KR20170075915A - Positive active material for rechargeable lithium battery, method of preparing the same, and rechargeable lithium battery including the same - Google Patents

Abstract

A cathode active material for a lithium secondary battery according to an embodiment of the present invention includes a core portion; Concentration gradient distribution; And a surface portion, wherein the content of Ni, Co, and Mn in the core portion is constant, and the content of Co in the core portion is about 100 mol% of Ni, Co, and Mn , The content of Ni, Co, and Mn in the surface portion is constant, and the content of Co is 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn, Gradually increases from the core portion toward the surface portion.

Description

TECHNICAL FIELD The present invention relates to a cathode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

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

Recently, with regard to the tendency to miniaturize and lighten portable electronic devices, there is an increasing need for high performance and large capacity of batteries used as power sources for these devices.

Cells generate electricity by using materials that can electrochemically react to the positive and negative electrodes. A representative example of such a battery is a lithium secondary battery that generates electrical energy by a change in chemical potential when the lithium ions are intercalated / deintercalated in the positive electrode and the negative electrode.

The lithium secondary battery is manufactured by using a material capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, and filling an organic electrolyte or a polymer electrolyte between the positive electrode and the negative electrode.

As a cathode active material of a lithium secondary battery, a lithium composite metal compound is used. For example, composite metal oxides such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ and LiMnO ₂ have been studied.

Of the above cathode active materials, Mn-based cathode active materials such as LiMn ₂ O ₄ and LiMnO ₂ are easy to synthesize and are relatively inexpensive and have excellent thermal stability compared to other active materials in overcharging, However, it has a disadvantage of low capacity.

LiCoO ₂ is a typical cathode active material commercially available and commercially available since it has good electric conductivity, high battery voltage of about 3.7 V, excellent cycle life characteristics, stability and discharge capacity. However, since LiCoO ₂ is expensive, it accounts for more than 30% of the battery price, which causes the price competitiveness to deteriorate.

LiNiO ₂ also exhibits the highest discharge capacity of the battery among the above-mentioned cathode active materials, but it is difficult to synthesize LiNiO ₂ . Also, the high oxidation state of nickel causes degradation of battery life and electrode life, and there is a problem that self discharge is severe and reversibility is low. In addition, it is difficult to commercialize it because the stability is not completely secured.

The present invention provides a positive electrode active material for a lithium secondary battery improved in structural stability and electrochemical characteristics, and a lithium secondary battery including the positive electrode active material.

A cathode active material for a lithium secondary battery according to an embodiment of the present invention includes a core portion; Concentration gradient distribution; And a surface portion, wherein the content of Ni, Co, and Mn in the core portion is constant, and the content of Co in the core portion is about 100 mol% of Ni, Co, and Mn , The content of Ni, Co, and Mn in the surface portion is constant, and the content of Co is 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn, Gradually increases from the core portion toward the surface portion.

The Co content in the core portion may be 0 to 15 mol%, relative to 100 mol% of Ni, Co, and Mn. More specifically, it may contain 75 to 85 mol% of Ni, 0 to 15 mol% of Co and 0 to 25 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the core portion.

The Co content in the surface portion may be 30 to 50 mol%, relative to 100 mol% of Ni, Co, and Mn. More specifically, it may contain 30 to 50 mol% of Ni, 30 to 50 mol% of Co and 10 to 40 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the surface portion.

And may further contain 0.001 to 0.01 molar percent of a metal selected from Mg, Cr, Zr, Ti and Al with respect to 100 molar parts of Ni, Co, and Mn in the positive electrode active material.

The thickness of the core portion may be 1 to 10 mu m.

The thickness of the surface portion may be 0.01 to 0.5 mu m.

The thickness of the concentration gradient portion may be 1 to 10 mu m.

A method of preparing a cathode active material for a lithium secondary battery according to an embodiment of the present invention includes: preparing a first precursor solution containing Ni, Co, and Mn; Preparing a second precursor solution comprising Ni, Co, and Mn; Injecting only the first precursor solution into the reactor to produce a core portion; Adding the first precursor solution and the second precursor solution while increasing the amount of the second precursor solution to prepare a concentration gradient; Wherein the Co content in the first precursor solution is less than 20 mol% based on 100 mol% of Ni, Co, and Mn, and the Co content in the second precursor solution is less than 20 mol% The content is 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn.

