Lithium secondary battery and method for manufacturing anode for lithium secondary battery

Abstract

A lithium secondary battery according to an exemplary embodiment includes a cathode, an isolation layer interposed between the cathode and the anode, and an anode opposite the cathode. The anode includes an anode current collector and an anode active material layer formed on the anode current collector. The anode active material layer includes an anode active material and a conductive additive. The conductive additive includes expandable graphite having a particle size of 10 [mu] m or less.

Description

[0001] This application is a divisional application of Chinese patent application filed on September 15, 2020, with Chinese patent application number 202010968165.3 and the invention title "Lithium secondary battery and method for manufacturing an anode for lithium secondary battery", and this application claims priority to Korean application with application number 10-2019-0113279. Technical Field

[0002] This invention relates to lithium secondary batteries and a method for manufacturing an anode for lithium secondary batteries. More specifically, this invention relates to lithium secondary batteries comprising graphite-based materials and a method for manufacturing an anode for the same lithium secondary battery. Background Technology

[0003] With the development of information and display technologies, rechargeable and dischargeable secondary batteries have been widely used as power sources for portable electronic devices such as portable cameras, mobile phones, and portable computers. Secondary batteries include, for example, lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Lithium-ion batteries have attracted attention due to their high operating voltage, energy density per unit weight, high charging rate, and compact size.

[0004] For example, a lithium secondary battery may include an electrode assembly comprising a cathode, an anode, a separator (diaphragm), and an electrolyte immersed in the electrode assembly. The lithium secondary battery may further include a casing having, for example, a pouch-like shape.

[0005] Recently, as the application of lithium-ion batteries has expanded from compact electronic devices to larger devices such as hybrid vehicles, conventional lithium-ion batteries may no longer be able to provide sufficient capacity and power.

[0006] For example, in lithium-ion batteries used in automobiles, repeated charging and discharging operations are required due to rapid power consumption.

[0007] Therefore, it may be necessary to develop lithium secondary batteries that can maintain mechanical and operational stability even after hundreds of repeated charge and discharge cycles. Summary of the Invention

[0008] According to one aspect of the present invention, an anode for a lithium secondary battery is provided, which has improved charging and discharging performance.

[0009] According to an exemplary embodiment, a method is provided for manufacturing an anode for a lithium secondary battery having improved charging and discharging performance.

[0010] According to one aspect of the present invention, a lithium secondary battery with improved charging and discharging performance is provided.

[0011] A lithium secondary battery according to an exemplary embodiment includes a cathode, an insulating layer, and an anode opposite the cathode, the insulating layer being intermediated between the cathode and the anode. The anode includes an anode current collector and an anode active material layer formed on the anode current collector. The anode active material layer includes an anode active material and a conductive additive. The conductive additive includes expandable graphite with a particle size of less than 10 μm.

[0012] In some implementations, the particle size of expandable graphite can be in the range of 1 μm to 7 μm.

[0013] In some implementations, the expansion rate of expandable graphite can be 150%-500%.

[0014] In some implementations, the anodic active material may include natural graphite and artificial graphite.

[0015] In some embodiments, the anolyte layer may further include at least one of a carbon-based conductive agent and a metal-based conductive agent.

[0016] In some implementations, the content of expandable graphite can range from 1% to 8% by weight, based on the total weight of the anodic active material layer.

[0017] In some embodiments, the anodic active material layer may include pores formed by the elastic deformation of expandable graphite.

[0018] In some implementations, the porosity of the anode active material layer can be from 17% to 29%.

[0019] In some implementations, the density of the anolyte active material layer is from 1.4 g / cc to 1.9 g / cc.

[0020] In a method for manufacturing an anode for a lithium secondary battery according to an exemplary embodiment, graphite particles, including natural or artificial graphite, can be ground to a particle size of less than 10 μm. The ground graphite particles can be acid-treated. The acid-treated graphite particles can be washed and dried to prepare expandable graphite. The expandable graphite can be mixed with an anode active material to prepare an anode slurry. The anode slurry can be coated onto a current collector to form an anode active material layer.

