Separator for lithium secondary battery, method for producing the same, and lithium secondary battery including the same

Abstract

To provide a separator for a lithium secondary battery that has an overhang area that inhibits thermal shrinkage after impregnation in an electrolyte and exposure to heat.SOLUTION: The present invention relates to a separator for a lithium secondary battery, a manufacturing method thereof, an electrode assembly for a lithium secondary battery including the same, and a lithium secondary battery including the same, and the separator for the lithium secondary battery includes a first region and a second region located at both ends of the first region, the second region satisfying the relationships of the following Equations 1 and 2. Thickness increase rate of the second region>MD heat shrinkage rate of the second region [Equation 1] Thickness increase rate of the second region>TD heat shrinkage rate of the second region [Equation 2].SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2024-0072491, filed on June 3, 2024, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a separator for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. [Background technology]

[0003] Recently, with the rapid spread of battery-powered electronic devices such as mobile phones, laptop computers, and electric vehicles, the demand for high-energy-density, high-capacity secondary batteries has been increasing rapidly. As a result, research and development efforts to improve the performance of lithium secondary batteries have been actively conducted.

[0004] A lithium secondary battery is a battery that includes a cathode and an anode, each containing an active material capable of intercalating and deintercalating lithium ions, and an electrolyte. Electrical energy is generated through oxidation and reduction reactions that occur when lithium ions are intercalated and deintercalated at the cathode and anode. Summary of the Invention [Problem to be solved by the invention]

[0005] One embodiment provides a separator for a lithium secondary battery having an overhang region that inhibits thermal shrinkage after impregnation in an electrolyte and exposure to heat.

[0006] Another embodiment provides a method for manufacturing the separator for a lithium secondary battery.

[0007] Another embodiment provides a lithium secondary battery including the separator for a lithium secondary battery. [Means for solving the problem]

[0008] One embodiment provides a separator for a lithium secondary battery, comprising a first region and second regions located at both ends of the first region, wherein the second regions satisfy the relationships of the following formulas 1 and 2: Thickness increase rate of the second region > MD heat shrinkage rate of the second region [Equation 1] Thickness increase rate of the second region > TD thermal shrinkage rate of the second region [Equation 2] (In the above formulas 1 and 2, the MD heat shrinkage rate of the second region, the TD heat shrinkage rate of the second region, and the thickness increase rate of the second region are heat shrinkage rates measured after the separator is immersed in an electrolyte solution at 140°C for 1 hour.)

[0009] Another embodiment provides a method for manufacturing a separator for a lithium secondary battery, including pressurizing only a portion of a separator film that forms a second region to manufacture the separator for a lithium secondary battery.

[0010] Yet another embodiment provides a lithium secondary battery including: a positive electrode; a negative electrode; and the lithium secondary battery separator positioned between the positive electrode and the negative electrode. [Effects of the Invention]

[0011] The separator for a lithium secondary battery according to one embodiment has an overhang region that suppresses thermal shrinkage after impregnation in an electrolyte and exposure to heat, thereby preventing short circuits between the separator and electrodes. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a plan view illustrating an overhang region of a lithium secondary battery separator. [Figure 2] FIG. 2 is a cross-sectional view of a separator for a lithium secondary battery according to one embodiment. [Figure 3] FIG. 3 is a cross-sectional view of a separator for a lithium secondary battery according to one embodiment. [Figure 4]FIG. 4 is a cross-sectional view of an electrode assembly for a lithium secondary battery according to an embodiment. [Figure 5] FIG. 5 is a photograph of the separator after pressure treatment (left side) and before pressure treatment (right side). [Figure 6] FIG. 6 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 7] FIG. 7 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 8] FIG. 8 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. [Figure 9] FIG. 9 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail, but these are presented by way of example only and do not limit the present invention, which is defined only by the scope of the claims that follow.

[0014] Unless otherwise specified in this specification, when a layer, film, region, plate, or other part is described as being "on" another part, this includes not only when it is "directly on" the other part, but also when there is another part in between.

[0015] Unless otherwise specified herein, singular references can also include plural references, and unless otherwise specified, "A or B" can mean "including A, including B, or including A and B."

[0016] As used herein, "combinations thereof" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0017] Unless otherwise defined herein, particle size may refer to the average particle size. Furthermore, particle size refers to the average particle size (D50), which refers to the diameter of particles with a cumulative volume of 50% by volume in a particle size distribution. The average particle size (D50) can be measured by methods well known to those skilled in the art, such as using a particle size analyzer or a transmission electron microscope (TEM) or scanning electron microscope (SEM) image. Alternatively, the average particle size (D50) can be measured using a measuring device that utilizes dynamic light scattering, and data analysis can be performed to count the number of particles in each particle size range, followed by calculation. Alternatively, the average particle size (D50) can be measured using a laser diffraction method. When measuring by the laser diffraction method, more specifically, the particles to be measured are dispersed in a dispersion medium, then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac's MT 3000), and irradiated with ultrasonic waves of approximately 28 kHz at an output of 60 W. The average particle size (D50) based on 50% of the particle size distribution in the measuring device can then be calculated.

