Positive electrode for secondary battery containing novel dispersant, electrode assembly containing the same, and secondary battery

Abstract

According to this embodiment, a positive electrode for a secondary battery is provided, the positive electrode includes a positive electrode mixture layer formed on at least one surface of a current collector and including a positive electrode active material, a conductive material, a binder, and a dispersant; The dispersant is a mixture of a first dispersant based on butadiene-acrylonitrile and a second dispersant based on styrene-ethylene oxide (styrene-EO), The dispersant is contained in an amount of 0.1 wt % or more and less than 0.3 wt % based on the total weight of the positive electrode mixture layer, and a positive electrode for a secondary battery, an electrode assembly including the same, and a secondary battery are provided.

Description

Technical Field

[0001] Cross-reference to Related Application(s) This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0131692 filed on Oct. 05, 2021 and Korean Patent Application No. 10-2022-0127179 filed on Oct. 05, 2022, and all of the contents disclosed in the documents of the Korean patent applications are incorporated herein by reference in their entirety.

[0002] The present invention relates to a positive electrode for a secondary battery including a novel dispersant, an electrode assembly including the same, and a secondary battery.

Background Art

[0003] Due to the rapid increase in the use of fossil fuels, the demand for alternative and clean energy has been increasing. As part of this, the fields of power generation and power storage using electrochemistry are the most actively studied.

[0004] Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery, and its usage area is increasingly expanding.

[0005] Recently, due to the development of technologies and the increasing demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has been rapidly increasing. Among such secondary batteries, many studies have been conducted on lithium secondary batteries that exhibit high charge / discharge characteristics and long life characteristics and are environmentally friendly, and they have also been commercialized and widely used.

[0006] Generally, a lithium secondary battery is manufactured by impregnating an electrode assembly including a positive electrode, a negative electrode, and a porous separator with a lithium non-aqueous electrolyte.

[0007] Such lithium secondary batteries generally include, depending on the form of the battery case, a cylindrical or rectangular secondary battery in which a stack / folding type or wound type electrode assembly is received in a can made of metal as the battery case, a pouch type battery having a structure in which a stack type or stack / folding type electrode assembly is built into a pouch-shaped battery case made of an aluminum laminate sheet, and the like.

[0008] On the other hand, since the cylindrical battery is used as the battery for the electric vehicle, the loading of the active material layer is increasing in accordance with the demand for improving the energy of the cylindrical battery.

[0009] However, when these laminates are wound to manufacture a wound type electrode assembly for use in a cylindrical battery after manufacturing an electrode with a high loading, there is a problem that cracks occur in the positive electrode located in the core part, deteriorating the battery performance.

[0010] Therefore, even if such problems are solved and a wound type electrode assembly with a small curvature is manufactured, there is an urgent need for the development of a positive electrode for a secondary battery in which cracks rarely occur.

Summary of the Invention

Problems to be Solved by the Invention

[0011] An object of the present invention is to solve the problems of the conventional technologies as described above and the technical problems that have been required for a long time.

[0012] Specifically, the problem to be solved by the present invention is to provide a positive electrode for a secondary battery with improved flexibility capable of preventing cracks in the innermost positive electrode during the manufacture of an electrode assembly having a wound structure, an electrode assembly including the same, and a secondary battery, in order to prevent a decrease in secondary battery performance.

Means for Solving the Problems

[0013] According to an embodiment of the present invention for achieving such an object, The positive electrode for a secondary battery according to the present invention includes a positive electrode mixture layer formed on at least one surface of a current collector and containing a positive electrode active material, a conductive material, a binder, and a dispersant. The dispersant is a mixture of a butadiene-acrylonitrile-based first dispersant and a styrene-ethylene oxide-based (styrene-EO-based) second dispersant. The dispersant is contained in an amount of 0.1% by weight or more and less than 0.3% by weight based on the total weight of the positive electrode mixture layer.

[0014] More specifically, the dispersant may be contained in an amount of 0.15% by weight to 0.25% by weight based on the total weight of the positive electrode mixture layer.

[0015] Here, the mixing ratio of the first dispersant and the second dispersant may be from 8:2 to 2:8 by weight.

