Manufacturing method of cathode active material and cathode active material manufactured therefrom

Abstract

To provide a cathode active material for lithium-concentration gradient lithium secondary batteries, a manufacturing method thereof, and a lithium secondary battery comprising the same.SOLUTION: A method for manufacturing a cathode active material comprises: a first step of preparing a mixture of a lithium precursor compound and a metal hydroxide; and a second step of sintering the mixture to prepare a sintered body, where a shell part of the sintered body comprises a compound represented by Chemical Formula 1 below, and a concentration of lithium in the shell part is higher than a concentration of lithium in a core part of the sintered body: Li1+x1(Nia1Cob1Mnc1)1-x1O2 (Chemical Formula 1), where x1, al, 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.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a positive electrode active material and a positive electrode active material manufactured therefrom.

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, voltage, long cycle life, and low self-discharge rate. The core materials of such lithium secondary batteries are the positive electrode material, negative electrode material, electrolyte, and separator. Recently, with the expanding demand for electric vehicles, the importance of the positive electrode active material of the lithium secondary battery for driving them has been increasing.

[0003] The positive electrode active material can be classified into LCO, NCM, NCA, LMO, LFP, etc. depending on the constituent materials. As a positive electrode active material for secondary batteries applied to medium and large-sized electronic devices, LiNi having a structure such as LCO x Co y Mn z O2 and LiNi x Co y Al z O2-based Ni-based layered positive electrode active materials are mainly used. The Ni-based layered positive electrode active material has the advantages of cost reduction by replacing a part of expensive cobalt with nickel, having relatively long characteristics of high energy density and long life, and being able to increase the reversible capacity of the lithium secondary battery.

[0004] However, despite the excellent electrical / chemical properties of such high-concentration Ni-based (Ni-rich) layered positive electrode active materials, compared with layered structure positive electrode active materials of other compositions, the nickel oxidation number has to be closer to trivalent, so it is structurally unstable, and there are difficulties in synthesis due to the tendency to maintain a stable divalent oxidation number. Therefore, the importance of the optimal synthesis temperature and lithium content has been emphasized together with a high oxygen partial pressure during the manufacture of Ni-rich layered positive electrode active materials.

[0005] During the production of Ni-rich layered cathode active materials, by manufacturing under a high oxidation atmosphere, the consumption of high-concentration oxygen (99.95 - 99.999%) is large (about 400 - 500 cc / min), the production cost can increase, and an excessively high synthesis temperature can cause Li / O loss on the surface and form a deteriorated structure such as NiO. Similarly, when the lithium content is insufficient, a Li-poor structure is formed on the surface, and when it is excessive, residual lithium compounds on the surface are formed, which can lead to side reactions and performance degradation.

[0006] From this, in previous studies, there were attempts to synthesize Ni-rich materials using air gas instead of high-concentration oxygen. However, as the oxygen partial pressure decreases, there are problems such as an increase in cation mixing due to Ni 2+ and electrochemical performance that cannot reach that of using high-concentration oxygen gas due to issues such as the formation of impurities.

[0007] Also, through the synthesis of a core-shell structure in which an NCM material (such as NCM111, 523, etc.) with a high Co and Mn content and a low nickel oxidation number is on the surface and a Ni-rich layered cathode active material with a high nickel oxidation number is in the core to suppress the deterioration of the surface structure due to the instability of Ni 3+ cations, or by forming a Li-excess structure through conditions of a high-purity oxygen atmosphere, a low synthesis temperature, and a high lithium content during the synthesis of Ni-rich layered cathode active materials, and doping inactive lithium into the transition metal layer to enhance the stability of Ni-rich layered cathode active materials, etc. have been reported. However, for the core-shell, due to the decrease in the oxidation number of nickel, and for Li-excess Ni-rich materials, since excessive lithium occupies the place of transition metals in the transition metal layer, there is a problem that a decrease in capacity is inevitably followed.

[0008] Therefore, while reducing the cost in the manufacturing process, the development of Ni-rich cathode active materials with excellent capacity characteristics and improved stability is still necessary.

Summary of the Invention

[0009] One of the various objects of the present invention is to provide a method for producing a positive electrode active material capable of improving the capacity characteristics of a secondary battery and a positive electrode active material produced therefrom. One of the various objects of the present invention is to provide a method for producing a positive electrode active material capable of improving the cycle performance of a secondary battery and a positive electrode active material produced therefrom. One of the various objects of the present invention is to provide a method for producing a positive electrode active material capable of enhancing the stability of a secondary battery positive electrode and a positive electrode active material produced therefrom. One of the various objects of the present invention is to provide a method for producing a positive electrode active material capable of reducing the cost in the manufacturing process and a positive electrode active material produced therefrom. [Means for Solving the Problems]

[0010] The present invention has been devised to solve the above problems, and a method for producing a positive electrode active material according to an embodiment of the present invention includes: a first step of producing a mixture of a lithium precursor compound and a metal hydroxide; and a second step of sintering the mixture to produce a sintered body. The shell portion of the sintered body contains a compound represented by the following Chemical Formula 1, and the concentration of lithium in the shell portion is higher than the concentration of lithium in the core portion of the sintered body, thereby providing a method for producing a positive electrode active material. [Chemical Formula 1] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O2 In 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.

[0011] At this time, the first step may be a step of mixing the metal hydroxide and the lithium precursor compound at a molar ratio of 1:0.9 to 1.5.

