Negative electrodes for lithium-ion secondary batteries

Abstract

The negative electrode according to the present invention contains a mixture of carbon-coated artificial graphite and uncoated artificial graphite as the negative electrode active material. The incorporation of the uncoated artificial graphite prevents deterioration of the carbon-coated artificial graphite due to deformation during rolling. The negative electrode maintains its electrochemical properties by preventing deterioration of the carbon-coated artificial graphite, making it suitable for the manufacture of fast-charging batteries. Meanwhile, the negative electrode according to the present invention is fabricated as a double layer, allowing the upper and lower layers of the electrode to have different compositions of negative electrode active materials. In particular, the content of carbon-coated artificial graphite in the upper layer may be increased to provide a negative electrode with better fast-charging characteristics. Furthermore, the dual-layer construction of the negative electrode with controlled binder content prevents binder migration in the negative electrode and improves adhesion between the electrode and current collector.

Description

[Technical Field]

[0001] This application claims priority based on Korean Patent Application No. 10-2020-0131460, dated October 12, 2021.

[0002] The present invention relates to a negative electrode for a lithium ion secondary battery and a secondary battery including the same, and more particularly, to a negative electrode that can be effectively used in a fast-charging battery and a secondary battery including the same. [Background technology]

[0003] As technological development and demand for mobile devices increases, the demand for rechargeable, compact, and high-capacity secondary batteries is rapidly increasing. Among secondary batteries, lithium secondary batteries, which have high energy density and voltage, have been commercialized and are widely used.

[0004] A lithium secondary battery has a structure in which an electrode assembly, which includes a positive electrode and a negative electrode, each of which has an active material coated on an electrode current collector, and a porous separator interposed between them, is impregnated with an electrolyte containing a lithium salt. The electrode is fabricated by coating a current collector with a slurry in which the active material, binder, and conductive material are dispersed in a solvent, followed by drying and pressing.

[0005] In addition, the basic performance characteristics of lithium secondary batteries, such as capacity, output, and lifespan, are significantly affected by the anode material. To maximize battery performance, the anode active material must have a potential for electrochemical reaction close to that of lithium metal, high reversibility in the reaction with lithium ions, and a high diffusion rate of lithium ions within the active material. Graphite is widely used as a material that meets these requirements.

[0006] To achieve diverse electrochemical properties of batteries, methods of combining various graphites and multilayered anodes have been proposed. In conventional multilayered electrodes, the anode active materials in the upper and lower layers are either different (upper layer a material / lower layer b material) or similar (upper layer a material / lower layer a material or upper layer b material / lower layer b material), meaning that each layer contains only one electrode active material. However, anodes with layers containing only a single material have encountered problems such as poor fast-charging characteristics or a rapid decline in high-temperature characteristics and capacity. Another problem is that cracks can occur in the electrode layers when the electrode dries.

[0007] In particular, when artificial graphite is used as the negative electrode active material in batteries for high-rate fast charging (2C rate or higher), low conductivity has been an issue. To improve this conductivity, a method of coating the surface of the artificial graphite with carbon (soft carbon or hard carbon) has been considered, but the carbon coating increases the strength of the electrode active material, which causes deformation of the electrode active material (increased orientation and breakage of the active material) during the battery manufacturing process (pressure process), resulting in side reactions and deterioration of high-temperature characteristics.

[0008] On the other hand, particularly as the electrode active material layer becomes thicker, there is a problem that the electrode binder migrates to the surface layer due to the evaporation of the solvent when the electrode is dried, and the electrode binder is distributed in a concentrated manner in the surface layer portion of the electrode, thereby reducing the binding strength between the electrode and the current collector. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention is intended to solve the above problems, and a first object of the present invention is to provide a multi-layered negative electrode including carbon-coated artificial graphite and uncoated artificial graphite as negative electrode active materials.

[0010] A second object of the present invention is to provide a lithium ion secondary battery including the negative electrode.

[0011] Meanwhile, other objects and advantages of the present invention will be understood from the following description, and it will be readily apparent that the objects and advantages of the present invention can be realized by the means or methods described in the claims, and combinations thereof. [Means for solving the problem]

[0012] A first aspect of the present invention relates to a negative electrode for a lithium-ion secondary battery, the negative electrode including a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, the negative electrode active material layer including a lower layer formed on the surface of the current collector and an upper layer formed on the lower layer, the lower layer and the upper layer each independently including a negative electrode composite including a negative electrode active material, a conductive material, and a binder, the upper layer and the lower layer each independently including a negative electrode active material a and a negative electrode active material b, the negative electrode active material a being artificial graphite whose surface is coated with a carbon material, and the negative electrode active material b being uncoated artificial graphite.

