Negative electrode active material, method for preparing negative electrode active material, negative electrode containing the same, and lithium secondary battery

Abstract

The present invention discloses a negative electrode active material comprising: a graphite core; a first carbon coating layer surrounding the graphite core; and a second carbon coating layer surrounding the first carbon coating layer, wherein the second carbon coating layer has a lower degree of 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. The present invention also discloses a negative electrode comprising the negative electrode active material and a lithium secondary battery comprising the negative electrode.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material, a method for preparing the negative electrode active material, a negative electrode containing the negative electrode active material, and a lithium secondary battery. More specifically, the present invention relates to a negative electrode active material exhibiting high initial efficiency and excellent rapid charging characteristics, a method for preparing the negative electrode active material, a negative electrode containing the negative electrode active material, and a lithium secondary battery.

[0002] This application claims priority from Korean Patent Application No. 10-2019-0121071 filed in Korea on September 30, 2019, the disclosure of which is incorporated herein by reference. Background Art

[0003] With the development of technology and the increase in demand for mobile devices, the demand for secondary batteries as the energy source of these mobile devices is increasing. Among these secondary batteries, lithium secondary batteries with high energy density and operating voltage, long cycle life and low discharge rate have been commercialized and widely used.

[0004] A lithium secondary battery has a structure comprising an electrode assembly and an electrolyte containing a lithium salt injected into the electrode assembly. The electrode assembly comprises a positive electrode, a negative electrode, and a porous separator disposed between the two electrodes. The positive electrode and the negative electrode each comprise an active material coated on an electrode current collector. The electrodes are obtained by applying a slurry comprising the active material, a binder, and a conductive material dispersed in a solvent to the current collector, followed by drying and rolling.

[0005] Until now, lithium metal has been used as the negative electrode of secondary batteries. However, because lithium metal is known to cause battery short circuits due to the formation of lithium dendrites, which poses a risk of explosion, it has been replaced by carbonaceous compounds that can reversibly intercalate and deintercalate lithium ions while maintaining structural and electrical properties.

[0006] This carbonaceous compound has a significantly low discharge potential of about -3 V based on the standard hydrogen electrode potential and exhibits excellent electrode cycle life due to a significantly reversible charge / discharge behavior derived from the uniaxial alignment of the graphene layer. In addition, the carbonaceous compound exhibits a relatively low Li / Li ratio when Li ions are intercalated. + The electrode potential is 0 V, which is similar to the electrode potential of pure lithium metal. Therefore, when lithium metal is combined with an oxide-based positive electrode to form a battery, higher energy can be obtained.

[0007] Natural graphite, conventionally used as a negative electrode, has high capacity per unit weight but exhibits increased alignment when the electrode is rolled, causing deterioration in lithium ion insertion / extraction characteristics, undesirably leading to deterioration in the fast charge characteristics of the battery. Summary of the Invention

[0008] Technical issues

[0009] The present invention aims to solve the problems of the related art, and thus the present invention aims to provide a negative electrode active material having high initial efficiency and excellent fast charging characteristics, and a method for preparing the negative electrode active material.

[0010] The present invention also provides a negative electrode comprising the negative electrode active material and a lithium secondary battery comprising the negative electrode.

[0011] Technical Solution

[0012] In one aspect of the present invention, a negative electrode active material according to any one of the following embodiments is provided.

[0013] According to a first embodiment of the present invention, there is provided a negative electrode active material, the negative electrode active material comprising:

[0014] Graphite core;

[0015] a first carbon coating surrounding an outside of the graphite core; and

[0016] a second carbon coating layer surrounding an outer side of the first carbon coating layer,

[0017] wherein the second carbon coating has a lower crystallinity than the first carbon coating, or

[0018] The second carbon coating layer includes hard carbon, and the first carbon coating layer includes soft carbon.

[0019] According to a second embodiment of the present invention, there is provided the negative electrode active material as described in the first embodiment, wherein the content of the first carbon coating layer and the content of the second carbon coating layer are each 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

[0020] According to a third embodiment of the present invention, there is provided the negative electrode active material as described in the first or second embodiment, which has an average particle size (D 50 ).

[0021] According to a fourth embodiment of the present invention, there is provided the negative electrode active material as described in any one of the first to third embodiments, wherein the graphite core has an average particle size (D 50 ).

[0022] According to a fifth embodiment of the present invention, a negative electrode active material as described in any one of the first to fourth embodiments is provided, wherein the FWHM (full width at half maximum) value of the D band of the second carbon coating layer corresponds to 1.3 times or more of the FWHM value of the D band of the first carbon coating layer.

[0023] According to a sixth embodiment of the present invention, a negative electrode active material as described in any one of the first to fifth embodiments is provided, wherein the FWHM (full width at half maximum) value of the D band of the second carbon coating layer corresponds to 1.3 to 3 times the FWHM value of the D band of the first carbon coating layer.

[0024] In another aspect of the present invention, a method for manufacturing a negative electrode active material according to any one of the following embodiments is provided.

[0025] According to a seventh embodiment of the present invention, there is provided a method for manufacturing the negative electrode active material according to the first embodiment, the method comprising the following steps:

[0026] mixing graphite with a first carbon precursor and subjecting the obtained mixture to a first heat treatment at a temperature of 1400° C. to 1600° C. to obtain a product including graphite as a graphite core and a first carbon coating layer surrounding the outside of the graphite core; and

[0027] The product of the aforementioned step is mixed with a second carbon precursor and the obtained mixture is subjected to a second heat treatment at a temperature of 1100° C. to 1300° C. to form a second carbon coating layer surrounding the outer side of the first carbon coating layer.

