Positive electrode for lithium-sulfur secondary battery having improved lifespan performance, method for manufacturing same, and lithium-sulfur secondary battery comprising same

Abstract

The present invention relates to: a positive electrode for a lithium-sulfur secondary battery, the positive electrode having improved lifespan performance; a method for manufacturing same; and a lithium-sulfur secondary battery comprising same. Specifically, the present invention relates to: a positive electrode for a lithium-sulfur secondary battery, the positive electrode having an improved cycle life and calendar life due to suppressing the shuttling of polysulfide compounds; a method for manufacturing same; and a lithium-sulfur secondary battery comprising same.

Description

Anode for a lithium-sulfur secondary battery with improved life performance, a method for manufacturing the same, and a lithium-sulfur secondary battery comprising the same

[0001] The present invention relates to a positive electrode for a lithium-sulfur secondary battery with improved life performance, a method for manufacturing the same, and a lithium-sulfur secondary battery comprising the same. Specifically, the invention relates to a positive electrode for a lithium-sulfur secondary battery with improved life performance by suppressing the dissolution of polysulfides, a method for manufacturing the same, and a lithium-sulfur secondary battery comprising the same.

[0002] Lithium-sulfur secondary batteries are secondary batteries that use inorganic or organic sulfur compounds as positive electrode active materials and materials capable of inserting / deleting lithium ions, such as carbon-based materials, or alkali metals such as lithium metal, as negative electrode active materials. Lithium-sulfur batteries use readily available, inexpensive sulfur compounds as positive electrode active materials, resulting in low unit costs and a high capacity of approximately 1,675 mAh·g. -1 High theoretical capacity of 2,600 Wh·kg -1 It is attracting attention as a next-generation secondary battery candidate that has high energy density and is advantageous for weight reduction.

[0003] The biggest problem limiting the commercialization of lithium-sulfur secondary batteries is the problem of polysulfide dissolution, which is the loss of polysulfide from the cathode.

[0004] In a lithium-sulfur secondary battery, when discharging, the negative active material lithium is oxidized by donating electrons and ionizing into lithium cations, and the positive active material sulfur series material is reduced by accepting electrons. Here, through the reduction reaction of the sulfur series material, the S-S bond accepts two electrons and is converted into the form of a sulfur anion. The lithium cation produced by the oxidation reaction of lithium is transferred to the positive electrode through the electrolyte, and this combines with the sulfur anion produced by the reduction reaction of the sulfur series compound to form a salt. Specifically, before discharge, sulfur has a cyclic S8 structure, which is converted into lithium polysulfide (Li2Sx) through the reduction reaction, and is completely reduced to produce lithium sulfide (Li2S).

[0005] In this way, sulfur used in the positive electrode active material is an insulator, so it is difficult for electrons generated by electrochemical reactions to move, and polysulfide (LiS) generated during the charge / discharge process x ) There is a problem of reduced battery life performance and rate performance due to slow kinetic activity of electrochemical reaction due to dissolution problem and low electrical conductivity of sulfur and lithium sulfide.

[0006] Accordingly, polysulfide (LiS) is used without or with minimal degradation of battery performance. x ) It is necessary to develop a method to suppress the phenomenon of melting.

[0007] The present invention has been conceived to solve the problems of the above-described prior art, and the problem to be solved by the present invention is to provide a method for manufacturing a positive electrode for a lithium-sulfur secondary battery that can improve the life performance of the battery by suppressing the polysulfide dissolution phenomenon in the lithium-sulfur secondary battery.

[0008] Another technical task of the present invention is to provide a positive electrode for a lithium-sulfur secondary battery having excellent life performance while minimizing the impact on output performance, and a lithium-sulfur secondary battery including the positive electrode.

[0009] Other objects, specific advantages and novel features of the present invention will become more apparent from the following detailed description and preferred embodiments in conjunction with the accompanying drawings.

[0010] In order to solve the above-described technical problem, the present invention comprises the steps of: 1) preparing a graphene dispersion solution; and

[0011] 2) A method for manufacturing a positive electrode for a lithium-sulfur secondary battery is provided, including a step of forming a graphene coating layer by spraying the graphene dispersion solution on a positive electrode active material layer.

[0012] In one embodiment of the present invention, the graphene dispersion solution in step 1) may contain graphene in an amount of 5 wt% to 15 wt%.

[0013] In one embodiment of the present invention, in step 1), the graphene dispersion solution can be dispersed by applying ultrasonic waves after putting graphene into a solvent.

[0014] In one embodiment of the present invention, the ultrasound may be applied at a frequency of 20 kHz to 150 kHz and an output of 100 W to 500 W.

[0015] In one embodiment of the present invention, in step 2), the graphene coating layer may be formed to a thickness of 1 μm to 20 μm.

[0016] In one embodiment of the present invention, a step of absorbing a liquid electrolyte onto the graphene coating layer may be included.

