Lithium secondary battery comprising an Si-based negative electrode active material

Abstract

A lithium secondary battery with reduced withstand voltage defects and improved capacity retention is provided. According to one aspect of the present disclosure, a lithium secondary battery is provided, comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, wherein the negative electrode comprises a Si-based negative electrode active material, and the separator comprises a separator substrate having a plurality of pores and comprising a polyolefin resin, wherein the polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2, an average pore size of 20 to 40 nm, and a maximum pore size of 50 nm or less.

Description

Technical Field

[0001] This disclosure claims the benefit of the filing date of Korean Patent Application No. 10-2022-0072073, filed on June 14, 2022, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a lithium secondary battery including a Si-based negative electrode active material as a negative electrode active material. Background Art

[0003] As a separator for lithium secondary batteries, a film based on a polymer resin such as polyolefin and having a plurality of pores is used. Typically, the electrode assembly is manufactured by a lamination process, wherein the separator and the electrode are combined by applying heat and pressure. The higher the heat and pressure applied to the separator, the higher the bonding force between the electrode and the separator. Recently, for the purpose of improving productivity, the process has been accelerated, so the time for applying heat to the separator is shortened, and therefore, adhesion is ensured by increasing the applied pressure. However, the increase in applied pressure may cause deformation of the electrode assembly. During the lamination process, the thickness of the polymer film substrate is significantly reduced, the damage to the pores is increased, resulting in a decrease in the performance of the battery and a decrease in the dielectric breakdown voltage of the separator, an increase in withstand voltage (Hi-pot) defects and a decrease in capacity retention rate (CRR).

[0004] In particular, when Si-based negative electrode active materials such as Si, SiO, or Si alloys are used as negative electrode active materials for lithium secondary batteries, the volume of the negative electrode expands significantly, thereby increasing the internal pressure of the battery and causing increased compressive deformation of the separator. In addition, compared with graphite negative electrode active materials, Si-based negative electrode active materials have greater granularity, roughness, and hardness, which can cause local damage to the separator when the negative electrode active material and the separator are laminated.

[0005] Therefore, when using a Si-based negative electrode active material, there is a need to develop a separator having improved compression resistance. Summary of the Invention

[0006] Technical issues

[0007] An object of the present disclosure is to provide a lithium secondary battery including a Si-based negative electrode active material, the lithium secondary battery having reduced voltage resistance defects and improved capacity retention.

[0008] Obviously, the objects and advantages of the present disclosure can be achieved by the means or methods recited in the claims and their combinations.

[0009] Technical Solution

[0010] According to a first aspect of the present disclosure, a lithium secondary battery is provided, comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, wherein the negative electrode comprises a Si-based negative electrode active material, the separator comprises a separator substrate, the separator substrate has a plurality of pores and comprises a polyolefin resin, the polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2, an average pore size of 20 to 40 nm, and a maximum pore size of 50 nm or less.

[0011] According to a second aspect of the present disclosure, there is provided a lithium secondary battery comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, wherein

[0012] The negative electrode comprises a Si-based negative electrode active material,

[0013] The separator includes a separator substrate having a plurality of pores and comprising a polyolefin resin.

[0014] The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2,

[0015] The pores have an average pore size of 20 to 40 nm and a maximum pore size of 50 nm or less, and

[0016] The diaphragm substrate is characterized in that

[0017] When a tensile stress of 15 MPa is applied at 60°C for 60 seconds, the strain is 25% or less, and

[0018] After a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed, it takes 200 seconds or less for the recovery rate to reach 70%.

[0019] According to a third aspect of the present disclosure, a lithium secondary battery is provided.

[0020] In the first or second aspect,

[0021] The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.0,

[0022] The pores have an average pore size of 20 to 39 nm and a maximum pore size of 48 nm or less, and

[0023] The diaphragm is characterized in that

[0024] When a tensile stress of 15 MPa is applied at 60°C for 60 seconds, the strain is 23% or less, and

[0025] It takes 190 seconds or less for the recovery rate to reach 70% after a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed.

[0026] A fourth aspect of the present disclosure provides a lithium secondary battery,

[0027] In the first or third aspect,

[0028] The polyolefin resin has a polydispersity index (PDI) of 2.6 to 3.9,

[0029] The pores have an average pore size of 21 to 38 nm and a maximum pore size of 46 nm or less, and

[0030] The diaphragm is characterized in that

[0031] When a tensile stress of 15 MPa is applied at 60°C for 60 seconds, the strain is 21% or less, and

[0032] It takes 180 seconds or less for the recovery rate to reach 70% after a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed.

[0033] A fifth aspect of the present disclosure provides a lithium secondary battery,

[0034] In the fourth aspect,

[0035] The average pore size of the pores is 22.2 to 36.1 nm,

[0036] The diaphragm is characterized in that

[0037] When a tensile stress of 15 MPa is applied at 60°C for 60 seconds, the strain is 20.1% or less, and

[0038] When a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed, it takes 178 seconds or less until the recovery rate reaches 70%.

[0039] A sixth aspect of the present disclosure provides a lithium secondary battery,

[0040] In any one of the first to fifth aspects, the polyolefin resin has a weight average molecular weight of 500,000 to 1,500,000 g / mole.

[0041] A seventh aspect of the present disclosure provides a lithium secondary battery,

[0042] In any one of the first to sixth aspects, the separator substrate includes a core portion made of a mixture of polyethylene and polypropylene, and a polyethylene skin portion provided on each of both surfaces of the core portion.

[0043] An eighth aspect of the present disclosure provides a lithium secondary battery,

[0044] In any one of the first to seventh aspects, the separator substrate is a separator substrate produced by a wet production method in which pores are formed by extracting a pore-forming agent.

[0045] A ninth aspect of the present disclosure provides a lithium secondary battery,

[0046] In any one of the first to eighth aspects, the Si-based negative electrode active material includes at least one selected from the group consisting of Si, SiO, and a Si alloy.

[0047] A tenth aspect of the present disclosure provides a lithium secondary battery,

[0048] In any one of the first to ninth aspects, the negative electrode further contains graphite.

