Positive electrode for lithium-sulfur battery, method for manufacturing the same, and lithium-sulfur battery including the same

Abstract

The present invention relates to a positive electrode for a lithium-sulfur battery, comprising a porous carbon material having a particle size (D10 standard) of less than 10 μm and a sulfur-carbon composite containing sulfur in at least a part of the inner and outer surfaces of the porous carbon material, and having a porosity of more than 75% and not more than 85%.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0151291 filed November 14, 2022 and Korean Patent Application No. 10-2023-0156911 filed November 14, 2023, and incorporates all contents disclosed in the documents of said Korean patent applications as part of this specification.

[0002] The present invention relates to a positive electrode for a lithium-sulfur battery, a method for producing the same, and a lithium-sulfur battery including the same. [Background technology]

[0003] Recently, interest in energy storage technology has been growing. Its applications have expanded to include cell phones, video cameras, laptops, and even electric vehicles, and efforts in the research and development of electrochemical devices have become increasingly concrete.

[0004] Electrochemical devices are the field that has attracted the most attention in this regard, and among them, the development of rechargeable secondary batteries has been the focus of attention. Recently, in developing such batteries, research and development has been conducted on new electrode and battery designs to improve capacity density and energy efficiency.

[0005] Among the secondary batteries currently in use, lithium secondary batteries, developed in the early 1990s, are attracting attention for their advantages of higher operating voltage and significantly greater energy density than conventional batteries that use aqueous electrolytes, such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries.

[0006] In particular, lithium-sulfur (Li-S) batteries are secondary batteries that use a sulfur-based material with a sulfur-sulfur bond (SS bond) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, the main material used for the positive electrode active material, has the advantages of being abundant, non-toxic, and low in weight per atom. Furthermore, the theoretical discharge capacity of lithium-sulfur batteries is 1675mAh / g-sulfur, and their theoretical energy density is 2,600Wh / kg, which is significantly higher than the theoretical energy densities of other battery systems currently under development (Ni-MH batteries: 450Wh / kg, Li-FeS batteries: 480Wh / kg, Li-MnO2 batteries: 1,000Wh / kg, and Na-S batteries: 800Wh / kg), making them the most promising battery currently under development.

[0007] During the discharge reaction of a lithium-sulfur battery, an oxidation reaction of lithium occurs at the negative electrode, and a reduction reaction of sulfur occurs at the positive electrode. Before discharge, sulfur has a cyclic S8 structure. During the reduction reaction (discharge), the S-SS bond is broken, decreasing the oxidation number of S. During the oxidation reaction (charge), the S-SS bond is re-formed, increasing the oxidation number of S. Electrical energy is stored and generated through an oxidation-reduction reaction. During this reaction, sulfur is reduced from the cyclic S8 to form a linear lithium polysulfide (Li2S x , x=8, 6, 4, 2), and finally when this lithium polysulfide is completely reduced, lithium sulfide (Li2S) is generated. Due to the process of reduction to each lithium polysulfide, the discharge behavior of lithium-sulfur batteries is characterized by a stepwise discharge voltage, which is different from that of lithium-ion batteries.

[0008] To improve the performance of such lithium-sulfur batteries, it is necessary to maximize the reactivity of the positive electrode active material. Because sulfur, which is used as a positive electrode active material in lithium-sulfur batteries, has low conductivity, a carbon material is used as a carrier to maximize the reactivity, and a sulfur-carbon composite, which is a mixture of the carbon material and sulfur, is commonly used.

[0009] In the sulfur-carbon composite, when a carbon material having a high specific surface area and porosity is mixed with sulfur, the amount of sulfur supported can be increased, thereby improving the energy density of the lithium-sulfur battery. However, there is a problem that the reactivity is reduced, resulting in a decrease in discharge capacity, which needs to be improved.

[0010] The particle size of the carbon material has a significant impact on the discharge capacity and energy density of lithium-sulfur batteries. If the carbon material contains fine particles, the porosity of the positive electrode cannot be ensured, resulting in a decrease in the reactivity of the lithium-sulfur battery. Therefore, a carbon material that does not contain fine particles must be used, but controlling the amount of fine particles is difficult due to the low density of carbon materials. Therefore, research is needed to develop a method for preventing the discharge capacity and energy density of lithium-sulfur batteries from decreasing even when the carbon material contains fine particles. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 10-2020-0109861 Summary of the Invention [Problem to be solved by the invention]

[0012] Therefore, the present inventors have conducted extensive research to solve the above problem. As a result, they have found that when a porous carbon material having a particle size (D10 standard) of less than 10 μm produced by centrifugal grinding is used as the carbon material of a sulfur-carbon composite, which is a cathode active material, the porosity of the cathode can be ensured even if the particle size (D10 standard) of the porous carbon material is less than 10 μm. As a result, the discharge capacity and reactivity of a lithium-sulfur battery can be improved and the energy density can be ensured, thereby completing the present invention.

