Negative active material, method for preparing the same, and lithium secondary battery comprising the same

Abstract

The present disclosure relates to a negative active material for a lithium secondary battery, a lithium secondary battery comprising the same, and a method of preparing the same. The negative active material comprises a coating layer self-bound to the surface of a carbonaceous material and containing porous carbon particles, thereby exhibiting reduced electrical resistance during intercalation of lithium ions on the surface of the carbonaceous material and providing improved surface reactivity and structural stability, thereby providing improved high-rate charge characteristics without causing deterioration of charge / discharge efficiency and life characteristics when used as a negative active material for a lithium secondary battery. The negative active material comprises a carbonaceous material and a carbon coating layer self-bound to the surface of the carbonaceous material. Here, the self-bound amorphous carbon coating layer can optionally have a controlled pore structure by chemical etching.

Description

Technical Field

[0001] This disclosure relates to a negative electrode active material for lithium secondary batteries, a method for preparing the same, and a lithium secondary battery comprising the negative electrode active material.

[0002] This application claims priority to Korean Patent Application No. 10-2020-0031553, filed in Korea on March 13, 2020, the disclosure of which is incorporated herein by reference. Background Technology

[0003] With the increasing availability of portable and compact electrical / electronic instruments, new types of secondary batteries, such as nickel-metal hydride batteries and lithium-ion batteries, have been actively developed.

[0004] Specifically, lithium-ion batteries are batteries that use lithium metal as the negative electrode active material and a non-aqueous solvent as the electrolyte. Because lithium is a metal with a very high ionization tendency, it can exhibit high voltage, thus enabling the development of batteries with high energy density. Lithium-ion batteries using lithium metal as the negative electrode active material have been used as next-generation batteries for a long time.

[0005] When carbonaceous materials are used as the negative electrode active material for such lithium-ion secondary batteries, the charge / discharge potential of lithium is below the stable range of existing non-aqueous electrolytes, causing electrolyte decomposition during charge / discharge. Therefore, a coating is formed on the surface of the carbonaceous negative electrode active material. In other words, the electrolyte decomposes before lithium ions intercalate into the carbonaceous material, thus forming a coating on the electrode surface. This coating allows lithium ions to permeate through it but interrupts electron conduction. Therefore, once the coating is formed, electrolyte decomposition caused by electron conduction between the electrode and the electrolyte is suppressed, and lithium ions are selectively intercalated / deintercalated. Such coatings are called solid electrolyte interphase (SEI) films.

[0006] For the reasons mentioned above, the resistance generated on the surface of carbonaceous materials is very high when lithium ions are intercalated during charging. As a result, lithium metal deposition occurs under high-rate charging conditions, which is considered the root cause of the low charge / discharge efficiency and deteriorated lifespan characteristics of currently available lithium secondary batteries using carbonaceous materials as the negative electrode active material during high-rate charging.

[0007] To address the aforementioned issues, methods have been proposed to improve lithium-ion conductivity by physically or chemically modifying the surface of carbonaceous anode active materials to ensure high-rate charging characteristics of lithium secondary batteries using carbonaceous materials. However, while such surface modification can improve lifetime characteristics, it does not solve the problems of lithium metal deposition and degradation of capacity, high-rate characteristics, and charge / discharge efficiency.

[0008] Therefore, although various methods for forming functional coatings have been studied in order to suppress lithium metal deposition during high-rate charging and reduce the resulting resistance, no method has yet been developed to suppress lithium metal deposition during high-rate charging. Summary of the Invention

[0009] Technical issues

[0010] This disclosure aims to address the problems in the related technologies. Therefore, this disclosure aims to provide a negative electrode active material for lithium secondary batteries. When used as a negative electrode active material, the negative electrode active material for lithium secondary batteries can ensure high-rate charging characteristics without reducing charging / discharging efficiency and lifespan characteristics, and provides improved high-rate charging characteristics.

[0011] This disclosure also aims to provide a lithium secondary battery comprising the above-mentioned negative electrode material for lithium secondary batteries.

[0012] Furthermore, this disclosure aims to provide a method for manufacturing the aforementioned negative electrode material for lithium secondary batteries.

[0013] Technical solution

[0014] In one aspect of this disclosure, a negative electrode active material for a lithium secondary battery is provided according to any of the following embodiments.

[0015] According to a first embodiment, a negative electrode active material for lithium secondary batteries is provided, comprising:

[0016] Carbonaceous materials; and

[0017] A porous carbon coating that self-bonds to the surface of the carbonaceous material.

[0018] According to the second embodiment, a negative electrode active material for a lithium secondary battery as defined in the first embodiment is provided, wherein the porous carbon coating comprises a metal element selected from or two or more of the following: Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe, and Al.

