Graphite-based negative electrode active material, method for preparing the same, and negative electrode and lithium secondary battery comprising the same

Abstract

This invention relates to a graphite-based negative electrode active material and its preparation method. The graphite-based negative electrode active material has excellent fast charging performance. The graphite-based negative electrode active material contains artificial graphite. The interplanar spacing d(002) of the (002) facet of the artificial graphite is 0.3360 nm to 0.3370 nm, and the crystallite size Lc in the c-axis direction is 30.0 nm to 38.5 nm.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2024-0096748, filed on July 22, 2024, the disclosure of which is incorporated herein by reference.

[0002] This invention relates to a graphite-based negative electrode active material with excellent fast charging performance, its preparation method, and a negative electrode and lithium secondary battery containing the same. Background Technology

[0003] Due to the rapid increase in the use of fossil fuels, the demand for alternative or clean energy is growing, and as part of this trend, the most active area of ​​research is the generation and storage of electricity using electrochemical reactions.

[0004] A typical example of an electrochemical device utilizing electrochemical energy is the secondary battery, and its application is trending towards greater scope. In recent years, the demand for secondary batteries as an energy source for electronic devices and electric vehicles has grown significantly. Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material in which lithium ions released from the positive electrode are inserted and extracted, and carbon-based active materials such as natural or artificial graphite are primarily used as negative electrode active materials.

[0005] With the increasing demand for electric vehicles in recent years, there is a need to develop secondary batteries with excellent fast-charging performance. Because the charging speed of secondary batteries is more affected by the negative electrode than the positive electrode, ongoing research focuses on improving the charging performance of the negative electrode material to enhance the fast-charging performance of secondary batteries.

[0006] To address this, a technology using silicon (Si)-based anode active materials with excellent fast-charging performance has been proposed. However, a problem exists: because Si-based anode active materials undergo significant volume changes during charge and discharge, their lifetime characteristics deteriorate when used. Furthermore, Si-based anode active materials are more expensive than carbon-based anode active materials, thus increasing the battery's cost.

[0007] Therefore, there is a need for a method to improve the charging performance of carbon-based anode active materials that exhibit small volume changes during charging and discharging and are relatively inexpensive. Summary of the Invention

[0008] Technical issues

[0009] One aspect of the present invention provides a graphite-based negative electrode active material and a method for preparing the same, wherein the graphite-based negative electrode active material has excellent fast charging performance by having small graphite crystal size and containing cavities in its crystal structure.

[0010] Furthermore, another aspect of the present invention provides a negative electrode and a lithium secondary battery, which have excellent fast charging performance by comprising the above-mentioned graphite-based negative electrode active material.

[0011] Technical solution

[0012] [1] The present invention provides a method for preparing a graphite-based negative electrode active material, the method comprising: forming a precursor with an oxygen content of 3% to 15% by crushing and granulating a carbon-based raw material; and preparing artificial graphite with an interface spacing d(002) of 0.3360 nm to 0.3370 nm and a crystallite size Lc of 30.0 nm to 38.5 nm in the c-axis direction by graphitizing the precursor.

[0013] [2] The present invention provides the method described in [1] above, wherein the carbon-based raw material comprises needle coke, embedded coke, coal tar pitch, resin pitch, soft carbon or a combination thereof.

[0014] [3] The present invention provides the method described in [1] or [2] above, wherein the formation of the precursor having an oxygen content of 3% to 15% by weight is carried out by at least one of pulverization and granulation while heating to 200°C to 300°C in an oxygen atmosphere.

[0015] [4] The present invention provides the method described in [3] above, wherein the heating is performed at a heating rate of 1°C / min to 20°C / min.

[0016] [5] The present invention provides the method described in [1] or [2] above, wherein the formation of the precursor with an oxygen content of 3% to 15% by weight is carried out by crushing and granulating the raw material and then supplying air at 100°C to 300°C to contact the air with the raw material.

[0017] [6] The present invention provides a method for at least one of [1] to [5] above, wherein the method further comprises, after preparing the artificial graphite, forming a carbon-based coating by mixing the artificial graphite with a carbonaceous material and carbonizing the mixture.

[0018] [7] The present invention provides the method described in [6] above, wherein the carbonaceous material is pitch.

[0019] [8] The present invention provides a method for at least one of [1] to [7] above, wherein the artificial graphite contains cavities in a lattice.

[0020] [9] The present invention provides a graphite-based negative electrode active material comprising artificial graphite, wherein the interplanar spacing d(002) of the (002) facet of the artificial graphite is 0.3360 nm to 0.3370 nm and the crystallite size Lc in the c-axis direction is 30.0 nm to 38.5 nm.

