WO2018084525A1 - Cathode active material for lithium secondary battery and lithium secondary battery comprising same - Google Patents

Abstract

The present invention relates to a cathode active material for a lithium secondary battery and a lithium secondary battery comprising the same. The cathode active material comprises a compound represented by formula 1, as follows, [Formula 1] Li_1-xNa_xCo_1-y-zMg_yTi_zO₂ (where 0 < x ≤ 0.05, 0 < y ≤ 0.01, and 0 ≤ z ≤ 0.01).

Description

Cathode active material for lithium secondary battery and lithium secondary battery comprising same

It relates to a cathode active material for a lithium secondary battery and a lithium secondary battery comprising the same.

Recently, lithium secondary batteries have been in the spotlight as power sources of portable small electronic devices.

The lithium secondary battery has a structure including a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator located between the positive electrode and the negative electrode, and an electrolyte.

As the negative electrode active material, various types of carbon-based materials or Si-based active materials including artificial, natural graphite, and hard carbon capable of inserting / desorbing lithium are used.

The cathode active material is mainly composed of an oxide composed of lithium and a transition metal having a structure capable of intercalation of lithium ions such as LiCoO ₂ , LiMn ₂ O ₄ , LiNi _{1 - x} Co _x O ₂ (0 <x <1), and the like. Used.

In the production of such a positive electrode active material, Na is mainly included as an impurity in the raw material, and Na is known as a material that lowers the sintering of the positive electrode active material produced during heat treatment, thereby causing poor contact between particles and reducing conductivity. It was intended to reduce the Na content contained in the active material to less than 100 ppm.

However, in recent years, when Na is replaced to a certain amount or less in the Li site, the layered structure is stabilized, and since the ion radius of Na is 25% larger than that of Li, the Li-O-Li interlayer spacing is increased to facilitate Li diffusion. There have been reports of improved retention and capacity characteristics, and research on a cathode active material replacing Na has been made. These studies have been limited to the NASICON structure compound is a low electrical conductivity, recently _{LiMn 2 O 4, LiNi 1/} 3 Co 1/3 Mn 1/3 O 2, LiNi 0.5 Co 0.2 Mn 0.3 O 2, LiNi 0. ₅ Co _{0 . 2} Al _{0 .} Studies have been attempted to dope Na in the composition, such as ₃ O ₂ .

As an example of the Na doping method, Na _x CoO ₂ is first prepared, followed by solution reaction of Na _x CoO ₂ and a lithium raw material such as Li ₂ CO ₃ to ion-exchange Na and Li to form a layered structure. It may be prepared in a form in which a certain amount of Li and Na coexist in the Li site of. The prepared active material is an active material having a prismatic structure that is not layered. This method can widen the Na content range, and as the charge and discharge are repeated, it can suppress the transition to the spinel structure to suppress the capacity decrease of the positive electrode active material. There is a problem that cannot be applied.

One embodiment of the present invention is to provide a positive electrode active material for a lithium secondary battery having excellent initial charge and discharge efficiency, excellent high voltage phase transition characteristics and high temperature standing cycle life retention.

Another embodiment is to provide a lithium secondary battery including the positive electrode active material.

One embodiment of the present invention provides a cathode active material for a lithium secondary battery including a compound represented by the following Chemical Formula 1.

[Formula 1]

Li _1-x Na _x Co _1-yz Mg _y Ti _z O ₂

(In Formula 1,

0 <x ≤ 0.01, 0 <y ≤ 0.01, 0 ≤ z ≤ 0.01)

In Formula 1, y may be 0 <y ≦ 0.006.

The positive electrode active material may be for a high voltage lithium secondary battery having an operating voltage of 4.3V or more, in another embodiment, 4.3V to 4.5V, and in another embodiment, 4.35V to 4.45V.

The positive electrode active material may be prepared by a solid phase method.

Another embodiment includes a cathode including the cathode active material; A negative electrode including a negative electrode active material; And it provides a lithium secondary battery comprising an electrolyte.

Other specific details of embodiments of the present invention are included in the following detailed description.

The cathode active material for a lithium secondary battery according to an embodiment may provide a lithium secondary battery having excellent initial charging and discharging efficiency, excellent high voltage phase transition characteristics, and high temperature standing life retention.

