KR102208279B1 - Preparation Method of Porous Si for anode material of lithium-ion battery - Google Patents

Abstract

The present invention relates to a method of manufacturing porous silicon for a negative electrode material of a lithium secondary battery, and in particular, a first step of forming a first mixed powder by mixing silicon oxide powder and metal oxide powder; A second step of pressing and firing the first mixed powder to form a sintered body; A third step of electrolytic reduction of the sintered body; And
It relates to a method of manufacturing porous silicon for a negative electrode material of a lithium secondary battery including a fourth step of manufacturing porous silicon by acid treatment.

Description

Preparation Method of Porous Si for Anode Material of Lithium-ion Battery}

The present invention relates to a method of manufacturing porous silicon for a negative electrode material of a lithium secondary battery.

As the demand for energy storage devices such as automobiles, displays, and portable electronic devices for mobile phones increases, many studies are being conducted to improve battery efficiency. Among lithium secondary battery anode materials, silicon-based anode materials are abundantly present in nature, have a very high theoretical capacity of 4200 mAh/g, and have a low potential difference with lithium (0.4V vs. Li/Li ⁺ ), so the next carbon is the next carbon. It is in the spotlight as a material to replace the cathode material. However, during charging and discharging, volume expansion of more than 300% of the existing volume occurs due to alloying with lithium ions, resulting in a large amount of irreversible capacity reduction.

Silicon cannot exist as a pure metal in nature, and generally exists in the form of a compound or an oxide of silica (SiO ₂ ). The most representative method of manufacturing silicon is a thermal reduction method in which a reducing agent such as carbon or magnesium is added in a certain ratio to reduce at high temperature. Among them, magnesium thermal reduction has a problem that a considerable amount of by-products such as magnesium silicate or magnesium silicide are generated, and the carbon thermal reduction method using a carbon reducing agent requires uniform mixing of carbon and silica. And, as it proceeds at a high temperature of 2000℃ or higher, selection of manufacturing equipment capable of high-temperature synthesis, safety issues, and high manufacturing costs are required. As a related art, Korean Patent Registration No. 10-1918637 discloses a method of manufacturing silicon by thermal reduction of magnesium.

Korean Patent Registration No. 10-1918637

An object of the present invention is to provide a method of manufacturing porous silicon for a negative electrode material for a lithium secondary battery.

To achieve the above object,

An embodiment of the present invention

A first step of mixing silicon oxide powder and metal oxide powder to form a first mixed powder;

A second step of pressing and firing the mixed powder to form a sintered body;

A third step of electrolytic reduction of the sintered body; And

It provides a silicon manufacturing method comprising; a fourth step of manufacturing porous silicon by acid treatment.

In addition, another embodiment of the present invention

It provides a lithium secondary battery negative electrode containing porous silicon prepared by the above manufacturing method.

Furthermore, another embodiment of the present invention

It provides a lithium secondary battery including the negative electrode.

The silicon manufacturing method of the present invention can perform reduction at a relatively lower temperature than the conventional thermal reduction method, so that manufacturing stability is high, and since the yield does not depend on a reducing agent, a constant yield can be maintained, thereby improving manufacturing efficiency.

In addition, when the porous silicon manufactured by the method of the present invention is used as a negative electrode of a lithium secondary battery, the volume expansion due to alloying with lithium is significantly reduced, so that the capacity retention rate is high and the cycle stability is high.

1 is a photograph showing a sintered body manufactured according to Example 1 of the present invention,
2 and 3 are graphs of the results of elemental analysis using Energy Dispersive Spectrometry (EDS) for porous silicon prepared according to Examples 1 and 2 of the present invention,
2 is a photograph of the porous silicon manufactured according to Example 1 of the present invention observed using a transmission electron microscope (TEM),
5 to 8 are graphs showing cycle characteristics in a lithium secondary battery using porous silicon prepared according to Examples 1 to 3 and Comparative Example 1 as a negative electrode.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

In addition, embodiments of the present invention are provided in order to more completely explain the present invention to those having average knowledge in the art. Accordingly, the shapes and sizes of elements in the drawings may be exaggerated for clearer explanation, and elements indicated by the same reference numerals in the drawings are the same elements. In addition, the same reference numerals are used throughout the drawings for portions having similar functions and functions. In addition, "including" certain elements throughout the specification means that other elements may be further included rather than excluding other elements unless specifically stated to the contrary.

