KR102083543B1 - Graphene nano sheet for negative electrode active material and preparing method thereof - Google Patents

Abstract

The present invention relates to a graphene nanosheet for an anode active material having an alkali chloride layer formed on its surface and a method for manufacturing the same. The graphene nanosheet for an anode active material of the present invention has a specific surface area because an alkali chloride layer is formed on its surface. By controlling the side reaction with the electrolyte is reduced, the negative electrode containing the same can form a stable solid electrolyte (Solid Electrolyte Interface (SEI)) coating compared to the conventional bare graphene (bare graphene) can be improved high temperature durability In addition, since the initial efficiency of the lithium secondary battery including the same may be improved, it may be usefully used in the production of a lithium secondary battery.

Description

Graphene nanosheet for anode active material and manufacturing method {GRAPHENE NANO SHEET FOR NEGATIVE ELECTRODE ACTIVE MATERIAL AND PREPARING METHOD THEREOF}

The present invention relates to a graphene nanosheet for negative electrode active material and a method for manufacturing the same, and more particularly, to a graphene nanosheet for negative electrode active material having an alkali chloride layer formed on the surface and a method for manufacturing the same.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing. Among them, lithium secondary batteries exhibiting high energy density and operating potential, long cycle life, and low self-discharge rate. Batteries have been commercialized and widely used.

A lithium secondary battery generally includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator, and an electrolyte, and is a secondary battery in which charge and discharge are performed by intercalation-decalation of lithium ions. Lithium secondary batteries have high energy density, high electromotive force, and high capacity, and thus have been applied to various fields.

Metal oxides such as LiCoO ₂ , LiMnO ₂ , LiMn ₂ O _4, or LiCrO ₂ are used as a positive electrode active material constituting the positive electrode of a lithium secondary battery, and metal lithium, graphite (graphite) as a negative electrode active material constituting the negative electrode ), Or carbon based materials such as activated carbon, or materials such as silicon oxide (SiO _x ) are used. Among the negative active materials, metal lithium was initially used, but as the charge and discharge cycles progress, lithium atoms grow on the surface of the metal lithium to damage the separator and damage the battery. Recently, carbon-based materials are mainly used.

Among these carbon-based materials, graphene has excellent electrical conductivity, and has a structure that is advantageous for the input / output of lithium ions compared to general graphite, and thus, many studies are being conducted as negative electrode materials of next-generation lithium secondary batteries. However, graphene has a problem of large side reaction with the electrolyte due to the large specific surface area, low initial efficiency, and deterioration of stage characteristics at high temperature.

Therefore, it is necessary to develop a technology that can solve the problems caused by the large specific surface area of the graphene as described above.

An object of the present invention is to provide a graphene nanosheet for a negative electrode active material, which can solve the problem caused by the large specific surface area of graphene.

Another object of the present invention is to provide a negative electrode active material for a lithium secondary battery including the graphene nanosheets for the negative electrode active material and a lithium secondary battery comprising the same.

In addition, another object of the present invention is to provide a method for producing the graphene nanosheets for the negative electrode active material and a method for producing a negative electrode active material for a lithium secondary battery comprising the same.

In order to solve the above problems, the present invention

Provided are graphene nanosheets for a negative electrode active material having an alkali chloride layer formed on its surface.

In order to solve the above other problem, the present invention

Provided is a negative electrode active material for a lithium secondary battery including the graphene nanosheets for the negative electrode active material, and a lithium secondary battery including the negative electrode active material for the lithium secondary battery.

Moreover, in order to solve the said another subject, this invention is

(1) preparing a graphene nanosheet;

(2) mixing and stirring the aqueous solution of graphene nanosheets and alkali chloride;

(3) filtering out the stirred mixture under reduced pressure; And

(4) drying the product obtained by the filtration in a vacuum oven at 60 to 200 ℃

It provides a method for producing a graphene nanosheets for a negative electrode active material comprising a, and a method for producing a negative electrode active material for a lithium secondary battery comprising the manufacturing method.

