KR20030006746A - Cathodic material and lithium-sulfur battery having the same - Google Patents

Abstract

PURPOSE: A conductive material for a positive electrode and a lithium-sulfur battery using the conductive material are provided, to improve the lifetime of a battery by increasing the energy per weight and per volume of a battery and the adhesive strength between a positive coating layer and a current collector. CONSTITUTION: The conductive material comprises 100 parts by weight of a conductive material mixture consisting of 5-95 wt% of carbon black and 5-95 wt% of at least one selected from carbon nano-fiber, fibrous graphite and carbon nanotube; and 0.5-60 parts by weight of graphite. Preferably the carbon black has a particle size less than 0.2 micrometers and a surface area less than 1,000 m¬2/g; the graphite is spherical, imperfect spherical or thin strip-shaped and has a particle size less than 50 micrometers and a surface area less than 200 m¬2/g; the carbon nano-fiber has a diameter less than 1.0 micrometers, a length less than 10 micrometers and a surface area less than 500 m¬2/g; and the fibrous graphite has a diameter less than 15 micrometers, a length less than 15 micrometers and a surface area of 50 m¬2/g. The lithium-sulfur battery comprises a positive electrode which consists of the conductive material, an active material and a binder.

Description

A conductive material for a positive electrode and a lithium-sulfur battery using the same {Cathodic material and lithium-sulfur battery having the same}

The present invention relates to a conductive material for a positive electrode and a lithium battery using the same, and more particularly, improves the mechanical properties of the positive electrode and at the same time increases the utilization of sulfur, that is, the impregnating ability of the positive electrode with increased capacity, that is, the active material The present invention relates to a conductive material for a positive electrode and a lithium-sulfur battery capable of producing a battery having an increased utilization of phosphorus sulfur and also an increase in the density of the positive electrode, that is, the energy per weight and the volume.

Portable power supplies are required for all portable electronic devices, including notebook computers as well as mobile devices such as mobile phones. Recently, lithium ion batteries using lithium metal oxide as a cathode material and carbon capable of intercalating lithium ions as a cathode material have been used as portable power sources. The lithium ion battery using these shows an energy density of about 100-180 Wh / kg. However, along with the development of high performance electronic devices and electric vehicles, the necessity of high capacity secondary batteries has emerged. As a result, the development of new materials having increased energy per weight and volume than conventional cathode materials and cathode materials has been required.

Sulfur has a high capacity of 1675mAh / g, and is being researched as a new anode material due to its economical and environmentally friendly advantages over metal oxides. However, since the sulfur particles are insulators, in order to use them as the active material of the positive electrode, a conductive material for imparting electronic conductivity to the sulfur particles is essential. The conductive material that can be used at this time is a metal particle, such as powder, fibrous, flake form, conductive polymer, carbon and the like. Among them, carbon is the most commonly used conductive material, and in particular, the carbon black-based conductive material is widely used for lithium-sulfur batteries as well as other kinds of batteries, supercapacitors and fuel cells, including lithium ion batteries.

However, carbon blacks represented by acetylene black and ketjen black have very small particle size (tens of nanometers), so the impregnation of the anode containing them is not large. Impregnation of the electrolyte is a very important factor in determining the usable capacity of the positive electrode. In the case of lithium-sulfur batteries, the utilization of sulfur is determined by the impregnation of the positive electrode with electrolyte.

A mixed conductive material of carbon black and carbon nanofibers is used to increase the impregnation of the positive electrode with the electrolyte. Due to the structural characteristics of the carbon nanofibers, the positive electrode including the mixed conductive material forms a porous structure, thereby improving the impregnation of the electrolyte, and consequently, the utilization rate of sulfur as an active material is increased. However, when the mixed conductive material containing carbon nanofibers is used, the electrode density of the positive electrode is reduced compared to the case of using only carbon black as the conductive material, thereby reducing energy per weight and energy per volume of the lithium-sulfur battery using the positive electrode. There is a problem. In addition, due to the structural characteristics of the carbon nanofibers, there is a problem in that the electrode state including the binding force of the manufactured positive electrode and the adhesive force with the current collector is lowered.

Accordingly, a first object of the present invention is to provide a conductive material for a positive electrode, which increases the impregnation property and electrode density in an electrolyte solution and improves an electrode state including adhesion and binding force with a current collector.

A second object of the present invention is to provide a lithium-sulfur battery in which energy per weight, energy per volume, and lifetime are increased by employing the positive electrode described above.

Figure 1 shows the ratio of the conductive material and the electrode density of the positive electrode prepared by Comparative Examples 1 and 2 and Examples 1 and 2 and the energy per weight, volume per volume of the lithium-sulfur battery composed of the positive electrode.