[0049] According to an eleventh aspect of the present disclosure, in any one of the first to tenth aspects,

[0050] The separator further includes an organic / inorganic composite coating layer disposed on at least one surface of the separator substrate, wherein the organic / inorganic composite coating layer includes a crystalline binder and an amorphous binder.

[0051] According to a twelfth aspect of the present disclosure, in the eleventh aspect,

[0052] The lithium secondary battery further includes an electrolyte, and the crystalline binder and the amorphous binder each independently have a concentration gradient in a thickness direction of the organic / inorganic composite coating layer.

[0053] According to the thirteenth aspect of the present disclosure, in the twelfth aspect,

[0054] The organic / inorganic composite coating may include a first portion adjacent to the separator substrate and a second portion opposite to the first portion, and a concentration of the crystalline binder in the second portion is higher than a concentration of the crystalline binder in the first portion.

[0055] Beneficial effects

[0056] The polyolefin separator provided in the lithium secondary battery according to the present disclosure has a polyolefin resin polydispersity index (PDI), an average pore size, and a maximum pore size controlled to fall within a predetermined range, and has a strain and recovery rate controlled to be no higher than predetermined values. Therefore, the separator has improved compression resistance.

[0057] As a result, during the lamination process for manufacturing an electrode assembly having a Si-based negative electrode with a high volume expansion rate and a high hardness index, the separator thickness reduction rate or separator damage caused by applied pressure is reduced. Consequently, withstand voltage defects are reduced and capacity retention is improved. Furthermore, the process speed can be increased, leading to improved workability. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The accompanying drawings illustrate preferred embodiments of the present disclosure and, in conjunction with the detailed description given below, illustrate the principles of the present disclosure, but the scope of the present disclosure is not limited thereto. On the other hand, the shapes, sizes, proportions or ratios of the elements shown in the accompanying drawings may be exaggerated to emphasize more clearly.

[0059] Figure 1 is a SEM image of a cross section of the separator of Example 1;

[0060] Figure 2 is a SEM image of a cross section of the separator of Comparative Example 1; and

[0061] Figure 3 : is the distribution of the crystalline binder, the amorphous binder, and the inorganic particles in the cross section of the separator according to one embodiment of the present disclosure injected with the electrolyte. DETAILED DESCRIPTION

[0062] Hereinafter, the present disclosure will be described. Before giving the following detailed description of the present disclosure, it should be noted that the terms and words used in this specification and claims should not be interpreted as limited to the ordinary meaning or dictionary definition, but should be interpreted as meanings and concepts consistent with the technical ideas of the present disclosure on the basis that the inventors are able to appropriately define the concepts of the terms so as to describe their invention in the best possible way. The exemplary embodiments described herein and the configurations shown in the accompanying drawings are presented for illustrative purposes and do not exhaustively represent the technical purport of the present disclosure. Therefore, it should be understood that various equivalents and variations that can replace the exemplary embodiments and configurations will exist when this application is filed.

[0063] It will also be understood that, unless the context clearly indicates otherwise, when used in this specification, the terms “comprises,” “includes,” or “has” specify the presence of an element but do not preclude the presence or addition of one or more other elements.

[0064] In this specification, the separator has a porous property with a plurality of pores and serves as a porous ion-conductive barrier that prevents electrical connection between the negative electrode and the positive electrode in a lithium secondary battery and enables ion transport. The feature "having pores" herein means that gaseous and / or liquid fluids can pass from one side of the object to the other through the pores formed in the object and the structure connecting the pores.

[0065] In the present disclosure, the polyolefin separator can be interpreted as the separator itself or a constituent element of the separator. Therefore, if required in terms of material or function, the polyolefin separator described in the present disclosure may further include other layers on at least one surface of the separator substrate. According to one embodiment of the present disclosure, the separator may have an organic / inorganic composite coating containing inorganic particles and / or a binder resin on at least one side or both sides of the porous substrate.

[0066] In the present disclosure, lithium secondary batteries use lithium ions as ion conductors. Examples of such lithium secondary batteries include, but are not limited to, non-aqueous electrolyte secondary batteries containing liquid electrolytes, all-solid-state batteries containing solid electrolytes, lithium polymer batteries containing gel-type polymer electrolytes, and lithium metal batteries using lithium metal as the negative electrode.

[0067] Hereinafter, a lithium secondary battery according to one embodiment of the present disclosure will be described in detail.

[0068] The lithium secondary battery according to the present disclosure includes a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, wherein

[0069] The negative electrode comprises a Si-based negative electrode active material,

[0070] The separator includes a separator substrate having a plurality of pores and comprising a polyolefin resin.

[0071] The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2,

[0072] The pores have an average pore size of 20 to 40 nm and a maximum pore size of 50 nm or less, and

[0073] The diaphragm is characterized in that

[0074] When a tensile stress of 15 MPa is applied at 60°C for 60 seconds, the strain is 25% or less, and

[0075] After a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed, it takes 200 seconds or less for the recovery rate to reach 70%.

[0076] In the present disclosure, the negative electrode includes a Si-based negative electrode active material. The Si-based negative electrode active material may include at least one material selected from the group consisting of Si, SiO, and a Si alloy, but is not limited thereto. In addition to including the Si-based negative electrode active material, the negative electrode may further include 10% to 90% by weight of another negative electrode active material, such as graphite, relative to the total weight of the negative electrode active material.

[0077] Si-based negative electrode active materials, such as Si, SiO, and Si alloys, are currently being researched and developed for capacity reasons. When used as negative electrode active materials for lithium secondary batteries, the negative electrode experiences significant volume expansion, increasing the internal pressure of the battery and causing increased compressive deformation of the separator. Furthermore, compared to graphite negative electrode active materials, Si-based negative electrode active materials exhibit greater granularity, roughness, and hardness, which can cause localized damage to the separator when laminated with it.

[0078] In order to solve the problems caused by using a Si-based negative electrode active material, the present disclosure applies a compression-resistant polyolefin-based separator having the above-mentioned characteristics.

