JP7533873B2 - Lithium-sulfur secondary battery and method for manufacturing the lithium-sulfur secondary battery - Google Patents

Description

The present invention relates to a lithium-sulfur secondary battery having a positive electrode covered with a monomer film, and a method for manufacturing a lithium-sulfur secondary battery having a positive electrode covered with a monomer film.

With the spread of mobile phone terminals and the research and development of electric vehicles, there is a demand for high-capacity secondary batteries. Lithium-ion secondary batteries are widely used as such secondary batteries.

Lithium-sulfur batteries, which have sulfur as a positive electrode active material, have been attracting attention as a secondary battery with a higher capacity than lithium-ion secondary batteries. The theoretical capacity of sulfur (1670 mAh/g) is about 10 times higher than the theoretical capacity (about 140 mAh/g) of _LiCoO2, a typical positive electrode active material of lithium-ion batteries. In addition, sulfur is a low-cost and abundant resource.

In lithium-sulfur batteries, elemental sulfur (S ₈ ) is reduced to polysulfide anions at the positive electrode during discharge, ultimately producing Li ₂ S. Meanwhile, at the negative electrode, lithium is released as lithium ions, which travel through the electrolyte to the positive electrode and become the Li source for the production of Li ₂ S.

Here, lithium polysulfide, a reaction intermediate consisting of sulfur polysulfides (polysulfide anions) and lithium, is easily soluble in organic solvents and dissolves into the battery's electrolyte. When lithium polysulfide dissolves, the positive electrode active material decreases, and the capacity retention rate decreases.

Furthermore, during charging, polysulfide anions dissolved in the electrolyte are reduced when they reach the surface of the negative electrode, and are oxidized when they reach the surface of the positive electrode. This causes a short circuit due to mass transfer in the electrolyte. This results in a significant decrease in charge/discharge efficiency due to the so-called shuttle effect, which means that the battery will not charge even if a charging current is continued to be applied.

JP 2005-79096 A discloses a lithium-sulfur secondary battery in which the surface of the positive electrode is covered with a polymer film by coating the surface with a monomer containing a non-aqueous electrolyte and then polymerizing the film.

However, in the above battery, the capacity retention rate, for example the capacity after 10 cycles relative to the initial capacity, was 60%. This is presumably because the above polymer film was unable to sufficiently suppress the elution of lithium polysulfide into the electrolyte.

JP 2005-79096 A

The present invention aims to provide a lithium-sulfur secondary battery with high charge/discharge efficiency and capacity retention, and a method for manufacturing a lithium-sulfur secondary battery with high charge/discharge efficiency and capacity retention.

The lithium-sulfur secondary battery according to an embodiment of the present invention comprises a current collector which is a porous metal containing aluminum or nickel as a main component and has a porosity of 50% or more and 98% or less, a layer of a positive electrode active material having a sulfur loading of 4 mg/cm2 or ^more disposed on the current collector, and a polymer film having a sulfonic acid group which covers the layer of the positive electrode active material, and the polymer film contains a chemically crosslinked polymer having any one monomer selected from acrylamidomethylpropanesulfonic acid, vinylsulfonic acid, allylsulfonic acid, ethylsulfonic acid acrylate, butylsulfonic acid acrylate, and isoprene sulfonic acid.

The method for manufacturing a lithium-sulfur secondary battery according to an embodiment of the present invention includes, in order, a step of preparing a positive electrode by disposing a positive electrode active material on a current collector, a step of applying a monomer solution containing a monomer having a sulfonic acid group, a crosslinking agent, and an initiator to the positive electrode, thereby covering the positive electrode active material with a monomer film, and a step of irradiating the monomer film with ultraviolet light.

According to an embodiment of the present invention, it is possible to provide a lithium-sulfur secondary battery having high charge/discharge efficiency and capacity retention rate, and a method for manufacturing a lithium-sulfur secondary battery having high charge/discharge efficiency and capacity retention rate.

FIG. 1 is a diagram illustrating the configuration of a lithium-sulfur secondary battery according to an embodiment. 1 is a photograph of a porous metal of a positive electrode of a lithium-sulfur secondary battery according to an embodiment. FIG. 11 is a graph showing charge/discharge characteristics of a lithium-sulfur secondary battery according to a second embodiment.