[0018] In this specification, the "thermal shrinkage rate" is measured on 10 randomly selected samples of the separator and then calculated as an average value. In this specification, each "thermal shrinkage rate" may be a value measured by the method in the experimental example below.

[0019] The "thickness increase rate" herein is determined by measuring 10 times at any position in the overhang region of 10 randomly selected samples of the separator, and calculating the average value. Each "thickness increase rate" herein may be a value measured by the method in the experimental example below.

[0020] In this specification, the "MD (machine direction)" and "TD (transverse direction)" of the separator may be substantially the same directions as the MD and TD of the porous substrate of the separator, respectively.

[0021] The following description will be focused on lithium secondary batteries, but the present invention can also be applied to secondary batteries other than lithium secondary batteries. According to one embodiment, the separator for a lithium secondary battery includes a first region and a second region located at both ends of the first region, the second region being located at both ends of the first region in the width direction of the separator.

[0022] In one embodiment, the first and second regions may be integrally formed, where "integrally formed" means that the first and second regions are not adhered by an adhesive or bonding layer and are not easily separated by physical force.

[0023] According to one embodiment, the second region may be an overhang region in an electrode assembly for a lithium secondary battery. The overhang region may refer to a region of the separator that does not contact the electrode.

[0024] The overhang region will be described with reference to FIGS.

[0025] 1 and 2 are plan views illustrating an overhang region of a separator for a lithium secondary battery. Fig. 1 is a plan view showing an electrode 40 partially stacked on a separator 30. Fig. 2 is a plan view showing an electrode 40 stacked on a separator 30, extending in one direction.

[0026] 1 and 2, the overhang region refers to the region of the separator 30 that is not in contact with the electrode (positive electrode or negative electrode) 40.

[0027] Because the overhang region is not in contact with the electrode, when the battery is exposed to a high temperature environment, the boundary between the electrode and the overhang region may gradually shorten due to thermal shrinkage. In particular, since the distance between both ends of the separator and the leading edge of the electrode is short as shown in Figures 1 and 2, when the battery is exposed to a high temperature environment, the boundary between the electrode and the overhang region may gradually shorten due to shrinkage, causing an internal short circuit and potentially affecting the safety and reliability of the battery.

[0028] In a separator according to one embodiment, the second region satisfies the relationships of Equation 1 and Equation 2 below: Thickness increase rate of the second region > MD heat shrinkage rate of the second region [Equation 1] Thickness increase rate of the second region > TD thermal shrinkage rate of the second region [Equation 2] (In Equation 1 and Equation 2, the MD heat shrinkage rate of the second region, the TD heat shrinkage rate of the second region, and the thickness increase rate of the second region are the heat shrinkage rates measured after the separator is immersed in an electrolyte at 140°C for 1 hour.)

[0029] As described below in the method for manufacturing a separator for a lithium secondary battery, the second region is manufactured by pressurizing the separator film. The separator film may include only a porous substrate, or may include a coating layer formed on one or both sides of the porous substrate.

[0030] The pressure treatment applies a predetermined pressure to the porous substrate in the thickness direction. Therefore, the pressure treatment can increase the residual stress in the thickness direction in the MD and TD due to the pressure applied to the porous substrate. This further increases the degree to which the residual stress in the thickness direction in the MD and TD of the second region is relieved when the separator is impregnated in an electrolyte and exposed to heat. As shown in Equations 1 and 2, this can increase the thickness increase rate of the second region compared to the MD heat shrinkage rate and TD heat shrinkage rate of the second region, respectively.

[0031] When a battery is left at high temperatures, it is preferable that the MD and TD shrinkage rates, especially the TD shrinkage rate, of the overhang region of the separator be low to prevent internal short circuits. Therefore, when the second region satisfies Equations 1 and 2, the MD and TD thermal shrinkage rates of the overhang region are significantly reduced when the battery is exposed to high temperatures, thereby preventing internal short circuits and improving safety.

[0032] In one embodiment, the second region can have a thickness increase of 5-70%, for example, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70%, or 12-57%.

[0033] In one embodiment, the second region has a lower TD heat shrinkage rate than the MD heat shrinkage rate, which may be advantageous for preventing internal short circuits and improving safety when the battery is exposed to high temperatures. For example, the second region may have a TD heat shrinkage rate of 3 to 20%, e.g., 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20%, or 5 to 15%, and an MD heat shrinkage rate of 5 to 25%, e.g., 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25%, or 7 to 20%.

