KR20150117172A - Negative electrode active material, methode for synthesis the same, and lithium rechargable battery including the same - Google Patents

Abstract

The present invention relates to a negative electrode active material, a method for preparing the same and a lithium secondary battery comprising the same wherein the negative electrode active material contains reduced graphene oxide aerogel having a structure constituting a porous network in which reduced graphene oxide sheets are interconnected in a three-dimensional way, and a silicon coating layer positioned on a surface of the reduced graphene oxide sheets.

Description

TECHNICAL FIELD [0001] The present invention relates to a negative electrode active material, a method for producing the same, and a lithium secondary battery including the negative active material,

A negative electrode active material, a production method thereof, and a lithium secondary battery comprising the same.

Lithium secondary batteries have attracted attention as power sources for driving electronic devices. Graphite is mainly used as a negative electrode material of a lithium secondary battery, but it is difficult to increase the capacity of a lithium secondary battery because the capacity per unit mass of graphite is as small as 372 mAh / g.

Examples of an anode material exhibiting a higher capacity than graphite include materials forming lithium and intermetallic compounds such as silicon, tin and their oxides. Particularly, materials such as silicon have an advantage of realizing a high capacity and enabling miniaturization of the battery.

However, these materials cause a change in the crystal structure when lithium is absorbed and stored, which causes a problem that the volume expands. In the case of silicon, the volumetric expansion by filling occurs. At room temperature, the volumetric growth rate expands to about 3.7 times that of the volume of silicon before volume expansion. As a result, there is a problem that the life of the battery is rapidly deteriorated. In addition, materials such as silicon have lower electric conductivity than graphite, and thus are difficult to be used alone, and there is a problem in that the resistance between the material and the coating film is large.

Active research is being conducted on a solution to the problems of the negative electrode active material such as silicon.

Provided is a negative electrode active material capable of alleviating the volume expansion of a negative electrode active material, having excellent electrical conductivity and being physically and chemically stable, a method for producing the same, and a lithium secondary battery having high reversibility capacity, high rate characteristics and excellent lifetime characteristics.

In one embodiment of the present invention, there is provided a reduced oxidized graphene aerogel having a structure in which reduced oxidized graphene sheets are three-dimensionally connected to each other to form a porous network, A negative electrode active material comprising a silicon coating layer is provided.

The silicon coating layer may be such that silicon particles are present in the form of iris on the surface of the reduced oxidized graphene sheets.

The silicon particles may be in the form of a hemisphere.

The average particle diameter of the silicon particles may be 5 nm to 20 nm.

The silicon particles may be uniformly distributed on the surfaces of the reduced oxidized graphene sheets.

In the silicon coating layer, the silicon may be amorphous silicon.

The reduced oxidized graphene aerogels may be included in an amount of 16.6 to 20.8% by weight based on the total amount of the negative electrode active material, and the silicon coating layer may be included in an amount of 79.2 to 83.4% by weight.

According to another embodiment of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: preparing an aqueous solution in which graphene oxide grains are dispersed; Freezing the aqueous solution in which the oxidized graphene sheets are dispersed; Freeze-drying the frozen material to obtain an oxidized graphene aerogel having a structure in which the oxidized graphene sheets are three-dimensionally connected to each other to form a porous network; Reducing the oxidized graphene aerogels; And coating silicon on the surface of the reduced oxidized graphene aerogel.

The step of preparing an aqueous solution in which the graphene oxide particles are dispersed may include oxidizing the graphite and then ultrasonically grinding to obtain oxidized graphene sheets, and dispersing the oxidized graphene sheets in water.

The step of freezing the aqueous solution in which the oxidized graphene sheets are dispersed may be rapid freezing using liquid nitrogen.

The step of lyophilizing the frozen material to obtain an oxidized graphene aerogel may be to sublimate the solvent under vacuum conditions.

The step of reducing the oxidized graphene aerogels may include increasing the temperature of the oxidized graphene aerogels in an inert gas atmosphere and reacting the oxidized graphene aerogels in a hydrogen atmosphere when the temperature reaches 1000 ° C.

The step of coating silicon on the surface of the reduced oxidized graphene aerogels may be by using chemical vapor deposition (CVD).

The step of coating the surface of the reduced oxidized graphene aerogel with silicon may include the chemical vapor deposition of silicon on the surface of the reduced oxidized graphene aerogel using a silane gas.

