JP7209780B2 - Positive electrode active material for secondary battery, method for producing the same, and lithium secondary battery including the same - Google Patents

Description

This application claims priority benefit from Korean Patent Application No. 10-2017-0016347 dated February 6, 2017 and Korean Patent Application No. 10-2017-0166956 dated December 6, 2017. , and all contents disclosed in the documents of the corresponding Korean patent application are incorporated as part of the present specification.

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

Demand for secondary batteries as an energy source is rapidly increasing along with technological development and increasing demand for mobile devices. Among such secondary batteries, lithium secondary batteries have been commercialized and widely used because of their high energy density and voltage, long cycle life and low self-discharge rate.

Lithium transition metal oxides are used as positive electrode active materials for lithium secondary batteries. Among them, lithium cobalt oxide such as LiCoO ₂ is mainly used because of its high working voltage and excellent capacity characteristics. However, LiCoO ₂ has very poor thermal properties due to destabilization of the crystal structure due to delithiation, and is expensive.

Lithium manganese oxides ₍ such as _LiMnO2 or LiMn2O4), lithium iron phosphate compounds ₍ such as _LiFePO4 ) or lithium nickel oxides ₍ such as _LiNiO2 ) have been developed as materials for use in place of LiCoO2. was done. In particular, research and development have been actively carried out on lithium nickel oxide, which has a high reversible capacity of about 200 mAh/g and thus facilitates the realization of large-capacity batteries. However, LiNiO ₂ is inferior to LiCoO ₂ in thermal stability, and if an internal short circuit occurs due to external pressure during charging, the positive electrode active material itself will decompose, causing the battery to explode and catch fire. be.

Therefore, as a method of maintaining the excellent reversible capacity of LiNiO ₂ and improving the low thermal stability, a method of substituting a portion of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed. rice field. However, in the case of LiNi _1-α Co _α O ₂ (α=0.1 to 0.3) in which a portion of nickel is replaced with cobalt, excellent charge/discharge characteristics and life characteristics are exhibited, but thermal stability is poor. There is the problem of low In addition, a nickel-manganese-based lithium composite metal oxide in which a portion of Ni is replaced with Mn having excellent thermal stability and a nickel-cobalt-manganese-based lithium composite metal oxide in which Mn and Co are substituted (hereinafter simply referred to as “NCM-based lithium oxide In the case of "product"), the output characteristics are low, and there is a risk of elution of metal elements and deterioration of battery characteristics due to this.

In order to solve this problem, a lithium transition metal oxide having a metal composition concentration gradient has been proposed. However, in this method, the cathode active material precursor can be synthesized so that the metal composition has a concentration gradient. In the process of synthesizing the lithium transition metal oxide, there is a problem that the diffusion rate of metal ions increases and the concentration gradient of the internal and external metal compositions disappears.

As another method, research is being conducted to increase the Ni content in NCM-based lithium oxides in order to increase the energy density of compact automobiles and power storage batteries. However, as the Ni content increases, the crystals grow rapidly when fired at a high temperature, making it difficult to raise the firing temperature and controlling the size of the crystals. When the amount is increased, there is a problem that the battery capacity and life characteristics are rapidly decreased.

The present invention provides a High-Ni positive electrode active material having a transition metal composition with a concentration gradient and a high nickel (Ni) concentration, wherein the concentration gradient of the transition metal composition can be maintained even after firing, and high firing It is an object of the present invention to provide a positive electrode active material for secondary batteries that can ensure high capacity, excellent high-temperature stability and life characteristics by suppressing the rapid increase in crystal size even at high temperatures. and

The present invention is a lithium transition metal oxide containing at least two or more transition metals selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn), and is contained in the lithium transition metal oxide. The content of nickel (Ni) is 60 mol% or more in the total transition metals obtained, and at least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the lithium transition metal oxide. has a concentration gradient in which the concentration difference between the center of the lithium transition metal oxide particle and the surface of the particle is 2 atom % or more, and the lithium transition metal oxide is composed of W, Mo, Ta, Nb, La and Bi A cathode active material for a secondary battery containing at least one doping element selected from the group consisting of 2,000 ppm or more of the doping element.

The present invention also includes mixing a positive electrode active material precursor, a lithium-containing source material and a doping element source material, and firing after said mixing to form a lithium transition metal oxide, wherein said positive electrode is The active material precursor contains at least two transition metals selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn), and all the transition metals contained in the positive electrode active material precursor The content of nickel (Ni) is 60 mol% or more, and at least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the positive electrode active material precursor is a positive electrode active material A positive electrode for a secondary battery having a concentration gradient within precursor particles, wherein the doping element raw material contains at least one doping element selected from the group consisting of W, Mo, Ta, Nb, La and Bi. A method for manufacturing an active material is provided.

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

In the present invention, a transition metal composition has a concentration gradient, and by doping a High-Ni positive electrode active material with a high nickel (Ni) concentration with a specific doping element, the transition metal concentration gradient in the particles during firing It is possible to suppress the disappearance of the transition metal composition, maintain the concentration gradient of the transition metal composition even after firing, and suppress the rapid increase in the crystal size even at a high firing temperature.

In addition, since high-temperature firing is possible, it is possible to produce a positive electrode active material with improved high-temperature stability and heat resistance, thereby providing a lithium secondary battery with high capacity and excellent life characteristics. be able to.

Hereinafter, the present invention will be described in more detail in order to facilitate understanding of the present invention. In this regard, the terms and words used in the specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventors believe that their invention should be implemented in the best possible manner. According to the principle that the concepts of terms can be appropriately defined for the purpose of explanation, the meanings and concepts should be interpreted in accordance with the technical idea of the present invention.

