Positive electrode for lithium secondary battery, method for producing same, and lithium secondary battery including same

Abstract

A positive electrode according to an exemplary embodiment includes a positive electrode current collector, a temperature control layer disposed on one or both sides of the positive electrode current collector and covering a partial region of the positive electrode current collector, and a positive electrode active material layer disposed on an exposed portion of the positive electrode current collector that is not covered by the temperature control layer and on the temperature control layer, and the temperature control layer includes a PTC (Positive Temperature Coefficient) material.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2023-0105428, filed on August 11, 2023.

[0002] The present invention relates to a positive electrode for a lithium secondary battery that can control the temperature of the battery without a separate control in response to changes in the temperature of the surrounding environment, a method for manufacturing the same, and a lithium secondary battery including the same. [Background technology]

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

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

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

[0006] However, lithium iron phosphate has high resistance, and battery cells using lithium iron phosphate have inferior capacity and output characteristics at low temperatures compared to battery cells using other positive electrode active materials. Specifically, battery cells using nickel-cobalt-manganese oxide (LNCMO) as the positive electrode active material have charge and discharge capacities at -10°C that are approximately 73% and 73% of the charge and discharge capacity at room temperature, respectively, while battery cells using lithium iron phosphate as the positive electrode active material have charge and discharge capacities at -10°C that are approximately 51% and 50% of the charge and discharge capacity at room temperature, respectively.

[0007] Therefore, in order to improve the capacity and output characteristics of a battery cell using lithium iron phosphate as a positive electrode active material in a low-temperature environment, technical attempts have been made to install a heater outside the battery cell separately and operate the heater in a low-temperature environment, etc. However, such conventional techniques are cumbersome because they require the addition of a separate heating film and controller.

[0008] Therefore, there is a need for technological development to improve the poor low-temperature characteristics of battery cells using lithium iron phosphate. Summary of the Invention [Problem to be solved by the invention]

[0009] The problem to be solved by the technical concept of the present invention is to improve the capacity and output characteristics that decrease at low temperatures in a battery using lithium iron phosphate. [Means for solving the problem]

[0010] According to one embodiment of the present invention, there is provided a positive electrode including a positive electrode current collector, a temperature control layer disposed on one or both sides of the positive electrode current collector and covering a portion of the positive electrode current collector, and a positive electrode active material layer disposed on an exposed portion of the positive electrode current collector not covered by the temperature control layer and on the temperature control layer, wherein the temperature control layer contains a PTC (Positive Temperature Coefficient) material.

[0011] In one embodiment, the positive electrode active material layer contains lithium iron phosphate as a positive electrode active material.

[0012] In one embodiment, the PTC material has a structure in which a conductive material is dispersed in a polymer material.

[0013] In one embodiment, the conductive material is a conductive carbon-based material.

[0014] In one embodiment, the self temperature control layer contains 1% to 60% by weight of a polymer material and 1% to 60% by weight of a conductive material.

[0015] In one embodiment, the self-temperature control layer has one pattern shape selected from a stripe shape in which multiple strips are arranged side by side at a distance, a grid shape in which multiple strips extending in the Y-axis direction intersect with multiple strips extending in the X-axis direction, and a radial pattern in which one strip spreads out like a tornado.

[0016] In one embodiment, the self-temperature control layer has the grid pattern.

[0017] In one embodiment, the width and length of the strip is selected from the range of 0.5 mm to 10 mm.

[0018] In one embodiment, the thickness of the temperature control layer is selected from the range of 3% to 30% of the thickness of the positive electrode active material layer.

[0019] In one embodiment, the thickness of the positive electrode active material layer is selected from the range of 50 μm to 200 μm.

[0020] In one embodiment, an area A of the positive electrode current collector covered by the temperature control layer is in a range of 50% or less of an area B of the entire positive electrode current collector.

[0021] In one embodiment, at least a portion of a side surface of the temperature control layer can be in contact with the positive electrode active material layer.

[0022] According to another embodiment of the present invention, there is provided a method for manufacturing a positive electrode for a lithium secondary battery, the method comprising the steps of: applying a PTC material composition onto a positive electrode current collector; applying a positive electrode slurry onto the positive electrode current collector on which the PTC material composition has been applied; and drying and rolling, wherein the step of applying the PTC material composition is characterized in that the PTC material composition is applied so as to cover a portion of the positive electrode current collector.

