WO2020231162A1 - Sulfur-carbon composite, and cathode and lithium secondary battery each comprising same - Google Patents

Abstract

The present invention relates to a sulfur-carbon composite and a cathode for a lithium secondary battery and a lithium secondary battery each comprising same. More specifically, carbons contained in the sulfur-carbon composite may include various shapes of carbons. Particularly, when applied as a cathode active material in a lithium battery, the sulfur-carbon composite containing a predetermined amount of planar-shaped carbons can prevent the elution of sulfur and improve the reaction rate in the cathode, thereby enhancing the performance of the lithium secondary battery.

Description

Sulfur-carbon composite, positive electrode and lithium secondary battery including the same

This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0056010 filed May 14, 2019 and Korean Patent Application No. 10-2020-0056606 filed May 12, 2020. All contents disclosed in the literature are included as part of this specification.

The present invention relates to a sulfur-carbon composite applicable as a positive electrode material of a lithium secondary battery, a positive electrode including the same, and a lithium secondary battery.

Recently, the demand for secondary batteries is increasing with the rapid development of electronic devices and electric vehicles. In particular, according to the trend of miniaturization and weight reduction of portable electronic devices, there is a growing demand for a secondary battery having a high energy density that can meet the trend.

Among the secondary batteries, a lithium-sulfur secondary battery uses a sulfur-based compound having a sulfur-sulfur bond as a positive electrode active material, and an alkali metal such as lithium or a carbon-based material in which metal ions such as lithium ions are inserted and deintercalated, or an alloy with lithium It is a secondary battery that uses silicon or tin to form a negative electrode active material. Specifically, electrical energy is stored by using an oxidation-reduction reaction in which sulfur-sulfur bonds are cut off during discharge, which is a reduction reaction, and the oxidation number of sulfur decreases, and during charging, which is an oxidation reaction, sulfur-sulfur bonds are formed again as the oxidation number of sulfur increases. And create it.

In particular, sulfur, which is used as a positive electrode active material in lithium-sulfur secondary batteries, has a theoretical energy density of 1675 mAh/g, and has a theoretical energy density that is 5 times higher than that of the positive electrode active material used in conventional lithium secondary batteries. It is a battery capable of expressing the density. In addition, sulfur is attracting attention as an energy source for mid- to large-sized devices such as electric vehicles as well as portable electronic devices because of its low cost, rich reserves, easy supply, and environmental friendliness.

However, since sulfur has no conductivity, it is applied as an electrochemical positive electrode active material by forming a porous carbon material and a sulfur-carbon composite. In the sulfur-carbon composite applied as a positive electrode active material as described above, sulfur (S) becomes Li ₂ S by the electrons transferred through the carbon in the lead wire and the sulfur-carbon composite and lithium ions transferred through the electrolyte from the negative electrode. Reduce.

Sulfur in its natural state exists in the form of an S ₈ ring, and in the process of becoming Li ₂ S with the discharge of the battery, Li ₂ S ₈ , Li ₂ S ₆ , Li ₂ S ₄ After passing through the form of lithium-polysulfide such as, etc., these lithium-polysulfides are well soluble in the electrolyte, so the lithium-polysulfide moves to the negative electrode in a dissolved state in the electrolyte, resulting in a reduction in life (shuttle effect). have.

In addition, in the case of minimizing the use of carbon in the sulfur-carbon composite and increasing energy density by increasing the sulfur content, an optimal carbon structure is required because the transfer resistance of electrons and lithium ions increases as the sulfur content increases.

Korean Patent Publication No. 2016-0051610, a patent that applies carbon having various types of structures in a sulfur-carbon composite, relates to a cathode material for a lithium-sulfur secondary battery, a mixture of a sulfur-carbon nanotube composite and a sulfur-graphene composite. Disclosed is a technology for using a sulfur-carbon composite comprising a cathode material.

However, since carbon materials have advantages and disadvantages depending on their shape, various types of carbon materials used in the manufacture of sulfur-carbon composites are optimal in order to maximize the advantages of the shapes of these carbon materials and minimize the disadvantages. It is necessary to develop a technology for producing a sulfur-carbon composite that is effective in improving the performance of a lithium-sulfur secondary battery by combining it under the conditions of.

[Prior technical literature]

[Patent Literature]

(Patent Document 1) Korean Patent Publication No. 2016-0051610

Accordingly, the present inventors, when manufacturing a sulfur-carbon composite applied as a positive electrode active material of a lithium secondary battery, contain planar carbon in a certain ratio, and also limit the elution of sulfur by including at least one of point carbon and linear carbon. , To provide a sulfur-carbon composite capable of improving battery performance by facilitating contact with an electrolyte and maintaining a reaction rate.