[0016] The first dispersant may contain hydrogenated butadiene, non-hydrogenated butadiene, and acrylonitrile. Specifically, the first dispersant may be a butadiene-acrylonitrile-based polymer in which 50 to 80% by weight of hydrogenated butadiene, 0.1 to 10% by weight of non-hydrogenated butadiene, and 15 to 40% by weight of acrylonitrile are polymerized, and the butadiene-acrylonitrile-based polymer may have a weight average molecular weight of 30,000 to 80,000.

[0017] The second dispersant may contain styrene, ethylene oxide, and an aliphatic compound. Specifically, the second dispersant may be a styrene-ethylene oxide-based polymer in which 20 to 40% by weight of styrene, 40 to 60% by weight of ethylene oxide, and 15 to 30% by weight of an aliphatic compound are polymerized, and the styrene-ethylene oxide-based polymer may have a weight average molecular weight of 9,000 to 12,000.

[0018] On the other hand, the positive electrode active material of the positive electrode may be a lithium transition metal oxide represented by the following chemical formula (1). Li 1+x Ni a Co bMn c M 1-(a+b+c) O 2-y A y (1) In the chemical formula (1), M is at least one selected from the group consisting of Cu, Ti, Mg, Al, Pt, and Zr, A is an oxygen-substituted halogen, 0 ≦ x ≦ 0.5, 0.8 ≦ a ≦ 1, 0 ≦ b ≦ 0.2, 0 ≦ c ≦ 0.2, 0.9 ≦ a + b + c ≦ 1, and 0 ≦ y ≦ 0.001.

[0019] Further, the conductive material may be a carbon nanotube.

[0020] Such a positive electrode containing the dispersant according to the present invention is preferably applied to a high-loading positive electrode. Specifically, the positive electrode mixture layer may have a loading amount of 26 mg / cm 2 to 30 mg / cm 2 .

[0021] The positive electrode for a secondary battery having such a configuration may have a breaking strength (brittleness) of 41 gf to 50 gf.

[0022] On the other hand, according to another embodiment of the present invention, the present invention also provides an electrode assembly including the positive electrode for a secondary battery, a negative electrode, and a separator, and having a structure in which the positive electrode, the negative electrode, and the separator are all wound together such that the positive electrode is located on the innermost side, and the innermost radius (r) is 1.4 mm to 2.0 mm.

[0023] Also, according to an embodiment of the present invention, there is provided a secondary battery in which the electrode assembly and an electrolytic solution are both installed in a battery case.

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] Hereinafter, for better understanding of the present invention, the present invention will be described in more detail.

[0025] The terms and words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings. Instead, based on the principle that the inventor can appropriately define the concept of the terms in order to best explain his or her invention, they should be construed in a meaning and concept that conforms to the technical idea of the present invention, rather than being construed in a way that limits the invention.

[0026] The terms used in this specification are merely used to illustrate exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0027] In this specification, terms such as "comprising," "including," or "having" are intended to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and should be understood not to preclude in advance the presence or addition possibility of one or more other features, numbers, steps, components, or combinations thereof.

[0028] According to one embodiment of the present invention, a positive electrode for a secondary battery, wherein the positive electrode includes a positive electrode mixture layer formed on at least one surface of a current collector and containing a positive electrode active material, a conductive material, a binder, and a dispersant, wherein the dispersant is a mixture of a butadiene-acrylonitrile-based first dispersant and a styrene-ethylene oxide-based (styrene-EO-based) second dispersant, there is provided a positive electrode for a secondary battery, wherein the dispersant is contained in an amount of 0.1% by weight or more and less than 0.3% by weight based on the total weight of the positive electrode mixture layer.

[0029] Conventionally, a dispersant has been used to improve the dispersibility of the positive electrode. However, the previously used dispersant could not have sufficient dispersibility due to an increase in the loading amount, and when manufacturing an electrode assembly having a structure in which the electrode is wound with low electrode flexibility, cracks occurred in the core part, continuously causing problems such as a decrease in the performance and stability of the secondary battery.

[0030] Therefore, the inventors of the present application repeated in-depth research, and finally, when using a high-loading cathode, a new dispersant capable of ensuring sufficient dispersibility was applied, and the content of the dispersant capable of exhibiting optimized secondary battery performance was derived.

