Lithium-ion secondary battery and its manufacturing method

Abstract

To provide a lithium ion secondary battery improved in the ionic conductivity, and a manufacturing method thereof.SOLUTION: In one embodiment, a lithium ion secondary battery includes: an electrode assembly including an anode, a cathode, and a separator disposed between the anode and the cathode; a case accommodating the electrode assembly; and an electrolyte filling the case. The separator includes a coating layer on at least one of the two sides of the separator, and the coating layer includes boron nitride nanotubes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a lithium ion secondary battery and a method for manufacturing the same. [Background technology]

[0002] Unlike disposable batteries that are discarded after use, secondary batteries can be reused through repeated charging and discharging. They are used in a variety of fields, including mobile phones, laptops, and electric vehicles, and in recent years have increasingly been used as energy storage devices. There are various types of such secondary batteries, with the lithium-ion secondary battery being a representative example. Lithium-ion secondary batteries have the advantages of high energy density, no memory effect, a relatively short charging time, and ease of charging and discharging. However, the charge / discharge efficiency and charging time of lithium-ion secondary batteries are affected by the ionic conductivity of lithium ions. Therefore, in order to improve the efficiency of lithium-ion secondary batteries, it is necessary to improve the ionic conductivity of lithium ions. Summary of the Invention [Problem to be solved by the invention]

[0003] SUMMARY OF THE INVENTION Embodiments of the present invention provide a lithium-ion secondary battery with improved ionic conductivity and a method for manufacturing the same. [Means for solving the problem]

[0004] One embodiment of the present invention discloses a lithium ion secondary battery including an electrode assembly including an anode, a cathode, and a separator disposed between the anode and the cathode, a case that houses the electrode assembly, and an electrolyte solution filled in the case, wherein the separator includes a coating layer on at least one of both surfaces of the separator, and the coating layer includes boron nitride nanotubes. In this embodiment, the separator may have an ionic conductivity of 1.7 mS / cm or less at 60°C and 0.4 mS / cm or more at -10°C. In this embodiment, the coating layer may have an average surface roughness of 50 nm to 2 μm. In this embodiment, the aspect ratio of the boron nitride nanotubes may be 20-6000. In this embodiment, the boron nitride nanotubes may be surface-treated to have hydrophilic or hydrophobic properties. In this embodiment, the surface-treated boron nitride nanotube may further include a first layer located on at least a portion of the boron nitride nanotube, the first layer including a hydroxyphenyl group and π-bonded to the boron nitride nanotube. In this embodiment, the surface-treated boron nitride nanotubes may have the hydrophilic property. In this embodiment, the surface-treated boron nitride nanotube may further include a second layer on the first layer, the second layer including an amine group or a thiol group as a hydrocarbon group, and the surface-treated boron nitride nanotube may have the hydrophobic property. In this embodiment, at least some of the boron nitride nanotubes may be attached to the separator surface at an angle of 1° to 30°.

[0005] Another embodiment of the present invention is a method for manufacturing a separator, comprising the steps of: mixing boron nitride nanotubes in a solvent to form a coating liquid; coating the coating liquid on at least one of both surfaces of a separator to form a coating layer; and a cathode and an anode disposed on either side of the separator to form an electrode assembly; and a case where the electrode assembly is disposed in a case and the case is filled with an electrolyte. In this embodiment, the coating layer may be formed by electrostatically spraying or spray coating the coating liquid onto at least one of both surfaces of the separator. In this embodiment, the method may further include a step of drying the coating layer after forming the coating layer. In this embodiment, the coating liquid may contain 0.01 wt % to 10 wt % of the boron nitride nanotubes. In this embodiment, the boron nitride nanotubes may be surface-treated to have hydrophilic or hydrophobic properties. In this embodiment, the coating layer may have an average surface roughness of 50 nm to 2 μm. [Effects of the Invention]

[0006] According to the embodiment of the present invention, the ionic conductivity of the lithium ion secondary battery is improved, and the performance of the lithium ion secondary battery can be improved. Furthermore, the boron nitride nanotubes attached to the surface of the separator reduce the tendency of the separator to shrink, efficiently dissipate generated heat, improve thermal stability, and provide excellent safety for the secondary battery. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating an example of a lithium ion secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 2 is a TEM image of boron nitride nanotubes contained in the coating layer of FIG. [Figure 3] FIG. 3 is an SEM image of the separator surface on which the coating layer of FIG. 1 is formed. [Figure 4] FIG. 4 is a flowchart that schematically shows a method for manufacturing the lithium ion secondary battery of FIG. [Figure 5] FIG. 5 is a perspective view schematically illustrating an example of the boron nitride nanotubes contained in the coating layer of FIG. [Figure 6]FIG. 6 is a flow chart that schematically illustrates the surface treatment method for the boron nitride nanotubes of FIG. [Figure 7] FIG. 7 is a perspective view schematically illustrating an example of the boron nitride nanotubes contained in the coating layer of FIG. [Figure 8] FIG. 8 is a flow chart that schematically illustrates the surface treatment method for the boron nitride nanotubes of FIG. [Figure 9] FIG. 9 is a diagram schematically showing an example of a method for forming a boron nitride nanotube coating layer on the separator of FIG. [Figure 10] FIG. 10 is a diagram showing the results of measuring the lithium ion conductivity of the separator. [Figure 11] FIG. 11 is a graph showing the results of measuring the initial charge / discharge capacity of a lithium ion secondary battery. [Figure 12] FIG. 12 is a diagram showing the characteristics (rate performance) of a lithium ion secondary battery depending on the temperature and charge / discharge rate. [Figure 13] FIG. 13 is a diagram showing the characteristics (rate performance) of a lithium ion secondary battery depending on the temperature and charge / discharge rate. [Figure 14] FIG. 14 is a diagram showing the results of measuring the temperature of a lithium ion secondary battery during charging and discharging. [Figure 15] FIG. 15 is a diagram showing the results of measuring the temperature of a lithium ion secondary battery during charging and discharging. [Figure 16] FIG. 16 is a graph showing the results of a CV (cyclic voltammetry) test on a lithium ion secondary battery. [Figure 17] FIG. 17 is a graph showing the results of a CV (cyclic voltammetry) test on a lithium ion secondary battery. [Figure 18] FIG. 18 is a diagram showing the characteristics (C-rate performance) of a lithium ion secondary battery depending on the charge / discharge rate. [Figure 19] FIG. 19 is a diagram showing the cycle retention results of the lithium ion secondary battery of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0008] Since the present invention can be modified in various ways and can have various embodiments, specific embodiments are shown in the drawings and will be described in detail from the detailed description. The effects and features of the present invention, and methods for achieving them, will become apparent by referring to the embodiments described in detail below together with the drawings. However, the present invention is not limited to the embodiments disclosed below, and can be implemented in various forms.

