Silicon-carbon composite material of egg yolk-shell structure, method for preparing same, and anode active material comprising said silicon-carbon composite material of egg yolk-shell structure

Abstract

The present invention relates to a silicon-carbon composite material of a yolk-shell structure, a method for preparing the same, and an anode active material comprising the silicon-carbon composite material of the yolk-shell structure. The silicon-carbon composite material of an egg yolk-shell structure according to one embodiment of the present invention can be prepared without using a strong acid, and can have uniform voids formed using an inorganic layer of uniform thickness as a sacrificial layer to accommodate silicon volume expansion when used as an anode active material, therefore, peeling of the outermost carbon thin film caused thereby can be suppressed. As a result, deterioration in battery performance and lifetime can be suppressed.

Description

Technical Field

[0001] The present invention relates to a silicon-carbon composite material with a yolk-shell structure, and more particularly to a silicon-carbon composite material with a yolk-shell structure, a preparation method thereof, and an anode active material comprising the silicon-carbon composite material with a yolk-shell structure. Background Art

[0002] With recent advancements in the information and communications industries, demand for electronic devices has surged, and as the electric vehicle market has rapidly expanded, demand for batteries used in these electronic devices and electric vehicles has also increased.

[0003] Secondary batteries, such as lithium secondary batteries and all-solid-state batteries containing liquid electrolytes, are most widely used in such applications because they have high energy density and exhibit low self-discharge when not in use. Secondary batteries typically consist of a cathode, an anode, and an electrolyte (liquid or solid), and carbon materials such as graphite are widely used as the anode active material in secondary batteries.

[0004] In recent years, efforts have focused on using silicon-based anode active materials to increase the capacity of secondary batteries. Silicon, due to its theoretically high energy density, has attracted attention as a next-generation battery anode active material to replace graphite. However, during charge and discharge, it reacts with lithium and undergoes volume expansion of up to approximately 300%, significantly reducing mechanical stability and causing cracking of silicon anode active materials during charge-discharge cycles.

[0005] To address this issue, a silicon-carbon composite material with a yolk-shell structure has been proposed. This material utilizes porous silicon particles with internal voids to accommodate silicon volume expansion, and a carbon film is formed on the particle surface. To prepare this composite material, an inorganic material is coated on the surface of the silicon particles, which serve as the yolk, and the coated layer is then etched using a strong acid such as hydrofluoric acid.

[0006] Therefore, there is a need to develop a new material that can be used as a sacrificial layer in a more environmentally friendly and simpler way. Summary of the Invention

[0007] Technical issues

[0008] The present invention provides a yolk-shell structured silicon-carbon composite material that can suppress battery performance degradation caused by volume expansion of silicon when used as an anode active material by forming uniform voids using an inorganic layer having a uniform thickness as a sacrificial layer.

[0009] The present invention also provides a method for efficiently preparing the silicon-carbon composite material with the yolk-shell structure.

[0010] The present invention also provides an anode active material comprising the silicon-carbon composite material with the yolk-shell structure.

[0011] Technical Solution

[0012] The present invention provides a method for preparing a silicon-carbon composite material with a yolk-shell structure, the method comprising: (1) supplying silicon particles into a reactor; (2) forming an inorganic layer on the silicon particles by atomic layer deposition (ALD) to prepare a silicon-inorganic layer composite material; (3) forming a carbon film on the silicon-inorganic layer composite material to obtain a silicon-inorganic layer-carbon composite material; and (4) removing at least a portion of the inorganic layer from the silicon-inorganic layer-carbon composite material to produce a silicon-carbon composite material with a yolk-shell structure.

[0013] According to one embodiment of the present invention, step (2) may include: (2-1) supplying a first precursor into the reactor to form a first precursor multilayer, wherein at least a portion of the first precursor is adsorbed on the silicon particles; (2-2) purging the interior of the reactor to remove unadsorbed first precursor from the first precursor multilayer, thereby forming a first precursor monolayer; (2-3) supplying a second precursor into the reactor so that at least a portion of the second precursor reacts with the first precursor monolayer to form a composite material layer; and (2-4) purging the interior of the reactor to remove unreacted second precursor from the composite material layer, thereby forming an inorganic layer, wherein steps (2-1) to (2-4) constitute one cycle, and the cycle can be performed one or more times.

[0014] In addition, the supply time of the first precursor and the supply time of the second precursor may each independently be 1 to 600 seconds.

[0015] In addition, the first precursor may include at least one selected from the group consisting of trimethylaluminum, diethylzinc, zinc acetate, tetrakis(dimethylamino)tin(IV), butoxytris(ethylmethylamino)hafnium, isopropoxytitanium, and diisopropylaminosilane.

[0016] In addition, the second precursor may include at least one selected from the group consisting of H2O, O3, H2O plasma, O3 plasma, and O2 plasma.

[0017] In addition, the gas used for purging in at least one of steps (2-2) and (2-4) may include nitrogen (N2) or argon (Ar).

[0018] Additionally, the cycle may be performed 1 to 300 times.

[0019] Additionally, the temperature of the silicon particles may be maintained at 90 to 400° C. during the cycle.

[0020] Alternatively, step (4) may be performed by etching the inorganic layer from the silicon-inorganic layer-carbon composite material.

[0021] In addition, the present invention provides a silicon-carbon composite material with a yolk-shell structure, which comprises: silicon particles; and a carbon film formed on the silicon particles, with a predetermined gap between the carbon film and the silicon particles.

[0022] According to one embodiment of the present invention, the average particle size of the silicon particles may be 10 nm to 50 μm, and the average thickness of the carbon thin film may be 1 nm to 1 μm.

[0023] In addition, the porosity of the yolk-shell structured silicon-carbon composite material may be 10% to 80%.

[0024] In addition, the present invention provides an anode active material, which comprises the silicon-carbon composite material with the yolk-shell structure.

[0025] In addition, the present invention provides an all-solid-state battery, which includes an SEI (solid electrolyte interface) layer, and the SEI layer contains the above-mentioned yolk-shell structured silicon-carbon composite material.

