Unit cell including thermochromic polymer and defect detection method using the same

Abstract

The present invention relates to a unit cell including a thermochromic polymer and a defect detection method using the same, and more particularly, to a unit cell including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator includes a thermochromic polymer configured such that a color of the thermochromic polymer changes according to a temperature, thereby easily checking whether the unit cell is abnormal.

Description

TECHNICAL FIELD

[0001] This application claims the priority benefit of Korean Patent Application No. 2020-0104833, filed on August 20, 2020, the disclosure of which is incorporated herein in its entirety by reference.

[0002] The present application relates to a unit cell including a thermochromic polymer and a defect detection method using the same. More particularly, the present application relates to a unit cell including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator includes a thermochromic polymer configured such that a color of the thermochromic polymer changes according to a temperature. BACKGROUND

[0003] In recent years, battery cells capable of charging and discharging have been widely used as energy sources for wireless mobile devices. In addition, secondary batteries have attracted attention as power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (Plug-In HEVs), which are proposed as alternatives to existing gasoline vehicles and diesel vehicles that use fossil fuels, which cause air pollution. In addition, secondary batteries are also used for high-output power tools, electric bicycles (E-bikes), electric scooters (E-scooters), electric golf carts, or energy storage systems.

[0004] According to the shape of the battery case, battery cells are classified as cylindrical battery cells configured such that an electrode assembly is installed in a cylindrical metal can, prismatic battery cells configured such that an electrode assembly is installed in a prismatic metal can, or pouch battery cells configured such that an electrode assembly is installed in a pouch-shaped case made of an aluminum laminate sheet. Among them, the cylindrical battery cell has an advantage in that the capacity of the cylindrical battery cell is relatively large and the structure is stable.

[0005] A cylindrical battery is manufactured by accommodating a jelly-roll type electrode assembly together with an electrolyte solution in a cylindrical case. Typically, the upwardly protruding positive tab of the jelly-roll type electrode assembly is coupled to a cap assembly, whereby the top cap serves as a positive terminal, and the downwardly protruding negative tab of the jelly-roll type electrode assembly is coupled to the battery case, whereby the battery case serves as a negative terminal.

[0006] The negative tab is attached to the bottom of the battery case by welding. During the welding process, molten metal can be scattered from the electrode rod or welding wire, or spatter / splatter can occur in which the molten metal partially melts and adheres or weakly adheres to the periphery of the negative tab in small pieces, whereby the electrode assembly can have a defect. In the case of using such a defective electrode assembly, the performance of the battery can be reduced, or a specific portion of the electrode assembly can be overheated, which can reduce the stability of the battery.

[0007] Conventionally, in order to solve the above problems, the separator is directly observed or analyzed using an SEM or a microscope to check the short-circuit portion of the separator, which takes a long time. In addition, in order to find the short-circuit portion, analysis and observation are performed using CT, which is inconvenient.

[0008] In Non-Patent Document 1, a material configured to change the volume of the material according to the temperature is added to the separator, whereby the closing characteristics of the separator are improved while preventing thermal runaway. However, there is a problem in that it is difficult to check the defect of a specific portion and the volume change of the separator must be measured to check the short-circuit portion.

[0009] Therefore, in order to improve the stability of the battery and improve the performance of the battery, there is a need for a configuration capable of directly identifying a short-circuit portion in a secondary battery or whether the secondary battery is abnormal.

[0010] (Prior Art Documents)

[0011] (Non-Patent Document 1) Temperature-responsive microspheres-coated separator for thermal shutdown protection of lithium ion batteries (RSC ADV. 2015, 5, 172) (Weixiao Ji et al.) (2014.11.21) SUMMARY

[0012] TECHNICAL PROBLEM

[0013] The present invention has been made in view of the above problems, and an object of the present invention is to provide a unit cell configured to add a material for indicating a short-circuit or whether a defect occurs in a separator in which a short-circuit is likely to occur, whereby damage or a defect of the separator can be indicated, and a defect detection method using the same.

