Lithium-ion secondary battery negative electrode and lithium-ion secondary battery containing the negative electrode

Abstract

This invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery comprising the negative electrode. The negative electrode comprises a negative electrode active material layer, which includes a first negative electrode active material and a second negative electrode active material. The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The second negative electrode active material comprises a second graphite and does not have a carbon layer on its surface. The weight ratio of the first negative electrode active material to the second negative electrode active material is 4:6 to 6:4, and the BET specific surface area of ​​the negative electrode active material layer is 1.1 to 1.7 m². 2 / g. Lithium secondary batteries containing the negative electrode for lithium secondary batteries according to the present invention can be charged quickly and have improved high-temperature characteristics and safety.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2021-0034291, filed in Korea on March 16, 2021.

[0002] This disclosure relates to a negative electrode for a lithium secondary battery and a lithium secondary battery comprising the negative electrode. In particular, this disclosure relates to a negative electrode capable of rapid charging and discharging and a lithium secondary battery comprising the negative electrode. Background Technology

[0003] In recent years, energy storage technology has received increasing attention. As its applications have expanded to power mobile phones, portable cameras, and laptops, and even electric vehicles, there is a growing need for high-energy-density batteries to power these electronic devices. Lithium-ion batteries best meet this need. Therefore, active research has been conducted on such lithium-ion batteries.

[0004] Typically, such lithium secondary batteries contain a positive electrode containing lithium metal oxide, a negative electrode containing carbonaceous materials, an electrolyte containing lithium salt and organic solvent, and a separator inserted between the positive and negative electrodes to electrically insulate the two electrodes from each other.

[0005] Typically, the negative electrode for lithium-ion rechargeable batteries is obtained by mixing a negative electrode active material, a conductive material, and a binder to form a slurry, coating the slurry onto a current collector, and then drying and pressing it. As a negative electrode active material, carbonaceous active materials capable of reversibly inserting / deintercalating lithium ions while maintaining structural and electrical properties are widely used. These carbonaceous active materials include various types of carbonaceous materials, such as artificial graphite, natural graphite, and hard carbon. Among them, graphite-based active materials, due to their high reversibility, are the most widely used, ensuring the lifespan characteristics of lithium-ion rechargeable batteries.

[0006] On the other hand, in order to improve the performance of lithium secondary batteries, there have been continuous attempts to manufacture lithium secondary batteries capable of rapid charging and discharging. The rapid charging and discharging performance of lithium secondary batteries mainly depends on their negative electrode. For this reason, graphite with carbon-coated surface is widely used as the negative electrode active material.

[0007] However, graphite coated with carbon exhibits high surface hardness, which can cause particle breakage during the pressing step in anode manufacturing. Furthermore, the increased specific surface area due to particle breakage leads to increased surface reactivity at high temperatures, resulting in undesirable side reactions. Additionally, lithium-ion batteries containing anodes using such active materials exhibit rapid performance degradation and high flammability at high temperatures, posing safety concerns. Summary of the Invention

[0008] Technical issues

[0009] This disclosure aims to address the problems of the prior art, and therefore aims to provide a fast charge / discharge negative electrode with improved high-temperature characteristics and safety.

[0010] This disclosure also aims to provide a lithium secondary battery including the negative electrode that is capable of rapid charging and discharging and has improved high-temperature characteristics and safety.

[0011] Technical solution

[0012] In one aspect of this disclosure, a negative electrode for a lithium secondary battery is provided according to any of the following embodiments.

[0013] According to a first embodiment, a negative electrode for a lithium secondary battery is provided, comprising:

[0014] Negative current collector; and

[0015] A negative electrode active material layer is disposed on at least one surface of the negative electrode current collector and comprises a first negative electrode active material, a second negative electrode active material, a conductive material, and a binder. The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The second negative electrode active material comprises a second graphite and does not have a carbon layer on the surface of the second graphite.

[0016] The weight ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:4, and

[0017] The BET specific surface area of ​​the negative electrode active material layer is 1.1-1.7 m². 2 / g.

[0018] According to the second embodiment, a negative electrode for a lithium secondary battery as defined in the first embodiment is provided, wherein the total pore volume of the negative electrode active material layer is 0.010-0.017 cm³. 3 / g.

[0019] According to the third embodiment, a negative electrode for a lithium secondary battery as defined in the first or second embodiment is provided, wherein the first graphite comprises natural graphite, artificial graphite, graphitized carbon fiber, graphitized mesophase carbon microspheres, or two or more thereof.

[0020] According to the fourth embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to third embodiments is provided, wherein the amorphous carbon layer comprises amorphous carbon derived from glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, polyurethane resin, polyimide resin, coal tar pitch, petroleum tar pitch, low molecular weight heavy oil, or two or more thereof.

[0021] According to the fifth embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to fourth embodiments is provided, wherein the amorphous carbon layer has an amount of 1-20% by weight based on 100% by weight of the total weight of the first negative electrode active material.

[0022] According to the sixth embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to fifth embodiments is provided, wherein the average particle size of the first negative electrode active material is 5-30 μm.

[0023] According to the seventh embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to sixth embodiments is provided, wherein the BET specific surface area of ​​the first negative electrode active material is 0.5-2m². 2 / g.

[0024] According to the eighth embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to seventh embodiments is provided, wherein the second graphite comprises natural graphite, artificial graphite, graphitized carbon fiber, graphitized mesophase carbon microspheres, or two or more thereof.

[0025] According to the ninth embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to eighth embodiments is provided, wherein the average particle size of the second negative electrode active material is 13-25 μm.

[0026] According to the tenth embodiment, a negative electrode for a lithium secondary battery as defined in any one of the first to ninth embodiments is provided, wherein the BET specific surface area of ​​the second negative electrode active material is 0.5-2m². 2 / g.

[0027] In another aspect of this disclosure, a lithium secondary battery according to the following embodiments is provided.

[0028] According to the eleventh embodiment, a lithium secondary battery is provided, comprising a negative electrode for a lithium secondary battery as defined in any one of the first to tenth embodiments.