[0013] A second aspect of the present invention is the first aspect, wherein the content of the negative electrode active material b in the upper layer is 40 to 60 wt % with respect to the total amount of the negative electrode active material a and the negative electrode active material b.

[0014] In a third aspect of the present invention, in the first or second aspect, the binder content of the negative electrode composite material of the lower layer is relatively higher than the binder content of the negative electrode composite material of the upper layer.

[0015] A fourth aspect of the present invention is directed to any one of the first to third aspects, wherein the artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have a degree of orientation (I of particles). 004 I against 110 The ratio of (a) to (b) is 3 to 25.

[0016] A fifth aspect of the present invention is any one of the first to fourth aspects, wherein the artificial graphite of the negative electrode active material a and the artificial graphite of the negative electrode active material b each independently have a specific surface area of ​​0.5 to 5 m 2 / g.

[0017] A sixth aspect of the present invention is any one of the first to fifth aspects, wherein the carbon material of the negative electrode active material a includes a low-crystalline carbon material and / or an amorphous carbon material.

[0018] A seventh aspect of the present invention is any one of the first to sixth aspects, wherein the negative electrode active material a includes artificial graphite and a carbon coating layer formed on a surface of the artificial graphite, and the carbon coating layer is included in an amount of 1 to 10 wt % relative to 100 wt % of the negative electrode active material a.

[0019] An eighth aspect of the present invention is any one of the first to seventh aspects, wherein the upper and lower layers contain the same negative electrode active material a and the same negative electrode active material b.

[0020] A ninth aspect of the present invention is a method for manufacturing a negative electrode according to any one of the first to eighth aspects, comprising: preparing a first negative electrode slurry containing a lower layer negative electrode composite and a second negative electrode slurry containing the upper layer negative electrode composite; sequentially or simultaneously applying the first and second negative electrode slurries; and drying the applied first and second negative electrode slurries.

[0021] A tenth aspect of the present invention relates to a secondary battery including the anode according to any one of the first to eighth aspects, wherein the cathode includes lithium cobalt oxide (LCO) or lithium nickel cobalt manganese oxide (NCM), the electrolyte has an ionic conductivity of 6.5 mS / cm or more, and the concentration of the lithium salt in the electrolyte is 0.8 to 1.4 M, and the separator is a polyethylene porous film (thickness 3 to 15 μm), optionally having an inorganic coating layer. [Effects of the Invention]

[0022] The anode according to the present invention is a mixture of carbon-coated artificial graphite and uncoated artificial graphite as an anode active material, and the introduction of uncoated artificial graphite prevents deterioration of the carbon-coated artificial graphite due to deformation that occurs during rolling.

[0023] The negative electrode is suitable for manufacturing a fast-charging battery because it prevents deterioration of the carbon-coated artificial graphite and maintains good electrochemical properties.

[0024] Meanwhile, the anode according to the present invention is fabricated in a double layer, and the composition of the anode active material may be different between the upper and lower layers of the electrode. In particular, the content of carbon-coated artificial graphite may be further increased in the upper layer, thereby providing an anode that is more suited to fast charging characteristics.

[0025] In addition, by adjusting the binder content of the negative electrode and manufacturing it in a double layer, migration of the binder in the negative electrode is prevented and the adhesion between the electrode and the current collector is improved.

[0026] The drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the above-described content of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to only the matters shown in such drawings. Meanwhile, the shape, size, scale, ratio, etc. of elements in the drawings attached to this specification are exaggerated to emphasize the description more clearly. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a graph showing experimental results of capacity retention and volume expansion rate at different temperatures and C rates in Example 1 and Comparative Examples 1 to 3. [Figure 2] 1 is a graph showing experimental results of capacity retention and volume expansion rate at different temperatures and C rates in Example 1 and Comparative Examples 1 to 3. [Figure 3]1 is a graph showing experimental results of capacity retention and volume expansion rate at different temperatures and C rates in Example 1 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0028] The terms and words used in the following specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that correspond to the technical idea of ​​the present invention, based on the principle that an inventor can appropriately define the concept of a term in order to best explain his / her invention.

[0029] The present invention relates to a negative electrode for a lithium ion secondary battery.

