Porous polymer substrate, method of manufacturing the same, and lithium secondary battery comprising the same

Abstract

The present invention provides a porous polymer substrate in which a first polymer comprises an olefin-derived first repeating unit and a second polymer comprises: a hard segment comprising an olefin-derived second repeating unit; and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, wherein the weight average molecular weight of the first polymer is at least three times greater than the weight average molecular weight of the second polymer.

Description

Porous polymer substrate, method for manufacturing the same, and lithium secondary battery including the same

[0001] The present invention relates to a porous polymer substrate, a method for producing the same, and a lithium secondary battery including the same.

[0002] Cross-citation with related applications

[0003] This application claims the benefit of priority to Korean Patent Application No. 10-2024-0019761, filed February 8, 2024, and Korean Patent Application No. 10-2024-0144265, filed October 21, 2024, the entire contents of which are incorporated herein by reference.

[0004] Interest in energy storage technology has been growing steadily. As applications expand to include energy storage for mobile phones, camcorders, laptops, and even electric vehicles, research and development efforts on batteries are becoming increasingly concrete. Electrochemical devices are receiving the most attention in this regard, and with the recent trend toward miniaturization and lightweighting of electronic devices, the development of secondary batteries, which are compact, lightweight, and capable of high-capacity recharge and discharge, is becoming a focus of attention.

[0005] Among the secondary batteries currently in use, lithium secondary batteries developed in the early 1990s are attracting attention due to their advantages of higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use aqueous electrolytes.

[0006] Meanwhile, the lithium secondary battery is used after an activation process is performed. The activation process is performed by applying a current up to a predetermined voltage to an electrode assembly impregnated with an electrolyte. At this time, the separator of the lithium secondary battery includes a material having a hydrophobic property, such as polyolefin, while the electrolyte is a material having a hydrophilic property. Therefore, there is a problem in that the separator is not quickly impregnated with the electrolyte during the activation process. As described above, if the separator is not quickly or sufficiently impregnated with the electrolyte, the electrode active material may not be sufficiently impregnated with the electrolyte, which may hinder the smooth movement of lithium ions.

[0007] The present invention has been invented to solve the above-described problems, and the present invention aims to provide a porous polymer substrate having improved electrolyte impregnation properties and excellent ionic conductivity during an activation process by including a repeating unit derived from an oxygen-containing monomer in the porous polymer substrate.

[0008] In addition, the present invention seeks to provide a method for manufacturing the porous polymer substrate by a wet method.

[0009] In addition, the present invention seeks to provide a method for manufacturing the porous polymer substrate by a dry method.

[0010] To achieve this purpose, according to one aspect of the present invention, a porous polymer substrate, a method for manufacturing the porous polymer substrate, and a lithium secondary battery including the same are provided.

[0011] According to a first embodiment, a porous polymer substrate is provided, wherein the first polymer comprises a first repeating unit derived from an olefin, the second polymer comprises a hard segment comprising a second repeating unit derived from an olefin, and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and the weight average molecular weight of the first polymer is at least three times that of the second polymer.

[0012] According to a second embodiment, in the first embodiment, the content of the second polymer may have a gradient that increases from the center of the porous polymer substrate to the surface.

[0013] According to a third embodiment, in any one of the first to second embodiments, the content of the second polymer on the surface of the porous polymer substrate may be 1.5 times or more than the content of the second polymer at the center of the porous polymer substrate.

[0014] According to a fourth embodiment, in any one of the first to third embodiments, the weight average molecular weight of the first polymer may be 150,000 to 2,000,000, and the weight average molecular weight of the second polymer may be 10,000 to 100,000.

[0015] According to a fifth embodiment, in any one of the first to fourth embodiments, the content of the second polymer may be 1 wt% to 15 wt% based on 100 wt% of the porous polymer substrate.

[0016] According to a sixth embodiment, in any one of the first to fifth embodiments, the weight ratio of the hard portion and the soft portion of the second polymer may be 1:0.1 to 5.

[0017] According to a seventh embodiment, in any one of the first to sixth embodiments, the first repeating unit may be derived from a monomer of ethylene, propylene, butylene, pentene, isoprene, or two or more thereof.

[0018] According to an eighth embodiment, in any one of the first to seventh embodiments, the second repeating unit may be derived from a monomer of ethylene, propylene, butylene, pentene, isoprene, or two or more thereof.

[0019] According to the ninth embodiment, in any one of the first to eighth embodiments, the third repeating unit may be derived from an epoxy group-containing monomer, a hydroxy group-containing monomer, a carboxyl group-containing monomer, or two or more monomers thereof.

[0020] According to a tenth embodiment, a method for manufacturing a porous polymer substrate is provided, comprising the steps of: kneading and extruding a mixture comprising a first polymer, a second polymer, and a plasticizer to obtain an extrudate; cooling the extrudate on a casting roll to obtain a sheet; and biaxially stretching the obtained sheet in a machine direction (MD) and a transverse machine direction (TD) to obtain a stretched sheet; and extracting the plasticizer from the stretched sheet with an organic solvent; wherein the first polymer comprises a first repeating unit derived from an olefin, the second polymer comprises a hard segment comprising a second repeating unit derived from an olefin, and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and wherein a weight average molecular weight of the first polymer is at least three times that of the second polymer.

[0021] According to the eleventh embodiment, in the tenth embodiment, the mixture may include 1 to 15 parts by weight of the second polymer and 200 to 300 parts by weight of the plasticizer based on 100 parts by weight of the first polymer.

[0022] According to the 12th embodiment, in any one of the 10th to 11th embodiments, the step of kneading the mixture may be a twin screw extruder having a length (L) / diameter (D) of 52 or more and 80 or less.

[0023] According to the 13th embodiment, in any one of the 10th to 12th embodiments, the step of obtaining the extrudate may use a die extruder.

[0024] According to a 14th embodiment, a method for producing a porous polymer substrate is provided, comprising: preparing a mixture comprising a first polymer and a second polymer; melt-extruding the mixture to prepare an unstretched sheet; and stretching the unstretched sheet to obtain an stretched sheet; wherein the first polymer comprises a first repeating unit derived from an olefin, the second polymer comprises a hard segment comprising a second repeating unit derived from an olefin, and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and wherein the weight average molecular weight of the first polymer is at least three times that of the second polymer.

[0025] According to the 15th embodiment, in the 14th embodiment, the crystallinity of the unstretched sheet may be 50% to 70%.

[0026] According to the 16th embodiment, in any one of the 14th to 15th embodiments, the step of obtaining the stretched sheet may include a step of stretching in the MD direction and the TD direction.

[0027] According to a 17th embodiment, a lithium secondary battery is provided, comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the separator comprises a porous polymer substrate according to any one of the first to ninth embodiments.

[0028] A porous polymer substrate according to one embodiment of the present invention has excellent electrolyte impregnation properties, thus having excellent ionic conductivity and shortening the activation process time.

[0029] According to one embodiment of the present invention, a porous polymer substrate has a higher content of a second polymer on the surface than on the center, so that the porous polymer substrate can have excellent mechanical properties and even better hydrophilicity.

[0030] The effects of the present invention are not limited to the effects described above, and should be understood to include all effects that can be inferred from the detailed description of the present invention or the composition of the invention described in the claims.

[0031] The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of ​​the present invention together with the contents of the invention described above, and therefore the present invention should not be interpreted as being limited to matters described in such drawings.

[0032] Figure 1 is a photograph comparing the impregnation properties of the porous substrates of Example 1 and Comparative Example 1.

[0033] Figure 2 is a photograph comparing the impregnation properties of the porous polymer substrates of Example 2-1 and Comparative Example 2-1.

[0034] Figure 3a is a photograph of the surface of the porous polymer substrate of Example 2-1 measured using SEM.

