KR101665099B1 - Anode materials for lithium rechargeable batteries including natural graphite and metal and a preparation method thereof - Google Patents

Abstract

The present invention relates to a negative electrode active material for a lithium ion secondary battery comprising natural graphite and a metal material and a method for producing the same. More specifically, a crystallized metal layer is formed on the outer surface and the inner pore surface of natural graphite, and the surface of the crystallized metal layer is coated with a carbon layer pyrolyzed by a chemical vapor deposition method, and has high capacity and high output characteristics. In addition, the crystallized metal layer is a nano-sized metal material, which has the advantage of alleviating the volumetric expansion of the metal material during charging and discharging of the lithium ion secondary battery and improving the lifetime characteristics.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode active material for a lithium ion secondary battery including a natural graphite and a metal material and an anode active material for a lithium ion secondary battery,

The present invention relates to a negative electrode active material for a lithium ion secondary battery comprising natural graphite and a metal material and a method for producing the same, which comprises forming a crystallized metal layer on a surface of natural graphite having an inner pore and thermally decomposing A negative electrode active material for a lithium ion secondary battery, and a method for manufacturing the negative electrode active material.

The rapid development of information communication and mobile communication technology has led to the birth of portable electronic devices with multifunctionality and high energy consumption. Lithium ion secondary batteries are the most widely used source of power for these portable electronic devices.

The lithium ion secondary battery includes lithium ions in an electrode assembly in which a microporous separation membrane is interposed between a positive electrode containing a positive electrode active material capable of intercalating and deintercalating lithium ions and a negative electrode containing a negative active material capable of intercalating and deintercalating lithium ions Containing nonaqueous electrolyte, charging and discharging are performed by using intercalation and deintercalation of lithium ions.

On the other hand, due to the development of memory, display, and thin technology, portable devices have become highly integrated and versatile in recent years. Such high integration and multifunctionality have become important factors for increasing energy consumption of portable devices. In order to satisfy such a condition, a battery system having a capacity higher than that of a currently used lithium ion secondary battery is required, so that research is being actively conducted.

[0005] Japanese Patent Registration No. 0738054 (published on July 24, 2007) discloses a negative electrode active material in which silicon microparticles and carbon fibers are coated on the surfaces of graphite core particles and graphite core particles, a negative electrode including the negative active material and a lithium ion battery .

Natural graphite used as a negative electrode active material of such a lithium ion secondary battery has a theoretical capacity density of 372 mAh / g. In order to operate high-energy portable equipment smoothly, a high-capacity active material is required.

The most excellent anode active material of a lithium ion secondary battery uses a lithium metal, and a lithium metal generates dendrite crystals upon charging and discharging, so that there is a problem in reactivity with high moisture and safety.

The active material other than graphite and lithium metal can be classified into a non-carbon-based metal, a non-carbon-based oxide, or a metal alloyed with a different kind of metal. Typical examples of non-carbon based and non-carbon based oxides include silicon (Si) and tin ).

Particularly, in the case of silicon, when _4.4 Li ions per silicon ion react with lithium ions to form Li _4.4 Si, the theoretical capacity density of 4200 mAh / g is obtained. Therefore, the theoretical capacity density of artificial graphite, 372 mAh / g. < / RTI >

However, due to repeated shrinkage and expansion of lithium silicide (Li _x Si) generated during charging and discharging, volume change occurs, and problems such as non-pulverization of the silicon active material powder and electrical contact failure between the silicon active material powder and the current collector occur In addition, as the charging and discharging cycle of the battery progresses, the capacity of the battery decreases sharply, thereby shortening the cycle life.

Therefore, it is necessary to develop a negative electrode active material having improved lifetime characteristics by minimizing the stress due to volume shrinkage and expansion of non-carbon materials generated during charging and discharging of the lithium ion secondary battery.

Patent Publication No. 0738054 (issued on July 24, 2007)

SUMMARY OF THE INVENTION The object of the present invention is to solve the problems of the conventional art by forming a crystallized metal layer on the surface of natural graphite having internal pores and coating a carbon layer pyrolyzed by chemical vapor deposition on the surface of the crystallized metal layer, A negative electrode active material for a lithium ion secondary battery comprising a natural graphite and a metal material having a high capacity and a high output characteristic when the negative electrode active material is applied to the secondary battery and having improved cycle life characteristics and less change in volume, Method.

