Porous carbon material, its manufacturing method, positive electrode for lithium secondary battery containing the carbon material as a positive electrode active material, and lithium secondary battery

Abstract

This paper discloses a porous carbon material that can improve the performance of SSE (sparingly solvating electrolyte-based) lithium secondary batteries by applying the porous carbon material, which has an increased pore volume and specific surface area through an activation process, to the positive electrode, a manufacturing method thereof, a positive electrode for a lithium secondary battery including the carbon material as a positive electrode active material, and a lithium secondary battery. The porous carbon material has a specific surface area of ​​1,700 m 2 / g or more, and the pore volume is 5 cm 3 / g or more.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0176685 filed on December 16, 2022, and incorporates all of the contents disclosed in the documents of that Korean patent application as part of this specification.

[0002] The present invention relates to a porous carbon material, a manufacturing method thereof, a positive electrode for a lithium secondary battery including the carbon material as a positive electrode active material, and a lithium secondary battery. More particularly, the present invention relates to a porous carbon material having an increased pore volume and specific surface area through an activation process, which can improve the performance of an SSE (sparingly solvating electrolyte based) lithium secondary battery by applying the porous carbon material to a positive electrode, a manufacturing method thereof, a positive electrode for a lithium secondary battery including the carbon material as a positive electrode active material, and a lithium secondary battery. [Background technology]

[0003] As interest in energy storage technology grows, its application fields expand to include mobile phones, tablets, laptops, and camcorders, as well as the energy sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs), and research and development of electrochemical devices is gradually increasing. Electrochemical devices are the field that has attracted the most attention in this regard, and the development of lithium-based secondary batteries, such as rechargeable lithium-sulfur batteries, has been a focus of attention. Recently, the development of such batteries has led to research and development of new electrode and battery designs to improve capacity density and specific energy.

[0004] These lithium-based secondary batteries, especially lithium-sulfur batteries, have a high energy density (or theoretical capacity) and are attracting attention as a next-generation secondary battery that can replace lithium-ion batteries. In these lithium-sulfur batteries, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur during discharge, during which sulfur changes from a cyclic structure of S8 to a linear structure of lithium polysulfide (LiPS). These lithium-sulfur batteries are characterized by their gradual discharge voltage until polysulfide is completely reduced to Li2S.

[0005] However, the biggest obstacle to the commercialization of lithium-sulfur batteries is their lifespan, as their charge / discharge efficiency decreases during the charge / discharge process, resulting in a deterioration in the battery's lifespan. This deterioration in the lifespan of lithium-sulfur batteries can be caused by a variety of factors, including electrolyte side reactions (accumulation of by-products due to electrolyte decomposition), instability of lithium metal (dendrites growing on the lithium anode, causing short circuits), and accumulation of by-products on the cathode (elution of lithium polysulfides from the cathode).

[0006] In batteries that use sulfur-based compounds as the positive electrode active material and alkali metals such as lithium as the negative electrode active material, lithium polysulfide leaching and shuttle phenomenon occurs during charging and discharging. The lithium polysulfide is transferred to the negative electrode, reducing the capacity of the lithium-sulfur battery and thereby reducing the battery's lifespan and reactivity. Polysulfides leached from the positive electrode have high solubility in organic electrolytes, which can cause unwanted migration to the negative electrode through the electrolyte (PS shuttling). This results in a reduction in capacity due to irreversible loss of the positive electrode active material and a reduction in battery life due to the deposition of sulfur particles on the lithium metal surface due to side reactions.

[0007] On the other hand, in order to build a high energy density of more than 400Wh / kg or 600Wh / L for lithium-sulfur batteries, high loading (about 4.0mAh / cm) is required. 2An electrolyte and positive electrode active material system that can operate even under conditions of low porosity (approximately 60% or less) and low porosity (approximately 60% or less) is required. In other words, the behavior of such lithium-sulfur batteries can vary greatly depending on the electrolyte. An electrolyte in which sulfur from the positive electrode is dissolved into the electrolyte in the form of lithium polysulfide (LiPS) is called a catholyte, while an electrolyte in which sulfur is hardly dissolved in the form of lithium polysulfide and is not dissolved is called an SSE (sparingly solvating electrolyte). Lithium-sulfur batteries that utilize conventional catholyte systems have been developed using Li2S x Because it relies on a liquid-phase reaction through the production of intermediate polysulfides (catholyte type), it is unable to fully utilize the high theoretical discharge capacity of sulfur (1,675mAh / g), and instead has the problem of battery degradation due to the elution of polysulfides, resulting in a rapid reduction in battery life.

[0008] Meanwhile, recently, a sparingly solvating electrolyte (SSE) system capable of suppressing polysulfide elution has been developed. It has been confirmed that when a carbon material with a high specific surface area (BET specific surface area) is used as a sulfur support, more than 80% of the theoretical sulfur discharge capacity can be utilized. However, a large specific surface area of ​​a carbon material used as a sulfur support does not necessarily result in a high discharge capacity. In particular, under high sulfur content conditions (e.g., >70 wt%), the pore volume of the carbon material, in addition to the specific surface area, significantly affects sulfur utilization. To achieve high energy density (e.g., >400 Wh / kg), a high-sulfur-content cathode active material and a high-loading electrode are required, so optimization by adjusting the specific surface area and porosity of the carbon material is essential.

