KR102272424B1 - Porous graphene-silicon aerogel composite containing graphene-wrapped silicon nanoparticles, preparation of the same and lithium secondary battery using the same - Google Patents

Abstract

In the present specification, reduced graphene oxide having a porous structure connected to each other sterically; and a spherical composite dispersed in the reduced graphene oxide, wherein the spherical composite is characterized in that the reduced graphene surrounds the silicon nanoparticles, the graphene-silicon airgel composite is disclosed.

Description

Porous graphene-silicon airgel composite comprising silicon nanoparticles surrounded by graphene, manufacturing method thereof, and lithium secondary battery comprising same BATTERY USING THE SAME}

The present invention relates to a porous graphene-silicon airgel composite comprising silicon nanoparticles surrounded by graphene, a manufacturing method thereof, and a lithium secondary battery including the same. More specifically, the present invention relates to reduced graphene oxide having a porous structure that is sterically connected to each other; and a spherical composite dispersed in the reduced graphene oxide; wherein the spherical composite is characterized in that the reduced graphene surrounds the silicon nanoparticles, graphene-silicon airgel composite, its manufacturing method, and its containing It relates to a lithium secondary battery.

In the 21st century, with the rapid development of the semiconductor industry, the information communication era of a new communication paradigm has arrived, in which small electric and electronic devices such as notebook computers, mobile phones, DMB phones, and portable communication devices are generalized from simple information reception to multimedia functions based on two-way communication. are doing In order to meet the needs of these multi-functional electrical and electronic devices, high-capacity, high-output secondary batteries are being researched and developed with a focus on battery materials.

In the related art, lithium metal was used as a negative electrode active material of a lithium secondary battery, but carbon-based negative electrode active material is widely used instead of lithium metal. Graphite among crystalline carbons is typically used as a carbon-based anode active material. Carbon-based anode active materials such as graphite have an upper limit of about 372 mAh/g in theoretical capacity, so it is suitable for mobile digital convergence devices that require high capacity. It is insufficient as a cathode material. In particular, in the automobile field, the development of electric vehicles is urgently required due to the depletion of fossil fuels. However, when the existing lithium secondary battery is applied, it is difficult to operate a long distance of 200 km or more with a single charge, and the existing lithium secondary battery is a renewable energy source. It is also not suitable for a long-term energy storage system for storing electric power generated by a source.

Therefore, a lot of new high-capacity materials have recently appeared to replace commercial graphite electrodes. Among them, silicon (Si)-based anode materials are the most promising because their theoretical capacity is about 4,200 mAh/g, which is more than 10 times larger than graphite materials. material is emerging. However, the silicon anode material causes a change in the crystal structure when absorbing and storing lithium, resulting in a large volume change of 300% or more. Graphite currently used as an anode material has a volume expansion rate of about 1.2 times due to lithium charging, whereas silicon absorbs and stores the maximum amount of lithium, Li _{4 . It} is converted to 4 Si and expands to about 4.12 times the volume of silicon before volume expansion. Due to this volume expansion, the structure of the electrode collapses and the coulombic efficiency decreases, making continued use difficult.

Recently, nanostructured electrodes are attracting attention because they are expected to greatly improve the energy density and velocity characteristics, and the relaxation and speed of volume change due to insertion/deintercalation of lithium ions. If the cathode or anode materials have a high specific surface area, the solid-phase diffusion path is reduced and the anode-separator-cathode interface is greatly increased. Therefore, it is predicted that high-rate discharge characteristics and high-output lithium-ion batteries can be realized through three-dimensional structuring of nanostructured materials. do.

On the other hand, graphene exists as a unique structure of a two-dimensional nanosheet composed of single-atom-thick sp ² shell carbon atoms arranged in a hexagonal shape, and is composed of carbon nanotubes (CNT) or carbon nanofibers (CNF). Together with their unique electrical conductivity and structural properties, they are being used in various fields. In addition, graphene can be prepared by direct exfoliation of graphite or reduction of graphene oxide (GO) nanosheets, and graphene reduced from graphene oxide is also called reduced graphene oxide (rGO) due to its characteristics. As such, recent graphene-based composite materials have attracted a lot of attention worldwide because of their improved material properties such as electrical conductivity, chemical properties and electrical properties, and many studies on the manufacture of graphene-based composites have been reported. .

In order to overcome the limitations of the existing electrode technology, if nanostructured materials such as graphene are used as electrode materials, not only can the electrode provide excellent electronic conductivity and reduce the diffusion path, but also the insertion and release process of a ^{large amount of Li +} A lithium secondary battery with high output and high energy density is expected because it can alleviate the stress generated in the However, most of the previously reported graphene/silicon composites are synthesized through a mechanical/physical mixing method of graphene and silicon, and for graphene/silicon composites with a structure that can effectively buffer the volume expansion of silicon, So far, there are few reports.

