阅读说明：本技术 二次电池用正极活性材料、其制备方法和包含其的锂二次电池 (Positive electrode active material for secondary battery, method of preparing the same, and lithium secondary battery comprising the same ) 是由 蔡和锡 朴商敃 朴信英 朴洪圭 姜成勋 于 2019-02-21 设计创作，主要内容包括：本发明涉及一种制备二次电池用正极活性材料的方法和由此制备的正极活性材料,所述方法包括以下步骤：准备含有镍(Ni)、钴(Co)以及选自锰(Mn)和铝(Al)中的至少一种的锂复合过渡金属氧化物；洗涤所述锂复合过渡金属氧化物以除去所述锂复合过渡金属氧化物的表面上存在的锂副产物；以及将所述经洗涤的锂复合过渡金属氧化物、含钴(Co)原料和含硼(B)原料混合,继而在600℃以上的温度下进行高温热处理。(The present invention relates to a method of preparing a positive electrode active material for a secondary battery, the method comprising the steps of: preparing a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and at least one selected from manganese (Mn) and aluminum (Al); washing the lithium composite transition metal oxide to remove lithium by-products present on the surface of the lithium composite transition metal oxide; and mixing the washed lithium composite transition metal oxide, a cobalt (Co) -containing raw material and a boron (B) -containing raw material, followed by high-temperature heat treatment at a temperature of 600 ℃ or higher.)

1. A method of preparing a positive electrode active material for a secondary battery, the method comprising the steps of:

preparing a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co), and containing at least one selected from manganese (Mn) and aluminum (Al);

removing lithium by-products present on the surface of the lithium composite transition metal oxide by washing the lithium composite transition metal oxide with water; and

mixing the washed lithium composite transition metal oxide, cobalt (Co) -containing raw material and boron (B) -containing raw material, and performing high-temperature heat treatment at a temperature of 600 ℃ or higher.

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the lithium composite transition metal oxide has a nickel (Ni) content of 60 mol% or more with respect to the total transition metal content.

3. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the boron (B) -containing feedstock comprises B₄C。

4. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the high temperature heat treatment is performed at a temperature of 600 ℃ to 900 ℃ in an oxidizing atmosphere.

5. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the cobalt (Co) -containing raw material is mixed in an amount of 0.001 parts by weight to 0.01 parts by weight with respect to 100 parts by weight of the lithium composite transition metal oxide.

6. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the boron (B) -containing raw material is mixed in an amount of 0.0001 to 0.001 parts by weight relative to 100 parts by weight of the lithium composite transition metal oxide.

7. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the lithium composite transition metal oxide is represented by the following formula 1:

[ formula 1]

Li_pNi_1-(x1+y1+z1)Co_x1M^a _y1M^b _z1M^c _q1O_2-aA_a

In the formula, M^aIs at least one selected from Mn and Al,

M^bis at least one selected from Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Ge, V, Si, Nb, Mo and Cr,

M^cis at least one selected from Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr,

a is at least one selected from P and F,

p is more than or equal to 0.9 and less than or equal to 1.05, x1 is more than 0 and less than or equal to 0.3, y1 is more than 0 and less than or equal to 0.2, z1 is more than or equal to 0 and less than or equal to 0.1, q1 is more than or equal to 0 and less than or equal to 0.1, a is more than or equal to 0 and less than or equal to 1, and x1+ y1+ z1 is more than or.

8. A positive electrode active material for a secondary battery, comprising:

a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co), and containing at least one selected from manganese (Mn) and aluminum (Al); and

a surface coating part formed on the particle surface of the lithium composite transition metal oxide,

wherein the surface coating portion includes a cobalt-rich layer having a higher cobalt content than the lithium composite transition metal oxide and a lithium boron oxide.

9. The positive electrode active material according to claim 8,

wherein the lithium composite transition metal oxide has a nickel (Ni) content of 60 mol% or more with respect to the total transition metal content.

10. The positive electrode active material according to claim 8,

wherein a difference between a ratio of the number of cobalt (Co) atoms to the total number of atoms of nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al) in the cobalt-rich layer and a ratio of the number of cobalt (Co) atoms to the total number of atoms of nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al) in the lithium composite transition metal oxide is 0.05 to 0.2.

11. The positive electrode active material according to claim 8,

wherein a content of boron (B) contained in the lithium boron oxide is 100ppm to 1,000ppm with respect to a total weight of the positive electrode active material.

12. The positive electrode active material according to claim 8,

wherein the surface coating has a thickness of 10nm to 100 nm.

13. The positive electrode active material according to claim 8,

wherein the content of the lithium by-product is 0.55 wt% or less with respect to the total weight of the positive electrode active material.

14. The positive electrode active material according to claim 8,

wherein the lithium composite transition metal oxide is represented by the following formula 1:

[ formula 1]

Li_pNi_1-(x1+y1+z1)Co_x1M^a _y1M^b _z1M^c _q1O_2-aA_a

In the above formula, M^aIs at least one selected from Mn and Al,

M^bis at least one selected from Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Ge, V, Si, Nb, Mo and Cr,

M^cis at least one selected from Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr,

a is at least one selected from P and F,

p is more than or equal to 0.9 and less than or equal to 1.05, x1 is more than 0 and less than or equal to 0.3, y1 is more than 0 and less than or equal to 0.2, z1 is more than or equal to 0 and less than or equal to 0.1, q1 is more than or equal to 0 and less than or equal to 0.1, a is more than or equal to 0 and less than or equal to 1, and x1+ y1+ z1 is more than or.

15. A positive electrode for a secondary battery, comprising the positive electrode active material according to any one of claims 8 to 14.

