SECONDARY BATTERY SEPARATOR AND SECONDARY BATTERY CONTAINING THIS SEPARATOR

Abstract

A separator for a secondary battery includes a substrate and a coating layer formed on a surface of the substrate and containing inorganic particles. The separator has a thermal shrinkage coefficient in the range of 5 kPa to 30 kPa. The thermal shrinkage rate, defined as a ratio of a thermal TD shrinkage rate of the separator measured after storage at 130°C for 1 hour to a thermal TD shrinkage rate of the substrate measured after storage at 130°C for 1 hour, is 0.3 to 0.5.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The embodiments of the present application relate to a separator for a secondary battery and a secondary battery including the same, and more particularly to a separator for a secondary battery including a substrate and a coating layer formed thereon, and a secondary battery including the separator. 2. Description of the state of the art

[0002] A secondary battery is a battery that can be repeatedly charged and discharged. With the rapid progress of the information and communications and display industries, secondary batteries have been used as a power source in various portable electronic telecommunications devices such as camcorders, mobile phones, and laptops. Recently, a battery pack containing a secondary battery has also been developed and used as a power source in environmentally friendly vehicles such as electric vehicles.

[0003] Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and the like. Lithium secondary batteries have high operating voltage and high energy density per unit weight, making them advantageous in terms of charging speed and lightweight construction, so their development is progressing in this regard.

[0004] The secondary battery may include an electrode assembly including a cathode, an anode, and a separator disposed between the cathode and the anode. Repeated charging and discharging of the secondary battery under high-temperature conditions may result in deformation of the separator. For example, a difference in shrinkage rates between an MD direction and a TD direction may lead to increased deformation of the separator.

[0005] In this case, partial damage to the separator may cause a short circuit between the cathode and the anode, which may lead to explosion or ignition.

[0006] The separator may include a substrate and a coating layer formed on the substrate. Depending on the different thermal shrinkage properties between the substrate and the coating, thermal damage to the separator may be accelerated. SUMMARY OF THE INVENTION

[0007] An object of the present disclosure is to provide a separator for a secondary battery having improved thermal and mechanical stability and reliability.

[0008] Another object of the present disclosure is to provide a secondary battery having improved thermal and mechanical stability and reliability incorporating the separator.

[0009] A separator for a secondary battery according to embodiments of the present disclosure includes: a substrate; and a coating layer formed on a surface of the substrate and containing inorganic particles. The separator has a thermal shrinkage coefficient in a range of 5 kPa to 30 kPa, defined by Equation 1 below. The ratio of the thermal shrinkage rate, defined as the ratio of the thermal TD shrinkage rate of the substrate measured after 1 hour of storage at 130°C, to the thermal TD shrinkage rate of the separator measured after 1 hour of storage at 130°C, is 0.3 to 0.5. Thermal shrinkage coefficient=(0.015N−0.01N) / A0(L11−L10) / L0 (in equation 1, A0 is an initial cross-sectional area (m 2 ) of a separator sample, L0 is an initial TD length of the separator sample, L 10 is a TD length of the separator sample after a 0.01N TMA measurement and L11 is a TD length of the separator sample after a 0.015N TMA measurement).

[0010] In some embodiments, L 10 and L 11 Lengths measured when the sample is TD-shrunk with a force of 0.01N and 0.015N to its maximum shrinkage while the temperature increases at a rate of 5°C / min from room temperature in a nitrogen (N2) atmosphere using a thermomechanical analysis (TMA) instrument.

[0011] In some embodiments, the maximum shrinkage at 0.01N TMA, as defined in Equation 2-1 below, may be 15% or less: 0.01N maximum shrinkage(%)={1−(L10 / L0)}×100

[0012] In some embodiments, the maximum shrinkage at 0.015N TMA as defined in Equation 2-2 below may be 10% or less: 0.015N maximum shrinkage(%)={1−(L11 / L0)}×100

[0013] In some embodiments, the thermal shrinkage coefficient may be 8 kPa to 28 kPa.

[0014] In some embodiments, the substrate may include a polyethylene resin having a melt flow index (MI) of 0.05 to 0.3 measured under conditions of 2.16 kg load at 190°C.

[0015] In some embodiments, the substrate film may have a TD stretch ratio of 3 to 8 times.

[0016] In some embodiments, the substrate may have a thermal TD shrinkage rate of 20% or less.

[0017] In some embodiments, the substrate may have a thermal TD shrinkage rate of 8% to 18%.

[0018] In some embodiments, the separator may have a thermal TD shrinkage rate of 12% or less.

[0019] In some embodiments, the separator may have a thermal TD shrinkage rate of 3% to 10%.

[0020] In some embodiments, the inorganic particles have a mean particle diameter (D50) of 0.4 µm to 1 µm.

[0021] In some embodiments, the coating layer may have a porosity of 40 to 60%.

[0022] In some embodiments, the coating layer may be formed only on one surface of the substrate.

[0023] A secondary battery according to embodiments of the present disclosure includes: a cathode and an anode that are repeatedly stacked; and the above-described separator for a secondary battery disposed between the cathode and the anode.

[0024] The separator for a secondary battery according to the above-described embodiments includes the substrate and the coating layer and can have a thermal shrinkage coefficient within a predetermined range. Within the above range, the tendency of the separator to curl can be suppressed while ensuring improved heat resistance.

[0025] In some embodiments, deformation of the separator can be suppressed by adjusting the thermal shrinkage rates of the substrate and the coating layer within a predetermined range. Accordingly, overcharging of the secondary battery or battery cell containing the separator can be suppressed, and mechanical stability and high-temperature charge / discharge stability can be further improved.

