KR100551002B1 - Conductive material used in positive electrode for lithium sulfur battery, positive electrode for lithium sulfur battery and lithium sulfur battery comprising same - Google Patents

Abstract

The present invention relates to a conductive material for a positive electrode of a lithium sulfur battery, a positive electrode for a lithium sulfur battery, and a lithium sulfur battery including the same, wherein the conductive material includes a carbon-based material having a dibutyl phthalate oil absorption greater than 400 ml / 100 g. Since the positive electrode of the present invention uses a conductive material having a high DBP oil absorption, the capacity characteristics of the battery can be improved.

DBP oil absorption, conductive material, lithium sulfur battery, anode

Description

A conductive material for a lithium sulfur battery positive electrode, a cathode for a lithium sulfur battery and a lithium sulfur battery including the same.

1 is a graph showing the discharge capacity according to the dibutyl phthalate oil absorption amount of the positive electrode of Examples 1 to 2 and Comparative Examples 1 to 2 of the present invention.

[Industrial use]

The present invention relates to a conductive material for a positive electrode of a lithium sulfur battery, a positive electrode for a lithium sulfur battery, and a lithium sulfur battery including the same, and more particularly, to a positive electrode for a lithium sulfur battery having excellent discharge capacity, and a lithium sulfur battery including the same. .

[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. As a secondary battery that satisfies these requirements, development of a lithium sulfur battery using a sulfur-based material as a cathode active material is being actively conducted.

The lithium sulfur 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 insertion / deintercalation of metal ions such as lithium ions occurs. It is a secondary battery used as a negative electrode active material. In the reduction reaction (discharged), the SS bond is broken and the oxidation number of S decreases. In the oxidation reaction (charged), the oxidation-reduction reaction of the SS bond is formed by increasing the oxidation number of S and the electrical energy is stored and stored. Create

Lithium sulfur battery has a theoretical energy density of 2800 Wh / kg (1675 mAh / g), which is much higher than other batteries, and the sulfur-based material used as a positive electrode active material has abundant resources, is inexpensive, and attracts attention as an environmentally friendly material. .

Sulfur, which is used as a positive electrode active material of a lithium sulfur battery, is an insulator and thus requires a conductive material for the movement of electrons generated by an electrochemical reaction. In other words, the role of the conductive material is important to actively generate the electrochemical reaction.

As the conductive material, carbon of carbon black, metal powder, or the like may be used, and carbon is generally used. Since the conductive material serves as a reaction site for the polysulfide present in the liquid phase during charging and discharging, the conductive material should have a large specific surface area and be able to impregnate a large amount of the electrolyte (or polysulfide solution).

In order to satisfy such physical properties, US Pat. No. 6,143,448 describes the adjustment of the physical properties of the conductive material to dibutyl phthalate (DBP) oil absorption and specific surface area. The DBP oil absorption of this electrically conductive material was 52-400 ml / 100g, the specific surface area was 250-1000 m <2>, and the particle size was 10-50 nm. However, research is still being conducted to increase battery capacity by controlling the properties of conductive materials.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a conductive material for a lithium sulfur battery positive electrode that can optimize the physical properties of the battery.

Another object of the present invention is to provide a positive electrode for a lithium sulfur battery using a conductive material having optimum physical properties.

Still another object of the present invention is to provide a lithium sulfur battery including the positive electrode.

In order to achieve the above object, the present invention provides a conductive material for a positive electrode of a lithium sulfur battery comprising a carbon-based material having a dibutyl phthalate oil absorption greater than 400ml / 100g.

The present invention also provides a positive electrode for a lithium sulfur battery comprising the conductive material and the positive electrode active material.

In addition, the present invention is the positive electrode; A negative electrode including a negative electrode active material; And it provides a lithium sulfur battery comprising an electrolyte solution.

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

The present invention relates to a conductive material in which the physical properties of the conductive material used for the positive electrode of a lithium sulfur battery are adjusted to obtain an optimum capacity.

In the lithium sulfur battery, a carbon-based material such as carbon or carbon black is generally used as the conductive material. The properties of such carbon-based materials may be expressed in terms of particle size, surface area, aggregate size (structure), surface chemistry, etc. The structure of the carbon-based material, Agglomerate size is a property of how much primary particles of carbon are aggregated and is one of the important variables affecting battery capacity since electrolyte impregnation varies depending on the structure, that is, the conductive material must transfer electrons well and polysulfide The structure of the carbon-based material satisfying these properties may include a structure of high structural black (primary particles) formed by gathering primary particles of carbon black. It refers to a large number of basic particles, and the structure also has several basic particles attached to each other. A large number of primary particles, composed of many branched chains with a large amount of void space, good conductivity, good dispersion, and high viscosity. There is this.