[0030] According to one aspect of the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a lower layer formed on the negative electrode current collector and an upper layer formed on the lower layer. The lower layer and upper layer each independently include a negative electrode composite (upper layer negative electrode composite) including a negative electrode active material, a conductive material, and a binder. The upper layer and lower layer each independently include a negative electrode active material a and a negative electrode active material b, where the negative electrode active material b is artificial graphite, and the negative electrode active material a includes the artificial graphite and a carbon coating layer formed on the surface of the artificial graphite. The negative electrode active material b does not include a carbon coating layer.

[0031] The artificial graphite can be produced by a graphitization method in which raw materials such as coal tar, coal tar pitch, and petroleum-based heavy oil are generally sintered at 2,500°C or higher. After the graphitization, the artificial graphite is used as an anode active material through particle adjustment such as pulverization and secondary particle formation.

[0032] Typically, artificial graphite has crystals randomly distributed within the particles, and is less spheroidized and has a somewhat sharper shape than natural graphite. The artificial graphite is available in powder, flake, block, plate, or rod form, and it is desirable for the crystal grains to have an isotropic orientation so that the migration distance of lithium ions can be shortened to improve output characteristics. Considering this, the artificial graphite is available in flake and / or plate form.

[0033] In addition, the artificial graphite has a degree of orientation (I of particles) 004 I against 110 The ratio of (ratio of porosity to volume ratio) is 3 to 25. If the degree of orientation of the artificial graphite is less than 3, there will be a large amount of voids in the particles, which will reduce the capacity per volume and increase the irreversible capacity. If the degree of orientation is 25 or more, there will be a large volume change during charge and discharge, which will reduce the life characteristics, which is undesirable. In a specific embodiment of the present invention, the degree of orientation may be in the range of 12 to 25, in order to appropriately control the porosity and irreversible capacity.

[0034] Here, the degree of orientation is measured by the peak intensity ratio of the (110) plane to the (004) plane by X-ray diffraction analysis (XRD). 004 is diffracted by the stacking planes in the C-axis direction (vertical direction) of graphite, and the greater the amount of diffraction, the higher and wider the peak that is formed. 110 corresponds to the A-axis direction (transverse direction). In this case, the degree of orientation is evaluated by the area ratio of the two peaks. Such methods for measuring the degree of orientation of graphite are widely known to those skilled in the art, and are measured by such methods.

[0035] In one embodiment of the present invention, the X-ray diffraction analysis is performed using a Bruker D4 Endeavor X-ray diffraction analyzer with Cu-Kα radiation, and the values ​​are corrected using the Topas3 fitting program, if necessary.

[0036] Specifically, the XRD measurement conditions are as follows: - Target: Cu (Kα line) graphite monochromator -Slit: Divergence slit = 1 degree, Receiving slit = 0.1 mm, Scattering slit = 1 degree - Measurement area and step angle / measurement time: (110) plane: 76.5 degrees < 2θ < 78.5 degrees, 0.01 degrees / 3 seconds (004) plane: 53.5 degrees < 2θ < 56.0 degrees, 0.01 degrees / 3 seconds, Here, 2θ represents the diffraction angle. The XRD measurement is just one example, and other measurement methods can also be used to measure the degree of orientation.

[0037] The measurement of the degree of orientation of natural graphite, which will be described later, is also performed under the above conditions.

[0038] The artificial graphite used in one embodiment of the present invention includes commercially available mesophase carbon microbeads (MCMB), mesophase pitch-based carbon fiber (MPCF), block-graphitized artificial graphite, and powder-graphitized artificial graphite, and preferably has a sphericity of 0.91 or less, preferably 0.6 to 0.91, and more preferably 0.7 to 0.9. The artificial graphite may have a particle size of 5 to 30 μm, preferably 10 to 25 μm.

[0039] The artificial graphite has a specific surface area of ​​0.5 to 5 m 2 / g, specifically, 0.6 to 4 m 2 / g, and within the range satisfying the above-mentioned range, it is preferably smaller than the specific surface area of ​​natural graphite. If the specific surface area of ​​the artificial graphite is too small outside the above-mentioned range, the output characteristics during charge and discharge will decrease, and conversely, if the specific surface area is too large, the initial efficiency will decrease, which is undesirable.

[0040] The specific surface area of ​​the artificial graphite can be measured by the Brunauer-Emmett-Teller (BET) method. For example, it can be measured by the BET 6-point method using a porosimetry analyzer (Belsorp-II mini, Bell Japan Inc.) and a nitrogen gas adsorption / flow method. The specific surface area of ​​natural graphite, which will be described later, is also measured under these conditions.

