Porous carbon material with impurities removed, method for producing the same, positive electrode for lithium-sulfur battery containing the carbon material as a positive electrode active material, and lithium-sulfur battery

Abstract

The porous carbon material from which impurities such as moisture have been removed through pretreatment is applied to the positive electrode of a lithium-sulfur battery, thereby improving the problem of overvoltage during charging of the battery. The porous carbon material from which impurities have been removed, its manufacturing method, and a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery including the carbon material as a positive electrode active material are disclosed. The porous carbon material has a specific surface area of ​​200 to 1,700 m2. 2 / g, and is characterized by the removal of impurities through pretreatment using microwaves.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0180557 filed on December 16, 2021, and Korean Patent Application No. 10-2022-0152348 filed on November 15, 2022, and all of the contents disclosed in the corresponding Korean patent applications are incorporated herein by reference in their entirety.

[0002] The present invention relates to a porous carbon material from which impurities have been removed, a method for manufacturing the same, a positive electrode for a lithium-sulfur battery including the carbon material as a positive electrode active material, and a lithium-sulfur battery. More specifically, the present invention relates to a porous carbon material from which impurities such as moisture have been removed through pretreatment and applied to a positive electrode for a lithium-sulfur battery, thereby improving the charging overvoltage problem of the battery, a method for manufacturing the same, a positive electrode for a lithium-sulfur battery including the carbon material as a positive electrode active material, and a lithium-sulfur battery.

Background Art

[0003] As interest in energy storage technologies has been increasing gradually, the application fields have been expanding to mobile phones, tablets, laptops, camcorders, and even to the energy of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and research and development on electrochemical devices have been gradually increasing. Electrochemical devices are the most spotlighted fields in this regard, and among them, the development of secondary batteries such as rechargeable lithium-sulfur batteries has become the focus of interest. Recently, in developing such batteries, research and development on new electrode and battery designs have been led to improve the capacity density and specific energy.

[0004] Among such electrochemical devices, a lithium-sulfur battery (Li-S battery) has attracted attention as a next-generation secondary battery having a high energy density (theoretical capacity) and capable of replacing lithium-ion batteries. In such a lithium-sulfur battery, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur during discharge. At this time, sulfur has a ring structure of S8 It forms a more linear structure of lithium polysulfide (LiPS), and such a lithium-sulfur battery is characterized by showing a stepwise discharge voltage until the polysulfide is completely reduced to Li 2 S.

[0005] However, the biggest obstacle in the commercialization of lithium-sulfur batteries is the lifespan. During the charge and discharge process, the charge / discharge efficiency decreases and the battery lifespan degrades. The reasons for the degradation of such lithium-sulfur battery lifespan are diverse, including side reactions of the electrolyte (deposition of by-products due to electrolyte decomposition), instability of lithium metal (growth of dendrites on the lithium anode causing short circuits), and deposition of cathode by-products (leakage of lithium polysulfide from the cathode).

[0006] That is, in a battery using a sulfur-based compound as the cathode active material and an alkali metal such as lithium as the anode active material, during charge and discharge, the leakage of lithium polysulfide and the shuttle phenomenon occur, and the lithium polysulfide is transmitted to the anode, reducing the capacity of the lithium-sulfur battery. As a result, the lithium-sulfur battery has a major problem of reduced lifespan and reactivity. That is, since the polysulfide leaked from the cathode has a high solubility in the organic electrolyte, unwanted movement (PS shuttling) to the anode may occur through the electrolyte. As a result, capacity reduction due to irreversible loss of the cathode active material and reduction of the battery lifespan due to deposition of sulfur particles on the lithium metal surface by side reactions occur.

[0007] On the other hand, in order for a lithium-sulfur battery to achieve a high energy density of about 400 Wh / kg or more or 600 Wh / L or more, a high loading (about 4.0 mAh / cm 2An electrolyte and a positive electrode active material system that can be driven even under conditions of high porosity (about 60% or more) and low porosity (about 60% or less) are required. That is, the behavior of such a lithium-sulfur battery can vary greatly depending on the electrolyte. When sulfur in the positive electrode emerges in the form of lithium polysulfide (LiPS) in the electrolyte, the electrolyte is called catholyte. When sulfur hardly emerges in the form of lithium polysulfide, the electrolyte is called SSE (sparingly solvating electrolyte). Lithium-sulfur batteries that utilize existing catholyte systems depend on liquid-phase reactions through the formation of intermediate polysulfides having a 2 S x Li form (catholyte type). Therefore, the high theoretical discharge capacity of sulfur (1,675 mAh / g) cannot be fully utilized, and conversely, there is a problem that the battery life is drastically reduced due to the degradation of the battery caused by the emergence of polysulfides.

[0008] On the other hand, recently, an SSE (sparingly solvating electrolyte) electrolyte system that can suppress the emergence of polysulfides has been developed. In particular, when a carbon material having a high specific surface area (BET Specific surface area) of 200 to 1,700 m 2 / g is applied as a carrier for sulfur, it has been confirmed that more than 90% of the theoretical discharge capacity of sulfur can be utilized. However, in the case of such a carbon material, due to the high specific surface area, relatively more impurities such as moisture are contained, so the electrode side reaction becomes large and a charging overvoltage phenomenon occurs, resulting in a problem of low utilization. To solve such a problem, in the industry, a carbon material with a high specific surface area is heat-treated in a furnace. In this case, the required time is long and it is difficult to effectively remove impurities.