[0009] In the following embodiments, terms such as first and second are not used in a limiting sense but to distinguish one component from another.

[0010] In the following embodiments, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0011] In the following embodiments, terms such as "comprise" or "have" mean that the features or components described in this specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0012] In the following embodiments, when a part such as a film, region, or component is said to be on or above another part, this includes not only the case where it is directly on top of the other part, but also the case where another film, region, component, etc. is interposed between them.

[0013] In the drawings, the size of components may be exaggerated or reduced for the sake of convenience of explanation. For example, the size and thickness of each component shown in the drawings are arbitrarily shown for the sake of convenience of explanation, and the present invention is not necessarily limited to those shown in the drawings.

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. When referring to the drawings, the same or corresponding elements will be designated by the same reference numerals.

[0015] FIG. 1 is a cross-sectional view schematically illustrating an example of a lithium-ion secondary battery according to one embodiment of the present invention, FIG. 2 is a TEM image of boron nitride nanotubes contained in the coating layer of FIG. 1, and FIG. 3 is an SEM image of the surface of a separator on which the coating layer of FIG. 1 is formed. Referring to FIG. 1, a lithium ion secondary battery 10 according to an embodiment of the present invention may include an electrode assembly 100, a case 200 that houses the electrode assembly, and an electrolyte 300 filled in the case 200.

[0016] The electrode assembly can include a cathode 110, an anode 120, and a separator 130 therebetween to prevent shorting between the cathode 110 and the anode 120. The cathode 110 includes a cathode film 112 and a 1 shows the cathode active material layer 114 applied to one side of the cathode film 112, but is not limited thereto, and the cathode active material layer 114 may be applied to both sides of the cathode film 112. The cathode film 112 may be metallic, formed from aluminum, stainless steel, titanium, silver, or a combination of materials selected from the above. The cathode active material layer 114 can include a cathode active material, a binder, and a conductive agent.

[0017] The cathode active material can be formed of a material capable of reversibly absorbing and releasing lithium ions. For example, the cathode active material can include at least one material selected from the group consisting of lithium transition metal oxides such as lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganate, lithium manganate, and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, and vanadium oxide.

[0018] The binder can include at least one substance selected from the group consisting of polyvinylidene fluoride-based binders such as polyvinylidene fluoride, vinylidene fluoride / hexafluoropropylene copolymer, and vinylidene fluoride / tetrafluoroethylene copolymer; carboxymethylcellulose-based binders such as sodium-carboxymethylcellulose and lithium-carboxymethylcellulose; acrylate-based binders such as polyacrylic acid, lithium-polyacrylic acid, acrylic, polyacrylonitrile, polymethyl methacrylate, and polybutyl acrylate; polyamide imide, polytetrafluoroethylene, polyethylene oxide, polypyrrole, lithium-Nafion, and styrene butadiene rubber-based polymers.

[0019] The conductive agent can include at least one substance selected from the group consisting of carbon-based conductive agents such as carbon black, carbon fiber, carbon nanotubes, and graphite; conductive fibers such as metal fibers; metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives.

[0020] The anode 120 may include an anode film 122 and an anode active material layer 124 coated on the anode film 122. Although FIG. 1 illustrates the anode active material layer 124 coated on one side of the anode film 122, the present invention is not limited thereto, and the anode active material layer 124 may be coated on both sides of the anode film 122. The anode film 122 can include at least one metal selected from the group consisting of copper, stainless steel, nickel, titanium, etc. The anode active material layer 124 can include an anode active material, a binder, and a conductive agent.

[0021] The anode active material can be formed of a material capable of alloying with lithium or reversibly absorbing and releasing lithium. For example, the anode active material can include at least one material selected from the group consisting of a metal, a carbon-based material, a metal oxide, and a lithium metal nitride.

[0022] The metal can include at least one material selected from the group consisting of lithium, silicon, magnesium, calcium, aluminum, germanium, tin, lead, arsenic, antimony, bismuth, silver, gold, zinc, cadmium, mercury, copper, iron, nickel, cobalt, and indium.

[0023] Carbon-based materials include graphite, graphite carbon fiber, carbon nanotubes, coke, and mesocarbons. The material may include at least one material selected from the group consisting of microbeads (MCMB), polyacene, pitch-based carbon fiber, and non-graphitizable carbon (hardcarbon).