[0023] According to another embodiment of the present invention, there is provided a lithium secondary battery comprising the above positive electrode, a negative electrode, a separator, and an electrolyte. [Effects of the Invention]

[0024] According to an exemplary embodiment of the present invention, a self-temperature control layer containing a PTC (Positive Temperature Coefficient) material can increase the amount of heat generated by the battery when it is operated in a low-temperature environment, thereby improving the capacity and output characteristics of the battery, which tend to decrease at low temperatures. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment. [Figure 2] 1 is a diagram illustrating an arrangement of a self-temperature regulating layer according to an exemplary embodiment; [Figure 3] 1 is a diagram illustrating an arrangement of a self-temperature regulating layer according to an exemplary embodiment; [Figure 4] 1 is a diagram illustrating an arrangement of a self-temperature regulating layer according to an exemplary embodiment; [Figure 5] 1 is a flowchart illustrating a method for manufacturing a positive electrode for a lithium secondary battery according to an exemplary embodiment. [Figure 6] 1 is a graph illustrating properties of a PTC material according to an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention will now be described in more detail to aid in its understanding.

[0027] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concepts of terms in order to best explain his own invention.

[0028] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0029] The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art, and therefore the shapes and sizes of components in the drawings may be exaggerated, omitted, or shown in a schematic manner for clearer explanation. Therefore, the sizes and proportions of each component do not completely reflect the actual sizes and proportions.

[0030] In this specification, terms such as "comprise," "comprise," or "have" are intended to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and may be understood as not precluding the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0031] As used herein, the term "combinations thereof" in Markush expressions means a mixture or combination of one or more components selected from the group of components described in the Markush expressions, and means including one or more components selected from the group of components described above.

[0032] In this specification, the phrase "A and / or B" means "A or B, or both."

[0033] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0034] In this specification, the overall length direction of the positive electrode is defined as the X-axis direction, the overall width direction of the positive electrode is defined as the Y-axis direction, and the direction perpendicular to the plane formed by the combination of the X-axis direction and the Y-axis direction is defined as the Z-axis direction.

[0035] <Positive electrodes for lithium secondary batteries> Fig. 1 is a cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment, and Fig. 2 is a diagram showing the arrangement of a temperature control layer according to an exemplary embodiment. The top view of Fig. 2 shows a top view of the positive electrode, and the bottom view of Fig. 2 shows a cross-sectional view of the positive electrode taken along the dotted line in the top view of Fig. 2.

[0036] Referring to these drawings, a positive electrode 100 for a lithium secondary battery (hereinafter referred to as "positive electrode") according to one embodiment of the present invention includes a positive electrode current collector 110, a temperature control layer 120, and a positive electrode active material layer 130, and the temperature control layer 120 may be arranged to cover a portion of the positive electrode current collector 110, and the positive electrode active material layer 130 may be arranged on the exposed positive electrode current collector portion 110E not covered by the temperature control layer and the temperature control layer 120.

[0037] The temperature control layer 120 includes a PTC (Positive Temperature Coefficient) material. FIG. 6 is a graph showing the characteristics of a PTC material according to an exemplary embodiment. Referring to FIG. 6, a PTC material has a positive temperature coefficient, meaning that its resistance is low at low temperatures and increases with increasing temperature. Because the PTC material exhibits relatively low resistance at low temperatures, a higher current flows through it, generating heat. When the temperature increases due to self-heating of the PTC material, the resistance of the PTC material increases. This increase in resistance reduces the current flow and reduces heat generation. Therefore, when a current flows through the temperature control layer 120 containing the PTC material in a low-temperature environment, the PTC material may self-heat as the heat generated by the PTC material increases, causing the temperature of the positive electrode to rise. Furthermore, when the temperature of the PTC material increases and reaches a certain temperature, the resistance of the PTC material increases dramatically, reducing the current flowing through the temperature control layer, thereby preventing overheating of the battery. As described above, the temperature control layer 120 has a self-heating effect when the battery is operated in a low-temperature environment, so that a separate battery heating means is not required to increase the temperature of the battery in a low-temperature environment.

[0038] Meanwhile, since the temperature control layer 120 can function as a resistor in room temperature and high temperature environments, it is preferable that the temperature control layer 120 be arranged to cover a portion of the positive electrode current collector rather than to cover the entire surface of the positive electrode current collector.

[0039] As described above, when the cathode 100 according to the exemplary embodiment is operated in a low-temperature environment, the PTC function of the temperature control layer increases the amount of heat generated, thereby increasing the temperature of the battery without a separate heating means. In particular, batteries using lithium iron phosphate as the cathode active material tend to have reduced capacity and output in low-temperature environments. However, according to the exemplary embodiment, even when lithium iron phosphate is used as the cathode active material, the temperature of the battery increases in low-temperature environments without a separate heating means, thereby improving capacity development and output performance.