First Embodiment
1, the lithium-sulfur secondary battery 10 (hereinafter also referred to as "battery") of this embodiment includes, as main components, a positive electrode 20 including a porous current collector 21, a separator 30, and a negative electrode 40 including a negative electrode active material that absorbs and desorbs lithium ions. The battery 10 includes a coin cell case 51/gasket 52/negative electrode 40/separator 30 including electrolyte 35/positive electrode 20/spacer 53/spring washer 54/top cover 55, which are arranged in this order.

The positive electrode 20 has a porous metal current collector 21 (Figure 2) and a sulfur-based positive electrode active material 22 arranged on the wall surface of the internal pores of the current collector 21.

The current collector 21, which is the base material of the positive electrode 20, is, for example, Celmet (registered trademark), which is foamed aluminum. The positive electrode active material 22 contains elemental sulfur (S), a binder, and a conductive agent.

The current collector 21 is a porous body with a large surface area, and therefore can support a large amount of positive electrode active material. For example, a current collector made of metal foil can support only about 0.3 mg/ ^cm2 of sulfur. In contrast, as described below, the amount of sulfur supported by the current collector 21 of the embodiment is 4 mg/ ^cm2 or more. Therefore, the battery 10 has a high energy density per unit area of the positive electrode.

The positive electrode active material 22 is covered with a polymer film 25. The polymer film 25 is polyAMPS (2-acrylamido-2-methylpropanesulfonic acid) with a molecular weight of 2,000,000, which is a linear polymer shown in (Formula 1). After the battery is assembled, the polymer film 25 becomes a gel electrolyte film impregnated with the electrolyte.

(Equation 1)

The negative electrode 40 is a metallic lithium plate with a thickness of 200 μm.

The negative electrode 40 may contain one or more negative electrode active materials selected from the group consisting of lithium, lithium alloys, and carbon capable of absorbing and desorbing lithium. The negative electrode active material contained in the negative electrode absorbs and desorbs lithium ions.

The separator 30 disposed between the positive electrode 20 and the negative electrode 40 is not an essential component, but the separator 30 has the function of shortening the interelectrode distance and carrying the electrolyte 35.

Examples of the separator 30 include a glass fiber separator that absorbs and retains the electrolyte 35, a porous sheet made of resin, and a nonwoven fabric. The porous sheet is made of, for example, a porous resin. In the battery 10, the separator 30 is a polypropylene porous sheet.

The electrolyte is an organic electrolyte solution in which 1 mole of lithium salt is dissolved in a solvent consisting of 1 mole of 1,2-dimethoxyethane (DME) and 1 mole of 1.3-dioxolane (DOL).

The lithium salt is Li-TFSI (lithium (trifluoromethanesulfonyl) imide).

The lithium salt may be, for example, LiBF ₄ , Li[FSA]: lithium bis(fluorosulfonyl)amide, or LiClO ₄ . The solvent of the electrolyte 35 may be ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), dimethoxyethane (DME), or a mixture thereof.

In addition, aprotic solvated ionic liquids in which an ether and a lithium salt form a complex are particularly preferably used for the electrolyte 35, since they have low volatility, low viscosity, high lithium ion concentration, and high lithium ion conductivity. In addition, an electrolyte obtained by adding a solvent to an aprotic solvated ionic liquid and diluting it can also be used. A solvent that does not destroy the structure of the complex is preferably used as the additive solvent. Examples of such additive solvents include hydrofluoroethers (HFEs) such as HF ₂ CF ₂ CH ₂ C-O-CF ₂ CF ₂ H and F ₃ CH ₂ C-O-CF ₂ CF ₂ H, which are fluorine-based solvents.

<Battery manufacturing method>
Next, a method for manufacturing the battery 10 will be described.

<Step S10: Positive electrode production process>

The Celmet used for the current collector 21 had a porosity of 98%, a specific surface area of 8500 m ² /m ³ , a pore diameter of 0.45 mm, and a thickness of 1.0 mm.

The specific surface area of the current collector 21 is preferably 1500 ^m2 / ^m3 or more, particularly preferably 3000 ^m2 / ^m3 or more. In order to obtain a large surface area, it is preferable that the current collector 21 has a porosity of 50% or more, has continuous pores, and contains almost no closed pores. Note that a current collector 21 with a porosity of more than 98% has a weak mechanical strength and is not easy to handle.