According to an aspect of the present invention, there is provided a method of manufacturing an anode active material for a lithium ion secondary battery, the method comprising the steps of: depositing a hemisphere on a surface of a natural graphite and an inner pore surface of the natural graphite, Forming an amorphous metal layer having an island structure of the type; And forming a carbon layer on the amorphous metal layer by secondary chemical vapor deposition, wherein the primary chemical vapor deposition is performed at a temperature of from 450 캜 to less than 600 캜, and the secondary chemical vapor deposition is performed at 600 To 900 < 0 > C, and a carbon layer is formed in the secondary chemical vapor deposition process for forming the carbon layer, and at the same time, at least a part of the amorphous metal layer is crystallized.

The natural graphite has an average particle diameter of more than 0 mu m and not more than 50 mu m and a surface area of 1 to 20 m < 2 > / g. The metal layer is contained in an amount of 1 to 20 parts by weight based on 100 parts by weight of natural graphite, has a thickness of 10 to 50 nm, and is characterized by being silicon (Si), antimony (Sb), or a combination thereof. The carbon layer has a thickness of 1 to 100 nm and is contained in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of natural graphite.

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The forming of the carbon layer may include forming a carbon layer and simultaneously crystallizing at least a portion of the amorphous metal layer. The forming of the amorphous metal layer may include forming an amorphous metal layer using SiH ₄ , SiHCl ₃ , Si ₂ H _6, or SiH ₂ Cl ₂ Based gas is used for chemical vapor deposition.

Wherein the first chemical vapor deposition is performed at a temperature of from 450 캜 to less than 600 캜, the second chemical vapor deposition is performed at a temperature of from 700 캜 to 900 캜, the second chemical vapor deposition is performed using C ₂ H ₂ , CH ₄ , C ₂ H ₄ , Toluene, or Xylene is used as a precursor, and citric acid, sucrose or glucose is used as a precursor material when a sol-gel method is used instead of the secondary chemical vapor deposition.

Another embodiment of the present invention is a lithium ion secondary battery comprising the above-described negative electrode active material or a negative electrode active material produced by the above method.

The negative electrode active material for a lithium ion secondary battery according to the present invention is characterized in that a crystallized metal layer is formed on a surface of a natural graphite and an inner pore and a carbon layer pyrolyzed by chemical vapor deposition is coated on the surface of the crystallized metal layer And has high capacity and high output characteristics when applied to a secondary battery. In addition, the crystallized metal layer is a nano-sized metal material, which has the effect of alleviating the volume expansion of the metal material and improving the lifetime characteristics during charging and discharging of the lithium ion secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing a cross section of a negative active material coated with crystalline silicon and a carbon layer on a natural graphite surface of the present invention. FIG.
2 is a cross-sectional image of a porous natural graphite surface having an internal pore of the present invention observed with a focused ion beam (FIB).
3 is a scanning electron microscope (SEM) photograph of a negative electrode active material coated with amorphous silicon on the natural graphite surface of the present invention.
4 is a high-magnification transmission electron microscope (SEM) photograph of the negative electrode active material coated with amorphous silicon on the natural graphite surface of the present invention.
5 is a transmission electron microscope (TEM) photograph of a negative electrode active material having crystalline silicon and a carbon layer formed on the natural graphite surface of the present invention.
6 is a graph showing the silicon crystal peaks of Examples and Comparative Examples of the present invention.
7 is a graph showing a discharge capacity of a lithium ion secondary battery produced from the negative active material of Example 2 of the present invention and Comparative Examples 1, 2 and 5. FIG.
8 is a graph showing the discharge capacities of lithium ion secondary batteries made from the negative electrode active materials of Examples 1 and 2 and Comparative Examples 3 to 5 of the present invention.
9 is a graph showing an output characteristic evaluation result of a lithium ion secondary battery produced from the negative active material of the example of the present invention and the comparative example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Prior to the description, terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary meanings and should be construed in accordance with the technical concept of the present invention.

Throughout this specification, when an element is referred to as "including" an element, it is understood that it may include other elements as well, without departing from the other elements unless specifically stated otherwise.

Each step may be performed differently than the order specified unless explicitly stated in the context of the specific order. That is, each of the steps may be performed in the same order as described, or may be performed substantially concurrently or in the reverse order.