Whether or not the conductive material, which is a carbon-based material, has such a dense structure can be measured by dibutyl phthalate (DBP) absorption rate. The conductive material of the present invention is a carbon-based material having a DBP oil absorption greater than 400 ml / 100 g. Preferred DBP oil absorption is 410 to 700 ml / 100 g. Since the carbonaceous material having a DBP oil absorption greater than 400ml / 100g of the conductive material is in a state in which a large number of primary particles are agglomerated, the dispersibility in the solvent is good when the active material composition is prepared, thereby improving the homogeneity of the electrode plate when manufacturing the positive electrode plate. In addition, since a large amount of voids exist between the structures of the carbon-based material in the manufactured anode, the contact area with the electrolyte is increased to increase the electrochemical reaction area, and the electrolyte impregnability is increased to improve the battery capacity. Can be. Therefore, when DBP oil absorption amount of a electrically conductive material is 400 ml / 100g or less, since a capacity | capacitance falls, it is not preferable.

The conductive material preferably has a pH of 6 or more, more preferably 6 to 10, and preferably an average particle size of 10 to 50 nm. Moreover, 800 m <2> / g or more is preferable and, as for the specific surface area of a electrically conductive material, 900-1500 m <2> / g is more preferable. When the average particle size of the conductive material is larger than 50 nm, the specific surface area naturally decreases, and thus, the reaction site at which the electrochemical reaction can occur is reduced, which adversely affects the performance of the battery. In addition, when the average particle size of the conductive material is less than 10 nm, it is expected that the specific surface area is large and the reaction site is increased, thereby affecting battery performance. However, in the case of a carbon-based material having a very small particle size and a large specific surface area, the slurry properties It is worsened, there are difficulties in the process, and in order to attach such a conductive material to the electrode plate, a large amount of binder is required, which causes a problem of reducing the amount of the active material on the electrode plate.

As such, when a conductive material having a large DBP oil absorption (unit: ml / 100g) is used for the positive electrode of the lithium sulfur battery, the dispersion is uniform during the preparation of the active material composition, thereby improving the quality of the active material composition, and thus, producing a homogeneous positive electrode plate. there was. In addition, the use of a carbon-based material having a large DBP oil absorption can further improve the impregnation of the electrolyte, thereby improving the performance of the battery.

The reason why the discharge capacity was high when using the carbon powder having a large DBP oil absorption was that the powder having a large DBP oil absorption had a lot of primary particles, so that the dispersibility in a solvent such as water was good. In the electrode plate, since a large amount of pores exist between the structures of carbon, the contact area with the electrolyte is widened and the electrochemical reaction area is widened, and the electrolyte impregnation is improved, which contributes to the improvement of the battery capacity.

The positive electrode for lithium sulfur battery of this invention contains the said electrically conductive material and positive electrode active material. As the cathode active material, elemental sulfur (S ₈ ), a sulfur-based compound, or a mixture thereof may be used. The sulfur-based compound is selected from the group consisting of Li ₂ S _n (n ≧ 1), an organic sulfur compound, and carbon-sulfur polymer ((C ₂ S _x ) _n : x = 2.5 to 50, n ≥ 2) Can be used.

In addition, the positive electrode active material may further include a binder that can be attached to the current collector. Representative examples of the binder include poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, crosslinked polyethylene oxide, polyvinyl ether, poly (methyl methacrylate), poly Vinylidene fluoride, copolymer of polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly (ethyl acrylate), polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile, polyvinylpyridine , Polystyrene, derivatives thereof, blends, copolymers and the like.

The content of the conductive material, the positive electrode active material and the binder in the positive electrode of the present invention is not a factor influencing the effects of the present invention, and may be appropriately mixed. The mixing ratio may be widely understood by those skilled in the art. to be.

The lithium sulfur battery of the present invention including the positive electrode includes a negative electrode and an electrolyte solution. The negative electrode active material in the negative electrode is a group consisting of a material capable of reversibly intercalating or deintercalating lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, a lithium metal and a lithium alloy. Can be selected from.

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, a representative example of a material capable of reacting with lithium ions to reversibly form a lithium-containing compound may include, but is not limited to, tin oxide (SnO ₂ ), titanium nitrate, silicon (Si), and the like. 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 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.

As said electrolyte solution, what contains an electrolyte salt and an organic solvent can be used.

As the organic solvent, 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 select one or more solvents 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 sulfur elements among aryl compounds, bicyclic ethers, and acyclic carbonates; strong polar solvents are bicyclic carbonates, sulfoxide compounds, lactone compounds Is defined as a solvent having a dielectric constant of greater than 15 that can dissolve lithium polysulfide among ketone compounds, ester compounds, sulfate compounds, and sulfite compounds, and lithium protective solvents are saturated ether compounds, unsaturated ether compounds, N, O And a charge / discharge cycle efficiency (cycle efficiency) of forming a SEI (Solid Electrolyte Interface) film stable on a lithium metal, such as a heterocyclic compound containing S, or a combination thereof, is defined as a solvent of 50% or more.

 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, dimethyl sulfoxide, dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, or ethylene glycol sulfite.

