Positive electrode for lithium secondary battery, method for manufacturing same, and lithium secondary battery comprising same

Abstract

The present disclosure provides a positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising: a positive electrode current collector; the automatic temperature control layer is arranged on one or two surfaces of the positive electrode current collector, but is arranged to cover one part of the positive electrode current collector; and a positive electrode active material layer disposed on the exposed positive electrode current collector region not covered by the self-temperature control layer and on the self-temperature control layer, in which the self-temperature control layer includes a positive temperature coefficient (PTC) material.

Description

Technical Field

[0001] This application claims the priority benefit of Korean Patent Application No. 10-2023-0105428, filed on August 11, 2023, the disclosure of which is incorporated herein by reference.

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

[0003] With the technological development and increasing demand for electric vehicles and energy storage systems (ESS), the demand for batteries as an energy source is rapidly growing, and various studies are being conducted on batteries that can meet various requirements. In particular, as a power source for these devices, active research is being conducted on lithium secondary batteries having a high energy density and excellent life and cycle characteristics.

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

[0005] Since lithium iron phosphate contains iron, a material that is rich in resources and low in cost, its cost is low. In addition, lithium iron phosphate has low toxicity, meaning that its use reduces environmental pollution. In addition, since lithium iron phosphate has an olivine structure, its active material structure can remain stable at high temperatures compared to lithium transition metal oxides having a layered structure. As a result, it has the advantages of excellent high-temperature stability and high-temperature life characteristics of the battery.

[0006] However, since lithium iron phosphate has a high resistance, a battery cell using lithium iron phosphate has characteristics of reduced capacity and power output at low temperatures compared to battery cells using other positive electrode active materials. Specifically, a battery cell using nickel-cobalt-manganese oxide (LNCMO) as a positive electrode active material has a charging capacity and a discharging capacity at minus 10 degrees Celsius of about 73% of the charging capacity at room temperature and about 73% of the discharging capacity at room temperature, respectively, while a battery cell using lithium iron phosphate as a positive electrode active material has a charging capacity and a discharging capacity at minus 10 degrees Celsius of about 51% of the charging capacity at room temperature and about 50% of the discharging 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, such as separately installing a heater outside the battery cell and driving the heater in a low-temperature environment. However, these existing technologies require adding a separate heating film and controller.

[0008] Therefore, technologies need to be developed to improve the poor low-temperature characteristics of battery cells using lithium iron phosphate. Summary of the Invention

[0009] Technical Problem

[0010] The technical idea of the present invention aims to solve the problems of reduced capacity and power output characteristics of batteries using lithium iron phosphate at low temperatures.

[0011] Technical Solution

[0012] According to an embodiment of the present invention, to solve the above problems, a positive electrode is provided. The positive electrode includes:

[0013] A positive electrode current collector;

[0014] A self-regulating temperature layer, the self-regulating temperature layer is disposed on one or two surfaces of the positive electrode current collector, but is arranged to cover a part of the positive electrode current collector; and

[0015] A positive electrode active material layer, the positive electrode active material layer is disposed on the exposed positive electrode current collector area not covered by the self-regulating temperature layer and on the self-regulating temperature layer, wherein

[0016] The self-regulating temperature layer includes a positive temperature coefficient (PTC) material.

[0017] In one embodiment, the positive electrode active material layer includes lithium iron phosphate as the positive electrode active material.

[0018] In one embodiment, the PTC material has a structure in which a conductive material is dispersed in a polymer material.

[0019] In one embodiment, the conductive material is a conductive carbon material.

[0020] In one embodiment, the self-regulating temperature layer includes 1 to 60% by weight of a polymer material and 1 to 60% by weight of a conductive material.

[0021] In one embodiment, the self-regulating temperature layer has a pattern shape selected from the following patterns: a strip pattern in which a plurality of strips are arranged side by side at intervals, a grid 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 a strip unfolds in a spiral pattern.

[0022] In one embodiment, the self-regulating temperature layer has a grid pattern.

[0023] In one embodiment, the width of the strip is selected in the range of 0.5 to 10 mm.

[0024] In one embodiment, the thickness of the self - temperature - controlling layer is selected within the range of 3% to 30% of the thickness of the positive electrode active material layer.

[0025] In one embodiment, the thickness of the positive electrode active material layer is selected within the range of 50 to 200 μm.

[0026] In one embodiment, the area A of the positive electrode current collector covered by the self - temperature - controlling layer is less than or equal to 50% of the total area B of the positive electrode current collector.

[0027] In one embodiment, at least a part of the side surface of the self - temperature - controlling layer can be in contact with the positive electrode active material layer.

[0028] 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:

[0029] Coating a PTC material composition on a positive electrode current collector;

[0030] Coating a positive electrode paste onto the positive electrode current collector coated with the PTC material composition; and

[0031] Drying and rolling, wherein

[0032] The step of coating the PTC material composition is characterized in that the PTC material composition is coated to cover a part of the positive electrode current collector.

[0033] According to still another embodiment of the present invention, there is provided a lithium secondary battery, the lithium secondary battery comprising:

[0034] A positive electrode;

[0035] A negative electrode;

[0036] A separator; and

[0037] An electrolyte.

