Method for preparing positive electrode active material and positive electrode active material prepared thereby

Abstract

The present invention relates to a method for preparing a positive electrode active material and a positive electrode active material prepared thereby. The method for preparing the positive electrode active material includes: a first step of preparing a mixture of a lithium precursor compound and a metal hydroxide; and a second step of preparing a sintered body by sintering the mixture, a shell portion of the sintered body including a compound represented by Chemical Formula 1, a lithium concentration of the shell portion of the sintered body being higher than a lithium concentration of a core portion of the sintered body, Chemical Formula 1: Li1 + x1 (Nia1Cob1Mnc1) 1-x1O2, in Chemical Formula 1, x1, a1, b1, and c1 being real numbers satisfying-0.05 < = x1 < = 0.2, 0.6 < = a1 < = 1.0, 0 < = b1 < = 0.3, and 0 < = c1 < = 0.4, respectively. The present invention uses excess lithium and low oxygen partial pressure at a low synthesis temperature, thereby forming secondary particles having a lithium concentration gradient in which excess lithium is gradually contained from the core to the surface while the entire structure reaches a stoichiometric equilibrium.

Description

Technical Field

[0001] The invention relates to a method for preparing a positive electrode active material and the positive electrode active material prepared thereby. Background Art

[0002] Lithium secondary batteries are used in various fields such as mobile devices, energy storage systems and electric vehicles due to their high energy density and voltage, long cycle life and low self-discharge rate. The core materials of lithium secondary batteries as mentioned above can be said to be positive electrode materials, negative electrode materials, electrolytes and separators. In recent years, with the increase in demand for electric vehicles, the importance of positive electrode active materials of lithium secondary batteries used to drive electric vehicles has become increasingly prominent.

[0003] The above positive electrode active materials can be divided into lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), lithium manganese oxide (LMO) and lithium iron phosphate (LFP) according to the constituent materials. As positive electrode active materials for secondary batteries suitable for medium and large electronic devices, LiNi x Co y Mn z O 2 and LiNi x Co y Al z O 2 The nickel-based positive electrode active material can reduce the cost by replacing part of the expensive cobalt with nickel, and has the characteristics of high energy density and relatively long life, so it has the advantage of being able to increase the reversible capacity of the lithium secondary battery.

[0004] However, despite the excellent electrical / chemical properties of this high-concentration nickel-rich layered cathode active material, the nickel oxidation number must be closer to trivalent compared to other layered cathode active materials, so its structure is unstable and difficult to synthesize because it tends to maintain a stable oxidation number of 2. Therefore, in the preparation of nickel-rich layered cathode active materials, in addition to high oxygen partial pressure, the optimal synthesis temperature and lithium content are also emphasized.

[0005] When preparing nickel-rich layered positive electrode active materials, due to the preparation under a highly oxidizing atmosphere, the consumption of high-concentration oxygen (99.95-99.999%) is large (about 400-500cc / minute), the manufacturing cost may increase, and excessive synthesis temperature will lead to surface Li / O loss and the formation of degraded structures such as NiO. Similarly, if the lithium content is insufficient, a lithium-poor structure will be formed on the surface. If the lithium content is excessive, residual lithium compounds will be formed on the surface, which may lead to side reactions and performance degradation.

[0006] Therefore, in previous studies, attempts were made to use air gas instead of high-concentration oxygen to synthesize nickel-rich materials. However, as the oxygen partial pressure decreases, the Ni 2+ This results in increased cation mixing, impurity formation, and other problems, and the electrochemical performance is lower than that when using high concentration oxygen.

[0007] In addition, it is reported that as a method for improving electrochemical performance, there is a method of suppressing the electrochemical performance of the nickel-rich layered positive electrode active material by arranging a core-shell structure in which an NCM material with a low nickel oxidation number and a high cobalt and manganese content (such as NCM111, 523, etc.) is arranged in the shell and a nickel-rich layered positive electrode active material with a high nickel oxidation number is arranged in the core. 3+ A method in which the surface structure deteriorates due to the instability of cations, or a method in which, during the synthesis of nickel-rich layered positive electrode active materials, a lithium-excess structure is formed under high-purity oxygen atmosphere, low synthesis temperature, and high lithium content conditions, thereby improving the stability of the nickel-rich layered positive electrode active material by doping inactive lithium into the transition metal layer, etc. have been reported. However, the oxidation number of nickel is reduced in the core-shell structure, and the lithium-excess nickel-rich material replaces the transition metal in the transition metal layer to occupy excess lithium, so there is an inevitable problem of capacity reduction.

[0008] Therefore, there is still a need to develop nickel-rich positive active materials having excellent capacity characteristics and improved stability while reducing manufacturing costs. Summary of the invention

[0009] Technical issues

[0010] One of the various objects of the present invention is to provide a method for preparing a positive electrode active material capable of improving the capacity characteristics of a secondary battery and the positive electrode active material prepared thereby.

[0011] One of the various objects of the present invention is to provide a method for preparing a positive electrode active material capable of improving the cycle performance of a secondary battery and the positive electrode active material prepared thereby.

[0012] One of the various objects of the present invention is to provide a method for preparing a positive electrode active material capable of improving the stability of a positive electrode of a secondary battery and the positive electrode active material prepared thereby.

[0013] One of the various objects of the present invention is to provide a method for preparing a positive electrode active material and a positive electrode active material prepared thereby, which can reduce the manufacturing process cost.

[0014] Solutions to the problem

[0015] The present invention is developed to solve the above-mentioned problems. One embodiment of the present invention can provide a method for preparing a positive electrode active material, which includes: a first step of preparing a mixture of a lithium precursor compound and a metal hydroxide; and a second step of preparing a sintered body by sintering the mixture, wherein the shell of the sintered body includes a compound represented by the following chemical formula 1, and the lithium concentration of the shell of the sintered body is higher than the lithium concentration of the core.

[0016] [Chemical formula 1]

[0017] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O 2

[0018] In the above chemical formula 1, x1, a1, b1 and c1 are real numbers satisfying -0.05≤x1≤0.2, 0.6≤a1≤1.0, 0≤b1≤0.3, and 0≤c1≤0.4, respectively.

[0019] At this time, the first step may be a step of mixing the metal hydroxide and the lithium precursor compound in a molar ratio of 1:0.9 to 1:1.5.