In addition, in the case of the currently reported graphene/metal or metal oxide composite, it is difficult to evenly disperse it in a metal or metal oxide state, and the metal particles agglomerate when exposed to the graphene surface due to the volume expansion of the metal particles on the surface of the metal particles. Problems such as continuous interface generation due to cracking, decrease in battery charging and discharging efficiency, continuous electrolyte decomposition reaction and consequent increase in electrolyte usage, increase in battery resistance and decrease in battery life due to interfacial generation and decrease in inter-particle conductivity, may occur. While relatively easy and mass-produced at a low cost, there is a need to develop a technology for manufacturing a silicon-containing composite, which can solve the problems caused by the conventional use of silicon.

Patent Publication No. 10-2014-0112451 Patent Publication No. 10-2015-0117172

Chem. Mater. 1999, 11(3), 771-778

Therefore, in order to prevent the metal nanoparticles from growing too large or becoming large particles by aggregation, by surrounding silicon with graphene oxide, the metal nanoparticles are uniformly dispersed in the graphene and the change in volume is alleviated at the same time. Thus, by improving electrical contact, we intend to develop a graphene-silicon airgel composite that can secure high capacity and cycle characteristics.

In addition, using a solution containing silicon nanoparticles surrounded by the graphene oxide and graphene oxide, a graphene-silicon airgel composite manufacturing method that can be mass-produced at low cost simply by a hydrothermal self-assembly synthesis method. would like to provide

In addition, by applying a three-dimensional graphene-silicon airgel composite, which is the final product, as an anode active material for a lithium secondary battery, in one reaction vessel without adding a conductive material or additional post-treatment, excellent charge/discharge characteristics, high capacity, and excellent volume stability were secured. An object of the present invention is to provide a lithium secondary battery.

In an exemplary embodiment of the present invention, reduced graphene oxide having a porous structure connected to each other sterically; and a spherical composite dispersed in the reduced graphene oxide, wherein the spherical composite is characterized in that the reduced graphene surrounds the silicon nanoparticles, graphene provides a silicon airgel composite.

In an exemplary embodiment of the present invention, spray drying a first solution comprising a first graphene oxide and silicon nanoparticles; heat-treating and reducing the spray-dried material to obtain a spherical composite; reducing and gelling a second solution including the spherical composite and second graphene oxide; and freeze-drying the reduced and gelled second solution to obtain a porous airgel composite comprising graphene/silicon having a three-dimensional structure; comprising, a graphene-silicon airgel composite manufacturing method do.

An exemplary embodiment of the present invention provides an anode active material comprising the above-described graphene-silicon airgel composite.

In an exemplary embodiment of the present invention, a negative electrode comprising the above-described negative active material; anode; and an electrolyte; provides a lithium secondary battery comprising.

The graphene-silicon airgel composite prepared in the present invention has excellent volume-to-volume specific surface area and electrical conductivity, and the constituent graphenes form a three-dimensional network, so that it can be used as an electrode material for secondary batteries without a binder when manufacturing an electrode. Furthermore, it is very useful as an electrode material for an electric double-layer supercapacitor, an electromagnetic wave shielding material, a high-conductivity material, a catalyst support, and a reinforcing material for a composite material.

In addition, in its manufacturing method, using a solution containing silicon nanoparticles surrounded by graphene oxide and graphene oxide, low-cost mass production is possible through a simple process such as a hydrothermal self-assembly synthesis method.