[0035] Figure 3b is a photograph of the surface of the porous polymer substrate of Comparative Example 2-1 measured using SEM.

[0036] Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0037] The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0038] Justice

[0039] In this specification, when it is said that a part “includes” a component, this does not mean that other components are excluded, but rather that other components may be included, unless otherwise specifically stated.

[0040] Certain terms used in this specification are for convenience and are not limiting. Terms such as "upper," "lower," "left," "right," "front," "back," "inner," and "outer" may be used to describe relative positions or directions between components, rather than absolute positions. These terms include, in addition to themselves, words containing them, derivatives thereof, and words with similar meanings.

[0041] In the present specification, the "thickness" of each layer included in the electrode used may refer to a value measured by a known method for measuring thickness. The thickness measurement method is not limited thereto, but may be, for example, a value measured using a thickness gauge (Mitutoyo, VL-50S-B).

[0042]

[0043] <Porous polymer substrate>

[0044] The present invention provides a porous polymer substrate.

[0045] The first polymer of the porous polymer substrate of the present invention includes a first repeating unit derived from an olefin, the second polymer includes a hard segment including a second repeating unit derived from an olefin and a soft segment including a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and the weight average molecular weight of the first polymer is characterized in that it is three times or more greater than the weight average molecular weight of the second polymer.

[0046]

[0047] In one embodiment of the present invention, the first repeating unit may be derived from a monomer selected from ethylene, propylene, butylene, pentene, isoprene, or two or more thereof. Preferably, the first repeating unit may be derived from a propylene monomer.

[0048] In one embodiment of the present invention, when the first repeating unit is derived from a propylene monomer, the first polymer may be polypropylene.

[0049] In one embodiment of the present invention, when the first polymer is polypropylene, it may exhibit superior heat resistance and chemical resistance compared to polyethylene. On the other hand, when the first polymer is polypropylene, electrolyte impregnation properties may be somewhat disadvantageous. However, as described below, when the porous polymer substrate includes the second polymer, the electrolyte impregnation properties of the porous polymer substrate may be excellent.

[0050] In one embodiment of the present invention, the polypropylene may include isotactic polypropylene, atactic polypropylene, or a combination thereof.

[0051] In one embodiment of the present invention, 'atactic polypropylene' means polypropylene in which stereochemistry for adjacent chiral centers is randomly present, and in the present specification, the ratio of such randomly arranged atactic stereochemistry is expressed as 'atactic index'.

[0052] In one embodiment of the present invention, the term "isotactic polypropylene" is understood to mean polypropylene consisting essentially of 100% meso dia. Meanwhile, in the present specification, the term "isotactic index" is understood to mean the meso dia ratio of polypropylene.

[0053] In one embodiment of the present invention, the polypropylene may be ultra-low density polypropylene, low density polypropylene, linear low density polypropylene, metallocene polypropylene, polypropylene-based elastomer, or two or more thereof.

[0054]

[0055] In one embodiment of the present invention, the second polymer may be a copolymer comprising a second repeating unit and a third repeating unit, and the second polymer may be a random copolymer, an alternating copolymer, or a block copolymer comprising the second repeating unit and the third repeating unit. Preferably, the second polymer may be a block copolymer comprising a block formed of the second repeating unit and a block formed of the third repeating unit.

[0056] In the present specification, a block composed of the second repeating unit is referred to as a hard segment, and a block composed of the third repeating unit is referred to as a soft segment. The hard segment includes a second repeating unit derived from an olefin, and the second repeating unit derived from an olefin has excellent linearity and crystallinity, so that a block including the second repeating unit exhibits hardness. The soft segment includes a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and the third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer has relatively poor linearity and crystallinity due to the hydrophilic group or the oxygen atom, so that a block including the third repeating unit exhibits softness.

[0057] In one embodiment of the present invention, the second repeating unit may be derived from a monomer selected from ethylene, propylene, butylene, pentene, isoprene, or two or more thereof. Preferably, the second repeating unit may be derived from a propylene monomer.

[0058] In one embodiment of the present invention, the oxygen-containing monomer may include an oxygen atom as part of the main chain or in a side chain, preferably in the main chain. When the hydrophilic group is included in the side chain, the main chain may be an olefin compound.

[0059] In one embodiment of the present invention, the third repeating unit may be derived from an epoxy group-containing monomer, a hydroxy group-containing monomer, a carboxyl group-containing monomer, or two or more monomers thereof.

[0060] In one embodiment of the present invention, the epoxy group-containing monomer may be ethylene oxide, propylene oxide, 1,2-butylene oxide, 2,3-butylene oxide, or a combination thereof.

[0061] In one embodiment of the present invention, the hydroxyl group-containing monomer may be, for example, an alkene containing an alcohol group having 1 to 6 carbon atoms. For example, the hydroxyl group-containing monomer can be vinyl alcohol, allyl alcohol, 3-buten-1-ol, 3-buten-2-ol, 4-penten-1-ol, 4-penten-2-ol, 1-penten-3-ol, 5-hexen-1-ol, 5-hexen-2-ol, 5-hexen-1-ol, 1-hexen-3-ol, ethylene glycol, propylene glycol, glycerol, or a combination thereof.

[0062] In one embodiment of the present invention, the carboxyl group-containing monomer may be, for example, (meth)acrylate, and the amine group-containing monomer may be, for example, vinyl pyrrolidone.

[0063]

[0064] In one embodiment of the present invention, the second repeating unit may be the same type as the first repeating unit, and in this case, the second polymer may have excellent compatibility with the first polymer.

[0065] In one embodiment of the present invention, the second polymer may include a polyolefin-b-polyethylene oxide-based compound, for example, polyethylene-b-polyethylene oxide, polypropylene-b-polyethylene oxide, polybutylene-b-polyethylene oxide, polypentene-b-polyethylene oxide, polyisoprene-b-polyethylene oxide, polyethylene-b-polypropylene-b-polyethylene oxide, ethylene propylene alternating copolymer-b-polyethylene oxide, and the like.

[0066]

[0067] In one embodiment of the present invention, the content of the second polymer may have a gradient that increases from the center of the porous polymer substrate to the surface.

[0068] In one embodiment of the present invention, the weight average molecular weight of the first polymer may be at least 4 times or at least 5 times the weight average molecular weight of the second polymer. Meanwhile, in one embodiment of the present invention, the first weight average molecular weight may be at most 50 times, at most 40 times, or at most 35 times the weight average molecular weight of the second polymer.

[0069]

[0070] The manufacturing process of the porous polymer substrate of the present invention is manufactured by a wet method using a plasticizer or a dry method without using a plasticizer, as described below, and a mixture of a first polymer and a second polymer is extruded to form an unstretched sheet. In the above process, the second polymer migrates to the surface of the unstretched sheet, and when the weight average molecular weight of the first polymer satisfies the above-described range, the miscibility of the second polymer and the first polymer is excellent, and the proportion of the second polymer on the surface is high, so that the hydrophilicity of the surface of the manufactured porous polymer substrate can be maximized, and therefore, the impregnation property of the porous polymer substrate for an electrolyte can be excellent.

[0071]

[0072] In one embodiment of the present invention, the content of the second polymer on the surface of the porous polymer substrate may be 1.5 times or more, 1.6 times or more, 1.7 times or more, or 1.8 times or more than the content of the second polymer at the center of the porous polymer substrate. By satisfying the above-described range of the content of the second polymer on the surface of the porous polymer substrate, the pore structure within the porous polymer substrate can be uniformly developed, thereby further improving the mechanical strength.

[0073] In one embodiment of the present invention, the surface of the porous polymer substrate may refer to a portion from one surface to 10% of the upper thickness based on the thickness direction of the porous polymer substrate. Alternatively, the surface of the porous polymer substrate may refer to a portion from one surface to 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% of the upper thickness based on the thickness direction of the porous polymer substrate.

