JP2020507894A - Positive electrode active material for secondary battery, method for producing the same, and lithium secondary battery including the same - Google Patents

Abstract

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), wherein the lithium transition metal oxide contains Nickel (Ni) content of at least 60 mol% of all the 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 particles and the surface of the particles is 2 atom% or more, and the lithium transition metal oxide is formed from W, Mo, Ta, Nb, La, and Bi. And at least one doping element selected from the group consisting of a positive electrode active material for a secondary battery containing 2,000 ppm or more. About the quality.

Description

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

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

  With the development of technology and the demand for mobile devices, the demand for secondary batteries as an energy source is increasing rapidly. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low self-discharge rate have been commercialized and widely used.

As a positive electrode active material of a lithium secondary battery, a lithium transition metal oxide is used. Among them, a lithium cobalt oxide of LiCoO ₂ having a high working voltage and excellent capacity characteristics is mainly used. However, LiCoO ₂ has extremely poor thermal characteristics due to destabilization of the crystal structure due to lithium removal, and is expensive, so that there is a limit to large-scale use as a power source in fields such as electric vehicles.

As materials to be used instead of LiCoO ₂ , lithium manganese oxide (such as LiMnO ₂ or LiMn ₂ O ₄ ), lithium iron phosphate compound (such as LiFePO ₄ ) or lithium nickel oxide (such as LiNiO ₂ ) have been developed. Was done. Above all, research and development on lithium nickel oxide, which has a high reversible capacity of about 200 mAh / g and can easily realize a large-capacity battery, has been actively conducted. However, LiNiO ₂ is inferior in thermal stability to LiCoO ₂ , and when an internal short circuit occurs due to external pressure or the like in a charged state, the positive electrode active material itself is decomposed, which causes the battery to burst and ignite. is there.

Therefore, as a method for maintaining excellent reversible capacity of LiNiO ₂ and improving low thermal stability, a method of replacing part of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed. Was. However, LiNi _1-α Co _α O ₂ (α = 0.1 to 0.3) in which nickel is partially substituted with cobalt exhibits excellent charge / discharge characteristics and life characteristics, but has poor thermal stability. There is a problem of low. Further, a nickel manganese-based lithium composite metal oxide in which a part of Ni is substituted by 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”) ), The output characteristics are low, and there is a possibility that the metal element is eluted and the battery characteristics are reduced.

  In order to solve such a problem, a lithium transition metal oxide having a concentration gradient of a metal composition has been proposed. However, in this method, when synthesizing the positive electrode active material precursor, the metal composition can be synthesized so as to have a concentration gradient, but the precursor having the metal composition concentration gradient and the lithium salt are mixed and fired. In the process of synthesizing the lithium transition metal oxide, there has been a problem that the diffusion speed of metal ions is increased and the concentration gradient of the metal composition inside and outside is eliminated.

  As still another method, research has been conducted to increase the content of Ni in an NCM-based lithium oxide in order to increase the energy density of a battery for a small vehicle or a power storage battery. However, as the content of Ni increases, the crystal grows rapidly and greatly when fired at a high temperature, it is difficult to increase the firing temperature, it is difficult to control the size of the crystal, and the size of the crystal is sharply increased. When it increases, there is a problem that the battery capacity and the life characteristics are rapidly reduced.

  The present invention relates to a High-Ni-based positive electrode active material having a transition metal composition having a concentration gradient and a high nickel (Ni) concentration, capable of maintaining a transition metal composition concentration gradient even after calcination, and having high calcination. An object of the present invention is to provide a positive electrode active material for a secondary battery that has a high capacity and can secure excellent high-temperature stability and life characteristics by suppressing a sudden increase in crystal size even at a temperature. 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), wherein the lithium transition metal oxide contains Nickel (Ni) content of at least 60 mol% of all the 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 particles and the surface of the particles is 2 atom% or more, and the lithium transition metal oxide is formed from W, Mo, Ta, Nb, La, and Bi. And at least one doping element selected from the group consisting of 2,000 ppm or more. To provide an active material.

  Further, the present invention includes a step of mixing a positive electrode active material precursor, a lithium-containing raw material and a doping element raw material, and after the mixing, firing to form a lithium transition metal oxide; The active material precursor includes at least two or more transition metals selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn), and all transition metals contained in the positive electrode active material precursor. Wherein 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 doping element raw material having a concentration gradient within the precursor particles, wherein the doping element raw material is at least one selected from the group consisting of W, Mo, Ta, Nb, La and Bi; It provides a method for producing a positive electrode active material for a secondary battery comprising the doping elements.

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

  The present invention provides a High-Ni-based positive electrode active material having a transition metal composition having a concentration gradient and a high nickel (Ni) concentration, by doping a specific doping element with the transition metal concentration gradient in a particle during firing. Can be suppressed, the concentration gradient of the transition metal composition can be maintained even after firing, and a sharp increase in crystal size can be suppressed even at a high firing temperature.