[0009] Therefore, a solution is required to improve the charging overvoltage problem by effectively removing impurities such as moisture contained in the carbon material having a high specific surface area while using the SSE electrolyte system.

Summary of the Invention

Problems to be Solved by the Invention

[0010] Therefore, an object of the present invention is to provide a porous carbon material from which impurities such as moisture have been removed through pretreatment, apply it to the positive electrode of a lithium-sulfur battery, and improve the charging overvoltage problem of the battery. The present invention also provides a porous carbon material from which impurities have been removed, a method for producing the same, a positive electrode for a lithium-sulfur battery containing the carbon material as a positive electrode active material, and a lithium-sulfur battery.

Means for Solving the Problems

[0011] To achieve the above object, the present invention provides a porous carbon material having a specific surface area of 200 to 1,700 m 2 / g, from which impurities have been removed through pretreatment using microwaves.

[0012] Further, the present invention provides a method for producing a porous carbon material from which impurities have been removed, the method including: (a) placing a porous carbon material having a specific surface area of 200 to 1,700 m 2 / g in a sealed container, injecting an inert gas, and purging; and (b) applying microwaves to the porous carbon material.

[0013] Further, the present invention provides a positive electrode for a lithium-sulfur battery including a sulfur-carbon composite in which sulfur is supported on the porous carbon material from which impurities have been removed as a positive electrode active material.

[0014] Further, the present invention provides a lithium-sulfur battery including the positive electrode for a lithium-sulfur battery; a negative electrode; a separator interposed therebetween; and an electrolyte including a first solvent containing a fluorine-based ether compound, a second solvent containing a glyme-based compound, and a lithium salt.

Advantages of the Invention

[0015] According to the present invention, a porous carbon material from which impurities have been removed, a method for producing the same, a positive electrode for a lithium-sulfur battery containing the carbon material as a positive electrode active material, and a lithium-sulfur battery, by applying a porous carbon material from which impurities such as moisture have been removed through pretreatment to a positive electrode for a lithium-sulfur battery, the problem of charging overvoltage of the battery can be improved. Further, by combining a positive electrode for a lithium-sulfur battery containing the porous carbon material from which impurities have been removed as a positive electrode active material with a SSE (sparingly solvating electrolyte) electrolyte system (discharge capacity: ~1,600 mAh / gs) instead of an existing Catholyte electrolyte system (discharge capacity: ~1,200 mAh / gs), the theoretical discharge capacity of sulfur (1,675 mAh / g) can be utilized at 90% or more, and it also has the advantage of maintaining a high energy density of about 400 Wh / kg or more or 600 Wh / L or more.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0017] Hereinafter, the present invention will be described in detail.

[0018] The porous carbon material according to the present invention has a specific surface area of 200 to 1,700 m 2 / g and is characterized in that impurities are removed through pretreatment using microwaves.

[0019] To construct a lithium-sulfur battery with a high energy density of about 400 Wh / kg or more or 600 Wh / L or more, an electrolyte and a positive electrode active material system that can be driven even under high loading (about 4.0 mAh / cm 2 or more) and low porosity (about 60% or less) conditions are required. And as such an electrolyte system, an SSE (sparingly solvating electrolyte) electrolyte system that complements the problems of the electrolyte (Catholyte) when sulfur in the positive electrode gushes out in the form of lithium polysulfide (LiPS) into the electrolyte (that is, can suppress the gushing out of polysulfide) has been developed. And in particular, when a carbon material having a high specific surface area (BET Specific surface area) of 200 to 1,700 m 2 / g is applied as a positive electrode active material with a sulfur carrier and combined with this SSE electrolyte system, it has been confirmed that more than 90% of the theoretical discharge capacity of sulfur can be utilized (that is, the sulfur utilization rate is more than 90%. The "sulfur utilization rate" specifically refers to the ratio of the discharge capacity (mAh) per gram of the weight of the sulfur element contained in the positive electrode of the corresponding battery to the theoretical capacity of 1,675 mAh / g per gram of sulfur weight. For example, when the discharge capacity per gram of the weight of the sulfur element 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, in the case of such a carbon material, due to the high specific surface area, relatively more impurities such as moisture (specifically, moisture contained in the carbon material and other impurities such as unnecessary functional groups existing inside and on the surface of the carbon material) are also contained. Therefore, the electrode side reaction becomes large and the charging overvoltage phenomenon occurs, resulting in a problem of low utilization. To solve such a problem, in the industry, a carbon material with a high specific surface area is heat-treated in a furnace, but in this case, the required time is long and effective removal of impurities is difficult.

[0021] Therefore, while using the SSE electrolyte system, the applicant effectively removed impurities such as moisture contained in the carbon material with a high specific surface area through pretreatment using microwaves, thereby enhancing the utilization rate of the carbon material with a high specific surface area. In addition, by improving the ability of the carbon material to support sulfur, the production of a uniform positive electrode active material was made possible, thereby improving the charging overvoltage problem.

[0022] As in the present invention, if a pretreatment of applying (or irradiating) microwaves to a porous carbon material with a high specific surface area is performed, rapid temperature increase and interruption are possible by energy transfer rather than the existing heat transfer. In particular, since the yield of the energy incident on the porous carbon material being converted into thermal energy is high, it has the advantage of being more efficient than existing methods, such as the time required to remove impurities being short.

