Cathode for lithium secondary battery, method for manufacturing same, and lithium secondary battery comprising same

Abstract

A cathode according to exemplary embodiments comprises: a cathode current collector; a self-temperature control layer disposed on one surface or both surfaces of the cathode current collector, the self-temperature control layer being arranged to cover a partial area of the cathode current collector; and a cathode active material layer disposed on the self-temperature control layer and a portion of the cathode current collector, which is not covered by the self-temperature control layer and thus exposed, wherein the self-temperature control layer comprises a positive temperature coefficient (PTC) material.

Description

Anode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery comprising the same

[0001] This application claims the benefit of priority from 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 capable of controlling the temperature of a battery without separate control in response to temperature changes in the surrounding environment, a method for manufacturing the same, and a lithium secondary battery including the same.

[0003] With the increasing technological development and demand for electric vehicles and energy storage systems (ESS), the demand for batteries as an energy source is rapidly increasing. Consequently, diverse research is being conducted to develop batteries that can meet these diverse needs. In particular, active research is being conducted on lithium secondary batteries, which possess high energy density and excellent lifespan and cycling characteristics as power sources for these devices.

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

[0005] Lithium iron phosphate is inexpensive because it contains iron, a resource-abundant and inexpensive material. Furthermore, its low toxicity reduces environmental pollution when used. Furthermore, because lithium iron phosphate has an olivine structure, its active material structure remains stable at high temperatures compared to layered lithium transition metal oxides. Consequently, the battery offers excellent high-temperature stability and high-temperature cycle life.

[0006] However, since lithium iron phosphate has high resistance, battery cells using lithium iron phosphate have inferior development 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 capacity and discharge capacity at -10 degrees Celsius of about 73% of the charge capacity at room temperature and about 73% of the discharge capacity at room temperature, respectively, but battery cells using lithium iron phosphate as the positive electrode active material have charge capacity and discharge capacity at -10 degrees Celsius of about 51% of the charge capacity at room temperature and about 50% of the discharge capacity at room temperature, respectively.

[0007] Accordingly, technological attempts are being made to improve the capacity and output characteristics of battery cells using lithium iron phosphate as the cathode active material in low-temperature environments, such as installing a separate heater external to the battery cell and operating the heater in low-temperature environments. However, these conventional technologies suffer from the inconvenience of requiring the separate addition of a 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.

[0009] The technical idea of ​​the present invention aims to solve a problem by improving the capacity and output characteristics that are reduced at low temperatures in a battery using lithium iron phosphate.

[0010] According to one embodiment of the present invention for solving the above-described problem, a positive electrode is provided. The positive electrode includes: a positive electrode current collector; a self-temperature control layer disposed on one or both sides of the positive electrode current collector, the self-temperature control layer being disposed so as to cover a portion of the positive electrode current collector; and a positive electrode active material layer disposed on a portion of the positive electrode current collector that is not covered by the self-temperature control layer and on the self-temperature control layer, wherein the self-temperature control layer includes a PTC (Positive Temperature Coefficient) material.

[0011] In one embodiment, the positive electrode active material layer includes lithium iron phosphate as the 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 comprises 1 to 60 wt% of a polymeric material; and 1 to 60 wt% of a conductive material.

[0015] In one embodiment, the self-temperature control layer has one type of pattern shape selected from a stripe shape in which a plurality of strips are spaced apart in parallel, a grid shape in which a plurality of strips extending in the Y-axis direction intersect a plurality of strips extending in the X-axis direction, and a radial pattern in which one strip spreads out in a swirl shape.

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

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

[0018] In one embodiment, the thickness of the self-temperature control layer is selected in 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 in the range of 50 to 200 μm.

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

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

[0022] According to another embodiment of the present invention, a method for manufacturing a positive electrode for a lithium secondary battery is provided, comprising: a step of applying a PTC material composition on a positive electrode current collector; a step of applying a positive electrode slurry on the positive electrode current collector to which the PTC material composition has been applied; and a step of 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, a lithium secondary battery is provided including the positive electrode; the negative electrode; the separator; and the electrolyte.

[0024] According to exemplary embodiments of the present invention, due to a self-temperature control layer including a PTC (Positive Temperature Coefficient) material, when the battery is driven in a low-temperature environment, the heat generation of the battery can be increased, thereby improving the capacity and output characteristics of the battery, which are deteriorated at low temperatures.

[0025] FIG. 1 is a cross-sectional view of a positive electrode for a lithium secondary battery according to exemplary embodiments.

[0026] FIG. 2 is a drawing showing the arrangement of a self-temperature control layer according to exemplary embodiments.

[0027] Figures 3 and 4 are drawings showing the arrangement of a self-temperature control layer according to exemplary embodiments.

[0028] FIG. 5 is a flowchart illustrating a method for manufacturing a positive electrode for a lithium secondary battery according to exemplary embodiments.

