Anode active material, method for producing anode active material, anode including same, and lithium secondary battery

Abstract

To provide: a negative electrode active material with high initial efficiency and excellent rapid charge characteristics; a method for preparing the negative electrode active material; a negative electrode including the negative electrode active material; and a lithium secondary battery including the negative electrode.SOLUTION: A negative electrode active material includes: a graphite core; a first carbon coating layer surrounding the outside of the graphite core; and a second carbon coating layer surrounding the outside of the first carbon coating layer, where the second carbon coating layer has a lower crystallinity than the first carbon coating layer, or the second carbon coating layer includes hard carbon and the first carbon coating layer includes soft carbon. Also presented are a negative electrode comprising the same, and a lithium secondary battery.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a negative electrode active material, a method for producing a negative electrode active material, a negative electrode including the same, and a lithium secondary battery, and more particularly to a negative electrode active material having high initial efficiency and excellent rapid charge performance, a method for producing a negative electrode active material, a negative electrode including the same, and a lithium secondary battery.

[0002] This application claims priority to Korean Patent Application No. 10-2019-0121071, filed on September 30, 2019, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings. [Background technology]

[0003] As technological development and demand for mobile devices increases, the demand for secondary batteries as an energy source is growing rapidly. Among such secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low discharge rate, have been commercialized and are widely used.

[0004] A lithium secondary battery is constructed by impregnating an electrolyte containing a lithium salt into an electrode assembly having a porous separator between a positive electrode and a negative electrode, each of which has an active material applied to a current collector. The electrode is manufactured by applying a slurry, in which the active material, a binder, and a conductive material are dispersed in a solvent, to the current collector, followed by drying and pressing.

[0005] Lithium metal has traditionally been used as the negative electrode of secondary batteries, but due to the risk of battery short circuit and explosion caused by the formation of dendrites, it has been replaced by carbon-based compounds that allow reversible intercalation and deintercalation of lithium ions while maintaining structural and electrical properties.

[0006] The carbonaceous compound has a very low discharge potential of about -3 V against the standard hydrogen electrode potential, and exhibits excellent electrode cycle life due to the highly reversible charge / discharge behavior caused by the uniaxial orientation of the graphene layer. In addition, when charging with Li ions, the electrode potential is 0 V Li / Li + Since it can show a potential almost similar to that of pure lithium metal, it has an advantage that a higher energy can be obtained when it is combined with an oxide-based positive electrode to form a battery.

[0007] Natural graphite, which has been widely used in the past as a negative electrode, has a large capacity per unit weight, but has the disadvantage that the degree of orientation becomes high during electrode rolling, which reduces the insertion / extraction characteristics of lithium ions, resulting in a decrease in the fast charging characteristics of the battery. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, the present invention has been made to solve the above problems, and an object of the present invention is to provide a negative electrode active material having high initial efficiency and excellent fast charging performance, and a method for manufacturing the negative electrode active material.

[0009] Another object of the present invention is to provide a negative electrode containing the negative electrode active material and a lithium secondary battery including the same. [Means for solving the problem]

[0010] In order to achieve the above object, one aspect of the present invention provides a negative electrode active material according to the following embodiment.

[0011] According to the first embodiment, A graphite core; a first carbon coating layer surrounding the outside of the graphite core; a second carbon coating layer surrounding the outside of the first carbon coating layer; the second carbon coating layer has a lower crystallinity than the first carbon coating layer, or The second carbon coating layer includes hard carbon, and the first carbon coating layer includes soft carbon, thereby providing an anode active material.

[0012] According to the second embodiment, in the first embodiment, The content of the first carbon coating layer and the content of the second carbon coating layer may be independently 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

[0013] According to the third embodiment, in the first or second embodiment, The negative electrode active material may have an average particle size (D50) of 7 to 25 μm.

[0014] According to the fourth embodiment, in any one of the first to third embodiments, The graphite core may have an average particle size (D50) of 5 to 20 μm.

[0015] According to the fifth embodiment, in any one of the first to fourth embodiments, The full width at half-maximum (FWHM) value of the D band of the second carbon coating layer may be 1.3 times or more the FWHM value of the D band of the first carbon coating layer.

[0016] According to the sixth embodiment, in any one of the first to fifth embodiments, The FWHM value of the D band of the second carbon coating layer may be 1.3 to 3 times the FWHM value of the D band of the first carbon coating layer.

[0017] Another aspect of the present invention provides a method for manufacturing a negative electrode active material according to the following embodiment.

[0018] According to the seventh embodiment, mixing the graphite with a first carbon precursor and subjecting the mixture to a first heat treatment at a temperature of 1,400 to 1,600°C to convert the graphite into a graphite core and forming a first carbon coating layer surrounding the graphite core; and mixing a result of the step of forming the first carbon coating layer with a second carbon precursor, and performing a second heat treatment at a temperature of 1,100 to 1,300°C to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0019] According to the eighth embodiment, in the seventh embodiment, The first and second carbon coating layers of the negative active material may be formed such that the content of the first and second carbon coating layers is independently 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

[0020] According to the ninth embodiment, A negative electrode including a current collector and a negative electrode active material layer located on at least one surface of the current collector, The negative electrode active material layer includes the negative electrode active material according to any one of the first to sixth embodiments.