1 is a view schematically showing the structure of a lithium secondary battery according to one embodiment of the present invention.

Figure 2 is a graph showing the X-ray diffraction measurement results of the positive electrode active material prepared in Examples 1 to 3 and Comparative Example 1.

3 is a graph showing the results of nuclear magnetic resonance of the positive electrode active material prepared in Examples 1 to 3 and Comparative Example 1.

4 is a scanning electron micrograph according to primary firing (A) and secondary firing (B) of the positive electrode active material prepared according to Example 2. FIG.

5 is a scanning electron micrograph according to primary firing (A) and secondary firing (B) of the positive electrode active material prepared according to Comparative Example 1.

6 is a graph showing the rate characteristics of the half-cell containing the positive electrode active material of Examples 1 to 3 and Comparative Example 1.

7 is a graph showing rate characteristics of a half cell including the positive electrode active materials of Example 2 and Comparative Example 2. FIG.

FIG. 8 is a graph showing room temperature cycle life characteristics of a half cell including the cathode active materials of Examples 1 to 3 and Comparative Examples 1 and 2. FIG.

9 is a graph showing the high temperature cycle life characteristics of the half cell including the cathode active materials of Examples 1 to 3 and Comparative Examples 1 and 2.

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, by which the present invention is not limited and the present invention is defined only by the scope of the claims to be described later.

The cathode active material for a rechargeable lithium battery according to one embodiment of the present invention may include a compound represented by the following Formula 1.

[Formula 1]

Li _{1 - x} Na _x Co _{1 -y- z} Mg _y Ti _z O ₂

In Formula 1, 0 <x ≦ 0.01, 0 <y ≦ 0.01, 0 ≦ z ≦ 0.01, and in one embodiment, 0 <y ≦ 0.006.

As described above, the cathode active material is a compound in which a part of Li is substituted with Na, and a part of Co is substituted with Ti with Mg, optionally Mg.

When a part of Li is substituted with Na, the ion radius of Na is much larger than Li, and thus the interval between Li-O-Li bonds in the c-axis direction increases, so that lithium is easily inserted and removed during charging and discharging. Is improved, and as a result, the charging and discharging efficiency can be improved. In general, only 1.02Å of Na ions cannot be replaced by 0.54Å of Co sites, since other types of ions can be substituted for the original ion site only when the size ratio between the two ions is about 25%. .

In the cathode active material, x value may be 0 <x ≤ 0.01, according to one embodiment may be 0 <x ≤ 0.005, and according to another embodiment, 0.002 ≤ x ≤ 0.005. When the x value is included in the above range, the lithium diffusion rate of the positive electrode active material is improved, so that the capacity may be maintained even at high C-rate, long life at room temperature, and high temperature life of 45 ° C. If the x value exceeds 0.01, a secondary phase other than the layered structure is produced, i.e., it cannot maintain a proper structure, and a large amount of Na can be precipitated to the cathode during charging and discharging, resulting in deterioration of the electrochemical properties. not.

When a part of Co is substituted with Mg, and optionally Mg, Ti may be stabilized, the layer structure of the positive electrode active material may be improved, and electrical conductivity may be improved, and lifespan retention may be improved at a high voltage of 4.4 V or more. If Mg and, optionally, Mg and Ti replace Li together, the amount of active Li participating in the electrochemical reaction may be reduced, and the capacity and life may be reduced by blocking a channel through which Li diffuses.

The y value in the cathode active material may be 0 <y ≤ 0.01, and according to one embodiment, 0 <y ≤ 0.006. When the y value is included in the above range, it may have an advantage of improving the life characteristics by exerting the pillar effect of maintaining the structure of Li—O—Li at high voltage. If the value of y is 0 or exceeds 0.01, Mg may inadvertently enter the Li site and deteriorate the capacity and life characteristics.

In the cathode active material, a z value may be 0 ≦ z ≦ 0.01, and according to one embodiment, 0 ≦ z ≦ 0.005 . When Ti is included in the cathode active material together with Mg, the structure can be stabilized at a high voltage, and at the same time, the electrical conductivity is improved, thereby improving the service life and rate characteristics. Such an effect can be effectively obtained when the z value is 0.01 at maximum, and if it exceeds 0.01, there may be a problem that TiO ₂ is left without entering the Co site.

