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

Abstract

The present invention relates to a cathode active material for a sodium secondary battery, the cathode active material being represented by chemical formula 1 and being formed of a single particle. (Chemical formula 1) NaxNiaFebMncMdO2 In chemical formula 1, M is at least one element selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo, 0.6 ≤ x ≤ 1, 0.01 ≤ a ≤ 0.9, 0.01 ≤ b ≤ 0.9, 0.01 ≤ c < 0.9, and 0 ≤ d ≤ 0.5.

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 cathode active material for a sodium secondary battery, a method for producing the same, 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 underway on various energy-storing 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 its potential for use with graphite as a cathode.

[0005] To solve this problem, sodium secondary batteries are being studied. Compared to lithium secondary batteries, sodium secondary batteries have abundant reserves and are therefore price competitive, which makes them advantageous for application in popular electric vehicles and large-scale energy storage devices in the future.

[0006] On the other hand, cathode active materials for sodium secondary batteries have problems such as low volumetric energy density due to small particle size and side reactions with the electrolyte during the charge-discharge process. Furthermore, cathode active materials for sodium secondary batteries, which are composed of nano-sized primary particles aggregated into micro-sized secondary particles, have problems with reduced cycle life characteristics due to complex structural changes caused by sodium ion insertion / deintercalation.

[0007] Therefore, in order to improve the energy density per volume in the cathode active material for sodium secondary batteries, synthesis of micro-unit particles is essential, and at the same time, various studies are being conducted to improve the life characteristics by solving the micro-crack problem of micro-unit particles.

[0008] Previous literature

[0009] Korean Patent No. 10-2061731 (December 26, 2019)

[0010] The present invention is intended to solve the above problems, and its specific purposes are as follows.

[0011] The purpose of the present invention is 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.

[0012] In addition, the present invention aims to provide a positive electrode active material for a sodium secondary battery manufactured in the form of a single particle but having improved mechanical properties, a method for manufacturing the same, and a sodium secondary battery including the same.

[0013]

[0014] In order to achieve the above-mentioned purpose, the present invention is implemented by an embodiment having the following configuration.

[0015] In one embodiment, a cathode active material for a sodium secondary battery is represented by the following chemical formula 1 and is composed of single particles.

[0016] (Chemical formula 1)

[0017] Na x Ni a Fe b Mn c M d O2

[0018] In the above chemical formula 1, M is at least one element selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo. 0.6 ≤ x ≤ 1, 0.01 ≤ a ≤ 0.9, 0.01 ≤ b ≤ 0.9, 0.01 ≤ c < 0.9, and 0 ≤ d ≤ 0.5.

[0019] In one embodiment, the single particle has an average particle size (D 50 ) may be 1 ㎛ to 50 ㎛.

[0020] 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°.

[0021] In one embodiment, in the chemical formula 1, M is a doping element, and as the content of M increases, particles can grow in the (010) plane direction.

[0022] In one embodiment, in the chemical formula 1, M is a doping element, and as the content of M increases, the movement path of sodium ions may increase.

[0023] In one embodiment, in the chemical formula 1, M may include at least one of Ti and Zn.

[0024] In one embodiment, the single particle is composed of a single grain boundary, and the grain boundary may include a hexagonal α-NaFeO2 structure having an R-3m spacing group.

[0025] In one embodiment, a sodium secondary battery is provided comprising the above-described positive electrode material.

[0026] In one embodiment, the present invention includes a method for producing a cathode active material for a sodium secondary battery, comprising the steps of: preparing a mixture by mixing a cathode active material precursor including a transition metal compound and a sodium compound; and calcining the mixture.

[0027] 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.

[0028] In one embodiment, the positive electrode active material precursor and the sodium compound may be mixed in a molar ratio of the transition metal compound to the sodium compound of 1:0.6 to 1.5.

[0029] In one embodiment, the transition metal compound includes nickel (Ni), iron (Fe), manganese (Mn) and an M element, and the M element may be at least one selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo.

[0030] In one embodiment, the molar ratio of nickel, iron, and manganese may be 1 to 9: 1 to 9: 1 to 9.

[0031] In one embodiment, the M element includes at least one of Ti and Zn as a doping element, and the M element can be added in an amount of 0.05 mol% to 0.4 mol%.

[0032] In one embodiment, prior to the firing step, the method may further include pressurizing the mixture at a first pressure to produce it in the form of pellets.

[0033] In one embodiment, the first pressure may be 20 MPa to 60 MPa.