[0021] Yet another aspect of the present invention provides a lithium secondary battery according to the following embodiment.

[0022] According to the tenth embodiment, A lithium secondary battery including the negative electrode according to the ninth embodiment is provided. Effect of the Invention

[0023] According to one aspect of the present invention, there is provided an anode active material in which natural graphite is coated with a double layer of carbon to increase the amount of carbon coating on the natural graphite, and the crystallinity of the double layer of carbon coating layer is controlled. When such an anode active material is applied to the anode of a secondary battery, a secondary battery having high initial efficiency and excellent fast charging performance can be provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, the terms and words used in this specification and claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept that corresponds to the technical idea of ​​the present invention, based on the principle that the inventor himself can appropriately define the concept of the term in order to explain the invention in the best possible way.

[0025] The negative electrode active material according to one embodiment of the present invention comprises: A graphite core; a first carbon coating layer surrounding the outside of the graphite core; a second carbon coating layer surrounding the outside of the first carbon coating layer; the second carbon coating layer has a lower crystallinity than the first carbon coating layer, or The second carbon coating layer comprises hard carbon and the first carbon coating layer comprises soft carbon.

[0026] The graphite core may be synthetic graphite, natural graphite, or a combination thereof, i.e., the graphite core may be made of crystalline graphite. Since natural graphite usually has a higher capacity than synthetic graphite, the use of natural graphite as the graphite core may be advantageous in terms of capacity.

[0027] The shape of the graphite core is not particularly limited, but may be spherical. The spheres may be produced by a common spheroidizing method known in the art using the graphite core as a raw material. For example, the raw material may be subjected to mechanical treatment such as impact compression, friction, or shear force, so that the particles constituting the graphite core are folded or mixed together, and the corners of the particles are cut off. The mechanical treatment can be carried out using a spheronization device commonly known in the art, for example, a pulverizer such as a Counter Jet Mill (Hosokawa Micron Corporation, Japan), an ACM Pulverizer (Hosokawa Micron Corporation, Japan), or a Current Jet (Nisshin Engineering Corporation, Japan), a granulator such as a SARARA (Kawasaki Heavy Industries, Japan), a GRANUREX (Freund Corporation, Japan), a New-Gra Machine (Seishin Enterprises, Japan), or an Agromaster (Hosokawa Micron Corporation, Japan), a kneader such as a pressure kneader (dispersion kneader) or a two-roll mill, a Mechano Micro System, an extruder, a ball mill, a planetary mill, a Mechano Fusion System, Nobilta, a hybridization device, or a compression shear processing device such as a rotary ball mill, can be used.

[0028] The graphite core may have an average particle size (D50) of 5 to 20 μm or 8 to 12 μm. When the graphite core is within the above average particle size range, a first carbon coating layer can be formed sufficiently uniformly on the surface of the graphite core, and when the graphite core is contained in an amount within the above range, the output characteristics and cycle characteristics of a lithium secondary battery using the negative electrode active material including the graphite core are excellent.

[0029] The negative active material of the present invention includes a first carbon coating layer surrounding the outside of the graphite core, and a second carbon coating layer surrounding the outside of the first carbon coating layer in sequence.

[0030] Generally, the faster the charge performance is increased, the larger the coating amount of amorphous carbon, which has a larger interplanar spacing and lower crystallinity (degree of crystallization) than graphite. However, if the amount of carbon coating is excessive, the coated carbon powders become entangled with each other, and the negative electrode active material cannot be sufficiently disintegrated when manufacturing the negative electrode active material, resulting in a very large average particle size (D50) of the negative electrode active material. This makes it difficult to perform the coating process when manufacturing the negative electrode, and there is a risk of problems such as a decrease in rate characteristics and a decrease in the capacity of the electrode at the optimal density.

[0031] Meanwhile, if a single carbon coating layer is formed at once to correspond to the total content of the first carbon coating layer (coating amount) and the second carbon coating layer (coating amount) of the present invention, a problem of aggregation of materials for forming the carbon coating layer may occur.

[0032] In order to solve this problem, the present invention includes a double carbon coating layer, that is, a first carbon coating layer surrounding the outside of the graphite core and a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0033] At this time, the first carbon coating layer and the second carbon coating layer may be selected separately based on the crystallinity or the material that constitutes them.

[0034] The second carbon coating layer has a lower crystallinity than the first carbon coating layer (first type), or the first carbon coating layer contains soft carbon and the second carbon coating layer contains hard carbon (second type).

[0035] The first carbon coating layer and the second carbon coating layer may be formed by mixing an amorphous carbon-based material corresponding to a carbon precursor with a material to be coated and baking the mixture. Specifically, the first carbon coating layer may be formed to surround the outside of the graphite core by mixing graphite with a first carbon precursor and performing a heat treatment (first heat treatment), and the second carbon coating layer may be formed to surround the outside of the first carbon coating layer by mixing the graphite core, the outside of which is surrounded by the first carbon coating layer, with a second carbon precursor and performing a heat treatment (second heat treatment).

