Positive electrode and lithium secondary battery containing it

Abstract

The present invention relates to a positive electrode and a lithium secondary battery manufactured using the same, which includes a first active material layer and a second active material layer sequentially stacked on a positive electrode current collector, in which the conductive material contained in the first active material layer is a linear conductive material, and the content of the linear conductive material contained in the first active material layer is higher than that in the second active material layer, thereby having the effect of increasing the adhesive strength between the positive electrode current collector and the positive electrode active material layer.

Description

[Technical Field]

[0001] This application claims priority under Korean Patent Application No. 10-2022-0166976 dated 2 December 2022, and all content disclosed in the said Korean Patent Application is incorporated herein by reference.

[0002] This invention relates to a positive electrode and a lithium secondary battery containing the same. [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, which are used as power sources for such devices.

[0004] Typically, in terms of battery shape, there is high demand for prismatic rechargeable batteries and pouch-type rechargeable batteries, which are thin and can be applied to products such as mobile phones. In terms of materials, there is high demand for lithium rechargeable batteries such as lithium-ion batteries and lithium-ion polymer batteries, which have advantages such as high energy density, discharge voltage, and output stability.

[0005] Generally, such rechargeable batteries have an electrode assembly inside the battery case, consisting of a positive electrode, a negative electrode, and a separator membrane placed between them, with the positive and negative electrode tabs welded to the two electrode tabs and sealed so that they are exposed to the outside of the battery case. These electrode tabs are electrically connected to an external device through contact, and the rechargeable battery either supplies power to the external device or receives power from the external device via the electrode tabs.

[0006] 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.

[0007] Lithium iron phosphate is inexpensive because it contains iron, a resource that is abundant and low-cost. Furthermore, its low toxicity means that its use can reduce environmental pollution. Additionally, 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.

[0008] However, lithium iron phosphate has the problem of reduced lithium mobility and low electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, conventionally, lithium iron phosphate with a small average particle size was used to shorten the lithium transport pathway, and the surface of the lithium iron phosphate was coated with carbon to improve electrical conductivity, resulting in the excessive use of conductive materials.

[0009] However, as the size of lithium iron phosphate particles decreases, the specific surface area increases, and lithium iron phosphate with a carbon coating on its surface exhibits reduced wettability with the solvent. As a result, severe aggregation of lithium iron phosphate particles occurs, reducing the stability of the cathode slurry and the processability of the coating. Since lithium iron phosphate and the binder are not effectively mixed, the adhesive strength between the cathode current collector and the cathode active material layer (hereinafter referred to as cathode adhesive strength) in the manufactured cathode decreases. This phenomenon can be exacerbated in high-loading cathodes.

[0010] A decrease in positive electrode adhesion can lead to delamination of the positive electrode active material layer during electrode manufacturing or charging / discharging, increasing battery resistance and reducing the capacity of the secondary battery.

[0011] As the demand for high-energy-density secondary batteries increases, lithium iron phosphate cathodes containing 550 mg / 25 cm³ are becoming more common. 2 While ensuring the above loading capacity, technology is required to improve the positive electrode adhesion strength. [Overview of the project] [Problems that the invention aims to solve]

[0012] The present invention aims to solve the above-mentioned problems and to provide a positive electrode with excellent positive electrode adhesion and a lithium secondary battery manufactured using the same, by differentiating the content ratio of conductive material in the first active material layer and the second active material layer in order to increase the adhesive strength of the positive electrode. [Means for solving the problem]

[0013] The present invention provides a positive electrode. In one example, the positive electrode according to the present invention includes a positive electrode current collector, a first active material layer formed on one or both sides of the positive electrode current collector, and a second active material layer formed on the first active material layer, wherein the first active material layer and the second active material layer each contain a positive electrode active material, a linear conductive material, a rubber-based binder, a fluorine-based binder, and a rubber-based dispersant, and the rubber-based binder contained in the first active material layer and the second active material layer respectively is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 to 500 kg / mol, and the rubber-based dispersant contained in the first active material layer and the second active material layer respectively is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 to 50 kg / mol, and the content of the linear conductive material contained in the first active material layer is greater than the content of the conductive material contained in the second active material layer.

[0014] In a specific example, the linear conductive material contained in the first active material layer and the second active material layer according to the present invention satisfies the following condition 1.

[0015] [Condition 1] 0.5 ≤ E2 / E1 ≤ 0.9

[0016] E1 represents the content (weight %) of the linear conductive material contained in the first active material layer, E2 represents the content (weight %) of the linear conductive material contained in the second active material layer.

[0017] In other specific examples, the linear conductive material of the present invention is contained in the first active material layer in the range of 1.0 wt% to 1.5 wt%, and is contained in the second active material layer in the range of 0.4 wt% to 0.95 wt%.

[0018] In other specific examples, the fluorine-based binders contained in the first active material layer and the second active material layer of the present invention satisfy the following Condition 2.

[0019] [Condition 2] 0.5 ≦ P2 / P1 ≦ 0.8

[0020] P1 represents the content (weight %) of the fluorine-based binder contained in the first active material layer, P2 represents the content (weight %) of the fluorine-based binder contained in the second active material layer.

[0021] In other specific examples, the fluorine-based binder of the present invention is contained in the first active material layer in the range of 2.0 wt% to 2.5 wt%, and is contained in the second active material layer in the range of 1.2 wt% to 1.8 wt%.

[0022] In other specific examples, the rubber-based binders contained in the first active material layer and the second active material layer satisfy the following Condition 3.

[0023] [Condition 3] 1.0 ≦ R2 / R1 ≦ 1.4

[0024] R1 represents the content (weight %) of the rubber-based binder contained in the first active material layer, R2 represents the content (weight %) of the rubber-based binder contained in the second active material layer.

