KR101692826B1 - Cathode Active Material with Modified Surface and Method for Preparing the Same - Google Patents

Abstract

The present invention relates to a cathode active material composition comprising at least one type A element having a property of enhancing the surface stability of the cathode active material and penetrating the center of the cathode active material and at least one type B element having a property of improving the structural stability of the cathode active material and diffusing to the surface of the cathode active material Element;
On the particle cross section of the cathode active material, the species A is distributed in a concentration gradient increasing from the center to the surface of the particle, and the species B is distributed in a concentration gradient increasing from the particle surface to the center A cathode active material for a secondary battery, and a method for manufacturing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-modified cathode active material,

The present invention relates to a surface-modified cathode active material and a method for producing the same.

Lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a cathode active material of the lithium secondary battery, and lithium-containing manganese oxides such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, The use of oxides (LiNiO ₂ ) is also used.

Among them, LiCoO ₂ is superior in properties such as excellent cycle characteristics and is widely used at present. However, LiCoO ₂ is relatively expensive, has a charge and discharge current as low as about 150 mAh / g, has a crystal structure unstable at a voltage of 4.3 V or higher, And there is a risk of ignition.

Lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have the advantage of using manganese which is abundant in resources and environmentally friendly as a raw material and thus attracts much attention as a cathode active material capable of replacing LiCoO ₂ , The oxide has a disadvantage in that it has a small capacity and poor cycle characteristics.

Lithium nickel oxide such as LiNiO ₂ has a lower discharge capacity than that of the cobalt oxide when it is charged at 4.3 V. The reversible capacity of the doped LiNiO ₂ is about 165 mAh / g of LiCoO ₂ ) ≪ / RTI > to about 200 mAh / g. However, the LiNiO ₂ based oxide has a problem in that a rapid phase transition of the crystal structure occurs depending on the volume change accompanied by the charge-discharge cycle, and an excessive amount of gas is generated during the cycle.

On the other hand, in general, the lithium secondary battery using the cathode active material has a problem that the safety of the battery is lowered due to the degeneration of the surface structure of the active material and the exothermic reaction accompanying the sudden structural collapse as the charging and discharging are repeated, . This problem is particularly serious at high temperatures. This is because the electrolyte is decomposed due to moisture and other influences inside the battery, the active material is deteriorated due to the instability of the surface of the anode, and the interface resistance between the electrode including the active material and the electrolyte is increased.

Conventionally, studies have been made to solve these problems and to improve the surface of an active material to improve battery performance. Typical examples thereof include coating the surface of an active material to prevent deterioration of the active material and increase interfacial stability between the electrolyte and the active material Technology.

However, when the surface of the active material is coated, the coating layer is difficult to be formed while uniformly covering the surface of the cathode material, and when the coating layer is peeled off during charging / discharging, the surface of the cathode material is exposed again to the electrolyte.

Due to these problems, there is a high need for a technique capable of stabilizing the surface of a cathode material and improving stability at a high voltage.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and the technical problems required from the past.

More specifically, the object of the present invention is to provide a positive electrode active material which has at least one or more A species elements which increase the surface stability of the positive electrode active material and which are distributed in a concentration gradient increasing from the center of the particle toward the surface, And the element is distributed in a concentration gradient increasing from the particle surface to the central portion, thereby ensuring the safety of the particle surface and providing the cathode active material with stabilized internal structure.

In order to accomplish the above object, the present invention provides a cathode active material,

At least one type A element having a property of increasing the surface stability of the positive electrode active material and penetrating into the center portion of the positive electrode active material and at least one type B element having a property of enhancing the structural stability of the positive electrode active material and diffusing to the surface of the positive electrode active material and;

On the particle cross section of the cathode active material, the species A is distributed in a concentration gradient increasing from the center to the surface of the particle, and the species B is distributed in a concentration gradient increasing from the particle surface to the center .