[0023] On the other hand, although the present invention is to apply microwaves to a porous carbon material to remove impurities contained in the carbon material, if microwaves are applied in a state where the porous carbon material is combined with another substance (for example, sulfur) separately, selective removal of the impurities contained in the carbon material is impossible. For example, when microwaves are applied to a sulfur-carbon composite in which a porous carbon material and sulfur are combined, sulfur is vaporized and volatilized, so it is impossible to remove only the impurities contained in the carbon material. This is because when microwaves are applied, the temperature increase rate of carbon is faster than that of sulfur, and the energy generated when the temperature of carbon rises is transmitted to the surrounding sulfur, and sulfur is vaporized from the low temperature before the impurities contained in the carbon are removed.

[0024] As described above, the porous carbon material of the present invention has a specific surface area of 200 to 1,700 m 2It is characterized in that impurities are removed through pretreatment using microwaves at / g. When the porous carbon material is used in other fields than the battery field, there is no particular limitation as long as impurities are removed by pretreatment using microwaves even if the specific surface area exceeds the above range. However, in a lithium-sulfur battery using an SSE electrolyte system to utilize 90% or more of the theoretical discharge capacity of sulfur, using a carbon material having a specific surface area within the above range can maximize the performance of the battery. Also, the pore volume of the porous carbon material may be 1.5 cm 3 / g or more. If the pore volume of the porous carbon material is less than 1.5 cm 3 / g, the sulfur loading amount may decrease, making it difficult to realize a high energy density battery.

[0025] And as the target carbon material to which microwaves are applied, for example, carbon nanotubes; graphene (especially, multilayer graphene flake, MGF); graphite; carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, lamp black, etc. of carbon black; carbon fiber; or a mixture containing two or more of these may be used.

[0026] Also, the pretreatment using microwaves is characterized in that microwaves are applied to the porous carbon material under the condition that MPPT according to the following formula 1 is 2,000 to 10,000 W*s / g.

[0027] [Formula 1] MPPT (W*s / g) = Microwave Power (W) × Times (s) / Carbon material mass (g)

[0028] In the above formula 1, Times is the time in seconds when microwaves are applied and exceeds 10 seconds. And the porous carbon material is preferably in powder form.

[0029] However, since each of the carbon materials before the application of microwaves has different contents of other impurities such as moisture and functional groups, excluding carbon, the MPPT (W*s / g) values in the above formula (1) are also different.

[0030] As an embodiment, the MPPT value of the carbon nanotubes is 2,000 to 10,000 W*s / g. However, when it exceeds about 2,800 W*s / g, there may be no practical benefit because no further removal of impurities is performed. And if the MPPT value of the carbon nanotubes is less than 2,000 W*s / g, it is impossible or insufficient to remove impurities, making it difficult to achieve the object of the present invention for improving the charging overvoltage. When the MPPT value of the carbon nanotubes exceeds 10,000 W*s / g, it may lead to ignition.

[0031] The MPPT value of the graphene (especially, multilayer graphene flake, MGF) is also 2,000 to 10,000 W*s / g. However, when it exceeds about 5,300 W*s / g, there may be no practical benefit because no further removal of impurities is performed. And if the MPPT value of the graphene is less than 2,000 W*s / g, it is impossible or insufficient to remove impurities, making it difficult to achieve the object of the present invention for improving the charging overvoltage. When the MPPT value of the graphene exceeds 10,000 W*s / g, it may lead to ignition.

[0032] The MPPT value of the carbon black is also 2,000 to 10,000 W*s / g. However, when it exceeds about 3,500 W*s / g, there may be no practical benefit because no further removal of impurities is performed. And if the MPPT value of the carbon black is less than 2,000 W*s / g, it is impossible or insufficient to remove impurities, making it difficult to achieve the object of the present invention for improving the charging overvoltage. When the MPPT value of the carbon black exceeds 10,000 W*s / g, it may lead to ignition.

[0033] The MPPT value of the Ketjen black is 2,000 to 10,000 W*s / g, preferably 5,000 to 9,900 W*s / g. When it exceeds 9,900 W*s / g, there may be no practical benefit because no further impurity removal is performed. If the MPPT value of the Ketjen black is less than 2,000 W*s / g, impurity removal may be impossible or insufficient, making it difficult to achieve the object of the present invention aimed at improving the charging overvoltage. When the MPPT value of the Ketjen black exceeds 10,000 W*s / g, it may lead to ignition. Therefore, when applying microwaves to the carbon material, microwave energy must be applied in accordance with the MPPT range corresponding to each carbon material.

[0034] The porous carbon material to which microwaves are applied and which has the specific surface area and pore volume as described above is characterized in that 90 to 100%, preferably 99 to 100% of the total impurities contained in the carbon material have been removed. The impurities include moisture, and specifically mean those containing moisture contained in the porous carbon material and other impurities such as unnecessary functional groups present inside and on the surface of the carbon material. On the other hand, it can be said that it is inevitable that moisture will be reabsorbed during storage even if microwaves are applied to the carbon material with such a high specific surface area to remove impurities.