[0029] Figure 6 is a graph showing the characteristics of PTC materials according to exemplary embodiments.

[0030] [Explanation of symbols]

[0031] 100: Bipolar

[0032] 110: Positive current collector

[0033] 120: Self-temperature control layer

[0034] 130: Positive electrode active material layer

[0035] Hereinafter, the present invention will be described in more detail to help understand the present invention.

[0036] Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0037] The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0038] Since the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art, the shapes and sizes of components in the drawings may be exaggerated, omitted, or schematically illustrated for clearer explanation. Accordingly, the sizes and proportions of each component do not fully reflect the actual sizes or proportions.

[0039] In this specification, it should be understood that the terms “include,” “comprising,” or “having” are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0040] In this specification, the term “combination(s) thereof” included in the surface of the Makushi format means one or more mixtures or combinations selected from the group consisting of the components described in the expression of the Makushi format, and means including one or more selected from the group consisting of the above components.

[0041] In this specification, the description of “A and / or B” means “A or B or both.”

[0042] In this specification, “%” means weight percent unless explicitly indicated otherwise.

[0043] In this specification, the direction of the positive electrode's electric field is defined as the X-axis direction, the direction of the positive electrode's full width is defined as the Y-axis direction, and the direction perpendicular to the plane obtained by combining the X-axis direction and the Y-axis direction is defined as the Z-axis direction.

[0044]

[0045] Cathode for lithium secondary batteries

[0046] Fig. 1 is a cross-sectional view of a positive electrode for a lithium secondary battery according to exemplary embodiments, and Fig. 2 is a drawing showing the arrangement of a self-temperature control layer according to exemplary embodiments. The upper drawing of Fig. 2 is a top view of the positive electrode, and the lower drawing of Fig. 2 is a cross-sectional view of the positive electrode taken along a dotted line in the upper drawing of Fig. 2.

[0047] Referring to these drawings, a positive electrode (100, hereinafter referred to as “positive electrode”) for a lithium secondary battery according to one embodiment of the present invention includes a positive electrode current collector (110), a self-temperature control layer (120), and a positive electrode active material layer (130). The self-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 a portion of the positive electrode current collector (110E) that is exposed and not covered by the self-temperature control layer and on the self-temperature control layer (120).

[0048] The self-temperature control layer (120) includes a PTC (Positive Temperature Coefficient) material. FIG. 6 is a graph showing the characteristics of the PTC material according to exemplary embodiments. Referring to FIG. 6, the PTC material has a positive temperature coefficient, so it has a characteristic of having low resistance in a low-temperature environment and increasing resistance as the temperature rises. Since the PTC material exhibits relatively low resistance at low temperatures, a higher current flows, generating heat. When the temperature rises due to the self-heating of the PTC material, the resistance of the PTC material increases, and the increase in resistance reduces the current flow, thereby reducing heat generation. Therefore, when current flows through the self-temperature control layer (120) including the PTC material in a low-temperature environment, the self-temperature control layer (120) can self-heat as the heat generation amount of the PTC material increases, thereby increasing the temperature of the positive electrode. Furthermore, when the temperature of the PTC material increases and reaches a specific temperature, the resistance of the PTC material dramatically increases, reducing the current flowing through the self-temperature control layer, thereby preventing overheating of the battery. As described above, the self-temperature control layer (120) has a self-heating effect when the battery is operated in a low-temperature environment, so a separate battery heating means is not necessary to increase the temperature of the battery in a low-temperature environment.

[0049] Meanwhile, since the self-temperature control layer (120) can act as a resistor in a room temperature or high temperature environment, it is preferable that the self-temperature control layer (120) be arranged to cover a part of the positive electrode current collector rather than being arranged to cover the entire front surface of the positive electrode current collector.

[0050] As described above, the positive electrode (100) according to exemplary embodiments can increase the temperature of the battery without a separate heating means by increasing the amount of heat generated by the PTC function of the self-temperature control layer when the battery is operated in a low-temperature environment. In particular, a battery using lithium iron phosphate as the positive electrode active material has a reduced capacity and output in a low-temperature environment, but according to exemplary embodiments, even if lithium iron phosphate is used as the positive electrode active material, the temperature of the battery increases in a low-temperature environment without a separate heating means, so that the capacity expression and output performance can be improved.

[0051] The positive electrode current collector is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. For example, the positive electrode current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

[0052] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance adhesion to the positive electrode active material layer. The positive electrode current collector may be used in various forms, such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

[0053] The surface roughness Ra of the positive electrode current collector is not particularly limited. In exemplary embodiments, the positive electrode current collector may have a surface roughness Ra of less than 1 μm, specifically, in the range of 50 to 900 nm, or 70 to 700 nm, or 85 to 500 nm, or 90 to 300 nm. When the surface roughness Ra of the positive electrode current collector is in the above range, the interfacial adhesive force between the positive electrode current collector and the positive electrode active material layer may be better.

