Positive electrode active material and lithium battery including the same

Abstract

The positive electrode active material for a lithium secondary battery according to the present invention comprises NCM particles, wherein the NCM particles are single-crystal particles; and a coating layer surrounding the surface of the NCM particles, wherein the average particle size of the NCM particles is 2 μm to 8 μm, the coating layer comprises a lithium-manganese composite, and the weight ratio of the coating layer is 0.5 wt% to 2.5 wt% with respect to the NCM particles.

Description

Technology Field

[0001] The present invention relates to a lithium battery, and more specifically, to a positive electrode active material of a lithium battery. Background Technology

[0002] As the demand for mobile devices expands, the demand for conventional lithium-ion batteries continues to rise. Lithium-ion batteries are utilized as representative rechargeable batteries due to their high energy density per unit weight, ability to operate at relatively high voltages, and excellent charge-discharge efficiency.

[0003] Recently, as the demand for lithium-ion batteries increases, the importance of the cathode, which determines the capacity of lithium-ion batteries, is growing. Therefore, research on cathode active materials is continuing to manufacture lithium-ion batteries that provide high reversible capacity. The problem to be solved

[0004] The technical problem to be solved by the present invention is to provide a positive electrode active material having improved thermal safety characteristics and electrochemical characteristics, and a lithium secondary battery including the same.

[0005] Another technical problem that the present invention aims to solve is to provide a lithium secondary battery having improved electrochemical characteristics. means of solving the problem

[0006] A positive electrode active material according to the concept of the present invention comprises NCM particles, wherein the NCM particles are single-crystal particles; and a coating layer surrounding the surface of the NCM particles, wherein the average particle size of the NCM particles is 2 μm to 8 μm, the coating layer comprises a lithium-manganese composite, and the weight ratio of the coating layer is 0.5 wt% to 2.5 wt% with respect to the NCM particles.

[0007] A method for manufacturing a positive electrode active material according to another concept of the present invention comprises: manufacturing a first powder containing NCM particles; manufacturing a coating solution by dispersing a transition metal oxide in a solvent; manufacturing a mixture by mixing the first powder with the coating solution; drying the mixture to evaporate the solvent; and performing a heat treatment process on the mixture to obtain a positive electrode active material, wherein the weight ratio of the coating solution is 0.5 wt% to 2.5 wt% with respect to the first powder. Effects of the invention

[0008] The positive electrode active material for a lithium secondary battery according to the present invention may include alloy particles and a coating layer surrounding the alloy particles. The alloy particles are single-crystal particles containing nickel and lithium, and the coating layer may include a lithium-manganese composite. The coating layer is provided on the surface of the alloy particles to prevent structural collapse and cracking of the alloy particles that occur during charging and discharging of the lithium secondary battery. As a result, the lifespan characteristics and electrical characteristics of the lithium secondary battery can be improved. Brief explanation of the drawing

[0009] FIG. 1 is a cross-sectional view of a lithium secondary battery according to an embodiment of the present invention. Figure 2 is an enlarged view according to the M region of Figure 1. FIG. 3a is a diagram showing a positive active material according to an embodiment of the present invention. FIG. 3b is a drawing showing a precursor according to an embodiment of the present invention. FIG. 4 is a diagram illustrating a method for manufacturing a positive electrode active material of the present invention. Figure 5a shows the FE-SEM scanning electron microscope (FE-SEM) analysis results according to each of the embodiments and comparative examples of the present invention. FIG. 5b shows the TEM-EDS analysis results according to each of the embodiments and comparative examples of the present invention. Figures 6a and 6b are XRD analysis results of the positive electrode active material according to the present invention. Figure 7 is a TEM-EDS analysis result according to an embodiment of the present invention. Figure 8 is a graph showing the distribution of elements according to an embodiment of the present invention. Figure 9 is an HR-TEM image result according to an embodiment of the present invention. FIGS. 10a and FIGS. 10b are X-ray photoelectron spectroscopy (XPS) analysis results according to each of the embodiments and comparative examples of the present invention. FIG. 11 is a graph showing the amount of residual lithium in the positive electrode active material according to each of the embodiments and comparative examples of the present invention. FIGS. 12a and FIGS. 13a are graphs showing the voltage profile of the first cycle in a lithium secondary battery according to each of the embodiments and comparative examples of the present invention. FIGS. 12b and FIGS. 13b are graphs showing the cycle performance of a lithium secondary battery according to each of the embodiments and comparative examples of the present invention. FIGS. 14a to 14c are HR-TEM image results after a cycle according to each of the embodiments and comparative examples of the present invention. Specific details for implementing the invention

[0010] In order to fully understand the structure and effects of the present invention, preferred embodiments of the present invention are described with reference to the attached drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various forms and various modifications can be made. The description of these embodiments is provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention.