16. A lithium secondary battery comprising the positive electrode according to claim 15.

Technical Field

Cross reference to related applications

This application claims the benefit of korean patent application No. 10-2018-0024858, filed on 28.2.2018 from the korean intellectual property office, the disclosure of which is incorporated herein by reference in its entirety.

Background

In recent years, with the rapid spread of electronic devices using batteries, such as mobile phones, notebook computers, and electric vehicles, the demand for secondary batteries that are small in size, light in weight, and relatively high in capacity has rapidly increased. In particular, the lithium secondary battery is lightweight and has high energy density, so that it has been spotlighted as a driving power source for portable devices. Therefore, research and development efforts for improving the performance of the lithium secondary battery have been actively made.

In a lithium secondary battery in which an organic electrolyte or a polymer electrolyte is filled between a positive electrode and a negative electrode respectively composed of active materials capable of intercalating and deintercalating lithium ions, electric energy is generated through oxidation and reduction reactions when lithium ions are intercalated and deintercalated in the positive electrode and the negative electrode.

Lithium cobalt oxide (LiCoO)₂) Lithium nickel oxide (LiNiO)₂) Lithium manganese oxide (LiMnO)₂、LiMn₂O₄Etc.), lithium iron phosphate compounds (LiFePO)₄) Etc. have been used as positive electrode active materials for lithium secondary batteries. Further, LiNiO is maintained as a solution₂Has been developed, in which a part of nickel (Ni) is replaced with cobalt (Co) and manganese (Mn)/aluminum (Al) (hereinafter, simply referred to as "NCM-based lithium composite transition metal oxide" or "NCA-based lithium composite transition metal oxide"). However, the conventionally developed NCM-based/NCA-based lithium composite transition metal oxide has limitations in application due to insufficient capacity characteristics.

In order to improve such limitations, studies have been recently conducted on increasing the content of Ni in the NCM-based/NCA-based lithium composite transition metal oxide. However, in the case of a high-concentration nickel positive electrode active material having a high nickel content, there are the following problems: the structural stability and chemical stability of the active material are deteriorated, and the thermal stability is rapidly deteriorated. In addition, as the nickel content in the active material increases, LiOH and Li are present on the surface of the positive electrode active material₂CO₃The residual amount of the lithium by-product present in the form of (a) increases, and thus, gas generation and swelling phenomena are caused, thereby causing a problem of deterioration in the life and stability of the battery.

Therefore, it is required to develop a positive electrode active material rich in nickel at a high concentration, which is suitable for high capacity, and also has a small residual amount of lithium by-product and excellent high temperature stability.

Disclosure of Invention

Technical problem

In order to overcome the above problems, an aspect of the present invention provides: a high nickel positive electrode active material in which the amount of residual lithium by-product is small, and at the same time, structural stability, excellent capacity characteristics and high temperature stability are achieved; a process for the preparation thereof; and a positive electrode for a secondary battery and a lithium secondary battery comprising the same.

Another aspect of the present invention also provides a method of preparing a positive electrode active material, which can simplify a coating process performed to overcome the thermal stability problem of a high nickel positive electrode active material and reduce production time and process costs.

Technical scheme

According to an aspect of the present invention, there is provided a method of preparing a positive electrode active material for a secondary battery, the method including the steps of: preparing a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co), and containing at least one selected from manganese (Mn) and aluminum (Al); removing lithium by-products present on the surface of the lithium composite transition metal oxide by washing the lithium composite transition metal oxide with water; and mixing the washed lithium composite transition metal oxide, cobalt (Co) -containing raw material and boron (B) -containing raw material and performing high-temperature heat treatment at a temperature of 600 ℃ or higher.

According to another aspect of the present invention, there is provided a positive electrode active material for a secondary battery, including: a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co), and containing at least one selected from manganese (Mn) and aluminum (Al); and a surface coating portion formed on a surface of a particle of the lithium composite transition metal oxide, wherein the surface coating portion includes a cobalt-rich layer having a higher cobalt content than the lithium composite transition metal oxide and a lithium boron oxide.

According to another aspect of the present invention, there is provided a positive electrode and a lithium secondary battery each including the positive electrode active material.

Advantageous effects

According to the present invention, it is possible to provide a cathode active material in which deterioration of structural/chemical stability caused by an increase in nickel (Ni) in a high-nickel cathode active material is improved, and high capacity and excellent thermal stability are achieved. In addition, the residual amount of lithium by-product of the high nickel positive electrode active material is reduced, and high temperature life characteristics and output characteristics are improved.

Further, according to the present invention, the surface-coated portions are simultaneously formed in the high-temperature heat treatment step after the water washing, thereby simplifying the process while overcoming the high-temperature stability problem and reducing the production time and process cost.

Drawings

Fig. 1 is a graph showing heat flows according to temperatures measured by using a differential scanning calorimeter (Sensys evo DSC of SETARAM instruments) of the positive electrode active materials in examples 1 and 2 and comparative examples 1 to 3;

fig. 2 is a graph showing capacity retention rates according to charge-discharge cycles of battery cells manufactured by using the positive electrode active materials in examples 1 and 2 and comparative examples 1 to 3; and

fig. 3 is a graph showing the resistance increase rate according to charge-discharge cycles of battery cells manufactured by using the positive electrode active materials in examples 1 and 2 and comparative examples 1 to 3.

Detailed Description

Hereinafter, the present invention will be described in more detail to enable the present invention to be more clearly understood. In this case, it should be understood that the words or terms used in the specification and claims should not be construed as meanings defined in a general dictionary, and it will be further understood that the words or terms should be construed as having meanings consistent with their meanings in the context of the related art and technical idea of the present invention on the basis of the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention.

