KR101913898B1 - Coating material for cathode active material, preparation method thereof, and lithium secondary battery comprising same - Google Patents

Abstract

The present invention provides a positive electrode active material coating material comprising an electrochromic polyimide which is a polyimide having a structure having excellent conductivity. When the positive electrode active material is coated on the surface of a nano coating using the coating material according to the present invention, By preventing the active material from being in direct contact with the electrolyte, side reactions that may occur between the positive electrode active material and the electrolyte can be suppressed, and the life and efficiency characteristics of the secondary battery can be remarkably improved. In particular, it is possible to provide a lithium ion secondary battery improved in life characteristics and conductivity at high temperature and high voltage conditions.

Description

TECHNICAL FIELD The present invention relates to a positive electrode active material coating material, a method for producing the same, and a lithium secondary battery comprising the same. BACKGROUND ART [0002]

The present invention relates to a cathode active material coating material, a method for producing the same, and a lithium secondary battery comprising the cathode active material coating material. More particularly, the present invention relates to a positive electrode active material coating material containing conductive polyimide (PI), a method for producing the same, and a lithium secondary battery comprising the same.

Lithium rechargeable batteries have been widely used as power sources for portable devices since they appeared in 1991 as small-sized, lightweight, and large-capacity batteries. In recent years, with the rapid development of the electronics, communication, and computer industries, camcorders, mobile phones, and notebook PCs have been remarkably developed and demand for lithium secondary batteries as a power source for driving these portable electronic information communication devices is increasing day by day .

However, the lithium secondary battery has a problem that its service life is rapidly deteriorated by repeated charging and discharging. This degradation in lifetime is caused by a side reaction between the anode and the electrolyte, and this phenomenon may become more serious under high voltage and high temperature conditions. Therefore, it is necessary to develop a secondary battery for a high voltage. For this purpose, it is very important to control the side reaction between the cathode active material and the electrolyte or the electrode interface reaction.

To solve these problems, a technique has been developed for coating a surface of a cathode active material with a metal oxide including Mg, Al, Co, K, Na, or Ca. In particular, it is generally known that oxides such as Al ₂ O ₃ , ZrO ₂ , and AlPO ₄ can be coated on the surface of the cathode active material on the surfaces of these cathode active materials. It is also true that the coating layer improves the safety characteristics of the cathode active material.

However, in the case of surface coating using the oxide coating layer, the oxide coating layer is finely dispersed in the form of nano-sized particles rather than entirely covering the surface of the cathode active material. As a result, the effect of surface modification of the cathode active material by the oxide coating layer is limited. In addition, the oxide coating layer is a kind of ion-insulating layer which is difficult to move lithium ions, and may cause deterioration of ionic conductivity.

In order to solve the above problem, the surface of the cathode material is coated with polyimide to improve the ionic conductivity of the oxide coating layer and to uniformly surround the surface of the cathode material, thereby preventing direct contact with the electrolyte, thereby reducing the side reaction with the electrolyte (Korean Patent Registration No. 1105342). However, since the coating layer composed of polyimide is difficult to have conductivity with respect to electrons, a polyimide coating layer contains a substance capable of providing electron conductivity such as carbon black as a supplement. However, there is a limitation in improving the electron conductivity by uniformly dispersing carbon black having a large size (10 nm or more) and forming secondary particles by coagulation in a coating layer composed of polyimide when viewed from a molecular point of view.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a positive electrode active material coating material capable of exhibiting excellent performance under high temperature and high voltage conditions by suppressing side reactions between a positive electrode active material and an electrolyte.

Another object of the present invention is to provide a method for manufacturing the cathode active material coating material.

Another object of the present invention is to provide a cathode active material coated with the coating material and a lithium ion secondary battery produced using the same.

In order to solve the above-described technical problem,

There is provided a positive electrode active material coating material comprising a polyimide having a repeating structure represented by the following formula (1).

[Chemical Formula 1]

 In the above formula,

X is a tetravalent organic group derived from tetracarboxylic acid,

Y is a divalent organic group derived from a diamine of the following formula (2)

n is an integer of 1 or more,

(2)

In the above formula,

R ₁ is a monovalent organic group containing hydrogen, a substituted or unsubstituted aromatic ring,

Q and Z are the same or different and are independently a divalent organic group containing a substituted or unsubstituted aromatic ring, and a and b are an integer of 1 or more.

And R < _{1 >} is a monovalent organic group selected from the group consisting of hydrogen, a substituted or unsubstituted C6-C24 aryl group or a substituted or unsubstituted aromatic ring or a secondary amine or a tertiary amine,

The Q and Z may be a divalent organic group selected from a substituted or unsubstituted secondary amine containing a C 6-24 arylene group or an aromatic ring or a tertiary amine.

The above R ₁ is selected from hydrogen or a monovalent organic group represented by the following general formulas (3) to (5)

(3)

[Chemical Formula 4]

[Chemical Formula 5]

R ₂ and R ₅ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by the following formula (6)

[Chemical Formula 6]

Each of R ₆ and R ₇ is independently hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms,

And Q and Z are each independently selected from divalent organic groups represented by the following formulas (7) to (9).

(7)

[Chemical Formula 8]

[Chemical Formula 9]

R ₁₁ and R ₁₄ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by Formula 6.

More specifically, the diamine of the formula (2) may be selected from the compounds of the following formulas (10a) to (10d).