[0011] In this specification, when a component is described as being on another component, it means that it may be formed directly on the other component or that a third component may be interposed between them. Additionally, in the drawings, the thicknesses of the components are exaggerated for the effective description of the technical content. Throughout the specification, parts indicated by the same reference numeral represent the same components.

[0012] The embodiments described herein will be described with reference to cross-sectional and / or plan views, which are exemplary illustrations of the invention. In the drawings, the thicknesses of films and regions are exaggerated for effective description of the technical content. Accordingly, the regions illustrated in the drawings are schematic in nature, and the shapes of the regions illustrated in the drawings are intended to illustrate specific forms of regions of the device and are not intended to limit the scope of the invention. Although terms such as first, second, third, etc., have been used to describe various components in the various embodiments of this specification, these components should not be limited by such terms. These terms are used merely to distinguish one component from another. The embodiments described and illustrated herein also include their complementary embodiments.

[0013] The terms used herein are for describing the embodiments and are not intended to limit the invention. In this specification, the singular form includes the plural form unless specifically stated otherwise in the text. As used herein, 'comprises' and / or 'comprising' do not exclude the presence or addition of one or more other components to the mentioned components.

[0015] FIG. 1 is a conceptual diagram briefly illustrating a lithium secondary battery according to embodiments of the present invention. Referring to FIG. 1, the lithium secondary battery may include a negative electrode (10), a positive electrode (20), an electrolyte (30), and a separator (40).

[0016] The cathode (10) and the anode (20) may be spaced apart from each other with a separator (40) in between. The cathode and the anode (20) may be positioned to face each other with the separator (40) in between. The cathode (10), the anode (20), and the separator (40) may come into contact with the electrolyte (30).

[0017] The electrolyte (30) may be a medium for transferring lithium ions between the negative electrode (10) and the positive electrode (20). Within the electrolyte (30), the lithium ions may pass through the separator (40) and move toward the negative electrode (10) or the positive electrode (20).

[0018] The positive electrode (20) may include a positive electrode active material that is a source of lithium ions. The positive electrode active material according to the present invention may include lithium composite oxide (LiNiCoMnO) particles containing a high amount of nickel. In the present invention, the particles are referred to as NCM particles (AP).

[0019] The NCM particles (AP) of the present invention can improve battery capacity by having a high Ni content. The NCM particles (AP) of the present invention can increase the energy density of the battery, making it economical.

[0020] Generally, in ternary cathode materials composed of Ni, Co, and Mn, an increase in Ni content leads to structural collapse, resulting in reduced structural stability. Furthermore, volume changes during cycling cause cracking, which leads to a rapid decrease in capacity. To address this issue, single-particle NCM particles can be used. However, due to the large particle size of single-particle NCM particles, structural collapse occurs within the particles.

[0021] To solve the above problems, the present invention uses single-crystal NCM particles, and a coating layer can be formed on the surface of the NCM particles. The coating layer and the NCM particles can have high lattice compatibility. By doing so, the resistance of the lithium secondary battery can be reduced, thereby improving reversible capacity and rate characteristics. By modifying the surface of the NCM particles through the coating layer, volume changes of the NCM particles can be mitigated.

[0022] A positive electrode (20) comprising a positive electrode active material according to embodiments of the present invention will be described in more detail. The positive electrode (20) may include a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector. The positive electrode active material layer may include the positive electrode active material of the present invention, namely NCM particles to be described later.

[0023] The above positive current collector may include a conductive material that does not cause chemical changes in the battery. For example, the above positive current collector may include stainless steel, aluminum, nickel, titanium, or calcined carbon.

[0024] The anode current collector may have a thickness of 3 μm to 500 μm. The anode current collector may have a surface with an uneven shape, thereby increasing the adhesion of the anode active material. The anode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, or nonwoven fabric.

