WO2020085811A1 - Lithium-sulfur secondary battery - Google Patents

Abstract

The present invention provides a lithium-sulfur secondary battery comprising a cathode, an anode, a separator and an electrolytic solution. The cathode has an SC factor, represented by mathematical formula 1, of 0.45 or greater. The electrolytic solution comprises a solvent and a lithium salt, and the solvent comprises a first solvent having a DV² factor represented by mathematical formula 2, of 1.7 or less, and a second solvent which is a fluorinated ether-based solvent.

Description

Lithium-sulfur secondary battery

This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0128729 filed on October 26, 2018 and Korean Patent Application No. 10-2019-0131538 filed on October 22, 2019. All content disclosed in the literature is included as part of this specification.

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

As the application area of secondary batteries is expanded to electric vehicles (EVs) or energy storage devices (ESSs), lithium-ion secondary batteries with relatively low weight-to-weight energy storage density (~ 250 Wh / kg) are used in these products. There is a limit to the application. On the other hand, lithium-sulfur secondary batteries are in the spotlight as the next generation secondary battery technology because they can theoretically realize a high weight-to-weight energy storage density (~ 2,600 Wh / kg).

The lithium-sulfur secondary battery refers to a battery system using a sulfur-based material having an S-S bond (Sulfur-Sulfur Bond) as a positive electrode active material and a lithium metal as a negative electrode active material. Sulfur, the main material of the positive electrode active material, has an advantage of having a large amount of resources worldwide, not toxic, and having a low weight per atom.

In the lithium-sulfur secondary battery, lithium, a negative electrode active material, is released and ionized during discharge, and is oxidized, and a sulfur-based material, a positive electrode active material, is absorbed and 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 combines with the sulfur anion generated by the reduction reaction of sulfur to form a salt. Specifically, sulfur before discharge has a cyclic S ₈ structure, which is converted to lithium polysulfide (LiS _x ) by a reduction reaction. When lithium polysulfide is completely reduced, lithium sulfide (Li ₂ S) is generated.

As a positive electrode active material, sulfur has a characteristic of low electrical conductivity, so it is difficult to secure reactivity with electrons and lithium ions in a solid phase form. In order to improve the reactivity of the sulfur, the existing lithium-sulfur secondary battery generates an intermediate polysulfide in the form of Li ₂ S _x to induce a liquid phase reaction and improve reactivity. In this case, an ether-based solvent such as dioxolane or dimethoxyethane, which is highly soluble in lithium polysulfide, is used as a solvent for the electrolyte. In addition, the existing lithium-sulfur secondary battery constructs a catholyte type lithium-sulfur secondary battery system to improve reactivity, in which case, due to the characteristics of lithium polysulfide dissolved in the electrolyte, the reactivity of sulfur according to the content of the electrolyte And lifetime characteristics are affected. In order to build a high energy density, a low content of electrolyte must be injected, but as the content of the electrolyte 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 and an increase in side reactions.

In order to build a high-energy density lithium-sulfur secondary battery, a battery system capable of driving an electrode with high loading and low porosity is required, and research on such a battery system has been continuously conducted in the related technical field.

[Advanced technical literature]

[Non-patent literature]

(Non-Patent Document 1) Abbas Fotouhi et al., Lithium-Sulfur Battery Technology Readiness and Applications―A Review, Energies 2017, 10, 1937.

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

According to a first aspect of the invention,

The present invention provides a lithium-sulfur secondary battery comprising an anode, a cathode, a separator, and an electrolyte.

In one embodiment of the present invention, the positive electrode has an SC factor value of 0.45 or more represented by Equation 1 below.

[Equation 1]

(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 / cm 2), and α is 10 (constant).)

In one embodiment of the present invention, the electrolyte solution includes a solvent and a lithium salt,

The solvent includes a first solvent having a DV ² factor value of 1.75 or less represented by Equation 2 below, and a second solvent being a fluorinated ether-based solvent.

