Lithium-ion secondary battery and manufacturing method therefor

Abstract

Disclosed in an embodiment of the present invention is a lithium-ion secondary battery comprising: an electrode assembly comprising a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode; a case in which the electrode assembly is accommodated; and an electrolyte filled in the case, wherein the separator comprises a coating layer on at least any one of both surfaces thereof and the coating layer contains boron nitride nanotubes.

Description

Lithium ion secondary battery and method for manufacturing the same

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

[0002] Secondary batteries, unlike disposable batteries that are discarded after use, are reusable through repeated charging and discharging. They are used in a variety of fields, including mobile phones, laptops, and electric vehicles, and their use as energy storage devices is also increasing recently.

[0003] There are various types of secondary batteries, but the most representative one is the lithium-ion secondary battery. Lithium-ion secondary batteries have high energy density, no memory effect, relatively short charging times, and easy charging and discharging. Meanwhile, the charge / discharge efficiency and charging time of lithium-ion secondary batteries are affected by the ionic conductivity of lithium ions. Therefore, to improve the efficiency of lithium-ion secondary batteries, the ionic conductivity of lithium ions needs to be improved.

[0004] Embodiments of the present invention provide a lithium ion secondary battery with improved ionic conductivity and a method for manufacturing the same.

[0005] One embodiment of the present invention discloses a lithium ion secondary battery, comprising: an electrode assembly including a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode; a case accommodating the electrode assembly; and an electrolyte 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.

[0006] According to embodiments of the present invention, the ionic conductivity of a lithium ion secondary battery can be increased, thereby improving the performance of the lithium ion secondary battery.

[0007] In addition, boron nitride nanotubes are attached to the surface of the separator, which reduces the shrinkage tendency of the separator and efficiently dissipates the generated heat, thereby improving thermal stability and thus improving the safety of the secondary battery.

[0008] 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.

[0009] Figure 2 is a TEM image of boron nitride nanotubes included in the coating layer of Figure 1.

[0010] Fig. 3 is an SEM image of the surface of the separator on which the coating layer of Fig. 1 is formed.

[0011] Figure 4 is a flowchart schematically illustrating a method for manufacturing the lithium ion secondary battery of Figure 1.

[0012] FIG. 5 is a perspective view schematically illustrating an example of boron nitride nanotubes included in the coating layer of FIG. 1.

[0013] Figure 6 is a flowchart schematically illustrating a method for surface treatment of boron nitride nanotubes of Figure 5.

[0014] FIG. 7 is a perspective view schematically illustrating an example of boron nitride nanotubes included in the coating layer of FIG. 1.

[0015] Figure 8 is a flowchart schematically illustrating a method for surface treatment of boron nitride nanotubes of Figure 7.

[0016] FIG. 9 is a schematic diagram illustrating an example of a method for forming a boron nitride nanotube coating layer on the separator of FIG. 1.

[0017] Figure 10 is a diagram showing the results of measuring the lithium ion conductivity of a separator.

[0018] Figure 11 is a graph showing the results of measuring the initial charge / discharge capacity of a lithium ion secondary battery.

[0019] Figures 12 and 13 are diagrams showing the characteristics (rate performance) of a lithium ion secondary battery according to temperature and charge / discharge rate.

[0020] Figures 14 and 15 are diagrams showing the results of measuring the temperature of a lithium ion secondary battery during charging and discharging.

[0021] Figures 16 and 17 are graphs showing the results of CV (cyclic vloltammetry) tests of lithium ion secondary batteries.

[0022] Figure 18 is a diagram showing the characteristics (C-rate performance) of a lithium ion secondary battery according to the charge / discharge rate.

[0023] Figure 19 is a diagram showing the cycle retention results of the lithium ion secondary battery of Figure 18.

[0024] One embodiment of the present invention discloses a lithium ion secondary battery comprising: an electrode assembly including a negative electrode, a positive electrode, and a separator disposed between the negative electrode and the positive electrode; a case accommodating the electrode assembly; and an electrolyte 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.

[0025] In this embodiment, the ionic conductivity of the setter may be 1.7 mS / cm or less at 60°C and 0.4 mS / cm or more at -10°C.

[0026] In this embodiment, the average surface roughness of the coating layer may be 50 nm to 2 μm.

[0027] In this embodiment, the aspect ratio of the boron nitride nanotube may be 20 to 6000.

[0028] In this embodiment, the boron nitride nanotubes may be surface-treated to have hydrophilicity or hydrophobicity.

[0029] In the present embodiment, the surface-treated boron nitride nanotube further includes a first layer positioned on at least a portion of the boron nitride nanotube, wherein the first layer includes a hydroxyphenyl group and is capable of forming a π bond with the boron nitride nanotube.

[0030] In this embodiment, the surface-treated boron nitride nanotubes may have the hydrophilicity.

[0031] In the present embodiment, the surface-treated boron nitride nanotube further includes a second layer on the first layer, the second layer is surface-treated to include an amine group or a thiol group as a hydrocarbon group, and the surface-treated boron nitride nanotube can have the hydrophobicity.

[0032] In the present embodiment, at least some of the boron nitride nanotubes may be attached to the surface of the separator at an angle of 1° to 30°.

