Silicon-based anode material and secondary battery comprising same

Abstract

The present invention relates to a silicon-carbon composite, and an anode active material for a secondary battery, comprising same. More specifically, the present invention relates to a silicon-carbon composite, and an anode active material for a secondary battery, comprising same, the silicon-carbon composite, in which the size of a silicon cluster is controlled by carbon atoms, being used as an anode active material for a secondary battery so as to minimize a change in volume of the anode active material and inhibit side reactions, thereby enabling the lifespan of a secondary battery to be improved.

Description

Silicon-based negative electrode material and secondary battery containing the same

[0001] The present invention relates to a silicon-based negative electrode material of a novel structure and a secondary battery including the same.

[0002] The performance improvement of secondary batteries is based on the components of positive electrode materials, negative electrode materials, and electrolyte.

[0003] Among the above components, graphite-based materials, which are mainly used as cathode materials, are commercially available due to their excellent electrochemical performance and low cost, but their theoretical capacity is limited to 370 mAh / g, which limits their application to high-capacity secondary batteries.

[0004] To overcome the above limitations, non-graphite anode active materials such as silicon, tin, and germanium are emerging as alternative materials. Silicon-based anode materials can store more than 10 times more lithium per unit mass than graphite, and are thus attracting great attention as next-generation battery materials. However, silicon-based anode materials experience volume changes of up to three times that of silicon particles during charge and discharge, which can lead to fracture due to mechanical damage to the silicon particles during repeated charge and discharge. This leads to the formation of an excessive solid electrolyte interphase layer (SEI) and a decrease in lithium ion conductivity, resulting in low initial efficiency and lifespan stability. Therefore, there are still many difficulties in popularizing silicon-based anode materials. A solution to the above-mentioned silicon particle fracture has been developed by very finely controlling the silicon particle size.

[0005] Prior art document 1 (KR10-1963164B1) discloses that the thickness of the entire silicon coating layer on the carbon base material layer is 5 to 50 nm and the thickness of each silicon coating layer is 1 to 15 nm. However, each layer is clearly identifiable, and the thickness of the entire silicon coating layer is thin, which has the disadvantage of low capacity.

[0006] Prior art document 2 (KR10-1628873B1) discloses that a carbon-based material is used as the base material, and a silicon coating layer is formed on the surface of the carbon-based base material. However, since this is limited to silicon being positioned on the carbon-based base material, there is a limitation that the silicon deposition amount is low and the capacity is slow.

[0007] Therefore, it is still necessary to develop a silicon-based negative electrode material for secondary battery negative electrode active materials that can overcome the limitations described above and achieve high capacity and excellent lifespan when applied to secondary batteries.

[0008] The purpose of the present invention is to provide a silicon-based negative electrode material that can significantly improve the capacity and lifespan of a secondary battery when used as a negative electrode material of a secondary battery, and a secondary battery including the same.

[0009] The purpose of the present invention is not limited to the aforementioned purposes, and other unmentioned purposes and advantages of the present invention can be understood through the following description and will be more clearly understood through the embodiments of the present invention. Furthermore, it will be readily apparent that the purposes and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0010] According to one aspect of the present invention, a silicon-based negative electrode material can be provided, comprising: a porous carbon structure; and silicon nanodot clusters deposited inside and outside the porous carbon structure; wherein the silicon nanodot clusters refer to an aggregate including one or more silicon nanodot particles, the silicon nanodot particles have a maximum particle diameter of 20 nm or less, and a carbon coating layer having a thickness of 5 nm or less is formed on at least a portion of the surface of the silicon nanodot particles.

[0011] The above silicon nanodot cluster may exist in an amorphous or semi-crystalline form.

[0012] When the total weight of the above silicon-based negative electrode material is 100 wt%, the content of the silicon nanodot cluster may be 50 wt% or more.

[0013] The average particle diameter of the above silicon-based negative electrode material may be 50 nm or more and 20 μm or less.

[0014] The thickness of the silicon nanodot cluster formed on the outside of the above porous carbon structure may be 10 nm to 1000 nm.

[0015] According to another aspect of the present invention, a negative electrode active material including the silicon-based negative electrode material of the present invention can be provided.

[0016] According to another aspect of the present invention, a secondary battery can be provided, which includes a negative electrode active material including a silicon-based negative electrode material of the present invention.

[0017] Since the silicon-based negative electrode material of the present invention can be formed with a total content of active silicon (active Si, meaning silicon that reacts with Li) of 50 wt% or more, when applied as a negative electrode material to a secondary battery, the lifespan can be improved and a high capacity satisfying an initial discharge capacity (IDC) standard of 1900 mAh / g or more can be achieved.

[0018] By controlling the maximum particle size of the silicon nanodot particles in the silicon-based negative electrode material of the present invention to 20 nm or less, when applied as a negative electrode material to a secondary battery, the silicon particles do not break during charging and discharging of the secondary battery, thereby contributing to improving the performance of the secondary battery. In addition, since the silicon-based negative electrode material of the present invention has silicon particles in the form of dot particles, voids exist between the silicon particles or between silicon nanodot clusters, which can exhibit an effect (buffer effect) that can alleviate the volume expansion phenomenon that is a problem during charging and discharging of the secondary battery.

[0019] Controlling the maximum particle size of silicon nanodot particles to 20 nm or less as aimed at in the present invention requires complex technology, and methods known to date include depositing silicon inside the micropores of a carbon structure having a fine pore distribution, or coating a silicon thin film layer with a thickness of 20 nm or less on the surface of carbon particles such as graphite without pores. However, the present invention has the advantage of being able to control the silicon nanodot particle size to 20 nm or less by placing a carbon-based matrix in a reactor and then supplying a silicon source gas into the reactor to form silicon in the form of dot particles rather than a thin film.