[0012] In addition, the lithium precursor compound may further contain one or more molten salts selected from the group consisting of LiNO3, Li2SO4, Li2CO3, LiCl, LiI, and LiBr.

[0013] Note that the second stage can be carried out at a temperature of 600 °C or higher and / or 900 °C or lower.

[0014] At this time, the second stage can also be carried out in an air atmosphere.

[0015] In addition, the second stage can be carried out for 8 hours or more and / or 24 hours or less with an air inflow rate of 0 sccm or more and / or 600 sccm or less.

[0016] In one embodiment of the present invention, the core part of the sintered body of the method for producing a positive electrode active material according to the present invention may contain a compound represented by the following Chemical Formula 2. [Chemical Formula 2] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O2 In 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.

[0017] In one example, it can have a concentration gradient in which the lithium content continuously increases from the core part to the surface of the shell part of the sintered body.

[0018] In another example, the oxidation number of nickel in the shell part of the sintered body may be higher than the oxidation number of nickel in the core part.

[0019] In another embodiment of the present invention, the present invention provides a positive electrode active material including 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 in the shell part is higher than the lithium concentration in the core part. [Chemical formula 1] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O2 In the 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. [Chemical formula 2] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O2 In the 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.

[0020] At this time, the positive electrode active material according to the present invention can have a concentration gradient in which the lithium content continuously increases from the core part to the surface of the shell part.

[0021] Further, the oxidation number of nickel (Ni) in the shell part may be higher than the oxidation number of nickel in the core part.

[0022] In one example, the d-spacing (d(003)-spacing) of the (003) plane in the lattice of the core part of the positive electrode active material according to the present invention may be larger than the d(003)-spacing of the shell part.

[0023] Also, the primary particle size of the shell part may be larger than the primary particle size of the core part.

Advantages of the Invention

[0024] One of the various effects of the present invention is that it can improve the capacity characteristics of a secondary battery. One of the effects of the present invention is that it can improve the cycle performance of a secondary battery. One of the various effects of the present invention is that it can enhance the stability of the positive electrode of a secondary battery. One of the various effects of the present invention is that it can reduce the manufacturing cost of the positive electrode active material of a secondary battery. However, the various beneficial advantages and effects of the present invention are not limited to the foregoing, and can be more easily understood in the process of describing specific embodiments of the present invention.

Brief Description of the Drawings

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BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Hereinafter, embodiments of the present invention will be described with reference to specific embodiments and the accompanying drawings. This is not intended to limit the technology described in this specification to specific embodiments, but should be understood to include various modifications, equivalents, and / or alternatives of the embodiments of the present invention. In connection with the description of the drawings, similar reference numerals may be used for similar components.

[0027] In this specification, expressions such as "having", "capable of having", "including", or "may include" indicate the presence of the feature (e.g., a component such as a numerical value, a function, an operation, or a part), and do not exclude the presence of further features.

[0028] 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" may 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 cases of (1) including at least one A, (2) including at least one B, or (3) including all of at least one A and at least one B.

[0029] The present invention relates to a method for manufacturing a positive electrode active material. The method for manufacturing a positive electrode active material according to the present invention may include a first step of manufacturing a mixture of a lithium precursor compound and a metal hydroxide; and a second step of sintering the mixture to manufacture a sintered body.

[0030] At this time, the shell portion of the sintered body may contain a compound represented by the following Chemical Formula 1, and the lithium concentration in the shell portion may be higher than the lithium concentration in the core portion of the sintered body. [Chemical Formula 1] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O2 In 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.

[0031] The Ni-based layered cathode active material used for the cathode active material of a secondary battery has characteristics of a high energy density and a relatively long life, and has an advantage that it can increase the reversible capacity of a lithium secondary battery. However, a high-concentration Ni-based (Ni-rich) layered cathode active material has problems of being structurally unstable and difficult to synthesize. Further, when manufacturing a Ni-rich layered cathode active material, a high oxygen partial pressure is required, the manufacturing cost increases, and an excessively high synthesis temperature causes Li / O loss on the surface and can form a deteriorated structure such as NiO. And when the lithium content is insufficient, there is a problem that a Li-poor structure is formed on the surface, and when it is excessive, residual lithium compounds on the surface are formed, which can cause side reactions and performance degradation.

[0032] On the other hand, the method for manufacturing a cathode active material according to the present invention can manufacture a sintered body having a compound represented by Chemical Formula 1 in the shell portion, and by having a Li-excess structure in which the lithium concentration in the shell portion is higher than the lithium concentration in the core portion of the sintered body, a Ni-rich cathode active material with improved stability can be manufactured.

[0033] In one example, when the method for manufacturing a positive electrode active material according to the present invention has a compound represented by the above Chemical Formula 1 as a shell part, the shell part may include lithium layers and transition metal layers. At this time, the lithium layer of the shell part may include a first lithium layer containing only lithium and a second lithium layer in which at least a part of the lithium in the first lithium layer is substituted with a transition metal. The transition metal in the second lithium layer of the shell part may be regularly substituted and arranged within the second lithium layer. Further, the transition metal layer of the shell part may include a first transition metal layer containing only a transition metal and a second transition metal layer configured such that at least a part of the transition metal has excess lithium (x1). The excess lithium (x1) in the second transition metal layer of the shell part may mean a component corresponding to x1 in the above-described Chemical Formula 1 and may be substituted and arranged with the transition metal in the second transition metal layer.