[0021] In some implementations, the acid-treated graphite particles can be heated before washing and drying.

[0022] In some implementations, the anode slurry coated on the current collector can be dried during the formation of the anode active material layer. The dried anode slurry can then be pressed.

[0023] In some implementations, the expansion rate of expandable graphite can be less than 500% before pressing, and the expansion rate of expandable graphite can be 110%-300% after pressing.

[0024] In some implementations, the content of expandable graphite can be from 1% to 8% by weight, depending on the solids content of the anode slurry.

[0025] According to an exemplary embodiment of the present invention, the anolyte active material layer may include expandable graphite with a particle size of less than 10 μm. The expandable graphite may have a predetermined elasticity and can form voids in the anolyte active material layer while elastically recovering after being compressed during anode formation. These voids can be effectively used as impregnation paths for the electrolyte and ion flow paths.

[0026] In an exemplary embodiment, expandable graphite can be formed using graphite particles with a particle size of less than 10 μm and can be used as a conductive additive for the anode. Therefore, voids can be effectively formed in the anode active material layer, and the cycle characteristics of the lithium secondary battery can be improved. Attached Figure Description

[0027] Figure 1 A top plan view is provided to illustrate a lithium secondary battery according to an exemplary embodiment.

[0028] Figure 2 A cross-sectional view of a lithium secondary battery according to an exemplary embodiment is shown.

[0029] Figure 3 A scanning electron microscope (SEM) image showing a cross-section of the anolyte layer according to an exemplary embodiment.

[0030] Figure 4 SEM image showing a cross section of the anolyte layer according to the comparative example. Detailed Implementation

[0031] According to an exemplary embodiment of the present invention, a lithium secondary battery is provided, comprising a cathode, an separator, and an anode, wherein the anode may comprise expandable graphite with a particle size of less than 10 μm. A method for manufacturing an anode for a lithium secondary battery is also provided. The lithium secondary battery may have improved charging and discharging performance.

[0032] The invention will now be described in detail with reference to the accompanying drawings. However, those skilled in the art will understand that such embodiments described with reference to the drawings are provided to further understand the spirit of the invention and do not limit the subject matter to be protected as set forth in the detailed description and the appended claims.

[0033] Figure 1 and Figure 2 These are schematic top plan views and schematic cross-sectional views illustrating a lithium secondary battery according to an exemplary embodiment. For example, Figure 2 For along Figure 1 A cross-sectional view of line I-I' taken along the thickness direction of the lithium secondary battery.

[0034] exist Figure 1 and Figure 2 In this context, two directions perpendicular to each other on a plane are defined as the first direction and the second direction. For example, the first direction could be the length direction of the lithium secondary battery, and the second direction could be the width direction of the lithium secondary battery.

[0035] For ease of description, omissions are made. Figure 1 Diagram of the cathode and anode.

[0036] refer to Figure 1 and Figure 2 The lithium secondary battery may include an electrode assembly 150 and a housing 160 that houses the electrode assembly 150. The electrode assembly 150 may include a cathode 100, an anode 130, and a separator 140.

[0037] The cathode 100 may include a cathode current collector 105 and a cathode active material layer 110 formed on at least one surface of the cathode current collector 105. In an exemplary embodiment, the cathode active material layer 110 may be formed on both surfaces of the cathode current collector 105 (e.g., an upper surface and a lower surface). For example, the cathode active material layer 110 may be coated on each of the upper and lower surfaces of the cathode current collector 105, and may be directly coated on the surface of the cathode current collector 105.

[0038] The cathode current collector 105 may comprise stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. Preferably, aluminum or alloys thereof may be used.

[0039] The cathode active material layer 110 may include lithium metal oxide as the cathode active material. In an exemplary embodiment, the cathode active material layer 110 may include lithium (Li)-nickel (Ni) based oxide.

[0040] In some embodiments, the lithium metal oxide included in the cathode active material layer 110 can be represented by the following chemical formula 1.