The surface area of a flat current collector having a two-dimensional surface is proportional to the planar dimensions. In contrast, the surface area of current collector 21 having a three-dimensional mesh structure is proportional to the planar dimensions (external surface area) and thickness. The surface area of current collector 21 is preferably at least twice the surface area of a current collector having a two-dimensional surface with the same planar dimensions, and more preferably at least five times. From a technical standpoint, the surface area of current collector 21 is no more than 100 times the surface area of a current collector having a two-dimensional surface with the same planar dimensions.

The specific surface area is measured, for example, by the capacitance method. The capacitance method is a measurement method that utilizes the fact that the capacitance of a sample is proportional to its surface area. Several metal plates or other objects with known surface areas are prepared, and the capacitance of each is measured. Then, by creating a calibration curve of "capacitance" versus "area," the surface area of the sample can be calculated by the calibration method. On the other hand, the porosity is calculated from the specific gravity and volume of the sample.

The current collector 21 may be a porous metal such as a metal foam, a nonwoven metal fabric, a metal fiber aggregate, or a metal particle aggregate. The metal surface may be coated with carbon.

The positive electrode active material 22 is a Ketjen Black (KB) complex containing elemental sulfur (S), which is a sulfur-based active material, and KB, which is a conductive agent. The weight ratio of the KB complex is (S/KB = 6/4). The binder contains CMC (carboxymethyl cellulose) and SBR (styrene butadiene rubber), and the weight ratio is (CMC/SBR = 2/1). The binder is not particularly limited as long as it does not react with the electrolyte 35, and may be, for example, PVDF (polyvinylidene fluoride) or polytetrafluoroethylene. The weight ratio of the positive electrode active material 22 is (KB complex/KB/binder = 60/30/3).

The positive electrode active material 22 is formed by adding an appropriate amount of N-methyl-2-pyrrolidone (NMP) to the S/KB composite and binder, kneading the mixture to obtain an active material slurry, dropping the slurry onto the current collector 21, and then drying the slurry.

When the active material slurry is dripped onto the surface (main surface) of the flat current collector 21 having a first main surface and a second main surface, the active material slurry is absorbed into the internal space of the current collector 21, which is a porous body. After dripping an appropriate amount of the slurry, a drying process is performed at 60°C for 24 hours, and the positive electrode active material 22 is disposed on the internal wall surface of the current collector 21.

The amount of sulfur (supported amount) in the positive electrode active material 22 was calculated by measuring the weight before and after disposing the active material. The amount of sulfur in the active material 22 disposed on the current collector 21 was 10.0 mg/ ^cm2 based on the area of the main surface.

<Step S20: Polymer Film Disposition Process>
A 15% by weight aqueous solution of the polymer having a sulfonic acid group shown in (Formula 1) was dip-coated onto the positive electrode 20 having the positive electrode active material 22 disposed thereon. Then, the resultant was dried under reduced pressure at 60° C. for 15 hours, thereby producing a positive electrode 20 in which the positive electrode active material 22 was covered with a polymer film 25.

The weight W2 of the positive electrode after the polymer film 25 was disposed was 119% by weight compared to the weight W1 of the positive electrode 20 before the polymer film 25 was disposed.

<Step S30: Assembly process>
In a glove box under an argon atmosphere, the electrolyte 35 is dropped onto the positive electrode 20, and then the separator 30 is disposed on the positive electrode 20. Furthermore, an appropriate amount of the electrolyte 35 is added to the separator 30, and the separator 30 is immersed in the electrolyte 35 by being held at 60° C. for 60 minutes.

The negative electrode 40 was placed on the separator 30 on the positive electrode 20, and the electrolyte 35 was poured in. The negative electrode 40 was then placed in a 2032-type coin cell case 51 (made of SUS304, 3.2 mm thick) and the separator 30 was placed on the negative electrode 40. A spring washer 54 was placed on the separator 30. The coin cell case 51 was sealed with a top cover 55 from above the spring washer 54, and a lithium-sulfur battery 10 with the structure shown in FIG. 1 was produced. A gasket 52 was placed on the side wall of the coin cell case 51.

In the assembly process, the positive electrode 20 containing the positive electrode active material and the negative electrode 40 containing the negative electrode active material are sealed inside the coin cell case 51 with the separator 30 containing the electrolyte 35 sandwiched between them.

<Battery characteristics>
The results of evaluating the characteristics of the battery 10 according to the embodiment produced by the above method are shown below. For comparison, a comparative battery 110 having the same configuration as the battery 10 but not including the polymer film 25 on the positive electrode 20 was also produced and evaluated in the same manner.