<Method for preparing positive electrode active material>

The present invention provides a method for preparing a positive active material for a secondary battery, the method comprising the steps of: preparing a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co), and containing at least one selected from manganese (Mn) and aluminum (Al); removing lithium by-products present on the surface of the lithium composite transition metal oxide by washing the lithium composite transition metal oxide with water; and mixing the washed lithium composite transition metal oxide, cobalt (Co) -containing raw material and boron (B) -containing raw material and performing high-temperature heat treatment at a temperature of 600 ℃ or higher. Hereinafter, each step of the present invention will be described in more detail.

First, a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co) and containing at least one selected from manganese (Mn) and aluminum (Al) is prepared.

The lithium composite transition metal oxide may be a high nickel NCM-based/NCA-based lithium composite transition metal oxide having a nickel (Ni) content of 60 mol% or more with respect to the total transition metal content. More preferably, the content of nickel (Ni) may be 70 mol% or more, and still more preferably, the content of nickel (Ni) may be 80 mol% or more, with respect to the total transition metal content. In the lithium composite transition metal oxide, the content of nickel (Ni) satisfies 60 mol% or more with respect to the total transition metal content, so that a high capacity can be secured.

More specifically, the lithium composite transition metal oxide may be represented by the following formula 1:

[ formula 1]

Li_pNi_1-(x1+y1+z1)Co_x1M^a _y1M^b _z1M^c _q1O_2-aA_a

In the above formula, M^aIs at least one selected from Mn and Al, M^bIs at least one selected from Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Ge, V, Si, Nb, Mo and Cr, M^cIs at least one selected from Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, A is at least one selected from P and F, P is more than or equal to 0.9 and less than or equal to 1.05, 0<x1≤0.3，0<y1≤0.2，0≤z1≤0.1，0≤q1≤0.1，0≤a<1, and 0<x1+y1+z1≤0.4。

In the lithium composite transition metal oxide of formula 1, Li may be contained in an amount corresponding to p, i.e., in an amount of 0.9. ltoreq. p.ltoreq.1.05. When p is less than 0.9, there is a possibility that the capacity is reduced, and when p exceeds 1.05, the particles are sintered in the firing process, so that it may be difficult to prepare the cathode active material. In view of the significant improvement effect of the capacity characteristics of the positive electrode active material due to the control of the Li content and the balance of the sintering property in the preparation of the active material, it may be more preferable to contain Li in an amount of 1.0. ltoreq. p.ltoreq.1.05.

In the lithium composite transition metal oxide of formula 1, Ni may be contained in an amount corresponding to 1- (x1+ y1+ z1), for example, in an amount of 0.6. ltoreq.1- (x1+ y1+ z1) < 1. When the content of Ni in the lithium composite transition metal oxide of formula 1 is 0.6 or more, a sufficient amount of Ni that can contribute to charge and discharge is secured, thereby realizing a high capacity. More preferably, Ni may be contained in an amount of 0.80. ltoreq.1- (x1+ y1+ z 1). ltoreq.0.99.

In the lithium composite transition metal oxide of formula 1, Co may be contained in an amount corresponding to x1, i.e., 0< x1 ≦ 0.3. When the content of Co in the lithium composite transition metal oxide of formula 1 exceeds 0.3, there is a possibility of an increase in cost. Co may be more specifically contained in an amount of 0.05. ltoreq. x 1. ltoreq.0.2 in consideration of a significant improvement effect of capacity characteristics due to the inclusion of Co.

In the lithium composite transition metal oxide of formula 1, M^aMay be Mn or Al, or may be Mn and Al, and such a metal element may improve the stability of the active material, and as a result, the stability of the battery may be improved. In view of the improvement effect of the life characteristics, an amount corresponding to y1, i.e., 0, may be contained<M in an amount of y1 ≤ 0.2^a. When y1 in the lithium composite transition metal oxide of formula 1 exceeds 0.2, the output characteristics and capacity characteristics of the battery may rather deteriorate, and M may be contained more specifically in an amount of 0.05. ltoreq. y 1. ltoreq.0.2^a。

In the lithium composite transition metal oxide of formula 1, M^bMay be a doping element contained in the crystal structure of the lithium composite transition metal oxide, and may contain an amount corresponding to z1, i.e., 0. ltoreq. z 1. ltoreq.0.1M of (A)^b。

In the lithium composite transition metal oxide of formula 1, the metal element M may not be contained in the structure of the lithium composite transition metal oxide^cHaving M doped on its surface^cThe lithium composite transition metal oxide of (1) may be prepared by a method in which M may be mixed with a lithium source and fired^cThe sources may also be mixed together and fired, or M may be added separately and fired after formation of the lithium composite transition metal oxide^cA source. May contain an amount of M corresponding to q1^cThat is, M may be contained in an amount not deteriorating the characteristics of the positive electrode active material in the range of 0. ltoreq. q 1. ltoreq.0.1^c。

In the lithium composite transition metal oxide of formula 1, the element A is an element that substitutes for a part of oxygen, and may be P and/or F, and the element A may substitute for oxygen in an amount corresponding to a, that is, in an amount of 0. ltoreq. a < 1.

The lithium composite transition metal oxide used in the present invention may be, for example, an NCM-based lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), or an NCA-based lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and aluminum (Al). Alternatively, the positive active material may be a four-component lithium composite transition metal oxide, which must include four components of nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al). In the case of the four-component positive electrode active material, stability can be improved and the life can be improved without deteriorating output characteristics and capacity characteristics, as compared to the NCM-based/NCA-based positive electrode active material.

