WO2021025370A1 - Cathode active material for lithium secondary battery - Google Patents

Abstract

The present invention provides a cathode active material for a lithium secondary battery, comprising: a core comprising a lithium transition metal oxide; and a plurality of coating layers, wherein the coating layers comprise a first coating layer and a second coating layer which are respectively formed so as to reduce different lithium by-products that exist near the surface of the core. The coating layers can effectively reduce remaining lithium, which can exist on the surface of lithium transition metal oxide particles, through a specific remaining-lithium reducing effect that can be provided by the respective coating layers.

Description

Positive active material for lithium secondary battery

The present invention provides a positive electrode active material for a lithium secondary battery including a plurality of coating layers capable of improving various operating performance and safety such as life characteristics, discharge capacity, cycle characteristics, and output characteristics of a secondary battery by effective removal of residual lithium. About.

Lithium secondary batteries are used in various fields such as mobile devices, energy storage systems, and electric vehicles due to their high energy density and voltage, long cycle life, and low self-discharge rate.

One of the main causes of problems such as deterioration in performance and safety of lithium secondary batteries is the presence of lithium by-products, that is, residual lithium present on the surface of the positive electrode active material. In this residual lithium, some lithium compounds cause gas generation in the battery operation process, and some lithium compounds increase the acidity (pH) of the positive electrode material, thereby causing gelation of the positive electrode slurry. If a gelation phenomenon of the electrode mixture (slurry) occurs in the manufacturing process of the electrode plate, not only is it disadvantageous in the manufacturing process, but also operation performance is deteriorated due to uneven distribution of electrode components.

The above-described problems are particularly serious in the case of a nickel (Ni)-based positive electrode active material, which causes a significant deterioration in performance of a lithium secondary battery due to many lithium by-products in the manufacturing process despite a high capacity.

Specifically, in the case of a Ni-based positive electrode active material, it is mainly prepared by reacting nickel oxide or nickel carbonate with an excessive amount of lithium oxide, and at this time, various lithium by-products remain in the finally prepared positive electrode active material. In addition, recent Ni-based positive electrode active materials are being researched and developed in the direction of increasing the Ni content in order to increase the capacity characteristics.In this case, due to the tendency of Ni to remain as Ni ^{2 +} , the residual amount of lithium by-products on the surface is further increased. There is a problem that it becomes.

Therefore, there is a high need to develop a technology capable of solving these problems.

An object of the present invention is to solve the problems of the prior art and technical problems that have been requested from the past.

After repeating various experiments and in-depth research, the inventors of the present application greatly reduced the amount of residual lithium when individual coating layers each capable of reducing residual lithium in the positive electrode active material were added to the core containing lithium transition metal oxide. While it was found that not only life characteristics but also various operating characteristics and safety were significantly improved, the present invention was completed.

Accordingly, the cathode active material for a lithium secondary battery of the present invention includes a core including a lithium transition metal oxide and a plurality of coating layers, and the coating layers are a first coating layer formed to reduce different lithium by-products existing near the surface of the core, respectively. And a second coating layer.

The residual lithium, for example, causes a side reaction at the interface between the lithium transition metal oxide particles capable of storing and discharging lithium and the electrolyte, and has a negative effect on the manufacturing process of the positive electrode plate by raising the pH. It degrades overall operating performance and safety.

Accordingly, the cathode active material for a lithium secondary battery according to the present invention includes a plurality of coating layers specialized for each type of residual lithium, thereby maximizing the effect of reducing residual lithium causing various problems as described above.

The core may be a lithium transition metal oxide including a transition metal such as lithium and nickel, and may include a composition represented by Formula 1 below.

Li[Li _x M _1-xy D _y ]O _2-z Q _z (1)

In the above formula,

M is one or more transition metal elements that are stable in the 4th or 6th configuration;

D is one or more elements selected from alkaline earth metal, transition metal, and non-metal as a dopant;

Q is one or more anions;

0≤x≤0.1, 0≤y≤0.1, 0≤z≤0.2.

For reference, when D is a transition metal, the transition metal defined in M may be excluded from these transition metals.