[0038] Advantageous Effects

[0039] According to an exemplary embodiment of the present invention, the self - temperature - controlling layer containing a positive temperature coefficient (PTC) material can increase the heat generation amount of the battery when the battery operates in a low - temperature environment, thereby improving the capacity and power output characteristics of the battery reduced at low temperatures. Brief Description of the Drawings

[0040] Figure 1 It is a cross - sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0041] Figure 2 It is a diagram showing the arrangement of the self - temperature - controlling layer according to an exemplary embodiment.

[0042] Figures 3 to 4A diagram showing the arrangement of a self-regulating temperature layer according to an exemplary embodiment.

[0043] Figure 5 A flowchart showing a method for manufacturing a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0044] Figure 6 A diagram showing the properties of a PTC material according to an exemplary embodiment.

[0045] [Description of Reference Numerals]

[0046] 100: Positive electrode

[0047] 110: Positive electrode current collector

[0048] 120: Self-regulating temperature layer

[0049] 130: Positive electrode active material layer Detailed Description

[0050] Hereinafter, the present invention will be described in more detail to help understand the present invention.

[0051] The terms and words used in this specification and claims should not be construed as limited to their ordinary or dictionary meanings, but should be interpreted on the basis that the inventor can appropriately define the concept of the terms to best explain the principles of his invention. Therefore, they should be understood in a meaning and sense consistent with the technical idea of the present invention.

[0052] The terms used in this specification are for the purpose of describing exemplary embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, singular expressions include plural expressions.

[0053] Since the embodiments of the present invention are provided to more comprehensively illustrate the present invention to those of ordinary skill in the art, for clarity, the shapes and sizes of the components in the drawings may be exaggerated, omitted, or shown schematically. Therefore, the dimensions or ratios of the components do not necessarily represent their actual dimensions or ratios.

[0054] In this specification, terms such as "comprising", "including", or "having" clearly indicate the presence of the stated features, numbers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0055] As used herein, the term "combinations thereof" in the Markush format means one or more mixtures or combinations selected from the components described in the Markush format, and includes one or more selected from the components.

[0056] In this specification, the reference to "A and / or B" shall mean "A or B or both".

[0057] In this specification, unless otherwise clearly stated, "%" shall mean weight %.

[0058] As used herein, the longitudinal direction of the entire positive electrode is defined as the X-axis direction, the lateral direction of the entire positive electrode is defined as the Y-axis direction, and the direction perpendicular to the plane combined with the X-axis direction and the Y-axis direction is defined as the Z-axis direction.

[0059] Positive electrode for lithium secondary battery

[0060] Figure 1 is a cross-sectional view of a positive electrode for a lithium secondary battery according to an exemplary embodiment, Figure 2 is a view showing the arrangement of a self-regulating temperature layer according to an exemplary embodiment. Figure 2 The upper figure of shows a top view of the positive electrode, Figure 2 The lower figure of shows along Figure 2 a cross-sectional view of the positive electrode taken along the dashed line in the upper figure.

[0061] Referring to these drawings, a positive electrode (100, hereinafter referred to as "positive electrode") for a lithium secondary battery according to an embodiment of the present invention includes a positive electrode current collector (110), a self-regulating temperature layer (120), and a positive electrode active material layer (130), wherein the self-regulating temperature layer (120) can be arranged to cover a part of the positive electrode current collector (110), and the positive electrode active material layer (130) can be arranged on the exposed positive electrode current collector region (110E) not covered by the self-regulating temperature layer and on the self-regulating temperature layer (120).

[0062] The self-regulating temperature layer 120 contains a positive temperature coefficient (PTC) material. Figure 6 is a view showing the properties of the PTC material according to an exemplary embodiment. Referring to Figure 6 , the PTC material has a constant temperature coefficient, characterized in that it has a low resistance in a low-temperature environment and its resistance increases as the temperature rises. Since the PTC material exhibits a relatively low resistance at low temperatures, a higher current can flow through them, generating heat.

[0063] 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 flow of current, thereby reducing heat generation. Therefore, when the self-regulating temperature layer 120 containing the PTC material passes an electric current in a low-temperature environment, the self-regulating temperature layer 120 can self-heat as the heat generation of the PTC material increases, thereby causing the temperature of the positive electrode to rise. In addition, when the temperature of the PTC material rises and reaches a certain temperature, the resistance of the PTC material increases sharply, which reduces the current flowing through the self-regulating temperature layer, thereby preventing the battery from overheating. As described above, the self-regulating temperature layer 120 has a self-heating effect when the battery operates in a low-temperature environment, so a separate battery heating device is not required to raise the temperature of the battery in a low-temperature environment.

[0064] On the other hand, since the self-regulating temperature layer 120 can act as a resistor in a room-temperature or high-temperature environment, the self-regulating temperature layer 120 is preferably arranged to cover a part of the positive electrode current collector, rather than covering the entire surface of the positive electrode current collector.

[0065] When the battery operates in a low-temperature environment, the positive electrode 100 according to the above exemplary embodiment can increase the heat generation through the PTC function of the self-regulating temperature layer, thereby raising the temperature of the battery without a separate heating device. In particular, the capacity and output of a battery having lithium iron phosphate as the positive electrode active material decrease in a low-temperature environment, but according to the exemplary embodiment, even when lithium iron phosphate is applied as the positive electrode active material, the temperature of the battery can be raised without a separate heating device in a low-temperature environment, thereby improving the capacity performance and output performance.