In order to accomplish the above object, the present invention provides a method for producing a graphite substrate, comprising the steps of: forming a crystallized metal layer formed on a natural graphite surface and an inner pore surface of the natural graphite in an island structure in the form of a hemisphere, The negative electrode active material for a lithium ion secondary battery as shown in Fig. 1 having a structure in which a pyrolyzed carbon layer is coated is presented.

The natural graphite used in the present invention preferably has an average particle diameter of 50 m or less and a surface area of 1 to 20 m < 2 > / g. Natural graphite serves as a matrix for supporting the metal material so as not to fall off, and sufficiently transfers electrons and controls volume expansion during charging and discharging. When the average particle size of natural graphite exceeds 50 μm, the specific surface area It is difficult to obtain high capacity and high output characteristics because the area for coating metal material and amorphous carbon is reduced.

The metal layer is included in an amount of 1 to 20 parts by weight based on 100 parts by weight of natural graphite, and preferably has a thickness of 10 to 50 nm. When the metal layer is included in the above range, the performance is the most excellent. When the metal layer is contained in an amount exceeding 20 parts by weight, a high capacity can be exhibited, but the volume expansion becomes worse and the lifetime characteristic deteriorates. When the metal layer has the thickness in the above range, the volume expansion of the metal can be most effectively controlled during charging and discharging. Since the metal layer is bonded to the artificial graphite in the form of a very thin film, the layer separation does not occur and rapid charging and discharging are possible .

In addition, the metal layer is uniformly formed on the surface of natural graphite. Due to the hemispherical island structure, the volume expansion of the metal can be more effectively controlled, and the physical and electrical characteristics of artificial graphite can be utilized as it is.

Preferably, the metal layer is made of silicon (Si), antimony (Sb) or a combination thereof, but may include metals capable of absorbing and releasing lithium electrochemically. More preferably, it may be crystalline silicon. The crystalline silicon has a small risk of breaking crystals according to volume expansion and can maintain a stable structure.

The carbon layer may have a thickness of 1 to 100 nm and may be included in an amount of 0.5 to 10 parts by weight based on 100 parts by weight of natural graphite. If the amount of the carbon layer is less than 0.5 parts by weight, the metal layer reacts with the electrolyte to form a by-product, thereby deteriorating the lifetime characteristics. If the carbon layer is more than 10 parts by weight, There is a problem of causing resistance to lithium movement.

A method of manufacturing an anode active material for a lithium ion secondary battery according to an embodiment of the present invention includes the steps of forming an amorphous metal layer having an island structure in the form of hemisphere through a primary chemical vapor deposition on a natural graphite surface and an inner pore surface of the natural graphite, ; And forming a carbon layer on the metal layer by a sol-gel method or a secondary chemical vapor deposition. When an amorphous metal layer is formed on the surface of natural graphite by the chemical vapor deposition, a very uniformly distributed amorphous metal layer can be obtained as compared with the conventional method.

At this time, in the step of forming the carbon layer, at least a part of the amorphous metal layer is crystallized while forming a carbon layer through a sol-gel method or chemical vapor deposition. The forming of the amorphous metal layer may be performed by chemical vapor deposition using a silane-based gas such as SiH ₄ , SiHCl ₃ , Si ₂ H _6, or SiH ₂ Cl ₂ , An amorphous metal layer having a very uniform distribution can be formed.

The primary chemical vapor deposition is preferably performed at a temperature of 450 ° C or more and less than 600 ° C. When the deposition is performed in the temperature range, a large amount of high purity amorphous metal can be coated. When the primary chemical vapor deposition proceeds at a temperature higher than 600 ° C, the crystalline metal is directly deposited and becomes vulnerable to stress or strain rather than being crystallized after deposition of the amorphous metal.

Also, the secondary chemical vapor deposition is performed at a temperature of 600 to 900 ° C., and the time required from 1 minute to less than 1 hour is required. In the secondary chemical vapor deposition, C ₂ H ₂ , CH ₄ , C ₂ H ₄ , Toluene or Xylene, and in the sol-gel method citric acid, sucrose or glucose is used.

A lithium ion secondary battery according to an embodiment of the present invention is a lithium ion secondary battery including the above-described anode active material or an anode active material manufactured by the above method.

The lithium ion secondary battery can be classified into a lithium ion battery and a lithium ion polymer battery depending on the kind of a separator and an electrolyte to be used. The lithium ion secondary battery can be classified into a cylindrical shape, a square shape, a coin shape, a pouch shape, Type and thin film type.