Herein, the element enhancing the surface stability of the cathode active material refers to an element exhibiting an effect of suppressing the cation mixing phenomenon of the surface when the elements are placed on the surface of the cathode active material.

In addition, when the lithium ion of the cathode active material is excessively desorbed, the element which improves the structural stability of the cathode active material may lose the reversibility of the lithium removal / insertion reaction due to the change of the crystal structure as the amount of lithium decreases, Refers to an element capable of stabilizing the crystal structure of the cathode active material by being substituted at the lithium position of the cathode active material particle.

In addition, the property of the cathode active material to permeate into the center portion of the cathode means that the element of the dopant intends to penetrate into the center of the cathode active material in the process of manufacturing the cathode active material, Means the property that the element of the dopant diffuses to the surface of the cathode active material in the process of manufacturing the active material.

On the other hand, the cathode active material according to the present invention is characterized in that the A-type element showing the surface stabilizing effect of the cathode active material on the particle cross section of the cathode active material is distributed in a concentration gradient in which the concentration increases from the center to the surface of the cathode active material, The B-type element which increases the structural stability may be distributed in a concentration gradient in which the concentration increases from the particle surface to the central portion. In this case, the effect of suppressing cation mixing of the A-type element mainly distributed in the surface layer of the positive electrode active material is exhibited to improve the safety of the surface of the positive electrode active material, and the B-type element is mainly distributed in the particles Thereby providing a cathode active material having an internal structure stabilized.

In one specific example, the species A may have a concentration gradient that decreases from 1 to 10% per 0.1 distance of the average particle size (R) of the cathode active material from the particle surface to the center, based on the concentration of the particle surface And the B-type element may have a concentration gradient decreasing by 1 to 10% with respect to a distance of 0.1 times the average particle diameter (R) of the cathode active material from the center of the particle to the surface based on the concentration of the center of the particle.

In one specific example, the A species element is not particularly limited as long as it is an element that inhibits the cation mixing phenomenon of the particle surface. For example, it may be one or more selected from the group consisting of Y, Mg and Al And more specifically Mg.

In one specific example, the B-type element is not particularly limited as long as it is an element capable of enhancing the stability of the crystal structure. For example, the B-type element may be at least one element selected from the group consisting of Sn, Nb, Zr, W, And more particularly Ti.

The species A and the species B may be contained in an amount of 1 to 20% on a molar basis. When the species A or B exceeds 20%, the content of the element exhibiting a relatively low capacity is reduced, so that the capacity of the battery can be reduced. Conversely, when the content is less than 1% It can not exhibit predetermined surface safety and structural safety of particles, which is not preferable.

The cathode active material may be a lithium transition metal oxide represented by the following general formula (1).

Li _x M _{y a a} B _b O ₂ (1)

In this formula,

M is one or more selected from the group consisting of Mn, Ni, Co, Cr, V, Fe, Cu and Zn;

A is at least one selected from the group consisting of Y, Mg and Al;

B is at least one selected from the group consisting of Sn, Nb, Zr, W, and Ti;

x + y + a + b = 2, and 0.95? x?

In detail, M may be at least one selected from the group consisting of Mn, Ni and Co, A may be Mg or Al, and B may be Ti, more particularly, M is Co , A is Mg, and B is Ti.

On the other hand, in the prior art, when the cathode active material including the A-type element and the B-type element is produced, the A-type element can improve the surface stability of the cathode active material, It is difficult to exhibit excellent surface stability because the surface of the positive electrode active material has a small amount. On the other hand, the B-type element can increase the structural stability, but has a property of lowering the surface stability of the positive electrode active material and diffusing to the surface of the particles, There is a problem that the surface stability of the cathode active material is further deteriorated because the B-type element is mainly located on the particle surface of the cathode active material.

In order to solve the above problems, the inventors of the present application have conducted intensive studies and experiments on a manufacturing method capable of locating the A species element on the particle surface and placing the B element in the center of the particle, When the B-type element is doped in the step of producing the precursor of the cathode active material and the A-type element is solid-phase doped to the prepared precursor, the B-type element is mainly located at the center of the particle and the A- It has been confirmed that the positive electrode active material can be produced by a structure mainly located on the particle surface, and thus the present invention has been accomplished.