[0066] The positive electrode current collector can be, but is not limited to, any material having conductivity and not causing chemical changes in the battery. For example, as the current collector, stainless steel, aluminum, nickel, titanium, calcined carbon can be used; or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

[0067] The thickness of the positive electrode current collector can be 3 μm to 500 μm, and fine irregularities can also be provided on the surface of the positive electrode current collector to increase the adhesion to the positive electrode active material layer. For example, it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0068] There is no particular limitation on the surface roughness Ra of the positive electrode current collector. In the exemplary embodiment, the surface roughness Ra of the positive electrode current collector can be less than 1 μm, more 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 within the above range, the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer can be better.

[0069] Here, the surface roughness refers to the degree of minute unevenness on the surface of the positive current collector, which can be represented by the arithmetic mean roughness. The surface roughness Ra is measured as follows. Using a laser microscope (VK-X100k, Keyence), after focusing on the surface of the positive current collector at a magnification of ×150, laser scanning is performed in the automatic measurement mode. The measurement standard is set according to JIS B0601:2001, and the measurement area is selected as the entire area to measure Ra over the entire area. The Ra value of each sample is determined by measuring Ra at 10 positions on the surface of the positive current collector while moving the measurement position of the sample, and is expressed as the average value.

[0070] Since the self-regulating temperature layer in the present invention functions to increase the temperature of the positive electrode by self-heating at low temperatures, the larger the area ratio of the area where the self-regulating temperature layer contacts the positive electrode active material layer, the greater the temperature increase effect. In this regard, even if the surface of the positive current collector has the minute uneven shape as described above, the thickness of the self-regulating temperature layer can be in a range where it can cover the concave and convex portions of the minute unevenness. For example, as Figure 1 shown, at least a part of the side surface of the self-regulating temperature layer 120 can be adjacent to the positive electrode active material layer 130. In the self-regulating temperature layer 120, the side surface refers to the other two surfaces except for the upper surface parallel to the X direction and adjacent to the positive electrode active material layer 130 and the lower surface parallel to the X direction and adjacent to the positive current collector 110, and can be the side surfaces in the thickness direction (Z direction). To increase the self-heating effect of the self-regulating temperature layer, it is desirable that the area where the self-regulating temperature layer 120 is adjacent to the positive electrode active material layer is larger.

[0071] The self-regulating temperature layer 120 can be provided on one or both surfaces of the positive current collector 110. In the case of a double-sided positive electrode in which the positive electrode active material layer is formed on both surfaces of the current collector, the self-regulating temperature layer can be provided on both surfaces of the positive current collector, and in the case of a single-sided positive electrode in which the positive electrode active material layer is formed on one surface of the current collector, the self-regulating temperature layer can be provided on one surface of the positive current collector.

[0072] The self-regulating temperature layer can contain a PTC material or can be composed of a PTC material. The PTC material has the property that its resistance decreases as the temperature of the battery decreases and its resistance increases as the temperature of the battery increases. The PTC material can be a structure in which a conductive material is dispersed in a polymer material having a low conductivity, and an electric conduction path is formed along the conductive particles. Because of the volume expansion of the polymer material and in some cases the flow of the conductive particles, which increases the distance between the conductive particles, the resistance of the PTC material can increase as the temperature rises. Conversely, when the temperature decreases, because the volume of the polymer material decreases, the distance between the conductive particles may decrease, which can reduce the resistance of the PTC material.

[0073] Since the self - regulating layer can be used as a resistor of the positive electrode at room temperature and high temperature because it contains PTC material, it is preferred to pattern the self - regulating layer to expose a part of the positive electrode current collector, as Figure 1 and 2 shown.

[0074] Referring to Figure 1 and 2 ,the self - regulating layer 120 can be arranged in a grid pattern, where a plurality of strips 121 extending in the y - axis direction intersect with a plurality of strips 121' extending in the x - axis direction. When the self - regulating layer is arranged in a grid pattern, it is preferred because the resistance of the electrode increases little at room temperature.

[0075] Figure 3 and 4 show various arrangements of the self - regulating layer according to exemplary embodiments. Referring to Figure 3 ,the self - regulating layer 120 can be arranged in a strip pattern, where a plurality of strips 121 are spaced apart from each other. Referring to Figure 4 ,the self - regulating layer 120 can be arranged radially, where one strip 121 unfolds in a cyclone pattern.

[0076] The plurality of strips can be spaced apart from each other at a certain interval. There is no particular limitation on the mutual interval between the strips, but it can be in the range of 5 to 40 mm, more particularly in the range of 7 to 35 mm, more particularly in the range of 10 to 30 mm, more particularly in the range of 12 to 25 mm.

[0077] In a specific example, the width W1 of the strip 121 can be in the range of 0.5 to 10 mm, more specifically in the range of 1 to 5 mm, even more specifically in the range of 1.5 to 4 mm. When the width of the strip is within the above - mentioned numerical range, it is preferred in terms of the capacity performance and output performance of the battery in a low - temperature environment.