Accordingly, an object of the present invention is to provide a sulfur-carbon composite containing planar carbon in a certain ratio.

Another object of the present invention is to provide a positive electrode for a lithium secondary battery comprising a sulfur-carbon composite containing the planar carbon in a certain ratio.

Another object of the present invention is to provide a lithium secondary battery comprising a sulfur-carbon composite containing the planar carbon in a predetermined ratio.

In order to achieve the above object, the present invention is a sulfur-carbon composite comprising planar carbon,

The planar carbon provides a sulfur-carbon composite containing more than 0% by weight and less than 50% by weight based on the total weight of the carbon.

The present invention also provides a positive electrode for a lithium secondary battery comprising the sulfur-carbon composite.

The present invention also provides a lithium secondary battery comprising the positive electrode.

The sulfur-carbon composite according to the present invention may improve the performance of a battery by including planar carbon or various types of carbon including planar carbon.

Since the sulfur-carbon composite contains planar carbon, it is possible to support a large amount of sulfur to increase energy density and prevent sulfur from eluting when applied to a positive electrode for a lithium secondary battery.

In addition, the sulfur-carbon composite includes a sulfur-carbon composite in which sulfur is exposed to the outside, such as a sulfur-carbon composite containing point-like carbon or a linear carbon-composite containing linear carbon, so that contact between sulfur and the electrolyte is possible. It is easy to maintain the reaction rate and prevent voltage drop.

In addition, when the sulfur-carbon composite is applied to a positive electrode of a lithium secondary battery, the discharge capacity and high rate characteristics of the lithium secondary battery can be improved.

1A to 1C are schematic diagrams of a planar sulfur-carbon composite, a point-type sulfur-carbon composite, and a linear sulfur-carbon composite, respectively, included in the sulfur-carbon composite according to the present invention.

2 is a schematic diagram of a positive electrode for a lithium secondary battery comprising a sulfur-carbon composite according to the present invention.

3A is a graph showing the initial discharge performance of a lithium-sulfur secondary battery in which the sulfur-carbon composites each prepared in Examples 1 to 4 and Comparative Example 1 were applied to a positive electrode, and FIG. 3B is a graph showing the initial discharge performance of Examples 1 to 4 and Comparative Example 1 Is a graph showing the results of measuring the high rate characteristics of a lithium-sulfur secondary battery in which the sulfur-carbon composites prepared in each were applied to a positive electrode.

4A is a graph showing the initial discharge performance of a lithium-sulfur secondary battery in which the sulfur-carbon composites each prepared in Examples 5 to 6 and Comparative Example 1 were applied to a positive electrode, and FIG. 4B is a graph showing the initial discharge performance of Examples 5 to 6 and Comparative Example 1 Is a graph showing the results of measuring the high rate characteristics of a lithium-sulfur secondary battery in which the sulfur-carbon composites prepared in each were applied to a positive electrode.

5 is a SEM (Scanning Electron Microscope) photograph of the sulfur-carbon composites prepared in Examples 2 to 4, respectively.

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

The term "point carbon" as used herein refers to carbon having a shape similar to a dot, and is also referred to as zero-dimensional carbon. In addition, the sulfur-carbon composite including the "point-type carbon" is referred to as "point-type sulfur-carbon composite".

The term "linear carbon" as used herein refers to carbon having a shape similar to a line and is also referred to as one-dimensional carbon. In addition, the sulfur-carbon composite including the "linear carbon" is referred to as a "linear sulfur-carbon composite".

The term "planar carbon" as used herein refers to carbon having a shape similar to that of cotton, and is also referred to as two-dimensional carbon. In addition, the sulfur-carbon composite including the "facet carbon" is referred to as "facet sulfur-carbon composite".

Sulfur-carbon complex

The present invention relates to a sulfur-carbon composite comprising carbon in various shapes. Since the shape of the sulfur-carbon composite formed according to the shape of the carbon may be determined, the sulfur-carbon composite may have various shapes according to the carbon shape.

For example, the shape of the sulfur-carbon composite may have the same shape as that of carbon. In other words, the sulfur-carbon composite including the planar carbon may also be a planar sulfur-carbon composite having a planar shape, and the sulfur-carbon composite including the point-like carbon may also be a point sulfur-carbon composite having a point shape, , The sulfur-carbon composite including the linear carbon may have a linear sulfur-carbon composite shape.

The sulfur-carbon composite according to the present invention may include planar carbon. In addition, the sulfur-carbon composite may further include at least one selected from point-type carbon and linear carbon in addition to planar carbon.