[0035] On the one hand, if a pretreatment using microwaves is performed on a porous carbon material having a high specific surface area together with the present invention (that is, in other words, if only the carbon material is microwave-treated before producing the sulfur-carbon composite contained in the positive electrode active material of the lithium-sulfur battery), the content of sulfur actually participating in the reaction during the driving of the lithium-sulfur battery can be controlled more quickly and accurately compared to the conventional method. For example, when sulfur and a carbon material without microwave pretreatment are mixed at a weight ratio of 70:30, if the carbon material contains impurities at a content of 5% by weight, the actual weight ratio of sulfur to the carbon material becomes 70:28.5 (that is, 70:(30×0.95)). That is, the discharge capacity of the lithium-sulfur battery is calculated based on the content of sulfur contained in the battery. However, if a pretreatment using microwaves is performed on a carbon material having a high specific surface area as in the present invention, the content of sulfur and the carbon material contained in the positive electrode active material can be grasped more accurately. That is, in other words, there are no impurities in the porous carbon material and there is no error in the content of sulfur and the carbon material contained in the positive electrode active material.

[0036] Next, a method for producing the porous carbon material from which the impurities described above have been removed will be described. The method for producing the porous carbon material from which the impurities have been removed includes: (a) a step of putting a porous carbon material having a specific surface area of 200 to 1,700 m 2 / g into a sealed container, injecting an inert gas, and purging; and (b) a step of applying microwaves to the porous carbon material.

[0037] Examples of the sealed container include ordinary containers such as glass jars that can be sealed and purged even when an inert gas is injected. The inert gas is a general inert gas such as nitrogen (N 2 ), and there are no special restrictions on the injection conditions of the inert gas. And the step (b) is characterized in that microwaves are applied to the porous carbon material under the condition that MPPT according to the following formula 1 is 2,000 to 10,000 W*s / g.

[0038] [Formula 1] MPPT (W*s / g) = Microwave Power (W) × Times (s) / Mass of carbon material (g)

[0039] In the above formula (1), Times exceeds 10 seconds as the time in seconds when microwaves are applied. And the porous carbon material is preferably in the form of powder (Powder).

[0040] However, since each of the carbon materials before microwaves are applied has different contents of other impurities such as moisture and functional groups excluding carbon, the MPPT (W*s / g) values in the above formula (1) may also be different. The explanation regarding this is replaced by what was described above as one embodiment.

[0041] Subsequently, the positive electrode for a lithium-sulfur battery according to the present invention will be described. The positive electrode for a lithium-sulfur battery contains a sulfur-carbon composite in which sulfur is supported on the porous carbon material from which the above impurities have been removed as a positive electrode active material.

[0042] The positive electrode for a lithium-sulfur battery includes a positive electrode active material, a binder, a conductive material, etc. And in the positive electrode active material, in addition to the porous carbon material from which the above-described impurities have been removed, it can contain elemental sulfur (Elemental sulfur, S 8 ), sulfur-based compounds or mixtures thereof. Specifically, the sulfur-based compounds may be Li 2 S n (n ≧ 1) or organic sulfur compounds, etc. And as described above, a sulfur-carbon composite ((C 2 S x ): x = 2.5 to 50, n ≧ 2) containing a porous carbon material from which impurities have been removed and sulfur is preferably used as the positive electrode active material. n : x = 2.5 to 50, n ≧ 2) is preferably used as the positive electrode active material.

[0043] The sulfur-carbon composite may have a particle size of 1 to 100 μm. When the particle size of the sulfur-carbon composite is less than 1 μm, the resistance between particles may increase, resulting in an overvoltage in the electrode of the lithium-sulfur battery. When it exceeds 100 μm, the surface area per unit weight becomes small, reducing the wetting area with the electrolyte in the electrode and the reaction sites with lithium ions. As a result, the amount of electron transfer decreases with respect to the size of the composite, the reaction slows down, and the discharge capacity of the battery may decrease.

[0044] The sulfur (S) can be contained in an amount of 60 to 90% by weight, preferably 65 to 85% by weight, more preferably 65 to 80% by weight, based on the total weight of the positive electrode active material. If the sulfur is used in an amount less than 60% by weight based on the total weight of the positive electrode, there may be a problem of decreasing the energy density of the battery. When used in an amount exceeding 90% by weight, there may be a problem that the conductivity in the electrode decreases and the safety of the electrode deteriorates.

[0045] The positive electrode active material containing sulfur and the carbon material as described above can 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 weight of the positive electrode. If the content of the positive electrode active material is less than 80 parts by weight based on 100 parts by weight of the total weight of the positive electrode, there may be a problem of decreasing the energy density of the battery. When it exceeds 99 parts by weight, the conductivity in the electrode may decrease and the stability of the electrode may deteriorate.

[0046] The binder contained in the positive electrode is a component that assists in binding the positive electrode active material and the conductive material, etc., and binding to the current collector. For example, polyvinylidene fluoride (PVdF), polyvinylidene fluoride - hexafluoropropylene 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, polyvinyl pyridine, polyvinyl pyrrolidone, styrene - butadiene rubber, acrylonitrile - butadiene rubber, ethylene - propylene - diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene - butylene rubber, fluorine rubber, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, and one or more selected from the group consisting of mixtures thereof can be used, but it is not necessarily limited thereto.

[0047] The binder is usually 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 weight of the positive electrode. If the content of the binder is less than 1 part by weight, the adhesive force between the positive electrode active material and the current collector may become insufficient. If it exceeds 50 parts by weight, although the adhesive force improves, the content of the positive electrode active material decreases accordingly, and the battery capacity may decrease.

