KR20040110863A - Positive electrode for rechargeable lithium battery and rechargeable lithium battery comprising same - Google Patents

Abstract

PURPOSE: Provided a positive electrode and a lithium secondary battery comprising the same, which has excellent cycle-life characteristics and capacity property. CONSTITUTION: The positive electrode for a lithium secondary battery includes a conductive material containing a carbonic material treated with an acid; and a positive electrode active material. Preferably, the carbonic material is treated with a solution of the acid selected from the group consisting of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid and boric acid, which has a concentration of 10 to 80 wt%. The positive electrode active material is a sulfur-series compound selected from the group consisting of an elemental sulfur(S8), Li2Sn(n1), an organic sulfur compound and a carbon-sulfur polymer((C2Sx)n, wherein x is 2.5 to 50, n 2), or a mixture thereof.

Description

A positive electrode for a lithium secondary battery and a lithium secondary battery including the same {POSITIVE ELECTRODE FOR RECHARGEABLE LITHIUM BATTERY AND RECHARGEABLE LITHIUM BATTERY COMPRISING SAME}

[Industrial use]

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to a positive electrode for a lithium secondary battery having improved battery characteristics and a lithium secondary 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 positive electrode active material is actively progressing.

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 or carbon powder of carbon blacks may be used. In general, carbon, particularly carbon black having a surface area of 20 to 2000 m ² / g, is most widely used. Since the conductive material serves as a reaction site for the polysulfides present in the liquid phase during charging and discharging, the conductive material should have a large specific surface area and be capable of impregnating a large amount of the electrolyte (or polysulfide solution).

However, since the carbon black conductive material has a large surface area, the area exposed to the electrolyte is relatively larger than that of the positive electrode active material, so that the electrolyte is denatured by the reaction between the carbon black and the electrolyte, thereby increasing the charge and discharge performance of the battery. There is a problem of deterioration.

In order to solve this problem, Korean Patent Publication No. 99-81129 discloses that carbon black is washed with an organic solvent, ozone treatment, or hydrogen gas to change the physical properties of carbon black to suppress electrolyte decomposition reactions, and A method of facilitating mixing and contact with a cell to improve battery performance is described. However, these methods do not have sufficient effect of inhibiting the reaction of carbon black and electrolyte solution, and the research on them continues.

In addition, J. of Korean Ind. & Eng. Chemistry Vol. 7 No. 4, 1996, 768-776 describe the treatment of carbon black with nitric acid to change surface properties to improve dispersibility in aqueous solutions. This method relates to the dispersion of carbon black, which is generally used for industrial purposes such as color pigments, when the carbon black is dispersed in water, an organic solvent or a rubber component, that is, an ink or a tire. The improvement of the dispersibility of carbon black as a conductive material when used with is not described at all.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a positive electrode for a lithium secondary battery that can provide a battery exhibiting improved battery performance by modifying the physical properties of the conductive material.

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

1 is an FT-IR graph of a conventional conductive material used in Comparative Example 1. FIG.

2 is an FT-IR graph of an acid-treated conductive material used in Example 1 of the present invention.

3 is a graph showing the cycle life characteristics of the battery of Example 2 and Comparative Example 2 of the present invention.

4 is a graph showing the discharge capacity characteristics of the battery of Example 2 and Comparative Example 2 of the present invention.

In order to achieve the above object, the present invention comprises a conductive material comprising a carbon-based material treated with an acid; And it provides a positive electrode for a lithium secondary battery comprising a positive electrode active material.

The present invention also the anode; A negative electrode including a negative electrode active material; And it provides a lithium secondary battery comprising an electrolyte solution.

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

The present invention is to improve the battery performance by modifying the surface of the conventional conductive material used to impart electrical conductivity to the positive electrode active material used for the positive electrode of the lithium secondary battery.

As the conventional conductive material usable in the present invention, any one can be used as long as it can impart electrical conductivity to the positive electrode active material such as carbon black, carbon, or Ketjen black, but mainly used is carbon black. Let's explain.