[0041] The tap density of the artificial graphite is 0.7 to 1.15 g / cc, more specifically, 0.8 to 1.1 g / cc. If the tap density is less than 0.7 g / cc, the contact area between particles is insufficient, resulting in reduced adhesive strength and reduced capacity per volume. If the tap density exceeds 1.15 g / cc, the tortuosity of the electrode and the wettability of the electrolyte are reduced, resulting in reduced output characteristics during charge and discharge, which is undesirable.

[0042] The tap density is measured by placing 50 g of precursor in a 100 cc tapping cylinder and tapping it 3,000 times using a COPLEY JV-1000 measuring instrument or a SEISHIN (KYT-4000) measuring instrument. The tap density of natural graphite, which will be described later, is also measured under these conditions.

[0043] The artificial graphite has an average particle size (D50) of 8 to 30 μm, specifically 12 to 25 μm. If the average particle size (D50) of the artificial graphite is less than 8 μm, the initial efficiency of the secondary battery decreases due to an increase in specific surface area, resulting in a decrease in battery performance. If the average particle size (D50) exceeds 30 μm, the adhesive strength decreases and the packing density decreases, resulting in a decrease in capacity.

[0044] The average particle size of the artificial graphite can be measured, for example, by a laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to several mm, and provides results with high reproducibility and high resolution. The average particle size (D50) of the artificial graphite can be defined as the particle size at 50% of the particle size distribution. The average particle size (D50) of the artificial graphite can be measured, for example, by dispersing the artificial graphite in an ethanol / water solution, introducing it into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and irradiating it with ultrasonic waves of about 28 kHz at an output of 60 W. The average particle size (D50) at 50% of the particle size distribution can then be calculated using the analyzer.

[0045] In the present invention, the negative electrode active material b can be selected and applied without any other limitations as long as it is an artificial graphite having the above characteristics.

[0046] Meanwhile, the negative electrode active material a has a core-shell structure having a core particle and a coating layer covering at least a part or all of the surface of the core particle, the core particle being artificial graphite having the above-mentioned properties, and the coating layer including a carbon material.

[0047] In a specific embodiment of the present invention, the coating layer may include a low-crystalline carbon material and / or an amorphous carbon material as the carbon material. In a specific embodiment, the content of the low-crystalline and / or amorphous carbon material is 70 wt % or more, 80 wt % or more, or 90 wt % or more of the total weight of the coating layer. The coverage area of ​​the coating layer is 70 wt % or more, 80 wt % or more, or 90 wt % or more of the surface area of ​​the artificial graphite particles. In a specific embodiment of the present invention, the thickness of the coating layer is 5 to 1,000 nm, and is appropriately adjusted within this range.

[0048] In one specific embodiment of the present invention, the low-crystalline carbon material may include at least one of soft carbon and / or soft carbon having a low-crystalline structure obtained by heat-treating the soft carbon at a temperature of about 1000° C. or less. Meanwhile, the amorphous carbon material may include at least one selected from hard carbon, carbon black, sulfur black, and acetylene black.

[0049] In one specific embodiment of the present invention, the coating layer may be formed by coating the artificial graphite particles with a carbon precursor material, such as a polymer material or pitch, and then heat-treating (carbonizing) the resulting mixture at a temperature of approximately 500 to 1000°C. In this regard, since an excessively high carbonization temperature can affect the crystalline structure of the artificial graphite, it is preferable to control the temperature within the above range. In yet another specific embodiment, the coating layer may be formed by directly coating conductive carbon particles onto the surfaces of the artificial graphite particles. Examples of such conductive carbon particles include carbon black, such as acetylene black, thermal black, furnace black, and channel black, as well as carbon fiber and carbon tubes. However, these are merely examples and are not intended to be limiting.

[0050] Meanwhile, in one specific embodiment of the present invention, the upper layer includes negative electrode active material a and negative electrode active material b, and the content of the negative electrode active material b may be in the range of 40 to 60 wt % relative to 100 wt % of the total weight of the negative electrode active material a and negative electrode active material b. Also, the lower layer includes negative electrode active material a and negative electrode active material b, and the content of the negative electrode active material b may be in the range of 40 to 60 wt % relative to 100 wt % of the total weight of the negative electrode active material a and negative electrode active material b.

[0051] Meanwhile, in one embodiment of the present invention, the upper and lower layers may contain the same negative electrode active material a and the same negative electrode active material b.