The step of coating the surface of the reduced oxidized graphene aerogel with silicon may be performed at a temperature of 450 ° C or more and less than 550 ° C.

In the step of coating silicon on the surface of the reduced oxidized graphene aerogels, the silicon may be amorphous silicon.

In the step of coating the surface of the reduced oxidized graphene aerogel with silicon, the weight ratio of the reduced oxidized graphene and the silicon may be 16.6: 83.4 to 20.8: 79.2.

In another embodiment of the present invention, there is provided a lithium secondary battery including the negative electrode, the positive electrode, and the electrolyte including the negative active material.

The negative electrode active material according to one embodiment alleviates the problem of volume expansion, is excellent in electric conductivity, and is physically and chemically stable. The lithium secondary battery according to another embodiment has a high reversible capacity and is excellent in high-rate characteristics and life characteristics.

FIG. 1 is a schematic view illustrating a process of manufacturing an anode active material according to one embodiment.
FIG. 2 is a diagram schematically showing the shape change of the negative electrode active material in the embodiment according to the lithium insertion and desorption process. FIG.
3 is a transmission electron micrograph of the negative electrode active material of the embodiment.
Figure 4 is a scanning electron micrograph of reduced oxidized graphene in an embodiment.
5 is a transmission electron micrograph of the reduced oxidized graphene of the example.
FIG. 6 is a transmission electron micrograph of a negative electrode active material of the example, a state in which lithium ions are implanted, and a state in which lithium ions are removed.
FIG. 7 is a transmission electron micrograph showing a state in which crystalline silicon and lithium ions are implanted, and a state in which lithium ions are removed. FIG.
8 is a scanning electron microscope (SEM) image of the electrode plate to which the negative electrode active material of the embodiment is applied, and the thickness of the electrode plate after the first cycle and after 100 cycles. FIG. 5 is a scanning electron microscope (SEM) image of the surface of the electrode plate and the electrode plate after the first cycle.
9 is a scanning electron microscope photograph of the electrode plate on which the crystalline silicon is applied and the electrode plate thickness after the first cycle and after 100 cycles. Also, it is a scanning electron microscope (SEM) image of the surface of the crystalline plate and the plate after the first cycle.
10 is an X-ray spectroscopic pattern for crystalline silicon particles (SiNPs), the negative electrode active material ( a- SBG) prepared in the above example, and reduced graphene oxide (rGO).
11 is a Raman spectrum of the negative active material (a-SBG), the oxidized graphene (GO) and the reduced oxidized graphene (rGO) prepared in the examples.
12 is an X-ray photoelectron spectroscopy graph for oxidized graphene (GO) and reduced oxidized graphene (rGO).
FIG. 13 is a graph showing electrochemical characteristics of a half cell of Example ( a- SBG) and a half cell of Comparative Example (SiNPs) having the same cell capacity.
14 is a graph showing electrochemical characteristics of full cells prepared in Examples.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited thereto, and the present invention is only defined by the scope of the following claims.

In one embodiment of the present invention, a reduced graphene oxide (rGO) aerogel having a structure in which the reduced oxidized graphene sheets are connected to each other in a three-dimensional manner to form a porous network, And a silicon coating layer positioned on the surface of the oxidized graphene sheets.

The reduced oxidized graphene aerogels have a structure in which the reduced oxidized graphene sheets have a structure in which they are interconnected while retaining a large number of voids, and when the electrode plates are formed, they also have an interconnected structure while retaining a void space .

Here, aerogels are terms that mean solid, having lost its original form.

The reduced graphene graphene aerogels serve as a matrix for supporting the silicon so as not to fall off, and can sufficiently transfer electrons and control the volume expansion during charging and discharging.

In addition, the reduced oxidized graphene sheets have a large amount of defects on the surface, and the defects can act as a channel for lithium ions. Normally, lithium ions can enter and exit only at the corners of the carbon layer. However, in the case of the reduced graphene sheet, the lithium ions can move not only to the corners but also to the surfaces, so that the fluidity of the lithium ions can be improved.

The silicon coating layer may be a silicon deposition layer, for example.

Specifically, the silicon coating layer may be such that silicon particles are present in an island form on the surface of the reduced oxidized graphene sheets. Accordingly, the anode active material can alleviate the problem of expansion of silicon due to charging and discharging, and can utilize physical and electrical characteristics of the reduced oxidized graphene as it is.

