KR20210003575A - Lithium-sulfur secondary battery - Google Patents

Abstract

The present invention provides a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte includes a solvent and a lithium salt, and the solvent includes a first solvent which has a DV^2 factor value of 1.75 or less, represented by equation 1: DV^2 factor = γ × μ/DV, a second solvent which is a fluorinated ether solvent, and a third solvent which is a cyclic ether-based solvent containing one oxygen atom. The solvent includes 1 wt% or more to less than 20 wt% of the third solvent based on the total weight of the first solvent and the second solvent.

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) and energy storage devices (ESS), lithium-ion secondary batteries with relatively low weight-to-energy storage density (~250 Wh/kg) are suitable for these products. There is a limitation in application to 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 refers to a battery system in which a sulfur-based material having an S-S bond is used as a positive electrode active material and lithium metal is used as a negative electrode active material. Sulfur, which is the main material of the positive electrode active material, has the advantage of having abundant resources, non-toxicity, and low weight per atom worldwide.

In a lithium-sulfur secondary battery, lithium, which is a negative electrode active material, is oxidized while ionizing and releasing electrons during discharge, and a sulfur-based material, which is a positive electrode active material, accepts electrons and is reduced. Here, 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 SS 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, before discharge, sulfur has a cyclic S ₈ structure, which is converted into lithium polysulfide (LiS _x ) by a reduction reaction. When lithium polysulfide is completely reduced, lithium sulfide (Li ₂ S) is generated.

Sulfur, which is a positive electrode active material, has low electrical conductivity, so it is difficult to secure reactivity with electrons and lithium ions in a solid state. Existing lithium-sulfur secondary batteries induce a liquid phase reaction and improve reactivity by generating Li ₂ S _x type intermediate polysulfide 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, the reactivity of sulfur depends on the content of the electrolyte due to the characteristics of lithium polysulfide dissolved in the electrolyte. And life characteristics are affected. In order to build a high energy density, it is necessary to inject a low content of electrolyte, but as the electrolyte content decreases, the concentration of lithium polysulfide in the electrolyte increases, and it is difficult to operate a normal battery due to a decrease in fluidity of the active material and an increase in side reactions.

In order to construct a lithium-sulfur secondary battery of 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.

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

In order to solve the above problem, the present invention is to provide a lithium-sulfur secondary battery capable of implementing a lithium-sulfur secondary battery having a high energy density by controlling a positive electrode and an electrolyte solution under specific conditions.

According to the first aspect of the invention,

The present invention is a lithium-sulfur secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte contains a solvent and a lithium salt, and the solvent is a first having a DV ² factor value of 1.75 or less represented by Equation 1 below. 1 solvent; A second solvent which is a fluorinated ether solvent; And it provides a lithium-sulfur secondary battery comprising a third solvent that is a cyclic ether-based solvent containing one oxygen atom.

[Equation 1]

Here, DV is the dipole moment per unit volume (D·mol/L), μ is the viscosity of the solvent (cP, 25°C), and γ is 100 (constant).

In one embodiment of the present invention, the solvent contains 80 to 99% by weight of the first solvent and the second solvent based on the total weight of the solvent, and the mixing weight ratio of the first solvent and the second solvent is 3:7 To 1:9.

In one embodiment of the present invention, the solvent contains 1% by weight or more and less than 20% by weight of the third solvent based on the total weight of the first solvent and the second solvent.

In one embodiment of the present invention, the first solvent is selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine, 1-iodopropane, and combinations thereof. .

In one embodiment of the present invention, the second solvent is 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2, 2,2-tetrafluoroethyl trifluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, 1H,1H,2'H,3H-decafluorodipropyl Ether, pentafluoroethyl 2,2,2-trifluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, and combinations thereof.

In one embodiment of the present invention, the third solvent is selected from the group consisting of tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and combinations thereof.

In one embodiment of the present invention, the first solvent has a DV ² factor value of 1.5 or less.

In one embodiment of the present invention, the positive electrode has an SC factor value of 0.45 or more, expressed by Equation 2 below.

[Equation 2]

Here, 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), and α is 10 (constant).

In one embodiment of the present invention, the lithium-sulfur secondary battery has an NS factor value of 3.5 or less represented by Equation 3 below.