2 is a graph showing the cycle characteristics of a lithium-sulfur battery constructed using a positive electrode prepared according to Comparative Example 1.

3 is a graph showing cycle characteristics of a lithium-sulfur battery constructed using a positive electrode prepared according to Comparative Example 2 of the present invention.

4 is a graph showing cycle characteristics of a lithium-sulfur battery constructed using a positive electrode manufactured according to Example 1 of the present invention.

5 is a graph showing cycle characteristics of a lithium-sulfur battery constructed using a positive electrode prepared according to Example 2 of the present invention.

In the present invention for achieving the above object is a lithium-sulfur battery formed by mixing in a proportion of 100 parts by weight of the mixed conductive material consisting of 9 to 95% by weight of carbon black and 95 to 5% by weight of carbon nanofibers and 0.5 to 60 parts by weight of graphite A conductive material for positive electrode is provided.

The present invention also provides a lithium-sulfur battery comprising the conductive material for the positive electrode described above.

The use of active materials such as sulfur or organic sulfur is increased due to the impregnation of the electrolyte with the electrolyte, and the energy per weight and energy per volume of the lithium-sulfur battery including the cathode are increased due to the increase in the anode electrode density. As the adhesion between the current collectors increases, the lifespan of the lithium battery including the positive electrode increases.

Hereinafter, the present invention will be described in detail.

The conductive material of the present invention is formed by mixing at least one selected from carbon black, carbon nanofiber carbon nanofiber fibrous graphite and carbon nanotube, and graphite as a main component.

The carbon black generally has a particle size of less than 0.2 μm, in particular less than 0.05 μm, and a surface area of less than 1000 m ² / g, especially less than 100 m ² / g, and serves to impart electronic conductivity to the sulfur particles which are insulators.

The carbon nanofibers have a diameter of less than 1.0 μm, in particular less than 0.2 μm, a length of less than 100 μm, in particular less than 30 μm, and a surface area of less than 500 m ² / g, especially less than 50 m ² / g, to improve the impregnation of the electrolyte of the positive electrode. do.

The graphite particles have the form of spherical or incomplete spheres or flakes, and have a particle size of less than 50 μm, in particular less than 15 μm, a surface area of less than 200 m ² / g, especially less than 30 m ² / g, and increase the electrode density of the anode, It is included to improve mechanical properties, including adhesion.

Examples of the carbon nanofibers include fibrous graphite or carbon nanotubes. The fibrous graphite is preferably less than 15 μm in diameter, especially less than 10 μm, less than 150 μm in length and less than 100 μm in length and less than 50 m ² / g, in particular less than 100 m ² / g.

In order to prepare the conductive material, carbon black 5 to 95% by weight, preferably 30 to 70% by weight, more preferably 40 to 60% by weight and carbon nanofibers, 95 to 5% by weight, preferably 70 To 30% by weight, more preferably 60 to 40% by weight is mixed to obtain a mixed conductive material. To the mixed conductive material is added 0.5 to 60% by weight, preferably 2 to 30% by weight, more preferably 4 to 15% by weight, such as 100 parts by weight of the mixed conductive material, to obtain a conductive material for a positive electrode according to the present invention. .

According to the present invention, there is also provided a lithium-sulfur battery comprising a positive electrode made of a conductive material for a positive electrode including carbon black, carbon nanofibers and graphite, an active material, and a binder.

The active material includes an organic sulfur compound capable of bonding / separation between sulfur-sulfur atoms during unit sulfur or charging / discharging.

The present invention relates to an energy storage device that employs a conductive material composed of carbon black, carbon nanofibers, and graphite to manufacture a positive electrode, and employs the positive electrode as the positive electrode, thereby improving energy per weight, energy per volume, and lifetime. .

The structure of the lithium-sulfur battery employing the conductive material will be described.

The positive electrode that can be used is an electrode in which sulfur or organic sulfur acts as an active material, and various auxiliary active materials such as a metal oxide capable of intercalating lithium, a metal itself, or a conductive polymer capable of oxidation / reduction during charge / discharge can be added. . Sulfur or an organic sulfur compound as an active material means a material capable of bonding / separating sulfur-sulfur atoms during charging / discharging. Active materials in this sense are sulfur molecules (S ₈ ), polysulfides (Li ₂ S _x , 1 <x), sulfur-carbon polymers, DMC (DMcT, 2,5-dimercapto-1,3,4-thiadiazole) And disulfide compounds including TTCA, 1,3,5-trithiocyanuic acid, and the like. Sulfur or organic sulfur active materials are insulators, and thus the mixed conductive materials described above are used to facilitate electron transfer to these active materials. And a binder is added for bonding the active material and the conductive material. It is also possible to prepare a positive electrode free of sulfur or organic sulfur compounds and to add active materials containing sulfur to the electrolyte. As a negative electrode which can be used, as a group 1 or group 2 element on a periodic table, lithium metal, a lithium metal sheet, lithium metal powder, a lithium alloy, sodium metal, an alloy, etc. can be used normally. The electrolyte solution is a method of injecting after assembling the battery in a laminated structure and a winding structure. In the case of using the gel-type polymer electrolyte, the battery is assembled using the polymer electrolyte containing the electrolyte solution. In this case, a polymer electrolyte may be added together with sulfur and carbon in the anode. Non-gel solid polymer electrolytes may also be used, in which case there is no additional injection of liquid electrolyte.