[0020] In addition, the lithium precursor compound may further include a compound selected from the group consisting of LiNO 3 , Li 2 SO 4 , Li 2 CO 3 , LiCl, LiI and LiBr.

[0021] On the other hand, the second step may be performed at a temperature of 600° C. or higher and 900° C. or lower.

[0022] At this time, the second step may be performed under an air atmosphere.

[0023] In addition, the second step may be performed for a period of time of 8 hours to 24 hours at an air inflow rate of 0 sccm to 600 sccm.

[0024] In one embodiment of the present invention, the core of the sintered body of the method for preparing a positive electrode active material of the present invention may include a compound represented by the following Chemical Formula 2.

[0025] [Chemical formula 2]

[0026] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O 2

[0027] In the above chemical formula 2, x2, a2, b2 and c2 are real numbers satisfying -0.2≤x2≤0.05, 0.6≤a2≤1.0, 0≤b2≤0.3, and 0≤c2≤0.4, respectively.

[0028] In one example, the sintered body may have a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion.

[0029] In another example, the oxidation number of nickel (Ni) in the shell portion of the sintered body may be higher than that of nickel in the core portion.

[0030] In another embodiment of the present invention, the present invention may provide a positive electrode active material including: a shell portion represented by the following Chemical Formula 1; and a core portion represented by the following Chemical Formula 2, wherein the lithium concentration of the shell portion is higher than that of the core portion.

[0031] [Chemical formula 1]

[0032] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O 2

[0033] In the above chemical formula 1, x1, a1, b1 and c1 are real numbers satisfying -0.05≤x1≤0.2, 0.6≤a1≤1.0, 0≤b1≤0.3, and 0≤c1≤0.4, respectively.

[0034] [Chemical formula 2]

[0035] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O 2

[0036] In the above chemical formula 2, x2, a2, b2 and c2 are real numbers satisfying -0.2≤x2≤0.05, 0.6≤a2≤1.0, 0≤b2≤0.3, and 0≤c2≤0.4, respectively.

[0037] At this time, the cathode active material according to the present invention may have a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion.

[0038] Furthermore, the oxidation number of nickel in the shell portion may be higher than the oxidation number of nickel in the core portion described above.

[0039] In one example, the d-spacing of (003) planes in the lattice of the core of other positive active materials according to the present invention may be greater than the d(003)-spacing of the shell.

[0040] Furthermore, the primary particle size of the shell portion may be larger than the primary particle size of the core portion.

[0041] Effects of the Invention

[0042] One of the effects of the present invention is that the capacity characteristics of a secondary battery can be improved.

[0043] One of the effects of the present invention is to improve the cycle performance of a secondary battery.

[0044] One of the effects of the present invention is that the stability of the positive electrode of the secondary battery can be improved.

[0045] One of the effects of the present invention is that the cost of the preparation process of the positive electrode active material of the secondary battery can be reduced.

[0046] However, various advantageous advantages and effects of the present invention are not limited to the above contents and can be more easily understood through the process of describing the embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram for explaining a case where the core portion and the shell portion are formed to have a lithium concentration gradient in the method for producing a positive electrode active material of the present invention.

[0048] Figure 2 The process of preparing the positive electrode active material in the embodiment of the present invention is schematically shown.

[0049] Figure 3 The XRD analysis results of the positive electrode active materials prepared in the examples are shown.

[0050] Figure 4 The change in lattice constant of the positive electrode active material prepared in the example after sintering is shown.

[0051] Figure 5 The change in the lattice volume of the positive electrode active material prepared in the example after sintering is shown.

[0052] Figure 6 The impurity generation phase fraction of the positive electrode active material prepared in the example after sintering is shown.

[0053] Figure 7 The neutron diffraction analysis results of Examples are shown.

[0054] Figure 8 The results of X-ray absorption near edge structure (XANES) analysis of the examples are shown.

[0055] Fig. 9 The results of soft X-ray absorption spectroscopy (XAS) analysis of Examples are shown.

[0056] Fig.10 The experimental results of the 700°C target and Li=1.029 sample are shown.

[0057] Fig.11 The TXM-XANES analysis results of the above samples are shown.

[0058] Fig.12 The TEM-EELS analysis results of the above samples are shown.

[0059] Fig.13 (a) and Fig.13 (b) shows the HR-TEM analysis results of samples synthesized at 700° C., target Li=1.029 composition conditions, and at 500 cc / min and 100 cc / min conditions, respectively.

[0060] Fig.14 The electrochemical cycle characteristics of the positive electrode active material synthesized at 700° C. and a target Li=1.029 composition condition as the atmospheric gas flow rate is reduced from 500 cc / min to 100 cc / min are shown.

[0061] Fig.15 A comparison of electrochemical multiple charge / discharge rate performance at 500 cc / min and 100 cc / min under target Li=1.029 composition conditions at 700°C is shown.

[0062] Fig.16 The electrochemical cycle characteristics of the positive electrode active material synthesized at 500 cc / min and 100 cc / min under the target composition conditions of Li=1.029 at 700° C. with impurities removed by a water washing process are shown.

[0063] Fig.17 (a) and Fig.17 (b) shows the real-time charge / discharge XRD analysis results of the sample synthesized at 700° C., target Li=1.029 composition conditions at 100 cc / min and the sample synthesized at 750° C., target Li=1.029 composition conditions at 100 cc / min, respectively.

[0064] Fig.18 XANES and soft XAS analysis results of target Li=1.029 samples at 800°C to 750°C according to atmospheric gas flow rates are shown.

[0065] Fig.19 The electrochemical cycling characteristics of the samples at 800°C to 750°C and with Li=1.0 to 1.029 targets according to the atmospheric gas flow rate are shown.

[0066] Fig. 20 The primary particle size distribution of the positive electrode active material synthesized at 700° C. and a target composition of Li=1.029 at 500 cc / min and 100 cc / min is shown. DETAILED DESCRIPTION

[0067] The following describes the embodiments of the present invention in combination with specific embodiments and drawings. However, this does not limit the technology described in this specification to a specific embodiment, and it should be understood that it includes various modifications, equivalents and / or alternatives of the embodiments of this specification. In terms of the description of the drawings, similar reference numerals can be used for similar components.