Therefore, the cathode active material according to the present invention may be doped in the step of preparing the precursor of the cathode active material, and the A-type element may be doped into the precursor in a solid phase.

Here, the solid phase doping is a method of doping the dopant by using the principle that solid particles are diffused at a high temperature. For example, the solid particles containing the A species, the lithium compound, and the prepared precursor are dry- The mixture is heat-treated at a high temperature to prepare a cathode active material doped with the species A of the element.

Meanwhile, in one specific example, the cathode active material according to the present invention can be produced by a wet precipitation method, and more specifically, can be divided into a co-precipitation method or an oxidative precipitation method depending on the method of forming the precursor.

Specifically, a method for producing a cathode active material by coprecipitation includes:

i) preparing a transition metal aqueous solution comprising a transition metal salt for producing a precursor and a salt including a B-element;

ii) adding a strong base to the transition metal aqueous solution and coprecipitating it to synthesize a precursor;

iii) washing the synthesized precursor with distilled water and drying it for at least 1 hour in an incubator at 60 ° C to 120 ° C;

vi) preparing a mixture by mixing the dried precursor, the lithium compound, and the salt containing the species A together; And

v) firing the mixture in an oxidizing atmosphere;

. ≪ / RTI >

However, when the precursor of the cathode active material is prepared by the coprecipitation method, the precursor is produced in a crystalline form such as a hydroxide (M-OH) or an oxide compound. Therefore, in order to produce a crystalline cathode active material, There was a need to perform a pretreatment process of oxidizing at 60 DEG C for about 1 hour before. In addition, ammonia is generally used as a pH regulator in the coexistence. Since the use of such ammonia in a large amount is limited, there is a great difficulty in controlling the pH control when such co-precipitation is applied in mass production.

Therefore, the cathode active material of the present invention may be prepared by an oxidative precipitation method which can overcome the disadvantage of the coprecipitation method. Specifically, a method of preparing a cathode active material by using an oxide precipitation method,

i) preparing a transition metal aqueous solution comprising a transition metal salt for producing a precursor and a salt including a B-element;

ii) a step of adding an oxidizing agent to the transition metal aqueous solution to oxidize and precipitate to form a transition metal precursor;

iii) preparing a mixture by mixing the transition metal precursor, the lithium compound, and the salt including the element A; And

vi) firing the mixture in an oxidizing atmosphere;

And a control unit.

In this oxidation precipitation method, after the prepared precursor is washed, a salt containing a lithium compound and an element A can be simultaneously fired without a pretreatment step, and a transition metal precursor of an amorphous phase can be generated, The cathode active material having an excellent crystallinity can be obtained even if the sintering process is performed at a lower temperature than that of the cathode active material.

On the other hand, the desired average particle diameter, particle diameter distribution, and particle density can be controlled by properly controlling the temperature, pH, reaction time, slurry concentration, ion concentration and the like in the above production method.

In one specific example, the pH value may be adjusted using an oxidizing agent or basic compound to the transition metal aqueous solution, for example, the oxidizing agent is _{NaClO, Na 2 S 2 O 8} , K 2 S 2 O 8, NaClO 2 , And (NH ₄ ) ₂ S ₂ O _8. Examples of the basic compound include ammonia, KOH, LiOH, NaOH, NH ₄ OH, CH ₃ COOH, or HCN have.

The transition metal salt for producing the precursor preferably has an anion which is easily decomposed and easily volatilized during firing, and may be a sulfate or nitrate, and in particular may be a sulfate. The sulfates may be, for example, but not limited to, one or more selected from the group consisting of nickel sulfate, cobalt sulfate, and manganese sulfate.