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

  Hereinafter, in order to facilitate understanding of the present invention, the present invention will be described in more detail. At this time, the terms and words used in the present specification and claims should not be interpreted as being limited to ordinary or lexical meanings, and the inventors consider their own invention in the best way. Should be interpreted to have meanings and concepts consistent with the technical idea of the present invention in accordance with the principle that the concept of terms can be defined as appropriate for explanation.

  The positive electrode active material for a secondary battery of 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), The content of nickel (Ni) in all the transition metals contained in the lithium transition metal oxide is 60 mol% or more, and nickel (Ni), cobalt (Co) and nickel (Ni) contained in the lithium transition metal oxide are contained. At least one of manganese (Mn) has a concentration gradient in which the concentration difference between the center of the lithium transition metal oxide particles and the surface of the particles is 2 atom% or more, and the lithium transition metal oxide is W, Mo, At least one doping element selected from the group consisting of Ta, Nb, La, and Bi is contained, and the doping element contains 2,000 ppm or more. It is.

  The doping element is a heavy element having an atomic weight of 92 or more. The present invention uses a heavy element having an atomic weight of 92 or more as a doping element to perform doping on the surface and inside of the positive electrode active material, thereby enabling high-temperature firing, and even after high-temperature firing, the concentration gradient of the transition metal composition of the positive electrode active material. Can be maintained, the crystal size can be prevented from increasing rapidly, and a high capacity and excellent life characteristics and high-temperature stability can be realized.

  When producing a positive electrode active material having a transition metal composition having a concentration gradient, generally, a positive electrode active material precursor having a concentration gradient is mixed with a lithium-containing raw material and then fired. At this time, conventionally, the diffusion rate of the transition metal ion is increased by the high sintering temperature, and in the finally generated positive electrode active material of the lithium transition metal oxide, the concentration gradient of the transition metal in the particles is smaller than the target value. There was a problem that it was almost gone.

  In the case of a High-Ni-based lithium transition metal oxide in which the content of nickel (Ni) is increased to 60 mol% or more, and further to 80 mol% or more, in order to increase the energy density, crystals are fired at high temperature. There is a problem that the capacity and life characteristics are rapidly reduced due to rapid growth, and the concentration gradient itself in the particles of the positive electrode active material precursor is smaller than when the content of nickel (Ni) is less than 60 mol%. Is formed so as not to be large, so that it is very difficult to maintain the transition metal concentration gradient of the positive electrode active material after firing.

  Therefore, in order to solve such a problem, the present invention provides a High-Ni-based positive electrode active material in which the transition metal composition has a concentration gradient and the concentration of nickel (Ni) is 60 mol% or more, and more preferably 80 mol% or more. By doping the material with a specific doping element having an atomic weight of 92 or more, the transition metal concentration gradient in the particles is prevented from disappearing during firing, and the concentration gradient of the transition metal composition is maintained even after firing, so that the capacity and life can be maintained. Characteristics and stability can be improved.

  Further, since 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 of suppressing grain growth, it is possible to prevent the crystal from growing rapidly even at a high firing temperature. In addition, the size of the crystal can be easily controlled, and the size of the crystal can be reduced even with a high content of nickel (Ni), so that excellent capacity and life characteristics are exhibited.

  Further, according to the present invention, since high-temperature sintering can be performed by doping a specific doping element having an atomic weight of 92 or more, a positive electrode active material having improved high-temperature stability and heat resistance can be manufactured. It is possible to provide a lithium secondary battery having a capacity and excellent life characteristics.

  The doping element according to an embodiment of the present invention has an atomic weight of 92 or more and an oxidation number of +3 or more. By doping with a doping element having an oxidation number as high as +3 or more, it can easily react with lithium at the time of sintering, whereby the rate of lithium diffusing into the inside can be adjusted, and thereby, the transition metal ion And a rod-like internal structure can be maintained. Further, such a concentration gradient and a rod-shaped internal structure can provide high-temperature life and stability of the positive electrode active material due to structural stability.

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

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

[Chemical formula 1]
_{Li a Ni x1 Mn y1 Co z1} M w1 O 2

  In Chemical Formula 1, M includes at least one 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 Chemical Formula 1 is an average composition of the lithium transition metal oxide in the positive electrode active material particles.

  In the positive electrode active material of the lithium transition metal oxide represented by Chemical Formula 1, Li may be included in a content corresponding to a, that is, 0.9 <a ≦ 1.1. If a is 0.9 or less, the capacity may decrease. If it exceeds 1.1, the particles may be sintered in the firing step, which may make it difficult to produce a 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 during the production of the positive electrode active material, the Li is more preferably 1.0 ≦ a ≦ 1. 05 may be included.

  In the positive electrode active material of the lithium transition metal oxide represented by Chemical Formula 1, Ni may be included in a content corresponding to x1, that is, 0.6 ≦ x1 ≦ 0.95. When the content of Ni in the positive electrode active material of Chemical Formula 1 is less than 0.6, there is a possibility that there is a limit in realizing high capacity. Therefore, it is not possible to secure an amount of Li sufficient to contribute to charge and discharge by being replaced by the metal, and the charge and discharge capacity may be reduced. The content of Ni sufficient for achieving high capacity may be 0.6 or more, preferably 0.8 or more, and most preferably, Ni is 0.8 ≦ x1 ≦ It may be contained at a content of 0.95.