[0031] Specifically, the dispersant used for the cathode of the secondary battery according to the present invention is a mixture of a butadiene-acrylonitrile-based first dispersant and a styrene-ethylene oxide-based (styrene-EO-based) second dispersant, and the content of the total dispersant is included in an amount of 0.1% by weight or more and less than 0.3% by weight based on the total weight of the cathode mixture layer. Specifically, it may be included in an amount of 0.15% by weight to 0.25% by weight, and more specifically, 0.15% by weight to 0.2% by weight.

[0032] When outside the above range, if the total content of the dispersant is outside the above range and is excessively small, the improvement in electrode flexibility according to the present invention, that is, the effect of preventing crack generation cannot be obtained. When it is excessively large, the dispersant acts as a resistance and the electrode resistance increases, which is not preferable.

[0033] Also, the mixing ratio of the first dispersant and the second dispersant may be from 9:1 to 1:9 by weight, specifically, it may be from 2:8 to 8:2, and more specifically, it may be from 4:6 to 6:4.

[0034] The first dispersant of the butadiene-acrylonitrile type plays a role in improving the dispersibility of the cathode active material and the like, and the second dispersant of the styrene-ethylene oxide type (styrene-EO type) plays a role in imparting flexibility to the electrode.

[0035] Therefore, when outside the above range and one side is contained in an excessively small amount, the effects intended by the present application cannot be appropriately exhibited.

[0036] Specifically, the first dispersant may contain hydrogenated butadiene, non-hydrogenated butadiene, and acrylonitrile. More specifically, it may be a butadiene-acrylonitrile copolymer in which 50 to 80% by weight of hydrogenated butadiene, 0.1 to 10% by weight of non-hydrogenated butadiene, and 15 to 40% by weight of acrylonitrile are polymerized.

[0037] Most specifically, the first dispersant may be a butadiene-acrylonitrile copolymer in which 60 to 70% by weight of hydrogenated butadiene, 0.1 to 1% by weight of non-hydrogenated butadiene, and 29 to 39% by weight of acrylonitrile are polymerized.

[0038] When included within the above ranges, the effect of the dispersibility can be fully exerted to ensure sufficient flexibility of the electrode.

[0039] Further, the butadiene-acrylonitrile copolymer may have a weight average molecular weight of 30,000 to 80,000, and more specifically, 40,000 to 60,000.

[0040] If it is outside the above ranges and has an excessively small molecular weight, there is a problem that layer separation occurs during positive electrode coating and drying without aggregation. If it has an excessively large molecular weight, the dispersibility in the solvent decreases, and a slurry aggregation phenomenon occurs, acting as resistance, which is not preferable.

[0041] Specifically, the second dispersant may contain styrene, ethylene oxide, and an aliphatic compound. More specifically, it may be a styrene-ethylene oxide copolymer in which 20 to 40% by weight of styrene, 40 to 60% by weight of ethylene oxide, and 15 to 30% by weight of the aliphatic compound are polymerized.

[0042] Most specifically, the second dispersant may be a styrene-ethylene oxide copolymer in which 20 to 30% by weight of styrene, 50 to 60% by weight of ethylene oxide, and 20 to 30% by weight of the aliphatic compound are polymerized.

[0043] When included within the above range, sufficient phase stability of the dispersion can be ensured, and a suitable range of viscosity can also be obtained.

[0044] Also, the styrene-ethylene oxide-based polymer may have a weight average molecular weight of 9,000 to 12,000, specifically 10,000 to 11,000.

[0045] If it is outside the above range and has an excessively small molecular weight, the viscosity of the slurry will be low and the solid content cannot be increased, and there is a problem of damaging the positive electrode during coating and drying. If it has an excessively large molecular weight, the dispersibility will decrease, a slurry aggregation phenomenon will occur and resistance will be generated, and also, the viscosity of the slurry will become excessively high and the phase stability will decrease. Therefore, it is not preferable.

[0046] When using a novel dispersant under the above conditions, sufficient flexibility can be given even to a high-loading positive electrode, so that even when wound with a small curvature, cracks will not occur in the core part.