[0025] The above positive active material layer may further include a conductive material and a binder in addition to the positive active material of the present invention. The positive active material may be included in an amount of 80 wt% to 99 wt%, more specifically 85 wt% to 98 wt%, based on the total weight of the positive active material layer.

[0026] The conductive material can impart conductivity to the positive active material layer. The conductive material may include at least one of a carbon-based material (e.g., graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber), metal powder, metal fiber, conductive whisker, conductive metal oxide, conductive polymer, and combinations thereof. The conductive material may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the positive active material layer.

[0027] The binder may include at least one of polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, and combinations thereof. The binder may be included in an amount of 1 wt% to 30 wt% with respect to the total weight of the anode active material layer.

[0028] The anode (20) can be manufactured according to a conventional anode manufacturing method, except that the anode active material of the present invention is used. Specifically, a mixture can be prepared by dissolving or dispersing the anode active material, binder, and conductive material of the present invention in a solvent. The anode (20) can be manufactured by applying the mixture onto an anode current collector, followed by drying and rolling.

[0029] The above solvent may be a solvent commonly used in the relevant technical field, and may include, for example, at least one of dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, and combinations thereof.

[0030] In another embodiment, the mixture can be cast onto a separate support to produce a film. An anode (20) can be produced by laminating the film onto an anode current collector.

[0032] FIG. 2 is an enlarged view of region M of FIG. 1. FIG. 3a is a diagram showing a positive electrode active material according to the present invention. With reference to FIG. 2 and FIG. 3a, the positive electrode active material will be described in more detail.

[0033] Referring to FIGS. 2 and FIGS. 3a, the positive active material layer (12) may include a positive active material (120). The positive active material (120) may include a coating layer (COL) on NCM particles (AP). In this case, the NCM particles (AP) may be single-crystal particles. The NCM particles (AP) may contain a high content of nickel. More specifically, the NCM particles (AP) may have a ratio of Ni atoms among transition metal atoms composed of Ni, Co, and Mn of 89 at% to 95 at%.

[0034] The average particle size (DM1) of the NCM particles (AP) can be 2 μm to 8 μm. The NCM particles (AP) can be single particles rather than multiply particles. Thus, the NCM particles (AP) can have a larger particle size than that of the oxide in the form of multiply particles. Thus, the single-crystal NCM particles (AP) can have a longer lithium ion transport pathway than the polycrystalline NCM particles.

[0035] The surface of the NCM particle (AP) can be surrounded by a coating layer (COL). The average thickness (TH1) of the coating layer (COL) may be 10 nm to 20 nm. The coating layer (COL) may include lithium and transition metals. As an example, the coating layer (COL) may include a lithium-manganese complex. The lithium in the coating layer (COL) may be derived from residual lithium on the surface of the NCM particle (AP) during the manufacturing process described later. Therefore, the amount of residual lithium on the surface of the NCM particle (AP) may be reduced by forming the coating layer (COL).

[0036] The transition metal of the coating layer (COL) may include at least one of copper (Cu), silver (Ag), gold (Au), nickel (Ni), chromium (Cr), manganese (Mn), cobalt (Co), zinc (Zn), and titanium (Ti). For example, the coating layer (COL) may be Li2MnO3. By coating the NCM particles (AP) with the coating layer (COL), the thermal stability, strength, and durability of the positive electrode active material (120) can be further improved.

[0037] The manganese (Mn) content in the coating layer (COL) may be greater than the manganese (Mn) content in the NCM particles (AP). Thus, the concentration of manganese (Mn) may increase as it moves closer to the coating layer (COL) from the NCM particles (AP). The manganese (Mn) content in the NCM particles (AP) may be smaller than the nickel (Ni) content in the NCM particles (AP). On the other hand, the manganese (Mn) content in the coating layer (COL) may be greater than the nickel (Ni) content in the coating layer (COL).

[0038] The weight ratio of the coating layer (COL) may be 0.5 wt% to 2.5 wt% relative to the NCM particles (AP). If the weight ratio of the coating layer (COL) is smaller than the above range, surface modification of the NCM particles (AP) does not occur, and structural collapse of the NCM particles (AP) may occur during charging and discharging of the lithium secondary battery. If the weight ratio of the coating layer (COL) is larger than the above range, a thick coating layer (COL) is formed, which may instead reduce the capacity of the lithium secondary battery.

