MANUFACTURING METHOD OF CATHODE ACTIVE MATERIAL, AND CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY MANUFACTURED
THEREBY
TECHNICAI. FIELD
[0001] The present invention ■ relates to a manufacturing method of a cathode active material, and cathode active material for lithium secondary-battery manufactured thereby. BACKGROUND ART
[0002] Lithium secondary batteries have been widely used as power sources of portable devices after they have emerged as small, lightweight, and high-capacity batteries since 1991. Recently, in line with the rapid development of electronics, communications, and computer industries, camcorders, mobile phones, and notebook PCs have appeared and undergone continuous and remarkable development. Accordingly, the demand for lithium secondary batterie.s as a power source for driving these portable electronic information and communication devices has increased day by day.
[0003] Lithium secondary batteries have limitations in that their lifetime rapidly decreases as charge and discharge are repeated. In particular, the above limitations are more severe at high temperature. The reason for this is due to a phenomenon that occurs when an'electrolyte is decomposed or an active material is degraded due to moisture in the battery or

other' effects, and the internal resistance of the battery increases.
[0004] In order to address the above limitations, a technique of coating, the surface of a cathode active material with an oxide of metal, such as magnesium (Mg), aluminum (Al), cobalt (Co), potassium (K) , sodium (Na) , and calcium (Ca) , by a heat treatment has developed. ■ Also, research to improve energy density and high-ratt? characteristics by adding- Ti02 .to a LiCo02 active material has been conducted.
[0005] However, limitations, such as lifetime degradation or gas generation due to the decomposition of the electrolyte during charge and discharge, have not been fully resolved.yet. [0006] In the case that impurities are present in the surface of a cathode active material during a process of fabricating an electrode of a lithium secondary battery, the impurities may not only affect aging .in a step of preparing an electrode slurry during the process of fabricating an electrode of a lithium secondary battery, but may also cause a swelling phenomenon in the lithium secondary battery by reacting with an electrolyte solution that' is injected into the lithium secondary battery.
[0007] In order to address the above limitations, a method of coating the surface of a cathode active material with H3BO3 has been developed.
[0008] Examples of the above method may include a method of coating bhe surface of a cathode active material by mixing the cathode active material with H3BO3 by shaking several times

using a shaker. However, in this case, H3BO3 particles may agglomerate on the surface of the cathode active material. [0009] As another example, there is a method of coating a cathode active material by mixing the cathode active material and H3BO3 using mechanical compositing equipment, for example, a Nobilta™ device. In this case, since an amount of a coating layer included in the cathode active material is- not increased when H3BO3 is added in a predubermined amount or'more, there may be limitations in the reaction process.
[0010] Therefore, there is an urgent need to develop a method of preparing a cathode active material which may improve, the performance of a lithium secondary . battery while addressing the above limitations. [0011] [Prior Art Documents] [0012] [Patent Docioment]
[0013] Japanese Patent Application Laid-Open Publication No. 2009-152214
DISCLOSURE OF THE INVENTION TECHNICAL PROBLEM
[0014] An aspect of the present invention provides a method of preparing a cathode active material which may transform lithium impurities present in a lithium transition metal oxide into a structurally stable lithium boron.oxide by performing a heat treatment using a boron-containing compound. [0015] Another aspect of the present invention provides a cathode active material, which includes a . coating layer including-a lithium boron oxide on the surface,of the lithium transition metal oxide, by the method of preparing a cathode

active material.
[0016] Another aspect of the present inventioh provides a cathode and a lithium secondary battery including the cathode active material. ' ■
TECHNICAL SOLUTION
[0017] According to an aspect of the present invention, there
is provided a w^ethod Qf prepariaq a. catb.ode a,ct,i.'^re. ma-tracial
including coating a surface of a lithium transition metal
oxide with a lithium boron oxide by dry mixing, the lithium
transition metal oxide and a boron-containing compound and
performing a heat treatment.
[0018] According to another aspect of the present invention, there is provided a cathode active material including: a lithium transition metal oxide; and a coating layer including a lithium boron oxide on a surface of the lithium transition metal oxide.
[0019] According to another aspect of the present invention, there is provided a cathode including the cathode active material.
[0020] According to another aspect of the present invention, there is provided a lithium secondary battery, including the cathode. ' • ADVANTAGEOUS EFFECTS
[0021] A method of preparing a cathode active material according to an embodiment of the present invention may easily transform lithium impurities present in a lithium transition metal oxide into a structurally stable lithium boron oxide by