[0014] TECHNICAL SOLUTION

[0015] To achieve the above object, the present application provides a unit cell including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator includes a thermochromic polymer configured such that a color of the thermochromic polymer changes according to a temperature.

[0016] The separator can include a separator substrate and a coating layer on at least one surface of the separator substrate, and the coating layer can include the thermochromic polymer.

[0017] The coating layer can have a thickness of 1 µm to 20 µm.

[0018] The thermochromic polymer can change color in a range of 60 °C to 120 °C.

[0019] The thermochromic polymer can be poly(N-isopropylacrylamide) (PNIPAAm).

[0020] The thermochromic polymer can be distributed in a surface of the separator substrate or a surface of the coating layer.

[0021] The thermochromic polymer can be distributed at a distance of 0 µm to 2 µm from a surface of the separator.

[0022] The thermochromic polymer can be distributed in a surface facing the positive electrode and a surface facing the negative electrode.

[0023] Further, the present application provides a defect detection method including: S1) manufacturing a unit cell, S2) housing the unit cell in a case and sealing the case, S3) checking a color change of the unit cell, and S4) determining that the unit cell that changes color is defective.

[0024] In step S3, the color change of the unit cell can be checked by at least one of ultraviolet measurement or infrared measurement.

[0025] An ultraviolet measurement device or an infrared measurement device can be installed in the case.

[0026] In the present application, one or more configurations not conflicting with each other can be selected and combined from the above configurations.

[0027] Advantageous effects

[0028] As is apparent from the above description, the unit cell according to the present application includes a thermochromic polymer configured such that a color of the thermochromic polymer changes according to a temperature, and thus it is possible to check a position of a metal scattered during welding, and thus it is possible to detect a defective portion of the unit cell.

[0029] Further, the thermochromic polymer is included in the separator, whereby the temperature of the separator can be identified. Accordingly, the power supply can be interrupted before a short circuit occurs in the separator, and thus the stability of the battery can be improved.

[0030] Further, the short-circuited portion of the separator can be clearly indicated, whereby research data that is helpful for subsequent welding or improvement of the performance of the separator can be obtained.

[0031] Further, the defective portion can be indicated using color, thereby reducing the detection and analysis time and also reducing the cost. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a schematic view of a defect detection method according to an embodiment of the present application. DETAILED DESCRIPTION

[0033] Now, preferred embodiments of the present application will be described in detail with reference to the accompanying drawings, so that a person of ordinary skill in the art to which the present application pertains can easily implement the preferred embodiments of the present application. However, when describing the operation principle of the preferred embodiments of the present application in detail, a detailed description of known functions and configurations included herein will be omitted when it can obscure the subject matter of the present application.

[0034] Further, the same reference numerals will be used to refer to components that perform similar functions or operations in all of the accompanying drawings. Throughout the specification, in the case where a component is referred to as being connected to another component, the one component can be directly connected to the other component, or the one component can be indirectly connected to the other component via yet another component. Further, the inclusion of certain elements does not mean the exclusion of other elements, but means that the elements can be further included unless otherwise stated.

[0035] Further, the description of elements embodied by limitation or addition can be applied to all inventions unless specifically limited, and does not limit a specific invention.

[0036] Further, in the description of the present application and the claims of the present application, the singular form is intended to include the plural form unless otherwise stated.

[0037] Further, in the description of the present application and the claims of the present application, "or" includes "and" unless otherwise stated. Accordingly, "including A or B" means three cases, i.e., a case including A, a case including B, and a case including A and B.

[0038] Hereinafter, the present application will be described with reference to embodiments. However, the embodiments are provided only to make the present application easier to understand, and should not be interpreted as limiting the scope of the present application. Hereinafter, the present application will be described in more detail.

[0039] The unit cell according to the present application includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator includes a thermochromic polymer, the color of which changes according to temperature.

[0040] positive electrode

[0041] For example, the positive electrode can be manufactured by applying a positive electrode mixture of a positive electrode active material composed of positive electrode active material particles, a conductive agent, and a binder to a positive electrode current collector. If necessary, a filler can be further added to the positive electrode mixture.

