Separator for lithium-sulfur battery, lithium-sulfur battery including separator, and method for manufacturing separator

Abstract

A separator for a lithium-sulfur battery according to one embodiment of the present invention includes a porous polymer substrate and a coating layer disposed on at least one surface of the porous polymer substrate, the coating layer including a polyamic acid compound having both carboxylic acid functional groups and amide functional groups, wherein the molar ratio of the carboxylic acid functional groups to the amide functional groups is 1:0.5 to 1:5. The coating layer including the polyamic acid compound on the separator prevents lithium polysulfides leached from the positive electrode of the lithium-sulfur battery from being transferred to the negative electrode. This prevents sulfur particles from being deposited on the surface of the lithium metal in the negative electrode, thereby maintaining the charge / discharge capacity of the lithium-sulfur battery and improving its lifespan.

Description

[Technical Field]

[0001] The present invention relates to a separator capable of suppressing the migration of polysulfides eluted from a positive electrode and improving the lifespan, and a secondary battery including the separator.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0110367, filed on August 31, 2022, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings thereof. [Background technology]

[0003] As technological development and demand for mobile devices increases, the demand for secondary batteries as an energy source is rapidly increasing. Among secondary batteries, lithium secondary batteries have become common and are widely used due to their high energy density, working potential, long cycle life, and low self-discharge rate.

[0004] In addition, in recent years, with growing interest in environmental issues, active research has been conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can replace vehicles that use fossil fuels such as gasoline and diesel vehicles, which are one of the main causes of air pollution. Lithium secondary batteries, which have high energy density, high discharge voltage, and output stability, are being primarily researched and used as power sources for such electric vehicles and hybrid electric vehicles.

[0005] A lithium secondary battery has a structure in which a porous separator is interposed between an electrode assembly including a positive electrode and a negative electrode, each of which has an active material coated on a current collector, and a non-aqueous electrolyte containing a lithium salt is impregnated in the electrode assembly.

[0006] Currently, the lithium secondary battery market is dominated by technology based on pairing a lithium cobalt oxide (LiCoO2) cathode with a graphite anode. The rated voltage of these batteries is approximately 3.6 V, compared to 1.5 V for most other battery types (e.g., Ni-CD, Ni-MH). Their volumetric and mass energy densities are approximately 300-500 Wh / l and 160-200 Wh / kg, respectively. These are the highest values ​​of all batteries currently on the market. These batteries also have low self-discharge and long lifespans (500 or 1,000 cycles). Despite these impressive performances, all current lithium-ion batteries have plateaued in performance, with limited potential for improvement.

[0007] Therefore, lithium-sulfur (Li-S) batteries are gaining attention as an alternative to lithium-ion batteries.

[0008] Like conventional lithium-ion secondary batteries, lithium-sulfur batteries operate by lithium ions moving within an electrolyte between a positive electrode and a negative electrode. However, because lithium-sulfur batteries use only simple sulfur, they operate based on a redox reaction between sulfur and lithium ions, unlike conventional lithium-ion secondary batteries, in which lithium ions enter the gaps between the molecules of the electrode active material, transforming the electrode structure to store energy. Therefore, lithium-sulfur batteries are not limited in electrode structure compared to conventional lithium-ion secondary batteries, and theoretically can have a larger capacity for the same volume. Due to these characteristics, in a lithium-sulfur battery consisting of a sulfur cathode and a lithium metal anode, assuming that the ring-structured monomeric sulfur (S8) reacts completely to form lithium polysulfide (Li2S), the theoretical capacity is 1,675mAh / g and the theoretical energy density is 2,600Wh / kg, which is three to six times higher than other conventional battery systems (Ni / MH battery: 450Wh / kg, Li / FeS: 480Wh / kg, Li / MnO2: 1,000Wh / kg, Na / S: 800Wh / kg).

[0009] On the other hand, conventional transition metal oxide-based lithium-ion secondary batteries are considered to contain heavy metal pollutants because their cathodes use oxides of nickel (Ni), cobalt (Co), and manganese (Mn), which have densities higher than those of heavy metals (metals of 5 g / mL or more). However, lithium-sulfur batteries are environmentally friendly because they eliminate these pollutants and use non-toxic materials. Furthermore, sulfur, the cathode material, has the advantage of being abundant and inexpensive.

[0010] Meanwhile, in lithium-sulfur batteries, sulfur reduction and lithium metal oxidation occur during discharge. During this process, sulfur converts from a ring-shaped S8 to a linear lithium polysulfide (LiPS). These batteries exhibit a gradual discharge voltage until the lithium polysulfide is completely reduced to Li2S. However, during the charge / discharge process, lithium-sulfur batteries experience reduced charge / discharge efficiency, resulting in a shortened battery life. This shortened life of lithium-sulfur batteries can be attributed to a variety of factors, including electrolyte side reactions, lithium metal instability, and the accumulation of by-products on the cathode (e.g., lithium polysulfide leaching from the cathode).

[0011] Lithium-sulfur batteries, which use sulfur-based compounds as the positive electrode active material and alkali metals such as lithium as the negative electrode active material, suffer from the problem of lithium polysulfide leaching during charging and discharging. The lithium polysulfide leached from the positive electrode is transferred to the negative electrode, reducing the capacity of the lithium-sulfur battery and shortening its lifespan. Because lithium polysulfide leached from the positive electrode has high solubility in the electrolyte, it passes through the separator membrane in the electrolyte and unintentionally migrates to the negative electrode, resulting in a decrease in capacity due to irreversible loss of the positive electrode active material and a decrease in battery life due to the deposition of sulfur particles on the lithium metal surface due to side reactions.

[0012] To solve the problem of reduced battery life due to lithium polysulfide, the industry has conducted research into adding lithium polysulfide adsorbents to the positive electrode composite, or adding reaction-reducing substances to the negative electrode to prevent side reactions on the surface of lithium metal, but no significant results have been achieved. Summary of the Invention [Problem to be solved by the invention]

[0013] An object of the present invention is to provide a separator that can improve the lifespan of a lithium-sulfur battery by preventing lithium polysulfide leached from a positive electrode from being transferred to a negative electrode.