[0029] Beneficial effects

[0030] According to one embodiment of the present disclosure, the negative electrode for a lithium secondary battery includes a negative electrode active material layer comprising a first negative electrode active material and a second negative electrode active material. The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The second negative electrode active material comprises a second graphite and does not have a carbon layer on the surface of the second graphite. Therefore, it is capable of rapid charging and discharging and shows improvement in the phenomenon of an increase in the total specific surface area of ​​the negative electrode due to the breakage of the negative electrode active material during the pressing step during the manufacturing of the negative electrode.

[0031] Specifically, an embodiment of the negative electrode for a lithium secondary battery according to this disclosure includes a negative electrode active material layer. This layer contains, within a predetermined weight ratio range, a first negative electrode active material comprising a first graphite and an amorphous carbon layer disposed on the surface of the first graphite, and a second negative electrode active material comprising a second graphite but without a carbon layer on its surface. This improves the interfacial resistance of the surface, thereby facilitating rapid charge and discharge. Furthermore, it minimizes the increase in the specific surface area of ​​the negative electrode active material caused by surface breakage of the negative electrode active material particles due to the pressing step during negative electrode manufacturing.

[0032] Furthermore, the negative electrode for a lithium secondary battery according to one embodiment of this disclosure has a specific range of total BET specific surface area of ​​the negative electrode active material layer, thereby providing the negative electrode with improved surface reactivity at high temperatures, thereby achieving improved high-temperature characteristics and safety.

[0033] According to one embodiment of this disclosure, a lithium secondary battery includes the aforementioned negative electrode for lithium secondary batteries, thus enabling rapid charging and discharging and possessing improved high-temperature characteristics and safety. Attached Figure Description

[0034] 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 is not to be construed as being limited to the accompanying drawings.

[0035] Figure 1 This is a graph showing the capacity retention of each lithium secondary battery according to Examples 1 and 2 and Comparative Examples 1-3, determined by performing the following cycle, in which each cycle includes charging each battery to 4.45V at a constant current (CC) mode at a rate of 1.5C at a high temperature (45°C), charging the battery to a charging cutoff current of 0.005C at a constant voltage (CV) mode, and discharging the battery to 3V at a rate of 1C at a constant current mode. Detailed Implementation

[0036] 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 based on the meaning and concepts corresponding to the technical aspects of the present disclosure, on the principle of allowing the inventors to appropriately define the terminology for the best interpretation.

[0037] Therefore, the description presented herein is merely a preferred example for illustrative purposes only 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.

[0038] In one aspect of this disclosure, a negative electrode for a lithium secondary battery is provided, comprising:

[0039] Negative current collector; and

[0040] A negative electrode active material layer is disposed on at least one surface of the negative electrode current collector and comprises a first negative electrode active material, a second negative electrode active material, a conductive material, and a binder. The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The second negative electrode active material comprises a second graphite and does not have a carbon layer on the surface of the second graphite.

[0041] The weight ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:4, and

[0042] The BET specific surface area of ​​the negative electrode active material layer is 1.1-1.7 m². 2 / g.

[0043] There are no particular limitations on the negative electrode current collector, as long as it has high conductivity and does not cause any chemical changes in the corresponding battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatments of carbon, nickel, titanium, silver, etc., aluminum-cadmium alloys, etc., can be used.

[0044] According to one embodiment of this disclosure, the thickness of the negative electrode current collector can be 3-500 μm.

[0045] The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The first negative electrode active material may have a core-shell structure, comprising a first graphite particle core and a shell formed by the amorphous carbon layer disposed on the surface of the core. The amorphous carbon layer formed on the surface of the first graphite facilitates the insertion / extraction of lithium ions at the particle surface and reduces the diffusion resistance of lithium ions. According to one embodiment of this disclosure, a negative electrode for a lithium secondary battery contains the first negative electrode active material comprising a first graphite and an amorphous carbon layer disposed on the surface of the first graphite, thus reducing the diffusion resistance of lithium ions and promoting rapid charge and discharge.

[0046] According to one embodiment of this disclosure, the first graphite may comprise natural graphite, artificial graphite, graphitized carbon fiber, graphitized mesophase carbon microspheres, or two or more thereof.

[0047] Natural graphite refers to graphite that is produced and mined in nature, while artificial graphite refers to graphite obtained by carbonizing coal and petroleum-based pitch at temperatures above 2500°C. Any artificial graphite can be used without particular limitation, as long as it is prepared by conventional methods known to those skilled in the art. Non-limiting examples of artificial graphite include artificial graphite obtained by calcining mesophase carbon microspheres (MCMB) at 2800-3000°C, embedding coke, etc. According to one embodiment of this disclosure, the first graphite may comprise artificial graphite. When the first graphite comprises artificial graphite, fewer crystal surface edges are exposed on the particle surface compared to natural graphite; therefore, such artificial graphite has a relatively small specific surface area, thereby easily reducing side reactions on the surface.

[0048] According to one embodiment of this disclosure, the average particle size of the first graphite can be 3-25 μm or 5-25 μm. When the average particle size of the first graphite meets the above-defined range, it is possible to prevent the problem of increased specific surface area of ​​the first graphite and reduced initial battery efficiency. Furthermore, it is easy to prevent problems such as reduced adhesion to the current collector and reduced battery capacity due to decreased packing density.

[0049] As used in this article, the term "average particle size" refers to D 50 Particle size, and "D 50 "Particle size" refers to the particle size at the 50% point in the cumulative particle number distribution based on particle size. The particle size D can be determined using laser diffraction. 50Specifically, the powder to be analyzed is dispersed in a dispersion medium and introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The differences in the diffraction pattern according to particle size are then determined as the particles pass through the laser beam, and the particle size distribution is calculated. The particle size at the 50% point of the cumulative particle number distribution based on particle size is then calculated to determine D. 50 .

[0050] According to one embodiment of this disclosure, the amorphous carbon layer may comprise amorphous carbon derived from glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, polyurethane resin, polyimide resin, coal tar pitch, petroleum tar pitch, low molecular weight heavy oil, or two or more of the above.

