Positive electrode and lithium secondary battery manufactured using the same

Abstract

A positive electrode according to an embodiment of the present invention includes a positive electrode active material layer disposed on at least one surface of a positive electrode current collector. The positive electrode active material layer includes a lithium transition metal phosphate and a fluorine-based binder. When a flexibility evaluation is performed by contacting a phi (Φ) measuring rod with the positive electrode active material layer and then lifting the positive electrode, a ratio (=P / D) of a porosity P of the positive electrode active material layer calculated according to Equation 1 below to a maximum phi value D of the measuring rod at which cracks occur may be 10 or more. [Formula 1] Porosity (P) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} x 100

Description

[Technical field]

[0001] This application claims the benefit of priority to Korean Patent Application No. 10-2022-0166703, filed on December 2, 2022.

[0002] The present invention relates to a positive electrode and a lithium secondary battery manufactured using the same, and more particularly to a positive electrode having excellent flexibility and positive electrode adhesion and a reduced risk of cracking of the positive electrode during the manufacturing process, and a lithium secondary battery having increased energy density. [Background technology]

[0003] As technological development and demand for electric vehicles and energy storage systems (ESS) increases, the demand for batteries as energy sources is rapidly increasing, and various researches on batteries that can meet various requirements are being conducted accordingly. In particular, researches on lithium secondary batteries that have high energy density as a power source for such devices and have excellent life and cycle characteristics are being actively conducted.

[0004] Lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (LNCMO), lithium transition metal phosphate, etc. are used as positive electrode active materials for lithium secondary batteries.

[0005] The lithium transition metal phosphate is low-cost because it contains low-cost materials that are abundant in resources. In addition, it has low toxicity, so that the use of the lithium transition metal phosphate can reduce environmental pollution. Furthermore, since the lithium transition metal phosphate has an olivine structure, the active material structure can be stably maintained at high temperatures compared to lithium transition metal oxides having a layered structure. As a result, the battery has excellent high-temperature stability and high-temperature life characteristics.

[0006] However, lithium transition metal phosphate has a lower lithium mobility and lower electrical conductivity than lithium transition metal oxides such as lithium cobalt oxide (LCO) and lithium nickel cobalt manganese oxide (LNCMO), so lithium transition metal phosphate with a small average particle size is mainly used as a positive electrode active material. However, when the size of the lithium transition metal phosphate particles is small, the specific surface area increases, which causes severe particle aggregation and the lithium transition metal phosphate and binder are not effectively mixed, resulting in a decrease in the positive electrode adhesive strength. As a result, there is a problem that the positive electrode active material layer is detached during electrode manufacturing or charging and discharging, increasing the battery resistance and decreasing the capacity of the secondary battery.

[0007] In addition, a positive electrode containing lithium transition metal phosphate as a positive electrode active material has a lower rolling density compared to positive electrodes containing lithium cobalt oxide (LCO) or lithium nickel cobalt manganese oxide (LNCMO) as a positive electrode active material, and when the rolling density is increased, flexibility is reduced and the positive electrode active material layer is detached during the rolling and slitting process, limiting the ability to increase the rolling density. This makes it difficult to increase the energy density of the battery, and especially when manufacturing a wound electrode assembly, there is a high risk of cracks occurring in the positive electrode due to the low flexibility.

[0008] In order to improve the flexibility of the lithium transition metal phosphate-based positive electrode, a technique of selecting a polymer binder with a high molecular weight as the binder contained in the positive electrode active material layer has been considered, but a binder with a high molecular weight can rapidly increase the viscosity of the positive electrode slurry after mixing, which leads to a decrease in the coating productivity of the slurry. However, if the solid content in the slurry is reduced to reduce the viscosity of the positive electrode slurry, there is a problem that it takes a long time to dry due to the increase in the amount of solvent.

[0009] Therefore, there is a need to develop a technology that can improve the flexibility and adhesive strength of the positive electrode without causing a decrease in the productivity of coating and drying the positive electrode. Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a positive electrode having improved flexibility and adhesive strength without causing a decrease in productivity of coating and drying the positive electrode, and a lithium secondary battery having improved energy density. [Means for solving the problem]

[0011] According to one embodiment of the present invention, there is provided a positive electrode comprising a positive electrode active material layer disposed on at least one surface of a positive electrode current collector, the positive electrode active material layer comprising a lithium transition metal phosphate and a fluorine-based binder, and characterized in that a ratio (P / D) of a porosity P of the positive electrode active material layer calculated according to the following Equation 1 to a maximum phi value D of the phi (Φ) measuring rod at which cracks occur during a flexibility evaluation in which the positive electrode is lifted up after contacting the positive electrode active material layer with the phi (Φ) measuring rod is 10 or more:

[0012] [Formula 1] Porosity (P) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} x 100

[0013] In the positive electrode according to one embodiment, the P / D value may be 12.5 to 36.

[0014] In the positive electrode according to an embodiment, the porosity P of the positive electrode active material layer calculated by Equation 1 above may be 27% or more.

[0015] In the positive electrode according to one embodiment, the porosity P of the positive electrode active material layer calculated by the above formula 1 may be 28% to 36%.

[0016] In the positive electrode according to one embodiment, the fluorine-based binder may include a first fluorine-based binder having a weight average molecular weight (Mw) of 500,000 g / mol to 750,000 g / mol and a second fluorine-based binder having a weight average molecular weight (Mw) of 800,000 g / mol or more.