[0039] Each of the NCM particles (AP) and the coating layer (COL) can have a layered structure. Since the NCM particles (AP) and the coating layer (COL) have similar structures, the transport of lithium ions can be accelerated.

[0040] According to the present invention, a coating layer (COL) is provided on the surface of an NCM particle (AP) to prevent structural collapse and cracking of the positive active material (120). In addition, since the coating layer (COL) is formed using residual lithium on the surface of the NCM particle (AP), adverse effects caused by residual lithium of the NCM particle (AP) can be prevented. As a result, the lifespan characteristics and electrical characteristics of the lithium secondary battery can be improved.

[0042] FIG. 3b is a diagram showing a precursor according to an embodiment of the present invention. FIG. 4 is a schematic diagram for explaining a method for manufacturing a positive electrode active material according to the present invention. With reference to FIG. 3b and FIG. 4, a method for manufacturing a positive electrode active material according to an embodiment of the present invention will be described in more detail. Technical features that overlap with FIG. 1 to FIG. 3a described above are omitted.

[0043] Referring to FIG. 3b and FIG. 4, a method for manufacturing a positive electrode active material according to the present invention may include manufacturing a first powder containing NCM particles (AP) (S1), manufacturing a coating solution by dispersing a transition metal oxide in a solvent (S2), manufacturing a mixture by mixing the first powder with the coating solution (S3), drying the mixture to evaporate the solvent (S4), and obtaining a positive electrode active material by performing a heat treatment process on the mixture (S5).

[0044] Manufacturing the first powder (S1) may include manufacturing a precursor (121) containing single-crystal NCM particles (AP). Manufacturing the precursor (121) may include manufacturing single-crystal NCM particles through molten salt synthesis (MSS), grinding the NCM particles and washing them with distilled water and ethanol, and drying the NCM particles. The precursor (121) obtained as described above may include NCM particles (AP) and residual lithium (RL) on the surface of the NCM particles.

[0045] Preparing the coating solution (S2) may include dispersing an amorphous transition metal oxide (MO) in a solvent. The transition metal oxide (MO) may exist in the solvent in a colloidal form. For example, the coating solution may be prepared by dispersing manganese oxide (MnO2) in ethanol (solvent). In the solvent, the transition metal oxide (MO) may have a micelle structure.

[0046] The process of preparing the mixture (S3) may include adding and dispersing the first powder into the coating solution. At this time, the ratio of the content of the transition metal oxide (MO) in the coating solution to the content of the NCM particles (AP) in the first powder may be 0.5:100 to 2.5:100.

[0047] By mixing the coating solution and the first powder, a coating layer (COL) can be coated on the surface of the NCM particle (AP). Referring to S4 in FIG. 4, drying the mixture (S4) can be performed at a temperature of 70°C to 100°C. At this time, as the solvent evaporates, the transition metal oxide (MO) of the coating solution can be attached to the surface of the NCM particle (AP) to form a preliminary coating layer (PCOL). Specifically, due to van der Waals forces between transition metal oxide (MO) micelles, the transition metal oxide (MO) can be attached to the surface of the NCM particle (AP). The preliminary coating layer (PCOL) can surround the surface of the NCM particle (AP).

[0048] A positive electrode active material (120) can be obtained by performing a heat treatment process (S5) on the mixture from which the solvent has been evaporated. The heat treatment process can be performed at a temperature of 600°C to 900°C. The heat treatment process can be performed for 2 to 10 hours.

[0049] Through the above heat treatment process, residual lithium (RL) of the pre-coating layer (PCOL) and the NCM particles (AP) can react to form a coating layer (COL). As a result, the lithium in the coating layer (COL) may originate from the NCM particles (AP).

[0050] Referring again to FIG. 4, the coating layer (COL) can be formed by evaporation-induced self-assembly (EISA). Specifically, by drying the mixture to evaporate the solvent, a transition metal oxide (MO) is attached to the NCM particles (AP), and the attached transition metal oxide (MO) reacts with residual lithium (RL) to self-assemble the coating layer (COL).

[0051] Accordingly, the method for manufacturing a positive electrode active material according to the present invention can form a coating layer (COL) simply and efficiently without additional processes such as a long-duration ultrasonic process. As a result, the method for manufacturing a positive electrode active material according to the present invention can save costs and time.

[0053] Examples and comparative examples of the present invention are described below. However, the following examples are merely one example of the present invention, and the present invention is not limited to the following examples.

