KR20040033678A - Binder and positive electrode for lithium sulfur battery - Google Patents

Abstract

PURPOSE: An organic binder for a lithium-sulfur battery and a positive electrode containing the binder for a lithium-sulfur battery are provided, to improve the stability (lifetime characteristic) of a positive electrode plate and a capacity per active mass. CONSTITUTION: The organic binder has a double bond and comprises a polymer capable of being crosslinked by elemental sulfur. Preferably the polymer is a natural rubber or a synthetic rubber. The positive electrode comprises elemental sulfur; a conductive material; an organic binder crosslinked by sulfur; and a vulcanization accelerator. Preferably the organic binder is a polyolefin rubber having a double bond in polymer structure; and the vulcanization accelerator is selected from the group consisting of an organic sulfur compound, zinc dithiocarbamate, zinc oxide and stearic acid.

Description

A binder for a lithium-sulfur battery and a cathode for a lithium-sulfur battery including the same {BINDER AND POSITIVE ELECTRODE FOR LITHIUM SULFUR BATTERY}

[Industrial use]

The present invention relates to a lithium-sulfur battery binder and a lithium-sulfur battery positive electrode including the same, and more particularly to a lithium-sulfur battery binder and a lithium-sulfur battery positive electrode including the same, which can provide a battery having excellent life characteristics. It is about.

[Prior art]

Recently, as the miniaturization, weight reduction, and high performance of electronic products, electronic devices, and communication devices have rapidly progressed, there is a great demand for improving the performance of secondary batteries to be used as power sources for these products. Lithium-sulfur cells have a theoretical energy density of 2800 Wh / kg (1675 mAh / g), which is significantly higher than other battery systems. Sulfur is also attracting attention as an inexpensive and environmentally friendly material due to its rich resources. Therefore, many researchers have tried to construct lithium secondary batteries using sulfur.

Sulfur, which is used as a cathode active material of a lithium-sulfur battery, is an insulator, and thus requires a conductive material to move electrons generated by an electrochemical reaction. Carbon blacks, metal powders, and the like are used as such conductive materials. However, in order to attach the positive electrode mixture composition to the current collector, the selection of an appropriate binder is important. At this time, the binder must have a small amount of addition to give physical strength to the electrode, so it should be easy to manufacture a high energy density positive electrode, have no reactivity with the electrolyte, and maintain a stable form in the battery operating temperature range. In addition, the binder used in the lithium-sulfur battery should be dissolved in the organic solvent used in the preparation of the slurry composition, which is a composition for producing the positive electrode plate, and should not be dissolved in the electrolyte solution. Therefore, as there are very few materials satisfying all of these properties, there is a problem that the choice of materials for use as a binder is too small. Therefore, only materials satisfying these properties can be used rather than excellent binding ability, which is the main purpose of the binder. .

The present invention has been made to solve the above problems, and an object of the present invention is to provide an organic binder for a lithium-sulfur battery capable of providing a lithium-sulfur battery having excellent life characteristics.

Another object of the present invention is to provide an organic binder for a lithium-sulfur battery which can provide a lithium-sulfur battery having a large capacity per weight of the mixture.

Still another object of the present invention is to provide a cathode for a lithium-sulfur battery which can provide a lithium-sulfur battery of high energy density using the organic binder.

1 is a perspective view of a lithium-sulfur battery prepared according to a preferred embodiment of the present invention.

Figure 2 is a graph showing the 0.5C discharge curve of the lithium-sulfur battery prepared by the method of Example 1 and Comparative Example 1 of the present invention.

In order to achieve the above object, the present invention provides an organic binder for a lithium-sulfur battery having a double bond and comprising a polymer that can be crosslinked by sulfur.

The present invention also provides a positive electrode for a lithium-sulfur battery comprising an organic binder crosslinked with the polymer, elemental sulfur, a conductive material and a vulcanization reaction accelerator.

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

The present invention relates to an organic binder that can be used for the positive electrode of a lithium-sulfur battery, and relates to an organic binder that can be crosslinked by sulfur.

