KR20200073120A - Lithium-sulfur secondary battery - Google Patents

Abstract

The present invention relates to a lithium-sulfur secondary battery. More specifically, a positive electrode includes a conductive material containing a high specific surface area carbon material. In addition, by specifying conditions of the positive electrode and an electrolyte, it is possible to improve energy density compared to the conventional lithium-sulfur secondary battery.

Description

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

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

Lithium having a relatively low weight-to-weight energy storage density (~250 Wh/kg) as the application area of the secondary battery expands to electric vehicles (EVs) or energy storage systems (ESSs), etc. Ion secondary batteries have limitations in application to these products. 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 is a battery system using a sulfur-based material having a sulfur-sulfur bond as a positive electrode active material and lithium metal as a negative electrode active material. The lithium-sulfur secondary battery has an advantage that sulfur, which is a main material of the positive electrode active material, has abundant resources worldwide, has no toxicity, and has a low weight per atom.

In the lithium-sulfur secondary battery, lithium, which is a negative electrode active material, is released and ionized during discharge, and is oxidized, and a sulfur-based material, which is a positive electrode active material, is absorbed and reduced. At this time, the oxidation reaction of lithium is a process in which lithium metal releases electrons and is converted into lithium cation form. In addition, the reduction reaction of sulfur is a process in which the sulfur-sulfur bond is converted into a sulfur anion form by accepting two electrons. 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 (lithium polysulfide, Li ₂ S _x , x = 8, 6, 4, 2) by a reduction reaction, and this lithium polysulfide When is completely reduced, lithium sulfide (Li ₂ S) is finally produced.

As a positive electrode active material, sulfur has low electrical conductivity, and thus it is difficult to secure reactivity with electrons and lithium ions in a solid-state 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-state reaction and improve reactivity. In this case, an ether-based solvent such as dioxolane and 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 in order to improve reactivity. In this case, due to the characteristics of lithium polysulfide that easily dissolves in the electrolyte, the sulfur content depends on the content of the electrolyte. Reactivity and life characteristics will be affected. In addition, in order to build a high energy density, a low content of electrolyte must be injected, but as the content of the electrolyte decreases, the concentration of lithium polysulfide in the electrolyte increases, leading to a decrease in fluidity of the active material and an increase in side reactions. It is difficult.

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

As an example, Republic of Korea Patent Publication No. 2016-0037084 is to prevent the dissolution of lithium polysulfide by using a carbon nanotube aggregate of a three-dimensional structure coated with graphene with a carbon material, the conductivity of the sulfur-carbon nanotube composite It is disclosed that it can be improved.

In addition, Korean Patent Registration No. 1379716 treats hydrofluoric acid on graphene to form pores on the graphene surface, and uses a graphene composite containing sulfur produced through a method of growing sulfur 유particles in the pores as a positive electrode active material. It is disclosed that it is possible to minimize the capacity decrease of the battery by suppressing the dissolution of lithium polysulfide through.

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

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

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

Accordingly, the present inventors completed the present invention by confirming that a lithium-sulfur secondary battery having a high energy density can be implemented by adjusting the positive electrode and the electrolyte to specific conditions as a result of various studies to solve the above problems.

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

In order to achieve the above object, the present invention is a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode includes a conductive material comprising a high specific surface area carbon material, and the positive electrode is represented by Equation 1 below. Provided is a lithium-sulfur secondary battery having an SC factor value of 0.45 or more, represented by:

[Equation 1]

(In Equation 1 above, P, L, and α follow what is described in the specification.).

The high specific surface area carbon material may have a specific surface area of 100 to 500 m 2 /g.

The high specific surface area carbon material may include one or more selected from the group consisting of carbon nanotubes, graphene, carbon black, and carbon fibers.

The high specific surface area carbon material may be included in an amount of 0.01 to 30% by weight based on the total weight of the positive electrode active material layer.

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

[Equation 2]

(In Equation 2 above, DV, μ, and γ follow what is described in the specification.).

