Cathode active material and lithium secondary battery including same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, to a positive electrode active material capable of inducing uniform particle growth of primary particles within secondary particles by a relatively simple process without additional additives or process design, and capable of improving the uniformity of the primary particles. The present invention relates to a positive electrode active material and a lithium secondary battery comprising the same, and more specifically, to a positive electrode active material and a lithium secondary battery comprising the same,

Description

Technical Field

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, to a positive electrode active material and a lithium secondary battery including the same, which can induce uniform particle growth of primary particles within secondary particles through a relatively simple process without additional additives or process design, thereby controlling atomic and nanoscale defects to ensure stability and exhibit excellent electrochemical properties. 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. As mentioned above, the core materials of lithium secondary batteries 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-mentioned 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 with the same structure as LCO is mainly used. 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, although nickel-based layered cathode active materials have excellent electrochemical properties, for commercial applications of nickel-based layered cathode active materials, their commercialization is limited due to the following problems such as limitations in cycle characteristics or structural stability.

[0005] First, the existing general nickel-based layered positive electrode active material is prepared by mixing a precursor synthesized by a coprecipitation method with a lithium source and synthesizing it into a solid phase. In this case, there is a problem of performance degradation of lithium secondary batteries due to microcracks. More specifically, the nickel-based layered positive electrode active material generally has a secondary particle structure of several μm in size formed by aggregation of submicron-sized fine primary particles. In the case of the nickel-based layered positive electrode active material synthesized by the existing coprecipitation method, microcracks may appear inside the secondary particles during long-term charge / discharge, resulting in new interface exposure, and the side reaction with the electrolyte at the interface may be accelerated, which may become the cause of the deterioration of the battery life characteristics.

[0006] Secondly, in order to fully utilize the excellent electrical / chemical properties of the nickel-based layered positive active material and achieve high energy density, it is necessary to increase the electrode density (>3.3 g / cc). In the case of the nickel-based positive active material synthesized by the existing coprecipitation method, as described above, the structural stability of the secondary particles is low, so the disintegration of the secondary particles is prone to occur. In this case, there is a problem of electrolyte depletion due to side reactions with the electrolyte. In other words, the nickel-based layered positive active material in the form of secondary particles synthesized by the existing coprecipitation method also has limitations in terms of exhibiting high energy density characteristics.

[0007] Third, the nickel-based layered positive electrode active material may cause a decrease in battery performance such as a decrease in capacity due to the instability of highly oxidized nickel cations and oxygen detachment in the main structure. In other words, oxygen detachment will cause excessive production of NiO with a rock salt structure in the layered structure of the positive electrode active material and increase lithium byproducts. At this time, NiO gradually increases due to repeated charge and discharge, resulting in increased resistance, and various side reactions may occur as lithium byproducts increase, resulting in deterioration of battery performance such as a decrease in capacity.

[0008] As mentioned above, atomic-level defects such as oxygen generation and cation mixing that occur in existing synthesis processes can lead to nanoscale defects such as nanopores. The sizes of these nanoscale defects range from intra-particle pores to inter-particle pores, which become factors that hinder the high capacity and stabilization of nickel-based layered positive electrode active materials.

[0009] Therefore, in order to fully utilize the excellent electro / chemical properties of Ni-based hierarchical cathode active materials, which have attracted attention as potential cathode materials due to their high energy density and increased lifespan, it is urgently necessary to study methods that can control atomic- and nanoscale defects through uniform reaction-induced particle growth of primary particles in secondary particles constituting Ni-based hierarchical cathode active materials.

[0010] Prior art literature

[0011] Patent Document

[0012] (Patent Document) Korean Patent Publication No. 2016 - 0094063 (August 9, 2016) Summary of the Invention

[0013] Technical Problem

[0014] The present invention was developed to solve the above problems. The problem to be solved by the present invention is to provide a positive electrode active material and a method for preparing the same, which induce particle growth through uniform reaction of primary particles in secondary particles by performing pretreatment at the temperature at which the metal hydroxide and the lithium precursor start to react and then finally synthesizing, so as to be able to control nano - scale defects, and thus can solve the problem of performance deterioration in terms of long - term life.

[0015] In addition, the problem to be solved by the present invention is to provide a positive electrode active material and a method for preparing the same, which can be synthesized by a single synthesis process without special additives, and thus can synthesize a positive electrode active material having improved performance compared to existing materials without changing the existing process design, and can be used in various industries.

[0016] Solution to the Problem

[0017] To solve the above problems, the present invention provides a method for preparing a positive electrode active material. The method for preparing the positive electrode active material includes: a first step of forming a precursor mixture by mixing a metal hydroxide and a lithium precursor; a second step of performing heat treatment on the precursor mixture; and a third step of preparing LMO by sintering the precursor mixture that has undergone the heat treatment. x .

[0018] Wherein, L is Li (lithium), M is at least one selected from Co (cobalt), Ni (nickel), Al (aluminum), Mn (manganese), and Mg (magnesium), and x is a constant of 0.5 to 2.5.

[0019] In addition, according to an embodiment of the present invention, the above LMO x may be a compound represented by the following Chemical Formula 1.

[0020] [Chemical Formula 1]

[0021] LiNi a Co b Mn c O 2

[0022] Wherein, a, b, and c are constants of 0.8 ≤ a < 1.0, 0 < b < 0.1, 0 < c < 0.1, respectively, and a + b + c = 1.

[0023] In addition, the lithium precursor in the first step may further include a molten salt.

[0024] In addition, through the heat treatment in the second step, an intermediate phase represented by the following chemical formula 2 is formed, which induces uniform growth of primary particles in the secondary particles.

[0025] [Chemical formula 2]

[0026] Li x M 2-x O 2

[0027] Here, M is at least one selected from Co (cobalt), Ni (nickel), Al (aluminum), Mn (manganese) and Mg (magnesium), and x is a constant ranging from 0.2 to 0.8.