The organic binder is a polymer having a double bond, that is, a polyolefin rubber having a double bond, and a polymer crosslinked by vulcanization. Examples include natural rubber and synthetic rubber, and examples of synthetic rubber include styrene-butadiene copolymer, butyl rubber, isobutylene-isoprene copolymer, acrylonitrile-butadiene-rubber (NBR) rubber, ethylene And ethylene propylene diene terpolymer (EPDM).

The positive electrode of the present invention using such an organic binder includes an organic binder crosslinked with the polymer, a positive electrode active material, a conductive material, and a vulcanization reaction accelerator.

In order to manufacture the positive electrode, a binder is first dissolved in a solvent for preparing a composition in a slurry form, and then a conductive material is dispersed.

The conductive material is not particularly limited as an electrically conductive conductive material that allows electrons to move smoothly in the positive electrode plate, but a conductive material or a conductive polymer such as a graphite material, a carbon material, or the like may be preferably used. The graphite material includes KS 6 (manufactured by Timcal) and the carbon material includes super P (MMM manufactured), ketjen black, denka black, acetylene black, and carbon black. . Examples of the conductive polymer include polyaniline, polythiophene, polyacetylene, polypyrrole, and the like. These conductive conductive materials may be used alone or in combination of two or more thereof.

As a solvent for preparing the composition, a positive electrode active material, a binder, and a conductive material may be uniformly dispersed, and it is preferable to use one that is easily evaporated. Typically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, dimethyl formamide and the like can be used.

Next, the positive electrode active material is uniformly dispersed again in the composition in which the conductive material and the binder are dispersed to prepare a positive electrode active material composition. In addition, organic sulfur compounds such as tetramethylthiuram disulfide, zinc salts of dithiocarbamic acid, vulcanization reactions such as zinc oxide, lead oxide or stearic acid in order to promote the vulcanization reaction which is a later process. An accelerator may be further added to the positive electrode active material composition.

Elemental sulfur (S ₈ ) is used as the cathode active material.

The amount of the binder in the composition is preferably 2 to 10% by weight, the amount of the vulcanization reaction accelerator is preferably 0.01 to 1% by weight. In the composition, when the amount of the binder is less than 2% by weight, the binding force of the electrode plate is lowered, and when the amount of the binder is more than 10% by weight, the capacity per weight of the mixture is not preferable. In addition, when the amount of the vulcanization reaction accelerator is less than 0.01% by weight, there is little effect of improving the reaction rate by using the vulcanization reaction accelerator, and the capacity of the positive electrode is lowered when the amount of the vulcanization reaction accelerator exceeds 1% by weight. Not desirable

In addition, the amount of the positive electrode active material, the conductive material and the solvent included in the composition does not have a particularly important meaning in the present invention, it is sufficient to have a suitable viscosity to facilitate the coating of the slurry.

The composition thus prepared is applied to a current collector, and the current collector to which the composition is applied is heat-treated to prepare a positive electrode. According to this heat treatment process, the binder contained in the composition is vulcanized by sulfur contained in the positive electrode active material, so that a crosslinking reaction occurs between the polymers of the binder so that the binders have a crosslinked structure. The heat treatment step is sufficient to be carried out at a temperature and time that the vulcanization reaction can occur, for example, it is carried out for 5 to 60 minutes at 70 to 130 ℃. The vulcanization process may be performed by passing an infrared tunnel or microwave tunnel in addition to the heat treatment. The coating thickness of the composition is sufficient to coat the current collector with an appropriate thickness depending on the viscosity and the thickness of the positive electrode plate to be formed. The current collector is not particularly limited, but conductive materials such as stainless steel, aluminum, copper, titanium, and the like are preferably used, and more preferably, a carbon-coated aluminum current collector is used. The use of an aluminum substrate coated with carbon has an advantage in that the adhesion to the active material is excellent, the contact resistance is low, and the corrosion of polysulfide of aluminum is prevented, compared with the non-carbon coated aluminum substrate.

The lithium-sulfur battery 1 of the present invention using the positive electrode includes a positive electrode 3, a negative electrode 4 and a separator located between the positive electrode 3 and the negative electrode 4, as shown in FIG. A battery can 5. An electrolyte is present between the positive electrode 3 and the negative electrode 4.