[0017] In the positive electrode according to one embodiment, the second fluorine-based binder may have a weight average molecular weight (Mw) of 850,000 g / mol to 2,000,000 g / mol.

[0018] In the positive electrode according to one embodiment, the ratio (=B / A) of the weight B of the lithium transition metal phosphate to the total weight A of the fluorine-based binder may be 27-98.

[0019] In the positive electrode according to one embodiment, the ratio (=B / A) of the weight B of the lithium transition metal phosphate to the total weight A of the fluorine-based binder may be 30-65.

[0020] In one embodiment of the positive electrode, when a flexibility evaluation is performed by contacting a phi (Φ) measuring rod with the positive electrode active material layer and then lifting the positive electrode, the maximum phi (Φ) value of the measuring rod at which cracks occur is 3 phi (Φ) or less.

[0021] In the positive electrode according to one embodiment, the lithium transition metal phosphate may be represented by the following Chemical Formula 1.

[0022] [Chemical formula 1] Li 1+a M1 1-x M2 x (PO y-b )D b

[0023] In the above Chemical Formula 1, M1 is one or more elements selected from Fe, Mn, Co, Ni, Cu, Zn, and Mg; M2 is one or more elements selected from any one of Groups 2 to 15, excluding element M1; D is at least one selected from the group consisting of F, S, and N; -0.5≦a≦+0.5, 0≦x≦0.8, 3.95≦y≦4.05, and 0≦b≦1.

[0024] In one embodiment of the positive electrode, the loading amount of the positive electrode active material layer is 400 mg / 25 cm 2 ~700mg / 25cm 2 The range may be:

[0025] In the positive electrode according to one embodiment, the positive electrode active material layer further includes a conductive material, and the conductive material may be carbon nanotubes.

[0026] In the positive electrode according to one embodiment, the conductive material may be contained in the positive electrode active material layer in an amount of 0.5% by weight to 1.3% by weight.

[0027] In the positive electrode according to one embodiment, the positive electrode active material layer may further include a hydrogenated nitrile butadiene rubber.

[0028] According to another embodiment of the present invention, there is provided a lithium secondary battery including the above-described positive electrode. Effect of the Invention

[0029] The positive electrode according to the present invention has an effect of reducing the risk of cracking when bent due to its improved flexibility.

[0030] In addition, the positive electrode according to the present invention has improved adhesive strength, and even without a decrease in adhesive strength, the weight ratio of the lithium transition metal phosphate contained in the positive electrode active material layer can be increased, thereby improving the energy density. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The advantages and features of the present invention, and the methods for achieving them, will become clear from the detailed description of the embodiments below in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms, and the present embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the present invention of the scope of the invention, and the present invention is only defined by the scope of the claims. The same reference numerals refer to the same elements throughout the specification.

[0032] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used as commonly understood by those having ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless otherwise clearly defined.

[0033] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular form includes the plural form unless otherwise specified in the phrase. The words "comprises" and / or "comprising" used in the specification do not exclude the presence or addition of one or more other components other than the components mentioned.

[0034] In this specification, when a part includes a certain component, this does not mean that it excludes other components, and it means that it may further include other components, unless specifically stated to the contrary.

[0035] In this specification, the phrase "A and / or B" means A or B, or A and B.

[0036] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0037] In this specification, D 50D means the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured, for example, by using a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to about several mm, and can provide results with high reproducibility and high resolution.

[0038] In this specification, "phi (Φ)" refers to the diameter of a measuring rod in millimeters (mm).

[0039] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorption at liquid nitrogen temperature (77K) using a BELSORP-mino II manufactured by BEL Japan.

[0040] In the present specification, "weight average molecular weight (Mw)" means a value converted to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the weight average molecular weight is a value converted from a value measured under the following conditions using GPC, and standard polystyrene from an Agilent system was used to create a calibration curve.

[0041] <Measurement conditions> Measuring instrument: Agilent GPC (Agulent 1200 series, USA) Column: PL Mixed B 2 pieces connected Column temperature: 40℃ Eluent: Tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0042] In this specification, the flexibility of the positive electrode can be measured by the following method. A measuring rod is manufactured for each phi (Φ) and measured at 600 mg / 25 cm. 2A positive electrode with a loading of 10 ...

[0043] In this specification, the porosity of the positive electrode active material layer may be defined by the following Equation 1, in which the "measured density of the positive electrode active material layer" is a value calculated from the density by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of a rolled positive electrode, and the "true density of the positive electrode active material" is the density with respect to the volume of only the positive electrode active material particles themselves excluding pores, and may be measured using a Pycnometer (AccuPycII 1340).

[0044] Porosity (P) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} x 100

[0045] The present invention will be specifically described below.

[0046] <Positive electrode> A positive electrode according to an embodiment of the present invention includes a positive electrode active material layer disposed on at least one surface of a positive electrode current collector. The positive electrode active material layer includes a lithium transition metal phosphate and a fluorine-based binder. When a flexibility evaluation is performed by contacting a phi (Φ) measuring rod with the positive electrode active material layer and then lifting the positive electrode, a ratio (=P / D) of a porosity P of the positive electrode active material layer calculated according to Equation 1 below to a maximum phi value D of the measuring rod at which cracks occur may be 10 or more.

[0047] [Formula 1] Porosity (P) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} x 100

[0048] Here, the method of evaluating the flexibility has been described above, so a duplicated description will be omitted.