[0042] In general, the positive electrode current collector is manufactured to have a thickness of 3 μm to 500 μm. The positive electrode current collector is not particularly limited as long as the positive electrode current collector exhibits high conductivity while the positive electrode current collector does not cause any chemical change in a battery to which the positive electrode current collector is applied. For example, the positive electrode current collector can be made of stainless steel, aluminum, nickel, or titanium. Alternatively, the positive electrode current collector can be made of aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, or silver. In particular, aluminum can be used. The positive electrode current collector can have a microscale uneven pattern formed on its surface to increase adhesion of the positive electrode active material. The current collector can be configured in any one of various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam body, and a nonwoven fabric body.

[0043] The positive electrode active material can be composed of, for example, a layered compound such as lithium nickel oxide (LiNiO2) or a compound substituted with one or more transition metals; a compound represented by the chemical formula Li 1+ x Mn 2-x LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxide such as LiV3O8, V2O5, or Cu2V2O7; Ni-site type lithium nickel oxide represented by the chemical formula LiNi 1-x M x O2(wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3); LiMn 2- x M xLi2Mn3MO8 (wherein M = Fe, Co, Ni, Cu, or Zn); LiMn2O4, wherein a part of Li in the chemical formula is substituted with an alkaline earth metal ion; a disulfide compound; or Fe2(MoO4)3. However, the present application is not limited thereto.

[0044] The conductive agent is generally added so as to account for 0.1 to 30% by weight, based on the total weight of the mixture including the positive electrode active material. The conductive agent is not particularly limited as long as the conductive agent exhibits high conductivity while not causing any chemical change in the battery to which the conductive agent is applied. For example, the following materials can be used as the conductive agent: graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyaniline derivatives.

[0045] The binder included in the positive electrode is a component that facilitates the binding between the active material and the conductive agent and the binding with the current collector. The binder is generally added in an amount of 0.1 to 30% by weight, based on the total weight of the mixture including the positive electrode active material. As examples of the binder, 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, fluoro rubber, and various copolymers can be used.

[0046] negative electrode

[0047] The negative electrode can be manufactured by applying the negative electrode active material to the negative electrode current collector and drying it. The above-described components can be further included as necessary, optionally.

[0048] The negative current collector is generally manufactured to have a thickness of 3 μm to 500 μm. The negative current collector is not particularly limited as long as the negative current collector exhibits high electrical conductivity while the negative current collector does not cause any chemical change in a battery to which the negative current collector is applied. For example, the negative current collector can be made of copper, stainless steel, aluminum, nickel, titanium, or calcined carbon. Alternatively, the negative current collector can be made of copper or stainless steel, or aluminum cadmium alloy, the surface of which is treated with carbon, nickel, titanium, or silver. In addition, the negative current collector can have a microscale uneven pattern formed on the surface thereof to increase the binding force of the negative active material in the same manner as the positive current collector. The negative current collector can be configured in any one of various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam body, and a nonwoven fabric body.

[0049] As the negative active material, for example, carbon such as non-graphitizable carbon or graphite-based carbon; metal complex oxides such as Li x Fe2O3(0≤x≤1), Li x WO2(0≤x≤1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, 2, and 3 elements of the periodic table, halogen; 0≤x≤1; 1≤y≤3; 1≤z≤8); lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxide such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, or Bi2O5; electrically conductive polymer such as polyacetylene; or Li-Co-Ni-based material.

[0050] separator

[0051] The separator according to the present application can be composed of only the separator substrate, or can include a coating layer on at least one surface of the separator substrate.

[0052] The thermochromic polymer can be included in at least one of the separator substrate and the coating layer.