[0020] Due to the excellent properties of the carbon composite of the silicon-based negative electrode material of the present invention, the initial discharge capacity, initial efficiency, and life characteristics of a secondary battery manufactured by including the composite as a negative electrode material can be improved.

[0021] In addition to the effects described above, the effects of the present invention are described together with the description of matters for carrying out the invention below.

[0022] Figure 1 shows a TEM photograph of a silicon-based negative electrode material manufactured in Example 1 of the present invention.

[0023] Figure 2 shows a TEM photograph (color inversion applied) of a silicon-based negative electrode material manufactured in Example 2 of the present invention.

[0024] Figure 3 shows an SEM cross-sectional photograph of a silicon-based negative electrode material manufactured in Example 1 of the present invention.

[0025] Figure 4 shows an SEM cross-sectional photograph of a silicon-based negative electrode material manufactured in Example 2 of the present invention.

[0026] Figure 5 shows an SEM cross-sectional photograph of a silicon-based negative electrode material manufactured in Comparative Example 1 of the present invention.

[0027] Figure 6 shows a TEM photograph of a silicon-based negative electrode material manufactured in Comparative Example 2 of the present invention.

[0028] Figure 7 shows an SEM cross-sectional photograph of a silicon-based negative electrode material manufactured in Comparative Example 3 of the present invention.

[0029] Figure 8 shows a graph of the life characteristics (cycle retention, %) measured in Experimental Example 4 of the present invention.

[0030] The above-described objects, features, and advantages will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can easily practice the technical idea of ​​the present invention. In describing the present invention, if it is determined that a detailed description of known technologies related to the present invention may unnecessarily obscure the gist of the present invention, a detailed description thereof will be omitted. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to indicate the same or similar components.

[0031] In this specification, when the terms "includes," "contains," "has," "consists of," "arranges," and "provides" are used for a component, other parts may be added unless "only" is used. When a component is expressed in the singular, it includes the plural unless otherwise explicitly stated.

[0032] Unless a unit is specifically limited for any numerical value in this specification, it is interpreted as being based on weight (wt).

[0033] In this specification, micropore means a pore having a size of 2 nm or less, mesopore means a pore having a size in the range of more than 2 nm and less than 50 nm, and macropore means a pore having a size in the range of more than 50 nm and less than 350 nm.

[0034] In this specification, the pore volume of micropores and mesopores was measured by the BET (Brunauer-Emmett-Teller) method (nitrogen adsorption / desorption method), and the pore volume of macropores was measured by the mercury porosimetry method.

[0035] In interpreting the components in this specification, even if there is no separate explicit description, it is interpreted to include the range of error.

[0036]

[0037] Hereinafter, the present invention will be described in more detail.

[0038] A silicon-based anode material according to the present invention comprises a porous carbon structure; and silicon nanodot clusters deposited on the inside and outside of the porous carbon structure. Here, the silicon nanodot cluster refers to an aggregate including one or more silicon nanodot particles, and the silicon nanodot particles preferably have a maximum particle diameter of 20 nm or less. By controlling the maximum particle diameter in this way, silicon breakage can be prevented when applied as an anode material of a secondary battery. In addition, a carbon coating layer having a thickness of 5 nm or less is formed on at least a portion of the surface of the silicon nanodot particles, thereby allowing the silicon particles to exist in the form of dots.

[0039] In the present invention, as a result of a preliminary study, it was found that silicon particles included in the negative electrode material expand and contract repeatedly during the charging and discharging of a secondary battery. Silicon particle structures having a zero-dimensional dot shape undergo width expansion, so the difference in size during contraction and expansion is relatively small, whereas silicon particle structures having a one-dimensional line or two-dimensional layer shape undergo length expansion, so the difference in size during contraction and expansion is relatively large. In order to solve the problem that the larger the size difference between the silicon particles included in the negative electrode material during the shrinkage and expansion during the charging and discharging of a secondary battery, the phenomenon of fracture of the silicon particles and the resulting deterioration in the performance of the secondary battery occur, the present invention was devised. That is, even if the silicon particles included in the negative electrode material have the same particle diameter (size), the degree of increase or decrease in length expansion is much greater than the width expansion in the dot shape, so in the silicon-based negative electrode material of the silicon-carbon composite in which silicon particles are deposited in the form of layers, the durability against cracks of the silicon particles is lowered. Therefore, since silicon particles in the form of zero-dimensional dot particles have the advantage of superior structural stability in directional growth compared to those in the form of one-dimensional lines or two-dimensional layers, the silicon-based negative electrode material of the present invention includes silicon nano-dot particles, and a state in which one or more silicon nano-dot particles are clumped together in a distinct state on a carbon layer is defined as a cluster.

[0040] In this way, by presenting a structure in which each silicon particle is coated with a carbon material to form a spherical cluster structure so that dot-shaped silicon particles can exist in a dot-shaped form, even if the silicon particle expands, directional growth can spread in all directions, thereby reducing the possibility of reaching a fracture critical point due to directional growth, thereby preventing cracking and fracture of the silicon particle.

[0041] The above silicon nanodot particles may have a maximum particle diameter of 20 nm or less. However, if the particle diameter becomes too small, the specific surface area increases, forming excessive SEI (Solid Electrolyte Interphase), which may reduce the initial efficiency when applied to a secondary battery. Therefore, it is preferable that the particle diameter be 1 nm or more.