A method of manufacturing porous silicon for a negative electrode material for a lithium secondary battery according to an embodiment of the present invention,

A first step of mixing silicon oxide powder and metal oxide powder to form a first mixed powder;

A second step of pressing and firing the mixed powder to form a sintered body;

A third step of electrolytic reduction of the sintered body; And

It may include; a fourth step of producing porous silicon by acid treatment.

Hereinafter, a method of manufacturing porous silicon for a negative electrode material of a lithium secondary battery according to an embodiment of the present invention will be described in detail for each step.

The first step is a step of forming a mixed powder, and a first mixed powder may be formed by mixing silicon oxide powder and metal oxide powder.

The silicon oxide powder may be represented by SiO _x (0<x≤2), but it may be more preferable to use silicon dioxide (SiO ₂ ) in that it can be used at a relatively low cost, and the silicon dioxide ( SiO ₂ ) may be one or more selected from the group consisting of quartz, silica sand, and amorphous silicon dioxide, but it may be more preferably amorphous silicon dioxide (SiO ₂ ) having a high reduction rate.

The silicon oxide powder may be particles having an average particle diameter of 20 nm to 100 μm. If the average particle diameter of the silicon oxide powder is less than 20 nm, the formation of the mixed powder including the silicon oxide powder is easily dispersed, so that a very high pressure is required during the production of the molded body, resulting in a decrease in manufacturing efficiency. When the average particle diameter of the oxide powder exceeds 100 μm, even if the silicon oxide powder is mixed with the metal oxide, there may be a problem in that the electrical conductivity of the silicon oxide powder is not improved during electrolytic reduction.

According to the size of the silicon oxide powder, the amount of the additive included in the first mixed powder may vary.

The metal oxide powder is a metal oxide powder having a smaller cast free energy value required for reduction than the silicon oxide powder, and in the electrolytic reduction step, the metal oxide is reduced earlier than the silicon oxide contained in the sintered body, and the metal oxide is reduced. Metals can accelerate the reduction of the silicon oxide.

The metal oxide is nickel oxide, cobalt oxide (Ⅲ), titanium oxide and copper oxide of at least one, but may be, may be more preferably in the nickel oxide powder represented by _x NiO, the NiO _x (0 <x≤1 Of the nickel oxides represented by ), it may be more preferable to use nickel monoxide (NiO) whose Gibbs free energy value required for reduction is significantly smaller than that of silicon oxide.

For example, when silicon oxide powder and metal oxide powder are used as silicon dioxide (SiO ₂ ) powder and nickel monoxide (NiO) powder, respectively, silicon dioxide (SiO ₂ ) and nickel monoxide (NiO) are Gibbs free energy required for reduction. Since (Gibbs free energy, △G) is 708.9KJ/mol and 137KJ/mol, respectively, in the electrolytic reduction step, nickel monoxide (NiO) having a relatively small Gibbs free energy value is first reduced, and then silicon dioxide (SiO ₂ ). Reduction can proceed. The nickel monoxide (NiO) is reduced to form nickel (Ni), and the nickel (Ni) has remarkably excellent electrical conductivity compared to silicon dioxide (SiO ₂ ), which is an electrical insulator, and includes silicon dioxide (SiO ₂ ). By increasing the electrical conductivity inside the sintered body, it is possible to improve the reduction efficiency of silicon dioxide (SiO ₂ ).

The metal oxide powder may be included in an amount of 1 to 20% by weight based on the weight of the silicon oxide powder, it may be preferable to include it in an amount of 3 to 10% by weight, and it may be more preferable to include it in an amount of 5% by weight. .

This is to facilitate the reduction of the silicon oxide but not remain as an impurity, and if the metal oxide powder contains less than 1% by weight of the silicon oxide powder, the metal oxide is the silicon oxide. The degree of reduction of the oxide may be insufficient, and when the metal oxide powder exceeds 20% by weight based on the weight of the silicon oxide powder, an alloy containing a large amount of metal and silicon is generated, and the yield of porous silicon is reduced. A problem of deterioration may occur.

The first mixed powder may further include a carbon material, and before the silicon oxide is reduced in the electrolytic reduction step, the carbon material is first reduced to accelerate the reduction of the silicon oxide powder.

