KR102194750B1 - Multi-shell Anode Active Material, Manufacturing Method thereof and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

The present invention relates to a negative electrode active material with a multi-layer structure capable of effectively suppressing volume expansion and, more specifically, to a negative electrode active material with a multi-layer structure comprising: a core portion including flaky graphite; a first shell portion surrounding the core portion and including a first amorphous carbon layer in which active particles and conductive fibers are dispersed; and a second shell portion surrounding the first shell and including a second amorphous carbon layer, wherein the active particles are at least one selected from the group consisting of nano metal particles and nano metal-containing compounds, and at least a part of the conductive fibers extend to the second amorphous carbon layer and are exposed on the surface of the second shell portion, to a manufacturing method thereof, and to a lithium secondary battery comprising the same.

Description

Multi-shell Anode Active Material, Manufacturing Method thereof and Lithium Secondary Battery Comprising the Same}

The present invention relates to a high-capacity negative active material formed in a multilayer structure and having excellent conductivity, a method for manufacturing the same, and a lithium secondary battery having excellent life characteristics, output characteristics, and safety including the same.

Lithium secondary batteries are widely used as power sources for mobile electronic devices including mobile phones, and their application fields are expanding as demand for large devices such as electric vehicles increases.

Meanwhile, most of the currently commercialized lithium secondary batteries use carbon-based materials as negative active materials. In particular, graphite exhibits a very reversible charge/discharge behavior due to the uniaxial orientation of the graphene layer, showing excellent life characteristics, and a potential similar to that of lithium metal, so when constructing lithium oxide-based positive electrodes and batteries There is an advantage that high energy can be obtained. However, despite these advantages, the low theoretical capacity (372mAh/g) of graphite is acting as a limitation at the present time when a high-capacity battery is required.

Accordingly, there is an attempt to use a metal material such as Si, Sn, or Al, which exhibits a relatively high capacity as a material that can replace the carbon-based negative active material. However, such metallic materials cause large volume expansion and contraction in the process of insertion and desorption of lithium, resulting in micronization and loss of conduction paths, resulting in poor lifespan characteristics, resulting in lower overall battery performance.

In order to solve these problems, various carbon materials are simply mixed with Si, or fine powdered Si is chemically fixed on the carbon surface using a silane coupling agent, or an effort to fix amorphous carbon on the Si surface through CVD. There is this.

However, in the case of a material that simply mixes a carbon material with Si, as the charging and discharging proceeds, the carbon is released from Si in the process of undergoing large volume expansion and contraction, and this leads to a problem that the life characteristics are greatly reduced due to the decrease in electrical conductivity. have.

In addition, materials in which fine powdered Si is chemically fixed to the carbon surface using a silane coupling agent or CVD, etc., have a long bonding duration due to a silane coupling agent and CVD, so the life characteristics may be deteriorated as the charge/discharge cycle proceeds. In addition, there is a problem in that it is difficult to obtain a stable negative electrode material by performing the physicochemical adhesion uniformly.

Even with such various attempts, the problem of damage to the electrode due to the expansion of Si during the discharge process was still raised.

Accordingly, there is a high need for a high-capacity negative electrode active material having excellent conductivity and a lithium secondary battery having excellent life characteristics and output characteristics, and high safety, including the same.

An object of the present invention is to solve the problems of the prior art and technical problems that have been requested from the past.

Specifically, it is an object of the present invention to provide a high-capacity negative active material having excellent conductivity while suppressing volume expansion during charging and discharging of a battery, including a negative active material having a multilayer structure.

An object of the present invention is to provide a method for preparing the negative active material.

Another object of the present invention is to provide a lithium secondary battery, including the negative active material, having excellent lifespan and output characteristics and excellent safety.

The present invention,

A core portion containing flaky graphite;

A first shell part surrounding the core part and including a first amorphous carbon layer in which active particles and conductive fibers are dispersed; And

Includes; a second shell portion surrounding the first shell and including a second amorphous carbon layer,

The active particles are at least one selected from the group consisting of nano metal particles and nano metal-containing compounds,

At least a portion of the conductive fibers extends to the second amorphous carbon layer to provide a negative active material having a multilayer structure that is exposed on the surface of the second shell portion.

The thickness of the flaky graphite may be 0.1 to 10 μm.

The average particle diameter (D50) of the flaky graphite may be 1 to 15 μm.

At least a portion of the active particles may be physically or chemically bonded to the surface of flaky graphite, conductive fibers, or flaky graphite and conductive fibers.

The nano metal particles may include Si, Sn, Sb, Al, Ge, Zn, Pb, or a combination thereof.