[0009] Therefore, there is a need to develop a battery that uses an SSE electrolyte system while also using a carbon material for the positive electrode active material that has a larger specific surface area and pore volume than conventional materials, and that has a high sulfur utilization rate even under conditions of high sulfur content. Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, an object of the present invention is to provide a porous carbon material that can improve the performance of SSE (sparingly solvating electrolyte based) lithium secondary batteries by applying the porous carbon material, which has an increased pore volume and specific surface area through an activation process, to the positive electrode, a manufacturing method thereof, a positive electrode for a lithium secondary battery including the carbon material as a positive electrode active material, and a lithium secondary battery. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a method for producing a granular material having a specific surface area of ​​1,700 m 2 / g or more, and the pore volume is 5 cm 3 / g or more.

[0012] The present invention also provides a method for producing the porous carbon material, which includes a step of physically activating a carbon material with a gas activator or chemically activating a carbon material with a chemical activator.

[0013] The present invention also provides a positive electrode for a lithium secondary battery, including the sulfur-carbon composite in which sulfur is supported on the porous carbon material, as a positive electrode active material.

[0014] The present invention also provides a lithium secondary battery comprising: the positive electrode for the lithium secondary battery; a negative electrode; a separator interposed therebetween; and an electrolyte comprising a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and a lithium salt. [Effects of the Invention]

[0015] The porous carbon material, its manufacturing method, and a positive electrode for a lithium secondary battery and a lithium secondary battery including the carbon material as a positive electrode active material according to the present invention have the advantage of improving the performance of SSE (sparingly solvating electrolyte based) lithium secondary batteries by applying a porous carbon material having an increased pore volume and specific surface area through an activation process to the positive electrode. [Brief explanation of the drawings]

[0016] [Figure 1] 1 shows nitrogen adsorption / desorption isotherms used to confirm the specific surface area of ​​porous carbon materials according to an embodiment of the present invention and a comparative example. [Figure 2] 1 is a pore size distribution diagram (BJH plot, mesopore / macropore analysis) used to confirm the pore size of porous carbon materials according to an example and a comparative example of the present invention. [Figure 3] 1 is a pore size distribution diagram (HK plot, micropore analysis) used to confirm the pore size of porous carbon materials according to an embodiment of the present invention and a comparative example. [Figure 4] 1 is a thermogravimetric analysis (TGA) graph of sulfur-carbon composites according to an example of the present invention and a comparative example. [Figure 5] 1 is a graph showing the discharge capacity of lithium secondary batteries according to an example of the present invention and a comparative example. [Figure 6] 1 is a graph showing discharge capacity and life characteristics of lithium secondary batteries according to an example of the present invention and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below.

[0018] The porous carbon material of the present invention has a specific surface area of ​​1,700 m 2 / g or more, and the pore volume is 5 cm 3 / g or more.

[0019] To build a lithium-sulfur battery with a high energy density of more than 400Wh / kg or 600Wh / L, a high loading (about 4.0mAh / cm) is required. 2 An electrolyte and cathode active material system that can operate even under conditions of low porosity (approximately 60% or less) is required. As such an electrolyte system, a sparingly solvating electrolyte (SSE) electrolyte system has been developed that overcomes the problem of cathode electrolytes (i.e., can suppress the dissolution of polysulfide) when sulfur from the cathode dissolves in the electrolyte in the form of lithium polysulfide (LiPS). They then applied a carbon material with a high specific surface area (BET specific surface area) as a sulfur support to the positive electrode active material and confirmed that by incorporating this into an SSE electrolyte system, 80% or more of the theoretical discharge capacity of sulfur could be utilized. (That is, the sulfur utilization rate is 80% or more. The "sulfur utilization rate" specifically refers to the ratio of the discharge capacity (mAh) per weight (gram) of elemental sulfur contained in the positive electrode of the battery to the theoretical capacity per weight of sulfur, which is 1,675 mAh / g. For example, when the discharge capacity per weight of elemental sulfur present in the positive electrode of a lithium-sulfur battery is 1,600 mAh / g, the sulfur utilization rate is 95.5% (1,600 / 1,675).)

[0020] However, a large specific surface area of ​​a carbon material used as a sulfur support does not necessarily mean that it exhibits high discharge capacity. In particular, under conditions of high sulfur content (e.g., >70 wt%), the pore volume of the carbon material, in addition to the specific surface area, significantly affects sulfur utilization. Therefore, even if a carbon material has a large specific surface area, a small pore volume results in insufficient pores to support sulfur, resulting in low sulfur support efficiency. This reduces the electron acceptance of non-conductive sulfur and increases the proportion of sulfur participating in the reaction, resulting in a significant decrease in the battery's discharge capacity and lifespan. In other words, achieving a high energy density (e.g., >400 Wh / kg) requires the use of a high-loading electrode by using a high-sulfur cathode active material, which requires optimization by controlling the specific surface area and porosity of the carbon material. Therefore, the present applicant invented a carbon material for a cathode active material with increased specific surface area and pore volume, and further improved battery performance by incorporating an SSE electrolyte system.