Among these lithium transition metal oxides, in particular, lithium transition metal oxides having a high nickel content have a large residual amount of lithium by-products, resulting in problems as described above. In one specific example, lithium transition metal oxides constituting the core contain nickel. It may be a material with a high nickel content having a composition of 80% or more, preferably 90% or more, based on the mole of the transition metal.

Specifically, in the case of a high nickel-based positive electrode active material among positive electrode active materials, Li ₂ O remaining on the surface of the active material after firing reacts with H ₂ O or CO ₂ in the atmosphere to form residual lithium (Li ₂ CO ₃ , LiOH). And, this can lead to various problems. In the case of Li ₂ CO- ₃ remaining on the surface of the positive electrode active material, there is a problem in that gas such as CO, CO ₂ , and O ₂ is generated by reacting with the electrolyte during the charging and discharging process. It may cause problems such as reduced safety. In the case of LiOH, it is not only involved in the side reaction between Li ₂ CO ₃ and the electrolyte solution mentioned above, but also increases the pH of the positive electrode active material because it is a basic material, and thus, when the positive electrode active material is slurried for positive electrode manufacturing, the binder crosslinking The slurry may be gelled while causing a decrease in processability during electrode manufacturing.

Accordingly, the present invention can be particularly preferably applied to a positive electrode active material for a lithium secondary battery having a high nickel content.

In the positive active material of the present invention, the average particle diameter (D50) of the core may be, for example, 2 μm to 20 μm, but is not particularly limited.

The coating layers may be present near the surface of the core, and any one of the first coating layer and the second coating layer is first formed on at least a part of the surface of the core, and then the remaining coating layers are formed, that is, the first coating layer and the first coating layer. 2 The coating layer may be sequentially formed, or the second coating layer and the first coating layer may be sequentially formed.

In one specific example, when the first coating layer and the second coating layer are sequentially formed on the surface of the core, the heat treatment temperature for forming the first coating layer may be relatively higher than the heat treatment temperature for forming the second coating layer. have. That is, by first forming the first coating layer having a relatively high optimum temperature for heat treatment on the surface of the core, it is possible to prevent the first coating layer from being deteriorated at the heat treatment temperature for forming the second coating layer, This will be described in more detail later.

In some cases, a coating layer other than the first coating layer and the second coating layer may be additionally located between the core and the coating layer located outside the core surface, and another coating layer is added between the first coating layer and the second coating layer. It can also be a shape located as.

The reason that the coating layer located in the outer region of the core is divided into a plurality of coating layers is to exert a different effect through each of the coating layers for the positive electrode active material for a lithium secondary battery according to the present invention. The first coating layer may be formed to mainly reduce Li ₂ CO ₃ present in the vicinity of the core, and the second coating layer may be formed to mainly reduce LiOH existing near the core.

Specifically, Li ₂ CO ₃ remaining on the core surface can be mainly reduced by the first coating layer, and the cathode active material for a lithium secondary battery according to an embodiment of the present invention is a carbon/sulfur analyzer (Carbon/Sulfur Determinator). The content of Li ₂ CO ₃ measured by may be 0.18% by weight or less based on the total weight of the positive electrode active material, thereby suppressing gas generation and improving performance of the lithium secondary battery to which the positive electrode active material is applied.

In addition, LiOH remaining on the core surface can be mainly reduced by the second coating layer, and the positive electrode active material for a lithium secondary battery according to an embodiment of the present invention may appear when the surface LiOH is excessively high. By suppressing slurry gelation, the processability of electrode manufacturing can be improved.

Here, measuring the content of LiOH remaining on the surface of the active material may have limitations in qualitative and quantitative analysis depending on the resolution of the analytical equipment, so it may be more preferable to measure the content of LiOH by measuring the pH of the active material. have. Therefore, in the present invention, the content of LiOH is determined through the pH of the active material. When the content of LiOH on the surface of the positive electrode active material is high, the -OH group is also increased, making the positive electrode active material basic and increasing the pH. When is high, it can be determined that the content of LiOH is high.

Specifically, the pH of the filtrate obtained by filtering the mixture of the positive electrode active material and reverse osmosis deionized water (RO water) according to an embodiment of the present invention may be 11.65 or less, through which the positive electrode active material is applied. When the cathode active material is slurried, gelation can be prevented due to a low pH.