[0036] Examples of the amorphous carbon-based active material include, but are not limited to, one or more amorphous carbon precursors selected from the group consisting of hard carbon raw materials such as sucrose, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, polyacrylonitrile resin, polyamide resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin, and vinyl chloride resin; and soft carbon raw materials such as coal-based pitch, petroleum-based pitch, polyvinyl chloride, mesophase pitch, tar, and heavy oil.

[0037] The crystallinity of the first and second carbon coating layers can be confirmed by comparing the FWHM value of G band and the FWHM value of D band in Raman spectroscopy.

[0038] The Raman spectroscopy is a method for analyzing the structure of the first carbon coating layer and the second carbon coating layer, and is characterized by a Raman spectrum of the carbon coating layer at a wave number of 1580 cm -1 The peak in the region around this band is called the G band, which is a peak that indicates the sp2 bonds in the carbon coating layer and represents carbon crystals without structural defects. -1The peak in the vicinity is called the D band, which indicates the sp3 bond in the carbon coating layer and increases when the atomic bond consisting of the sp2 bond is broken and becomes an sp3 bond. Such a D band increases when disorder or defects are generated in the carbon coating layer.

[0039] In the present invention, the G band of the Raman spectrum for the carbon coating layer has a wave number of 1550 cm -1 ~1620cm -1 and the D band is a peak in the region of wavenumber 1330 cm -1 ~1370cm -1 The wave number range for the G band and the D band corresponds to the range that can be shifted by the laser light source used in the Raman analysis. The Raman value used in the present invention is not particularly limited, but can be measured at a laser wavelength of 532 nm using a DXR Raman spectrometer (manufactured by Thermo Electron Scientific).

[0040] According to an embodiment of the present invention, the FWHM value of the D band of the second carbon coating layer may be 1.3 or more times larger than the FWHM value of the D band of the first carbon coating layer, specifically 1.3 to 3 times larger, more specifically 1.3 to 2.7 times larger, and even more specifically 1.3 to 2 times larger. When the ratio of the FWHM value of the D band of the second carbon coating layer to the FWHM value of the D band of the first carbon coating layer satisfies this range, more defects are generated in the second carbon coating layer, and the crystallinity (degree of crystallinity) of the second carbon coating layer becomes lower than that of the first carbon coating layer.

[0041] In the case of carbon materials with a high degree of crystallinity, the lattice spacing of the carbon layers is small, while in the case of amorphous carbon materials with a low degree of crystallinity, the lattice spacing of the carbon layers is large. In a secondary battery using such a carbon material as a negative electrode active material, when charging, lithium ions are difficult to penetrate from the electrolyte into the graphene layers at once because the lattice spacing of the carbon layers is small in carbon materials with a high degree of crystallinity such as graphite, but in the case of amorphous carbon materials with a lower degree of crystallinity than graphite, the lattice spacing of the carbon layers is large, so that lithium ions can easily penetrate, and the intercalation speed of lithium ions into the carbon layers is faster.

[0042] The negative active material of the present invention is designed so that the crystallinity of the carbon material increases in the order from the second carbon coating layer, which is the outermost layer, through the first carbon coating layer, to the graphite core in the center. That is, the second carbon coating layer, which is first in contact with the electrolyte, has the lowest crystallinity, followed by the first carbon coating layer, which has a relatively higher crystallinity than the second carbon coating layer, and then the graphite core, which has the highest crystallinity. This allows lithium ions to easily penetrate the negative active material in the early stages and quickly insert into the carbon layer, thereby providing excellent fast charging properties.

[0043] In the second type, the first carbon coating layer contains soft carbon, and the second carbon coating layer contains hard carbon.

[0044] The soft carbon (easily graphitizable carbon) is obtained by heating coke, needle coke or coal tar pitch, which are by-products generated during the refining process of crude oil, petroleum pitch, or a mixture of two or more of these, at about 1000°C.

[0045] The hard carbon (non-graphitizable carbon) may include carbonized carbonaceous materials such as sucrose, phenolic resin, furan resin, furfuryl alcohol, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, or a mixture of two or more of these.

[0046] In the case of hard carbon, the carbon layers are tightly intertwined with each other, the crystallites are very small, and the precursor has a high degree of structural disorder, making it difficult to rearrange the crystal structure for graphitization even by high-temperature firing at 2,500° C. or more. On the other hand, in the case of soft carbon, which is easy to graphitize, a structure is formed in which the planes of the graphite layers are arranged relatively parallel to each other, making it easy to graphitize the crystalline structure.

