KR20070079980A - Cathode material containing ag nano particle as conductive material and lithium secondary battery comprising the same - Google Patents

Abstract

Provided are a positive electrode active mass which can improve the capacity of a battery even by adding a small amount of a conductive material by using a silver nanoparticle, and a lithium secondary battery containing the positive electrode using the positive electrode active mass. The positive electrode active mass comprises a positive electrode active material; 0.2-2.0 wt% of a silver nanoparticle having an average particle size of 5-100 nm as a conductive material; and a binder. Preferably the silver nanoparticle is prepared by reacting AgNO3 and NaBH4 by sol-gel method; and sintering the obtained one at a temperature of 500-700 deg.C under nitrogen atmosphere of high purity to remove the impurities.

Description

Cathode Material Containing Ag Nano Particle as Conductive Material and Lithium Secondary Battery Comprising the Same

1 is a graph showing the results of the discharge characteristics test for the batteries of Examples 1 and 3 and Comparative Examples 1 and 2 in Experimental Example 2 of the present invention;

2 is a graph showing the results of the cycle characteristics test for the batteries of Examples 1 and 3 and Comparative Examples 1 and 2 in Experimental Example 3 of the present invention.

The present invention relates to a positive electrode mixture containing silver nanoparticles as a conductive material and a lithium secondary battery composed therefrom. More specifically, the present invention relates to a positive electrode mixture containing silver nanoparticles. Providing a positive electrode mixture that can provide the life characteristics and rate characteristics corresponding to a battery using a conventional conductive material and can provide further increased battery capacity by using a small amount of the conductive material, and a lithium secondary battery of excellent performance consisting of do.

Recently, due to the explosive increase in the demand for portable electronic devices, the demand for secondary batteries is also rapidly increasing, and among them, lithium secondary batteries having high energy density, high discharge voltage, and output stability have made great progress.

As electronic devices become highly functional and miniaturized, secondary batteries are also required to be miniaturized and transformed into various forms at the same time as high performance. For example, in the case of a notebook computer, since the size of the secondary battery has a great influence on the thickness of the notebook computer, structural changes have been attempted to reduce the thickness of the notebook in the form of the battery with high capacity and high performance. In addition, as a serious environmental problem, as a solution for global warming, development of electric vehicles, hybrid electric vehicles, etc., which use secondary batteries, is being accelerated.

In general, a lithium secondary battery has a structure in which a lithium electrolyte is impregnated into an electrode assembly including a cathode including a lithium transition metal oxide, a cathode including a carbon-based active material, and a separator. The positive electrode is prepared by coating a positive electrode mixture containing a lithium transition metal oxide on an aluminum foil, and the negative electrode is prepared by coating a negative electrode mixture including a carbon-based active material on a copper foil. In the electrode of such a structure, when the amount of the electrode mixture is coated on the current collector for the purpose of increasing the capacity of the secondary battery, there may be a phenomenon that the performance of the battery is reduced. In addition, when the disproportionation of the inside of the electrode is intensified, the electrode reaction is locally concentrated, and in this case, lithium metal may be precipitated, which may cause problems in safety.

On the other hand, a conductive material is generally added to the positive electrode mixture and the negative electrode mixture for the purpose of improving the electrical conductivity of the active material. In particular, since the lithium transition metal oxide used as the positive electrode active material has low electrical conductivity, a conductive material is essentially added to the positive electrode mixture.

As such a conductive material, a carbon-based material is mainly used, such as graphite such as natural graphite or artificial graphite, carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, or summer black, In some cases, conductive fibers such as carbon fibers and metal fibers are used. Specific examples of commercially available conducting agents include acetylene black Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack, EC series (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

The conductive material is added in an amount of about 3 to 15% by weight based on the weight of the positive electrode mixture, and when too little is used, the performance of the battery is degraded due to an increase in the internal resistance of the electrode. Since the content of the binder must be increased together, a problem such as a decrease in battery capacity due to a decrease in the electrode active material is caused.

Therefore, there is a high need for a technology capable of exerting a desired effect while reducing the content of the conductive material. In the present invention to solve the above problems by adding a very small amount of silver nanoparticles having a predetermined particle size range, some prior art of adding metal particles to the internal configuration of the battery will be described below.

