KR101409837B1 - Poly-dopamine assisted surface coating methode of cathode powder for lithium rechargeable batteries - Google Patents

Abstract

Disclosed is a method for surface modification of a cathode active material for a lithium secondary battery, which comprises coating a nano particle for a lithium secondary battery or a cathode active material powder having an irregular shape with a stable material to improve electrochemical characteristics, thermal stability and the like. (A) dissolving dopamine in a buffer solution; (b) adding a positive electrode active material to the buffer solution in which the dopamine is dissolved and reacting to form a poly dopamine thin film on the surface of the positive electrode active material; (c) adding a cathode active material having the polypodamine thin film formed thereon to a coating solution containing a coating precursor material containing a metal element, and reacting the ionic group or hydrate of the coating precursor with the cathode active material To the surface; And (d) heat treating the reactant in the step (c) to remove the polypodamine thin film and form a coating layer containing a compound containing the metal element on the surface of the cathode active material. A method for surface modification of an active material is provided.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for surface modification of a cathode active material for lithium secondary batteries using polydodamine. BACKGROUND ART < RTI ID = 0.0 > [0002]

The present invention relates to a cathode active material for a lithium secondary battery, and more particularly, to a method for improving a surface of a cathode active material for a lithium secondary battery for improving electrochemical characteristics, thermal stability and the like.

The present invention is derived from the research carried out with the support of the Nano-based Information & Energy Business Division-New Technology Fusion Growth Power Project, funded by the government (Ministry of Education, Science and Technology)

[Assignment number: 2012K001263, Research title: Lithium-rich high-capacity anode material development]

Lithium rechargeable batteries, commercialized by Sony in 1992, have rapidly grown to become a core component of personal electronic devices such as mobile phones, PDAs, and notebook computers. BACKGROUND ART In recent years, automotive rechargeable batteries for use in hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), electric vehicles (EV) a smart grid, and the like, have been researched for commercialization of a secondary battery suitable for a newly created customer.

The cathode material, which is one of the core materials of the secondary battery, is an important component that determines the core performance such as the capacity and cycle characteristics of the lithium secondary battery. Various researches have been carried out to improve the electrochemical properties of the cathode material. The surface modification method is one of the examples of commercialization since its utility is recognized.

An important factor that deteriorates the cycle characteristics, high rate characteristics, and thermal stability of the lithium secondary battery is the reaction between the electrode interface and the electrolyte. The electrolyte reacts with moisture to form acid ions, which react with the surface of the cathode active material to elute the transition metal and form an undesired interface layer, resulting in deterioration of electrochemical characteristics. Such as _{Al 2 O 3, ZrO 2,} La 2 O 3 with high stability in order to prevent this, oxide, AlPO _4, coated nanoparticles, such as a fluoride of phosphate, such as LiCoPO _4, such as AlF _3, LaF ₃ in the positive electrode surface Thereby improving the effect of preventing the direct contact between the anode and the electrolyte, reducing the unnecessary reaction at the interface, and reducing the elution of the transition metal. (HJ Lee et al., J. Power Sources, vol.195, p6122, 2010; J. Liu et al., J. Electrochem. , Vol.156, A66, 2009; ST Myung et al., Mater., Vol.17, p3695, 2005; GTK Fey et al., J. Power Sources, vol.146, p65; SH Kang et al., Electrochem. JM Zheng et al., Electrochemical. Soc., Vol.155, A775, 2008, SH, Vol. 42, p. Yun et al., J. Power Sources, vol.195, p6108, 2010).

The surface modification method through such a coating needs to control the method of efficiently improving the characteristics of the anode, and the coating conditions. If the coating on the surface is non-uniform or too thick, the coating material itself acts as an element that interferes with lithium ion and electron movement, and precise control over thickness and coating improvement is needed. Until now, the surface coating layer has been formed by mixing the coating material with the cathode active material in the state of dissolving the coating material in the solution, drying and heat-treating. Recently, a method of forming a surface coating layer by a mechanical milling method and a heat treatment process without using a liquid phase has been used. However, the conventional methods can be applied to a cathode material having a particle size of several micrometers or more, and it is very difficult to apply when the size of the cathode powder is reduced to nanoparticles.