[0047] The weight average molecular weight can be measured by Mw using gel permeation chromatography (GPC). Specifically, a Polymer Laboratories PLgel MIX-B column with a length of 300 mm was used, and evaluation was carried out using a Waters PL-GPC220 instrument. The evaluation temperature was 160 °C, 1,2,4-trichlorobenzene was used as the solvent, and the flow rate was measured at a speed of 1 mL / min. The sample was prepared at a concentration of 10 mg / 10 mL and then supplied in an amount of 200 μL. The value of Mw was derived using a calibration curve formed using polystyrene standards. Nine types of molecular weights of polystyrene standards, namely 2,000 / 10,000 / 30,000 / 70,000 / 200,000 / 700,000 / 2,000,000 / 4,000,000 / 10,000,000, were used.

[0048] On the one hand, each of the positive electrode active materials may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x O4 (where x is from 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M x O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga and x = 0.01 to 0.3), Ni-site type lithium nickel oxide represented by; chemical formula LiMn 2-x M x O2 (where M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or lithium manganese composite oxide represented by Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); LiNi x Mn 2-x O4, spinel-structured lithium manganese composite oxide; LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions; lithium iron phosphate represented by LiFePO4; disulfide compounds; Fe2(MoO4)3, etc. may be included, but are not limited thereto.

[0049] However, for the high-loading positive electrode to which the present invention is applied, improvement of energy density is important. Therefore, the positive electrode active material may be a Ni-high-content lithium transition metal oxide represented by the following chemical formula (1). Li 1+x Ni a Co b Mn c M 1-(a+b+c) O 2-y A y (1) In the chemical formula (1), M is at least one selected from the group consisting of Cu, Ti, Mg, Al, Pt, and Zr, A is an oxygen-substituted halogen, 0 ≦ x ≦ 0.5, 0.8 ≦ a ≦ 1, 0 ≦ b ≦ 0.2, 0 ≦ c ≦ 0.2, 0.9 ≦ a + b + c ≦ 1, and 0 ≦ y ≦ 0.001.

[0050] More specifically, the a may be 0.88 ≦ a < 1.

[0051] Also, an active material different from the lithium transition metal oxide represented by the chemical formula (1) may be mixed and used.

[0052] Also, the conductive material is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. are used. Specifically, carbon nanotubes may be used as the carbon fibers.

[0053] The carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0054] Such a conductive material may be contained in an amount of 0.1 to 30% by weight, specifically 0.1 to 10% by weight, more specifically 0.5 to 5% by weight based on the total weight of the positive electrode mixture layer.

[0055] If it is outside the above range and the content is excessively large, the content of the active material relatively decreases and the capacity decreases. If it is excessively small, the conductivity and output characteristics may decrease, which is not preferable.

[0056] As specific examples, the binder may include polyvinylidene fluoride (PVDF), vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene monomer (EPDM), sulfonated - EPDM, styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc.

[0057] Such a binder may be contained in an amount of 0.1 to 30% by weight, specifically 0.1 to 10% by weight, more specifically 0.5 to 5% by weight, based on the total weight of the positive electrode active material layer.

[0058] If it is outside the above range and the content of the binder is excessively high, the content of the active material will decrease and the capacity will decrease. If it is excessively low, the adhesive force will decrease and the life performance, etc. may decrease, which is not preferable.

[0059] On the other hand, the present invention is more effective for high - loading positive electrodes. Therefore, the positive electrode active material layer may have a loading amount of 26 mg / cm 2 to 30 mg / cm 2 The application of the dispersant according to the present invention is most effective in the high - loading positive electrode within the above range.

[0060] The positive electrode for a secondary battery having such a configuration may have increased flexibility and a breaking strength of 41 gf to 50 gf.

[0061]

[0062] ​The breaking strength is a value measured by placing the positive electrode on a support table such that both ends of the manufactured positive electrode with a width of 3 cm or less are positioned on the support table, and then allowing the middle portion to be separated from the ground. After that, the central portion of the positive electrode is pressed from above with a plate of 3 cm × 10 cm × 10 mm, and the thinnest part presses the central portion of the positive electrode at a speed of 1 mm / sec until the positive electrode breaks. This value is not affected by the area of the positive electrode.

[0063] If it deviates from the above range and the breaking strength of the positive electrode is excessively large, the flexibility of the positive electrode decreases. When manufacturing an electrode assembly with a structure wound using the positive electrode, there is a problem that cracks occur in the core part. If it is excessively large, the resistance increases due to an increase in the content of the dispersant, which is not preferable.