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

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

[Equation 3]

(In Equation 3 above, the SC factor and the DV ² factor follow what is described in the specification.).

The lithium-sulfur secondary battery may have an ED factor value of 850 or more represented by Equation 4 below:

[Equation 4]

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

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

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

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

The solvent may include a second solvent in an amount of 50 to 99% by weight based on the total weight of the solvent.

The solvent may include 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 exhibits 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 scanning electron microscope image of Example 1 according to Experimental Example 1 of the present invention.
2 is a scanning electron microscope image of Comparative Example 1 according to Experimental Example 1 of the present invention.
3 is a graph showing the capacity characteristics of Example 1 according to Experimental Example 2 of the present invention
4 is a graph showing cycle characteristics of Example 1 according to Experimental Example 2 of the present invention.
5 is a graph showing the capacity characteristics of Comparative Example 1 according to Experimental Example 2 of the present invention
6 is a graph showing cycle characteristics of Comparative Example 1 according to Experimental Example 2 of the present invention.

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

The terms or words used in the present specification and claims should not be interpreted as being limited to ordinary or lexical meanings, and the inventor can appropriately define the concept of terms in order to best describe his or her invention. Based on the principle that it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention.

The terms used in the present invention are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present invention, terms such as'include' or'have' are intended to designate the existence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and one or more other features. It should be understood that the existence or addition possibilities of fields or numbers, steps, operations, components, parts or combinations thereof are not excluded in advance.

As used herein, the terms “polysulfide” are “polysulfide ions (S _x ^2- , x = 8, 6, 4, 2))” and “lithium polysulfide (Li ₂ S _x. Or LiS _x ^- x = 8, 6, 4, 2)”.

For the 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.

In a lithium-sulfur secondary battery, the oxidation number of sulfur decreases when the sulfur-sulfur bond of the sulfur-based compound is cut off during the reduction reaction discharge, and the sulfur-sulfur bond is regenerated during the oxidation reaction charge and the oxidation number of sulfur increases. Electric energy is generated using a reduction reaction.

Lithium-sulfur secondary batteries have a high discharge capacity and energy density among various secondary batteries, and sulfur used as a positive electrode active material has a large amount of reserves and is inexpensive, so the manufacturing cost of the battery can be lowered, and the next generation secondary due to its environmentally friendly advantages It is spotlighted as a battery.

Despite these advantages, since the positive electrode active material sulfur is a non-conductor, a sulfur-carbon composite compounded with a conductive carbon material is generally used to compensate for this.

However, in the case of the existing lithium-sulfur secondary battery system, the loss of sulfur occurs because it cannot suppress the elution of lithium polysulfide as described above, and as a result, the amount of sulfur participating in the electrochemical reaction is rapidly reduced, and the theoretical discharge capacity in actual driving And not all of the theoretical energy densities. In addition, in addition to being suspended or precipitated in the electrolyte, the eluted lithium polysulfide reacts directly with lithium and adheres to the surface of the anode in the form of Li ₂ S, thereby causing a problem of corrosion of the lithium metal anode.

In the prior art, carbon materials of various structures or materials, such as introducing a coating layer or forming pores in a carbon material to suppress the dissolution of lithium polysulfide, have not been effectively improved.

Accordingly, the present invention is a lithium-sulfur secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte, and includes a high specific surface area carbon material as a conductive material, a low porosity, and a positive electrode having a high loading amount of sulfur as a positive electrode active material. It provides a lithium-sulfur secondary battery. In the case of a general secondary battery, when the porosity is decreased at the positive electrode and the content of the positive electrode active material is increased, the energy density of the secondary battery including the positive electrode increases. However, when 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, so that the above-mentioned anode is a target when applied to the lithium-sulfur secondary battery. It is difficult to achieve one performance. Therefore, in the present invention, a carbon material having a higher specific surface area than the conductive material of the carbon black and carbon fiber series used in the present invention is introduced as a conductive material in the positive electrode, and the sulfur-related conditions are defined in the positive electrode, and appropriate electrolyte conditions are specified. By doing so, it provides a lithium-sulfur secondary battery having a higher energy density than an existing lithium-sulfur secondary battery in actual driving.