[0047] The second type of negative electrode active material is designed to include soft carbon in the first carbon coating layer located on the inside in contact with the graphite core, and hard carbon in the second carbon coating layer, which is the outermost layer of the negative electrode active material. As described in the first type above, in the second type of negative electrode active material, the second carbon coating layer, which is first in contact with the electrolyte, also includes hard carbon with very small crystallite size and strong structural disorder in the precursor, and the interplanar spacing of the carbon layer in such hard carbon is relatively large, so that the penetration of lithium ions contained in the electrolyte is quite easy. Meanwhile, in the first carbon coating layer, the planes of the graphene layers are arranged relatively parallel to each other, and the interplanar spacing of the carbon layers is small, so that the liquid electrolyte is less likely to penetrate between the graphene layers at once compared to the second carbon coating layer. As a result, the second carbon coating layer that the electrolyte first comes into contact with contains hard carbon with a greater degree of structural disorder, followed by the first carbon coating layer containing soft carbon in which the planes of the graphene layers are arranged relatively more parallel than in the second carbon coating layer, and then the graphite core in which the graphene layers are closely spaced and regularly stacked. This allows the lithium ions contained in the electrolyte initially to easily penetrate the anode active material and quickly insert into the carbon layer, thereby demonstrating excellent characteristics in terms of fast charging.

[0048] According to an embodiment of the present invention, the content of the first carbon coating layer and the content of the second carbon coating layer may be, independently, 3 to 6 parts by weight or 4 to 5 parts by weight based on 100 parts by weight of the graphite core.

[0049] When the contents of the first carbon coating layer and the second carbon coating layer satisfy these ranges, the graphite core is sufficiently coated to effectively prevent direct contact with the electrolyte, and an appropriate amount of amorphous carbon having a low crystallinity is coated on the surface of the active material to improve output characteristics and fast charging characteristics. In addition, problems such as difficulty in charging and discharging a lithium secondary battery including the negative electrode active material due to an excessive carbon coating layer being formed, or a decrease in the absolute amount of the negative electrode space into which lithium is inserted, resulting in a decrease in the capacity of the lithium secondary battery, can be prevented.

[0050] The negative electrode active material may have an average particle size (D50) of 7 to 25 μm or 8 to 22 μm. When the negative electrode active material has an average particle size (D50) within this range, handling such as mixing during slurry production is easy, processability is improved, excellent rapid charging performance can be achieved, and a decrease in the capacity of the electrode can be prevented.

[0051] According to another aspect of the present invention, there is provided a method for producing the negative electrode active material.

[0052] According to an embodiment of the present invention, a method for manufacturing a negative electrode active material includes: mixing the graphite with a first carbon precursor and subjecting the mixture to a first heat treatment at a temperature of 1,400 to 1,600°C to convert the graphite into a graphite core and forming a first carbon coating layer surrounding the graphite core; and mixing a result of the step of forming the first carbon coating layer with a second carbon precursor and performing a second heat treatment at a temperature of 1,100 to 1,300°C to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0053] First, graphite and a first carbon precursor are mixed and subjected to a first heat treatment at a temperature of 1,400 to 1,600° C. to turn the graphite into a graphite core and form a first carbon coating layer surrounding the outside of the graphite core.

[0054] The graphite can be synthetic graphite, natural graphite, or a combination thereof.

[0055] The first carbon precursor may be an amorphous carbon-based material, for example, one or more amorphous carbon precursors selected from the group consisting of hard carbon raw materials such as sucrose, phenol resin, naphthalene resin, polyvinyl alcohol resin, furfuryl alcohol resin, polyacrylonitrile resin, polyamide resin, furan resin, cellulose resin, styrene resin, polyimide resin, epoxy resin, or vinyl chloride resin; and soft carbon raw materials such as coal-based pitch, petroleum-based pitch, polyvinyl chloride, mesophase pitch, tar, or heavy oil, but is not limited thereto.

[0056] The method of mixing the graphite and the first carbon precursor is not particularly limited, and may be a method known in the art, such as a mechanochemical method using a kneader such as a two-roll mill, a blade, a mechanomicrosystem, an extruder, a ball mill, a planetary mill, a mechanofusion system, Nobilta, hybridization, a rotary ball mill, or a spray dry method, an emulsification method, or the like.

[0057] The first heat treatment temperature is 1,400 to 1,600° C., and according to an embodiment of the present invention, may be 1,450 to 1,550° C. When the first heat treatment temperature satisfies this range, the micropores of the graphite core can be maintained, and the amorphous carbon material precursor can be sufficiently carbonized.

[0058] Next, the resultant of the step of forming the first carbon coating layer is mixed with a second carbon precursor, and a second heat treatment is performed at a temperature of 1,100 to 1,300°C to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0059] The second carbon precursor may be an amorphous carbon-based material similar to the first carbon precursor. In this case, the first carbon precursor and the second carbon precursor may be made of the same type of amorphous carbon-based material or different types of amorphous carbon-based materials.

[0060] In addition, a method for mixing the result of the step of forming the first carbon coating layer with the second carbon precursor may also be selected from the above-mentioned methods for mixing the graphite core with the first carbon precursor.

[0061] The second heat treatment temperature may be 1,100 to 1,300°C, and may be 1,150 to 1,250°C according to an embodiment of the present invention.

[0062] If a single carbon coating layer having the total content of the first carbon coating layer (coating amount) and the second carbon coating layer (coating amount) is formed at once, a problem of aggregation of materials for forming the carbon coating layer may occur. In one embodiment of the present invention, the first carbon coating layer and the second carbon coating layer are formed separately to prevent such a problem.