Korean Patent Registration No. 515,029 discloses a technique of forming a cluster or a thin film having a thickness of 1 to 300 nm by singly or mixing a layer of a conductive material and a metal oxide on the surface of a cathode active material. However, in order to form the metal cluster or thin film as described above, a gas dispersion coating technology is required. For this purpose, a gas dispersion coating tank, a heater, a coating solution supply pump, a coating solution storage tank, a baffle, a gas supply system, and a high speed Since a separate device such as a rotary rotor, a filter, and a gas aftertreatment system is required, the manufacturing cost is greatly increased despite good physical properties. In addition, since the amount of the conductive material applied to the surface of the positive electrode active material is confirmed by the embodiment of the patent, etc. up to 12% by weight, the battery capacity is greatly reduced as described above.

Korean Patent No. 519,013 discloses a technique of using cobalt metal powder instead of a carbonaceous material as a negative electrode active material. However, although the patent adds metal powder to the electrode, carbon black is separately used as the conductive material, and since the amount of the conductive material is 25% by weight and the amount of the binder is very high, 15% by weight, Reduction of the battery capacity due to the decrease in the content of the active material is inevitable.

Korean Patent Application Publication No. 2002-68782 discloses an impregnation method, deposition method, initial wetting method, ion exchange method, and colo in a carbon secondary battery using sulfur or a sulfur compound as a cathode active material. This month, the technique of including by adding in the adsorption method is disclosed. However, although the above application adds the metal fine particles to the battery, carbon is used as the conductive material, and since the metal fine particles are added to the inside of the carbon, the amount of the conductive material is not reduced. In addition, since a separate addition process and purification process are required to include the metal fine particles in the carbon, which is a conductive material, the overall economic efficiency of the process is very low.

Japanese Patent No. 2925612 discloses a technique in which a metal powder is used as a conductive material for a negative electrode of a hydrogen storage alloy in a metal-hydrogen alkali storage battery rather than a lithium secondary battery. However, the above technique reduces the content of the conductive material by adding about 30 μm of the metal powder at 5 wt%, when compared to the conventional technology using a carbon-based material as the conductive material when it is applied to a lithium secondary battery as it is. No effect is obtained.

Therefore, there is a high necessity for developing a technology for a battery capable of exhibiting excellent physical properties while reducing the content of a conductive material for a positive electrode in a lithium secondary battery.

The present invention aims to solve the problems of the prior art and the technical problems that have been requested from the past.

After extensive research and various experiments, the inventors of the present invention, in the case of using silver nanoparticles having a predetermined particle diameter as a conductive material, add a relatively small amount of carbon nanoconductive material to a conventional battery. The present invention has been shown to show comparable or better life characteristics and rate characteristics, and in particular, that the capacity of a battery can be increased by the addition of a trace amount of a conductive material.

Accordingly, the positive electrode mixture for lithium secondary batteries according to the present invention is a positive electrode mixture for lithium secondary batteries comprising a positive electrode active material, a conductive material and a binder, wherein the silver nanoparticles having an average particle diameter of 5 to 100 nm are mixed as the conductive material. It is composed of 0.2 to 2.0% by weight based on the total weight.

The use of such a small amount of conductive material can meet such demands by increasing the amount of the electrode active material used in the positive electrode mixture, considering the recent development trend for secondary batteries requiring a more compact structure and high output and large capacity. That is, a small amount of silver particles added in a small amount improves the mobility of lithium ions in the positive electrode to help uniform charging and discharging throughout the electrode. The improvement effect of the lifetime characteristic by this can be exhibited.

In particular, since the positive electrode mixture according to the present invention exhibits excellent life characteristics and rate characteristics at high speed charging and discharging, the secondary battery to which it is applied provides excellent power.

As described above, the silver nanoparticles used as the conductive material in the cathode mixture of the present invention have a particle diameter of 5 to 100 nm, and when the particle diameter is too small, it is not easy to manufacture and uniform dispersion in the cathode mixture is difficult. On the contrary, if it is too large, it is not preferable because its content must be increased to provide a desired level of electrical conductivity.

The silver nanoparticles may be used as long as they have the same size as described above. For example, AgNO ₃ and NaBH ₄ may be prepared by sol-gel synthesis to remove impurities. The silver nanoparticles having a size of 5 nm or more may be prepared by firing with high purity nitrogen gas at ˜700 ° C. The size of the silver nanoparticles can be synthesized in the size of 5 ~ 100 nm depending on the reaction conditions.

As described above, the silver nanoparticles may be added at 0.2 to 2% by weight, preferably 0.5 to 1.5% by weight based on the total weight of the positive electrode mixture. When the content is too small, it is difficult to improve the performance of the battery, in particular, high-rate discharge characteristics due to the low electrical conductivity of the electrode, on the contrary, when too large, it is difficult to expect a high battery capacity compared to the same standard because the amount of the binder is increased Not desirable

The positive electrode mixture is composed of the above-mentioned conductive material and a binder in the positive electrode active material, and a filler may be further added as necessary.