[0019] According to a third embodiment, a negative electrode active material for a lithium secondary battery as defined in the first or second embodiment is provided, wherein the porous carbon coating comprises the metal element Zn, Co, or a combination thereof.

[0020] According to the fourth embodiment, a negative electrode active material for a lithium secondary battery as defined in any one of the first to third embodiments is provided, wherein the content of the porous carbon coating is 50% by weight or less based on the total weight of the negative electrode active material.

[0021] According to the fifth embodiment, a negative electrode active material for a lithium secondary battery as defined in any one of the first to fourth embodiments is provided, wherein the average particle size of the carbonaceous material is 25 μm or less.

[0022] According to a sixth embodiment, a lithium secondary battery is provided, the lithium secondary battery having a negative electrode comprising a negative electrode active material for a lithium secondary battery as defined in any one of the first to fifth embodiments.

[0023] According to a seventh embodiment, a method for preparing a negative electrode active material for lithium secondary batteries is provided, comprising the following steps:

[0024] Prepare carbonaceous materials; and

[0025] Metal-organic frameworks (MOFs) are grown directly on the surface of the carbonaceous material;

[0026] The carbonaceous material on which the MOF has grown is dried; and

[0027] The dried carbonaceous material on which MOF has been grown is heat-treated to form a porous carbon coating containing metal elements on the surface of the carbonaceous material.

[0028] According to the eighth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in the seventh embodiment is provided, wherein the step of directly growing a metal-organic framework (MOF) on the surface of a carbonaceous material includes:

[0029] The step of mixing a precursor solution containing a metal compound, an organic compound, and hydrogen peroxide with the carbonaceous material to directly grow the metal-organic framework on the surface of the carbonaceous material; or

[0030] The step of mixing a carbonaceous material composition containing the carbonaceous material dispersed in hydrogen peroxide with a metal compound solution and an organic compound solution to grow a metal-organic framework directly on the surface of the carbonaceous material.

[0031] According to the ninth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in the eighth embodiment is provided, wherein the metal compound comprises metal acetate, metal nitrate, metal carbonate, metal hydroxide, or two or more thereof.

[0032] According to the tenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in the eighth or ninth embodiment is provided, wherein the metal compound comprises Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe, Al or two or more thereof.

[0033] According to the eleventh embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any one of the eighth to tenth embodiments is provided, wherein the metal of the metal compound comprises Zn, Co, or a combination thereof.

[0034] According to the twelfth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any one of the eighth to eleventh embodiments is provided, wherein the organic compound comprises a carboxylic acid compound, an imidazole compound, or two or more thereof.

[0035] According to the thirteenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any one of the eighth to twelfth embodiments is provided, wherein the metal compound is zinc acetate, cobalt acetate, or a mixture thereof, and the organic compound is 2-methylimidazole.

[0036] According to the fourteenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any one of the eighth to thirteenth embodiments is provided, wherein the amount of hydrogen peroxide in the precursor solution is 1 to 50% by weight to initiate the direct growth of the MOF on the surface of the carbonaceous material.

[0037] According to the fifteenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any one of the seventh to fourteenth embodiments is provided, wherein the drying step is performed at 25 to 120°C.

[0038] According to the sixteenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any one of the seventh to fifteenth embodiments is provided, wherein the heat treatment step is performed for 1 to 10 hours in an inert gas atmosphere at 800°C to 1500°C.

[0039] According to the seventeenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in any of the seventh to sixteenth embodiments is provided, the method further comprising a chemical etching step for removing the metal element after the step of forming a porous carbon coating.

[0040] According to the eighteenth embodiment, a method for preparing a negative electrode active material for lithium secondary batteries as defined in the seventeenth embodiment is provided, wherein the chemical etching step is performed by stirring the negative electrode active material in an acid solution with a concentration of 0.5 to 3 M for 1 to 10 hours, followed by drying at 25 to 120°C.

[0041] Beneficial effects

[0042] According to this disclosure, lithium metal deposition on the surface of a carbonaceous material used as the negative electrode active material in a lithium secondary battery can be suppressed by forming a porous carbon coating that self-bonds to the surface of the carbonaceous material, thereby inducing more stable high-rate charging characteristics.

[0043] Furthermore, when a negative electrode active material containing a porous carbon coating is used as the negative electrode active material for a lithium secondary battery, excellent lifetime characteristics can be provided by reducing the resistance generated when lithium is embedded on the surface of the negative electrode active material. Attached Figure Description

[0044] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, serve to provide a further understanding of the technical features of the present disclosure; therefore, the present disclosure should not be construed as being limited to the drawings.

