KR100551005B1 - Positive electrode for lithium-sulfur battery and lithium-sulfur battery comprising same - Google Patents

Abstract

The present invention is a lithium-sulfur battery positive electrode and a lithium-sulfur battery comprising the same, the positive electrode is a sulfur-based positive electrode active material; bookbinder; Conducting agents; And carbon nanotube (CNT), graphite nano fiber (GNF), carbon nano fiber (CNF), activated carbon fiber (ACF) and mixtures thereof. It includes an electrically conductive linear fiber having an average diameter of 50nm or less selected from the group.

The positive electrode including the electrically conductive linear fiber has excellent durability of the internal structure of the electrode so that the structure is not collapsed during charge and discharge, and the electron transfer is excellent, thereby improving the performance of the battery.

Linear fiber, electrode structure support, lithium sulfur battery, carbon nanotube, graphite nanofiber, carbon nanofiber, activated carbon fiber

Description

A positive electrode for a lithium-sulfur battery, and a lithium-sulfur battery including the same {POSITIVE ELECTRODE FOR LITHIUM-SULFUR BATTERY AND LITHIUM-SULFUR BATTERY COMPRISING SAME}

1 is a perspective view of a lithium sulfur battery.

2 is a graph showing the cycle life of the lithium-sulfur battery of Examples 1 to 3 and Comparative Examples 1 to 4 of the present invention.

3 and 4 are electron micrographs before and after the charge and discharge of the positive electrode prepared according to Example 2 of the present invention, respectively.

[Industrial use]

The present invention relates to a lithium-sulfur battery positive electrode and a lithium-sulfur battery comprising the same, and more particularly, to a lithium-sulfur battery positive electrode having a superior durability of an electrode structure and having excellent cycle life characteristics, and a lithium-sulfur battery including the same. It is about.

[Prior art]

With the rapid development of portable electronic devices, the demand for secondary batteries is increasing. In particular, there is a continuous demand for the emergence of high energy density batteries that can meet the trend toward smaller, lighter, thinner and smaller portable electronic devices, and batteries that must satisfy inexpensive, safe and environmentally friendly aspects. It is required.

Lithium-sulfur batteries are inexpensive and environmentally friendly materials, and the energy density of lithium is 3830 mAh / g in terms of energy density, and the energy density of sulfur is expected to be high at 1675 mAh / g. It is emerging as the most promising battery which satisfies the above conditions.

The lithium-sulfur battery uses a sulfur-based compound having a sulfur-sulfur combination as a positive electrode active material, and an alkali metal such as lithium or a carbon-based material in which insertion and deintercalation of metal ions such as lithium ions occur. As a negative electrode active material, wherein the oxidation number of S decreases as the SS bond is broken during the reduction reaction (discharge), and the oxidation of the SS bond is formed again when the oxidation number of S increases during the oxidation reaction (charging). Reduction reactions are used to store and generate electrical energy.

The lithium-sulfur battery has an energy density of 3830 mAh / g when using lithium metal as a negative electrode active material, and an energy density of 1675 mAh / g when using elemental sulfur (S ₈ ) as a positive electrode active material. It is the most promising battery in terms of quality. In addition, the sulfur-based material used as the positive electrode active material has the advantage that it is cheap and environmentally friendly material.

However, there are no examples of successful commercialization with lithium-sulfur battery systems. The reason why the lithium-sulfur battery has not been commercialized is that when sulfur is used as an active material, the utilization rate indicating the amount of sulfur participating in the electrochemical redox reaction in the battery relative to the amount of sulfur input is low. Because it represents.

Therefore, studies to increase the capacity by increasing the electrochemical redox reaction are ongoing, but the satisfactory effect has not yet been obtained.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a positive electrode for a lithium-sulfur battery that can improve sulfur performance and improve battery performance.

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

In order to achieve the above object, the present invention is a sulfur-based cathode active material; bookbinder; Conducting agents; And carbon nanotube (CNT), graphite nano fiber (GNF), carbon nano fiber (CNF), activated carbon fiber (ACF), and mixtures thereof. It includes a positive electrode for a lithium-sulfur battery comprising an electrically conductive linear fiber having an average diameter of 50nm or less selected from the group.