[0028] According to an eighth embodiment of the present invention, there is provided a method for manufacturing a negative electrode active material as described in the seventh embodiment, wherein the first carbon coating layer and the second carbon coating layer are formed in such a manner that the content of the first carbon coating layer and the content of the second carbon coating layer can each be 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

[0029] In still another aspect of the present invention, a negative electrode according to the following embodiment is provided.

[0030] According to a ninth embodiment of the present invention, there is provided a negative electrode including a current collector and a negative electrode active material layer provided on at least one surface of the current collector, wherein the negative electrode active material layer includes the negative electrode active material according to any one of the first to sixth embodiments.

[0031] In still another aspect of the present invention, a lithium secondary battery according to the following embodiment is provided.

[0032] According to a tenth embodiment of the present invention, there is provided a lithium secondary battery including the negative electrode according to the ninth embodiment.

[0033] Beneficial effects

[0034] According to one embodiment of the present invention, a negative electrode active material comprising natural graphite is provided, wherein the natural graphite is coated with a double carbon coating layer to increase the carbon coating amount of the natural graphite, wherein the double carbon coating layer has a controllable crystallinity. When the negative electrode active material is applied to the negative electrode of a secondary battery, a secondary battery having high initial efficiency and excellent fast charging characteristics can be provided. DETAILED DESCRIPTION

[0035] Hereinafter, preferred embodiments of the present invention will be described in detail. Before the description, it should be understood that the terms used in the specification and the appended claims should not be interpreted as limited to the general and dictionary meanings, but should be interpreted according to the meanings and concepts corresponding to the technical aspects of the present invention on the basis of the principle that the inventor appropriately defines the terms to obtain the best interpretation.

[0036] In one aspect of the present invention, there is provided a negative electrode active material, the negative electrode active material comprising:

[0037] Graphite core;

[0038] a first carbon coating surrounding an outside of the graphite core; and

[0039] a second carbon coating layer surrounding an outer side of the first carbon coating layer,

[0040] wherein the second carbon coating has a lower crystallinity than the first carbon coating, or

[0041] The second carbon coating layer includes hard carbon, and the first carbon coating layer includes soft carbon.

[0042] The graphite core may be artificial graphite, natural graphite, or a combination thereof. In other words, the graphite core may include crystalline graphite. Because natural graphite generally exhibits higher capacity than artificial graphite, using natural graphite as the graphite core is advantageous in terms of capacity.

[0043] The shape of the graphite core is not particularly limited, but may be spherical. Spherical graphite may be obtained by subjecting the graphite core used as a raw material to a spheroidization process generally known to those skilled in the art. For example, spherical graphite may be obtained by subjecting the raw material to a mechanical treatment such as impact compression, friction, or shearing force, so that the particles forming the graphite core are bent or mixed and the corners of the particles are cut off. Mechanical treatment can be carried out by using spheroidization devices generally known to those skilled in the art, and specific examples of these devices include: mills such as a countercurrent jet mill (Hosokawa Micron, JP), an ACM pulverizer (Hosokawa Micron, JP) or a current jet (Nissin, JP); particle assemblers such as SARARA (Kawasaki Heavy Industries, Ltd., JP), GRANUREX (Freund Corporation, JP), New-Gra Machine (Seishin, JP) or Acromaster (Kosokawa Micron, JP); kneaders such as a dispersion kneader or a twin roll; compression shear processing devices such as a mechanical microsystem, an extruder, a ball mill, a planetary mill, a mechanical fusion system, Nobilta, a hybrid or rotary ball mill, etc.

[0044] The graphite core may have an average particle size (D 50 When the average particle size of the graphite core is within the above range, the first carbon coating layer can be sufficiently and uniformly formed on the surface of the graphite core. When the content of the graphite core falls within the above range, a lithium secondary battery using a negative electrode active material including the graphite core can have excellent output and cycle characteristics.

[0045] The negative electrode active material according to the present invention includes a first carbon coating layer surrounding the outside of a graphite core, and further includes a second carbon coating layer sequentially surrounding the outside of the first carbon coating layer.

[0046] Generally, when the coating amount of amorphous carbon, which has a larger interplanar spacing and lower crystallinity (degree of crystallinity) than graphite, increases, the fast charging characteristics are enhanced. However, when the coating amount of carbon is too large, the coated carbon powder agglomerates, making it difficult to break the negative electrode active material particles during the preparation process of the negative electrode active material, resulting in an average particle size (D 50 This makes it difficult to carry out the coating process during the preparation of the negative electrode active material and causes problems such as degradation of rate characteristics and degradation of the capacity of the electrode at the optimal density.

[0047] On the other hand, when a single carbon coating layer is formed at once in such a manner that the coating amount of the single carbon coating layer can correspond to the total content of the content (coating amount) of the first carbon coating layer and the content (coating amount) of the second carbon coating layer, the material forming the carbon coating layer may cause an agglomeration problem.

[0048] In order to solve the above problems, the negative electrode active material according to the present invention includes a double carbon coating layer including a first carbon coating layer surrounding the outside of a graphite core and a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0049] Here, the first carbon coating and the second carbon coating can be selected in such a way that they can differ from one another in terms of crystallinity or composition.

[0050] 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).

[0051] The first carbon coating layer and the second carbon coating layer can be formed by mixing an amorphous carbonaceous material corresponding to a carbon precursor with a material to be coated and firing the resulting mixture. In particular, the first carbon coating layer can be formed by mixing graphite with a first carbon precursor and heat-treating the mixture so that the first carbon coating layer surrounds the outside of the graphite core (first heat treatment), and the second carbon coating layer can be formed by mixing the graphite core surrounded by the first carbon coating layer with a second carbon precursor and heat-treating the mixture so that the second carbon coating layer surrounds the outside of the first carbon coating layer (second heat treatment).