[Equation 3]

Here, the DV ² factor is the same as the value defined by Equation 1, and the SC factor is the same as the value defined by Equation 2 above.

In one embodiment of the present invention, the lithium-sulfur secondary battery has an ED factor value of 1600 or higher represented by Equation 4 below.

[Equation 4]

Where V is the nominal discharge voltage for Li/Li ⁺ (V), D is the density of the electrolyte (g/cm 3 ), C is the discharge capacity (mAh/g) when discharging at a 0.1C rate, and the SC factor is It is the same as the value defined by Equation 2 above.

The lithium-sulfur secondary battery according to the present invention has a high energy density that 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 graph showing the measurement of ED factor values of lithium-sulfur secondary batteries according to Examples 1 to 6 and Comparative Examples 1 to 5.
2 is a graph showing the measurement of ED factor values of lithium-sulfur secondary batteries according to Reference Examples 1 and 2 and Reference Comparative Examples 1 to 8.
3 is a graph showing the measurement of ED factor values of lithium-sulfur secondary batteries according to Reference Examples 1 and 3 to 7 and Reference Comparative Examples 9 to 14.

The specific examples provided according to the present invention can all be achieved by the following description. The following description is to be understood as describing preferred embodiments of the present invention, and it is to be understood that the present invention is not necessarily limited thereto.

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.

The present invention provides a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator and an electrolyte. The lithium-sulfur secondary battery according to the present invention includes a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material. When the porosity in the positive electrode is lowered and the content of the positive electrode active material is increased, the energy density of a battery including the positive electrode increases. However, in a lithium-sulfur secondary battery, if the porosity of the positive electrode is reduced to a minimum and the sulfur content is increased to the maximum, the proportion of the electrolyte solution per unit sulfur content decreases. Therefore, when the positive electrode is applied to a lithium-sulfur secondary battery It is difficult to implement one performance. In the present invention, it is intended to provide a lithium-sulfur secondary battery having a high energy density compared to a conventional lithium-sulfur secondary battery in actual implementation by limiting the conditions related to sulfur in the positive electrode and specifying an appropriate electrolyte condition therefor.

In the present invention, the electrolytic solution 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 may be at least one selected from the group consisting of. 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 8.0M, 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 5.0M, more preferably 1.0 to 3.0M. 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, a second solvent and a third solvent. The first solvent has the highest dipole moment per unit volume among the constituents contained in the solvent at least 1% by weight, and thus the first solvent is characterized by having 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 a DV ² factor represented by Equation 1 below.

[Equation 1]

Here, DV is the dipole moment per unit volume (debye(D)·mol/L), μ is the viscosity of the solvent (cP, 25°C), and γ is 100 (constant). According to an embodiment of 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 value of the DV ² factor is not particularly limited, but considering an embodiment of an actual lithium-sulfur secondary battery, the value of the DV ² factor may be 0.1 or more. When a solvent having a DV ² factor of 1.75 or less is mixed, such as the first solvent, 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. , The battery performance is not deteriorated.

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 be selected from the group consisting of butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine), 1-iodopropane (1-iodopropane), and combinations thereof.

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. 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 low porosity positive electrode. Accordingly, 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 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), 1H,1H,2'H,3H-decafluorodipropyl ether (1H,1H,2'H,3H-Decafluorodipropyl ether), pentafluoroethyl 2,2,2-trifluoroethyl ether (Pentafluoroethyl 2,2,2-trifluoroethyl ether), 1H,1H,2'H-perfluorodipropyl ether (1H,1H,2'H-Perfluorodipropyl ether) and a combination thereof may be selected from the group consisting of. The first solvent and the second solvent may contain 70% by weight or more, preferably 80 to 99% by weight, based on the total weight of the solvent. 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 an embodiment of 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). I can.

In the present invention, the third solvent is a cyclic ether solvent containing one oxygen atom. The third solvent can improve the performance of the battery when used together with the first solvent and the second solvent, and an ether system such as 1,3-Dioxolane containing two or more oxygens in the ring The compound is not suitable as a third solvent. If the third solvent is a cyclic ether solvent containing one oxygen atom generally used in the relevant technical field, the type is not particularly limited, but tetrahydrofuran, 2-methyltetrahydrofuran (2 -Methyltetrahydrofuran), 2,5-dimethyltetrahydrofuran (2,5-Dimethyltetrahydrofuran), and may be selected from the group consisting of a combination thereof. According to an embodiment of the present invention, the third solvent may contain 1% by weight or more and less than 20% by weight, preferably 2 to 15% by weight, based on the total weight of the first solvent and the second solvent, In the above range, the effect of improving the performance of the battery may be more excellent.