[0034] As in this example, when the shell part of the positive electrode active material according to the present invention has a lithium layer and a transition metal layer, the shell part can have a structure in which excess lithium is included on the transition metal layer, and the transition metal may include any one or more of nickel, cobalt, and manganese, and the concentration (a1) of nickel in the transition metal may be 60 mol% or more. Thus, the method for manufacturing a positive electrode active material according to the present invention may have a shell part that may include excess lithium and can have a Li-excess structure in which the lithium concentration of the shell part is higher than the lithium concentration of the core part in the sintered body. Further, the concentration (a1) of nickel in the transition metal layer of the shell part may be 60 mol% or more, enabling the realization of a high-concentration Ni-rich layered positive electrode active material.

[0035] In an exemplary embodiment of the present invention, the compound represented by Chemical Formula 1 may further include one or more compounds selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb), and ruthenium (Ru), if necessary. When the compound represented by Chemical Formula 1 contains the above components, the compound represented by Chemical Formula 1 may be represented by the following Chemical Formula 1-1. [Chemical Formula 1-1] Li 1+x1 (Ni a1 Co b1 Mn c1 Y d1 ) 1-x1 O2

[0036] In Chemical Formula 1-1, Y may represent one or more 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. The method for manufacturing a cathode active material according to the present invention may include a first step of manufacturing a mixture of a lithium precursor compound and a metal hydroxide. 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 1.5.

[0037] 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. Also, the lithium precursor compound may be LiOH, LiOH·H2O, or Li2CO3, but is not limited thereto, and various lithium-containing compounds capable of supplying lithium may be used.

[0038] In one exemplary embodiment, the lithium precursor compound of the method for manufacturing a positive electrode active material according to the present invention may further include one or more molten salts selected from the group consisting of LiNO3, Li2SO4, Li2CO3, LiCl, LiI, and LiBr. The molten salt can lower the melting point of the lithium precursor compound and induce rapid lithium diffusion. Further, when the lithium precursor compound contains the molten salt, a positive electrode active material with few nano-defects and high density can be manufactured.

[0039] At this time, the lithium precursor compound of the method for manufacturing a positive electrode active material according to this exemplary embodiment and the molten salt can be mixed at a ratio of 1:0.5 to 2, and more preferably, can be mixed at a molar ratio of 1:0.9 to 1.5. When outside the above range, the eutectic temperature of the molten salt may be outside the range, and sufficient lithium diffusion may not occur.

[0040] In one embodiment of the present invention, the second step of the method for manufacturing a positive electrode active material according to the present invention can be carried out at a temperature of 600 °C or higher and / or 900 °C or lower. The sintering temperature may be, for example, 600 °C or higher, 610 °C or higher, 620 °C or higher, 630 °C or higher, 640 °C or higher, or 650 °C or higher, and may be 900 °C or lower, 860 °C or lower, 820 °C or lower, 780 °C or lower, or 750 °C or lower, but is not limited thereto. When the second step of the method for manufacturing a positive electrode active material according to this embodiment satisfies the above temperature range, a Li-excess Ni-rich positive electrode active material with a high lithium concentration in the positive electrode active material can be manufactured, and the stability of the manufactured positive electrode active material can be enhanced. In particular, when the second step is carried out at a temperature of 650 °C or higher and 780 °C or lower, the stability of the manufactured positive electrode active material is significantly high, and its initial capacity is very high and excellent.

[0041] In one example, the second step of the method for manufacturing the positive electrode active material according to the present invention can also be carried out in an air atmosphere. In the case of conventional Ni-rich positive electrode active materials, in order to increase the oxidation number of nickel, the sintering of the active material was advanced in a high-concentration oxygen atmosphere. However, there was a problem that a large amount of high-concentration oxygen was used, increasing the manufacturing cost, and an excessively high synthesis temperature caused surface Li / O loss and could form a deteriorated structure such as NiO. On the other hand, the method for manufacturing the positive electrode active material according to this example can reduce the manufacturing cost by performing sintering at a low synthesis temperature and in an air atmosphere, and as will be described later, it is possible to provide a positive electrode active material having excellent stability and improved electrochemical performance.

[0042] In another example, the second step of the method for manufacturing the positive electrode active material according to the present invention can be carried out for 8 hours or more and / or 24 hours or less with an air inflow rate of 600 sccm or less. The air inflow rate may be, for example, 600 sccm or less, 550 sccm or less, 500 sccm or less, 450 sccm or less, 400 sccm or less, 350 sccm or less, 300 sccm or less, 250 sccm or less, or 200 sccm or less, but is not limited thereto. The lower limit of the air inflow rate is not particularly limited and may be, for example, 0 sccm or more or more than 0 sccm. At this time, when the air inflow rate is 0 sccm or more and 300 sccm or less, the stability of the manufactured positive electrode active material is significantly high, and its initial capacity is very high and excellent.

[0043] The air means general air containing 79% nitrogen (N2) and 21% oxygen (O2), or may mean a gas mixed at a ratio of 10 to 99% oxygen: 90 to 1% nitrogen.