The first mixed powder may further include at least one additive of a binder material and a lubricant. The binder may improve adhesion between powders contained in the first mixed powder during the firing process of the first mixed powder. The binder is 1 selected from the group consisting of poly methyl methacrylate, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinyl alcohol (PVA). More than one kind of aqueous binder may be used, but is not limited thereto.

The binder may be included in an amount of 0.2 to 5% by weight based on the weight of the silicon oxide powder, and may be preferably included in an amount of 0.2 to 3% by weight, in order to prevent the formation of a residue by being removed in the firing step. , It may be more preferable to include in an amount of 0.5 to 1% by weight. However, the content of the binder is not limited thereto, and may vary depending on the particle size of the silicon oxide powder.

The lubricant may be for forming a compact having a uniform density by reducing friction between a mold and the powder in the process of forming a compact by pressing the first mixed powder.

The lubricants include magnesium stearate, calcium stearate, zinc stearate, lithium stearate, sodium stearate, and aluminum stearate. ) One or more materials selected from the group consisting of may be used.

The lubricant may be included in an amount of 0.2 to 5% by weight based on the weight of the silicon oxide powder, and may be preferably contained in an amount of 0.2 to 3% by weight, in order not to form a residue by being removed in the firing step. , It may be more preferable to include in an amount of 0.5 to 1% by weight. However, the content of the lubricant is not limited thereto, and may vary depending on the particle size of the silicon oxide powder.

The mixing in the first step may be liquid mixing, and the first mixed powder may be formed by mixing the silicon oxide powder and the metal oxide powder in a solvent, followed by drying.

Thus, the first step may further include the step of mixing the silicon oxide powder and the metal oxide powder into a solvent, and then drying and recovering, and the drying and recovering may include evaporating or evaporating the solvent to form the powder. It may be a recovery step.

In this case, ethanol or distilled water may be used as the solvent, but it may be preferable to use ethanol, which has a lower boiling point and easier to dry.

For example, the silicon oxide powder and the metal oxide powder may be added to ethanol and mixed, and then dried in an oven at 80 to 99° C. for 1 to 3 hours to form a first mixed powder.

In the first step, the silicon oxide powder and the metal oxide powder are mixed in a solvent, and then dried and recovered are repeated 5 to 10 times to uniformly mix the raw material powders such as the silicon oxide powder and the metal oxide powder. Mixed powder can be formed.

The second step is a step of forming a sintered body by pressing and firing the first mixed powder. By forming a sintered body including silicon powder, loss of powder during the electrolytic reduction process can be minimized.

If the first mixed powder is not formed as a sintered body, the silicon oxide powder is separated from the electrode and distributed in the electrolyte in the electrolytic reduction step, so that recovery may be difficult.

The second step may be a step of filling a die or a mold with the first mixed powder and pressing to form a molded body, followed by firing to form a sintered body.

In this case, the shape of the die or the mold may be a cylindrical or rectangular shape, but is not limited thereto.

The pressurization is for densifying the mixed powder by applying pressure to the first mixed powder to increase the density, and a method of applying pressure in a uniaxial or biaxial direction may be used.

The pressure may be preferably applied to a pressure of 50 to 150 bar size, it may be more preferable to apply a pressure of 80 to 150 bar size, it may be more preferable to apply a pressure of 80 to 100 bar size.

If the pressure is less than 50 bar, the degree of compression of the first mixed powder in the die may be insufficient, so that a molded body may not be formed, and when the pressure exceeds 150 bar, Plastic deformation may occur in the process of compression due to the pressure, and mechanical defects such as cracks may occur in the molded body, and thus the molded body may not be manufactured in a desired shape, or the molded body may be formed in a subsequent firing step due to the crack. There may be a problem of being broken into pieces.

The molded body may be a die having a predetermined shape or a pellet shape formed by compressing the first mixed powder filled in a mold, and may be a cylindrical shape or a rectangular shape, but is not limited thereto.

For example, the molded body may be a cylindrical pellet formed by filling the 1 mixed powder into a cylindrical die having a diameter of 1 to 3 cm and then applying uniaxial pressure.

