Positive electrode and lithium secondary battery manufactured using the same

Abstract

A positive electrode according to one embodiment of the present invention includes a positive electrode active material layer located on at least one surface of a positive electrode current collector, the positive electrode active material layer including a lithium transition metal phosphate, a fluorine-based binder, and a conductive material, the lithium transition metal phosphate including a carbon coating layer formed on a surface thereof, and a ratio (B / A) of a total weight B of the fluorine-based binder to a total weight A of the conductive material and carbon of the lithium transition metal phosphate in the positive electrode active material layer is 0.7 to 1.7.

Description

[Technical Field]

[0001] This application claims priority rights based on Korean Patent Application No. 10-2022-0145389, filed on November 3, 2022.

[0002] This invention relates to a positive electrode and lithium secondary battery that exhibit excellent adhesive strength and resistance characteristics even with a reduced binder content. [Background technology]

[0003] As technological development and demand for electric vehicles and energy storage systems (ESS) increase, the demand for batteries as an energy source is rapidly growing, and accordingly, diverse research is being conducted on batteries that can meet a variety of requirements. In particular, there is a lot of research being done on lithium-ion secondary batteries that have high energy density while also possessing excellent lifespan and cycle characteristics as power sources for such devices.

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

[0005] Lithium iron phosphate is inexpensive because it contains iron, a resource that is abundant and inexpensive. Furthermore, its low toxicity can reduce environmental pollution when used. In addition, because lithium iron phosphate has an olivine structure, its active material structure can be stably maintained at high temperatures compared to layered lithium transition metal oxides. This results in superior high-temperature stability and high-temperature lifespan characteristics for batteries.

[0006] However, due to its structural characteristics, lithium iron phosphate oxide has the disadvantage of having a low lithium ion diffusion rate and high electrode resistance compared to lithium cobalt oxides (LCO) and lithium nickel cobalt manganese oxides (LNCMO), which have layered or spinel structures, because lithium ions can only move along one-dimensional paths.

[0007] Attempts have been made to improve such high electrode resistance by coating the surface of lithium iron phosphate with carbon or increasing the content of conductive material. However, such attempts can lead to a decrease in the content of the positive electrode active material, resulting in a low energy density for the battery.

[0008] Another method to improve positive electrode resistance is to reduce the binder content; however, there are limits to how much the binder content can be reduced in order to manufacture a positive electrode with a certain level of adhesion.

[0009] Therefore, in lithium iron phosphate cathodes, there is a need to develop technologies that improve resistance characteristics without reducing cathode adhesion. [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention aims to provide a lithium secondary battery with a positive electrode that has reduced positive electrode resistance while possessing a suitable level of adhesive performance, and with improved resistance characteristics. [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 located on at least one surface of a positive electrode current collector, wherein the positive electrode active material layer comprises a lithium transition metal phosphate, a fluorine-based binder and a conductive material, the lithium transition metal phosphate comprises a carbon coating layer formed on a surface thereof, and a ratio (B / A) of a total weight B of the fluorine-based binder to a total weight A of carbon from the conductive material and the lithium transition metal phosphate in the positive electrode active material layer is 0.7 to 1.7.

[0012] In one embodiment of the present invention, a ratio (B / A) of a weight B of the fluorine-based binder to a total weight A of carbon from the conductive material and the lithium transition metal phosphate in the positive electrode active material layer may be 0.72 to 1.3.

[0013] In one embodiment of the present invention, the fluorine-based binder may be contained in the positive electrode active material layer in an amount of 3% by weight or less.

[0014] In one embodiment of the present invention, the fluorine-based binder may be contained in the positive electrode active material layer in an amount of 1.6 to 2.7% by weight.

[0015] In one embodiment of the present invention, the positive electrode active material layer may have a porosity according to the following Formula 1 in the range of 25 to 30%.

[0016] [Formula 1] Porosity (%) = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0017] In one embodiment of the present invention, the positive electrode active material layer may have a porosity according to Formula 1 in the range of 26 to 29%.

[0018] In one embodiment of the present invention, 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.

[0019] In one embodiment of the present invention, the first fluorine-based binder may be a homopolymer of polyvinylidene fluoride (PVDF).

[0020] In one embodiment of the present invention, the second fluorine-based binder may be polyvinylidene fluoride (PVDF) containing a polar functional group.

[0021] In one embodiment of the present invention, the second fluorine-based binder may have a weight average molecular weight (Mw) of 900,000 g / mol to 2,000,000 g / mol.

[0022] In one embodiment of the present invention, the weight ratio of the first fluorine-based binder to the second fluorine-based binder may be 2:3 to 3:1.

[0023] In one embodiment of the present invention, the ratio (D / C) of the weight D of the lithium transition metal phosphate to the total weight C of the fluorine-based binder in the positive electrode active material layer may be 32 to 82.

[0024] In one embodiment of the present invention, the lithium transition metal phosphate may be a compound represented by the following Chemical Formula 1.

[0025] [Chemical Formula 1] Li 1+a M1 1-x M2 x (PO y-b )D b

[0026] In Chemical Formula 1 above, M1 is one or more elements selected from the group consisting of Fe, Mn, Co, Ni, Cu, Zn and Mg, M2 is selected from any one of Groups 2 to 15, and is one or more elements excluding the element M1, D is one or more selected from the group consisting of F, S and N, where -0.5 ≤ a ≤ +0.5, 0 ≤ x ≤ 0.5, 3.95 ≤ y ≤ 4.05, and 0 ≤ b ≤ 1.

[0027] In one embodiment of the present invention, in the above chemical formula 1, M1 is Fe, and M2 may be one or more selected from the group consisting of Al, Mg, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y.

[0028] In one embodiment of the present invention, the loading amount of the positive electrode active material layer is 450 to 650 mg / 25 cm². 2 It could be within the range.

[0029] In one embodiment of the present invention, the conductive material may be a carbon nanotube.

[0030] In one embodiment of the present invention, the positive electrode active material layer may further contain hydrogenated nitrile-based butadiene rubber.

[0031] According to another embodiment of the present invention, a lithium secondary battery including the positive electrode described above is provided. [Effects of the Invention]

[0032] The positive electrode according to the present invention has the effect of reducing positive electrode resistance while maintaining excellent adhesion by controlling the value of B / A, which is the ratio of the total weight B of the fluorine-based binder to the total weight A of carbon that contributes to the conductive performance in the positive electrode active material layer, to an optimal range.