[0017] In one embodiment of the present invention, 3) after forming a graphene coating layer, a step of applying a slurry containing a conductive ceramic compound, a binder, and a solvent on the graphene coating layer to allow the conductive ceramic compound to penetrate the graphene coating layer, and then removing the solvent to form a composite film layer may be further included.

[0018] The present invention provides a positive electrode for a lithium-sulfur secondary battery, comprising: a current collector; a positive electrode active material layer provided on the current collector and including an organic or inorganic sulfur compound; and a graphene coating layer formed on the positive electrode active material layer.

[0019] According to the manufacturing method of the present invention, the polysulfide dissolution suppression effect is excellent, so that the life performance of the battery can be improved.

[0020] According to the manufacturing method of the present invention, a positive electrode for a lithium-sulfur secondary battery having excellent life performance while minimizing the impact on output performance and a lithium-sulfur secondary battery including the same can be provided.

[0021] Figure 1 is a schematic diagram showing the layered structure of a lithium-sulfur secondary battery positive electrode according to the prior art.

[0022] Figure 2 is a schematic diagram showing the layered structure of a lithium-sulfur secondary battery positive electrode of the present invention including a graphene coating layer.

[0023] Figure 3 is a schematic diagram showing the layered structure of a lithium-sulfur secondary battery positive electrode of the present invention including a graphene coating layer and a composite film layer.

[0024] FIG. 4 is a graph comparing the cycle life of a lithium-sulfur secondary battery including a positive electrode according to a preferred embodiment of the present invention and a lithium-sulfur secondary battery including a positive electrode according to the prior art.

[0025] In this specification, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0026] The terms "comprise" and / or "comprising" as used herein specify the presence of stated features, steps, numbers, operations, elements, elements and / or groups thereof, but do not preclude the presence or addition of one or more other features, steps, numbers, operations, elements, elements and / or groups thereof.

[0027] If a term expressed as a component in this specification includes a functional expression, it may be defined to encompass not only the function in question but also other functions that can be clearly understood by a person skilled in the art.

[0028] When multiple embodiments are described in this specification, the effects of the present invention may be defined to include not only the operational effects derived from each embodiment itself, but also the effects resulting from the organic combination of the embodiments. For example, even if Embodiments 1 and 2 are described independently in this specification, unless the context clearly indicates otherwise, the effects resulting from the organic combination of Embodiments 1 and 2 may also be included in the effects of the present invention.

[0029] The numerical range indicated by the term "to" in this specification refers to a numerical range that includes the values ​​described before and after the term as the lower limit and the upper limit, respectively. When multiple numerical values ​​are disclosed as the upper and lower limits of an arbitrary numerical range, the numerical range disclosed in this specification can be understood as any numerical range that has any one of the multiple lower limit values ​​and any one of the multiple upper limit values ​​as the lower limit and the upper limit, respectively. For example, when a to b, or c to d is described in the specification, it can be understood that a or more and b or less, a or more and d or less, c or more and d or less, or c or more and b or less is described.

[0030] As used herein, terms such as "about" or "substantially" mean a reasonable amount of variation from the term that does not significantly alter the final result. These terms may be interpreted to include a variation of at least ±5% or at least ±10%, provided that such variation does not alter the meaning of the term and render it invalid.

[0031] Hereinafter, each configuration and effect of the lithium-sulfur secondary battery according to the present invention will be described in more detail.

[0032] anode

[0033] As described above, lithium-sulfur secondary batteries have the advantage of being able to reduce the manufacturing cost of secondary batteries by using inorganic or organic sulfur compounds with lower unit prices as cathode active materials compared to lithium-ion batteries, lithium polymer batteries, etc., and of being able to reduce weight due to high energy density, but have the disadvantage of reduced life performance due to polysulfide elution.

[0034] Accordingly, the present invention provides a method for manufacturing a positive electrode for a lithium-sulfur secondary battery, comprising the steps of 1) manufacturing a graphene dispersion solution; and 2) spraying the graphene dispersion solution onto a positive electrode active material layer to form a graphene coating layer. This method can suppress polysulfide from dissolving into an electrolyte and being lost, and significantly improve the life performance of the secondary battery.

[0035] The positive electrode for a lithium-sulfur secondary battery manufactured by the method according to the present invention can effectively prevent polysulfide from being dissolved and lost from the positive electrode active material layer into an electrolyte by forming a uniform and dense graphene coating layer on the surface of the positive electrode active material layer.

[0036] In the above step 1), the graphene dispersion solution can be prepared by uniformly dispersing graphene in a solvent.

[0037] Specifically, the solvent may be an organic solvent. The organic solvent may be, for example, one or a mixture of two or more selected from the group consisting of N,N-dimethylformamide (DMF), dimethylacetamide (DMAc), tetrahydrofuran (THF), dimethylsulfoxide (DMSO), γ-butyrolactone (GBL), N-methylpyrrolidone (NMP), chloroform, toluene, acetone, pentane, hexane, and heptane. Preferably, the organic solvent may be N-methylpyrrolidone. However, the type of solvent is not necessarily limited thereto, and a person skilled in the art may select without limitation any solvent that can stably disperse graphene particles without departing from the purpose of the present invention.