[0040] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery, and examples of the current collector include stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

[0041] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0042] The surface roughness Ra of the positive electrode current collector is not particularly limited. In an exemplary embodiment, the surface roughness Ra of the positive electrode current collector may be less than 1 μm, specifically in the range of 50 nm to 900 nm, 70 nm to 700 nm, 85 nm to 500 nm, or 90 nm to 300 nm. When the surface roughness Ra of the positive electrode current collector is in the above range, the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer can be more excellent.

[0043] Here, surface roughness refers to the degree of minute irregularities on the surface of a positive electrode current collector and can be expressed as an arithmetic mean roughness. The surface roughness Ra was measured as follows: Using a laser microscope (VK-X100k, Keyence Corporation), a measurement magnification of ×150 was used to focus on the surface of the positive electrode current collector, and laser scanning was performed in automatic measurement mode. The measurement standard was set according to JIS B0601:2001, and the Ra for the entire area was measured by selecting all areas as the measurement region. The Ra value for each sample was expressed as the average of Ra measurements at 10 points on the surface of the positive electrode current collector while moving the measurement position of the sample.

[0044] In the present invention, the temperature control layer functions to self-heat at low temperatures and increase the temperature of the positive electrode. Therefore, the temperature increase effect can be enhanced by increasing the area ratio of the temperature control layer in contact with the positive electrode active material layer. Even if the surface of the positive electrode current collector has a finely textured shape as described above, the thickness of the temperature control layer can be large enough to cover all of the protrusions and recesses of the finely textured shape. For example, as shown in FIG. 1 , at least a portion of the side surface of the temperature control layer 120 can be in contact with the positive electrode active material layer 130. Here, the side surface refers to the two remaining surfaces of the temperature control layer 120, excluding the upper surface parallel to the X direction and in contact with the positive electrode active material layer 130, and the lower surface parallel to the X direction and in contact with the positive electrode current collector 110. In other words, the side surface can be a surface in the thickness direction (Z direction). To enhance the self-heating effect of the temperature control layer, the larger the area of ​​the temperature control layer 120 in contact with the positive electrode active material layer, the more preferable.

[0045] The temperature control layer 120 may be disposed on one or both sides of the positive electrode current collector 110. In the case of a double-sided positive electrode in which a positive electrode active material layer is formed on both sides of the current collector, the temperature control layer may also be disposed on both sides of the positive electrode current collector, and in the case of a cross-sectional positive electrode in which a positive electrode active material layer is formed on one side of the current collector, the temperature control layer may also be disposed on one side of the positive electrode current collector.

[0046] The temperature control layer may include or be made of a PTC material. PTC materials have the property of decreasing resistance as the battery temperature decreases and increasing resistance as the battery temperature increases. A PTC material may have a structure in which a conductive material is dispersed in a polymeric material with low electrical conductivity, and an electrical path is formed along the conductive particles. As the temperature increases, the volume of the polymeric material expands and, in some cases, the conductive particles flow, increasing the distance between the conductive particles, which can increase the resistance of the PTC material. Conversely, as the temperature decreases, the volume of the polymeric material decreases, reducing the distance between the conductive particles, which can decrease the resistance of the PTC material.

[0047] Since the temperature control layer contains a PTC material, it can act as a resistor for the positive electrode in both normal and high temperature environments. Therefore, the temperature control layer is preferably arranged in a pattern such that a portion of the positive electrode current collector is exposed, as shown in Figures 1 and 2.

[0048] 1 and 2, the temperature control layer 120 may be arranged in a lattice pattern, with a plurality of strips 121 extending in the Y-axis direction intersecting with a plurality of strips 121' extending in the X-axis direction. Arranging the temperature control layer in a lattice pattern is preferable because the increase in the resistance of the electrode at room temperature is not large.

[0049] 3 and 4 show various arrangements of the temperature control layer according to exemplary embodiments. Referring to Fig. 3, the temperature control layer 120 may be arranged in a stripe pattern, with multiple strips 121 spaced apart from one another. Referring to Fig. 4, the temperature control layer 120 may be arranged in a radial pattern, with each strip 121 spreading out like a tornado.

[0050] The plurality of strips may be spaced apart at regular intervals, and the intervals between the strips are not particularly limited, but may be in the range of 5 mm to 40 mm, more specifically 7 mm to 35 mm, more specifically 10 mm to 30 mm, and even more specifically 12 mm to 25 mm.

[0051] In one specific example, the width W1 of the strip 121 may be in the range of 0.5 mm to 10 mm, specifically 1 mm to 5 mm, and more specifically 1.5 mm to 4 mm. When the width W1 of the strip is within the above range, it is preferable in terms of capacity development and output performance of the battery in a low-temperature environment.