[Equation 2]

(Where 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 first solvent has a DV ² factor value of 1.5 or less.

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

[Equation 3]

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

In one embodiment of the present invention, the lithium-sulfur secondary battery has an ED factor value of 850 or more represented by Equation 4 below.

[Equation 4]

(Wherein, 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 at discharge at 0.1C rate (mAh / g), and SC factor Is the same as the value defined by Equation 1 above.)

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 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 invention, the solvent comprises 1 to 50% by weight of the first solvent.

In one embodiment of the invention, the solvent comprises 50 to 99% by weight of the second solvent.

In one embodiment of the invention, the solvent comprises a first solvent and a second solvent in a weight ratio of 3: 7 to 1: 9.

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 to specific conditions.

1 is a graph showing the measured ED factor values of the lithium-sulfur secondary batteries according to Examples 1 and 2 and Comparative Examples 1 to 8.

2 is a graph showing the measured ED factor values of the lithium-sulfur secondary batteries according to Examples 1 and 3 to 7 and Comparative Examples 9 to 14.

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

For the properties described in the present specification, unless the measurement conditions and methods are specifically described, the properties are measured according to measurement conditions and methods generally used by those skilled in the art.

The present invention provides a lithium-sulfur secondary battery comprising an anode, a cathode, 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 is lowered at the positive electrode and the content of the positive electrode active material is increased, the energy density of the battery containing the positive electrode increases. However, if the porosity of the anode is minimized and the sulfur content is maximized in the lithium-sulfur secondary battery, the ratio of the electrolyte solution per unit sulfur content decreases. Therefore, when the anode is applied to the lithium-sulfur secondary battery, the target is It is difficult to achieve one performance. In the present invention, it is intended to provide a lithium-sulfur secondary battery having a higher energy density than an existing lithium-sulfur secondary battery in actual implementation by limiting conditions related to sulfur at the positive electrode and specifying appropriate electrolyte conditions.

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 the current collector. If, when the positive electrode is a positive electrode active material layer is formed on one surface of the current collector, the positive electrode may be prepared by applying a positive electrode active material slurry containing a positive electrode active material on one surface of the current collector and drying it. The slurry may further include additives such as a binder and a conductive material, a filler, and a dispersant in addition to the positive electrode active material.

The positive 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 organic sulfur compound, or a carbon-sulfur polymer ((C ₂ S _x ) _n : x = 2.5 to 50, n≥2). They are applied in combination with a conductive material because the sulfur material alone does not have electrical conductivity.

The binder is a component that assists the bonding of the positive electrode active material and the conductive material and bonding to the current collector, and may be usually added in an amount of 1% to 30% by weight based on the total amount of the positive electrode active material slurry. The binder is not particularly limited, for example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacryl Late (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 monomers (EPDM), sulfonated EPDM, styrene-butylene rubber (SBR) and fluorine rubber.

The conductive material is not particularly limited, 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 powder, aluminum powder, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; And conductive materials such as polyphenylene derivatives. The conductive material may be a content of 0.05% to 5% by weight based on the total weight of the positive electrode active material slurry.

The filler may be used as a component that suppresses the expansion of the positive electrode, if necessary, whether or not to use it, and does not cause any chemical change in the battery, but is not particularly limited if it is a fibrous material, for example, olefin polymers such as polyethylene polypropylene; It may be a fibrous material such as glass fiber, carbon fiber.

The dispersant (dispersion) 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, the positive electrode active material slurry is distributed on one side of the positive electrode current collector and then uniformly dispersed using a doctor blade or the like. You can. In addition, it may be performed through methods 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 positive electrode of the present invention manufactured by the above-described material and method is divided by the SC factor value represented by the following equation (1).

