Positive electrode and lithium secondary battery manufactured using the same

Abstract

To provide a positive electrode and a lithium secondary battery in which electrode detachment is prevented, battery resistance is reduced and battery capacity is improved by improving adhesive force between a positive electrode current collector and a positive electrode active material layer.SOLUTION: A positive electrode includes a positive electrode active material layer. The positive electrode active material layer includes a first lithium iron phosphate and a second lithium iron phosphate as a positive electrode active material. The first lithium iron phosphate has an average particle diameter D50 greater than that of the second lithium iron phosphate, and includes at least one facet. When the cross section of the positive electrode is observed with a scanning electron microscope (SEM), the cross section of the first lithium iron phosphate has at least one side having a length of 2 μm or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0187035, dated December 24, 2021.

[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 including lithium iron phosphate having a flat surface and a lithium secondary battery manufactured using the same. [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 are being conducted on batteries that can meet various requirements. In particular, researches on lithium secondary batteries, which have high energy density and excellent life and cycle characteristics as a power source for such devices, are being actively conducted.

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

[0005] Lithium iron phosphate is inexpensive because it contains iron, a resource-rich and low-cost material. Furthermore, its low toxicity reduces environmental pollution when used. Furthermore, because lithium iron phosphate has an olivine structure, its active material structure can be stably maintained at high temperatures compared to lithium transition metal oxides with a layered structure. This provides the battery with excellent high-temperature stability and high-temperature life characteristics.

[0006] However, lithium iron phosphate has the drawback of low lithium mobility and low electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, conventional lithium iron phosphates have been used by reducing the average particle size to shorten the lithium migration path and improve lithium ion mobility. However, as the size of lithium iron phosphate particles decreases, the specific surface area increases, which leads to significant particle aggregation and ineffective mixing of the lithium iron phosphate and binder, resulting in a decrease in the adhesive strength between the positive electrode current collector and the positive electrode active material layer (hereinafter referred to as "positive electrode adhesive strength").

[0007] A decrease in the positive electrode adhesive strength can lead to detachment of the positive electrode active material layer during electrode manufacturing or charge / discharge, resulting in increased battery resistance and reduced secondary battery capacity. Therefore, there is a need for a technology to improve the positive electrode adhesive strength in positive electrodes containing lithium iron phosphate. Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide a positive electrode and a lithium secondary battery in which the adhesive strength between the positive electrode current collector and the positive electrode active material layer is improved, thereby preventing electrode detachment, reducing battery resistance, and improving battery capacity. [Means for solving the problem]

[0009] According to one embodiment of the present invention, a positive electrode active material layer is provided, the positive electrode active material layer including first lithium iron phosphate and second lithium iron phosphate as positive electrode active materials, the first lithium iron phosphate having a larger average particle diameter D than the second lithium iron phosphate. 50 When a cross section of the positive electrode is observed with a scanning electron microscope (SEM), the cross section of the first lithium iron phosphate has at least one side with a length of 2 μm or more.

[0010] In one embodiment of the present invention, the lithium iron phosphate monobasic and the lithium iron phosphate dibasic may each independently be a compound represented by the following Chemical Formula 1:

[0011] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0012] (In the above chemical formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0013] In one embodiment of the present invention, the monolithic lithium iron phosphate is a primary particle having a monolith structure and may include at least one or more flat facets on the surface.

[0014] In one embodiment of the present invention, the average particle diameter D of the first lithium iron phosphate 50 can be 2 μm to 8 μm.

[0015] In one embodiment of the present invention, the lithium iron phosphate may be in the form of polygonal pillar particles.

[0016] In one embodiment of the present invention, the positive electrode active material layer may contain the monolithium iron phosphate and the dilithium iron phosphate in a volume ratio of 10:90 to 40:60.

[0017] In one embodiment of the present invention, the average particle size D of the lithium iron phosphate dihydrate 50 can be 1 μm or less.

[0018] In one embodiment of the present invention, the lithium iron phosphate may be secondary particles formed by agglomeration of primary particles.

[0019] A positive electrode according to an embodiment of the present invention includes a positive electrode current collector and the positive electrode active material layer formed on at least one surface of the positive electrode current collector, and the adhesive strength between the positive electrode current collector and the positive electrode active material layer measured by a 90-degree peel test may be 70 gf / 20 mm to 200 gf / 20 mm.

[0020] In one embodiment of the present invention, the positive electrode active material layer may further include a conductive material, a binder, and a dispersant.

[0021] In one embodiment of the present invention, the positive electrode active material layer may contain 94.9 to 97.0 wt % of the positive electrode active material, 0.8 to 1.2 wt % of the conductive material, 2.0 to 3.5 wt % of the binder, and 0.2 to 0.4 wt % of the dispersant.