The charge/discharge evaluation was performed with a cutoff potential of 1.5 V-3.0 V (vs. Li/Li ⁺ ) and a charge/discharge rate of 0.1 C. The cyclic voltammetry measurement (CV) was performed with a cutoff potential of 1.5 V-3.0 V (vs. Li/Li ⁺ ) and a scan rate of 0.1 mV/s. The charge/discharge rate of 0.1 C is a 10-hour rate (the current value at which the theoretical capacity of the electrode active material is discharged or charged in n hours is represented as C/n).

The second cycle capacities of both batteries 10 and 110 were approximately 900 mAh/g-S. However, after 10 cycles, battery 10 had a capacity retention rate of 83% and a charge/discharge efficiency of 90%, whereas battery 110 had a capacity retention rate of 77% and a charge/discharge efficiency of less than 80%.

Battery 10 has better capacity retention and charge/discharge efficiency than battery 110 because the polymer film 25 prevents sulfur from leaching from the positive electrode 20.

In particular, in the battery 10, since the polymer membrane 25 has sulfonic acid groups, the negatively charged polysulfide anions are prevented from eluting from the positive electrode 20 due to repulsion between the sulfonic acid groups and the strongly negatively charged sulfonic acid groups. In other words, the polymer membrane 25 having sulfonic acid groups suppresses the elution of polysulfide anions into the electrolyte more than a polymer membrane without sulfonic acid groups.

Second Embodiment
The battery 10A of the second embodiment differs from the polymer film 25 of the battery 10 only in the polymer film 25A. Therefore, a description of the other components will be omitted.

The polymer film 25A of the battery 10A is produced by ultraviolet curing of a monomer film containing a monomer having a sulfonic acid group, a crosslinking agent, and an initiator.

The manufacturing method of the battery 10A is substantially the same as the manufacturing method of the battery 10 in terms of step S10 (positive electrode fabrication step) and step S30 (assembly step). On the other hand, the polymer film disposing step (step S20) includes a monomer film disposing step (step S22) and an ultraviolet curing step (step S24).

<Step S22: Monomer film disposing step>
A monomer solution containing a monomer having a sulfonic acid group, a crosslinking agent, and an initiator is applied to the positive electrode on which the positive electrode active material 22 is disposed, and the positive electrode active material 22 is covered with a monomer film.

The monomer is 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as shown in formula 2.
(Equation 2)

The crosslinking agent is N-N'-methylenebisacrylamide (MBAA) shown in formula 3.
(Equation 3)

The initiator is 2,2'-azobis(2- methylpropionamidine ) dihydrochloride (AMPA) shown in formula 4.
(Equation 4)

The monomer solution was prepared by dissolving 1.8 mmol of monomer, 0.2 mmol of crosslinker, and 0.09 mmol of initiator in 3 mL of water. That is, the molar ratio of the monomer solution (monomer:crosslinker:initiator) was (9:1:0.45).

The current collector 21 on which the active material 22 was disposed was impregnated with the monomer solution, and then vacuum dried (40°C) for 15 hours to produce a monomer film.

<Step S24: UV curing process>
By irradiating the monomer film with ultraviolet light, a polymer film 25A having sulfonic acid groups, which covers the positive electrode active material 22, was produced.

For example, the monomer film was irradiated with ultraviolet light having a wavelength of 365 nm and an intensity of 1080 mW/cm ² for 20 seconds.

The weight W2 of the positive electrode with the polymer film 25A disposed thereon was 225% of the weight W1 of the positive electrode before the monomer film was disposed thereon. In other words, the weight of the polymer film 25A was 125% by weight of the positive electrode before the polymer film 25A was disposed thereon.

Figure 3 shows the cycle characteristics of battery 10A. The initial capacity was approximately 900 mAh/g-S. After 10 cycles, the capacity retention rate was 88%, and although not shown, the charge/discharge efficiency was 95%. Furthermore, after 25 cycles, the capacity retention rate was 78%, and the charge/discharge efficiency was 95%.

The reason why battery 10A has higher performance than battery 10 is the difference in the structure of polymer membranes 25, 25A. Polymer membrane 25A is a chemically cross-linked type, and is a sulfonic acid network polymer that has many cross-linking points that are connections between molecules and forms a three-dimensional dense mesh structure.