[0026] Beneficial effects

[0027] The yolk-shell silicon-carbon composite material according to one embodiment of the present invention is manufactured based on an inorganic layer formed using atomic layer deposition, which enables precise control of the thickness of the sacrificial layer. Furthermore, by using an inorganic layer of uniform thickness as the sacrificial layer to form uniform voids, the yolk-shell silicon-carbon composite material can accommodate the volume expansion of silicon when used as an anode active material, thereby suppressing the peeling of the outermost carbon film caused by this expansion. As a result, degradation of battery performance and lifespan can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a transmission electron microscope (TEM) image of a silicon-inorganic layer composite material that is an intermediate of the silicon-carbon composite material (Example 1) according to one embodiment of the present invention.

[0029] Figure 2 is a transmission electron microscope-energy dispersive spectrometer (TEM-EDS) image of a silicon-inorganic layer composite material that is an intermediate of the silicon-carbon composite material (Example 1) according to one embodiment of the present invention.

[0030] Figure 3 is a transmission electron microscope (TEM) image of a silicon-inorganic layer-carbon composite material that is an intermediate of the silicon-carbon composite material (Example 1) according to one embodiment of the present invention.

[0031] Figure 4 is a TEM-EDS image of a silicon-inorganic layer-carbon composite material which is an intermediate of the silicon-carbon composite material (Example 1) according to one embodiment of the present invention.

[0032] Figure 5 is a TEM image of a silicon-carbon composite material (Example 1) according to one embodiment of the present invention.

[0033] Figure 6 is a TEM-EDS image of a silicon-carbon composite material according to one embodiment of the present invention (Example 1).

[0034] Figure 7 is an image of an atomic distribution curve of a silicon-carbon composite material (Example 1) according to one embodiment of the present invention.

[0035] Figure 8 is a TEM image of a silicon-inorganic layer composite material which is an intermediate of the silicon-carbon composite material (Example 2) according to one embodiment of the present invention.

[0036] Best Mode for Carrying Out the Invention

[0037] The present invention is not limited to the embodiments disclosed below, and may be changed in various forms as long as the spirit of the invention is not changed.

[0038] In the present specification, unless otherwise specified, the term "comprise" or "include" means that other parts / components may further be included.

[0039] Unless otherwise indicated, all numerical values ​​and expressions expressing amounts of parts / components, reaction conditions, and so forth described in this specification are to be understood as being modified in all instances by the term "about."

[0040] In this specification, when a component is described as being formed on, below, connected to, or coupled to another component, such description is intended to include both: direct formation, connection, or coupling between the components, and indirect formation, connection, or coupling via another component.

[0041] Hereinafter, the present invention will be described in more detail.

[0042] Silicon-carbon composite material with yolk-shell structure

[0043] The present invention provides a silicon-carbon composite material with a yolk-shell structure. The silicon-carbon composite material with a yolk-shell structure comprises silicon particles and a carbon film formed on the silicon particles, with a predetermined gap between the silicon particles and the carbon film.

[0044] According to one embodiment of the present invention, a silicon-carbon composite material with a yolk-shell structure is obtained using a silicon-inorganic layer composite material and a silicon-inorganic layer-carbon composite material as intermediates, wherein the silicon-inorganic layer composite material comprises silicon particles and an inorganic layer formed on the silicon particles, and the silicon-inorganic layer-carbon composite material comprises a carbon thin film formed on the silicon-inorganic layer composite material.

[0045] Therefore, the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention will be described below in conjunction with intermediates: a silicon-inorganic layer composite material and a silicon-inorganic layer-carbon composite material.

[0046] Silicon particles

[0047] In one embodiment of the present invention, the intermediates (ie, the silicon-inorganic layer composite material and the silicon-inorganic layer-carbon composite material) and the final product (ie, the yolk-shell structured silicon-carbon composite material) contain silicon particles.

[0048] When the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention is used as an anode active material, since the silicon particles in the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention are used to store lithium, the silicon particles can play the role of the main anode active material.

[0049] Silicon particles can be crystalline or amorphous, and from the perspective of expansion / contraction during the charge and discharge of secondary batteries and overall battery performance, they can preferably be amorphous or in a similar phase. When silicon particles are crystalline, smaller crystallite size allows the formation of a denser composite material, thereby improving the strength of the matrix and preventing cracking. Therefore, the initial efficiency and cycle life characteristics of the secondary battery can be improved. On the other hand, when silicon particles are amorphous or in a similar phase, the expansion or contraction during the charge and discharge of the secondary battery can be reduced, and battery performance (such as capacity characteristics) can be improved.

[0050] The size of the silicon particles can be selected to be suitable for the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention, which is to be used as an anode active material. Specifically, the average particle size of the silicon particles can be 10 nm to 50 μm, and preferably 10 nm to 25 μm. When the silicon particles meet the above average particle size range, they can more advantageously achieve the objectives of the present invention.

[0051] The silicon particles may also contain a silicon oxide compound. The silicon oxide compound can be represented by the general formula SiOx, where 0.5 ≤ x ≤ 2. Here, when the value of x is less than 0.5, expansion and contraction during charging and discharging of the secondary battery may become significant, resulting in deterioration of life characteristics. When x exceeds 2, the amount of inactive oxide increases, which may lead to a decrease in the initial efficiency of the secondary battery.

[0052] The content of the silicon oxide compound in the silicon particles may be 50 wt % or less based on the total weight of the silicon particles. When the content of the silicon oxide compound in the silicon particles exceeds 50 wt %, initial efficiency of the secondary battery may deteriorate.

[0053] In addition to silicon and oxygen, the silicon particles may also contain elements such as carbon or magnesium, but the silicon content in the silicon particles may be selected so that the total silicon content in the silicon-carbon composite material is 1% to 80% by weight, and preferably 5% to 50% by weight. When the total silicon content in the silicon-carbon composite material is less than 1% by weight, the amount of active material capable of absorbing and releasing lithium may be insufficient, which may result in a decrease in the charge / discharge capacity of the secondary battery. When the total silicon content in the silicon-carbon composite material exceeds 80% by weight, the charge / discharge capacity of the secondary battery can be increased; however, the expansion and contraction of the electrode during charge and discharge may become excessive, and the anode active material powder may be further crushed, which may result in deterioration of the cycle characteristics of the secondary battery.