Carbon black is an ultrafine particle composed of more than 95% of amorphous carbonaceous material, and small spherical particles (fine particles are spherical) are collected to form a minimum aggregate, and the aggregates are aggregated again to form an irregular chain branch complex. In addition, carbon atom radicals in the carbon black surface layer (outermost surface layer) are easily oxidized to form various functional groups. Representative functional groups of the carbon black thus formed are -OH, -O, -COOH, -OCO and the like.

These carbon blacks generally have various properties such as reinforcement, coloring, weather resistance, chemical resistance, and electrical conductivity, and when used as a conductive material for batteries, dispersion in a medium such as water or an organic solvent may affect the quality of the product. It has a big impact. In addition, carbon black is a fine powder having a large surface area, and is not easily dispersed in other media because of its greater cohesion than affinity with other materials.

In the present invention, the conductive material was treated with an acid to perform surface modification. More specifically, the process of treating with acid is a surface-modified conductive material treated by adding a conductive material to the acid solution, mixing for a predetermined time, and then removing the acid solution. The acid solution is at least one acid solution selected from the group consisting of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid and boric acid, it is preferable to use an acid aqueous solution having a concentration of 10 to 80% by weight. When the concentration of the acid solution is less than 10% by weight, the acid treatment effect is insignificant, and it is difficult to show improved performance. When the concentration of the acid solution exceeds 80% by weight, the pore walls of the micropore are eroded by the acid and the pores are depressed. The phenomenon occurs. In this case, the pore size of the carbon black is changed, so that the specific surface area of the original carbon black is reduced, and there is a problem in that the reaction site is reduced.

The mixing time is preferably carried out for 5 minutes to 12 hours, more preferably for 3 to 5 hours. If the mixing time is less than 5 minutes, it is difficult to obtain an acid treatment effect, and even if the mixing process is carried out for more than 12 hours, the acid treatment effect does not increase any longer, so it is not necessary to mix for more than 12 hours.

As described above, according to the acid treatment process, the conductive material of the present invention can increase affinity with a medium used in the production of the positive electrode, thereby producing a homogeneous electrode plate. In addition, the impurities contained in the periphery of the conductive material are removed, and micropores are eroded by acid, and as the pore size increases, a large amount of active material present in the liquid phase in the lithium sulfur battery may be impregnated between the pores. Since the number of reaction sites where the electrochemical reaction occurs smoothly during charging and discharging increases, the performance of the battery can be improved.

In the positive electrode active material for a lithium sulfur battery using the conductive material of the present invention, 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.

When lithium metal is used as the negative electrode active material, an inorganic protective layer, an organic protective film, or a material in which these are laminated on the lithium metal surface may also be used as the negative electrode. As the inorganic protective film, Mg, Al, B, Sn, Pb, Cd, Si, In, Ga, lithium silicate, lithium borate, lithium phosphate, lithium phosphoronide, lithium silicosulfide, lithium borosulfide, lithium aluminosulfide And it is made of a material selected from the group consisting of 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 those having a dielectric constant of less than 15 that can dissolve elemental sulfur among aryl compounds, bicyclic ethers, and acyclic carbonates; 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, 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 Surface Treatment with Nitric Acid

After dilution by adding 200 g of water to 100 g of 60% HNO ₃ solution, this dilution was added to a beaker containing 20 g of ketjen black and mixed. At this time, the mixing time was 3 hours. After mixing, the Ketjen Black was separated under reduced pressure using a filtration apparatus, and the filtrate was washed with distilled water gradually to remove nitric acid remaining on the surface of the separated Ketjen Black, until the filtrate was neutral. Thus obtained sample was dried for 24 hours in an 80 ℃ vacuum oven to prepare a surface-modified conductive material.

(Comparative Example 1)

Ketjen Black, which was not treated with nitric acid, that is, without surface modification, was used as the conductive material.