Up to now, efforts have been made to improve the high-rate characteristics by introducing a nanoparticle-sized cathode material. The nanoparticle size anode material has the advantage that it can react with lithium ion at a high speed because the distance of lithium and electrons to travel is short and the contact area with the electrolyte is wide. However, the wide contact area means the area of reaction with the electrolyte to such an extent that the elution of the transition metal and the unwanted interfacial reaction with the electrolyte are activated, resulting in deterioration of cycle characteristics and the like.

As a solution to this problem, the surface coating method can be very effective. However, it is very difficult to uniformly coat the surface of the nanoparticles with an oxide or the like. Since the particle size of the oxide to be coated and the particle size of the cathode active material do not differ much, uneven coating tends to occur. In addition, in the case of a one-dimensional shape such as a nanorod or a nanofiber, Uniform coatings are very difficult due to the imbalance of surface energy. Therefore, the surface modification through the coating of the nanoparticle anode material has hardly been attempted yet.

The present invention aims to improve the electrochemical characteristics, thermal stability, and the like by coating a nano particle for a lithium secondary battery or a non-uniform active material powder with a stable material.

As a means for solving the above-mentioned technical problems, the present invention provides a method for preparing a compound of formula (I), comprising: (a) dissolving dopamine in a buffer solution; (b) adding a positive electrode active material to the buffer solution in which the dopamine is dissolved and reacting to form a poly dopamine thin film on the surface of the positive electrode active material; (c) adding a cathode active material having the polypodamine thin film formed thereon to a coating solution containing a coating precursor material containing a metal element, and reacting the ionic group or hydrate of the coating precursor with the cathode active material To the surface; And (d) heat treating the reactant in the step (c) to remove the polypodamine thin film and form a coating layer containing a compound containing the metal element on the surface of the cathode active material. A method for surface modification of an active material is provided.

Also, the cathode active material has a size (based on an average diameter) of 50 to 1000 nm. The present invention also provides a method for surface modification of a cathode active material for a lithium secondary battery.

Also, the cathode active material is any one selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 14:

[Chemical Formula 1]

Li _x Mn _{1 - y} M _y A ₂

(2)

Li _x Mn _{1 - y} M _y O _{2 - z} B _z

(3)

Li _x Mn ₂ O _{4 - z} B _z

[Chemical Formula 4]

Li _x Mn _{2 - y} M _y A ₄

[Chemical Formula 5]

Li _x Mn _{1- y- z} Ni _y M _{z a a}

[Chemical Formula 6]

Li _x Mn _{1 - y - z} Ni _y M _z O _{2 - a} B _a

(7)

Li _x Co _{1 - y} M _y A ₂

[Chemical Formula 8]

Li _x CoO _{2 - z} B _z

[Chemical Formula 9]

Li _x Ni _{1 - y} M _y A ₂

[Chemical formula 10]

Li _x Ni _{1 - y} M _y O _{2 - z} B _z

(11)

Li _x Ni _{1- y z} Co _y M _{z a a}

[Chemical Formula 12]

Li _x Ni _{1 - y - z} Co _y M _z O _{2 - a} B _a

[Chemical Formula 13]

Li _x Ni _{1- y- z} Mn _y M _{z a a}

[Chemical Formula 14]

Li _x Ni _{1- y- z} Mn _y M _z O _{2 - a} B _a

Mg, Al, Co, K, Na, Ca, Si, Ti, Sn (where 0 & Wherein A is any one selected from the group consisting of O, F, S, and P, and is selected from the group consisting of V, Ge, Ga, B, As, Zr, Mn, Cr, Ni, And B is any one selected from the group consisting of F, S and P.)

Also, the reaction of step (b) is performed for 1 to 100 minutes so that the poly-dopamine thin film is formed to a thickness of 1 to 100 nm. The present invention also provides a method for surface modification of a cathode active material for a lithium secondary battery.

The metal element may be at least one element selected from the group consisting of Li, Na, K, Mg, Ca, Cu, Cd, Fe, Mn, Ni, Pb, Sn, Sr, Xe, Zn, Al, B, Bi, Wherein the metal element is at least one selected from the group consisting of Nd, V, Y, Ge, Si, Sn, Nb, Ti, Zr, Sb, Ta, Wherein the positive electrode active material is any one selected from the group consisting of phosphorus compounds and metal fluorides.