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

At this time, the operating voltage of the lithium secondary battery can range from a minimum of 2.5V to 4.25V and a maximum of 2.5V to 4.6V. This indicates that the structural stability of the lithium-excess transition metal oxide represented by the above formula (1) can operate at a relatively high voltage as the safety of the battery is improved. Therefore, the upper limit value of the operating voltage of the lithium secondary battery according to the present invention may be 4.25 V to 4.6 V, and may be 4.3 V to 4.35 V in view of the stability of the lithium complex transition metal oxide represented by the general formula (1).

On the other hand, the lithium secondary battery generally comprises the positive electrode, the negative electrode, the separator, and a non-aqueous electrolyte containing a lithium salt.

The positive electrode is prepared, for example, by applying a slurry of a mixture of a positive electrode active material, a conductive material and a binder according to the present invention on a positive electrode current collector, followed by drying. If necessary, the positive electrode active material, The mixture (electrode mixture) of the binder and the like may further contain at least one substance selected from the group consisting of a viscosity adjusting agent and a filler.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery, and may be formed of a material such as stainless steel, aluminum, nickel, titanium, sintered carbon, or a surface of aluminum or stainless steel Treated with carbon, nickel, titanium, silver or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The conductive material is a component for further improving the conductivity of the electrode active material and may be added in an amount of 0.01 to 30% by weight based on the total weight of the electrode mixture. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon fibers such as carbon nanotubes and fullerene; conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 50 wt% based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is optionally used as an auxiliary component for suppressing the expansion of the electrode. The filler is not particularly limited as long as it is a fibrous material without causing chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The negative electrode is manufactured by applying a negative electrode material onto the negative electrode collector and drying the same, and may further include components such as a conductive material and a binder as described above, if necessary.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt. As the non-aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte and the like are used.

Examples of the nonaqueous electrolytic solution include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, But are not limited to, lactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, Nitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives , Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving charge / discharge characteristics, flame retardancy, etc., non-aqueous electrolytes may be used in the form of, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride and the like are added It is possible. In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene carbonate, PRS (propene sultone), FEC (fluoro-ethylene carbonate), and the like.

The present invention also provides a battery module including the secondary battery as a unit cell, and a battery pack including the battery module.

The battery pack can be used as a power source for a device requiring a high capacity, excellent output characteristics, and battery safety. Specific examples of the battery pack include a compact device such as a computer, a mobile phone, a power tool, A power tool powered and moved by the power tool; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart; Power storage systems, and the like, but are not limited thereto.

As described above, the positive electrode active material according to the present invention is characterized in that at least one A-type element having the property of increasing the surface stability of the positive electrode active material and penetrating the center of the positive electrode active material is mainly located on the surface of the positive electrode active material, At least one or more B-type elements having the property of improving the structural stability and diffusing to the surface of the particles are mainly located in the center of the particles of the cathode active material, thereby ensuring the stability of the particle surface and stabilizing the internal structure.

In addition, when the cathode active material according to the present invention is prepared by an oxidative precipitation method, not only the process efficiency can be improved but also a crystalline cathode active material can be produced as compared with the conventional wet process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the results of measurement of surface energy (E _surf ) of cathode active materials according to various preparation examples of the present invention;
2 is a graph showing the results of measuring energy (E _mix ) which suppresses cation mixing of the cathode active materials according to various preparation examples of the present invention;
3 is a schematic view showing a concentration gradient of Mg and Ti of a cathode active material particle according to an embodiment of the present invention;
4 is a schematic view showing a concentration gradient of Mg and Ti of a cathode active material particle according to one comparative example of the present invention.

Hereinafter, the present invention will be described with reference to Examples. However, the following Examples are intended to illustrate the present invention, and the scope of the present invention is not limited thereto.

≪ Preparation Example 1 &

Preparation of cathode active material

The tank for 3 L wet reactor was filled with 2 L of distilled water, and nitrogen gas was continuously injected into the tank at a rate of 1 L / min to remove dissolved oxygen. At this time, the temperature of the distilled water in the tank was maintained at 45 to 50 ° C by using a temperature holding device. The distilled water in the tank was stirred at a speed of 1000 to 1200 rpm using an impeller connected to a motor provided outside the tank.