[0021]

[10] The present invention provides the graphite-based negative electrode active material described above [9], wherein the graphite-based negative electrode active material has a tap density of 0.90 g / cc or higher.

[0022]

[11] The present invention provides the graphite-based negative electrode active material described in [9] or

[10] above, wherein the artificial graphite is prepared by graphitizing a precursor with an oxygen content of 3% to 15% by weight.

[0023]

[12] The present invention provides a negative electrode comprising a graphite-based negative electrode active material of any one of [9] to

[11] above.

[0024]

[13] The present invention provides the negative electrode described in

[12] above, wherein when a half cell prepared using the negative electrode and a lithium metal counter electrode is charged at 3C, the state of charge (SOC) value at which lithium deposition occurs is 35% or more.

[0025]

[14] The present invention provides a lithium secondary battery, wherein the lithium secondary battery comprises the negative electrode described in

[12] or

[13] above.

[0026] Beneficial effects

[0027] This invention enables the preparation of artificial graphite with small crystal size and cavities in its crystal structure by controlling the oxygen content of the precursor during the preparation of artificial graphite to suppress the transformation of the crystal structure to sp2 structure within the carbonization temperature range during graphitization.

[0028] Furthermore, in the case of preparing artificial graphite by using a precursor with a relatively high oxygen content as in this invention, since high-density artificial graphite can be prepared by reducing the amount of highly volatile organic components in the precursor to reduce volatile substances within the carbonization temperature range, graphite-based negative electrode active materials with excellent mechanical properties and less particle breakage can be prepared.

[0029] Because the graphite-based anode active material according to the present invention comprises artificial graphite with smaller crystallite size than conventional anode active materials and cavities in its crystal structure, lithium ions have high mobility within the artificial graphite, and the short lithium ion movement path facilitates lithium ion insertion / extraction, thereby exhibiting excellent charging performance. Therefore, by using the graphite-based anode active material of the present invention, a lithium secondary battery with excellent fast-charging performance can be realized. Detailed Implementation

[0030] It will be understood that the words or terms used in the specification and claims should not be interpreted as having the meaning defined in a common dictionary, and it will also be understood that the words or terms should be interpreted as having a meaning consistent with their meaning in the context of the relevant field and technical ideas of the invention, based on the principle that the inventor can appropriately define the meaning of the words or terms to best interpret the invention.

[0031] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to limit the invention. In the specification, unless otherwise indicated, singular terms may include plural forms.

[0032] It will be further understood that, when used in this specification, the terms “comprising,” “including,” or “having” are used to explicitly state the presence of the said feature, number, step, element, or combination thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

[0033] Methods for preparing graphite-based anode active materials

[0034] First, the method for preparing graphite-based negative electrode active materials according to the present invention will be described.

[0035] The method for preparing graphite-based anode active materials according to the present invention includes the following steps: (1) forming a precursor with an oxygen content of 3% to 15% by crushing and granulating a carbon-based raw material; (2) preparing a graphite-based anode active material by graphitizing the precursor, wherein, as needed, the method may further include (3) forming a carbon coating by mixing the precursor with a carbonaceous material and carbonizing the mixture after graphitization.

[0036] The various steps of the preparation method according to the present invention will be described in detail below.

[0037] (1) Precursor formation steps

[0038] First, prepare carbon-based raw materials, and crush and granulate the carbon-based raw materials to form a precursor with an oxygen content of 3% to 15% by weight, preferably 3% to 10% by weight.

[0039] Specifically, by controlling the temperature and atmosphere of the crushing and / or granulation process of the carbon-based raw materials, or by performing an additional surface modification process after the crushing and granulation of the carbon-based raw materials, a precursor with an oxygen content of 3% to 15% by weight, preferably 3% to 10% by weight, can be formed.

[0040] When the oxygen content of the precursor meets the above-mentioned range, artificial graphite with excellent fast-charging performance and capacity characteristics can be prepared. More specifically, when the oxygen content of the precursor is less than 3% by weight, the formation of cavities during the granulation process is reduced and the surface density may decrease, and when the oxygen content is greater than 15% by weight, there is a problem of reduced granulation strength.

[0041] When the precursor has a high oxygen content, the transformation from the amorphous sp3 structure to the crystalline sp2 structure is suppressed by oxygen within the temperature range of 900℃ to 1300℃ (carbonization temperature range). Therefore, when a precursor with a high oxygen content is used, crystallization occurs with a relatively large amount of amorphous structure compared to when a precursor with a low oxygen content is used, resulting in the formation of cavities in the crystalline structure and the formation of graphite with small crystal sizes.