[0025] In other specific examples, the rubber-based binder is included in the first active material layer in an amount ranging from 0.3% to 0.7% by weight, and in the second active material layer in an amount ranging from 0.4% to 0.8% by weight.

[0026] In other specific examples, the total content (by weight) of fluorine-based binders and rubber-based binders contained in the first and second active material layers is in the range of 1.5 or more.

[0027] In another example, the positive electrode has a porosity in the range of 26% to 34% (v / v).

[0028] In another example, in a vacuum-dried positive electrode, the adhesive strength measured when peeling the positive electrode active material layer, including the first and second active material layers, from the positive electrode current collector at a 90° angle is 45 gf / 20 mm or more.

[0029] In a specific example, the positive electrode active material contains lithium iron phosphate, which is a compound represented by the following chemical formula 1.

[0030] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0031] In the above chemical formula 1, M comprises 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 comprises 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.

[0032] In another specific example, the linear conductive material is carbon nanotubes.

[0033] Furthermore, the present invention provides a lithium secondary battery manufactured using the positive electrode described above. [Effects of the Invention]

[0034] The positive electrode according to the present invention can improve energy density by differentiating the content of linear conductive material contained in the first active material layer and the second active material layer, thereby decreasing the binder content and increasing the positive electrode active material content as the positive electrode adhesion strength improves. [Modes for carrying out the invention]

[0035] The present invention is subject to various modifications and may take many forms; therefore, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to any particular disclosure, but rather should be understood to include all modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0036] In this application, terms such as “includes” and “have” are intended to specify the presence of features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, without prejudice to the existence or possibility of adding one or more other features, numbers, steps, actions, components, parts, or combinations thereof. Furthermore, when a part such as a layer, film, region, or plate is said to be “on top” of another part, this includes not only when it is “directly on top” of the other part, but also when there is another part in between. Conversely, when a part such as a layer, film, region, or plate is said to be “below” another part, this includes not only when it is “directly below” the other part, but also when there is another part in between. Furthermore, in this application, being “on top” may include being located not only at the top but also at the bottom.

[0037] In this specification, D 50 This refers to the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve. 50This 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.

[0038] 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-mini II from BEL Japan.

[0039] 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.

[0040] <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)

[0041] 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 using double-sided tape in the longitudinal direction. 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 the load cell of the UTM device. 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 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 is calculated. This process is repeated a total of five times, and the average value is evaluated as the positive electrode adhesion strength (gf / 20mm) of each sample.

[0042] This invention provides a positive electrode and a lithium secondary battery manufactured using the same.

[0043] Lithium iron phosphate, commonly used as a positive electrode active material, exhibits lower lithium mobility and electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, lithium iron phosphate with a small average particle size is primarily used as a positive electrode active material. However, when lithium iron phosphate particles are small, the specific surface area increases, leading to severe particle aggregation. This prevents effective mixing of lithium iron phosphate and the binder, resulting in reduced positive electrode adhesion. This can cause desorption of the positive electrode active material layer during electrode manufacturing or charging / discharging, increasing battery resistance and reducing the capacity of the secondary battery.

[0044] Therefore, the present invention relates to a positive electrode in which a first active material layer is formed on the positive electrode current collector, and a second active material layer is formed on the first active material layer, wherein the rubber-based binder contained in the first active material layer and the second active material layer is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 to 500 kg / mol, and the rubber-based dispersant is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 to 50 kg / mol, and the content of linear conductive material contained in the first active material layer is greater than the content of conductive material contained in the second active material layer. In this case, the dispersibility of lithium iron phosphate particles is improved to suppress their aggregation, the migration of the binder is suppressed, and the adhesive strength of the positive electrode is greatly improved.

[0045] The positive electrode according to the present invention and the lithium secondary battery manufactured using it will be described in detail below.

[0046] <Positive electrode>

[0047] A positive electrode according to one example of the present invention includes a positive electrode current collector, a first active material layer formed on one or both sides of the positive electrode current collector, and a second active material layer formed on the first active material layer, wherein the first active material layer and the second active material layer each contain a positive electrode active material, a linear conductive material, a rubber-based binder, a fluorine-based binder, and a rubber-based dispersant, and the rubber-based binder contained in the first active material layer and the second active material layer respectively is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 kg / mol to 500 kg / mol, and the rubber-based dispersant contained in the first active material layer and the second active material layer respectively is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 kg / mol to 50 kg / mol, and the content of the linear conductive material contained in the first active material layer is greater than the content of the conductive material contained in the second active material layer.

[0048] The positive electrode according to one embodiment of the present invention has a structure in which the positive electrode active material layer (including both the first and second active material layers) is in direct contact with the positive electrode current collector due to excellent interfacial adhesion between the positive electrode current collector and the positive electrode active material layer, and does not require a separate layer to improve adhesion between the positive electrode active material layer and the positive electrode current collector. In other words, the positive electrode according to one embodiment of the present invention can exhibit excellent interfacial adhesion 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.

[0049] A positive electrode according to one embodiment of the present invention includes a positive electrode active material layer composed of a first active material layer and a second active material layer. Specifically, the positive electrode according to the present invention includes a positive electrode current collector, a first active material layer formed on one or both sides of the positive electrode current collector, and a second active material layer formed on the first active material layer.

[0050] When the positive electrode active material layer is formed as a single layer, during the drying process after applying the positive electrode slurry to the positive electrode current collector, some of the binder in areas adjacent to the current collector may move toward the electrode surface as the solvent vaporizes. As a result, the binder content in the areas where the positive electrode active material layer contacts the positive electrode current collector decreases compared to before drying, and the adhesive strength of the resulting positive electrode (adhesion between the positive electrode active material layer and the positive electrode current collector) may also decrease.