[0014] Another problem to be solved by the present invention is to provide a lithium-sulfur battery including the separator.

[0015] A problem to be solved by a still further aspect of the present invention is to provide a method for producing the separation membrane.

[0016] Furthermore, other objects and advantages of the present invention can be realized by the means or methods set forth in the claims, and combinations thereof. [Means for solving the problem]

[0017] To achieve the above object, the present invention provides a separator according to the following embodiments, a lithium-sulfur battery including the separator, and a method for manufacturing the separator.

[0018] According to the first embodiment, a porous polymer substrate having a large number of voids; a coating layer located on at least one surface of the porous polymer substrate, the coating layer comprising a polyamide compound having both carboxylic acid functional groups and amide functional groups; The separator for a lithium-sulfur battery may have a molar ratio of the carboxylic acid functional group to the amide functional group of 1:0.5 to 1:5.

[0019] According to the second embodiment, in the first embodiment, The molar ratio of the carboxylic acid functional groups to the amide functional groups may be 1:0.75 to 1:3.

[0020] According to the third embodiment, in the first or second embodiment, The polyamic acid compound may be a polymer having, as a repeating unit, a unit having both the carboxylic acid functional group and the amide functional group.

[0021] According to the fourth embodiment, in any one of the first to third embodiments, The molar concentration (mmol / L) of the carboxylic acid functional groups may be 5-15 and the molar concentration of the amide functional groups may be 8-20.

[0022] According to the fifth embodiment, in any one of the first to fourth embodiments, The polyamic acid compound may be an aromatic polyamic acid compound.

[0023] According to the sixth embodiment, in any one of the first to fifth embodiments, The polyamic acid may have a PDI (Poly Dispersity Index, molecular weight distribution) of 2.6 to 3.2.

[0024] According to the seventh embodiment, in any one of the first to sixth embodiments, The polyamic acid compound having both carboxylic acid functional groups and amide functional groups may be present inside the pores of the porous polymer substrate.

[0025] According to the eighth embodiment, The battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The separator is a separator for a lithium-sulfur battery according to any one of the first to sixth embodiments, thereby providing a lithium-sulfur battery.

[0026] According to the ninth embodiment, providing a porous polymer substrate having a number of voids; forming a coating layer on at least one surface of the porous polymer substrate, the coating layer including a polyamic acid compound having both a carboxylic acid functional group and an amide functional group; The step of forming a coating layer containing the polyamic acid compound includes Immersing the porous polymer substrate in a first solution containing a carboxylic acid dianhydride compound; immersing the porous polymer substrate immersed in the first solution into a second solution containing a diamine compound; The molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.5 to 1:5.

[0027] According to the tenth embodiment, in the ninth embodiment, When a coating layer containing a polyamic acid compound is formed on at least one surface of the porous polymer substrate, the polyamic acid compound may be present inside the pores while the coating layer of the polyamic acid compound is formed on at least one surface of the porous polymer substrate.

[0028] According to the eleventh embodiment, in the ninth or tenth embodiment, The molar ratio of the carboxylic acid functional groups to the amide functional groups may be 1:0.75 to 1:3.

[0029] According to the twelfth embodiment, in any one of the ninth to eleventh embodiments, The content of the carboxylic acid dianhydride compound in the first solution may be 0.2 wt% to 3 wt%, and the content of the diamine compound in the second solution may be 0.2 wt% to 3 wt%.

[0030] According to the thirteenth embodiment, in any one of the ninth to twelfth embodiments, The carboxylic acid functionality may be derived from a carboxylic dianhydride compound, and the amide functionality may be derived from a diamine compound.

[0031] According to the fourteenth embodiment, in the thirteenth embodiment, The carboxylic acid dianhydride compound includes 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-diphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic acid dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 1,2,3,4-benzenetetracarboxylic acid dianhydride, methylcyclohexenetetracarboxylic acid dianhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, ethylenetetracarboxylic acid dianhydride, or two or more thereof; The diamine compound may include p-phenylenediamine, m-phenylenediamine, 2,2'-bis(trifluoromethyl)-4,4'-biphenyldiamine, 2,2'-dimethyl-4,4'-diaminobenzidine, 4,4'-diaminodiphenyl sulfone, 2,7-diaminofluorene, 4,4-diaminooctafluorobiphenyl, 4,4'-oxydianiline, 2,2'-dimethyl-4,4'-diaminobiphenyl, m-xylylenediamine, p-xylylenediamine, 4,4'-diaminobenzanilide, or two or more thereof.

[0032] According to the 15th embodiment, in any one of the 9th to 14th embodiments, The steps of immersing the porous polymer substrate in the first solution and the second solution may each be performed for 40 to 80 minutes.

[0033] According to the sixteenth embodiment, in the fifteenth embodiment, After immersing the porous polymer substrate in the second solution, washing the porous polymeric substrate; The method may further include the step of drying the washed porous polymer substrate under vacuum. [Effects of the Invention]

[0034] According to one embodiment of the present invention, a separator for a lithium-sulfur battery is formed by immersing a porous polymer substrate in a first solution containing a carboxylic acid dianhydride compound and then in a second solution containing a diamine compound, thereby forming a coating layer containing a polyamic acid compound on at least one surface of the porous polymer substrate. The polyamic acid compound contains a carboxylic acid functional group and an amide functional group, and the molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.5 to 1:5. By coating the separator with the coating layer containing the polyamic acid compound, lithium polysulfides leached from the positive electrode of the lithium-sulfur battery are prevented from being transferred to the negative electrode. This prevents the deposition of sulfur particles on the surface of the lithium metal in the negative electrode, thereby maintaining the charge / discharge capacity of the lithium-sulfur battery and improving its battery life.

[0035] In addition, the polyamic acid of the coating layer is prepared through in-situ polymerization. The monomolecular carboxylic acid dianhydride compound is first provided to the porous polymer substrate, and the carboxylic acid dianhydride compound is applied to the porous polymer substrate, even to the interior of the pores. The diamine compound is then provided, and a polymerization reaction occurs within the pores, thereby coating the interior of the pores with the polyamic acid compound. In addition, the interfacial resistance between the coating layer and the porous polymer substrate is reduced, resulting in better electrochemical performance.