[0033] Another embodiment of the present invention discloses a method for manufacturing a lithium ion secondary battery, comprising: forming a coating solution by mixing boron nitride nanotubes in a solvent; forming a coating layer by coating the coating solution on at least one of both surfaces of a separator; forming an electrode assembly by arranging a negative electrode and a positive electrode on each of both surfaces of the separator on which the coating layer is formed; and positioning the electrode assembly in a case and filling the case with an electrolyte.

[0034] In this embodiment, the coating layer can be formed by electrostatically spraying or spray coating the coating liquid on at least one of the two surfaces of the separator.

[0035] In this embodiment, after forming the coating layer, a step of drying the coating layer may be further included.

[0036] In the present embodiment, the coating solution may contain 0.01 wt% to 10 wt% of the boron nitride nanotubes.

[0037] In this embodiment, the boron nitride nanotubes may be surface-treated to have hydrophilicity or hydrophobicity.

[0038] In this embodiment, the average surface roughness of the coating layer may be 50 nm to 2 μm.

[0039] The present invention is capable of various modifications and embodiments. Specific embodiments are illustrated in the drawings and described in detail in the detailed description. The effects and features of the present invention, as well as the methods for achieving them, will become clearer with reference to the embodiments described in detail below, along with the drawings. However, the present invention is not limited to the embodiments disclosed below and can be implemented in various forms.

[0040] In the examples below, the terms first, second, etc. are not used in a limiting sense, but are used for the purpose of distinguishing one component from another.

[0041] In the examples below, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0042] In the examples below, terms such as “include” or “have” mean that a feature or component described in the specification is present, and do not preclude the possibility that one or more other features or components may be added.

[0043] In the following examples, when a part such as a film, region, component, etc. is said to be on or above another part, it 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 in between.

[0044] For convenience of explanation, the sizes of components in the drawings may be exaggerated or reduced. For example, the sizes and thicknesses of each component shown in the drawings are arbitrarily indicated for convenience of explanation, and thus the present invention is not necessarily limited to what is shown.

[0045] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings, and when describing with reference to the drawings, identical or corresponding components will be given the same drawing reference numerals.

[0046] FIG. 1 is a cross-sectional view schematically illustrating an example of a lithium ion secondary battery according to an embodiment of the present invention, FIG. 2 is a TEM image of boron nitride nanotubes included in the coating layer of FIG. 1, and FIG. 3 is a SEM image of the surface of a separator on which the coating layer of FIG. 1 is formed.

[0047] Referring to FIG. 1, a lithium ion secondary battery (10) according to one embodiment of the present invention may include an electrode assembly (100), a case (200) that accommodates the electrode assembly, and an electrolyte (300) filled within the case (200).

[0048] The electrode assembly may include a positive electrode (110), a negative electrode (120), and a separator (130) between them to prevent short circuiting of the positive electrode (110) and the negative electrode (120).

[0049] The positive electrode (110) may include a positive electrode film (112) and a positive electrode active material layer (114) applied on the positive electrode film (112). In FIG. 1, the positive electrode active material layer (114) is illustrated as being applied to one side of the positive electrode film (112), but this is not limited thereto, and the positive electrode active material layer (114) may be applied to both sides of the positive electrode film (112).

[0050] The anode film (112) may be a metal formed of aluminum, stainless steel, titanium, silver, or a combination of materials selected from these.

[0051] The positive electrode active material layer (114) may include a positive electrode active material, a binder, and a conductive agent.

[0052] The cathode active material may be formed of a material capable of reversibly absorbing and releasing lithium ions. For example, the cathode active material may 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 manganese oxide, lithium manganese oxide, and lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, and vanadium oxide.

[0053] The binder may include at least one material selected from the group consisting of polyvinylidene fluoride-based binders such as polyvinylidene fluoride, vinylidene fluoride / hexafluoropropylene copolymer, vinylidene fluoride / tetrafluoroethylene copolymer, carboxymethylcellulose-based binders such as sodium-carboxymethylcellulose, lithium-carboxymethylcellulose, acrylate-based binders such as polyacrylic acid, lithium-polyacrylic acid, acrylic, polyacrylonitrile, polymethyl methacrylate, polybutylacrylate, polyamideimide, polytetrafluoroethylene, polyethylene oxide, polypyrrole, lithium-Nafion, and styrene butadiene rubber-based polymers.

[0054] The conductive agent may include at least one material selected from the group consisting of carbon-based conductive agents such as carbon black, carbon fibers, carbon nanotubes, and graphite, conductive fibers such as metal fibers, metal powders such as fluorocarbon 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.

[0055] The negative electrode (120) may include a negative electrode film (122) and a negative electrode active material layer (124) applied on the negative electrode film (122). In FIG. 1, the negative electrode active material layer (124) is illustrated as being applied to one side of the negative electrode film (122), but this is not limited thereto, and the negative electrode active material layer (124) may be applied to both sides of the negative electrode film (122).

[0056] The negative electrode film (122) may include at least one metal selected from the group consisting of copper, stainless steel, nickel, titanium, etc. The negative electrode active material layer (124) may include a negative electrode active material, a binder, and a conductive agent.

[0057] The negative electrode active material may be formed of a material capable of alloying with lithium or reversibly absorbing and releasing lithium. For example, the negative electrode active material may include at least one material selected from the group consisting of metals, carbon-based materials, metal oxides, and lithium metal nitrides.

[0058] The metal may 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.

[0059] The carbon-based material may include at least one material selected from the group consisting of graphite, graphite carbon fibers, carbon nanotubes, coke, mesocarbon microbeads (MCMB), polyacene, pitch-based carbon fibers, and hard carbon.