[0052] For example, the amorphous carbonaceous material can be obtained from at least one amorphous carbon precursor selected from the following: a hard carbon material including sucrose, a phenolic resin, a naphthalene resin, a polyvinyl alcohol resin, a furfuryl alcohol resin, a polyacrylonitrile resin, a polyamide resin, a furan resin, a cellulose resin, a styrene resin, a polyimide resin, an epoxy resin, or a vinyl chloride resin; and a soft carbon material including coal tar, petroleum pitch, polyvinyl chloride, mesophase pitch, tar, or heavy oil. However, the scope of the present invention is not limited thereto.

[0053] The crystallinity of each of the first carbon coating layer and the second carbon coating layer can be determined by comparing the FWHM (full width at half maximum) values ​​of the G band and the D band according to the Raman spectrum.

[0054] Raman spectroscopy is a method for analyzing the structures of the first and second carbon coating layers, wherein the Raman spectrum is obtained by spectroscopy at a wave number of 1580 cm -1 The peak appearing in the vicinity is called the G band, which represents the sp 2 The peak of the bond and corresponds to the carbon crystal without structural defects. On the other hand, the peak at wave number 1360 cm in the Raman spectrum -1 The peak appearing in the nearby area is called D band, which represents the sp3 The peak of the bond and when the sp 2 The atomic bonds forming the bond break and transform into sp 3 When any disorder or defect is created in the carbon coating, this D band increases.

[0055] According to the present invention, the G band of the Raman spectrum of the carbon coating may be present at a wave number of 1550 cm -1 to 1620cm -1 The peak in the region, and the D band may be present at wave number 1330 cm -1 to 1370cm -1 The wavenumber range of the G band and the D band corresponds to the region where a shift may occur depending on the laser source used for Raman spectroscopy. Although there is no particular limitation on the Raman values ​​used herein, the values ​​can be determined using a DXR Raman microscope (Thermo Electron Scientific Instruments LLC) at a wavelength of 532 nm.

[0056] According to one embodiment of the present invention, the FWHM value of the D band of the second carbon coating layer may correspond to 1.3 times or more, particularly 1.3 to 3 times, more particularly 1.3 to 2.7 times, and even more particularly 1.3 to 2 times the FWHM value of the D band of the first carbon coating layer. 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 the above range, more defects are generated in the second carbon coating layer, and the second carbon coating layer has a lower degree of crystallinity (crystallization) than the first carbon coating layer.

[0057] Regarding carbon coating materials, the interplanar spacing between the carbon layers of carbon materials with high crystallinity is small, while the interplanar spacing between the carbon layers of amorphous carbon materials with low crystallinity is large. When charging a secondary battery using such a carbon material as a negative electrode active material, in the case of a carbon material with high crystallinity, such as graphite, the interplanar spacing between the carbon layers is small, and lithium ions find it difficult to quickly penetrate from the electrolyte into the gaps between the graphene layers. On the other hand, in the case of an amorphous carbon material with lower crystallinity than graphite, lithium ions can easily penetrate into the gaps due to the large interplanar spacing between the carbon layers, resulting in an increase in the rate at which lithium ions are embedded in the carbon layers.

[0058] The negative electrode active material according to the present invention is designed so that the crystallinity of the carbon material gradually increases in the order of the outermost layer (the second carbon coating layer), the first carbon coating layer, and the centrally located graphite core. In other words, according to the present invention, the second carbon coating layer, which initially encounters 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 the graphite core, which has the highest crystallinity. In this way, lithium ions can easily penetrate the negative electrode active material in the initial stage and then can be quickly embedded in the carbon layers, resulting in excellent fast charging characteristics.

[0059] In the second type, the first carbon coating layer comprises soft carbon, and the second carbon coating layer comprises hard carbon.

[0060] Soft carbon (graphitizable carbon) can be prepared by heating by-products produced during crude oil refining, such as coke, needle coke, coal tar pitch, petroleum pitch, or mixtures thereof, to 1000°C.

[0061] The hard carbon (non-graphitizable carbon) may include a carbonization product of a carbonaceous material selected from the group consisting of sucrose, phenolic resin, furan resin, furfuryl alcohol, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, or a mixture thereof.

[0062] In the case of hard carbon, the carbon layers are strongly entangled with each other, the crystal grains are very small, and the degree of structural disorder in the precursor is high. Therefore, even by using high temperatures of 2500°C or above, it is difficult to rearrange the crystal structure for the purpose of graphitization. On the other hand, in the case of graphitizable soft carbon, the graphite layer planes form a structure in which they are arranged parallel to each other, thereby allowing crystallization and graphitization to proceed easily.