The average particle diameter (D50) of the nano-metal particles may be 10 to 200 nm.

The conductive fiber may be at least one selected from the group consisting of graphene, carbon fiber, carbon nanotube, conductive polymer fiber, and metal filament.

The average particle diameter of the core portion may be 1 to 30 μm.

The thickness of the first shell portion may be 1 to 15 μm, and the thickness of the second shell portion may be 0.1 to 15 μm.

The porosity of the negative active material may be 0.1 to 5%.

Compared to the second shell portion, the first shell portion may include a relatively large number of voids.

The core portion may include a third amorphous carbon layer capable of suppressing volume expansion by being collapsed during the charging and discharging process of the battery.

The present invention also,

(A) preparing core particles by mixing flaky graphite and a solvent;

(B) preparing a first shell part by mixing nano active particles, conductive fibers, and a first amorphous carbon precursor with the core part particles and then performing a first heat treatment; And

(C) coating a second amorphous carbon precursor on the surface of the first shell and then performing a second heat treatment to prepare a second shell.

Based on 100 parts by weight of the flaky graphite, 50 to 200 parts by weight of nanoactive particles, 1 to 10 parts by weight of conductive fibers, and 10 to 60 parts by weight of the first amorphous carbon precursor may be used.

The first heat treatment may be performed at 300 to 900°C.

Based on 100 parts by weight of the flaky graphite, the second amorphous carbon precursor may be 10 to 60 parts by weight.

The second heat treatment may be performed at 800 to 1300°C.

In the step (a), after mixing the third amorphous carbon precursor and the flaky graphite, the core portion including the flaky graphite surrounding the third amorphous carbon layer may be manufactured by heating at 200 to 900°C.

The negative active material according to the present invention is formed in a multi-layered structure of the core part, the first shell part, and the second shell part, so even if the active particles located in the first shell part are repeatedly contracted and expanded during the charging and discharging process of the battery, such a multi-layered structure has a buffering effect. By doing so, it is possible to effectively suppress the volume expansion, and at the same time, it is possible to block contact between the active particles and the electrolyte, thereby ensuring the safety of the lithium secondary battery.

In addition, according to the present invention, since the active particles include nano-silicon particles having a high capacity, the life characteristics of the lithium secondary battery may be improved.

In addition, according to the present invention, at least a portion of the conductive fiber is exposed to the surface of the second shell portion to form a conductive path, so that the lithium secondary battery accordingly has a high capacity and excellent output characteristics.

1 is a schematic diagram showing a cross section of a negative active material according to an embodiment of the present invention;
2 is a schematic diagram showing a cross section of a negative active material according to another embodiment of the present invention;
3 is a SEM photograph of the negative active material according to Experimental Example 1; And
4 is a graph showing the capacity retention rate according to the progress of the cycle according to Experimental Example 4.

Negative active material

1 is a schematic diagram schematically showing a cross section of a negative active material according to an embodiment of the present invention, but is not limited to the above form.

Referring to Figure 1, the negative active material according to the present invention,

A core portion including the flaky graphite 100;

A first shell portion surrounding the core portion and including a first amorphous carbon layer 130 in which active particles 110 and conductive fibers 120 are dispersed; And

Includes; a second shell portion surrounding the first shell portion and including a second amorphous carbon layer 140,

The active particles 110 are at least one selected from the group consisting of nano metal particles and nano metal-containing compounds,

At least a portion of the conductive fiber 120 is in the form of a multilayer structure extending to the second amorphous carbon layer 140 and exposing the surface of the second shell portion.

Flaky graphite (100) constituting the core portion is crystalline graphite and belongs to natural graphite. Flaky graphite (100) exhibits flexible properties as it has a van der Waals bond in the vertical direction in a state in which several broad plate-like particles are stacked, and has a strong covalent bond in the parallel direction of crystals, so the volume generated during the charging and discharging process It can act as a buffer against swelling.

In the present invention, the thickness of the flaky graphite 100 may be 0.1 to 10 μm, and the average particle diameter (D50) may be 1 to 15 μm. If the thickness of the flaky graphite 100 is less than 0.1 µm or the average particle diameter is less than 1 µm, the filling density is lowered and a large amount of flaky graphite is required and the specific surface area is too large, which is undesirable due to difficulties in the manufacturing process. When the thickness of the phase graphite is more than 10 µm or the average particle diameter is more than 15 µm, the flaky graphite 100 is too large and thick, so that the flexibility is lowered and the buffering effect against volume expansion is lowered, which is not preferable. Specifically, the thickness of the flaky graphite 100 may be 0.3 to 3 μm and the average particle diameter D50 may be 3 to 10 μm.

