Negative electrode for secondary battery, negative electrode slurry, and method for manufacturing negative electrode

Abstract

Provided are a negative electrode for a secondary battery, a slurry for a negative electrode, and a method for manufacturing a negative electrode, each of which has improved life characteristics. The negative electrode for a secondary battery contains at least a graphite-based material and a silicon-based material as negative electrode active materials, and a conductive material, the graphite-based material includes a granulated product of artificial graphite and natural graphite, at least a part of the natural graphite in the granulated product is partially flaked to form a flaked portion, and the flaked portion is in contact with the silicon-based material or other granulated product of artificial graphite and natural graphite, or is complexed with the silicon-based material.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a secondary battery, a negative electrode paste, and a method for manufacturing a negative electrode.

[0002] This application claims priority to Japanese Patent Application No. 2020-216840 filed on December 25, 2020, the disclosure of which is incorporated herein by reference. Background Art

[0003] With the increase in the development of technologies and demands for mobile devices and electric vehicles, the demand for secondary batteries as an energy source has been increasing. Among such secondary batteries, lithium-ion secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate have been commercialized and widely used. Recently, active research has been conducted to provide such lithium-ion secondary batteries having a high capacity.

[0004] Compared with carbonaceous materials such as graphite which are mainly used currently, silicon-based materials such as silicon oxides or silicon-based alloys have a higher theoretical capacity density, and thus have been studied as negative electrode materials for improving the energy density of lithium-ion secondary batteries. In particular, silicon oxides (SiO x (0 < x < 2), such as SiO) have been partially commercialized due to a relatively low expansion rate. However, such silicon oxides show an initial efficiency that is at least 20% lower than that of graphite, and when used alone, the difference in irreversible capacity compared to the positive electrode increases. Therefore, such silicon oxides are actually used by mixing with carbonaceous materials at a content of several percent. In addition, silicon-based alloys having a lower irreversible capacity than silicon oxides show problems in terms of expansion rate, and thus the use in combination with graphite has been examined so that the expansion rate of the entire electrode can be reduced.

[0005] However, carbonaceous materials and silicon-based materials show different powder properties, conductivity, expansion rate, etc. Therefore, when using a carbonaceous material in combination with a silicon-based material, it is difficult to maintain the conduction path between the carbonaceous material and the silicon-based material under the conditions of expansion and contraction caused by repeated charge and discharge while forming a structure in which all active materials are utilized. In addition, in this case, the capacity of the carbonaceous material and the capacity of the silicon-based material having a higher expansion rate become less utilized.

[0006] Furthermore, when artificial graphite is used as a carbonaceous material, it exhibits the advantage of a lower expansion rate compared to natural graphite. However, it is hard and difficult to deform even under pressure. Moreover, it is prone to forming voids in the gaps between silicon-based materials from the beginning of electrode manufacturing. As a result, small silicon-based material particles are isolated, leading to degradation of the initial capacity or capacity degradation after repeated charge and discharge.

[0007] Therefore, to ensure conductive pathways in anodes containing both carbonaceous and silicon-based materials, methods have been proposed for forming composites of non-conductive silicon-based materials and conductive carbonaceous materials (e.g., Patent Document 1 or Patent Document 2), or for adding nanomaterials such as graphene or carbon nanotubes as conductive materials (e.g., Patent Document 3). However, in the former case, it is difficult to ensure conductive pathways between particles. Furthermore, in the latter case, it is difficult to uniformly disperse the nanomaterials throughout the anode, and nanomaterials are very expensive.

[0008] [Existing technical documents]

[0009] [Patent Literature]

[0010] Patent Document 1: Japanese Patent Application Publication No. 2019-067579

[0011] Patent Document 2: International Publication No. 2012 / 140790

[0012] Patent Document 3: Japanese Patent Application Publication No. 2020-013718 Summary of the Invention

[0013] Technical issues

[0014] This disclosure aims to address problems in the relevant field, and therefore aims to provide a negative electrode for secondary batteries that can be stably and repeatedly charged and discharged by forming a high level and robust conductive path in the negative electrode, thereby providing improved life characteristics; and to provide a negative electrode slurry and a method for manufacturing the negative electrode.

[0015] Technical solution

[0016] According to a first embodiment of this disclosure, a negative electrode for a secondary battery is provided, comprising at least a graphite-based material and a silicon-based material as negative electrode active materials, and a conductive material, wherein the graphite-based material comprises a granulated product of artificial graphite and natural graphite, at least a portion of the natural graphite in the granulated product is partially flaked to form a flake portion, and the flake portion is in contact with the silicon-based material or other granulated products of artificial graphite and natural graphite, or forms a composite with the silicon-based material.

[0017] According to a second embodiment of this disclosure, a negative electrode for a secondary battery as defined in the first embodiment is provided, wherein the silicon-based material comprises any one or both of silicon oxide and silicon alloy.

[0018] According to a third embodiment of this disclosure, a negative electrode for a secondary battery as defined in the second embodiment is provided, wherein the silicon alloy comprises at least one selected from titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu).

[0019] According to a fourth embodiment of this disclosure, a negative electrode for a secondary battery as defined in any one of the first to third embodiments is provided, wherein the conductive material is carbon black.

[0020] According to a fifth embodiment of this disclosure, a negative electrode for a secondary battery as defined in any one of the first to fourth embodiments is provided, wherein the weight ratio of the graphite material to the silicon material is 98:2-50:50.

[0021] According to a sixth embodiment of this disclosure, a negative electrode for a secondary battery as defined in any of the first to fifth embodiments is provided, wherein the granulated product comprises artificial graphite and natural graphite in a weight ratio of 60:40 to 90:10.