The lithium composite transition metal oxide represented by formula 1 may be prepared, for example, by a method in which a lithium composite transition metal oxide precursor containing nickel (Ni) and cobalt (Co) and containing at least one selected from manganese (Mn) and aluminum (Al) is mixed with a lithium-containing raw material, and then the mixture is fired at 600 to 900 ℃, but the method is not limited thereto.

The positive active material precursor may be an NCM-based compound containing nickel (Ni), cobalt (Co), and manganese (Mn), or may be an NCA-based compound containing nickel (Ni), cobalt (Co), and aluminum (Al),or may be a four-component positive active material precursor that must contain four components of nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al). Alternatively, it may contain M in addition to nickel (Ni), cobalt (Co), manganese (Mn) and/or aluminum (Al)^bThe positive electrode active material precursor of (1). The cathode active material precursor may use a commercially available cathode active material precursor, or may be prepared according to a method for preparing a cathode active material precursor well known in the art.

For example, a nickel-cobalt-manganese precursor can be prepared by: a complex-forming agent containing an ammonium cation and a basic compound are added to a transition metal solution containing a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material, and then a coprecipitation reaction is performed.

The nickel-containing feedstock may be, for example, nickel-containing acetates, nitrates, sulfates, halides, sulfides, hydroxides, oxides or oxyhydroxides, and may in particular be Ni (OH)₂、NiO、NiOOH、NiCO₃·2Ni(OH)₂·4H₂O、NiC₂O₂·2H₂O、Ni(NO₃)₂·6H₂O、NiSO₄、NiSO₄·6H₂O, a fatty acid nickel salt, a nickel halide, or a combination thereof, but the embodiment is not limited thereto.

The cobalt-containing feedstock may be a cobalt-containing acetate, nitrate, sulphate, halide, sulphide, hydroxide, oxide or oxyhydroxide, and may in particular be Co (OH)₂、CoOOH、Co(OCOCH₃)₂·4H₂O、Co(NO₃)₂·6H₂O、CoSO₄、Co(SO₄)₂·7H₂O, or a combination thereof, but the embodiment is not limited thereto.

The manganese-containing feedstock can be, for example, a manganese-containing acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide, or combination thereof, and can specifically be a manganese oxide, such as Mn₂O₃、MnO₂Or Mn₃O₄(ii) a Manganese salts, e.g. MnCO₃、Mn(NO₃)₂、MnSO₄B, BManganese acid, manganese dicarboxylate, manganese citrate, and manganese fatty acid salts; manganese oxyhydroxide; manganese chloride; or a combination thereof, but the embodiment is not limited thereto.

The transition metal solution may be prepared by adding a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material to a solvent, specifically, for example, water or a mixed solvent of water and an organic solvent (for example, alcohol, etc.) that can be uniformly mixed with water; or may be prepared by mixing an aqueous solution of a nickel-containing feedstock, an aqueous solution of a cobalt-containing feedstock, and a manganese-containing feedstock.

The complex-forming agent containing an ammonium cation may be, for example, NH₄OH、(NH₄)₂SO₄、NH₄NO₃、NH₄Cl、CH₃COONH₄、(NH₄)₂CO₃Or a combination thereof, but the embodiment is not limited thereto. On the other hand, the complex-forming agent containing an ammonium cation may be used in the form of an aqueous solution, and as the solvent, water or a mixture of water and an organic solvent (specifically, alcohol or the like) that can be uniformly mixed with water may be used.

The basic compound may be an alkali or alkaline earth metal hydroxide, such as NaOH, KOH or Ca (OH)₂A hydrate thereof, or a combination thereof. The basic compound may also be used in the form of an aqueous solution, and as the solvent, water or a mixture of water and an organic solvent (specifically, alcohol or the like) which can be uniformly mixed with water may be used.

Meanwhile, although not necessary, the basic compound in which the anionic compound containing the element a (i.e., P and/or F) is dissolved may be used if desired. In this case, the element a derived from the anionic compound is partially substituted at the oxygen site of the precursor, and therefore, the effect of suppressing oxygen desorption and reaction with the electrolyte during charge and discharge of the secondary battery can be obtained.

The basic compound is added to adjust the pH of the reaction solution, and may be added in an amount such that the pH of the metal solution becomes 11 to 13.

On the other hand, the coprecipitation reaction may be performed at a temperature of 40 to 70 ℃ in an inactive atmosphere such as a nitrogen or argon atmosphere.

Through the above-described procedure, particles of nickel-cobalt-manganese hydroxide are formed and precipitated in the reaction solution. The precipitated nickel cobalt manganese hydroxide particles may be separated and dried by conventional methods to obtain a nickel cobalt manganese precursor.

The positive active material precursor prepared by the above-described method may be mixed with a lithium-containing raw material, or the positive active material precursor, the lithium-containing raw material, and the M-containing compound may be mixed^cThe raw materials are mixed and then fired at 600 to 900 c, preferably 600 to 800 c, to obtain a lithium composite transition metal oxide.

Said group containing M^cThe starting material may be a mixture containing the element M^cAnd when M is an acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, oxyhydroxide, or combination thereof, and^cwhen is Al, said compound contains M^cThe starting material may be, for example, Al₂O₃、Al₂(SO₄)₃、AlCl₃Aluminum isopropoxide, Al (NO)₃)₃Or a combination thereof, but the embodiment is not limited thereto.

The lithium-containing raw material may be a sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide or oxyhydroxide containing lithium, and is not particularly limited as long as it is dissolved in water. The lithium source may in particular be Li₂CO₃、LiNO₃、LiNO₂、LiOH、LiOH·H₂O、LiH、LiF、LiCl、LiBr、LiI、CH₃COOLi、Li₂O、Li₂SO₄、CH₃COOLi or Li₃C₆H₅O₇And any one or a mixture of two or more thereof may be used.