Such a positive electrode active material may be suitably used in a high voltage lithium secondary battery having an operating voltage of 4.3 V or higher, and according to one embodiment, the operating voltage is 4.3 V to 4.5 V, and according to another embodiment, the operating voltage is 4.3 V to 4.45. It can be suitably used for the high voltage lithium secondary battery which is V. In Formula 1, if x is a positive electrode active material of 0, y is 0 in a high-voltage lithium secondary battery of 4.4V or more, there may be a problem that the layer structure collapses during charging and discharging and capacity is greatly reduced at the beginning of the life cycle Not appropriate

Such a positive electrode active material may be manufactured by a solid phase method. This solid state method is explained below.

Lithium raw material, cobalt raw material, sodium raw material and magnesium raw material are mixed. At this time, the titanium raw material may be further added.

The lithium raw material may include lithium acetate, lithium nitrate, lithium hydroxide, lithium carbonate, lithium acetate, hydrates thereof, or a combination thereof. The cobalt raw material may include cobalt nitrate, cobalt hydroxide, cobalt oxide, cobalt acetate, cobalt carbonate, hydrates thereof, or a combination thereof. The sodium raw material may include sodium nitrate, sodium hydroxide, sodium carbonate, sodium oxide, sodium acetate, or a combination thereof. The magnesium raw material may include magnesium nitrate, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium acetate, or a combination thereof. The titanium raw material may include titanium nitrate, titanium hydroxide, titanium carbonate, Titanium oxide, titanium acetate, or a combination thereof.

The mixing ratio of the lithium raw material, the cobalt raw material, the sodium raw material and the magnesium raw material, and optionally the titanium raw material may be properly adjusted to obtain the composition of Chemical Formula 1.

The resulting mixture is subjected to primary heat treatment. The primary heat treatment process may be carried out at a temperature of 1000 ℃ to 1050 ℃, wherein the heat treatment time may be 4 hours to 16 hours. In addition, the heat treatment atmosphere may be an air atmosphere, an oxygen atmosphere, or a combination thereof.

After the first heat treatment, a second heat treatment is performed on the first heat treatment product to prepare a cathode active material. The secondary heat treatment process may be further subjected to a secondary heat treatment, a post heat treatment for 4 hours to 8 hours at a temperature of 900 ℃ to 1000 ℃. If the secondary heat treatment process is not performed, Na may precipitate on the surface of the positive electrode active material, and in this case, since it becomes a secondary phase having a structure other than a layered structure, the standard capacity of the active material decreases, and electrical conductivity decreases, thereby improving the life characteristics. It can be adversely affected and not appropriate.

Another embodiment includes a cathode including the cathode active material; cathode; And it provides a lithium secondary battery comprising an electrolyte.

The positive electrode includes a current collector and a positive electrode active material formed on the current collector and including a positive electrode active material.

In the cathode active material layer, the content of the cathode active material may be 90% by weight to 98% by weight based on the total weight of the cathode active material layer. In addition, the cathode active material layer may further include a binder and a conductive material. The binder and the conductive material may be included in amounts of 1 wt% to 5 wt% based on the total weight of the positive electrode active material layer.

The binder is polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, polyvinyl fluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride , Polyethylene, polypropylene, styrene-butadiene rubber, acrylic styrene butadiene rubber, epoxy resin, nylon and the like can be used, but is not limited thereto.

Examples of the conductive material include carbonaceous materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, metal powders such as copper, nickel, aluminum, and silver, and polyphenylene derivatives such as metallic fibers such as metal fibers. A conductive material containing a conductive polymer or a mixture thereof can be used.

Al foil may be used as the current collector, but is not limited thereto.

The negative electrode includes a negative electrode active material layer including a current collector and a negative electrode active material formed on the current collector.

The anode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, a lithium metal, an alloy of lithium metal, a material doped and undoped with lithium, or a transition metal oxide.