[0022] According to a seventh embodiment of the present disclosure, a secondary battery is provided, comprising a negative electrode as defined in any one of the first to sixth embodiments, and further comprising a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte.

[0023] According to the eighth embodiment of this disclosure, a negative electrode slurry for a secondary battery is provided, comprising: a graphite-based material and a silicon-based material as negative electrode active materials; a conductive material; a solvent; and at least one of a thickener and a binder, wherein the solid content of the slurry is 60% by weight or more relative to the weight of the slurry, wherein the graphite-based material comprises a granulated product of artificial graphite and natural graphite.

[0024] According to a ninth embodiment of this disclosure, a negative electrode slurry as defined in the eighth embodiment is provided, wherein at least a portion of the natural graphite in the granulation product is partially flaked to form a flake portion.

[0025] According to a tenth embodiment of this disclosure, a negative electrode slurry as defined in an eighth or ninth embodiment is provided, wherein the solid content in the slurry is 65-75% by weight relative to the weight of the slurry.

[0026] According to the eleventh embodiment of this disclosure, a method for manufacturing a negative electrode for a secondary battery is provided, comprising the following steps: mixing at least one of a graphite-based material and a silicon-based material, a conductive material, a solvent, a thickener, and a binder as the negative electrode active material to prepare a slurry with a solid content of 60% by weight or more relative to the weight of the slurry; performing hard mixing of the slurry; and applying the slurry to a current collector to obtain a negative electrode, wherein the graphite-based material is a granulated product of artificial graphite and natural graphite.

[0027] According to the twelfth embodiment of this disclosure, a method as defined in the eleventh embodiment is provided, wherein in the step of hard mixing of the slurry, at least a portion of the natural graphite in the granulation product is partially flaked to form a flake portion.

[0028] According to a thirteenth embodiment of this disclosure, a method as defined in the eleventh or twelfth embodiment is provided, which, after the step of hard mixing the slurry, further includes the step of adding an adhesive and a solvent to the slurry.

[0029] Beneficial effects

[0030] According to this disclosure, since a granulated product of artificial graphite and natural graphite is used as a graphite-based material for use as a negative electrode active material, and at least a portion of the natural graphite in the granulated product is partially flaked to form a flake portion, the silicon-based material and the graphite-based material are in contact with each other over a large area through the flake portion, or the silicon-based material forms a composite with the partially flaked portion. In this way, a high level and strong conductive path with graphite can be formed to stably repeat charge and discharge, thereby providing improved lifetime characteristics. Attached Figure Description

[0031] Figure 1 To show a scanning electron microscope (SEM) image of the surface of the negative electrode according to Example 3.

[0032] Figure 2 Another SEM image of the surface of the negative electrode according to Example 3 is shown. Detailed Implementation

[0033] Preferred embodiments of this disclosure will be described in detail below, but the scope of this disclosure is not limited thereto.

[0034] Throughout this specification, unless otherwise stated, the term "average particle size" refers to the particle size at the 50% cumulative value in the particle size distribution determined by laser diffraction scattering, i.e., the median diameter (D). 50Additionally, the symbol "-" is used to indicate numbers at both ends of the range it refers to. For example, the expression "1-2" means "equal to or greater than 1 and equal to or less than 2".

[0035] [Non-aqueous electrolyte secondary battery]

[0036] In one aspect, this disclosure relates to a non-aqueous electrolyte secondary battery. An embodiment of the non-aqueous electrolyte secondary battery according to this disclosure includes a negative electrode, a positive electrode, a separator inserted between the negative and positive electrodes, and a non-aqueous electrolyte. Specific examples of such secondary batteries include lithium-ion secondary batteries, which have several advantages such as high energy density, discharge voltage, and output stability.

[0037] The present disclosure will be explained below using such a lithium-ion secondary battery as an example. However, the scope of the present disclosure is not limited to lithium-ion secondary batteries, but can be applied to various types of non-aqueous electrolyte secondary batteries.

[0038] A lithium-ion secondary battery according to an embodiment of this disclosure includes a negative electrode, a positive electrode, a separator inserted between the negative electrode and the positive electrode, and a non-aqueous electrolyte. Additionally, the lithium-ion secondary battery may optionally include a battery case configured to house an electrode assembly comprising the negative electrode, the positive electrode, and the separator, and a sealing member for sealing the battery case.

[0039] [negative electrode]

[0040] The negative electrode includes a negative electrode current collector and a layer of negative electrode active material formed on one or both surfaces of the negative electrode current collector. The negative electrode active material layer may be formed on the entire surface or a portion of the surface of the negative electrode current collector.

[0041] (Negative electrode current collector)

[0042] There are no particular restrictions on the negative electrode current collector used for the negative electrode, as long as it is conductive and does not cause any chemical changes in the battery. Specific examples of the negative electrode current collector include: copper; stainless steel; aluminum; nickel; titanium; sintered carbon; copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloys, etc.

[0043] The thickness of the negative electrode current collector can be 3-500 μm. Fine surface irregularities can be formed on the surface of the negative electrode current collector, thereby enhancing adhesion to the negative electrode active material. For example, the negative electrode current collector can have various shapes, such as membranes, sheets, foils, meshes, porous bodies, foams, nonwoven fabrics, etc.

[0044] (Negative electrode active material layer)

[0045] The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry comprising a mixture of a negative electrode active material, a binder, and a conductive material dissolved or dispersed in a solvent onto a negative electrode current collector, followed by drying and pressing. In one variation, the negative electrode active material layer can be formed by casting the negative electrode slurry onto another support, peeling the film off the support, and laminating the film layer onto the negative electrode current collector. If desired, the mixture may further contain dispersants, fillers, or other optional additives.