[0079] The polyolefin separator according to the present disclosure is made using a polyolefin resin as a base resin. Examples of polyolefin-based resins include, but are not limited to, polyethylene, polypropylene, polypentene, and the like. A porous separator made from such a polyolefin resin as a base resin, i.e., a separator having a plurality of pores, is advantageous in terms of imparting a closed-cell (shutdown) function at an appropriate temperature. In particular, when both polyethylene and polypropylene are included as the polyolefin resin, both closed-cell characteristics and physical properties such as mechanical strength can be improved.

[0080] Generally, the higher the molecular weight of a resin, the more advantageous it is in terms of compression resistance. Therefore, the weight average molecular weight of the polyolefin resin may be 500,000 to 1,500,000 g / mole. When different types of polyolefin resins are used in a mixed form or a multilayer separator made of different types of polyolefin resins is formed, the weight average molecular weight of the polyolefin resin is calculated by adding the weight average molecular weights while applying the content ratio of each polyolefin resin.

[0081] In addition to the above-mentioned polyolefin base resin, other resin components may be further mixed as needed. In addition to the resin components, for example, filler particles may also be included. The filler particles may be included to serve as a pressure barrier so that the thickness, pore size and porosity of the diaphragm substrate are not excessively reduced due to the high pressure applied in the lamination process described later. The filler particles may include organic filler particles or inorganic filler particles having a predetermined particle size. The composition of the filler particles is not limited as long as they exhibit a strength equal to or greater than that of the polyolefin resin.

[0082] In the present disclosure, the polydispersity index (PDI) of the polyolefin resin is 2.5 to 4.2, the average pore size of the pores is 20 to 40 nm, and the maximum pore size of the pores is 50 nm or less. That is, in the present disclosure, the polydispersity index of the polyolefin resin is low, and the average pore size and the maximum pore size are small. When these ranges are met at the same time, the compression resistance is improved. When the polydispersity index is less than 2.5, there are problems of reduced processability and reduced film uniformity. On the other hand, when the polydispersity index exceeds 4.2, there is a problem of reduced compression resistance. In addition, when the average pore size of the pores is less than 20 nm, the permeability is reduced, and the small pores are blocked by by-products generated during battery charging and discharging. When the average pore size exceeds 40 nm, the thickness of the diaphragm is uneven, thickness variations occur, and the compression resistance of the diaphragm is reduced due to local thickness variations. In addition, when the maximum pore size of the pores exceeds 50 nm, there is a problem of reduced compression resistance.

[0083] The size of the pores can be calculated from the pore size distribution measured using a capillary flow porometer. For example, first, a diaphragm to be measured is moistened with a wetting agent such as a Galwick solution, and then the air pressure on one side of the substrate is gradually increased. At this point, when the applied air pressure is greater than the capillary attraction of the wetting agent present in the pores, the wetting agent in the pores is pushed out. The pore size and distribution are measured from the air pressure and flow rate at the moment the wetting agent is pushed out of the pores. In this way, the average pore size and maximum pore size can be determined.

[0084] In this regard, the polyolefin resin may have a polydispersity index (PDI) in the range of 2.5 to 4.0, more specifically in the range of 2.6 to 3.9. Furthermore, the average pore size of the pores may be in the range of 20 to 39 nm, more specifically in the range of 21 to 38 nm, and most specifically in the range of 22.2 to 36.1 nm. Furthermore, the maximum pore size of the pores may be 48 nm or less, more specifically 46 nm or less.

[0085] On the other hand, in the present disclosure, the strain of the diaphragm substrate when a tensile stress of 15 MPa is applied at 60°C for 60 seconds is 25% or less, and the recovery time until the recovery rate reaches 70% when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed is 200 seconds or less.

[0086] When the strain under the above conditions exceeds 25% or the recovery time until the recovery rate reaches 70% exceeds 200 seconds, the compression resistance of the separator decreases after the separator is laminated with the Si-based negative electrode.

[0087] In this regard, the polyolefin separator substrate is characterized in that,

[0088] The separator substrate has a strain of 23% or less when a tensile stress of 15 MPa is applied at 60°C for 60 seconds, and a recovery time of 190 seconds or less until a recovery rate reaches 70% when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed.

[0089] More specifically, the separator substrate has a strain of 21% or less when a tensile stress of 15 MPa is applied at 60°C for 60 seconds, and a recovery time of 180 seconds or less until a recovery rate reaches 70% when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed, and

[0090] Most specifically, the separator substrate has a strain of 20.1% or less when subjected to a tensile stress of 15 MPa at 60°C for 60 seconds, and a recovery time of 178 seconds or less until the recovery rate reaches 70% when subjected to a tensile stress of 2 MPa at 70°C for 180 seconds and then removed.

[0091] The polyolefin separator substrate may be prepared by the following method, but is not limited thereto.

[0092] In one embodiment of the present disclosure, the separator can be prepared by the following steps: kneading a polyolefin resin with a diluent at a high temperature to form a single phase, performing phase separation into a polymer material and a diluent during a cooling process, and performing elongation and heat fixing treatment (wet method). In particular, the polyolefin separator can include a core made of a polymer of polyethylene and polypropylene, and a polyethylene surface layer portion laminated on both sides of the core. However, the polyolefin separator is not limited thereto.

[0093] The average pore size and the maximum pore size of the pores of the separator substrate can be easily controlled to fall within the range of the present disclosure by adjusting the mixing ratio of the diluent, the stretching ratio, and the heat treatment temperature.

[0094] In the present disclosure, the polyolefin separator substrate prepared by the above method has a thickness of 5 μm to 30 μm.

[0095] In one embodiment of the present disclosure, the separator may further include an organic / inorganic composite coating layer formed on at least one surface of the polyolefin separator substrate.

[0096] The organic / inorganic composite coating comprises a binder resin and inorganic particles and has a porous property. In one embodiment of the present disclosure, the binder resin and the inorganic particles of the organic / inorganic composite coating may be included in a weight ratio of 1:99 to 30:70. This ratio may be appropriately adjusted within this range. For example, relative to the sum of 100 weight % of the binder resin and the inorganic particles, the binder resin may be 1 weight % or more, 5 weight % or more, or 10 weight % or more, and the inorganic particles may be 80 weight % or more, 85 weight % or more, 90 weight % or more, or 95 weight % or more.