[0026] The secondary battery containing the separator of the present disclosure can be used in a wide range of environmentally friendly technologies, such as electric vehicles, battery charging stations, solar power generation, wind power generation, and the like, which utilize the batteries. Furthermore, the lithium secondary battery containing the separator of the present disclosure can be used in environmentally friendly electric vehicles, hybrid vehicles, and the like, which aim to mitigate climate change by reducing air pollution and greenhouse gas emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above and other objects, features and advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig.1 is a schematic cross-sectional view showing a separator for a secondary battery according to exemplary embodiments; and Fig. 2 and Fig. 3 are a plan view and a cross-sectional view showing a secondary battery according to exemplary embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0028] The embodiments disclosed in the present application provide a separator for a secondary battery including a substrate and a coating layer formed thereon. Furthermore, a secondary battery including the separator is provided.

[0029] The exemplary embodiments will be described in more detail with reference to the drawings. However, since the drawings accompanying the present disclosure and the exemplary embodiments are only intended to better understand the technical spirit of the present invention, they should not be construed as limiting the present invention to the contents illustrated and described in the drawings and exemplary embodiments.

[0030] The terms "first," "second," "upper part," "upper layer," "lower part," "lower layer," etc., used here do not denote an absolute position, but are used in a relative sense. For example, the terms are used relatively to denote a different area relative to a specific reference plane.

[0031] Fig.1 is a schematic cross-sectional view showing a separator for a secondary battery according to exemplary embodiments. Fig. The machine direction (MD) shown in Figure 1 indicates a direction in which a process for manufacturing the separator is carried out and may correspond to a longitudinal direction of the separator. The transverse direction (TD) is a direction perpendicular to the MD and may correspond to a width direction of the separator.

[0032] Referring to Fig. 1, a separator for a secondary battery (hereinafter abbreviated as separator) 140 may include a substrate 142 and a coating layer 145 formed on a surface of the substrate 142.

[0033] The substrate 142 may include a polyolefin film. The substrate 142 may include, for example, a porous polyolefin film. Accordingly, a short circuit between the cathode and the anode may be blocked by the separator 140 while facilitating the flow of ions.

[0034] The substrate 142 may, for example, comprise a copolymer of two or more of the following: polyethylene, polypropylene, polybutylene, polypentene, polyhexene, polyoctene, ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene, or a mixture thereof.

[0035] Examples of polyethylene may include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and the like. In one embodiment, high-density polyethylene with high crystallinity and a high melting point of the resin may be used.

[0036] In some embodiments, the substrate 142 may also include a resin such as polyether, polyacetal, polyamide, polycarbonate, polyimide, polyamideimide, polyetherimide, etc.

[0037] In some embodiments, the raw resins of the polyolefin film described above can be melted and blended, then cooled and cut to produce resin pellets. The resin pellets can be extruded at a temperature of 200°C or higher, e.g., with a T-die extruder, to produce a resin sheet. The resin sheet can be cooled to produce an unstretched sheet.

[0038] The substrate 142 can be manufactured by stretching the unstretched sheet. The stretching process can include simultaneous biaxial stretching, sequential biaxial stretching, multi-stage stretching, multiple stretching, etc.

[0039] In order to ensure the strength of the substrate 142, the uniformity of the film properties, the isotropy, etc., it is possible to form the substrate 142 by simultaneous biaxial stretching.

[0040] Simultaneous biaxial stretching can refer to a stretching method in which MD stretching and TD stretching are performed simultaneously, where the stretching ratios in each direction can be different.

[0041] In exemplary embodiments, the MD stretch ratio and the TD stretch ratio can each be 3 to 8 times. For example, if the stretch ratio is less than 3 times, the unevenness of the film properties of the substrate 142 may increase, and if it is more than 8 times, thermal shrinkage may become excessive.

[0042] In some embodiments, the MD stretch ratio or the TD stretch ratio may be 3 to 6 times, in one embodiment 3 to 5 times or 3 to 4 times.

[0043] In some embodiments, the TD stretch ratio can be adjusted within the range described above to easily control the thermal shrinkage properties described below. The MD ratio can be adjusted within a range that does not cause deterioration of the thermal shrinkage properties.

[0044] In exemplary embodiments, the melt flow index (MI) of the polyolefin resin contained in the substrate 142 may be 0.05 to 0.3. Within the above range, sufficient heat resistance can be ensured while the extrusion process can be carried out smoothly.

[0045] In some embodiments, the MI of the polyolefin resin may be 0.05 to 0.25. In one embodiment, the MI of the polyolefin resin may be 0.07 to 0.25 or 0.08 to 0.23. Within the above range, the desired thermal shrinkage properties of the substrate 142 and the separator 140, described further below, may be better achieved.

[0046] The MI can be measured as the weight extruded with a load of 2.16 kg at 190°C for 10 minutes. Accordingly, the MI can be expressed in units of grams per 10 minutes (g / 10 min).

[0047] The coating layer 145 may be formed on one surface of the substrate 142. According to exemplary embodiments, the coating layer 145 may be formed on only one of the top and bottom surfaces of the substrate 142. In this case, the separator 140 may be implemented as a single-sided coating separator.

[0048] By using the single-side coated separator, while ensuring the heat resistance, ignition stability, and penetration stability of the separator 140 through the coating layer 145, a reduction in the capacity of the battery due to an increase in the thickness of the separator 140 can be prevented. For example, by increasing the number of stacks of the cathode and anode in a limited space, sufficient capacity characteristics can be ensured, and an ion transfer distance between the cathode and the anode can be reduced.