[0013] Therefore, an object of the present invention is to provide a positive electrode for a lithium-sulfur battery in which the discharge capacity and energy density of the lithium-sulfur battery are not reduced even when the particle size (D10 standard) of the porous carbon material is less than 10 μm.

[0014] Another object of the present invention is to provide a method for manufacturing the positive electrode for a lithium-sulfur battery and a lithium-sulfur battery including the same. [Means for solving the problem]

[0015] In order to achieve the above purpose, The present invention relates to a current collector; and a positive electrode active material layer disposed on at least one surface of the current collector; the positive electrode active material layer includes, as a positive electrode active material, a porous carbon material and a sulfur-carbon composite containing sulfur in at least a portion of the inside and the outer surface of the porous carbon material; The particle size (D10 standard) of the porous carbon material is less than 10 μm, The present invention provides a positive electrode for a lithium-sulfur battery, wherein the porosity of the positive electrode is greater than 75% and not greater than 85%.

[0016] The present invention also provides a method for producing a porous carbon material having a particle size (D10 standard) of less than 10 μm by centrifugal pulverization of the porous carbon material; (2) mixing the porous carbon material with sulfur and heat-treating the mixture to prepare a sulfur-carbon composite; (3) preparing a positive electrode slurry containing the sulfur-carbon composite; and (4) applying the positive electrode slurry to a current collector.

[0017] The present invention also provides a lithium-sulfur battery comprising the positive electrode of the present invention; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte. [Effects of the Invention]

[0018] If the particle size (D10) of the porous carbon material of the sulfur-carbon composite is less than 10 μm, the carbon material may block the pores of the positive electrode, reducing the porosity of the positive electrode, thereby resulting in reduced reactivity, discharge capacity, and energy density of the lithium-sulfur battery.

[0019] In the positive electrode for a lithium-sulfur battery according to the present invention, even if the particle size (D10) of the porous carbon material of the sulfur-carbon composite, which is the positive electrode active material, is less than 10 μm, the tap density of the carbon material is low and the pores of the positive electrode are not blocked, thereby ensuring the porosity of the positive electrode. Therefore, even if the particle size (D10) of the porous carbon material is less than 10 μm, the energy density of the lithium-sulfur battery is prevented from decreasing and the reactivity of the lithium-sulfur battery is ensured. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will now be described in more detail.

[0021] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best explain his / her invention.

[0022] The term "composite" as used in this specification means a substance in which two or more materials are combined to form physically and chemically distinct phases, thereby exhibiting more effective functions.

[0023] Lithium-sulfur batteries use sulfur as the positive electrode active material and lithium metal as the negative electrode active material. When a lithium-sulfur battery is discharged, an oxidation reaction of lithium occurs at the negative electrode, and a reduction reaction of sulfur occurs at the positive electrode. At this time, the reduced sulfur combines with lithium ions transferred from the negative electrode and is converted into lithium polysulfide, which ultimately leads to the formation of lithium sulfide.

[0024] Lithium-sulfur batteries have attracted attention as next-generation batteries due to their significantly higher theoretical energy density than conventional lithium secondary batteries, and the fact that sulfur, which is used as the positive electrode active material, is abundant and inexpensive, which allows for lower battery manufacturing costs.

[0025] Despite these advantages, it is difficult to achieve the full theoretical energy density in actual operation due to the low electrical conductivity of sulfur, the cathode active material, and the lithium ion conductivity.

[0026] To improve the electrical conductivity of sulfur, sulfur-carbon composites containing sulfur in at least a portion of the interior and exterior surfaces of the porous carbon material are most commonly used as positive electrode active materials in lithium-sulfur batteries. However, the discharge capacity and energy density of lithium-sulfur batteries are affected by the particle size of the porous carbon material. The more fine particles contained in the porous carbon material, the more likely it is that the fine particles will clog the pores of the positive electrode, slowing down the reaction that generates the discharge capacity and reducing the reactivity of the lithium-sulfur battery. Therefore, while it is essential to control the fine particles of the porous carbon material in the sulfur-carbon composite, controlling the fine particles is difficult due to the low density of the porous carbon material.

[0027] Therefore, the present invention aims to provide a positive electrode for a lithium-sulfur battery that can ensure the porosity of the positive electrode by not blocking the pores of the positive electrode even if the porous carbon material contains fine powder.

[0028] In the present invention, the fine powder may mean a powder having a particle size (D10 standard) of less than 10 μm.

[0029] Positive electrodes for lithium-sulfur batteries The present invention relates to a current collector; and a positive electrode active material layer disposed on at least one surface of the current collector; the positive electrode active material layer includes, as a positive electrode active material, a porous carbon material and a sulfur-carbon composite containing sulfur in at least a portion of the inside and the outer surface of the porous carbon material; The particle size (D10 standard) of the porous carbon material is less than 10 μm, The positive electrode for a lithium-sulfur battery has a porosity of more than 75% and not more than 85%.