[0042] The above silicon nanodot clusters are preferably amorphous or semi-crystalline. Crystalline silicon expands approximately 300% during charging and discharging due to lithium insertion, posing a significant risk of structural collapse. Conversely, amorphous or semi-crystalline silicon is structurally disordered, offering the advantage of being able to more flexibly absorb expansion stress.

[0043] When the total weight of the silicon-based negative electrode material is 100 wt%, the content of the silicon nanodot cluster is preferably 50 wt% or more, and since it is formed on a carbon structure, it may be, for example, 60 wt% or more, for example, 70 wt% or more, for example, 80 wt% or more based on the silicon-based negative electrode material. In particular, the silicon nanodot cluster of the present invention does not exist in a state where silicon is combined with oxygen or another metal, and thus has an advantage in that it is active silicon (Active Si), which does not simply refer to silicon in the silicon-based negative electrode material, but actually refers to silicon that can react with lithium to store lithium.

[0044] In the present invention, a silicon source gas is injected into a carbon structure as a base material to deposit silicon particles, and a carbon source gas is injected in the middle of the silicon injection to form a carbon coating layer. The carbon coating layer plays an important role in enabling the formation of spherical silicon nanodot particles. It is important to form a carbon coating layer as minimal as possible to perform this role. From this perspective, the thickness of the carbon coating layer is preferably 5 nm or less, and when the total weight of the silicon nanodot particles in the silicon-based negative electrode material is 100 wt%, the total carbon content contained in the carbon coating layer formed on the surface of the silicon nanodot particles is preferably 5 wt% or less. In this case, the carbon coating layer may be substantially formed of a single carbon material.

[0045] The silicon source gas for forming the above silicon nanodot particles may include at least one selected from the group consisting of monosilane (SiH4) gas, disilane (Si2H6) gas, monochlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2) gas, trichlorosilane (SiHCl3) gas, and trimethylsilane (SiH(CH3)3) gas, but is not limited thereto. In addition, the carbon source gas for forming the carbon coating layer may include at least one selected from the group consisting of methane gas, ethane gas, propane gas, butane gas, acetylene gas, and ethylene gas, but is not limited thereto.

[0046] The average particle size of the silicon-based anode material depends on the size of the porous carbon structure as the base material and can be controlled by the amount of silicon particles to be deposited. If the size of the silicon-based anode material becomes too small, a large amount of SEI layer (Solid Electrolyte Interphase layer) may be formed on the electrode surface during charging and discharging of the secondary battery, which may cause a problem of lowering the initial efficiency. On the other hand, if the average particle size of the silicon-based anode material particles becomes too large, there may be a problem of aggravated reduction in the lifespan due to shrinkage and expansion of the silicon-based anode material during charging and discharging of the secondary battery. Therefore, the average particle size (D) of the silicon-based anode material 50 ) may be, for example, 50 nm or more and 20 μm or less, for example, 100 nm or more and 10 μm or less, for example, 500 nm or more and 10 μm or less, for example, 1 μm or more and 5 μm or less.

[0047] Since the above porous carbon structure has pores, when silicon is deposited, silicon is deposited inside the pores, but silicon nanodot clusters are also formed on the outside of the porous carbon structure, and the thickness of the silicon nanodot clusters formed on the outside of the porous carbon structure can be controlled as desired through the amount and number of silicon depositions, and can be, for example, 10 nm to 1000 nm, for example, 10 nm to 500 nm, for example, 20 nm to 100 nm.

[0048] It is preferable that the porous carbon structure of the present invention have a pore volume including micropores, mesopores, and macropores. This can contribute to mitigating the expansion of silicon-based anode materials by creating voids capable of accommodating silicon expansion during secondary battery charging and discharging. In addition, it can provide nucleation sites for silicon growth, thereby increasing the amount of silicon deposition and ultimately achieving high capacity.

[0049] Based on 100 vol% of the total pore volume of the porous carbon structure, the micropores may be 10 to 30 vol%, the mesopores may be 20 to 40 vol%, and the macropores may be 40 to 60 vol%. When the porous carbon structure of the present invention satisfies the pore distribution, silicon can be evenly impregnated into the porous carbon structure, and thus silicon expansion can be efficiently controlled during secondary battery charging, and there can be an advantage of high lifespan maintenance.

[0050] The total porosity, which is the combined pore volume of micropores, mesopores, and macropores of the above porous carbon structure, is to provide sufficient silicon deposition sites, and the total porosity is 0.5 to 2.5 cm 3 / g can be, for example, 0.8 to 1.5 cm 3 / g. The specific surface area (BET) of the above porous carbon structure is 100 to 2000 m 2 / g can be, for example, 150~500 m 2 / g. It is advantageous to use a porous carbon structure that satisfies such pore and surface area characteristics in producing the negative electrode material desired in the present invention.

[0051] The silicon-based negative electrode material of the present invention corresponds to a silicon-carbon composite and can be used as a negative electrode active material for a secondary battery. At least one of a binder and a conductive agent, preferably both a binder and a conductive agent, is mixed in a solvent to prepare a negative electrode active material slurry, which is then formed into a specific shape or applied onto a negative electrode current collector to prepare a negative electrode. The binder, conductive agent, and solvent are not particularly limited as long as they are commonly used in the present technical field.