Further, although not necessary, a raw material containing a may be further mixed during firing to substitute a part of oxygen in the lithium composite transition metal oxide with the element a to dope. In this case, the A-containing raw material may be, for example, Na₃PO₄、K₃PO₄、Mg₃(PO₄)₂、AlF₃、NH₄F or LiF, but the embodiment is not limited thereto. When a part of oxygen is replaced by the element a as described above, the effect of suppressing oxygen desorption and reaction with the electrolyte during charge and discharge of the secondary battery can be obtained.

Next, lithium by-products present on the surface of the lithium composite transition metal oxide are removed by washing the lithium composite transition metal oxide with water.

Since the lithium composite transition metal oxide containing nickel at a high concentration is structurally unstable compared to the lithium composite transition metal oxide containing nickel at a low concentration, lithium by-products such as unreacted lithium hydroxide and lithium carbonate are more generated in the manufacturing process. For example, when the lithium composite transition metal oxide has a nickel fraction of less than 80 mol%, the amount of lithium by-products after synthesis is about 0.5 wt% to about 0.6 wt%, and when the lithium composite transition metal oxide has a nickel fraction of 80 mol% or more, the amount of lithium by-products after synthesis is up to about 1 wt%. On the other hand, when a large amount of lithium by-product exists in the positive electrode active material, the lithium by-product and the electrolyte react with each other to generate gas and swell, thereby significantly deteriorating high-temperature stability. Therefore, a water washing step for removing lithium by-products from the lithium composite transition metal oxide containing nickel at a high concentration is essentially required.

The water washing step may be performed, for example, by adding the lithium composite transition metal oxide to ultrapure water and then stirring the mixture. At this time, the washing temperature may be 20 ℃ or less, preferably 10 ℃ to 20 ℃, and the washing time may be 10 minutes to 1 hour. When the washing temperature and the washing time satisfy the above ranges, the lithium by-product can be effectively removed.

Thereafter, the washed lithium composite transition metal oxide, cobalt (Co) -containing raw material, and boron (B) -containing raw material are mixed and subjected to high-temperature heat treatment. In this case, the high-temperature heat treatment may be performed at a temperature of 600 ℃ or higher, more preferably 600 ℃ to 900 ℃, and still more preferably 700 ℃ to 900 ℃. The high temperature heat treatment step further removes lithium by-products by the high temperature heat treatment and recrystallizes metal elements in the positive electrode active material to improve structural stability and thermal stability. In the case of a lithium composite transition metal oxide containing nickel at a high concentration, water washing is performed to remove residual lithium by-products, and during the water washing, lithium in the crystal structure is also desorbed in addition to the lithium by-products, thereby deteriorating crystallinity and stability. Therefore, the metal element in the lithium composite transition metal oxide can be recrystallized by performing high-temperature heat treatment on the washed lithium composite transition metal oxide, thereby filling the voids of lithium and improving surface stability.

In the heat treatment of the present invention, a cobalt (Co) -containing raw material and a boron (B) -containing raw material are mixed together and subjected to a high-temperature heat treatment. Conventionally, in order to improve the thermal stability of a lithium composite transition metal oxide containing nickel at a high concentration, a coating process is separately performed at a low temperature after a high-temperature heat treatment, but there is a problem in that the production time and process cost are increased due to an increase in process steps. Meanwhile, in the present invention, the cobalt (Co) -containing raw material and the boron (B) -containing raw material are mixed together in a high-temperature heat treatment step after water washing to simultaneously form the surface-coated portion, thereby simplifying the process and reducing the production time and process cost. Further, it was confirmed that, due to the cathode active material having the surface coating portion manufactured as described above, thermal stability, high-temperature lifespan characteristics, and output characteristics were improved while problems of increasing process time and cost were overcome.

The cobalt (Co) -containing feedstock may be a cobalt-containing acetate, nitrate, sulphate, halide, sulphide, hydroxide, oxide or oxyhydroxide, and may in particular be Co (OH)₂、CoOOH、Co(OCOCH₃)₂·4H₂O、Co(NO₃)₂·6H₂O、Co(SO₄)₂·7H₂O, or a combination thereof, but the embodiment is not limited thereto.

The cobalt (Co) -containing raw material may be mixed in an amount of 0.001 to 0.01 parts by weight, preferably 0.002 to 0.008 parts by weight, with respect to 100 parts by weight of the lithium composite transition metal oxide. When the content of the cobalt (Co) -containing raw material satisfies the above range, the output characteristics can be effectively improved without inhibiting the capacity characteristics of the lithium composite transition metal oxide. Specifically, when the amount is less than 0.001 parts by weight, the output improving effect is insignificant, and when the amount exceeds 0.01 parts by weight, nickel in the lithium composite transition metal oxide may be substituted with cobalt, thereby deteriorating the capacity characteristics.

The boron (B) -containing raw material may contain B₄C and B₂O₃And B may be more preferably used₄C. Due to B in which carbon and boron are covalently bonded₄C has a relatively high melting point, and thus can effectively form lithium boron oxide without decomposition even when heat treatment is performed at 600 ℃. On the other hand, since H is generally used as a raw material containing boron (B)₃BO₃Has a low melting point, and thus may not form lithium boron oxide due to decomposition reaction when heat treatment is performed at 400 ℃ or higher.