[0038] The above graphene dispersion solution may preferably contain graphene in an amount of 5 wt% to 15 wt%. The content of graphene can be specifically 5 wt% to 14 wt%, 5 wt% to 13 wt%, 5 wt% to 12 wt%, 5 wt% to 11 wt%, 6 wt% to 15 wt%, 6 wt% to 14 wt%, 6 wt% to 13 wt%, 6 wt% to 12 wt%, 7 wt% to 15 wt%, 7 wt% to 14 wt%, 7 wt% to 13 wt%, 7 wt% to 12 wt%, 8 wt% to 15 wt%, 8 wt% to 14 wt%, 8 wt% to 13 wt%, or 8 wt% to 12 wt%. If the graphene content is less than 5 wt%, the density of the formed graphene coating layer is low, making it difficult to prevent polysulfide from dissolving from the anode. If the graphene content exceeds 15 wt%, the graphene particles may clump together within the graphene dispersion solution, making it difficult to form a graphene coating layer with a uniform density. If the graphene coating layer does not have a uniform thickness and density, the polysulfide dissolution phenomenon cannot be effectively suppressed.

[0039] In addition, the graphene dispersion solution can preferably be manufactured by dispersing graphene in a solvent and then more effectively dispersing it using ultrasonic waves from a sonicator.

[0040] In one embodiment of the present invention, the ultrasound may be applied to the graphene dispersion solution at a temperature of 10°C to 50°C for 10 minutes to 2 hours. The time for applying the ultrasound may vary depending on the type of solvent. For solvents with high viscosity, a long time is required, while for solvents with low viscosity, the graphene may be dispersed by applying the ultrasound for a short time.

[0041] Additionally, if the temperature at which the ultrasonic dispersion method is performed is below 10°C, the viscosity of the solvent may increase, resulting in a longer processing time. Conversely, if the temperature exceeds 50°C, the solvent may significantly evaporate and decrease during the ultrasonic dispersion method, so it is preferable to control the temperature range within 10°C to 50°C.

[0042] In addition, it is preferable that the ultrasound be applied at a frequency of 20 kHz to 150 kHz and an output of 100 W to 500 W. If the frequency of the ultrasound is less than 20 kHz or the output is less than 100 W, the mixing of the solution by the ultrasound may not be sufficient and may take a long time. In addition, if it exceeds 150 kHz, the energy is excessive, which increases the cost and the mixing may proceed excessively violently, which may cause loss of graphene. Therefore, it is preferable to apply the ultrasound at a frequency and output within the above range.

[0043] Additionally, the above ultrasonic range can effectively remove microbubbles present in the graphene dispersion. If the ultrasonic frequency exceeds the above-described range, the cavitation effect is too large, which may cause physical and chemical alteration of the graphene dispersion solution. If the ultrasonic frequency is less than 20 kHz, the cavitation effect is too small, and the dissolved gas cannot be effectively removed.

[0044] The above step 2) is a step of forming a graphene coating layer by spraying the graphene dispersion solution prepared in the above step 1) onto the positive electrode active material layer.

[0045] The above positive electrode active material layer may preferably be formed by applying a positive electrode slurry containing a positive electrode active material, a conductive material, a positive electrode binder, and a solvent onto a current collector.

[0046] The current collector can support the positive electrode active material layer and can be selected from materials having high conductivity without causing chemical changes in the secondary battery. Examples of current collectors that can be used include copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, or silver (Ag), and aluminum-cadmium alloys.

[0047] The above-mentioned collector can form fine irregularities on its surface to strengthen the bonding strength with the positive electrode active material, and can be used in various forms such as a film, sheet, foil, mesh, net, porous body, foam, and non-woven fabric.

[0048] The cathode active material is inorganic sulfur (S8), lithium polysulfide (Li2Sn, 1≤n≤8), and carbon sulfur polymer (C2S x ) m (2.5≤x≤50, 2≤m) or a mixture of two or more thereof, but is not limited thereto. Preferably, inorganic sulfur (S8) can be used.

[0049] Since the sulfur compound contained in the above-mentioned positive electrode active material has little electrical conductivity on its own, it can be used in combination with a conductive material such as a carbon-based material. Accordingly, the above-mentioned positive electrode active material can be included in the form of a sulfur-carbon complex.

[0050] The carbon included in the above sulfur-carbon complex is a porous carbon material that provides a framework in which the sulfur can be uniformly and stably fixed, and complements the low electrical conductivity of sulfur to enable the electrochemical reaction to proceed smoothly.

[0051] The above porous carbon material can generally be manufactured by carbonizing precursors of various carbon materials. The average diameter of the pores is in the range of 1 to 200 nm, and the porosity or porosity can be in the range of 10 to 90% of the total volume of the porous carbon material. If the average diameter of the pores is less than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if it exceeds the above range, the mechanical strength of the porous carbon material is weakened, making it undesirable for application to the electrode manufacturing process.