performing a heat treatment near the melting point of a boron-containing compound.
[0022] Also, a coating layer may be formed in which the lithium boron oxide is uniformly coated on the surface of the lithium transition metal oxide in an amount proportional to the used amount of the boron-containing compound even at a low heat treatment temperature. BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The following drawings attached to the specification illustrate preferred examples of the present invention by example, and serve to enable technical concepts of the present invention to be further understood together with detailed description of the invention .given below, and therefore the present invention should not be interpreted only with matters in such drawings.
[0024] FIG. 1 is a schematic diagram illustrating a method of
preparing a cathode active material according to an embodiment
of the present invention; ■
[0025] FIG. 2 is a graph illustrating the results of pH titration to investigate the amounts of lithium impurities of cathode active materials prepared in Example 1 of the present invention and Comparative Examples 1 and 2;
[0026] FIG. -3 is a. graph illustrating the results of measuring capacity characteristics after high-temperature storage (60°C) of lithium secondary batteries of Example 5 and Comparative Example 4 according to Experimental Example 3; and [0027] FIG. 4 is a graph illustrating the ■ results of measuring high-temperature (45°C) cycle . characteristics of

lithium secondary batteries of Example 5 and ■ Comparative Example 4 according to Experimental Example A. MODE FOR CARRYING OUT THE INVENTION
[0028] Hereinafter, the present invention will be described in more detail to allow for a clearer understanding of the present invention.
[0029] It will be understood that words or terms used in the specification and claims shall not be interpreted as the meaning defined in commonly used dictionaries. It will, be further understood that ' the words or terms. should be interpreted as having a meaning that is consistent with .their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention.
[0030] As illustrated in FIG. 1, a method of preparing a cathode active material according to an embodiment of the present invention may include coating a surface of a lithium transition metal oxide with a lithium boron- oxide by dry mixing the lithium transition metal oxide and a boron-containing compound and performing a heat treatment. [0031] The method of preparing a cathode active material according to the embodiment ' of thf» present invention may easily transform lithium impurities present in a lithium transition metal oxide into a structurally stable lithium boron oxide by dry. mixing the lithium transition metal oxide and a boron-containing compound and performing a heat

treatment, in particular, near a melting point of the boron-containing compound. Also, a coating layer may be formed in which the lithium boron oxide is uniformly coated on the surface of the lithium transition metal oxide .in an amount proportional to the used amount of the boron-containing compound even-at a low heat treatment temperature. [0032] In the method of preparing a cathode active material according to the embodiment of the. present invention, the boron-containing compound may be any one selected from the group consisting of H3BO3, B2O3, C6H5B(OH)2,- (CeHsOsB, [CH3{CH2) aOlsB, C13H19BO3, C3H9B3O6, and (C3H70)3B, or a mixture- of two or more thereof.
[0033] In general, a .method of forming a coating layer on the surface of the lithium transition metal oxide may include a dry mixing method and a wet mixing method. In the case that the wet mixing method is used, a more uniform coating layer formed on the surface of the lithium transition metal oxide may be obtained. However, - with respect to the wet mixing method, the boron-containing compound must be used in a state of an aqueous solution, ' and in this case, .there is a possibility that damage to the. lithium transition metal'oxide occurs due to the aqueous solution.
[0034] Thus, according to the method of preparing a cathode active material according to the . embodiment of the present invention, Since the boron-containing compound is dry mixed with the lithium transition metal oxide' and a heat treatment is performed near the melting point of the boron-containing