  In the positive electrode active material of the lithium transition metal oxide represented by Chemical Formula 1, Mn improves the structural stability of the active material, and as a result, can improve the stability of the battery. Considering the effect of improving the life characteristics by containing Mn, the Mn may be contained at a content corresponding to y1, that is, 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 the capacity characteristics of the battery may be deteriorated.

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

  In the positive electrode active material of the lithium transition metal oxide represented by Chemical Formula 1, M may be one or more selected from the group consisting of W, Mo, Ta, Nb, La, and Bi, and the structure of the positive electrode active material Stability can be improved, the concentration gradient of Ni, Mn or Co can be maintained, and rapid growth of crystals can be prevented. The M doping element is contained at a content corresponding to w1, that is, a content of 0 <w1 ≦ 0.1 within a range that achieves the intended effect and does not deteriorate the characteristics of the positive electrode active material. Is also good.

  In the positive electrode active material of the lithium transition metal oxide of Chemical Formula 1, the elements Ni, Co, and Mn are other elements for improving the battery characteristics by adjusting the distribution of the transition metal element in the positive electrode active material. May be partially substituted or doped. Specifically, it may be partially substituted or doped with at least one element 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 particles and the surface of the particles. It has a concentration gradient of 2 atom% or more.

  Since the lithium transition metal oxide according to the present invention contains, as a doping element, at least one selected from the group consisting of W, Mo, Ta, Nb, La, and Bi, the concentration of the transition metal composition after firing is maintained. The 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 of the lithium transition metal oxide particles toward the surface. Can have.

  In the present invention, "showing 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 in which the transition metal changes gradually throughout the particles. . 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, there may be a difference of 0.1 to 3 atom%, and more specifically, a difference of 1 to 2 atom%.

  Further, in the present invention, the concentration gradient structure and the concentration of the transition metal in the positive electrode active material particles may be determined by an electron probe micro analyzer (EPMA) analysis method, an inductively coupled plasma-atom emission spectroscopy (Inductively Coupled Plasma) method. -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 can be measured while moving from the center of the positive electrode active material to the surface using EPMA. Alternatively, the atomic ratio of each metal can be measured while etching from the surface of the positive electrode active material toward the center by XPS.

  As described above, by varying the concentration of each transition metal in the positive electrode active material particles due to the concentration gradient, the characteristics of the transition metal can be easily used, and the effect of improving the battery performance of the positive electrode active material can be further improved. Can be done. In addition, by having a concentration gradient in which the concentration of the transition metal changes continuously according to the position in the positive electrode active material particles, a sharp phase boundary region does not exist from the center to the surface, and the crystal structure is reduced. Can be stabilized and the thermal stability can be increased. When the concentration gradient of the metal is constant, the effect of improving the structural stability can be further improved.

  Conventionally, even if the cathode active material precursor has a transition metal concentration gradient, the diffusion rate of transition metal ions is increased by the high sintering temperature during sintering, and the positive electrode active material of the finally generated lithium transition metal oxide is used. Although the substance has a problem that the concentration gradient of the transition metal in the particles becomes smaller than or almost disappears from a target value, the present invention relates to a group consisting of W, Mo, Ta, Nb, La, and Bi as doping elements. By containing at least one selected from among the above, 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) included 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 concentration of Ni is maintained at a high concentration at the center of the particle, and the concentration of Ni decreases toward the surface, so that thermal stability is exhibited and a decrease in capacity can be prevented.

  The positive electrode active material according to an embodiment of the present invention, by being doped with a specific doping element, prevents diffusion of a transition metal having a concentration gradient formed during firing, and the concentration gradient is also reduced in the fired positive electrode active material. In order to maintain the concentration, the nickel (Ni) may have a concentration difference of 2 atom% or more, more preferably 3 atom% or more, between the center of the particle and the surface of the particle.

  As in the case of the embodiment of the present invention, when the average content of nickel (Ni) is 60 mol% or more of all transition metals in the positive electrode active material particles, firing is performed more than when the average content is less than 60 mol%. It is difficult for the subsequent positive electrode active material particles to maintain the concentration gradient. Further, when the content of nickel (Ni) is 80 mol% or more, it is more likely that the positive electrode active material particles after firing maintain the concentration gradient than when the content is less than 80 mol% (for example, 50 mol%). difficult.

  For example, when the average content of nickel (Ni) is 50 mol%, the transition metal concentration at the center of the particle and the surface of the particle without doping of the specific doping element according to an embodiment of the present invention. The difference can be realized to be about 10 atom%, specifically, about 5 to 8 atom%, but when the average content of nickel (Ni) is 60 mol% or more, the positive electrode active material precursor Since the particles have a high concentration of nickel (Ni) as a whole, the concentration gradient of nickel (Ni) in the positive electrode active material precursor particles before firing is not large. Therefore, the concentration gradient after firing is maintained. Becomes more difficult. However, when a specific doping element having an atomic weight of 92 or more is doped 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 not removed. A good concentration gradient can be maintained.

