WO2019066585A1 - Method for preparing cathode active material for secondary battery, cathode active material prepared thereby, and lithium secondary battery comprising same - Google Patents

Abstract

The present invention provides a method for preparing a cathode active material for a secondary battery, comprising the steps of: preparing a lithium transition metal oxide; forming a mixture by mixing the lithium transition metal oxide, a coating polymer and a carbide; and heating the mixture so as to form, on the surface of the lithium transition metal oxide particles, a coating layer comprising a carbonated coating polymer and the carbide.

Description

A method for producing a cathode active material for a secondary battery, a cathode active material thus prepared, and a lithium secondary battery comprising the same

Mutual citation with related application

This application claims the benefit of priority based on Korean Patent Application No. 10-2017-0127757 filed on September 29, 2017, and Korean Patent Application No. 10-2018-0115214 filed on September 27, 2018, The entire contents of which are incorporated herein by reference.

Technical field

The present invention relates to a method for producing a cathode active material for a secondary battery, a cathode active material thus prepared, and a lithium secondary battery comprising the cathode active material.

2. Description of the Related Art In recent years, with the rapid spread of electronic devices using batteries such as mobile phones, notebook computers, electric vehicles, and the like, the demand for secondary batteries of small size and light weight and relatively high capacity has been rapidly increasing. Particularly, the lithium secondary battery is light in weight and has a high energy density, and is attracting attention as a driving power source for portable devices. Accordingly, research and development efforts for improving the performance of the lithium secondary battery have been actively conducted.

The lithium secondary battery has a structure in which an organic electrolyte or a polymer electrolyte is filled between a positive electrode and a negative electrode, which are made of an active material capable of intercalating and deintercalating lithium ions, and oxidized when lithium ions are inserted / And electrical energy is produced by the reduction reaction.

As the positive electrode active material of the lithium secondary battery, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ or LiMn ₂ O ₄ ), lithium iron phosphate compound (LiFePO ₄ ) do. Further, as a method for improving low thermal stability while maintaining excellent reversible capacity of LiNiO ₂ , a method of replacing a part of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed. However, LiNi _{1 - α} Co _α O ₂ (α = 0.1 ~ 0.3), in which nickel is partially replaced by cobalt, exhibits excellent charge / discharge characteristics and lifetime characteristics, but has low thermal stability. On the other hand, a nickel manganese-based lithium composite metal oxide in which a part of nickel (Ni) is substituted with manganese (Mn) excellent in thermal stability and a nickel-cobalt manganese-based lithium composite metal oxide in which manganese (Mn) (Hereinafter referred to as " NCM-based lithium oxide ") has an advantage in that it has excellent cycle characteristics and thermal stability.

Recently, the demand for higher capacity and higher energy density has been increasing, and attempts have been made to achieve the target energy density by increasing the driving voltage range to higher voltage. Accordingly, development of a cathode active material for high voltage and high voltage having reliability and stability at a charge voltage higher than 4.35 V, which is higher than that of the conventional 4.3 V, is required.

Particularly, in the high voltage state, there is a problem of an increase in side reaction with the electrolyte and degradation of lifetime characteristics and resistance increase due to the formation of a SEI (solid electrolyte interface) film on the surface of the cathode active material, and development of a cathode active material is needed.

Disclosed is a cathode active material for high voltage and high voltage secondary batteries, which suppresses side reaction with an electrolyte under high voltage and formation of a solid electrolyte interface (SEI) film on the surface of a cathode active material, and has improved resistance characteristics and life characteristics.

The present invention provides a method of manufacturing a lithium secondary battery including: providing a lithium transition metal oxide; Mixing the lithium transition metal oxide, a coating polymer, and a carbide to form a mixture; And heat-treating the mixture to form a coating layer comprising a carbonated coating polymer and a carbide on the surface of the lithium transition metal oxide. The present invention also provides a method of manufacturing a cathode active material for a secondary battery.

The present invention also relates to a lithium transition metal oxide; And a coating layer formed on the particle surface of the lithium-transition metal oxide, wherein the coating layer is in the form of a film, and the coating layer comprises a carbonated coating polymer and a carbide.