[Equation 1]

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 / cm 2), 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 cathode, separator, and electrolyte as well as the above-described positive electrode, and according to an embodiment of the present invention, the lithium-sulfur secondary battery has high energy To implement 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 the embodiment of the 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 a conventional lithium-sulfur secondary battery, performance such as energy density of the battery decreases in actual implementation, but in the case of a lithium-sulfur secondary battery according to the present invention, even in actual implementation Its performance is maintained without deterioration.

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 electrode 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 Can be used. 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 ion (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 ( Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

The binder is not limited to the above-described binder, and any material that can be used as a binder in the related art is possible.

Materials such as a current collector other than the negative electrode active material and the conductive material, and materials and methods used in the positive electrode described above may be used.

In the present invention, the separator is a physical separator having a function of physically separating an electrode, and can be used without particular limitation as long as it is used as a normal separator. desirable.

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

Specifically, 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. It is possible to use a non-woven fabric made of high melting point glass fiber or the like. Among them, a porous polymer film is preferably used.

If a polymer film is used as both the buffer layer and the separation membrane, the amount of electrolyte impregnation and ion conduction characteristics decreases, and the effect of reducing overvoltage and improving capacity characteristics is negligible. Conversely, when all non-woven materials are used, mechanical stiffness cannot be secured, resulting in a short circuit of the battery. However, when the film-type separator and the polymer nonwoven fabric buffer layer are used together, the mechanical performance can be secured together with the battery performance improvement effect 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. At this time, the polyethylene polymer film preferably has a thickness of 10 to 25 μm and a porosity of 40 to 50%.

In the present invention, the electrolyte solution is a non-aqueous electrolyte solution containing a lithium salt, and is composed of a lithium salt and a solvent. The electrolyte 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 realize a high energy density of the lithium-sulfur secondary battery due to an increase in the 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 , chloro borane lithium, lower aliphatic lithium carboxylate, lithium tetraphenyl borate, and lithium imide. In one embodiment of the present invention, the lithium salt may be preferably a lithium imide such as LiTFSI.

The concentration of the lithium salt is 0.1 to 8.0M, depending on several factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the conditions for charging and discharging the battery, the working temperature and other factors known in the field of lithium secondary batteries. , Preferably 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 solution may be lowered to deteriorate the battery performance, and if it exceeds the above range, the mobility of the lithium ion (Li ⁺ ) may decrease due to an increase in the viscosity of the electrolyte solution. It is desirable to select an appropriate concentration.

The solvent includes a first solvent and a second solvent. The first solvent has the highest dipole moment per unit volume among the components contained in the solvent by 1% by weight or more, and thus 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 phase 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]

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 DV ² factor value is not particularly limited, but considering the embodiment of the actual lithium-sulfur secondary battery, the DV ² factor value may be 0.1 or more. When mixing a solvent having a DV ² factor value of 1.75 or less, such as a first solvent, 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 functionality of the electrolyte can be maintained. , The performance of the battery does not deteriorate.

If the first solvent in the present invention is included in the range of the above-mentioned DV ² factor value, the type is not particularly limited, but propionitrile (Propionitrile), dimethylacetamide (Dimethylacetamide), dimethylformamide (Dimethylformamide), gamma- Butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine), can be selected from the group consisting of 1-iodopropane (1-iodopropane) and combinations thereof. According to an embodiment of the present invention, the first solvent may include 1 to 50% by weight, preferably 5 to 40% by weight, more preferably 10 to 30% by weight based on the solvent constituting the electrolyte solution. When the solvent according to the present invention includes the first solvent within the above-mentioned weight percent range, the performance of the battery may be improved even when used with a positive electrode having a low porosity and a high loading amount of sulfur as a positive electrode active material.

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

[Equation 3]

Here, the SC factor is the same as the value defined by Equation 1, and the DV ² factor is the same as the value defined by Equation 2. According to an embodiment of the present invention, the NS factor value 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 NS factor value is not particularly limited, but considering the embodiment of the actual lithium-sulfur secondary battery, the NS factor value may be 0.1 or more. When the NS factor value is adjusted within the above range, the performance improvement effect of the lithium-sulfur secondary battery may be more excellent.