[0063] In the second type of negative electrode active material, the first carbon coating layer, which is disposed internally and in contact with the graphite core, comprises soft carbon, while the outermost layer of the negative electrode active material, the second carbon coating layer, comprises hard carbon. As described above in connection with the first type, in the case of the second type of negative electrode active material, the second carbon coating layer, which initially encounters the electrolyte, exhibits very small grain sizes and contains hard carbon with a high degree of structural disorder in the precursor. Therefore, in the case of this hard carbon, the interplanar spacing between the carbon layers is relatively large, thereby facilitating the penetration of lithium ions contained in the electrolyte. On the other hand, the first carbon coating layer comprises graphene layer planes arranged relatively parallel to each other and exhibits a small interplanar spacing between the carbon layers. Therefore, compared to the second carbon coating layer, it is difficult for the liquid electrolyte to immediately penetrate into the gaps between the graphene layers. As a result, when the second carbon coating layer that initially encounters the electrolyte contains hard carbon with a high degree of structural disorder, followed by the first carbon coating layer (the first carbon coating layer contains soft carbon in which the planes of the graphene layers are arranged relatively parallel to each other compared to the second carbon coating layer) and the graphite core (in which the graphene layers are regularly and very closely stacked with each other), the lithium ions originally contained in the electrolyte can easily penetrate into the negative electrode active material and can be quickly embedded in the carbon layer, thereby obtaining excellent fast charging characteristics.

[0064] According to one embodiment of the present invention, based on 100 parts by weight of the graphite core, the content of the first carbon coating layer and the content of the second carbon coating layer may be each independently 3 parts by weight to 6 parts by weight, or 4 parts by weight to 5 parts by weight.

[0065] When the content of the first carbon coating layer and the content of the second carbon coating layer meet the above ranges, they can fully cover the graphite core to prevent the graphite core from directly contacting the electrolyte. In addition, because the surface of the active material is coated with an appropriate amount of amorphous carbon with low crystallinity, the output and fast charging characteristics can be improved. It is also possible to prevent the problem of deterioration of the charge / discharge characteristics of lithium secondary batteries containing negative electrode active materials with excessively high carbon coating contents, or to prevent the absolute amount of space in which lithium can be inserted and the capacity of lithium secondary batteries from decreasing.

[0066] The negative electrode active material may have an average particle size (D 50 When the average particle size of the negative electrode active material (D 50 ) satisfies the above range, slurry formation processability can be improved due to easy handling, such as mixing, excellent fast charging characteristics can be achieved, and a decrease in electrode capacity can be prevented.

[0067] In another aspect of the present invention, there is provided a method for manufacturing a negative electrode active material, the method comprising the steps of:

[0068] mixing graphite with a first carbon precursor and subjecting the obtained mixture to a first heat treatment at a temperature of 1400° C. to 1600° C. to obtain a product including graphite as a graphite core and a first carbon coating layer surrounding the outside of the graphite core; and

[0069] The product of the aforementioned step is mixed with a second carbon precursor and the obtained mixture is subjected to a second heat treatment at a temperature of 1100° C. to 1300° C. to form a second carbon coating layer surrounding the outer side of the first carbon coating layer.

[0070] First, graphite is mixed with a first carbon precursor and the resulting product is subjected to a first heat treatment at a temperature of 1400° C. to 1600° C. to obtain a product including graphite as a graphite core and a first carbon coating layer surrounding the outside of the graphite core.

[0071] The graphite may be artificial graphite, natural graphite or a combination thereof.

[0072] The first carbon precursor may include an amorphous carbonaceous material. For example, the first carbon precursor may be obtained from at least one amorphous carbon precursor selected from the following: a hard carbon material including sucrose, a phenolic resin, a naphthalene resin, a polyvinyl alcohol resin, a furfuryl alcohol resin, a polyacrylonitrile resin, a polyamide resin, a furan resin, a cellulose resin, a styrene resin, a polyimide resin, an epoxy resin, or a vinyl chloride resin; and a soft carbon material including coal tar, petroleum pitch, polyvinyl chloride, mesophase pitch, tar, or heavy oil. However, the scope of the present invention is not limited thereto.

[0073] There is no particular limitation on the method for mixing the graphite with the first carbon precursor, and any method generally known to those skilled in the art may be used. For example, mixing may be performed using mechanical and chemical processes, for example, using a kneader such as a twin-roll, blade, mechanical microsystem, extruder, ball mill, planetary mill, mechanical-fusion system, Nobilta, mixing or rotary ball mill; or a spray drying process or an emulsification process.

[0074] The first heat treatment temperature may be 1400° C. to 1600° C. According to one embodiment of the present invention, the first heat treatment temperature may be 1450° C. to 1550° C. When the first heat treatment temperature satisfies the above range, the graphite core can maintain its micropores and the amorphous carbonaceous precursor can be fully carbonized.

[0075] Next, the product of the step of forming the first carbon coating layer is mixed with a second carbon precursor, and the obtained mixture is subjected to a second heat treatment at a temperature of 1100° C. to 1300° C. to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0076] The second carbon precursor may include an amorphous carbonaceous material in the same manner as the first carbon precursor. Here, the first carbon precursor and the second carbon precursor may include the same kind of amorphous carbonaceous material or different kinds of amorphous carbonaceous materials.

[0077] In addition, the method of mixing the product in the step of forming the first carbon coating layer with the second carbon precursor may be selected from the above-mentioned method of mixing graphite with the first carbon precursor.

[0078] The second heat treatment may be performed at a temperature of 1100° C. to 1300° C. According to one embodiment of the present invention, the second heat treatment may be performed at a temperature of 1150° C. to 1250° C.

[0079] When a single carbon coating layer is formed at once so that the coating amount of the single carbon coating layer corresponds to the total content (coating amount) of the first carbon coating layer and the second carbon coating layer, the materials forming the carbon coating layer may cause agglomeration. Therefore, according to one embodiment of the present invention, the first and second carbon coating layers are formed separately to prevent the above problem.