[0051] According to one embodiment of this disclosure, based on 100% by weight of the total weight of the first negative electrode active material, the amorphous carbon layer may be present in an amount of 1-20% by weight, 1-10% by weight, 1-3% by weight, or 3-10% by weight. When the content of the amorphous carbon layer meets the above-defined range, the diffusion resistance of lithium ions can be reduced, and the capacity of the negative electrode active material can be easily ensured.

[0052] For example, the content of the amorphous carbon layer can be calculated by heat-treating the first negative electrode active material at 1000°C and determining the weight reduction ratio, but is not limited thereto.

[0053] According to one embodiment of this disclosure, the thickness of the amorphous carbon layer can be 1-1000 nm, 5-800 nm, 5-500 nm, or 500-800 nm. When the thickness of the amorphous carbon layer meets the above-defined range, a solid electrolyte interface (SEI) layer can be easily and stably formed. Furthermore, lithium-ion conductivity is ensured, thus easily ensuring fast charge / discharge characteristics.

[0054] The thickness of the amorphous carbon layer can be determined, for example, by using a transmission electron microscope (TEM), but is not limited thereto.

[0055] According to one embodiment of this disclosure, the average particle size of the first negative electrode active material can be 5-30 μm, 5-20 μm, 9-17 μm, 9-15 μm, or 15-17 μm. When the average particle size of the first negative electrode active material meets the above-defined range, it is possible to prevent the problem of increased specific surface area of ​​the first negative electrode active material and decreased initial battery efficiency. Furthermore, it is easy to prevent problems such as reduced adhesion to the current collector and decreased battery capacity due to reduced filler density. When the average particle size of the first negative electrode active material meets the above-defined range, the negative electrode active material layer can easily have a particle size of 1.1-1.7 μm. 2 / g of total BET specific surface area.

[0056] According to one embodiment of this disclosure, the BET specific surface area of ​​the first negative electrode active material can be 0.5-2m². 2 / g, 0.7-1.5m 2 / g, 0.7-0.9m 2 / g or 0.9-1.5m 2 / g. When the BET specific surface area of ​​the first negative electrode active material meets the above-defined range, problems such as deterioration of output characteristics during charging and discharging, reduction of initial efficiency, and increase of side reactions on the surface can be prevented. When the first negative electrode active material meets the above-defined BET specific surface area range, the negative electrode active material layer can easily have a thickness of 1.1-1.7m. 2 / g of total BET specific surface area.

[0057] The BET specific surface area can be determined using the Brunauer-Emmett-Teller (BET) method. For example, the BET specific surface area can be determined as follows: The first negative electrode active material is introduced into a standard sample cell (glass) and dried under vacuum at 130°C for 2 hours. Liquid nitrogen is introduced into a liquid nitrogen container for measurement at 77K. The standard sample cell is cooled and then fixed to the BELSORP-mino II testing system (BEL Japan) for measurement in liquid nitrogen. The test procedure is then run to determine the BET specific surface area by adsorption / desorption processes on the sampled surface of the first negative electrode active material under a nitrogen atmosphere.

[0058] The second negative electrode active material comprises a second graphite and has no carbon layer on its surface. Because the second negative electrode active material lacks a carbon layer on its surface, it exhibits low surface hardness, thereby minimizing particle breakage during the pressing step in the manufacture of the negative electrode. Therefore, since the negative electrode for a lithium secondary battery according to embodiments of this disclosure comprises a second negative electrode active material in addition to the first negative electrode active material, rapid charge and discharge are promoted, while the increase in specific surface area caused by breakage on the surface of the negative electrode active material is mitigated.

[0059] According to one embodiment of this disclosure, the second graphite may comprise natural graphite, artificial graphite, graphitized carbon fibers, graphitized mesophase carbon microspheres, or two or more thereof. According to one embodiment of this disclosure, the second graphite may comprise artificial graphite. When the second graphite comprises artificial graphite, compared to natural graphite, fewer edges of the crystal surface are exposed on the particle surface; therefore, such artificial graphite has a relatively small specific surface area, thereby easily reducing side reactions on the surface.

[0060] According to one embodiment of this disclosure, the average particle size of the second negative electrode active material can be 13-25 μm or 14-18 μm. When the average particle size of the second negative electrode active material meets the above-defined range, it is possible to prevent the problem of increased specific surface area of ​​the second negative electrode active material and decreased initial battery efficiency. Furthermore, it is easy to prevent problems such as reduced adhesion to the current collector and decreased battery capacity due to reduced filler density. When the average particle size of the second negative electrode active material meets the above-defined range, the negative electrode active material layer can easily have a particle size of 1.1-1.7 μm. 2 / g of total BET specific surface area.

[0061] According to one embodiment of this disclosure, the BET specific surface area of ​​the second negative electrode active material can be 0.5-2m². 2 / g, 0.9-1.8m 2 / g, 0.9-1.3m 2 / g or 1.3-1.8m 2 / g. When the BET specific surface area of ​​the second negative electrode active material meets the above-defined range, problems such as deterioration of output characteristics, reduction of initial efficiency, and increase of side reactions on the surface during charge and discharge can be prevented. When the BET specific surface area of ​​the second negative electrode active material meets the above-defined range, the negative electrode active material layer can easily have a thickness of 1.1-1.7m. 2 / g of total BET specific surface area.

[0062] The BET specific surface area can be determined using the Brunol-Emmett-Teller (BET) method. For example, the BET specific surface area can be determined as follows: The second negative electrode active material is introduced into a standard sample cell (glass) and dried under vacuum at 130°C for 2 hours. Liquid nitrogen is introduced into a liquid nitrogen container for measurement at 77K. The standard sample cell is cooled and then fixed to a BELSORP-mino II testing system (BEL Japan) for measurement in liquid nitrogen. The test procedure is then run to determine the BET specific surface area by adsorption / desorption processes on the surface of the sampled second negative electrode active material under a nitrogen atmosphere.