[0053] separator substrate

[0054] The separator substrate electrically insulates the positive electrode and the negative electrode from each other, thereby preventing a short circuit, and provides a movement path of lithium ions. A porous film having high resistance to an electrolyte solution as an organic solvent and a very small pore diameter can be used. The separator substrate is not particularly limited as long as the separator substrate can be generally used as a material for a separator of a secondary battery. For example, the separator substrate can include a resin such as a polyolefin-based resin (polyethylene, polypropylene, polybutylene, or polyvinyl chloride), or a mixture or copolymer thereof, or can include a resin such as polyethylene terephthalate, polycycloolefin, polyethersulfone, polyamide, polyimide, polyimide-amide, polyaramid, nylon, or polytetrafluoroethylene. Among them, it is preferable to use a polyolefin-based resin because of high applicability of a slurry for a porous coating layer and a decrease in thickness of a separator for a secondary battery, thereby increasing a percentage of an electrode active material layer in a battery, and thus increasing a capacity per unit volume thereof.

[0055] The thickness of the separator substrate can be 1 μm to 100 μm, and preferably 1 μm to 30 μm. The pore diameter of the separator substrate can be generally 0.01 μm to 10 μm.

[0056] coating

[0057] Although the thickness of the coating layer is not particularly limited, it is basically preferable that the coating layer has a sufficient thickness to exhibit a coating layer addition effect when the capacity of the secondary battery is high. In consideration of this, the thickness of the coating layer according to the present application can be 1 μm to 20 μm. If the coating layer is thinner than 1 μm, a sufficient amount of the thermochromic polymer is not included, thereby making it difficult to indicate a defective portion, which is undesirable. If the coating layer is thicker than 20 μm, it is difficult to accurately identify the temperature applied to the separator, which is also undesirable.

[0058] The coating layer can be located on at least one surface of the separator substrate. In order to accurately identify the temperature of the separator, the coating layer including the thermochromic polymer according to the present application is preferably located on opposite surfaces of the separator substrate.

[0059] thermochromic polymer

[0060] In the secondary battery according to the present application, the thermochromic polymer can be included in the separator substrate or in the coating layer located on at least one surface of the separator substrate. Any thermochromic polymer can be used as long as the color of the thermochromic polymer is changed by heat while not affecting the performance of the battery. The thermochromic polymer can be reversibly or irreversibly colored. In the case of using a thermochromic polymer configured to be reversibly colored, a color change of the separator can be perceived, thereby interrupting the use of the battery at a certain temperature or higher, and when the color of the separator is restored, the battery can be used again.

[0061] In the case of using a thermochromic polymer configured to be irreversibly colored, the overheated portion of the color change separator can be confirmed, and thus the problematic portion can be clearly identified.

[0062] The thermochromic polymer can be poly(N-isopropylacrylamide) (PNIPAAm). In addition, the thermochromic polymer can be manufactured by adding a thermochromic dye such as triphenylmethane colorant, phenol pyridinium betaine, sulphophthalein, Reichardt dye, thyranine, indicator colorant, azo pigment, or fluorescein colorant (for example, 2-chloro-6-dimethylamino-3-methylfluoran) to a polymer such as polyethylene, polypropylene, polyester, polyamide, and / or acrylonitrile-butadiene-styrene copolymer.

[0063] At least one thermochromic polymer can be included in the separator substrate or coating. The reason for this is that various colors are exhibited according to a temperature change of the separator substrate or coating, and thus it can be determined whether the battery is abnormal according to the temperature.

[0064] The thermochromic polymer can be colored in the range of 60℃ to 120℃. Since the above temperature range is set based on a temperature higher than a normal operating temperature of the battery cell and a temperature before the battery is about to heat, the temperature can vary according to the use environment of the battery.

[0065] In addition, the thermochromic polymer can be configured to exhibit different colors according to the position of the separator substrate. As described above, in the case where the thermochromic polymer is configured to exhibit different colors according to the position of the separator substrate, it is possible to simply check whether the separator is abnormal by detecting a color change having a specific wavelength through laser irradiation without separating the separator.

[0066] The thermochromic polymer can be distributed in the surface of the separator substrate or the surface of the coating. The thermochromic polymer can be distributed at a distance of 0μm to 2μm from the surface of the separator substrate or the surface of the coating. The surface of the separator substrate or the surface of the coating can refer to the entire outer surface of the separator substrate or the coating that contacts the outside or a portion thereof. As described above, in the case where the thermochromic polymer is located in the surface of the separator substrate or the surface of the coating, it is possible to more rapidly perceive a color change of the thermochromic polymer and rapidly identify heat applied to the separator through the thermochromic polymer, and thus it is possible to more rapidly identify contraction of the separator or short circuit of the separator.

