KR102587556B1 - Method of preparing nanoporous silicon and anode active material for lithium secondary battery comprising nanoprous silicon prepared thereby - Google Patents

Abstract

The present invention provides a manufacturing method capable of mass producing porous silicon optimized for improving the capacity and lifespan characteristics of lithium secondary batteries, a negative electrode containing porous silicon produced by the method as a negative electrode active material, and a lithium secondary battery containing the negative electrode. provides.
The method for producing porous silicon according to the present invention includes the steps of (S1) injecting a silicon precursor and a heat dispersant into a reactor and mixing them while depressurizing the interior of the reactor; (S2) performing a first heat reduction reaction for 20 minutes to 2 hours by injecting a metal reducing agent in vapor form while heating the reactor to a temperature of 600 to 750°C; (S3) performing a secondary heat reduction reaction for 1 to 4 hours while reheating the reactor to maintain the temperature when the internal temperature of the reactor is lowered to 250 to 400°C; and (S4) acid-treating and washing the silicon precursor reduced in the above step.

Description

Method for producing porous nanosilicon and negative electrode active material for lithium secondary battery containing porous nanosilicon produced by this method

The present invention relates to a method for manufacturing porous nanosilicon suitable for use as a negative electrode active material for lithium secondary batteries, and more specifically, to a method for continuously manufacturing porous nanosilicon with controlled specific surface area and mesopore ratio.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. In particular, lithium secondary batteries with high energy density per unit weight and long cycle life have been commercialized and are widely used.

A lithium secondary battery has a structure in which an electrolyte containing lithium salt is impregnated in an electrode assembly with a porous separator interposed between the positive electrode and the negative electrode, each of which has an active material applied to the electrode current collector. The electrode consists of an active material and a binder. and a slurry in which a conductive material is dispersed in a solvent is applied to a current collector, dried, and rolled.

The basic performance characteristics of these lithium secondary batteries, such as capacity, output, and lifespan, are greatly affected by the anode material. In order to maximize the performance of the battery, the negative electrode active material must have an electrochemical reaction potential close to that of lithium metal, the reaction reversibility with lithium ions must be high, and the diffusion rate of lithium ions within the active material must be fast. Carbon-based materials are widely used as materials that meet these requirements. Although the carbon-based active material has excellent stability and reversibility, it has limitations in terms of capacity. Therefore, recently, Si-based materials with high theoretical capacity have been applied as negative electrode active materials in fields that require high-capacity batteries, such as electric vehicles and hybrid electric vehicles.

However, when Si-based negative electrode active materials absorb and store lithium, their crystal structure changes and their volume expands. This volume expansion causes large stress inside the active material, causing cracks, which causes poor contact between the active material and the current collector, shortening the charge/discharge cycle life of the battery.

In order to solve the problem caused by such volume expansion, attempts are being made to manufacture silicon in a porous structure to have a buffering effect against volume changes. For example, Korean Chem. Eng. Res ., 54(4), 459-464 (2016); In "Electrochemical Characteristics of Porous Silicon/Carbon Composite Anode Using Spherical Nano Silica", spherical nano silica (SiO ₂ ) with a size of 100 to 500 nm obtained from tetraethyl orthosilicate (TEOS) was mixed with magnesium (Mg) and incubated at 700°C. A technology is disclosed in which porous silicon is obtained by thermal reduction and then acid treatment, and a porous silicon/carbonization composite synthesized by reacting the porous silicon with a carbon precursor is used as a negative electrode active material. In addition, in the literature [ Electrochimica Acta, (2018): "High-rate and long-cycle life performance of nano-porous nano-silicon derived from mesoporous MCM-41 as an anode for lithium-ion battery"], mesoporous silica (MCM) -41) is mixed with Mg powder and pulverized, the mixture is thermally reduced at 650°C for 7 hours, and then acid treated to remove MgO by-products, thereby producing porous silicon with a size of several tens of nm.

In the above methods, porous silicon is manufactured by reducing nano-sized silica (SiO ₂ ) using a metal reducing agent such as magnesium (Mg) and treating it with acid to remove MgO, a by-product, but the magnesium thermal reduction process takes about 650 hours. Since the reaction is performed in one step at 700°C, the specific surface area and pore size of the final synthesized porous silicon are not sufficient to meet the capacity and lifespan characteristics of the secondary battery. In addition, the magnesium heat reduction process is performed discontinuously in a closed reactor, which is disadvantageous in terms of manufacturing efficiency.