1 is a graphene-silicon airgel composite (GSiGA) composite manufacturing process schematic diagram and an airgel electrode photograph including a spherical composite (GSi) using a hydrothermal synthesis method according to the present invention.
2a-2c are scanning microscope (SEM) pictures of the spherical complex (GSi) according to the present invention.
Figure 3a is a low magnification scanning microscope (SEM) photograph of the graphene-silicon airgel composite (GSiGA) according to the present invention.
Figure 3b is a high-magnification scanning microscope (SEM) photograph of the graphene-silicon airgel composite (GSiGA) according to the present invention.
Figure 3c is a low magnification scanning microscope (SEM) photograph of the graphene-silicon airgel composite (SiGA) containing silicon nanoparticles instead of the spherical composite according to the present invention.
Figure 3d is a high magnification scanning microscope (SEM) image of the graphene-silicon airgel composite (SiGA) containing silicon nanoparticles instead of the spherical composite according to the present invention.
Figure 4a is a transmission electron microscope (TEM) photograph of the graphene-silicon airgel composite (GSiGA) according to the present invention.
Figure 4b is a graphene-silicon airgel composite (GSiGA) according to the present invention a high-magnification transmission electron microscope (TEM) photograph and a diffraction pattern by limited-field electron diffraction (SAED).
5A to 5D are micro-tomography (Micro-CT) photographs, FIG. 5A is a porous graphene airgel structure of GSiGA, FIG. 5B is a silicon nanoparticle distribution state of GSiGA, and FIG. 5C is a graphene-containing silicon microparticle. A three-dimensional image of a porous graphene airgel composite (GSiGA), FIG. 5d is a photograph showing the distribution state of silicon nanoparticles in the GSiGA side.
6a is an X-ray of the graphene-silicon airgel composite (GSiGA) according to the present invention, the graphene-silicon airgel composite (SiGA) containing only silicon nanoparticles instead of the spherical composite, and the spherical composite (GSi) according to the present invention diffraction pattern (XRD).
6b is a graphene-silicon airgel composite (GSiGA) according to the present invention, a graphene-silicon airgel composite (SiGA) containing only silicon nanoparticles instead of a spherical composite, and a graphene airgel composite (GA) without a spherical composite ) is a Raman spectrum.
7 is a graphene-silicon airgel composite (GSiGA) according to the present invention, a graphene-silicon airgel composite (SiGA) containing only silicon nanoparticles instead of a spherical composite, and a graphene airgel composite without a spherical composite (GA) ) is the nitrogen adsorption/desorption isotherm.
8a is a graphene-silicon airgel composite (GSiGA) according to the present invention, a graphene-silicon airgel composite (SiGA) containing only silicon nanoparticles instead of a spherical composite, and a spherical composite (GSi) according to the present invention. analysis (TGA) graph and differential thermogravimetry (DTG) graph.
8b is a graphene-silicon airgel composite according to the present invention (GSiGA), a graphene-silicon airgel composite (SiGA) containing only silicon nanoparticles instead of a spherical composite, and the thermal weight of a spherical composite (GSi) according to the present invention The weight ratio of residues after analysis is shown.
9 is a graphene-silicon airgel composite (GSiGA) according to the present invention, a graphene-silicon airgel composite (SiGA) containing only silicon nanoparticles instead of a spherical composite, Si2p X of a spherical composite (GSi) according to the present invention Line photoelectron spectroscopy (XPS) graph.
10a to 10d are a graphene-silicon airgel composite (GSiGA) according to the present invention, a constant current charge/discharge curve (10a), a cyclic voltamogram (Cyclic voltamogram) graph (10b), a rate characteristic (10c), and a rate characteristic graph (10d) )to be.
11a is a top-view high-magnification scanning micrograph of the graphene-silicon airgel composite (GSiGA) according to the present invention before charging and discharging.
Figure 11b is a top-view low-magnification scanning micrograph of the graphene-silicon airgel composite (GSiGA) according to the present invention before charging and discharging.
11c is a top-view high-magnification scanning micrograph of the graphene-silicon airgel composite (GSiGA) according to the present invention after 50 cycles of charging and discharging.
11d is a top-view low-magnification scanning micrograph of the graphene-silicon airgel composite (GSiGA) according to the present invention after 50 cycles of charging and discharging.
Figure 12a is a view schematically showing the change in the shape of the SiGA anode composite active material according to the insertion and desorption process of lithium, Figure 12b is a view schematically showing the change in the shape of the GSiGA anode composite active material according to the insertion and desorption process of lithium to be.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The embodiments of the present invention disclosed in the text are illustrated for the purpose of explanation only, and the embodiments of the present invention may be embodied in various forms and should not be construed as being limited to the embodiments described in the text. .

The present invention is not intended to limit the present invention to a specific disclosed form, but is not intended to limit the present invention to a specific disclosed form, as various changes may be made and may have various forms, and all changes, equivalents or substitutes included in the spirit and scope of the present invention should be understood as including

In the present specification, when a part "includes" a certain component, this means that other components may be further included, rather than excluding other components, unless otherwise stated.

graphene -silicon airgel complex

In exemplary embodiments of the present invention, reduced graphene oxide having a porous structure connected to each other sterically; and a spherical composite dispersed in the reduced graphene oxide, wherein the spherical composite is characterized in that the reduced graphene surrounds the silicon nanoparticles, graphene provides a silicon airgel composite.

1 is a conceptual diagram of the graphene-silicon airgel composite of the present invention. Referring to this, the graphene-silicon airgel composite according to the present invention is three-dimensionally connected to each other and has a porous structure in reduced graphene oxide (GSiGA). The spherical composite (GSi) is uniformly dispersed, and each dispersed spherical composite has a structure in which silicon nanoparticles are surrounded by reduced graphene oxide.

Therefore, there is an effect that the silicon nanoparticles are coated with double-layer graphene, so that the silicon nanoparticles are not exposed to the outside of the graphene-silicon airgel composite, and the large volume of silicon nanoparticles according to the charging and discharging of the conventional lithium ion battery. In addition to buffering the change, it is possible to induce an effective electrochemical reaction by preventing the aggregation of silicon nanoparticles and reducing the resistivity of the electrode surface of the lithium ion battery.

In exemplary embodiments, the reduced graphene oxide in the spherical composite may be in the form of a net, and may be sterically connected to each other, and the silicon nanoparticles in the spherical composite may be separated from each other.

In exemplary embodiments, the size of the silicon nanoparticles may be 10 to 200 nm, for example, 20 nm or more, 30 nm or more, 40 nm or more, or 50 nm or more, 190 nm or less, 180 nm or less , 170 nm or less, 160 nm or less, 150 nm or less, 140 nm or less, 130 nm or less, 120 nm or less, 110 nm or less, 100 nm or less, 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less. If the size of the silicon nanoparticles is less than 10 nm, oxidation of the particles may increase, high manufacturing cost and difficulty in process control may become problems, and an unstable solid electrolyte interface may cause a decrease in battery performance. , and if it is more than 200 nm, it is difficult to accommodate the large volume change of silicon particles generated during repeated charging and discharging.