Examples of electrolytes that can be used in lithium-sulfur batteries include ethers such as tetrahydrofuran, tetrahydropyrane, dioxane, dioxolane, tetraethylene glycol dimethyl ether, diglyme, poly (ethylene glycol) dimethyl ether, and dimethoxyethane. Preferred are ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate series carbonate and acetonitrile, gamma-butylolactone and the like. Various mixed uses of these are also possible, to which ethylene glycol, dimethylsulfoxide, dimethylformamide, hexamethylphosphoamide, sulfolane, toluene, benzene, xylene and the like can be added. As the salt to be used, salts generally used in lithium batteries can be used. For example, the lithium salt may be LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiAsCl ₆ , LiCF ₃ SO ₃ , LiN (SO ₂ CF 3) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) _2, and the like.

Batteries manufactured to have the above-described configuration exhibit 50% or more sulfur utilization during primary discharge and 40% or more sulfur utilization even when repeated charging / discharging.

Hereinafter, the present invention will be described in more detail with reference to specific examples and comparative examples.

Comparative Example 1

Highly pure 60% by weight of sulfur particles, 25% by weight of carbon black-based Super-P (MMM Carbon) as a conductive material, 3% by weight of carboxymethylcellulose as a binder and 9% by weight of butadiene-styrene as tetrafluoride, a kind of fluorine-based After uniformly mixing 9% by weight of ethylene to form a mixture, the mixture was coated on an aluminum current collector to prepare a positive electrode containing sulfur. The prepared positive electrode was dried under vacuum at 60 ° C. for 1 hour. The electrode density of the prepared anode was 0.78. Using a lithium metal as a negative electrode and Celgard 3501 (trade name of Hoechst Celanese Corp) as a separator to prepare a winding cell, the ether-based electrolyte was impregnated and packaged. After the battery was left for 5 hours, a cycle was performed using a cycler.

The ratio of the conductive material and the electrode density of the produced positive electrode and the energy per weight and volume of the lithium-sulfur battery composed of the positive electrode are shown in FIG. 1.

2 is a graph showing the cycle performance of the lithium-sulfur battery prepared by Comparative Example 1.

The battery was charged / discharged at a rate of 0.1 C, in which case the current density was 0.42 mA / cm ² . The highest sulfur utilization during discharge is about 42%, with energy per weight and energy per volume of 140 Wh / kg and 229 Wh / L, respectively.

Comparative Example 2

First, as a mixed conductive material, a conductive material obtained by mixing carbon black-based super-pi and carbon nanofibers VWCF (Vapour Grown Carbon Fiber) of 83:17, 50:50 and 17:83 in weight ratio Prepared. The mixed conductive material and the sulfur powder as the active material and the binder were mixed in the same manner as in Comparative Example to prepare a positive electrode. A battery was prepared in the same manner as in the comparative example, and after aging at room temperature, a charge / discharge experiment was performed. Charge and discharge were performed at a rate of 0.1C, respectively.

The ratio of the conductive material and the electrode density of the produced positive electrode and the energy per weight and volume of the lithium-sulfur battery composed of the positive electrode are shown in FIG. 1.

3 is a graph showing the cycle characteristics of a lithium-sulfur battery constructed using a positive electrode prepared according to Comparative Example 2.

According to FIG. 3, the highest utilization rate of sulfur during discharge is 53.1% for the SV1 electrode, which is an increase of about 31% when compared to Comparative Example 1. This is because the impregnation of the electrolyte in the positive electrode increased as described above. However, as shown in FIG. 1, the addition of VGCF decreases the electrode density of the anode so that the increase in energy per volume is smaller than the increase in utilization of sulfur.