The sintering is to increase strength by applying heat to a compact formed by compression after pressing, and may be performed by heating at a temperature of less than 900 to 1600 °C. The sintering may be performed by heating at a temperature of less than 900 to 1600° C. for 3 to 5 hours.

By heating the molded body to a temperature of less than 900 to 1600 °C, the amorphous silicon oxide contained in the molded body can be phase-changed into a crystalline silicon oxide. For example, the molded body including the amorphous silicon dioxide may be changed to 900 °C or higher. By heating to a temperature, the amorphous silicon dioxide can be phase-changed into silicon dioxide (SiO ₂ ) having a quartz crystal structure. Through this, the strength of the silicon oxide powder is improved, and the shape of the molded body can be maintained even under a high temperature molten salt condition.

If the sintering temperature is less than 900° C., the density of the sintered body is formed non-uniformly, and in the subsequent electrolytic reduction step, a problem may occur in which the reaction is uneven, and the strength of the silicon oxide powder contained in the molded body It is low, and there may be a problem that the silicon oxide powder contained in the molded body is dispersed in the electrolyte containing the molten salt.

In addition, when the sintering temperature is 1600° C. or higher, a problem of melting the silicon oxide powder contained in the sintered body may occur.

It may be preferable that the density of the sintered body is 1 to 5 g/cm ³ .

If the density of the sintered body is less than 1 g/cm ³ , there may be a problem that a uniform reaction does not occur in the sintered body, and when the density of the sintered body exceeds 5 g/cm ³ , having the density It may be difficult to produce a molded body for manufacturing a sintered body.

Although the thickness or weight of the sintered body is not limited, it may be desirable to have a thickness or weight capable of causing a uniform reaction inside and outside the sintered body during the electrolytic reduction process.

The third step may be a step of forming silicon by reducing silicon oxide using a molten salt reduction method. The method of forming silicon by reducing silicon oxide using the molten salt reduction method is a difficult process because the reduction proceeds at a relatively lower temperature than the method of producing silicon by thermal reduction of silicon oxide, and uniform mixing with a reducing agent is not required. Conditions can be omitted.

In addition, the third step is a step of electrolytic reduction of silicon oxide and metal oxide contained in the sintered body, and the metal formed by reduction of the metal oxide during electrolytic reduction improves the electrical conductivity inside the sintered body, thereby electrolytic reduction reaction of silicon oxide. Can be further improved.

The third step may be performed using an electrolytic reduction device.

Accordingly, the third step may include configuring an electrolytic reduction device; Heating a solid salt to a melting point or higher to form a molten salt; And reducing the sintered body by applying a cell voltage of 2.5V to 2.8V.

The electrolytic reduction device may be a three-electrode electrolytic reduction device including a working electrode, a counter electrode, a reference electrode, and a reaction vessel, wherein the working electrode may be a cathode comprising a sintered body, and the counter electrode is an anode electrode The reference electrode may be Ag/AgCl, and the reaction vessel may be a reaction vessel containing a liquid electrolyte containing a molten salt.

In addition, in order to form a molten salt by heating a liquid electrolyte containing a solid salt in the electrolytic reduction reaction above the melting point of the salt, the reaction A heating device for heating the container and a current power supply device for energizing the cathode and anode may be further included.

As the working electrode, an electrode in which the sintered body is connected to a metal having high temperature stability and high electrical conductivity may be used. For example, a sintered body may be brought into contact with a stainless steel rod, and a working electrode may be formed by wrapping it with a nickel (Ni) mesh.

Further, the counter electrode may be an electrode in which graphite is connected to a metal having high stability at high temperature and high electrical conductivity. For example, a counter electrode may be formed by connecting a graphite rod to a stainless steel rod.

The reaction vessel forms molten salt by filling the reaction vessel with a solid salt at the time of the electrolytic reduction and then heating the reaction vessel with a heating device such as a heater. It may be desirable to be a container made of a material usable at a high temperature above the melting point. For example, the reaction vessel may be a ceramic material such as alumina (Al ₂ O ₃ ) or magnesium oxide (MgO), or stainless steel of SUS (Steel Use Stainless) series, but is not limited thereto.

The molten salt may include cationic species (M) and anionic species (Y).