[0052] According to an example, the method for manufacturing a silicon-based negative electrode material of the present invention is as follows:

[0053] (S1) A step of placing a porous carbon structure in a chemical vapor deposition (CVD) reactor;

[0054] (S2) A step of injecting purge gas into the chemical vapor deposition reactor to purge the reactor and raise the temperature to a temperature range of 400°C to 550°C (based on atmospheric pressure);

[0055] (S3) A step of simultaneously supplying a mixed gas of silicon source gas and hydrogen gas to deposit silicon nanodot clusters on at least the outside of the porous carbon structure;

[0056] (S4) A step of stopping the injection of the silicon source gas and hydrogen gas and injecting purge gas to purge;

[0057] (S5) A step of simultaneously supplying a mixed gas of carbon source gas and hydrogen gas to form a carbon coating layer with a thickness of 5 nm or less on a silicon nanodot cluster; and

[0058] (S6) It may include a step of stopping the injection of the carbon source gas and hydrogen gas and injecting a purge gas to purge.

[0059] At this time, by sequentially performing the above steps (S3), (S4), (S5) and (S6) n times (n is 10 or more, preferably 20 or more), a silicon-based negative electrode material in which silicon nanodot clusters including a carbon coating layer are formed on the porous carbon structure can be obtained.

[0060] In particular, in the present invention, rather than injecting silicon all at once as in the prior art, it is preferable to deposit silicon nanodot particles of a desired size by controlling the maximum amount of silicon injected at a time. At this time, the amount of silicon injected at a time can be controlled by considering a number of factors, such as the weight of the silicon-based anode material to be finally manufactured, the silicon content of the silicon-based anode material, the weight and specific surface area of ​​the porous carbon structure used as the base material, and the diameter of the silicon nanodot cluster to be manufactured.

[0061] In the above step (S2), the temperature of the reactor can be set in consideration of the temperature range at which a precursor gas, such as silane gas for depositing silicon, decomposes. If the temperature is lower than the above range, the precursor gas may not be sufficiently decomposed, and if the temperature is higher than the above range, silicon deposition may be excessive or silicon may be aggregated into lumps, which is also disadvantageous in terms of process efficiency. Therefore, the temperature can be appropriately selected within the temperature range of 400°C to 550°C.

[0062] During the process of the above step (S2), a purge gas may be additionally injected for purposes such as removing impurities, removing residual gases, preventing oxidation, and ensuring safety. The type of purge gas may be one or a mixture of two or more gases selected from hydrogen gas, nitrogen gas, argon gas, and helium gas, but the type of purge gas is not limited thereto.

[0063] The purpose of mixing hydrogen gas into the silicon source gas in the above step (S3) is to suppress the reactivity of Si radicals generated when Si-H bonds are broken. If hydrogen gas is not mixed, the thermal decomposition products of the silicon source gas may be concentrated mostly at the front of the reactor, which may cause a problem of uneven silicon deposition distribution.

[0064] In the above step (S3), the overall reaction rate may be controlled and a uniform homogeneous reaction product may be obtained by performing the process under an inert gas carrier gas atmosphere. The carrier gas may include at least one of nitrogen gas and argon gas, but is not necessarily limited thereto.

[0065] In the above step (S5), it is preferable to simultaneously supply a mixed gas of carbon source gas and hydrogen gas to form a carbon coating layer on the deposited silicon nanodot particles, and to control the amount of carbon injected to form a carbon coating layer with a thickness of 5 nm or less.

[0066] Hereinafter, the present invention will be described in more detail 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.

[0067]

[0068] [Manufacturing example]

[0069] Manufacturing Example 1 - Manufacturing of porous carbon structure A

[0070] Carbon black (OCI, DC3501) was steam activated at 950°C for 2 hours and then pulverized using an air pressure (IA) jet mill at 6.5 bar. The pulverized carbon black and phenol resin were mixed in isopropyl alcohol (IPA) at a weight ratio of 55:45 and dried by spray drying (flow rate: 70 ml, N2 gas: 200 ㎥ / min) to form spherical powder particles. The spheroidized particles were carbonized at a temperature of 1000°C for 1 hour to produce porous carbon structure A.

[0071] Manufacturing Example 2 - Manufacturing of porous carbon structure B

[0072] A porous carbon structure was manufactured using the same method as in Manufacturing Example 1, but carbon black (OCI, N326) was used instead of the starting material carbon black (OCI, DC3501) to manufacture porous carbon structure B.

[0073] For the porous carbon structures A and B above and the carbon structure C (AOTELEC, CP5), which is a graphite used in Comparative Example 3 below, the specific surface area (BET), pore volume and particle size (measured by a particle size analyzer (PSA), D50, unit: ㎛) were analyzed and measured, and are shown in Table 1 below. The specific surface area, micropore volume and mesopore volume were measured by a nitrogen adsorption method (BET, measuring equipment manufacturer: Micromeritics, measuring equipment model name: ASAP2460), and the macropore volume was measured by a mercury porosimetry (measuring equipment: Porosimeter, measuring equipment manufacturer: Micromeritics, measuring equipment model name: AutoPoreIV). In Table 1 below, 'micro' means micropore, 'meso' means mesopore, and 'macro' means macropore.

[0074] Specific surface area (m 2 / g)Pore volume (cm) 3 / g) Particle size (㎛) micromesomacro Porous carbon structure A 3 8 7 0.13 0.17 1.05 4.8 Porous carbon structure B 1 9 7 0.06 0.06 0.6 9 4.5 Carbon structure C (graphite) 2.4 0 0 4.6

[0075] Example 1

[0076] (1) 50 g of the porous carbon structure A manufactured in Manufacturing Example 1 as a base material was placed inside a rotary kiln, which is a reaction device, and purged by flowing argon gas at 1.5 slm for 1 hour. Then, the rotary kiln was rotated at 1.0 rpm and heated at a rate of 10 ℃ / min to the target temperature of 480 ℃.