[0074] In one embodiment of the present invention, the porous polymer substrate may be in the form of a sheet having a rectangular shape, and the longer length of the separator may be referred to as the longitudinal direction, long-side direction, or machine direction (MD), and the shorter length of the separator may be referred to as the traverse direction (TD). In this case, the 'thickness direction' may refer to a direction perpendicular to the longitudinal direction and the traverse direction.

[0075]

[0076] In one embodiment of the present invention, the weight average molecular weight of the first polymer may be 150,000 to 2,000,000, and the weight average molecular weight of the second polymer may be 10,000 to 100,000.

[0077] In one embodiment of the present invention, the weight average molecular weight of the first polymer may be 150,000 to 2,000,000, 170,000 to 1,500,000, or 200,000 to 1,200,000. When the weight average molecular weight of the first polymer satisfies the above-described range, the mechanical strength of the porous polymer substrate may be excellent.

[0078] In one embodiment of the present invention, the weight average molecular weight of the second polymer may be 10,000 to 100,000, 20,000 to 80,000, or 25,000 to 60,000. When the weight average molecular weight of the second polymer satisfies the above-described range, the porous polymer substrate may have excellent mechanical strength and excellent hydrophilicity.

[0079] At this time, the weight average molecular weight can be measured through gel permeation chromatography (GPC). For example, the gel permeation chromatography analysis device may be PL GPC220 from Agilent Technologies, and the measurement conditions may be as follows, for example.

[0080] - Column (maker, model no.): 2 x TSKgel SupermultiporeHZ-M + TSKgel SuperHZ-2500

[0081] - Eleunt: THF

[0082] - Temperature: 40℃

[0083] - Flow rate: 1.0 mL / min

[0084] - Injection volume, sample concentration: 30 ㎕, 1~10 mg / mL

[0085] - Standard: Polystyrene

[0086] - Detector: RI

[0087]

[0088] In one embodiment of the present invention, the content of the second polymer may be 1 wt% to 15 wt% based on 100 wt% of the porous polymer substrate. The content of the second polymer may be, for example, 3 wt% to 10 wt% or 4 wt% to 7 wt% based on 100 wt% of the porous polymer substrate. When the content of the second polymer satisfies the above-described range, the pore structure of the porous polymer substrate can be uniformly developed, and the mechanical strength can be further improved.

[0089]

[0090] In one embodiment of the present invention, the weight ratio of the hard portion and the soft portion of the second polymer may be 1:0.1 to 5. That is, the weight ratio of the second repeating unit and the third repeating unit may be 1:0.1 to 5. When the hard portion and the soft portion of the second polymer satisfy the above-described range, the compatibility of the second polymer and the first polymer may be excellent while imparting hydrophilicity to the porous polymer substrate.

[0091] In one embodiment of the present invention, the mole ratio of the second repeating unit and the third repeating unit in the second polymer may be 3:7 to 7:3, 4:6 to 6:4, or 5:5. When the mole ratio of the second repeating unit and the third repeating unit satisfies the above-described range, the porous polymer substrate may have excellent hydrophilicity and excellent mechanical strength.

[0092]

[0093] Meanwhile, methods for manufacturing porous polymer substrates used in membranes include a dry method in which an extruded substrate is stretched at a low temperature to generate microcracks at the lamellar crystal interface, thereby making the substrate porous, and a wet method in which a polymer and a plasticizer are uniformly mixed at a high temperature, then cooled to induce phase separation, and then the plasticizer is removed to obtain a porous membrane. Hereinafter, methods for manufacturing porous polymer substrates using the dry method and the wet method, and the unique physical properties of the porous polymer substrates manufactured thereby will be described in detail.

[0094]

[0095] <Method for manufacturing a porous polymer substrate using a wet method and a porous polymer substrate manufactured thereby>

[0096] According to one aspect of the present invention, a method for manufacturing a porous polymer substrate is provided, comprising: mixing and extruding a mixture comprising a first polymer, a second polymer, and a plasticizer to obtain an extrudate; cooling the extrudate on a casting roll to obtain a sheet; and biaxially stretching the obtained sheet in a machine direction (MD) and a transverse machine direction (TD) to obtain a stretched sheet; and extracting a plasticizer from the stretched sheet with an organic solvent; wherein the first polymer comprises a first repeating unit derived from an olefin, the second polymer comprises a hard segment comprising a second repeating unit derived from an olefin, and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and wherein the weight average molecular weight of the first polymer is at least three times that of the second polymer.

[0097] At this time, the contents regarding the first polymer and the second polymer are replaced with those described above.

[0098]

[0099] First, a mixture comprising a first polymer, a second polymer, and a plasticizer is kneaded and extruded to obtain an extrudate. At this time, additives commonly used in the art may be added to the mixture, including nucleating agents, antioxidants, UV stabilizers, and the like.

[0100] In one embodiment of the present invention, the plasticizer is not particularly limited as long as it is commonly used in the art. The plasticizer may be, for example, liquid paraffin, paraffin oil; phthalic acid esters such as dibutyl phthalate, dihexyl phthalate, and dioctyl phthalate; aromatic ethers such as diphenyl ether and benzyl ether; fatty acids having 10 to 20 carbon atoms such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; fatty acid alcohols having 10 to 20 carbon atoms such as palmitic alcohol, stearic alcohol, and oleic alcohol; It may be a fatty acid ester in which one or more fatty acids having 4 to 26 carbon atoms in a saturated and unsaturated fatty acid group, or an unsaturated fatty acid double bond substituted with epoxy, such as palmitic acid mono-, di-, or triester, stearic acid mono-, di-, or triester, oleic acid mono-, di-, or triester, or linoleic acid mono-, di-, or triester, are ester-bonded with an alcohol having 1 to 8 hydroxyl groups and 1 to 10 carbon atoms.

[0101]

[0102] In one embodiment of the present invention, in the step of obtaining the extrudate, the mixture may further include a compatibilizer.

[0103] In one embodiment of the present invention, the compatibilizer may be, for example, polyethylene grafted maleic anhydride (PE-g-MAH), poly(ethylene-co-vinyl acetate), poly(ethylene-co-glycidyl methacrylate), poly(ethylene-co-methacrylic acid), or two or more thereof.

[0104]

[0105] In one embodiment of the present invention, the mixture may contain 1 to 15 parts by weight, or 3 to 10 parts by weight, of a second polymer and 200 to 300 parts by weight, or 230 to 270 parts by weight, based on 100 parts by weight of the first polymer. When the content of the second polymer satisfies the above-described range, the hydrophilicity of the porous polymer substrate may be excellent. In addition, when the content of the plasticizer satisfies the above-described range, the hardness of the porous polymer substrate may be appropriate.

[0106]

[0107] In one embodiment of the present invention, the mixing of the mixture comprising the first polymer, the second polymer, the plasticizer, and optionally the compatibilizer may be performed using a twin-screw extruder, a mixer, or a Vanvai mixer. Preferably, the step of mixing the mixture may use a twin-screw extruder having a length (L) / diameter (D) of 52 to 80. If the length (L) / diameter (D) is less than the lower limit of the numerical range, mixing properties may be insufficient, and if the length (L) / diameter (D) is more than the upper limit of the numerical range, mechanical stability may be insufficient.

[0108]

[0109] In one embodiment of the present invention, the step of obtaining the extrudate may use a die extruder. The die extruder may be a T-die extruder or an annular die extruder. When a die extruder is used as described above in the present invention, oxygen-containing repeating units are arranged on the surface of the extrudate of the porous polymer substrate produced, thereby improving the hydrophilicity of the porous polymer substrate.