[0078] In a specific example, the thickness of the self - regulating layer can be 3% to 30% of the thickness of the positive electrode active material layer, more specifically 4% to 25%, more specifically 5% to 20%. In addition, in some embodiments, the thickness of the self - regulating layer can 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 - regulating layer is within the above - mentioned range, it is preferred because the battery can have appropriate resistance characteristics at room temperature and high temperature.

[0079] The area A of the positive current collector 110 covered by the self - controlled temperature layer 120 can be 50% or less of the total area B of the positive current collector, preferably 5% to 40%, more preferably 10% to 30%. When the proportion of the area A of the positive current collector 110 covered by the self - controlled temperature layer 120 satisfies the above range, it is expected to improve the capacity performance and output performance of the battery in a low - temperature environment, while still having appropriate resistance characteristics at room temperature and high - temperature environments.

[0080] In a specific example, the self - controlled temperature layer contains a PTC material to exhibit PTC properties, where the PTC material can be a conductive material dispersed in a polymer material.

[0081] The polymer material can be, but is not particularly limited to, for example, a thermoplastic polymer, provided that it has low electrical conductivity and its volume changes with temperature, thereby causing a change in the conductive network of the conductive material.

[0082] The thermoplastic polymer can be a semi - crystalline material because it is easier to obtain PTC properties in semi - crystalline materials compared to amorphous thermoplastic materials. The crystallinity of the semi - crystalline thermoplastic material can be 5% or more, more particularly 10% or more, more particularly 15% or more.

[0083] For example, the thermoplastic polymer can be, but is not particularly limited to, one or more selected from the following: high - density polyethylene; linear low - density polyethylene; low - density polyethylene; medium - density polyethylene; maleic anhydride - functionalized polyethylene; maleic anhydride - functionalized elastomer; ethylene copolymers (such as EXXELOR VA1801 and VA1803 from ExxonMobil); ethylene - butene copolymer; ethylene - octene copolymer; 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; polyethylene acetyl; acrylic resins; syndiotactic polystyrene (sPS); polyamides, including but not limited to PA6, PA66, PA11, PA12, PA6T, PA9T; polytetrafluoroethylene (polytetrafluoroethylene: PTFE); polybutylene terephthalate (PBT); polyphenylene sulfide (PPS); polyamideimide; polyimide; ethylene - vinyl acetate (EVA); glycidyl methacrylate - modified ethylene - vinyl acetate; polymethyl methacrylate (PMMA); and polyisobutylene; polyvinylidene chloride; polyvinylidene fluoride (PVDF); poly(methyl acrylate); polyacrylonitrile; polybutadiene; polyethylene terephthalate (PET); poly(8 - aminocaprylic acid); polyvinyl alcohol (PVA); and polycaprolactone.

[0084] The base material is only for example. Of course, in addition to thermoplastic polymers, thermosetting polymers can also be used to manufacture PTC materials.

[0085] Based on the total weight of the self-regulating temperature layer, the content of the polymer material can be 1 to 60% by weight, more specifically 5 to 50% by weight.

[0086] If the content of the polymer material is less than 1% by weight, the volume expansion of the PTC material when the temperature rises is not large, and it is difficult to effectively block the current. On the other hand, if the content of the polymer material is greater than 60% by weight, the polymer materials do not exist in the form of particles dispersed, but agglomerate with each other and exist in the form of large blocks, which leads to the deterioration of PTC performance.

[0087] The effective operating temperature of the PTC material can be appropriately selected within the range that does not damage the capacity performance and output characteristics of the battery and does not damage the general use of the battery. For example, it can be within the range of -20°C to +100°C.

[0088] In a specific example, the PTC material can include a polymer material and a conductive material, and can also optionally include an adhesive.

[0089] There is no particular limitation on the conductive materials as long as they are conductive without causing chemical changes. For example, graphite such as natural graphite or artificial graphite; carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, etc.; conductive fibers such as carbon fibers or metal fibers; fluorocarbons; metal powders such as aluminum powder, nickel powder; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. Conductive carbon materials are preferred. Conductive carbon materials such as carbon black and conductive fibers can be preferably used as the conductive materials contained in the self-regulating temperature layer because they have excellent conductivity and are beneficial to forming a conductive network with the conductive materials of the positive electrode active material layer.

[0090] In a specific example, based on the total weight of the self-regulating temperature layer, the content of the conductive material can be 1 to 60% by weight, more specifically 5 to 50% by weight.

[0091] When the content of the conductive material is less than 1% by weight, the content of the polymer material and the adhesive increases relatively, increasing the resistance of the electrode. When the content of the conductive material is greater than 60% by weight, the content of the polymer material and the adhesive decreases relatively, reducing the adhesion of the self-regulating temperature layer.

[0092] There is no particular limitation on the binder as long as it can provide sufficient adhesiveness without causing chemical changes. For example, it can be one or more selected from the following substances: polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, and various copolymers.