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 nitrite-based compound has an effect of forming a stable film on the lithium electrode and improving charging and discharging efficiency. The nitric acid or nitrous acid-based compound is 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 non-aqueous electrolyte may further include other additives for the purpose of improving charge/discharge characteristics and flame retardancy. 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 Dinon, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS), vinylene carbonate ( VC) and the like.

In the present invention, the positive electrode is not particularly limited, but may be a lithium thin film or a positive electrode active material layer formed on one surface of a current collector. If the positive electrode has a positive electrode active material layer formed on one surface of the current collector, the positive electrode may be prepared by applying a positive electrode active material slurry including a positive electrode active material on one surface of the current collector and then drying it. In addition to the positive electrode active material, the slurry may further include additives such as a binder, a conductive material, a filler, and a dispersant.

The positive electrode active material may include elemental sulfur (S ₈ ), a sulfur-based compound, or a mixture thereof. Specifically, the sulfur-based compound may be Li ₂ S _n (n≥1), an organosulfur compound or a carbon-sulfur polymer ((C ₂ S _x ) _n : x=2.5 to 50, n≥2). These are applied in combination with a conductive material because the sulfur material alone has no electrical conductivity.

The binder is a component that aids in bonding of the positive electrode active material and the conductive material to the current collector, and may be generally added in an amount of 1% to 30% by weight based on the total amount of the positive electrode active material slurry. Such a binder is not particularly limited, for example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, and polymethylmethacryl. Rate (polymethylmethacrylate), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene It may be one or a mixture of two or more selected from the group consisting of monomer (EPDM), sulfonated EPDM, styrene-butylene rubber (SBR), and fluorine rubber.

The conductive material is not particularly limited, but, for example, graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black (super-p), acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and denka black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; It may be a conductive material such as a polyphenylene derivative. The conductive material may generally be in an amount of 0.05% to 20% by weight based on the total weight of the positive electrode active material slurry.

The filler is a component that suppresses the expansion of the positive electrode and may be used as necessary, and it is not particularly limited as long as it is a fibrous material without causing chemical changes to the battery, for example, an olipine polymer such as polyethylene polypropylene; It may be a fibrous material such as glass fiber or carbon fiber.

The dispersant (dispersion liquid) is not particularly limited, but may be, for example, isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or the like.

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 within 1 day in a vacuum oven at 50°C to 200°C.

The anode of the present invention manufactured by the above-described material and method is classified by the SC factor value represented by Equation 2 below.

[Equation 2]

Here, 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), and α is 10 (constant). The lithium-sulfur secondary battery according to the present invention realizes a high energy density by organic bonding of the above-described positive electrode as well as a negative electrode, a separator, and an electrolyte, and according to a specific embodiment of the present invention, a lithium-sulfur secondary battery In order to implement the density, the SC factor value may be 0.45 or more, preferably 0.5 or more. In the present invention, the upper limit of the SC factor value is not particularly limited, but considering an embodiment of an actual lithium-sulfur secondary battery, the SC factor value may be 4.5 or less. When the SC factor value is 0.45 or more, in the case of the existing lithium-sulfur secondary battery, performance such as the energy density of the battery is deteriorated in actual implementation, but in the case of the lithium-sulfur secondary battery according to the present invention, the battery Is maintained without compromising its 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]

Here, the DV ² factor is the same as the value defined by Equation 1, and the SC factor is the same as the value defined by Equation 2 above. According to an embodiment of 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 considering an embodiment 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 negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

The negative active material layer includes a negative active material, a binder, and a conductive material. As the negative active material, a material capable of reversibly intercalating or deintercalating lithium ions (Li ⁺ ), a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions, lithium metal or lithium alloy You can use The material capable of reversibly occluding or releasing lithium ions (Li ⁺ ) 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 (Li ⁺ ) 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 binder is not limited to the above-described binder, and any binder that can be used in the relevant technical field may be used.