[0033] Furthermore, the positive electrode according to the present invention can increase the weight proportion of lithium transition metal phosphoroxide even with a smaller binder content than conventional positive electrodes, resulting in improved energy density. [Modes for carrying out the invention]

[0034] The advantages and features of the present invention, and the methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and may be embodied in a variety of different forms, and these embodiments are provided merely to complete the disclosure of the present invention and to fully inform those who are ordinary skill in the art to which the invention pertains, and the present invention is defined only by the scope of the claims. Throughout the specification, the same reference numerals refer to the same components.

[0035] Unless otherwise defined, all terms used herein (including technical and scientific terms) should be used in a way that is commonly understood by those skilled in the art to which this invention pertains. Furthermore, terms defined in commonly used dictionaries should not be interpreted ideally or excessively unless explicitly defined otherwise.

[0036] The terms used herein are for illustrative purposes only and are not intended to limit the invention. In this specification, singular terms include plural terms unless otherwise specified in the statement. The terms “comprises” and / or “comprising” as used in this specification do not preclude the presence or addition of one or more other components in addition to those mentioned.

[0037] In this specification, when a part is said to include a component, this does not exclude other components, unless otherwise stated, but rather means that it may include other components.

[0038] In this specification, the term "A and / or B" means A or B, or A and B.

[0039] In this specification, "%" means weight percent unless otherwise explicitly indicated.

[0040] In this specification, D 50This refers to the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50 This can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can yield highly reproducible and high-resolution results.

[0041] In this specification, "specific surface area" is measured by the BET method, and specifically, it can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77K) using BELSORP-miniII from BEL Japan.

[0042] In this specification, the total weight A of carbon in the conductive material and positive electrode active material is the total weight of carbon material contributing to the conductive performance of the positive electrode, and means the weight obtained by subtracting the weight of carbon contained in the fluorine-based binder from the total weight of carbon contained in the positive electrode active material layer. Specifically, the total weight of carbon contained in the positive electrode active material layer can be calculated from the total amount of CO2 generated by burning the positive electrode active material layer using a CS Analyzer after separating the positive electrode active material layer from the positive electrode current collector. The weight of carbon contained in the fluorine-based binder can be calculated from the measured fluorine content and the composition of the fluorine-based binder by collecting the gas generated by burning the positive electrode active material layer in a combustion tube in an absorbent solution, diluting the collected absorbent solution, and then measuring the fluorine content using ion chromatography.

[0043] In this specification, "weight-average molecular weight (Mw)" refers to the converted value relative to standard polystyrene measured by gel permeation chromatography (GPC). Specifically, the above weight-average molecular weight is a converted value obtained by measuring the value using GPC under the following conditions, and standard polystyrene from the Agilent system was used to prepare the calibration curve.

[0044] <Measurement conditions> Measurement instrument: Agilent GPC (Agilent 1200 series, USA) Column: PL Mixed B (2 concatenated columns) Column temperature: 40℃ Eluent: Tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0045] The positive electrode adhesion strength in this specification may be measured by the following method. A positive electrode cut to a length of 150 mm and a width of 20 mm is prepared, and the positive electrode active material layer is placed facing a slide glass measuring 75 mm in length and 25 mm in width, and the positive electrode is attached to the slide glass longitudinally using double-sided tape. That is, the slide glass is attached to a region corresponding to half of the positive electrode in the longitudinal direction. Then, a roller is rotated 10 times to ensure that the double-sided tape is uniformly attached to produce an evaluation sample. Next, the slide glass portion of the evaluation sample is fixed to the sample stage of a Universal Testing Machine (UTM) (LS5, AMETEK), and the half of the positive electrode without the slide glass attached is connected to a load cell equipped with the UTM. A force of 90° is applied to the load cell at a speed of 100 mm / min, and the load applied to the load cell is measured while moving it to a distance of 50 mm. At this time, the average value of the load measured in the 20mm to 40mm section of the running distance is calculated, and this is repeated a total of five times. The average value is then evaluated as the positive electrode adhesion strength (gf / 20mm) of each sample.

[0046] The present invention will be described in detail below.

[0047] <Positive electrode> A positive electrode according to one embodiment of the present invention includes a positive electrode active material layer located on at least one surface of a positive electrode current collector, the positive electrode active material layer includes a lithium transition metal phosphate oxide, a fluorine-based binder, and a conductive material, the lithium transition metal phosphate oxide includes a carbon coating layer formed on its surface, and the ratio (B / A) of the total weight B of the fluorine-based binder to the total weight A of the carbon in the conductive material and lithium transition metal phosphate within the positive electrode active material layer may be in the range of 0.7 to 1.7.

[0048] The inventors of the present invention conducted extensive research to improve the resistance characteristics of a positive electrode containing a lithium transition metal phosphate-based positive electrode active material without reducing adhesive strength. As a result, they discovered that when the ratio of the total weight B of the fluorine-based binder to the total weight A of the conductive material and carbon in the positive electrode active material (B / A) is between 0.7 and 1.7, the positive electrode resistance decreases while maintaining suitable adhesive performance, even if the total content of the fluorine-based binder in the positive electrode active material layer decreases. This led to the present invention.

[0049] The positive electrode according to one embodiment of the present invention has a suitable level of adhesive performance even with a lower binder content than that contained in conventional positive electrodes, by controlling the B / A value, which is the ratio of the total weight B of the fluorine-based binder to the total weight A of carbon that contributes to the conductive performance, to an acceptable range. Furthermore, the resistance characteristics are improved as the binder content contained in the positive electrode active material layer decreases, and the weight ratio of the positive electrode active material can be increased in proportion to the amount of binder that decreases, so the energy density per unit weight of the positive electrode can be increased. The B / A value is a parameter that controls not only the electrical performance of the electrode, such as conductivity, but also the physical performance of the electrode, such as adhesive strength. Therefore, by suitably controlling and limiting the B / A value, it is possible to manufacture an electrically / physically balanced positive electrode, an effect that cannot be achieved by methods that control electrical performance and physical performance separately.

[0050] The positive electrode according to the present invention will be described in detail.

[0051] A positive electrode according to one 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.

[0052] The positive electrode current collector is not particularly limited, as long as it is conductive without inducing a chemical change in the battery. For example, the current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc.

[0053] The positive electrode current collector can have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material layer. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0054] The positive electrode active material layer may contain a positive electrode active material. In addition, the positive electrode active material layer may further contain, as necessary, conductive materials, binders, and dispersants, in addition to the positive electrode active material.

[0055] The following provides a detailed explanation of each component included in the positive electrode active material layer.