[0049] However, in a single-sided coated separator, the difference in thermal shrinkage characteristics between the substrate 142 and the coating layer 145 may be greater than in a double-sided coated separator. For example, due to the difference in thermal shrinkage rate between the substrate 142 and the coating layer 145, roll bending may occur, and thickness deformation may easily occur.

[0050] According to the embodiments of the present disclosure described below, the thermal shrinkage rates of the substrate layer 142 and the coating layer 145 can be adjusted and the thermal shrinkage coefficient of the entire separator 140 can be controlled, thereby suppressing mechanical failure due to the thermal shrinkage discrepancy of the one-sided coating separator.

[0051] The coating layer 145 may contain inorganic particles (or ceramic particles). The inorganic particles may include aluminum hydroxide, boehmite, aluminum oxide, titanium dioxide, zirconium dioxide, barium sulfate, magnesium oxide, silicon dioxide, and the like. These may be used alone or in combination of two or more thereof.

[0052] In some embodiments, the coating layer 145 may not contain organic particles (e.g., polyethylene particles).

[0053] For example, the inorganic particles can be mixed with a binder in a solvent (e.g., water) to form a coating slurry. The coating slurry can be applied to a surface of the substrate 142 and then dried to form the coating layer 145.

[0054] Non-limiting examples of the binder include polymethyl methacrylate, polybutyl acrylate, polyvinylidene fluoride hexafluoropropylene, polyvinyl alcohol (PVA), polyimide, cellulose acetate, carboxymethyl cellulose (CMC), and the like. These can be used alone or in combination with two or more of them.

[0055] The content of inorganic particles may be 88 to 99 weight percent (wt%) based on the solid content of the slurry. In one embodiment, the content of inorganic particles may be 90 to 96 wt%. Within the above-mentioned range, it is possible to avoid excessive thermal shrinkage differential with the substrate 142 while ensuring sufficient heat resistance.

[0056] In some embodiments, the inorganic particles may have an average particle diameter (D50) of 0.4 µm to 1 µm. In one embodiment, the inorganic particles may have an average particle diameter (D50) of 0.4 µm or more and less than 1 µm, for example, 0.4 µm to 0.9 µm or 0.5 µm to 0.9 µm.

[0057] Within the above-mentioned range, the inorganic particles can facilitate the formation of a porous structure while simultaneously preventing side reactions with the electrolyte due to an excessive increase in the specific surface area. Furthermore, the contact / adhesion properties of the substrate 142 can be improved to prevent detachment of the coating layer 145 at high temperatures.

[0058] The term “mean particle diameter (D50)” used here can refer to the mean diameter of the particles at a point corresponding to 50% in a cumulative distribution (distribution based on the number of particles) in which the pores are arranged in order of size.

[0059] In some embodiments, the coating layer 145 may have a porosity of 40% to 60%. In one embodiment, the coating layer 145 may have a porosity of 45% to 55%. Within the above range, excessive thermal shrinkage can be prevented while facilitating ion transfer through the coating layer 145.

[0060] In exemplary embodiments, the separator 140 may have a total thickness of 5 µm to 30 µm. In some embodiments, the separator 140 may have a total thickness of 10 µm to 20 µm.

[0061] The substrate 142 may have a thickness of 4 µm to 25 µm. In some embodiments, the substrate 142 may have a thickness of 9 µm to 15 µm.

[0062] The coating layer 145 may have a thickness of 1 µm to 10 µm. In some embodiments, the coating layer 142 may have a thickness of 1 µm to 5 µm or 3 µm to 5 µm.

[0063] According to exemplary embodiments, the substrate 142 may have a thermal TD shrinkage rate of 20% or less. In some embodiments, the substrate 142 may have a thermal TD shrinkage rate of 18% or less, and in one embodiment, 16% or less.

[0064] For example, the substrate 142 may have a thermal TD shrinkage rate of 8 to 20%, 8 to 18%, 8 to 16%, 10 to 20%, 10 to 18%, or 10 to 16%.

[0065] Within the above range, the occurrence of roll bending or thickness deformation due to the thermal shrinkage difference with the coating layer 145 can be suppressed while ensuring the thermal shrinkage prevention properties by the substrate 142.

[0066] The thermal TD shrinkage rate of separator 140 may be less than the thermal shrinkage rate of substrate 142. In exemplary embodiments, separator 140 may have a thermal TD shrinkage rate of 12% or less. In some embodiments, separator 140 may have a thermal TD shrinkage rate of 10% or less, 9% or less, 8% or less, or 7% or less.

[0067] For example, the separator 140 may have a thermal TD shrinkage rate of 3% to 12%, 3% to 10%, 3% to 9%, 3% to 8%, 3% to 7%, 4% to 12%, 4% to 10%, 4% to 9%, 4% to 8%, or 4% to 7%.

[0068] Within the above range, deformation of the separator 140 at high temperatures due to the difference in thermal shrinkage of the substrate 142 can be suppressed, while at the same time, the occurrence of wrinkles is prevented by the coating layer 145.

[0069] The term “thermal shrinkage rate” used here refers to the ratio between the reduced TD length and the initial TD length after 1 hour of storage in a chamber at 130 °C.

[0070] In exemplary embodiments, the ratio between the thermal shrinkage rate of the separator 140 and the thermal shrinkage rate of the substrate 142 may be 0.3 to 0.5. If the thermal shrinkage rate is less than 0.3, a discrepancy in the thermal shrinkage rate between the substrate 142 and the coating layer 145 may increase excessively. Accordingly, under high-temperature conditions, the separator 140 may easily warp and deform, and a rapid temperature rise and ignition of the battery may occur due to an electrode short circuit.

