Separator for secondary battery and method for manufacturing same

Abstract

The present invention relates to: a separator comprising a porous substrate, a ceramic coating layer provided on at least one of respective surfaces of the porous substrate, and a positively charged self-assembled monolayer provided on the ceramic coating layer; a method for manufacturing the separator; and a secondary battery comprising the separator.

Description

Separator for secondary battery and method for manufacturing the same The present invention relates to a secondary battery separator including a self-assembled monolayer, a method for manufacturing the same, and a secondary battery including the separator. Due to the development of the information society, personal IT devices and computer networks have developed, and as a result, the overall dependence of society on electric energy has increased, and the development of technology to efficiently store and utilize electric energy is required. Among the technologies developed for this purpose, the technology most suitable for various purposes is secondary battery-based technology, and the types of secondary batteries widely used today include lithium secondary batteries, nickel cadmium batteries, nickel hydrogen batteries, and nickel zinc batteries. Among these, lithium secondary batteries, which theoretically have the highest energy density, are attracting attention as a power source for small electronic devices such as mobile phones and laptops, as well as electric vehicles and energy storage systems. Lithium secondary batteries mainly use graphite-based materials as anode materials, but since the theoretical capacity limit level has already been utilized, the anode material is a major limitation in implementing lithium secondary batteries with higher energy density. Therefore, there have been numerous attempts to use lithium metal, which has a capacity per mass more than 10 times greater, as anode materials. However, lithium metal electrodes have a problem in that lithium ions are unevenly deposited and eluted on the lithium metal surface during repeated charging and discharging processes, thereby generating lithium dendrites and dead lithium. The grown dendrites can not only cause internal short circuits, but also increase the contact area with the electrolyte, causing repeated generation of SEI (solid electrolyte interphase), which can lead to depletion of the electrolyte and lithium. In addition, during the lithium eluted process, lithium metal falls off, generating dead lithium that is electrically short-circuited. If this process is repeated, reaction products with the electrolyte accumulate on the lithium surface, which can cause increased resistance. Research has been actively conducted to solve these problems, and a representative method is to add additives to the electrolyte to form a reinforced SEI at the interface between the electrolyte and the cathode. However, this method can reduce the uniformity of the SEI, and if the amount of additive added is not properly controlled, there is a risk that the additive will cause a side reaction and lower the performance. Another method is to introduce a protective layer onto the surface of the lithium metal. However, in the process of forming the protective layer, if the slurry is directly applied to the surface of the lithium metal, there is a problem that the organic solvent contained in the slurry reacts with the lithium metal and lowers the stability. On the other hand, even if an aprotic solvent that does not react with the lithium metal is used as the slurry solvent, there is a limitation in that the stability of the lithium metal cannot be fully guaranteed due to the remaining moisture. Therefore, it is necessary to develop a method to overcome these limitations. The present invention is intended to solve the above problems, and provides a separator capable of contributing to stable SEI formation without adding a separate additive to an electrolyte or pretreating an electrode by including a positively charged self-assembled monolayer, a method for manufacturing the same, and a secondary battery having improved cycle characteristics by including the separator. [1] The present invention provides a secondary battery separator including a porous substrate; a ceramic coating layer provided on at least one of both surfaces of the porous substrate; and a self-assembled monolayer having a positive charge provided on the ceramic coating layer. [2] The present invention provides a secondary battery separator in which, when the O 1s orbital of the self-assembled monolayer is analyzed by X-ray photoelectron spectroscopy in the above [1], a peak is observed at 531.8±1.0 eV. [3] The present invention provides a secondary battery separator in which, when the N 1s orbital of the self-assembled monolayer is analyzed by X-ray photoelectron spectroscopy in the above [1] or [2], a peak is observed at 402.6±1.0 eV. [4] The present invention provides a secondary battery separator, wherein in at least one of the above [1] to [3], the self-assembled monolayer comprises a structure derived from a trialkoxysilyl ammonium salt or an aminoalkyl trialkoxysilane. [5] The present invention provides a secondary battery separator, wherein in at least one of the above [1] to [4], the self-assembled monolayer comprises a compound represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, * is a portion that binds to a compound represented by chemical formula 1 in a self-assembled monolayer or binds to the ceramic coating layer. E is -NR1R2R3 + or -NH2, R1 to R3 are each independently hydrogen or an alkyl group, L1 is an alkylene group. [6] The present invention is a method of applying voltage to a lithium symmetric battery having the separator between two lithium metal electrodes to induce polarization and stabilize the current until it reaches a steady state, in at least one of the above [1] to [5], and measuring the lithium ion transference number (t) according to the following equation 1. Li+ ) provides a secondary battery separator having a molecular weight of 0.5 or higher. [Formula 1] In the above equation 1, I0 is the current value (unit: A) in the initial state before polarization is induced. I ss is the current value (unit: A) in steady state, R0 is the interface resistance (unit: Ω) in the initial state before polarization is induced. R ss is the interface resistance value (unit: Ω) in normal state. △V is the voltage value (unit: V) applied to the battery. [7] The present invention provides a secondary battery separator, wherein in at least one of the above [1] to [6], the ceramic coating layer includes at least one ceramic selected from the group consisting of aluminum oxide (Al2O3), boehmite, cerium oxide (CeO2), calcium carbonate hydroxide (CaCO3), barium carbonate (BaCO3), magnesium oxide (MgO), magnesium (Mg(OH)2), aluminum hydroxide (Al(OH)3), clay, silica (SiO2), zirconia (ZrO2), titanium dioxide (TiO2), and zinc oxide (ZnO). [8] The present invention provides a method for manufacturing a separator for a secondary battery, comprising: a first step of preparing a separator substrate having a ceramic coating layer on at least one of both surfaces of a porous substrate; and a second step of forming a self-assembled monolayer by impregnating the surface of the ceramic coating layer with a monolayer forming solution containing a trialkoxysilyl ammonium salt, an aminoalkyl trialkoxysilane, or a combination thereof. [9] The present invention provides a method for manufacturing a separator for a secondary battery, wherein, in the above [8], the trialkoxysilyl ammonium salt is represented by the following chemical formula 2. [Chemical formula 2] In the above chemical formula 2, R1 to R6 are each independently hydrogen or an alkyl group, L2 is an alkylene group, A - is a monovalent anion.