[0093] In a specific example, based on the total weight of the self-regulating temperature layer, the content of the binder can be 20% by weight or less, more specifically 1 to 20% by weight, and even more specifically 2 to 15% by weight. The inclusion of the binder can improve the adhesiveness between the self-regulating temperature layer and the current collector and / or the adhesiveness between the self-regulating temperature layer and the positive electrode active material layer. However, if the content of the binder is greater than 20% by weight, the resistance of the positive electrode may increase, which may lead to an increase in the internal resistance of the battery, thereby reducing the overall performance of the battery.

[0094] The positive electrode active material layer 130 can be disposed above the exposed positive electrode current collector portion 110E not covered by the self-regulating temperature layer 120 and the self-regulating temperature layer 120. That is, a part of the positive electrode active material layer can be in contact with the surface of the positive electrode current collector, and the remaining part can be in contact with the surface of the self-regulating temperature layer.

[0095] The positive electrode active material layer can contain a positive electrode active material, a positive electrode binder, and can optionally contain a positive electrode conductive material, a filler, or a positive electrode dispersant as needed.

[0096] The positive electrode active material can contain lithium iron phosphate. Lithium iron phosphate can be used alone as the positive electrode active material, but a small amount, for example 0.01 to 3% by weight, of lithium nickel cobalt manganese oxide can also be used in admixture.

[0097] The lithium iron phosphate can include a form in which the lithium iron phosphate is doped with a metal and a structure in which the lithium iron phosphate is coated with carbon. Specifically, the lithium iron phosphate can have a composition represented by Chemical Formula 1 below.

[0098] [Chemical Formula 1]

[0099] Li 1+a Fe 1-x M x (PO 4-b )X b

[0100] In Chemical Formula 1,

[0101] M contains any one or more elements selected from the following: Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y,

[0102] X contains one or more elements selected from the following: F, S, and N, and

[0103] a, b, and x are respectively in the ranges of -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, and 0 ≤ x ≤ 0.8.

[0104] For example, lithium iron phosphate can be LiFePO 4 . As another example, lithium iron phosphate can have a structure in which LiFePO 4 is doped with Mn and / or has a structure with a carbon coating formed thereon. By applying 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.

[0105] Lithium iron phosphate can include a carbon coating formed on its surface. When a carbon coating is formed on the surface of lithium iron phosphate, the conductivity can be improved, thereby improving the resistance characteristics of the positive electrode. The carbon coating can be formed from at least one of the following substances: glucose, sucrose, lactose, starch, oligosaccharide, polysaccharide, fructose, cellulose, furfuryl alcohol polymer, block copolymer of ethylene and ethylene oxide, vinyl resin, cellulose resin, phenolic resin, asphalt resin, and tar resin. Specifically, the carbon coating can be formed by mixing the raw materials with lithium iron phosphate and then performing heat treatment.

[0106] The positive electrode binder is used to assist the binding of the positive electrode active material to the conductive material and the binding of the conductive material to the current collector. Specific examples include: polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, or various copolymers thereof, and any one of them can be used alone or a mixture of two or more of the foregoing can be used.

[0107] There is no particular limitation on the positive electrode conductive material as long as it is conductive and does not cause chemical changes in the battery. For example, the positive electrode conductive material can be: graphite; carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, etc.; conductive fibers such as carbon fibers or metal fibers; fluorocarbons; metal powders such as aluminum powder, nickel powder; conductive whiskers such as zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide; or polyphenylene derivatives. Examples of commercially available conductive materials include: Acetylene Black series (products from Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenback, EC series (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

[0108] The positive electrode dispersant inhibits the agglomeration of the positive electrode active material and can effectively disperse the positive electrode active material such as lithium iron phosphate in the positive electrode active material layer. The dispersant can include hydrogenated nitrile copolymers, and more particularly, the dispersant can be a hydrogenated nitrile copolymer.

[0109] Specifically, the hydrogenated nitrile copolymer can 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. As the α,β-unsaturated nitrile monomer, for example, acrylonitrile or methacrylonitrile can be used, and one of them can be used alone or a mixture of two or more of them can 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-methylbutadiene can be used, and one of them can be used alone or a mixture of two or more of them can be used.

[0110] More specifically, the hydrogenated nitrile copolymer can be hydrogenated nitrile rubber (H-NBR).

[0111] Based on the entire positive electrode active material layer, the content of the positive electrode dispersant can be 0.1 wt% to 2.0 wt%, more particularly 0.2 wt% to 1.8 wt%, and more particularly 0.3 wt% to 1.6 wt%. When the content of the positive electrode dispersant satisfies the above range, it is possible to inhibit the agglomeration of the positive electrode active material while preventing the gelation of the positive electrode slurry.

[0112] In a specific example, the average loading of the positive electrode active material layer can be in the range of 300 mg / 25 cm 2 to 600 mg / 25 cm 2 The thickness of the positive electrode active material layer can be selected in the range of 50 to 200 μm, more particularly in the range of 60 to 180 μm, and more particularly in the range of 70 to 160 μm.

[0113] Method for manufacturing a positive electrode for a lithium secondary battery

[0114] Figure 5 FIG. is a flowchart showing a method for manufacturing a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0115] Referring to Figure 5 , the method for manufacturing a positive electrode according to an exemplary embodiment may include:

[0116] Coating a PTC material composition onto a positive electrode current collector (P110);

[0117] Coating a positive electrode slurry onto the positive electrode current collector (P120) coated with the PTC material composition; and

[0118] Drying and rolling (P130).

