Silicon-carbon composite for anode active material and preparing method thereof

Abstract

The present invention relates to a silicon-carbon composite and a method for manufacturing the same. More specifically, the invention relates to a silicon-carbon composite in which n silicon layers and n carbon layers are alternately deposited on a carbon structure, and 2n-1 silicon carbide (SiC) protective layers are formed at each interface between the silicon layers and the carbon layers. By removing impurities remaining in the silicon-carbon composite and forming a robust SiC protective layer, the lifespan of the secondary battery can be improved when used as a negative electrode active material for the secondary battery.

Description

Technology Field

[0001] The present invention relates to a silicon-carbon composite for a negative electrode active material and a method for manufacturing the same. More specifically, the invention relates to a silicon-carbon composite for a negative electrode active material and a method for manufacturing the same, which can improve the lifespan of a secondary battery when used as a negative electrode active material by removing impurities remaining on the surface of the silicon-carbon composite and forming a robust protective layer. Background Technology

[0003] The performance improvement of secondary batteries is based on the components of the cathode material, anode active material, and electrolyte.

[0004] Among the above components, graphite-based materials, which are mainly used as negative electrode active materials, are commercially available due to their excellent electrochemical performance and low cost, but they have limitations in being applied to high-capacity secondary batteries because their theoretical capacity is limited to 370 mAh / g.

[0005] To overcome the aforementioned limitations, non-graphite-based anode active materials such as silicon, tin, and germanium are emerging as alternative materials. Among these, silicon-based anode active materials are receiving significant attention as candidates for next-generation anode active materials because they can store 10 times the amount of lithium per unit mass (3,592 mAh / g) compared to graphite, a commercial anode active material. However, silicon-based anode active materials undergo structural degradation of silicon particles during charging and discharging due to a volume change of approximately 300%, which can eventually lead to electrical failure caused by mechanical damage.

[0006] As one of the methods to solve this problem, silicon-carbon composites have been developed that can suppress the volume expansion of silicon particles using carbon-based materials. Since the structural deformation of silicon particles during charging and discharging can be fundamentally suppressed by minimizing the size of the silicon particles within the silicon-carbon composite, this can help improve the performance of secondary batteries.

[0007] Among various methods for manufacturing silicon-carbon composites, many methods have been proposed for manufacturing silicon-carbon composites containing silicon particles with small particles, using Chemical Vapor Deposition (CVD) to deposit silicon particles on the inside and / or outside of a carbon structure. The problem to be solved

[0009] The present invention aims to provide a silicon-carbon composite and a method for manufacturing the same, wherein a silicon carbide (SiC) layer is formed as a protective layer for silicon particles deposited on the inside and / or outside of a carbon structure while removing impurities contained in the carbon source gas used in the manufacture of a silicon-carbon composite for a negative electrode active material of a secondary battery.

[0010] The objective of the present invention is to provide a silicon-carbon composite as a negative electrode active material for a secondary battery that can improve the capacity, efficiency, capacity retention rate, and lifespan of the secondary battery when applied as a negative electrode active material for a secondary battery, and an efficient method for manufacturing the same.

[0011] The objectives of the present invention are not limited to those mentioned above, and other objectives and advantages of the present invention not mentioned may be understood from the following description and will be more clearly understood by the embodiments of the present invention. Furthermore, it will be readily apparent that the objectives and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims. means of solving the problem

[0013] According to one aspect of the present invention, a silicon-carbon composite is provided in which n silicon layers and n carbon layers are alternately deposited on a carbon structure, wherein 2n-1 silicon carbide (SiC) protective layers are formed at each interface of the silicon layers and carbon layers, and n is an integer from 10 to 30.

[0014] Each thickness of the silicon layer may be 1 to 20 nm, and each thickness of the carbon layer may be 0.5 to 3 nm.

[0015] The silicon particles contained in the above silicon-composite may have a crystal grain size of 8 nm or less.

[0016] The above carbon structure may have a particle size of 3 to 7 μm.

[0017] The above carbon structure may be a porous carbon structure.

[0018] When the total weight of the silicon-carbon composite is 100 weight%, the total weight of the 2n-1 silicon carbide (SiC) protective layer may be 1 to 7 weight%.

[0019] According to another aspect of the present invention, as a method for manufacturing the silicon-carbon composite,

[0020] (S1) A step of placing a carbon structure in a chemical vapor deposition (CVD) reactor;

[0021] (S2) a step of increasing the temperature to atmospheric pressure and 400°C to 550°C while injecting purge gas into the chemical vapor deposition reactor;

[0022] (S3) A step of depositing a silicon layer on the carbon structure by simultaneously supplying a silicon source gas and a hydrogen gas;

[0023] (S4) A flushing step in which only purge gas is injected to remove gases other than the purge gas;

[0024] (S5) A step of depositing a carbon layer on the silicon layer by simultaneously supplying a carbon source gas and a carrier gas;

[0025] (S6) A flushing step in which only purge gas is injected to remove gases other than the purge gas; and

[0026] (S7) repeating the steps (S3) to (S6) n times to alternately stack n silicon layers and n carbon layers on a carbon structure, and then annealing at a temperature of 500°C to 650°C for 30 minutes to 3 hours to form 2n-1 silicon carbide (SiC) protective layers at each interface of the silicon layers and carbon layers; comprising

[0027] A method for manufacturing a silicon-carbon composite can be provided, wherein n is an integer from 10 to 30.