[0021] Specific surface area is 1,700m 2 / g or more, and the pore volume is 5 cm 3 The porous carbon material of the present invention, which has a carbon content of 0.1 to 1.0 wt % or more, is characterized in that it is a porous carbon material that has been activated with an activator and is used as a positive electrode active material for a lithium secondary battery to which an SSE (sparingly solvating electrolyte) electrolyte system is applied.

[0022] That is, conventionally, carbonized porous carbon materials have been used. However, carbonization of carbon materials removes only organic compounds other than carbon (C), making it difficult to form additional micropores in the porous carbon material. However, the present invention physically activates the porous carbon material with a gas activator or chemically activates it with a chemical activator, forming micropores through a process in which the solid carbon reacts with the gas or chemical to gasify it. However, a further carbonization process is not performed during the physical or chemical activation. Therefore, the porous carbon material of the present invention is activated with an activator, but an additional carbonization process (or treatment) must not be performed during the activation process (or treatment).

[0023] The porous carbon material of the present invention exhibits its greatest effect when used as a positive electrode active material in a lithium secondary battery employing an SSE (sparingly solvating electrolyte) electrolyte system, and can utilize more than 80% of the theoretical discharge capacity of sulfur. Therefore, when the porous carbon material of the present invention is used as a positive electrode active material in a lithium secondary battery employing an SSE electrolyte system, rather than as a positive electrode active material in a lithium secondary battery employing a conventional electrolyte (Catholyte), the battery performance can be maximized. In other words, the porous carbon material is characterized as being used as a positive electrode active material for a lithium secondary battery employing an electrolyte containing a first solvent including a fluorine-based ether compound, a second solvent including a glyme-based compound, and a lithium salt.

[0024] Such a porous carbon material of the present invention can be used for a long time. 2 / g or more. If the porous carbon material has a specific surface area of ​​1,700 m 2 If the specific surface area is less than 1 / g, the sulfur loading efficiency is low, which increases electrode side reactions and causes charging overvoltage, resulting in significant reductions in the discharge capacity and life characteristics of the battery.

[0025] The porous carbon material of the present invention is 5 cm 3 / g or more, preferably 7cm 3 / g or more, more preferably 7 to 10 cm 3 / g. If the porous carbon material has a pore volume of 5 cm 3 If the porous carbon material has a pore volume of less than 7 cm / g, the sulfur loading efficiency will be low (i.e., the amount of sulfur loaded will be reduced), which will result in a low electron acceptance rate of non-conductive sulfur and a high proportion of sulfur participating in the reaction, which may cause a significant decrease in the discharge capacity and life characteristics of the battery. 3 / g or more, the pore volume is 7 cm 3 / g。 On the other hand, although the present invention mainly deals with the case where the porous carbon material is applied to a battery, it is obvious that the porous carbon material can be used in other fields that require a porous carbon material other than the battery field.

[0026] As mentioned above, the specific surface area is 1,700m 2 / g or more, and the pore volume is 5 cm 3 The porous carbon material of the present invention having a specific surface area of ​​1,700 m / g or more may be, for example, carbon nanotubes; graphene (particularly, multilayer graphene flakes (MGF)); graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon fiber; or a mixture containing two or more of these. Among these, the porous carbon material having a specific surface area of ​​1,700 m / g or more may be, for example, carbon nanotubes; graphene (particularly, multilayer graphene flakes (MGF)); graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon fiber; or a mixture containing two or more of these. 2 / g or more, and the pore volume is 5 cm 3 / g or more of Ketjenblack may be most advantageous in terms of improving battery performance.

[0027] Next, a method for manufacturing the porous carbon material will be described. The method for manufacturing the porous carbon material includes a step of physically activating a carbon material with a gas activator or chemically activating a carbon material with a chemical activator. The method is characterized in that an additional carbonization process is not performed during the physical or chemical activation.

[0028] The carbon material may be any of carbon nanotubes; graphene (particularly, multilayer graphene flakes (MGF)); graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon fiber; and a mixture containing two or more of these. The activated carbon material may be a carbon material that has not been subjected to any carbonization treatment except for the activation treatment. That is, an additional carbonization step may not be performed during the physical activation or chemical activation, and may not be performed before or after the activation.

[0029] Conventionally, carbon materials have only been carbonized, but carbonization of carbon materials removes only organic matter other than carbon (C), making it difficult to form additional micropores in the carbon material. However, if the carbon material is physically activated with a gas activator or chemically activated with a chemical activator, the carbon material reacts with gas or chemicals and the solid carbon is gasified, allowing additional micropores to form in the carbon material.

[0030] Therefore, the method for producing a porous carbon material of the present invention includes a step of activating a carbon material using an activator, but is characterized in that no additional carbonization step is performed during the physical or chemical activation, and no carbonization step may be performed before or after the activation treatment (physical or chemical activation).