[0022] In one embodiment of the present invention, the conductive material may be carbon nanotubes.

[0023] In one embodiment of the present invention, the dispersant may be a hydrogenated nitrile butadiene rubber.

[0024] In one embodiment of the present invention, the positive electrode active material layer may directly face the positive electrode current collector.

[0025] According to another embodiment of the present invention, there is provided a lithium secondary battery including the above-described positive electrode. [Effects of the Invention]

[0026] The positive electrode according to the present invention is characterized in that it uses large-particle lithium iron phosphate (dilithium iron phosphate) having flat surfaces of a certain size or larger together with small-particle lithium iron phosphate (dilithium iron phosphate) as the positive electrode active material. When large-particle lithium iron phosphate having flat surfaces of a certain size or larger is contained in the positive electrode active material layer as in the present invention, the flat surfaces come into face-to-face contact with the positive electrode current collector, resulting in a larger contact area with the positive electrode current collector and significantly improved positive electrode adhesion.

[0027] In addition, when the positive electrode slurry is rolled, the edges and corners of the first lithium iron phosphate particles are fitted into the positive electrode current collector, increasing the adhesion strength with the positive electrode current collector, thereby achieving the effect of improving the positive electrode adhesive strength.

[0028] Therefore, the positive electrode and the lithium secondary battery including the same can prevent the positive electrode active material layer from being detached during the manufacture of the electrode or during charge and discharge, thereby reducing the resistance of the battery and increasing the capacity of the battery. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is an SEM photograph of a cross section of the positive electrode produced in Example 1. [Figure 2] 1 is a SEM photograph of the positive electrode slurry prepared in Example 1. [Figure 3] 1 is an SEM photograph of a cross section of a positive electrode produced in Comparative Example 1. [Figure 4] 1 is an SEM photograph of a cross section of a positive electrode produced in Comparative Example 2. [Figure 5] 1 is a SEM photograph of the positive electrode slurry prepared in Comparative Example 2. [Figure 6] 1 is an SEM photograph of a cross section of a positive electrode produced in Comparative Example 3. [Figure 7] 1 is a SEM photograph of the positive electrode slurry prepared in Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0030] The advantages and features of the present invention, as well as methods for achieving them, will become apparent from the following detailed description of the embodiments accompanied by the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. The present embodiments are provided so that this disclosure will be complete and will fully convey the scope of the invention to those skilled in the art. The present invention is defined solely by the claims. The same reference numerals refer to the same elements throughout the specification.

[0031] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the sense that they can be commonly understood by a person of 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 they are clearly and specifically defined.

[0032] 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. As used in this specification, "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements other than the elements mentioned.

[0033] In this specification, when a part is said to include a certain element, this does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified to the contrary.

[0034] In this specification, the expression "A and / or B" means A or B, or A and B.

[0035] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0036] In this specification, D50 D means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using a laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution. For example, 50 can be calculated from the particle size corresponding to 50% of the cumulative volume measured by a laser diffraction particle size measuring device (e.g., Microrac MT 3000) after irradiating the active material with ultrasonic waves of about 28 kHz at an output of 60 W.

[0037] As used herein, a "facet crystal surface" refers to a flat surface of a crystal, and specifically may be a crystal surface. For example, a flat surface may be formed on the surface of a primary particle having a monolithic structure. When a cross section of an electrode including particles with flat surfaces on their surfaces is observed using a scanning electron microscope (hereinafter "SEM"), the cross section of the particle is observed as a polygon, and the flat surfaces may correspond to the sides of the polygon. The area of ​​the flat surface is proportional to the length of the side. Therefore, by measuring the length of the side, it can be inferred whether at least one flat surface included on the surface of the primary lithium iron phosphate has an area of ​​a certain size or more.

[0038] In this specification, the length of a side of a particle cross section can be measured by the following method. When the cross section of the electrode is observed with an SEM, the length of the straight line forming the angle of a polygon is measured according to the general definition of a side. Alternatively, if the distance between an imaginary line segment connecting any two points on the edge of the particle cross section and a part of the edge of the particle cross section connecting the two points is 50 nm or less, the length of the imaginary line segment corresponds to an approximation of the length of the side. Therefore, the length of the side can also be calculated by measuring the length of the imaginary line segment.