[0045] Figure 1 To illustrate a schematic flowchart of the process of manufacturing a negative electrode active material for lithium secondary batteries according to one embodiment of the present disclosure, the negative electrode active material for lithium secondary batteries comprises a porous carbon coating that is self-bonded to the graphite surface.

[0046] Figure 2 Scanning electron microscope (SEM) images of the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1 are shown.

[0047] Figure 3 The images show photographs of the negative electrode active materials according to Examples 1 and 2 obtained by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS).

[0048] Figure 4a and Figure 4b The X-ray diffraction (XRD) patterns of the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1 are shown.

[0049] Figure 5 The photographs show the BET specific surface area analysis results of the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1.

[0050] Figure 6 The graph shows the results of the charge / discharge characteristics of the lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1.

[0051] Figure 7a and Figure 7b This is a graph showing the results of the charging characteristics of the lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1 as a function of rate.

[0052] Figure 8 The graph shows the test results of the life characteristics evaluation of the lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1.

[0053] Figure 9 SEM images of the surface and cross-section of each electrode are shown after testing the lithium secondary batteries of Examples 1 and 2 and Comparative Example 1 to evaluate their lifetime characteristics. Detailed Implementation

[0054] In the following, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terminology used in the specification and appended claims should not be construed as limited to its general or dictionary meaning, but rather should be interpreted according to the meaning and concept corresponding to the technical aspects of the present disclosure, based on the principle that the inventors are allowed to appropriately define the terminology for the best interpretation.

[0055] Therefore, the description presented herein is merely a preferred example for illustrative purposes and is not intended to limit the scope of this disclosure. It should be understood that other equivalents and variations may be made thereto without departing from the scope of this disclosure.

[0056] In one aspect of this disclosure, a negative electrode active material for lithium secondary batteries is provided, comprising: a carbonaceous material; and a porous carbon coating that is self-bonded to the surface of the carbonaceous material.

[0057] The carbonaceous material may comprise at least one material selected from the following: materials containing crystalline or amorphous carbon, such as artificial graphite, natural graphite, graphitized carbon fibers, graphitized mesophase carbon microspheres, petroleum coke, calcined resin, carbon fibers, pyrolytic carbon, etc. The carbonaceous material may have an average particle size of less than 25 μm, 5–25 μm, or 8–20 μm. When the carbonaceous material has an average particle size of less than 25 μm, it can provide improved room temperature and low temperature output characteristics and may be advantageous in terms of fast charging.

[0058] As used in this article, “particle size (D)” n ")" refers to the particle size at the n% point in the cumulative particle number distribution based on particle size. In other words, D 50 (Average particle size) refers to the particle size at the 50th percentile point in the cumulative particle number distribution based on particle size, D 90 This refers to the particle size at the 90th percentile point in the cumulative particle number distribution based on particle size, and D 10 It refers to the particle size at the 10th percentile point in the cumulative distribution of particle numbers based on particle size.

[0059] Here, D includes the aforementioned average particle size. nThis can be determined using laser diffraction. Specifically, the material to be tested is dispersed in a dispersion medium, and the resulting dispersion is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500) to determine the difference in the diffraction pattern with particle size as particles pass through the laser beam, thereby providing the particle size distribution. Then, by calculating the particle size at the 10%, 50%, and 90% points of the cumulative particle number distribution based on particle size, D can be determined. 10 D 50 and D 90 .

[0060] According to one embodiment of this disclosure, the porous carbon coating may contain two or more metallic elements selected from Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe, Al, or the like. In other words, the porous carbon coating may contain any one metallic element selected from Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe, and Al, or may contain two or more different metallic elements in combination.

[0061] As described below, a porous carbon coating according to one embodiment of this disclosure can be formed by heat-treating a metal-organic framework (MOF) containing various metal compounds and organic compounds. Therefore, the type and amount of the metal elements contained in the porous carbon coating can be varied depending on the structure of the metal-organic framework (MOF). Preferably, the porous carbon coating may contain Zn or Co.

[0062] The porous carbon coating self-bonds to the surface of the carbonaceous material. Here, "self-bonding" refers to carbon bonding obtained by initiating a chemical bond between the activated carbonaceous material and the carbon coating precursor, followed by carbonization of the precursor.

[0063] Furthermore, the porous carbon coating can be physically or chemically bonded to the carbonaceous material. Here, "physical or chemical bonding" refers to the bonding between the surface of the carbonaceous material and the carbon coating. The presence of a carbon coating bonded to the surface of the carbonaceous material can be determined before and after the formation of the carbon coating on the surface of the carbonaceous material by analytical methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy.