[0056] (1) Positive electrode active material The present invention includes lithium transition metal phosphate as the positive electrode active material. Since lithium transition metal phosphate has an olivine structure, the active material structure is stably maintained at high temperatures compared to lithium transition metal oxides with a layered structure. As a result, when lithium transition metal phosphate is used as the positive electrode active material, the high-temperature stability and high-temperature life characteristics of the positive electrode are significantly improved, which can reduce the risk of fire in lithium secondary batteries containing the above positive electrode.

[0057] The above lithium transition metal phosphorus oxide may be a compound of the following chemical formula 1.

[0058] [Chemical formula 1] Li1+a M1 1-x M2 x (PO y-b )D b

[0059] In Chemical Formula 1 above, M1 is one or more elements selected from the group consisting of Fe, Mn, Co, Ni, Cu, Zn and Mg, M2 is selected from any one of Groups 2 to 15, and is one or more elements excluding the element M1, D is one or more selected from the group consisting of F, S and N, wherein -0.5≦a≦+0.5, 0≦x≦0.5, 3.95≦y≦4.05, and 0≦b≦1.

[0060] According to a specific embodiment of the present invention, in the lithium transition metal phosphate oxide, M1 is Fe, and M2 may be one or more 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 oxide may be lithium iron phosphate (LiFePO4).

[0061] The lithium transition metal phosphate oxide may be secondary particles formed by aggregation of primary particles with each other, or may have a monolithic structure composed of primary particles.

[0062] In the present invention, the "monolith structure" refers to a structure in which particles exist as independent phases that do not aggregate with each other in terms of morphology. As a particle structure contrasted with such a monolith structure, a structure in which small-sized particles ("primary particles") physically and / or chemically aggregate to form a relatively large-sized particle morphology ("secondary particle") is mentioned.

[0063] When the lithium transition metal phosphorus oxide has a single-component structure consisting of primary particles, the binder migration phenomenon during the drying process of the positive electrode slurry can be mitigated, thereby improving the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0064] On the other hand, when the lithium transition metal phosphate is a secondary particle, the lithium transition metal phosphate may further contain a hollow core within the secondary particle. By intentionally weakening the mechanical strength by forming a hollow inside the secondary particle, the secondary particle may break down and become a primary particle during the rolling process, thereby reducing the springback phenomenon.

[0065] In one embodiment, the diameter of the hollow core portion may be 4 μm to 12 μm, preferably 5 μm to 11 μm. When the diameter of the hollow core portion is within this range, excessive springback does not occur, the disintegration rate of secondary particles is reduced, and processability and productivity are not reduced. In the present invention, the measurement of the diameter of the hollow core portion is not particularly limited, but it can be measured, for example, by scanning electron microscopy (SEM).

[0066] Lithium transition metal phosphates may contain a carbon coating layer on their surface. When a carbon coating layer is formed on the surface of a lithium transition metal phosphate, electrical conductivity can be improved, and the resistance characteristics of the positive electrode can be enhanced.

[0067] 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, polymers of furfuryl alcohol, block copolymers of ethylene and ethylene oxide, vinyl resins, cellulosic resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed through a process in which the raw material is mixed with the lithium transition metal phosphorus oxide and then heat-treated.

[0068] Average particle size D of lithium transition metal phosphate50 The average particle size D of the positive electrode active material can be 0.8 μm to 20.0 μm, more specifically 0.9 μm to 10.0 μm, and more specifically 0.9 μm to 3.0 μm. 50 When the above range is met, the mobility of lithium within the lithium transition metal phosphate oxide is improved, and the charge-discharge characteristics of the battery may be improved.

[0069] The BET specific surface area of ​​lithium transition metal phosphorus oxide is 5 m². 2 / g~20m 2 It could be / g, specifically 7m 2 / g~18m 2 / g, more specifically 9m 2 / g~16m 2 It may be / g. When the above range is satisfied, aggregation of lithium transition metal phosphorus oxides can be effectively suppressed even in cathode slurry compositions with a relatively low dispersant content.

[0070] Lithium transition metal phosphate can be present in the positive electrode active material layer at a concentration of 95% to 99% by weight, specifically 95.5% to 98.5% by weight, and more specifically 96% to 98% by weight. When the lithium transition metal phosphate content satisfies the above range, the battery capacity of the positive electrode can be improved by ensuring sufficient positive electrode energy density. The energy density of a cell containing the positive electrode according to the present invention may be 300 Wh / L or more, more specifically 320 to 400 Wh / L, and more specifically 330 to 400 Wh / L. Here, the above energy density is the energy density of a full cell.

[0071] (2) Binder The present invention includes a fluorine-based binder as the binder, and the fluorine-based binder provides 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.

[0072] The above-mentioned fluorine-based binder may be a PVDF-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the above-mentioned PVDF-based polymer include PVDF monopolymer, 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)).

[0073] The positive electrode according to the present invention has a level of positive electrode adhesion equivalent to other conventional positive electrodes, while reducing electrode resistance. To achieve this, the content of the fluorine-based binder contained in the positive electrode active material layer is controlled in relation to the conductivity of the positive electrode active material layer. Specifically, the ratio (B / A) of the total weight B of the fluorine-based binder to the total weight A of the conductive material and carbon in the positive electrode active material, which contribute to the conductive performance in the positive electrode active material layer, is controlled to satisfy 0.7 to 1.7.

[0074] In a preferred embodiment of the present invention, the B / A value may be 0.72 to 1.3, and more preferably 0.8 to 1.25. When the B / A value is within the above range, it is possible to have excellent resistance characteristics while having adhesive performance that does not pose a risk of delamination of the positive electrode active material layer.

[0075] In one embodiment of the present invention, the fluorine-based binder may include a first fluorine-based binder and a second fluorine-based binder having different numerical ranges of weight-average molecular weight (Mw).

[0076] In one embodiment of the present invention, the first fluorine-based binder may have a weight-average molecular weight (Mw) of 500,000 g / mol to 750,000 g / mol, preferably 550,000 g / mol to 730,000 g / mol, and more preferably 580,000 g / mol to 700,000 g / mol. The first fluorine-based binder provides adhesive force between the positive electrode active material and the conductive material, and adhesive force between the current collector and the positive electrode active material layer.