[0071] If the thermal shrinkage ratio exceeds 0.5, the improvement in heat resistance by the coating 145 cannot be sufficiently achieved. Therefore, the high-temperature charge and discharge stability of the battery may deteriorate.

[0072] In some embodiments, the thermal shrinkage rate ratio may be 0.3 to 0.45, 0.31 to 0.45, or 0.32 to 0.45.

[0073] The thermal shrinkage coefficient of the separator 140 according to embodiments of the present disclosure, defined by Equation 1 below, may be 5 kPa to 30 kPa. Thermal shrinkage coefficient=(0.015N−0.01N) / A0(L11−L10) / L0

[0074] In equation 1, A0 represents an initial cross-sectional area (m 2 ) of a separator sample (width × thickness of the separator sample). L0 stands for the initial length (TD length) of the separator sample, L 10 for the length of the separator sample after the 0.01N TMA measurement and L 11 for the length of the separator sample after the 0.015N TMA measurement.

[0075] For example, L 10represents the length measured when the sample is shrunk with a force of 0.01 N to its maximum shrinkage while the temperature is increased at a rate of 5°C / min from room temperature in a nitrogen (N2) atmosphere using a thermomechanical analysis (TMA) device. L 11 represents the length measured when the sample is shrunk to its maximum shrinkage with a force of 0.015 N while the temperature is increased at a rate of 5°C / min from room temperature in a nitrogen (N2) atmosphere using a thermomechanical analysis (TMA) apparatus.

[0076] The value calculated according to equation 1 has a unit of N / m 2 and can be converted into Pa.

[0077] When the thermal shrinkage coefficient defined by Equation 1 is less than 5 kPa, the difference in shrinkage rates between 0.01 N TMA and 0.015 N TMA increases excessively, leading to dimensional deformation sensitivity under high-temperature conditions. Accordingly, the heat resistance and ignition resistance of the separator and battery may deteriorate.

[0078] If the thermal shrinkage coefficient defined by Equation 1 exceeds 30 kPa, the total shrinkage stress of the separator 140 may increase excessively. Accordingly, mechanical deformation of the battery cell may be caused by the pressure and temperature increase within the battery cell during high-temperature charge and discharge cycles.

[0079] In some embodiments, the thermal shrinkage coefficient may be 8 kPa to 28 kPa, 10 kPa to 28 kPa, 15 kPa to 28 kPa, or 20 kPa to 28 kPa.

[0080] In exemplary embodiments, the maximum shrinkage at 0.01N TMA may be 15% or less. The maximum shrinkage at 0.01N TMA may be defined by Equation 2-1 below. 0.01N maximum shrinkage(%)={1−(L10 / L0)}×100

[0081] In equation 2-1, L 10 and L0 as defined in Equation 1 and can be measured under the TMA conditions described above.

[0082] In some embodiments, the maximum shrinkage at 0.01N TMA may be 5% to 15%, 5% to 14%, 6% to 14%, or 6% to 12%.

[0083] In exemplary embodiments, the maximum shrinkage at 0.015N TMA may be 10% or less, or less than 10%. The maximum shrinkage at 0.015N TMA may be defined by Equation 2-2 below. 0.015N maximum shrinkage(%)={1−(L11 / L0)}×100

[0084] In equation 2-2, L11 and L0 as defined in Equation 1 and can be measured under the TMA conditions described above.

[0085] In some embodiments, the maximum shrinkage at 0.015 N TMA may be 9% or less, or 1% to 9%.

[0086] If the thermal shrinkage coefficient is adjusted within the maximum shrinkage described above, the heat resistance and stability of the separator with single-sided coating structure can be effectively improved, while reducing the thermal shrinkage discrepancy due to changing conditions.

[0087] Shrinkage-related properties, such as the thermal shrinkage rate described above, the thermal shrinkage coefficient, etc., can be adjusted by the factors described in the present disclosure. Furthermore, these properties can also be adjusted by the process conditions (such as drying, coating, and stretching rates, drying, coating, and stretching temperatures, etc.) for forming the substrate and coating.

[0088] Fig. 2 and Fig. 3 are a schematic plan view and a cross-sectional view, respectively, showing a secondary battery according to exemplary embodiments. For example, Fig. 3 is a cross-sectional view taken on line II' of Fig. 2 in the direction of the thickness of the secondary battery.

[0089] The Fig. 2 and Fig.3 is shown schematically for the convenience of description, and the structural configuration of the secondary battery of the present disclosure is not limited to that shown in Fig. 2 and Fig. 3 shown structure.

[0090] The secondary battery includes a cathode (100) and an anode (130) as described above, and the separator (140) as described in the above-described embodiments of the present disclosure, which is arranged between the cathode (100) and the anode (130). In Fig. 3, the separator 140 is shown in a single-layer form for the sake of simplicity, but as described above with reference to Fig. 1, the separator 140 includes the substrate 142 and the coating layer 145, and may have the one-sided coating structure.

[0091] The cathode 100 may include a cathode active material layer (110) formed by applying a cathode active material to a cathode current collector (105). The cathode active material may contain a compound that can reversibly intercalate and deintercalate lithium ions. In this case, the secondary battery may be embodied as a lithium secondary battery.

[0092] The cathode current collector 105 may comprise stainless steel, nickel, aluminum, titanium, or an alloy thereof. The cathode current collector 105 may also comprise aluminum or stainless steel that has been surface treated with carbon, nickel, titanium, or silver.