[0068] In this specification, expressions such as “having”, “can have”, “including” or “can include” refer to the existence of corresponding features (for example, constituent elements such as numbers, functions, actions or parts), and do not exclude the existence of additional features.

[0069] In this specification, expressions such as "A or B", "at least one of A or / and B", or "one or more of A or / and B" can include all possible combinations of the items listed together. For example, "A or B", "at least one of A and B", or "at least one of A or B" can all refer to the following situations: (1) at least one A is included; (2) at least one B is included; or (3) at least one A and at least one B are included.

[0070] The present invention relates to a method for preparing a positive electrode active material, which may include: a first step of preparing a mixture of a lithium precursor compound and a metal hydroxide; and a second step of preparing a sintered body by sintering the mixture.

[0071] In this case, the shell portion of the sintered body includes a compound represented by the following Chemical Formula 1, and the lithium concentration of the shell portion of the sintered body may be higher than that of the core portion.

[0072] [Chemical formula 1]

[0073] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O 2

[0074] In the above chemical formula 1, x1, a1, b1 and c1 are real numbers satisfying -0.05≤x1≤0.2, 0.6≤a1≤1.0, 0≤b1≤0.3, and 0≤c1≤0.4, respectively.

[0075] The nickel-based layered positive active material used as the positive active material of the secondary battery has high energy density and relatively long life characteristics, and therefore has the advantage of being able to increase the reversible capacity of the lithium secondary battery. However, there is a problem that the high-concentration nickel-rich layered positive active material is structurally unstable and difficult to synthesize. In addition, when preparing the nickel-rich layered positive active material, a high oxygen partial pressure is required, so the manufacturing cost increases, and too high a synthesis temperature can cause surface Li / O loss and form a degraded structure such as NiO. In addition, if the lithium content is insufficient, a lithium-poor structure may be formed on the surface, and if the lithium content is too much, residual lithium compounds may be formed on the surface, which may cause side reactions and performance degradation.

[0076] On the contrary, according to the preparation method of the positive electrode active material of the present invention, a sintered body including the compound represented by the above Chemical Formula 1 as the shell portion can be prepared, and a nickel-rich positive electrode active material with improved stability can be prepared by having a lithium concentration in the shell portion of the above sintered body being higher than the lithium concentration in the core portion.

[0077] In one example, when the preparation method of the positive electrode active material according to the present invention includes a compound represented by the above chemical formula 1 as a shell portion, the above shell portion may include a lithium layer (Li layers) and a transition metal layer (transition metal layers). At this time, the lithium layer of the above shell portion may include a first lithium layer containing only lithium and a second lithium layer in which at least a portion of the lithium in the above first lithium layer is replaced by a transition metal. The transition metal of the second lithium layer of the above shell portion may be regularly replaced and arranged in the above second lithium layer. In addition, the above transition metal layer of the shell portion may include a first transition metal layer containing only a transition metal and a second transition metal layer in which at least a portion of the transition metal has excess lithium (x1). The excess lithium (x1) in the second transition metal layer of the above shell portion may refer to a component corresponding to x1 in the above chemical formula 1, and may be arranged to replace the transition metal in the above second transition metal layer.

[0078] As in this example, when the shell portion of the positive electrode active material according to the present invention has a lithium layer and a transition metal layer, the shell portion may have a structure containing excess lithium on the transition metal layer, and the transition metal may include at least one of nickel, cobalt and manganese, and the concentration (a1) of nickel in the transition metal may be 60 mol% or more. As described above, according to the preparation method of the positive electrode active material of the present invention, the shell portion can contain excess lithium, so that it can have a lithium-over structure in which the lithium concentration of the shell portion is higher than the lithium concentration of the core portion. In addition, the nickel concentration (a1) of the transition metal layer of the shell portion may be 60 mol% or more, so a high-concentration nickel-rich layered positive electrode active material can be achieved.

[0079] In one example of the present invention, as required, the compound represented by the above chemical formula 1 may further include at least one compound selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb) and ruthenium (Ru). When the compound represented by the above chemical formula 1 includes the above components, the compound represented by the chemical formula 1 may be represented by the following chemical formula 1-1.

[0080] [Chemical formula 1-1]

[0081] Li 1+x1 (Ni a1 Co b1 Mn c1 Y d1 ) 1-x1 O 2

[0082] In the above chemical formula 1-1, Y may refer to at least one selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb) and ruthenium (Ru), and d1 may satisfy 0≤d1≤0.1.

[0083] The method for preparing a positive electrode active material according to the present invention may include a first step of preparing a mixture of a lithium precursor compound and a metal hydroxide. In this case, the first step may be a step of mixing the metal hydroxide and the lithium precursor compound in a molar ratio of 1:0.9 to 1:1.5.

[0084] The metal hydroxide may be a transition metal hydroxide containing nickel, and the transition metal may be, for example, one selected from the group consisting of Co, Mn, Al, Mg and V, but is not limited thereto. In addition, the lithium precursor compound may be LiOH, LiOH·H 2 O or Li 2 CO 3 , but not limited thereto, and various lithium-containing compounds capable of supplying lithium may be used.

[0085] In one example, the lithium precursor compound of the method for preparing the positive electrode active material according to the present invention may further include a compound selected from the group consisting of LiNO 3 , Li 2 SO 4 , Li 2 CO 3 At least one molten salt selected from the group consisting of LiCl, LiI and LiBr. The molten salt can reduce the melting point of the lithium precursor compound, thereby inducing rapid diffusion of lithium. In addition, when the lithium precursor compound includes the molten salt, a positive electrode active material with fewer nano defects and higher density can be prepared.

[0086] At this time, the lithium precursor compound and the above-mentioned molten salt in the preparation method of the positive electrode active material according to this example can be mixed in a molar ratio of 1:0.5 to 1:2, more preferably, in a molar ratio of 1:0.9 to 1:1.5. If it is outside the above range, it may exceed the eutectic temperature of the molten salt, so that sufficient lithium diffusion cannot be achieved.

