阅读说明：本技术 二次电池用正极活性材料、其制备方法及包含其的二次电池正极 (Positive electrode active material for secondary battery, method for preparing same, and secondary battery positive electrode comprising same ) 是由 卢恩帅 郑王谟 姜玟锡 李尚昱 白韶螺 王文秀 于 2019-11-28 设计创作，主要内容包括：本发明涉及二次电池用正极活性材料,所述正极活性材料是包括镍(Ni)和钴(Co)并且包括锰(Mn)和铝(Al)中的至少一种的锂复合过渡金属氧化物颗粒,其中,所述锂复合过渡金属氧化物颗粒在除锂之外的全部金属中包含60摩尔％以上的镍(Ni),掺杂元素掺杂在锂复合过渡金属氧化物颗粒上,并且锂复合过渡金属氧化物颗粒的颗粒强度为210MPa至290MPa。(The present invention relates to a positive electrode active material for a secondary battery, which is a lithium composite transition metal oxide particle including nickel (Ni) and cobalt (Co) and including at least one of manganese (Mn) and aluminum (Al), wherein the lithium composite transition metal oxide particle contains 60 mol% or more of nickel (Ni) in all metals except lithium, a doping element is doped on the lithium composite transition metal oxide particle, and the particle strength of the lithium composite transition metal oxide particle is 210MPa to 290 MPa.)

1. A positive electrode active material for a secondary battery, which is lithium composite transition metal oxide particles containing nickel (Ni) and cobalt (Co) and at least one of manganese (Mn) and aluminum (Al), wherein

The lithium composite transition metal oxide particle contains 60 mol% or more of nickel (Ni) in all metals except lithium,

a doping element is doped on the lithium composite transition metal oxide particles, and

the particle strength of the lithium composite transition metal oxide particles is 210MPa to 290 MPa.

2. The positive electrode active material according to claim 1, wherein the doping element is doped to 6 to 10 mol% with respect to all metals except lithium.

3. The cathode active material according to claim 1, wherein the doping element comprises at least one selected from the group consisting of P, B, Al, Si, W, Zr, and Ti.

4. The positive electrode active material according to claim 1, wherein the lithium composite transition metal oxide particles comprise a compound represented by the following formula 1:

[ formula 1]

Li_pNi_1-x-y-zCo_xM^a _yM^b _zO₂

Wherein, in formula 1, p is more than or equal to 1.0 and less than or equal to 1.5, x is more than 0 and less than or equal to 0.2, y is more than 0 and less than or equal to 0.2, z is more than or equal to 0.06 and less than or equal to 0.1, and x + y + z is more than 0 and less than or equal to 0.4,

M^ais at least one selected from the group consisting of Mn and Al, and

M^bis at least one selected from the group consisting of P, B, Al, Si, W, Zr and Ti.

5. The positive electrode active material according to claim 1, wherein the average particle diameter (D)₅₀) Is 8 μm to 30 μm.

6. The positive electrode active material according to claim 1, wherein the doping element is doped such that the concentration of the doping element decreases from the surface of the lithium composite transition metal oxide particle toward the center thereof.

7. The positive electrode active material according to claim 1, wherein a doping amount of a doping element is 70 mol% or more of a total number of moles of the doping element in a corresponding region from 60% to 100% of a center of the particle with respect to a radius of the lithium composite transition metal oxide particle.

8. The positive electrode active material according to claim 1, wherein the lithium composite transition metal oxide particle has a value of 0.7 to 1, which is obtained by substituting a doping element concentration of a particle surface and a doping element concentration of a particle center, which are obtained by Energy Dispersive Spectroscopy (EDS) analysis, into the following equation 1:

[ equation 1]

(Hs-Hc)/Hs

Wherein, in formula 1, H_sIs a doping element concentration, H, at the particle surface when the lithium composite transition metal oxide particles are analyzed by EDS_cIs a doping element concentration at the center of the particle when the lithium composite transition metal oxide particle is analyzed by EDS.

9. A method of manufacturing the positive electrode active material for a secondary battery according to claim 1, the method comprising:

mixing a transition metal hydroxide containing nickel (Ni) and cobalt (Co) and containing at least one of manganese (Mn) and aluminum (Al), in which nickel (Ni) is 60 mol% or more of the total metals, with a lithium compound, and performing a first firing on the mixture to prepare lithium composite transition metal oxide particles; and

mixing the lithium composite transition metal oxide particles with a doping source including a doping element, and subjecting the mixture to a second firing to dope the doping element on the lithium composite transition metal oxide particles.

10. The method of claim 9, wherein the dopant source comprises at least one dopant element selected from the group consisting of P, B, Al, Si, W, Zr, and Ti.

11. The method of claim 9, wherein the first firing is performed for 10 hours to 25 hours.

12. The method of claim 9, wherein the first firing is performed at 750 ℃ to 1000 ℃.

13. The method of claim 9, wherein the second firing is performed for 3 to 12 hours.

14. The method of claim 9, wherein the second firing is performed at 700 ℃ to 900 ℃.

15. A positive electrode for a secondary battery, the positive electrode comprising:

a positive current collector; and

a positive electrode active material layer formed on the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material for a secondary battery according to claim 1.

Technical Field

The present invention relates to a positive electrode active material for a secondary battery, a method for preparing the same, and a positive electrode for a secondary battery and a lithium secondary battery including the positive electrode active material.

Background

Recently, due to rapid spread of electronic devices (e.g., mobile phones, notebook computers, electric vehicles, etc.) using batteries, demand for small and lightweight secondary batteries of relatively high capacity is rapidly increasing. In particular, lithium secondary batteries are lightweight and have high energy density, and thus are receiving attention as driving power sources for portable devices. Accordingly, research and development work for improving the performance of the lithium secondary battery has been actively conducted.

When an organic electrolyte or a polymer electrolyte is charged between a positive electrode and a negative electrode made of an active material capable of intercalating and deintercalating lithium ions, the lithium secondary battery generates electric energy through oxidation and reduction reactions when lithium ions are intercalated/deintercalated from the positive electrode and the negative electrode.