[0119] According to an exemplary embodiment, the step (P110) of coating the PTC material composition may include coating the PTC material composition to cover a part of the positive electrode current collector. The PTC material composition is dried and rolled to become the above-mentioned self-regulating temperature layer.

[0120] The PTC material composition may be a composition in which the aforementioned PTC material is mixed / stirred in a solvent. The PTC substance has been described in detail previously, so redundant descriptions are omitted.

[0121] The solvent is used to mix the above polymer material, conductive material, and binder. The solvent can be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, acetonitrile, or water, and can be used alone or as a mixture of two or more of the foregoing. The solids concentration of the PTC material composition can be in the range of 50 wt% to 90 wt%, preferably 60 wt% to 80 wt%.

[0122] According to one embodiment, in the step P110 of coating the PTC composition, the PTC composition is coated so that certain regions of the current collector are exposed, but preferably coated in a patterned shape to prevent non-uniform resistance of the electrode.

[0123] The shape of the pattern can be selected from one of the strip shape, grid shape, and radial shape as described above.

[0124] The thickness of the PTC material composition, the ratio of the area covered by the PTC composition on the entire surface of the current collector, the width of the strip, etc. have been described in detail previously, and thus will not be repeated.

[0125] After coating the PTC material composition, the step P120 of coating the positive electrode paste can be carried out without drying the PTC material composition, or the step P120 of coating the positive electrode paste can be carried out after drying the PTC material composition.

[0126] The positive electrode paste can be a paste in which the above-mentioned positive electrode active material, positive electrode binder, positive electrode conductive material, and dispersant are mixed / stirred in a solvent. Since the positive electrode active material, positive electrode binder, positive electrode conductive material, and dispersant have been described in detail previously, redundant descriptions will be omitted.

[0127] The solvent can be any solvent commonly used in the art, and can be, for example, one or a mixture of two or more selected from the following substances: dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, and water.

[0128] The content of the solvent can provide appropriate viscosity and solid content for the positive electrode paste. For example, the content of the solvent can be such that the solid content in the paste is 50% to 75% by weight, more particularly 50% to 70% by weight, and more particularly 55% to 70% by weight.

[0129] Drying and rolling P130 can include passing the positive electrode paste through a drying device to remove the solvent, and pressing the electrode to an appropriate density.

[0130] There is no particular limitation on the method of the drying and rolling step, and it can be carried out by any method known in the field of positive electrodes for secondary batteries.

[0131] Even when lithium iron phosphate is used as the positive electrode active material, the positive electrode manufactured in this way does not require a separate device for heating the battery, because the self-regulating temperature layer with PTC characteristics can keep the capacity and output performance of the battery at a specific level in a low-temperature environment.

[0132] Lithium secondary battery

[0133] Next, the lithium secondary battery according to the present invention will be described.

[0134] The 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.

[0135] In a 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 can be arranged to cover a part of the positive electrode current collector 110, and the positive electrode active material layer 130 can be arranged on the exposed positive electrode current collector portion 110E not covered by the self-temperature control layer and on the self-temperature control layer 120.

[0136] For example, the negative electrode can be prepared by preparing a negative electrode forming composition containing a negative electrode active material, a negative electrode binder, and a negative electrode conductive material, and then coating the composition onto a negative electrode current collector.

[0137] There is no particular limitation on the negative electrode active material, and generally any compound capable of reversibly intercalating and deintercalating lithium can be used. Specific examples include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; (semi) metallic materials capable of forming an alloy with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; or composite materials containing (semi) metallic materials and carbonaceous materials. Low crystalline carbon includes soft carbon and hard carbon, and highly crystalline carbon includes natural graphite, condensated graphite, and pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, mesophase pitch, and high-temperature calcined carbon such as coke derived from petroleum or coal tar pitch. One of these materials can be used alone or a mixture of two or more of these materials can be used, and a thin film of metallic lithium can also be used as the negative electrode active material.

[0138] The negative electrode conductive material is used to impart conductivity to the electrode and can be used without any particular limitation as long as it is conductive and does not cause a chemical change in the battery to be constructed. Specific examples include: graphite such as natural graphite or artificial graphite; carbonaceous materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, carbon nanotube, etc.; metal powders or metal fibers such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc. Any one of them can be used alone or a mixture of two or more of the foregoing can be used. Based on the total weight of the negative electrode active material layer, the content of the negative electrode conductive material can generally be 1 to 30% by weight, more particularly 1 to 20% by weight, and more particularly 1 to 10% by weight.

[0139] The negative electrode binder is used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples include: polyvinylidene fluoride (PVDF), vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof. Any one of them can be used alone or a mixture of two or more of the foregoing can be used. Based on the total weight of the negative electrode active material layer, the content of the negative electrode binder can be 1 to 30% by weight, more particularly 1 to 20% by weight, and more particularly 1 to 10% by weight.

[0140] There is no particular limitation on the negative electrode current collector as long as it has high electrical conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon can be used; or copper or stainless steel surface - treated with carbon, nickel, titanium, silver, etc.; aluminum - cadmium alloy, etc.