[0034] In one embodiment, in the firing step, the firing may be performed at a temperature range of 800°C to 1100°C for 10 to 20 hours.

[0035] The present invention can obtain the following effects through the above-described examples.

[0036] The present invention can provide a cathode active material for a sodium secondary battery having high structural stability and improved electrochemical properties, a method for producing the same, and a sodium secondary battery including the same.

[0037] In addition, the present invention can provide a cathode active material for a sodium secondary battery, which is manufactured in the form of a single particle and has high mechanical strength, has a high packing density when manufacturing a cathode, and has improved life characteristics, a method for manufacturing the same, and a sodium secondary battery including the same.

[0038] Figure 1 is a single particle image of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.

[0039] Figure 2 is a drawing showing the crystal direction of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.

[0040] Figure 3 shows XRD data of the positive electrode active materials of Examples 1, 3, 4 and Comparative Example 1.

[0041] Figure 4 is an SEM image of the positive electrode active material of Comparative Example 1 and Example 1.

[0042] Figure 5 shows XRD data of the positive electrode active materials of Examples 1 and 2.

[0043] Figure 6 is an SEM image of the positive electrode active materials of Examples 1 and 2.

[0044] Figure 7 is an SEM image of the positive electrode active material of Example 3.

[0045] Figure 8 is an SEM image of the positive electrode active material of Examples 2 and 3.

[0046] Figure 9 is a cross-sectional SEM image of the positive electrode active material of Example 3.

[0047] Figure 10 is an SEM image of the positive electrode active material of Example 4.

[0048] Figure 11 is a cross-sectional SEM image of the positive electrode active material of Example 4.

[0049] Figure 12 shows electrochemical characteristic data of Examples 1, 2 and Comparative Example 1.

[0050] Figure 13 shows electrochemical characteristic data of Examples 3 and 4.

[0051] Figure 14 shows the life characteristic data of Examples 1 to 4 and Comparative Example 1.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057]

[0058] Fig. 1 is a single particle image of a cathode active material for a sodium secondary battery according to an embodiment of the present invention. Fig. 2 is a drawing showing the crystal orientation of a cathode active material for a sodium secondary battery according to an embodiment of the present invention.

[0059] A cathode active material for a sodium secondary battery according to one embodiment of the present invention is represented by the following chemical formula 1 and may be formed as a single particle.

[0060] (Chemical formula 1)

[0061] Na x Ni a Fe b Mn c M d O2

[0062] In the above chemical formula 1, M is at least one element selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo. 0.6 ≤ x ≤ 1, 0.01 ≤ a ≤ 0.9, 0.01 ≤ b ≤ 0.9, 0.01 ≤ c < 0.9, 0 ≤ d ≤ 0.5. Specifically, 0.01 ≤ d ≤ 0.5 may be satisfied.

[0063] Typically, cathode active materials for sodium secondary batteries are formed in the form of primary particles having a size of nanoparticles or smaller, or secondary particles formed by agglomeration of such primary particles. The cathode active materials for sodium secondary batteries having a size of nanoparticles or smaller have problems such as low volumetric energy density of the secondary battery and reduced lifespan characteristics due to side reactions with the electrolyte that occur during the charge / discharge process. In addition, even in the case of secondary particles formed by agglomeration of primary particles having a size of nanoparticles or smaller, complex structural deformation occurs due to insertion and desorption of sodium ions by the primary particles even though the secondary particles have a micro-size, which causes problems such as expansion of the cathode active material, microcracks, and reduced lifespan characteristics.

[0064] In the above chemical formula 1, M is a doping element, and as the content of M increases, particles can grow in the direction of the (010) plane. In addition, as the content of M increases, the migration path of sodium ions can increase. In the present embodiment, M may include at least one of Ti and Zn, and specifically, M may include both Ti and Zn.

[0065] In this embodiment, the cathode active material for a sodium secondary battery is in the form of a single particle having only one grain boundary, and the growth direction of the grain boundary can be controlled by the addition of M. Referring to FIG. 2, the cathode active material for a sodium secondary battery includes a pair of (001) planes facing each other, and a (100) plane, a (110) plane, and a (010) plane connecting the pair of (001) planes, wherein the sodium ion (Na) + ) passes through the (110) plane and the (010) plane, thereby forming a migration path of sodium ions. As the content of M increases, the particles grow in the direction of the (010) plane, thereby expanding the migration path of sodium ions, thereby improving the efficiency of the electrochemical reaction. Therefore, in Chemical Formula 1, it is preferable that M is included in the range of 0.01 ≤ d ≤ 0.5. If d is less than 0.01, the content of M is too low, so that the effect is insignificant, and if d is more than 0.5, the specific capacity may decrease.