[0028] In addition, the heat treatment in the second step may be performed at 200° C. to 400° C. for 2 hours to 8 hours.

[0029] In addition, the metal hydroxide and the lithium precursor in the first step may be mixed at a molar ratio of 1:0.9 to 1.1.

[0030] In addition, the third step may be performed at 600° C. to 900° C. for 6 hours to 16 hours.

[0031] In addition, the molten salt may be selected from LiNO 3 , Li 2 SO 4 , Li 2 CO 3 , LiCl, LiI and LiBr.

[0032] Alternatively, the lithium precursor may be LiOH and LiOH·H 2 O and the above-mentioned molten salt are mixed at a ratio of 1:0.5 to 2.

[0033] In addition, the content of the intermediate phase may be 40 wt % to 80 wt % relative to the total weight of the precursor mixture.

[0034] In addition, the present invention provides a positive electrode active material in the form of secondary particles with minimized atomic and nano-scale defects, the positive electrode active material is represented by the following chemical formula 3, and comprises: a first pore formed at a mean radius less than between particles; and a second pore having a size smaller than The average radius is formed within the particles.

[0035] [Chemical formula 3]

[0036] LMO x

[0037] Among them, L is Li (lithium), M is at least one selected from Co (cobalt), Ni (nickel), Al (aluminum), Mn (manganese), and Mg (magnesium), and x is a constant from 0.5 to 2.5.

[0038] In addition, according to an embodiment of the present invention, the above positive electrode active material may be a compound represented by the following Chemical Formula 4.

[0039] [Chemical Formula 4]

[0040] LiNi a Co b Mn c O 2

[0041] Among them, a, b, and c are constants of 0.8 ≤ a < 1.0, 0 < b < 0.1, and 0 < c < 0.1, respectively, and a + b + c = 1.

[0042] In addition, the above first pore has a distribution of less than 0.012 cm 3 / g.

[0043] In addition, the above second pore has a distribution of less than 0.0009 cm 3 / g.

[0044] In addition, the present invention provides a lithium secondary battery including the above positive electrode active material.

[0045] Effects of the Invention

[0046] According to the present invention, it is possible to induce grain growth to control nano-scale defects through the uniform reaction of primary grains within secondary grains constituting the positive electrode active material, solve the problem of performance degradation in terms of long-term life, and easily synthesize a positive electrode active material with high stability and high capacity through a single synthesis process without special additives. Therefore, the utilization rate in various industries using lithium secondary batteries can be greatly improved. Description of the Drawings

[0047] Figure 1 It is a SEM image of observing the cross-section of secondary grains after 150 cycles of an existing general nickel-based layered positive electrode active material.

[0048] Figure 2 It is a SEM image of observing the cross-section of secondary grains after 150 cycles of the nickel-based layered positive electrode active material according to the present invention.

[0049] Figure 3FIG. 1 is an image showing a thermogravimetric analysis result of a positive electrode active material precursor mixture according to an embodiment of the present invention.

[0050] Figure 4a FIG. 4 is a graph showing the phase fraction of the intermediate phase obtained by Rietveld analysis of an X-ray diffraction (XRD) pattern of a positive electrode active material according to an embodiment of the present invention.

[0051] Figure 4b FIG. 4 is a graph showing the result of Rietveld analysis of a neutron diffraction pattern of a mesophase of a positive electrode active material according to an embodiment of the present invention.

[0052] Figure 5 FIG. 4 is a graph showing the phase fraction analysis result of an intermediate product during the sintering process of a positive electrode active material according to an embodiment of the present invention.

[0053] Figure 6 The graph is a graph analyzing the changes in the phase fraction and the lattice constant during the primary sintering of the positive electrode active material according to one embodiment of the present invention.

[0054] Figure 7 FIG. 4 is a graph showing XAS analysis results for confirming the particle growth uniformity of secondary particles before and after primary sintering of a positive electrode active material according to an embodiment of the present invention.

[0055] Figure 8 The figures are images and graphs showing the results of XANES analysis and transmission electron microscope (TEM) analysis of a positive electrode active material according to an embodiment of the present invention.

[0056] Fig. 9 FIG. 5 is a graph showing analysis results of XRD and ND patterns of the intermediate phase of the positive electrode active material according to an embodiment of the present invention.

[0057] Fig.10 and Fig.11 FIG. 1 is a SEM image showing a cross-section of a particle of a positive electrode active material according to an embodiment of the present invention.

[0058] Figure 12 to Figure 14 FIG. 5 is a graph showing SAXS and SANS analysis results of a positive electrode active material according to an embodiment of the present invention.

[0059] Figures 15 to 19 XRD and ND pattern analysis, SAXS and SANS analysis, and TEM and SEM cross sections of a positive electrode active material according to an embodiment of the present invention are shown.

[0060] Fig. 20 FIG. 5 is a graph showing the electrochemical performance of a positive electrode active material according to an embodiment of the present invention.

[0061] Fig.21 and Fig. 22 FIG. 5 is a graph showing the electrochemical performance of a positive electrode active material according to another embodiment of the present invention. DETAILED DESCRIPTION

[0062] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the present invention. The present invention can be implemented in a variety of different implementations and is not limited to the embodiments described in this specification.

[0063] As mentioned above, the material with the highest capacity among the existing layered positive electrode active materials is a high nickel-based layered positive electrode active material, but the stoichiometric material synthesis is difficult due to the instability of nickel cations, and there are problems such as oxygen generation and cation mixing in the synthesis process to form atomic-level defects. In addition, atomic-level defects lead to nano-level defects such as nanopores, and the size of these nano-level defects varies from intra-particle pores to inter-particle pores, which become the cause of the degradation of life characteristics. In other words, there are limitations in commercialization due to limitations in cycle characteristics or structural stability.

