KR101665656B1 - Cathode Material for secondary battery, and Lithium secondary battery manufactured therefrom - Google Patents

Abstract

The present invention relates to a cathode which includes a cathode active material represented by Chemical Formula 1 below, a carbon fiber, a binder, and a conductive material and has a loading level of 1-100 mg/cm^2, wherein the cathode is a cathode for a secondary battery and has a porosity of 30-70 volume%. In addition, the present invention relates to a lithium ion battery which includes the cathode for the secondary battery. [Chemical Formula 1] Li_aNi_1-x-yCo_xMn_yM_bO^2 In Chemical Formula 1, 0.98 <= a <= 1.1, 0.05 <= x <= 0.5, 0.1 <= y <= 0.5, 0 <= b <= 0.05, and M can be any one selected from a transition metal or a lanthanoid selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, Zn, and combinations thereof.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a positive electrode for a secondary battery and a lithium secondary battery manufactured from the same,

The present invention relates to a positive electrode for a secondary battery and a lithium secondary battery produced therefrom.

Recently, interest in energy storage technology is increasing. The application of energy storage technology to mobile phones, camcorders and notebook PCs, and also to electric vehicles and energy storage systems has expanded and become medium and large, and the high energy density of lithium secondary batteries, which can store more electricity (energy) . In order to increase the energy density of the lithium secondary battery, it is necessary to use a high-voltage high-capacity cathode active material, manufacture a cathode having a high loading level, and also to manufacture a secondary battery comprising a high-capacity anode active material, a functional electrolyte material and a separator.

Generally, a lithium secondary battery has a structure in which a lithium electrolyte is impregnated in an electrode assembly composed of a positive electrode containing a positive electrode active material, a negative electrode including a negative electrode active material, and a porous separator. The positive electrode is prepared by coating a positive electrode mixture containing a positive electrode active material on an aluminum foil, and the negative electrode is prepared by coating a copper foil with a negative electrode mixture containing a negative electrode active material.

As a negative electrode active material of such a lithium secondary battery, a carbon material is mainly used, and use of lithium metal, a sulfur compound, a silicon compound, a tin compound and the like is also considered. In addition, lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as the positive electrode active material, and a lithium-containing manganese oxide such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure and a lithium- ₂ ) is also considered.

LiCoO ₂ has excellent properties such as excellent cycle characteristics and is widely used at present. However, LiCoO ₂ is low in safety, is expensive due to the resource limit of cobalt as a raw material, and has a limitation in mass use as a power source for fields such as electric vehicles. LiNiO ₂ is difficult to apply to actual mass production process at a reasonable cost due to its characteristics depending on the production method thereof, and lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have poor cycle characteristics.

Japanese Patent Publication No. 4124694 (May 16, 2008) is proposed as a similar prior art document.

Japanese Patent Publication No. 4124694 (May 16, 2008)

In order to solve the above problems, it is an object of the present invention to provide a positive electrode for a secondary battery having excellent high rate characteristics and a cycle life, and a lithium secondary battery produced therefrom.

In order to accomplish the above object, the present invention provides a positive electrode comprising a positive electrode active material satisfying the following formula (1), carbon fiber, a binder and a conductive material, wherein the positive electrode has a porosity of 30 to 75% by volume .

[Chemical Formula 1]

Li _a Ni _1-xy Co _x Mn _y M _b O ₂

Wherein M is at least one element selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, Zn, and combinations thereof, or a transition metal selected from the group consisting of combinations thereof.

Still another aspect of the present invention relates to a lithium secondary battery including the positive electrode for the secondary battery.

The positive electrode for a secondary battery according to the present invention can have excellent high rate characteristics and cycle life by producing a positive electrode having a high porosity by adding carbon fibers to a specific positive electrode active material.

A positive electrode for a secondary battery according to an exemplary embodiment of the present invention can have excellent high rate characteristics and cycle life by adding carbon fibers to a specific positive electrode active material and producing the positive electrode with a high porosity.

More specifically, a cathode for a secondary battery according to an exemplary embodiment of the present invention is a cathode made of a cathode active material, a carbon fiber, a binder, and a conductive material satisfying the following Chemical Formula 1, and has a porosity of 30 to 75% And may be an anode for a secondary battery.