Materials and methods used in the above-described positive electrode may be used as a configuration of a current collector excluding the negative active material and the conductive material.

In the present invention, the separator is a physical separator having a function of physically separating an electrode, 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 excellent electrolyte moisture impregnation ability. desirable.

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. This separator has a porosity of 30-50% 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 as 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 stiffness cannot be secured, resulting in 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.

According to a preferred embodiment of 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 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 prismatic 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.

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

[Equation 4]

Here, V is the nominal discharge voltage (V) for Li/Li ⁺ , D is the density of the electrolyte (g/cm 3 ), C is the discharge capacity at the time of discharge of the battery (mAh/g), and the SC factor is the above mathematical It is the same as the value defined by Equation 2. 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 an embodiment of the present invention, the ED factor value may be 1,580 or more, preferably 1,590 or more, more preferably 1,600 or more. In the present invention, the upper limit of the ED factor value is not particularly limited, but considering an embodiment of an actual lithium-sulfur secondary battery, the ED factor value may be 10,000 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.

Hereinafter, preferred embodiments are presented to aid in the understanding of the present invention, but the following examples are provided for easier understanding of the present invention, and the present invention is not limited thereto.

Example

Example One

Water was used as a solvent, sulfur, Super-P, SP, a conductive material, and a binder were mixed with a ball mill to prepare a composition for forming a positive electrode active material layer. At this time, Denka Black was used as the conductive material, and a binder in the form of a mixture of SBR and CMC was used as the binder, and the mixing ratio was sulfur and SP (9:1 ratio) by weight ratio: conductive material: binder is 90:10:10. I made it possible. The prepared composition for forming a positive electrode active material layer was applied to an aluminum current collector and dried to prepare a positive electrode (energy density of the positive electrode: 6.18 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 5.17 mg/cm 2. The SC factor value calculated based on this was 0.86.

After positioning the positive electrode and negative electrode prepared by the above-described method to face each other, a polyethylene separator having a thickness of 20 μm and a porosity of 45% was interposed between the positive electrode and the negative electrode.

After that, an electrolyte was injected into the case 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 3M in an organic solvent, wherein the organic solvent was propionitrile (first solvent) and 1H,1H,2 After mixing'H,3H-decafluorodipropyl ether (second solvent) in a 3:7 weight ratio, 2% by weight of tetrahydrofuran (third solvent) was mixed and used based on the total weight of the mixture. 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.

Example 2

A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that the preparation conditions of the electrolyte solution were changed in Example 1, and 2-methylterahydrofuran was used instead of tetrahydrofuran as the third solvent.

Example 3

A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 2,5-dimethyltetrahydrofuran was used instead of tetrahydrofuran as a third solvent by changing the preparation conditions of the electrolyte solution in Example 1 .

Example 4

A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 5% by weight of tetrahydrofuran, which is a third solvent, was used instead of 2% by weight by changing the preparation conditions of the electrolyte solution in Example 1 I did.

Example 5

A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 10% by weight of tetrahydrofuran was used instead of 2% by weight by changing the preparation conditions of the electrolyte solution in Example 1 I did.

Example 6

A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 15% by weight of tetrahydrofuran, which is a third solvent, was used instead of 2% by weight by changing the preparation conditions of the electrolyte solution in Example 1 I did.

Comparative example One

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that the preparation conditions of the electrolyte solution were changed in Example 1, and the third solvent was not used at all.

Comparative example 2

A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 1,3-dioxolane was used instead of tetrahydrofuran as a third solvent by changing the manufacturing conditions of the electrolyte solution in Example 1.

Comparative example 3

In Example 1, a lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 2-methyl-1,3-dioxolane was used instead of tetrahydrofuran as a third solvent by changing the manufacturing conditions of the electrolyte solution. Was prepared.

Comparative example 4

Lithium-sulfur secondary in the same manner as in Example 1, except that 2,2-dimethyl-1,3-dioxolane was used instead of tetrahydrofuran as a third solvent by changing the preparation conditions of the electrolyte solution in Example 1 The battery was prepared.

Comparative example 5

A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 20% by weight of tetrahydrofuran, which is a third solvent, was used instead of 2% by weight by changing the preparation conditions of the electrolyte solution in Example 1 I did.

The conditions of Examples 1 to 6 and Comparative Examples 1 to 5 are summarized and shown in Table 1 below.