[0087] In one embodiment of the present invention, the second step of the method for preparing a positive electrode active material according to the present invention can be performed at a temperature of 600°C or more and / or 900°C or less. For example, the sintering temperature may be 600°C or more, 610°C or more, 620°C or more, 630°C or more, 640°C or more, or 650°C or more, and may be 900°C or less, 860°C or less, 820°C or less, 780°C or less, or 750°C or less, but is not limited thereto. When the second step of the method for preparing a positive electrode active material according to the present embodiment satisfies the above temperature range, a lithium-rich nickel-rich positive electrode active material having a high lithium concentration in the positive electrode active material can be prepared, thereby improving the stability of the prepared positive electrode active material, and in particular, when the second step is performed at a temperature of 650°C or more and 780°C or less, the stability of the prepared positive electrode active material is significantly high and its initial capacity is very high, and therefore can be excellent.

[0088] In one example, the second step of the method for preparing a positive electrode active material according to the present invention can be carried out under an air atmosphere. In the case of existing nickel-rich positive electrode active materials, in order to increase the oxidation number of nickel, the active material is sintered under a high concentration oxygen atmosphere. However, there is a problem of increased manufacturing cost due to the use of a large amount of high concentration oxygen, and excessively high synthesis temperatures lead to Li / O loss on the surface, resulting in the formation of degraded structures such as NiO. On the contrary, the method for preparing a positive electrode active material according to this example can reduce manufacturing costs by sintering at a low synthesis temperature and an air atmosphere, and can provide a positive electrode active material having both excellent stability and improved electrochemical properties as described below.

[0089] In another example, the second step of the method for preparing a positive electrode active material according to the present invention can be performed for more than 8 hours and / or less than 24 hours at an air inflow of less than 600 sccm. For example, the inflow of the above-mentioned air can be less than 600 sccm, less than 550 sccm, less than 500 sccm, less than 450 sccm, less than 400 sccm, less than 350 sccm, less than 300 sccm, less than 250 sccm or less than 200 sccm, but is not limited thereto. There is no particular limitation on the lower limit of the above-mentioned air inflow, for example, it can be more than 0 sccm or greater than 0 sccm. At this time, when the above-mentioned air inflow is more than 0 sccm and less than 300 sccm, the stability of the prepared positive electrode active material is significantly high and its initial capacity is very high, so it can be excellent.

[0090] The above-mentioned air may refer to air containing 79% nitrogen (N 2 ) and 21% oxygen (O 2 ) may refer to general air, or may refer to a gas mixed with oxygen and nitrogen in a ratio of 10-99%:90-1%.

[0091] At this time, the oxygen partial pressure of the air may be 2.4 kPa or less. Figure 1 This is a schematic diagram showing a case where the core and shell parts are formed to have a lithium concentration gradient in the method for preparing a positive electrode active material according to the present invention. Figure 1 When the oxygen partial pressure is high, sufficient lithium diffuses into the core, so that the core and shell of the positive electrode active material cannot have a lithium concentration gradient. On the other hand, when the oxygen partial pressure is low at a low synthesis temperature, the reaction continues on the surface without sufficient lithium diffusion from the surface of the secondary particles to the core. Therefore, a relatively lithium-excessive Li+ layer can be formed on the surface. 1+x M 1-x O 2 structure, and as the relatively lithium-deficient Li 1-x M 1+x O 2 Structure (0≤x≤0.09). Thus, a positive electrode active material with a lithium concentration gradient can be prepared.

[0092] In one embodiment of the present invention, the core of the sintered body of the method for preparing a positive electrode active material of the present invention may include a compound represented by the following Chemical Formula 2.

[0093] [Chemical formula 2]

[0094] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O 2

[0095] In the above chemical formula 2, x2, a2, b2 and c2 are real numbers satisfying -0.2≤x2≤0.05, 0.6≤a2≤1.0, 0≤b2≤0.3, and 0≤c2≤0.4, respectively.

[0096] As described above, the sintered body of the method for preparing the positive electrode active material according to the present invention may have a shell portion of a lithium-permeable structure containing excess lithium. On the contrary, the lithium concentration of the core portion of the sintered body may be lower than that of the shell portion, and may contain the compound represented by the above chemical formula 2.

[0097] At this time, the sintered body of the method for preparing the positive active material according to the present invention may have a concentration gradient in which the lithium content continuously increases from the core to the surface of the shell. As described above, the sintered body of the method for preparing the positive active material according to the present invention may have a structure in which the lithium concentration of the core is lower than the lithium concentration of the shell. This structure is formed by the diffusion of external lithium into the interior of the sintered body during the sintering process. At this time, when the existing preparation method is used, an over-lithium structure with a high lithium concentration is formed in the core of the sintered body. On the other hand, the method for preparing the positive active material according to the present invention can reduce the amount of lithium diffused into the core of the sintered body, and thus can prepare a sintered body in which the lithium content of the shell is higher than the lithium content of the core and has a continuous concentration gradient.

[0098] In one example, when the preparation method of the positive electrode active material according to the present invention includes a compound represented by the above chemical formula 2 as a core, the core may include a lithium layer and a transition metal layer. At this time, the lithium layer of the core may include a first lithium layer containing only lithium and a second lithium layer in which at least a portion of the lithium in the first lithium layer is replaced by a transition metal. The transition metal of the second lithium layer of the core may be regularly replaced and arranged in the second lithium layer. In addition, the transition metal layer of the core may include a first transition metal layer containing only transition metal and a second transition metal layer in which at least a portion of the transition metal is replaced by lithium (x2). The lithium (x2) in the second transition metal layer of the core may refer to a component corresponding to x2 in the above chemical formula 2.

[0099] As in this example, when the core of the positive active material according to the present invention has a lithium layer and a transition metal layer, the core may have a structure in which the transition metal is additionally contained on the lithium layer due to the lack of lithium on the lithium layer, and the transition metal may include at least one of nickel, cobalt and manganese, and the concentration (a2) of nickel in the transition metal may be 60 mol% or more. As described above, the preparation method of the positive active material according to the present invention has a structure in which the lithium concentration of the shell of the sintered body is higher than the lithium concentration of the core, and at the same time, the nickel concentration (a1) in the transition metal layer of the core may be 60 mol% or more, so a high concentration of nickel-rich layered positive active material can be achieved. The transition metal content of the core and the shell does not change, and a uniform high nickel concentration (a1≥0.6) can be achieved in the core and the shell, so that a high capacity can be maintained, and the over-lithium layered structure of the shell makes the structure stable during the overall electrochemical (charge / discharge) reaction process to be able to maintain a high lifespan.

