Lithium secondary battery, method for preparing negative electrode active material, and method for manufacturing lithium secondary battery

Abstract

The invention provides a lithium secondary battery, a method for preparing a negative electrode active material, and a method for manufacturing the lithium secondary battery. A lithium secondary battery according to an embodiment of the present disclosure comprises: a positive electrode; a negative electrode including a carbon-based negative electrode active material coated with reduced graphene oxide (r-GO); and an electrolyte including a propylene carbonate-based compound. The lithium secondary battery has excellent low-temperature performance and can prevent damage of the negative electrode active material.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to a lithium secondary battery, a method for preparing a negative electrode active material, and a method for manufacturing a lithium secondary battery. Background Art

[0002] With the development of electronics, communications and aerospace industries, the demand for lithium secondary batteries as energy sources is increasing rapidly. In particular, with the emphasis on the importance of global environmental protection policies, the electric vehicle market is growing rapidly, and research and development of lithium secondary batteries is being actively carried out both at home and abroad.

[0003] A lithium secondary battery includes a positive electrode (cathode), a negative electrode (anode) and a separator therebetween. The positive electrode and the negative electrode each have an active material capable of inserting and extracting lithium ions.

[0004] Generally, two- / three-component electrolytes based on ethylene carbonate (EC) are used as electrolytes for lithium secondary batteries. However, ethylene carbonate has a high melting point, so the operating temperature is limited, which can cause a significant decrease in battery performance at low temperatures. Therefore, propylene carbonate (PC) is mixed and used together for the purpose of improving low-temperature performance, but when propylene carbonate is used together as an electrolyte, there is a problem that it causes the interlayer exfoliation (exfoliation) of carbon-based negative electrode active materials, especially graphite, to destroy it instead of forming a stable SEI film on the surface of the carbon-based negative electrode active material.

[0005] Therefore, it is necessary to develop a new technology that can not only use propylene carbonate in the electrolyte to improve the low-temperature performance, but also prevent the carbon-based negative electrode active material from being destroyed by the addition of propylene carbonate. Summary of the invention

[0006] Technical issues

[0007] Embodiments of the present disclosure can provide a lithium secondary battery having excellent low-temperature performance and capable of preventing destruction of a negative electrode active material, a method for preparing a negative electrode active material, and a method for manufacturing a lithium secondary battery.

[0008] The lithium secondary battery according to an embodiment of the present disclosure can be widely used in green technology fields such as solar power generation and wind power generation using batteries, in addition to electric vehicles and battery charging stations.

[0009] Technical Solution

[0010] A lithium secondary battery according to an embodiment of the present disclosure includes: a positive electrode; a negative electrode including a carbon-based negative electrode active material coated with reduced graphene oxide (r-GO); and an electrolyte including a propylene carbonate-based compound.

[0011] In one embodiment, the electrolyte may further include a lithium salt.

[0012] In one embodiment, the carbon-based negative electrode active material coated with reduced graphene oxide may be a core-shell structure, wherein the core includes the carbon-based negative electrode active material and the shell includes reduced graphene oxide.

[0013] In one embodiment, the carbon-based negative electrode active material may include a negative electrode active material with a plate-like structure.

[0014] In one embodiment, the carbon-based negative electrode active material may include artificial graphite, natural graphite or a combination thereof.

[0015] In one embodiment, the carbon-based negative electrode active material may include an edge-plane, and the reduced graphene oxide (r-GO) surrounds the edge-plane of the carbon-based negative electrode active material.

[0016] In one embodiment, the negative electrode active material may include nitrogen.

[0017] The method for preparing a negative electrode active material according to an embodiment of the present disclosure includes: a step of nitrogen-doping a carbon-based negative electrode active material; a step of coating graphene oxide on the surface of the carbon-based negative electrode active material; and a step of reducing the graphene oxide.

[0018] In one embodiment, the step of nitrogen-doping the carbon-based negative electrode active material may include: a step of heat-treating the carbon-based negative electrode active material in an atmosphere where ammonia gas flows.