[0042] For carbonaceous raw materials such as coke or soft carbon, rapid volatilization of highly volatile organic hydrocarbons within the temperature range of 600°C to 900°C causes swelling, which leads to a decrease in the density of the artificial graphite, resulting in deterioration of mechanical properties and particle breakage. However, in the case of a high oxygen content in the precursor as described in this invention, the amount of highly volatile organic hydrocarbons in the raw material is reduced, thus suppressing the rapid increase in volatilization within the temperature range below 1000°C, and therefore enabling the preparation of high-density artificial graphite.

[0043] The carbon-based raw materials may include needle coke, embedded coke, coal tar pitch, resin pitch, soft carbon, or combinations thereof, and may preferably include needle coke, embedded coke, soft carbon, or combinations thereof.

[0044] When preparing carbon-based raw materials, the raw materials are pulverized. Pulverization can be carried out using pulverization methods commonly known in the art, such as jet mills, hammer mills, roller mills, pin mills, vibratory mills, or impact mills, and there are no particular limitations on the methods used. During the pulverization process, the type of pulverizer and the pulverization method can be appropriately selected according to the type of carbon-based raw material.

[0045] The pulverization can reduce the average particle size (D) of the carbon-based raw materials. 50 The process is carried out in the range of 1 µm to 20 µm, preferably 5 µm to 15 µm. If the particle size of the pulverized raw material is too small, the discharge capacity may decrease, while if the particle size is too large, the fast charging performance may deteriorate.

[0046] The average particle size (D) of the carbon-based raw material 50The following method can be used to measure the cumulative particle size distribution: prepare a sample by diluting the carbon-based raw material with deionized water to 1% by weight, and then obtain the volume cumulative particle size distribution by laser diffraction using a particle size analyzer (Microtrac S3500).

[0047] Then, once the carbon-based raw materials are pulverized to the appropriate size, a granulation process is performed. This granulation process can be carried out, for example, by introducing the pulverized particles into a mixing device such as Mechanofusion or Nobilta and applying mechanical compression and / or shear force. During granulation, binders such as asphalt, polyvinyl alcohol, starch, and carboxymethyl cellulose can be added to the mixing device as needed.

[0048] According to one embodiment, the pulverizing and / or granulation process can be carried out simultaneously with heating in an oxygen atmosphere. In this case, the oxygen atmosphere refers to a gaseous atmosphere containing more than 20% by weight of oxygen, including an air atmosphere.

[0049] The heating can be carried out at 200°C to 300°C, and it is desirable that the heating rate is in the range of 1°C / min to 20°C / min, preferably about 2°C / min to about 10°C / min.

[0050] If the pulverization and / or granulation are carried out under oxygen atmosphere and heating conditions as described above, a precursor with a higher oxygen content than conventional precursors can be formed because the particle surface is modified by the oxidation reaction. Specifically, a precursor with an oxygen content of 3% to 15% by weight, preferably 3% to 10% by weight, can be formed.

[0051] According to another embodiment, after the crushing and granulation process of the carbon-based raw material, an additional process is performed by supplying high-temperature air (blowing air) to contact the granulated particles (precursor), which can also form a precursor with a higher oxygen content than conventional precursors. Specifically, a precursor with an oxygen content of 3% to 15% by weight, preferably 3% to 10% by weight, can be formed.

[0052] In cases where the additional contact with high-temperature air is performed as described above, the crushing and / or granulation process can be carried out under conditions of oxygen atmosphere and heating as in the above embodiments, or it can be carried out at room temperature.

[0053] During contact with hot air, the air temperature can be in the range of 100°C to 300°C, preferably 130°C to 250°C. If the hot air is brought into contact with the precursor as described above, a precursor with a high oxygen content will be formed because the precursor surface is oxidized. When the temperature of the air in contact with the precursor is below 100°C, the oxygen content of the precursor is unlikely to reach more than 3% by weight, while when the temperature of the air is above 300°C, problems such as the reduction or removal of binders like asphalt may occur.

[0054] In cases where a binder such as asphalt is used during the granulation process, the air temperature can be lower than the softening point of the binder. For example, the air temperature can be 10°C to 60°C lower than the softening point of the binder, preferably 30°C to 50°C lower.