[0030] The current collector is not particularly limited as long as it supports the positive electrode active material, does not cause chemical changes in the battery, and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, baked carbon, copper or stainless steel surface-treated with carbon, nickel, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0031] The positive electrode current collector may have fine irregularities on its surface to strengthen the bonding force with the positive electrode active material, and may be in various forms such as a film, sheet, foil, mesh, net, porous material, foam, or nonwoven fabric.

[0032] The positive electrode active material layer is disposed on at least one surface of a current collector, and may include, as a positive electrode active material, a porous carbon material and a sulfur-carbon composite containing sulfur in at least a part of the inside and the outside surface of the porous carbon material.

[0033] The sulfur may be inorganic sulfur (S8), Li-S n (n≧1), organic sulfur compounds and carbon-sulfur polymers [(C2S x ) n, x=2.5 to 50, n≧2]. Preferably, inorganic sulfur (S8) may be used.

[0034] Furthermore, the sulfur may be located not only inside the pores of the porous carbon material but also on the surface, and in this case, it may be present in an area of ​​less than 100%, preferably 1 to 95%, and more preferably 60 to 90% of the entire exterior surface of the porous carbon material. When the sulfur is present on the surface of the porous carbon material within this range, it can exhibit maximum effects in terms of electron transfer area and electrolyte wettability. Specifically, within this range, sulfur is impregnated thinly and uniformly on the surface of the porous carbon material, thereby increasing the contact area for electron transfer during charge and discharge. If the sulfur is located on 100% of the surface area of ​​the porous carbon material, the porous carbon material will be completely covered with sulfur, reducing the wettability of the electrolyte and reducing contact with the conductive material contained in the electrode, preventing electron transfer and preventing participation in the reaction.

[0035] The porous carbon material can generally be prepared by carbonizing various carbon precursors. The porous carbon material contains pores with varying internal sizes, with the average pore diameter ranging from 1 to 200 nm, and the porosity or void ratio ranging from 10 to 90% of the total volume of the porous carbon material. If the average pore diameter is less than this range, the pore size is merely at the molecular level, making sulfur impregnation impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material is weakened, making it unsuitable for use in electrode manufacturing processes.

[0036] The shape of the porous carbon material may be spherical, rod-like, needle-like, plate-like, tubular or bulk, and any shape commonly used in lithium-sulfur batteries may be used without limitation.

[0037] The porous carbon material may be any material commonly used in the art that 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 blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal 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); graphites such as natural graphite, artificial graphite, and expanded graphite; and activated carbon. Preferably, the porous carbon material may be carbon nanotubes.

[0038] In the present invention, the particle size (D10 standard) of the porous carbon material of the sulfur-carbon composite may be a fine powder of less than 10 μm. Conventionally, when the porous carbon material of a sulfur-carbon composite contains fine powder, the fine powder blocks the pores of the positive electrode, reducing the porosity of the positive electrode. This makes it difficult to ensure the reactivity of the lithium-sulfur battery, resulting in reduced discharge capacity and energy density. In other words, the discharge capacity and energy density of a lithium-sulfur battery are affected by the particle size of the porous carbon material. In the present invention, even if the porous carbon material contains fine powder with a particle size (D10 standard) of less than 10 μm, the porosity of the positive electrode can be ensured, thereby achieving discharge capacity, energy density, and reactivity at levels comparable to those of a lithium-sulfur battery containing a porous carbon material without fine powder. Therefore, the positive electrode for a lithium-sulfur battery and a lithium-sulfur battery including the same according to the present invention are not affected by the particle size of the porous carbon material, and therefore do not require adjustment of the particle size of the porous carbon material.

[0039] The porous carbon material having a particle size (D10 standard) of less than 10 μm may be produced by centrifugal grinding. The centrifugal grinding may involve grinding large particles against the blades of a centrifugal grinder to produce small particles. For example, carbon nanotube particles having entangled carbon nanotube chains may be ground into small particles by breaking the cavities between the entangled carbon nanotubes with the blades of the grinder. In this case, the degree of entanglement is reduced by breaking the entangled carbon nanotube chains, resulting in a low tap density. Specifically, the tap density of the porous carbon material having a particle size (D10 standard) of less than 10 μm produced by centrifugal grinding may be 0.01 to 0.1 g / cc, preferably 0.03 to 0.07 g / cc. Therefore, even if the particle size (D10 standard) of the porous carbon material is less than 10 μm, the positive electrode may have high porosity. However, porous carbon materials with particle sizes (D10 standard) of less than 10 μm that are not produced by centrifugal grinding have a tap density of more than 0.1 g / cc, and the high tap density can clog the pores of the positive electrode, making it impossible to provide a positive electrode with high porosity, which can reduce the reactivity of lithium-sulfur batteries.