Compared to the existing methods, the present invention maximizes the distribution of mesopores of 10 to 50 nm in porous silicon, increases the specific surface area, and applies process conditions that enable continuous reaction and production, thereby improving capacity and lifespan characteristics in lithium secondary batteries. The aim is to provide a manufacturing method that can mass-produce porous silicon optimized to improve .

Another object of the present invention is to provide a negative electrode containing porous silicon prepared by the above method as a negative electrode active material and a lithium secondary battery including the same.

According to one aspect of the present invention, (S1) injecting a silicon precursor and a heat dispersant into a reactor and mixing them while depressurizing the interior of the reactor; (S2) performing a first heat reduction reaction for 20 minutes to 2 hours by injecting a metal reducing agent in vapor form while heating the reactor to a temperature of 600 to 750°C; (S3) performing a secondary heat reduction reaction for 1 to 4 hours while reheating the reactor to maintain the temperature when the internal temperature of the reactor is lowered to 250 to 400°C; and (S4) acid-treating and washing the silicon precursor reduced in the above step. A method for producing porous silicon is provided.

According to another aspect of the present invention, a negative electrode comprising porous silicon prepared by the above method as a negative electrode active material, wherein the porous silicon has a proportion of mesopores of 10 to 50 nm accounting for more than 5% to 80% of the total pores and a specific surface area of A cathode having a weight of 120 to 600 m ² /g is provided.

According to another aspect of the present invention, a lithium secondary battery including the negative electrode, the capacity after 250 charge/discharge cycles at a voltage of 0.01V to 1.5V and a current of 2,100mA/g is 54% or more compared to the initial capacity. A secondary battery that is maintained is provided.

According to the present invention, the thermal reduction reaction of the silicon precursor is performed in two or more stages by injecting a metal reducing agent in a vapor state into the reduced pressure reactor, thereby maintaining the particle shape of the silicon precursor used as a reactant and reducing the diameter of the reduced silicon. The specific surface area can be increased while maximizing the ratio of mesopores of 10 to 50 nm, and the porous silicon finally obtained from this can contribute to improving capacity and lifespan performance when used as a negative electrode active material for secondary batteries.

Additionally, the method for producing porous silicon according to the present invention is performed in a continuous reactor, and the reaction heat generated during thermal reduction of the silicon precursor can be controlled by measuring the temperature, pressure, and concentration inside the reactor, and through this, a large amount of porous silicon can be produced. It is advantageous in terms of process efficiency as it can be produced.

The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention along with the contents of the above-described invention. Therefore, the present invention is limited to the matters described in such drawings. It should not be interpreted in a limited way.
Figure 1 schematically illustrates a reaction system in which a method for producing porous silicon according to an embodiment of the present invention was performed.
Figure 2 shows the charge and discharge capacity of a lithium secondary battery containing a negative electrode using porous silicon prepared in Example 1 over 250 charge and discharge cycles.
Figure 3 shows the charge and discharge capacity of a lithium secondary battery containing a negative electrode using porous silicon prepared in Comparative Example 1 over 250 charge and discharge cycles.
Figures 4a to 4c show the charge and discharge capacity of a lithium secondary battery including a negative electrode using porous silicon as a negative electrode active material prepared in Examples 1, 2, and 4, respectively, according to 100 charge/discharge cycles.

Hereinafter, terms or words used in this specification and claims should not be construed as limited to their usual or dictionary meanings, and the inventor appropriately defines the concept of terms in order to explain his or her invention in the best way. It must be interpreted with meaning and concept consistent with the technical idea of the present invention based on the principle that it can be done.

One embodiment of the present invention relates to a method for producing porous silicon used as a negative electrode active material for lithium secondary batteries, characterized in that porous silicon is produced by performing a thermal reduction reaction of a silicon precursor and a metal reducing agent in two or more steps. do.

Figure 1 schematically illustrates a reaction system in which the method for producing porous silicon according to an embodiment of the present invention is performed, and includes a continuous stirred tank reactor (CSTR) or a continuous rotary reactor. can be used

As shown in Figure 1, the reaction system has a metal reducing agent supply part, a silicon precursor supply part, and a heat dispersant supply part connected to the reactor by a line, and a sensor is installed inside the reactor to measure the temperature, pressure, and concentration of the reactants. The measurement results can be transmitted to an external control unit to control the reaction heat generated during thermal reduction of the silicon precursor. Additionally, a vent line equipped with a silicon carbide (SiC) or silica filter can be installed at the top of the reactor to recover unreacted excess magnesium vapor back to the chamber. Using this reactor system, mass production of the final product is possible, which is advantageous in terms of process efficiency.