[0046] The negative electrode active material may be present in an amount of 70-99% by weight relative to the total weight of the negative electrode active material layer.

[0047] (Negative electrode active material)

[0048] In the lithium-ion secondary battery according to embodiments of the present disclosure, the negative electrode active material comprises at least graphite-based materials and silicon-based materials.

[0049] The graphite-based material comprises a granulated product of artificial graphite and natural graphite. For example, the granulated product is formed by attaching natural graphite particles to artificial graphite particles that serve as a matrix (core). Granulation can be carried out by known methods, such as a dry method in which artificial graphite and natural graphite are mixed under an ambient atmosphere and the resulting mixture is granulated under the application of mechanical / physical force, or a wet method in which artificial graphite and natural graphite are dispersed and mixed in a solvent and the solvent is removed.

[0050] Artificial graphite is graphite industrially manufactured by calcining (i.e., graphitizing) graphitable carbonaceous materials such as coke or coal tar pitch at high temperatures (e.g., about 2800°C). Known examples of artificial graphite include mesophase carbon microspheres, mesophase carbon fibers, and bulk artificial graphite. However, there are no particular limitations on the types of artificial graphite that can be used according to this disclosure. Generally, artificial graphite is harder than natural graphite and is known to exhibit less expansion during charge and discharge compared to natural graphite when used as the negative electrode in lithium-ion secondary batteries.

[0051] Natural graphite is produced by mining graphite ore and subsequently processing it through methods such as ore concentration and purification. Known examples of natural graphite include flake natural graphite, block natural graphite, and amorphous graphite. However, there are no particular limitations on the types of natural graphite that can be used according to this disclosure.

[0052] The sizes of artificial graphite and natural graphite are selected such that the granulated product formed by granulating them can have a desired size. Additionally, the average particle size (D) of the granules obtained by granulating artificial and natural graphite is also considered. 50)It can be 3 - 30 μm, preferably 5 - 25 μm, more preferably 15 - 20 μm. For example, the average particle size of the granulated product can be 20 μm.

[0053] The weight ratio of artificial graphite to natural graphite in the granulated product is selected in such a way that a sufficient amount of natural graphite can be present on the surface of the granulated body. For example, the weight ratio of artificial graphite : natural graphite can be 60 : 40 - 90 : 10, such as 80 : 20.

[0054] The silicon - based material can include silicon oxide, silicon - based alloy, silicon (Si) powder, silicon nanoparticles, silicon nanowires, etc. Such silicon - based materials can be used alone or in combination. Preferably, either or both of silicon oxide and silicon - based alloy can be used.

[0055] Generally, silicon oxide is represented by the general formula SiO x where 0 < x < 2, and can be exemplified as SiO (x = 1). For example, silicon oxide can have a structure in which Si microparticles are dispersed in an amorphous silicon oxide matrix in a microcrystalline form or an amorphous form. Silicon oxide can contain only SiO x with a specific x value, or can be a mixture of two or more SiO x substances with different x values.

[0056] The silicon - based alloy can be a granulated product having a particle structure in which transition metal silicide microparticles are dispersed in an amorphous silicon matrix in a microcrystalline form or an amorphous form. For example, such a granulated product of silicon - based alloy is obtained by atomizing (preferably gas atomizing) silicon and a transition metal to obtain a silicon alloy powder, and converting silicon into amorphous silicon by mechanical alloying treatment. For example, the content of amorphous silicon in the granulated product of silicon - based alloy can be 10 - 60 wt%, preferably 20 - 40 wt%. The transition metal of the silicide can be at least one selected from titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Specific examples of combinations of two or more transition metals include Cr, Ti, and Fe, preferably Cr and Ti. When Cr and Ti are used, SiCrTi alloy powder is obtained, and specific examples of silicides theoretically formed in the powder include binary silicides such as CrSi, CrSi2, TiSi2, or TiSi, and ternary silicides such as Cr x Si y Ti zIn the case of SiCrTi alloy powder, Si is the main component, comprising 70-90 atomic percent, with the balance being transition metal components. For example, the atomic weight ratio of Si:Cr:Ti can be 84 atomic percent: 8 atomic percent: 8 atomic percent, or 82.4 atomic percent: 8.8 atomic percent: 8.8 atomic percent.

[0057] The silicon-based material can be in a particle state, and the average particle size (D) of the silicon-based material 50 The micrometer can be 0.1-10 μm, 1-5 μm (e.g., 5 μm), or 1-2 μm.

[0058] The average particle size of the silicon-based material can be selected in such a way that it can be smaller than the average particle size of the graphite-based material. When the silicon-based material has such a small particle size, the silicon-based material, which exhibits a higher expansion rate compared to the carbonaceous material, can be disposed in the voids (e.g., the voids between carbonaceous material particles), or disposed on the surface or inside the carbonaceous material.

[0059] The weight ratio of graphite-based materials to silicon-based materials in the negative electrode active material can be selected by taking into account the expansion rate and capacity of the silicon-based materials. For example, the weight ratio of graphite-based materials to silicon-based materials can be 98:2-50:50, especially 98:2-70:30 (e.g., 90:10).

[0060] (Adhesive)

[0061] The adhesive is a component that promotes adhesion between active and conductive materials, as well as adhesion to current collectors. Specific examples of such adhesives include, but are not limited to: polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), polyacrylic acid, acrylamide, fluororubber, and various copolymers thereof. Such adhesives can be used alone or in combination.