Since the graphene nanosheets for the negative electrode active material of the present invention have an alkali chloride layer formed on the surface thereof, side reactions with the electrolyte are reduced as the specific surface area is controlled, and the negative electrode including the negative electrode is added to a conventional untreated graphene (bare graphene). Compared to the stable solid electrolyte (SEI) film can be formed, high temperature durability can be improved, and the initial efficiency of the lithium secondary battery including the same can be improved, and thus it can be usefully used in the production of lithium secondary batteries. Can be.

FIG. 1 is a diagram schematically illustrating a form of a graphene nanosheet for an anode active material in which an alkali chloride layer is formed on a surface by coating alkali chloride on a bare graphene nanosheet.

Hereinafter, the present invention will be described in more detail to aid in understanding the present invention.

The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concepts of terms in order to best describe their invention. It should be interpreted as meanings and concepts corresponding to the technical idea of the present invention based on the principle that the present invention.

In the graphene nanosheets for the negative electrode active material of the present invention, an alkali chloride layer is formed on the surface of the graphene nanosheets.

The alkali chloride may be at least one selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), and rubidium chloride (RbCl), and specifically, lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl) may be one or more selected from the group consisting of.

The alkali chloride layer is formed on the graphene nanosheets to reduce the large surface area of graphene, thereby reducing the side reaction between the graphene and the electrolyte, thereby improving the initial efficiency of the lithium secondary battery including the same. Through the reaction with the electrolyte to form a more stable and solid solid electrolyte (Solid Electrolyte Interface; SEI) coating, the high temperature durability of the lithium secondary battery including the same can be improved.

FIG. 1 schematically shows a form made of graphene nanosheets for a negative electrode active material in which an alkali chloride layer is coated on a bare graphene nanosheet to form an alkali chloride layer on a surface thereof.

As can be seen in Figure 1, when the alkali chloride is coated on the untreated graphene nanosheet, the alkali chloride is chemically bonded to the edge and / or defect portion of the graphene nanosheet By covering the portions, the specific surface area of the graphene nanosheet is reduced, and reacts with the electrolyte during initial charging to form a solid solid electrolyte film on the graphene surface as compared with the untreated graphene.

The alkali chloride layer may have a thickness of 0.1 to 15 nm, specifically, may have a thickness of 0.2 to 10 nm, and more specifically, may have a thickness of 0.4 to 5 nm.

When the thickness of the alkali chloride layer is 0.1 nm or more, an appropriate surface area reduction effect of the graphene may be expected according to the formation of the alkali chloride layer. When the thickness of the alkali chloride layer is 15 nm or less, the alkali chloride layer may be The thickness of the alkali chloride layer may be so thick that it prevents the graphene nanosheet from decreasing the electrical conductivity or preventing the lithium ions from occluding and releasing during charging and discharging.

The alkali chloride layer may be 3 to 15% by weight, specifically 5 to 10% by weight, based on the total weight of the graphene nanosheets on which the alkali chloride layer is formed, and the alkali chloride layer is 3% by weight. In the case of% or more, the surface of the graphene nanosheets may be covered with 0.1 nm or more, which is a sufficient thickness range as a whole, and in the case of 15% by weight or less, the thickness of the alkali chloride layer may satisfy the range of 5 nm or less, which is an appropriate thickness range. Can be.

The graphene nanosheet may be a graphene having a sheet shape, not only a graphene composed of one layer, but a concept including graphene composed of several layers, and specifically, the graphene nanosheets may be 1 to 50. It may be made of a dog graphene layer, more specifically may be made of 1 to 30 layers, more specifically may be made of 1 to 10 layers.

The space occupied by one individual graphene layer in the graphene nanosheet, that is, the thickness is about 0.34 nm, and the graphene nanosheet on which the alkali chloride layer is formed may have a thickness of 0.4 to 22 nm. Specifically, it may have a thickness of 0.5 to 15 nm.

The graphene nanosheets may be nanosheets having a sheet shape, and the graphene nanosheets are placed on a plane, and the size of the graphene nanosheets when viewed in a direction perpendicular to the plane is 0.1 to 30 μm, specifically 0.5 to 15 μm. More specifically, it may be 1 to 15 ㎛. The size when viewed in a direction perpendicular to the plane represents the longest length assuming a line connecting one point to another point in the plane of the graphene nanosheets.