[0052] Meanwhile, in one specific embodiment of the present invention, the carbon coating layer may have a content ratio of 1 to 10 wt % relative to 100 wt % of the negative electrode active material a, for example, the carbon coating layer may be included in a content ratio of 2 to 6 wt %.

[0053] Meanwhile, in one specific embodiment of the present invention, the content of the carbon coating layer in the upper layer depends on the content of the negative electrode active material (a), for example, 1 to 5 wt% relative to 100 wt% of the upper layer negative electrode active material, and the content of the carbon coating layer in the lower layer is 1 to 4 wt% relative to 100 wt% of the lower layer negative electrode active material. Increasing the proportion of the negative electrode active material coated in the upper layer is advantageous for fast charging due to improved electrical conductivity.

[0054] Meanwhile, in a specific embodiment of the present invention, the binder content of the lower layer negative electrode composite is relatively higher than that of the upper layer negative electrode composite. For example, the lower layer contains a binder content of 2.4 to 3 wt% relative to 100 wt% of the lower layer negative electrode composite, and the binder content of the upper layer is set to be lower than that of the lower layer.

[0055] In the electrode manufacturing process, when the electrode slurry is dried, the solvent migrates to the surface of the electrode and volatilizes, and the binder resin also migrates to the electrode surface, which tends to result in uneven distribution of the binder resin on the surface of the electrode. In the present invention, by designing the binder content ratio between the upper and lower layers in this way, uneven distribution of the binder resin on the surface of the electrode can be prevented, thereby reducing the charge transfer resistance (Rct) on the electrode surface, and the binder resin remaining in the lower layer prevents a decrease in the binding strength between the electrode active material layer and the current collector.

[0056] Meanwhile, in the present invention, the negative electrode may be fabricated by forming a lower layer on a negative electrode current collector and then forming an upper layer on the lower layer. The methods for forming the lower and upper layers may be dry-on-wet or wet-on-wet. The dry-on-wet method involves coating a first negative electrode slurry containing a lower layer negative electrode composite on a current collector and drying it, followed by coating a second negative electrode slurry containing an upper layer negative electrode composite and drying it. The wet-on-wet method involves coating a second negative electrode slurry after coating the first negative electrode slurry but before drying it, and then simultaneously drying the lower and upper layers. For example, in one embodiment of the present invention, the negative electrode may be fabricated by a wet-on-wet method. Specifically, two types of slurries may be simultaneously coated and dried using an apparatus such as a double slot die to form the lower and upper negative electrode active material layers.

[0057] The method for coating the slurry is not particularly limited as long as it is a method commonly used in the art, such as a slot die coating method, or a Mayer bar coating method, gravure coating method, dip coating method, spray coating method, etc.

[0058] In the method according to an embodiment of the present invention, the negative electrode current collector is not particularly limited as long as it is conductive and does not induce a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0059] The thickness of the current collector is not particularly limited, but may be a commonly applied thickness of 3 to 500 μm.

[0060] As the binder, various binder polymers such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butadiene rubber (SBR), fluororubber, and various copolymers are used.

[0061] As the solvent, N-methylpyrrolidone, acetone, water, etc. can be used.

[0062] The conductive material is one that is conductive without inducing a chemical change in the battery, and examples thereof include carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; SWCNTs (single-walled carbon nanotubes), MWCNTs (multi-walled carbon nanotubes); metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0063] In one embodiment of the present invention, the upper and lower negative electrode active material layers may further contain a thickener, if necessary, such as carboxymethyl cellulose (CMC), carboxyethyl cellulose, polyvinylpyrrolidone, or the like, at least one of which may be used.

[0064] On the one hand, in one embodiment of the present invention, a pressing process is further performed on the dried negative electrode. The pressing is performed by a method commonly used in the art, such as roll pressing. On the other hand, the pressing is performed under heating conditions.

[0065] Yet another embodiment of the present invention relates to a lithium secondary battery including the negative electrode manufactured as described above. Specifically, the lithium secondary battery can be manufactured by injecting a lithium salt-containing electrolyte into an electrode assembly including a positive electrode, a negative electrode as described above, and a separator interposed therebetween.

[0066] The positive electrode can be manufactured by mixing a positive electrode active material, a conductive material, a binder, and a solvent to produce a slurry, and then directly coating it on a metal current collector, or casting it on a separate support and laminating a positive electrode active material film peeled off from this support on the metal current collector.