[0110]

[0111] In one embodiment of the present invention, the extrusion temperature during extrusion should be higher than the temperature at which the first polymer and the second polymer can be made into a single phase, and lower than the temperature at which thermal oxidation can occur and deteriorate the physical properties of the polymer. A preferred extrusion temperature is 170°C or higher and 230°C or lower. When processed within the above temperature range, sufficient mixing of the mixture can occur, and decomposition of the polymer can not occur. The mixture comprising the first polymer, the second polymer, a plasticizer, and optionally a compatibilizer can be blended and introduced into the compound, or can be individually introduced from separate feeders and then kneaded with an appropriate screw configuration in an extruder.

[0112]

[0113] Next, the extrudate is cooled on a casting roll to obtain a sheet.

[0114] In a specific embodiment of the present invention, any of the common casting, blowing, calendering, or water-cooling methods can be used to obtain a sheet-shaped molded article or sheet from an extrudate. The cooling rate of the extrudate is crucial. During the melt extrusion process, the extrudate, which thermodynamically forms a single phase, undergoes phase separation during the cooling process. At this time, phase separation of the first polymer and the second polymer occurs.

[0115]

[0116] Next, the obtained sheet is biaxially stretched in the machine direction (MD) and the transverse machine direction (TD). Stretching of the sheet can be performed by sequential or simultaneous stretching in a roll method or a tenter type. The stretch ratio is preferably 3 times or more in each of the mechanical direction (MD) and the transverse direction (TD), and the total stretch ratio is preferably 25 to 170 times. If the stretch ratio in one direction is less than 3 times, the orientation in one direction is insufficient, and at the same time, the balance of physical properties between the longitudinal and transverse directions is broken, resulting in a decrease in tensile strength and puncture strength, etc. In addition, if the total stretch ratio is less than 25 times, under-stretching occurs, and if it exceeds 170 times, there is a high possibility of breakage during stretching, and there is a disadvantage in that the shrinkage rate of the final film increases.

[0117]

[0118] Next, the plasticizer is extracted from the stretched sheet using an organic solvent. As the stretched sheet passes through the organic solvent, the plasticizer is extracted, thereby producing a porous polymer substrate having pores formed therein.

[0119] It is preferable that the organic solvent be an antisolvent for the first polymer and the second polymer, a good solvent for the plasticizer, and have a boiling point lower than the melting points of the first polymer and the second polymer, thereby allowing for rapid drying. Examples of the organic solvent include hydrocarbons such as n-hexane or cyclohexane; halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, fluorocarbons, and dichloromethane; alcohols such as ethanol or isopropanol; and ketones such as acetone or 2-butanone.

[0120]

[0121] In one embodiment of the present invention, the method for manufacturing the porous polymer substrate may further include a step of drying using a method such as hot air, an infrared heater, or vacuum heating after the biaxial stretching step.

[0122]

[0123] In one embodiment of the present invention, a step of heat-setting the thin-film porous polymer substrate after the drying step may be further included to finally remove residual stress and reduce shrinkage of the final film.

[0124] In one embodiment of the present invention, the heat-setting step is to fix the porous polymer substrate and apply heat to forcibly hold the porous polymer substrate, which is about to shrink, to remove residual stress. A high heat-setting temperature is advantageous in reducing the shrinkage rate, but if it is too high, the porous polymer substrate will partially melt, which will clog the formed micropores and reduce the permeability. It is preferable that the heat-setting temperature be selected within a temperature range where 10 to 30 wt% of the crystalline portion of the porous polymer substrate melts. If the heat-setting temperature is selected within a temperature range lower than the melting temperature of 10 wt% of the crystalline portion of the porous polymer substrate, the rearrangement of polyolefin molecules within the porous polymer substrate will be insufficient, and the residual stress relieving effect of the porous polymer substrate will not be achieved. On the other hand, if the heat-setting temperature is selected within a temperature range higher than the melting temperature of 30 wt% of the crystalline portion of the porous polymer substrate, the partial melting will clog the micropores and reduce the permeability. Here, the heat setting time should be relatively short when the heat setting temperature is high, and can be relatively long when the heat setting temperature is low. When using a tenter-type continuous heat setting device, the heat setting time is preferably about 20 seconds to 2 minutes. Most preferably, in the temperature range where 10 to 15 wt% of the crystalline portion of the porous polymer substrate melts, 1 to 2 minutes is appropriate, and in the temperature range where 15 to 30 wt% melts, 20 seconds to 1 minute is appropriate.

[0125]

[0126] In one embodiment of the present invention, a porous polymer substrate manufactured by a wet method as described above can be provided.

[0127] In one embodiment of the present invention, the porous polymer substrate for a separation membrane may further include a compatibilizer to prevent aggregation between the second polymer including a third repeating unit derived from an oxygen-containing monomer and to improve compatibility between the first polymer and the second polymer. In particular, since the first polymer is generally hydrophobic and the second polymer is hydrophilic, a compatibilizer containing both polar and nonpolar properties that facilitates simultaneous mixing of the two is preferred.

[0128]

[0129] In one embodiment of the present invention, the compatibilizer may be, for example, polyethylene grafted maleic anhydride (PE-g-MAH), poly(ethylene-co-vinyl acetate), poly(ethylene-co-glycidyl methacrylate), poly(ethylene-co-methacrylic acid), or two or more thereof.

[0130]

[0131] In one embodiment of the present invention, the compatibilizer may be included in an amount of 0.5 wt% to 5 wt% or 1 wt% to 3 wt% based on 100 wt% of the porous polymer substrate. When the content of the compatibilizer satisfies the above-described range, the interface between the first polymer and the second polymer may not be separated and may help form a co-continuous structure.

[0132]

[0133] In one embodiment of the present invention, the surface tension of the porous polymer substrate may be 30 dyne / cm to 75 dyne / cm or 32 dyne / cm to 70 dyne / cm.

[0134] In one embodiment of the present invention, the shape of the porous polymer substrate may be a film or a nonwoven fabric.

[0135]

[0136] In one embodiment of the present invention, the porous polymer substrate can exhibit a shutdown function at a temperature of, for example, 80 to 150°C. The shutdown temperature can be determined by measuring the time (sec) it takes for 100 ml of air to pass through the membrane at a constant pressure of 0.05 Mpa when the temperature is increased by 5°C per minute using a Wangyeon-style air permeability device, thereby determining the temperature at which the air permeability of the membrane rapidly increases. By satisfying the shutdown temperature, overcharge stability can be secured, and the high-temperature stability of the membrane can be significantly increased.

[0137]

[0138] In one embodiment of the present invention, the thickness of the porous polymer substrate is not particularly limited, but is specifically 1 to 100 μm, more specifically 5 to 50 μm, and as high output / high capacity batteries have been developed recently, it is advantageous to use a thin film as the porous polymer substrate.

[0139] In one embodiment of the present invention, the pore diameter present in the porous polymer substrate may be 10 nm to 100 nm, or 10 nm to 70 nm, or 10 nm to 50 nm, or 10 nm to 35 nm.

[0140] In one embodiment of the present invention, the porosity can be formed to be 5% to 90%, preferably 20% to 80%.

[0141] However, in the present invention, this numerical range can be easily modified according to specific embodiments or needs.

[0142] In this specification, the term "porosity" means the ratio of the volume occupied by pores to the total volume of a certain structure, and its unit volume % (vol%) is used, and can be used interchangeably with terms such as porosity, porosity, etc. In the present invention, the measurement of the porosity is not particularly limited, and according to an embodiment of the present invention, for example, it can be measured according to the BET (Brunauer-Emmett-Teller) measurement method using nitrogen gas or the mercury penetration method (Hg porosimeter) and ASTM D-2873. Alternatively, the true density of the porous polymer substrate can be calculated from the apparent density of the porous polymer substrate and the composition ratio of materials included in the porous polymer substrate and the density of each component, and the porosity of the porous polymer substrate can be calculated from the difference between the apparent density and the true density.