[0063] In addition, even when the first and second carbon coating layers are formed separately, if the heat treatment temperatures in the formation steps are both set to 1,100 to 1,300°C, which corresponds to the second heat treatment temperature, there is a concern that the initial efficiency of the negative active material may decrease. Also, if the heat treatment temperatures in the formation steps are both set to 1,400 to 1,600°C, which corresponds to the first heat treatment temperature, there is a concern that the fast charging performance of the negative active material may decrease.

[0064] According to another embodiment of the present invention, a method for manufacturing a negative electrode active material includes: mixing the graphite with a first carbon precursor and subjecting the mixture to a first heat treatment to convert the graphite into a graphite core and forming a first carbon coating layer surrounding the graphite core; and mixing the resultant of the previous step with a second carbon precursor and subjecting the mixture to a second heat treatment to form a second carbon coating layer surrounding the first carbon coating layer. Here, the first carbon precursor is a material that becomes soft carbon after heat treatment, and the second carbon precursor is a material that becomes hard carbon after heat treatment.

[0065] First, graphite and a first carbon precursor are mixed and subjected to a first heat treatment to turn the graphite into a graphite core, and a first carbon coating layer is formed surrounding the outside of the graphite core.

[0066] The first carbon precursor may be any material that becomes soft carbon after heat treatment, and may be, for example, coke, needle coke, coal tar pitch, petroleum pitch, or a mixture of two or more of these.

[0067] The method for mixing the graphite and the first carbon precursor may be selected from the methods for mixing the graphite and the first carbon precursor described above.

[0068] The first heat treatment temperature is 1,400 to 1,600° C., and according to an embodiment of the present invention, may be 1,450 to 1,550° C. When the first heat treatment temperature satisfies this range, the micropores of the graphite core can be maintained, and the amorphous carbon material precursor can be sufficiently carbonized.

[0069] Then, the resultant of the previous step is mixed with a second carbon precursor and subjected to a second heat treatment to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0070] The second carbon precursor may be any material that becomes hard carbon after heat treatment, and may be, for example, sucrose, phenol resin, furan resin, furfuryl alcohol, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, or a mixture of two or more of these.

[0071] The method of mixing the resultant of the previous step with the second carbon precursor may be selected from the methods of mixing the graphite with the first carbon precursor described above.

[0072] At this time, the second heat treatment temperature may be 1,100 to 1,300°C, and may be 1,150 to 1,250°C according to one embodiment of the present invention.

[0073] According to an embodiment of the present invention, the first carbon coating layer and the second carbon coating layer of the negative active material may be formed such that the content of the first carbon coating layer and the content of the second carbon coating layer are independently 3 to 6 parts by weight or 4 to 5 parts by weight based on 100 parts by weight of the graphite core.

[0074] When the contents of the first carbon coating layer and the second carbon coating layer satisfy these ranges, the graphite core is sufficiently coated to effectively prevent direct contact with the electrolyte, and an appropriate amount of amorphous carbon having a low degree of crystallinity is coated on the surface of the active material to improve output characteristics and fast charging characteristics. In addition, problems such as difficulty in charging and discharging a lithium secondary battery including the negative active material due to an excessive carbon coating layer being formed, or a decrease in the absolute amount of the negative electrode space into which lithium is inserted, resulting in a decrease in the capacity of the lithium secondary battery, can be prevented.

[0075] According to yet another aspect of the present invention, there is provided a negative electrode including the negative electrode active material.

[0076] Specifically, the anode according to an embodiment of the present invention includes a current collector and a negative electrode active material layer including the negative electrode active material according to the present invention on at least one surface of the current collector.

[0077] The negative electrode may be manufactured by coating at least one surface of a current collector with a slurry for a negative electrode active material layer obtained by dispersing the negative electrode active material, a binder, and a conductive material in a solvent, followed by drying and rolling.

[0078] The current collector is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity, and examples of the current collector include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, aluminum-cadmium alloy, etc. The thickness of the current collector is not particularly limited, but may be a commonly used thickness of 3 to 500 μm.

[0079] The negative electrode active material may be included in an amount of 80 wt % to 99 wt % based on the total weight of the negative electrode slurry composition.

[0080] The binder is a component that assists in binding between the conductive material and the active material or binding to the current collector, and is usually included in an amount of 0.1 to 20 wt % based on the total weight of the negative electrode slurry composition. Examples of such binders include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene-butylene rubber (SBR), lithium-substituted polyacrylate (Li-PAA), etc.

[0081] The conductive material is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity, and examples of the conductive material include carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. The conductive material may be added in an amount of 0.1 to 20 wt % based on the total weight of the negative electrode slurry composition.

[0082] The dispersion medium may include water or an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a suitable viscosity when the negative electrode slurry includes a negative electrode active material, and optionally a binder and a conductive material.

[0083] The method for coating the negative electrode slurry is not particularly limited as long as it is a method commonly used in the art, for example, a coating method using a slot die, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, etc. may also be used.

[0084] In yet another aspect of the present invention, there is provided a lithium secondary battery comprising the negative electrode, which may be fabricated by injecting a lithium salt-containing electrolyte into an electrode assembly including a positive electrode, the negative electrode, and a separator interposed therebetween.