[0067] As the active material used for the positive electrode, LiCoO2, LiNiO2, LiMn2O4, LiCoPO 1-x-y-z Co x M1 y M2 z O₂ (where M1 and M2 are each independently selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are each independently the atomic fraction of the oxide composition elements, where \(0\leq x\lt0.5\), \(0\leq y\lt0.5\), \(0\leq z\lt0.5\), and \(0\lt x + y + z\leq1\)) can include any one active material particle selected from the group consisting of or a mixture of two or more of these.

[0068] On the other hand, the conductive material, binder, and solvent are used in the same manner as those used during the manufacture of the negative electrode.

[0069] The separator may be a conventional porous polymer film, such as a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in combination. Alternatively, a thin insulating membrane with high ion permeability and mechanical strength may be used. The separator may include a safety reinforced separator (SRS), in which the surface of the separator is thinly coated with an inorganic coating layer, such as a ceramic material. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used, but is not limited thereto.

[0070] The electrolyte solution has an ionic conductivity of 6.5 mS / cm or more and contains a lithium salt as an electrolyte and an organic solvent for dissolving the lithium salt.

[0071] The lithium salt may be any one that is commonly used in electrolytes for secondary batteries. For example, the anion of the lithium salt may be F - , Cl - , 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 - , and (CF3CF2SO2)2N - In one embodiment of the present invention, the lithium salt is contained in the electrolyte in a range of 0.8 to 1.4 M.

[0072] The organic solvent contained in the electrolytic solution may be any commonly used organic solvent without limitation, and may typically be at least one selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfide, and tetrahydrofuran.

[0073] In particular, among the carbonate-based organic solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferably used because they are high-viscosity organic solvents with high dielectric constants and can easily dissociate lithium salts in the electrolyte. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte solution with high electrical conductivity can be prepared, making them even more preferable.

[0074] Optionally, the electrolyte stored according to the present invention may further contain additives such as overcharge inhibitors that are typically included in electrolytes.

[0075] In accordance with one embodiment of the present invention, a lithium secondary battery is fabricated by disposing a separator between a positive electrode and a negative electrode to form an electrode assembly, and then placing the electrode assembly in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then injecting an electrolyte. Alternatively, the electrode assemblies are stacked, impregnated with an electrolyte, and then placed in a battery case and sealed to form a lithium secondary battery.

[0076] According to one embodiment of the present invention, the lithium ion secondary battery is a stack type, a wound type, a stack and folding type, or a cable type.

[0077] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery for medium-sized and large-sized battery modules containing a number of battery cells. Preferred examples of the medium-sized and large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, and is particularly useful in hybrid electric vehicles and renewable energy storage batteries, which require high output.

[0078] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples according to the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0079] (1) Example 1 1) Preparation of negative electrode active material a D 50 is 15 μm, and the specific surface area is 0.9 m 2 Artificial graphite with a molecular weight of 1.5g and a degree of orientation of 13 was prepared, pitch-coated, and then carbonized at 1,100 to 1,300°C to form a carbon material coating layer on the surface of the artificial graphite. The thickness of the coating layer was about 800nm, and the content of the coating layer was about 4wt% relative to 100wt% of negative electrode active material particles a.

[0080] 2) Preparation of negative electrode active material b D 50 is 18 μm, and the specific surface area is 1.3 m 2 An artificial graphite having a particle size of 1 / g and a degree of orientation of 18.1 was prepared.

[0081] 3) Manufacturing of negative electrodes Preparation of the lower layer slurry A lower layer slurry was prepared by adding the negative electrode active material, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener to distilled water in a weight ratio of 95.35:0.5:3.0:1.15. For the negative electrode active material, negative electrode active material A and negative electrode active material B were mixed in a weight ratio of 50:50. Meanwhile, for the lower layer, the total amount of the carbon coating layer was 2 wt% relative to 100 wt% of the lower layer negative electrode active material.

[0082] Preparation of upper layer slurry A slurry for the upper layer was prepared by adding the negative electrode active material, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener to distilled water in a weight ratio of 97.35:0.5:1.0:1.15. For the negative electrode active material, negative electrode active material A and negative electrode active material B were mixed in a weight ratio of 50:50. Meanwhile, the total amount of the carbon coating layer in the upper layer was 2 wt% relative to 100 wt% of the upper layer negative electrode active material.

[0083] The lower layer slurry was then applied to the surface of a negative electrode current collector (copper thin film 8 μm thick), followed immediately by the upper layer slurry, and the resulting electrode was dried with hot air in a dryer. The dryer was controlled at a temperature of approximately 120-130°C. The resulting electrode had a thickness of 150 μm, with the lower layer accounting for approximately 50% of the thickness. The porosity was 25.2 vol%, and the electrode orientation was 10.7.