As a material capable of reversibly intercalating / deintercalating the lithium ions, any carbon-based negative electrode active material generally used in a lithium ion secondary battery may be used, and representative examples thereof include crystalline carbon. , Amorphous carbon or these can be used together. Examples of the crystalline carbon include graphite such as amorphous, plate, flake, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon ( hard carbon), mesophase pitch carbide, calcined coke, and the like.

Examples of the alloy of the lithium metal include lithium and Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn. Alloys of the metals selected may be used.

Examples of materials that can be doped and undoped with lithium include Si, SiO _x (0 <x <2), and Si-Q alloys (wherein Q is an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, and 16). An element selected from the group consisting of group elements, transition metals, rare earth elements, and combinations thereof, not Si), Sn, SnO ₂ , Sn-R alloys (wherein R is an alkali metal, an alkaline earth metal, a Group 13 element, 14 element, an element selected from group 15 elements, group 16 elements, transition metals, rare earth elements and combinations thereof, Sn may be mentioned not), and a mixture of at least one and SiO ₂ of which Can also be used. The elements Q and R include Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Sb, Bi, One selected from the group consisting of S, Se, Te, Po, and a combination thereof can be used.

Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide or lithium titanium oxide.

The content of the negative electrode active material in the negative electrode active material layer may be 95% by weight to 99% by weight with respect to the total weight of the negative electrode active material layer.

In one embodiment of the present invention, the negative electrode active material layer includes a binder, and optionally may further include a conductive material. The content of the binder in the negative electrode active material layer may be 1% by weight to 5% by weight based on the total weight of the negative electrode active material layer. In addition, when the conductive material is further included, 90 wt% to 98 wt% of the negative electrode active material, 1 wt% to 5 wt% of the binder, and 1 wt% to 5 wt% of the conductive material may be used.

The binder adheres the anode active material particles to each other well, and also serves to adhere the anode active material to the current collector well. As the binder, a non-aqueous binder, an aqueous binder, or a combination thereof may be used.

The non-aqueous binder may include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride , Polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

The aqueous binder may include styrene-butadiene rubber, acrylated styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluorine rubber, ethylene propylene copolymer, polyepichlorohydrin , Polyphosphazene, polyacrylonitrile, polystyrene, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol and combinations thereof It may be selected.

When using an aqueous binder as the negative electrode binder, it may further include a cellulose-based compound that can impart viscosity as a thickener. As this cellulose type compound, carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, these alkali metal salts, etc. can be used in mixture of 1 or more types. Na, K or Li may be used as the alkali metal. The amount of the thickener used may be 0.1 parts by weight to 3 parts by weight based on 100 parts by weight of the negative electrode active material.

Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; Metal materials such as metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive polymers such as polyphenylene derivatives; Or a conductive material containing a mixture thereof.

The current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

The electrolyte includes a non-aqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

As the non-aqueous organic solvent, a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvent may be used.

Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC), and ethylene carbonate ( EC), propylene carbonate (PC), butylene carbonate (BC) and the like can be used. The ester solvent may be methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, and caprolactone. And the like can be used. Dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc. may be used as the ether solvent. In addition, cyclohexanone may be used as the ketone solvent. In addition, ethyl alcohol, isopropyl alcohol, etc. may be used as the alcohol solvent, and the aprotic solvent may be R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms. Nitriles such as a double bond aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolane, and the like can be used. .

The organic solvents may be used alone or in combination of one or more, and the mixing ratio in the case of mixing one or more may be appropriately adjusted according to the desired battery performance, which can be widely understood by those skilled in the art. have.

In addition, in the case of the carbonate solvent, it is preferable to use a mixture of cyclic carbonate and chain carbonate. In this case, the cyclic carbonate and the chain carbonate may be mixed and used in a volume ratio of 1: 1 to 1: 9, so that the performance of the electrolyte may be excellent.

The organic solvent may further include an aromatic hydrocarbon organic solvent in the carbonate solvent. In this case, the carbonate solvent and the aromatic hydrocarbon organic solvent may be mixed in a volume ratio of 1: 1 to 30: 1.

As the aromatic hydrocarbon-based organic solvent, an aromatic hydrocarbon compound of Formula 1 may be used.