[0052] In one specific example, the thickness of the temperature control layer may be 3% to 30%, more specifically 4% to 25%, and even more specifically 5% to 20% of the thickness of the positive electrode active material layer. In some embodiments, the thickness of the temperature control layer may be in the range of 1 μm to 50 μm, 2 μm to 40 μm, 3 μm to 30 μm, or 5 μm to 20 μm. A temperature control layer thickness within the above range is preferred because it allows the battery to have appropriate resistance characteristics in both room temperature and high temperature environments.

[0053] The area A of the positive electrode current collector 110 covered by the temperature control layer 120 can be 50% or less, preferably 5% to 40%, and more preferably 10% to 30% of the area B of the entire positive electrode current collector. When the ratio of the area A of the positive electrode current collector 110 covered by the temperature control layer 120 satisfies the above range, it is possible to improve the capacity development and output performance of the battery in a low-temperature environment while maintaining appropriate resistance characteristics in normal temperature and high-temperature environments, which is preferable.

[0054] In one specific example, the self-temperature control layer includes a PTC material to exhibit PTC characteristics, and such a PTC material may be a polymer material in which a conductive substance is dispersed.

[0055] The polymer material is not particularly limited as long as it has low electrical conductivity and changes in volume in response to temperature changes, causing a change in the conductive network of the conductive substance. For example, it may be a thermoplastic polymer.

[0056] The thermoplastic polymer can be a semi-crystalline material, as it may be easier to obtain PTC properties with a semi-crystalline material compared to an amorphous thermoplastic material. The semi-crystalline thermoplastic material can have a crystallinity of 5% or more, particularly 10% or more, and more particularly 15% or more.

[0057] The thermoplastic polymer is not particularly limited as long as it satisfies the above-mentioned properties, and examples thereof include high-density polyethylene, linear low-density polyethylene, low-density polyethylene, medium-density polyethylene, maleic anhydride functionalized polyethylene, maleic anhydride functionalized elastomer, ethylene copolymers (e.g., Exxon Mobil's EXXELOR VA1801 and VA1803), ethylene-butene copolymers, ethylene-octene copolymers, ethylene-acrylate copolymers such as ethylene-methyl acrylate, ethylene-ethyl acrylate, and ethylene-butyl acrylate copolymers, polyethylene (PE) including glycidyl methacrylate modified polyethylene, polypropylene (PP), maleic anhydride functionalized polypropylene, glycidyl methacrylate modified polypropylene, polyvinyl chloride (PVC), polyvinyl acetate, polyvinyl acetyl, acrylic resins, syndiotactic polystyrene, and the like. The polymer may be one or more selected from the group consisting of polyamide, including but not limited to, polystyrene (sPS), PA6, PA66, PA11, PA12, PA6T, and PA9T, polytetrafluoroethylene (PTFE), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), polyamideimide, polyimide, polyethylene vinyl acetate (EVA), glycidyl methacrylate-modified polyethylene vinyl acetate, polyvinyl alcohol, polymethyl methacrylate (PMMA), polyisobutylene, polyvinylidene chloride, polyvinylidene fluoride (PVDF), polymethyl acrylate, polyacrylonitrile, polybutadiene, polyethylene terephthalate (PET), poly8-aminocaprylic acid, polyvinyl alcohol (PVA), and polycaprolactone.

[0058] The above description is merely an example, and it goes without saying that a PTC material can be manufactured using a thermosetting polymer in addition to the thermoplastic polymers mentioned above.

[0059] The content of the polymer material may be 1% by weight to 60% by weight, specifically 5% by weight to 50% by weight, based on the weight of the entire temperature control layer.

[0060] If the polymer content is less than 1% by weight, the volume expansion of the PTC material is insufficient when the temperature rises, making it difficult to effectively interrupt current.On the other hand, if the polymer content exceeds 60% by weight, the polymer material will not exist as dispersed particles but will aggregate into large clumps, resulting in a decrease in PTC performance.

[0061] The effective operating temperature of the PTC material can be appropriately selected within a range that does not interfere with the general use of the battery and does not inhibit the capacity development and output characteristics of the battery, and can be, for example, in the range of -20°C to +100°C.

[0062] In one specific example, the PTC material may include the polymer material and a conductive material, and may optionally further include a binder.

[0063] The conductive material may be any material that exhibits conductivity without undergoing chemical changes, and may include, for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fiber or metal fiber; metal powders such as carbon fluoride, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives. Conductive carbon materials are preferred. Conductive carbon materials such as carbon black and conductive fibers are preferred as conductive materials for the temperature control layer because they have excellent conductivity and are advantageous for forming a conductive network with the conductive material of the positive electrode active material layer.

[0064] In one specific example, the content of the conductive material may be 1 wt % to 60 wt %, specifically 5 wt % to 50 wt %, based on the total weight of the temperature control layer.