The positive electrode active material for secondary batteries of the present invention is a lithium transition metal oxide containing at least two transition metals selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn), The content of nickel (Ni) is 60 mol% or more among the total transition metals contained in the lithium transition metal oxide, and the lithium transition metal oxide contains nickel (Ni), cobalt (Co) and At least one of manganese (Mn) has a concentration gradient in which the concentration difference between the center of the lithium transition metal oxide particle and the surface of the particle is 2 atom % or more, and the lithium transition metal oxide includes W, Mo, At least one doping element selected from the group consisting of Ta, Nb, La and Bi is contained, and the doping element is contained in an amount of 2,000 ppm or more.

The doping element is a heavy element having an atomic weight of 92 or more. In the present invention, a heavy element having an atomic weight of 92 or more is used as a doping element to dope the surface and inside of the positive electrode active material, thereby enabling high temperature firing, and the concentration gradient of the transition metal composition of the positive electrode active material even after high temperature firing. can be maintained, a rapid increase in crystal size can be prevented, and high capacity as well as excellent life characteristics and high-temperature stability can be achieved.

When manufacturing a positive electrode active material having a transition metal composition with a concentration gradient, generally, a positive electrode active material precursor having a concentration gradient is mixed with a lithium-containing raw material and then baked. At this time, conventionally, the diffusion rate of transition metal ions increases due to the high firing temperature, and the finally produced positive electrode active material of lithium transition metal oxide has a transition metal concentration gradient in the particles smaller than the target value. There was a problem that it would be almost gone.

In addition, in the case of a high-Ni-based lithium transition metal oxide in which the nickel (Ni) content is increased to 60 mol% or more, or even 80 mol% or more, in order to increase the energy density, crystals are formed during high-temperature firing. There is a problem that the capacity and life characteristics deteriorate sharply due to rapid growth, and compared to the case where the nickel (Ni) content is less than 60 mol%, the concentration gradient itself within the particles of the positive electrode active material precursor is reduced. Therefore, it is very difficult to maintain the transition metal concentration gradient of the positive electrode active material after firing.

Therefore, in order to solve this problem, the present invention provides a high-Ni positive electrode active material having a transition metal composition with a concentration gradient and a nickel (Ni) concentration of 60 mol% or more, further 80 mol% or more. By doping the substance with a specific doping element having an atomic weight of 92 or more, the disappearance of the transition metal concentration gradient in the particles during firing is suppressed, and the concentration gradient of the transition metal composition is maintained even after firing, resulting in improved capacity and life. Improved properties and stability.

In addition, a specific doping element having an atomic weight of 92 or more is present at the crystal grain boundary of the transition metal and plays a role in suppressing grain growth, so it is possible to prevent the rapid growth of crystals even at high firing temperatures. , the crystal size can be easily controlled, and even with a high content of nickel (Ni), the crystal size can be formed small, and excellent capacity and life characteristics are exhibited.

In addition, in the present invention, since high-temperature firing is possible by doping with a specific doping element having an atomic weight of 92 or more, it is possible to produce a positive electrode active material with improved high-temperature stability and heat resistance. It is possible to provide a lithium secondary battery having a large capacity and excellent life characteristics.

The doping element according to one embodiment of the present invention is a doping element having an atomic weight of 92 or more and an oxidation number of +3 or more. By doping with a doping element having a high oxidation number of +3 or higher, it is possible to easily react with lithium during firing. concentration gradient and a rod-like internal structure can be maintained. In addition, the concentration gradient and the rod-shaped internal structure can provide stability of the positive electrode active material due to high temperature life and structural stability.

The doping element according to one embodiment of the present invention is at least one selected from the group consisting of W, Mo, Ta, Nb, La and Bi, more preferably W, La or Mo. It may be W, most preferably W.

The lithium transition metal oxide according to one embodiment of the present invention may be represented by Chemical Formula 1 below.

[Chemical Formula 1]
_{LiaNix1Mny1Coz1Mw1O2 _ _ _ _ _}

In Chemical Formula 1, M includes one or more selected from the group consisting of W, Mo, Ta, Nb, La and Bi, and 0.9<a≤1.1, 0.6≤x1≤0. 95, 0≤y1≤0.4, 0≤z1≤0.4, 0<w1≤0.1, x1+y1+z1=1.

The composition of Formula 1 is the average composition of the lithium transition metal oxide in the positive active material particles.

In the lithium transition metal oxide cathode active material of Formula 1, Li may be included in a content corresponding to a, that is, in a content of 0.9<a≦1.1. If a is 0.9 or less, the capacity may decrease, and if it exceeds 1.1, the particles may be sintered in the firing process, making it difficult to produce the positive electrode active material. Considering the remarkable effect of improving the capacity characteristics of the positive electrode active material by controlling the Li content and the balance of the sinterability at the time of manufacturing the positive electrode active material, the Li is more preferably 1.0≦a≦1.0. 05 content.

In the lithium transition metal oxide cathode active material of Chemical Formula 1, Ni may be included in a content corresponding to x1, that is, in a content of 0.6≦x1≦0.95. If the Ni content in the positive electrode active material of Chemical Formula 1 is less than 0.6, there may be a limit to the realization of a high capacity. A sufficient amount of Li to contribute to charge/discharge cannot be ensured, and the charge/discharge capacity may decrease. The content of Ni sufficient to achieve high capacity may be 0.6 or more, preferably 0.8 or more, and most preferably, Ni is 0.8≦x1≦ It may be included at a content of 0.95.

In the lithium transition metal oxide cathode active material of Chemical Formula 1, Mn can improve the structural stability of the active material, thereby improving the stability of the battery. Considering the effect of improving the life characteristics by including Mn, the Mn may be included in a content corresponding to y1, that is, in a content of 0≤y1≤0.4. If y1 in the positive electrode active material of Chemical Formula 1 exceeds 0.4, the output characteristics and capacity characteristics of the battery may deteriorate.