In exemplary embodiments, the diameter of the spherical composite may be 1 to 10 μm, for example 3 μm or more, 4 μm or more, 5 μm or more, or 6 μm or more, 9 μm or less, 8 μm or less, It may be 7 μm or less, or 6 μm or less. When the diameter is less than 1 μm, it may be difficult to control the size of the spherical composite obtained through spray drying, the yield may be low, and when the diameter is more than 10 μm, the binding force according to the formation of an unstable network with graphene when synthesizing graphene airgel This can fall.

In exemplary embodiments, the weight ratio of silicon nanoparticles and reduced graphene in the spherical composite may be 1:1 to 5, for example 1:1 to 4, 1:1 to 3, or 1:1 to It can be two. If the weight ratio is less than 1:1, complex formation may be difficult due to the influence of the spray-drying solution concentration, and if the weight ratio is more than 1:1, electrochemical properties may be deteriorated due to cohesiveness of silicone.

In exemplary embodiments, based on the total weight of the graphene-silicon airgel composite, the silicon nanoparticles may be included in 30% to 60% by weight, for example, 33% by weight or more, 35% by weight or more, 37 weight % or greater, 39 weight % or greater, or 41 weight % or greater, and 57 weight % or less, 55 weight % or less, 53 weight % or less, 51 weight % or less, 49 weight % or less, or 47 weight % or less. If the content is less than 30% by weight, the effect of increasing the capacity of the lithium secondary battery due to silicon may be insignificant, and if it is more than 60% by weight, the volume expansion rate is excessive, so the structure of the electrode may collapse and the Coulombic efficiency may be lowered.

graphene -silicon airgel Composite manufacturing method

In exemplary embodiments of the present invention, spray drying a first solution comprising a first graphene oxide and silicon nanoparticles; heat-treating and reducing the spray-dried material to obtain a spherical composite; reducing and gelling a second solution containing the spherical composite and second graphene oxide; and freeze-drying the reduced and gelled second solution to obtain a graphene-silicon airgel composite having a three-dimensional structure; comprising, a graphene-silicon airgel composite manufacturing method.

1 is a flow chart showing a graphene-silicon airgel composite manufacturing method according to the present invention.

In an exemplary embodiment, the first solution may be prepared by dispersing the silicon nanoparticles in a solution containing the first graphene oxide. Specifically, graphene oxide (GO) prepared using the Hummer's method may be supported in a solvent such as distilled water, and silicon nanoparticles may be dispersed in this solution.

In exemplary embodiments, the size of the silicon nanoparticles may be 10 to 200 nm, for example, 20 nm or more, 30 nm or more, 40 nm or more, or 50 nm or more, 190 nm or less, 180 nm or less , 170 nm or less, 160 nm or less, 150 nm or less, 140 nm or less, 130 nm or less, 120 nm or less, 110 nm or less, 100 nm or less, 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less. If the size of the silicon nanoparticles is less than 10 nm, oxidation of the particles may increase, high manufacturing cost and difficulty in process control may become problems, and an unstable solid electrolyte interface may cause a decrease in battery performance, If it is more than 200 nm, it is difficult to accommodate the large volume change of silicon particles generated during repeated charging and discharging.

Thereafter, the first solution is spray-dried using a spray device, and the spray-dried material is reduced by heat treatment in an inert atmosphere such as argon or nitrogen to obtain a spherical composite (GSi).

In an exemplary embodiment, the spherical composite may have a form in which reduced graphene surrounds silicon nanoparticles, and the reduced graphene is in the form of a net, and may be three-dimensionally connected to each other, and the silicon nanoparticles are may be separated.

In an exemplary embodiment, the diameter of the spherical composite may be 1 to 10 μm, such as 3 μm or more, 4 μm or more, 5 μm or more, or 6 μm or more, and 9 μm or less, 8 μm or less, 7 μm or more. or less, or 6 μm or less. When the diameter is less than 1 μm, it may be difficult to control the size of the spherical composite obtained through spray drying, the yield may be low, and when the diameter is more than 10 μm, the binding force according to the formation of an unstable network with graphene when synthesizing graphene airgel This can fall.

In exemplary embodiments, the weight ratio of silicon nanoparticles and reduced graphene in the spherical composite may be 1:1 to 5, for example 1:1 to 4, 1:1 to 3, or 1:1 to It can be two. If the weight ratio is less than 1:1, complex formation may be difficult due to the influence of the spray-drying solution concentration, and if the weight ratio is more than 1:1, electrochemical properties may be deteriorated due to cohesiveness of silicone.

In an exemplary embodiment, the second solution may be prepared by dispersing the spherical composite and the second graphene oxide in distilled water using ultrasonic waves. Specifically, the above-described spherical composite may be placed in a solvent together with a separate second graphene oxide, and the spherical composite and the second graphene oxide may be uniformly dispersed in the solvent using ultrasonic waves.