The present invention also provides a positive electrode and a lithium secondary battery including the positive electrode active material.

According to the present invention, a cathode active material for high voltage and high voltage secondary batteries can be produced which suppresses a side reaction with an electrolyte and a solid electrolyte interface (SEI) film formation on the surface of a cathode active material under a high voltage and has improved resistance characteristics and life characteristics.

The positive electrode active material for a secondary battery according to the present invention can prevent mechanical damage of a positive electrode active material generated by repetition of charging / discharging in a high voltage state. In addition, uniform coating can be performed on the entire surface of the cathode active material particles, and the initial resistance and the rate of resistance increase can be reduced by securing excellent electrical conductivity.

1 and 2 are SEM (Scanning Electron Microscope) photographs of a cathode active material manufactured according to Example 1 of the present invention.

FIGS. 3 and 4 are SEM (Scanning Electron Microscope) photographs of the cathode active material prepared according to Comparative Example 2 on an enlarged scale.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention. Herein, terms and words used in the present specification and claims should not be construed to be limited to ordinary or dictionary meanings, and the inventor may appropriately define the concept of the term to describe its own invention in the best way. It should be construed as meaning and concept consistent with the technical idea of the present invention.

A method of manufacturing a cathode active material for a secondary battery according to the present invention includes the steps of: providing a lithium transition metal oxide; Mixing the lithium transition metal oxide, a coating polymer, and a carbide to form a mixture; And heat-treating the mixture to form a coating layer comprising a carbonated coating polymer and carbide on the surface of the lithium transition metal oxide particle.

The present invention relates to a method of coating a lithium transition metal oxide particle by using a mixture of a coating polymer and a carbide to suppress side reactions with an electrolyte and formation of a solid electrolyte interface (SEI) film on the surface of a cathode active material under high voltage, And life characteristics can be improved. By using a polymer for coating a soft material, it is possible to prevent mechanical breakage of a cathode active material generated by repetition of charging / discharging in a high voltage state, and by using a carbide excellent in electrical conductivity, And the resistance increase rate can be reduced. Further, the coating polymer may be carbonized through heat treatment during the coating process to ensure additional electrical conductivity.

 The method for producing the cathode active material for a secondary battery of the present invention will be described in detail below.

First, a lithium transition metal oxide is provided.

The lithium transition metal oxide may be a lithium transition metal oxide which is typically used as a cathode active material and more preferably at least one transition metal selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn) A lithium-transition metal oxide including a cation can be used. For example, a layered compound such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) or a layered compound such as Li _{1 + x 1} Mn _{2-x 1} O ₄ (where x 1 is 0 to 0.33), LiMnO ₃ , LiMn ₂ O _3, LiMnO formula LiNi _{1 2} such as lithium manganese oxide, of _{- x2} M ¹ _x2 O ₂ (where, M ¹ = Co, Mn, and Al, Cu, Fe, Mg, B or Ga, x2 = 0.01 ~ 0.3 ) Ni site type lithium nickel oxide, the formula ^{_{LiMn 2-x3 M 2 x3 O}} 2 ( here, represented as M ² = Co, Ni, Fe, Cr, and Zn, or Ta, x3 = 0.01 ~ 0.1) or Li ₂ Mn A lithium manganese composite oxide expressed by ₃ M ³ O ₈ (where M ³ = Fe, Co, Ni, Cu or Zn), a spinel represented by LiNi _{x 4} Mn _{2 - x 4} O ₄ (where x ₄ = 0.01 to 1) (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiNiO ₂ ), and lithium manganese composite oxide (LiFePO ₄ ) Oxide (LiMn ₂ O ₄ ), Lithium iron phosphate compound (LiFePO ₄ ), and the like.

Alternatively, the cathode active material may include a lithium-transition metal composite oxide represented by the following general formula (1).