[0085] The positive electrode may be prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent to prepare a slurry, and then directly coating the slurry on a metal current collector or by casting the slurry on a separate support and laminating the positive electrode active material film peeled off from the support onto a metal current collector.

[0086] The active material used in the positive electrode is LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , LiCoPO 4 , LiFePO 4 and LiNi 1-x-y-z Co x M1 y M2 z O 2(M1 and M2 are each independently any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are atomic fractions of oxide composition elements independent of each other, where 0 ≦ x < 0.5, 0 ≦ y < 0.5, 0 ≦ z < 0.5, and 0 < x + y + z < 1), and may include any one of the active material particles or a mixture of two or more of these.)

[0087] On the other hand, the conductive material, binder, and solvent may be used in the same manner as those used in the production of the negative electrode.)

[0088] The separator may be a conventional porous polymer film used as a separator, for example, a porous polymer film made of a polyolefin-based polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, and may be used alone or in a laminated form. Further, an insulating thin film having high ion permeability and mechanical strength may be used. The separator may include a stability-reinforced separator (SRS, safety reinforced separator) in which a ceramic substance is thinly coated on the surface of the separator. In addition, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used, but is not limited thereto.)

[0089] The electrolytic solution contains a lithium salt as an electrolyte and an organic solvent for dissolving the same.)

[0090] The lithium salt can be used without limitation as long as it is normally used in an electrolytic solution for a secondary battery. For example, as an anion of the lithium salt, 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 - 及び(CF 3 CF 2 SO 2 ) 2 N- One selected from the group consisting of:

[0091] The organic solvent contained in the electrolytic solution may be any commonly used organic solvent without any limitation, and typically, one or more selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran may be used.

[0092] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are highly viscous organic solvents having a high dielectric constant and are therefore preferred for use since they can easily dissociate lithium salts in the electrolyte. It is more preferred to mix such cyclic carbonates with linear carbonates having low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio to produce an electrolyte solution having high electrical conductivity.

[0093] Optionally, the electrolyte used in accordance with the present invention may further contain additives such as overcharge inhibitors that are typically included in electrolytes.

[0094] The lithium secondary battery according to an embodiment of the present invention may be completed by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, housing the electrode assembly in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then injecting an electrolyte. Alternatively, the electrode assemblies may be stacked, impregnated with an electrolyte, and the resulting product may be housed in a battery case and sealed to complete the lithium secondary battery.

[0095] According to one embodiment of the present invention, the lithium secondary battery may be of a stacked type, a wound type, a stacked / folded type, or a cable type.

[0096] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a plurality of battery cells. Preferred examples of the medium- to large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, and is particularly useful for hybrid electric vehicles and renewable energy storage batteries, which are areas requiring high output.

[0097] The present invention will be described in detail below with reference to specific examples. However, the examples according to the present invention can be modified into many other forms, and the scope of the present invention should not be construed as being limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0098] Example 1 (1) Manufacture of negative electrode active material Natural graphite having an average particle size (D50) of 11 μm as a graphite core was mixed with pitch, and the pitch was coated on the natural graphite, followed by a first heat treatment at 1,500° C. to form a first carbon coating layer surrounding the outside of the natural graphite. At this time, the content (coating amount) of the first carbon coating layer was 5 parts by weight based on 100 parts by weight of the graphite core. Thereafter, the result of the first heat treatment to form the first carbon coating layer was mixed with pitch, and the pitch was coated on the first carbon coating layer of the natural graphite on which the first carbon coating layer was formed, followed by a second heat treatment at 1,200° C. to form a second carbon coating layer surrounding the outside of the first carbon coating layer. At this time, the content (coating amount) of the second carbon coating layer was 5 parts by weight based on 100 parts by weight of the natural graphite as the graphite core. A negative active material was prepared in which the sum of the content of the first carbon coating layer and the content of the second carbon coating layer was 10 parts by weight based on 100 parts by weight of natural graphite as a graphite core.

[0099] (2) Manufacturing of negative electrodes The negative electrode active material particles, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in a weight ratio of 96:1:2:1, and water was added to prepare a negative electrode slurry.

[0100] The negative electrode slurry was applied to a copper foil (current collector) at 3.6 mAh / cm 2 The negative electrode slurry was applied to the current collector by a loading amount of 1000 g / m2. The negative electrode slurry was then applied to the current collector by rolling, and then vacuum-dried at about 130° C. for 8 hours to prepare a negative electrode.

[0101] Example 2 A negative electrode active material and a negative electrode were prepared in the same manner as in Example 1, except that the heat treatment temperatures for forming the first and second carbon coating layers were changed as shown in Table 1 below.

[0102] Example 3 A negative electrode active material and a negative electrode were prepared in the same manner as in Example 1, except that the heat treatment temperatures for forming the first and second carbon coating layers were changed as shown in Table 1 below.

[0103] Comparative Example 1 Natural graphite as a graphite core was mixed with pitch, and the pitch was coated on the natural graphite, followed by heat treatment at 1,200° C. to form a first carbon coating layer surrounding the outside of the natural graphite. At this time, the content (coating amount) of the first carbon coating layer was 5 parts by weight based on 100 parts by weight of the graphite core, and a negative electrode was prepared in the same manner as in Example 1, except that a second carbon coating layer was not formed.