In an exemplary embodiment, the weight ratio of the spherical composite and the second graphene oxide may be 1:0.5 to 2, for example, 1:0.6 or more, 1:0.7 or more, or 1:0.8 or more, and 1:1.7 or less, 1:1.5 or less, or 1:1.3 or less. When the weight ratio is less than 1:0.5, an unstable network between graphene can be formed in the preparation of a porous airgel, and when the weight ratio is more than 1:2, various side reactions from high porosity and unstable solid electrolyte interface formed in the initial lithiation reaction, etc. This may lead to lower initial coulombic efficiency and lower cell performance.

Thereafter, the second solution can be gelled at a high temperature of 50 to 100° C. together with a reducing agent such as phosphoric acid or iodine, and the gelled second solution is washed with distilled water repeatedly until impurities are completely removed. After that, when it is completely dried using a freeze-drying method, a three-dimensional graphene-silicon airgel composite can be obtained.

Specifically, since the graphene-silicon airgel composite having a three-dimensional structure can be manufactured by hydrothermal self-assembly synthesis in the above process, it is possible to mass-produce the graphene-silicon airgel composite having a three-dimensional structure at a low cost by a simple process.

In an exemplary embodiment, the three-dimensional graphene-silicon airgel composite may be a spherical composite dispersed in reduced graphene oxide having a porous structure that is sterically connected to each other.

graphene -silicon airgel a negative active material comprising a composite; and lithium secondary battery

In exemplary embodiments of the present invention, the above-described graphene-a negative active material comprising a silicon airgel composite is provided. The graphene-silicon airgel composite according to the present invention is a lithium secondary battery negative electrode comprising a porous airgel composite containing graphene silicon (or silicon oxide) having a three-dimensional structure as a final product in one reaction vessel without adding a conductive material and additional post-treatment By applying it as an active material, it is possible to secure excellent charge/discharge characteristics, high capacity, and excellent volume stability.

In an exemplary embodiment, the negative active material is one or more metals selected from the group consisting of Sn, Al, Ge, Pb, Zn, Co, Cu, Ti, Ni, Li, Ag and Au, or alloys and oxides thereof. may include

In exemplary embodiments of the present invention, a negative electrode comprising the above-described negative active material; anode; and an electrolyte; provides a lithium secondary battery comprising.

The present invention will be described in more detail through the following practice. However, the examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example

Example One: GSi Composite Manufacturing

H ₂ SO ₄ H ₂ SO ₄ with Using KMn ₂ O ₄ Graphene oxide in which a carboxyl group (COOH), a hydroxyl group (OH), an epoxy group (1,2-ether), and a carbonyl group (CO) were introduced into graphite was synthesized by the Hamers method (Chem. Mater. 1999, 11) (3), 771-778).

Referring to FIG. 1 , the first graphene oxide made by the Hummus method was supported in distilled water at a concentration of 1 mg/mL, and 500 mg of silicon nanoparticles having an average particle diameter of 100 nm were dispersed in this solution. Then, the dispersion solution of the first graphene oxide and silicon nanoparticles was spray-dried using a spray device (80° C., 300 mL/h). The spray-dried material was put into a heat treatment furnace in an inert atmosphere (Ar, N ₂ ) and maintained at 300° C. for 1 hour at a temperature increase rate of 5° C. per minute to prepare a spherical composite (GSi) containing silicon nanoparticles and reduced graphene do.

The GSi composite obtained in Example 1 was observed with an electron microscope, and photographs of the results are shown in FIGS. 2A and 2B. In the scanning microscope (Figs. 2a and 2b), it can be seen that the microparticles are efficiently assembled in the form of spherical microparticles with a diameter of 5 to 10 μm, and the silicon nanoparticles are coated with a graphene sheet so that the graphene layers form the silicon nanoparticles. It can be seen that it is well surrounded. In addition, the graphene layers do not agglomerate adjacent silicon nanoparticles, and at the same time, the separated particles are well connected through the graphene layer, and it is confirmed through analysis that the silicon nanoparticles in the spherical composite are highly supported. did.

Example 2: GSi reduced containing micro-particles graphene airgel ( GSiGA ) complex

Referring to FIG. 1 , 100 mg of the spherical composite (hereinafter referred to as GSi) prepared in Example 1 and 100 mg of second graphene oxide were added to 50 mL of distilled water, and then sonicated to uniformly disperse in distilled water. _{10 g of phosphoric acid (H 2} PO ₄ ) and 1 g of iodine (I ₂ ) were added to the mixed solution as a reducing agent, and gelation was performed at 80° C. for 8 hours. After that. The airgel composite was washed with distilled water and washed repeatedly until impurities were completely removed, and then completely dried using a freeze-drying method, thereby preparing a graphene-silicon airgel composite having a three-dimensional structure (hereinafter, GSiGA).

comparative example One

In the same manner as in Example 2, except that silicon nanoparticles were used instead of the spherical composite according to the present invention when preparing the graphene airgel, graphene containing only silicon nanoparticles having a silicon content of 44 wt% based on the total weight of the composite A fin-silicone airgel composite (hereinafter, SiGA) was prepared.

comparative example 2

A graphene airgel (hereinafter, GA) was obtained in the same manner as in Example 2, except that only graphene oxide was not used without using the spherical composite (GSi) according to the present invention when preparing the graphene airgel.