[0055] [Example 1]

[0056] 1. Powder manufacturing

[0057] Ni 0.9 Co 0.05 Mn 0.05 A molten salt synthesis method using (OH)2 was performed to obtain single-crystal LiNi 0.9 Co 0.05 Mn 0.05 O2 (NCM particles) were prepared. The synthesized NCM particles were washed twice each with distilled water and ethanol. Afterward, NCM powder was prepared by drying in a vacuum oven at 80°C for 12 hours.

[0059] 2. Preparation of coating solution

[0060] (1) Preparation of amorphous manganese oxide (MnO2)

[0061] 41g of KMnO was dissolved in 10ml of distilled water, and 100ml of ethyl alcohol was added and stirred for 2 hours to prepare a colloidal solution. The colloidal solution was centrifuged at 5000rpm for 10 minutes to obtain amorphous manganese oxide (MnO2). The obtained amorphous manganese oxide (MnO2) was washed twice with distilled water and ethyl alcohol, and dried in a vacuum oven at 80°C for 12 hours to obtain manganese oxide (MnO2) powder.

[0062] (2) Dispersed in a solvent

[0063] The above manganese oxide (MnO2) powder was added to 25 ml of ethanol, and sonication was performed at 40 kHz for 20 minutes.

[0065] 3. Preparation of the mixture

[0066] Subsequently, the above NCM powder was mixed with the above coating solution to prepare a mixture. At this time, the ratio of the content of manganese oxide (MnO2) to the content of NCM particles of the first powder was 1:100.

[0068] 4. Drying and Heat Treatment Processes

[0069] The above mixture was dried by stirring at a temperature of 80°C for 4 hours. Subsequently, a heat treatment process was performed on the dried mixture at a temperature of 750°C for 5 hours. Thus, the positive electrode active material according to the present invention was prepared.

[0071] [Comparative Example 1]

[0072] The coating solution was omitted, and the positive active material was prepared using NCM particles. Except for this, it is identical to Example 1.

[0074] [Comparative Example 2]

[0075] A positive electrode active material was prepared by setting the ratio of the content of manganese oxide to the content of NCM particles (AP) of the first powder to 3:100. Except for this, it is identical to Example 1.

[0077] Examples and comparative examples of the present invention manufactured as described above are shown in Table 1 below.

[0078] Content ratio Manganese oxide (MnO2) NCM particles Example 1 1 100 Comparative Example 1 0 100 Comparative Example 2 3 100

[0079] Evaluation Example 1: Front-Eye Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), and Energy Dispersive Spectroscopy (EDS) Analysis

[0080] To confirm the morphology and surface of the cathode active material particles prepared according to Example 1, Comparative Example 1, and Comparative Example 2, they were analyzed using a field-effect scanning electron microscope (FE-SEM), a transmission electron microscope (TEM), and an energy dispersive spectrometer (EDS). Figure 5a shows the FE-SEM scanning electron microscope (FE-SEM) analysis results for each of the examples and comparative examples of the present invention. Figure 5b shows the TEM-EDS analysis results for each of the examples and comparative examples of the present invention.

[0081] Referring to Fig. 5a, it was confirmed that the NCM particles are single particles. Additionally, it was confirmed that the average particle size of the NCM particles is 5 μm to 6 μm. Referring to Fig. 5b, it can be seen that manganese is distributed on the surface of the NCM particles in the case of Example 1 and Comparative Example 2. Thus, it can be confirmed that a coating layer is formed on the NCM particles in Example 1 and Comparative Example 2, thereby modifying the surface of the NCM particles. Furthermore, it can be seen that the coating layer of Comparative Example 2 is formed thicker than the coating layer of Example 1. Thus, it can be confirmed that as the content of manganese oxide in the coating solution increases, the coating layer is formed thicker and denser.

[0083] Evaluation Example 2: X-ray Diffraction (XRD) Analysis

[0084] X-ray diffraction (XRD) analysis was performed to confirm the crystal structure of the positive electrode active material prepared according to Example 1, Comparative Example 1, and Comparative Example 2. Figures 6a and 6b are the XRD analysis results of the positive electrode active material according to the present invention.

