Positive electrode active material for sodium secondary battery, preparation method therefor, and sodium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material for a sodium secondary battery, the single-crystal positive electrode active material being structured with O3-type layers and represented by chemical formula 1. (Chemical formula 1) Nax[NiyCozMn1-y-z]O2. In chemical formula 1, x is 0.7-1.2, y is 0.55-0.65, and z is 0.15-0.25.

Description

Cathode active material for sodium secondary batteries, method for producing the same, and sodium secondary batteries containing the same

[0001] The present invention relates to a single particle type positive electrode active material for a sodium secondary battery manufactured by a novel manufacturing method, a manufacturing method thereof, and a sodium secondary battery including the same.

[0002] The development of portable electronic devices such as smartphones, MP3 players, and tablet PCs has led to an explosive increase in demand for secondary batteries capable of storing electrical energy. In particular, the emergence of electric vehicles, medium- to large-scale energy storage systems, and portable devices requiring high energy density is driving the demand for lithium secondary batteries.

[0003] Lithium secondary batteries pose safety concerns due to lithium's high reactivity. Furthermore, lithium is expensive, and extensive research is being conducted to address these issues. Lithium reserves are limited, raw material prices continue to rise, and continued demand is exacerbating supply instability. Furthermore, during the high-temperature firing process, cathode active materials for lithium secondary batteries experience oxygen desorption and lithium and nickel site substitution, which degrade battery performance. Therefore, the development of new materials is urgently needed.

[0004] As one such solution, research is being conducted on various energy storage ion carriers, including sodium, potassium, magnesium, aluminum, and calcium, which are abundant on Earth. Among these, sodium is actively being researched due to its low reduction potential and the potential for use with graphite as an anode. Furthermore, sodium secondary batteries are being studied. Compared to lithium secondary batteries, sodium secondary batteries have abundant reserves and are therefore price-competitive, making them attractive for future applications in mass-market electric vehicles and large-scale energy storage devices.

[0005] On the other hand, since the ionic radius of Na ion (1.02 Å) is larger than that of Li ion (0.76 Å), Na in the host structure + During the insertion / de-insertion process, significant volume changes occur. As a result, during the process of performing multiple cycles, the crystal structure becomes relatively unstable, and there is a problem that many side reactions with the electrolyte occur on the surface. In addition, since the electrode potential of Na ions (-2.71 V vs SHE) is slightly higher than that of Li ions (-3.01 V vs SHE), sodium secondary batteries have the disadvantage of exhibiting a lower energy density than lithium secondary batteries. Therefore, various studies are being conducted to increase the energy density of sodium secondary batteries while improving the cycle characteristics.

[0006] Korean Patent No. 10-2061731 (December 26, 2019)

[0007] The present invention is intended to solve the above problems, and its specific purposes are as follows.

[0008] The purpose of the present invention is to improve process efficiency by using a novel manufacturing method and to improve the structural stability of a cathode active material containing sodium, thereby improving the electrochemical characteristics and life characteristics of a cathode active material for a sodium secondary battery.

[0009] In addition, the present invention aims to provide a cathode active material for a sodium secondary battery having a single particle form with high particle strength and reduced surface residual sodium, a method for producing the same, and a sodium secondary battery including the same.

[0010] In order to achieve the above-mentioned purpose, the present invention is implemented by an embodiment having the following configuration.

[0011] In one embodiment, the positive electrode active material for the sodium secondary battery has an O3-type layered structure represented by the following chemical formula 1 and may be formed as a single particle.

[0012] (Chemical formula 1) Na x[Ni y Co z Mn 1-y-z ]O2

[0013] In chemical formula 1, X is 0.7 to 1.2, y is 0.55 to 0.65, and z is 0.15 to 0.25.

[0014] In one embodiment, a transition metal compound including at least one of Ni, Co, and Mn and a sodium compound including Na are physically mixed, heated, and then naturally cooled to room temperature, thereby producing a compound. The molar ratio of the transition metal compound to the sodium compound may be 1:1.4 to 2.

[0015] In one embodiment, the single particle has a porosity of 0.001 cm 3 / g or less, and the average particle diameter may be 1 μm to 10 μm.

[0016] In one embodiment, in X-ray diffraction (XRD) analysis, a main peak (104) peak may appear in a range of 2θ of 40° to 45°, three peaks may appear in a range of 2θ of 30° to 40°, and the intensity of the (104) peak may be greater than the intensity of each of the three peaks appearing in the range of 2θ of 30° to 40°.

[0017] In one embodiment, the compressive rupture strength of the single particle may be 100 MPa or greater.

[0018] In one embodiment, after performing 200 cycles, a surface layer having rock salt crystals is formed on the surface, wherein the average thickness of the surface layer may be 10 nm or less.

[0019] In one embodiment, the single particle may be formed of a single grain boundary, and the grain boundary may include a hexagonal α-NaFeO2 structure having an R-3m spacing group.

[0020] In one embodiment, a sodium secondary battery is provided comprising the above-described positive electrode material.

[0021] In one embodiment, the method for producing a positive electrode active material for a sodium secondary battery described above may include a step of producing a mixture by physically mixing a transition metal compound and a sodium compound at room temperature; a step of calcining the mixture; and a step of naturally cooling the mixture at room temperature after calcination is completed.

[0022] In one embodiment, in the step of preparing the mixture, the molar ratio of the transition metal compound and the sodium compound may be 1:1.4 to 2.

[0023] In one embodiment, in the firing step, the temperature may be raised from room temperature to 650°C to 830°C at a heating rate of 2°C / min to 10°C / min, and heat treatment may be performed for 12 to 30 hours.

[0024] In one embodiment, in the natural cooling step, after the firing is completed, the material may be maintained at room temperature in the air for 3 to 24 hours to allow for recrystallization.

[0025] In one embodiment, the sodium compound may be at least one selected from the group consisting of sodium carbonate, sodium nitrate, sodium acetate, sodium hydroxide, sodium hydroxide hydrate, and sodium oxide, and the transition metal compound may be at least one selected from the group consisting of transition metal oxides, transition metal nitrates, transition metal hydroxides, and transition metal hydroxides.