The silicon particles may be, for example, in the form of a hemisphere.

The average particle diameter of the silicon particles may be 5 nm to 20 nm, specifically 10 nm to 20 nm, and 5 nm to 15 nm.

The silicon particles may be uniformly distributed on the surfaces of the reduced oxidized graphene sheets.

In the silicon coating layer, the silicon may be amorphous silicon. In this case, the silicon coating layer is less likely to break crystals due to volume expansion and can maintain a stable structure. Unlike crystalline silicon, amorphous silicon has a uniform volume expansion due to the lack of directionality of lithium absorption and high lithium migration rate. In addition, amorphous silicon has less stress or strain in lithium absorption and desorption than crystalline silicon, so that it can stably maintain its structure.

The conventional silicon powder has a problem that the dispersion is not uniformly uniform, but the anode active material according to one embodiment is very uniformly distributed on the surface of the oxidized graphene sheets in which the silicon particles are reduced. Also, the silicon particles are present in the island form on the surfaces of the reduced graphene graphene sheets, so that the physical and electrical characteristics of the reduced graphene graphene can be maximized without being disturbed by the silicon.

The reduced oxidized graphene aerogels may be included in an amount of 16.6 to 20.8% by weight based on the total amount of the negative electrode active material, and the silicon coating layer may be included in an amount of 79.2 to 83.4% by weight.

The anode active material according to one embodiment may contain the silicon coating layer at a level of 82 wt% even though the silicon coating layer is not thickly coated on the reduced oxide graphene. This is because the reduced graphene graphene has a very high specific surface area and is ultralight so that a large amount of silicon can be coated on the surface thereof.

According to another embodiment of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: preparing an aqueous solution in which graphene oxide grains are dispersed; Freezing the aqueous solution in which the oxidized graphene sheets are dispersed; Freeze-drying the frozen material to obtain an oxidized graphene aerogel having a structure in which the oxidized graphene sheets are three-dimensionally connected to each other to form a porous network; Reducing the oxidized graphene aerogels; And coating silicon on the surface of the reduced oxidized graphene aerogel.

The above-described negative electrode active material can be produced through the above-described production method.

The step of preparing the aqueous solution in which the oxidized graphene sheets are dispersed may include, for example, oxidizing the graphite and then ultrasonically grinding to obtain oxidized graphene sheets, and dispersing the oxidized graphene sheets in water .

The step of freezing the aqueous solution in which the oxidized graphene sheets are dispersed may be rapid freezing using liquid nitrogen.

The step of lyophilizing the frozen material to obtain an oxidized graphene aerogel may be to sublimate the solvent under vacuum conditions.

The step of reducing the oxidized graphene airgel may include, for example, increasing the temperature of the oxidized graphene airgel in an inert gas atmosphere, and then reacting in a hydrogen atmosphere for about 1 hour when the temperature reaches about 1000 ° C .

Thus many -OH, -COO that by reducing the lyophilized graphene oxide airgel is present on the surface of the graphene oxide ^- by removing, functional group such as -COOH may significantly increase the electrical conductivity.

The step of coating silicon on the surface of the reduced oxidized graphene aerogels may be by using chemical vapor deposition (CVD). Specifically, the step of coating the surface of the reduced oxidized graphene aerogel with silicon may include the chemical vapor deposition of silicon on the surface of the reduced oxidized graphene aerogel using a silane gas.

Since the reduced oxidized graphene aerogels have a large amount of voids, silane gas can penetrate well on the surfaces of the reduced oxidized graphene sheets and can be uniformly coated.

Conventional silicon powders have a problem of low dispersion, but very uniformly distributed silicon particles can be obtained when silicon is deposited on the surface of the reduced oxidized graphene aerogels by chemical vapor deposition. Also, by using the chemical vapor deposition method, the reduced graphene graphene error can coat the surface of the gel with the silicon particles in the island shape.

At this time, the step of coating the silicon may be performed at a temperature higher than 450 ° C and lower than 550 ° C. In this case, a high-purity amorphous silicon can be deposited in a large amount.

Crystalline silicon is formed when a high temperature of 700 ° C or higher is used, and crystalline silicon is less susceptible to stress or strain than amorphous silicon, and lithium absorption or desorption rate is relatively low.