As described above, in the cathode active material for a lithium secondary battery according to the present invention, through the introduction of the first coating layer and the second coating layer, Li ₂ CO ₃ is reduced to suppress gas generation, and LiOH is reduced to reduce -OH groups. That is, since the pH is lowered, gelation of the electrode slurry is prevented, thereby improving processability. In addition, by reducing the total residual lithium, characteristics such as output characteristics and capacity of the battery can be improved, and the positive electrode active material is prevented from dissolving in the electrolyte by reacting with hydrofluoric acid derived from the electrolyte, thereby improving the cycle characteristics of the battery. .

The first coating layer and the second coating layer capable of exerting these effects, in one specific example, are composed of different compositions, respectively, W, Ti, P, B, Zr, Mo, Cr, Co, Al, Mg, Ta , Nb, F, Na, S. Bar may include any one or more elements, preferably, the first coating layer may include tungsten (W), the second coating layer may include boron (B).

More specifically, the first coating layer may include a composition of the formula represented by Li _a WO _{(a+b)/ 2} ( ₂ ≦a≦6, b is an oxidation number of W), and the second coating layer is Li _c B _d O _e (1≤c≤10, 1≤d≤10, e=(c+f)/g, where f is the absolute value of the oxidation number of B, and g is the absolute value of oxidation number of O) It may contain the composition of the formula.

Accordingly, the first coating layer may include at least one selected from the group consisting of, for example, Li ₂ WO ₄ , Li ₄ WO ₅ and Li ₆ WO ₆ , and the second coating layer is LiBO ₂ , LiBO _{4 ,} Li ₂ B ₄ O ₇ and Li ₂ O-2B ₂ O ₃ It may include one or more selected from the group consisting of.

The content of tungsten (W) included in the first coating layer may be 0.4 to 1.6% by weight relative to the total weight of the positive electrode active material. If the content of W is too small, the reduction of residual lithium due to the formation of the coating layer and The effect of improving battery performance may be insufficient. Conversely, if the content of W is excessive, the unit cost of the positive electrode active material may increase as the content of W, which is relatively expensive among various coating materials, is excessively high. It is not desirable because it cannot be said to be economical in terms of effectiveness.

The content of boron (B) contained in the second coating layer may be 0.03% by weight to 0.12% by weight based on the total weight of the positive electrode active material.If the content of B is too small, the reduction of residual lithium due to the formation of the coating layer and The effect of improving battery performance may be insufficient. Conversely, if the content of B is excessive, problems such as an increase in resistance and deterioration of battery performance may occur, which is not preferable.

Here, the first coating layer and the second coating layer may be designed not only in consideration of the optimum element content, but also may be designed according to the element content ratio between the coating layers. For example, the content ratio (B/W: molar basis) of B included in the second coating layer and W included in the first coating layer may be 0.075 to 0.3, and if the content ratio (B/W) is too small , Since the content of B is very insufficient, the reduction of LiOH, that is, the effect of reducing the pH of the positive electrode active material, may be small. On the contrary, if the content ratio (B/W) is too large, the content of W is insufficient, so the reduction effect of Li ₂ CO ₃ Can be less.

As described above, the coating layers mainly aim to reduce the residual lithium present near the surface of the core, and each of the coating layers may be preferably formed in a region occupying 25 to 100% based on the surface of the core.

The thickness of the coating layers may be appropriately set in consideration of the content range and content ratio range as described above.

When the first coating layer is formed in the outer area of the core, and then the second coating layer is formed in the outer area of the first coating layer, the second coating layer is formed in the outer area of the first coating layer and the outer area of the first coating layer and In addition, it may be formed in contact with the surface area of the core. When the first coating layer is formed in contact with a partial area of the core surface, the second coating layer is formed in the outer area of the first coating layer and the core This is because it can be formed on a region of the surface where the first coating layer is not formed.

Therefore, in the present specification, that the second coating layer is formed on the outer region of the first coating layer should be interpreted as that the second coating layer may be formed on the core surface as well as the outer region of the first coating layer.