Referring to FIG. 1, the silicon precursor and heat dispersant can be injected into the reactor through each supply part and mixed, and at this time, the interior of the reactor is depressurized (S1).

The pressure reduction may be performed below atmospheric pressure, thereby minimizing oxygen inflow and side reactions inside the reactor, thereby suppressing the possibility of explosion of the metal reducing agent due to oxygen and moisture during the heat reduction reaction using the metal reducing agent in the subsequent step. . In addition, when performing such pressure reduction, there is no need to use an inert gas such as argon during the heat reduction reaction.

The silicon precursor includes silicon dioxide, silicon monoxide, silica gel, metal silicate, zeolite, glass, quartz, and fumed silica ( It may contain one or more Si-containing materials selected from fumed silica, and among these, silicon dioxide can be preferably used because it is advantageous for conversion of the porous structure by thermal reduction.

These silicon precursors may be amorphous or crystalline, and may be particles with an average size (D ₅₀ ) of 20 to 1,000 nm, for example, 30 to 500 nm.

The heat dispersant is added to remove the reaction heat accompanying the thermal reduction reaction of the silicon precursor, and can prevent agglomeration of the porous silicon particles finally obtained. Examples of such heat dispersants may include one or more selected from sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl ₂ ), and magnesium chloride (MgCl ₂ ).

The heat dispersant may be used in solid or liquid form and may be used in an amount of 1 to 200 parts by weight based on 100 parts by weight of the silicon precursor.

Referring again to FIG. 1, the reactor and the supply part of the metal reducing agent are equipped with a heating jacket (gray part), so that the metal reducing agent can be injected in a vapor state while heating the reactor into which the silicon precursor has been previously injected (S2). . If necessary, the heat reducing agent may be injected into the reactor together with the metal reducing agent.

The metal reducing agent may include one or more selected from sodium (Na), magnesium (Mg), and aluminum (Al). In particular, magnesium can be advantageously used in terms of reduction efficiency of the silicon precursor.

The silicon precursor and metal reducing agent may react at a weight ratio of 1:0.8 to 1:1.5 during the thermal reduction reaction.

In the present invention, continuous injection is possible by injecting the metal reducing agent in a vapor state. For example, it can be continuously injected at a flow rate of 50 to 1,000 ml/min based on the 1L capacity of the reactor. When a metal reducing agent, such as magnesium, is injected in the gaseous state and reacts with solid silica particles, the thermal reduction reaction can be initiated at a rapid rate, improving the yield of the final porous silicon while using a minimal amount of metal reducing agent. .

The heating in step (S2) is to initiate a thermal reduction reaction between the silicon precursor and the metal reducing agent, and may be performed to a temperature of 600 to 750 °C at a temperature increase rate of 5 to 20 °C/min.

The heat reduction reaction is an exothermic reaction, and a metal reducing agent, such as magnesium, is injected into the reactor in a vaporous state and initiates a reaction with a solid silicon precursor (e.g., silica particles), causing a rapid increase in internal temperature. As a result, there is a risk that macro pores may be formed or the shape of the spherical silica particles may be deformed or destroyed, thereby deteriorating its function as a negative electrode active material. Therefore, by measuring and controlling the temperature inside the reactor and the pressure and concentration of the metal reducing agent, it is necessary to increase the ratio of mesopores with a diameter of 10 to 50 nm in the porous silicon obtained by reduction while maintaining the shape of the spherical silica particles. desirable. To achieve this, the temperature of the heated reactor is in the range of 600 to 750°C. After a certain period of time, for example, 20 minutes to 4 hours, heating due to heat input from the outside is temporarily cut off to prevent the heat reduction reaction from proceeding rapidly. It is necessary to prevent it.

Thereafter, when the temperature of the reactor is lowered to 250 to 400° C., the reactor is reheated by an external power source to maintain the temperature to perform a secondary heat reduction reaction (S3).

The secondary heat reduction reaction can maximize the ratio of mesopores of 10 to 50 nm by suppressing the creation of macro pores while maintaining the spherical particle shape in the silicon precursor reduced by the first reaction, and at the same time increase the specific surface area. You can.