[0040] The sulfur-carbon composite may contain 70 to 90 wt %, preferably 70 to 85 wt %, and more preferably 72 to 80 wt %, of sulfur, based on 100 wt % of the sulfur-carbon composite. If the sulfur content is below this range, the content of the porous carbon material in the sulfur-carbon composite increases relatively, resulting in an increased specific surface area and an increased binder content during cathode preparation. This increased binder content ultimately increases the sheet resistance of the cathode and acts as an insulator that inhibits electron transfer, potentially reducing battery performance. Conversely, if the sulfur content exceeds the above range, sulfur that cannot bond with the porous carbon material may aggregate or be re-eluted on the surface of the porous carbon material, making it difficult to accept electrons and unable to participate in electrochemical reactions, potentially resulting in a loss of battery capacity.

[0041] The positive electrode active material may be contained in an amount of 70 to 95 wt %, preferably 75 to 90 wt %, and more preferably 80 to 90 wt %, based on the total weight of the positive electrode active material layer. If the positive electrode active material is contained in an amount less than 70 wt %, the battery performance may be reduced, whereas if the amount exceeds 95 wt %, the content of conductive materials and binders other than the positive electrode active material may be relatively reduced, which may result in a reduction in properties such as conductivity, durability, and securing of lithium ion transport paths.

[0042] Furthermore, the sulfur content may be 65 to 80 wt %, preferably 67 to 75 wt %, based on the total weight of the positive electrode active material layer. If the sulfur content is less than 65 wt %, the positive electrode active material is insufficient, making it impossible to ensure the energy density of the lithium-sulfur battery. If the sulfur content exceeds 80 wt %, the content of the conductive material and binder is relatively reduced, which may result in a decrease in properties such as conductivity, durability, and securing of lithium ion transport paths.

[0043] The positive electrode active material layer may further contain a conductive material or a binder.

[0044] The conductive material electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from a current collector to the positive electrode active material. Any conductive material may be used without limitation as long as it does not undergo chemical change in a lithium-sulfur battery and has porosity and conductivity.

[0045] For example, the conductive material may be a porous carbon-based material, such as carbon black (CB), graphite, graphene, activated carbon, carbon nanotubes (CNT), carbon fiber, metallic fiber such as metal mesh, metallic powder such as copper, silver, nickel, and aluminum, or organic conductive material such as polyphenylene derivative. The conductive materials may be used alone or in combination.

[0046] Currently, commercially available conductive materials include acetylene black (products of Chevron Chemical Company and Gulf Oil Company, etc.), Ketjen Black EC (product of Armak Company), Vulcan XC-72 (product of Cabot Company), and Super P (product of MMM). Examples include acetylene black, carbon black, and graphite.

[0047] The conductive material may be contained in an amount of 1 to 20 wt %, preferably 3 to 18 wt %, and more preferably 5 to 15 wt %, based on the total weight of the positive electrode active material layer. If the conductive material is contained in an amount less than 1 wt %, the conductivity of the positive electrode cannot be ensured, and if it is contained in an amount more than 20 wt %, the contents of the positive electrode active material and binder are relatively reduced, which may result in a deterioration in battery performance.

[0048] The binder serves to support the positive electrode active material on the positive electrode current collector and to bind the positive electrode active material together. The binder may include, for example, one or more selected from the group consisting of styrene-butadiene rubber, carboxymethyl cellulose, poly(vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated polyethylene oxide, cross-linked polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), polyvinylidene fluoride, copolymer of polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly(ethyl acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, polyacrylic acid, derivatives thereof, blends thereof, and copolymers thereof.

[0049] The binder may be contained in an amount of 1 to 20 wt %, preferably 3 to 18 wt %, and more preferably 5 to 15 wt %, based on the total weight of the positive electrode active material layer. If the binder content is less than 1 wt %, the bonding strength between the positive electrode active materials or between the positive electrode active material and the current collector will be significantly reduced, and if the binder content exceeds 20 wt %, the battery capacity may be reduced.

[0050] The porosity of the positive electrode for a lithium-sulfur battery of the present invention may be greater than 75% and less than or equal to 85%. That is, even if the particle size (D10 standard) of the porous carbon material of the sulfur-carbon composite is less than 10 μm, the positive electrode for a lithium-sulfur battery of the present invention can have a porosity of greater than 75% and less than or equal to 85%. Therefore, a lithium-sulfur battery including the positive electrode can maintain its reactivity without experiencing a decrease in reactivity.

[0051] If the porosity of the positive electrode is less than 75%, the pores are insufficient to provide a lithium-sulfur battery with excellent reactivity, and if the porosity of the positive electrode is more than 85%, the increase in energy density of the lithium-sulfur battery is limited. Also, the electrolyte may not be able to reside in all the pores of the positive electrode, and some pores may not be utilized in the reaction, which may result in a loss of the positive electrode active material.