[0051] In batteries using positive electrodes with such low adhesive strength, detachment of the positive electrode current collector and positive electrode active material layer may occur during the charging and discharging process. This can lead to a swelling phenomenon where the thickness of the positive electrode and the battery containing it gradually increases, potentially causing a gradual decrease in the battery's electrochemical properties such as capacity and efficiency.

[0052] Therefore, the present invention is designed to solve the problems that appear in single-layer positive electrodes, by forming the positive electrode active material layer into a double-layer structure and further increasing the binder content of the active material slurry for forming the lower layer (first active material layer) compared to the active material slurry for forming the upper layer (second active material layer).

[0053] Specifically, the first active material layer and the second active material layer each independently contain a positive electrode active material, a binder, a conductive material, and a dispersant, and the binder content in the first active material layer is designed to be higher than the binder content in the second active material layer.

[0054] On the other hand, 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, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc., can be used as the positive electrode current collector.

[0055] 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.

[0056] The first active material layer and the second active material layer may each contain a positive electrode active material, a linear conductive material, a rubber-based binder, a fluorine-based binder, and a rubber-based dispersant, respectively.

[0057] The following provides a detailed explanation of each component included in the positive electrode active material layer.

[0058] (1) Positive electrode active material

[0059] This invention includes lithium iron phosphate as the positive electrode active material. Since lithium iron phosphate has an olivine structure, its active material structure is stably maintained at high temperatures compared to lithium transition metal oxides with a layered structure. As a result, when lithium iron 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-mentioned positive electrode.

[0060] The lithium iron phosphate mentioned above may be the compound of the following chemical formula 1.

[0061] [Chemical Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0062] In the above Chemical Formula 1, M contains any one or two 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 contains any one or two or more elements selected from the group consisting of F, S, and N; and a, b, and x are each -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, and 0 ≤ x ≤ 0.5.

[0063] For example, the above lithium iron phosphate can be LiFePO4.

[0064] In the present invention, the "monolith structure" means a structure in which particles exist as independent phases that do not aggregate with each other morphologically. As a particle structure contrasted with such a monolith structure, there is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form a relatively large particle form ("secondary particles").

[0065] When lithium iron phosphate has a monolith structure composed of primary particles, compared with the case of being secondary particles, the possibility of the occurrence of cracking phenomena of lithium iron phosphate particles in the rolling process is small, so it is preferable that the capacity reduction due to the detachment of cracked particles is small. Further, when lithium iron phosphate is primary particles of a monolith structure, the migration phenomenon of the binder can be alleviated in the drying process of the positive electrode slurry, so it can also be said to be preferable from the aspect of the interfacial adhesion force between the positive electrode current collector and the positive electrode active material layer.

[0066] Lithium iron phosphate may include a carbon coating layer on its surface. When a carbon coating layer is formed on the surface of lithium iron phosphate, the electrical conductivity is improved, and the resistance characteristics of the positive electrode can be improved.

[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 by a process in which the raw material is mixed with lithium iron phosphate and then heat-treated.

[0068] Average particle size D of lithium iron phosphate 50 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 lithium iron phosphate is improved, and the charge-discharge characteristics of the battery may be improved.

[0069] The BET specific surface area of ​​lithium iron phosphate 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 can be as low as / g. The above range corresponds to a lower value compared to ordinary lithium iron phosphate. When the above range is satisfied, aggregation of lithium iron phosphate can be effectively suppressed even in cathode slurry compositions with a relatively low dispersant content.

[0070] The lithium iron phosphate of the present invention is characterized by being included in the first active material layer and the second active material layer in different amounts. In a specific example, lithium iron phosphate may be included in the first active material layer in an amount of 95.0% to 96.3% by weight based on the total solid content of the positive electrode slurry composition. It may also be included in the second active material layer in an amount of 96.5% to 97.5% by weight based on the total solid content of the positive electrode slurry composition. Preferably, it may be included in the first active material layer in an amount of 95.5% to 96.0% by weight based on the total solid content of the positive electrode slurry composition, and in the second active material layer in an amount of 96.7% to 97.0% by weight based on the total solid content of the positive electrode slurry composition. When the content of lithium iron phosphate satisfies the above range, the battery capacity of the positive electrode can be improved by ensuring a sufficient positive electrode energy density.

[0071] (2) Conductive material

[0072] The conductive material of the present invention is a linear conductive material. Here, linear (needle type) means that the particles have a needle-like shape, for example, an aspect ratio (length / diameter value) in the range of 50 to 650, more specifically 60 to 300, or 100 to 300.

[0073] Linear conductive materials have higher electrical conductivity than point conductive materials and are distributed in a form where the active material layer encloses the surface of the active material, which is advantageous for the formation of a conductive network and can reduce the amount of conductive material required. Furthermore, they do not block the pores, which are the spaces between the active material, thus promoting pore development and facilitating the diffusion of lithium ions, which can reduce diffusion resistance.

[0074] The linear conductive material may be carbon nanotubes (CNT), vapor-grown carbon fibers (VGCF), carbon nanofibers (CNF), or a mixture of two or more of these, and is preferably carbon nanotubes. The conductive network of carbon nanotubes is preferable as a conductive material included in the positive electrode of the present invention because it can mitigate the binder lifting phenomenon during the drying process of the positive electrode slurry.

[0075] The present invention is characterized in that the amount of linear conductive material contained in the first active material layer is greater than the amount of conductive material contained in the second active material layer.

[0076] As described above, when the positive electrode active material layer is formed as a single layer, during the drying process after coating the positive electrode slurry, some of the binder in the area adjacent to the positive electrode current collector may move toward the electrode surface along with the vaporization of the solvent. As a result, the binder content in the area in contact with the current collector after drying decreases compared to before drying after coating the active material slurry, and the adhesive strength of the resulting positive electrode may also decrease.