[0064] Therefore, the present invention attempts to solve the above problems by providing a method for preparing a positive electrode active material, the method comprising: a first step of forming a precursor mixture by mixing a metal hydroxide and a lithium precursor; a second step of heat-treating the precursor mixture; and a third step of preparing LMO by sintering the heat-treated precursor mixture. x .

[0065] Therefore, the present invention can induce particle growth to control nanoscale defects through the uniform reaction of primary particles within secondary particles constituting the positive electrode active material, which can solve the problem of performance degradation in terms of long-term life. Positive electrode active materials with guaranteed stability and high capacity can be easily synthesized through a single synthesis process without special additives, thereby greatly improving the use of lithium secondary batteries in various industries.

[0066] Hereinafter, the present invention is described in detail with reference to the accompanying drawings.

[0067] Method for preparing positive electrode active material

[0068] The first step of the method for preparing a positive electrode active material according to the present invention is a step of forming a precursor mixture by mixing a metal hydroxide and a lithium precursor, and the second step is a step of heat-treating the precursor mixture.

[0069] In general, the positive electrode active material used in the lithium secondary battery usually has a secondary particle structure of several μm in size formed by the aggregation of submicron-sized fine primary particles. The positive electrode active material having the secondary particle structure breaks as the aggregated primary particles separate during repeated charging and discharging, so there is a problem of deterioration of battery characteristics. The above problem is caused by the structural characteristics of the secondary particles. In particular, the positive electrode active material can reduce the cost by replacing part of the expensive cobalt with nickel. For the nickel-based positive electrode active material, which has the advantage of increasing the reversible capacity of the lithium secondary battery due to high energy density and relatively long life, there is a problem of forming atomic-level defects such as oxygen generation and cation mixing. Therefore, when atomic-level defects and nano-level defects such as nanopores occur, the size of these nano-level defects varies from intra-particle pores to inter-particle pores, which becomes the cause of deterioration of life characteristics.

[0070] More specifically, refer to Figure 1 , the cross section of the secondary particles was observed after 150 cycles of the existing nickel-based layered positive active material, and as a result, it was confirmed that the rapid rupture of the secondary particles was observed due to the penetration of the electrolyte between the macropores generated during the synthesis process and the uneven shrinkage / expansion during the charging and discharging process. Therefore, measures for suppressing the formation of atomic and nano defects are being mentioned. For example, it is reported that there are attempts to synthesize positive electrode active materials with stable atomic structures by doping with various dopants (Al, B, Zr, Nb, Ta, etc.) or various methods of designing and controlling synthesis conditions such as synthesis temperature, pretreatment time, lithium content, etc. as a method of suppressing defects formed during the synthesis process. However, the existing research as described above mainly focuses on atomic-level defect control, and the correlation between defects of different scales and methods for suppressing them have never been introduced.

[0071] Therefore, in the present invention, the precursor mixture prepared in the above-mentioned first step is pretreated at a temperature at which the precursor mixture starts to react by the second step, and then finally synthesized to induce particle growth by uniform reaction of primary particles within secondary particles to control nano-scale defects, thereby solving the above-mentioned performance degradation problem in terms of long-term life occurring in the positive electrode active material in the form of secondary particles. That is, referring to Figure 2 , when the pretreatment according to the second step of the present invention to be described later is performed, it can be confirmed that the secondary particles are well maintained, thereby minimizing the formation of pores within the particles in the synthesis process to enable minimization of atomic defects and nano defects.

[0072] To this end, the present invention first determines the temperature at which the precursor mixture begins to react and form an intermediate phase, which can trigger rapid changes in atomic defects that can cause nano-defects, and then confirms that sufficient formation of a uniform intermediate phase based on the above temperature can suppress rapid changes at the atomic scale.

[0073] More specifically, refer to Figure 3 Thermogravimetric analysis (TGA) shows that the precursor mixture has a maximum weight loss between about 200°C and 250°C, a relatively small weight loss between about 250°C and 400°C, and the reaction begins to occur at about 200°C.

[0074] In addition, refer to Figure 4a When the phase fraction of the product was compared by Rietveld analysis of the X-ray diffraction (XRD) pattern, although the precursor mixture still existed after the temperature reached 250°C from room temperature, when sintered at 250°C for 6 hours, all the precursor mixture was decomposed and formed an intermediate phase of about 80% by weight or more. The results show that the reaction of the precursor mixture started at 250°C, similar to the TGA results. In addition, referring to Figure 4b The Rietveld analysis results of the neutron diffraction (ND) spectrum showed that the composition of the intermediate product was Li x TM 2-x O 2 (x=~0.4).

[0075] Similarly, refer to Figure 5 and Figure 6 By analyzing the phase fraction of the intermediate product in the synthesis process of the precursor mixture by X-ray diffraction (XRD) pattern analysis, it can be confirmed that the precursor mixture immediately starts to react after reaching 250°C without a single sintering, and all transition metal precursors are decomposed at 400°C, and most of the reactions are completed. In addition, unlike the case where the synthesis temperature is synthesized to 780°C without a single sintering, when the synthesis temperature is synthesized to 780°C after a single sintering at 250°C for 6 hours, it can be seen that the change in lattice constant along the c-axis is alleviated, which confirms that rapid changes on the atomic scale can be suppressed by sufficiently forming a uniform intermediate phase at 250°C, which is the time point when the reaction occurs.

[0076] Summarizing these results, it can be seen that the present invention forms 40 wt % to 80 wt % of the intermediate phase represented by the following Chemical Formula 2 relative to the total weight of the precursor mixture through the heat treatment in the second step, thereby inducing uniform growth of primary particles within secondary particles.

[0077] [Chemical formula 2]

[0078] Li x M 2-x O 2

[0079] Here, M is at least one selected from Co (cobalt), Ni (nickel), Al (aluminum), Mn (manganese) and Mg (magnesium), and x is a constant ranging from 0.2 to 0.8.