Examples of the positive electrode active material include a layered compound such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + x} Mn _2-x O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO _2, and the like; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ , Cu ₂ V ₂ O ₇ and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-x M _x O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 to 0.3; Formula LiMn _2-x M _x O ₂ (wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (wherein M = Fe, Co, Lithium manganese composite oxide represented by Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; Fe ₂ (MoO ₄ ) ₃ and the like, but are not limited to these.

The binder is a component that assists in bonding the active material and the conductive agent to the current collector, and is usually added in an amount of 5 to 30 wt% based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, Polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers, and the like.

The filler is optionally used as a component for inhibiting expansion of the positive electrode, and is not particularly limited as long as it is a fibrous material without causing chemical change in the battery. Examples of the filler include olefinic polymers such as polyethylene and polypropylene; Fibrous materials, such as glass fiber and carbon fiber, are used.

The positive electrode is prepared by, for example, adding a positive electrode mixture to a solvent such as NMP to prepare a slurry, and then applying the same to a current collector, drying and compressing the same. Since the silver nanoparticles according to the present invention have a very small specific surface area and are not easily dispersed, it is preferable to disperse the binder, the solvent and the silver nanoparticles, and further disperse the active material therein to prepare the slurry.

The current collector for the positive electrode is generally made to a thickness of 3 to 500 ㎛. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The present invention also provides a lithium secondary battery comprising a positive electrode made of the positive electrode mixture.

The lithium secondary battery according to the present invention is composed of a positive electrode, a negative electrode, a separator, and a lithium salt-containing nonaqueous electrolyte prepared by the above method.

The negative electrode is manufactured by applying, drying, and compressing a negative electrode material on a current collector, and, as necessary, may further include components as described above.

The current collector for the cathode is generally made of a thickness of 3 to 500 ㎛. Such a negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, the surface of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver, and the like, aluminum-cadmium alloy, and the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

As said negative electrode material, For example, carbon, such as hardly graphitized carbon and graphite type carbon; Li _x Fe ₂ O ₃ (0 ≦ _x ≦ 1), Li _x WO ₂ (0 ≦ _x ≦ 1), Sn _x Me _1-x Me _y O _z (Me: Mn, Fe, Pb, Ge; Me: Al Metal complex oxides such as B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen, 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , and metal oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte consists of a nonaqueous electrolyte and a lithium salt. As the nonaqueous electrolyte, a nonaqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, aceto Nitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative Aprotic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a good material to be dissolved in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, etc. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, in order to impart nonflammability, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics.

In one preferred embodiment, lithium salts such as LiPF ₆ , LiClO ₄ , LiBF ₄ , LiN (SO ₂ CF ₃ ) _2, and the like, may be prepared by cyclic carbonate of EC or PC as a highly dielectric solvent and DEC, DMC or EMC as a low viscosity solvent. Lithium salt-containing non-aqueous electrolyte can be prepared by adding to a mixed solvent of linear carbonate.

The electrode assembly composed of the positive electrode, the negative electrode, the separator and the like as described above may be manufactured in various forms such as jelly-roll type (wound type) and stack type (lamination type).

Hereinafter, the present invention will be described in more detail based on the following examples, but the scope of the present invention is not limited thereto.

Example 1

After mixing 0.3 wt% of silver particles having a particle size in the range of 5 to 100 nm as a conductive material to LiCoO ₂ as a cathode active material and 3 wt% of PVdF as a binder, the mixture was stirred with NMP as a solvent. It was coated on aluminum foil, which is a metal current collector. This was dried in a vacuum oven at 120 ° C. for 2 hours or more to prepare a positive electrode.

In addition, an electrode assembly was prepared using a cathode and a polypropylene porous separator coated with MCMB artificial graphite on the anode and the copper foil prepared above. After the electrode assembly was placed in a rectangular aluminum can, 1 M LiPF ₆ salt dissolved in a volume ratio of 1: 1 ethylene carbonate (EC) and dimethyl carbonate (DMC) solution was injected into the electrolyte, and then sealed and lithium secondary. The cell was assembled.

Example 2

A positive electrode and a battery were manufactured in the same manner as in Example 1, except that 0.5 wt% of silver particles and 3 wt% of PVdF were mixed.