[0049] Since the lithium transition metal phosphate used as the positive electrode active material is a fine particle compared to the lithium nickel cobalt manganese oxide, the positive electrode slurry containing the lithium transition metal phosphate has a higher viscosity than the positive electrode slurry containing the lithium nickel cobalt manganese oxide. Therefore, in the past, in consideration of the production process during the manufacture of the positive electrode slurry containing the lithium transition metal phosphate as the positive electrode active material, it was common to select a medium / low molecular weight fluorine-based binder with a weight average molecular weight (Mw) of 750,000 g / mol or less as the binder.

[0050] On the other hand, a positive electrode that contains lithium transition metal phosphate as a positive electrode active material has a lower rolling density compared to positive electrodes that contain lithium cobalt oxide (LCO) or lithium nickel cobalt manganese oxide (LNCMO) as a positive electrode active material, and increasing the rolling density reduces flexibility, which poses a risk of cracking the positive electrode, especially when manufactured as a jelly roll type.

[0051] The inventors of the present invention have found that controlling the value of P / D to a suitable value, for example, 10 or more, improves the flexibility and adhesive strength of the positive electrode. In some embodiments, the value of P / D can be controlled within a range of 12.5 to 36. The larger the value of P / D, the more preferable the flexibility of the positive electrode is, but if it is too large, the energy density of the positive electrode is too low, which is undesirable. Conversely, the smaller the value of P / D, the less flexible the electrode is, and in a situation where the positive electrode is subjected to a physical external force such as bending deformation, cracks may occur in the positive electrode, which may reduce the capacity and adhesive strength. Therefore, by adjusting the value of P / D, a balance between energy characteristics and physical durability can be achieved, and a positive electrode with both advantageous energy density and flexibility can be manufactured.

[0052] A positive electrode according to an embodiment of the present invention may include a positive electrode active material layer. Specifically, the positive electrode according to the present invention may include a positive electrode current collector and the positive electrode active material layer disposed on at least one surface of the positive electrode current collector.

[0053] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector may be stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like.

[0054] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0055] In the positive electrode according to the present invention, the porosity P of the positive electrode active material layer calculated by the above formula 1 may be 27% or more, preferably 28% to 36%, in order to improve flexibility. If the porosity P of the positive electrode active material layer is less than 27%, the flexibility of the positive electrode may decrease and cracks may occur in the positive electrode during the manufacturing process of the jelly-roll type electrode assembly, and if the porosity P of the positive electrode active material layer exceeds 36%, the energy density of the positive electrode may be reduced, which is undesirable.

[0056] In the rolling process of the positive electrode manufacturing process, the thickness change rate of the positive electrode active material layer before / after rolling and the loading amount of the positive electrode active material layer are appropriately controlled, so that the positive electrode active material layer may have a porosity within the above-mentioned numerical range. Specifically, the larger the thickness change rate before / after rolling or the larger the loading amount of the positive electrode active material layer, the smaller the porosity of the positive electrode active material layer may be, and conversely, the smaller the thickness change rate before / after rolling or the smaller the loading amount of the positive electrode active material layer, the larger the porosity of the positive electrode active material layer may be.

[0057] In one embodiment, the loading amount of the positive electrode active material layer is 400 mg / 25 cm 2 ~750mg / 25cm 2 , preferably 450 mg / 25 cm 2 ~700mg / 25cm 2The loading amount in the above range is larger than the loading amount of a general positive electrode and is suitable as a high-loading positive electrode.

[0058] The positive electrode according to the present invention has such flexibility that, during a flexibility test in which a measuring rod of different diameters is brought into contact with the positive electrode active material layer and then the positive electrode is lifted up, cracks occur with a measuring rod of 3 phi (Φ) or less, but no cracks occur with a measuring rod of more than 3 phi (Φ).

[0059] The positive electrode according to the present invention has a P / D value of 10 or more, and therefore has excellent flexibility, which reduces the rate of cracking during bending deformation of the positive electrode, and can prevent detachment of the positive electrode during the production process of the positive electrode.

[0060] In the positive electrode according to the present invention, the adhesive strength between the positive electrode current collector and the positive electrode active material layer, measured by a 90° peel test, can be 17 gf / 20 mm or more, specifically 20 gf / 20 mm or more, and more specifically in the range of 20 gf / 20 mm to 100 gf / 20 mm.

[0061] As described above, the positive electrode according to the present invention has excellent positive electrode adhesion and flexibility, making it possible to manufacture a high-loading positive electrode, and preventing detachment of the positive electrode, thereby reducing the cell resistance of the secondary battery, improving the capacity and output characteristics of the battery, and reducing defects occurring during the manufacturing process.

[0062] Hereinafter, each component contained in the positive electrode active material layer will be specifically described.

[0063] (1) Positive electrode active material The present invention includes a lithium transition metal phosphate as a positive electrode active material. Since the lithium transition metal phosphate has an olivine structure, the active material structure is more stable at high temperatures than lithium transition metal oxides having a layered structure. As a result, when the lithium transition metal phosphate is used as a positive electrode active material, the high temperature stability and high temperature life characteristics of the positive electrode are significantly improved, thereby reducing the risk of fire in a lithium secondary battery including the positive electrode.

