KR20210010334A - Lithium-sulfur secondary battery - Google Patents

Abstract

The present invention relates to a lithium-sulfur secondary battery and, more specifically, to a lithium-sulfur secondary battery, wherein a positive electrode includes a sulfur-carbon composite containing a porous carbon material into which a catalyst point is introduced, and the conditions of the positive electrode and an electrolyte are specified, thereby capable of realizing high energy density compared to a conventional lithium-sulfur battery.

Description

Lithium-sulfur secondary battery {LITHIUM-SULFUR SECONDARY BATTERY}

The present invention relates to a lithium-sulfur secondary battery.

As the application area of secondary batteries expands to electric vehicles (EV) or energy storage systems (ESS), lithium-ion has a relatively low energy storage density (~250 Wh/kg) compared to its weight. Ion secondary batteries have limitations in their application to these products. In contrast, lithium-sulfur secondary batteries are in the spotlight as a next-generation secondary battery technology because they can theoretically realize a high energy storage density (~2,600 Wh/kg) to weight.

The lithium-sulfur secondary battery is a battery system using a sulfur-based material having a sulfur-sulfur bond as a positive electrode active material and lithium metal as a negative electrode active material. This lithium-sulfur secondary battery has the advantage that sulfur, which is the main material of the positive electrode active material, has an abundance of resources worldwide, is non-toxic, and has a low weight per atom.

In a lithium-sulfur secondary battery, lithium, which is a negative electrode active material, releases electrons and is oxidized by ionization during discharge, and a sulfur-based material, which is a positive electrode active material, accepts electrons and is reduced. At this time, the oxidation reaction of lithium is a process in which lithium metal releases electrons and is converted into lithium cation form. In addition, the reduction reaction of sulfur is a process in which the sulfur-sulfur bond accepts two electrons and is converted into a sulfur anion form. The lithium cation generated by the oxidation reaction of lithium is transferred to the positive electrode through the electrolyte, and forms a salt by combining with the sulfur anion generated by the reduction reaction of sulfur. Specifically, sulfur before discharge has a cyclic S ₈ structure, which is converted into lithium polysulfide (Li ₂ S _x , x = 8, 6, 4, 2) by a reduction reaction, and this lithium polysulfide When is completely reduced, lithium sulfide (Li ₂ S) is finally produced.

Sulfur, which is a positive active material, has low electrical conductivity, so it is difficult to secure reactivity with electrons and lithium ions in a solid-state form. Existing lithium-sulfur secondary batteries induce a liquid-state reaction and improve reactivity by generating an intermediate polysulfide in the form of Li ₂ S _x to improve the reactivity of sulfur. In this case, an ether solvent such as dioxolane and dimethoxyethane having high solubility in lithium polysulfide is used as a solvent for the electrolyte solution. In addition, the existing lithium-sulfur secondary battery builds a catholyte-type lithium-sulfur secondary battery system to improve reactivity. In this case, due to the characteristic of lithium polysulfide, which is easily soluble in the electrolyte, the amount of sulfur Reactivity and lifetime characteristics will be affected. In addition, in order to build a high energy density, a low content of electrolyte must be injected, but as the electrolyte content decreases, the concentration of lithium polysulfide in the electrolyte increases. It is difficult.

Since the elution of lithium polysulfide adversely affects the capacity and life characteristics of the battery, various techniques have been proposed for suppressing the elution of lithium polysulfide.

For example, Korean Patent Laid-Open No. 2016-0037084 uses a carbon nanotube aggregate of a three-dimensional structure coated with graphene with a carbon material to block the melting of lithium polysulfide and improve the conductivity of the sulfur-carbon nanotube composite. It discloses that it can be improved.

In addition, Korean Patent Registration No.1379716 uses a graphene composite containing sulfur prepared by treating graphene with hydrofluoric acid to form pores on the surface of graphene and growing sulfur particles in the pores as a positive electrode active material. It is disclosed that lithium polysulfide elution can be suppressed to minimize a decrease in capacity of the battery.

These patents prevented the elution of lithium polysulfide by varying the structure or material of the sulfur-carbon composite used as the positive electrode active material to some extent improve the performance degradation problem of the lithium-sulfur secondary battery, but the effect is not sufficient. Therefore, in order to build a lithium-sulfur secondary battery with high energy density, a battery system capable of driving an electrode with high loading and low porosity is required, and research on such a battery system is continuously conducted in the relevant technical field. .

Republic of Korea Patent Publication No. 2016-0037084 (2016.04.05), sulfur-carbon nanotube composite, manufacturing method thereof, cathode active material for lithium-sulfur battery including the same, and lithium-sulfur battery including the same Republic of Korea Patent Registration No. 1379716 (2014.03.25), a lithium-sulfur secondary battery including a graphene composite anode containing sulfur and a method for manufacturing the same

Abbas Fotouhi et al., Lithium-Sulfur Battery Technology Readiness and Applications-A Review, Energies 2017, 10, 1937.

Accordingly, the present inventors conducted various studies to solve the above problem. As a result of applying a sulfur-carbon composite including a porous carbon material into which a catalyst point was introduced as a positive electrode active material, and controlling the positive electrode and the electrolyte under specific conditions, high energy The present invention was completed by confirming that a lithium-sulfur secondary battery having a density can be implemented.

Accordingly, an object of the present invention is to provide a lithium-sulfur secondary battery having excellent energy density.

In order to achieve the above object, the present invention is a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode includes a sulfur-carbon composite including a porous carbon material into which a catalyst point is introduced, and the following math Provides a lithium-sulfur secondary battery with an SC factor of 0.45 or more represented by Equation 1:

[Equation 1]

(In Equation 1, P, L, and α follow the description in the specification.).

The catalyst point may include a transition metal complex.

The catalyst point may include at least one selected from the group consisting of iron phthalocyanine, nickel phthalocyanine, manganese phthalocyanine, copper phthalocyanine, cobalt phthalocyanine, and zinc phthalocyanine.

The catalyst point may be included in an amount of 1 to 20% by weight based on 100% by weight of the total porous carbon material into which the catalyst point is introduced.

The catalyst point may be located on at least one of an outer surface and an inner surface of the pores of the porous carbon material.

The catalyst point may be bonded to the surface of the porous carbon material through π electron interaction.

The porous carbon material may include at least one selected from the group consisting of graphite, graphene, reduced graphene oxide, carbon black, carbon nanotubes, carbon fibers, graphite, and activated carbon.

The electrolyte solution includes a solvent and a lithium salt, and the solvent may include a first solvent having a DV ² factor value of 1.75 or less represented by Equation 2 below, and a second solvent that is a fluorinated ether solvent:

[Equation 2]

(In Equation 2, DV, μ, and γ follow the description in the specification.).

The first solvent may have a DV ² factor of 1.5 or less.

The lithium-sulfur secondary battery may have an NS factor value of 3.5 or less represented by Equation 3 below:

[Equation 3]

(In Equation 3, the SC factor and the DV ² factor follow the description in the specification.).