[0143] In one embodiment of the present invention, the pore diameter of the porous polymer substrate may be measured using a capillary flow porometry (CFP) device.

[0144]

[0145] <Method for manufacturing a porous polymer substrate using a dry method and a porous polymer substrate manufactured thereby>

[0146] According to another aspect of the present invention, a method for producing a porous polymer substrate is provided, comprising: preparing a mixture comprising a first polymer and a second polymer; melt-extruding the mixture to prepare an unstretched sheet; and stretching the unstretched sheet to obtain an stretched sheet; wherein the first polymer comprises a first repeating unit derived from an olefin, the second polymer comprises a hard segment comprising a second repeating unit derived from an olefin, and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer, and wherein the weight average molecular weight of the first polymer is at least three times that of the second polymer.

[0147] At this time, the contents regarding the first polymer and the second polymer are replaced with those described above.

[0148]

[0149] In one embodiment of the present invention, the porous polymer substrate may be a porous polymer film prepared by a dry method. The dry method refers to a method of melting a first polymer and a second polymer using an extruder, extruding the molten resin into a sheet, and then stretching it to form pores. Unlike the wet method that uses a plasticizer, the dry method forms pores by utilizing the crystallinity of a polymer resin such as polyolefin. Therefore, the mixing and extraction processes of plasticizers and diluents used in the wet method are eliminated, making it environmentally friendly and advantageous in terms of cost reduction.

[0150]

[0151] First, a mixture containing a first polymer and a second polymer is prepared.

[0152] Thereafter, the above mixture is melt-extruded to prepare an unstretched sheet.

[0153] In one embodiment of the present invention, one or more extruders may be used in the melt extrusion method. The extruder may be, but is not limited to, a flat die such as a T-die or a coat hanger die.

[0154] In one embodiment of the present invention, the melt extrusion may be performed using a single-screw or twin-screw extruder to extrude the mixture comprising the first polymer and the second polymer into a sheet or film form. At this time, the extrusion temperature may be about 170 to about 280°C.

[0155] In one embodiment of the present invention, a casting process may be performed after the extrusion process. This casting process may be performed at a temperature of about 120°C or lower. For example, it may be performed at a temperature of about 100°C or lower. Furthermore, the casting process may include a method of contacting the film with a cooling medium, or a method of contacting the film with a refrigerant-cooled roll or press.

[0156] In one embodiment of the present invention, the extruded unstretched sheet may be annealed to stabilize the crystal structure. The annealing process may be performed at a temperature of about 100°C to about 165°C. The annealing process may be performed in an oven at a temperature of about 100°C to about 165°C or about 125°C to about 160°C for a residence time of about 1 to about 60 minutes or about 3 to about 30 minutes.

[0157] In this specification, the term "unstretched sheet" is used to collectively refer to intermediate products, etc., in a state where no additional force is applied to cause significant dimensional deformation of the film other than the tension applied for film running. In this state, the film exists as a non-porous membrane without pores. This is used to distinguish it from the porous polymer substrate ultimately obtained after the stretching process.

[0158]

[0159] In one embodiment of the present invention, the crystallinity of the unstretched sheet may be 50% to 70%. If the crystallinity of the unstretched sheet is 50% or less, pore formation may not be easy, which may increase the resistance of the porous polymer substrate and deteriorate the life characteristics of the lithium secondary battery. On the other hand, if the crystallinity of the unstretched sheet is 70% or more, it may be difficult to ensure processability due to film tearing during stretching in the TD direction due to excessive crystallization.

[0160]

[0161] Thereafter, the above unstretched sheet is stretched to obtain a stretched sheet.

[0162] In one embodiment of the present invention, the stretching process can be performed at a temperature range of 100 to 150°C, for example, at a temperature range of 120 to 140°C.

[0163] Optionally, the stretched film may be heat-set at a temperature of 120 to 160°C for about 5 to about 300 seconds. After heat-setting, it may be relaxed at a temperature of 120 to 160°C, preferably 130 to 150°C, with a relaxation rate of 10 to 40%, preferably 10 to 25%.

[0164] In one embodiment of the present invention, the porous polymer substrate may be biaxially stretched in the MD direction and the TD direction. Specifically, in the case of biaxial stretching, the stretching may be performed by stretching 3 to 6 times in the longitudinal and transverse directions, respectively.

[0165] In one embodiment of the present invention, the stretching processes in the MD direction and the TD direction can be performed sequentially. In terms of productivity, it is more effective to shrink the width of the sheet through stretching in the MD direction and then perform stretching in the TD direction sequentially than to perform them in the opposite order. First, when the unstretched sheet is stretched in the MD direction, cracks may occur between the crystalline and amorphous regions of the extruded sheet, forming pores. After this stretching process in the MD direction, the stretching process in the TD direction can be performed sequentially. In particular, it is preferable that the stretching process in the TD direction be performed multiple times, two or more times, in multiple stages under different stretching ratios and temperature conditions. Since the rigidity in the TD direction is very weak after stretching in the MD direction, tearing or partial overstretching may occur during stretching in the TD direction. Therefore, a multi-stage stretching process can be performed according to the characteristics of the sheet.

[0166] In one embodiment of the present invention, the stretching process is not particularly limited. Examples of the stretching process include a method in which a plurality of rollers have different peripheral speeds and the rollers are stretched by utilizing the difference in peripheral speeds between them, a method in which both ends of an unstretched sheet are fixed with clips or pins and the gap between the clips or pins is widened in the direction of travel to stretch, and the like. These methods may be used in combination.

[0167]

[0168] In one embodiment of the present invention, the method for manufacturing the porous polymer substrate may further include a step of drying using a method such as hot air, an infrared heater, or vacuum heating after the stretching step.

[0169]

[0170] In another embodiment of the present invention, a porous polymer substrate manufactured by a wet method as described above may be provided.

[0171] In one embodiment of the present invention, the porosity of the porous polymer substrate may be 20% to 60% by volume, or 45% to 55% by volume.

[0172] In one embodiment of the present invention, the pore diameter of the porous polymer substrate may be 10 nm to 70 nm.

[0173] In one embodiment of the present invention, the crystallinity of the porous polymer substrate may be 60% to 85%. When the crystallinity of the porous polymer substrate is 60% or less, pore formation is difficult, which increases the resistance of the separator and deteriorates the life characteristics of the lithium secondary battery. On the other hand, when the crystallinity of the porous polymer substrate is 85% or more, it is difficult to ensure processability due to the film tearing during stretching in the TD direction due to excessive crystallization.

[0174] In one embodiment of the present invention, the crystallinity refers to the ratio of crystals contained in the polymer. The crystallinity can be measured by wide-angle X-ray scattering (WAXS), densitometry, and thermometry.

[0175] For example, the above thermal measurement method can be performed using a differential scanning calorimetry (DSC) device. In this case, the crystallinity x c is calculated by the following formula.

[0176] x c (%) = (ΔHm ÷ ΔHm 0 ) x 100

[0177] In the above equation, ΔHm represents the heat of fusion of the sample measured by the DSC device, and Hm 0 represents the equilibrium heat of fusion. Hm 0 The values ​​used in the published literature can be used. For example, according to the Polymer Handbook, for iPP, Hm 0 =8.7(kJ / mol).

[0178] In one embodiment of the present invention, the porous polymer substrate may be biaxially stretched in the MD direction and the TD direction.

[0179] In one embodiment of the present invention, the porous polymer substrate may be manufactured by a dry manufacturing method and may not contain a plasticizer. The porous polymer substrate may be manufactured by a dry manufacturing method and may substantially not contain a plasticizer. For example, the porous polymer substrate may contain less than 3 wt%, less than 2 wt%, less than 1 wt%, less than 0.5 wt%, or less than 0.1 wt% of the plasticizer based on 100 wt% of the porous polymer substrate.