The sulfur-carbon composite according to the present invention may contain 40 to 95% by weight of sulfur and 5 to 60% by weight of carbon. The sulfur content of the sulfur-carbon composite may be 40% by weight or more, 45% by weight or more, 50% by weight or more, 55% by weight or more, or 60% by weight or more, and also 75% by weight or less, 80% by weight or less, 85 It may be less than or equal to 90% by weight, or less than or equal to 95% by weight. When the sulfur is included in the prescribed range, the energy density of the lithium secondary battery can be improved. Therefore, if the sulfur content is less than 40% by weight, the energy density may be lowered, and if the content is more than 95% by weight, the electron and lithium ion transfer resistance may increase.

In addition, the content of carbon contained in the sulfur-carbon composite may be 5% by weight or more, 10% by weight or more, 15% by weight or more, 20% by weight or more, or 25% by weight or more, and also 40% by weight or less, 45% by weight It may be less than, 50% by weight or less, 55% by weight or less, or 60% by weight or less. When the carbon is included in the prescribed range, electronic conductivity and lithium ion conductivity of a lithium secondary battery may be improved. Therefore, if the carbon content is less than 5% by weight, electronic conductivity and lithium ion conductivity may be lowered, and if it is more than 60% by weight, the sulfur content is relatively lowered, thereby lowering energy density.

In the present invention, the sulfur is sulfur (S ₈ ), Li ₂ S _n (n ≥ 1), an organic sulfur compound and a carbon-sulfur polymer [(C ₂ S _x ) _n , x is an integer of 2.5 to 50, n ≥ 2] may be one or more selected from the group consisting of.

In the present invention, the carbon may further include at least one selected from point-type carbon and linear carbon in addition to planar carbon, and the present invention will be described in more detail below with reference to the drawings.

1A is a schematic diagram of a planar sulfur-carbon composite according to the present invention.

1A, the planar sulfur-carbon composite 11 is a form in which sulfur (S2) is inserted between planar carbon (C2), in other words, planar carbon (C2) supports and wraps sulfur (S2). Form. Due to such morphological characteristics, when the planar sulfur-carbon composite 11 is applied as a positive electrode active material of a lithium secondary battery, elution of sulfur can be prevented.

Planar carbon (C2) may be contained in an amount greater than 0% by weight and less than 50% by weight based on the total weight of carbon included in the sulfur-carbon composite according to the present invention. Specifically, the content of the planar carbon (C2) may be more than 0 wt%, 5 wt% or more, or 10 wt% or more, and 30 wt% or less, 35 wt% or less, 40 wt% or less, based on the total weight of the carbon , 45% or less or less than 50% by weight. If the content of the planar carbon (C2) is 0% by weight, the effect of preventing the elution of sulfur from the positive electrode is insignificant and there may be no effect of improving the performance by the planar carbon. Since is lowered, a voltage drop occurs, and sufficient battery capacity may not be implemented.

Planar carbon (C2) is selected from the group consisting of non-oxide graphene, graphene oxide, reduced graphene oxide, doped graphene, and carbon nanoribbon. It may be one or more, preferably reduced graphene oxide.

The specific surface area of the planar carbon (C2) may be greater than the sum of the specific surface areas of other carbons included in the sulfur-carbon composite. For example, when the sulfur-carbon composite includes planar carbon, point carbon and linear carbon, the specific surface area of the planar carbon may be greater than the sum of the specific surface areas of the point carbon and the linear carbon.

The specific surface area of the planar carbon (C2) may be 200 m2/g to 1000 m2/g, specifically 200 m2/g or more, 300 m2/g or more, 400 m2/g or more, or 500 m2/g or more, and , 700 m2/g or less, 800 m2/g or less, 900 m2/g or less, or 1000 m2/g or less. When planar carbon (C2) having such a specific surface area is used, it is possible to support more sulfur to improve the capacity of the battery and to suppress the elution of sulfur.

The content of sulfur (S2) contained in the planar sulfur-carbon composite 11 may be 10 to 45% by weight based on the total weight of sulfur contained in the sulfur-carbon composite, and specifically 10% by weight or more, 15 It may be greater than or equal to 20% by weight, or less than or equal to 35%, less than or equal to 40%, or less than or equal to 45% by weight. If it is less than the above range, the sulfur content in the battery decreases and the battery capacity is excessively reduced, and if it exceeds the above range, the electrical conductivity in the electrode excessively decreases, thereby increasing the resistance.

1B is a schematic diagram of a point-type sulfur-carbon composite according to the present invention, and FIG. 1C is a schematic diagram of a linear sulfur-carbon composite according to the present invention.