[0077] In batteries using positive electrodes with such low adhesive strength, detachment of the positive electrode current collector and positive electrode active material layer may occur during the charging and discharging process. This can lead to a swelling phenomenon where the thickness of the positive electrode and the battery containing it gradually increases, potentially causing a gradual decrease in the battery's electrochemical properties such as capacity and efficiency.

[0078] To address the problems that arise in single-layer positive electrodes, the positive electrode active material layer of the present invention is formed into a double-layer structure, with the content of linear conductive material in the positive electrode slurry forming the lower layer (first active material layer) being higher than that in the positive electrode slurry forming the upper layer (second active material layer). In this configuration, the adhesive force between the positive electrode current collector and the positive electrode active material layer (including the first and second active material layers) increases compared to the case where the content of linear conductive material in the positive electrode slurry forming the upper layer (second active material layer) is higher than that in the positive electrode slurry forming the lower layer (first active material layer). This is because the lifting phenomenon (migration) of the binder due to the conductive network structure of the linear conductive material, particularly carbon nanofibers, is mitigated during the electrode drying process.

[0079] Furthermore, a comparison of the thickness change that occurs during charging and discharging between a positive electrode in which the linear conductive material content of the positive electrode slurry forming the lower layer (first active material layer) is higher than that of the positive electrode slurry forming the upper layer (second active material layer) and a positive electrode in which the linear conductive material content of the positive electrode slurry forming the upper layer (second active material layer) is higher than that of the positive electrode slurry forming the lower layer (first active material layer) shows that the thickness change is smaller in the former case. This is because, in the former case, the relatively higher adhesive force between the positive electrode current collector and the positive electrode active material layer (including the first and second active material layers) prevents electrode detachment and suppresses the swelling phenomenon.

[0080] The linear conductive material of the present invention is characterized by being contained in a first active material layer and a second active material layer in different amounts. In a specific example, the linear conductive material contained in the first active material layer and the second active material layer, respectively, may be configured to satisfy the following condition 1.

[0081] [Condition 1] 0.5 ≤ E2 / E1 ≤ 0.9

[0082] E1 represents the content (by weight) of linear conductive material contained in the first active material layer. E2 represents the content (by weight) of linear conductive material contained in the second active material layer.

[0083] More specifically, the linear conductive material may be included in the first active material layer in an amount of 1.0% to 1.5% by weight and in the second active material layer in an amount of 0.4% to 0.95% by weight, preferably in an amount of 1.1% to 1.4% by weight and in the second active material layer in an amount of 0.6% to 0.90% by weight. When the content of the conductive material in the first and second active material layers satisfies the above ranges, the binder migration phenomenon due to the conductive network structure can be mitigated, increasing the adhesion between the positive electrode current collector and the positive electrode active material layer (including the first and second active material layers), and improving the electrical conductivity of the positive electrode by ensuring a positive electrode conductive network.

[0084] (3) Binder

[0085] The binder of the present invention includes both a fluorine-based binder and a rubber-based binder. The fluorine-based binder includes a PVDF-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the 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)).

[0086] The fluorine-based binder, together with the rubber-based binder, provides adhesive strength between the positive electrode active material and the conductive material, and between the current collector and the positive electrode active material layer.

[0087] The weight-average molecular weight (Mw) of the fluorine-based binder according to the present invention is 800 kg / mol to 1,500 kg / mol, preferably 900 kg / mol to 1,400 kg / mol, and more preferably 950 kg / mol to 1,200 kg / mol. If the weight-average molecular weight of the fluorine-based binder is below the above range, the adhesive strength improvement effect may not be sufficient. Conversely, if it exceeds the above range, the viscosity of the positive electrode slurry may increase with the same solid content, which may reduce coating stability and is therefore undesirable.

[0088] Furthermore, the fluorine-based binder may be contained in the first active material layer in an amount of 2.0% to 2.5% by weight and in the second active material layer in an amount of 1.2% to 1.8% by weight, preferably in an amount of 2.1% to 2.4% by weight in the first active material layer and in an amount of 1.3% to 1.6% by weight in the second active material layer.

[0089] On the other hand, rubber-based binders improve the flexibility of the positive electrode, increase the loading capacity of the positive electrode, and enhance rolling performance. Furthermore, the improved flexibility during physical impacts on the outside of the positive electrode helps maintain the adhesive strength of the positive electrode, thus contributing to improved adhesion.

[0090] The rubber binder of the present invention may be hydrogenated nitrile butadiene rubber (HNBR). The above-mentioned hydrogenated nitrile butadiene rubber (HNBR) is formed by hydrogenating nitrile butadiene rubber (NBR), which means that the double bonds originally contained in nitrile butadiene rubber (NBR) have been converted into single bonds.

[0091] Specifically, the rubber-based binders contained in the first and second active material layers are hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 kg / mol to 500 kg / mol. When the weight-average molecular weight (Mw) of the hydrogenated nitrile butadiene rubber (HNBR) is within the above range, it is possible to improve the adhesion and flexibility of the first and second active material layers, increase the rolling density of the positive electrode, and as the rolling density increases, the positive electrode resistance decreases, and the adhesion between the positive electrode current collector and the positive electrode active material layer may increase.

[0092] On the other hand, the rubber-based binder contained in the first active material layer and the second active material layer may be configured to satisfy the following condition 3.

[0093] [Condition 3] 1.0 ≤ R² / R1 ≤ 1.4

[0094] R1 represents the content (by weight) of the rubber-based binder contained in the first active material layer. R2 represents the content (by weight) of the rubber-based binder contained in the second active material layer.