Also, the present invention provides a method for modifying the surface of a cathode active material for a lithium secondary battery, wherein the reaction of step (c) is performed at 30 to 70 ° C.

Also, the heat treatment in the step (d) may be performed at 200 to 700 ° C for 1 to 10 hours to form the coating layer with a thickness of 1 to 100 nm. The present invention also provides a method for surface modification of a cathode active material for a lithium secondary battery.

According to the present invention, it is possible to form a polydodamine preliminary coating layer on the surface of a cathode active material using the properties of polydodamine and form a surface coating layer of a uniform inorganic material by using the preliminary coating layer, thereby being very effective in coating various shapes of cathode active materials including spherical to be.

Also, it is possible to provide a positive electrode active material having improved electrochemical characteristics and high thermal stability by allowing the surface of a positive electrode active material having a nanoparticle size that can not be coated due to uneven surface energy and small particle size to be modified through coating .

1 is a schematic view for explaining a surface modification method of a cathode active material for a lithium secondary battery according to the present invention,
Fig. 2 is a TEM photograph of the surface of the cathode active material (c, d, e) after surface coating using the cathode active material (a, b) and the polydodamine thin film formation before surface coating in the embodiment of the present invention,
FIG. 3 is a photograph showing TEM-EDS analysis results of a cathode active material after surface coating using a polydopamine thin film according to an embodiment of the present invention,
4 is a graph comparing charge / discharge cycle characteristics of a cathode active material after surface coating using a cathode active material before surface coating and a polydodamine thin film formation in an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. Throughout the specification, when an element is referred to as "including " an element, it means that it can include other elements, not excluding other elements, unless specifically stated otherwise.

Conventionally, a method of coating a cathode material has been known. However, an effective surface modification method for uniformly coating a non-uniform cathode active material having a nanoparticle size or a non-spherical shape as a method of modifying a powder surface having a size of several micrometers or more It was not presented.

In the present invention, unlike a conventional method of combining a nanoparticle-sized cathode active material and a coating material, in order to uniformly and easily bond the surface of a nanoparticle with a coating material, a poly Dopamine layer is used.

Polydodamine is a substance found in the process of studying the property of mussel which can easily attach to irregular places such as rocks. The reason why mussels can be easily and firmly adhered to any place (irrespective of moisture or uneven shape) is that the organic substances (4-dihydroxy-L-phenylalanine (DOPA), Mytilus lysine peptides contained in edulis foot protein 5 (Mefp-5), and is known to be similar to the polymerisation process of dopamine (H. Lee et al., Nature, vol.448, p338, 2007; Vol.38, p426, 2007; SM Kang et al., Angew. Chem. Int. Ed., Vol.49, p9401, 2010; J. Ryu et al., Adv. Funct. ). Dopamine, which is an environmentally friendly and easily obtainable organic substance, spontaneously polymerizes under a buffer solution of pH 8.5 and polydopamine formed through this process is very reactive and can easily form a new bond on its surface.

In the present invention, a polypodamine pre-coating layer is formed on the surface of the nanoparticulate cathode material and a uniform coating layer of an inorganic material is formed by using the pre-coating layer on the surface of the nanoparticle cathode material to develop a cathode active material for a lithium secondary battery .

1 is a schematic view for explaining a surface modification method of a cathode active material for a lithium secondary battery according to the present invention.

Referring to FIG. 1, the method for surface modification of a cathode active material for a lithium secondary battery according to the present invention comprises the steps of: (a) dissolving dopamine in a buffer solution; (b) adding a positive electrode active material to the buffer solution in which the dopamine is dissolved and reacting to form a poly dopamine thin film on the surface of the positive electrode active material; (c) adding a cathode active material having the polypodamine thin film formed thereon to a coating solution containing a coating precursor material containing a metal element, and reacting the ionic group or hydrate of the coating precursor with the cathode active material To the surface; And (d) heat treating the reactant in the step (c) to remove the polypodamine thin film, and forming a coating layer containing a compound containing the metal element on the surface of the cathode active material.