compound to melt and flow the boron-containing compound, the advantage of the wet mixing method, i.e., a uniform coating layer, may be realized without causing the damage to the lithium transition metal oxide, i.e., the limitation of the wgt mixing method.
[0035], Specifically, the boron-containing compound, for example, .H3BO3, begins to be melted while being softened at low. temperature,, for example, in- a temperature range of about 130°C to about 160°C.
[0036] Since the boron-containing compound may react with at least a portion of lithium impurities present in the lithium transition metal oxide while being melted and flowing by the heat treatment, the boron-containing compound may be easily t;:ansformed into a lithium boron oxide to be coated on the surface of the lithium transition metal oxide. Thus, the lithium impurities present in the lithium transition metal oxide may be reduced by the transformation of the lithium impurities into the lithium boron oxide.
[0037] According to the method of preparing a cathode active material according to the embodiment of the present invention, the heat treatment is performed in a temperature range of 130°C to 300°C, near the melting point of the boron-containing compound, and may be performed in a temperature r.ange of 130°C to 200°C, for example, for 3 hours to 10 hours. [003.8] In the case that the heat treatment temperature is less than 130°C, since the boron-containing compound is not sufficiently melted, the boron-containing compound may remain

as it is or may not form a uniform coating layer even if it is transformed into the lithium boron oxide. In the case in which the heat treatment temperature is greater than 300°C, since the reaction may be excessively fast due to the , high temperature, a uniform coating layer may not be formed on the surface of the lithium transition metal oxide.
[0039] According to the method of preparing a cathode active material according to the embodiment of the present invention, since the heat treatment is performed at a specific temperature, a coating layer may be formed in which, the lithium boron oxide is uniformly' coated on the surface of the lithium transition metal oxide in an amount proportional to the used amount of the boron-containing compound. [0040] In the preparation method according to the embodiment of the present invention, the dry mixing may be performed by a mortar grinder mixing method and a mechanical milling method. For example, it may be desirable to use the mechanical milling method to form a uniform coating layer.
[0041] Specifically, in the mortar grinder mixing method, lithium transition metal oxide and boron-containing compound are uniformly mixed using a- mortar, and a heat treatment may then be performed in the above heat treatment temperature range.
[0042] Also, the mechanical milling method, for' example, may mix lithium transition metal oxide and boron-containing compound by mechanical attrition using a roll mill, ball mill, high energy ball mill, planetary mill, stirred ball mill.

vibrating mill, or jet mill, and for example, compressive stress may be mechanically applied by. rotating at a speed of 100 rpm to 1,500 rpm.
[0043] In the case that the mechanical milling method is used, the lithium transition metal oxide and boron-containing compound are mixed by the mechanical milling method, and then the mixture may be heat treated in the above temperature range or mixing and heat treatment may be simultaneously performed in the above'milling device. According to an embodiment of the present invention, the mechanical milling method, instead of the mortar grinder mixing method, may be used to form a uniform coating layer.
[004.4] In the method of preparing a cathode active material
according to the embodiment of the present invention, an
amount of the boron-containing compound used is in a range of
0'. 05 wt% to 1 wt% and may be in a range of 0.1 wt% to 0.8 wt%
based on a total weight of the lithium transition metal oxide.
[0045] According to an embodiment of the present invention,
an amount of elemental boron (B) included in the coating layer
of the cathode active material may be increased as the amount
of the boron-containing compound used is increased within the
above range. .
[0046] Also, according to an embodiment of the present invention, a portion of the elemental B of the lithium boron oxide may be doped into the lithium transition metal oxide by the heat treatment,' and the' amount of the B' may have a concentration gradient gradually decreasing from the surface