[0039] The positive electrode adhesive strength in this specification is measured by a 90-degree peel test and can be measured as follows. A positive electrode cut to a length of 150 mm and a width of 20 mm is prepared. The positive electrode active material layer is placed facing a 75 mm long and 25 mm wide glass slide, and the positive electrode is attached to the glass slide in the longitudinal direction using double-sided tape. That is, the glass slide is attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, a test sample is prepared by rolling the double-sided tape 10 times with a roller to ensure uniform adhesion. Next, the glass slide portion of the test sample is fixed to the sample stage of a Universal Testing Machine (UTM) (product name: LS5, manufacturer: LLOYD), and the half of the positive electrode not attached to the glass slide is connected to the load cell of the UTM equipment. The load cell is moved 50 mm at a speed of 100 mm / min, and the load applied to the load cell is measured. At this time, the minimum load measured in a section of 20 mm to 40 mm of the running section is determined. This is repeated five times, and the average value is evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0040] The present invention will be specifically described below.

[0041] <Positive electrode> A positive electrode according to one embodiment of the present invention includes a positive electrode active material layer containing first lithium iron phosphate and second lithium iron phosphate as positive electrode active materials. In this case, the first lithium iron phosphate has a larger average particle diameter D than the second lithium iron phosphate. 50 When a cross section of the positive electrode is observed with a scanning electron microscope (SEM), the cross section of the first lithium iron phosphate has at least one side with a length of 2 μm or more.

[0042] Lithium iron phosphate, which has a small average particle size, is commonly used as a positive electrode active material because it has low lithium mobility and electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. However, when the lithium iron phosphate particle size is small, the specific surface area increases, which leads to significant particle aggregation and ineffective mixing of the lithium iron phosphate and binder, resulting in reduced positive electrode adhesion. This can lead to detachment of the positive electrode active material layer during electrode fabrication or charge / discharge, increasing battery resistance and reducing secondary battery capacity.

[0043] The present inventors conducted extensive research to solve these problems and discovered that the positive electrode adhesion can be significantly improved by using a combination of a large particle size lithium iron phosphate and a small particle size lithium iron phosphate, each of which has at least one flat surface of a certain size or larger on its surface. This led to the completion of the present invention. When a large particle size lithium iron phosphate with a large flat surface is used in combination, as in the present invention, the flat surface comes into face-to-face contact with the positive electrode current collector, resulting in a larger contact area with the current collector and improved positive electrode adhesion. This will be described in detail below.

[0044] 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 located on at least one surface of the positive electrode current collector.

[0045] 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 made of stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0046] 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. It may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0047] The positive electrode active material layer may include a positive electrode active material. Additionally, the positive electrode active material layer may further include a conductive material, a binder, and a dispersant, as necessary, in addition to the positive electrode active material.

[0048] Hereinafter, each component contained in the positive electrode active material layer will be specifically described.

[0049] (1) Positive electrode active material The present invention includes lithium iron phosphate dihydrate and lithium iron phosphate dihydrate as positive electrode active materials. Because lithium iron phosphate has an olivine structure, its active material structure is more stable at high temperatures than lithium transition metal oxides with a layered structure. As a result, when lithium iron 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 lithium secondary batteries containing the positive electrode.

[0050] On the other hand, the above lithium iron phosphate and lithium iron phosphate have an average particle size D 50 may be different, and specifically, the average particle size D of the first lithium iron phosphate 50 is the average particle size of lithium iron phosphate D 50 It can be larger.

[0051] The lithium iron phosphate monobasic and lithium iron phosphate dibasic may each independently be a compound represented by the following Chemical Formula 1:

[0052] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0053] (In the above chemical formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0054] For example, the lithium iron phosphate monobasic and lithium iron phosphate dibasic may each be LiFePO4.

[0055] The first lithium iron phosphate and the second lithium iron phosphate may each independently include a carbon coating layer on the surface thereof. When a carbon coating layer is formed on the surface of the lithium iron phosphate, electrical conductivity is improved, and the resistance characteristics of the positive electrode may be improved.

[0056] 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-ethylene oxide block copolymers, vinyl resins, cellulose resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing the raw material with the lithium iron phosphate and then heat-treating the mixture.

[0057] Preferably, the monolithic lithium iron phosphate may be a primary particle having a monolithic structure. In the present invention, the term "monolithic structure" refers to a morphological phase in which particles exist as independent phases that are not aggregated with each other. The monolithic lithium iron phosphate, which is a primary particle having a monolithic structure, may have at least one or more facet crystal surfaces on its surface.

[0058] The flat surface refers to a flat surface of a crystal, and specifically may correspond to a crystal surface. For example, the flat surface may be a flat surface having an area of ​​a certain size or more formed on the surface of monolithium iron phosphate, which is a primary particle with a single structure. The edges and corners of the monolithium iron phosphate particles with this structure can be fitted into the positive electrode current collector during rolling, thereby increasing the adhesion strength with the positive electrode current collector.