The lithium-sulfur secondary battery may have an ED factor value of 950 or more represented by Equation 4 below:

[Equation 4]

(In Equation 4, V, SC factors, C and D follow the description in the specification.).

The first solvent may include at least one selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine, and 1-iodopropane.

The second solvent is 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl tri Fluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H -It may contain one or more selected from the group consisting of perfluorodipropyl ether.

The solvent may include 1 to 50% by weight of the first solvent based on 100% by weight of the total solvent.

The solvent may include 50 to 99% by weight of the second solvent based on 100% by weight of the total solvent.

The solvent may include the first solvent and the second solvent in a weight ratio of 3:7 to 1:9.

The lithium-sulfur secondary battery according to the present invention exhibits a high energy density, which was difficult to implement with a conventional lithium-sulfur secondary battery by controlling the positive electrode and the electrolyte under specific conditions.

1 is a schematic diagram showing a longitudinal section of a porous carbon material into which a catalyst point is introduced according to an embodiment of the present invention.
2 is a schematic diagram showing a method of manufacturing a carbon material into which a catalyst point is introduced according to an embodiment of the present invention.
3 is a schematic diagram showing a longitudinal section of a sulfur-carbon composite according to an embodiment of the present invention.
4 is a graph showing the performance evaluation results at 25° C. of the lithium-sulfur secondary battery according to Experimental Example 1 of the present invention.
5 is a graph showing a performance evaluation result at 45° C. of a lithium-sulfur secondary battery according to Experimental Example 1 of the present invention.

Hereinafter, the present invention will be described in more detail.

The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The terms used in the present invention are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present invention, terms such as'include' or'have' are intended to designate the presence of features, numbers, steps, actions, components, parts, or a combination thereof described in the specification, but one or more other features It is to be understood that the presence or addition of elements or numbers, steps, actions, components, parts, or combinations thereof, does not preclude in advance.

The term “composite” as used herein refers to a material that exhibits more effective functions while forming different phases physically and chemically by combining two or more materials.

The term “polysulfide” as used herein refers to “polysulfide ion (S _x ^2- , x = 8, 6, 4, 2))” and “lithium polysulfide (Li ₂ S _x Or LiS _x ^- x = 8, 6, 4, 2)”.

For the physical properties described in the present specification, if the measurement conditions and methods are not specifically described, the physical properties are measured according to the measurement conditions and methods generally used by those skilled in the art.

Lithium-sulfur secondary batteries have a high discharge capacity and energy density among many secondary batteries, and sulfur used as a positive electrode active material has abundant reserves and is cheap, so the manufacturing cost of the battery can be lowered. It is in the spotlight as a battery.

However, in the case of the existing lithium-sulfur secondary battery system, the loss of sulfur occurs due to the inability to suppress the elution of lithium polysulfide as described above, and the amount of sulfur participating in the electrochemical reaction rapidly decreases. And not all of the theoretical energy density. In addition, the eluted lithium polysulfide, in addition to being suspended or precipitated in the electrolyte, reacts directly with lithium and is fixed in the form of Li ₂ S on the surface of the negative electrode, causing a problem of corroding the lithium metal negative electrode.

In the prior art, methods such as introducing a material capable of inhibiting the elution of lithium polysulfide into a positive electrode or separator in the form of an additive or protective layer, changing the structure or material of the positive electrode active material, and changing the composition of the electrolyte have been proposed. Not only the dissolution improvement effect was insignificant, but also the amount of sulfur, which is the positive electrode active material, is limited (ie, the loading amount), and it causes serious problems in the stability of the battery or is inefficient in terms of the process.

Therefore, in the present invention, in a lithium-sulfur secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, the porosity of the positive electrode active material layer (or a sulfur-carbon composite containing a porous carbon material into which a catalyst point is introduced as a positive electrode active material) is used. Porosity) is low, and a sulfur loading amount is high.

In general, when the porosity of the positive electrode is lowered and the loading amount of sulfur is increased, the energy density of a secondary battery including the same increases. However, in a lithium-sulfur secondary battery, if the porosity of the positive electrode is reduced to a minimum and the loading amount of sulfur is increased to the maximum, the proportion of electrolyte per unit sulfur content decreases. Therefore, when the positive electrode is applied to a lithium-sulfur secondary battery, the target It is difficult to implement performance.

Therefore, in the present invention, in order to improve the kinetics of the electrochemical reaction of sulfur, which is a positive electrode active material during charging and discharging of a lithium-sulfur secondary battery, a sulfur-carbon composite including a porous carbon material into which a catalyst point is introduced as a support for sulfur is prepared. By using as a positive electrode active material and specifying conditions related to sulfur in the positive electrode, a lithium-sulfur secondary battery having a higher energy density compared to the existing lithium-sulfur secondary battery when actually implemented is provided. In addition, the above-described energy density improvement effect is further increased by controlling the electrolyte solution to meet specific conditions together with the above configuration.

The positive electrode according to the present invention may include a positive electrode current collector and a positive electrode active material layer formed on one or both surfaces of the positive electrode current collector.

The positive electrode current collector supports the positive electrode active material layer, and is not particularly limited as long as it has high conductivity without causing chemical changes to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treated with carbon, nickel, silver, etc., aluminum-cadmium alloy, and the like may be used.

The positive electrode current collector may form fine irregularities on its surface to enhance the bonding strength with the positive electrode active material, and various forms such as films, sheets, foils, meshes, nets, porous bodies, foams, and nonwoven fabrics may be used.

The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 3 to 500 μm.

The positive electrode active material layer may include a positive electrode active material and optionally a conductive material and a binder.

The positive electrode active material includes a sulfur-based compound. The sulfur-based compound is inorganic sulfur (S ₈ ), Li ₂ S _n (n≥1), 2,5-dimercapto-1,3,4-thiadiazole (2,5-dimercapto-1,3,4 -thiadiazole), disulfide compounds such as 1,3,5-trithiocyanuic acid, organosulfur compounds, and carbon-sulfur polymers ((C ₂ S _x ) _n , x=2.5 To 50, n≥2) may be one or more selected from the group consisting of. Preferably, inorganic sulfur (S ₈ ) may be used.

Since the sulfur-based compound alone does not have electrical conductivity, it is applied in combination with a conductive material, a carbon material.

In the present invention, the positive electrode active material is a sulfur-carbon composite obtained by combining the sulfur-based compound and a porous carbon material, wherein the porous carbon material is a catalyst point showing a catalytic effect on the reduction reaction of sulfur, a positive electrode active material, on the inner and outer surfaces. It is characterized in that it is a porous carbon material containing. The catalytic point includes a transition metal complex, and since the catalytic activity for the reaction rate of the sulfur reduction reaction can be controlled by the transition metal complex, it may be referred to as a catalytic site.

Hereinafter, a porous carbon material into which a catalyst point according to the present invention is introduced will be described in more detail with reference to the drawings.

1 is a schematic diagram showing a longitudinal section of a porous carbon material into which a catalyst point is introduced according to an embodiment of the present invention.

