Separator for lithium secondary batteries, method of manufacturing the separator therefrom, and lithium secondary batteries including the separator therefrom.

Abstract

This disclosure relates to a separator for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same. Specifically, the separator includes a porous coating divided into a top layer, an intermediate layer, and a bottom layer, wherein the top layer contains a higher content of a particulate binder polymer than the bottom layer, and the porous coating includes a dispersant having carboxyl and ethylene glycol groups.

Description

Technical Field

[0001] This disclosure relates to a separator applicable to electrochemical devices such as lithium secondary batteries, a method of manufacturing the separator thereto, and a lithium secondary battery including the separator thereto.

[0002] This application claims priority to Korean Patent Application No. 10-2020-0127233 and Korean Patent Application No. 10-2020-0127237, filed in Korea on September 29, 2020, the disclosures of which are incorporated herein by reference. Background Technology

[0003] Energy storage technology has recently received increasing attention. Efforts to develop electrochemical devices are increasingly being realized, as applications of energy storage technology have expanded to power mobile phones, cameras, and laptops, and even electric vehicles. In this context, electrochemical devices have garnered the most attention. Among these electrochemical devices, the development of rechargeable secondary batteries has been a focus. Recently, active research has been conducted on designing novel electrodes and batteries to improve capacity density and specific energy in the development of these batteries.

[0004] Among commercially available rechargeable batteries, lithium-ion batteries, developed in the early 1990s, have attracted considerable attention due to their higher operating voltage and significantly higher energy density compared to conventional batteries such as Ni-MH, Ni-Cd, and lead-sulfuric acid batteries that utilize aqueous electrolytes.

[0005] Although these electrochemical devices have been manufactured by many companies, their safety characteristics have shown varying degrees of variation. Evaluating and ensuring the safety of these electrochemical devices is crucial. For example, the separator prevents short circuits between the positive and negative electrodes and provides a pathway for lithium-ion transport. Therefore, this separator is a critical factor affecting the battery's safety and output characteristics.

[0006] Such separators frequently incorporate polyolefin-based porous polymer substrates. To prevent thermal shrinkage of the porous polymer substrate and enhance adhesion to the electrodes, separators with a porous coating comprising inorganic particles and a binder polymer on at least one surface of the porous polymer substrate have been frequently used.

[0007] These separators can be broadly classified into aqueous coated separators using water-based solvents and organic coated separators using organic solvents, depending on the porous coating. In particular, aqueous coated separators allow for uniform film coating and exhibit high heat resistance.

[0008] However, there is still a need for a separator that demonstrates increased adhesion to the electrodes (Lami Strength) while maintaining high porosity in porous coatings. Summary of the Invention

[0009] Technical issues

[0010] This disclosure is designed to address problems in the related art, and therefore relates to providing an aqueous coated separator with excellent heat resistance, including a porous coating with high porosity and exhibiting improved adhesion to the electrodes, a method of manufacturing the same, and a lithium secondary battery including the same.

[0011] However, the inventive objectives to be achieved by this disclosure are not limited to the problems mentioned above, and other inventive objectives of this disclosure will be understood from the following detailed description and will become more apparent from the exemplary embodiments of this disclosure.

[0012] Technical solution

[0013] In one aspect of this disclosure, a separator for a lithium secondary battery, a method of manufacturing the separator therefrom, and a lithium secondary battery including the separator are provided according to any of the following embodiments.

[0014] According to a first embodiment, a separator for a lithium secondary battery is provided, comprising:

[0015] Porous polymer substrates; and

[0016] A porous coating formed on at least one surface of the porous polymer substrate, comprising inorganic particles, particulate binder polymer, and dispersant.

[0017] The dispersant comprises carboxyl and glycol groups, and

[0018] The porous coating is divided into a top layer, an intermediate layer, and a bottom layer in the thickness direction, wherein the content of the particulate adhesive polymer in the top layer is higher than that in the bottom layer.

[0019] According to the second embodiment, a separator for a lithium secondary battery as defined in the first embodiment is provided.

[0020] When the porous coating is divided into n blocks in the thickness direction, the top layer is the outermost layer;

[0021] When the porous coating is divided into n blocks in the thickness direction, the bottom layer is the layer facing the porous polymer substrate;

[0022] The intermediate layer is the remaining layer in the porous coating excluding the top and bottom layers; and

[0023] n is an integer from 3 to 10.

[0024] According to the third embodiment, a separator for a lithium secondary battery as defined in the first or second embodiment is provided.

[0025] The content of the particulate adhesive polymer has a concentration gradient that increases from the bottom layer to the top layer based on the thickness direction of the porous coating, and the content of the inorganic particles has a concentration gradient that increases from the top layer to the bottom layer based on the thickness direction of the porous coating.

[0026] According to the fourth embodiment, a separator for a lithium secondary battery as defined in any of the first to third embodiments is provided.

[0027] The intermediate layer in the porous coating may have a gradient concentration gradient of the particulate binder polymer, or may not have a specific concentration gradient.

[0028] According to the fifth embodiment, a separator for a lithium secondary battery as defined in any of the first to fourth embodiments is provided.

[0029] The weight ratio of the inorganic particles to the dispersant is from 99.5:0.5 to 95:5.

[0030] According to the sixth embodiment, a separator for a lithium secondary battery as defined in any of the first to fifth embodiments is provided.

[0031] The content of the particulate adhesive polymer is 10 to 50 parts by weight based on 100 parts by weight of the porous coating.

[0032] According to the seventh embodiment, a separator for a lithium secondary battery as defined in any of the first to sixth embodiments is provided.

[0033] The dispersant has a ratio of 0.05 to 0.25 of the equivalent number of ethylene glycol groups to the equivalent number of carboxyl groups.

[0034] According to the eighth embodiment, a separator for a lithium secondary battery as defined in any of the first to seventh embodiments is provided.