[Chemical Formula 1]

Li _p Ni _1-xy Co _x M ^a _y M ^b _z O ₂

Wherein M ^a is at least one element selected from the group consisting of Mn, Al and Zr and M ^b is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, W and Cr Or more, and 0.9? P? 1.5, 0? X? 0.5, 0? Y? 0.5, and 0? Z? More preferably 0? X + y? 0.7, and most preferably 0 <x + y? 0.4, and the content of nickel (Ni) in the total transition metal is 60 mol% Lt; / RTI &gt; For example, the cathode active material is more preferably LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , LiNi _{0 . 9} Co _{0 . 05} Mn _{0 . 05} O _{2 ,} or LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . 2} O ₂ Or a high-Ni NCM-based lithium transition metal oxide.

Next, the lithium transition metal oxide, the polymer for coating and the carbide are mixed to form a mixture.

The coating polymer can be used as long as it can coat the particle surface of the lithium transition metal oxide and does not deteriorate the electrochemical performance. Examples of the polymer include polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone Polyvinylpyrrolidone), polyethylene, polystyrene, polyethylene terephthalate, polyvinyl chloride, polyvinylidene chloride, epoxy resin, amino resin, phenol resin and polyester resin, and more preferably at least one of polyvinylidene At least one selected from the group consisting of polyvinylpyrrolidone (PVDF) and polyvinylpyrrolidone, polyethylene terephthalate and polyvinylidene chloride may be used.

By using the polymer for coating of a soft material, it is possible to prevent mechanical breakage of the positive electrode active material generated by repeating charging / discharging in a high voltage state. In addition, the formation of the coating layer through the conventional dry coating method of metal oxide is difficult to coat uniformly in the form of an island and is difficult to coat uniformly in the form of an island. However, in the case of the present invention in which the coating is coated with a mixture of the polymer for coating and a carbide It is possible to easily form a film-like uniform coating layer through temperature control during the coating process based on the melting point of the polymer for coating, and the carbide mixed with the coating polymer can uniformly coat the surface of the cathode active material particle Can be distributed. In addition, the coating polymer may be carbonized through a heat treatment during the coating process, thereby providing additional electrical conductivity.

The polymer for coating may be included in an amount of 0.001 to 10 parts by weight, more preferably 0.005 to 5 parts by weight, based on 100 parts by weight of the lithium-transition metal oxide contained in the mixture. The coating polymer is mixed in an amount of 0.001 to 10 parts by weight based on 100 parts by weight of the lithium-transition metal oxide, whereby a coating layer can be uniformly formed on the entire surface of the cathode active material particle. It is possible to prevent mechanical breakage of the positive electrode active material that occurs while repeating the above-described process.

The carbide is a compound composed of carbon and one element. For example, at least one selected from the group consisting of B ₄ C, Al ₄ C ₃ , TiC, TaC, WC, NbC, HfC, VC and ZrC Or more, more preferably B ₄ C or Al ₄ C ₃ .

The carbide is covalently bonded to carbon and other elements and has a relatively high melting point and can remain as it is without being decomposed into CO ₂ or CO even during high temperature heat treatment in an oxidizing atmosphere during the coating process, It can be coated with the polymer for uniform distribution on the surface of the cathode active material particles. As a result, excellent electrical conductivity can be ensured and excellent electrical conductivity can be secured to reduce initial resistance and resistance increase rate.

The carbide may be included in an amount of 0.001 to 10 parts by weight, more preferably 0.002 to 2 parts by weight, based on 100 parts by weight of the lithium-transition metal oxide contained in the mixture. Since the carbide is mixed in an amount of 0.001 to 10 parts by weight based on 100 parts by weight of the lithium transition metal oxide, it can be uniformly distributed over the surface of the positive electrode active material particles, and the initial resistance and the rate of increase in resistance can be decreased have.

On the other hand, the mixture may be prepared by adding and stirring a polymer for coating to a lithium-transition metal oxide, then adding and stirring the carbide, and simultaneously adding a coating polymer and a carbide, And the order of the manufacturing process is not particularly limited. When the mixture is formed, a stirring process can be selectively performed, and the stirring speed may be 2,000 rpm at 100 rpm.