[0031] Hereinafter, methods for physically activating and chemically activating carbon materials will be described in detail. First, the physical activation method is a method in which activation is performed at high temperatures using a gas activator (or activation gas). Examples of the gas activator include steam, carbon monoxide (CO), carbon dioxide (CO2), and oxygen (O2). In the physical activation method, the activation temperature may be 500 to 1,000°C, preferably 800 to 1,000°C. The activation time may be 1 to 10 hours, preferably 3 to 10 hours. If the activation temperature and time are outside the above ranges, micropores may not be properly formed in the carbon material.

[0032] The chemical activation method involves activating a chemical at high temperatures using a chemical activator. When the chemical activator is immersed in a carbon material and heated, dehydration and oxidation reactions of the chemical occur, forming micropores in the porous carbon material. Examples of chemical activators include KOH, K2CO3, NaOH, Na2CO3, AlCl3, MgCl3, and H3PO4, which are compounds with strong dehydrating and oxidizing properties. Prior to chemical activation, the material may be purged with an inert gas, such as nitrogen (N2) or argon, without any particular restrictions on the inert gas injection conditions. A sealed container may be used for the chemical activation. Examples of such a sealed container include glass jars, which are commonly used containers that can be sealed and purged after injection of an inert gas.

[0033] The immersion ratio of the carbon material before activation to the chemical activator may be 1:8 to 1:1 by weight. When the chemical activation method is used, the activation temperature may be 400 to 1,000°C, preferably 500 to 900°C. The activation time may be 1 to 10 hours, preferably 1 to 5 hours. If the activation temperature and time are outside the above ranges, micropores may not be properly formed in the porous carbon material.

[0034] Next, the positive electrode for a lithium secondary battery according to the present invention will be described. The positive electrode for a lithium secondary battery is a sulfur-carbon composite (C2S) in which sulfur is supported on the porous carbon material. x ) n (x=2.5-50, n≧2) as a positive electrode active material. The positive electrode for the lithium secondary battery further includes a binder and a conductive material in addition to the positive electrode active material. The sulfur may be elemental sulfur (S8), a sulfur-based compound, or a mixture thereof. Specifically, the sulfur-based compound may be Li2Sn (n≧1) or an organic sulfur compound.

[0035] The sulfur-carbon composite may have a particle size of 1 to 100 μm. If the particle size of the sulfur-carbon composite is less than 1 μm, the interparticle resistance may increase, possibly generating overvoltage in the electrode of a lithium-sulfur battery. If the particle size of the sulfur-carbon composite is more than 100 μm, the surface area per unit weight may decrease, resulting in a decrease in the wetting area with the electrolyte in the electrode and in the reaction sites with lithium ions. This may result in a decrease in the amount of electron transfer relative to the size of the composite, slowing down the reaction, and potentially reducing the discharge capacity of the battery.

[0036] The sulfur (S) contained in the positive electrode may be in a content of 60 to 90 wt %, preferably 65 to 85 wt %, and more preferably 70 to 80 wt %, based on the total weight of the positive electrode. If the sulfur is used in an amount less than 60 wt %, based on the total weight of the positive electrode, the energy density of the battery may decrease, while if the sulfur is used in an amount more than 90 wt %, the conductivity within the electrode may decrease, resulting in a decrease in the stability of the electrode.

[0037] The cathode active material containing sulfur and a carbon material may be contained in an amount of 80 to 99 parts by weight, preferably 90 to 95 parts by weight, based on 100 parts by weight of the total cathode. If the amount of the cathode active material is less than 80 parts by weight based on 100 parts by weight of the total cathode, the energy density of the battery may decrease, whereas if the amount is more than 99 parts by weight, the conductivity within the electrode may decrease, resulting in a decrease in the stability of the electrode.

[0038] The binder contained in the positive electrode is a component that assists in bonding the positive electrode active material to the conductive material and the like and in bonding to the current collector. For example, one or more binders selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF / HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl (meth)acrylate, polyethyl (meth)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof may be used, but are not necessarily limited to these.

[0039] The binder is typically added in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, based on 100 parts by weight of the total positive electrode. If the binder content is less than 1 part by weight, the adhesive strength between the positive electrode active material and the current collector may be insufficient. If the binder content exceeds 50 parts by weight, the adhesive strength is improved, but the content of the positive electrode active material is reduced accordingly, which may result in a lower battery capacity.

[0040] The conductive material contained in the positive electrode is not particularly limited as long as it does not cause side reactions in the internal environment of the battery, does not cause chemical changes in the battery, and has excellent electrical conductivity. Representative examples include graphite or conductive carbon. Examples include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon-based materials having a graphene or graphite crystal structure; carbon nanotubes; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These may be used alone or in combination of two or more, but are not necessarily limited to these.

[0041] The conductive material may be added in an amount of 0.5 to 10 parts by weight, preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the total positive electrode weight, but may not be included in the positive electrode of the present invention. If the content of the conductive material exceeds 10 parts by weight, based on 100 parts by weight of the total positive electrode weight, the amount of positive electrode active material will be relatively small, which may result in reduced capacity and energy density. The method for incorporating the conductive material into the positive electrode is not particularly limited, and conventional methods known in the art, such as coating the positive electrode active material, may be used. If necessary, a conductive second coating layer may be added to the positive electrode active material instead of adding the conductive material.