In the lithium transition metal oxide cathode active material of Formula 1, Co may be included in a content corresponding to z1, that is, in a content of 0≦z1≦0.4. If the Co content in the cathode active material of Formula 1 exceeds 0.4, the cost may increase. Considering the conspicuous effect of improving capacity characteristics by containing Co, the Co may be contained in a content of 0.10≦z1≦0.35, more specifically.

In the lithium transition metal oxide cathode active material of Formula 1, M may be at least one selected from the group consisting of W, Mo, Ta, Nb, La and Bi, and the structure of the cathode active material It can improve stability, maintain Ni, Mn or Co concentration gradients, and prevent rapid growth of crystals. The doping element M is included in a content corresponding to w1 within a range that achieves the intended effect and does not deteriorate the characteristics of the positive electrode active material, that is, the content of 0 < w1 ≤ 0.1. good too.

In the cathode active material of the lithium transition metal oxide of Chemical Formula 1, the elements Ni, Co, and Mn may be mixed with other elements to improve battery characteristics by controlling the distribution of the transition metal elements in the cathode active material. may be partially substituted or doped with Specifically, it may be partially substituted or doped with one or more elements selected from the group consisting of Al, Zr, Ti, Mg, Zn, Y, Fe and Ti.

At least one of nickel (Ni), cobalt (Co) and manganese (Mn) in the positive electrode active material of the lithium transition metal oxide of the present invention has a concentration difference between the center of the lithium transition metal oxide particle and the surface of the particle. It has a concentration gradient of 2 atom % or more.

Since the lithium transition metal oxide according to the present invention contains at least one or more selected from the group consisting of W, Mo, Ta, Nb, La and Bi as a doping element, the concentration of the transition metal composition is maintained even after firing. A gradient is maintained and the concentration difference between the center of the particle and the surface of the particle satisfies 2 atom % or more.

At least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the lithium transition metal oxide has a concentration gradient that gradually changes from the center to the surface of the lithium transition metal oxide particles. can have

In the present invention, the phrase "exhibits a concentration gradient in which the transition metal concentration gradually changes (increases or decreases)" means that the transition metal concentration exists in a concentration distribution that gradually changes throughout the particle. . Specifically, the concentration distribution is such that the change in the concentration of the transition metal per 1 μm in the particles is 0.1 to 5 atom %, based on the total number of moles of the metal contained in the positive electrode active material. Specifically, the difference may be 0.1 to 3 atom %, more specifically 1 to 2 atom %.

Further, in the present invention, the concentration gradient structure and concentration of the transition metal in the positive electrode active material particles can be determined by Electron Probe Micro Analyzer (EPMA) analysis, Inductively Coupled Plasma-Atomic Emission Spectroscopy. -Atomic Emission Spectrometer (ICP-AES), Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), or X-ray photoelectron spectroscopy (XPS). Specifically, the atomic ratio of each metal is measured while moving from the center to the surface of the cathode active material using EPMA, or the atomic ratio of each metal is measured from the surface to the center of the cathode active material by XPS. The atomic ratio of each metal can be measured while etching.

As described above, by varying the concentration of each transition metal in the positive electrode active material particles according to the concentration gradient, the characteristics of the transition metal can be easily utilized to further improve the effect of improving the battery performance of the positive electrode active material. can be made In addition, by having a concentration gradient in which the concentration of the transition metal continuously changes according to the position in the positive electrode active material particle, there is no sharp phase boundary region from the center to the surface, and the crystal structure can be stabilized and thermal stability can be increased. In addition, when the metal concentration gradient is constant, the effect of improving the structural stability can be further enhanced.

Conventionally, even if the positive electrode active material precursor has a transition metal concentration gradient, the transition metal ion diffusion rate increases due to the high firing temperature during firing, and the finally formed lithium transition metal oxide has a positive electrode activity. However, the present invention uses the group consisting of W, Mo, Ta, Nb, La and Bi as doping elements. By containing at least one or more selected from, the concentration gradient of the transition metal composition is maintained even after firing, and the effect of improving battery performance can be improved.

Specifically, in the cathode active material according to an embodiment of the present invention, nickel (Ni) contained in the cathode active material has a concentration gradient that gradually decreases from the center of the particle toward the surface of the particle. be able to. In this case, the Ni concentration is maintained at a high concentration at the center of the particle, and the Ni concentration decreases toward the surface, thereby exhibiting thermal stability and preventing a decrease in capacity.

The cathode active material according to an embodiment of the present invention is doped with a specific doping element to prevent the diffusion of transition metals having a concentration gradient during firing. In order to maintain the concentration of nickel (Ni), the concentration difference between the center of the particle and the surface of the particle may be 2 atom % or more, more preferably 3 atom % or more.

When the average content of nickel (Ni) is 60 mol % or more of all transition metals in the positive electrode active material particles, as in one embodiment of the present invention, the firing rate is higher than when it is less than 60 mol %. It is difficult for subsequent positive electrode active material particles to maintain a concentration gradient. In addition, when the nickel (Ni) content is 80 mol% or more, the concentration gradient of the positive electrode active material particles after firing is more likely to be maintained than when the content is less than 80 mol% (for example, 50 mol%). difficult.

For example, if the average nickel (Ni) content is 50 mol %, the concentration of the transition metal at the center of the grain and at the surface of the grain without doping with a specific doping element according to one embodiment of the present invention The difference can be about 10 atom %, specifically about 5 to 8 atom %. Since the particles have a high concentration of nickel (Ni) as a whole, the concentration gradient itself of nickel (Ni) in the positive electrode active material precursor particles before firing is not large, so the concentration gradient after firing can be maintained. becomes more difficult. However, when doping with a specific doping element having an atomic weight of 92 or more according to the present invention, even when the average content of nickel (Ni) is 60 mol %, more preferably 80 mol % or more, the positive electrode active material after firing is reduced. Concentration gradients can be well maintained.