Third solvent Content of third solvent
(weight %) Example 1 Tetrahydrofuran 2 Example 2 2-Methyltetrahydrofuran 2 Example 3 2,5-Dimethyltetrahydrofuran 2 Example 4 Tetrahydrofuran 5 Example 5 Tetrahydrofuran 10 Example 6 Tetrahydrofuran 15 Comparative Example 1 - - Comparative Example 2 1,3-Dioxolane 2 Comparative Example 3 2-Methyl-1,3-dioxolane 2 Comparative Example 4 2,2-Dimethyl-1,3-dioxolane 2 Comparative Example 5 Tetrahydrofuran 20

Experimental example

Experimental example One

The ED factor values of the lithium-sulfur secondary batteries according to Examples 1 to 6 and Comparative Examples 1 to 5 were measured and shown in Table 2 and FIG. 1.

Third solvent Content of third solvent
(weight %) ED factor Example 1 Tetrahydrofuran 2 1621 Example 2 2-Methyltetrahydrofuran 2 1644 Example 3 2,5-Dimethyltetrahydrofuran 2 1634 Example 4 Tetrahydrofuran 5 1628 Example 5 Tetrahydrofuran 10 1618 Example 6 Tetrahydrofuran 15 1605 Comparative Example 1 - - 1561 Comparative Example 2 1,3-Dioxolane 2 1550 Comparative Example 3 2-Methyl-1,3-dioxolane 2 1567 Comparative Example 4 2,2-Dimethyl-1,3-dioxolane 2 1574 Comparative Example 5 Tetrahydrofuran 20 1552

According to Table 2 and FIG. 1, the lithium-sulfur secondary battery according to Examples 1 to 3 uses a cyclic ether-based solvent containing one oxygen atom as a third solvent, so that a third solvent is not used. The lithium-sulfur secondary battery according to Example 1 and the cyclic ether-based solvent containing two oxygen atoms as a third solvent are used as a third solvent, which is higher than that of the lithium-sulfur secondary battery according to Comparative Examples 2 to 4. I can.

In addition, when about 2 to 15% by weight of the third solvent is added based on the total weight of the first solvent and the second solvent (lithium-sulfur secondary batteries according to Examples 3 to 6), an ED factor value of 1600 or higher can be achieved. In contrast, when about 20% by weight or more of the third solvent is added based on the total weight of the first solvent and the second solvent (lithium-sulfur secondary battery according to Comparative Example 5), the third solvent is added. The effect of improving the ED factor is not confirmed.

Reference Example

Reference Example One

Water was used as a solvent, sulfur, Super-P, SP, a conductive material, and a binder were mixed with a ball mill to prepare a composition for forming a positive electrode active material layer. At this time, Denka Black was used as the conductive material, and a binder in the form of a mixture of SBR and CMC was used as the binder, and the mixing ratio was sulfur and SP (9:1 ratio) by weight ratio: conductive material: binder is 90:10:10. I made it possible. The prepared composition for forming a positive electrode active material layer was applied to an aluminum current collector and dried to prepare a positive electrode (energy density of the positive electrode: 6.18 mAh/cm 2 ). By measuring the electrode weight and electrode thickness (using TESA-μHITE equipment) in the prepared positive electrode, the porosity of the positive electrode active material layer was 74%, and the mass of sulfur per unit area of the positive electrode active material layer was 3.75 mg/cm 2. The SC factor value calculated based on this was 0.50.

After positioning the positive electrode and negative electrode prepared by the above-described method to face each other, a polyethylene separator having a thickness of 20 μm and a porosity of 45% was interposed between the positive electrode and the negative electrode.

After that, an electrolyte was injected into the case 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 3M in an organic solvent, wherein the organic solvent was propionitrile (first solvent) and 1H,1H,2 A solvent obtained by mixing'H,3H-decafluorodipropyl ether (second solvent) 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.

Reference Example 2

By changing the manufacturing conditions of the positive electrode in Reference Example 1, a positive electrode having a porosity of 74% of the positive electrode active material layer, a mass of sulfur per unit area of the positive electrode active material layer of 3.33 mg/cm2, and a calculated SC factor value of 0.45 was prepared. A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except for one.