[0010] The present invention provides a method for producing a separator for a secondary battery, wherein, in the above [8] or [9], the aminoalkyl trialkoxysilane is represented by the following chemical formula 3. [Chemical Formula 3] In the above chemical formula 3, R7 to R9 are each independently an alkyl group, L3 is an alkylene group.

[0011] The present invention provides a method for manufacturing a secondary battery separator, wherein the solvent of the solution for forming the monolayer film is alcohol, water, or a combination thereof, in at least one of the above [8] to

[0010] .

[0012] The present invention provides a method for manufacturing a secondary battery separator, wherein the content of the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane in at least one of the above [8] to

[0011] is 0.1 vol% to 1.0 vol% based on the total volume of the solution for forming a monolayer.

[0013] The present invention provides a method for manufacturing a separator for a secondary battery, wherein, in at least one of the above [8] to

[0012] , the impregnation in the second step is performed for 12 to 48 hours.

[0014] The present invention provides a secondary battery including: a positive electrode; a negative electrode; a separator according to at least one of [1] to [7] interposed between the positive electrode and the negative electrode; and an electrolyte.

[0015] The present invention, in the above

[0014] , the negative electrode is lithium metal, or natural graphite, artificial graphite, SiO as the negative electrode active material. x A secondary battery is provided, which comprises (0≤x<2) or a mixture thereof. The secondary battery separator according to the present invention includes a self-assembled monolayer having a positive charge, so that PF6 is formed at the interface in contact with the electrolyte. - It can create an environment with high anion content in lithium salts, and thus contribute to the formation of a LiF-rich SEI layer on the cathode. That is, when the above separator is used, the problems of occurrence of lithium dendrites and dead lithium resulting from negative electrode degeneration, and increased resistance and decreased durability of secondary batteries due to these can be solved without a separate process of adding electrolyte additives or pretreating electrodes. Ultimately, a secondary battery including the above separator has the effect of improving cycle characteristics. Figure 1 is a diagram showing the change in cell voltage over time of a lithium symmetric cell using the separator manufactured in Example 1 and Comparative Example 2. Figure 2 is a diagram showing the change in cell voltage over time of a lithium symmetric cell using the separator manufactured in Example 1 and Comparative Example 2. Figure 3 is a diagram showing the XPS analysis results for the surface of the membrane manufactured in Example 1. Figure 4 is a diagram showing the XPS analysis results for the surface of the membrane manufactured in Comparative Example 2. Figure 5 is a diagram showing the XPS analysis results for the surface of the membrane manufactured in Comparative Example 3. Figure 6 is a diagram showing the results of measuring the water contact angle on the surface of the membrane manufactured in Example 1. Figure 7 is a diagram showing the results of measuring the water contact angle on the surface of the membrane manufactured in Comparative Example 2. Figure 8 is a diagram showing the results of measuring the water contact angle on the surface of the membrane manufactured in Comparative Example 3. Figure 9 is a diagram showing the XPS analysis results for the SEI layer of lithium metal recovered from a lithium symmetric cell using the separator manufactured in Example 1. Figure 10 is a diagram showing the XPS analysis results for the SEI layer of lithium metal recovered from a lithium symmetric cell using the separator manufactured in Comparative Example 2. Figure 11 is a diagram showing the results of XPS analysis of the SEI layer of lithium metal recovered from a lithium symmetric cell using the separator manufactured in Comparative Example 3. Hereinafter, the present invention will be described in more detail. In the present invention, “X-ray photoelectron spectroscopy (XPS)” is 5.0×10 by Axis Supra X-ray photoelectron spectrometer (Kratos Analytical Ltd). -10 It was performed using a monochromatic Al-Kα source under ultra-high vacuum of less than 10 Torr. As the electric vehicle market grows rapidly, the demand for high-capacity batteries is increasing, and in response, lithium metal anodes are attracting attention as a high-energy-density anode. Since lithium metal exhibits a much larger theoretical capacity and lower operating voltage than graphite, which is currently the most commercialized anode material, it is suitable for implementing a high-energy-density secondary battery. However, as explained above, there is a problem in that dendrites easily grow on the surface of the lithium metal when charge and discharge are repeated. These dendrites not only cause damage to the separator, but also rapidly increase the reactivity between the lithium metal and the electrolyte, which becomes a factor in the generation of a polymer film without electrical conductivity, which can lead to a decrease in battery performance. To overcome these problems, the inventors of the present invention introduced a self-assembled monolayer carrying a positive charge into the separator, and found that this can complement the limitations of the lithium metal anode by forming a LiF-rich SEI layer on the anode. Below, each component of the present