The polymer membrane 25A is physically impermeable to polysulfide anions due to its three-dimensional dense mesh structure. Furthermore, the electrical repulsion between polysulfide anions and sulfonic acid groups in the polymer membrane 25A is also strengthened by the three-dimensional dense mesh structure.

The polysulfide anions generated in the positive electrode active material 22 are unlikely to dissolve into the electrolyte that is impregnated into the narrow gaps of the polymer membrane 25A, in which the sulfonic acid groups form a dense network structure.

However, there is no wording that identifies polymer film 25A as a film, which was produced by ultraviolet curing of a monomer solution containing acrylamidomethylpropanesulfonic acid, a crosslinking agent, and an initiator. Furthermore, it is extremely difficult and impractical to identify it through structural analysis, etc.

In a battery in which the weight W2 of the polymer film 25A is 150% by weight of the positive electrode before the polymer film 25A is disposed, the cycle characteristics are not as good as those of the battery 10A. In addition, in a battery in which the weight W2 of the polymer film 25A is 250% by weight of the positive electrode before the polymer film 25A is disposed, the resistance is high and the rate characteristics are not as good as those of the battery 10A. For this reason, it is preferable that the weight of the polymer film 25 is more than 50% by weight and less than 150% by weight of the positive electrode before the polymer film 25 is disposed.

The monomer having a sulfonic acid group is not limited to AMPS as long as it has a sulfonic acid group. For example, the monomer may be vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, ethyl acrylate sulfonic acid, butyl acrylate sulfonic acid, or isoprene sulfonic acid.

The cross-linking agent is not limited to MBAA. For example, the characteristics (capacity retention rate, charge/discharge efficiency) of battery 10B having polymer membrane 25B using N,N'-tetraethylene glycol diacrylate as the cross-linking agent shown in (Formula 5) were approximately the same as those of battery 10A.

(Equation 5)

On the other hand, the characteristics (capacity retention rate, charge/discharge efficiency) of battery 10C, which has polymer film 25C using the compound shown in formula 6 as a crosslinking agent, were slightly lower than those of batteries 10A and 10B.

(Equation 6)
N,N'-{[(2-acrylamido-2-[(3-acrylamidopropoxy)methyl]propane-1,3-diyl)bis(oxy)]bis(propane-1,3-diyl)}diacrylamide

The characteristics (capacity retention rate, charge/discharge efficiency) of battery 10D, which has polymer film 25D using the compound shown in formula 7 as a crosslinking agent, were comparable to those of battery 10C.

N,N'-diacryloyl-4,7,10-trioxa-1,13-tridecanediamine (Equation 7)

N,N'-diacrylloyl-4,7,10-trioxa-1,13-tridecanediamine

Note that batteries 10C and 10D both had better characteristics than battery 10.

The crosslinking agent is preferably one having a radically polymerizable functional group, particularly a vinyl group, as the functional group. The number of functional groups possessed by the crosslinking agent may be two or more. For example, the crosslinking agent may be an aromatic vinyl monomer, an unsaturated nitrile monomer such as acrylonitrile or methacrylonitrile, an ester monomer such as an acrylic acid ester monomer or a methacrylic acid ester monomer, an unsaturated carboxylic acid monomer such as an acrylic acid monomer or a methacrylic acid monomer, or a maleic anhydride monomer.

Specific examples of crosslinking agents include divinylbenzene, triallyl cyanurate, diacetone diacrylamide, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, diethylene glycol dimethacrylate, diisopropenylbenzene, P-quinone dioxime, P,P'-dibenzoylquinone dioxime, phenylmaleimide, allyl methacrylate, N,N'-m-phenylene bismaleimide, diallyl phthalate, tetraallyloxyethane, and 1,2-polybutadiene.

The initiator may be any compound that decomposes when exposed to light to generate an initiating radical. For example, the initiator may be one or more selected from acylphosphine oxide, benzoin, benzoin alkyl ether, benzil, benzophenone, anthraquinone, and water-soluble azo initiators.

Specific examples of initiators include acetophenone, acetophenone benzyl ketal, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy-1,2-diphenylethan-1-one, xanthone, fluorenone, benzaldehyde, fluorene, anthraquinone, triphenylamine, carbazole, 3-methylacetophenone, benzophenone, 4-chlorobenzophenone, 4,4'-dimethoxybenzophenone, 4,4'-diaminobenzophenone, benzoin propyl ether, benzoin ethyl ether, benzil dimethyl ketal, 1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one, 2-hydroxy-2-methyl-1-phenylpropan Pan-1-one, thioxanthone, diethylthioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone).