[0064] On the other hand, according to another embodiment of the present invention, there is provided an electrode assembly including the positive electrode for the secondary battery, a negative electrode, and a separator, and having a structure in which the positive electrode, the negative electrode, and the separator are all wound such that the positive electrode is located on the innermost side, and the innermost radius (r) is from 1.4 mm to 2.0 mm.

[0065] When using the positive electrode manufactured as described above, the electrode flexibility increases, and even if the innermost radius (r) is formed as small as within the above range, cracks do not occur in the core part, and it is possible to manufacture an electrode assembly with a higher energy density per volume.

[0066] Furthermore, according to another embodiment of the present invention, there is provided a secondary battery in which the electrode assembly and an electrolytic solution are both installed in a battery case.

[0067] Additional components such as the negative electrode, the separator, and the electrolytic solution are known in the conventional art. Therefore, specific details are omitted in this specification, and the conventional configurations are included in the present invention.

[0068] Hereinafter, preferred embodiments of the present invention, comparative examples for comparison therewith, and experimental examples for evaluating these will be described. However, the above embodiments are merely illustrative of the present description, and it is obvious to those skilled in the art that various changes and modifications can be made within the scope of the present description and the scope of the technical idea. Needless to say, such variations and modifications belong to the scope of the appended claims.

[0069] <Example 1> As the positive electrode active material, LiNi 0.88 Co 0.07 Mn 0.04 Al 0.01 O2, an active material mixture in which Li2NiO2 is mixed at a weight ratio of 95:5 as a sacrificial positive electrode material, carbon nanotubes as a conductive material, and as a dispersant, a butadiene-acrylonitrile-based dispersant having a weight average molecular weight of 50,000: a styrene-ethylene oxide-based dispersant having a weight average molecular weight of 10,000 are mixed at a weight ratio of 40:60, and PVDF as a binder are mixed in an N-methylpyrrolidone solvent at a weight ratio of 98.02:0.6:0.1:1.28 to produce a composition for forming a positive electrode, and this is applied to an aluminum current collector so that the loading amount is 28.9 mg / cm 2 to produce positive electrode A.

[0070] <Example 2> Positive electrode B was produced in the same manner as in Example 1, except that the positive electrode active material: conductive material: dispersant: binder was mixed at a weight ratio of 97.94:0.6:0.18:1.28 in an N-methylpyrrolidone solvent.

[0071] <Example 3> Positive electrode C was produced in the same manner as in Example 1, except that in Example 1, the positive electrode active material: conductive material: dispersant: binder was mixed at a weight ratio of 97.94:0.6:0.18:1.28 in an N-methylpyrrolidone solvent and a butadiene-acrylonitrile-based dispersant having a weight average molecular weight of 20,000 was used.

[0072] <Example 4> In Example 1, except that the positive electrode active material: conductive material: dispersant: binder was mixed at a weight ratio of 97.94:0.6:0.18:1.28 in an N-methylpyrrolidone solvent and a butadiene-acrylonitrile-based dispersant with a weight average molecular weight of 90,000 was used, a positive electrode D was produced in the same manner as in Example 1.

[0073] <Example 5> In Example 1, except that the positive electrode active material: conductive material: dispersant: binder was mixed at a weight ratio of 97.94:0.6:0.18:1.28 in an N-methylpyrrolidone solvent and a styrene-ethylene oxide-based dispersant with a weight average molecular weight of 5,000 was used, a positive electrode E was produced in the same manner as in Example 1.

[0074] <Example 6> In Example 1, except that the positive electrode active material: conductive material: dispersant: binder was mixed at a weight ratio of 97.94:0.6:0.18:1.28 in an N-methylpyrrolidone solvent and a butadiene-acrylonitrile-based dispersant with a weight average molecular weight of 15,000 was used, a positive electrode F was produced in the same manner as in Example 1.

[0075] <Comparative Example 1> In Example 1, except that the positive electrode active material: conductive material: dispersant: binder was mixed at a weight ratio of 97.82:0.6:0.3:1.28 in an N-methylpyrrolidone solvent, a positive electrode G was produced in the same manner as in Example 1.