  In the cathode active material according to one embodiment of the present invention, at least one of manganese (Mn) and cobalt (Co) contained in the cathode 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, since the concentration of Mn is maintained at a low concentration at the center of the particle and the concentration of Mn increases toward the surface of the particle, excellent thermal stability can be obtained without reducing the capacity, By maintaining the low concentration of Co at the center and increasing the Co concentration toward the surface of the particles, it is possible to reduce the amount of used Co and prevent the capacity from decreasing.

  Similarly, the cathode 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 to prevent diffusion of a transition metal having a concentration gradient formed during firing, and At least one of the manganese (Mn) and the cobalt (Co) has a concentration difference of 2 atom% or more between the center of the particle and the surface of the particle because the concentration gradient is maintained even in the subsequent positive electrode active material. , More preferably 3 atom% or more.

  In the cathode active material according to one embodiment of the present invention, the concentration of nickel (Ni) included in the cathode 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 be increased from the center of the particle toward the surface of the particle with a concentration gradient that is complementary to the concentration gradient of the nickel (Ni). .

  Thus, in the positive electrode active material, the concentration of nickel (Ni) 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 the concentration gradient of, the thermal stability can be exhibited while maintaining the capacity characteristics.

  On the other hand, in the cathode active material according to an embodiment of the present invention, the doping element doped in 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. The positive electrode active material according to one embodiment of the present invention is obtained by mixing a positive electrode active material precursor and a lithium-containing raw material, and baking, mixing together a specific doping element having an atomic weight of 92 or more, and doping by heat treatment. Therefore, the doping element is in a form in which the doping element is diffused from the surface of the positive electrode active material to the inside of the positive electrode active material particles and is doped, and the concentration of the doping element gradually decreases from the surface of the particle toward the center of the particle. Can be.

Meanwhile, the lithium transition metal oxide cathode active material according to an embodiment of the present invention may have a crystal size of _{50 to} 200 nm, more preferably 80 to 180 nm, in average particle size (D50).

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

When the size of the crystal of the positive electrode active material is less than ₅₀ nm in average particle diameter (D ₅₀ ), the crystallinity is low, so that the storage property at a high temperature is rapidly deteriorated, or the gas is generated by a side reaction with the electrolyte due to a high specific surface area. The generation may increase, or the stability of the positive electrode active material may be deteriorated due to the structural instability of the positive electrode active material. When the thickness exceeds 200 nm, the capacity and life characteristics may be significantly reduced.

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

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

  The doping element is contained in an amount of 2,000 ppm or more based on 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.

  When the doping element is contained within the range of the content, it is possible to effectively suppress the disappearance of the transition metal concentration gradient in the particles at the time of firing, and to maintain the transition metal composition concentration gradient even after the firing. Thus, it is possible to suppress a sudden increase in the crystal size even at a high firing temperature.

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

  In the positive electrode active material according to an embodiment of the present invention, the maximum peak at which the heat flow rate is maximum is measured at 235 ° C. or more during thermal analysis by differential scanning calorimetry (DSC).

  The positive electrode active material according to one embodiment of the present invention can be fired at a high temperature, for example, at a firing temperature of 850 ° C. or more by doping 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 the high-temperature sintering, and a rapid increase in the crystal size can be suppressed.

  The positive electrode active material thus fired at a high temperature can be measured at a maximum peak at a heat flow rate of 235 ° C. or higher at the time of thermal analysis by differential scanning calorimetry (DSC). Stability and heat resistance can be improved.

  The method for producing a positive electrode active material for a secondary battery according to the present invention includes a step of mixing a positive electrode active material precursor, a lithium-containing raw material and a doping element raw material, and after the mixing, firing to produce a lithium transition metal oxide. Forming the positive electrode active material precursor, wherein the positive electrode active material precursor comprises at least two or more transition metals selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn); The content of nickel (Ni) among all transition metals contained in the precursor is 60 mol% or more, and nickel (Ni), cobalt (Co), and manganese (Mn) contained in the positive electrode active material precursor are contained. At least one has a concentration gradient in the positive electrode active material precursor particles, and the doping element raw material is selected from the group consisting of W, Mo, Ta, Nb, La, and Bi. Is-option including at least one doping element.

  According to the present invention, a cathode active material precursor having a transition metal concentration gradient and a nickel (Ni) content of 60 mol% or more is mixed with a lithium-containing raw material and a specific doping element raw material having an atomic weight of 92 or more. Then, firing is performed 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 positive electrode active material precursor.

  In the present invention, when the positive electrode active material precursor and the lithium-containing raw material are mixed and fired, the doping element raw materials are mixed together and doped by heat treatment. From the positive electrode active material particles.