[0035] The dispersant has a weight-average molecular weight of 100 to 10,000.

[0036] According to the ninth embodiment, a separator for a lithium secondary battery as defined in any of the first to eighth embodiments is provided.

[0037] The dispersant is a copolymer of polyacrylic acid and polyethylene glycol.

[0038] According to the tenth embodiment, a separator for a lithium secondary battery as defined in the ninth embodiment is provided.

[0039] The dispersant is a block copolymer of polyacrylic acid and polyethylene glycol.

[0040] According to the eleventh embodiment, a separator for a lithium secondary battery as defined in any of the first to tenth embodiments is provided.

[0041] The porous coating has a thickness of 2 μm to 10 μm.

[0042] According to the twelfth embodiment, a separator for a lithium secondary battery as defined in any of the first to eleventh embodiments is provided.

[0043] The inorganic particles mentioned therein are flaky inorganic particles.

[0044] According to the thirteenth embodiment, a separator for a lithium secondary battery as defined in the twelfth embodiment is provided.

[0045] The flaky inorganic particles have an aspect ratio of 3.5 or greater.

[0046] According to the fourteenth embodiment, a separator for a lithium secondary battery as defined in the twelfth embodiment is provided.

[0047] The flaky inorganic particles include at least one of Al(OH)3, AlO(OH), Mg(OH)2, and BaTiO3.

[0048] According to the fifteenth embodiment, a method for manufacturing a separator for a lithium secondary battery as defined in any of the first to fourteenth embodiments is provided.

[0049] The method includes the steps of applying a slurry containing a solvent, inorganic particles, a particulate binder polymer, and a dispersant for forming a porous coating to at least one surface of a porous polymer substrate, followed by drying.

[0050] The slurry used to form the porous coating has a viscosity of 200 cP or less, and

[0051] The slurry used to form the porous coating has a solid content of 10 to 40 parts by weight per 100 parts by weight of the slurry.

[0052] According to the sixteenth embodiment, a method for manufacturing a separator for a lithium secondary battery as defined in the fifteenth embodiment is provided.

[0053] The solvent mentioned is water.

[0054] The adhesive polymer is in the form of particles dispersed in the solvent, and

[0055] Phase separation is performed during the drying step.

[0056] According to the seventeenth embodiment, a method for manufacturing a separator for a lithium secondary battery, as defined in the fifteenth or sixteenth embodiment, is provided.

[0057] The partition plate thereon satisfies at least three of the following equations:

[0058] The increase rate of air infiltration time is ≤100%.

[0059] Heat shrinkage rate ≤10%

[0060] Adhesion to electrodes ≥70gf / 25mm

[0061] The viscosity of the slurry used to form the porous coating is ≤20cp.

[0062] (The "increase rate of air permeation time" is calculated using the formula [(air permeation time of porous polymer substrate - air permeation time of separator) / (air permeation time of porous polymer substrate)] × 100, and)

[0063] "Heat shrinkage rate" is defined as the smaller of the heat shrinkage rate in the longitudinal direction (MD) and the heat shrinkage rate in the transverse direction (TD), and is calculated by the formula [(initial length - length after heat shrinkage at 150°C for 30 minutes) / (initial length)] × 100.

[0064] In another aspect of this disclosure, a lithium secondary battery according to the following embodiments is provided.

[0065] According to the eighteenth embodiment, a lithium secondary battery is provided, including a positive electrode, a negative electrode, and a separator inserted between the positive electrode and the negative electrode.

[0066] The partition is the same as that defined in any of the first to fourteenth embodiments.

[0067] Beneficial effects

[0068] According to embodiments of this disclosure, a separator with improved heat resistance and adhesion to the electrode (Lami strength) by using a dispersant with predetermined properties is provided, and a lithium secondary battery including said separator is provided.

[0069] In particular, according to embodiments of this disclosure, a dispersant with predetermined properties is used to induce the separation of an intermediate layer between inorganic particles and a particulate binder polymer in a slurry used to form a porous coating. Therefore, a separator can be provided in which a porous coating adjacent to a porous polymer substrate is rich in inorganic particles, and the outermost portion of the porous coating is rich in a particulate binder polymer.

[0070] Since the inorganic particles are mainly distributed on this side of the porous polymer substrate, the shrinkage of the porous polymer substrate can be suppressed, resulting in improved heat resistance of the separator.

[0071] Meanwhile, since the particulate binder polymer is mainly distributed on the surface of the porous coating, it can improve the adhesion to the electrode.

[0072] In addition, according to embodiments of this disclosure, the porosity of the porous coating can be increased by using sheet-like inorganic particles with predetermined properties in the separator according to this disclosure. Specifically, according to embodiments of this disclosure, a low-viscosity slurry for forming the porous coating is applied to a porous polymer substrate. When such a low-viscosity slurry is used, the packing density of the sheet-like inorganic particles decreases. Therefore, when the solvent in the slurry dries, the sheet-like inorganic particles exhibit increased migration. As a result, the sheet-like inorganic particles can be arranged more randomly in the porous coating, and the porosity of the porous coating can be increased. Attached Figure Description

[0073] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the following disclosure, are used to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure should not be construed as limited to the drawings. Furthermore, the shape, size, scale, or proportion of some components in the drawings may be exaggerated for the purpose of clearer description.

[0074] Figure 1 The image is a scanning electron microscope (SEM) image of the septum according to Example 1.

[0075] Figure 2 and Figure 3 The images are SEM images of the partitions from Comparative Examples 1 and 2.

[0076] Figure 4This is a schematic cross-sectional view illustrating a partition according to an embodiment of the present disclosure. Detailed Implementation

[0077] In the following description, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terminology used in the specification and appended claims should not be construed as limited to its common or dictionary meaning, but rather should be interpreted based on its meaning and concept in relation to the technical aspects of the present disclosure, on the principle that the inventors are allowed to appropriately define the terminology for the best interpretation. Therefore, the descriptions presented herein are merely preferred examples for illustrative purposes and are not intended to limit the scope of the present disclosure; thus, it should be understood that other equivalents and modifications may be made thereto without departing from the scope of the present disclosure.