[0077] In a preferred embodiment of the present invention, the first fluorine-based binder may be a homopolymer of polyvinylidene fluoride (PVDF). The homopolymer polyvinylidene fluoride (PVDF) contains fewer polar functional groups such as -COOH, which reduces the number of hydrogen bonds between hydrogen atoms on the carbon coating layer coated on the surface of the lithium transition metal phosphoroxide particles. This prevents gelation of the positive electrode slurry, improves the coating processability of the positive electrode slurry, and results in a uniform thickness and / or surface of the coated positive electrode active material layer, thus providing the advantage of superior output performance and life characteristics of lithium secondary batteries manufactured using the positive electrode.

[0078] The above-mentioned second fluorine-based binder may have a weight-average molecular weight (Mw) of 800,000 g / mol or more, preferably 900,000 g / mol to 2,000,000 g / mol, and more preferably 1,000,000 g / mol to 1,500,000 g / mol. The second fluorine-based binder exhibits a synergistic effect with the first fluorine-based binder, and can reduce the total content of fluorine-based binders contained in the positive electrode active material layer without inducing a decrease in adhesive strength. That is, if the total weight percentage of fluorine-based binders contained in the positive electrode active material layer is the same, a positive electrode containing both the first and second fluorine-based binders will have superior adhesive performance compared to a positive electrode containing only the first fluorine-based binder. Therefore, when the first fluorine-based binder and the second fluorine-based binder are used in combination, even if the total weight percentage of fluorine-based binder contained in the positive electrode active material layer is reduced, the adhesive performance does not deteriorate, and the reduction in binder content further enhances the effect of improving the resistance characteristics.

[0079] In a preferred embodiment of the present invention, the second fluorine-based binder may be polyvinylidene fluoride (PVDF) containing polar functional groups. When the second fluorine-based binder contains polar functional groups, the crystallinity of the second fluorine-based binder is reduced, which can improve the flexibility of the positive electrode active material layer. 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 external stress is applied, and as a result, the adhesion strength of the positive electrode can be improved.

[0080] In a preferred embodiment of the present invention, the fluorine-based binder may be contained in the positive electrode active material layer in an amount of 3% by weight or less, more preferably 1.6 to 2.7% by weight. From the viewpoint of the resistance characteristics of the positive electrode, it is preferable that the content of the fluorine-based binder be within the above numerical range.

[0081] In a preferred embodiment of the present invention, the weight ratio of the first fluorine-based binder to the second fluorine-based binder may be 2:3 to 3:1, preferably 1:1 to 2:1. If the content of the first fluorine-based binder is excessively high, it may be undesirable in terms of adhesive performance, and if the content of the second fluorine-based binder is excessively high, it may induce gelation of the positive electrode slurry, which is undesirable.

[0082] In a preferred embodiment of the present invention, the ratio of the weight D of lithium transition metal phosphorus oxide, which is the positive electrode active material, to the total weight C of the first fluorine-based binder and the second fluorine-based binder may be 32 to 82, preferably 34 to 72, and more preferably 36 to 60. When the ratio of the weight D of lithium transition metal phosphorus oxide, which is the positive electrode active material, to the total weight C of the first fluorine-based binder and the second fluorine-based binder is within the above numerical range, both the capacitance characteristics and adhesion performance of the positive electrode can be maintained at a suitable level.

[0083] (3) Conductive material The positive electrode active material layer of the present invention may further contain a conductive material.

[0084] The conductive material mentioned above is not particularly limited as long as it is conductive without inducing a chemical change in the battery. Examples of such materials include graphite; carbon black such as carbon black, acetylene black, Ketjenblack, channel black, furnace black, lamp black, and 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; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available conductive materials include acetylene black series from Chevron Chemical Company, Denka Black (Denka Singapore Private Limited), Gulf Oil Company (products, etc.), Ketjenblack, EC series (products from Armak Company), Vulcan XC-72 (products from Cabot Company), and Super P (products from Timcal). Preferably, the conductive material may be carbon nanotubes. When the conductive material is carbon nanotubes, the excellent conductive network of carbon nanotubes can suppress the binder migration phenomenon, in which the binder attempts to move to the electrode surface during electrode drying, and further improve the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0085] The conductive material may be included in the positive electrode active material layer in an amount of 0.3% to 2.0% by weight, more specifically 0.4% to 1.5% by weight, and more specifically 0.5% to 1.3% by weight. When the content of the conductive material in the positive electrode active material layer satisfies the above range, the electrical conductivity of the positive electrode can be improved by ensuring a positive electrode conductive network.

[0086] According to one embodiment of the present invention, the positive electrode active material layer may contain 96-98% by weight of positive electrode active material, 0.8-1.3% by weight of conductive material, and 1.2-3.0% by weight of binder. When the composition within the positive electrode active material layer satisfies the above range, the adhesion and conductivity of the electrode are ensured, and at the same time, by increasing the active material content, the capacity and resistance performance of the lithium secondary battery containing the positive electrode are improved.

[0087] (4) Dispersant The positive electrode active material layer of the present invention may further contain a dispersant.

[0088] The above-mentioned dispersant suppresses the phenomenon of lithium transition metal phosphorus oxides excessively agglomerating within the positive electrode slurry composition, and enables the lithium transition metal phosphorus oxides to be effectively dispersed and present in the manufactured positive electrode active material layer.

[0089] The above dispersant may contain a hydrogenated nitrile copolymer, and more specifically, the above dispersant may be a hydrogenated nitrile copolymer.

[0090] Specifically, the above-mentioned hydrogenated nitrile copolymer may be a copolymer containing structural units derived from α,β-unsaturated nitrile and structural units derived from hydrogenated conjugated diene, or a copolymer containing structural units derived from α,β-unsaturated nitrile, structural units derived from conjugated diene, and structural units derived from hydrogenated conjugated diene. As the above-mentioned α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile may be used, and one of these alone or a mixture of two or more may be used. As the above-mentioned 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 of these alone or a mixture of two or more may be used.

[0091] More specifically, the above-mentioned hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR). In this case, the hydrogenated nitrile butadiene rubber may have a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, preferably 15,000 g / mol to 90,000 g / mol, and more preferably 20,000 g / mol to 50,000. When the hydrogenated nitrile butadiene rubber satisfies the above numerical range, it is superior in its effect of suppressing the aggregation of conductive material. Even if the conductive material aggregates, it aggregates into a spherical shape rather than a linear shape, thereby minimizing the specific surface area of ​​the aggregated conductive material compared to the case where the conductive material aggregates into a linear shape. As a result, the surface area of ​​the positive electrode active material adjacent to the aggregated conductive material that cannot participate in the lithium insertion / desorption reaction is minimized, thus lowering the discharge resistance of the lithium secondary battery.