The doping element raw 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 material may include at least one transition metal selected from the group consisting of W, Mo, Ta, Nb, La, and Bi, and more preferably may include tungsten (W). The doping element 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 ) And at least one selected from the group consisting of ammonium tungstate and ammonium molybdate.

  The positive electrode active material precursor may be represented by the following Chemical Formula 2.

[Chemical formula 2]
_{Ni x2 Mn y2 Co z2 (OH} ) 2

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

  The composition of Chemical Formula 2 is an average composition of the transition metal in the positive electrode active material precursor particles.

  In the positive electrode active material precursor of Formula 2, the content of nickel (Ni), manganese (Mn), and cobalt (Co) is applied in the same manner as the content of the above-described lithium transition metal oxide in the positive electrode active material. Is also good.

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

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

  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 be provided.

  In the cathode active material precursor according to an embodiment of the present invention, the concentration of nickel (Ni) included in the cathode 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) decreases and increases from the center of the particle to the surface of the particle with a concentration gradient that is complementary to the concentration gradient of the nickel (Ni). be able to.

  Thus, in the positive electrode active material precursor, the concentration of nickel (Ni) 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 the concentration gradient of the combination, it is possible to manufacture 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 the concentration gradient as described above can maintain the concentration gradient even after firing. Further, not only the concentration gradient is maintained even at a high firing temperature, but also a sharp increase in the crystal size can be prevented, and the primary particles can maintain a rod (Rod) structure. Therefore, the present invention enables high-temperature firing, and the high-temperature firing can be performed at a temperature of 850 to 950 ° C. More preferably, the firing for manufacturing the cathode active material according to an embodiment of the present invention may be performed at a temperature of 850 to 900C.

  When the firing temperature is lower than 850 ° C., the manufactured positive electrode active material is subjected to thermal analysis by differential scanning calorimetry (DSC). Insufficient reaction may cause raw materials to remain in the particles, resulting in a decrease in high-temperature stability and heat resistance of the battery, a decrease in volume density and crystallinity, and a decrease in structural stability. If the temperature exceeds 950 ° C., the change in the concentration gradient of the transition metal during the firing process may be insufficient, and uneven growth of particles may occur.

  In addition, the positive electrode active material according to an exemplary embodiment of the present invention may be manufactured to further include a step of forming the lithium transition metal oxide by firing as described above, and then washing with water.

  As in the embodiment of the present invention, in the case of a High-Ni-based lithium transition metal oxide containing nickel at a high concentration, it is structurally unstable as compared with a lithium transition metal oxide having a small nickel content. Therefore, a large amount of unreacted lithium by-products such as lithium hydroxide and lithium carbonate is generated in the production process. As described above, when a large amount of lithium by-product is present in the positive electrode active material, the lithium by-product reacts with the electrolytic solution to cause gas generation and swelling, and therefore, high-temperature stability is significantly reduced. Will be. Therefore, a water washing step for removing lithium by-products from the lithium transition metal oxide containing high concentration of nickel can be performed.

  By the water washing, lithium by-products remaining in the lithium transition metal oxide can be removed, and after a part of the positive electrode active material has been doped, the remaining doping element material can be removed. By removing the remaining doping element raw material by the water washing step, the phenomenon in which the doping element, for example, W is eluted in the electrolyte solution or eluted in the negative electrode as the cycle proceeds can be suppressed.

  The water washing can be performed using 80 to 200 parts by weight of pure water, more preferably 100 to 150 parts by weight, based on 100 parts by weight of the lithium transition metal oxide. After performing the water washing, the content of the doping element doped in the positive electrode active material may be 20 to 40 parts by weight based on the initial input amount of 100 parts by weight, and the remainder is included in the washing water and removed. Can be done. Specifically, after the water washing, the doping element contained in the lithium transition metal oxide may be contained in an amount of 2,000 ppm or more, more preferably from 2,500 ppm to 6,500 ppm, more preferably from 3,000 ppm to 6,500 ppm may be contained.

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

  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 including 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, for example, stainless steel, aluminum, nickel, titanium, calcined carbon or aluminum or A stainless steel surface treated with carbon, nickel, titanium, silver or the like can be used. In addition, the positive electrode current collector may have a thickness of usually 3 to 500 μm, and may form fine irregularities on the surface of the current collector to increase the adhesive force of the positive electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

  Further, the positive electrode active material layer may include a conductive material and a binder together with the above-described positive electrode active material.

  At this time, the conductive material is used for imparting conductivity to the electrode, and in the configured battery, it does not cause a chemical change and can be used without particular limitation as long as it has electronic conductivity. It 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; and conductive polymers such as polyphenylene derivatives. Or a mixture of two or more may be used. The conductive material may usually be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

  In addition, the binder serves to improve the adhesion between the positive electrode active material particles and the adhesive force between the positive electrode 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, and the like. , One of these or a mixture of two or more thereof can be used. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

  The positive electrode can be manufactured by an ordinary positive electrode manufacturing method except that the above-described positive electrode active material is used. Specifically, it can be manufactured by applying the above-described positive electrode active material and optionally a composition for forming a positive electrode active material layer containing a binder and a conductive material on a positive electrode current collector, followed by drying and rolling. 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 generally used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol (isopropyl alcohol), N-methylpyrrolidone (NMP), and acetone (acetone). Or water and the like, and one of these alone or a mixture of two or more thereof may be used. The amount of the solvent used is determined by dissolving or dispersing the positive electrode active material, the conductive material and the binder in consideration of the slurry application thickness and the production yield, and then, when coating for the production of the positive electrode, an excellent thickness. It is sufficient to have a viscosity that can show uniformity.