[0060] 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.

[0061] The binder and the conductive agent may be the same as those included in the positive electrode active material layer, respectively.

[0062] The separator (130) separates the positive electrode (110) and the negative electrode (120) to prevent a short circuit between the positive electrode (110) and the negative electrode (120). The separator (130) may be manufactured by coating a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP co-polymer) on any one substrate selected from the group consisting of, for example, polyethylene (PE), polystyrene (PS), polypropylene (PP), and a copolymer of polyethylene (PE) and polypropylene (PP), but is not limited thereto.

[0063] The separator (130) may include a coating layer (132) formed on at least one of both surfaces of the separator (130). FIG. 1 illustrates an example in which the coating layer (132) is formed on both surfaces of the separator (130), but is not limited thereto, and the coating layer (132) may be formed on only one surface of the separator (130).

[0064] The coating layer (132) can improve the conductivity of lithium ions in a lithium ion secondary battery (10) by including boron nitride nanotubes (BNNT).

[0065] Boron nitride nanotubes (BNNTs) are one-dimensional hexagonal nanotubes in which nitrogen (N) and boron (B) are alternately arranged, and may have open ends, as illustrated in Fig. 2. Such boron nitride nanotubes (BNNTs) may have a length of 5 μm to 10 μm and an aspect ratio of 20 to 6000, and the outer surface and inner space of the one-dimensional boron nitride nanotubes (BNNTs) can be utilized as a migration path for lithium ions.

[0066] More specifically, the electron deficiency (Lewis acid) at the boron center of boron nitride nanotubes (BNNTs) can interact with electrolyte molecules of an oxygen-rich electrolyte (300) to induce desolvation of lithium ions, thereby improving lithium ion transport through the surface and interior of the boron nitride nanotubes (BNNTs). That is, the Lewis acid interaction between the anion / solvent and the boron nitride nanotubes (BNNTs) can promote the dissociation of lithium ions and accelerate the transport of lithium ions, thereby inducing a high lithium ion transport rate.

[0067] In addition, as illustrated in FIG. 3, at least some of the boron nitride nanotubes (BNNTs) of the coating layer (132) may be attached to form a certain angle with the surface of the separator (130) rather than being completely laid down and attached to 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 some of the boron nitride nanotubes (BNNTs) and the surface of the separator (130) may be 1° to 30°. In this way, at least some of the boron nitride nanotubes (BNNTs) may be attached to form a certain angle with the surface of the separator (130), thereby acting as a passage through which lithium ions can move directionally between the positive electrode (110) and the negative electrode (120). Here, the boron nitride nanotubes (BNNT) attached to the surface of the separator (130) at a constant angle may be 30% to 70% of the total boron nitride nanotubes (BNNT).

[0068] In addition, the ionic conductivity of the setter (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, maintaining excellent ionic conductivity at high and low temperatures, and preventing or minimizing rapid performance degradation of the lithium ion arch battery (10) due to temperature changes.

[0069] In addition, when boron nitride nanotubes (BNNT) are attached to the surface of the separator (130), the thermal shrinkage tendency of the separator (130) when the separator (130) is heated can be reduced. Therefore, by forming a coating layer (132) including boron nitride nanotubes (BNNT) on at least one of the two surfaces of the separator (130), the ion conductivity of the lithium ion secondary battery (10) can be improved, and the thermal stability of the separator (130) can also be improved.

[0070] The case (200) may be made of 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 accordingly, the case (200) may be rigid coin-shaped, square-shaped, or cylindrical, or flexible pouch-shaped.

[0071] An electrolyte (300) is filled inside the case (200), and the electrode assembly (100) can be immersed in the electrolyte (300).

[0072] The electrolyte (300) may be a non-aqueous electrolyte containing a lithium salt, an organic solvent, etc. However, the present invention is not limited thereto, and the electrolyte (300) may include an aqueous electrolyte, a gel polymer electrolyte, etc.

[0073] Figure 4 is a flowchart schematically illustrating a method for manufacturing the lithium ion secondary battery of Figure 1.

[0074] Referring to FIGS. 1 and 4 together, a method for manufacturing a lithium ion secondary battery (10) according to an embodiment of the present invention may include a step (S10) of forming a coating solution by mixing boron nitride nanotubes (BNNT) in a solvent, a step (S20) of forming a coating layer (132) by coating the coating solution on at least one side of a separator (130) on both sides, a step (S30) of forming an electrode assembly (100) by arranging a positive electrode (110) and a negative electrode (120) on each side of the separator (130) on which the coating layer (132) is formed, and a step (S40) of manufacturing a lithium ion secondary battery (10) by positioning the electrode assembly (100) in a case (200) and filling the case (200) with an electrolyte (300).

[0075] The coating solution can be formed by dispersing boron nitride nanotubes (BNNTs) in a solvent. Dispersion can be achieved by ultrasonic dispersion, stirring, etc.

[0076] The solvent may be hydrophobic or hydrophilic. In the case of hydrophobicity, the solvent may include a nitrile solvent, an ether solvent, an ester solvent, etc., and as the nitrile solvent, for example, acetonitrile may be included.

[0077] For hydrophilicity, the solvent may include acetic acid, acetylacetone, 2-aminoethanol, anisole, isopropyl alcohol, benzyl alcohol, 1-butanol, 2-butanol, 2-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.