[0041] [Chemical Formula 1] Li 1+a Ni 1-(x+y) Co x M y O2 In Formula 1, -0.05 ≤ α ≤ 0.15, 0.01 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2, and M may include at least one selected from Mg, Sr, Ba, B, Al, Si, Mn, Ti, Zr, and W. In one embodiment, in Formula 1, 0.01 ≤ x ≤ 0.20 and 0.01 ≤ y ≤ 0.15.

[0042] Preferably, in Formula 1, M can be manganese (Mn). In this case, nickel-cobalt-manganese (NCM)-based lithium oxide can be used as the cathode active material.

[0043] For example, nickel (Ni) can be used as a metal related to the capacity of lithium-ion batteries. Higher amounts of nickel can improve the capacity and power of the lithium-ion battery. However, excessive nickel can reduce battery life and may be detrimental to the battery's mechanical and electrical stability. For example, cobalt (Co) can be used as a metal related to the conductivity or resistance of lithium-ion batteries. In one embodiment, M includes manganese (Mn), and Mn can be used as a metal related to the mechanical and electrical stability of the lithium-ion battery.

[0044] High capacity and power, low resistance and lifetime stability, which depend on the cathode active material layer 110, can usually be improved by the interaction of nickel, cobalt and manganese as described above.

[0045] For example, the cathode active material can be mixed with a binder, conductive agent, and / or dispersant in a solvent and stirred to form a slurry. The slurry can be coated onto the cathode current collector 105, dried, and pressed to form the cathode active material layer 110.

[0046] Adhesives may include organic-based adhesives, such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or water-based adhesives, such as styrene-butadiene rubber (SBR), which may be used with thickeners such as carboxymethyl cellulose (CMC)

[0047] For example, a PVDF-based binder can be used as the cathode binder. In this case, the amount of binder used to form the cathode active material layer 110 can be reduced, and the amount of cathode active material can be relatively increased. Therefore, the capacity and power of the lithium secondary battery can be improved.

[0048] Conductive agents can be added to facilitate electron migration between active material particles. For example, conductive agents may include carbon-based materials such as graphite, carbon black, graphene, carbon nanotubes, etc., and / or metal-based materials such as tin, tin oxide, titanium oxide, perovskite minerals (e.g., LaSrCoO3 or LaSrMnO3).

[0049] In some embodiments, the electrode density of the cathode can be from 3.0 g / cc to 3.9 g / cc, preferably from 3.2 g / cc to 3.8 g / cc.

[0050] In an exemplary embodiment, the cathode active material layer 110 may have a multilayer structure.

[0051] The anode 130 may include an anode current collector 125 and an anode active material layer 120 formed on at least one surface of the anode current collector 125. In an exemplary embodiment, the anode active material layer 120 may be formed on both surfaces of the anode current collector 125 (e.g., an upper surface and a lower surface). For example, the anode active material layer 120 may be coated on each of the upper and lower surfaces of the anode current collector 125, and may be coated directly on the surface of the anode current collector 125.

[0052] The anode current collector 125 may include gold, stainless steel, nickel, aluminum, titanium, copper, or alloys thereof, preferably copper or copper alloys.

[0053] In an exemplary embodiment, the anodic active material layer 120 may include expandable graphite.

[0054] Expandable graphite can refer to a substance formed by expanding natural or synthetic graphite to a predetermined volume. Expandable graphite can be formed by heat treatment following the embedding of acid between layers of a natural or synthetic graphite crystal structure. The acid can be rapidly removed during heat treatment, thereby allowing the interlayer bonds in the graphite to be at least partially broken by the expansion pressure.

[0055] When natural or synthetic graphite is exposed to acid, the acid can penetrate into the interlayer spaces of the graphite's layered structure. The interlayer bonds of graphite can be at least partially weakened by the penetrating acid. The overall volume of the graphite increases due to the expansion pressure of the acid vapor generated as it escapes through the edges of the interlayer spaces during rapid heat treatment. The expanded graphite can be washed and dried to obtain expandable graphite.

[0056] The particle size of expandable graphite contained in the anodic active material layer 120 can be less than 10 μm.