[0141] In addition, the negative electrode current collector generally can have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, fine irregularities can be formed on the surface of the negative electrode current collector to enhance the binding force of the negative electrode active material. For example, it can be used in various forms such as film, sheet, foil, net, porous body, foam body, non - woven fabric body, etc.

[0142] On the other hand, in the lithium secondary battery, the separator can be used without any particular limitation as long as it is generally used as a separator in the lithium secondary battery. Particularly preferably, it has low resistance to ion migration of the electrolyte and has excellent electrolyte wetting ability. Specifically, a porous polymer film can be used, for example, a porous polymer film made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer; or a laminated structure of two or more layers thereof. In addition, conventional porous non - woven fabrics can be used, for example, non - woven fabrics made of high - melting - point glass fibers, polyethylene terephthalate fibers, etc. The separator can also be a porous thin film with a pore size of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0143] On the other hand, in the lithium secondary battery, the electrolyte can include but is not particularly limited to organic solvents and lithium salts conventionally used in electrolytes.

[0144] An organic solvent can be used without limitation as long as it can serve as a medium in which ions participating in the electrochemical reaction of the battery can move. Specifically, the organic solvents include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; 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 dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), etc.

[0145] Among them, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) with high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery and a linear carbonate compound with low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate, etc.).

[0146] A lithium salt can be used without limitation as long as it is a compound that can provide lithium ions used in the lithium secondary battery. Specifically, the lithium salt can be LiPF 6 、LiClO 4 、LiAsF 6 、LiBF 4 、LiSbF 6 、LiAlO 4 、LiAlCl 4 、LiCF 3 SO 3 、LiC 4 F 9 SO 3 、LiN(C 2 F 5 SO 3 ) 2 、LiN(C 2 F 5 SO 2 ) 2 、LiN(CF 3 SO 2 ) 2 、LiCl、LiI or LiB(C 2 O 4 ) 2 。Preferably, the lithium salt is included in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0147] For the purposes of improving the life characteristics of the battery, suppressing the reduction of the battery capacity, improving the discharge capacity of the battery, etc., in addition to the above electrolyte components, the electrolyte may further contain, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (poly)ethylene glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. Herein, based on the total weight of the electrolyte, the content of the additive may be from 0.1 to 5% by weight.

[0148] The lithium secondary battery of the present invention can be manufactured by disposing a separator between the positive electrode and the negative electrode to form an electrode assembly, placing the electrode assembly in a cylindrical battery case or a prismatic battery case, and then injecting an electrolyte. Alternatively, the lithium secondary battery can be manufactured by laminating the electrode assembly, impregnating it with an electrolyte, and sealing the resulting product in a battery case.

[0149] The battery case can be any battery case conventionally used in the art, and there is no limitation on the shape according to the intended use of the battery, and it can be, for example, cylindrical, prismatic, pouch-type, or coin-type using a can.

[0150] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and thus can be used in portable devices such as mobile phones, laptop computers, digital cameras; energy storage systems (ESS); and electric vehicles such as hybrid electric vehicles (HEV).

[0151] The present invention will now 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.

[0152] Example 1

[0153] A current collector sheet was prepared, in which a self-regulating temperature layer containing a PTC material was coated on an aluminum foil with a thickness of 20 μm in a grid pattern as shown in Figure 1 and 2 . The thickness of the self-regulating temperature layer was 10 μm, the width of one strip was 2 mm, the mutual spacing between multiple strips was 20 mm, and the area of the aluminum foil covered by the self-regulating temperature layer was 22%. The positive electrode paste was coated on the current collector sheet, dried, and calendered to complete the preparation of the positive electrode. The positive electrode paste was prepared by mixing LiFePO 4 as the positive electrode active material, PVDF as the binder, and carbon black as the conductive material in a weight ratio of 96:2:2 in an NMP solvent, and the thickness of the positive electrode active material layer after drying the positive electrode paste on the positive electrode active material layer was 100 μm.

[0154] Example 2

[0155] Except that the coating pattern of the self - regulating temperature layer is strip - shaped as shown in Figure 3 , the positive electrode is prepared by the same method as in Example 1. In this case, the area of the aluminum foil covered by the self - regulating temperature layer is 22%, and the thickness of the self - regulating temperature layer is 10 μm.

[0156] Example 3

[0157] A current collector sheet is prepared, in which a self - regulating temperature layer containing a PTC material is coated on an aluminum foil with a thickness of 20 μm in a grid pattern, and the area of the aluminum foil covered by the self - regulating temperature layer is 50%. Then, a positive electrode paste having the same composition as in Example 1 is coated, and a positive electrode is prepared by drying and rolling under the same conditions as in Example 1.

[0158] Comparative Example

[0159] A positive electrode is prepared by preparing a current collector in which the self - regulating temperature layer is not formed in Example 1, coating a positive electrode paste having the same composition as in Example 1 on the current collector, and drying and rolling under the same conditions as in Example 1.

[0160] Experimental Example 1: Evaluation of Low - Temperature Capacity

[0161] Artificial graphite is used as the negative electrode active material, carbon black is used as the conductive material, and styrene - butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are used as binders. A negative electrode paste is prepared by mixing and stirring the negative electrode active material, conductive material, and binder in a weight ratio of 95.9:0.8:3.3 in water. The negative electrode paste is coated on a copper foil with a thickness of 20 μm, dried, and rolled to prepare a negative electrode.