invention is described in more detail. Membrane and method for producing same Specifically, a separation membrane according to one embodiment of the present invention comprises a porous substrate; a ceramic coating layer provided on at least one of both surfaces of the porous substrate; and a positively charged self-assembled monolayer provided on the ceramic coating layer. The self-assembled monolayer has a strong positive charge, so it can trap the anions of lithium salts in the electrolyte by electrostatic attraction. For example, LiPF6, the most widely used lithium salt, is PF6 - As the self-assembled monolayer is distributed around the self-assembled monolayer by electrostatic attraction, an environment similar to a high-concentration electrolyte is created at the interface, and the LUMO LEVEL of the lithium salt in the electrolyte is lowered, so that the formation of the SEI layer is caused by the decomposition reaction of the lithium salt anion (PF6) rather than the decomposition reaction of the organic solvent. - →PF5+F -) is formed. That is, a LiF-rich SEI is formed on the cathode, and since LiF exhibits chemically stable characteristics compared to other SEI components, the SEI layer can maintain a stable structure even during repeated charge / discharge processes. Therefore, not only can dendrites be physically inhibited from growing through the SEI layer, but by inhibiting the growth of dendrites, the isolation phenomenon of lithium can also be prevented, thereby suppressing the generation of dead lithium. Since this is an effect due to the electrostatic attraction between a positively charged self-assembled monolayer and lithium salt anions, the present invention has a great advantage in that it can be applied regardless of the type of lithium salt. Meanwhile, in order to manufacture the above membrane, A first step of preparing a membrane substrate having a ceramic coating layer on at least one of both surfaces of a porous substrate; and A second step of forming a self-assembled monolayer by impregnating the surface of the ceramic coating layer with a solution containing a trialkoxysilyl ammonium salt, an aminoalkyl trialkoxysilane, or a combination thereof must be performed. Specifically, in the second step, the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane can bind to the ceramic coating layer by the alkoxy group included in the structure, and more specifically, the alkoxy group and the surface of the ceramic coating layer can undergo a dehydration condensation reaction to form a self-assembled monolayer. For example, when the ceramic coating layer includes boehmite or alumina, water molecules (H2O) are adsorbed to the surface of the ceramic coating layer due to the polarity of the -OH or -O functional group. As a result, the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane undergoes a hydrolysis reaction to have -OH at the terminal, and this can undergo a dehydration condensation reaction with the nearby -OH or -O to form a self-assembled monolayer. Below, each component of the membrane and the first and second steps are described in more detail. (a) Ceramic coating layer (step 1) First, a membrane substrate having a ceramic coating layer on at least one of the two sides of a porous substrate is prepared. The above membrane substrate can be obtained and used as a commercially available product, or can be manufactured by coating a ceramic-containing slurry on at least one side of a porous substrate. The above porous substrate may be a polyolefin-based porous polymer substrate. The polyolefin-based porous polymer substrate may be a substrate including one or more polymers selected from the group consisting of polyethylene, polypropylene, ethylene vinyl acetate, ethylene butylacrylate, and ethylene ethyl acrylate, or a copolymer therebetween, and preferably may include polyethylene. Specifically, the thickness of the porous substrate may be 5 ㎛ to 20 ㎛, preferably 5 ㎛ to 15 ㎛. The above ceramic coating layer may include one or more ceramics selected from the group consisting of aluminum oxide (Al2O3), boehmite, cerium oxide (CeO2), calcium carbonate hydroxide (CaCO3), barium carbonate (BaCO3), magnesium oxide (MgO), magnesium (Mg(OH)2), aluminum hydroxide (Al(OH)3), clay, silica (SiO2), zirconia (ZrO2), titanium dioxide (TiO2), and zinc oxide (ZnO), and specifically may include aluminum oxide (Al2O3) or boehmite, and more specifically may include boehmite. When the ceramic coating layer includes boehmite, it is preferable in that it is rich in -OH functional groups that can bond with molecules constituting the self-assembled monolayer, for example, structures derived from trialkoxysilyl ammonium salts or aminoalkyl trialkoxysilanes. The above ceramic-containing slurry may include a binder together with the ceramic, and specifically may include a polyvinylidene fluoride (PVDF)-based binder. Meanwhile, the content of the ceramic based on the total weight of the ceramic coating layer, that is, the content of the ceramic based on the total weight of the solid content in the ceramic-containing slurry, may be 50 wt% to 90 wt%, specifically 60 wt% to 90 wt%. By adjusting the content of the ceramic within the above-described range, the heat resistance, durability, and stability of the separator can be simultaneously improved. Meanwhile, the content of the binder based on the total weight of the ceramic coating layer, that is, the content of the binder based on the total weight of the solid content in the ceramic-containing slurry, may be 10 wt% to 50 wt%, specifically 10 wt% to 40 wt%. The above PVDF-based binder may be at least one selected from