The second solvent in the present invention is a fluorinated ether-based solvent. Conventionally, in order to control the viscosity of the electrolyte, a solvent such as dimethoxyethane or dimethylcarbonate was used as a diluent. 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 low porosity. Therefore, in the present invention, a second solvent is added together with the first solvent to drive the positive electrode according to the present invention. If the second solvent is a fluorinated ether-based solvent generally used in the art, 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 combinations thereof. According to an embodiment of the present invention, the second solvent may include 50 to 99% by weight, preferably 60 to 95% by weight, more preferably 70 to 90% by weight based on the solvent constituting the electrolyte solution. When the solvent according to the present invention includes the second solvent within the above-mentioned weight percent range, the performance of the battery is improved even when used with a positive electrode having a low porosity and a positive loading amount of sulfur as a positive electrode active material, as in the first solvent. Can have When mixing the first solvent and the second solvent, considering the effect of improving the performance of the battery, the second solvent may be included in the electrolyte in an amount equal to or higher than the first solvent. According to an embodiment of the present invention, the solvent comprises 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). You can.

The non-aqueous electrolyte solution for a lithium-sulfur battery of the present invention may further include a nitric acid or nitrous acid compound 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 nitrite-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 acid such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, octyl nitrite Or nitrite compounds; One type selected from the group consisting of organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, and combinations thereof. Uses lithium nitrate.

In addition, the non-aqueous electrolyte may further include 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, hexatriphosphate 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 sulfone (PRS), vinylene carbonate ( VC) and the like.

The lithium-sulfur secondary battery of the present invention may be manufactured by placing a separator between an anode and a cathode to form an electrode assembly, and the electrode assembly is placed in a cylindrical battery case or a square battery case and then injected with electrolyte. Alternatively, after laminating the electrode assembly, it may be prepared by impregnating it with an electrolyte and sealing the resulting product in a battery case.

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

[Equation 4]

Here, 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 at discharge at 0.1C rate (mAh / g), and SC factor is It is the same as the value defined by Equation 1 above. The higher the ED factor, the higher the energy density in a real lithium-sulfur secondary battery. According to an embodiment of the present invention, the ED factor value may be 850 or more, preferably 870 or more, and more preferably 891 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 existing lithium-sulfur secondary battery.

Hereinafter, preferred examples are provided to help understanding of the present invention, but the following examples are provided only to more easily understand the present invention and the present invention is not limited thereto.

Example

Example  One

A composition for forming a positive electrode active material layer was prepared by mixing water as a solvent, and mixing sulfur, super-P, SP, a conductive material, and a binder with a ball mill. At this time, Denka Black was used as the conductive material, and a mixed binder 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 was 90:10:10 It was made possible. The prepared positive electrode active material layer-forming composition 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 electrode weight and electrode thickness (using TESA-μHITE equipment manufactured by TESA) in the prepared positive electrode 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 the positive electrode and the negative electrode prepared by the above-described method were placed to face each other, a polyethylene separator with a thickness of 20 μm and a porosity of 45% was interposed between the positive electrode and the negative electrode.

Thereafter, an electrolyte was injected into the case to prepare a lithium-sulfur secondary battery. At this time, the electrolyte solution was prepared by dissolving 3M concentration of lithium bis (trifluoromethyl sulfonyl) imide (LiTFSI) in an organic solvent, wherein the organic solvent was propionitrile (first solvent) and 1H, 1H, 2 A solvent in which '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 (25 ° C.) of the solvent measured using a BROOKFIELD AMETEK LVDV2T-CP viscometer was 0.38 cP. The DV ² factor value calculated based on this was 0.39.

Example  2

By changing the manufacturing conditions of the positive electrode, except that a positive electrode having a porosity of 74%, a mass of sulfur per unit area of the positive electrode active material layer of 3.33 mg / cm 2, and a SC factor value calculated on the basis of this was 0.45, was prepared. A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1.