[0036] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, and the present invention should not be interpreted as being limited only to the matters described in the drawings. [Brief explanation of the drawings]

[0037] [Figure 1] 1 is a diagram illustrating a method for manufacturing a separator for a lithium-sulfur battery according to an embodiment of the present invention. [Figure 2] 1 is a diagram illustrating a method for manufacturing a separator for a lithium-sulfur battery according to Example 1. FIG. [Figure 3] 1 is a graph illustrating the discharge capacity of a lithium-sulfur battery including a separator for a lithium-sulfur battery obtained by a manufacturing method according to an embodiment of the present invention and a lithium-sulfur battery including a separator for a lithium-sulfur battery according to a comparative example. [Figure 4] 1 is a graph illustrating the discharge capacity of a lithium-sulfur battery including a separator for a lithium-sulfur battery obtained by a manufacturing method according to an embodiment of the present invention and a lithium-sulfur battery including a separator for a lithium-sulfur battery according to a comparative example. [Figure 5] 1 is an image illustrating the results of thermal stability evaluation of the separation membrane according to Comparative Example 1. [Figure 6] 1 is an image illustrating the results of thermal stability evaluation of the separator according to Example 1. [Figure 7] 1 is a graph for confirming the suppression of polysulfide elution from separation membranes according to Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention will be described in more detail below.

[0039] The terms and words used in this specification and claims should not be interpreted limited to their ordinary and dictionary meanings, but should be interpreted in terms and concepts that correspond to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of the term in order to explain the invention in the best way.

[0040] Throughout this specification, when a part is described as "comprising" or "having" a certain component, it does not mean that other components are excluded, but that other components may also be included, unless otherwise specified.

[0041] Furthermore, the term "about" used throughout this specification, when inherent manufacturing and material tolerances are given, is used to mean a numerical value or a value close to the numerical value mentioned, in order to prevent unconscionable infringers from unfairly using the disclosure content in which an exact or absolute numerical value is mentioned to facilitate understanding of the present application.

[0042] Furthermore, throughout the specification, when a part is said to "include" a certain component, unless otherwise specified, it does not exclude other components, but means that it may further include other components.

[0043] The present invention relates to a separator, an electrochemical battery including the separator, and a method for manufacturing the same. In the present invention, the electrochemical battery may include any battery that performs an electrochemical reaction. Specific examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors such as supercapacitors. In particular, the electrochemical battery may be a secondary battery, and the secondary battery may be a lithium-ion secondary battery. The lithium-ion secondary battery may be a lithium-metal battery, a lithium-sulfur battery, an all-solid-state battery, a lithium polymer battery, or the like, among which a lithium-sulfur battery is preferred.

[0044] Lithium-sulfur batteries are attracting attention as next-generation secondary batteries due to their high discharge capacity and theoretical energy density, as well as the advantages of reducing battery manufacturing costs and being environmentally friendly, as sulfur, which is used as a positive electrode active material, is abundant and inexpensive.

[0045] In the present invention, the positive electrode active material includes a carbon-sulfur composite, and the carbon-sulfur composite includes a porous carbon material. In lithium-sulfur batteries, sulfur, which is a positive electrode active material, is a non-conductor, so to compensate for its low electrical conductivity, a sulfur-carbon composite is generally used, which is composited with a conductive carbon material.

[0046] According to one aspect of the present invention, a porous polymer substrate having a large number of voids; a coating layer located on at least one surface of the porous polymer substrate, the coating layer including a polyamic acid compound having both a carboxylic acid functional group and an amide functional group; The separator for a lithium-sulfur battery is provided, wherein the molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.5 to 1:5.

[0047] The porous polymer substrate may be any material that can be used as a separator for an electrochemical device, such as a porous polymer film or nonwoven fabric containing at least one polymer resin, including polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalate.

[0048] The coating layer is located on at least one surface of the porous polymer substrate and includes a polyamic acid compound having both a carboxylic acid functional group and an amide functional group, the carboxylic acid functional group being derived from a carboxylic dianhydride compound, and the amide functional group being derived from a diamine compound.

[0049] During discharge, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur in lithium-sulfur batteries. During this reaction, sulfur converts from a ring-shaped S8 to a linear lithium polysulfide (LiPS). This lithium-sulfur battery exhibits a gradual discharge voltage until the lithium polysulfide is completely reduced to Li2S.

[0050] However, the charge / discharge efficiency of lithium-sulfur batteries decreases during the charge / discharge process, resulting in a shortened battery life. This shortened battery life can be attributed to a variety of factors, including side reactions in the electrolyte, instability of lithium metal, and the accumulation of by-products on the cathode (e.g., elution of lithium polysulfides from the cathode).

[0051] Lithium-sulfur batteries, which use sulfur-based compounds as the positive electrode active material and alkali metals such as lithium as the negative electrode active material, suffer from the problem of lithium polysulfide leaching during charging and discharging. The lithium polysulfide leached from the positive electrode is transferred to the negative electrode, reducing the capacity of the lithium-sulfur battery and shortening its lifespan. Because lithium polysulfide leached from the positive electrode has high solubility in the electrolyte, it passes through the separator membrane via the electrolyte and unintentionally migrates to the negative electrode, resulting in a decrease in capacity due to irreversible loss of the positive electrode active material and a decrease in battery life due to the deposition of sulfur particles on the lithium metal surface due to side reactions.

[0052] To solve the problem of reduced battery life due to lithium polysulfide, research has been conducted in the industry to add a lithium polysulfide adsorbent to the positive electrode composite or to add a reaction-reducing substance to the negative electrode to prevent side reactions on the surface of lithium metal, but no significant results have been achieved.