[0034] In one embodiment, the first region may have a different thickness increase rate and heat shrinkage rate relative to the second region.

[0035] The second region may have a smaller thickness than the first region because the second region is manufactured by a pressure process.

[0036] According to one embodiment, the thickness of the second region may be 30 to 90% of the thickness of the first region, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90%, or 60 to 70%. This range prevents the separator from breaking due to an excessively thin second region, prevents rupture at the boundary between the first and second regions, and prevents problems such as an excessively thick second region hindering the effect of reducing the MD and TD thermal shrinkage.

[0037] According to one embodiment, the thickness of the first region may be 1 to 100 μm, for example, 5 to 20 μm, and the thickness of the second region may be 0.3 to 90 μm, for example, 3.5 to 18 μm.

[0038] FIG. 3 is a cross-sectional view of a separator for a lithium secondary battery according to one embodiment.

[0039] 3, a separator 30 for a lithium secondary battery may include a first region 31 and a second region 32 integrally formed with the first region 31 and positioned at both ends of the first region 31. The thickness of the second region 32 may be thinner than the thickness of the first region 31.

[0040] According to one embodiment, the width (L11 or L12) of the second region 32 of the overall width (L21 + L11 + L12) of the separator 30 may be, for example, more than 0% and 10% or less, for example, 1 to 5%. Within this range, the width L21 of the first region 31 facing the electrode is widened to improve the economic efficiency of the battery, and the thermal shrinkage rate of the second region is reduced to reduce internal short circuits when the battery is exposed to high temperatures.

[0041] According to one embodiment, the first region may be a region facing the electrode.

[0042] According to one embodiment, the first and second regions may have different hazes. This is because the second region is manufactured by a pressure treatment described below, which closes the pores of the porous substrate and makes it transparent, while the first region is not pressure-treated, and the pores cause light scattering, making it opaque. This can be seen in Figure 4.

[0043] Figure 5 is a photograph of the separator after pressure treatment (left side) and before pressure treatment (right side). Referring to Figure 5, it can be seen that the separator was opaque due to high haze before pressure treatment, but after pressure treatment, the haze was reduced and the separator became more transparent than before pressure treatment.

[0044] According to one embodiment, the separator may consist solely of a porous substrate.

[0045] According to another embodiment, the separator may include a porous substrate and a heat-resistant layer formed on at least one surface of the porous substrate.

[0046] According to another embodiment, the separator may include a porous substrate, a heat-resistant layer formed on at least one surface of the porous substrate, and an adhesive layer formed on one surface of the heat-resistant layer.

[0047] According to another embodiment, the separator may include a porous substrate and a heat-resistant adhesive layer formed on at least one surface of the porous substrate.

[0048] The porous substrate may be a substrate having a large number of pores and typically used in electrochemical devices. The porous substrate may be, but is not limited to, any one polymer selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ether ketone, polyaryl ether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a polymer membrane formed from a copolymer or mixture of two or more of these.

[0049] The porous substrate may be, for example, a polyolefin-based substrate containing polyolefin. Polyolefin-based substrates have excellent shutdown function and can contribute to improving the safety of the battery. The polyolefin-based substrate may be selected from, for example, a polyethylene single film, a polypropylene single film, a polyethylene / polypropylene double film, a polypropylene / polyethylene / polypropylene triple film, and a polyethylene / polypropylene / polyethylene triple film. Furthermore, the polyolefin-based substrate may contain a non-olefin resin in addition to the olefin resin, or may contain a copolymer of an olefin and a non-olefin monomer.

[0050] The porous substrate can have a thickness of 1 μm to 40 μm, for example, 1 μm to 30 μm, 1 μm to 20 μm, or 5 μm to 15 μm.

[0051] According to one embodiment, the porous substrate may have a porosity of 5 to 95%. For example, the porous substrate may have a porosity of 30 to 50%, e.g., 30 to 40%. Within this range, when subjected to the pressure treatment described below, the residual stress in the thickness direction can be further increased, making it easier to achieve formulas 1 and 2.

[0052] The "porosity" may be a value measured by a conventional method known to those skilled in the art. For example, a porous substrate is cut into samples with a width x length (10 cm x 10 cm) and the volume (cm) of each sample is measured. 3 ) and mass (g), and the volume, mass, and density (g / cm 3 ) of the porous substrate were calculated. 3 ) and the porosity was calculated using the following equation 3.