[0077] (2) After reaching the target temperature, a mixture of SiH4 gas and H2 gas (SiH4: H2 = 0.5 slm: 1.5 slm) was injected into the rotary kiln for 7.5 minutes, and a chemical vapor deposition (CVD) reaction was performed while injecting 3.75 L / time of SiH4, thereby depositing silicon dot particles with an average particle diameter of approximately 14 to 15 nm on the porous carbon structure.

[0078] (3) After the above reaction was completed, the supply of SiH4 and H2 mixed gas was stopped, and Ar gas was injected at 1.5 slm for 10 minutes to flush out the remaining gas.

[0079] (4) In order to protect the surface of the silicon dot particles and suppress the growth of a silicon layer rather than a silicon dot particle, a chemical vapor deposition (CVD) reaction was performed by injecting a mixture of C2H2 gas and Ar gas (C2H2: Ar = 0.5 slm: 1.5 slm) for 15 minutes, thereby forming a carbon protective layer with a thickness of approximately 1.7 nm on the surface of the silicon dot particles.

[0080] (5) After the above reaction was completed, the supply of C2H2 and Ar mixed gas was stopped, and Ar gas was injected at 1.5 slm for 10 minutes to flush out the remaining gas.

[0081] The above processes (2) to (5) were repeated 23 times, divided into 24 times in total, and silicon gas was injected. Then, the rotary kiln was cooled to room temperature, and the silicon-based anode material (final weight of approximately 120 g) was recovered. Based on this, it can be seen that approximately 70 g of silicon is contained, excluding 50 g of the base material, and it is calculated that approximately 58.3 wt% of silicon dot particles are contained. The amount and number of silicon injections and the measured specific surface area of ​​the manufactured silicon-based anode material are summarized and shown in Table 2 below.

[0082] Example 2

[0083] A silicon-based negative electrode material was manufactured in the same manner as in Example 1, but 50 g of porous carbon structure B was used as a base material instead of porous carbon structure A, and there was a design difference in that the total number of silicon injections was reduced from 24 to 20 by changing the injection time per silicon injection from 7.5 minutes to 9 minutes, as shown in Table 2 below. The silicon-based negative electrode material manufactured in Example 2 was also recovered in an amount of 120 g. In addition, it was confirmed to contain about 58.5% of silicon dot particles, in the same manner as in Example 1, and it was possible to manufacture a silicon-based negative electrode material in which the silicon dot particles were deposited in a cluster-forming state with an average particle diameter of 16 to 18 nm.

[0084] Comparative Example 1

[0085] The same method as in Example 1 was carried out, but as shown in Table 2 below, the SiH4: H2 mixed gas of (2) (SiH4: H2 = 0.5 slm: 1.5 slm) was injected for 3.0 hours, thereby injecting a total of 90 L of silicon in the same manner as in Example 1. The processes (2) to (5) were performed without repetition. As a result, it was observed that a single silicon layer, rather than large silicon particles or dot particles having a particle size of 100 to 200 nm inside and outside the porous structure, was formed, and a silicon-carbon composite in the form of silicon dot particles having a silicon content of about 60 wt% was obtained as a silicon-based negative electrode material. The silicon-based negative electrode material manufactured in Comparative Example 1 was also recovered in an amount of 120 g, and it was confirmed to contain about 60 wt% of silicon dot particles in the same manner as in Example 1.

[0086] Comparative Example 2

[0087] The same method as (2) to (5) of the above Example 1 was carried out, but 50 g of 'porous carbon structure B' was used as a base material instead of 'porous carbon structure A', and the SiH4: H2 mixed gas (SiH4: H2 = 0.5 slm: 1.5 slm) of (2) was injected for 30 minutes, and the silicon injection amount and method are shown in Table 2 below. As a result, it was observed that the silicon included in the silicon-based negative electrode material manufactured in Comparative Example 2 was not silicon dot particles, but rather one or more silicon layers (Si layers) were cross-deposited while overlapping silicon dot particles. The silicon-based negative electrode material manufactured in Comparative Example 2 was also recovered in an amount of 120 g, and it was confirmed to contain about 59.7 wt% of silicon dot particles, similar to Example 1.

[0088] Comparative Example 3

[0089] 20 g of 'Carbon Structure C', graphite as a parent material, was placed inside a rotary kiln, which was a reaction device, and purged by flowing argon gas at 1.5 slm for 1 hour. Then, the rotary kiln was rotated at 1.0 rpm and heated at a rate of 10 ℃ / min to the target temperature of 480 ℃.

[0090] After reaching the target temperature, a chemical vapor deposition (CVD) reaction was performed by injecting a mixture of SiH4 gas and H2 gas (SiH4: H2 = 0.3 slm: 0.9 slm) into the rotary kiln for 10 minutes, forming a silicon layer (Si layer) with a thickness of about 20 nm. After the rotary kiln was cooled to room temperature, the composite was obtained as a silicon-based anode material (final weight: about 22 g). A silicon-carbon composite with a silicon content of about 8.9 wt% was obtained as a silicon-based anode material. In Comparative Example 3, a carbon structure rather than a porous structure was used as a base material, and in order to deposit silicon particles with a size similar to that of Examples 1 and 2, the silicon injection amount was adjusted to be smaller than in Examples 1 and 2 as described above.