[0054] Here, surface roughness refers to the degree of fine unevenness on the surface of the positive electrode collector, and can be expressed as the arithmetic mean roughness. The method for measuring surface roughness Ra is as follows. Using a laser microscope (VK-X100k, Keyence), the focus was placed on the surface of the positive electrode collector at a measurement magnification of Х150, and laser scanning was performed in Auto measure mode. The measurement standard was set according to JIS B0601:2001, and the measurement area was selected as the entire area to measure Ra for the entire area. The Ra value for each sample was expressed as the average value by measuring Ra at 10 locations on the surface of the positive electrode collector while moving the measurement position of the sample.

[0055] In the present invention, since the self-temperature control layer functions to raise the temperature of the positive electrode by self-heating at a low temperature, the temperature-raising effect can be greater as the area ratio of the portion where the self-temperature control layer comes into contact with the positive electrode active material layer is higher. In this respect, even if the surface of the positive electrode current collector has a finely uneven shape as described above, the thickness of the self-temperature control layer can have a range that can cover not only the concave portion but also the convex portion of the fine unevenness. For example, as illustrated in FIG. 1, the self-temperature control layer (120) may have at least a portion of a side surface in contact with the positive electrode active material layer (130). Here, the side surface refers to two surfaces of the self-temperature control layer (120) excluding the upper surface that is parallel to the X direction and comes into contact with the positive electrode active material layer (130) and the lower surface that is flat to the X direction and comes into contact with the positive electrode current collector (110). That is, it can be a thickness direction (Z direction) surface. In order to increase the self-heating effect by the self-temperature control layer, it is preferable that the area of ​​the self-temperature control layer (120) in contact with the positive electrode active material layer be larger.

[0056] The self-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 positive electrode current collector, the self-temperature control layer may also be disposed on both sides of the positive electrode current collector, and in the case of a single-sided positive electrode in which a positive electrode active material layer is formed on one side of the positive electrode current collector, the self-temperature control layer may also be disposed on one side of the positive electrode current collector.

[0057] The above self-temperature control layer may include a PTC material, or may be composed of a PTC material. The PTC material has a characteristic in which the resistance decreases as the temperature of the battery decreases, and the resistance increases as the temperature of the battery increases. The PTC material may have a structure in which a conductive material is dispersed in a polymer material having low electrical conductivity, and an electrical path is formed along the conductive particles. When the temperature of the PTC material increases, the volume of the polymer material expands, and in some cases, the distance between the conductive particles increases due to the flow of the conductive particles, which may increase the resistance of the PTC material. Conversely, when the temperature decreases, the volume of the polymer material decreases, which may shorten the distance between the conductive particles, which may decrease the resistance of the PTC material.

[0058] Since the self-temperature control layer can act as a resistor of the positive electrode in both room temperature and high temperature environments as it includes a PTC material, it is preferable that the self-temperature control layer be patterned so that a portion of the positive electrode current collector is exposed as shown in FIGS. 1 and 2.

[0059] Referring to FIGS. 1 and 2, the self-temperature control layer (120) may be arranged in a grid shape in which a plurality of strips (121) extending in the Y-axis direction and a plurality of strips (121') extending in the X-axis direction intersect. When the self-temperature control layer is arranged in a grid shape, it is preferable in that the resistance increase of the electrode at room temperature is not large.

[0060] Figures 3 and 4 illustrate various arrangement configurations of a self-temperature control layer according to exemplary embodiments. Referring to Figure 3, the self-temperature control layer (120) may be arranged in a stripe shape in which a plurality of strips (121) are arranged in parallel and spaced apart from each other. Referring to Figure 4, the self-temperature control layer (120) may also be arranged in a radial shape in which a single strip (121) spreads out in a vortex shape.

[0061] The above plurality of strips may be arranged at regular intervals. The interval between the strips is not particularly limited, but may be specifically 5 to 40 mm, more specifically 7 to 35 mm, even more specifically 10 to 30 mm, and even more specifically 12 to 25 mm.

[0062] In one specific example, the width length (W1) of the strip (121) may be in the range of 0.5 to 10 mm, specifically 1 to 5 mm, and more specifically 1.5 to 4 mm. When the width length of the strip is within the above numerical range, it is preferable in terms of capacity expression and output performance of the battery in a low-temperature environment.

[0063] In one specific example, the thickness of the self-temperature control layer may be 3 to 30%, specifically 4 to 25%, and more specifically 5 to 20% of the thickness of the positive electrode active material layer. Further, in some embodiments, the thickness of the self-temperature control layer may be in the range of 1 to 50 μm, or 2 to 40 μm, or 3 to 30 μm, or 5 to 20 μm. When the thickness of the self-temperature control layer is within the above range, the battery can have appropriate resistance characteristics in room temperature and high temperature environments, which is preferable.