[0080] More specifically, refer to Figure 7 By comparing the bulk and surface oxidation numbers of the intermediate phase sample sintered at 250°C for 6 hours and the intermediate phase sample observed immediately after sintering at 400°C with the same phase fraction, it can be seen that the bulk oxidation numbers of the two samples are the same, but in terms of surface oxidation numbers, the NiL 3 -edge has higher photon energy and thus has a higher surface oxidation number than the sample sintered at 250°C for 6 hours. Figure 8 , through XANES analysis and transmission electron microscopy analysis, it can be found that the nickel oxidation number distribution of the secondary particles in the sample sintered at 250°C for 6 hours and the sample observed immediately after 400°C is as follows, that is, the nickel oxidation number in the secondary particles of the sample observed immediately after 400°C gradually increases from the core to the surface, while the sample sintered at 250°C for 6 hours has a uniform nickel oxidation number as a whole.

[0081] As described above, the present invention can be final synthesized by pre-treating at a temperature at which the precursor mixture starts to react, thereby inducing particle growth through a uniform reaction of primary particles within secondary particles, so that nano-scale defects can be controlled. To this end, the pre-treatment of the second step is performed at 200°C to 400°C for 2 hours to 8 hours, more preferably, at 230°C to 350°C for 2 hours to 6 hours. At this time, if the execution temperature of the second step is less than 200°C, then Figure 4a As shown, the formed Li x M 2-x O 2 The content of the intermediate phase is less than 10 wt %, so there may be a problem that nano defects cannot be effectively controlled. If the execution temperature of the second step exceeds 400°C, it will cause rapid changes in atomic defects during the sintering process, resulting in the formation of nano defects. In addition, if the execution time of the second step is less than 2 hours, the Li x M 2-x O 2 The intermediate phase content is less than 50% by weight, so there may be a problem of reduced nano-defect control effect. If the execution time of the second step exceeds 8 hours, it may be disadvantageous in terms of process operation.

[0082] At this time, the transition metal source of the metal hydroxide in the precursor mixture may be at least one compound selected from oxides or hydroxides of Ni, Co, Mn, Al, Mg and V, and the lithium precursor in the precursor mixture 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 elements may be used.

[0083] In addition, according to one embodiment of the present invention, the metal hydroxide and the lithium precursor may be mixed in a molar ratio of 1:0.9 to 1.1. At this time, if the molar ratio of the metal hydroxide to the lithium precursor is less than 1:0.9, the formation of NiO with a rock salt structure in the positive electrode active material may lead to a problem of electrochemical performance degradation. In addition, if the molar ratio of the metal hydroxide to the lithium precursor is greater than 1:1.1, a large amount of Li 2 CO 3 Or the problem of residual lithium compounds such as LiOH.

[0084] On the other hand, in the present invention, in order to induce more uniform synthesis of primary particles within secondary particles, the lithium precursor may further include a molten salt to perform the above-mentioned first step and second step.

[0085] More specifically, refer to Fig. 9 When the lithium precursor also contains a molten salt, by analyzing the XRD and ND spectra of the precursor mixture during the synthesis process to compare the phase fraction of the product, it can be confirmed that it has a lower melting point than when only LiOH is used, so the reaction starts at 200°C, which is lower than when only LiOH is used. As a result, all the precursor mixture is decomposed after sintering at 200°C for 6 hours, and about 80% of the intermediate phase is formed. This means that faster lithiation can be induced compared to the existing solid phase synthesis using only LiOH, so that rapid lithium diffusion during the synthesis process can be achieved.

[0086] Similarly, refer to Fig.10 By comparing the cross-sections of the secondary particles finally synthesized when synthesized using LiOH as a lithium precursor and when synthesized by further including a molten salt in the lithium precursor, it can be confirmed that secondary particles with a higher density close to zero pores are formed when synthesized by further including a molten salt in the lithium precursor. It can be seen that when a positive electrode active material is synthesized by adding a molten salt to a lithium precursor, a positive electrode active material with close to zero nano defects can be prepared.

[0087] To this end, the molten salt may be selected from LiNO 3 , Li 2 SO 4 , Li2 CO 3 、 at least one of the group consisting of LiCl, LiI and LiBr, and more preferably, it may be LiNO 3 or Li 2 SO 4 。 In this case, before the transition metal compound decomposes while releasing gas, Li containing a large amount of lithium is effectively generated at a lower temperature x M 2-x O 2 (x > 0.6) mesophase, so it is more conducive to reducing nano-defects during the sintering process.

[0088] In addition, the lithium precursor can be prepared by mixing any one of LiOH and LiOH·H 2 O and the above molten salt at a ratio of 1:0.5 to 2, and more preferably, at a molar ratio of 1:0.9 to 1.05. At this time, if the above lithium precursor and the above molten salt are mixed at a molar ratio less than 1:0.5 or greater than 1:2, there may be a problem of a higher secondary sintering temperature that requires exceeding the eutectic temperature of the molten salt.

[0089] Next, the third step of the method for preparing the positive electrode active material according to the present invention is to sinter the precursor mixture that has undergone the above heat treatment to prepare LMO x wherein, the above L is Li (lithium), M is at least one selected from Co (cobalt), Ni (nickel), Al (aluminum), Mn (manganese) and Mg (magnesium), and x is a constant from 0.5 to 2.5.

[0090] The above third step meets the purpose of the present invention, and known conventional methods and conditions for preparing the positive electrode active material by sintering the precursor mixture that has undergone the above first step and second step can be used, so it is not particularly limited. Preferably, the above third step is carried out at 600 °C to 900 °C for 6 hours to 16 hours, so that the final positive electrode active material can be prepared.

[0091] According to the preferred embodiment described above, the above LMO x can obtain a compound represented by the following Chemical Formula 1.

[0092] [Chemical Formula 1]

[0093] LiNi a Co b Mn c O 2

[0094] Among them, a, b, and c are constants of 0.8 ≤ a < 1.0, 0 < b < 0.1, 0 < c < 0.1, respectively, and a + b + c = 1.