[0180]

[0181] In one embodiment of the present invention, the porous polymer substrate may be characterized by being biaxially stretched in the MD direction and the TD direction.

[0182] In one embodiment of the present invention, the porous polymer substrate may include linear lamellae having a predetermined width, the lamellae being spaced apart at a predetermined interval and two adjacent lamellae being connected by a plurality of microfibrils, such that the pores may have a rectangular planar shape due to the lamellae and microfibrils.

[0183] A porous polymer substrate according to one embodiment of the present invention may exhibit a pore type due to a unique cleft structure that is different from conventional porous polymer films. The porous polymer substrate may include linear lamellae having a predetermined width, wherein the lamellae are spaced apart at a predetermined interval, and adjacent two lamellae are connected by a plurality of microfibrils. According to these structural characteristics, the pores included in the porous polymer substrate may exhibit a planar shape of a rectangle or a rectangle-like shape. This unique pore shape can be preferably realized by the method for manufacturing the porous polymer substrate of the present invention, wherein when an unstretched sheet is stretched in the MD direction, the interlamellar portions between the lamellae are cleaved, the lamellae are spaced apart at a predetermined interval, and the adjacent lamellae can be connected by a plurality of microfibrils derived from the interlamellar portions.

[0184] In addition, through TD direction stretching performed after MD direction stretching, the orientation of the lamellae formed in a certain direction by MD direction stretching is partially relaxed, thereby alleviating the asymmetry of the tensile strength in the MD direction and the TD direction, and at the same time, the resistance of the membrane can be lowered by additionally forming pores between a large number of microfibrils derived from the lamella portion.

[0185]

[0186] Lithium secondary battery

[0187] The present invention provides a lithium secondary battery.

[0188] According to one aspect of the present invention, the present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte. The separator comprises the porous polymer substrate described above.

[0189] In one embodiment of the present invention, the positive electrode can be manufactured by coating a composition for forming a positive electrode including a positive electrode active material, a binder, a conductive agent, a solvent, etc. on a positive electrode current collector.

[0190] The above-mentioned cathode active material may be a conventional cathode active material that can be used in the cathode of a conventional electrochemical device. Non-limiting examples of such cathode active materials include, but are not limited to, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide comprising these.

[0191] Meanwhile, the positive electrode active material may be included in an amount of 80 to 99 parts by weight, for example, 85 to 98 parts by weight, based on the total weight of the solid content of the composition for forming the positive electrode. When the positive electrode active material is included in the above-described content range, the positive electrode including it may exhibit excellent capacity characteristics.

[0192] The above-mentioned positive electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used.

[0193] The above binder is a component that assists in the bonding of the active material and the conductive material and the bonding to the current collector, and can typically be added in an amount of 1 to 30 wt% based on the total solid weight of the composition for forming the positive electrode. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers, etc.

[0194] The above-mentioned conductive agent can typically be added in an amount of 1 wt% to 30 wt% based on the total solid weight of the composition for forming the anode.

[0195] There are no particular limitations on the conductive material, as long as it is conductive and does not cause a chemical change in the battery, and conventional conductive materials can be used. Examples of such conductive materials include: graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers or metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powders; conductive whiskies such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specific examples of commercially available conductive agents include acetylene blacks (Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company), Ketjenblack, EC series (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

[0196] In addition, the positive electrode active material layer may optionally further include a dispersant as needed.

[0197] The above dispersant can be used without any special restrictions as long as it is used as a dispersant of the anode, and for example, an aqueous dispersant or an organic dispersant can be selectively used as needed. Preferably, the dispersant is a cellulose compound, polyalkylene oxide, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetal, polyvinyl ether, polyvinyl sulfonic acid, polyvinyl chloride (PVC), polyvinylidene fluoride, chitosan, starch, amylose, polyacrylamide, poly-N-isopropylacrylamide, poly-N,N-dimethylacrylamide, polyethyleneimine, polyoxyethylene, poly(2-methoxyethoxyethylene), poly(acrylamide-co-diallyldimethylammonium chloride), acrylonitrile / butadiene / styrene (ABS) polymer, acrylonitrile / styrene / acrylate ester (ASA) polymer, a mixture of acrylonitrile / styrene / acrylate ester (ASA) polymer and propylene carbonate, styrene / acrylonitrile (SAN) Examples thereof include copolymers, methyl methacrylate / acrylonitrile / butadiene / styrene (MABS) polymers, styrene butadiene rubber, nitrile butadiene rubber, and fluoroelastomers, and any one or a mixture of two or more thereof may be used. Hydrogenated nitrile butadiene rubber (H-NBR) may be used. When the positive electrode active material layer further includes a dispersant, the dispersibility of the components of the positive electrode active material layer, particularly the conductive material, may be increased, but is not limited thereto.

[0198] In addition, the solvent may be a solvent generally used in the relevant technical field, such as dimethylsulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these may be used alone or as a mixture of two or more. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder in consideration of the coating thickness and manufacturing yield of the slurry, and to have a viscosity that can exhibit excellent thickness uniformity when applied thereafter for manufacturing the positive electrode.

[0199]

[0200] The negative electrode according to the present invention can be manufactured by coating a negative electrode forming composition including the above-described negative electrode active material, binder, conductive agent, solvent, etc. on a negative electrode current collector. In addition, the negative electrode forming composition may optionally further include a dispersant as needed.

[0201] As the above negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Non-limiting examples of such negative electrode active materials include silicon-based negative electrode active materials exhibiting high-capacity characteristics, carbon-based negative electrode active materials, and Li x Fe2O3(0≤x≤1), Li x WO2(0≤x≤1), Sn x Me 1-x Me' y O z(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of group 1, 2, and 3 of the periodic table, halogens; 0 <x≤1; 1≤y≤3; 1≤z≤8) 등의 금속 복합 산화물; 리튬 금속; 리튬 합금; 주석계 합금; SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, 및 Bi2O5 등의 금속 산화물; 폴리아세틸렌 등의 도전성 고분자; Li-Co-Ni 계 재료; 티타늄 산화물; 리튬 티타늄 산화물 등의 음극 활물질을 들 수 있다. 상기 규소계 음극 활물질은 Si, SiOx(0.1<x<5), Si-금속 합금, Mg와 같은 금속이 도핑 또는 화학 결합된 실리콘 산화물 입자(SiOx, 0.1<x<5) 및 Si와 SiOx(0.1<x<5)의 합금으로 이루어진 군에서 선택된 하나 이상을 포함할 수 있다. 상기 탄소계 음극 활물질은 천연 흑연, 인조 흑연, 비정질 하드카본(hard carbon), 저결정질 소프트카본(soft carbon), 카본 블랙, 아세틸렌 블랙, 케첸 블랙, 수퍼 P, 그래핀 (graphene), 및 섬유상 탄소로 이루어진 군으로부터 선택되는 하나 이상을 포함할 수 있다.

[0202] The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 ㎛ to 500 ㎛, and like the positive electrode current collector, fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

[0203] The conductive material, binder, solvent or dispersant included in the above-described composition for forming the cathode may be applied without any special limitation as long as it is generally usable in a composition for forming an electrode. For example, the conductive material, binder, solvent or dispersant described in the above-described composition for forming the anode may be applied.

[0204]

[0205] In addition, the lithium secondary battery may further include an electrolyte. The electrolyte may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of a lithium secondary battery.

[0206] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0207] The organic solvent may be used without any particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent may include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; Carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R represents a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used. Among these, a carbonate solvent is preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and a linear carbonate compound having low viscosity (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) is more preferable.In this case, the performance of the electrolyte may be improved when the cyclic carbonate and the chain carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9.