As a positive electrode active material of a lithium secondary battery, lithium cobaltate (LiCoO) is mainly used₂). In addition, it is also contemplated to use LiMnO having a layered crystal structure₂Lithium manganese oxides (e.g., LiMn) having spinel-like crystal structures₂O₄) And lithium nickel oxide (LiNiO)₂)。

Recently, it has been proposed to use a lithium composite transition metal oxide in a form in which a part of nickel is substituted with another transition metal (e.g., manganese and cobalt). In particular, the lithium composite transition metal oxide containing a high content of nickel has an advantage of relatively excellent capacity characteristics.

However, in the case of the above-described cathode active material, the roll pressing treatment performed in the manufacture of the electrode may generate cracks on the particles, or structural collapse may occur due to repeated intercalation/deintercalation of lithium. These problems are urgently required to be solved because the battery performance is degraded due to particle breakage, structural collapse, and the like of the positive electrode active material.

Korean patent laid-open publication No. 10-2016 0053849 discloses a positive active material and a secondary battery including the same.

Disclosure of Invention

[ problem ] to

An aspect of the present invention provides a positive electrode active material for a secondary battery, which is capable of preventing breakage of particles in the positive electrode active material and improving structural stability thereof.

Another aspect of the present invention provides a positive electrode active material for a secondary battery, which has significantly improved life characteristics at high temperatures.

Another aspect of the present invention provides a method of preparing the above-described positive electrode active material for a secondary battery.

Still another aspect of the present invention provides a positive electrode for a secondary battery and a lithium secondary battery, each of which comprises the above-described positive electrode active material for a secondary battery.

[ solution ]

According to an aspect of the present invention, there is provided a positive electrode active material for a secondary battery, the positive electrode active material being lithium composite transition metal oxide particles including nickel (Ni) and cobalt (Co) and including at least one of manganese (Mn) and aluminum (Al), wherein the lithium composite transition metal oxide particles include 60 mol% or more of nickel (Ni) in all metals except lithium, the lithium composite transition metal oxide particles are doped with a doping element, and a particle strength of the lithium composite transition metal oxide particles is 210MPa to 290 MPa.

According to another aspect of the present invention, there is provided a method of preparing the above-described positive electrode active material for a secondary battery, the method comprising: mixing a transition metal hydroxide containing nickel (Ni) and cobalt (Co) and containing at least one of manganese (Mn) and aluminum (Al), in which nickel (Ni) is 60 mol% or more of the total metals, with a lithium compound; the mixture is subjected to a first firing to prepare lithium composite transition metal oxide particles, the lithium composite transition metal oxide particles are mixed with a doping source containing a doping element, and the mixture is subjected to a second firing to dope the doping element on the lithium composite transition metal oxide particles.

According to still another aspect of the present invention, there is provided a positive electrode for a secondary battery, including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, wherein the positive electrode active material layer includes the above-described positive electrode active material for a secondary battery.

According to still another aspect of the present invention, there is provided a lithium secondary battery including: the above-mentioned positive electrode for a secondary battery, a negative electrode opposed to the positive electrode for a secondary battery, a separator interposed between the positive electrode for a secondary battery and the negative electrode, and an electrolyte.

[ advantageous effects ]

The positive electrode active material for a secondary battery of the present invention is doped with a doping element and has a specific particle strength range. Therefore, the positive electrode active material may have significantly improved structural stability of particles, and may prevent a cracking problem in the particles.

In addition, since the positive electrode active material for a secondary battery of the present invention is excellent in structural stability and the particle strength is adjusted to a specific range, the positive electrode for a secondary battery and a lithium secondary battery including the above positive electrode active material for a secondary battery may have significantly improved life characteristics at high temperatures.

Description of the drawings

Fig. 1 is a graph showing capacity retention ratio of a lithium secondary battery including a positive electrode active material for a secondary battery of each of examples 1 to 4 and comparative examples 1 to 6, which is a ratio of discharge capacity per charge/discharge cycle to initial discharge capacity;

fig. 2 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of example 1, to identify breakage of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 3 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of example 2, to identify breakage of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 4 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of example 3, to identify breakage of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 5 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of example 4, to identify breakage of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 6 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of comparative example 1, to identify cracking of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 7 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of comparative example 2 to identify cracking of positive electrode active material particles after 400 times of charge/discharge at a high temperature (45 ℃);

fig. 8 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of comparative example 3 to identify cracking of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 9 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of comparative example 4 to identify cracking of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 10 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of comparative example 5 to identify cracking of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 11 is a Scanning Electron Microscope (SEM) photograph of a lithium secondary battery including the positive electrode active material for a secondary battery of comparative example 6, to identify cracking of positive electrode active material particles after 400 times of charge/discharge at high temperature (45 ℃);

fig. 12 is a graph showing analysis of distribution and content of a doping element in a positive electrode active material for a secondary battery of example 1 according to location, obtained by an Energy Dispersive Spectrometer (EDS); and

fig. 13 is a graph showing analysis of distribution and content of a doping element in the positive electrode active material for a secondary battery of comparative example 4 according to location, obtained by EDS.

Detailed Description

It will be understood that the words or terms used in the specification and claims of this invention should not be construed as limited to having the meanings defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the present invention, based on the principle that the inventor can appropriately define the meaning of the words or terms that best explain the present invention.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the invention. Terms in the singular may include the plural unless the context clearly dictates otherwise.

In this specification, it will be understood that the terms "comprises," "comprising," or "having" are intended to specify the presence of stated features, amounts, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, amounts, steps, elements, or combinations thereof.

In the present specification, the average particle diameter (D)₅₀) May be defined as the cumulative particle size corresponding to 50% of the volume in the particle size distribution curve for the particles. Average particle diameter (D)₅₀) Can be measured by, for example, laser diffraction. Laser diffraction methods are generally capable of measuring particle sizes from the submicron region to several millimeters, so that highly reproducible and high resolution results can be obtained.