[0095] At this time, if the execution temperature of the third step is lower than 600°C, Figure 5 As shown, there is a problem that a pseudo-layered structure phase in which lithium and transition metal are partially disordered is formed instead of a layered positive electrode material synthesized by separating the lithium layer and the transition metal layer. If the execution temperature of the third step exceeds 900°C, there may be a problem of forming a NiO phase with a rock salt structure. In addition, if the execution time of the third step is less than 4 hours, there may be a problem of disordered arrangement of lithium and transition metal in the crystal structure in the interlayer part, and if the execution time of the third step is more than 16 hours, lithium sublimation may occur, resulting in the synthesis of some lithium-deficient crystal structures.

[0096] Positive electrode active material

[0097] Next, the cathode active material according to the present invention will be described. However, in order to avoid repetition, the description of the part having the same technical concept as the method for preparing the cathode active material will be omitted.

[0098] The positive active material according to the present invention is a positive active material in the form of secondary particles with minimized atomic and nano-scale defects, and may be a compound represented by the following Chemical Formula 3, or according to a more preferred embodiment, may be a compound represented by the following Chemical Formula 4.

[0099] [Chemical formula 3]

[0100] LMO x

[0101] Wherein, the above L is Li (lithium), M is at least one selected from Co (cobalt), Ni (nickel), Al (aluminum), Mn (manganese) and Mg (magnesium), and x is a constant ranging from 0.5 to 2.5.

[0102] [Chemical formula 4]

[0103] LiNi a Co b Mn c O 2

[0104] At this time, the purpose of the present invention is to control nano-scale defects by inducing particle growth through uniform reaction of primary particles within secondary particles by pre-treating at a temperature at which the metal hydroxide and the lithium precursor begin to react, and then performing final synthesis. The above-mentioned positive electrode active material may include: a first pore having a size smaller than The average radius of the second pore is formed between the particles; and the second pore has a radius smaller than The average radius is formed within the particles.

[0105] More specifically, the average radius of the second pores formed in the particles is preferably smaller than More preferably less than Most preferably less than The average radius of .

[0106] At this time, the first pore (macropore) may refer to a pore with a radius of The above pores (pores), the above second pores (mesopores) may refer to the pores with a radius of to The third pore (micropore) can refer to the pores with a radius of The following pores.

[0107] During sintering, as the reaction proceeds, micropores are formed due to reasons such as precursor separation. At this time, as the sintering temperature gradually increases, the micropores expand and merge to form mesopores. At this time, the micropores and mesopores are mainly present inside the primary particles. Thereafter, as the sintering temperature further increases, some of the micropores and mesopores originally present inside the primary particles move to the particle surface and merge, and the pores formed on the surface due to this movement merge with the pores on the surface of the adjacent particles, thereby forming macropores as inter-particle pores.

[0108] At this time, since micropores and mesopores exist inside the particles, they can become nanoscale defects in the primary particles. The presence of these defects may lead to additional exposure of the interface and side reactions with the electrolyte, ultimately resulting in a shortened service life. Therefore, the smaller the average radius and distribution value of micropores and mesopores, the more beneficial it is for maintaining battery life and preventing degradation. Therefore, if the average radius of the second pore corresponding to the mesopore is If the second pore size is 0.0009 cm, the battery capacity may decrease rapidly due to the high defect level, which may be disadvantageous in terms of battery performance. 3 / g or more, it may be disadvantageous in terms of battery performance for the same reason.

[0109] In the case of macropores as interparticle pores, if the macropores are contained in the positive electrode active material at a content lower than a certain level, they can play a buffering effect during battery operation, which is beneficial in terms of battery performance. However, if the macropores are contained in the positive electrode active material at a content higher than a certain level, it will also have an adverse effect on battery performance. Therefore, if the average radius of the first pore is If the first pore size is 0.013 cm, the battery capacity may decrease rapidly due to the high defect level, which may be disadvantageous in terms of battery performance. 3 / g or more, then for the same reason, it may be disadvantageous in terms of battery performance such as a rapid decrease in battery capacity. However, if the distribution of the first pores is less than 0.013 cm 3 / g, then the degree of uniformity of the distribution becomes a more important factor than the distribution value. The more uniform the distribution, the smaller the risk of structural collapse, and a stable buffering effect can be shown, which is also advantageous in terms of battery performance.

[0110] More specifically, referring to Fig.11 , it can be confirmed that when synthesized without primary sintering, rapid changes in the lattice structure and non-uniform reactions are caused, and many defects of several hundred nanometers are formed centered on the nuclei. The rapid lattice changes occurring at the atomic scale can continue to the nanoscale defects. In addition, it is known that forming a uniform intermediate phase can suppress rapid changes not only at the atomic scale but also at the nanoscale.

[0111] On the other hand, referring to the Figure 12 to Figure 14 comparative analysis of the size and distribution changes of the first, second, and third pores, it can be seen that due to the primary particles gradually growing during the synthesis process, the macropores decrease as the synthesis temperature increases. And regarding the distribution of the second and third pores, micropores start to appear from 300 °C after the start of the reaction, and the largest number of pores is observed at 400 °C when the reaction occurs sharply. At this time, it can be seen that the rapid increase in micropores is induced by the sharp reaction. In addition, starting from 500 °C, as the mesopores merge, the number of macropores increases, which is consistent with the Fig.10 many macropores observed in the SEM image of the particle cross-section. At this time, Fig.12 and Fig.13 show the results of the particle size distribution measured during the synthesis of the cathode active material without primary sintering.