[0097] The organic / inorganic composite coating layer may be formed such that the inorganic particles are bound and integrated into a layer by a binder resin. The pores in the organic / inorganic composite coating layer are caused by empty spaces, ie, interstitial volumes, between the inorganic particles.

[0098] In one embodiment of the present disclosure, the porosity of the organic / inorganic composite coating and the porosity of the heat-resistant layer can be 30% to 70% by volume. When the porosity is 70% by volume or less, the mechanical properties can be sufficient to withstand the conditions of the pressing process during bonding to the electrode. In addition, since the surface opening rate is not too high, the porosity level is suitable for ensuring adhesion strength. On the other hand, when the porosity is 30% by volume or more, it is advantageous in terms of ion permeability.

[0099] The thickness of the organic / inorganic composite coating on either side of the separator substrate may be in the range of 1 μm to 20 μm. However, the thickness of the organic / inorganic composite coating is not particularly limited thereto. Those skilled in the art may adjust the thickness to fall within a suitable range, taking into account heat resistance or electrical resistance.

[0100] In the present disclosure, non-limiting examples of the binder resin suitable for the organic / inorganic composite coating may be any one polymer resin or a mixture of two or more selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan and carboxymethyl cellulose. However, it is not particularly limited thereto.

[0101] In one embodiment of the present disclosure, the inorganic particles that can be used in the organic / inorganic composite coating are not particularly limited, as long as they are electrochemically stable. That is, as long as they do not exceed the operating voltage range of the electrochemical element (e.g., for Li / Li +In the case of 0 to 5 V), oxidation and / or reduction reactions occur, otherwise the inorganic particles used in the present disclosure are not particularly limited.

[0102] Non-limiting examples of the inorganic particles include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT,0 <x<1,0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnium dioxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, SiC, Al(OH)3, TiO2, aluminum peroxide, zinc tin hydroxide (ZnSn(OH)6), zinc tin oxide (Zn2SnO4, ZnSnO3), antimony trioxide (Sb2O3), antimony tetroxide (Sb2O4), antimony pentoxide (Sb2O5), etc. Any one of these or a mixture of two or more of these can be used.

[0103] In addition, the average diameter D of the inorganic particles 50 There is no particular limitation, but it is preferably in the range of 0.3 μm to 1 μm to form a uniform coating and obtain a suitable porosity. 50 When the average diameter D of the inorganic particles is less than 0.3 μm, the dispersibility of the inorganic particles in the slurry prepared for manufacturing the heat-resistant layer may be reduced. 50 Above 1 μm, the thickness of the coating may be too large.

[0104] In one embodiment of the present disclosure, the organic / inorganic composite coating is prepared by the following method. First, a polymer solution is prepared by dissolving the binder resin in a suitable organic solvent. It is desired that the solubility coefficient of the solvent is similar to that of the binder polymer to be used and has a low boiling point. This is to promote uniform mixing and subsequent solvent removal. Non-limiting examples of available solvents include acetone, tetrahydrofuran, dichloromethane, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water or any mixture thereof.

[0105] Next, inorganic particles are added to the prepared polymer solution and dispersed. In the present disclosure, the content ratio of the inorganic particles to the binder is appropriately determined according to the thickness, pore size, and porosity of the heat-resistant layer to be formed as described above.

[0106] Next, the prepared slurry of inorganic particles is applied to at least one surface of the prepared diaphragm substrate and dried. The method of applying the slurry to the surface of the diaphragm substrate is not particularly limited to any one method, and conventional methods known in the art can be used. For example, various methods such as dip coating, die coating, roller coating, comma coating, or a combination thereof can be used.

[0107] For the drying process, the temperature and time conditions are appropriately set to minimize the occurrence of surface defects of the organic / inorganic composite coating. Drying can be performed using a drying auxiliary device such as a drying oven or hot air under appropriate conditions.

[0108] When the separator includes the organic / inorganic composite porous layer, damage caused by inorganic particles pressing against the surface of the separator substrate facing the organic / inorganic composite porous layer during a lamination process can be reduced.

[0109] The crystallinity herein can be expressed as the percentage of the melting enthalpy value measured by differential scanning calorimetry (DSC) relative to the melting enthalpy value of a theoretical perfect crystal (crystallinity 100%). According to the present disclosure, an amorphous adhesive refers to an adhesive having a crystallinity of 30% or less, and a crystalline adhesive refers to an adhesive having a crystallinity of more than 30%.

[0110] According to another embodiment of the present disclosure, the diaphragm further includes an organic / inorganic composite coating provided on at least one surface of the diaphragm substrate, wherein the organic / inorganic composite coating includes a crystalline adhesive and an amorphous adhesive. According to a further embodiment of the present disclosure, since the organic / inorganic composite coating includes a crystalline adhesive and an amorphous adhesive, when there is an electrolyte, the adhesion (wet adhesion) of the organic / inorganic composite coating to the electrode is ensured by the crystalline adhesive. For example, when there is no electrolyte, the crystalline adhesive and the amorphous adhesive can be randomly distributed on the surface of the diaphragm substrate. When the electrode is combined with the diaphragm, the amorphous adhesive with a relatively low glass transition temperature Tg and a relatively large surface area may be deformed due to the temperature and pressure during the combination, thereby being present in greater quantities on the surface, causing an increase in the dry adhesion to the electrode. When an electrolyte is introduced, the amorphous adhesive with high solubility and high fluidity to the electrolyte may swell and may move to the inside of the organic / inorganic composite coating or the inside of the electrode. For example, when using an organic solvent, an amorphous polymer adhesive can swell more than an adhesive with high crystallinity. Due to the expansion of the polymer adhesive, fluidity may occur, thereby allowing the polymer adhesive to move to the inside of the composite coating or the electrode. Since the crystalline binder mainly exists on the surface of the organic / inorganic composite coating, wet adhesion of the separator to the electrode may occur due to the crystalline binder when an electrolyte is present.