[0078] Throughout the specification, the statement "a portion includes an element" does not exclude the presence of any other elements, but rather implies that the portion may further include other elements.

[0079] As used in this article, the expression "A and / or B" means "A, B, or both of them".

[0080] The specific terms used in the following description are for illustrative purposes and are not restrictive. Terms such as “right” and “left” indicate direction in the accompanying drawings in which they are mentioned. These terms include the words listed above, their derivatives, and words with similar meanings.

[0081] In electrochemical devices such as lithium-ion batteries, separators typically use porous polymer substrates, which thus present the problem of exhibiting shrinkage behavior. Therefore, porous coatings have been introduced to reduce the thermal shrinkage rate of the separators.

[0082] Although such a separator can ensure heat resistance through the inorganic particles contained in the porous coating, as the energy density of the electrode assemblies being developed gradually increases, the separator needs to further ensure safety.

[0083] In particular, attempts have been made to improve heat resistance while enhancing adhesion to the electrodes in the case of waterborne porous coatings that have high adhesion to the electrodes even without a separate adhesive layer.

[0084] To address the aforementioned problems, the inventors of this disclosure use a dispersant with predetermined properties to induce the separation of an intermediate layer between inorganic particles and a particulate binder polymer in a slurry used to form a porous coating. In this way, this disclosure relates to providing a separator in which a porous coating adjacent to a porous polymer substrate is rich in inorganic particles, and the outermost portion of the porous coating is rich in a particulate binder polymer.

[0085] Since the inorganic particles are mainly distributed on this side of the porous polymer substrate as described above, the shrinkage of the porous polymer substrate can be suppressed, resulting in improved heat resistance of the separator.

[0086] Meanwhile, since the particulate binder polymer is mainly distributed on the surface of the porous coating, it can improve the adhesion to the electrode.

[0087] In one aspect of this disclosure, a method for manufacturing a separator for a lithium secondary battery is provided, comprising the following steps:

[0088] Porous polymer substrates; and

[0089] A porous coating formed on at least one surface of the porous polymer substrate, comprising inorganic particles, particulate binder polymer, and dispersant.

[0090] The dispersant comprises carboxyl and glycol groups, and

[0091] The porous coating is divided into a top layer, an intermediate layer, and a bottom layer in the thickness direction, wherein the content of the particulate adhesive polymer in the top layer is higher than that in the bottom layer.

[0092] The separator for lithium secondary batteries according to embodiments of the present disclosure will be explained in more detail below.

[0093] The separator for a lithium secondary battery according to embodiments of this disclosure includes inorganic particles, particulate binder polymer, and dispersant having predetermined properties in its porous coating.

[0094] The dispersant includes carboxyl and ethylene glycol groups as functional groups.

[0095] Carboxyl groups effectively disperse inorganic particles, while glycol groups reduce the surface tension of slurries used to form porous coatings, facilitate the formation of pores in the porous coating, and reduce the viscosity of the slurry. If the dispersant does not contain either carboxyl or glycol groups, it is difficult to form a porous coating, or the resulting separator exhibits poor physical properties in terms of adhesion and thermal shrinkage.

[0096] When a dispersant is introduced along with inorganic particles and a particulate binder polymer, the dispersant reduces the viscosity of the slurry used to form the porous coating, thereby maximizing the density difference between the particulate binder polymer and the inorganic particles. Therefore, a separator can be provided in which the porous coating adjacent to the porous polymer substrate is rich in inorganic particles, and the outermost portion of the porous coating is rich in particulate binder polymer.

[0097] If a dissolved binder, rather than a particulate binder, is introduced along with inorganic particles, the dissolved binder will be present in the portion of the porous coating adjacent to the porous substrate. Therefore, unlike the introduction of particulate binders, the introduction of a dissolved binder can hardly ensure adhesion to the electrodes.

[0098] According to embodiments of this disclosure, the dispersant may be a copolymer of polyacrylic acid and polyethylene glycol. In particular, the dispersant may be a block copolymer of polyacrylic acid and polyethylene glycol.

[0099] According to embodiments of this disclosure, the equivalence ratio of ethylene glycol groups to carboxyl groups (i.e., the ratio of the equivalence of ethylene glycol groups to the equivalence of carboxyl groups) can be from 0.05 to 0.25. The dispersant according to this disclosure is used to disperse inorganic particles and can have a carboxyl group content of 80% or greater.

[0100] According to embodiments of this disclosure, the dispersant may have a weight-average molecular weight of 100 or greater, 200 or greater, or 300 or greater, and 10,000 or less, 9,000 or less, or 8,000 or less. For example, from the perspective of ensuring processability, heat resistance, and adhesive properties, the dispersant may have a weight-average molecular weight of 100 to 10,000.

[0101] Here, the weight-average molecular weight of the dispersant can be determined using gel permeation chromatography (GPC, PL GPC220, Agilent Technologies).

[0102] Specifically, the weight-average molecular weight of the dispersant can be determined under the following analytical conditions:

[0103] - Column: PL MiniMixed B x 2

[0104] Solvent: DMF

[0105] - Flow rate: 0.3 mL / min

[0106] - Sample concentration: 2.0 mg / mL

[0107] Injection volume: 10μL

[0108] - Column temperature: 40℃

[0109] - Detector: Agilent RI detector

[0110] - Standard: Polystyrene (corrected using a third-order function)

[0111] -Data Processing: ChemStation

[0112] According to embodiments of this disclosure, the weight ratio of inorganic particles to dispersant can be from 99.5:0.5 to 95:5.