1, a porous carbon material into which a catalyst point is introduced according to an embodiment of the present invention includes a porous carbon material 10; And it may include a catalyst point 20 bonded to the inner and outer surfaces of the porous carbon material.

In the porous carbon material into which the catalyst point according to the present invention is introduced, the porous carbon material 10 provides a skeleton in which the above-described sulfur-based compound can be uniformly and stably fixed, and the electrochemical reaction is smooth by supplementing the low electrical conductivity of sulfur. To be able to proceed.

The porous carbon material 10 is a structure in the form of particles including a plurality of pores 11 on the inner and outer surfaces, and is made of a material having high electrical conductivity and has a pore volume and ratio sufficient to promote the electrochemical reaction of sulfur. It has a surface area and serves as a support for the catalyst point 20 to maintain or improve the performance, durability, and efficiency of the catalyst point.

The porous carbon material 10 is manufactured by carbonizing precursors of various carbon materials, and any one that is commonly used in the relevant technical field may be used. For example, as the porous carbon material, graphite; Graphene; Reduced graphene oxide (rGO); Carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); Carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); It may be one or more selected from the group consisting of graphite, such as natural graphite, artificial graphite, and expanded graphite, and activated carbon, but is not limited thereto.

The porous carbon material 10 may have a spherical shape, a rod shape, a needle shape, a plate shape, a tube shape, or a bulk shape, and may be used without limitation as long as it is commonly used in the relevant technical field.

The pores 11 formed in the porous carbon material 10 are partially opened, and the positive electrode active material sulfur, specifically, the above-described sulfur-based compound may be supported in the pores 11. .

The porous carbon material 10 may be particles having an average particle diameter of 1 to 50 μm, preferably 5 to 30 μm. If it is less than the above range, the lithium ion transfer efficiency may be lowered due to penetration and wetting of the electrolyte, and if it exceeds the above range, the electrode pores relative to the electrode weight may increase, thereby increasing the volume.

The pores 11 formed in the porous carbon material 10 may be mesopores having an average diameter of 2 to 50 nm, preferably 2 to 45 nm, and more preferably 2 to 40 nm.

The volume of the pores 11 included in the porous carbon material 10 may be 0.5 to 3.5 cc/g, preferably 1.0 to 3.0 cc/g, and more preferably 1.5 to 2.5 cc/g.

If the average diameter and volume of the pores 11 included in the porous carbon material 10 are less than the above range, the pore clogging phenomenon during the impregnation of sulfur may not be uniformly supported in each pore 11, and, Due to the limitation of the pore volume, the amount of sulfur supported in the pores 11 may be reduced. Conversely, when the average diameter and volume of the pores 11 included in the porous carbon material 10 exceed the above range, it becomes a macro-pore, and lithium polysulfide, which is an intermediate product along with the reactant in the sulfur reduction reaction. May elute.

In addition, as the specific surface area of the porous carbon material 10 increases, it is advantageous for the reactivity with sulfur and the catalytic activity of the catalyst point 20 to be described later, and specifically 100 to 1200 m 2 /g, preferably 150 to It can be 500 ㎡/g. If it is less than the above range, not only the reactivity decreases due to a decrease in the contact area with sulfur, but also the catalytic activity may decrease. If it exceeds the above range, there is a problem of an increase in side reactions due to an excessive specific surface area and a problem of an increase in the amount of binder required for preparing the positive electrode slurry. There may be.

The catalyst point 20 is a transition metal complex formed by bonding a nitrogen atom to a transition metal, and serves as a catalyst for a reduction reaction of sulfur, thereby improving a reaction rate (kinetic).

The catalyst point 20 includes a transition metal complex including a bond between a transition metal and a nitrogen atom, and further, the catalyst point 20 is a bond between a transition metal and a nitrogen atom and a bond between the nitrogen atom and a carbon atom. Includes. In the catalyst point 20, it is preferable that the transition metal complex includes a transition metal and four nitrogen atoms bonded to the transition metal. If the number of nitrogen atoms bonded to the transition metal is less than 4, the activity as a catalyst may be deteriorated. When it exceeds 4, structural stability may be deteriorated and thus catalytic activity may be lowered. As described above, when nitrogen is bonded to a transition metal, its structure is stable and exhibits excellent catalytic properties. Therefore, it has very high stability and catalyst compared to the catalyst point formed by bonding of atoms other than nitrogen to the transition metal. It can have an effect.

The catalyst point 20 contains at least one transition metal selected from the group consisting of iron (Fe), nickel (Ni), manganese (Mn), copper (Cu), cobalt (Co) and zinc (Zn). However, it is not limited thereto as long as it is a transition metal capable of exhibiting catalytic activity for a reduction reaction of sulfur.

Specifically, the catalyst point 20 is a metal-phthalocyanine (MePc), for example, iron phthalocyanine (FePc), nickel phthalocyanine (NiPc), manganese phthalocyanine (manganese phthalocyanine). MnPc), copper phthalocyanine (CuPc), cobalt phthalocyanine (CoPc), and zinc phthalocyanine (ZnPc).

In the catalyst point 20, the molar ratio of the transition metal and nitrogen may be 1: 2 to 10, preferably 1: 2 to 8, more preferably 1: 3 to 5. If it is less than the above range, the inner and outer surfaces of the porous carbon material 10 cannot be sufficiently doped as needed, and if the amount exceeds the above range, the amount of nitrogen per unit weight of the porous carbon material 10 increases and the catalyst Activity may be reduced.

The catalyst point 20 is a molecular-level composite having a size of 0.1 to 1 nm, preferably 0.1 to 0.9 nm, more preferably 0.1 to 0.8 nm, and the inside of the pores 11 of the porous carbon material 10 There is no reduction in the volume and size of the pores 11 even if they are bonded on the surface, so that even if the active material is supported inside the pores 11, it is possible to prevent pore clogging.

The catalyst point 20 may be adsorbed or bonded to at least one of the outer surface of the porous carbon material 10 and the inner surface of the pore 11. Specifically, the catalyst point 20 may be adsorbed and bonded to the inner and outer surfaces of the porous carbon material 10 by π electron interaction (π-π interaction). The π-electron interaction is not a bond between a specific element, but a bond between a surface and a surface, so that it can exhibit stronger adsorption than other types of bonds, and accordingly, the catalyst point 20 is the porous carbon material 10 Even if it is bonded to the inner and outer surfaces of the porous carbon material 10, it is possible to maintain the original characteristics.

The catalyst point 20 may be included in an amount of 1 to 20% by weight, preferably 4 to 16% by weight, based on 100% by weight of the total porous carbon material into which the catalyst point of the present invention is introduced. If the content of the catalyst point 20 is out of the above range, the effect of improving the reaction rate of the sulfur reduction reaction is lowered, so that the effect of improving the battery performance may be insignificant or may not increase any more.