In the present invention, a cathode active material in the form of powder is reacted in a buffer solution in which dopamine is dissolved to form a thin film of polypodamine on the surface of the cathode active material. Then, a coating precursor material, which can be a source of the coating material, and then reacted with a liquid formed in a solution or colloid form to induce a reaction with polydopamine having high reactivity, followed by heat treatment to form a composite of the cathode active material and the coating material. In the heat treatment process, dopamine is removed and only the coating material that remains on the surface of the cathode active material remains. At this time, the organic dopamine is removed by heat treatment at a temperature of about 400 ° C in the air.

Therefore, by using the surface modification method of the cathode active material for a lithium secondary battery according to the present invention, anodic oxide powder having a nanoparticle size, which has been difficult to coat, can be easily coated. Polydodamine can be formed on the surface of the cathode active material to a very thin thickness of several nanometers to several tens of nanometers because it is formed by spontaneous polymerization reaction in a buffer solution in which dopamine is dissolved. It acts as an efficient adhesive that reacts easily with metal ion hydrates. Therefore, it is possible to effectively coat nanoparticles such as nanorods and nanofibers having a one-dimensional structure, which were difficult to coat due to a small size of cathode active material or uneven surface energy, and to provide a nanoparticle-sized cathode active material Can exhibit higher thermal stability and improved electrochemical properties.

In the step (a) of the present invention, the buffer solution having a pH of about 8 to 9 and the alcohol such as methanol or ethanol are mixed at a molar ratio of about 1: 0.5 to 1: 1.5, At a concentration of about 5 to 15 mM, and thoroughly stirring.

In the present invention, the step (b) is a step of forming a poly dopamine thin film on the surface of the cathode active material, and is performed by reacting the cathode active material with the buffer solution in which the dopamine is dissolved.

The cathode active material can be applied to all kinds of cathode active materials that can be used in lithium secondary batteries at present and can be effectively applied to a cathode active material having a size of nanoparticles of 50 to 500 nm on the basis of an average diameter, Can be more easily coated using the method according to the present invention in the case of a large cathode active material. Also, it is an advantage of the present invention that the surface coating layer can be easily formed regardless of the size and shape of the cathode active material.

The positive electrode active material may be any one selected from the group consisting of compounds represented by the following general formulas (1) to (14).

[Chemical Formula 1]

Li _x Mn _{1 - y} M _y A ₂

(2)

Li _x Mn _{1 - y} M _y O _{2 - z} B _z

(3)

Li _x Mn ₂ O _{4 - z} B _z

[Chemical Formula 4]

Li _x Mn _{2 - y} M _y A ₄

[Chemical Formula 5]

Li _x Mn _{1- y- z} Ni _y M _{z a a}

[Chemical Formula 6]

Li _x Mn _{1 - y - z} Ni _y M _z O _{2 - a} B _a

(7)

Li _x Co _{1 - y} M _y A ₂

[Chemical Formula 8]

Li _x CoO _{2 - z} B _z

[Chemical Formula 9]

Li _x Ni _{1 - y} M _y A ₂

[Chemical formula 10]

Li _x Ni _{1 - y} M _y O _{2 - z} B _z

(11)

Li _x Ni _{1- y z} Co _y M _{z a a}

[Chemical Formula 12]

Li _x Ni _{1 - y - z} Co _y M _z O _{2 - a} B _a

[Chemical Formula 13]

Li _x Ni _{1- y- z} Mn _y M _{z a a}

[Chemical Formula 14]

Li _x Ni _{1- y- z} Mn _y M _z O _{2 - a} B _a

Mg, Al, Co, K, Na, Ca, Si, Ti, Sn (where 0 & Wherein A is any one selected from the group consisting of O, F, S, and P, and is selected from the group consisting of V, Ge, Ga, B, As, Zr, Mn, Cr, Ni, And B is any one selected from the group consisting of F, S and P.)

The cathode active material may be prepared in powder form through a simple combustion method using a sol-gel method. For example, a metal hydrate precursor may be used as a base material to form a solution using acetic acid and distilled water, and the solution may be heated to a gel state and then heat-treated to produce a powdery cathode active material.