[0066] As described above, the cathode active material for a sodium secondary battery according to an embodiment of the present invention can control the shape of the single particles in the process of synthesizing the cathode active material in the form of single particles according to the content of M, thereby improving the sodium ion diffusivity and energy density per volume.

[0067] The cathode active material for a sodium secondary battery according to the present embodiment is provided in the form of single particles, and the single particles have an average particle size (D 50) may be 1 ㎛ to 50 ㎛. If the size of the single particle is less than 1 ㎛, a side reaction with the electrolyte and structural displacement may occur during the charge / discharge process, which may deteriorate the electrochemical characteristics of the sodium secondary battery. If the size exceeds 50 ㎛, the specific capacity may decrease and the cathode may not be densely packed during the manufacturing process, which may be problematic. The cathode active material for the sodium secondary battery is provided in the form of single particles having a micro-unit size in the aforementioned range, thereby having a high energy density per volume, and at the same time, solving the problem of micro-cracks in micro-unit particles may improve the life characteristics of the sodium secondary battery.

[0068] 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.

[0069] 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.

[0070] According to another aspect of the present invention, the present invention includes a sodium secondary battery including the above-described positive electrode active material.

[0071] [Manufacturing method_Solid-state synthesis method]

[0072] According to another aspect of the present invention, the present invention includes a method for producing the cathode active material for a sodium secondary battery as described above. An embodiment of the present invention can obtain a cathode material for a sodium secondary battery in the form of a single particle through a solid-state synthesis method.

[0073] Solid-state synthesis (SST) is a method for synthesizing a cathode active material for sodium secondary batteries with a desired composition by heat-treating a transition metal oxide and a sodium compound at high temperatures. This method is applied to manufacture a cathode active material satisfying the composition of chemical formula 1 (NaxNiaFebMncMdO2), and by controlling the optimal sintering temperature, microcracks can be prevented and life characteristics can be improved.

[0074] The method for manufacturing the positive electrode active material for the above sodium secondary battery may include a step of manufacturing a mixture by mixing a positive electrode active material precursor including a transition metal compound and a sodium compound; and a step of calcining the mixture.

[0075] 1. Precursor mixing step

[0076] The method for manufacturing the positive electrode active material for the above sodium secondary battery includes a step of manufacturing a mixture by mixing a positive electrode active material precursor including a transition metal compound and a sodium compound.

[0077] 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.

[0078] 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.

[0079] Specifically, the transition metal compound may include nickel (Ni), iron (Fe), manganese (Mn), and M elements. The M elements may be at least one selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo. The M elements may be included as one or more doping elements.

[0080] The molar ratio of nickel, iron, and manganese may be 1 to 9: 1 to 9: 1 to 9. By providing nickel, iron, and manganese in the above-mentioned range, the specific capacity can be efficiently increased, and the crystal structure of the positive electrode active material can be stably formed. Specifically, the molar ratio of nickel, iron, and manganese may be 1 to 5: 1 to 5: 1 to 5, and more specifically, 4: 2: 4.

[0081] The above-described positive electrode active material precursor and the sodium compound may be mixed at a molar ratio of the transition metal compound and the sodium compound of 1:0.6 to 1.5. Specifically, the molar ratio of the transition metal compound and the sodium may be 1:1.04 to 0.5. By maintaining the molar ratio of the transition metal compound and the sodium within the above-described range, the sodium and transition metal lost during the process of forming and firing the pellets can be supplemented, thereby maintaining the molar ratio thereof, and further, the positive electrode active material in the form of single particles can be formed with a uniform size.

[0082] The above M element is a doping element and includes at least one of Ti and Zn, and the M element can be added at 0.05 mol% to 0.4 mol%. By including the M element within the above-mentioned range, the growth direction of a single particle crystal formed by a single grain boundary can be controlled, thereby improving the mobility of sodium ions. Specifically, the M element can be at 0.05 mol% to 0.2 mol%, or 0.05 mol% to 0.15 mol%.

[0083] 2. Molding and pelletizing step (optional)

[0084] Before the above-described firing step, the mixture may further be pressurized to a first pressure to produce pellets. The first pressure may be 20 MPa to 60 MPa. After mixing the sodium compound and the transition metal compound, the powder-form mixture may be fired as is, or the powder mixture may be produced into pellets and then fired. Specifically, the cathode active material for a sodium secondary battery according to an embodiment of the present invention may be produced into pellets before firing.