In the first shell portion surrounding the core portion, active particles 110 and conductive fibers 120 are dispersed on the first amorphous carbon layer 130.

Specifically, the active particles 110 are located on the first amorphous carbon layer 130, and at least some of them are flaky graphite 100, conductive fibers 120, or flaky graphite 100 and conductive fibers 120. ) Can be physically attached to the surface or chemically bonded. Accordingly, since the active particles can be uniformly distributed, electrical conductivity is improved during the charging and discharging process, thereby improving lifespan characteristics.

The active particles 110 may include a metal element capable of alloying with lithium, and in detail, may be at least one selected from the group consisting of nano metal particles and nano metal-containing compounds.

The nano metal particles may include Si, Sn, Sb, Al, Ge, Zn, Pb, or a combination thereof, and specifically, may be Si.

The nano-metal-containing compound may be an oxide of a metal including Si, Sn, Sb, Al, Ge, Zn, Pb, or a combination thereof, a composite of the metal and carbon, or a combination thereof, and specifically, a silicon oxide. I can.

The average particle diameter of the nano metal particles may be 10 to 200 nm. If the average particle diameter of the nano metal particles is within the above range, volume expansion during reaction with lithium may be suppressed, thereby improving the overall characteristics of the battery.If it is smaller or larger than this, handling may be difficult in the process or the present invention may exhibit the intended effect. It is not desirable. Specifically, it may be 50 to 150 nm.

The conductive fiber 120 is located on the first amorphous carbon layer 130 and at least a part of the surface of the flaky graphite 100, the active particles 110, or the flaky graphite 100 and the active particles 110 Since it may be physically attached to or chemically bonded to the negative electrode active material, it is possible to secure the mechanical rigidity of the negative electrode active material, while suppressing volume expansion during the charging and discharging process of the battery.

In addition, at least a portion of the conductive fiber 120 extends to the second amorphous carbon layer 140 and is exposed to the surface of the second shell to form a conductive path, thereby increasing electrical conductivity and reducing resistance to further improve output characteristics. Can be.

The conductive fiber 120 is not limited as long as it exhibits conductivity, but may be at least one selected from the group consisting of graphene, carbon fiber, carbon nanotube, conductive polymer fiber, and metal filament. The metal is stainless steel, aluminum, nickel, copper, titanium, platinum, gold, silver, ruthenium, tantalum, niobium, hafnium, zirconium, vanadium, indium, cobalt, tungsten, tin, beryllium, molybdenum, or alloys or laminates thereof. However, it is not limited thereto.

The diameter of the cross section of the conductive fiber 120 may be 1 to 50 μm, and the length may be 0.1 to 100 μm. The conductive fiber 120 may have a substantially uniform or variable diameter, and at least a portion of its long axis may be straight, curved or bent, or branched.

The first amorphous carbon layer 130 serves as a kind of binder, and thus, physicochemical bonding between the flaky graphite 100, the active particles 110, and the conductive fiber 120 may be improved.

The first amorphous carbon layer 130 is not limited as long as it is known in the art, for example, one or two selected from the group consisting of soft carbon, hard carbon, pitch carbide, mesophase pitch carbide, and calcined coke. Each of the above combinations can be used.

The negative electrode active material according to the present invention includes a second shell portion including the second amorphous carbon layer 140 to more effectively suppress the volume expansion of the active material during the charging and discharging process of the battery, and at the same time preventing contact between the active particles and the electrolyte. The safety of the lithium secondary battery can be secured because it can be blocked.

The second amorphous carbon layer 140 is not limited as long as it is known in the art, for example, one or two selected from the group consisting of soft carbon, hard carbon, pitch carbide, mesophase pitch carbide, and calcined coke. Each of the above combinations can be used.

The average particle diameter of the core portion may be 1 to 30 μm, and in detail, it may be 2 to 10 μm. When the average particle diameter of the core portion is within the above range, formation of the core portion may be facilitated and output characteristics may be improved.

The thickness of the first shell portion may be 1 to 15 μm, and in detail, it may be 2 to 10 μm. The volume expansion can be minimized within the above range, and the reversible capacity can be increased due to smooth movement of lithium ions.

The thickness of the second shell portion may be 0.1 to 15 µm, and in detail, may be 0.1 to 5 µm. Within the above range, volume expansion of the first shell portion may be minimized, and lithium ions may move smoothly, thereby increasing reversible capacity.