[0104] Comparative Example 2 Natural graphite as a graphite core was mixed with pitch, and the pitch was coated on the natural graphite, followed by heat treatment at 1,200° C. to form a first carbon coating layer surrounding the outside of the natural graphite. At this time, the content (coating amount) of the first carbon coating layer was 10 parts by weight based on 100 parts by weight of the graphite core, and a negative electrode was prepared in the same manner as in Example 1, except that a second carbon coating layer was not formed.

[0105] Comparative Example 3 Natural graphite as a graphite core was mixed with pitch, and the pitch was coated on the natural graphite, followed by heat treatment at 1,500° C. to form a first carbon coating layer surrounding the outside of the natural graphite. At this time, the content (coating amount) of the first carbon coating layer was 10 parts by weight based on 100 parts by weight of the graphite core, and a negative electrode was prepared in the same manner as in Example 1, except that a second carbon coating layer was not formed.

[0106] Comparative Example 4 Natural graphite having an average particle size (D50) of 11 μm as a graphite core was mixed with pitch, and the pitch was coated on the natural graphite, and then a first heat treatment was performed at 1,200° C. to form a first carbon coating layer surrounding the outside of the natural graphite. At this time, the content (coating amount) of the first carbon coating layer was 5 parts by weight based on 100 parts by weight of the graphite core. Thereafter, in order to form a second carbon coating layer, the result of the first heat treatment to form the first carbon coating layer was mixed with pitch, and the first carbon coating layer of the result of the first heat treatment was coated with pitch, and then a second heat treatment was performed at 1,500° C. to form a second carbon coating layer. At this time, the content (coating amount) of the second carbon coating layer was 5 parts by weight based on 100 parts by weight of the graphite core. A negative electrode active material and a negative electrode were manufactured in the same manner as in Example 1, except that the first carbon coating layer and the second carbon coating layer were formed on the graphite core.

[0107] Comparative Example 5 A negative electrode active material and a negative electrode were produced in the same manner as in Example 1, except that the first heat treatment temperature was 1,350°C and the second heat treatment temperature was 1,450°C. The characteristics of the negative electrodes produced in the above-mentioned Examples 1 to 3 and Comparative Examples 1 to 5 are summarized in Table 1 below.

[0108] [Table 1]

[0109] Experimental Example 1: Evaluation of battery capacity and fast charging performance The negative electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were used to prepare batteries in the following manner.

[0110] 1.7671cm 2 A lithium (Li) metal thin film cut into a circular shape was used as the positive electrode. A porous polyethylene separator was interposed between the positive electrode and the negative electrode, and 0.5 wt % vinylene carbonate was dissolved in a mixed solution of ethyl methyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 7:3 to prepare a 1M LiPF 6 An electrolyte solution in which the above was dissolved was injected to prepare a coin-shaped lithium half cell.

[0111] The fabricated half cell was charged at CC (constant current) / CV (constant voltage) (current rate of 0.2C, 5mV, 0.005C current cutoff) for the first three cycles and discharged at CC to 1.0V. The negative electrode capacity and initial efficiency confirmed at this time are shown in Table 2. Then, the cell was charged at a current rate of 1.5C to 80% SOC, and the SOC at which lithium was deposited (SOC at which Li plating occurs) was confirmed and is shown in Table 2.

[0112] Experimental Example 2: Analysis of Raman Spectra Raman spectroscopy was performed to examine the degree of crystallinity of the first and second carbon coating layers of the negative active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 5. Raman spectroscopy was performed using a Renishaw 2000 Raman spectrometer system and 532 nm laser excitation. To avoid the thermal effect of the laser, measurements were taken using a 100x optical lens with a low laser power density and an exposure time of 30 seconds. To reduce deviations at each position, a total of 25 points were measured over a 5 μm x 5 μm area, and the average values ​​are shown in Table 2.

[0113] [Table 2] Referring to Table 2, the G band (1580 cm) of the negative electrode active materials prepared in Examples 1 to 3 was -1 FWHM value of the D band (peak around 1350cm -1 As a result of comparing the FWHM values ​​of the D bands of the second carbon coating layers heat-treated at 1,150 to 1,250°C and the first carbon coating layers heat-treated at 1,450 to 1,550°C, it can be seen that the degree of crystallinity is relatively smaller in the second carbon coating layers heat-treated at 1,150 to 1,250°C than in the first carbon coating layers heat-treated at 1,450 to 1,550°C. Specifically, it can be seen that the FWHM values ​​of the D bands of the second carbon coating layers of the negative electrode active materials prepared in Examples 1 to 3 are respectively about 1.66 times, 1.34 times, and 2.65 times larger than the FWHM value of the D band of the first carbon coating layer. As a result, it can be seen that the secondary batteries using the negative electrode active materials prepared in Examples 1 to 3 exhibit high initial efficiency and excellent fast charging characteristics.

[0114] Compared with Examples 1 to 3, the negative active material of Comparative Example 1 was coated with one carbon coating layer at 5 parts by weight at a time, resulting in a small total carbon coating amount and a decrease in fast charging performance.