the group consisting of, for example, polyvinylidene fluoride homopolymer, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-trichloroethylene (PVDF-TCE), and polyvinylidene fluoride-trifluoroethylene (PVDF-CTFE). More specifically, polyvinylidene fluoride homopolymer or polyvinylidene fluoride-hexafluoropropylene copolymer can be used. The above binder may have a weight average molecular weight (Mw) of 500,000 g / mol to 1,500,000 g / mol, specifically 1,000,000 g / mol to 1,500,000 g / mol, and two or more types having different weight average molecular weights may be mixed and used. By using a PVDF-based binder within the above molecular weight range, the adhesive strength between the porous substrate and the ceramic coating layer is strengthened, so that the porous substrate, which is weak to heat, can be effectively suppressed from shrinking due to heat, and also, a separator having sufficiently improved electrolyte impregnation property can be manufactured, so that there is an advantage in that a battery having efficient electrical output can be produced by utilizing the same. Meanwhile, the thickness of the ceramic coating layer may be 1 ㎛ to 10 ㎛, preferably 1 ㎛ to 5 ㎛. In addition, the coating of the first step can be performed through dip coating, die coating, gravure coating, comma coating or inkjet printing, and a drying process can be performed at a temperature of 25°C to 100°C after coating. (b) Self-assembled monolayer (2nd stage) Next, the surface of the ceramic coating layer is impregnated with a solution containing a trialkoxysilyl ammonium salt, an aminoalkyl trialkoxysilane, or a combination thereof to form a self-assembled monolayer. In this case, since the self-assembled monolayer contains Si-O bonds, when the O 1s orbital of the self-assembled monolayer is analyzed by X-ray photoelectron spectroscopy, a peak can be observed at 531.8±1.0 eV. Meanwhile, when using trialkoxysilyl ammonium salt in the second step, the self-assembled monolayer is N + -C bond may be included, and accordingly, when the N 1s orbital of the self-assembled monolayer is analyzed, a peak may be observed at 402.6±1.0 eV. In addition, when an aminoalkyl trialkoxysilane is used in the second step, the self-assembled monolayer may include a C-NH2 bond, and accordingly, when the self-assembled N 1s orbital is analyzed, a peak may be observed at 399.4±1.0 eV. When a mixture of trialkoxysilyl ammonium salt and aminoalkyl trialkoxysilane is used in the second step, both peaks can be observed. In addition, since the hydrophilicity of the membrane changes during the formation of the self-assembled monolayer, the contact angle with water also changes, and thus the formation of the self-assembled monolayer can also be confirmed by a method of measuring the contact angle of the membrane with water. For example, the contact angle between the membrane and water according to one embodiment of the present invention may be 25° or more, specifically 30° to 55°, and more specifically 40° to 50°. Meanwhile, the self-assembled monolayer may include a structure derived from a trialkoxysilyl ammonium salt or an aminoalkyl trialkoxysilane, and preferably may include a structure derived from a trialkoxysilyl ammonium salt. The trialkoxysilyl ammonium salt is more preferable because it can interact more strongly with the anion of the lithium salt because the polarity of the terminal group is stronger. As a preferred example, the self-assembled monolayer may include a compound represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, * is a portion that binds to a compound represented by chemical formula 1 in a self-assembled monolayer or binds to the ceramic coating layer. E is -NR1R2R3 + or -NH2, R1 to R3 are each independently hydrogen or an alkyl group, L1 is an alkylene group. Meanwhile, the trialkoxysilyl ammonium salt may be represented by the following chemical formula 2, and more specifically, may be N-[3-(trimethoxysilyl)propyl]-N,N,N-trimethylammonium chloride. [Chemical formula 2] In the above chemical formula 2, R1 to R6 are each independently hydrogen or an alkyl group, L2 is an alkylene group, A - is a monovalent anion. In addition, when the self-assembled monolayer includes a structure derived from an aminoalkyl trialkoxysilane, the amino group located at the terminal may be partially positively charged due to a dipole moment. Specifically, the aminoalkyl trialkoxysilane may be represented by the following chemical formula 3, and more specifically, may be 3-aminopropyltrimethoxysilane. [Chemical Formula 3] In the above chemical formula 3, R7 to R9 are each independently an alkyl group, L1 is an alkylene group. The above chemical formula 1 is a structure derived from a trialkoxysilyl ammonium salt or an aminoalkyl trialkoxysilane, i.e., a structure derived from chemical formula 2 or 3. In the above chemical formulas 1 and 2, R1 to R3 may each independently be an alkyl group having 1 to 3 carbon atoms, and preferably may each be a methyl group. In the above chemical formula 2, R4 to R6 may each independently be an alkyl group having 1 to 3 carbon atoms, and preferably may each be a methyl group. In the above chemical formulas 1 to 3, L1 to L3 may each be an alkylene group having 1 to 10 carbon atoms, preferably an alkylene group having 1 to 5 carbon atoms, and more preferably a propylene group. In the above chemical formula 2, A - is Cl - , Br - , I - , F - Or OH - It can be, preferably Cl - It could be. In the above chemical formula 3, R7 to R9 can each independently be an alkyl group having 1 to 3 carbon atoms, and preferably, can each be a methyl group. That is, the self-assembled