The cationic species (M) include calcium (Ca), lithium (Li), sodium (Na), potassium (K), cesium (Cs), magnesium (Mg), strontium (Sr), francium (Fr), rubidium ( It may contain at least one of Rb), aluminum (Al), thorium (Th), and barium (Ba).

The anionic species (Y) may include at least one of chlorine (Cl), fluorine (F), iodine (I), and bromine (Br).

Thus, the molten salt is LiCl, KCl, NaCl, RbCl, CsCl, FrCl, CaCl ₂ , MgCl ₂ , SrCl ₂ , BaCl ₂ , AlCl ₃ , ThCl ₃ , LiF, KF, NaF, RbF, CsF, FrF, CaF _{2 , MgF 2, SrF 2, BaF} 2, AlF 3, ThF 3, LiPF 6, LiBr, NaBr, KBr, RbBr, CsBr, FrBr, LiI, NaI, KI, RbI, one selected from the group consisting of CsI, and FrI It may be more than that, it may be more preferable to use CaCl ₂ among them.

The CaCl ₂ molten salt has a reduction potential of Ca ²⁺ ions in the molten salt lower than that of silicon oxide, making it suitable as an electrolyte during electrolysis, and has excellent chemical reaction and electrical conductivity. In addition, there is an advantage of small concentration polarization because product diffusion and ion supply are fast. As a result, oxygen removed by the reduction reaction makes porous silicon.

The electrolytic reduction may be performed for 1 to 20 hours while maintaining a constant voltage of 2.6V to 2.8V between the cathode and the anode.

If the constant voltage is less than 2.6V, there may be a problem that the reaction does not proceed due to a low reduction potential at the voltage, and if the voltage exceeds 2.8V, the reduction potential is exceeded and contained in the molten salt. There may be a problem that unnecessary cationic species (M) are precipitated by reaching the reduction potential of the cationic species (M).

The electrolytic reduction may be performed in an inert atmosphere, for example, may be performed in an argon (Ar) glove box.

After the electrolytic reduction, the sintered body may include silicon and other impurities formed by reduction of silicon oxide, and the other impurities include at least one of unreduced silicon oxide, metal formed by reduction of metal oxide, and metal-silicon alloy. can do.

The method of manufacturing porous silicon for a negative electrode material for a lithium secondary battery according to an embodiment of the present invention may further include washing and drying the sintered body after the third step. The washing is to remove the molten salt that may be contained in the sintered body during the electrolytic reduction process, and the sintered body can be washed using distilled water or ethanol, and the drying is 1 to 5 in an oven at 50 to 80°C. It can be carried out by drying for hours.

The method of manufacturing porous silicon for a negative electrode material of a lithium secondary battery according to an embodiment of the present invention may further include, after the third step, pulverizing the sintered body to form a second mixed powder.

Through the pulverization process, the particle size of the second mixed powder can be adjusted and can be formed in a uniform size.

The second mixed powder preferably has an average particle diameter of 10 nm to 100 μm, and it may be more preferable to have an average particle diameter of 10 to 500 nm.

If the average particle diameter of the second mixed powder is less than 10 nm, difficulties in the pulverizing process may occur, and if the particle size of the second mixed powder exceeds 100 μm, the porous silicon to be produced Due to its large size, when used as a negative electrode material, a problem may arise that the effect of reducing the volume expansion of the negative electrode is insufficient.

The pulverization may be pulverized by ball milling at a speed of 200 rpm or more. The pulverization may be preferably ball milled at a speed of 200 to 1000 rpm, and more preferably ball milled at a speed of 200 to 500 rpm.

The pulverization time is not limited, but it is preferable to pulverize for 1 to 20 minutes so that oxidation of silicon does not occur during pulverization, and pulverization for 5 to 15 minutes may be more preferable.

The pulverization may be performed by a wet ball milling method in which the sintered body is put in a solvent and ball milled, and a step of drying after ball milling may be further performed.

At this time, the solvent may be at least one selected from the group consisting of 2-propanol (isopropyl alchohol), ethanol, methanol and tetrahydrofuran, and the drying may be performed in an oven at 50 to 80°C. It may be carried out by drying for a period of time, but is not limited thereto.

The fourth step is a step of acid-treating the second mixed powder formed by pulverization, and the metal contained in the second mixed powder or the metal contained in the metal-silicon alloy may be removed, and in the electrolytic reduction step Reduced silicon oxide can be removed.