[0042] A filler may be selectively added to the positive electrode of the present invention as a component for suppressing its expansion. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without causing chemical changes in the battery. For example, olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fiber and carbon fiber; etc. may be used.

[0043] The positive electrode can be manufactured by dispersing and mixing a positive electrode active material, a binder, a conductive material, etc. in a dispersion medium (solvent) to prepare a slurry, applying the slurry to a positive electrode current collector, and then drying and rolling the slurry. The dispersion medium may be, but is not limited to, NMP (N-methyl-2-pyrrolidone), DMF (dimethyl formamide), DMSO (dimethyl sulfoxide), ethanol, isopropanol, water, or a mixture thereof.

[0044] The positive electrode current collector may be, but is not limited to, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In-doped SnO), FTO (F-doped SnO), alloys thereof, or aluminum (Al) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag). The positive electrode current collector may be in the form of a foil, film, sheet, punched, porous, foam, or the like.

[0045] Finally, a lithium secondary battery according to the present invention will be described. The lithium secondary battery includes the above-described positive electrode for lithium secondary battery, a negative electrode, a separator interposed therebetween, and an electrolyte including a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and a lithium salt. When the positive electrode for lithium secondary battery contains sulfur, the lithium secondary battery is a lithium-sulfur battery.

[0046] The lithium secondary battery of the present invention contains an SSE (sparingly solvating electrolyte) and has a specific surface area of ​​1,700 m 2 / g or more, and the pore volume is 5 cm 3 / g or more of porous carbon material as a positive electrode active material, the sulfur utilization rate is 80% or more, preferably 85% or more of the theoretical discharge capacity, and the battery has a high energy density of about 400 Wh / kg or more or 600 Wh / L or more.

[0047] The first solvent containing a fluorine-based ether compound, the second solvent containing a glyme-based compound, and the lithium salt contained in the electrolyte of the lithium secondary battery according to the present invention will be specifically described below.

[0048] The first solvent is an electrolyte solvent containing a fluorine-based ether compound, and it dissolves polysulfides and inhibits solvent decomposition, thereby improving the coulombic efficiency (CE) of the battery and ultimately improving the battery's lifespan. More specifically, the first solvent containing a fluorine-based ether compound has excellent structural stability due to fluorine substitution compared to general organic solvents containing alkanes, and therefore is highly stable. Therefore, when this first solvent is used in an electrolyte for a lithium secondary battery, the stability of the electrolyte can be significantly improved, thereby improving the lifespan of the lithium secondary battery.

[0049] Examples of the fluorine-based ether compound include one or more hydrofluoroether-based (HFE type) compounds selected from the group consisting of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), bis(fluoromethyl)ether, 2-fluoromethyl ether, bis(2,2,2-trifluoroethyl)ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, and 1H,1H,2'H-perfluorodipropyl ether.

[0050] The second solvent is an electrolyte solvent containing a glyme-based compound (but not containing fluorine), and not only dissolves lithium salt to allow the electrolyte to have lithium ion conductivity, but also dissolves sulfur, a positive electrode active material, to facilitate the electrochemical reaction with lithium.

[0051] Specific examples of the glyme-based compound include one or more selected from the group consisting of dimethoxyethane, diethoxyethane, methoxyethoxyethane, 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 are not limited to these. Of these, the use of dimethoxyethane may be preferred.

[0052] The lithium salt is an electrolyte salt used to increase ionic conductivity, and any salt commonly used in the art may be used without limitation. Specific examples of the lithium salt include LiCl, LiBr, LiI, LiClO4, LiBF4, and 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 carboxylates having 4 or less carbon atoms, lithium tetraphenylborate, and lithium imide.

[0053] The concentration of the lithium salt can be determined taking into consideration ionic conductivity and the like, and may be, for example, 0.1 to 2 M, preferably 0.5 to 1 M, and more preferably 0.5 to 0.75 M. If the concentration of the lithium salt is below this range, it may be difficult to ensure ionic conductivity suitable for battery operation, whereas if the concentration exceeds this range, the viscosity of the electrolyte may increase, reducing the mobility of lithium ions, or the decomposition reaction of the lithium salt itself may increase, resulting in a decrease in battery performance.

[0054] In the electrolyte containing the first solvent, the second solvent, and the lithium salt, the molar ratio of the lithium salt to the second solvent to the first solvent may be 1:0.5 to 3:4.1 to 15. In one embodiment of the present invention, the molar ratio of the lithium salt to the second solvent to the first solvent may be 1:2:4 to 13, 1:3:3 to 10, or 1:4:5 to 10. The electrolyte contained in the lithium secondary battery of the present invention may contain a first solvent containing a fluorinated ether compound at a higher content ratio than the second solvent containing a glyme-based compound. When the first solvent containing a fluorinated ether compound is contained at a higher content ratio than the second solvent containing a glyme-based compound, this is advantageous in that it suppresses polysulfide formation, enables a battery capacity close to the theoretical capacity of sulfur, and suppresses a decrease in battery capacity during battery use. Therefore, it is preferable to contain the first solvent containing a fluorinated ether compound at a higher content ratio than the second solvent containing a glyme-based compound.