The boron (B) -containing raw material may be mixed in an amount of 0.0001 to 0.001 parts by weight, preferably 0.0002 to 0.0008 parts by weight, based on 100 parts by weight of the lithium composite transition metal oxide. When the content of the boron (B) -containing raw material satisfies the above range, the capacity and high-temperature life characteristics of the positive electrode active material can be effectively improved. Specifically, when the amount is less than 0.0001 parts by weight, the capacity improving effect may not be significant, and when the amount exceeds 0.001 parts by weight, reactivity with lithium may increase, thereby rather deteriorating capacity and high-temperature lifespan characteristics.

As described above, when the cobalt (Co) -containing raw material and the boron (B) -containing raw material are further mixed to perform the high-temperature heat treatment, the surface of the lithium composite transition metal oxide is coated with a cobalt component during the high-temperature heat treatment to form a cobalt-rich layer having a relatively higher cobalt content than the interior of the lithium composite transition metal oxide, and the lithium byproduct of the lithium composite transition metal oxide reacts with boron to form lithium boron oxide. When the surface coating portion including the cobalt-rich layer and the lithium boron oxide is formed on the surface of the lithium composite transition metal oxide as described above, the effect of improving the output characteristics and the thermal stability can be obtained.

Meanwhile, the heat treatment is performed in an oxidizing atmosphere, for example, an oxygen atmosphere. Specifically, the heat treatment may be performed while supplying oxygen at a flow rate of 0.5 to 10 liters/minute, preferably 1 to 5 liters/minute. When the heat treatment is performed in an oxidizing atmosphere as in the present invention, the lithium by-product can be effectively removed. According to the studies of the present inventors, when the heat treatment is performed in the atmosphere, the effect of removing the lithium by-product is remarkably deteriorated, and particularly, when the heat treatment is performed at 700 ℃ or more in the atmosphere, the amount of the lithium by-product is rather increased as compared with before the heat treatment.

Further, the high-temperature heat treatment may be performed at a temperature of 600 ℃ or higher, for example, 600 ℃ to 900 ℃, more preferably 700 ℃ to 900 ℃ for 10 hours or less, for example, 1 hour to 10 hours. When the heat treatment temperature and time satisfy the above ranges, the effect of improving thermal stability may be excellent. According to the study of the present inventors, when the heat treatment temperature is lower than 600 ℃, the thermal stability improving effect is hardly exhibited.

<Positive electrode active material for secondary battery>

Next, a positive electrode active material for a secondary battery according to the present invention will be described.

The positive electrode active material for a secondary battery prepared according to the present invention comprises: a lithium composite transition metal oxide containing nickel (Ni) and cobalt (Co), and containing at least one selected from manganese (Mn) and aluminum (Al); and a surface portion formed on a surface of the lithium composite transition metal oxide particle, wherein the surface portion comprises a cobalt-rich layer having a higher cobalt content than the lithium composite transition metal oxide and a lithium boron oxide.

The lithium composite transition metal oxide may be a high nickel NCM-based/NCA-based lithium composite transition metal oxide having a nickel (Ni) content of 60 mol% or more with respect to the total transition metal content. More preferably, the content of nickel (Ni) may be 70 mol% or more, and still more preferably, the content of nickel (Ni) may be 80 mol% or more, with respect to the total transition metal content. In the lithium composite transition metal oxide, the content of nickel (Ni) satisfies 60 mol% or more with respect to the total transition metal content, so that a high capacity can be secured.

More specifically, the lithium composite transition metal oxide may be represented by the following formula 1:

[ formula 1]

Li_pNi_1-(x1+y1+z1)Co_x1M^a _y1M^b _z1M^c _q1O_2-aA_a

In the above formula, M^aIs at least one selected from Mn and Al, M^bIs at least one selected from Zr, W, Mg, Al, Ce, Hf, Ta, La, Ti, Sr, Ba, Ge, V, Si, Nb, Mo and Cr, M^cIs at least one selected from Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, A is at least one selected from P and F, P is more than or equal to 0.9 and less than or equal to 1.05, 0<x1≤0.3，0<y1≤0.2，0≤z1≤0.1，0≤q1≤0.1，0≤a<1, and 0<x1+ y1+ z1 is less than or equal to 0.4. From the above [ formula 1]]The specific specification of the represented lithium composite transition metal oxide is the same as that described in the above-described preparation method, and thus, a detailed description thereof will be omitted.

The cobalt-rich layer is formed by mixing a lithium composite transition metal oxide and a cobalt (Co) -containing raw material, and coating the surface of the lithium composite transition metal oxide with a cobalt component derived from the cobalt-containing raw material in a high-temperature heat treatment process, and contains a relatively large amount of cobalt compared to the lithium composite transition metal oxide.

Specifically, the difference between the ratio of the number of cobalt atoms to the total number of metal element atoms other than lithium (i.e., the sum of the numbers of atoms of nickel, cobalt, manganese, and aluminum) (hereinafter, referred to as "cobalt atomic fraction") in the cobalt-rich layer and the cobalt atomic fraction of the lithium composite transition metal oxide may be about 0.05 to about 0.2, preferably about 0.05 to about 0.15. More specifically, the atomic fraction of cobalt in nickel, cobalt, manganese and aluminum (i.e., the ratio of the number of cobalt atoms to the sum of the number of atoms of nickel, cobalt, manganese and aluminum) in the cobalt-rich layer may be 0.05 to 0.45, preferably 0.05 to 0.35. When the cobalt atomic fraction in the cobalt-rich layer satisfies the above range, the output characteristics of the lithium composite transition metal oxide can be effectively improved without inhibiting the capacity characteristics thereof.

The lithium boron oxide is formed by mixing a lithium composite transition metal oxide with a boron-containing (B) raw material and then reacting the lithium by-product of the lithium composite transition metal oxide with boron in a high-temperature heat treatment process.