[0093] The cathode active material may contain a compound that can reversibly intercalate and deintercalate lithium ions.

[0094] According to exemplary embodiments, the cathode active material may contain a lithium nickel metal oxide. The lithium nickel metal oxide may also contain at least one of the elements cobalt (Co), manganese (Mn), and aluminum (Al).

[0095] In some embodiments, the cathode active material or the lithium nickel metal oxide may contain a layered or crystal structure of Formula 1 below. Li x Ni a M b O 2+z [Formula 1]

[0096] In Formula 1, x, a, b, and z can satisfy 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, -0.5≤z≤0.1. As described above, M can contain Co, Mn, and / or Al.

[0097] The chemical structure represented by Formula 1 indicates a bonding relationship between elements contained in the layered or crystal structure of the cathode active material and does not exclude other additional elements. For example, M includes Co and / or Mn, and Co and / or Mn may be included as the main active elements of the cathode active material along with Ni. It should be understood here that Formula 1 is used to express the bonding relationship between the main active elements and is a formula that includes the introduction and substitution of additional elements.

[0098] In one embodiment, the cathode active material may further contain auxiliary elements added to the main active elements to improve their chemical stability or the layered / crystal structure. The auxiliary element may be incorporated into the layered / crystal structure to form a bond, and it should be understood that this case also falls within the chemical structure range represented by Formula 1.

[0099] The auxiliary element may, for example, contain at least one of the elements Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may act as an active auxiliary element, which, together with Co or Mn, such as Al, contributes to the capacity / output activity of the cathode active material.

[0100] The cathode active material or the lithium nickel metal oxide may, for example, contain a layered structure or a crystal structure of the following formula 1-1. Li x Ni a M1 b1 M2 b2 O 2+z [Formula 1-1]

[0101] In Formula 1, M1 may contain Co, Mn, and / or Al. M2 may contain the auxiliary elements described above. In Formula 1-1, x, a, b1, b2, and z may satisfy 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤bl+b2≤0.4, -0.5≤z≤0.1.

[0102] The cathode active material may further contain a coating element or a doping element. For example, elements that are substantially identical or similar to the auxiliary elements described above can be used as the coating element or doping element. For example, the elements described above can be used alone or in combination of two or more of them as the coating element or doping element.

[0103] The coating element or the doping element may be present on the surface of the lithium nickel metal oxide particles or penetrate through the surface of the lithium nickel metal composite oxide particles to be incorporated into the bonding structure represented by the above formula 1 or formula 1-1.

[0104] The cathode active material may contain a nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide with an increased nickel content can be used.

[0105] Nickel, as a transition metal, can be used in conjunction with the performance and capacity of the lithium secondary battery. Therefore, as described above, by using a high-Ni composition in the cathode active material, a high-capacity cathode and a high-capacity lithium secondary battery can be provided.

[0106] As the Ni content increases, the long-term stability and lifetime of the cathode or secondary battery may decrease significantly, and side reactions with the electrolyte may also increase. However, in exemplary embodiments, electrical conductivity can be maintained by adding Co, and lifetime stability and capacity retention can be improved by adding Mn.

[0107] The Ni content (e.g., the molar fraction of nickel relative to the total moles of nickel, cobalt, and manganese) in the NCM-based lithium oxide may be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the Ni content may be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0108] In some embodiments, the cathode active material may also include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate-based active material (e.g., LiFePO4).

[0109] In some embodiments, the cathode active material may include, for example, a manganese (Mn)-rich active material, a lithium-rich layered oxide (LLO) / over-lithium oxide (OLO)-based active material, or a cobalt-free (Co) active material having a chemical structure or crystal structure according to Formula 2 below. p[Li2MnO3]·(1-p)[Li q JO2] [Formula 2]

[0110] In formula 2, p and q can satisfy the conditions 0 <p<1, 0,9≤q≤1,2 erfüllen, und J kann mindestens ein Element aus Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg und B enthalten.

[0111] For example, the cathode active material can be mixed in a solvent to form a cathode slurry. The cathode current collector 105 can be coated with the cathode slurry and then dried and pressed to form the cathode active material layer 110.

[0112] The cathode active material layer (110) may further contain a binder and optionally a conductive agent, a thickener, etc.

[0113] Materials substantially identical or similar to the binders and conductive agents described above can be used as the binder and conductive agent for a cathode. In some embodiments, polyvinylidene fluoride (PVDF) can be used as the binder for a cathode.

[0114] The anode 130 may include an anode current collector 125 and an anode active material layer 120.

[0115] The anode current collector 155 may contain the metals or alloys mentioned in connection with the cathode current collector. In some embodiments, the anode current collector 155 may contain copper or a copper alloy.

[0116] The anode active material layer 120 may be formed on a top and / or bottom surface of the anode current collector 125. The anode active material layer 120 may be formed on the top or bottom surface, respectively.

[0117] The anode active material layer 120 may contain an anode active material and an anode binder.

[0118] Carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc., a lithium alloy, a silicon (Si) compound, or tin, etc. can be used as the anode active material.

[0119] Examples of amorphous carbon may include hard coal, coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers (MPCF), or similar.

[0120] Examples of crystalline carbon may include graphite-based carbon such as natural graphite, artificial graphite, graphite coke, graphite MCMB, graphite MPCF or similar.

[0121] Other elements contained in the lithium alloy may include, for example, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium, etc.