In the step of coating the surface of the reduced oxidized graphene aerogel with silicon, the weight ratio of the reduced oxidized graphene and the silicon may be 16.6: 83.4 to 20.8: 79.2. As described above, according to one embodiment, the silicon may not be coated thickly on the surface of the reduced oxidized graphene but may be coated with a high content of 82 wt%.

In another embodiment of the present invention, there is provided a lithium secondary battery including the negative electrode, the positive electrode, and the electrolyte including the negative active material.

The lithium secondary battery can be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery depending on the type of the separator and electrolyte used. The lithium secondary battery can be classified into a cylindrical shape, a square shape, a coin shape, Depending on the size, it can be divided into bulk type and thin type. The structure and the manufacturing method of these cells are well known in the art, and detailed description thereof will be omitted.

The negative electrode includes a current collector and a negative active material layer formed on the current collector, and the negative active material layer includes a negative active material.

The description of the anode active material is omitted since it is as described above.

The negative electrode active material layer also includes a binder, and may further include a conductive material.

The binder serves to adhere the anode active material particles to each other and to adhere the anode active material to the current collector. As the binder, a water-insoluble binder, a water-soluble binder, or a combination thereof may be used.

Examples of the water-insoluble binder include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride , Polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

Examples of the water-soluble binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, propylene and olefin copolymers having 2 to 8 carbon atoms, (meth) acrylic acid and (meth) Copolymers or combinations thereof.

When a water-soluble binder is used as the negative electrode binder, it may further include a cellulose-based compound capable of imparting viscosity. As the cellulose-based compound, carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, alkali metal salts thereof or the like may be used in combination. As the alkali metal, Na, K or Li can be used. The content of the thickener may be 0.1 to 3 parts by weight based on 100 parts by weight of the binder.

The conductive material is used for imparting conductivity to the electrode. Any conductive material can be used without causing any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, Carbon-based materials such as black and carbon fiber; Metal powders such as copper, nickel, aluminum, and silver, or metal-based materials such as metal fibers; Conductive polymers such as polyphenylene derivatives; Or a mixture thereof may be used.

The collector may be selected from the group consisting of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foil, a polymer substrate coated with a conductive metal, and a combination thereof.

The anode includes a current collector and a cathode active material layer formed on the current collector. As the cathode active material, a compound capable of reversibly intercalating and deintercalating lithium (a lithiated intercalation compound) can be used. Concretely, at least one of complex oxides of metal and lithium selected from cobalt, manganese, nickel, and combinations thereof may be used. As a more specific example, a compound represented by any one of the following formulas may be used.

Li _a A _{1 - b} X _b D ₂ (0.90? A? 1.8, 0? B? 0.5); Li _a A _{1 - b} X _b O _{2 - c} D _c (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05); LiE _{1 - b} X _b O _{2 - c} D _c (0? B? 0.5, 0? C? 0.05); LiE _{2 - b} X _b O _{4 - c} D _c (0? B? 0.5, 0? C? 0.05); Li _a Ni _{1 -b- c} Co _b X _c D _? (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <?? 2); Li _a Ni _{1 -b- c} Co _b X _c O _{2 -α} T _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2); Li _a Ni _{1 -b- c} Co _b X _c O _{2 - ?} T ₂ (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <? <2); Li _a Ni _{1 -b- c} Mn _b X _c D _? (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0 <?? 2); Li _a Ni _{1 -b- c} Mn _b X _c O _{2- α} T _α (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2); Li _a Ni _{1 -bc} Mn _b X _c O _2-α T ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, 0 <α <2); Li _a Ni _b E _c G _d O ₂ (0.90? A? 1.8, 0? B? 0.9, 0? C? 0.5, 0.001? D? 0.1); Li _a Ni _b Co _c Mn _d G _e O ₂ (0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1); Li _a NiG _b O ₂ (0.90? A? 1.8, 0.001? B? 0.1); Li _a CoG _b O ₂ (0.90? A? 1.8, 0.001? B? 0.1); Li _a MnG _b O ₂ (0.90? A? 1.8, 0.001? B? 0.1); Li _a Mn ₂ G _b O ₄ (0.90? A? 1.8, 0.001? B? 0.1); Li _a MnG _b PO ₄ (0.90? A? 1.8, 0.001? B? 0.1); QO _2; QS ₂ ; LiQS ₂ ; V ₂ O ₅ ; LiV ₂ O ₅ ; LiZO ₂ ; LiNiVO _4; Li _(3-f) J ₂ (PO ₄ ) ₃ (0? F? 2); Li _(3-f) Fe ₂ (PO ₄ ) ₃ (0? F? 2); LiFePO ₄