Reference Comparative example One

By changing the manufacturing conditions of the positive electrode in Reference Example 1, a positive electrode having a porosity of 78% of the positive electrode active material layer, a mass of sulfur per unit area of the positive electrode active material layer of 2.33 mg/cm2, and a calculated SC factor value of 0.30 was prepared. A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except for one.

Reference Comparative example 2

By changing the manufacturing conditions of the positive electrode in Reference Example 1, a positive electrode having a porosity of 76% of the positive electrode active material layer, a mass of sulfur per unit area of the positive electrode active material layer of 2.67 mg/cm2, and a calculated SC factor value of 0.35 was prepared. A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except for one.

Reference Comparative example 3

By changing the manufacturing conditions of the positive electrode in Reference Example 1, a positive electrode having a porosity of 75% of the positive electrode active material layer, a mass of sulfur per unit area of the positive electrode active material layer of 3.1 mg/cm 2, and an SC factor value of 0.41 calculated based on this was prepared. A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except for one.

Reference Comparative example 4

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, 1M concentration of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in an organic solvent in which 1,3-dioxolane and dimethyl ether were mixed in a 1:1 volume ratio. ) And 1 wt% of LiNO ₃ were dissolved in an electrolyte solution, and a lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1.

Reference Comparative example 5

By changing the manufacturing conditions of the electrolyte solution in Reference Example 2, 1M concentration of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in an organic solvent in which 1,3-dioxolane and dimethyl ether were mixed in a 1:1 volume ratio. ) And 1 wt% of LiNO ₃ were dissolved in an electrolyte, and a lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 2.

Reference Comparative example 6

By changing the manufacturing conditions of the electrolyte solution in Reference Comparative Example 1, 1M concentration of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in an organic solvent in which 1,3-dioxolane and dimethyl ether were mixed in a 1:1 volume ratio. ) And 1 wt% of LiNO ₃ were dissolved in an electrolyte, and a lithium-sulfur secondary battery was manufactured in the same manner as in Reference Comparative Example 1.

Reference Comparative example 7

By changing the preparation conditions of the electrolyte solution in Reference Comparative Example 2, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) at a concentration of 1M in an organic solvent in which 1,3-dioxolane and dimethyl ether were mixed in a 1:1 volume ratio. ) And 1 wt% of LiNO ₃ were dissolved in an electrolyte, and a lithium-sulfur secondary battery was manufactured in the same manner as in Reference Comparative Example 2.

Reference Comparative example 8

By changing the preparation conditions of the electrolyte solution in Reference Comparative Example 3, 1M concentration of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in an organic solvent in which 1,3-dioxolane and dimethyl ether were mixed in a 1:1 volume ratio. ) And 1 wt% of LiNO ₃ were dissolved in an electrolyte, and a lithium-sulfur secondary battery was manufactured in the same manner as in Reference Comparative Example 3.

The conditions of Reference Examples 1 and 2 and Reference Comparative Examples 1 to 8 are summarized and shown in Table 3 below.

Electrolyte composition SC factor Reference Example 1 Composition of the first electrolyte ^{1 )} 0.50 Reference Example 2 0.45 Reference Comparative Example 1 0.30 Reference Comparative Example 2 0.35 Reference Comparative Example 3 0.41 Reference Comparative Example 4 Second electrolyte composition ^{2 )} 0.50 Reference Comparative Example 5 0.45 Reference Comparative Example 6 0.30 Reference Comparative Example 7 0.35 Reference Comparative Example 8 0.41 ¹⁾ Composition of first electrolyte = Propionitrile: 1H,1H,2'H,3H-Decafluorodipropyl ether (3:7, w/w) solvent, 3.0M LiTFSI
²⁾ Second electrolyte composition = 1,3-Dioxolan:Dimethyl ether (1:1, v/v) solvent, 1.0M LiTFSI, 1.0 wt% LiNO ₃

Reference Example 3

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 59.29 D·mol/L, the viscosity of the solvent was 0.61 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 1.02 of dimethylacetamide was used.

Reference Example 4

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 71.04 D·mol/L, the viscosity of the solvent was 0.51 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 0.71 of dimethylformamide was used.

Reference Example 5

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 84.91 D·mol/L, the viscosity of the solvent was 1.03 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 1.21 of gamma-butyrolactone was used.