[0048] The conductive material contained in the positive electrode is not particularly limited as long as it has excellent electrical conductivity without causing side reactions in the internal environment of the battery and without causing chemical changes to the battery. Typically, graphite or conductive carbon can be used. For example, 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 substances with a crystal structure of graphene or graphite; 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 can be used alone or in combination of two or more, but it is not necessarily limited thereto.

[0049] The conductive material can usually 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 weight of the positive electrode, but it may not be included in the positive electrode of the present invention. If the content of the conductive material exceeds 10 parts by weight and is too much, the amount of the positive electrode active material may be relatively reduced, and the capacity and energy density may be decreased. The method of including the conductive material in the positive electrode is not greatly limited, and ordinary methods known in the art such as coating on the positive electrode active material can be used. Also, if necessary, the addition of the conductive material as described above can be replaced by adding a conductive second coating layer to the positive electrode active material.

[0050] In addition, a filler can be selectively added to the positive electrode of the present invention as a component for suppressing its swelling. Such a filler is not particularly limited as long as it can suppress the swelling of the electrode without causing chemical changes to the battery. For example, olefin-based polymers such as polyethylene and polypropylene; fibrous substances such as glass fibers and carbon fibers can be used.

[0051] 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 form a slurry, applying this slurry onto a positive electrode current collector, and then drying and rolling. In the dispersion medium, NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof can be used, but it is not necessarily limited thereto.

[0052] As the positive electrode current collector, 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 2 )、FTO (F doped SnO 2 )、and alloys thereof, and those obtained by surface-treating the surface of aluminum (Al) or stainless steel with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag) can be used, but it is not necessarily limited thereto. The form of the positive electrode current collector may be in the form of a foil, film, sheet, punched-out piece, porous body, foam, etc.

[0053] Finally, the lithium-sulfur battery according to the present invention will be described. The lithium-sulfur battery includes the positive electrode for a lithium-sulfur battery, the negative electrode, a separator interposed therebetween, and a first solvent containing a fluorine-based ether compound, a second solvent containing a glyme-based compound, and an electrolyte containing a lithium salt, as described above.

[0054] The lithium-sulfur battery of the present invention utilizes an SSE (sparingly solvating electrolyte) electrolyte system, but 200 to 1,700 m 2It contains a porous carbon material with a high specific surface area (BET Specific surface area) per g and to which microwaves are applied to remove impurities as a positive electrode active material, and can utilize 90% or more, preferably 94 to 100% of the theoretical discharge capacity of sulfur, and at the same time has a high energy density of about 400 Wh / kg or more or 600 Wh / L or more.

[0055] Hereinafter, 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-sulfur battery according to the present invention will be specifically described.

[0056] The first solvent is an electrolyte solvent containing a fluorine-based ether compound, and by having an effect of suppressing the dissolution and solvent decomposition of polysulfide, it serves to improve the coulombic efficiency (C.E.) of the battery and ultimately improve the battery life. More specifically, the first solvent containing the fluorine-based ether compound has extremely high stability because the structural stability of the solvent is excellent compared to a general organic solvent containing an alkene due to fluorine substitution. By using this in the electrolyte of a lithium-sulfur battery, the stability of the electrolyte can be greatly improved, and thereby the life performance of the lithium-sulfur battery can be improved.

[0057] Examples of the fluorine-based ether compound include one or more hydrofluoroether (HFE type) compounds selected from the group consisting of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (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.

[0058] The second solvent serves as an electrolyte solvent containing a glyme-based compound (however, it does not contain fluorine). It not only dissolves a lithium salt so that the electrolyte solution has lithium ion conductivity, but also plays a role in causing sulfur, which is the positive electrode active material, to exude and smoothly proceed with the electrochemical reaction with lithium.

[0059] 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 thereto. Among these, the use of dimethoxyethane is preferred.

[0060] The lithium salt can be used without limitation as long as it is a commonly used electrolyte salt in the industry for increasing ionic conductivity. Specific examples of the lithium salt include LiCl, LiBr, LiI, LiClO 4 , LiBF 4 , LiB 10 Cl 10 , LiPF 6 , LiCF 3 SO 3 , LiCF 3 CO 2 , LiC 4 BO 8 , LiAsF 6 , LiSbF 6 , LiAlCl 4 , CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2 ) 2 NLi, (C 2 F 5 SO 2 ) 2 NLi, (SO 2 F) 2 NLi, (CF 3 SO 2 ) 3 CLi, lithium chloroborane, lithium lower aliphatic carboxylate having 4 or less carbon atoms, lithium tetraphenylborate, and lithium imide, and one or more selected from the group consisting thereof can be exemplified.

[0061] The concentration of the lithium salt can be determined in consideration of ionic conductivity and the like. For example, it may be 0.1 to 2 M, preferably 0.5 to 1 M, more preferably 0.5 to 0.75 M. When the concentration of the lithium salt is less than the above range, it may be difficult to ensure ionic conductivity suitable for battery driving. When it exceeds the above range, the viscosity of the electrolyte may increase and the mobility of lithium ions may decrease, or the decomposition reaction of the lithium salt itself may increase and the performance of the battery may decrease.