Example 1

First, a conductive material mixed with carbon black series, carbon nanofibers, and spherical graphite particles (KS6, Timrex) in a weight ratio of 80: 16: 4, 76: 15: 9, 73:14:13 was prepared as a mixed conductive material. It was. A positive electrode was prepared by mixing sulfur powder and a binder as the mixed conductive material and the active material in the same manner as in Comparative Example 1. After the battery was prepared in the same manner as in the comparative example, a charge / discharge experiment was performed after room temperature aging. Charging and discharging were performed at a rate of 0.1C, respectively.

The ratio of the conductive material and the electrode density of the produced positive electrode and the energy per weight and volume of the lithium-sulfur battery composed of the positive electrode are shown in FIG. 1.

4 is a graph showing cycle characteristics of a lithium-sulfur battery constructed using a positive electrode manufactured according to Example 1;

Referring to FIG. 4, the highest utilization rate of sulfur during discharge is 52.2% for the SVK1 electrode, which is higher than that of the comparative example but lower than that of the first example. This is because the electrode density is increased as shown in FIG. In other words, the electrolyte impregnation is slightly decreased due to the increase of the electrode density, thereby reducing the utilization of sulfur. However, it can be seen that the energy per volume and the energy per weight have increased because the amount of sulfur supported per volume increases due to the increase in electrode density. The increase in electrode density is due to the fact that spherical graphite particles can fit into many voids formed by very long fibrous VGCFs. In addition, when the spherical graphite particles are added together with the carbon nanofibers as in the present embodiment, the adhesion and binding force of the electrode are improved. Thus, the positive electrode does not have a peeling phenomenon even after a long charge / discharge cycle, and thus the life of the lithium-sulfur battery including the positive electrode may be improved.

Example 2

First, instead of carbon black-based super-pi and carbon nano-arbor VGCF as a mixed conductive material, fibrous graphite MPCF (Meso Pitch based Graphite Fiber, Petoka) and spherical graphite particles (KS6, Timrex) in weight ratio of 80: A conductive material mixed with 16: 4, 76: 15: 9, and 73:14:13 was prepared. A positive electrode was prepared by mixing sulfur powder and a binder as the mixed conductive material and the active material in the same manner as in Comparative Example 1. After the battery was prepared in the same manner as in the comparative example, a charge / discharge experiment was performed after room temperature aging. Charge and discharge were performed at a rate of 0.1C, respectively.

The ratio of the conductive material and the electrode density of the produced positive electrode and the energy per weight and volume of the lithium-sulfur battery composed of the positive electrode are shown in FIG. 1.

FIG. 5 is a graph showing cycle characteristics of a lithium-sulfur battery constructed using a positive electrode prepared according to Example 2. FIG.

As can be seen in Figure 1 it can be seen that the electrode density increased compared to the SV electrode. Sulfur utilization is slightly reduced compared to that of the SV electrode, but the energy per volume is rather increased. In addition, the electrode of this embodiment also has excellent adhesion and binding force due to the addition of graphite. Therefore, the life of the lithium-sulfur battery increases.

As described above, according to the present invention, the utilization rate of an active material such as sulfur or organic sulfur is increased due to the increase in impregnation of the electrolyte with the electrolyte, and the energy per weight of the lithium-sulfur battery including the positive electrode, The energy per volume increases and the life span of the energy storage device including the anode is increased by increasing the adhesion between the anode coating layer and the current collector.

In addition, it can be seen that the conductive material of the present invention can be used for all other batteries including lithium ion batteries, fuel cells, electric double layer capacitors using electric double layer charge / discharge reactions, and capacitors using Faraday reactions including electron transfer.

Claims (6)

A battery conductive material formed by mixing at a ratio of 100 parts by weight of a mixed conductive material consisting of 5 to 95% by weight of carbon black and at least one selected from 95 to 5% by weight of carbon nanofiber fibrous graphite and carbon nanotubes and 0.5 to 60 parts by weight of graphite. The method of claim 1, wherein the carbon black has a particle size of less than 0.2㎛ and the surface area is less than 1000m ² / g, the graphite is a particle in the form of a spherical or incomplete sphere or flakes and the particle size is less than 50㎛ A battery conductive material, characterized in that the surface area is less than 200 m ² / g. The conductive material for a battery according to claim 1, wherein the carbon nanofibers are less than 1.0 µm in diameter, less than 100 µm in length, and have a surface area of less than 500 m ² / g. 4. The battery conductive material according to claim 3, wherein the fibrous graphite has a diameter of less than 15 µm and a length of less than 150 µm, and a surface area of less than 50 m ² / g. A lithium-sulfur battery comprising a positive electrode made of a battery conductive material and an active material and a binder according to claim 1. The lithium-sulfur battery of claim 5, wherein the active material comprises a compound including a sulfur molecule (S ₈ ), polysulfide (Li ₂ S _x , 1 <x), or a sulfur-sulfur atom.