The negative electrode 4 is formed of a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to form a lithium-containing compound reversibly, lithium metal and a lithium alloy. It includes a negative electrode active material selected.

As a material capable of reversibly intercalating / deintercalating the lithium ions, any carbon-based negative electrode active material generally used in a lithium ion secondary battery may be used, and representative examples thereof include crystalline carbon. , Amorphous carbon or these can be used together. In addition, representative examples of the material capable of reacting with the lithium ions to form a lithium-containing compound reversibly include tin oxide (SnO 2), titanium nitrate, and silicon (Si), but are not limited thereto. As the lithium alloy, an alloy of a metal selected from the group consisting of lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn may be used.

An inorganic protective layer, an organic protective layer, or a material in which these layers are stacked on a lithium metal surface may also be used as a cathode. As the inorganic protective film, Mg, Al, B, C, Sn, Pb, Cd, Si, In, Ga, lithium silicate, lithium borate, lithium phosphate, lithium phosphoride nitride, lithium silicosulfide, lithium borosulfide, lithium aluminium It consists of a material selected from the group consisting of nosulfide and lithium phosphosulfide. The organic protective film is poly (p-phenylene), polyacetylene, poly (p-phenylene vinylene), polyaniline, polypyrrole, polythiophene, poly (2,5-ethylene vinylene), acetylene, poly (ferry) Naphthalene), polyacene, and poly (naphthalene-2,6-diyl), and a monomer, oligomer or polymer having conductivity selected from the group consisting of.

In addition, in the process of charging and discharging the lithium-sulfur battery, sulfur used as the positive electrode active material may be changed into an inert material and adhered to the surface of the lithium negative electrode. As described above, inactive sulfur refers to sulfur in which sulfur is no longer able to participate in the electrochemical reaction of the anode through various electrochemical or chemical reactions, and inert sulfur formed on the surface of the lithium cathode is a protective layer of the lithium cathode. There is also an advantage to act as. Therefore, lithium metal and inert sulfur formed on the lithium metal, for example lithium sulfide, may be used as the negative electrode.

As the organic solvent used in the electrolyte solution of the present invention, a single solvent may be used, or two or more mixed organic solvents may be used. When using two or more mixed organic solvents, it is preferable to use one or more solvents selected from two or more groups among the weak polar solvent group, the strong polar solvent group, and the lithium metal protective solvent group.

Weak polar solvents are defined as solvents with a dielectric constant of less than 15 that can dissolve elemental sulfur among aryl compounds, bicyclic ethers, and acyclic carbonates, and strong polar solvents include acyclic carbonates, sulfoxide compounds, lactone compounds, Among ketone compounds, ester compounds, sulfate compounds, and sulfite compounds, a dielectric constant capable of dissolving lithium polysulfide is defined as greater than 15, and lithium protective solvents are saturated ether compounds, unsaturated ether compounds, N, O, It is defined as a solvent having a charge and discharge cycle efficiency (cycle efficiency) of 50% or more to form a SEI (Solid Electrolyte Interface) film stable on a lithium metal, such as a heterocyclic compound containing S or a combination thereof.

Specific examples of weak polar solvents include xylene, dimethoxyethane, 2-methyltetrahydrofuran, diethyl carbonate, dimethyl carbonate, toluene, dimethyl ether, diethyl ether, diglyme, tetraglyme and the like.

Specific examples of strong polar solvents include hexamethyl phosphoric triamide, gamma-butyrolactone, acetonitrile, ethylene carbonate, propylene carbonate, N-methylpyrrolidone, 3-methyl-2-oxazolidone , Dimethyl formamide, sulfolane, dimethyl acetamide or dimethyl sulfoxide, dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, ethylene glycol sulfite and the like.

Specific examples of lithium protective solvents include tetrahydrofuran, ethylene oxide, dioxolane, 3,5-dimethyl isoxazole, 2,5-dimethyl furan, furan, 2-methyl furan, 1,4-dioxane and 4-methyldioxane. Solan and others.