The graphene nanosheets may be prepared through known graphene manufacturing methods, such as physical peeling, chemical peeling, SiC crystal pyrolysis, peel-reinsertion-expansion, or chemical vapor deposition.

Graphene nanosheets having an alkali chloride layer formed on the surface may have a specific surface area of 50 to 1,000 m ² / g, specifically, may have a specific surface area of 100 to 700 m ² / g, and more specifically, It may have a specific surface area of 150 to 500 m ² / g. In the present invention, the specific surface area can be measured by the Brunauer-Emmett-Teller (BET) method. For example, using a porosimetry analyzer (Bell Japan Inc, Belsorp-II mini) can be measured by the BET 6 point method by the nitrogen gas adsorption distribution method.

When the specific surface area is less than 50 m ² / g, there is no special output improvement effect compared to the conventional carbon-based active material, when the specific surface area is more than 1,000 m ² / g, the alkali chloride layer is formed on the surface Since the specific surface area of the fin nanosheets is not small enough, the irreversible capacity due to side reactions with the electrolyte is large and thus the initial efficiency is deteriorated, and the battery capacity is also lowered.

As such, the graphene nanosheets for the negative electrode active material according to an example of the present invention may be used as a negative electrode active material of a lithium secondary battery, and thus the present invention includes the negative electrode active material for a lithium secondary battery, including the graphene nanosheet for the negative electrode active material. to provide.

The graphene nanosheets for the negative electrode active material may be used as a negative electrode active material, and may be used alone or in combination with other negative electrode active materials. Therefore, the negative active material for a lithium secondary battery according to an exemplary embodiment of the present invention may include 10 to 100 wt% of the graphene nanosheet for the negative active material based on the total weight of the negative active material for the lithium secondary battery, and specifically, 30 to 100 wt% It may be included, more specifically may comprise 50 to 100% by weight.

When the graphene nanosheets for the negative electrode active material are included in an amount of 10% by weight or more based on the total weight of the negative electrode active material for the lithium secondary battery, it has excellent electrical conductivity and excellent occlusion / release of lithium ions, which is an advantage of using graphene as a negative electrode active material. The effect according to the point, that is, the electrical conductivity and the conductivity of lithium ions is very excellent can be expected to provide a role (path) to react with the lithium ions in the electrode.

As the negative electrode active material other than the graphene nanosheets, a carbon material, lithium metal, silicon, tin, or the like, in which lithium ions which are commonly used may be occluded and released may be used. Specifically, a carbon material may be used, and as the carbon material, both low crystalline carbon and high crystalline carbon may be used. Soft crystalline carbon and hard carbon are typical low crystalline carbon, and high crystalline carbon is natural graphite, kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber. High-temperature calcined carbon such as (mesophase pitch based carbon fiber), meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes.

The negative electrode active material for a lithium secondary battery may be used in a lithium secondary battery, and thus the present invention provides a lithium secondary battery including the negative electrode active material for a lithium secondary battery.

The lithium secondary battery may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode.

The positive electrode can be prepared by conventional methods known in the art. For example, a slurry may be prepared by mixing and stirring a solvent, a binder, a conductive material, and a dispersant in a cathode active material, if necessary, and then applying (coating) to a current collector of a metal material, compressing, and drying the same to prepare a cathode. have.

The current collector of the metal material is a metal having high conductivity, and is a metal to which the slurry of the positive electrode active material can easily adhere, and is particularly limited as long as it has high conductivity without causing chemical change in the battery in the voltage range of the battery. For example, the surface-treated with stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon, nickel, titanium, silver, or the like on the surface of aluminum or stainless steel may be used. In addition, fine unevenness may be formed on the surface of the current collector to increase the adhesion of the positive electrode active material. The current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric, and may have a thickness of 3 to 500 μm.