[0019] In one embodiment, the step of coating graphene oxide on the surface of the carbon-based negative electrode active material may include: dispersing the carbon-based negative electrode active material in a solvent; and adding the graphene oxide into the solvent in which the carbon-based negative electrode active material is dispersed.

[0020] In one embodiment, the graphene oxide may be coated on the surface of the carbon-based negative electrode active material by electrostatic attraction.

[0021] In one embodiment, the step of reducing the graphene oxide may include: a step of heat treating the carbon-based negative electrode active material coated with the graphene oxide in an atmosphere of hydrogen flow.

[0022] According to an embodiment of the present invention, a method for manufacturing a lithium secondary battery includes: preparing a carbon-based negative electrode active material coated with reduced graphene oxide (r-GO); manufacturing an electrode assembly including a positive electrode and a negative electrode including the negative electrode active material; and injecting an electrolyte including a propylene carbonate-based compound into a accommodating space for accommodating the electrode assembly.

[0023] In one embodiment, the step of preparing the carbon-based negative electrode active material coated with reduced graphene oxide may include: a step of nitrogen doping the carbon-based negative electrode active material; a step of coating graphene oxide on the surface of the carbon-based negative electrode active material; and a step of reducing the graphene oxide.

[0024] Technical Effects

[0025] According to the present disclosure, a lithium secondary battery having excellent low-temperature performance and capable of preventing destruction of a negative electrode active material, a method for preparing a negative electrode active material, and a lithium secondary battery can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A diagram for explaining a negative electrode active material according to an embodiment of the present disclosure;

[0027] Figure 2 is a flow chart for illustrating a method for manufacturing a lithium secondary battery according to an embodiment of the present disclosure;

[0028] Figure 3 is a flow chart for illustrating a method for preparing a negative electrode active material according to an embodiment of the present disclosure;

[0029] Figure 4 : is a graph showing the capacity retention rate of a lithium secondary battery according to a comparative example of an embodiment of the present disclosure.

[0030] Description of Reference Numerals

[0031] 11: Carbon-based negative electrode active material

[0032] 12: Reduced graphene oxide DETAILED DESCRIPTION

[0033] The present disclosure is described in detail below, but this is merely exemplary, and the present disclosure is not limited to the specific embodiments described exemplarily.

[0034] Lithium secondary battery

[0035] One aspect of the present disclosure provides a lithium secondary battery including: a positive electrode; a negative electrode including a carbon-based negative electrode active material coated with reduced graphene oxide (r-GO); and an electrolyte including a propylene carbonate-based compound.

[0036] The lithium secondary battery provided in one aspect of the present disclosure will be described below with respect to each constituent element.

[0037] First, one aspect of the present disclosure provides a lithium secondary battery including a positive electrode.

[0038] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer may be formed on at least one side of the positive electrode current collector. That is, the positive electrode active material layer may be formed on one side of the positive electrode current collector, or on both sides of the positive electrode current collector.

[0039] The positive electrode active material layer may include a positive electrode active material. The positive electrode active material may be a material in which lithium ions can be embedded and de-embedded. In one embodiment, the positive electrode active material may be a lithium metal oxide. For example, the positive electrode active material may be one of lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate compound, lithium manganese phosphate compound, lithium cobalt phosphate compound, and lithium vanadium phosphate compound, but is not limited to a specific example.

[0040] Also, an aspect of the present disclosure provides a lithium secondary battery including a negative electrode.

[0041] The negative electrode may include a negative electrode current collector and a negative electrode active material layer.

[0042] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the secondary battery and has conductivity, and can be, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. In addition, the binding force of the negative electrode active material can be strengthened by forming tiny concave and convex on the surface, and can be used in various forms such as film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

[0043] The negative electrode active material layer may be formed on at least one side of the negative electrode current collector, that is, the negative electrode active material layer may be formed on one side of the negative electrode current collector, or may be formed on both sides of the negative electrode current collector.