[0113] According to this disclosure, there are no particular limitations on the inorganic particles, as long as they are electrochemically stable. That is, there are no particular limitations on the inorganic particles that can be used herein, provided they are within the operating voltage range of the applicable electrochemical device (e.g., based on Li / Li). + It should not cause oxidation and / or reduction in the 0-5V range. In particular, when using inorganic particles with high dielectric constants, the ionic conductivity of the electrolyte can be improved by increasing the degree of dissociation of electrolyte salts such as lithium salts in the liquid electrolyte.

[0114] For the reasons mentioned above, inorganic particles can be inorganic particles with a dielectric constant of 5 or greater, inorganic particles with lithium-ion transport capability, or mixtures thereof.

[0115] Inorganic particles with a dielectric constant of 5 or greater may include those selected from Al2O3, SiO2, ZrO2, AlO(OH), Al(OH)3, TiO2, BaTiO3, and Pb(ZrO2). x Ti 1-x O3(PZT, where 0 < x < 1), Pb 1-x La x Zr 1-y Ti y O3(PLZT, where 0 < x < 1, 0 < y < 1), (1-x)Pb(Mg) 1 / 3 Nb 2 / 3 The group consisting of O3PbTiO3 (PMN-PT, where 0 < x < 1), hafnium oxide (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, SiC, or a mixture of two or more of them.

[0116] Inorganic particles with lithium-ion transport capabilities can be selected from lithium phosphate (Li3PO4) and lithium titanium phosphate (Li... x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) x O y Base glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La yTiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Ge y P z S w 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), and P2S5-based glass (Li x P y S z Any one of the groups consisting of 0 < x < 3, 0 < y < 3, 0 < z < 7, or a mixture of two or more of them.

[0117] In addition, there are no particular limitations on the average particle diameter of the inorganic particles. However, the inorganic particles preferably have an average particle diameter of 0.001 μm to 10 μm in order to form a coating with uniform thickness and provide suitable porosity. The average particle diameter of the inorganic particles is preferably 100 nm to 2 μm, more preferably 150 nm to 1 μm.

[0118] Inorganic particles can be added after they have been pre-crushed to a predetermined average particle diameter. Alternatively, the inorganic particles can be added to the binder polymer solution, then crushed and dispersed, while controlling them to have a predetermined diameter using a ball milling process or similar method.

[0119] According to embodiments of this disclosure, the inorganic particles can be flake-shaped inorganic particles. When using flake-shaped inorganic particles, there is an advantage that the rate of increase in air permeation time can be reduced compared to spherical inorganic particles. Furthermore, according to embodiments of this disclosure, the porosity of the porous coating can be increased by using flake-shaped inorganic particles with a high aspect ratio.

[0120] As used herein, "flaky inorganic particles" refers to particles having two planar surfaces facing or opposite each other and exhibiting a longer diameter (lateral width) greater than their thickness. Flaky inorganic particles, as used in accordance with this disclosure, will be explained in detail below.

[0121] (1) Longer diameter, thickness

[0122] The plate-like inorganic particles may have a relatively long diameter (lateral width) of 0.5 μm or larger, 0.6 μm or larger, or 0.7 μm or larger, and 1.5 μm or smaller, 1.3 μm or smaller, or 1.2 μm or smaller.

[0123] Here, the sheet-like inorganic particles may have a thickness of 0.05 μm or larger, 0.1 μm or larger, or 0.15 μm or larger, and 0.3 μm or smaller, 0.2 μm or smaller, or 0.1 μm or smaller. When the sheet-like particles satisfy both the above-defined longer diameter range and thickness range, a desired separator with high porosity can be formed.

[0124] Here, methods for measuring longer diameters and thicknesses may include: 1) calculating the arithmetic mean of the transverse width and thickness measurements of any 10 determinants in a scanning electron microscope (SEM) image of the sheet-like inorganic particles.

[0125] In addition, the method may include: 2) calculating the longer diameter and thickness from the average particle diameter of the secondary particles determined by laser diffraction scattering and the specific surface area determined by the BET method.

[0126] Furthermore, the method may include: 3) actually measuring the longer diameter and thickness using an atomic force microscope.

[0127] (2) Aspect Ratio

[0128] As used in this article, "aspect ratio" refers to the ratio of the arithmetic mean of the longer diameter to the arithmetic mean of the thickness (arithmetic mean of the longer diameter / arithmetic mean of the thickness). The longer diameter and thickness can be obtained using the methods mentioned above.

[0129] According to this disclosure, the sheet-like inorganic particles may have an aspect ratio of 3.5 or greater, or 4 or greater and 6 or less, 5.5 or less, or 5 or less. When the sheet-like inorganic particles meet the aspect ratios defined above, a desired separator with high porosity can be formed.

[0130] (3) Content

[0131] According to this disclosure, the flake-like inorganic particles may be used in amounts of 20 parts by weight or more, 25 parts by weight or more, or 30 parts by weight or more, and 50 parts by weight or less, 45 parts by weight or less, or 40 parts by weight or less, based on 100 parts by weight of the slurry for forming the porous coating.

[0132] (4) Type

[0133] According to embodiments of this disclosure, the sheet-like inorganic particles may include Al(OH)3, AlO(OH), Mg(OH)2, BaTiO3, or two or more of them.

[0134] Furthermore, according to embodiments of this disclosure, a particulate binder polymer is used instead of a non-particulate binder polymer, and this particulate binder polymer has a density of 200 nm or less. 50 Diameter. When using a binder polymer with a granular shape, rather than a non-granular shape, the porosity in the porous coating can be ensured. Furthermore, when using a granular binder polymer with a predetermined diameter range, the granular binder polymer cannot penetrate into the pores of the porous polymer substrate, thus advantageously preventing an increase in resistance.

[0135] For this purpose, the particulate binder polymer has a D that is larger than the diameter of the pores in the porous polymer substrate. 50 Particle diameter. For example, particulate adhesive polymers may have a diameter of 80 nm or larger, 90 nm or larger, or 100 nm or larger. 50 diameter.