The positive electrode active material may be, for example, substituted with a layered compound such as lithium cobalt oxide [Li _x CoO ₂ (0.5 <x <1.3)], lithium nickel oxide [Li _x NiO ₂ (0.5 <x <1.3)], or an additional transition metal. compound; Formula _{Li 1 + x Mn 2 - x} O 4 ( where, x is from 0 to _{0.33), LiMnO 3, LiMn 2 O} 3, or _{[Li x MnO 2 (0.5 <} x <1.3)] lithium manganese oxide and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , or Cu ₂ V ₂ O ₇ ; Ni-site type lithium nickel oxide represented by the formula LiNi _{1 - x} M _x O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3; Formula LiMn _{2 - x} M _x O ₂ , wherein M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) _{3 and the} like.

The solvent for forming the positive electrode includes an organic solvent such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, dimethyl acetamide or water, and these solvents are used alone or in combination of two or more. Can be used by mixing. The amount of the solvent used is sufficient to dissolve and disperse the positive electrode active material, the binder, and the conductive material in consideration of the coating thickness of the slurry and the production yield.

The binder may be polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid and polymers in which hydrogen thereof is replaced with Li, Na or Ca, or the like, or Various kinds of binder polymers such as various copolymers can be used.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as acetylene black, Ketjen black, channel black, farnes black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powders; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. The conductive material may be used in an amount of 1 wt% to 20 wt% with respect to the total weight of the positive electrode slurry.

The dispersant may be an aqueous dispersant or an organic dispersant such as N-methyl-2-pyrrolidone.

The negative electrode may be manufactured by a conventional method known in the art, and examples of the conventional method of manufacturing the negative electrode may include a negative electrode active material, and a negative electrode active material slurry by mixing and stirring additives such as a binder and a conductive material. After that, a method of applying the same to a current collector, drying, and compressing may be used.

In the preparation of the negative electrode, when the graphene nanosheets for the negative electrode active material of the present invention is used as a negative electrode active material, since the graphene nanosheets themselves may serve as a conductive material, a conventional method using a separate conductive material Unlike the conductive material may not be used.

The solvent for forming the negative electrode includes an organic solvent such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, dimethyl acetamide or water, and these solvents alone or in combination of two or more. Can be used by mixing. The amount of the solvent used is sufficient to dissolve and disperse the negative electrode active material, the binder, and the conductive material in consideration of the coating thickness of the slurry and the production yield.

The binder may be used to bind the negative electrode active material particles to maintain the molded body, and is not particularly limited as long as it is a conventional binder used when preparing a slurry for the negative electrode active material, and for example, polyvinyl alcohol, carboxymethyl cellulose, and hydroxy, which are non-aqueous binders. Propylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene or polypropylene, and the like. Any one or a mixture of two or more selected from the group consisting of ronitrile-butadiene rubber, styrene-butadiene rubber and acrylic rubber can be used. Aqueous binders are economical and environmentally friendly compared to non-aqueous binders, are harmless to the health of workers, and have a superior binding effect than non-aqueous binders. Preferably styrene-butadiene rubber may be used.

The binder may be included in less than 10% by weight of the total weight of the slurry for the negative electrode active material, specifically, may be included in 0.1% by weight to 10% by weight. If the content of the binder is less than 0.1% by weight, the effect of using the binder is insignificant and undesirable. If the content of the binder is more than 10% by weight, the capacity per volume may decrease due to the decrease in the relative content of the active material due to the increase in the content of the binder. not.

The negative electrode current collector used for the negative electrode according to an embodiment of the present invention may have a thickness of 3 ㎛ to 500 ㎛. The negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, carbon, carbon, on the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, it is possible to enhance the bonding strength of the negative electrode active material by forming fine irregularities on the surface, it can be used in various forms such as film, sheet, foil, net, porous body, foam, non-woven fabric.

In addition, as the separator, conventional porous polymer films conventionally used as separators, such as polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer and ethylene-methacrylate copolymer, etc. The porous polymer film prepared by using a single or a lamination thereof may be used, or a conventional porous nonwoven fabric, such as a high melting point glass fiber, polyethylene terephthalate fibers and the like can be used, but is not limited thereto.