[0044] The negative electrode active material layer may include a negative electrode active material. The negative electrode active material may include a carbon-based negative electrode active material, and the surface of the carbon-based negative electrode active material may include reduced graphene oxide (r-GO). Figure 1Understanding the structure of the negative electrode active material according to an embodiment of the present disclosure In an embodiment, the negative electrode active material may be a core-shell structure consisting of a core of a carbon-based negative electrode active material 11 and a shell of reduced graphene oxide 12 .

[0045] In the embodiment, for the carbon-based negative electrode active material 11, in one embodiment, the above-mentioned carbon-based negative electrode active material may include one or more selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, conductive carbon black (super-P) and graphene.

[0046] Specifically, in the embodiment, the carbon-based negative electrode active material 11 may be a plate-like negative electrode active material, for example, a graphite-based negative electrode active material, for example, may include one or more selected from the group consisting of artificial graphite and natural graphite, but is not limited thereto.

[0047] Although not intending to be limited to a particular theory, when the electrolyte includes a propylene carbonate-based compound, the carbon-based negative electrode active material 11 may be peeled off due to the high binding energy between the propylene carbonate-based compound and lithium ions and the resulting clusters formed by the propylene carbonate-based compound and lithium ions. However, for the negative electrode active material according to one embodiment, since the reduced graphene oxide 12 is coated on the surface of the carbon-based negative electrode active material 11 as described above, the peeling of the carbon-based negative electrode active material can be suppressed even if the electrolyte includes a propylene carbonate-based compound.

[0048] Specifically, when the carbon-based negative electrode active material 11 has a plate-like structure, the carbon-based negative electrode active material may include an edge plane, and the reduced graphene oxide (r-GO) surrounds the edge plane of the carbon-based negative electrode active material, thereby preventing the peeling of the negative electrode active material caused by the propylene carbonate-based compound.

[0049] The negative electrode active material may include nitrogen. Specifically, the carbon-based negative electrode active material 11 may be doped with nitrogen, so that graphene oxide may be coated on the surface of the carbon-based negative electrode active material by utilizing electrostatic attraction.

[0050] In an embodiment, the negative electrode active material layer may further include a binder. The binder may be a material that improves the bonding force between the negative electrode current collector and the negative electrode active material and / or the bonding force between the negative electrode active materials. In an embodiment, the binder may include one or more selected from the group consisting of styrene-butadiene rubber (SBR)-based binders, carboxymethyl cellulose (CMC), polyacrylic acid (polyacrylic acid)-based binders, poly (3,4-ethylenedioxythiophene) ((poly (3,4-ethylenedioxythiophene), PEDOT)-based binders, polyvinylidene fluoride (polyvinylidenefluoride, PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (Poly (vinylidene fluoride-co-hexafluoropropylene)), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate (polymethylmethacrylate), acrylonitrile-butadiene rubber (NBR) and polybutadiene rubber (BR), but is not limited to a specific example.

[0051] In an embodiment, the negative electrode active material layer may further include a conductive material that can impart conductivity to the negative electrode. The conductive material may include, for example, one or more selected from the group consisting of a metal-based conductive material, a carbon-based conductive material, and a conductive polymer. The metal-based conductive material may be, for example, a metal powder or metal fiber such as copper, nickel, aluminum, silver, etc.; a conductive whisker such as zinc oxide, potassium titanate, etc.; or a conductive metal oxide such as titanium oxide. The carbon-based conductive material may be, for example, graphite, carbon black, graphene, or carbon nanotubes. The conductive polymer may be, for example, a polyphenylene derivative, etc.

[0052] In an embodiment, the negative electrode active material layer may further selectively include a thickener to ensure advantages in the manufacturing process. The thickener may improve the problem of cracks on the negative electrode surface by strengthening the cohesive force of the binder. The thickener may include, for example, one or more selected from the group consisting of carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose and cellulose gum.

[0053] Furthermore, a lithium secondary battery provided in one aspect of the present disclosure may include an electrolyte. Specifically, the electrolyte may include an electrolytic solution.