[0085] Referring to FIGS. 6a and 6b, the positive active materials according to Example 1, Comparative Example 1, and Comparative Example 2 all exhibit a hexagonal structure similar to NCM particles. Additionally, the ratio of the peak of the (003) plane to the peak of the (104) plane was 1.62 for Comparative Example 1, 1.69 for Example 1, and 1.71 for Comparative Example 2. That is, the ratio of the peak of the (003) plane to the peak of the (104) plane increased as the thickness of the coating layer increased. This indicates that the coating layer reduces the mixing phenomenon of nickel and lithium ions and prevents structural collapse within the NCM particles.

[0086] On the other hand, the thickness of the coating layer according to Example 1 and Comparative Example 2 was small, so it could not be confirmed by the XRD pattern. However, the peaks of the (110) and (108) planes and the peaks of the (102) plane and (006) plane were clearly separated, confirming that a crystalline layer structure was formed. In addition, since the position of the peak of the (003) plane was the same for Example 1, Comparative Example 2, and Comparative Example 1, it can be seen that the lattice constant of the NCM particles did not change due to the coating layer.

[0088] Evaluation Example 3: Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) Analysis

[0089] To confirm the shape and surface of the cathode active material particles prepared according to Example 1 in detail, they were analyzed using a transmission electron microscope (TEM) and an energy dispersive spectrometer (EDS). Figure 7 shows the TEM-EDS analysis results according to an embodiment of the present invention. Figure 8 is a graph showing the distribution of elements according to an embodiment of the present invention. Figure 9 is a High Resolution TEM (HR-TEM) image result according to an embodiment of the present invention. Specifically, a first direction (D1) may be formed from the coating layer into the interior of the NCM particle. Figure 8 is a graph showing the distribution of elements according to the first direction (D1).

[0090] Referring to FIGS. 7 and 8, in the case of Example 1, a coating layer was uniformly formed on the surface of the NCM particles. At this time, the thickness of the coating layer was 15 nm. The manganese content in the coating layer was higher than the manganese content in the NCM particles. On the other hand, the nickel content in the coating layer was lower than the nickel content in the NCM particles. The manganese content in the coating layer may be greater than the nickel content in the coating layer.

[0091] Referring to FIG. 9, the first region (Region 1) is part of the NCM particle, and the second region (Region 2) is part of the coating layer. Looking at the FET pattern of the first region (Region 1), the (003) plane and the (012) plane can be identified, thereby confirming that the surface of the NCM particle has a hexagonal layered structure.

[0092] Looking at the FET pattern of Region 2, the (001) plane and the (331) plane can be identified. Additionally, the white straight line (indicated by the yellow straight line) in the FET pattern of Region 2 represents a stacking fault phenomenon, and a coating layer exhibiting stacking faults can exhibit excellent performance. Thus, it can be confirmed that the coating layer has a monoclinic layered structure. Consequently, it can be seen that the positive electrode active material according to Example 1 has a coating layer that is well synthesized on the NCM particles without structural collapse of the NCM particles.

[0094] Evaluation Example 4: X-ray Photoelectron Spectroscopy (XPS) Analysis

[0095] FIGS. 10a and 10b show the results of X-ray photoelectron spectroscopy (XPS) analysis of a positive electrode active material according to one embodiment. XPS measurements were performed using monochromated Al Kα radiation (hν = 1486.6 eV) at a pressure of 2.0 x 10⁻⁸ mBar. The binding energy was analyzed after calibration based on the C 1s 284.6 eV peak. For precise analysis, a scan pass energy of 50.0 eV was used, and prior to sample analysis, a sputtering process was performed for 20 seconds at 2 kV energy in an argon (Ar+) atmosphere to remove surface contaminants with a thickness of 5 to 10 nm, after which measurements were taken.

[0096] Figure 10a is a graph showing the nickel (Ni) 2p spectrum, and Figure 10b is a graph showing the manganese (Mn) 2p spectrum.

[0097] Referring to Fig. 10a, Ni 3+ Peak, Ni 2+ Peaks and saturation (Sat) peaks can be observed. At this time, Ni 3+ Ni for the peak 2+ The peak ratios are 1.23 for Example 1, 0.86 for Comparative Example 1, and 1.69 for Comparative Example 2. Thus, as the thickness of the coating layer increases, Ni 3+ Ni for the peak 2+ It can be observed that the ratio of peaks increases. In other words, it can be confirmed that as the thickness of the coating layer increases, the mixing phenomenon between nickel and lithium ions decreases. This is because a coating layer containing manganese (Mn), which is relatively more heat-resistant than nickel (Ni), was formed.