[0054] Inorganic layer

[0055] In one embodiment of the present invention, the intermediate silicon-inorganic layer composite material includes an inorganic layer formed on the surface of silicon particles. The inorganic layer is formed by an atomic layer deposition (ALD) process to be described later.

[0056] The inorganic layer is formed by ALD and serves as a sacrificial layer for forming the carbon thin film. The material of the inorganic layer is not particularly limited as long as it can be sufficiently removed by the etching process described later to enable the production of a yolk-shell structured silicon-carbon composite material.

[0057] In a specific embodiment of the present invention, the inorganic layer may include at least one selected from the group consisting of aluminum oxide (Al2O3), zinc oxide (ZnO), tin oxide (SnO2), hafnium oxide (HfO), zirconium oxide (ZrO2), titanium oxide (TiO2) and silicon oxide (SiO2), but is not particularly limited thereto. In a preferred specific embodiment of the present invention, the inorganic layer may include aluminum oxide (Al2O3).

[0058] In one embodiment of the present invention, the thickness of the inorganic layer may be 1 nm to 10 μm, preferably 1 nm to 1 μm, and more preferably 1 to 100 nm. When the thickness of the inorganic layer is within this range, the inorganic layer can appropriately function as a sacrificial layer during the etching step to be described later, thereby enabling the formation of suitable gaps between the silicon particles and the carbon film.

[0059] In one embodiment of the present invention, during the etching step to be described later, the inorganic layer as an intermediate on the surface of the silicon particles in the silicon-inorganic layer composite material can be at least partially removed, and the amount removed is preferably 10 wt % to 100 wt %, and more preferably 50 wt % to 100 wt %, thereby appropriately forming gaps between the silicon particles and the carbon film.

[0060] In the final product according to one embodiment of the present invention (i.e., the yolk-shell structured silicon-carbon composite material), the residue of the inorganic layer left after etching may be present between the silicon particles and the carbon film. Therefore, the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention may contain the residue of the inorganic layer between the silicon particles and the carbon film. However, the present invention is not limited thereto, and if the inorganic layer is completely removed by etching, the yolk-shell structured silicon-carbon composite material may not contain any residue of the inorganic layer between the silicon particles and the carbon film.

[0061] carbon film

[0062] A yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention includes a carbon thin film formed on silicon particles.

[0063] By including a carbon thin film, the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention can ensure appropriate conductivity and enable appropriate adjustment of the specific surface area, thereby further improving the life characteristics and capacity of the secondary battery when used as an anode active material.

[0064] The conductivity of the anode active material is an important factor that facilitates electron transfer during electrochemical reactions. By including a carbon film, the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention can improve the charge / discharge capacity, initial charge efficiency, and capacity retention of secondary batteries, provide excellent conductivity, inhibit side reactions of the electrolyte, and further enhance the performance of secondary batteries.

[0065] In the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention, the thickness of the carbon film may be 1 nm to 1 μm, preferably 3 to 150 nm, and more preferably 5 to 100 nm. When the thickness of the carbon film is within this range, improved conductivity can be achieved while suppressing a decrease in the capacity of the secondary battery.

[0066] The carbon thin film may include one or more selected from graphene, carbon nanotubes, carbon nanofibers, and graphite. Specifically, the carbon thin film may include graphene, and may also include graphite, but is not particularly limited thereto.

[0067] gap

[0068] In the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention, gaps exist between silicon particles and the carbon thin film.

[0069] As described above, in the silicon-inorganic layer composite material which is an intermediate for preparing a silicon-carbon composite material of a yolk-shell structure according to one embodiment of the present invention, the inorganic layer on the surface of the silicon particles serves as a sacrificial layer and is at least partially removed during the etching step to be described later, and the amount removed is preferably 10 wt % to 100 wt %, and more preferably 50 wt % to 100 wt %, thereby forming a gap between the silicon particles and the carbon film.

[0070] In a silicon-carbon composite material with a yolk-shell structure according to one embodiment of the present invention, the gaps between the silicon particles and the carbon film can accommodate the volume expansion of silicon when the silicon-carbon composite material with a yolk-shell structure is used as an anode active material, thereby inhibiting the peeling of the outermost carbon film caused by such expansion.

[0071] The porosity of the yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention may be 10% to 80%, preferably 10% to 50%, and more preferably 20% to 50%. Here, the porosity can be measured by the Brunauer-Emmett-Teller (BET) method, but is not particularly limited thereto. When the yolk-shell structured silicon-carbon composite material meets the above porosity range, it can advantageously achieve the purpose of the present invention.

[0072] Method for preparing silicon-carbon composite material with yolk-shell structure

[0073] A silicon-carbon composite material with a yolk-shell structure according to one embodiment of the present invention is prepared by a method comprising the following steps: (1) supplying silicon particles into a reactor; (2) forming an inorganic layer on the silicon particles by atomic layer deposition (ALD) to prepare a silicon-inorganic layer composite material; (3) forming a carbon thin film on the silicon-inorganic layer composite material to obtain a silicon-inorganic layer-carbon composite material; and (4) removing at least a portion of the inorganic layer from the silicon-inorganic layer-carbon composite material to produce a silicon-carbon composite material with a yolk-shell structure.

[0074] Atomic layer deposition (ALD) refers to a deposition method for growing thin films in atomic layers. In a typical chemical vapor deposition (CVD) method, reactants are introduced simultaneously, and both gas phase reaction and surface reaction occur simultaneously. In contrast, atomic layer deposition (ALD) suppresses gas phase reaction by sequentially introducing reactants, and deposition is carried out by surface reaction based on self-limiting adsorption on the target surface to be deposited. Since the total thickness of the deposited film is controlled by the number of deposition cycles, ALD has the advantage of being able to easily control thickness at the atomic layer level. In addition, due to its excellent performance such as compatibility with various materials, good thin film properties and processability at relatively low temperatures, ALD has been mainly used in semiconductor dielectrics, magnetic materials, etc. With the recent progress of nanotechnology, atomic layer deposition of inorganic materials is no longer limited to its traditional focus on integrated circuit devices, and is now used in a variety of fields.