Example 3

A positive electrode and a battery were manufactured in the same manner as in Example 1, except that 1.0 wt% of silver particles and 3 wt% of PVdF were mixed.

Example 4

A positive electrode and a battery were manufactured in the same manner as in Example 1, except that the silver particles were mixed at 1.5 wt% and the PVdF was mixed at 3 wt%.

Comparative Example 1

A positive electrode and a battery were manufactured in the same manner as in Example 1, except that carbon black was mixed at 1.5 wt% and PVdF was mixed at 3 wt% as the conductive material.

Comparative Example 2

A positive electrode and a battery were manufactured in the same manner as in Example 1, except that 3 wt% of carbon black and 3 wt% of PVdF were mixed as the conductive material.

Experimental Example 1

The electrical conductivities of the cathodes prepared in Examples 1 to 4 and Comparative Examples 1 and 2, respectively, were measured, and charge and discharge of the prepared lithium secondary batteries, respectively, were performed at room temperature, and the results are shown in Table 1 below.

TABLE 1

As shown in Table 1, the battery of Examples 1 to 4 according to the present invention is a battery of Comparative Examples 1 and 2 having a conductive material content of at least 3% by weight, even though the content of the conductive material is 1.5% by weight or less. It can be seen that the charge and discharge characteristics are comparable.

Experimental Example 2

The high-rate discharge characteristics of the batteries prepared in Examples 1 and 3 and Comparative Examples 1 and 2, respectively, were measured using a charge / discharge tester. The charging was carried out under the conditions of 0.2 C and the discharge under the conditions of 0.5 C / 1.0 C. The graph of FIG. 1 shows the discharge behavior as a result of this experiment.

As shown in FIG. 1, it can be seen that the batteries of Examples 1 and 3 are superior to those of Comparative Examples 1 and 2 or rather have higher rate discharge characteristics. In other words, it can be seen that the use of a small amount of conductive material improves the electrical conductivity of the positive electrode, thereby improving the charge and discharge characteristics of the battery.

Experimental Example 3

For the batteries prepared in Examples 1 and 3 and Comparative Examples 1 and 2, respectively, the life characteristics of the batteries were measured using a charge / discharge tester. Charging and discharging were repeatedly performed under charging / discharging conditions of 0.8C / 0.5C.

The graph of FIG. 2 shows discharge cycle characteristics as a result of this experiment. As shown in Figure 2, it can be seen that the battery of Examples 1 and 3 shows the cycle characteristics comparable to the batteries of Comparative Examples 1 and 2. That is, Comparative Example 1 and Example 1 show the same level of high rate discharge characteristics and lifetime characteristics. Thus, it can be seen that even when a small amount of conductive material is used, the electrical conductivity of the positive electrode is improved to obtain a desired level of cycle characteristics.

In addition, as shown in Table 2 below, by reducing the amount of the conductive material used, the amount of the active material may be increased to increase the capacity of the battery. Table 2 below shows the ratio of the active material relative to the battery of Examples 1-3 based on the amount of active material of Example 4 battery.

TABLE 2

Therefore, it can be seen that the battery according to the present invention can be manufactured as a battery having a high capacity compared to the same standard.

As described above, the positive electrode mixture according to the present invention shows a lifespan characteristic and a rate characteristic that are comparable to or better than that of a conventional battery to which a relatively large amount of carbon-based conductive material is added, even when an extremely small amount of the conductive material is added. With the addition of a small amount of conductive material, the capacity of the battery can be increased.

Those skilled in the art to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above contents.

Claims (5)

In the positive electrode mixture for a lithium secondary battery comprising a positive electrode active material, a conductive material and a binder, silver nanoparticles having an average particle diameter of 5 to 100 nm is added as 0.2 to 2.0% by weight based on the total weight of the mixture as the conductive material. Anode mixture. The method of claim 1, wherein the silver nanoparticles are prepared by sintering AgNO ₃ and NaBH ₄ by sol-gel (sol-gel) synthesis method with a high purity nitrogen gas of 500 ~ 700 ℃ to remove impurities. Anode mixture. The cathode mixture of claim 1, wherein the silver nanoparticles are added in an amount of 0.5 wt% to 1.5 wt%. A lithium secondary battery comprising a positive electrode made of the positive electrode mixture according to any one of claims 1 to 4. The lithium secondary battery of claim 4, wherein the cathode is prepared by dispersing a binder, a solvent, and silver nanoparticles, and then dispersing a cathode active material therein and applying a slurry to a current collector.

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