Separator for secondary battery and secondary battery comprising same

Abstract

The present invention relates to a separator for a secondary battery and a secondary battery comprising same. The separator for a secondary battery comprises a porous substrate and a coating layer positioned on at least one surface of the porous substrate. The coating layer comprises: a crosslinked product of a first binder, a second binder, a crosslinking agent, and a chain extender; and a filler, wherein the first binder is an aqueous binder, the second binder is a carboxyalkylcellulose-based compound or a salt thereof, the crosslinking agent includes citric acid, the citric acid is included in an amount of 5 to 50 parts by weight with respect to 100 parts by weight of the total amount of the first binder and the second binder, and the crosslinking agent and the chain extender in the crosslinked product are crosslinked at a weight ratio of 1:0.2 to 1:2.

Description

Separator for secondary batteries and secondary battery including the same

[0001] This invention relates to a separator for a secondary battery and a secondary battery including the same.

[0002] Recently, accompanied by the rapid proliferation of battery-powered electronic devices such as mobile phones, laptop computers, and electric vehicles, the demand for high-energy-density, high-capacity rechargeable batteries is rapidly increasing. Accordingly, research and development to improve the performance of lithium-ion batteries is actively underway.

[0003] A lithium secondary battery is a battery comprising a positive electrode and a negative electrode containing an active material capable of lithium ion intercalation and deintercalation, and an electrolyte, and produces electrical energy through oxidation and reduction reactions when lithium ions are intercalated and deintercalated from the positive electrode and the negative electrode. The lithium secondary battery may include a separator between the positive electrode and the negative electrode.

[0004] Recently, there has been an ongoing need to suppress battery ignition and explosion caused by external impacts. Meanwhile, the separator is manufactured, wound onto a winding unit, unwound, and laminated with the electrode plate. If the coating layer detaches from the separator during the battery manufacturing process, production may be halted, potentially leading to reduced manufacturing processability. Furthermore, in jelly-roll type batteries, since the separator is wound in a roll form, battery performance may also decline if the coating layer detaches from the separator.

[0005] One embodiment provides a separator for a secondary battery capable of suppressing ignition and explosion of the battery in response to external collisions.

[0006] Another embodiment provides a separator for a secondary battery having a low dry shrinkage rate and a low shrinkage rate in the electrolyte.

[0007] Another embodiment provides a separator for a secondary battery that does not detach the coating layer from the porous substrate.

[0008] According to one embodiment, a separator for a secondary battery is provided.

[0009] 1. The separator for the secondary battery comprises a porous substrate and a coating layer located on at least one surface of the porous substrate, wherein the coating layer comprises a crosslinked material of a first binder, a second binder, a crosslinking agent, and a chain extender; and a filler, wherein the first binder is a water-based binder, the second binder is a carboxyalkylcellulose-based compound or a salt thereof, the crosslinking agent comprises citric acid, wherein the citric acid is included in an amount of 5 to 50 parts by weight per 100 parts by weight of the total of the first binder and the second binder, and the crosslinked material is crosslinked in a weight ratio of the crosslinking agent to the chain extender of 1:0.2 to 1:2.

[0010] 2. A separator for a secondary battery, wherein the chain extender in 1 is a difunctional or higher alcohol or a difunctional or higher amine.

[0011] 3. A separator for a secondary battery, wherein the chain extender in 1 to 2 comprises one or more of polyethylene glycol and polypropylene glycol.

[0012] 4. A separator for a secondary battery, wherein the chain extender in 1 to 3 has a weight-average molecular weight of 500 to 2000 g / mol.

[0013] 5. A separator for a secondary battery, wherein, in 1 to 4, the chain extender is included in an amount of 1 to 20 parts by weight per 100 parts by weight of the total sum of the first binder and the second binder.

[0014] 6. A separator for a secondary battery according to 1 to 5, wherein the first binder : the second binder is included in a weight ratio of 80 : 20 to 25 : 75 among 100 parts by weight of the total sum of the first binder and the second binder.

[0015] 7. A separator for a secondary battery, wherein the second binder in 1 to 6 is carboxymethylcellulose or an alkali metal salt thereof.

[0016] 8. A separator for a secondary battery, wherein the carboxyalkylcellulose-based compound or salt thereof in accordance with 1 to 7 has a weight-average molecular weight of 500 to 2000 g / mol.

[0017] 9. A separator for a secondary battery, wherein, in 1 to 8, the citric acid is included in an amount of 5 to 200 parts by weight per 100 parts by weight of the first binder, and the citric acid is included in an amount of 5 to 200 parts by weight per 100 parts by weight of the second binder.

[0018] 10. In 1 to 9, the filler is a separator for a secondary battery having a particle size D100 of 0.7 μm or less.

[0019] 11. A separator for a secondary battery, wherein the filler in 1 to 10 is boehmite and is plate-shaped.

[0020] 12. A separator for a secondary battery according to 1 to 10, wherein the first binder comprises a crosslinking unit with the citric acid, and the crosslinking unit with the citric acid comprises one or more of the following: a unit derived from (meth)acrylic acid or a salt thereof, a unit derived from (meth)acrylamide, a unit derived from hydroxyalkyl (meth)acrylate, a unit derived from (meth)acrylonitrile, a unit derived from (meth)acrylamido-2-methylpropanesulfonic acid and a salt thereof, and a unit derived from ethyleneimine.

[0021] 13. A separator for a secondary battery, wherein, in 1 to 12, the crosslinking unit with citric acid is included in the (meth)acrylic binder at a concentration of 10 to 100 mol%.

[0022] 14. A separator for a secondary battery, wherein the (meth)acrylic binder comprises a sulfonate group-containing structural unit in accordance with 1 to 13.

[0023] 15. A separator for a secondary battery according to 1 to 14, wherein the (meth)acrylic binder further comprises one or more of a structural unit derived from (meth)acrylate or (meth)acrylic acid, a cyano group-containing structural unit, and a structural unit derived from (meth)acrylamide.

[0024] 16. A mixture of the first binder and the second binder in 1 to 15: the filler is included in a mass ratio of 1:10 to 1:50, for a separator for a secondary battery.

[0025] Another example of an implementation provides a secondary battery.

[0026] The above secondary battery includes a positive electrode, a negative electrode, and a separator for the secondary battery located between the positive electrode and the negative electrode.

[0027] A separator for a secondary battery according to one embodiment can suppress ignition and explosion of the battery in response to external impacts, thereby increasing the reliability and stability of the battery. In addition, the separator can extend the lifespan of the battery by having a low dry shrinkage rate and a low shrinkage rate within the electrolyte. Furthermore, the separator can increase the reliability of the battery by preventing the detachment of the coating layer from the substrate.

[0028] FIG. 1 is a cross-sectional view showing a separator for a lithium secondary battery according to one embodiment.

[0029] FIGS. 2 to 5 are cross-sectional views schematically showing a lithium secondary battery according to one embodiment.

[0030] Hereinafter, embodiments of the present invention will be described in detail. However, these are presented as examples and are not intended to limit the present invention, and the present invention is defined only by the scope of the claims set forth below.

[0031] Unless otherwise specifically stated in this specification, when a part such as a layer, film, region, plate, etc. is described as being "on" another part, this includes not only cases where it is "immediately on" another part, but also cases where there is another part in between.

[0032] Unless otherwise specified in this specification, a singular form may also include a plural form. Additionally, unless otherwise specified, "A or B" may mean "including A, including B, or including A and B."

[0033] In this specification, "combination of these" may mean a mixture of components, a laminate, a composite, a copolymer, an alloy, a blend, and a reaction product, etc.

[0034] In this specification, 'size D100' refers to the size of a particle in which the cumulative volume of the particle size distribution is 100 volume%. The particle size distribution can be measured by methods widely known to those skilled in the art. For example, the particle size distribution may be measured by a particle size analyzer, or by a transmission electron microscope (TEM) image or a scanning electron microscope (SEM) image. Alternatively, the D100 value may be obtained by measuring using a measuring device utilizing dynamic light scattering, performing data analysis to count the number of particles for each particle size range, and then calculating from this. Alternatively, it may be measured using a laser diffraction method. When measuring by laser diffraction, more specifically, after dispersing the particles to be measured in a dispersion medium, they are introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000) and irradiated with ultrasound of about 28 kHz at an output of 60 W, and then D100 can be calculated based on 100% of the particle size distribution in the measuring device.