of the lithium transition metal oxide to the- inside thereof.
[0047] Furthermore, the present invention provides a cathode active material which includes a • lithium transition metal oxide; and a coating layer including a lithium boron oxide on a surface of the lithium transition metal oxide. [0048] The coating layer may include elemental B. in an amount of 100 ppm to 2,000 ppm, for example, 250 ppm to 1,100 ppm. [0049] Also,• in the cathode active material according to the embodiment of the present- invention, the lithium- boron oxide included in the coating layer may be included in an amount of 0.05 wt% to 1 wt%, for example, 0.1 wt% to 0.8 wt% based on a total weight of the cathode active material.
[0050] In the case that the amount of the lithium boron oxide
is less than 0.05 wt%, since a thickness of the coating'layer
formed on the surface of the lithium transition metal oxide
may be decreased, an effect of suppressing side reactions
between electrolytes during charge and discharge may be
insignificant. In the case in which the amount of the lithium
boron oxide is greater than 1 wt%, since the thickness of the
coating layer may be increased due to the excessive amount of
the lithium boron oxide, electrochemical properties of a
lithium secondary battery tnay be reduced due to the ■ resulting
increase in resistance. ; ' .
[0051] The lithium boron oxide may be LiBOa, Li2B407, or a mixture thereof. [0052] Also, the thickness of the. coating layer may be in a

range of 10 nm to 1,000 nm.
[0053] In the cathode active material according to the embodiment of the present invention, a typically used lithium transition metal oxide may be used as the lithium transition metal oxide, and examples of the lithium transition metal oxide may be any one selected from the group consisting of a lithium-cobalt-basGd oxide, a lithium-manganese-based oxide, a lithium-nickel-manganese-based oxide, a lithium-manganese-coba;lt-based oxide,- and a •lithium-nickel-manganese-cobalt-based oxide, or a mixture of two or, more .thereof. In. particular, a layered-structure lithium transition meal oxide having high capacity characteristics' may be used and may be represented by Chemical Formula 1. below: [0054] '
Lii+a [NixMnyCOzMv] G2-cAc [0055] where M is any one selected from the group consisting of aluminum (Al), zirconium (Zr), zinc (Zn), titanium (Ti) , magnesium (Mg) , gallium (Ga) , .and indium (In), or two .or more elements thereof; A is- at least one selected from the group consisting of phosphorus (P) , fluorine (F)-, sulfur (S) , ■ and nitrogen (N), and 0
(1-s-t) [Li(LiaMn(i_a-x-z)NixCOz)02] -slLizCOa] -tlLiOH] [0059] where 0 •
Lii+a [NixMnyCOjBvjMv] 02-cAc [0066] where M is any one selected from the group consisting of Al, Zr, Zn, Ti, Mg, Ga, and In, or two or more elements thereof; A is at least one selected from the group consisting of P, F, S, and N, and 0^x<1.0, 0 [0086] Example 1
[0087] MOOH (M=Nio.78Mno.iiCoo.il) was used as a mixed transition metal precursor, the mixed transition metal precursor and Li2C03 were mixed at a stoichiometric ratio (Li :M=.l. 00 :1) , and LiNio.78Mno.iiCoo.ii02 was prepared by sintering the mixture, in a temperature range of about 800°C to about 900°C for 10 hours in air.
[0088] LiNio.78Mno.iiCoo,iiD2 and H3BO3 were weighed at a weight ratio of 100:0.17 and mixed with a dry mixer (CYCLOMIX,' HOSOKAWA Micron Corporation) to obtain' mixed powder. The powder thus obtained was heat treated at 150°C for 5 hours in an oxygen atmosphere. A cathode active material including

LiB02 and Li2B407 in the surface of LiNio.78Mno.11Coo.11O2 was obtained by the above method. A thickness of the coating layer was 150 nm.
[0089] Example 2
[0090] A cathode active material was prepared in the same, manner as in Example 1 except that LiNio.78Mno.11Coo.11O2 and H3BO3 were used at a weight ratio nf 100:0.34. .
[0091] In the cathode active material, a thickness of the coating layer was 230 nm.
[0092] Example 3
[0093] A cathode active material was prepared . in the same manner as in Example 1 except that LiNio.78Mno.11Coo.11O2 and H3BO3 were used at a weight ratio of 100:0.68.
[0094] In the cathode active material, a- thickness of the coating layer was 300 nm.
[0095] Comparative Example 1
[0096] A cathode active material was prepared In the same manner as in Example 1 except that LiNio.78Mno.11Coo.11O2 and H3BO3 were used at a weight, ratio of 100:0.0 9 and a heat treatment was not performed.
[0097] In the cathode active material, a thickness of the coating layer was 100 nm. .
[0098] Comparative Example 2