[0092] The above-mentioned dispersant may be included in an amount of 0.1% to 2.0% by weight, specifically 0.2% to 1.8% by weight, or more specifically 0.3% to 1.6% by weight, based on the total solid content of the 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.

[0093] The positive electrode according to the present invention can be manufactured according to a conventional method for manufacturing a positive electrode. Specifically, the positive electrode can be manufactured by first producing a positive electrode slurry composition containing the positive electrode active material, conductive material, binder and / or dispersant described above, then applying the positive electrode slurry composition onto a positive electrode current collector, followed by drying and rolling.

[0094] Alternatively, the positive electrode can also be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0095] In one embodiment of the present invention, the loading amount of the positive electrode active material layer is 450 to 650 mg / 25 cm². 2 Preferably 500-650 mg / 25 cm 2Most preferably 600-650 mg / 25 cm 2 It can fall within this range. The loading amount within the above range is greater than the loading amount of a typical positive electrode, and is suitable as a high-loading positive electrode.

[0096] In one embodiment of the present invention, the porosity of the positive electrode active material layer after rolling may be in the range of 25 to 30%, preferably 26 to 29%. Here, the porosity can be calculated according to the following formula 1. In formula 1, true density refers to the density of only the portion completely filled with material, excluding the gaps between the positive electrode active material particles.

[0097] [Formula 1] Porosity (%) = {1 - (Measured density of positive electrode active material layer / True density of positive electrode active material)} × 100

[0098] When the porosity of the positive electrode active material layer satisfies the above numerical range, it is preferable in terms of the adhesive performance of the positive electrode and the capacity of the positive electrode.

[0099] The positive electrode according to one embodiment of the present invention has excellent positive electrode adhesion and resistance characteristics, which enables the manufacture of a high-loading positive electrode. By preventing the positive electrode from detaching, the cell resistance of the secondary battery is reduced, improving the battery's capacity and output characteristics, and reducing defects that occur during the manufacturing process.

[0100] The positive electrode of the present invention may have an adhesive strength between the positive electrode current collector and the positive electrode active material layer measured by a 90° peel test of 19 gf / 20 mm or more, specifically 20 gf / 20 mm or more, specifically 20 to 45 gf / 20 mm, and specifically 20 to 40 gf / 20 mm. It is preferable that the positive electrode has an adhesive strength of at least the above value, as this can prevent the delamination of the positive electrode active material layer during the manufacturing process.

[0101] The positive electrode of the present invention may have a DC resistance (DCIR) of 1.91 Ω or less, more specifically 1.5 to 1.85 Ω, and more specifically 1.6 to 1.82 Ω, when measured at a temperature of 25°C, with a state of charge of 30%, and a discharge pulse of 0.5C for 10 seconds. Furthermore, the DC resistance (DCIR) of 1.7 Ω or less, more specifically 1.4 to 1.69 Ω, and more specifically 1.45 to 1.68 Ω, when measured at a temperature of 25°C, with a state of charge of 30%, and a discharge pulse of 2C for 10 seconds. The method for measuring DC resistance is as described in Experimental Example 3 below.

[0102] <Lithium-ion secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0103] A lithium secondary battery according to one embodiment of the present invention may include a positive electrode, a negative electrode, a separation membrane interposed between the negative electrodes, and an electrolyte.

[0104] The positive electrode in the lithium secondary battery described above is as stated above. For example, the positive electrode includes a positive electrode active material layer located on at least one surface of the positive electrode current collector, the positive electrode active material layer includes a lithium transition metal phosphate oxide, a fluorine-based binder, and a conductive material, the lithium transition metal phosphate oxide includes a carbon coating layer formed on its surface, the ratio (B / A) of the total weight B of the fluorine-based binder to the total weight A of the conductive material and the carbon of the lithium transition metal phosphate in the positive electrode active material layer is 0.7 to 1.7, and preferably the fluorine-based binder may include a first fluorine-based binder with a weight-average molecular weight (Mw) of 500,000 g / mol to 750,000 g / mol, and a second fluorine-based binder with a weight-average molecular weight (Mw) of 800,000 g / mol or more.

[0105] Additionally, the positive electrode may further contain a conductive material and a dispersant.

[0106] The above-mentioned negative electrode can be manufactured, for example, by first producing 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 applying it to the negative electrode current collector.

[0107] The negative electrode active material is not particularly limited, and compounds capable of reversible intercalation and deintercalation of lithium can usually be used. 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, or Al alloys; or composites containing metallic compounds and carbonaceous materials. Examples of low-crystallinity carbon include soft carbon and hard carbon, while examples of high-crystallinity carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes. One of these can be used alone or in mixtures of two or more, and a metallic lithium thin film can also be used as the negative electrode active material.

[0108] The above-mentioned negative electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations 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 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 powders or metal fibers 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 of these alone or a mixture of two or more can be used. The above-mentioned negative electrode conductive material can usually be included in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, and more specifically 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0109] The above-mentioned negative electrode binder plays a role in improving the adhesion between negative electrode active material particles and the adhesion 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, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more may be used. The above-mentioned negative electrode binder may be included in an amount of 1 to 30% by weight, specifically 1 to 20% by weight, and more specifically 1 to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0110] On the other hand, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used.

[0111] Furthermore, the negative electrode current collector can typically have a thickness of 3 to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to strengthen the bonding force of the negative electrode active material. For example, it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0112] On the other hand, in the lithium secondary battery described above, the separation membrane can be used without particular limitations as long as it is normally used as a separation membrane in lithium secondary batteries, and it is especially preferable that it has low resistance to ion movement of the electrolyte while having excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, for example, porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. In addition, ordinary porous nonwoven fabrics, for example, nonwoven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc., can also be used. Furthermore, the separation membrane 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.

[0113] On the other hand, in the lithium secondary battery described above, the electrolyte may include, but is not particularly limited to, organic solvents and lithium salts commonly used as electrolytes.

[0114] The above organic solvents can be used without particular limitations, as long as they can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvents can include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; and carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0115] Among these, carbonate-based solvents are preferred, and more preferably are mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate).

[0116] The lithium salt described above can be used without particular limitation as long as it is a compound capable of providing lithium ions for use in lithium secondary batteries. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. It is preferable that the lithium salt is contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0117] In addition to the components of the electrolyte, the above electrolyte may also contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the above additive may be included in an amount of 0.1 to 5% by weight relative to the total weight of the electrolyte.