[0024] The metal oxide may include at least one selected from the group consisting of lithium titanium oxide, titanium oxide, molybdenum oxide, niobium oxide, iron oxide, tungsten oxide, tin oxide, amorphous tin composite oxide, silicon monoxide, cobalt oxide, and nickel oxide.

[0025] The binder and conductive agent may be the same as those contained in the cathode active material layer.

[0026] The separator 130 separates the cathode 110 and the anode 120 and prevents a short circuit between the cathode 110 and the anode 120. The separator 130 can be manufactured by, for example, but not limited to, coating a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP copolymer) onto any one substrate selected from the group consisting of polyethylene (PE), polystyrene (PS), polypropylene (PP), and a copolymer of polyethylene (PE) and polypropylene (PP).

[0027] The separator 130 may include a coating layer 132 formed on at least one of both surfaces of the separator 130. While FIG. 1 illustrates an example in which the coating layer 132 is formed on both surfaces of the separator 130, the present invention is not limited thereto, and the coating layer 132 may be formed on only one surface of the separator 130. The coating layer 132 includes boron nitride nanotubes (BNNTs) and can improve the conductivity of lithium ions in the lithium ion secondary battery 10 .

[0028] Boron nitride nanotubes (BNNTs) are one-dimensional hexagonal nanotubes in which nitrogen (N) and boron (B) are alternately arranged, and can have open ends as shown in Figure 2. Such boron nitride nanotubes (BNNTs) can have lengths of 5 μm to 10 μm and aspect ratios of 20 to 6000, and the outer surfaces and internal spaces of the one-dimensional boron nitride nanotubes (BNNTs) can be used as paths for lithium ion migration.

[0029] More specifically, the electron-deficient (Lewis acid) boron centers of BNNTs can interact with the electrolyte molecules of the oxygen-rich electrolyte 300 to induce desolvation of lithium ions, improving lithium ion transport on the surface and through the interior of the BNNTs. That is, the Lewis acid interaction between the anion / solvent and the BNNTs can promote lithium ion dissociation, accelerating lithium ion transport and inducing a high lithium ion transport rate.

[0030] 3, at least a portion of the boron nitride nanotubes (BNNTs) of the coating layer 132 are not attached perfectly flush to the surface of the separator 130, but can be attached so as to form a certain angle with the surface of the separator 130. As an example, the average surface roughness (rms) of the coating layer 132 can be 50 nm to 2 μm, and the angle formed between at least a portion of the boron nitride nanotubes (BNNTs) and the surface of the separator 130 can be 1° to 30°. In this way, by attaching at least a portion of the boron nitride nanotubes (BNNTs) to the surface of the separator 130 so as to form a certain angle, lithium ions can move directionally between the cathode 110 and the anode 120. Here, the boron nitride nanotubes (BNNTs) attached to the surface of the separator 130 at a certain angle may account for 30% to 70% of the total boron nitride nanotubes (BNNTs).

[0031] In addition, the ionic conductivity of the separator 130 on which the coating layer 132 is formed is 1.7 mS / cm or less at 60°C and 0.4 mS / cm or more at -10°C, and thus the separator 130 maintains excellent ionic conductivity at high and low temperatures, thereby preventing or minimizing a sudden performance degradation of the lithium ion secondary battery 10 due to temperature changes.

[0032] Furthermore, when boron nitride nanotubes (BNNTs) are attached to the surface of separator 130, the tendency of separator 130 to thermally shrink when heated can be reduced. Therefore, by forming coating layer 132 containing boron nitride nanotubes (BNNTs) on at least one of both surfaces of separator 130, the ionic conductivity of lithium-ion secondary battery 10 can be improved, and the thermal stability of separator 130 can also be improved.

[0033] The case 200 can be configured in various shapes and materials depending on the type of lithium ion secondary battery 10. The lithium ion secondary battery 10 may be coin-shaped, square-shaped, pouch-shaped, or cylindrical, and therefore the case 200 may be rigid coin-shaped, square-shaped, cylindrical, or flexible pouch-shaped.

[0034] The case 200 is filled with an electrolyte 300 , and the electrode assembly 100 may be immersed in the electrolyte 300 .

[0035] The electrolyte solution 300 may be a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, etc. However, the electrolyte solution 300 is not limited thereto, and may include a water-based electrolyte, a gel polymer electrolyte, etc.

[0036] FIG. 4 is a flowchart that schematically shows a method for manufacturing the lithium ion secondary battery of FIG.

[0037] 1 and 4, a method for manufacturing a lithium-ion secondary battery 10 according to an embodiment of the present invention may include the steps of: mixing boron nitride nanotubes (BNNTs) in a solvent to form a coating solution (S10); coating the coating solution on at least one of both surfaces of a separator 130 to form a coating layer 132 (S20); forming an electrode assembly 100 by arranging a cathode 110 and an anode 120 on both surfaces of the separator 130 on which the coating layer 132 has been formed (S30); and manufacturing a lithium-ion secondary battery 10 by placing the electrode assembly 100 in a case 200 and filling the case 200 with an electrolyte 300 (S40).

[0038] The coating liquid can be prepared by dispersing boron nitride nanotubes (BNNTs) in a solvent. Dispersion can be achieved by ultrasonic dispersion, stirring, or the like.

[0039] The solvent may be hydrophobic or hydrophilic. If the solvent is hydrophobic, the solvent may include a nitrile solvent, an ether solvent, an ester solvent, etc. The nitrile solvent may include, for example, acetonitrile.