[0052] The shape of the above porous carbon material may be spherical, rod-shaped, needle-shaped, plate-shaped, tubular or bulk-shaped, and may be used without limitation as long as it is a shape commonly used in lithium-sulfur batteries.

[0053] The porous carbon material may be any material that is commonly used in the art and has a porous structure or a high specific surface area. For example, the porous carbon material may be at least one selected from the group consisting of graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and graphite and activated carbon such as natural graphite, artificial graphite, and expanded graphite, but is not limited thereto.

[0054] The above cathode slurry may further include one or more additives selected from transition metal elements, group ⅢA elements, group ⅣA elements, sulfur compounds of these elements, and alloys of these elements and sulfur.

[0055] The above transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg, etc., the above group ⅢA elements include Al, Ga, In, Ti, etc., and the above group ⅣA elements may include Ge, Sn, Pb, etc.

[0056] The method for producing the above sulfur-carbon complex is not particularly limited in the present invention, and a method commonly used in the art can be used.

[0057] The above conductive material is a material that electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from the current collector to the positive electrode active material. Any conductive material can be used without limitation.

[0058] For example, the conductive material may be graphite such as natural graphite or artificial graphite; carbon black such as Super-P, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or summer black; carbon derivatives such as carbon nanotubes or fullerene; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powders such as aluminum or nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, or polypyrrole, which may be used alone or in combination.

[0059] The above positive electrode binder maintains the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active materials to further increase the bonding strength between them, and any binder known in the industry can be used.

[0060] For example, the positive electrode binder may be a fluororesin binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); a rubber binder including styrene butadiene rubber (SBR), acrylonitrile-butidiene rubber, or styrene-isoprene rubber; a cellulose binder including carboxyl methyl cellulose (CMC), starch, hydroxy propyl cellulose, or regenerated cellulose; a polyalcohol binder including polyvinyl alcohol (PVA); a polyacrylic binder including polyacrylic acid (PAA); a polyolefin binder including polyethylene or polypropylene; a polyimide binder; or a polyester binder. and a silane-based binder; a mixture or copolymer of one or more selected from the group consisting of;

[0061] In a preferred embodiment of the present invention, the content of the positive electrode active material included in the positive electrode slurry may be 50 wt% to 70 wt%, the content of the conductive material may be 25 wt% to 40 wt%, and the content of the binder may be 5 wt% to 15 wt%.

[0062] The above positive electrode slurry may preferably be a solvent in which a positive electrode active material, a conductive agent, and a positive electrode binder are uniformly dispersed. A solvent capable of uniformly dispersing the positive electrode active material, the conductive agent, and the binder is used. The solvent is most preferably an aqueous solvent such as water, and the water may be distilled water or deionized water. However, the present invention is not limited thereto, and if necessary, a lower alcohol that is easily mixed with water may be used. The lower alcohol includes methanol, ethanol, propanol, isopropanol, and butanol, and preferably, these may be mixed with water and used.

[0063] In the above step 2), a graphene coating layer can be formed with a thin thickness by forming a graphene coating layer by spraying, and the polysulfide dissolution phenomenon can be effectively suppressed without significantly reducing the output performance of the secondary battery.

[0064] In a preferred embodiment of the present invention, the graphene coating layer in step 2) may be formed to a thickness of 1 μm to 20 μm. If the thickness of the graphene coating layer is less than 1 μm, the polysulfide dissolution phenomenon may not be sufficiently suppressed. In addition, the durability is reduced, and the graphene coating layer is likely to be damaged. Conversely, if the thickness exceeds 20 μm, the graphene coating layer may act as a resistor, which may reduce the output performance of the secondary battery.

[0065] In one embodiment of the present invention, the process of manufacturing a battery can be simplified by absorbing a liquid electrolyte into the positive electrode layer on which the graphene coating layer is formed.

[0066] Specifically, the liquid electrolyte can be dropped onto an anode having a graphene coating layer formed thereon to be completely absorbed, and a separator and a cathode can be sequentially laminated on the anode to manufacture a final battery.

[0067] Additionally, according to one embodiment of the present invention, a liquid electrolyte can be dropped and absorbed onto the composite film layer described below.

[0068] Meanwhile, a method for manufacturing a positive electrode for a lithium-sulfur secondary battery according to a preferred embodiment of the present invention may further include the step of 3) forming a graphene coating layer, then applying a slurry containing a conductive ceramic compound, a binder, and a solvent on the graphene coating layer to allow the conductive ceramic compound to penetrate the graphene coating layer, and then removing the solvent to form a composite film layer.

[0069] The present invention further comprises a composite film layer including a conductive ceramic compound in addition to a graphene coating layer, thereby more effectively suppressing the elution of polysulfide from the anode.

