aggregate, and as a result, during lithiation, they will exhibit volume expansion as aggregated micron-sized powder rather than as nanosilicon.

[0105] As a result, it can be confirmed that when dispersibility is high, the absolute value of the volume change of nanosilicon due to lithiumization is small, and the electrode expansion rate decreases as the expansion buffering effect by the uniformly formed small pores inside and the volume expansion suppression effect by the graphene layer increase.

[0106] <Experimental Example 4> Variance

[0107] Figure 14 is an SEM image of the silicon-graphite fusion precursor.

[0108] Referring to Figure 14, it can be seen that the silicon-graphite fusion precursor has a structure in which multiple silicon particles are located on the upper and lower parts of a plate-like graphite, and nanosilicon particles are unevenly distributed between the expanded graphite.

[0109] Figures 15 and 16 show (a) an SEM image of silicon particles that are not uniformly dispersed on the graphite surface but aggregated, (b) an EDS mapping image of silicon, and (c) an EDS mapping image of graphite.

[0110] Figure 17 shows (a) an SEM image of silicon particles uniformly dispersed on a graphite surface, (b) an EDS mapping image of silicon, and (c) an EDS mapping image of graphite.

[0111] Comparing Figures 15(a), 16(a), and 17(a), it can be seen that in Figure 17(a), the silicon of the silicon-graphene composite is uniformly dispersed on the graphite surface. Comparing Figures 15(b), 16(b), and 17(b), and Figures 15(c), 16(c), and 17(c), it can be seen that in Figures 17(b) and 17(c), the silicon of the silicon-graphene composite is uniformly dispersed on the graphene surface.

[0112] Figure 18 is an image showing a cross-section of the negative electrode active material fabricated using the silicon-graphene composite shown in Figures 15 and 16, and Table 3 shows the EDS map sum spectrum. The fabricated spherical active material was dispersed using an ion beam milling machine, and SEM and EDS mapping were performed.

[0113] [Table 3]

[0114] Referring to Figure 18, we can observe that there are numerous coarse silicon aggregates of the micrometer scale between the graphite layers.

[0115] Figure 19 is an image showing a cross-section of the negative electrode active material manufactured using the silicon-graphene composite shown in Figure 17, and Table 4 shows the EDS map sum spectrum.

[0116] [Table 4]

[0117] Referring to Figure 19, we can observe that graphite and silicon are uniformly dispersed inside.

[0118] Figure 20 is a flowchart showing the steps from the plasma treatment stage to the final physical property evaluation.

[0119] Table 5 shows the results of the electrochemical evaluation of the comparative example and production example 1 and production example 2, which were manufactured as shown in Table 1.

[0120] Figures 21, 22, and 23 are graphs showing the results of the electrochemical evaluation for the comparative example, production example 1, and production example 2, respectively.

[0121] Figure 24 is a graph showing the cycle retention for the comparative example, manufacturing example 1, and manufacturing example 2.

[0122] [Table 5]

[0123] Referring to Table 5 and Figures 21-24, it can be seen that in the comparative example, compared to Manufacturing Example 1 and Manufacturing Example 2, the nanosilicon aggregated to resemble micron-grade silicon, resulting in significantly lower initial charge-discharge characteristics and initial efficiency. Furthermore, a sharp decline in cycle retention was also observed.

[0124] Figures 25 and 26 show the results of observing the surface condition after cleaning the cathode electrode surface with DMC solution from the charged state after cycle retention.

[0125] Referring to Figures 25 and 26, in the comparative example where cycle retention was measured to be low in relation to the electrode surface after the 50th cycle evaluation, it was observed that the spheroidized negative electrode active material surface was cracked. In contrast, in manufacturing example 1, where the cycle retention characteristics were good, it was observed that the negative electrode active material was maintained in the same condition as the initial state even after cycling.