[0064] The area A of the positive electrode current collector (110) covered by the self-temperature control layer (120) may be 50% or less, preferably 5 to 40%, and more preferably 10 to 30% of the total area B of the positive electrode current collector. When the ratio of the area A of the positive electrode current collector (110) covered by the self-temperature control layer (120) satisfies the above range, it is preferable because the capacity expression and output performance of the battery can be improved in a low-temperature environment, while also having appropriate resistance characteristics in room temperature and high-temperature environments.

[0065] In one specific example, the self-temperature control layer comprises a PTC material to exhibit PTC characteristics, and the PTC material may be a conductive material dispersed in a polymer material.

[0066] The above polymer material is not particularly limited, but may be, for example, a thermoplastic polymer, as long as it has low electrical conductivity and changes in volume with temperature change, causing a change in the conductive network by the conductive material.

[0067] The thermoplastic polymer may be a semi-crystalline material, as it may be easier to obtain PTC properties in a semi-crystalline material compared to an amorphous thermoplastic material. The semi-crystalline thermoplastic material may have a crystallinity of at least 5%, specifically at least 10%, and more specifically at least 15%.

[0068] The thermoplastic polymer is not particularly limited as long as it satisfies the above-mentioned properties, and includes, but is not limited to, high-density polyethylene, linear low-density polyethylene, low-density polyethylene, medium-density polyethylene, maleic anhydride functionalized polyethylene, maleic anhydride functionalized elastomers, ethylene copolymers (e.g., EXXELOR VA1801 and VA1803 of ExxonMobil), ethylene butene copolymers, ethylene octene copolymers, 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, and the like. Polyamides including but not limited to polyvinyl chloride (PVC), polyvinyl acetate, polyvinyl acetyl, acrylic resin, syndiotactic polystyrene (sPS), PA6, PA66, PA11, PA12, PA6T, 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 It may be at least one selected from the group consisting of alcohol (PVA), and polycaprolactone.

[0069] The above description is only one example, and it is of course possible to manufacture a PTC material using a thermosetting polymer in addition to the thermoplastic polymer.

[0070] The content of the polymer material may be 1 to 60 wt%, specifically 5 to 50 wt%, based on the total weight of the self-temperature control layer.

[0071] If the content of the polymer material is less than 1 wt%, the volume expansion of the PTC material is not large when the temperature rises, making it difficult to effectively block the current. On the other hand, if the content of the polymer material exceeds 60 wt%, the polymer material does not exist in a dispersed particle form, but rather aggregates together to exist in the form of large lumps, which causes the PTC characteristics to deteriorate.

[0072] The effective operating temperature of the above PTC material can be appropriately selected within a range that does not hinder the general use of the battery and does not impair the capacity expression and output characteristics of the battery, and may be, for example, in the range of -20°C to +100°C.

[0073] In one specific example, the PTC material may include the polymer material and the conductive material, and optionally further include a binder.

[0074] The conductive material is not particularly limited as long as it is conductive without causing a chemical change, and 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, summer black; conductive fibers such as carbon fiber or metal fiber; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc., may be used, and preferably, a conductive carbon-based material may be used. The conductive carbon-based material such as the carbon black or conductive fiber may be preferable as a conductive material included in the self-temperature control layer because it has excellent conductivity and is advantageous in forming a conductive network with the conductive material of the positive electrode active material layer.

[0075] In one specific example, the content of the conductive material may be 1 to 60 wt%, specifically 5 to 50 wt%, based on the total weight of the self-temperature control layer.

[0076] When the content of the above-mentioned conductive material is less than 1 wt%, the content of the polymer material and the binder relatively increases, thereby increasing the resistance of the electrode. When it exceeds 60 wt%, the content of the binder relatively decreases, thereby reducing the adhesive strength of the self-temperature control layer.

[0077] The above binder is not particularly limited as long as it can provide sufficient adhesive strength without causing chemical change, 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.

[0078] In one specific example, the content of the binder may be 20 wt% or less, specifically 1 to 20 wt%, and more specifically 2 to 15 wt%, based on the total weight of the self-temperature control layer. When the binder is included, the adhesive strength between the self-temperature control layer and the current collector and / or the adhesive strength between the self-temperature control layer and the positive electrode active material layer may be improved. However, when 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, resulting in a problem of deterioration in the overall performance of the battery.

[0079]

[0080] The above-described positive electrode active material layer (130) can be arranged over the positive electrode current collector portion (110E) that is exposed and not covered by the self-temperature control layer; and the self-temperature control layer (120); That is, a portion of the positive electrode active material layer can be in contact with the positive electrode current collector, and the remaining portion can be in contact with the self-temperature control layer.