monolayer may include a structure derived from N-[3-(trimethoxysilyl)propyl]-N,N,N-trimethylammonium chloride or 3-aminopropyltrimethoxysilane. Meanwhile, the above separator has a lithium ion transference number (t) according to the following equation 1.Li+ ) is 0.5 or more, preferably 0.6 or more, and more preferably 0.7 or more. However, since the anion cannot be completely fixed in the electrolyte, it may be less than 1.0, specifically 0.8 or less. [Formula 1] In the above equation 1, I0 is the current value (unit: A) in the initial state before polarization is induced. I ss is the current value (unit: A) in steady state, R0 is the interface resistance (unit: Ω) in the initial state before polarization is induced. R ss is the interface resistance value (unit: Ω) in normal state. △V is the voltage value (unit: V) applied to the battery. Specifically, the lithium ion yield is measured by applying voltage to a lithium symmetrical battery equipped with the separator between two lithium metal electrodes to induce polarization and stabilize the current until it reaches a steady state, and does not represent the absolute mobility of lithium ions. + Since it is an indicator showing the relative mobility between anions and anions, it is suitable for representing the performance of the membrane in limiting the movement of anions. Meanwhile, the solvent of the solution for forming the monolayer may be alcohol, water or a combination thereof, and the alcohol may be ethanol or methanol. Preferably, the solvent may be alcohol, more preferably ethanol. In addition, the content of the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane may be 0.1 vol% to 1.0 vol%, preferably 0.1 vol% to 0.5 vol%, more preferably 0.1 vol% to 0.3 vol%, based on the total volume of the solution for forming the monolayer. When the content of the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane is 0.1 vol% or more, it is easy to form a self-assembled monolayer capable of implementing the above-described effect. However, if the content of the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane is excessive, a monolayer film may not be formed but the material may be irregularly stacked, so it is preferable that it does not exceed 1.0 vol%. Meanwhile, the impregnation of the second step may be performed for 12 to 48 hours, preferably 20 to 30 hours, and after the impregnation, a washing process with ethanol at room temperature may be performed, and after the washing, a drying process may be performed at a temperature of 25 to 40°C. In order to prevent damage to the separator, the temperature of the drying process is preferably 40°C or lower. When a self-assembled monolayer is formed according to the present invention, a self-assembled monolayer having a thickness of several nm is formed, so it does not act as a resistor that impedes the movement of lithium ions. Secondary battery In addition, a secondary battery according to the present invention includes a cathode, an anode, a separator interposed between the cathode and the anode, and an electrolyte, wherein the separator is according to the present invention described above. The secondary battery of the present invention can be manufactured according to a conventional method known in the art, and components excluding the separator, such as the positive electrode, negative electrode, and electrolyte, can be used in the present invention without limitation as long as they are conventionally used in secondary batteries; however, reference may be made to the following description as a preferred example. The above positive electrode may include a positive electrode current collector made of a metal sheet having excellent conductivity, for example, aluminum foil, and a positive electrode active material layer coated on one or both surfaces thereof. The positive electrode active material layer may include a lithium metal oxide including lithium and a transition metal such as cobalt, manganese, and / or nickel as a positive electrode active material, and may further include a conductive material and / or a binder as necessary. The positive electrode active material, conductive material, and binder may be various materials commonly used in the manufacture of secondary batteries without limitation. Meanwhile, considering the effect of the aforementioned separator, the negative electrode is preferably lithium metal, but regardless of the type of the negative electrode, lithium dendrite is the cause of performance degradation and the common goal is to form a reinforced SEI layer, so it is not limited to a lithium metal negative electrode. That is, the negative electrode may be lithium metal, or may be natural graphite, artificial graphite, or SiO as the negative electrode active material. x (0≤x<2) or mixtures thereof. For example, the negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector and including a negative electrode active material. The above negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 ㎛ to 500 ㎛, and fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc. The above negative electrode active material layer optionally includes a binder and a conductive material together with the negative electrode active material. As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; SiO x (0 <x< 2), SnO2, 바나듐 산화물, 리튬 바나듐 산화물과 같이 리튬을 도프 및 탈도프할 수 있는 금속산화물; 또는 Si-C 복합체 또는 Sn-C 복합체과 같이 상기 금속질 화합물과 탄소질 재료를 포함하는 복합물 등을 들 수 있으며, 이들 중 어느 하나 또는 둘 이상의 혼합물이 사용될 수 있다. Specifically, the above negative electrode active material layer is made of natural graphite, artificial graphite, SiO as a negative electrode active material. x (0≤x<2) or a