[Chemical Formula 10a]

[Chemical Formula 10b]

[Chemical Formula 10c]

[Chemical Formula 10d]

In the above formula,

Each of R ₂₁ to R ₂₇ independently represents hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms.

X in Formula 1 may be a divalent organic group having 6 to 24 carbon atoms and an aromatic or alicyclic group having 3 to 24 carbon atoms.

In order to solve the other problems of the present invention,

a) preparing a mixed solution of a polyamic acid having a repeating structure represented by the following formula (12) and an organic solvent;

[Chemical Formula 12]

In the above formula,

X is a tetravalent organic group derived from tetracarboxylic acid,

Y is a divalent organic group derived from a diamine of the following formula (2)

n is an integer of 1 or more,

(2)

In the above formula,

R1 is a monovalent organic group containing hydrogen, a substituted or unsubstituted aromatic ring,

Q and Z are the same or different and are independently a divalent organic group containing a substituted or unsubstituted aromatic ring,

a and b are integers of 1 or more,

b) forming a coating film containing the polyamic acid on the surface of the cathode active material by dispersing the cathode active material in the mixed solution; And

c) Imidizing the polyamic acid contained in the coating film formed on the surface of the cathode active material to form a polyimide coating layer.

The thickness of the coating layer may be 1 nm to 200 nm.

In addition, the cathode active material may be any one selected from the group consisting of V ₂ O ₅ , TiS, and MoS, or two or more of them selected from the group consisting of a lithium transition metal oxide of spinel having a multilayer rock salt structure, an olivine structure and a cubic structure, Complex oxide.

Specifically, the cathode active material may be any one selected from the group consisting of oxides of the structural formulas 1 to 3 and V ₂ O ₅ , TiS and MoS, or a mixture of two or more thereof.

<Structure 1>

Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂ (-0.5? X? 0.6, 0? A, b, c? 1, x + a + b + c = 1);

&Lt; Formula 2 >

LiMn _{2 - x} M _x O ₄ (M is at least one element selected from the group consisting of Ni, Co, Fe, P, S, Zr, Ti and Al, 0? X? 2);

<Formula 3>

_{Li 1 + a Fe 1 - x} M x (PO 4 -b) X b (M is Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn , and Y X is at least one element selected from the group consisting of F, S and N, and -0.5? A? + 0.5, 0? X? 0.5, and 0? B?

More specifically, the positive electrode active material is _{LiCoO 2, LiNiO 2, LiMnO 2} , LiMn 2 O 4, Li [Ni a Co b Mn c] O 2 (0 <a, b, c = 1, a + b + c = 1) and LiFePO ₄ , or a mixture of two or more thereof.

According to one embodiment, the mixed solvent in which the polyamic acid and the organic solvent are mixed may include at least one metal ion selected from the group consisting of Mg, Al, Zr, Zn, Ru, Cs, Cu, Fe, Cr, It can be more inclusive.

Also, the polyimide coating layer can be formed by imidizing the coated cathode active material containing the polyamic acid at a temperature of 300 ° C to 400 ° C.

The present invention provides a cathode active material coated with the coating method described above.

The present invention also provides a lithium secondary battery produced using the coated cathode active material.

Further, when the C-rate efficiency of the lithium secondary battery is represented by Equation (1), the C-rate efficiency may be 90% or more.

[Equation 1]

C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] x 100

Also, the initial charging efficiency of the lithium secondary battery may be 97% or more.

In addition, the initial charge capacity of the coated lithium secondary battery cathode active material may have a value of 195 mAh / g or more.

The present invention provides a positive electrode active material coating material comprising an electrochromic polyimide that is a polyimide having a structure having excellent conductivity, thereby preventing direct contact of the positive electrode active material with an electrolyte solution, This can significantly improve the lifetime and efficiency characteristics of the secondary battery particularly at high temperature and high voltage conditions.

1A is a SEM image of a surface of a cathode active material before polyimide coating.
1B is a SEM image of the surface of a cathode active material after coating with a coating material according to an embodiment of the present invention.
FIG. 2 is a graph comparing the cycle life of a secondary battery made of the cathode active material of Example 1 and Comparative Example 1. FIG.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is capable of various modifications and various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The present invention provides a positive electrode active material coating material comprising a polyimide having a repeating structure represented by the following general formula (1).

[Chemical Formula 1]

In the above formula,

X is a tetravalent organic group derived from tetracarboxylic acid,

Y is a divalent organic group derived from a diamine of the following formula (2)

n is an integer of 1 or more or 2 or more,

(2)

In the above formula,

R ₁ is a monovalent organic group containing hydrogen, a substituted or unsubstituted aromatic ring,

Q and Z are the same or different from each other and independently represent a divalent organic group containing a substituted or unsubstituted aromatic ring, and a and b are integers of 1 or more, preferably 1 or more and 4 or less.

According to one embodiment, R &lt; _{1 &gt;} is a monovalent organic group selected from a hydrogen or a substituted or unsubstituted C6-C24 aryl group or a secondary amine or a tertiary amine containing a substituted or unsubstituted aromatic ring ,

The Q and Z may be a divalent organic group selected from a substituted or unsubstituted secondary amine containing a C 6-24 arylene group or an aromatic ring or a tertiary amine.