In addition, in the positive electrode active material according to one embodiment of the present invention, at least one of manganese (Mn) and cobalt (Co) contained in the positive electrode active material gradually increases from the center of the particle toward the surface of the particle. It can have an increasing concentration gradient. In this case, the concentration of Mn remains low at the center of the particle and increases toward the surface of the particle, thus obtaining excellent thermal stability without a decrease in capacity. By maintaining a low Co concentration at the center of the particles and increasing the Co concentration toward the surface of the particles, it is possible to reduce the amount of Co used and prevent a decrease in capacity.

Similarly, the positive electrode active material of the present invention is doped with a specific doping element having an atomic weight of 92 or more according to an embodiment of the present invention, thereby preventing diffusion of the transition metal having a concentration gradient during firing, and At least one of manganese (Mn) and cobalt (Co) has a concentration difference of 2 atom% or more between the center of the particle and the surface of the particle so that the concentration gradient is maintained in the subsequent positive electrode active material. , more preferably 3 atom % or more.

In the positive electrode active material according to an embodiment of the present invention, the concentration of nickel (Ni) contained in the positive electrode active material decreases with a continuous concentration gradient from the center of the particle toward the surface layer of the particle, At least one of manganese (Mn) and cobalt (Co) can increase from the center of the particle toward the surface of the particle with a continuous concentration gradient complementary to the concentration gradient of nickel (Ni). .

In this way, the concentration of nickel (Ni) in the positive electrode active material gradually decreases from the center of the particle toward the surface of the particle, and the concentration of manganese (Mn) and/or cobalt (Co) gradually increases. By having a concentration gradient of , it is possible to maintain the capacity characteristics and exhibit thermal stability.

Meanwhile, in the cathode active material according to an embodiment of the present invention, the doping element doped into the cathode active material has a concentration gradient that gradually decreases from the surface of the lithium transition metal oxide particles toward the center of the particles. can be done. The cathode active material according to an embodiment of the present invention is doped by mixing a cathode active material precursor and a lithium-containing raw material, mixing them with a specific doping element having an atomic weight of 92 or more, and heat-treating them when firing. Therefore, the doping element is doped by diffusing from the surface of the positive electrode active material into the inside of the positive electrode active material particle, and the concentration of the doping element gradually decreases from the surface of the particle toward the center of the particle. can be done.

Meanwhile, the lithium transition metal oxide cathode active material according to an embodiment of the present invention may have an average crystal size (D ₅₀ ) of 50 to 200 nm, more preferably 80 to 180 nm.

According to one embodiment of the present invention, by doping with a specific doping element having an atomic weight of 92 or more, even with a high content of nickel (Ni) (High-Ni system), the crystal size is reduced when fired at high temperature. A rapid increase can be prevented, and control of the crystal size can be facilitated.

When the crystal size of the positive electrode active material is less than 50 nm in average particle size (D ₅₀ ), the crystallinity is low, so the storage stability at high temperatures is rapidly deteriorated, or the side reaction with the electrolyte due to the high specific surface area causes gas. Generation may increase, or the stability of the positive electrode active material may be poor due to structural instability of the positive electrode active material, and if it exceeds 200 nm, the capacity and life characteristics may be significantly reduced.

The crystal size of the cathode active material according to an embodiment of the present invention may have an average particle diameter (D ₅₀ ) of 50-200 nm, and the average particle diameter (D ₅₀ ) of the cathode active material may be 3-50 μm. There may be.

In the present invention, the average particle size (D ₅₀ ) can be defined as the particle size corresponding to 50% of the volume cumulative amount on the particle size distribution curve. The average particle size ( _D50 ) can be measured, for example, using a laser diffraction method. For example, the average particle diameter (D ₅₀ ) of the positive electrode active material is measured by dispersing the particles of the positive electrode active material in a dispersion medium and then using a commercially available laser diffraction particle size distribution analyzer (for example, Microtrac MT 3000). ) and irradiated with an ultrasonic wave of about 28 kHz with an output of 60 W, the average particle diameter (D ₅₀ ) corresponding to 50% of the volume cumulative amount in the measuring device can be calculated.

The doping element is contained in an amount of 2,000 ppm or more with respect to the lithium transition metal oxide. More preferably 2,500 ppm to 6,500 ppm, more preferably 3,000 ppm to 6,500 ppm may be contained.

By containing the doping element within the above content range, it is possible to effectively suppress the disappearance of the transition metal concentration gradient in the particles during firing, and the concentration gradient of the transition metal composition is maintained even after firing. It is possible to suppress the rapid increase in crystal size even at a high firing temperature.

On the other hand, the positive electrode active material of the present invention is a high-content nickel (Ni)-based (High-Ni-based) positive electrode active material in which the content of nickel (Ni) is 60 mol% or more of all transition metals. In addition, a large amount of lithium by-products such as unreacted lithium hydroxide and lithium carbonate are generated, and a water washing process is performed to remove them. The content of the doping element means the content of the doping element finally contained in the lithium transition metal oxide after the water washing process.

The positive active material according to an embodiment of the present invention has a maximum peak of heat flow measured at 235° C. or higher in thermal analysis by differential scanning calorimetry (DSC).

The cathode active material according to one embodiment of the present invention can be sintered at a high temperature, for example, at a temperature of 850° C. or higher by doping with a specific doping element having an atomic weight of 92 or more. This is because the concentration gradient of the transition metal composition can be maintained even after high-temperature firing, and a rapid increase in crystal size can be suppressed.

The cathode active material sintered at a high temperature can be measured at a maximum peak of 235° C. or higher in thermal analysis by differential scanning calorimetry (DSC). Stability and heat resistance can be improved.

A method for producing a positive electrode active material for a secondary battery according to the present invention comprises the steps of: mixing a positive electrode active material precursor, a lithium-containing raw material, and a doping element raw material; wherein the positive electrode active material precursor contains at least two transition metals selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn), and the positive electrode active material The content of nickel (Ni) is 60 mol% or more among the total transition metals contained in the precursor, and nickel (Ni), cobalt (Co) and manganese (Mn) contained in the positive electrode active material precursor has a concentration gradient within the positive electrode active material precursor particles, and the doping element raw material is at least one or more selected from the group consisting of W, Mo, Ta, Nb, La and Bi Contains doping elements.