[0136] The granular adhesive polymer may include acrylic granular adhesives (e.g., copolymers of butyl acrylate and ethylhexyl acrylate, copolymers of methyl methacrylate and ethyl acrylate, polyacrylonitrile, polycyanoacrylate, etc.), acrylonitrile-butadiene-styrene rubber, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, polyvinyl chloride, polyvinylidene fluoride, polyvinyl alcohol, polystyrene, or mixtures of two or more thereof.

[0137] The particulate adhesive polymer can be used in amounts of 10 to 50 parts by weight based on 100 parts by weight of porous coating.

[0138] Meanwhile, in the porous coating according to the embodiments of the present disclosure, when the porous coating is divided into a top layer, an intermediate layer and a bottom layer in the thickness direction, the content of particulate binder polymer in the top layer is higher than the content of particulate binder polymer in the bottom layer.

[0139] In particular, when the porous coating is divided into n blocks in the thickness direction, the top layer is the outermost layer; when the porous coating is divided into n blocks in the thickness direction, the bottom layer is the layer facing the porous polymer substrate; the middle layer is the remaining layer in the porous coating excluding the top and bottom layers; and n is an integer from 3 to 10.

[0140] Here, the content of the particulate binder polymer has a concentration gradient that increases from the bottom layer to the top layer along the thickness direction of the porous coating.

[0141] Furthermore, the content of inorganic particles exhibits a concentration gradient that increases from the top layer to the bottom layer along the thickness direction of the porous coating.

[0142] Here, the intermediate layer in the porous coating has a gradient concentration gradient of particulate binder polymer, or it does not have a specific concentration gradient.

[0143] In other words, the content of particulate binder polymer increases towards the top layer of the porous coating, while the content of inorganic particles increases towards the bottom layer of the porous coating.

[0144] In other words, in the partition according to the embodiments of the present disclosure, the ratio of the content of particulate binder polymer (A) to the content of inorganic particles (B) in the porous coating (content of particulate binder polymer / content of inorganic particles (A / B)) increases toward the surface portion of the porous coating.

[0145] According to embodiments of this disclosure, a porous coating may be formed on one or both surfaces of a porous polymer substrate.

[0146] According to this disclosure, the porous polymer substrate is a porous membrane that provides channels for lithium-ion transport while electrically insulating the negative and positive electrodes from each other to prevent short circuits. Any material can be used without particular limitation, as long as it is a material conventionally used as a separator in electrochemical devices.

[0147] In particular, the porous polymer substrate can be a porous polymer film substrate or a porous polymer nonwoven mesh substrate.

[0148] The porous polymer membrane substrate can be a porous polymer membrane comprising a polyolefin such as polyethylene or polypropylene. This polyolefin porous polymer membrane substrate can achieve a shut-off function at temperatures ranging from 80°C to 130°C.

[0149] Here, the polyolefin porous polymer membrane substrate may be formed by a polymer alone or a combination of two or more of the following: a polyolefin polymer including polyethylene, polypropylene, polybutene, or polypentene, which contain high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene.

[0150] In addition, porous polymer membrane substrates can be obtained by molding various polymers other than polyolefins, such as polyesters, into a membrane shape. Furthermore, porous polymer membrane substrates can have a stacked structure of two or more membrane layers, wherein each membrane layer can be formed by a polymer, including the polymers mentioned above such as polyolefins or polyesters, alone or in combination of two or more of them.

[0151] In addition to the polyolefins mentioned above, porous polymer membrane substrates and porous polymer nonwoven substrates can also be formed from polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, or polyethylene naphthalene, either alone or in combination.

[0152] While there are no particular limitations on the thickness of the porous polymer substrate, it can have a thickness of 1 μm to 100 μm, particularly 5 μm to 50 μm. Since batteries have recently offered high output / high capacity, it is advantageous to use thin films as porous polymer substrates. The porous polymer substrate can have pore diameters of 10 nm to 100 nm, 10 nm to 70 nm, 10 nm to 50 nm, or 10 nm to 35 nm, and a porosity of 5% to 90%, preferably 20% to 80%. However, according to this disclosure, these numerical ranges can be easily changed depending on the specific embodiment or, if necessary.

[0153] The pores in a porous polymer substrate can include various types of pore structures. A substrate falls within the scope of this disclosure when either the average pore size measured using a porosimeter or the average pore size observed using a field emission scanning electron microscope (FE-SEM) meets the ranges defined above.

[0154] In this paper, in the case of dry separators with a commonly known uniaxial orientation, the median pore size, measured by FE-SEM in the transverse (TD) direction rather than the longitudinal (MD) direction, is taken as the standard pore size. In the case of other porous polymer substrates with a network structure (e.g., wet polyethylene (PE) separators), the pore size measured by a porosimeter is taken as the standard pore size.

[0155] While there are no particular limitations on the thickness of the porous coating, it can range from 1 μm to 10 μm, particularly from 1.5 μm to 6 μm. Furthermore, there are no particular limitations on the porosity of the porous coating, but it is preferably between 35% and 65%.

[0156] The partition according to the embodiments of this disclosure may further include other additives as components of the porous coating, in addition to the inorganic particles and adhesive polymers described above.

[0157] In another aspect of this disclosure, an electrochemical device is provided, including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator is the separator described above according to the embodiments of this disclosure.

[0158] Electrochemical devices include any device that performs an electrochemical reaction, and specific examples include all types of primary cells, secondary cells, fuel cells, solar cells, or capacitors such as supercapacitors. In particular, lithium secondary cells, including lithium metal secondary cells, lithium-ion secondary cells, lithium polymer secondary cells, or lithium-ion polymer batteries, are preferred among secondary cells.