[0064] According to one embodiment of this disclosure, the porous carbon coating may be uniformly formed on the surface of the carbonaceous material, or may partially cover a portion of the surface of the carbonaceous material.

[0065] The content of the porous carbon coating can be less than 50% by weight, 1 to 50% by weight, 1 to 30% by weight, or 1 to 10% by weight. Here, after measuring the weight of the initially introduced carbonaceous material (a) and the final negative electrode active material (b) respectively, the content of the porous carbon coating can be calculated according to the following formula: content of porous carbon coating (by weight) = [(ba) / b] × 100.

[0066] When the content of the porous carbon coating meets the above range, it can improve the output characteristics at room temperature and high temperature and the fast charging performance.

[0067] In another aspect of this disclosure, a method for preparing a negative electrode active material for lithium secondary batteries is provided, comprising the following steps:

[0068] Prepare carbonaceous materials; and

[0069] Metal-organic frameworks (MOFs) are grown directly on the surface of the carbonaceous material;

[0070] The carbonaceous material on which the MOF has grown is dried; and

[0071] The dried carbonaceous material on which MOF has been grown is heat-treated to form a porous carbon coating containing metal elements on the surface of the carbonaceous material.

[0072] According to one embodiment of this disclosure, the step of directly growing a metal-organic framework (MOF) on the surface of a carbonaceous material may include mixing a precursor solution containing a metal compound, an organic compound, and hydrogen peroxide with the carbonaceous material to directly grow the MOF on the surface of the carbonaceous material. In this case, a precursor solution containing a metal compound, an organic compound, and hydrogen peroxide is prepared, and then the precursor solution is mixed with the carbonaceous material to directly grow the MOF on the surface of the carbonaceous material.

[0073] According to another embodiment of this disclosure, the step of directly growing a metal-organic framework (MOF) on the surface of the carbonaceous material may include: mixing a carbonaceous material composition comprising the carbonaceous material dispersed in hydrogen peroxide with a metal compound solution and an organic compound solution to directly grow the MOF on the surface of the carbonaceous material. In this case, the carbonaceous material is dispersed in hydrogen peroxide to prepare the carbonaceous material composition, and the metal compound and organic compound are dissolved in a solvent (e.g., water) to prepare the metal compound solution and organic compound solution, respectively. The carbonaceous material composition can then be mixed with the metal compound solution and the organic compound solution to directly grow the MOF on the surface of the carbonaceous material.

[0074] Here, based on the total content of the carbonaceous material composition, the content of the carbonaceous material can be 0.1 to 15% by weight or 2 to 8% by weight. When the content of the carbonaceous material meets the above range, the initial efficiency, capacity retention characteristics, and output characteristics of the secondary battery can be improved when the obtained product is used as a negative electrode active material for lithium secondary batteries.

[0075] Furthermore, the metal compound solution and the organic compound solution may each have a concentration of 1–25% by weight or 3–17% by weight. When the metal compound solution and the organic compound solution each meet the above-defined concentrations, the formation of the metal-organic framework can be promoted.

[0076] The metal compound may include metal acetate, metal nitrate, metal carbonate, metal hydroxide, or two or more of them.

[0077] The metal compound may contain Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe, Al, or two or more thereof. According to one embodiment of this disclosure, the metal element contained in the porous carbon coating may be Zn, Co, or a combination thereof.

[0078] The organic compound may include a carboxylic acid compound, an imidazole compound, or two or more thereof.

[0079] According to one embodiment, the metal compound may be zinc acetate, cobalt acetate, or a mixture thereof, and the organic compound may be 2-methylimidazole.

[0080] To initiate the direct growth of the MOF on the surface of the carbonaceous material, the precursor solution may further contain 1–50 wt% or 1–10 wt% of hydrogen peroxide (H2O2). When the surface of the carbonaceous material is treated with H2O2, the surface of the carbonaceous material is oxidized and activated into heteroatoms such as oxygen formed on the surface, thereby allowing growth to occur at the corresponding sites.

[0081] Here, "direct growth of MOF on the surface of the carbonaceous material" refers to the growth of the precursor while it is chemically bonded to the surface of the carbonaceous material.

[0082] The drying step can be carried out at 25–120°C or 100–120°C.

[0083] The heat treatment step can be carried out for 1 to 10 hours or 3 to 8 hours in an inert gas atmosphere at 800 to 1500°C or 900 to 1300°C.

[0084] According to one embodiment of this disclosure, the method may further include a chemical etching step for removing the metal elements after the step of forming the porous carbon coating.