[0040] For hydrophilic solvents, the solvents are acetic acid, acetylacetone, 2-aminoethanol, anisole, isopropyl alcohol, benzyl alcohol, 1-butanol, 2-butanol, 2- These may include butanone, t-butyl alcohol, cyclohexanol, cyclohexanone, di-n-butyl phthalate, diethyl glycol, diglyme, dimethoxyethane, dimethylformamide, dimethyl phthalate, dimethyl sulfoxide, dioxane, ethanol, ether, ethyl acetate, ethyl acetoacetate, ethyl benzoate, ethylene glycol, glycerin, N-methyl-2-pyrrolidone, tetrahydrofuran, and the like.

[0041] The boron nitride nanotubes (BNNTs) may be contained in the coating liquid at 0.01 wt % to 10 wt %.

[0042] When the content of boron nitride nanotubes (BNNTs) is 0.01 wt% or more relative to the total coating liquid (100 wt%), when the coating liquid is sprayed onto at least one side of the separator 130 to form the coating layer 132, the lithium ion conductivity of the separator 130 increases, the thermal stability improves, and the performance and stability of the lithium ion secondary battery 10 can be improved.

[0043] On the other hand, if the content of boron nitride nanotubes (BNNTs) is greater than 10 wt% relative to the total 100 wt% of the electrolyte 300, aggregation of the boron nitride nanotubes (BNNTs) may occur on the surface of the separator 130, which may reduce the charging performance and capacity of the lithium-ion secondary battery.

[0044] Meanwhile, as will be described later, the coating layer 132 can be formed by electrostatically spraying a coating liquid. Therefore, it is necessary to uniformly disperse boron nitride nanotubes (BNNTs) in the coating liquid for forming the coating layer 132. However, since boron nitride nanotubes (BNNTs) generally have low dispersibility in organic and aqueous solvents, the dispersibility of the boron nitride nanotubes (BNNTs) can be improved by surface-treating the boron nitride nanotubes (BNNTs) to be hydrophilic or hydrophobic. This will be described with reference to FIGS. 5 to 8.

[0045] FIG. 5 is a perspective view schematically showing an example of surface-treated boron nitride nanotubes dispersed in the electrolyte of FIG. 1, and FIG. 6 is a flowchart schematically showing a method for surface-treating the boron nitride nanotubes of FIG. 5. 5 shows an example of a surface-treated boron nitride nanotube (BNNT'). Referring to FIG. 5, the surface-treated boron nitride nanotube (BNNT') can include a first layer 410 on at least a portion of the surface of the boron nitride nanotube (BNNT).

[0046] For example, the first layer 410 may contain hydroxyphenyl groups, which may make the boron nitride nanotubes (BNNTs) hydrophilic, thereby enabling the surface-treated boron nitride nanotubes (BNNTs') to disperse well in a polar electrolyte.

[0047] For example, the first layer 410 may include a hydroxyphenyl group and may be π-bonded with a boron nitride nanotube (BNNT). As an example, the first layer 410 may be a polyphenol group and may include at least one of tannic acid, gallic acid, catechol, epigallocatechin, pyrogallol, hexahydroxydiphenic acid, ellagic acid, and chlorogenic acid.

[0048] The polyphenol groups of the first layer 410 are oxidized to highly reactive quinone oligomers and attached to the surface of the boron nitride nanotubes (BNNTs), and the first layer 410 can be formed on the boron nitride nanotubes (BNNTs) through strong interactions between the catechol molecules of the polyphenol groups and the boron nitride nanotubes (BNNTs) via van der Waals bonds and π-π stacking.

[0049] In this way, by forming the first layer 410 on at least a portion of the surface of the boron nitride nanotube (BNNT), hydroxy groups are present on the surface of the boron nitride nanotube (BNNT), and the surface-treated boron nitride nanotube (BNNT') can have hydrophilic properties, thereby improving its dispersibility in hydrophilic solvents.

[0050] As shown in FIG. 6, a method for forming the first layer 410 may include the steps of: mixing a first surface treatment agent in water to form a mixture (S110); dispersing boron nitride nanotubes (BNNTs) in the mixture to form a dispersion (S120); and washing and drying the boron nitride nanotubes (BNNTs) from the dispersion (S130).

[0051] The first surface treatment agent is a material capable of forming the first layer 410 and may include a hydroxyphenyl group. As an example, the first surface treatment agent is a polyphenol group and may include at least one of tannic acid, gallic acid, catechol, epigallocatechin, pyrogallol, hexahydroxydiphenic acid, ellagic acid, and chlorogenic acid.

[0052] The first surface treatment agent may be contained in an amount of 0.1 wt% to 0.2 wt% based on the total mixture. If the content of the first surface treatment agent in the mixture is less than 0.1 wt%, it is difficult to form the first layer 410 that can impart hydrophilicity to the boron nitride nanotubes (BNNTs). On the other hand, even if the content of the first surface treatment agent is more than 0.2 wt%, the surface treatment effect of the boron nitride nanotubes (BNNTs) does not increase continuously. Therefore, the content of the first surface treatment agent is preferably 0.1 wt% to 0.2 wt% based on the total mixture.

[0053] Next, boron nitride nanotubes (BNNTs) are dispersed in the mixture by ultrasonic dispersion, stirring, or the like. In this case, the dispersion may have a weak alkaline property, which allows the polyphenol groups to be oxidized by dissolved oxygen in the dispersion to highly reactive quinone oligomers, which are then attached to the surface of the boron nitride nanotubes (BNNTs), forming the first layer 410.