[0126] Therefore, by dispersing the silicon-graphite fusion precursor to graphene, an optimal dispersion state is achieved with exfoliated graphite and uniformly dispersed silicon particles. This allows for the production of a negative electrode active material, enabling the manufacture of a secondary battery exhibiting high charge-discharge characteristics and stable cycle characteristics.

[0127] The above description of the present invention is illustrative, and a person with ordinary skill in the art to which the invention pertains will understand that it can be easily modified into other specific forms without altering the technical idea or essential features of the invention. Therefore, the embodiments described above should be understood in all respects as illustrative and not limiting. For example, each component described in a single form may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined manner.

[0128] The scope of the present invention is defined by the claims described below, and all modifications or alterations derived from the meaning and scope of the claims, as well as the concept of equivalents thereof, should be interpreted as being included within the scope of the present invention.

Claims

1. The cross-section including the center has multiple paired layers arranged in a unit circle, and multiple such unit circles are arranged concentrically. The aforementioned paired layer is Graphene layer, The invention is characterized by comprising a particle layer containing a plurality of nanosilicon particles uniformly located on the graphene layer, Silicon-graphene composite.

2. The paired layers are characterized by forming a circle when connected or disconnected. The silicon-graphene composite according to claim 1.

3. The thickness of the paired layer is characterized by being 50 nm to 500 nm. The silicon-graphene composite according to claim 1.

4. The thickness of the graphene layer is characterized by being 1 nm to 300 nm. The silicon-graphene composite according to claim 1.

5. A step comprising a silicon-graphite fusion precursor, The steps include: Plasma treatment of the silicon-graphite fusion precursor, The process includes the step of dispersing the plasma-treated silicon-graphite fusion precursor in a wet grinding step to form a silicon-graphene composite. A method for dispersing silicon-graphite fusion precursors.

6. The silicon-graphite fusion precursor is characterized by having a structure in which multiple silicon particles are located on the upper and lower parts of a plate-shaped graphite. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

7. The silicon content is greater than 0 and less than or equal to 70% by wt%, The silicon content is characterized in that it is the case of application of the fusion precursor alone, application of a mixture of the fusion precursor and silicon, or a composite in which expanded graphite and silicon are applied together. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

8. The step of plasma treatment of the silicon-graphite fusion precursor involves applying nitrogen (N) to the silicon-expanded graphite mixed powder, which is the silicon-graphite fusion precursor. ２ The process is characterized by purging the material and then performing DC plasma treatment. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

9. The solvent used in the wet grinding process is characterized by having a boiling point of 60 to 160°C. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

10. The solvent used in the wet grinding process has a Hansen solubility parameter of 17 to 23 MPa. １ / ２ or 45-50 MPa １ / ２ Characterized by, A method for dispersing a silicon-graphite fusion precursor according to claim 5.

11. The wet grinding step is characterized by adding a dispersant or coupling agent that can chemically activate the fusion precursor. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

12. The wet grinding process is characterized by homogenizing the fusion precursor by applying shear stress and cavitation. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

13. The silicon-graphene composite is characterized by having a structure in which multiple silicon particles are located on the surface of a single exfoliated graphene. A method for dispersing a silicon-graphite fusion precursor according to claim 5.

14. The silicon particles are characterized in that they are uniformly distributed on the upper and lower parts of the single graphene. A method for dispersing a silicon-graphite fusion precursor according to claim 13.

15. A step comprising a silicon-graphite fusion precursor, The steps include: Plasma treatment of the silicon-graphite fusion precursor, The steps include: dispersing the plasma-treated silicon-graphite fusion precursor in a wet grinding process to form a silicon-graphene composite in a detached form; The steps include: coating the exfoliated silicon-graphene composite with a binder; The process comprises the step of granulating the binder-coated and exfoliated silicon-graphene composite to produce a granulated silicon-graphene composite, A method for manufacturing silicon-graphene composites.

16. The thickness of the graphene layer is characterized by being 1 nm to 300 nm. A method for producing a silicon-graphene composite according to claim 15.

17. A silicon-graphene composite according to claim 1, Anode active material for lithium secondary batteries.

Images