[0057] If the particle size of expandable graphite exceeds 10 μm, the distance between the outer edge (edge ​​portion or edge region) of the graphite particle and the center may increase. In this case, since the area of ​​the edge region is smaller than the area of ​​the graphite layer during the heat treatment process used to remove acid embedded between the graphite layers, the expansion pressure of the acid vapor may increase. Therefore, the graphite particles may over-expand, and the interlayer bonds may be excessively damaged. Over-expanded graphite particles may be almost completely deformed, resulting in, for example, a loss of elasticity.

[0058] In an exemplary embodiment, the particle size of the expandable graphite can be less than 10 μm. In this case, the area of ​​the edge portion used as the outlet for acid vapor can be adjusted to an appropriate range relative to the area of ​​the graphite crystal layers. Therefore, the expansion pressure of the acid vapor during heat treatment may become relatively small, thereby maintaining the degree of expansion of the graphite particles at an appropriate level and also appropriately preserving the interlayer bonding of the graphite crystals. Furthermore, the expandable graphite can be elastic. For example, when pressures below a threshold are applied and removed, the expandable graphite according to the exemplary embodiment can revert to its original expanded form.

[0059] Preferably, the particle size of expandable graphite can be 3 μm to 7 μm, or 5 μm to 7 μm. Within this range, the elasticity of expandable graphite can be improved. If the particle size is less than 3 μm, elastic recovery may not be achieved because the graphite may not expand sufficiently.

[0060] In an exemplary embodiment, the particle size of expandable graphite may refer to the volume median diameter (D50) of the expandable graphite particles.

[0061] Expandable graphite can be pressed during the formation of the anolyte layer 120, and its volume can be reduced during pressing. When pressing is complete, the volume of the expandable graphite may increase due to elastic deformation, a phenomenon known as springback (SB). Pores can be formed in the anolyte layer 120 through elastic deformation. These pores can provide, for example, pathways for electrolyte permeation. Therefore, the anolyte can be more uniformly impregnated in the electrolyte, and ions from the electrolyte can easily move through the pores.

[0062] In an exemplary embodiment, the expansion rate of expandable graphite can be 150%-500% relative to its state before expansion (e.g., before acid treatment). The term "expansion rate" as used herein can refer to the percentage of the volume of expandable graphite after expansion and / or extrusion deformation relative to its volume before expansion. Within this expansion rate range, pores with appropriate volumes can be formed in the anolyte layer 120. For example, if the expansion rate is less than the aforementioned range, it may be insufficient to form pores. If the expansion rate exceeds the aforementioned range, the anolyte layer 120 may be over-deformed, the adhesion between the anolyte layer 120 and the anode current collector 125 may be weakened, or the operational stability of the lithium secondary battery may be reduced.

[0063] In an exemplary embodiment, the content of expandable graphite can be from 1% to 8% by weight, based on the total weight of the anode active material layer 120. If the amount of expandable graphite is less than the above range, sufficient pore formation may not be possible. Therefore, the power and lifespan of the lithium secondary battery may be reduced. If the amount of expandable graphite exceeds the above range, excessive pore formation may occur. Therefore, the volume of the anode active material layer 120 may increase excessively, and the mechanical and chemical stability may also decrease, thereby reducing the lifespan. Preferably, the content of expandable graphite can be from 1% to 6% by weight, 1% to 5% by weight, 2% to 5% by weight, or 3% to 5% by weight, based on the total weight of the anode active material layer 120.

[0064] In the manufacture of expandable graphite according to an exemplary embodiment, graphite particles may be ground to have a particle size of less than 10 μm. The graphite particles may include natural graphite or artificial graphite.

[0065] The ground graphite particles can be acid-treated. In this case, nitric acid or sulfuric acid can be used, and a mixture of nitric acid and sulfuric acid is preferred. Nitric acid or sulfuric acid molecules can embed between the crystalline layers of the ground graphite particles.

[0066] Acid-treated graphite particles can be heat-treated after the first washing / drying. The graphite particles can be expanded and fixed through heat treatment. Heat treatment can be performed at temperatures ranging from 500°C to 1200°C. The expansion rate of the expandable graphite can be controlled by adjusting the temperature.