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

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

The positive electrode current collector can form a fine unevenness on its surface to enhance the bonding force with the positive electrode active material, and various forms such as a film, sheet, foil, mesh, net, porous body, foam, and nonwoven fabric can be used.

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

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

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

Since the sulfur-based compound is not electrically conductive alone, it is used in combination with a carbon material that is a conductive material. Preferably, the positive electrode active material may be a sulfur-carbon composite. At this time, the term “composite” refers to a substance that combines two or more materials to form physically and chemically different phases and express more effective functions.

In the sulfur-carbon composite, the carbon material provides a skeleton in which the positive electrode active material sulfur can be fixed uniformly and stably, and compensates for the low electrical conductivity of sulfur so that the electrochemical reaction can proceed smoothly.

The carbon material can be produced by carbonizing precursors of various carbon materials. The carbon material includes a plurality of pores that are not constant on the surface and inside, and the average diameter of the pores is in the range of 1 to 500 nm, and the porosity (or porosity) may range from 10 to 90% of the total volume of the carbon material. . If the average diameter of the pores is less than the above range, impregnation of sulfur is impossible because the pore size is only at the molecular level. On the contrary, when it exceeds the above range, the mechanical strength of the carbon material is weakened, which is preferable for application in the electrode manufacturing process. Do not.

The shape of the carbon material may be spherical, rod-shaped, needle-shaped, plate-shaped, tube-shaped, or bulk-type if used in a lithium-sulfur battery without limitation.

The carbon material has a porous structure and may be any of those commonly used in the art. For example, the carbon material may include graphite; Graphene; Carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); Carbon fibers such as graphite nanofiber (GNF), carbon nanofiber (CNF), and activated carbon fiber (ACF); It may be at least one selected from the group consisting of natural graphite, artificial graphite, expanded graphite and activated carbon, but is not limited thereto.

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

In the sulfur-carbon composite of the present invention, the carbon material may be included in an amount of 10 to 50% by weight, preferably 20 to 40% by weight, based on the total weight of the sulfur-carbon composite.

Accordingly, the weight ratio of the carbon material and sulfur in the sulfur-carbon composite may be 1:1 to 1:9, preferably 1:1.5 to 1:4. If it is less than the above weight ratio range, as the content of the carbon material increases, the amount of binder addition required in preparing the positive electrode active material slurry containing the positive electrode active material increases. The increase in the amount of the binder added eventually increases the sheet resistance of the electrode and acts as an insulator preventing an electron pass, thereby degrading cell performance. Conversely, when the weight ratio range is exceeded, sulfur may be aggregated between them, and it may be difficult to directly participate in the electrode reaction due to difficulty receiving electrons.

The sulfur-carbon composite may be compounded by simply mixing the aforementioned sulfur and carbon materials, or may have a core-shell structured coating form or a supported form. The core-shell structure may be formed by coating one of sulfur or carbon materials with another material, for example, the surface of the carbon material may be wrapped with sulfur or vice versa. In addition, the supported form may be a form in which sulfur is supported inside the carbon material. The form of the sulfur-carbon composite may be any form as long as it satisfies the content ratios of the sulfur and carbon materials, and is not limited in the present invention.

The average diameter of the sulfur-carbon composite according to the present invention is not particularly limited in the present invention and may vary, but is 0.5 to 100 μm, preferably 1 to 50 μm. When the above range is satisfied, there is an advantage that a high loading electrode can be manufactured.

The positive electrode active material may further include at least one additive selected from transition metal elements, group IIIA elements, group IVA elements, sulfur compounds of these elements, and alloys of these elements with sulfur in addition to the above-described composition.

The transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg, and the like, and the group IIIA elements include Al, Ga, In, and Ti, and the group IVA elements include Ge, Sn, and Pb.

The content of the positive electrode active material may be included in 5 to 40% by weight based on the total weight of the positive electrode active material slurry.

In the present invention, the conductive material is characterized by having a higher specific surface area than the conventional carbon black and carbon fiber-based conductive materials.

The conductive material is a material that electrically connects the positive electrode active material components to help electrons move from the current collector to the positive electrode active material, and serves to promote a smooth electrochemical reaction of the positive electrode active material, sulfur.

In the present invention, a high specific surface area carbon material is used as a conductive material to improve the reactivity of a positive electrode active material in a lithium-sulfur battery secondary battery system including an electrolyte to be described later, thereby exhibiting improved capacity characteristics.

The conductive material used in the existing lithium-sulfur secondary battery, for example, vapor grown carbon fiber (VGCF), typically has a specific surface area in the range of 10 to 50 m 2 /g, whereas the carbon material of the present invention has a specific surface area Since this is high, the contact area with the positive electrode active material is high, it is effective in improving the reactivity of the positive electrode as described above.

The high specific surface area carbon material may have a specific surface area of 100 to 500 m 2 /g, preferably 150 to 400 m 2 /g. At this time, the specific surface area can be measured by a conventional method of BET (Brunauer & Emmett & Teller). When the specific surface area of the carbon material is less than the above range, it is difficult to form a conductive structure in the electrode, and thus the electrode conductivity decreases. On the contrary, when the specific surface area is exceeded, the amount of binder added during the preparation of the positive electrode active material slurry due to the increase in the specific surface area of the solid material There may be a growing problem.

The high specific surface area carbon material can be used without limitation as long as it has conductivity. For example, the high specific surface area carbon material has the above specific surface area range, and may include one or more selected from the group consisting of carbon nanotubes, graphene, carbon black, and carbon fibers. Preferably, the high specific surface area carbon material may be at least one selected from the group consisting of carbon nanotubes and graphene.

In the present invention, when the conductive material includes carbon nanotubes and graphene among high specific surface area carbon materials, the weight ratio of the carbon nanotubes and graphene is 0:10 to 10:0, preferably 1:9 to 9 It can be :1. Graphene increases the adhesion between the positive electrode active material, the carbon nanotubes are small in diameter, so it is advantageous to form a conductive structure in a small space, so it is advantageous to mix within the above-mentioned weight ratio range.

The content of the conductive material may be included in 0.01 to 30% by weight based on the total weight of the positive electrode active material slurry.

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

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

The content of the binder may be added in 0.5 to 30% by weight based on the total weight of the positive electrode active material slurry. If the content of the binder is less than 0.5% by weight, the physical properties of the positive electrode may deteriorate, and the active material and the conductive material in the positive electrode may drop off, and when it exceeds 30% by weight, the ratio of the active material and the conductive material in the positive electrode is relatively reduced, thereby reducing battery capacity. can do.

The positive electrode can be produced by a conventional method known in the art. For example, a positive electrode active material is prepared by mixing and stirring additives such as a solvent, a conductive material, and a binder, a filler, if necessary, to prepare a positive electrode active material slurry, and then applying (coating) it to a current collector of a metal material, compressing it, and drying it. An anode can be produced.

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

The coating may be performed by a method commonly known in the art, but for example, 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. Can. In addition, it may be performed through a method such as die casting, comma coating, or screen printing.

The drying is not particularly limited, but may be performed within 1 day in a vacuum oven at 50 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]

(In Equation 1 above,

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 ),

α is 10 (constant).

The lithium-sulfur secondary battery according to the present invention realizes high energy density by organic bonding of the cathode, separator and electrolyte as well as the above-described positive electrode, and according to the present invention, the lithium-sulfur secondary battery realizes high energy density. In order to do this, 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 driving 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, the performance of the existing lithium-sulfur secondary battery, such as the energy density of the battery, decreases during actual driving, but in the case of the lithium-sulfur secondary battery according to the present invention, the battery performance even during actual driving It is maintained without deterioration.