(Molar ratio) of nickel sulfate (NiSO ₄ ), manganese sulfate (MnSO ₄ ), and cobalt sulfate (CoSO ₄ ) to prepare a 1.5M transition metal aqueous solution. Separately, a 3M aqueous solution of sodium hydroxide Were prepared. The transition metal aqueous solution was continuously pumped by a metering pump into the tank for the wet reactor at 0.18 L / hr. The aqueous sodium hydroxide solution was pumped with a control device to adjust the pH of the distilled water in the tank so that the distilled water in the wet reactor tank was maintained at a pH of 11.0 to 11.5. At this time, a 30% ammonia solution as an additive was continuously pumped into the reactor at a rate of 0.035 L to 0.04 L / hr.

The average retention time in the wet reactor tank of the solution was adjusted to 5 to 6 hours by adjusting the flow rate of the transition metal aqueous solution, sodium hydroxide aqueous solution and ammonia solution. After the reaction in the tank reached a steady state, The nickel-cobalt-manganese complex transition metal precursor prepared by continuously reacting the transition metal ion of the metal aqueous solution, the hydroxide ion of sodium hydroxide, and the ammonia solution of the ammonia solution for 20 hours is supplied to the overflow pipe Lt; / RTI >

The complex transition metal precursor thus obtained was washed several times with distilled water and dried in a constant temperature drier at 120 ° C for 24 hours to obtain a nickel-cobalt-manganese complex transition metal precursor ((Ni _0.6 Mn _0.2 Co _0.2 ) OOH).

The nickel-cobalt-manganese composite transition by dry mixing Li ₂ CO ₃ with a metal precursor and a dopant Al (Al ₂ O _3), thereby forming a positive electrode active material in Centigrade 910 degree for 10 hours _{((Li (Ni 0.6 Mn 0.2} Co _0.2 ) _0.998 Al _0.002 ) O ₂ ).

≪ Production Examples 2 to 12 >

Preparation of cathode active material

Except that dopant Mg, Si, Y, Ti, F, Sn, Zr, B, Nb, W or Mo was doped in place of the dopant Al in Production Example 1. The cathode active material powders were prepared.

<Experimental Example 1>

Analysis of the energy changes of the particle surfaces of the cathode active materials of Production Examples 1 to 12 was performed to predict the preferred position of the dopant. The results of the analysis are shown in Table 1 and FIG.

Dopant ㅿ Surface Energy (eV) Dopant ㅿ Surface Energy (eV) (100) (100) Al 0.720 B -1,296 Mg 0.384 Si 0.138 Ti -0.633 Y 0.223 W -2.072 Nb -1.629 Zr -1.074 Mo -2,273 F -0.896 Sn -0.906

When the amount of energy change (E _surf ) on the surface of the cathode active material shows a positive value with respect to 0, it means that the element of the dopant has a property of being located at the center of the particles of the cathode active material. On the contrary, When the amount of surface energy change (E _surf ) shows a negative value with respect to 0, it means that the element of the dopant has a property of being positioned on the surface of the particle of the cathode active material.

The amount of energy change (E _surf ) of the cathode active material surface in the production examples is represented by Al>Mg>Si>Y>0.0>Ti>F>Sn>Zr>B>Nb> Al, Mg, Si, and Y, which have a positive value based on 0, have a property of being located at the center of the particle of the cathode active material, and have negative It can be confirmed that Ti, F, Sn, Zr, B, Nb, W and Mo representing the value have a property of being positioned on the surface of the particles of the cathode active material.

<Experimental Example 2>

Cation mixing phenomenon was observed on the cathode active material surfaces of Production Examples 1, 2, 5, 7, 8, 11, and 12 in which each of Al, Mg, Sn, Zr, W and Ti was doped. (E _mix ), and the results of the analysis are shown in the graph of FIG. 2 below.