[Formula 1]

(In Chemical Formula 1, R ₁ to R ₆ are the same as or different from each other and are selected from the group consisting of hydrogen, halogen, an alkyl group having 1 to 10 carbon atoms, a haloalkyl group, and a combination thereof.)

Specific examples of the aromatic hydrocarbon organic solvent include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-tri Fluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1 , 2,4-trichlorobenzene, iodobenzene, 1,2-dioodobenzene, 1,3-dioiobenzene, 1,4-dioiobenzene, 1,2,3-triiodobenzene, 1, 2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluoro Rotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2, 3,5-trichlorotoluene, iodotoluene, 2,3-dioodotoluene, 2,4-diaodotoluene, 2 , 5-diaodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

The electrolyte may further include vinylene carbonate or an ethylene carbonate-based compound of Formula 2 as a life improving additive to improve battery life.

[Formula 2]

In Formula 2, R ₇ and R ₈ are the same as or different from each other, and are selected from the group consisting of hydrogen, a halogen group, a cyano group (CN), a nitro group (NO ₂ ), and an alkyl group having 1 to 5 fluorinated carbon atoms. At least one of R ₇ and R ₈ is selected from the group consisting of a halogen group, a cyano group (CN), a nitro group (NO ₂ ) and a fluorinated C1-5 alkyl group, provided that both R ₇ and R ₈ are all Not hydrogen.)

Representative examples of the ethylene carbonate-based compound include difluoro ethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate or fluoroethylene carbonate. Can be. In the case of further using such life improving additives, the amount thereof can be properly adjusted.

The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions in the battery to enable the operation of a basic lithium secondary battery and to promote the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of such lithium salts are LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN (SO ₂ C ₂ F ₅ ) ₂ , Li (CF ₃ SO ₂ ) ₂ N, LiN (SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), where x and y are natural numbers, for example Supporting one or more selected from the group consisting of LiCl, LiI and LiB (C ₂ O ₄ ) ₂ (lithium bis (oxalato) borate (LiBOB)); It is preferable to use the concentration of lithium salt within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt is included in the above range, since the electrolyte has an appropriate conductivity and viscosity, it can exhibit excellent electrolyte performance, Lithium ions can move effectively.

Depending on the type of lithium secondary battery, a separator may exist between the positive electrode and the negative electrode. As the separator, polyethylene, polypropylene, polyvinylidene fluoride or two or more multilayer films thereof may be used, and polyethylene / polypropylene two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / poly It goes without saying that a mixed multilayer film such as a propylene three-layer separator can be used.

1 is an exploded perspective view of a rechargeable lithium battery according to one embodiment of the present invention. Although a lithium secondary battery according to an embodiment is described as an example of being rectangular, the present invention is not limited thereto, and may be applied to various types of batteries, such as a cylindrical shape and a pouch type.

Referring to FIG. 1, the lithium secondary battery 100 according to the exemplary embodiment includes an electrode assembly 40 and an electrode assembly 40 interposed between the positive electrode 10 and the negative electrode 20 with a separator 30 interposed therebetween. It may include a case 50 is built. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown).

Hereinafter, examples and comparative examples of the present invention are described. Such following examples are only examples of the present invention, and the present invention is not limited to the following examples.

(Example 1)

Lithium carbonate (Li ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), cobalt oxide (Co ₃ O ₄ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ) in the final product Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z O ₂ were mixed in the composition (x = 0.002, y = 0.006 , z = 0.005) the mixing ratio (molar ratio) that.

The resulting mixture was first calcined at 1025 ° C. for 6 hours in an air atmosphere, and the calcined product was calcined at 950 ° C. for 6 hours, followed by secondary calcining, followed by Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z 0 ₂ (x = 0.002, y = 0.006, z = 0.005) positive electrode active material was prepared.

(Example 2)

Lithium carbonate (Li ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), cobalt oxide (Co ₃ O ₄ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ) in the final product Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z O ₂ were mixed in the composition (x = 0.005, y = 0.006 , z = 0.005) the mixing ratio (molar ratio) that.

The resulting mixture was first calcined at 1025 ° C. for 6 hours in an air atmosphere, and the calcined product was calcined at 950 ° C. for 6 hours, followed by secondary calcining, followed by Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z O ₂ (x = 0.005, y = 0.006, z = 0.005) positive electrode active material was prepared.