[0052] Method for manufacturing positive electrodes for lithium-sulfur batteries The present invention also relates to a method for producing a positive electrode for a lithium-sulfur battery, the method comprising: (1) centrifugal grinding of a porous carbon material to produce a porous carbon material having a particle size (D10 standard) of less than 10 μm; (2) mixing the porous carbon material with sulfur and heat-treating the mixture to prepare a sulfur-carbon composite; (3) preparing a positive electrode slurry containing the sulfur-carbon composite; and (4) applying the positive electrode slurry to a current collector.

[0053] The step (1) is a step of centrifugal pulverizing a porous carbon material to produce a porous carbon material having a particle size (D10 standard) of less than 10 μm.

[0054] The porous carbon material is as described above.

[0055] The centrifugal grinding may be carried out at 6000 to 18000 rpm and a linear velocity of 80 to 200 m / s, preferably at 6000 to 18000 rpm and a linear velocity of 90 to 160 m / s.

[0056] Specifically, the centrifugal grinding may involve centrifugally grinding the porous carbon material at the rpm and linear velocity, and then adjusting the mesh size of a sieve installed in the centrifugal grinder to adjust the particle size of the porous carbon material. Furthermore, a cyclone process may be performed to capture the porous carbon material that has passed through the sieve, thereby adjusting the particle size of the porous carbon material. The porous carbon material obtained through this process may have a particle size (D10 standard) of less than 10 μm.

[0057] As described above, by pulverizing the porous carbon material using the centrifugal pulverization, the porous carbon material having a particle size (D10 basis) of less than 10 μm can have a low tap density. Specifically, the tap density of the porous carbon material may be 0.01 to 0.1 g / cc, preferably 0.03 to 0.07 g / cc. On the other hand, when the porous carbon material is not prepared by centrifugal pulverization, the tap density may exceed 0.1 g / cc even if the particle size (D10 basis) of the porous carbon material is less than 10 μm. Therefore, the positive electrode for a lithium-sulfur battery according to the present invention can have high porosity even if the particle size (D10 basis) of the porous carbon material is less than 10 μm. On the other hand, when the porous carbon material is not prepared by centrifugal pulverization, the high tap density can clog the pores of the positive electrode, making it impossible to prepare a positive electrode with high porosity.

[0058] The step (2) is a step of mixing the porous carbon material with sulfur and heat-treating the mixture to produce a sulfur-carbon composite.

[0059] The porous carbon material having a particle size (D10 standard) of less than 10 μm produced in step (1) is mixed with sulfur, and the mixing may be simple mixing.

[0060] The heat treatment may be performed at a temperature of 130°C to 170°C, and the heat treatment time is not particularly limited, but may be performed for 15 minutes to 2 hours. If the temperature exceeds 170°C, sulfur may evaporate, and if it is below 130°C, sulfur may not melt and be uniformly distributed. If the heat treatment time is less than 15 minutes, sulfur may not be sufficiently dissolved and impregnated, and if it exceeds 2 hours, some sulfur may evaporate or be unevenly impregnated.

[0061] The step (3) is a step of preparing a positive electrode slurry containing the sulfur-carbon composite prepared in the step (2).

[0062] The positive electrode slurry may form a positive electrode active material layer. The positive electrode slurry may include a sulfur-carbon composite as a positive electrode active material, and may further include a binder or a conductive material. The binder and the conductive material are as described above.

[0063] The positive electrode slurry may also contain a solvent to uniformly disperse the sulfur-carbon composite. The solvent is preferably water, which may be distilled water or deionized water. However, the solvent is not limited to these, and a lower alcohol that is easily miscible with water may be used as needed. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol, and these may be mixed with water.

[0064] The content of the solvent may be at a concentration that allows the positive electrode slurry to be easily applied to the current collector, and the specific content varies depending on the application method and device.

[0065] The positive electrode slurry may further contain, as needed, substances commonly used in the art for the purpose of improving its performance, such as a viscosity modifier, a fluidizing agent, and a filler.

[0066] The step (4) is a step of applying the positive electrode slurry prepared in the step (3) onto a current collector.

[0067] The method for applying the positive electrode slurry to the current collector is not particularly limited in the present invention, and examples thereof include a doctor blade, die casting, comma coating, screen printing, etc. Alternatively, the positive electrode slurry may be applied to the current collector by molding on a separate substrate and then pressing or lamination.

[0068] After step (4), a drying step may be performed to remove the solvent from the positive electrode slurry. The drying step is performed at a temperature and time sufficient to sufficiently remove the solvent. The conditions vary depending on the type of solvent, and are not particularly limited in the present invention. Examples include drying with warm air, hot air, or low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. The drying rate is usually adjusted so that the solvent can be removed as quickly as possible, within a range that does not cause cracks in the positive electrode active material layer due to stress concentration or peeling of the positive electrode active material layer from the positive electrode current collector.