[0063] According to this disclosure, the weight ratio of the first negative electrode active material to the second negative electrode active material is 4:6 to 6:4. When the weight ratio of the first negative electrode active material to the second negative electrode active material meets the above-defined range, the conductivity is improved and the interfacial resistance on the surface is improved, thereby facilitating rapid charge and discharge. Furthermore, the increase in the specific surface area of ​​the negative electrode active material caused by the breakage of negative electrode active material particles during the pressing step in negative electrode manufacturing can be minimized.

[0064] When the weight ratio of the first negative electrode active material to the second negative electrode active material is less than 4:6, the interfacial resistance on the entire surface of the negative electrode active material layer increases, so lithium ions cannot easily embed into the negative electrode active material, which is not conducive to rapid charging and discharging.

[0065] When the weight ratio of the first negative electrode active material to the second negative electrode active material is greater than 6:4, the cracking of the active material becomes severe during the pressing step. As a result, the total BET specific surface area of ​​the negative electrode active material increases excessively, leading to a reduction in the lifespan and safety of the lithium secondary battery containing the negative electrode at high temperatures.

[0066] According to one embodiment of this disclosure, the weight ratio of the first negative electrode active material to the second negative electrode active material can be 5:5. When the weight ratio of the first negative electrode active material to the second negative electrode active material is 5:5, the surface reactivity of the negative electrode at high temperatures can be further enhanced, thereby further improving the high-temperature characteristics and safety of the negative electrode and the lithium secondary battery containing the negative electrode.

[0067] According to one embodiment of this disclosure, based on 100% by weight of the total weight of the negative electrode active material layer, the total content of the first and second negative electrode active materials can be 80-98% by weight or 90-98% by weight. When the content of the first and second negative electrode active materials meets the above-defined range, it is easy to ensure conductivity while fully realizing battery capacity and preventing an increase in resistance. Furthermore, it is easy to ensure adhesion to the current collector while ensuring processability.

[0068] According to one embodiment of this disclosure, based on 100% by weight of the negative electrode active material layer, the amount of the conductive material added can be 1-10% by weight. There are no particular limitations on the conductive material, as long as it is conductive and does not cause chemical changes in the corresponding battery. Specific examples of the conductive material include: graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; conductive fibers, such as carbon fibers or metal fibers; fluorocarbons; metal powders, such as aluminum or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives.

[0069] The adhesive is a component that facilitates the bonding between the active material and the conductive material, as well as the bonding with the current collector. Typically, the amount of adhesive added can be 1-10% by weight, based on the total weight of the negative electrode active material layer. Specific examples of the adhesive include polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0070] As the BET specific surface area of ​​the negative electrode active material layer increases, surface reactivity increases, leading to an increase in the content of the solid electrolyte interphase (SEI) layer. The SEI layer formed on the surface of the negative electrode decomposes and generates heat and gas at high temperatures, potentially causing an internal short circuit in the battery and ultimately significantly affecting the thermal safety of the negative electrode and the battery. Furthermore, with the increase in the BET specific surface area of ​​the negative electrode active material layer, the SEI layer content increases, thus further increasing the heat released by SEI layer decomposition, resulting in a significant decrease in the thermal safety of the negative electrode and the battery.

[0071] According to this disclosure, the BET specific surface area of ​​the negative electrode active material layer is 1.1-1.7 m². 2 / g. In this document, the BET specific surface area of ​​the negative electrode active material layer refers to the BET specific surface area of ​​the entire negative electrode active material layer comprising the mixture of the first negative electrode active material, the second negative electrode active material, the conductive material, and the binder, and is different from the individual BET specific surface areas of the first and second negative electrode active materials. The BET specific surface area of ​​the negative electrode active material layer refers to the BET specific surface area of ​​the pressed negative electrode active material layer. When the BET specific surface area of ​​the negative electrode active material layer meets the above-defined range, the interfacial resistance on the surface of the negative electrode active material layer is improved, thereby ensuring fast charging characteristics while reducing side reactions at high temperatures and exothermic reactions caused by SEI layer decomposition. Lithium secondary batteries having a negative electrode comprising the negative electrode active material layer exhibit improved lifetime characteristics and capacity retention at high temperatures and ensure safety against high-temperature fires. In this document, "high temperature" refers to a temperature above 45°C.

[0072] When the BET specific surface area of ​​the negative electrode active material layer is less than 1.1m² 2 At / g, the interfacial resistance on the surface of the negative electrode active material layer is high, leading to a deterioration in fast charging characteristics.

[0073] When the BET specific surface area of ​​the negative electrode active material layer is greater than 1.7m² 2 At a concentration of / g, surface reactivity increases, and side reactions also increase at high temperatures. Furthermore, the increased exothermic reaction from SEI layer decomposition could lead to battery ignition, thus compromising battery lifespan and stability at high temperatures.

[0074] The BET specific surface area of ​​the negative electrode active material layer can be determined based on the type and content of the first graphite, the amorphous carbon layer, and the second negative electrode active material, as well as the electrode pressing density.

[0075] According to one embodiment of this disclosure, the BET specific surface area of ​​the negative electrode active material layer may be affected not only by its composition but also by the electrode pressing density. For example, as the electrode pressing density increases, the electrode exhibits lower porosity, and therefore the BET specific surface area of ​​the negative electrode active material layer may change.

[0076] According to one embodiment of this disclosure, the porosity of the negative electrode active material layer can be 20%-30%, 20%-26%, or 26%-30%. When the porosity of the negative electrode active material layer meets the above-defined range, the negative electrode active material layer easily has a porosity of 1.1-1.7 μm. 2 / g BET specific surface area. In other words, when the weight ratio of the first negative electrode active material to the second negative electrode active material is in the range of 4:6-6:4, and the porosity of the negative electrode active material layer is 20%-30%, then the negative electrode active material layer easily has a BET specific surface area of ​​1.1-1.7m. 2 / g BET specific surface area. Specifically, when the weight ratio of the first negative electrode active material to the second negative electrode active material is 5:5, and the porosity of the negative electrode active material layer is 20%-30%, then the negative electrode active material layer easily has a surface area of ​​1.1-1.7m. 2 / g BET specific surface area.