The present invention mixes a positive electrode active material precursor having a transition metal concentration gradient and a nickel (Ni) content of 60 mol % or more with a lithium-containing raw material and a specific doping element raw material having an atomic weight of 92 or more. and calcined to produce a positive electrode active material. More preferably, the content of nickel (Ni) may be 80 mol% or more of all transition metals contained in the cathode active material precursor.

In the present invention, when the positive electrode active material precursor and the lithium-containing raw material are mixed and sintered, the doping element raw material is mixed together and heat-treated for doping. can be manufactured in a form of being doped by diffusing from the inside of the positive electrode active material particles.

The doping element source material may include a transition metal having an atomic weight of 92 or more and an oxidation number of +3 or more. The doping element source material may include at least one transition metal selected from the group consisting of W, Mo, Ta, Nb, La and Bi, and more preferably tungsten (W). The doping element source material may be, for example, an oxide, an organic compound or a composite oxide of the transition metal, specifically tungsten oxide (WO ₃ ), sodium tungstate (Na ₂ WO ₄ ). , ammonium tungstate and ammonium molybdate.

The cathode active material precursor may be represented by Formula 2 below.

[Chemical Formula 2]
_{Nix2Mny2Coz2 ( OH} ) ₂

In Formula 2, 0.6≦x2≦0.95, 0≦y2≦0.4, 0≦z2≦0.4, and x2+y2+z2=1.

The composition of Formula 2 is the average composition of transition metals in the cathode active material precursor particles.

The contents of nickel (Ni), manganese (Mn) and cobalt (Co) in the positive electrode active material precursor of Chemical Formula 2 are the same as the contents of the lithium transition metal oxide in the positive electrode active material. good too.

At least one transition metal element of nickel (Ni), manganese (Mn), and cobalt (Co) in the cathode active material precursor of Chemical Formula 2 has a concentration gradient in which the concentration gradually changes within the cathode active material precursor particles. can be shown.

Specifically, in the cathode active material precursor, nickel (Ni) contained in the cathode active material precursor may have a concentration gradient that gradually decreases from the center of the particle toward the surface of the particle.

Further, in the positive electrode active material precursor, at least one of manganese (Mn) and cobalt (Co) contained in the positive electrode active material precursor has a concentration gradient that gradually increases from the center of the particle toward the surface of the particle. can have

In the positive electrode active material precursor according to one embodiment of the present invention, the concentration of nickel (Ni) contained in the positive electrode active material precursor has a continuous concentration gradient from the center of the particle toward the surface layer of the particle. At least one of manganese (Mn) and cobalt (Co) increases from the center of the particle toward the surface of the particle with a continuous concentration gradient complementary to the concentration gradient of nickel (Ni). be able to.

Thus, the concentration of nickel (Ni) gradually decreases from the center of the particle toward the surface of the particle within the cathode active material precursor, and the concentration of manganese (Mn) and/or cobalt (Co) gradually increases. By having a combination of concentration gradients, it is possible to produce a positive electrode active material that exhibits thermal stability while maintaining capacity characteristics.

According to the present invention, by doping a specific doping element having an atomic weight of 92 or more, the positive electrode active material precursor having a concentration gradient as described above can maintain the concentration gradient even after firing. In addition, the concentration gradient can be maintained even at a high firing temperature, the rapid increase in crystal size can be prevented, and the primary particles can maintain a rod structure. Therefore, the present invention is capable of high-temperature firing, and high-temperature firing can be performed at a temperature of 850-950.degree. More preferably, the calcination for manufacturing the cathode active material according to an embodiment of the present invention may be performed at a temperature of 850-900°C.

When the firing temperature is less than 850°C, the produced positive electrode active material has a maximum heat flow rate of less than 235°C during thermal analysis by differential scanning calorimetry (DSC), Insufficient reaction can result in residual raw materials in the particles, which can reduce the high temperature stability and heat resistance of the battery, and can reduce the volume density and crystallinity, which can reduce structural stability. If the temperature exceeds 950° C., the transition metal concentration gradient may not change sufficiently during the firing process, and non-uniform growth of particles may occur.

In addition, the cathode active material according to an embodiment of the present invention may be manufactured by further including washing with water after baking to form the lithium transition metal oxide.

As in one embodiment of the present invention, in the case of a High-Ni-based lithium transition metal oxide containing a high concentration of nickel, it is structurally unstable compared to a lithium transition metal oxide containing a small amount of nickel. Therefore, more lithium by-products such as unreacted lithium hydroxide and lithium carbonate are generated in the manufacturing process. Thus, when a large amount of lithium by-products are present in the positive electrode active material, the lithium by-products react with the electrolyte to cause gas generation and swelling, which significantly reduces high-temperature stability. It will be. Therefore, a water washing step can be performed to remove lithium by-products from lithium transition metal oxides containing high concentrations of nickel.

The washing with water can remove lithium by-products remaining in the lithium transition metal oxide, and can remove doping element source materials remaining after a portion of the lithium transition metal oxide is doped into the cathode active material. By removing the remaining doping element raw material by the water washing process, it is possible to prevent the doping element such as W from being eluted into the electrolyte or into the negative electrode as the cycle progresses.

The water washing can be carried out using 80 to 200 parts by weight, more preferably 100 to 150 parts by weight of pure water with respect to 100 parts by weight of the lithium transition metal oxide. After the washing, the content of the doping element doped in the cathode active material may be 20 to 40 parts by weight with respect to 100 parts by weight of the initial charge, and the rest is contained in the washing water and removed. can be Specifically, after washing with water, the doping element contained in the lithium transition metal oxide may contain 2,000 ppm or more, more preferably 2,500 ppm to 6,500 ppm, more preferably 3,000 ppm to 6,500 ppm may be contained.