Specifically, R 1 is selected from hydrogen or a monovalent organic group represented by the following general formulas (3) to (5)

(3)

[Chemical Formula 4]

[Chemical Formula 5]

R ₂ and R ₅ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by the following formula (6)

[Chemical Formula 6]

Each of R ₆ and R ₇ is independently hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms,

And Q and Z are each independently selected from divalent organic groups represented by the following formulas (7) to (9).

(7)

[Chemical Formula 8]

[Chemical Formula 9]

R ₁₁ and R ₁₄ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by Formula 6.

More specifically, the diamine of the formula (2) may be selected from the compounds of the following formulas (10a) to (10d).

[Chemical Formula 10a]

[Chemical Formula 10b]

[Chemical Formula 10c]

[Chemical Formula 10d]

In the above formula,

Each of R ₂₁ to R ₂₇ independently represents hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms.

According to one embodiment, X in Formula 1 may be a tetravalent organic group derived from a tetracarboxylic acid having 6 to 24 carbon atoms and an aromatic or alicyclic group having 3 to 24 carbon atoms,

More specifically, X may be a tetravalent organic group selected from the group consisting of the following formulas (11a) to (11r).

The cycloaliphatic ring and the aromatic ring included in the general formulas (11a) to (11b) may be substituted with a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms.

Such a polyimide structure has a structure in which a pair of non-covalent electron is present, and thus has a property of excellent charge transfer. Particularly, the polyimide structure has a characteristic that a color is changed by an oxidation-reduction reaction, It belongs to the polyimide structure used. By applying polyimide having the above-described structure to a cathode active material coating, it is possible not only to suppress the occurrence of side reactions between the cathode material and the electrolyte, but also to use polyimide with improved electrical conductivity, A cathode active material capable of improving the lifetime and performance of the cathode active material can be produced. Especially, the elution that can occur under high temperature and high pressure conditions and the reaction between the electrolyte and the anode can be more efficiently suppressed.

Specifically, the conductive polyimide serves as a protective film for preventing direct contact of the positive electrode active material with the electrolyte, and also has a non-covalent electron pair and a π-bond widely delineated in the main chain, The electrons can move smoothly, and the electric conductivity is very good, and it is possible to smoothly provide a path through which electrons can pass through the film, so that the current and voltage distribution in the electrode are uniformly maintained during the charge / discharge cycle The cycle characteristics can be greatly improved.

The present invention also relates to

Preparing a mixed solution of a polyamic acid having a repeating structure represented by the following formula (12) and an organic solvent;

[Chemical Formula 12]

In the above formula,

X is a tetravalent organic group derived from tetracarboxylic acid,

Y is a divalent organic group derived from a diamine of the following formula (2)

n is an integer of 1 or more or 2 or more,

(2)

In the above formula,

R ₁ is a monovalent organic group containing hydrogen, a substituted or unsubstituted aromatic ring,

Q and Z are the same or different and are independently a divalent organic group containing a substituted or unsubstituted aromatic ring,

a and b are integers of 1 or more, preferably 1 or more and 4 or less,

Forming a coating film containing the polyamic acid on the surface of the cathode active material by dispersing the cathode active material in the mixed solution; And

Forming a polyimide coating layer by imidizing a polyamic acid contained in the coating film formed on the surface of the cathode active material

The present invention also provides a method for coating a cathode active material.

The polyamic acid of Formula 12 may be synthesized by reacting tetracarboxylic acid and diamine in the same equivalent amount. The aromatic anhydride may be a tetracarboxylic acid including a tetravalent organic group represented by X in Formula 12, and X , The diamine may be a diamine containing a divalent organic group represented by Y in the formula (12), and the description of Y is as described above.

The polyamic acid may be used in an amount of 0.1 part by weight to 1 part by weight based on 100 parts by weight of the organic solvent.

In the case of synthesizing the polyamic acid or polyimide of the present invention, in order to inactivate the excess polyamino group or acid anhydride group, a terminal endblock which encapsulates the end of the polyimide by reacting the molecular end with a dicarboxylic anhydride or a monoamine Can be added.

Examples of the dicarboxylic anhydride used for sealing the ends of the polyimide or polyamic acid include phthalic anhydride, 2,3-benzophenone dicarboxylic anhydride, 3,4-benzophenone dicarboxylic anhydride, 2,3-dicarboxyphenylphenyl Biphenyldicarboxylic anhydride, 3,4-biphenyldicarboxylic anhydride, 2,3-dicarboxyphenylphenylsulfonic anhydride, 3,4-dicarboxyphenylphenylsulfonic anhydride, 2,3-di Carboxyphenylphenylsulfide anhydride, 1,2-naphthalenedicarboxylic anhydride, 2,3-naphthalenedicarboxylic anhydride, 1,8-naphthalenedicarboxylic anhydride, 1,2-anthracenedicarboxylic anhydride, 2,3-anthracenedicarboxylic anhydride, Anhydride, 1,9-anthracenedicarboxylic anhydride, and the like. These dicarboxylic anhydrides may be those having a group which is not reactive with an amine or a dicarboxylic anhydride in the molecule.