The Co content in the first precursor solution may be 0 to 15 mol%, based on 100 mol% of Ni, Co, and Mn. More specifically, it may contain 75 to 85 mol% of Ni, 0 to 15 mol% of Co and 0 to 25 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the first precursor solution.

The Co content in the second precursor solution may be 30 to 50 mol%, based on 100 mol% of Ni, Co, and Mn. More specifically, it may contain 30 to 50 mol% of Ni, 30 to 50 mol% of Co and 10 to 40 mol% of Mn, based on 100 mol% of Ni, Co and Mn in the second precursor solution.

And mixing and firing a precursor having a surface portion and a lithium source material.

Precipitating and doping the precursor in which the surface portion is formed and a metal oxide solution selected from MgO, Cr ₂ O ₃ , ZrO ₂ , TiO _2, and Al ₂ O ₃ .

According to an embodiment of the present invention, a cathode active material for a lithium secondary battery improved in structural stability and electrochemical characteristics can be provided. Particularly, the life characteristics are improved.

1 is a scanning electron microscope (SEM) photograph of the cathode active material prepared according to Example 1. Fig.
2 is an SEM photograph of the cathode active material prepared according to Example 2. Fig.
3 is a SEM photograph of the cathode active material prepared according to Comparative Example 1. FIG.
4 is a graph showing the initial charge and discharge of 4.25 V of the coin battery manufactured in Examples 1 and 2 and Comparative Example 1. Fig.
5 is a graph showing various rate characteristics of the coin battery manufactured in Examples 1 and 2 and Comparative Example 1 at 4.25 V. FIG.
6 is a graph showing lifetime characteristics at 4.3 V of the coin battery manufactured in Examples 1 and 2 and Comparative Example 1. FIG.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid an undesirable interpretation of the present invention. Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Whenever a component is referred to as "including" an element throughout the specification, it is to be understood that the element may include other elements, not the exclusion of any other element, unless the context clearly dictates otherwise. Also, singular forms include plural forms unless the context clearly dictates otherwise.

A cathode active material for a lithium secondary battery according to an embodiment of the present invention includes a core portion; Concentration gradient distribution; And a surface portion, wherein the content of Ni, Co, and Mn in the core portion is constant, and the content of Co in the core portion is about 100 mol% of Ni, Co, and Mn , The content of Ni, Co, and Mn in the surface portion is constant, and the content of Co is 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn, Gradually increases from the core portion toward the surface portion.

First, the core portion is located at the center of the cathode active material for a lithium secondary battery. The Co content in the core portion may be less than 20 mol%, based on 100 mol% of Ni, Co, and Mn. Specifically, the Co content in the core portion may be 0 to 15 mol% based on 100 mol% of Ni, Co, and Mn. More specifically, it may contain 75 to 85 mol% of Ni, 0 to 15 mol% of Co and 0 to 25 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the core portion. The thickness of the core portion may be 1 to 10 mu m. In this case, the thickness of the core portion means the length from the center portion of the cathode active material to the outermost portion of the core portion. In other words, it means the radius of core equivalent to the core.

The concentration gradient portion is located at the outer periphery of the central portion, and the content of Co in the concentration gradient portion gradually increases from the core portion toward the surface portion. The content of Mn as well as the content of Co in the concentrate portion can be gradually increased from the core portion toward the surface portion. The content of Ni can be gradually reduced from the core portion toward the surface portion.

The thickness of the concentration gradient portion may be 1 to 10 mu m. The thickness of the concentration gradient section means the length from the boundary between the concentration gradient section and the core section to the boundary between the concentration gradient section and the surface section.

The Co content at the boundary between the concentration gradient section and the core section may be the same as the Co content of the core section. Conversely, the Co content at the interface between the concentration gradient and the surface portion may be the same as the Co content at the surface portion.

The slope of the concentration gradient at the concentration gradient can be more than one.

The surface portion is located at the outer periphery of the concentration gradient portion. The Co content in the surface portion may be 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn. When the Co content in the core portion is less than 30 mol%, the initial discharge capacity may be reduced and the output characteristics may be poor. Specifically, the Co content in the surface portion may be 30 to 50 mol% based on 100 mol% of Ni, Co, and Mn. More specifically, it may contain 30 to 50 mol% of Ni, 30 to 50 mol% of Co and 10 to 40 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the surface portion. The thickness of the surface portion may be 0.01 to 0.5 mu m. In this case, the thickness of the surface portion means the length from the boundary between the concentration gradient portion and the surface portion to the outermost surface portion.