[0028] According to another aspect of the present invention, a negative electrode active material for a secondary battery comprising the silicon-carbon composite can be provided.

[0029] According to another aspect of the present invention, a secondary battery can be provided comprising a negative electrode active material for a secondary battery comprising the silicon-carbon composite. Effects of the invention

[0031] Since the silicon-carbon composite according to the present invention can effectively remove impurities remaining due to carbon source gas, it can solve the problems of reduced initial charging efficiency and reduced secondary battery life caused by impurity gas when used as a negative electrode active material for a secondary battery.

[0032] The silicon-carbon composite according to the present invention has a silicon carbide (SiC) layer having high strength and toughness as a protective layer, while maintaining the amorphous state of silicon. Therefore, when used as a negative electrode active material for a secondary battery, it reduces the decrease in initial charging efficiency and suppresses cracking of the negative electrode active material and the continuous excessive formation of the solid electrolyte interface (SEI) layer due to shrinkage and expansion during the charging and discharging process of the secondary battery, thereby improving the lifespan of the secondary battery.

[0033] According to the method for manufacturing a silicon-carbon composite of the present invention, the silicon-carbon composite can be manufactured efficiently.

[0034] In addition to the effects described above, the effects of the present invention are described together with the details for implementing the invention below. Brief explanation of the drawing

[0036] Figure 1 is a graph showing the XRD analysis results of silicon contained in the silicon-carbon composites of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 of the present invention. FIG. 2 shows a graph measuring the lifespan evaluation (capacity retention rate, %) according to charge and discharge cycles in a coin pool-cell test using the silicon-carbon composites of Example 1, Comparative Example 1, and Comparative Example 2 of the present invention as negative electrode active materials for secondary batteries. Specific details for implementing the invention

[0037] The aforementioned objectives, features, and advantages are described in detail below with reference to the attached drawings, thereby enabling those skilled in the art to easily implement the technical concept of the present invention. In describing the present invention, detailed descriptions of known technologies related to the present invention are omitted if it is determined that such descriptions would unnecessarily obscure the essence of the invention. Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the attached drawings. In the drawings, the same reference numerals are used to indicate the same or similar components.

[0038] In this specification, where terms such as "includes," "contains," "has," "is made up of," "arranges," or "is equipped" are used for a component, other parts may be added unless "only" is used. Where a component is expressed in the singular, it includes cases where it is in the plural unless specifically stated otherwise.

[0039] Unless otherwise specifically defined for any numerical value in this specification, it shall be interpreted as being based on weight (wt).

[0040] In interpreting the components in this specification, they are interpreted to include an error range even if there is no separate explicit description.

[0042] The present invention will be described in more detail below.

[0044] silicon-carbon composite

[0045] According to one example of the present invention, a silicon-carbon composite having n silicon layers and n carbon layers alternately deposited on a carbon structure,

[0046] A 2n-1 silicon carbide (SiC) protective layer is formed at each interface of the silicon layer and the carbon layer, and

[0047] A silicon-carbon composite can be provided, wherein n is an integer from 10 to 30.

[0048] In particular, the silicon-carbon composite of the present invention is characterized by a structure in which a silicon layer and a carbon layer are alternately deposited on a carbon structure, and a silicon carbide (SiC) protective layer is formed at the interface between the silicon layer and the carbon layer. Since n silicon layers and n carbon layers are formed, a total of 2n-1 silicon carbide protective layers are formed at the interface.

[0049] Each thickness of the silicon layer may be 1 to 20 nm, and each thickness of the carbon layer may be in the range of 0.5 to 3 nm.

[0050] The above silicon carbide protective layer maintains the amorphous state of silicon and can perform the role of a protective layer due to the high strength and toughness of the silicon carbide material. When the above SiC protective layer is formed at the interface, it has the advantages of improving initial charging efficiency, suppressing excessive formation of the SEI layer, and exhibiting characteristics as a high-life anode material.

[0051] However, if excessive heat treatment is performed, the amorphous silicon may fail to maintain its amorphous state, and the SiC layer may be excessively formed, reducing the amount of active Si and potentially causing a significant decrease in capacity. In this regard, it is desirable for the silicon grain size to be 8 nm or less. Additionally, when the total weight of the silicon-carbon composite is 100 wt%, it is desirable to control the total weight of the 2n-1 silicon carbide (SiC) protective layer to be within the range of 1 to 7 wt%. If the total weight of the silicon carbide (SiC) protective layer is less than 1 wt%, the silicon carbide protective layer may not be sufficiently formed, and if it exceeds 7 wt%, the capacity reduction rate may become too large, as confirmed through experiments.