The average particle diameter (D50) of the negative active material according to the present invention may be 1 to 500 um, and in detail, 1 to 100 um. The optimum effect according to the present invention can be exhibited within the above range.

The porosity of the negative active material according to the present invention is 0.1 to 5%, and it is possible to effectively buffer volume expansion due to charging and reversing of a battery including a plurality of voids.

In particular, the first shell portion using the first amorphous carbon layer 130 includes a relatively large number of voids compared to the second shell portion using the second amorphous carbon layer 140 to effectively expand the volume of the active particles 110. Can be buffered. As will be described later, these voids can be formed by adjusting the heat treatment conditions during the manufacturing process. The pores provide a buffer space by reaction between the active particles 110 and the lithium ions during charging and discharging, have excellent lithium ions conductivity, and have a high filling density, so that the capacity per volume of the negative electrode increases, resulting in cycle life characteristics. It can be improved.

Meanwhile, FIG. 2 is a schematic diagram schematically showing a cross section of an anode active material according to another embodiment of the present invention, but is not limited to the above form.

2, in the negative active material according to the present invention

A core portion including the flaky graphite 200 and a third amorphous carbon layer 350 capable of suppressing volume expansion by collapsing during the charging and discharging process of the battery;

A first shell portion surrounding the core portion and including a first amorphous carbon layer 230 in which active particles 210 and conductive fibers 220 are dispersed; And

Includes; a second shell portion surrounding the first shell and including a second amorphous carbon layer 240,

The active particles 210 are at least one selected from the group consisting of nano metal particles and nano metal-containing compounds,

At least a portion of the conductive fiber 220 may extend to the second amorphous carbon layer 240 and be exposed on the surface of the second shell portion.

Accordingly, since the core portion includes the third amorphous carbon layer 350 that collapses during the charging and discharging process of the battery, volume expansion can be more effectively suppressed.

The third amorphous carbon layer 350 may be formed of one or a combination of two or more selected from the group consisting of soft carbon, hard carbon, pitch carbide, mesophase pitch carbide, and fired coke, respectively, and the first amorphous carbon The layer 230 and the second amorphous carbon layer 240 may be the same or different.

Method for producing negative active material

The present invention, as a method for producing the negative active material,

(A) preparing core particles by mixing flaky graphite and a solvent;

(B) preparing a first shell part by mixing nano active particles, conductive fibers, and a first amorphous carbon precursor with the core part particles and then performing a first heat treatment; And

(C) applying a second amorphous carbon precursor to the first shell portion and then performing a second heat treatment to prepare a second shell portion; It provides a method for manufacturing a negative active material having a multilayer structure comprising.

In step (a), flaky graphite is mixed with a solvent to obtain core particles in which flaky graphite is aggregated.

The solvent is N-methylpyrrolidone, dimethylformamide, toluene, ethylene, dimethylacetamide, acetone, methyl ethyl ketone, hexane, tetrahydrofuran, decane, ethanol, methanol, isopropanol, water, ethyl acetate, or a combination thereof It may include.

In the step (b), the nano-active particles, the conductive fibers, and the first amorphous carbon precursor are mixed, spray-dried on the core particles prepared above, and then subjected to a first heat treatment to prepare a first shell portion surrounding the core portion.

The spray drying may be performed by a drying method including rotational spraying, nozzle spraying, ultrasonic spraying, or a combination thereof.

In this case, 50 to 200 parts by weight of nano-active particles, 1 to 10 parts by weight of conductive fibers, and 10 to 60 parts by weight of the first amorphous carbon precursor may be mixed based on 100 parts by weight of the flaky graphite. If the raw materials are out of the above-defined content, the resistance increases and the output characteristics decrease, or the micronization phenomenon may be intensified, which is not preferable.

The mixing may be a dry or wet mixing method, and although not particularly limited in the present invention, dry mixing is preferred.

Dry mixing may be a method used for preparing a conventional mixed powder, and mechanical milling may be performed for uniform particle mixing. Mechanical milling is a method of pulverizing a sample while imparting pre-mechanical energy to the sample.For example, a roll mill, a ball mill, a mechanofusion or a jet mill can be used, and in detail, a ball mill that efficiently uses high impact energy can be used. have.

The mechanical milling treatment conditions can be appropriately adjusted according to the equipment used, and the faster the rotation speed, the faster the product generation speed, and the longer the rotation time, the higher the conversion rate of the product to the raw material composition. For example, in the case of using a general ball mill, it is preferable that the rotational speed per minute is 1000 rpm to 3000 rpm, and under the conditions of 0.5 to 5 minutes, specifically, the rotation speed per minute is 1500 rpm to 2500 rpm, and the conditions of 1 minute to 3 minutes Do.