1B and 1C, the point-type sulfur-carbon composite 12 has a core-shell form by forming sulfur (S0) on the surface of the point-type carbon (C0), and the linear sulfur-carbon composite 13 is linear carbon. Since sulfur (S1) is formed inside and/or on the surface of (C1) to have a tube shape, sulfur (S0, S1) is exposed to the outside. Due to these morphological features, when the point-shaped sulfur-carbon composite 12 or the linear sulfur-carbon composite 13 is applied as a positive electrode active material of a lithium secondary battery, the sulfur (S0, S1) exposed to the surface is It is easy to contact the bar, it is possible to improve the battery performance by preventing the voltage drop.

At least one carbon selected from point-like carbon (C0) and linear carbon (C1) may be 50% by weight or more and less than 100% by weight based on the total weight of carbon contained in the sulfur-carbon composite according to the present invention, and specifically , 50% by weight or more, 55% by weight or more, 60% by weight or more, 65% by weight or more, or 70% by weight or more, and also 80% by weight or less, 85% by weight or less, 90% by weight or less, 95% by weight or less or It may be less than 100% by weight. If the content of one or more of the carbons selected from point-like carbon (C0) and linear carbon (C1) is less than 50% by weight, the electrolyte is not smoothly in and out of the positive electrode and lithium ion conductivity is lowered, resulting in a voltage drop and sufficient battery capacity. In the case of 100% by weight, since there is no surface carbon (C2), the effect of preventing the elution of sulfur from the positive electrode is insignificant, and the discharge capacity and life characteristics of the lithium secondary battery may be deteriorated.

Point carbon (C0) may be at least one selected from the group consisting of ketjen black, denka black, acetylene black, super-p, and fullerene. And, preferably, it may be Ketjen Black.

Linear carbon (C1) may be one or more selected from the group consisting of carbon nanotubes (CNT) and carbon fibers, and preferably carbon nanotubes.

The content of sulfur (S0, S1) contained in at least one selected from the point-type sulfur-carbon composite 12 and the linear sulfur-carbon composite 13 is 55 to the total weight of sulfur contained in the sulfur-carbon composite. It may be 90% by weight, specifically 55% by weight or more, 60% by weight or more, or 65% by weight or more, and also 85% by weight or less, 90% by weight or less, or 95% by weight or less. If it is less than the above range, the sulfur content in the battery decreases and the battery capacity is excessively reduced, and if it exceeds the above range, the electrical conductivity in the electrode excessively decreases, thereby increasing the resistance.

Method for producing sulfur-carbon composite

In the present invention, a method for preparing a sulfur-carbon composite is not particularly limited, and a method for producing a sulfur-carbon composite commonly used in the art may be used.

In addition, the form of carbon contained in the sulfur-carbon composite according to the present invention, that is, planar carbon, point-like carbon, or linear carbon may all be applied to the same method for preparing the sulfur-carbon composite.

For example, the sulfur-carbon composite may be prepared by a melt diffusion method. The melt diffusion method is a manufacturing method in which sulfur penetrates into carbon particles by melting sulfur through heating. In this case, the heat treatment may include various direct or indirect heating methods.

The sulfur-carbon composite according to the present invention comprises the steps of (S1) mixing sulfur and carbon; And heat-treating the mixture of sulfur and carbon formed in the step (S1).

The amounts and types of sulfur and carbon in the step (S1) are as described above.

In addition, the heat treatment temperature in the step (S2) is a temperature at which sulfur is dissolved and permeated into the carbon to be supported, and may be higher than the melting point of sulfur.

Specifically, the temperature during the heat treatment may be 100 to 200°C, specifically, 100°C or more, 105°C or more, 110°C or more, 115°C or more, or 120°C or more, and 180°C or less, 185°C or less , 190°C or less, 195°C or less, or 200°C or less, and may be heat treated by a melt diffusion method. If it is less than the above range, the sulfur-carbon composite itself may not be manufactured because the process of dissolving sulfur and seeping into the carbon does not proceed.If it exceeds the above range, the loss rate increases due to the vaporization of sulfur, and the sulfur-carbon composite is denatured to become a cathode material of a lithium secondary battery. When applied, the effect of improving the performance of the battery may be insignificant.

In addition, the planar sulfur-carbon composite, point-type sulfur-carbon composite, and linear sulfur-carbon composite according to the present invention may be prepared respectively according to the method for preparing a sulfur-carbon composite as described above, or may be prepared simultaneously.

Lithium secondary battery

The present invention also relates to a lithium secondary battery comprising the sulfur-carbon composite as described above. In this case, the sulfur-carbon composite may preferably be included as a positive electrode active material.