A lithium salt which can be included as an electrolyte used in the present invention can be used without limitation, those which are commonly used in a lithium secondary battery electrolyte, for example the lithium salt of the anion is ^{F -, Cl -, Br -} , 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 may be any one selected from the group consisting of _{^{-, CH 3 CO 2 -,}} SCN - , and _{(CF 3 CF 2 SO 2)} 2 N.

Examples of the electrolyte used in the present invention include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, and the like, which can be used in manufacturing a lithium secondary battery. no.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be cylindrical, square, pouch type or coin type using a can.

The lithium secondary battery according to the present invention may not only be used in a battery cell used as a power source of a small device, but also preferably used as a unit battery in a medium-large battery module including a plurality of battery cells.

Preferred examples of the medium-to-large device include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

The present invention also provides a method for producing the graphene nanosheets for the negative electrode active material.

The graphene nanosheets for the negative electrode active material include (1) preparing graphene nanosheets; (2) mixing and stirring the aqueous solution of graphene nanosheets and alkali chloride; (3) filtering out the stirred mixture under reduced pressure; And (4) drying the product obtained by the filtration in a vacuum oven at 60 to 200 ° C.

The graphene nanosheets may be in the form of graphene powder, and the graphene nanosheets may be prepared in known graphene methods such as physical exfoliation, chemical exfoliation, SiC crystal pyrolysis, exfoliation-reinsertion-expansion, Or it may be prepared through a chemical vapor deposition method.

Before the graphene nanosheets are mixed with the aqueous solution of the alkali chloride in the step (2) and stirred, a heat treatment step for lowering the content of impurities in the graphene nanosheets may be further performed, and the heat treatment may be performed at 600 ° C. To 1,500 ° C., specifically, 800 ° C. to 1,200 ° C., for 0.5 to 5 hours, specifically 1 to 3 hours. The heat treatment step may be performed under an H ₂ / Ar gas atmosphere.

By mixing and stirring the aqueous solution of graphene nanosheets and alkali chloride, a layer of alkali chloride may be formed on the surface of the graphene nanosheets.

The stirring speed may be 100 to 1,200 rpm, specifically 100 to 800 rpm, and more specifically 200 to 500 rpm. When the stirring speed is 100 rpm or more, the alkali chloride can be smoothly bonded to the surface of the graphene nanosheets, and when the stirring speed is 1,200 rpm or less, the layer of alkali chloride is appropriately on the surface of the graphene nanosheets. While it can be formed, the excessive stirring speed can prevent the graphene nanosheets from being damaged and broken into too small pieces.

The stirring may be performed for 30 minutes to 12 hours, specifically for 30 minutes to 6 hours, and more specifically for 1 hour to 3 hours.

If the stirring time is too short, the formation of an alkali chloride layer on the surface of the graphene nanosheets may be insufficient, and if the stirring time is too long, problems may occur due to evaporation of the aqueous solution of alkali chloride in the process, It is preferable that the said stirring is made in the said time range.

The stirring may be performed in a temperature range of 20 to 60 ° C, specifically 25 to 45 ° C, more specifically in a temperature range of 25 to 35 ° C.

When the stirring temperature is less than 20 ℃ the surface of the graphene nanosheets and the reaction rate of the alkali chloride is slow to make the coating smoothly, when the stirring temperature exceeds 60 ℃ the graphene nanosheets The aqueous alkali chloride solution may evaporate or change to a water vapor state before it is sufficiently wetted with the aqueous alkali chloride solution.

The alkali chloride may be at least one selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), and rubidium chloride (RbCl), and specifically, lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl) may be one or more selected from the group consisting of.

The aqueous alkali chloride solution may be 1 to 10% (w / v) concentration, specifically 1 to 7% (w / v) concentration, more specifically 2 to 5% (w / v) concentration may be have.

When the concentration of the alkali chloride aqueous solution is 1% (w / w) or more, the alkali chloride is smoothly bonded to the surface of the graphene nanosheets, thereby preventing the problem of excessively long stirring time, 10% ( If less than w / w), it is possible to prevent the formation of the alkali chloride layer is excessive to reduce the conductivity between the graphene nanosheets.