Hereinafter, the present invention will be described in more detail.

< Positive electrode active Material for Secondary Battery >

The present invention relates to a positive electrode active material for a secondary battery, and particularly, to a positive electrode active material for a lithium secondary battery.

The positive electrode active material for a secondary battery of the present invention is a lithium composite transition metal oxide particle comprising nickel (Ni) and cobalt (Co) and at least one of manganese (Mn) and aluminum (Al), wherein the lithium composite transition metal oxide particle comprises 60 mol% or more of nickel (Ni) in all metals except lithium, the lithium composite transition metal oxide particle is doped with a doping element thereon, and the particle strength of the lithium composite transition metal oxide particle is 210MPa to 290 MPa.

The positive electrode active material for a secondary battery has lithium composite transition metal oxide particles having a high nickel content and doped with a doping element, and has a specific particle strength range. Therefore, the positive electrode active material for a secondary battery has a high level of particle strength, and may have high structural stability. Therefore, the positive electrode active material for a secondary battery can significantly prevent cracks or particle fracture during roll pressing of the electrode. In addition, the positive electrode active material for a secondary battery has improved particle strength and structural stability, and thus may have significantly improved life characteristics at high temperatures.

The particle strength of the positive electrode active material for a secondary battery is 210MPa to 290 MPa. The positive electrode active material for a secondary battery has the above particle strength range, and thus can effectively prevent a particle fracture phenomenon during roll pressing, prevent collapse of a particle structure caused by intercalation/deintercalation of lithium at the time of charge/discharge of a battery, and impart excellent durability to particles.

When the particle strength of the positive electrode active material for a secondary battery is less than 210MPa, it is difficult to achieve the above-described effects of preventing particle breakage and improving particle durability. When more than 290MPa, there is a risk of suppressing the output of the battery, and since the strength of the particles becomes too high, it may be difficult to roll the particles. In severe cases, the electrodes may be damaged, which is undesirable from a lifetime perspective.

The particle strength of the positive electrode active material for a secondary battery is preferably 215MPa to 275MPa, more preferably 223MPa to 250MPa, and even more preferably 232MPa to 245MPa, in terms of further improving the durability, structural stability, and particle life characteristics.

The above particle strength can be measured by: particles of the positive electrode active material were dropped onto the plate by a micropressure tester, and then pressure was gradually applied to the point where the particles broke by the tester, and then the force at that point was quantified.

The range of the particle strength of the positive electrode active material for a secondary battery can be controlled by the type of the doping element, the content of the doping element, the degree of distribution of the doping element in the particles, and the average particle diameter (D) of the positive electrode active material₅₀) Is achieved.

The positive electrode active material for a secondary battery is a lithium composite transition metal oxide particle containing nickel (Ni) and cobalt (Co) and containing at least one of manganese (Mn) and aluminum (Al).

The lithium composite transition metal oxide particles include nickel (Ni) and cobalt (Co) and include at least one of manganese (Mn) and aluminum (Al).

The lithium composite transition metal oxide particles may be high-Ni lithium composite transition metal oxide particles containing 60 mol% or more of nickel (Ni) in all metals except lithium. Preferably, the lithium composite transition metal oxide particle may include 61 mol% or more of nickel (Ni) in all metals except lithium. As in the present invention, when high Ni lithium composite transition metal oxide particles having a nickel (Ni) content in the above range in all metals are used, an even higher capacity can be ensured.

The lithium composite transition metal oxide particles may be a lithium composite transition metal oxide in which the ratio of the number of moles of lithium (Li) to the number of moles of all metals except lithium (Li/Me) is 1 or more, whereby the capacity characteristics and output characteristics of the battery can be improved.

Specifically, the ratio of the number of moles of lithium (Li) to the number of moles of all metals except lithium (Li/Me) of the lithium composite transition metal oxide may be 1 to 1.3, particularly 1.01 to 1.25, more particularly 1.02 to 1.2. When (Li/Me) is within the above range, it is good in exhibiting excellent battery capacity and output characteristics.

The lithium composite transition metal oxide particles may be doped with a doping element. The doping element may be doped inside the lithium composite transition metal oxide particle.

The doping element may be doped on the lithium composite transition metal oxide particle to improve the structural stability and particle strength of the lithium composite transition metal oxide, particularly, the lithium composite transition metal oxide particle having a high nickel content.

The doping element may be doped to 6 to 10 mol%, preferably 6.5 to 9.8 mol%, more preferably 7.5 to 9.5 mol% with respect to all metals except lithium in the lithium composite transition metal oxide particle. When the above range is satisfied, the following effects can be more preferably achieved: preventing breakage of particles during rolling due to improvement of particle strength, preventing structural collapse due to improvement of structural stability, and improving life property at room temperature and high temperature.

The doping element may include at least one doping element selected from the group consisting of P, B, Al, Si, W, Zr, and Ti, preferably at least one doping element selected from the group consisting of B, W, Zr and Ti, more preferably at least one doping element selected from the group consisting of W and Zr, and further preferably a doping element W. The doping element is a component capable of achieving more desirable effects in improving the particle strength and structural stability, and can expand the movement path of lithium to an appropriate level, which is preferable in terms of resistance reduction and output characteristics.

The doping of the doping element may be such that the content of the doping element decreases from the surface of the lithium composite transition metal oxide particle toward the center thereof. That is, the content of the doping element may become high from the central portion of the particle to the surface portion thereof, and therefore, the improvement of the strength and structural stability of the particle may be more preferably achieved. This distribution of the tendency of the doping element in the particles can be achieved, for example, by the following method: the method of first firing the transition metal hydroxide and the lithium compound to prepare a first fired material, and then second firing the first fired material and the doping source to further dope the doping element on the surface portion, instead of firing all of the transition metal hydroxide, the lithium compound, and the doping source at once when preparing the positive electrode active material.

The doping element may be doped to 70 mol% or more, preferably 85 mol% or more of the total number of moles of the doping element in a region corresponding to 60% to 100% from the center of the particle with respect to the radius of the lithium composite transition metal oxide particle. When the above range is satisfied, the doping element is more doped in a region closer to the particle surface than the center thereof, so that the above effects of improving the particle strength and preventing the structure from collapsing can be more preferably achieved.