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

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

In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. Examples include stainless steel, aluminum, nickel, titanium, baked carbon, aluminum, and A stainless steel surface treated with carbon, nickel, titanium, silver, or the like can be used. In addition, the cathode current collector may generally have a thickness of 3 to 500 μm, and fine unevenness may be formed on the surface of the current collector to increase adhesion of the cathode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

In addition, the positive electrode active material layer may contain a conductive material and a binder together with the positive electrode active material described above.

At this time, the conductive material is used to provide conductivity to the electrode, and can be used without particular limitations as long as it does not cause chemical changes in the battery and has electronic conductivity. is. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; copper, nickel, aluminum, metal powders or metal fibers such as silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives; Or a mixture of two or more can be used. The conductive material may generally be contained in an amount of 1 to 30% by weight with respect to the total weight of the positive electrode active material layer.

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

The positive electrode may be manufactured by a normal positive electrode manufacturing method except for using the positive electrode active material described above. Specifically, it can be produced by coating a positive electrode current collector with a positive electrode active material layer-forming composition containing the above positive electrode active material and optionally a binder and a conductive material, followed by drying and rolling. At this time, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone. or water, etc., and a single kind or a mixture of two or more kinds thereof can be used. The amount of the solvent used is such that the positive electrode active material, the conductive material and the binder are dissolved or dispersed in consideration of the coating thickness of the slurry and the manufacturing yield. It is sufficient to have a viscosity that can exhibit uniformity.

As another method, the positive electrode is obtained by casting the positive electrode active material layer-forming composition on another support, and then peeling the film from the support and laminating the resulting film on the positive electrode current collector. (lamination).

According to still another embodiment of the present invention, an electrochemical device including the positive electrode is provided. Specifically, the electrochemical device may be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. It is as follows. In addition, the lithium secondary battery may optionally further include a battery container for housing the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

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

The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Examples include copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper, and A stainless steel surface treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, or the like can be used. In addition, the negative electrode current collector can usually have a thickness of 3 to 500 μm, and, similarly to the positive electrode current collector, fine unevenness is formed on the surface of the current collector to bond the negative electrode active material. You can also increase your strength. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

The negative active material layer optionally includes a binder and a conductive material along with a negative active material. For example, the negative electrode active material layer is formed by applying a negative electrode forming composition containing a negative electrode active material and optionally a binder and a conductive material on a negative electrode current collector and drying it, or applying the negative electrode forming composition onto a negative electrode collector. on another support and then peeled from the support to laminate the resulting film on the negative electrode current collector.

A compound capable of reversible intercalation and deintercalation of lithium may be used as the negative active material. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn Metallic compounds that can be alloyed with lithium, such as alloys or Al alloys; can be doped and undoped with lithium, such as SiOβ (0< _β < ₂ ), SnO2, vanadium oxide, lithium vanadium oxide; or a composite containing the metal compound 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 these can be used. Also, a metallic lithium thin film may be used as the negative active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon, and examples of highly crystalline carbon include amorphous, plate-like, scale-like, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum High temperature fired carbons such as petroleum or coal tar pitch derived cokes are typical.

Also, the binder and the conductive material may be as described above for the positive electrode.

On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. In particular, those having low resistance to ion migration of the electrolyte and excellent electrolyte-liquid-holding ability are preferable. Specifically, porous polymer films, for example, polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer and ethylene/methacrylate copolymer. A manufactured porous polymeric film or a laminated structure of two or more layers thereof may be used. Ordinary porous nonwoven fabrics such as nonwoven fabrics made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may also be used. Also, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric material may be used, and optionally a single layer or multi-layer structure may be used. .

The electrolyte used in the present invention includes organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic Examples include, but are not limited to, electrolytes.

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

Any organic solvent can be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent includes an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone; dibutyl ether; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethylcarbonate (DMC) ), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (propylene carbonate, PC), etc. alcohol-based solvents such as ethanol and isopropyl alcohol; ); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes, and the like. Among them, carbonate-based solvents are preferable, and a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge-discharge performance of the battery, and a chain carbonate-based compound with a bottom viscosity ( For example, ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) are more preferred. In this case, when the cyclic carbonate and the linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9, excellent electrolytic solution performance can be exhibited.

The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salts include LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN _{( C2F5SO2} ) ₂ , LiN _{( CF3SO2} ) ₂ , LiCl, LiI, or _{LiB ( C2O4} ) ₂ , etc. may be used. The concentration of the lithium salt is preferably in the range of 0.1-2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that excellent electrolyte performance can be exhibited and lithium ions can effectively move.

In addition to the components of the electrolyte, the electrolyte may include haloalkylene carbonate such as difluoroethylene carbonate for improving battery life characteristics, suppressing battery capacity decrease, and improving battery discharge capacity. compound, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, One or more additives such as ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum trichloride may also be included. At this time, the additive may be included in an amount of 0.1-5% by weight based on the total weight of the electrolyte.

As described above, the lithium secondary battery containing the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate. , and hybrid electric vehicles (HEV).

Accordingly, another embodiment of the present invention provides a battery module including the lithium secondary battery as a unit cell and a battery pack including the same.

The battery modules or battery packs are widely used in Power Tools; Electric Vehicles (EV), Hybrid Electric Vehicles (HEV), and Plug-in Hybrid Electric Vehicles (PHEV). ); or any one or more of a system for power storage.

Hereinafter, embodiments of the present invention will be described in detail so that a person having ordinary knowledge in the technical field to which the present invention belongs can easily implement the present invention. This invention may, however, be embodied in various different forms and is not limited to the illustrative embodiments set forth herein.