[0095] Specifically, the rubber-based binder may be included in the first active material layer in an amount of 0.3% to 0.7% by weight, and in the second active material layer in an amount of 0.4% to 0.8% by weight. Preferably, it may be included in the first active material layer in an amount of 0.4% to 0.6% by weight, and in the second active material layer in an amount of 0.5% to 0.7% by weight.

[0096] On the other hand, the total content of fluorine-based binders and rubber-based binders may be configured to be even greater in the first active material layer than in the second active material layer. Specifically, the positive electrode is characterized by having a double-layer structure for the positive electrode active material layer, consisting of a first active material layer and a second active material layer, with a higher binder content in the slurry forming the lower first active material layer than in the upper second active material layer.

[0097] Furthermore, in the positive electrode according to one embodiment of the present invention, the total content (by weight) of the fluorine-based binder and rubber-based binder contained in the first active material layer and the second active material layer may be in the range of 1.5 or more. When the total content of the binder (including the fluorine-based binder and the rubber-based binder) is less than 1.5% by weight, there is a problem that the adhesive strength between the positive electrode active material layer (including the first active material layer and the second active material layer) and the positive electrode current collector decreases.

[0098] When the positive electrode active material layer is formed as a single layer, during the drying process, some of the binder in the area adjacent to the current collector may migrate toward the electrode surface as the solvent vaporizes. As a result, the binder content in the area in contact with the current collector after drying decreases compared to before drying after coating the positive electrode slurry, and the adhesive strength of the resulting positive electrode (adhesion between the active material layer and the current collector) may also decrease.

[0099] In batteries using a positive electrode with such low adhesive strength, detachment of the positive electrode current collector and the positive electrode active material layer may occur during the charging and discharging process, and a swelling phenomenon may occur in which the thickness of the positive electrode and the battery containing it gradually increases, causing a gradual decrease in the electrochemical properties of the battery, such as capacity and efficiency. Therefore, in order to solve the problem of reduced adhesive strength of the positive electrode that occurs in such single-layer structures, the present invention is characterized by having a double-layer structure for the positive electrode active material layer, and increasing the binder content of the first active material layer, which is the lower layer, compared to the second active material layer, which is the upper layer of the positive electrode active material layer.

[0100] More specifically, the total binder content (including fluorine-based binders and rubber-based binders) in the first active material layer may be in the range of 1.0 to 1.7 times, preferably in the range of 1.2 to 1.5 times, the total binder content in the second active material layer. When the total binder content is within the above range, even if some of the binder in the lower layer, the first active material layer, moves toward the electrode surface during the drying process, the first active material layer of the positive electrode ultimately obtained after drying will have an excess amount of binder remaining compared to a single-layer positive electrode, thus preventing a decrease in the adhesive strength of the positive electrode that occurs during the drying process. Therefore, secondary batteries using a positive electrode with improved adhesion can suppress detachment of the positive electrode current collector and positive electrode active material layer during the charge-discharge process, swelling of the positive electrode and the battery containing it, and a decrease in electrochemical properties during the charge-discharge process.

[0101] (4) Dispersant

[0102] Furthermore, the first active material layer and the second active material layer of the present invention contain a dispersant. The dispersant is used to increase the dispersibility of the constituent components of the active material layer. In particular, it increases the dispersibility of the conductive material within the active material. On the other hand, the dispersant may also act as a thickening agent.

[0103] The dispersant of the present invention may be hydrogenated nitrile butadiene rubber (HNBR). As described above, hydrogenated nitrile butadiene rubber (HNBR) can improve the adhesion and flexibility of the first and second active material layers, increase the rolling density, lower the positive electrode resistance, and increase the adhesion between the positive electrode current collector and the positive electrode active material layer (including the first and second active material layers).

[0104] The dispersant may be present in the first active material layer in an amount of 0.2% to 0.5% by weight and in the second active material layer in an amount of 0.1% to 0.4% by weight, preferably in an amount of 0.3% to 0.4% by weight in the first active material layer and in an amount of 0.2% to 0.3% by weight in the second active material layer. Since the amount of binder (including fluorine-based binders and rubber-based binders) present in the first active material layer is greater than the amount of binder (including fluorine-based binders and rubber-based binders) present in the second active material layer, a higher content of binder is added to the first active material layer than to the second active material layer, taking this into consideration.

[0105] On the other hand, the hydrogenated nitrile butadiene rubber (HNBR) that constitutes the dispersant has the same chemical structure as the hydrogenated nitrile butadiene rubber (HNBR) that constitutes the binder, but its weight-average molecular weight (Mw) is different.

[0106] In a specific example, the rubber-based binder contained in the first and second active material layers, respectively, may be hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 kg / mol to 500 kg / mol, and the rubber-based dispersant contained in the first and second active material layers, respectively, may be hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 kg / mol to 50 kg / mol. The smaller the weight-average molecular weight (Mw) of the hydrogenated nitrile butadiene rubber (HNBR), the smaller the particle size may be. By making the weight-average molecular weight (Mw) of the hydrogenated nitrile butadiene rubber (HNBR) used as a conductive material smaller than the weight-average molecular weight (Mw) of the hydrogenated nitrile butadiene rubber (HNBR) used as a dispersant, the dispersibility of the fluorine-based and rubber-based binders can be improved. Furthermore, even if the particle size of the lithium iron phosphate-based cathode active material decreases, aggregation between the cathode active material particles can be prevented, thereby solving the dispersibility problem.