And may further contain 0.001 to 0.01 molar percent of a metal selected from Mg, Cr, Zr, Ti and Al with respect to 100 molar parts of Ni, Co, and Mn in the positive electrode active material.

Hereinafter, a method for producing a cathode active material for a lithium secondary battery according to an embodiment of the present invention will be described.

A method of preparing a cathode active material for a lithium secondary battery according to an embodiment of the present invention includes: preparing a first precursor solution containing Ni, Co, and Mn; Preparing a second precursor solution comprising Ni, Co, and Mn; Injecting only the first precursor solution into the reactor to produce a core portion; Adding the first precursor solution and the second precursor solution while increasing the amount of the second precursor solution to prepare a concentration gradient; Wherein the Co content in the first precursor solution is less than 20 mol% based on 100 mol% of Ni, Co, and Mn, and the Co content in the second precursor solution is less than 20 mol% The content is 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn.

First, a first precursor solution containing Ni, Co, and Mn is prepared. The Co content in the first precursor solution may be less than 20 mol%, based on 100 mol% Ni, Co, and Mn. Specifically, the Co content in the first precursor solution may be 0 to 15 mol%, based on 100 mol% of Ni, Co, and Mn. More specifically, it may contain 75 to 85 mol% of Ni, 0 to 15 mol% of Co and 0 to 25 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the first precursor solution.

Next, a second precursor solution containing Ni, Co, and Mn is prepared. The Co content in the second precursor solution may be 30 mol% or more, relative to 100 mol% of Ni, Co, and Mn. When the Co content in the first precursor solution is less than 30 mol%, the initial discharge capacity may be reduced and the output characteristics may be poor. Specifically, the Co content in the second precursor solution may be 30 to 50 mol%, based on 100 mol% of Ni, Co, and Mn. More specifically, it may contain 30 to 50 mol% of Ni, 30 to 50 mol% of Co and 10 to 40 mol% of Mn, based on 100 mol% of Ni, Co and Mn in the second precursor solution.

Next, only the first precursor solution is charged into the reactor to prepare the core portion. At this time, it is possible to adjust the thickness of the core portion by appropriately adjusting the time for injecting only the first precursor solution.

Next, the first precursor solution and the second precursor solution are added, and the second precursor solution is added while increasing the amount of the precursor solution to prepare a concentration gradient portion. At this time, the amount of the first precursor solution to be introduced can be relatively reduced, and the addition of the first precursor solution can be stopped when the concentration of the precursor solution is completed. At this time, the concentration gradient can be adjusted by changing the amount of increase of the amount of the second precursor solution.

Next, only the second precursor solution is injected to prepare the surface portion. At this time, the thickness of the surface portion can be adjusted by appropriately adjusting the time for injecting only the second precursor solution.

And mixing and firing a precursor having a surface portion and a lithium source material.

Precipitating and doping the precursor in which the surface portion is formed and a metal oxide solution selected from MgO, Cr ₂ O ₃ , ZrO ₂ , TiO _2, and Al ₂ O ₃ .

The positive electrode active material according to one embodiment of the present invention can be usefully used as a positive electrode of a lithium secondary battery. The lithium secondary battery includes a cathode and an electrolyte including an anode active material together with a cathode.

The positive electrode is prepared by preparing a positive electrode active material composition by mixing a positive electrode active material according to an embodiment of the present invention, a conductive material, a binder and a solvent, and then directly coating and drying on the aluminum current collector. Or by casting the positive electrode active material composition on a separate support, then peeling the support from the support, and laminating the resulting film on an aluminum current collector.

The conductive material may be carbon black, graphite, metal powder, and the binder may include vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene And mixtures thereof. Further, N-methylpyrrolidone, acetone, tetrahydrofuran, decane and the like are used as the solvent. At this time, the content of the cathode active material, the conductive material, the binder and the solvent is used at a level normally used in a lithium secondary battery.

The negative electrode is prepared by mixing the negative electrode active material, the binder and the solvent in the same manner as the positive electrode. The anode active material composition is coated directly on the copper current collector or cast on a separate support, and the negative active material film, Laminated. At this time, the negative electrode active material composition may further contain a conductive material if necessary.