[0064] The lithium transition metal phosphate may be a compound represented by the following Formula 1:

[0065] [Chemical formula 1] Li 1+a M1 1-x M2 x (PO y-b )D b

[0066] In the above Chemical Formula 1, M1 is one or more elements selected from Fe, Mn, Co, Ni, Cu, Zn, and Mg; M2 is one or more elements selected from any one of Groups 2 to 15, excluding element M1; D is at least one selected from the group consisting of F, S, and N; -0.5≦a≦+0.5, 0≦x≦0.8, 3.95≦y≦4.05, and 0≦b≦1.

[0067] According to a specific embodiment of the present invention, in the lithium transition metal phosphate, M1 is Fe, and M2 is at least one selected from the group consisting of Al, Mg, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. Specifically, the lithium transition metal phosphate is lithium iron phosphate (LiFePO 4 ).

[0068] The lithium transition metal phosphate may be in the form of secondary particles formed by agglomeration of primary particles, and preferably has a monolithic structure consisting of primary particles.

[0069] In the present invention, the term "monolith structure" refers to a structure in which particles are not aggregated with each other in terms of morphology and exist in independent phases. A particle structure that is contrasted with such a monolith structure is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form relatively large particles ("secondary particles").

[0070] When the lithium transition metal phosphate has a monolithic structure made of primary particles, the possibility of the lithium transition metal phosphate particles cracking during rolling is small compared to when the lithium transition metal phosphate is made of secondary particles, which is preferable because the capacity loss due to the detachment of the cracked particles is small. Also, when the lithium transition metal phosphate is made of monolithic primary particles, the migration of the binder during the drying process of the positive electrode slurry can be mitigated, which is preferable in terms of the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0071] The lithium transition metal phosphate may include a carbon coating layer on the surface. When a carbon coating layer is formed on the surface of the lithium transition metal phosphate, the electrical conductivity is improved and the resistance characteristics of the positive electrode may be improved.

[0072] The carbon coating layer may be formed using at least one raw material selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, furfuryl alcohol polymers, ethylene and ethylene oxide block copolymers, vinyl resins, cellulose resins, phenol resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing the raw material with the lithium transition metal phosphate and then heat treating the mixture.

[0073] Average particle size of lithium transition metal phosphate D 50 The average particle diameter D of the positive electrode active material may be 0.8 μm to 20.0 μm, specifically 0.9 μm to 10.0 μm, and more specifically 0.9 μm to 3.0 μm. 50 When satisfies the above range, the mobility of lithium in the lithium transition metal phosphate is improved, and the charge / discharge characteristics of the battery can be improved.

[0074] The BET specific surface area of ​​lithium transition metal phosphate is 5m 2 / g~20m 2 / g, specifically 7m 2 / g~18m2 / g, more specifically 9m 2 / g~16m 2 / g. This range corresponds to a lower value compared to typical lithium transition metal phosphates. When this range is satisfied, aggregation of the lithium transition metal phosphate can be effectively suppressed even in a positive electrode slurry composition having a relatively low content of dispersant.

[0075] The lithium transition metal phosphate may be contained in an amount of 93% by weight to 99% by weight, specifically 94% by weight to 98% by weight, more specifically 95% by weight to 98% by weight, based on the total solid content of the positive electrode slurry composition. When the content of the lithium transition metal phosphate satisfies the above range, sufficient positive electrode energy density is ensured, thereby improving the battery capacity of the positive electrode.

[0076] (2) Binder The positive electrode according to the present invention may contain a fluorine-based binder as a binder, and may preferably contain two types of fluorine-based binders having different weight average molecular weights.

[0077] Specifically, it may contain a first fluorine-based binder having a weight average molecular weight (Mw) of 500,000 g / mol to 750,000 g / mol and a second fluorine-based binder having a weight average molecular weight (Mw) of 800,000 g / mol or more.

[0078] The first fluorine-based binder and the second fluorine-based binder may each independently include a PVDF-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the PVDF-based polymer include a PVDF single polymer, PVDF-HFP (Poly(vinylidene fluoride-co-hexafluoropropylene)), PVDF-CTFE (Poly(vinylidene fluoride-co-chlorotrifluoroethylene)), PVDF-TFE (Poly(vinylidene tetrafluoroethylene)), and PVDF-TrFE (Poly(vinylidene trifluoroethylene)).

[0079] The first fluorine-based binder may have a weight average molecular weight (Mw) of 500,000 g / mol to 750,000 g / mol, preferably 530,000 g / mol to 730,000 g / mol, and more preferably 550,000 g / mol to 650,000 g / mol. The first fluorine-based binder may provide stability to the positive electrode slurry and may play a role in providing adhesive strength between the positive electrode active material and the conductive material and adhesive strength between the current collector and the positive electrode active material layer.

[0080] In a preferred embodiment, the first fluorine-based binder may be a homopolymer of polyvinylidene fluoride (PVDF). Since the homopolymer polyvinylidene fluoride (PVDF) contains a small number of polar functional groups such as -COOH, the functional groups of the binder may reduce the number of hydrogen bonds between hydrogen on the carbon coating layer coated on the surface of the lithium transition metal phosphate particles, thereby preventing gelation of the positive electrode slurry, improving the coating processability of the positive electrode slurry, and providing a uniform thickness and / or surface of the coated positive electrode active material layer, which is advantageous in that the output performance and life characteristics of a lithium secondary battery manufactured using the positive electrode are excellent.