As described above, among the various forms in which the coating layers are formed, in the form in which the first coating layer is first formed in the outer region of the core and the second coating layer is formed in the outer region of the first coating layer, the heat treatment temperature of the first coating layer is the second coating layer. It may be desirable to be relatively higher than the heat treatment temperature of.

In a specific example, each of the first coating layer containing tungsten (W) and the second coating layer containing boron (B) may be formed under different optimal heat treatment conditions. Optimal heat treatment of tungsten (W) for forming a coating layer The temperature (eg, about 400° C.) may be higher than the optimum heat treatment temperature (eg, about 300° C.) of boron (B). For example, the first coating layer may be formed by mixing a lithium transition metal oxide and a tungsten compound as a core and heat treatment in an oxygen atmosphere at a temperature of 300 to 500° C. for 9 to 12 hours, and the second coating layer is the first coating layer It may be formed by mixing the formed lithium transition metal oxide with a boron compound and heat treatment in an oxygen atmosphere at a temperature of 200 to 400° C. for 9 to 12 hours under a condition that satisfies a temperature lower than the above temperature.

Examples of tungsten compounds for the heat treatment include WO ₃ , H ₂ WO ₄ , (NH ₄ ) ₆ H ₂ W ₁₂ O ₄₀ , WCl ₄ And the like, and examples of the boron compound include H ₃ BO ₃ , B ₂ O _{3 ,} HBPO _4, and the like, but are not limited thereto. It goes without saying that the optimum heat treatment conditions may vary slightly depending on the types of the tungsten compound and the boron compound used for heat treatment for forming the coating layer.

Therefore, when the first coating layer is formed after the second coating layer is first formed on the outer region of the core, boron (B) is included in the process of forming the first coating layer containing tungsten (W) having a relatively high heat treatment temperature. Since the structure of the second coating layer may be changed or damaged, the desired effect of the coating layers may not be properly exhibited.

Therefore, the coating layers may be in a form in which the first coating layer is first formed on the outer region of the core, and then the second coating layer is subsequently formed, based on the optimum heat treatment temperature of the material contained therein. With the desired effect, that is, effective reduction of Li ₂ CO ₃ through the first coating layer and effective reduction of LiOH through the second coating layer may be possible.

The present invention also relates to a lithium secondary battery including the positive electrode active material.

Since the configuration of a lithium secondary battery and a method of manufacturing the same are known in the art, detailed descriptions thereof will be omitted in this specification.

As described above, in the cathode active material for a lithium secondary battery according to the present invention, Li ₂ CO ₃ is reduced by a plurality of coating layers of a specific composition to suppress the generation of gas in the battery, LiOH is reduced to reduce -OH groups, That is, since the pH is lowered, gelation of the electrode slurry can be prevented, thereby improving processability, and characteristics such as output characteristics and capacity of the battery can be improved by reducing the total residual lithium, and the positive electrode active material reacts with hydrofluoric acid derived from the electrolyte. It is prevented from dissolving in the electrolyte solution, so that the cycle characteristics and safety of the battery can be improved.

1 is a graph showing a volume increase rate according to gas generation performed in Experimental Example 2;

2 is a table showing the viscosity measurement results for each period performed in Experimental Example 3.

Hereinafter, the present invention will be described in more detail with reference to the drawings according to an embodiment of the present invention, but the scope of the present invention is not limited thereto.

[Example 1: Preparation of positive electrode active material]

LiNi _{0 . 90} Mn _{0 . 06} Co _{0 . 04 The} lithium transition metal oxide represented by O ₂ is mixed with about 0.4% by weight of WO ₃ based on the total weight of the lithium transition metal oxide, and heat-treated at about 400° C. for about 10 hours in an oxygen atmosphere to contain a lithium tungsten compound A positive electrode active material having the first coating layer formed thereon was prepared.

In the positive electrode active material on which the first coating layer is formed, about 0.12% by weight of H ₃ BO ₃ is mixed based on the total weight of the lithium transition metal oxide, and heat-treated at about 300° C. for about 10 hours in an oxygen atmosphere to include a lithium boron compound. A positive electrode active material having a second coating layer formed thereon was prepared.