[0054] The electrolyte may include a propylene carbonate-based compound, wherein the propylene carbonate-based compound may be a substituted or unsubstituted propylene carbonate. Specifically, the propylene carbonate-based compound may be (unsubstituted) propylene carbonate. The electrolyte may be a non-aqueous electrolyte, and propylene carbonate may be included in the electrolyte as a solvent. In an embodiment, the electrolyte may further include a lithium salt.

[0055] The electrolyte includes the propylene carbonate-based compound, and thus a lithium secondary battery provided in one aspect of the present disclosure may have excellent low-temperature characteristics.

[0056] In an embodiment, a lithium secondary battery provided in one aspect of the present disclosure may further include a separator.

[0057] The separator may be between the negative electrode and the positive electrode. The separator is configured to prevent an electrical short circuit between the negative electrode and the positive electrode so that an ion flow is generated. The separator may include a porous polymer film or a porous non-woven fabric. Among them, the porous polymer film may be configured to include a single layer or multiple layers of polyolefin polymers such as ethylene polymers, propylene polymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. The porous non-woven fabric may include high melting point glass fibers and polyethylene terephthalate fibers. However, it is not limited to this. According to an embodiment, the separator may be a high heat-resistant separator (CCS; Ceramic Coated Separator) including ceramic.

[0058] The lithium secondary battery provided in one aspect of the present disclosure may include an electrode assembly, which may be manufactured by winding, laminating, folding or zigzag stacking processes of the above-mentioned negative electrode, positive electrode and separator. In addition, the electrode assembly may be provided together with the above-mentioned electrolyte to manufacture the secondary battery according to the present disclosure. The secondary battery may be any one of a cylindrical, square, pouch and coin shape using a can, but is not limited thereto.

[0059] Negative electrode active material manufacturing method and lithium secondary battery manufacturing method

[0060] In another aspect of the present disclosure, a method for preparing a negative electrode active material is provided, comprising: a step of nitrogen-doping a carbon-based negative electrode active material; a step of coating graphene oxide on the surface of the carbon-based negative electrode active material; and a step of reducing the graphene oxide.

[0061] In another aspect of the present disclosure, a method for manufacturing a lithium secondary battery is provided, comprising: a step of preparing a carbon-based negative electrode active material coated with reduced graphene oxide (r-GO); a step of manufacturing an electrode assembly comprising a positive electrode and a negative electrode comprising the above-mentioned negative electrode active material; and a step of injecting an electrolyte comprising a propylene carbonate-based compound into a accommodating space accommodating the above-mentioned electrode assembly.

[0062] In the following description, the negative electrode active material, the carbon-based negative electrode active material, the reduced graphene oxide, the positive electrode, the negative electrode, the propylene carbonate-based compound, the electrolyte, etc. are respectively as specifically described above.

[0063] Figure 2 is a flowchart for illustrating a method for manufacturing a lithium secondary battery according to an embodiment of the present disclosure. Figure 3 FIG. 1 is a flow chart for illustrating a method for preparing a negative electrode active material according to an embodiment of the present disclosure. Figure 2 and Figure 3 , describing a method for preparing a negative electrode active material and a method for manufacturing a lithium secondary battery according to an embodiment of the present disclosure.

[0064] refer to Figure 2 First, the method for manufacturing a lithium secondary battery according to an embodiment of the present disclosure can prepare a negative electrode active material through the operation of S100. The negative electrode active material can be a carbon-based negative electrode active material coated with reduced graphene oxide.

[0065] Available through Figure 3 More details Figure 2 Operation of the S100. Reference Figure 3 ,exist Figure 2 In the operation of preparing the negative electrode active material (S100), the carbon-based negative electrode active material may be nitrogen-doped by the operation of S110. In one embodiment, the carbon-based negative electrode active material may be nitrogen-doped by heat-treating the carbon-based negative electrode active material under an atmosphere of ammonia flow. Therefore, the nitrogen-doped carbon-based negative electrode active material may include nitrogen element.