[0053] Therefore, the present invention discloses a separator for a lithium-sulfur battery, which includes a coating layer containing the polyamic acid compound having both the carboxylic acid functional group and the amide functional group on at least one surface of the porous polymer substrate to prevent lithium polysulfide leached from the positive electrode from migrating to and accumulating on the negative electrode. By coating the porous polymer substrate with the polyamic acid compound having the carboxylic acid functional group and the amide functional group, lithium polysulfide leached from the positive electrode does not migrate to the negative electrode, thereby improving the life of the lithium-sulfur battery.

[0054] According to one embodiment of the present invention, not only is a coating layer including a polyamic acid compound formed on the porous polymer substrate, but the polyamic acid compound having both a carboxylic acid functional group and an amide functional group may also be present within the pores of the porous polymer substrate.

[0055] In one embodiment, the polyamic acid compound may be a polymer having repeating units each having both the carboxylic acid functional group and the amide functional group. The polyamic acid compound may include an aromatic polyamic acid compound, an aliphatic polyamic acid compound, an aliphatic cyclic polyamic acid compound, etc. In the present invention, an aromatic polyamic acid compound having a phenol group capable of forming a π-π bond with the pores of the porous polymer substrate may be selected to more uniformly and smoothly coat the porous polymer substrate.

[0056] According to one embodiment of the present invention, the polyamic acid compound may have a chemical structure represented by the following Chemical Formula 1. Alternatively, the polyamic acid compound may include a repeating unit represented by the following Chemical Formula 1.

[0057] [ka]

[0058] In Chemical Formula 1, A1 is a tetravalent organic group having 1 to 20 carbon atoms, and A2 is a divalent organic group having 1 to 20 carbon atoms.

[0059] For example, A1 can be a tetravalent aliphatic organic group having 1 to 20 carbon atoms, a tetravalent alicyclic organic group having 3 to 20 carbon atoms, or a tetravalent aromatic organic group having 6 to 20 carbon atoms.

[0060] For example, A2 can be a divalent aliphatic organic group having 1 to 20 carbon atoms, a divalent alicyclic organic group having 3 to 20 carbon atoms, or a divalent aromatic organic group having 6 to 20 carbon atoms.

[0061] According to one embodiment of the present invention, the polyamic acid compound may have a chemical structure represented by the following Chemical Formula 2. Alternatively, the polyamic acid compound may include a repeating unit represented by the following Chemical Formula 2.

[0062] [ka]

[0063] In Chemical Formula 2, Ar1 is a tetravalent aromatic organic group having 6 to 20 carbon atoms, and Ar2 is a divalent aromatic organic group having 6 to 20 carbon atoms.

[0064] According to one embodiment of the present invention, the polyamic acid compound may be benzophenone polyamic acid (BPAA) having the structure of the following Chemical Formula 3, or may be naphthalene polyamic acid, pyrene polyamic acid, or the like.

[0065] [ka]

[0066] In Chemical Formula 3, n can be an integer of about 3 to 12, or about 5 to 10.

[0067] In the polyamic acid compound of the present invention, the molar ratio of the carboxylic acid functional groups to the amide functional groups is 1:0.5 to 1:5. According to one embodiment of the present invention, the molar ratio of the carboxylic acid functional groups to the amide functional groups may be about 1:0.75 to 1:3, about 1:0.8 to 1:2, or about 1:1 to 1:1.5.

[0068] When the molar ratio of the carboxylic acid functional group to the amide functional group satisfies this range, lithium polysulfide eluted from the positive electrode is prevented from being transferred to the negative electrode, which is advantageous for improving the life of the lithium-sulfur battery.When the molar ratio of the carboxylic acid functional group to the amide functional group is less than 1:0.5 or more than 1:5, lithium polysulfide eluted from the positive electrode is not sufficiently blocked from being transferred to the negative electrode, which is undesirable because it does not sufficiently improve the life of the lithium-sulfur battery.

[0069] According to one embodiment of the present invention, the molar concentration (mmol / L) of the carboxylic acid functional group may be about 5 to 15 or about 5 to 8, and the molar concentration (mmol / L) of the amide functional group may be about 8 to 20 or about 8 to 12. When the molar concentrations (mmol / L) of the carboxylic acid functional group and the amide functional group satisfy these ranges, permeation of lithium polysulfide is prevented, which is advantageous for improving the life characteristics of the battery.

[0070] The molar concentrations and molar ratios of the carboxylic acid functional groups and the amide functional groups may be measured by various methods known in the art. For example, the molar concentrations and molar ratios of the carboxylic acid functional groups and the amide functional groups may be measured using a H-NMR (nuclear magnetic resonance) spectrometer under the following conditions:

[0071] - Equipment: AVANCE AC 400FT NMR spectrometer (Bruker) - Measurement frequency: 400MHz - Measurement solvent: DMSO-d6 - Measurement temperature: 25℃

[0072] The polyamic acid compound may have a weight-average molecular weight of about 1,000 to 5,000 or about 4,000 to 5,000, and a PDI (molecular weight distribution) of about 2.6 to 3.2 or about 2.7 to 2.9. When the weight-average molecular weight and molecular weight distribution of the polyamic acid compound satisfy these ranges, permeation of lithium polysulfide is prevented, which is advantageous for improving the life characteristics of the battery.

[0073] The weight-average molecular weight and PDI can be measured using gel permeation chromatography (GPC), etc. Specifically, the number-average molecular weight (Mn) and weight-average molecular weight (Mw) can be measured using gel permeation chromatography (PL GPC220, manufactured by Agilent Technologies) under the following conditions, and the molecular weight distribution can be calculated by dividing the weight-average molecular weight by the number-average molecular weight.

[0074] -Column: PL MiniMixed Bx2 Solvent: tetrahydrofuran (THF) -Flow rate: 0.3ml / min -Sample concentration: 2.0mg / ml -Injection volume: 10μl -Column temperature: 40℃ -Detector: Agilent RI detector -Standard: Polystyrene (corrected by a cubic function) -Data processing: ChemStation

[0075] According to a specific embodiment of the present invention, the separator for a lithium-sulfur battery may further include an organic / inorganic composite porous coating layer disposed on at least one surface of the porous polymer substrate and including a plurality of inorganic particles and a binder.