[0085] Here, the chemical etching step can be performed by stirring the negative electrode active material in an acid solution with a concentration of 0.5–3M, 0.7–2M, or 1–1.5M for 1–10 hours, and then drying it at 25–120°C or 30–100°C. Specific examples of the acid solution may include hydrochloric acid solution, sulfuric acid solution, hydrofluoric acid solution, aqua regia (a mixed solution of hydrochloric acid and nitric acid), etc.

[0086] Figure 1 This is a flowchart illustrating a process for manufacturing a carbonaceous material as a negative electrode active material for a lithium secondary battery according to one embodiment of the present disclosure, the carbonaceous material comprising a porous carbon coating containing Zn or Co that is self-bonded to its surface.

[0087] refer to Figure 1 Materials such as carbonaceous materials and zinc acetate and 2-methylimidazole as precursors for porous carbon are prepared. Although zinc acetate and 2-methylimidazole are exemplified as precursors for forming porous carbon coatings according to one embodiment of this disclosure, the scope of this disclosure is not limited thereto. For example, various types of precursors can be provided depending on the specific type of MOF used to form the porous carbon coating. As mentioned above, according to one embodiment of this disclosure, carbonaceous materials with an average particle size of 25 μm or less are preferably used. Although various materials can be used as carbonaceous materials, graphitic materials are preferred in consideration of their combination with porous carbon coatings containing Zn or Co.

[0088] First, in order to form a porous carbon coating, the carbonaceous material is dispersed and mixed in H2O2 solvent to prepare a carbonaceous material composition.

[0089] Next, zinc acetate and the 2-methylimidazole precursor are dissolved in water to prepare a porous carbon precursor aqueous solution. Here, the porous carbon precursor aqueous solution can be prepared from a 1:1 volume ratio of 2-methylimidazole aqueous solution (solution 1) and zinc acetate aqueous solution (solution 2).

[0090] According to this disclosure, various materials can be used as porous carbon precursors, depending on the specific type of MOF to be obtained.

[0091] Then, the carbonaceous material composition is mixed with the 2-methylimidazole aqueous solution (solution 1) so that the carbonaceous material can be coated with the 2-methylimidazole organic compound. Subsequently, the zinc acetate aqueous solution (solution 2) is mixed to initiate the growth of a metal-organic framework (MOF) through the reaction of the 2-methylimidazole coated on the surface of the carbonaceous material with zinc acetate. Here, zinc acetate and 2-methylimidazole are preferably mixed such that each component exists at a concentration of 10-30% by weight in the combined solution of the zinc acetate aqueous solution and the 2-methylimidazole aqueous solution.

[0092] The carbonaceous material, comprising the MOF particles that have self-bonded to the surface of the carbonaceous material through precipitation, is then dried. This drying step can be performed for 24 hours at a temperature of 25–100°C (e.g., 100°C).

[0093] Subsequently, the dried carbonaceous material is heat-treated to form a porous carbon coating containing metallic elements (e.g., Zn or Co) that are self-bonded to the surface of the carbonaceous material, thereby providing a negative electrode active material according to one embodiment of the present disclosure. Here, the heat treatment can be performed for 1 to 10 hours in an inert gas atmosphere at 500–1000°C, for example, for 6 hours in an inert gas atmosphere at 900°C.

[0094] As described above, the negative electrode active material according to this disclosure includes a porous carbon coating containing metal elements and formed on the surface of a carbonaceous material, thereby enabling more stable lithium-ion conduction and preventing lithium metal from depositing on the surface of the carbonaceous material during high-rate charging.

[0095] Furthermore, the negative electrode active material exhibits improved surface reactivity and structural stability by introducing such a functional coating, thereby ensuring high-rate charging characteristics when used as a negative electrode active material for lithium secondary batteries while suppressing lithium metal deposition and preventing degradation of lifetime characteristics.

[0096] Embodiments will be described more fully below to facilitate a better understanding of this disclosure. However, the following embodiments may be embodied in many different forms and should not be construed as limited to the exemplary implementations set forth herein. Rather, these exemplary implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art.

[0097] In Examples 1 and 2 and Comparative Example 1, the number-average particle size (Da) of the carbonaceous materials was determined by using laser diffraction. 50Specifically, the powder to be tested is dispersed in water as the dispersion medium, and the resulting dispersion is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500) to determine the difference in the diffraction pattern with particle size as particles pass through the laser beam, thereby providing a particle size distribution. Then, D is determined by calculating the particle size at the 10%, 50%, and 90% points of the cumulative particle number distribution based on particle size. 10 D 50 and D 90 .