In the above formula, A is selected from the group consisting of Ni, Co, Mn, and combinations thereof; X is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from the group consisting of O, F, S, P, and combinations thereof; E is selected from the group consisting of Co, Mn, and combinations thereof; T is selected from the group consisting of F, S, P, and combinations thereof; G is selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and combinations thereof; Q is selected from the group consisting of Ti, Mo, Mn, and combinations thereof; Z is selected from the group consisting of Cr, V, Fe, Sc, Y, and combinations thereof; J is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

Of course, a compound having a coating layer on the surface of the compound may be used, or a compound having a coating layer may be mixed with the compound. The coating layer may comprise at least one coating element compound selected from the group consisting of oxides, hydroxides of coating elements, oxyhydroxides of coating elements, oxycarbonates of coating elements, and hydroxycarbonates of coating elements. The compound constituting these coating layers may be amorphous or crystalline. The coating layer may contain Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof. The coating layer forming step may be any coating method as long as it can coat the above compound by a method which does not adversely affect the physical properties of the cathode active material (for example, spray coating, dipping, etc.) by using these elements, It will be understood by those skilled in the art that a detailed description will be omitted.

The cathode active material layer also includes a binder and a conductive material.

The binder serves to adhere the positive electrode active material particles to each other and to adhere the positive electrode active material to the current collector. Typical examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl Polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-acrylonitrile, styrene-butadiene rubber, Butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like, but not limited thereto.

The conductive material is used for imparting conductivity to the electrode. Any conductive material can be used without causing any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, Metal powders such as black, carbon fiber, copper, nickel, aluminum, and silver, metal fibers, and the like, and conductive materials such as polyphenylene derivatives may be used alone or in combination.

As the current collector, Al may be used, but the present invention is not limited thereto.

The negative electrode and the positive electrode are prepared by mixing an active material, a conductive material and a binder in a solvent to prepare an active material composition and applying the composition to an electric current collector. The method of manufacturing the electrode is well known in the art, and therefore, a detailed description thereof will be omitted herein. As the solvent, N-methylpyrrolidone or the like can be used, but it is not limited thereto.

In the non-aqueous liquid electrolyte secondary battery according to an embodiment of the present invention, the electrolyte includes a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

Depending on the type of the lithium secondary battery, a separator may exist between the positive electrode and the negative electrode. The separator may be a polyethylene / polypropylene double layer separator, a polyethylene / polypropylene / polyethylene triple layer separator, a polypropylene / polyethylene / poly It is needless to say that a mixed multilayer film such as a propylene three-layer separator and the like can be used.

Hereinafter, examples and comparative examples of the present invention will be described. The following embodiments are only examples of the present invention, and the present invention is not limited to the following embodiments.

Example

(Preparation of negative electrode active material)

The graphite flakes are sufficiently oxidized through the Hummers method, washed several times with deionized water, and sonicated to remove graphene oxide (GO) by sonication.

The graphene oxide thus obtained is diluted with deionized water to a concentration of about 1 mg / ml to prepare a GO aqueous suspension. At this time, the graphene grains are negatively charged on the surface and therefore dispersed fairly well in polar water.

Thereafter, the oxidized graphene water dispersion is rapidly frozen using liquid nitrogen.

The frozen oxidized graphene water dispersion is then completely dried for several days using the freeze dry method. The lyophilized graphene grains thus have a considerably porous structure.

Thereafter, the freeze-dried graphene oxide is placed in a heat treatment furnace (Tubefurnace) of an inert atmosphere (Ar, N ₂ ) and the temperature is increased. When the temperature reaches about 1000 ° C., Reduced graphene graphene (rGO) can be obtained in which the shape remains unchanged.

Subsequently, silane gas is deposited (generated at about 420 캜) at a temperature of about 500 캜 by using a chemical vapor deposition (CVD) process on the obtained porous reduced oxidized graphene to obtain a large amount of high purity amorphous silicon Is evenly deposited on the surface of the reduced graphene oxide in island form.

Thus, an anode active material was prepared. The weight content composition of the negative active material for a lithium secondary battery was as follows: reduced graphene: silicon = 18 (%): 82 (%).