[0035] In this specification, 'size D50' refers to a size that signifies the diameter of a particle whose cumulative volume in a particle size distribution is 50 volume%. The particle size distribution can be obtained by referring to the method described in 'size D100'.

[0036] If the above particle is spherical, the above size may refer to the particle diameter.

[0037] In this specification, '(meth)acrylic' means acrylic and / or methacrylic.

[0038] Unless otherwise defined below, "substitution" means that a hydrogen in a compound is a C1 to C30 alkyl group, C2 to C30 alkenyl group, C2 to C30 alkynyl group, C6 to C30 aryl group, C7 to C30 alkylaryl group, C1 to C30 alkoxy group, C1 to C30 heteroalkyl group, C3 to C30 heteroalkylaryl group, C3 to C30 cycloalkyl group, C3 to C15 cycloalkenyl group, C6 to C30 cycloalkynyl group, C2 to C30 heterocycloalkyl group, halogen (F, Cl, Br, or I), hydroxyl group (-OH), nitro group (-NO2), cyano group (-CN), amino group (-NRR') (wherein R and R' are independently hydrogen or C1 to C6 alkyl groups), or sulfobetaine group (-RR'N + (CH2) n SO3 - , n is a natural number from 1 to 10), carboxybetaine group (-RR'N + (CH2) n COO - , n is a natural number from 1 to 10) (where R and R' are independently C1 to C20 alkyl groups), azido group (-N3), amido group (-C(=NH)NH2), hydrazino group (-NHNH2), hydrazono group (=N(NH2), carbamoyl group (-C(O)NH2), thiol group (-SH), acyl group (-C(=O)R, where R is hydrogen, a C1 to C6 alkyl group, a C1 to C6 alkoxy group, or a C6 to C12 aryl group), carboxyl group (-COOH) or its salt (-C(=O)OM, where M is an organic or inorganic cation), sulfonic acid group (-SO3H) or its salt (-SO3M, where M is an organic or inorganic cation), phosphate group (-PO3H2) or its salt (-PO3MH or -PO3M2, Here, M is an organic or inorganic cation and means being substituted with a substituent selected from a combination thereof.

[0039] In the following, C1 to C3 alkyl groups refer to methyl groups, ethyl groups, or propyl groups. C1 to C10 alkylene groups may be, for example, C1 to C6 alkylene groups, C1 to C5 alkylene groups, or C1 to C3 alkylene groups, and may be, for example, methylene groups, ethylene groups, or propylene groups. C3 to C20 cycloalkylene groups may be, for example, C3 to C10 cycloalkylene groups or C5 to C10 cycloalkylene groups, and may be, for example, cyclohexylene groups. C6 to C20 arylene groups may be, for example, C6 to C10 arylene groups, and may be, for example, phenylene groups. C3 to C20 heterocyclic groups may be, for example, C3 to C10 heterocyclic groups, and may be, for example, pyridine groups.

[0040] In the following, "hetero" means containing one or more heteroatoms selected from N, O, S, Si, and P. In the chemical formulas, the * symbol indicates a part connected to the same or different atoms, groups, or structural units. Unless specifically stated otherwise in the chemical formulas described herein, hydrogen may be considered to be bonded in the structure of the chemical formulas.

[0041] In the following, "alkali metal" refers to elements belonging to Group 1 of the periodic table, such as lithium, sodium, potassium, rubidium, cesium, or francium, and may exist in a positive or neutral state.

[0042] In this specification, when describing a numerical range, 'X to Y' means 'X or greater and Y or less (X ≤ and ≤ Y).'

[0043] According to one embodiment, a separator for a secondary battery comprises a porous substrate and a coating layer located on at least one surface of the porous substrate, wherein the coating layer comprises a crosslinked product of a first binder, a second binder, a crosslinking agent, and a chain extender; and a filler, wherein the first binder is a water-based binder, the second binder is a carboxyalkylcellulose-based compound or a salt thereof, the crosslinking agent comprises citric acid, and the citric acid is included in an amount of 5 to 50 parts by weight per 100 parts by weight of the total of the first binder and the second binder, and the crosslinked product is crosslinked in a weight ratio of the crosslinking agent to the chain extender of 1:0.2 to 1:2.

[0044] A crosslinked product of a composition comprising only citric acid as a crosslinking agent in a mixture comprising the first binder and the second binder has high tensile strength, which can suppress ignition and explosion of the battery against external impacts, thereby increasing the reliability and stability of the battery. A crosslinked product obtained by applying citric acid as a crosslinking agent and a chain extender together to a mixture comprising the first binder and the second binder simultaneously improves the tensile strength and toughness of the crosslinked product, thereby providing a low dry shrinkage rate and a low shrinkage rate in the electrolyte, and further increasing the reliability of the battery by preventing the detachment of the coating layer from the substrate. If the coating layer does not detach from the substrate, the coating layer does not detach from the separator during the battery manufacturing process, thereby improving the processability of the battery manufacturing process, and in the case of a jellyroll type battery, the separator is wound in a roll form, which can also increase battery performance.

[0045] The above citric acid is one of the polycarboxylic acids having a hydroxyl group as shown in Chemical Formula 9 below.

[0046] [Chemical Formula 9]

[0047]

[0048] The above citric acid may be crosslinked with the first binder and the carboxyalkylcellulose-based compound or its salt. The above citric acid has high dispersibility in a mixture containing the first binder and the carboxyalkylcellulose-based compound or its salt, and thus the crosslinking to both the first binder and the carboxyalkylcellulose-based compound or its salt is high, which may facilitate increasing the tensile strength of the crosslinked material. By increasing the tensile strength of the crosslinked material, it may be easier to suppress ignition and explosion of the battery in response to external impact.

[0049] The above-mentioned chain extender is used to control the glass transition temperature of conventional copolymers. On the other hand, the present invention achieved the effect of the aforementioned separation membrane by using a chain extender.

[0050] The chain extender does not crosslink with the first binder and the carboxyalkylcellulose-based compound or its salt. Instead, the chain extender may crosslink with the citric acid. The chain extender can simultaneously improve the tensile strength and toughness of the crosslinked material by crosslinking the first binder and the carboxyalkylcellulose-based compound or its salt without affecting the crosslinking by the citric acid when the citric acid crosslinks the first binder and the carboxyalkylcellulose-based compound or its salt together. As will be described below, the chain extender has a longer main chain length relative to the citric acid. Therefore, the chain extender is thought to be effective in increasing the toughness of the crosslinked material, but the present invention is not limited thereto. The above chain extender can suppress ignition and explosion of the battery upon external impact by simultaneously improving the tensile strength and toughness of the crosslinked material, provide a low dry shrinkage rate and a low shrinkage rate within the electrolyte, and prevent the coating layer from detaching from the substrate. Furthermore, since the length of the main chain forming the molecule can be easily controlled by the chain extender, it is easy to control the content of the first binder, the second binder, and citric acid in the coating layer, thereby improving the manufacturing processability of the separator.

[0051] In one embodiment, the tensile strength of the crosslinked material may be 900 MPa or more, for example 1000 MPa or more, or 1000 to 5000 MPa, and the toughness of the crosslinked material may be 500 MPa or more, for example 500 to 1500 MPa. Within the above range, the effect of the above-described separation membrane can be easily achieved.

[0052] The above tensile strength is Young's modulus, and the tensile strength and toughness can be measured by the method described below.

[0053] The citric acid and the chain extender are crosslinked in the crosslinked material in a weight ratio of 1:0.2 to 1:2.

[0054] If the above weight ratio is less than 1:0.2, the amount of chain extender added is low, so the toughness of the crosslinked material is low, which may make it easy for the coating layer to detach from the separator.

[0055] If the above weight ratio exceeds 1:2, the chain extender is included in excess within the crosslinker and remains unable to participate in the crosslinking reaction, which may degrade the physical properties of the membrane.