The mixture may contain the coating polymer and the carbide in a weight ratio of 1:99 to 99: 1, more preferably 20:80 to 80:20. By including the coating polymer and the carbide in the weight ratio range, a uniform film-like coating layer can be formed on the entire surface of the cathode active material particle, and the carbide can be uniformly distributed. Also, it is possible to suppress the formation of a SEI (solid electrolyte interface) film on the surface of a cathode active material and a side reaction with an electrolyte under a high voltage, and a cathode active material for high voltage and high resistance and life characteristics can be produced.

Next, the mixture is subjected to heat treatment to form a coating layer containing a carbonated coating polymer and a carbide on the surface of the lithium transition metal oxide.

The heat treatment for forming the coating layer may be performed in an oxidizing atmosphere such as air or oxygen or an inert atmosphere such as nitrogen, but may be performed in an oxidizing atmosphere.

The heat treatment may be performed at 200 to 800 ° C for 0.5 to 5 hours, more preferably at 300 to 600 ° C for 0.5 to 5 hours.

The cathode active material for a secondary battery of the present invention thus produced is a lithium transition metal oxide; And a coating layer formed on the particle surface of the lithium transition metal oxide, wherein the coating layer is in the form of a film, and the coating layer comprises a carbonated coating polymer and a carbide.

The coating polymer used in the coating process can be formed into a carbonized form through heat treatment during the coating process. The carbonized coating polymer can ensure additional electrical conductivity.

The carbide may remain as it is without being decomposed into CO ₂ or CO even during high-temperature heat treatment in an oxidizing atmosphere during the coating process. A coating layer may be formed together with the coating polymer and may be uniformly distributed on the surface of the cathode active material particle. have.

The coating layer may contain 0.001 to 10 parts by weight, more preferably 0.002 to 2 parts by weight, of the carbide relative to 100 parts by weight of the lithium-transition metal oxide particles. Carbide is not decomposed and removed even during the heat treatment during the coating process so that the above content can be satisfied and carbide is contained within the above range to secure excellent electrical conductivity and reduce initial resistance and resistance increase rate, And can be suitably applied as a high-voltage positive electrode active material.

The coating polymer and the carbide are applied in the same manner as described above for the method of manufacturing the cathode active material for a secondary battery of the present invention.

The coating layer is in the form of a film surrounding the particle surface of the lithium transition metal oxide, and the thickness of the coating layer may be 5 to 2,000 nm, more preferably 10 to 500 nm. If the thickness of the coating layer is less than 5 nm, the coating layer may be too thin to form a uniform coating layer. If the thickness of the coating layer is more than 2,000 nm, the ion conductivity may be lowered.

According to the present invention, a cathode active material for a secondary battery manufactured by coating a coating polymer and a carbide as a hybrid coating suppresses a side reaction with an electrolyte under a high voltage and forms a solid electrolyte interface (SEI) film on the surface of the cathode active material And thus, it is possible to remarkably improve the high-voltage undervoltage resistance characteristic and the life characteristic.

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

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

In the anode, the cathode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and for example, a metal such as stainless steel, aluminum, nickel, titanium, sintered carbon, , Nickel, titanium, silver, or the like may be used. In addition, the cathode current collector may have a thickness of 3 to 500 탆, and fine unevenness may be formed on the surface of the cathode current collector to increase the adhesive force of the cathode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

In addition, the cathode active material layer may include a conductive material and a binder together with the cathode active material described above.

At this time, the conductive material is used for imparting conductivity to the electrode. The conductive material can be used without particular limitation as long as it has electron conductivity without causing chemical change. Specific examples thereof include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black and carbon fiber; Metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; And polyphenylene derivatives. These may be used alone or in admixture of two or more. The conductive material may be typically contained in an amount of 1 to 30% by weight based on the total weight of the cathode active material layer.

In addition, the binder serves to improve adhesion between the positive electrode active material particles and adhesion between the positive electrode active material and the positive electrode collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose ), Starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, and various copolymers thereof. One kind or a mixture of two or more kinds of them may be used. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the cathode active material layer.