[0062] The content of the binder relative to the total weight of the negative electrode active material can be 0.1-30% by weight. The content of the binder is preferably 0.5-20% by weight, more preferably 1-10% by weight. When the content of the binder polymer meets the above range, sufficient adhesion can be imparted to the electrode while preventing capacity degradation of the battery.

[0063] (Conductive materials)

[0064] There are no particular limitations on the conductive materials mentioned, as long as they are conductive materials that do not cause chemical changes. Specific examples of such conductive materials include, but are not limited to: carbonaceous materials (carbonaceous materials added separately from the carbonaceous material used as the negative electrode active material), such as artificial graphite, natural graphite, carbon nanotubes, graphene, carbon black, acetylene black, Ketjen black, Denka black, thermally cracked carbon black, channel black, furnace black, lamp black, carbon fiber, etc.; metal powders or metal fibers, such as aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver, gold, lanthanum, ruthenium, platinum, iridium, etc.; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; conductive polymers, such as polyaniline, polythiophene, polyacetylene, polypyrrole, polyphenylene derivatives, etc. Such conductive materials can be used alone or in combination.

[0065] Advantageously, carbonaceous materials, such as carbon black, acetylene black, Ketjen black, Danka black, thermal cracking carbon black, channel black, furnace black, or lamp black, are chosen as conductive materials in terms of dispersibility or cost. However, this does not preclude the use of artificial graphite, natural graphite, carbon nanotubes, or graphene as conductive materials.

[0066] The content of the conductive material relative to the total weight of the negative electrode active material layer can be 0.1-30% by weight. The content of the conductive material relative to the total weight of the negative electrode active material layer is preferably 0.5-15% by weight, more preferably 0.5-10% by weight. When the content of the conductive material meets the above ranges, it has the advantage of providing sufficient conductivity without reducing the amount of negative electrode active material, thereby ensuring battery capacity.

[0067] (Thickener)

[0068] The negative electrode slurry may further contain a thickener. In particular, the thickener may be a cellulose-based compound. Specific examples of cellulose-based compounds include carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl cellulose (HPC), methyl hydroxypropyl cellulose (MHPC), ethyl hydroxyethyl cellulose (EHEC), methyl ethyl hydroxyethyl cellulose (MEHEC), etc. Such cellulose-based compounds may be used alone or in combination. For example, the thickener may be used in an amount of 0.5-10% by weight relative to the total weight of the negative electrode active material layer.

[0069] (solvent)

[0070] There are no particular restrictions on the solvents used in the negative electrode slurry, as long as they are commonly used in the manufacture of the negative electrode. Specific examples of such solvents include, but are not limited to: pure water, N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropanol, acetone, etc. Such solvents can be used alone or in combination.

[0071] [Method for manufacturing the negative electrode]

[0072] A method for manufacturing a negative electrode for a lithium-ion secondary battery according to an embodiment of the present disclosure may include the following steps: (1) preparing a negative electrode slurry; (2) performing hard mixing of the slurry; and (3) manufacturing a negative electrode from the negative electrode slurry.

[0073] (1) Steps for preparing negative electrode slurry

[0074] Prepare graphite-based materials (granulated products of artificial and natural graphite) and silicon-based materials as negative electrode active materials, as well as conductive materials, thickeners, binders, and solvents. If necessary, prepare dispersants or fillers, or other optional additives. Thickeners can be readily used when pre-dissolved in a solvent (water, NMP, etc.). Then, mix the above components to prepare the negative electrode slurry. Specifically, first add the conductive material, thickener, and dispersant to the solvent and mix with it, then introduce the silicon-based and graphite-based materials into it. When using a binder that does not require the combined use of a thickener, a portion of the binder is introduced to replace the thickener.

[0075] The solids content in the negative electrode slurry is controlled to be at least 60% by weight relative to the total weight of the slurry. This solids content can be controlled by adjusting the amount of solvent added to the slurry. Preferably, the negative electrode slurry is prepared with a solids content of 65% by weight or more. When the solids content is less than 60% by weight, sufficient shear stress cannot be obtained in the subsequent hard mixing step due to the low viscosity of the slurry. Furthermore, sufficient interaction (collision, impact, friction, etc.) between the graphite-based and silicon-based materials is not possible. There is no particular upper limit to the solids content in the slurry, as long as the subsequent hard mixing step can be performed appropriately. However, when the solids content is too high, the components in the slurry exhibit low dispersibility; therefore, the solids content can be set to 75% by weight or less, preferably 70% by weight or less.

[0076] (2) Step of hard mixing of negative electrode slurry

[0077] Next, the negative electrode slurry prepared in step (1) is subjected to hard mixing (also known as kneading). For example, the hard mixing can be carried out using known methods, such as by using a planetary centrifugal mixer.

[0078] During the hard mixing process, due to the high solids content (over 60% by weight) in the slurry, the silicon-based materials collide violently with the graphite-based materials. Here, natural graphite is generally softer than synthetic graphite or silicon-based materials. Therefore, in at least a portion of the natural graphite present in the granulated product of the graphite-based material, the surface portion of the natural graphite is partially flaked by surface collisions caused by the hard silicon-based materials, thereby forming flake-like portions. Here, "partial flake-like" means that the surface portion of the natural graphite is not completely peeled and separated from the natural graphite, but rather that the surface portion of the natural graphite is partially peeled, curled, and flake-like, i.e., cracked and roughened. The flake-like portions can be monolayer graphene. In this case, flake-like refers to grapheneization. In one variant, the number of graphene layers in the flake-like portions can be more than two layers, about tens of layers, about hundreds of layers, or less than about thousands of layers, and can exhibit a thickness of less than a few nanometers, or less than tens of nanometers, or a thickness on the submicron scale (e.g., less than about 300 nm, or less than about 100 nm).