Examples of the monoamines include aniline, o-toluidine, m-toluidine, p-toluidine, 2,3-xylidine, 2,4-xylidine, 2,5- Chloro-aniline, p-chloroaniline, o-nitroaniline, o-bromoaniline, m-bromoaniline, Aniline, o-nitroaniline, m-nitroaniline, p-nitroaniline, o-aminophenol, m-aminophenol, p-aminophenol, o-anilidine, m- Aminobenzonitrile, p-aminobenzonitrile, 2-aminobenzonitrile, p-aminobenzonitrile, p-aminobenzonitrile, Aminophenoxyphenyl ether, 4-aminophenolphenyl ether, 2-aminobenzophenone, 3-aminobenzophenone, 4-aminobenzophenone, - aminobenzophenone, 2-aminophene Aminophenolphenylsulfone, 4-aminophenolphenylsulfone,? -Naphthylamine,? -Naphthylphenylsulfone, 3-aminophenolphenylsulfone, Amino-1-naphthol, 5-amino-1-naphthol, 5-amino-1-naphthol, Naphthol, 7-amino-2-naphthol, 8-amino-2-naphthol, 1-aminoanthracene, 2-aminoanthracene and 9-aminoanthracene. These monoamines may have a group which is not reactive with an amine or a dicarboxylic anhydride in the molecule.

Examples of the isocyanate include monoisocyanate compounds such as phenyl isocyanate and naphthyl isocyanate.

As a method for further encapsulating the aromatic diamine compound and the tetracarboxylic acid dianhydride and the terminal of the obtained polyimide by using the terminal sealing agent, the tetracarboxylic acid dianhydride and the diamine are reacted and then the terminal sealing agent is added to continue the reaction A method in which a diamine adduct is reacted with a dicarboxylic anhydride end capping agent followed by addition of a tetracarboxylic acid dianhydride and the reaction is further continued, a method in which a monoamine end-capping agent is added to a tetracarboxylic acid dianhydride, A method in which the reaction is further continued, a method in which a tetracarboxylic acid dianhydride, a diamine, and the terminal sealing agent are simultaneously added and reacted.

The end-capping agent may be added in an amount of 20 parts by weight or less, preferably 1 to 10 parts by weight, more preferably 1 to 5 parts by weight based on 100 parts by weight of the tetracarboxylic dianhydride and the diamine.

The production of the polyamic acid through the polymerization of the acid dianhydride and the diamine can be carried out according to a conventional method for producing a polyamic acid such as solution polymerization. Specifically, the diamine can be prepared by dissolving the diamine in an organic solvent, and then adding an acid anhydride to the resultant mixed solution to effect polymerization reaction. The reaction may be carried out under anhydrous conditions, and the temperature during the polymerization may be 25 to 50 ° C, preferably 40 to 45 ° C. Specific examples of the organic solvent include ketones such as methyl ethyl ketone and cyclohexanone; Aromatic hydrocarbons such as toluene, xylene, and tetramethylbenzene; Ethylene glycol monoethyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether , Glycol ethers (cellosolve) such as dipropylene glycol diethyl ether and triethylene glycol monoethyl ether; Examples of the solvent include ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, dipropylene glycol monomethyl ether acetate, ethanol, propanol, ethylene glycol, propylene glycol, N-dimethylacetamide (DMAc), N, N-diethylacetamide, dimethylformamide (DMF), diethylformamide (DEF) , N-ethylpyrrolidone (NEP), 1,3-dimethyl-2-imidazolidinone, N, N-dimethylmethoxyacetamide, dimethylsulfoxide, pyridine, dimethylsulfone, hexamethylphosphoramide, tetra (2-methoxyethyl) ether, 1,2-bis (2-methoxyethyl) ether, 1,2-dimethoxyethane, Methoxyethoxy) ethane, bis [2- (2-methoxyethoxy)] ether and Mixture to be used is selected from the group consisting of.

The mixed solution of the polyamic acid and the organic solvent may further contain a metal ion such as Mg, Al, Zr, Zn, Ru, Cs, Cu, Fe, Cr, Ti, or La, May be contained at a concentration of about 30 parts by weight to 60 parts by weight relative to 100 parts by weight.

The metal ion may be doped with an electron donating pair of a nitrogen atom and a metal ion in the structure of the polyimide having an amine structure to improve the electron conductivity and improve the conductivity of the lithium ion.

According to one embodiment, the thickness of the coating layer formed by the above method may be nano-sized, and the nanofilm thus formed is covered with the entire surface of the cathode active material in the form of a thin film. As a result, the surface-coated cathode active material can further improve not only the normal voltage condition but also the life characteristic and conductivity at high temperature and high voltage conditions.

In the surface-coated cathode active material according to an embodiment of the present invention, the thickness of the nanocapsule may be 1 nm to 200 nm, preferably 5 nm to 50 nm. When the thickness of the nanocapsules is less than 1 nm, the effect of preventing the side reactions between the positive electrode active material and the electrolyte due to the nanocapsules may be insufficient. If the thickness of the nanocapsules is more than 200 nm, the thickness of the nanocapsules may excessively increase, resulting in an obstacle to the mobility of the lithium ions, thereby increasing the resistance.

According to an embodiment of the present invention, the cathode active material may be applied to a general voltage or a high voltage, and may be used without limitation as long as it is a compound capable of reversibly intercalating / deintercalating lithium.

Specifically, the surface-coated cathode active material according to one embodiment of the present invention includes a lithium-transition metal oxide of spinel having a cubic system layered rock salt structure, an olivine structure, a cubic structure having high capacity characteristics, V ₂ O ₅ , TiS , MoS, or a composite oxide of two or more of them.

More specifically, the cathode active material has the following structural formulas 1 to 3

Oxide, and any one selected from the group consisting of V ₂ O ₅ , TiS, and MoS, or a mixture of two or more thereof.