[0056] For example, the above weight ratios are 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, 1:0.5, 1:0.55, 1:0.6, 1:0.65, 1:0.7, 1:0.75, 1:0.8, 1:0.85, 1:0.9, 1:0.95, 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2, 1:1.25, 1:1.3, 1:1.35, 1:1.4, 1:1.45, 1:1.5, 1:1.55, 1:1.6, 1:1.65, 1:1.7, 1:1.75, It can be 1:1.8, 1:1.85, 1:1.9, 1:1.95, 1:2, for example 1:0.25 to 1:2, for example 1:0.5 to 1:1.

[0057] The citric acid is included in an amount of 5 to 50 parts by weight relative to 100 parts by weight of the total of the first binder and the second binder. If the citric acid is less than 5 parts by weight relative to the total of 100 parts by weight, the increase in the modulus of the coating layer is negligible, which may be disadvantageous in suppressing ignition and explosion of the battery in response to external impact. If the citric acid is more than 50 parts by weight relative to the total of 100 parts by weight, the bonding strength of the separator to the substrate is lowered, causing separation between the coating layer and the substrate in the separator, which may result in reduced reliability. In one embodiment, the citric acid may be included in an amount of 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50 parts by weight, 10 to 50 parts by weight, for example, 10 to 20 parts by weight, based on the total amount of 100 parts by weight. Within this range, the effect of the separation membrane may be significant.

[0058] In one embodiment, the chain extender may be included in an amount of 1 to 20 parts by weight, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 parts by weight, 1 to 10 parts by weight, or 2 to 10 parts by weight, based on 100 parts by weight of the total amount of the first binder and the second binder. Within the above range, the above weight ratio can be easily reached, and the remainder may not affect the physical properties of the separation membrane.

[0059] In one embodiment, the thermal shrinkage rate in the mechanical direction (MD) and transverse direction (TD) measured after being left in an electrolyte at 150°C for 1 hour may be 7% or less, for example, 5% or less. The MD and TD may each be in the same direction as the MD and TD of the porous substrate.

[0060] In one embodiment, the total of the first binder and the second binder may be 95% by weight or more of the mixture, for example, 95 to 100% by weight, or 99 to 100% by weight, or 100% by weight. Within this range, it may be easy to implement the effect of the separation membrane described above.

[0061] In one embodiment, the citric acid may be included in an amount of 95% by weight or more of the crosslinking agent, for example, 95 to 100% by weight, 99 to 100% by weight, or 100% by weight. Within this range, it may be easy to implement the effect of the separation membrane described above.

[0062] In one embodiment, the crosslinker may be a thermal crosslinker.

[0063] In one embodiment, the coating layer may be formed from a composition for a coating layer comprising: the first binder; the second binder, i.e., a carboxyalkylcellulose-based compound or a salt thereof; the crosslinking agent, i.e., citric acid; a chain extender; and a filler.

[0064] Hereinafter, each component of the above-mentioned coating layer composition will be described in detail.

[0065] 1st binder

[0066] The first binder above must be included in an appropriate amount relative to the second binder above. Among 100 parts by weight of the total of the first binder and the second binder, the first binder : the second binder above may be included in a weight ratio of 80 : 20 to 25 : 75. Within this range, the dry heat shrinkage rate of the separator and the heat shrinkage rate in the electrolyte may be lowered, the bonding strength of the substrate may be increased, and the stability of the cell against external impact may be enhanced. For example, the weight ratio of the first binder : the second binder above may be 80:20, 75:25, 70:30, 65:35, 60:40, 55:45, 50:50, 45:55, 40:60, 35:65, 30:70, 25:75, or 80 : 20 to 30 : 70. Within the above range, the effect of the aforementioned separation membrane can be significant.

[0067] The first binder above is a water-based binder. The water-based binder has good dispersibility in a composition for a coating layer containing carboxyalkylcellulose or a salt thereof and citric acid, so it can be easy to implement the effect of the separation membrane.

[0068] The first binder above may be a water-based heat-resistant binder and a (meth)acrylic binder.

[0069] The first binder above may include one or more of the following: a crosslinking unit with the citric acid, for example, a unit derived from (meth)acrylic acid or a salt thereof, a unit derived from (meth)acrylamide, a unit derived from hydroxyalkyl (meth)acrylate, a unit derived from (meth)acrylonitrile, a unit derived from (meth)acrylamido-2-methylpropanesulfonic acid and a salt thereof, and a unit derived from ethyleneimine.

[0070] In one embodiment, the crosslinking unit with citric acid may be included in the first binder in an amount of 10 to 100 mol%, for example, 10 to 95 mol%, for example, 30 to 95 mol%. Within this range, the modulus of the coating layer can be increased, and the air permeability can be prevented from increasing due to the modulus of the coating layer becoming excessively high.

[0071] The above (meth)acrylic binder may include a sulfonate group-containing structural unit. The sulfonate group-containing structural unit may be effective in improving the heat resistance and membrane resistance of the separator.

[0072] The above sulfonate group-containing structural unit may be included in the above (meth)acrylic binder in an amount of 0.1 to 60 mol%, for example 0.1 to 20 mol%, 0.1 to 10 mol%, 1 to 20 mol%, for example 1 to 10 mol%, for example 20 to 65 mol%, or 30 to 65 mol%. When the above sulfonate group-containing structural unit is included in the above range, the separator may exhibit excellent adhesion, heat resistance, air permeability, and oxidation resistance.

[0073] The above (meth)acrylic binder may further include one or more of structural units derived from (meth)acrylate or (meth)acrylic acid, cyano group-containing structural units, and structural units derived from (meth)acrylamide.

[0074] Preferably, the (meth)acrylic binder may comprise a structural unit derived from (meth)acrylate or (meth)acrylic acid, more preferably a structural unit derived from (meth)acrylic acid. The structural unit derived from (meth)acrylic acid may provide a crosslinking site with the citric acid.

[0075] The structural unit derived from the above (meth)acrylate or (meth)acrylic acid may be included in the above (meth)acrylic binder in an amount of 0 mol% to 70 mol%, for example, 10 mol% to 70 mol%, 10 to 60 mol%, 20 to 60 mol%, 10 to 50 mol%, 30 to 60 mol%, 10 to 40 mol%, or 40 to 55 mol%. Within the above range, the separator may exhibit excellent adhesion, heat resistance, air permeability, and oxidation resistance.

[0076] The above cyano group-containing structural unit may be included in the above (meth)acrylic binder in an amount of 0 to 85 mol%, for example, 30 to 85 mol%, 30 to 70 mol%, 30 to 60 mol%, 35 to 60 mol%, or 35 to 55 mol%. Within the above range, the separator can secure excellent oxidation resistance and exhibit adhesion, heat resistance, and air permeability.

[0077] The structural unit derived from the above (meth)acrylamide may be included in the above (meth)acrylic binder in an amount of 0 mol% to 95 mol%, for example, 40 to 85 mol%, 50 to 85 mol%, 55 to 95 mol%, 60 to 85 mol%, 75 to 95 mol%, or 80 to 95 mol%. Within the above range, the separator can secure excellent oxidation resistance and exhibit adhesion, heat resistance, and air permeability.

[0078] According to one embodiment, the (meth)acrylic binder may have a sulfonate group-containing structural unit, for example, 0.1 to 30 mol%, for example, 0.1 to 10 mol%; a structural unit derived from (meth)acrylate or (meth)acrylic acid, for example, 10 to 60 mol%, for example, 10 to 40 mol%; and a cyano group-containing structural unit, for example, 30 to 85 mol%, for example, 30 to 80 mol% (referred to as the 1-1 binder). In one embodiment, the total sum of the sulfonate group-containing structural unit, the structural unit derived from (meth)acrylate or (meth)acrylic acid, and the cyano group-containing structural unit may be 95 mol% or more, for example, 95 to 100 mol%, or 100 mol% of the 100 mol% of the (meth)acrylic binder.

[0079] According to another embodiment, the (meth)acrylic binder may have a sulfonate group-containing structural unit, for example, 1 mol% to 20 mol%, for example, 5 to 20 mol%, and a structural unit derived from (meth)acrylamide, for example, 80 mol% to 99 mol%, for example, 80 to 95 mol% (referred to as the first-second binder). In one embodiment, the total sum of the sulfonate group-containing structural unit and the structural unit derived from (meth)acrylamide may be 95 mol% or more, for example, 95 to 100 mol%, or 100 mol% of the 100 mol% of the (meth)acrylic binder.