[0107] According to one embodiment of the present invention, the positive electrode may have excellent positive electrode adhesion and flexibility. Specifically, in the positive electrode, the positive electrode active material layer may have improved adhesion to the positive electrode current collector. As a result, it is possible to manufacture a high-loading positive electrode, and by preventing the detachment of the positive electrode, the cell resistance of the secondary battery is reduced, the capacity and output characteristics of the battery are improved, and defects occurring in the manufacturing process can be reduced.

[0108] On the other hand, according to one embodiment of the present invention, the positive electrode may have a porosity in the range of 26% to 34% (v / v). Since the positive electrode active material, such as lithium iron phosphate, having a porosity in the above range can move freely between electrodes, the cell resistance of the secondary battery can be reduced, and the battery capacity and output characteristics can be improved.

[0109] Furthermore, according to one embodiment of the present invention, in a vacuum-dried positive electrode, the adhesive strength measured when evaluating the adhesive strength by peeling the first active material layer and the second active material layer from the positive electrode current collector at a 90° angle is 45 gf / 20 mm or more, specifically 50 gf / 20 mm or more, or in the range of 45 to 60 gf / 20 mm.

[0110] <Lithium-ion secondary battery>

[0111] Next, the lithium secondary battery according to the present invention will be described.

[0112] 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 positive electrode and the negative electrode, and an electrolyte.

[0113] In lithium secondary batteries, the positive electrode is as described above. For example, a positive electrode according to one embodiment of the present invention includes a positive electrode current collector, a first active material layer formed on one or both sides of the positive electrode current collector, and a second active material layer formed on the first active material layer, wherein the first active material layer and the second active material layer each contain a positive electrode active material, a linear conductive material, a rubber-based binder, a fluorine-based binder, and a rubber-based dispersant, and the rubber-based binder contained in the first active material layer and the second active material layer, respectively, is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 kg / mol to 500 kg / mol, and the rubber-based dispersant contained in the first active material layer and the second active material layer, respectively, is hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 kg / mol to 50 kg / mol, and the content of the linear conductive material contained in the first active material layer is greater than the content of the conductive material contained in the second active material layer.

[0114] The 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 coating the negative electrode current collector with this composition.

[0115] 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. Furthermore, examples of low-crystalline carbon include soft carbon and hard carbon, while examples of high-crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes. One of these may be used alone or in mixtures of two or more, and a metallic lithium thin film may also be used as the negative electrode active material.

[0116] The 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 chemical changes in the battery it is composed of. 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. One of these may be used alone or in mixtures of two or more. The above negative electrode conductive material can typically be included in an amount of 1% to 30% by weight, specifically 1% to 20% by weight, or more specifically 1% to 10% by weight, relative to the total weight of the negative electrode active material layer.

[0117] The 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 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.

[0118] 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.

[0119] Furthermore, the negative electrode current collector can generally have a thickness of 3 μm 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.

[0120] On the other hand, in lithium secondary batteries, the above-mentioned 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 above-mentioned 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.

[0121] On the other hand, in the lithium secondary battery described above, the electrolyte may include, and is not particularly limited to, organic solvents and lithium salts commonly used as electrolytes.

[0122] As an organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move is not particularly limited and can be used. Specifically, the above organic solvents may 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), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0123] 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 chain-type carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate).

[0124] The lithium salt can be any compound capable of providing lithium ions for use in lithium secondary batteries, without any particular limitations. Specifically, the lithium salt can 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. The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0125] In addition to the components of the electrolyte described above, the 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 derivatives, 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 additives may be included in an amount of 0.1% to 5% by weight relative to the total weight of the electrolyte.

[0126] 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.

[0127] 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 that 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 as the electrolyte, the step of drying the electrode assembly may be omitted.

[0128] 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.

[0129] The battery case can 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 tin can.

[0130] 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).

[0131] 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.

[0132] <Example 1: Manufacturing of a positive electrode>

[0133] (1) Manufacturing of positive electrode slurry

[0134] The cathode active material was lithium iron phosphate (S2O), and the conductive material was carbon nanotubes (CNT). A fluorine-based binder (Kynar®, HSV900), a rubber-based binder was hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) of 200 kg / mol, and a dispersant was hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) of 20 kg / mol was mixed with the first cathode slurry and the second cathode slurry.

[0135] In the first cathode slurry, lithium iron phosphate, conductive material, dispersant, fluorine-based binder, and rubber-based binder were present in a weight ratio of 95.74:1.2:0.36:2.2:0.5.

[0136] Furthermore, in the second positive electrode slurry, lithium iron phosphate, conductive material, dispersant, fluorine-based binder, and rubber-based binder were present in a weight ratio of 96.96:0.8:0.24:1.4:0.6.

[0137] (2) Manufacturing of the positive electrode

[0138] After applying the first positive electrode slurry to a 20 μm thick aluminum thin film, the film was hot-air dried at 130°C for 5 minutes so that the solid content of the positive electrode slurry was 99.0% by weight or more, and then rolled using a roll-pressing method to form the first active material layer.

[0139] Next, the second cathode slurry was coated onto the first active material layer, hot-air dried at 130°C for 5 minutes, and then rolled using a roll-pressing method to form the first active material layer, thereby producing a cathode with a double layer. At this time, the total loading amount of the cathode was 584 mg / 25 cm 2 The porosity was 29% (v / v).

[0140] <Examples 2-3 and Comparative Examples 1-3: Cathode Manufacturing>

[0141] The cathodes were manufactured in the same manner as in Example 1, except that the weight ratios of lithium iron phosphate, conductive material, dispersant, fluorine-based binder, and rubber-based binder in the first cathode slurry and second cathode slurry were changed as shown in Table 1.

[0142] <Examples 4-6 and Comparative Examples 4-7: Cathode Manufacturing>

[0143] The cathodes were manufactured in the same manner as in Example 1, except that the weight-average molecular weights of the rubber-based binder and dispersant in the first cathode slurry and the second cathode slurry were changed as shown in Table 1.