[0069] Furthermore, after the drying, the current collector can be pressed to increase the density of the positive electrode active material in the positive electrode. Examples of pressing methods include die pressing and roll pressing.

[0070] The porosity of the lithium-sulfur battery positive electrode manufactured by the above manufacturing method may be more than 75% and not more than 85%. Generally, if the particle size (D10 standard) of the porous carbon material is a fine powder of less than 10 μm, the fine powder of the porous carbon material clogs the pores of the positive electrode, making it impossible to manufacture a positive electrode with high porosity. This leads to a problem of reduced reactivity of the lithium-sulfur battery. However, by centrifugal pulverization of the porous carbon material in step (1) above, the present invention can manufacture a lithium-sulfur battery positive electrode with a porosity of more than 75% and not more than 85%, even if the particle size (D10 standard) of the porous carbon material is less than 10 μm.

[0071] Therefore, the present invention can provide a method for manufacturing a positive electrode for a lithium-sulfur battery in which the porosity of the positive electrode is not affected by fine particles of a porous carbon material.

[0072] Lithium-sulfur battery The present invention relates to a lithium-sulfur battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0073] The positive electrode may be the positive electrode for a lithium-sulfur battery of the present invention described above.

[0074] The negative electrode may include a current collector and a negative electrode active material layer coated on one or both sides of the current collector, or may be a lithium metal plate.

[0075] The current collector is for supporting the negative electrode active material layer, and is the same as the current collector used in the positive electrode.

[0076] The negative electrode active material layer may include, in addition to the negative electrode active material, a conductive material, a binder, etc. In this case, the conductive material and the binder are as described above.

[0077] The negative electrode active material is a lithium ion (Li + The lithium ion-containing compound may include a material capable of reversible intercalation or deintercalation of lithium ions, a material capable of reversibly reacting with lithium ions to form a lithium-containing compound, lithium metal, or a lithium alloy.

[0078] The lithium ion (Li + The material capable of reversibly inserting or de-inserting lithium ions (Li) may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. + The material capable of reacting with lithium (Li) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanate, or silicon. The lithium alloy may 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).

[0079] Preferably, the negative electrode active material may be lithium metal, specifically in the form of a lithium metal thin film or lithium metal powder.

[0080] A separator may be interposed between the positive electrode and the negative electrode.

[0081] The separator separates or insulates the positive electrode and the negative electrode from each other and allows lithium ions to be transported between the positive electrode and the negative electrode. The separator may be made of a porous non-conductive or insulating material, and any material commonly used as a separator in lithium secondary batteries may be used without any particular limitation. The separator may be an independent member such as a film, or may be a coating layer attached to the positive electrode and / or the negative electrode.

[0082] The separation membrane preferably has low resistance to the ion migration of the electrolyte and has excellent moisture-absorbing ability for the electrolyte.

[0083] The separator may be made of a porous substrate. The porous substrate may be any porous substrate commonly used in secondary batteries. A porous polymer film may be used alone or in combination. For example, a nonwoven fabric such as a high-melting-point glass fiber or polyethylene terephthalate fiber, or a polyolefin-based porous membrane may be used, but is not limited to these.

[0084] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate that is generally used in electrochemical elements may be used. For example, the porous substrate may be made of a material selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalates, polytetrafluoroethylenes, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, and poly(p-phenylene benzobisoxazole). The adhesive layer may comprise one or more materials selected from the group consisting of benzobisoxazole and polyarylate.

[0085] The thickness of the porous substrate is not particularly limited, and may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness of the porous substrate is not limited to the above range, if the thickness is too thin below the lower limit, the mechanical properties may be deteriorated, and the separator may be easily damaged during use of the battery.

[0086] The average diameter and porosity of the pores present in the porous substrate are not particularly limited, and may be 0.001 to 50 μm and 10 to 95%, respectively.

[0087] The electrolyte contains lithium ions and serves as a medium for causing an electrochemical oxidation or reduction reaction between the positive electrode and the negative electrode.

[0088] The electrolyte can be a non-aqueous liquid electrolyte or a solid electrolyte that does not react with lithium metal, but is preferably a non-aqueous electrolyte that includes an electrolyte salt and an organic solvent.

[0089] The electrolyte salt contained in the non-aqueous electrolyte solution is a lithium salt. The lithium salt may be any salt commonly used in electrolyte solutions for lithium secondary batteries, and examples thereof include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, CH3SO3Li, (CF3SO2)2NLi, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carbonate, lithium 4-phenylborate, lithium imide, etc. may also be used.