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

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

The negative active material is a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, lithium metal or lithium alloy Can be used.

The material capable of reversibly intercalating or deintercalating the lithium ion 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 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 conductive material is a material that electrically connects the electrolytic solution and the negative electrode active material, and any conductive material may be used without limitation.

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

The negative electrode active material and the conductive material, such as a current collector, a binder, and a method of manufacturing the negative electrode may be the materials and methods used in the positive electrode.

The separation membrane according to the present invention has a function of physically separating the positive electrode and the negative electrode, and can be used without particular limitation as long as it is used as a normal separation membrane. Particularly, it is preferable that it has low resistance to ion movement of the electrolyte and has excellent electrolyte moisture absorption ability. Do.

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 such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer and an 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 separator, the amount of electrolyte impregnation and ion conduction properties decrease, and the effect of reducing overvoltage and improving capacity characteristics is negligible. Conversely, when all non-woven materials are used, mechanical stiffness is not secured, resulting in a short circuit of the battery. However, when the film-type separation membrane and the polymer nonwoven fabric buffer layer are used together, mechanical strength can be secured together with an effect of improving battery performance due to the adoption of the buffer layer.

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

The electrolytic solution according to the present invention is a non-aqueous electrolytic solution containing a lithium salt and is composed of a lithium salt and a solvent. The electrolyte 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 8.0 M, 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 5.0 M, more preferably 1.0 to 3.0 M. If the concentration of the lithium salt is less than the above range, the conductivity of the electrolyte solution may be lowered to deteriorate the battery performance, and if it exceeds the above range, the viscosity of the electrolyte solution may increase and mobility of lithium ions (Li ⁺ ) may decrease, so that 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 divided by the DV ² factor represented by Equation 2 below.

[Equation 2]

(In Equation 2 above,

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

μ is the viscosity of the solvent (cP, 25℃),

γ is 100 (constant).

According to the present invention, the DV ² factor value may be 1.75 or less, preferably 1.5 or less. In the present invention, the lower limit of the DV ² factor value is not particularly limited, but considering the driving of an 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 as in the 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. There is, the performance of the battery is not lowered.

In the present invention, if the first solvent is included in the range of the above-mentioned DV ² factor value, the type is not particularly limited, but propionitrile, dimethylacetamide, dimethylformamide, gamma- Butyrolactone (Gamma-Butyrolactone), triethylamine (Triethylamine) and may include one or more selected from the group consisting of 1-iodopropane (1-iodopropane).

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 total weight of 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 can 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]

(In Equation 3 above,

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

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

In 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 driving of an 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 solution, a solvent such as dimethoxyethane or dimethylcarbonate was used as a diluent, and 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 an anode having a 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), pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H-perfluorodipropyl ether (1H,1H, 2′H-Perfluorodipropyl ether).

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 total weight of the solvent constituting the electrolyte solution. When the solvent according to the present invention includes a 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 greater than the first solvent. According to the present invention, the solvent may include the first solvent and the second solvent in a weight ratio of 1:1 to 1:9, preferably 3:7 to 1:9 (first solvent:second solvent).

The non-aqueous electrolyte solution for a lithium-sulfur battery of the present invention may further include 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 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 amide, nitrobenzene derivative, sulfur, quinone imine dye, and N-substituted oxazoli Dinon, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sulfone (PRS), vinylene carbonate ( VC) and the like.

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

[Equation 4]

(In Equation 4 above,

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

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

C is the discharge capacity (mAh/g) when discharging at 0.1C rate,

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

The higher the ED factor, the higher the energy density in a real lithium-sulfur secondary battery. According to 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 the driving of the 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.

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 manufactured by impregnating it in an electrolyte and sealing the obtained result in a battery case.