[0079] On the other hand, the artificial graphite in the granules is hardly damaged during hard mixing due to its rigidity. As a result, the entire granule of graphite-based materials is able to suppress undesirable damage or pulverization during hard mixing due to the presence of artificial graphite as the matrix. Similarly, rigid silicon-based materials are hardly damaged during hard mixing.

[0080] As hard mixing proceeds, the silicon-based material dispersed in the slurry comes into contact with the granules, particularly with the flakes formed in natural graphite, and thus can form a complex with the flakes of natural graphite (e.g., through physical and / or chemical adhesion / bonding) by shear or impact forces applied during hard mixing.

[0081] During the hard mixing process, the slurry temperature is set to a high level (e.g., 60-80°C, preferably 65-75°C) to promote hard mixing. As a result, some of the natural graphite in the granules can be partially flaked.

[0082] (3) Steps for manufacturing a negative electrode from negative electrode slurry

[0083] After the hard mixing is complete, further mixing is performed by introducing all the binder and adding solvent to control the solids content to approximately 50% by weight. Finally, solvent is added to control the slurry to an easily applyable solids content, followed by gentle mixing. The resulting negative electrode slurry is applied to a negative electrode current collector, then dried and pressed to obtain a negative electrode containing a layer of negative electrode active material formed on the negative electrode current collector. Before applying the negative electrode slurry, solvent is further added to the hard-mixed slurry to promote application, and then the slurry can be applied.

[0084] In one variation, the negative electrode slurry can be cast onto another support, and a film layer obtained by peeling it off from the support can be laminated onto the negative electrode current collector to obtain a negative electrode. Alternatively, any alternative method other than those described above can be used to form a layer of negative electrode active material on the negative electrode current collector.

[0085] The resulting negative electrode comprises at least a graphite-based material (granulated material of artificial and natural graphite) and a silicon-based material as the negative electrode active material, as well as a conductive material. At least a portion of the natural graphite in the granulated product, particularly the surface portion of the natural graphite, is partially flaked to form flake portions. Since these flake portions are not separated from the natural graphite, they exist on the surface of the granulated product, oriented in all directions, and in contact with adjacent silicon-based materials or other granulated products. Furthermore, a portion of these flake portions further forms a complex with the silicon-based material to ensure an isotropic conductive path in the negative electrode. The conductive path formed by these flake portions does not cause any dispersion-related problems because it originates from the granulated material as the negative electrode active material. Additionally, the conductive path can be further improved by adding conductive material. Moreover, since the smaller-particle-size silicon-based material contacts the granulated material over a large area through these flake portions, or forms a complex with the flake portions, a high level and robust conductive path is formed throughout the negative electrode.

[0086] [positive electrode]

[0087] In a lithium-ion secondary battery according to an embodiment of the present disclosure, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on one or both surfaces of the positive electrode current collector. The positive electrode active material layer may be formed on the entire surface or a portion of the surface of the positive electrode current collector.

[0088] (Positive current collector)

[0089] There are no particular restrictions on the positive electrode current collector used for the positive electrode, as long as it is conductive and does not cause any chemical changes in the battery. Specific examples of such positive electrode current collectors include: stainless steel; aluminum; nickel; titanium; sintered carbon; aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.; aluminum-cadmium alloys, etc.

[0090] The thickness of the positive electrode current collector can be 3-500 μm. Fine surface irregularities can be formed on the surface of the positive electrode current collector, thereby enhancing adhesion to the positive electrode active material. For example, the positive electrode current collector can have various shapes, such as membranes, sheets, foils, meshes, porous bodies, foams, nonwovens, etc.

[0091] (Positive electrode active material layer)

[0092] The positive electrode active material layer can be formed, for example, by applying a positive electrode paste containing a mixture of a positive electrode active material, a binder, and a conductive material dissolved or dispersed in a solvent onto a positive electrode current collector, followed by drying and pressing. If necessary, the mixture may further contain a dispersant, a filler, or other optional additives.

[0093] Based on the total weight of the positive electrode active material layer, the positive electrode active material may be present in an amount of 80-99% by weight.

[0094] (Positive electrode active material)

[0095] The positive electrode active material may include a compound capable of reversibly inserting / extracting lithium. Specific examples of the positive electrode active material include lithium metal composite oxides containing at least one metal such as cobalt, manganese, nickel, copper, vanadium, or aluminum and lithium. More particularly, the lithium metal composite oxides may include: lithium-manganese oxides (such as LiMnO2, LiMn2O4, etc.); lithium-cobalt oxides (such as LiCoO2, etc.); lithium-nickel oxides (such as LiNiO2, etc.); lithium-copper oxides (such as Li2CuO2, etc.); lithium-vanadium oxides (such as LiV3O8, etc.); lithium-nickel-manganese oxides (such as, LiNi 1-z Mn z O2 (where 0 < z < 1), LiMn 2-z Ni z O4 (where 0 < z < 2)); lithium-nickel-cobalt oxides (such as, LiNi 1-y Co y O2 (where 0 < y < 1)); lithium-manganese-cobalt oxides (such as, LiCo 1-z Mn z O2 (where 0 < z < 1), LiMn 2-y Co y O4 (where 0 < y < 2)); lithium-nickel-manganese-cobalt oxides (such as, Li(Ni x Co y Mn z )O2 (0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1) or Li(Ni x Co y Mn z )O4 (0 < x < 2, 0 < y < 2, 0 < z < 2, x + y + z = 2)); lithium-nickel-cobalt-metal (M) oxides (such as, Li(Ni x Co y Mn z M w)O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and 0 < x < 1, 0 < y < 1, 0 < z < 1, 0 < w < 1 and x + y + z + w = 1)); those compounds in which the transition metal element is partially replaced by at least one metal element; etc. The positive electrode active material layer may contain any one of these compounds or two or more compounds. However, the scope of the present disclosure is not limited thereto.