[0075] Hereinafter, a step-by-step description will be given of a method for preparing a yolk-shell structured silicon-carbon composite material using atomic layer deposition (ALD).

[0076] Step (1)

[0077] In step (1), silicon particles are supplied to a reactor.

[0078] The specific details of the silicon particles used in step (1) are as described above in the section on the silicon-carbon composite material with a yolk-shell structure.

[0079] In the method for preparing a yolk-shell structured silicon-carbon composite material according to one embodiment of the present invention, the structure of the reactor is not particularly limited as long as it can form an inorganic layer on the surface of silicon particles using atomic layer deposition (ALD).

[0080] Step (2)

[0081] In step (2), an inorganic layer is formed on the silicon particles by atomic layer deposition (ALD) to prepare a silicon-inorganic layer composite material.

[0082] In this case, step (2) may include: (2-1) supplying a first precursor into a reactor to form a first precursor multilayer, wherein at least a portion of the first precursor is adsorbed on silicon particles; (2-2) purging the interior of the reactor to remove unadsorbed first precursor from the first precursor multilayer, thereby forming a first precursor monolayer; (2-3) supplying a second precursor into the reactor to form a composite material layer by reacting the first precursor monolayer with at least a portion of the second precursor; and (2-4) purging the interior of the reactor to remove unreacted second precursor from the composite material layer, thereby forming an inorganic layer, wherein steps (2-1) to (2-4) constitute one cycle, and such a cycle may be performed one or more times.

[0083] First, in step (2-1), a first precursor is supplied to the reactor to form a first precursor multilayer, wherein at least a portion of the first precursor is adsorbed on silicon particles.

[0084] In a specific embodiment of the present invention, the first precursor may include at least one selected from the group consisting of trimethylaluminum, diethylzinc, zinc acetate, tetrakis(dimethylamino)tin(IV), butoxytris(ethylmethylamino)hafnium, isopropoxytitanium and diisopropylaminosilane. In a preferred specific embodiment of the present invention, the first precursor may include trimethylaluminum.

[0085] In a specific embodiment of the present invention, the supply time of the first precursor can be 1 to 600 seconds. In a preferred specific embodiment of the present invention, the supply time of the first precursor can be more than 1 second, more than 2 seconds, more than 5 seconds, more than 10 seconds or more than 15 seconds, and is less than 600 seconds, less than 300 seconds, less than 100 seconds, less than 50 seconds or less than 20 seconds. When the supply time of the first precursor is within the above range, a layer of the first precursor can be appropriately formed on the silicon particles. However, if the supply time of the first precursor is less than 1 second, it may be difficult to form an inorganic layer with uniform thickness due to incomplete atomic layer deposition, and if the supply time exceeds 600 seconds, the inorganic layer may become too thick, and the purge time may also increase, resulting in an increase in the overall process time.

[0086] Next, in step (2-2), the supply of the first precursor is stopped, and the interior of the reactor is purged to remove the non-adsorbed first precursor from the first precursor multilayer, thereby forming a first precursor monolayer.

[0087] In one embodiment of the present invention, step (2-2) may be performed by purging with an inert gas, and the inert gas may include nitrogen (N2) or argon (Ar).

[0088] In a specific embodiment of the present invention, the purge time in step (2-2) can be 1 to 300 seconds. In a preferred specific embodiment of the present invention, the purge time can be more than 1 second, more than 2 seconds, more than 5 seconds, more than 10 seconds or more than 15 seconds, and is less than 300 seconds, less than 200 seconds, less than 100 seconds, less than 50 seconds or less than 20 seconds. If the purge time is too short, residues of the first precursor may be left, resulting in the formation of an inorganic oxide film thicker than expected. On the other hand, if the purge time is too long, the process efficiency may be reduced.

[0089] Next, in step (2-3), a second precursor is supplied to the reactor to react the first precursor monolayer with at least a portion of the second precursor, thereby forming a composite material layer.

[0090] In one embodiment of the present invention, the second precursor may include at least one selected from the group consisting of: H2O, O3, H2O plasma, O3 plasma, and O2 plasma. In a preferred embodiment of the present invention, the second precursor may include H2O.

[0091] The layer of the first precursor and the second precursor can react to form an inorganic layer on the particle, wherein the inorganic layer comprises at least one selected from the group consisting of: aluminum oxide (Al2O3), zinc oxide (ZnO), tin oxide (SnO2), hafnium oxide (HfO), zirconium oxide (ZrO2), titanium oxide (TiO2) and silicon oxide (SiO2).

[0092] In a specific embodiment of the present invention, the supply time of the second precursor can be 1 to 600 seconds. In a preferred specific embodiment of the present invention, the supply time of the second precursor can be more than 1 second, more than 2 seconds, more than 5 seconds, more than 10 seconds or more than 15 seconds, and is less than 600 seconds, less than 300 seconds, less than 100 seconds, less than 50 seconds or less than 20 seconds. When the supply time of the second precursor is within the above range, an inorganic layer can be appropriately formed on the silicon particles. However, if the supply time of the second precursor is less than 1 second, it may be difficult to form an inorganic layer with uniform thickness due to incomplete atomic layer deposition, and if the supply time exceeds 600 seconds, the inorganic layer may become too thick, and the purge time may also increase, resulting in an increase in the overall process time.

[0093] Next, in step (2-4), the supply of the second precursor is stopped, and the interior of the reactor is purged to remove the unreacted second precursor from the composite material layer, thereby forming an inorganic layer.

[0094] In one embodiment of the present invention, steps (2-4) may be performed by purging with an inert gas, and the inert gas may include nitrogen (N2) or argon (Ar).