[0118] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly by placing a separation membrane between the positive electrode and the negative electrode, and then injecting an electrolyte after placing the electrode assembly in a cylindrical or rectangular battery case. Alternatively, the electrode assembly can be manufactured by stacking the electrode assemblies, impregnating them with an electrolyte, and then sealing the resulting product in a battery case.

[0119] When manufacturing 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 during the production of the positive electrode. If an electrolyte with the same components as the organic solvent used during the production of the positive electrode is used, the step of drying the electrode assembly may be omitted.

[0120] Unlike the lithium secondary batteries described above, a lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0121] The battery case described above may be one of those commonly used in this field, and there are no restrictions on its external shape depending on the battery's application. For example, it could be cylindrical, rectangular, pouch-type, or coin-type, using a can.

[0122] The lithium secondary battery according to the present invention exhibits excellent discharge capacity, output characteristics, and capacity retention rate stably, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in energy storage systems (ESS) and electric vehicles such as hybrid electric vehicles (HEVs).

[0123] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and do not limit the scope of the present invention.

[0124] <Example 1: Manufacturing of a positive electrode>

[0125] (1) Manufacturing of positive electrode slurry As lithium transition metal phosphate, average particle size D 50 is 1.0 μm, Having a carbon coating layer formed on its surface,LiFePO4, a single-component primary particle, carbon nanotubes (CNTs) as a conductive material, polyvinylidene fluoride (PVDF), a homopolymer with a weight-average molecular weight (Mw) of 670,000 g / mol, as the first fluorine-based binder, and polyvinylidene fluoride (PVDF), containing polar functional groups and with a weight-average molecular weight (Mw) of 1,000,000 g / mol, as the second fluorine-based binder, were added to N-methylpyrrolidone (NMP) solvent. A cathode slurry was prepared by mixing at 2500 rpm for 90 minutes using a homo-disperse.

[0126] In the above positive electrode slurry, lithium transition metal phosphate oxide, conductive material, primary fluorine-based binder, and secondary fluorine-based binder were present in a weight ratio of 96.7:0.8:1.25:1.25, and the solid content of the above positive electrode slurry was 62% by weight.

[0127] (2) Manufacturing of the positive electrode The above positive electrode slurry is applied to a 20 μm thick aluminum thin film. The loading amount of the positive electrode active material layer 600mg / 25cm 2 After coating the material in this manner, the cathode slurry was hot-air dried at 130°C for 5 minutes so that its solid content was 99.0% by weight or more. Subsequently, the dried cathode slurry was rolled to produce a cathode so that the porosity of the cathode active material layer was 27%.

[0128] At this time, the porosity was measured by the method shown in Equation 1 below.

[0129] [Formula 1] Porosity (%) = {1 - (Measured density of positive electrode active material layer / True density of positive electrode active material)} × 100

[0130] <Examples 2-4: Manufacturing of the positive electrode>

[0131] The positive electrode was manufactured in the same manner as in Example 1, except that the weight ratios of lithium transition metal phosphate oxide, conductive material, primary fluorine-based binder, and secondary fluorine-based binder in the positive electrode slurry were changed as shown in Table 1.

[0132] <Example 5: Manufacturing of the positive electrode>

[0133] As lithium transition metal phosphate, average particle size D 50 is 1.0 μm, Having a carbon coating layer formed on its surface, LiFePO4, a single-structure primary particle, carbon nanotubes (CNTs) as a conductive material, and polyvinylidene fluoride (PVDF), a homopolymer with a weight-average molecular weight (Mw) of 670,000 g / mol, as a binder were added to N-methylpyrrolidone (NMP) solvent. A cathode slurry was prepared by mixing the mixture at 2500 rpm for 90 minutes using a homo-disperse.

[0134] In the above positive electrode slurry, lithium transition metal phosphate, conductive material, and binder were present in a weight ratio of 96.2:0.8:3, and the solid content of the positive electrode slurry was 62% by weight.

[0135] <Example 6: Manufacturing of the positive electrode>

[0136] The positive electrode was manufactured in the same manner as in Example 5, except that the weight ratio of lithium transition metal phosphate, conductive material, and binder in the positive electrode slurry was changed as shown in Table 1.

[0137] <Comparative Example 1: Manufacturing of the positive electrode>

[0138] The positive electrode was manufactured in the same manner as in Example 1, except that the weight ratios of lithium transition metal phosphate oxide, conductive material, primary fluorine-based binder, and secondary fluorine-based binder in the positive electrode slurry were changed as shown in Table 1.

[0139] <Comparative Example 2: Manufacturing of the positive electrode>

[0140] As lithium transition metal phosphate, average particle size D50 A 1.0 μm, single-structure primary particle LiFePO4, carbon nanotubes (CNTs) as a conductive material, and polyvinylidene fluoride (PVDF) with a weight-average molecular weight (Mw) of 1,000,000 g / mol and containing polar functional groups were added to an N-methylpyrrolidone (NMP) solvent.

[0141] In the above positive electrode slurry, lithium transition metal phosphate, conductive material, and binder were present in a weight ratio of 96.2:0.8:3, and the solid content of the positive electrode slurry was 62% by weight.

[0142] However, the viscosity of the positive electrode slurry was too high, preventing the target loading amount (600 mg / 25 cm) from reaching the positive electrode current collector. 2 We were unable to coat the positive electrode slurry with the material, and therefore could not manufacture the positive electrode.

[0143] <Comparative Example 3>

[0144] The positive electrode was manufactured in the same manner as in Example 1, except that the weight ratios of lithium transition metal phosphate oxide, conductive material, primary fluorine-based binder, and secondary fluorine-based binder in the positive electrode slurry were changed as shown in Table 1.

[0145] <Experimental Example 1: Calculation of the ratio (B / A) of the total weight B of the fluorine-based binder to the total weight A of carbon contained in the conductive material and lithium transition metal phosphorus oxide in the positive electrode active material layer>

[0146] For the cathodes manufactured in Examples 1-6 and Comparative Examples 1-3, the ratio of the total weight B of the fluorine-based binder to the total weight A of carbon in the cathode active material layer (B / A) was calculated and the results are shown in Table 1.

[0147] First, the total weight A of carbon in the conductive material and positive electrode active material was calculated by subtracting the weight of carbon contained in the fluorine-based binder from the total weight of carbon contained in the positive electrode active material layer.

[0148] The total weight of carbon contained in the positive electrode active material layer was calculated using a CS Analyzer after separating the positive electrode active material layer from the current collector. Specifically, the total weight of carbon contained in the positive electrode active material layer was calculated from the total amount of CO2 generated by burning the positive electrode active material layer using a CS Analyzer.