[0096] Particularly, from the viewpoint of improving the capacity characteristics and stability of the battery, it is preferably LiCoO2, LiMnO2, LiMn2O4, lithium nickel manganese cobalt oxide (such as Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.4 Mn 0.3 Co 0.3 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), lithium nickel cobalt aluminum oxide (such as Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), etc.

[0097] (Binder and conductive material)

[0098] The types and contents of the binder and conductive material for the positive electrode paste may be the same as those described above for the negative electrode.

[0099] (Solvent)

[0100] There is no particular limitation on the solvent for the positive electrode paste as long as it is commonly used in the manufacture of the positive electrode. Specific examples of the solvent include, but are not limited to: pure water; amine solvents such as N,N-dimethylaminopropylamine, diethylenetriamine, N,N-dimethylformamide (DMF), etc.; ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; amide solvents such as dimethylacetamide, N-methyl-2-pyrrolidone (NMP), etc.; dimethyl sulfoxide (DMSO), etc. Such solvents may be used alone or in combination.

[0101] The solvent can be used in an amount such that, taking into account the slurry coating thickness or yield, the positive electrode active material, conductive material, and binder are dissolved or dispersed in the solvent, and a viscosity is provided that enables excellent thickness uniformity when applied to the positive electrode current collector.

[0102] [Manufacturing method of the positive electrode]

[0103] Similar to the manufacturing method of the negative electrode, the manufacturing method of the positive electrode for a lithium-ion secondary battery according to the embodiments of the present disclosure may include the following steps: dissolving or dispersing a positive electrode active material and optionally a binder, conductive material, thickener, etc. in a solvent to obtain a positive electrode slurry; and, for example, forming a positive electrode active material layer on a positive electrode current collector by applying the positive electrode slurry to the positive electrode current collector to obtain a positive electrode.

[0104] [Septum]

[0105] In the lithium-ion secondary battery according to embodiments of this disclosure, the separator serves to separate the negative and positive electrodes and provide a lithium-ion transport channel. Any separator can be used without limitation, as long as it is generally used as a separator for lithium-ion secondary batteries. In particular, the separator preferably exhibits low resistance to electrolyte ion migration and high wettability with the electrolyte. Specific examples of the separator may include porous polymer membranes, such as porous polymer membranes made of polyolefin polymers including ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, or ethylene / methacrylate copolymers, or laminates having two or more such porous polymer membranes. Alternatively, conventional porous nonwoven fabrics can be used, such as nonwoven fabrics made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength.

[0106] [Non-aqueous electrolytes]

[0107] In the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure, the non-aqueous electrolyte may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, etc., which can be used to manufacture secondary batteries.

[0108] The non-aqueous electrolyte may contain organic solvents and lithium salts, and may further contain additives if desired. In the following text, the liquid electrolyte is also referred to as an "electrolyte".

[0109] There are no particular limitations on the organic solvents mentioned, as long as they can serve as a medium through which ions participating in the electrochemical reactions of the battery can move. Specific examples of such organic solvents include, but are not limited to: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether and tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic solvents, such as benzene and fluorobenzene; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents, such as ethanol and isopropanol; nitrile solvents, such as R-CN (where R is a C2-C20 straight-chain, branched, or cyclic hydrocarbon group, which may optionally contain a double-bonded aromatic ring or ether bond); amide solvents, such as dimethylformamide; dioxolane solvents, such as 1,3-dioxolane; sulfolane solvents, etc. Such solvents can be used alone or in combination. In particular, carbonate-based solvents are preferred. Furthermore, a mixture that enhances the charge-discharge characteristics of the battery is more preferred, wherein the mixture comprises a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, and a linear carbonate compound with low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate). In this case, excellent electrolyte quality can be achieved when the cyclic carbonate and linear carbonate are mixed in a volume ratio of approximately 1:1 to 1:9.

[0110] There are no particular limitations on the lithium salt, as long as it is a compound capable of providing lithium ions for use in lithium-ion secondary batteries. Specific examples of such lithium salts include, but are not limited to: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. These lithium salts can be used alone or in combination. For example, the lithium salt can be present in the electrolyte at a concentration of 0.1-2 mol / L. When the concentration of the lithium salt falls within the above range, the electrolyte has suitable conductivity and viscosity and exhibits excellent electrolyte quality, thus enabling efficient transport of lithium ions.

[0111] Additives may be optionally used to improve battery life characteristics, suppress battery capacity degradation, and improve battery discharge capacity. Specific examples of such additives include, but are not limited to: alkylene carbonate halide compounds, such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); pyridine; triethyl phosphite; triethanolamine; cyclic ethers; ethylenediamine; n-glycol dimethyl ether; hexamethylphosphoryltriamine; nitrobenzene derivatives; sulfur; quinone imine dyes; N-substituted compounds. Alzolidinediones; N,N-substituted imidazolidinedions; ethylene glycol dialkyl ethers; ammonium salts; pyrroles; 2-methoxyethanol; aluminum trichloride, etc. These additives can be used alone or in combination. The additives can be used in amounts of 0.1-15% by weight relative to the total weight of the electrolyte.