<Structure 1>

Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂ (-0.5? X? 0.6, 0? A, b, c? 1, x + a + b + c = 1);

&Lt; Formula 2 >

LiMn _{2 - x} M _x O ₄ where M is selected from the group consisting of Ni, Co, Fe, P, S, Zr, Ti and Al

One or more elements, 0 &lt; = x &lt; = 2);

<Formula 3>

_{Li 1 + a Fe 1 - x} M x (PO 4 -b) X b (M is Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce,

In, Zn and Y, X is at least one element selected from the group consisting of F, S and N, and -0.5? A? + 0.5, 0? X? 0.5, 0? b? 0.1)

More specifically, the cathode active material may be LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li [Ni _a Co _b Mn _c ] O ₂ (0 <a, b, c ≤ 1, a + b + c = 1) and LiFePO ₄ , or a mixture of two or more thereof.

The dispersion of the cathode active material is preferably dispersed in the mixed solution for 1 hour or more using a high-speed stirrer after the cathode active material is uniformly dispersed in the mixed solution. When the solvent is removed by heating and concentration after confirming uniform dispersion of the cathode active material, a cathode active material coated with a coating film containing polyamic acid can be obtained.

In the method of manufacturing a surface-coated cathode active material according to an embodiment of the present invention, a polyamic acid having a coating formed on the surface of the cathode active material may be converted into polyimide through an imidation reaction.

The method may further include the step of doping the electron donating pairs existing in the polyimide chain formed on the surface of the cathode active material with the metal ions to improve the electron conductivity.

The imidization reaction is carried out by raising the temperature of the cathode active material containing the coating obtained in the step ii) to about 300 ° C to 400 ° C at an interval of 50 to 100 ° C at a rate of 3 ° C / For 10 minutes to 120 minutes. After the temperature is raised at an interval of 50 to 100 캜, the temperature may be maintained for 10 to 120 minutes, for example. More specifically, the positive electrode active material containing the coating was heated at 60 ° C, 120 ° C, 200 ° C, 300 ° C and 400 ° C at a rate of 3 ° C / min, The imidization reaction can be allowed to proceed by maintaining at 200 캜 for 60 minutes, 300 캜 for 60 minutes, and 400 캜 for 10 minutes.

The present invention can improve the lifetime characteristics in both the normal voltage and high voltage regions by suppressing the direct reaction between the cathode material and the electrolyte by forming the coating layer on the surface of the cathode material with the nano coating on the surface of the polyimide having excellent conductivity and oxidation- In particular, by effectively suppressing the interfacial reaction between the electrolyte and the anode, such as deterioration of the cathode material due to the phase transition of the cathode material due to the elution of metal ions, which may occur under high temperature and high pressure conditions, the lifetime characteristics and the C- Can be improved. For example, the initial discharge capacity may be 190 mAh / g or more, the capacity retention ratio at 30 cycles or more may be 70% or more, preferably 75%, more preferably 80% or more, 60% or more, preferably 65% or more, more preferably 70% or more.

As used herein, the term " normal voltage " means the case where the charging voltage of the lithium secondary battery is in the range of 3.0 V to less than 4.2 V, and the term " high voltage " means that the charging voltage is in the range of 4.2 V to 5.0 V , And the term " high temperature " may mean in the range of 45 ° C to 65 ° C.

The present invention also provides a positive electrode comprising the surface-coated positive electrode active material.

The anode may be prepared by a conventional method known in the art.

For example, a slurry is prepared by mixing and stirring a solvent, a binder, a conductive agent, and a dispersant, if necessary, on the surface-coated cathode active material, coating the same on a current collector of a metal material, Can be prepared.

The current collector of the metal material is a metal having high conductivity and can be easily adhered to the slurry of the cathode active material, and any material can be used as long as it is not reactive in the voltage range of the battery. Non-limiting examples of the positive electrode current collector include foil produced by aluminum, nickel, or a combination thereof.

Examples of the solvent for forming the positive electrode include organic solvents such as NMP (N-methylpyrrolidone), DMF (dimethylformamide), acetone, and dimethylacetamide, and water. These solvents may be used alone or in combination of two or more Can be mixed and used. The amount of the solvent used is sufficient to dissolve and disperse the cathode active material, the binder and the conductive agent in consideration of the coating thickness of the slurry and the production yield.

Examples of the binder include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) Various kinds of binder polymers such as sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid and polymers substituted with Li, Na or Ca, or various copolymers thereof are used .

The conductive agent is not particularly limited as long as it has electrical conductivity without causing any chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon black such as carbon black, acetylene black, Ketjen black, channel black, panes black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber;

Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The dispersing agent may be an aqueous dispersing agent or an organic dispersing agent such as N-methyl-2-pyrrolidone.

The present invention also provides a secondary battery including the positive electrode, the negative electrode, and the separator interposed between the positive electrode and the negative electrode.

As the negative electrode active material used for the negative electrode according to an embodiment of the present invention, a carbon material, lithium metal, silicon, or tin which lithium ions can be occluded and released can be used. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high-temperature sintered carbon such as mesophase pitch based carbonfiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch derived cokes.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m.

Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used.

In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

As the binder and the conductive agent used for the cathode, those which can be commonly used in the art can be used as the anode. The negative electrode may be prepared by mixing and stirring the negative electrode active material and the additives, and then applying the negative electrode active material slurry to the current collector and compressing the negative electrode active material slurry.