[0065] If the content of the conductive material is less than 1 wt%, the content of the polymer material and binder will increase relatively, resulting in an increase in the resistance of the electrode. If the content of the conductive material is more than 60 wt%, the content of the binder will decrease relatively, which may result in a decrease in the adhesive strength of the temperature control layer.

[0066] The binder is not particularly limited as long as it can provide sufficient adhesive strength without inducing chemical changes, and may be, for example, one or more selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0067] In one specific example, the content of the binder may be 20 wt % or less, specifically 1 wt % to 20 wt %, and more specifically 2 wt % to 15 wt % based on the total weight of the temperature control layer. When the binder is included, the adhesive strength between the temperature control layer and the current collector and / or the adhesive strength between the temperature control layer and the positive electrode active material layer may be improved. However, if the content of the binder exceeds 20 wt %, the resistance of the positive electrode increases, which leads to an increase in the internal resistance of the battery and a decrease in overall battery performance.

[0068] The positive electrode active material layer 130 may be disposed over the positive electrode current collector portion 110E that is not covered by the temperature control layer and the temperature control layer 120. That is, a portion of the positive electrode active material layer may be in contact with the positive electrode current collector, and the remaining portion may be in contact with the temperature control layer.

[0069] The positive electrode active material layer includes a positive electrode active material and a positive electrode binder, and may optionally include a positive electrode conductive material, a filler, or a positive electrode dispersant, as needed.

[0070] The positive electrode active material may contain lithium iron phosphate. Although lithium iron phosphate may be used alone as the positive electrode active material, it may also be mixed with a small amount of lithium nickel cobalt manganese oxide, for example, 0.01% by weight to 3% by weight.

[0071] The lithium iron phosphate may include a metal-doped lithium iron phosphate and a carbon-coated lithium iron phosphate. Specifically, the lithium iron phosphate may have a composition represented by the following Chemical Formula 1:

[0072] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b In the above chemical formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X contains one or more elements selected from the group consisting of F, S, and N; a, b, and x are in the ranges of −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.8, respectively.

[0073] For example, the lithium iron phosphate may be LiFePO4. As another example, the lithium iron phosphate may have a structure in which LiFePO4 is doped with Mn and / or a carbon coating layer is formed. By using the lithium iron phosphate as the positive electrode active material, the safety of the battery can be improved and the manufacturing cost can be significantly reduced.

[0074] The lithium iron phosphate may include a carbon coating layer formed on its surface. When a carbon coating layer is formed on the surface of the lithium iron phosphate, electrical conductivity is improved, thereby improving the resistance characteristics of the positive electrode. The carbon coating layer may be formed of at least one selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, furfuryl alcohol polymers, ethylene-ethylene oxide block copolymers, vinyl resins, cellulose resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing raw materials with lithium iron phosphate and then heat-treating the mixture.

[0075] The positive electrode binder functions to bond the positive electrode active material and conductive material, etc., and to aid in bonding to the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers thereof, and these may be used alone or in combination.

[0076] The positive electrode conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the conductive material may be graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fiber such as carbon fiber or metal fiber; metal powder such as carbon fluoride, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives. Examples of commercially available conductive materials include the acetylene black series from Chevron Chemical Company and Denka Singapore Private Limited, products of Gulf Oil Company (e.g., Gulf Oil Company), Ketjenblack, the EC series (products of Armak Company), Vulcan XC-72 (products of Cabot Company), and Super P (products of Timcal).

[0077] The positive electrode dispersant suppresses aggregation of the positive electrode active material, thereby effectively dispersing the positive electrode active material, such as lithium iron phosphate, in the positive electrode active material layer. The dispersant may include a hydrogenated nitrile copolymer, and more specifically, the dispersant may be a hydrogenated nitrile copolymer.

[0078] Specifically, the hydrogenated nitrile copolymer may be a copolymer containing α,β-unsaturated nitrile-derived structural units and hydrogenated conjugated diene-derived structural units, or a copolymer containing α,β-unsaturated nitrile-derived structural units, conjugated diene-derived structural units, and hydrogenated conjugated diene-derived structural units. Examples of the α,β-unsaturated nitrile monomer include acrylonitrile and methacrylonitrile, and these may be used alone or in combination. Examples of the conjugated diene monomer include conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene, and these may be used alone or in combination.

[0079] More specifically, the hydrogenated nitrile copolymer may be hydrogenated nitrile butadiene rubber (H-NBR).

[0080] The positive electrode dispersant may be contained in an amount of 0.1 wt % to 2.0 wt %, specifically 0.2 wt % to 1.8 wt %, more specifically 0.3 wt % to 1.6 wt %, based on the total weight of the positive electrode active material layer. When the content of the positive electrode dispersant satisfies the above range, it is possible to prevent gelation of the positive electrode slurry while suppressing aggregation of the positive electrode active material.