[0159] There are no particular limitations on the positive and negative electrodes used in conjunction with the separator according to this disclosure, and they can be obtained by incorporating electrode active materials into the electrode current collector using methods generally known in the art. Non-limiting examples of positive electrode active materials include conventional positive electrode active materials that can be used as positive electrodes in conventional electrochemical devices. In particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or lithium composite oxides containing combinations thereof are preferred. Non-limiting examples of negative electrode active materials include conventional negative electrode active materials that can be used as negative electrodes in conventional electrochemical devices. In particular, materials such as lithium metal or lithium alloys (which intercalate lithium), carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials are preferred. Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel, or combinations thereof. Non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, copper alloys, or combinations thereof.

[0160] The electrolyte that can be used in the electrochemical device according to this disclosure is having A + B - Salts of structure, in which A + Including, for example, Li + Na + K + alkali metal cations such as B, or combinations thereof, - Including PF6, etc. - BF4 - Cl - ,Br - I - ClO4 - AsF6 - CH3CO2 - CF3SO3 - N(CF3SO2)2 - C(CF2SO2)3 - Anions such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma-butyrolactone (γ-butyrolactone), or combinations thereof, are present in organic solvents, including but not limited to these.

[0161] Depending on the manufacturing process of the final product and the desired properties of the final product, electrolyte injection can be performed at an appropriate step during the process used to manufacture the battery. That is, electrolyte injection can be performed before battery assembly or as a final step in battery assembly.

[0162] There are no particular limitations on the methods for manufacturing separators for lithium secondary batteries according to this disclosure. However, the separators can be obtained by methods known to those skilled in the art or by any conventional method.

[0163] According to embodiments of this disclosure, a separator can be obtained by introducing a particulate binder polymer into a solution containing inorganic particles dispersed in a solvent and a dispersant to prepare a slurry for forming a porous coating, applying the slurry onto a porous polymer substrate, and then drying it to form a porous coating.

[0164] For example, according to embodiments of this disclosure, phase separation can occur when a slurry for forming a porous coating is applied to a porous polymer substrate and the solvent is dried. In particular, phase separation occurs during solvent drying due to density differences within a single porous coating, thus providing a separator in which the portion adjacent to the porous polymer substrate is rich in inorganic particles and the outermost portion of the porous coating is rich in binder polymer.

[0165] Meanwhile, when a multilayer porous coating is formed by sequentially applying a slurry for forming a porous coating onto a porous polymer substrate, the coating solution may come into contact with a dried coating, causing separation from the coating and resulting in a significant decrease in adhesion.

[0166] According to this disclosure, the slurry used to form the porous coating may have a viscosity of 200 cP or less, 150 cP or less, 100 cP or less, or 20 cP or less, and 5 cP or more, 8 cP or more, or 10 cP or more. Within the above-defined ranges, the separation of the intermediate layer between the inorganic particles and the binder polymer can occur smoothly, thereby forming a separator with high porosity.

[0167] The viscosity of the slurry can be measured using a vibratory viscometer or an E-type viscometer.

[0168] The slurry may include a dispersant that is appropriately added to it to prevent the aggregation of inorganic particles.

[0169] Inorganic particles can be dispersed using methods commonly known to those skilled in the art. For example, ultrasonic dispersers, ball mills, bead mills, dispersers, mixers, or the like can be used; in particular, ball mills or bead mills are preferred. The dispersion time can vary depending on the volume to be processed, but can suitably be from 1 hour to 20 hours. The particle size of the pulverized particles can be controlled depending on the size of the beads used in the ball mill or bead mill, or the ball milling (or bead milling) time.

[0170] While there are no particular limitations on the process of applying a slurry for forming a porous coating onto a porous polymer substrate, slot coating or dip coating processes are preferred. Slot coating involves applying a slurry supplied via a slot die to the entire surface of the substrate, and the coating thickness can be controlled depending on the flow rate supplied from a metering pump. Dip coating involves immersing the substrate in a tank containing a slurry to perform the coating, and the coating thickness can be controlled depending on the concentration of the composition and the rate at which the substrate is removed from the tank. Furthermore, for more precise control of the coating thickness, post-metering can be performed after immersion using a Mayer bar or similar method.

[0171] The porous polymer substrate coated with a slurry for forming a porous coating can then be dried in a dryer such as an oven to form a porous coating on at least one surface of the porous polymer substrate.

[0172] Drying can be carried out in a drying chamber, where the conditions are not particularly limited due to the use of non-solvents.

[0173] The drying step can be carried out at a relative humidity of 30% or greater, 35% or greater, or 40% or greater, and 80% or less, 75% or less, or 70% or less. For example, the drying step can be carried out at a relative humidity of 40% to 80%. Alternatively, the drying step can be carried out at a temperature of 20°C to 70°C for 0.1 to 2 minutes.

[0174] According to this disclosure, the solid content (solvent-free slurry for forming a porous coating) can be 10 parts by weight or more, 15 parts by weight or more, or 20 parts by weight or more, and 40 parts by weight or less, 35 parts by weight or less, or 30 parts by weight or less based on 100 parts by weight of the slurry for forming a porous coating.

[0175] According to embodiments of this disclosure, the solvent may be an aqueous solvent or an organic solvent.

[0176] The solvents used herein preferably have solubility parameters similar to those of the particulate binder polymer to be used and a low boiling point. This is because such solvents promote homogeneous mixing and subsequent solvent removal.

[0177] For example, when the solvent is an organic solvent, the adhesive polymer dissolves in the organic solvent.

[0178] In this context, non-limiting examples of organic solvents include any one of acetone, tetrahydrofuran, dichloromethane, chloroform, dimethylformamide, N-methyl-2-pyrrolidone, methyl ethyl ketone, and cyclohexane, or mixtures of two or more of them.

[0179] For example, when the solvent is an aqueous solvent, the binder polymer can be particles dispersed in the solvent, and the separator can further include a separate binder layer on the porous coating to enhance adhesion to the electrodes.