[0080] According to another embodiment, the (meth)acrylic binder may have a structural unit containing a sulfonate group, a structural unit derived from (meth)acrylate or (meth)acrylic acid, and a structural unit derived from (meth)acrylamide (referred to as the 1st-3rd binder). In one embodiment, the total sum of the structural units containing the sulfonate group, the structural unit derived from (meth)acrylate or (meth)acrylic acid, and the structural unit derived from (meth)acrylamide may be 95 mol% or more of the 100 mol% of the (meth)acrylic binder, for example, 95 to 100 mol%, or 100 mol%.

[0081] For example, the above (meth)acrylic binder may be a 1-1 binder or a 1-2 binder. The above 1-1 binder or 1-2 binder may have a superior effect in improving film resistance and heat resistance.

[0082] Each structural unit of the above (mat)acrylic binder is described in detail.

[0083] The structural unit derived from the above (meth)acrylate or (meth)acrylic acid may be represented, for example, by the following Chemical Formula 1, Chemical Formula 2, Chemical Formula 3, or a combination thereof:

[0084] [Chemical Formula 1]

[0085]

[0086] [Chemical Formula 2]

[0087]

[0088] [Chemical Formula 3]

[0089]

[0090] (In the above Chemical Formulas 1 to 3,

[0091] R 1 to R 6 Each is independently a hydrogen or methyl group, and

[0092] In the above chemical formula 2,

[0093] M is an alkali metal).

[0094] The above alkali metal may be, for example, lithium, sodium, potassium, rubidium, or cesium.

[0095] For example, the structural unit derived from the above (meth)acrylate or (meth)acrylic acid may include the structural unit represented by the above chemical formula 2 and the structural unit represented by the above chemical formula 3, and in this case, the structural unit represented by the above chemical formula 2 and the structural unit represented by the above chemical formula 3 may be included in a molar ratio of 10:1 to 1:2, or 10:1 to 1:1, or 5:1 to 1:1.

[0096] The above cyano group-containing structural unit can be represented, for example, by the following chemical formula 4.

[0097] [Chemical Formula 4]

[0098]

[0099] (In the above chemical formula 4,

[0100] R 7 and R 8 Each is independently hydrogen or a C1 to C3 alkyl group, and

[0101] L 1 is -C(=O)-, -C(=O)O-, -OC(=O)-, -O- or -C(=O)NH-, and

[0102] x is an integer from 0 to 2, and

[0103] L 2 is a substituted or unsubstituted C1 to C10 alkylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C6 to C20 arylene group, or a substituted or unsubstituted C3 to C20 heterocyclic group, and

[0104] y is an integer from 0 to 2).

[0105] The above cyano group-containing structural unit may be a structural unit derived from, for example, (meth)acrylonitrile, alkenitrile, cyanoalkyl (meth)acrylate, or 2-(vinyloxy)alkanitrile. Here, the alkene may be a C1 to C20 alkene, a C1 to C10 alkene, or a C1 to C6 alkene, the alkyl may be a C1 to C20 alkyl, a C1 to C10 alkyl, or a C1 to C6 alkyl, and the alkane may be a C1 to C20 alkane, a C1 to C10 alkane, or a C1 to C6 alkane.

[0106] The above alkenitrile may be, for example, allyl cyanide, 4-pentene nitrile, 3-pentene nitrile, 2-pentene nitrile, or 5-hexenitrile. The above cyanoalkyl (meth)acrylate may be, for example, cyanomethyl (meth)acrylate, cyanoethyl (meth)acrylate, cyanopropyl (meth)acrylate, or cyanooctyl (meth)acrylate. The above 2-(vinyloxy)alkanitrile may be, for example, 2-(vinyloxy)ethanenitrile or 2-(vinyloxy)propanenitrile.

[0107] The above sulfonate group-containing structural unit may be a structural unit containing a conjugate base of sulfonic acid, a sulfonate, sulfonic acid, or a derivative thereof. For example, the above sulfonate group-containing structural unit may be represented by the following chemical formulas 5, 6, 7, or a combination thereof.

[0108] [Chemical Formula 5]

[0109]

[0110] [Chemical Formula 6]

[0111]

[0112] [Chemical Formula 7]

[0113]

[0114] (In the above chemical formulas 5 to 7,

[0115] R 9 to R 14 Each is independently hydrogen or a C1 to C3 alkyl group, and

[0116] L 3 , L 5 and L 7 Each is independently -C(=O)-, -C(=O)O-, -OC(=O)-, -O- or -C(=O)NH-, and

[0117] L 4 , L 6 and L 8 Each is independently a substituted or unsubstituted C1 to C10 alkylene group, a substituted or unsubstituted C3 to C20 cycloalkylene group, a substituted or unsubstituted C6 to C20 arylene group, or a substituted or unsubstituted C3 to C20 heterocyclic group, and

[0118] a, b, c, d, e, and f are each independently integers from 0 to 2, and

[0119] In the above chemical formula 6,

[0120] M is an alkali metal).

[0121] For example, in the above chemical formulas 5 to 7,

[0122] L 3 , L 5 and L 7 Each is independently -C(=O)NH-, and

[0123] L 4 , L 6 and L 8 Each is independently a C1 to C10 alkylene group, and

[0124] a, b, c, d, e, and f can be integers of 1.

[0125] The above sulfonate group-containing structural unit may include only one of the structural unit represented by Chemical Formula 5, the structural unit represented by Chemical Formula 6, and the structural unit represented by Chemical Formula 7, or it may include two or more types. For example, the above sulfonate group-containing structural unit may include the structural unit represented by Chemical Formula 6, and for another example, the above sulfonate group-containing structural unit may include the structural unit represented by Chemical Formula 6 and the structural unit represented by Chemical Formula 7.

[0126] The above sulfonate group-containing structural unit may be a structural unit derived from, for example, vinyl sulfonic acid, allyl sulfonic acid, styrene sulfonic acid, anethole sulfonic acid, (meth)acrylamidoalkane sulfonic acid, sulfoalkyl (meth)acrylate, or salts thereof. Here, the alkane may be a C1 to C20 alkane, a C1 to C10 alkane, or a C1 to C6 alkane, and the alkyl may be a C1 to C20 alkyl, a C1 to C10 alkyl, or a C1 to C6 alkyl. The above salt refers to a salt composed of the aforementioned sulfonic acid and a suitable ion. The ion may be, for example, an alkali metal ion, in which case the salt may be an alkali metal sulfonic acid salt.

[0127] The above (meth)acrylamidoalkane sulfonic acid may be, for example, 2-(meth)acrylamido-2-methylpropanesulfonic acid, and the above sulfoalkyl (meth)acrylate may be, for example, 2-sulfoethyl (meth)acrylate, 3-sulfopropyl (meth)acrylate, etc.

[0128] The structural unit derived from the above (meth)acrylamide can be represented by the following chemical formula 8.

[0129] [Chemical Formula 8]

[0130]

[0131] (In the above chemical formula 8,

[0132] R15 and R 16 Each is independently a hydrogen or methyl group).

[0133] The above (meth)acrylic binder may include an alkali metal. The alkali metal may exist in a cation form and may be, for example, lithium, sodium, potassium, rubidium, or cesium. For example, the alkali metal may be combined with the above (meth)acrylic binder and exist in the form of a salt. The alkali metal can assist in the synthesis of the above (meth)acrylic binder in an aqueous solvent, improve the adhesion of the coating layer, and improve the heat resistance, air permeability, and oxidation resistance of the separator.

[0134] The alkali metal may be included in the alkali metal and the (meth)acrylic binder in an amount of 1% to 40% by weight, for example, 1% to 30% by weight, or 1% to 20% by weight, or 10% to 20% by weight. For example, the (meth)acrylic binder and the alkali metal may be included in a weight ratio of 99:1 to 60:40, a weight ratio of 99:1 to 70:30, for example, a weight ratio of 99:1 to 80:20, or for example, a weight ratio of 90:10 to 80:20.

[0135] In addition, the alkali metal may be included in an amount of 0.1 mol% to 1.0 mol% relative to the total content of the alkali metal and the (meth)acrylic binder. When the alkali metal is included within the above range, the coating layer may have excellent adhesion, and the separator containing it may exhibit excellent heat resistance, air permeability, and oxidation resistance.