[0098] Referring to FIG. 10b, peaks corresponding to the Li2MnO3 coating layer in Example 1 and Comparative Example 2 can be observed. This confirms that manganese (Mn) is well coated on the NCM particles.

[0100] Evaluation Example 5: Residual Lithium Analysis

[0101] To determine the amount of residual lithium in the cathode active materials according to Example 1, Comparative Example 1, and Comparative Example 2, analysis was performed using an acid-base titration method. The content of lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) was calculated according to the following Equations 1 and 2. The results are shown in the graph of Figure 11.

[0102] [Equation 1]

[0103]

[0104] [Equation 2]

[0105]

[0106] In the above Equations 1 and 2, V1 is the volume of HCl solution injected until pH 8.4 is reached, and V2 is the volume of HCl solution injected until pH 4.6 is reached. A1 is the weight of distilled water (Diwater), and A1 is the weight of the HCl solution. C HCl is the concentration of HCl, and M LiOH is the molecular weight of lithium hydroxide (LiOH). M Li2CO3 ≡ is the molecular weight of lithium carbonate (Li2CO3). W1 is the weight of the positive electrode active material.

[0107] Referring to FIG. 11, when a coating layer is formed (Example 1 and Comparative Example 2), the amount of residual lithium is reduced compared to when there is no coating layer (Comparative Example 1). Specifically, Comparative Example 1 had the highest residual lithium content at 0.50 wt%. The residual lithium content of Example 1 was 0.32 wt%, and the residual lithium content of Comparative Example 2 was 0.29 wt%. This indicates that the coating layer is formed using the residual lithium of the NCM particles. Furthermore, by forming a coating layer, the amount of residual lithium is reduced, thereby preventing adverse effects caused by residual lithium.

[0109] Evaluation Example 6: Cycle Performance Evaluation

[0110] To evaluate the electrochemical performance of the cathode active materials prepared according to Example 1, Comparative Example 1, and Comparative Example 2, a coin cell type lithium secondary battery was prepared and charged and discharged under a current density of 0.1 A / g. Figures 12a and 13a are graphs showing the voltage profile of the first cycle. Figures 12b and 13b are graphs showing the cycle performance of the lithium secondary battery.

[0111] Referring to Figures 12a and 12b, charging and discharging were performed under high temperature conditions of 50°C. Although Li2MnO3 has electrochemically inactive characteristics, the coating layer exhibited low resistance. Furthermore, as the thickness of the coating layer increased, the capacity retention rate after 100 cycles increased. This is because the coating layer contains manganese (Mn), which has high resistance to heat. In other words, by forming a coating layer containing manganese (Mn), the thermal stability of the cathode active material can be improved.

[0112] Referring to FIGS. 13a and 13b, charging and discharging were performed under high voltage conditions of 4.5V. In the case of Comparative Example 2, the capacity decreased after 100 cycles because it had a thicker coating layer compared to Example 1. Thus, it was confirmed that the capacity retention rate may decrease when the thickness of the coating layer is 20nm or more.

[0113] On the other hand, Example 1 exhibited a capacity similar to that of the case without a coating layer (Comparative Example 1) even under high pressure conditions. In addition, the capacity retention rate of Example 1 was significantly higher than that of Comparative Examples 1 and 2. Thus, it can be seen that when a coating layer of appropriate thickness is formed, cycle performance is improved without a decrease in capacity.

[0115] Evaluation Example 7: HR-TEM analysis after cycle performance evaluation

[0116] After evaluating the cycle performance according to Evaluation Example 6 above, the cathode active material was analyzed using HR-TEM. Figures 14a to 14c show the HR-TEM image results after cycling according to Example 1 and Comparative Example 1, respectively. Specifically, Figure 14a is an HR-TEM photograph of the cathode active material according to Example 1 and Comparative Example 1, respectively. Figure 14b shows the FET pattern analysis results of the cathode active material according to Example 1 and Comparative Example 1, respectively. Figure 14c shows the SAED (Selected Area Electron Diffraction) pattern analysis results of the cathode active material according to Example 1 and Comparative Example 1, respectively.