[0085] When manufactured in the above pellet form and then fired, heat can be uniformly transferred during firing and the size of the manufactured single particles can be manufactured to be uniform. The pellet form can be formed by applying a first pressure. If the pressure is less than 20 MPa, the pellet form cannot be maintained and problems such as powder dispersion or pellet breakage during the firing process may occur. If the first pressure exceeds 60 MPa, the manufactured single particles of the positive electrode active material can be manufactured in a size smaller than a micron unit, which is problematic.

[0086] 3. Heat treatment stage

[0087] In the above-mentioned calcining step, the calcination may be performed at a temperature range of 800°C to 1100°C for 10 to 20 hours. If the heat treatment temperature is less than 800°C, it may be lower than the melting points of the sodium and transition metals included in the cathode active material, so unreacted metal particles may exist in the manufactured cathode active material, and a layered crystal phase may not be formed well. If it exceeds 1100°C, the sodium source may volatilize, so additional sodium source needs to be added, and the probability of residual sodium and particle agglomeration increases, and the secondary battery performance may deteriorate due to the formation of oxide and hydrate films caused by the residual sodium. In addition, if the calcination time is less than 10 hours, the calcination is not sufficiently performed, which is problematic, and if it exceeds 20 hours, agglomeration of single particles may occur.

[0088] 4. Cooling and post-processing

[0089] After heat treatment, cool slowly to prevent cracks from occurring due to rapid shrinkage. The fired sample is crushed and sieved to achieve the desired particle size (D50 = 2–5 μm).

[0090]

[0091] [Manufacturing method_Coprecipitation method]

[0092] In another embodiment of the present invention, a co-precipitation method may be used. The co-precipitation method is a method capable of synthesizing a positive electrode active material precursor with a uniform composition and fine particles. This method involves co-precipitating transition metal ions and sodium ions from a solution to form a precursor, and then heat-treating the precursor to produce a positive electrode active material having a desired composition.

[0093] In the co-precipitation method according to the embodiment of the present invention, a solution containing transition metal ions and sodium ions is prepared for precursor synthesis, and then precipitation can be induced using an appropriate pH regulator.

[0094] To this end, first prepare a raw material solution by dissolving a transition metal precursor containing a transition metal ion to produce a solution. The transition metal precursor that can be used here is sulfate (SO4 2- ), chloride (Cl - ), acetate (CH₃COO - ) can be used. (Example: NiSO₄·6H2O, FeSO₄7H2O, MnSO₄H2O, etc.) Here, the molar ratio is preferably adjusted to Ni:Fe:Mn = 4:2:4 and mixed.

[0095] Next, prepare a sodium compound solution by dissolving the sodium source. Available sodium compounds include Na₂CO₃ (sodium carbonate), NaOH (sodium hydroxide), and NaNO₃ (sodium nitrate). Mix at the target molar ratio (1:0.6–1.5) to ensure uniform precipitation of sodium ions during the coprecipitation reaction.

[0096] Next, a coprecipitation reaction is performed by mixing the reaction solution at 60-90℃ and stirring at low speed (300-800 rpm). The pH is adjusted to 8-12. A pH adjuster (NH₄OH, NaOH) is added to induce co-precipitation of the transition metal and sodium. Maintaining an appropriate pH enables uniform particle formation. The reaction time is maintained for approximately 10-24 hours, and after precipitation, impurities are removed through sufficient washing (using DI water + ethanol), and the precursor powder is secured through filtering and drying.

[0097]

[0098] By applying this co-precipitation method, sodium and transition metal ions can be uniformly distributed in the precursor stage, maintaining a uniform composition of the final cathode active material. In addition, the particle size formed within the solution can be controlled according to factors such as pH, reaction time, and temperature. Furthermore, since the precursor formed through the co-precipitation method has a fine and uniform particle size, it is possible to synthesize high-quality single particles that minimize the occurrence of microcracks during subsequent heat treatment. In addition, compared to the solid-state synthesis method, there is less sodium loss and more precise composition control.

[0099] Co-precipitation allows for the manufacture of a precursor by uniformly mixing transition metals and sodium ions. Subsequent heat treatment effectively synthesizes a cathode active material for sodium secondary batteries with the desired composition. While maintaining the same chemical composition as conventional solid-state synthesis methods, it offers advantages such as particle size control, improved crystallinity, and prevention of sodium loss, enabling the production of superior cathode active materials.