The reaction of the cathode active material in the solution in which the dopamine is dissolved can be carried out at room temperature for several minutes to several hundreds of minutes, preferably for 1 to 100 minutes, more preferably for 5 to 50 minutes. The thickness of the pre-coated layer of polypodamine is determined according to the reaction time and the temperature during the reaction. If the thickness is too thick, the polypodamine layer may not be sufficiently removed during the heat treatment, but if the thickness is too thin, It is necessary to control the reaction time depending on the type, size, and shape of the cathode active material. At the end of the reaction, a polydodamine precoat layer is formed on the surface of the cathode active material in the form of a thin film, which acts as an adhesive promoting the reaction of the coating material with the source. The thickness of the pre-coating layer of polypodamine is preferably 10 to 1000 nm, more preferably 10 to 100 nm. After reacting the cathode active material, it may be dried several times using distilled water, ethanol, etc., and then dried at a temperature of 80 to 100 ° C. for 5 to 50 hours.

In the present invention, the step (c) is a step of adhering a source of a coating material, that is, a coating precursor to a surface of a cathode active material on which a polyphenolic thin film is formed, Is performed by reacting the prepared cathode active material with the prepared polypodamine thin film.

The thing which can be used as a coating material oxide, such as _{Al 2 O 3, ZrO 2,} La 2 O 3 is present in the coating material to an existing, AlPO _4, phosphate, such as LiCoPO _4, AlF _3, LaF _3, such as a fluoride of And so on. In a coating solution containing a coating precursor containing a metal element such as a solution state of these materials or a colloidally dispersed state in a solution, polydopamine is reacted with a cathode active material precoated with. When the reaction is completed, the ionic groups or hydrates of the precursor of the coating are attached to the surface of the cathode active material via the poly dopamine thin film layer.

As such a metal element, for example, Li, Na, K, Mg, Ca, Cu, Cd, Fe, Mn, Ni, Pb, Sn, Sr, Xe, Zn, Al, B, Bi, La, Nd, V, Y, Ge, Si, Sn, Nb, Ti, Zr, Sb, Ta, Mo, Re and W.

The coating solution may be prepared, for example, by adding a coating precursor containing a metal element to ethanol at a concentration of about 0.5 to 5 wt% to dissolve the coating precursor, and adding a small amount of diluted ammonia water to form a colloidal coating solution.

The reaction with the cathode active material in the coating solution may be carried out at 30 to 70 ° C, preferably at 40 to 60 ° C until all of the ethanol in the coating solution has evaporated. If the reaction temperature is too low, the reaction activity may be low and the reaction time may be long. If the reaction temperature is too high, the formation of the coating layer may be uneven in a subsequent heat treatment process. After completion of the reaction, the reaction product can be dried at a temperature of 80 to 100 ° C. for 5 to 50 hours.

In the step (d) of the present invention, the polypodamine thin film is removed and a coating layer containing a desired coating material, that is, a compound containing a metal element, is finely formed on the surface of the cathode active material. .

The heat treatment is preferably performed at a temperature of 200 to 700 ° C. and for 1 to 10 hours in consideration of the physical properties of the polydodamine to be removed and the formation of a uniform coating layer, and the annealing is performed at 300 to 500 ° C. for 2 to 5 hours More preferable. At this time, the thickness of the formed coating layer is 1 to 100 nm, preferably 5 to 30 nm, so that the coating material can be very finely dispersed on the surface of the positive electrode active material even when applied to the nanoparticulate active material powder. The compound containing the metal element constituting the coating layer formed by the heat treatment is in the form of a metal oxide, metal oxide, metal fluoride, or the like containing various metal elements described above.

Example

(1) Production of nanoparticle-sized cathode active material

Nanoparticles (Li [Ni _{0 .2} Li _{0 .2} Mn _{0 .6} ] O ₂ ) were prepared in powder form through a simple combustion method using a sol-gel method. A base material is _{manganese acetate tetrahydrate (Mn (CH 3} CO 2) 2 · 4H 2 O), nickel (Ⅱ) nitrate hexahydrate (Ni (NO 3) 2 · 6H 2 O), lithium nitrate (LiNO 3) and a lithium acetate dihydrate (CH ₃ CO ₂ Li · 2H ₂ O) was used. The solution was prepared using basic materials, acetic acid (CH ₃ COOH) and distilled water according to the composition ratio, and then heated to 80 to 90 ° C to form a gel state. Then, it was heat treated at about 400 ° C in a hot plate to form a powder of the same shape as ash while causing a vigorous combustion reaction. The reformed powders were grinded and then subjected to a primary heat treatment at 500 ° C for 4 hours, followed by a heat treatment at 800 ° C for 6 hours, followed by rapid cooling at room temperature to produce a cathode active material having a size of 100 to 500 nm.