[0081] The above-mentioned positive electrode active material layer includes a positive electrode active material, a positive electrode binder, and may optionally include a positive electrode conductive material, a filler, or a positive electrode dispersant, as needed.

[0082] The above positive electrode active material may include lithium iron phosphate. While lithium iron phosphate may be used alone as the positive electrode active material, it is also possible to use a small amount of lithium nickel cobalt manganese oxide mixed therewith, for example, in an amount of 0.01 to 3 wt%.

[0083] The above lithium iron phosphate may include a structure in which lithium iron phosphate is doped with a metal and a structure in which lithium iron phosphate is coated with carbon. Specifically, the lithium iron phosphate may have a composition represented by the following chemical formula 1.

[0084] [Chemical Formula 1]

[0085] Li 1+a Fe 1-x M x (PO4-b )X b

[0086] In the above chemical formula 1,

[0087] M contains 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,

[0088] X contains one or more elements selected from the group consisting of F, S and N,

[0089] a, b, and x are in the ranges of -0.5≤a≤0.5, 0≤b≤0.1, and 0≤x≤0.8, respectively.

[0090] For example, the lithium iron phosphate may be LiFePO4. As another example, the lithium iron phosphate may have a structure in which Mn is doped in LiFePO4 and / or a structure in which a carbon coating layer is formed. By applying the lithium iron phosphate as a cathode active material, battery safety can be improved and manufacturing costs can be significantly reduced.

[0091] The lithium iron phosphate may include a carbon coating layer formed on the 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 a polymer of glucose, sucrose, lactose, starch, oligosaccharide, polyoligosaccharide, fructose, cellulose, furfuryl alcohol, a block copolymer of ethylene and ethylene oxide, a vinyl resin, a cellulose resin, a phenol resin, a pitch resin, and a tar resin. Specifically, the carbon coating layer may be formed through a process of mixing raw materials with lithium iron phosphate and then heat-treating them.

[0092] The above-mentioned positive electrode binder plays a role in assisting in the bonding of the positive electrode active material and the conductive material, and in the bonding to the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof may be used.

[0093] The above-mentioned positive electrode conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery. For example, the conductive material may be a carbon black such as graphite, acetylene black, Ketjen black, channel black, furnace black, lamp black, or summer black; a conductive fiber such as carbon fiber or metal fiber; a metal powder such as fluorinated carbon, aluminum, or nickel powder; a conductive whiskey such as zinc oxide or potassium titanate; a conductive metal oxide such as titanium oxide; or a conductive material such as a polyphenylene derivative. Examples of commercially available challenge agents include the acetylene black series (Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company), Ketjenblack, the EC series (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

[0094] The above-mentioned positive electrode dispersant suppresses the phenomenon of positive electrode active material agglomeration and effectively disperses the positive electrode active material, such as lithium iron phosphate, in the positive electrode active material layer. The above-mentioned dispersant may include a hydrogenated nitrile copolymer, and specifically, the above-mentioned dispersant may be a hydrogenated nitrile copolymer.

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

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

[0097] The above positive electrode dispersant may be included in an amount of 0.1 wt% to 2.0 wt%, specifically 0.2 wt% to 1.8 wt%, and more specifically 0.3 wt% to 1.6 wt%, based on the entire positive electrode active material layer. When the content of the positive electrode dispersant satisfies the above range, it is possible to suppress aggregation of the positive electrode active material while preventing gelation of the positive electrode slurry.

[0098] In one specific example, the positive electrode active material layer may have a loading amount ranging from an average of 300 mg / 25㎠ to 600 mg / 25㎠. And the thickness of the positive electrode active material layer may be selected from a range of 50 to 200 ㎛, specifically from a range of 60 to 180 ㎛, and more specifically from a range of 70 to 160 ㎛.

[0099]

[0100] Method for manufacturing a cathode for a lithium secondary battery

[0101] FIG. 5 is a flowchart illustrating a method for manufacturing a positive electrode for a lithium secondary battery according to exemplary embodiments.

[0102] Referring to FIG. 5, a method for manufacturing a positive electrode according to exemplary embodiments may include a step of applying a PTC material composition onto a positive electrode current collector (P110); a step of applying a positive electrode slurry onto the positive electrode current collector to which the PTC material composition has been applied (P120); and a step of drying and rolling (P130). According to exemplary embodiments, the step of applying the PTC material composition (P110) may be such that the PTC material composition covers a portion of the positive electrode current collector. The PTC material composition becomes the self-temperature control layer described above through the drying and rolling processes.

[0103] The above PTC material composition may be a composition in which the above-described PTC material is mixed / stirred in a solvent. Since the PTC material has been described in detail above, a duplicate description will be omitted.