[0067] Further, the thermochromic polymer according to the present application can be distributed in the surface facing the positive electrode and the surface facing the negative electrode. The reason for this is that in the case where the separator separating the positive electrode and the negative electrode from each other is defective, the separator can be quickly identified and the problem can be solved.

[0068] In addition to the thermochromic polymer, the separator substrate or coating according to the present application can further include a wavelength conversion material.

[0069] Preferably, the wavelength conversion material is a phosphorescent material including a nanomaterial such as a quantum dot and a phosphor. Although other materials can be used depending on the detection means, it is preferable to use a fluorescent material as the wavelength conversion material.

[0070] inorganic material

[0071] In addition to the thermochromic polymer, an inorganic material can be further included in the coating to increase the mechanical strength of the separator. The inorganic material is not particularly limited as long as the inorganic material provides a uniform thickness to the coating and does not undergo oxidation and / or reduction in the operating voltage range of the secondary battery to which the present application is applied. Specifically, in the case where inorganic particles having ion transport ability are used, the ion conductivity of the electrochemical device can be improved, and thus the performance of the battery can be improved. Further, in the case where inorganic particles having a high dielectric constant are used as the inorganic particles, the degree of dissociation of an electrolyte salt (e.g., a lithium salt) in a liquid electrolyte can be increased, and thus the ion conductivity of the electrolyte solution can be improved.

[0072] An example of the inorganic material can be an inorganic material having at least one of lithium ion transport ability, piezoelectricity, and flame retardancy.

[0073] The inorganic material having high lithium ion transport ability refers to an inorganic material containing a lithium element but moving lithium ions without storing lithium. Since the inorganic material having lithium ion transport ability is capable of transporting and moving lithium ions due to a kind of defect present in the particle structure thereof, the lithium ion conductivity in the battery can be improved, and thus the performance of the battery can be improved.

[0074] The inorganic material having lithium ion transport ability can be, for example, selected from the group consisting of lithium phosphate, lithium titanium phosphate, lithium aluminum titanium phosphate, (LiAlTiP) x O y base glass, lithium lanthanum titanate, lithium germanium thiophosphate, lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass (Li x Si y S zP2S5-based glass (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7), and mixtures thereof. However, the present application is not limited thereto.

[0075] An inorganic material having piezoelectricity refers to a material that is a non-conductor under atmospheric pressure, but has physical properties such as electrical conductivity due to a change in its internal structure when a predetermined pressure is applied thereto, is a material having a dielectric constant of 100 or more, i.e., a high dielectric constant, and is configured to have one surface positively charged and the other surface negatively charged when a predetermined pressure is applied thereto to be tensioned or compressed, thereby generating a potential difference between the opposite surfaces thereof.

[0076] In the case of using an inorganic material having the above-described properties, when an internal short circuit occurs between two electrodes due to an external impact caused by a local crush or a nail, etc., the positive electrode and the negative electrode do not directly contact each other due to the inorganic particles formed on the separator by coating, and due to the piezoelectricity of the inorganic particles, a potential difference is generated in the inorganic particles, thereby electrons move between the two electrodes, i.e., a micro-current flows therebetween, so that the voltage of the battery slowly decreases, thereby improving the safety of the battery.

[0077] The inorganic particles having piezoelectricity can be, for example, BaTiO3, Pb(Zr,Ti)O3(PZT), Pb(Mg 1-x La x Zr 1-y Ti y O3(PLZT), Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3(PMN-PT), hafnia (HfO2), or mixtures thereof. However, the present application is not limited thereto.

[0078] The flame-retardant inorganic material can be at least one selected from the group consisting of an antimony-containing compound, a metal hydroxide or metal hydrate, a guanidino compound, a boron-containing compound, and a zinc stannate compound.