[0067] A second washing and drying process can be performed on acid-treated or heat-treated graphite particles. This releases acid molecules embedded between the layers of the graphite crystals, and expandable graphite according to an exemplary embodiment can be obtained.

[0068] When graphite particles larger than 10 μm are expanded and then ground to a size below 10 μm, the degree of expansion may increase excessively. This can lead to a loss of elasticity in expandable graphite. Furthermore, grinding expanded graphite may result in delamination, leading to weakened interlayer bond breaking, rather than the breaking of covalent bonds in the 2D graphite network structure, or it may cause agglomeration of delaminated graphite. Therefore, particles larger than 10 μm that may not have been ground to the desired size (large particles) may still exist. When preparing electrode slurries, large particles may reduce dispersion performance, and elastic deformation may not occur within them. Large particles may also cause pore closure or filling in the anolyte layer.

[0069] In an exemplary embodiment, the anode active material layer 120 may include an anode active material capable of absorbing and releasing lithium ions.

[0070] The anode active material may include at least one of carbon-based active materials, silicon-based active materials, and lithium composite active materials.

[0071] Carbon-based active materials may include at least one of natural graphite, artificial graphite, hard carbon, soft carbon, carbon nanotubes, carbon fibers, coke, and pitch. Artificial graphite and hard carbon may include primary and / or secondary particle shapes. Preferably, natural graphite and / or artificial graphite can be used as anode active materials.

[0072] Compared to natural graphite, synthetic graphite has a relatively longer lifespan. Natural graphite has a larger specific surface area than synthetic graphite, resulting in relatively lower electrical resistance, which is advantageous from a power consumption perspective.

[0073] In one embodiment, the particle size or crystal size of the carbon-based active material can be from about 5 μm to about 30 μm.

[0074] In exemplary embodiments, silicon-based active materials may include silicon, silicon oxide, silicon-carbon composites, silicon oxide-carbon composites, or silicon-silicon oxide-carbon composites. Lithium composite active materials may include lithium titanate (LTO).

[0075] For example, expandable graphite and anolyte can be mixed and stirred in a solvent with a binder, conductive agent and / or dispersant to prepare an anolyte slurry. The anolyte slurry can be coated onto an anolyte current collector 125, then dried and pressed to form an anolyte active material layer 120.

[0076] In an exemplary embodiment, the content of expandable graphite can be from 1% to 8% by weight, based on the solids content of the anode slurry. If the amount of expandable graphite is less than the above range, sufficient pore formation may not be possible. Therefore, the power and efficiency of the lithium secondary battery may be reduced. If the amount of expandable graphite exceeds the above range, excessive pore formation may occur. Therefore, the mechanical and chemical operational stability of the anode active material layer 120 may be reduced, and its lifetime may be reduced. Preferably, the content of expandable graphite can be from 1% to 6% by weight, 1% to 5% by weight, 2% to 5% by weight, or 3% to 5% by weight, based on the solids content of the anode slurry.

[0077] In an exemplary embodiment, the expansion rate of expandable graphite before pressing can be 150%-500% relative to its volume before expansion. Furthermore, the expansion rate after pressing can be 110%-300% relative to its volume before expansion. "Expansion rate before pressing" and "expansion rate after pressing" can refer to the volume ratio (percentage) of graphite relative to its volume before expansion. If the expansion rates before and after pressing are within the above ranges, the anolyte active material can be effectively impregnated, and the electrolyte and ions can be effectively transferred. Furthermore, the internal bonding force of the anolyte active material layer 120 and the adhesion force between the anolyte active material layer 120 and the current collector 125 can be maintained.

[0078] The adhesives and conductive agents that are substantially the same as or similar to those used for the cathode 100 described above can be used. In some embodiments, the adhesive for the anode 130 may include an aqueous adhesive, such as styrene-butadiene rubber (SBR) or an acrylic adhesive, which may be used with a thickener (such as carboxymethyl cellulose (CMC)) to improve compatibility with carbon-based active materials.