[0070] During the process of forming the composite film layer, the conductive ceramic compound included in the slurry can penetrate into the pores of the graphene coating layer, thereby improving the conductivity of the graphene coating layer. Furthermore, the conductive ceramic compound can penetrate into the pores of the graphene coating layer, thereby making the graphene coating layer denser and more effectively suppressing the polysulfide dissolution phenomenon.

[0071] The above conductive ceramic compound may be at least one selected from the group consisting of lithium oxide, lithium sulfide, lithium phosphate, amorphous ion conductive material, NASICON, sodium sulfide, sodium oxide or sodium phosphate.

[0072] The above lithium oxide is, for example, Li-α-Al2O3, Li-β-Al2O3, Li-TiO2, Li-BaTiO3, Li-SiO2, LaTiO3, LiTiO3, Li5La3Ta2O 12 , Li6La2CaTa2O 12 , Li4SiO4, Li3BO 2.5 N 0.5 , Li9SiAlO8, Li6La2CaNb2O 12 , Li6La2SrNb2O12 , Li2Nd3TeSbO 12 , Li7La3Zr2O 12 (LLZO), Li5La3Ta2O 12 and may be at least one selected from the group consisting of Li9SiAlO8.

[0073] The above lithium sulfide is, for example, Li 10 GeP2S 12 , Li7P2S 11 , Li 3.25 Ge 0.25 P 0.75 S4(LGPS), Li2S-Si2S5, Li2S-Ga2S3-GeS2, Li2S-Sb2S3-GeS2, Li2S-P2S5, Li2S-P2S5-Li4SiO4, and Li 3.25 -Ge 0.25 -P 0.75 It may be one or more selected from the group consisting of S4 (Thio-LISICON).

[0074] The above lithium phosphate is, for example, LAGP (Li 1+x Al x Ge 2-x (PO4)3)(O <x<2), LTAP(Li 1+x Ti 2-x Al x (PO4)3)(0 <x<2), Li 1+x Ti 2-x Al x Si y (PO4) 3-y (0 <x<2, 0<y<3), LiAl x Zr 2-x (PO4)3(0 <x<2), 및 LiTi x Zr 2-x (PO4)3(0 <x<2)로 이루어진 군에서 선택된 하나 이상일 수 있다.

[0075] The above amorphous ion-conducting material may be at least one selected from the group consisting of phosphorous-based glass, oxide-based glass, and oxide-sulfide based glass.

[0076] The binder included in the composite film layer may be at least one selected from the group consisting of, for example, polyvinylidene fluoride (PVdF), poly[(vinylidene fluoride-co-trifluoroethylene]], polyethylene glycol (PEO), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), polyvinyl chloride (PVC), polyvinylpyrrolidone (PVP), polyimide (PI), polyethylene (PE), polyurethane (PU), polypropylene (PP), poly(propylene oxide) (PPO), poly(ethyleneimine) (PEI), poly(ethylene sulfide) (PES), poly(vinyl acetate) (PVAc), poly(ethylene succinate) (PESc), polyester, polyamine, polysulfide, siloxane, styrene butadiene rubber (SBR), and carboxymethylcellulose (CMC).

[0077] In a preferred embodiment of the present invention, the thickness of the composite film layer may be 5 μm to 30 μm. If the thickness of the composite film layer is less than 5 μm, the effect of inhibiting polysulfide dissolution may not be sufficient. Conversely, if the thickness exceeds 30 μm, it may act as a resistance to the movement of lithium ions.

[0078] In a preferred embodiment of the present invention, step 3) may be performed before the graphene coating layer is completely dried. Performing step 3) before the graphene coating layer is completely dried means that step 3) is performed while the solvent of the graphene dispersion solution remains.

[0079] In a preferred embodiment of the present invention, the thicknesses of the graphene coating layer and the composite film layer may have a ratio of 1:1.1 to 1:1.5.

[0080] The present invention provides a positive electrode for a lithium-sulfur secondary battery, comprising: a current collector; a positive electrode active material layer provided on the current collector and including an organic or inorganic sulfur compound; and a graphene coating layer formed on the positive electrode active material layer.

[0081] Since each component that constitutes the current collector, positive electrode active material, and graphene coating layer has been described above, the description of overlapping parts is omitted.

[0082] cathode

[0083] The negative electrode may include a negative electrode current collector and a negative electrode active material layer applied to one surface of the negative electrode current collector. Alternatively, the negative electrode may be lithium metal.

[0084] The above negative electrode current collector is for supporting the negative electrode active material layer, and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and may be selected from the group consisting of copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, alloys thereof, and combinations thereof. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and the alloy may be an aluminum-cadmium alloy. In addition, calcined carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer, etc. may be used. Generally, a copper foil is applied as the negative electrode current collector.

[0085] In addition, the form can be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc. with / without fine unevenness formed on the surface.

[0086] The above-mentioned negative electrode active material layer may include, in addition to the negative electrode active material, a conductive material, a negative electrode binder, etc. In this case, the conductive material and the negative electrode binder may be selected from the same materials as the conductive material and the positive electrode binder of the positive electrode.