[0079] The antimony-containing compound is one selected from the group consisting of antimony trioxide (Sb2O3), antimony tetroxide (Sb2O4), and antimony pentoxide (Sb2O5). The metal hydroxide or metal hydrate is one selected from the group consisting of aluminum hydroxide (Al(OH)3), aluminum oxyhydroxide (AIO(OH)), and CaO AI2O3 6H2O, but excluding magnesium hydroxide (Mg(OH)2). The guanidino compound is one selected from the group consisting of guanidine nitrate, guanidine sulfamate, guanidine phosphate, and guanidine phosphate urea. The boron-containing compound is H3BO3 or HBO2. The zinc stannate compound is one selected from the group consisting of Zn2SnO4, ZnSnO3, and ZnSn(OH)6.

[0080] Preferably, the flame-retardant inorganic material is at least one selected from the group consisting of magnesium hydroxide (Mg(OH)2), aluminum hydroxide (Al(OH)3), aluminum oxyhydroxide (AIO(OH)), and CaO AI2O3 6H2O.

[0081] As a result of adding the flame-retardant inorganic material, overcharging can be prevented, flame-retardant properties can be added to the separator, or a sudden increase in battery temperature can be prevented. In the flame-retardant inorganic material according to the present application, the metal hydroxide decomposes as the temperature increases, and a dehydration reaction, which is an endothermic reaction, occurs. At this time, due to the endothermic reaction and the generated water, an additional flame-retardant effect can be obtained.

[0082] To improve the flame retardancy of the flame-retardant inorganic material, a flame-retardant synergist can be further included. The flame-retardant synergist can be a silicon-based additive, zinc oxide, tin oxide, a nickel compound, zinc borate, a melamine compound, or a mixture of two or more thereof.

[0083] The flame-retardant synergist can be used in various combinations according to the characteristics of the flame-retardant inorganic material. As a material capable of improving the efficiency of the flame-retardant inorganic material, a flame-retardant synergist other than the above-described flame-retardant synergist can be further included. Also, a halogen can be further added, and phosphorus or a phosphorus compound can be added. The flame-retardant synergist can be provided to the extent that the efficiency of the flame-retardant inorganic material can be improved. The flame-retardant synergist can be added in an amount of 0.001 times to 0.1 times the total weight of the flame-retardant inorganic material.

[0084] Further, the absorbent is configured to absorb moisture molecules generated by the flame-retardant inorganic material; however, the absorbent is limited only to a material that does not reduce battery performance. Any ordinary material can be used as the absorbent without limitation. For example, zeolite, porous silica, or porous alumina can be used. However, the present application is not limited thereto.

[0085] Although the particle diameter of the inorganic material is not particularly limited, the D50 can have a range of 20 nm to 10 µm, specifically 100 nm to 1 µm, in consideration of the purpose of forming a coating layer having a uniform thickness and providing appropriate porosity.

[0086] In the particle size distribution curve of the particles, D50 means the particle size of the particles corresponding to 50% of the cumulative number of particles, and the average particle size of the inorganic particles is measured using a particle size analyzer (product name: MASTERSIZER 3000; manufacturer: Malvern).

[0087] The inorganic material can be included in 10 to 99% by weight based on the weight of the total solid content of the coating. If the content of the inorganic material is less than 10% by weight, it is difficult to achieve the effect obtained by adding the inorganic material, which is undesirable. If the content of the inorganic material is greater than 99% by weight, the content of the binder is too small, so the adhesion between the inorganic particles can be reduced, and thus the inorganic coating can be separated from the separator substrate or an uncoated area can occur at the time of coating, which is also undesirable.

[0088] In addition to the inorganic material, the coating can further include a binder. The inorganic material can be uniformly distributed in the coating together with the binder. Since the inorganic material is uniformly distributed in the binder, resistance in the secondary battery can be uniformly formed, thereby preventing short circuit from occurring in a specific portion.