Specifically, the content of boron (B) contained in the lithium boron oxide may be 100ppm to 1,000ppm, preferably 200ppm to 500ppm, with respect to the total weight of the positive electrode active material. When the content of boron (B) satisfies the above range, high-temperature stability can be effectively improved, and capacity and high-temperature life characteristics can be improved.

When the surface coating portion including the cobalt-rich layer and the lithium boron oxide is formed on the surface of the lithium composite transition metal oxide as described above, the output characteristics and the thermal stability are improved.

The surface coating may have a thickness of 10nm to 100nm, preferably 30nm to 70 nm. When the thickness of the surface coating portion exceeds 100nm, the initial discharge capacity may be reduced and the surface coating portion may act as a resistive layer blocking movement of lithium, and when the thickness of the surface coating portion is less than 30nm, output, thermal stability, and cycle characteristics may be deteriorated.

The positive active material according to the present invention is prepared by: the high-temperature heat treatment is performed in an oxidizing atmosphere after washing with water, and the surface coating portion including the cobalt-rich layer and the lithium boron oxide is formed during the high-temperature heat treatment, and therefore, the residual amount of lithium by-products is significantly small, and excellent high-temperature stability can be achieved, as compared to a conventional positive electrode active material containing nickel at a high concentration.

In the cathode active material according to the present invention, the content of the lithium by-product may satisfy 0.55 wt% or less, preferably 0.53 wt% or less, more preferably 0.50 wt% or less, relative to the total weight of the cathode active material. Therefore, when a secondary battery is manufactured by using the cathode active material according to the present invention, gas generation and swelling phenomena can be effectively suppressed during charge and discharge.

The positive electrode active material according to the present invention may have a main peak in a temperature range of 220 ℃ to 250 ℃, preferably 230 ℃ to 240 ℃, more preferably 234 ℃ to 240 ℃ when measuring a heat flow by Differential Scanning Calorimetry (DSC), and the heat flow thereof may satisfy 2,000W/g or less, preferably 1,800W/g or less, more preferably 1,750W/g or less. When the high-temperature heat treatment is not performed after the water washing, when the heat treatment temperature and atmosphere do not satisfy the conditions of the present invention even if the high-temperature heat treatment is performed, or when the surface coating portion is not formed, a peak occurs at a relatively low temperature, and a high heat flow value exceeding 2,000W/g occurs. When such a cathode active material having a peak in a low temperature range and having a high heat flow is used, if the internal temperature of the battery is increased due to overcharge or the like, the heat flow may rapidly increase and explosion may occur. Meanwhile, the positive electrode active material of the present invention has a relatively high temperature range in which a peak occurs and a small amount of heat flow, and therefore, even when the internal temperature of the battery rises due to overcharge or the like, the possibility of explosion is low.

<Positive electrode and secondary battery>

According to another embodiment of the present invention, there are provided a positive electrode for a lithium secondary battery and a lithium secondary battery including the positive electrode active material.

Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing the positive electrode active material.

In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing any chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface is surface-treated with carbon, nickel, titanium, silver, or the like may be used. In addition, the cathode current collector may conventionally have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the cathode current collector to enhance adhesion of the cathode active material. Various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric can be used.

Further, the positive electrode active material layer may include a conductive material and a binder in addition to the above-described positive electrode active material.

The conductive material is used to impart conductivity to the electrode, and may be used without particular limitation so long as it has electron conductivity without causing any chemical change in the battery constructed. Specific examples thereof may include graphite (e.g., natural graphite or synthetic graphite); carbon-based materials (e.g., carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber); metal powder or metal fiber (e.g., copper, nickel, aluminum, or silver); conductive whiskers (e.g., zinc oxide or potassium titanate); conductive metal oxides (e.g., titanium oxide); or a conductive polymer (e.g., polyphenylene derivative), and either one alone or a mixture of two or more thereof may be used. The conductive material may be conventionally contained in an amount of 1 wt% to 30 wt% with respect to the total weight of the positive electrode active material layer.

In addition, the binder serves to improve the adhesion between the particles of the positive electrode active material and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples thereof may include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, Ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one alone or a mixture of two or more thereof may be used. The binder may be contained in an amount of 1 wt% to 30 wt% with respect to the total weight of the positive electrode active material layer.

The positive electrode may be manufactured according to a typical positive electrode manufacturing method, except that the positive electrode uses the above-described positive electrode active material. Specifically, the positive electrode may be manufactured by coating a composition for forming a positive electrode active material layer, which includes the above-described positive electrode active material and optionally a binder and a conductive material, on a positive electrode current collector, followed by drying and rolling. Here, the types and contents of the positive electrode active material, the binder, and the conductive material are the same as those described above.

As the solvent, a solvent generally used in the art may be used, and examples thereof may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, and the like, and any one alone or a mixture of two or more thereof may be used. The solvent to be used is in an amount sufficient to dissolve or disperse the positive electrode active material, the conductive material, and the binder in consideration of the coating thickness of the slurry and the manufacturing yield, and has a viscosity capable of exhibiting excellent thickness uniformity when coating for manufacturing the positive electrode is subsequently performed.

Alternatively, the positive electrode may be manufactured by casting the composition for forming the positive electrode active material layer on a separate support, and then laminating a film obtained by peeling off from the support on the positive electrode current collector.

According to another embodiment of the present invention, there is provided an electrochemical device including the positive electrode. The electrochemical device may be specifically a battery, a capacitor, or the like, and more specifically, may be a lithium secondary battery.

The lithium secondary battery specifically includes: a positive electrode; a negative electrode disposed to face the positive electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, and the positive electrode is the same as the positive electrode described above. In addition, the lithium secondary battery may further optionally include: a battery container for storing an electrode assembly of the positive electrode, the negative electrode and the separator; and a sealing member for sealing the battery container.