[Chemical Formula 1]

Li _a Ni _1-xy Co _x Mn _y M _b O ₂

In the formula 1, 0.98? A? 1.1, 0.05? X? 0.5, 0.1? Y? 0.5, 0? B? 0.05, A transition metal selected from the group consisting of combinations thereof, and a lanthanide group. More preferably, a = 1, 0.1? X? 0.3, and 0.15? Y? 0.25. By satisfying this range, the crystal structure of the cathode active material can be more stabilized and more excellent high-rate characteristics can be obtained.

The positive electrode active material satisfying the formula (1) can increase the content of manganese (Mn) to 0.1 or more (y value), so that the crystal structure of the positive electrode active material can be more stably maintained. On the other hand, when manganese is less than 0.1 (y value), since the crystal structure of the positive electrode active material is unstable and the porosity is high, the positive electrode active material tends to swell during charging and discharging, And the cycle life may be low.

Thus, by adding carbon fibers to the cathode active material satisfying the formula (1) and subjecting the carbon fibers to a compression process so as to have a high porosity (apparent porosity), excellent high-rate characteristics and cycle life can be obtained. On the other hand, in the case of using a cathode active material which does not satisfy the formula (1), but has a very low content of manganese, the cathode active material is liable to be swollen due to a high porosity, Performance and cycle life can be lowered.

Briefly, a cathode for a secondary battery according to an exemplary embodiment of the present invention is manufactured by adding a carbon fiber, a binder, a conductive material, or the like to a specific cathode active material satisfying Formula 1, Can be prepared. In the case of a positive electrode prepared so as to have a high loading level, if the porosity is low, the wettability with the electrolyte decreases, diffusion of lithium ions is limited, and charging / discharging performance may be deteriorated. In addition, when the carbon fiber is not added although having a high loading level and a high porosity, the connectivity and / or electrical conductivity between the cathode active material particles may be lowered. That is, by adding a carbon fiber to a specific cathode active material, and by using the carbon fiber to produce a positive electrode having a high loading level and a high porosity, it is possible to further improve the wetting property of the electrolytic solution, .

Specifically, the positive electrode made of the positive electrode active material satisfying the formula (1) has a high loading level of 100 mg / cm ² or less and a high porosity of 30 to 75 vol%, and the carbon fibers uniformly mixed and dispersed with the positive electrode active material By serving as a conductor, it is possible to provide a uniform current distribution from the surface of the anode to the bulk, maintain the electrochemical activity of the cathode active material, and improve the high-performance and cycle life. More specifically, the anode for a secondary battery according to an exemplary embodiment of the present invention may have a loading level of 0.1 to 100 mg / cm 2, more preferably 2 to 50 mg / cm 2, and most preferably 7 to 25 mg / cm 2 Lt; / RTI &gt;

The carbon fiber according to an exemplary embodiment of the present invention has a high porosity to improve the wetting property of the electrolytic solution. Thus, it is possible to provide a sufficient transport path of the positive charge carrier as the charge carrier during charging and discharging, Thereby increasing the inter-particle connectivity and electrical conductivity. The carbon fiber for use herein is not particularly limited as long as it can be used together with the cathode active material satisfying the above formula (1) and exhibits a porosity of 30 to 75% by volume. For example, one or more selected from carbon nanofibers, carbon micro fibers, carbon nanowires, and carbon nanotubes can be used. The average diameter of the carbon fibers may be from 1 nm to 20 占 퐉, more preferably from 10 nm to 1 占 퐉, and most preferably from 50 to 500 nm. And can be effectively dispersed within the above range to have a high porosity. The content of the carbon fiber may be 0.1 to 10 parts by weight, more preferably 0.5 to 8 parts by weight, and most preferably 1 to 5 parts by weight based on 100 parts by weight of the cathode active material. Within the above range, the connectivity and electrical conductivity of the cathode active material particles can be improved, and a high porosity of 30 to 75% by volume can be obtained. In addition, the carbon fiber according to an example may be one whose surface has been subjected to oxidation treatment, and the surface-treated carbon fiber can inhibit entanglement of the carbon fiber to a great extent. The oxidation treatment can be carried out by heat treatment in air or immersion in an oxidizing solution, and the specific method can be carried out using a conventional method.