[0055] The contact time between the high-temperature air and the precursor can be adjusted appropriately according to the air temperature, and can be, for example, 20 to 150 minutes, 30 to 140 minutes, or 60 to 120 minutes. The higher the air temperature, the shorter the contact time can be. For example, when the air temperature is 50°C lower than the softening point of the adhesive, the contact time can be approximately 100 to 140 minutes, and when the air temperature is 30°C lower than the softening point of the adhesive, the contact time can be approximately 20 to 40 minutes. When the contact time with high-temperature air meets the above ranges, it is easy to control the oxygen content of the precursor to 3% to 15% by weight. If the contact time is too short, it will be difficult to achieve the target oxygen content due to insufficient surface modification, while if the contact time is too long, problems such as the reduction or removal of adhesives such as bitumen may occur.

[0056] (2) Graphitization process

[0057] When a precursor with an oxygen content of 3% to 15% by weight is prepared by the above process, the precursor is graphitized to prepare artificial graphite.

[0058] Graphitization can be carried out using graphitization apparatus and methods known in the art. For example, the graphitization can be carried out by introducing the precursor into a graphitization furnace such as an Acheson graphitization furnace, a box-type graphitization furnace, or a longitudinal graphitization furnace and heating it to a temperature range of 2000°C to 3500°C, 2500°C to 3500°C, or 2800°C to 3500°C.

[0059] As described above, when graphitization is performed using a precursor with an oxygen content of 3% to 15% by weight, the transformation from amorphous to crystalline structure within the carbonization temperature range is suppressed by the oxygen contained in the precursor. As a result, because crystallization occurs at a relatively high amorphous content, cavities are formed in the crystalline structure and the resulting crystallites are relatively small.

[0060] Specifically, the artificial graphite according to the present invention can contain cavities in its crystal structure. When cavities are formed in the crystal structure, the fast-charging performance can be improved because the mobility of lithium ions inside the artificial graphite increases.

[0061] When a cavity is present in the crystal structure, the interplanar spacing d(002) of the (002) plane increases relatively, and the crystallite size Lc in the c-axis direction decreases relatively. Therefore, the presence of a cavity in the crystal structure can be inferred from the values ​​of d(002) and Lc. The interplanar spacing d(002) of the (002) plane of the artificial graphite prepared according to the method of the present invention, as measured by X-ray diffraction (XRD), can be 0.3360 nm or more, preferably 0.3360 nm to 0.3370 nm, more preferably 0.3360 nm to 0.3365 nm, and the crystallite size Lc in the c-axis direction, as measured by XRD, can be 38.5 nm or less, preferably 30.0 nm to 38.5 nm, more preferably 34 nm to 38.5 nm.

[0062] (3) Carbonization process

[0063] Although not mandatory, after the preparation of artificial graphite, an additional step can be performed to form a carbon-based coating by mixing the artificial graphite with carbonaceous materials and carbonizing the mixture.

[0064] In cases where a carbon-based coating is formed on the surface of artificial graphite, the strong bonding of the artificial graphite particles constituting the granulated particles can improve the stability of the granulated particles during the charging and discharging process and further improve the fast charging performance.

[0065] The carbonaceous material can be, for example, asphalt, and can be either commonly used solid or liquid asphalt. Solid asphalt can be obtained by pulverizing coal tar pitch, petroleum pitch, synthetic pitch, or wood tar pitch. Liquid asphalt can be prepared by dissolving liquid resin or solid asphalt in a solvent, coating the solution, and then carbonizing the solution. In this case, hexane, toluene, tetrahydrofuran (THF), quinoline, N-methylpyrrolidone (NMP), or ethanol can be used as the solvent.

[0066] Based on 100 parts by weight of artificial graphite, the mixing amount of the carbonaceous material can be from 1 to 10 parts by weight, from 1 to 5 parts by weight, or from 3 to 5 parts by weight. When the mixing amount of the carbonaceous material meets the above ranges, the granulation stability of the graphite-based negative electrode active material is improved, and the fast charging performance is improved. If the amount of carbonaceous material is too small, the effect of improving granulation stability and fast charging performance is not significant, while if the amount of carbonaceous material is too large, the electrochemical performance may be degraded due to the formation of an excessively thick carbon coating.

[0067] The mixing of artificial graphite with carbonaceous materials can be carried out by methods generally known in the art, and there are no particular limitations. For example, mixing can be carried out using mechanochemical methods, such as kneaders, blades, mechanical microsystems, extruders, ball mills, planetary mills, mechanofusion systems, Nobilta, hydridization, and rotary ball mills; or spray drying or emulsion methods.