[0081] The second fluorine-based binder may have a weight average molecular weight (Mw) of 800,000 g / mol or more, preferably 850,000 g / mol to 2,000,000 g / mol, and more preferably 900,000 g / mol to 1,500,000 g / mol. The second fluorine-based binder may improve the flexibility and adhesive strength of the positive electrode.

[0082] The second fluorine-based binder may be a homopolymer or copolymer of polyvinylidene fluoride (PVDF), and in one embodiment of the present invention, the second fluorine-based binder may be polyvinylidene fluoride (PVDF) containing a polar functional group. When the second fluorine-based binder contains a polar functional group, the crystallinity of the second fluorine-based binder is reduced, which can improve the flexibility of the positive electrode active material layer. In addition, due to the improved flexibility of the positive electrode active material layer, the positive electrode active material layer does not detach from the positive electrode current collector even when an external stress is applied, and as a result, the adhesive strength of the positive electrode can be improved.

[0083] The polar functional group may be one or more selected from the group consisting of a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, an acid anhydride group, and a hydroxyl group and salts thereof, but is not necessarily limited thereto, and any group that can be used as a polar functional group in the art is possible.

[0084] When a first fluorine-based binder having a medium / low molecular weight with a weight average molecular weight (Mw) in the range of 500,000 g / mol to 750,000 g / mol is used together with a second fluorine-based binder having a high molecular weight with a weight average molecular weight (Mw) of 800,000 g / mol or more, the flexibility of the positive electrode can be improved compared to the conventional technology in which only a medium / low molecular weight fluorine-based binder is selected as the binder.

[0085] In a preferred embodiment, the weight ratio of the first fluorine-based binder to the second fluorine-based binder may be 65:35 to 1:4, preferably 1:1 to 1:4, and more preferably 1:1 to 1:3. If the weight ratio of the first fluorine-based binder to the second fluorine-based binder deviates from 65:35 and the content of the first fluorine-based binder is excessively high, this is not preferable in terms of the flexibility of the positive electrode, and if the weight ratio of the first fluorine-based binder to the second fluorine-based binder deviates from 1:4 and the content of the second fluorine-based binder is excessively high, this is not preferable because gelation of the positive electrode slurry may be induced.

[0086] The total weight of the first fluorine-based binder and the second fluorine-based binder may be 1.0 wt % to 4.0 wt %, preferably 1.2 wt % to 3.5 wt %, and more preferably 1.4 wt % to 3.0 wt %, in the positive electrode active material layer.

[0087] Furthermore, the ratio of weight B of the lithium transition metal phosphate as the positive electrode active material to the total weight A of the first fluorine-based binder and the second fluorine-based binder may be 27 to 98, preferably 30 to 65, and more preferably 32 to 48.5. The positive electrode of the present invention contains fluorine-based binders having different weight-average molecular weight ranges, which improves the adhesive strength of the positive electrode and increases the weight ratio of the positive electrode active material contained in the positive electrode active material layer, thereby increasing the energy density per weight of the positive electrode.

[0088] (3) Conductive material The positive electrode active material layer of the present invention may further contain a conductive material.

[0089] The conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and may be, for example, graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer 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. Specific examples of commercially available conductive materials include acetylene black series products of Chevron Chemical Company, Denka Black (Denka Singapore Private Limited), Gulf Oil Company (Gulf Oil Company products, etc.), Ketjenblack, EC series (Armak Company products), Vulcan XC-72 (Cabot Company products), and Super P (Timcal Company products). Preferably, the conductive material may be carbon nanotubes. When the conductive material is carbon nanotubes, the excellent conductive network of the carbon nanotubes can suppress the binder migration phenomenon in which the binder moves to the electrode surface when the electrode is dried, and can further improve the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0090] The conductive material may be contained in the positive electrode active material layer at 0.3 wt% to 2.0 wt%, specifically 0.4 wt% to 1.5 wt%, more specifically 0.5 wt% to 1.3 wt%. When the content of the conductive material in the positive electrode active material layer satisfies the above range, the positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0091] According to one embodiment of the present invention, the positive electrode active material layer may contain 94.9 to 97.2 wt % of positive electrode active material, 0.8 to 1.3 wt % of conductive material, and 2.0 to 3.5 wt % of binder. When the composition in the positive electrode active material layer satisfies the above-mentioned range, the adhesive strength and conductivity of the electrode are ensured, and at the same time, the content of the active material is increased, thereby optimizing the capacity and resistance performance of a lithium secondary battery including the positive electrode.

[0092] (4) Dispersants The positive electrode active material layer of the present invention may further contain a dispersant.

[0093] The dispersant prevents the lithium transition metal phosphate from being excessively aggregated in the positive electrode slurry composition, allowing the lithium transition metal phosphate to be effectively dispersed and present in the prepared positive electrode active material layer.

[0094] The dispersant may include a hydrogenated nitrile-based copolymer, and specifically, the dispersant may be a hydrogenated nitrile-based copolymer.

[0095] Specifically, the hydrogenated nitrile copolymer may be a copolymer containing an α,β-unsaturated nitrile-derived structural unit and a hydrogenated conjugated diene-derived structural unit, or a copolymer containing an α,β-unsaturated nitrile-derived structural unit, a conjugated diene-derived structural unit, and a hydrogenated conjugated diene-derived structural unit. As the α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile may be used, and one or a mixture of two or more of these may be used. As the conjugated diene monomer, for example, a conjugated diene monomer having 4 to 6 carbon atoms such as 1,3-butadiene, isoprene, or 2,3-methylbutadiene may be used, and one or a mixture of two or more of these may be used.