[0078] Boron nitride nanotubes (BNNTs) may be included in the coating solution in an amount of 0.01 wt% to 10 wt%.

[0079] When the content of boron nitride nanotubes (BNNT) is 0.01 wt% or more with respect to the total 100 wt% of the coating solution, when the coating solution is sprayed on at least one side of the separator (130) to form a coating layer (132), the lithium ion conductivity of the separator (130) increases and the thermal stability is improved, so that the performance and stability of the lithium ion secondary battery (10) can be improved.

[0080] On the other hand, if the content of boron nitride nanotubes (BNNT) is greater than 10 wt% with respect to the total 100 wt% of the electrolyte (300), aggregation of boron nitride nanotubes (BNNT) may occur on the surface of the separator (130), which may reduce the charging performance and capacity of the lithium ion secondary battery.

[0081] Meanwhile, as described below, the coating layer (132) can be formed by electrostatically spraying the coating solution. Therefore, boron nitride nanotubes (BNNT) need to be uniformly dispersed in the coating solution for forming the coating layer (132). However, since boron nitride nanotubes (BNNT) generally have low dispersibility in organic and aqueous solvents, the dispersibility of the boron nitride nanotubes (BNNT) can be improved by surface-treating the surface of the boron nitride nanotubes (BNNT) to be hydrophilic or hydrophobic. This will be described with reference to FIGS. 5 to 8.

[0082] FIG. 5 is a perspective view schematically illustrating an example of surface-treated boron nitride nanotubes dispersed in the electrolyte of FIG. 1, and FIG. 6 is a flowchart schematically illustrating a method for surface-treating the boron nitride nanotubes of FIG. 5.

[0083] FIG. 5 illustrates an example of a surface-treated boron nitride nanotube (BNNT'). Referring to FIG. 5, the surface-treated boron nitride nanotube (BNNT') may include a first layer (410) on at least a portion of the boron nitride nanotube (BNNT) surface.

[0084] The first layer (410) can, for example, include a hydroxyphenyl group to make the boron nitride nanotube (BNNT) hydrophilic, thereby enabling the surface-treated boron nitride nanotube (BNNT') to be well dispersed in a polar electrolyte.

[0085] For example, the first layer (410) may include a hydroxyphenyl group and may form a bond with boron nitride nanotubes (BNNTs). As an example, the first layer (410) may include a polyphenol group, such as at least one of tannic acid, gallic acid, catechol, epigallocatechin, pyrogallol, hexahydroxydiphenic acid, ellagic acid, and chlorogenic acid.

[0086] The polyphenol group of the first layer (410) can be oxidized into a highly reactive quinone oligomer and attached to the surface of a boron nitride nanotube (BNNT), and the first layer (410) can be formed on the boron nitride nanotube (BNNT) by a strong interaction between the catechol molecules of the polyphenol group and the boron nitride nanotube (BNNT) through van der Waals bonding and π-ð stacking.

[0087] In this way, since the first layer (410) is formed on at least a portion of the surface of the boron nitride nanotube (BNNT), thereby allowing hydroxyl groups to exist on the surface of the boron nitride nanotube (BNNT), the surface-treated boron nitride nanotube (BNNT') can have hydrophilicity, and thus, dispersibility in a hydrophilic solvent can be improved.

[0088] The method for forming the first layer (410) may include, as illustrated in FIG. 6, a step (S110) of forming a mixture by mixing a first surface treatment agent in water, a step (S120) of dispersing boron nitride nanotubes (BNNTs) in the mixture to form a dispersion, and a step (S130) of washing and drying boron nitride nanotubes (BNNTs) from the dispersion.

[0089] The first surface treatment agent is a material capable of forming the first layer (410) and may include a hydroxyphenyl group. For example, the first surface treatment agent may include a polyphenol group and at least one of tannic acid, gallic acid, catechol, epigallocatechin, pyrogallol, hexahydroxydiphenic acid, ellagic acid, and chlorogenic acid.

[0090] The first surface treatment agent may be included in an amount of 0.1 wt% to 0.2 wt% based on the entire mixture. If the content of the first surface treatment agent in the mixture is less than 0.1 wt%, it is difficult to form a first layer (410) capable of imparting hydrophilicity to boron nitride nanotubes (BNNTs). On the other hand, even if the content of the first surface treatment agent is greater than 0.2 wt%, the surface treatment effect of the boron nitride nanotubes (BNNTs) does not continuously increase. Therefore, the content of the first surface treatment agent is preferably 0.1 wt% to 0.2 wt% based on the entire mixture.

[0091] Next, boron nitride nanotubes (BNNTs) are dispersed in the mixture. Dispersion can be achieved by ultrasonic dispersion, stirring, etc.

[0092] At this time, the dispersion may have a weak alkaline property. Since the dispersion has a weak alkaline property, the polyphenol group can be oxidized into a highly reactive quinone oligomer by the dissolved oxygen in the dispersion, and as a result, it can be attached to the surface of boron nitride nanotubes (BNNTs), thereby forming a first layer (410).

[0093] The pH of the dispersion can be adjusted using a base such as sodium hydroxide. For 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 is difficult to form quinone oligomers, so the pH of the dispersion is preferably 8 to 9.