The present invention also comprises a sulfur-based positive electrode active material containing a sulfur compound; bookbinder; Conducting agents; And an anode including an electrically conductive linear fiber having an average diameter of 50 nm or less selected from the group consisting of carbon nanotubes, graphite nanofibers, carbon nanofibers, activated carbon fibers and mixtures thereof; A negative electrode including a negative electrode active material selected from the group consisting of a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of forming a compound reversibly with lithium, a lithium metal and a lithium alloy; And it provides a lithium-sulfur battery comprising an electrolyte comprising a lithium salt and an organic solvent.

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

The positive electrode for a lithium-sulfur battery of the present invention comprises a sulfur-based positive electrode active material; bookbinder; Conducting agents; And electroconductive linear fibers having an average diameter of 50 nm or less selected from the group consisting of carbon nanotubes, graphite nanofibers, carbon nanofibers, activated carbon fibers and mixtures thereof.

In the present invention, an electrically conductive linear fiber is used for the anode to maintain the structure in the electrode during charging and discharging and to form a conductive network together with the conductive agent so that electron transfer can be performed smoothly. The electrically conductive linear fiber preferably has an average diameter of 50 nm or less, preferably 10 to 50 nm, even more preferably 10 to 30 nm. The linear fiber forms a network by wrapping the sulfur-based cathode active material in the electrode plate, but when the average diameter of the linear fiber exceeds 50 nm, there is a problem in forming such a network.

The content of the linear fiber is preferably 2 to 20% by weight based on the positive electrode. When the content of the linear fiber is less than 2% by weight, it is difficult to show the effect in the electrode plate. When the amount of the linear fiber exceeds 20% by weight, the amount of the active material in the electrode plate should be relatively reduced, thereby reducing the total electrode capacity.

The conductive agent and the linear fiber may be used in a weight ratio of 10:90 to 90:10, and more preferably the weight ratio of the conductive agent and the linear fiber is 15:85 to 85:15. If the content of the linear fiber is too high, even though the structure of the electrode is solid, since the linear fiber having a relatively small specific surface area becomes larger than that of the conductive agent, the reaction site of the electrode is reduced, thereby reducing the discharge capacity.

The cathode active material may be a cathode, an organic sulfur compound, and a carbon-sulfur polymer in which sulfur (elemental sulfur, S ₈ ) solid Li ₂ S _n (n ≧ 1), Li ₂ S _n (n ≧ 1) is dissolved [(C ₂ S _x ) _n , x = 2.5 to 50, n ≥ 2] may be used at least one sulfur-based material selected from the group consisting of.

The conductive agent for allowing electrons to move smoothly in the positive electrode active material together with the positive electrode active material is not particularly limited, but a conductive material such as a graphite material, a carbon material, or a conductive polymer may be preferably used. The graphite material is KS 6 (manufactured by Timcal) and the carbon material is super P (MMM company), ketjen black, denka black, acetylene black, carbon black, and the like. . Examples of the conductive polymer include polyaniline, polythiophene, polyacetylene, polypyrrole, and the like. These conductive conductive agents may be used alone or in combination of two or more thereof.

In addition, binders that serve to attach the positive electrode active material to the current collector include poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, crosslinked polyethylene oxide, polyvinyl ether , Poly (methyl methacrylate), polyvinylidene fluoride, copolymer of polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly (ethyl acrylate), polytetrafluoroethylene, polyvinyl Chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, derivatives thereof, blends, copolymers and the like can be used.

The anode of the present invention may also further comprise a coating layer composed of a polymer, an inorganic material, an organic material or a mixture thereof.

The polymers are polyvinylidene fluoride, copolymers of polyvinylidene fluoride and hexafluoropropylene, poly (vinyl acetate), poly (vinyl butyral-co-vinyl alcohol-co-vinyl acetate), poly (methylmetha) Acrylate-co-ethyl acrylate), polyacrylonitrile, polyvinyl chloride-co-vinyl acetate, polyvinyl alcohol, poly (1-vinylpyrrolidone-co-vinyl acetate), cellulose acetate, polyvinylpyrroly Don, polyacrylate, polymethacrylate, polyolefin, polyurethane, polyvinyl ether, acrylonitrile-butadiene rubber, styrene-butadiene rubber, acrylonitrile-butadiene styrene, sulfonated styrene / ethylene-butylene / styrene Triblock copolymers, polyethylene oxides and mixtures thereof can be used.

The inorganic materials include colloidal silica, amorphous silica, surface treated silica, colloidal alumina, amorphous alumina, tin oxide, titanium oxide, titanium sulfide (TiS ₂ ), vanadium oxide, zirconium oxide (ZrO ₂ ), iron oxide (Iron). Oxide), iron sulfide (Iron Sulfide, FeS), iron titanate (Iron titanate, FeTiO ₃ ), barium titanate (Vanadium titanate, BaTiO ₃ ) and a mixture thereof may be used, and the organic material may be conductive Carbon can be used.