[0079] For example, the anolyte active layer 120 and / or the anolyte slurry may contain conductive additives. Expandable graphite according to an exemplary embodiment may be provided as a conductive additive.

[0080] In an exemplary embodiment, the anolyte layer 120 may have a porosity of 17%-29%. For example, since the expandable graphite according to the exemplary embodiment recovers after pressing the anolyte layer 120, the porosity of the anolyte layer 120 can be adjusted within the above range. If the porosity is less than the above range, the impregnation of the anolyte by the electrolyte and the formation of flow paths for the electrolyte and ions may be insufficient. If the porosity exceeds the above range, the mechanical and chemical operational stability of the anolyte layer 120 may deteriorate. Preferably, the porosity may be 18-29%, 19-24%, or 21-24%.

[0081] In an exemplary embodiment, the density of the anode active material layer 120 can be from 1.4 g / cc to 1.9 g / cc. If the density is less than 1.4 g / cc, the power and capacity of the lithium secondary battery may be reduced. If the density exceeds 1.9 g / cc, excessive stress may be applied to the expandable graphite during pressing, resulting in a loss of elasticity in the expandable graphite. Therefore, pores formed by the elastic deformation of the expandable graphite may not be generated. Preferably, the density of the anode active material layer 120 can be from 1.5 g / cc to 1.8 g / cc.

[0082] In some embodiments, the area and / or volume of the anode 130 (e.g., the contact area with the separator 140) can be larger than the area and / or volume of the cathode 100. Therefore, lithium ions generated from the cathode 100 can be readily transferred to the anode 130 without loss due to, for example, precipitation or deposition, thereby further improving power and capacity.

[0083] In an exemplary embodiment, the anode active material layer 120 may have a multilayer structure.

[0084] The isolation layer 140 may be located between the cathode 100 and the anode 130. The isolation layer 140 may include a porous polymer membrane made of, for example, a polyolefin-based polymer, such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc. The isolation layer 140 may also be formed of a nonwoven fabric, including high-melting-point glass fiber, polyethylene terephthalate fiber, etc.

[0085] The separator 140 can extend between the cathode 100 and the anode 130, and can be folded and wound along the thickness direction of the lithium secondary battery. Therefore, multiple cathodes 100 and anodes 130 can be stacked in the thickness direction through the separator 140.

[0086] In an exemplary embodiment, an electrode cell may be defined by a cathode 100, an anode 130, and an insulating layer 140, and multiple electrode cells may be stacked to form an electrode assembly 150. The electrode assembly 150 may have, for example, a jelly roll shape.

[0087] The electrode assembly 150 can be housed in the housing 160, and the electrolyte can be injected into the housing. The housing 160 may include, for example, a bag, a can, etc.

[0088] In an exemplary embodiment, the electrolyte may include a non-aqueous electrolyte solution.

[0089] Non-aqueous electrolyte solutions can contain lithium salts and organic solvents. Lithium salts can be derived from Li... + X - This indicates that the lithium salt anion X - It can include, for example, F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6- (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - (CF3CF2SO2)2N - wait.

[0090] Organic solvents can include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, etc. These organic solvents can be used alone or in combination.

[0091] like Figure 1 As shown, the tabs (cathode tabs and anode tabs) can protrude from the cathode current collector 105 and anode current collector 125 included in each electrode unit to extend to one side of the housing 160. The tabs can be welded to the side of the housing 160 to form electrode leads (cathode lead 107 and anode lead 127) extending or protruding to the outside of the housing 160.

[0092] Figure 1 The cathode lead 107 and anode lead 127 are shown to be formed on the same side of the housing 160 or the lithium secondary battery. However, the cathode lead 107 and anode lead 127 may be formed on opposite sides of the housing 160 or the lithium secondary battery.

[0093] For example, cathode lead 107 can be formed at one end of housing 160, while anode lead 127 can be formed at the other end of housing 160.

[0094] Lithium secondary batteries can be manufactured in shapes such as cylindrical (canned), square, pouch, and coin-shaped.