The first heat treatment may be performed at 300 to 900°C. If the heat treatment temperature is lower or higher than this, the first amorphous carbon is not sufficiently softened, so that active particles and conductive fibers cannot be evenly distributed, and voids are not sufficiently formed, making it difficult for the present invention to exhibit the intended effect, which is not preferable. . Specifically, it may be carried out at 400 to 600 ℃.

The first amorphous carbon precursor is a coal-based pitch, petroleum-based pitch, mesophase pitch, tar, low molecular weight heavy oil, glucose, gelatin, sugars, phenol resin, naphthalene resin, furfuryl alcohol resin, polyamide resin, furan Resin, polyimide resin, epoxy resin, vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethylcellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, Tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylonitrile, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride It may be one or two or more selected from the group consisting of.

In the step (c), a second amorphous carbon precursor may be applied to the surface of the first shell and then subjected to a second heat treatment to prepare a second shell.

The second amorphous carbon precursor is a coal-based pitch, petroleum-based pitch, mesophase pitch, tar, low molecular weight heavy oil, glucose, gelatin, sugars, phenol resin, naphthalene resin, furfuryl alcohol resin, polyamide resin, furan Resin, polyimide resin, epoxy resin, vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethylcellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, Tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylonitrile, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride It may be one or two or more selected from the group consisting of.

Based on 100 parts by weight of the flaky graphite, the second amorphous carbon precursor may be 10 to 60 parts by weight, and in detail, 10 to 40 parts by weight. If the raw materials are out of the above-defined content, the resistance increases and the output characteristics decrease, or the pulverization phenomenon may be intensified, which is undesirable.

The second heat treatment may be performed at 800 to 1300°C. A high-density second amorphous carbon layer is formed within the above temperature range to minimize the structural change of the active material due to volume expansion of the active particles during the charging and discharging process of the battery, and it is possible to block the contact of the electrolyte, so that the overall performance of the battery This can be improved. Specifically, it may be carried out at 1000 to 1300 °C.

In the present invention, the step (a) may be performed by mixing the third amorphous carbon precursor and the flaky graphite and then heating at 200 to 900°C to prepare a core portion including flaky graphite surrounding the third amorphous carbon layer. The third amorphous carbon layer collapses during the charging and discharging process of the battery and acts as a buffer to suppress volume expansion of the core portion.

The third amorphous carbon precursor is a coal-based pitch, petroleum-based pitch, mesophase pitch, tar, low molecular weight heavy oil, glucose, gelatin, sugars, phenol resin, naphthalene resin, furfuryl alcohol resin, polyamide resin, furan Resin, polyimide resin, epoxy resin, vinyl chloride resin, gum arabic, citric acid, stearic acid, sucrose, polyvinylidene fluoride, carboxymethylcellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, Tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, starch, polyacrylic acid, sodium polyacrylonitrile, polyacrylonitrile, cellulose, styrene, polyvinyl alcohol, and polyvinyl chloride It may be one or two or more selected from the group consisting of.

When the heat treatment temperature is lower or higher than the above-defined range, voids are not sufficiently formed, and thus it is difficult to collapse during the charging/discharging process of the battery, which is not preferable. Specifically, it may be performed at 200 to 500°C.

Based on 100 parts by weight of the flaky graphite, the third amorphous carbon precursor may be 10 to 30 parts by weight, and in detail, 10 to 20 parts by weight. If the raw materials are out of the above-defined content, the resistance increases and the output characteristics decrease, or the micronization phenomenon may be intensified, which is not preferable.

Lithium secondary battery

In addition, the present invention provides a lithium secondary battery including the negative active material.

The lithium secondary battery includes a positive electrode including a positive electrode active material; It includes a negative electrode including the negative electrode active material, and an electrolyte.

The positive electrode is formed by applying a positive electrode mixture including a positive electrode active material to a current collector, and the positive electrode mixture may further include a binder and a conductive material as necessary.

The positive electrode active material is, for example, LiNi _0.8-x Co _0.2 AlxO ₂ , LiCo _x Mn _y O ₂ , LiNi _x Co _y O ₂ , LiNi _x Mn _y O ₂ , LiNi _x Co _y Mn _z O ₂ , LiCoO ₂ , Lithium metal oxides such as LiNiO ₂ , LiMnO ₂ , LiFePO ₄ , LiCoPO ₄ , LiMnPO ₄ and Li ₄ Ti ₅ O ₁₂ (0<x<1, 0<y<1); Chalcogenides such as Cu ₂ Mo ₆ S ₈ , FeS, CoS and MiS, scandium, ruthenium, titanium, vanadium, molybdenum, chromium, manganese, iron, cobalt, nickel, copper, zinc, etc. It can be used, more specifically, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , TiS ₂ , ZrS ₂ , RuO ₂ , Co ₃ O ₄ , Mo ₆ S ₈ , V ₂ O ₅ and the like may be used, but are not limited thereto.