[0136] The above (meth)acrylic binder may be in various forms, such as an alternating polymer in which the structural units are alternately distributed, a randomly distributed polymer, or a graft polymer in which some structural units are grafted.

[0137] The weight-average molecular weight (Mw) of the above (meth)acrylic binder may be 200,000 g / mol to 700,000 g / mol, for example, 200,000 g / mol to 600,000 g / mol, or 300,000 g / mol to 600,000 g / mol. Within the above range, the separator may exhibit excellent adhesion, heat resistance, air permeability, and oxidation resistance. In this specification, the weight-average molecular weight may be the polystyrene-equivalent average molecular weight measured using gel permeation chromatography.

[0138] The glass transition temperature of the above (meth)acrylic binder may be 200°C to 280°C, for example, 210°C to 270°C, or 210°C to 260°C. Within this range, the separator may exhibit excellent adhesion, heat resistance, air permeability, and oxidation resistance. The glass transition temperature may be a value measured by differential scanning calorimetry.

[0139] The above (meth)acrylic binder may have a melting point (Tm) of 160°C or higher.

[0140] The above (meth)acrylic binder can be manufactured by a solution polymerization method.

[0141] According to one embodiment, the (met)acrylic binder may be included in the coating layer of the separator in the form of a film.

[0142] 2nd Binder

[0143] The second binder above is a carboxyalkylcellulose-based compound or a salt thereof. When combined with the first binder and citric acid described above, the carboxyalkylcellulose-based compound or its salt may facilitate high substrate binding strength and suppress ignition and explosion of the battery due to external impact.

[0144] Preferably, the second binder may be carboxymethylcellulose or an alkali metal salt thereof, for example, a sodium salt of carboxymethylcellulose.

[0145] The above carboxyalkylcellulose-based compound or salt thereof has a weight-average molecular weight of 50,000 to 1,000,000 g / mol, for example, 50,000, 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000, 550,000, 600,000, 650,000, 700,000, 750,000, 800,000, 850,000, 900,000, 950,000, 1,000,000 g / mol, 100,000 to 600,000 It may be g / mol. Within the above range, the viscosity of the coating layer composition does not become excessively high, making it easy to ensure the dispersibility of citric acid.

[0146] citric acid

[0147] Citric acid may be included in an amount of 5 to 200 parts by weight, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 parts by weight, 5 to 50 parts by weight, for example, 10 to 40 parts by weight, based on 100 parts by weight of the first binder. Within the above range, it is easy to suppress ignition and explosion of the battery against external impacts, provide a low thermal shrinkage rate, and prevent the coating layer from detaching from the separator.

[0148] Citric acid may be included in an amount of 5 to 200 parts by weight, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200 parts by weight, 5 to 50 parts by weight, for example, 10 to 30 parts by weight, based on 100 parts by weight of the second binder. Within the above range, it is easy to suppress ignition and explosion of the battery against external impacts, provide a low thermal shrinkage rate, and prevent the coating layer from detaching from the separator.

[0149] Chain extender

[0150] The above chain extender can increase the toughness of the crosslink by reacting with the citric acid as a difunctional or multifunctional alcohol or a difunctional amine to connect the chains to each other.

[0151] In one embodiment, the chain extender is a difunctional or higher alcohol and may include, but is not limited to, polyethylene glycol, polypropylene glycol, etc.

[0152] The above chain extender may have a weight-average molecular weight of 500 to 2000 g / mol, for example, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000 g / mol, or 1000 to 2000 g / mol. Within the above range, flexibility may be achieved, and the effect of improving the toughness of the crosslinked material may be high.

[0153] filler

[0154] The above filler may have a size D100 of 0.7 μm or less. Within this range, the separator may easily reach the above dry shrinkage rate and shrinkage rate in the electrolyte. For example, the above filler may have a size D100 of 0.55 μm or less, for example, 0.3 to 0.5 μm.

[0155] According to one embodiment, the filler may have a size D50 of 0.4 μm or less, for example, 0.3 μm or less, for example, 0.1 μm to 0.3 μm. Within this range, there may be an effect of improving the heat resistance of the separator.

[0156] The above filler may be, for example, an inorganic filler, an organic filler, an organic-inorganic composite filler, or a combination thereof. The above inorganic filler may be a ceramic material capable of improving heat resistance. The above inorganic filler may include, for example, a metal oxide, a metal metal oxide, a metal fluoride, a metal hydroxide, or a combination thereof. The above inorganic filler may include, for example, Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, or a combination thereof, but is not limited thereto. The above organic filler may include an acrylic compound, an imide compound, an amide compound, or a combination thereof, but is not limited thereto. The above organic filler may have a core-shell structure, but is not limited thereto.

[0157] The above filler may be spherical, plate-shaped, cubic, or amorphous. Preferably, the filler may be plate-shaped or cubic.

[0158] The above filler may be included in the mixture of the first binder and the second binder in a mass ratio of 1:10 to 1:50, for example, 1:20 to 1:30. Within this range, the adhesion of the separator may increase.

[0159] The above filler may be included in an amount of 50% to 99% by weight, for example 70% to 99% by weight, for example 75% to 99% by weight, for example 80% to 99% by weight, for example 85% to 99% by weight, for example 90% to 99% by weight, for example 95% to 99% by weight, of the total amount of the coating layer. When the above filler is included within the above range, excellent heat resistance, durability, oxidation resistance, and stability may be exhibited.

[0160] The coating layer may have a thickness of 0.01㎛ to 20㎛, and within the above range, may have a thickness of 1㎛ to 10㎛, or 1㎛ to 5㎛, or 1㎛ to 3㎛.

[0161] The ratio of the thickness of the coating layer to the thickness of the porous substrate may be 0.05 to 0.5, for example, 0.05 to 0.4, or 0.05 to 0.3, or 0.1 to 0.2. Within the above range, the separator may exhibit excellent air permeability, heat resistance, and adhesion. Here, 'thickness of the coating layer' refers to the thickness of one coating layer when the coating layer is formed only on one side of the porous substrate, and refers to the total thickness of two coating layers when the coating layer is formed on both sides of the porous substrate.

[0162] The above coating layer can be formed by coating the porous substrate with the composition for the coating layer to a predetermined thickness and then aging it at 85 to 120°C for 15 hours or more, for example, 15 to 24 hours.

[0163] porous substrate

[0164] The porous substrate may be a substrate having a number of pores and typically used in electrochemical devices. The porous substrate may be, but is not limited to, any one polymer selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyetheretherketone, polyaryletherketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a polymer membrane formed from two or more copolymers or mixtures of these.

[0165] The porous substrate may be a polyolefin-based substrate including, for example, a polyolefin, and the polyolefin-based substrate may contribute to improving the safety of the battery by having an excellent shutdown function. The polyolefin-based substrate may be selected from, for example, a polyethylene single membrane, a polypropylene single membrane, a polyethylene / polypropylene double membrane, a polypropylene / polyethylene / polypropylene triple membrane, and a polyethylene / polypropylene / polyethylene triple membrane. In addition, the polyolefin-based resin may include a non-olefin resin in addition to the olefin resin, or may include a copolymer of an olefin and a non-olefin monomer.

[0166] The porous substrate may have a thickness of 1 μm to 40 μm, and, for example, may have a thickness of 1 μm to 30 μm, 1 μm to 20 μm, or 5 μm to 15 μm.

[0167] A separator for a lithium secondary battery according to one embodiment may exhibit excellent air permeability and may have an air permeability value of, for example, less than 200 sec / 100cc, for example, 190 sec / 100cc or less, or 180 sec / 100cc or less. That is, it may have an air permeability value of less than 40 sec / 100cc·1㎛ per unit thickness, for example, 30 sec / 100cc·1㎛ or less, or 25 sec / 100cc·1㎛ or less. Here, air permeability refers to the time (in seconds) required for 100cc of air to pass through a unit thickness of the separator. The air permeability per unit thickness can be obtained by measuring the air permeability over the entire thickness of the separator and then dividing by the thickness. The air permeability can be measured by using an air permeability measuring device (Asahi Seiko, EG01-55-1MR) to measure the time (in seconds) required for 100cc of air to pass through.

[0168] FIG. 1 is a cross-sectional view showing a separator for a lithium secondary battery according to one embodiment.