[0095] Preferred embodiments are presented below to describe the invention in more detail. However, the following examples are merely illustrative, and those skilled in the art will clearly understand that various changes and modifications can be made within the scope and spirit of the invention. Such changes and modifications should be included within the appended claims.

[0096] Preparation example: Preparation of micro-expandable graphite Natural graphite was ground to an average particle size of 5 μm and then soaked in a 9:1 mixed acid solution of sulfuric acid and nitric acid at 25 °C for 2 hours. The acid-treated graphite was filtered to remove the acid solution and washed once with deionized water to prepare a wet powder. Subsequently, the wet powder was rapidly heated in a microwave oven at 900 °C, washed with water, and dried to prepare micro-expandable graphite with a volume ratio of approximately 400% compared to its unexpanded volume.

[0097] Examples 1-4 (1) Manufacturing of the anode The micro-expandable graphite from the above preparation example was used as a conductive additive, and 92 wt% of spherical natural graphite as the anode active material, 1.8 wt% of SBR-based binder, 1.2 wt% of CMC as a thickener, and 5 wt% of flake amorphous graphite as a conductive agent were mixed to form an anode slurry. The content of the micro-expandable graphite in the preparation example is shown in Table 1 below. The amounts of anode active material and conductive agent varied with the amount of expandable graphite. The anode slurry was coated on a copper substrate, dried, and pressed to a density of 1.7 g / cc to obtain an anode plate with a thickness of 70 μm.

[0098] (2) Preparation of cathode LiNi, used as the cathode active material, was mixed in a weight ratio of 92:5:3. 0.8 Co 0.1 Mn 0.1 The cathode paste is prepared using O2, carbon black as a conductive agent, and PVDF as a binder. The cathode paste is coated onto an aluminum substrate, dried, and pressed to form a cathode plate.

[0099] (3) Manufacturing of secondary batteries The cathode and anode obtained as described above are arranged with a polyethylene diaphragm (25 μm) between them to form an electrode unit, and the electrode units are stacked to form an electrode assembly. The electrode assembly is inserted into a bag, and the tab portions are welded. Electrolyte is injected to form a secondary battery.

[0100] An electrolyte was prepared by dissolving 1M LiPF6 in a mixed solvent of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (EC / EMC / DEC) (25 / 45 / 30; volume ratio), followed by the addition of 1% by weight of vinylene carbonate, 1,3-propensultone (PRS) and 0.5% by weight of lithium bis(oxalate)borate (LiBOB).

[0101] Examples 5-8 Expandable graphite was prepared using the same method as in Example 1, except that natural graphite with average diameters of 1 μm and 7 μm was used. The expandable graphite was used as a conductive additive to obtain a secondary battery using the same method as in Example 1.

[0102] Comparative Examples 1-5 Using plate-shaped natural graphite or Super P (carbon black) instead of the amounts of micro-expandable graphite shown in Table 1, lithium secondary batteries were manufactured using the same method as in Example 1.

[0103] Comparative Example 6 Expandable graphite was prepared using the same method as in Example 1, except that the expandable graphite (expansion rate of approximately 800%) was obtained from natural graphite with an average diameter of 20 μm. The expandable graphite was used as a conductive additive to obtain a secondary battery using the same method as in Example 1.

[0104] Experimental Example (1) Evaluation of SEM (Scanning Electron Microscopy) images The anodes of Example 3 and Comparative Example 3 were cut along the thickness direction, and the following results were obtained: Figure 3 and Figure 4 The SEM image of the cross-section shown.

[0105] refer to Figure 3 A sufficient number of pores are formed through the elastic deformation of the pressed expandable graphite.

[0106] refer to Figure 4 In the anode of Comparative Example 3, which was pressed at a density of 1.7 gg / cc, the flow path through the anode active material was essentially closed, and no pores were formed.

[0107] (2) Measurement of porosity The porosity of the anodes in the examples and comparative examples was measured. The porosity was measured using an AutoPore V 9600 mercury-injected porosimeter under the conditions described below.