[0091] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Base material 50g 50g 50g 50g 20g Amount of negative electrode material yield 120g 120g 120g 120g 22g Total SiH4 input (L) 90 90 90 90 3 Total Si input (g) 112.8 112.8 112.8 112.8 3.8 1 times Si input (g) 4.3 5.1 10 2.9 17.13.4 SiH4 input method 0.5 L x 7.5 min x 24 times 0.5 L x 9 min x 20 times 0.5 L x 180 min x 1 time 0.5 L x 30 min x 6 times 0.3 L x 10 min x 1 time

[0092] * At 0℃ and 1 atm, 1L of SiH4 gas contains 1.253g of Si element.

[0093] [Experimental Example]

[0094] 1) Experimental Example 1: Surface Observation

[0095] SEM and TEM images for observing the surface of the manufactured silicon-based negative electrode material were obtained by the following method.

[0096] <SEM (Scanning Electron Microscope) 사진>

[0097] A slurry was made by mixing a silicon-based negative electrode material, a CMC / SBR binder, and water in a weight ratio of 8:1:1, and the slurry was coated on a copper electrode plate with a thickness of approximately 100 μm using a doctor blade, and then dried at 100°C to produce an analytical electrode plate. A cross-section was created by irradiating the copper electrode plate with an ion beam in the thickness direction using an ion milling device, and SEM photographs were taken.

[0098] <TEM (Transmission Electron Microscope) 사진>

[0099] After attaching the silicon-based cathode material to the TEM grid, a cross-sectional specimen with a thickness of approximately 1 μm was created using FIB (Focused Ion Beam) equipment, and TEM images were taken.

[0100] Cross-sectional TEM images of the surface of the silicon-based negative electrode material obtained in Examples 1 and 2 are shown in FIGS. 1 and 2, respectively, and cross-sectional SEM images are shown in FIGS. 3 and 4, respectively. The SEM image of the surface of the silicon-based negative electrode material obtained in Comparative Example 1 is shown in FIG. 5. The cross-sectional TEM image of the surface of the silicon-based negative electrode material obtained in Comparative Example 2 is shown in FIG. 6. The cross-sectional SEM image of the surface of the silicon-based negative electrode material obtained in Comparative Example 3 is shown in FIG. 7.

[0101] Referring to FIGS. 1 to 4, which are TEM and SEM photographs showing a portion of the surface of the negative electrode material of Examples 1 and 2, it was confirmed that a carbon coating layer was deposited on a silicon cluster having a dot shape on a porous carbon structure and that there were voids between the silicon clusters. In addition, it was clearly confirmed from FIGS. 1 to 4 that silicon dot particles having a diameter of 20 nm or less did not overlap and formed clusters.

[0102] Specifically, in the TEM image of Example 1, the gray area marked as “A” in FIG. 1 exemplarily shows a part of the carbon structure, the dark black area marked as “B” exemplarily shows a silicon nanodot cluster, and the white area marked as “C” exemplarily shows a void where the silicon nanodot cluster is not formed. In addition, when looking at the silicon nanodot cluster, an almost white coating layer is observed between the silicon dot-shaped particles, which corresponds to the carbon coating layer. As a result of randomly selecting and measuring the silicon nanodot particles of Example 1, their particle sizes were measured to be 14.79 nm, 12.64 nm, and 16.07 nm, respectively, and it can be observed with the naked eye that most of the silicon nanodot particles have similar sizes. In addition, as a result of randomly selecting and measuring the thickness of the silicon nanodot cluster deposited while accumulating on the outside of the carbon structure of the silicon-based anode material of Example 1, the values ​​were 26.65 nm and 46.13 nm.

[0103] Fig. 2, a TEM photograph of Example 2, is a color-inverted representation to clearly show the structure of the silicon-based anode material. The gray portion indicated by “a” in Fig. 2 exemplarily shows a portion of the carbon structure, the white portion indicated by “b” exemplarily shows a silicon nanodot cluster, and the dark black portion indicated by “c” exemplarily shows a void where a silicon nanodot cluster is not formed. In addition, when examining the silicon nanodot cluster, an almost black coating layer is observed between the silicon dot-shaped particles, which corresponds to a carbon coating layer.

[0104] Fig. 3, which is an SEM photograph of Example 1, is a cross-sectional photograph of the manufactured silicon-based negative electrode material. The central part of Fig. 3 shows silicon nanodot clusters formed in the internal pores of the silicon-based negative electrode material, and silicon nanodot particles are exemplarily indicated by “solid circles.” As a result of randomly selecting and measuring the silicon nanodot particles of Example 1, their particle sizes were measured to be 15.04 nm and 15.00 nm, respectively, and it can be observed with the naked eye that most of the silicon nanodot particles have similar sizes.

[0105] Fig. 4, which is an SEM photograph of Example 2, is a cross-sectional photograph of the manufactured silicon-based negative electrode material. The upper left part of Fig. 4 shows silicon nanodot clusters formed in the internal pores of the silicon-based negative electrode material, and silicon nanodot particles are exemplarily indicated by “solid circles.”

[0106] In Fig. 5, which is an SEM photograph of Comparative Example 1, the white portion is filled with silicon, the dark gray portion represents a carbon structure, and the black portion represents empty space. Referring to Fig. 5, which is an SEM cross-sectional photograph of the silicon-based negative electrode material obtained in Comparative Example 1, it was confirmed that silicon was injected all at once rather than in parts and that silicon was deposited on the carbon structure without forming a carbon coating layer, resulting in the formation of a single silicon layer, and that the silicon layer or silicon dot particles could not be distinguished.