The lithium secondary battery according to the present invention may include a positive electrode, a negative electrode, a separator and an electrolyte interposed therebetween.

2 is a schematic diagram of a positive electrode for a lithium secondary battery comprising a sulfur-carbon composite according to the present invention.

Referring to FIG. 2, the positive electrode 1 for a lithium secondary battery may include a positive electrode current collector 20 and a positive electrode active material layer 10 having a positive electrode active material formed on the positive electrode current collector 20.

The positive electrode active material may include a sulfur-carbon composite, and the sulfur-carbon composite may include a planar sulfur-carbon composite 11, and a point-shaped sulfur-carbon composite 12 and a linear sulfur-carbon composite ( It may contain one or more selected from among 13).

In addition, a lithium-containing transition metal oxide may be used as the positive electrode active material, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li(Ni _a Co _b Mn _c )O ₂ (0<a<1,0<b<1,0<c<1, a+b+c=1), LiNi _{1 - y} Co _y O ₂ , LiCo _{1 - y} Mn _y O ₂ , LiNi _{1 - y} Mn _y O ₂ (O≤y<1), Li(Ni _a Co _b Mn _c )O ₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn _{2 - z Ni z O 4, LiMn} 2 - z Co z O 4 (0 <z <2), LiCoPO may be used any one or a mixture of two or more of these _four and is selected from the group consisting of LiFePO _4. In addition, in addition to these oxides, sulfide, selenide, and halide may be used.

In addition, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes to the battery, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or on the surface of aluminum or stainless steel. Carbon, nickel, titanium, silver or the like surface-treated may be used. In this case, the positive electrode current collector may be in various forms such as a film, sheet, foil, net, porous material, foam, non-woven fabric having fine irregularities formed on the surface so as to increase adhesion to the positive electrode active material.

In the present invention, the negative electrode of the lithium secondary battery may include a negative electrode current collector and a negative electrode active material layer having a negative electrode active material formed on the negative electrode current collector.

As the negative active material, lithium metal or a carbon material through which lithium ions can be occluded and released may be used, such as silicon or tin. Preferably, a carbon material may be used, and both low crystalline carbon and high crystalline carbon may be used as the carbon material. As low crystalline carbon, soft carbon and hard carbon are typical, and high crystalline carbon is natural graphite, kish graphite, pyrolytic carbon, liquid crystal pitch-based carbon fiber (mesophase pitch based carbon fiber), meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbons such as petroleum or coal tar pitch derived cokes are typical. At this time, the negative electrode may include a binder, and as the binder, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride (PVDF), and polyacrylonitrile ), polymethylmethacrylate, etc., various kinds of binder polymers may be used.

In addition, the negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, or the like may be used. In addition, the negative electrode current collector, like the positive electrode current collector, may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric having fine irregularities on the surface thereof.

In this case, the positive electrode active material layer or the negative electrode active material layer may further include a binder resin, a conductive material, a filler, and other additives.

The binder resin is used for bonding of an electrode active material and a conductive material and bonding to a current collector. Examples of such binder resins include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetra Fluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber, and various copolymers thereof.

The conductive material is used to further improve the conductivity of the electrode active material. Such a conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Polyphenylene derivatives and the like can be used. Preferably, the conductive material may be a vapor grown carbon fiber (VGCF).

The filler is selectively used as a component that suppresses the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing chemical changes to the battery, and examples thereof include olefin-based polymers such as polyethylene and polypropylene; Fibrous materials such as glass fiber and carbon fiber are used.

In the present invention, the separator may be made of a porous substrate, and the porous substrate may be used as long as it is a porous substrate commonly used in an electrochemical device, for example, a polyolefin-based porous membrane or a nonwoven fabric. It can be used, but is not particularly limited thereto.

Examples of the polyolefin-based porous membrane include polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, polyolefin-based polymers such as polypropylene, polybutylene, and polypentene, respectively, or a mixture of them. There is one membrane.

As the nonwoven fabric, in addition to the polyolefin nonwoven fabric, for example, polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate ), polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, and polyethylenenaphthalene, respectively, alone or Nonwoven fabrics formed of polymers obtained by mixing them are exemplified. The structure of the nonwoven fabric may be a spunbond nonwoven fabric composed of long fibers or a melt blown nonwoven fabric.

The thickness of the porous substrate is not particularly limited, but may be 1 μm to 100 μm, or 5 μm to 50 μm.

The size and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.001 μm to 50 μm and 10% to 95%, respectively.