[0094] The mixing ratio of boron nitride nanotubes (BNNTs) dispersed in the dispersion and the first surface treatment agent may be 1:1 to 1:0.1 in wt%. If the content of boron nitride nanotubes (BNNTs) dispersed in the dispersion exceeds 10 times that 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 1 time that of the first surface treatment agent, the amount of the first surface treatment agent discarded in the subsequent washing process increases rapidly.

[0095] After dispersing boron nitride nanotubes (BNNT) in a mixture to form a dispersion, the boron nitride nanotubes (BNNT) in which the first layer (410) is formed are washed and dried.

[0096] In the washing step, the surface-treated boron nitride nanotubes (BNNTs) are washed with water to remove any remaining polyphenol groups. Subsequently, only the surface-treated boron nitride nanotubes (BNNTs) are collected through centrifugation or filtering, and then dried to obtain a powder of surface-treated boron nitride nanotubes (BNNTs) that have been imparted with hydrophilicity.

[0097] The above method can be performed at room temperature and in an air atmosphere. Therefore, according to the present invention, boron nitride nanotubes can be made hydrophilic by a simple method of dispersing the boron nitride nanotubes in a mixture of the first surface treatment agent and water, without creating a specific environment. Furthermore, tannic acid and the like contained in the first layer are environmentally friendly substances derived from nature, so the surface-treated boron nitride nanotubes may not cause environmental pollution.

[0098] FIG. 7 is a perspective view schematically illustrating another example of surface-treated boron nitride nanotubes dispersed in the electrolyte of FIG. 1, and FIG. 8 is a flowchart schematically illustrating a method for surface-treating the boron nitride nanotubes of FIG. 7.

[0099] Fig. 7 illustrates another example of a surface-treated boron nitride nanotube (BNNT''). Referring to Fig. 7, the surface-treated boron nitride nanotube (BNNT'') may 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, by further including the second layer (420), the boron nitride nanotube (BNNT) may be made hydrophobic.

[0100] The second layer (420) can be formed on the first layer (410). The first layer (410) is the same as that illustrated and described in FIG. 5, and thus will not be described again.

[0101] The second layer (420) is a layer for imparting hydrophobicity to 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 an alkyl amine, an alkyl thiol, an aryl amine, an aryl thiol, a benzyl amine, and a benzyl thiol.

[0102] Meanwhile, the polyphenol group of the first layer (410) can be oxidized into a highly reactive quinone oligomer and attached to the surface of a boron nitride nanotube (BNNT), and the quinone structure attached in this way can anchor the primary amine group of the second layer (420) by a Michael addition mechanism. More specifically, the reaction of the amine or thiol of the second layer (420) and the hydroxyl group of the first layer (410) is a reaction in which the amine or thiol of the second layer (420) undergoes a Michael addition to the hydroxyl group of the first layer (410), thereby allowing the second layer (420) to be formed on the first layer (410).

[0103] By including a second layer (420) like this, the surface-treated boron nitride nanotube (BNNT'') becomes hydrophobic, and thus can have excellent dispersibility in a hydrophobic solvent.

[0104] Meanwhile, the first layer (410) is omitted and the second layer (420) is not formed directly on the surface of the boron nitride nanotube (BNNT). Therefore, in order to form the second layer (420) to impart hydrophobicity to the boron nitride nanotube (BNNT), the first layer (410) must be formed first.

[0105] A method for forming a second layer (420) may include, as illustrated in FIG. 7, a step of forming a first mixture by mixing a first surface treatment agent in water (S210), a step of dispersing boron nitride nanotubes (BNNTs) in the first mixture to form a dispersion (S220), a step of forming a second mixture by mixing a second surface treatment agent in the dispersion (S230), and a step of washing and drying boron nitride nanotubes (BNNTs) from the second mixture (S240).

[0106] The step of forming the first mixture (S210) and the step of forming the dispersion (S220) are the same as the step of forming the mixture (S110 of FIG. 6) and the step of forming the dispersion (S120 of FIG. 6) of FIG. 6, and therefore, they will not be described repeatedly, and only the differences from FIG. 6 will be described.

[0107] Referring to Fig. 8, after forming a dispersion, a second surface treatment agent is additionally mixed into the dispersion to form a second mixture (S230). The second surface treatment agent is a substance for imparting hydrophobicity to boron nitride nanotubes (BNNTs) and may include an amine group or a thiol group as a hydrocarbon group capable of undergoing Michael addition to the hydroxyl group of the first layer (410).

[0108] For example, the second surface treatment agent may include at least one of an alkyl amine, an alkyl thiol, an aryl amine, an aryl thiol, a benzyl amine, and a benzyl thiol.

[0109] The amount of the second surface treatment agent mixed may be 0.5 to 2 times the amount of the boron nitride nanotubes (BNNTs). If the amount of the second surface treatment agent mixed is less than 0.5 times the amount of the boron nitride nanotubes (BNNTs), it is difficult for the second layer to be effectively formed, making it difficult for the boron nitride nanotubes (BNNTs) to become hydrophobic. On the other hand, if the amount of the second surface treatment agent mixed is greater than 2 times the amount of the boron nitride nanotubes (BNNTs), the amount of the second surface treatment agent discarded during the subsequent washing process increases rapidly.

[0110] After dispersing 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).

[0111] In the washing step, the surface-treated boron nitride nanotubes (BNNTs) are sequentially washed with water and ethanol to remove any remaining polyphenol and hydrocarbon groups. Subsequently, only the boron nitride nanotubes (BNNTs) are collected through centrifugation or filtering, and then dried to obtain a powder of boron nitride nanotubes (BNNTs) that have been surface-treated to impart hydrophobicity.