[0108] i) Sample weight: 1g ± 0.1g ii) Electrode sampling: 1cm × 5cm, suitable for sample weight iii) Measuring medium: mercury iv) Measurement conditions: from 0.2 psig to 33,000 psig, approximately 150 points. iv) Mercury injection interval: 10 seconds v) Mercury contact angle: 130 degrees (3) Measure the initial efficiency The secondary batteries used in the examples and comparative examples were charged (CC / CV 0.5C 4.3V 0.05CA cutoff) and discharged (CC 1.0C 3.0V cutoff), and the initial charge / discharge capacity was measured. Furthermore, the initial reversible / irreversible capacity and efficiency were calculated.

[0109] (4) Evaluation of capacity retention Using the secondary batteries from the examples and comparative examples, 200 and 250 cycles of charge (1C 4.2V 0.1C cutoff) and discharge (1C 3.0V cutoff) were repeated. The discharge capacity of the 200th cycle and the discharge capacity of the 250th cycle were divided by the discharge capacity of the first cycle to evaluate the lifetime retention performance, expressed as a percentage.

[0110] The results are shown in Table 1 below.

[0111] [Table 1]

[0112] Referring to Table 1 above, in the examples where micro-expandable graphite prepared from graphite with a diameter of less than 10 μm was used as a conductive additive, a secondary battery with improved lifetime characteristics was obtained.

Claims

1. A lithium secondary battery, comprising: cathode; Isolation layer; and An anode opposite the cathode, wherein the insulating layer is located between the cathode and the anode, wherein the anode comprises: Anode current collector; and An anodic active material layer is formed on the anode current collector, the anodic active material layer comprising an anodic active material and conductive additives. The conductive additive includes expandable graphite with a particle size of 3 μm to 10 μm.

2. The lithium secondary battery according to claim 1, wherein, The expandable graphite has a particle size in the range of 3 μm to 7 μm.

3. The lithium secondary battery according to claim 1, wherein, The expansion rate of the expandable graphite is 110% to 300% relative to its volume before expansion.

4. The lithium secondary battery according to claim 1, wherein, The anodic active material includes natural graphite or artificial graphite.

5. The lithium secondary battery according to claim 1, wherein, The anodic active material layer further includes at least one of a carbon-based conductive agent and a metal-based conductive agent.

6. The lithium secondary battery according to claim 1, wherein, Based on the total weight of the anodic active material layer, the content of the expandable graphite is in the range of 1% to 8% by weight.

7. The lithium secondary battery according to claim 1, wherein, The anodic active material layer includes pores formed by the elastic deformation of the expandable graphite.

8. The lithium secondary battery according to claim 1, wherein, The porosity of the anodic active material layer is 17% to 29%.

9. The lithium secondary battery according to claim 1, wherein, The density of the anodic active material layer is from 1.4 g / cc to 1.9 g / cc.

10. A method for manufacturing an anode for a lithium secondary battery, comprising: Graphite particles, including natural or artificial graphite, are ground to a particle size of 3 μm to 10 μm. The ground graphite particles were then subjected to acid treatment. Acid-treated graphite particles are washed and dried to prepare expandable graphite; The expandable graphite is mixed with an anode active material to prepare an anode slurry; and The anode slurry is coated onto the current collector to form an anode active material layer.

11. The method of manufacturing an anode for a lithium secondary battery according to claim 10, further comprising heating the acid-treated graphite particles prior to washing and drying.

12. The method of manufacturing an anode for a lithium secondary battery according to claim 10, wherein forming the anode active material layer comprises: Dry the anode slurry coated on the current collector; and Press the dried anode slurry.

13. The method for manufacturing an anode for a lithium secondary battery according to claim 12, wherein, The expansion rate of the expandable graphite before compression is 100% to 500% relative to its volume before expansion.

14. The method for manufacturing an anode for a lithium secondary battery according to claim 12, wherein, The expansion rate of the expanded graphite after pressing is 110% to 300% relative to its volume before expansion.

15. The method for manufacturing an anode for a lithium secondary battery according to claim 10, wherein, Based on the solids content of the anode slurry, the content of expandable graphite is from 1% to 8% by weight.

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