[0044] At this time, the oxygen partial pressure of the air may be 2.4 kPa or less. FIG. 1 is a schematic diagram showing that in the method for producing a positive electrode active material according to the present invention, the core portion and the shell portion are formed to have a lithium concentration gradient. Referring to FIG. 1, when the oxygen partial pressure is high, a sufficient amount of lithium diffuses to the core, and the core portion and the shell portion of the positive electrode active material do not have a lithium concentration gradient. On the other hand, when the oxygen partial pressure is low at a low synthesis temperature, the reaction continues to occur on the surface in a state where sufficient lithium cannot diffuse from the surface of the secondary particles to the core. Therefore, a Li 1+x M 1-x O2 structure with relatively excessive lithium on the surface and a Li 1-x M 1+x O2 structure with relatively insufficient lithium toward the core can be formed (0 ≦ x ≦ 0.09). Through this, a positive electrode active material having a lithium concentration gradient can be produced.

[0045] In one embodiment of the present invention, the core portion of the sintered body of the method for producing a positive electrode active material according to the present invention may contain a compound represented by the following Chemical Formula 2. [Chemical Formula 2] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O2

[0046] In 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.

[0047] As described above, the sintered body of the method for producing a positive electrode active material according to the present invention can have a shell portion having a Li-excess structure in which lithium is contained in excess. In contrast, the core portion of the sintered body may have a lower lithium concentration than the shell portion and may contain a compound represented by Chemical Formula 2.

[0048] At this time, the sintered body of the method for manufacturing a positive electrode active material according to the present invention can have a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion. As described above, the sintered body of the method for manufacturing a positive electrode active material according to the present invention can have a structure in which the lithium concentration in the core portion is lower than the lithium concentration in the shell portion. Such a structure is formed by the diffusion of external lithium into the interior of the sintered body during the sintering process. At this time, in the case of the conventional manufacturing method, a lithium-excess structure with a high lithium concentration in the core portion of the sintered body is formed. On the other hand, the method for manufacturing a positive electrode active material according to the present invention can reduce the amount of lithium diffusing into the core portion of the sintered body, and can manufacture a sintered body having a continuous concentration gradient while the lithium content in the shell portion is higher than that in the core portion.

[0049] In one example, when the method for manufacturing a positive electrode active material according to the present invention has the compound represented by the above Chemical Formula 2 as the core portion, the core portion may include lithium layers and transition metal layers. At this time, the lithium layer of the core portion may include a first lithium layer containing only lithium and a second lithium layer in which at least a part of the lithium in the first lithium layer is substituted with a transition metal. The transition metal in the second lithium layer of the core portion may be regularly substituted and arranged within the second lithium layer. Further, the transition metal layer of the core portion may include a first transition metal layer containing only a transition metal and a second transition metal layer in which at least a part of the transition metal is substituted with lithium (x2). The lithium (x2) in the second transition metal layer of the core portion may mean a component corresponding to x2 in the above Chemical Formula 2.

[0050] As exemplified in this case, when the core part of the positive electrode active material according to the present invention has a lithium layer and a transition metal layer, the core part may have a structure in which lithium is deficient on the lithium layer and the transition metal is further contained on the lithium layer. The transition metal may contain any one or more of nickel, cobalt, and manganese, and the concentration (a2) of nickel in the transition metal may be 60 mol% or more. Thus, the method for manufacturing a positive electrode active material according to the present invention has a structure in which the lithium concentration in the shell part is higher than the lithium concentration in the core part of the sintered body, and at the same time, the concentration (a1) of nickel in the transition metal layer in the core part may be 60 mol% or more, enabling the realization of a high-concentration Ni-rich layered positive electrode active material. There is no change in the transition metal content between the core part and the shell part, and a high nickel concentration (a1≧0.6) can be realized uniformly in the core part and the shell part, maintaining a high capacity, and the lithium-excess layered structure in the shell part stabilizes the structure during the overall electrochemical (charge and discharge) reaction, enabling the maintenance of a long life.

[0051] In one embodiment of the present invention, the compound represented by Chemical Formula 2 may further contain one or more compounds selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb), and ruthenium (Ru) as necessary. When the compound represented by Chemical Formula 1 contains the above components, the compound represented by Chemical Formula 2 may be represented by the following Chemical Formula 2-1. [Chemical Formula 2-1] Li 1+x2 (Ni a2 Co b2 Mn c2 Y d2 ) 1-x2 O2

[0052] In Chemical Formula 2-1, Y may mean one or more selected from the group consisting of aluminum (Al), zirconium (Zr), magnesium (Mg), tungsten (W), tantalum (Ta), niobium (Nb), and ruthenium (Ru), and d2 can satisfy 0≦d2≦0.1. In one example, in the sintered body of the method for manufacturing a positive electrode active material according to the present invention, the oxidation number of nickel (Ni) in the shell part may be higher than the oxidation number of nickel in the core part. There is a problem of cation mixing occurring in a general high-concentration Ni-based (Ni-rich) positive electrode active material. Cation mixing is a phenomenon in which divalent nickel ions (Ni 2+ ) are introduced into the lithium sites, which can reduce the capacity of the secondary battery. On the other hand, the method for manufacturing a positive electrode active material according to the present invention can form a sintered body in which the oxidation number of nickel (Ni) in the shell part is higher than the oxidation number of nickel in the core part, and by having a Li-excess structure containing an excessive amount of lithium, divalent nickel ions (Ni 2+ ) can be reduced, and cation mixing can be prevented.