[0112] Specifically, ethylene fluoride carbonate and ethylene difluorocarbonate can act as film-forming agents to form a coating film at the interface between the electrode and the electrolyte. For example, when at least one of ethylene fluoride carbonate and ethylene difluorocarbonate is used, a high-quality solid electrolyte interface (SEI) film is formed during the alloying of the silicon-based material with lithium in a negative electrode containing a silicon-based material, thereby enabling stable charge and discharge. The content of the film-forming agent relative to the total weight of the electrolyte can be 0.1-15% by weight, preferably 0.5-10% by weight, more preferably 1-7% by weight. The film-forming agent may include at least one of ethylene fluoride carbonate and ethylene difluorocarbonate.

[0113] [Manufacturing method of non-aqueous electrolyte secondary battery]

[0114] The non-aqueous electrolyte secondary battery according to embodiments of this disclosure can be obtained by inserting a separator and an electrolyte between the negative and positive electrodes obtained as described above. In particular, the non-aqueous electrolyte secondary battery can be obtained by placing a separator between the negative and positive electrodes to form an electrode assembly, introducing the electrode assembly into a battery case, such as a cylindrical battery case or a prismatic battery case, injecting electrolyte therein, and sealing the battery case.

[0115] The battery casing can be any type commonly used in the art. For example, the shape of the battery casing can include a cylindrical, prismatic, bag-shaped, or coin-shaped casing, similar to a can.

[0116] The lithium-ion secondary batteries according to embodiments of this disclosure can be used not only as power sources for small devices, but also as unit cells in medium to large-sized battery modules comprising multiple battery cells. Preferred examples of such medium to large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0117] Pattern used for the content of this disclosure

[0118] The present disclosure will be explained in more detail below with reference to embodiments and comparative examples. However, the scope of the present disclosure is not limited thereto.

[0119] [Example 1]

[0120] First, 90 wt% of a granulated product of artificial and natural graphite (average particle size: 20 μm, artificial graphite to natural graphite weight ratio = 8:2) was mixed with 10 wt% of silica (SiO) with an average particle size of 5 μm. Then, 1 wt% of carbon black as a conductive material and 1.7 wt% of carboxymethyl cellulose (CMC) as a thickener were added to the mixture relative to the total weight of the mixture. Pure water was then added to bring the solids content to 65 wt% to prepare a slurry. The slurry was then hard-mixed using a planetary centrifugal mixer. The temperature of the slurry during hard mixing was 70°C. Subsequently, 1.5 wt% of styrene-butadiene rubber (SBR) as a binder was added to the resulting slurry, and pure water was further added to promote application, thereby providing a solids content controlled at 50 wt%. The slurry was then applied to a copper foil, dried at 60°C for 30 minutes and then vacuum dried at 120°C for 12 hours. The resulting structure was pressed to an electrode density of 1.65 g / cc to obtain a negative electrode.

[0121] [Example 2]

[0122] The negative electrode was obtained in the same manner as in Example 1, except that the amount of pure water was controlled so that the solids content of the slurry during hard mixing could be 60% by weight. The temperature of the slurry during hard mixing was 65°C.

[0123] [Example 3]

[0124] The negative electrode was obtained in the same manner as in Example 1, except that a granulated product of a SiCrTi alloy with an average particle size of 2 μm (atomic weight ratio (Si:Cr:Ti) = 82.4:8.8:8.8) was used instead of silicon monoxide (SiO) and mixed with granulated products of artificial and natural graphite. The slurry temperature was 72°C during hard mixing.

[0125] [Comparative Example 1]

[0126] The negative electrode was obtained in the same manner as in Example 1, except that the amount of pure water was controlled so that the solids content of the slurry during hard mixing could be 55% by weight. The temperature of the slurry during hard mixing was 57°C.

[0127] [Comparative Example 2]

[0128] The negative electrode was obtained in the same manner as in Example 3, except that the amount of pure water was controlled so that the solids content of the slurry during hard mixing could be 55% by weight. The temperature of the slurry during hard mixing was 58°C.

[0129] [Comparative Example 3]

[0130] The negative electrode was obtained in the same manner as in Example 1, except that artificial graphite powder with an average particle size of 20 μm was used instead of the granulated product of artificial and natural graphite. The slurry temperature was 36°C during hard mixing.

[0131] [Comparative Example 4]

[0132] The negative electrode was obtained in the same manner as in Example 1, except that a powder mixture containing artificial graphite powder with an average particle size of 20 μm and natural graphite powder with an average particle size of 5 μm in a weight ratio of 8:2 was used instead of the granulated product of artificial and natural graphite. The slurry temperature was 50°C during hard mixing.

[0133] [Comparative Example 5]

[0134] The negative electrode was obtained in the same manner as in Comparative Example 3, except that 1% by weight of graphene was used as the conductive material instead of 1% by weight of carbon black. The slurry temperature was 36°C during hard mixing.

[0135] [Comparative Example 6]

[0136] The negative electrode was obtained in the same manner as in Comparative Example 1, except that natural graphite powder with an average particle size of 20 μm was used instead of the granulated product of artificial graphite and natural graphite. The slurry temperature was 57°C during hard mixing.

[0137] [Experimental Example 1: SEM Analysis]

[0138] The negative electrode according to Example 3 was observed using a scanning electron microscope (SEM). An SEM image showing the natural graphite portion of the negative electrode is shown. Figure 1 and Figure 2 In the SEM image, the surface of natural graphite is partially flaked, fractured, and roughened. Specifically, in... Figure 2 In the SEM images, the surface portion of natural graphite is partially curled and grapheneized, or the granulated product of Si alloy is dispersed on or in contact with the surface of the flake portion.