[Example 2: Preparation of positive electrode active material]

LiNi ₀ as a lithium transition metal oxide _{. 82} Mn _{0 . 11} Co _{0 . A} positive active material was prepared in the same manner as in Example 1, except that ₀₇ O ₂ was used.

[Comparative Example 1: Preparation of positive electrode active material]

A positive electrode active material was prepared in the same manner as in Example 1, except that a coating layer was not formed.

[Comparative Example 2: Preparation of cathode active material]

A positive electrode active material was prepared in the same manner as in Example 1, except that a coating layer was formed by simultaneously heat-treating WO ₃ and H ₃ BO ₃ at 300 to 400°C.

[Comparative Example 3: Preparation of cathode active material]

A positive electrode active material was prepared in the same manner as in Example 1, except that the first coating layer including the lithium tungsten compound was not formed.

[Comparative Example 4: Preparation of cathode active material]

A positive electrode active material was prepared in the same manner as in Example 1, except that H ₃ BO ₃ was first heat-treated to form a first coating layer, and then WO ₃ was heat-treated to form a second coating layer.

[Comparative Example 5: Preparation of positive electrode active material]

A positive electrode active material was prepared in the same manner as in Example 1, except that the coating layer including the lithium boron compound was not formed.

[Comparative Example 6: Preparation of cathode active material]

A positive electrode active material was prepared in the same manner as in Example 2, except that a coating layer was not formed.

[Production Example 1: Preparation of lithium secondary battery]

A lithium secondary battery was manufactured by using the positive active materials of the Examples and Comparative Examples, respectively.

Specifically, the positive electrode active material of each of the Examples and Comparative Examples, a carbon black conductive material, and a PVDF binder were mixed in an N-methylpyrrolidone solvent in a weight ratio of 96:2.5:1.5 to prepare a slurry for forming a positive electrode. Then, it was coated on an aluminum current collector, dried at 80 to 120° C., and then rolled to prepare a positive electrode.

In addition, a negative electrode using lithium metal as a negative electrode active material was prepared.

An electrode assembly was manufactured by interposing a porous polyethylene separator between the positive electrode and the negative electrode prepared as described above, and after placing the electrode assembly in the case, an electrolyte was injected into the case to prepare a lithium secondary battery. At this time, the electrolyte is lithium hexafluorophosphate (LiPF ₆ ) having a concentration of 1.0 M in an organic solvent consisting of ethylene carbonate/ethyl methyl carbonate (mixed volume ratio of EC/EMC = 1/2) and vinylene carbonate of 2% by weight of the total electrolyte. (Vinylene Carbonate) was prepared by dissolving.

[Experimental Example 1: Analysis of Physical Properties of Positive Active Material and Battery Evaluation]

After obtaining a filtrate by filtering a mixture of the positive electrode active material and reverse osmosis deionized water (Ro water) of the Examples and Comparative Examples, the pH of the filtrate was measured (using Metrohm's 827 pH lab equipment), The pH of the positive active material of each of Examples and Comparative Examples was analyzed.

In addition, the content of Li ₂ CO ₃ in the positive electrode active material of the Examples and Comparative Examples was measured (using ELTRA's Carbon/Sulfur Determinator), and in a crucible of a certain size, A certain amount of the positive electrode active material was added and the carbon (C) compound obtained by firing in an oxygen atmosphere was analyzed to measure the content of Li ₂ CO ₃ in the positive electrode active material.

In addition, a coin half cell (using Li metal negative electrode) manufactured using the positive electrode active material of each of the above Examples and Comparative Examples was charged at 25° C. with a constant current (CC) of 0.1 C until it reached 4.3 V, Thereafter, it was charged with a constant voltage (CV) of 4.3V and charged once until the charging current reached a constant current value of 0.05C. After leaving for 10 minutes, it was discharged with a constant current of 0.1C until it reached 3.0V, and the charge and discharge capacity in one cycle was measured.

Thereafter, the life characteristics were evaluated for each cell. Specifically, charging/discharging was performed 50 times for the lithium secondary battery under the condition of 0.5C/1C within a driving voltage range of 3.0V to 4.3V at 25°C. As a result, cycle capacity retention, which is the ratio of the discharge capacity at 50 cycles to the initial capacity after 50 charging and discharging at room temperature, was measured, respectively.