In step (2), the graphene powder and the alkali chloride may be mixed in a weight ratio of 100: 1 to 100: 30, specifically, in a weight ratio of 100: 3 to 100: 20, and more specifically, 100: It may be mixed in a weight ratio of 5 to 100: 10.

The agitated mixture is filtered to collect graphene nanosheets for negative electrode active material in which the product, ie, the alkali chloride, forms a layer on the graphene nanosheets. In this case, the filtration may be performed under reduced pressure. After the filtration, the obtained graphene nanosheets are not washed with distilled water.

Next, the collected product obtained by the filtration may be dried in a vacuum oven at 60 to 200 ° C. to prepare graphene nanosheets for the negative electrode active material. The drying time is not particularly limited, but may be made for 3 hours or more, specifically 6 hours to 24 hours.

The method of manufacturing the graphene nanosheets for the negative electrode active material may be used for the preparation of the negative electrode active material for lithium secondary batteries, and thus the present invention includes a method of manufacturing the negative electrode active material for lithium secondary batteries, including the method for producing the graphene nanosheets for the negative electrode active material. To provide.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples, but the present invention is not limited to these Examples and Experimental Examples. Embodiments according to the present invention can be modified in many different forms, the scope of the invention should not be construed as limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1 Preparation of Graphene Nanosheets for Anode Active Materials

5 g of a powder of graphene nanosheets having a thickness of 0.4 to 5 nm and a width of 1 to 15 μm was heat-treated at 1000 ° C. for 120 minutes in a H ₂ / Ag gas atmosphere. The heat-treated graphene nanosheet powder was mixed with 200 ml of a 2% (w / v) NaCl aqueous solution, stirred at 30 ° C. for 30 minutes, and the stirred mixture was filtered under elevated pressure.

The product obtained through the filtration was dried for 24 hours at a temperature of 100 ℃ using a vacuum oven to prepare a graphene nanosheet for negative electrode active material having an alkali chloride layer formed on the surface.

Example 2 Fabrication of Anode and Lithium Secondary Battery

<Production of Cathode>

Using the graphene nanosheets for the negative electrode active material prepared in Example 1 as a negative electrode active material, using a polyvinylidene (PVdF) as a binder, these were mixed in a weight ratio of 90:10, and then water (H ₂ O) as a solvent Mixing together to prepare a uniform negative electrode active material slurry.

The prepared negative electrode active material slurry was coated on one surface of a copper current collector to a thickness of 65 μm, dried and rolled, and then punched to a predetermined size to prepare a negative electrode.

<Production of Lithium Secondary Battery>

Li metal was used as a counter electrode, and a polyolefin separator was interposed between the negative electrode and the Li metal, and then 1M in a solvent in which ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were mixed at a volume ratio of 30:70. A coin-type half cell was prepared by injecting an electrolyte in which LiPF ₆ was dissolved.

Comparative Example 1

A negative electrode and a nose were prepared in the same manner as in Example 2, except that the untreated graphene nanosheet powder (product name, manufacturer) was used as the negative electrode active material in place of the graphene nanosheet for the negative electrode active material prepared in Example 1. A doll half cell was prepared.

Comparative Example 2

<Production of Anode Active Material>

In place of the graphene nanosheet powder, except that natural graphite having an average particle diameter (D ₅₀ ) dl 11 μm was used, graphite having an alkali chloride layer formed on the surface was prepared in the same manner as in Example 1.

<Production of Cathode and Lithium Secondary Battery>

Instead of the graphene nanosheets for the negative electrode active material prepared in Example 1, except for using graphite having an alkali chloride layer formed on the surface prepared above, the negative electrode and coin-type halves through the same method as in Example 2 The battery was prepared.

Comparative Example 3

<Production of Anode Active Material>

An alkali carbonate layer was formed on the surface of the graphene nanosheets instead of an alkali chloride layer by the same method as Example 1, except that an aqueous solution of Na ₂ CO ₃ , which is an alkali carbonate, was used instead of the aqueous NaCl solution as the alkali chloride. The graphene nanosheets for the negative electrode active material having an alkali carbonate layer formed on the surface thereof were prepared.