[0090] The concentration of the lithium salt may be 0.2 to 2 M, specifically 0.4 to 2 M, more specifically 0.4 to 1.7 M, depending on various factors such as the exact composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charge and discharge conditions of the battery, the operating temperature, and other factors known in the lithium battery field. If the lithium salt is used at a concentration less than 0.2 M, the conductivity of the electrolyte may decrease, which may degrade the performance of the electrolyte, while if it is used at a concentration greater than 2 M, the viscosity of the electrolyte may increase, which may reduce the mobility of the lithium ions.

[0091] The organic solvent contained in the nonaqueous electrolyte may be any organic solvent commonly used in electrolytes for lithium secondary batteries, and may be, for example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc., which may be used alone or in combination of two or more. Among these, ether compounds may be typically used.

[0092] The ether-based compound may include an acyclic ether and a cyclic ether.

[0093] For example, the acyclic ether may be one or more selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethyl methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether, but is not limited to these.

[0094] For example, the cyclic ether may be one or more selected from the group consisting of 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methylfuran, 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, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether, but is not limited thereto.

[0095] The ester in the organic solvent 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.

[0096] Specific examples of the linear carbonate compound include, but are not limited to, any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, or a mixture of two or more thereof.

[0097] Specific examples of the cyclic carbonate compound include any one or mixtures of two or more 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. Examples of halides thereof include, but are not limited to, fluoroethylene carbonate (FEC).

[0098] The electrolyte may further include a nitric acid or nitrite-based compound as an additive in addition to the electrolyte salt and organic solvent. The nitric acid or nitrite-based compound forms a stable film on the lithium metal electrode, which serves as the negative electrode, thereby improving charge / discharge efficiency.

[0099] The nitric acid or nitrite compound is not particularly limited in the present invention, but may be one selected from the group consisting of inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrate, propyl nitrate, butyl nitrate, pentyl nitrate, and octyl nitrate; and organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, and combinations thereof. Preferably, lithium nitrate is used.

[0100] The electrolyte injection may be performed at an appropriate step during the manufacturing process of the electrochemical device depending on the manufacturing process and required properties of the final product, i.e., before assembly of the electrochemical device or at the final step of assembly of the electrochemical device.

[0101] The lithium secondary battery according to the present invention can be manufactured by a lamination or stacking process of the separator and the electrodes and a folding process in addition to the general winding process.

[0102] The shape of the lithium secondary battery is not particularly limited, and may be in various shapes such as a cylindrical shape, a laminated shape, or a coin shape.

[0103] Preferred examples will be described below to aid in understanding the present invention. However, the following examples are merely illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical idea of ​​the present invention. Naturally, such changes and modifications also fall within the scope of the appended claims.

[0104] Examples 1 to 5. Preparation of positive electrodes for lithium-sulfur batteries Carbon nanotubes with a particle size (D10 standard) of less than 10 μm were prepared by injecting them into a centrifugal mill (Netzsch, CSM). The carbon nanotubes were mixed with sulfur (Sigma-Aldrich product) and then heat-treated at 155°C for 30 minutes to prepare a sulfur-carbon composite (S:CNT = 70:30).

[0105] The sulfur-carbon composite and the binder were mixed in a weight ratio of 96:4, and then added to DI water as a solvent to prepare a positive electrode slurry.

[0106] The positive electrode slurry was coated on a 20 μm thick aluminum foil current collector and dried at 80° C. for 2 hours to prepare a positive electrode for a lithium-sulfur battery.

[0107] In Examples 1 to 5, the carbon nanotubes were pulverized at different rpm and linear speeds during the centrifugal pulverization, and the rpm, linear speed, particle size of the carbon nanotubes (based on D10), porosity of the positive electrode, and tap density of the carbon nanotubes are shown in Table 1 below.

[0108] [Table 1]

[0109] The carbon nanotubes of Examples 1 to 5 contain fine powder having a particle size (D10 standard) of less than 10 μm, and have a tap density of 0.01 to 0.1 g / cc. Even with the fine powder, the positive electrode has a porosity of more than 75%.

[0110] Comparative Examples 1 to 3. Production of positive electrodes for lithium-sulfur batteries Carbon nanotubes and sulfur (Sigma-Aldrich product) were mixed and then heat-treated at 155°C for 30 minutes to prepare a sulfur-carbon composite (S:CNT = 70:30).

[0111] The sulfur-carbon composite and the binder were mixed in a weight ratio of 96:4, and then added to DI water as a solvent to prepare a positive electrode slurry.

[0112] The positive electrode slurry was coated on a 20 μm thick aluminum foil current collector and dried at 80° C. for 2 hours to prepare a positive electrode for a lithium-sulfur battery.