[0112] Meanwhile, unlike the above method, when a second surface treatment agent was mixed in an organic solvent such as toluene and boron nitride nanotubes (BNNTs) were dispersed, a second layer (420) was not formed on the surface of the boron nitride nanotubes (BNNTs), and thus hydrophobicity could not be imparted to the boron nitride nanotubes (BNNTs). That is, in order to form the second layer (420), a first layer (410) must be formed in advance on the surface of the boron nitride nanotubes (BNNTs) by the first surface treatment agent.

[0113] The above method can be performed at room temperature and in an air atmosphere. Therefore, according to the present invention, boron nitride nanotubes (BNNTs) can be made hydrophobic by a simple method of dispersing BNNTs in a mixture of a first surface treatment agent and water, and then further mixing a second surface treatment agent, without creating a specific environment. In addition, according to the present method, since water is used as a dispersion medium in the process of surface-treating boron nitride nanotubes (BNNTs) to make them hydrophobic, the problem of environmental pollution caused by the use of organic solvents does not arise.

[0114] Referring again to FIG. 4, the manufactured coating solution is coated on at least one surface of the separator (130) to form a coating layer (132) (S20).

[0115] FIG. 9 is a schematic diagram illustrating an example of a method for forming a boron nitride nanotube coating layer on the separator of FIG. 1.

[0116] Fig. 9 schematically illustrates an electrostatic spraying method as an example of a method for forming a coating layer (132). Electrostatic spraying is a method of splitting a liquid into fine droplets by an electric force and spraying them. For example, a liquid passing through a nozzle (N) can form a Taylor Cone by an electromagnetic force, and split into fine droplets by the repulsive force between liquid particles after passing through a short liquid column section. At this time, the fine droplets being sprayed are in a charged state. Therefore, during electrostatic spraying, voltage is applied to the nozzle (N), and an electrode body such as a drum can be in a grounded state.

[0117] Referring to Fig. 9, a separator (130) is positioned on an electrode body such as a drum, and a coating layer (132) can be formed by electrostatically spraying a coating liquid onto the separator (130) from a nozzle (N) having a hole with a diameter in micrometer units. At this time, the drum is grounded and formed in a cylindrical shape so as to be able to rotate.

[0118] In Fig. 9, electrostatic spraying is illustrated and described as an example of a method for forming a 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 and ultrasonic spraying.

[0119] In addition, a material for forming a separator (130) may be sprayed onto a rotating conveyor belt to first form the separator (130), and a coating liquid for forming a coating layer (132) may be sprayed onto the formed separator (130) to continuously 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.

[0120] After spraying the coating solution, it is dried to remove the solvent and form a coating layer (132). Drying can be performed using various methods such as vacuum drying, hot air drying, freeze drying, and spray drying.

[0121] Meanwhile, the coating layer (132) may be formed by repeating the electrostatic spraying multiple times. For example, the coating layer (132) may be formed by repeating the electrostatic spraying and drying processes two to four times. When the coating layer (132) is formed by repeating the electrostatic spraying multiple times, the amount of boron nitride nanotubes included in the coating layer (132) increases, so that more boron nitride nanotubes are used as a migration path for lithium ions, thereby improving the overall ionic conductivity.

[0122] Specifically, when a coating layer (132) is formed using a coating solution containing 0.1 wt% of boron nitride nanotubes, the ionic conductivity of the separator (130) when the coating layer (132) is formed through one electrostatic spraying is 1.34 mS / cm, but when the coating layer (132) is formed through two electrostatic sprayings, the ionic conductivity of the separator (130) increases to 1.48 mS / cm, and when the coating layer (132) is formed through three electrostatic sprayings, the ionic conductivity of the separator (130) increases to 1.78 mS / cm. However, when the number of electrostatic sprayings is 5 or more, aggregation of boron nitride nanotubes (BNNT) may occur on the surface of the separator (130), which may instead decrease the ionic conductivity of the separator (130).

[0123] Meanwhile, at least some of the boron nitride nanotubes (BNNT) included in the coating layer (132) may be attached to form a certain angle with the surface of the separator (130) rather than being completely laid down and attached to 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 some of the boron nitride nanotubes (BNNT) and the surface of the separator (130) may be 1° to 30°. In this way, at least some of the boron nitride nanotubes (BNNT) may be attached to form a certain angle with the surface of the separator (130), thereby acting as a passage through which lithium ions can move directionally between the positive electrode (110) and the negative electrode (120).

[0124] Next, an electrode assembly (110) is formed by placing a negative electrode (120) and a positive electrode (110) on each side of a separator (130) on which a coating layer (132) is formed (S30), and the electrode assembly (110) is positioned in a case (200), and then the case (200) is filled with an electrolyte to manufacture a lithium secondary battery (10) (S40).

[0125] Figure 10 is a diagram showing the results of measuring the lithium ion conductivity of a separator.

[0126] Figure 10 (A) shows the lithium ion conductivity of a PE separator, and (B) shows the lithium ion conductivity when a coating layer is formed on the surface of a PE separator according to the present invention.

[0127] In Fig. 10 (B), the coating layer was formed by electrostatically spraying a coating solution containing 0.5 wt% of boron nitride nanotubes dispersed in IPA onto both sides of a PE separator and drying the coating solution at a temperature of 40°C for 24 hours to remove the solvent of the coating solution.