[0089] The binder is generally referred to as a polymer binder, and can have a characteristic of being gelled when impregnated with a liquid electrolyte solution for a binder, thereby exhibiting a high electrolyte solution impregnation rate. In fact, in the case of a binder polymer that has a high electrolyte solution impregnation rate, the polymer can be impregnated with an electrolyte solution injected after the battery is assembled, in which the polymer absorbing the electrolyte solution exhibits an electrolyte ion conduction ability. In addition, the wettability with respect to the electrolyte solution for a battery is improved compared to a conventional hydrophobic polyolefin-based separator, and the use of a polar electrolyte solution for a battery, which has been conventionally difficult to use, also becomes possible. Therefore, if possible, the solubility parameter of the polymer is preferably 15 to 45 MPa 1 / 2 , more preferably 15 to 25 MPa 1 / 2 , and 30 to 45 MPa 1 / 2 . If the solubility parameter is less than 15 MPa 1 / 2 and greater than 45 MPa 1 / 2 , it is difficult to achieve swelling by a general liquid electrolyte solution for a battery.

[0090] Specifically, the binder is at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-co-hexafluoropropylene, polyvinylpyrrolidone, polyacrylonitrile, polyvinylidene fluoride-co-trichloroethylene, polyvinylidene fluoride-co-trifluorochloroethylene (PVdF-CTFE), polymethyl methacrylate, polyvinyl acetate, ethylene-co-vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile butadiene styrene copolymer, polyacrylonitrile-styrene copolymer, gelatin, polyethylene glycol, polyethylene glycol dimethyl ether, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), tetrafluoroethylene (TFE), fluoroelastomer, and polyimide. Preferably, the binder is at least one selected from the group consisting of PVdF, TFE, and polyimide.

[0091] The binder material can further include at least one of baicalin, luteolin, taxifolin, myricetin, quercetin, rutin, catechin, epigallocatechin gallate, butein, coniferyl alcohol, a phenolic compound including tannic acid, pyrogallol, amylose, amylopectin, xanthan gum, and a fatty acid-based hydrophilic or non-hydrophilic polymer. Such a binder material includes a large number of OH groups, thereby improving adhesion between the binder and the inorganic material and between the substrate and the binder, preventing internal short circuit by a self-repairing function of a portion damaged like a separator, increasing adhesion between the separator and the positive electrode and between the separator and the negative electrode, and preventing elution of transition metals of the positive electrode.

[0092] dispersant

[0093] The coating layer can further include a dispersant to further improve dispersibility of the inorganic material including the thermochromic polymer. The dispersant is used to maintain a state in which the thermochromic polymer is uniformly dispersed in the binder when a coating layer slurry is manufactured. For example, an anionic surfactant can be used to maintain uniform dispersion while increasing dispersibility.

[0094] The anionic component including at least one selected from the group consisting of a carboxylate, a phosphate, a sulfonate, and a sulfate can constitute a head of the anionic surfactant. Among them, the sulfonate is preferably used as the head of the anionic surfactant.

[0095] A material having nonionic surfactant properties can be used as a tail of the anionic surfactant. Although the material having nonionic surfactant properties is not particularly limited, a material including an alkyl group can be used. The material including an alkyl group can be a polyalkylene oxide having 5 to 200 repeating units of an alkylene oxide. At this time, the polyalkylene oxide can be at least one selected from the group consisting of a polyethylene oxide, a polypropylene oxide, and a polyethylene oxide-polypropylene oxide copolymer.

[0096] A typical example of the anionic surfactant is carboxyl methylcellulose (CMC).

[0097] In addition, at least one selected from the group consisting of an oil-soluble polyamine, an oil-soluble amine compound, a fatty acid, a fatty alcohol, a sorbitan fatty acid ester, tannic acid, and pyrogallol can be used as the dispersant.

[0098] The content of the dispersant can be 0.2 parts by weight to 10 parts by weight based on 100 parts by weight of the inorganic material. If the dispersant is included in less than 0.2 parts by weight based on 100 parts by weight of the inorganic material, the inorganic material can be easily precipitated. If the dispersant is included in more than 10 parts by weight based on 100 parts by weight of the inorganic material, the adhesion of the coating layer to the separator substrate can be reduced, or impurities can be generated due to a reaction with an electrolyte solution when a secondary battery is manufactured.