In one embodiment of the present invention, in step (S2), the internal temperature of the reactor is maintained at 650 ° C. for 40 minutes for the first heat reduction reaction, and in step (S3), the internal temperature of the reactor is maintained for the second heat reduction reaction. It can be maintained at 400°C for 4 hours for reaction.

Through the first and second thermal reductions, part or all of the silicon precursor is reduced to porous silicon, part or all of the metal reducing agent is oxidized to metal oxide, and the resulting porous silicon has 10 to 10% in total pores. The proportion of mesopores of 50 nm may range from more than 5% to 80%, specifically 12 to 45%, and the specific surface area may range from 120 to 600 m ² /g.

Next, the produced porous silicon is recovered, and acid treatment and washing are performed to etch metal oxides, which are by-products, and remove reaction residues.

The acid treatment is performed for 1 to 5 hours using at least one acid selected from hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ), phosphoric acid (H ₃ PO ₄ ), and acetic acid (CH ₃ COOH). It can be performed during

The cleaning can be performed using water or alcohol.

When porous silicon manufactured through the above process is used as a negative electrode active material for lithium secondary batteries, it can prevent deterioration of the electrode by suppressing volume expansion through pores present in the particles.

Moreover, the porous silicon obtained by performing heat treatment by primary and secondary heating according to the present invention has a high ratio of mesopores of 10 to 50 nm and the specific surface area is expanded to the range of 120 to 600 m ² /g, When charging a secondary battery, lithium (Li) ions desorbed from the positive electrode active material pass through the electrolyte and are inserted into silicon (Si) particles of the negative electrode active material, thereby shortening the distance between Li ions and Si. This shortening of the distance between Li ions and Si facilitates the insertion/extraction of Li ions into Si particles, thereby increasing the particle size to overcome the low diffusion rate of Si (1.7x10 ¹¹ cm ² /sec) when using a non-porous Si active material. It is advantageous in terms of energy consumption and cost because the process that reduced it can be avoided. In addition, the number of sites where Li ions are inserted into Si particles increases, thereby reducing the resistance of the electrode due to charge transfer, thereby improving capacity retention and lifespan performance according to charge and discharge cycles.

Accordingly, another embodiment of the present invention provides a negative electrode containing porous silicon produced by the above method as a negative electrode active material.

The negative electrode according to one embodiment of the present invention is prepared by coating a current collector and at least one surface of the current collector with a slurry obtained by dispersing a negative electrode active material containing porous silicon prepared according to the present invention, a binder, and a conductive material in a solvent. , can be manufactured by drying and rolling.

The current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, carbon or nickel on the surface of copper or stainless steel. , surface treated with titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. The thickness of the current collector is not particularly limited, but may have a commonly applied thickness of 3 to 500 ㎛.

The negative electrode active material may be included in an amount of 60% by weight to 99% by weight based on the total weight of the slurry composition.

The binder is a component that assists in bonding between the conductive material, the active material, or the current collector, and is typically included in an amount of 0.1 to 20% by weight based on the total weight of the negative electrode slurry composition. Examples of such binders include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidenefluoride, polyacrylonitrile, and polymethylmethacrylate. , polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butyrene rubber (SBR), etc. I can hear it.

The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, carbon black, acetylene black, Ketjen black, channel black, panel black, lamp black, thermal black, etc. black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as fluorocarbon, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used. The conductive material may be added in an amount of 0.1 to 20% by weight based on the total weight of the anode slurry composition.

The dispersion medium may contain an organic solvent such as water or NMP (N-methyl-2-pyrrolidone), and is used in an amount to achieve a desirable viscosity when the negative electrode slurry contains a negative electrode active material and optionally a binder and a conductive material. can be used

Additionally, the coating method of the slurry is not particularly limited as long as it is a method commonly used in the field. For example, a coating method using a slot die may be used, and in addition, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, etc. may be used.

Another embodiment of the present invention relates to a lithium secondary battery including the negative electrode. Specifically, the lithium secondary battery can be manufactured by injecting a lithium salt-containing electrolyte into an electrode assembly including a positive electrode, a negative electrode as described above, and a separator interposed therebetween.

The positive electrode is prepared by mixing the positive electrode active material, conductive material, binder, and solvent to prepare a slurry, and then coating it directly on the metal current collector, or casting it on a separate support and lamination of the positive electrode active material film peeled from this support to the metal current collector. Thus, the anode can be manufactured.