[0100]

[0101]

[0102] 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.

[0103] 1. Manufacturing of positive electrode active material

[0104] Example 1

[0105] NaNi 0.4 Fe 0.2 Mn 0.4As precursors for synthesizing O2 single crystal cathode active material, NiO (Aldrich), Fe2O3 (Aldrich), and MnO2 (Aldrich) materials satisfying the molar ratio of Ni, Fe, and Mn of 4:2:4 were mixed using a mortar at room temperature. Here, sodium raw material Na2CO3 (Aldrich) was added so that the molar ratio with the transition metals (Ni, Fe, and Mn) contained in the mortar was 1.05:1. After that, the temperature was increased from room temperature to 1000 ℃ at a heating rate of 5 ℃ / min and maintained for 12 hours to obtain Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O2 was prepared as a single particle type cathode active material and is shown in Table 1.

[0106]

[0107] Comparative Example 1

[0108] Na[Ni] was prepared in the same manner as in Example 1, except that the heat treatment temperature was 900 ℃ and the ratio of Na2CO3 and transition metal was changed to 1.03:1. 0.4 Fe 0.2 Mn 0.4 ]O2 was prepared and shown in Table 1.

[0109]

[0110] Example 2

[0111] As precursors for synthesizing cathode active materials, NiO (Aldrich), Fe2O3 (Aldrich), and MnO2 (Aldrich) materials satisfying the molar ratio of Ni, Fe, and Mn of 4:2:4 were mixed using a mortar at room temperature. Here, sodium raw material Na2CO3 (Aldrich) was added to the mortar so that the molar ratio of the transition metals (Ni, Fe, and Mn) contained in the mortar was 1.05:1 to prepare a mixture. 1.5 g of the prepared mixture was added to a 14 MM Die (model name, place of purchase) and pressurized at 40 MPa for 10 minutes to prepare a pellet. The prepared pellet was heated from room temperature to 1000 ℃ at a heating rate of 5 ℃ / min and maintained at the temperature for 12 hours to prepare Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O2 was prepared and shown in Table 1.

[0112]

[0113] Example 3

[0114] As precursors for synthesizing cathode active materials, NiO (Aldrich), Fe2O3 (Aldrich), MnO2 (Aldrich), and TiO2 (Aldrich) materials satisfying the molar ratio of Ni, Fe, Mn, and Ti of 4:2:3:1 were mixed using a mortar. Here, sodium raw material Na2CO3 (Aldrich) was added to the transition metals (Ni, Fe, Mn, Ti) contained in the mortar so that the molar ratio was 1.05:1 to prepare a mixture. 1.5 g of the prepared mixture was added to a 14 MM Die (model name, place of purchase) and pressurized at 40 MPa for 10 minutes to prepare a pellet. The prepared pellet was heated from room temperature to 1000 ℃ at a heating rate of 5 ℃ / min and maintained at the temperature for 12 hours to prepare Na[Ni 0.4 Fe 0.2 Mn 0.3 Ti 0.1 ]O2 was prepared as a single particle type cathode active material and is shown in Table 1.

[0115]

[0116] Example 4

[0117] As precursors for synthesizing cathode active materials, NiO (Aldrich), ZnO (Aldrich), Fe2O3 (Aldrich), MnO2 (Aldrich), and TiO2 (Aldrich) materials satisfying the molar ratio of Ni, Zn, Fe, Mn, and Ti of 35:5:20:30:10 were mixed using a mortar. Here, sodium raw material Na2CO3 (Aldrich) was added to the transition metals (Ni, Zn, Fe, Mn, Ti) contained in the mortar so as to have a molar ratio of 1.05:1 to prepare a mixture. 1.5 g of the prepared mixture was added to a 14 MM Die (model name, place of purchase) and pressurized at 40 MPa for 10 minutes to prepare a pellet. The prepared pellet was heated from room temperature to 1000 ℃ at a heating rate of 5 ℃ / min and maintained at the temperature for 12 hours to prepare Na[Ni 0.35 Zn 0.05 Fe 0.2 Mn 0.3 Ti 0.1 A single particle type cathode active material represented by [O2] was manufactured and shown in Table 1.