[0100] In one embodiment of the present invention, as required, the compound represented by the above chemical formula 2 may further include at least one compound selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb) and ruthenium (Ru). When the compound represented by the above chemical formula 1 includes the above components, the compound represented by the chemical formula 2 may be represented by the following chemical formula 2-1.

[0101] [Chemical formula 2-1]

[0102] Li 1+x2 (Ni a2 Co b2 Mn c2 Y d2 ) 1-x2 O 2

[0103] In the above chemical formula 2-1, Y may refer to at least one selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb) and ruthenium (Ru), and d2 may satisfy 0≤d2≤0.1.

[0104] In one example, in the sintered body of the method for preparing the positive electrode active material according to the present invention, the oxidation number of the nickel in the shell can be higher than the oxidation number of the nickel in the core. In general high-concentration nickel-based (Ni-rich) positive electrode active materials, there may be a problem of cation mixing. Mixed cations are divalent nickel ions (Ni 2+ ) is introduced into the lithium site, which reduces the capacity of the secondary battery. On the other hand, according to the method for preparing the positive electrode active material of the present invention, a sintered body can be formed in which the oxidation number of nickel in the shell is higher than that of nickel in the core, and has a lithium-excess structure containing excess lithium, thereby reducing the amount of divalent nickel ions (Ni 2+ ) to prevent cation mixing.

[0105] The present invention also relates to a positive electrode active material. The positive electrode active material according to the present invention comprises: a shell part represented by the following chemical formula 1; and a core part represented by the following chemical formula 2, wherein the lithium concentration of the shell part may be higher than that of the core part.

[0106] [Chemical formula 1]

[0107] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O 2

[0108] In the above chemical formula 1, x1, a1, b1 and c1 are real numbers satisfying -0.05≤x1≤0.2, 0.6≤a1≤1.0, 0≤b1≤0.3, and 0≤c1≤0.4, respectively.

[0109] [Chemical formula 2]

[0110] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O 2

[0111] In the above chemical formula 2, x2, a2, b2 and c2 are real numbers satisfying -0.2≤x2≤0.05, 0.6≤a2≤1.0, 0≤b2≤0.3, and 0≤c2≤0.4, respectively.

[0112] At this time, the sintered body of the cathode active material according to the present invention may have a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion.

[0113] On the other hand, the oxidation number of nickel in the shell portion of the sintered body of the cathode active material according to the present invention may be higher than the oxidation number of nickel in the core portion.

[0114] In one example, the d-spacing (d(003)-spacing) of the (003) planes in the lattice of the core of the sintered body of the positive electrode active material according to the present invention may be greater than the d(003) spacing of the shell. The above-mentioned d-spacing may refer to the interplanar spacing within the lattice and may be confirmed by XRD analysis. As shown in this example, when the d-spacing of the (003) planes in the lattice of the core of the positive electrode active material of the sintered body of the positive electrode active material is greater than the d-spacing of the lattice of the shell, a high-capacity secondary battery may be achieved, and the lithium-excess layered structure of the shell may stabilize the structure and maintain a high lifespan throughout the electrochemical (charge / discharge) reaction process.

[0115] In addition, the shell and core of the sintered body of the positive electrode active material according to the present invention may each contain primary particles, and the size of the primary particles of the shell may be larger than the size of the primary particles of the core. The size of the primary particles may refer to the average particle size of the primary particles. As described above, when the size of the primary particles of the shell is larger than the size of the primary particles of the core, the mechanical strength of the sintered body can be improved, and the contact area between the shell and the electrolyte can be reduced to suppress the side reactions during the charge / discharge process, thereby improving the service life.

[0116] The contents regarding the sintered body of the above-mentioned positive electrode active material and the like are the same as those described above and thus will be omitted.

[0117] Hereinafter, preferred embodiments are proposed to help understand the present invention. However, the following embodiments are provided to make it easier to understand the present invention, and the content of the present invention is not limited to the following embodiments.

[0118] Example 1: Preparation of positive electrode active material

[0119] The positive electrode active material of the present invention is Figure 2 In order to prepare the positive electrode active material according to the present invention, Ni 0.92 Co 0.03 Mn 0.05 (OH) 2 +LiOH·H 2 O precursor mixed sample. At this time, the precursor mixture was sintered for 12 hours under the conditions of transition metal: lithium compound = 1: 0.97 ~ 1.30 molar ratio, gas atmosphere of 100mL / min air (air), and final synthesis temperature of 700 ° C to 800 ° C, and finally synthesized positive electrode active material Li x Me 2-x O 2 (x=0.97,1.0,1.029,1.074,1.13).

[0120] Examples 2 and 3: Preparation of positive electrode active materials

[0121] A positive electrode active material was prepared in the same manner as in Example 1 except that the gas atmosphere during sintering was changed to 0 mL / min and 250 mL / min of air.

[0122] Examples 4 and 5: Preparation of positive electrode active materials

[0123] The positive electrode active material was prepared by adding the gas atmosphere at 350 mL / min and 500 mL / min of air during sintering.

[0124] Table 1

[0125]

[0126]

[0127] Table 1 above summarizes the ratios of the precursor mixture samples prepared in the examples, the gas atmosphere and the synthesis temperature conditions.

[0128] Experimental Example 1: X-ray Diffraction (XRD) Pattern

[0129] Figure 3 The XRD analysis results of the positive electrode active materials prepared in the examples are shown. Figure 3As shown, after the precursor mixture was synthesized under each condition, the conditions for forming a layered structure and impurities under each synthesis condition were confirmed by X-ray diffraction (XRD) patterns. As the atmospheric gas inflow rate was reduced from 500cc / min to 0cc / min and as the synthesis temperature was increased, it was qualitatively observed that the lithium compound increased and the impurity content also increased.

[0130] Experimental Example 2: X-ray diffraction (XRD) pattern Rietveld structure analysis

[0131] Figure 4 shows the change in the lattice constant of the positive electrode active material prepared in the example after sintering, Figure 5 shows the change in the lattice volume of the positive electrode active material prepared in the example after sintering, Figure 6 The impurity generation phase fraction of the positive electrode active material prepared in the embodiment after sintering is shown. The lattice constant change, lattice volume change and impurity generation phase fraction are confirmed by Rietveld analysis of the X-ray diffraction (XRD) pattern. As the synthesis temperature increases and the atmospheric gas inflow rate slows down, the overall lattice constant increases. And, regardless of the conditions, as the lithium content increases, the lattice constant shows a decreasing trend.