[0111] Specifically, the lithium secondary battery further comprises an electrolyte, and the crystalline binder and the amorphous binder each independently have a concentration gradient in the thickness direction of the organic / inorganic composite coating. For example, a slurry can be applied to at least one surface of the diaphragm substrate, and then the diaphragm substrate can be transferred to a heating zone so that the slurry can be dried to form an organic / inorganic composite coating. The diaphragm substrate applied with the slurry can be dried while moving through a heating zone heated to a predetermined temperature at a predetermined speed to form a diaphragm with an organic / inorganic composite coating. According to one example, the heating temperature of the heating zone can be in the range of 45°C to 65°C. The porous diaphragm substrate can be moved through the heating zone at a speed of 25 to 150 meters per minute, specifically 40 to 60 meters per minute. The drying conditions of the organic / inorganic composite coating can be determined based on the morphological differences between the binder and the inorganic particles. The drying conditions can determine the concentration gradients of the binder and the inorganic particles distributed in the thickness direction of the coating.

[0112] According to a further embodiment of the present disclosure, the organic / inorganic composite coating may include a first portion adjacent to the diaphragm substrate and a second portion opposite to the first portion. Specifically, the concentration of the crystalline binder in the second portion may be higher than the concentration of the crystalline binder in the first portion. The first portion adjacent to the diaphragm substrate may be relatively close to the diaphragm substrate relative to the middle surface of the organic / inorganic composite coating cut along the surface direction. Specifically, the first portion may be in contact with the diaphragm substrate. For example, the second portion may be closer to the electrode than the first portion relative to the middle surface of the organic / inorganic composite coating cut along the surface direction. Specifically, the second portion may be in contact with the electrode. According to one example, the average thickness of the first portion may be in the range of 0.1 to 10 μm, 0.1 to 5 μm, 0.1 to 1.5 μm, 0.1 to 1.0 μm, 0.1 to 0.5 μm or 0.1 to 0.3 μm. The average thickness of the second portion may be the same as the average thickness of the first portion.

[0113] According to still further embodiments of the present disclosure, the crystalline binder and the amorphous binder may each independently include an aqueous emulsion-type binder.

[0114] For example, the crystalline adhesive may be a polyvinylidene fluoride-based adhesive, and the amorphous adhesive may be an acrylic-based adhesive.

[0115] Specifically, the crystalline binder may be one or more selected from the group consisting of polyvinylidene fluoride, polyhexafluoropropylene, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-TCE), and polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE). More particularly, the crystalline binder may be a copolymer containing polyvinylidene fluoride.

[0116] According to yet further embodiments of the present disclosure, the average particle size D of the crystalline binder is 50 It may be in the range of 200 nm or more, preferably in the range of 200 nm to 350 nm.

[0117] According to still further embodiments of the present disclosure, the weight average molecular weight (M w ) can be in the range of 10,000 to 10,000,000 g / mole. The density of the crystalline binder can be in the range of 1.1 to 1.5 g / cm 3 within the range.

[0118] According to still further embodiments of the present disclosure, the glass transition temperature of the crystalline binder may be in the range of 80° C. to 200° C., in the range of 80° C. to 150° C., in the range of 110° C. to 145° C., in the range of 80° C. to 150° C. The crystalline binder may exhibit high oxidation resistance to the electrolyte.

[0119] According to yet further embodiments of the present disclosure, the amorphous adhesive may include an acrylic polymer, specifically a copolymer including acrylic acid / acrylate as a monomer.

[0120] If necessary, the amorphous binder may further include one or more binder materials selected from the group consisting of styrene-butadiene rubber, nitrile rubber, acrylonitrile-butadiene rubber, and acrylonitrile-butadiene-styrene rubber.

[0121] The average particle size D of the amorphous binder 50 It can be above 350nm. 50The average particle size of the amorphous binder is preferably in the range of 350nm to 500nm, more preferably in the range of 350nm to 450nm. If the average particle size of the amorphous binder exceeds 500nm, in the absence of an electrolyte, the mobility toward the interface between the electrode and the coating is reduced, and the adhesion between the electrode and the diaphragm is reduced, making it difficult to form an electrode assembly. In addition, when forming a coating, it is difficult to obtain a uniform coating quality. In addition, since the number of binder particles decreases for the same binder content, the number of contact points between adjacent inorganic particles decreases. Therefore, the possibility of inorganic material detachment increases, the gap volume decreases, and the resistance decreases.

[0122] The amorphous binder has a larger average particle size D than the crystalline binder 50 The amorphous adhesive has a larger contact area with the electrode than the crystalline adhesive, thereby providing better dry adhesion.

[0123] The weight average molecular weight of the amorphous binder may be in the range of 10,000 to 10,000,000 g / mole. The density of the amorphous binder may be in the range of 0.5 to 1.1 g / cm 3 within the range.

[0124] The glass transition temperature of the amorphous binder may be in the range of 40°C or more, preferably in the range of 45°C to 60°C, and more preferably in the range of 48°C to 60°C. In the past, acrylic binder particles having a relatively small particle size of 100 to 150 nm were used. Typically, acrylic binder particles contain diene butadiene rubber and have a low glass transition temperature. Due to the low glass transition temperature and the presence of double bonds, the acrylic binder particles react with the electrolyte, thereby generating gas. Therefore, instead of using an acrylic binder having a glass transition temperature below 0°C, the amorphous binder may use a copolymer having an increased glass transition temperature produced by copolymerization of an acrylic monomer and styrene, thereby preventing side reactions with the electrolyte. In addition, when the glass transition temperature is within the above range, the amorphous binder maintains its shape even during the initial drying of the coating and exhibits dry adhesion to the electrode.

[0125] In a specific example, the crystalline binder may have characteristics similar to those of a binder for a positive electrode, and the amorphous binder may have characteristics similar to those of a binder for a negative electrode.