The above-described doping content distribution tendency of the doping element can be indirectly evaluated by performing Energy Dispersive Spectrometer (EDS) analysis on a particle section of the positive electrode active material. Specifically, the doping degree of the doping element can be predicted by performing EDS analysis on each region from the center of the positive electrode active material particle to the surface thereof.

In addition, a value of the lithium composite transition metal oxide particle obtained by substituting a doping element concentration at the surface of the particle and a doping element concentration at the center of the particle, which are obtained by Energy Dispersive Spectroscopy (EDS) analysis, into the following equation 1 may be 0.7 to 1.

[ equation 1]

(H_s-H_c)/H_s

In equation 1, H_sIs a doping element concentration, H, on the surface of the lithium composite transition metal oxide particles when the particles are analyzed by EDS_cIs the concentration of the doping element at the center of the particle when the lithium composite transition metal oxide particle is analyzed by EDS.

When the value obtained by the above equation 1 satisfies 0.7 to 1, the doping element may be more concentrated on the surface portion of the positive electrode active material, thereby further improving the particle strength, and thus the durability and the lifetime characteristics may be improved to more excellent levels.

Preferably, the value of the lithium composite transition metal oxide particle obtained by substituting the doping element concentration at the particle surface and the doping element concentration at the particle center (obtained by EDS analysis) into formula 1 may be 0.9 to 1, and when the above range is satisfied, the above effect may be more preferably achieved.

The lithium composite transition oxide particles may include a compound represented by formula 1 below.

[ formula 1]

Li_pNi_1-x-y-zCo_xM^a _yM^b _zO₂

In formula 1, p is not less than 1.0 and not more than 1.5, 0<x≤0.2，0<y is not more than 0.2, z is not less than 0.06 and not more than 0.1, and 0<x+y+z≤0.4，M^aIs at least one selected from the group consisting of Mn and Al, and M^bIs at least one selected from the group consisting of P, B, Al, Si, W, Zr and Ti.

In the compound represented by the above formula 1, the content of Li may correspond to p, wherein 1. ltoreq. p.ltoreq.1.5, particularly 1.01. ltoreq. p.ltoreq.1.25, more particularly 1.02. ltoreq. p.ltoreq.1.2. When the above range is satisfied, the output and capacity characteristics of the battery can be significantly improved.

In the compound represented by the above formula 1, the content of Ni may correspond to 1- (x + y + z), for example, 0.6. ltoreq.1- (x + y + z) <1, particularly 0.61. ltoreq.1- (x + y + z) <1, whereby a high capacity of the battery can be secured.

In the lithium composite transition metal oxide of the above formula 1, the content of Co may correspond to x, i.e., 0< x.ltoreq.0.2. When x of formula 1 is greater than 0.2, cost may increase.

In the compound represented by the above formula 1, M^aIs a component capable of improving the stability of an active material to improve the stability of a battery, and may be at least one selected from the group consisting of Mn and Al. When considering the effect of improving the life characteristics, M^aMay correspond to y, i.e. 0<y is less than or equal to 0.2. When y of the above formula 1 is greater than 0.2, the output characteristics and capacity characteristics of the battery may deteriorate conversely.

In the compound represented by the above formula 1, M^bIs a doping element, and may be at least one selected from the group consisting of P, B, Al, Si, W, Zr, and Ti. M²The content of (b) may correspond to z, i.e. 0.06. ltoreq. z.ltoreq.0.1, preferably 0.065. ltoreq. z.ltoreq.0.098, more preferably 0.075. ltoreq. z.ltoreq.0.095.

Average particle diameter (D) of positive electrode active material for secondary battery₅₀) It may be 8 μm to 30 μm, preferably 10 μm to 20 μm. When the above range is satisfied, the rolling can be smoothly performed, the structural stability can be improved, and the above effects of improving the output characteristics and the life characteristics can be achieved to an excellent level.

< method for producing Positive electrode active Material for Secondary Battery >

In addition, the present invention provides a method of preparing the above-described positive electrode active material for a secondary battery.

The method of preparing a positive active material for a secondary battery includes: a transition metal hydroxide including nickel (Ni) and cobalt (Co) and including at least one of manganese (Mn) and aluminum (Al), wherein nickel (Ni) is 60 mol% or more in the entire metal, is mixed with a lithium compound, and the mixture is subjected to a first firing to prepare lithium composite transition metal oxide particles, the lithium composite transition metal oxide particles and a doping source including a doping element are mixed, and the mixture is subjected to a second firing to dope the doping element on the lithium composite transition metal oxide particles.

The above-described cathode active material for a secondary battery may be prepared by the above-described method, and more specifically, a cathode active material having a particle strength of 210MPa to 290MPa may be prepared.

In addition, according to the preparation method of the present invention, the lithium composite transition metal oxide is not prepared by firing the transition metal hydroxide, the lithium compound, and the doping source at one time. Instead, the transition metal hydroxide and the lithium compound are first fired to prepare a first fired material (lithium composite transition metal oxide), and then the first fired material and the doping source are second fired to prepare a positive electrode active material for a secondary battery. Thus, the doping of the doping element of the doping source may be such that the content of the doping element decreases from the surface of the particle towards its center. Therefore, the particle strength of the positive electrode active material can be further improved, so that the effects of improving the durability of the particles and preventing cracks during rolling or charging/discharging can be more preferably achieved. In addition, since the structural stability of the particles can be improved by doping the element by the above-described method, structural collapse and deterioration of life characteristics due to intercalation/deintercalation of lithium can be effectively prevented.

The method for preparing the positive electrode active material for the secondary battery comprises the following steps: a transition metal hydroxide including nickel (Ni) and cobalt (Co) and including at least one of manganese (Mn) and aluminum (Al) is mixed with a lithium compound, and the mixture is first calcined to prepare lithium composite transition metal oxide particles.

