Positive electrode for lithium-sulfur secondary battery and lithium-sulfur secondary battery comprising the same

Abstract

Provided is a positive electrode for a lithium-sulfur secondary battery, the positive electrode for a lithium-sulfur secondary battery including a positive electrode active material, a conductive material, a binder, and a polyvalent metal salt. The polyvalent metal salt includes a cation of a metal selected from metals having an effective nuclear charge number of 3 to 6 for the outermost electrons in the 3rd period and the 4th period. The positive electrode for a lithium-sulfur secondary battery can improve the performance of a lithium-sulfur secondary battery by incorporating a polyvalent metal salt, and thus effectively suppresses the elution of lithium polysulfide when applied to a battery, while not significantly increasing the weight of the electrode and not significantly reducing the conductivity of the electrode.

Description

Technical Field

[0001] This invention relates to a positive electrode for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery comprising the same. Specifically, this invention relates to a positive electrode for a lithium-sulfur secondary battery comprising a polyvalent metal salt and a lithium-sulfur secondary battery comprising the positive electrode.

[0002] This application claims priority to Korean Patent Application No. 10-2020-0137322, filed on October 22, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0003] As the application of secondary batteries expands to electric vehicles (EVs) or energy storage devices (ESS), the use of lithium-ion secondary batteries, with their relatively low energy density relative to weight (approximately 250 Wh / kg), faces numerous limitations. Instead, lithium-sulfur secondary batteries are attracting significant attention as a next-generation secondary battery technology because they can achieve theoretically high energy density relative to weight (approximately 2,600 Wh / kg).

[0004] Lithium-sulfur secondary batteries refer to battery systems that use sulfur-sulfur materials with SS bonds (sulfur-sulfur bonds) as the positive electrode active material and lithium metal as the negative electrode active material. The advantages of sulfur as the main positive electrode active material are its abundant resources, non-toxicity, and low atomic weight.

[0005] In lithium-sulfur secondary batteries, during discharge, lithium, the negative electrode active material, is oxidized and ionized while releasing electrons, while sulfur, the positive electrode active material, is reduced while accepting electrons. In this case, the lithium oxidation reaction is the process by which lithium metal releases electrons and converts into lithium cations. Conversely, the sulfur reduction reaction is the process by which the S-S bond accepts two electrons and converts into sulfur anions. The lithium cations generated by the lithium oxidation reaction are transferred to the positive electrode via the electrolyte and combine with the sulfur anions generated by the sulfur reduction reaction to form a salt. Specifically, sulfur has a cyclic S8 structure before discharge, which is converted into lithium polysulfide (LiS) through a reduction reaction. x When lithium polysulfides are completely reduced, lithium sulfide (Li2S) is produced.

[0006] Lithium-sulfur secondary batteries may suffer from problems such as low conductivity of sulfur, dissolution and volume expansion of lithium polysulfides during charging / discharging, and low coulombic efficiency, resulting in a rapid decrease in capacity as charging / discharging progresses. Therefore, these problems must be solved to improve battery performance.

[0007] Porous carbon materials are widely used in lithium-sulfur secondary batteries to impart conductivity through recombination with sulfur, which is the active material in these batteries. Research is ongoing to improve battery performance by controlling the size and volume of the pores formed within these porous carbon materials. In the case of metal-organic frameworks (MOFs), the advantage lies in the ability to synthesize pores with sizes ranging from 1000 to 4000 μm by forming pores of different sizes depending on the type of organic molecules or metal atoms constituting the MOF. 2 Porous carbon materials with high specific surface area per g. However, while carbonizing existing MOF materials and applying them to lithium-sulfur secondary batteries can improve the dissolution problem of lithium polysulfides to some extent, it may also increase the overall weight of the electrode and reduce its conductivity due to the additional use of low-conductivity MOF materials. Furthermore, the aforementioned techniques require complex fabrication processes.

[0008] Therefore, research is constantly being conducted in related technical fields to improve the performance of lithium-sulfur secondary batteries.