[0077] According to one embodiment of this disclosure, the BET specific surface area of ​​the negative electrode active material layer can be 1.12-1.69 m². 2 / g. When the BET specific surface area of ​​the negative electrode active material layer meets the above-defined range, the interfacial resistance on the surface of the negative electrode active material layer is improved, thereby reducing side reactions at high temperatures and thus improving fast charging characteristics. Furthermore, it easily reduces the heat release caused by SEI layer decomposition. Therefore, high-temperature characteristics and safety can be easily improved.

[0078] The BET specific surface area of ​​the negative electrode active material layer can be determined by the following method: A completely dried negative electrode (including the negative electrode current collector) is cut into 8mm × 8mm pieces, and 30 such negative electrode samples are prepared. In this paper, the thickness of the negative electrode active material layer is approximately 80 μm, and its surface area is controlled at 3.6 mAh / cm². 2 The loading was determined. The sampled negative electrode active material layer was introduced into a standard sample cell (glass) and dried under vacuum at 130°C for 2 hours. Liquid nitrogen was introduced into a liquid nitrogen container for measurement at 77K. The standard sample cell was cooled and then fixed to the testing system BELSORP-minoII (BEL Japan) for measurement in liquid nitrogen. The test program was then run to determine the BET specific surface area by adsorption / desorption processes on the surface of the sampled electrode under a nitrogen atmosphere. In this paper, when calculating the BET specific surface area, the weight of the negative electrode active material layer excluding the negative electrode current collector is used as the sample weight input.

[0079] According to one embodiment of this disclosure, the total pore volume of the negative electrode active material layer can be 0.010-0.017 cm³. 3 / g.

[0080] In this document, the total pore volume of the negative electrode active material layer refers to the pore volume of the entire negative electrode active material layer comprising the mixture of the first negative electrode active material, the second negative electrode active material, the conductive material, and the binder, and is different from the individual pore volumes of the first and second negative electrode active materials. The total pore volume of the negative electrode active material layer refers to the total pore volume of the pressed negative electrode active material layer. For example, the total pore volume of the negative electrode active material layer can be measured simultaneously with the BET specific surface area determination, or it can be measured using mercury intrusion porosimetry, micro-CT, etc.

[0081] According to one embodiment of this disclosure, the negative electrode for a lithium secondary battery includes a negative electrode active material layer comprising a first negative electrode active material and a second negative electrode active material. The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The second negative electrode active material comprises a second graphite and does not have a carbon layer on its surface. Therefore, it shows an improvement in the phenomenon of an increase in the total specific surface area of ​​the negative electrode due to the breakage of the negative electrode active material during the pressing step during negative electrode manufacturing, and enables rapid charge and discharge. Thus, high-temperature characteristics and safety can be ensured.

[0082] According to one embodiment of this disclosure, the negative electrode of a lithium secondary battery exhibits a charging rate of 1.5C and a charging level of over 75% within 25 minutes.

[0083] In a lithium secondary battery negative electrode according to one embodiment of the present disclosure, the negative electrode active material layer has a BET specific surface area within a predetermined range after pressing, thus exhibiting improved surface reactivity, thereby providing improved high-temperature characteristics and safety.

[0084] The negative electrode for lithium secondary batteries can be manufactured by the following methods, but the manufacturing method of the negative electrode is not limited to these.

[0085] In another aspect of this disclosure, a method for manufacturing a negative electrode according to one embodiment of this disclosure is provided, comprising the following steps:

[0086] A first negative electrode active material comprising a first graphite and an amorphous carbon layer disposed on the surface of the first graphite is prepared;

[0087] A negative electrode active material slurry is prepared, comprising a first negative electrode active material prepared in the above steps, a second negative electrode active material containing a second graphite but without a carbon layer on the surface of the second graphite, a conductive material, a binder, and a dispersion medium; and

[0088] The negative electrode active material slurry is coated on at least one surface of the negative electrode current collector, and then pressed and dried.

[0089] In the following, a method for manufacturing a negative electrode according to one embodiment of the present disclosure will be described in detail with reference to its main parts.

[0090] First, a first negative electrode active material is prepared comprising a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. Refer to the description above for the first graphite and the amorphous carbon layer.

[0091] There are no particular limitations on the method for forming an amorphous carbon layer on the surface of the first graphite; any method known to those skilled in the art can be used. For example, a carbonaceous precursor material can be directly coated onto the first graphite, followed by heat treatment. In particular, the first graphite can be mixed with a carbonaceous precursor material or immersed in a carbonaceous precursor material, followed by heat treatment at 700-1300°C. When heat treatment is performed within the aforementioned temperature range, an amorphous carbon layer can be formed without affecting the crystal structure of the first graphite. In one variation, the amorphous carbon layer can be formed by chemical vapor deposition of the carbonaceous precursor material.

[0092] Next, a negative electrode active material slurry is prepared, which includes a first negative electrode active material, a second negative electrode active material containing a second graphite and having no carbon layer on the surface of the second graphite, a conductive material, a binder, and a dispersion medium.

[0093] For the first negative electrode active material, the second negative electrode active material, the conductive material, and the adhesive, please refer to the description above.

[0094] Depending on the specific type of adhesive, the dispersion medium can be used as a solvent capable of dissolving the adhesive, or as a dispersion medium that cannot dissolve the adhesive but can disperse it.

[0095] According to one embodiment of this disclosure, the dispersion medium may include N-methyl-2-pyrrolidone, acetone, methyl ethyl ketone, dimethylformamide, dimethylacetamide, methanol, ethanol, isopropanol, or two or more of these organic solvents, or water.

[0096] Next, the negative electrode active material slurry is coated onto at least one surface of the negative electrode current collector, and then pressed and dried.

[0097] Regarding the negative current collector, please refer to the description above.

[0098] The negative electrode active material slurry can be coated onto at least one surface of the negative electrode current collector. Non-limiting examples of coating methods include dip coating, die coating, roll coating, comma coating, doctor blade coating, reverse roll coating, direct roll coating, etc.