[0052] In the present invention, a porous carbon structure can be used as the carbon structure, and the capacity of the secondary battery can be improved by depositing silicon and carbon inside the pores of the carbon structure, and consequently, a silicon layer and a carbon layer can be stacked inside and outside the pores.

[0053] According to one example, the porous carbon structure has a specific surface area of ​​200 to 400 m² 2 / g, and the volume of pores with a pore size of 50 nm or less is 0.01 to 0.4 cc / g, and the volume of pores with a pore size greater than 50 nm and 350 nm or less is 0.5 to 2 cc / g. It may be used, but is not limited thereto.

[0054] While a high specific surface area of ​​the above porous carbon structure allows for the realization of high capacity characteristics by containing a large amount of high-energy-density silicon nanoparticles, excessively high specific surface area may lead to problems such as deterioration of the mechanical properties of the carbon structure; therefore, the porous carbon structure has a specific surface area of ​​200 to 400 m² 2 It may be / g, preferably 300~350 m 2 It can be / g.

[0055] In the porous carbon structure described above, pores with a pore size of 50 nm or less are micropores. Since the impregnation or deposition of silicon particles into the micropores alleviates volume expansion and thus offers advantages in terms of lifespan, it is desirable to include a certain number of micropores. On the other hand, if the number of micropores becomes too high, a surface area remains unfilled with silicon, resulting in the formation of a large amount of SEI (solid electrolyte interphase layer). Consequently, when applied as a negative electrode material in a secondary battery, this can cause a problem where the initial efficiency of the secondary battery decreases. Therefore, the volume of pores with a pore size of 50 nm or less in the porous carbon structure may be 0.01 to 0.4 cc / g, and preferably 0.1 to 0.3 cc / g. In the present invention, the volume of micropores is measured by the nitrogen adsorption method and analyzed by density-functional theory (DFT).

[0056] In the porous carbon structure above, pores with a pore size greater than 50 nm and less than or equal to 350 nm are macropores, which are relatively large pores. It is desirable to include a certain amount or more to provide space for stacking silicon layers and carbon layers, but if too much is included, it may degrade the mechanical properties of the carbon structure or create unwanted large deposition lumps. Therefore, the volume of pores with a pore size greater than 50 nm and less than or equal to 350 nm in the porous carbon structure may be 0.5 to 2 cc / g, and preferably 1 to 3 cc / g.

[0057] The silicon-carbon composite of the present invention can be used as a negative electrode active material for a secondary battery, and a negative electrode can be manufactured by preparing a negative electrode active material slurry by mixing one or more of a binder and a conductive material, preferably both the binder and the conductive material, in a solvent, and then forming the slurry into a specific shape or coating it onto a negative electrode current collector. The binder, conductive material, and solvent are not particularly limited as long as they are commonly used in the art.

[0059] Method for manufacturing a silicon-carbon composite

[0060] According to one embodiment of the present invention, a method for manufacturing a silicon-carbon composite is provided, and the manufactured silicon-carbon composite can satisfy the characteristics of the silicon-carbon composite of the present invention described above.

[0061] According to one aspect of the present invention,

[0062] (S1) A step of placing a carbon structure in a chemical vapor deposition (CVD) reactor;

[0063] (S2) a step of increasing the temperature to atmospheric pressure and 400°C to 550°C while injecting purge gas into the chemical vapor deposition reactor;

[0064] (S3) A step of depositing a silicon layer on the carbon structure by simultaneously supplying a silicon source gas and a hydrogen gas;

[0065] (S4) A flushing step in which only purge gas is injected to remove gases other than the purge gas;

[0066] (S5) A step of depositing a carbon layer on the silicon layer by simultaneously supplying a carbon source gas and a carrier gas;

[0067] (S6) A flushing step in which only purge gas is injected to remove gases other than the purge gas; and

[0068] (S7) repeating the steps (S3) to (S6) n times to alternately stack n silicon layers and n carbon layers on a carbon structure, and then annealing at a temperature of 500°C to 650°C for 30 minutes to 3 hours to form 2n-1 silicon carbide (SiC) protective layers at each interface of the silicon layers and carbon layers; comprising

[0069] A method for manufacturing a silicon-carbon composite can be provided, wherein n is an integer from 10 to 30.

[0071] According to one example, the reactor used in step (S1) above may be a CVD reactor equipped with a tube furnace or a rotary tube furnace.