[0059] When a cross section of a positive electrode including a first lithium iron phosphate having a flat surface on its surface is observed using an SEM, the cross section of the first lithium iron phosphate may be observed as a polygon. In this case, the flat surface may correspond to the sides of the polygon, and the sides corresponding to the flat surface may be observed as straight lines. The area of ​​the flat surface is proportional to the length of the side. Therefore, by measuring the length of the side, it can be inferred whether at least one flat surface included on the surface of the first lithium iron phosphate has an area of ​​a certain size or more.

[0060] The length of the side of the cross section of the first lithium iron phosphate can be measured by observing the cross section of the positive electrode with an SEM. Specifically, when the cross section of the positive electrode is observed with an SEM, if the distance between an imaginary line segment connecting any two points on the edge of the cross section of the first lithium iron phosphate and a part of the edge of the cross section of the first lithium iron phosphate connecting the two points is 50 nm or less, the length of the imaginary line segment approximates the length of the side. Therefore, the length of the side can be calculated by measuring the length of the imaginary line segment.

[0061] When the cross section of the positive electrode is observed with a scanning electron microscope (SEM), the cross section of the first lithium iron phosphate may have at least one side with a length of 2 μm or more. If the length of the side is less than 2 μm, the area of ​​the flat surface that can come into face-to-face contact with the positive electrode current collector is small, making it difficult to ensure a sufficiently high positive electrode adhesive strength.

[0062] Specifically, when a cross section of the positive electrode is observed with an SEM, the cross section of the first lithium iron phosphate may have at least one side with a length of 2 μm to 8 μm, more specifically, 2 μm to 4 μm. When the side length satisfies the above range, at least one flat surface included on the surface of the first lithium iron phosphate may have an area of ​​a certain size or more, and problems such as reduced lithium mobility and electrical conductivity due to an excessively large average particle size of the first lithium iron phosphate, increased battery resistance, and reduced output performance may be prevented.

[0063] The primary lithium iron phosphate may be particles in the shape of polygonal pillars. The polygonal pillars may be, for example, square pillars, pentagonal pillars, hexagonal pillars, octagonal pillars, etc., but are not limited thereto. The corners and vertices of the polygonal pillars may be angular and / or rounded. When the primary lithium iron phosphate is in the shape of polygonal pillars, the corners and vertices of the particles may be fitted into the positive electrode current collector during rolling of the positive electrode slurry, thereby easily adhering the positive electrode active material to the positive electrode current collector and increasing the positive electrode adhesive strength.

[0064] When the first lithium iron phosphate is in the form of polygonal pillar particles, the cross section of the first lithium iron phosphate in the cross section of the positive electrode containing the first lithium iron phosphate may be polygonal, for example, triangular, rectangular, pentagonal, hexagonal, octagonal, etc., but is not limited thereto.

[0065] Average particle size D of lithium iron phosphate 50 The average particle diameter D of the first lithium iron phosphate may be 2 μm to 8 μm, specifically 3 μm to 7 μm, and more specifically 4 μm to 6 μm. 50 When the thickness satisfies the above range, the area of ​​the flat surface that can come into face-to-face contact with the positive electrode current collector is increased, and thus the positive electrode adhesive strength can be improved.

[0066] The average particle size of the dilute lithium iron phosphate is D 50The combined use of dilithium iron phosphate having a smaller average particle size than dilithium iron phosphate has the effect of improving the electrical resistance and output characteristics of a lithium secondary battery including the positive electrode.

[0067] The lithium iron phosphate may be a secondary particle formed by agglomeration of primary particles, and therefore may not include a flat surface on the surface of the particle.

[0068] Average particle size D of dilithium iron phosphate 50 The average particle diameter D of the lithium iron phosphate dibasic may be 1 μm or less, specifically 0.5 μm to 1 μm, and more specifically 0.8 μm to 1 μm. 50 When the above range is satisfied, the mobility of lithium in the dilithium iron phosphate is improved, and the charge / discharge characteristics of the battery can be improved.

[0069] In the positive electrode, the dilithium iron phosphate and the dilithium iron phosphate may be contained in a volume ratio of 10:90 to 40:60, specifically 15:85 to 40:60, and more specifically 20:80 to 35:65. When the volume ratio of the dilithium iron phosphate and the dilithium iron phosphate satisfies the above range, sufficient positive electrode adhesive strength can be ensured to prevent electrode detachment, and lithium mobility can be improved to improve the charge / discharge characteristics of the battery.

[0070] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 90% to 99% by weight, specifically 92% to 97% by weight, and more specifically 94% to 97% by weight. When the content of the positive electrode active material satisfies the above range, sufficient energy density of the positive electrode can be ensured, thereby improving the battery capacity of the positive electrode.