[0112] In addition, in the case of final synthesis without primary sintering and in the case of synthesis after primary sintering at 200 °C to 400 °C, through the Figures 15 to 19 structural analysis results, when synthesized without primary sintering, many macropores and mesopores of the largest size are observed, pores inside and between particles are observed, and the unevenly distributed pores gather together and macropores are locally observed. On the contrary, in the case of synthesis after primary sintering, fewer macropores and mesopores of the smallest size are observed, and it is known that high-density secondary particles with uniformly distributed pores between small particles are formed. This means that when low-temperature primary sintering is carried out during the synthesis process, a slow and uniform reaction is triggered, and Li x M 2-x O 2 (0.2 < x < 0.8) intermediate phase is formed uniformly, and small-sized, uniform pores are formed, which continue until the final synthesis.

[0113] As described above, the present invention can induce particle growth to control nanoscale defects through a uniform reaction of primary particles within secondary particles constituting the positive electrode active material, can solve the problem of performance degradation in terms of long-term life, and can easily synthesize positive electrode active materials with high capacity and stability through a single synthesis process without special additives. Therefore, the use of lithium secondary batteries containing the positive electrode active materials according to the present invention in various industries can be greatly improved.

[0114] The present invention will be described in more detail by the following examples, but the following examples should not be construed as limiting the scope of the present invention but should be construed as helping to understand the present invention.

[0115] Example 1 - Preparation of positive electrode active material

[0116] 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 mixture sample. At this time, the precursor mixture is a transition metal compound: lithium compound = 1:1 to 1.03 molar ratio, at 400-500 mL / min O 2 The synthesis was carried out under the conditions of gas atmosphere and final synthesis temperature of 780°C. After the first sintering at 200°C for 6 hours, the second sintering was carried out at 780°C for 12 hours to produce LiNi as the final positive electrode active material. 0.92 Co 0.03 Mn 0.05 O 2 .

[0117] Examples 2 to 4 - Preparation of positive electrode active materials

[0118] A positive electrode active material was prepared in the same manner as in Example 1 except that the primary sintering temperature was changed as shown in Table 1 below.

[0119] Example 5 - Preparation of positive electrode active material

[0120] Using Ni 0.92 Co 0.03 Mn 0.05 (OH) 2 +LiOH·H 2 O:LiNO 3 or Li 2 SO 4 The precursor mixture samples of (2:3 ratio) were prepared in the same manner as in Example 1 except that the primary sintering temperature was changed as shown in Table 1 below.

[0121] Comparative Example 1 - Preparation of Cathode Active Material

[0122] A cathode active material was prepared in the same manner as in Example 1 above, except that a primary sintering was not performed as shown in Table 1 below.

[0123] Table 1

[0124]

[0125] Experimental Example 1 - Thermogravimetric Analysis

[0126] As Figure 3 shown, thermogravimetric analysis (TGA) was performed to set the primary sintering temperature for homogeneous synthesis. The results showed that the precursor mixture had the largest weight loss between approximately 200°C and 250°C, a relatively small weight loss between approximately 250°C and 400°C, and the reaction started to occur at approximately 200°C.

[0127] Experimental Example 2 - Rietveld Analysis of X-ray Diffraction (XRD) Patterns

[0128] Referring Figure 4a , during the synthesis of the precursor mixture, Rietveld analysis of the X-ray diffraction (XRD) patterns was performed to compare the phase fractions of the products. After reaching 250°C from room temperature, only Ni 0.92 Co 0.03 Mn 0.05 (OH) 2 and LiOH were present. After sintering at 250°C for 6 hours, all Ni 0.92 Co 0.03 Mn 0.05 (OH) 2 decomposed, and approximately 80 wt% of Li x M 2-x O 2 (0.2 < x < 0.8) R-3m intermediate phase was formed. After reaching 300°C, some of the precursor mixture had reacted, forming approximately 30 wt% of Li x M 2-x O 2 (0.2 < x < 0.8). After sintering at 300°C for 6 hours, the same phase fraction as in the case of sintering at 250°C for 6 hours was confirmed. It was verified that all transition metal precursors decomposed after reaching 400°C. As a result, similar to the TGA results, it was shown that the reaction of the precursor mixture started at 250°C.

[0129] Experimental Example 3 - Rietveld Analysis of Neutron Diffraction Patterns

[0130] Reference Figure 4b , in order to quantify the lithium content of the intermediate product, neutron diffraction patterns of the primary sintered intermediate phase sintered at 250 °C for 6 hours once and the intermediate phase at 400 °C with the same phase fraction were analyzed by the Rietveld analysis method. As a result, it was found that both phases had an R-3m layered block structure of Li x M 2-x O 2 (x = ~0.4).

[0131] Experimental Example 4 - Analysis of the Phase Fraction of the Intermediate Product

[0132] Reference Figure 5 , it was confirmed that when comparing the phase fractions of the intermediate products by analyzing the X-ray diffraction (XRD) patterns during the real-time synthesis process of the precursor mixture, similarly, after reaching 250 °C without primary sintering, the precursor mixture started to react, and when reaching 400 °C, all transition metal precursors decomposed and most of the reactions were completed.

[0133] Experimental Example 5 - Analysis of the Changes in Phase Fraction and Lattice Constant during Primary Sintering

[0134] As Figure 6 shown, the XRD patterns during the process of primary sintering in real time at 250 °C as the reaction temperature were further analyzed. After sintering at 250 °C for 3 hours, all transition metal precursors decomposed. Different from the synthesis temperature of 780 °C without primary sintering, when the synthesis temperature reached 780 °C after primary sintering at 250 °C for 6 hours, it was further confirmed that the change in the lattice constant along the c-axis was alleviated. This means that by fully forming a uniform Li x M 2-x O 2 (0.2 < x < 0.8) intermediate phase at 250 °C as the time point when the reaction occurs, the sharp change at the atomic scale can be suppressed.