[0118]

[0119]

[0120] Classification Chemical formula Sodium: Transition metal (molar ratio) Heat treatment temperature Form before heat treatment Example 1 Na [Ni 0.4 Fe 0.2 Mn 0.4 ]O21.05:11000 ℃Powder comparison example 1Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O21.03:1900 ℃Powder Example 2Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O21.05:11000 ℃Pellet Example 3Na[Ni 0.4 Fe 0.2 Mn 0.3 Ti 0.1 ]O21.05:11000 ℃Pellet Example 4Na[Ni0.35 Zn 0.05 Fe 0.2 Mn 0.3 Ti 0.1 ]O21.05:11000 ℃ pellet

[0121]

[0122] Manufacturing of half-cells

[0123] In a glove box with an argon atmosphere, the cathode active materials manufactured according to Comparative Example 1 and Examples 1 to 4 were used, respectively, and the cathode active materials were mixed with a conductive agent (Super-P, KS-6) and a binder (Poly vinylidene fluoride) in an organic solvent (NMP (N-Methyl-2-Pyrrolidone)) at a weight ratio of 80:10:10, and then coated on an aluminum current collector and pressed to form a cathode. Sodium metal was used as a cathode, a glass filter was used as a separator, and a half-cell was manufactured using a non-aqueous electrolyte containing electrolyte NaPF6 (1 M) in a mixed organic solvent of ethylene carbonate (EC, 50 wt. %), diethyl carbonate (DEC, 50 wt. %), and fluoroethylene carbonate (FEC 5 wt. %) as an additive.

[0124]

[0125] 2. Evaluation of positive electrode active materials

[0126]

[0127] X-Ray diffraction spectrum evaluation method

[0128] The X-ray diffraction spectra of the cathode active materials of the comparative examples and examples were evaluated under the following conditions.

[0129] An XRD analyzer (Smart lab, Rigaku / Power 3kW) was used, and the X-ray source used was CuK-alpha characteristic X-ray with a wavelength of 1.541 Å. Diffraction angles were measured in the range of 10 to 90°, and the scan speed was 0.013 sec / step. The measured data were fitted with a Pseudo-Voigt function to calculate the full width at half maximum (FWHM) of each crystal plane.

[0130]

[0131] SEM measurement

[0132] Field-emission scanning electron microscopy (FESEM, Helios) was used for the cathode active materials of comparative examples and examples, and measurements were made at a distance of 8 to 9 mm from the material with an acceleration voltage of 15 kV.

[0133]

[0134]

[0135] Electrochemical properties evaluation method

[0136] For the previously manufactured half cells, the charge-discharge characteristics and coulombic efficiency were confirmed by charging them to 4.2 V at 1 C in CC-CV mode at 30 °C and discharging them to 2 V in CC-CV mode at 1 C. The 1C current density of each half cell was 150 mAh / g. In addition, the life (cycle) characteristics were evaluated at 1C current density in the voltage range of 2.0 V to 4.2 V at 45 °C.

[0137]

[0138] 3. Characteristics of positive electrode active materials

[0139]

[0140] Figure 3 shows XRD data of the positive electrode active materials of Examples 1, 3, 4 and Comparative Example 1.

[0141] Referring to FIG. 3, it can be confirmed that in the XRD data, the main peak (104) peak appears in the range of 2θ of 40° to 45°, and three peaks appear in the range of 2θ of 30° to 40°. At this time, the intensity of the (104) peak was greater than the intensity of each of the three peaks appearing in the range of 2θ of 30° to 40°. That is, it was confirmed that Examples 1, 3, and 4, which are positive electrode active materials manufactured in the form of single particles according to the present embodiment, exhibited the same crystal structure as Comparative Example 1, which had a different molar ratio of sodium and transition metal.

[0142]

[0143] FIG. 4 is an SEM image of the cathode active materials of Comparative Example 1 and Example 1. Table 2 below shows the average particle size, particle strength, and specific surface area for the cathode active materials of Examples 1 to 4 and Comparative Example 1. The average particle size was measured using SEM images, and is the average value obtained by producing a total of 10 samples for each cathode active material and measuring 10 at various locations for each sample. The particle strength was converted to a pressure unit (MPa) by measuring the point at which cracks occurred in the particles while gradually increasing the pressure for each sample. The specific surface area was measured by the BET method, and was calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77 K) using BELSORP-mini II from BEL Japan.