[0080] Furthermore, even if the first and second carbon coating layers are formed separately, the negative electrode active material may undesirably exhibit low initial efficiency when the heat treatment temperature of each step is 1100° C. to 1300° C., corresponding to the second heat treatment temperature. Furthermore, the negative electrode active material may undesirably exhibit poor rapid charge characteristics when the heat treatment temperature of each step is 1400° C. to 1600° C., corresponding to the first heat treatment temperature.

[0081] In one embodiment, a method for preparing a negative electrode active material is provided, the method comprising the steps of:

[0082] mixing graphite with a first carbon precursor and subjecting the obtained mixture to a first heat treatment to obtain a product including graphite as a graphite core and a first carbon coating layer surrounding the outside of the graphite core; and

[0083] The product of the above step is mixed with a second carbon precursor and the obtained mixture is subjected to a second heat treatment to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0084] The first carbon precursor is a material that is converted into soft carbon after heat treatment, and the second carbon precursor is a material that is converted into hard carbon after heat treatment.

[0085] First, graphite is mixed with a first carbon precursor and the obtained mixture is subjected to a first heat treatment to obtain a product including graphite as a graphite core and a first carbon coating layer surrounding the outside of the graphite core.

[0086] There is no particular limitation on the first carbon precursor as long as it can be converted into soft carbon after heat treatment. Specific examples of the first carbon precursor include coke, needle coke, coal tar pitch, petroleum pitch, or a mixture thereof.

[0087] The method of mixing graphite with the first carbon precursor may be selected from the above-mentioned methods of mixing graphite with the first carbon precursor.

[0088] The first heat treatment temperature may be 1400° C. to 1600° C. According to one embodiment of the present invention, the first heat treatment temperature may be 1450° C. to 1550° C. When the first heat treatment temperature satisfies the above range, the graphite core can maintain its micropores and the amorphous carbonaceous precursor can be fully carbonized.

[0089] Next, the product of the aforementioned step is mixed with a second carbon precursor, and the obtained mixture is subjected to a second heat treatment to form a second carbon coating layer surrounding the outside of the first carbon coating layer.

[0090] The second carbon precursor is not particularly limited as long as it can be converted into hard carbon after heat treatment. Specific examples of the second carbon precursor include sucrose, phenolic resin, furan resin, furfuryl alcohol, polyacrylonitrile, polyimide, epoxy resin, cellulose, styrene, or mixtures thereof.

[0091] The method of mixing the product of the aforementioned step with the second carbon precursor may be selected from the above-mentioned method of mixing graphite with the first carbon precursor.

[0092] Here, the second heat treatment may be performed at a temperature of 1100° C. to 1300° C. According to one embodiment of the present invention, the second heat treatment may be performed at a temperature of 1150° C. to 1250° C.

[0093] According to one embodiment of the present invention, the first carbon coating layer and the second carbon coating layer are formed in such a manner that the content of the first carbon coating layer and the content of the second carbon coating layer may be each independently 3 to 6 parts by weight, or 4 to 5 parts by weight, based on 100 parts by weight of the graphite core.

[0094] When the content of the first carbon coating layer and the content of the second carbon coating layer meet the above ranges, they can fully cover the graphite core to prevent the graphite core from directly contacting the electrolyte. In addition, because the surface of the active material is coated with an appropriate amount of amorphous carbon with low crystallinity, the output and fast charging characteristics can be improved. It is also possible to prevent the problem of deterioration of the charge / discharge characteristics of lithium secondary batteries containing negative electrode active materials with excessively high carbon coating contents, or to prevent the absolute amount of space in which lithium can be inserted and the capacity of lithium secondary batteries from decreasing.

[0095] In yet another aspect of the present invention, a negative electrode including the negative electrode active material is provided.

[0096] Specifically, a negative electrode according to an embodiment of the present invention includes: a current collector; and a negative electrode active material layer provided on at least one side of the current collector and including the negative electrode active material according to an embodiment of the present invention.

[0097] The electrode layer may be formed by coating a slurry for a negative active material layer prepared by dispersing the negative active material according to the present invention, a binder, and a conductive material in a solvent on at least one side of a current collector, followed by drying and rolling.

[0098] There are no particular limitations on the current collector, as long as it is conductive and does not cause any chemical changes in the corresponding battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like; and aluminum-cadmium alloys can be used. While there are no particular limitations on the thickness of the current collector, it can be between 3 μm and 500 μm.

[0099] The negative electrode active material may be used in an amount of 80 wt % to 99 wt % based on the total weight of the negative electrode slurry composition.

[0100] The binder is a component that helps to bond between the conductive material and the active material or to the current collector, and is generally used in an amount of 0.1% to 20% by weight based on the total weight of the negative electrode slurry composition. Specific examples of the binder include polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylate, styrene-butadiene rubber (SBR), lithium polyacrylate (Li-PAA), etc.

[0101] There are no particular limitations on the conductive material, as long as it does not cause chemical changes in the corresponding battery and has conductivity. Specific examples of the conductive material include: carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; fluorocarbons; metal powders such as aluminum or nickel powders; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. The conductive material can be added in an amount of 0.1% to 20% by weight based on the total weight of the negative electrode slurry composition.

[0102] The dispersion medium may include water or an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that the negative electrode slurry containing the negative electrode active material and optionally a binder and a conductive material may have a desired level of viscosity.

[0103] In addition, there is no particular limitation on the coating method of the negative electrode slurry, as long as it is a method currently used in the art. For example, a coating method using a slit die can be used. In addition, a Meyer bar coating method, a gravure coating method, a dip coating method, a spray coating method, etc. can also be used.