[0076] <Comparative Example 2> As the positive electrode active material, LiNi 0.88 Co 0.07 Mn 0.04 Al 0.1O2, an active material mixture in which Li2NiO2 is mixed as a sacrificial cathode material at a weight ratio of 95:5, carbon nanotubes as a conductive material, a butadiene-acrylonitrile-based dispersant with a weight average molecular weight of 220,000 as a dispersant, and PVDF as a binder were mixed in an N-methylpyrrolidone solvent at a ratio of 98:0.6:0.12:1.28 by weight to produce a composition for forming a cathode, and this was applied to an aluminum current collector so that the loading amount was 28.9 mg / cm 2 to produce a cathode H.

[0077] <Comparative Example 3> In Example 1, except that the content of the dispersant was reduced and the cathode active material:conductive material:dispersant:binder was mixed at a weight ratio of 98.03:0.6:0.07:1.28 in an N-methylpyrrolidone solvent, a cathode I was produced in the same manner as in Example 1.

[0078] <Experimental Example 1> Brittleness measurement Three each of the cathodes and the like produced in Examples 1 to 6 and Comparative Examples 1 to 3 were prepared in a size of 10 mm × 150 mm. Using the Texture Analysis equipment of Stable Micro Systems, the cathodes and the like prepared were placed on the support table so that both ends were positioned on the support table, and after making the middle part away from the ground, the central part of the cathode was pressed with a plate of 3 cm × 10 cm × 10 mm from above, and the thinnest part pressed the central part of the cathode, and the strength at which the cathode was broken was measured 3 times. The descending speed of the plate was set to 1 mm / s.

[0079] The results are shown in Table 1 below. The larger the value in Table 1 below, the more the flexibility decreases.

[0080]

Table 1

[0081] Referring to Table 1 above, in the case of the present invention, it can be confirmed that when the breaking strength is from 41 gf to 50 gf or less, appropriate flexibility is ensured. Conventionally, in Comparative Example 2 using a dispersant, the breaking strength becomes excessively high and the flexibility decreases. It can also be confirmed that when the content of the dispersant is excessively high or low, the breaking strength is excessively low or high.

[0082] <Experimental Example 2> Crack Measurement The positive electrodes etc. produced in Examples 1 to 6 and Comparative Examples 1 to 3 above were wound using a 3.2 pi core. After 1 hour, the positive electrode was unwound and the presence or absence of cracks was visually confirmed, and the results are shown in Table 2 below.

[0083]

Table 2

[0084] Referring to Table 2 above, it can be confirmed that cracks occurred in Comparative Example 2 where the breaking strength was excessively high as confirmed in Table 1 above.

[0085] <Experimental Example 3> DCIR Measurement The positive electrodes etc. produced in Examples 1 to 6 and Comparative Examples 1 to 3 above were prepared.

[0086] As the cathode active material, a mixture of graphite:SiO mixed at a weight ratio of 95:5 was used. As the binder, styrene-butadiene rubber (SBR), as the thickener, sodium carboxymethyl cellulose (CMC), and as the conductive material, carbon black were mixed at a weight ratio of 97:1:1:1, and then added to the solvent water to produce a cathode-forming composition, which was applied to a 10 μm copper current collector to produce a cathode.

[0087] After interposing a polyethylene separator (thickness: 15 μm) between the positive electrode etc. manufactured above and the negative electrode, a solvent in which ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 20:10:70 was contained, and an electrolytic solution containing 1.3 M LiPF6 of 1 M was injected into the total amount of the electrolytic solution to manufacture a secondary battery.

[0088] When the secondary battery was charged to SOC 50% at room temperature and discharged for 10 seconds at 0.5 C, the voltage drop generated was recorded, and the resistance value (mOhm) of the secondary battery was measured using R = V / I, and the results are shown in Table 3 below. Charge: 0.5C, CC / CV, 4.25V, 1 / 20C cut-off Discharge: 0.5C, CC, 2.5V, cut-off

[0089]

Table 3

[0090] Referring to Table 3, it can be confirmed that when the positive electrode according to the present invention is used, a resistance value at a similar level is shown as compared with the case where an existing dispersant is used. However, when it is contained at 0.3% by weight or more, the resistance increases rapidly, and it can also be confirmed that the resistance increases rather when a dispersant with a small content is used.

[0091] On the other hand, when the weight average molecular weight of the dispersant is excessively small or large (Examples 3 to 6), the stability of the slurry decreases, and the dispersant acts as a resistor, etc., and it can be seen that the resistance tends to increase somewhat compared to Example 2.