[0162] Electrode assemblies are manufactured by opposing each of the positive electrodes and negative electrodes prepared in Examples 1 to 3 and the Comparative Example and inserting a separator made of 18 - μm - thick polypropylene therebetween. Each of the prepared electrode assemblies is inserted into a pouch - type battery case, and a secondary battery is prepared by injecting an electrolyte composition into the battery case and sealing the case. As the electrolyte composition, a solution in which lithium hexafluorophosphate (LiPF 6 , 1.0 M) and vinylene carbonate (VC, 2 wt%) are mixed in a mixture of ethylene carbonate (EC):dimethyl carbonate (DMC):diethyl carbonate (DEC)=1:1:1 (volume ratio) is used.

[0163] For each of the fabricated secondary batteries, at a temperature of -10°C, it was charged at a constant current of 0.33C until the voltage reached 4.2 V, and then discharged at a constant current of 0.33C until the voltage reached 3.0 V. Here, "C" is the unit of the charge and discharge rate, which is the current (A) during charging and discharging divided by the rated capacity (Ah) of the secondary battery. For example, 1C is the charge / discharge rate required to fully charge or discharge the secondary battery in 1 hour.

[0164] The discharge capacity during discharge was measured and the results are shown in Table 1. The values in Table 1 are expressed as relative percentages, with the capacity of the secondary battery including the positive electrode of Example 1 being 100%.

[0165] Experimental Example 2: Evaluation of Resistance

[0166] For each of the secondary batteries fabricated in Example 1, the resistance was measured at a temperature of -10°C in the following manner. It was charged from SOC 0% to SOC 30% at a current of 0.05C. Then, at SOC 30%, a pulse was applied to it by applying a current of 0.5C for 30 seconds. Here, SOC (state of charge) is the percentage of the charge capacity of the secondary battery to the total capacity, and "C" in 0.5C is the unit of the charge / discharge rate as described in Example 1 above. The DCIR was calculated and the results are shown in Table 1.

[0167] At a temperature of 25°C, the resistance was measured using the same method as above, and the results are shown in Table 1.

[0168]

[0169] The capacity and resistance characteristics at low temperature of the secondary battery fabricated using the positive electrode of the comparative example that does not include the self - temperature - controlling layer according to the present invention are worse than those of the secondary battery fabricated using the positive electrode of any one of Examples 1 to 3 at low temperature.

[0170] It was found that the secondary battery fabricated using the positive electrode of Example 3, in which the self - temperature - controlling layer covers 50% of the area of the positive electrode current collector, has the best capacity and low - temperature characteristics at low temperature, but the worst resistance characteristics at room temperature. Therefore, in the present invention, it is desirable to adjust the coating area of the self - temperature - controlling layer to an appropriate level.

[0171] Therefore, the positive electrode and secondary battery according to the present invention have shown excellent performance in terms of capacity performance at low temperature without a separate device for heating the battery, and the resistance characteristics at room temperature are not significantly reduced compared with conventional positive electrodes.

[0172] The present invention has been described in more detail above with reference to the accompanying drawings and embodiments. However, it should be understood that the configurations shown in the accompanying drawings or embodiments described herein are only examples of the present invention and do not represent all the technical ideas of the present invention. It should also be understood that there may be various equivalents and variations that can be substituted for it when the present invention is filed.

Claims

1. A positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising: positive electrode current collector; A self-temperature-regulating layer, the self-temperature-regulating layer being disposed on one or both surfaces of the positive electrode current collector but being disposed so as to cover a portion of the positive electrode current collector; and A positive electrode active material layer, the positive electrode active material layer is arranged on the exposed positive electrode current collector area not covered by the self-temperature controlling layer and on the self-temperature controlling layer, wherein The self-temperature controlling layer comprises a positive temperature coefficient (PTC) material.

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

3. The positive electrode for a lithium secondary battery according to claim 1, wherein The PTC material has a conductive material dispersed in a polymer material.

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

5. The positive electrode for lithium secondary battery according to claim 3, wherein The self-temperature controlling layer comprises: 1 wt % to 60 wt % polymeric material; and 1 wt % to 60 wt % conductive material.

6. The positive electrode for a lithium secondary battery according to claim 1, wherein The self-controlling temperature layer has a pattern shape selected from the following patterns: a strip-shaped pattern in which a plurality of strips are spaced side by side, a grid-shaped pattern in which a plurality of strips extending along the y-axis direction intersect with a plurality of strips extending along the x-axis direction, and a radial pattern in which a strip is spread out in a whirlwind-shaped pattern.

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

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

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

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

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

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

13. A method for manufacturing a positive electrode for a lithium secondary battery, the method comprising the following steps: coating the PTC material composition on the positive electrode current collector; applying a positive electrode slurry onto the positive electrode current collector coated with the PTC material composition; and Drying and calendering, where The step of applying the PTC material composition is characterized in that the PTC material composition is applied to cover a portion of the positive electrode current collector.

14. A lithium secondary battery, comprising: The positive electrode according to any one of claims 1 to 12; negative electrode; Diaphragm; and Electrolytes.

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