[0009] [Existing Technical Documents]

[0010] [Patent Literature]

[0011] (Non-patent literature 1) Jianming Zheng et al., Lewis acid-base interactions between polysulfides and metal organic framework in lithium-sulfur batteries, Nano Letters, 2014, 14, 2345-2352

[0012] (Non-Patent Literature 2) Junwen Zhou et al., Rational Design of a Metal-Organic Framework Host for Sulfur Storage in Fast, Long-Cycle Li-S Batteries, Energy & Environmental Science, 2014, 7, 2715 Summary of the Invention

[0013] Technical issues

[0014] To address the aforementioned problems, the present invention aims to provide a positive electrode for lithium-sulfur secondary batteries, wherein the positive electrode can improve the cycle performance of the battery by introducing a polyvalent metal salt into the positive electrode.

[0015] Technical solution

[0016] According to a first aspect of the present invention, the present invention provides a positive electrode for a lithium-sulfur secondary battery, the positive electrode comprising a positive electrode active material, a conductive material, a binder, and a multivalent metal salt.

[0017] In one embodiment of the invention, the multivalent metal salt comprises a cation of a metal selected from those with an effective nuclear charge of 3 to 6 in the outermost electrons of the 3rd and 4th periods.

[0018] In one embodiment of the invention, the multivalent metal salt comprises a metal selected from Mg. 2+ And Al 3+ The cations in and selected from OH - CO3 2- NO3 - and SO4 2- Anions in the solution.

[0019] In one embodiment of the invention, the adhesive is a polymer having oxygen- or nitrogen-containing functional groups.

[0020] In one embodiment of the invention, the adhesive is selected from polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and combinations thereof.

[0021] In one embodiment of the invention, the content of the polyvalent metal salt is from 30 mol% to 100 mol% based on the molar number of the polymer monomers in the adhesive.

[0022] In one embodiment of the present invention, the positive electrode active material is selected from elemental sulfur (S8), sulfur-carbon composite materials, sulfur compounds, and combinations thereof.

[0023] In one embodiment of the invention, the binder content is 5% to 20% by weight, based on the total weight of the positive electrode active material.

[0024] In one embodiment of the present invention, the conductive material is selected from graphite, carbon black, conductive fibers, metal powder, fluorinated carbon, conductive whiskers, conductive metal oxides, polyphenylene derivatives, and combinations thereof.

[0025] In one embodiment of the invention, the content of the conductive material is from 5% to 20% by weight, based on the total weight of the positive electrode active material.

[0026] In one embodiment of the invention, in the case of the positive electrode, the multivalent metal salt is added in an aqueous solution, then dried and introduced.

[0027] In one embodiment of the invention, the multivalent metal salt in aqueous solution is added before drying the mixture of positive electrode active material, conductive material and binder, or coated after drying.

[0028] According to a second aspect of the present invention, a lithium-sulfur secondary battery comprising the above-described positive electrode is provided.

[0029] Beneficial effects

[0030] The positive electrode for lithium-sulfur secondary batteries according to the present invention can improve the performance of lithium-sulfur secondary batteries by simply introducing polyvalent metal salts. Therefore, when applied to batteries, it effectively suppresses the dissolution of lithium polysulfides without significantly increasing the weight of the electrode or significantly reducing the conductivity of the electrode. Attached Figure Description

[0031] Figure 1 The figure shows the results of thermogravimetric analysis of the sulfur-carbon composite material according to Experimental Example 1.

[0032] Figure 2 The figure shows the thermogravimetric analysis results of the polyacrylic acid adhesive with immobilized polyvalent metal ions according to Experimental Example 2.

[0033] Figure 3 The graph shows the analysis results of the capacity retention of the lithium-sulfur secondary battery based on Experimental Example 3. Detailed Implementation

[0034] The embodiments provided by the present invention can all be implemented through the following description. It will be understood that the following description is intended to depict preferred embodiments of the present invention, and the present invention is not necessarily limited thereto.

[0035] With respect to the physical properties described herein, unless otherwise specified, the physical properties are measured according to the measurement conditions and methods commonly used by those skilled in the art.

[0036] This invention provides a positive electrode for a lithium-sulfur secondary battery and a lithium-sulfur secondary battery comprising the same. The positive electrode for a lithium-sulfur secondary battery and the remaining components (negative electrode, separator, electrolyte) of the lithium-sulfur secondary battery according to the invention will be described in detail below.