The GSiGA obtained in Example 2 and the SiGA obtained in Comparative Example 1 were observed under a scanning microscope, and the resulting photos are shown in FIGS. 3A to 3D .

Specifically, FIG. 3a is a low magnification scanning microscope image of the graphene-silicon airgel composite (GSiGA) according to the present invention, FIG. 3b is a graphene-silicon airgel composite according to the present invention (GSiGA), a high magnification scanning microscope image, FIG. 3c is A low magnification scanning microscope image of a graphene-silicon airgel composite (SiGA) containing silicon nanoparticles instead of a spherical composite according to the present invention, FIG. 3D is a graphene containing silicon nanoparticles instead of a spherical composite according to the present invention - This is a high-magnification scanning microscope image of the silicon airgel composite (SiGA).

From Figs. 3a-3d, 5 to 10 μm spherical microparticles are well dispersed in a porous structure in which graphene sheets are three-dimensionally interconnected (Figs. 3a, 3b), whereas in SiGA, silicon nanoparticles are aggregated to form airgel pores and graphene. It was possible to observe the load (Fig. 3c, 3d) exposed on the pin surface.

As a result of transmission electron micrographs of GSiGA ( FIGS. 4a and b ), it was confirmed that silicon nanoparticles having a size of 100 nm or less were well dispersed in the graphene matrix ( FIG. 4a ). In order to investigate the internal structure of the material, superlattice diffraction points of the form (110), (220), and (311) are shown in the diffraction pattern of the silicon nanoparticles present in GSiGA by the limited-field electron diffraction method (Fig. 4b), which Through this, the crystalline diffraction pattern of the silicon nanoparticles was confirmed.

5a-5d are results taken using a non-destructive method micro tomography to confirm and analyze the constituent materials and distribution states present in the graphene-silicon airgel composite (GSiGA). Specifically, micro tomography is a tomography technique using X-rays, and can obtain a three-dimensional cross-sectional image having a pixel size of micro units without external damage to the object to be observed.

Figure 5a is a porous graphene airgel structure of GSiGA, Figure 5b is a silicon nanoparticle distribution state of GSiGA, Figure 5c is a three-dimensional image of a porous graphene airgel composite (GSiGA) including graphene-silicon microparticles, Figure 5d is This is a picture showing the side image of GSiGA. Through this, it was observed that silicon was uniformly distributed in the graphene matrix.

In order to examine the crystallinity of Si, X-ray diffraction analysis of GSi, SiGA, and GSiGA is shown in FIG. 6a. The crystallinity peaks of silicon nanoparticles having (110), (220), and (311) planes at Si 2θ = 28, 48, and 57°, respectively, were obtained from SiGA and GSiGA, which are silicon-based graphene airgel composites, as well as silicon-complexed GSi observed. In the case of GSiGA, a wide peak indicating carbon with low crystallinity can be seen centered at 2θ = 23°.

Raman analysis of the resulting spectrum (Figure 6b) 1360 cm ^{- 1} and 1580 cm ^-1 in each of the D-band and G-band of carbon was observed, a strong peak appears at 498 cm ^-1 has the nano-sized crystalline (crystallline) It is due to the E _2g mode of Si, and the peak of crystalline Si having a face-centred cubic (fcc) diamond structure type near ^{300 cm -1 was confirmed in both SiGA and GSiGA.}

_{7 is an N 2} adsorption/desorption isotherm of SiGA and GA of Comparative Examples 1 and 2 and GSiGA composite of Example 2, and the corresponding specific surface area values are tabulated within the isotherm. From this, the results calculated by Brunauer-Emmett-Teller (BET) analysis showed that the specific surface area of GA was 86 m ² g ^-1 , SiGA 105 m ² g ^-1 , and GSiGA 90 m ² g ^-1 . can be checked That is, it can be confirmed that the GSiGA according to the present invention is a three-dimensional porous structure.

8A and 8B are graphs of the results of thermogravimetric analysis, in which the amount of silicon and graphene present in the composite were accurately measured through the thermogravimetric analysis. As a result of the experiment, the weight of the spherical composite (GSi) was reduced by about 35% due to the thermal decomposition of graphene at 500 ° C. From this, the weight ratio of graphene present in the microparticles was 35% by weight, and the weight ratio of silicon was 65 It was confirmed that the weight %. In the case of GSiGA, it was confirmed that the thermal decomposition of reduced graphene oxide near 600 ° C. was due to the thermal stability of the composite, and the weight ratio of silicon was 42 wt% and 44 wt% of GSiGA and SiGA, respectively. .

9a to 9c, X-ray photoelectron spectroscopy (XPS) was used as a surface analysis to examine the constituent elements and chemical bonding states of the surface and the interface. As a result of separating the Si peaks of SiGA, GSiGA, and GSi using X-ray photoelectron spectroscopy, two peaks were observed at binding energies of 99.5 eV and 104 eV, corresponding to ^{metallic Si (Si 0} ) and silicon oxide (Si ^{4+ ).} .