In the present invention, the electrolyte may be a nonaqueous electrolyte, and the electrolyte salt contained in the nonaqueous electrolyte is a lithium salt. The lithium salt may be used without limitation, those commonly used in an electrolyte for a lithium secondary battery. For example, the lithium salt is LiFSI, LiPF ₆ , LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiPF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, it may be one or more selected from the group consisting of lithium chloroborane and lithium 4-phenyl borate.

As organic solvents included in the above-described non-aqueous electrolyte, those commonly used in electrolytes for lithium secondary batteries can be used without limitation, and for example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc. can be used alone or in two or more types. It can be mixed and used. Among them, representatively, a cyclic carbonate, a linear carbonate, or a carbonate compound that is a slurry thereof may be included.

Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, Any one selected from the group consisting of 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, or two or more of these slurries. These halides include, for example, fluoroethylene carbonate (FEC), but are not limited thereto.

In addition, a specific example of the linear carbonate compound is any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, or Two or more of these slurries may be representatively used, but are not limited thereto. Particularly, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are organic solvents of high viscosity and have high dielectric constants, so that lithium salts in the electrolyte can be more easily dissociated.These cyclic carbonates include dimethyl carbonate and diethyl carbonate. If a low viscosity, low dielectric constant linear carbonate is mixed in an appropriate ratio and used, an electrolyte solution having a higher electrical conductivity can be prepared.

In addition, the ether of the organic solvent is selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether, ethylpropyl ether, dimethoxyethane (DME) and dioxolane (DOL). Any one or two or more of these slurries may be used, but the present invention is not limited thereto.

In addition, esters in the organic solvent include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, Any one selected from the group consisting of σ-valerolactone and σ-caprolactone, or two or more types of slurry may be used, but is not limited thereto.

The injection of the non-aqueous electrolyte may be performed at an appropriate step in the manufacturing process of the electrochemical device, depending on the manufacturing process and required physical properties of the final product. That is, it can be applied before assembling the electrochemical device or at the final stage of assembling the electrochemical device.

In the lithium secondary battery according to the present invention, in addition to winding, which is a general process, lamination, stacking, and folding of a separator and an electrode are possible.

Further, the shape of the battery case is not particularly limited, and may be in various shapes such as a cylindrical shape, a stacked type, a square shape, a pouch type, or a coin type. The structure and manufacturing method of these batteries are widely known in this field, and thus detailed descriptions are omitted.

In addition, the lithium secondary battery can be classified into various batteries, such as lithium-sulfur secondary batteries, lithium-air batteries, lithium-oxide batteries, and lithium all-solid batteries, depending on the material of the positive electrode/cathode used.

The present invention also provides a battery module including the lithium secondary battery as a unit cell.

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

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

Lithium-sulfur secondary battery

The sulfur-carbon composite according to the present invention can be applied to a positive electrode of a lithium-sulfur secondary battery among lithium secondary batteries.

In this case, the lithium-sulfur secondary battery may be a battery including the sulfur-carbon composite as a positive electrode active material.

The sulfur-carbon composite can exhibit high ionic conductivity by securing the path of lithium ions to the inside of the pores, and acts as a sulfur carrier to increase reactivity with sulfur, which is a positive electrode active material, to increase the initial discharge capacity of a lithium-sulfur secondary battery. And high rate performance can be improved at the same time.

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

Comparative Example 1

(1) Preparation of sulfur-carbon composite

(1-1) Mixing of carbon and sulfur

A mixture of 25% by weight of carbon powder and 75% by weight of sulfur powder was obtained. In this case, the carbon powder is linear carbon and is a carbon nanotube (CNT) powder.

(1-2) Heat treatment

The mixture obtained in (1-1) was heat-treated at 155° C. to prepare a sulfur-carbon composite through a melt diffusion method. In this case, the sulfur-carbon complex includes a linear sulfur-carbon complex.

(2) anode manufacturing

The sulfur-carbon composite obtained in (1) above as a positive electrode active material, VGCF (Vapor grown carbon fiber) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 8:1:1, and a concentration of 20% Disperse in water to prepare a positive electrode slurry.

The positive electrode slurry was coated on Al foil and dried to prepare a positive electrode.

(3) Manufacture of lithium-sulfur secondary battery

50 μm thick lithium foil as the negative electrode, the positive electrode prepared in (2) above, the electrolyte was DOL/DME (1:1, v/v) as a solvent, and 1M LiTFSI and 3% by weight of LiNO ₃ were included. A lithium-sulfur secondary battery in the form of a coin cell was manufactured using the electrolyte solution and polyolefin separator prepared as the composition. In this case, DOL means dioxolane, and DME means dimethoxyethane.