[0115] In addition, the negative active materials of Comparative Examples 2 and 3 were prepared by coating 10 parts by weight with one carbon coating layer at a time, and the secondary batteries using such negative active materials did not have good fast charging performance. This is because coating a large amount of pitch (10 parts by weight) at a time caused the natural graphite particles corresponding to the graphite core to aggregate and increase the average particle size D50, resulting in an increase in the diffusion resistance of lithium ions within the natural graphite particles, which resulted in a decrease in fast charging performance.

[0116] In addition, Comparative Example 2 was coated with 10 parts by weight of pitch and then heat-treated at a relatively low temperature (1,200°C) compared to Comparative Example 3, resulting in the production of a relatively large amount of carbon coating layer with low crystallinity, and showed initial efficiency characteristics that were 1.1 percentage points lower than Comparative Example 3.

[0117] In the case of Comparative Example 3, the average particle size (D50) became larger for the same reason as in Comparative Example 2, resulting in a decrease in the fast charging performance. In addition, because the heat treatment temperature was 1,500°C, which was higher than in Comparative Example 2, the crystallinity of the generated carbon coating layer became higher, resulting in a decrease in the fast charging performance compared to Comparative Example 2.

[0118] In the case of Comparative Example 4, a first carbon coating layer of 5 parts by weight was formed by heat treatment at 1,200°C, and then a second carbon coating layer of 5 parts by weight was formed by heat treatment at 1,500°C. The first carbon coating layer that was initially coated was also heat-treated again at 1,500°C by heat treatment for forming the second carbon coating layer. As a result, the ratio of the FWHM value of the D band of the second carbon coating layer of the negative active material of Comparative Example 4 to the FWHM value of the D band of the first carbon coating layer was about 0.61. The negative active material of Comparative Example 4 had a further increased crystallinity of the outermost carbon coating layer (second carbon coating layer) compared to the negative active materials of Examples 1 to 3, and the secondary battery using the negative active material of Comparative Example 4 had a lower rapid charging performance than the secondary battery using the negative active materials of Examples 1 to 3. Meanwhile, the negative electrode active material of Comparative Example 4 has the same carbon coating layer content as Comparative Example 3, but the average particle size is relatively smaller due to the process of forming two layers. Therefore, when applied to a secondary battery, the fast charging performance is partially improved, but is still significantly lower than the secondary batteries using the negative electrode active materials of Examples 1 to 3.

[0119] In the negative electrode active material of Comparative Example 5, the ratio of the FWHM value of the D band of the second carbon coating layer to the FWHM value of the D band of the first carbon coating layer was about 0.86. The secondary battery using the negative electrode active material of Comparative Example 5 had significantly lower rapid charging performance than the secondary battery using the negative electrode active material of Examples 1 to 3 for the same reason as in Comparative Example 4. In the negative electrode active material of Comparative Example 5, the heat treatment temperature during the formation of the second carbon coating layer was lower than that of Comparative Example 4, and the crystallinity of the second carbon coating layer (outermost coating layer) was lower than that of the negative electrode active material of Comparative Example 4. Therefore, the rapid charging characteristics of the secondary battery using the negative electrode active material of Comparative Example 5 were somewhat improved compared to the secondary battery using the negative electrode active material of Comparative Example 4, but were still significantly lower than those of the secondary battery using the negative electrode active material of Examples 1 to 3.

Claims

1. A graphite core; a first carbon coating layer surrounding the outside of the graphite core; a second carbon coating layer surrounding the outside of the first carbon coating layer; the second carbon coating layer has a lower crystallinity than the first carbon coating layer; The content of the first carbon coating layer and the content of the second carbon coating layer are each independently 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

2. The negative electrode active material according to claim 1, wherein the negative electrode active material has an average particle size (D50) of 7 to 25 μm.

3. 2. The negative electrode active material according to claim 1, wherein the graphite core has an average particle size (D50) of 5 to 20 μm.

4. 2 . The negative electrode active material according to claim 1 , wherein the FWHM value of the D band of the second carbon coating layer is 1.3 times or more the FWHM value of the D band of the first carbon coating layer.

5. 2. The negative electrode active material according to claim 1, wherein the FWHM value of the D band of the second carbon coating layer is 1.3 to 3 times the FWHM value of the D band of the first carbon coating layer.

6. A method for producing the negative electrode active material according to any one of claims 1 to 5, mixing the graphite with a first carbon precursor and subjecting the mixture to a first heat treatment at a temperature of 1,400 to 1,600° C. to convert the graphite into a graphite core and forming a first carbon coating layer surrounding the graphite core; and mixing a result of the step of forming the first carbon coating layer with a second carbon precursor, and performing a second heat treatment at a temperature of 1,100 to 1,300° C. to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

7. 7. The method of claim 6, wherein the first carbon coating layer and the second carbon coating layer of the negative active material are formed so that their contents are each independently 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

8. A negative electrode including a current collector and a negative electrode active material layer located on at least one surface of the current collector, The negative electrode active material layer comprises the negative electrode active material according to claim 1 .

9. A lithium secondary battery comprising the negative electrode according to claim 8.

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