[0208] The lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2. LiCl, LiI, or LiB(C2O4)2. It is preferable that the concentration of the lithium salt be used within the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte can exhibit excellent electrolyte performance because it has appropriate conductivity and viscosity, and lithium ions can move effectively.

[0209] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. At this time, the additives may be contained in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the total weight of the electrolyte.

[0210] The lithium secondary battery according to the present invention can be included in a battery module as a unit battery, and the battery module can be used in a battery pack and a device that includes the battery pack as a power source. Specific examples of the device include, but are not limited to, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage system.

[0211]

[0212] Hereinafter, the present invention will be described in more detail through examples, but the following examples are intended to illustrate the present invention, and the scope of the present invention is not limited to these examples.

[0213] <Example>

[0214] <Example 1: Preparation of porous polymer substrate using wet method>

[0215] As a first polymer, a high-density polypropylene having a first repeating unit derived from an ethylene monomer was prepared (weight average molecular weight: 1,000,000), and as a second polymer, a polypropylene-polyethylene oxide copolymer having a second repeating unit derived from a propylene monomer and a third repeating unit derived from an ethylene oxide monomer (weight average molecular weight: 30,000) was introduced at 10 parts by weight based on 100 parts by weight of the first polymer. At this time, the molar ratio of the second repeating unit and the third repeating unit in the second polymer was 5:5. In addition, as a plasticizer, liquid paraffin oil was introduced into a twin screw extruder at 230 parts by weight based on 100 parts by weight of the first polymer. The extrudate discharged from an extruder set to a temperature of 150°C to 250°C was passed through a casting roll to cool, thereby obtaining a sheet, and the obtained sheet was biaxially stretched using a tenter-type sequential stretching machine for MD stretching and then TD stretching. The MD stretching ratio and the TD stretching ratio were both 3 to 12 times. The obtained stretched sheet was used as an organic solvent to extract the plasticizer, thereby preparing a porous polymer substrate. The content of the second polymer in the prepared porous polymer substrate was 10 wt% based on 100 wt% of the porous polymer substrate.

[0216]

[0217] <Comparative Example 1>

[0218] A sheet was obtained by passing the extrudate discharged from the extruder through a casting roll in the same manner as in Example 1, except that the second polymer was not used. Thereafter, a porous polymer substrate was manufactured in the same manner as in Example 1.

[0219]

[0220] Experimental Example 1

[0221] Experimental Example 1: Measurement of impregnation properties of Example 1 and Comparative Example 1

[0222] A 1M LiPF6 non-aqueous electrolyte was prepared by adding LiPF6 to a non-aqueous electrolyte solvent prepared by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 30:70 as an electrolyte.

[0223] The prepared electrolyte was dripped onto one side of the porous polymer substrate of Example 1 and Comparative Example 1 using a dropper, and the degree of electrolyte impregnation after 5 minutes is shown in Figure 1.

[0224] Specifically, the horizontal direction is A, the vertical direction is B, and the size in the A direction and the B direction of the impregnated electrolyte 5 minutes later compared to the initial size in Example 1 and Comparative Example 1 were measured to calculate the increase rate. (The A direction and the B direction are orthogonal.)

[0225] According to Fig. 1, when looking at the front side (the side where the electrolyte is dripped), Example 1 showed a size increase rate in the A direction that was 24 percentage points higher and a size increase rate in the B direction that was 81 percentage points higher than Comparative Example 1. In addition, when looking at the back side, the size increase rate in the A direction was 24 percentage points higher and a size increase rate in the B direction that was 98 percentage points higher. That is, it was confirmed that Example 1 had improved impregnation properties compared to Comparative Example 1.

[0226]

[0227] <Example 2: Preparation of a porous polymer substrate using a dry method>

[0228] Example 2-1

[0229] A mixture containing polypropylene having a first repeating unit derived from propylene as the first polymer (Daehan Yuhwa Co., Ltd., S802M, weight average molecular weight: 230,000) and a polypropylene-b-polyethylene oxide copolymer having a second repeating unit derived from propylene and a third repeating unit derived from ethylene oxide monomer as the second polymer (PP-b-PEO, Sanyo Chemical Co., Ltd., Pelestat230, weight average molecular weight: 40,000) was prepared. At this time, the weight ratio of the second repeating unit (hard part) and the third repeating unit (soft part) was 1:1. In order to increase the mixing property, the first polymer and the second polymer were compounded at a ratio of 5:5, and the first polymer was additionally added to the compounded pellets, and the extruder was fed with the first polymer so that the final weight ratio of the first polymer and the second polymer became 95:5. The mixture was extruded into a sheet form using a T-die extruder at a temperature of 230°C. The above extruded unstretched sheet was crystallized by contacting it with a chill roll maintained at 126°C. The crystallinity of the above unstretched sheet was confirmed to be approximately 58.6%.

[0230] Afterwards, MD direction stretching was performed, and this was performed by utilizing the difference in peripheral speed of multiple stretching rollers. To this end, multiple continuous rollers maintained at 120℃ were provided, and the speed of the rollers was continuously increased, and stretching was performed in the MD direction until the speed increased by 300% compared to the initial speed.

[0231]

[0232] Example 2-2

[0233] The membrane manufactured in Example 2-1 was placed in an oven maintained at 161°C after clipping both ends, and stretched in the TD direction until the width became 185% of the initial width, while simultaneously reducing the speed in the MD direction to 55% to manufacture a membrane.

[0234]

[0235] Comparative Example 2-1

[0236] A membrane was manufactured in the same manner as in Example 2-1 except that the second polymer was not used.

[0237]

[0238] Comparative Example 2-2

[0239] A separator was manufactured in the same manner as in Example 2-1, except that the raw material input ratio was adjusted so that the weight ratio of the first polymer and the second polymer was 80:20. The crystallinity of the unstretched sheet was reduced to approximately 41.3%, the porosity of the separator was formed to be less than 30%, and the puncture strength was implemented to be less than 200 gf, so that the targeted properties could not be implemented.

[0240]

[0241] The thickness, porosity, pore size, puncture strength, weight ratio of the first polymer and the second polymer in the center and surface of the membrane, and impregnation test results of the membrane substrates of Examples 2-1 to 2-2 and Comparative Example 2-1 obtained in this manner are shown in Table 1.

[0242] Thickness (㎛) Porosity (vol%) Pore size (nm) Puncture strength (gf) Weight ratio of first polymer:second polymer in the center of the membrane Weight ratio of first polymer:second polymer in the surface of the membrane Impregnation test results Example 2-1 10.9 34.6 32 25 2.8 9 7.6:2.4 9 2.2:7.8 2 0.18% increase Example 2-2 10.8 3 7.7 3 8 27 1.1 9 7.8:2.2 9 1.9:8.1 2 5.22% increase Comparative Example 2-1 11.9 3 9.3 3 23 0 1.3 1 00:01 0:01 0.89% increase

[0243] Examples 2-1 and 2-2 had superior electrolyte impregnation properties compared to Comparative Example 2-1. Specifically, according to FIG. 2, in Example 2-1, the area occupied by the electrolyte increased to 20.18% of the initial area after 5 minutes, whereas in Comparative Example 2-1, the area occupied by the electrolyte increased to 10.89% of the initial area after 5 minutes, confirming that Example 2-1 had improved electrolyte impregnation properties, i.e., better hydrophilicity, compared to Comparative Example 2-1. Meanwhile, it was confirmed that in Examples 2-1 and 2-2, the mechanical properties such as porosity and puncture strength were appropriate even when the second polymer was included, compared to Comparative Example 2-1.

[0244] In addition, it was confirmed that the membranes of Examples 2-1 and 2-2 used a predetermined amount of the second polymer by distributing a larger amount of the second polymer on the surface compared to the center of the membrane, thereby maximizing the degree of hydrophilic modification of the membrane.