Example 1

(1) Preparation of sulfur-carbon composite

(1-1) Mixing of carbon and sulfur

A mixture of 25% by weight of carbon powder and 75% by weight of sulfur powder was obtained. At this time, the carbon powder includes 10% by weight of reduced graphene oxide powder as planar carbon and 90% by weight of CNT powder as linear carbon. The specific surface area of the reduced graphene oxide is 600 m 2 /g.

(1-2) Heat treatment

The mixture obtained in (1-1) was heat-treated at 155° C., so that sulfur was supported on the carbon through a melt diffusion method to prepare a sulfur-carbon composite. At this time, the prepared sulfur-carbon composite includes a planar sulfur-carbon composite and a linear sulfur-carbon composite.

(2) anode manufacturing

The sulfur-carbon composite obtained in (1) above as a positive electrode active material, and polyvinylidene fluoride (PVDF) as a VGCF (Vapor grown carbon fiber) binder as a conductive material were mixed in a weight ratio of 8:1:1, and at a concentration of 20%. Disperse in water to prepare a positive electrode slurry.

The positive electrode slurry was coated on Al foil and dried to prepare a positive electrode.

(3) Manufacture of lithium-sulfur secondary battery

50 μm thick lithium foil as the negative electrode, the positive electrode prepared in (2) above, the electrolyte was DOL/DME (1:1, v/v) as a solvent, and 1M LiTFSI and 3% by weight of LiNO ₃ were included. A lithium-sulfur secondary battery in the form of a coin cell was manufactured using the electrolyte solution and polyolefin separator prepared as the composition. In this case, DOL means dioxolane, and DME means dimethoxyethane.

Examples 2 to 4

It was carried out in the same manner as in Example 1, but as shown in Table 1 below, based on the total weight of carbon contained in the sulfur-carbon composite, the planar carbon content was 20% by weight, 30% by weight, and 40% by weight, respectively, and sulfur- A carbon composite, a positive electrode, and a lithium-sulfur secondary battery were prepared.

Examples 5 to 6

In the same manner as in Example 1, a sulfur-carbon composite was prepared in the composition as shown in Table 1 below. At this time, as shown in Table 1 below, the surface carbon content is 26% by weight and 35% by weight, respectively, based on the total weight of carbon contained in the sulfur-carbon composite, and the sulfur-carbon composite, the positive electrode, and the lithium-sulfur secondary battery are Was prepared.

Unit:% by weight	Sulfur-carbon complex		carbon		sulfur
	Sulfur content	Carbon content	Cotton type carbon content	Linear carbon content	Cotton sulfur content	Linear sulfur content
Comparative Example 1	75	25	0	100	0	100
Example 1	75	25	10	90	10	90
Example 2	75	25	20	80	20	80
Example 3	75	25	30	70	30	70
Example 4	75	25	40	60	40	60
Example 5	76.5	23.5	26	74	31	69
Example 6	77	23	35	65	42	58

Experimental example 1: Analysis of performance improvement effect of lithium-sulfur secondary battery

An experiment was conducted on the performance of a lithium-sulfur secondary battery in which the sulfur-carbon composites each prepared in Examples 1 to 6 and Comparative Example 1 were applied to a positive electrode. The lithium-sulfur secondary battery manufactured in the form of a coin cell was repeatedly charged/discharged at room temperature to conduct a charge/discharge test, and after the first discharge was performed at 0.1C, charging/discharging was repeated two more times in the same manner. Charging/discharging was repeated three times at 0.2c, and then charging/discharging was continued at 0.3C/0.5C. Through this, a capacity-voltage graph was obtained at the first discharge to evaluate the initial discharge performance, and a capacity change graph according to cycle repetition was obtained to evaluate high rate performance (0.3C/0.5C).

3A is a graph showing the initial discharge performance of a lithium-sulfur secondary battery in which the sulfur-carbon composites each prepared in Examples 1 to 4 and Comparative Example 1 were applied to a positive electrode, and FIG. 3B is a graph showing the initial discharge performance of Examples 1 to 4 and Comparative Example 1 Is a graph showing the results of measuring the high rate characteristics of a lithium-sulfur secondary battery in which the sulfur-carbon composites prepared in each were applied to a positive electrode.

Referring to FIG. 3A, the discharge capacity per weight of sulfur is increased in the lithium-sulfur secondary batteries of Examples 1 to 4 including both planar carbon and linear carbon compared to Comparative Example 1 that does not include planar carbon and includes only linear carbon. I can see that. Among Examples 1 to 4, as the content of planar carbon increased, the discharge capacity increased, and it was confirmed that the discharge capacity of Example 3 in which the content of planar carbon was 30% by weight was the highest. On the other hand, it was confirmed that the discharge capacity did not increase any more when the content of planar carbon was 40% by weight as in Example 4 exceeding 30% by weight.