[0037] Positive electrode for lithium-sulfur secondary batteries

[0038] The positive electrode for a lithium-sulfur secondary battery of the present invention comprises a positive electrode active material, a conductive material, a binder, and a polyvalent metal salt. The positive electrode can be divided into a positive electrode current collector and a layer of positive electrode active material coated on the positive electrode current collector, wherein the aforementioned positive electrode active material, conductive material, binder, and polyvalent metal salt are contained within the positive electrode active material layer.

[0039] The positive electrode active material constituting the positive electrode for a lithium-sulfur secondary battery according to the present invention can be selected from elemental sulfur (S8), sulfur-carbon composite materials, sulfur compounds, or mixtures thereof, but is not necessarily limited thereto. Specifically, the sulfur compound can be Li2S. n (n≥1), organic sulfur compounds or carbon-sulfur polymers (C2S) x ) n (e.g., x = 2.5–50, n ≥ 2). These sulfur materials are used in combination with conductive materials because they are not conductive on their own. Furthermore, the sulfur-carbon composite material can be a positive electrode active material formed by mixing carbon and elemental sulfur or sulfur compounds to reduce sulfur dissolution into the electrolyte and increase the conductivity of the sulfur-containing electrode. The ratio of positive electrode active material in the positive electrode active material layer can be adjusted according to the performance of the target battery. According to one embodiment of the invention, based on 100 parts by weight of the positive electrode active material layer, the positive electrode active material layer contains 60 to 95 parts by weight of positive electrode active material.

[0040] The carbon material constituting the sulfur-carbon composite material can be crystalline carbon or amorphous carbon, and can be conductive carbon. Specifically, the carbon material can be selected from graphite, graphene, Super P, carbon black, Danka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, carbon nanofiber, carbon nanotube, carbon nanowire, carbon nanoring, carbon fabric, and fullerene (C 60 ).

[0041] Exemplary sulfur-carbon composite materials include sulfur-carbon nanotube composite materials. Specifically, the sulfur-carbon nanotube composite material contains sulfur or sulfur compounds within carbon nanotube aggregates having a three-dimensional structure and in at least a portion of the inner and outer surfaces of the carbon nanotube aggregates. Because the sulfur-carbon nanotube composite material according to one embodiment of the present invention contains sulfur present within the three-dimensional structure of the carbon nanotubes, even if soluble polysulfides are generated due to electrochemical reactions, if the soluble polysulfides can be located within the carbon nanotubes, structural collapse of the positive electrode can be suppressed even when the polysulfides are dissolved, by maintaining the three-dimensional entangled structure. Therefore, the advantage of lithium-sulfur secondary batteries containing sulfur-carbon nanotube composite materials is that high capacity can be achieved even under high loads. Furthermore, sulfur or sulfur compounds can be contained within the internal pores of the carbon nanotube aggregates. Carbon nanotubes refer to linearly conductive carbon; specifically, carbon nanotubes can be carbon nanotubes (CNTs), graphite nanofibers (GNFs), carbon nanofibers (CNFs), or activated carbon fibers (ACFs). Both single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) can be used.

[0042] According to one embodiment of the invention, a sulfur-carbon composite material is prepared by impregnating the outer surface and interior of carbon with sulfur or sulfur compounds, and the diameter of the carbon can optionally be adjusted before, after, or both before and after the impregnation step. The impregnation step can be carried out by mixing carbon with powdered sulfur or sulfur compounds and then heating to impregnate the carbon with molten sulfur or sulfur compounds, and this mixing can be carried out by dry ball milling, dry spray milling, or dry bead milling. According to one embodiment of the invention, based on 100 parts by weight of the sulfur-carbon composite material, the sulfur-carbon composite material contains 60 to 90 parts by weight of sulfur or sulfur compounds. When the content of sulfur or sulfur compounds in the sulfur-carbon composite material is adjusted within the above-mentioned range, electrons are appropriately supplied to the non-conductive sulfur or sulfur compounds while maintaining a high ratio of sulfur or sulfur compounds. The sulfur-carbon composite material has a porosity exceeding a certain level because it has the form in which sulfur or sulfur compounds are supported on porous carbon. Therefore, even if the positive electrode active material layer is rolled to the extent that the structure of the sulfur-carbon composite material is still maintained, the impregnation of the electrolyte in the positive electrode active material layer will not deteriorate.