[0087] The above negative electrode active material may include a material capable of reversibly intercalating or deintercalating lithium ions (Li+), a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, lithium metal, or a lithium alloy.

[0088] The above lithium ion (Li + ) can be reversibly inserted or removed, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The lithium ion (Li + ) can be, for example, tin oxide, titanium nitrate or silicon. The lithium alloy can be, for example, an alloy of lithium (Li) and a metal selected from the group consisting of 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).

[0089] Preferably, the negative active material may be lithium metal, and specifically, may be in the form of a lithium metal thin film or lithium metal powder.

[0090] The method for forming the above-mentioned negative active material is not particularly limited, and any method for forming a layer or film commonly used in the art can be used. For example, methods such as compression, coating, and deposition can be used. In addition, a case in which a battery is assembled without a lithium thin film on the current collector and then a metallic lithium thin film is formed on the metal plate through initial charging is also included in the negative electrode of the present invention.

[0091] membrane

[0092] The separator separates or insulates the positive and negative electrodes from each other and enables lithium ion transport between the positive and negative electrodes. It may be made of a porous non-conductive or insulating material, and can be used without any special restrictions as long as it is commonly used as a separator in lithium secondary batteries. The separator may be an independent member such as a film, or may be a coating layer added to the positive and / or negative electrodes.

[0093] It is preferable that the above separator have low resistance to ion movement of the electrolyte and excellent moisture absorption capacity for the electrolyte.

[0094] The above separator may be formed of a porous substrate. Any porous substrate commonly used in secondary batteries may be used as the porous substrate. A porous polymer film may be used alone or in a laminated manner. For example, a nonwoven fabric or a polyolefin porous film made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used, but is not limited thereto.

[0095] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate may be a polyolefin such as polyethylene, polypropylene, polyester such as polyethyleneterephthalate, polybutyleneterephthalate, polyamide, polyacetal, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, polyethylenenaphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, polyethylene It may include at least one material selected from the group consisting of poly(p-phenylene benzobisoxazole) and polyarylate.

[0096] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. The thickness range of the porous substrate is not limited to the above-mentioned range, but if the thickness is excessively thinner than the above-mentioned lower limit, the mechanical properties may deteriorate, and the separator may be easily damaged during battery use.

[0097] The average diameter and porosity of the pores present in the porous substrate are not particularly limited, but may be 0.001 to 50 ㎛ and 10 to 95%, respectively.

[0098] The shape of the above lithium secondary battery is not particularly limited and can be made into various shapes such as cylindrical, stacked, and coin-shaped.

[0099] electrolyte

[0100] The electrolyte used in the present invention refers to a liquid electrolyte, and may be a non-aqueous electrolyte or a solid electrolyte that does not react with lithium metal, but is preferably a non-aqueous electrolyte and includes an electrolyte salt and an organic solvent.

[0101] The electrolyte salt included in the above non-aqueous electrolyte is a lithium salt. The lithium salt may be used without limitation as long as it is commonly used in electrolytes for lithium secondary batteries. For example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium 4-phenylborate, lithium imide, etc. can be used.

[0102] The concentration of the lithium salt may be appropriately determined by considering ionic conductivity, solubility, etc., and may be, for example, 0.1 to 4.0 M, preferably 0.5 to 2.0 M. If the concentration of the lithium salt is below the above range, it is difficult to secure ionic conductivity suitable for battery operation, and conversely, if it exceeds the above range, the viscosity of the electrolyte increases, which reduces the mobility of lithium ions and increases the decomposition reaction of the lithium salt itself, which may deteriorate the performance of the battery. Therefore, it is appropriately controlled within the above range.

[0103] As the organic solvent included in the above non-aqueous electrolyte, those commonly used in electrolytes for lithium secondary batteries can be used without limitation, and for example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc. can be used singly or in a mixture of two or more.

[0104] The above ether compound may include an acyclic ether and a cyclic ether.

[0105] For example, the acyclic ether may be at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether, ethylpropyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethylmethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methylethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methylethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methylethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, polyethylene glycol methylethyl ether, but is not limited thereto.

[0106] For example, the cyclic ether is 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, At least one selected from the group consisting of 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether may be used, but is not limited thereto.

[0107] Among the organic solvents, the ester may be any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0108] Specific examples of the linear carbonate compound include, but are not limited to, one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, or a mixture of two or more thereof.

[0109] In addition, specific examples of the cyclic carbonate compound include one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, or a mixture of two or more thereof. Examples of the halides thereof include, but are not limited to, fluoroethylene carbonate (FEC).

[0110] The injection of the above electrolyte may be performed at an appropriate stage during the manufacturing process of a lithium-sulfur battery, depending on the manufacturing process and required properties of the final product. That is, it may be applied before or during the final assembly stage of the lithium-sulfur battery.

[0111] The present invention also provides a lithium-sulfur secondary battery comprising the positive electrode; the negative electrode; and a separator interposed between the positive electrode and the negative electrode, wherein the separator is impregnated with an electrolyte.