[0122] The silicon compound can e.g. B. silicon (Si), silicon oxide (e.g. SiOx, 0 <x<2) oder eine Silizium-Kohlenstoff- Verbindung enthaltend Siliziumkarbid (SiC) enthalten.

[0123] For example, an anode slurry can be prepared by mixing the anode active material with the binder, conductive agent, thickener, and the like in a solvent and then stirring. The anode slurry can then be applied to at least one surface of the anode current collector 125 and subsequently dried and pressed to produce the anode 130.

[0124] Materials substantially the same or similar to the above-described material used in the cathode active material layer 110 can be used as the binder and conductive agent. In some embodiments, the binder for an anode, for example, may include an aqueous binder such as styrene-butadiene rubber (SBR) in accordance with the carbon-based active material and may be used together with a thickener such as carboxymethylcellulose (CMC).

[0125] According to exemplary embodiments, an electrode cell is defined by the cathode 100, the anode 130, and the separator 140, and a plurality of electrode cells may be stacked to form, for example, an electrode assembly 150. The electrode assembly 150 may be a wound, stacked, Z-folded, or stacked-folded electrode assembly.

[0126] In one embodiment, the electrode assembly 150 may be housed in the housing 160 together with an electrolyte. According to exemplary embodiments, a non-aqueous electrolyte may be used as the electrolyte.

[0127] The non-aqueous electrolyte contains a lithium salt of an electrolyte and an organic solvent, the lithium salt being replaced, for example, by Li + X - and is represented as an anion (X - ) of the lithium salt F - , Cl - , Br - , I- , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - , etc. can be cited as examples.

[0128] As the organic solvent, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulforane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, and the like can be used. These compounds can be used alone or in combination with two or more of them.

[0129] As in Fig. 3, electrode tabs (a cathode tab and an anode tab) may protrude from each cathode current collector 105 and each anode current collector 125, respectively, associated with each electrode cell and extend to an end portion of the housing 160.

[0130] The electrode tabs may be fused to the one end portion of the housing 160 to be connected to electrode leads (a cathode lead 107 and an anode lead 127) extending to or exposed outside the housing 160.

[0131] In Fig. In Figure 3, the cathode lead 107 and the anode lead 127 are illustrated as projecting from a top surface of the housing 160 in a planar direction, but the positions of these electrode leads are not limited thereto. For example, the electrode leads may project from at least one of the two sides of the housing 160 or from a bottom surface of the housing 160. Alternatively, the cathode lead 107 and the anode lead 127 may be formed to project from different sides of the housing 160, respectively.

[0132] The lithium secondary battery can be manufactured in the shape of a cylindrical container, a square container, a bag container, or a coin container.

[0133] The secondary battery described above may include the separator 140 according to the embodiments of the present disclosure. For example, by using the separator 140 with the single-sided coating structure, the number of stacks of the cathode 100 and the anode 130 can be relatively increased.

[0134] Therefore, the capacity of the secondary battery can be further increased while maintaining a limited size. Furthermore, the separator 140 is designed to exhibit the above-described thermal shrinkage properties even in the single-sided coating structure, so that defects such as curling, deformation, and wrinkling can be suppressed, thereby improving the operating stability of the secondary battery even at high charging and discharging temperatures.

[0135] Preferred examples are suggested below to facilitate understanding of the present disclosure. However, the following examples are only illustrative of the present disclosure, and those skilled in the art will naturally understand that various changes and modifications are possible within the scope and spirit of the present disclosure. Such changes and modifications are properly encompassed in the appended claims. Examples and comparison examples

[0136] Pellets prepared using a polyethylene resin with the MI values ​​described in Tables 1 and 2 were mixed with oil in a ratio of 3:7 (volume ratio).

[0137] The mixture was extruded onto a sheet using a T-die extruder at 200 °C and then solidified by a chill roll to produce an unstretched sheet.

[0138] The unstretched sheet was TD-stretched at 120 °C in the ratios described in Tables 1 and 2 (the MD stretch ratio was equally 5-fold) and then thermally consolidated at 130 °C to prepare substrates.

[0139] Boehmite with the average particle diameters (D50) described in Tables 1 and 2 and a polyacrylate binder were mixed with water in the weight ratios described in Tables 1 and 2 to prepare a slurry. The slurry was applied to the substrate and then dried to form coating layers with the porosity values ​​described in Table 1.

[0140] The thicknesses of the coating layer and the substrate are described in Tables 1 and 2.

[0141] The MI was measured by applying a load of 21.6 kg at 190°C for 10 minutes according to JIS K7210:1999.

[0142] The porosity (%) of the coating was calculated using the following equation. Porosity of the coating layer (%) = {1 − (weight of the coating layer / volume of the coating layer) / solid density of the coating layer} × 100

[0143] The solid density of the coating layer was calculated by adjusting the density of the solid components such as inorganic substances and binders, etc. used in the preparation of the slurry to the ratio.

[0144] The volume of the coating layer was calculated by measuring the thickness, width and length of the coating layer.