[0043] The conductive material constituting the positive electrode for a lithium-sulfur secondary battery according to the present invention can be selected from: graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermally cracked carbon black; conductive fibers, such as carbon fibers or metal fibers; fluorinated carbon; metal powder, such as aluminum powder or nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives, but not necessarily limited thereto. The ratio of conductive material in the positive electrode active material layer can be adjusted with consideration of the performance of the target battery. According to one embodiment of the present invention, based on 100 parts by weight of the positive electrode active material layer, the positive electrode active material layer contains 0.05 to 20 parts by weight, preferably 5 to 15 parts by weight of conductive material. The ratio of conductive material is also adjusted based on the positive electrode active material. According to one embodiment of the present invention, based on the total weight of the positive electrode active material, the content of conductive material can be 5% to 20% by weight, preferably 10% to 15% by weight.

[0044] In this invention, the binder constituting the positive electrode for a lithium-sulfur battery according to the invention essentially functions to assist in the bonding of the positive electrode active material and the conductive material, as well as their bonding to the current collector, and also has the additional function of immobilizing the metal cations of a polyvalent metal salt, along with the aforementioned functions. To possess the above functions, the binder can be a polymer having oxygen- or nitrogen-containing functional groups. In this polymer, oxygen or nitrogen atoms, which have high electronegativity, are relatively rich in electrons and therefore tend to be negatively charged, and the metal cations are immobilized to the binder by electrostatic attraction with the metal cations. According to one embodiment of the invention, the binder can be selected from polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and combinations thereof, and is preferably polyacrylic acid.

[0045] To enhance the bonding strength between the components constituting the positive electrode, the binder can be used in combination with at least one selected from the following: vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polymethyl methacrylate, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluororubber. The binder ratio in the positive electrode active material layer can be adjusted with consideration for desired battery performance. According to one embodiment of the invention, based on 100 parts by weight of the positive electrode active material layer, the positive electrode active material layer contains 1 to 30 parts by weight, preferably 3 to 20 parts by weight, more preferably 5 to 15 parts by weight of binder. The binder ratio can also be adjusted based on the positive electrode active material. According to one embodiment of the invention, based on the total weight of the positive electrode active material, the binder content is 5% to 20% by weight, preferably 10% to 15% by weight.

[0046] The polyvalent metal salt constituting the positive electrode of the lithium-sulfur secondary battery according to the present invention contains divalent or higher polyvalent metal cations. These polyvalent metal cations can bind to the negative charge generated when sulfur or sulfur-based materials are reduced by accepting electrons during discharge of the lithium-sulfur secondary battery via electroattraction. However, since the polyvalent metal cations are not macromolecules, they may dissolve from the positive electrode into the electrolyte; therefore, to fix the negative charge, the polyvalent metal cations also need to be fixed to macromolecules. The polyvalent metal cations can be fixed to the aforementioned binder. In polymers having oxygen- or nitrogen-containing functional groups as binders, the highly electronegative oxygen or nitrogen moieties can have a negative charge, and the polyvalent metal cations can be fixed via electroattraction. Because the polyvalent metal cations have a polyvalent positive charge, even when combined with the binder via electroattraction, the polyvalent metal cations can retain some positive charge, thus enabling fixation onto sulfur or sulfur-based materials that have a negative charge after discharge.

[0047] Because multivalent metal salts lack the high functionality to improve the performance of lithium-sulfur secondary batteries between negatively charged positive electrode active materials and binders, even if multivalent metal salts contain multivalent metal cations, it is necessary to appropriately select the types of cations and anions constituting the multivalent metal salt. According to one embodiment of the invention, the multivalent metal salt may contain cations of metals selected from those with an effective nuclear charge of 3 to 6, preferably 3 to 4.5, in the outermost shell of metals in the 3rd and 4th periods. According to one embodiment of the invention, the multivalent metal salt may contain cations selected from Mg. 2+ And Al 3+ The cations in and selected from OH - CO3 2- NO3 - and SO4 2 middle - The anion.