Comparative example  One

By changing the manufacturing conditions of the positive electrode, except that a positive electrode having a porosity of 78%, a mass of sulfur per unit area of the positive electrode active material layer of 2.33 mg / cm 2, and a SC factor value calculated based on this was prepared, A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1.

Comparative example  2

By changing the manufacturing conditions of the positive electrode, except that a positive electrode having a porosity of 76%, a mass of sulfur per unit area of the positive electrode active material layer of 2.67 mg / cm 2, and a SC factor value calculated on the basis of this was 0.35, was prepared. A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1.

Comparative example  3

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

Comparative example  4

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

Comparative example  5

By changing the preparation conditions of the electrolytic solution, 1,3-dioxolane and dimethyl ether in an organic solvent mixed in a 1: 1 volume ratio of 1M concentration of lithium bis (trifluoromethyl sulfonyl) imide (LiTFSI) and 1 wt% A lithium-sulfur secondary battery was manufactured in the same manner as in Example 2, except that the electrolyte solution in which LiNO ₃ was dissolved was used.

Comparative example  6

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

Comparative example  7

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

Comparative example  8

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

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

	Electrolyte composition	SC factor
Example 1	First electrolyte composition 1 )	0.50
Example 2		0.45
Comparative Example 1		0.30
Comparative Example 2		0.35
Comparative Example 3		0.41
Comparative Example 4	Second electrolyte composition 2 )	0.50
Comparative Example 5		0.45
Comparative Example 6		0.30
Comparative Example 7		0.35
Comparative Example 8		0.41
1) First electrolyte composition = 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

Example  3

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume was 59.29D · mol / L, the solvent viscosity (25 ° C) was 0.61cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 1.02 dimethylacetamide was used.

Example  4

By changing the manufacturing conditions of the electrolyte solution, the dipole moment per unit volume instead of propionitrile as the first solvent was 71.04D · mol / L, the solvent viscosity (25 ° C) was 0.51cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 0.71 dimethylformamide was used.

Example  5

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume is 84.91D · mol / L, the solvent viscosity (25 ° C) is 1.03cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that gamma-butyrolactone of 1.21 was used.

Example  6

By changing the manufacturing conditions of the electrolytic solution, the dipole moment per unit volume is 136.8 D · mol / L instead of propionitrile as the first solvent, the solvent viscosity (25 ° C.) is 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 Example 1, except that 0.31 triethylamine was used.

Example  7

By changing the manufacturing conditions of the electrolytic solution, the dipole moment per unit volume is 32.42D · mol / L instead of propionitrile as the first solvent, the solvent viscosity (25 ° C) is 0.45cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 1.40 i-iodopropane was used.

Comparative example  9

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume is 33.66D · mol / L, the solvent viscosity (25 ° C) is 0.7cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 2.07, 1,3-dioxolane (1,3-Dioxolane) was used.

Comparative example  10

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume was 20.54 D · mol / L, the solvent viscosity (25 ° C.) 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 Example 1, except that 2.33, 1,2-dimethoxyethane was used.

Comparative example  11

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume is 25.79 D · mol / L, the solvent viscosity (25 ° C.) is 0.58 cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 2.24 tetrahydrofuran was used.

Comparative example  12

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume was 59.43D · mol / L, the solvent viscosity (25 ° C) was 1.16cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 1.95 dimethyl sulfoxide was used.

Comparative example  13

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume is 96.13D · mol / L, the solvent viscosity (25 ° C) is 1.71cP, and the DV ² factor value calculated based on this A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that 1.77 propylene carbonate was used.

Comparative example  14

By changing the manufacturing conditions of the electrolyte, instead of propionitrile as the first solvent, the dipole moment per unit volume was 5.74 D · mol / L, the solvent viscosity (25 ° C.) 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 Example 1, except that 9.93 dimethyl carbonate was used.