[0104] In another aspect of the present invention, a lithium secondary battery comprising the negative electrode is provided. In particular, the lithium secondary battery can be obtained by injecting a lithium salt-containing electrolyte into an electrode assembly comprising a positive electrode, the negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0105] The positive electrode can be obtained by mixing a positive electrode active material, a conductive material, a binder and a solvent to form a slurry and applying the slurry directly to a metal current collector; or by casting the slurry onto a separate support, peeling the positive electrode active material film from the support and laminating the film on a metal current collector.

[0106] The active material used in the positive electrode can be any one active material particle selected from the following: LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4 and LiNi 1-x-y-z Co x M1 y M2 z O2 (wherein M1 and M2 each independently represent 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 each independently represent an atomic ratio of an element forming an oxide, and 0≤x<0.5, 0≤y<0.5, 0≤z<0.5, and 0 <x+y+z≤1)。

[0107] On the other hand, the same conductive material, binder, and solvent as those used to produce the negative electrode can be used.

[0108] Separator can be conventional porous polymer film used as separator.For example, porous polymer film can be a porous polymer film made of polyolefin polymer, and the polyolefin polymer is such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene / hexene copolymer or ethylene / methacrylate copolymer.This porous polymer film can be used alone or in the form of laminate.In addition, insulating film with high ion permeability and mechanical strength can be used.Separator can include safety enhanced separator (SRS), and the safety enhanced separator (SRS) includes ceramic material coated on separator surface with little thickness.In addition, conventional porous nonwoven fabrics such as nonwoven fabrics made of high melting point glass fiber or polyethylene terephthalate fiber can be used, but the scope of the present invention is not limited thereto.

[0109] The electrolyte contains a lithium salt as an electrolyte salt and an organic solvent for dissolving the lithium salt.

[0110] Any lithium salt conventionally used for electrolytes for secondary batteries can be used without particular limitation. For example, the anion of the lithium salt can be any one selected from the following: - 、Cl - Br - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - .

[0111] The organic solvent contained in the electrolyte may be any conventionally used organic solvent without particular limitation. Typical examples of the organic solvent include at least one selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran.

[0112] Especially, in carbonate organic solvent, ethylene carbonate and propylene carbonate as cyclic carbonate are organic solvents with high viscosity and high dielectric constant, thus can preferably be used, because they can be easily dissociated in electrolyte with lithium salts.When using described cyclic carbonate after this cyclic carbonate is mixed with straight-chain carbonate such as dimethyl carbonate or diethyl carbonate with low viscosity and low dielectric constant, can prepare the higher electrolyte of conductivity, this is preferred.

[0113] Optionally, the electrolyte used according to the present invention may further contain additives contained in conventional electrolytes such as an overcharge preventer and the like.

[0114] A lithium secondary battery according to one embodiment of the present invention can be obtained by inserting a separator between a positive electrode and a negative electrode to form an electrode assembly, introducing the electrode assembly into a pouch-shaped battery case, a cylindrical battery case, or a prismatic battery case, and then injecting an electrolyte therein. In one variation, a lithium secondary battery can be obtained by stacking the electrode assemblies, impregnating the stack with an electrolyte, and introducing the resulting product into a battery case, which is then sealed.

[0115] According to one embodiment of the present invention, the lithium secondary battery may be a stacked type battery, a wound type battery, a stack-folded type battery, or a cable type battery.

[0116] The lithium secondary battery according to the present invention can be used as a battery cell for a power source of a small device, and can preferably be used as a unit cell of a medium or large battery module containing a plurality of battery cells. Specific examples of such medium or large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, and the like. In particular, the lithium secondary battery can be used for batteries of hybrid electric vehicles and new renewable energy storage batteries that require high output.

[0117] Hereinafter, the embodiments will be described more fully so that the present invention can be easily understood. However, the following embodiments can be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth herein. On the contrary, these exemplary embodiments are provided to make the present invention thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0118] Example 1

[0119] (1) Preparation of negative electrode active materials

[0120] The average particle size (D 50) is mixed with asphalt as a natural graphite core having a diameter of 11 μm so that the natural graphite can be coated with asphalt. Next, the obtained product is subjected to a first heat treatment at 1500°C to form a first coating layer around the outside of the natural graphite. Here, based on 100 parts by weight of the graphite core, the content (coating amount) of the first carbon coating layer is 5 parts by weight. Then, the product of the first heat treatment with the first carbon coating layer is mixed with asphalt so that the first carbon coating layer formed on the natural graphite can be coated with asphalt. Thereafter, the obtained product is subjected to a second heat treatment at 1200°C to form a second carbon coating layer around the outside of the first carbon coating layer. Here, based on 100 parts by weight of the graphite core, i.e., natural graphite, the content (coating amount) of the second carbon coating layer is 5 parts by weight. In this way, a negative electrode active material is obtained, wherein the total content of the first carbon coating layer and the second carbon coating layer is 10 parts by weight based on 100 parts by weight of the graphite core, i.e., natural graphite.

[0121] (2) Manufacturing of negative electrode

[0122] The negative electrode active material particles obtained as described above, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed at a weight ratio of 96:1:2:1, and water was added thereto to prepare a negative electrode slurry.

[0123] The prepared slurry was heated at 3.6 mAh / cm 2 The negative electrode slurry was coated on a copper foil (current collector) in an amount of 1000 nm. The current collector coated with the negative electrode slurry was then rolled and vacuum dried at a temperature of about 130° C. for 8 hours to obtain a negative electrode.

[0124] Example 2

[0125] A negative electrode active material and a negative electrode were obtained in the same manner as in Example 1, except that the heat treatment temperature for forming each of the first carbon coating layer and the second carbon coating layer was changed as shown in Table 1 below.