[0098] Example 1

[0099] <Preparation of Negative Electrode Active Materials>

[0100] The number average particle size (D) 50 Artificial graphite with a thickness of 17 μm and no coating is used as a carbonaceous material.

[0101] First, artificial graphite is dispersed and stirred in hydrogen peroxide (H2O2) to prepare an artificial graphite composition. Here, the content of artificial graphite in the artificial graphite composition is 8.6% by weight. Furthermore, 2-methylimidazole and zinc acetate are dissolved in water respectively to prepare an aqueous solution of 2-methylimidazole and an aqueous solution of zinc acetate. Here, the concentration of the 2-methylimidazole aqueous solution is 16.3% by weight and the concentration of the zinc acetate aqueous solution is 4.5% by weight.

[0102] Next, the artificial graphite composition is mixed with the 2-methylimidazole aqueous solution, then stirred, and the resulting mixture is further mixed with the zinc acetate aqueous solution and stirred to uniformly coat the graphite surface. The resulting product is dried at 100°C and finally heat-treated at 900°C to obtain a negative electrode active material for lithium secondary batteries containing a Zn-containing porous carbon coating on the surface of the artificial graphite as a carbonaceous material.

[0103] Manufacturing of Secondary Batteries

[0104] The negative electrode active material obtained according to Example 1 is used to manufacture a lithium secondary battery.

[0105] First, a slurry was prepared in N-methyl-2-pyrrolidone (NMP) as a solvent using 95.6 wt% of the negative electrode active material according to Example 1, 1.0 wt% of Super-P as a conductive material, and 3.4 wt% of polyvinylidene fluoride (PVDF) as a binder. The slurry was coated onto copper foil and subsequently dried to obtain an electrode. Here, the electrode has a strength of 5 mg / cm³. 2The loading level was [not specified], and the electrode mixture had a density of 1.5 g / cc. A half-cell was fabricated using lithium metal as the counter electrode, and its electrochemical properties were evaluated. The electrolyte used herein comprised 1 M LiPF6 dissolved in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:7 volume ratio.

[0106] Example 2

[0107] The negative electrode active material according to Example 1 was chemically etched with a 1M hydrochloric acid solution and then dried at 100°C to obtain the negative electrode active material.

[0108] A lithium secondary battery was obtained in the same manner as in Example 1, except that the obtained negative electrode active material was used.

[0109] Comparative Example 1

[0110] Using the number average particle size (D) 50 Artificial graphite with a thickness of 17 μm and no coating was used as the negative electrode active material.

[0111] A lithium secondary battery was obtained in the same manner as in Example 1, except that such uncoated artificial graphite was used as the negative electrode active material.

[0112] Table 1 below shows the content and preparation conditions of each negative electrode active material according to Examples 1 and 2 and Comparative Example 1.

[0113] [Table 1]

[0114]

[0115] Figure 2 Scanning electron microscope (SEM) images showing the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1 are displayed. Reference Figure 2 It can be seen that each of the negative electrode active materials according to Examples 1 and 2 has a porous carbon coating that is self-bonded to the graphite surface, and the porous carbon coating has the shape of amorphous particles and contains primary particles with a size of tens to hundreds of nanometers.

[0116] Figure 3 The images shown are obtained by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) analysis of the negative electrode active materials according to Examples 1 and 2.

[0117] refer to Figure 3 In Examples 1 and 2, it can be seen that, unlike Comparative Example 1, a Zn-containing porous carbon coating with a size of tens to hundreds of nanometers was formed on the graphite surface.

[0118] EDS analysis revealed that the porous carbon particles contained Zn and N, and that N was spontaneously incorporated into the carbonaceous structure through the decomposition of 2-methylimidazole as a precursor.

[0119] Even after chemical etching according to Example 2, the structural changes of the porous carbon coating bonded to the graphite surface were small, indicating that the Zn-containing porous carbon coating is physically or chemically bonded to the graphite surface.

[0120] Figure 4a and Figure 4b The X-ray diffraction (XRD) patterns of the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1 are shown.

[0121] like Figure 4a and Figure 4b As shown, the XRD analysis results indicate that Comparative Example 1 exhibits a peak pattern substantially different from that of Examples 1 and 2. In other words, in the case of the negative electrode active materials of Examples 1 and 2 according to the present disclosure, unlike the negative electrode active material according to Comparative Example 1, peaks originating from typical amorphous carbon structures are detected at 2θ = 20° or lower.

[0122] Patterns corresponding to graphite and Zn were observed. In particular, in the cases of Examples 1 and 2, peaks were observed at approximately 2θ = 26°, 42°, 44°, 54° and 77° corresponding to the characteristic peaks of graphite, and peaks were also observed at approximately 2θ = 36°, 39°, 43° and 70° corresponding to the characteristic peaks of Zn, as shown in Table 2 below.