Referring to the graph of FIG. 2, it can be seen that only Preparation Examples 1 and 2 in which Al and Mg are doped have an inhibitory energy (E _mix ) exceeding a critical energy value preventing cation mixing phenomenon . That is, it has been analyzed that the elements Al and Mg can effectively suppress the cationic mixing on the surface of the cathode active material, thereby improving the surface stability of the cathode active material.

As a result, although Al or Mg is a material capable of improving the surface stability of the cathode active material, it has a property of being positioned at the center of the cathode active material particle, thereby effectively preventing the cation mixing phenomenon on the surface of the cathode active material Ti is a substance capable of causing cation mixing on the surface but has a property of being positioned on the surface of the particle. Therefore, the cation mixing of the surface is further intensified, It can be expected that the risk that the metal may be located may increase.

&Lt; Example 1 >

Preparation of cathode active material

First, cobalt sulfate hexahy-drate (CoSO _{4 ·} 7H ₂ O) was dissolved in distilled water to prepare a cobalt sulfate solution having a cobalt concentration of 3.0 g / L. To this cobalt sulfate solution, dopant Ti (TiO ₂ ) Solution. Sodium persulfate, an oxidant, was prepared by dissolving Na ₂ S ₂ O ₈ .7H ₂ O in distilled water at a concentration of 10% (w / v). Then, 5M NaOH solution was used to adjust the pH of the mixed solution, and the first reagent was used for all the chemicals used.

The pre-prepared mixed solution was put in a reaction vessel, and after the solution reached a predetermined temperature, a certain amount of oxidizing agent was added while stirring at a constant stirring speed. In order to keep the pH constant, 5M NaOH was automatically added through the pump connected to the pH controller.

The mixed solution in the reaction tank reacted with the oxidizing agent to cause an oxidative precipitation reaction, and a cobalt transition metal precursor containing Ti, which is solid solution, was obtained from these precipitates.

The precursor thus obtained is washed with water to remove impurities, and then the precursor and the dopant Mg (MgCO ₃ ) are mixed with Li ₂ CO ₃ at a molar ratio of each composition, and then heated at an elevating rate of 3 ° C / Followed by baking at 550 DEG C for 10 hours to prepare a cathode active material ((LiCo _0.996 Ti _0.002 Mg _0.002 ) O ₂ ) powder.

&Lt; Comparative Example 1 &

Preparation of cathode active material

Cobalt transition metal precursor (Co (OH) ₂ ) _, dopant Mg (MgCO ₃ ), dopant Ti (TiO ₂ ) and Li ₂ CO ₃ were mixed in a dry state and then fired at 910 ° C. for 10 hours to obtain a cathode active material (LiCo _0.996 Ti _0.002 Mg _0.002 ) O ₂ ).

<Experimental Example 3>

The structures of the cathode active material particles according to Example 1 and Comparative Example 1 were analyzed, and schematic diagrams of the results are shown in FIGS. 3 and 4, respectively.

3 and 4, the particles of the cathode active material prepared by the oxidative precipitation method of Example 1 of FIG. 3 are distributed in such a concentration gradient that the Mg element increases in concentration from the center of grains to the surface, and the Ti element Is distributed in a concentration gradient increasing from the particle surface to the center.

On the other hand, the particles of the cathode active material produced by the dry calcination method of Comparative Example 1 of FIG. 4 were distributed in a concentration gradient in which the concentration of Ti element increased from the center of the particle to the surface, and the Mg element decreased from the particle surface to the center And the concentration gradient is increased.