The binder according to an embodiment of the present invention plays a role of attaching the positive electrode active material particles to each other well and attaching the positive electrode active material to the current collector well and is not particularly limited as long as it is commonly used in the art, For example, polyvinylidene fluoride (PVdF), hexafluoro propylene (HFP), polyvinylidene fluoride-co-hexafluoro propylene, polyvinylidene fluoride But are not limited to, polyvinylidene fluoride-trichlorethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate polyvinylacetate, polyethylene-co-vinyl cellulose acetate propionate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, pullulan, cyanoethylsucrose, carboxyl methyl cellulose, acrylonitrile styrene butadiene, Acrylonitrile-styrene-butadiene copolymer, polyimide, and the like, but is not limited thereto. The content of the binder may be 0.1 to 30 parts by weight, more preferably 1 to 20 parts by weight, and most preferably 5 to 15 parts by weight based on 100 parts by weight of the cathode active material. Within the above range, the positive electrode active material particles can be adhered well to each other, and the porosity is not lowered, so that excellent high-rate characteristics can be obtained.

The conductive material according to an exemplary embodiment of the present invention may be used as an electron conductive material that does not cause a chemical change in an electrochemical material, and may be used without any particular limitation. Specifically, for example, graphite such as natural graphite or artificial graphite; Carbon black such as Super-P carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Metal powders such as carbon fluoride, aluminum, nickel, and stainless steel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; And a conductive material such as a polyphenylene derivative, or the like may be used. The conductive material may be contained in an amount of 0.1 to 20 parts by weight, more preferably 1 to 10 parts by weight, and most preferably 3 to 8 parts by weight based on 100 parts by weight of the cathode active material. The electron can be effectively transported together with the carbon fibers within the above range, and the high porosity is not lowered, so that the high-rate characteristics can be obtained.

The positive electrode for a secondary battery according to an exemplary embodiment of the present invention may be manufactured by an electrode manufacturing process that is manufactured by mixing the positive electrode active material, the binder, the conductive material, and the carbon fiber in a solvent to prepare a slurry, have. Since the electrode manufacturing process is well known in the art, a detailed description thereof will be omitted herein.

However, the positive electrode for a secondary battery according to an exemplary embodiment of the present invention may be manufactured by a manufacturing method described later, rather than a typical manufacturing method, and the high-rate characteristics of the positive electrode for a secondary battery can be further improved through such a method.

Specifically, a cathode for a secondary battery according to an embodiment of the present invention includes: a) preparing a first slurry by mixing a cathode active material, a first binder, a first conductive material, and a first carbon fiber in a first solvent; b) dispersing the second slurry in a non-solvent to produce and separate composite particles; c) dissolving the second binder in a second solvent to prepare a saturated solution; and d) mixing the prepared composite particles, the second conductive material and the second carbon fiber with the saturated solution to prepare a second slurry.

Through the above-described method, composite particles having a high porosity can be prepared first, a second slurry can be prepared again using the prepared composite particles, and then coated on a current collector to produce a positive electrode for a secondary battery, The positive electrode can be manufactured, and more excellent high-rate characteristics can be obtained. However, although the above method can produce a cathode having an improved high-rate characteristic, the method may have a disadvantage in that the process proceeds in several steps compared with the conventional method.

In the manufacturing method according to an example, the first binder and the second binder, the first conductive material and the second conductive material, and the first carbon fiber and the second carbon fiber may be the same or different. The first solvent and the second solvent are not particularly limited and can be used as long as they are conventionally used in the art to produce a slurry. Examples of the solvent include dimethyl acetamide (N, N-dimethylacetamide, DMAc), dimethylformamide Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), triethyl phosphate (TEP), acetone, methyl ethyl Methylethyl ketone, and tetrahydrofuran (THF). The first solvent and the second solvent may be the same or different. In the case of the second solvent, if the step of applying the second slurry on the current collector is not smooth, it is possible to add a small amount to the extent that application is possible. The non-solvent may be any solvent that does not dissolve the binder. For example, water, hexane, pentane, methanol, ethanol, or the like may be used. However, in order to smoothly disperse the first slurry, it is preferable to use an excess amount of the non-solvent in an amount of 5 times or more, preferably 10 times or more, the volume of the slurry. The dispersion method is not particularly limited, but may be carried out by spraying the first slurry using a spraying method or the like.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a cathode for a secondary battery and a lithium secondary battery manufactured using the same according to the present invention will be described in more detail with reference to the following examples. It should be understood, however, that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention.

Unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

Also, the singular forms as used in the specification and the appended claims are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[Example 1]

Cathode active material: Binder: Conductive material: Carbon fiber was added at a weight ratio of 84: 8: 5: 3 to prepare a positive electrode for a secondary battery.

Specifically, 95.2 mg of polyvinylidene fluoride having a weight average molecular weight of 534,000 g / mol, 59.5 mg of carbon black, and 35.7 mg of carbon nanofibers were dissolved in about 4 ml of methylpyrrolidone (NMP), 1 g of LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , And then uniformly mixed to prepare a slurry. The prepared slurry was coated on an aluminum foil as a current collector, followed by drying and rolling to prepare a cathode for a secondary battery having a loading level of 11 mg / cm 2 and a porosity of about 50% by volume.

Ethylene carbonate (EC): ethyl methyl carbonyl marinade (EMC) 3: was LiPF ₆ / EC dissolving LiPF ₆ in a mixed solvent in a volume ratio of 7 with 1M: an EMC electrolytic solution and a high voltage 0.1㎖ additives methyl (2, 2 (2-trifluoroethyl) carbonate was added in an amount of 10% by weight based on 100% by weight of the electrolytic solution to prepare a mixed electrolytic solution. (LiPF ₆ / EC: 100% (2,2,2-trifluoroethyl) carbonate 10% by weight)

Subsequently, a coin-lithium secondary battery was fabricated using the positive electrode, the lithium metal negative electrode, and the liquid electrolyte prepared above.

[Comparative Example 1]

Except that carbon fiber was not added and the cathode active material: binder: conductive material was added in a weight ratio of 84: 8: 8, a loading level of 10 mg / cm 2 and a porosity of about 50 volume % And a coin-lithium secondary battery were fabricated.

The coin-lithium battery prepared in Example 1 and Comparative Example 1 was charged at 0.2 C in a voltage range of 3.0-4.6 V and discharged at three cycles at C-rates of 0.2 C, 0.5 C, 1 C, 2 C and 5 C The results are shown in Table 1.

cycle Discharge capacity (%) 0.2C 0.5 C 1C 2C 5C Example 1 1st 100 95 90 84 68 2nd 101 94 90 83 66 3rd 99 94 89 82 65 Comparative Example 1 1st 100 98 94 86 22 2nd 103 98 94 81 17 3rd 102 98 93 76 15

As shown in Table 1, the coin lithium secondary battery of Example 1 and Comparative Example 1 exhibited similar discharge capacities at low rates of 0.2C, 0.5C, and 1C, while at high rate discharges of 2C and 5C, It can be seen that the discharge capacity is higher than the discharge capacity of Comparative Example 1 and the capacity retention rate at the same C-rate is significantly improved. Particularly, the discharge capacity at 5C in Example 1 was 68%, and it was found that Example 1 having a specific porosity and specific cathode active material and carbon fiber showed a remarkably improved high rate performance than that of Comparative Example 1 at 22% have.

Claims (7)

A positive electrode comprising a positive electrode active material satisfying the following formula 1, a carbon fiber, a binder and a conductive material, wherein the positive electrode has a porosity of 30 to 75% by volume and a loading level of 0.1 to 100 mg /
[Chemical Formula 1]
Li _a Ni _1-xy Co _x Mn _y M _b O ₂
Wherein M is at least one element selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, Zn, A transition metal selected from the group consisting of combinations thereof, and a lanthanide group.
The method according to claim 1,
Wherein the carbon fiber is one or more selected from carbon nanofibers, carbon micro fibers, carbon nanowires, and carbon nanotubes.
The method according to claim 1,
Wherein the carbon fiber is contained in an amount of 0.1 to 20 parts by weight based on 100 parts by weight of the positive electrode active material.
The method according to claim 1,
Wherein the average diameter of the carbon fibers is 1 nm to 20 占 퐉.
The method according to claim 1,
Wherein the surface of the carbon fiber is oxidized.
delete A lithium secondary battery comprising a positive electrode for a secondary battery according to any one of claims 1 to 5.