[0081] In one specific example, the positive electrode active material layer has an average thickness of 300 mg / 25 cm 2 ~600mg / 25cm 2The thickness of the positive electrode active material layer may be selected from the range of 50 μm to 200 μm, specifically from the range of 60 μm to 180 μm, and more specifically from the range of 70 μm to 160 μm.

[0082] <Method of manufacturing a positive electrode for a lithium secondary battery> FIG. 5 is a flowchart illustrating a method for manufacturing a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0083] 5 , a method for manufacturing a positive electrode according to an exemplary embodiment may include a process P110 of applying a PTC material composition onto a positive electrode current collector, a process P120 of applying a positive electrode slurry onto the positive electrode current collector on which the PTC material composition has been applied, and a drying and rolling process P130. According to an exemplary embodiment, the process P110 of applying the PTC material composition may be performed so that the PTC material composition covers a portion of the positive electrode current collector. The PTC material composition becomes the self-temperature control layer through the drying and rolling processes.

[0084] The PTC material composition may be a composition obtained by mixing / stirring the PTC material described above in a solvent. The PTC material has been described in detail above, so a duplicate description will be omitted.

[0085] The solvent is used to mix the polymer material, conductive material, and binder. The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, acetonitrile, or water. One or more of these solvents may be used alone or in combination. The solids concentration of the PTC material composition may be in the range of 50% to 90% by weight, preferably 60% to 80% by weight.

[0086] According to one embodiment, in the step P110 of applying the PTC composition, the PTC composition is preferably applied so that a portion of the current collector is exposed, and also in a pattern to prevent uneven resistance of the electrode.

[0087] The shape of the pattern may be one selected from the group consisting of stripe, grid, and radial, as described above.

[0088] The thickness of the PTC material composition, the proportion of the area of ​​the entire current collector to which the PTC composition is applied, and the width and length of the strip have been described in detail above, so a duplicated description will be omitted.

[0089] After applying the PTC material composition, the process P120 of applying the positive electrode slurry can be performed without drying the PTC material composition, or the process P120 of applying the positive electrode slurry can be performed after drying the PTC material composition.

[0090] The cathode slurry may be a slurry prepared by mixing and stirring the above-described cathode active material, cathode binder, cathode conductive material, dispersant, etc. in a solvent. The cathode active material, cathode binder, cathode conductive material, and dispersant have been described in detail above, and therefore, a repeated description will be omitted.

[0091] The solvent may be a solvent commonly used in the art, such as one or a mixture of two or more selected from the group consisting of dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, and water.

[0092] The solvent may be included in an amount that allows the positive electrode slurry to have an appropriate viscosity and solid content. For example, the solvent may be included in an amount that allows the slurry to have a solid content of 50 wt % to 75 wt %, specifically 50 wt % to 70 wt %, more specifically 55 wt % to 70 wt %.

[0093] The drying and rolling P130 may include a process of passing the electrode through a drying device to remove the solvent in the positive electrode slurry, and pressing the electrode to have an appropriate density.

[0094] The method for the drying and rolling process is not particularly limited, and may be performed by a method well known in the field of positive electrodes for secondary batteries.

[0095] The positive electrode manufactured in this manner does not require a separate battery heating means because the self-temperature control layer with PTC properties can maintain a constant level of battery capacity and output performance in a low-temperature environment, even when lithium iron phosphate is used as the positive electrode active material.

[0096] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0097] A lithium secondary battery according to an embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0098] The positive electrode of the lithium secondary battery is as described above. For example, the positive electrode 100 includes a positive electrode current collector 110, a temperature control layer 120, and a positive electrode active material layer 130. The temperature control layer 120 may be disposed to cover a portion of the positive electrode current collector 110, and the positive electrode active material layer 130 may be disposed on the exposed positive electrode current collector portion 110E not covered by the temperature control layer and the temperature control layer 120.

[0099] The negative electrode may be manufactured, for example, by preparing a negative electrode-forming composition including a negative electrode active material, a negative electrode binder, and a negative electrode conductive material on a negative electrode current collector, and then coating the composition on the negative electrode current collector.

[0100] The negative electrode active material is not particularly limited, and may generally be a compound capable of reversible lithium intercalation and deintercalation. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and composites containing metallic compounds and carbonaceous materials. In addition, 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, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes. One or a mixture of two or more of these may be used, and a metallic lithium thin film may also be used as the negative electrode active material.

[0101] The negative electrode conductive material is used to impart conductivity to the electrode and can be any material that exhibits electronic conductivity without causing chemical changes in the battery. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powder or fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The negative electrode conductive material may typically be present in an amount of 1 wt % to 30 wt %, specifically 1 wt % to 20 wt %, and more specifically 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer.