Third Embodiment
Since the battery 10E of the third embodiment is similar to the battery 10A and has the same effects, components having the same functions are given the same reference numerals and descriptions thereof are omitted.

In battery 10E, the monomer solution before polymerization contains lithium salt. That is, the polymer film 25E of battery 10E contains lithium salt even before the assembly process.

The monomer solution for producing the polymer membrane 25E of the battery 10E was prepared by dissolving 1.8 mmol of AMPS, 0.2 mmol of MBAA, 0.09 mmol of AMPA, and 0.5 mmol of Li-TFSI in 3 mL of water.

The charge/discharge efficiency of battery 10E after 10 cycles was 98%. Furthermore, the charge/discharge efficiency after 50 cycles was also 98%.

Furthermore, the discharge capacity of battery 10E at 1 C was 64% of the discharge capacity at 0.1 C, which was approximately the same rate characteristic as battery 110. The resistance value (DC) of battery 10E was 2/3 that of battery 10B.

The amount of lithium salt added to the monomer solution is preferably 10 mol % or more relative to the monomer. If the amount of lithium salt added is less than this range, the effect is not significant. The upper limit of the amount of lithium salt added is about 1 mol %.

In the above explanation, the battery 10 and the like have been described as having a simple structure. However, the battery of the present invention may be an assembled battery having a structure in which a plurality of unit cells such as the battery 10 are stacked, or a battery having a structure in which cells having the same stacked structure are wound and housed in a case.

The present invention is not limited to the above-described embodiments, and various modifications, combinations, and applications are possible without departing from the spirit of the invention.

10, 10A to 10E... Lithium-sulfur secondary battery 20... Positive electrode 21... Current collector 22... Positive electrode active material 25, 25A to 25E... Polymer film 30... Separator 35... Electrolyte 40... Negative electrode 51... Coin cell case 52... Gasket 54... Spring washer 55... Top cover

Claims (11)

a current collector that is a porous metal containing aluminum or nickel as a main component and has a porosity of 50% or more and 98% or less;
a layer of a positive electrode active material having a sulfur loading of 4 mg/cm2 or ^more disposed on the current collector;
a polymer film having a sulfonic acid group covering the layer of the positive electrode active material ;
The lithium-sulfur secondary battery is characterized in that the polymer membrane contains a chemically crosslinked polymer having any one monomer selected from the group consisting of acrylamidomethylpropanesulfonic acid, vinylsulfonic acid, allylsulfonic acid, ethyl acrylate sulfonic acid, butyl acrylate sulfonic acid, and isoprene sulfonic acid. 2. The lithium-sulfur secondary battery according to claim 1, wherein the polymer is poly(2-acrylamido-2-methylpropanesulfonic acid). 2. The lithium-sulfur secondary battery according to claim 1, wherein the polymer membrane is a gel electrolyte membrane impregnated with an electrolyte. 4. The lithium-sulfur secondary battery according to claim 1, wherein the polymer film contains tetraethylene glycol diacrylate or methylene bisacrylamide as a crosslinking agent. The lithium-sulfur secondary battery according to any one of claims 1 to 3, characterized in that the polymer film contains a lithium salt. forming a positive electrode by disposing a positive electrode active material on a current collector;
applying a monomer solution containing a monomer having a sulfonic acid group, a crosslinking agent, and an initiator to the positive electrode to cover the positive electrode active material with a monomer film;
and a step of irradiating the monomer film with ultraviolet light. 7. The method for producing a lithium-sulfur secondary battery according to claim ⁶ , wherein the current collector is a porous metal containing aluminum or nickel as a main component and having a porosity of 50% to 98%, and the amount of sulfur supported in the positive electrode is 4 mg/cm2 or more. The method for producing a lithium-sulfur secondary battery according to claim 7, characterized in that the monomer is acrylamidomethylpropanesulfonic acid. The method for producing a lithium-sulfur secondary battery according to claim 8, characterized in that the crosslinking agent is tetraethylene glycol diacrylate or methylene bisacrylamide. 10. The method for producing a lithium-sulfur secondary battery according to claim 9, wherein the initiator is 2,2'-azobis(2- methylpropionamidine ) dihydrochloride. The method for producing a lithium-sulfur secondary battery according to claim 9 or 10, characterized in that the monomer solution contains a lithium salt.

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