[0095] In a specific embodiment of the present invention, the purge time in step (2-4) can be 1 to 300 seconds. In a preferred specific embodiment of the present invention, the purge time can be more than 1 second, more than 2 seconds, more than 5 seconds, more than 10 seconds or more than 15 seconds, and is less than 300 seconds, less than 200 seconds, less than 100 seconds, less than 50 seconds or less than 20 seconds. If the purge time is too short, residues of the second precursor may be left, resulting in the formation of an inorganic oxide film thicker than expected. On the other hand, if the purge time is too long, the process efficiency may be reduced.

[0096] In one embodiment of the present invention, the temperature of the silicon particles during the above cycle can be maintained at 90 to 400° C. In a preferred embodiment of the present invention, the temperature of the silicon particles during the above cycle can be maintained at 120 to 300° C. If the temperature of the silicon particles is lower than 90° C., defects may exist in the formed inorganic layer due to incomplete reaction, and if the temperature exceeds 400° C., there may be a problem of excessive formation of an inorganic thin film on the silicon particles.

[0097] In one embodiment of the present invention, the above cycle can be performed at a pressure of 0.5 to 10 Torr. In a preferred embodiment of the present invention, the above cycle can be performed at a pressure of 1 to 5 Torr.

[0098] In one embodiment of the present invention, the above cycle can be repeated 1 to 300 times. In a preferred embodiment of the present invention, the above cycle can be repeated 50 to 300 times. If the cycle is repeated more than 300 times, an excessive amount of inorganic thin film may be formed on the silicon particles, and the process time may be prolonged.

[0099] Step (3)

[0100] In step (3), a carbon thin film is formed on the silicon-inorganic layer composite material to obtain a silicon-inorganic layer-carbon composite material.

[0101] The step of forming a carbon film on the particles of the silicon-inorganic composite material as an intermediate can be performed using apparatus and methods known in the art of the present invention (such as chemical pyrolytic deposition), but is not particularly limited thereto. For example, the carbon film can be formed by carbonizing a polymer film formed by initiator-based chemical vapor deposition (iCVD).

[0102] In one embodiment of the present invention, the step of forming a carbon thin film on the intermediate silicon-inorganic layer composite material can be performed by heat-treating the intermediate silicon-inorganic layer composite material at a temperature of 400 to 1,400° C. in the presence of a gaseous carbon source.

[0103] In a preferred embodiment of the present invention, the carbon source may include at least one selected from the group consisting of methane, ethane, propane, butane, methanol, ethanol, propanol, propylene glycol, butanediol, ethylene, propylene, butylene, butadiene, cyclopentene, acetylene, benzene, toluene, xylene, ethylbenzene, naphthalene, anthracene and butylated hydroxytoluene, but is not particularly limited thereto.

[0104] The step of forming a carbon thin film on the silicon-inorganic layer composite material as an intermediate may be performed in the presence of at least one inert gas selected from the group consisting of hydrogen, nitrogen, helium, and argon, in addition to the above-mentioned carbon source.

[0105] The reaction time (heat treatment time) for forming the carbon thin film can be appropriately adjusted according to the heat treatment temperature, the pressure during the heat treatment, the composition of the gas mixture, and the desired carbon coating amount. For example, the reaction time may be 10 minutes to 100 hours, specifically 30 minutes to 90 hours, and more preferably 50 minutes to 40 hours, but is not particularly limited to these ranges.

[0106] In one specific embodiment of the present invention, the step of forming a carbon thin film on a silicon-inorganic layer composite material as an intermediate can be performed by mixing the silicon-inorganic layer composite material with such a solution in which a carbon source is dispersed in a solvent as needed, and then drying and heat-treating at a temperature of 400 to 1,400°C.

[0107] In a preferred embodiment of the present invention, the carbon source can be selected from the group consisting of: pitch, hydrocarbon-based material and petroleum-based material. More specifically, the pitch can be petroleum-based pitch, coal-based pitch or a mixture thereof; the hydrocarbon-based material can be furfuryl alcohol or phenolic resin; and the petroleum-based material can be pyrolysis fuel oil (PFO), naphtha cracking residue (NCB), ethylene cracking residue (EBO), vacuum residue (VR), deasphalted oil (DAO), atmospheric residue (AR), fluidized catalytic cracking decant oil (FCC-DO), residual oil fluidized catalytic cracking decant oil (RFCC-DO) or heavy aromatic oil. The solvent can be tetrahydrofuran (THF) or alcohol.

[0108] In addition, when the carbon thin film is formed by carbonizing a polymer thin film, the polymer thin film may be formed by polymerizing a monomer (a volatile substance capable of forming a polymer when activated with an initiator) by activation with an initiator, and then the formed polymer thin film may be carbonized.

[0109] In this case, the monomer may be one or more of a vinyl monomer or an acrylate monomer containing at least one of the following groups: a siloxane group, an amine group, a fluoride group, and an aromatic hydrocarbon group.

[0110] The polymer film formed by polymerizing monomers can be, for example, polymerized from at least one monomer selected from the group consisting of: 4-vinylpyridine (4VP); 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl methacrylate (PFDMA); 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4D4); 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane (V3D3); hexavinyldisiloxane (HVDS); glycidyl methacrylate; divinyl Benzene; diethylene glycol divinyl ether; diethylene glycol diacrylate (DEGDA); ethylene glycol dimethacrylate; dimethylaminoethyl methacrylate; methacrylic acid; 1,3-divinyl-1,1,3,3-tetramethyldisiloxane; 1H,1H,2H,2H-perfluorodecyl acrylate; perfluorodecyl methacrylate; dodecafluoroheptyl acrylate; pentafluorophenyl methacrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,9-pentadecafluorononyl acrylate; 2-methyl-3,3,4,4,5,5,6,6,7,7,8,8,9,9,9-pentadecafluorononyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,9-pentadecafluorononyl acrylate 8,8-tridecafluorooctyl acrylate; 2-methyl-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate; 3,3,4,4,5,5,6,6,7,7,7-undecafluoroheptyl acrylate; 2-methyl-3,3,4,4,5,5,6,6,7,7,7-undecafluoroheptyl acrylate; 3,3,4,4,5,5,6,6,6-nonafluorohexyl acrylate; 2-methyl-3,3,4,4,5,5,6,6,6-nonafluorohexyl acrylate; 3,3,4,4,5,5,6,6,6-nonafluorohexyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-nonadecafluoroundecyl acrylate; 2-methyl-3,3, 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-Nonadecafluoroundecyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-henonecafluorododecyl acrylate; 2-methyl-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-henonecafluorododecyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,13-tridecylfluorotridecyl acrylate;2-Methyl-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,13-tricosylfluorotridecyl acrylate; 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,14-pentacofluorotetradecyl acrylate; and 2-Methyl-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,14-pentacofluorotetradecyl acrylate; dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl methacrylate, and diethylaminoethyl acrylate.