[0123] [Table 2]

[0124]

[0125] Figure 5 The photographs show the BET specific surface area analysis results of the negative electrode active materials according to Examples 1 and 2 and Comparative Example 1.

[0126] Here, the specific surface area of ​​each negative electrode active material was determined by the BET method. In particular, the specific surface area was calculated by the amount of nitrogen adsorbed at liquid nitrogen temperature (77K) using BELSORP-mino II, which is available from BEL Japan.

[0127] like Figure 5 As shown, the negative electrode active material according to Example 1 exhibits an increase in specific surface area (15.0 m²) by introducing a self-bonded porous carbon coating. 2 / g). In Example 2, which used additional chemical etching, the specific surface area increased by an additional 19.6 m² due to Zn deintercalation. 2 / g).

[0128] Figure 6 This is a graph showing the results of the initial charge / discharge characteristics of the lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1. Here, the results are compared between 0.005–1.5V and Li / Li. + The initial charge / discharge characteristics of the lithium secondary batteries were evaluated by charging / discharging each of Examples 1 and 2 and Comparative Example 1 three times with a constant current of 0.1C (35mA / g) within the potential range.

[0129] refer to Figure 6 It can be seen that, compared with Comparative Example 1, Examples 1 and 2, which contain a porous carbon coating on the surface of artificial graphite, show an increase in reversible capacity.

[0130] Figure 7a and Figure 7b This is a graph showing the results of the charging characteristics of the lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1 as a function of rate.

[0131] Here, by comparing 0.005–1.5V with Li / Li + Within the potential range, each lithium secondary battery according to Examples 1 and 2 and Comparative Example 1 was charged / discharged three times with a constant current of 0.1C (35mA / g). Then, each battery was charged with a constant current of 1C (350mA / g), 3C (1050mA / g), and 5C (1750mA / g) and discharged with a constant current of 0.5C (350mA / g). The charging characteristics of the lithium secondary batteries as a function of the rate were evaluated.

[0132] refer to Figure 7a and Figure 7b As can be seen, compared to Comparative Example 1, the lithium secondary batteries containing self-bonded porous carbon coatings according to Examples 1 and 2 exhibit improved initial charge / discharge characteristics (charge capacity) and high-rate charging characteristics. It is believed that because the Zn-containing porous carbon coating is introduced onto the graphite surface, the resistance during lithium-ion intercalation can be effectively reduced, and more stable lithium-ion conduction can be initiated during high-rate charging, thereby providing improved initial charge / discharge characteristics and high-rate charging characteristics.

[0133] Figure 8 This is a graph showing the test results for evaluating the lifespan characteristics of lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1. Here, the comparison is made at 0.005–1.5V vs. Li / Li. +The lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1 were charged / discharged 3 times at a constant current of 0.1C (35mA / g) within the potential range, and then charged at a constant current of 3C (1050mA / g) and discharged 100 times at a constant current of 1C (350mA / g) to evaluate the life characteristics of the lithium secondary batteries.

[0134] refer to Figure 8 Examples 1 and 2 exhibited excellent lifetime characteristics after 100 charge / discharge cycles. It is believed that the lithium-ion secondary batteries according to Examples 1 and 2 utilize a negative electrode active material comprising a Zn-containing porous carbon coating introduced therein, thereby effectively reducing resistance during lithium-ion intercalation and providing improved charging characteristics.

[0135] Figure 9 SEM images of the surface and cross-section of each electrode are shown after testing the lithium secondary batteries according to Examples 1 and 2 and Comparative Example 1 to evaluate their lifetime characteristics.

[0136] refer to Figure 9 After evaluating the lifetime characteristics, it can be seen that Comparative Example 1 shows a thick coating film formed on the electrode surface, while Examples 1 and 2 show relatively thin coating films, indicating that lithium metal deposition is not significant. This suggests that Examples 1 and 2 provide improved high-rate charging characteristics.

[0137] The present disclosure has been described in detail. However, it should be understood that the detailed description and specific examples shown are for illustrative purposes only, while illustrating preferred embodiments of the present disclosure, as various variations and modifications within the scope of the disclosure will be apparent to those skilled in the art from this detailed description.