[0144]

[0145] Classification Chemical formula Average particle size (D) 50 ) Example 1 Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O23 ㎛Comparative example 1Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O21 ㎛Example 2Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O23 ㎛Example 3Na[Ni 0.4 Fe0.2 Mn 0.3 Ti 0.1 ]O23.2 ㎛Example 4Na[Ni 0.35 Zn 0.05 Fe 0.2 Mn 0.3 Ti 0.1 ]O23,1 ㎛

[0146] Referring to Table 2, Examples 1 to 4 of the present invention were in the form of single particles with an average particle diameter exceeding 1 ㎛, and the size of the single particles was also uniform. On the other hand, Comparative Example 1 was in the form of small particles less than 1 ㎛, and the shape of the particles was also in the form of small particles being aggregated. Therefore, in the case of Comparative Example 1, the particle strength was lower and the specific surface area was higher than in Examples 1 to 4. In the case of Comparative Example 1, problems such as particle collapse or breakage occurred during the rolling process of manufacturing the positive electrode due to the low particle strength. In addition, since the surface area was large, the side reaction with the electrolyte was also relatively high.

[0147]

[0148] Fig. 5 is XRD data of the positive electrode active materials of Examples 1 and 2. Fig. 6 is an SEM image of the positive electrode active materials of Examples 1 and 2. Fig. 7 is an SEM image of the positive electrode active material of Example 3.

[0149] Referring to Figure 5, when comparing Example 1, a cathode active material manufactured by heat treatment in a powder state, and Example 2, a cathode active material manufactured by heat treatment in a pellet form, it was confirmed that there was no difference in crystal structure due to the difference in manufacturing method when the molar ratio of sodium and transition metal and the heat treatment temperature were the same.

[0150] Referring to FIGS. 6 and 7, it was confirmed that Example 2 had a larger and more uniform particle size compared to Example 1. That is, it was confirmed that when manufacturing using a pellet form rather than a powder form, the particle size of the cathode active material in the form of a single particle was provided larger and more uniformly, thereby improving the performance of the secondary battery. Specifically, the cathode active materials according to Examples 1 to 4 had an average particle diameter of 2 ㎛ and a single grain boundary. On the other hand, the particles synthesized through Comparative Example 1 had an average diameter of 1 ㎛ or less and multiple grain boundaries.

[0151]

[0152] Figure 8 is an SEM image of the positive electrode active material of Examples 2 and 3.

[0153] Referring to Fig. 8, in the case of Example 3, it was confirmed that the Ti doping element was further included, and when Ti was doped, the (010) plane crystal grew further (see Fig. 2). In Fig. 8, in the case of Examples 2 and 3 of the present invention, the average particle size was 1.0 ㎛ or more in both cases, and in the particle thickness of the (010) plane, Example 2 was 0.7 to 0.8 ㎛, and Example 3 was 1.1 to 1.2 ㎛. Specifically, sodium ions (Na) in the (010) plane direction + ) moves, and the movement path of sodium ions increases due to the growth of the (010) crystal. As a result, the electrochemical characteristics in the secondary battery are further improved.

[0154]

[0155] Fig. 9 is a cross-sectional SEM image of the positive electrode active material of Example 3. Fig. 10 is a SEM image of the positive electrode active material of Example 4. Fig. 11 is a cross-sectional SEM image of the positive electrode active material of Example 4.

[0156] Referring to FIGS. 9 to 11, cross-sectional SEM images of the positive electrodes manufactured using Example 3 (Ti doping) and Example 4 (Ti, Zn doping) were confirmed, and as a result, the positive electrode active materials were in the form of single particles and no separate grain boundaries were observed in each, and as a result, it was confirmed that the positive electrode active materials according to Examples 3 and 4 were formed of a single grain boundary. In addition, it was confirmed that the single particles of the positive electrode active materials were formed with a high packing density without any breakage. The positive electrode active material according to this example is formed in the form of single particles with a single grain boundary having high particle strength, thereby preventing side reactions with the electrolyte in a secondary battery and improving performance.

[0157]

[0158] Figure 12 shows electrochemical characteristic data of Examples 1 and 2 and Comparative Example 1. Figure 13 shows electrochemical characteristic data of Examples 3 and 4. Table 3 shows the results of confirming the capacity for each current density.

[0159]

[0160] Classification Chemical formula 0.1C Cap (mAh / g) 0.2C Cap (mAh / g) 0.5C Cap (mAh / g) 1C Cap (mAh / g) Example 1 Na [Ni 0.4 Fe 0.2 Mn 0.4 ]O2168161152138Comparative Example 1Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O2171164157150Example 2Na[Ni 0.4 Fe 0.2 Mn 0.4 ]O2174166159153Example 3Na[Ni 0.4 Fe 0.2 Mn 0.3 Ti 0.1 ]O2180171164152Example 4Na[Ni 0.35 Zn 0.05 Fe 0.2 Mn 0.3 Ti 0.1 ]O2167164153149

[0161] In FIGS. 12 and 13, the charge / discharge profiles were confirmed while changing the current density from 0.1 C -> 0.2 C -> 0.5 C -> 1 C. Example 2 showed a stable capacity even when the current density increased compared to Example 1 and Comparative Example 1. In addition, it was confirmed that Examples 3 and 4 showed high discharge capacities without being significantly affected by the current density compared to Example 2. In addition, Examples 3 and 4 showed stable discharge profiles compared to Examples 1 and 2. That is, in the cathode active material according to this example, the electrochemical stability of the cathode active material was improved by manufacturing it as a pellet and adding a doping element.