[0135] Experimental Example 6 - XAS Analysis

[0136] Reference Figure 7 , in order to confirm the particle growth uniformity of the secondary particles before and after primary sintering, the oxidation numbers of the bulk and surface of the intermediate phase samples sintered at 250 °C for 6 hours and the intermediate phase samples at 400 °C with the same phase fraction were compared by hard / soft X-ray absorption spectroscopy (XAS) analysis. As a result, it was confirmed that the bulk oxidation numbers of the two samples were the same, but in the case of the surface oxidation number, the Ni L 3 -edge of the 400 °C sample had a higher photon energy and a higher surface oxidation number than the sample sintered at 250 °C for 6 hours.

[0137] Experimental Example 7 - XANES Analysis and Transmission Electron Microscopy Analysis

[0138] Reference Figure 8 , the nickel oxidation number distribution in the secondary particles of the sample sintered at 250 °C for 6 hours and the sample at 400 °C was confirmed by transmission X-ray microscopy (TXM)-X-ray absorption near edge structure (XANES) analysis and transmission electron microscopy (TEM)-electron energy loss spectroscopy (EELS) analysis. As can be seen from the figure, the oxidation number of nickel in the sample after reaching 400 °C gradually increases from the core to the surface, while the sample sintered at 250 °C for 6 hours has a uniform nickel oxidation number overall.

[0139] Experimental Example 8 - SEM Cross-Section Analysis

[0140] Reference Fig.11 , in the 400 °C intermediate phase without primary sintering, the larger the number of macropores observed towards the inside of the secondary particles, while the cross-section of the secondary particles of the intermediate phase sintered at 250 °C for the first time is very uniform. This indicates that when synthesizing without primary sintering, it will lead to a sharp change in the lattice structure and an inhomogeneous reaction, forming a large number of defects of several hundred nanometers centered on the nucleus. At the same time, it is also confirmed that the sharp lattice change observed in the previous diffraction analysis may continue to the nano-scale defects. In addition, this indicates that the formation of a uniform Li x M 2-x O 2 (0.2 < x < 0.8) intermediate phase can suppress sharp changes not only at the atomic scale but also at the nano-scale.

[0141] Experimental Example 9 - SAXS and SANS Analysis

[0142] Reference Figure 12 to Figure 14 , in order to quantitatively analyze the nano-pores of the secondary particles occurring during the synthesis process, X-ray and neutron small-angle scattering spectroscopy (SAXS and SANS) were first used to compare and analyze the pore size and distribution changes of the average nano-pores of the secondary particles. At this time, Fig.12 and Fig.13 show the results of the particle size distribution measured during the synthesis of the cathode active material without primary sintering. From room temperature to 500 °C, the size of the micropores gradually increases through the reaction of the precursor mixture, while from 600 °C to 780 °C, the size of the mesopores gradually increases with the merging of the micropores. Due to the gradually growing primary particles during the synthesis process, the macropores decrease with the increase of the synthesis temperature. In addition, regarding the pore distribution, micropores start to appear at 300 °C after the start of the reaction, and the largest number of pores is observed at 400 °C when the reaction proceeds rapidly. At this time, the sharp increase in macropores is caused by the rapid reaction, which is consistent with the previous Figure 8The results are consistent with the large number of macropores observed in the cross-sectional SEM images of the 400°C particles. In addition, it was confirmed that starting from 500°C, the macropores increased as the mesopores merged. When comparing the particle distribution of the sample after 400°C, where the largest macropores were formed, and a similar sample sintered at 250°C for 6 hours, the sample sintered at 250°C for 6 hours had a smaller and more uniform micropore distribution. This suggests that the low temperature heat treatment process can induce a uniform reaction within the secondary particles, despite the similar overall reaction extent of the two samples.

[0143] Experimental Example 10 - Radius and content distribution of the first pore and the second pore of the positive electrode active material

[0144] Tables 2 and 3 below show the radius and content distribution of the first pores and the second pores of the positive electrode active material prepared without primary sintering and the positive electrode active material prepared after primary sintering.

[0145] Table 2

[0146]

[0147] Table 3

[0148]

[0149] Experimental Example 11 – SEM cross section, XRD and ND pattern analysis

[0150] Reference Figures 15 to 19 , Figures 15 to 19 A comparison between the final phase samples synthesized without a single sintering and the final phase samples sintered once at 200 to 400 degrees is shown. The bulk structure analysis by XRD and ND analysis was similar for all samples, but the pore quantitative analysis by SAXS / SANS analysis showed differences. In the sample synthesized without a single sintering (P-NCM), many macropores and mesopores of the largest size were observed, while fewer macropores and mesopores of the smallest size were observed in the sample synthesized after a single sintering at 250 degrees (P250-NCM). This means that when a low-temperature single sintering is performed during the synthesis process, a slow and uniform reaction is induced, resulting in the uniform formation of Li x M 2-x O 2An intermediate phase (0.2 < x < 0.8) is formed, and small-sized, uniform pores are formed, which continue until the final synthesis. To confirm the pore information obtained by SAXS / SANS analysis, the TEM and SEM cross-sections of representative samples P-NCM and P250-NCM were compared. In P-NCM, pores were observed inside and between the particles, and large pores were locally observed due to the aggregation of unevenly distributed pores. In contrast, in P250-NCM, it was confirmed that high-density secondary particles with uniformly distributed small inter-particle pores were formed, which was consistent with the results of SAXS / SANS analysis.

[0151] Experimental Example 12 - Comparison of Electrochemical Properties

[0152] Reference Fig. 20 , the electrochemical properties of samples synthesized directly without primary sintering and samples sintered once at 200 to 400 degrees were compared. Except for P-NCM, all samples after primary sintering had improved electrochemical cycling characteristics, and it was confirmed that the electrochemical properties (cycling and capacity) of the most uniformly synthesized P250-NCM were improved the most.