As the separator, a conventional porous polymer film conventionally used as a separator, for example, a polyolefin such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer A porous polymer film made of a high molecular weight polymer may be used alone or in a laminated manner, or a nonwoven fabric made of a conventional porous nonwoven fabric such as a glass fiber having a high melting point, a polyethylene terephthalate fiber or the like may be used. It is not.

The lithium salt that can be used as the electrolyte used in the present invention may be any of those conventionally used in an electrolyte for a lithium secondary battery. For example, the anion of the lithium salt may include F ^- , Cl ^- , Br ^- , I ^{- _{NO 3 -, N (CN)}} 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF 2 -, ^{_{(CF 3) 5 PF -,}} (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 SO 2) 2 N -, (FSO 2) 2 N -, CF 3 _{CF 2 (CF 3) 2 CO} -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3 -, CF ₃ CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^-, and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

Examples of the electrolyte used in the present invention include an organic-based liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the production of a lithium secondary battery. no.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

The lithium secondary battery according to the present invention can be used not only in a battery cell used as a power source of a small device but also as a unit cell in a middle- or large-sized battery module including a plurality of battery cells.

Preferable examples of the above medium and large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and electric power storage systems.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Manufacturing example  One: Polyamic acid  Precursor ( PMDA - DDA )

4.704 g (23.606 mmol) of 4,4'-diaminodiphenylamine (DDA) was dissolved in 40 g of dimethylacetamide (DMAc) as a reaction solvent while nitrogen gas was slowly passed through a 250-mL reactor equipped with a stirrer and a nitrogen- After dissolving, 4.737 g (21.717 mmol) of pyromellitic dianhydride (PMDA) and 0.559 g (3.777 mmol) of phthalic anhydride (PA), which is an end capper, were added while nitrogen gas was passed through and 50 g . And polymerized at 0 DEG C for 12 hours to obtain 10 wt% of polyamic acid (PMDA + DDA). The intrinsic viscosity of the polyamic acid (PMDA + DDA) was found to be 1.87 dL / g at a concentration of 0.5 dL / g in DMAc solution at 30 ° C. The weight average molecular weight (Mw) was 55,000 g / mol.

Manufacturing example  2: Polyamic acid  Precursor ( PMDA -PDA)

3.252 g (30.074 mmol) of paraphenylenediamine (PDA) was dissolved in 40 g of dimethyl acetamide (DMAc) as a reaction solvent while nitrogen gas was slowly passed through a 250-mL reactor equipped with a stirrer and a nitrogen introducing apparatus. , 6.035 g (27.668 mmol) of pyromellitic dianhydride (PMDA) and 0.559 g (3.777 mmol) of phthalic anhydride (PA) were added to the flask and 50 g of the remaining solvent was further added thereto. And polymerized at 0 DEG C for 12 hours to obtain 10 wt% of polyamic acid (BPDA-PDA). The intrinsic viscosity of the polyamic acid (BPDA-PDA) was found to be 1.87 dL / g at a concentration of 0.5 dL / g in DMAc solution at 30 ° C. The weight average molecular weight (Mw) was 67,000 g / mol.

&Lt; Preparation of surface-coated cathode active material >

Example 1

The polyamic acid prepared in Preparation Example 1 and the organic solvent were diluted with dimethylacetamide to a concentration of 0.5% by weight of polyamic acid to prepare a mixed solution.

20 g of LiCoO ₂ particles were charged into the mixed solution as a cathode active material and stirred for 1 hour using a high-speed stirrer. The temperature was raised to the boiling point of the solvent while stirring was continued to evaporate the solvent, thereby preparing a cathode active material coated with a polyamic acid coating on the surface.

The positive electrode active material coated with the polyamic acid-coated surface was heated at 60 ° C, 120 ° C, 200 ° C, 300 ° C and 400 ° C at a rate of 3 ° C / min, Min at 200 캜 for 60 minutes, at 300 캜 for 60 minutes, and at 400 캜 for 10 minutes to carry out the imidization reaction.

Upon completion of the imidization reaction, a LiCoO ₂ cathode active material coated with a nanocapsule containing polyimide (PMDA-DDA) was prepared. FIG. 1 shows SEM images of the surface states of the cathode active material before coating (a) and after coating (b).

Comparative Example 1

The polyamic acid prepared in Preparation Example 2 and the organic solvent were diluted with dimethylacetamide to a concentration of 0.5 wt% of polyamic acid to prepare a mixed solution.

20 g of LiCoO ₂ particles were charged into the mixed solution as a cathode active material and stirred for 1 hour using a high-speed stirrer. The temperature was raised to the boiling point of the solvent while stirring was continued to evaporate the solvent, thereby preparing a cathode active material coated with a polyamic acid coating on the surface.

The positive electrode active material coated with the polyamic acid-coated surface was heated at 60 ° C, 120 ° C, 200 ° C, 300 ° C and 400 ° C at a rate of 3 ° C / min, Min at 200 캜 for 60 minutes, at 300 캜 for 60 minutes, and at 400 캜 for 10 minutes to carry out the imidization reaction.

Upon completion of the imidation reaction (PMDA-PDA), a LiCoO ₂ cathode active material coated with a nano-coating containing polyimide was prepared.

Comparative Example 2

Pure LiCoO ₂ cathode active material containing no polyamic acid was used.