The conditions of Examples 1 and 3 to 7 and Comparative Examples 9 to 14 are summarized in Table 2 below. NS factor values are calculated and are shown in Table 2 below.

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

Experimental example

Experimental example  One

The ED factor values of the lithium-sulfur secondary batteries according to Examples 1 and 2 and Comparative Examples 1 to 8 were measured and are shown in Table 3 and FIG. 1.

	Electrolyte composition	SC factor	ED factor
Example 1	First electrolyte composition 1 )	0.50	1004.6
Example 2		0.45	893.0
Comparative Example 1		0.30	593.1
Comparative Example 2		0.35	695.6
Comparative Example 3		0.41	819.4
Comparative Example 4	Second electrolyte composition 2 )	0.50	877.9
Comparative Example 5		0.45	890.8
Comparative Example 6		0.30	654.4
Comparative Example 7		0.35	761.4
Comparative Example 8		0.41	882.5
1) First electrolyte composition = 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 Table 3 and FIG. 1, the lithium-sulfur secondary batteries according to Examples 1 and 2 have an ED factor value of 891 or more that cannot be realized by a lithium-sulfur secondary battery having a second electrolyte composition or an SC factor of 0.41 or less. Can have This means that the lithium-sulfur secondary battery according to the present invention can realize a higher energy density that could not be realized in the existing lithium-sulfur secondary battery.

Experimental example  2

The ED factor values of the lithium-sulfur secondary batteries according to Examples 1 and 3 to 7 and Comparative Examples 9 to 14 were measured and are shown in Table 4 and FIG. 2.

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

According to Tables 4 and 2, when sulfur is highly loaded with an SC factor value of 0.5, the energy of a lithium-sulfur secondary battery is less than 1.75 of DV ² or NS factor of less than 3.50 as in Examples 1 and 3 to 7. Density can be significantly improved.

All simple modifications and variations of the present invention belong to the scope of the present invention, and the specific protection scope of the present invention will become apparent by the appended claims.

Claims (10)

1. A lithium-sulfur secondary battery comprising an anode, a cathode, a separator, and an electrolyte,

   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]

   

   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 / cm 2), and α is 10 (constant).
2. The method according to claim 1,

   The electrolyte solution includes a solvent and a lithium salt,

   The solvent,

   A first solvent having a DV ² factor value of 1.75 or less represented by Equation 2 below; And

   Lithium-sulfur secondary battery comprising a fluorinated ether-based second solvent:

   [Equation 2]

   

   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).
3. The method according to claim 2,

   The first solvent is a lithium-sulfur secondary battery, characterized in that it has a DV ² factor value of 1.5 or less.
4. The method according to claim 2,

   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 SC factor is the same as the value defined by Equation 1, and the DV ² factor is the same as the value defined by Equation 2.
5. The method according to claim 1,

   The lithium-sulfur secondary battery is a lithium-sulfur secondary battery characterized in that the ED factor value represented by the following equation (4) is 850 or more:

   [Equation 4]

   

   Here, 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 at discharge at 0.1C rate (mAh / g), and SC factor is It is the same as the value defined by Equation 1 above.
6. The method according to claim 2,

   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. .
7. The method according to claim 2,

   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 Lithium-sulfur secondary battery, characterized in that it is selected from the group consisting of, 2-trifluoroethyl ether, 1H, 1H, 2'H-perfluorodipropyl ether and combinations thereof.
8. The method according to claim 2,

   The solvent is a lithium-sulfur secondary battery, characterized in that it contains 1 to 50% by weight of the first solvent.
9. The method according to claim 2,

   The solvent is a lithium-sulfur secondary battery, characterized in that it contains 50 to 99% by weight of the second solvent.
10. The method according to claim 2,

    The solvent is 3: 7 to 1: 9 lithium-sulfur secondary battery, characterized in that it comprises a first solvent and a second solvent in a weight ratio.

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