In particular, in the present invention, by using the porous carbon material into which the catalyst point as described above is introduced as a support for sulfur, the reaction rate of the reduction reaction of sulfur is improved, and thus the high performance of the lithium-sulfur secondary battery including the same can be realized. In addition, as a catalyst point including a relatively inexpensive transition metal is introduced into the surface of a porous carbon material instead of expensive platinum (Pt), which has been used as a conventional electrochemical catalyst, it may be advantageous for commercialization due to low cost. In addition, in a lithium-sulfur secondary battery system including an electrolyte to be described later, as a solid-state reactivity between sulfur and an electrolyte can be secured, the lithium-sulfur secondary battery exhibits improved discharge capacity characteristics. It makes it possible to achieve high energy density of secondary batteries.

Next, a method of manufacturing a porous carbon material into which a catalyst point according to the present invention is introduced will be described.

2 is a schematic diagram showing a method of manufacturing a carbon material into which a catalyst point is introduced according to an embodiment of the present invention.

Referring to FIG. 2, by adding carbon nanotubes as a porous carbon material to a catalyst point dispersion in which a transition metal composite is dispersed in an organic solvent, a porous carbon material having a catalyst point bonded to the surface of the carbon nanotubes is prepared. I can.

Specifically, the method of manufacturing the porous carbon material into which the catalyst point is introduced includes the steps of (S1) dispersing the transition metal composite in a solvent to prepare a catalyst point dispersion; (S2) adding and mixing a porous carbon material to the catalyst point dispersion obtained in step (S1); (S3) filtering the mixed solution obtained in the (S2) step, and (S4) drying the powder obtained in the filtering of the mixed solution after the (S3) step.

In the step (S1), a catalyst point dispersion may be prepared by dispersing (or dissolving) a transition metal complex including a transition metal and nitrogen in a solvent. Preferably, the transition metal complex is dispersed in a solvent and subjected to ultrasonic treatment to prepare a catalyst point dispersion.

The concentration of the catalyst point dispersion may be 5 to 15%, preferably 5 to 12%, and more preferably 5 to 10% based on the weight of the solid content. If it is less than the above range, the weight of the transition metal composite contained in the catalyst point dispersion is reduced, resulting in poor catalytic activity, and if it exceeds the above range, the weight of the transition metal composite increases, thereby clogging the pores of the porous carbon material.

The transition metal complex used in the catalyst point dispersion is metal-phthalocyanine (MePC), and the specific type is as described above.

The metal-phthalocyanine is a type of macrocyclic compound having a structure in which rings of a nitrogen atom and a carbon atom cross, and has a chemical structure in which metal ions are coordinated in the center. Since the metal-phthalocyanine is used as a catalyst point, it is possible to prepare a catalyst point including a transition metal complex having a stable structure in which four nitrogen atoms are bonded to the transition metal. In general, in order to bond four nitrogen atoms to the transition metal, it is necessary to undergo several steps such as reacting with a precursor material containing nitrogen atoms, and further reacting under ammonia (NH ₃ ) atmosphere. In the present invention, by using a transition metal complex with a metal-phthalocyanine having the chemical structure as described above, a catalyst point including a transition metal complex having a stable structure in which four nitrogen atoms are bonded to the transition metal as described above is obtained in a simple process. Can be manufactured.

The solvent used in the step (S1) is dimethyl carbonate, dimethyl formamide, N-methyl formamide, sulfolane (tetrahydrothiophene-1,1-dioxide), 3-methylsulfolane, N-butyl sulfone, dimethyl Sulfoxide, pyrolidinone (HEP), dimethylpiperidone (DMPD), N-methylpyrrolidinone (NMP), N-methyl acetamide, dimethyl acetamide (DMAc), N,N-dimethylformamide (DMF) , Diethyl acetamide (DEAc) Dipropylacetamide (DPAc), ethanol, propanol, butanol, hexanol, ethylene glycol, tetrachloroethylene, propylene glycol, toluene, trpentin, methyl acetate, ethyl acetate, petroleum ether, acetone , Cresol and glycerol may be one or more organic solvents selected from the group consisting of, preferably, the solubility of the transition metal complex may be high when using N,N-dimethylformamide as the solvent.

Subsequently, in the step (S2), a porous carbon material may be added to and mixed with the catalyst point dispersion obtained in the step (S1).

The porous carbon material used in step (S2) is as described above.

Through the step (S2), a process in which catalyst points are adsorbed and bonded (or catalyst points are introduced) on the surface of the porous carbon material is performed. That is, the catalyst points including the transition metal complex on the inner and outer surfaces of the porous carbon material are adsorbed and bonded through the π electron interaction.

When the porous carbon material is added to the catalyst point dispersion and mixed, if necessary, ultrasonic treatment is performed and then stirred to obtain a mixed solution.

At this time, the content of the catalyst point and the porous carbon material in the carbon material into which the produced catalyst point is introduced may be appropriately adjusted and used in the manufacturing process so that the content of the catalyst point and the porous carbon material satisfy the weight range as described above.

Subsequently, in step (S3), impurities may be removed by filtering the mixed solution obtained in step (S2). For the filtration, a general filtration method such as a vacuum pump may be applied mutatis mutandis, and after the filtration process is performed, a washing process using alcohol such as ethanol or the like may be additionally performed if necessary.

Subsequently, in the step (S4), a porous carbon material into which the catalyst point according to the present invention is introduced is finally prepared by drying the powder of the mixed solution obtained from the filtration in the step (S3).

The porous carbon material into which the catalyst point of the present invention is introduced is a structure in which catalyst points including a transition metal composite are bonded to the inner and outer surfaces of the porous carbon material, and in order to improve the bonding strength between the porous carbon material and the catalyst point, the drying is performed at 60 To 100° C., preferably 65 to 95° C., more preferably at a temperature of 70 to 90° C. for 10 to 14 hours, preferably 10.5 to 13.5 hours, and more preferably 11 to 13 hours.

Accordingly, the sulfur-carbon composite of the present invention, as shown in FIG. 3, includes the porous carbon material 10 and the sulfur-based compound 30 into which the catalyst point 20 is introduced.

The sulfur-carbon composite may include the sulfur-based compound in at least a portion of the porous carbon material into which the catalyst point is introduced and the porous carbon material into which the catalyst point is introduced.

In the sulfur-carbon composite, the porous carbon material into which the sulfur-based compound and the catalyst point are introduced may be simply mixed to form a composite, or may have a core-shell structure coated or supported form. The coating form of the core-shell structure is that any one of a sulfur-based compound or a porous carbon material into which a catalyst point is introduced is coated with another material, for example, the surface of a porous carbon material into which the catalyst point is introduced is wrapped with a sulfur-based compound or vice versa. Can be. In addition, the supported form may be a form in which a sulfur-based compound is supported inside the porous carbon material into which the catalyst point is introduced. The form of the sulfur-carbon composite may be any form as long as it satisfies the content ratio of the sulfur-based compound and the porous carbon material into which the catalyst point is introduced, which will be described later, and is not limited in the present invention.

In the sulfur-carbon composite, the sulfur-based compound may be included in an amount of 50 to 90% by weight, preferably 60 to 80% by weight, based on the total weight of the sulfur-carbon composite.