The negative electrode active material includes a binder, and may further include a conductive material. The binder serves to adhere the anode active material particles to each other and 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, ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene Fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

Examples of the water-soluble binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, propylene and an olefin copolymer having 2 to 8 carbon atoms, a copolymer of methacrylic acid and methacrylic acid- . ≪ / RTI >

On the other hand, when a water-soluble binder is used as the binder of the negative electrode active material, it may further include a cellulose-based compound as a thickener capable of imparting viscosity. Cellulose, methyl cellulose, alkali metal salts of these, and the like may be used in combination. As the alkali metal, Na, K or Li can be used. The content of the thickener is preferably 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, Black, or carbon fiber, a metal powder such as copper, nickel, aluminum, or silver or a metal powder such as metal fiber, a conductive polymer such as polyphenylene derivative, or a mixture thereof may be used .

The current collector may be selected from the group consisting of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foil, 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, at least one of a composite oxide of a metal selected from cobalt, manganese, nickel, and a combination thereof, which is a compound capable of reversibly intercalating and deintercalating lithium, may be used. More specifically, a compound represented by any one of the following formulas can 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 _a (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0? A? 2); Li _a Ni _{1- b c} Co _b X _c O _2-a T _a (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0? A? 2); Li _a Ni _{1- b c} Co _b X _c O _2-a T ₂ (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0? A? 2); Li _a Ni _1-bc Mn _b X _c D _a (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0? A? 2); Li _a Ni _1-bc Mn _b X _c O _2-a T _a (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0? A? 2); Li _a Ni _1-bc Mn _b X _c O _2-a T ₂ (0.90? A? 1.8, 0? B? 0.5, 0? C? 0.05, 0? A? 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) J 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. Finally, J is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

At this time, 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. As the element contained in the coating layer, Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr or a mixture thereof may be used.

The coating layer forming step may be performed by spraying or dipping, which does not adversely affect the physical properties of the cathode active material, by using these elements in the above compound, but the present invention is not limited thereto.

The cathode active material layer may include a binder. The binder serves to adhere the positive electrode active material particles to each other well and to adhere the positive electrode active material to the current collector efficiently. The binder includes polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, polyvinyl chloride, carboxylated polyvinyl Polyvinylidene fluoride, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, Butadiene rubber, epoxy resin, nylon, and the like may be used, but the present invention is not limited thereto.

When the cathode active material layer includes a binder, it may further include a conductive material. The conductive material is used for imparting conductivity to the electrode and may be a metal powder such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, copper, nickel, And one or more conductive materials such as polyphenylene derivatives may be used in combination. However, any electronic conductive material can be used without causing any chemical change of the battery.

The current collector may be aluminum (Al), but 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.

Depending on the type of the lithium ion secondary battery, a separator may exist between the positive electrode and the negative electrode. As the separator, polyethylene, polypropylene, polyvinylidene fluoride or a multilayer film of two or more thereof may be used, or a mixed multilayer film obtained by mixing the above components may be used.

Hereinafter, the technical features of the present invention will be described in detail with reference to embodiments and drawings. 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 invention as defined by the appended claims and their equivalents. shall.

[ Experimental Example  One]

Seven kinds of anode active materials of Examples 1 and 2 and Comparative Examples 1 to 5 were prepared in the following manner in order to examine the effect of the composition of the anode active material for a lithium ion secondary battery.

First, in Comparative Example 1, natural graphite having an average diameter of 20 占 퐉 was put into a rotary kiln, and the inside air was removed in a vacuum state and nitrogen was introduced. Thereafter, the silicon layer was coated by a first chemical vapor deposition in which the rotary furnace was operated at 6 rpm and the SiH ₄ was maintained at 550 sccm (Standard cubic centimeter per minute) at 550 ° C for 1 hour. SEM images of the negative electrode active material of Comparative Example 1 thus prepared are shown in FIGS. 3 to 4.

Comparative Example 2 was prepared by dissolving 50 parts by weight of citric acid in 500 parts by weight of methanol based on 100 parts by weight of natural graphite coated with the silicon layer prepared in Example 1, mixing the graphite with the graphite deposited thereon, -gel method. Dried, put into a box furnace, and heat-treated at 650 ° C for 5 hours in a nitrogen atmosphere.