[0053] The present invention also relates to a positive electrode active material. The positive electrode active material according to the present invention includes a shell part represented by the following Chemical Formula 1 and a core part represented by the following Chemical Formula 2, and the lithium concentration in the shell part may be higher than the lithium concentration in the core part. [Chemical Formula 1] Li 1+x1 (Ni a1 Co b1 Mn c1 ) 1-x1 O2 In 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. [Chemical Formula 2] Li 1-x2 (Ni a2 Co b2 Mn c2 ) 1+x2 O2 In 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.

[0054] At this time, the sintered body of the positive electrode active material according to the present invention can have a concentration gradient in which the lithium content continuously increases from the core portion to the surface of the shell portion.

[0055] In addition, the oxidation number of nickel in the shell portion of the sintered body of the positive electrode active material according to the present invention may be higher than the oxidation number of nickel in the core portion.

[0056] In one example, the d-spacing (d(003)-spacing) with respect to the (003) plane within the lattice of the core portion of the sintered body of the positive electrode active material according to the present invention may be larger than the d(003)-spacing of the shell portion. The d-spacing may mean the interplanar distance within the lattice and can be confirmed through XRD analysis. As in this example, when the d-spacing (d(003)-spacing) with respect to the (003) plane within the lattice of the core portion of the sintered body of the positive electrode active material is larger than the d(003)-spacing of the shell portion, it is possible to embody a high-capacity secondary battery, and the lithium-excess layered structure of the shell portion stabilizes the structure during the overall electrochemical (charge-discharge) reaction and can maintain a long life.

[0057] Also, the shell portion and the core portion of the sintered body of the positive electrode active material according to the present invention may each contain primary particles, and the primary particle size of the shell portion may be larger than the primary particle size of the core portion. The primary particle size may mean the average particle diameter of the primary particles. When the primary particle size of the shell portion is larger than the primary particle size of the core portion in this way, the mechanical strength of the sintered body can be improved, the contact area with the electrolyte in the shell portion can be reduced, side reactions can be suppressed during charge and discharge, and the life can be improved. Since the content regarding the sintered body of the positive electrode active material and the like is the same as described above, it will be omitted.

Examples

[0058] The following presents preferred embodiments to facilitate the understanding of the present invention. However, the following embodiments are merely provided to more easily understand the present invention, and the content of the present invention is not limited by the following embodiments.

[0059] Example 1: Production of the positive electrode active material The positive electrode active material according to the present invention was produced through the process shown in FIG. 2. To produce the positive electrode active material according to the present invention, Ni 0.92 Co 0.03 Mn 0.05 (OH)2 + LiOH·H2O precursor mixed sample was prepared. At this time, the precursor mixture was sintered for 12 hours under the conditions of a transition metal compound:lithium compound = 1:0.97 to 1.30 molar ratio and a final synthesis temperature of 700 to 800 °C under a gas atmosphere of 100 mL / min Air, to synthesize the final positive electrode active material Li x Me 2-x O2 (x = 0.97, 1.0, 1.029, 1.074, 1.13).

[0060] Examples 2 and 3: Production of the positive electrode active material The positive electrode active material was produced in the same manner as in Example 1, except that the gas atmosphere was changed to 0 mL / min and 250 mL / min Air during sintering.

[0061] Examples 4 and 5: Production of the positive electrode active material During sintering, conditions of 350 mL / min and 500 mL / min Air in the gas atmosphere were added to produce the positive electrode active material.

[0062] [Table 1] Table 1 above organizes the ratios, gas atmospheres, and synthesis temperature conditions of the precursor mixed samples produced in the examples.

[0063] Experimental Example 1: X-ray diffraction (XRD) pattern Figure 3 is a diagram showing the XRD analysis results for the positive electrode active material manufactured in the examples. As shown in Figure 3, after synthesis under each condition of the precursor mixture, the conditions for forming the layered structure and impurities under each synthesis condition were confirmed through the X-ray diffraction (XRD) pattern. Qualitatively, it was confirmed that as the atmospheric gas inflow rate decreased from 500 cc / min to 0 cc / min, and as the synthesis temperature increased, the impurity content increased due to the increase in the lithium compound.

[0064] Experimental Example 2: Rietveld Structure Analysis of X-ray Diffraction (XRD) Pattern Figure 4 is a diagram showing the change in the lattice constant after sintering of the positive electrode active material manufactured in the examples, Figure 5 is a diagram showing the change in the lattice volume of the examples, and Figure 6 is a diagram showing the phase fraction of impurity generation in the examples. The change in the lattice constant, the change in the lattice volume, and the phase fraction of impurity generation were confirmed through the Rietveld analysis of the X-ray diffraction (XRD) pattern. Generally, the lattice constant increased as the synthesis temperature increased and the atmospheric gas inflow rate decreased. And regardless of the conditions, as the lithium content increased, it led to a decrease in the lattice constant. This is because as the lithium compound increases under all synthesis conditions, the lithium contained in the layered crystal structure increases, and through this, it can be indirectly confirmed that this led to an increase in the oxidation number of the transition metal. Also, as the atmospheric gas inflow rate decreased and the synthesis temperature increased, the residual lithium compounds (such as LiOH, Li2O, Li2CO3, etc.) increased up to 7.5%, which indicates the characteristic that lithium is less likely to be contained in the structure due to insufficient oxygen and high synthesis temperature.