The shape of the positive electrode active material is not particularly limited, and may be in a particle shape, such as a spherical shape, an oval shape, or a rectangular parallelepiped shape. The average particle diameter of the positive electrode active material may be in the range of 1 to 50 μm, but is not limited thereto. The average particle diameter of the positive electrode active material can be obtained, for example, by measuring the particle diameter of the active material observed with a scanning electron microscope and calculating the average value thereof.

The binder is not particularly limited, and a fluorine-containing binder such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) may be used, but is not limited thereto.

The content of the binder is not particularly limited as long as it can fix the positive electrode active material, and may be in the range of 0 to 10% by weight with respect to the entire positive electrode.

The conductive material is not particularly limited as long as it can improve the conductivity of the anode, and examples thereof include nickel powder, cobalt oxide, titanium oxide, and carbon. The carbon may be, in detail, any one selected from the group consisting of Ketjen black, acetylene black, furnace black, graphite, carbon fiber, and fullerene, or at least one of them.

The content of the conductive material may be selected in consideration of other battery conditions such as the type of the conductive material, and may be, for example, in the range of 1 to 10% by weight based on the entire positive electrode.

The thickness of the positive electrode mixture layer in which the positive electrode mixture including the positive electrode active material, the binder, and the conductive material is applied to a current collector may be, for example, 0.1 micrometers to 1000 micrometers.

In some cases, the positive electrode mixture may contain 0.1% to 60% by weight, specifically 10% to 50% by weight, based on the total weight of the positive electrode mixture, the solid electrolyte according to the present invention.

The thickness of the positive electrode mixture layer may be, for example, 0.1 micrometers to 1000 micrometers.

The positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes to the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel, Surface-treated with nickel, titanium, silver, or the like may be used. In addition, various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics having fine irregularities formed on the surface may be used.

The negative electrode may be a negative electrode mixture coated with a negative electrode active material on the negative electrode current collector. The negative active material may be the negative active material according to the present invention, but in some cases, metal oxide, metal, lithium composite oxide, crystalline carbon, amorphous carbon, and the like may be used together. If necessary, the negative electrode mixture may further include a binder and a conductive material having the above-described configuration.

At this time, the negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the lithium secondary battery battery. For example, it is applied to the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, or the like may be used. In addition, the negative electrode current collector, like the positive electrode current collector, may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric having fine irregularities on the surface thereof.

The electrolyte solution is composed of an organic solvent and an electrolyte.

The organic solvent is not limited as long as it is commonly used, and for example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxy One or more selected from the group consisting of ethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran may be used.

So long as it is a lithium salt which can be included in the electrolyte in a conventional not restricted, for example, as the lithium salt, the anion is ^{F -, Cl -, I -} , NO 3-, N (CN) 2-, BF 4- ^{, ClO 4-, PF 6-, (} CF 3) 2 PF 4-, (CF 3) 3 PF 3-, (CF 3) 4 PF 2-, (CF 3) 5 PF -, (CF 3) 6 P _{^{-, CF 3 SO 3-, CF}} 3 CF 2 SO 3-, (CF 3 SO 2) 2 N -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO ^{_{2) 2 CH -, (SF}} 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3-, CF 3 CO 2-, CH 3 CO 2-, SCN - and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- One selected from the group consisting of can be used

A separator is disposed between the positive electrode and the negative electrode to form a battery structure, and the battery structure is wound or folded and placed in a cylindrical battery case or a rectangular battery case, and then an electrolyte is injected to complete a secondary battery. Alternatively, a lithium secondary battery is completed by stacking the battery structure in a bi-cell structure, impregnating the battery structure into an electrolyte, and sealing the resulting product in a pouch.

Although described with reference to the following examples, the following examples are for illustrating the present invention, and the scope of the present invention is not limited thereto.

<Example 1>

A core material was prepared by mixing flaky graphite and N-methylpyrrolidone having an average thickness of 0.5 μm and an average particle diameter of 5 μm.