[0066] and, Figure 2 The operation of preparing the negative electrode active material (S100) can coat graphene oxide on the surface of the carbon-based negative electrode active material through the operation of S120. In an embodiment, the operation of coating graphene oxide on the surface of the carbon-based negative electrode active material may include the operation of dispersing the carbon-based negative electrode active material in a solvent and the operation of adding graphene oxide to the solvent in which the carbon-based negative electrode active material is dispersed. In one embodiment, the solvent may be water, but is not limited thereto. In the operation of S120, the carbon-based negative electrode active material may be a carbon-based negative electrode active material doped with nitrogen through the operation of S110.

[0067] In an embodiment, graphene oxide can be coated on the surface of the carbon-based negative electrode active material by electrostatic attraction. In an embodiment, as a solution containing the carbon-based negative electrode active material and graphene oxide in a solvent is stirred, graphene oxide can be coated on the surface of the carbon-based negative electrode active material by electrostatic attraction.

[0068] In an embodiment, the nitrogen element doped in the carbon-based negative electrode active material may have a + polarity, and the hydroxyl group and the carboxyl group included in the graphene oxide may have a - polarity. Therefore, the part of the graphene oxide having a - polarity is combined with the part of the nitrogen-doped carbon-based negative electrode active material having a + polarity through electrostatic attraction, thereby, the graphene oxide can be coated on the surface of the carbon-based negative electrode active material.

[0069] and, Figure 2 The operation of preparing the negative electrode active material (S100) can reduce the graphene oxide coated on the surface of the carbon-based negative electrode active material through the operation of S130. In one embodiment, the carbon-based negative electrode active material coated with graphene oxide can be heat-treated under an atmosphere of hydrogen flow, thereby reducing the graphene oxide. Thus, at least a portion of the graphene oxide coated on the surface of the carbon-based negative electrode active material can be converted into reduced graphene oxide (Reduced Graphene Oxide, r-GO).

[0070] Next, the method for manufacturing a lithium secondary battery according to an embodiment of the present disclosure can manufacture an electrode assembly through the operation of S200. As described above, the electrode assembly may include a positive electrode and a negative electrode, and a separator may be included between the positive electrode and the negative electrode. The electrode assembly may be manufactured by performing a winding, lamination, folding or zigzag stacking process on the negative electrode, the positive electrode and the separator.

[0071] Next, according to the manufacturing method of a lithium secondary battery of one embodiment of the present disclosure, an electrolyte including a propylene carbonate-based compound can be injected into the accommodation space through the operation of S300. The accommodation space can be a space for accommodating an electrode assembly manufactured through the operation of S200. In one embodiment, the accommodation space can be configured as a space inside a shell of various shapes. In one embodiment, the electrolyte can be injected into the accommodation space after the electrode assembly is configured in the accommodation space, but it is not limited thereto. In another embodiment, the electrolyte can be injected into the accommodation space first and then the electrode assembly can be configured in the accommodation space.

[0072] As described above, the electrolyte may further include a lithium salt, and the solvent of the non-aqueous electrolyte solution that may be used as the electrolyte may include propylene carbonate. When propylene carbonate is used as the solvent of the electrolyte solution, the low temperature characteristics of the manufactured secondary battery may be improved.

[0073] Example

[0074] The embodiments of the present invention are further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only used to illustrate the present invention, and do not limit the scope of the attached claims. Various changes and modifications can be made to the embodiments within the scope of the present invention and the technical concept, which is obvious to those skilled in the art. Obviously, such deformation and modification belong to the scope of the attached claims.

[0075] <Manufacturing Example 1>-Preparation of negative electrode active material

[0076] Graphite was placed in a crucible and heat treated at 600° C. for 5 hours in an atmosphere in which 400 ppm of NH 3 gas was passed. Thus, the surface of the heat-treated graphite was doped with nitrogen.

[0077] Next, nitrogen-doped graphite and graphene oxide were placed in water at a weight ratio of 99:1, and stirred at 400 rpm for 2 hours at room temperature using a magnetic bar. With the help of the electrostatic attraction between the graphene oxide dispersed in water and the nitrogen-doped graphite, the graphene oxide was evenly coated on the surface of the nitrogen-doped graphite.