(2) Preparation of cathode active material having polydopamine thin film

First, a mixture was prepared by mixing buffer solution (pH 8.5) and methanol at a molar ratio of 1: 1 to dissolve 10 mM of dopamine. Then, the prepared cathode active material was added to a mixture of a buffer solution in which dopamine was dissolved and methanol and reacted to form a poly dopamine thin film on the surface of the cathode active material. At this time, the reaction was carried out for 30 minutes. After completion of the reaction, the reaction was repeated several times, followed by drying at 90 ° C for about 24 hours.

(3) Production of cathode active material coated with metal oxide

A coating solution in which a coating material is dissolved in a colloidal form is prepared in order to react the coating material with the cathode active material having the polypodamine thin film formed thereon. The coating solution was prepared by adding 1 wt% of zirconium (IV) oxynitrate hydrate (N ₂ O ₇ Zr xH ₂ O) to ethanol (99.9%), dissolving at room temperature for 1 hour, diluting with distilled water, Was prepared in colloidal form. Then, a cathode active material having a thin film of polypodamine was added to the coating solution in which ZrO ₂ was dissolved, and the reaction was continued at 50 ° C. until all of the ethanol solution evaporated, followed by drying at 90 ° C. for 24 hours. Finally, the reaction product was heat-treated at 400 ° C for 3 hours to remove the polypodamine thin film, and ZrO ₂ was finely dispersed on the surface of the cathode active material to form a coating layer. The nanoparticle-sized cathode active material Li [Ni _{0 .2} Li _{0 . 2} Mn _{0 .6} ] O ₂ ).

2 is a surface TEM photograph of the cathode active material (c, d, e) after surface coating using the cathode active material (a, b) and the polydodamine thin film formation before surface coating in the embodiment of the present invention. Referring to FIG. 2, it can be seen that the cathode active material having no coating layer has a smooth surface, while the cathode active material having a ZrO ₂ coating layer is uniformly coated with fine particles having a size of several nanometers.

3 is a photograph showing TEM-EDS analysis results of a cathode active material after surface coating using a polydodamine thin film according to an embodiment of the present invention. 3 (a) is a surface TEM photograph for TEM-EDS analysis, in which a quadrangular part is analyzed, and FIGS. 3 (b) to 3 (d) EDS spots. As a result of the analysis, it can be confirmed that Zr, which is a surface coating material, is dispersed very uniformly together with Ni and Mn which constitute the positive electrode powder.

The size of the powder of the cathode active material shown in FIG. 2 and FIG. 3 is about 100 to 500 nm, which is very fine nanoparticles. Although the nanoparticles have various shapes other than spherical shapes, And it can be confirmed that it is finely coated on the surface.

FIG. 4 is a graph comparing charge-discharge cycle characteristics of a cathode active material after surface coating using a cathode active material before surface coating and a polydodamine thin film in an embodiment of the present invention. In FIG. 4, the results for the cathode active material of the micrometer size level are shown for comparison. The current density was measured at the initial two times of 40 mA / g and then 100 mA / g, 2.0V. ≪ / RTI > Fig. 4 (a) shows the measurement result at 30 DEG C, and Fig. 4 (b) shows the measurement result at 45 DEG C.

Referring to FIG. 4, it can be seen that a positive electrode powder having a smaller particle size than a positive electrode powder having a relatively large particle size at a micrometer level exhibits a high discharge capacity. This can be interpreted as a decrease in the distance that electrons and ions move as the particle size decreases and the contact area with the electrolyte increases, facilitating insertion / removal of lithium ions. However, the small particle size of the anode powder causes a relatively large reduction in the discharge capacity as the cycle progresses due to undesired side reactions (elution of the transition metal and formation of undesired interface layers at the interface) due to the area of contact with the wide electrolyte Able to know. On the other hand, it can be seen that the positive electrode powder formed with the ZrO ₂ coating layer exhibits a very stable cycle characteristic in addition to the capacity increase due to the nanoparticle effect. From this, it can be interpreted that the ZrO ₂ coating layer effectively protects the surface of the nanoparticle-sized cathode active material from the electrolyte having high reactivity.