[0102] The negative electrode binder serves to improve adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The negative electrode binder may be included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, more specifically 1 wt% to 10 wt%, based on the total weight of the negative electrode active material layer.

[0103] Meanwhile, 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-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0104] The negative electrode current collector may have a thickness of typically 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have a surface with fine irregularities to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0105] Meanwhile, in the lithium secondary battery, the separator can be any separator typically used in lithium secondary batteries, with no particular limitations. It is particularly preferred that the separator exhibits low resistance to electrolyte ion migration and excellent electrolyte humidification. Specifically, porous polymer films, such as those made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminate structures of two or more of these layers, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. The separator can be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0106] Meanwhile, in the lithium secondary battery, the electrolyte may contain an organic solvent and a lithium salt that are commonly used in electrolytes, and is not particularly limited.

[0107] The organic solvent may be any organic solvent without particular limitation, as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0108] Among these, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred.

[0109] The lithium salt may be any compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0110] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the battery's life characteristics, suppressing battery capacity reduction, improving the battery's discharge capacity, etc. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0111] The lithium secondary battery of the present invention may be fabricated by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then inserting the electrode assembly into a cylindrical or prismatic battery case and then injecting an electrolyte thereinto, or by stacking the electrode assemblies, impregnating them with an electrolyte, and then inserting the resulting assembly into a battery case and sealing it.

[0112] The battery case may be one commonly used in the art, and the shape of the battery case is not limited depending on the intended use of the battery. For example, the battery case may be a cylindrical type using a can, a square type, a pouch type, or a coin type.

[0113] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, laptops, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0114] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0115] Example 1 A current collector sheet was prepared by coating a 20 μm-thick aluminum foil with a temperature control layer containing a PTC material in a grid pattern as shown in Figures 1 and 2. The temperature control layer was 10 μm thick, with a 2 mm strip width and length and a 20 mm spacing between strips. The temperature control layer covered 22% of the aluminum foil surface. The cathode was fabricated by coating the current collector sheet with a positive electrode slurry, drying, and rolling. The positive electrode slurry was prepared by mixing LiFePO4 (positive electrode active material), PVDF (binder), and carbon black (conductive material) in NMP solvent in a weight ratio of 96:2:2. The dried positive electrode active material layer was 100 μm thick.

[0116] <Example 2> A positive electrode was prepared in the same manner as in Example 1, except that the coating pattern of the temperature control layer was striped as shown in Figure 3. In this case, the area of ​​the aluminum foil covered by the temperature control layer was 22%, and the thickness of the temperature control layer was 10 µm.

[0117] Example 3 A current collector sheet was prepared in which a temperature control layer containing a PTC material was coated in a grid pattern on a 20 μm-thick aluminum foil, and the area of ​​the aluminum foil covered by the temperature control layer was 50%. A positive electrode slurry with the same composition as in Example 1 was then applied, and the positive electrode was produced by drying and rolling under the same conditions as in Example 1.

[0118] <Comparative Example> In Example 1, a current collector on which no temperature control layer was formed was prepared, and a positive electrode slurry having the same composition as in Example 1 was applied to the current collector. The positive electrode was then dried and rolled under the same conditions as in Example 1 to produce a positive electrode.

[0119] <Experimental Example 1: Low-Temperature Capacity Evaluation> The negative electrode active material was artificial graphite, the conductive material was carbon black, and the binders were styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC). The negative electrode active material, conductive material, and binder were mixed and stirred in water in a weight ratio of 95.9:0.8:3.3 to prepare a negative electrode slurry. The negative electrode slurry was applied to a 20 μm-thick copper foil, dried, and rolled to prepare a negative electrode.

[0120] The positive electrode and the negative electrode prepared in Examples 1 to 3 and the Comparative Example were placed opposite each other, with an 18 μm polypropylene separator interposed between them, to fabricate electrode assemblies. Each electrode assembly was inserted into a pouch-type battery case, and an electrolyte composition was injected into the battery case, after which the case was sealed to fabricate a secondary battery. The electrolyte composition used was a solution prepared by mixing lithium hexafluorophosphate (LiPF6, 1.0 M) and vinylene carbonate (VC, 2 wt %) in a mixture of ethylene carbonate (EC): dimethyl carbonate (DMC): diethyl carbonate (DEC) = 1:1:1 (volume ratio).

[0121] Each of the fabricated secondary batteries was charged at a constant current of 0.33 C at a temperature of -10°C up to 4.2 V, and then discharged at a constant current of 0.33 C until the voltage reached 3.0 V. Here, "C" is a unit of charge / discharge rate, and is the value obtained by dividing the current (A) during charge / discharge by the rated capacity (Ah) of the secondary battery. For example, 1 C means a charge / discharge rate at which it takes one hour to fully charge or fully discharge the secondary battery.