[0065] Experimental Example 3: Rietveld Analysis of Neutron Diffraction Pattern To quantify the lithium content under various synthesis conditions, neutron diffraction analysis was carried out. Figure 7 is a diagram showing the results of neutron diffraction analysis for the examples. Referring to Figure 7, it can be seen that as the synthesis temperature decreases to 700 °C at an air gas flow rate of 500 cc / min, an excessive amount of lithium can be contained up to a maximum of 8%. However, as the inflow rate of the air gas decreases, the oxygen required when the transition metal precursor (Me(OH)2) reacts with the lithium compound becomes insufficient, so it becomes difficult for the structure to contain an excessive amount of lithium, and it was confirmed that the excessive lithium decreased to a maximum of 5%. Also, when the synthesis temperature becomes excessively high at 800 °C with an air gas flow rate of 500 cc / min, an excessive amount of lithium is contained up to a maximum of 3%, and when the air gas inflow decreases to 0 cc / min, it was confirmed that an excessive amount of lithium cannot be contained at more than a maximum of 1%. Also, under the synthesis conditions with an excessive amount of lithium, it was confirmed that the ratio of cation mixing can be reduced by the decrease of Ni 2+ .

[0066] Experimental Example 4: XANES Analysis Figure 8 is a diagram showing the results of X-ray absorption near-edge structure (XANES) analysis for the examples. Through XANES analysis, the change in the Ni oxidation number of the bulk structure showing the entire secondary particles due to the content of excessive lithium shown in Experimental Example 3 was confirmed. Referring to Figure 8, as the air gas increases from a low flow rate to a high flow rate, and as the synthesis temperature decreases, the increase in the oxidation number of Ni due to the increase in the lithium amount further increases significantly, which means that the content of excessive lithium contained in the structure in Experimental Example 3 increases further as it approaches the above conditions.

[0067] Experimental Example 5: XAS Analysis Figure 9 is a diagram showing the results of soft X-ray absorption spectroscopy (XAS) analysis for the examples. Through the soft X-ray absorption spectroscopy analysis, the change in the Ni oxidation number of the average surface structure of the secondary particles due to the content of excessive lithium shown in Experimental Example 3 was confirmed. Referring to Figure 9, similar to Experimental Example 4, as the atmospheric gas increased from a low flow rate to a high flow rate, and as the synthesis temperature decreased, the increase in the oxidation number of Ni due to the increase in the lithium amount further increased significantly. This means that in Experimental Example 3, the closer the content of excessive lithium contained in the structure is to the above conditions, the further it increases.

[0068] Experimental Example 6: Comparative analysis of the surface and bulk oxidation numbers of secondary particles according to the atmospheric gas flow rate of a 700 °C, target Li = 1.029 sample Figure 10 is a diagram showing the experimental results for a 700 °C, target Li = 1.029 sample. Figure 10(a) is a diagram showing the Ni oxidation number of the entire bulk structure of the secondary particles analyzed through XANES analysis, and Figure 10(b) is a diagram showing the change in the Ni oxidation number of the surface structure of the secondary particles analyzed through soft X-ray absorption spectroscopy analysis. Referring to Figure 10(a), it can be confirmed that as the atmospheric gas flow rate increased, the Ni oxidation number of the bulk structure decreased. On the other hand, referring to Figure 10(b), the Ni oxidation number of the surface structure gradually increased while the atmospheric gas flow rate decreased from 500 cc / min to 100 cc / min, excluding 0 cc / min. In particular, it was confirmed that the Ni oxidation number of the surface structure was the largest at 100 cc / min. Figure 11 is a diagram showing the results of transmission X-ray microscopy (TXM)-XANES analysis of the sample, and Figure 12 is a diagram showing the results of transmission electron microscopy (TEM)-electron energy loss spectroscopy (EELS) analysis of the sample. Referring to Figures 11 and 12, in the case of the 700 °C, target Li = 1.029 sample, it can be confirmed that at 100 cc / min, Ni on the surface structure is further oxidized, and a more reduced structure is formed towards the core. This is a result contrary to the fact that when the atmospheric gas flow rate is very fast at 500 cc / min, the entire bulk structure has the same Ni oxidation number. The difference in the Ni oxidation number between the surface and the core structure means that the lithium distribution excessively contained in the transition metal layer is different. Therefore, in the sample synthesized at 700 °C, target Li = 1.029, and 100 cc / min, the surface structure has an excess of lithium, and it shows the formation of secondary particles with a lithium concentration gradient approaching the stoichiometric composition towards the core.

[0069] Experimental Example 7: Comparative analysis of the d-spacing of the (003) plane from the core to the surface of secondary particles according to the atmospheric gas flow rate of the 700 °C, target Li = 1.029 sample Figures 13(a) and 13(b) are diagrams respectively showing the results of high-resolution (HR)-TEM analysis for the atmospheric gas flow rates of 500 cc / min and 100 cc / min of the 700 °C, target Li = 1.029 sample. Referring to Figures 13(a) and 13(b), for the sample synthesized at an atmospheric gas flow rate of 500 cc / min, the d-spacing of the (003) plane (d(003)-spacing) is about 4.64 Å and is constant from the core to the surface of the secondary particles, while for the sample synthesized at an atmospheric gas flow rate of 100 cc / min, a difference in d-spacing can be confirmed where d(003)-spacing is about 5.00 Å at the core and about 4.64 Å at the surface. This is consistent with the difference in oxidation number shown in Figures 10, 11, and 12, that is, when the atmospheric gas flow rate is 100 cc / min at 700 °C and target Li = 1.029, the Ni oxidation number gradually increases from the core to the surface.