In Comparative Example 3, the natural graphite coated with the silicon layer prepared in Example 1 was put into a rotary furnace and operated at 4 rpm in a nitrogen atmosphere, and a second step of supplying C ₂ H ₂ gas at 1.5 L / min at 600 ° C. for 10 minutes The carbon was coated by chemical vapor deposition.

In Example 1, the temperature of Comparative Example 3 was maintained at 700 ° C to coat carbon and crystallize all or part of the amorphous silicon.

In Example 2, the temperature of Comparative Example 3 was maintained at 900 占 폚 to coat carbon, and all or part of the amorphous silicon was crystallized. The negative electrode active materials of Examples 1 and 2 thus prepared were observed with a transmission electron microscope (TEM).

5 is an image obtained by EDX (Energy Dispersive X-ray) analysis in an electron transmission microscope (TEM) after cutting a cross section of a material with FIB (Focused Ion Beam), and the element detected at the lower left is indicated. The yellow Ti is a coated layer for transmission electron microscopy analysis. The thin layer below it is a mixture of carbon and silicon, and below it is silicon coated with pores of natural graphite.

In Comparative Example 4, only the amorphous silicon was crystallized without being coated with carbon by heat treatment at 900 ° C without supplying C ₂ H ₂ gas in Comparative Example 3.

Comparative Example 5 is natural graphite having an average diameter of 20 占 퐉 without any treatment.

The laminated structure of the negative electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 5 thus prepared is shown in Table 1 below.

division Laminated structure of negative electrode active material Remarks Example 1 Natural graphite + crystalline silicon + carbon layer 700 ° C CVD Example 2 Natural graphite + crystalline silicon + carbon layer 900 ° C CVD Comparative Example 1 Natural graphite + amorphous silicon - Comparative Example 2 Natural graphite + amorphous silicon + carbon layer left-hand Comparative Example 3 Natural graphite + amorphous silicon + carbon layer 600 ° C CVD Comparative Example 4 Natural graphite + crystalline silicon - Comparative Example 5 Natural graphite -

As shown in Table 1, in Examples 1 and 2, after the amorphous silicon was formed in the pores of the natural graphite and on the surface in the first chemical vapor deposition step, the amorphous silicon was subjected to a high-temperature secondary chemical vapor deposition step, And corresponds to a negative active material for a lithium ion secondary battery of the present invention in which silicon is crystallized.

On the other hand, Comparative Examples 1 to 5 are for comparison with Examples 1 and 2, in which the second chemical vapor deposition process is not performed (Comparative Example 1, Comparative Example 4), the low temperature heat treatment (Comparative Example 2) Or a low temperature secondary chemical vapor deposition process (Comparative Example 3), whereby the amorphous silicon formed in the first chemical vapor deposition step is not crystallized, or the natural graphite itself (Comparative Example 5).

[ Experimental Example  2]

The lithium ions of Examples 1 and 2 and Comparative Examples 1 to 5 prepared according to Experimental Example 1 The specific surface area of the negative electrode active material for the secondary battery was measured using nitrogen gas as an adsorbate under a liquid nitrogen atmosphere of 77 K. After the nitrogen adsorption isothermal test, the specific surface area of BET (Brunauer-Emmett-Teller) The results are shown in Table 2 below.

In this case, BET refers to the specific surface area of a powder or mass, regardless of the material of the sample, by utilizing the phenomenon of mass adsorption and chemisorption. By using BET by gas adsorption, the surface area and the porosity of particles existing in the sample Can be measured. The molecules of the adsorbed gas are physically adsorbed on the surface of the particles, including any pores or crystallite surfaces, under controlled conditions in a vacuum chamber.

For example, the BET Specific Surface Area can be measured by observing the nitrogen adsorption of the sample using a Tristar II nitrogen adsorption machine. A multi-point BET measurement can be performed using a partial pressure range of 0.01-0.3 P / P _o . The adsorption isotherm can be obtained by measuring the gas pressure on the sample as a function of the volume of gas introduced into the chamber. The linear region of the adsorption isotherm can then be used to determine the volume of gas required to form a monolayer over the surface area of the available particles, using the BET theory, as described by equation (1) below .