The positive electrode may be manufactured according to a conventional positive electrode manufacturing method, except that the positive electrode active material described above is used. Specifically, the composition for forming a cathode active material layer containing the above-mentioned cathode active material and optionally a binder and a conductive material may be coated on the cathode current collector, followed by drying and rolling. At this time, the types and contents of the cathode active material, the binder, and the conductive material are as described above.

Examples of the solvent include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, and the like. Water and the like, and one kind or a mixture of two or more kinds can be used. The amount of the solvent to be used is sufficient to dissolve or disperse the cathode active material, the conductive material and the binder in consideration of the coating thickness of the slurry and the yield of the slurry, and then to have a viscosity capable of exhibiting excellent thickness uniformity Do.

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

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

Specifically, the lithium secondary battery includes a positive electrode, a negative electrode disposed opposite to the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, as described above. The lithium secondary battery may further include a battery container for storing the positive electrode, the negative electrode and the electrode assembly of the separator, and a sealing member for sealing the battery container.

In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the negative electrode current collector may be formed on the surface of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, Carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like may be used. In addition, the negative electrode collector may have a thickness of 3 to 500 탆, and similarly to the positive electrode collector, fine unevenness may be formed on the surface of the collector to enhance the binding force of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The anode active material layer optionally includes a binder and a conductive material together with the anode active material. The negative electrode active material layer may be formed by applying and drying a composition for forming a negative electrode including a negative electrode active material on the negative electrode collector and, optionally, a binder and a conductive material, or by casting the composition for forming a negative electrode on a separate support , And a film obtained by peeling from the support may be laminated on the negative electrode collector.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon; Metal compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys; _{SiO β (0 <β <2} ), SnO 2, vanadium oxide, which can dope and de-dope a lithium metal oxide such as lithium vanadium oxide; Or a composite containing the metallic compound and the carbonaceous material such as Si-C composite or Sn-C composite, and any one or a mixture of two or more thereof may be used. Also, a metal lithium thin film may be used as the negative electrode active material. The carbon material may be both low-crystalline carbon and high-crystallinity carbon. Examples of the low-crystalline carbon include soft carbon and hard carbon. Examples of the highly crystalline carbon include natural graphite, artificial graphite, artificial graphite or artificial graphite, Kish graphite graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar coke derived cokes).

In addition, the binder and the conductive material may be the same as those described above for the anode.

Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a moving path of lithium ions. The separator can be used without any particular limitation as long as it is used as a separator in a lithium secondary battery. Particularly, It is preferable to have a low resistance and an excellent ability to impregnate the electrolyte. Specifically, porous polymer films such as porous polymer films made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, May be used. Further, a nonwoven fabric made of a conventional porous nonwoven fabric, for example, glass fiber of high melting point, polyethylene terephthalate fiber, or the like may be used. In order to secure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may be optionally used as a single layer or a multilayer structure.

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. It is not.

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

The organic solvent may be used without limitation as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate,? -Butyrolactone and? -Caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethyl carbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate PC) and the like; Alcohol solvents such as ethyl alcohol and isopropyl alcohol; R-CN (R is a straight, branched or cyclic hydrocarbon group of C2 to C20, which may contain a double bond aromatic ring or an ether bond); Amides such as dimethylformamide; Dioxolanes such as 1,3-dioxolane; Or sulfolane may be used. Among these, a carbonate-based solvent is preferable, and a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant, for example, such as ethylene carbonate or propylene carbonate, For example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1: 1 to about 1: 9, the performance of the electrolytic solution may be excellent.

The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium _{salt, LiPF 6, LiClO 4, LiAsF} 6, LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO 3) 2 _{, LiN (C 2 F 5 SO} 2) 2, LiN (CF 3 SO 2) 2. LiCl, LiI, or LiB (C ₂ O ₄ ) ₂ may be used. The concentration of the lithium salt is preferably in the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has an appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and the lithium ion can effectively move.