[0055] The negative electrode included in the lithium secondary battery of the present invention may be a lithium-based metal, and may further include a negative electrode current collector on one side of the lithium-based metal. The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity, 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. Alternatively, calcined carbon, a non-conductive polymer or a conductive polymer surface-treated with a conductive material, or the like may be used. Typically, a copper thin plate is used as the negative electrode current collector.

[0056] The negative electrode current collector may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric, with or without fine irregularities on the surface. The negative electrode current collector has a thickness of 3 to 50 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collection effect decreases, while if the thickness exceeds 50 μm, the processability decreases when the cell is folded and assembled.

[0057] The lithium-based metal may be lithium or a lithium alloy. In this case, the lithium alloy contains an element that can be alloyed with lithium, and specifically may be an alloy of lithium with one or more elements selected from the group consisting of Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al.

[0058] The lithium-based metal may be in the form of a sheet or foil, and in some cases, may be in the form of lithium or a lithium alloy deposited or coated on a current collector by a dry process, or may be in the form of metal or alloy deposited or coated on particles by a wet process, etc.

[0059] A conventional separator may be interposed between the positive electrode and the negative electrode. The separator is a physical separator having a function of physically separating the electrodes, and any separator commonly used may be used without particular limitation. In particular, a separator having low resistance to ion migration of the electrolyte and excellent humidifying ability for the electrolyte is preferred.

[0060] The separator separates or insulates the positive electrode and the negative electrode from each other while allowing lithium ions to be transported between them. The separator may be made of a porous, non-conductive, or insulating material. The separator may be an independent member such as a film, or a coating layer attached to the positive electrode and / or the negative electrode.

[0061] Examples of polyolefin-based porous membranes that can be used as the separation membrane include membranes formed from polyolefin-based polymers such as polyethylene (e.g., high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutylene, and polypentene, either alone or in combination. Examples of nonwoven fabrics that can be used as the separator include nonwoven fabrics made of polymers such as polyphenylene oxide, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, and polyester, either alone or in combination. Such nonwoven fabrics may be spunbond or meltblown, consisting of long fibers, as the fiber form that forms the porous web.

[0062] The thickness of the separator is not particularly limited, but is preferably in the range of 1 to 100 μm, and more preferably in the range of 5 to 50 μm. If the thickness of the separator is less than 1 μm, it will not be able to maintain its mechanical properties, and if it exceeds 100 μm, it will act as a resistance layer, resulting in reduced battery performance. The pore size and porosity of the separator are not particularly limited, but the pore size is preferably 0.1 to 50 μm and the porosity is preferably 10 to 95%. If the pore size of the separator is less than 0.1 μm or the porosity is less than 10%, it will act as a resistance layer, and if the pore size is more than 50 μm or the porosity is more than 95%, it will not be able to maintain its mechanical properties.

[0063] The lithium secondary battery of the present invention, including the above-described positive electrode, negative electrode, separator, and electrolyte, can be manufactured by placing the positive electrode and the negative electrode opposite each other, interposing a separator therebetween, and then injecting an electrolyte solution.

[0064] Meanwhile, the lithium secondary battery according to the present invention can be applied not only to battery cells used as power sources for small devices, but also particularly suitably used as unit batteries for battery modules, which are power sources for medium- to large-sized devices. In this regard, the present invention also provides a battery module including two or more lithium secondary batteries electrically connected (in series or parallel). The number of lithium secondary batteries included in the battery module can be adjusted in various ways, taking into account the application and capacity of the battery module. Furthermore, the present invention also provides a battery pack in which the battery modules are electrically connected according to conventional techniques in the art. The battery module and battery pack may be used as a power source for one or more medium- to large-sized devices, including, but not limited to, power tools; electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric trucks; electric commercial vehicles; or power storage systems. However, the lithium secondary battery of the present invention may be preferably used as an aircraft battery for use in Urban Air Mobility (UAM).

[0065] 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, these changes and modifications also fall within the scope of the appended claims.

[0066] [Example 1] Manufacturing of porous carbon materials 20 g of Ketjen Black was placed in a quartz tube using a fluidized bed reactor, and after purging with nitrogen gas for 1 minute, CO 2 gas was injected at a temperature of 900°C for 5 hours to activate the material.

[0067] [Comparative Example 1] Porous carbon material The same Ketjen black as that used in Example 1 was prepared, but was not subjected to the activation treatment.

[0068] [Experimental Example 1] Evaluation of specific surface area and pore volume of porous carbon materials The specific surface area and pore volume of each of the porous carbon materials of Example 1 and Comparative Example 1 were measured, and the results are shown in Table 1. The measurement was performed using a porosity analyzer (model: Belsorp-max, manufacturer: Bel Japan), and the specific surface area and pore volume values ​​were calculated from nitrogen adsorption / desorption isotherms at 77K.