[0144] <Experimental Example 1: Viscosity Measurement of Cathode Slurry>

[0145] The viscosity of the positive electrode slurries produced in Examples 1-6 and Comparative Examples 1-7 was measured, and the results are shown in Table 2.

[0146] Specifically, the cathode slurries prepared in Examples 1-6 and Comparative Examples 1-7 were cooled for 1 hour at room temperature and 1% relative humidity. The viscosity of the cathode slurry composition was then measured using a Brookfield viscometer at 25°C with a shear rate of 2.5 / s. Viscosity measurements were performed within 2 hours of the preparation of the cathode slurry composition, including the cooling time.

[0147] <Experiment Example 2: Adhesion Test of the Positive Electrode>

[0148] The positive electrodes manufactured in Examples 1-6 and Comparative Examples 1-7 were vacuum-dried at 130°C for 2 hours, and then the adhesive strength between the positive electrode active material layer and the positive electrode current collector was measured. The results are shown in Table 2.

[0149] Specifically, the positive electrodes manufactured in Examples 1-6 and Comparative Examples 1-7 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 a region 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.

[0150] 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 the load cell of the UTM device. 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 determine the positive electrode adhesion strength (gf / 20 mm) of each sample.

[0151] [Table 1-1] [Table 1-2]

[0152] [Table 2]

[0153] In Example 1, the positive electrode contains more conductive material in the first active material layer than in the second active material layer. On the other hand, in Comparative Examples 1 to 3, less conductive material is contained in the first active material layer than in the second active material layer. It can be confirmed that the positive electrode adhesive strength of Example 1 is superior to that of Comparative Examples 1 to 3. Furthermore, although the proportion of conductive material contained in the first and second active material layers differs from that of Example 1, the positive electrodes of Examples 2 to 3 still contain more conductive material in the first active material layer than in the second active material layer, and their positive electrode adhesive strength is superior to that of Comparative Examples 1 to 3. It can be confirmed that the difference is small but not significant compared to Example 1. In other words, when more conductive material is contained in the first active material layer than in the second active material layer, as in Examples 1 to 3, the positive electrode adhesive strength is superior to when less conductive material is contained in the first active material layer than in the second active material layer, as in Comparative Examples 1 to 3.

[0154] Specifically, assuming that E1 represents the content (weight %) of linear conductive material contained in the first active material layer and E2 represents the content (weight %) of linear conductive material contained in the second active material layer, Examples 1 to 3 satisfy condition 1 below, but Comparative Examples 1 to 3 do not satisfy condition 1 below.

[0155] [Condition 1] 0.5 ≤ E2 / E1 ≤ 0.9

[0156] Therefore, when the content of linear conductive material in the positive electrode slurry forming the first active material layer (the lower layer) is higher than that of the positive electrode slurry forming the second active material layer (the lower layer), the adhesive strength between the positive electrode current collector and the positive electrode active material layer (including the first and second active material layers) increased compared to when the content of conductive material in the positive electrode slurry forming the second active material layer (the upper layer) was higher than that of the positive electrode slurry forming the first active material layer (the lower layer). This is judged to be because the binder migration phenomenon caused by the conductive network structure of the conductive material, especially linear conductive material such as carbon nanofibers, was mitigated during the electrode drying process, thus improving the adhesive strength between the positive electrode current collector and the positive electrode active material layer (including the first and second active material layers). Batteries using a positive electrode with improved adhesive strength are expected to enhance electrochemical properties such as battery capacity and efficiency by preventing the detachment of the positive electrode current collector and positive electrode active material layer during the charging and discharging process, and by preventing the swelling phenomenon in which the thickness of the positive electrode and the battery containing it gradually increases.

[0157] In Examples 1 and 4-6, the positive electrodes contained a rubber-based binder in the first and second active material layers, respectively, which was hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 kg / mol to 500 kg / mol. The rubber-based dispersant contained in the first and second active material layers, respectively, was also hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 kg / mol to 50 kg / mol. On the other hand, Comparative Examples 4-7 do not fall within the above range.

[0158] The adhesive strength of the positive electrodes in Examples 1 and 4-6 was shown to be far superior to that of the positive electrodes in Comparative Examples 4-7. This is analyzed to be because, when the hydrogenated nitrile butadiene rubber (HNBR) rubber-based binder and rubber-based dispersant contained in the first and second active material layers, respectively, have a weight-average molecular weight (Mw) within the above range, aggregation between the constituent particles of the positive electrode is prevented, resolving dispersibility, and the phenomenon of the binder (including fluorine-based binder and rubber-based binder) moving to the electrode surface during the drying process (migration) is prevented, thereby improving the adhesive strength.

[0159] As described above, according to the embodiments of the present invention, a positive electrode in which the hydrogenated nitrile butadiene rubber (HNBR) rubber-based binder and rubber-based dispersant contained in the first active material layer and the second active material layer, respectively, have a certain range of weight-average molecular weight (Mw), and the content of the linear conductive material contained in the first active material layer is greater than the content of the conductive material contained in the second active material layer, can have excellent adhesive strength even while containing a large amount of positive electrode active material.

[0160] <Experimental Example 4: Measurement of change in positive electrode thickness>

[0161] The thickness change of the positive electrodes manufactured in Example 1 and Comparative Example 1 was measured.

[0162] Specifically, lithium secondary batteries manufactured using the positive electrodes of Example 1 and Comparative Example 1 were charged at 25°C in CC / CV mode by applying a current of 1 / 3C until the State of Charge (SOC) reached 30% and 100%, respectively. After disassembling the lithium secondary batteries of Example 1 and Comparative Example 1, the thickness of the positive electrode was measured, and the rate of change was measured, as shown in Table 3 below.