[0149] The weight of carbon contained in a fluorinated binder can be derived by measuring the weight of fluorine contained in the positive electrode active material layer, converting the measured weight of fluorine to the total weight (B) of the fluorinated binder contained in the positive electrode active material layer, and then calculating the weight of carbon contained in the fluorinated binder using the weight percentage of carbon in the fluorinated binder.

[0150] The method for measuring the weight of fluorine contained in the positive electrode active material layer is as follows: The positive electrode active material layer was separated from the current collector, the gas generated by burning the separated positive electrode active material layer was collected in an absorbent solution, and after diluting the collected absorbent solution, the weight of fluorine contained in the positive electrode active material layer was measured using ion chromatography.

[0151] The total weight B of the fluorine-based binder was calculated using the total weight (B) of the fluorine-based binder contained in the positive electrode active material layer, which was determined in the process of deriving the weight of carbon contained in the above-mentioned fluorine-based binder.

[0152] Table 1 shows the total weight B of the fluorine-based binder relative to the total weight A of carbon in the conductive material and positive electrode active material, as calculated in this manner.

[0153] <Experimental Example 2: Evaluation of Adhesion Strength of the Positive Electrode>

[0154] The positive electrodes manufactured in Examples 1-6 and Comparative Examples 1-3 were vacuum-dried at 130°C for 2 hours, and 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.

[0155] Specifically, the positive electrodes manufactured in Examples 1-6 and Comparative Examples 1-3 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 glass slide measuring 75 mm in length and 25 mm in width using double-sided tape in the longitudinal direction. In other words, the glass slide was attached to an area corresponding to half of the longitudinal direction of the positive electrode. Then, a roller was rotated 10 times to ensure uniform adhesion of the double-sided tape, and evaluation samples were manufactured.

[0156] Next, the slide glass 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 slide glass was not attached, was connected to a load cell equipped with the UTM. The load cell was moved at a speed of 100 mm / min with a 90° force applied, and the load applied to the load cell was measured while moving it up to 50 mm. At this time, the average value of the load measured in the 20 mm to 40 mm section of the travel distance was calculated, and this was repeated a total of 5 times. The average value was then used to evaluate the positive electrode adhesion strength (gf / 20 mm) of each sample.

[0157] <Experimental Example 3: Resistance Evaluation>

[0158] <Manufacturing of secondary batteries> A negative electrode slurry was prepared by mixing and stirring artificial graphite as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, carboxymethylcellulose (CMC) as the thickener, and carbon nanotubes (CNT) as the conductive material in water in a weight ratio of 95:3:1:1.

[0159] The above negative electrode slurry was applied to a copper thin film at a rate of 295 mg / 25 cm². 2 After coating the material in this manner, the negative electrode slurry was dried so that its solid content was 99.0% by weight or more, and then rolled to produce the negative electrode.

[0160] In each of the examples 1-6 and comparative examples 1-3, a polyolefin separation membrane was interposed between the positive and negative electrodes, and then the electrolyte was injected to produce the secondary batteries of the examples and comparative examples. The electrolyte used was a non-aqueous electrolyte solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, to which vinylene carbonate (VC) was added at 2% by weight relative to the solvent, and LiPF6 was dissolved at 1M.

[0161] Each manufactured secondary battery was activated by charging it with a constant current of 0.1C, and then degassing was performed.

[0162] <Initial resistance measurement> Each of the secondary batteries manufactured as described above was charged to 100% SOC at 1C, then charged to 3.7V at 0.33C CC under constant current-constant voltage (CC-CV) charging conditions at 25°C, followed by a 0.05C current cut-off, and discharged to 2.5V at 0.33C under CC conditions. After completing three cycles of the above charge-discharge process, the voltage drop measured while the battery was charged to 30% SOC and a discharge pulse (0.5C) was applied for 10 seconds was calculated according to the following formula, and the results are shown in Table 1. DCIR was also calculated for the voltage drop measured while a discharge pulse (2.0C) was applied for 10 seconds, and the results are also shown in Table 1.

[0163] DCIR = (V0 - V1) / I (V0 = voltage before pulse, V1 = voltage after 10 seconds of pulse, I = applied current)

[0164] <Increase in resistance after high-temperature storage> The secondary battery was stored in a high-temperature environment of 60°C for two weeks. Afterward, it was charged to a State of Charge (SOC) of 30% at a temperature of 25°C. The voltage drop measured while a discharge pulse (0.5C) was applied for 10 seconds was then calculated using the above formula to determine the DCIR (Digital Voltage Intensity). The percentage increase in resistance after high-temperature storage, calculated by substituting this value into formula (1) below, is shown in Table 1.

[0165] Equation (1): Resistance increase rate (%) = {(Resistance after high-temperature storage - Initial resistance) / Initial resistance} × 100

[0166] [Table 1]

[0167] The positive electrodes of Examples 1 to 6 demonstrated superior resistance characteristics compared to the electrodes of the Comparative Examples, despite having excellent adhesive strength, due to their B / A values ​​being within the range of 0.7 to 1.7. The positive electrode of Comparative Example 1, whose B / A value fell outside the range of 0.7 to 1.7, was shown to have low binder content and low adhesive strength. Furthermore, despite having the smallest total binder content, the positive electrode of Comparative Example 1 showed a larger initial DCIR and a greater rate of resistance increase at high temperatures compared to the positive electrodes of Examples 1 to 6. This is interpreted as the adhesive strength between the positive electrode current collector and the positive electrode active material layer being too low, increasing interfacial resistance, and the rate of resistance increase significantly due to electrode detachment under high-temperature storage conditions. In addition, the positive electrode of Comparative Example 3 had the best adhesive strength, but its resistance characteristics were inferior due to its B / A value exceeding 1.7.

[0168] Furthermore, the positive electrodes of Examples 1 and 2, where the B / A value was within the range of 0.72 to 1.3, showed a smaller initial DCIR and a smaller rate of increase in high-temperature resistance compared to the positive electrodes of Examples 3 to 5. Therefore, it is preferable for the B / A value to be within the range of 0.72 to 1.3 in order to improve the resistance characteristics of the positive electrode.

[0169] <Example 7: Manufacturing of the positive electrode>

[0170] The positive electrode was manufactured using the same method as in Example 1, except that a positive electrode slurry of the same composition as that produced in Example 1 was prepared, and the porosity of the positive electrode active material layer was changed to 26% during rolling.