Active materials used in the positive electrode include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ and LiNi _1-xyz Co _x M1 _y M2 _z O ₂ (M1 and M2 are independently selected from Al, Ni, Co, is any one selected from the group consisting of Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently the atomic fraction of the oxide composition elements, 0≤x<0.5, 0≤ y<0.5, 0≤z<0.5, 0<x+y+z≤1) or a mixture of two or more of them.

Meanwhile, the conductive material, binder, and solvent may be used in the same manner as those used in manufacturing the anode.

The separator is a conventional porous polymer film used as a separator, for example, a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer. The prepared porous polymer film can be used alone or by stacking them. Additionally, a thin insulating film with high ion permeability and mechanical strength can be used. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on the surface of the separator. In addition, conventional porous nonwoven fabrics, for example, nonwoven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc., can be used, but are not limited thereto.

The electrolyte solution contains lithium salt as an electrolyte and an organic solvent for dissolving it.

The lithium salt may be used without limitation as long as it is commonly used in electrolytes for secondary batteries. For example, the anions of the lithium salt include F ^- , Cl ^- , I ^- , NO ₃ ^- , N(CN) ₂ ^- , BF ₄ ^- , ClO ₄ ^- , PF ₆ ^- , (CF ₃ ) ₂ PF ₄ ^- , (CF ₃ ) ₃ PF ₃ ^- , (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF ^- , (CF ₃ ) ₆ P ^- , CF ₃ SO ₃ ^- , CF ₃ CF ₂ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- , (FSO ₂ ) ₂ N ^- , CF ₃ CF ₂ (CF ₃ ) ₂ CO ^- , ( CF ₃ SO ₂ ) ₂ CH ^- , (SF ₅ ) ₃ C ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ (CF ₂ ) ₇ SO ₃ ^- , CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , One type selected from the group consisting of SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- can be used.

The organic solvent contained in the electrolyte solution may be any commonly used solvent without limitation, and representative examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropyl carbonate, and dimethyl sulfoxide. One or more types selected from the group consisting of side, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran may be used.

In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high viscosity organic solvents and have a high dielectric constant, so they can be preferably used because they easily dissociate lithium salts in the electrolyte. These cyclic carbonates include dimethyl carbonate and diethyl. If low viscosity, low dielectric constant linear carbonate such as carbonate is mixed and used in an appropriate ratio, an electrolyte solution with high electrical conductivity can be made and can be used more preferably.

Optionally, the electrolyte solution stored according to the present invention may further include additives such as overcharge prevention agents included in conventional electrolyte solutions.

In a lithium secondary battery according to an embodiment of the present invention, an electrode assembly is formed by placing a separator between the positive electrode and the negative electrode, the electrode assembly is placed in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then an electrolyte is added. By injection, a secondary battery can be completed. Alternatively, a lithium secondary battery can be completed by stacking the electrode assemblies, impregnating them with an electrolyte, and sealing the resulting product in a battery case.

According to one embodiment of the present invention, the lithium secondary battery may be a stack type, a wound type, a stack and fold type, or a cable type.

The lithium secondary battery according to the present invention can not only be used in battery cells used as a power source for small devices, but can also be preferably used as a unit cell in medium to large-sized battery modules containing a plurality of battery cells. Preferred examples of the medium-to-large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc., and are particularly useful for hybrid electric vehicles and batteries for renewable energy storage, which are areas requiring high output. It can be used effectively.

Hereinafter, the present invention will be described in detail through examples to aid understanding. However, the embodiments according to the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the following embodiments. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1:

As shown in Figure 1, 5.0 mg of silicon dioxide solid particles (SiO ₂ , D ₅₀ =50 nm) and 1.0 mg of sodium chloride (NaCl) powder were injected into a continuous stirred tank reactor (CSTR) and mixed. Meanwhile, before heating the reactor, in order to suppress side reactions, the pressure was reduced using a vacuum pump to remove oxygen in the air, and the initial internal pressure of the reactor was set to 20 mmHg.

To ensure uniform mixing of the reactants, the reactor was heated at a temperature increase rate of 20°C/min while rotating the stirrer at 500 rpm. When the internal temperature of the reactor reached 650°C, Mg vapor (vapor pressure 0.021 atm) at 750°C was injected at a flow rate of 50 ml/min for 15 minutes through the inlet installed in the reactor to perform the first heat reduction reaction. . At this time, the first heating was performed at 650°C for 20 minutes, and then the heating was stopped to prevent the heat reduction reaction from proceeding rapidly.