[0169] Referring to FIG. 1, a separator for a lithium secondary battery comprises a porous substrate 1; and a coating layer 2 located on both sides of the porous substrate 1. The coating layer 2 comprises a filler 3; and a crosslinked material 4 comprising a first binder, a second binder, a crosslinking agent, and a chain extender.

[0170] lithium secondary battery

[0171] Another embodiment provides a lithium secondary battery comprising a separator for a lithium secondary battery according to one embodiment; a positive electrode; and a negative electrode. The separator for the lithium secondary battery is described above. The separator for the lithium secondary battery may be positioned between the positive electrode and the negative electrode.

[0172] anode

[0173] A positive electrode for a lithium secondary battery may include a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer may include a positive electrode active material and may further include a binder and / or a conductive material. As an example, the positive electrode may further include an additive capable of acting as a sacrificial electrode.

[0174] positive electrode active material

[0175] As a positive electrode active material, a compound capable of reversible intercalation and deintercalation of lithium (a lithated intercalation compound) may be used. Specifically, one or more composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0176] The above composite oxide may be a lithium transition metal composite oxide, and specific examples include a lithium nickel-based oxide, a lithium cobalt-based oxide, a lithium manganese-based oxide, a lithium iron phosphate-based compound, a cobalt-free nickel-manganese-based oxide, or a combination thereof.

[0177] As an example, a compound represented by any one of the following chemical formulas may be used. Li a A 1-b X b O 2-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a Mn 2-b X b O 4-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a Ni 1-b-c Co b X c O 2-α D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α<2); Li a Ni 1-b-c Mn b X c O 2-α Dα (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α<2); Li a Ni b Co c L 1 d G e O2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0≤e≤0.1); Li a NiG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a CoG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-b G b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn2G b O4(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-g G g PO4(0.90≤a≤1.8, 0≤g≤0.5); Li (3-f) Fe2(PO4)3(0≤f≤2); Li a FePO4(0.90≤a≤1.8).

[0178] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; L 1 is Mn, Al, or a combination thereof.

[0179] For example, the above-mentioned positive electrode active material may be a high-nickel positive electrode active material in which the nickel content relative to 100 mol% of the metal excluding lithium in the lithium transition metal composite oxide is 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more and 99 mol% or less. The high-nickel positive electrode active material can achieve high capacity and can be applied to high-capacity, high-density lithium secondary batteries.

[0180] The content of the above positive active material is 90% to 99.5% by weight of 100% by weight of the positive active material layer, and the content of the binder and the conductive material may each be 0.5% to 5% by weight with respect to 100% by weight of the positive active material layer.

[0181] The above binder serves to adhere the positive active material particles well to each other and also to adhere the positive active material well to the current collector. Representative examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc.

[0182] The above conductive material is used to impart conductivity to the electrode, and any electronically conductive material that does not cause chemical changes can be used in the battery being constructed. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, carbon nanotube; metal-based materials in the form of metal powder or metal fibers containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0183] Al may be used as the current collector mentioned above, but is not limited thereto.

[0184] cathode

[0185] A negative electrode for a lithium secondary battery comprises a current collector and a negative electrode active material layer located on the current collector. The negative electrode active material layer comprises a negative electrode active material and may further comprise a binder and / or a conductive material.

[0186] For example, the negative electrode active material layer may comprise 90% to 99% by weight of negative electrode active material, 0.5% to 5% by weight of binder, and 0% to 5% by weight of conductive material.

[0187] cathode active material

[0188] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, or a transition metal oxide.

[0189] A material capable of reversibly intercalating / deintercalating the above lithium ions may be a carbon-based negative electrode active material, such as crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0190] As the above lithium metal alloy, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn may be used.

[0191] As a material capable of doping and undoping the above lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material may be used. The Si-based negative electrode active material may be silicon, a silicon-carbon composite, SiOx (0 < x < 2), a Si-Q alloy (wherein Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination thereof. The Sn-based negative electrode active material may be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0192] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, it may include a secondary particle (core) assembled from silicon primary particles and an amorphous carbon coating layer (shell) located on the surface of the secondary particle. The amorphous carbon may also be located between the silicon primary particles, so that, for example, the silicon primary particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0193] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core comprising crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of the core.

[0194] The above Si-based or Sn-based negative electrode active material can be used in combination with a carbon-based negative electrode active material.

[0195] The above binder serves to effectively bond the negative electrode active material particles to each other and also to effectively bond the negative electrode active material to the current collector. As the binder, a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof may be used.

[0196] Examples of the above-mentioned non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

[0197] The above-mentioned water-based binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylenepropylenediene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0198] When a water-based binder is used as the above-mentioned cathode binder, a cellulose-based compound capable of imparting viscosity may be further included. As this cellulose-based compound, one or more types such as carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof may be mixed and used. Na, K, or Li may be used as the alkali metal.

[0199] The above dry binder is a polymer material capable of fiberization, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0200] The above conductive material is used to impart conductivity to the electrode, and any electronically conductive material that does not cause chemical changes can be used in the battery being constructed. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fiber, carbon nanofiber, carbon nanotube; metal-based materials in the form of metal powder or metal fibers including copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0201] As the above-mentioned cathode current collector, a material selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof may be used.

[0202] Lithium secondary batteries may contain additional electrolyte.

[0203] electrolyte

[0204] The electrolyte for a lithium secondary battery may include a non-aqueous organic solvent and a lithium salt.

[0205] The above-mentioned non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0206] The above-mentioned non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, a non-protic solvent, or a combination thereof.

[0207] The above carbonate-based solvents may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc.

[0208] Ester-based solvents such as methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methylpropionate, ethylpropionate, decanolide, mevalonolactone, valerolactone, and caprolactone may be used.

[0209] As ether-based solvents, dibutyl ether, tetraglame, diglame, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, etc. may be used. Additionally, as ketone-based solvents, cyclohexanone, etc. may be used. As alcohol-based solvents, ethyl alcohol, isopropyl alcohol, etc. may be used, and as aprotic solvents, nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, an aromatic ring, or an ether group); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane, 1,4-dioxolane; sulfolanes, etc. may be used.

[0210] The above-mentioned non-aqueous organic solvent can be used alone or in a mixture of two or more types.

[0211] In addition, when using a carbonate-based solvent, a mixture of cyclic carbonates and chain carbonates can be used, and the cyclic carbonates and chain carbonates can be mixed in a volume ratio of 1:1 to 1:9.

[0212] The above lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions within the battery, enabling the basic operation of a lithium secondary battery and facilitating the movement of lithium ions between the anode and cathode. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1It may include one or more selected from SO2)(x and y are integers from 1 to 20), lithium trifluoromethane sulfonate, lithium tetrafluoroethane sulfonate, lithium difluorobis(oxalate)phosphate (LiDFOB), and lithium bis(oxalate)borate (LiBOB).

[0213] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch, coin, etc., depending on their shape. FIGS. 2 to 5 are schematic diagrams illustrating a lithium secondary battery according to one embodiment. FIG. 2 can be described as a cylindrical battery, FIG. 3 as a prismatic battery, and FIGS. 4 and 5 as a pouch battery. Referring to FIGS. 2 to 5, the lithium secondary battery 100 may include an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is housed. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50 as in FIG. 2. In addition, in FIG. 3, the lithium secondary battery 100 may include a positive lead tab 11 and a positive terminal 12, a negative lead tab 21 and a negative terminal 22. As shown in FIG. 4 and FIG. 5, the lithium secondary battery 100 may include electrode tabs 70, namely a positive tab 71 and a negative tab 72, which serve as electrical paths for inducing current formed in the electrode assembly 40 to the outside.

[0214] A lithium secondary battery according to one embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electric devices, etc., but the present invention is not limited thereto.

[0215] Examples and comparative examples of the present invention are described below. However, the following examples are merely one example of the present invention, and the present invention is not limited to the following examples.

[0216] Preparation Example 1

[0217] Distilled water (6361 g), acrylamide (9.5 mol), potassium persulfate (0.01 mol), 2-acrylamido-2-methylpropanesulfonic acid (0.5 mol), and 5N aqueous lithium hydroxide solution (1.05 equivalents relative to the total amount of 2-acrylamido-2-methylpropanesulfonic acid) were added to a 10 L 4-neck flask equipped with a stirrer, thermometer, and condenser, and then the internal pressure was reduced to 10 mmHg using a diaphragm pump and the internal pressure was returned to atmospheric pressure using nitrogen, and this operation was repeated 3 times.