[0069] [Table 1]

[0070] FIG. 1 shows nitrogen adsorption / desorption isotherms used to determine the specific surface area of ​​the porous carbon materials according to an example and a comparative example. FIG. 2 shows a pore size distribution diagram (BJH plot, mesopore / macropore analysis) used to determine the pore size of the porous carbon materials according to an example and a comparative example. FIG. 3 shows a pore size distribution diagram (HK plot, micropore analysis) used to determine the pore size of the porous carbon materials according to an example and a comparative example. As described above, the specific surface area and pore volume of each of the porous carbon materials according to Example 1 and Comparative Example 1 were measured. As shown in Table 1 and as shown in FIGS. 1 to 3, it was confirmed that the activated porous carbon material according to Example 1 had significantly increased specific surface area and pore volume compared to the unactivated porous carbon material according to Comparative Example 1.

[0071] [Experimental Example 2] Measurement of tap density of porous carbon materials The tap density of each of the porous carbon materials of Example 1 and Comparative Example 1 was measured, and the results are shown in Table 2. A tap density measuring device (model name: KYT-5000K, manufacturer: SEISHIN) was used for the measurement, and the results of tapping a 100 ml sample 1,000 times using this device are shown below.

[0072] [Table 2]

[0073] As described above, the tap density was measured for each of the porous carbon materials of Example 1 and Comparative Example 1. As a result, as shown in Table 2, the porous carbon material of Example 1, which was subjected to activation treatment, exhibited a lower tap density than the porous carbon material of Comparative Example 1, which was not subjected to activation treatment. Therefore, this also confirms the advantage of activating carbon materials.

[0074] [Example 2, Comparative Example 2] Production of sulfur-carbon composites To load sulfur on the porous carbon materials of Example 1 and Comparative Example 1, sulfur and the porous carbon materials were mixed in a weight ratio of 70:30 and then heated to prepare sulfur-carbon composites.

[0075] Comparative Example 3 Production of sulfur-carbon composites In order to load sulfur on the porous carbon material of Comparative Example 1, sulfur and the porous carbon material were mixed in a weight ratio of 60:40 and then heated to prepare a sulfur-carbon composite.

[0076] [Experimental Example 2] Thermogravimetric analysis (TGA) of sulfur-carbon composites Thermogravimetric analysis (heating condition: 20°C to 900°C at 10°C / min, air atmosphere) was performed on each of the sulfur-carbon composites prepared in Example 2, Comparative Example 2, and Comparative Example 3. The results of observing the mass changes are shown in Figure 4. A thermogravimetric analyzer (manufactured by Mettler Toledo) was used for the thermogravimetric analysis.

[0077] Figure 4 shows thermogravimetric analysis (TGA) graphs of the sulfur-carbon composites according to an example and a comparative example. The results of the TGA analysis of the sulfur-carbon composites prepared in Example 2, Comparative Example 2, and Comparative Example 3 indicate that sulfur volatilization occurs at a relatively higher temperature in Example 2 than in Comparative Examples 2 and 3 due to the sulfur loading inside the pores of the carbon material. This indicates that the sulfur loading efficiency increases when the carbon material is activated.

[0078] [Example 3] Lithium secondary battery manufacturing Cathode manufacturing 90 parts by weight of the sulfur-carbon composite prepared in Example 2 as a positive electrode active material (sulfur content was 63 wt % based on the total weight of the positive electrode), 5 parts by weight of Denka Black as a conductive material, and 5 parts by weight of styrene butadiene rubber / carboxymethyl cellulose (SBR:CMC=7:3) as a binder were mixed to prepare a positive electrode slurry composition. The prepared slurry composition was then coated on a current collector (Al foil), dried at 50°C for 12 hours, and pressed using a roll press to prepare a positive electrode (at this time, the sulfur loading was 3.0 mAh / cm). 2 (We decided to do so.)

[0079] Electrolyte production First, LiTFSI (concentration: 0.65 M), dimethoxyethane (second solvent), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE, first solvent) were mixed at room temperature in a molar ratio of 1:2:9 to prepare an electrolyte (SSE).

[0080] Lithium secondary battery manufacturing The prepared positive electrode and a 150 μm-thick lithium metal negative electrode were positioned facing each other, a polyethylene (PE) separator was interposed between them, and the prepared electrolyte (SSE) was injected to fabricate a coin cell-type lithium secondary battery. Meanwhile, in the battery fabrication, the positive electrode was punched out to a circular electrode of 14 phi, the polyethylene separator was punched out to 19 phi, and the lithium metal was punched out to 16 phi.

[0081] Comparative Example 4 Lithium secondary battery manufacturing A coin cell-type lithium secondary battery was fabricated in the same manner as in Example 3, except that the sulfur-carbon composite prepared in Example 2 was replaced with the sulfur-carbon composite prepared in Comparative Example 2 (the content of sulfur alone was 63 wt % based on the total weight of the cathode).

[0082] Comparative Example 5 Lithium secondary battery manufacturing A coin cell-type lithium secondary battery was fabricated in the same manner as in Example 3, except that the sulfur-carbon composite prepared in Example 2 was replaced with the sulfur-carbon composite prepared in Comparative Example 3 (the content of sulfur alone was 54 wt % based on the total weight of the cathode).