(Preparation of negative electrode)

The binder used for preparing the battery using the negative electrode active material is a mixture of polyacrylic acid (PAA) and sodium carboxymethyl cellulose (CMC). The weight ratio of the slurry to the active material is PAA : CMC was 8: 1: 1.

The slurry was uniformly coated on a copper foil having a thickness of 18 탆, dried in an oven at 80 캜 for about 30 minutes, and further dried in a vacuum oven at 150 캜 for 2 hours or more to prepare a negative electrode.

(Preparation of half cell)

LiPF ₆ was added to the prepared cathode and lithium foil as a counter electrode, and 10 wt% (%) fluoroethylene carbonate was contained in the solvent in which the porous polyethylene film was used as a separator, and ethylene carbonate and diethyl carbonate were mixed in a volume ratio of 3: A 2032R half cell was prepared according to a conventionally known manufacturing process using a liquid electrolyte dissolved in a concentration of 1.3M.

(Preparation of full cell)

A full cell was prepared in the same manner as in the above half cell except that the prepared negative electrode was used and LiCoO ₂ active material was used as the positive electrode.

Comparative Example

Except that crystalline silicon was used as the negative electrode active material and a negative electrode plate was produced together with the binder by adding the super-P conductive agent in an amount equal to the amount of reduced graphene graphene used in the negative active material of the Example. A half cell was fabricated in the same manner as in Example.

Test Example  1: An overview of the manufacturing process

FIG. 1 schematically shows a manufacturing process of an anode active material according to an embodiment to facilitate understanding.

Test Example  2: Analysis of shape of anode active material

FIG. 2 is a diagram schematically showing the shape change of the negative electrode active material in the embodiment according to the lithium insertion and desorption process. FIG.

As shown in the left-hand side of FIG. 2, the cathode plate made in the embodiment will have a structure in which the reduced graphene graphenes are interconnected with each other, and on the reduced graphene grains, .

When the lithium ion is inserted by driving the cell, the silicon expands in volume, and the reduced graphene grains including silicon are deformed by themselves. When the lithium ion is removed, the amorphous silicon returns to its original size with elastic behavior, and the reduced graphene grains including silicon are deformed to maintain its shape. This will be described in detail in Experimental Example 4 below.

3 is a transmission electron micrograph of the negative electrode active material of the embodiment. 3 (b), 3 (c) and 3 (d) are transmission electron microscopes from low magnification to high magnification.

FIG. 3E is a Fourier transform image showing that silicon coated with reduced oxidized graphene is amorphous.

It can be seen from FIG. 3 that the amorphous silicon of less than about 20 nm is uniformly distributed in the island shape on the surface of the reduced graphene oxide.

Test Example  3: Reduced Oxidized graphene  Shape analysis

The thermally reduced graphene grains produced while maintaining the porosity in the above example were sampled on a carbon tape and then photographed by a scanning electron microscope and the photograph is shown in FIG.

The reduced graphene oxide grains were dissolved in water and ethanol mixed solution to form an ink, and a drop was dropped on a carbon grid. The dried graphene grains were dried and then photographed by a transmission electron microscope (HRTEM). The photograph is shown in FIG.

4 and 5, the specific surface area is wide and very thin, so that the shape of the transparent reduced oxide graphene can be confirmed.

Test Example  4: On charge and discharge  The negative active material and Plate  Change analysis

Each of the anode active material (initial material), lithium ion-implanted state, and lithium ion-removed state was sampled on a carbon grid and photographed using a transmission electron microscope (HRTEM). The photograph is shown in FIG.

FIG. 6 is a photograph of a coating of oxidized graphene on which amorphous silicon of less than 20 nm is reduced. When silicon ions are injected by charging and discharging lithium ions after discharging lithium ions, there is no difference from the original size Of the time.

As data to be compared with this, crystalline silicon was sampled on a carbon grid in the same manner as above and photographed by a transmission electron microscope (TEM). The photograph is shown in FIG.

FIG. 7 shows that when the lithium ions are injected and removed in the above manner, the reaction is not elastic. As a result, it can be seen that the amorphous silicon is advantageous in that the size of the material is important for the elastic behavior and the residual stress is relatively small in the interior.

Also, changes in the thickness of the electrode plate after the first electrode plate and after the first cycle and after 100 cycles using the negative electrode active material of the above embodiment were observed through a scanning electron microscope (SEM), and the photograph is shown in FIG.