[0068] When artificial graphite and carbonaceous materials are uniformly mixed, a carbon coating is formed by carbonizing the mixture at a temperature of 1000°C to 1600°C, preferably 1200°C to 1400°C. In this case, the carbonization time can be, for example, in the range of 18 to 30 hours or 20 to 26 hours. When the temperature and time of the carbonization treatment meet the above ranges, the carbon is sufficiently stabilized, impurities in the carbonaceous materials are almost completely removed, and changes in the properties of the coated surface due to excessively high temperatures are prevented.

[0069] The carbon coating formed as described above can be formed from amorphous carbon or crystalline carbon.

[0070] Graphite-based anode active materials

[0071] The graphite-based negative electrode active material according to the present invention comprises artificial graphite, wherein the interplanar spacing d(002) of the (002) plane, as measured by X-ray diffraction (XRD), is 0.3360 nm or more, preferably 0.3360 nm to 0.3370 nm, more preferably 0.3360 nm to 0.3365 nm, and the crystallite size Lc in the c-axis direction, as measured by XRD, is 38.5 nm or less, preferably 30.0 nm to 38.5 nm, more preferably 34 nm to 38.5 nm.

[0072] When the d(002) and Lc of the artificial graphite satisfy the above-mentioned ranges, excellent fast charging performance can be achieved. Specifically, when the d(002) and Lc of the artificial graphite satisfy the above-mentioned ranges, it can be considered that cavities are formed between the crystal lattice, and when cavities are included in the crystal structure, the fast charging performance is improved because the mobility of lithium ions inside the artificial graphite increases.

[0073] The artificial graphite in which d(002) and Lc satisfy the above range can be prepared by graphitizing a precursor with an oxygen content of 3% to 15% by weight, specifically by the method described above according to the present invention.

[0074] The graphite-based negative electrode active material may also include a carbon-based coating on the surface of the artificial graphite. The carbon-based coating can be formed by mixing artificial graphite with a carbonaceous material such as pitch, and then carbonizing the mixture. Since the method for forming the carbon-based coating is the same as described above, detailed descriptions are omitted.

[0075] The tap density of the graphite-based negative electrode active material can be 0.90 g / cc or higher, preferably 1.00 g / cc to 1.30 g / cc, and more preferably 1.00 g / cc to 1.20 g / cc. If the tap density meets the above range, it will exhibit excellent capacity characteristics because a high electrode density can be achieved.

[0076] negative electrode

[0077] Next, the negative electrode according to the present invention will be described.

[0078] The negative electrode according to the present invention comprises the graphite-based negative electrode active material according to the present invention as the negative electrode active material. For example, the negative electrode may comprise a negative electrode active material layer containing the graphite-based negative electrode active material according to the present invention, and the negative electrode may specifically comprise a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0079] The negative electrode active material can be formed from the graphite-based negative electrode active material according to the present invention, or it can be mixed with other types of negative electrode active materials (e.g., natural graphite, silicon-based negative electrode active materials, etc.) and used.

[0080] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 90% to 99.5% by weight, 93% to 99% by weight, or 95% to 98.5% by weight.

[0081] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and does not cause chemical changes in the battery. The negative electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics, and the materials used can be, for example, copper, nickel, stainless steel, titanium, sintered carbon, or combinations thereof. The thickness of the negative electrode current collector is typically from 3 μm to 500 μm. If necessary, fine irregularities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material.

[0082] The negative electrode active material layer can be disposed on the current collector. The negative electrode active material layer can be disposed on at least one surface of the current collector, specifically, on one or two surfaces of the current collector.

[0083] The negative electrode active material layer may further include at least one of an adhesive and a conductive agent, in addition to the negative electrode active material.

[0084] The adhesive may include at least one selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials whose hydrogen is replaced by lithium (Li), sodium (Na), or calcium (Ca), or may include various copolymers thereof.

[0085] Based on the total weight of the negative electrode active material layer, the content of the adhesive can be from 0.1% to 8% by weight, from 0.5% to 5% by weight, or from 0.8% to 4% by weight.

[0086] There are no particular limitations on the conductive agent, as long as it is conductive and does not cause chemical changes in the battery. For example, conductive materials such as: graphite, such as natural or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0087] Based on the total weight of the negative electrode active material layer, the content of the conductive agent can be from 0.1% to 8% by weight, from 0.5% to 5% by weight, or from 0.8% to 4% by weight.