[0099] A cathode active material was prepared in the same
manner as in Comparative Example 1 ■ except that
LiNio.78Mno.11Coo.11O2 and H3BO3 were used at a weight ratio of
10.0:0.17.
[00100] In the cathode active material, a thickness of the
coating layer was 150 nm.
[00101] Compara'kive Example 3
[00102] A cathode active material was prepared in the same manner as in Comparative ■ Example 1 except that LiNio.78Mno.11Coo.11O2 and H3BO3 were used at a weight ratio of 100:0.34.
[00103] In the cathode active material, a thickness of the cpating layer was 160 nm.
[00104] <.Preparation of Lithium Secondary Battery>
[00105] Example 4
[00106] Cathode Preparation
[00107] The cathode active material .prepared in Example 1,
which included a coating layer containing LiB02 and Li2B407 in
the surface of LiNio.78Mno.11Coo.11O2, was used.
[00108] A cathode mixture slurry was prepared by adding 94 wt%
o.f the cathode active material, 3 wt% of carbon black as a
conductive agent, and 3 wt% of. PVdF as a binder to N-methyl-2-
pyrrolidone (NMP) as a -solvent. An about 20 yim thick aluminum
(Al) thin film as a cathode collector was coated with the
cathode mixture slurry and dried, and the Al thin film was

then roll-pressed to prepare a cathode.
[00109] Anode Preparation
[00110] An anode active material slurry was prepared by mixing 96.3 wt% of carbon powder as an anode active material, 1.0 wt% of super-p as a conductive agent, and 1.5 wt% of styrene-butadiene rubber (SBR) and 1.2 wt%, of carboxymethyl cellulose (CMC) as a binder, and adding the mixture to NMP as a solvent. A 10 pm thick copper (Cu) thin film as an anode collector was coated with the anode active material slurry and' dried, and the Cu thin film was then roll-pressed to prepare an anode.
[00111] Non-Aqueous Electrolyte Solution Preparation [00112] AIM LiPFg non-aqueous electrolyte solution was prepared by adding LiPFe to a non-aqueous electrolyte solvent that was prepared by mixing ethylene carbonate and diethyl carbonate, as an electrolyte, at a volume ratio of 30:70.
[00113] Lithium Secondary Battery Preparation
[00114] A mixed separator of polyethylene and polypropylene
was disposed between- the cathode and anode thus prepared, and
a polymer type battery was then prepared by a typical method.
Then, the preparation of each lithium secondary battery was
completed by injecting the prepared non-aqueous electrolyte
solution.
[00115] Examples 5 and 6

[00116] Lithium secondary batteries were prepared in the same manner as in Example 4 except that the cathode active materials prepared in Examples 2 and- 3 were respectively used.
[00117] Comparative Examples 4 to 6
[00118] Lithium secondary batteries were prepared in the same manner as in Example 4 . except that the cathode active materials prepared in. Comparative• Examples 1 to 3 were respectively used.
[00119] Experimental Example 1: Inductively Coupled Plasma (TCP) Mass Analysis
[00120] In order to investigate the amounts of 'elemental B included in the coating layers of the cathode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3, the coating layers were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).
[00121] Specifically, 0.1. g of each of the cathode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 was sampled, 2 ml .of distilled water and 3 m( of concentrated nitric acid were added thereto, and each sample was dissolved after closing a lid. After the. sample was completely dissolved, the solution was diluted by adding.50 mf of ultrapure water thereto. Thereafter, the diluted solution was again diluted in 10 times and then analyzed with an inductively coupled plasma-atomic emission spectrometer (ICP-AES) . The ICP-AES (ICP 5300DV, PerkinElmer Inc.) was operated.