[0062] In the electrolyte containing the first solvent, the second solvent, and the lithium salt as described above, the molar ratio of the lithium salt, the second solvent, and the first solvent may be 1:0.5 to 3:4.1 to 15. Further, as an embodiment of the present invention, the molar ratio of the lithium salt, the second solvent, and the first solvent may be 1:2:4 to 13 or 1:3:3 to 10 or 1:4:5 to 10. In the electrolyte contained in the lithium-sulfur battery of the present invention, the first solvent containing the fluorine-based ether compound is contained in a higher content ratio than the second solvent containing the glyme-based compound. Thus, when the first solvent containing the fluorine-based ether compound is contained in a higher content ratio than the second solvent containing the glyme-based compound, it has an advantage in suppressing the generation of polysulfide and enabling the realization of a battery capacity close to the theoretical capacity of sulfur, and suppressing the decrease in battery capacity due to battery use. Therefore, it is preferable to set the first solvent containing the fluorine-based ether compound to be contained in a higher content ratio than the second solvent containing the glyme-based compound.

[0063] The negative electrode contained in the lithium-sulfur battery of the present invention is a lithium-based metal, and a current collector can be further included on one side of the lithium-based metal. The current collector can be a negative electrode current collector. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, and can 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 can be surface-treated with carbon, nickel, titanium, or silver. In the alloy, an aluminum-cadmium alloy can be used. In addition, fired carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer can also be used. Generally, a copper thin plate is applied as the negative electrode current collector.

[0064] In addition, the form can be various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc. with or without fine irregularities formed on the surface. Further, the negative electrode current collector having a thickness in the range of 3 to 50 μm is applicable. If the thickness of the negative electrode current collector is less than 3 μm, the current collection effect deteriorates. On the other hand, if the thickness exceeds 50 μm, there is a problem that the workability deteriorates when the cell is folded and assembled.

[0065] The lithium-based metal may be lithium or a lithium alloy. At this time, the lithium alloy contains an element alloyable with lithium. Specifically, it may be an alloy of lithium and one or more 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.

[0066] The lithium-based metal may be in the form of a sheet or foil. In some cases, lithium or a lithium alloy is deposited or coated on the current collector by a dry process, or particulate metal and alloy are deposited or coated by a wet process or the like.

[0067] A normal separator can 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 can be used without particular limitation as long as it is a separator commonly used. In particular, a separator having a low resistance to ion movement of the electrolytic solution and excellent moisture retention ability of the electrolytic solution is preferable.

[0068] In addition, the separator allows lithium ions to be transported between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. Such a separator can 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 added to the positive electrode and / or the negative electrode.

[0069] Examples of polyolefin-based porous membranes that can be used as the separation membrane include membranes formed of polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, polypropylene, polybutylene, polypentene, etc., either alone or as a mixture of these polymers. Examples of non-woven fabrics that can be used as the separation membrane include non-woven fabrics formed of polyphenyleneoxide, polyimide, polyamide, polycarbonate, polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc., either alone or as a mixture of these polymers. Such non-woven fabrics can be in the form of fibers forming a porous web and include spunbond or meltblown forms composed of long fibers.

[0070] The thickness of the separation membrane is not particularly limited, but a range of 1 to 100 μm is preferred, and more preferably a range of 5 to 50 μm. When the thickness of the separation membrane is less than 1 μm, the mechanical properties cannot be maintained. When it exceeds 100 μm, the separation membrane acts as a resistance layer and the performance of the battery deteriorates. The pore size and porosity of the separation membrane 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 separation membrane is less than 0.1 μm or the porosity is less than 10%, the separation membrane acts as a resistance layer. When the pore size exceeds 50 μm or the porosity exceeds 95%, the mechanical properties cannot be maintained.

[0071] The lithium-sulfur battery of the present invention including the positive electrode, negative electrode, separation membrane, and electrolyte as described above can be manufactured through a process of injecting an electrolytic solution after facing the positive electrode to the negative electrode with a separation membrane interposed therebetween.

[0072] On the one hand, the lithium-sulfur battery according to the present invention can be applied not only to battery cells used as power sources for small devices, but also can be particularly suitably used as unit cells of battery modules that are power sources for medium and large-sized devices. In this regard, the present invention also provides a battery module in which two or more lithium-sulfur batteries are electrically connected (in series or in parallel). Needless to say, the number of lithium-sulfur batteries included in the battery module can be variously adjusted in consideration of the use and capacity of the battery module and the like. Furthermore, the present invention provides a battery pack in which the battery modules are electrically connected based on ordinary techniques in the art. The battery module and the battery pack can be used as power sources for one or more medium and large-sized devices among power tools; electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV); electric trucks; electric commercial vehicles; or power storage systems, but are not necessarily limited thereto. However, the lithium-sulfur battery of the present invention is preferably an aircraft battery used for urban air mobility (UAM).

[0073] Hereinafter, preferred embodiments are presented to assist in understanding the present invention. However, the following embodiments are merely illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications can be made within the scope of the present invention and the scope of the technical idea, and such changes and modifications naturally belong to the appended claims.

[0074] [Examples 1 to 8, Comparative Examples 1 to 4] Manufacture of Porous Carbon Material After placing the porous carbon material in a glass jar, nitrogen gas was injected and purging was carried out for 1 minute, and then the porous carbon material was irradiated with microwaves. At this time, the porous carbon materials used in Examples 1 to 8 and Comparative Examples 1 to 4 were as follows. And the microwave energy values (MPPT) received by each unit carbon material used in Examples 1 to 8 and Comparative Examples 1 to 4 are shown in Table 1 below.