[0053] Porosity (%) = (volume - mass / sample density) / volume x 100 [Equation 3]

[0054] (Example: Density of sample = Density of polyethylene)

[0055] According to one embodiment, the porous substrate may be an MD-stretched film, a TD-stretched film, or an MD- and TD-stretched film.

[0056] According to one embodiment, the porous substrate may include a polymer having a weight-average molecular weight of 3,000 to 500,000 g / mol. This range may facilitate the formation of the overhang region. The weight-average molecular weight may be determined using gel permeation chromatography (GPC) in terms of polystyrene.

[0057] The heat-resistant layer may contain binders known to those skilled in the art and commonly contained in heat-resistant layers of separators. The heat-resistant layer may further contain organic and / or inorganic ceramics. The organic and / or inorganic ceramics may be of the type known to those skilled in the art and commonly contained in heat-resistant layers of separators.

[0058] The adhesive layer may include a conventional adhesive binder known to those skilled in the art and included in adhesive layers of separators, and may include one or more of a core-shell adhesive binder and a non-core-shell adhesive binder.

[0059] The heat-resistant adhesive layer is known to those skilled in the art and can include conventional heat-resistant binders and adhesive binders that are included in heat-resistant adhesive layers of separators.

[0060] An electrode assembly for a lithium secondary battery according to one embodiment includes a separator for a lithium secondary battery and an electrode stacked on a first region of the separator for a lithium secondary battery.

[0061] FIG. 4 is a cross-sectional view of an electrode assembly for a lithium secondary battery according to an embodiment.

[0062] Referring to FIG. 4, the electrode assembly for a lithium secondary battery may include a separator 30 for a lithium secondary battery including a first region 31 and second regions 32 formed integrally with the first region 31 and located at both ends of the first region 31, and an electrode 40 stacked on the first region 31.

[0063] Electrode 40 may be a positive or negative electrode as described below.

[0064] The electrode 40 may be formed by coating an electrode slurry on the first region 31 of the lithium secondary battery separator 30 and drying the coating. Alternatively, although not shown in FIG. 3 , the electrode 40 may be laminated on the first region 31 with an adhesive layer.

[0065] According to an embodiment, a method for manufacturing a separator for a lithium secondary battery includes pressurizing only a portion of a separator film corresponding to a second region to manufacture a separator for a lithium secondary battery.

[0066] According to one embodiment, the separator film may be formed using the above-described porous substrate alone.

[0067] According to another embodiment, the separator film may include a porous substrate and a heat-resistant layer formed on at least one surface of the porous substrate.

[0068] According to another embodiment, the separator film may include a porous substrate, a heat-resistant layer formed on at least one surface of the porous substrate, and an adhesive layer formed on one surface of the heat-resistant layer.

[0069] The detailed description thereof is omitted since it is the same as that described above.

[0070] The pressure treatment is carried out only on the portion of the separator film that corresponds to the second region, and the portion that is not pressure treated can become the first region of the separator.

[0071] The pressure treatment can vary depending on the thickness of the separator film, particularly the porous substrate, but can be performed by pressing only the portion corresponding to the second region in the thickness direction under a linear pressure of 0.01 to 10 ton / cm. The linear pressure can be determined according to definitions known to those skilled in the art. The linear pressure can be applied by controlling a roll press, as described below, to press only the portion of the separator film that will become the overhang region.

[0072] In one embodiment, the maximum pressure, e.g., linear pressure, during the pressurization may be set so that the thickness of the separator before the pressurization and the thickness of the overhang region satisfy the following formula:

[0073] Thickness of overhang area = Thickness of separator before pressure treatment × (1-(porosity / 100)) [Equation 4] (In Equation 4, porosity is the porosity of the separator before pressure treatment (unit: %))

[0074] When Equation 4 is satisfied, the pores in the separator are removed and the separator appears transparent, which indicates whether the overhang region has been well formed.

[0075] In one embodiment, the porosity may be between 5 and 95%.

[0076] The pressure treatment can be carried out using a conventional roll press. The cross-sectional pattern of the roll press is not limited, but may be angular or have a curved surface to prevent the boundary between the first and second regions from breaking.

[0077] In one embodiment, the pressure treatment may be carried out at room temperature, for example, 20 to 25°C.

[0078] In other embodiments, the pressure treatment may be performed at an elevated temperature, which may further reduce the MD heat shrinkage and TD heat shrinkage of the second region.

[0079] The high temperature can be determined by the melting point of the porous substrate. Generally, the high temperature can be performed at a temperature below the melting point of the porous substrate. For example, the high temperature can be performed at a temperature below the melting point of the porous substrate, for example, (the melting point of the porous substrate - 10 ° C) or below, for example, (the melting point of the porous substrate - 20 ° C) or below.