As the negative electrode active material, a material capable of intercalating / deintercalating lithium is used. For example, lithium metal, lithium alloy, coke, artificial graphite, natural graphite, organic polymeric compound combustion material, . The conductive material, the binder and the solvent are used in the same manner as in the case of the above-mentioned anode.

The separator may be polyethylene, polypropylene, polyvinylidene fluoride or a multilayer film of two or more thereof. The separator may be a polyethylene / polypropylene double-layer separator, It is needless to say that a mixed multilayer film such as a polyethylene / polypropylene / polyethylene three-layer separator, a polypropylene / polyethylene / polypropylene three-layer separator and the like can be used.

As the electrolyte to be charged into the lithium secondary battery, a non-aqueous electrolyte or a known solid electrolyte may be used, and a lithium salt dissolved therein may be used.

The solvent of the non-aqueous electrolyte is not particularly limited, but cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; Chain carbonates such as dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate; Esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and? -Butyrolactone; Ethers such as 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane and 2-methyltetrahydrofuran; Nitriles such as acetonitrile; Amides such as dimethylformamide and the like can be used. These may be used singly or in combination. Particularly, a mixed solvent of cyclic carbonate and chain carbonate can be preferably used.

As the electrolyte, a gelated polymer electrolyte in which an electrolyte solution is impregnated with a polymer electrolyte such as polyethylene oxide or polyacrylonitrile, or an inorganic solid electrolyte such as LiI or Li ₃ N can be used.

Wherein the lithium salt is selected from the group consisting of LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCl, and LiI.

Hereinafter, the embodiment will be described in detail. The following examples are illustrative of the present invention only and are not intended to limit the scope of the present invention.

[ Example ] - Preparation of cathode active material

12 liters of distilled water was placed in a coprecipitation reactor (capacity 100 L, output of the rotary motor was 2.2 kW or more), N ₂ gas was supplied to the reactor at a rate of 5 liters / minute, and the reactor was stirred at 350 rpm while maintaining the temperature at 45 ° C .

The first precursor solution in which Ni _, Co _, and Mn were mixed at a mixture concentration of 2.5 M of nickel sulfate, cobalt sulfate, and manganese sulfate at the molar ratios set forth in Table 1 below _, and Ni _, Co _, and Mn were mixed at a molar ratio And the mixture of the first precursor solution and the second precursor solution was changed from 0 to 100 while being supplied at a rate of 1.5 liters / hour while supplying a 17 molar ammonia solution at a rate of 0.2 liters / hour Were continuously added to the reactor to prepare a precursor having a concentration gradient. The pH was adjusted to 11.4 by feeding a 6 molar sodium hydroxide solution for pH adjustment. At this time, the impeller speed was adjusted to 350 rpm.

The second precursor solution was mixed with the second precursor solution while changing the mixing ratio from 0 to 100, and then the second precursor solution was supplied at a fixed time of 30 minutes after the addition of the first precursor solution to prepare a core portion having a predetermined size , And the completion time of the first precursor solution was 30 minutes before the completion of the addition, thereby forming a metal complex hydroxide having a constant final concentration.

The first precursor solution (mol%) The second precursor solution (mol%) Core part thickness (radius, 탆) Concentration bulb thickness (㎛) Surface part thickness (탆) Ni Co Mn Ni Co Mn Comparative Example 1 80 20 0 40 20 40 4 4 0.05-0.1 Example 1 80 10 10 40 30 30 4 4 0.05-0.1 Example 2 80 0 20 40 40 20 3 5 0.05-0.1 Comparative Example 2 60 20 20 - - - - - -

The resulting metal complex hydroxide was filtered, washed with water, and then dried in a hot air dryer at 130 ° C for 12 hours. The metal composite hydroxide and LiOH were mixed at a molar ratio of 1.0: 1.05, heated at a heating rate of 2 ° C / min, pre-baked at 450 ° C and 700 ° C for 5 hours, Followed by firing to obtain a cathode active material powder.

[ Experimental Example ] - Cell Chemical Evaluation

For the cell chemical evaluation, the slurry was prepared by using the cathode active material prepared in Examples 1 and 2 and Comparative Examples 1 and 2 in a ratio of 92.5: 3.5: 4 in the ratio of the cathode active material, binder and carbon, After drying, press was performed. Vacuum-dried for 1 hour after pressing. The prepared electrode plate was used as an anode and a lithium metal was used as a cathode. A coin cell was fabricated to evaluate charge / discharge characteristics at 4.25 V according to various rates. Life characteristics were evaluated at 4.3 V and the results are shown in FIGS. . As shown in FIGS. 4 to 6, it can be confirmed that the cathode active materials prepared in Examples 1 and 2 are superior to those of Comparative Example 1 in charge / discharge and lifetime characteristics.