[0088] After preparing a negative electrode slurry by mixing the negative electrode active material with an optional binder and conductive agent in a solvent such as N-methyl-2-pyrrolidone (NMP) or water, a negative electrode current collector can be prepared by coating the negative electrode slurry with the negative electrode current collector and drying and calendering the coated negative electrode current collector.

[0089] Because the negative electrode according to the present invention comprises a graphite-based negative electrode active material, which contains artificial graphite with a crystallite size smaller than that of conventional artificial graphite and cavities in its crystal structure, the lithium ion mobility in the negative electrode active material is excellent, and the lithium ion insertion / extraction is easy, thereby resulting in excellent fast charging performance.

[0090] The fast-charging performance of the negative electrode can be evaluated by the state of charge (SOC) value at which lithium deposition occurs when a half-cell prepared using the negative electrode and a lithium metal counter electrode is charged at 3C. The SOC of a half-cell prepared using a conventional graphite-based negative electrode active material during 3C charging is less than 35%, while the SOC of a half-cell prepared using the negative electrode according to the present invention during 3C charging is greater than 35%, which is higher than that of conventional half-cells. This shows that the fast-charging performance of the negative electrode using the negative electrode active material according to the present invention is superior to that of conventional negative electrodes.

[0091] Lithium secondary batteries

[0092] Next, the lithium secondary battery according to the present invention will be described.

[0093] The lithium secondary battery according to the present invention includes the negative electrode described above. Specifically, the secondary battery according to the present invention may include: the aforementioned negative electrode; a positive electrode opposite to the negative electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

[0094] Since the negative electrode has already been described above, the following will describe the other components besides the negative electrode.

[0095] The positive electrode may include: a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector.

[0096] There are no particular limitations on the positive electrode current collector, as long as it has high conductivity and does not cause chemical changes in the battery. The positive electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, nonwoven fabrics, etc., and the material of the positive electrode current collector can be, for example, aluminum, stainless steel, nickel, titanium, sintered carbon, aluminum-cadmium alloys, or combinations thereof. The thickness of the positive electrode current collector is typically from 3 μm to 500 μm. If necessary, fine irregularities can be formed on the surface of the positive electrode current collector to improve the adhesion of the positive electrode active material.

[0097] The positive electrode active material layer contains a positive electrode active material, and may also contain a conductive agent and a binder if necessary.

[0098] As the positive electrode active material, various positive electrode active materials used in the art can be used without limitation, and there is no particular limitation on the positive electrode active material. For example, the positive electrode active material may include: layered compounds such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or compounds replaced by one or more transition metals; lithium phosphate compounds such as LiFe 1-a Mn a PO4 (0≤a≤1); lithium manganese oxides such as Li 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5 and Cu2V2O7; and LiNi 1-c2 M c2 Lithium nickel oxide represented by O2 (where M is at least one of the following: cobalt (Co), manganese (Mn), aluminum (Al), zirconium (Zr), yttrium (Y), tungsten (W), molybdenum (Mo), copper (Cu), iron (Fe), magnesium (Mg), boron (B), and gallium (Ga), and c2 satisfies 0.01≤c2≤0.3); and lithium nickel oxide represented by the chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxides represented by O2 (where M is selected from at least one of the following groups: Co, Ni, Fe, chromium (Cr), zinc (Zn) and tantalum (Ta), and c3 satisfies 0.01≤c3≤0.1) or Li2Mn3M'O8 (where M' is selected from at least one of the following groups: Fe, Co, Ni, Cu and Zn), but the positive electrode active material is not limited to these.

[0099] The adhesive is a component that facilitates the bonding between the active material and the conductive agent, as well as the bonding with the current collector. Specifically, the adhesive may comprise at least one of the following: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0100] The conductive agent can be used in secondary batteries to assist and improve conductivity, and there are no particular limitations, as long as it is conductive and does not cause chemical changes. Specifically, the conductive agent may include at least one selected from the group consisting of: graphite such as natural or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0101] After preparing a positive electrode slurry by mixing the positive electrode active material with an optional binder and conductive agent in an organic solvent such as N-methyl-2-pyrrolidone (NMP), a positive electrode can be prepared by coating a positive electrode current collector with the positive electrode slurry and then drying and calendering the coated positive electrode current collector.

[0102] The separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is commonly used in lithium-ion secondary batteries. For example, the separator may include: polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure having two or more layers, and / or a nonwoven fabric formed from high-melting-point glass fiber or polyethylene terephthalate fiber as the substrate. To ensure heat resistance or mechanical strength, a coated separator containing ceramic components and / or polymer materials on the substrate can be used.