In addition to the electrolyte components, the electrolyte may contain, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate or the like, pyridine, triethanolamine, or the like for the purpose of improving lifetime characteristics of the battery, Ethyl phosphite, triethanol amine, cyclic ether, ethylenediamine, glyme, hexametriamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, At least one additive such as benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, benzyl alcohol, The additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

As described above, since the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, it can be used in portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles hybrid electric vehicle (HEV)).

According to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the same.

The battery module or the battery pack may include a power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); Or a power storage system, as shown in FIG.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Example 1

LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . B 2} O ₂ and polyvinylidene fluoride (PVDF) as carbides, B ₄ C as a carbide in a weight ratio of 100: 2: 0.1, and heat-treated at 400 ° C. for 3 hours in an oxygen atmosphere to form the LiNi _0.6 Co _0.2 Mn _On the particle surface of _0.2 O ₂ To form a coating layer.

Example 2

Was prepared in the same manner as in Example 1, except that Al ₄ C ₃ was used instead of B ₄ C as a carbide.

Example 3

(PVP) instead of polyvinylidene fluoride (PVDF) as a polymer for coating was prepared in the same manner as in Example 1.

Example 4

LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . B 2} O ₂ and polyvinylidene fluoride (PVDF) as carbides, B ₄ C as a carbide in a weight ratio of 100: 0.2: 0.5, and heat-treated at 400 ° C. for 3 hours in an oxygen atmosphere to form the LiNi _0.6 Co _0.2 Mn _A coating layer was formed on the surface of _0.2 O ₂ particles.

Comparative Example 1

LiNi _0.6 Co _0.2 Mn _0.2 O ₂ in which no coating layer was formed was prepared.

Comparative Example 2

LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . 2} O ₂ and H ₃ BO ₃ 100: a mixture in a weight ratio of 0.1 and heat-treated for about 3 hours in an oxygen atmosphere to 400 ℃ the LiNi _{0. 6} Co _{0 . 2} Mn _{0 . 2} O ₂ Of the particle surface.

Comparative Example 3

LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . 2} O ₂ , polyvinylidene fluoride (PVDF) and carbon black were mixed at a weight ratio of 100: 2: 0.1 and heat-treated at 400 ° C for about 3 hours in an oxygen atmosphere to form the particle surface of LiNi _0.6 Co _0.2 Mn _0.2 O ₂ To form a coating layer.

In the case of the coating layer thus formed, the thickness was very uneven, and no carbon black was detected in the coating layer. It is thought that carbon black was decomposed and removed into CO ₂ or CO during the heat treatment process.

Comparative Example  4

LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . 2} O ₂ and B ₄ C 100: a mixture in a weight ratio of 0.1 and heat-treated for about 3 hours in an oxygen atmosphere to 400 ℃ the LiNi _{0. 6} Co _{0 . 2} Mn _{0 . 2} O ₂ Of the particle surface.

Comparative Example  5

LiNi _{0 . 6} Co _{0 . 2} Mn _{0 . 2} O ₂ and polyvinylidene fluoride (PVDF) 100: mixed in a weight ratio of 2, by heat treatment for about 3 hours in an oxygen atmosphere to 400 ℃ the LiNi _{0. 6} Co _{0 . 2} Mn _{0 . 2} O ₂ Of the particle surface.

[Experimental Example 1: Observation of cathode active material]

1 and FIG. 2 (Example 1) and FIGS. 3 and 4 (Comparative Example 2) are photographs of a cathode active material produced according to Example 1 and Comparative Example 2, which are observed by a scanning electron microscope (SEM) .

1 and 2, Example 1 prepared by coating with a coating polymer and a hybrid coating of carbide shows that a film-like coating layer surrounding the active material particle surface is formed to a thickness of 1,000 nm on average I could confirm.

Referring to FIGS. 3 and 4, in Comparative Example 2 coated with H ₃ BO ₃ , which is a conventional coating material, it can be confirmed that an island-shaped coating portion is formed on the surface of the active material particle.