[0026] The present invention can obtain the following effects through the above-described examples.

[0027] The present invention can provide a cathode active material for a sodium secondary battery having high structural stability and improved long-term cycle characteristics, a method for producing the same, and a sodium secondary battery including the same.

[0028] In addition, the present invention can provide a cathode active material for a sodium secondary battery having a single particle form with high particle strength and reduced side reactions with an electrolyte, a method for producing the same, and a sodium secondary battery including the same.

[0029] Figure 1 is a drawing schematically showing a method for synthesizing a cathode active material according to an embodiment of the present invention.

[0030] Figure 2 is an XRD pattern of Example 1 and Manufacturing Examples 1 to 5.

[0031] Figure 3 is an SEM image of Manufacturing Examples 4 and 5.

[0032] Figure 4 is a Rietveld refined XRD pattern of Example 1 and Comparative Examples 1 and 2.

[0033] Figure 5 shows SEM and EDX mapping images of Example 1 and Comparative Example 1.

[0034] Figure 6 shows the results of the pore distribution of the particles of Example 1 and Comparative Example 1.

[0035] Figure 7 shows the particle strength test method of Example 1 and Comparative Example 1 and optical images before and after the test.

[0036] Figure 8 is a graph showing the particle strength experiments of Example 1 and Comparative Example 1.

[0037] Figure 9 shows the results of confirming the electrochemical characteristics of a sodium secondary battery using Example 1 and Comparative Example 1.

[0038] Figure 10 shows dQ dV in each cycle of the sodium secondary battery using Example 1 and Comparative Example 1. -1 It's a curve.

[0039] Figure 11 shows the xanes results of Ni K-edge and Co K-edge of Example 1 and Comparative Example 1.

[0040] Figure 12 is an SEM image of the positive electrode active material after performing a cycle for Example 1 and Comparative Example 1.

[0041] Figure 13 is a STEM image after 200 cycles of Example 1 and Comparative Example 1.

[0042] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. However, the technical concept of the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided to ensure that the disclosure is thorough and complete and to sufficiently convey the spirit of the present invention to those skilled in the art.

[0043] In this specification, when a component is referred to as being on another component, it means that it can be formed directly on the other component, or a third component may be interposed between them. In addition, in the drawings, the thicknesses of films and regions are exaggerated for the purpose of effectively explaining the technical contents.

[0044] Also, although terms such as first, second, and third have been used to describe various components in various embodiments of this specification, these components should not be limited by these terms. These terms are only used to distinguish one component from another. Thus, what is referred to as a first component in one embodiment may be referred to as a second component in another embodiment. Each embodiment described and illustrated herein also includes its complementary embodiments. Also, the term "and / or" has been used herein to mean including at least one of the components listed before and after.

[0045] In the specification, singular expressions include plural expressions unless the context clearly dictates otherwise. Furthermore, terms such as "comprises" or "has" are intended to specify the presence of a feature, number, step, component, or combination thereof described in the specification, but should not be construed as excluding the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0046] In addition, when describing the present invention below, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

[0047] Figure 1 is a drawing schematically showing a method for synthesizing a cathode active material according to an embodiment of the present invention.

[0048] The present invention relates to a cathode active material for a sodium secondary battery, wherein the cathode active material for a sodium secondary battery has an O3-type layered structure and is represented by the following chemical formula 1, and may be formed as a single particle.

[0049] (Chemical formula 1) Na x [Ni y Co z Mn 1-y-z ]O2

[0050] In chemical formula 1, X is 0.7 to 1.2, y is 0.55 to 0.65, and z is 0.15 to 0.25.

[0051] In addition, the cathode active material for the sodium secondary battery can be manufactured as a sodium cathode active material by using a transition metal compound as is, rather than by first manufacturing a precursor containing a transition metal by a co-precipitation method or the like, as in the past, and then calcining the manufactured precursor with a sodium compound.

[0052] In order to increase the discharge capacity in the O3-NaNCM cathode active material having a layered structure, a method of increasing the Ni content is used. However, when the Ni content exceeds 60 mol% (based on 100 mol% of the total transition metal), side reactions with the electrolyte, which is polar and acidic, increase, resulting in a rapid decrease in capacity. At this time, as a method of increasing the Ni content, a precursor containing nickel is prepared using a co-precipitation method, and the cathode active material prepared using the precursor is prepared in the shape of a spherical secondary particle composed of a group of a plurality of primary particles.

[0053] The secondary particles, which are composed of a group of these multiple primary particles, are polycrystalline, and during the charge / discharge cycle, large-sized sodium ions are easily broken at the particle boundaries between the primary particles where they are inserted / deintercalated, thereby forming microcracks within the secondary particles. The microcracks formed within the secondary particles extend from the surface to the inside of the secondary particles to create a new interface, thereby promoting side reactions between the electrolyte and the interface. In particular, the polycrystalline O3-NaNCM cathode active material has a high charge voltage range of Ni 4+ The structural deterioration and instability of the electrolyte increase side reactions at the interface, which is problematic.

[0054] According to an embodiment of the present invention, a cathode active material for a sodium secondary battery can be prepared in the form of a single particle using a novel method. The cathode active material for a sodium secondary battery prepared in the form of a single particle can suppress cracking in the particles during multiple charge / discharge cycles, thereby improving cycle characteristics and structural stability. In addition, by using a novel manufacturing method, the molten salt method, the manufacturing process can be simplified and a low sintering temperature can be applied, thereby improving process efficiency and enabling mass production.

[0055] The cathode active material according to an embodiment of the present invention can be manufactured by physically mixing a transition metal compound containing at least one of Ni, Co, and Mn and a sodium compound containing Na, heating the mixture, and then naturally cooling the mixture to room temperature. For example, a process of manufacturing a transition metal precursor using the transition metal compound by a coprecipitation method or the like can be omitted. In addition, by applying a method of naturally cooling the mixture to room temperature after heating rather than the quenching method used in the past, the process cost can be reduced and the difference in physical properties per batch that may occur in a process with a time difference can be reduced.