[0104] The solvent is for mixing the above-described polymer material, conductive material, and binder. The solvent may be a solvent commonly used in the relevant technical field, and examples thereof include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, acetonitrile, or water, and one of these may be used alone or as a mixture of two or more thereof. In addition, the solid content concentration of the PTC material composition may be in the range of 50 wt% to 90 wt%, preferably 60 wt% to 80 wt%.

[0105] According to one embodiment, in the process of applying the PTC composition (P110), the PTC composition is applied so that a portion of the current collector is exposed, and it is preferable to apply the PTC composition in a pattern shape to prevent uneven resistance of the electrode.

[0106] The shape of the above pattern may be one selected from stripe, grid, and radial shapes as described above.

[0107] The coating thickness of the PTC material composition, the ratio of the area on which the PTC composition is applied on the entire surface of the current collector, the width and length of the strip, etc. have been described in detail above, so a duplicate description will be omitted.

[0108] After applying the above PTC material composition, a process (P120) of applying the slurry for the positive electrode can be performed without drying the PTC material composition, and a process (P120) of applying the slurry for the positive electrode can be performed after drying the PTC material composition.

[0109] The above-mentioned positive electrode slurry may be a slurry in which the above-mentioned positive electrode active material, positive electrode binder, positive electrode conductive material, and dispersant are mixed / stirred in a solvent. The positive electrode active material, positive electrode binder, positive electrode conductive material, and dispersant have been described in detail above, so a duplicate description will be omitted.

[0110] The solvent may be a solvent commonly used in the relevant technical field, and for example, 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 may be used.

[0111] The solvent may be included in an amount such that the slurry for the positive electrode has an appropriate viscosity and solids content. For example, the solvent may be included in an amount such that the solids content in the slurry is 50 wt% to 75 wt%, specifically 50 wt% to 70 wt%, and more specifically 55 wt% to 70 wt%.

[0112] The above drying and rolling (P130) may include a process of passing the slurry for the positive electrode through a drying device to remove the solvent, and pressing the electrode to have an appropriate density.

[0113] The method of the above drying and rolling process is not particularly limited, and can be performed by a method widely known in the field of positive electrodes of secondary batteries.

[0114] A cathode manufactured in this manner can maintain a certain level of battery capacity and output performance in a low-temperature environment due to a self-temperature control layer having PTC characteristics, even when lithium iron phosphate is used as the cathode active material, so a separate battery heating means is unnecessary.

[0115]

[0116] lithium secondary battery

[0117] Next, a lithium secondary battery according to the present invention will be described.

[0118] A lithium secondary battery according to one 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.

[0119] In the lithium secondary battery, the positive electrode is as described above. For example, the positive electrode (100) includes a positive electrode current collector (110), a self-temperature control layer (120), and a positive electrode active material layer (130). The self-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 a portion of the positive electrode current collector (110E) that is exposed and not covered by the self-temperature control layer and on the self-temperature control layer (120).

[0120] The above negative electrode can 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 applying the composition on the negative electrode current collector.

[0121] The negative electrode active material is not particularly limited, and a compound capable of reversible intercalation and deintercalation of lithium can be typically 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 alloy, Sn alloy, or Al alloy; or a composite including a metallic compound and a carbonaceous material. In addition, examples of low-crystalline carbon include soft carbon and hard carbon, and examples of high-crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes. One of these may be used alone or a mixture of two or more thereof, and a metallic lithium thin film may also be used as the negative electrode active material.

[0122] The above-described negative electrode conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without any particular restrictions. 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 nanotube; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like. One of these may be used alone or a mixture of two or more may be used. The negative electrode conductive material may typically be included in an amount of 1 to 30 wt%, specifically 1 to 20 wt%, and more specifically 1 to 10 wt%, based on the total weight of the negative electrode active material layer.

[0123] The above negative electrode binder serves to improve adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the negative electrode current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof. The above negative electrode binder may be included in an amount of 1 to 30 wt%, specifically 1 to 20 wt%, and more specifically 1 to 10 wt%, based on the total weight of the negative electrode active material layer.

[0124] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used.

[0125] In addition, the negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and, like the positive electrode current collector, may have fine irregularities formed on the surface of the negative electrode current collector to enhance the bonding strength of the negative electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0126]

[0127] Meanwhile, in the lithium secondary battery, the separator may be used without any special restrictions as long as it is a separator typically used in lithium secondary batteries, and in particular, one having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, may be used. In addition, a typical porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may also be used. In addition, the separator may be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0128]

[0129] Meanwhile, in the lithium secondary battery, the electrolyte may include an organic solvent and lithium salt commonly used in electrolytes, and is not particularly limited.

[0130] The organic solvent may be used without any particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent 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; Carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC) can be used.

[0131] Among these, a carbonate solvent is preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and a linear carbonate compound having low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) is more preferable.