[0145] The thicknesses of the substrate and the coating were measured using a thickness gauge (VL-50S-B, Mitutoyo). [TABLE 1] Example 1 Example 2 Example 3 Example 4 Example 5 MI (g / 10 min) 0,08 0,08 0,17 0,23 0,45 TD expansion ratio 4 4 6 6 7 Total thickness (µm) 12 14 16 19 13 Thickness of the substrate (µm) 9 9 11 14 10 Thickness of the coating layer (µm) 3 5 5 5 3 Average particle diameter of the inorganic particles (µm) 0,5 0,9 0,9 0,7 0,5 Ratio of inorganic particles in the coating (wt%) 96 93 93 90 96 Porosity of the coating layer (%) 46,0 53,0 53,0 46 46,0 [TABLE 2] Comparison example 1 Comparison example 2 Comparison Example 3 Comparison Example 4 Comparison Example 5 MI (g / 10 min) 0,21 0,17 0,45 0,08 0,23 TD expansion ratio 11 6 7 4 6 Total thickness (µm) 12 16 15 11 19 Thickness of the substrate (µm) 9 11 10 9 14 Thickness of the coating layer (µm) 3 5 5 2 5 Average particle diameter 0,5 0,3 0,9 0,7 0,6 of the inorganic particles (um) Ratio of inorganic particles in the coating (wt%) 96 95 93 90 96 Porosity of the coating layer (%) 46,0 44,0 53 46,0 48.0 Measurement of separator properties(1) Thermal TD shrinkage rate

[0146] After the formation of the substrate and coating, a separator sample (TD length: 30 cm, MD length: 30 cm) was stored in a chamber at 130 °C for 1 hour, and the change in TD length was measured to calculate the thermal shrinkage rate using the following equation. TD thermal shrinkage rate(%)=[(initial length−length after storage) / (initial length)]×100 (2) Thermal TMA shrinkage rate

[0147] The separator sample was subjected to shrinkage with a force of 0.01 N and 0.015 N while the temperature increased at a rate of 5°C / min from room temperature in a nitrogen (N2) atmosphere. Subsequently, the TD length after maximum shrinkage was measured using a TMA device (TMA Q400, TA).

[0148] Using Equations 2-1 and 2-2 described above, the maximum shrinkage rate at 0.01 N TMA and the maximum shrinkage rate at 0.015 N TMA were measured. (3) Thermal shrinkage coefficient

[0149] Based on the results measured under the TMA conditions mentioned in (2) above, the thermal shrinkage coefficient was determined using Equation 1 described above.

[0150] The measurement results are shown in Tables 3 and 4 below. Experimental example(1) Production of secondary batteries

[0151] A cathode slurry was prepared by adding 94 wt% lithium cobalt oxide as the cathode active material, 2.5 wt% polyvinylidene fluoride (PVDF) as the binder, and 3.5 wt% Super-P as the conductive agent to N-methylpyrrolidone (NMP). The cathode slurry was evenly coated on a 12 µm thick aluminum substrate and then dried and pressed to prepare a cathode.

[0152] An anode slurry was prepared by adding 95 wt% artificial graphite as the graphite-based active material and 5 wt% styrene-butadiene rubber (SBR)-carboxymethylcellulose (CMC) as the binder to water. The anode slurry was evenly coated on a copper substrate with a thickness of 8 µm and then dried and pressed to prepare an anode.

[0153] An electrode assembly was prepared by notching the cathode and anode to a specific size and stacking them with the separators of Examples and Comparative Examples in between.

[0154] The electrode assembly was placed in a bag and sealed on three sides, except for the electrolyte injection section. The portion containing the electrode tabs was enclosed in the sealing section. Electrolyte was injected through the electrolyte injection section, and the electrolyte injection section was also sealed. The assembly was then impregnated for another 12 hours to produce a 2000 mAh lithium secondary battery.

[0155] A 1 M LiPF6 solution was prepared with a mixed solvent of EC / EMC / DMC (3 / 5 / 2; volume ratio) and used as electrolyte. (2) Measurement of the rolling bending ratio

[0156] As described above, when measuring the thermal shrinkage rate, the length at which roll bending occurred in the TD direction was measured, and the ratio between the length at which roll bending occurred and the total shrinkage length was calculated. (3) Evaluation of storage properties at high temperatures

[0157] The secondary battery prepared as above (1) was stored at 60°C for 20 weeks to check whether the bag was opened.

[0158] If the bag was not opened after 20 weeks of storage, it was rated “◯” and if it was opened, it was rated “X”. (3) Assessment of overload

[0159] After charging the secondary battery prepared above (1) to 5.5 V at 1 C, ignition and smoke evolution were observed for up to 2 hours and evaluated as follows. ◯: No ignition or smoke development △: Partial appearance of smoke X: Ignition event (4) Assessment of heat load

[0160] The secondary battery prepared as above (1) was exposed to 140°C for 1 hour, and the ignition and smoke occurrence were evaluated as follows. ◯: No ignition or smoke development △: Partial appearance of smoke X: Ignition event

[0161] The evaluation results are presented together in Tables 3 and 4 below. [TABLE 3] Example 1 Example 2 Example 3 Example 4 Example 5 Thermal shrinkage rate of the substrate (%) 12 12 16 12 29 Thermal shrinkage rate of the separator (%) 4 5 7 5 14 Ratio of thermal 0,33 0,42 0,44 0,44 0,48 Shrinkage rate between separator and substrate 0.01N TMA maximum shrinkage rate (%) 8 9 12 6 21 0.015N TMA maximum shrinkage rate (%) 5 6 9 0 15 Thermal shrinkage coefficient (kPa) 27,8 23,8 20,8 8,8 12,8 Roll bending ratio (%) 4 6 6 4 9 Storage at high temperatures ◯ ◯ ◯ ◯ ◯ Overload ◯ ◯ ◯ ◯ Δ Heat exposure ◯ ◯ ◯ ◯ Δ [TABLE 4] Comparison example 1 Comparison example 2 Comparison example 3 Comparison example 4 Comparison example 5 Thermal shrinkage rate of the substrate (%) 22 16 29 12 12 Thermal shrinkage rate of the separator (%) 9 4 20 5 7 Ratio of thermal shrinkage rate between separator and substrate 0,41 0,25 0,69 0,42 0,58 0.01N TMA maximum shrinkage rate (%) 18 6 29 11 9 0.015N TMA maximum shrinkage rate (%) 16 3 15 8 7 Thermal shrinkage coefficient (kPa) 41,7 20,8 4,8 30,3 26,3 Roll bending ratio (%) 6 17 4 6 14 Storage at high temperatures ◯ X ◯ ◯ X Overload ◯ X X X X Thermal shrinkage rate of the substrate (%) X X X X X