mixture thereof, and more specifically, a mixture of artificial graphite and natural graphite may be included. The negative electrode active material may be included in an amount of 80 wt% to 99 wt% based on the total weight of the negative electrode active material layer. The above binder is a component that assists in bonding between the conductive agent, the active material, and the current collector, and is typically added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, various copolymers thereof, and the like. The conductive agent is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. The conductive agent is not particularly limited as long as it has conductivity and does not induce a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fiber or metal fiber; fluorinated carbon; metal powder such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. may be used. The above negative active material layer can be manufactured by applying and drying a negative electrode mixture prepared by dissolving or dispersing a negative electrode active material and, optionally, a binder and a conductive material in a solvent on a negative electrode current collector, or by casting the negative electrode mixture on a separate support and then laminating the resulting film on a negative electrode current collector by peeling it off from the support. Meanwhile, the lithium metal as the cathode may have a thickness of 100 ㎛ to 500 ㎛, preferably 200 ㎛ to 400 ㎛. In addition, since the SEI layer formed on the cathode is rich in LiF components, when the Li 1s orbital of the SEI layer is analyzed by XPS, a strong peak corresponding to the Li-F binding energy may appear. Meanwhile, as the electrolyte, any electrolyte capable of moving lithium ions generated by an electrochemical reaction at the electrode during charge and discharge can be used without limitation, and for example, it can be a liquid electrolyte in which a lithium salt is dissolved in an organic solvent. The lithium salt may be used without any particular limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery and fluorine ions required for LiF production. Specifically, the lithium salt may be at least one selected from LiPF6, LiAsF6, LiBF4, LiSbF6, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, and LiN(CF3SO2)2. The concentration of the lithium salt may be appropriately changed within a generally usable range, but is preferably used within a range of 0.1 M to 5.0 M, and preferably 0.1 M to 3.0 M. However, when the impregnation property of the above-mentioned separator is taken into consideration, the viscosity of the electrolyte may be 0.5 cP to 6.0 cP, preferably 2.0 cP to 5.0 cP, and more preferably 3.0 cP to 4.0 cP. Any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without any special restrictions. For example, cyclic carbonate-based solvents such as ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate, linear carbonate-based organic solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, and ethyl methyl carbonate (EMC), or mixed organic solvents thereof can be used. Meanwhile, the present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same. The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as, for example, a power tool; a means of transportation such as an electric bicycle, an electric car, or a hybrid electric vehicle (HEV); and a power storage system. Hereinafter, the present invention will be described in more detail through specific examples. [Example: Manufacturing of a membrane] Example 1. A membrane substrate was prepared in which a boehmite-containing coating layer having a thickness of 3 ㎛ was provided on each side of a polyethylene porous film having a thickness of 11 ㎛. Meanwhile, a solution for forming a monolayer was prepared by dissolving N-[3-(trimethoxysilyl)propyl]-N,N,N-trimethylammonium chloride in ethanol at a concentration of 0.2 vol%. The above membrane substrate was completely immersed in the above monolayer forming solution for 24 hours, washed using ethanol at room temperature, and dried at a temperature of 40°C or lower to complete a membrane in which a self-assembled monolayer was formed on both sides of the above membrane substrate. Comparative example 1. A polyethylene porous film with a thickness of 11 μm was prepared as a separator. Comparative example 2. In the above Example 1, a membrane substrate was prepared as a membrane without forming a self-assembled monolayer, that is, the membrane substrate used in Example 1. Comparative example 3. A membrane was prepared in the same manner as in Example 1, except that trichloro(hexyl)silane was used instead of N-[3-(trimethoxysilyl)propyl]-N,N,N-trimethylammonium chloride. [Experimental example] Experimental Example 1. Confirmation of Formation of Self-Assembly Monolayer (1) XPS analysis The results of analyzing the N 1s orbitals of the membranes manufactured in Example 1, Comparative Example 2, and Comparative Example 3 by X-ray photoelectron spectroscopy are shown in Figures 3 to 5, respectively. Looking at Figure 3, which is the XPS analysis result for Example 1, the membrane of Example 1 is CN + It can be confirmed that a peak appears at 402.6 eV corresponding to the binding energy. However, in Figs. 4 and 5, which are the XPS analysis results for Comparative Examples 2 and 3, it can be confirmed that no peak appears at the corresponding binding energy. Through this, it can be confirmed that a self-assembled monolayer with a positive charge was formed on the separator of Example 1. Meanwhile, the peak corresponding to the CN binding energy appeared due to the material used as a binder in the ceramic