(1)

(One)

In the above equation (1), v is the volume of the gas, P is the pressure of the gas, and P ₀ is the saturation pressure. V _m is the volume of gas required to form the monolayer and c is the BET constant.

division Specific surface area (m ² / g) Example 1 3.58 Example 2 3.49 Comparative Example 1 2.93 Comparative Example 2 16.62 Comparative Example 3 3.68 Comparative Example 4 3.36 Comparative Example 5 5.28

According to Table 2, the specific surface area of the negative electrode active materials of Examples 1 and 2 and Comparative Examples 2 to 5 except for Comparative Example 1 has a high specific surface area of 3 m ² / g or more.

[ Experimental Example  3]

X-ray diffraction (XRD) analysis was performed on the negative active material for lithium ion secondary batteries of Examples 1 and 2 and Comparative Examples 1 to 5 prepared according to Experimental Example 1 to determine crystallization of amorphous silicon formed on the natural graphite phase. The results of the X-ray diffraction analysis are shown in FIG. 6. At this time, Cu-Kαray was used as a light source.

As can be seen from the measurement results of FIG. 6, it was confirmed that the crystal peaks of silicon (denoted by a symbol) were observed in the negative electrode active materials of Examples 1 and 2 and Comparative Example 4 in which a secondary chemical vapor deposition process or heat treatment was performed at a high temperature It can be confirmed that the amorphous silicon formed on the pores of the natural graphite or the surface thereof through the primary chemical vapor deposition process is at least partially changed to crystalline silicon through a subsequent secondary chemical vapor deposition process or heat treatment process.

On the other hand, in Comparative Example 3, although the anode active material was prepared in the same manner as in Examples 1 and 2, since the temperature of the secondary chemical vapor deposition process was 600 ° C., the chemical vapor deposition It can be seen that Examples 1 and 2 did not change to crystalline silicon in different ways.

In the case of Comparative Example 4, the second chemical vapor deposition process was not performed. However, after the amorphous silicon was formed through the first chemical vapor deposition process, the amorphous silicon was subjected to a high-temperature heat treatment (900 ° C) In the case of Comparative Example 2, the amorphous silicon was formed through the sol-gel method and then was not crystallized by heat treatment at a low temperature of 650 ° C. However, if the thermal annealing step is performed at a high temperature, the amorphous silicon may be crystallized Can be predicted.

[ Experimental Example  4]

In order to compare the capacity characteristics of the lithium ion secondary batteries produced in Examples 1 and 2 and Comparative Examples 1 to 5 prepared in accordance with Experimental Example 1 for the lithium ion secondary battery for a lithium ion secondary battery, Active Material: Super-P (Sigma Aldrich) as a conductive material: SBR: binder, carboxymethyl cellulose (CMC) as a binder was mixed at a weight ratio of 95.8: 1: 1.5: 1.7 to prepare a slurry.

The slurry was coated on a copper foil, dried at 80 DEG C for 2 hours, rolled and further dried in a vacuum oven at 110 DEG C for 12 hours to prepare an electrode plate. The cycle life of the CR2016 coin half cell manufactured by this method was 50 charge / discharge cycles of 0.005-1.0 V at 0.5 C, and the cycle life was evaluated. The results are shown in Tables 3, 7 and 8 below.

division 1st cycle
Discharge capacity (mAh / g) 50th cycle
Discharge capacity (mAh / g) Capacity retention rate (%) Example 1 503 470 93.4 Example 2 513 491 95.8 Comparative Example 1 508 440 86.5 Comparative Example 2 502 463 92.3 Comparative Example 3 526 469 89.2 Comparative Example 4 506 459 90.5 Comparative Example 5 371 366 98.7

7 and 8 are graphs showing discharge capacities of coin half cells using the negative electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 5. The discharge capacity after repeating the charge and discharge cycles 50 times Respectively.

On the other hand, in Table 3, the capacity retention rate calculated from the first cycle discharge capacity and the 50th cycle discharge capacity value, which is calculated through the following equation (2), is additionally indicated.

(2)

(2)

According to Table 3, it was confirmed that the capacity retention ratio of Examples 1 and 2 and Comparative Examples 2, 4, and 5 was higher than 90%, and the capacity retention ratio of Comparative Example 5 was the highest. However, in the case of Comparative Example 5, the discharge capacity value is remarkably lower than that of the other negative electrode active materials (Examples 1 and 2 and Comparative Examples 1 to 4), indicating that it is not suitable for use as a secondary battery.