[0080] Another embodiment provides a lithium secondary battery including a positive electrode, a negative electrode, and a separator for a lithium secondary battery located between the positive electrode and the negative electrode.

[0081] The separator for a lithium secondary battery is substantially the same as that described above.

[0082] Positive electrodes for lithium secondary batteries

[0083] The positive electrode for a lithium secondary battery may include a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes a positive electrode active material, and may further include a binder and / or a conductive material.

[0084] As an example, the positive electrode may further include an additive that can act as a sacrificial positive electrode.

[0085] The content of the positive electrode active material may be 90% by weight to 99.5% by weight relative to 100% by weight of the positive electrode active material layer, and the contents of the binder and conductive material may be 0.5% by weight to 5% by weight each relative to 100% by weight of the positive electrode active material layer.

[0086] The positive electrode active material may be a compound capable of reversibly inserting and extracting lithium (lithiated intercalation compound). Specifically, one or more of composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0087] The composite oxide may be a lithium transition metal composite oxide, and specific examples thereof include lithium nickel-based oxide, lithium cobalt-based oxide, lithium manganese-based oxide, lithium iron phosphate-based compound, cobalt-nickel-free manganese-based oxide, or a combination thereof.

[0088] As an example, a compound represented by any one of the following chemical formulas can be used: Li a A 1-b X b O 2-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Mn 2-b X b O 4-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05);Li a Ni 1-b-c Co b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni 1-b-c Mn b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2);Li a Ni b Co c L 1 d G e O2(0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, 0≦e≦0.1);Li a NiG bO2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a CoG b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-b G b O2(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8, 0.001≦b≦0.1);Li a Mn 1-g G g PO4(0.90≦a≦1.8, 0≦g≦0.5);Li (3-f) Fe2(PO4)3(0≦f≦2);Li a FePO4(0.90≦a≦1.8). In the formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; L 1 is Mn, Al or a combination thereof.

[0089] For example, the positive electrode active material may be a high-nickel positive electrode active material having a nickel content of 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more, but not more than 99 mol%, relative to 100 mol% of metals (excluding lithium) in the lithium transition metal complex oxide. Because the high-nickel positive electrode active material can provide high capacity, it can be applied to high-capacity, high-density lithium secondary batteries.

[0090] The binder serves to firmly adhere the positive electrode active material particles to each other and to firmly adhere the positive electrode active material to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, and nylon.

[0091] The conductive material is used to impart conductivity to the electrode, and any material that is electronically conductive without causing chemical changes in the battery that is constructed can be used. Examples of the conductive material include carbon-based materials such as natural black smoke, artificial black smoke, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0092] The current collector may be made of Al, but is not limited to this.

[0093] Negative electrodes for lithium secondary batteries

[0094] A negative electrode for a lithium secondary battery includes a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material layer includes a negative electrode active material and may further include a binder and / or a conductive material.

[0095] For example, the negative electrode active material layer can contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0096] The negative electrode active material includes a material capable of reversibly inserting / desorbing lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping and undoping lithium, or a transition metal oxide.

[0097] Examples of the material capable of reversibly inserting / desorbing lithium ions include carbon-based negative electrode active materials, which can include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon can include soot such as amorphous, plate-shaped, flaky, spherical, or fibrous natural soot or artificial soot, and examples of amorphous carbon can include soft carbon or hard carbon, mesophase pitch carbide, plasticized coke, and the like.

[0098] As the alloy of lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0099] As the material capable of doping and undoping lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. The Si-based negative electrode active material may be silicon, a silicon-carbon composite, SiOx (0 < x < 2), a Si-Q alloy (Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination thereof. The Sn-based negative electrode active material may be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0100] The silicone-carbon composite may be a composite of silicone and amorphous carbon. According to one embodiment, the silicone-carbon composite may be in the form of silicone particles and the surfaces of the silicone particles coated with amorphous carbon. For example, it may include secondary particles (cores) formed by assembling silicone primary particles and an amorphous carbon coating layer (shell) located on the surfaces of the secondary particles. Amorphous carbon may also be located between the silicone primary particles; for example, the silicone primary particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0101] The silicone-carbon composite may further comprise crystalline carbon. For example, the silicone-carbon composite may comprise a core comprising crystalline carbon and silicone particles and an amorphous carbon coating layer located on the core surface.

[0102] The Si-based negative electrode active material or the Sn-based negative electrode active material can be used in combination with a carbon-based negative electrode active material.

[0103] The binder serves to firmly adhere the negative electrode active material particles to each other and to the current collector. The binder may be a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0104] Non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymers, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

[0105] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0106] When an aqueous binder is used as the negative electrode binder, it may further contain a cellulose-based compound that can impart viscosity. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li.