Lithium trifluoromethanesulfonimide, lithium triflate, lithium perchlorate, LiPF ₆ or LiBF ₄ may be used as the electrolytic salt used in the electrolyte.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Example 1)

80% by weight of elemental sulfur (S ₈ ), 15% by weight of ketjen black as conductive material, 5% by weight of styrene-butadiene copolymer, a kind of synthetic rubber as a binder, and 0.5% by weight of lead oxide as a vulcanization reaction accelerator, acetonitrile / N Methylpyrrolidone (volume ratio 8: 2) was mixed in a mixed solvent to prepare a positive electrode mixture slurry for a lithium-sulfur battery. The slurry was coated on a carbon-coated Al current collector, and the slurry-coated current collector was heat-treated at 100 ° C. for 1 hour to prepare a positive electrode plate.

A lithium-sulfur battery was manufactured using the prepared positive electrode plate, lithium negative electrode, and electrolyte solution. As the electrolyte, a mixed electrolyte of dioxolane, dimethoxyethane, diglyme, and sulfolane (5: 2: 2: 1 volume ratio) in which 1M LiSO ₃ CF ₃ was dissolved was used.

(Comparative Example 1)

80% by weight of elemental sulfur (S ₈ ), 15% by weight of ketjen black as a conductive material, 3.5% by weight of styrene-butadiene rubber as a binder and 1.5% by weight of carboxymethyl cellulose as a thickener were mixed in water to prepare a positive electrode mixed slurry for a lithium-sulfur battery. Prepared. This slurry was coated on a carbon-coated Al current collector, and the slurry-coated current collector was dried under vacuum for at least 12 hours to prepare a positive electrode plate.

A lithium-sulfur battery was manufactured using the prepared cathode plate, lithium metal anode, and electrolyte solution. As the electrolyte, a mixed electrolyte of dioxolane, dimethoxyethane, diglyme, and sulfolane (5: 2: 2: 1 volume ratio) in which 1M LiSO ₃ CF ₃ was dissolved was used.

Charge / discharge graph: Increased capacity per combined weight.

The lithium-sulfur battery prepared by the method of Example 1 and Comparative Example 1 was evaluated at room temperature using the following charge / discharge protocol. At the first discharge, the second discharge, the third discharge, the fourth discharge, and the fifth discharge, the current densities are 0.2 mA / cm 2 (0.1C), 0.2 mA / cm 2 (0.1C), 0.4 mA / cm 2 (0.2C), 1 mA / cm 2 (0.5C) and 2 mA / cm 2 (1C), and the current density during charging was equally 0.4 mA / cm 2 (0.2C) in all cycles. At this time, the charge and discharge cut-off voltages were 2.8 and 1.5V, respectively. If the voltage did not increase due to the shuttle phenomenon during charging, the battery was charged at 110% of the theoretical capacity. The sulfur utilization rate here defines the sulfur utilization rate of the battery having a capacity of 837.5 mAh / g as 100%. The discharge curves of the fourth discharge (0.5C) of the batteries of Example 1 and Comparative Example 1 measured and evaluated in this way are shown in FIG. 2. As shown in FIG. 2, it can be seen that the sulfur utilization rate and the average voltage of the battery of Example 1 are significantly improved compared to those of Comparative Example 1.

The positive electrode including the organic binder crosslinked by sulfur of the present invention is excellent in stability (life characteristic) of the electrode plate and has a large capacity per mixture weight.

Claims (6)

An organic binder for a lithium-sulfur battery having a double bond and comprising a polymer capable of crosslinking by elemental sulfur (S ₈ ). The organic binder of claim 1, wherein the polymer is natural rubber or synthetic rubber. Elemental sulfur; Conductive material; Organic binders crosslinked by sulfur; And Vulcanization Reaction Accelerator A positive electrode for a lithium-sulfur battery comprising a. The positive electrode for a lithium-sulfur battery according to claim 3, wherein a polyolefin rubber having a double bond in a polymer structure is used as the organic binder. The positive electrode for a lithium-sulfur battery according to claim 3, wherein the organic binder is natural rubber or synthetic rubber. The positive electrode for a lithium-sulfur battery according to claim 3, wherein the vulcanization reaction accelerator is selected from the group consisting of an organosulfur compound, a zinc salt of dithiocarbamic acid, zinc oxide and stearic acid.