[0056] The above transition metal compound includes a nickel compound as a source of Ni, a manganese compound as a source of Mn, and a cobalt compound as a source of Co, and the nickel compound, the manganese compound, and the cobalt compound can be physically mixed with the sodium compound. When the total mole number of the nickel compound, the manganese compound, and the cobalt compound is 1, the mole number of the sodium compound can be included as 1.4 to 2. When the mole number of the sodium compound is less than 1.4, impurities are formed inside the sodium cathode active material, and when it is more than 2, the sodium compound remaining after the reaction reacts on the surface of the sodium cathode active material and attaches to the surface of the single particle in the form of fine powder to act as an impurity.

[0057] The above single particle has a porosity of 0.001 cm 3 / g or less, and the average particle diameter may be 1 μm to 10 μm. The porosity of the single particle is 0.001 cm 3 If it exceeds / g, the mechanical strength of the single particle is reduced, and problems such as microcracks forming inside the particle during the insertion / de-insertion process of sodium ions may occur. Specifically, the single particle has a porosity of 0.00001 cm 3 / g to 0.001 cm 3 / g, or 0.00001 cm 3 / g to 0.003 cm3 / g, or 0.00001 cm 3 / g to 0.005 cm 3 / g may be.

[0058] The above single particles may have an average particle size of 1 μm to 10 μm. If the size of the single particles is less than 1 μm, a side reaction with the electrolyte may occur during the charge / discharge process, and process loss may increase during the manufacturing process of the positive electrode of the sodium secondary battery, which is problematic. In addition, if the average particle size of the single particles exceeds 10 μm, the specific capacity may decrease, and the positive electrode may not be densely packed during the manufacturing process, which is problematic. The positive electrode active material for the sodium secondary battery is provided in the form of single particles having a micro-unit size in the above-described range, thereby having a high energy density per volume, and at the same time, solving the problem of micro-cracks of micro-unit particles, thereby improving the life characteristics of the sodium secondary battery. Specifically, the average particle size of the single particles may be 1 μm to 8 μm, or 1 μm to 7 μm, or 2 μm to 8 μm.

[0059] The above single particle is composed of a single grain boundary, and the grain boundary may include a hexagonal α-NaFeO2 structure having an R-3m space group.

[0060] In the cathode active material for a sodium secondary battery according to the present embodiment, in X-ray diffraction (XRD) analysis, a main peak (104) peak appears in the range of 2θ of 40° to 45°, three peaks appear in the range of 2θ of 30° to 40°, and the intensity of the (104) peak may be greater than the intensity of each of the three peaks appearing in the range of 2θ of 30° to 40°. The cathode active material for a sodium secondary battery according to the present embodiment is manufactured by a novel method, and as a result, although it is provided in a different form from the prior art in the form of micro-sized single particles, peaks related to sodium crystal spheres can be confirmed in XRD analysis.

[0061] The compressive rupture strength of the single particle may be 100 MPa or more. If the compressive rupture strength of the single particle is less than 100 MPa, problems such as microcracks forming in the single particle during the charge / discharge cycle, which reduces long-term cycle characteristics, and the particle being destroyed by the pressure applied during the manufacturing process of the positive electrode may occur. Specifically, the compressive rupture strength of the single particle may be 100 MPa to 5000 MPa, or 200 MPa to 5000 MPa, or 300 MPa to 5000 MPa.

[0062] In addition, the cathode active material for a sodium secondary battery according to an embodiment of the present invention can have improved long-term cycle characteristics. For example, after manufacturing a secondary battery using the cathode active material for a sodium secondary battery, even if multiple cycles are performed, almost no side reaction with the electrolyte occurs, and thus, no significant change occurs in the crystal structure of the surface layer.

[0063] Typically, in positive electrode materials for sodium secondary batteries, the rock salt crystal structure is generated due to the oxygen loss phenomenon of the particles, and the rock salt crystal structure can develop on the particle surface due to side reactions with the electrolyte in the as-prepared state. In addition, structural changes occur in layered positive electrode materials as they are cycled multiple times, and changes can be formed particularly on the surface of positive electrode material particles that are in direct contact with the electrolyte. In particular, in the case of a positive electrode material with a layered structure (R-3m) that has undergone deterioration, crystals having a rock salt (Fm3m) structure can be formed on the particle surface.

[0064] On the other hand, the cathode active material for a sodium secondary battery according to an embodiment of the present invention hardly forms a rock salt structure on the surface even in a freshly manufactured state. In addition, it has high structural stability even after performing multiple cycles, so that hardly any change in the crystal structure on the particle surface occurs. For example, after performing 200 cycles on the cathode active material for a sodium secondary battery, a surface layer having a rock salt (Fm3m) structure crystal may hardly be formed on the surface of a single particle of the cathode active material for a sodium secondary battery with an average thickness of 10 nm or less. The average thickness of the surface layer may mean the length in the direction from the outermost surface of the single particle toward the inner center. Specifically, the surface layer may have an average thickness of 8 nm or less, or 5 nm or less, or 0.05 nm to 5 nm, or 0.01 nm to 4 nm.

[0065] According to another aspect of the present invention, the present invention can provide a sodium secondary battery using the positive electrode active material for a sodium secondary battery described above. The sodium secondary battery can include a positive electrode using the positive electrode active material for a sodium secondary battery, a negative electrode corresponding to the positive electrode, and an electrolyte. If the electrolyte is an all-solid, it can be provided by being interposed between the positive electrode and the negative electrode, and if the electrolyte is a liquid, a separate separator can be provided between the positive electrode and the negative electrode. In the sodium secondary battery, the negative electrode, the electrolyte, the separator, and the assembly method thereof can utilize various methods known in the art.