[0076] According to one aspect of the present invention, providing a porous polymer substrate having a number of voids; forming a coating layer on at least one surface of the porous polymer substrate, the coating layer including a polyamic acid compound having both a carboxylic acid functional group and an amide functional group; The step of forming a coating layer containing the polyamic acid compound includes Immersing the porous polymer substrate in a first solution containing a carboxylic acid dianhydride compound; immersing the porous polymer substrate immersed in the first solution into a second solution containing a diamine compound; The molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.75 to 1:3.

[0077] According to one embodiment of the present invention, when a coating layer containing a polyamic acid compound is formed on at least one surface of the porous polymer substrate, the polyamic acid compound may be present inside pores while the coating layer of the polyamic acid compound is formed on at least one surface of the porous polymer substrate.

[0078] Hereinafter, a method for manufacturing a separator for a lithium-sulfur battery according to a specific embodiment of the present invention will be described.

[0079] FIG. 1 is a diagram illustrating a method for manufacturing a separator for a lithium-sulfur battery according to an embodiment of the present invention.

[0080] 1, a porous polymer substrate 100 may be prepared. As described above, the porous polymer substrate 100 may be a porous polymer film or nonwoven fabric containing one or more polymer resins such as polyolefin, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalate, etc.

[0081] Thereafter, the porous polymer substrate 100 may be immersed in a first solution (SOL1). For example, the porous polymer substrate 100 may be immersed in the first solution (SOL1) for about 40 to 80 minutes. The first solution (SOL1) may contain the carboxylic acid dianhydride compound, specifically the carboxylic acid dianhydride compound and a first solvent, which may be n-methylpyrrolidone (NMP), tetrahydrofuran (THF), dimethylformamide (DMF), acetone, ethanol, water, or a mixture of two or more of these. The content of the carboxylic acid dianhydride compound in the first solution (SOL1) may be about 0.2 wt% to 3 wt% or about 0.5 wt% to 1 wt%.

[0082] In one example, the carboxylic acid dianhydride compound may include 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-diphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic acid dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 1,2,3,4-benzenetetracarboxylic acid dianhydride, methylcyclohexenetetracarboxylic acid dianhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, ethylenetetracarboxylic acid dianhydride, or two or more thereof.

[0083] For example, the carboxylic acid dianhydride compound may include 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BPTCDA).

[0084] The porous polymer substrate 100 immersed in the first solution (SOL1) may be immersed in the second solution (SOL2). For example, the porous polymer substrate 100 may be immersed in the second solution (SOL2) for about 40 to 80 minutes. In one example, the second solution (SOL2) may contain the diamine compound, specifically the diamine compound and a second solvent, which may be n-methylpyrrolidone (NMP), tetrahydrofuran (THF), dimethylformamide (DMF), acetone, ethanol, water, or a mixture of two or more of these. The content of the diamine compound in the second solution (SOL2) may be about 0.2 wt% to 3 wt% or about 0.5 wt% to 1 wt%.

[0085] In one example, the diamine compound may include p-phenylenediamine, m-phenylenediamine, 2,2'-bis(trifluoromethyl)-4,4'-biphenyldiamine, 2,2'-dimethyl-4,4'-diaminobenzidine, 4,4'-diaminodiphenyl sulfone, 2,7-diaminofluorene, 4,4-diaminooctafluorobiphenyl, 4,4'-oxydianiline, 2,2'-dimethyl-4,4'-diaminobiphenyl, m-xylylenediamine, p-xylylenediamine, 4,4'-diaminobenzanilide, or two or more thereof.

[0086] For example, the diamine compound may include p-phenylene diamine (PDA).

[0087] The polyamic acid (e.g., benzophenone polyamic acid (BPAA) in FIG. 1) of the coating layer according to an embodiment of the present invention is prepared by in-situ polymerization. In this case, the process of immersing the porous polymer substrate 100 in the first solution (SOL1) and the second solution (SOL2) can be appropriately set in consideration of the coating properties of the compound on the porous polymer substrate 100.

[0088] In one example, the porous polymer substrate 100 may be immersed in a first solution (SOL1), and then the second solution (SOL2) may be injected into the porous polymer substrate 100 immersed in the first solution (SOL1) and immersed in the second solution (SOL2).

[0089] In another example, after the porous polymer substrate 100 is immersed in the first solution (SOL1), the porous polymer substrate 100 coated with the first solution (SOL1) can be removed and immersed in the second solution (SOL2).

[0090] In addition, the order in which the porous polymer substrate 100 is immersed in the first solution (SOL1) and the second solution (SOL2) can also be set appropriately.

[0091] In one example, the porous polymer substrate 100 may be immersed in a first solution (SOL1) and then in a second solution (SOL2). In this case, the carboxylic acid dianhydride compound contained in the first solution (SOL1) has a relatively large number of phenol groups, which may form a π-π bond with the porous polymer substrate 100. Therefore, by first immersing the porous polymer substrate 100 in the first solution (SOL1) having a relatively large number of phenol groups, the bonding strength between the porous polymer substrate 100 and the first solution (SOL1) may be increased.

[0092] However, the present invention is not limited to this, and as another example, the porous polymer substrate 100 may be immersed in the second solution (SOL2) first.

[0093] Thereafter, the porous polymer substrate 100 may be washed and dried for about 24 hours to remove any unreacted material remaining on the surface of the porous polymer substrate 100. For example, the porous polymer substrate 100 may be washed with a cleaning agent such as ethanol and dried under vacuum.

[0094] According to an embodiment of the present invention, the polyamic acid of the coating layer is prepared through in-situ polymerization. For example, the carboxylic acid dianhydride compound having the carboxylic acid functional group may be first provided to the porous polymer substrate 100, followed by the diamine compound having the amine functional group. By providing the carboxylic acid dianhydride compound in a monomolecular state to the porous polymer substrate 100 first, the carboxylic acid dianhydride compound can be uniformly coated even inside the voids of the porous polymer substrate 100. The diamine compound is then provided, and a polymerization reaction occurs inside the voids, thereby allowing the polyamic acid compound to be more uniformly coated inside the voids. Furthermore, the interfacial resistance between the coating layer and the porous polymer substrate 100 is reduced, resulting in better electrochemical performance.