[0054] The pH of the dispersion can be adjusted with a base such as sodium hydroxide. In one example, the pH of the dispersion can be 8 to 9. If the pH of the dispersion is lower than 8 or higher than 9, it becomes difficult to form quinone oligomers, so the pH of the dispersion is preferably 8 to 9.

[0055] The mixing ratio of the boron nitride nanotubes (BNNTs) dispersed in the dispersion to the first surface treatment agent can be 1:1 to 1:0.1 by wt %. If the content of the boron nitride nanotubes (BNNTs) dispersed in the dispersion exceeds 10 times the content of the first surface treatment agent, it may be difficult to effectively form the first layer 410 on the surface of the boron nitride nanotubes (BNNTs). On the other hand, if the content of boron nitride nanotubes (BNNTs) dispersed in the dispersion is less than one-fold relative to the first surface treatment agent, the amount of the first surface treatment agent discarded in the subsequent washing step increases sharply.

[0056] After dispersing boron nitride nanotubes (BNNTs) in the mixed liquid to form a dispersion, the boron nitride nanotubes (BNNTs) on which the first layer 410 is formed are washed and dried.

[0057] In the washing process, the surface-treated boron nitride nanotubes (BNNT') are washed with water to remove any remaining polyphenol groups. The surface-treated boron nitride nanotubes (BNNT') are then collected by centrifugation or filtration, and dried to obtain a powder of hydrophilic surface-treated boron nitride nanotubes (BNNT').

[0058] The above method can be carried out at room temperature and in the atmosphere. Therefore, according to the present invention, boron nitride nanotubes can be made hydrophilic by simply dispersing them in a mixture of water and a first surface treatment agent, without creating a specific environment. Furthermore, tannic acid and other substances contained in the first layer are naturally occurring, environmentally friendly substances, and the surface-treated boron nitride nanotubes may not cause environmental pollution.

[0059] FIG. 7 is a perspective view schematically showing another example of surface-treated boron nitride nanotubes dispersed in the electrolyte of FIG. 1, and FIG. 8 is a flowchart schematically showing a method for surface-treating the boron nitride nanotubes of FIG. 7. FIG. 7 shows another example of a surface-treated boron nitride nanotube (BNNT''). Referring to FIG. 7, the surface-treated boron nitride nanotube (BNNT'') can include a first layer 410 formed on at least a portion of the surface of the boron nitride nanotube (BNNT) and a second layer 420 on the first layer 410. That is, compared to the surface-treated boron nitride nanotube (BNNT') of FIG. 5, the boron nitride nanotube (BNNT) can be made hydrophobic by further including the second layer 420.

[0060] The second layer 420 can be formed on the first layer 410. The first layer 410 is the same as that shown and described in Figure 5 and will not be described again.

[0061] The second layer 420 is a layer for imparting hydrophobicity to the boron nitride nanotubes (BNNTs) and may include an amine group or a thiol group as a hydrocarbon group. For example, the second layer 420 may include at least one of alkylamine, alkylthiol, arylamine, arylthiol, benzylamine, and benzylthiol.

[0062] Meanwhile, the polyphenol groups of the first layer 410 are oxidized to reactive quinone oligomers and attached to the surface of the boron nitride nanotubes (BNNTs), and the attached quinone structures can immobilize the primary amine groups of the second layer 420 through a Michael addition mechanism. More specifically, the reaction between the amine or thiol of the second layer 420 and the hydroxyl groups of the first layer 410 is a Michael addition reaction of the amine or thiol of the second layer 420 to the hydroxyl groups of the first layer 410, and thus the second layer 420 can be formed on the first layer 410.

[0063] When the second layer 420 is further included, the surface-treated boron nitride nanotubes (BNNTs'') have hydrophobicity and can have excellent dispersibility in a hydrophobic solvent.

[0064] On the other hand, the first layer 410 is omitted and the second layer 420 is not formed directly on the surface of the boron nitride nanotubes (BNNTs). Therefore, in order to form the second layer 420 to impart hydrophobicity to the boron nitride nanotubes (BNNTs), the first layer 410 must be formed first.

[0065] As shown in FIG. 7, a method for forming the second layer 420 may include the steps of: mixing a first surface treatment agent in water to form a first mixture (S210); dispersing boron nitride nanotubes (BNNTs) in the first mixture to form a dispersion (S220); mixing a second surface treatment agent in the dispersion to form a second mixture (S230); and washing and drying the boron nitride nanotubes (BNNTs) from the second mixture (S240).

[0066] The step of forming a first mixture (S210) and the step of forming a dispersion (S220) are the same as the step of forming a mixture (S110 in FIG. 6) and the step of forming a dispersion (S120 in FIG. 6) in FIG. 6, so they will not be described repeatedly, and only the differences from FIG. 6 will be described.

[0067] 8, after forming the dispersion, a second surface treatment agent is further mixed with the dispersion to form a second mixture (S230). The second surface treatment agent is a material for imparting hydrophobicity to the boron nitride nanotubes (BNNTs) and may include an amine group or a thiol group as a hydrocarbon group capable of Michael addition to the hydroxyl group of the first layer 410.

[0068] For example, the second surface treatment agent may include at least one of alkylamine, alkylthiol, arylamine, arylthiol, benzylamine, and benzylthiol.

[0069] The amount of the second surface treatment agent mixed can be 0.5 to 2 times the amount of the boron nitride nanotubes (BNNTs) mixed. If the amount of the second surface treatment agent mixed is less than 0.5 times the amount of the boron nitride nanotubes (BNNTs) mixed, the second layer will not be formed effectively and the boron nitride nanotubes (BNNTs) will not be easily converted to hydrophobicity. If the amount of the second surface treatment agent mixed is more than twice the amount of the boron nitride nanotubes (BNNTs) mixed, the amount of the second surface treatment agent discarded in the subsequent cleaning process will increase dramatically.