[0066] According to another aspect of the present invention, an embodiment of the present invention may include a method for manufacturing a positive electrode active material for a sodium secondary battery as described above.

[0067] The method for manufacturing the positive electrode active material for the above sodium secondary battery may include a step of manufacturing a mixture by physically mixing a transition metal compound and a sodium compound at room temperature; a step of calcining the mixture; and a step of naturally cooling at room temperature after the calcination is completed.

[0068] In the step of preparing the above mixture, the molar ratio of the transition metal compound and the sodium compound may be 1:1.4 to 2.

[0069] The above transition metal compound and the sodium compound may each be provided in powder form, and may be physically stirred and mixed at room temperature. The transition metal compound and the sodium compound may be heat-treated after being uniformly mixed with each other. In the process of gradually increasing the temperature for the heat treatment, the sodium compound having a relatively low melting point may first undergo a phase change into a liquid state. The sodium compound that has been changed into a liquid state may smoothly diffuse along the migration path formed by the powdered transition metal compound, so that the liquid sodium compound may be uniformly dispersed in the powdered transition metal compound in the same form as the substrate. In the process of gradually increasing the heat treatment temperature, the internal stress in the transition metal compound may be reduced, and crystal grain nuclei may be formed and grown into single particles, thereby recrystallizing. The Na ions of the sodium compound may diffuse into the interior of the single particles.

[0070] In the process of performing such dissolution and recrystallization, the sodium compound may be partially volatilized due to its higher volatility compared to the transition metal compound. If the mole number of the sodium compound is less than 1.4 based on 1 mole of the entire transition metal compound, the content of the sodium compound may be small, so that impurities may be formed inside the single particle, or the sodium element may be relatively insufficient, so that the layered structure may be formed incompletely. On the other hand, if the mole number of the sodium compound is more than 2 based on 1 mole of the entire transition metal compound, the sodium compound added in excess may act as an excess impurity, so that small-sized particles may be formed in the form of clumps on the outer surface of the single particle, which may lower the electrochemical performance of the sodium secondary battery.

[0071] In addition, in the above-mentioned calcining step, the sodium compound may be transformed into a liquid state and a dissolution and recrystallization process may be performed in the process of gradually increasing the temperature by applying heat. In the above-mentioned calcining step, the temperature may be increased from room temperature to 650°C to 830°C at a heating rate of 2°C / min to 10°C / min, and heat treatment may be performed for 12 to 30 hours.

[0072] If the heating rate is less than 2 ℃ / min, the process efficiency is reduced, and the rate at which the liquid sodium compound diffuses between the powdered transition metal compound is too slow, so the reaction is not performed uniformly overall. If it exceeds 10 ℃ / min, the liquid sodium compound is not uniformly mixed with the powdered transition metal compound, which is problematic. Specifically, the heating rate may be 2 ℃ / min to 8 ℃ / min, or 2 ℃ / min to 7 ℃ / min, or 4 ℃ / min to 7 ℃ / min.

[0073] In addition, the sintering step can be performed by raising the temperature to 650 ℃ to 830 ℃ for 12 to 30 hours. By performing the above-mentioned temperature range and time, the sodium compound can sufficiently diffuse inside the transition metal and recrystallize into a single particle form made of a single crystal. If the temperature in the sintering step is lower than 650 ℃, recrystallization is not sufficiently performed, which is problematic, and if it exceeds 830 ℃, the process efficiency is lowered and it is necessary to additionally perform unnecessary processes such as quenching thereafter. Specifically, the sintering step can be 650 ℃ to 800 ℃, or 700 ℃ to 800 ℃, or 700 ℃ to 770 ℃.

[0074] According to an embodiment of the present invention, the cathode active material for a sodium secondary battery can be recrystallized by maintaining it at room temperature in the air for 3 to 24 hours after the calcination is completed in the natural cooling step. By maintaining it in air containing oxygen and naturally cooling it without performing a rapid cooling process requiring energy such as conventional quenching, it can be manufactured into a cathode active material for a secondary battery in the form of a single particle, which is a single crystal with high particle strength.

[0075] If the time in the above natural cooling step is less than 3 hours, the positive electrode active material for the sodium secondary battery is not sufficiently cooled, and since 24 hours is sufficient, if it takes longer than that, the process efficiency may be reduced.

[0076] The cathode active material for a sodium secondary battery according to an embodiment of the present invention can be prepared by natural cooling without a quenching process, and even when prepared by natural cooling, impurities such as a Na2CO3 compound phase are not formed, and can be formed as a single crystal grain boundary including a hexagonal α-NaFeO2 structure having an R-3m space group.

[0077] In addition, by using a novel manufacturing method, the heat treatment can be performed at a lower temperature than the temperature at which it is performed in the process of manufacturing conventional single crystal particles, and by using the dissolution-recrystallization mechanism of the molten salt method, the diffusion of sodium ions is increased, so that single crystal particles can be formed at a relatively low temperature.

[0078] The above sodium compound may be at least one selected from the group consisting of sodium carbonate, sodium nitrate, sodium acetate, sodium hydroxide, sodium hydroxide hydrate, and sodium oxide.

[0079] The above transition metal compound may be at least one selected from the group consisting of transition metal oxides, transition metal nitrates, transition metal hydroxides, and transition metal hydroxides.

[0080]

[0081] Hereinafter, examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the scope of the present invention is not limited by the following examples.

[0082]

[0083] 1. Manufacturing of sodium cathode active material

[0084] Example 1_Na[Ni 0.6 Co 0.2 Mn 0.2 ] Manufacture of O2 single crystal cathode active material

[0085] NiO, Co3O4, and MnO2 were mixed using a mortar so that the molar ratio of Ni, Co, and Mn was 6:2:2. In addition, NaOH, a sodium raw material, was added so that the molar ratio with the transition metal (total moles of Ni, Co, and Mn) was 1.4:1. After that, the temperature was increased from room temperature to 720 ℃ at a heating rate of 5 ℃ / min, and heat treatment was performed at 720 ℃ for 24 hours in an oxygen atmosphere. After the heat treatment was completed, the sodium cathode active material in the form of single particles was manufactured by naturally cooling it while maintaining it at room temperature in the air. The manufactured sodium cathode active material was used directly without an additional grinding process.