[0072] According to one example, in order to control the overall reaction rate and obtain a uniform reaction product, the purge gas is injected together with the increase in the temperature of the reactor so that the step (S2) is performed under a purge gas atmosphere. The purge gas may be an inert gas and may include, for example, one or more of helium gas, neon gas, krypton gas, xenon gas, radon gas, nitrogen gas and argon gas, but is not necessarily limited thereto.

[0073] According to one example, the temperature condition of step (S2) above is for performing a deposition reaction and may be, for example, 400°C to 550°C, for example, 450°C to 500°C, and the temperature raised in step (S2) is maintained until the reaction is finished.

[0074] According to one example, the silicon source gas used in step (S3) above may include one or more selected from monosilane (SiH4) gas, disilane (Si2H6) gas, monochlorosilane (SiH3Cl) gas, dichlorosilane (SiH2Cl2) gas, trichlorosilane (SiHCl3) gas, and trimethylsilane (SiH(CH3)3) gas, but other types of silicon source gases may be used if the silicon layer is to be deposited. When depositing the silicon layer in step (S3), the silicon source gas and hydrogen gas are supplied simultaneously. This is because if hydrogen gas is not supplied simultaneously, the thermal decomposition products of the silicon gas may be mostly concentrated at the front of the reactor, and there may be a problem of causing an uneven distribution of Si.

[0075] In order to satisfy the characteristics of silicon particles in the silicon-carbon composite of the present invention, particularly the characteristics of crystallinity and crystallite size, the ratio of the silicon source and hydrogen gas can be adjusted. For example, in step (S3), the silicon source gas and hydrogen gas can be supplied in a ratio of silicon source gas to hydrogen gas of 1:2 to 1:20 (unit: sccm), preferably in a ratio of silicon source gas to hydrogen gas of 1:5 to 1:15 (unit: sccm), and more preferably in a ratio of silicon source gas to hydrogen gas of 1:7 to 1:10 (unit: sccm), but are not necessarily limited thereto.

[0076] The above step (S4) is a flushing step that removes other gases other than the purge gas by injecting only the purge gas, and this is a necessary step to ensure that the silicon layer and the carbon layer are stacked without interference with each other. That is, if the silicon layer and the carbon layer are not stacked without interference, they are deposited in a silicon-carbon combined silicon carbide (SiC) state, which reduces reactivity with lithium ions, and consequently, may cause problems such as reduced capacity, lifespan, and efficiency of the secondary battery.

[0077] The above purge gas may include an inert gas and may be one or more of helium gas, neon gas, krypton gas, xenon gas, radon gas, nitrogen gas, and argon gas.

[0078] In the above step (S5), a carbon source gas and a carrier gas can be supplied simultaneously to deposit a carbon layer on the silicon layer of the porous carbon structure.

[0079] The aforementioned carrier gas plays the role of moving the carbon source gas on its own, depending on the type of gas, when the deposition source gas flows into the reactor.

[0081] The above carbon source gases are methane (CH4) gas, ethane (C2H6) gas, propane (C3H8) gas, and butane (C4H 10 It may include one or more selected from ) gas, acetylene (C2H2) gas and ethylene (C2H4) gas, but other types of carbon source gases may be used if the purpose is to deposit a carbon layer.

[0082] The above carrier gas can be selected from nitrogen gas or argon gas.

[0083] In the above step (S5), the carbon source gas and the carrier gas can be supplied in a ratio of 1:2 to 1:20 (unit: sccm), preferably in a ratio of 1:5 to 1:15 (unit: sccm), and more preferably in a ratio of 1:7 to 1:10 (unit: sccm), but are not necessarily limited thereto.

[0084] The above step (S6) is identical to step (S4) and is a flushing step that removes gases other than the purge gas by injecting only the purge gas. In order to stack a pure silicon layer and a pure carbon layer without interference between the silicon layer and the carbon layer, it is desirable to perform a flushing step in which only the purge gas is injected to remove the carbon source gas, hydrogen gas, etc. that were injected to stack the carbon layer remaining in the reactor.

[0085] Steps (S3) to (S6) above can be repeated at least 6 times to alternately stack at least 6 silicon layers and carbon layers each within a porous carbon structure, preferably at least 6 to 15 silicon layers and carbon layers each.

[0086] At this time, since the pores within the porous carbon structure are large, a silicon layer or a carbon layer is first deposited inside the pores, and after the inside of the pores is filled with a silicon layer or a carbon layer, a silicon layer or a carbon layer may be partially deposited outside the porous carbon structure. In the present invention, the deposition conditions of steps (S3) and (S5) are adjusted so that the thickness of each carbon layer is controlled to 10 nm or less and the thickness of each silicon layer is controlled to 20 nm or less, and the flushing steps of steps (S4) and (S6) are performed so that a pure silicon layer and a carbon layer can be alternately deposited.

[0087] The steps (S3) to (S6) above can be repeated n times to alternately stack n silicon layers and carbon layers each within the porous carbon structure, for example, 10 to 30 silicon layers and carbon layers each can be alternately stacked.