[0099] The coated negative electrode active material slurry can be dried using methods conventionally used for drying negative electrodes. For example, the coated negative electrode active material slurry can be dried with air at 80-150°C, 80-110°C, or 110-150°C for 30 seconds to 5 minutes, 30 seconds to 2 minutes, or 2-5 minutes. When the drying time is within the above-defined range, residual solvents can be removed without adversely affecting productivity.

[0100] The negative electrode for the lithium secondary battery can be used together with the positive electrode and the separator to manufacture the lithium secondary battery.

[0101] The lithium secondary battery according to this disclosure exhibits improved lifespan characteristics and capacity retention at high temperatures, as well as improved safety against fire at high temperatures, by using a negative electrode for a lithium secondary battery according to one embodiment of this disclosure.

[0102] Lithium secondary batteries can include lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0103] There are no particular limitations on the positive electrode used in combination with the negative electrode for lithium secondary batteries according to this disclosure, and the positive electrode can be obtained by using methods known in the art to bond a layer of positive active material comprising positive active material, conductive material and binder to a positive current collector.

[0104] The positive electrode active material can be any positive electrode active material conventionally used in lithium secondary batteries, and lithium-containing transition metal oxides can be used. Specific examples of lithium-containing transition metal oxides include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and Li(Ni) a Co b Mn c )O2(0 <a<1,0<b<1,0<c<1,a+b+c=1)、LiNi 1-y Co y O2, LiCo 1-y Mn y O2, LiNi 1-y Mn y O2 (0≤y<1), Li (Ni) a Co b Mn c )O4(0 <a<2,0<b<2,0<c<2,a+b+c=2)、LiMn 2-z Ni z O4, LiMn 2-z Co zO4 (0 < z < 2), LiCoPO4, LiFePO4, or two or more of them. In addition to such oxides, sulfides, selenides, and halides can also be used.

[0105] Non-limiting examples of the positive electrode current collector include foils made of aluminum, nickel, or a combination thereof.

[0106] Based on the total weight of the positive electrode active material layer, the addition amount of the conductive material used in the positive electrode active material layer can be 1 - 30% by weight. There is no particular limitation on the conductive material as long as it does not cause chemical changes in the corresponding battery and has conductivity. Specific examples of the conductive material include: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal cracking carbon black; conductive fibers, such as carbon fibers or metal fibers; fluorocarbons; metal powders, such as aluminum or nickel powders; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; conductive materials, such as polyphenylene derivatives, etc.

[0107] The binder contained in the positive electrode active material layer is a component that helps the adhesion of the active material and the conductive material and the adhesion to the current collector. Generally, based on the total weight of the active material layer, the addition amount of the binder can be 1 - 30% by weight. Specific examples of the binder include polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene terpolymer (EPDM), sulfonated EPDM, styrene - butadiene rubber, fluororubber, various copolymers, etc.

[0108] There is no particular limitation on the separator used in the lithium secondary battery according to the present disclosure, and the separator may include: a porous polymer substrate; and an organic / inorganic composite porous layer formed on at least one surface of the porous polymer substrate and containing a plurality of inorganic particles and a binder polymer. The separator is inserted between the positive electrode and the negative electrode and serves to insulate the positive electrode and the negative electrode from each other.

[0109] Any porous polymer substrate can be used as long as it is a substrate commonly used in the art. For example, the porous polymer substrate may include a polyolefin porous polymer membrane or non - woven fabric, but is not limited thereto.

[0110] Non - limiting examples of the polyolefin porous polymer membrane include membranes made of polyolefin polymers, such as polyethylene alone (including high - density polyethylene, linear low - density polyethylene, low - density polyethylene, or ultra - high - molecular - weight polyethylene), polypropylene, polybutene, or polyisopentene, or a combination of two or more of them.

[0111] In addition to polyolefin non-woven fabrics, the non-woven fabric may further include a non-woven fabric formed of polyethylene terephthalate, polybutylene terephthalate, polyester, polyoxymethylene, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, or polyethylene naphthalate alone or in combination. The non-woven fabric may have a structure of a spunbond non-woven fabric or a melt-blown non-woven fabric containing long fibers.

[0112] Although there is no particular limitation on the thickness of the porous polymer substrate, the thickness of the porous polymer substrate may be 3 - 50 μm or 3 - 15 μm. Although there is no particular limitation on the pore size and porosity in the porous polymer substrate, the pore size and porosity may be 0.01 - 50 μm and 10% - 95%, respectively.

[0113] There is no particular limitation on the inorganic particles as long as they are electrochemically stable. In other words, there is no particular limitation on the inorganic particles that can be used in this article as long as they do not cause oxidation and / or reduction within the operating voltage range of the applied battery (for example, 0 - 5V based on Li / Li + ). According to one embodiment of the present disclosure, the inorganic particles may include high dielectric constant inorganic particles having a dielectric constant of 5 or more, or 10 or more, inorganic particles having lithium ion transport ability, or two or more of them. Non-limiting examples of inorganic particles having a dielectric constant of 5 or more may include any one selected from BaTiO3, BaSO4, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3 (PLZT, where 0 < x < 1 and 0 < y < 1), Pb(Mg 1 / 3Nb 2 / 3 )O3 - PbTiO3 (PMN - PT), hafnium dioxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, Y2O3, SiO2, Al2O3, γ - AlOOH, Al(OH)3, SiC, TiO2, etc., or a mixture of two or more of them. However, the scope of the present disclosure is not limited thereto.

[0114] According to one embodiment of this disclosure, while there is no particular limitation on the particle size of the inorganic particles, the inorganic particles may have a particle size of about 0.01-10 μm or about 0.05-1.0 μm to form a coating with uniform thickness and appropriate porosity. In this document, the average particle size of the inorganic particles refers to the particle size (D50) corresponding to the 50% cumulative value from the smallest particles, calculated based on the particle size distribution determination of the graded particles using a general particle size distribution analyzer. The particle size distribution can be determined by laser diffraction.