[0107] The dry binder may be a fiberizable polymeric material such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0108] The conductive material is used to impart conductivity to the electrode and may be any material that is electronically conductive without causing chemical changes in the battery. Specific examples include carbon-based materials such as natural black carbon, artificial black carbon, carbon black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0109] The negative electrode current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0110] The lithium secondary battery may further include an electrolyte.

[0111] electrolyte

[0112] The electrolyte for the lithium secondary battery includes a non-aqueous organic solvent and a lithium salt.

[0113] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reactions of the battery can migrate.

[0114] The non-aqueous organic solvent may be a carbonate, ester, ether, ketone or alcohol solvent, an aprotic solvent, or a combination thereof.

[0115] Examples of carbonate solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC).

[0116] Examples of ester solvents that can be used include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, and caprolactone.

[0117] Examples of ether solvents that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Examples of ketone solvents that can be used include cyclohexanone. Examples of alcohol solvents that can be used include ethyl alcohol and isopropyl alcohol. Examples of aprotic solvents that can be used include nitriles such as R-CN (R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double bond, an aromatic ring, or an ether group); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane and 1,4-dioxolane; and sulfolanes.

[0118] The non-aqueous organic solvents can be used alone or in combination of two or more.

[0119] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate can be mixed and used, and the cyclic carbonate and the chain carbonate can be mixed in a volume ratio of 1:1 to 1:9.

[0120] Lithium salts are substances dissolved in organic solvents and act as a source of lithium ions in the battery, enabling basic lithium secondary battery operation and facilitating the movement of lithium ions between the positive and negative electrodes. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1SO2) (x and y are integers of 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato)borate (LiBOB).

[0121] Lithium secondary battery

[0122] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch-shaped, coin-shaped, and other shapes depending on their shape. FIGS. 6 to 9 are schematic diagrams illustrating lithium secondary batteries according to embodiments, with FIG. 6 illustrating a cylindrical battery, FIG. 7 illustrating a prismatic battery, and FIGS. 8 and 9 illustrating pouch-shaped battery shapes. Referring to FIGS. 5 to 8, a lithium secondary battery 100 may include an electrode assembly 40 including a positive electrode 10 and a negative electrode 20 with a separator 30 interposed therebetween, and a case 50 housing the electrode assembly 40. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). As shown in FIG. 6, the lithium secondary battery 100 may include a sealing member 60 that seals the case 50. Also, as shown in FIG. 7, the lithium secondary battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 8 and 9, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical paths for conducting the current generated in the electrode assembly 40 to the outside.

[0123] The lithium secondary battery according to an embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electrical appliances, but the present invention is not limited thereto.

[0124] Examples and comparative examples of the present invention will be described below. However, the examples described below are merely examples of the present invention, and the present invention is not limited to these examples.

[0125] <Measurement of MD and TD thermal shrinkage after impregnation of overhang area in electrolyte and heat exposure>

[0126] The separators including the first and second regions manufactured in the following examples and comparative examples were cut into rectangular shapes with an MD of 100 mm to prepare samples. The overhang region of the sample was MD x TD (100 mm x 10 mm).

[0127] A positive electrode slurry was prepared by mixing 97 wt% LiCoNiAl as a positive electrode active material, 1.5 wt% carbon nanotubes as conductive materials, and 1.5 wt% polyvinyl fluoride, and adding water. The prepared positive electrode slurry was applied to aluminum foil, dried, and rolled to prepare a positive electrode. A negative electrode active material slurry was prepared by mixing 97.4 wt% negative electrode active material, 1.0 wt% carboxymethyl cellulose, 1.5 wt% styrene-butadiene rubber, and 0.1 wt% carbon nanotubes as a conductive material. A silicone-based negative electrode active material was used as the negative electrode active material. The prepared negative electrode slurry was applied to copper foil, dried, and rolled to prepare a negative electrode.

[0128] One sample was placed between the positive and negative electrodes, and three sets of positive-sample-negative electrode stacks were created and placed in pouches. Two grams of electrolyte (a solution of 1.5M LiPF6 dissolved in ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (30:50:20 volume ratio)) was poured into the stack to completely impregnate it. After sealing, the stack was left at 25°C for 12 hours and then placed in an oven at 140°C for 1 hour. The samples were removed from the pouches, and the MD and TD thermal shrinkages were calculated using the following formulas. The MD and TD thermal shrinkages were calculated as average values ​​after measurements were taken on 10 randomly selected samples from the separator.

[0129] Heat shrinkage ratio = (L0-L1) / L0 x 100 [mathematical formula] (L0 is the initial length of the overhang region, and L1 is the length of the overhang region after leaving it in the electrolyte at 140°C for 1 hour.)