  In another method, the positive electrode is obtained by casting the composition for forming a positive electrode active material layer on another support, and then laminating a film obtained by peeling the composition from the support on a positive electrode current collector. (Lamination).

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

  Specifically, the lithium secondary battery includes a positive electrode, a negative electrode positioned opposite to the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. It is as follows. The lithium secondary battery may further include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

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

  The negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, fired carbon, and copper. A stainless steel surface treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like may be used. In addition, the negative electrode current collector may have a thickness of usually 3 to 500 μm, and similarly to the positive electrode current collector, by forming fine irregularities on the surface of the current collector, binding of the negative electrode active material is performed. You can also strengthen your power. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

  The negative electrode active material layer optionally includes a binder and a conductive material together with the negative electrode active material. The negative electrode active material layer is, for example, a negative electrode active material on a negative electrode current collector, and selectively coated with a negative electrode forming composition containing a binder and a conductive material, and dried, or the negative electrode forming composition Can be manufactured by casting a film on another support and then laminating a film obtained by peeling off the support on a negative electrode current collector.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. 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 alloy, and Sn alloy. Metallic compounds that can be alloyed with lithium, such as alloys or Al alloys; doping and undoping of lithium such as SiO _β (0 <β <2), SnO ₂ , vanadium oxide, and lithium vanadium oxide A metal oxide that can be used; 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. In addition, a metal lithium thin film may be used as the negative electrode active material. Further, as the carbon material, both low-crystalline carbon and high-crystalline carbon can be used. As the low crystalline carbon, a soft carbon (soft carbon) and a hard carbon (hard carbon) are typical, and as the high crystalline carbon, an amorphous, plate-like, flaky, spherical or fibrous natural graphite or Artificial graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch-based carbon fiber, meso-carbon and petroleum pitches, meso-carbon and mesos High temperature calcined carbon such as coal-based coke (petroleum or coal tar pitch driven cokes) is typical.

  Further, 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 moving lithium ions. Usually, the separator is not particularly limited as long as it is used as a separator in the lithium secondary battery. It is particularly preferable to use those which have low resistance to ion migration of the electrolyte and have excellent electrolyte humidification ability. Specifically, a porous polymer film, for example, a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer. A manufactured porous polymer film or a laminated structure of two or more layers thereof can be used. In addition, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fiber, polyethylene terephthalate fiber, or the like may be used. In order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, or may be selectively used in a single-layer or multilayer structure. .

  Examples of the electrolyte used in the present invention include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the production of a lithium secondary battery. Examples of the electrolyte include, but are not limited to, an electrolyte.

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

  The organic solvent can be used without any 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, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; and dibutyl ether. Ether solvents such as (dibutyl ether) or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate and dimethylcarbonate. ), Diethyl carbonate (diethylcarbona) e, DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (ethyl methyl carbonate, EMC), ethylene carbonate (ethylene carbonate, EC), propylene carbonate, isopropyl carbonate, ethanol, isopropyl carbonate, ethanol, and the like; Nitriles such as R-CN (R is a C2-C20 linear, branched or cyclic hydrocarbon group and may contain a double bond aromatic ring or an ether bond); Amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; and sulfolanes are used. It may be. Among them, a carbonate-based solvent is preferable, and a cyclic carbonate (for example, ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant capable of enhancing the charge / discharge performance of a battery, and a chain carbonate-based compound having a bottom viscosity ( For example, a mixture of ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed and used in a volume ratio of about 1: 1 to about 1: 9, excellent performance of the electrolytic solution can be exhibited.

The lithium salt can be used without particular limitation as long as it is a compound that can provide lithium ions used in a lithium secondary battery. Specifically, the lithium _{salt, LiPF 6, LiClO 4, LiAsF} 6, LiBF 4, LiSbF 6, LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO ₃ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiCl, LiI, or LiB (C ₂ O ₄ ) ₂ may be used. The concentration of the lithium salt is preferably in the range of 0.1 to 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 move effectively.

  In the electrolyte, in addition to the components of the electrolyte, in order to improve the life characteristics of the battery, suppress reduction in the capacity of the battery, improve the discharge capacity of the battery, and the like, for example, haloalkylene carbonate such as difluoroethylene carbonate System compound, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme (glyme), hexaphosphoric triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, One or more additives such as ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride may be further included. At this time, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

  As described above, the lithium secondary battery including the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore suitable for portable devices such as mobile phones, notebook computers, and digital cameras. And electric vehicles such as hybrid electric vehicles (HEVs).