The transition metal hydroxide is a component that is a precursor of a positive electrode active material for a secondary battery, includes nickel (Ni) and cobalt (Co) and includes at least one of manganese (Mn) and aluminum (Al).

The transition metal hydroxide may be a high Ni transition metal hydroxide in which the content of nickel (Ni) in all metals contained in the transition metal hydroxide is 60 mol% or more. More preferably, the content of nickel (Ni) in the entire metal may be 61 mol% or more. When the content of nickel (Ni) in the entire metal is controlled within the above range, a high capacity of the prepared cathode active material can be ensured.

The transition metal hydroxide may be a compound represented by the following formula 2.

[ formula 2]

Ni_1-x1-y1Co_x1M^a1 _y1(OH)₂

In formula 2, 0<x1≤0.2，0<y1 is 0.2 or less and 0<x1+ y1 is less than or equal to 0.4, and M^a1Is at least one selected from the group consisting of Mn and Al.

In formula 2 above, x1 and y1 may be the same as x and y, respectively, described in formula 1 above.

Together with the transition metal hydroxide, the lithium compound is also a precursor of a positive electrode active material for a secondary battery.

The lithium compound can be a carbonate containing lithium (e.g., Li)₂CO₃Etc.), hydrates (e.g., lithium hydroxide hydrate (LiOH. H)₂O), hydroxides (e.g., lithium hydroxide), nitrates (e.g., lithium nitrate (LiNO), etc.)₃) Etc.) and chlorides (e.g., lithium chloride (LiCl), etc.), and any one or a mixture of two or more thereof may be used.

The first firing may be performed at a temperature of 750 ℃ to 1000 ℃, preferably 800 ℃ to 900 ℃, and preferably satisfies the above range since a positive electrode active material of a layered structure having a stable structure may be formed.

The first firing may be performed for 10 hours to 25 hours, preferably 13 hours to 18 hours, and preferably satisfies the above range since a positive electrode active material having a stable layered structure may be formed.

The first firing may be performed in an oxygen atmosphere in terms of preventing excessive generation of lithium impurities and producing a first fired material excellent in grain development.

The method for preparing the positive electrode active material for the secondary battery comprises the following steps: the lithium composite transition metal oxide particles are mixed with a doping source including a doping element, and the mixture is subjected to a second firing to dope the doping element on the lithium composite transition metal oxide particles.

The positive electrode active material for a secondary battery of the present invention may be prepared by second firing the lithium composite transition metal oxide particles and the doping source.

The doping source is a material for providing a doping element to be doped on the positive electrode active material, and includes the doping element.

The doping source may include at least one doping element selected from the group consisting of P, B, Al, Si, W, Zr, and Ti, preferably includes at least one doping element selected from the group consisting of B, W, Zr and Ti, more preferably includes at least one doping element selected from the group consisting of W and Zr, and even more preferably includes the doping element W. Specifically, the doping source may include an oxide of the above-described doping element.

The input amount of the doping source may be appropriately controlled in consideration of the content (mol%) of the doping element in the positive electrode active material.

The second firing may be performed at 700 to 900 c, preferably 750 to 880 c, and preferably satisfies the above range because the distribution of the doping element in the positive electrode active material may be more concentrated on the surface of the particle than in the center of the particle, whereby the above particle strength range may be achieved.

The second firing may be performed for 3 to 12 hours, preferably 5 to 8 hours, and preferably satisfies the above range because the distribution of the doping element in the positive electrode active material may be more concentrated on the surface of the particles than in the center of the particles, whereby the above range of particle strength may be achieved.

In terms of smooth doping, the second firing may be performed in an oxygen atmosphere.

< Positive electrode for Secondary Battery and Secondary Battery >

In addition, the present invention provides a positive electrode for a secondary battery, including the positive electrode active material for a secondary battery.

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

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like can be used. In addition, the thickness of the positive electrode current collector is generally 3 to 500 μm, and minute irregularities may be formed on the surface of the positive electrode current collector to improve the adhesiveness of the positive electrode active material. For example, the positive electrode collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric body.

The positive electrode active material layer may include a conductive material and a binder, in addition to the above-described positive electrode active material for a secondary battery.

The conductive material is used to impart conductivity to the electrode, and any conductive material may be used without particular limitation so long as it has electronic conductivity without causing chemical changes in the battery to be constructed. Specific examples thereof may include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as potassium titanate; conductive metal oxides such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and any one or a mixture of two or more of them may be used. Generally, the content of the conductive material may be 1 to 30 wt% based on the total weight of the positive electrode active material layer.

The binder serves to improve the bonding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples thereof may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, Styrene Butadiene Rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more of them may be used. The binder may be contained in an amount of 1 to 30 wt% based on the total weight of the positive electrode active material layer.

In addition to using the above-described cathode active material, a cathode for a secondary battery may be manufactured according to a typical method of manufacturing a cathode. Specifically, the positive electrode can be manufactured by the following steps: a composition for forming a positive electrode active material layer, which includes the above-described positive electrode active material and optionally a binder and a conductive material, is coated on a positive electrode current collector, followed by drying and roll-pressing. At this time, the types and contents of the positive electrode active material, the binder, and the conductive material are as described above.

The solvent may be a solvent commonly used in the art, and may be dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, or the like. Any one or a mixture of two or more of them may be used. The amount of the solvent is sufficient if the solvent can dissolve and disperse the positive electrode active material, the binder, and the conductive material, and then has a viscosity that can exhibit excellent thickness uniformity in a coating process at the time of manufacturing the positive electrode, in consideration of the coating thickness and the manufacturing yield of the slurry.

In addition, in another method, a positive electrode for a secondary battery may be manufactured by casting the composition for forming a positive electrode active material layer on a separate support, and then laminating a film obtained by peeling off from the support on a positive electrode current collector.

In addition, the present invention provides an electrochemical device including the positive electrode for a secondary battery. The electrochemical device may specifically be a secondary battery, a capacitor, or the like, and more specifically, may be a lithium secondary battery.