Claims

1. A negative electrode active material for lithium secondary batteries, comprising: Carbonaceous materials; and The porous carbon coating is self-bonded to the surface of the carbonaceous material. The "porous carbon coating that self-bonds to the surface of the carbonaceous material" is obtained through carbon bonding, which is achieved by initiating chemical bonding between the activated carbonaceous material and the carbon coating precursor, followed by carbonization of the precursor. The negative electrode active material for lithium secondary batteries is prepared by a method comprising the following steps: Prepare carbonaceous materials; Metal-organic frameworks are grown directly on the surface of the carbonaceous material. The carbonaceous material on which the metal-organic framework has grown is dried; and The dried carbonaceous material on which a metal-organic framework has grown is heat-treated to form a porous carbon coating containing metal elements on the surface of the carbonaceous material. The step of directly growing the metal-organic framework on the surface of the carbonaceous material includes: The step of mixing a precursor solution containing a metal compound, an organic compound, and hydrogen peroxide with the carbonaceous material to grow the metal-organic framework directly on the surface of the carbonaceous material; or The step of mixing a carbonaceous material composition containing the carbonaceous material dispersed in hydrogen peroxide with a metal compound solution and an organic compound solution to grow a metal-organic framework directly on the surface of the carbonaceous material.

2. The negative electrode active material for lithium secondary batteries according to claim 1, wherein the porous carbon coating comprises a metal element selected from or two or more of the following: Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe and Al.

3. The negative electrode active material for lithium secondary batteries according to claim 1, wherein the porous carbon coating comprises the metal elements Zn, Co, or a combination thereof.

4. The negative electrode active material for lithium secondary batteries according to claim 1, wherein the content of the porous carbon coating is less than 50% by weight based on the total weight of the negative electrode active material.

5. The negative electrode active material for lithium secondary batteries according to claim 1, wherein the carbonaceous material has a number-average particle size of less than 25 μm.

6. A lithium secondary battery having a negative electrode comprising a negative electrode active material for a lithium secondary battery as defined in any one of claims 1 to 5.

7. A method for preparing a negative electrode active material for lithium secondary batteries, comprising the following steps: Prepare carbonaceous materials; and Metal-organic frameworks are grown directly on the surface of the carbonaceous material. The carbonaceous material on which the metal-organic framework has grown is dried; and The dried carbonaceous material on which a metal-organic framework has grown is heat-treated to form a porous carbon coating containing metal elements on the surface of the carbonaceous material. The step of directly growing the metal-organic framework on the surface of the carbonaceous material includes: The step of mixing a precursor solution containing a metal compound, an organic compound, and hydrogen peroxide with the carbonaceous material to grow the metal-organic framework directly on the surface of the carbonaceous material; or The step of mixing a carbonaceous material composition containing the carbonaceous material dispersed in hydrogen peroxide with a metal compound solution and an organic compound solution to grow a metal-organic framework directly on the surface of the carbonaceous material.

8. The method for preparing negative electrode active material for lithium secondary batteries according to claim 7, wherein the metal compound comprises metal acetate, metal nitrate, metal carbonate, metal hydroxide, or two or more thereof.

9. The method for preparing a negative electrode active material for lithium secondary batteries according to claim 7, wherein the metal compound comprises Zn, Co, Cu, Ti, Hf, Zr, Ni, Mg, Ti, V, Cr, Fe, Al or two or more thereof.

10. The method for preparing a negative electrode active material for lithium secondary batteries according to claim 7, wherein the metal of the metal compound comprises Zn, Co, or a combination thereof.

11. The method for preparing a negative electrode active material for lithium secondary batteries according to claim 7, wherein the organic compound comprises a carboxylic acid compound, an imidazole compound, or two or more thereof.

12. The method for preparing a negative electrode active material for lithium secondary batteries according to claim 7, wherein the metal compound is zinc acetate, cobalt acetate, or a mixture thereof, and the organic compound is 2-methylimidazole.

13. The method for preparing a negative electrode active material for lithium secondary batteries according to claim 7, wherein the amount of hydrogen peroxide in the precursor solution is 1 to 50% by weight to initiate the direct growth of the metal-organic framework on the surface of the carbonaceous material.

14. The method for preparing negative electrode active material for lithium secondary batteries according to claim 7, wherein the drying step is carried out at 25–120°C.

15. The method for preparing negative electrode active material for lithium secondary batteries according to claim 7, wherein the heat treatment step is carried out in an inert gas atmosphere at 800°C to 1500°C for 1 to 10 hours.

16. The method for preparing a negative electrode active material for lithium secondary batteries according to claim 7, wherein the method further comprises a chemical etching step for removing the metal element after the step of forming the porous carbon coating.

17. The method for preparing negative electrode active material for lithium secondary batteries according to claim 16, wherein the chemical etching step is performed by stirring the negative electrode active material in an acid solution with a concentration of 0.5 to 3 M for 1 to 10 hours, followed by drying at 25 to 120°C.

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