[0117] Referring to FIGS. 14a to 14c, in the case of Example 1, both the exterior of the positive electrode active material and the interior of the NCM particle formed a hexagonal structure even after cycling. Example 1 maintained the structure prior to cycling even after cycling. However, in the case of Comparative Example 1, the exterior of the positive electrode active material formed a rock salt structure, and the interior of the NCM particle formed a spinel structure. That is, it can be seen that structural collapse occurred in the positive electrode active material according to Comparative Example 1 due to cycling. Consequently, it can be confirmed that the positive electrode active material forming the coating layer has improved structural stability without structural collapse of the positive electrode active material.

[0119] Although embodiments of the present invention have been described above with reference to the attached drawings, those skilled in the art will understand that the present invention may be implemented in other specific forms without changing its technical concept or essential features. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

Claim 1 A positive electrode active material for a lithium secondary battery comprising: NCM particles, wherein the NCM particles are single-crystal particles; and a coating layer surrounding the surface of the NCM particles, wherein the average particle size of the NCM particles is 2 μm to 8 μm, the coating layer comprises a lithium-manganese composite, the weight ratio of the coating layer is 0.5 wt% to 2.5 wt% relative to the NCM particles, and wherein the NCM particles and the coating layer each comprise manganese and nickel, wherein the content of the manganese in the coating layer is greater than the content of the manganese in the NCM particles, the content of the nickel in the coating layer is less than the content of the nickel in the NCM particles, and the content of the manganese in the coating layer is greater than the content of the nickel in the coating layer. Claim 2 A positive electrode active material for a lithium secondary battery according to claim 1, wherein the NCM particles contain nickel, and the content of the nickel within the NCM particles is 89 at% to 95 at% relative to the NCM particles. Claim 3 In claim 1, the lithium in the coating layer is a positive electrode active material for a lithium secondary battery derived from the NCM particles. Claim 4 In claim 1, the coating layer and the NCM particles each have a layered structure and are positive active materials for a lithium secondary battery. Claim 5 delete Claim 6 A positive electrode active material for a lithium secondary battery according to claim 1, wherein the thickness of the coating layer is 10 nm to 20 nm. Claim 7 A lithium secondary battery comprising a positive electrode; a negative electrode; and an electrolyte between the positive electrode and the negative electrode, wherein the positive electrode comprises a positive current collector and a positive active material layer on the positive current collector, and the positive active material layer comprises a positive active material according to any one of claims 1 to 4 and 6. Claim 8 A method for manufacturing a positive electrode active material for a lithium secondary battery, comprising: preparing a first powder containing NCM particles; preparing a coating solution by dispersing a transition metal oxide in a solvent; preparing a mixture by mixing the first powder with the coating solution; drying the mixture to evaporate the solvent; and performing a heat treatment process on the mixture to obtain a positive electrode active material, wherein the weight ratio of the coating solution is 0.5 wt% to 2.5 wt% with respect to the first powder, and the positive electrode active material comprises the NCM particles and a coating layer surrounding the NCM particles, wherein the NCM particles and the coating layer each comprise manganese and nickel, wherein the content of manganese in the coating layer is greater than the content of manganese in the NCM particles, the content of nickel in the coating layer is less than the content of nickel in the NCM particles, and the content of manganese in the coating layer is greater than the content of nickel in the coating layer. Claim 9 A method for manufacturing a positive electrode active material for a lithium secondary battery according to claim 8, wherein the NCM particles are single-crystal particles and the average particle size of the NCM particles is 2 μm to 8 μm. Claim 10 delete Claim 11 A method for manufacturing a positive electrode active material for a lithium secondary battery according to claim 8, wherein the coating layer comprises lithium, and the lithium in the coating layer is derived from the NCM particles. Claim 12 A method for manufacturing a positive electrode active material for a lithium secondary battery according to claim 8, wherein the thickness of the coating layer is 10 nm to 20 nm. Claim 13 A method for manufacturing a positive electrode active material for a lithium secondary battery, wherein, in claim 8, the coating layer is formed by an evaporation-induced self-assembly process (EISA). Claim 14 A method for manufacturing a positive electrode active material for a lithium secondary battery according to claim 8, wherein drying the mixture comprises attaching the transition metal oxide to the surface of the NCM particles. Claim 15 A method for manufacturing a positive electrode active material for a lithium secondary battery according to claim 8, wherein the heat treatment process comprises the transition metal oxide reacting with residual lithium on the surface of the NCM particles to form a coating layer.

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