The secondary battery specifically includes the above-described positive electrode for a secondary battery, a negative electrode opposed to the positive electrode for a secondary battery, a separator interposed between the positive electrode and the negative electrode for a secondary battery, and an electrolyte. In addition, the secondary battery may optionally further include: a battery case for accommodating an electrode assembly composed of a positive electrode, a negative electrode, and a separator; and a sealing member for sealing the battery case.

In the secondary battery, the anode includes an anode current collector and an anode active material layer on the anode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like, aluminum-cadmium alloy, and the like can be used. In addition, the thickness of the negative electrode current collector may be generally 3 to 500 μm, and as in the case of the positive electrode current collector, minute irregularities may be formed on the surface of the negative electrode current collector, thereby improving the adhesion of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, a non-woven fabric body, and the like.

The negative electrode active material layer may optionally include a binder and a conductive material in addition to the negative electrode active material. As an example, the negative electrode active material layer may be prepared by coating a composition for forming a negative electrode, which includes a negative electrode active material and optionally a binder and a conductive material, on a negative electrode current collector after drying. Alternatively, the negative electrode active material layer may be manufactured by casting the composition on a separate support, and then laminating a film obtained by being peeled off from the support on the negative electrode current collector.

As the negative electrode active material, a compound capable of reversibly intercalating and deintercalating lithium may be used. Specific examples of the anode active material may include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; a carbon nanotube; a metal substance alloyable with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides which can be doped and dedoped with lithium, e.g. SiO_β(0<β<2)，SnO₂Vanadium oxide and lithium vanadium oxide; or a composite comprising a metal substance and a carbonaceous material, such as a Si-C composite or a Sn-C composite, and any one or a mixture of two or more of them may be used. In addition, a metallic lithium thin film may be used as a negative electrode active material. In addition, low crystalline carbon, high crystalline carbon, and the like can be used as the carbon material. Representative examples of the low crystalline carbon may include soft carbon and hard carbon, high junctionRepresentative examples of crystalline carbon may include irregular, planar, flake, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microbeads, mesophase pitch, and high temperature sintered carbon, such as petroleum or coal tar pitch-derived coke.

In addition, the binder and the conductive material may be the same as those described above when the positive electrode is described.

Meanwhile, in the secondary battery, a separator is used to separate the anode and the cathode and provide a moving path of lithium ions. Any separator may be used without particular limitation so long as it is generally used as a separator in a lithium secondary battery. In particular, a separator having a high moisture retention capacity for an electrolyte and a low resistance to movement of electrolyte ions is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer (e.g., an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer), or a laminated structure having two or more layers thereof may be used. Further, a common porous nonwoven fabric, for example, a nonwoven fabric formed of high-melting glass fibers, polyethylene terephthalate fibers, or the like, may be used. In addition, a coated separator including a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure may be selectively used.

In addition, the electrolyte used in the present invention may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a melt-type inorganic electrolyte that may be used to prepare a lithium secondary battery, but the present invention is not limited thereto.

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

Any organic solvent may be used without particular limitation so long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, there can be used: ester solvents such as methyl acetate, ethyl acetate, γ -butyrolactone and ∈ -caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; or carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), Ethyl Methyl Carbonate (EMC), Ethylene Carbonate (EC), and Propylene Carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R — CN (where R is a linear, branched, or cyclic C2 to C20 hydrocarbyl group, and may include a double bond aromatic ring or ether bond); amides, such as dimethylformamide; dioxolanes, such as 1, 3-dioxolane; or sulfolanes. Among these solvents, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge/discharge performance of a battery, and a low-viscosity linear carbonate-based compound (e.g., ethylene carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution may be excellent.

Any compound may be used as the lithium salt without particular limitation so long as it can provide lithium ions used in the lithium secondary battery. Specifically, LiPF may be used₆、LiClO₄、LiAsF₆、LiBF₄、LiSbF₆、LiAlO₄、LiAlCl₄、LiCF₃SO₃、LiC₄F₉SO₃、LiN(C₂F₅SO₃)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)₂LiCl, LiI or LiB (C)₂O₄)₂As the lithium salt. The lithium salt may be used in a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity to exhibit excellent performance, and lithium ions can be efficiently moved.

In the electrolyte, in order to improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, and increase the discharge capacity of the battery, one or more of the following additives may be further included: for example, halogenated alkylene carbonate-based compounds (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, N-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be contained in an amount of 0.1 to 5 wt% based on the total weight of the electrolyte.

As described above, the secondary battery including the positive electrode active material for a secondary battery of the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and thus the secondary battery is suitably used in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as Hybrid Electric Vehicles (HEVs).

Therefore, according to another embodiment of the present invention, there are provided a battery module (battery module) including the secondary battery as a cell unit and a battery pack (battery pack) including the battery module.

The battery module or the battery pack may be used as a power source for at least one of medium-and large-sized devices: for example, power tools; electric vehicles, including Electric Vehicles (EVs), Hybrid Electric Vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); or an electrical power storage system.

Best mode for carrying out the invention

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

Examples

Example 1: preparation of positive electrode active material for secondary battery

A transition metal hydroxide Ni_0.7Co_0.1Mn_0.2(OH)₂And lithium hydroxide LiOH. At this time, the transition metal hydroxide and lithium hydroxide are mixed so that lithium (Li) is presentThe ratio of the number of moles to the total number of moles of transition metal of the transition metal hydroxide, Li/Me, was controlled to be 1.03.

First roasting the mixture at 850 ℃ for 15 hours in an oxygen atmosphere to obtain a first roasted material Li_1.03Ni_0.7Co_0.1Mn_0.2O₂。

Compounding lithium with transition metal oxide and doping source WO₃And (4) mixing. At this time, the lithium composite transition metal oxide and the doping source were mixed so that the doping element W contained in the doping source was doped to 7 mol% with respect to all metals except lithium in the prepared cathode active material. The mixture is subjected to secondary roasting for 7 hours at 800 ℃ in an oxygen atmosphere to prepare a positive electrode active material Li_1.03Ni_0.63Co_0.1Mn_0.2W_0.07O₂(average particle diameter (D)₅₀) 11 μm) in which the doping element W is doped on the lithium composite transition metal oxide.