[0163] On the other hand, the negative electrode constituting the lithium secondary battery described above was prepared by adding a mixture of artificial graphite (GT), natural graphite (AGP8), and SiO (KSC6027D) (85:10:5, wt%) as the active material, a mixture of carbon black (Super-C65) and CNT (NCL-H5) as the conductive material, SBR (BM-L302) as the binder, and CMC (Daicel 2200) as the thickener in a ratio of 95.1:1.5:2.3:1.1 to distilled water, which was the solvent, to produce a negative electrode slurry. The negative electrode slurry was coated onto one side of a 6 μm thick copper foil, and the negative electrode was produced by drying and rolling under the same conditions as the positive electrode described above.

[0164] Furthermore, a non-aqueous electrolyte was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 30:70 (volume ratio) organic solvent with 0.1% by weight of tetravinylsilane (VS2), 1% by weight of ethylene sulfate (ESa), 0.5% by weight of 1,3-propanesultone (PS), 1% by weight of lithium difluorophosphate (DFP), and 40.2% by weight of LiBF as additives, and dissolving 0.7M LiPF6 and 0.5M LiFSI in it. A lithium secondary battery was manufactured by interposing a separator (DB0905 / BA 1.8μm) between the positive and negative electrodes and then injecting the above electrolyte.

[0165] [Table 3]

[0166] Under the same charging conditions, the positive electrode of Example 1 exhibits less thickness change during charging and discharging compared to the positive electrode of Comparative Example 1. Since Example 1 has a higher adhesive strength than Comparative Example 1, electrode detachment can be prevented, and this is analyzed to have prevented the swelling phenomenon. Therefore, by configuring the positive electrode so that the content of linear conductive material in the first active material layer is greater than the content of conductive material in the second active material layer, the adhesive strength of the positive electrode can be improved while simultaneously suppressing the swelling phenomenon due to charging and discharging.

[0167] While preferred embodiments of the present invention have been described above, a person skilled in the art or with ordinary knowledge of the art will understand that the present invention can be modified and altered in various ways without departing from the spirit and art domain set forth in the claims.

[0168] Therefore, the technical scope of the present invention is not limited to what is described in the summary of the invention in the specification, but is defined by the claims.

Claims

1. Positive electrode current collector and A first active material layer formed on one or both sides of the positive electrode current collector, It includes a second active material layer formed on a first active material layer, The first active material layer and the second active material layer each contain a positive electrode active material, a linear conductive material, a rubber-based binder, a fluorine-based binder, and a rubber-based dispersant, and The positive electrode active material contained in the first active material layer and the second active material layer, respectively, contains lithium iron phosphate. The rubber-based binders contained in the first active material layer and the second active material layer, respectively, are hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 100 kg / mol to 500 kg / mol. The rubber-based dispersants contained in the first active material layer and the second active material layer, respectively, are hydrogenated nitrile butadiene rubber (HNBR) with a weight-average molecular weight (Mw) in the range of 10 kg / mol to 50 kg / mol. The amount of linear conductive material contained in the first active material layer is greater than the amount of conductive material contained in the second active material layer. The linear conductive material contained in the first active material layer and the second active material layer respectively satisfies the following condition 1 for the positive electrode: [Condition 1] 0.5 ≤ E2 / E1 ≤ 0.9 E1 represents the content (by weight) of linear conductive material contained in the first active material layer. E2 represents the content (by weight) of linear conductive material contained in the second active material layer.

2. The linear conductive material is contained in the first active material layer in an amount ranging from 1.0% to 1.5% by weight. The positive electrode according to claim 1, which is contained in the second active material layer in an amount of 0.4% to 0.95% by weight.

3. The fluorine-based binder contained in the first active material layer and the second active material layer respectively satisfies the following condition 2, the positive electrode according to claim 1: [Condition 2] 0.5 ≤ P2 / P1 ≤ 0.8 P1 represents the content (by weight) of the fluorine-based binder contained in the first active material layer. P2 represents the content (by weight) of the fluorine-based binder contained in the second active material layer.

4. The fluorine-based binder is contained in the first active material layer in an amount ranging from 2.0% to 2.5% by weight. The positive electrode according to claim 1, which is contained in the second active material layer in an amount of 1.2% to 1.8% by weight.

5. The rubber-based binder contained in the first active material layer and the second active material layer satisfies the following condition 3, and is the positive electrode according to claim 1: [Condition 3] 1.0 ≤ R² / R1 ≤ 1.4 R1 represents the content (by weight) of the rubber-based binder contained in the first active material layer. R2 represents the content (by weight) of the rubber-based binder contained in the second active material layer.

6. The rubber-based binder is contained in the first active material layer in an amount ranging from 0.3% to 0.7% by weight. The positive electrode according to claim 1, which is contained in the second active material layer in an amount of 0.4% to 0.8% by weight.

7. The positive electrode according to claim 1, wherein the total content (by weight) of the fluorine-based binder and the rubber-based binder contained in the first active material layer and the second active material layer is in the range of 1.5 or more.

8. The positive electrode according to claim 1, wherein the porosity is in the range of 26% to 34% (v / v).

9. The positive electrode according to claim 1, wherein the adhesive strength measured during the evaluation of the adhesive strength when peeling the positive electrode active material layer, including the first active material layer and the second active material layer, from the positive electrode current collector at a 90° angle in a vacuum-dried positive electrode is 45 gf / 20 mm or more.

10. The positive electrode according to claim 1, wherein lithium iron phosphate is 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 the above chemical formula 1, M comprises 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 comprises 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.

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

12. A lithium secondary battery comprising a positive electrode according to any one of claims 1 to 11.

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