[0071] (2) Conductive material The conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples include graphite; carbon black, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; conductive fibers, such as carbon fiber and metal fiber; 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; and conductive materials, such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black products from Chevron Chemical Company, Denka Singapore Private Limited, and Gulf Oil Company; Ketjenblack, EC series (products of Armak Company), Vulcan XC-72 (products of Cabot Company), and Super P (products of Timcal). Preferably, the conductive material is carbon nanotubes. A conductive network of carbon nanotubes is particularly preferred as a conductive material contained in the positive electrode of the present invention because it can mitigate the migration phenomenon of the binder during the drying process of the positive electrode slurry composition.

[0072] The conductive material may be contained in the positive electrode active material layer in an amount of 0.3 wt % to 2.0 wt %, specifically 0.6 wt % to 1.5 wt %, more specifically 0.8 wt % to 1.2 wt %. When the content of the conductive material in the positive electrode active material layer satisfies the above range, a positive electrode conductive network is ensured, thereby improving the electrical conductivity of the positive electrode.

[0073] (3) Binder The binder functions to bind the positive electrode active material and conductive material, etc., and to assist in binding to the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers thereof, and these may be used alone or in combination.

[0074] The binder may be contained in the positive electrode active material layer in an amount of 1.0 wt % to 5.0 wt %, specifically 1.5 wt % to 4.0 wt %, more specifically 2.0 wt % to 3.5 wt %. When the binder content satisfies the above range, the contact area between the binder and lithium iron phosphate is increased, thereby ensuring excellent positive electrode adhesive strength.

[0075] (4) Dispersant The dispersant prevents excessive aggregation of lithium iron phosphate in the positive electrode slurry composition, thereby allowing the lithium iron phosphate to be effectively dispersed and present in the prepared positive electrode active material layer.

[0076] The dispersant may include a hydrogenated nitrile copolymer, and specifically, the dispersant may be a hydrogenated nitrile copolymer.

[0077] Specifically, the hydrogenated nitrile copolymer may be a copolymer containing structural units derived from an α,β-unsaturated nitrile and structural units derived from a hydrogenated conjugated diene, or a copolymer containing structural units derived from an α,β-unsaturated nitrile, structural units derived from a conjugated diene, and structural units derived from a hydrogenated conjugated diene. Examples of the α,β-unsaturated nitrile monomer include acrylonitrile and methacrylonitrile, and these may be used alone or in combination. Examples of the conjugated diene monomer include conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene, and these may be used alone or in combination.

[0078] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR).

[0079] The dispersant may be contained in the positive electrode active material layer in an amount of 1.5 wt % or less, specifically 0.1 wt % to 0.8 wt %, more specifically 0.2 wt % to 0.4 wt %. When the content of the dispersant satisfies the above range, aggregation of the conductive material in the positive electrode active material layer can be suppressed, thereby improving the positive electrode conductive network.

[0080] According to one embodiment of the present invention, the positive electrode active material layer may contain 94.9 to 97.0 wt % of positive electrode active material, 0.8 to 1.2 wt % of conductive material, 2.0 to 3.5 wt % of binder, and 0.2 to 0.4 wt % of dispersant. When the composition of the positive electrode active material layer satisfies the above ranges, the adhesive strength and conductivity of the electrode are ensured, and at the same time, by increasing the content of the active material, the capacity and resistance performance of a lithium secondary battery including the positive electrode can be optimized.

[0081] The positive electrode may be manufactured according to a conventional method for manufacturing a positive electrode, except that the positive electrode active material described above is used. Specifically, the positive electrode may be manufactured by preparing a positive electrode slurry composition containing the positive electrode active material described above, a conductive material, a binder, and / or a dispersant, applying the positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling the composition.

[0082] Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry composition on a separate support, peeling the resulting film from the support, and laminating the resulting film on a positive electrode current collector.

[0083] A positive electrode according to an embodiment of the present invention may have excellent positive electrode adhesion. Specifically, in the positive electrode, the positive electrode active material layer may have improved adhesion to the positive electrode current collector. As a result, detachment of the positive electrode is prevented, 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.

[0084] In a positive electrode according to an embodiment of the present invention, the positive electrode active material layer directly faces the positive electrode current collector, and a separate layer for improving adhesion between the positive electrode active material layer and the positive electrode current collector may not be included. That is, the positive electrode according to the present invention may have excellent interfacial adhesion between the positive electrode current collector and the positive electrode active material layer without including a separate layer, such as a binder layer, adhesive layer, bonding layer, or primer coating layer, that may be interposed between the positive electrode current collector and the positive electrode active material layer to improve adhesion.