[0084] 4) Comparative Example 1 A lower layer slurry was prepared by dissolving negative electrode active material b, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener in a weight ratio of 95.35:0.5:3.0:1.15 in distilled water. A 97.35:0.5:1.0:1.15 weight ratio of negative electrode active material a, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener in distilled water to prepare an upper layer slurry. The lower layer slurry was then applied to the surface of a negative electrode current collector (copper thin film 8 μm thick), followed immediately by the upper layer slurry. The electrode was then dried with hot air in a dryer. The dryer was controlled at a temperature range of approximately 120–130°C. The obtained electrode had a thickness of 150 μm, the lower layer had a thickness ratio of about 50%, the porosity was 25.2 vol%, and the electrode orientation degree was 10.7.

[0085] 5) Comparative Example 2 Spheroidized natural graphite (D50 9μm, specific surface area 2.1m 2 A lower layer slurry was prepared by dissolving negative electrode active material (a) (anode active material a), Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener in a weight ratio of 95.35:0.5:3.0:1.15 in distilled water. A lower layer slurry was prepared by dissolving negative electrode active material (a), Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener in a weight ratio of 97.35:0.5:1.0:1.15 in distilled water. The lower layer slurry was then applied to the surface of a negative electrode current collector (copper thin film 8 μm thick), followed immediately by the upper layer slurry. The electrode was then dried with hot air in a dryer. The dryer was controlled at a temperature range of approximately 120 to 130°C. The obtained electrode had a thickness of 150 μm, the lower layer had a thickness ratio of about 50%, the porosity was 25.2 vol%, and the electrode orientation degree was 10.7.

[0086] 6) Comparative Example 3 A lower layer slurry was prepared by dissolving negative electrode active material A, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener in a weight ratio of 95.35:0.5:3.0:1.15 in distilled water. A higher layer slurry was prepared by dissolving negative electrode active material A, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder polymer, and carboxymethyl cellulose (CMC) as a thickener in a weight ratio of 97.35:0.5:1.0:1.15 in distilled water. The lower layer slurry was then applied to the surface of a negative electrode current collector (copper thin film 8 μm thick), followed immediately by the upper layer slurry. The electrode was then dried with hot air in a dryer. The dryer was controlled at a temperature range of approximately 120–130°C. The obtained electrode had a thickness of 150 μm, the lower layer had a thickness ratio of about 50%, the porosity was 25.2 vol%, and the electrode orientation degree was 10.7.

[0087] (2) Battery manufacturing Batteries were fabricated using the negative electrodes prepared in Example 1 and Comparative Examples 1 to 3.

[0088] The positive electrode was prepared as follows.

[0089] A positive electrode active material (LiCoO2), a binder (PVDF), and a conductive material (acetylene black) were mixed in NMP in a weight ratio of 96.5:1.5:2 to prepare a positive electrode active material layer slurry (solid content 70 wt%). This slurry was applied to an aluminum thin film (thickness approximately 10 μm) and dried at 60°C for 6 hours to prepare a positive electrode.

[0090] A porous polyethylene film (10 μm) was prepared as a separator, and the positive electrode / separator / negative electrode were sequentially stacked and pressurized at 80° C. to obtain an electrode assembly.

[0091] The electrode assembly was placed in a pouch-type battery exterior material, and an electrolyte solution was poured to fabricate a battery. The electrolyte solution was a mixture of ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate in a mass ratio of 2:1:2.5:4.5, and was added to a concentration of LiPF61.4M.

[0092] (3) Evaluation of capacity retention rate and expansion rate 1) 1.5C room temperature cycle Each battery of Example 1 and Comparative Examples 1 to 3 was charged at a constant current (CC) of 1.5 C up to 4.45 V, then charged at a constant voltage (CV) until the charging current reached 0.005 C (cutoff current), and discharged at a constant current of 1 C down to 3 V. The battery was charged and discharged 1,000 times to evaluate its capacity retention. This experiment was carried out at room temperature (25°C). The results are shown in Figure 1 below. It was confirmed that the battery of Example 1 had a superior capacity retention compared to the batteries of Comparative Examples 1 to 3.

[0093] Meanwhile, as a result of checking the volume expansion rate of the battery, it was confirmed that the volume expansion of the battery of Example 1 was smaller than that of the batteries of Comparative Examples 1 to 3.