Reference Example 6

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 136.8 D·mol/L, the viscosity of the solvent was 0.42 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 0.31 of triethylamine was used.

Reference Example 7

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 32.42 D·mol/L, the viscosity of the solvent was 0.45 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 1-iodopropane of 1.40 was used.

Reference Comparative example 9

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 33.66 D·mol/L, the viscosity of the solvent was 0.7 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 2.07 of 1,3-Dioxolane was used.

Reference Comparative example 10

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 20.54 D·mol/L, the viscosity of the solvent was 0.48 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that the 2.33 1,2-dimethoxyethane was used.

Reference Comparative example 11

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 25.79 D·mol/L, the viscosity of the solvent was 0.58 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was prepared in the same manner as in Reference Example 1, except that the 2.24 tetrahydrofuran was used.

Reference Comparative example 12

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 59.43 D·mol/L, the viscosity of the solvent was 1.16 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 1.95 of dimethyl sulfoxide was used.

Reference Comparative example 13

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 96.13 D·mol/L, the viscosity of the solvent was 1.71 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that propylene carbonate, which is 1.77, was used.

Reference Comparative example 14

By changing the manufacturing conditions of the electrolyte solution in Reference Example 1, instead of propionitrile as the first solvent, the dipole moment per unit volume was 5.74 D·mol/L, the viscosity of the solvent was 0.57 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Reference Example 1, except that 9.93, dimethyl carbonate was used.

The conditions of Reference Examples 1 and 3 to 7 and Reference Comparative Examples 9 to 14 are summarized and shown in Table 4 below. NS factor values were calculated and shown in Table 4 below.

First solvent DV ² factor NS factor Reference Example 1 Propionitrile 0.39 0.78 Reference Example 3 Dimethylacetamide 1.02 2.04 Reference Example 4 Dimethylformamide 0.71 1.42 Reference Example 5 Gamma-Butyrolactone 1.21 2.42 Reference Example 6 Triethylamine 0.31 0.62 Reference Example 7 1-iodopropane 1.40 2.80 Reference Comparative Example 9 1,3-Dioxolane 2.07 4.14 Reference Comparative Example 10 1,2-Dimethoxyethane 2.33 4.66 Reference Comparative Example 11 Tetrahydrofuran 2.24 4.48 Reference Comparative Example 12 Dimethyl sulfoxide 1.95 3.90 Reference Comparative Example 13 Propylene carbonate 1.77 3.54 Reference Comparative Example 14 Dimethyl carbonate 9.93 19.86

Reference Experimental example

Reference Experimental example One

The ED factor values of lithium-sulfur secondary batteries according to Reference Examples 1 and 2 and Reference Comparative Examples 1 to 8 were measured and shown in Tables 5 and 2.

Electrolyte composition SC factor ED factor Reference Example 1 Composition of the first electrolyte ^{1 )} 0.50 1004.6 Reference Example 2 0.45 893.0 Reference Comparative Example 1 0.30 593.1 Reference Comparative Example 2 0.35 695.6 Reference Comparative Example 3 0.41 819.4 Reference Comparative Example 4 Second electrolyte composition ^{2 )} 0.50 877.9 Reference Comparative Example 5 0.45 890.8 Reference Comparative Example 6 0.30 654.4 Reference Comparative Example 7 0.35 761.4 Reference Comparative Example 8 0.41 882.5 ¹⁾ Composition of first electrolyte = Propionitrile: 1H,1H,2'H,3H-Decafluorodipropyl ether (3:7, w/w) solvent, 3.0M LiTFSI
²⁾ Second electrolyte composition = 1,3-Dioxolan:Dimethyl ether (1:1, v/v) solvent, 1.0M LiTFSI, 1.0 wt% LiNO ₃

According to Tables 5 and 2, the lithium-sulfur secondary battery according to Reference Examples 1 and 2 has an ED factor of 891 or more, which cannot be implemented by a lithium-sulfur secondary battery having a second electrolyte composition or an SC factor of 0.41 or less. Can have a value. This means that the lithium-sulfur secondary battery according to the present invention can implement a higher energy density that was not possible in the conventional lithium-sulfur secondary battery.

Reference Experimental example 2

The ED factor values of the lithium-sulfur secondary batteries according to Reference Examples 1 and 3 to 7 and Reference Comparative Examples 9 to 14 were measured and shown in Tables 6 and 3.