[0048] The content of polyvalent metal salts can be determined based on the content of the binder in the positive electrode active material layer. When the binder is a polymer with oxygen- or nitrogen-containing functional groups, the number of functional groups with functionality is important; therefore, when determining the content of polyvalent metal salts, the standard is the molar number of monomers in the polymer binder, not the molar number of the polymer binder itself. According to one embodiment of the invention, the content of polyvalent metal salts can be from 30 mol% to 100 mol% based on the molar number of polymer monomers in the binder. Specifically, based on the molar number of polymer monomers in the binder, the content of polyvalent metal salts is 30 mol% or more, 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more and less than 100 mol%, and 30 to 100 mol%, 50 to 100 mol%, and 70 to 100 mol%. If the content of polyvalent metal salts is greater than 100 mol% relative to the molar number of polymer monomers in the binder, the functional groups of the binder cannot sufficiently immobilize the polyvalent metal cations. Furthermore, if the content of the polyvalent metal salt is less than 30 mol% relative to the number of moles of polymer monomers in the adhesive, multiple adhesive functional groups are fixed to a single polyvalent metal cation, thus the polyvalent metal cation cannot adequately fix the negatively charged positive electrode active material.

[0049] The positive electrode is prepared by coating a slurry containing a positive electrode active material onto one or both surfaces of a positive electrode current collector to form a layer of positive electrode active material. The coating can be performed by methods commonly known in the art, such as doctor blade coating, die casting, comma coating, and screen printing. A polyvalent metal salt is added to the positive electrode in an aqueous solution. The aqueous solution polyvalent metal salt is separated and maintained as polyvalent metal cations and anions, and the separated polyvalent metal cations are readily fixed to a binder. According to one embodiment of the invention, the concentration of the polyvalent metal salt in the aqueous solution can be from 0.05 M to 0.5 M, preferably from 0.1 M to 0.3 M. The aqueous solution polyvalent metal salt can be directly added to a slurry for the positive electrode active material, which consists of a positive electrode active material, a conductive material, and a binder, or coated onto a dried layer after the slurry has been coated onto the positive electrode current collector and dried. If the aqueous solution polyvalent metal salt is directly added to the slurry for the positive electrode active material, the functional polyvalent metal salt can be uniformly dispersed in the positive electrode active material, which is more beneficial for improving the performance of lithium-sulfur secondary batteries.

[0050] Lithium-sulfur secondary batteries

[0051] The lithium-sulfur secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is as described above, and the negative electrode, separator, and electrolyte will be described in detail below.

[0052] The negative electrode constituting the lithium-sulfur secondary battery according to the present invention comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

[0053] The negative electrode active material layer comprises a negative electrode active material, a binder, and a conductive material. The negative electrode active material can be capable of reversibly inserting or deintercalating lithium ions (Li). + Materials that can react with lithium ions to reversibly form lithium-containing compounds, lithium metal, or lithium alloys.

[0054] The ability to reversibly insert or de-intercalate lithium ions (Li) + The material can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The material capable of reacting with lithium ions (Li...) + The material used to reversibly form the lithium-containing compound in the reaction can be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy can be, for example, an alloy of lithium (Li) and metals selected from the following: sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0055] The adhesive, conductive material, and negative electrode current collector can be selected with reference to the configuration described for the positive electrode, but in the implementation, they are not necessarily selected to be the same as those for the positive electrode. Furthermore, the method for forming the negative electrode active material layer on the negative electrode current collector can be selected with reference to the method used for the positive electrode, but in the implementation, this method is not necessarily selected to be the same as that used for the positive electrode.

[0056] The lithium-sulfur secondary battery separator according to the present invention is a physical separator that functions to physically separate the electrodes. The separator can be used without particular limitation, as long as it is used as a conventional separator. In particular, a separator exhibiting low resistance to ion migration in the electrolyte while possessing excellent electrolyte wetting ability is preferred. The separator enables the transport of lithium ions between the positive and negative electrodes while separating or isolating the positive and negative electrodes from each other.