[0086]

[0087] Comparative Example 1

[0088] Ni using coprecipitation method 0.6 Co 0.2 Mn 0.2 (OH)2 precursor was prepared. A transition metal solution, a chelating agent, and sodium hydroxide as a precipitating agent were mixed in a co-precipitation reactor to obtain Ni 0.6 Co 0.2 Mn 0.2 OH)2 was manufactured, and then through filtration and drying processes, finally Ni 0.6 Co 0.2 Mn 0.2 (OH)2 precursor was prepared. The prepared precursor was mixed with NaOH using a caustic. NaOH was added so that the molar ratio with the transition metal (total moles of Ni, Co, and Mn) was 1.15:1. After that, the temperature was increased from room temperature to 720°C at a heating rate of 5°C / min, and heat treatment was performed at 720°C for 24 hours in an oxygen atmosphere. After completion of the heat treatment, quenching (rapid cooling process) was performed in a vacuum environment to prepare a sodium cathode active material in the form of multiple particles.

[0089]

[0090] Comparative Example 2

[0091] A positive electrode active material was manufactured in the same manner as in Comparative Example 1, except that instead of performing quenching in a vacuum environment after heat treatment, the material was maintained at room temperature in the air and cooled naturally.

[0092]

[0093] Manufacturing Example 1

[0094] A positive electrode active material was manufactured in the same manner as in Example 1, except that the molar ratio of NaOH and transition metal was changed to 1.25:1.

[0095]

[0096] Manufacturing Example 2

[0097] A positive electrode active material was manufactured in the same manner as in Example 1, except that the molar ratio of NaOH and transition metal was changed to 1.3:1.

[0098]

[0099] Manufacturing Example 3

[0100] A positive electrode active material was manufactured in the same manner as in Example 1, except that the molar ratio of NaOH and transition metal was changed to 1.35:1.

[0101]

[0102] Manufacturing Example 4

[0103] A positive electrode active material was manufactured in the same manner as in Example 1, except that the molar ratio of NaOH and transition metal was changed to 1.45:1.

[0104]

[0105] Manufacturing Example 5

[0106] A positive electrode active material was manufactured in the same manner as in Example 1, except that the molar ratio of NaOH and transition metal was changed to 1.5:1.

[0107]

[0108] Table 1 below shows the molar ratio of sodium hydroxide and transition metal in the cathode active material according to the previously manufactured examples, comparative examples, and manufacturing examples.

[0109] Classification Na: Total moles of transition metals Example 11.4:1 Comparative example 11.15:1 Comparative example 21.15:1 Manufacturing example 11.25:1 Manufacturing example 21.3:1 Manufacturing example 31.35:1 Manufacturing example 41.45:1 Manufacturing example 51.5:1

[0110]

[0111] 2. Evaluation of sodium cathode active material

[0112]

[0113] Impurity assessment of sodium anode materials according to sodium hydroxide content

[0114] By comparing Example 1 and Manufacturing Examples 1 to 5, sodium cathode active materials manufactured according to the sodium hydroxide content were evaluated. Figure 2 shows XRD patterns of Example 1 and Manufacturing Examples 1 to 5. Figure 3 shows SEM images of Manufacturing Examples 4 and 5.

[0115] Referring to Fig. 2, it was confirmed in the XRD pattern that impurities such as NiO and Na3Mn4 were not formed when the mole number of NaOH was 1.4 or more with respect to the total mole number of transition metals. That is, the XRD pattern showed that there were no impurities inside the positive electrode active materials in Example 1, Manufacturing Example 4, and Manufacturing Example 5.

[0116] Referring to Fig. 3, in the SEM image, in the case of Manufacturing Example 4, there were no impurities on the surface of the positive electrode active material, but in the case of Manufacturing Example 5, impurities (dotted lines) appeared on the surface of the positive electrode active material. That is, when the mole number of NaOH was 1.5 or more with respect to the total mole number of transition metals, it was confirmed that residual sodium was formed on the surface of the positive electrode active material. That is, when the mole number of NaOH was 1.4 or 1.45 with respect to the total mole number of transition metals, no impurities appeared on either the inside or the outside of the positive electrode active material.

[0117]

[0118] The cathode active materials manufactured by different manufacturing methods were compared between Example 1 and Comparative Example 1. Fig. 4 shows the Rietveld refined XRD patterns of Example 1 and Comparative Examples 1 and 2. Fig. 5 shows SEM and EDX mapping images of Example 1 and Comparative Example 1. Table 2 shows the structural parameters of the particles of Example 1 and Comparative Example 1. Table 3 shows the results of confirming the chemical composition of Example 1 and Comparative Example 1.

[0119] Example 1Space group: R-3mRwp = 4.12 %α-axis: 2.9476 Åc-axis: 15.7764 ÅVolume: 118.411 Å 3 AtomsitexyzUisoOccNa13a0002.4390.083Ni13b000.51.3210.050Co13b000.51.3210.017Mn13b000.51.3210.017O16c000.23472.3050.167Comparative example 1Space group: R-3mRwp = 2.7618 %α-axis: 2.9500 Åc-axis: 15.7830 ÅVolume: 118.955 Å 3 AtomsitexyzUisoOccNa13a0002.6000.083Ni13b000.51.5360.050Co13b000.51.5360.017Mn13b000.51.5360.017O16c000.24682.8080.167

[0120] NiCoMnExample 10.60570.19100.2032Comparative Example 10.59830.20010.2016

[0121] Referring to FIG. 4 and Table 3, in the XRD data analysis of Example 1 and Comparative Example 1 through Rietveld refinement, both Example 1 and Comparative Example 1 exhibited an O3-type α-NaFeO2 structure (R-3m space group) with no impurity phases detected. In other words, it was confirmed that Example 1 and Comparative Example 1 did not have a significant difference in the O3-type crystal structure, regardless of the process of synthesizing the positive electrode active material or the shape of the particles.