[0103] Next, the electrolyte may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes or molten inorganic electrolytes that can be used to prepare secondary batteries, but the present invention is not limited thereto.

[0104] For example, the electrolyte may contain an organic solvent and a lithium salt.

[0105] As the organic solvent, any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions participating in the battery electrochemical reaction can move. Specifically, as the organic solvent, the following can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group and may contain double-bonded aromatic rings or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate-based solvents are preferred, and more preferably, mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of the battery, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate).

[0106] The lithium salt can be used without particular limitation, as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, as the lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, or combinations thereof can be used. The lithium salt can be used in a concentration range of 0.1 M to 4.0 M, preferably 0.5 M to 3.0 M, and more preferably 0.8 M to 2.0 M. Because when the concentration of the lithium salt is within the above range, the electrolyte can have suitable conductivity and viscosity, thus obtaining excellent electrolyte performance and efficient lithium ion movement.

[0107] The secondary battery can be prepared by placing a separator between the negative and positive electrodes and then injecting an electrolyte, following conventional methods for preparing secondary batteries.

[0108] The lithium secondary battery according to the present invention is particularly suitable for use as a power source for electric vehicles because it has excellent fast-charging performance by using a negative electrode active material with excellent charging performance.

[0109] Hereinafter, embodiments of the invention will be described in detail in a manner that enables those skilled in the art to readily implement it. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[0110] Comparative Example 1

[0111] The coke raw material is introduced into a planetary mill and pulverized until the average particle size (D) is reached. 50 The particle size reaches 8 μm. The pulverized coke raw material and pitch are introduced into a granulator and granulated to prepare the precursor. In this case, pulverization and granulation are carried out at room temperature, atmospheric pressure, and in an air atmosphere.

[0112] The prepared precursor was placed in a graphitization furnace and heated to 3,000°C at a heating rate of 0.5°C / min, held for 12 hours, and then cooled to prepare artificial graphite.

[0113] Comparative Example 2

[0114] Except for the air blowing process at 100°C for 1 hour after crushing and granulation, before placing the precursor into the graphitization furnace, the precursor and artificial graphite were prepared in the same manner as in Comparative Example 1.

[0115] Example 1

[0116] Except for the air blowing process at 170°C for 1 hour after crushing and granulation, before placing the precursor into the graphitization furnace, the precursor and artificial graphite were prepared in the same manner as in Comparative Example 1.

[0117] Example 2

[0118] Except for the air blowing process at 220°C for 1 hour after crushing and granulation, before placing the precursor into the graphitization furnace, the precursor and artificial graphite were prepared in the same manner as in Comparative Example 1.

[0119] Example 3

[0120] Except for the air blowing process at 300°C for 1 hour after crushing and granulation, before placing the precursor into the graphitization furnace, the precursor and artificial graphite were prepared in the same manner as in Comparative Example 1.

[0121] Experimental Example 1 - Measurement of Precursor Oxygen Content

[0122] The precursors prepared in Examples 1 to 3 and Comparative Examples 1 and 2 were collected, and the elemental content of each precursor was measured using the following method. The measurement results are shown in Table 1 below.

[0123] <Methods for measuring elemental content>

[0124] Each precursor is introduced into an elemental analyzer and combusted while the temperature is increased. During the combustion process of the precursor, CO2, H2O, N2 and SO2 gases are mainly generated until the temperature reaches about 1000°C, and CO gas is generated due to thermal decomposition of oxygen (O) at around 1060°C. The generated gases are separated by a gas chromatography (GC) column, and a thermal conductivity detector (TCD) is used to quantitatively analyze the contents of C, H, N, O and S.

[0125] [Table 1]

[0126] Experimental Example 2

[0127] The tap density, d(002) and Lc(002) of each artificial graphite prepared in Examples 1 to 3 and Comparative Examples 1 and 2 were measured by the following method. The measurement results are shown in Table 2 below.

[0128] (1) Tap density (cc / g): 40 g of artificial graphite was collected, then placed into a sample container with volume graduations for tap density measurement, and the volume after 1000 times of tapping was measured to calculate the tap density.

[0129] (2) d(002) and Lc(002): The artificial graphite sample was placed in a powder measurement holder, and the sample surface was irradiated with X-rays under the following conditions using a Bruker D8 XRD equipment (Cu, λ=1.5418 Å) for measurement.