[0128] Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using a stainless steel disk as a blocking electrode, and the electrolyte used was a 1:1 mixture of EC and DMC with 1 M LiPF6 dispersed in it.

[0129] Referring to Figure 10, it can be seen that in case of (B) where a coating layer is formed, the lithium ion conductivity is increased compared to (A) not only at room temperature (25°C) and high temperature (60°C), but also at low temperature (-10°C).

[0130] Specifically, in the case of (A) at a low temperature (-10℃), the ionic conductivity was 0.31 mS / cm, whereas in the case of (B) with a coating layer formed, it was 0.44 mS / cm, indicating an approximately 42% increase. In addition, in the case of (A) at a high temperature (60℃), the ionic conductivity was 1.20 mS / cm, whereas in the case of (B) with a coating layer formed, it was 1.65 mS / cm, indicating an approximately 37.5% increase. Therefore, by forming a coating layer including boron nitride nanotubes, rapid performance degradation of lithium ion secondary batteries at high and low temperatures can be prevented or minimized.

[0131] Additionally, when the temperature increases from 45°C to 60°C, the ionic conductivity of (A) increases by 19%, while (B) shows a result of increasing by 25%. This indicates that the thermal stability of the separator is improved by forming a coating layer on the separator.

[0132] Figure 11 is a graph showing the results of measuring the initial charge / discharge capacity of a lithium ion secondary battery.

[0133] Figure 11 shows the results of precycling a lithium ion secondary battery including an NCM523 positive electrode and a graphite negative electrode from 2.7 V to 4.2 V at a charge / discharge rate of 0.1 C. Figure 11 (A) includes the separator of Figure 10 (A), and Figure 11 (B) includes the separator of Figure 10 (B).

[0134] As can be seen in Fig. 11, in case of (A), the charge capacity decreased from 383.6 mAh to 380.4 mAh, and the coulombic efficiency was measured to be 99.19%, but in case of (B), which includes a separator having a coating layer including boron nitride nanotubes formed thereon, the charge capacity changed from 413.6 mAh to 410.9 mAh, and the coulombic efficiency was measured to be 99.34%. Therefore, it can be seen that the efficiency of the lithium ion secondary battery is improved as the lithium ion conductivity increases due to the formation of a coating layer on the separator.

[0135] Figures 12 and 13 are diagrams showing the characteristics (rate performance) of a lithium ion secondary battery according to temperature and charge / discharge rate.

[0136] In FIGS. 12 and 13, (A) is the result using the lithium ion secondary battery of FIG. 11 (A), and (B) is the result using the lithium ion secondary battery of FIG. 11 (B). In addition, (B2) of FIGS. 12 and 13 is different 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 same as (B) in that it is the result using a lithium ion secondary battery including a separator on which a coating layer including boron nitride nanotubes was formed.

[0137] Figures 12 and 13 illustrate the capacity of a lithium ion secondary battery measured at different charge and discharge rates. Here, Figure 12 illustrates the capacity measured at 25°C, and Figure 13 illustrates the capacity measured at -10°C.

[0138] First, referring to Fig. 12, it can be seen that (B) and (B2) have better characteristics according to charge and discharge rates than (A) at 0.5C, 0.7C, 1C, 3C, and 5C. In particular, referring to Fig. 13, which shows the results measured at -10℃, it can be seen that (B) and (B2) have superior characteristics according to charge and discharge rates than (A) at 0.5C, 1C, and 3C.

[0139] As can be seen from the above results, when a coating layer including boron nitride nanotubes is formed on the separator, the characteristics according to the charge / discharge rate are improved not only at room temperature but also at low temperatures due to an increase in lithium ion conductivity compared to the case where a coating layer including boron nitride nanotubes is not formed.

[0140] Figures 14 and 15 are diagrams showing the results of measuring the temperature of a lithium ion secondary battery during charging and discharging. Figure 14 shows the results using the lithium ion secondary battery of Figure 11 (A), and Figure 15 shows the results using the lithium ion secondary battery of Figure 11 (B). In Figures 14 and 15, the temperatures of the lithium ion secondary batteries were measured while repeatedly charging and discharging at a charge / discharge rate of 0.5 C at room temperature (25°C).

[0141] Referring to FIGS. 14 and 15, it can be seen that (B), in which a coating layer including boron nitride nanotubes is formed on the separator, has a lower average temperature than (A). In particular, the maximum temperature of (B) was measured to be 27.96°C, while (A) was measured to be 28.85°C. From this, it can be seen that when a coating layer including 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 it is also expected to be due to the low internal resistance resulting from the high lithium ion conductivity.

[0142] Figures 16 and 17 are graphs illustrating the results of CV (cyclic voltammetry) tests of lithium ion secondary batteries. Figures 16 and 17 are tests to confirm the possibility of a chemical reaction between an electrode and an electrolyte that occurs during charging and discharging of a lithium ion secondary battery. In a configuration having a separator between a lithium metal electrode and a stainless steel electrode, the results confirm whether a chemical reaction occurred between the lithium metal electrode and the stainless steel electrode.

[0143] Fig. 16 shows the results of the lithium ion secondary battery of Fig. 11 (A), and Fig. 17 shows the results of the lithium ion secondary battery of Fig. 11 (B). In Figs. 16 and 17, the scan rate was set to conditions of -0.2 to 5 V and 0.1 mV / s.