[0057] This membrane can be made of a porous, non-conductive, or insulating material with a porosity of 30% to 50%. Specifically, porous polymer membranes can be used, such as those made from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers), and nonwoven fabrics made from high-melting-point glass fibers can also be used. Among these, porous polymer membranes are preferred.

[0058] If both the buffer layer and the separator are made of polymer membranes, the electrolyte impregnation and ion conduction characteristics will be reduced, and the effects of reducing overvoltage and improving capacity characteristics will become less significant. Conversely, if both the buffer layer and the separator are made of nonwoven fabric, mechanical stiffness cannot be ensured, thus leading to battery short circuits. However, if both a membrane-type separator and a polymer nonwoven fabric buffer layer are used, the improved battery performance and mechanical strength can be ensured due to the use of the buffer layer.

[0059] According to one embodiment of the invention, an ethylene homopolymer (polyethylene) polymer membrane is used as the separator, and a polyimide nonwoven fabric is used as the buffer layer. In this case, it is preferred that the polyethylene polymer membrane has a thickness of 10 μm to 25 μm and a porosity of 40% to 50%.

[0060] The electrolyte constituting the lithium-sulfur secondary battery according to the present invention is a non-aqueous electrolyte containing lithium salt and comprising lithium salt and solvent.

[0061] The lithium salt is a substance that is readily soluble in non-aqueous organic solvents, and may be, for example, at least one selected from the following: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 The lithium salts include LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lower aliphatic carboxylic acids, and lithium imide. In one embodiment of the invention, the lithium salt may preferably be lithium imide, such as LiTFSI.

[0062] The concentration of the lithium salt can be from 0.1M to 8.0M, preferably from 0.5M to 5.0M, and more preferably from 1.0M to 3.0M, depending on various factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium secondary batteries. If the concentration of the lithium salt is below the above range, the conductivity of the electrolyte may decrease, and therefore the performance of the battery may be reduced. If the concentration of the lithium salt exceeds the above range, the viscosity of the electrolyte may increase, and the lithium ions (Li) may... + The migration rate of ) may decrease. Therefore, it is preferable to select an appropriate concentration within the above range.

[0063] Solvents can be used without particular restrictions, as long as they are non-aqueous solvents that act as a medium through which ions participating in the electrochemical reaction of the battery can move. According to one embodiment of the invention, the solvent can be a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, or an aprotic solvent.

[0064] Specific examples of carbonate solvents may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), or butyl carbonate (BC).

[0065] Examples of ester solvents may include methyl acetate, ethyl acetate, n-propyl acetate, ethyl 1,1-dimethylacetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valproic acid lactone, mevalonate lactone, caprolactone, etc.

[0066] Specific examples of ether solvents may include diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and polyethylene glycol dimethyl ether, etc.

[0067] Specific examples of ketone solvents include cyclohexanone, etc.

[0068] Examples of alcohol solvents may include ethanol and isopropanol.

[0069] Examples of aprotic solvents may include: nitriles, such as acetonitrile; amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane (DOL); and sulfolane, etc.

[0070] Non-aqueous organic solvents can be used alone or in combination of two or more. The mixing ratio can be appropriately adjusted when using two or more in combination, depending on the desired battery performance. Specifically, a 1:1 volume ratio mixture of 1,3-dioxolane and dimethoxyethane is preferred.

[0071] The lithium-sulfur secondary battery of the present invention can be manufactured by arranging a separator between a positive electrode and a negative electrode to form an electrode assembly, inserting the electrode assembly into a cylindrical or rectangular battery case, and then injecting an electrolyte. Alternatively, the lithium-sulfur secondary battery of the present invention can be manufactured by laminating the electrode assembly, impregnating the electrode assembly with an electrolyte, placing the resulting product into a battery case, and then sealing it.

[0072] Preferred embodiments will be described below to aid in understanding the invention. However, the following embodiments are provided to aid in understanding the invention, but the invention is not limited thereto.