<Production of Cathode and Lithium Secondary Battery>

In place of the graphene nanosheets for the negative electrode active material prepared in Example 1, except that the graphene nanosheets with an alkali carbonate layer formed on the surface prepared above, the negative electrode and the A coin-type half cell was prepared.

Experimental Example 1: Initial Efficiency Evaluation

Each of the batteries prepared in Example 2 and Comparative Examples 1 to 3 was charged at 25 ° C. with a constant current (CC) until 5 mV, and then charged at a constant voltage (CV) to charge a current of 0.005 C ( The first charge was performed until cut-off current). Thereafter, it was left for 20 minutes and then discharged until it became 1.5 V with a constant current (CC) of 0.1 C. The efficiency at this time was measured and shown in Table 1 below.

Experimental Example 2 Evaluation of High Temperature Storage

Each of the batteries prepared in Example 2 and Comparative Examples 1 to 3 was subjected to one cycle charge / discharge in the same manner as in Experiment 1 with a current of 0.1 C, and then the initial discharge capacity was measured. Then, charge it with a constant current (CC) of 0.1 C until it reaches 5 mV, and then charge it with a constant voltage (CV) until the charging current reaches 0.005 C (cut-off current), and then, this temperature is 45 degrees. The battery was stored for 1 week, and then repeatedly charged / discharged with a current of 0.1 C to measure the capacity retention rate of the battery. The results are shown in Table 1 below.

Experimental Example 3: Rate Characteristic Evaluation

Each of the batteries prepared in Example 2 and Comparative Examples 1 to 3 measured the discharge capacity (0.2 C rate charging) at 0.1 C and 10 C rate, and the discharge capacity at 10 C-rate compared to 0.1 C discharge capacity. The ratio was calculated and shown in Table 1 together.

Initial efficiency 45, 1 week storage capacity retention 10C / 0.1C capacity retention rate Example 2 69.7% 90.1% 86% Comparative Example 1 51.3% 82.3% - Comparative Example 2 90% 91.7% 43% Comparative Example 3 65.2% 87.5% 83.2%

As shown in Table 1, in the case of the battery of Comparative Example 2 using natural graphite instead of graphene nanosheet powder as the negative electrode active material, the high temperature storage characteristics were excellent, but the rate characteristic is an example using graphene nanosheet powder It was confirmed that it is significantly lower than the battery of 2 and Comparative Example 3. On the other hand, in terms of initial efficiency, the battery of Comparative Example 2 using graphite having an alkali chloride layer formed on its surface as a negative electrode active material was superior to the batteries of Comparative Examples 1 and 3 using graphene-based materials. It is believed that this is because the reaction with the electrolyte was also large due to the large specific surface area. In addition, the battery of Example 2 of the present invention uses a graphene nanosheet with an alkali chloride layer formed on its surface as a negative electrode active material, and an untreated graphene nanosheet without a separate layer formed on its surface as a negative electrode active material. Compared with the cell of Example 1 used and the cell of Comparative Example 3 using the graphene nanosheet in which an alkali carbonate layer was formed in place of the alkali chloride layer on the surface, the initial efficiency was higher, and the comparison using natural graphite as a negative electrode active material. The fall value of efficiency compared with the battery of Example 2 was not relatively large.

On the other hand, when looking at the high temperature storage properties, in the case of the battery of Example 2 using the graphene nanosheets with an alkali chloride layer formed on the surface as a negative electrode active material, a comparative example using the graphene nanosheets not subjected to surface coating treatment Compared with the battery of 1 and the battery of Comparative Example 3 using the graphene nanosheets with the alkali carbonate layer formed on the surface, it was confirmed that the high temperature storage performance was excellent. When compared to the battery of Example 2, a small capacity retention difference was shown. The difference in capacity retention rate between the battery of Example 2 and the batteries of Comparative Examples 1 and 3 is that when the graphene nanosheet having an alkali chloride layer formed on the surface is used as the negative electrode active material, the surface coating is not treated. Compared to the graphene nanosheets in which an alkali carbonate layer is formed on the fin nanosheets and the surface, it is believed that the film forms a more stable and solid solid electrolyte (SEI) film.