[0130] <Measurement of thickness increase rate after impregnation of overhang area in electrolyte and heat exposure>

[0131] The thickness of the overhang area was measured. After performing the same method as for measuring the thermal shrinkage of the overhang area, the thickness of the overhang area was measured. The thickness increase rate was calculated using the following mathematical formula. The thickness increase rate was calculated by measuring 10 times at any position in the overhang area for each of 10 randomly selected samples of the separator and calculating the average value.

[0132] Thickness increase rate = (T1-T0) / T1x100 [mathematical formula] (T0 is the initial thickness of the overhang region, and T1 is the thickness of the overhang region after leaving it in the electrolyte at 140°C for 1 hour.)

[0133] The thickness was measured using a Litematic measuring instrument (VL-50, Mitutoyo).

[0134] In the following examples, "Pressure 1" means 0.5 ton / cm, "Pressure 2" means 1 ton / cm, and "Pressure 3" means 2 ton / cm, and these are linear pressures.

[0135] Example 1

[0136] A polyethylene film (PE, W-SCOPE, porosity: 30%, thickness: 15 μm) was pressed at room temperature (25°C) under the condition of "Pressure 1" using a roll press only in the area where the overhang region was to be formed (the area from one end of the TD of the polyethylene film to 10 mm), to produce a separator with the first and second regions (overhang regions) formed.

[0137] <Examples 2 to 12>

[0138] A separator having a first region and a second region (overhang region) formed thereon was manufactured in the same manner as in Example 1, except that the treatment conditions for the polyethylene film in Example 1 were changed as shown in Table 1 below.

[0139] <Comparative Example 1>

[0140] In Example 1, a polyethylene film (PE, W-SCOPE, porosity: 30%, thickness: 15 μm) was used as a separator, but the polyethylene film was not pressurized with "Pressure 1" in the area where the overhang region was to be formed.

[0141] <Comparative Example 2>

[0142] A commercially available separator was used, in which coating layers were formed on both sides of a polyethylene film (thickness: 14 μm, polyethylene film thickness: 7 μm, total thickness of coating layer: 7 μm, coating layer contains ceramics and a heat-resistant binder).

[0143] <Comparative Example 3>

[0144] A commercially available product separator was used in which coating layers were formed on both sides of a polyethylene film (thickness: 14 μm, thickness of polyethylene film: 7 μm, total thickness of coating layer: 7 μm, coating layer included a heat-resistant layer containing ceramics and a binder, and an adhesive layer containing an adhesive binder and laminated on the heat-resistant layer).

[0145] In Table 1 below, the units of TD and MD are mm, and the unit of thickness is μm.

[0146] [Table 1]

[0147] As shown in Table 1, the separators for lithium secondary batteries of the examples satisfy Equations 1 and 2. This suggests that the separators of the examples have overhanging regions that suppress MD and TD thermal shrinkage after impregnation in an electrolyte and heat exposure, thereby preventing short circuits between the separator and electrodes.

[0148] On the other hand, it can be seen that the separator of the comparative example did not satisfy Equations 1 and 2. This suggests that the separator of the comparative example has an overhang region in which the MD thermal shrinkage and TD thermal shrinkage after impregnation in an electrolyte and heat exposure are relatively less suppressed compared to the examples, which may increase the probability of short-circuiting between the separator and the electrode.

[0149] Example 13

[0150] Only the area of ​​a polyethylene film (porosity: 30%, thickness: 7 μm) where the overhang region was to be formed (the area from one end of the TD of the polyethylene film to 10 mm) was pressed using a roll press at 80°C under the condition of "Pressure 1" to produce a separator in which the first and second regions (overhang regions) were formed.

[0151] <Examples 14 to 24>

[0152] In Example 13, a separator was manufactured in the same manner as in Example 1, except that the treatment conditions for the overhang region of the polyethylene film were changed as shown in Table 2 below.

[0153] In Table 2 below, the units of TD and MD are mm, and the unit of thickness is μm.

[0154] [Table 2]

[0155] As shown in Table 2, the separators for lithium secondary batteries of the examples satisfy Equations 1 and 2. This suggests that the separators of the examples have overhanging regions that suppress MD and TD thermal shrinkage after impregnation in an electrolyte and heat exposure, thereby preventing short circuits between the separator and electrodes.

[0156] Example 25

[0157] A commercially available separator with coating layers formed on both sides of a polyethylene film (polyethylene film thickness: 7 μm, total coating layer thickness: 7 μm, coating layer includes ceramics and a heat-resistant binder) was treated at 140°C under a pressure of 1 to produce a separator. The separator was produced in the same manner as in Example 1.