under the following conditions: Forward. Power 1,300 W; Torch Height 15 mm, Plasma Gas Flow Rate 15.00 L/min; Sample Gas Flow Rate 0.8 L/min; Auxiliary Gas Flow Rate 0.20 L/min; and Pumping Speed 1.5 mf/min. As a result, the amounts of elemental B included in the coating layers of the cathode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 are presented in Table 1 below. [00122] [Table 1]
[00123] As illustrated in Table 1, there ■ was a difference between the. amounts of the elemental B included in the examples subjected to the heat treatment at 150°C and the comparative examples not subjected to a heat treatment even if the same amount of H3BO3 was used.
[00124] Specifically, with respect to Examples 1. to 3 subjected to the heat treatment at 150°C, i.e., near the melting point of H3BO3, the amount of the elemental- B included in the cathode active material was also increased from 275 ppm

to 1,110 ppm- similar to the theoretical value as the amount of the H3BO3 used was increased from 0.17 wt% to 0.68 wt%. [00125] However, with respect to Comparative Examples 1 to 3 not subjected to a heat treatment, as a result of changing the amount of the H3BO3 used to 0.09 wt%, 0.17 wt%,- and. 0.34 wt%, the amount of the elemental B included in the cathode active material was increased similar to the Lheoretical value in the case that the amount of the H3BO3 used was 0.09 w.t% and 0.17 wt%. However, in the case in which the amount of the H3BO3 used' was ■ 0.3.4%, the amount'of the elemental B was the same as that of the case of using 0.17'wt%.
[00126] In the case that the heat treatment was not performed as in the comparative examples, it may be understood that the amount of the elemental B included in the cathode active material was not increased when adding H3BO3 in a predetermined amount or more.
[00127] In contrast, in the case that the heat treatment was performed as in Examples 1 to 3, it may be understood that the amount of the elemental' B was detected from the cathode active, material by the amount of the H3BO3 used.
[00128] Experimental Example 2: . pH Titration Experiment to Investigate the Amount of Lithiiom Impurities
[00129] In order to investigate the amounts of lithium impurities in the cathode active materials prepared in Example 1. and Comparative Examples 1 and 2, pH titrations were performed, and the results thereof are presented in FIG. 2.

Metrohm 794 was used as a pH meter and pH values were recorded while titrating in 0.02 ml. increments.
[00130] FIG. 2 is a graph comparing the amounts of lithium impurities reduced in respective cathode active materials of Example 1 and Comparative Examples 1 and 2.
[00131] That is, as a result of comparing the amounts of 0.1 M HCl for 10 g of the respective cathode active materials,- when comparing the amounto of hydrochloric acid used for hydrochloric acid titration with, reference to FIG. 2, Example 1 was about 11.6 ml, Comparative , Example 1 was 15 ml, and Comparative 'Example 2 was about 14.2 ml. Thus, it may be understood that Example 1 was decreased' by about 20% or more in comparison to Comparative Examples 1 and 2.
[00132] Experimental Example 3: Output Characteristics After High-Temperature Storage and Resistance Increase Rate Evaluation Tests
[00133] FIG. 3 is a graph illustrating the results of measuring capacity characteristics after high-temperature storage (60°C) of the lithium secondary batteries of Example 5 and Comparative Example 4 according to Experimental Example 3. [00134] The lithium secondary batteries of Example 5 (cathode active material of Example 2)' and' Comparative Example 4 (cathode active material of Comparative Example. 1) were stored at 60 °C and then charged at 1 C to 4.15 V/30 mA under a constant current/constant voltage (CC/CV) condition. Then, the lithium secondary batteries were discharged at a constant

current (CC) of 1 C to a voltage of 2.5 V to measure discharge capacities. The results thereof are presented in FIG. 3. [00135] Referring to FIG. 3, a slope of a capacity retention ratio of the lithium secondary battery of Example 5 of the present invention was slower than ■ that of the lithium secondary battery of Comparative Example 4 to a storage time of 4 weeks. Specifically, it may be understood that the capacity retention ratio of the lithium secondary battery of Example 5 was increased by about 3% at a storage time of 4 weeks in comparison to that of the lithium secondary battery of Comparative Example 4.
[00136] Also, it may be understood that a resistance increase rate of the lithium secondary battery of Example 5 was decreased by about 25% in comparison to that of the lithium secondary battery of Comparative Example 4. It may be understood that the decrease in the resistance increase rate to about 25% may also affect output characteristics. That is, it may be understood that the lithium secondary battery of Example 5 may exhibit excellent output characteristics due to the decrease in the resistance increase rate.
[00137] Experimental Example 4: High-Temperature (45°C) Cycle Characteristics and Resistance Increase Rate Evaluation Tests [00138] The lithium secondary batteries of Example 5 and Comparative Example 4 were- charged at 1 C to 4.15 V/30 mA at 45°C under a constant, current/constant voltage (CC/CV) condition and then discharged at a constant current (CC) of 2