[0245] Experimental Example 2

[0246] Experimental Example 2-1: Measurement of the thickness of the membrane of Examples 2-1, 2-2 and Comparative Example 2-1

[0247] In Examples 2-1, 2-2 and Comparative Example 2-1, the thickness of the separator was measured using a thickness measuring device (Mitutoyo, VL-50S-B).

[0248]

[0249] Experimental Example 2-2: Measurement of porosity of membranes of Examples 2-1, 2-2 and Comparative Example 2-1

[0250] The porosity of the membrane in Examples 2-1, 2-2 and Comparative Example 2-1 was measured by collecting a specimen of a certain area and measuring the weight using the formula: Porosity (%) = 100 - [(Weight of membrane) / (Area of ​​specimen * Thickness of specimen * Density of membrane material)] * 100.

[0251]

[0252] Experimental Example 2-3: Pore size of the membrane of Examples 2-1, 2-2 and Comparative Example 2-1

[0253] In Examples 2-1, 2-2 and Comparative Example 2-1, the size of the membrane pores was measured according to the capillary flow porometry (CFP) method, which collects a specimen of a certain area and then measures the diameter of the smallest pore in the thickness direction.

[0254]

[0255] Experimental Example 2-4: Puncture strength of the membranes of Examples 2-1, 2-2 and Comparative Example 2-1

[0256] The strength of the membranes of Examples 2-1, 2-2 and Comparative Example 2-1 was measured when they were broken at a speed of 2 mm / sec using a needle with a diameter of 1 mm (radius of curvature of 0.5 mm), and the value was calculated by dividing the value by the thickness of the membrane. An Instron 3342 (manufacturer: Instron) device was used to measure the puncture strength.

[0257]

[0258] Experimental Example 2-5: Weight ratio of the first polymer and the second polymer in the central and surface portions of the membranes of Examples 2-1, 2-2 and Comparative Example 2-1

[0259] IR analysis was performed on each of the first and second polymers and the mixing ratio, to prepare a calibration curve for each content of the first and second polymers. The membranes of each of Examples 2-1 and 2-2 and Comparative Example 2-1 were cut obliquely at an angle of 5° to collect samples. The collected samples were examined using a Micro-ATR (Attenuated Total Reflectance) device to identify characteristic peaks at the center and surface of the membrane. The measured characteristic peaks were interpolated to calculate the content ratio by comparing them with the previously prepared calibration curve.

[0260]

[0261] Experimental Example 2-6: Measurement of impregnation properties of the membranes of Example 2-1 and Comparative Example 2-1

[0262] A 1M LiPF6 non-aqueous electrolyte was prepared by adding LiPF6 to a non-aqueous electrolyte solvent prepared by mixing ethylene carbonate and diethyl carbonate in a volume ratio of 30:70 as an electrolyte.

[0263] The prepared electrolyte was dripped onto one side of the porous polymer substrate of Example 2-1 and Comparative Example 2-1 using a dropper, and the degree of electrolyte impregnation after 5 minutes is shown in Figure 2.

[0264] Specifically, after using a dropper to drop the electrolyte onto one side of the porous polymer substrate of Example 2-1 and Comparative Example 2-1, the initial area was measured, and then the area occupied by the electrolyte was measured after 5 minutes to calculate the increase rate. The impregnation increase data of Example 2-1 and Comparative Example 2-1 are shown in Fig. 2.

[0265]

[0266] Experimental Example 2-7: SEM image measurement of the membrane of Example 2-1 and Comparative Example 2-1

[0267] The surfaces of the porous polymer substrates of Example 2-1 and Comparative Example 2-1 were measured using SEM and are shown in Figs. 3a and 3b, respectively.

Claims

1. The first polymer contains a first repeating unit of olefin origin, The second polymer comprises a hard segment comprising a second repeating unit derived from an olefin and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer. A porous polymer substrate wherein the weight average molecular weight of the first polymer is at least three times that of the second polymer.

2. In claim 1, A porous polymer substrate characterized in that the content of the second polymer has a gradient that increases from the center of the porous polymer substrate to the surface.

3. In claim 1, A porous polymer substrate, characterized in that the content of the second polymer on the surface of the porous polymer substrate is 1.5 times or more than the content of the second polymer at the center of the porous polymer substrate.

4. In claim 1, The weight average molecular weight of the first polymer is 150,000 to 2,000,000, A porous polymer substrate, characterized in that the weight average molecular weight of the second polymer is 10,000 to 100,000.

5. In claim 1, A porous polymer substrate, characterized in that the content of the second polymer is 1 wt% to 15 wt% based on 100 wt% of the porous polymer substrate.

6. In claim 1, A porous polymer substrate characterized in that the weight ratio of the hard portion and the soft portion of the second polymer is 1:0.1 to 5.

7. In claim 1, A porous polymer substrate, characterized in that the first repeating unit is derived from ethylene, propylene, butylene, pentene, isoprene or two or more monomers thereof.

8. In claim 1, A porous polymer substrate, characterized in that the second repeating unit is derived from ethylene, propylene, butylene, pentene, isoprene or two or more monomers thereof.

9. In claim 1, A porous polymer substrate characterized in that the third repeating unit is derived from an epoxy group-containing monomer, a hydroxy group-containing monomer, a carboxyl group-containing monomer, or two or more monomers thereof.

10. A method for manufacturing a porous polymer substrate, A step of mixing and extruding a mixture comprising a first polymer, a second polymer, and a plasticizer to obtain an extrudate; A step of cooling the extruded product on a casting roll to obtain a sheet; and A step of biaxially stretching the obtained sheet in the machine direction (MD) and the transverse machine direction (TD) to obtain a stretched sheet; and A step of extracting a plasticizer from the above-mentioned stretched sheet with an organic solvent; The first polymer comprises a first repeating unit derived from olefins, The second polymer comprises a hard segment comprising a second repeating unit derived from an olefin and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer. A method for manufacturing a porous polymer substrate, characterized in that the weight average molecular weight of the first polymer is at least three times the weight average molecular weight of the second polymer.

11. In claim 10 A method for producing a porous polymer substrate, characterized in that the mixture comprises 1 to 15 parts by weight of a second polymer and 200 to 300 parts by weight of a plasticizer based on 100 parts by weight of a first polymer.

12. In claim 10 A method for manufacturing a porous polymer substrate, characterized in that the step of mixing the above mixture uses a twin screw extruder having a length (L) / diameter (D) of 52 or more and 80 or less.

13. In claim 10 A method for producing a porous polymer substrate, characterized in that the step of obtaining the extrudate uses a die extruder.

14. A method for manufacturing a porous polymer substrate, A step of preparing a mixture comprising a first polymer and a second polymer; A step of preparing an unstretched sheet by melt-extruding the above mixture; and A step of obtaining a stretched sheet by stretching the above unstretched sheet; The first polymer comprises a first repeating unit derived from olefins, The second polymer comprises a hard segment comprising a second repeating unit derived from an olefin and a soft segment comprising a third repeating unit derived from a hydrophilic group-containing or oxygen-containing monomer. A method for manufacturing a porous polymer substrate, characterized in that the weight average molecular weight of the first polymer is at least three times the weight average molecular weight of the second polymer.

15. In claim 14 A method for manufacturing a porous polymer substrate, characterized in that the crystallinity of the above-mentioned unstretched sheet is 50% to 70%.

16. In claim 14 A method for manufacturing a porous polymer substrate, characterized in that the step of obtaining the above-mentioned stretched sheet includes a step of stretching in the MD direction and the TD direction.

17. A lithium secondary battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, A lithium secondary battery, characterized in that the separator comprises a porous polymer substrate as described in any one of claims 1 to 9.

Images