[0073] Preferred implementation scheme

[0074] Example

[0075] Example 1

[0076] 3 g of Ketjen black (manufacturer: MICHIBISHI CHEMICAL, specification: 600JD) and 9 g of sulfur were dispersed in 200 ml of toluene and heated and stirred at 60 °C for 3 hours. The powder obtained by evaporating toluene was heated at 160 °C for 20 hours under an argon atmosphere (condition: 100 sccm) to carry the molten sulfur within the pores of the porous carbon material. The resulting product was then heated in a vacuum oven at 50 °C (condition: below 1 Pa) for 3 hours to prepare a sulfur-carbon composite material.

[0077] The prepared sulfur-carbon composite material (positive electrode active material) was combined with Super P carbon black (manufacturer: TIMCAL, conductive material) and polyacrylic acid (molecular weight: M). w 450k (adhesive) was mixed in an 8:1:1 weight ratio and further mixed with a 0.1M Al2(SO4)3 aqueous solution in a 1:1 molar ratio of polyacrylic acid polymer monomers to Al2(SO4)3 to prepare a slurry (mixing conditions: Elim Global 8000D, zirconia balls (6mm and 3mm in diameter mixed in a 1:1 ratio), polypropylene vials). The slurry was then coated onto aluminum foil using a doctor blade (coating amount: based on sulfur weight of 1.7 mg / cm³). 2 (approximately 2mAh / cm) 2 Then, it is dried in air at 50°C and in vacuum at 50°C in sequence to produce the positive electrode (effective nuclear charge number of the outermost electrons of Al: 4.066).

[0078] Example 2

[0079] Except that a 0.2M MgSO4 aqueous solution was used instead of a 0.1M Al2(SO4)3 aqueous solution as the aqueous solution of the polyvalent metal salt, the positive electrode was manufactured in the same manner as in Example 1 (effective nuclear charge of the outermost electrons of Mg: 3.308).

[0080] Example 3

[0081] Except that a 0.2M ZnSO4 aqueous solution was used instead of a 0.1M Al2(SO4)3 aqueous solution as the aqueous solution of the polyvalent metal salt, the positive electrode was manufactured in the same manner as in Example 1 (effective nuclear charge number of the outermost electrons of Zn: 5.965).

[0082] Comparative Example 1

[0083] The positive electrode was manufactured in the same manner as in Example 1, except that the slurry was prepared without additional mixing of 0.1M Al2(SO4)3 aqueous solution.

[0084] Experimental Example

[0085] Experimental Example 1: Thermogravimetric Analysis of Sulfur-Carbon Composite Materials

[0086] To confirm the weight of sulfur in the sulfur-carbon composite material prepared in Example 1, thermogravimetric analysis was performed, and the results are as follows: Figure 1 As shown. The analytical apparatus and analytical conditions are as follows.

[0087] <Thermogravimetric Analysis>

[0088] Analytical apparatus: Shimadzu TA 50

[0089] Analytical conditions: argon gas 10 mL / min, heating rate 5 °C / min, maximum temperature 600 °C

[0090] according to Figure 1 It was confirmed that the weight ratio of porous carbon material (Ketjen Black) to sulfur in the sulfur-carbon composite material (S / KB) prepared in Example 1 was approximately 33:67.

[0091] Experimental Example 2: Thermogravimetric Analysis of Polyacrylic Acid Adhesives with Immobilized Polyvalent Metal Ions

[0092] Polyacrylic acid (PAA) with a molar ratio (moles of polyacrylic acid monomers: moles of Al2(SO4)3 or ZnSO4) of 1:1 was stirred at room temperature and then dried at 60°C after dialysis for 2 weeks. Thermogravimetric analysis was performed to check the residual amount of polyvalent metal ions in the adhesive. The results are as follows: Figure 2 As shown. The analytical apparatus and analytical conditions are as follows.