[0107] In Fig. 6, which is a TEM photograph of Comparative Example 2, the white part is the silicon part, and it can be seen that the particles are indistinguishable and overlap to form a layer phase, and also, the black line between the white parts corresponds to the carbon coating layer. The gray part indicated by the solid line in Fig. 6 exemplarily indicates a part of the carbon structure, and the black part indicated by the dotted line exemplarily indicates a pore. Referring to Fig. 6, which is a TEM cross-sectional photograph of the silicon-based anode material obtained in Comparative Example 2, although a carbon coating layer was formed, since the number of times silicon was divided and injected was reduced, silicon giant dot particles (>20 nm) distinguished by the internal carbon coating layer were observed even when the same amount of silicon was deposited.

[0108] The dark black part on the left side of Fig. 7, which is an SEM photograph of Comparative Example 3, represents graphite, which is a carbon structure, and the light gray layer formed on top of it means that a silicon layer has been formed. The clumped structure shown on the right side and bottom of Fig. 7 is a lump of binder that was mixed during the SEM photographing. Referring to Fig. 7, which is an SEM cross-sectional photograph of the silicon-based negative electrode material obtained in Comparative Example 3, it was confirmed that since graphite, not a porous carbon structure, was used as the structure, silicon was slightly deposited only on the outside of the structure, not the inside.

[0109]

[0110] 2) Experimental Example 2: Half-Cell Test - Initial Capacity Verification

[0111] The negative electrode materials manufactured in Examples 1 and 2 and Comparative Examples 1 to 3 were prepared as negative electrode active materials. Separately, conductive carbon black (Super P) and carbon nanotube conductive material (SWCNT) were mixed (Super P: SWCNT = 9.9:0.1, weight ratio) to prepare a conductive material.

[0112] Separately, a binder was prepared by mixing CMC (sodium salt of carboxymethyl cellulose) and SBR (styrene butadiene rubber) (CMC: SBR = 3:7, weight ratio). The prepared negative electrode material, conductive material, and binder were mixed using a sinky mixer (negative electrode material: conductive material: binder = 8:1:1, weight ratio) to obtain a negative electrode slurry. The negative electrode slurry was applied to a copper foil with a thickness of 18 μm and dried at 80°C for 1 hour. Then, after heat treatment in a vacuum oven at 100°C for 12 hours, it was punched to 14 mmØ to manufacture a negative electrode plate. The manufactured negative electrode plate was manufactured into a coin cell using Li metal as a counter electrode, and the electrochemical characteristics were confirmed. The charge / discharge conditions were charge CC / CV: 0.005V / 0.005C, discharge CC 1.0V, and the rate was 0.1C. Using TOSCAT-3100 equipment, charge / discharge was performed, and the initial charge capacity (ICC, mAh / g), initial efficiency (ICE, %), and initial discharge capacity (IDC, mAh / g) were measured. The results are shown in Table 3 below.

[0113]

[0114] 3) Experimental Example 3: Discharge Curve Experiment (Plateau Verification) - Silicon Particle Size Verification

[0115] According to the half-cell manufactured in Experimental Example 1 and the charge / discharge conditions, the initial discharge curve experimental graph of one cycle for measuring the initial discharge capacity was plotted, and then the plateau slope was calculated based on this according to the following equation (1). Since a plateau occurs in the 0.4 to 0.5 V region in the discharge curve of a secondary battery when silicon particles with a particle size of 20 nm or more are included, the occurrence of a plateau was determined by whether the plateau slope value was 2.0 or more. It was determined that the plateau occurrence was suppressed only when the plateau slope value calculated according to the following equation (1) was 2.0 or more. In addition, if the plateau slope value was 2.0 or more, it means that the average silicon particle size did not exceed 20 nm. In addition, as shown in the following Experimental Example 3, it was confirmed that when applied as an actual negative electrode material, the lifespan of a secondary battery was significantly increased, which could contribute to the improvement of the performance of the secondary battery.

[0116] Equation (1): (Capacity of total cathode material) / (Specific capacity at 0.5 V - Specific capacity at 0.4 V)

[0117] The plateau slope calculated according to the above equation (1) is shown in Table 1 below. In the present invention, the “plateau slope” is a value calculated according to the above equation (1) and is a value for normalization by dividing the specific capacity at 1.0 V by (specific capacity at 0.5 V - specific capacity at 0.4 V).

[0118] Since the discharge voltage under the conditions of this experimental example is 1.0 V, the specific capacity at 1.0 V means the capacity of the entire negative electrode material. The value according to the above equation (1) is proportional to the capacity of the entire negative electrode material.

[0119] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Initial Charge Capacity (ICC) (mAh / g) 2 2 4 1 2 1 8 1 2 4 2 8 2 2 9 7 6 5 9 Initial Efficiency (ICE) (%) 8 7.1 8 9.6 8 8.6 8 8.9 8 8.9 Initial Discharge Capacity (IDC) (mAh / g) 1 9 5 3 1 9 5 1 2 1 5 2 2 0 7 2 5 8 5 Plateau Slope 5.7 3 3.5 6 1.7 8 2.9 6 1.2

[0120] The silicon-based negative electrode materials manufactured in Examples 1 and 2 and Comparative Examples 1 to 3 had an initial discharge capacity (IDC) of 1900 mAh / g or more, but the plateau slope and TEM and SEM images in Table 3 confirmed that the silicon dot particle size formed in Examples 1 and 2 was 20 nm or less, while that of Comparative Examples 1 and 2 was 20 nm or more. Comparative Example 3, which had a low silicon deposition amount, was measured to have significantly lower charge and discharge capacities than that of Example 1. In order to confirm the effect of the silicon dot particle size and the structure in which carbon is coated on the silicon dot particle on the lifespan of a secondary battery, a coin full cell was manufactured and evaluated as in Experimental Example 4 below.