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

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

Examples of the medium-to-large-sized device include a power tool that moves under power by an all-electric motor; Electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like; Electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters); Electric golf carts; A power storage system, and the like, but is not limited thereto.

Hereinafter, a preferred embodiment is provided to help the understanding of the present invention, but the following examples are only illustrative of the present invention, and it is apparent to those skilled in the art that various changes and modifications are possible within the scope and technical scope of the present invention. It is no wonder that such variations and modifications fall within the scope of the appended claims.

Examples and comparative examples

[Example 1]

Carbon nanotubes and sulfur (S ₈ ) were evenly mixed in a weight ratio of 3:1, crushed by mortar mixing, and placed in an oven at 155° C. for 30 minutes to prepare a sulfur-carbon composite.

A cathode active material slurry was prepared by mixing the sulfur-carbon composite, conductive material, and binder prepared above. At this time, as the conductive material, a conductive material in the form of a mixture of carbon nanotubes and graphene having a specific surface area of 150 m 2 /g or more in a weight ratio of 9:1 was used, and the mixing ratio was sulfur-carbon composite: conductive material: binder in a weight ratio. Was set to 90:5:5.

The positive electrode active material slurry thus prepared was coated on a current collector of 20 μm thick aluminum foil and dried to prepare a positive electrode (energy density of the positive electrode: 5.45 mAh/cm 2 ). The porosity of the positive electrode active material layer calculated by measuring the electrode weight and electrode thickness (using TESA-μHITE equipment manufactured by TESA) in the prepared positive electrode was 68%, and the mass of sulfur per unit area of the positive electrode active material layer was 4.54 mg/cm 2. The SC factor value calculated based on this was 0.668.

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

Subsequently, after placing the electrode assembly inside the case, an electrolyte was injected to prepare a lithium-sulfur secondary battery. At this time, the electrolyte was prepared by dissolving 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. In the first solvent, the dipole moment per unit volume 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. Charging and discharging of the produced battery was performed at 45°C.

[Comparative Example 1]

By changing the manufacturing conditions of the positive electrode, vapor-grown carbon fibers (VGCF) were used as the conductive material, the energy density of the positive electrode was 5.67 mAh/cm 2, the porosity of the positive electrode active material layer was 68%, and the mass of sulfur per unit area of the positive electrode active material layer was A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that a positive electrode having an SC factor value of 0.700 was calculated based on this value of 4.73 mg/cm 2.

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

Electrolyte composition SC factor DV ² factor NS factor ED factor Example 1 Composition of the first electrolyte ¹⁾ 0.668 0.39 0.58 1188 Comparative Example 1 0.700 0.39 0.56 1072 ¹⁾ First electrolyte composition = Propionitrile: 1H,1H,2'H,3H-Decafluorodipropyl ether(3:7, w/w) solvent, 3.0M LiTFSI

Experimental Example 1. Scanning electron microscope analysis

The surface shape of the anode prepared in Example 1 and Comparative Example 1 was observed using a scanning electron microscope (SEM). The results obtained at this time are shown in FIGS. 1 and 2 below.

1 and 2, it can be confirmed that the positive electrode of Comparative Example 1 is not densely formed in a conductive structure by a conductive material compared to the positive electrode of Example 1.

Experimental Example 2. Battery performance evaluation

The batteries prepared in Example 1 and Comparative Example 1 were measured for capacity from 1.0 to 3.6 V using a charge/discharge measuring device. In addition, the discharge capacity was measured by performing a cycle of sequentially discharging at 0.1C and 0.2C rate CC (CC: Constant Current). The results obtained at this time are shown in FIGS. 3 to 6.

3 to 6, it can be confirmed that the capacity characteristics of the battery including the positive electrode according to the present invention are superior to the comparative example.

3 and 5, it can be seen that Comparative Example 1 using an existing conductive material exhibits a significantly lower capacity compared to the discharge capacity of the battery according to Example 1.