[0132] This is because, under all synthesis conditions, as the amount of lithium compounds increases, the amount of lithium contained in the layered crystal structure increases, which can be indirectly confirmed to lead to an increase in the oxidation number of the transition metal. In addition, as the atmospheric gas inflow rate decreases and the synthesis temperature increases, the residual lithium compounds (LiOH, Li 2 O. Li 2 CO 3 This indicates that lithium is difficult to be included in the structure under insufficient oxygen and high synthesis temperature.

[0133] Experimental Example 3: Rietveld Analysis of Neutron Diffraction Patterns

[0134] In order to quantify the lithium content according to the respective synthesis conditions, neutron diffraction analysis was performed. Figure 7 The neutron diffraction analysis results of the examples are shown. Figure 7 It is known that when the synthesis temperature is reduced to 700°C under the condition of 500cc / min atmospheric gas, an excess lithium of up to 8% can be contained. However, it is confirmed that as the atmospheric gas inflow rate decreases, the transition metal precursor (Me(OH) 2 ) becomes insufficient in oxygen required for reacting with lithium compounds, making it difficult for the structure to contain excess lithium, thereby reducing the excess lithium by up to 5%.

[0135] In addition, it was confirmed that if the synthesis temperature is too high to 800°C under the condition of 500cc / min atmospheric gas, the excess lithium content can only reach 3% at most, and if the inflow of atmospheric gas is reduced to 0cc / min, the excess lithium content is less than 1% at most. In addition, it was confirmed that under the synthesis conditions of adding excess lithium, by reducing Ni 2+ content to reduce the cationic mixing ratio.

[0136] Experimental Example 4: XANES Analysis

[0137] Figure 8 The results of X-ray absorption near edge structure (XANES) analysis of the examples are shown. Through X-ray XANES analysis, it was confirmed that the oxidation number of nickel in the bulk structure of the secondary particles as a whole changed due to the excessive lithium content in Experimental Example 3. Figure 8 , as the atmospheric gas flow rate increases from low to high flow rate, and as the synthesis temperature decreases, the oxidation number of nickel increases more significantly due to the increase in the amount of lithium, which means that the closer the content of excess lithium in the structure of Experimental Example 3 is to the above conditions, the more it increases.

[0138] Experimental Example 5: XAS Analysis

[0139] Fig. 9 The results of soft X-ray absorption spectroscopy (XAS) analysis of the examples are shown. Through soft X-ray XAS analysis, it was confirmed that the oxidation number of nickel in the average surface structure of the secondary particles as a whole changed due to the excessive lithium content in Experimental Example 3. Fig. 9 ,Similar to Experimental Example 4, as the atmospheric gas flow rate increases from a low flow rate to a high flow rate, and as the synthesis temperature decreases, the oxidation number of nickel increases more significantly due to the increase in the amount of lithium. This means that the closer the content of excess lithium in the structure of Experimental Example 3 is to the above conditions, the more it increases.

[0140] Experimental Example 6: Comparative Analysis of Secondary Particle Surface and Bulk Oxidation Numbers of 700°C Target Li=1.029 Samples Based on Atmospheric Gas Flow Rate

[0141] Fig.10 The experimental results of the sample at 700°C and target Li=1.029 are shown. Fig.10 Part (a) shows the nickel oxidation number of the overall bulk structure of the secondary particles analyzed by XANES analysis, Fig.10 Part (b) shows the change in the nickel oxidation number of the secondary particle surface structure analyzed by soft XAS analysis. Fig.10 In part (a), it can be confirmed that the nickel oxidation number of the bulk structure decreases with the increase of the atmospheric gas flow rate. Fig.10In part (b), when the atmospheric gas flow rate other than 0 cc / min was reduced from 500 cc / min to 100 cc / min, the nickel oxidation number of the surface structure gradually increased. In particular, it was confirmed that the nickel oxidation number of the surface structure was the largest at 100 cc / min.

[0142] Fig.11 The TXM-XANES analysis results of the above samples are shown. Fig.12 The TEM-EELS analysis results of the above samples are shown. Fig.11 and Fig.12 It can be confirmed that in the case of the sample with a temperature of 700°C and a target Li=1.029, the nickel in the surface structure is further oxidized at a rate of 100cc / min, forming a more reduced structure as it approaches the core. This is the opposite result to the case where the entire bulk structure has the same nickel oxidation number when the atmospheric gas flow rate is very high (500cc / min). The difference in Ni oxidation number between the surface and core structures means that the distribution of excess lithium in the transition metal layer is different. Therefore, the sample synthesized at 700°C, target Li=1.029, and 100cc / min shows the formation of secondary particles with a lithium concentration gradient in which the surface structure is more inclined to lithium excess and the core is closer to the stoichiometric composition.

[0143] Experimental Example 7: Comparative Analysis of the (003) Plane D-Spacing from the Core to the Surface of Secondary Particles in 700°C, Target Li=1.029 Samples According to the Atmospheric Gas Flow Rate

[0144] Fig.13 (a) and Fig.13 (b) shows the high resolution (HR)-TEM analysis results of the target Li=1.029 sample at 700°C and 500cc / min and 100cc / min respectively. Fig.13 (a) and Fig.13 (b) of the present invention confirms that the sample synthesized at an atmospheric gas flow rate of 500 cc / min has a The d-spacing of the (003) planes (d(003) spacing) of the secondary particles is constant from the core to the surface, whereas the d(003) spacing of the sample synthesized at an atmospheric gas flow rate of 100 cc / min is about And on the surface is about There is a difference in d spacing. This is consistent with Fig.10 , Fig.11 and Fig.12 The difference in oxidation number shown is consistent with the fact that the oxidation number of nickel gradually increases from the core to the surface at 700°C, target Li=1.029, and an atmospheric gas flow rate of 100 cc / min.