[0144] Referring to FIGS. 16 and 17, FIGS. 16 and 17 have peaks at the same locations, but FIG. 17 shows a larger current peak than FIG. 16, which means that the ionic conductivity is larger.

[0145] In addition, as can be seen in Fig. 17, the CV (cyclic vloltammetry) test results showed that no significant peaks were generated during reduction at voltages lower than 0 and oxidation at voltages higher than 0. In other words, since no chemical reaction occurs between the lithium metal electrode and the stainless steel electrode, it demonstrates electrochemical stability and practical battery applications.

[0146] Fig. 18 is a diagram showing the characteristics (C-rate performance) of a lithium ion secondary battery according to the charge / discharge rate, and Fig. 19 is a diagram showing the cycle retention results of the lithium ion secondary battery of Fig. 18.

[0147] In FIGS. 18 and 19, the lithium ion secondary battery is a pouch type, includes an LFP positive electrode material, a graphite negative electrode material, and a LiPF6 electrolyte, and a PE separator is used as the separator.

[0148] In FIGS. 18 and 19, (A) is a case where a coating layer according to the present invention is not applied to a separator, and (B) is a case where a coating layer including boron nitride nanotubes is formed on a separator. The coating layer was formed using the same method as in FIG. 10.

[0149] Referring to Fig. 18, it can be seen that the discharge capacity of (B) is greater than that of (A) at 0.1C, 0.2C, and 0.3C. This is because the lithium ion conductivity is improved by forming a coating layer including boron nitride nanotubes on the separator surface.

[0150] In addition, Fig. 19 shows the discharge capacity when charging and discharging were repeated at a rate of 0.1C, and it can be seen that the discharge capacity of (B) in which a coating layer was formed on the surface of the separator was improved compared to (A) in which a coating layer was not formed.

[0151] That is, even in the case of secondary batteries that use various cathode materials, such as LFP, rather than NCM type cathode materials, the performance of the secondary battery can be improved by forming a coating layer containing boron nitride nanotubes on the surface of the separator, thereby improving lithium ion conductivity.

[0152] While the present invention has been described with reference to the embodiments illustrated in the drawings, these are merely exemplary, and those skilled in the art will appreciate that various modifications and variations of the embodiments are possible. Therefore, the true scope of technical protection of the present invention should be determined by the technical spirit of the appended claims.

Claims

1. An electrode assembly comprising a cathode, an anode, and a separator disposed between the cathode and the anode; a case accommodating the electrode assembly; and Containing an electrolyte filled in the case; The separator includes a coating layer on at least one of the two surfaces of the separator, A lithium ion secondary battery wherein the coating layer comprises boron nitride nanotubes.

2. In paragraph 1, A lithium ion secondary battery having an ionic conductivity of the above-mentioned setter of 1.7 mS / cm or less at 60°C and 0.4 mS / cm or more at -10°C.

3. In paragraph 1, A lithium ion secondary battery having an average surface roughness of the coating layer of 50 nm to 2 μm.

4. In paragraph 1, A lithium ion secondary battery wherein the aspect ratio of the above boron nitride nanotubes is 20 to 6000.

5. In paragraph 1, The above boron nitride nanotubes are surface-treated to have hydrophilic or hydrophobic properties, and are lithium ion secondary batteries.

6. In paragraph 5, The surface-treated boron nitride nanotube further comprises a first layer positioned on at least a portion of the boron nitride nanotube, A lithium ion secondary battery wherein the first layer contains a hydroxyphenyl group and forms a π bond with the boron nitride nanotube.

7. In paragraph 6, The surface-treated boron nitride nanotubes are a lithium ion secondary battery having the hydrophilic properties.

8. In paragraph 6, The surface-treated boron nitride nanotube further comprises a second layer on the first layer, The second layer is surface-treated to include an amine group or a thiol group as a hydrocarbon group, and the surface-treated boron nitride nanotube has the hydrophobicity of a lithium ion secondary battery.

9. In paragraph 1, A lithium ion secondary battery, wherein at least some of the boron nitride nanotubes are attached to the surface of the separator at an angle of 1° to 30°.

10. A step of forming a coating solution by mixing boron nitride nanotubes into a solvent; A step of forming a coating layer by coating the coating solution on at least one side of the separator; A step of forming an electrode assembly by arranging a cathode and an anode on each side of the separator on which the coating layer is formed; and A method for manufacturing a lithium ion secondary battery, comprising the steps of positioning the electrode assembly within a case and filling the case with an electrolyte.

11. In paragraph 10, A method for manufacturing a lithium ion secondary battery, wherein the coating layer is formed by electrostatically spraying or spray coating the coating liquid on at least one of the two surfaces of the separator.

12. In paragraph 10, A method for manufacturing a lithium ion secondary battery, further comprising a step of drying the coating layer after forming the coating layer.

13. In paragraph 10, A method for manufacturing a lithium ion secondary battery, wherein the coating solution contains 0.01 wt to 10 wt% of the boron nitride nanotubes.

14. In paragraph 10, A method for manufacturing a lithium ion secondary battery in which the above boron nitride nanotubes are surface-treated to have hydrophilic or hydrophobic properties.

15. In paragraph 10, A method for manufacturing a lithium ion secondary battery, wherein the average surface roughness of the coating layer is 50 nm to 2 μm.

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