[0096] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR).

[0097] The above dispersant may be contained in an amount of 0.1% by weight to 2.0% by weight, specifically 0.2% by weight to 1.8% by weight, more specifically 0.3% by weight to 1.6% by weight, based on the total solid content of the above positive electrode slurry composition. When the content of the dispersant satisfies the above range, aggregation of the positive electrode active material can be suppressed and gelation of the positive electrode slurry composition can be prevented.

[0098] The positive electrode according to the present invention can be manufactured according to the usual method for manufacturing a positive electrode. Specifically, after manufacturing a positive electrode slurry composition containing the above-described positive electrode active material, conductive material, binder and / or dispersant, the positive electrode can be manufactured by applying the positive electrode slurry composition onto a positive electrode current collector and then drying and rolling.

[0099] In another method, the above positive electrode can also be manufactured by casting the above positive electrode slurry composition onto a separate support, peeling the film obtained by peeling from the support, and laminating the film onto a positive electrode current collector.

[0100] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0101] The lithium secondary battery according to an embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and an electrolyte.

[0102] The positive electrode in the above lithium secondary battery is as described above. For example, the positive electrode is a positive electrode including a positive electrode active material layer disposed on at least one surface of a positive electrode current collector, the positive electrode active material layer includes a lithium transition metal phosphate and a fluorine-based binder, and when lifting the positive electrode after contacting a phi (Φ) measuring rod on the positive electrode active material layer, the ratio value (=P / D) of the porosity P of the positive electrode active material layer calculated by the following formula 1 with respect to the maximum phi value D of the measuring rod at which cracks occur can be 10 or more.

[0103] [Formula 1] Porosity (P) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0104] Additionally, the positive electrode may further include a conductive material and a dispersant.

[0105] The negative electrode can be manufactured, for example, by preparing a composition for forming a negative electrode, which contains a negative electrode active material, a negative electrode binder, and a negative electrode conductive material, on a negative electrode current collector, and then coating the composition on the negative electrode current collector.

[0106] The negative electrode active material is not particularly limited, and may be a compound capable of reversible intercalation and deintercalation of lithium.Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; or a composite containing a metallic compound and a carbonaceous material. In addition, examples of low crystalline carbon include soft carbon and hard carbon, and examples of high crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature fired carbon such as petroleum or coal tar pitch derived cokes. Among these, one type alone or a mixture of two or more types may be used, and a metallic lithium thin film may also be used as the negative electrode active material.

[0107] The negative electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing a chemical change in the battery that is constructed. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, carbon fiber, and carbon nanotubes; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one or more of these may be used alone or in combination. The negative electrode conductive material may be generally contained in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, and more specifically 1 to 10% by weight, based on the total weight of the negative electrode active material layer.

[0108] The negative electrode binder plays a role in improving the adhesion between the negative electrode active material particles and the adhesive strength between the negative electrode active material and the negative electrode current collector. Specific examples 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 rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used alone or in combination. The negative electrode binder may be included in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, more specifically 1 to 10% by weight, based on the total weight of the negative electrode active material layer.

[0109] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0110] In addition, the negative electrode current collector may typically have a thickness of 3 to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0111] Meanwhile, in the lithium secondary battery, the separator may be any one that is generally used as a separator in a lithium secondary battery, and is preferably one that has low resistance to ion migration of the electrolyte and has excellent electrolyte humidification ability. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, etc. may be used. In addition, the separator may be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0112] Meanwhile, in the lithium secondary battery, the electrolyte may include an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0113] The organic solvent may be any organic solvent without particular limitation, so long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0114] Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of the battery, and low-viscosity linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) are more preferred.

[0115] The lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF 6 , LiClO 4 , LiAsF 6 , LiBF 4 , LiSbF 6, LiAlO 4 , LiAlCl 4 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN(C 2 F 5 SO 3 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 , LiCl, LiI, or LiB(C 2 O 4 ) 2 The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol % to 2 mol %.

[0116] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing the decrease in the capacity of the battery, improving the discharge capacity of the battery, etc. In this case, the additives may be contained in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

[0117] The lithium secondary battery of the present invention may be manufactured by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then inserting the electrode assembly into a cylindrical or prismatic battery case and then injecting an electrolyte into the battery case. Alternatively, the electrode assemblies may be stacked, impregnated with an electrolyte, and then inserted into a battery case and sealed.

[0118] When manufacturing the lithium secondary battery of the present invention, the electrode assembly may be dried to remove at least one organic solvent used in manufacturing the positive electrode, selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate. If an electrolyte having the same composition as the organic solvent used in manufacturing the positive electrode is used as the electrolyte, the process of drying the electrode assembly may be omitted.

[0119] Unlike the above-described lithium secondary batteries, the lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0120] The battery case may be one commonly used in the art, and the shape of the battery case is not limited depending on the application of the battery. For example, the battery case may be a cylindrical type using a can, a square type, a pouch type, a coin type, or the like.

[0121] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in portable devices such as mobile phones, notebook computers, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0122] The present invention will be described in more detail below with reference to examples. However, the following examples are merely for the purpose of illustrating the present invention, and the scope of the present invention is not limited to these examples alone.