[0088] Since the carbon structure used in the present invention is a porous carbon structure, when the deposition reaction is carried out, a silicon layer or a carbon layer is first deposited inside the pores, and after the inside of the pores is filled with a silicon layer or a carbon layer, a silicon layer or a carbon layer may be partially deposited outside the porous carbon structure. In the present invention, the deposition conditions of steps (S3) and (S5) are adjusted so that the thickness of each carbon layer is controlled to 0.5 to 3 nm and the thickness of each silicon layer is controlled to 1 to 20 nm, and the flushing steps of steps (S4) and (S6) are performed so that a pure silicon layer and a carbon layer can be deposited alternately.

[0089] When a process of depositing silicon and carbon on a carbon structure using a silicon source gas and a carbon source gas is performed as described in steps (S1) to (S6) above, trace amounts of impurities generated during the process of depositing silicon and carbon remain. For example, when carbon is deposited using a carbon source gas such as acetylene gas, impurities such as unknown organic carbon compounds, hydrogen (H), and oxygen (O) may remain due to incomplete carbonization.

[0090] According to the present invention, there is an advantage in that it can solve the problem of reduced initial charge efficiency (ICE) and reduced battery life caused by impurities contained in the carbon source gas when applied as a negative electrode active material of a secondary battery.

[0091] Specifically, the method for manufacturing a silicon-carbon composite of the present invention is characterized by forming a silicon carbide (SiC) protective layer at the interface between the silicon layer and the carbon layer by performing a step (S7) of heat treatment (annealing) at a temperature of 500°C to 650°C on the silicon-carbon composite precursor obtained through steps (S1) to (S6).

[0092] As the heat treatment temperature of step (S7) above increases, the strength of the silicon carbide protective layer increases, resulting in a more robust effect. Consequently, when used as a negative electrode active material for a secondary battery, this can contribute to improving the lifespan of the secondary battery and preventing a decrease in initial efficiency. During the heat treatment process of step (S7), the silicon in the silicon layer reacts with the carbon in the carbon layer to form a SiC protective layer at the interface. This results in a relative decrease in the content of active Si, which may lead to a decrease in the capacity of the negative electrode active material; however, it is important to control the temperature so that the decrease in discharge capacity compared to the pre-heat treatment precursor composite does not become too low.

[0093] Meanwhile, as the heat treatment temperature of step (S7) increases, the size of the silicon crystal grains within the silicon-carbon composite increases, which may lead to a problem that actually reduces the lifespan of the secondary battery. More specifically, the silicon-carbon composite precursor obtained according to steps (S1) to (S6) without heat treatment contains amorphous silicon, but depending on the heat treatment temperature of step (S7), the silicon crystal grain size may grow and become crystalline, which may cause a reduction in lifespan. This is because silicon particles in an amorphous state are structurally more flexible and can better respond to volume changes occurring during charging and discharging, thus exhibiting more advantageous characteristics than in a crystalline state. From this perspective, it is desirable that the silicon crystal grain size be controlled to 8 nm or less even after performing up to step (S7). In the present invention, the crystal grain size of the silicon particles is determined by calculating the full width at half maximum (FWHM) of the peak near 2θ = 28° after performing XRD analysis.

[0094] Therefore, it is important to achieve all of the following: ensuring that the silicon grain size is 8 nm or less to increase the lifespan of the secondary battery, while simultaneously controlling the amount of silicon carbide protective layer to prevent capacity degradation and appropriately improving the strength of the silicon carbide protective layer.

[0095] Accordingly, in the present invention, it was experimentally confirmed that when the heat treatment temperature of step (S7) is in the range of 500°C to 650°C, which is much lower than the post-treatment temperature of silicon-carbon composites generally in the prior art, the lifespan of the secondary battery can be improved and the problem of initial efficiency degradation can be solved when used as a negative electrode active material, and the heat treatment temperature may be in the range of, for example, 600°C to 650°C, and may be, for example, 650°C.

[0096] According to one example, the heat treatment time of step (S7) may be 10 minutes to 3 hours, for example, 30 minutes to 2 hours, for example, 1 hour. If the heat treatment is performed for a shorter time than the lower limit, a SiC protective layer may not be formed, and if the heat treatment time is longer, even at the same temperature, the silicon crystal grain size may increase, causing a decrease in lifespan, and if the heat treatment is performed for a longer time than the upper limit, a problem of excessive crystal grain growth may occur, but it is not limited thereto, and the heat treatment temperature is more important than the heat treatment time.

[0097] According to one example, after step (S7) above, the method may further include step (S8) of forming an outer carbon coating layer. The description regarding the outer carbon coating layer applies in the same way as the description regarding the silicon-carbon composite, and the step of forming the outer carbon coating layer may be performed under an inert gas atmosphere. For example, the inert gas may be argon gas or nitrogen gas, but is not limited thereto.