[Experimental Example 2: Evaluation of high-voltage cycle characteristics]

Using the cathode active materials prepared in Examples 1 to 4 and Comparative Examples 1 to 5, carbon black and PVDF binder were mixed in a N-methylpyrrolidone solvent in a weight ratio of 96: 2: 2, Was coated on one side of the aluminum current collector, dried at 130 캜, and rolled to prepare a positive electrode.

In addition, as a negative electrode active material, natural graphite, a carbon black conductive material and a PVdF binder were mixed in a N-methylpyrrolidone solvent in a weight ratio of 85: 10: 5 to prepare a composition for forming an anode, To prepare a negative electrode.

A lithium secondary battery was prepared by preparing an electrode assembly between a positive electrode and a negative electrode manufactured as described above through a separator of porous polyethylene, positioning the electrode assembly inside a case, and then injecting an electrolyte into the case. The electrolyte solution was prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF ₆ ) in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixed volume ratio of EC / DMC / EMC = 3/4/3) Respectively.

Each of the prepared lithium secondary battery cells was charged at a rate of 0.7 C and 4.4 V in a CCCV mode at 25 ° C and 45 ° C, cut off at 0.55C, The capacity retention (%) was measured while discharging to 3.0 V and performing charge / discharge 100 times. The results are shown in Table 1.

	Capacity retention after 100 cycles (%)
	25 ℃	45 ° C
Example 1	98.6	95.7
Example 2	95.3	93.0
Example 3	97.7	94.8
Example 4	98.1	95.6
Comparative Example 1	90.4	88.1
Comparative Example 2	91.4	90.5
Comparative Example 3	91.0	88.4
Comparative Example 4	91.2	90.6
Comparative Example 5	92.7	90.8

Referring to Table 1, Examples 1 to 4, which were prepared by coating a coating polymer and a hybrid coating of carbide, were compared with Comparative Example 1 which was not coated or coated with H ₃ BO _{3 which} is a conventional coating material (25 ° C.) and high temperature (45 ° C.) under high voltage compared to Example 2. In addition, the cycle characteristics of Examples 1 to 4 were remarkably superior to those of Comparative Example 3 in which a polymer and carbon (carbon black) were coated as a coating material.

Further, in Comparative Example 4 in which B ₄ C was coated as a single coating material, the cyclotoxicity at room temperature (25 ° C) and high temperature (45 ° C) was lower than those in Examples 1 to 4 and was similar to that in Comparative Example 2 . Also, in Comparative Example 5 in which PVDF was coated as a single coating material, cycle characteristics at room temperature (25 ° C) and high temperature (45 ° C) were significantly lower than those of Examples 1 to 4.

[Experimental Example 3: Evaluation of high voltage cell resistance]

Each of the prepared lithium secondary battery cells was charged at a rate of 0.7 C and 4.4 V in a CCCV mode at 25 ° C and 45 ° C, cut off at 0.55C, And the resistance increase rate (DCIR [%]) was measured while charging / discharging was performed 100 times. The results are shown in Table 2.

	Resistance increase rate after 100 cycles (%)
	25 ℃	45 ° C
Example 1	14	25
Example 2	13	27
Example 3	18	30
Example 4	17	22
Comparative Example 1	32	51
Comparative Example 2	29	48
Comparative Example 3	31	49
Comparative Example 4	25	43
Comparative Example 5	35	52

Referring to Table 2, Examples 1 to 4 prepared by coating with a coating polymer and a hybrid coating of carbide were compared with Comparative Example 1 which was not coated or coated with H ₃ BO _{3 which} is a conventional coating material The resistance increase rate at room temperature and high temperature (45 ° C) under a high voltage was remarkably reduced compared to Example 2. In addition, the resistance characteristics of Examples 1 to 4 were remarkably superior to those of Comparative Example 3 in which a polymer and carbon (carbon black) were coated as a coating material.