[0095] In addition, the carboxylic acid functional group and the amide functional group remain in the polyamic acid of the coating layer according to an embodiment of the present invention. In other words, while polyimide can be synthesized by imidizing polyamic acid, the coating layer of the present invention includes polyamic acid that has not undergone imidization. As a result, the carboxylic acid functional group and the amide functional group remain in the polyamic acid of the coating layer, and a separator coated with the coating layer can inhibit the migration of lithium polysulfide, thereby improving the life of the lithium-sulfur battery.

[0096] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples according to the present invention can be modified into many other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0097] Example 1 positive electrode A carbon-sulfur composite was used as the positive electrode active material, which was prepared by mixing carbon nanotubes (CNTs) and sulfur (S) at a weight ratio of 1:3. The carbon-sulfur composite was mixed with a PAA binder at a weight ratio of 96:4 to prepare a positive electrode slurry. The positive electrode slurry was then uniformly coated on aluminum foil to a thickness of 300 μm, dried at 55°C, and cut to a diameter (Φ) of 11 mm to be used as a positive electrode.

[0098] Anode and electrolyte Using lithium metal as the negative electrode, an electrolyte for a lithium-sulfur secondary battery was prepared by adding 1.0M LiNO3 to a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethyl ether in a volume ratio (v / v) of 50:50.

[0099] separation membrane FIG. 2 is a schematic diagram showing the process for producing the separation membrane according to Example 1.

[0100] Referring to FIG. 2, a polyethylene porous film (Celgard 2320) was prepared as a porous polymer substrate, and a first solution (SOL1) was prepared using n-methylpyrrolidone (NMP) as a first solvent and containing 1 wt% of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTCDA) as a carboxylic dianhydride compound. Furthermore, a second solution (SOL2) was prepared using ethanol as a second solvent and containing 1 wt% of p-phenylenediamine (p-PDA) as a diamine compound.

[0101] First, the porous polymer substrate was immersed in the prepared first solution (SOL1) for 1 hour, and then the second solution (SOL2) was poured in. As a result, a separator having a coating layer containing in-situ polymerized benzophenone polyamic acid (BPAA) on both sides of the porous polymer substrate was fabricated.

[0102] In this case, the molar concentration (mmol / L) of the carboxylic acid functional group in the benzophenone polyamic acid (BPAA) was 6.27, and the molar concentration (mmol / L) of the amide functional group was 9.34, so the molar ratio of the carboxylic acid functional group to the amide functional group was 1:1.5.

[0103] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0104] <Example 2> Positive electrode, negative electrode, and electrolyte A positive electrode, a negative electrode, and an electrolyte were prepared in the same manner as in Example 1.

[0105] separation membrane A polyethylene porous film (Celgard 2320) was prepared as a porous polymer substrate. A first solution containing 0.5 wt% of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTCDA) as a carboxylic dianhydride compound was prepared using n-methylpyrrolidone (NMP) as a first solvent. A second solution containing 1 wt% of p-phenylenediamine (p-PDA) as a diamine compound was prepared using ethanol as a second solvent.

[0106] First, the porous polymer substrate was immersed in the first solution for 1 hour, and then the second solution was poured into the porous polymer substrate, resulting in the preparation of a separator having a coating layer containing in-situ polymerized benzophenone polyamic acid (BPAA) on both sides of the porous polymer substrate.

[0107] In this case, the molar concentration (mmol / L) of the carboxylic acid functional group in the benzophenone polyamic acid (BPAA) was 6.27, and the molar concentration (mmol / L) of the amide functional group was 18.68, so the molar ratio of the carboxylic acid functional group to the amide functional group was 1:3.

[0108] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0109] Example 3 Positive electrode, negative electrode, and electrolyte A positive electrode, a negative electrode, and an electrolyte were prepared in the same manner as in Example 1.

[0110] separation membrane A polyethylene porous film (Celgard 2320) was prepared as a porous polymer substrate. A first solution containing 1 wt% of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTCDA) as a carboxylic dianhydride compound was prepared using n-methylpyrrolidone (NMP) as a first solvent. A second solution containing 0.5 wt% of p-phenylenediamine (p-PDA) as a diamine compound was prepared using ethanol as a second solvent.

[0111] First, the porous polymer substrate was immersed in the first solution for 1 hour, and then the second solution was poured into the porous polymer substrate, resulting in the preparation of a separator having a coating layer containing in-situ polymerized benzophenone polyamic acid (BPAA) on both sides of the porous polymer substrate.

[0112] In this case, the molar concentration (mmol / L) of the carboxylic acid functional group in the benzophenone polyamic acid (BPAA) was 12.54, and the molar concentration (mmol / L) of the amide functional group was 9.34, so the molar ratio of the carboxylic acid functional group to the amide functional group was 1:0.75.

[0113] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0114] <Comparative Example 1> Positive electrode, negative electrode, and electrolyte A positive electrode, a negative electrode, and an electrolyte were prepared in the same manner as in Example 1.

[0115] separation membrane A polyethylene porous film (Celgard 2320) was used as the porous polymer substrate, and a separator without a coating layer containing polyamic acid was used.

[0116] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0117] <Comparative Example 2> Positive electrode, negative electrode, and electrolyte A positive electrode, a negative electrode, and an electrolyte were prepared in the same manner as in Example 1.

[0118] separation membrane A polyethylene porous film (Celgard 2320) was prepared as a porous polymer substrate. A first solution containing 5 wt% of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTCDA) as a carboxylic dianhydride compound was prepared using n-methylpyrrolidone (NMP) as a first solvent. A second solution containing 1 wt% of p-phenylenediamine (p-PDA) as a diamine compound was prepared using ethanol as a second solvent.

[0119] First, the porous polymer substrate was immersed in the first solution for 1 hour, and then the second solution was poured into the porous polymer substrate, resulting in the preparation of a separator having a coating layer containing in-situ polymerized benzophenone polyamic acid (BPAA) on both sides of the porous polymer substrate.