Example 2: preparation of positive electrode active material for secondary battery

The positive electrode active material Li for the secondary battery of example 2 was prepared in the same manner as in example 1_1.03Ni_0.61Co_0.1Mn_0.2W_0.09O₂(average particle diameter (D)₅₀) 11 μm) except that the lithium composite transition metal oxide and the dopant source were mixed such that the dopant element W contained in the dopant source was doped to 9 mol% with respect to all metals except lithium in the cathode active material.

Example 3: preparation of positive electrode active material for secondary battery

The positive electrode active material Li for the secondary battery of example 3 was prepared in the same manner as in example 1_1.03Ni_0.63Co_0.1Mn_0.2Zr_0.07O₂(average particle diameter (D)₅₀) 11 μm) except that ZrO was used₂As a doping source.

Example 4: preparation of positive electrode active material for secondary battery

The secondary battery of example 4 was prepared in the same manner as in example 1Positive electrode active material Li_1.03Ni_0.6Co_0.1Mn_0.2W_0.1O₂(average particle diameter (D)₅₀) 11 μm) except that the lithium composite transition metal oxide and the dopant source were mixed so that the doping element W contained in the dopant source was doped so as to be 10 mol% with respect to all metals except lithium in the cathode active material.

Comparative example 1: preparation of positive electrode active material for secondary battery

The first fired material Li prepared in example 1 was added_1.03Ni_0.7Co_0.1Mn_0.2O₂(average particle diameter (D)₅₀) 11 μm) was used as a positive electrode active material (average particle diameter (D) for a secondary battery of comparative example 1₅₀) 11 μm).

Comparative example 2: preparation of positive electrode active material for secondary battery

A positive electrode active material Li for a secondary battery of comparative example 2 was prepared in the same manner as in example 1_1.03Ni_0.69Co_0.1Mn_0.2Zr_0.01O₂(average particle diameter (D)₅₀) 11 μm) except that ZrO was used₂As a doping source, and the lithium composite transition metal oxide and the doping source were mixed so that the doping element Zr contained in the doping source was doped to 1 mol% with respect to the entire metal except lithium in the positive electrode active material.

Comparative example 3: preparation of positive electrode active material for secondary battery

A positive electrode active material Li for a secondary battery of comparative example 3 was prepared in the same manner as in example 1_1.03Ni_0.69Co_0.1Mn_0.2W_0.01O₂(average particle diameter (D)₅₀) 11 μm) except that WO was used₃As a doping source, and the lithium composite transition metal oxide and the doping source were mixed so that the doping element W contained in the doping source was doped to 1 mol% with respect to the entire metal except lithium in the positive electrode active material.

Comparative example 4: preparation of positive electrode active material for secondary battery

A transition metal hydroxide Ni_0.7Co_0.1Mn_0.2(OH)₂Lithium hydroxide LiOH and doping source WO₃And (4) mixing.

At this time, the transition metal hydroxide and lithium hydroxide were mixed so that the ratio of the number of moles of lithium (Li) to the total number of moles of transition metal of the transition metal hydroxide, Li/Me, was controlled to 1.03. The lithium composite transition metal oxide and the doping source were mixed so that the doping element W contained in the doping source was doped to 7 mol% with respect to all metals except lithium in the positive electrode active material.

Firing the mixture at 850 ℃ for 15 hours in an oxygen atmosphere to obtain an average particle diameter (D)₅₀) Li of 11 μm_1.03Ni_0.63Co_0.1Mn_0.2W_0.07O₂(average particle diameter (D)₅₀) 11 μm) as a positive electrode active material, wherein a doping element W is doped in the lithium composite transition metal oxide.

Comparative example 5: preparation of positive electrode active material for secondary battery

A positive electrode active material Li for a secondary battery of comparative example 5 was prepared in the same manner as in example 1_1.03Ni_0.65Co_0.1Mn_0.2W_0.05O₂(average particle diameter (D)₅₀) 11 μm) except that WO was used₃As a doping source, and the lithium composite transition metal oxide and the doping source were mixed so that the doping element W was doped to 5 mol% with respect to all metals except lithium in the positive electrode active material.

Comparative example 6: preparation of positive electrode active material for secondary battery

Mixed transition metal hydroxide Ni_0.7Co_0.1Mn_0.2(OH)₂And lithium hydroxide LiOH. At this time, the transition metal hydroxide and lithium hydroxide were mixed so that the ratio of the number of moles of lithium (Li) to the total number of moles of transition metals of the transition metal hydroxide, Li/Me, was controlled to 1.03.

First roasting the mixture at 950 deg.C for 20 hr in oxygen atmosphere to obtain first roasted material Li_1.03Ni_0.7Co_0.1Mn_0.2O₂。

Mixed lithium composite transition metal oxide and doping source WO₃. At this time, the lithium composite transition metal oxide and the doping source were mixed so that the doping element W contained in the doping source was doped to 20 mol% with respect to all metals except lithium in the prepared cathode active material. The mixture was subjected to a second firing at 900 ℃ for 10 hours in an oxygen atmosphere to prepare a positive electrode active material Li_1.03Ni_0.5Co_0.1Mn_0.2W_0.2O₂(average particle diameter (D)₅₀) 11 μm), wherein the doping element W is doped on the lithium composite transition metal oxide.

Examples of the experiments

Experimental example 1: measurement of particle Strength

The particle strength of the cathode active material of each of examples 1 to 4 and comparative examples 1 to 6 was measured and shown in table 1 below.

< measurement of pellet Strength >

The particle strength of the positive electrode active material of example 1 was measured as follows: particles of the positive electrode active material of example 1 were dropped on the plate by a micropressure tester (apparatus name: MCT-W500, Shimadzu co., ltd. manufactured), and then pressure was gradually applied to the point where the particles broke by the tester, and then the force at the point was quantified.