The porous carbon material into which the catalyst point is introduced in the sulfur-carbon composite may be included in an amount of 10 to 50% by weight, preferably 20 to 40% by weight, based on the total weight of the sulfur-carbon composite.

Accordingly, in the sulfur-carbon composite, the weight ratio of the porous carbon material into which the catalyst point is introduced and the sulfur-based compound may be 1:1 to 1:9, preferably 1:1.5 to 1:4. If the weight ratio is less than the above range, as the content of the porous carbon material into which the catalyst point is introduced increases, the amount of binder required for preparing the positive electrode slurry increases. Such an increase in the amount of the binder added eventually increases the sheet resistance of the electrode and acts as an insulator to prevent electron pass, thereby reducing cell performance. On the contrary, when the weight ratio is exceeded, the sulfur-based compounds are aggregated with each other, and it is difficult to receive electrons, so that it may be difficult to directly participate in the electrode reaction.

The conductive material included in the positive electrode active material layer is a material that serves as a path through which electrons move from a current collector to the positive electrode active material by electrically connecting the electrolyte solution to the positive electrode active material, and may be used without limitation as long as it has conductivity. .

For example, the conductive material includes carbon black such as Super-P (Super-P), Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon black; Carbon derivatives such as carbon nanotubes, graphene, and fullerene; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Alternatively, conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole may be used alone or in combination.

The content of the conductive material may be added in an amount of 0.01 to 30% by weight based on the total weight of the positive electrode active material layer.

The binder maintains the positive electrode active material in the positive electrode current collector, and organically connects the positive electrode active materials to increase the binding force therebetween, and any binder known in the art may be used.

For example, the binder may include a fluororesin binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); A rubber-based binder including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; Cellulose-based binders including carboxyl methyl cellulose (CMC), starch, hydroxy propyl cellulose, and regenerated cellulose; Poly alcohol-based binder; Polyolefin-based binders including polyethylene and polypropylene; Polyimide binder; Polyester binder; And a silane-based binder; one, two or more mixtures or copolymers selected from the group consisting of may be used.

The content of the binder may be added in an amount of 0.5 to 30% by weight based on the total weight of the positive electrode active material layer. If the content of the binder is less than 0.5% by weight, the physical properties of the positive electrode are deteriorated and the active material and the conductive material in the positive electrode may fall off, and if it exceeds 30% by weight, the ratio of the active material and the conductive material in the positive electrode is relatively reduced, resulting in a decrease in battery capacity can do.

The positive electrode can be manufactured by a conventional method known in the art. For example, after preparing a slurry by mixing and stirring additives such as a solvent and, if necessary, a binder, a conductive material, and a filler in the positive electrode active material, it is applied (coated) to the current collector of a metal material, compressed, and dried to form a positive electrode. Can be manufactured.

Specifically, first, the binder is dissolved in a solvent for preparing a slurry, and then a conductive material is dispersed. As a solvent for preparing the slurry, a positive electrode active material, a binder, and a conductive material can be uniformly dispersed, and it is preferable to use an easily evaporated solvent. Typically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, iso You can use propyl alcohol or the like. Next, a positive electrode active material, or optionally together with an additive, is uniformly dispersed again in a solvent in which the conductive material is dispersed to prepare a positive electrode slurry. The amount of a solvent, a positive electrode active material, or optionally an additive contained in the slurry does not have a particularly important meaning in the present application, and it is sufficient to have an appropriate viscosity to facilitate the application of the slurry. The slurry thus prepared is applied to a current collector and dried to form a positive electrode. The slurry may be applied to the current collector in an appropriate thickness according to the viscosity of the slurry and the thickness of the positive electrode to be formed.

The coating may be performed by a method commonly known in the art, but for example, after distributing the positive electrode active material slurry to the upper surface of one side of the positive electrode current collector, it may be uniformly dispersed using a doctor blade or the like. I can. In addition, it may be performed through a method such as die casting, comma coating, and screen printing.

The drying is not particularly limited, but may be performed in a vacuum oven at 50 to 200° C. within 1 day.

The positive electrode of the present invention manufactured by the above-described composition and method is classified by the SC factor value represented by Equation 1 below.

[Equation 1]

(In Equation 1,

P is the porosity (%) of the positive electrode active material layer in the positive electrode,

L is the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode (mg/cm2),

α is 10 (constant)).

The lithium-sulfur secondary battery according to the present invention realizes a high energy density by organic combination of not only the above-described positive electrode but also the negative electrode, separator, and electrolyte, and according to the present invention, the lithium-sulfur secondary battery realizes high energy density. To do this, the SC factor value may be 0.45 or higher, preferably 0.5 or higher. In the present invention, the upper limit of the SC factor value is not particularly limited, but considering the actual driving of the lithium-sulfur secondary battery, the SC factor value may be 4.5 or less. When the SC factor value is 0.45 or more, the performance of the existing lithium-sulfur secondary battery, such as energy density, decreases during actual driving, but in the case of the lithium-sulfur secondary battery according to the present invention, the performance of the battery even during actual driving. It is kept without deterioration.

The negative electrode according to the present invention may include a negative electrode current collector and a negative active material layer formed on one or both sides thereof. Alternatively, the negative electrode may be a lithium metal plate.

The negative active material layer may include a negative active material and optionally a conductive material and a binder.

The negative active material is a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, lithium metal or lithium alloy You can use

The material capable of reversibly intercalating or deintercalating lithium ions may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof.

A material capable of reversibly forming a lithium-containing compound by reacting with the lithium ions may be, for example, tin oxide, titanium nitrate, or silicon.

The lithium alloy is, for example, lithium (Li) and sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium ( It may be an alloy of a metal selected from the group consisting of Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

The configuration of a current collector, a conductive material, a binder, and the like, except for the negative active material, and a method of manufacturing the negative electrode may include materials and methods used in the positive electrode described above.

The separator according to the present invention is a physical separator having a function of physically separating an anode and a cathode, and if it is used as a conventional separator, it can be used without particular limitation. In particular, it has a low resistance against ion migration of the electrolyte and has an electrolyte moisture-absorbing ability. It is desirable to be excellent.

In addition, the separator separates or insulates the positive electrode and the negative electrode from each other and enables transport of lithium ions between the positive electrode and the negative electrode. Such a separator has a porosity of 30 to 50% and may be made of a non-conductive or insulating material.

Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer may be used. In addition, nonwoven fabrics made of high melting point glass fibers or the like can be used. Among these, a porous polymer film is preferably used.

If a polymer film is used for both the buffer layer and the separator, the impregnation amount of the electrolyte solution and the ion conduction characteristics decrease, and the effect of reducing the overvoltage and improving the capacity characteristics becomes insignificant. Conversely, when all non-woven materials are used, mechanical rigidity cannot be secured, resulting in a problem of a battery short circuit. However, when a film-type separator and a polymer nonwoven buffer layer are used together, it is possible to secure mechanical strength as well as an effect of improving battery performance due to the adoption of the buffer layer.