[0132] The lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2. LiCl, LiI, or LiB(C2O4)2. It is preferable that the lithium salt be included in the electrolyte at a concentration of approximately 0.6 mol% to 2 mol%.

[0133] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. At this time, the additives may be included in an amount of 0.1 to 5 wt% based on the total weight of the electrolyte.

[0134]

[0135] The lithium secondary battery of the present invention can be manufactured by forming an electrode assembly by placing a separator between a positive electrode and a negative electrode, placing the electrode assembly in a cylindrical or square battery case, and then injecting an electrolyte. Alternatively, the electrode assembly can be manufactured by stacking the electrode assembly, impregnating it with an electrolyte, and placing the resulting product in a battery case and sealing it.

[0136] The above battery case can be adopted as one commonly used in the field, and there is no limitation on the external shape according to the use of the battery. For example, it can be a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape.

[0137] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in electric vehicles such as energy storage systems (ESS) and hybrid electric vehicles (HEVs).

[0138]

[0139] Hereinafter, the present invention will be described in more detail by way of examples. However, the following examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention solely to these examples.

[0140]

[0141] Example 1

[0142] A current collector sheet was prepared by pattern-coating a self-temperature control layer containing a PTC material in a grid shape on a 20-μm-thick aluminum foil as shown in FIGS. 1 and 2. The self-temperature control layer had a thickness of 10 μm, a width of 2 mm, a gap between multiple strips of 20 mm, and an area of ​​aluminum foil covered by the self-temperature control layer of 22%. A cathode slurry was applied onto the current collector sheet, dried, and rolled to complete the manufacture of the cathode. The cathode slurry was prepared by mixing LiFePO4 as a cathode active material, PVDF as a binder, and carbon black as a conductive material in a weight ratio of 96:2:2 in an NMP solvent, and the thickness of the cathode active material layer after drying the cathode slurry was 100 μm.

[0143]

[0144] Example 2

[0145] An anode was manufactured in the same manner as in Example 1, except that the coating pattern of the self-temperature control layer was striped as shown in Fig. 3. At this time, the area of ​​the aluminum foil covered by the self-temperature control layer was 22%, and the thickness of the self-temperature control layer was 10 μm.

[0146]

[0147] Example 3

[0148] A current collector sheet was prepared in which a self-temperature control layer containing a PTC material was pattern-coated in a grid shape on a 20 ㎛ thick aluminum foil, and the current collector sheet was prepared in which the area of ​​the aluminum foil covered by the self-temperature control layer was 50%. Thereafter, a positive electrode slurry having the same composition as in Example 1 was applied, and dried and rolled under the same conditions as in Example 1 to manufacture a positive electrode.

[0149]

[0150] Comparative example

[0151] In Example 1, a current collector without a self-temperature control layer formed was prepared, and a positive electrode slurry having the same composition as in Example 1 was applied on the current collector, and dried and rolled under the same conditions as in Example 1 to manufacture a positive electrode.

[0152]

[0153] Experimental Example 1: Low-Temperature Capacity Evaluation

[0154] Artificial graphite was used as the negative active material, carbon black as the conductive material, and styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) as the binder. The negative active material, conductive material, and binder were mixed and stirred in water at 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.

[0155] The positive and negative electrodes manufactured in Examples 1 to 3 and Comparative Examples were respectively placed in opposition to each other, and a separator made of 18 μm polypropylene was interposed therebetween to manufacture an electrode assembly. Each manufactured electrode assembly was inserted into a pouch-type battery case, an electrolyte composition was injected into the battery case, and the case was sealed to manufacture a secondary battery. At this time, as the electrolyte composition, a solution was used in which lithium hexafluorophosphate (LiPF6, 1.0 M) and vinyl carbonate (VC, 2 wt%) were mixed in a mixture of ethylene carbonate (EC): dimethyl carbonate (DMC): diethyl carbonate (DEC) = 1:1:1 (volume ratio).

[0156] For each manufactured secondary battery, it was charged at a temperature of -10℃ with a constant current of 0.33C until the voltage reached 4.2V, and then discharged at a constant current of 0.33C until the voltage reached 3.0V. Here, the "C" is a unit of charge / discharge rate, which is the current (A) during charge / discharge divided by the rated capacity (Ah) of the secondary battery. For example, 1C means a charge / discharge rate that takes 1 hour to fully charge or fully discharge the secondary battery.

[0157] The discharge capacity during discharge was measured and the results are shown in Table 1. The values ​​listed in Table 1 are expressed as relative ratios based on the capacity of the secondary battery including the positive electrode of Example 1 as 100%.