FIG. 1 shows FT-IR analysis of ketjen black without nitric acid treatment of Comparative Example 1, and FIG. 2 shows FT-IR analysis after nitric acid treatment of Example 1. FIG. Looking at the graph of Figure 2, it can be seen that after the nitric acid treatment -OH, -CH, -COOH, -CH ₃ groups are detected. In addition, when comparing the graph of FIG. 1 and FIG. 2, compared with the electrically conductive material of the comparative example 1 which the nitric acid process did not carry out, the -OH group of 3400cm <^-1> vicinity has an intensity after nitric acid treatment compared with the electrically conductive material of the comparative example 1 which was not nitric acid treated It can be seen that the peak shifted as the COOH peak appearing near 1627 cm ⁻¹ diverged into a doublet. In addition, it can be seen that the hydrocarbon peak appears strongly around 1384 cm ^-1 . As a result, the surface treatment of carbon black may cause decomposition and conversion to other functional groups due to reaction with nitric acid, and it can be seen that the amount of functional groups is reduced as a result of decreasing the intensity of the peak.

(Example 2)

A positive electrode active material slurry was prepared by mixing an elemental sulfur (S ₈ ) positive electrode active material and the surface modified Ketjen black and polyethylene oxide binder prepared in Example 1 in an isopropyl solvent at a ratio of 74: 18: 8 by weight. Using this slurry, the positive electrode for lithium sulfur batteries was manufactured by a conventional method.

(Comparative Example 2)

The same procedure as in Example 2 was conducted except that Ketjen Black, which was not surface-modified in Comparative Example 1, was used as the conductive material.

The cycle life characteristics obtained by manufacturing a lithium sulfur battery by a conventional method using the positive electrode of Example 2 and Comparative Example 2, and charging and discharging with 0.2C charge and 0.5C discharge using this battery are shown in FIG. 3. . As shown in FIG. 3, the battery using the positive electrode of Comparative Example 2 using the non-surface treated conductive material rapidly decreased its discharge capacity at 45 cycles of charging and discharging, while using carbon having a surface treatment of 65 cycles. It can be seen that the discharge capacity is maintained until.

4 shows discharge capacities obtained by charging and discharging the batteries of Example 2 and Comparative Example 2 with 0.2C charge and 1.0C discharge. As can be seen in FIG. 4, the discharge capacity of the battery of Comparative Example 2 using the surface-treated conductive material was 15.3 mAh, but the discharge capacity of the surface-treated battery of Example 2 was 20.6 mAh, 30% compared to Comparative Example 2. Increased over.

As described above, the positive electrode of the present invention using an acid-treated conductive material can provide a battery excellent in cycle life characteristics and capacity characteristics.

Claims (11)

A conductive material comprising a carbon-based material treated with an acid; And Positive electrode active material A positive electrode for a lithium secondary battery comprising a. The positive electrode for a lithium secondary battery of claim 1, wherein the acid is selected from the group consisting of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, and boric acid. The positive electrode for a lithium secondary battery according to claim 1, wherein the acid treatment is performed using an acid solution. The positive electrode for a rechargeable lithium battery of claim 3, wherein the acid solution has a concentration of 10 to 80 wt%. The method of claim 1, 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 secondary 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 conductive material including a carbon-based material treated with an acid and a positive electrode active material; A negative electrode including a negative electrode active material; And Electrolyte Lithium secondary battery comprising a. The lithium secondary battery of claim 6, wherein the acid is selected from the group consisting of nitric acid, hydrochloric acid, phosphoric acid, sulfuric acid, acetic acid, and boric acid. The lithium secondary battery according to claim 6, wherein the acid treatment is performed using an acid solution. The lithium secondary battery of claim 8, wherein the acid solution has a concentration of about 10 wt% to about 80 wt%. The method of claim 6, 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 lithium secondary 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 6, 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 The lithium secondary battery which becomes.