Based on 100 parts by weight of flaky graphite, 110 parts by weight of nano-silicon particles having an average particle diameter of 115 nm, 2 parts by weight of carbon nanofibers, and 15 parts by weight of polyacrylonitrile are mixed and sprayed onto the core material to coat 600 parts. It was heat-treated at °C. The heat treatment copper polyacrylonitrile forms a porous structure to prepare a first shell portion surrounding the core portion.

20 parts by weight of pitch carbon was applied to the first shell and then heat-treated at 1150° C. to prepare a negative active material having a core part, a first shell part, and a second shell part structure.

<Example 2>

A negative active material was prepared in the same manner as in Example 1, except that heat treatment was performed at 400° C. when manufacturing the first shell part.

<Comparative Example 1>

A negative active material was prepared in the same manner as in Example 1, except that amorphous carbon powder was used instead of flaky graphite in the manufacturing process of the core part.

<Comparative Example 2>

A negative active material was prepared in the same manner as in Example 1, except that heat treatment was performed at 1000°C in the process of manufacturing the first shell part.

<Experimental Example 1>

SEM photographs of the negative active material prepared in Examples 1 and 2 and Comparative Example 1 are shown in FIGS. 3(a), 3(b), and 3(c), respectively.

<Experimental Example 2>

The porosity of the negative active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 was calculated. The porosity (P) is calculated as follows.

Using this, the volume of the internal pores of the negative active material powder prepared in Example 1 is 0.0042 mL/g, and the volume of the internal pores of the negative active material powder prepared in Comparative Example 1 is 0.0589 mL/g. Since the density of the powder is equally 2.35 g/cc, P = 0.0042/((1/2.35) + 0.0042) = 0.98% for Example 1 and P = 12.16% for Comparative Example 1. Similarly, using this, the volume of the internal pores of the negative active material powder of Example 2 is 0.0068 mL/g, and the volume of the internal pores of the negative active material powder of Comparative Example 2 is 0.031 mL/g, so the porosity is calculated and shown in Table 1 below. Done.

Sample Porosity
(%) Example 1 0.98 Example 2 1.57 Comparative Example 1 12.16 Comparative Example 2 6.8

In general, when the porosity is high, the contact area of the electrolyte increases, which greatly affects the lifespan. Therefore, when using the negative electrode active material of Example 1, it is expected to exhibit a relatively high lifespan compared to the case of the negative electrode active material of Comparative Example 1. I can.

<Experimental Example 3>

The average particle diameter (D50) of the core portion, the thickness of the first shell portion, the thickness of the second shell portion, and the average particle diameter (D50) of the powder of the negative electrode active material according to Example 1 were measured and shown in Table 2 below.

1st core part (D50) 1st shell thickness 2nd shell thickness Powder D50 3 um 4 um 1.5 um 14 um

<Experimental Example 4>

A negative electrode plate prepared by using the negative electrode active materials according to Examples 1 and 2 and Comparative Examples 1 and 2, respectively, was cut into a circular shape of 1.4875 cm ² and used as a negative electrode, and a lithium (Li) metal cut into a circular shape of 1.4875 cm ² The thin film was used as the anode. Here, in the design of the negative electrode plate, the rolling density was 1.55 g/cc, the current density was 2.95 mA/cm ² , and the electrode plate capacity was 510 mAh/g. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, and vinylene carbonate dissolved in 0.5% by weight was dissolved in a mixed solution having a volume ratio of methyl ethyl carbonate (EMC) and ethylene carbonate (EC) of 7:3, A lithium coin half-cell was prepared by injecting an electrolyte solution in which 1M concentration of LiPF ₆ was dissolved.

Using the lithium coin half cell, the thickness change rate, discharge capacity, initial efficiency, and capacity retention rate are shown in Table 3 below, and the capacity retention rate according to 50 cycles is shown in FIG. 4.

One cycle (chemical cycle) and two cycles were charged and discharged at 0.1 C (standard cycle), and charging and discharging were performed at 0.5 C/discharge 1.0 C from the third cycle to the 49 cycle. 50 cycles are completed in the state of charging (lithium is contained in the negative electrode), the battery is disassembled and the thickness is measured, and the electrode thickness after 50 cycles is compared to the average of the charge thickness in the initial charge (average value of 5 samples). The rate of change was calculated.

The basic total discharge conditions are as follows.

-Charging condition: CC(constant current)/CV(constant voltage)(0.01V/0.01C, current cut-off)

-Discharge condition: CC (constant current) condition 1.5V cut off

Through the results of one charge and discharge, the discharge capacity (mAh/g) and initial efficiency (%) were derived. Specifically, the initial efficiency (%) was derived by the following calculation.