In the method for manufacturing porous silicon according to an embodiment of the present invention, silicon may be formed by reducing silicon oxide through the electrolytic reduction in the third step, and the metal-silicon alloy included in the acid treatment in the fourth step Metal can be removed and porous silicon can be formed.

The porous silicon may have a porosity of 5 to 90%, and due to the void, when the porous silicon is used as a negative electrode active material for a secondary battery, the degree of volume expansion is significantly smaller than when using silicon as a negative electrode active material. It can solve the problem of reducing the capacity.

The acid treatment may include at least one of a hydrochloric acid treatment step and a hydrofluoric acid treatment step, and if both the hydrochloric acid treatment step and the hydrofluoric acid treatment step are performed, the hydrofluoric acid treatment step may be performed after the hydrochloric acid treatment step, and after the hydrofluoric acid treatment step The hydrochloric acid treatment step can be performed.

The hydrochloric acid treatment step is for removing the metal component contained in the second mixed powder, and may be a step of drying the pulverized second mixed powder after treating it with a hydrochloric acid solution. At this time, the hydrochloric acid solution may contain hydrochloric acid and a solvent, and the solvent may be one selected from the group consisting of distilled water, ethanol, isopropanol, isopropanol, and a mixed solution thereof.

The hydrochloric acid treatment step is carried out by putting the second mixed powder in a 0.1M to 1M hydrochloric acid solution, acid treatment for 1 to 3 hours, vacuum filtration, and drying in a vacuum oven at 50 to 80°C for 1 to 5 hours. However, it is not limited thereto.

The hydrofluoric acid treatment step is to remove unreduced silicon oxide contained in the second mixed powder, and may be a step of drying the pulverized second mixed powder after treating it with a hydrofluoric acid solution. At this time, the hydrofluoric acid solution may contain hydrofluoric acid and a solvent, and the solvent may be one selected from the group consisting of distilled water, ethanol, isopropanol, isopropanol, and a mixed solution thereof.

The hydrofluoric acid treatment step may be performed by putting the second mixed powder in a 0.1M to 1M hydrofluoric acid solution, acid treatment for 1 to 3 hours, vacuum filtration, and drying in an oven at 50 to 80°C for 1 to 5 hours. However, it is not limited thereto.

Another embodiment of the present invention may provide a negative electrode for a lithium secondary battery including porous silicon manufactured by the above manufacturing method.

The negative electrode may include the porous silicon as a negative electrode active material, and may have a form in which the porous silicon is coated on a current collector.

Another embodiment of the present invention may provide a lithium secondary battery including the negative electrode.

The lithium secondary battery may include a negative electrode containing the porous silicon as a negative electrode active material and lithium (Li) as a positive electrode.

Hereinafter, the present invention will be described in detail through examples and experimental examples.

However, the following examples and experimental examples are merely illustrative of the present invention, and the contents of the present invention are not limited by the following examples.

<Example 1> Preparation of porous silicon (1)

Step 1: After mixing 1 g of silicon dioxide (SiO ₂ ) powder, 0.01 g of nickel oxide (NiO) powder, 0.01 g of PVA, 0.01 g of zinc stearate and ethanol in a mortar, in an oven at 80°C. It was dried for 3 hours to form a mixed powder. Thereafter, the mixed powder was added to ethanol again, mixed and dried in an oven at 80° C. for 3 hours 5 times to form a first mixed powder.

Step 2: Put the first mixed powder in a pellet die having a diameter of 1.3 cm, and pressurized at a pressure of 80 bar in the sunrise direction using a hydraulic press to prepare a pellet. Thereafter, the pellets were heated at 900° C. for 3 hours to form a sintered body in the shape of FIG. 1.

Step 3: Connect the sintered body to the end of a stainless steel rod using nickel gauze to form a working electrode, connect a graphite rod to the stainless steel rod to form a counter electrode, and a three-electrode using Ag/AgCl as a reference electrode A liquid electrolyte containing CaCl ₂ was placed in the reaction vessel of the cell apparatus.

Put the three-electrode cell device in a high-purity argon glove box, heat the reaction vessel to 850° C. using a heating device to form a CaCl ₂ molten salt, and place the working electrode, counter electrode, and reference electrode 3 cm in the molten salt. After being immersed in the depth, electrolytic reduction was performed by passing an electric current.