[0111] In addition, the initiator may be, for example, a peroxide. Specifically, the initiator may include at least one selected from the group consisting of di-tert-butyl peroxide, tert-butyl perbenzoate, benzoyl peroxide, methyl ethyl ketone peroxide, lauryl peroxide, and benzophenone, but is not limited thereto.

[0112] In this case, the flow rate of the monomer supplied to the reactor may be 0.1 sccm to 10 sccm. Specifically, the monomer flow rate may be 0.1 sccm or more or 0.2 sccm or more and 10 sccm or less, 5 sccm or less, or 4 sccm or less.

[0113] In addition, the flow rate of the initiator supplied to the reactor may be 0.1 sccm to 5 sccm. Specifically, the initiator flow rate may be 0.1 sccm or more and 5 sccm or less, 3 sccm or less, or 2 sccm or less.

[0114] In addition, the initiator may be activated by a predetermined heat treatment, which may be performed at a temperature of 135 to 350° C., preferably 140 to 340° C., and satisfying this temperature condition may be advantageous in preventing changes in the properties of reactants.

[0115] On the other hand, it is preferred to keep the surface temperature of the silicon-inorganic layer composite material at a low level to increase the adsorption rate of monomers and free radicals. Specifically, the surface temperature of the silicon-inorganic layer composite material is preferably in the range of 10 to 50° C., and more preferably in the range of 13 to 45° C., but is not particularly limited to these ranges.

[0116] Step (4)

[0117] In step (4), at least a portion of the inorganic layer in the silicon-inorganic layer-carbon composite material is removed to produce a silicon-carbon composite material having a yolk-shell structure.

[0118] In this case, step (4) can be performed by etching the inorganic layer from the silicon-inorganic layer-carbon composite material. Etching can be performed by immersing the silicon-inorganic layer-carbon composite material in an alkaline or acidic aqueous solution and treating it at a temperature of room temperature to 80°C for 1 minute to 1 hour.

[0119] In a specific embodiment of the present invention, the base may include at least one selected from the group consisting of sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH) 2), but is not particularly limited thereto. Here, the concentration of the alkaline aqueous solution may be 0.1 to 3 M, although it is not particularly limited to this range.

[0120] In a specific embodiment of the present invention, the acid may include at least one selected from the group consisting of hydrofluoric acid (HF), hydrochloric acid, sulfuric acid and nitric acid, but is not particularly limited thereto. Here, the concentration of the acidic aqueous solution may be 0.1 to 2 M, although it is not particularly limited to this range.

[0121] By etching the silicon-inorganic layer-carbon composite material as described above, at least a portion of the inorganic layer can be removed, preferably 10% to 100% by weight, and more preferably 50% to 100% by weight, so that a suitable gap can be formed between the silicon particles and the carbon film. In the final product according to one embodiment of the present invention (i.e., a silicon-carbon composite material with a yolk-shell structure), the residue of the inorganic layer left after the removal in step (4) may be present between the silicon particles and the carbon film.

[0122] In step (4), when voids are formed between the silicon particles and the carbon thin film, the extent of void formation and the extent of the inorganic layer remaining can be appropriately controlled by adjusting factors such as the size of the silicon particles, the etching time, and the temperature of the alkaline aqueous solution. When the resulting yolk-shell structured silicon-carbon composite material is used as an anode active material, the conductivity of the anode active material can be improved without structural changes.

[0123] One or more additional steps

[0124] In the method for preparing a yolk-shell silicon-carbon composite material according to one embodiment of the present invention, the obtained yolk-shell silicon-carbon composite material can be crushed or pulverized and classified. By classification, the particle size distribution of the composite material can be made uniform. Classification can be performed by dry classification, wet classification, or classification using a sieve.

[0125] Anode active material

[0126] According to another embodiment of the present invention, there is provided an anode active material comprising a silicon-carbon composite material having a yolk-shell structure.

[0127] In addition to the yolk-shell structure of the silicon-carbon composite material, the anode active material according to one embodiment of the present invention may also include a carbon-based anode material, specifically a graphite-based anode material. For example, the anode active material can be obtained by mixing the yolk-shell structure of the silicon-carbon composite material according to one embodiment of the present invention with a carbon-based anode material (such as a graphite-based anode material).

[0128] Here, the carbon-based anode material may include at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon, carbon fiber, carbon nanotube, pyrolytic carbon, coke, a calcined product of an organic polymer compound, and carbon black, but is not particularly limited thereto.

[0129] The content of the carbon-based anode material in the anode active material according to one embodiment of the present invention may be 2 to 80 wt %, preferably 5 to 70 wt %, and more preferably 30 to 70 wt % based on the total weight of the anode active material.

[0130] The anode active material according to one embodiment of the present invention can be effectively used to manufacture an anode of a secondary battery, specifically, an anode of a lithium secondary battery and an all-solid-state battery.

[0131] All-solid-state batteries

[0132] According to another embodiment of the present invention, an all-solid-state battery is provided. The all-solid-state battery includes an SEI (solid electrolyte interface) layer, and the SEI layer includes a silicon-carbon composite material with a yolk-shell structure.