-Initial efficiency (%) = (Capacity after one discharge / capacity for one charge) Х 100

The capacity retention rate and the electrode thickness change rate were derived by the following calculations, respectively.

-Capacity maintenance rate (%) = (49 discharge capacity / 1 discharge capacity) Х 100

-Electrode thickness change rate (%) = (electrode thickness change after 50 cycles/2 cycles electrode thickness after charging) Х 100

Sample Electrode thickness
Rate of change (%) Discharge capacity
(mAh/g) Initial efficiency
(%) Capacity retention rate
(% @50 Cycle) Porosity
(%) Example 1 25.1 512 91.1 91.8 0.98 Example 2 24.2 514 90.6 92.7 1.57 Comparative Example 1 33.7 508 89.4 89.3 12.16 Comparative Example 2 29.5 511 89.7 88.4 6.8

According to Table 3, the battery using the negative electrode active material of Examples 1 and 2 according to the present invention has a smaller electrode thickness change rate compared to the battery using the negative electrode active material of Comparative Examples 1 and 2, and has excellent discharge capacity and initial efficiency, as well as capacity. It can be seen that the retention rate is excellent.

100, 200 flaky graphite
110, 210 active particles
120, 220 conductive fiber
130, 230 first amorphous carbon layer
140, 240 second amorphous carbon layer
350 third amorphous carbon layer

Claims (19)

As a negative active material of a multilayer structure,
A core portion containing flaky graphite;
A first shell portion surrounding the core portion and including a first amorphous carbon layer in which active particles and conductive fibers are dispersed; And
Includes; a second shell portion surrounding the first shell and including a second amorphous carbon layer,
The active particles are at least one selected from the group consisting of nano metal particles and nano metal-containing compounds,
At least a portion of the conductive fiber extends to the second amorphous carbon layer and is exposed on the surface of the second shell portion,
The negative active material has a porosity of 0.1 to 5%,
The negative active material of the multilayer structure, characterized in that the first shell portion comprises a plurality of voids relatively compared to the second shell portion. The cathode active material of claim 1, wherein the thickness of the flaky graphite is 0.1 to 10 μm. The negative active material of claim 1, wherein the average particle diameter (D50) of the flaky graphite is 1 to 15 μm. The negative active material of claim 1, wherein at least some of the active particles are physically or chemically bonded to the surface of flaky graphite, conductive fibers, or flaky graphite and conductive fibers. The negative active material of claim 1, wherein the nano-metal particles include Si, Sn, Sb, Al, Ge, Zn, Pb, or a combination thereof. The negative active material of claim 1, wherein the average particle diameter (D50) of the nano-metal particles is 10 to 200 nm. The negative active material of claim 1, wherein the conductive fiber is at least one selected from the group consisting of graphene, carbon fiber, carbon nanotube, conductive polymer fiber, and metal filament. The negative active material of claim 1, wherein the average particle diameter of the core part is 1 to 30 µm. The negative active material of claim 1, wherein the first shell part has a thickness of 1 to 15 µm, and the second shell part has a thickness of 0.1 to 15 µm. delete delete The negative active material of claim 1, wherein the core part comprises a third amorphous carbon layer capable of suppressing volume expansion by being collapsed during charging and discharging of the battery. (A) preparing core particles by mixing flaky graphite and a solvent;
(B) preparing a first shell part by mixing the core part particles with nano active particles, conductive fibers, and a first amorphous carbon precursor and then performing a first heat treatment at 400 to 600°C; And
(C) applying a second amorphous carbon precursor to the surface of the first shell portion and then performing a second heat treatment at 1000 to 1300° C. to prepare a second shell portion; including
Porosity is 0.1 to 5%,
The method of manufacturing a negative active material having a multilayer structure, wherein the first shell portion includes a plurality of pores relatively compared to the second shell portion. The multilayer structure of claim 13, wherein, based on 100 parts by weight of the flaky graphite, 50 to 200 parts by weight of nanoactive particles, 1 to 10 parts by weight of conductive fibers, and 10 to 60 parts by weight of the first amorphous carbon precursor. Method for producing a negative active material. delete 14. The method of claim 13, wherein the second amorphous carbon precursor is 10 to 60 parts by weight based on 100 parts by weight of the flaky graphite. delete The method of claim 13, wherein the step (a) comprises mixing the third amorphous carbon precursor and the flaky graphite and heating at 200 to 900°C to prepare a core part comprising flaky graphite surrounding the third amorphous carbon layer. Method for producing a negative active material having a multilayer structure, characterized in that. A lithium secondary battery comprising the negative active material according to claim 1.

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