[0115] According to one embodiment of this disclosure, the adhesive polymer contained in the diaphragm may include, but is not limited to: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polyvinylidene fluoride-co-chlorotrifluoroethylene, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-styrene-butadiene copolymer, imide, or two or more thereof.

[0116] According to one embodiment of this disclosure, the weight ratio of inorganic particles to adhesive polymer contained in the diaphragm can be 20:80-99.9:0.1, 50:50-99.5:0.5, or 70:30-80:20. When the weight ratio of inorganic particles to adhesive polymer falls within the above-defined range, sufficient adhesion between the inorganic particles can be ensured while also ensuring sufficient voids formed between the inorganic particles.

[0117] According to one embodiment of this disclosure, the organic / inorganic composite porous layer may have a structure in which the inorganic particles are bonded together by the binder polymer while filling each other in contact. In this way, interstitial volumes are formed between the inorganic filler particles, which become voids and thus form pores.

[0118] According to one embodiment of this disclosure, the lithium secondary battery includes an electrolyte, which may contain an organic solvent and a lithium salt. Furthermore, the electrolyte may include an organic solid electrolyte or an inorganic solid electrolyte.

[0119] Specific examples of the organic solvents include aprotic organic solvents such as N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, ethyl propionate, etc.

[0120] The lithium salt is a material that is readily soluble in non-aqueous electrolytes, and specific examples include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB. 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylic acids, lithium tetraphenylborate, lithium imino, etc.

[0121] In addition, the electrolyte may also contain pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, and N-substituted compounds. The electrolyte may contain azole ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, and aluminum trichloride to improve charge-discharge characteristics, flame retardancy, etc. Optionally, the electrolyte may also contain a halogen-containing solvent, such as carbon tetrachloride or trifluoroethylene, to impart flame retardancy. The electrolyte may also contain carbon dioxide gas to improve high-temperature storage characteristics.

[0122] Specific examples of the organic solid electrolyte may include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polyagitation lysine, polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ionizable dissociable groups, etc.

[0123] Specific examples of the inorganic solid electrolyte may include nitrides, halides and sulfates of Li, such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH and Li3PO4-Li2S-SiS2.

[0124] Depending on the manufacturing process of the final product and the required performance of the final product, the electrolyte can be injected at an appropriate step during the battery manufacturing process. In other words, the electrolyte can be injected before battery assembly or as a final step in battery assembly.

[0125] According to one embodiment of this disclosure, the application of a separator for lithium secondary batteries to a battery can be performed not only through a conventional winding process, but also by stacking and folding the separator and electrodes.

[0126] According to one embodiment of this disclosure, the separator for the lithium secondary battery can be inserted between the positive and negative electrodes of the lithium secondary battery. When an electrode assembly is formed by assembling multiple cells or electrodes, the separator can be inserted between adjacent cells or electrodes. The electrode assembly can have various structures, such as simple stacked type, jelly roll type, stack-folded type, layer-stacked type, etc.

[0127] Although there are no particular restrictions on the shape of the lithium secondary battery, it can be cylindrical, pouch-shaped or coin-shaped, and can be used as a can.

[0128] Methods of implementing the invention

[0129] The embodiments will be described more fully below to facilitate a clear understanding of this disclosure. However, the following embodiments may be implemented in many different forms and should not be construed as limited to the exemplary implementations set forth herein. Rather, these exemplary implementations are provided to make this disclosure thorough and complete, and to fully convey the scope of this disclosure to those skilled in the art.

[0130] Example 1

[0131] Manufacturing of negative electrodes for lithium secondary batteries

[0132] Artificial graphite (obtained from Shanshan Company) was coated with pitch and heat-treated at 1000-1300℃ to prepare the first negative electrode active material (average particle size: 15μm, BET specific surface area: 0.9m²). 2 / g), which contains an amorphous carbon layer on the surface of the first graphite.

[0133] Based on 100% by weight of the first negative electrode active material, the content of the amorphous carbon layer in the first negative electrode active material is 3% by weight, and the amorphous carbon layer has a thickness of about 500 nm.

[0134] Then, the first negative electrode active material was mixed with artificial graphite (average particle size: 18 μm, BET specific surface area: 1.3 m²). 2The negative electrode active material ( / g) was mixed in a weight ratio of 5:5, and the mixed negative electrode active material, carbon black as a conductive material, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as a binder were mixed in a weight ratio of 96.05:0.5:2.3:1.15 to prepare a negative electrode active material slurry.

[0135] The negative electrode active material slurry was coated onto one surface of an 8 μm thick copper foil, pressed using a roller press, and the resulting structure was dried under vacuum to obtain a negative electrode for lithium secondary batteries. Here, the porosity of the obtained negative electrode is 26%.

[0136] Manufacturing of lithium secondary batteries

[0137] A slurry of positive electrode active material, prepared by mixing LiCoO2 (as the positive electrode active material), PVDF (as the binder), and acetylene black (as the conductive material) in a weight ratio of 96.5:1.5:2, is coated onto a surface of an aluminum foil with a thickness of 10 μm, and then dried and pressed to obtain the positive electrode.

[0138] A 10 μm thick polyethylene substrate is inserted between the positive and negative electrodes obtained as described above, and then pressure is applied to obtain the electrode assembly.

[0139] An electrolyte (EC:PC:EP:PP = 2:1:2.5:4.5, LiPF6 1.4M) (ionic conductivity ≥6.5 mS / cm) was injected into the electrode assembly obtained as described above to obtain a lithium secondary battery.

[0140] Example 2

[0141] A negative electrode for a lithium secondary battery and a lithium secondary battery were obtained in the same manner as in Example 1, except that the first negative electrode active material and the second negative electrode active material used in Example 1 were mixed at a weight ratio of 6:4. Here, the porosity of the resulting negative electrode was 26%.

[0142] Comparative Example 1

[0143] A negative electrode for a lithium secondary battery and a lithium secondary battery were obtained in the same manner as in Example 1, except that the first negative electrode active material according to Example 1 was used alone. Here, the porosity of the obtained negative electrode is 26%.