In addition, referring to FIG. 3B, it can be seen that even if the charge/discharge rate is changed, the discharge capacity of Example 3 in which the content of planar carbon is 30% by weight is the highest as in the initial discharge performance curve of FIG. Similarly, as in Example 4, when the content of planar carbon was 40% by weight, it was confirmed that the discharge capacity did not increase any more.

4A is a graph showing the first initial discharge performance of a lithium-sulfur secondary battery in which the sulfur-carbon composites each prepared in Examples 5 to 6 and Comparative Example 1 were applied to a positive electrode, and FIG. 4B is A graph showing the results of measuring the high rate characteristics of a lithium-sulfur secondary battery in which the sulfur-carbon composites each prepared in 1 were applied to a positive electrode.

Referring to FIG. 4A, although the sulfur content of the planar carbon composite was higher in Examples 5 to 6 and thus the total sulfur content was increased, it can be seen that the discharge capacity was higher than that of Comparative Example 1. However, as in Example 6, it can be seen that the increase in discharge capacity stops when the content of planar carbon is increased and the total sulfur content is further increased. Therefore, it can be seen that a higher energy density can be realized when the sulfur content of the planar sulfur-carbon composite is higher than that of the linear sulfur-carbon composite based on the total weight of sulfur contained in the sulfur-carbon composite.

Further, referring to FIG. 4B, it can be seen that the discharge capacity of Example 5 is the highest at a high rate, similar to the initial discharge performance curve of FIG. 4A.

Experimental Example 2: Observation of the shape of a sulfur-carbon composite according to the content of planar carbon

5 is a SEM (Scanning Electron Microscope, JEOL, JSM7200) photographs of sulfur-carbon composites prepared in Examples 2 to 4, respectively.

The sulfur-carbon composites of Example 2, Example 3, and Example 4 are the case where the content of reduced graphene oxide, which is planar carbon, is 20% by weight, 30% by weight, and 40% by weight, respectively, based on the total weight of carbon.

Referring to FIG. 5, since both Examples 2, 3, and 4 contain an appropriate amount of planar carbon (indicated in white color), sulfur elution can be prevented, but in the case of Example 4, the content of planar carbon is in Example 2 And it can be seen that the structure is relatively more difficult to transfer ions than 3.

[Explanation of code]

1: anode

10: positive active material layer

20: current collector

11: Faceted sulfur-carbon complex

12: point type sulfur-carbon complex

13: linear sulfur-carbon complex

S0, S1, S2: sulfur

C0: point carbon, C1: linear carbon, C2: planar carbon

Claims (11)

1. As a sulfur-carbon composite containing facet carbon,

   The planar carbon is contained in an amount greater than 0% by weight and less than 50% by weight based on the total weight of the carbon, sulfur-carbon composite.
2. The method of claim 1,

   The planar carbon is contained in an amount of 10 to 30% by weight based on the total weight of the carbon, sulfur-carbon composite.
3. The method of claim 1,

   The sulfur-carbon composite is a planar carbon; And one or more carbons selected from point-like carbon and linear carbon; to a sulfur-carbon composite.
4. The method of claim 1,

   The planar carbon is selected from the group consisting of non-oxide graphene, graphene oxide, reduced graphene oxide, doped graphene, and carbon nanoribbon, Sulfur-carbon complex.
5. The method of claim 3,

   The point-like carbon is selected from the group consisting of Ketjen Black, Denka Black, acetylene black, super-p and fullerene, sulfur-carbon composite.
6. The method of claim 3,

   The linear carbon is selected from the group consisting of carbon nanotubes and carbon fibers, sulfur-carbon composite.
7. The method of claim 3,

   The sulfur-carbon composite including the planar carbon is a type supported between the planar carbon,

   The sulfur-carbon composite including the point-like carbon is a form in which sulfur is formed on the surface of the point-like carbon,

   The sulfur-carbon composite including the linear carbon is in a form in which sulfur is formed on the inside, the surface, or the inside and the surface of the linear carbon.
8. The method of claim 1,

   The sulfur-carbon composite is a sulfur-carbon composite containing 40 to 95% by weight of sulfur and 5 to 60% by weight of carbon.
9. A positive electrode for a lithium secondary battery comprising the sulfur-carbon composite of any one of claims 1 to 8.
10. A lithium secondary battery comprising the positive electrode according to claim 9.
11. The method of claim 10,

    The lithium secondary battery is a lithium-sulfur secondary battery, a lithium secondary battery.

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