At this time, a voltage of 2.6V was formed between the working electrode and the counter electrode and maintained for 1 to 20 hours.

Thereafter, the sintered body was washed with distilled water, filtered through an aspirator, and dried in a vacuum oven at 50° C. for 3 hours.

Thereafter, the sintered body was pulverized by ball milling at a speed of 200 rpm for about 20 minutes, and then the powder pulverized with an aspirator was filtered and dried in a vacuum oven at 50° C. for 3 hours.

Step 4: Put the pulverized powder in 0.1M hydrochloric acid solution and acid-treat for 1 hour to remove nickel contained in the nickel-silicon alloy, and then filter it with an aspirator for 3 hours in a vacuum oven at 50°C. It was dried during. Thereafter, the pulverized powder was put in a 0.1M hydrofluoric acid solution and acid-treated for 1 hour to remove unreduced silicon oxide, filtered through an aspirator, and dried in a vacuum oven at 50° C. for 3 hours to obtain porous silicon. Was prepared.

<Example 2> Preparation of porous silicon (2)

In Example 1, porous silicon was manufactured by performing the same method as in Example 1, except that the fourth step was performed differently as follows.

Step 4: Put the pulverized powder in 0.1M hydrochloric acid solution and acid-treat for 1 hour to remove nickel contained in the nickel-silicon alloy, and then filter it with an aspirator for 3 hours in a vacuum oven at 50°C. During drying, porous silicon was prepared.

<Example 3> Preparation of porous silicon (3)

In Example 1, porous silicon was manufactured by performing the same method as in Example 1, except that the fourth step was performed differently as follows.

Step 4: Put the pulverized powder in 0.1 M hydrofluoric acid solution and acid-treat for 1 hour to remove nickel contained in the nickel-silicon alloy, and then filter it with an aspirator for 3 hours in a vacuum oven at 50°C. During drying, porous silicon was prepared.

<Comparative Example 1>

In Example 1, porous silicon was manufactured by performing the same method as in Example 1, except that the fourth step was not performed.

<Experimental Example 1>

Elemental analysis was performed using Energy Dispersive Spectrometry (EDS) to confirm the components of the porous silicon prepared in Examples 1 to 3 and Comparative Example 1, and the results are shown in Tables 1, 2 and 3 Shown in.

EDS analysis result Example 1 Removed both nickel and unreduced silicon oxide Example 2 Nickel was removed, but unreduced silicon oxide was not removed. Example 3 Some nickel and unreduced silicon oxides are not removed Comparative Example 1 Nickel and unreduced silicon oxides are not removed

FIG. 2 is a result of EDS analysis of the porous silicon prepared in Example 1. As shown in FIG. 2, the porous silicon prepared according to Example 1 contains 99.29% by weight of silicon and 0.71% by weight of oxygen, and It was found not to contain nickel or nickel oxide. Through this, it can be seen that the porous silicon prepared in Example 1 does not contain unreduced silicon oxide, metal oxide, and metal, and most of the silicon oxide is reduced to produce silicon, and metal impurities are removed through acid treatment. It can be seen that it has been removed.

In addition, FIG. 3 is a result of EDS analysis of the porous silicon prepared in Example 2. As shown in FIG. 3, the porous silicon prepared according to Example 2 contains 89.03% by weight of silicon and 10.97% by weight of oxygen. , Nickel and nickel oxide were not included. Through this, it can be seen that the porous silicon prepared in Example 2 contains unreduced silicon oxide.

<Experimental Example 2>

In order to confirm the shape of the porous silicon prepared in Example 1, shape analysis was performed using a transmission electron microscope (TEM), and the results are shown in FIG. 4.

As shown in Figure 4, it can be seen that the porous silicon particles of 500 nm or less are aggregated. It can be seen that lattice oxygen is removed from the silicon oxide due to electrochemical reduction in the electrolytic reduction process, and impurities are removed through the subsequent acid treatment process to form a fine porous structure on the surface.