[0218] The reaction is carried out for 12 hours while controlling the temperature of the reaction solution to stabilize at 65°C to 70°C. After cooling to room temperature, the pH of the reaction solution is adjusted to 7 to 8 using a 25% aqueous ammonia solution.

[0219] Poly(acrylamide-co-2-acrylamido-2-methylpropanesulfonic acid)lithium salt was prepared using this method. The molar ratio of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid lithium salt was 95:5. About 10 mL of the reaction solution (reaction product) was taken and the non-volatile component was measured, resulting in 9.5 wt% (theoretical value: 10 wt%).

[0220] Preparation Example 2

[0221] Distilled water (968 g), acrylic acid (0.39 mol), ammonium persulfate (2.85 mmol), 2-acrylamido-2-methylpropanesulfonic acid (0.02 mol), and a 20% aqueous solution of lithium hydroxide (0.8 equivalents relative to the total amount of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid) were added to a 3 L four-necked flask equipped with a stirrer, thermometer, and condenser. After reducing the internal pressure to 10 mmHg using a diaphragm pump and returning the internal pressure to atmospheric pressure with nitrogen, the operation was repeated three times, and then acrylonitrile (0.59 mol) was added.

[0222] The reaction is carried out for 18 hours while controlling the temperature of the reaction solution to stabilize at 65°C to 70°C, and after adding ammonium persulfate (0.95 mmol) a second time, the temperature is raised to 80°C and the reaction is carried out again for 4 hours. After cooling to room temperature, the pH of the reaction solution is adjusted to 7 to 8 using a 25% aqueous ammonia solution.

[0223] Poly(acrylic acid-co-lithium acrylate-co-acrylonitrile-co-2-acrylamido-2-methylpropanesulfonate lithium salt) was prepared as an acrylic binder using this method. The molar ratio of acrylic acid + lithium acrylate, acrylonitrile, and 2-acrylamido-2-methylpropanesulfonate lithium salt was 39:59:2. About 10 mL of the reaction solution (reaction product) was taken and the non-volatile component was measured, resulting in 9.0 wt% (theoretical value: 10 wt%).

[0224] Example 1

[0225] An acrylic binder (10 wt%) prepared in Preparation Example 1 and boehmite (size D100: 500 nm, size D50: 200 nm, plate-like) as a filler were mixed, added to water as a solvent, and then milled and dispersed using a bead mill at 25°C for 30 minutes to prepare a dispersion.

[0226] A sodium salt of carboxymethylcellulose (CMC-Na, weight-average molecular weight: 200,000 g / mol), citric acid, and polyethylene glycol (weight-average molecular weight: 500 g / mol) were added to the above dispersion, and water was added so that the total solid content was 20% by weight to prepare a composition for a coating layer.

[0227] The above coating layer composition comprises 50 parts by weight of an acrylic binder; 50 parts by weight of a sodium salt of carboxymethylcellulose; 10 parts by weight of citric acid and 5 parts by weight of polyethylene glycol, based on a total of 100 parts by weight of the acrylic binder and carboxymethylcellulose. The weight ratio of the total of the acrylic binder and the sodium salt of carboxymethylcellulose to the filler in the above coating layer composition is 1:20.

[0228] As a porous substrate, a polyethylene-based film (thickness: 8㎛, SK, air permeability: 120 sec / 100cc, puncture strength: 480kgf) was coated with the above coating layer composition to a thickness of 2㎛ on each side using a die-coating method, and then dried and aged in an oven at 100℃ for 16 hours to produce a separator.

[0229] Examples 2 to 8

[0230] A separation membrane was prepared in the same manner as in Example 1, except that the content of the acrylic binder, sodium salt of carboxymethylcellulose, citric acid, and polyethylene glycol, and / or the weight-average molecular weight of polyethylene glycol were changed as shown in Table 1 below.

[0231] Example 9

[0232] A separator was prepared in the same manner as in Example 1, except that the binder of Preparation Example 2 was used instead of the binder of Preparation Example 1 in Example 1.

[0233] Comparative Example 1

[0234] A separation membrane was prepared in the same manner as in Example 1, except that the sodium salt of carboxymethylcellulose was not included in Example 1.

[0235] Comparative Example 2

[0236] A separator was prepared in the same manner as in Example 1, except that polyethylene glycol was not included in Example 1.

[0237] Comparative Example 3

[0238] A separation membrane was prepared in the same manner as in Example 1, except that citric acid was not included in Example 1.

[0239] Comparative Example 4

[0240] A separation membrane was prepared in the same manner as in Example 1, except that 80 parts by weight of citric acid and 40 parts by weight of polyethylene glycol were included for every 100 parts by weight of the total of the acrylic binder and the sodium salt of carboxymethylcellulose in Example 1.

[0241] Comparative Example 5 to Comparative Example 6

[0242] A separator was prepared in the same manner as in Example 1, except that the weight ratio of citric acid to polyethylene glycol in Example 1 was changed as shown in Table 1 below.

[0243] Comparative Example 7

[0244] A separation membrane was prepared in the same manner as in Example 1, except that adipic acid was used instead of citric acid in Example 1.

[0245] It was evaluated using examples and comparative examples.

[0246] Tensile strength (unit: MPa) and toughness (unit: MPa) of crosslinks

[0247] In the examples and comparative examples, an aqueous solution was prepared in which the total solid content was 10% by weight by excluding the filler from the composition for the coating layer and additionally including water.

[0248] 2 to 3 ml of the above-prepared aqueous solution was poured into a mold with width x length (1 cm x 10 cm), dried in an oven at 50°C for 3 hours, and then aged in an oven at 85°C for 16 hours to produce a film with a thickness of 40 μm. A specimen with width x length (1 cm x 2 m) was taken from the prepared film, and the tensile strength and toughness of the specimen were measured using a universal testing machine (UTM) at 25°C. Toughness was determined as the area under curve (AUC) of the stress-strain curve derived by the UTM.

[0249] Thermal shrinkage rate in electrolyte (Unit: %)

[0250] Samples were prepared by cutting the separators of the examples and comparative examples into pieces measuring 5 cm × 5 cm. An anode slurry was prepared by mixing 97 wt% lithium cobalt nickel aluminum-based oxide as an anode active material, 1.5 wt% carbon nanotubes as a conductive material, and 1.5 wt% polyvinylidene fluoride as a binder, and adding N-methyl-2-pyrrolidone. An anode was manufactured by coating the prepared anode slurry onto aluminum foil, drying it, and rolling it. An anode was manufactured by mixing 97.4 wt% graphite, 1.0 wt% carboxymethylcellulose, 1.5 wt% styrene-butadiene-based rubber as a cathode active material, and 0.1 wt% carbon nanotubes as a conductive material, and adding distilled water. An anode was manufactured by coating the prepared anode slurry onto copper foil, drying it, and rolling it.

[0251] One of the above samples was placed between the anode and the cathode to create three sets of anode-sample-cathode stacks, which were then placed in a pouch. 2.5 g of electrolyte (ethylene carbonate, ethylmethyl carbonate, and dimethyl carbonate (30:50:20 volume ratio) dissolved in 1.5 M LiPF6) was injected to completely immerse the stacks in the electrolyte, and the pouch was sealed. The pouches were then left at 25°C for 12 hours and placed in an oven at 150°C for 1 hour. Afterward, the samples were removed from the pouches, and the thermal shrinkage rates for MD and TD were calculated. The thermal shrinkage rates are calculated according to Equation 1 below.

[0252] [Mathematical Formula 1]

[0253] Thermal shrinkage rate = (L0 - L1) / L0 x 100.

[0254] (L0 is the initial length of the membrane, L1 is the length of the membrane after standing at 150°C for 1 hour).