[0120] In this case, the molar concentration (mmol / L) of the carboxylic acid functional group in the benzophenone polyamic acid (BPAA) was 62.69, and the molar concentration (mmol / L) of the amide functional group was 18.68, so the molar ratio of the carboxylic acid functional group to the amide functional group was 1:0.3.

[0121] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0122] <Comparative Example 3> Positive electrode, negative electrode, and electrolyte A positive electrode, a negative electrode, and an electrolyte were prepared in the same manner as in Example 1.

[0123] separation membrane A first solution containing 1 wt% of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTCDA) as a carboxylic dianhydride compound in n-methylpyrrolidone (NMP) as a first solvent was mixed with a second solution containing 1 wt% of p-phenylenediamine (p-PDA) as a diamine compound in ethanol as a second solvent to prepare a mixture. The mixture was then applied to both sides of a porous polymer substrate, a polyethylene porous film (Celgard 3230), to prepare a separator having a coating layer containing ex-situ polymerized benzophenone polyamic acid (BPAA) on both sides of the porous polymer substrate.

[0124] In this case, the molar concentration (mmol / L) of the carboxylic acid functional group in the benzophenone polyamic acid was 6.27, and the molar concentration (mmol / L) of the amide functional group was 9.34, so the molar ratio of the carboxylic acid functional group to the amide functional group was 1:1.5.

[0125] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0126] <Comparative Example 4> Positive electrode, negative electrode, and electrolyte A positive electrode, a negative electrode, and an electrolyte were prepared in the same manner as in Example 1.

[0127] separation membrane A polyethylene porous film (Celgard 2320) was prepared as a porous polymer substrate. A first solution containing 5 wt% of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BPTCDA) as a carboxylic dianhydride compound was prepared using n-methylpyrrolidone (NMP) as a first solvent. A second solution containing 1 wt% of p-phenylenediamine (p-PDA) as a diamine compound was prepared using ethanol as a second solvent.

[0128] First, the porous polymer substrate was immersed in the first solution for 1 hour, and then the second solution was poured into the porous polymer substrate, resulting in the preparation of a separator having a coating layer containing in-situ polymerized benzophenone polyamic acid (BPAA) on both sides of the porous polymer substrate.

[0129] In this case, the molar concentration (mmol / L) of the carboxylic acid functional group in the benzophenone polyamic acid (BPAA) was 6.27, and the molar concentration (mmol / L) of the amide functional group was 37.62, so the molar ratio of the carboxylic acid functional group to the amide functional group was 1:6.

[0130] A 2032 coin-type lithium-sulfur battery was fabricated using the above-described positive electrode, negative electrode, electrolyte, and separator.

[0131] <Evaluation results> 1. Measurement of the molar concentration and molar ratio of carboxylic acid functional groups and amide functional groups The molar concentrations and molar ratios of the carboxylic acid functional group and the amide functional group of the polyamic acid compound contained in the coating layer of the separator prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were measured using a H-NMR spectrometer under the following conditions.

[0132] - Equipment: AVANCE AC 400FT NMR spectrometer (Bruker) - Measurement frequency: 400MHz - Measurement solvent: DMSO-d6 - Measurement temperature: 25℃

[0133] The results are shown in Table 1 below.

[0134] [Table 1]

[0135] 2. Evaluation of life characteristics (cycle characteristics) Evaluation method The lithium-sulfur batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were charged and discharged at 60° C. under the following conditions, and the initial discharge capacity and the discharge capacity after 100 cycles were evaluated.

[0136] 1 to 3 cycles Charging conditions: CC (Constant Current) / CV (Constant Voltage), (4.25V, 0.1C Current Cut-Off) Discharge conditions: CC (constant current) conditions 3V, 0.1C 4 to 100 cycles Charging conditions: CC (constant current) / CV (constant voltage), (4.25V, 0.2C current cutoff) Discharge conditions: CC (constant current) conditions 3V, 0.2C The results are shown in Figures 3 and 4.

[0137] 3, the lithium-sulfur battery according to Example 1 exhibited a higher initial discharge capacity than the lithium-sulfur battery according to Comparative Example 1. For example, the initial discharge capacity of the lithium-sulfur battery according to Example 1 was approximately 800 mAh / g, while the initial discharge capacity of the lithium-sulfur battery according to Comparative Example 1 was approximately 650 mAh / g.

[0138] Furthermore, it was measured that the lithium-sulfur battery according to Example 1 had a higher discharge capacity at about 10 to 100 cycles than the lithium-sulfur battery according to Comparative Example 1. After about 100 cycles, the lithium-sulfur battery according to Example 1 maintained a discharge capacity of about 600 mAh / g or more, while the discharge capacity of the lithium-sulfur battery according to Comparative Example 1 decreased to about 200 mAh / g.

[0139] Referring to FIG. 4, it was measured that the lithium-sulfur batteries according to Examples 1, 2, and 3 continuously maintained a discharge capacity of about 750 mAh / g or more from about 1 cycle to about 100 cycles.

[0140] Meanwhile, the lithium-sulfur battery according to Comparative Example 1 was measured to have a relatively low discharge capacity of approximately 600 mAh / g from approximately 1 to 100 cycles. The lithium-sulfur battery according to Comparative Example 2 maintained a discharge capacity of over 750 mAh / g up to approximately 70 cycles, but the discharge capacity rapidly decreased to approximately 520 mAh / g from approximately 70 to 100 cycles. The lithium-sulfur battery according to Comparative Example 3 was measured to have a discharge capacity of approximately 800 mAh / g from approximately 1 to 3 cycles, then rapidly dropped, and had a low discharge capacity of approximately 250 mAh / g or less from approximately 4 to 40 cycles. However, the discharge capacity rapidly increased to approximately 250 mAh / g around approximately 25 and 30 cycles, showing unstable results. The lithium-sulfur battery according to Comparative Example 4 maintained a discharge capacity of approximately 750 mAh / g up to approximately 70 cycles, but rapidly dropped after 70 cycles, making it impossible to measure.