[0123] <Example 1: Production of positive electrode> (1) Preparation of cathode slurry

[0124] Average particle size D of lithium transition metal phosphate 50 The diameter is 1.0 μm, and the LiFePO is a primary particle with a single structure. 4Carbon nanotubes (CNTs) as a conductive material, polyvinylidene fluoride (PVDF) as a homopolymer with a weight average molecular weight (Mw) of 630,000 g / mol as a first fluorine-based binder, and polyvinylidene fluoride (PVDF) as a second fluorine-based binder with a weight average molecular weight (Mw) of 1,000,000 g / mol were added to an N-methylpyrrolidone (NMP) solvent. The mixture was mixed at 2500 rpm for 90 minutes using a homo-disperse (mixer) to prepare a positive electrode slurry.

[0125] In the positive electrode slurry, the lithium transition metal phosphate, the conductive material, the first fluorine-based binder, and the second fluorine-based binder were present in a weight ratio of 96.5:1.0:1.25:1.25, and the solid content of the positive electrode slurry was 62 wt %.

[0126] (2) Manufacturing of positive electrodes The above positive electrode slurry was applied at 600 mg / 25 cm on a 20 μm thick aluminum thin film. 2 After coating, the positive electrode slurry was dried with hot air at 130° C. for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt % or more. Then, the dried positive electrode slurry was rolled to prepare a positive electrode so that the porosity of the positive electrode active material layer was 29%.

[0127] Here, the porosity P is a value calculated by the following formula, the "measured density of the positive electrode active material layer" is a value calculated from the density by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of a rolled positive electrode, and the "true density of the positive electrode active material" is the density with respect to the volume of only the positive electrode active material particles themselves excluding pores, and is a value measured using a Pycnometer (AccuPycII 1340).

[0128] Porosity (P) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} x 100

[0129] <Example 2: Production of positive electrode> A positive electrode was manufactured in the same manner as in Example 1, except that the dried positive electrode slurry was rolled so that the porosity of the positive electrode active material layer was 32%.

[0130] <Example 3: Production of positive electrode> A positive electrode was prepared in the same manner as in Example 1, except that the second fluorine-based binder was changed to polyvinylidene fluoride (PVDF) having a weight average molecular weight (Mw) of 880,000 when preparing the positive electrode slurry.

[0131] <Example 4: Production of positive electrode> A positive electrode was prepared in the same manner as in Example 1, except that the first fluorine-based binder was changed to polyvinylidene fluoride (PVDF), which is a homopolymer having a weight average molecular weight (Mw) of 700,000, when preparing the positive electrode slurry.

[0132] <Comparative Example 1: Production of Positive Electrode> (1) Preparation of cathode slurry

[0133] Average particle size D of lithium transition metal phosphate 50 The diameter is 1.0 μm, and the LiFePO is a primary particle with a single structure. 4 Carbon nanotubes (CNTs) as a conductive material and polyvinylidene fluoride (PVDF), a homopolymer with a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, were added to N-methylpyrrolidone (NMP) solvent. The mixture was mixed at 2500 rpm for 90 minutes using a homo-disperse (mixer) to prepare a positive electrode slurry.

[0134] In the positive electrode slurry, the lithium transition metal phosphate, the conductive material, and the fluorine-based binder were present in a weight ratio of 96.5:1.0:2.5, and the solid content of the positive electrode slurry was 62 wt %.

[0135] (2) Manufacturing of positive electrodes The above positive electrode slurry was applied at 600 mg / 25 cm on a 20 μm thick aluminum thin film. 2After coating, the positive electrode slurry was dried with hot air at 130° C. for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt % or more. Then, the dried positive electrode slurry was rolled to prepare a positive electrode so that the porosity of the positive electrode active material layer was 29%.

[0136] <Comparative Example 2: Production of Positive Electrode> A positive electrode was manufactured in the same manner as in Example 1, except that the dried positive electrode slurry was rolled so that the porosity of the positive electrode active material layer was 26%.

[0137] <Experimental Example 1: Evaluation of Positive Electrode Flexibility> The positive electrodes produced in Examples 1 to 4 and Comparative Examples 1 and 2 were vacuum dried at a temperature of 130° C. for 2 hours, and then their flexibility was measured. The results are shown in Table 1.

[0138] Specifically, measuring rods with diameters of 1Φ, 1.5Φ, 2Φ, 2.5Φ, 3Φ, 4Φ, 5Φ, 6Φ, and 7Φ are prepared, and the positive electrodes manufactured in Examples 1 to 4 and Comparative Examples 1 to 2 are cut into 10 cm wide and 30 cm long. The cut positive electrodes are bent in half to contact the measuring rods, and then both ends of the positive electrodes are lifted at a speed of 10 mm per minute. At this time, the electrodes are lifted until the force measured by the UTM reaches 5 N. The electrodes are measured for each diameter and observed with an optical microscope to see if cracks occur. If no cracks are found, the test is continued with a measuring rod with a smaller diameter. The maximum diameter (Φ) value of the measuring rod at which cracks occur is shown in Table 1.

[0139] The ratio (P / D) of the porosity P of the positive electrode active material layer to the maximum phi value D of the measuring rod at which cracks occur was calculated. The results are shown in Table 1.

[0140] <Experimental Example 2: Positive electrode adhesion test> The positive electrodes produced in Examples 1 to 4 and Comparative Examples 1 and 2 were each vacuum-dried at 130° C. for 2 hours, and then the adhesive strength between the positive electrode active material layer and the positive electrode current collector was measured. The results are shown in Table 1.