Example 3: graphene airgel using a complex lithium secondary battery cathode

After cutting the graphene-silicon/graphene airgel composite GSiGA obtained in Example 2 and SiGA and GA obtained in Comparative Examples 1 and 2, the prepared negative electrode and positive electrode lithium hexafluorophosphate (Lithium Hexafluorophosphate, LiPF ₆ ) and 1 Lithium manufactured by manufacturing a coin cell composed of a :1 vol% ethylene carbonate (Ethylene Carbonate, EC)/dimethyl carbonate (DMC) liquid electrolyte and conducting a charge/discharge test for the coin cell using a charge/discharger. The charge/discharge capacity and cycle characteristics were investigated as the negative electrode of the secondary battery.

For the lithium secondary batteries prepared in Example 2 and Comparative Examples 1 and 2, a charging/discharging experiment was performed using a WBCS3000L charging/discharging device manufactured by Won-A Tech. Charging and discharging were performed in a voltage range of 0.01 to 2.0 V with a current of 200 mAh/g.

The charging and discharging results of the lithium secondary batteries according to Example 2 and Comparative Examples 1 and 2 are shown in FIG. 10A . As shown in FIG. 10a, the GSiGA of Example 2 showed an initial negative electrode capacity of 1217 mAh/g, whereas the SiGA and GA of Comparative Examples 1 and 2 showed an initial negative electrode capacity of 806 mAh/g and 274 mAh/g. It was. In addition, the coulombic efficiency was 71% for GA, 89% for SiGA, and 92% for GSiGA after charging and discharging twice. Therefore, it can be seen that the initial capacity of GSiGA and the coulombic efficiency after two charging and discharging are high.

FIG. 10b shows a cyclic voltamogram graph of GSiGA. During the first discharge, near 0.64 V, before lithium ions are inserted into the graphite, the electrolyte is decomposed to form a film SEI (Solid Electrolyte Interface or Solid Electrolyte Interphase) on the electrode surface, and through this SEI, electrons between the electrode and the electrolyte Electrolyte decomposition due to migration is suppressed, and only insertion/desorption of lithium ions is selectively possible. From the first cycle due to the reaction of silicon with lithium by the electrochemical reaction determines the secret Si is a Li _x Si is generated which is purified, particularly low voltage of 0.1 V in Li ₂ Si and Li ₄ in the vicinity of 0.2 _V. It can be seen that the reaction peak of _{4 Si appears.} From the second cycle, three reduction peaks of 0.3, 0.24, and 0.08 V during discharge and two oxidation peaks at 0.29 and 0.45 V during charge reaction were observed, which led to lithiation/delithiation of amorphous Si. indicated.

10c shows the cycle characteristics of the electrodes of Example 2 and Comparative Examples 1 and 2. As can be seen from the results, the SiGA electrode of Comparative Example 1 shows a high initial negative electrode capacity, but after 50 charge/discharge cycles, it shows a similar capacity to Comparative Example 2 and shows unstable cycle characteristics. On the other hand, the GSiGA electrode maintains a capacity about 3 times higher than that of graphite with a theoretical capacity of 372 mAh/g, and shows a stable capacity decrease as the cycle increases.

Figure 10d is a graph showing the rate capability (Rate Capabilities) of the battery for a lithium secondary battery containing GSiGA as an anode active material according to this embodiment, the current density at the time of charging in the range of 0.01 V ~ 2 V 0.2 C (200 mAhg ^-1 ), and the current density during discharge is in the range of 0.1, 0.2, 0.5, 1, 2, 5, 10, 0.2 C (a, b, c, d, e, f, g, h) It is a graph measuring the storage capacity of the secondary battery as it changes.

According to FIG. 10D , when the negative active material of Example 2 is applied, it can be confirmed that the output characteristics are excellent. Again, it can be inferred that graphene not only protects the silicon surface from exposure to electrolyte, but also provides excellent electrical conductivity.

Since the SiGA composite is agglomerated due to the attraction between silicon and particles, it is exposed to the surface and pores of the graphene airgel and used as a negative electrode for a lithium secondary battery. did. This has a problem in that it is difficult to evenly disperse in the state of silicon nanoparticles because the miscibility between graphene and silicon nanoparticles is low and metal aggregation occurs.

On the other hand, the reason why GSiGA, a sample prepared by adding GSi particles, shows a stable capacity reduction, is because graphene surrounds silicon and suppresses agglomeration of silicon particles to become larger particles, so that they are evenly dispersed in the graphene matrix. This is because it not only buffers the large volume change of silicon nanoparticles due to the charging and discharging of lithium ion batteries, but also reduces the specific resistance of the electrode surface of the lithium ion battery to induce an effective electrochemical reaction during charging and discharging of the battery. I think. Therefore, in the GSiGA electrode, graphene plays a role in buffering the aggregation of silicon particles, so that silicon is dispersed in the graphene airgel matrix, increasing electrochemically active sites, and graphene airgel maintains electrical conductivity. It is considered that the electrochemical properties are very good.

11A to 11D are scanning electron microscope results of graphene-silicon airgel composites (GSiGA) before and after 50 charge/discharge cycles. As can be seen from the surface high magnification (Fig. 11a) and low magnification (Fig. 11b) images of GSiGA obtained before charging and discharging, and high magnification (Fig. 11c) and low magnification (Fig. 11d) images of GSiGA after 50 cycles of charging and discharging, 3 It was confirmed that the dimensional spherical microparticle shape was well maintained in the graphene airgel matrix.