[0112] A lithium-sulfur secondary battery according to a preferred embodiment of the present invention may have the electrolyte impregnated into the composite film layer.

[0113] <Example>

[0114] Example 1: Manufacturing of a lithium-sulfur secondary battery with a graphene coating layer

[0115] Sulfur (S8) was prepared as a cathode active material, graphene powder as a conductive agent, and polyvinylidene fluoride (PVDF) as a cathode binder.

[0116] The positive electrode active material, conductive agent, and binder were mixed in a weight ratio of 6:3:1 and ground for approximately 20 minutes. The mixture was then dissolved in an N-methylpyrrolidone (NMP) solvent. The solution, in which the mixture was dissolved in the NMP solvent, was stirred for approximately 10 minutes to adjust the viscosity by adjusting the amount of solvent, thereby dispersing the positive electrode active material, thereby preparing a positive electrode slurry.

[0117] The above positive electrode slurry was applied onto aluminum foil and dried at about 60°C for more than 9 hours to form a positive electrode active material layer.

[0118] Graphene powder and NMP solvent were mixed at a weight ratio of 1:9, and ultrasonic waves having a frequency of 100 kHz and an output of 400 W at about 30°C were applied for about 60 minutes to prepare a graphene dispersion solution in which graphene powder was uniformly dispersed in the NMP solution.

[0119] The prepared graphene dispersion solution was sprayed onto the positive electrode active material layer using a sprayer to form a graphene coating layer having a thickness of approximately 10 μm. Thereafter, the graphene coating layer was dried to manufacture a positive electrode for a lithium-sulfur secondary battery, including a current collector, a positive electrode active material layer, and a graphene coating layer.

[0120] A solution of LiPF6 salt dissolved in diethyl ether at a concentration of 1 M was used as the electrolyte. A porous polyethylene film was used as the separator, and a lithium metal foil was used as the cathode.

[0121] Afterwards, the electrolyte was dropped onto the positive electrode to completely absorb it, a separator and a negative electrode were sequentially stacked on the positive electrode, and the separator was again impregnated with the electrolyte to manufacture a lithium-sulfur secondary battery.

[0122] Example 2: Manufacturing of a lithium-sulfur secondary battery with a graphene coating layer

[0123] The same procedure as Example 1 was followed, but the graphene dispersion solution was prepared by dispersing the graphene content to be approximately 3 wt%, and the thickness of the graphene coating layer was set to be approximately 500 nm.

[0124] Example 3: Manufacturing of a lithium-sulfur secondary battery with a graphene coating layer

[0125] The same procedure as Example 1 was followed, but the graphene dispersion solution was prepared by dispersing the graphene content to be approximately 25 wt%, and the thickness of the graphene coating layer was formed to be approximately 30 ㎛.

[0126] Example 4: Manufacturing of a lithium-sulfur secondary battery having a graphene coating layer and a composite film layer formed thereon.

[0127] The same procedure as Example 1 was followed, except that a composite film layer was additionally formed on the graphene coating layer as described below.

[0128] Li, a conductive ceramic compound 1.3 Al 0.3 Ge 1.7 (PO4)3 and PVDF were mixed in a weight ratio of approximately 3:1 and dispersed in an NMP solvent to prepare a slurry. The slurry was applied to the graphene coating layer to a thickness of approximately 15 μm. The application was performed before the graphene coating layer was completely dry. That is, the application was performed at the point when any moisture on the surface was visually removed.

[0129] After applying the above slurry, the applied laminate was placed on an ultrasonic generator and ultrasonic waves of 100 kHz at an output of 200 W were applied for about 15 minutes to form Li on the interface. 1.3 Al 0.3 Ge 1.7 (PO4)3 was infiltrated into the graphene coating layer.

[0130] Afterwards, the laminate was dried to remove all solvents, thereby manufacturing a positive electrode for a lithium-sulfur secondary battery sequentially including a current collector, a positive electrode active material layer, a graphene coating layer, and a composite film layer.

[0131] Afterwards, a lithium-sulfur secondary battery was manufactured by configuring a separator, a cathode, and an electrolyte using the same process as in Example 1.

[0132] Example 5: Manufacturing of a lithium-sulfur secondary battery having a graphene coating layer and a composite film layer formed thereon.

[0133] The same procedure as Example 4 was followed, except that the composite film layer was formed with a thickness of 50 μm.

[0134] Example 6: Manufacturing of a lithium-sulfur secondary battery having a graphene coating layer and a composite film layer formed thereon.

[0135] The same procedure as Example 4 was followed, except that the slurry was applied to a thickness of 5 μm to form a composite film layer.

[0136] Comparative Example 1: Manufacturing of a lithium-sulfur secondary battery including a cathode without a graphene coating layer

[0137] A lithium-sulfur secondary battery was manufactured in the same manner as Example 1, except that a graphene coating layer was not formed after forming the positive electrode active material layer.