[0141] Specifically, the positive electrodes produced in Examples 1 to 4 and Comparative Examples 1 to 2 were cut to a length of 150 mm and a width of 20 mm, and the surface of the positive electrode was attached to a slide glass having a length of 75 mm and a width of 25 mm in the longitudinal direction using double-sided tape. That is, the slide glass was attached to an area corresponding to half of the longitudinal direction of the positive electrode. Then, a roller was rotated 10 times to uniformly attach the double-sided tape to produce an evaluation sample.

[0142] Next, the glass slide portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM) (product name: LS5, manufacturer: LLOYD), and the positive electrode half to which the glass slide was not attached was connected to the load cell of the UTM equipment. The load cell was subjected to a 90° force at a speed of 100 mm / min, and the load applied to the load cell was measured as it was moved up to 50 mm. The average load measured in the 20 mm to 40 mm section of the travel section was calculated, and this was repeated a total of five times, and the average value was evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0143] [Table 1]

[0144] Referring to Table 1, the positive electrodes according to Examples 1, 3, and 4 and the positive electrode according to Comparative Example 1 all have the same binder content and porosity values, but the positive electrodes according to the Examples have a P / D value of 10 or more, and are therefore evaluated to be superior in flexibility as well as adhesive strength.

[0145] The positive electrode according to Comparative Example 2 had a small porosity value of 26, which indicated the best adhesive strength, but was rated as having the worst flexibility.

[0146] On the other hand, the positive electrode of Example 2 was shown to have inferior adhesive strength due to a decrease in rolling density compared to the positive electrodes of Examples 1, 3, and 4, but was evaluated to have the best flexibility.

[0147] As described above, the positive electrode according to the present invention is evaluated to be able to minimize the risk of crack generation during the battery manufacturing process as a result of improved flexibility.

Claims

1. A positive electrode including a positive electrode active material layer disposed on at least one surface of a positive electrode current collector, The positive electrode active material layer includes a lithium transition metal phosphate and a fluorine-based binder, a ratio (=P / D) of a porosity P of the positive electrode active material layer calculated according to Equation 1 below to a maximum phi value D of the measuring rod at which a crack occurs when a flexibility evaluation is performed by contacting a phi (Φ) measuring rod with the positive electrode active material layer and then lifting the positive electrode is 10 or more. [Formula 1] Porosity (P)={1-(measured density of positive electrode active material layer / true density of positive electrode active material)}×100

2. 2. The positive electrode of claim 1, wherein the P / D value is 12.5 or more and 36 or less.

3. The positive electrode according to claim 1 , wherein the porosity P of the positive electrode active material layer calculated by Equation 1 is 27% or more.

4. The positive electrode according to claim 1 , wherein the porosity P of the positive electrode active material layer calculated by Equation 1 is 28% or more and 36% or less.

5. 2. The positive electrode according to claim 1, wherein a ratio (=B / A) of a weight B of the lithium transition metal phosphate to a total weight A of the fluorine-based binder is 27 or more and 98 or less.

6. 2. The positive electrode according to claim 1, wherein a ratio (=B / A) of a weight B of the lithium transition metal phosphate to a total weight A of the fluorine-based binder is 30 or more and 65 or less.

7. 2. The positive electrode according to claim 1, wherein, when a flexibility evaluation is performed by contacting a measuring rod with different phi (Φ) with the positive electrode active material layer and then lifting the positive electrode, the maximum phi (Φ) value of the measuring rod at which cracks occur is 3 phi (Φ) or less.

8. The positive electrode of claim 1 , wherein the lithium transition metal phosphate is represented by the following Chemical Formula 1: [Chemical formula 1] <h2 style=";text-align:left;direction:ltr">Li<h2 style=";text-align:left;direction:ltr"> １＋ａ <h2 style=";text-align:left;direction:ltr"> 11<h2 style=";text-align:left;direction:ltr"> １－ｘ <h2 style=";text-align:left;direction:ltr"> 12<h2 style=";text-align:left;direction:ltr"> ｘ <h2 style=";text-align:left;direction:ltr"> ((PO<h2 style=";text-align:left;direction:ltr"> ｙ－ｂ <h2 style=";text-align:left;direction:ltr"> )D<h2 style=";text-align:left;direction:ltr"> ｂ In the above Chemical Formula 1, M1 is one or more elements selected from Fe, Mn, Co, Ni, Cu, Zn, and Mg; M2 is one or more elements selected from any one of Groups 2 to 15, excluding element M1; D is at least one selected from the group consisting of F, S, and N; -0.5≦a≦+0.5, 0≦x≦0.8, 3.95≦y≦4.05, and 0≦b≦1.

9. The loading amount of the positive electrode active material layer is 400 mg / 25 cm ２ Above, 700mg / 25cm ２ 2. The positive electrode of claim 1, wherein the positive electrode has a molecular weight of 1.0 or less.

10. 10. The positive electrode of claim 1, wherein the positive electrode active material layer further comprises a conductive material, the conductive material being a carbon nanotube.

11. The positive electrode according to claim 10 , wherein the conductive material is contained in the positive electrode active material layer in an amount of 0.5% by weight or more and 1.3% by weight or less.

12. 10. The positive electrode of claim 1, wherein the positive electrode active material layer further comprises a hydrogenated nitrile butadiene rubber.

13. A lithium secondary battery comprising the positive electrode and the negative electrode according to any one of claims 1 to 12.

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