[0130] <XRD measurement conditions>

[0131] Divergence slit: 0.5°

[0132] Sample fixing distance: 3 mm

[0133] Step size for 2θ=20° to 80°: 0.00428°

[0134] Total scanning time = about 75 minutes

[0135] Experimental Example 3

[0136] A separator was arranged between a lithium counter electrode and each negative electrode prepared in Examples 1 to 4 and Comparative Examples 1 to 3 to prepare an electrode assembly. The electrode assembly was placed into a battery case, and then an electrolyte was injected to prepare a coin half-cell.

[0137] The coin half-cell was charged at 3.0C in a constant current (CC) mode to obtain a charging curve according to SOC, and the inflection point in the corresponding charging curve was evaluated as the point where lithium precipitation occurs to measure the SOC value when lithium precipitation occurs.

[0138] In addition, the coin half-cell was charged to 0.005 V at 0.1C in constant current / constant voltage (CCCV) mode with a cutoff current of 0.005C, and discharged to 1.5 V at 0.1C in CC mode to measure the discharge capacity and initial efficiency.

[0139] The measurement results are shown in Table 2 below.

[0140] [Table 2]

[0141] According to Table 2, the artificial graphite in Examples 1 to 3, prepared using precursors with an oxygen content of 3% to 15% by weight, has an interplanar spacing d(002) of 0.3360 nm or more on the (002) facets and a crystallite size Lc of 38.5 nm or less in the c-axis direction, confirming that cavities are formed between the crystal lattices. In contrast, the artificial graphite in Comparative Examples 1 and 2, prepared using precursors with an oxygen content of less than 3% by weight, has an interplanar spacing d(002) of less than 0.3360 nm on the (002) facets and a crystallite size Lc of greater than 38.5 nm in the c-axis direction.

[0142] Furthermore, it was found that the tap density of the artificial graphite in Examples 1 to 3 was higher than that of the artificial graphite in Comparative Examples 1 and 2.

[0143] Furthermore, compared with the coin half-cells using artificial graphite from Comparative Examples 1 and 2, the coin half-cells using artificial graphite from Examples 1 to 3 exhibited higher SOC values ​​when lithium deposition occurred during 3C charging, indicating that fast charging performance was improved when using artificial graphite from Examples 1 to 3.

Claims

1. A method for preparing graphite-based negative electrode active materials, the method comprising: A precursor with an oxygen content of 3% to 15% by weight is formed by crushing and granulating carbon-based raw materials; and Artificial graphite with a face spacing d(002) of 0.3360 nm to 0.3370 nm and a crystallite size Lc of 30.0 nm to 38.5 nm in the c-axis direction is prepared by graphitizing the precursor.

2. The method according to claim 1, wherein the carbon-based raw material comprises needle coke, embedded coke, coal tar pitch, resin pitch, soft carbon, or a combination thereof.

3. The method according to claim 1, wherein the formation of the precursor with an oxygen content of 3% to 15% by weight is carried out by at least one of the pulverization and granulation while heating to 200°C to 300°C in an oxygen atmosphere.

4. The method according to claim 3, wherein the heating is performed at a heating rate of 1°C / min to 20°C / min.

5. The method of claim 1, wherein the formation of the precursor with an oxygen content of 3% to 15% by weight is carried out by crushing and granulating the raw material, and then supplying air at 100°C to 300°C to contact the air with the raw material.

6. The method of claim 1, further comprising, after preparing the artificial graphite, forming a carbon-based coating by mixing the artificial graphite with a carbonaceous material and carbonizing the mixture.

7. The method according to claim 6, wherein the carbonaceous material is pitch.

8. The method of claim 1, wherein the artificial graphite comprises cavities in a lattice.

9. A graphite-based negative electrode active material comprising artificial graphite, wherein the interplanar spacing d(002) of the (002) facet of the artificial graphite is 0.3360 nm to 0.3370 nm and the crystallite size Lc in the c-axis direction is 30.0 nm to 38.5 nm.

10. The graphite-based negative electrode active material according to claim 9, wherein the graphite-based negative electrode active material has a tap density of 0.90 g / cc or higher.

11. The graphite-based negative electrode active material according to claim 9, wherein the artificial graphite is prepared by graphitizing a precursor with an oxygen content of 3% to 15% by weight.

12. A negative electrode comprising the graphite-based negative electrode active material according to any one of claims 9 to 11.

13. The negative electrode according to claim 12, wherein, When the half-cell prepared using the negative electrode and lithium metal counter electrode is charged at 3C, the state of charge (SOC) value at which lithium deposition occurs is 35% or higher.

14. A lithium secondary battery comprising the negative electrode as described in claim 12.

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