[0078] Thereafter, the graphite coated with graphene oxide was filtered and put back into the crucible, and heat treated at 800° C. for 8 hours in an atmosphere of 1000 ppm H 2 gas flowing therethrough, thereby reducing the graphene oxide on the graphite surface to reduced graphene oxide.

[0079] <Example 1> - Fabrication of coin half-cell

[0080] The negative electrode active material of Preparation Example 1, multi-walled carbon nanotubes, carboxymethyl cellulose, and styrene butadiene rubber were mixed in water at a weight ratio of 97.1:0.4:1.2:1.3 to prepare a negative electrode active material composition.

[0081] The prepared negative electrode active material composition was coated on a copper thin film as a negative electrode current collector, and then dried under vacuum at 130° C. for 1 hour, thereby manufacturing a negative electrode having a negative electrode active material layer formed on the copper thin film.

[0082] Then, a coin half-cell of the CR2032 type using the negative electrode manufactured by the above process was manufactured, using metallic lithium as the counter electrode, a PE separator as the separator, and a substance containing 1.0M LiPF6 dissolved in propylene carbonate solvent as the electrolyte.

[0083] <Comparative Example 1>

[0084] A coin half cell was manufactured in the same manner as in Example 1, except that uncoated graphite was used instead of the negative electrode active material of Manufacturing Example 1.

[0085] <Comparative Example 2>

[0086] A coin half cell was produced in the same manner as in Comparative Example 1, except that ethylene carbonate was used as the solvent of the electrolyte solution instead of propylene carbonate.

[0087] <Experimental Example 1> Confirmation of charge and discharge performance

[0088] For the half-cells of Example 1, Comparative Example 1 and Comparative Example 2, the first charge / discharge capacity was confirmed using an electrochemical charger / discharger, and the initial efficiency was calculated based on the capacity.

[0089] The capacity per gram of the half-cells of Example 1, Comparative Example 1 and Comparative Example 2 was calculated under the conditions of charge (0.1C CC / CV charge 0.005V 0.005C cut) and discharge (0.1C CC discharge, 1.5V cut), and the initial efficiency was calculated by calculating the discharge capacity / charge capacity. The results are recorded in the following Table 1.

[0090]

Table 1

[0091] Initial efficiency (%) Example 1 92 Comparative Example 1 78 Comparative Example 2 92

[0092] That is, referring to Table 1, it can be confirmed that Example 1 using the negative electrode active material of Production Example 1 has an initial efficiency at a level similar to that of Comparative Example 2 using ethylene carbonate as the solvent of the electrolyte even when propylene carbonate is used as the solvent of the electrolyte.

[0093] Furthermore, it was confirmed that Example 1 using the negative electrode active material in which the graphite surface was coated with reduced graphene oxide as described in Preparation Example 1 had significantly better initial efficiency than Comparative Example 1 using uncoated graphite as the negative electrode active material.

[0094] <Experimental Example 2> Confirmation of Capacity Retention Rate

[0095] The capacity retention rate of the half-cells of Example 1, Comparative Example 1 and Comparative Example 2 at 25° C. was evaluated using an electrochemical charger / discharger.

[0096] The capacity changes of the half-cells of Example 1, Comparative Example 1 and Comparative Example 2 were observed under the conditions of charge (0.5C CC / CV charge 0.005V 0.005C cut) and discharge (0.5C CC discharge 1.0V cut). The results are shown in Figure 4 middle.

[0097] refer to Figure 4It can be confirmed that Example 1 using the negative electrode active material of Preparation Example 1 has a capacity retention rate at a level similar to that of Comparative Example 2 using ethylene carbonate as the solvent of the electrolyte even when propylene carbonate is used as the solvent of the electrolyte.

[0098] Furthermore, it was confirmed that Example 1 using the negative electrode active material in which the graphite surface was coated with reduced graphene oxide as described in Preparation Example 1 had significantly better capacity retention than Comparative Example 1 using uncoated graphite as the negative electrode active material.