[0085] The adhesive strength between the positive electrode current collector and the positive electrode active material layer measured by a 90° peel test can be 70 gf / 20 mm to 200 gf / 20 mm, specifically 80 gf / 20 mm to 170 gf / 20 mm, and more specifically 90 gf / 20 mm to 150 gf / 20 mm. This range corresponds to a higher level of adhesive strength than that of conventional positive electrodes using lithium iron phosphate. This is achieved by using a mixture of large particle diameter lithium iron phosphate and small particle diameter lithium iron phosphate, with the primary lithium iron phosphate having at least one flat surface of a certain size or larger on the particle surface.

[0086] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0087] A 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 electrode and the negative electrode, and an electrolyte.

[0088] The positive electrode in the lithium secondary battery is as described above. For example, the positive electrode includes a positive electrode active material layer containing first lithium iron phosphate and second lithium iron phosphate as positive electrode active materials, and the first lithium iron phosphate has a larger average particle diameter D than the second lithium iron phosphate. 50 The particle surface has at least one flat surface of a certain size or larger. In this case, when a cross section of the positive electrode is observed with a scanning electron microscope (SEM), the cross section of the first lithium iron phosphate has at least one side with a length of 2 μm or larger.

[0089] Additionally, the positive electrode may further comprise a conductive material, a binder, and a dispersant.

[0090] The negative electrode can be manufactured, for example, by preparing a negative electrode-forming composition containing 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.

[0091] The negative electrode active material is not particularly limited, and may generally be a compound capable of reversible lithium intercalation and deintercalation. 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 alloys, Sn alloys, and Al alloys; and composites containing metallic compounds and carbonaceous materials. Low-crystalline carbon includes soft carbon and hard carbon, while highly crystalline carbon includes natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes. These materials may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material.

[0092] The negative electrode conductive material is used to impart conductivity to the electrode and can be any material that is electronically conductive without causing chemical changes in the resulting battery. Specific examples include graphite, such as natural graphite or 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 fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The negative electrode conductive material is typically present in an amount of 1 to 30 wt %, specifically 1 to 20 wt %, and more specifically 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0093] The negative electrode binder serves to improve adhesion between negative electrode active material particles and 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, and various copolymers thereof. 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 wt %, specifically 1 to 20 wt %, more specifically 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0094] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes 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.

[0095] 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 material, foam, or nonwoven fabric.

[0096] Meanwhile, in the lithium secondary battery, the separator can be any material commonly used as a separator in lithium secondary batteries. In particular, a material that exhibits low resistance to electrolyte ion migration and excellent electrolyte humidification is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. The separator can 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.

[0097] 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.

[0098] The organic solvent may be any solvent capable of serving as a medium through which ions involved in the electrochemical reaction of the battery can migrate. Specific examples of the organic solvent 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 dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylenecarbonate (EC), and propylenecarbonate (PC).

[0099] Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0100] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0101] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric 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 battery's life characteristics, suppressing battery capacity reduction, improving the battery's discharge capacity, etc. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0102] 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 placing the electrode assembly in a cylindrical or prismatic battery case and injecting an electrolyte thereinto, or by stacking the electrode assemblies, impregnating them with an electrolyte, and then placing the resulting assembly in a battery case and sealing it.

[0103] When fabricating the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents selected from the group consisting of N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate, which were used in fabricating the positive electrode. If an electrolyte having the same organic solvent as that used in fabricating the positive electrode is used, the step of drying the electrode assembly may be omitted.

[0104] Unlike the lithium secondary batteries described above, the lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0105] The battery case may be any battery case commonly used in the art, and may have any shape depending on the intended use of the battery, such as a cylindrical can, a square can, a pouch, or a coin.

[0106] 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 the fields of portable devices such as mobile phones, laptops, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0107] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0108] <Example 1: Production of positive electrode> (1) Preparation of Positive Electrode Slurry Composition Average particle size D as first lithium iron phosphate 50 The average particle size D is 5 μm, and the single-body structure primary particle LiFePO4 is used as dilithium iron phosphate. 50The particle size was 0.9 μm, and secondary particles of LiFePO4 were used. The lithium iron phosphate, the lithium iron phosphate, carbon nanotubes (CNTs), polyvinylidene fluoride (PVdF), and hydrogenated nitrile butadiene rubber (H-NBR) were added to an N-methylpyrrolidone (NMP) solvent and stirred to prepare a positive electrode slurry composition. In the positive electrode slurry composition, the positive electrode active material, conductive material, binder, and dispersant were present in a weight ratio of 95.7:1.0:3.0:0.3. The solids content of the positive electrode slurry composition was 60 wt %, and the lithium iron phosphate and lithium iron phosphate were contained in a volume ratio of 3:7.