[0145] Experimental Example 8: Comparison of electrochemical performance at 700°C, target Li=1.029 composition, when atmospheric gas flow rate is reduced from 500cc / min to 100cc / min

[0146] Fig.14 Table 2 shows the electrochemical cycle characteristics of the synthesized positive electrode active material under the conditions of 700°C and target Li=1.029 composition as the atmospheric gas flow rate is reduced from 500cc / min to 100cc / min. Fig.14 As shown in Table 2, the sample synthesized under the conditions of 700°C, target Li=1.029 composition, and 100 cc / min to form lithium concentration gradient secondary particles has a maximum initial capacity of 227 mAh / g and exhibits excellent cycle characteristics.

[0147] Experimental Example 9: Comparison of electrochemical charge / discharge rate performance at 700°C, target Li=1.029 composition, when atmospheric gas flow rate is reduced from 100cc / min to 500cc / min

[0148] Fig.15 A comparison of electrochemical multiple charge / discharge rate performance under conditions of 500cc / min and 100cc / min at 700°C and target Li=1.029 composition is shown. After 5 cycles of charge / discharge at 0.1C rate to 0.2C, 0.5C, 1C, and 2C rates, when charged / discharged again at 0.1C rate, the sample synthesized at 500cc / min showed a capacity ratio of about 76%, while the sample synthesized at 100cc / min had a capacity ratio of about 85%, that is, it showed excellent discharge capacity and speed performance. The excellent speed performance of 100cc / min means that the relatively wide d(003) spacing of the core improves the mobility of lithium.

[0149] Table 2

[0150] 0.2C rate 500cc / min 350cc / min 250cc / min 100cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 162.5 182.86 201.52 226.73 <![CDATA[Capacity at the 100th cycle (mAh g -1 )]]> 152.6 185.02 180.89 206.74 Capacity retention rate (%) 93.90 101.18 89.76 91.18

[0151] Experimental Example 10: Comparison of electrochemical performance of water-washed 700°C, target Li=1.029 composition when atmospheric gas flow rate is reduced from 500cc / min to 100cc / min

[0152] Fig.16 Table 3 shows the electrochemical cycle characteristics of the positive electrode active material synthesized under the conditions of 700°C, target Li=1.029 composition, and atmospheric gas flow rate reduced from 500cc / min to 100cc / min under the conditions of 700°C, target Li=1.029 composition, and 100cc / min, which remove impurities by a water washing process. Similarly, among the samples with impurities removed, the sample synthesized under the conditions of 700°C, target Li=1.029 composition, and 100cc / min to form lithium concentration gradient secondary particles showed the most stable cycle characteristics of 204mAh / g.

[0153] Table 3

[0154] 0.2C ratio (water washed sample) 500cc / min 350cc / min 250cc / min 100cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 173.74 180.8 198.65 204.37 <![CDATA[Capacity at the 200th cycle (mAh g -1 )]]> 150.20 158.82 151.09 178.28 Capacity retention rate (%) 86.45 87.84 76.06 87.23

[0155] Experimental Example 11: Real-time charge / discharge XRD analysis comparison

[0156] Fig.17 (a) and Fig.17 (b) shows the real-time charge / discharge XRD analysis results of the sample synthesized at 700°C, target Li=1.029 composition, 100cc / min and the sample synthesized at 750°C, target Li=1.029 composition, 100cc / min. Fig.17 Although the samples synthesized at 700°C, target Li=1.029 composition, and 100cc / min conditions are Fig.13 and Fig.14 The maximum capacity is shown, and the change in the lattice constant c-axis is 0.72% less than that of the sample synthesized at 750°C, target Li=1.029 composition, and 100cc / min, which has a similar capacity. This proves that the material with lithium concentration gradient secondary particles has excellent electrochemical performance.

[0157] Experimental Example 12: Comparison of changes in secondary particle bulk and surface oxidation number according to atmospheric gas flow rate for samples at 800℃ to 750℃ and target Li=1.029

[0158] Fig.18 The XANES and soft XAS analysis results of the target Li=1.029 sample at 800°C to 750°C according to the atmospheric gas flow rate are shown. Fig.18 (a) and Fig.18 (b) shows the bulk oxidation number and surface oxidation number of the sample at 800°C and target Li=1.029, respectively. Fig.18 (c) and Fig.18 (d) shows the bulk oxidation number and surface oxidation number of the sample at 750°C and target Li=1.029, respectively. Fig.18 It was confirmed that in the 700℃, target Li=1.029 sample, the nickel oxidation numbers of the surface and the bulk showed opposite trends with the decrease of the atmospheric gas flow rate, while when the synthesis temperature was increased to 750℃ to 800℃, the changes in the nickel oxidation numbers of the surface and the bulk caused by the decrease in the atmospheric gas flow rate gradually showed similar trends, and finally at the synthesis temperature of 800℃, the nickel oxidation numbers of the surface and the bulk decreased in the same trend with the decrease of the atmospheric gas flow rate.

[0159] Experimental Example 13: Comparison of electrochemical performance of samples at 800°C and 750°C, target Li=1.0 and 1.029 according to atmospheric gas flow rate

[0160] Fig.19 Tables 4 to 8 show the electrochemical cycle characteristics of the samples at 800°C to 750°C and with target Li=1.0 to 1.029 according to the atmospheric gas flow rate. Fig.19 As shown in Tables 4 to 8, at a temperature of 750°C, as at 700°C, as the gas flow rate decreases from 500cc / min to 100cc / min, the initial capacity increases, but the cycle characteristics decrease. At a high temperature of 800°C, as the gas flow rate decreases, both the initial capacity and the cycle characteristics decrease. This proves that the most ideal lithium concentration gradient secondary particles are formed under the low gas flow rate condition of 700°C.