[0126] Example 3

[0127] A negative electrode active material and a negative electrode were obtained in the same manner as in Example 1, except that the heat treatment temperature for forming each of the first carbon coating layer and the second carbon coating layer was changed as shown in Table 1 below.

[0128] Comparative Example 1

[0129] A negative electrode active material and a negative electrode were obtained in the same manner as in Example 1, except that natural graphite as a graphite core was mixed with asphalt so that the natural graphite could be coated with asphalt, and the obtained product was heat-treated at 1200°C to form a first carbon coating layer surrounding the outside of the natural graphite, wherein 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 no second carbon coating layer was formed.

[0130] Comparative Example 2

[0131] A negative electrode active material and a negative electrode were obtained in the same manner as in Example 1, except that natural graphite as a graphite core was mixed with asphalt so that the natural graphite could be coated with asphalt, and the obtained product was heat-treated at 1200°C to form a first carbon coating layer surrounding the outside of the natural graphite, wherein 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 no second carbon coating layer was formed.

[0132] Comparative Example 3

[0133] A negative electrode active material and a negative electrode were obtained in the same manner as in Example 1, except that natural graphite as a graphite core was mixed with asphalt so that the natural graphite could be coated with asphalt, and the obtained product was heat-treated at 1500°C to form a first carbon coating layer surrounding the outside of the natural graphite, wherein 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 no second carbon coating layer was formed.

[0134] Comparative Example 4

[0135] The average particle size (D 50 ) is mixed with asphalt as a graphite core with a thickness of 11 μm so that the natural graphite can be coated with asphalt. Next, the obtained product is subjected to a first heat treatment at 1200°C to form a first coating around the outside of the natural graphite. Here, based on 100 parts by weight of the graphite core, the content (coating amount) of the first carbon coating is 5 parts by weight. Then, the product of the first heat treatment with the first carbon coating is mixed with asphalt so that the first carbon coating formed on the natural graphite can be coated with asphalt. Thereafter, the obtained product is subjected to a second heat treatment at 1500°C to form a second carbon coating around the outside of the first carbon coating. Here, based on 100 parts by weight of the graphite core, the content (coating amount) of the second carbon coating is 5 parts by weight. In other words, a negative electrode active material and a negative electrode are obtained in the same manner as in Example 1, except that the first carbon coating and the second carbon coating are formed on the graphite core as described above.

[0136] Comparative Example 5

[0137] A negative electrode active material and a negative electrode were obtained in the same manner as in Example 1, except that the first heat treatment temperature was 1350°C and the second heat treatment temperature was 1450°C.

[0138] The characteristics of various negative electrodes obtained according to Examples 1 to 3 and Comparative Examples 1 to 5 are shown in Table 1 below.

[0139] [Table 1]

[0140]

[0141]

[0142] Experimental Example 1: Evaluation of Battery Capacity and Battery Fast Charging Characteristics

[0143] Batteries were manufactured using the respective negative electrodes according to Examples 1 to 3 and Comparative Examples 1 to 5 in the manner described below.

[0144] Cut into pieces with an area of ​​1.7671cm 2 Li metal was used as the positive electrode. In addition, a porous polyethylene separator was inserted between the positive electrode and the negative electrode to form an electrode assembly. Then, 0.5 wt% of vinylene carbonate (VC) was dissolved in a mixed solvent containing ethyl methyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 7:3, and 1M LiPF6 was added thereto to prepare an electrolyte. The electrolyte was injected into the electrode assembly to obtain a lithium coin-type half-cell.

[0145] During the first three cycles, each half-cell was charged in constant current (CC) / constant voltage (CV) mode (current rate 0.2C, 5mV, 0.005C current cutoff) and discharged in CC mode to 1.0V. The negative electrode capacity and initial efficiency determined at this time are shown in Table 2 below. Then, when the half-cell was charged to an SOC (state of charge) of 80% at a current rate of 1.5C, the SOC at which lithium plating occurred (Li plating SOC) was determined. The results are shown in Table 2 below.

[0146] Experimental Example 2: Raman spectroscopy

[0147] Raman spectroscopy was performed to determine the crystallinity of the first and second carbon layers in the various negative electrode active materials according to Examples 1 to 3 and Comparative Examples 1 to 5. Raman spectroscopy was performed using a Renishaw 2000 Raman microscope system with 532 nm laser excitation. To avoid laser thermal effects, a 100x optical lens was used, the laser output density was low, and the exposure time was 30 seconds. To reduce position-dependent deviations, a total of 25 points were measured per 5 μm × 5 μm area, and the results are presented as average values ​​in Table 2 below.

[0148] [Table 2]

[0149]

[0150] Referring to Table 2, when the G band (at about 1580 cm -1 peak at about 1350 cm) and D band (at about 1350 cm -1 When the FWHM values ​​of the peak at (at ) are compared, it can be seen that the second carbon coating layer heat-treated at 1150°C to 1250°C exhibits relatively lower crystallinity than the first carbon coating layer heat-treated at 1450°C to 1550°C. In particular, it can be seen that the FWHM values ​​of the D band of the second carbon coating layer of the negative electrode active materials according to Examples 1 to 3 are approximately 1.66 times, 1.34 times, and 2.65 times the FWHM value of the D band of the first carbon coating layer, respectively. As a result, the secondary batteries using the negative electrode active materials according to Examples 1 to 3 exhibit high initial efficiency and excellent fast charging characteristics.

[0151] Compared to Examples 1 to 3, the negative active material according to Comparative Example 1 including the single carbon coating layer coated in a coating amount of 5 parts by weight in a single pass exhibited lower rapid charge characteristics due to such a low carbon coating amount.