[0070] Experimental Example 8: Comparison of Electrochemical Performance by Decreasing the Flow Rate of Atmospheric Gas from 500 cc / min to 100 cc / min at 700 °C with a Target Li = 1.029 Composition Figures 14 and Table 2 are diagrams showing the electrochemical cycle characteristics of the synthesized positive electrode active material by decreasing the flow rate of atmospheric gas from 500 cc / min to 100 cc / min at 700 °C with a target Li = 1.029 composition. Referring to Figures 14 and Table 2, the sample synthesized at 700 °C, with a target Li = 1.029 composition and at 100 cc / min, in which secondary particles with a lithium concentration gradient were formed, showed excellent cycle characteristics while having the largest initial capacity of 227 mAh / g.

[0071] Experimental Example 9: Performance Comparison of Electrochemical Charge and Discharge Rates by Decreasing the Flow Rates of Atmospheric Gas to 500 cc / min and 100 cc / min at 700 °C with a Target Li = 1.029 Composition Figure 15 is a diagram showing the performance comparison of the electrochemical multiple charge and discharge rates for 500 cc / min and 100 cc / min at 700 °C with a target Li = 1.029 composition. When charging and discharging 5 cycles each from a 0.1C rate to 0.2C, 0.5C, 1C, and 2C rates and then charging and discharging at a 0.1C rate, the sample synthesized at 500 cc / min showed a capacity ratio of about 76%, while the sample synthesized at 100 cc / min showed excellent discharge capacity and rate performance with a capacity ratio of about 85%. The excellent rate performance at 100 cc / min means that the mobility of lithium was improved due to the relatively wide d(003)-spacing in the core part.

[0072] [Table 2]

[0073] Experimental Example 10: Comparison of Electrochemical Performance by Decreasing the Flow Rate of Atmospheric Gas from 500 cc / min to 100 cc / min at 700 °C with a Target Li = 1.029 Composition after Washing Figures 16 and Table 3 show the electrochemical cycle characteristics of the positive electrode active material synthesized at 700 °C with a target Li = 1.029 composition from 500 cc / min to 100 cc / min, in which impurities were removed through the water washing process. Similarly, even in the sample with impurities removed, the sample synthesized at 700 °C with a target Li = 1.029 composition and 100 cc / min, in which secondary particles with a lithium concentration gradient were formed, had 204 mAh / g and showed the most stable cycle characteristics.

[0074]

Table 3

[0075] Experimental Example 11: Comparison of Real-Time Charge-Discharge XRD Analysis Figures 17(a) and 17(b) are diagrams showing the results of real-time charge-discharge XRD analysis for the sample synthesized at 700 °C with a target Li = 1.029 composition and 100 cc / min and the sample synthesized at 750 °C with a target Li = 1.029 composition and 100 cc / min, respectively. Referring to Figure 17, it was confirmed that the sample synthesized at 700 °C with a target Li = 1.029 composition and 100 cc / min had a 0.72% smaller change in the lattice constant c-axis than the sample synthesized at 750 °C with a target Li = 1.029 composition and 100 cc / min, which exhibited the existing similar capacity, despite the former showing the largest capacity as shown in Figures 13 and 14. This proves that the material with secondary particles having a lithium concentration gradient has excellent electrochemical properties.

[0076] Experimental Example 12: Comparison of Changes in Secondary Particle Bulk and Surface Oxidation Number due to Atmospheric Gas Flow Rate for Samples at 800 - 750 °C with a Target Li = 1.029 Figure 18 shows the XANES and Soft-XAS analysis results according to the atmospheric gas flow rate for the target Li = 1.029 samples at 800 - 750 °C. Figures 18(a) and 18(b) are diagrams showing the bulk oxidation number and surface oxidation number, respectively, for the sample with a target Li = 1.029 at 800 °C, and Figures 18(c) and 18(d) are diagrams showing the bulk oxidation number and surface oxidation number, respectively, for the sample with a target Li = 1.029 at 750 °C. Referring to Figure 18, unlike the tendency where the surface and bulk Ni oxidation numbers show opposite trends due to the decrease in the atmospheric gas flow rate for the 700 °C, target Li = 1.029 sample, it was confirmed that as the synthesis temperature increases from 750 to 800 °C and the atmospheric gas flow rate decreases, the changes in the Ni oxidation numbers of the surface and bulk have a similar tendency. As a result, it was confirmed that at the 800 °C synthesis temperature, the tendency for the oxidation numbers to decrease due to the decrease in the atmospheric gas flow rate of the surface and bulk is consistent.

[0077] Experimental Example 13: Comparison of Electrochemical Performance According to Atmospheric Gas Flow Rate for Samples with Target Li = 1.0 and 1.029 at 800 °C and 750 °C Figures 19 and Tables 4 - 8 show the electrochemical cycle characteristics according to the atmospheric gas flow rate for the target Li = 1.0 - 1.029 samples at 800 - 750 °C. Referring to Figures 19 and Tables 4 - 8, at the 750 °C temperature, similar to 700 °C, as the gas flow rate decreased from 500 cc / min to 100 cc / min, the initial capacity increased, but the cycle characteristics decreased. At the high temperature of 800 °C, as the gas flow rate decreased, both the initial capacity and the cycle characteristics decreased. This proves that the most ideal secondary particles with a lithium concentration gradient were formed under the low gas flow rate conditions at 700 °C.