In addition, the B ₄ C single coating of Comparative Example 4 exhibited a higher resistance increase rate than Examples 1 to 4. This is presumably because the resistance increase rate was reduced due to the improvement of the surface electrical conductivity and the inhibition of the formation of side reactants by the uniform coating of the polymer-carbide in Examples 1 to 4, and as shown in Comparative Example 4, It is considered that uniform coating is not formed on the surface of the cathode active material.

In the case of the PVDF single coating of Comparative Example 5, a relatively high resistance increase rate was exhibited, which was accompanied by an increase in the initial resistance of the polymer itself and a continuous increase in the resistance increase rate during charging and discharging.

Claims (17)

1. Providing a lithium transition metal oxide;

   Mixing the lithium transition metal oxide, a coating polymer, and a carbide to form a mixture; And

   Heat-treating the mixture to form a coating layer comprising carbonized coating polymer and carbide on the particle surface of the lithium-transition metal oxide;

    Wherein the positive electrode active material is a positive electrode active material.
2. The method according to claim 1,

   The coating polymer may be selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylpyrrolidone, polyethylene, polystyrene, polyethylene terephthalate, polyvinyl chloride, polyvinylidene chloride, epoxy resins, amino resins, And an ester resin. The method for producing a cathode active material for a secondary battery according to claim 1,
3. The method according to claim 1,

   Wherein the carbide is at least one selected from the group consisting of B ₄ C, Al ₄ C ₃ , TiC, TaC, WC, NbC, HfC, VC and ZrC.
4. The method according to claim 1,

   Wherein the mixture contains 0.001 to 10 parts by weight of the coating polymer per 100 parts by weight of the lithium transition metal oxide.
5. The method according to claim 1,

   Wherein the mixture contains 0.001 to 10 parts by weight of the carbide relative to 100 parts by weight of the lithium transition metal oxide.
6. The method according to claim 1,

   Wherein the mixture comprises the polymer for coating and the carbide in a weight ratio of 1:99 to 99: 1.
7. The method according to claim 1,

   The lithium transition metal oxide may be at least one selected from the group consisting of lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), lithium iron phosphate compound (LiFePO ₄ ) And a metal oxide. The method for manufacturing a cathode active material for a secondary battery according to claim 1,

   [Chemical Formula 1]

   Li _p Ni _{1 -x- y} Co _x M ^a _y M ^b _z O ₂

   Wherein M ^a is at least one element selected from the group consisting of Mn, Al and Zr and M ^b is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo, W and Cr Or more, and 0.9? P? 1.5, 0? X? 0.5, 0? Y? 0.5, and 0? Z?
8. The method according to claim 1,

   Wherein the heat treatment is performed at 200 to 800 ° C.
9. Lithium transition metal oxide; And

   And a coating layer formed on the particle surface of the lithium transition metal oxide,

   Wherein the coating layer is in the form of a film,

   Wherein the coating layer comprises a carbonated coating polymer and a carbide.
10. 10. The method of claim 9,

    The coating polymer may be selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylpyrrolidone, polyethylene, polystyrene, polyethylene terephthalate, polyvinyl chloride, polyvinylidene chloride, epoxy resins, amino resins, And an ester resin.
11. 10. The method of claim 9,

    Wherein the polymer for coating is at least one or more selected from the group consisting of polyvinylidene fluoride (PVDF) and polyvinylpyrrolidone, polyethylene terephthalate and polyvinylidene chloride.
12. 10. The method of claim 9,

    Wherein the carbide is at least one selected from the group consisting of B ₄ C, Al ₄ C ₃ , TiC, TaC, WC, NbC, HfC, VC and ZrC.
13. 10. The method of claim 9,

    Wherein the carbide is B ₄ C or Al ₄ C ₃ .
14. 10. The method of claim 9,

    The coating layer is 5 to 2,000 nm thick.
15. 10. The method of claim 9,

    Wherein the coating layer comprises 0.001 to 10 parts by weight of the carbide relative to 100 parts by weight of the lithium-transition metal oxide particles.
16. A positive electrode for a secondary battery comprising the positive electrode active material according to any one of claims 9 to 15.
17. 17. A lithium secondary battery comprising the positive electrode according to claim 16.

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