coating, and therefore appeared in both Example 1 and Comparative Examples 2 and 3. (2) Contact angle measurement The contact angle of the membranes manufactured in Example 1, Comparative Examples 2, and 3 for water was measured using a contact angle measuring device (Drop shape analysis system, DSA100) at 25°C. Specifically, 5 μl of distilled water was dropped on the surface of each membrane, and the angle between the membrane surface and the water droplet was measured, and the results are shown in FIGS. 6 to 8. As shown in FIG. 6, the contact angle of the membrane of Example 1 was measured to be about 46°, as shown in FIG. 7, the contact angle of the membrane of Comparative Example 2 was measured to be about 22°, and as shown in FIG. 8, the contact angle of the membrane of Comparative Example 3 was measured to be about 30°. Since the hydroxyl group having a high affinity for water adsorbed on the surface by the hydrolysis reaction was replaced with chemical formula 2 when the self-assembled monolayer was formed on the surface of the membrane of Example 1, it was less hydrophilic and showed a larger contact angle than the membrane of Comparative Example 2. In the case of Comparative Example 3, it can be confirmed that this effect did not occur because a positively charged self-assembled monolayer was not formed, and thus the contact angle did not increase significantly. Experimental Example 2. Membrane Performance Evaluation (1) Manufacturing of Li symmetrical cell First, a solution was prepared by dissolving 1 M LiPF6 in an organic solvent containing ethylene carbonate (EC): diethyl carbonate (DEC): dimethyl carbonate (DMC) as a liquid electrolyte in a volume ratio of 1:1:1. Two 300 μm thick lithium metal sheets were prepared as the negative and counter electrodes, the separator manufactured in Example 1 was interposed between the two lithium metals, and the liquid electrolyte was injected to manufacture a 2032 coin-type lithium symmetrical cell. A lithium symmetric cell was also manufactured in the same manner, inserting the separators manufactured in Comparative Examples 1 to 3 instead of the separator manufactured in Example 1. (2) Lithium ion yield evaluation At 25°C, a voltage of 10 mV was applied to each of the lithium symmetric cells manufactured above to induce polarization in the electrodes, and the current was stabilized until it reached a steady state. The current and interface resistance values ​​before polarization induction and at the steady state were measured using a potentiometer, and the lithium ion yield was calculated by substituting the values ​​into Equation 1, as shown in Table 1 below. △V (V)I0(A)I ss (A)R0(Ω)R ss (Ω)t Li+ Example 10.010.00004560.00004261651600.726Comparative Example 10.010.00004380.00002502003000.283Comparative Example 20.010.00004500.00004101751400.454Comparative Example 30.010.00004400.00002601902900.394 Through the results in Table 1, it can be confirmed that the separator of Example 1 including a positively charged self-assembled monolayer has the effect of lowering the interfacial resistance and increasing the lithium ion yield compared to the separators of Comparative Examples 1 and 2 which do not include a self-assembled monolayer and the separator of Comparative Example 3 which includes a non-positively charged self-assembled monolayer. (3) Check voltage change The lithium symmetrical cells of each of the above-mentioned manufactured examples 1, 2 and 3 were charged and discharged at 25°C with a charge / discharge capacity of 1 mAh / cm. 2 and current density 1mA / cm 2 By performing lithium deposition / desorption cycles, a voltage profile as shown in Fig. 1 was obtained. Charge / discharge capacity 2mAh / cm 2 and current density 1mA / cm 2 Even under the conditions, lithium deposition / desorption cycles were performed to obtain a voltage profile as shown in Fig. 2. Generally, in voltage profiles such as those in FIGS. 1 and 2, the point at which the voltage increases is judged as the point at which the cathode degrades. Looking at FIGS. 1 and 2, it can be seen that the voltage of the cell to which the separator of Example 1 was applied is maintained without significant change over time, whereas the voltage of the cell to which the separators of Comparative Examples 2 and 3 were applied increases rapidly from a certain point in time. (4) Analysis of SEI layer components The lithium symmetrical cells of each of the above-mentioned manufactured examples 1, 2 and 3 were each charged at 1 mA / cm at 30°C. -2 1 mAh / cm with a current density of (2.011 mA) -2(2.011 mAh) capacity was charged under CC conditions, and the cell was activated (formed) by CC discharge with the same current density and capacity. This type of charge / discharge was considered one cycle, and 25 cycles were repeated. Then, the lithium metal electrode was separated from the cell, and the Li 1s orbital of the SEI layer formed on the electrode was analyzed by X-ray photoelectron spectroscopy. The results are shown in Figures 9 to 11. Comparing FIG. 9 (Example 1) with FIG. 10 (Comparative Example 2) and FIG. 11 (Comparative Example 3), it can be confirmed that the peak corresponding to the ROOC-Li binding energy, which is a decomposition product of the carbonate solvent, in the electrode included in the cell of Example 1 decreased compared to the electrodes included in the cells of Comparative Examples 2 and 3, but the peak corresponding to the Li-F binding energy significantly increased. Through this, it can be confirmed that the electrode of Example 1 includes a more LiF component in the SEI layer than the electrodes of Comparative Examples 2 and 3.