[0070] After dispersing the boron nitride nanotubes (BNNTs) in the second mixture to form a dispersion, the boron nitride nanotubes on which the second layer is formed are washed and dried (S240).

[0071] In the washing process, the surface-treated boron nitride nanotubes (BNNTs) are washed sequentially with water and ethanol to remove any remaining polyphenol and hydrocarbon groups. The boron nitride nanotubes (BNNTs) are then collected by centrifugation or filtration, and dried to obtain a powder of boron nitride nanotubes (BNNTs) that have been surface-treated to have hydrophobic properties.

[0072] On the other hand, unlike the above method, when the second surface treatment agent was mixed in an organic solvent such as toluene and the boron nitride nanotubes (BNNTs) were dispersed, the second layer 420 was not formed on the surface of the boron nitride nanotubes (BNNTs), and therefore the hydrophobicity of the boron nitride nanotubes (BNNTs) could not be imparted. In other words, in order to form the second layer 420, A first layer 410 must first be formed on the surface of the boron nitride nanotubes (BNNTs) using a first surface treatment agent.

[0073] The method can be carried out at room temperature and in the atmosphere. Therefore, according to the present invention, boron nitride nanotubes (BNNTs) can be made hydrophobic without creating a specific environment by a simple method of dispersing boron nitride nanotubes (BNNTs) in a mixture obtained by mixing a first surface treatment agent with water, and then further mixing in a second surface treatment agent. Furthermore, according to this method, by using water as a dispersion medium in the process of surface treating the boron nitride nanotubes (BNNTs) to make them hydrophobic, the problem of environmental pollution caused by using organic solvents does not occur.

[0074] Referring again to FIG. 4, the prepared coating liquid is coated on at least one surface of the separator 130 to form a coating layer 132 (S20).

[0075] FIG. 9 is a diagram schematically showing an example of a method for forming a boron nitride nanotube coating layer on the separator of FIG. FIG. 9 is a schematic diagram of an electrostatic spraying method, which is an example of a method for forming the coating layer 132. Electrostatic spraying is a method of atomizing a liquid by splitting it into fine droplets using electrical force. For example, liquid passing through a nozzle N forms a Taylor cone using electromagnetic force, and after passing through a short liquid column, the liquid is split by the repulsive force between the liquid particles and sprayed as fine droplets. At this time, the sprayed fine droplets are electrically charged. Therefore, during electrostatic spraying, a voltage is applied to the nozzle N, and an electrode body such as a drum may be grounded.

[0076] 9, a separator 130 is placed on an electrode body such as a drum, and a coating liquid is electrostatically sprayed onto the separator 130 from a nozzle N having holes with a diameter of microns to form a coating layer 132. In this case, the drum is cylindrical and can rotate while being grounded.

[0077] In FIG. 9, electrostatic spraying is shown and described as an example of a method for forming the coating layer 132, but the present invention is not limited thereto, and the coating layer 132 can be formed by various methods such as air spraying, ultrasonic spraying, etc.

[0078] Alternatively, the separator 130 may be first formed by spraying a material for forming the separator 130 onto a rotating conveyor belt, and then a coating liquid for forming the coating layer 132 may be sprayed onto the formed separator 130 to successively form the separator 130 and the coating layer 132. Here, the material for forming the separator 130 may be polyethylene (PE), polystyrene (PS), polypropylene (PP), polyvinylidene fluoride-hexafluoropropylene copolymer, etc.

[0079] After spraying the coating liquid, the coating liquid is dried to remove the solvent, thereby forming the coating layer 132. Drying can be performed by various methods such as vacuum drying, hot air drying, freeze drying, spray drying, and the like.

[0080] Meanwhile, the coating layer 132 may be formed by repeating the electrostatic spraying multiple times. For example, the electrostatic spraying and drying process may be repeated two to four times to form the coating layer 132. When the coating layer 132 is formed by repeating the electrostatic spraying multiple times, the amount of boron nitride nanotubes contained in the coating layer 132 increases, and more boron nitride nanotubes are used as paths for lithium ions to move, thereby improving the overall ionic conductivity.

[0081] Specifically, when coating layer 132 was formed using a coating liquid containing 0.1 wt% boron nitride nanotubes, the ionic conductivity of separator 130 was 1.34 mS / cm when coating layer 132 was formed by one electrostatic spraying, but when coating layer 132 was formed by two electrostatic sprayings, the ionic conductivity of separator 130 increased to 1.48 mS / cm, and when coating layer 132 was formed by three electrostatic sprayings, the ionic conductivity of separator 130 increased to 1.78 mS / cm. However, when electrostatic spraying was performed five or more times, aggregation of boron nitride nanotubes (BNNTs) occurred on the surface of separator 130, and the ionic conductivity of separator 130 may actually decrease.

[0082] Meanwhile, at least a portion of the boron nitride nanotubes (BNNTs) included in the coating layer 132 may not be completely attached to the surface of the separator 130, but may be attached so as to form a certain angle with the surface of the separator 130. For example, the average surface roughness (rms) of the coating layer 132 may be 50 nm to 2 μm, and the angle formed between at least a portion of the boron nitride nanotubes (BNNTs) and the surface of the separator 130 may be 1° to 30°. In this way, at least a portion of the boron nitride nanotubes (BNNTs) are attached so as to form a certain angle with the surface of the separator 130, and can serve as a path for directional movement of lithium ions between the cathode 110 and the anode 120.