  Thus, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

  The battery module or the battery pack may be a power tool; an electric vehicle (EV); an electric hybrid vehicle (Hybrid Electric Vehicle) (HEV); and a plug-in hybrid electric vehicle (Plug-in Hybrid Electric Vehicle). ), Or any one or more of the power storage systems.

  Hereinafter, embodiments of the present invention will be described in detail so that those having ordinary knowledge in the technical field to which the present invention belongs can be easily implemented. However, the invention may be embodied in various different forms and is not limited to the embodiments described herein.

[Production Example 1: Production of positive electrode active material precursor]
In a batch batch type 20 L reactor set at 60 ° C., the amounts of NiSO ₄ , CoSO ₄ , MnSO ₄ are adjusted so that the molar ratio of nickel: cobalt: manganese is 90: 5: 5. To prepare a first metal-containing solution having a concentration of 2M by mixing in water, and to prepare NiSO ₄ , CoSO ₄ and MnSO ₄ at a molar ratio of nickel: cobalt: manganese of 60:20:20. A second metal-containing solution having a concentration of 2M was prepared by mixing in water in such an amount. The container filled with the first metal-containing solution is connected to enter the reactor, and the container filled with the second metal-containing solution is connected to the container filled with the first metal-containing solution. Connected to enter. Further, a 25% by weight aqueous solution of NaOH and a 15% by weight aqueous solution of NH ₄ OH were prepared and connected to the reactor, respectively.

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

Next, the first metal-containing solution was charged at a rate of 6 ml / hr, an aqueous NaOH solution at a rate of 1 ml / hr, and an aqueous NH ₄ OH solution at a rate of 2 ml / hr, and reacted for 30 minutes. The product seed was formed. 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 nickel cobalt manganese composite metal hydroxide particles, Was induced to produce a concentration gradient. 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 produce a positive electrode active material precursor having a concentration gradient.

  The prepared positive electrode active material precursor has an average molar ratio of Ni: Co: mn of 80:10:10, and Ni has a concentration gradient that gradually decreases from the center of the particle toward the surface of the particle. , And Co and Mn showed a concentration gradient that gradually increased from the center of the particle toward the surface of the particle.

Example 1
The positive electrode active material precursor prepared in Preparation Example 1 was mixed with lithium hydroxide (LiOH) at a molar ratio of 1: 1.03 as a lithium raw material, and as a doping element raw material for doping 8,000 ppm of tungsten. after mixing the _WO 3 0.6 g) of WO ₃ to 1 wt% (ex) precursor 60g relative to the weight of the precursor, the positive electrode active material of lithium transition metal oxides performed about 13 hours firing at 890 ° C. Was formed. After baking, the mixture was stirred for 5 minutes using 100 parts by weight of pure water with respect to 100 parts by weight of the positive electrode active material, and then subjected to a water washing step of passing through a filter and drying at 130 ° C. for 24 hours. An active material (W6, 100 ppm dope) was produced.

Example 2
A positive electrode active material (W5, W5) was prepared in the same manner as in Example 1 except that WO ₃ was mixed as a doping element raw material at 1% by weight based on the weight of the precursor, and then calcined at 880 ° C. for about 13 hours. 900 ppm dope).

Example 3
As a doping element raw material, La ₂ O ₃ was mixed with 0.55 wt% (ex) of precursor 60 g of La ₂ O ₃ 0.33 g), and calcined at 890 ° C. for about 15 hours. A positive electrode active material (La 3,000 ppm doped) was produced in the same manner as in Example 1 except that the experiment was performed.

Comparative Example 1
The positive electrode 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 a doping element raw material was not added. 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 1.32% by weight of Al ₂ O ₃ as a doping element raw material was mixed with the precursor, and then baked at 880 ° C. for about 13 hours. Was manufactured.

Comparative Example 3
Performed in the same manner as in Example 1, except that ZrO ₂ was mixed at 0.27% by weight with respect to the precursor in order to dope 2,000 ppm of Zr as a doping element raw material, and then baked at 880 ° C. for about 13 hours. As a result, 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, followed by baking at 780 ° C. for about 13 hours. .

[Experimental example 1: Analysis of positive electrode active material]
In order to confirm the concentration gradient of Ni, Co, and Mn in the lithium transition metal oxide and the distribution of the doped doping element, the positive electrode active material manufactured in Example 1 was subjected to ion milling to form a cross section of the particle, The composition from the center to the surface was confirmed by electron beam microanalyzer (EPMA) analysis. For comparison, the active materials produced in Comparative Examples 1 to 3 were also confirmed. 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 maintained as good as 3 atom% or more from the center of the particle to the surface of the particle even after firing. On the other hand, in Comparative Example 1 which was not doped and Comparative Examples 2 and 3 which were doped with Al and Zr, the concentration gradients of Ni, Co, and Mn after firing were almost completely eliminated or showed an insufficient level.