The positive electrode of the present invention is prepared by coating and drying a composition in which a positive electrode active material, a conductive agent, a binder, and a linear fiber are dispersed in a solvent. The solvent for preparing the slurry can uniformly disperse the sulfur-based active material, the binder and the conductive agent, and it is preferable to use one that is easily evaporated. Typically, acetonitrile, methanol, ethanol, tetrahydrofuran, water and iso Propyl alcohol, dimethyl formamide and the like can be used. Next, the sulfur-based active material and the additive are uniformly dispersed again in the slurry in which the conductive agent is dispersed to prepare a cathode active material slurry. The amount of solvent, sulfur compound or optionally additives included in the slurry does not have a particularly important meaning in the present invention, it is sufficient only to have a suitable viscosity to facilitate the coating of the slurry.

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 Al 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 by polysulfide of aluminum can be prevented, compared with the non-carbon coated Al substrate.

A lithium-sulfur cell 1 comprising the positive electrode is shown in FIG. 1. As shown in FIG. 1, a lithium-sulfur battery includes a battery can 5 including a positive electrode 3, a negative electrode 4, and a separator positioned between the positive electrode 3 and the negative electrode 4.

As the negative electrode, a material made of a negative electrode active material including a material capable of reversibly intercalating lithium ions, a material capable of reversibly forming a compound with lithium metal, or a lithium metal or a lithium alloy is used.

As a material capable of reversibly intercalating lithium ions, any carbon negative active material generally used in a lithium ion secondary battery may be used, and representative examples thereof include crystalline carbon, amorphous carbon, or a combination thereof. Can be used. 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, silicon (Si) and the like, but is 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. The inorganic protective film is Mg, Al, B, C, Sn, Pb, Cd, Si, In, Ga, lithium silicate, lithium borate, lithium phosphate, lithium phosphoronide, lithium silicon sulfide, 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.

The electrolyte used with the positive electrode of the present invention includes a lithium salt as a supporting electrolytic salt and a non-aqueous organic solvent. The organic solvent of the electrolyte used in the lithium-sulfur battery is suitably the elemental sulfur (S ₈ ), lithium sulfide (Li ₂ S), lithium polysulfide (Li ₂ S _n , n = 2, 4, 6, 8 ...) Dissolve well. The organic solvent may be benzene, fluorobenzene, toluene, trifluorotoluene, xylene, cyclohexane, tetrahydrofuran, 2-methyl tetrahydrofuran, cyclohexanone, ethanol, isopropyl alcohol, dimethyl carbonate, ethyl Methyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, propyl acetate, dimethoxy ethane, 1,3-dioxolane, diglyme, tetraglyme, ethylene carbonate At least one solvent selected from the group consisting of propylene carbonate, γ-butyrolactone and sulfolane.

The electrolytic salt lithium salt is lithium trifluoromethansulfonimide (lithium trifluoromethansulfonimide), lithium triflate (lithium triflate), lithium perchlorate (lithium perclorate), lithium hexafluoro azenate (LiAsF ₆ ), lithium trifluor Romethanesulfonate (CF ₃ SO ₃ Li), LiPF ₆ , LiBF ₄ or tetraalkylammonium, for example tetrabutylammonium tetrafluoroborate, or a liquid salt at room temperature, for example 1-ethyl-3-methyl One or more imidazolium salts such as imidazolium bis (perfluoroethyl sulfonyl) imide and the like can be used. The electrolyte contains a lithium salt in a concentration of 0.5 to 2.0M.

The electrolyte may be used as a liquid electrolyte, or may be used in the form of a solid electrolyte separator. When used as a liquid electrolyte, a physical separator having a function of physically separating an electrode further includes a separator made of porous glass, plastic, ceramic, or polymer.

The electrolyte separator functions as a physical separation of the electrode and a transfer medium for moving metal ions, and both electrochemically stable electric and ion conductive materials may be used. As such an electrically and ionically conductive material, a glass electrolyte, a polymer electrolyte, or a ceramic electrolyte may be used. As a particularly preferred solid electrolyte, the supported electrolyte salt is mixed with a polymer electrolyte such as polyether, polyimine, polythioether, or the like. The solid electrolyte separator may include less than about 20% by weight of the non-aqueous organic solvent, and in this case, may further include a suitable gelling agent to reduce the fluidity of the organic solvent.