Preferably, in the present invention, an ethylene homopolymer (polyethylene) polymer film is used as a separator, and a polyimide nonwoven fabric is used as a buffer layer. In this case, the polyethylene polymer film preferably has a thickness of 10 to 25 µm and a porosity of 40 to 50%.

The electrolytic solution according to the present invention is a non-aqueous electrolytic solution containing a lithium salt and is composed of a lithium salt and a solvent. The electrolyte solution has a density of less than 1.5 g/cm 3. When the electrolyte has a density of 1.5 g/cm 3 or more, it is difficult to achieve a high energy density of a lithium-sulfur secondary battery due to an increase in weight of the electrolyte.

The lithium salt is a material that can be easily dissolved in a non-aqueous organic solvent, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiB(Ph) ₄ , LiC ₄ BO ₈ , LiPF ₆ , _{LiCF 3 SO 3, LiCF 3 CO} 2, LiAsF 6, LiSbF 6, LiAlCl 4, LiSO 3 CH 3, LiSO 3 CF 3, LiSCN, LiC (CF 3 SO 2) 3, LiN (CF 3 SO 2) 2, LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN(SO ₂ F) ₂ , lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium tetraphenylborate, and lithium imide. In one embodiment of the present invention, the lithium salt may be a lithium imide such as LiTFSI.

The concentration of the lithium salt is 0.1 to 8.0 M, depending on various factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, working temperature and other factors known in the lithium secondary battery field. , Preferably it may be 0.5 to 5.0 M, more preferably 1.0 to 3.0 M. If the concentration of the lithium salt is less than the above range, the conductivity of the electrolyte may be lowered and battery performance may be deteriorated. If the concentration of the lithium salt is less than the above range, the viscosity of the electrolyte may increase and the mobility of lithium ions (Li ⁺ ) may decrease. It is desirable to select an appropriate concentration.

The solvent includes a first solvent and a second solvent. The first solvent is characterized by having the highest dipole moment per unit volume among the constituents contained in the solvent at least 1% by weight, and thus has a high dipole moment and a low viscosity. When a solvent having a high dipole moment is used, it has an effect of improving the solid state reactivity of sulfur, and this effect can be excellently expressed when the solvent itself has a low viscosity. In the present invention, the first solvent is classified by the DV ² factor represented by Equation 2 below.

[Equation 2]

(In Equation 2,

DV is the dipole moment per unit volume (D·㏖/L),

μ is the viscosity of the solvent (cP, 25 °C),

γ is 100 (constant)).

According to the present invention, the DV ² factor value may be 1.75 or less, preferably 1.5 or less. In the present invention, the lower limit of the DV ² factor value is not particularly limited, but when considering the actual driving of the lithium-sulfur secondary battery, the DV ² factor value may be 0.1 or more. When a solvent having a DV ² factor of 1.75 or less, such as the first solvent, is mixed, the functionality of the electrolyte can be maintained even when a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material is applied to a lithium-sulfur battery. So, the performance of the battery does not deteriorate.

In the present invention, if the first solvent is included in the range of the DV ² factor, the type is not particularly limited, but propionitrile, dimethylacetamide, dimethylformamide, gamma- It may contain at least one selected from the group consisting of butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine), and 1-iodopropane (1-iodopropane).

The first solvent may be included in an amount of 1 to 50% by weight, preferably 5 to 40% by weight, and more preferably 10 to 30% by weight, based on the total weight of the solvent constituting the electrolyte. When the solvent according to the present invention contains the first solvent within the above-described weight% range, even when used with a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material may have an effect of improving battery performance.

The lithium-sulfur secondary battery of the present invention may be further classified by an NS factor obtained by combining the SC factor and the DV ² factor. The NS factor is represented by Equation 3 below.

[Equation 3]

(In Equation 3,

SC factor is the same as the value defined by Equation 1,

The DV ² factor is the same as the value defined by Equation 2.).

In the present invention, the value of the NS factor may be 3.5 or less, preferably 3.0 or less, and more preferably 2.7 or less. In the present invention, the lower limit of the value of the NS factor is not particularly limited, but when considering the driving of an actual lithium-sulfur secondary battery, the value of the NS factor may be 0.1 or more. When the NS factor value is adjusted within the above range, the effect of improving the performance of the lithium-sulfur secondary battery may be more excellent.

In the present invention, the second solvent is a fluorinated ether solvent. Conventionally, a solvent such as dimethoxyethane and dimethylcarbonate has been used as a diluent to control the viscosity of the electrolyte solution.When such a solvent is used as a diluent, high loading as in the present invention , It is not possible to drive a battery containing a positive electrode having a low porosity. Therefore, in the present invention, the second solvent is added together with the first solvent to drive the anode according to the present invention. If the second solvent is a fluorinated ether-based solvent generally used in the relevant technical field, the type is not particularly limited, but 1H,1H,2'H,3H-decafluorodipropyl ether (1H,1H,2' H,3H-Decafluorodipropyl ether), difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl trifluoro Methyl ether (1,2,2,2-Tetrafluoroethyl trifluoromethyl ether), 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether (1,1,2,3,3,3- Hexafluoropropyl difluoromethyl ether), pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H-perfluorodipropyl ether (1H,1H, 2′H-Perfluorodipropyl ether) may include at least one selected from the group consisting of.

The second solvent may be included in an amount of 50 to 99% by weight, preferably 60 to 95% by weight, more preferably 70 to 90% by weight, based on the total weight of the solvent constituting the electrolyte. When the solvent according to the present invention contains the second solvent within the above-described weight% range, the effect of improving the performance of the battery even when used with a positive electrode having a low porosity and a high loading amount of sulfur, a positive electrode active material, like the first solvent. Can have When mixing the first solvent and the second solvent, the second solvent may be included in the electrolyte in an amount equal to or greater than the first solvent in consideration of an effect of improving the performance of the battery. According to the present invention, the solvent may contain a first solvent and a second solvent in a weight ratio of 1:1 to 1:9, preferably 3:7 to 1:9 (first solvent: second solvent).

The non-aqueous electrolyte solution for a lithium-sulfur battery of the present invention may further include nitric acid or nitrous acid-based compounds as an additive. The nitric acid or nitrous acid-based compound has an effect of forming a stable film on the lithium electrode and improving charging/discharging efficiency. Such nitric acid or nitrite-based compounds are not particularly limited in the present invention, but lithium nitrate (LiNO ₃ ), potassium nitrate (KNO ₃ ), cesium nitrate (CsNO ₃ ), barium nitrate (Ba(NO ₃ ) ₂ ), ammonium nitrate Inorganic nitric acid or nitrite compounds such as (NH ₄ NO ₃ ), lithium nitrite (LiNO ₂ ), potassium nitrite (KNO ₂ ), cesium nitrite (CsNO ₂ ), and ammonium nitrite (NH ₄ NO ₂ ); Organic nitric acids such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite Or nitrous acid compounds; One selected from the group consisting of organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitro pyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations thereof is possible, and preferably Uses lithium nitrate.