On the other hand, when the temperature is increased, the side reaction with the electrolyte becomes more active, so that the capacity decreases at a higher rate than at room temperature. Particularly, in the case of the positive electrode powder having a small particle size and no coating, it is confirmed that the cell loses its function at about 40 cycles. On the other hand, according to the embodiment of the present invention, it can be seen that the positive electrode powder coated with the surface exhibits a relatively stable cycle characteristic due to the surface protective effect of the coating layer.

The preferred embodiments of the present invention have been described in detail with reference to the drawings. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.

Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing detailed description, and all changes or modifications derived from the meaning, range, and equivalence of the claims are included in the scope of the present invention Should be interpreted.

Claims (7)

(a) dissolving dopamine in a buffer solution;
(b) adding a positive electrode active material to the buffer solution in which the dopamine is dissolved and reacting to form a poly dopamine thin film on the surface of the positive electrode active material;
(c) adding a cathode active material having the polypodamine thin film formed thereon to a coating solution containing a coating precursor material containing a metal element, and reacting the ionic group or hydrate of the coating precursor with the cathode active material To the surface; And
(d) heat treating the reactant in the step (c) to remove the polypodamine thin film, and forming a coating layer containing a compound containing the metal element on the surface of the cathode active material;
Wherein the surface of the positive electrode active material for lithium secondary battery is a surface of the positive electrode active material. The method according to claim 1,
Wherein the positive electrode active material has a size (based on an average diameter) of 50 to 1000 nm. The method according to claim 1,
Wherein the cathode active material is any one selected from the group consisting of compounds represented by the following formulas (1) to (14).
[Chemical Formula 1]
Li _x Mn _{1 - y} M _y A ₂
(2)
Li _x Mn _{1 - y} M _y O _{2 - z} B _z
(3)
Li _x Mn ₂ O _{4 - z} B _z
[Chemical Formula 4]
Li _x Mn _{2 - y} M _y A ₄
[Chemical Formula 5]
Li _x Mn _{1- y- z} Ni _y M _{z a a}
[Chemical Formula 6]
Li _x Mn _{1 - y - z} Ni _y M _z O _{2 - a} B _a
(7)
Li _x Co _{1 - y} M _y A ₂
[Chemical Formula 8]
Li _x CoO _{2 - z} B _z
[Chemical Formula 9]
Li _x Ni _{1 - y} M _y A ₂
[Chemical formula 10]
Li _x Ni _{1 - y} M _y O _{2 - z} B _z
(11)
Li _x Ni _{1- y z} Co _y M _{z a a}
[Chemical Formula 12]
Li _x Ni _{1 - y - z} Co _y M _z O _{2 - a} B _a
[Chemical Formula 13]
Li _x Ni _{1- y- z} Mn _y M _{z a a}
[Chemical Formula 14]
Li _x Ni _{1- y- z} Mn _y M _z O _{2 - a} B _a
Mg, Al, Co, K, Na, Ca, Si, Ti, Sn (where 0 & Wherein A is any one selected from the group consisting of O, F, S, and P, and is selected from the group consisting of V, Ge, Ga, B, As, Zr, Mn, Cr, Ni, And B is any one selected from the group consisting of F, S and P.) The method according to claim 1,
Wherein the reaction of step (b) is performed for 1 to 100 minutes so that the polypodamine thin film is formed to a thickness of 1 to 100 nm. The method according to claim 1,
Wherein the metal element is selected from the group consisting of Li, Na, K, Mg, Ca, Cu, Cd, Fe, Mn, Ni, Pb, Sn, Sr, Xe, Zn, Al, B, Bi, Ce, Cr, In, La, Wherein the compound containing at least one element selected from the group consisting of V, Y, Ge, Si, Sn, Nb, Ti, Zr, Sb, Ta, Mo, Re and W is a metal oxide, And a metal fluoride. The method for surface modification of a cathode active material for a lithium secondary battery according to claim 1, The method according to claim 1,
Wherein the reaction of step (c) is carried out at 30 to 70 ° C. The method according to claim 1,
Wherein the heat treatment in step (d) is performed at 200 to 700 ° C for 1 to 10 hours so that the coating layer is formed to a thickness of 1 to 100 nm.

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