[0180] In this context, non-limiting examples of aqueous solvents include at least one selected from water, methanol, ethanol, propanol, butanol, butanediol, ethylene glycol, propylene glycol, and tripropylene glycol.

[0181] Furthermore, the partition according to this disclosure satisfies at least three of the following equations:

[0182] The increase rate of air infiltration time is ≤100%.

[0183] Heat shrinkage rate ≤10%

[0184] Adhesion to electrodes ≥70gf / 25mm

[0185] The viscosity of the slurry used to form the porous coating is ≤20cp.

[0186] (The "increase rate of air permeation time" is calculated using the formula [(air permeation time of porous polymer substrate - air permeation time of separator) / (air permeation time of porous polymer substrate)] × 100, and)

[0187] "Heat shrinkage rate" is defined as the smaller of the heat shrinkage rate in the longitudinal direction (MD) and the heat shrinkage rate in the transverse direction (TD), and is calculated by the formula [(initial length - length after heat shrinkage at 150°C for 30 minutes) / (initial length)] × 100.

[0188] The embodiments will then be described in more detail so that this disclosure can be readily understood. However, the following embodiments may be embodied in many different forms and should not be construed as limited to the exemplary implementations set forth herein. Rather, these exemplary implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art.

[0189] Example 1

[0190] First, flake-shaped aluminum hydroxide (average particle diameter: 800 nm, available from Huber) as inorganic particles and poly(ethylene glycol)-block-poly(acrylic acid) (available from BYK) as a dispersant were introduced into water and homogenized therein. Then, an acrylic granular binder (available from Zeon) as a granular binder polymer was sequentially introduced to prepare a slurry for forming a porous coating. The slurry had an inorganic particle to granular binder weight ratio of 70:30. Furthermore, the inorganic particle to dispersant weight ratio was 99.5:0.5. The slurry was applied to one surface of a porous polyethylene substrate using a doctor blade and then dried to prepare a separator with a porous coating. This is shown in Table 1. Figure 1 A scanning electron microscope (SEM) image of the septum according to Example 1 is shown.

[0191] Comparative Example 1

[0192] The separator was obtained in the same manner as in Example 1, except that carboxymethyl cellulose (CMC, available from GL Chem) was used instead of poly(ethylene glycol)-block-poly(acrylic acid) as the dispersant. This is shown in Table 1. Figure 2 SEM images of the partition according to Comparative Example 1 are shown.

[0193] Comparative Example 2

[0194] The separator was obtained in the same manner as in Example 1, except that alumina (Al2O3, average particle diameter: 600 nm, available from Alteo) was used as spherical inorganic particles instead of sheet-like inorganic particles, and carboxymethyl cellulose (CMC, available from GL Chem) was used instead of poly(ethylene glycol)-block-poly(acrylic acid) as the dispersant. This is shown in Table 1. Figure 3 SEM images of the partition according to Comparative Example 2 are shown.

[0195] Comparative Example 3

[0196] The separator was obtained in the same manner as in Example 1, except that polyethylene glycol (available from Aldrich) having only ethylene glycol functional groups was used instead of poly(ethylene glycol)-block-poly(acrylic acid) as the dispersant. This is shown in Table 1.

[0197] Comparative Example 4

[0198] The separator was obtained in the same manner as in Example 1, except that polyacrylic acid (available from Aldrich) with only carboxyl functional groups was used instead of poly(ethylene glycol)-block-poly(acrylic acid) as the dispersant. This is shown in Table 1.

[0199] [Table 1]

[0200]

[0201]

[0202] When a dispersant containing both carboxyl and glycol groups is used in the case of Example 1 in Table 1, the separator exhibits a low air permeation time increase rate of 73%, high adhesion to the electrode, and low thermal shrinkage.

[0203] In contrast, when carboxymethyl cellulose was used as a dispersant in Comparative Examples 1 and 2, each separator showed a significantly higher rate of increase in air permeation time compared to Example 1, and showed reduced adhesion to the electrodes and differential thermal shrinkage compared to Example 1.

[0204] When a dispersant containing only ethylene glycol groups was used in Comparative Example 3, the inorganic particles could not be dispersed and could not be coated.

[0205] When a dispersant containing only carboxyl groups was used in Comparative Example 4, the separator exhibited lower adhesion to the electrode and lower differential thermal shrinkage compared to Example 1.

[0206] Test methods

[0207] 1) Methods for measuring thickness

[0208] The thickness of the partition was measured using a thickness gauge (VL-50S-B, available from Mitutoyo).

[0209] 2) Method for determining the viscosity of the slurry used to form the porous coating

[0210] The viscosity of 100cc of slurry used to form a porous coating was determined by using a DV 2T viscometer (Brookfield viscometer) at 25°C.

[0211] 3) Methods for determining air infiltration time

[0212] An air permeation time meter (EG01-55-1MR, available from Asahi Seiko) was used to determine the time (sec) required for 100 mL of air to pass through a partition under constant pressure (0.05 MPa). The air permeation time was recorded as the average of the values ​​measured at three points, including one point each on the left, middle, and right sides.

[0213] 4) Method for determining the adhesion (Lami strength) between the electrode and the separator.

[0214] To determine the adhesion (Lami strength) between the electrode and the separator, the negative electrode was prepared as follows.

[0215] First, artificial graphite, carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed with water in a weight ratio of 96:1:2:2 to prepare a negative electrode slurry. This negative electrode slurry was then subjected to a reaction at 3.5 mAh / cm³. 2 The capacity is applied to the copper foil and dried at 130°C for 3 hours or longer, followed by pressing to obtain the negative electrode.