[0161] Table 4

[0162] 800℃, target Li=1.0, 0.2C rate 500cc / min 100cc / min 0cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 216.19 211.83 185.22 <![CDATA[Capacity at the 100th cycle (mAh g -1 )]]> 168.72 137.58 122.56 Capacity retention rate (%) 78.04 64.95 66.17

[0163] Table 5

[0164] 800℃, target Li=1.03, 0.2C rate 500cc / min 100cc / min 0cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 226.00 218.09 217.52 <![CDATA[Capacity at the 100th cycle (mAh g -1 )]]> 181.49 157.40 143.27 Capacity retention rate (%) 80.30 72.17 65.87

[0165] Table 6

[0166] 750℃, target Li=1.0, 0.2C rate 500cc / min 100cc / min 0cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 212.86 225.57 207.54 <![CDATA[Capacity at the 100th cycle (mAh g -1 )]]> 190.18 176.69 161.77 Capacity retention rate (%) 89.34 78.33 77.95

[0167] Table 7

[0168] 750℃, target Li=1.03, 0.2C rate 500cc / min 100cc / min 0cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 185.81 215.90 209.72 <![CDATA[Capacity at the 100th cycle (mAh g -1 )]]> 173.67 164.78 168.26 Capacity retention rate (%) 93.47 76.32 80.23

[0169] Table 8

[0170] 700℃, target Li=1.0, 0.2C rate 500cc / min 100cc / min 0cc / min <![CDATA[Capacity of the first cycle (mAh g -1 )]]> 177.77 205.34 205.75 <![CDATA[Capacity at the 100th cycle (mAh g -1 )]]> 151.40 182.87 163.92 Capacity retention rate (%) 85.17 89.06 79.66

[0171] Experimental Example 14: Comparison of particle sizes of positive electrode active materials synthesized at 700°C, target Li=1.029 composition, 500cc / min and 100cc / min

[0172] Fig. 20 The particle size distribution of the positive active material synthesized under the conditions of 700°C, target Li=1.029 composition, 500cc / min and 100cc / min is shown. In the case of the positive active material synthesized at a gas flow rate of 500cc / min, it can be confirmed that the primary particle size from the shell to the core is uniformly maintained at 200nm to 300nm, but in the case of the positive active material synthesized at 100cc / min, the shell particle size is 500nm to 1000nm, which is larger than the core particle size. This shows that the crystallization of the shell structure is more significant due to excess lithium compared to the core. The increase in the primary particle size of the shell within the secondary particle means that the interface exposure of the positive active material to the electrolyte can be reduced, and can help improve the life by reducing the interfacial side reactions caused by electrochemical cycles.

[0173] Although the embodiments of the present invention have been described in detail above, the present invention is limited by the attached claims, and is not limited to the above-mentioned embodiments and drawings. Therefore, those skilled in the art can perform various substitutions, deformations and changes without departing from the technical concept of the present invention recorded in the claims, and this belongs to the scope of the present invention.

Claims

1. A method for preparing a positive electrode active material, characterized in that: include: The first step is to prepare a mixture of a lithium precursor compound and a metal hydroxide; and In the second step, a sintered body is prepared by sintering the above mixture. The shell portion of the sintered body includes a compound represented by the following Chemical Formula 1: The lithium concentration in the shell of the sintered body is higher than that in the core: [Chemical formula 1] Li 1+x1 (Ni a1 Co b1 Mr c1 ) 1-x1 O2 In the above chemical formula 1, x1, a1, b1 and c1 are real numbers satisfying -0.05≤x1≤0.2, 0.6≤a1≤1.0, 0≤b1≤0.3, and 0≤c1≤0.4, respectively.

2. The method for preparing a positive electrode active material according to claim 1, characterized in that: The first step is to mix the metal hydroxide and the lithium precursor compound in a molar ratio of 1:0.9 to 1:1.

5. The lithium precursor compound further includes at least one molten salt selected from the group consisting of LiNO3, Li2SO4, Li2CO3, LiCl, LiI and LiBr.

3. The method for preparing a positive electrode active material according to claim 1, characterized in that: The second step is performed at a temperature of 600° C. or higher and 900° C. or lower.

4. The method for preparing a positive electrode active material according to claim 1, characterized in that: The second step is performed at a temperature of 650°C or higher and 780°C or lower.

5. The method for preparing a positive electrode active material according to claim 1, characterized in that: The above second step is performed under air atmosphere.

6. The method for preparing the positive electrode active material according to claim 5, characterized in that: The second step is performed for a period of time of 8 hours to 24 hours at an air inflow rate of 0 sccm to 600 sccm.

7. The method for preparing a positive electrode active material according to claim 5, characterized in that: The second step is performed for a period of time of 8 hours to 24 hours at an air inflow rate of 0 sccm to 300 sccm.

8. The method for preparing a positive electrode active material according to claim 1, characterized in that: The core of the sintered body includes a compound represented by the following Chemical Formula 2: [Chemical formula 2] Li 1-x2 (Ni a2 Co b2 Mr c2 ) 1+x2 O2 In the above chemical formula 2, x2, a2, b2 and c2 are real numbers satisfying -0.2≤x2≤0.05, 0.6≤a2≤1.0, 0≤b2≤0.3, and 0≤c2≤0.4, respectively.

9. The method for preparing a positive electrode active material according to claim 1, characterized in that: The sintered body has a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion.

10. The method for preparing a positive electrode active material according to claim 1, characterized in that: The oxidation number of nickel in the shell portion of the sintered body is higher than the oxidation number of nickel in the core portion.

11. A positive electrode active material, characterized in that: include: A shell portion represented by the following chemical formula 1; and The core is represented by the following chemical formula 2: The lithium concentration in the shell is higher than that in the core as described above: [Chemical formula 1] Li 1+x1 (Ni a1 Co b1 Mr c1 ) 1-x1 O2 In the above chemical formula 1, x1, a1, b1 and c1 are real numbers satisfying -0.05≤x1≤0.2, 0.6≤a1≤1.0, 0≤b1≤0.3, 0≤c1≤0.4, respectively. [Chemical formula 2] Li 1-x2 (Ni a2 Co b2 Mr c2 ) 1+x2 O2 In the above chemical formula 2, x2, a2, b2 and c2 are real numbers satisfying -0.2≤x2≤0.05, 0.6≤a2≤1.0, 0≤b2≤0.3, and 0≤c2≤0.4, respectively.

12. The positive electrode active material according to claim 11, characterized in that There is a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion.

13. The positive electrode active material according to claim 11, characterized in that The oxidation number of nickel in the shell portion is higher than the oxidation number of nickel in the core portion.

14. The positive electrode active material according to claim 11, characterized in that The d spacing of the (003) plane in the crystal lattice of the core portion, that is, the d(003) spacing is larger than the d(003) spacing of the shell portion.

15. The positive electrode active material according to claim 11, characterized in that The primary particle size of the shell portion is larger than the primary particle size of the core portion.

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