[0109] (2) Manufacturing of the positive electrode The cathode slurry composition was coated on a 15 μm-thick aluminum thin film and then vacuum-dried at 130° C. for 20 minutes to achieve a solid content of 99.0 wt % or more. The dried cathode slurry composition was then rolled to fabricate a cathode active material layer with a porosity of 29%. The cathode active material layer had a thickness of 91 μm and a loading capacity of 3.5 mAh / cm. 2 It was.

[0110] <Example 2> Average particle size D as first lithium iron phosphate 50 A positive electrode was produced in the same manner as in Example 1, except that the thickness was 7 μm.

[0111] Example 3 A positive electrode was prepared in the same manner as in Example 1, except that the volume ratio of lithium iron phosphate to lithium iron phosphate was changed to 2:8.

[0112] <Comparative Example 1: Production of Positive Electrode> A positive electrode was prepared in the same manner as in Example 1, except that no lithium iron phosphate was used.

[0113] <Comparative Example 2: Production of Positive Electrode> Average particle size D as first lithium iron phosphate 50The positive electrode was manufactured in the same manner as in Example 1, except that the particle diameter was 12 μm and secondary particles of LiFePO 4 were used.

[0114] <Comparative Example 3: Production of Positive Electrode> Average particle size D of lithium iron phosphate 50 A positive electrode was produced in the same manner as in Example 1, except that the thickness was 3 μm.

[0115] [Table 1]

[0116] <Experimental Example 1 - Checking the presence or absence of flat surfaces on lithium iron phosphate with the naked eye> The cross sections of the positive electrode slurry compositions and positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 3 were observed with an SEM, and it was confirmed with the naked eye whether or not a flat surface was present on the surface of the lithium iron phosphate.

[0117] Specifically, when a cross section of a positive electrode is observed using an SEM, the cross section of the dilithium iron phosphate having a flat surface on the surface can be observed as a polygonal shape with sides of a certain length or more, and when a positive electrode slurry is observed using an SEM, the dilithium iron phosphate having a flat surface on the surface can be observed as a polygonal column with sides of a certain area or more.

[0118] FIG. 1 is an SEM photograph of a cross section of a positive electrode prepared in Example 1, FIG. 2 is an SEM photograph of a positive electrode slurry prepared in Example 1, FIG. 3 is an SEM photograph of a cross section of a positive electrode prepared in Comparative Example 1, FIG. 4 is an SEM photograph of a cross section of a positive electrode prepared in Comparative Example 2, FIG. 5 is an SEM photograph of a positive electrode slurry prepared in Comparative Example 2, FIG. 6 is an SEM photograph of a cross section of a positive electrode prepared in Comparative Example 3, and FIG. 7 is an SEM photograph of a positive electrode slurry prepared in Comparative Example 3.

[0119] The presence or absence of flat surfaces on the surface of the lithium iron phosphate was confirmed with the naked eye, and the results are shown in Table 2 below.

[0120] O: Flat surface exists X: No flat surface exists

[0121] <Experimental Example 2 - Confirmation of the side length of the cross section of lithium iron phosphate> In order to estimate the area of ​​the flat surface contained in the dilute lithium iron phosphate, the cross section of each of the positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 was observed using an SEM, and the length of the side of the cross section of the dilute lithium iron phosphate was confirmed.

[0122] Specifically, the cross sections of the positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were surface-treated by ion milling, and then a 50 μm × 50 μm area of ​​the positive electrode cross section was observed using an SEM. In the SEM photograph of the positive electrode cross section, the lithium iron phosphate was observed with bright contrast, and the binder was observed with dark contrast. Furthermore, the cross section of the dilithium iron phosphate was observed as a polygon, and the edges of the cross section of the dilithium iron phosphate were observed due to the contrast difference between the positive electrode active material and the binder.

[0123] In this case, when the distance between an imaginary line segment connecting any two points on the edge of the cross section of the lithium iron phosphate monobasic in an SEM photograph of the cross section of the positive electrode and a part of the edge connecting the two points is 50 nm or less, the length of the imaginary line segment corresponds to an approximation of the length of the side. Therefore, the length of the imaginary line segment was measured to calculate the length of the side, and it was confirmed whether the length of the side was 2 μm or more, and the results are shown in Table 2 below.

[0124] O: The cross section of the first lithium iron phosphate has a side with a length of 2 μm or more. X: No side with a length of 2 μm or more exists in the cross section of lithium iron phosphate

[0125] <Experimental Example 3 - Evaluation of Positive Electrode Adhesion Strength> The adhesive strength between the positive electrode active material layer and the positive electrode current collector in each of the positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 3 was compared.