[0075] [Table 1]

[0076] - Examples 1, 5, Comparative Example 1: Carbon nanotubes with a specific surface area of 270 m 2 / g - Examples 2, 6, Comparative Example 2: Multi-layer graphene flakes with a specific surface area of 1,600 m 2 / g - Examples 3, 7, Comparative Example 3: Carbon black with a specific surface area of 1,550 m 2 / g - Examples 4, 8, Comparative Example 4: Ketjen black with a specific surface area of 1,350 m 2 / g

[0077] [Comparative Example 5] Porous Carbon Material Carbon nanotubes with the same specific surface area of 270 m 2 / g as those used in Examples 1, 5 and Comparative Example 1 were prepared. That is, unlike Examples 1 to 8 and Comparative Examples 1 to 4, the carbon material was not irradiated with microwaves.

[0078] [Experimental Example 1] Evaluation of Removal Rate of Impurities Contained in Porous Carbon Material The content of impurities removed from each of the porous carbon materials produced in Examples 1 to 8 and Comparative Examples 1 to 4 was measured, and the results are shown in Table 2 below. That is, it was confirmed how much the content of impurities contained in the carbon material decreased after irradiation with microwaves. For the measurement, a precision balance (model: ML204T / 00, manufacturer: Mettler Toledo) with a measurement unit of 0.1 mg was used.

[0079]

Table 2

[0080] As a result of measuring the content of impurities removed from each of the porous carbon materials produced in Examples 1 to 8 and Comparative Examples 1 to 4 as described above, as shown in Table 1, it was confirmed that the impurities in the porous carbon materials of Examples 1 to 8, in which the microwave energy value received per unit carbon material was set to 2,000 to 10,000 W*s / g, were completely removed. On the other hand, it was found that the removal of impurities in the porous carbon materials of Comparative Examples 1 to 3, in which the microwave energy value received per unit carbon material was set to less than 2,000 W*s / g, was insufficient. Further, in the case of Comparative Example 4, in which the microwave energy value received per unit carbon material was set to exceed 10,000 W*s / g, although the impurities were completely removed as in Examples 1 to 8, a firing phenomenon also occurred.

[0081] [Experimental Example 2] Confirmation of Presence or Absence of Impurity Removal through Thermogravimetric Analysis (TGA) To confirm the presence or absence of impurity removal in the porous carbon material produced in Example 1, TGA analysis (heating rate of 10 °C / min (R.T to 500 °C), nitrogen atmosphere) was performed together with the porous carbon material of Comparative Example 5 (untreated with microwaves).

[0082] Figure 1 is a TGA analysis graph for confirming the presence or absence of impurity removal of the porous carbon material according to an embodiment of the present invention and a comparative example. As a result of TGA analysis of the porous carbon material produced in Example 1 and the porous carbon material of Comparative Example 5, the porous carbon material of Example 1 with the microwave energy value received by the unit carbon material set at 2,000 to 10,000 W*s / g showed significantly less weight loss due to temperature increase compared to the porous carbon material of Comparative Example 5 without microwave irradiation as shown in Figure 1. Therefore, it can be seen that impurities such as moisture are removed when a microwave energy value of 2,000 to 10,000 W*s / g is applied to the porous carbon material (on the other hand, the reason for the weight loss up to 100°C in Figure 1 is due to moisture being absorbed during the storage of the carbon material).

[0083] [Experimental Example 3] Confirmation of Presence or Absence of Impurity Removal through Elemental Analysis (EA) To confirm the presence or absence of impurity removal of the porous carbon material produced in Example 1, an EA analysis was performed using an elemental analyzer (Model: Flash 2000, Manufacturer: Thermo Scientific TM ) together with the porous carbon material of Comparative Example 5 (untreated with microwaves), and the results are shown in Table 3 below.

[0084] [Table 3]

[0085] As a result of EA analysis of the porous carbon material produced in Example 1 and the porous carbon material of Comparative Example 5, it was confirmed that the content of carbon (C) increased due to the removal of impurities such as moisture in the porous carbon material of Example 1 with the microwave energy value received by the unit carbon material set at 2,000 to 10,000 W*s / g.

[0086] [Comparative Example 6] Manufacture of Sulfur-Carbon Composite The carbon nanotubes of Comparative Example 5 that had not been microwave-treated and sulfur (S) were mixed at a weight ratio of 75:25, and then dried to produce a sulfur-carbon composite. Subsequently, the produced sulfur-carbon composite was placed in a glass jar, and nitrogen gas was injected to conduct purging for 1 minute, and then microwave irradiation was performed (MPPT: 2,160 W*s / g).

[0087] [Experimental Example 4] Evaluation of Temperature Profile by Microwave Irradiation Time While irradiating the porous carbon material produced in Example 1 and the sulfur-carbon composite produced in Comparative Example 6 with microwaves (MPPT: 2,160 W*s / g), the temperature over time was measured with a Thermocouple (306 data logger, Conrad Electronics, Hirschau, Germany), and the results are shown in Figure 2. Figure 2 is a graph showing the temperature profile over time when irradiating the porous carbon material and the sulfur-carbon composite with microwaves under the same conditions.

[0088] As a result of measuring the temperature in 5-second units while irradiating the porous carbon material of Example 1 and the sulfur-carbon composite of Comparative Example 6 with microwaves, as shown in Figure 2, it was confirmed that the heating rate of the porous carbon material (Example 1) was much faster than the heating rate of the sulfur-carbon composite (Comparative Example 6). That is, when irradiated with microwaves, the heating rate of carbon is faster than that of sulfur, and the temperature of carbon rises first. The energy generated when the temperature of carbon rises is transmitted to sulfur and converted into energy for sulfur to vaporize. At this time, sulfur vaporizes from a lower temperature before the impurities contained in carbon are removed.