[0141] 3. Evaluation of thermal stability Evaluation method The separators prepared in Example 1 and Comparative Example 1 were visually observed for color change and shrinkage as the temperature was increased in the range of 100°C to 150°C.

[0142] The results are shown in Figures 5 and 6.

[0143] Referring to FIG. 5, in the case of the separator according to Comparative Example 1, shrinkage and warping occurred with increasing temperature, but the color of the separator did not change.

[0144] Referring to FIG. 6, in the case of the separator according to Example 1, shrinkage and warping hardly occurred even when the temperature increased, and the color changed as the polyamic acid coated on the separator was oxidized.

[0145] 4. Confirmation of suppression of polysulfide elution Evaluation method The separators prepared in Example 1 and Comparative Example 1 were placed on the lids of perforated minivials, and polysulfide-containing electrolyte was placed inside the minivials. The minivials were then placed inside a chamber containing bare electrolyte without polysulfide, and the minivials were then inverted so that the lids were in contact with the electrolyte.

[0146] After leaving it for 24 hours to allow diffusion through the separator, the separator was removed and the surface that had come into contact with the polysulfide-free electrolyte was analyzed using UV / Vis spectroscopy. This analysis confirmed whether polysulfides inside the vial had leaked or eluted through the separator to the outside.

[0147] The bare electrolyte was prepared by adding 1.0M LiNO3 to a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethyl ether in a volume ratio (v / v) of 50:50.

[0148] The results are shown in Figure 7.

[0149] In the case of the separation membrane of Example 1, the polysulfides present inside the minivial were prevented from eluting to the outside, so that the peaks were difficult to observe in the UV / Vis spectrum or had low intensity, whereas in the case of the separation membrane of Comparative Example 1, the polysulfides were eluted, so that the peaks were observed in the UV / Vis spectrum and had high intensity.

Claims

1. a porous polymer substrate having a plurality of voids; a coating layer located on at least one surface of the porous polymer substrate, the coating layer including a polyamic acid compound having both a carboxylic acid functional group and an amide functional group; The separator for a lithium-sulfur battery has a molar ratio of the carboxylic acid functional group to the amide functional group of 1:0.5 to 1:

5.

2. 2. The separator for a lithium-sulfur battery according to claim 1, wherein the molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.75 to 1:

3.

3. 2. The separator for a lithium-sulfur battery according to claim 1, wherein the polyamic acid compound is a polymer having, as a repeating unit, a unit having both the carboxylic acid functional group and the amide functional group.

4. the molar concentration (mmol / L) of the carboxylic acid functional group is 5 or more and 15 or less; 2. The separator for a lithium-sulfur battery according to claim 1, wherein the molar concentration of the amide functional group is 8-20.

5. 2. The separator for a lithium-sulfur battery according to claim 1, wherein the polyamic acid compound is an aromatic polyamic acid compound.

6. 2. The separator for a lithium-sulfur battery according to claim 1, wherein the polyamic acid has a PDI (molecular weight distribution) of 2.6 or more and 3.2 or less.

7. The separator for a lithium-sulfur battery according to any one of claims 1 to 6, wherein the polyamic acid compound having both a carboxylic acid functional group and an amide functional group is present inside the pores of the porous polymer substrate.

8. The battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, A lithium-sulfur battery, wherein the separator is the lithium-sulfur battery separator according to any one of claims 1 to 6.

9. providing a porous polymeric substrate having a plurality of voids; forming a coating layer on at least one surface of the porous polymer substrate, the coating layer including a polyamic acid compound having both a carboxylic acid functional group and an amide functional group; The step of forming a coating layer containing the polyamic acid compound includes Immersing the porous polymer substrate in a first solution containing a carboxylic acid dianhydride compound; immersing the porous polymer substrate immersed in the first solution into a second solution containing a diamine compound; The method for producing a separator for a lithium-sulfur battery, wherein the molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.5 to 1:

5.

10. 10. The method for manufacturing a separator for a lithium-sulfur battery according to claim 9, wherein when a coating layer containing a polyamic acid compound is formed on at least one surface of the porous polymer substrate, the polyamic acid compound is present in the pores while the coating layer containing the polyamic acid compound is formed on at least one surface of the porous polymer substrate.

11. 10. The method for manufacturing a separator for a lithium-sulfur battery according to claim 9, wherein the molar ratio of the carboxylic acid functional group to the amide functional group is 1:0.75 to 1:

3.

12. the content of the carboxylic acid dianhydride compound in the first solution is 0.2 wt % or more and 3 wt % or less; 10. The method of claim 9, wherein the content of the diamine compound in the second solution is 0.2 wt % or more and 3 wt % or less.

13. 10. The method of claim 9, wherein the carboxylic acid functional group is derived from a carboxylic dianhydride compound, and the amide functional group is derived from a diamine compound.

14. The carboxylic acid dianhydride compound includes 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-diphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenylsulfonetetracarboxylic acid dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 1,2,3,4-benzenetetracarboxylic acid dianhydride, methylcyclohexenetetracarboxylic acid dianhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, ethylenetetracarboxylic acid dianhydride, or two or more thereof; 14. The method of claim 13, wherein the diamine compound comprises p-phenylenediamine, m-phenylenediamine, 2,2'-bis(trifluoromethyl)-4,4'-biphenyldiamine, 2,2'-dimethyl-4,4'-diaminobenzidine, 4,4'-diaminodiphenylsulfone, 2,7-diaminofluorene, 4,4-diaminooctafluorobiphenyl, 4,4'-oxydianiline, 2,2'-dimethyl-4,4'-diaminobiphenyl, m-xylylenediamine, p-xylylenediamine, 4,4'-diaminobenzanilide, or two or more thereof.

15. 15. The method of claim 9, wherein the immersion of the porous polymer substrate in the first solution and the immersion of the porous polymer substrate in the second solution are performed for 40 minutes or more and 80 minutes or less, respectively.

16. After immersing the porous polymer substrate in the second solution, washing the porous polymeric substrate; The method of claim 15, further comprising: drying the washed porous polymer substrate in a vacuum.

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