[0099] The present invention will be explained in more detail below through examples and experimental examples. However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0101] Comparative Example 1

[0102] Silicon-carbon composite A was prepared without heat treatment.

[0103] The method for manufacturing silicon-carbon composite A is as follows.

[0104] 1. Carbon black (DC3501, OCI) and phenolic resin were mixed in isopropyl alcohol at a weight ratio of 55:45 and then dried by spray drying to form spherical powder particles. The spherical particles were then subjected to carbonization at a temperature of approximately 1000–1100°C and steam activation at 950°C to produce 'porous carbon structure a'. 'Porous carbon structure a' had a specific surface area (BET) of 354 m² 2 / g, the volume of pores with a pore size of 50 nm or less was 0.27 cc / g, and the volume of pores with a pore size greater than 50 nm and 350 nm or less was 1.26 cc / g.

[0105] 2. Silicon-carbon composite A was prepared using the above 'porous carbon structure a' in the following manner.

[0106] (1) 50 g of the above 'porous carbon structure a' was introduced into a rotary kiln at a temperature of 520°C.

[0107] (2) A reaction to deposit a silicon layer was carried out by introducing a mixed gas of SiH4 gas and H2 gas (SiH4: H2 = 0.5 slm : 1.5 slm) for 15 minutes.

[0108] (3) After the above silicon layer deposition reaction was finished, the supply of the SiH4 and H2 mixed gas was stopped, and Ar gas was supplied at 1.5 slm for 10 minutes to flush out the remaining gas.

[0109] (4) Then, a reaction to deposit a carbon layer was carried out by introducing a mixture of C2H2 gas and Ar gas (C2H2: Ar = 0.5 slm : 1.5 slm) for 15 minutes.

[0110] (5) After the carbon layer deposition reaction was finished, the supply of the C2H2 and Ar mixed gas was stopped, and Ar gas was supplied at 1.5 slm for 10 minutes to flush out the remaining gas.

[0111] The above process (2) to (5) was repeated 11 more times to produce a silicon-carbon composite A containing about 60 weight% silicon.

[0113] Example 1

[0114] The silicon-carbon composite A of Comparative Example 1 was introduced into a kiln and heat-treated at a temperature of 600°C for 1 hour to produce the silicon-carbon composite of Example 1.

[0116] Example 2

[0117] Silicon-carbon composite A of Comparative Example 1 was introduced into a kiln and heat-treated at a temperature of 650°C for 1 hour to produce the silicon-carbon composite of Example 2.

[0119] Comparative Example 2

[0120] Silicon-carbon composite A of Comparative Example 1 was introduced into a kiln and heat-treated at a temperature of 700°C for 1 hour to produce the silicon-carbon composite of Comparative Example 2.

[0122] Comparative Example 3

[0123] Silicon-carbon composite B containing about 60 wt% silicon was prepared by using the ‘porous carbon structure a’ prepared in Comparative Example 1 above in the same manner as the method of preparing silicon-carbon composite A, but by repeating the process of (2) to (5) 23 times additionally.

[0125] Example 3

[0126] The silicon-carbon composite B of Comparative Example 3 above was introduced into a kiln and heat-treated at a temperature of 600°C for 1 hour to produce the silicon-carbon composite of Example 3.

[0128] Comparative Example 4

[0129] The silicon-carbon composite B of Comparative Example 3 was introduced into a kiln and heat-treated at a temperature of 700°C for 1 hour to produce the silicon-carbon composite of Comparative Example 4.

[0131] <Experimental Example 1: XRD Analysis>

[0132] XRD analysis was performed on each of the silicon-carbon composites of Comparative Examples 1 and 2 and Examples 1 and 2, respectively, and the resulting graph is shown in Fig. 1. The crystal grain size of the silicon particles was calculated from the XRD analysis graph and is shown in Table 1 below. When the crystal grain size of silicon at half maximum width of the XRD graph is 2 nm or less, it is determined to be amorphous.

[0134] Silicon grain size Comparative Example 1 (No heat treatment) 1.8 nm Example 1 (600℃ heat treatment) 1.7 nm Example 2 (650℃ heat treatment) 5.8 nm Comparative Example 2 (700℃ heat treatment) 8.5 nm Comparative Example 3 (No heat treatment) 1.5 nm Example 3 (600℃ heat treatment) 5.2 nm Comparative Example 4 (700℃ heat treatment) 8.2 nm

[0136] Referring to Table 1, when heat treatment was performed at 700°C exceeding 650°C, a sharp peak occurred near 2θ = 28°, and it was confirmed that the crystallite size increased from 1.8 nm to 5.8 nm. This indicates that the silicon particles of Comparative Example 1, which were not heat-treated, were in an amorphous state, but transformed into a crystallite state after heat treatment. The amorphous state is structurally more flexible and can better respond to volume changes occurring during charging and discharging, thus exhibiting more advantageous characteristics than the crystallite state; this was experimentally confirmed in the cell evaluations of Experimental Examples 2 and 3 below. Furthermore, as a result of heat treatment at 700°C in Comparative Example 2, the silicon crystallite size was measured to be 8.5 nm, which is larger than the crystallite size intended in the present invention; it was experimentally confirmed in the cell evaluations of Experimental Examples 2 and 3 below that a decrease in lifespan occurred.