[0171] <Example 8: Manufacturing of the positive electrode>

[0172] The cathode was manufactured using the same method as in Example 1, except that a cathode slurry of the same composition as that produced in Example 1 was prepared, and the porosity of the cathode active material layer was changed to 28% during rolling.

[0173] <Example 9: Manufacturing of the positive electrode>

[0174] The cathode was manufactured using the same method as in Example 1, except that a cathode slurry of the same composition as that produced in Example 1 was prepared, and the porosity of the cathode active material layer was changed to 30% during rolling.

[0175] <Comparative Example 4: Manufacturing of the positive electrode>

[0176] The cathode was manufactured using the same method as in Example 1, except that a cathode slurry of the same composition as that produced in Example 1 was prepared, and the porosity of the cathode active material layer was changed to 24% during rolling.

[0177] <Comparative Example 5: Manufacturing of the positive electrode>

[0178] The cathode was manufactured using the same method as in Example 1, except that a cathode slurry of the same composition as that produced in Example 1 was prepared, and the porosity of the cathode active material layer was changed to 32% during rolling.

[0179] <Experimental Example 4: Evaluation of Adhesion Strength>

[0180] The adhesive strength was measured for each positive electrode of Examples 7 to 9 and Comparative Examples 4 to 5 using the same method as in Experimental Example 2, and the results are shown in Table 2.

[0181] <Experimental Example 5: Volume Evaluation>

[0182] <Manufacturing of secondary batteries> A lithium metal counter electrode was prepared as the negative electrode.

[0183] In Examples 7-9 and Comparative Examples 4-5, a polyolefin separation membrane was interposed between the positive and negative electrodes, and then the electrolyte was injected to produce the secondary batteries of the Examples and Comparative Examples. The electrolyte used was a non-aqueous electrolyte solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 2:8, to which vinylene carbonate (VC) was added at 0.5% by weight relative to the solvent, and LiPF6 was dissolved at 1M.

[0184] Each manufactured secondary battery was activated by charging it with a constant current of 0.1C, and then degassed. Subsequently, the capacity of each battery was measured by initial charging and discharging under the following conditions, and the results are shown in Table 2.

[0185] Charging: 0.1C, CC / CV, 3.7V, 0.05C cut-off

[0186] Discharge: 0.1C, CC, 2.5V, cut-off

[0187] [Table 2]

[0188] Referring to Table 2, the positive electrode of Comparative Example 4, which had a porosity of less than 25%, showed the best capacity characteristics, but had an adhesive strength of less than 20 gf / 20 mm, and was evaluated as having a risk of delamination of the positive electrode active material layer. The positive electrode of Comparative Example 5, which had a porosity exceeding 30%, showed the best adhesive strength, but had a capacity of less than 157 mAh / g, and was evaluated as unsuitable as a finished product. Therefore, it is preferable that the porosity of the positive electrode active material layer in the positive electrode according to the present invention be in the range of 25 to 30%.

Claims

1. It includes a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, The positive electrode active material layer comprises a lithium transition metal phosphate oxide, a fluorine-based binder, and a conductive material. The lithium transition metal phosphorus oxide includes a carbon coating layer formed on its surface. The ratio (B / A) of the total weight B of the fluorine-based binder to the total weight A of the carbon in the conductive material and lithium transition metal phosphorus oxide within the positive electrode active material layer is 0.7 to 1.

7. The loading amount of the positive electrode active material layer is 450 to 650 mg / 25 cm for each face of the positive electrode current collector. ２ The positive electrode, within the specified range.

2. The positive electrode according to claim 1, wherein the ratio of the weight B of the fluorine-based binder to the total weight A of the conductive material and lithium transition metal phosphorus oxide carbon in the positive electrode active material layer (B / A) is 0.72 to 1.

3.

3. The positive electrode according to claim 1, wherein the fluorine-based binder is contained in the positive electrode active material layer in an amount of 3% by weight or less.

4. The positive electrode according to claim 1, wherein the fluorine-based binder is contained in the positive electrode active material layer in an amount of 1.6 to 2.7% by weight.

5. The positive electrode according to claim 1, wherein the positive electrode active material layer has a porosity in the range of 25 to 30% according to the following formula 1: [Formula 1] Porosity (%) = {1 - (Measured density of positive electrode active material layer / True density of positive electrode active material)} × 100.

6. The positive electrode according to claim 5, wherein the positive electrode active material layer has a porosity in the range of 26 to 29% according to formula 1.

7. The positive electrode according to claim 1, wherein the fluorine-based binder comprises 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.

8. The cathode according to claim 7, wherein the first fluorine-based binder is a homopolymer of polyvinylidene fluoride (PVDF).

9. The positive electrode according to claim 7, wherein the second fluorine-based binder is polyvinylidene fluoride (PVDF) containing polar functional groups.

10. The positive electrode according to claim 7, wherein the second fluorine-based binder has a weight-average molecular weight (Mw) of 900,000 g / mol to 2,000,000 g / mol.

11. The positive electrode according to claim 7, wherein the weight ratio of the first fluorine-based binder to the second fluorine-based binder is 2:3 to 3:

1.

12. The positive electrode according to claim 1, wherein the ratio of the weight D of lithium transition metal phosphorus oxide to the total weight C of the fluorine-based binder in the positive electrode active material layer (D / C) is 32 to 82.

13. The lithium transition metal phosphorus oxide is represented by the following chemical formula 1, and is the positive electrode according to claim 1: [Chemical formula 1] Li １＋ａ M11 １－ｘ M2 ｘ (PO ｙ－ｂ )D ｂ In the aforementioned chemical formula 1, M1 is one or more elements selected from Fe, Mn, Co, Ni, Cu, Zn, and Mg. M2 is selected from any one of groups 2 through 15, and is one or more elements excluding element M1. D is one or more selected from the group consisting of F, S, and N. -0.5 ≤ a ≤ +0.5, 0 ≤ x ≤ 0.5, 3.95 ≤ y ≤ 4.05, and 0 ≤ b ≤ 1.

14. The positive electrode according to claim 13, wherein in the chemical formula 1, M1 is Fe and M2 is one or more selected from the group consisting of Al, Mg, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y.

15. The positive electrode according to claim 1, wherein the conductive material is a carbon nanotube.

16. The positive electrode according to claim 1, wherein the positive electrode active material layer further comprises hydrogenated nitrile butadiene rubber.

17. A lithium secondary battery comprising a positive electrode as described in any one of claims 1 to 16.

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