C'. to a voltage of 2.5 V to measure discharge capacities. The charge and discharge were repeated 1 to 400- cycles and the measured discharge capacities are presented in FIG. 4. [00139] Referring to FIG. 4, the lithium secondary battery of Example 5 of the present invention exhibited the slope of the capacity retention ratio similar to that of Comparative Example 4 to the 4 00^^ cycle.
[00140] However, it may be understood that the resistance increase rate of the lithium secondary battery of Example 5 was decreased by about 7% at the 400*^^ cycle in 'comparison to that of the lithium secondary battery of Comparative Example 4. It may be understood ■that the decrease in the resistance increase rate may also affect output characteristics. That is, it may be understood that the lithium secondary battery of Example 5 may exhibit excellent output characteristics due to the decrease in the resistance increase rate.

CLAIMS
1. A method of preparing a cathode active material, the method comprising coating a surface of a lithium transition metal oxide with a lithium boron oxide by dry mixing the lithium transition metal oxide and a boron-containing compound and performing a heat treatment.
2i The method of claim 1, wherein the heat treatment is performed in a temperature range'of 130°C to 300°C.
3. The method of claim 2, ' wherein the heat treatment is performed in. a temperature range of 130°Cto 200°C.
4. The method of claim 2, wherein the boron-containing compound is transformed into a lithium boron oxide through a reaction with at least a portion of lithium impurities in the lithium transition metal oxide by the heat treatment.
5-. The method of claim 2, wherein a portion of elemental boron (B) of the lithium boron- oxide is doped into the lithium transition metal oxide by the heat treatment, and an amount of the B has a concentration gradient gradually decreasing from the surface of the lithium transition metal oxide to inside thereof. .
6. The method of claim 4, wherein the lithium impurities

comprise LiOH, LiaCOa, or a mixture thereof.
7. The method of claim 1, wherein the mixing is performed by a mortar grinder mixing method or a'mechanical milling method.
8. The method of claim 7, wherein the mixing by the mechanical, milling method is-' performed by using a roll mill, ball mill, high energy ball mill, planetary mill, stirred ball mill-, vibrating mill, or jet mill.
9. The method of claim 1, wherein the boron-containing compound comprises any one selected from the group consisting of H3BO3, B2O3, C6H5B(OH)2, (CeHsOJsB, ' [CH3 (CH2) 3O] 3B, C13H19BO3, C3H9B3O6, and' (C3H70)3B, or a mixture of two or more thereof.
10. The method of claim .1, wherein the boron-containing compound is used in an amount of 0.05 wt% to 1 wt% based on a total weight of the lithium transition metal oxide.
11. The method of claim 1, wherein the lithium boron oxide is LiB02,. Li2B407, or a mixture thereof.
.12. The' method of claim 1, wherein the lithium transition' metal oxide is represented by Chemical Formula 1:

Lii+a [NixMnyCOzMv] 02-cAc
where M is any - one selected from the group consisting of

aluminum (Al), zirconium (Zr), zinc (Zn), titanium (Ti) , magnesium (Mg), gallium (Ga) , and indium (In), or two or more elements thereof; A is at least one selected from the group consisting of phosphorus (P),. fluorine (F) , sulfur (S) , and nitrogen (N),and 0:^x ■ .
Lilt;, rNixMnyCo.B^MjOi^cAc
where M is any one selected from- the group consisting of aluminum (Al), zirconium (Zr), zinc (Zn), titanium (Ti), magnesium (Mg), gallium' (Ga), and indium (In), or two or more elements thereof; A is at least one selected from the group consisting of phosphorus (P) , fluorine (F) , sulfur (S) , and nitrogen (N) , and 0