In addition, the electrolyte may further contain other additives for the purpose of improving charge/discharge characteristics, flame retardancy, and the like. Examples of the additives include pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazoli Dione, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS), vinylene carbonate ( VC) and the like.

The lithium-sulfur secondary battery according to the present invention is classified by the ED factor value represented by Equation 4 below.

[Equation 4]

(In Equation 4,

V is the discharge nominal voltage (V) for Li/Li ⁺ ,

SC factor is the same as the value defined by Equation 1,

C is the discharge capacity (mAh/g) at the time of discharge at 0.1 C rate,

D is the density of the electrolyte solution (g/cm 3 ).

The higher the value of the ED factor, the higher the energy density can be realized in the actual lithium-sulfur secondary battery. According to the present invention, the ED factor value may be 950 or more, preferably 1100 or more, and more preferably 1150 or more. In the present invention, the upper limit of the ED factor value is not particularly limited, but when considering the actual driving of the lithium-sulfur secondary battery, the ED factor value may be 10000 or less. The range of the ED factor value means that the lithium-sulfur secondary battery according to the present invention can realize an improved energy density than the conventional lithium-sulfur secondary battery.

The lithium-sulfur secondary battery of the present invention may be prepared by disposing a separator between a positive electrode and a negative electrode to form an electrode assembly, and the electrode assembly may be placed in a cylindrical battery case or a rectangular battery case, and then an electrolyte is injected. Alternatively, after laminating the electrode assembly, the electrode assembly may be impregnated with an electrolyte, and the resulting product may be sealed by putting it in a battery case.

In addition, the present invention provides a battery module including the lithium-sulfur secondary battery as a unit cell.

The battery module can be used as a power source for medium and large-sized devices that require high temperature stability, long cycle characteristics, and high capacity characteristics.

Examples of the medium and large-sized devices include a power tool that is powered by an omniscient motor and moves; Electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; Electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters); Electric golf cart; Power storage systems, etc., but are not limited thereto.

Hereinafter, a preferred embodiment is presented to aid the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention, It is natural that such modifications and modifications fall within the appended claims.

Manufacturing example

[Production Example 1]

After dispersing 60 mg of iron phthalocyanine (FePc, manufactured by Sigma Aldrich) as a catalyst point in 500 ml of N,N-dimethylformamide (DMF) as a solvent, bath sonication was performed for 10 minutes to perform iron phthalocyanine ( FePc) dispersion was prepared.

Subsequently, after adding 940 mg of carbon nanotubes (manufactured by CNano) to the iron phthalocyanine (FePc) dispersion prepared above, ultrasonic waves were performed in a bath for 10 minutes, and the mixture was stirred at room temperature at a speed of 500 rpm for 4 hours. Got it.

Subsequently, the mixed solution prepared above was filtered with a vacuum pump, washed with 1000 ml of ethanol, and the upper powder of the mixed solution obtained from the filtration and washing was dried at 80° C. for 12 hours, and iron phthalocyanine (FePc) as a catalyst point The introduced carbon nanotubes (FePc-CNT) were prepared.

Examples and Comparative Examples

[Example 1]

Sulfur (S ₈ ) and carbon nanotubes (FePc-CNT) into which the catalyst point prepared in Preparation Example 1 was introduced were evenly mixed at 70:30 parts by weight to prepare a sulfur-carbon mixture, and then in an oven at 155° C. Leave for a minute to prepare a sulfur-carbon composite.

A slurry for forming a positive active material layer was prepared by mixing the prepared sulfur-carbon composite, a conductive material, and a binder. At this time, vapor-grown carbon fiber as a conductive material and styrene butadiene rubber/carboxymethyl cellulose (SBR/CMC 7:3) were used as a binder, respectively. At this time, the mixing ratio was such that the sulfur-carbon composite: conductive material: binder was 90:5:5 by weight.

The thus-prepared slurry was applied to a current collector of 20 μm-thick aluminum foil and dried to prepare a positive electrode (energy density of the positive electrode: 4.3 mAh/cm 2 ). The porosity of the positive electrode active material layer calculated by measuring the electrode weight and the electrode thickness (using TESA-μHITE equipment) in the prepared positive electrode was 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 3.0 mg/cm 2. The SC factor value calculated based on this was 0.5.

After positioning the positive electrode and the negative electrode prepared by the above-described method to face each other, an electrode assembly was manufactured by interposing a separator therebetween. At this time, a lithium foil having a thickness of 60 μm was used as the negative electrode, and polyethylene having a thickness of 20 μm and a porosity of 45% was used as a separator.

Subsequently, after placing the electrode assembly inside the case, an electrolyte was injected to prepare a lithium-sulfur secondary battery. At this time, the electrolyte was prepared by dissolving lithium bis (trifluoromethyl sulfonyl) imide (LiTFSI) at a concentration of 3 M in an organic solvent, wherein the organic solvent was propionitrile (first solvent) and 1H,1H, A solvent in which 2'H,3H-decafluorodipropyl ether (second solvent) was mixed in a 3:7 weight ratio was used. The dipole moment per unit volume in the first solvent was 97.1 D·mol/L, and the viscosity of the solvent measured using a BROOKFIELD AMETEK LVDV2T-CP viscometer was 0.38 cP (25° C.). The DV ² factor value calculated based on this was 0.39. Charging and discharging of the prepared battery was performed at two temperatures of 25°C and 45°C.

[Comparative Example 1]

When preparing a sulfur-carbon composite, lithium was used in the same manner as in Example 1, except that a conventional carbon nanotube (manufactured by CNano) was used instead of the carbon nanotube into which the catalyst point of Preparation Example 1 was introduced. A secondary battery was prepared. At this time, the porosity of the positive electrode active material layer calculated by measuring the electrode weight and the electrode thickness (using TESA-μHITE equipment) in the prepared positive electrode was 60%, and the mass of sulfur per unit area of the positive electrode active material layer was 3.0 mg/cm 2. The SC factor value calculated based on this was 0.5.

[Comparative Example 2]

A lithium secondary battery was manufactured in the same manner as in Example 1, except that an electrolyte solution including the second electrolyte composition was used. The electrolyte solution containing the second electrolyte composition was prepared by dissolving lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) and 1% by weight of lithium nitrate at a concentration of 1 M in an organic solvent, wherein the organic solvent is 1, A solvent obtained by mixing 3-dioxolane (first solvent) and dimethoxyethane (second solvent) at a weight ratio of 5:5 was used. In the first solvent, the dipole moment per unit volume was 97.1 D·mol/L, and the viscosity of the solvent measured by using a BROOKFIELD AMETEK LVDV2T-CP viscometer was 0.38 cP (25° C.). The DV ² factor value calculated based on this was 1.77. Charging and discharging of the prepared battery was performed at 25°C.

The conditions of Examples and Comparative Examples are summarized and shown in Table 1 below.