[0122] Also, referring to FIG. 4, while Example 1 and Comparative Example 1 showed similar crystal structures, it was confirmed that many impurities were generated in the case of Comparative Example 2. Comparative Example 2 was manufactured in the same manner as Comparative Example 1, but the cooling was performed naturally instead of quenching in the same manner as Example 1, and the XRD results showed that many Na2CO3 compound phases were formed and also showed unclear peaks. That is, it can be confirmed that when manufacturing a cathode active material by a conventional method, the quenching process is essential to form a complete phase without impurities and to reduce the generation of carbonate species.

[0123] Referring to FIG. 5, Example 1 was in the form of single particles with an average particle diameter of approximately 4 ㎛ to 6 ㎛, and Comparative Example 1 was in the form of secondary particles in which multiple primary particles were aggregated, and the average particle diameter of the secondary particles was approximately 10 ㎛. It was confirmed that Example 1 was manufactured as single particles with a smooth surface, with an average particle diameter of approximately 50% of that of Comparative Example 1. In addition, it was confirmed that the manufactured single particle type cathode active material of Example 1 was not aggregated but was dispersed. In Example 1, the cathode active material was manufactured by naturally cooling after heat treatment without performing an additional pulverization process, and it was confirmed that it was manufactured in the form of single particles with a uniform average particle diameter.

[0124] In the SEM image of Fig. 5, Example 1 appeared in the form of plate-shaped single particles of 3-5 μm without grain boundaries, and Comparative Example 1 was confirmed to be composed of spherical secondary particles of 10-15 μm composed of nano-sized primary particles. In addition, in the EDX mapping data (the last drawings of Example 1 and Comparative Example 1 in Fig. 5), both sides showed a uniform distribution of Na, Ni, Co, Mn, and O throughout the particle (Example 1 based on single particles, Comparative Example 1 based on secondary particles).

[0125] The chemical compositions of Example 1 and Comparative Example 1 were confirmed using ICP-OES in Table 3. The chemical compositions of the transition metals of Example 1 and Comparative Example 1 were close to the composition of the metals added during the manufacturing process, Ni:Co:Mn = 6:2:2, and the contents of Ni, Co, and Mn in Example 1 and Comparative Example 1 were similar.

[0126] In Example 1, it was confirmed that even when the precursor was manufactured using a method other than the coprecipitation method of Comparative Example 1, the content of the final transition metal included in the cathode active material remained similar. Referring to Fig. 4, the crystal structures of Example 1 and Comparative Example 1 were found to be identical. That is, even when manufactured using a novel method, such as Example 1 of the present invention, the content and crystal structure of the transition metal were found to be similar to those of the prior art.

[0127] Figure 6 shows the results of the pore distribution of the particles of Example 1 and Comparative Example 1.

[0128] Referring to Figure 6, the pore volumes of Example 1 and Comparative Example 1 were confirmed through BET analysis. In Example 1, pores were almost absent in the form of single particles (0.000619 cm 3 / g), Comparative Example 1 showed a large number of pores (0.003178 cm 3 / g) The pores that appeared were mainly distributed in the meso size range of 2-6 nm. That is, Comparative Example 1 had a higher porosity than Example 1, and thus it was confirmed that the mechanical strength of the particles was low. In addition, due to the high porosity distributed in Comparative Example 1, sodium ions (Na) having a larger size than conventional lithium ions were formed. + ) was confirmed to accelerate the internal cracking of the particle during the insertion / removal process.

[0129] Figure 7 shows the particle strength test method of Example 1 and Comparative Example 1 and optical images before and after the test. Figure 8 is a graph showing the particle strength test of Example 1 and Comparative Example 1. The particle strength test was performed using a Microcompression test (MCT, MCT-W 500, SHIMADZU), and a loading speed of 10 (3.8736 mN / s) was used to apply a compressive force to each particle until the particle broke.

[0130] In Fig. 7, the particle strength was calculated by applying increasing pressure to the tip using each particle of Example 1 and Comparative Example 1, and the applied strength at the point when the particle was destroyed was calculated. Referring to Fig. 8, the compressive rupture strength of the particle was calculated using the following equation. Table 4 shows the results of confirming the particle strength of Fig. 8.

[0131] (Equation) Strength = (ax F) / (πd 2 )

[0132] Strength: Compression rupture strength (MPa)

[0133] a: 2.8

[0134] F: test force (N)

[0135] d: particle diameter (mm)

[0136] Here, Force refers to the force applied to the particle when a crack occurs in the particle, Displacement is the distance the tip in contact with the particle surface moves from its original position due to the crack occurrence, Diameter is the measured particle size, and Strength is the force applied when the diameter decreases by 10% after the crack occurs.

[0137] Example 1 Comparative Example 1 Force 16.35696 mN 4.559077 mN Strength 478.73 MPa 45.08 MPa Displacement 0.5148 μm 0.7584 μm

[0138] Referring to FIGS. 7, 8, and Table 4, when the same force was applied to the particles of Example 1 and Comparative Example 1, it was confirmed that in the case of Example 1, there was almost no difference before and after applying the force, whereas in the case of Comparative Example 1, a damaged area was formed in the particle. According to the compression test results, Example 1 showed a mechanical strength of 478.73 MPa, which was more than 10 times the force withstood by Comparative Example 1 (45.08 MPa).

[0139]

[0140] Figure 9 shows the results of confirming the electrochemical characteristics of the sodium secondary battery using Example 1 and Comparative Example 1. In Figure 9, each of Example 1 and Comparative Example 1 was used as a half-cell using Na metal as a negative electrode, electrolyte (1M NaPF6in EC:DEC + 5 wt% FEC, 150 ㎕), Separator 1, and Glass fiber 1, and charge / discharge was performed under the conditions of 1.5 - 4.0 V (1C = 150 mAh / g) at a 0.5 C rate. Figure 10 shows the dQ dV in each cycle of the sodium secondary battery using Example 1 and Comparative Example 1. -1 It is a curve. In Fig. 10, it is shown for 1 cycle, 200 cycles, and 350 cycles, respectively.