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)

An elemental sulfur (S ₈ ) powder, a carbon conductive agent, a binder, and a linear fiber were added to water in a weight ratio of 84: 2: 4: 10 and then ball milled for 24 hours to prepare a cathode active material slurry. Styrene-butadiene rubber / carboxymethyl cellulose was used as the binder, and carbon nanotubes having an average diameter of 25 nm were used as the linear fiber. The prepared positive electrode active material slurry was coated on a carbon-coated Al current collector, and then dried in a hot air oven at 60 ° C. The dried electrode plate was cut into a size of 25 mm 50 mm to prepare a battery for evaluation having a battery capacity of 25 mAh. As the negative electrode, an unoxidized lithium metal foil (thickness 200 mu m) was used.

 (Example 2)

A battery was manufactured in the same manner as in Example 1, except that carbon nanotubes having an average diameter of 10 nm were used as the linear fibers.

(Example 3)

A battery was manufactured in the same manner as in Example 1, except that graphite nanofibers (GNF) having an average diameter of 10 nm were used as the linear fibers.

(Comparative Example 1)

A battery was manufactured in the same manner as in Example 1, except that a positive electrode prepared without adding a linear fiber was used.

(Comparative Example 2)

A battery was manufactured in the same manner as in Example 1, except that an activated carbon fiber having an average diameter of 9 μm was used as the linear fiber.

(Comparative Example 3)

A battery was manufactured in the same manner as in Example 1, except that carbon nanofibers having an average diameter of 0.5 μm were used as the linear fiber.

(Comparative Example 4)

A battery was manufactured in the same manner as in Example 1, except that graphite nanofibers having an average diameter of 125 nm were used as the linear fibers.

The test cells of Examples 1 to 2 and Comparative Examples 1 to 4 were 1.0C discharged and 0.5C charged in a cut-off voltage range of 2.8-1.5V to measure 50 charge and discharge cycle life characteristics, and are shown in FIG. 2. . As shown in Figure 2, it can be seen that the battery of Examples 1 to 3 made of a positive electrode including a linear fiber having an average diameter range according to the present invention has excellent life characteristics in the battery of Comparative Examples 1 to 4. .

 3 and 4 show electron micrographs before and after charging and discharging the positive electrode prepared according to Example 2, respectively. As shown in FIG. 3, the surface of the electrode plate before charging and discharging is uniformly distributed by a binder together with carbon nanotube linear fibers used as a conductive agent. 4 is a result of observing the inside of the electrode through an electron microscope by washing the electrode plate after the charge and discharge reaction after performing the charge and discharge reaction. Sulfur, which is present in the liquid phase during charging and discharging, is removed in the washing process, and carbon and carbon nanotubes used as a conductive agent are entangled and present in a solid form. It can also be observed that this leaves the carbon matrix intact. The carbon nanotubes distributed in this way facilitate the transfer of electrons, improve the physical properties of the electrode plate, and maintain the electrochemical reaction site. In addition, the sulfur, which existed in the initial state in the solid state due to the characteristics of the sulfur battery, plays a role in enduring the structural change of the electrode by generating a phase change in the liquid phase during charge and discharge.

The positive electrode including the electroconductive linear fiber of the present invention has excellent durability of the internal structure of the electrode so that the structure is not collapsed during charging and discharging and excellent electron transfer can improve the performance of the battery.

Claims (17)