[0126] Specifically, each positive electrode produced in Examples 1 to 3 and Comparative Examples 1 to 3 was cut to a length of 150 mm and a width of 20 mm, and the surface of the positive electrode was attached longitudinally to a slide glass 75 mm long and 25 mm wide 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 rolled 10 times to ensure uniform adhesion of the double-sided tape, producing an evaluation sample.

[0127] 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 half of the positive electrode without the glass slide was connected to the load cell of the UTM equipment. The load cell was moved up to 50 mm at a speed of 100 mm / min, and the load applied to the load cell was measured. The minimum load measured in the 20 mm to 40 mm section of the moving section was recorded as the positive electrode adhesive strength (gf / 20 mm) of each sample. A total of five measurements were taken for each positive electrode, and the average values ​​are shown in Table 2 below.

[0128] [Table 2]

[0129] From Table 2 above, it can be seen that the positive electrodes of Comparative Examples 1 and 2, which do not include a flat surface on the surface of the lithium iron phosphate, have significantly lower positive electrode adhesive strength than the positive electrode of Example 1.

[0130] It was also confirmed that the positive electrode of Comparative Example 3, in which the length of the side of the cross section of the first lithium iron phosphate was less than 2 μm, had significantly lower positive electrode adhesive strength than the positive electrode of Example 1. This is believed to be because in the positive electrode of Comparative Example 3, the flat surface formed on the surface of the first lithium iron phosphate had an area less than a certain size.

[0131] Meanwhile, the positive electrodes of Examples 1 to 3, in which the cross section of the first lithium iron phosphate has at least one side with a length of 2 μm or more, were found to have significantly higher positive electrode adhesive strength than the positive electrodes of Comparative Examples 1 to 3. This is believed to be because the flat surface formed on the surface of the first lithium iron phosphate in the positive electrodes of the Examples has an area of ​​a certain size or more. Furthermore, as shown in FIG. 1, this is believed to be because the corners and / or vertices of the first lithium iron phosphate particles in the positive electrodes of the Examples are fitted into the positive electrode current collector.

Claims

1. A positive electrode including a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector, the positive electrode active material layer contains lithium iron phosphate and lithium iron phosphate as positive electrode active materials, The first lithium iron phosphate is The surface includes at least one flat facet; The average particle size D ５０ is large, A positive electrode, wherein when a cross section of the positive electrode is observed with a scanning electron microscope (SEM), the cross section of the dilute lithium iron phosphate has at least one side having a length of 2 μm or more.

2. 2. The positive electrode of claim 1, wherein the monolithic lithium iron phosphate and the dilithium lithium iron phosphate each have an olivine structure.

3. 2. The positive electrode of claim 1, wherein the first lithium iron phosphate and the second lithium iron phosphate are each independently a compound represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ In Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.

4. 2. The positive electrode according to claim 1, wherein the first lithium iron phosphate is a primary particle having a monolith structure.

5. The average particle diameter D of the first lithium iron phosphate ５０ The positive electrode according to claim 1, wherein the thickness is 2 μm to 8 μm.

6. 2. The positive electrode according to claim 1, wherein the first lithium iron phosphate is in the form of polygonal pillar particles.

7. In the positive electrode active material layer, 2. The positive electrode according to claim 1, wherein the lithium iron phosphate and the lithium iron phosphate are contained in a volume ratio of 10:90 to 40:

60.

8. The average particle diameter D of the lithium iron phosphate dibasic ５０ The positive electrode of claim 1 , wherein the thickness is 1 μm or less.

9. The positive electrode according to claim 1 , wherein the secondary lithium iron phosphate is a secondary particle formed by agglomeration of a plurality of primary particles.

10. The positive electrode according to claim 1 , wherein corners or vertices of some of the first lithium iron phosphate particles are fitted into the positive electrode current collector.

11. The positive electrode according to claim 1 , wherein the positive electrode current collector has a surface having minute irregularities.

12. The positive electrode according to claim 1 , wherein the positive electrode active material layer further comprises a conductive material, a binder, and a dispersant.

13. The positive electrode active material layer is 94.9 to 97.0% by weight of the positive electrode active material, 0.8 to 1.2% by weight of the conductive material; 2.0 to 3.5% by weight of the binder, and 13. The positive electrode of claim 12, comprising 0.2 to 0.4 wt. % of the dispersant.

14. The positive electrode of claim 12 , wherein the conductive material is a carbon nanotube.

15. A lithium secondary battery comprising the positive electrode according to any one of claims 1 to 14.

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