[0083] [Experimental Example 3] Evaluation of discharge capacity and life characteristics of lithium secondary batteries The lithium secondary batteries prepared in Example 3, Comparative Example 4, and Comparative Example 5 were subjected to 0.1C charge / 0.1C discharge three times, followed by 0.2C charge / 0.3C discharge to evaluate the discharge capacity and lifespan characteristics of the batteries, and the discharge capacity evaluation results are shown in Table 3 below. The voltage range used was 1.2 to 3.6V, and the evaluation temperature was room temperature. Figure 5 is a graph showing the discharge capacity of the lithium secondary batteries according to an example and a comparative example of the present invention, and Figure 6 is a graph showing the discharge capacity and lifespan characteristics of the lithium secondary batteries according to an example and a comparative example of the present invention.

[0084] [Table 3]

[0085] The discharge capacity and lifespan of the lithium secondary batteries manufactured in Example 3, Comparative Example 4, and Comparative Example 5 were measured. As a result, it was found that the SSE electrolyte was used and the specific surface area was 1,700 m 2 / g or more, and the pore volume is 5 cm 3 The battery of Example 3, in which the sulfur-carbon composite contained a carbon material of 63% or more by weight, exhibited a discharge capacity of 80% or more of the theoretical capacity and had excellent life characteristics, even though the sulfur content was as high as 63% based on the total weight of the cathode, as shown in Table 3 and as shown in FIGS. 5 and 6.

[0086] On the other hand, even though the SSE electrolyte was applied, the specific surface area was 1,700 m 2 / g and the pore volume is less than 5 cm 3 The batteries of Comparative Examples 4 and 5, in which the carbon material was added to the sulfur-carbon composite at less than 1000 W / g, were found to be inferior in both discharge capacity and lifespan characteristics to those of Example 3, as shown in Table 3 and as shown in Figures 5 and 6. In particular, in Comparative Example 4, the sulfur content was high at 63% based on the total weight of the cathode, as in Example 3. However, the high sulfur content adversely affected the cell overvoltage, resulting in decreased discharge capacity and nominal voltage, as well as decreased lifespan characteristics.

Claims

1. Specific surface area is 1,700m ２ / g or more, and the pore volume is 5 cm ３ / g or more.

2. The pore volume of the porous carbon material is 7 cm ３ The porous carbon material according to claim 1, wherein the porous carbon material has a molecular weight of 1 / g or more.

3. 2. The porous carbon material according to claim 1, wherein the porous carbon material is selected from the group consisting of carbon nanotubes; graphene (multilayer graphene flake, MGF); graphite; carbon black selected from the group consisting of carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, and lamp black; carbon fiber; and a mixture containing two or more of these.

4. The porous carbon material according to claim 3, wherein the porous carbon material is Ketjen black.

5. The porous carbon material according to claim 1, wherein the porous carbon material is activated.

6. 2. The porous carbon material according to claim 1, wherein the porous carbon material is a porous carbon material for a positive electrode active material of a lithium secondary battery, the porous carbon material comprising a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and an electrolyte containing a lithium salt.

7. A method for producing the porous carbon material according to any one of claims 1 to 6, comprising: A method for producing a porous carbon material, comprising physically activating the carbon material with a gas activator or chemically activating the carbon material with a chemical activator.

8. 8. The method for producing a porous carbon material according to claim 7, wherein the carbon material before activation is reacted with a gas or a chemical to gasify solid carbon, thereby forming micropores in the carbon material.

9. The method for producing a porous carbon material according to claim 7, wherein no carbonization step is performed during the physical activation or chemical activation.

10. The method for producing a porous carbon material according to claim 7, wherein no carbonization step is performed before or after the physical or chemical activation.

11. 8. The method of claim 7, wherein the gas activator is selected from the group consisting of water vapor, carbon monoxide, carbon dioxide, and oxygen, and the physical activation using the gas activator is performed at a temperature of 500 to 1,000°C for 1 to 10 hours.

12. The chemical activator is KOH, K ２ CO ３ , NaOH, Na ２ CO ３ , AlCl ３ , MgCl ３ and H ３ P.O. ４ and the chemical activation using the chemical activator is performed at a temperature of 400° C. to 1,000° C. for 1 hour to 10 hours.

13. 8. The method for manufacturing a porous carbon material according to claim 7, wherein the immersion ratio of the carbon material before activation to the chemical activator is 1:8 to 1:1 by weight.

14. A positive electrode for a lithium secondary battery, comprising the sulfur-carbon composite in which sulfur is supported on the porous carbon material according to claim 1 as a positive electrode active material.

15. 15. A lithium secondary battery comprising: the positive electrode for a lithium secondary battery according to claim 14; a negative electrode; a separator interposed therebetween; and an electrolyte comprising a first solvent containing a fluorinated ether compound, a second solvent containing a glyme compound, and a lithium salt.

16. 16. The lithium secondary battery of claim 15, wherein the sulfur content of the positive electrode is 60 wt % or more and 90 wt % or less based on the total weight of the positive electrode.

17. 16. The lithium secondary battery according to claim 15, wherein the utilization rate of sulfur contained in the positive electrode is 80% or more of the theoretical discharge capacity.

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