[0078] [Table 4]

[0079] [Table 5]

[0080]

Table 6

[0081]

Table 7

[0082]

Table 8

[0083] Experimental Example 14: Comparison of Particle Sizes of Cathode Active Materials Synthesized at 500 cc / min and 100 cc / min with a Target Li = 1.029 Composition at 700°C Figure 20 is a diagram showing the particle size distribution of cathode active materials synthesized at 500 cc / min and 100 cc / min with a target Li = 1.029 composition at 700°C. In the case of the cathode active material synthesized at a gas flow rate of 500 cc / min, it can be confirmed that the primary particle size is uniform at 200 - 300 nm from the shell part to the core part. However, in the case of the cathode active material synthesized at 100 cc / min, it was confirmed that the particle size of the shell part is 500 - 1000 nm, which is larger than that of the core part. This indicates that the crystallization of the shell part occurred more significantly due to lithium excess compared to the core part. The increase in the primary particle size of the shell part within the secondary particle means that the interfacial exposure of the cathode active material to the electrolyte can be reduced, which can reduce side reactions at the interface due to electrochemical cycling and contribute to an improvement in the lifespan.

[0084] As described in detail above regarding the embodiments of the present invention, the present invention is not limited by the aforementioned embodiments and the attached drawings, but is intended to be limited by the attached claims. Therefore, various forms of substitution, modification, and change are possible by those having ordinary knowledge in the technical field within the scope not departing from the technical idea of the present invention described in the claims, and this can also be said to belong to the scope of the present invention.

Claims

1. A first step of preparing a mixture of a lithium precursor compound and a metal hydroxide; a second step of sintering the mixture to produce a sintered body; The shell portion of the sintered body contains a compound represented by the following chemical formula 1: A method for producing a positive electrode active material having a higher lithium concentration in a shell portion than in a core portion of the sintered body: [Chemical formula 1] Li １＋ｘ１ (N ａ１ Co ｂ１ Mn ｃ１ ) １－ｘ１ O ２ In the above Chemical Formula 1, x1, a1, b1, and c1 are real numbers that satisfy −0.05≦x1≦0.2, 0.6≦a1≦1.0, 0≦b1≦0.3, and 0≦c1≦0.4, respectively.

2. The first step is mixing a metal hydroxide and a lithium precursor compound in a molar ratio of 1:0.9 to 1.5; The lithium precursor compound is LiNO ３ , Li ２ SO ４ , Li ２ CO ３ 2. The method of claim 1, further comprising the step of: adding one or more molten salts selected from the group consisting of LiCl, LiI, and LiBr.

3. The method of claim 1 , wherein the second step is performed at a temperature of 600° C. or more and 900° C. or less.

4. The method of claim 1 , wherein the second step is performed at a temperature of 650° C. or more and 780° C. or less.

5. The method of claim 1 , wherein the second step is performed in an air atmosphere.

6. 6. The method of claim 5, wherein the second step is performed for 8 hours to 24 hours at an air inflow rate of 0 sccm to 600 sccm.

7. 6. The method of claim 5, wherein the second step is performed for 8 hours to 24 hours at an air inflow rate of 0 sccm to 300 sccm.

8. 2. The method for producing the positive electrode active material according to claim 1, wherein the core portion of the sintered body contains a compound represented by the following chemical formula 2: [Chemical formula 2] Li １－ｘ２ (N ａ２ Co ｂ２ Mn ｃ２ ) １＋ｘ２ O ２ In the above Chemical Formula 2, x2, a2, b2, and c2 are real numbers that satisfy the following conditions: -0.2≦x2≦0.05, 0.6≦a2≦1.0, 0≦b2≦0.3, and 0≦c2≦0.4, respectively.

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

10. The method for producing a positive electrode active material according to claim 1 , wherein an oxidation number of nickel (Ni) in the shell portion of the sintered body is higher than an oxidation number of nickel in the core portion of the sintered body.

11. A shell portion represented by the following chemical formula 1; A core portion represented by the following chemical formula 2; A positive electrode active material having a higher lithium concentration in the shell portion than in the core portion: [Chemical formula 1] Li １＋ｘ１ (N ａ１ Co ｂ１ Mn ｃ１ ) １－ｘ１ O ２ In the above Chemical Formula 1, x1, a1, b1, and c1 are real numbers that satisfy −0.05≦x1≦0.2, 0.6≦a1≦1.0, 0≦b1≦0.3, and 0≦c1≦0.4, respectively. [Chemical formula 2] Li １－ｘ２ (N ａ２ Co ｂ２ Mn ｃ２ ) １＋ｘ２ O ２ In the above Chemical Formula 2, x2, a2, b2, and c2 are real numbers that satisfy the following conditions: -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 , having a concentration gradient in which the lithium content increases continuously from the core portion to the surface of the shell portion.

13. The positive electrode active material according to claim 11 , wherein the oxidation number of nickel (Ni) 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, wherein the d-spacing (d(003)-spacing) of the core portion relative to the (003) plane in the lattice is greater than the d(003)-spacing of the shell portion.

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

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