[0138] <Experimental Example 2: Coin Half-Cell Evaluation (Coin Half-Cell Test)>

[0139] Silicon-carbon composites of Examples 1 to 3 and Comparative Examples 1 to 4 were prepared as negative electrode active materials.

[0140] To prepare a cathode slurry, a conductive material was prepared by mixing conductive carbon black (Super P) and carbon nanotubes (SWCNT) (Super P : SWCNT = 9.9 : 0.1, weight ratio), and a binder was prepared by mixing CMC (sodium salt of carboxymethyl cellulose) and SBR (styrene-butadiene rubber) (CMC : SBR = 3 : 7, weight ratio). The cathode active material, conductive material, and binder prepared above were mixed using a sinker mixer (cathode active material : conductive material : binder = 8 : 1 : 1, weight ratio) to obtain a cathode slurry.

[0141] The above cathode slurry was applied to a cathode current collector (copper foil, 18 μm), dried at 80°C for 1 hour, and then heat-treated in a vacuum oven at 100°C for 12 hours to obtain a cathode plate.

[0142] The fabricated negative electrode plate was punched to 16 mmΦ to obtain a negative electrode plate for a coin cell, and coin cell fabrication and electrochemical evaluation were carried out using the negative electrode plate and a Li-metal counter electrode. The charge and discharge conditions for the initial 1-2 cycles (Formation cycle) were charge CC / CV: 0.005 V / 0.005 C, discharge CC: 1.0 V, and the rate limit was 0.1 C.

[0143] Using the TOSCAT-3100 half-cell device, charging and discharging were performed to measure the initial charge capacity (ICC, mAh / g), initial discharge capacity (IDC, mAh / g), and initial efficiency (ICE, %), which are shown in Tables 2 and 3 below.

[0144] The amount of SiC layer formed at the interface between the silicon layer and the carbon layer was calculated as shown in Equation 1 below. When a SiC layer is formed at the interface between the silicon layer and the carbon layer of the silicon-carbon composite after heat treatment, the amount of active Si in the silicon layer decreases, and thus the ICC capacity decreases. The ICC value expressed by silicon (active Si) can be calculated by assuming it to be 4000 mAh / g (theoretical value).

[0145] [Equation 1]

[0146]

[0147] Based on the silicon content of the silicon-carbon composite without heat treatment, the remaining silicon excluding the silicon content (weight%) calculated according to Equation 1 above can be considered to be formed of SiC, and the calculated values ​​are shown in Tables 2 and 3 below.

[0149] ICC ICE(@1.0V) IDC(@1.0V) SiC production amount (calculated value) Comparative Example 1 (No heat treatment) 2419 89.2% 2158 0% Example 1 (600℃ heat treatment) 2335 89.6% 2100 2.8% Example 2 (650℃ heat treatment) 2304 89.4% 2058 3.9% Comparative Example 2 (700℃ heat treatment) 2207 89.0% 1971 7.4%

[0151] ICC ICE(@1.0V) IDC(@1.0V) SiC production amount (calculated value) Comparative Example 3 (No heat treatment) 2318 86.5% 2005 0% Example 3 (600℃ heat treatment) 2281 87.5% 1996 2.4% Comparative Example 4 (700℃ heat treatment) 1800 87.6% 1577 19.5%

[0153] Referring to Tables 2 and 3 above, the initial charge efficiency (ICE) was improved when heat treatment was performed compared to when heat treatment was not performed. However, when silicon and carbon combine to form silicon carbide (SiC) during the heat treatment process, the content of active silicon (Active Si) decreases, resulting in a decrease in active capacity and consequently a drop in ICC and IDC values. In particular, when heat treatment was performed at 700°C, exceeding 650°C, a problem occurred in which the discharge capacity (IDC) decreased by more than 9% compared to the case without heat treatment. However, when heat treatment was performed at 600–650°C, it was confirmed that the discharge capacity decreased relatively less (within 5%) compared to the case without heat treatment.

[0155] < Experimental Example 3: Coin Full-Cell Evaluation (Coin Full-Cell Test)

[0156] A cathode material was prepared by mixing 9 wt% of the silicon-carbon composite of Examples 1 and 2 and Comparative Examples 1 and 2 with 91 wt% of graphite by weight. Separately, a conductive material was prepared by mixing conductive carbon black (Super P) and carbon nanotube conductive material (SWCNT) (Super P : SWCNT = 9 : 1 by weight ratio). Separately, a binder was prepared by mixing CMC (sodium salt of carboxymethyl cellulose) and SBR (styrene-butadiene rubber) (CMC : SBR = 3 : 7 by weight ratio).