[0093] <Thermogravimetric Analysis>

[0094] Analytical apparatus: Shimadzu TA 50

[0095] Analytical conditions: Oxygen 10 mL / min, heating rate 5 °C / min, maximum temperature 600 °C

[0096] according to Figure 2 Following thermogravimetric analysis (TGA), the residual amount of Al2(SO4)3-PAA was confirmed to be 6.9%, and the residual amount of ZnSO4-PAA was confirmed to be 8.3%. Considering that the residual amount is related to the amount of metal that was not removed when equilibrium was reached with excess solvent, and that the TGA was performed under an oxygen atmosphere, the amount of metal was measured in the form of oxides. That is, the 6.9% residual amount in Al2(SO4)3-PAA refers to the amount of Al2O3, and the 8.3% residual amount in ZnSO4-PAA refers to the amount of ZnO. The results of the TGA confirmed that polyacrylic acid has fixed polyvalent metals (Al or Zn) to a certain level through carboxylic acid functional groups.

[0097] Experimental Example 3: Analysis of Capacity Retention Rate of Lithium-Sulfur Secondary Batteries

[0098] The positive electrode prepared according to the examples and comparative examples was fabricated to have a diameter of 14 mm, and a separator was laminated ( A coin cell was fabricated by placing a 2400 (19 mm diameter) electrode and a 16 mm diameter (160 μm thick) anode and a 160 μm thick cathode into a coin cell casing (2032 type coin cell (Welcos)) and adding an electrolyte. The electrolyte was prepared by adding 2 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.1 M LiNO3 to a solvent obtained by mixing 1,3-dioxolane (DOL) and dimethoxyethane (DME).

[0099] The capacity retention rate was analyzed by charging / discharging the manufactured coin batteries, and the results are as follows: Figure 3 As shown. The analytical apparatus and analytical conditions are as follows.

[0100] <Analysis of Cyclic Stability>

[0101] Analytical apparatus: Wonatech WBCS3000L

[0102] Analysis conditions: For the first 3 cycles, charge / discharge was performed at 0.5 A / g, and thereafter at 1 A / g.

[0103] according to Figure 3 It was confirmed that adding polyvalent metal salts according to Examples 1 to 3 improved the capacity retention of lithium-sulfur secondary batteries during cycling. Specifically, it was confirmed that using Al2(SO4)3 of Example 1 and MgSO4 of Example 2 improved the medium- and long-term cycling performance, and that using ZnSO4 of Example 3 improved the initial cycling performance.

[0104] All simple modifications and variations of this invention fall within the scope of this invention, and the specific scope of protection of this invention will be clearly defined by the appended claims.

Claims

1. A positive electrode for a lithium-sulfur secondary battery, the positive electrode for a lithium-sulfur secondary battery comprising a positive electrode active material, a conductive material, a binder, and a polyvalent metal salt, wherein said multivalent metal salt consists of Mg 2+ cations and anions selected from OH - , CO3 2- , NO3 - and SO4 2- , the positive electrode active material is a sulfur-carbon composite material, wherein the binder is a polymer having an oxygen- or nitrogen-containing functional group, wherein the content of the polyvalent metal salt is 30 to 100 mole% based on the number of moles of the polymer monomer of the binder.

2. The positive electrode for a lithium-sulfur secondary battery according to claim 1, wherein the binder is selected from the group consisting of polyacrylic acid, polyvinyl alcohol, polyacrylonitrile, and combinations thereof.

3. The positive electrode for a lithium-sulfur secondary battery according to claim 1, wherein the content of the binder is 5 to 20 wt% based on the total weight of the positive electrode active material.

4. The positive electrode for a lithium-sulfur secondary battery according to claim 1, wherein the conductive material is selected from the group consisting of graphite, carbon black, conductive fibers, metal powders, carbon fluoride, conductive whiskers, conductive metal oxides, polyphenylene derivatives, and combinations thereof.

5. The positive electrode for a lithium-sulfur secondary battery according to claim 1, wherein the content of the conductive material is 5 to 20 wt% based on the total weight of the positive electrode active material.

6. The positive electrode for a lithium-sulfur secondary battery according to claim 1, wherein in the positive electrode, the polyvalent metal salt is introduced by adding the polyvalent metal salt in an aqueous solution state, followed by drying.

7. The positive electrode for a lithium-sulfur secondary battery according to claim 6, wherein the polyvalent metal salt in an aqueous solution state is added before drying a mixture of the positive electrode active material, the conductive material, and the binder, or is coated after drying.

8. A lithium-sulfur secondary battery comprising the positive electrode of claim 1.

Images