The left electrode plate of Fig. The electrode plate was made by using the active material of the embodiment without a conductive agent in a ratio of 8: 2. The electrode plate was once charged and discharged, and a scanning electron microscope photograph was taken. After 100 cycles, a scanning electron microscope photograph was taken at the same position. At this time, the thickness of the plate is considerably reduced.

One of the many characteristics of graphene is flexibility. Initially, grapins are interconnection, forming a very porous electrode plate, and as the cycle progresses, the grains expand on the surface of the silicon on the surface of the graphene, causing the graphene to deform between the porous plate spaces, So that a compact and stable electrode can be formed later. The right surface photograph shows that the porous surface is contraction after one cycle and there is no crack.

Then, a crystalline silicon electrode plate was fabricated to have the same cell capacity as the comparison data, and the thickness change of the electrode plate was observed through a scanning electron microscope photograph through the same experiment. A conductive agent called Super-P was added to the electrode plate in the same manner as the graphene weight ratio of the negative electrode active material of the Example.

The thickness of the initial plate is about 9 mu m, and it can be observed with a scanning electron microscope image that the change in thickness after one cycle and after 100 cycles is significant. And cracking of the surface occurred.

This shows that no matter how well the material is modified, it can not be commercialized if the plates are not intact during the cycle.

Test Example  5: Example  Analysis of x-ray spectroscopy for negative electrode active materials

X-ray spectroscopic patterns of the crystalline silicon particles (SiNPs), the negative active material (a-SBG) prepared in the above example, and the reduced graphene oxide (rGO) are shown in Fig. 10, amorphous silicon information deposited on crystalline silicon, reduced oxide graphene, and reduced oxide graphene can be confirmed.

The Raman spectra of the negative active material (a-SBG), the oxidized graphene (GO) and the reduced oxidized graphene (rGO) prepared in the above examples are shown in FIG. As a result, it was confirmed that the defect in the carbon body was reduced when the graphene oxide was formed from the reduced graphene oxide, and that silicon was deposited on the reduced oxide graphene in the anode active material of the example.

The results of x-ray photoelectron spectroscopy for oxidized graphene (GO) and reduced oxidized graphene (rGO) are shown in FIG. It can be seen from FIG. 12 that when the graphene oxide is reduced, the bonds between the carbon atoms and the oxygen atoms are greatly reduced, which means that the reduction of the oxidized graphene is very good.

Test Example  6: Electrochemical Characterization of Half Cells

The electrochemical characteristics of the half cell of the Example and the half cell of the Comparative Example were evaluated, and the results are shown in FIG.

FIG. 13A is a graph showing the capacity of a half cell of the embodiment, and it can be confirmed that the reversible capacity is about 2800 mAh / g or more.

FIG. 13B is a graph showing the charging characteristics for the cells of the half cell of the embodiment and the cell of the comparative example. It can be seen from FIG. 13 (b) that the characteristics of the battery according to the embodiment are superior to those of the comparative example.

FIG. 13C is a graph showing the cycle characteristics at the time of 5C (Current density: 14A.g ^-1 ) of the half cell of the embodiment, showing the cycle stability maintaining the average lithium capacity 1126mAh / g during 1000 cycles.

13 and 13 are graphs showing the data of FIG. 13B as a voltage profile. At this time, the discharge is fixed to 0.5 C (CCCV methode), and the cut-off voltage is 0.01 to 1.5 V. FIG.

FIG. 13F is a graph of the voltage profile of the cycle data of FIG. 13C, wherein the discharge is fixed at 0.5C (CCCV methode), and the cutoff voltage is 0.01 to 1.2V.

FIG. 13 (g) is a graph showing the internal resistance measured by electrochemical impedance analysis (EIS) when the half cell of the embodiment and the half cell of the comparative example were reacted with the lithium ion for the first time. As a result, it can be seen that the internal resistance of the embodiment is much smaller than that of the comparative example.

Test Example  7: Evaluation of electrochemical characteristics of full cell

14 is a graph showing electrochemical characteristics of full cells prepared in Examples.

14 (a) shows the formation capacity of the full-cell of Example, and the reversible capacity of the positive electrode is about 170 mAh / g or more.

FIG. 14B is a graph showing the capacity of the full-cell of Example according to the charging and discharging rate. The discharge and charge were fixed at 0.5 C (CCCV methode). 14 (b), it can be seen that the characteristics are excellent even in the case of the full cell.