[0144] Comparative Example 2

[0145] A negative electrode for a lithium secondary battery and a lithium secondary battery were obtained in the same manner as in Example 1, except that the first negative electrode active material and the second negative electrode active material used in Example 1 were mixed at a weight ratio of 7:3. Here, the porosity of the resulting negative electrode was 26%.

[0146] Comparative Example 3

[0147] A negative electrode for a lithium secondary battery and a lithium secondary battery were obtained in the same manner as in Example 1, except that the first negative electrode active material and the second negative electrode active material used in Example 1 were mixed at a weight ratio of 3:7. Here, the porosity of the resulting negative electrode was 26%.

[0148] Test Example 1: Determination of BET specific surface area and total pore volume of the negative electrode active material layer

[0149] In the negative electrodes for lithium secondary batteries according to Examples 1 and 2 and Comparative Examples 1-3, the BET specific surface area and total pore volume of the negative electrode active material layer were measured. The results are shown in Table 1 below.

[0150] The BET (Brunor-Emmett-Teller) specific surface area of ​​the negative electrode active material layer was determined by the following method.

[0151] Thirty completely dried negative electrodes (including the negative electrode current collector) were prepared by cutting them into 8mm × 8mm pieces. The sampled negative electrode active material layer was introduced into a standard sample cell (glass) and dried under vacuum at 130°C for 2 hours. Liquid nitrogen was introduced into a liquid nitrogen container for measurement at 77K. The standard sample cell was cooled and then fixed to the BELSORP-mino II testing system (BEL Japan) for measurement in liquid nitrogen. The test program was then run to determine the BET specific surface area by the adsorption / desorption process on the surface of the sampled electrode under a nitrogen atmosphere. In this paper, the weight of the negative electrode active material layer excluding the negative electrode current collector was used as the sample weight input when calculating the BET specific surface area.

[0152] The total pore volume of the negative electrode active material layer was measured simultaneously with the BET specific surface area of ​​the negative electrode active material layer.

[0153] [Table 1]

[0154]

[0155] Test Example 2: Evaluation of Capacity Retention of Lithium-ion Secondary Batteries at High Temperatures

[0156] The lithium secondary batteries according to Examples 1 and 2 and Comparative Examples 1 to 3 were charged at 45°C in constant current (CC) mode at a rate of 1.5C to 4.45V, and then charged in constant voltage (CV) mode to a charging cutoff current of 0.005C. They were then discharged in CC mode at a rate of 1C to 3V. Each of these charge-discharge cycles was considered one cycle, and the battery capacity retention was measured after repeated cycles at 45°C.

[0157] Calculate the capacity retention rate using the following formula:

[0158] Capacity retention rate = (Discharge capacity / Initial discharge capacity) × 100 (%)

[0159] The results are shown in Figure 1 middle.

[0160] Depend on Figure 1 It can be seen that, compared with the lithium secondary batteries according to Comparative Examples 1-3, the lithium secondary batteries according to Examples 1 and 2 respectively have a high capacity retention rate.

[0161] Test Example 3: Analysis of Hot Box Test Results

[0162] The lithium secondary batteries according to Examples 1 and 2 and Comparative Examples 1-3 were introduced into an oven and heated from room temperature at a rate of 5°C / min, and then left to stand at 140°C for 30 minutes. After reaching 140°C, the ignition rate within 30 minutes was determined. The results are shown in Table 2 below.

[0163] Here, the ignition rate is obtained by the ratio of batteries that, when heated and held at a certain temperature, experience a rapid temperature rise and voltage drop due to a short circuit in the battery, which simultaneously causes ignition.

[0164] [Table 2]

[0165] Ignition rate (%) at 140°C in a hot chamber. Example 1 20 Example 2 30 Comparative Example 1 100 Comparative Example 2 100 Comparative Example 3 60

[0166] As can be seen from Table 2, the ignition rates of the lithium secondary batteries according to Examples 1 and 2 at 140°C are significantly lower than those of the lithium secondary batteries according to Comparative Examples 1-3 at 140°C.

Claims

1. A negative electrode for a lithium secondary battery, comprising: Negative current collector; and A negative electrode active material layer is disposed on at least one surface of the negative electrode current collector and comprises a first negative electrode active material, a second negative electrode active material, a conductive material, and a binder. The first negative electrode active material comprises a first graphite and an amorphous carbon layer disposed on the surface of the first graphite. The second negative electrode active material comprises a second graphite and does not have a carbon layer on the surface of the second graphite. The weight ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:4, and The BET specific surface area of ​​the negative electrode active material layer is 1.1-1.7 m². 2 / g, The first graphite comprises artificial graphite. The second type of graphite comprises artificial graphite. The negative electrode active material layer has a porosity of 20% to 30%.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein the total pore volume of the negative electrode active material layer is 0.010-0.017 cm³. 3 / g.

3. The negative electrode for a lithium secondary battery according to claim 1, wherein the amorphous carbon layer comprises amorphous carbon derived from glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resin, naphthalene resin, polyvinyl alcohol resin, polyurethane resin, polyimide resin, coal tar pitch, petroleum tar pitch, low molecular weight heavy oil, or two or more thereof.

4. The negative electrode for a lithium secondary battery according to claim 1, wherein, Based on 100% of the total weight of the first negative electrode active material, the amorphous carbon layer has an amount of 1-20% by weight.

5. The negative electrode for a lithium secondary battery according to claim 1, wherein the average particle size of the first negative electrode active material is 5-30 μm.

6. The negative electrode for a lithium secondary battery according to claim 1, wherein the BET specific surface area of ​​the first negative electrode active material is 0.5-2 m². 2 / g.

7. The negative electrode for a lithium secondary battery according to claim 1, wherein the average particle size of the second negative electrode active material is 13-25 μm.

8. The negative electrode for a lithium secondary battery according to claim 1, wherein the BET specific surface area of ​​the second negative electrode active material is 0.5-2 m². 2 / g.

9. A lithium secondary battery comprising a negative electrode for a lithium secondary battery according to any one of claims 1 to 8.

Images