[0126] Specifically, the positive electrode can use a polyvinylidene fluoride homopolymer with a crystallinity of 95% or more. For example, homopolymer PVDF, which can be used as a crystalline binder, can be included in the positive electrode mixture. The negative electrode can use an aqueous emulsion type amorphous acrylic binder, which can be included in the negative electrode mixture.

[0127] As described above, using a binder with similar characteristics to those used for the negative and positive electrodes for the separator coating improves adhesion between the negative electrode and separator, and between the positive electrode and separator. For example, the same binder can be used for the coating and the corresponding negative and positive electrodes. Alternatively, the crystalline binder can have the same polymer backbone as the positive electrode binder, and the amorphous binder can have the same polymer backbone as the negative electrode binder. For example, the binder used for the negative or positive electrode can be a PVdF homopolymer, while the binder used for the separator can be a PVdF-HFP binder with a 10% HFP substitution ratio.

[0128] According to still further embodiments of the present disclosure, the weight ratio of the crystalline binder to the amorphous binder may be in the range of 1:9 to 9:1, more particularly in the range of 3:7 to 7:3. When the weight ratio of the crystalline binder to the amorphous binder relative to the total weight of the crystalline binder and the amorphous binder falls within the above numerical range, both the adhesion in the absence of an electrolyte, i.e., dry adhesion, and the adhesion in the presence of an electrolyte, i.e., wet adhesion, are good.

[0129] According to still further embodiments of the present disclosure, the wet adhesion of the separator to the electrode may be in the range of 10 to 15 gf / 20 mm. For example, the wet adhesion of the separator to the electrode may be measured by cutting each of the electrode and the separator into sections having predetermined sizes, placing each section into a bag, and injecting an electrolyte into the bag; pressing the bag containing the electrolyte to prepare a specimen for measuring wet adhesion; and performing a 90° peel test at a speed of 200 mm / min using an Instron UTM apparatus to measure the adhesion of the separator to the electrode.

[0130] The diaphragm prepared by the above method is placed between the negative electrode and the positive electrode containing the Si-based negative electrode active material, and heat and / or pressure is applied to the stacked structure so that the electrode and the diaphragm are bonded. This process is called a lamination process for forming an electrode assembly. In one embodiment of the present disclosure, a rolling device comprising a pair of pressure rollers can be used to perform the lamination process. That is, the negative electrode, the diaphragm, and the positive electrode are stacked in sequence, and the stacked structure is inserted between the pressure rollers to achieve interlayer bonding. In this case, the lamination process can be performed by hot pressing.

[0131] In the present disclosure, the negative electrode may include a negative electrode current collector and a negative electrode active material layer on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a Si-based negative electrode active material, a binder resin, and optionally a conductive material. As the negative electrode active material, the Si-based negative electrode active material may be used alone or in combination with a carbon-based negative electrode active material such as graphite.

[0132] The positive electrode comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer comprises a positive electrode active material, a conductive material and a binder resin. The positive electrode active material may be one material or a mixture of two or more materials selected from the following: a layered compound, for example, a lithium manganese composite oxide (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) or a compound substituted with one or more transition metals; a compound having the chemical formula Li 1+x Mn 2-x Lithium manganese oxides represented by O4 (wherein x is 0 to 0.33) such as LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxides such as Li2CuO2; vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7, etc.; 1-x M x Ni-site lithium nickel oxide represented by O2 (wherein M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3); represented by the chemical formula LiMn 1-x M x Lithium manganese composite oxides represented by LiMn2O4 (wherein M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (wherein M = Fe, Co, Ni, Cu or Zn); LiMn2O4, in which Li ions are partially replaced by alkaline earth metal ions; and disulfide compounds; and Fe2(MoO4)3.

[0133] In a specific embodiment of the present disclosure, the conductive material may be one or a mixture of two or more selected from the group consisting of graphite, carbon black, carbon fiber or metal fiber, metal powder, conductive whisker, conductive metal oxide, activated carbon, and polyphenylene derivatives. More specifically, the conductive material may be one or a mixture of two or more selected from the group consisting of natural graphite, synthetic graphite, super-p, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, Danka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide.

[0134] As the binder resin, a polymer commonly used in the art for electrodes can be used. Non-limiting examples of such binder resins include, but are not limited to, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, etc.

[0135] As described above, the prepared electrode assembly is housed in an appropriate case, and an electrolyte is injected into the case to manufacture a battery cell.

[0136] In the present disclosure, the electrolyte comprises + B - "Salts of structure, wherein A + Including alkali metal cations such as Li + 、Na + , K + or a combination thereof, B - Including anions such as PF6 - 、BF4 - 、Cl - Br - , I - 、ClO4 - 、AsF6 - 、CH3CO2 - CF3SO3 - 、N(CF3SO2)2 - 、C(CF2SO2)3 - or a combination thereof. The salt may be dissolved or dissociated in an organic solvent, wherein the organic solvent is selected from one or a mixture of two or more of the following: propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC) and gamma-butyrolactone (γ-butyrolactone), but is not limited thereto.

[0137] In addition, the present disclosure provides a battery module, a battery pack including the battery module, and a device including the battery pack as a power source. The battery module includes a battery including the electrode assembly as a unit cell. Specific examples of the device include: a power tool driven by a battery-powered motor; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); and electric two-wheeled vehicles, including electric bicycles (E-bikes), electric scooters (E-scooters), electric golf carts, and power storage systems, but examples are not limited thereto.

[0138] Hereinafter, the present disclosure will be described in detail with reference to embodiments. The embodiments of the present disclosure described later can be changed into various forms, so that the scope of the present disclosure should not be interpreted as being limited to the embodiments described later. Providing embodiments enables those skilled in the art to more easily understand the present disclosure.

[0139] [Example 1]

[0140] Preparation of diaphragm

[0141] Preparation steps of the diaphragm:

[0142] Polyethylene (weight-average molecular weight of 900,000 g / mol) and polypropylene (weight-average molecular weight of 350,000 g / mol) were mixed in a weight ratio of 93:7 to form the core. A polyethylene resin (weight-average molecular weight of 900,000 g / mol) was coextruded to form skin layers on both surfaces of the core. A polyolefin separator substrate (total thickness: approximately 9 μm, core thickness: 7 μm, combined thickness of both skin layers: 2 μm) was prepared using a wet process while setting the elongation temperature to 105°C and the heat setting temperature to 130°C.