FIG. 14C is a graph showing lifetime characteristics during 100 cycles at the time of 5 C, 10 C, 15 C, and 20 C rapid discharging of the full cell of the embodiment. The charge was fixed at 0.5C (CCCV methode) and the cutoff voltage was 2.5 ~ 4.3V.

FIG. 14D is a graph showing lifetime characteristics at 100 C cycles at 5 C and 7 C rapid charging of the full cell of the embodiment. At this time, the discharge is fixed to 0.5 C (CCCV methode), and the cutoff voltage is 2.5 to 4.3 V.

14E and 14F are voltage profiles for the 20C discharge 7C charge cycle in FIGS. 14C and 14D. As mentioned above, the charge and discharge are fixed at 0.5C (CCCV methode), and the cutoff voltage is 2.5 ~ 4.3V, respectively.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. As will be understood by those skilled in the art. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

Claims (18)

A reduced oxidized graphene aerogel having a structure in which the reduced oxidized graphene sheets are connected to each other in a three-dimensional manner to form a porous network, and
And a silicon coating layer positioned on the surface of the reduced oxidized graphene sheets. The method of claim 1,
Wherein the silicon coating layer is in the form of an island in the surface of the reduced oxidized graphene sheets. 3. The method of claim 2,
Wherein the silicon particles are in a hemisphere form. 3. The method of claim 2,
Wherein the average particle diameter of the silicon particles is 5 nm to 20 nm. 3. The method of claim 2,
Wherein the silicon particles are uniformly distributed on a surface of the reduced oxidized graphene sheets. The method of claim 1,
Wherein the silicon in the silicon coating layer is amorphous silicon. The method of claim 1,
With respect to the total amount of the negative electrode active material,
The reduced oxidized graphene aerogels include 16.6 to 20.8 wt%
Wherein the silicon coating layer contains 79.2 to 83.4 wt% of the negative electrode active material. Preparing an aqueous solution in which oxidized graphene sheets are dispersed;
Freezing the aqueous solution in which the oxidized graphene sheets are dispersed;
Freeze-drying the frozen material to obtain an oxidized graphene aerogel having a structure in which the oxidized graphene sheets are three-dimensionally connected to each other to form a porous network;
Reducing the oxidized graphene aerogels; And
And coating silicon on the surface of the reduced oxidized graphene airgel. 9. The method of claim 8,
The step of preparing an aqueous solution in which the graphene oxide particles are dispersed includes a step of oxidizing the graphite and ultrasonically pulverizing the same to obtain oxidized graphene sheets, and dispersing the oxidized graphene sheets in water to prepare an anode active material Way. 9. The method of claim 8,
Wherein the step of freezing the aqueous solution in which the oxidized graphene sheets are dispersed is rapidly frozen using liquid nitrogen. 9. The method of claim 8,
Wherein the step of lyophilizing the frozen material to obtain an oxidized graphene aerogel having a structure in which oxidized graphene sheets are three-dimensionally connected to each other to form a porous network, sublimates the solvent under vacuum conditions . 9. The method of claim 8,
Wherein the step of reducing the graphene graphene airgel comprises increasing the temperature of the graphene graphene airgel in an inert gas atmosphere and reacting the oxidized graphene airgel in a hydrogen atmosphere when the temperature reaches 1000 ° C. 9. The method of claim 8,
Wherein the step of coating silicon on the surface of the reduced oxidized graphene aerogels uses chemical vapor deposition (CVD). The method of claim 13,
The step of coating silicon on the surface of the reduced oxidized graphene airgel
And silicon is chemically vapor-deposited on the surface of the reduced graphene graphene aerogel using silane gas. The method of claim 14,
Wherein the step of coating silicon on the surface of the reduced oxidized graphene airgel is performed at a temperature of 450 ° C or more and less than 550 ° C. 9. The method of claim 8,
Wherein the silicon is amorphous silicon in the step of coating silicon on the surface of the reduced oxidized graphene airgel. 9. The method of claim 8,
In the step of coating silicon on the surface of the reduced oxidized graphene aerogels,
Wherein the weight ratio of the reduced graphene grapnel airgel and the silicon is 16.6: 83.4 to 20.8: 79.2. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 7,
Anode, and
A lithium secondary battery comprising an electrolyte.

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