[0138] Experimental Example: Battery Life Performance and Output Performance Evaluation

[0139] The life performance and output of lithium-sulfur secondary batteries according to Examples 1 to 6 and Comparative Example 1 were compared. Specifically, charging and discharging were repeated approximately 100 times at a rate of 0.1 C, and the discharge capacity in the first cycle and the discharge capacity in the 100th cycle were compared to calculate the capacity retention rate according to the following mathematical equation 1.

[0140] [Mathematical Formula 1]

[0141] Capacity retention rate = (discharge capacity of the 100th cycle) / (discharge capacity of the 1st cycle) × 100 (%)

[0142] The discharge capacity and capacity retention rate in the first and 100th cycles of the lithium-sulfur secondary batteries according to the examples and comparative examples are shown in Table 1 below. In addition, the cycle life performance of the lithium-sulfur secondary batteries according to the examples 1 and comparative examples 1 is compared in Fig. 4.

[0143] In addition, the discharge voltage in the first cycle and the discharge voltage in the second cycle of the lithium-sulfur secondary batteries of the examples and comparative examples were measured, respectively, and are shown in Table 1 below.

[0144] 1st cycle discharge voltage (V)100th cycle discharge voltage (V)1st cycle discharge capacity (㎃h / g)100th cycle discharge capacity (㎃h / g)Capacity retention rate (%)Example 12.382.101,00480880.5Example 22.401.181,13664056.4Example 32.291.9685667478.8Example 42.352.2497386288.6Example 52.121.9383968281.3Example 62.371.441,04264361.8Comparative example 12.411.031,16151144.0

[0145] Referring to Table 1 above, it can be confirmed that the lithium-sulfur secondary batteries of Examples 1 to 6 having a graphene coating layer formed thereon exhibited superior capacity retention compared to the lithium-sulfur secondary battery of Comparative Example 1, and that the discharge capacity at the 100th cycle was also superior.

[0146] Referring to Figure 4, which compares the change in capacity according to the progress of the charge / discharge cycle of Example 1 (orange) and Comparative Example 1 (blue), it can be confirmed that Comparative Example 1 has a rapid decrease in capacity from the beginning, resulting in a deterioration in life performance, whereas Example 1 has a gradual decrease in capacity, resulting in an improvement in life performance.

[0147] In addition, it can be seen that Examples 3 and 5, in which the thickness of the graphene coating layer or composite film layer is excessively thick, have lower output and discharge capacity than Example 1.

[0148] Example 4 is a lithium-sulfur secondary battery including a positive electrode having an additional composite film layer in Example 1. Although it exhibited a discharge voltage and a first cycle discharge capacity that were somewhat lower than those of Example 1, it was found that the discharge voltage and capacity retention rate were excellent when charge and discharge were repeated 100 times.

[0149] [Explanation of symbols]

[0150] 100: Whole house

[0151] 200: Active material layer

[0152] 300: Graphene coating layer

[0153] 310: Penetration layer

[0154] 400: Composite film layer

Claims

1. 1) A step for preparing a graphene dispersion solution; and 2) a step of forming a graphene coating layer by spraying the graphene dispersion solution onto the positive electrode active material layer; The above graphene dispersion solution is prepared by adding graphene to an organic solvent in an amount of 5 wt% to 15 wt% and dispersing it using an ultrasonic dispersion method. Method for manufacturing a cathode for a lithium-sulfur secondary battery.

2. In paragraph 1, In the above step 1), the graphene dispersion solution contains graphene in an amount of 5 wt% to 15 wt%. Method for manufacturing a cathode for a lithium-sulfur secondary battery.

3. In paragraph 1, In the above step 1), the graphene dispersion solution is prepared by dispersing graphene by putting it into a solvent and then applying ultrasonic waves. Method for manufacturing a cathode for a lithium-sulfur secondary battery.

4. In paragraph 3, The above ultrasound is applied at a frequency of 20 kHz to 150 kHz and an output of 100 W to 500 W. Method for manufacturing a cathode for a lithium-sulfur secondary battery.

5. In paragraph 1, In the above step 2), the graphene coating layer is formed to a thickness of 1 ㎛ to 20 ㎛. Method for manufacturing a cathode for a lithium-sulfur secondary battery.

6. In paragraph 1, Comprising a step of absorbing a liquid electrolyte onto the graphene coating layer, Method for manufacturing a cathode for a lithium-sulfur secondary battery.

7. In paragraph 1, 3) A step of forming a graphene coating layer, and then applying a slurry containing a conductive ceramic compound, a binder, and a solvent on the graphene coating layer to allow the conductive ceramic compound to penetrate the graphene coating layer, and then removing the solvent to form a composite film layer; further comprising; Method for manufacturing a cathode for a lithium-sulfur secondary battery.

8. Whole house; A positive electrode active material layer provided on the above-mentioned collector and including an organic or inorganic sulfur compound; and A graphene coating layer formed on the positive electrode active material layer; Cathode for lithium-sulfur secondary batteries.

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