Specific examples of the lithium protective solvent include tetrahydrofuran, ethylene oxide, dioxolane, 3,5-dimethyl isoxazole, 2,5-dimethyl furan, furan, 2-methyl furan, 1,4-oxane, 4-methyldioxolane Etc.

The electrolytic salt lithium salt is lithium trifluoromethansulfonimide (lithium trifluoromethansulfonimide), lithium triflate (lithium triflate), lithium perchlorate (lithium perclorate), LiPF ₆ , LiBF ₄ or tetraalkylammonium, for example tetra One or more butylammonium tetrafluoroborate, or liquid salts at room temperature, such as imidazolium salts such as 1-ethyl-3-methylimidazolium bis- (perfluoroethyl sulfonyl) imide and the like Can be.

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)

Inorganic sulfur (S ₈ ) positive electrode active material; A carbon black conductive material: polyethylene oxide binder was mixed in an isopropyl solvent at a ratio of 84: 12: 4 by weight to prepare a positive electrode active material slurry. The DBP oil absorption amount of the said electrically conductive material was 425 ml / 100 g. Moreover, pH was 6-9, particle size 30nm, and specific surface area was 950m <^2> .

The positive electrode was manufactured by the conventional method using the positive electrode active material slurry.

(Example 2)

The same procedure as in Example 1 was carried out except that a carbon black conductive material having a DBP oil absorption of 495 ml / 100 g was used. The carbon black conductive material had a pH of 8 to 10, a particle size of 33.9 nm, and a specific surface area of 1270 m ² / g.

(Comparative Example 1)

The same procedure as in Example 1 was carried out except that a carbon black conductive material having a DBP oil absorption of 130 ml / 100 g was used. The carbon black conductive material had a pH of 8, a particle size of 19 nm, and a specific surface area of 150 m ² / g.

(Comparative Example 2)

The same procedure as in Example 1 was carried out except that a carbon black conductive material having a DBP oil absorption of 330 ml / 100 g was used.

A lithium sulfur battery was manufactured using the positive electrode and the lithium foil negative electrode of Examples 1 to 3 and Comparative Examples 1 to 2 above. The produced battery was charged and discharged at a 0.2C charge rate and a 0.2C discharge rate to measure discharge capacity, and the results are shown in FIG. 1. In FIG. 1, the discharge capacity% is a value compared to the design capacity. As shown in FIG. 1, the batteries of Examples 1 to 3 using a conductive material having a DBP oil absorption greater than 400 ml / 100 g were 57.6%, 64.8%, and 65%, respectively, based on the design capacity, and the conductive material of 400 ml / 100 g or less was used. It can be seen that the discharge capacity% is superior to the batteries of Comparative Examples 1 and 2.

As described above, the present invention can provide a battery having excellent capacity characteristics by using a conductive material having a high DBP oil absorption in the positive electrode.

Claims (17)

delete delete Positive electrode active material; And Carbonaceous conductive material having dibutyl phthalate oil absorption of 410 to 700 ml / 100 g and an average particle size of 10 to 50 nm A positive electrode for a lithium sulfur battery comprising a. delete The positive electrode for a lithium sulfur battery according to claim 3, wherein the conductive material has a pH of 6-11. delete delete The positive electrode for lithium sulfur battery of Claim 3 whose specific surface area of the said electrically conductive material is 900-1500m <2> / g. The method of claim 3, wherein the positive electrode active material is inorganic sulfur (elemental sulfur, S ₈ ), Li ₂ S _n (n≥1), organic sulfur compound, and carbon-sulfur polymer ((C ₂ S _x ) _n : x = A positive electrode for a lithium sulfur battery, which is a sulfur-based compound selected from the group consisting of 2.5 to 50 and n ≧ 2) or a mixture thereof. A positive electrode including a positive electrode active material and a dibutylphthalate oil absorption amount of 410 to 700 ml / 100 g, and a carbon-based conductive material having an average particle size of 10 to 50 nm; A negative electrode including a negative electrode active material; And Electrolyte Lithium sulfur battery comprising a. delete The lithium sulfur battery of claim 10, wherein the conductive material has a pH of 6 to 11. 11. delete delete The lithium sulfur battery of claim 10, wherein a specific surface area of the conductive material is 900 to 1500 m 2 / g. The method of claim 10, wherein the positive electrode active material is inorganic sulfur (elemental sulfur, S ₈ ), Li ₂ S _n (n≥1), organic sulfur compound, and carbon-sulfur polymer ((C ₂ S _x ) _n : x = Lithium sulfur battery which is a sulfur-based compound selected from the group consisting of 2.5 to 50, n≥2) or a mixture thereof. The method of claim 10, wherein the negative electrode active material is selected from the group consisting of a material capable of reversibly intercalating lithium ions, a material capable of reacting with lithium ions to form a lithium-containing compound reversibly, lithium metal and a lithium alloy It is a lithium sulfur battery.

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