Sulfur-based positive electrode active material; bookbinder; Conducting agents; And Carbon nanotube (CNT), Graphite nano fiber (GNF), Carbon nano fiber (CNF), Activated carbon fiber (ACF) and mixtures thereof Electroconductive linear fiber with an average diameter of 10 to 50 nm selected from A positive electrode for a lithium-sulfur battery comprising a. delete The positive electrode for a lithium-sulfur battery of claim 1, wherein the linear fiber has an average diameter of 10 to 30 nm. The positive electrode for a lithium-sulfur battery of claim 1, wherein the linear fiber is used in an amount of 2 to 20 wt% based on the positive electrode. The positive electrode for a lithium-sulfur battery of claim 1, wherein the conductive agent: electrically conductive linear fiber is used in a weight ratio of 10:90 to 90:10. The cathode active material of claim 1, wherein the cathode active material is a cathode, an organic sulfur compound, and carbon in which elemental sulfur (S ₈ ), solid Li ₂ S _n (n ≧ 1), and Li ₂ S _n (n ≧ 1) are dissolved A positive electrode for a lithium-sulfur battery comprising at least one sulfur compound selected from the group consisting of sulfur polymers [(C ₂ S _x ) _n , x = 2.5 to 50, n ≧ 2]. The positive electrode of claim 1, wherein the positive electrode further comprises a coating layer selected from the group consisting of a polymer coating layer, an inorganic coating layer, and an organic coating layer. The method of claim 7, wherein the polymer is a polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, poly (vinyl acetate), poly (vinyl butyral-co-vinyl alcohol-co-vinyl acetate ), Poly (methylmethacrylate-co-ethyl acrylate), polyacrylonitrile, polyvinyl chloride-co-vinyl acetate, polyvinyl alcohol, poly (1-vinylpyrrolidone-co-vinyl acetate), cellulose Acetate, polyvinylpyrrolidone, polyacrylate, polymethacrylate, polyolefin, polyurethane, polyvinyl ether, acrylonitrile-butadiene rubber, styrene-butadiene rubber, acrylonitrile-butadiene styrene, sulfonated styrene / Lithium-sulfur former selected from the group consisting of ethylene-butylene / styrene triblock copolymers, polyethylene oxides and mixtures thereof Positive for. The method of claim 7, wherein the inorganic material is colloidal silica, amorphous silica, surface-treated silica, colloidal alumina, amorphous alumina, tin oxide, titanium oxide, titanium sulfide (TiS ₂ ), vanadium oxide, zirconium oxide (ZrO ₂ ), Iron oxide (Iron Oxide), iron sulfide (Iron Sulfide, FeS), iron titanate (Iron titanate, FeTiO ₃ ), barium titanate (Vanadium titanate, BaTiO ₃ ) and mixtures thereof, the organic material is conductive Positive electrode for lithium-sulfur battery which is carbon. Sulfur-based positive electrode active material; bookbinder; Conducting agents; And Carbon nanotube (CNT), Graphite nano fiber (GNF), Carbon nano fiber (CNF), Activated carbon fiber (ACF) and mixtures thereof Conductive linear fiber with an average diameter of 50 nm or less selected from Anode comprising a; A negative electrode including a negative electrode active material selected from the group consisting of a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of forming a compound reversibly with lithium, a lithium metal and a lithium alloy; And Electrolyte Containing Lithium Salt And Organic Solvent Lithium-sulfur battery comprising a. The lithium-sulfur battery of claim 10, wherein the linear fiber has an average diameter of 10 to 50 nm. The lithium-sulfur battery of claim 10, wherein the linear fiber is used in an amount of 2 to 20 wt% based on the positive electrode. The lithium-sulfur battery of claim 10, wherein the conductive agent: electrically conductive linear fiber is used in a weight ratio of 10:90 to 90:10. The cathode active material of claim 10, wherein the cathode active material is a cathode, an organic sulfur compound, and a carbon-sulfur polymer in which elemental sulfur, solid Li ₂ S _n (n ≧ 1), Li ₂ S _n (n ≧ 1) are dissolved [(C ₂ S _x ) _n , x = 2.5 to 50, n ≥ 2) containing at least one sulfur compound selected from the group consisting of lithium-sulfur battery. The method of claim 10, wherein the organic solvent is benzene, fluorobenzene, toluene, trifluorotoluene, xylene, cyclohexane, tetrahydrofuran, 2-methyl tetrahydrofuran, cyclohexanone, ethanol, isopropyl alcohol , Dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, methylpropionate, ethylpropionate, methyl acetate, ethyl acetate, propyl acetate, dimethoxy ethane, 1,3-dioxolane, diglyme, tetra Lithium-sulfur battery, which is at least one solvent selected from the group consisting of glyme, ethylene carbonate, propylene carbonate, γ-butyrolactone and sulfolane. The method of claim 10, wherein the lithium salt is lithium trifluoromethansulfonimide (lithium trifluoromethansulfonimide), lithium triflate (lithium triflate), lithium perclorate (lithium perclorate), lithium hexafluoro azenate (LiAsF ₆ ), lithium At least one compound selected from the group consisting of trifluoromethanesulfonate (CF ₃ SO ₃ Li), LiPF ₆ , LiBF ₄ , tetraalkylammonium, and a liquid salt at room temperature. The lithium-sulfur battery of claim 16, wherein the electrolyte comprises lithium salt at a concentration of 0.5 to 2.0M.

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