[0216] The resulting negative electrode was cut into 25mm × 100mm dimensions. Additionally, each separator obtained according to the examples and comparative examples was cut into 15mm × 100mm dimensions. The separators were stacked with the negative electrode, and this stack was inserted between 100μm thick PET films and adhered using a flatbed press. Here, the flatbed press was heated and pressurized at 60°C for 1 second at a pressure of 6.5MPa. The adhered separators and negative electrodes were attached to a glass slide using double-sided tape. The ends of the separators (10mm or less from the end of the adhered surface) were peeled off and attached to the 25mm × 100mm PET film using single-sided tape, allowing them to be joined longitudinally. The glass slide was then mounted to the lower fixture of the UTM instrument (LLOYD Instrument LFPlus), and the PET film adhered to the separators was mounted to the upper fixture of the UTM instrument. Force was then applied at 180° and a rate of 300mm / min. The force required to separate the negative electrode from the porous coating facing the negative electrode is measured.

[0217] 5) Methods for determining heat shrinkage rate

[0218] The heat shrinkage rate of the partition is calculated using the formula [(initial length - length after heat shrinkage at 150℃ / min) / (initial length)] × 100.

[0219] Here, the thermal shrinkage rate in the longitudinal direction (MD) and the thermal shrinkage rate in the transverse direction (TD) were measured.

[0220] 6) Average particle diameter (D50) of inorganic particles contained in the slurry.

[0221] The average particle diameter of the inorganic particles contained in the slurry was determined using a particle size analyzer (MASTERSIZER3000, available from Malvern).

[0222] 7) The proportion of particulate binder polymer in porous coatings

[0223] The proportion of particulate binder polymer in the porous coating was determined by X-ray diffraction and energy-dispersive X-ray analysis (EDX).

[0224] [Description of main components]

[0225] 100: partition

[0226] 10: Porous polymer substrate

[0227] 20: Porous coating

[0228] 21: Inorganic particles

[0229] 22: Particulate adhesive polymer

Claims

1. A separator for a lithium secondary battery, comprising: Porous polymer substrate; and A porous coating formed on at least one surface of the porous polymer substrate, comprising inorganic particles, particulate binder polymer, and dispersant. The dispersant comprises carboxyl and glycol groups, and The porous coating is divided into a top layer, an intermediate layer, and a bottom layer in the thickness direction, wherein the content of the particulate binder polymer in the top layer is higher than that in the bottom layer. The top layer is the outermost layer, the bottom layer is the layer facing the porous polymer substrate, and the middle layer is the remaining layer in the porous coating excluding the top layer and the bottom layer.

2. The separator for a lithium secondary battery according to claim 1, wherein the content of the particulate binder polymer has a concentration gradient that increases from the bottom layer to the top layer based on the thickness direction of the porous coating, and the content of the inorganic particles has a concentration gradient that increases from the top layer to the bottom layer based on the thickness direction of the porous coating.

3. The separator for a lithium secondary battery according to claim 1, wherein the intermediate layer in the porous coating has a gradient concentration gradient of the particulate binder polymer, or does not have a specific concentration gradient.

4. The separator for a lithium secondary battery according to claim 1, wherein the weight ratio of the inorganic particles to the dispersant is from 99.5:0.5 to 95:

5.

5. The separator for a lithium secondary battery according to claim 1, wherein the content of the particulate binder polymer is from 10 parts by weight to 50 parts by weight based on 100 parts by weight of the porous coating.

6. The separator for a lithium secondary battery according to claim 1, wherein the dispersant has a ratio of 0.05 to 0.25 of the equivalent amount of ethylene glycol groups to the equivalent amount of carboxyl groups.

7. The separator for a lithium secondary battery according to claim 1, wherein the dispersant has a weight-average molecular weight of 100 to 10,000.

8. The separator for a lithium secondary battery according to claim 1, wherein the dispersant is a copolymer of polyacrylic acid and polyethylene glycol.

9. The separator for a lithium secondary battery according to claim 8, wherein the dispersant is a block copolymer of polyacrylic acid and polyethylene glycol.

10. The separator for a lithium secondary battery according to claim 1, wherein the porous coating has a thickness of 2 μm to 10 μm.

11. The separator for a lithium secondary battery according to claim 1, wherein the inorganic particles are flaky inorganic particles.

12. The separator for a lithium secondary battery according to claim 11, wherein the sheet-like inorganic particles have an aspect ratio of 3.5 or greater.

13. The separator for a lithium secondary battery according to claim 11, wherein the sheet-like inorganic particles comprise at least one of Al(OH)3, AlO(OH), Mg(OH)2, and BaTiO3.

14. A method for manufacturing a separator for a lithium secondary battery as defined in claim 1, comprising the steps of applying a slurry containing a solvent, inorganic particles, a particulate binder polymer, and a dispersant for forming a porous coating to at least one surface of a porous polymer substrate, followed by drying. The slurry used to form the porous coating has a viscosity of 200 cP or less, and The slurry used to form the porous coating has a solid content of 10 to 40 parts by weight per 100 parts by weight of the slurry.

15. The method of manufacturing a partition according to claim 14, wherein the solvent is water. The adhesive polymer is in the form of particles dispersed in the solvent, and Phase separation is performed during the drying step.

16. The method of manufacturing a partition according to claim 14, wherein the partition satisfies at least three of the following formulas: The increase rate of air infiltration time is ≤100%. Heat shrinkage rate ≤10% Adhesion to electrodes ≥70gf / 25mm The viscosity of the slurry used to form the porous coating is ≤20cp. The "increase rate of air permeation time" is calculated using the formula [(air permeation time of porous polymer substrate - air permeation time of separator) / (air permeation time of porous polymer substrate)] × 100. "Heat shrinkage rate" is defined as the smaller of the heat shrinkage rate in the longitudinal direction (MD) and the heat shrinkage rate in the transverse direction (TD), and is calculated by the formula [(initial length - length after heat shrinkage at 150°C for 30 minutes) / (initial length)] × 100.

17. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator is the same as that defined in any one of claims 1 to 13.

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