The above experimental results are shown in Table 1 below.

The pH value increases as the content of residual LiOH on the surface of the positive electrode active material increases. Therefore, a large pH value may mean that the amount of residual LiOH is large on the surface of the positive electrode active material. When the content of LiOH in the positive electrode active material is high, the electrode slurry is gelled, causing a problem in the manufacturing process of an electrode for a lithium secondary battery.

As shown in Table 1, the positive electrode active material of Example 1 having a composition of 90% Ni was found to have a lower pH than Comparative Examples 1 to 5, and the positive active material of Example 2 having a composition of 82% Ni was also found to have a pH compared to Comparative Example 6. Was found to be low.

On the other hand, if the content of residual Li ₂ CO _{3 on} the surface of the positive electrode active material is high, gas is generated inside the battery by reacting with the electrolyte injected into the lithium secondary battery, which may cause swelling in the lithium secondary battery. For this reason, a major problem may arise in the performance and safety of the battery.

As shown in Table 1, the positive electrode active material of Example 1 having a composition of 90% Ni was found to have a lower content of Li ₂ CO ₃ compared to the positive electrode active materials of Comparative Examples 1 to 5, and Example 2 having an 82% Ni composition The positive active material of was also found to have a lower content of Li ₂ CO ₃ compared to the positive active material of Comparative Example 6.

In addition, with respect to the performance of the battery, as shown in Table 1, the coin half cell including the positive electrode active material of Example 1 has capacity characteristics compared to the coin half cell including the positive electrode active material of Comparative Examples 1 to 5 , It can be seen that it shows more improved effects in terms of initial efficiency and cycle characteristics.

In the evaluation of applying the positive electrode active material of Example 2 with an 82% Ni composition, it can be seen that more improved effects in terms of capacity characteristics, initial efficiency, and cycle characteristics compared to Comparative Example 6.

[Experimental Example 2: Evaluation of gas generation of positive electrode plate]

A single plate full cell of a certain size (using a graphite negative electrode) prepared using each of the positive electrode active materials of Example 1 and Comparative Examples 1 to 5 was at 25°C until 4.2V with a constant current (CC) of 0.1C After charging, it was charged with a constant voltage (CV) of 4.2V, and charged once until the charging current reached a constant current value of 0.05C. Then, it was discharged until it became 3.0V with a constant current (CC) of 0.1C. Finally, the cell was charged with a constant current (CC) of 0.1C at 25°C until it reached 4.2V, and then the cell was disassembled while being charged with a constant voltage (CV) of 4.2V to extract the positive electrode. The extracted positive electrode was again enclosed in an aluminum pouch of a predetermined size with a predetermined amount of electrolyte and stored in a high temperature chamber at 60° C. to measure the increased pouch volume by date. The contents of the evaluation are shown in FIG. 1.

As shown in FIG. 1, it can be seen that the positive electrode to which the positive electrode active material of Example 1 is applied has suppressed gas generation compared to the positive electrode to which the positive electrode active material of Comparative Examples 1 to 5 is applied.

Referring to Table 1 and FIG. 1 together, it was confirmed that the Li ₂ CO ₃ content was also high in Comparative Example 1, Comparative Example 3 and Comparative Example 2 in which gas was generated the most, and in Example 1, the Li ₂ CO ₃ content It is judged that gas generation was suppressed due to the small amount.

Based on such evaluation results, it was confirmed that the content of Li ₂ CO ₃ has a great influence on gas generation.

[Experimental Example 3: Viscosity evaluation of positive electrode slurry]

In Experimental Example 2, the viscosity evaluation of the positive electrode slurry (using Brookfield's DV 2T Viscometer equipment) for the positive electrode active materials of Example 1, which generated the least gas, and Comparative Examples 4 and 5, which generated relatively little gas, was conducted. I did. Here, Comparative Example 1, which is a bare form of Example 1, was also included in this evaluation object.

The viscosity of the mixed slurry of the positive electrode active material of Example 1 and Comparative Examples 1, 4, and 5, the binder, the conductive agent, and the NMP was measured, and after storage for 10 days, the viscosity of the slurry was measured again. The experimental results are shown in FIG. 2.