[0122] The discharge capacity during discharge was measured, and the results are shown in Table 1. The values ​​shown in Table 1 are expressed as relative percentages, with the capacity of the secondary battery including the positive electrode of Example 1 taken as 100%.

[0123] <Experimental Example 2: Resistance Evaluation> Resistance was measured at -10°C for each secondary battery fabricated in Experimental Example 1 as follows: The secondary batteries were charged at a current of 0.05C from 0% SOC until they reached 30% SOC. Then, at 30% SOC, a current of 0.5C was applied for 30 seconds, providing a pulse. Here, SOC (State of Charge) is the percentage of charge capacity relative to the total capacity of the secondary battery, and the "C" in 0.5C is the unit of charge / discharge rate, as described in Experimental Example 1. DCIR was calculated, and the results are shown in Table 1.

[0124] The resistance was also measured at a temperature of 25°C in the same manner as above, and the results are shown in Table 1.

[0125] [Table 1]

[0126] The secondary battery manufactured using the comparative example positive electrode that does not include the temperature control layer according to the present invention has inferior capacity and resistance characteristics at low temperatures compared to the secondary batteries manufactured using the positive electrodes of Examples 1 to 3.

[0127] The secondary battery manufactured using the positive electrode of Example 3, in which the temperature control layer covered 50% of the positive electrode current collector area, was shown to have the best low-temperature capacity and low-temperature characteristics, but the worst resistance characteristics at room temperature. Therefore, in the present invention, it is preferable to adjust the coating area of ​​the temperature control layer to an appropriate level.

[0128] As described above, the positive electrode and secondary battery according to the present invention are excellent in terms of capacity development even at low temperatures without a separate battery heating means, and the resistance characteristics at room temperature are not significantly reduced compared to conventional positive electrodes.

[0129] The present invention has been described in more detail above through the drawings and embodiments, etc. However, the configurations shown in the drawings or embodiments in this specification are merely one embodiment of the present invention and do not represent all of the technical ideas of the present invention, and therefore, there may be various equivalents and modifications that can replace them at the time of filing this application. [Explanation of symbols]

[0130] 100: Positive electrode 110: Positive electrode current collector 120: Self-temperature control layer 130: Positive electrode active material layer

Claims

1. a positive electrode current collector; a temperature control layer disposed on one or both surfaces of the positive electrode current collector so as to cover a partial region of the positive electrode current collector; a positive electrode current collector portion that is exposed and not covered by the temperature control layer, and a positive electrode active material layer that is disposed on the temperature control layer; The positive electrode for a lithium secondary battery, wherein the self-temperature control layer contains a PTC material.

2. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode active material layer contains lithium iron phosphate as a positive electrode active material.

3. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the PTC material is a polymer material in which a conductive substance is dispersed.

4. The positive electrode for a lithium secondary battery according to claim 3 , wherein the conductive material is a conductive carbon-based material.

5. The self-temperature control layer is 1% by weight or more and 60% by weight or less of a polymer material; 1% by weight or more and 60% by weight or less of a conductive material; The positive electrode for a lithium secondary battery according to claim 3 , comprising:

6. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the temperature control layer has one pattern selected from the group consisting of a stripe pattern in which a plurality of strips are arranged side by side at a distance from one another, a lattice pattern in which a plurality of strips extending in the Y-axis direction intersect with a plurality of strips extending in the X-axis direction, and a radial pattern in which one strip spreads out like a tornado.

7. The positive electrode for a lithium secondary battery according to claim 6 , wherein the temperature control layer has the grid pattern.

8. 7. The positive electrode for a lithium secondary battery according to claim 6, wherein the width of the strip is selected from the range of 0.5 mm to 10 mm.

9. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the thickness of the temperature control layer is selected from the range of 3% to 30% of the thickness of the positive electrode active material layer.

10. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the thickness of the positive electrode active material layer is selected from the range of 50 μm or more and 200 μm or less.

11. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein an area A of the positive electrode current collector covered by the temperature control layer is 50% or less of an area B of the entire positive electrode current collector.

12. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein at least a portion of a side surface of the temperature control layer is in contact with the positive electrode active material layer.

13. applying a PTC material composition onto a positive electrode current collector; applying a positive electrode slurry onto the positive electrode current collector on which the PTC material composition has been applied; drying and rolling, In the step of applying the PTC material composition, the PTC material composition is applied to a portion of a positive electrode current collector so as to cover the portion.

14. The positive electrode according to any one of claims 1 to 12, a negative electrode; A separation membrane; and an electrolyte.

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