Electrolyte composition SC factor DV ² factor NS factor ED factor
(0.1C) Example 1
(25 ℃) Composition of the first electrolyte ¹⁾ 0.5 0.39 0.78 1254 Example 1
(45 ℃) 0.5 0.39 0.78 1480 Comparative Example 1 (25°C) 0.5 0.39 0.78 1218 Comparative Example 1 (45 °C) 0.5 0.39 0.78 1472 Comparative Example 2
(25 ℃) Second electrolyte composition ²⁾ 0.42 1.77 4.21 914 1) Composition of the first electrolyte = Propionitrile: 1H,1H,2'H,3H-Decafluorodipropyl ether (3:7, w/w) solvent, 3.0 M LiTFSI
2) Second electrolyte composition = 1,3-Dioxolane (DOL):Dimethyl ether (DME) (5:5, v/v) solvent, 1.0 M LiTFSI, 1% by weight LiNO ₃

Experimental example 1. Performance evaluation of lithium-sulfur secondary battery

The performance of the lithium-sulfur secondary batteries prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, was evaluated using a charge/discharge measuring device (LAND CT-2001A, manufactured by Wuhan).

For each lithium-sulfur secondary battery, after 2.5 cycles of 0.1C (0.55 mA·cm ^-2 ) charge/ 0.1C (0.55 mA·cm ^-2 ) discharge, 0.2C (1.1 mA·cm ^-2 ) charge/ 0.2C ( 1.1 mA·cm ^{- 2} ) After 3 cycles of discharge, 0.3C (1.65 mA·cm ^-2 ) charge/ 0.5C (2.65 mA·cm ^-2 ) discharge condition is used for 3 times at 25°C and 45°C. It was measured at eggplant temperature. The results obtained at this time are shown in Table 2, FIGS. 4 and 5.

Example 1 Comparative Example 1 Comparative Example 2 △V _n (%) Discharge voltage (V) 25 ℃ 1.90 1.85 2.13 2.7 45 ℃ 2.08 2.07 - 0.5

As shown in FIGS. 4 and 5, it can be confirmed that the lithium-sulfur secondary battery including the positive electrode according to the present invention has excellent discharge capacity and overvoltage characteristics compared to Comparative Examples 1 and 2.

Referring to FIG. 4, as a result of evaluating the performance of the lithium-sulfur secondary batteries prepared in Example 1 and Comparative Example 1 at 25° C., the lithium-sulfur secondary battery of Example 1 was lithium-sulfur of Comparative Example 1. Compared to the secondary battery, it can be seen that the overvoltage is greatly improved in the initial discharge and the discharge capacity is also improved.

In addition, as a result of evaluating the performance of the lithium-sulfur secondary battery of Example 1 and Comparative Example 1 at 45° C., the lithium-sulfur secondary battery of Example 1 was shown in FIG. -Compared to the sulfur secondary battery, it can be seen that the voltage drop that occurs around 350 mAh/g is improved.

In addition, referring to Table 2, it can be seen that the nominal discharge voltage (V _n ) increases by 2.7% in Example 1 compared to Comparative Example 1 at 25°C, and 0.5% at 45°C. Through this, it can be seen that the effect of the catalyst point can be maximized at room temperature.

From these results, the lithium-sulfur secondary battery of the present invention includes a sulfur-carbon composite including a porous carbon material into which a catalyst point is introduced in the positive electrode, and the positive electrode and the electrolyte satisfy certain conditions, thereby remarkably excellent overvoltage improvement effect, It can be seen that a higher energy density, which was not possible in the existing lithium-sulfur secondary battery, can be stably implemented.

10: porous carbon material
11: Qigong
20: catalyst point
30: sulfur compound

Claims (16)

As a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator and an electrolyte,
The anode includes a sulfur-carbon composite comprising a porous carbon material into which a catalyst point is introduced,
The positive electrode is a lithium-sulfur secondary battery having an SC factor value of 0.45 or more represented by Equation 1 below:
[Equation 1]

(In Equation 1,
P is the porosity (%) of the positive electrode active material layer in the positive electrode,
L is the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode (mg/cm2),
α is 10 (constant)). The method of claim 1,
The catalyst point comprises a transition metal composite, lithium-sulfur battery. The method of claim 1,
The catalyst point includes at least one selected from the group consisting of iron phthalocyanine, nickel phthalocyanine, manganese phthalocyanine, copper phthalocyanine, cobalt phthalocyanine and zinc phthalocyanine, lithium-sulfur battery. The method of claim 1,
The catalyst point is contained in 1 to 20% by weight based on the total 100% by weight of the porous carbon material into which the catalyst point is introduced, lithium-sulfur battery. The method of claim 1,
The catalyst point is located on any one or more of the outer surface of the porous carbon material and the inner surface of the pores, lithium-sulfur battery. The method of claim 1,
The catalyst point is a lithium-sulfur battery that is bonded through a π electron interaction with the surface of the porous carbon material. The method of claim 1,
The porous carbon material comprises at least one selected from the group consisting of graphite, graphene, reduced graphene oxide, carbon black, carbon nanotubes, carbon fibers, graphite, and activated carbon, lithium-sulfur battery. The method of claim 1,
The electrolyte solution contains a solvent and a lithium salt,
The solvent includes a first solvent having a DV ² factor of 1.75 or less and a second solvent that is a fluorinated ether-based solvent represented by Equation 2 below:
[Equation 2]

(In Equation 2,
DV is the dipole moment per unit volume (D·㏖/L),
μ is the viscosity of the solvent (cP, 25 °C),
γ is 100 (constant)). The method of claim 8,
The first solvent is a DV ² factor value of 1.5 or less, the lithium-sulfur secondary battery. The method of claim 8,
The lithium-sulfur secondary battery has an NS factor value of 3.5 or less represented by Equation 3 below:
[Equation 3]

(In Equation 3,
SC factor is the same as the value defined by Equation 1,
The DV ² factor is the same as the value defined by Equation 2.). The method of claim 1,
The lithium-sulfur secondary battery has an ED factor value of 950 or more represented by Equation 4 below:
[Equation 4]

(In Equation 4,
V is the discharge nominal voltage (V) for Li/Li ⁺ ,
SC factor is the same as the value defined by Equation 1,
C is the discharge capacity (mAh/g) at the time of discharge at 0.1 C rate,
D is the density of the electrolyte solution (g/cm 3 ). The method of claim 8,
The first solvent is a lithium-sulfur secondary battery comprising at least one selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine and 1-iodopropane . The method of claim 8,
The second solvent is 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl tri Fluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H -A lithium-sulfur secondary battery comprising at least one selected from the group consisting of perfluorodipropyl ether. The method of claim 8,
The solvent comprises 1 to 50% by weight of the first solvent based on the total 100% by weight of the solvent, lithium-sulfur secondary battery. The method of claim 8,
The solvent comprises 50 to 99% by weight of the second solvent based on the total 100% by weight of the solvent, lithium-sulfur secondary battery. The method of claim 8,
The solvent comprises a first solvent and a second solvent in a weight ratio of 3:7 to 1:9, lithium-sulfur secondary battery.

Images