[0255] Cell properties:

[0256] Cylindrical cells containing the separators of the examples and comparative examples and having an SOC of 50% were placed on an evaluation stand (SS41 steel, flat, thickness 70 mm), and a cylindrical weight of 9.1 ± 0.1 kg with a diameter of 15.8 ± 0.1 mm was dropped from a height of 630 ± 25 mm onto the cells to check for explosion or ignition using the international standard UN crash evaluation method. The number of cases where the crash evaluation was passed (cases where there was no explosion, rupture, or ignition of the cells) out of the total number of evaluated samples (10) is shown. If the number of cases that failed was 2 or fewer, it was evaluated as OK, and if the number of cases that failed was 3 or more, it was evaluated as NG.

[0257] Detachment of coating layer from separator:

[0258] The separation membrane was cut into 1 cm x 1 cm pieces and immersed in 25°C water for 24 hours, and the peeling of the coating layer was evaluated by visually checking whether it occurred in the water.

[0259] First Binder Second Binder Weight Ratio 1 Crosslinking Agent, Content Chain Extender Content (Mw) Weight Ratio 2 Weight Ratio 3 Example 1 Preparation Example 1 CMC-Na 50:50 Citric Acid, 105(500) 1:0.51:20 Example 2 Preparation Example 1 CMC-Na 60:40 Citric Acid, 105(500) 1:0.51:20 Example 3 Preparation Example 1 CMC-Na 50:50 Citric Acid, 105(1000) 1:0.51:20 Example 4 Preparation Example 1 CMC-Na 50:50 Citric Acid, 102(1000) 1:0.21:20 Example 5 Preparation Example 1 CMC-Na 50:50 Citric Acid, 205(1000)1:0.251:20 Example 6 Preparation Example 1 CMC-Na 50:50 citric acid, 102(2000)1:0.21:20 Example 7 Preparation Example 1 CMC-Na 50:50 citric acid, 105(2000)1:0.51:20 Example 8 Preparation Example 1 CMC-Na 50:50 citric acid, 1020(2000)1:21:20 Example 9 Preparation Example 2 CMC-Na 50:50 citric acid, 105(1000)1:0.51:20 Comparative Example 1 Preparation Example 1-100:0 citric acid, 105(1000)1:0.51:20 Comparative Example 2 Preparation Example 1CMC-Na50:50 Citric Acid, 100-1:20 Comparative Example 3 Preparation Example 1CMC-Na50:50-5(1000)-1:20 Comparative Example 4 Preparation Example 1CMC-Na50:50 Citric Acid, 8040(2000)1:0.51:20 Comparative Example 5 Preparation Example 1CMC-Na50:50 Citric Acid, 101(1000)1:0.11:20 Comparative Example 6 Preparation Example 1CMC-Na50:50 Citric Acid, 1025(1000)1:2.51:20 Comparative Example 7 Preparation Example 1CMC-Na50:50 Adipic Acid, 105(1000)1:0.51:20

[0260] Crosslinking Material Thermal Shrinkage Rate Cell Characteristics Coating Layer Detachment Tensile Strength Toughness MDTD Example 1 194 21 18 12.5 3.5 OK None Example 2 24 30 12 0 23 3.5 OK None Example 3 17 48 13 50 3.5 3.5 OK None Example 4 18 61 69 4 3.5 OK None Example 5 4 10 25 0 3 2.5 OK None Example 6 16 36 5 18 3.5 4 OK None Example 7 14 12 8 19 3.5 3.5 OK None Example 8 9 10 85 36 7 OK None Example 9 10 0 69 5 4 5 4.5 OK None Comparative Example 1 13 87 17 27.5 NG None Comparative Example 2 16 34 5 3.5 4.0 OK With Comparative Example 3 15 9 51 43 38 NG With Comparative Example 412423011013NG No Comparison Example 51987666OK With Comparison Example 613024981419NG With Comparison Example 71211523548NG With

[0261]

[0262] In Table 1,

[0263] Weight ratio 1: Weight ratio of the first binder to the second binder

[0264] Weight ratio 2: Weight ratio of citric acid to chain extender

[0265] Weight ratio 3: Total of 1st Binder + 2nd Binder : Weight ratio of Filler

[0266]

[0267] As shown in Table 2 above, the separator of the example has strong heat resistance and simultaneously secures rigidity and elongation, which can be advantageous for cell characteristics and coating layer detachment. The separator of the example has a low thermal shrinkage rate, so it has excellent heat resistance and simultaneously secures both tensile strength and toughness, thereby improving cell characteristics against external impacts and preventing the coating layer from detaching from the porous substrate, which can increase the reliability of the battery.

[0268] However, as shown in Table 2 above, the separator of the comparative example that does not satisfy the composition of the coating layer of the present case had poor cell characteristics or poor heat resistance and had poor reliability due to peeling of the coating layer.

[0269]

[0270] Although preferred embodiments of the present invention have been described above, the present invention is not limited thereto and can be implemented with various modifications within the scope of the claims, the detailed description of the invention, and the attached drawings, and it is obvious that such modifications also fall within the scope of the present invention.

Claims

1. A porous substrate and a coating layer located on at least one surface of the porous substrate, and The above coating layer comprises a first binder, a second binder, a crosslinking agent and a chain extender crosslinked; and a filler, The first binder is a water-based binder, the second binder is a carboxyalkylcellulose-based compound or a salt thereof, and the crosslinking agent comprises citric acid. The citric acid is included in an amount of 5 to 50 parts by weight per 100 parts by weight of the total of the first binder and the second binder, and A separator for a secondary battery, wherein the crosslinking agent among the above crosslinking materials: the chain extender is crosslinked in a weight ratio of 1:0.2 to 1:

2.

2. A separator for a secondary battery according to claim 1, wherein the chain extender is a difunctional or higher alcohol or a difunctional or higher amine.

3. A separator for a secondary battery according to claim 1, wherein the chain extender comprises one or more of polyethylene glycol and polypropylene glycol.

4. In claim 1, the chain extender is a separator for a secondary battery having a weight-average molecular weight of 500 to 2000 g / mol.

5. A separator for a secondary battery according to claim 1, wherein the chain extender is included in an amount of 1 to 20 parts by weight per 100 parts by weight of the total of the first binder and the second binder.

6. A separator for a secondary battery according to claim 1, wherein the first binder : the second binder is included in a weight ratio of 80 : 20 to 25 : 75 among 100 parts by weight of the total sum of the first binder and the second binder.

7. A separator for a secondary battery according to claim 1, wherein the second binder is carboxymethylcellulose or an alkali metal salt thereof.

8. A separator for a secondary battery according to claim 1, wherein the carboxyalkylcellulose-based compound or salt thereof has a weight-average molecular weight of 500 to 2000 g / mol.

9. A separator for a secondary battery according to claim 1, wherein the citric acid is included in an amount of 5 to 200 parts by weight per 100 parts by weight of the first binder, and the citric acid is included in an amount of 5 to 200 parts by weight per 100 parts by weight of the second binder.

10. In claim 1, the filler is a separator for a secondary battery having a size D100 of 0.7㎛ or less.

11. A separator for a secondary battery according to claim 1, wherein the filler is boehmite and is plate-shaped.

12. A separator for a secondary battery according to claim 1, wherein the first binder comprises a crosslinking unit with the citric acid, and the crosslinking unit with the citric acid comprises one or more of the following: a unit derived from (meth)acrylic acid or a salt thereof, a unit derived from (meth)acrylamide, a unit derived from hydroxyalkyl (meth)acrylate, a unit derived from (meth)acrylonitrile, a unit derived from (meth)acrylamido-2-methylpropanesulfonic acid and a salt thereof, and a unit derived from ethyleneimine.

13. A separator for a secondary battery according to claim 12, wherein the crosslinking unit with citric acid is included in the (meth)acrylic binder in an amount of 10 to 100 mol%.

14. A separator for a secondary battery according to claim 1, wherein the (meth)acrylic binder comprises a sulfonate group-containing structural unit.

15. A separator for a secondary battery according to claim 14, wherein the (meth)acrylic binder further comprises one or more of the following: a structural unit derived from (meth)acrylate or (meth)acrylic acid, a cyano group-containing structural unit, and a structural unit derived from (meth)acrylamide.

16. A mixture of the first binder and the second binder according to claim 1: the filler is included in a mass ratio of 1:10 to 1:50, for a separator for a secondary battery.

17. A secondary battery comprising: a positive electrode; a negative electrode; and a separator for a secondary battery according to any one of claims 1 to 16 located between the positive electrode and the negative electrode.

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