12A and 12B are views schematically illustrating changes in the shape of the anode composite active material according to the lithium insertion and desorption processes.

Specifically, FIG. 12a is a schematic diagram of a SiGA electrode of Comparative Example 1 including silicon nanoparticles, and FIG. 12b is a schematic diagram of a graphene airgel composite (Example 2 GSiGA) electrode including reduced graphene oxide/silicon composite. Example GSiGA prepared by the above method suppressed exposure of the porous graphene airgel surface and pores by enclosing silicon with a double-layer graphene coating technology, and improved dispersibility of silicon nanoparticles in graphene matrix and insertion of lithium ions. The performance of the lithium secondary battery was improved by minimizing the subsequent volume expansion.

As described above in detail a specific part of the content of the present invention, for those of ordinary skill in the art, it is clear that this specific description is only a preferred embodiment, and the scope of the present invention is not limited thereby. something to do. Accordingly, it is intended that the substantial scope of the present invention be defined by the appended claims and their equivalents.

Claims (21)

A graphene-silicon airgel composite comprising:
reduced graphene oxide that is sterically connected and has a porous structure; and
Including; spherical composite dispersed in the reduced graphene oxide;
The spherical composite is a reduced graphene surrounding silicon nanoparticles,
Based on the total weight of the graphene-silicon airgel composite, the graphene-silicon airgel composite, characterized in that it comprises 30% to 57% by weight of the silicon nanoparticles. According to claim 1,
The reduced graphene oxide in the spherical composite is in the form of a net, characterized in that they are three-dimensionally connected to each other, the graphene-silicon airgel composite. 3. The method of claim 2,
The silicon nanoparticles in the spherical composite are characterized in that they are separated from each other, graphene-silicon-airgel composite. According to claim 1,
The size of the silicon nanoparticles is characterized in that 10 to 200 nm, graphene-silicon-airgel composite. According to claim 1,
The spherical composite has a diameter of 1 to 10 μm, characterized in that the graphene-silicon airgel composite. According to claim 1,
The weight ratio of silicon nanoparticles and reduced graphene in the spherical composite is 1: 1 to 5, characterized in that the graphene-silicon airgel composite. delete The graphene of any one of claims 1 to 6 as a method for producing a silicon airgel composite,
Spray drying a first solution containing a first graphene oxide and silicon nanoparticles;
heat-treating and reducing the spray-dried material to obtain a spherical composite;
reducing and gelling a second solution containing the spherical composite and second graphene oxide; and
By freeze-drying the reduced and gelled second solution, obtaining a porous airgel composite comprising graphene/silicon having a three-dimensional structure; Containing, graphene-silicon airgel composite manufacturing method. 9. The method of claim 8,
The first solution, the graphene-silicon airgel composite manufacturing method, characterized in that prepared by dispersing the silicon nanoparticles in a solution containing the first graphene oxide. 9. The method of claim 8,
The size of the silicon nanoparticles, characterized in that 50 to 200 nm, graphene-silicon-airgel composite manufacturing method. 9. The method of claim 8,
The spherical composite is in the form of silicon nanoparticles surrounded by reduced graphene,
The reduced graphene is in the form of a net, characterized in that they are three-dimensionally connected to each other, graphene-silicon airgel composite manufacturing method. 12. The method of claim 11,
The silicon nanoparticles are characterized in that they are separated from each other, graphene-silicon airgel composite manufacturing method. 9. The method of claim 8,
The spherical composite has a diameter of 1 to 10 μm, characterized in that the graphene-silicon airgel composite manufacturing method. 9. The method of claim 8,
The weight ratio of silicon nanoparticles and reduced graphene in the spherical composite is 1:1 to 5, graphene-silicon airgel composite manufacturing method. 9. The method of claim 8,
The second solution, the graphene-silicon airgel composite manufacturing method, characterized in that prepared by dispersing the spherical composite and the second graphene oxide using ultrasonic waves in distilled water. 9. The method of claim 8,
The weight ratio of the spherical composite and the second graphene oxide is 1:0.5 to 2, the graphene-silicon-airgel composite manufacturing method. 9. The method of claim 8,
The graphene-silicon airgel composite of the three-dimensional structure is characterized in that it is synthesized by hydrothermal self-assembly, the graphene-silicon-airgel composite manufacturing method. 9. The method of claim 8,
The three-dimensional graphene-silicon airgel composite is sterically connected to each other, characterized in that the spherical composite is dispersed in reduced graphene oxide having a porous structure, the graphene-silicon-airgel composite manufacturing method. A negative active material comprising the graphene-silicon airgel composite according to any one of claims 1 to 6. 20. The method of claim 19,
The anode active material further comprises one or more metals selected from the group consisting of Sn, Al, Ge, Pb, Zn, Co, Cu, Ti, Ni, Li, Ag and Au, or alloys and oxides thereof. , negative active material. A negative electrode comprising the negative active material according to claim 19;
anode; and
Lithium secondary battery comprising; electrolyte.

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