&Lt; Production of lithium secondary battery >

Anode manufacturing

The surface-coated LiCoO ₂ cathode active material prepared in Example 1 and Comparative Example 1 was used.

The positive electrode active material, carbon black as a conductive agent and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 95: 3: 2 and added to a solvent of N-methyl-2-pyrrolidone (NMP) To prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied to an aluminum (Al) thin film having a thickness of about 20 탆 and dried at 130 캜 for 2 hours to prepare a positive electrode, followed by roll pressing to produce a positive electrode Respectively.

Cathode manufacture

Lithium metal foil was used as the cathode.

Electrolytic solution manufacturing

LiPF ₆ was added to a nonaqueous electrolyte solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as electrolytes in a volume ratio of 1: 2 to prepare a 1M LiPF ₆ nonaqueous electrolyte solution.

Lithium secondary battery manufacturing

After the thus prepared positive electrode and negative electrode were fabricated by using a polyethylene separator (Dornensa, F2OBHE, thickness = 20 占 퐉) and a mixed separator of an electrolytic solution and a polypropylene interposed therebetween by a conventional method, Aqueous electrolyte was injected to prepare a coin cell type lithium secondary battery.

< Experimental Example  One : Charging and discharging  Capacity assessment>

The lithium secondary batteries (battery capacity: 4.3 mAh) of Example 1 and Comparative Example 1 were charged at 0.5 C in a voltage range of 3 to 4.5 V at 45 캜 and discharged at 1 C to perform charge and discharge.

< Experimental Example  2: Evaluation of C-rate efficiency characteristic @ 45 ℃>

The C-rate efficiency of the lithium secondary battery (battery capacity: 4.3 mAh) of Example 1 and Comparative Example 1 was measured in a high-temperature environment (45 ° C). The C-rate efficiency can be defined as a ratio of the capacity when the battery is charged to 0.5 C to the capacity when the battery is discharged to 0.1 C and the capacity when the battery is discharged to 2 C, as shown in Equation (1).

[Equation 1]

C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] * 100

The evaluation values of the charge-discharge capacity and the C-rate efficiency characteristic are shown in Table 1 below.

< Experimental Example  3: Evaluation of high temperature lifetime characteristics>

The cycle life of the lithium secondary batteries of Example 1 and Comparative Example 1 (battery capacity: 4.3 mAh) was measured under 0.5 C charging and 1 C discharging conditions. The capacity retention rate according to the number of cycles is shown in FIG.

1 ^st Charging Capacity
(mAh / g) 1 ^st
Discharge capacity
(mAh / g) 1 ^st
Charge / discharge efficiency
(%) C-rate
(%) Capacity retention rate (%)
30 ^th cycle Capacity retention rate (%)
50 ^th cycle Example 1 196.8 192.2 98.8 98.9 81 75 Comparative Example 1 194.9 192.9 98.7 98.6 63 49 Comparative Example 2 199.2 194.5 97.5 97.2 59 45

From the above results, it can be seen that the lithium secondary battery according to the present invention has an excellent initial charge / discharge efficiency of 98% or more, an initial discharge capacity of 190 mAh / g or more, a capacity retention rate of 30 cycles / Able to know.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, those skilled in the art will appreciate that such specific embodiments are merely preferred embodiments and that the scope of the present invention is not limited thereby. something to do. It is therefore intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims (23)

A positive electrode active material coating material comprising a polyimide having a repeating structure represented by the following formula (1)
[Chemical Formula 1]

In the above formula,
X is a tetravalent organic group derived from tetracarboxylic acid,
Y is a divalent organic group derived from a diamine of the following formula (2)
n is an integer of 1 or more,
(2)

In the above formula,
R1 is a monovalent organic group containing hydrogen, a substituted or unsubstituted aromatic ring,
Q and Z are the same or different and are independently a divalent organic group containing a substituted or unsubstituted aromatic ring,
a and b are an integer of 1 or more. The method according to claim 1,
Wherein R &lt; _{1 &gt;} is a monovalent organic group selected from a hydrogen or a substituted or unsubstituted aryl group having 6 to 24 carbon atoms or a substituted or unsubstituted aromatic ring or a secondary amine or a tertiary amine,
Wherein Q and Z are a divalent organic group selected from a substituted or unsubstituted secondary amine or a tertiary amine containing an arylene group or an aromatic ring having 6 to 24 carbon atoms. The method according to claim 1,
Wherein R &lt; _{1 &gt;} is hydrogen or a monovalent organic group represented by the following general formulas (3) to (5)
(3)

[Chemical Formula 4]

[Chemical Formula 5]

Wherein R ₂ to R ₅ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by the following formula (6)
[Chemical Formula 6]

Each of R ₆ and R ₇ is independently hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms,
And Q and Z are each independently selected from divalent organic groups represented by the following general formulas (7) to (9):
(7)

[Chemical Formula 8]

[Chemical Formula 9]

R ₁₁ to R ₁₄ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by the above formula (6). The method according to claim 1,
Wherein the diamine of formula (2) is selected from compounds of the following formulas (10a) to (10d): &lt; EMI ID =
[Chemical Formula 10a]

[Chemical Formula 10b]

[Chemical Formula 10c]

[Chemical Formula 10d]