Thereafter, when the temperature of the reactor was lowered to 250°C, the reactor was reheated for 4 hours at a temperature increase rate of 5°C/min to maintain the above temperature to perform a secondary heat reduction reaction. After the secondary reaction, Mg vapor was discharged through a vent line equipped with a carborundum filter made of silicon carbide. The discharged Mg vapor can be recovered and injected back into the reactor.

Thereafter, the reaction product was acid treated with 1M HCl for 4 hours and then washed with water to prepare porous silicon.

Example 2:

Porous silicon was prepared in the same manner as in Example 1, except that secondary heating was performed so that the temperature of the reactor was maintained at 400°C for 4 hours.

Comparative Example 1

Porous silicon was manufactured in the same process as Example 1, except that secondary heating was not performed.

Example 3

Porous silicon was prepared in the same manner as in Example 1, except that the first heating was performed at 650°C for 40 minutes, and the second heating was performed so that the temperature of the reactor was maintained at 250°C for 4 hours.

Example 4

Porous silicon was prepared in the same manner as in Example 1, except that primary heating was performed at 650°C for 40 minutes and secondary heating was performed so that the temperature of the reactor was maintained at 400°C for 4 hours.

Example 5

Porous silicon was prepared in the same manner as in Example 1, except that the first heating was performed at 650°C for 40 minutes, and the second heating was performed so that the temperature of the reactor was maintained at 400°C for 2 hours.

Example 6

Porous silicon was prepared in the same manner as in Example 1, except that the first heating was performed at 650°C for 40 minutes, and the second heating was performed so that the temperature of the reactor was maintained at 400°C for 1 hour.

Comparative Example 2

Porous silicon was manufactured in the same process as Example 1, except that the first heating was performed at 650°C for 40 minutes and the second heating was not performed.

The pore size and specific surface area of the porous silicon prepared in the above examples and comparative examples were confirmed by measuring the amount of physically adsorbed gas according to the method of Brunauer, Emmett and Teller (BET method), and the results are shown in Table 1 below.

Heat reduction conditions porous silicon primary heating secondary heating pore size
(nm) Distribution ratio of mesopores (10-50nm) specific surface area
( ^m2 /g) Example 1 650℃, 20 minutes 250℃, 4 hours 5.0 ~ 25.0 12% to 15% 158 Example 2 650℃, 20 minutes 400℃, 4 hours 6.9 ~ 45.0 15% to 20% 255 Comparative Example 1 650℃, 20 minutes - 4.0~6.0 5% or less 72 Example 3 650℃, 40 minutes 250℃, 4 hours 4.5 ~ 45.0 15% to 20% 254 Example 4 650℃, 40 minutes 400℃, 4 hours 6.0 ~ 48.0 40% to 45% 358 Example 5 650℃, 40 minutes 400℃, 2 hours 6.9 ~ 45.0 15% to 20% 224 Example 6 650℃, 40 minutes 400℃, 1 hour 6.9 ~ 22.0 12% to 15% 127 Comparative Example 2 650℃, 40 minutes - 4.0~6.0 5% or less 88

As can be seen in Table 1, a comparative example in which the porous silicon particles prepared by performing the magnesium thermal reduction reaction of the SiO ₂ particles by primary/secondary multi-stage heating according to the example were not subjected to secondary heating. Compared to 1 and 2, it can be seen that the specific surface area is increased by maximizing the distribution of mesopores.

In addition, the porous silicon prepared in Example 4, in which the first heating was performed at 650 ° C. for 40 minutes and the second heating was performed at 400 ° C. for 4 hours, had a distribution and specific surface area of mesopores with a diameter of 10 to 40 nm. The most excellent effect was realized.

Experimental example: Performance evaluation of lithium secondary battery

Each of the porous silicon prepared in Examples and Comparative Examples was used as a negative electrode active material, and polyacrylic acid (PAA, M _W : about 5,000 or less) as a binder and carbon black as a conductive material were mixed at a weight ratio of 65:20:15. , these were added to distilled water as a dispersion medium to obtain a slurry, and then applied on a copper thin film and dried and rolled to prepare a negative electrode.