[0094] 2) 1.5C high temperature cycle Each battery of Example 1 and Comparative Examples 1 to 3 was charged at a constant current (CC) of 1.5 C to 4.45 V, then charged at a constant voltage (CV) until the charging current reached 0.005 C (cutoff current), and then discharged at a constant current of 1 C to 3 V. The charge-discharge cycle was repeated 700 times to evaluate the capacity retention. This experiment was conducted at a high temperature (45°C). The results are shown in Figure 2 below. It was confirmed that the battery of Example 1 had a superior capacity retention rate compared to Comparative Examples 1 to 3. Meanwhile, the volume expansion rate of the battery was confirmed to be less for the battery of Example 1 than for the batteries of Comparative Examples 1 to 3.

[0095] 3) 2.0C high temperature cycle Each battery of Example 1 and Comparative Examples 1 to 3 was charged at a constant current (CC) of 2.0 C to 4.45 V, then charged at a constant voltage (CV) until the charging current reached 0.005 C (cutoff current), and discharged at a constant current of 1 C to 3 V. The battery was then charged and discharged 1,000 times to evaluate its capacity retention. This experiment was conducted at room temperature (25°C). The results are shown in Figure 3 below. It was confirmed that the battery of Example 1 had a superior capacity retention rate compared to the batteries of Comparative Examples 1 to 3. Meanwhile, the volume expansion rate of the battery was confirmed to be less for the battery of Example 1 than for the batteries of Comparative Examples 1 to 3.

Claims

1. A negative electrode for a lithium ion secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, the negative electrode active material layer includes a lower layer formed on a surface of the negative electrode current collector and an upper layer formed on the lower layer, and the lower layer and the upper layer each independently include a negative electrode composite including a negative electrode active material, a conductive material, and a binder; the lower layer and the upper layer each independently include a negative electrode active material a and a negative electrode active material b, the negative electrode active material a being artificial graphite whose surface is coated with a carbon material, and the negative electrode active material b being uncoated artificial graphite; The content of the negative electrode active material b in the lower layer is 40 to 60% by mass based on the total amount of the negative electrode active material a and the negative electrode active material b, The negative electrode active material a includes artificial graphite and a carbon coating layer formed on the surface of the artificial graphite, and the carbon coating layer is included in an amount of 1 to 10 wt % based on 100 wt % of the negative electrode active material a; a binder content ratio of the lower layer negative electrode composite is relatively higher than a binder content ratio of the upper layer negative electrode composite, and the lower layer contains a binder content in the range of 2.4 to 3 wt % relative to 100 wt % of the lower layer negative electrode composite.

2. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the content of the negative electrode active material b in the upper layer is 40 to 60 mass % based on the total amount of the negative electrode active material a and the negative electrode active material b.

3. The artificial graphite of the negative electrode active material a and the negative electrode active material b each independently has a degree of orientation (I of particles). ００４ I against １１０ 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the ratio of (a) to (b) is 3 to 25.

4. The artificial graphite of the negative electrode active material a and the negative electrode active material b each independently has a degree of orientation (I of particles). ００４ I against １１０ 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the ratio of (a) to (b) is 12 to 25.

5. The negative electrode active material a and the negative electrode active material b each independently have a specific surface area of ​​0.5 to 5 m ２ The negative electrode for a lithium ion secondary battery according to claim 1, wherein the SiO2 content is 1 / g.

6. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the carbon material of the negative electrode active material a includes a low-crystalline carbon material and / or an amorphous carbon material.

7. 2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the upper layer and the lower layer contain the same negative electrode active material a and the same negative electrode active material b.

8. A method for producing a negative electrode for a lithium ion secondary battery according to claim 1, A method for manufacturing a negative electrode for a lithium ion secondary battery, comprising: preparing a first negative electrode slurry containing a lower layer negative electrode composite and a second negative electrode slurry containing an upper layer negative electrode composite; sequentially or simultaneously applying the first negative electrode slurry and the second negative electrode slurry; and drying the first negative electrode slurry and the second negative electrode slurry.

9. A lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to claim 1, a positive electrode, a separator, and an electrolyte solution, the positive electrode comprises lithium cobalt oxide (LCO) or lithium nickel cobalt manganese oxide (NCM); The electrolyte has an ionic conductivity of 6.5 mS / cm or more, and the concentration of lithium salt in the electrolyte is 0.8 to 1.4 M; The separator is a porous polyethylene film having a thickness of 3 to 15 μm, and optionally has an inorganic coating layer.

Images