[0121]

[0122] 4) Experimental Example 4: Coin full-cell manufacturing and evaluation

[0123] In Example 1 and Comparative Examples 1 and 2, 8.8 wt% of the silicon-based negative electrode material was prepared by mixing 91.2 wt% of graphite with a weight ratio to prepare a negative electrode active material. Separately, conductive carbon black (Super P) and carbon nanotube conductive material (SWCNT) were mixed (Super P: SWCNT = 9: 1, weight ratio) to prepare a conductive material. Separately, sodium salt of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were mixed (CMC: SBR = 3: 7, weight ratio) to prepare a binder. The negative electrode material, conductive material, and binder prepared above were mixed using a sinky mixer (negative electrode material: conductive material: binder = 95.8: 1: 3.2, weight ratio) to obtain a negative electrode slurry. The above cathode slurry was applied to a copper foil having 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 mmØ to manufacture a cathode plate.

[0124] As confirmed in Experimental Example 2, the silicon-based negative electrode material manufactured in Comparative Example 3 had a small discharge capacity, so in order to prepare a negative electrode active material having the same capacity conditions, 64 wt% of the silicon-based negative electrode material of Comparative Example 3 was mixed with 36 wt% of graphite to prepare a negative electrode active material, and the negative electrode plate was manufactured in the same manner as in Example 1.

[0125] The positive electrode was manufactured by mixing the positive electrode material NCM811, conductive carbon black (Super P), and PVDF in a weight ratio of 96:2:2, and punching the plate to 14 mmØ to manufacture the positive electrode. Using the above negative and positive electrodes, a coin full cell (CR2032) was manufactured and the electrochemical characteristics were confirmed. The charge and discharge conditions for the initial 1-2 cycles (cycle formation) were as follows: charge CC / CV was 4.25 V / 0.05 C, discharge CC was 2.5 V, and the rate was 0.1 C. After that, for 100 cycles, only the rate was changed to 0.5 C, and charge and discharge were performed to measure the life characteristics (cycle retention, %), and the resulting graph is shown in Fig. 8. The lifespan (%) is expressed as a percentage by dividing the discharge capacity of the nth cycle by the discharge capacity of the 3rd cycle based on the discharge capacity of the 3rd cycle performed at a rate of 0.5C, as shown in Table 4 below. The lifespan in Table 4, which is the experimental result, actually refers to the lifespan of the 103rd cycle. In addition, the lifespan (%) characteristics according to the charge / discharge cycle are plotted and shown in Figure 8.

[0126] In the case of Examples 1 and 2, since the silicon within the silicon-carbon composite is separated by a carbon coating layer so that it has a small dot shape of 20 nm or less, the lifespan reduction due to volume expansion of the silicon is prevented, and the lifespan characteristics were measured to be the highest.

[0127] In the case of Comparative Example 1, since there is no carbon coating layer inside the complex, silicon exists in the form of large dots (>20 nm) or small dots are connected so that the silicon giant particles appear like a silicon layer, making it difficult to prevent the volume expansion of silicon, resulting in the lowest lifespan.

[0128] In the case of Comparative Example 2, although a carbon coating layer exists, the lifespan was reduced compared to Example 1 because each silicon dot particle separated by the carbon coating layer was 20 nm or larger.

[0129] In Comparative Example 3, since silicon was deposited on the graphite surface with a small BET surface area, a silicon layer (thickness >20 nm) was formed that surrounded the graphite like a giant silicon particle. Although the thickness of the silicon layer itself may be less than that of Comparative Example 1, since the silicon existing only on the surface forms a huge lump, it exhibits the behavior of a giant silicon particle, making it difficult for the graphite to prevent the volume expansion of the silicon, resulting in the fastest decrease in lifespan.

[0130] Coin Full Cell Life (%) (@100 Cycles) Example 192.0% Example 289.3% Comparative Example 179.0% Comparative Example 284.0% Comparative Example 340.8%

[0131] Although the embodiments of the present specification have been described in more detail with reference to the attached drawings, the present specification is not necessarily limited to these embodiments, and various modifications may be implemented without departing from the technical spirit of the present specification. Therefore, the embodiments disclosed in this specification are not intended to limit the technical spirit of the present specification, but to explain, and the scope of the technical spirit of the present specification is not limited by these embodiments. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of protection of this specification should be interpreted by the claims, and all technical ideas within a scope equivalent thereto should be interpreted as being included in the scope of rights of this specification.

Claims

1. Porous carbon structure; and including silicon nanodot clusters deposited inside and outside the porous carbon structure; The above silicon nanodot cluster means an aggregate containing one or more silicon nanodot particles, and the silicon nanodot particles have a maximum particle diameter of 20 nm or less. A carbon coating layer having a thickness of 5 nm or less is formed on at least a portion of the surface of the silicon nanodot particles. Silicon-based cathode material.

2. In paragraph 1, The above silicon nanodot cluster exists in an amorphous or semi-crystalline form. Silicon-based cathode material.

3. In paragraph 1, When the total weight of the above silicon-based negative electrode material is 100 wt%, the content of the silicon nanodot cluster is 50 wt% or more. Silicon-based cathode material.

4. In paragraph 1, The average particle diameter of the above silicon-based negative electrode material is 50 nm or more and 20 μm or less. Silicon-based cathode material.

5. In paragraph 1, The thickness of the silicon nanodot cluster formed on the outside of the porous carbon structure is 10 nm to 1000 nm. Silicon-based cathode material.

6. A negative electrode active material comprising a silicon-based negative electrode material according to Article 1.

7. A secondary battery comprising a negative electrode active material according to Article 6.

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