After manufacturing a half cell using the cathode, charge/discharge was performed at room temperature (25°C) at a voltage of 0.01V to 1.5V and a current of 2,100mA/g, and then charge/discharge for each cycle. The capacity was measured.

Figures 2 and 3 show the charge and discharge capacity according to 250 charge and discharge cycles of a lithium secondary battery including a negative electrode in which porous silicon prepared in Example 1, Example 4, and Comparative Example 1 was used as the negative electrode active material, respectively. .

From Figures 2 and 3, in Example 1, the charge and discharge capacity was maintained at the level of 1500 mA while repeating 250 cycles as porous silicon with a large pore size and specific surface area was used in the negative electrode, whereas in Comparative Example 1, the pore size was maintained at the level of 1500 mA. And, as the specific surface area is relatively small, it can be seen that the charge/discharge capacity drops to 1200mA after 250 cycles.

That is, when using the porous silicon manufactured according to the present invention as a negative electrode active material, the capacity after 250 charge/discharge cycles was maintained at more than 54% of the initial capacity in the battery performance evaluation.

In addition, Figures 4a to 4c show the charge and discharge capacity according to 100 charge and discharge cycles of a lithium secondary battery containing a negative electrode using porous silicon prepared in Examples 1, 2, and 4, respectively, as a negative electrode active material, and the initial It can be seen that the capacity maintenance rate of 100 cycles is high compared to other cycles.

Claims (15)

(S1) Depressurizing the interior of the reactor while injecting and mixing the silicon precursor and heat dispersant into the reactor;
(S2) performing a first heat reduction reaction for 20 minutes to 2 hours by injecting a metal reducing agent in vapor form while heating the reactor to a temperature of 600 to 750°C;
(S3) performing a secondary heat reduction reaction for 1 to 4 hours while reheating the reactor to maintain 250 to 400 ℃ when the internal temperature of the reactor is lowered to 250 to 400 ℃; and
(S4) A method for producing porous silicon, comprising the steps of acid-treating and washing the silicon precursor reduced in the above step. The method of claim 1, wherein the silicon precursor is silicon dioxide, silicon monoxide, silica gel, metal silicate, zeolite, glass, quartz ( A method for producing porous silicon, comprising at least one Si-containing material selected from quartz) and fumed silica. The method of claim 1, wherein the silicon precursor is used in the form of particles with a size of 20 to 1,000 nm. The method of claim 1, wherein the metal reducing agent includes at least one selected from sodium (Na), magnesium (Mg), and aluminum (Al). The method of claim 1, wherein the heat dispersant includes at least one selected from sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl ₂ ), and magnesium chloride (MgCl ₂ ). The method of claim 1, wherein the pressure reduction in step (S1) is performed below atmospheric pressure. The method of claim 1, wherein the heating in step (S2) is performed at a temperature increase rate of 5 to 20 °C/min. The method of claim 1, wherein the secondary thermal reduction reaction by reheating in step (S3) maximizes the ratio of mesopores with a diameter of 10 to 50 nm in the reduced silicon while maintaining the particle shape of the silicon precursor used as a reactant. A method of manufacturing porous silicon. The method of claim 8, wherein the mesopores account for more than 5% to 80% of the total pores. The method of claim 1, wherein in step (S2), the internal temperature of the reactor is maintained at 650° C. for 40 minutes for the first heat reduction reaction,
A method for producing porous silicon in which the internal temperature of the reactor in step (S3) is maintained at 400°C for 4 hours for the secondary heat reduction reaction. The method of claim 1, wherein the reactor is a continuous mixing or rotating continuous reactor. The method of claim 1, wherein the reactor is equipped with a sensor for measuring the temperature, pressure, and concentration of internal reactants, and is connected to an external control unit. The method of claim 1, wherein a vent line equipped with a silicon carbide (SiC) or silica filter is additionally installed at the top of the reactor. A negative electrode comprising porous silicon manufactured by the method according to any one of claims 1 to 13 as a negative electrode active material, wherein the porous silicon has a proportion of mesopores of 10 to 50 nm exceeding 5% to 80% of the total pores. A cathode that occupies and has a specific surface area of 120 to 600 m ² /g. A lithium secondary battery containing the negative electrode according to claim 14, wherein the capacity after 250 charge/discharge cycles is maintained at more than 54% of the initial capacity under a voltage of 0.01V to 1.5V and a current of 2,100mA/g. .