[0083] Next, an anode 120 and a cathode 110 are disposed on both sides of the separator 130 on which the coating layer 132 is formed, respectively, to form an electrode assembly 100 (S30). The electrode assembly 100 is then placed in a case 200, and the case 200 is then filled with an electrolyte solution, thereby manufacturing a lithium-ion secondary battery 10 (S40).

[0084] FIG. 10 is a diagram showing the results of measuring the lithium ion conductivity of the separator. In FIG. 10, A shows the lithium ion conductivity of the PE separator, and B shows the lithium ion conductivity when a coating layer is formed on the surface of the PE separator according to the present invention.

[0085] In Figure 10B, the coating layer was formed by electrostatically spraying a coating liquid, in which 0.5 wt% of boron nitride nanotubes were dispersed in IPA, onto both sides of the PE separator, and then drying it at a temperature of 40°C for 24 hours to remove the solvent from the coating liquid.

[0086] The ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using a stainless steel disk as a blocking electrode, and the electrolyte was a 1:1 mixture of EC and DMC with 1M LiPF6 dispersed in it.

[0087] Referring to FIG. 10, it can be seen that in the case of B in which a coating layer was formed, the lithium ion conductivity was increased compared to A not only at room temperature (25°C) and high temperature (60°C) but also at low temperature (-10°C).

[0088] Specifically, at low temperatures (-10°C), the ionic conductivity of sample A was 0.31 mS / cm, while that of sample B with a coating layer was 0.44 mS / cm, indicating an increase of approximately 42%. Furthermore, at high temperatures (60°C), the ionic conductivity of sample A was 1.20 mS / cm, while that of sample B with a coating layer was 1.65 mS / cm, indicating an increase of approximately 37.5%. Therefore, by forming a coating layer containing boron nitride nanotubes, it is possible to prevent or minimize the rapid performance degradation of lithium-ion secondary batteries at high and low temperatures.

[0089] In addition, when the temperature increased from 45 to 60°C, the ionic conductivity of A increased by 19%. In contrast, B shows a 25% increase, which indicates that the thermal stability of the separator is improved by forming a coating layer on it.

[0090] FIG. 11 is a graph showing the results of measuring the initial charge / discharge capacity of a lithium ion secondary battery. Figure 11 shows the results of precycling a lithium ion secondary battery including an NCM523 cathode and a graphite anode from 2.7 V to 4.2 V at a charge / discharge rate of 0.1 C. Figure 11A includes the separator of Figure 10A, and Figure 11B includes the separator of Figure 10B.

[0091] As can be seen from Figure 11, in case A, the charge capacity decreased from 383.6 mAh to 380.4 mAh, and the Coulombic efficiency was measured to be 99.19%, while in case B, which includes a separator with a coating layer containing boron nitride nanotubes, the charge capacity changed from 413.6 mAh to 410.9 mAh, and the Coulombic efficiency was measured to be 99.34%. This shows that the efficiency of lithium-ion secondary batteries improves as the lithium ion conductivity increases due to the formation of a coating layer on the separator.

[0092] 12 and 13 are diagrams showing the characteristics (rate performance) of a lithium ion secondary battery depending on the temperature and charge / discharge rate. 12 and 13, A is the result using the lithium ion secondary battery of A in Fig. 11, and B is the result using the lithium ion secondary battery of B in Fig. 11. Note that B2 in Fig. 12 and 13 differs from B only in that a coating solution containing 0.7 wt% of boron nitride nanotubes was used when forming the coating layer, and the rest is the result using a lithium ion secondary battery including a separator on which a coating layer containing boron nitride nanotubes was formed, just like B.

[0093] 12 and 13 show the capacity of a lithium-ion secondary battery measured at different charge / discharge rates, where Fig. 12 shows the capacity measured at 25°C and Fig. 13 shows the capacity measured at -10°C.

[0094] First, referring to Figure 12, it can be seen that B and B2 have better charge / discharge rate characteristics than A at 0.5C, 0.7C, 1C, 3C, and 5C. In particular, referring to Figure 13 showing the results measured at -10°C, it can be seen that B and B2 have better charge / discharge rate characteristics than A at 0.5C, 1C, and 3C.

[0095] As can be seen from the above results, when a separator is coated with a coating layer containing boron nitride nanotubes, the lithium ion conductivity increases, resulting in improved charge / discharge rate characteristics at low temperatures as well as room temperature.

[0096] Figures 14 and 15 are diagrams showing the results of measuring the temperature of a lithium-ion secondary battery during charge and discharge. Figure 14 shows the results for the lithium-ion secondary battery of A in Figure 11, and Figure 15 shows the results for the lithium-ion secondary battery of B in Figure 11. In Figures 14 and 15, the temperature of the lithium-ion secondary battery was measured while repeatedly charging and discharging at a charge and discharge rate of 0.5 C at room temperature (25°C).

[0097] 14 and 15, it can be seen that B, in which a coating layer containing boron nitride nanotubes is formed on the separator, has a lower average temperature than A. In particular, the maximum temperature of B was measured at 27.96°C, while A was measured at 28.85°C. This shows that when a coating layer containing boron nitride nanotubes is formed on the separator, the heat dissipation characteristics of the lithium ion secondary battery are also improved. This is because boron nitride nanotubes are a material with very high thermal conductivity and also have high lithium ion conductivity, which results in This is expected to be due to the low internal resistance.