[0158]

[0159] Experimental Example 2: Resistance Evaluation

[0160] For each secondary battery manufactured in the above Experimental Example 1, the resistance was measured at a temperature of -10℃ by the following method. The battery was charged with a current of 0.05C from SOC 0% to SOC 30%. Thereafter, a pulse was given by applying a current of 0.5C for 30 seconds at SOC 30%. Here, SOC (State of Charge) is the percentage of the charge capacity with respect to the total capacity of the secondary battery, and the "C" of 0.5C is the unit of charge / discharge rate as described in the above Experimental Example 1. The DCIR was calculated and the results are shown in Table 1.

[0161] Resistance was measured using the same method as above at a temperature of 25℃, and the results are shown in Table 1.

[0162] Low Temperature Discharge Capacity (%, vs. Example 1) DCIR (mΩ) Low Temperature (-10°C) Room Temperature (25°C) Example 1 100% 9.6 2.01 Example 2 90% 11.2 1.97 Example 3 120% 8.1 2.19 Comparative Example 50% 19.8 1.76

[0163] A secondary battery manufactured using a comparative positive electrode that does not include a self-temperature control layer according to the present invention has inferior low-temperature capacity and resistance characteristics compared to the secondary batteries manufactured using the positive electrodes of Examples 1 to 3.

[0164] The secondary battery manufactured using the positive electrode of Example 3, in which the self-temperature control layer covers 50% of the positive electrode current collector area, exhibited the best low-temperature capacity and low-temperature characteristics, but the worst room-temperature resistance characteristics. Therefore, in the present invention, it is desirable to adjust the application area of ​​the self-temperature control layer to an appropriate level.

[0165] In this way, the positive electrode and secondary battery according to the present invention were found to be excellent in terms of capacity expression even at low temperatures without a separate battery heating means, and the resistance characteristics at room temperature were not significantly reduced compared to the conventional positive electrode.

[0166]

[0167] The present invention has been described in more detail through the drawings and examples. However, the configurations described in the drawings or examples described in this specification are merely embodiments of the present invention and do not represent all of the technical ideas of the present invention. Therefore, it should be understood that various equivalents and modified examples may exist as of the time of this application.

Claims

1. Bipolar collector; A self-temperature control layer disposed on one or both sides of the positive electrode collector, so as to cover a portion of the positive electrode collector; and A positive electrode current collector portion exposed and not covered by the self-temperature control layer; and a positive electrode active material layer disposed on the self-temperature control layer; A positive electrode for a lithium secondary battery, characterized in that the self-temperature control layer includes a PTC (Positive Temperature Coefficient) material.

2. In claim 1, the cathode active material layer is a cathode for a lithium secondary battery, which contains lithium iron phosphate as the cathode active material.

3. A positive electrode for a lithium secondary battery according to claim 1, wherein the PTC material is characterized by a conductive material dispersed in a polymer material.

4. A positive electrode for a lithium secondary battery, characterized in that in claim 3, the conductive material is a conductive carbon-based material.

5. In claim 3, The self-temperature control layer is 1 to 60 wt% of a polymeric material; and 1 to 60 wt% of a challenging material; A cathode for a lithium secondary battery comprising:

6. A positive electrode for a lithium secondary battery according to claim 1, wherein the self-temperature control layer has one type of pattern shape selected from a stripe shape in which a plurality of strips are spaced apart in parallel, a lattice shape in which a plurality of strips extending in the Y-axis direction and a plurality of strips extending in the X-axis direction intersect, and a radial pattern in which one strip spreads out in a spiral shape.

7. A positive electrode for a lithium secondary battery according to claim 6, characterized in that the self-temperature control layer has the grid-shaped pattern.

8. A positive electrode for a lithium secondary battery according to claim 6, wherein the width and length of the strip are selected in a range of 0.5 to 10 mm.

9. In claim 1, A lithium secondary battery cathode, wherein the thickness of the self-temperature control layer is selected in a range of 3 to 30% of the thickness of the cathode active material layer.

10. In claim 1, A positive electrode for a lithium secondary battery, wherein the thickness of the positive electrode active material layer is selected in the range of 50 to 200 ㎛.

11. In claim 1, A positive electrode for a lithium secondary battery, wherein the area A of the positive electrode current collector covered by the self-temperature control layer is 50% or less of the total area B of the positive electrode current collector.

12. In claim 1, A positive electrode for a lithium secondary battery, characterized in that at least a portion of a side surface of the self-temperature control layer is in contact with the positive electrode active material layer.

13. Process of applying a PTC material composition on a positive electrode collector; A process of applying a slurry for a cathode on a cathode current collector to which the above PTC material composition is applied; and Including the process of drying and rolling, A method for manufacturing a positive electrode for a lithium secondary battery, characterized in that the process of applying the above PTC material composition is such that the PTC material composition covers a portion of a positive electrode current collector.

14. An anode according to any one of claims 1 to 12; cathode; Membrane; and A lithium secondary battery containing an electrolyte.

Images