[0139] [Experimental Example 2: Capacity Retention]

[0140] The capacity retention rate, representing the lifetime characteristics, was evaluated as follows. Coin cells (half-cells) were obtained using negative electrodes obtained according to Examples 1-3 and Comparative Examples 1-6 and lithium metal as the counter electrode (i.e., positive electrode). Each coin cell was charged and discharged for the first cycle (first cycle) at a constant current of 0.2C with a cutoff voltage of 1.5V. Under the same conditions, each coin cell was charged and discharged for the second cycle (second cycle). Then, the same charge and discharge cycle was repeated for 48 cycles (48 times) at a constant current of 0.5C to determine the discharge capacity (mAh). In other words, a total of 50 charge and discharge cycles were repeated, including the first and second cycles. The capacity retention rate was calculated as the ratio of the discharge capacity at the 50th charge and discharge cycle to the discharge capacity at the 3rd cycle (discharge capacity at the 50th cycle / discharge capacity at the 3rd cycle). The results are shown in Table 1 below.

[0141] [Table 1]

[0142]

[0143] As can be seen from Table 1, Examples 1-3, which use granulated products of artificial and natural graphite as graphite-based materials and have a slurry solids content of 60% by weight or more during hard mixing, provide high capacity retention. This is believed to be because a high level and robust conductive path is formed throughout the negative electrode through partial flake formation of natural graphite, and this conductive path is maintained even when the negative electrode active material (especially silicon-based materials) repeatedly expands during charge and discharge. Furthermore, it is believed that using a relatively high slurry temperature of 65°C or higher during hard mixing according to Examples 1-3 also promotes hard mixing.

[0144] On the other hand, in the cases of Comparative Examples 1 and 2, even when the granulated product of artificial graphite and natural graphite was used as a graphite-based material, the capacity retention rate decreased due to the low slurry solids content of 55% by weight during hard mixing of each electrode. It is believed that because the viscosity of the slurry is low during hard mixing, the natural graphite does not flake in the granulated product. Similarly, it is believed that the natural graphite powder does not flake in the case of Comparative Example 6.

[0145] The reason why Comparative Examples 3 and 5 each provide low capacity retention is that artificial graphite is relatively rigid compared to natural graphite. Artificial graphite does not undergo flake formation during hard mixing, and therefore cannot form a high level and robust conductive path like the conductive paths in Examples 1-3. Furthermore, referring to Comparative Example 5, adding graphene alone does not allow both the graphitic and silicon-based materials to contact the graphene. Additionally, in this case, it is difficult to uniformly disperse the graphene in the negative electrode. Moreover, since graphene tends to align parallel to the current collector, an isotropic conductive path like the conductive paths in Examples 1-3 is not formed, especially in the direction perpendicular to the current collector surface, where the conductive path is insufficient.

[0146] In Comparative Example 4, since only a mixture of artificial graphite and natural graphite was used, the artificial graphite did not flake during hard mixing, and the soft natural graphite was over-crushed, resulting in a lower capacity retention rate compared to Examples 1-3, where the presence of artificial graphite as a matrix suppressed the over-crushing of natural graphite.

Claims

1. A negative electrode for a secondary battery, comprising at least a graphite-based material and a silicon-based material as negative electrode active materials, and a conductive material. The graphite-based materials mentioned above include granulated products of artificial graphite and natural graphite. At least a portion of the natural graphite in the granulated product is partially flaked to form flake portions, and The thinned portion comes into contact with the silicon-based material or other granulated products of artificial and natural graphite, or forms a composite with the silicon-based material. "Partial flake formation" refers to the fact that the surface portion of natural graphite is not completely peeled off and separated from natural graphite, but rather that the surface portion of natural graphite is partially peeled off, curled, and flaked.

2. The negative electrode for a secondary battery according to claim 1, wherein the silicon-based material comprises any one or both of silicon oxide and silicon alloy.

3. The negative electrode for a secondary battery according to claim 2, wherein the silicon alloy comprises at least one selected from titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper.

4. The negative electrode for a secondary battery according to claim 1, wherein the conductive material is carbon black.

5. The negative electrode for a secondary battery according to claim 1, wherein the weight ratio of the graphite material to the silicon material is 98:2-50:

50.

6. The negative electrode for a secondary battery according to claim 1, wherein the granulated product comprises artificial graphite and natural graphite in a weight ratio of 60:40 to 90:

10.

7. A secondary battery comprising a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode as defined in any one of claims 1 to 6, and an electrolyte.

8. A negative electrode slurry for a secondary battery, used to manufacture a negative electrode for a secondary battery according to any one of claims 1 to 6, wherein the negative electrode slurry comprises: Graphite-based and silicon-based materials as negative electrode active materials; conductive materials; solvents; and at least one of thickeners and binders. The solids content in the slurry is 60% by weight or more relative to the weight of the slurry. The graphite-based materials mentioned above include granulated products of artificial graphite and natural graphite. In the granulation product, at least a portion of the natural graphite is partially flaked to form a flake portion.

9. The negative electrode slurry according to claim 8, wherein the solid content in the slurry is 65-75% by weight relative to the weight of the slurry.

10. A method for manufacturing a negative electrode for a secondary battery according to any one of claims 1 to 6, comprising the following steps: A slurry having a solid content of 60% by weight or more relative to the weight of the slurry is prepared by mixing at least one of graphite-based materials and silicon-based materials, conductive materials, solvents, thickeners and binders as negative electrode active materials. The slurry is hard-mixed, wherein during the hard-mixing step, at least a portion of the natural graphite in the granulation product is partially flaked to form flake portions; and The slurry is applied to the current collector to obtain the negative electrode. The graphite-based material mentioned above is a granulated product of artificial graphite and natural graphite.

11. The method of claim 10, further comprising, after the step of hard mixing the slurry, adding an adhesive and a solvent to the slurry.

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