(Example 3)

Lithium carbonate (Li ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), cobalt oxide (Co ₃ O ₄ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ) in the final product Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z O ₂ were mixed in the composition (x = 0.01, y = 0.006 , z = 0.005) the mixing ratio (molar ratio) that.

The resulting mixture was first calcined at 1025 ° C. for 6 hours in an air atmosphere, and the calcined product was calcined at 950 ° C. for 6 hours, followed by secondary calcining, followed by Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z 0 ₂ (x = 0.01, y = 0.006, z = 0.005) positive electrode active material was prepared.

(Comparative Example 1)

Lithium carbonate (Li ₂ CO ₃ ), cobalt oxide (Co ₃ O ₄ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ) was added to Li _{1 - x} Na _x Co _{1 -y- z} Mg _y Ti _z in the final product. O ₂ were mixed in the composition (x = 0, y = 0.006 , z = 0.005) the mixing ratio (molar ratio) that.

The resulting mixture was calcined first for 6 hours at 1025 ℃ temperature in an air atmosphere, the firing of the product in 950 ℃ temperature for 6 hours post - heating the second fired to _{Li 1 .02-x Na x Co} 1-yz Mg y A Ti _z O ₂ (x = 0, y = 0.006, z = 0.005) positive electrode active material was prepared.

(Comparative Example 2)

Lithium carbonate (Li ₂ CO _3), sodium carbonate (Na ₂ CO _3), cobalt oxide (Co ₃ O ₄₎ in the final product the Li _{1 - x Na x Co 1 -y- z} Mg y Ti z O 2 Composition ( It mixed at the mixing ratio (molar ratio) which becomes x = 0.005, y = 0, z = 0).

The resulting mixture was first calcined at 1025 ° C. for 6 hours in an air atmosphere, and the calcined product was calcined at 950 ° C. for 6 hours, followed by secondary calcining, followed by Li _{1 - x} Na _x Co _{1 -yz} Mg _y Ti _z 0 ₂ (x = 0.005, y = 0, z = 0) A positive electrode active material was prepared.

* Characterization of positive electrode active material

(i) crystal structure analysis

_{X Na x Co 1 -y- z Mg} y Ti z O 2 Composition _- Example 1 to 3 and Comparative Example 1, a cathode active material for a lithium secondary battery the X- ray diffraction analyzer (XRD) manufactured by using the Li ₁ In (0≤x≤0.01, y = 0.006, z = 0.005), the formation of secondary phase and the crystal structure change according to Na content were confirmed. As a result of X-ray diffraction analysis, CuKα rays were used. The results are shown in Figure 2 (in Figure 2, the y-axis is intensity).

As shown in Figure 2, it can be seen that the secondary phase peak associated with Na does not appear.

In addition, the lattice constant of the positive electrode active material was obtained by X-ray diffraction measurement using CuKα rays. The measured a-axis length, c-axis length, and the distance ratio between the crystal axes (c / a axis ratio) are shown in Table 1 together. In Table 1 below, V (Å ³ ) represents the volume of a unit cell.

	a-axis length (Å)	b-axis length	c axis length (Å)	Distance ratio (c / a ratio)	V (Å 3 )
Comparative Example 1	2.8166	2.8166	14.055	4.990	96.57
Example 1	2.8167	2.8167	14.057	4.991	96.58
Example 2	2.8169	2.8169	14.058	4.991	96.61
Example 3	2.8173	2.8173	14.062	4.991	96.67

As shown in Table 1, when the Na content was increased to 1.0 mol%, the a and b axis lengths increased linearly from 2.8166 ms to 2.8173 ms and the c axis lengths from 14.055 ms to 14.062 ms and c / a The ratio was 4.991, yielding almost similar results.

That is, the lattice constant increases with increasing Na content, but a similar result was obtained for c / a. From this result, Na was well doped into Li sites, and the crystallinity of the cathode active materials of Examples 1 to 4 was prepared. You can see that they are all similar.