[Production Example 1: Production of positive electrode active material precursor]
Amounts of _NiSO4 , _CoSO4 , _MnSO4 to provide a nickel:cobalt:manganese molar ratio of 90:5:5 in a batch 20 L reactor set at 60°C. to provide a first metal-containing solution at a concentration of 2M by mixing in water with _NiSO4 , _CoSO4 , _MnSO4 in a nickel:cobalt:manganese molar ratio of 60:20:20. A second metal-containing solution at a concentration of 2M was prepared by mixing in water in amounts such as A container filled with a first metal-containing solution is connected into the reactor and a container filled with a second metal-containing solution is connected to a container filled with the first metal-containing solution. connected to enter. Further, an aqueous NaOH solution with a concentration of 25% by weight and an aqueous NH ₄ OH solution with a concentration of 15% by weight were prepared and connected to the reactor respectively.

After 4 liters of deionized water was put into a coprecipitation reactor (capacity 20 L), nitrogen gas was purged into the reactor at a rate of 2 liters/minute to remove dissolved oxygen in the water, and a non-oxidizing atmosphere was created in the reactor. made it Next, 10 ml of a 25% by weight NaOH aqueous solution and 200 ml of a 15% by weight ammonia aqueous solution were added, and the mixture was stirred at a temperature of 60° C. and a stirring speed of 400 rpm to maintain a pH of 12.0.

Next, 6 ml/hr of the first metal-containing solution, 1 ml/hr of an aqueous NaOH solution, and 2 ml/hr of an aqueous NH ₄ OH solution are added at a rate of 30 minutes to react for 30 minutes to form a nickel-manganese-cobalt-based composite metal hydroxide. Formed the seed of the thing. Next, the second metal-containing solution is introduced into the container of the first metal-containing solution at a rate of 6 ml/hr to induce the growth of the particles of the nickel-cobalt-manganese-based composite metal hydroxide, and was induced to generate a concentration gradient in Next, the reaction was maintained for 40 hours to grow a nickel-manganese-cobalt-based composite metal hydroxide. The resulting nickel-manganese-cobalt-based composite metal-containing hydroxide particles were separated, washed with water, and dried in an oven at 120° C. to prepare a cathode active material precursor having a concentration gradient.

The cathode active material precursor thus produced has an average molar ratio of Ni:Co:mn of 80:10:10, and the concentration of Ni gradually decreases from the center of the particle toward the surface of the particle. , and Co and Mn exhibited concentration gradients that gradually increased from the center of the grain toward the surface of the grain.

Example 1
The cathode active material precursor prepared in Preparation Example 1 and lithium hydroxide (LiOH) as a lithium raw material were mixed at a molar ratio of 1:1.03, and the doping element raw material was used to dope 8,000 ppm of tungsten. WO ₃ is 1% by weight based on the weight of the precursor (ex) 60 g of the precursor is mixed with 0.6 g of WO ₃ , and then sintered at 890° C. for about 13 hours to obtain a positive electrode active material of lithium transition metal oxide. formed. After firing, 100 parts by weight of the positive electrode active material is stirred for 5 minutes using 100 parts by weight of pure water, passed through a filter, and dried at 130° C. for 24 hours to perform a water washing process, and the final positive electrode is obtained. An active material (W6, 100 ppm doped) was produced.

Example 2
A cathode active material (W5 _, 900 ppm dope) was produced.

Example 3
0.55% by weight of La2O3 as a doping element raw material based on the weight of the precursor ₍ ex) ₆₀ g of precursor was mixed with 0.33 g of _La2O3 ) _, and then sintered at 890° C. for about 15 hours. A positive electrode active material (La 3,000 ppm doped) was manufactured in the same manner as in Example 1, except that it was carried out.

Comparative example 1
The cathode active material precursor prepared in Preparation Example 1 and lithium hydroxide (LiOH) as a lithium raw material were mixed at a molar ratio of 1:1.03, and the doping element raw material was not added, and the mixture was heated at 890° C. for about 13 hours. Firing was performed to produce a positive electrode active material.

Comparative example 2
A cathode active material was prepared in the same manner as in Example 1, except that Al ₂ O ₃ as a doping element raw material was mixed with 1.33% by weight of the precursor and then sintered at 880° C. for about 13 hours. manufactured.

Comparative example 3
In order to dope Zr 2,000 ppm as a doping element raw material, 0.27% by weight of ZrO ₂ was mixed with the precursor, and then calcined at 880 ° C. for about 13 hours. Then, a positive electrode active material was produced.

Comparative example 4
A cathode active material was manufactured in the same manner as in Example 1, except that 0.27% by weight of ZrO ₂ as a doping element raw material was mixed with the precursor and then sintered at 780° C. for about 13 hours. .

[Experimental Example 1: Analysis of Positive Electrode Active Material]
In order to confirm the concentration gradients of Ni, Co, and Mn in the lithium transition metal oxide and the distribution of the doped doping elements, the positive electrode active material prepared in Example 1 was subjected to ion milling to cross-section the particles, The center-to-surface composition was confirmed by electron probe microanalyzer (EPMA) analysis. For comparison, the active materials produced in Comparative Examples 1 to 3 were also checked. The results are shown in Tables 1 to 4 below.

Referring to Tables 1 to 4, in Example 1 doped with tungsten (W), the concentration gradient of Ni, Co, and Mn was well maintained at 3 atom% or more from the center of the particle to the surface of the particle even after firing. On the other hand, the non-doped Comparative Example 1 and the Al and Zr doped Comparative Examples 2 and 3 showed little or insufficient concentration gradients of Ni, Co, and Mn after firing.

[Experimental Example 2: Measurement of crystal size]
In order to measure the crystal size of the positive electrode active materials produced in Examples 1 to 3 and Comparative Examples 1 to 3, the crystal size was measured using a laser diffraction particle size distribution analyzer (Malvern, Mastersizer 3000). Table 5 shows the results.

Referring to Table 5, Examples 1 to 3 had crystal sizes of 200 nm or less in average particle size (D ₅₀ ), while Comparative Examples 1 to 3 exhibited very large crystal sizes.