[Experimental example 2: Measurement of crystal size]
In order to measure the size of the crystals of the positive electrode active materials manufactured in Examples 1 to 3 and Comparative Examples 1 to 3, the size of the crystals was measured using a laser diffraction particle size distribution analyzer (manufactured by Malvern, Mastersizer 3000). The measurement was performed and the results are shown in Table 5.

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

[Production Example 2: Production of lithium secondary battery]
Lithium secondary batteries were manufactured using the positive electrode active materials manufactured in Examples 1 to 3 and Comparative Examples 1 to 4, respectively.

  Specifically, the respective positive electrode active materials, carbon black conductive materials and PVdF binders produced in Examples 1 to 4 and Comparative Examples 1 to 4 were mixed in an N-methylpyrrolidone solvent at 95: 2.5: 2. The mixture was mixed at a weight ratio of 5 to produce a positive electrode forming composition (viscosity: 5000 mPa · s), which was applied to an aluminum current collector, dried at 130 ° C., and then 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, and the electrode assembly is disposed inside the case. This was injected to produce a lithium secondary battery. At this time, the electrolyte was mixed with an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (mixing volume ratio of EC / DMC / EMC = 3/4/3) in lithium hexafluorophosphate (1.0 M concentration). LiPF ₆ ).

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

  For DSC evaluation, the battery was decomposed in a state of charge of 100% SOC, a positive electrode and a new electrolytic solution were charged into a DSC measurement cell, and the temperature was measured from 10 ° C. per minute to 400 ° C. from normal temperature. As a result, the temperature at which the maximum peak (Main peak) at which the heat flow rate is maximum is shown in Table 6 below.

  Referring to Table 6, it can be seen that Examples 1 to 3 have significantly improved high-temperature cycle characteristics as compared with Comparative Examples 1 to 3. In the case of Comparative Example 4 fired at 780 ° C., which is a relatively low temperature, the maximum peak (Main peak) of the heat flow rate was shown at 230 ° C. in the DSC measurement, and Example 1 fired at a high temperature of 850 ° C. or more. It can be seen that the high-temperature stability and the heat resistance are significantly lower than those of Nos. 1 to 3.

Claims (20)

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);
The content of nickel (Ni) among all transition metals contained in the lithium transition metal oxide is 60 mol% or more;
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 particles and the surface of the particles. Having a concentration gradient
The lithium transition metal oxide contains at least one doping element selected from the group consisting of W, Mo, Ta, Nb, La, and Bi, and the doping element contains 2,000 ppm or more. , Positive electrode active material for secondary batteries.   2. The secondary battery according to claim 1, wherein the positive electrode active material for a secondary battery has a maximum peak having a maximum heat flow rate measured at 235 ° C. or higher during thermal analysis by differential scanning calorimetry (DSC). For positive electrode active material.   The positive electrode active material for a secondary battery according to claim 1, wherein the doping element is tungsten (W).   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 of the lithium transition metal oxide particles toward the surface. The positive electrode active material for a secondary battery according to claim 1, comprising:   The positive electrode 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 positive electrode 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.   The positive electrode active material for a secondary battery according to claim 1, wherein the nickel (Ni) has a concentration difference between the center of the lithium transition metal oxide particles and the surface of the particles of 3 atom% or more.   The secondary battery according to claim 1, wherein at least one of the manganese (Mn) and the 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. For positive electrode active material.   The positive electrode active material for a secondary battery according to claim 1, wherein the content of nickel (Ni) among all transition metals contained in the lithium transition metal oxide is 80 mol% or more. 2. The positive electrode active material for a secondary battery according to claim 1, wherein the lithium transition metal oxide has an average crystal grain size (D ₅₀ ) of 50 to 200 nm. Mixing the positive electrode active material precursor, the lithium-containing source material and the doping element source material,
After the mixing, baking to form a lithium transition metal oxide,
The positive electrode active material precursor includes at least two or more transition metals selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn),
Nickel (Ni) content of all transition metals contained in the positive electrode active material precursor is 60 mol% or more,
At least one of nickel (Ni), cobalt (Co) and manganese (Mn) contained in the positive electrode active material precursor has a concentration gradient in the positive electrode active material precursor particles,
The method of manufacturing a positive electrode active material for a secondary battery, wherein the doping element material includes at least one doping element selected from the group consisting of W, Mo, Ta, Nb, La, and Bi.   The method of claim 11, wherein the firing is performed at a temperature of 850 to 950 ° C. 12.   The method of claim 11, wherein the doping element material includes tungsten (W).   The method of manufacturing a positive electrode active material for a secondary battery according to claim 11, 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.   The secondary battery according to claim 11, wherein at least one of the manganese (Mn) and the 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.   The method for producing a positive electrode active material for a secondary battery according to claim 11, wherein the content of nickel (Ni) among all transition metals contained in the positive electrode active material precursor is 80 mol% or more. After the calcination to form a lithium transition metal oxide,
The method of claim 11, further comprising washing the lithium transition metal oxide with water.   The method according to claim 17, wherein the doping element contained in the lithium transition metal oxide after the water washing 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 according to claim 19.