In the lithium secondary battery, the anode includes an anode current collector and an anode active material layer disposed on the anode current collector.

The anode current collector is not particularly limited as long as it has high conductivity without causing any chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. In addition, the anode current collector may conventionally have a thickness of 3 μm to 500 μm, and similar to the cathode current collector, fine irregularities may be formed on the surface of the current collector to enhance adhesion of the anode active material. Various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric can be used.

The negative electrode active material layer optionally includes a binder and a conductive material in addition to the negative electrode active material. As one embodiment, the anode active material layer may be prepared by: coating a composition for forming an anode, which includes an anode active material and optionally a binder and a conductive material, on an anode current collector, and drying; or alternatively, the composition for forming the anode is cast on a separate support, and then a film obtained by peeling from the support is laminated on an anode current collector.

A compound capable of reversibly intercalating and deintercalating lithium may be used as the negative electrode active material. Specific examples thereof may include carbonaceous materials (e.g., artificial graphite, natural graphite, graphitized carbon fiber, or amorphous carbon); (semi) metallic material (e.g. Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy or Al alloy) capable of alloying with lithium; metal oxides capable of doping and dedoping lithium (e.g. SiO)_α(0<α<2)、SnO₂Vanadium oxide or lithium vanadium oxide); or a composite (e.g., Si — C composite or Sn — C composite) containing a (semi) metal-based material and a carbonaceous material, and either one alone or a mixture of two or more thereof may be used. A metallic lithium thin film may also be used as the negative active material. In addition, both low crystalline carbon and high crystalline carbon can be used as the carbon material. As typical examples of the low crystalline carbon, soft carbon or hard carbon may be used, and as typical examples of the high crystalline carbon, natural graphite or artificial graphite in an amorphous, planar, flaky, spherical or fibrous form; condensing graphite; pyrolytic carbon; mesophase pitch-based carbon fibers; mesocarbon microbeads; mesophase pitch; or high temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

Further, the binder and the conductive material may be the same as those in the positive electrode described above.

Meanwhile, in the lithium secondary battery, a separator is used to separate an anode and a cathode from each other and provide a transfer path of lithium ions, and any separator may be used as the separator without particular limitation as long as it is conventionally used in the lithium secondary battery. In particular, a separator having excellent electrolyte retaining ability and low resistance to the transfer of electrolyte ions may be preferably used. Specifically, a porous polymer film formed of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, or an ethylene/methacrylate copolymer, or having a laminated structure of two or more layers thereof may be used. It is also possible to use a conventional porous nonwoven fabric, such as a nonwoven fabric formed of glass fibers or polyethylene terephthalate fibers having a high melting point. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and a single layer structure or a multi-layer structure may be optionally used.

In addition, as an example of the electrolyte used in the present invention, an organic-based liquid electrolyte, an inorganic-based liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a melt-type inorganic electrolyte, which may be used to manufacture a lithium secondary battery, may be used, but the embodiment is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be used without particular limitation so long as the organic solvent can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as examples of the organic solvent, ester solvents (e.g., methyl acetate, ethyl acetate, γ -butyrolactone, or-caprolactone); ether solvents (e.g., dibutyl ether or tetrahydrofuran); ketone solvents (e.g., cyclohexanone); aromatic hydrocarbon solvents (e.g., benzene or fluorobenzene); a carbonate-based solvent (e.g., dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), Ethyl Methyl Carbonate (EMC), Ethylene Carbonate (EC), or Propylene Carbonate (PC)); alcohol solvents (e.g., ethanol or isopropanol); nitriles (e.g., R — CN, where R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, and may contain double bonds, aromatic rings, or ether linkages); amides (e.g., dimethylformamide); dioxolane (e.g., 1, 3-dioxolane); or sulfolane. Among these examples, a carbonate-based solvent may be preferably used, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) and a low-viscosity linear carbonate-based compound (e.g., ethylene carbonate, dimethyl carbonate, or diethyl carbonate) having high ion conductivity and high dielectric constant to increase charge and discharge characteristics of a battery may be more preferably used. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the electrolyte may exhibit excellent properties.

The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions for a lithium secondary battery. Specifically, LiPF may be used₆、LiClO₄、LiAsF₆、LiBF₄、LiSbF₆、LiAlO₄、LiAlCl₄、LiCF₃SO₃、LiC₄F₉SO₃、LiN(C₂F₅SO₃)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)₂LiCl, LiI or LiB (C)₂O₄)₂As the lithium salt. The lithium salt may be preferably used in a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt is included in the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent electrolytic performance and efficiently transferring lithium ions.

In addition to the electrolyte component, in order to improve the life characteristics of the battery, suppress the capacity decrease of the battery, and improve the discharge capacity of the battery, the electrolyte may further contain, for example, at least one additive of: halogenated alkylene carbonate compounds (e.g. difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (di) glymes, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substitutedOxazolidinones, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, and the like. In this caseThe additive may be included in an amount of 0.1 wt% to 5 wt% with respect to the total weight of the electrolyte.

As described above, since the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, the lithium secondary battery may be used in portable devices, such as mobile phones, notebook computers, or digital cameras, and in the electric vehicle industry, such as Hybrid Electric Vehicles (HEVs).

Therefore, according to another embodiment of the present invention, there are provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

The battery module or the battery pack may be used as a power source for a middle-or large-sized device for at least one of: an electric tool; electric vehicles, including Electric Vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or an electrical storage system.

Hereinafter, the present invention will be described in more detail according to examples. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.