[0089] In other words, when irradiating microwaves under the same MPPT conditions, in the case of the porous carbon material (Example 1), it rises to about 400 °C based on the time point of 30 seconds of microwave irradiation, and impurities in the carbon material are removed. On the other hand, for the sulfur-carbon composite (Comparative Example 6), based on the time point of 30 seconds of microwave irradiation, it is about 200 °C (the temperature at which sulfur volatilization occurs rapidly), and sulfur is volatilized together with impurities in the carbon material. Therefore, if microwaves are applied to the sulfur-carbon composite (Comparative Example 6) itself, not only is it impossible to selectively remove only the impurities in the carbon material, but on the contrary, it will cause sulfur loss and only have an adverse effect on battery performance. Therefore, it can be seen that even if microwaves are applied to the carbon material, the object of the present invention can be achieved by applying them only to the carbon material itself that is not in a state of being complexed with sulfur.

[0090] [Experimental Example 5] Thermogravimetric Analysis (TGA) In the Experimental Example 2, it has been confirmed through thermogravimetric analysis (TGA) that impurities such as moisture are removed from the porous carbon material of Example 1. And for comparison and contrast with this, TGA analysis (heating rate of 10 °C / min (R.T~500 °C), nitrogen atmosphere) was also carried out on the sulfur-carbon composite of Comparative Example 6.

[0091] Figure 3 is a TGA analysis graph showing the degree of weight loss due to temperature increase by performing TGA analysis on the porous carbon material according to an embodiment of the present invention and the sulfur-carbon composite according to the comparative example. As a result of performing TGA analysis on the porous carbon material of Example 1 and the sulfur-carbon composite of Comparative Example 6, as shown in Figure 3, it can be confirmed that the porous carbon material of Example 1 shows significantly less weight loss due to temperature increase compared to the sulfur-carbon composite of Comparative Example 6. This is because, as explained in Experimental Example 4, even when irradiating microwaves under the same MPPT conditions to the porous carbon material (Example 1) and the sulfur-carbon composite (Comparative Example 6), the sulfur-carbon composite (Comparative Example 6) volatilizes sulfur together with the impurities in the carbon material.

[0092] That is, even through these experimental results, it can be confirmed that it is impossible to selectively remove only the impurities contained in the carbon material in the sulfur-carbon composite, and instead, it only causes sulfur loss and has an adverse effect on battery performance. Therefore, it can be understood that the object of the present invention can be achieved by applying microwaves only to the carbon material itself that is not in a state of being compounded with sulfur even when microwaves are applied to the carbon material.

Claims

1. (a) placing a porous carbon material having a specific surface area of 200 to 1,700 m ２ / g in a sealed container and then injecting an inert gas for purging; and (b) applying microwaves to the porous carbon material; A method for producing a porous carbon material from which impurities have been removed, comprising: In the step (b), microwaves are applied to the porous carbon material under the condition that the MPPT according to the following formula 1 is 2,000 W*s / g to 10,000 W*s / g: A method for producing a porous carbon material from which impurities have been removed: [Formula 1] MPPT (W*s / g) = Microwave Power (W) × Times (s) / Carbon material mass (g) In the formula 1, the time (Times) exceeds 10 seconds as the time in seconds when microwaves are applied.

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

3. The porous carbon material is selected from the group consisting of carbon nanotubes, graphene (multilayer graphene flake, MGF), carbon black, and ketjen black. The production method according to claim 2, characterized in that it is selected from the group consisting of carbon black; carbon fiber; and a mixture containing two or more of these.

4. The production method according to claim 1, characterized in that the pore volume of the porous carbon material is 1.5 cm3 / g or more.

5. The production method according to claim 1, characterized in that 90% to 100% of the total impurities contained in the porous carbon material have been removed.

6. The production method according to claim 5, characterized in that 99% to 100% of the total impurities contained in the porous carbon material have been removed.

7. The production method according to claim 1, characterized in that the impurities include moisture and functional groups present inside and on the surface of the carbon material.

8. Including the production method according to claim 1, A method for producing a positive electrode for a lithium-sulfur battery, which includes a sulfur-carbon composite in which sulfur is supported on the porous carbon material from which impurities have been removed as a positive electrode active material.

9. The method for manufacturing a positive electrode for a lithium-sulfur battery according to claim 8, characterized in that there is no error in the content of sulfur and carbon material contained in the positive electrode active material.

10. Including the manufacturing method according to claim 8, The method for manufacturing a lithium-sulfur battery, comprising: the positive electrode for a lithium-sulfur battery; a negative electrode; a separator interposed therebetween; and an electrolyte containing a first solvent containing a fluorine-based ether compound, a second solvent containing a glyme-based compound, and a lithium salt.

11. The method for manufacturing a lithium-sulfur battery according to claim 10, characterized in that the sulfur utilization rate contained in the positive electrode is 90% or more of the theoretical discharge capacity.

12. The method for manufacturing a lithium-sulfur battery according to claim 10, characterized in that the energy density of the lithium-sulfur battery is 400 Wh / kg or more or 600 Wh / L or more.

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