[0143] Preparation steps of the composition for forming an organic / inorganic composite coating:

[0144] The average particle size (D 50 ) is 500nm and the density is 4g / cm 3 , aspect ratio of 1.3 and BET specific surface area of 8m 2 / g alumina product (Sumitomo, AES11) was used for inorganic particles. The average particle size (D 50 ) of 200 nm fluorine-based aqueous dispersion (Arkema, Aquatec 9530, 30% by weight of solid) was used as the crystalline polymer. 50 ) is 350 nm and has a density of 1 g / cm 3An acrylic acid aqueous dispersion (LGC SA22, 30% solids) was used as the amorphous polymer. Carboxymethyl cellulose (Gl Chem SG-L02) was used as the dispersant. These components were mixed in a weight ratio of 79:18:2:1 (inorganic particles: crystalline polymer: amorphous polymer: dispersant) and added to a solvent (water) to prepare an organic / inorganic composite coating composition with a solids content of 30% by weight (viscosity at 20°C = 50 cP).

[0145] Steps for forming an organic / inorganic coating on each side of the separator substrate:

[0146] The organic / inorganic composite coating composition was applied to both surfaces of the polyolefin separator substrate using a rod coater. The polyolefin separator substrate, with the organic / inorganic composite coating composition applied to both surfaces, was moved at a speed of 40 m / min and introduced into seven heating zones at temperatures of 65°C, 65°C, 60°C, 55°C, 50°C, 45°C, and 45°C, respectively. The coating was dried for 2 to 3 minutes to form an organic / inorganic composite coating layer (each layer having a thickness of 3 μm). Thus, a separator was obtained.

[0147] Preparation of positive electrode

[0148] The positive electrode active material (LiNi 0.8 Mn 0.1 Co 0.1 A positive electrode active material slurry was prepared by mixing a conductive material (carbon black), a dispersant, and a binder resin (a mixture of PVDF-HFP and PVDF) with water at a weight ratio of 97.5:0.7:0.14:1.66. The concentration of the components other than water was 50% by weight. This slurry was then applied to the surface of an aluminum thin film (10 μm thick) and dried to produce a positive electrode having a positive electrode active material layer (120 μm thick).

[0149] Preparation of negative electrode

[0150] A Si-based negative electrode active material (20% SiO, 80% graphite), a conductive material (carbon black), a dispersant, and a binder resin (a mixture of PVDF-HFP and PVDF) were mixed with water at a weight ratio of 97.5:0.7:0.14:1.66 to prepare a negative electrode active material slurry. The concentration of the components other than water was 50% by weight. This slurry was then applied to the surface of a 10 μm thick copper film and dried to produce a negative electrode having a 120 μm thick negative electrode active material layer.

[0151] Lamination process

[0152] An electrode assembly was obtained by inserting one of the separators of the example and the comparative example between the positive electrode and the negative electrode and performing a lamination process. The lamination process was performed using a hot press at 70° C. and 5.2 MPa for 10 seconds.

[0153] Electrolyte injection process

[0154] An electrolyte having a composition of LiPF6 1.0 M, EC / EMC=3 / 7, and VC=2.0 was injected into each electrode assembly to manufacture a single cell.

[0155] [Example 2]

[0156] A single cell was manufactured in the same manner as in Example 1, except that the elongation temperature was adjusted to 120°C and the heat fixing temperature was adjusted to 115°C when the separator was manufactured.

[0157] [Example 3]

[0158] A single cell was manufactured in the same manner as in Example 1, except that the heat fixing temperature was adjusted to 115° C. when the separator was manufactured.

[0159] [Comparative Example 1]

[0160] A single cell was manufactured in the same manner as in Example 1, except that a mixture of polyethylene (average molecular weight 900,000 g / mol) and polypropylene (average molecular weight 350,000 g / mol) in a weight ratio of 97:3 was used as a resin to form the surface layer portion when preparing the separator, and the elongation temperature was adjusted to 120°C.

[0161] [Comparative Example 2]

[0162] A single cell was manufactured in the same manner as in Example 1, except that, when preparing the separator, a mixture of polyethylene (average molecular weight 900,000 g / mole) and polypropylene (average molecular weight 350,000 g / mole) in a weight ratio of 97:3 was used as a resin to form the surface layer portion, and the heat fixing temperature was adjusted to 115°C.

[0163] [Comparative Example 3]

[0164] A single cell was manufactured in the same manner as in Example 1, except that a mixture of polyethylene (average molecular weight 900,000 g / mol) and polypropylene (average molecular weight 350,000 g / mol) in a weight ratio of 97:3 was used as the resin to form the surface layer portion when preparing the separator.

[0165] [Comparative Example 4]

[0166] A single cell was manufactured in the same manner as in Example 1, except that, when preparing the separator, a mixture of polyethylene (average molecular weight 900,000 g / mol) and polypropylene (average molecular weight 350,000 g / mol) in a weight ratio of 97:3 was used as a resin to form the surface layer portion, the elongation temperature was adjusted to 120°C, and the heat fixing temperature was adjusted to 115°C.

[0167] [Experimental Example 1: Evaluation of Physical Properties of Separator Substrate]

[0168] For each polyolefin membrane substrate manufactured, PDI, average pore size, maximum pore size, strain when a tensile stress of 15 MPa is applied at 60°C for 60 seconds (referred to as recovery strain), and the time required until the recovery rate reaches 70% when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed (referred to as recovery time) are shown in Table 1 below.

[0169] [Formula 1: PDI measurement]

[0170] PDI = (weight average molecular weight) / (number average molecular weight)

[0171] In this case, the values of the weight average molecular weight and the number average molecular weight are obtained by subjecting a separator cut into a predetermined size to gel permeation chromatography (GPC) analysis.