The output characteristics of the cathode active material powders prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated and the results are shown in Table 2 below.

0.1 / 0.1C 0.5 / 0.1C 0.5 / 1.0C 0.5 / 2.0C Comparative Example 1 174.8 175.1 161.6 155.8 Example 1 178.7 178.7 163.2 155.4 Example 2 178.0 178.1 164.0 157.2 Comparative Example 2 174.4 174.5 160.1 151.8

As shown in Table 2, the cathode active materials of Examples 1 and 2 exhibited improved initial discharge capacity and increased output characteristics as compared with the cathode active materials of Comparative Examples 1 and 2.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (16)

A core portion; Concentration gradient distribution; And a surface portion, and is a lithium composite oxide containing Ni, Co, and Mn,
The content of Ni, Co, and Mn in the core portion is constant, and the content of Co is less than 20 mol% based on 100 mol% of Ni, Co, and Mn,
The content of Ni, Co, and Mn in the surface portion is constant, and the content of Co is 30 mol% or more with respect to 100 mol% of Ni, Co, and Mn,
Wherein the content of Co in the concentration gradient portion gradually increases from the core portion toward the surface portion. The method according to claim 1,
Wherein the Co content in the core portion is 0 to 15 mol% based on 100 mol% of Ni, Co, and Mn. 3. The method of claim 2,
Wherein the core portion contains 75 to 85% by mole of Ni, 0 to 15% by mole of Co and 0 to 25% by mole of Mn relative to 100% by mole of Ni, Co and Mn in the core portion. The method according to claim 1,
Wherein the Co content in the surface portion is 30 to 50 mol% based on 100 mol% of Ni, Co, and Mn. 5. The method of claim 4,
, 30 to 50 mol% of Ni, 30 to 50 mol% of Co and 10 to 40 mol% of Mn relative to 100 mol% of Ni, Co and Mn in the surface portion. 6. The method of claim 5,
Further comprising 0.001 to 0.01 molar parts of a metal selected from Mg, Cr, Zr, Ti and Al with respect to 100 molar parts of Ni, Co and Mn in the positive electrode active material. The method according to claim 1,
And the thickness of the core portion is 1 to 10 占 퐉. The method according to claim 1,
Wherein the thickness of the surface portion is 0.01 to 0.5 占 퐉. The method according to claim 1,
Wherein the thickness of the concentration gradient portion is 1 to 10 占 퐉. Preparing a first precursor solution comprising Ni, Co, and Mn;
Preparing a second precursor solution comprising Ni, Co, and Mn;
Injecting only the first precursor solution into a reactor to produce a core;
Injecting the first precursor solution and the second precursor solution while increasing the amount of the second precursor solution to produce a concentration gradient; And
And injecting only the second precursor solution to prepare a surface portion,
The Co content in the first precursor solution is less than 20 mol%, based on 100 mol% Ni, Co, and Mn,
Wherein the Co content in the second precursor solution is 30 mol% or more based on 100 mol% of Ni, Co, and Mn. 11. The method of claim 10,
Wherein the Co content in the first precursor solution is 0 to 15 mol% based on 100 mol% of Ni, Co, and Mn. 12. The method of claim 11,
Wherein the first precursor solution contains 75 to 85% by mole of Ni, 0 to 15% by mole of Co and 0 to 25% by mole of Mn relative to 100% by mole of Ni, Co and Mn in the first precursor solution. 11. The method of claim 10,
Wherein the Co content in the second precursor solution is 30 to 50 mol% based on 100 mol% of Ni, Co, and Mn. 14. The method of claim 13,
Wherein the second precursor solution contains 30 to 50 mol% of Ni, 30 to 50 mol% of Co, and 10 to 40 mol% of Mn based on 100 mol% of Ni, Co, and Mn in the second precursor solution. 11. The method of claim 10,
And mixing and firing the precursor having the surface portion and the lithium source material. 11. The method of claim 10,
Further comprising the step of precipitating and doping the precursor formed with the surface portion and a metal oxide solution selected from MgO, Cr ₂ O ₃ , ZrO ₂ , TiO ₂ and Al ₂ O ₃ for doping.

Images