[0099] Combining Experimental Example 1 and Experimental Example 2, when graphite is used as the negative electrode active material in Comparative Example 1 and propylene carbonate is used as the solvent of the electrolyte, propylene carbonate and lithium ions form clusters, and when these clusters are embedded in the interlayer structure of graphite, graphite is peeled off, thereby generating irreversible capacity. For this reason, the initial efficiency and capacity retention rate of Comparative Example 1 are low.

[0100] On the contrary, when graphite coated with reduced graphene oxide is used as the negative electrode active material as in Example 1, even if propylene carbonate is used as the solvent of the electrolyte, the negative electrode active material can be prevented from being peeled off due to the cluster embedding of propylene carbonate and lithium ions. Thus, Example 1 can have an initial efficiency and capacity retention rate similar to those of Comparative Example 2 in which no clusters are generated.

[0101] Furthermore, it is expected that the low-temperature characteristics of Example 1 in which propylene carbonate is used as a solvent for the electrolyte solution are superior to those of Comparative Example 2 in which ethylene carbonate is used as a solvent for the electrolyte solution.

Claims

1. A lithium secondary battery, comprising: positive electrode; A negative electrode comprising a carbon-based negative electrode active material coated with reduced graphene oxide; as well as The electrolyte includes a propylene carbonate-based compound. 2 . The lithium secondary battery according to claim 1 , wherein the electrolyte further comprises a lithium salt. 3 . The lithium secondary battery according to claim 1 , wherein the carbon-based negative electrode active material coated with reduced graphene oxide is a core-shell structure, wherein the core comprises the carbon-based negative electrode active material and the shell comprises reduced graphene oxide. 4 . The lithium secondary battery according to claim 1 , wherein the carbon-based negative electrode active material comprises a negative electrode active material having a plate-like structure. 5 . The lithium secondary battery according to claim 1 , wherein the carbon-based negative electrode active material comprises artificial graphite, natural graphite or a combination thereof.

6. The lithium secondary battery according to claim 1, wherein the carbon-based negative electrode active material comprises an edge plane, The reduced graphene oxide surrounds the edge plane of the carbon-based negative electrode active material. 7 . The lithium secondary battery according to claim 1 , wherein the negative electrode active material comprises nitrogen.

8. A method for preparing a negative electrode active material, comprising: The step of nitrogen doping the carbon-based negative electrode active material; A step of coating graphene oxide on the surface of the carbon-based negative electrode active material; as well as The step of reducing the graphene oxide.

9. The method for preparing a negative electrode active material according to claim 8, wherein the step of nitrogen doping the carbon-based negative electrode active material comprises: The step of heat treating the carbon-based negative electrode active material in an atmosphere of ammonia flow.

10. The method for preparing a negative electrode active material according to claim 8, wherein the step of coating graphene oxide on the surface of the carbon-based negative electrode active material comprises: The step of dispersing the carbon-based negative electrode active material in a solvent; as well as The step of adding the graphene oxide into a solvent in which the carbon-based negative electrode active material is dispersed. 11 . The method for preparing a negative electrode active material according to claim 10 , wherein the graphene oxide is coated on the surface of the carbon-based negative electrode active material by electrostatic attraction.

12. The method for preparing a negative electrode active material according to claim 8, wherein the step of reducing the graphene oxide comprises: The step of heat treating the carbon-based negative electrode active material coated with the graphene oxide in an atmosphere of hydrogen flow.

13. A method for manufacturing a lithium secondary battery, comprising: The step of preparing a carbon-based negative electrode active material coated with reduced graphene oxide; The step of manufacturing an electrode assembly including a positive electrode and a negative electrode including the negative electrode active material; as well as The step of injecting an electrolyte including a propylene carbonate-based compound into a housing space for housing the electrode assembly.

14. The method for manufacturing a lithium secondary battery according to claim 13, wherein the step of preparing a carbon-based negative electrode active material coated with reduced graphene oxide comprises: The step of nitrogen doping the carbon-based negative electrode active material; A step of coating graphene oxide on the surface of the carbon-based negative electrode active material; as well as The step of reducing the graphene oxide.

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