[0157] The cathode material, conductive material, and binder prepared above were mixed with a sinker (cathode material : conductive material : binder = 95.8 : 1 : 3.2, weight ratio) to obtain a cathode slurry.

[0158] The above cathode slurry was applied to a copper foil with a thickness of 18 μm and dried at 80°C for 1 hour. Then, it was rolled and heat-treated in a vacuum oven at 100°C for 12 hours, and then punched to 16 mmΦ to manufacture a cathode plate.

[0159] The anode was manufactured by mixing the anode material NCM622, conductive carbon black (Super P), and PVDF in a weight ratio of 92:4:4 and punching the electrode plate to 14mmΦ.

[0160] A coin full cell (CR2032) was manufactured using the above cathode and anode, and its electrochemical characteristics were verified. The charge / discharge conditions for the initial 1-2 cycles (cycle formation) were set with a charge CC / CV of 4.25 V / 0.05 C, a discharge CC of 2.5 V, and a rate limit of 0.1 C. Subsequently, for 100 cycles, charge and discharge were performed with only the rate limit changed to 0.5 C to measure the life characteristics.

[0162] Capacity Retention Rate (CRR) (@100 cycle) Comparative Example 1 (No heat treatment) 80.1% Example 1 (600℃ heat treatment) 90.4% Example 2 (650℃ heat treatment) 91.6% Comparative Example 2 (700℃ heat treatment) 85.9%

[0164] Referring to Table 4, it was confirmed that the capacity retention rate was improved and the lifespan was improved when heat treatment was performed compared to when heat treatment was not performed. This is interpreted as being due to the lifespan improvement effect resulting from the removal of impurities in the carbon source gas and the formation of a SiC protective layer. When heat treatment was performed at 600°C to 650°C containing amorphous silicon, the lifespan performance was even better than when heat treatment was performed at 700°C.

[0166] Although the embodiments of this specification have been described in more detail with reference to the attached drawings, this specification is not necessarily limited to these embodiments and may be modified in various ways within the scope of the technical spirit of this specification. Accordingly, the embodiments disclosed in this specification are intended to explain, not limit, the technical spirit of this specification, and the scope of the technical spirit of this specification is not limited by these embodiments. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. The scope of protection of this specification shall be interpreted by the claims, and all technical spirits within an equivalent scope shall be interpreted as being included within the scope of rights of this specification.

Claims

Claim 1 A silicon-carbon composite having n silicon layers and n carbon layers alternately deposited on a carbon structure, wherein 2n-1 silicon carbide (SiC) protective layers are formed at each interface of the silicon layers and carbon layers, and n is an integer from 10 to 30. Claim 2 A silicon-carbon composite according to claim 1, wherein each thickness of the silicon layer is 1 to 20 nm and each thickness of the carbon layer is 0.5 to 3 nm. Claim 3 In claim 1, the silicon particles contained in the silicon-composite are a silicon-carbon composite having a crystal grain size of 8 nm or less. Claim 4 A silicon-carbon composite according to claim 1, wherein the carbon structure has a particle size of 3 to 7 μm. Claim 5 In claim 1, the carbon structure is a silicon-carbon composite that is a porous carbon structure. Claim 6 A silicon-carbon composite according to claim 1, wherein, when the total weight of the silicon-carbon composite is 100 weight%, the total weight of the 2n-1 silicon carbide (SiC) protective layer is 1 to 7 weight%. Claim 7 A method for manufacturing a silicon-carbon composite according to any one of claims 1 to 6, comprising: (S1) placing a carbon structure in a chemical vapor deposition (CVD) reactor; (S2) raising the temperature to atmospheric pressure and 400°C to 550°C while injecting a purge gas into the chemical vapor deposition reactor; (S3) simultaneously supplying a silicon source gas and a hydrogen gas to deposit a silicon layer on the carbon structure; (S4) a flushing step in which only a purge gas is injected to remove other gases other than the purge gas; (S5) simultaneously supplying a carbon source gas and a carrier gas to deposit a carbon layer on the silicon layer; (S6) a flushing step in which only a purge gas is injected to remove other gases other than the purge gas; and (S7) repeating steps (S3) to (S6) n times to alternately stack n silicon layers and n carbon layers on a carbon structure, and then annealing at a temperature of 500°C to 650°C for 30 minutes to 3 hours to form 2n-1 silicon carbide (SiC) protective layers at each interface of the silicon layers and carbon layers; wherein n is an integer from 10 to 30. Claim 8 A negative electrode active material for a secondary battery comprising a silicon-carbon composite according to any one of claims 1 to 6. Claim 9 A secondary battery comprising a negative electrode active material for a secondary battery comprising a silicon-carbon composite according to any one of claims 1 to 6.

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