Therefore, it can be seen that Example 2 having a relatively high discharge capacity value and a high capacity retention rate of 95.8% at the same time is suitable for use as an anode active material of a secondary battery.

[ Experimental Example  5]

In order to compare the degree of volume change in the charging and discharging process of the lithium ion secondary battery manufactured using the negative electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 5 prepared according to Experimental Example 1, The (initial) thickness including the electrode plate and the negative electrode active material was measured. After the completion of the 50 cycle cycle life evaluation, the CR2016 coin battery was disassembled, and the expanded thickness of the electrode plate and the active material was measured in micrometers. For the thickness values thus measured, the expansion ratios of the examples and the comparative examples were compared using the following equation (3), and the results are summarized in Table 4.

division Expansion ratio (%) Example 1 54 Example 2 38 Comparative Example 1 50 Comparative Example 2 44 Comparative Example 3 56 Comparative Example 4 44 Comparative Example 5 19

According to Table 4, the expansion rates of Examples 2 and 5 were as low as 38% and 19%, respectively. The low expansion rate of the second embodiment of the present invention is due to the continuous shrinkage of lithium silicide generated during charging and discharging, It is insensitive to the volume change due to expansion and has a small capacity reduction of the secondary battery as the charge and discharge cycle progresses.

(3)

(3)

[ Experimental Example  6]

The output characteristics of the lithium ion secondary battery fabricated in Experimental Example 4 were evaluated by varying the applied current from 0.005 to 1.0 V at 0.1 to 7.0 C as shown in FIG.

According to the measurement results of FIG. 9, the output characteristics of the lithium ion secondary battery including the negative electrode active material of Example 2 of the present invention are the most excellent.

As a result, the negative electrode active material of Comparative Example 5 in which natural graphite was used as it is, has excellent values in BET specific surface area, capacity retention rate and expansion ratio comparison test as compared with Examples 1 and 2 of the present invention However, the discharge capacity (Table 3) and the output characteristics (FIG. 9), which are the most important physical properties of the secondary battery, were low.

As a result, the negative electrode active material of Comparative Example 5 has a limited battery capacity because it is applied to a lithium ion secondary battery. As in Comparative Examples 1 to 4, only the silicon layer is coated on the natural graphite Compared with the negative active material coated with amorphous silicon layer (Comparative Examples 2 and 3), as in Examples 1 and 2 of the present invention, the negative active material containing crystalline silicon and the carbon layer in the natural graphite has a high capacity , It has a high output characteristic and a high capacity retention ratio, which is also advantageous in terms of life characteristics.

Claims (14)

delete delete delete delete delete delete delete Forming an amorphous metal layer having an island structure in the form of a hemisphere through a primary chemical vapor deposition on a natural graphite surface and an inner pore surface of the natural graphite; And
And forming a carbon layer on the amorphous metal layer through secondary chemical vapor deposition,
Wherein the primary chemical vapor deposition is performed at a temperature of from 450 캜 to less than 600 캜,
The secondary chemical vapor deposition is performed at a temperature of 600 to 900 DEG C,
Wherein a carbon layer is formed in the secondary chemical vapor deposition process for forming the carbon layer, and at the same time, at least a part of the amorphous metal layer is crystallized. 9. The method of claim 8,
Wherein the natural graphite has an average particle diameter of more than 0 占 퐉 and not more than 50 占 퐉 and a surface area of 1 to 20 m2 / g. 9. The method of claim 8,
Wherein the forming of the amorphous metal layer is performed by chemical vapor deposition using a silane-based gas of SiH ₄ , SiHCl ₃ , Si ₂ H _6, or SiH ₂ Cl ₂ . Way. 9. The method of claim 8,
The metal layer is contained in an amount of 1 to 20 parts by weight based on 100 parts by weight of the natural graphite,
Wherein the content of the carbon layer is 0.5 to 10 parts by weight based on 100 parts by weight of the natural graphite. 9. The method of claim 8,
Wherein the metal layer has a thickness of 10 to 50 nm and the carbon layer has a thickness of 1 to 100 nm. 9. The method of claim 8,
Wherein the second chemical vapor deposition is performed using C ₂ H ₂ , CH ₄ , C ₂ H ₄ , Toluene or Xylene as a reaction gas and citric acid, sucrose or glucose as a sol-gel method. Gt; 14. A lithium ion secondary battery comprising a negative electrode active material produced by the method of any one of claims 8 to 13.

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