[0162] From Tables 3 and 4, it is clear that in the examples in which a separator satisfying the above-described range of the thermal shrinkage ratio between the separator and the substrate and the thermal shrinkage coefficient was used, the curling tendency was suppressed overall and the stability at high temperatures and overload was improved.

[0163] In Comparative Example 1, as the substrate's expansion ratio increased, the substrate's thermal shrinkage rate and the thermal shrinkage coefficient also increased. Accordingly, the battery ignition occurred due to heat exposure.

[0164] In Comparative Example 2, a coating layer containing boehmite with a particle diameter of 0.3 µm was formed, so the thermal shrinkage rate of the separator was improved, but roll bending increased significantly when the thermal shrinkage ratio between the separator and the substrate decreased excessively. Furthermore, the stability of the battery also deteriorated due to an increase in side reactions between the components in the coating layer.

[0165] In Comparative Example 3, as the MI of the PE resin used in the substrate increased, the thermal shrinkage rate of the substrate and separator also increased. Accordingly, the thermal shrinkage coefficient decreased, but the thermal shrinkage ratio between the separator and substrate increased, resulting in a deterioration in battery stability.

[0166] In Comparative Example 4, in which the thermal shrinkage coefficient increased excessively, and in Comparative Example 5, in which the thermal shrinkage rate increased significantly, both the high-temperature and overcharge stability deteriorated.

[0167] In Example 5, with increasing MI of the PE resin used in the substrate, the heat transfer rate ratio between the separator and substrate increased relatively compared to the other examples. Accordingly, a small amount of gas was observed during the overload and heat impact evaluation.

Claims

[1] A separator for a secondary battery, comprising: a substrate; and a coating layer formed on a surface of the substrate and containing inorganic particles, wherein the separator has a thermal shrinkage coefficient in a range of 5 kPa to 30 kPa, which is defined by the following equation 1, and a thermal shrinkage rate ratio, defined as the ratio of a thermal TD shrinkage rate of the substrate measured after one hour of storage at 130°C to a thermal TD shrinkage rate of the separator measured after one hour of storage at 130°C, is 0.3 to 0.5: Thermal shrinkage coefficient=(0.015N−0.01N) / A0(L11−L10) / L0 where, in equation 1, A0 is an initial cross-sectional area (m 2 ) of a separator sample, L0 is an initial TD length of the separator sample, L 10is a TD length of the separator sample after a 0.01N TMA measurement and L 11 is a TD length of the separator sample after a 0.015N TMA measurement. [2] A separator for a secondary battery according to claim 1, wherein L 10 and L 11 Lengths are measured when the sample is TD-shrunk with a force of 0.01N and 0.015N to its maximum shrinkage while increasing the temperature at a rate of 5°C / min from room temperature in a nitrogen (N2) atmosphere using a thermomechanical analysis (TMA) apparatus. [3] A separator for a secondary battery according to claim 2, wherein the maximum shrinkage at 0.01N TMA defined by Equation 2-1 is 15% or less: 0.01 maximum shrinkage(%)={1−(L10 / L0)}×100 [4] A separator for a secondary battery according to claim 2 or 3, wherein the maximum shrinkage at 0.015N TMA defined by equation 2-2 is 10% or less: 0.015N maximum shrinkage(%)={1−(L11 / L0)}×100 [5] A separator for a secondary battery according to any one of claims 1 to 4, wherein the thermal shrinkage coefficient is 8 kPa to 28 kPa. [6] A separator for a secondary battery according to any one of claims 1 to 5, wherein the substrate comprises a polyethylene resin having a melt flow index (MI) of 0.05 to 0.3 as measured under conditions of 2.16 kg load at 190°C. [7] A separator for a secondary battery according to any one of claims 1 to 6, wherein the substrate film has a TD stretch ratio of 3 times to 8 times. [8] A separator for a secondary battery according to any one of claims 1 to 7, wherein the substrate has a thermal TD shrinkage rate of 20% or less. [9] A separator for a secondary battery according to any one of claims 1 to 8, wherein the substrate has a thermal TD shrinkage rate of 8% to 18%. [10] A separator for a secondary battery according to any one of claims 1 to 9, wherein the separator has a thermal TD shrinkage rate of 12% or less. [11] A separator for a secondary battery according to any one of claims 1 to 10, wherein the separator has a thermal TD shrinkage rate of 3% to 10%. [12] A separator for a secondary battery according to any one of claims 1 to 11, wherein the inorganic particles have an average particle diameter (D50) of 0.4 µm to 1 µm. [13] A separator for a secondary battery according to any one of claims 1 to 12, wherein the coating layer has a porosity of 40% to 60%. [14] A separator for a secondary battery according to any one of claims 1 to 13, wherein the coating layer is formed only on one surface of the substrate. [15] Secondary battery comprising: a cathode and an anode that are repeatedly stacked; and the separator for a secondary battery according to any one of claims 1 to 14, which is arranged between the cathode and the anode.

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