[0158] Example 26

[0159] A commercially available separator was manufactured by treating a polyethylene film with a coating layer formed on both sides (polyethylene film thickness: 7 μm, total coating layer thickness: 7 μm, coating layer including a heat-resistant layer containing ceramics and a binder, and an adhesive layer containing an adhesive binder and laminated on the heat-resistant layer) at 140°C and a pressure of 1.

[0160] <Comparative Example 4>

[0161] A separator in which coating layers were formed on both sides of the polyethylene film in Example 25 (product name, manufacturer, polyethylene film thickness: 7 μm, total thickness of coating layer: 7 μm, coating layer contains ceramics and a heat-resistant binder) was used.

[0162] <Comparative Example 5>

[0163] In Example 26, a separator was used in which coating layers were formed on both sides of the polyethylene film (thickness of the polyethylene film: 7 μm, total thickness of the coating layer: 7 μm, the coating layer included a heat-resistant layer containing ceramics and a binder, and an adhesive layer containing an adhesive binder and laminated on the heat-resistant layer).

[0164] The separators of the examples and comparative examples were evaluated for the thermal shrinkage rate and thickness increase rate of the overhang region using the same method.

[0165] In Table 3 below, Heat Shrinkage Rate 1 and Thickness Increase Rate 1 are measured values ​​after leaving the sample at 130°C for 1 hour in the heat shrinkage measurement, and Heat Shrinkage Rate 2 and Thickness Increase Rate 2 are measured values ​​after leaving the sample at 140°C for 1 hour in the heat shrinkage measurement.

[0166] [Table 3]

[0167] As shown in Table 3, the separators for lithium secondary batteries of the examples satisfy Equations 1 and 2. This suggests that the separators of the examples have overhanging regions that suppress MD and TD thermal shrinkage after impregnation in an electrolyte and heat exposure, thereby preventing short circuits between the separator and electrodes.

[0168] Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto and can be embodied in various modifications within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it is to be understood that these also fall within the scope of the present invention.

Claims

1. a first region and a second region located at both ends of the first region, The second region satisfies the relationships of the following formulas 1 and 2: Thickness increase rate of the second region>MD heat shrinkage rate of the second region [Equation 1] Thickness increase rate of the second region>TD heat shrinkage rate of the second region [Equation 2] (In the above formulas 1 and 2, the MD heat shrinkage rate of the second region, the TD heat shrinkage rate of the second region, and the thickness increase rate of the second region are heat shrinkage rates measured after the separator is immersed in an electrolyte at 140°C for 1 hour.)

2. The separator for a lithium secondary battery according to claim 1 , wherein the first region is a region facing an electrode, and the second region is an overhang region.

3. 2. The separator for a lithium secondary battery according to claim 1, wherein the second region has a thickness increase rate of 5 to 70%.

4. 2. The separator for a lithium secondary battery according to claim 1, wherein the second region has a TD heat shrinkage of 3 to 20% and an MD heat shrinkage of 5 to 25%.

5. 2. The separator for a lithium secondary battery according to claim 1, wherein the thickness of the second region is 30 to 90% of the thickness of the first region.

6. The separator for a lithium secondary battery according to claim 1, wherein the first region and the second region have different hazes.

7. The separator for a lithium secondary battery according to claim 1, wherein the first region has a thickness increase rate and a thermal shrinkage rate different from those of the second region.

8. The separator for a lithium secondary battery according to claim 1 , wherein the first region and the second region include a porous substrate.

9. The separator for a lithium secondary battery according to claim 8, wherein the porous substrate has a porosity of 5 to 95%.

10. The separator for a lithium secondary battery according to claim 8, wherein the porous substrate is an MD-stretched film, a TD-stretched film, or an MD- and TD-stretched film.

11. The separator for a lithium secondary battery according to claim 8, wherein at least one of a heat-resistant layer and an adhesive layer is further formed on at least one surface of the porous substrate.

12. 2. The method of claim 1, further comprising the step of pressurizing only a portion of the separator film corresponding to the second region to manufacture the separator for the lithium secondary battery.

13. 13. The method for producing a separator for a lithium secondary battery according to claim 12, wherein the pressure treatment is carried out under a linear pressure of 0.01 to 10 ton / cm.

14. The separator film includes a porous substrate, The method for manufacturing a separator for a lithium secondary battery according to claim 12, wherein the pressure treatment is performed at a temperature equal to or lower than the melting point of the porous substrate.

15. A lithium secondary battery comprising: a positive electrode; a negative electrode; and a lithium secondary battery separator according to any one of claims 1 to 11, or a lithium secondary battery separator produced by the production method according to any one of claims 12 to 14, located between the positive electrode and the negative electrode.

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