[0133] The all-solid-state battery may include a cathode, an anode, and a solid electrolyte between the cathode and the anode. The anode may include an anode active material layer, and the anode active material layer may include an SEI (solid electrolyte interface) layer on at least a portion of the surface of the anode active material particles, the SEI layer comprising the above-mentioned yolk-shell structured silicon-carbon composite material.

[0134] On the other hand, the anode active material particles may be a carbon-based anode material, and in this case, the description may be the same as that provided above with respect to the anode active material; therefore, a detailed description thereof will be omitted.

[0135] In addition, except for the anode active material particles and SEI layer of the all-solid-state battery, the anode configuration, cathode configuration and solid electrolyte configuration may be based on those of known all-solid-state batteries and are therefore not particularly limited in the present invention. DETAILED DESCRIPTION

[0136] Example

[0137] The present invention will now be described in more detail with reference to the following examples.The following examples are provided for illustrative purposes only and are not intended to limit the scope of the present invention.

[0138] <Example 1>

[0139] Preparation of silicon-inorganic layer composite materials (intermediates)

[0140] A conventional ALD reactor was used to prepare a silicon-inorganic layer composite material. 5 g of silicon powder with an average particle size of 20 μm and 2 μm, respectively, was evenly spread and arranged on a circular silicon wafer in the reactor. The silicon-inorganic layer composite material was prepared by forming an inorganic layer on the surface of the silicon particles under the following conditions. The specific materials and process conditions used in Example 1 are as follows.

[0141] - Silicon particles: average particle size of 20 μm and 2 μm (REC silicon)

[0142] - First precursor: trimethylaluminum (EG Chem)

[0143] - Second precursor: H2O (Millpore, Milli-Q)

[0144] - Precursor carrier gas: N2 300 sccm

[0145] - Average flow rate and supply time of the first precursor: 2 seconds

[0146] - Purge gas and time after supplying the first precursor: N2, 60 seconds

[0147] - Average flow rate and supply time of the second precursor: 2 seconds

[0148] - Purge gas and time after supplying the second precursor: N2, 60 seconds

[0149] - Pressure inside the reaction chamber: 1.2 Torr

[0150] - Surface temperature of the mounting unit (silicon wafer) in the reactor: 150°C

[0151] - Number of cycles: 100 cycles

[0152] - Inorganic layer formed: Al2O3

[0153] Test Example 1-1

[0154] (1) X-ray photoelectron spectroscopy

[0155] The silicon particles before and after the inorganic layer was formed were subjected to elemental analysis of the particle surface using X-ray photoelectron spectroscopy (XPS) (Multilab 2000, Thermo).

[0156] [Table 1]

[0157]

[0158] As shown in Table 1, aluminum was hardly detected on the surface of the silicon particles before the inorganic layer was formed, but a significant amount of aluminum was detected after the inorganic layer was formed on the silicon particles by the production method of the present invention. Therefore, it was confirmed that such a silicon-inorganic layer composite material in which an aluminum oxide inorganic layer was formed on the surface of the silicon particles was produced by the production method of the present invention.

[0159] (2) Transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS)

[0160] The silicon particles with an average particle size of 20 μm on which the Al2O3 inorganic layer had been formed were observed using a transmission electron microscope (TEM) and an energy dispersive spectrometer (EDS) (Tecnai G2 F30 S-Twin, FEI Company). Figure 1 The EDS analysis results are shown in Figure 2 and shown in Table 2.

[0161] [Table 2]

[0162]

[0163] Depend on Figure 1 As can be seen from the TEM images of A and 1B, the silicon-inorganic layer composite material obtained in Example 1 substantially maintains its original shape without problems such as severe agglomeration or particle breakage, and forms an inorganic layer having a uniform thickness of about 5 nm. Figure 2 A and 2B and Table 2 show the results of EDS analysis of silicon-inorganic layer composite materials. Figure 2 As shown in A and 2B and Table 2, aluminum, a component of the inorganic layer, was observed on the silicon-inorganic layer composite material. Therefore, it was qualitatively confirmed that such a silicon-inorganic layer composite material in which an inorganic layer was formed on the surface of silicon particles was prepared by the preparation method of the present invention.

[0164] Preparation of silicon-inorganic layer-carbon composite materials

[0165] The silicon-inorganic layer composite material (intermediate) prepared above 5 g is spread evenly and is placed on the circular silicon chip in the reactor. The monomer and initiator of evaporation are supplied to the reaction chamber by inlet to form polymer film (pDVB) on the surface of silicon-inorganic layer composite material, thereby produce silicon-inorganic layer-polymer composite material. The concrete materials and processing conditions employed in this embodiment are as follows.

[0166] - Monomer: divinylbenzene (Aldrich, 80%)

[0167] - Initiator: di-tert-butyl peroxide (Aldrich, 98%)

[0168] - Average initiator supply flow rate: 0.26 sccm

[0169] - Average monomer supply flow rate: 0.425 sccm

[0170] - Filament temperature: 140°C

[0171] - Pressure inside the reaction chamber: 450 mTorr

[0172] - Surface temperature of the mounting unit (silicon wafer) in the reactor: 23°C

[0173] Subsequently, the silicon-inorganic layer-polymer composite material was carbonized at 700° C. for 30 minutes to carbonize the polymer film, thereby producing a silicon-inorganic layer-carbon composite material.

[0174] Test Example 1-2

[0175] (1) Transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS)

[0176] Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) (Tecnai G2 F30 S-Twin, FEI Company) were used to observe the silicon-inorganic layer-carbon composite material prepared above. Figure 3 The EDS analysis results are shown in Figure 4 Shown in.

[0177] As a result, Figure 3 The TEM images in the figure confirm that a uniform thin film with a thickness of about 20 nm is formed on the silicon particles in the silicon-inorganic layer-carbon composite material obtained in the embodiment. Figure 4The EDS analysis results shown in 4A, 4B, and 4C confirmed that both aluminum and carbon were uniformly observed throughout the entire area of ​​the silicon-inorganic layer-carbon composite material. Therefore, it was qualitatively confirmed that such a silicon-inorganic layer-carbon composite material in which an inorganic layer and a carbon thin film were formed on the surface of silicon particles was produced by the production method of the present invention.