[0113] In Comparative Examples 1 to 3, the carbon nanotubes were not centrifuged, and positive electrodes for lithium-sulfur batteries were manufactured using carbon nanotubes having different particle sizes (D10 standard). The particle sizes (D10 standard) of the carbon nanotubes and the porosity of the positive electrodes are shown in Table 2 below.

[0114] [Table 2]

[0115] In Comparative Example 1, the particle size (D10 standard) of the carbon nanotubes exceeded 10 μm, and the porosity of the positive electrode exceeded 75%, but the tap density exceeded 0.1 g / cc.

[0116] In Comparative Examples 2 and 3, the particle size (D10 standard) of the carbon nanotubes was fine powder of less than 10 μm, and the fine powder blocked the pores of the positive electrode, resulting in a porosity of 75% or less and a tap density of more than 0.1 g / cc.

[0117] Experimental Example 1: Measurement of discharge capacity and relative energy density of lithium-sulfur battery The positive electrodes of Examples 1 to 5 and Comparative Examples 1 to 3 were used to fabricate lithium-sulfur batteries.

[0118] Specifically, the positive electrode and the negative electrode were placed facing each other, and a porous polyethylene was interposed between them as a separator, and then an electrolyte was injected to fabricate a lithium-sulfur battery.

[0119] In this case, lithium metal was used as the negative electrode, and a mixture of 1M LiTFSI and 3 wt% lithium nitrate (LiNO3) dissolved in an organic solvent consisting of 2-methylfuran and dimethoxyethane (33:77 (volume ratio)) was used as the electrolyte.

[0120] Each of the lithium-sulfur batteries of Examples 1 to 5 and Comparative Examples 1 to 3 was discharged at a constant current of 0.5 C in CC mode at 25° C. to 1.8 V, and then charged at a constant current of 0.2 C to 2.5 V, and the discharge capacity was measured.

[0121] The relative energy density of each of the lithium-sulfur batteries of Examples 1 to 5 and Comparative Examples 1 to 3 was measured, and the discharge capacity and relative energy density are shown in the following Table 3. The relative energy density was calculated based on that of Comparative Example 2.

[0122] [Table 3]

[0123] From the results in Table 3, when the particle size (D10 basis) of the carbon nanotubes of Comparative Example 1 exceeds 10 μm, Examples 1 to 5 of the present invention can be seen to be capable of producing positive electrodes with a low tap density of 0.01 to 0.1 g / cc and a porosity exceeding 75%, even when the particle size (D10 basis) of the carbon nanotubes is less than 10 μm, and as a result, discharge capacity and relative energy density were comparable to or superior to those of Comparative Example 1.

[0124] In Comparative Examples 2 and 3, the particle size (D10 standard) of the carbon nanotubes was less than 10 μm, but because centrifugal pulverization was not performed, it was not possible to manufacture a positive electrode with a tap density exceeding 0.1 g / cc and a porosity exceeding 75%, resulting in very low discharge capacity and relative energy density.

[0125] Therefore, it can be seen that centrifugal grinding of a porous carbon material can ensure the porosity of the positive electrode even when the particle size (D10 standard) of the porous carbon material is less than 10 μm, and can improve the discharge capacity and relative energy density of a lithium-sulfur battery.

Claims

1. a current collector; and a positive electrode active material layer disposed on at least one surface of the current collector; the positive electrode active material layer includes, as a positive electrode active material, a porous carbon material and a sulfur-carbon composite containing sulfur in at least a portion of the inside and the outer surface of the porous carbon material; The particle size (D10 standard) of the porous carbon material is less than 10 μm, A positive electrode for a lithium-sulfur battery, wherein the porosity of the positive electrode active material layer is more than 75% and not more than 85%, The positive electrode for a lithium-sulfur battery, wherein the porous carbon material comprises at least one material selected from the group consisting of carbon nanotubes and carbon fibers.

2. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the sulfur is contained in an amount of 65 to 80 wt % based on the total weight of the positive electrode active material layer.

3. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the tap density of the porous carbon material is 0.01 to 0.1 g / cc.

4. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the positive electrode active material layer further contains a binder or a conductive material.

5. (1) centrifugal crushing a porous carbon material to produce a porous carbon material having a particle size (D10 standard) of less than 10 μm; (2) mixing the porous carbon material with sulfur and heat treating the mixture to prepare a sulfur-carbon composite; (3) preparing a positive electrode slurry containing the sulfur-carbon composite; and (4) applying the positive electrode slurry to a current collector.

6. 6. The method for producing a positive electrode for a lithium-sulfur battery according to claim 5, wherein the step (1) comprises centrifugal pulverization at 6,000 to 18,000 rpm and a linear velocity of 80 to 200 m / s.

7. The method for producing a positive electrode for a lithium-sulfur battery according to claim 5, wherein the positive electrode active material layer has a porosity of more than 75% and not more than 85%.

8. A lithium-sulfur battery comprising: the positive electrode according to any one of claims 1 to 4; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

Images