In addition, in the case of the battery of Example 1 using the graphene nanosheets with an alkali chloride layer formed on the surface, 10C / 0.1C was higher than the battery of Comparative Example 2 using graphite having an alkali chloride layer formed on the surface as the negative electrode active material. The capacity retention rate was shown, and it was confirmed that the graphene nanosheets exhibited better capacity (normalized capacity) at higher C-rate than graphite.

Through these experimental results, when the graphene nanosheets with the alkali chloride layer formed on the surface of the present invention are used as the negative electrode active material, the graphene nanosheets exhibit excellent rate characteristics compared to graphite, but have low initial efficiency and low efficiency of conventional graphene. Since problems such as high temperature storage capacity retention rate can be solved, it was confirmed that a battery capable of exhibiting excellent initial efficiency, high temperature storage property, and rate characteristics can be manufactured.

Claims (17)

An alkali chloride layer is formed on the surface,
Graphene nanosheets for negative electrode active material having a thickness of 0.4 to 22 nm.
The method of claim 1,
The alkali chloride is at least one graphene nanosheet for negative electrode active material selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), and rubidium chloride (RbCl).
The method of claim 1,
Graphene nanosheets for the negative electrode active material having a thickness of 0.1 to 15 nm the alkali chloride layer.
The method of claim 1,
Graphene nanosheets for the negative electrode active material that the graphene nanosheets consisting of 1 to 50 graphene layers.
delete The method of claim 1,
The graphene nanosheets are nanosheets having a sheet shape,
The graphene nanosheets are placed on a plane, and the size of the graphene nanosheets is 0.1 to 30 μm when viewed in a direction perpendicular to the plane.
In this case, the size when viewed in a direction perpendicular to the plane is a graphene nano for the negative electrode active material that represents the longest length assuming a line connecting one point to another point in the plane of the graphene nanosheets Sheet.
The method of claim 1,
Graphene nanosheets for negative electrode active material having a specific surface area of 50 to 1,000 m ² / g of the graphene nanosheets with an alkali chloride layer formed on the surface.
The negative electrode active material for lithium secondary batteries containing the graphene nanosheet for negative electrode active materials in any one of Claims 1-4, 6, and 7.
The method of claim 8,
The negative active material for a lithium secondary battery includes 10 to 100 wt% of the graphene nanosheet for the negative electrode active material based on the total weight of the negative electrode active material for a lithium secondary battery.
A lithium secondary battery comprising the anode active material for lithium secondary battery according to claim 8.
(1) preparing a graphene nanosheet;
(2) mixing and stirring the aqueous solution of graphene nanosheets and alkali chloride;
(3) filtering out the stirred mixture under reduced pressure; And
(4) drying the product obtained by the filtration in a vacuum oven at 60 to 200 ℃
Method for producing a graphene nanosheets for negative electrode active material according to claim 1 comprising a.
The method of claim 11,
Method for producing a graphene nanosheets for negative electrode active material having the stirring speed of 100 to 1,200 rpm.
The method of claim 11,
After the step (1), before the step (2), further comprises a heat treatment step for lowering the content of impurities in the graphene nanosheets,
The heat treatment is a method for producing a graphene nanosheets for negative electrode active material is made for 0.5 to 5 hours at a temperature of 600 ℃ to 1,500 ℃ under H ₂ / Ar gas atmosphere.
The method of claim 11,
The alkali chloride is at least one selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), and rubidium chloride (RbCl) graphene nanosheets for negative electrode active material.
The method of claim 11,
Method for producing a graphene nanosheets for the negative electrode active material is the aqueous alkali chloride solution of 1 to 10% (w / v) concentration.
The method of claim 11,
Graphene powder and alkali chloride in the step (2) is a method for producing a graphene nanosheets for negative electrode active material is mixed in a weight ratio of 100: 1 to 100: 30.
A method of manufacturing a negative active material for a lithium secondary battery, comprising the manufacturing method of claim 11.

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