First solvent DV ² factor NS factor ED factor Reference Example 1 Propionitrile 0.39 0.78 1004.6 Reference Example 3 Dimethylacetamide 1.02 2.04 939.4 Reference Example 4 Dimethylformamide 0.71 1.42 968.8 Reference Example 5 Gamma-Butyrolactone 1.21 2.42 924.7 Reference Example 6 Triethylamine 0.31 0.62 1019.5 Reference Example 7 1-iodopropane 1.40 2.80 917.4 Reference Comparative Example 9 1,3-Dioxolane 2.07 4.14 567.6 Reference Comparative Example 10 1,2-Dimethoxyethane 2.33 4.66 667.0 Reference Comparative Example 11 Tetrahydrofuran 2.24 4.48 580.5 Reference Comparative Example 12 Dimethyl sulfoxide 1.95 3.90 376.0 Reference Comparative Example 13 Propylene carbonate 1.77 3.54 352.4 Reference Comparative Example 14 Dimethyl carbonate 9.93 19.86 287.6

According to Table 6 and FIG. 3, when the SC factor value is 0.5 and sulfur is highly loaded, as in Reference Examples 1 and 3 to 7, when the DV ² factor is 1.75 or less or the NS factor is 3.50 or less, the lithium-sulfur secondary battery Energy density can be significantly improved.

All simple modifications or changes of the present invention belong to the scope of the present invention, and the specific scope of protection of the present invention will be made clear by the appended claims.

Claims (10)

As a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator and an electrolyte,
The electrolyte solution contains a solvent and a lithium salt,
The solvent is,
A first solvent having a DV ² factor value of 1.75 or less represented by Equation 1 below;
A second solvent which is a fluorinated ether solvent; And
Lithium-sulfur secondary battery comprising a third solvent, which is a cyclic ether-based solvent containing one oxygen atom:
[Equation 1]

Here, DV is the dipole moment per unit volume (D·mol/L), μ is the viscosity of the solvent (cP, 25°C), and γ is 100 (constant). The method according to claim 1,
The solvent comprises 80 to 99% by weight of the first solvent and the second solvent based on the total weight of the solvent, and the mixing weight ratio of the first solvent and the second solvent is 3:7 to 1:9. Lithium-sulfur secondary battery. The method according to claim 1,
The solvent is a lithium-sulfur secondary battery, characterized in that it contains 1% by weight or more and less than 20% by weight of the third solvent based on the total weight of the first solvent and the second solvent. The method according to claim 1,
The first solvent is a lithium-sulfur secondary battery, characterized in that it is selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine, 1-iodopropane, and combinations thereof . The method according to claim 1,
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, 1H,1H,2'H,3H-decafluorodipropyl ether, pentafluoroethyl 2,2 ,2-trifluoroethyl ether, 1H,1H,2'H-perfluorodipropyl ether, and a lithium-sulfur secondary battery, characterized in that selected from the group consisting of a combination thereof. The method according to claim 1,
The third solvent is a lithium-sulfur secondary battery, characterized in that selected from the group consisting of tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and combinations thereof. The method according to claim 1,
The lithium-sulfur secondary battery, characterized in that the first solvent has a DV ² factor value of 1.5 or less. The method according to claim 1,
The positive electrode is a lithium-sulfur secondary battery, characterized in that the SC factor value represented by Equation 2 below is 0.45 or more:
[Equation 2]

Here, 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), and α is 10 (constant). The method of claim 8,
The lithium-sulfur secondary battery is a lithium-sulfur secondary battery, characterized in that the NS factor value represented by Equation 3 below is 3.5 or less:
[Equation 3]

Here, the DV ² factor is the same as the value defined by Equation 1, and the SC factor is the same as the value defined by Equation 2 above. The method of claim 8,
The lithium-sulfur secondary battery is a lithium-sulfur secondary battery, characterized in that the ED factor value represented by Equation 4 below is 1600 or higher:
[Equation 4]

Where V is the nominal discharge voltage for Li/Li ⁺ (V), D is the density of the electrolyte (g/cm 3 ), C is the discharge capacity (mAh/g) when discharging at a 0.1C rate, and the SC factor is It is the same as the value defined by Equation 2 above.

Images