As shown in FIG. 2, it was confirmed that Example 1 had the lowest viscosity of the slurry compared to Comparative Examples 1, 4, and 5, but it was confirmed that the viscosity was not gelled and maintained a relatively low viscosity even after storage for 10 days.

On the other hand, Comparative Examples 1, 4, and 5 exhibited higher slurry viscosity compared to Example 1, and a gelation phenomenon occurred after storage for 10 days.

Those of ordinary skill in the field to which the present invention belongs will be able to make various modifications and improvements based on the above contents, which should be interpreted as belonging to the scope of the present invention.

Claims (15)

1. Including a core comprising a lithium transition metal oxide and a plurality of coating layers,

   The coating layers include a first coating layer and a second coating layer formed to reduce different lithium by-products existing near the surface of the core, respectively.
2. The cathode active material for a lithium secondary battery according to claim 1, wherein a first coating layer and a second coating layer are sequentially formed on the surface of the core.
3. The cathode active material of claim 2, wherein a heat treatment temperature for forming the first coating layer is relatively higher than a heat treatment temperature for forming the second coating layer.
4. The lithium of claim 1, wherein the first coating layer is formed to reduce Li ₂ CO ₃ present near the surface of the core, and the second coating layer is formed to reduce LiOH present near the surface of the core. Positive active material for secondary batteries.
5. The cathode active material according to claim 4, wherein the cathode active material has a Li ₂ CO ₃ content of 0.18 wt% or less based on the total weight of the cathode active material as measured by a carbon/sulfur determinator.
6. The cathode active material for a lithium secondary battery according to claim 1, wherein a pH of the filtrate obtained by filtering the mixture of the cathode active material and reverse osmosis deionized water (Ro water) is 11.65 or less.
7. The method of claim 1, wherein the first coating layer and the second coating layer are composed of different compositions, respectively, W, Ti, P, B, Zr, Mo, Cr, Co, Al, Mg, Ta, Nb, F, Na , A cathode active material for a lithium secondary battery, characterized in that it contains any one or more elements of the S element.
8. The cathode active material for a lithium secondary battery according to claim 7, wherein the first coating layer contains tungsten (W) and the second coating layer contains boron (B).
9. The method of claim 8,

   The first coating layer comprises _a composition of the formula represented by Li _a WO _{(a+b)/ 2} ( ₂ ≦a≦6, b is an oxidation number of W);

   The second coating layer is Li _c B _d O _e (1≦ _c ≦10, 1≦d≦10, e=(c+f)/g, where f is the absolute value of the oxidation number of B, and g is the A positive electrode active material for a lithium secondary battery, characterized in that it comprises a composition of a chemical formula represented by an absolute value of oxidation number).
10. The method of claim 8,

    The first coating layer includes at least one selected from the group consisting of Li ₂ WO ₄ , Li ₄ WO ₅ and Li ₆ WO ₆ ;

    The second coating layer is characterized in that it comprises at least one selected from the group consisting of LiBO ₂ , LiBO _{4 ,} Li ₂ B ₄ O ₇ and Li ₂ O-2B ₂ O ₃ , a cathode active material for a lithium secondary battery.
11. The method of claim 8,

    The content of tungsten (W) included in the first coating layer is 0.4 to 1.6% by weight based on the total weight of the positive electrode active material,

    The content of boron (B) included in the second coating layer is 0.03 to 0.12% by weight based on the total weight of the positive electrode active material.
12. The cathode active material for a lithium secondary battery according to claim 8, wherein the content ratio (B/W) of boron (B) included in the second coating layer and tungsten (W) included in the first coating layer is 0.075 to 0.3. .
13. The cathode active material for a lithium secondary battery according to claim 1, wherein each of the first coating layer and the second coating layer is formed in an area occupying 25 to 100% based on a surface portion of the core.
14. The cathode active material for a lithium secondary battery according to claim 1, wherein the lithium transition metal oxide has a nickel content of 80% or more based on the mole of the transition metal.
15. A lithium secondary battery comprising the positive active material according to any one of claims 1 to 14.

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