[0092] A person having ordinary knowledge in the field to which the present invention pertains would be able to make various applications and modifications within the scope of the present invention based on the above content.

Industrial Applicability

[0093] As described above, the positive electrode for a secondary battery according to an embodiment of the present invention contains a novel dispersant, and thus, even when manufactured at a high loading, it is possible to prevent cracks in the core part during the manufacture of a wound electrode assembly, and there is an effect of preventing a decrease in the performance of a secondary battery including the same.

Claims

1. A positive electrode for a secondary battery, wherein the positive electrode for the secondary battery is formed on at least one surface of a current collector and includes a positive electrode mixture layer containing a positive electrode active material, a conductive material, a binder, and a dispersant, wherein the dispersant is a mixture of a butadiene-acrylonitrile-based first dispersant and a styrene-ethylene oxide-based (styrene-EO-based) second dispersant, wherein the first dispersant is a butadiene-acrylonitrile-based polymer obtained by polymerizing hydrogenated butadiene, non-hydrogenated butadiene, and acrylonitrile, and the second dispersant is a styrene-ethylene oxide-based polymer obtained by polymerizing styrene, ethylene oxide, and an aliphatic compound, wherein the dispersant is contained in an amount of 0.1% by weight or more and less than 0.3% by weight based on the total weight of the positive electrode mixture layer, a positive electrode for a secondary battery.

2. The positive electrode for a secondary battery according to claim 1, wherein the dispersant is contained in an amount of 0.15% by weight to 0.25% by weight based on the total weight of the positive electrode mixture layer.

3. The positive electrode for a secondary battery according to claim 1, wherein the mixing ratio of the first dispersant and the second dispersant is 8:2 to 2:8 based on the weights of the first dispersant and the second dispersant.

4. The positive electrode for a secondary battery according to claim 1, wherein the first dispersant is a butadiene-acrylonitrile-based polymer obtained by polymerizing 50 to 80% by weight of hydrogenated butadiene, 0.1 to 10% by weight of non-hydrogenated butadiene, and 15 to 40% by weight of acrylonitrile.

5. The positive electrode for a secondary battery according to claim 4, wherein the butadiene-acrylonitrile-based polymer has a weight average molecular weight of 30,000 to 80,000.

6. The positive electrode for a secondary battery according to claim 1, wherein the second dispersant is a styrene-ethylene oxide-based polymer obtained by polymerizing 20 to 40% by weight of styrene, 40 to 60% by weight of ethylene oxide, and 15 to 30% by weight of an aliphatic compound.

7. The positive electrode for a secondary battery according to claim 6, wherein the styrene-ethylene oxide-based polymer has a weight average molecular weight of 9,000 to 12,000.

8. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode active material is a lithium transition metal oxide represented by the following chemical formula (1). Li １+ｘ Ni ａ Co ｂ Mn ｃ M １－（ａ+ｂ+ｃ） O ２－ｙ A ｙ (1) In the chemical formula (1), M is at least one selected from the group consisting of Cu, Ti, Mg, Al, Pt, and Zr, A is an oxygen-substituted halogen, 0 ≤ x ≤ 0.5, 0.8 ≤ a ≤ 1, 0 ≤ b ≤ 0.2, 0 ≤ c ≤ 0.2, 0.9 ≤ a + b + c ≤ 1, and 0 ≤ y ≤ 0.

001.

9. The positive electrode for a secondary battery according to claim 1, wherein the conductive material is a carbon nanotube.

10. The positive electrode mixture layer has a loading amount of 26 mg / cm ２ to 30 mg / cm ２ The positive electrode for a secondary battery according to claim 1.

11. The positive electrode for a secondary battery according to claim 1, wherein the positive electrode for a secondary battery has a brittleness of 41 gf to 50 gf.

12. A positive electrode for a secondary battery, a negative electrode, and a separator according to any one of claims 1 to 11, having a structure in which the positive electrode for a secondary battery, the negative electrode, and the separator are all wound together such that the positive electrode for a secondary battery is present on the innermost side, An electrode assembly having an innermost radius (r) of 1.4 mm to 2.0 mm.

13. A secondary battery in which the electrode assembly according to claim 12 and an electrolytic solution are both installed in a battery case.

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