<Experimental Example 3>

In order to confirm the battery characteristics of a lithium secondary battery using porous silicon prepared according to Examples 1 to 3 and Comparative Example 1 as a negative electrode active material of the present invention, a half coin cell of a lithium secondary battery was manufactured by the following method, and cut- Charging and discharging experiments were performed under a rate-control condition of 0.1C based on 1C = 4200mAh/g with an Off voltage of 0.01V to 1.5V, and cycle characteristics were confirmed, and the results are shown in Tables 1 and 5 to 8.

(Manufacture of half coin cell of lithium secondary battery)

For each of the porous silicon prepared according to Examples 1 to 3 and Comparative Example 1, super P carbon black as a conductive agent and polyacryl acid as a binder were blocked at a weight ratio of 50:30:20. Put in a bowl and mixed to prepare a slurry.

Using a copper foil as a current collector, the slurry was coated to a thickness of 0.02 mm on the copper foil using a doctor blading technique, and then dried in an air atmosphere at 80° C. for about 1 hour.

The dried slurry was compressed using a roll press until the thickness of the dried slurry became 0.016 mm, which is 80% of 0.02 mm, and then dried in a vacuum oven at 120° C. for 5 hours to prepare a copper foil having a porous silicon thin film.

Thereafter, the copper foil on which the porous silicon thin film was formed in a high purity argon (Ar) glove box blocked from moisture was cut into a coin shape having a diameter of 16 mm and used as a negative electrode, and lithium metal was used as a diameter of 16 mm and a thickness of 3.2 mm. And used as a positive electrode, and a CR2032 type lithium secondary battery cell was formed using an electrolyte made of 1.15M LiPF ₆ in EC/EMC=3/7(V/V)+10% FEC.

5 to 8 are graphs showing the cycle characteristics of a lithium secondary battery using porous silicon prepared according to Examples 1 to 3 and Comparative Example 1 as a negative electrode active material, and the capacity retention rates based on 20 cycles are about 91.8% and 60%, respectively. , 48.62% and 22.4%, and it can be seen that when the porous silicon of Example 1 is used as an anode active material, the highest capacity retention rate appears.

Claims (14)

A first step of mixing silicon oxide powder and metal oxide powder to form a first mixed powder;
A second step of forming a sintered body by pressing the first mixed powder at a pressure of 50 to 150 bar and firing at a temperature of 900 to 1600°C;
A third step of electrolytic reduction of the sintered body; And
Including; a fourth step of producing porous silicon by acid treatment,
The metal oxide powder is at least one of nickel oxide, cobalt oxide (III), titanium oxide, and copper oxide,
The acid treatment is characterized in that it includes both a hydrochloric acid treatment step and a hydrofluoric acid treatment step.
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
The method of claim 1,
The first mixed powder is
It characterized in that it comprises the metal oxide powder in an amount of 1 to 20% by weight based on the weight of the silicon oxide powder
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
delete The method of claim 1,
The first mixed powder, characterized in that it further comprises at least one of a binder material and a lubricant
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
The method of claim 1,
The first mixed powder is
The silicon oxide powder and the metal oxide powder, characterized in that formed by drying after mixing with a solvent
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
delete delete The method of claim 1,
Electrolytic reduction in the third step
A cathode comprising the sintered body;
An anode; And
A liquid electrolyte containing a molten salt; characterized in that it is carried out in an electrolytic reduction device containing
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
The method of claim 8,
The molten salt is LiCl, KCl, NaCl, RbCl, CsCl, FrCl, CaCl ₂ , MgCl ₂ , SrCl ₂ , BaCl ₂ , AlCl ₃ , ThCl ₃ , LiF, KF, NaF, RbF, CsF, FrF, CaF ₂ , MgF _{2, SrF 2, BaF 2,} AlF 3, ThF 3, LiPF 6, LiBr, NaBr, KBr, RbBr, CsBr, FrBr, LiI, NaI, KI, any one selected from the group consisting of RbI, CsI, and FrI or more Characterized by
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
The method of claim 1,
The method of manufacturing the porous silicon for a negative electrode material of the lithium secondary battery further comprises: after the third step, pulverizing the sintered body to form a second mixed powder;
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
delete The method of claim 1,
The porous silicon is characterized in that the porosity is 5 to 90%
Method of manufacturing porous silicon for negative electrode material of lithium secondary battery.
A lithium secondary battery negative electrode comprising porous silicon manufactured by the method of claim 1.
A lithium secondary battery comprising the negative electrode of claim 13.

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