Further, the particle strength of the positive electrode active material of each of examples 2 to 4 and comparative examples 1 to 6 was measured in the same manner as described above.

For each of examples and comparative examples, experiments were repeated a total of 10 times to obtain average values of examples and comparative examples, and the values are shown in table 1 below.

[ Table 1]

Experimental example 2: evaluation of high temperature Life characteristics

The high-temperature life characteristics of the cathode active materials of each of examples 1 to 4 and comparative examples 1 to 6 were evaluated and shown in fig. 1 and table 2.

< production of lithium Secondary Battery >

The cathode active material, the carbon black conductive material, and the PVdF binder of example 1 were mixed in an N-methylpyrrolidone solvent at a weight ratio of 96.5:1.5:2 to prepare a cathode slurry, and the cathode slurry was coated on one surface of an aluminum current collector, dried at 130 ℃, and then rolled to a porosity of 25% to manufacture a cathode for a secondary battery.

Next, a mixture as a negative active material in which artificial graphite and natural graphite were mixed in a ratio of 5:5, super c as a conductive material, and SBR/CMC as a binder were mixed in a weight ratio of 96:1:3 to prepare a negative slurry, and the negative slurry was coated on one surface of a copper current collector, dried at 130 ℃, and then rolled to a porosity of 30% to manufacture a negative electrode.

A porous polyethylene separator was disposed between the positive electrode and the negative electrode for a secondary battery manufactured as described above to manufacture an electrode assembly, and then the electrode assembly was placed in a case. After that, an electrolyte was injected into the case to manufacture the lithium secondary battery of example 1. At this time, the electrolyte was prepared as follows: mixing lithium hexafluorophosphate (LiPF)₆) Dissolved in an organic solvent mixed with Ethylene Carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) in a volume ratio of 3:4:3 at a concentration of 1.0M.

Further, lithium secondary batteries of each of examples 2 to 4 and comparative examples 1 to 5 were manufactured in the same manner as in example 1, except that the positive electrode active material for a secondary battery of each of examples 2 to 4 and comparative examples 1 to 6 was used instead of the positive electrode active material for a secondary battery of example 1.

< method for evaluating high temperature Life characteristics >

The lithium secondary batteries of examples and comparative examples manufactured as described above were charged/discharged at a high temperature (45 ℃) in a driving voltage range of 3.0V to 4.25V under the condition of 1C/1C.

For each of examples and comparative examples, capacity retention rates, which are ratios of discharge capacity per charge/discharge cycle to initial capacity, were measured and shown in fig. 1, and capacity retention rates of 400 cycles, which are ratios of discharge capacity at 400 th cycle after performing charge/discharge 400 times to initial capacity, were determined and shown in table 2.

[ Table 2]

	Capacity retention (%), at 45 ℃ for 400 cycles
		Example 1	87.7
Example 2	90.4
		Example 3	86.2
Example 4	85.3
		Comparative example 1	64.2
Comparative example 2	75.9
		Comparative example 3	80.2
Comparative example 4	83.9
		Comparative example 5	82.4
Comparative example 6	73.8

Referring to fig. 1 and table 2, it can be confirmed that the lithium secondary battery manufactured using each of the cathode active materials of the examples satisfying the pellet strength of the present invention shows excellent capacity retention at high temperature and thus high temperature life characteristics, as compared to the comparative examples.

Experimental example 3: observation of particle breakage

The breakage of the positive electrode active material particles after conducting charge/discharge at a high temperature (45 ℃) 400 times under the conditions of experimental example 2 was observed by a Scanning Electron Microscope (SEM).

SEM photographs of examples 1 to 4 are shown in fig. 2 to 5, respectively, and SEM photographs of comparative examples 1 to 6 are shown in fig. 5 to 11, respectively.

Referring to fig. 2 to 11, the positive electrode active materials of the respective examples satisfying the particle strength of the present invention were almost free from particle breakage. However, the particles of the comparative example were broken much after the charge/discharge cycle was performed. Therefore, structural stability and durability were poor, and thus, it was confirmed that life characteristics would be poor.

Experimental example 4: EDS Observation

The distribution and content of the doping element of each of example 1 and comparative example 4 were analyzed by an Energy Dispersive Spectrometer (EDS) (apparatus name: FE-SEM, manufactured by Bruker co., ltd.).

As EDS analysis, the sectional shape of each of the particles of example 1 and comparative example 4 was analyzed using SEM, and the doping element content of a specific region of the particle was quantitatively analyzed using EMAX program.

The contents of the doping element W from the surface to the center of each of example 1 and comparative example 4 are shown in fig. 12 and 13, respectively. According to fig. 12 and 13, in a region having a distance of 60% to 100% from the center of the particle with respect to the radius of the lithium composite transition metal oxide particle, the doping amount of the corresponding region was about 99 wt% of the total doping amount for example 1, and about 40 wt% for comparative example 4.

Further, when the particle cross section of the cathode active material of example 1 was analyzed by EDS, a value obtained by substituting the doping element concentrations at the particle surface and the particle center into the following equation 1 was 1. In addition, when the particle section of the cathode active material of comparative example 1 was analyzed by EDS, a value obtained by substituting the doping element concentrations at the particle surface and the particle center into the following equation 1 was 0.25.

[ equation 1]

(Hs-Hc)/Hs

(in equation 1, H_sIs a doping element concentration, H, on the surface of the lithium composite transition metal oxide particles when the particles are analyzed by EDS_cIs the doping element concentration at the center of the particle when the lithium composite transition metal oxide particle is analyzed by EDS).

When the above results were examined, the cathode active material of example 1 was formed to have a large amount of doping elements distributed on the surface thereof, thereby having high particle strength, and as shown in experimental examples 2 and 3, effects of excellent durability, prevention of particle breakage, and improvement of life characteristics were achieved.

However, the doping element of the positive electrode active material of comparative example 4 is uniformly distributed throughout the active material particles, and thus it is difficult to satisfy the desired particle strength, durability, and life characteristics with poor effects.