(ii) nuclear magnetic resonance (NMR) evaluation

Na doping of the cathode active material for lithium secondary batteries prepared in Examples 1 to 3 was additionally measured using a nuclear magnetic resonance device (NMR). The magnetic resonance apparatus was ²³ Na-NMR (Bruker Avance III), the results are shown in FIG. In addition, for comparison, NMR of sodium carbonate (Na ₂ CO ₃ ), which is a Na raw material, was also measured, and the results are also shown in FIG. 3 (in FIG. 2, the y-axis is intensity).

As shown in FIG. 3, the chemically active signal of Na did not appear in the cathode active material prepared in Comparative Example 1. On the other hand, the positive electrode active materials prepared in Examples 1 to 3 showed a chemical shift at -8 ppm and 58 ppm, and were completely different from the spectrum of Na ₂ CO ₃ used as a raw material. In particular, the two chemical signals in Examples 2 to 3 it can be seen that the relative strength gradually increases as the doping amount of Na increases.

From the two results of XRD and NMR shown in FIGS. 2 and 3, it was confirmed that Na was well doped in Li sites.

(iii) morphology observation

In order to confirm the morphology change according to the primary and secondary firing treatments of the positive electrode active material 0.5 doped Na prepared according to Example 2 and the positive electrode active material prepared according to Comparative Example 1 (FE- SEM) was observed.

The results of Example 2 are shown in FIG. 4 (A: primary firing, B: secondary firing), and the results of Comparative Example 1 are shown in FIG. 4 (A: primary firing, B: secondary firing).

As shown in FIG. 4, in Example 2, Na was precipitated on the surface of the product when the primary firing was performed at 1025 ° C., but Na precipitates were formed on the surface of the cathode active material prepared by secondary firing at 950 ° C. It was confirmed that there was no.

On the other hand, as shown in Figure 5, in Comparative Example 1, it can be seen that there is no change in the surface morphology according to the first and second firing.

From this result, it can be seen that when Na-doped LiCoO ₂ is produced by the solid phase method, performing the primary and secondary firing steps can prevent Na precipitation.

* Battery characteristic evaluation

(i) Preparation of Anode

The cathode active material slurry prepared by mixing the cathode active material, the carbon black conductive material, and the polyvinylidene fluoride binder prepared according to Examples 1 to 3 and Comparative Example 1 in an N-methyl pyrrolidone solvent at a weight ratio of 96: 2: 2. Was prepared.

The prepared positive electrode active material slurry was uniformly coated on Al foil, and vacuum dried at 120 ° C. to prepare a positive electrode.

(ii) coin cell manufacturing

Coin cell of the 2032 standard by a conventional method using the positive electrode, a lithium metal electrode, a polyethylene separator (Celgard2300), and a mixed solvent (1: 1 volume ratio) electrolyte solution of ethylene carbonate and dimethyl carbonate in which 1M LiPF ₆ is dissolved. A coin cell was prepared.

(iii) evaluation of electrochemical properties

A. Rate Characterization

In order to evaluate the rate characteristics of the prepared battery, using the electrochemical analyzer (H0, HC0105R, Korea), the half cells of Examples 1 to 3 and Comparative Examples 1 and 2 at 25 ℃ and 45 ℃ , Charge and discharge in the potential region of 3.0V to 4.3V, 0.2C, 0.5C, 1C, 2C and 3C, and measured the discharge capacity (discharge capacity). The results of Examples 1 to 3 and Comparative Example 1 are shown in FIG. 6, and the results of Comparative Example 2 are shown in FIG. 7. In addition, for the sake of comparison, the results of Example 2 are also shown in FIG. 7 together.

As shown in FIG. 6, it can be seen that the half-cell using the cathode active material of Comparative Example 1, which is not doped with Na, has a significantly reduced capacity at a current density of 2C or more. On the other hand, in the case of the half-cell manufactured using the cathode active materials of Examples 1 to 3 doped with Na, the capacity does not significantly decrease under various current density conditions from 0.2C to 3C, that is, the high rate characteristics are excellent. It can be seen.