[0141] Referring to Figure 9, the initial discharge capacity of Example 1 (108.26 mAh g -1 ) is comparative example 1 (117.94 mAh g -1 ) was lower than that of Example 1, which is because Example 1 is in the form of a single particle and is larger in size than the primary particles of Comparative Example 1, and thus Na + This is because the diffusion path is relatively long. On the other hand, when checking the cycle characteristics, Example 1 showed a stable capacity retention rate of 73% after 350 cycles, whereas Comparative Example 1 showed a continuous decrease in capacity after 200 cycles and then showed an unstable coulombic efficiency and a rapid capacity decrease (capacity retention rate of 30.08% after 350 cycles). Example 1 initially showed a slightly lower discharge capacity, but it was confirmed that the voltage resistance change was minimized while performing 200 and 350 cycles compared to Comparative Example 1.

[0142] Referring to Fig. 10, the charge-discharge curves of Example 1 and Comparative Example 1 contain several voltage plateaus, which indicate typical several phase transformations (O3hex. → O3'mon. → P3hex. → P3'mon. → O3hex + O'3mon.) that occur with the gliding of the TMO2 layer (transition metal oxide layer). dQ dV at 200 and 350 cycles -1 Referring to the curves, Example 1 showed a stable reversible reaction during the Na ion insertion / de-insertion process with a slight peak shift, while Comparative Example 1 showed a dQ dV -1 In all oxidations and reductions, the resistance gradually increased, and the oxidation and reduction peak intensities decreased rapidly during the cycle. That is, it was confirmed that Example 1 showed a minimum of irreversible phase transitions during the cycle compared to Comparative Example 1, and mostly showed reversible oxidation-reduction reactions.

[0143] Figure 11 shows the xanes results of Ni K-edge and Co K-edge of Example 1 and Comparative Example 1.

[0144] The difference in the reversible redox reactions of Example 1 (SC-NCM) and Comparative Example 1 (PC-NCM) was confirmed using XANES at the Ni and Co K-edges. Within the voltage range of 1.5-4.0 V, O3-type Li[Ni 1-x-y Co x Mn y ]Similar to the O2 anode, Ni 2+ / 3+ / 4+ The redox reaction of O3 type Na[Ni 1-x-y Co x Mn y ]O2 is the main element that provides reversible capacity at the anode. Referring to Fig. 11, the change in the oxidation state of Ni ions can be confirmed. Before performing the cycle, the Ni ions of Example 1 and Comparative Example 1 in the pristine state showed a mixed state of 2+ and 3+. After charging to 4.0 V, the XANES spectra of Example 1 and Comparative Example 1 shifted to higher energy, showing that the Ni ions were 4+.

[0145] In Comparative Example 1, Ni during the cycle process 2+ / 4+ To confirm the reversible redox reaction, the valence states of Ni ions were compared after charging for 1 and 200 cycles, respectively. In Comparative Example 1, a distinct energy difference was observed in the 1st and 200th cycles (Ni K-edge spectrum) compared to Example 1, and Ni was observed to change during repeated cycling. 2+ / 4+ The reversible redox reaction was relatively low. Comparative Example 1 had a higher pore volume than Example 1, and thus, the reactive Ni in the charged state was highly 4+ Many side reactions occurred with the electrolyte, forming a rock salt phase similar to inactive NiO from the surface to the inside of the particle (see Fig. 13 below). In Example 1 and Comparative Example 1, the Co K-edge spectrum showed that Co was well formed without any change in Co oxidation number even after charging.3+ was well maintained. The XANES data were consistent with the electrochemical charge-discharge test results, and during the cycling process, Example 1 showed Ni 2+ / 3+ / 4+ It was confirmed that the reversible redox reaction was excellent and showed high capacity characteristics.

[0146]

[0147] Figure 12 is an SEM image of the cathode active material after cycling for Example 1 and Comparative Example 1. In Figure 12, the particle morphology of the pristine before performing the charge / discharge cycle and after performing 1 cycle of charge and discharge and 200 cycles for each of Example 1 and Comparative Example 1 is confirmed. The cycle was performed under the conditions of 0.5-4.0 V, 0.5 C.

[0148] Comparative Example 1 appeared in the form of spherical secondary particles composed of densely packed nano-sized primary particles in a pristine state. In the 1-cycle charge state (4.0 V), microcracks occurred in Comparative Example 1, and noticeable gaps appeared between adjacent primary particles within the secondary particles. Even after the discharge process at 1.5 V, cracks within the secondary particles of Comparative Example 1 were still maintained, and it was confirmed that the microcracks did not close reversibly. After 200 cycles (discharge state at 1.5 V), Comparative Example 1 was mechanically ruptured from the surface to the inside of the secondary particles, and severe microcracks occurred and the surface area also increased significantly. In other words, Na in the crystal structure of Comparative Example 1 + This means that morphological damage has occurred due to repeated insertion / de-insertion of ions.

[0149] In Example 1, it was confirmed that the size and shape of the particles did not change even after 1 cycle of charging and discharging, and that even after 200 cycles of charging and discharging, no cracks occurred in the particles and the shape was maintained similar to before performing the charge and discharge cycle. On the other hand, in Comparative Example 1, cracks occurred inside the particles from the 1st charging cycle, and the cracks gradually expanded over the course of 200 cycles, resulting in problems such as splitting of the secondary particles.

[0150] O3 type Na[Ni as in Comparative Example 1 1-x-y Co x Mn y ]O2 cathode active material was found to undergo significant expansion and contraction of the particle volume during several phase changes, which caused structural stress, resulting in serious microcracks within the particles and consequently accelerated capacity loss. In the case of Example 1, the mechanical strength of the cathode was improved, which could suppress the formation of microcracks within the particles, and thus, reversible Na during multiple cycles + It was possible to prevent the deterioration of storage characteristics.