[0152] In addition, in the case of the negative electrode active materials according to Comparative Examples 2 and 3, the negative electrode active materials were prepared by applying a single carbon coating layer in an amount of 10 parts by weight at a time, and the secondary battery materials using the negative electrode active materials showed poor fast charging characteristics. Because the pitch was applied in a large amount of 10 parts by weight at a time, the natural graphite particles corresponding to the graphite cores agglomerated with each other, so that the average particle size D 50 The increase leads to the problem that the diffusion resistance of lithium ions in natural graphite particles increases and the fast charging characteristics deteriorate.

[0153] Furthermore, in the case of Comparative Example 2, after applying 10 parts by weight of pitch, heat treatment was performed at a relatively lower temperature (1200° C.) compared to Comparative Example 3. Therefore, a relatively large amount of low-crystallinity carbon coating was formed, thereby providing an initial efficiency that was 1.1% lower than that of Comparative Example 3.

[0154] In the case of Comparative Example 3, the average particle size D 50 The increase was due to the same reason as in Comparative Example 2, and the secondary battery showed deteriorated fast charging characteristics. In addition, because a higher heat treatment temperature (1500°C) than that in Comparative Example 2 was used, the obtained carbon coating showed a higher degree of crystallinity, and thus the secondary battery showed further deteriorated fast charging characteristics compared to Comparative Example 2.

[0155] In the case of Comparative Example 4, a heat treatment was performed at 1200°C to form a first carbon coating layer in an amount of 5 parts by weight, and then a further heat treatment was performed at 1500°C to form a second carbon coating layer in an amount of 5 parts by weight. The pre-applied first carbon coating layer was additionally heat treated at 1500°C to form a second carbon coating layer. Therefore, in the case of the negative electrode active material according to Comparative Example 4, 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 approximately 0.61. Compared with the negative electrode active materials according to Examples 1 to 3, the negative electrode active material according to Comparative Example 4 showed an outermost carbon coating layer (second carbon coating layer) with higher crystallinity. As a result, the secondary battery using the negative electrode active material according to Comparative Example 4 showed lower rapid charging characteristics compared to the secondary battery using the negative electrode active materials according to Examples 1 to 3. On the other hand, the negative electrode active material according to Comparative Example 4 had a relatively smaller average particle size than the negative electrode active material according to Comparative Example 3 because it underwent a process of forming two carbon coating layers despite the same content of the carbon coating layers. As a result, when the negative electrode active material according to Comparative Example 4 was applied to the secondary battery, the secondary battery showed slightly improved rapid charging characteristics, but the rapid charging characteristics were significantly reduced compared to the secondary batteries using the negative electrode active materials according to Examples 1 to 3.

[0156] In the case 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 is about 0.86. Due to the same reasons as in Comparative Example 4, the secondary battery using the negative electrode active material according to Comparative Example 5 shows significantly deteriorated fast charging characteristics than the secondary battery using the negative electrode active material according to Examples 1 to 3. In the negative electrode active material according to Comparative Example 5, the heat treatment temperature for forming the second carbon coating layer is lower than that of Comparative Example 4, whereby the second carbon coating layer (outermost coating layer) shows lower crystallinity than the negative electrode active material according to Comparative Example 4. As a result, the secondary battery using the negative electrode active material according to Comparative Example 5 shows slightly improved fast charging characteristics compared to the secondary battery using the negative electrode active material according to Comparative Example 4, but the fast charging characteristics are significantly reduced compared to the secondary battery using the negative electrode active material according to Examples 1 to 3.

Claims

1. A negative electrode active material, comprising: Graphite core; a first carbon coating surrounding an outside of the graphite core; and a second carbon coating layer surrounding an outer side of the first carbon coating layer, wherein the second carbon coating has a lower crystallinity than the first carbon coating, wherein the first carbon coating layer and the second carbon coating layer are both amorphous carbon layers, wherein the first carbon coating and the second carbon coating are both soft carbon or hard carbon, wherein the negative electrode active material is in the form of particles, The first carbon coating layer and the second carbon coating layer each contain 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, having an average particle size D of 7 μm to 25 μm. 50 .

3. The negative electrode active material according to claim 1, wherein the graphite core has an average particle size D of 5 μm to 20 μm. 50 . 4 . The negative active material according to claim 1 , wherein the FWHM (full width at half maximum) value of the D band of the second carbon coating layer corresponds to 1.3 times or more the FWHM value of the D band of the first carbon coating layer. 5 . The negative active material according to claim 1 , wherein the FWHM (full width at half maximum) value of the D band of the second carbon coating layer corresponds to 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 claim 1, comprising the following steps: mixing graphite with a first carbon precursor and subjecting the obtained mixture to a first heat treatment at a temperature of 1400° C. to 1600° C. to obtain a product including graphite as a graphite core and a first carbon coating layer surrounding the outside of the graphite core; and The product of the above step is mixed with a second carbon precursor and the obtained mixture is subjected to a second heat treatment at a temperature of 1100° C. to 1300° C. to form a second carbon coating layer surrounding the outer side of the first carbon coating layer. The first carbon coating layer and the second carbon coating layer each contain 3 to 6 parts by weight based on 100 parts by weight of the graphite core.

7. A negative electrode comprising a current collector and a negative electrode active material layer provided on at least one surface of the current collector, The negative electrode active material layer comprises the negative electrode active material according to any one of claims 1 to 5. 8 . A lithium secondary battery comprising the negative electrode according to claim 7 .

Images