In the above formula,
Each of R ₂₁ to R ₂₇ independently represents hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms. The method according to claim 1,
Wherein X in the formula (1) is a polyimide selected from tetravalent organic materials having 6 to 24 carbon atoms and aromatic groups having 3 to 24 carbon atoms. A cathode active material coated with the coating of any one of claims 1 to 5. Preparing a mixed solution of a polyamic acid having a repeating structure represented by the following formula (12) and an organic solvent;
[Chemical Formula 12]

In the above formula,
X is a tetravalent organic group derived from tetracarboxylic acid,
Y is a divalent organic group derived from a diamine of the following formula (2)
n is an integer of 1 or more,
(2)

In the above formula,
R ₁ is a monovalent organic group containing hydrogen, a substituted or unsubstituted aromatic ring,
Q and Z are the same or different and are independently a divalent organic group containing a substituted or unsubstituted aromatic ring,
a and b are integers of 1 or more,
Forming a coating film containing the polyamic acid on the surface of the cathode active material by dispersing the cathode active material in the mixed solution; And
Forming a polyimide coating layer by imidizing a polyamic acid contained in the coating film formed on the surface of the cathode active material
Wherein the positive electrode active material is a lithium salt. 8. The method of claim 7,
Wherein R &lt; _{1 &gt;} is a hydrogen or a substituted or unsubstituted C6-C24 aryl group or a secondary amine or a tertiary amine containing a substituted or unsubstituted aromatic ring,
Wherein Q and Z are a substituted or unsubstituted secondary amine or tertiary amine containing an arylene group or an aromatic ring having 6 to 24 carbon atoms. 8. The method of claim 7,
Wherein R &lt; _{1 &gt;} is hydrogen or a monovalent organic group represented by the following general formulas (3) to (5)
(3)

[Chemical Formula 4]

[Chemical Formula 5]

R ₂ and R ₅ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by the following formula (6)
[Chemical Formula 6]

Each of R ₆ and R ₇ is independently hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms or an alkoxy group having 1 to 6 carbon atoms,
And Q and Z are each independently selected from divalent organic groups represented by the following general formulas (7) to (9):
(7)

[Chemical Formula 8]

[Chemical Formula 9]

R ₁₁ and R ₁₄ are hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or a monovalent organic group represented by Formula 6. 8. The method of claim 7,
Wherein the diamine of the formula (2) is selected from the compounds of the following formulas (10a) to (10d):
[Chemical Formula 10a]

[Chemical Formula 10b]

[Chemical Formula 10c]

[Chemical Formula 10d]

In the above formula,
Each of R ₂₁ to R ₂₇ independently represents hydrogen, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, or an alkoxy group having 1 to 6 carbon atoms. 8. The method of claim 7,
Wherein X is a tetravalent organic group having 6 to 24 carbon atoms, aromatic, cycloalkane having 3 to 24 carbon atoms, or cycloalkene having 4 to 24 carbon atoms. 8. The method of claim 7,
Wherein the thickness of the coating layer is 1 nm to 200 nm. 8. The method of claim 7,
Wherein the cathode active material is a lithium transition metal oxide of spinel having a multilayer rock salt structure, an olivine structure, and a cubic structure, and a composite oxide of any one selected from the group consisting of V ₂ O ₅ , TiS, and MoS, Lt; / RTI &gt; 8. The method of claim 7,
Wherein the cathode active material is any one selected from the group consisting of oxides of the structural formulas 1 to 3 and V ₂ O ₅ , TiS, and MoS, or a mixture of two or more thereof,
<Structure 1>
Li _{1 + x} [Ni _a Co _b Mn _c ] O ₂ (-0.5? X? 0.6, 0? A, b, c? 1, x + a + b + c = 1);
&Lt; Formula 2 >
LiMn _{2 - x} M _x O ₄ (M is at least one element selected from the group consisting of Ni, Co, Fe, P, S, Zr, Ti and Al, 0? X? 2);
<Formula 3>
_{Li 1 + a Fe 1 - x} M x (PO 4 -b) X b (M is Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn , and Y X is at least one element selected from the group consisting of F, S and N, and -0.5? A? + 0.5, 0? X? 0.5, and 0? B? . 8. The method of claim 7,
Wherein the cathode active material is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li [Ni _a Co _b Mn _c ] O ₂ (0 <a, b, c ≤ 1, a + b + LiFePO ₄ , or a mixture of two or more thereof. 8. The method of claim 7,
Wherein the mixed solvent in which the polyamic acid and the organic solvent are mixed further comprises at least one metal ion selected from the group consisting of Mg, Al, Zr, Zn, Ru, Cs, Cu, Fe, Cr, A method of coating an active material. 8. The method of claim 7,
Wherein the polyimide coating layer is formed by imidizing the coated cathode active material containing the polyamic acid at a temperature of 300 ° C to 400 ° C. 18. A cathode active material coated by the method of any one of claims 7 to 17. A lithium secondary battery produced by using the cathode active material of claim 18. 20. The method of claim 19,
A lithium secondary battery having a C-rate efficiency of 90% or more represented by the following formula:
[Equation 1]
C-rate (%) = [(2C discharge capacity) / (0.1C discharge capacity)] x100. 20. The method of claim 19,
A lithium secondary battery having an initial charge / discharge efficiency of 98% or more. 20. The method of claim 19,
Wherein the initial discharge capacity is 190 mAh / g or more, and the capacity retention ratio at 30 cycles or more is 70% or more. 20. The method of claim 19,
Wherein the initial discharge capacity is 190 mAh / g or more, and the capacity retention ratio at 50 cycles or more is 60% or more.

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