[0162]

[0163] Figure 14 shows the life characteristic data of Examples 1 to 4 and Comparative Example 1.

[0164] In Fig. 14, the reversible capacity and cycle life characteristics were evaluated at a current density of 1C and within an operating voltage range of 2.0 V to 4.2 V using the positive electrode active materials according to Examples 1 to 4 and Comparative Example 1. The positive electrode active material according to Example 1 in the form of a single crystal showed better cycle characteristics than that of Comparative Example 1 in the form of a polycrystal. In the case of Example 2, the positive electrode active material manufactured into a pellet and then heat-treated showed better cycle characteristics than that of Example 1 in the form of a powder. That is, although both Examples 1 and 2 were provided in the form of single particles, Example 2 in the form of a pellet showed better cycle characteristics. Examples 3 and 4, which further include a doping element, showed better cycle characteristics than Example 2, and compared to Example 3, which was doped only with patented Ti, when doped with a heterogeneous element such as Ti and Zn, showed better cycle characteristics.

[0165]

[0166] 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. A cathode active material for a sodium secondary battery, represented by the following chemical formula 1 and composed of single particles: (Chemical formula 1) So x Ni a Feb b Mr c M d O2 In the above chemical formula 1, M is one or more elements selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo. 0.6 ≤ x ≤ 1, 0.01 ≤ a ≤ 0.9, 0.01 ≤ b ≤ 0.9, 0.01 ≤ c < 0.9, 0 ≤ d ≤ 0.

5.

2. In paragraph 1, The above single particles have an average particle size (D 50 ) A cathode active material for a sodium secondary battery having a particle size of 1 ㎛ to 50 ㎛.

3. 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°.

4. In paragraph 1, In the above chemical formula 1, M is a doping element, and as the content of M increases, particles grow in the direction of the (010) plane, thereby increasing the movement path of sodium ions. A positive electrode active material for a sodium secondary battery.

5. In paragraph 1, In the above chemical formula 1, M is a positive electrode active material for a sodium secondary battery containing at least one of Ti and Zn.

6. 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.

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

8. A step of preparing a mixture by mixing a positive electrode active material precursor including a transition metal compound and a sodium compound; and A step of calcining the above mixture; including, 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 according to any one of claims 1 to 7, wherein the 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.

9. In paragraph 8, The above cathode active material precursor and sodium compound, A method for producing a cathode active material for a sodium secondary battery, wherein the above transition metal compound and the sodium compound are mixed at a molar ratio of 1:0.6 to 1.

5.

10. In paragraph 8, The above transition metal compound is, Contains nickel (Ni), iron (Fe), manganese (Mn) and M elements, A method for manufacturing a positive electrode active material for a sodium secondary battery, wherein the above M element is at least one selected from Co, Cr, Zr, Nb, Cu, V, Ti, Zn, Al, Ga, Mg, B, Zn, Sn, Sb, Hf, Ce, Bi, and Mo.

11. In paragraph 10, A method for manufacturing a cathode active material for a sodium secondary battery, wherein the molar ratio of nickel, iron, and manganese is 1 to 9: 1 to 9: 1 to 9.

12. In paragraph 10, The above M element contains at least one of Ti and Zn as a doping element, A method for producing a positive electrode active material for a sodium secondary battery, wherein the above M element is added in an amount of 0.05 mol% to 0.4 mol%.

13. In paragraph 8, A method for producing a cathode active material for a sodium secondary battery, further comprising, before the above-mentioned calcining step, pressurizing the mixture at a first pressure to produce it in the form of a pellet.

14. In paragraph 13, A method for producing a positive electrode active material for a sodium secondary battery, wherein the first pressure is 20 MPa to 60 MPa.

15. In paragraph 8, In the above firing step, A method for producing a positive electrode active material for a sodium secondary battery, wherein the above-mentioned calcination is performed at a temperature range of 800°C to 1100°C for 10 to 20 hours.

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