Therefore, in the method for producing a positive electrode active material according to the present invention, Ti is diffused to the particle surface of the positive electrode active material, and Mg has a property of penetrating into the center of the particle of the positive electrode active material, , Mg is doped mainly in the surface lattice of the cathode active material, and Ti is formed mainly in the center of the particles by doping the dopant Ti in advance in the manufacturing step of the cathode active material and then doping the prepared precursor with the dopant Mg in a solid phase. This structure maximizes the effect of suppressing cation mixing of the particle surface of the cathode active material and improves the internal structure stability of the particles.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (25)

A method for producing a positive electrode active material,
i) preparing a transition metal aqueous solution comprising a transition metal salt for producing a precursor and a salt including a B-element having a property of enhancing the structural stability of the cathode active material and diffusing to the surface of the cathode active material;
ii) adding a strong base to the transition metal aqueous solution and coprecipitating it to synthesize a precursor;
iii) washing the synthesized precursor with distilled water and drying it for at least 1 hour in an incubator at 60 ° C to 120 ° C;
vi) preparing a mixture by mixing the dried precursor, the lithium compound, and the salt including the element A having the property of increasing the surface stability of the cathode active material and penetrating the center of the cathode active material, together; And
v) firing the mixture in an oxidizing atmosphere;
Wherein the species A is distributed in a concentration gradient in which the concentration increases from the center of grains to the surface and the concentration of the species B in the concentration gradient increases from the surface of the particles to the center thereof And a positive electrode active material,
Wherein the element A is at least one element selected from the group consisting of Y, Mg and Al, the element B is at least one element selected from the group consisting of Sn, Nb, Zr, W and Ti, And at least one metal salt selected from the group consisting of Mn, Ni, Co, Cr, V, Fe, Cu, and Zn. delete delete delete delete delete delete delete delete delete delete delete delete delete A method for producing a positive electrode active material,
i) preparing a transition metal aqueous solution comprising a transition metal salt for producing a precursor and a salt including a B-element having a property of enhancing the structural stability of the cathode active material and diffusing to the surface of the cathode active material;
ii) a step of adding an oxidizing agent to the transition metal aqueous solution to oxidize and precipitate to form a transition metal precursor;
iii) preparing a mixture by mixing the transition metal precursor, the lithium compound, and the salt including the element A having the property of increasing the surface stability of the cathode active material and penetrating the center of the cathode active material; And
vi) firing the mixture in an oxidizing atmosphere;
Wherein the species A is distributed in a concentration gradient in which the concentration increases from the center of grains to the surface and the concentration of the species B in the concentration gradient increases from the surface of the particles to the center thereof And a positive electrode active material,
Wherein the element A is at least one element selected from the group consisting of Y, Mg and Al, the element B is at least one element selected from the group consisting of Sn, Nb, Zr, W and Ti, And at least one metal salt selected from the group consisting of Mn, Ni, Co, Cr, V, Fe, Cu, and Zn. The method of claim 15 wherein the oxidizing agent is characterized in that at least one selected from the group consisting of _{NaClO, Na 2 S 2 O 8} , K 2 S 2 O 8, NaClO 2, and _{(NH 4) 2 S 2 O} 8 A method for producing a cathode active material. 16. The method of claim 1 or 15, wherein the transition metal salt for preparing the precursor is a sulfate. 18. The method of claim 17, wherein the sulfate is one or more selected from the group consisting of nickel sulfate, cobalt sulfate, and manganese sulfate. delete delete The method according to any one of claims 1 to 15, wherein the concentration of the species A is 1 to 10% of the average particle size (R) of the cathode active material, Wherein the positive electrode active material has a slope of at least 20 占 퐉. The method according to any one of claims 1 to 15, wherein the B-type element has a concentration decreasing from 1% to 10% of the average particle diameter (R) of the cathode active material Wherein the positive electrode active material has a slope of at least 20 占 퐉. The method of claim 1 or 15, wherein the transition metal salt for preparing the precursor comprises at least one metal salt selected from the group consisting of Mn, Ni and Co. The method for producing a cathode active material according to claim 1 or 15, wherein the species A is Mg or Al. The method of producing a cathode active material according to claim 1 or 15, wherein the B element is Ti.

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