As shown in FIG. 7, the half-cell using the positive electrode active material of Example 2 doped with 0.5 mol% of Na does not significantly reduce its capacity under various current density conditions from 0.2C to 3C, that is, excellent in high rate characteristics. On the other hand, it can be seen that the half cell using the positive electrode active material of Comparative Example 2 is significantly reduced from 1C. From this result, when Mg and Ti are not doped, the capacity starts to decrease from the current density of 0.5C which is relatively low in the voltage range of 4.3V, and at 3C, the capacity almost does not work as 0. Therefore, Na It can be seen that even if doped, Mg and Ti are not included, the high rate characteristic is seriously degraded.

B. Cycle Life Characterization

The half-cells prepared by using the positive electrode active materials prepared according to Examples 1 to 3 and Comparative Examples 1 and 2 were charged and discharged at 0.2C at room temperature (25 ° C.) for 40 times to measure the discharge capacity according to the cycle. One result is shown in FIG.

As shown in FIG. 8, it can be seen that the room temperature cycle life characteristics of the half cells using the cathode active materials of Examples 1 to 3 are superior to those of the half cells using the cathode active materials of Comparative Examples 1 and 2.

In addition, it can be seen from the results of FIG. 8 that the cycle life characteristics are closely correlated with the amount of Na doping. That is, the battery using the positive electrode active material of Example 2 doped with Na 0.5 mol%, Example 3, the Na doping amount increased to 1.0 mol%, the Na doped amount reduced to 0.2 mol% using the positive electrode active material of Example 1 Compared to the cell, the result with the least capacity reduction up to 40 cycles was obtained.

In addition, even in the case of containing Na, the battery using the positive electrode active material of Comparative Example 2 that does not contain Mg and Ti, the discharge capacity is remarkably deteriorated, it can be seen that the discharge capacity hardly comes out, especially in 30 cycles.

In addition, the half-cells manufactured using the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 and 2 were charged and discharged at 0.2 C at a high temperature (45 ° C.) for 40 times to measure discharge capacity according to cycles. The results are shown in FIG.

As shown in FIG. 9, it can be seen that the high temperature cycle life characteristics of the half cells using the positive electrode active materials of Examples 1 to 3 are superior to the half cells using the positive electrode active materials of Comparative Examples 1 and 3.

In addition, similar to the results of FIG. 8, it can be seen that the high temperature cycle life results shown in FIG. 9 are greatly affected by the Na content. In particular, in the case of the battery using the positive electrode active material of Example 2 doped with 0.5 mol% of Na, the battery using the positive electrode active material of Example 1 having a Na doping amount of 0.2 mol% and Example 3 having a Na doping amount of 1.0 mol% In comparison, results with the least capacity reduction up to 40 cycles were obtained.

In addition, even in the case of containing Na, the battery using the positive electrode active material of Comparative Example 2 not containing Mg and Ti, the capacity was reduced from the initial five cycles, the result was the lowest high temperature life retention characteristics.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and changes can be made within the scope of the claims and the detailed description of the invention and the accompanying drawings. Naturally, it belongs to the scope of the invention.

Claims (7)

1. To include a compound represented by the formula

   Cathode active material for lithium secondary battery.

   [Formula 1]

   Li _1-x Na _x Co _1-yz Mg _y Ti _z O ₂

   (In Formula 1,

   0 <x ≤ 0.01, 0 <y ≤ 0.01, 0 ≤ z ≤ 0.01)
2. The method of claim 1,

   The positive electrode active material is a positive electrode active material for lithium secondary batteries which is for high voltage lithium secondary batteries having an operating voltage of 4.3V or more.
3. The method of claim 1,

   The cathode active material is a cathode active material for a lithium secondary battery is a high voltage lithium secondary battery having an operating voltage of 4.3V to 4.5V.
4. The method of claim 1,

   The positive electrode active material is a positive electrode active material for a lithium secondary battery for a high voltage lithium secondary battery having an operating voltage of 4.35V to 4.45V.
5. The method of claim 1,

   Wherein y is 0 <y ≤ 0.006 positive electrode active material for lithium secondary batteries.
6. The method of claim 1,

   The cathode active material is a cathode active material for a lithium secondary battery prepared by a solid phase method.
7. A positive electrode comprising the positive electrode active material of any one of claims 1 to 6;

   A negative electrode including a negative electrode active material; And

   Electrolyte

   Lithium secondary battery comprising a.

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