[Production Example 2: Production of lithium secondary battery]
Lithium secondary batteries were manufactured using the cathode active materials prepared in Examples 1-3 and Comparative Examples 1-4.

Specifically, the cathode active material, the carbon black conductive material, and the PVdF binder prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were mixed in an N-methylpyrrolidone solvent at a ratio of 95:2.5:2. A positive electrode forming composition (viscosity: 5000 mPa·s) was produced by mixing at a weight ratio of 5, applied to an aluminum current collector, dried at 130° C. and rolled to produce a positive electrode. Li metal was used as the negative electrode.

An electrode assembly is manufactured by interposing a porous polyethylene separator between the positive electrode and the negative electrode manufactured as described above. A lithium secondary battery was manufactured by injection. At this time, the electrolytic solution was prepared by adding 1.0M lithium hexafluorophosphate ( LiPF ₆ ) was dissolved.

[Experimental Example 3: Evaluation of life characteristics and DSC]
A coin cell (using Li metal negative electrode) was manufactured using the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 4, and the life at high temperature was confirmed. Specifically, the coin cell was charged/discharged 50 times under conditions of 0.3C/0.3C within a driving voltage range of 2.5V to 4.25V at a temperature of 45°C. Table 6 shows the cycle capacity retention, which is the ratio of the discharge capacity to the initial capacity after 50 cycles of charging and discharging at a high temperature (45° C.).

In addition, for DSC evaluation, the battery was disassembled in a charged state of SOC 100%, the positive electrode and a new electrolytic solution were put into the DSC measurement cell, and the temperature was raised from room temperature to 400° C. by 10° C. per minute. As a result, the temperature at which the maximum heat flow (main peak) is shown is shown in Table 6 below.

Referring to Table 6, it can be seen that the high-temperature cycle characteristics of Examples 1-3 were significantly improved compared to Comparative Examples 1-3. In addition, in the case of Comparative Example 4, which was sintered at a relatively low temperature of 780° C., the main peak of the heat flow was shown at 230° C. in the DSC measurement, and Example 1, which was sintered at a high temperature of 850° C. or higher. It can be seen that the high temperature stability and heat resistance are greatly reduced compared to ~3.

Claims (17)

A lithium transition metal oxide containing at least two transition metals selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn),
The content of nickel (Ni) is 60 mol% or more of all transition metals contained in the lithium transition metal oxide,
At least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the lithium transition metal oxide has a concentration difference of 2 atom% or more between the center of the lithium transition metal oxide particle and the surface of the particle. has a concentration gradient of
The positive electrode active material for a secondary battery, wherein the lithium transition metal oxide contains at least one doping element selected from the group consisting of La and Bi, and the doping element contains 2,000 ppm or more. . 2. The secondary battery according to claim 1, wherein the cathode active material for a secondary battery has a maximum peak of heat flow measured at 235° C. or higher during thermal analysis by differential scanning calorimetry (DSC). positive electrode active material. At least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the lithium transition metal oxide has a concentration gradient that gradually changes from the center to the surface of the lithium transition metal oxide particles. The positive electrode active material for a secondary battery according to claim 1, comprising: The cathode active material for a secondary battery according to claim 1, wherein the doping element has a concentration gradient that gradually decreases from the surface of the lithium transition metal oxide particles toward the center of the particles. The cathode active material for a secondary battery according to claim 1, wherein the nickel (Ni) has a concentration gradient that gradually decreases from the center of the lithium transition metal oxide particles toward the surface of the particles. 2. The cathode active material for a secondary battery according to claim 1, wherein the nickel (Ni) has a concentration difference of 3 atom % or more between the center of the lithium transition metal oxide particles and the surface of the particles. The secondary battery of claim 1, wherein at least one of manganese (Mn) and cobalt (Co) has a concentration gradient that gradually increases from the center of the lithium transition metal oxide particles toward the surface of the particles. positive electrode active material. The cathode active material for a secondary battery according to claim 1, wherein the content of nickel (Ni) is 80 mol% or more of all transition metals contained in the lithium transition metal oxide. mixing a cathode active material precursor, a lithium-containing source material and a doping element source material;
after said mixing, calcining to form a lithium transition metal oxide;
The positive electrode active material precursor contains at least two transition metals selected from the group consisting of nickel (Ni), cobalt (Co) and manganese (Mn),
The content of nickel (Ni) is 60 mol% or more of all transition metals contained in the positive electrode active material precursor,
At least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the positive electrode active material precursor has a concentration gradient within the positive electrode active material precursor particles,
A method for producing a positive electrode active material for a secondary battery, wherein the doping element raw material contains at least one doping element selected from the group consisting of La and Bi. The method for producing a positive electrode active material for a secondary battery according to claim 9 , wherein the baking is performed at a temperature of 850 to 950°C. 10. The method of claim 9 , wherein the nickel (Ni) has a concentration gradient that gradually decreases from the center of the positive electrode active material precursor particles toward the surface of the particles. 10. The secondary battery according to claim 9 , wherein at least one of manganese (Mn) and cobalt (Co) has a concentration gradient that gradually increases from the center of the positive electrode active material precursor particles toward the surface of the particles. A method for producing a positive electrode active material. 10. The method of manufacturing a positive electrode active material for a secondary battery according to claim 9 , wherein the content of nickel (Ni) in all transition metals contained in the positive electrode active material precursor is 80 mol% or more. After the firing to form the lithium transition metal oxide,
10. The method of manufacturing a positive electrode active material for a secondary battery according to claim 9 , further comprising the step of washing the lithium transition metal oxide with water. 15. The method of claim 14 , wherein after washing with water, the doping element contained in the lithium transition metal oxide is 2,000 ppm or more. A positive electrode for a secondary battery, comprising the positive electrode active material according to claim 1 . A lithium secondary battery comprising the positive electrode for a secondary battery according to claim 16 .