[0153] Experimental Example 13 - SEM, XRD, and ND Pattern Analysis of Example 5

[0154] Reference Fig. 9 and Fig.10 , the phase fractions of the products were compared by analyzing the XRD and ND patterns during the synthesis of the eutectic precursor mixture. When using the eutectic molten salt, its melting point is lower than that when only using LiOH, so the reaction starts at a temperature of 200 degrees, which is lower than before. As a result, after sintering at 200 degrees for 6 hours, all Ni 0.92 Co 0.03 Mn 0.05 (OH) 2 decomposed to form approximately 80% of Li x M 2-x O 2 (0.6 < x < 0.8). This indicates that faster lithiation can be induced compared to the existing solid-phase synthesis using only LiOH, so the eutectic synthesis process enables rapid lithium diffusion. The cross-sections of the finally synthesized secondary particles were compared when using LiOH as the existing lithium precursor and when using the eutectic molten salt. The results confirmed that the PE200-NCM synthesized by the eutectic process had the highest density of secondary particles with nearly zero pores. This indicates that the eutectic synthesis process is closest to the method of synthesizing cathode active materials with nearly zero nano-defects.

[0155] Experimental Example 14 - Comparison of Electrochemical Properties of Example 5

[0156] Reference Fig.21 and Fig. 22, the electrochemical cycle characteristics of the three samples were compared. As shown in the left figure, there was no decrease in cycle characteristics when 50 cycles were performed at a rate of 0.15C, and in the long-term life test of 200 cycles, it showed better characteristics than the samples (P-NCM, P250-NCM) using LiOH as the existing lithium precursor. In addition, by observing the results of the cross-section of the secondary particles after 150 cycles, it was found that the P-NCM synthesized in the existing synthesis process had sharp cracks in the secondary particles due to the penetration of the electrolyte between the macropores formed during the synthesis process and the uneven shrinkage / expansion during the charge / discharge process, while the secondary particles of the P250-NCM synthesized after one sintering and the PE200-NCM synthesized by the eutectic process remained as good as before, proving that minimizing the formation of pores within the particles during the synthesis process is an important factor in synthesizing stable positive active materials.

Claims

1. A method for preparing a positive electrode active material, characterized in that: Comprising: The first step of forming a precursor mixture by mixing a metal hydroxide and a lithium precursor; The second step of heat-treating the above precursor mixture; And The third step is to prepare LMO by sintering the precursor mixture after the above heat treatment. x , Wherein, L is lithium, M is at least one selected from cobalt, nickel, aluminum, manganese, and magnesium, and x is a constant from 0.5 to 2.

5.

2. The method for preparing a cathode active material according to claim 1, characterized in that The above LMO x It is a compound represented by the following chemical formula 1: [Chemical formula 1] LiNi a Co b Mr c O2 Wherein, a, b, and c are constants of 0.8 ≤ a < 1.0, 0 < b < 0.1, and 0 < c < 0.1 respectively.

3. The method for preparing a cathode active material according to claim 1, characterized in that The lithium precursor in the first step further contains a molten salt.

4. The method for preparing a cathode active material according to claim 1, characterized in that Through the heat treatment in the second step, an intermediate phase represented by the following chemical formula 2 is formed, inducing uniform growth of primary particles within secondary particles: [Chemical formula 2] <h2 style=";text-align:left;direction:ltr">Li<h2 style=";text-align:left;direction:ltr"> x <h2 style=";text-align:left;direction:ltr"> M<h2 style=";text-align:left;direction:ltr"> 2-x <h2 style=";text-align:left;direction:ltr"> O2 Wherein, M is at least one selected from cobalt, nickel, aluminum, manganese, and magnesium, and x is a constant from 0.2 to 0.

8.

5. The method for preparing a cathode active material according to claim 1, characterized in that The heat treatment in the second step is carried out at 200 °C to 400 °C for 2 hours to 8 hours.

6. The method for preparing a cathode active material according to claim 1, characterized in that The heat treatment in the second step is carried out at 230 °C to 350 °C for 2 hours to 6 hours.

7. The method for preparing a cathode active material according to claim 1, characterized in that The metal hydroxide and the lithium precursor in the first step are mixed at a molar ratio of 1:0.9 to 1.

1.

8. The method for preparing a cathode active material according to claim 1, characterized in that The third step is carried out at 600 °C to 900 °C for 4 hours to 16 hours.

9. The method for preparing a cathode active material according to claim 3, characterized in that The molten salt is at least one selected from the group consisting of LiNO3, Li2SO4, Li2CO3, LiCl, LiI, and LiBr, and the lithium precursor is formed by mixing any one of LiOH and LiOH·H2O and the molten salt at a molar ratio of 1:0.5 to 2.

10. The method for preparing a positive electrode active material according to claim 4, characterized in that: Relative to the total weight of the precursor mixture, the content of the intermediate phase is 40 wt% to 80 wt%.

11. A cathode active material, which is a cathode active material in the form of secondary particles with minimized atomic and nanoscale defects. The cathode active material is characterized by being represented by the following chemical formula 3 and comprising: The first pore has a size smaller than The average radius of the particles is formed between them; And The second pore has a size smaller than The average radius of the particles is formed inside the particles. [Chemical formula 3] LMO x Wherein, L is lithium, M is at least one selected from cobalt, nickel, aluminum, manganese, and magnesium, and x is a constant from 0.5 to 2.

5.

12. The cathode active material according to claim 11, characterized in that The cathode active material is a compound represented by the following chemical formula 4: [Chemical formula 4] LiNi a Co b Mr c O2 Wherein, a, b, and c are constants of 0.8 ≤ a < 1.0, 0 < b < 0.1, and 0 < c < 0.1 respectively.

13. The cathode active material according to claim 11, characterized in that The first pore has a size less than 0.013 cm 3 / g distribution, the second pore size is less than 0.0009cm 3 / g distribution.

14. The positive electrode active material according to claim 13, characterized in that The second pore size is less than 0.0007 cm 3 / g distribution.

15. A lithium secondary battery, characterized in that: The positive electrode active material according to claim 11 is included.

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