In addition, it can be seen through FIG. 4 and FIG. 6 that the discharge capacity of the battery including the positive electrode of Example 1 is higher than that of Comparative Example 1 throughout the cycle, and thus the discharge capacity retention rate is excellent. From the above results, it can be confirmed that the lithium-sulfur secondary battery of the present invention can realize a higher energy density that could not be realized in the existing lithium-sulfur secondary battery.

Claims (14)

A lithium-sulfur secondary battery comprising an anode, a cathode, a separator and an electrolyte,
The positive electrode includes a conductive material including a high specific surface area carbon material,
The positive electrode is a lithium-sulfur secondary battery having an SC factor value of 0.45 or more represented by Equation 1 below:
[Equation 1]

(In Equation 1 above,
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 ),
α is 10 (constant). According to claim 1,
The high specific surface area carbon material has a specific surface area of 100 to 500 m 2 /g, a lithium-sulfur secondary battery. According to claim 2,
The high specific surface area carbon material includes at least one selected from the group consisting of carbon nanotubes, graphene, carbon black, and carbon fibers, a lithium-sulfur battery. According to claim 3,
The high specific surface area carbon material is a mixture of carbon nanotubes and graphene in a weight ratio of 0:10 to 10:0, lithium-sulfur battery. According to claim 1,
The high specific surface area carbon material is contained in 0.01 to 30% by weight based on the total weight of the positive electrode active material slurry, lithium-sulfur battery. According to claim 1,
The electrolyte solution includes a solvent and a lithium salt,
The solvent is a lithium-sulfur secondary battery comprising a first solvent having a DV ² factor value of 1.75 or less and a second solvent being a fluorinated ether-based solvent represented by Equation 2 below:
[Equation 2]

(In Equation 2 above,
DV is the dipole moment per unit volume (D·mol/L),
μ is the viscosity of the solvent (cP, 25℃),
γ is 100 (constant). The method of claim 6,
The first solvent has a DV ² factor value of 1.5 or less, lithium-sulfur secondary battery. The method of claim 6,
The lithium-sulfur secondary battery, the NS factor value represented by Equation 3 below is 3.5 or less, lithium-sulfur secondary battery:
[Equation 3]

(In Equation 3 above,
SC factor is the same as the value defined by Equation 1 above,
The DV ² factor is the same as the value defined by Equation 2 above.). According to claim 1,
The lithium-sulfur secondary battery is a lithium-sulfur secondary battery having an ED factor value of 850 or more represented by Equation 4 below:
[Equation 4]

(In Equation 4 above,
V is the discharge nominal voltage (V) for Li/Li ⁺ ,
SC factor is the same as the value defined by Equation 1 above,
C is the discharge capacity (mAh/g) when discharging at 0.1C rate,
D is the density of the electrolyte (g/cm 3 ).) The method of claim 6,
The first solvent is a lithium-sulfur secondary battery comprising at least one member selected from the group consisting of propionitrile, dimethylacetamide, dimethylformamide, gamma-butyrolactone, triethylamine and 1-iodopropane. . The method of claim 6,
The second solvent is 1H,1H,2'H,3H-decafluorodipropyl ether, difluoromethyl 2,2,2-trifluoroethyl ether, 1,2,2,2-tetrafluoroethyl tri Fluoromethyl ether, 1,1,2,3,3,3-hexafluoropropyl difluoromethyl ether, pentafluoroethyl 2,2,2-trifluoroethyl ether and 1H,1H,2′H -A lithium-sulfur secondary battery comprising at least one member selected from the group consisting of perfluorodipropyl ether. The method of claim 6,
The solvent includes a first solvent in an amount of 1 to 50% by weight based on the total weight of the solvent, lithium-sulfur battery. The method of claim 6,
The solvent comprises a second solvent in an amount of 50 to 99% by weight based on the total weight of the solvent, lithium-sulfur battery. The method of claim 6,
The solvent comprises a first solvent and a second solvent in a weight ratio of 3:7 to 1:9, lithium-sulfur battery.

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