[0099] defect detection method

[0100] The defect detection method according to the present application can include: S1) a step of manufacturing the above-described unit cell, S2) a step of accommodating the unit cell in a case and sealing the case, S3) a step of checking a color change of the unit cell, and S4) a step of determining that the unit cell having the color change is defective.

[0101] In the step S3), the color change of the unit cell can be detected by ultraviolet measurement, infrared measurement, or the naked eye.

[0102] The ultraviolet measurement or the infrared measurement can be performed using a measuring device installed in the case. Figure 1 is a schematic diagram of the defect detection method according to the embodiment of the present application, and is a schematic diagram of ultraviolet measurement or ultraviolet spectrometry.

[0103] As Figure 1As shown, the defect detection method according to the present application can be performed using a detection device 200 such as a silicon diode or a UV spectrometer, which is disposed near at least one of the upper and lower portions of the cylindrical secondary battery 100 using the unit cell according to the present application. The defect detection method according to the present application can be performed by applying infrared or ultraviolet light to the separator without disassembling the cylindrical secondary battery 100. The infrared or ultraviolet light passes through the separator of the cylindrical secondary battery 100, detects discoloration of the separator, and reports to the outside whether the secondary battery is abnormal. The detection device 200 can be connected to a battery management module, and when the secondary battery is abnormal, use of the secondary battery can be interrupted.

[0104] In addition, the defect detection method can be used for various other batteries such as pouch-shaped secondary batteries and prismatic secondary batteries, in addition to the cylindrical secondary battery 100 using the unit cell according to the present application.

[0105] In addition, the defect detection method according to the present application can be performed with the naked eye. In this case, the method can be used to determine and analyze abnormal portions of the unit cell according to the present application. In the defect detection method according to the present application, a sealed unit cell can be disassembled and a color change of the unit cell can be checked with the naked eye to detect defects.

[0106] At this time, an image of the unit cell can be captured using a digital camera, and a color change of the unit cell can be analyzed using a predetermined program for convenience of detection.

[0107] Those skilled in the art to which the present application pertains will appreciate that, based on the above description, various applications and modifications can be made within the scope of the present application.

Claims

1. A unit cell, comprising: a positive electrode; a negative electrode; and a separator disposed between the positive electrode and the negative electrode, wherein the separator includes a thermochromic polymer configured such that a color of the thermochromic polymer changes according to a temperature; wherein the separator includes a separator substrate and a coating layer on at least one surface of the separator substrate, and the coating layer includes the thermochromic polymer and a wavelength conversion material; and wherein the thermochromic polymer is poly(N-isopropylacrylamide), or wherein the thermochromic polymer is manufactured by adding one or more thermochromic dyes selected from the group consisting of triphenylmethane colorants, phenol pyridinium betaine, sulfonphthalein, Reichardt dye, thyronine, indicator colorants, azo pigments, and fluorescein colorants to a polymer selected from the group consisting of polyethylene, polypropylene, polyester, polyamide, and acrylonitrile-butadiene-styrene copolymer, wherein the wavelength conversion material is a phosphorescent material including a nanomaterial.

2. The unit cell according to claim 1, wherein the coating layer has a thickness of 1 pm to 20 pm.

3. The unit cell according to claim 1, wherein the thermochromic polymer changes color in a range of 60°C to 120°C.

4. The unit cell according to claim 1, wherein the thermochromic polymer is distributed at a distance of 0 pm to 2 pm from a surface of the separator.

5. The unit cell according to claim 1, wherein the thermochromic polymer is distributed in a surface facing the positive electrode and a surface facing the negative electrode.

6. A defect detection method, comprising: S1) manufacturing a unit cell according to any one of claims 1 to 5; S2) housing the unit cell in a housing and sealing the housing; S3) checking a color change of the unit cell, and 7. The defect detection method of claim 6, wherein, S4) determining that a unit cell that changes color is defective. In step S3, the color change of the unit cell is checked by at least one of ultraviolet measurement or ultraviolet spectroscopic measurement.

8. The defect detection method according to claim 7, wherein an ultraviolet measurement device or an infrared measurement device is installed in the housing.

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