[0178] Preparation of Silicon-Carbon Composite Materials with Yolk-Shell Structure

[0179] The silicon-inorganic layer-carbon composite material (intermediate) prepared above was immersed in H 3 PO 4 (85%, Aldrich) at 50° C. for 100 seconds to etch the inorganic layer, thereby producing a yolk-shell structured silicon-carbon composite material.

[0180] Test Examples 1-3

[0181] (1) Transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS)

[0182] Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) (Tecnai G2 F30 S-Twin, FEI Company) were used to observe the yolk-shell structured silicon-carbon composite material prepared above. Figure 5 The EDS analysis results are shown in Figure 6 and 7 Shown in.

[0183] As a result, Figure 5 The TEM images in the figure confirm that a silicon-carbon composite material with a yolk-shell structure having uniform voids is formed. Figure 6 and 7 The results confirmed that a carbon film was formed on the silicon particles, and in particular Figure 7 The presence of defined voids was confirmed as evidenced by the presence of regions (approximately 110 nm) where no atoms were detected between regions where carbon atoms and silicon atoms were detected.

[0184] <Example 2>

[0185] Preparation of silicon-inorganic layer composite materials (intermediates)

[0186] A silicon-inorganic layer composite material was prepared in the same manner as in Example 1 except that the following conditions were changed.

[0187] - Silicon particles: average particle size 20 μm (REC silicon)

[0188] - First precursor: titanium(IV) isopropoxide (TTIP)

[0189] - Second precursor: H2O (Millpore, Milli-Q)

[0190] - Precursor carrier gas: N2 300 sccm

[0191] - Average flow rate and supply time of the first precursor: 15 seconds

[0192] - Purge gas and time after supplying the first precursor: N2, 600 seconds

[0193] - Average flow rate and supply time of the second precursor: 15 seconds

[0194] - Purge gas and time after supplying the second precursor: N2, 600 seconds

[0195] - Pressure inside the reaction chamber: 1.2 Torr

[0196] - Surface temperature of the mounting unit (silicon wafer) in the reactor: 150°C

[0197] - Number of cycles: 100 cycles

[0198] - Inorganic layer formed: TiO2

[0199] Test Example 2

[0200] (1) Transmission electron microscopy analysis

[0201] Transmission electron microscopy (TEM) analysis was performed on the silicon particles on which the TiO2 inorganic layer had been formed. Figure 8 Shown in.

[0202] Depend on Figure 8 As can be seen from the TEM images of 8A and 8B, the silicon-inorganic layer composite material obtained in Example 2 substantially maintains its original shape without problems such as severe agglomeration or particle breakage, and forms an inorganic layer with a uniform thickness of about 6 nm.

Claims

1. A method for preparing a silicon-carbon composite material with a yolk-shell structure, the method comprising: (1) supplying silicon particles into a reactor; (2) forming an inorganic layer on the silicon particles by atomic layer deposition (ALD) to prepare a silicon-inorganic layer composite material; (3) forming a carbon film on the silicon-inorganic layer composite material to obtain a silicon-inorganic layer-carbon composite material; as well as (4) removing at least a portion of the inorganic layer from the silicon-inorganic layer-carbon composite material to produce a silicon-carbon composite material having a yolk-shell structure.

2. The method according to claim 1, wherein step (2) comprises: (2-1) supplying a first precursor into the reactor to form a first precursor multilayer, wherein at least a portion of the first precursor is adsorbed on the silicon particles; (2-2) purging the interior of the reactor to remove unadsorbed first precursor from the first precursor multilayer, thereby forming a first precursor monolayer; (2-3) supplying a second precursor into the reactor so that at least a portion of the second precursor reacts with the first precursor monolayer to form a composite material layer; as well as (2-4) purging the interior of the reactor to remove unreacted second precursor from the composite material layer, thereby forming an inorganic layer, Steps (2-1) to (2-4) constitute one cycle, and the cycle is performed one or more times. 3 . The method according to claim 2 , wherein the supply time of the first precursor and the supply time of the second precursor are each independently 1 to 600 seconds.

4. The method of claim 2, wherein the first precursor comprises at least one selected from the group consisting of trimethylaluminum, diethylzinc, zinc acetate, tetrakis(dimethylamino)tin(IV), butoxytris(ethylmethylamino)hafnium, isopropoxytitanium, and diisopropylaminosilane. 5 . The method of claim 2 , wherein the second precursor comprises at least one selected from the group consisting of H 2 O, O 3 , H 2 O plasma, O 3 plasma, and O 2 plasma.

6. The method according to claim 2, wherein the gas used for purging in at least one of steps (2-2) and (2-4) comprises nitrogen (N2) or argon (Ar). The method of claim 2 , wherein the cycling is performed 1 to 300 times.

8. The method of claim 2, wherein the temperature of the silicon particles is maintained at 90 to 400°C during the cycling.

9. The method according to claim 1, wherein step (4) is performed by etching the inorganic layer from the silicon-inorganic layer-carbon composite material.

10. A silicon-carbon composite material with a yolk-shell structure, comprising: silicon particles; and A carbon thin film is formed on the silicon particles, with a predetermined gap between the silicon particles and the carbon thin film.

11. The silicon-carbon composite material with yolk-shell structure according to claim 10, wherein the average particle size of the silicon particles is 10 nm to 50 μm, and The average thickness of the carbon film is 1 nm to 1 μm. 12 . The silicon-carbon composite material with a yolk-shell structure according to claim 10 , wherein the porosity of the silicon-carbon composite material with a yolk-shell structure is 10% to 80%. 13 . An anode active material comprising the yolk-shell structured silicon-carbon composite material according to claim 10 .

14. An all-solid-state battery, comprising a solid electrolyte interface (SEI) film, wherein the solid electrolyte interface (SEI) film comprises the yolk-shell structured silicon-carbon composite material according to any one of claims 10 to 12.

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