[0172] [Measurement of average pore size and maximum pore size]

[0173] The average pore size and the maximum pore size are measured by pore size distribution using the capillary flow porosimetry (CFP) method.

[0174] [Measurement of strain when a tensile stress of 15 MPa is applied for 60 seconds at 60°C]

[0175] The strain was measured by dynamic mechanical analysis creep evaluation by applying a stress of 15 MPa at 60°C for 60 seconds.

[0176] [Measurement of the time required for the recovery rate to reach 70% when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed]

[0177] Dynamic mechanical analysis was used to measure the time required for the recovery rate to reach 70% after a stress of 2 MPa was applied at 70°C for 180 seconds.

[0178] [Table 1]

[0179]

[0180] [Experimental Example 2: Evaluation of Single Cell Characteristics]

[0181] [Capacity retention rate]

[0182] The cell capacity was measured by performing 200 cycles of 2C / 2C charge and discharge and 3 cycles of 0.33C charge and discharge at 25° C. The change in cell capacity before and after evaluation was calculated.

[0183] [Withstand voltage evaluation pass rate]

[0184] Ten of the manufactured cells were tested under conditions of 50 V and <0.5 mA (light on: 0 s, time: 0.1 s), and it was confirmed whether the cells were defective.

[0185] The measurement results are shown in Table 2 below.

[0186] [Table 2]

[0187]

[0188] [Example 4: Preparation of a separator comprising an organic / inorganic composite coating composed only of an amorphous polymer]

[0189] A separator was manufactured in the same manner as in Example 1, except that the weight ratio was changed from 79:18:2:1 (inorganic particles:crystalline polymer:amorphous polymer:dispersant) to 79:20:1 (inorganic particles:amorphous polymer:dispersant).

[0190] [Experimental Example 3: Measurement of Wet Adhesion Force of Separator-Electrode Assembly Samples According to Examples 1 and 4]

[0191] The positive electrode of Example 1 and the separators produced by the methods of Examples 1 and 4 were each cut into 20 mm widths, placed in bags, and a carbonate-based electrolyte was injected into the bags. Two separate separator-electrode assembly samples for wet adhesion measurement were prepared by pressing the electrolyte-containing bags under conditions of 5 kgf, 70°C, and 4 minutes. To measure the adhesion of the separator to the positive electrode, a 90° peel test was performed at a speed of 200 mm / min using an Instron UTM device.

[0192] [Table 3]

[0193]

[0194] Referring to Table 3, compared with Example 4, in the case of the membrane-electrode assembly according to Example 1, since the organic / inorganic composite coating contains both a crystalline binder and an amorphous binder, the adhesion of the membrane to the electrode in a wet state in the presence of an electrolyte is increased to a suitable level of 10 to 15 gf / 20 mm.

Claims

1. A lithium secondary battery comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, wherein the negative electrode comprises a Si-based negative electrode active material, The separator includes a separator substrate having a plurality of pores and comprising a polyolefin resin. The polyolefin resin has a polydispersity index of 2.5 to 4.2, and The average pore size of the pores is 20 nm to 40 nm and the maximum pore size is 50 nm or less, The separator substrate has a strain of 25% or less when a tensile stress of 15 MPa is applied at 60°C for 60 seconds, and takes 200 seconds or less until a recovery rate reaches 70% after a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then removed. The polydispersity index refers to the weight average molecular weight / number average molecular weight of the polyolefin resin. The separator substrate includes a core portion made of a mixture of polyethylene and polypropylene, and a polyethylene surface layer portion provided on each of both surfaces of the core portion.

2. The lithium secondary battery according to claim 1, wherein the polydispersity index of the polyolefin resin is 2.5 to 4.0, The pores have an average pore size of 20 nm to 39 nm and a maximum pore size of 48 nm or less, and The separator substrate has a strain of 23% or less when subjected to a tensile stress of 15 MPa at 60°C for 60 seconds, and a recovery time of 190 seconds or less until the recovery rate reaches 70% when subjected to a tensile stress of 2 MPa at 70°C for 180 seconds and then removed.

3. The lithium secondary battery according to claim 1, wherein the polydispersity index of the polyolefin resin is 2.6 to 3.9, The pores have an average pore size of 21 nm to 38 nm and a maximum pore size of 46 nm or less, and The separator substrate has a strain of 21% or less when subjected to a tensile stress of 15 MPa at 60°C for 60 seconds, and a recovery time of 180 seconds or less until the recovery rate reaches 70% when subjected to a tensile stress of 2 MPa at 70°C for 180 seconds and then removed.

4. The lithium secondary battery according to claim 3, wherein the average pore size of the pores is 22.2 nm to 36.1 nm, and The separator substrate has a strain of 20.1% or less when subjected to a tensile stress of 15 MPa at 60°C for 60 seconds, and a recovery time of 178 seconds or less until the recovery rate reaches 70% when subjected to a tensile stress of 2 MPa at 70°C for 180 seconds and then removed. 5 . The lithium secondary battery according to claim 1 , wherein the polyolefin resin has a weight average molecular weight of 500,000 g / mol to 1,500,000 g / mol.

6. The lithium secondary battery according to claim 1, wherein the separator further comprises an organic / inorganic composite coating on at least one surface of the separator substrate, and The organic / inorganic composite coating comprises a crystalline binder and an amorphous binder.

7. The lithium secondary battery according to claim 6, wherein the lithium secondary battery further comprises an electrolyte, The crystalline binder and the amorphous binder each independently have a concentration gradient in a thickness direction of the organic / inorganic composite coating.

8. The lithium secondary battery according to claim 6, wherein the organic / inorganic composite coating comprises a first portion adjacent to the separator substrate and a second portion opposite to the first portion, and The concentration of the crystalline binder in the second portion is higher than the concentration of the crystalline binder in the first portion. 9 . The lithium secondary battery according to claim 1 , wherein the Si-based negative electrode active material comprises at least one selected from the group consisting of Si, SiO, and a Si alloy. 10 . The lithium secondary battery according to claim 1 , wherein the negative electrode further comprises graphite.

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