[0098] Figures 16 and 17 are graphs showing the results of CV (cyclic voltammetry) tests on lithium-ion secondary batteries. Figures 16 and 17 are tests to confirm the possibility of chemical reactions occurring between the electrodes and the electrolyte during charging and discharging of the lithium-ion secondary battery, and show the results of confirming whether a chemical reaction occurred between the lithium metal electrode and the stainless steel electrode in a configuration with a separator between them.

[0099] Fig. 16 shows the results for the lithium ion secondary battery A in Fig. 11, and Fig. 17 shows the results for the lithium ion secondary battery B in Fig. 11. In Fig. 16 and Fig. 17, the scan rate was set to -0.2 to 5 V and 0.1 mV / s.

[0100] Referring to Figures 16 and 17, Figures 16 and 17 have peaks at the same positions, but Figure 17 shows a larger current peak than Figure 16, which means that the ionic conductivity is larger.

[0101] In addition, as can be seen from Figure 17, the results of the CV (cyclic voltammetry) test showed that no distinct peaks were generated during reduction below 0 voltage or oxidation above 0 voltage. This indicates that no chemical reaction occurs between the lithium metal electrode and the stainless steel electrode, demonstrating electrochemical stability and feasibility in practical battery applications.

[0102] FIG. 18 is a diagram showing the characteristics (C-rate performance) of a lithium ion secondary battery depending on the charge / discharge rate, and FIG. 19 is a diagram showing the cycle retention results of the lithium ion secondary battery of FIG.

[0103] 18 and 19, the lithium ion secondary battery was a pouch type, contained an LFP cathode material, a graphite anode material, and an LiPF6 electrolyte, and used a PE separator. 18 and 19, A is the case where the coating layer according to the present invention is not applied to the separator, and B is the case where the coating layer containing boron nitride nanotubes is formed on the separator. The coating layer was formed in the same manner as in FIG.

[0104] 18, it can be seen that the discharge capacity of B at 0.1C, 0.2C, and 0.3C is greater than that of A. This is because the lithium ion conductivity is improved by forming a coating layer containing boron nitride nanotubes on the separator surface.

[0105] Furthermore, Figure 19 shows the discharge capacity when charging and discharging were repeated at a rate of 0.1 C, and it can be seen that B, which has a coating layer formed on the separator surface, has an improved discharge capacity compared to A, which does not.

[0106] In other words, even in the case of secondary batteries that use various cathode materials such as LFP instead of NCM-type cathode materials, forming a coating layer containing boron nitride nanotubes on the separator surface can improve lithium ion conductivity and secondary battery performance.

[0107] Although the present invention has been described with reference to one embodiment shown in the drawings, it will be understood that this is merely an example, and that various modifications and variations of the embodiment are possible by those skilled in the art. Therefore, the true technical scope of protection of the present invention should be determined by the technical spirit of the appended claims. [Explanation of symbols]

[0108] 10 Lithium-ion secondary battery 100 Electrode Assembly 110 cathode 112 Cathode Film 114 Cathode active material layer 120 anodes 122 Anode Film 124 Anode active material layer 130 Separator 132 Coating Layer 200 cases 300 Electrolyte

Claims

1. an electrode assembly including an anode, a cathode, and a separator disposed between the anode and the cathode; a case for housing the electrode assembly; an electrolyte filled in the case; the separator includes a coating layer on at least one of both surfaces of the separator; the coating layer comprises boron nitride nanotubes; The coating layer has an average surface roughness of 50 nm to 2 μm.

2. 2. The lithium ion secondary battery according to claim 1, wherein the aspect ratio of the boron nitride nanotubes is 20 to 6000.

3. 2. The lithium ion secondary battery according to claim 1, wherein the boron nitride nanotubes are surface-treated to have hydrophilic or hydrophobic properties.

4. the surface-treated boron nitride nanotubes further comprising a first layer overlying at least a portion of the boron nitride nanotubes; The lithium ion secondary battery according to claim 3 , wherein the first layer contains a hydroxyphenyl group and forms a π bond with the boron nitride nanotube.

5. The lithium ion secondary battery according to claim 4 , wherein the surface-treated boron nitride nanotubes have the hydrophilic property.

6. the surface-treated boron nitride nanotubes further comprise a second layer on the first layer; 5. The lithium ion secondary battery according to claim 4, wherein the second layer contains an amine group or a thiol group as the hydrocarbon group, and the surface-treated boron nitride nanotube has the hydrophobic property.

7. At least some of the boron nitride nanotubes are in contact with the separator surface by 1° to 30°. The lithium ion secondary battery according to claim 1, wherein the battery is mounted at an angle of

8. mixing boron nitride nanotubes in a solvent to form a coating solution; coating the coating liquid on at least one of both surfaces of a separator to form a coating layer; forming an electrode assembly by disposing an anode and a cathode on both sides of the separator on which the coating layer is formed; placing the electrode assembly in a case and filling the case with an electrolyte; The coating layer has an average surface roughness of 50 nm to 2 μm.

9. The method for manufacturing a lithium ion secondary battery according to claim 8 , wherein the coating layer is formed by electrostatically spraying or spray coating the coating liquid onto at least one of both surfaces of the separator.

10. The method for manufacturing a lithium ion secondary battery according to claim 8 , further comprising the step of drying the coating layer after forming the coating layer.

11. 9. The method for producing a lithium ion secondary battery according to claim 8, wherein the coating liquid contains 0.01 wt % to 10 wt % of the boron nitride nanotubes.

12. The method for manufacturing a lithium ion secondary battery according to claim 8 , wherein the boron nitride nanotubes are surface-treated to have hydrophilicity or hydrophobicity.

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