[0151] That is, in Comparative Example 1, microcracks generated in the particles caused side reactions with the electrolyte, and undesirable side reactions occurred in the microcracks with the electrolyte solution, accelerating the formation of electrically separated particles within the electrode during repeated cycles. On the other hand, in Example 1, microcracks did not occur in the particles during multiple cycles, and as a result, side reactions with the electrolyte were prevented.

[0152]

[0153] Fig. 13 is a STEM image after 200 cycles of Example 1 and Comparative Example 1. In Fig. 13, 200 cycles were performed under the conditions of 1.5-4.0 V, 0.5 C, and areas a, b, and c in the first image of Example 1 were enlarged, and areas d, e, and f in the first image of Comparative Example 1 were enlarged. For c in Example 1 and f in Comparative Example 1, the area corresponding to the surface was confirmed using Fourier filtered transforms.

[0154] The secondary particles of the cycled comparative example 1 were almost pulverized, and it was confirmed that a thick NiO-like rock salt layer (approximately 17 nm) was formed on the surface in a damaged form. This change in particle morphology facilitates the penetration of the electrolyte into the particle, resulting in the formation of unstable Ni not only on the particle surface but also inside the exposed particle. 4+ Accelerated the reduction of the species. As a result, this is Na + An insulating NiO-like rock salt phase was formed, which impeded the movement of the electrolyte and increased the resistance of the secondary battery during long-term cycling.

[0155] Example 1 exhibited a rock salt phase similar to NiO that was approximately eight times thinner than Comparative Example 1, and had a smooth surface with no noticeable morphological damage. Furthermore, in the case of Example 1, almost no distortion occurred even at the edges where the probability of reaction with the electrolyte is high. In other words, it was confirmed that the single crystal particles manufactured according to the manufacturing method of Example 1 suppressed the formation of microcracks and mitigated side reactions with the electrolyte, thereby exhibiting stable electrochemical performance.

[0156]

[0157] As described above, the positive electrode according to Example 1 of the present invention was manufactured using a molten salt synthesis method rather than a method of separately synthesizing a precursor using a coprecipitation method and then calcining it with a sodium compound. The manufactured Example 1 was found to have high strength with almost no pores in the form of single particles, and it was confirmed that almost no microcracks occurred during the cycle, indicating almost no side reaction with the electrolyte. In addition, unlike the conventional method, it does not require complex fixing such as quenching, so it can be advantageously applied to commercialization with low production cost and high efficiency.

[0158]

[0159] Those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering its technical spirit or essential characteristics. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims below rather than the detailed description above, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

Claims

1. 3 Type layered structure, represented by the following chemical formula 1, and a cathode active material for a sodium secondary battery consisting of single particles: (Chemical formula 1) Na x [Ni y Co z Mn 1-y-z ]O2 In chemical formula 1, X is 0.7 to 1.2, y is 0.55 to 0.65, and z is 0.15 to 0.

25.

2. In paragraph 1, A transition metal compound containing at least one of Ni, Co, and Mn and a sodium compound containing Na are physically mixed, and then heated and then naturally cooled to room temperature, thereby producing a A cathode active material for a sodium secondary battery, wherein the molar ratio of the above transition metal compound and the above sodium compound is 1:1.4 to 2.

3. In paragraph 1, The above single particle has a porosity of 0.001 cm 3 A cathode active material for a sodium secondary battery having a particle size of / g or less and an average particle diameter of 1 μm to 10 μm.

4. In paragraph 1, A cathode active material for a sodium secondary battery, wherein in X-ray diffraction (XRD) analysis, a main peak (104) peak appears in the range of 2θ of 40° to 45°, three peaks appear in the range of 2θ of 30° to 40°, and the intensity of the (104) peak is greater than the intensity of each of the three peaks appearing in the range of 2θ of 30° to 40°.

5. In paragraph 1, A cathode active material for a sodium secondary battery, wherein the compressive rupture strength of the above single particle is 100 MPa or more.

6. In paragraph 1, A cathode active material for a sodium secondary battery, wherein a surface layer having rock salt crystals is formed on the surface after 200 cycles, and the average thickness of the surface layer is 10 nm or less.

7. In paragraph 1, The above single particle is composed of one grain boundary, The above crystal grain boundary is a cathode active material for a sodium secondary battery including a hexagonal α-NaFeO2 structure having an R-3m space group.

8. A sodium secondary battery comprising a positive electrode active material according to any one of claims 1 to 7.

9. A step of preparing a mixture by physically mixing a transition metal compound and a sodium compound at room temperature; and a step of calcining the above mixture; and A method for manufacturing a positive electrode active material for a sodium secondary battery according to any one of claims 1 to 7, comprising a step of naturally cooling at room temperature after completion of firing.

10. In paragraph 9, In the step of preparing the above mixture, A method for producing a cathode active material for a sodium secondary battery, wherein the molar ratio of the above transition metal compound and the above sodium compound is 1:1.4 to 2.

11. In paragraph 9, In the above firing step, A method for producing a cathode active material for a sodium secondary battery, comprising heat-treating a temperature of 650°C to 830°C at a heating rate of 2°C / min to 10°C / min from room temperature for 12 to 30 hours.

12. In paragraph 9, In the above natural cooling step, A method for manufacturing a cathode active material for a sodium secondary battery, comprising: after the above-mentioned calcination is completed, maintaining the cathode active material at room temperature in the air for 3 to 24 hours to cause recrystallization.

13. In paragraph 9, The above sodium compound is at least one selected from the group consisting of sodium carbonate, sodium nitrate, sodium acetate, sodium hydroxide, sodium hydroxide hydrate, and sodium oxide, A method for producing a cathode active material for a sodium secondary battery, wherein the above transition metal compound is at least one selected from the group consisting of a transition metal oxide, a transition metal nitrate, a transition metal hydroxide, and a transition metal hydroxide.

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