Claims

1. Porous substrate; A ceramic coating layer provided on at least one of the two sides of the porous substrate; and A separator for a secondary battery, comprising a self-assembled monolayer having a positive charge provided on the ceramic coating layer.

2. In claim 1, A secondary battery separator, wherein a peak is observed at 531.8±1.0 eV when the O 1s orbital of the self-assembled monolayer is analyzed by X-ray photoelectron spectroscopy.

3. In claim 1, A secondary battery separator, wherein a peak is observed at 402.6±1.0 eV when the N 1s orbital of the self-assembled monolayer is analyzed by X-ray photoelectron spectroscopy.

4. In claim 1, A secondary battery separator, wherein the self-assembled monolayer comprises a structure derived from a trialkoxysilyl ammonium salt or an aminoalkyl trialkoxysilane.

5. In claim 1, The above self-assembled monolayer is a secondary battery separator comprising a compound represented by the following chemical formula 1: [Chemical Formula 1] In the above chemical formula 1, * is a portion that binds to a compound represented by chemical formula 1 in a self-assembled monolayer or binds to the ceramic coating layer. E is -NR1R2R3 + or -NH2, R1 to R3 are each independently hydrogen or an alkyl group, L1 is an alkylene group.

6. In claim 1, The lithium ion transference number (t) according to the following equation 1 was measured by applying voltage to a lithium symmetric battery equipped with the above separator between two lithium metal electrodes to induce polarization and stabilize the current until it reaches a steady state. Li+ ) is 0.5 or higher, a separator for secondary batteries: [Formula 1] In the above equation 1, I0 is the current value (unit: A) in the initial state before polarization is induced. I ss is the current value (unit: A) in steady state, R0 is the interface resistance (unit: Ω) in the initial state before polarization is induced. R ss is the interface resistance value (unit: Ω) in normal state. △V is the voltage value (unit: V) applied to the battery.

7. In claim 1, A secondary battery separator, wherein the ceramic coating layer comprises at least one ceramic selected from the group consisting of aluminum oxide (Al2O3), boehmite, cerium oxide (CeO2), calcium carbonate hydroxide (CaCO3), barium carbonate (BaCO3), magnesium oxide (MgO), magnesium (Mg(OH)2), aluminum hydroxide (Al(OH)3), clay, silica (SiO2), zirconia (ZrO2), titanium dioxide (TiO2), and zinc oxide (ZnO).

8. A first step of preparing a membrane substrate having a ceramic coating layer on at least one of the two sides of a porous substrate; and A method for manufacturing a separator for a secondary battery, comprising a second step of forming a self-assembled monolayer by impregnating the surface of the ceramic coating layer with a monolayer forming solution containing a trialkoxysilyl ammonium salt, an aminoalkyl trialkoxysilane, or a combination thereof.

9. In claim 8, The above trialkoxysilyl ammonium salt is represented by the following chemical formula 2, a method for producing a separator for a secondary battery: [Chemical formula 2] In the above chemical formula 2, R1 to R6 are each independently hydrogen or an alkyl group, L2 is an alkylene group, A - is a monovalent anion.

10. In claim 8, A method for producing a separator for a secondary battery, wherein the aminoalkyl trialkoxysilane is represented by the following chemical formula 3: [Chemical Formula 3] In the above chemical formula 3, R7 to R9 are each independently an alkyl group, L3 is an alkylene group.

11. In claim 8, A method for manufacturing a separator for a secondary battery, wherein the solvent of the solution for forming the above monolayer is alcohol, water or a combination thereof.

12. In claim 8, A method for producing a separator for a secondary battery, wherein the content of the trialkoxysilyl ammonium salt or the aminoalkyl trialkoxysilane is 0.1 vol% to 1.0 vol% based on the total volume of the solution for forming the monolayer.

13. In claim 8, A method for manufacturing a separator for a secondary battery, wherein the impregnation in the second step is performed for 12 to 48 hours.

14. A secondary battery comprising: a cathode; a separator according to claim 1 interposed between the cathode and the anode; and an electrolyte.

15. In claim 14, The above negative electrode is lithium metal, or natural graphite, artificial graphite, SiO as the negative electrode active material. x A secondary battery comprising (0≤x<2) or a mixture thereof.

Images